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+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #67282 (https://www.gutenberg.org/ebooks/67282)
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-The Project Gutenberg eBook of The Principles of Biology, Volume 2
-(of 2), by Herbert Spencer
-
-This eBook is for the use of anyone anywhere in the United States and
-most other parts of the world at no cost and with almost no restrictions
-whatsoever. You may copy it, give it away or re-use it under the terms
-of the Project Gutenberg License included with this eBook or online at
-www.gutenberg.org. If you are not located in the United States, you
-will have to check the laws of the country where you are located before
-using this eBook.
-
-Title: The Principles of Biology, Volume 2 (of 2)
-
-Author: Herbert Spencer
-
-Release Date: January 30, 2022 [eBook #67282]
-
-Language: English
-
-Produced by: Karin Spence, Tim Lindell, Adrian Mastronardi, Josep Cols
-             Canals and the Online Distributed Proofreading Team at
-             https://www.pgdp.net (This file was produced from images
-             generously made available by The Internet Archive/American
-             Libraries.)
-
-*** START OF THE PROJECT GUTENBERG EBOOK THE PRINCIPLES OF BIOLOGY,
-VOLUME 2 (OF 2) ***
-
-
-
-
-
-                           THE PRINCIPLES OF
-                                BIOLOGY
-
-
-                                  BY
-                            HERBERT SPENCER
-
-
-                           _IN TWO VOLUMES_
-                                VOL. II
-
-                     REVISED AND ENLARGED EDITION
-                                 1899
-
-                               NEW YORK
-                        D. APPLETON AND COMPANY
-                                 1900
-
-
-
-
-                        COPYRIGHT, 1867, 1899,
-                      BY D. APPLETON AND COMPANY.
-
-
-
-
-                                PREFACE
-
-            TO THE REVISED AND ENLARGED EDITION OF VOL. II.
-
-
-To the statements made in the preface to the first volume of this
-revised edition, there must here be added a few having special
-reference to this second volume.
-
-One of them is that the revision has not been carried out in quite the
-same way, but in a way somewhat less complete. When reviewing the first
-volume a friendly critic, Prof. Lloyd Morgan, said:--
-
-   “But though the intellectual weight has also been augmented,
-   it is an open question whether it would not have been wiser
-   to leave intact a treatise, &c... relegating corrections and
-   additions to notes and appendices.”
-
-I think that Prof. Morgan is right. Though at the close of the preface
-to volume I, I wrote:--“in all sections not marked as new, the
-essential ideas set forth are the same as they were in the original
-edition of 1864,” yet the reader who has not read this statement,
-or does not bear it in mind, will suppose that all or most of the
-enunciated conceptions are of recent date, whereas only a small part of
-them are. I have therefore decided to follow, in this second volume, a
-course somewhat like that suggested by Prof. Morgan--somewhat like, I
-say, because in sundry cases the amendments could not be satisfactorily
-made by appended notes.
-
-But there has been a further reason for this change of method. An
-invalid who is nearly eighty cannot with prudence enter upon work which
-will take long to complete. Hence I have thought it better to make the
-needful alterations and additions in ways requiring relatively moderate
-time and labour.
-
-The additions made to this volume are less numerous and less important
-than those made to the first volume. A new chapter ending Part V, on
-“The Integration of the Organic World,” serves to round off the general
-theory of Evolution in its application to living things. Beyond a new
-section (§ 289_a_) and the various foot-notes, serving chiefly the
-purpose of elucidation, there are notes of some significance appended
-to Chapters I, III, IV, and V, in Part IV, Chapters V and VIII, in Part
-V, and Chapters IX, X, and XII in Part VI. Moreover there are three
-further appendices, D^2, F, and G, which have, I think, considerable
-significance as serving to make clearer some of the views expressed in
-the body of the work.
-
-Turning from the additions to the revisions, I have to say that the
-aid needed for bringing up to date the contents of this volume, has
-been given me by the gentlemen who gave me like aid in revising the
-first volume: omitting Prof. Perkin, within whose province none of the
-contents of this volume fall. Plant-Morphology and Plant-Physiology
-have been overseen by Mr. A. G. Tansley. Criticisms upon parts dealing
-with Animal Morphology I owe to Mr. J. T. Cunningham and Prof. E. W.
-MacBride. And the statements included under Animal Physiology have been
-checked by Mr. W. B. Hardy.
-
-For reasons like those named in the preface to the first volume, I
-have not submitted the proofs of this revised second volume to these
-gentlemen: a fact which it is needful to name, since one or other of
-them might else be held responsible for some error which is not his but
-mine. It is the more requisite to say this because while, in respect
-of matters of fact, I have, save in one or two cases, accepted their
-corrections as not to be questioned, I have not always done this in
-respect of matters of inference, but in sundry places have adhered to
-my own interpretations.
-
-Perhaps I may be excused for expressing some satisfaction that I
-have not been obliged to relinquish the views set forth in 1864–7.
-The hypothesis of physiological units--or, as I would now call them,
-constitutional units--has been adopted by several zoologists under
-modified forms. So far as I am aware, the alleged general law of
-organic symmetry has not called forth any manifestations of dissent.
-The suggested theory of vertebrate structure appears to have become
-current; and from the investigations of the late Prof. Cope, has
-received verification. The conclusions drawn in Part VI on “The Laws
-of Multiplication,” have not, I believe, been controverted. And though
-only some works on botany have given currency to the doctrine set forth
-in Appendix C, “On Circulation and the Formation of Wood in Plants,”
-yet I have met with no attempt to disprove it. The only views contested
-by certain of the gentlemen above named, are those concerning the
-origin of the two great phænogamic types of plants, and the origin of
-the annulose type of animals. I have not, however,--perhaps because
-of natural bias--found myself compelled to surrender these views. My
-reasons for adhering to them will be found in notes to the ends of
-Chapters III and IV in Part IV, and in Appendix D^2.
-
-On now finally leaving biological studies, it remains only to say that
-I am glad I have survived long enough to give this work its finished
-form.
-
-    BRIGHTON,
-        _October, 1899_.
-
-
-
-
-                          PREFACE TO VOL. II.
-
-
-The proof sheets of this volume, like those of the last volume, have
-been looked through by Dr. Hooker and Prof. Huxley; and I have, as
-before, to thank them for their valuable criticisms, and for the
-trouble they have taken in checking the numerous statements of fact on
-which the arguments proceed. The consciousness that their many duties
-render time extremely precious to them, makes me feel how heavy is my
-obligation.
-
-Part IV., with which this volume commences, contains numerous figures.
-Nearly one half of them are repetitions, mostly altered in scale and
-simplified in execution, of figures, or parts of figures, contained
-in the works of various Botanists and Zoologists. Among the authors
-whom I have laid under contribution, I may name Berkeley, Carpenter,
-Cuvier, Green, Harvey, Hooker, Huxley, Milne-Edwards, Ralfs, Smith.
-The remaining figures, numbering 150, are from original sketches and
-diagrams.
-
-The successive instalments which compose this volume, were issued
-to the Subscribers at the following dates:--No. 13 (pp. 1–80) in
-January, 1865; No. 14 (pp. 81–160) in June, 1865; No. 15 (pp. 161–240)
-in December, 1865; No. 16 (pp. 241–320) in June, 1866; No. 17 (pp.
-321–400) in November, 1866; and No. 18 (pp. 401–566) in March, 1867.
-
-   LONDON, _March 23rd, 1867_.
-
-
-
-
-                         CONTENTS OF VOL. II.
-
-
-                 PART IV.--MORPHOLOGICAL DEVELOPMENT.
-
-    CHAP.                                                          PAGE
-
-       I.--THE PROBLEMS OF MORPHOLOGY                                 3
-
-      II.--THE MORPHOLOGICAL COMPOSITION OF PLANTS                   17
-
-     III.--THE MORPHOLOGICAL COMPOSITION OF PLANTS--_Continued_      37
-
-      IV.--THE MORPHOLOGICAL COMPOSITION OF ANIMALS                  85
-
-       V.--THE MORPHOLOGICAL COMPOSITION OF ANIMALS--_Continued_    111
-
-      VI.--MORPHOLOGICAL DIFFERENTIATION IN PLANTS                  128
-
-     VII.--THE GENERAL SHAPES OF PLANTS                             134
-
-    VIII.--THE SHAPES OF BRANCHES                                   145
-
-      IX.--THE SHAPES OF LEAVES                                     152
-
-       X.--THE SHAPES OF FLOWERS                                    161
-
-      XI.--THE SHAPES OF VEGETAL CELLS                              175
-
-     XII.--CHANGES OF SHAPE OTHERWISE CAUSED                        178
-
-    XIII.--MORPHOLOGICAL DIFFERENTIATION IN ANIMALS                 183
-
-     XIV.--THE GENERAL SHAPES OF ANIMALS                            186
-
-      XV.--THE SHAPES OF VERTEBRATE SKELETONS                       209
-
-     XVI.--THE SHAPES OF ANIMAL CELLS                               228
-
-    XVII.--SUMMARY OF MORPHOLOGICAL DEVELOPMENT                     231
-
-
-                  PART V.--PHYSIOLOGICAL DEVELOPMENT.
-
-       I.--THE PROBLEMS OF PHYSIOLOGY                               239
-
-      II.--DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES
-             OF PLANTS                                              244
-
-     III.--DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS       251
-
-      IV.--DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS       272
-
-       V.--PHYSIOLOGICAL INTEGRATION IN PLANTS                      292
-
-      VI.--DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES
-             OF ANIMALS                                             299
-
-     VII.--DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS      309
-
-    VIII.--DIFFERENTIATIONS AMONG THE INNER TISSUES OF ANIMALS      323
-
-      IX.--PHYSIOLOGICAL INTEGRATION IN ANIMALS                     373
-
-       X.--SUMMARY OF PHYSIOLOGICAL DEVELOPMENT                     384
-
-     X^A.--THE INTEGRATION OF THE ORGANIC WORLD                     396
-
-
-                   PART VI.--LAWS OF MULTIPLICATION.
-
-       I.--THE FACTORS                                              411
-
-      II.--_À PRIORI_ PRINCIPLE                                     417
-
-     III.--OBVERSE _À PRIORI_ PRINCIPLE                             424
-
-      IV.--DIFFICULTIES OF INDUCTIVE VERIFICATION                   432
-
-       V.--ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS            439
-
-      VI.--ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS             448
-
-     VII.--THE ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS,
-             ASEXUAL AND SEXUAL                                     461
-
-    VIII.--ANTAGONISM BETWEEN EXPENDITURE AND GENESIS               467
-
-      IX.--COINCIDENCE BETWEEN HIGH NUTRITION AND GENESIS           475
-
-       X.--SPECIALITIES OF THESE RELATIONS                          486
-
-      XI.--INTERPRETATION AND QUALIFICATION                         497
-
-     XII.--MULTIPLICATION OF THE HUMAN RACE                         506
-
-    XIII.--HUMAN POPULATION IN THE FUTURE                           522
-
-
-                              APPENDICES.
-
-    A.--SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS           541
-
-    B.--A CRITICISM ON PROF. OWEN’S THEORY OF THE VERTEBRATE
-          SKELETON                                                  548
-
-    C.--ON CIRCULATION AND THE FORMATION OF WOOD IN PLANTS          567
-
-    D.--ON THE ORIGIN OF THE VERTEBRATE TYPE                        599
-
-  D^2.--THE ANNULOSE TYPE                                           602
-
-    E.--THE SHAPES AND ARRANGEMENTS OF FLOWERS                      608
-
-    F.--PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS                     612
-
-    G.--THE INHERITANCE OF FUNCTIONALLY-CAUSED MODIFICATIONS        618
-
-
-
-
-                               PART IV.
-
-                      MORPHOLOGICAL DEVELOPMENT.
-
-
-
-
-                              CHAPTER I.
-
-                      THE PROBLEMS OF MORPHOLOGY.
-
-
-§ 175. The division of Morphology from Physiology, is one which may
-be tolerably-well preserved so long as we do not carry our inquiries
-beyond the empirical generalizations of their respective phenomena; but
-it is one which becomes in great measure nominal, when the phenomena
-are to be rationally interpreted. It would be possible, after analyzing
-our Solar System, to set down certain general truths respecting the
-sizes and distances of its primary and secondary members, omitting
-all mention of their motions; and it would be possible to set down
-certain other general truths respecting their motions, without
-specifying their dimensions or positions, further than as greater
-or less, nearer or more remote. But on seeking to account for these
-general truths, arrived at by induction, we find ourselves obliged to
-consider simultaneously the relative sizes and places of the masses,
-and the relative amounts and directions of their motions. Similarly
-with organisms. Though we may frame sundry comprehensive propositions
-respecting the arrangements of their organs, considered as so many
-inert parts; and though we may establish several wide conclusions
-respecting the separate and combined actions of their organs, without
-knowing anything definite respecting the forms and positions of these
-organs; yet we cannot reach such a rationale of the facts as the
-hypothesis of Evolution aims at, without contemplating structures and
-functions in their mutual relations. Everywhere structures in great
-measure determine functions; and everywhere functions are incessantly
-modifying structures. In Nature the two are inseparable co-operators;
-and Science can give no true interpretation of Nature without keeping
-their co-operation constantly in view. An account of organic evolution,
-in its more special aspects, must be essentially an account of the
-interactions of structures and functions, as perpetually altered by
-changes of conditions.
-
-Hence, when treating apart Morphological Development and Physiological
-Development, all we can do is to direct our attention mainly to the
-one or to the other, as the case may be. In dealing with the facts of
-structure, we must consider the facts of function only in such general
-way as is needful to explain the facts of structure; and conversely
-when dealing with the facts of function.
-
-
-§ 176. The problems of Morphology fall into two distinct classes,
-answering respectively to the two leading aspects of Evolution. In
-things which evolve there go on two processes--increase of mass
-and increase of structure. Increase of mass is primary, and in
-simple evolution takes place almost alone. Increase of structure is
-secondary, accompanying or following increase of mass with more or
-less regularity, wherever evolution rises above that form which small
-inorganic bodies, such as crystals, present to us. As the fundamental
-antagonism between Dissolution and Evolution consists in this, that
-while the one is an integration of motion and disintegration of matter,
-the other is an integration of matter and disintegration of motion; and
-as this integration of matter accompanying disintegration of motion,
-is a necessary antecedent to the differentiation of the matter so
-integrated; it follows that questions concerning the mode in which the
-parts are united into a whole, must be dealt with before questions
-concerning the mode in which these parts become modified.[1]
-
-This is not obviously a morphological question. But an illustration or
-two will make it manifest that fundamental differences may be produced
-between aggregates by differences in the degrees of composition of the
-increments: the ultimate units of the increments being the same. Thus
-an accumulation of things of a given kind may be made by adding one
-at a time. Or the things may be tied up into bundles of ten, and the
-tens placed together. Or the tens may be united into hundreds, and a
-pile of hundreds formed. Such unlikenesses in the structures of masses
-are habitually seen in our mercantile transactions. Articles which
-the consumer recognizes as single, the retailer keeps wrapped up in
-dozens, the wholesaler sends in gross, and the manufacturer supplies
-in packages of a hundred gross. That is, they severally increase
-their stocks by units of simple, of compound, and of doubly-compound
-kinds. Similarly result those differences of morphological composition
-which we have first to consider. An organism consists of units.
-These units may be aggregated into a mass by the addition of unit
-to unit. Or they may be united into groups, and the groups joined
-together. Or these groups of groups may be so combined as to form a
-doubly-compound aggregate. Hence there arises respecting each organic
-form the question--is its composition of the first, second, third,
-or fourth order?--does it exhibit units of a singly-compounded kind
-only, or are these consolidated into units of a doubly-compounded
-kind, or a triply-compounded kind? And if it displays double or triple
-composition, the homologies of its different parts become problems.
-Under the disguises induced by the consolidation of primary, secondary,
-and tertiary units, it has to be ascertained which answer to which, in
-their degrees of composition.
-
-Such questions are more intricate than they at first appear; since,
-besides the obscurities caused by progressive integration, and those
-due to accompanying modifications of form, further obscurities
-result from the variable growths of units of the different orders.
-Just as an army may be augmented by recruiting each company, without
-increasing the number of companies; or may be augmented by making up
-the full complement of companies in each regiment, while the number
-of regiments remains the same; or may be augmented by putting more
-regiments into each division, other things being unchanged; or may be
-augmented by adding to the number of its divisions without altering
-the components of each division; or may be augmented by two or three
-of these processes at once; so, in organisms, increase of mass may
-result from additions of units of the first order, or those of the
-second order, or those of still higher orders; or it may be due to
-simultaneous additions to units of several orders. And this last mode
-of integration being the general mode, puts difficulties in the way of
-analysis. Just as the structure of an army would be made less easy to
-understand if companies often outgrew regiments, or regiments became
-larger than brigades; so these questions of morphological composition
-are complicated by the indeterminate sizes of the units of each
-kind: relatively-simple units frequently becoming more bulky than
-relatively-compound units.
-
-
-§ 177. The morphological problems of the second class are those having
-for their subject-matter the changes of shape which accompany changes
-of aggregation. The most general questions respecting the structure
-of an organism, having been answered when it is ascertained of what
-units it is composed as a whole, and in its several parts; there come
-the more special questions concerning its form--form in the ordinary
-sense. After the contrasts caused by variations in the process of
-integration, we have to consider the contrasts caused by variations
-in the process of differentiation. To speak specifically--the shape
-of the organism as a whole, irrespective of its composition, has to
-be accounted for. Reasons have to be found for the unlikeness between
-its general outlines and the general outlines of allied organisms. And
-there have to be answered kindred inquiries respecting the proportions
-of its component parts:--Why, among such of these as are homologous
-with one another, have there arisen the differences that exist? And how
-have there been produced the contrasts between them and the homologous
-parts of organisms of the same type?
-
-Very numerous are the heterogeneities of form presenting themselves
-for interpretation under these heads. The ultimate morphological
-units combined in any group, may be differentiated individually, or
-collectively, or both: each of them may undergo changes of shape;
-or some of them may be changed and others not; or the group may be
-rendered multiform by the greater growth of some of its units than of
-others. Similarly with the compound units arising by union of these
-simple units. Aggregates of the second order may be made relatively
-complex in form, by inequalities in the rates of multiplication of
-their component units in diverse directions; and among a number of such
-aggregates, numerous unlikenesses may be constituted by differences in
-their degrees of growth, and by differences in their modes of growth.
-Manifestly, at each higher stage of composition the possible sources of
-divergence are multiplied still further.
-
-That facts of this order can be accounted for in detail is not to
-be expected--the data are wanting. All that we may hope to do is to
-ascertain their general laws. How this is to be attempted we will now
-consider.
-
-
-§ 178. The task before us is to trace throughout these phenomena
-the process of evolution; and to show how, as displayed in them, it
-conforms to those first principles which evolution in general conforms
-to. Two sets of factors have to be taken into account. Let us look at
-them.
-
-The factors of the first class are those which tend directly to
-change an organic aggregate, in common with every other aggregate,
-from that more simple form which is not in equilibrium with incident
-forces, to that more complex form which is in equilibrium with them.
-We have to mark how, in correspondence with the universal law that
-the uniform lapses into the multiform, and the less multiform into
-the more multiform, the parts of each organism are ever becoming
-further differentiated; and we have to trace the varying relations
-to incident forces by which further differentiations are entailed.
-We have to observe, too, how each primary modification of structure,
-induced by an altered distribution of forces, becomes a parent of
-secondary modifications--how, through the necessary multiplication of
-effects, change of form in one part brings about changes of form in
-other parts. And then we have also to note the metamorphoses constantly
-being induced by the process of segregation--by the gradual union
-of like parts exposed to like forces, and the gradual separation of
-like parts exposed to unlike forces. The factors of the second class
-which we have to keep in view throughout our interpretations, are
-the formative tendencies of organisms themselves--the proclivities
-inherited by them from antecedent organisms, and which past processes
-of evolution have bequeathed. We have seen it to be inferable from
-various orders of facts (§§ 65, 84, 97–97_g_), that organisms are
-built up of certain highly-complex molecules, which we distinguished as
-physiological units [or constitutional units as they might otherwise
-be called]--each kind of organism being built up of units peculiar to
-itself. We recognized in these units, powers of arranging themselves
-into the forms of the organisms to which they belong; analogous
-to the powers which the molecules of inorganic substances have of
-aggregating into specific crystalline forms. We have consequently
-to regard this proclivity of the physiological units, as producing,
-during the development of any organism, a combination of internal
-forces that expend themselves in working out a structure in equilibrium
-with the forces to which ancestral organisms were exposed; but not in
-equilibrium with the forces to which the existing organism is exposed,
-if the environment has been changed. Hence the problem in all cases is,
-to ascertain the resultant of internal organizing forces, tending to
-reproduce the ancestral form, and external modifying forces, tending
-to cause deviations from that form. Moreover, we have to take into
-account, not only the characters of immediately-preceding ancestors,
-but also those of their ancestors, and ancestors of all degrees of
-remoteness. Setting out with rudimentary types, we have to consider
-how, in each successive stage of evolution, the structures acquired
-during previous stages have been obscured by further integrations
-and further differentiations; or, conversely, how the lineaments of
-primitive organisms have all along continued to manifest themselves
-under the superposed modifications.
-
-
-§179. Two ways of carrying on the inquiry suggest themselves. We may
-go through the several great groups of organisms, with the view of
-reaching, by comparison of parts, certain general truths respecting
-the homologies, the forms, and the relations of their parts; and then,
-having dealt with the phenomena inductively, may retrace our steps with
-the view of deductively interpreting the general truths reached. Or,
-instead of thus separating the two investigations, we may carry them on
-hand in hand--first establishing each general truth empirically, and
-then proceeding to the rationale of it. This last method will, I think,
-conduce to both brevity and clearness. Let us now thus deal with the
-first class of morphological problems.
-
-
-[NOTE.--In preparation for treating of morphological development,
-sundry other general considerations should have been included in the
-foregoing chapter when originally published. This seems the most
-appropriate place for now naming them. Some were implicitly contained
-in the first volume, but it will be well definitely to state these, as
-well as the others not yet implied.
-
-Interpretation of the forms of organisms and the forms of their parts,
-must depend mainly on the conclusions previously drawn respecting
-their phylogeny; and the drawing of such conclusions must be guided
-by recognition of the various factors of Evolution, as well as by
-recognition of certain extremely general results of Evolution and
-certain concomitants of Evolution.
-
-A primary one among these is that no existing species can exhibit
-more than approximately the ancestral structure of any other existing
-species. As all ancestors have disappeared, so, in a greater or less
-degree, the traits, specific, generic, or ordinal, which distinguished
-the earlier of them have disappeared. Setting out with the familiar
-symbol, a tree, let us regard its peripheral twigs as representing
-extant species; let us assume that the interior of the tree is filled
-up with some supporting substance, leaving only the ends of the
-living twigs projecting; and let us suppose the trunk, main branches,
-secondary branches, tertiary branches, &c., have decayed away.
-Then if we take these decayed parts to stand for the divergent and
-re-divergent lines of evolution which are represented by fossils in the
-Earth’s crust, it will be manifest, first, that no one of the living
-superficial twigs (or species) exhibits the ancestral organization
-whence any other of the living superficial twigs (or species) has been
-developed; it will be manifest, second, that the generic structure
-inherited by any existing species must be a structure out of which
-came sundry allied species--the fork, as it were, at which adjacent
-twigs diverged; and third, that the ancestor of an order must, in like
-manner, be sought at some point deeper down in the symbolic tree--a
-place of divergence of the sub-branches representing allied genera.
-Similarly with the ancestral types of classes, still deeper down in
-the tree or further back in time. So that phylogeny becomes more and
-more speculative as its questions become more and more radical. And
-the difficulty is made greater by the deficiency of palæontological
-evidence.
-
-One obvious corollary is that an ancestral type from which sundry
-allied types now existing diverged, was, speaking generally, simpler
-than these; since the divergent types became different by the
-superposing of modifications, adding to their complexities. There is
-a further reason for inferring that the least specialized member of
-any group is more like the remote ancestor than any of the others; for
-every adaptation stands in the way of subsequent re-adaptations: it
-presents a greater amount of structure to be undone. To get some idea
-of the ancestral type where no extant member of the group is manifestly
-simpler than the rest, the method must be to take all its extant
-members and, after letting their differences mutually cancel, observe
-what remains common to them all.
-
-But there are difficulties standing in the way of phylogeny, and
-consequently of morphology, much greater than these. Returning to
-our symbolic tree, it is clear that it would be far from easy to say
-of any one twig which extinct sub-branch, branch, and main branch it
-belonged to, even supposing that the growths of all parts had been
-uniformly outwards. Immensely more perplexing, then, must be the
-affiliation if various of the branches, sub-branches, &c., have sent
-out backward-growing shoots which have come to the surface only after
-prolonged retrograde courses, and if other branches have sent shoots
-into regions occupied by alien branches--shoots bearing twigs which
-come to the surface along with those to which they are but remotely
-allied. The problems of origin and of structure which organisms
-present, are met by both of the difficulties thus symbolized.
-
-One of them arises from the prevalence of retrograde metamorphoses.
-Throughout the animal world these are variously displayed by parasites,
-multitudinous in their kinds; for most of them belong to types
-much higher in organization. Changed habits and consequent changed
-structures have so transferred them that only by study of their
-embryonic stages can their kinships be made out. And these retrograde
-metamorphoses, conspicuous among parasites, have, in the course of
-evolution, affected some members of all groups; for in all groups the
-struggle for existence has compelled some to adopt careers less trying
-but less profitable.
-
-Not only by forcing on many kinds of organisms simpler ways of living,
-and consequent degeneracy, has the universal competition caused
-obscuring transformations. It has done this also by tempting many other
-kinds of organisms to adopt ways of life not simpler than before but
-merely different. Pressure continually prompts every type to intrude on
-other types’ spheres of activity; and so causes it to assume certain
-structural characters of the types whose spheres it invades, masking
-its previous characters. Modifications hence arising have, in the great
-mass of cases, been superposed one on another time after time. The
-aquatic animal becomes through several transitions a land-animal, and
-then the land-animal through other transitions becomes now an aërial
-animal like the bat and now an aquatic animal like the whale. Certain
-kinds of birds furnish extreme illustrations. There was the change from
-the fish to the water-breathing amphibian and then to the air-breathing
-amphibian; thence to the reptile living on the Earth’s surface; thence
-to the flying reptile and the bird; then came the diving birds, joining
-with their aërial life a life passed partly in the water; and finally
-came a type like the penguin, in which the power of flight has been
-lost and the water has again become the almost exclusive medium, except
-for breathing. Of course the mouldings and re-mouldings of structure
-resulting from these successive unlike modes of life, in many cases put
-great difficulties in the way of ascertaining which are the original
-corresponding parts. Some parts have become abnormally large; others
-have dwindled or disappeared; and the relative positions of parts
-have often been greatly changed. A bat’s wing and a bird’s wing are
-analogous organs, but their frameworks are but partially homologous.
-While in the bird the terminal parts of the fore-limb do little towards
-supporting the wing, in the bat the wing is mainly supported by
-enormously-developed terminal parts.
-
-The effects of the struggle to survive, which here prompts a simpler
-life with resulting degeneracy and there a different life with
-resulting new developments, are far from being the only causes of
-morphological obscurations. Fulfilment of certain highly general
-requirements gives certain common traits to plants of widely divergent
-classes; and fulfilment of certain other highly general requirements
-gives certain common traits to animals of widely divergent classes.
-It was remarked in the first volume (§ 54_f_) that the cardinal
-distinction between the characters of plants and animals arises from
-the fact that while the chief food of plants is universally present the
-food of animals is scattered. Here it has to be added that to utilize
-the universally distributed food the ordinary plant needs the aid of
-light, and has to acquire structures enabling it to get that aid;
-while the ordinary animal, to utilize the scattered food, must acquire
-the structures needful for locomotion. Let us contemplate separately
-the traits hence resulting in the vegetal world and the traits hence
-resulting in the animal world.
-
-The familiar plantain meets the requirement by growing stiff leaves
-enabling it to press down the competing grasses around which would
-else shade it; but the great majority of ordinary plants meet the
-requirement by raising themselves into the air. Hence the need for a
-stem, and hence the fact that plants of widely unlike natures similarly
-form stems which, in achieving strength enough to support the foliage
-and resist the wind, acquire certain adaptive structures having a
-general similarity. Here from the edge of a pool is a reed, and here
-from the adjacent copse is a hemlock: the one having grown tall in
-escaping the shade of its companions and the other in escaping the
-shade of the surrounding brushwood. On being cut across each discloses
-a tube, and each exhibits septa dividing this tube into chambers. In
-either case by the tubular structure is gained the greatest strength
-with the least material; but there is no morphological kinship between
-the tubes nor between the septa. Still more marked is the simulation
-of homology by analogy in another plant which the adjacent ditch may
-furnish--the common Horsetail. In this, again, we see an elongated
-vertical-growing part, raising the foliage into the air; and, as
-before, this is tubular and divided by septa. A type utterly alien from
-the other two has, by survival of the fittest, been similarly moulded
-to meet mechanical needs.
-
-Passing now to the obscurations in the animal world caused by
-alterations favouring locomotion, we note first that the locomotive
-power is at the outset very slight. Among many orders of _Protozoa_,
-as also among many low types of _Metazoa_, vibratile cilia are the
-most general agents of locomotion--necessarily feeble locomotion.
-Regarded in the mass, the _Cœlenterata_, when not stationary like
-the _Hydra_ or higher types in the hydroid stage, usually possess
-only such small self-mobility as the slow rhythmical contractions of
-their umbrella-disks effect, or else such as is effected by bands of
-cilia or of vibratile plates, as in the _Beroe_. Even among these low
-tpes of _Metazoa_, however, in which ordinarily the radial structure
-is conspicuous, or but slightly obscured by an ovoid form as in the
-_Ctenophora_, we find, in the _Cestus veneris_, extreme obscuration
-caused by an elongation which facilitates movement through the water;
-alike by the actions of its vibratile plates and by its undulations,
-which simulate those of sundry higher animals.
-
-And here we come upon the essential fact to be recognized. Elongation
-favours locomotion in various ways that are severally taken advantage
-of by different types of creatures. (1) To a given mass of moving
-matter the resistance of the medium decreases along with decrease
-in the area of its transverse section, and this implies increase of
-length: a given force will move the lengthened mass along with greater
-facility. (2) Reaching a certain point the elongated form enables an
-animal to progress by undulations, as in the water fish do, and even
-some cœlenterates and turbellarians do, and as on land snakes do:
-lateral resistances serving in either case as fulcra. (3) Lengthening
-of the body serves otherwise to aid locomotion in the creeping or
-burrowing worm, which, utilizing the statical resistance of its hinder
-part thrusts onwards its fore part, and then, holding fast its fore
-part by the aid of minute _setæ_, draws the hinder part after it. But
-elongation, doubly advantageous at first, while the body is itself the
-chief instrument of locomotion, gradually loses its advantageousness
-as special instruments of locomotion are developed. (4) This we
-see in that locomotive action effected by limbs, which, many and
-small in the lower _Arthropoda_ and becoming few and larger in the
-higher, at length give great activity to a shortened and consolidated
-body: a stage reached only through stages of decreasing elongation
-accompanying increase of limb-power. (5) In the _Vertebrata_ locomotion
-by undulations comes, along certain lines of evolution, to be replaced
-by that limb locomotion which accompanies the rise from water-life
-to land-life: the evolution of Amphibians exhibiting the transition.
-(6) Further, we see among mammals that as limbs become efficient the
-elongated body ceases to be itself instrumental in locomotion, but that
-still some elongation remains a characteristic. (7) Finally, where
-limb locomotion reaches its highest degree, as in birds, elongation
-disappears.
-
-These classes of familiar facts I have recalled to show that, in
-the course of evolution, achievement by plants of the all-essential
-elevation into the air and by animals of the all-essential power of
-movement have developed this trait of elongation in various types;
-and that in each kingdom acquisition of the common trait has had a
-tendency now to obscure morphological equivalence, and now to give the
-appearance of kinship where there is none. A further purpose has been
-to prepare the way for a question hereafter to be discussed--whether,
-in the various types of either kingdom, the elongation is effected in
-the same ways or in different ways. We shall have to ask whether the
-vertically-growing part is always, like that of _Lessonia_, a simple
-individual, or whether, as possibly in Phænogams, it is a united series
-of individuals; and similarly whether the elongated body is always
-single, like that of a mollusc, or whether, as possibly in annulose
-animals, it is a series of united individuals.]
-
-
-
-
-                              CHAPTER II.
-
-               THE MORPHOLOGICAL COMPOSITION OF PLANTS.
-
-
-§ 180. Evolution implies insensible modifications and gradual
-transitions, which render definition difficult--which make it
-impossible to separate absolutely the phases of organization from
-one another. And this indefiniteness of distinction, to be expected
-_à priori_, we are compelled to recognize _à posteriori_, the moment
-we begin to group morphological phenomena into general propositions.
-Thus, on inquiring what is the morphological unit, whether of plants
-or of animals, we find that the facts refuse to be included in any
-rigid formula. The doctrine that all organisms are built up of cells,
-or that cells are the elements out of which every tissue is developed,
-is but approximately true. There are living forms of which cellular
-structure cannot be asserted; and in living forms that are for the
-most part cellular, there are nevertheless certain portions which
-are not produced by the metamorphosis of cells. Supposing that clay
-were the only material available for building, the proposition that
-all houses are built of bricks, would bear about the same relation to
-the truth, as does the proposition that all organisms are composed
-of cells. This generalization respecting houses would be open to
-two criticisms:--first, that certain houses of a primitive kind are
-formed, not of bricks, but out of unmoulded clay; and second, that
-though other houses consist mainly of bricks, yet their chimney-pots,
-drain-pipes, and ridge-tiles, do not result from combination or
-metamorphosis of bricks, but are made directly out of the original
-clay. And of like natures are the criticisms which must be passed
-on the generalization, that cells are the morphological units of
-organisms. To continue the simile, the truth turns out to be, that the
-primitive clay or protoplasm out of which organisms are built, may be
-moulded either directly, or with various degrees of indirectness, into
-organic structures. The physiological units which we are obliged to
-assume as the components of this protoplasm, must, as we have seen, be
-the possessors of those proclivities which result in the structural
-arrangements of the organism. The assumption of such structural
-arrangements may go on, and in many cases does go on, by the shortest
-route; without the passage through what we call metamorphoses. But
-where such structural arrangements are reached by a circuitous route,
-the first stage is the formation of these small aggregates which, under
-the name of cells, are currently regarded as morphological units.
-
-The rationale of these truths appears to be furnished by the hypothesis
-of evolution. We set out with molecules some degrees higher in
-complexity than those molecules of nitrogenous colloidal substance
-into which organic matter is resolvable; and we regard these very much
-more complex molecules as having the implied greater instability,
-greater sensitiveness to surrounding influences, and consequent
-greater mobility of form. Such being the primitive physiological
-units, organic evolution must begin with the formation of a minute
-aggregate of them--an aggregate showing vitality by a higher degree
-of that readiness to change its form of aggregation which colloidal
-matter in general displays; and by its ability to unite the nitrogenous
-molecules it meets with, into complex molecules like those of which
-it is composed. Obviously, the earliest forms must have been minute;
-since, in the absence of any but diffused organic matter, no form
-but a minute one could find nutriment. Obviously, too, it must have
-been structureless; since, as differentiations are producible only
-by the unlike actions of incident forces, there could have been no
-differentiations before such forces had had time to work. Hence,
-distinctions of parts like those required to constitute a cell were
-necessarily absent at first. And we need not therefore be surprised
-to find, as we do find, specks of protoplasm manifesting life, and
-yet showing no signs of organization. A further stage of evolution
-is reached when the imperfectly integrated molecules forming one of
-these minute aggregates, become more coherent; at the same time as
-they pass into a state of heterogeneity, gradually increasing in its
-definiteness. That is to say, we may look for the assumption by them,
-of some distinctions of parts, such as we find in cells and in what
-are called unicellular organisms. They cannot retain their primordial
-uniformity; and while in a few cases they may depart from it but
-slightly, they will, in the great majority of cases, acquire a decided
-multiformity: there will result the comparatively integrated and
-comparatively differentiated _Protophyta_ and _Protozoa_.
-The production of minute aggregates of physiological units being
-the first step, and the passage of such minute aggregates into more
-consolidated and more complex forms being the second step, it must
-naturally happen that all higher organic types, subsequently arising
-by further integrations and differentiations, will everywhere bear the
-impress of this earliest phase of evolution. From the law of heredity,
-considered as extending to the entire succession of living things
-during the Earth’s past history, it follows that since the formation
-of these small, simple organisms must have preceded the formation
-of larger and more complex organisms, the larger and more complex
-organisms must inherit their essential characters. We may anticipate
-that the multiplication and combination of these minute aggregates or
-cells, will be conspicuous in the early developmental stages of plants
-and animals; and that throughout all subsequent stages, cell-production
-and cell-differentiation will be dominant characteristics. The
-physiological units peculiar to each higher species will, speaking
-generally, pass through this form of aggregation on their way towards
-the final arrangement they are to assume; because those primordial
-physiological units from which they are remotely descended, aggregated
-into this form. And yet, just as in other cases we found reasons for
-inferring (§ 131) that the traits of ancestral organization may,
-under certain conditions, be partially or wholly obliterated, and the
-ultimate structure assumed without passing through them; so, here,
-it is to be inferred that the process of cell-formation may, in some
-cases, be passed over. Thus the hypothesis of evolution prepares us for
-those two radical modifications of the cell-doctrine which the facts
-oblige us to make. It leads us to expect that as structureless portions
-of protoplasm must have preceded cells in the process of general
-evolution; so, in the special evolution of each higher organism, there
-will be an habitual production of cells out of structureless blastema.
-And it leads us to expect that though, generally, the physiological
-units composing a structureless blastema, will display their inherited
-proclivities by cell-development and metamorphosis; there will
-nevertheless occur cases in which the tissue to be formed, is formed by
-direct transformation of the blastema.[2]
-
-Interpreting the facts in this manner, we may recognize that large
-amount of truth which the cell-doctrine contains, without committing
-ourselves to the errors involved by a sweeping assertion of it. We
-are enabled to understand how it happens that organic structures are
-usually cellular in their composition, at the same time that they are
-not universally so. We are shown that while we may properly continue to
-regard the cell as the morphological unit, we must constantly bear in
-mind that it is such only in a qualified sense.
-
-
-§ 181. These aggregates of the lowest order, each formed of
-physiological units united into a group that is structurally single and
-cannot be divided without destruction of its individuality, may, as
-above implied, exist as independent organisms. The assumption to which
-we are committed by the hypothesis of evolution, that such so-called
-unicellular plants were at first the only kinds of plants, is in
-harmony with the fact that habitats not occupied by plants of higher
-orders, commonly contain these protophytes in great abundance and great
-variety. The various species of _Pleurococcaceæ_, of _Desmidiaceæ_,
-and _Diatomaceæ_, supply examples of morphological units living and
-propagating separately, under numerous modifications of form and
-structure. Figures 1, 2, and 3, represent a few of the commonest types.
-
-  [Illustration: Figs. 1, 2, 3.]
-
-  [Illustration: Figs. 4, 5, 6.]
-
-Mostly, simple plants are too small to be individually visible
-without the microscope. But, in some cases, these vegetal aggregates
-of the first order grow to appreciable sizes. In the mycelium of some
-fungi, we have single cells developed into long branched filaments,
-or ramified tubules, that are of considerable lengths. An analogous
-structure characterizes certain tribes of _Algæ_, of which _Codium
-adhærens_, Fig. 4, may serve as an example. In _Botrydium_, another
-alga, Fig. 5, we have a structure which is described as simulating a
-higher plant, with root, stem, bud, and fruit, all produced by the
-branching of a single cell. And among fungi the genus _Mucor_, Fig. 6,
-furnishes an example of allied kind.[3] Here, though the size attained
-is much greater than that of many organisms which are morphologically
-compound, we are compelled to consider the morphological composition
-as simple; since the whole can no more be separated into minor wholes,
-than can the branched vascular system of an animal. In these cases we
-have considerable bulk attained, not by a number of aggregates of the
-first order being united into an aggregate of the second order, but by
-the continuous growth of an aggregate of the first order.
-
-
-§ 182. The transition to higher forms begins in a very unobtrusive
-manner. Among these aggregates of the first order, an approach towards
-that union by which aggregates of the second order are produced, is
-indicated by mere juxtaposition. Protophytes multiply rapidly; and
-their rapid multiplication sometimes causes crowding. When, instead
-of floating free in the water, they form a thin film on a moist
-surface, or are imbedded in a common matrix of mucilage; the mechanical
-obstacles to dispersion result in a kind of feeble integration, vaguely
-shadowing forth a combined group. Somewhat more definite combination is
-shown us by such plants as _Palmella botryoides_. Here the members of a
-family of cells, arising by the spontaneous fission of a parent-cell,
-remain united by slender threads of that jelly-like substance which
-envelops their surfaces. In some _Diatomaceæ_ several individuals,
-instead of completely separating, hold together by their angles; and
-in other _Diatomaceæ_, as the _Bacillaria_, a variable number of units
-cohere so slightly, that they are continually moving in relation to one
-another.
-
-This formation of aggregates of the second order, faintly indicated
-in feeble and variable unions like the above, may be traced through
-phases of increasing permanence and definiteness, as well as increasing
-extent. In the yeast-plant, Fig. 7, we have cells which may exist
-singly, or joined into groups of several; and which have their shapes
-scarcely at all modified by their connexion. Among the _Desmidiaceæ_,
-it happens in many cases that the two individuals produced by division
-of a parent-individual, part as soon as they are fully formed; but in
-other cases, instead of parting they compose a group of two. Allied
-kinds show us how, by subsequent fissions of the adherent individuals
-and their progeny, there result longer groups; and in some species,
-a continuous thread of them is thus produced. Figs. 8, 9, 11, exhibit
-these several stages. Fig. 10 represents a _Scenedesmus_ in which the
-individuation of the group is manifest. Instead of linear aggregation,
-many protophytes illustrate central aggregation; as shown in Figs. 12,
-13, 14, 15. Other instances are furnished by such forms as the _Gonium
-pectorale_, Fig. 16 (_a_ being the front view, and _b_ the edge view),
-and the _Sarcina ventriculi_, Fig. 17. Further, we have that spherical
-mode of aggregation of which the _Volvox globator_ furnishes a familiar
-instance.
-
-  [Illustration: Figs. 7–17.]
-
-  [Illustration: Figs. 18–23.]
-
-Thus far, however, the individuality of the secondary aggregate is
-feebly pronounced: not simply in the sense that it is small; but also
-in the sense that the individualities of the primary aggregates are
-very little subordinated. But on seeking further, we find transitions
-towards forms in which the compound individuality is more dominant,
-while the simple individualities are more obscured. Obscuration of one
-kind accompanies mere increase of size in the secondary aggregate.
-In proportion to the greater number of the morphological units held
-together in one mass, becomes their relative insignificance as
-individuals. We see this in the irregularly-spreading lichens that
-form patches on rocks; and in such creeping fungi as grow in films or
-laminæ on decaying wood and the bark of trees. In these cases, however,
-the integration of the component cells is of an almost mechanical
-kind. The aggregate of them is scarcely more individuated than a lump
-of inorganic matter: as witness the way in which the lichen extends
-its curved edges in this or that direction, as the surface favours;
-or the way in which the fungus grows round and imbeds the shoots and
-leaves that lie in its way, just as so much plastic clay might do.
-Though here, in the augmentation of mass, we see a progress towards the
-evolution of a higher type, we have as yet none of that definiteness
-required to constitute a compound unit, or true aggregate of the
-second order. Another kind of obscuration of the morphological units,
-is brought about by their more complete coalescence into the form of
-some structure made by their union. This is well exemplified among
-the _Confervoideæ_ and _Conjugatæ_. In Fig. 18, there are represented
-the stages of a growing _Mougeotia genuflexa_, in which this merging
-of the simple individualities into the compound individuality, is
-shown in the history of a single plant; and in Figs. 19, 20, 21, 22,
-23, are represented a series of species from this group, and that of
-_Cladophora_,[4] in which we see a progressing integration. While,
-in the lower types, the primitive spheroidal forms of the cells
-are scarcely altered, in the higher types the cells are so fused
-together as to constitute cylinders divided by septa. Here, however,
-the indefiniteness is still great. There are no specific limits to
-the length of any thread thus produced, and there is none of that
-differentiation of parts required to give a decided individuality to
-the whole.
-
-To constitute something like a true aggregate of the second order,
-capable of serving as a compound unit that may be combined with others
-like itself into still higher aggregates, there must exist both mass
-and definiteness.
-
-
-§ 183. An approach towards plants which unite these characters, may be
-traced in such forms as _Bangia ciliaris_, Fig. 24. The multiplication
-of cells here takes place, not in a longitudinal direction only, but
-also in a transverse direction; and the transverse multiplication being
-greater towards the middle of the frond, there results a difference
-between the middle and the two extremities--a character which, in
-a feeble way, unites all the parts into a whole. Even this slight
-individuation is, however, very indefinitely marked; since, as shown by
-the figures, the lateral multiplication of cells does not go on in a
-precise manner.
-
-  [Illustration: Fig. 24.]
-
-From some such type as this there appear to arise, through slight
-differences in the modes of growth, two closely-allied groups of
-plants, having individualities somewhat more pronounced. If, while
-the cells multiply longitudinally, their lateral multiplication goes
-on in one direction only, there results a flat surface, as in the
-genus _Ulva_ (Sea-lettuce) or in the upper part of the thallus of
-_Enteromorpha Linza_, Fig. 25; or where the lateral multiplication is
-less uniform in its rate, in types like Fig. 26. But where the lateral
-multiplication occurs in two directions transverse to one another, a
-hollow frond may be produced--sometimes irregularly spheroidal, and
-sometimes irregularly tubular; as in _Enteromorpha intestinalis_,
-Fig. 27. And often, as in _Enteromorpha compressa_, Fig. 28, and
-other species, this tubular frond becomes branched. Figs. 29 and 30
-are magnified portions of such fronds, showing the simple cellular
-aggregation which allies them with the preceding forms.
-
-  [Illustration: Figs. 25–30.]
-
-In the common _Fuci_ of our coasts, other and somewhat higher stages
-of this integration are displayed. We have fronds preserving something
-like constant breadths and dividing dichotomously with approximate
-regularity. Though the subdivisions so produced are not to be regarded
-as separate fronds, but only as extensions of one frond, they
-foreshadow a higher degree of composition; and by the comparatively
-methodic way in which they are united, give to the aggregate a more
-definite, as well as a more complex, individuality. Many of the higher
-lichens exhibit an analogous advance. While in the lowest lichens, the
-different parts of the thallus are held together only by being all
-attached to the supporting surface, in the higher lichens the thallus
-is so far integrated that it can support itself by attachment to such
-surface at one point only. And then, in still more developed kinds,
-we find the thallus assuming a dichotomously-branched form, and so
-gaining a more specific character as well as greater size.
-
-Where, as in types like these, the morphological units show an inherent
-tendency to arrange themselves in a manner which is so far constant
-as to give characteristic proportions, we may say that there is a
-recognizable compound individuality. Considering the Thallophytes which
-grow in this way apart from their kinships, and wholly with reference
-to their morphological composition, we might not inaptly describe them
-as pseudo-foliar.
-
-
-§ 184. Another mode in which aggregation is so carried on as to produce
-a compound individuality of considerable definiteness, is variously
-displayed among other families of _Algæ_. When the cells, instead
-of multiplying longitudinally alone, and instead of all multiplying
-laterally as well as longitudinally, multiply laterally only at
-particular places, they produce branched structures.
-
-Indications of this mode of aggregation occur among the _Confervoideæ_,
-as shown in Figs. 22, 23. Though, in some of the more-developed
-_Algæ_ which exhibit the ramified arrangement in a higher degree, the
-component cells are, like those of the lower _Algæ_, united together
-end to end, in such way as but little to obscure their separate
-forms, as in _Cladophora Hutchinsiæ_, Fig. 31; they nevertheless
-evince greater subordination to the whole of which they are parts,
-by arranging themselves more methodically. Still further pronounced
-becomes the compound individuality when, while the component cells of
-the branches unite completely into jointed cylinders, the component
-cells of the stem form an axis distinguished by its relative thickness
-and complexity. Such types of structures are indicated by Figs. 32,
-33--figures representing small portions of plants which are quite
-tree-like in their entire outlines. On examining Figs. 34, 35, 36,
-which show the structures of the stems in these types, it will be
-seen, too, that the component cells in becoming more coherent,
-have undergone changes of form which obscure their individualities
-more than before. Not only are they much elongated, but they are so
-compressed as to be prismatic rather than cylindrical. This structure,
-besides displaying integration of the morphological units carried on
-in two directions instead of one; and besides displaying this higher
-integration in the greater merging of the individualities of the
-morphological units in the general individuality; also displays it in
-the more pronounced subordination of the branches and branchlets to the
-main stem. This differentiation and consolidation of the stem, brings
-all the secondary growths into more marked dependence; and so renders
-the individuality of the aggregate more decided.
-
-  [Illustration: Figs. 31–36.]
-
-We might not inappropriately call this type of structure pseud-axial.
-It simulates that of the higher plants in certain superficial
-characters. We see in it a primary axis along which development may
-continue indefinitely, and from which there bud out, laterally,
-secondary axes of like nature, bearing like tertiary axes; and this is
-a mode of growth with which Phænogams make us familiar.
-
-
-§ 185. Some of the larger _Algæ_ supply examples of an integration
-still more advanced; not simply inasmuch as they unite much greater
-numbers of morphological units into continuous masses, but also
-inasmuch as they combine the pseudo-foliar structure with the
-pseud-axial structure. Our own shores furnish an instance of this in
-the common _Laminaria_; and certain gigantic _Laminariaceæ_ of the
-Antarctic seas, furnish yet better instances. In _Necrocystis_ the
-germ develops a very long slender stem, which eventually expands into
-a large bladder-like or cylindrical air-vessel; and the surface of
-this bears numerous leaf-shaped expansions. Another kind, _Lessonia
-fuscescens_, Fig. 37, shows us a massive stem growing up through water
-many feet deep--a stem which, bifurcating as it approaches the surface,
-flattens out the ends of its subdivisions into fronds like ribands.
-These, however, are not true foliar appendages, since they are merely
-expanded continuations of the stem. In _Egregia_ branches of the
-thallus not only take the form of leaves, but these are differentiated
-into several categories in accordance with a division of labour. In
-any of these _Laminariaceæ_ the whole plant, great as may be its size,
-and made up though it seems to be of many groups of morphological
-units, united into a compound group by their marked subordination to a
-connecting mass, is nevertheless a single thallus, which is added to by
-intercalary growth at the “transition place,” at the junction of the
-stem-like and leaf-like portions. The aggregate is still an aggregate
-of the second order.
-
-  [Illustration: Fig. 37.]
-
-  [Illustration: Figs. 38–40.]
-
-But among certain of the highest _Algæ_, we _do_ find something more
-than this union of the pseud-axial with the pseudo-foliar structure.
-In addition to pseud-axes of comparative complexity; and in addition
-to pseudo-folia that are like leaves, not only in their general shapes
-but in having mid-ribs and even veins; there are the beginnings of
-a higher stage of integration. Figs. 38, 39, and 40, show some of
-the steps. In _Rhodymenia palmata_, Fig. 38, the parent-frond is
-comparatively irregular in form, and without a mid-rib; and along with
-this very imperfect integration, we see that the secondary fronds
-growing from the edges are distributed very much at random, and are by
-no means specific in their shapes. A considerable advance is displayed
-by _Phyllophora rubens_, Fig. 39. Here the frond, primary, secondary,
-or tertiary, betrays some approach towards regularity in both form
-and size; by which, as also by its partially-developed mid-rib, there
-is established a more marked individuality; and at the same time,
-the growth of the secondary fronds no longer occurs anywhere on the
-edge, in the same plane as the parent-frond, but from the surface at
-specific places. _Delesseria sanguinea_, Fig. 40, illustrates a much
-more definite arrangement of the same kind. The fronds of this plant,
-quite regularly shaped, have their parts decidedly subordinated to the
-whole; and from their mid-ribs grow other fronds which are just like
-them. Each of these fronds is an organized group of those morphological
-units which we distinguish as aggregates of the first order. And in
-this case, two or more such aggregates of the second order, well
-individuated by their forms and structures, are united together; and
-the plant composed of them is thus rendered, in so far, an aggregate of
-the third order.
-
-Just noting that in certain of the most developed _Algæ_, as the
-_Sargassum_, or common gulf-weed, this tertiary degree of composition
-is far more completely displayed, so as to produce among Thallophytes a
-type of structure closely simulating that of the higher plants, let us
-now pass to the consideration of these higher plants.
-
-
-§ 186. Having the surface of the soil for a support and the air for a
-medium, terrestrial plants are mechanically circumstanced in a manner
-widely different from that in which aquatic plants are circumstanced.
-Instead of being buoyed up by a surrounding fluid of specific gravity
-equal to their own, they have to erect themselves into a rare fluid
-which yields no appreciable support. Further, they are dissimilarly
-conditioned in having two sources of nutriment in place of one. Unlike
-the _Algæ_, which derive all the materials for their tissues from
-the water bathing their entire surfaces, and use their roots only
-for attachment, most of the plants which cover the Earth’s surface,
-absorb part of their food through their imbedded roots and part through
-their exposed leaves. These two marked unlikenesses in the relations
-to surrounding conditions, profoundly affect the respective modes of
-growth. We must duly bear them in mind while studying the further
-advance of composition.
-
-  [Illustration: Figs. 41–44.]
-
-  [Illustration: Figs. 45–49.]
-
-The class of plants to which we now turn--that of the
-_Archegoniatæ_--is nearly related by its lower members to the classes
-above dealt with: so much so, that some of the inferior liverworts are
-quite licheniform, and are often mistaken for lichens. Passing over
-these, let us recommence our synthesis with such members of the class
-as repeat those indications of progress towards a higher composition,
-which we have just observed among the more-developed _Algæ_. The
-_Jungermanniaceæ_ furnish us with a series of types, clearly indicating
-the transition from an aggregate of the second order to an aggregate
-of the third order. Figs. 41 and 42, indicate the structure among the
-lowest of this group. Here there is but an incomplete development of
-the second order of aggregate. The frond grows as irregularly as the
-thallus of a lichen: it is indefinite in size and outline, spreading
-hither or thither as the conditions favour. Moreover, it lacks the
-differentiations required to subordinate its parts to the whole: it
-is uniformly cellular, having neither mid-rib nor veins; and it puts
-out rootlets indifferently from all parts of its under surface. In
-Fig. 43, _Pellia epiphylla_, we have an advance on this type. There
-is here, as shown in the transverse section, Fig. 44, a thickening
-of the frond along its central portion, producing something like an
-approach towards a mid-rib; and from this the rootlets are chiefly
-given off. The outline, too, is much less irregular; whence results
-greater distinctness of the individuality. A further step is displayed
-in _Metzgeria furcata_, Fig. 45. The frond of this plant, comparatively
-well integrated by the distribution of its substance around a decided
-mid-rib, and by its comparatively-definite outlines, produces secondary
-fronds. There is what is called proliferous growth; and occasionally,
-as shown in Fig. 46, representing an enlarged portion, the growth is
-doubly-proliferous. In these cases, however, the tertiary aggregate, so
-far as it is formed, is but very feebly integrated; and its integration
-is but temporary. For not only do these younger fronds that bud out
-from the mid-ribs of older fronds, develop rootlets of their own; but
-as soon as they are well grown and adequately rooted, they dissolve
-their connexions with the parent-fronds, and become quite independent.
-From these transitional forms we pass, in the higher _Jungermanniaceæ_,
-to forms composed of many fronds that are permanently united by a
-continuous stem. A more-developed aggregate of the third order is
-thus produced. But though, along with increased definiteness in
-the secondary aggregates, there is here an integration of them so
-extensive and so regular, that they are visibly subordinated to the
-whole they form; yet the subordination is really very incomplete.
-In some instances, as in _Radula complanata_, Fig. 47, the leaflets
-develop roots from their under surfaces, just as the primitive frond
-does; and in the majority of the group, as in _J. capitata_, Fig. 48,
-roots are given off all along the connecting stem, at the spots where
-the leaflets or frondlets join it: the result being that though the
-connected frondlets form a physical whole, they do not form, in any
-decided manner, a physiological whole; since successive portions of
-the united series, carry on their functions independently of the rest.
-Finally, the most developed members of the group, whether lineally
-descended from the less developed or from an early type common to the
-two, present us with tertiary aggregates which are physiologically as
-well as physically integrated.[5] Not lying prone like the kinds thus
-far described, but growing erect, the stem and attached leaflets become
-dependent upon a single root or group of roots; and being so prevented
-from carrying on their functions separately, are made members of a
-compound individual: there arises a definitely-established aggregate of
-the third degree of composition.
-
-The facts as arranged in the above order are suggestive. Minute
-aggregates, or cells, the grouping of which we traced in § 182, showed
-us analogous phases of indefinite union, which appeared to lead the
-way towards definite union. We see here among compound aggregates, as
-we saw there among simple aggregates, the establishment of a specific
-form, and a size that falls within moderate limits of variation. This
-passage from less definite extension to more definite extension,
-seems in the one case, as the other, to be accompanied by the result,
-that growth exceeding a certain rate, ends in the formation of a new
-aggregate, rather than an enlargement of the old. And on the higher
-stage, as on the lower, this process, irregularly carried out in the
-simpler types, produces in them unions that are but temporary; while in
-the more-developed types, it proceeds in a systematic way, and ends in
-the production of a permanent aggregate that is doubly compound.
-
-Must we then conclude that as cells, or morphological units, are
-integrated into a unit of a higher order, which we call a thallus
-or frond; so, by the integration of fronds, there is evolved a
-structure such as the above-delineated species possess? Whether this
-is the interpretation to be given of these plants, we shall best see
-when considering whether it is the interpretation to be given of
-plants which rank above them. Thus far we have dealt only with the
-Cryptogamia. We have now to deal with the Phanerogamia or Phænogamia.
-
-
-
-
-                             CHAPTER III.
-
-               THE MORPHOLOGICAL COMPOSITION OF PLANTS,
-                             _CONTINUED_.
-
-
-§ 187. That advanced composition arrived at in the _Archegoniatæ_, is
-carried still further in the Flowering Plants. In these most-elevated
-vegetal forms, aggregation of the third order is always distinctly
-displayed; and aggregates of the fourth, fifth, sixth, &c., orders are
-very common.
-
-Our inquiry into the morphology of these flowering plants, may be
-advantageously commenced by studying the development of simple leaves
-into compound leaves. It is easy to trace the transition, as well as
-the conditions under which it occurs; and tracing it will prepare us
-for understanding how, and when, metamorphoses still greater in degree
-take place.
-
-
-§ 188. If we examine a branch of the common bramble, when in flower
-or afterwards, we shall not unfrequently find a simple or undivided
-leaf, at the insertion of one of the lateral flower-bearing axes,
-composing the terminal cluster of flowers. Sometimes this leaf is
-partially lobed; sometimes cleft into three small leaflets. Lower down
-on the shoot, if it be a lateral one, occur larger leaves, composed
-of three leaflets; and in some of these, two of the leaflets may be
-lobed more or less deeply. On the main stem the leaves, usually still
-larger, will be found to have five leaflets. Supposing the plant to be
-a well-grown one, it will furnish all gradations between the simple,
-very small leaf, and the large composite leaf, containing sometimes
-even seven leaflets. Figs. 50 to 64, represent leading stages of the
-transition. What determines this transition? Observation shows that
-the quintuple leaves occur where the materials for growth are supplied
-in greatest abundance; that the leaves become less and less compound,
-in proportion to their remoteness from the main currents of sap; and
-that where an entire absence of divisions or lobes is observed, it is
-on leaves within the flower-bunch: at the place, that is, where the
-forces which cause growth are nearly equilibrated by the forces which
-oppose growth; and where, as a consequence, gamogenesis is about to
-be set in (§ 78). Additional evidence that the degree of nutrition
-determines the degree of composition of the leaf, is furnished by
-the relative sizes of the leaves. Not only, on the average, is the
-quintuple leaf much larger in its total area than the triple leaf;
-but the component leaflets of the one, are usually much larger than
-those of the other. The like contrasts are still more marked between
-triple leaves and simple leaves. This connection of decreasing size
-with decreasing composition, is conspicuous in the series of figures:
-the differences shown being not nearly so great as may be frequently
-observed. Confirmation may be drawn from the fact that when the leading
-shoot is broken or arrested in its growth, the shoots it gives off
-(provided they are given off after the injury), and into which its
-checked currents of sap are thrown, produce leaves of five leaflets
-where ordinarily leaves of three leaflets occur. Of course incidental
-circumstances, as variations in the amounts of sunshine, or of rain,
-or of matter supplied to the roots, are ever producing changes in the
-state of the plant as a whole; and by thus affecting the nutrition of
-its leaf-buds at the times of their formation, cause irregularities
-in the relations of size and composition above described. But taking
-these causes into account, it is abundantly manifest that a leaf-bud of
-the bramble will develop into a simple leaf or into a leaf compounded
-in different degrees, according to the quantity of assimilable matter
-brought to it at the time when the rudiments of its structure are
-being fixed. And on studying the habits of other plants--on observing
-how annuals that have compound leaves usually bear simple leaves at
-the outset, when the assimilating surface is but small; and how, when
-compound-leaved plants in full growth bear simple leaves in the midst
-of compound ones, the relative smallness of such simple leaves shows
-that the buds from which they arose were ill-supplied with sap; it
-will cease to be doubted that a foliar organ may be metamorphosed into
-a group of foliar organs, if furnished, at the right time, with a
-quantity of matter greater than can be readily organized round a single
-centre of growth. An examination of the transitions through which
-a compound leaf passes into a doubly-compound leaf, as seen in the
-various intermediate forms of leaflets in Fig. 65, will further enforce
-this conclusion.
-
-  [Illustration: Figs. 50–64.]
-
-  [Illustration: Fig. 65.]
-
-Here we may advantageously note, too, how in such cases the leaf-stalk
-undergoes concomitant changes of structure. In the bramble-leaves above
-described, it becomes compound simultaneously with the leaf--the veins
-become mid-ribs while the mid-ribs become petioles. Moreover, the
-secondary stalks, and still more the main stalks, bear thorns similar
-in their shapes, and approaching in their sizes, to those on the stem;
-besides simulating the stem in colour and texture. In the petioles of
-large compound leaves, like those of the common _Heracleum_, we see
-still more distinctly both internal and external approximations in
-character to axes. Nor are there wanting plants whose large, though
-simple, leaves, are held out far from the stems by foot-stalks that
-are, near the ends, sometimes so like axes that the transverse sections
-of the two are indistinguishable; as instance the _Calla palustris_.
-
-One other fact respecting the modifications which leaves undergo,
-should be set down. Not only may leaf-stalks assume to a great degree
-the characters of stems, when they have to discharge the functions
-of stems, by supporting many leaves or very large leaves; but they
-may assume the characters of leaves, when they have to undertake the
-functions of leaves. The Australian Acacias furnish a remarkable
-illustration of this. Acacias elsewhere found bear pinnate leaves;
-but the majority of those found in Australia bear what appear to be
-simple leaves. It turns out, however, that these are merely leaf-stalks
-flattened out into foliar shapes: the laminæ of the leaves being
-undeveloped. And the proof is that in young plants, showing their
-kinships by their embryonic characters, these leaf-like petioles bear
-true leaflets at their ends. A metamorphosis of like kind occurs in
-_Oxalis bupleurifolia_, Fig. 66. The fact most deserving of notice,
-however, is that these leaf-stalks, in usurping the general aspects and
-functions of leaf-blades, have, to some also usurped their structures:
-though their venation is not like that of the leaf-blades they replace,
-yet they have veins, and in some cases mid-ribs.
-
-  [Illustration: Fig. 66.]
-
-Reduced to their most general expression, the truths above shadowed
-forth are these:--That group of morphological units, or cells, which
-we see integrated into the compound unit called a leaf, has, in
-each higher plant, a typical form, due to the special arrangement
-of these cells around a mid-rib and veins. If the multiplication of
-morphological units, at the time when the leaf-bud is taking on its
-main outlines, exceeds a certain limit, these units begin to arrange
-themselves round secondary centres, or lines of growth, in such ways as
-to repeat, in part or wholly, the typical form: the larger veins become
-transformed into imperfect mid-ribs of partially independent leaves;
-or into complete mid-ribs of quite separate leaves. And as there goes
-on this transition from a single aggregate of cells to a group of
-such aggregates, there simultaneously arises, by similarly insensible
-steps, a distinct structure which supports the several aggregates thus
-produced, and unites them into a compound aggregate. These phenomena
-should be carefully studied; since they give us a key to more involved
-phenomena.[6]
-
-
-§ 189. Thus far we have dealt with leaves ordinarily so-called: briefly
-indicating the homologies between the parts of the simple and the
-compound. Let us now turn to the homologies among foliar organs in
-general. These have been made familiar to readers of natural history
-by popularized outlines of _The Metamorphosis of Plants_--a title, by
-the way, which is far too extensive; since the phenomena treated of
-under it, form but a small portion of those it properly includes.
-
-Passing over certain vague anticipations which have been quoted from
-ancient writers, and noting only that some clearer recognitions
-were reached by Joachim Jung, a Hamburg professor, in the middle
-of the 17th century; we come to the _Theoria Generationis_, which
-Wolff published in 1759, and in which he gives definite forms to the
-conceptions that have since become current. Specifying the views of
-Wolff, Dr. Masters writes:--“After speaking of the homologous nature
-of the leaves, the sepals and petals, an homology consequent on their
-similarity of structure and identity of origin, he goes on to state
-that the ‘pericarp is manifestly composed of several leaves, as in the
-calyx, with this difference only, that the leaves which are merely
-placed in close contact in the calyx, are here united together’; a
-view which he corroborates by referring to the manner in which many
-capsules open and separate ‘into their leaves.’ The seeds, too, he
-looks upon as consisting of leaves in close combination. His reasons
-for considering the petals and stamens as homologous with leaves, are
-based upon the same facts as those which led Linnæus, and, many years
-afterwards, Goethe, to the same conclusion. ‘In a word,’ says Wolff,
-‘we see nothing in the whole plant, whose parts at first sight differ
-so remarkably from each other, but leaves and stem, to which latter
-the root is referrible.’” It appears that Wolff, too, enunciated the
-now-accepted interpretation of compound fruits: basing it on the same
-evidence as that since assigned. In the essay of Goethe, published
-thirty years after, these relations among the parts of flowering plants
-were traced out in greater detail, but not in so radical a way; for
-Goethe did not, as did Wolff, verify his hypothesis by dissecting buds
-in their early stages of development. Goethe appears to have arrived
-at his conclusions independently. But that they were original with
-him, and that he gave a more variously-illustrated exposition of them
-than had been given by Wolff, does not entitle him to anything beyond
-a secondary place, among those who have established this important
-generalization.
-
-Were it not that these pages may be read by some to whom Biology, in
-all its divisions, is a new subject of study, it would be needless to
-name the evidence on which this now-familiar generalization rests. For
-the information of such it will suffice to say, that the fundamental
-kinship existing among all the foliar organs of a flowering plant, is
-shown by the transitional forms which may be traced between them, and
-by the occasional assumption of one another’s forms. “Floral leaves, or
-bracts, are frequently only to be distinguished from ordinary leaves
-by their position at the base of the flower; at other times the bracts
-gradually assume more and more of the appearance of the sepals.” The
-sepals, or divisions of the calyx, are not unlike undeveloped leaves:
-sometimes assuming quite the structure of leaves. In other cases, they
-acquire partially or wholly the colours of the petals--as, indeed, the
-bracts and uppermost stem-leaves occasionally do. Similarly, the petals
-show their alliances to the foliar organs lower down on the axis, and
-to those higher up on the axis. On the one hand, they may develop into
-ordinary leaves that are green and veined; and, on the other hand, as
-so commonly seen in double flowers, they may bear anthers on their
-edges. All varieties of gradation into neighbouring foliar organs may
-be witnessed in stamens. Flattened and tinted in various degrees, they
-pass insensibly into petals, and through them prove their homology with
-leaves; into which, indeed, they are transformed in flowers that become
-wholly foliaceous. The style, too, is occasionally changed into petals
-or into green leaflets; and even the ovules are now and then seen to
-take on leaf-like forms. Thus we have clear evidence that in Phænogams,
-all the appendages of the axis are homologues: they are all modified
-leaves.
-
-Wolff established, and Goethe further illustrated, another general law
-of structure in flowering plants. Each leaf commonly contains in its
-axil a bud, similar in structure to the terminal bud. This axillary bud
-may remain undeveloped; or it may develop into a lateral shoot like
-the main shoot; or it may develop into a flower. If a shoot bearing
-lateral flowers be examined, it will be found that the internode, or
-space which separates each leaf with its axillary flower from the leaf
-and axillary flower above it, becomes gradually less towards the upper
-end of the shoot. In some plants, as in the fox-glove, the internodes
-constitute a regularly-diminishing series. In other plants, the series
-they form suddenly begins to diminish so rapidly, as to bring the
-flowers into a short spike: instance the common orchis. And again, by
-still more sudden dwarfing of the internodes, the flowers are brought
-into a cluster; as they are in the cowslip. On contemplating a clover
-flower, in which this clustering has been carried so far as to produce
-a compact head; and on considering what must happen if, by a further
-arrest of axial development, the foot-stalks of the florets disappear;
-it will be seen that there must result a crowd of flowers, seated
-close together on the end of the axis. And if, at the same time, the
-internodes of the upper stem-leaves also remain undeveloped, these
-stem-leaves will be grouped into a common involucre: we shall have
-a composite flower, such as the thistle. Hence, to modifications in
-the developments of foliar organs, have to be added modifications in
-the developments of axial organs. Comparisons disclose the gradations
-through which axes, like their appendages, pass into all varieties
-of size, proportion, and structure. And we learn that the occurrence
-of these two kinds of metamorphosis, in all conceivable degrees
-and combinations, furnishes us with a proximate interpretation of
-morphological composition in Phænogams.
-
-I say a proximate interpretation, because there remain to be solved
-certain deeper problems; one of which at once presents itself to be
-dealt with under the present head. Leaves, petals, stamens, &c., being
-shown to be homologous foliar organs; and the part to which they are
-attached, proving to be an indefinitely-extended axis of growth, or
-axial organ; we are met by the questions,--What is a foliar organ? and
-What is an axial organ? The morphological composition of a Phænogam is
-undetermined, so long as we cannot say to what lower structures leaves
-and shoots are homologous; and how this integration of them originates.
-To these questions let us now address ourselves.
-
-
-§ 190–1. Already, in § 78, reference has been made to the occasional
-development of foliar organs into axial organs: the special case there
-described being that of a fox-glove, in which some of the sepals were
-replaced by flower-buds. The observation of these and some analogous
-monstrosities, raising the suspicion that the distinction between
-foliar organs and axial organs is not absolute, led me to examine into
-the matter; and the result has been the deepening of this suspicion
-into a conviction. Part of the evidence is given in Appendix A.
-
-Some time after having reached this conviction, I found on looking
-into the literature of the subject, that analogous irregularities had
-suggested to other observers, beliefs similarly at variance with the
-current morphological creed. Difficulties in satisfactorily defining
-these two elements, have served to shake this creed in some minds.
-To others, the strange leaf-like developments which axes undergo in
-certain plants, have afforded reasons for doubting the constancy of
-this distinction which vegetal morphologists usually draw. And those
-not otherwise rendered sceptical, have been made to hesitate by such
-cases as that of the Nepaul-barley, in which the glume, a foliar
-organ, becomes developed into an axis and bears flowers. In his
-essay--“Vegetable Morphology: its History and Present Condition,”[7]
-whence I have already quoted, Dr. Masters indicates sundry of the
-grounds for thinking that there is no impassable demarcation between
-leaf and stem. Among other difficulties which meet us if we assume that
-the distinction is absolute, one is implied by this question:--“What
-shall we say to cases such as those afforded by the leaves of _Guarea_
-and _Trichilia_, where the leaves after a time assume the condition
-of branches and develop young leaflets from their free extremities,
-a process less perfectly seen in some of the pinnate-leaved kinds of
-_Berberis_ or _Mahonia_, to be found in almost every shrubbery?”
-
-A class of facts on which it will be desirable for us here to dwell
-a moment, before proceeding to deal with the matter deductively, is
-presented by the _Cactaceæ_. In this remarkable group of plants,
-deviating in such varied ways from the ordinary phænogamic type, we
-find many highly instructive modifications of form and structure. By
-contemplating the changes here displayed within the limits of a single
-order, we shall greatly widen our conception of the possibilities of
-metamorphosis in the vegetal kingdom, taken as a whole. Two different,
-but similarly-significant, truths are illustrated. First, we are shown
-how, of these two components of a flowering plant, commonly regarded
-as primordially distinguished, one may assume, throughout numerous
-species, the functions, and to a great degree the appearance, of the
-other. Second, we are shown how, in the same individual, there may
-occur a re-metamorphosis: the usurped function and appearance being
-maintained in one part of the plant, while in another part there is a
-return to the ordinary appearance and function. We will consider these
-two truths separately. Some of the _Euphorbiaceæ_, which simulate
-Cactuses, show us the stages through which such abnormal structures are
-arrived at. In _Euphorbia splendens_, the lateral axes are considerably
-swollen at their distal ends, so as often to be club-shaped: still,
-however, being covered with bark of the ordinary colour, and still
-bearing leaves. But in kindred plants, as _Euphorbia neriifolia_,
-this swelling of the lateral axes is carried to a far greater extent;
-and, at the same time, a green colour and a fleshy consistence have
-been acquired: the typical relations nevertheless being still shown
-by the few leaves that grow out of these soft and swollen axes. In
-the _Cactaceæ_, which are thus resembled by plants not otherwise
-allied to them, we have indications of a parallel transformation.
-Some kinds, not commonly brought to England, bear leaves; but in the
-species most familiar to us, the leaves are undeveloped and the axes
-assume their functions. Passing over the many varieties of form and
-combination which these green succulent growths display, we have to
-note that in some genera, as in _Phyllocactus_, they become flattened
-out into foliaceous shapes, having mid-ribs and something approaching
-to veins. So that here, and in the genus _Epiphyllum_, which has this
-character still more marked, the plant appears to be composed of fleshy
-leaves growing one upon another. And then, in _Rhipsalis_, the same
-parts are so leaf-like, that an uncritical observer would regard them
-as leaves. These which are axial organs in their homologies, have
-become foliar organs in their analogies. When, instead of comparing
-these strangely-modified axes in different genera of Cactuses, we
-compare them in the same individual, we meet with transformations no
-less striking. Where a tree-like form is produced by the growth of
-these foliaceous shoots, one on another; and where, as a consequence,
-the first-formed of them become the main stem that acts as support
-to secondary and tertiary stems; they lose their green, succulent
-character, acquire bark, and become woody. In resuming the functions
-of axes they resume the structures of axes, from which they had
-deviated. In Fig. 71 are shown some of the leaf-like axes of _Rhipsalis
-rhombea_ in their young state; while Fig. 72 represents the oldest
-portion of the same plant, in which the foliaceous characters are
-quite obliterated, and there has resulted an ordinary stem-structure.
-One further fact is to be noted. At the same time that their
-leaf-like appearances are lost, the axes also lose their separate
-individualities. As they become stem-like, they also become integrated;
-and they do this so effectually that their original points of junction,
-at first so strongly marked, are effaced, and a consolidated trunk is
-produced.
-
-  [Illustration: Figs. 71–72.]
-
-Joined with the facts previously specified, these facts help us to
-conceive how, in the evolution of flowering plants in general, the
-morphological components that were once distinct, may become extremely
-disguised. We may rationally expect that during so long a course of
-modification, much greater changes of form, and much more decided
-fusions of parts, have taken place. Seeing how, in an individual
-plant, the single leaves pass into compound leaves, by the development
-of their veins into mid-ribs while their petioles begin to simulate
-axes; and seeing that leaves ordinarily exhibiting definitely-limited
-developments, occasionally produce other leaves from their edges; we
-are led to suspect the possibility of still greater changes in foliar
-organs. When, further, we find that within the limits of one natural
-order, petioles usurp the functions and appearances of leaves, at
-the same time that in other orders, as in _Ruscus_, lateral axes so
-simulate leaves that their axial nature would by most not be suspected,
-did they not bear flowers on their mid-ribs or edges; and when, among
-Cactuses, we perceive that such metamorphoses and re-metamorphoses
-take place with great facility; our suspicion that the morphological
-elements of Phænogams admit of profound transformations, is deepened.
-And then, on discovering how frequent are the monstrosities which do
-not seem satisfactorily explicable without admitting the development
-of foliar organs into axial organs; we become ready to entertain
-the hypothesis that during the evolution of the phænogamic type, the
-distinction between leaves and axes has arisen by degrees.
-
-With our preconceptions loosened by such facts, and carrying with us
-the general idea which such facts suggest, let us now consider in what
-way the typical structure of a flowering plant may be interpreted.
-
-
-§ 192. To proceed methodically, we must seek a clue to the structures
-of Phanerogams, in the structures of those inferior plants that
-approach to them--_Archegoniatæ_. The various divisions of this
-class present, along with sundry characters which ally them with
-Thallophytes, other characters by which the phænogamic structure is
-shadowed forth. While some of the inferior _Hepaticæ_ or Liverworts,
-severally consist of little more than a thallus-like frond, among
-the higher members of this group, and still more among the Mosses
-and Ferns, we find a distinctly marked stem.[8] Some Archegoniates
-(or rather Rhizoids) have foliar expansions that are indefinite in
-their forms; and some have quite definitely-shaped leaves. Roots are
-possessed by all the more-developed genera of the class; but there are
-other genera, as _Sphagnum_, which have no roots. Here the fronds are
-formed of only a single layer of cells; and there a double layer gives
-them a higher character--a difference exhibited between closely-allied
-genera of one group, the Mosses. Equally varied are the developments
-of the foliar organs in their detailed structures: now being without
-mid-ribs or veins; now having mid-ribs but no veins; now having
-both mid-ribs and veins. Nor must we omit the similarly-significant
-circumstance, that whereas in the lower Archegoniates the reproductive
-elements are immersed here and there in the thallus-like frond, they
-are, in the higher orders, seated in well-specialized and quite
-distinct fructifying organs, having analogies with the flowers of
-Phænogams. Thus, many facts imply that if the Phænogamic type is to
-be analyzed at all, we must look among the Archegoniates for its
-morphological components, and the manner of their integration.
-
-Already we have seen among the lower _Cryptogamia_, how, as they became
-integrated and definitely limited, aggregates acquire the habit of
-budding out other aggregates, on reaching certain stages of growth.
-Cells produce other cells endogenously or exogenously; and fronds
-give origin to other fronds from their edges or surfaces. We have
-seen, too, that the new aggregates so produced, whether of the first
-order or the second order, may either separate or remain connected.
-Fissiparously-multiplying cells in some cases part company, while in
-other cases they unite into threads or laminæ or masses; and fronds
-originating proliferously from other fronds, sometimes when mature
-disconnect themselves from their parents, and sometimes continue
-attached to them. Whether they do or do not part, is clearly determined
-by their nutrition. If the conditions are such that they can severally
-thrive better by separating after a certain development is reached,
-it will become their habit then to separate; since natural selection
-will favour the propagation of those which separate most nearly at
-that time. If, conversely, it profits the species for the cells or
-fronds to continue longer attached, which it can only do if their
-growths and subsequent powers of multiplication are thereby increased,
-it must happen, through the continual survival of the fittest, that
-longer attachment will become an established characteristic; and,
-by persistence in this process, permanent attachment will result
-when permanent attachment is advantageous. That disunion is really
-a consequence of relative innutrition, and union a consequence of
-relative nutrition, is clear _à posteriori_. On the one hand, the
-separation of the new individuals, whether in germs or as developed
-aggregates, is a dissolving away of the connecting substance; and
-this implies that the connecting substance has ceased to perform its
-function as a channel of nutriment. On the other hand, where, as
-we see among Phænogams, there is about to take place a separation
-of new individuals in the shape of germs, at the point where the
-nutrition is the lowest, a sudden increase of nutrition will cause the
-impending separation to be arrested; and the fructifying elements,
-reverting towards the ordinary form, thereupon develop in connexion
-with the parent. Turning to the Archegoniates, we find among them
-many indications of this transition from discontinuous development
-to continuous development. Thus the Liverworts give origin to new
-plants by cells which they throw off from their surfaces; as, indeed,
-we have seen that much higher plants do. “According to Bischoff,”
-says Schleiden, “both the cells of the stem (_Jungermannia_ [now
-_Lophocolea_] _bidentata_) and those of the leaves (_J. exsecta_)
-separate themselves as propagative cells from the plant, and isolated
-cells shoot out and develop while still connected with the parent
-plant into small cellular bodies (_Metzgeria furcata_), which separate
-from the plant, and grow into new plants, as in _Mnium androgynum_
-among the Mosses.” Now in the way above explained, these propagative
-cells and proliferous buds, may continue developing in connexion with
-the parent to various degrees before separating; or the buds which
-are about to become fructifying organs may similarly, under increased
-nutrition, develop into young fronds. As Sir W. Hooker says of the male
-fructification in _Metzgeria furcata_,--“It has the appearance of being
-a young shoot or innovation (for in colour and texture I can perceive
-no difference) rolled up into a spherical figure.” On finding in this
-same plant, that sometimes the proliferously-produced frond buds out
-from itself another frond before separating from the parent, as shown
-in Fig. 46, it becomes clear that this long-continued connexion may
-readily pass into permanent connexion. And when we see how, even among
-Phænogams, buds may either detach themselves as bulbils, or remain
-attached and become shoots; we can scarcely doubt that among inferior
-plants, less definite in their modes of organization, such transitions
-must continually occur.
-
-  [Illustration: Figs. 73–76.]
-
-Let us suppose, then, that Fig. 73 is the frond of some primitive
-Archegoniate, similar in general characters to _Pellia epiphylla_,
-Fig. 43; bearing, like it, the fructifying buds on its upper surface,
-and having a slightly-marked mid-rib and rootlets. And suppose
-that, as shown, a secondary frond is proliferously produced from
-the mid-rib, and continues attached to it. Evidently the ordinary
-discontinuous development, can thus become a continuous development,
-only on condition that there is an adequate supply, to the secondary
-frond, of such materials as are furnished by the rootlets: the
-remaining materials being obtainable by itself from the air. Hence,
-that portion of the mid-rib lying between the secondary frond and
-the chief rootlets, having its function increased, will increase in
-bulk. An additional consequence will be a greater concentration of
-the rootlets--there will be extra growth of those which are most
-serviceably placed. Observe, next, that the structure so arising
-is likely to be maintained. Such a variation implying, as it does,
-circumstances especially favourable to the growth of the plant, will
-give to the plant extra chances of leaving descendants; since the area
-of frond supported by a given area of the soil, being greater than
-in other individuals, there may be a greater production of spores.
-And then, among the more numerous descendants thus secured by it, the
-variation will give advantages to those in which it recurs. Such a
-mode of growth having, in this manner, become established, let us ask
-what is next likely to result. If it becomes the habit of the primary
-frond to bear a secondary frond from its mid-rib, this secondary frond,
-composed of physiological units of the same kind, will inherit the
-habit; and supposing that the supply of mineral matters obtained by
-the rootlets suffices for the full development of the secondary frond,
-there is a likelihood that the growth from it of a tertiary frond,
-will become an habitual characteristic of the variety. Along with the
-establishment of such a tertiary frond, as shown in Fig. 74, there
-must arise a further development of mid-rib in the primary frond, as
-well as in the secondary frond--a development which must bring with it
-a greater integration of the two; while, simultaneously, extra growth
-will take place in such of the rootlets as are most directly connected
-with this main channel of circulation. Without further explanation it
-will be seen, on inspecting Figs. 75 and 76, that there may in this
-manner result an integrated series of fronds, placed alternately on
-opposite sides of a connecting vascular structure. That this connecting
-vascular structure will, as shown in the figures, become more distinct
-from the foliar surfaces as these multiply, is no unwarranted
-assumption; for we have seen in compound-leaved plants, how, under
-analogous conditions, mid-ribs become developed into separate
-supporting parts, which acquire some of the characters of axes while
-assuming their functions. And now mark how clearly the structure thus
-built up by integration of proliferously-growing fronds, corresponds
-with the structure of the more-developed _Jungermanniaceæ_. Each of
-the fronds successively produced, repeating the characters of its
-parent, will bear roots; and will bear them in homologous places, as
-shown. Further, the united mid-ribs having but very little rigidity,
-will be unable to maintain an erect position. Hence there will result
-the recumbent, continuously-rooted stem, which these types exhibit: an
-embryo phænogam having the weakness of an embryo.[9]
-
-A natural concomitant of the mode of growth here described, is that
-the stem, while it increases longitudinally, increases scarcely at
-all transversely: hence the old name Acrogens. Clearly the transverse
-development of a stem is the correlative, partly of its function as
-a channel of circulation, and partly of its function as a mechanical
-support. That an axis may lift its attached leaves into the air,
-implies thickness and solidity proportionate to the mass of such
-leaves; and an increase of its sap-vessels, also proportionate to the
-mass of such leaves, is necessitated when the roots are all at one end
-and the leaves at the other. But in the generality of Acrogens, these
-conditions, under which arises the necessity for transverse growth
-of the axis, are absent wholly or in great part. The stem habitually
-creeps below the surface, or lies prone upon the surface; and where
-it grows in a vertical or inclined direction, does this by attaching
-itself to a vertical or inclined object. Moreover, throwing out
-rootlets, as it mostly does, at intervals throughout its length, it
-is not called upon in any considerable degree, to transfer nutritive
-materials from one of its ends to the other. Hence this peculiarity
-which gives their name to the Acrogens, now called Archegoniates, is a
-natural accompaniment of the low degree of specialization reached in
-them. And that it is an incidental and not a necessary peculiarity, is
-demonstrated by two converse facts. On the one hand, in those higher
-Acrogens which, like the tree-ferns, lift large masses of foliage into
-the air, there is just as decided a transverse expansion of the axis
-as in dicotyledonous trees. On the other hand, in those Dicotyledons
-which, like the common Dodder, gain support and nutriment from the
-surfaces over which they creep, there is no more lateral expansion of
-the axis than is habitual among Acrogens or Archegoniates. Concluding,
-as we are thus fully justified in doing, that the lateral expansion
-accompanying longitudinal extension, which is a general characteristic
-of Phanerogams as distinguished from Archegoniates, is nothing more
-than a concomitant of their usually-vertical growth;[10] let us now
-go on to consider how vertical growth originates, and what are the
-structural changes it involves.
-
-
-§ 193. Plants depend for their prosperity mainly on air and light: they
-dwindle where they are smothered, and thrive where they can expand
-their leaves into free space and sunshine. Those kinds which assume
-prone positions, consequently labour under disadvantages in being
-habitually interfered with by one another--they are mutually shaded
-and mutually injured. Such of them, however, as happen, by variations
-in mode of growth, to rise higher than others, are more likely to
-flourish and leave offspring than others. That is to say, natural
-selection will favour the more upright-growing forms. Individuals
-with structures which lift them above the rest, are the fittest for
-the conditions; and by the continual survival of the fittest, such
-structures must become established. There are two essentially-different
-ways in which the integrated series of fronds above described, may
-be modified so as to acquire the stiffness needful for maintaining
-perpendicularity. We will consider them separately.
-
-  [Illustration: Figs. 77, 78.]
-
-A thin layer of substance gains greatly in power of resisting a
-transverse strain, if it is bent round so as to form a tube: witness
-the difference between the pliability of a sheet of paper when
-outspread, and the rigidity of the same sheet of paper when rolled up.
-Engineers constantly recognize this truth, in devising appliances by
-which the greatest strength shall be obtained at the smallest cost of
-material; and among organisms, we see that natural selection habitually
-establishes structures conforming to the same principle, wherever
-lightness and stiffness are to be combined. The cylindrical bones of
-mammals and birds, and the hollow shafts of feathers, are examples.
-The lower plants, too, furnish cases where the strength needful for
-maintaining an upright position, is acquired by this rolling up of a
-flat thallus or frond. In Fig. 77 we have an _Alga_ which approaches
-towards a tubular distribution of substance; and which has a consequent
-rigidity. Sundry common forms of lichen, having the thallus folded
-into a branched tube, still more decidedly display the connexion
-between this structural arrangement and this mechanical advantage.
-And from the particular class of plants we are here dealing with--the
-Archegoniates--a type is shown in Fig. 78, _Riella helicophylla_,
-similarly characterized by a thin frond that is made stiff enough to
-stand, by an incurving which, though it does not produce a hollow
-cylinder, produces a kindred form. If, then, as we have seen, natural
-selection or survival of the fittest will favour such among these
-recumbent Archegoniates as are enabled, by variations in their
-structures, to maintain raised postures; it will favour the formation
-of fronds that curve round upon themselves, and curve round upon the
-fronds growing out of them. What, now, will be the result should such a
-modification take place in the group of proliferous fronds represented
-in Fig. 76? Clearly, the result will be a structure like that shown
-in Fig. 79. And if this inrolling becomes more complete, a form like
-_Jungermannia cordifolia_, represented in Fig. 80, will be produced.
-
-  [Illustration: Figs. 79, 80.]
-
-  [Illustration: Figs. 81–89.]
-
-  [Illustration: Figs. 90, 91.]
-
-When the successive fronds are thus folded round so completely that
-their opposite edges meet, these opposite edges will be apt to unite:
-not that they will grow together after being formed, but that they will
-develop in connexion; or, in botanical language, will become “adnate.”
-That foliar surfaces which, in their embryonic state, are in close
-contact, often join into one, is a familiar fact. It is habitually
-so with sepals or divisions of the calyx. In all campanulate flowers
-it is so with petals. And in some tribes of plants it is so with
-stamens. We are therefore well warranted in inferring that, under the
-conditions above described, the successive fronds or leaflets will,
-by union of their remote edges, first at their points of origin and
-afterwards higher up, form sheaths inserted one within another, and
-including the axis. This incurving of the successive fronds, ending
-in the formation of sheaths, may be accompanied by different sets of
-modifications. Supposing Fig. 81 to be a transverse section of such
-type (_a_ being the mid-rib, and _b_ the expansion of an older frond;
-while _c_ is a younger frond proliferously developed within it), there
-may begin two divergent kinds of changes, leading to two contrasted
-structures. If, while frond continues to grow out of frond, the
-series of united mid-ribs continues to be the channel of circulation
-between the uppermost fronds and the roots--if, as a consequence,
-the compound mid-rib, or rudimentary axis, continues to increase in
-size laterally; there will arise the series of transitional forms
-represented by the transverse sections 82, 83, 84, 85; ending in the
-production of a solid axis, everywhere wrapped round by the foliar
-surface of the frond, as an outer layer or sheath. But if, on the
-other hand, circumstances favour a form of plant which maintains its
-uprightness at the smallest cost of substance--if the vascular bundles
-of each succeeding mid-rib, instead of remaining concentrated, become
-distributed all round the tube formed by the infolded frond; then the
-structure eventually reached, through the transitional forms 86, 87,
-88, 89, will be a hollow cylinder.[11] And now observe how the two
-structures thus produced, correspond with two kinds of Monocotyledons.
-Fig. 90 represents a species of _Dendrobium_, in which we see clearly
-how each leaf is but a continuation of the external layer of a solid
-axis--a sheath such as would result from the infolded edges of a frond
-becoming adnate; and on examining how the sheath of each leaf includes
-the one above it, and how the successive sheaths include the axis, it
-will be manifest that the relations of parts are just such as exist
-in the united series of fronds shown in Fig. 79--the successive nodes
-answering to the successive points of origin of the fronds. Conversely,
-the stem of a grass, Fig. 91, displays just such relations of parts,
-as would result from the development of the type shown in Fig. 79,
-if instead of the mid-ribs thickening into a solid axis, the matter
-composing them became evenly distributed round the foliar surfaces,
-at the same time that the incurved edges of the foliar surfaces
-united. The arrangements of the tubular axis and its appendages, thus
-resulting, are still more instructive than those of the solid axis.
-For while, even more clearly than in the _Dendrobium_, we see at the
-point _b_, a continuity of structure between the substance of the
-axis below the node, and the substance of the sheath above the node:
-we see that this sheath, instead of having its edges united as in
-_Dendrobium_, has them simply overlapping, so as to form an incomplete
-hollow cylinder which may be taken off and unrolled; and we see that
-were the overlapping edges of this sheath united all the way from the
-node _a_ to the node _b_, it would constitute a tubular axis, like that
-which precedes it or like that which it includes. And then, giving an
-unexpected conclusiveness to the argument, it turns out that in one
-family of grasses, the overlapping edges of the sheaths _do_ unite:
-thus furnishing us with a demonstration that tubular structures _are_
-produced by the incurving and joining of foliar surfaces; and that so,
-hollow axes may be interpreted as above, without making any assumption
-unwarranted by fact. One further correspondence between the type thus
-ideally constructed, and the monocotyledonous type, must be noted. If,
-as already pointed out, the transverse growth of an axis arises when
-the axis comes to be a channel of circulation between all the roots at
-one of its extremities and all the leaves at the other; and if this
-lateral bulging must increase as fast as the quantity of foliage to
-be brought in communication with the roots increases--especially if
-such foliage has at the same time to be raised high above the earth’s
-surface; what must happen to a plant constructed in the manner just
-described? The elder fronds or foliar organs, ensheathing the younger
-ones, as well as the incipient axis serving as a bond of union, are
-at first of such circumference only as suffices to inclose these
-undeveloped parts. What, then, will take place when the inclosed
-parts grow--when the axis thickens while it elongates? Evidently the
-earliest-formed sheaths, not being large enough for the swelling axis,
-must burst; and evidently each of the later-formed sheaths must, in
-its turn, do the like. There must result a gradual exfoliation of
-the successive sheaths, like that indicated as beginning in the above
-figure of _Dendrobium_; which, at _a_, shows the bud of the undeveloped
-parts just visible above the enwrapping sheaths, while at _b_, and _c_,
-it shows the older sheaths in process of being split open. That is to
-say, there must result the mode of growth which helped to give the name
-Endogens to this class.
-
-  [Illustration: Figs. 92–94.]
-
-  [Illustration: Figs. 95–99.]
-
-The other way in which an integrated series of fronds may acquire
-the rigidity needful for maintaining an erect position, has next to
-be considered. If the successive fronds do not acquire such habit
-of curling as may be taken advantage of by natural selection, so as
-to produce the requisite stiffness; then, the only way in which the
-requisite stiffness appears producible, is by the thickening and
-hardening of the fused series of mid-ribs. The incipient axis will not,
-in this case, be inclosed by the rolled-up fronds; but will continue
-exposed. Survival of the fittest will favour the genesis of a type, in
-which those portions of the successive mid-ribs that enter into the
-continuous bond, become more bulky than the disengaged portions of the
-mid-ribs: the individuals which thrive and have the best chances of
-leaving offspring, being, by the hypothesis, individuals having axes
-stiff enough to raise their foliage above that of their fellows. At
-the same time, under the same influences, there will tend to result an
-elongation of those portions of the mid-ribs, which become parts of the
-incipient axis; seeing that it will profit the plant to have its leaves
-so far removed from one another, as to prevent mutual interferences.
-Hence, from the recumbent type there will evolve, by indirect
-equilibration (§ 167), such modifications as are shown in Figs. 92,
-93, 94; the first of which is a slight advance on the ideal type
-represented in Fig. 76, arising in the way described; and the others
-of which are actual plants--_Haplomitrium Hookeri_, and _Plagiochila
-decipiens_. Thus the higher Archegoniates show us how, along with
-an assumption of the upright attitude, there does go on, as we see
-there must go on, a separation of the leaf-producing parts from the
-root-producing parts; a greater development of that connecting portion
-of the successive fronds, by which they are kept in communication with
-the roots, and raised above the ground; and a consequent increased
-differentiation of such connecting portion from the parts attached
-to it. And this lateral bulging of the axis, directly or indirectly
-consequent on its functions as a support and a channel, being here
-unrestrained by the early-formed fronds folded round it, goes on
-without the bursting of these. Hence arises a leading character of
-what is called exogenous growth--a growth which is, however, still
-habitually accompanied by exfoliation, in flasks, of the outermost
-layers, continually being cracked and split by the accumulation of
-layers within them. And now if we examine plants of the exogenous type,
-we find among them many displaying the stages of this metamorphosis. In
-Fig. 95, is shown a form in which the continuity of the axis with the
-mid-rib of the leaf, is manifest--a continuity that is conspicuous in
-the common thistle. Here the foliar expansion, running some distance
-down the axis, makes the included portion of the axis a part of its
-mid-rib; just as in the ideal types above drawn. By the greater growth
-of the internodes, which are very variable, not only in different
-plants but in the same plant, there results a modification like that
-delineated in Fig. 96. And then, in such forms as Fig. 97, there is
-shown the arrangement that arises when, by more rapid development of
-the proximal end of the mid-rib, the distal part of the foliar surface
-is separated from the part which embraces the axis: the wings of the
-mid-rib still serving, however, to connect the two portions of the
-foliar surface. Such a separation is, as pointed out in § 188, an
-habitual occurrence; and in some compound leaves, an actual tearing
-of the inter-venous tissue is caused by extra growth of the mid-rib.
-Modifications like this, and the further one in Fig. 98, we may expect
-to be established by survival of the fittest, among those plants
-which produce considerable masses of leaves; since the development of
-mid-ribs into foot-stalks, by throwing the leaves further away from
-the axes, will diminish the shading of the leaves, one by another. And
-then, among plants of bushy growth, in which the assimilating surfaces
-become still more liable to intercept one another’s light, natural
-selection will continue to give an advantage to those which carry their
-assimilating surfaces at the ends of the petioles, and do not develop
-assimilating surfaces close to the axis, where they are most shaded.
-Whence will result a disappearance of the stipules and the foliar
-fringes of the mid-ribs; ending in the production of the ordinary
-stalked leaf, Fig. 99, which is characteristic of trees. Meanwhile, the
-axis thickens in proportion to the number of leaves it has to carry,
-and to put in communication with the roots; and so there comes to be a
-more marked contrast between it and the petioles, severally carrying a
-leaf each.[12]
-
-
-§ 194. When, in the course of the process above sketched out, there has
-arisen such community of nutrition among the fronds thus integrated
-into a series, that the younger ones are aided by materials which the
-older ones have elaborated; the younger fronds will begin to show, at
-earlier and earlier periods of development, the structures about to
-originate from them. Abundant nutrition will abbreviate the intervals
-between the successive prolifications; so that eventually, while each
-frond is yet imperfectly formed, the rudiment of the next will begin to
-show itself. All embryology justifies this inference. The analogies it
-furnishes lead us to expect that when this serial arrangement becomes
-organic, the growing part of the series will show the general relations
-of the forthcoming parts, while they are very small and unspecialized.
-What will in such case be the appearances they assume? We shall have no
-difficulty in perceiving what it will be, if we take a form like that
-shown in Fig. 92, and dwarf its several parts at the same time that we
-generalize them. Figs. 100, 101, 102, and 103, will show the result;
-and in Fig. 104, which is the bud of a dicotyledon, we see how clear is
-the morphological correspondence: _a_ being the rudiment of a foliar
-organ beginning to take shape; _b_ being the almost formless rudiment
-of the next foliar organ; and _c_ being the quite-undifferentiated
-part whence the rudiments of subsequent foliar organs are to arise.
-
-  [Illustration: Figs. 100–104.]
-
-  [Illustration: Figs. 105–106.]
-
-And now we are prepared for entering on a still-remaining question
-respecting the structure of Phænogams--what is the origin of axillary
-buds? As the synthesis at present stands, it does not account for
-these; but on looking a little more closely into the matter, we shall
-find that the axillary buds are interpretable in the same manner as the
-terminal buds. So to interpret them, however, we must return to that
-process of proliferous growth with which we set out, for the purpose
-of observing some facts not before named. _Delesseria hypoglossum_,
-Fig. 105, represents a seaweed of the same genus as one outlined in
-Fig. 40; but of a species in which proliferous growth is carried much
-further. Here, not only does the primary frond bud out many secondary
-fronds from its mid-rib; but most of the secondary fronds similarly bud
-out several tertiary fronds; and even by some of the tertiary fronds,
-this prolification is repeated. Besides being shown that the budding
-out of several fronds from one frond, may become habitual; we are also
-shown that it may become a habit inherited by the fronds so produced,
-and also by the fronds they produce: the manifestation of the tendency
-being probably limited only by failure of nutrition. That under fit
-conditions an analogous mode of growth will occur in fronds of the
-acrogenic type, like those we set out with, is shown by the case of
-_Metzgeria furcata_, Figs. 45, 46, in which such compound prolification
-is partially displayed. Let us suppose, then, that the frond _a_,
-Fig. 106, produces not only a single secondary frond _b_, but also
-another such secondary frond _b′_. Let us suppose, further, that the
-frond _b_ is in like manner doubly proliferous: producing both _c_
-and _c′_. Lastly, let us suppose that in the second frond _b′_ which
-_a_ produces, as well as in the second frond _c′_ which _b_ produces,
-the doubly-proliferous habit is manifested. If, now, this habit grows
-organic--if it becomes, as it naturally will become, the characteristic
-of a plant of luxuriant growth, the unfolding parts of which can be
-fed by the unfolded parts; it will happen with each lateral series,
-as with the main series, that its successive components will begin to
-show themselves at earlier and earlier stages of development. And in
-the same way that, by dwarfing and generalizing the original series, we
-arrive at a structure like that of the terminal bud; by dwarfing and
-generalizing a lateral series, as shown in Figs. 107–110, we arrive at
-a structure answering in nature and position to the axillary bud.
-
-  [Illustration: Figs. 107–110.]
-
-Facts confirming these interpretations are afforded by the structure
-and distribution of buds. The phænogamic axis in its primordial form,
-being an integrated series of folia; and the development of that part
-by which these folia are held together at considerable distances
-from one another, taking place afterwards; it is inferable from the
-general principles of embryology, that in its rudimentary stages, the
-phænogamic shoot will have its foliar parts more clearly marked out
-than its axial parts. This we see in every bud. Every bud consists
-of the rudiments of leaves packed together without any appreciable
-internodal spaces; and the internodal spaces begin to increase with
-rapidity, only when the foliar organs have been considerably developed.
-Moreover, where nutrition falls short, and arrest of development
-takes place--that is, where a flower is formed--the internodes remain
-undeveloped: the unfolding ceases before the later-acquired characters
-of the phænogamic shoot are assumed. Lastly, as the hypothesis leads us
-to expect, axillary buds make their appearances later than the foliar
-organs which they accompany; and where, as at the ends of shoots, these
-foliar organs show failure of chlorophyll, the axillary buds are not
-produced at all. That these are inferable traits of structure, will be
-manifest on inspecting Figs. 106–110; and on observing, first, that
-the doubly-proliferous tendency of which the axillary bud is a result,
-implies abundant nutrition; and on observing, next, that the original
-place of secondary prolification, is such that the foliar surface on
-which it occurs, must grow to some extent before the bud appears.
-
-On thus looking at the matter--on contemplating afresh the ideal type
-shown in Fig. 106, and noting how, by the conditions of the case, the
-secondary prolifications must cease before that primary prolification
-which produces the main axis; we are enabled to reconcile all the
-phenomena of axillary gemmation. We see harmony among the several
-facts--first, that the axillary bud becomes a lateral, leaf-bearing
-axis if there is abundant material for growth; second, that its
-development is arrested, or it becomes a flower-bearing axis, if the
-supply of sap is but moderate; third, that it is absent when the
-nutrition is failing. We are no longer committed to the gratuitous
-assumption that, in the phænogamic type, there must exist an axillary
-bud to each foliar organ; but we are led to conclude, _à priori_,
-that which we find, _à posteriori_, that axillary buds are as normally
-absent in flowers as they are normally present lower down the axis. And
-then, to complete the argument, we are prepared for the corollary that
-axillary prolification may naturally arise even at the ends of axes,
-should the failing nutrition which causes the dwarfing of the foliar
-organs to form a flower, be suddenly changed into such high nutrition
-as to transform the components of the flower into appendages that are
-green, if not otherwise leaf-like--a condition under which only, this
-phenomenon is proved to occur.
-
-
-§ 195. One more question presents itself, when we contrast the early
-stages of development in the two classes of Phænogams; and a further
-answer, supplied by the hypothesis, gives to the hypothesis a further
-probability. It is characteristic of a monocotyledon, to have a single
-seed-leaf or cotyledon; and it is characteristic of a dicotyledon, to
-have at least two cotyledons, if not more than two. That is to say, the
-monocotyledonous mode of germination everywhere co-exists with the
-endogenous mode of growth; and along with the exogenous mode of growth, there always goes either a dicotyledonous or
-polycotyledonous germination. Why is this? Such correlations cannot be
-accidental--cannot be meaningless. A true theory of the phænogamic
-types in their origin and divergence, should account for the connexion
-of these traits. Let us see whether the foregoing theory does this.
-
-The higher plants, like the higher animals, bequeath to their offspring
-more or less of nutriment and structure. Superior organisms of either
-kingdom do not, as do all inferior organisms, cast off their progeny
-in the shape of minute portions of protoplasm, unorganized and without
-stocks of material for them to organize; but they either deposit
-along with the germs they cast off, certain quantities of albuminoid
-substance to be appropriated by them while they develop themselves,
-or else they continue to supply such substance while the germs
-partially develop themselves before their detachment. Among plants
-this constitutes one distinction between seeds and spores. Every seed
-contains a store of food to serve the young plant during the first
-stages of its independent life; and usually, too, before the seed is
-detached, the young plant is so far advanced in structure, that it
-bears to the attached stock of nutriment much the same relation that
-the young fish bears to the appended yelk-bag at the time of leaving
-the egg. Sometimes, indeed, the development of chlorophyll gives the
-seed-leaves a bright green, while the seed is still contained in the
-parent-pod. This early organization of the phænogam must be supposed
-rudely to indicate the type out of which the phænogamic type arose.
-On the foregoing hypothesis, the seed-leaves therefore represent the
-primordial fronds; which, indeed, they simulate in their simple,
-cellular, unveined structures. And the question here to be asked is--do
-the different relations of the parts in young monocotyledons and
-dicotyledons correspond with the different relations of the primordial
-fronds, implied by the endogenous and the exogenous modes of growth? We
-shall find that they do.
-
-  [Illustration: Figs. 111–122.]
-
-Starting, as before, with the proliferous form shown in Fig. 111, it
-is clear that if the strength required for maintaining the vertical
-attitude, is obtained by the rolling up of the fronds, the primary
-frond will more and more conceal the secondary frond within it. At the
-same time, the secondary frond must continue to be dependent on the
-first for its nutrition; and, being produced within the first, must
-be prevented by defective supply of light and air, from ever becoming
-synchronous in its development with the first. Hence, this infolding
-which leads to the endogenous mode of growth, implies that there must
-always continue such pre-eminence of the first-formed frond or its
-representative, as to make the germination monocotyledonous. Figs.
-111 to 115, show the transitional forms that would result from the
-infolding of the fronds. In Fig. 116 (a vertical section of the form
-represented in Fig. 115) are exhibited the relations of the successive
-fronds to each other. The modified relations that would result, if
-the nutrition of the embryo admitted of anticipatory development of
-the successive fronds, are shown in Fig. 117. And how readily the
-structure may pass into that of the monocotyledonous germ, will be
-seen on inspecting Fig. 118; which is a vertical section of an actual
-monocotyledon at an early stage--the incomplete lines at the left of
-its root, indicating its connexion with the seed.[13] Contrariwise,
-where the strength required for maintaining an upright attitude is not
-obtained by the rolling up of the fronds, but by the strengthening
-of the continuous mid-rib, the second frond, so far from being less
-favourably circumstanced than the first, becomes in some respects even
-more favourably circumstanced: being above the other, it gets a greater
-share of light, and it is less restricted by surrounding obstacles.
-There is nothing, therefore, to prevent it from rapidly gaining an
-equality with the first. And if we assume, as the truths of embryology
-entitle us to do, an increasing tendency towards anticipation in the
-development of subsequent fronds--if we assume that here, as in other
-cases, structures which were originally produced in succession will,
-if the nutrition allows and no mechanical dependence hinders, come to
-be produced simultaneously; there is nothing to prevent the passage of
-the type represented in Fig. 111, into that represented in Fig. 122.
-Or rather, there is everything to facilitate it; seeing that natural
-selection will continually favour the production of a form in which the
-second frond grows in such way as not to shade the first, and in such
-way as allows the axis readily to assume a vertical position.
-
-Thus, then, is interpretable the universal connexion between
-monocotyledonous germination and endogenous growth; as well as
-the similarly-universal connexion between exogenous growth and
-the development of two or more cotyledons. That it explains these
-fundamental relations, adds very greatly to the probability of the
-hypothesis.
-
-
-§ 196. While we are in this manner enabled to discern the kinship that
-exists between the higher vegetal types themselves, as well as between
-them and the lower types; we are at the same time supplied with a
-rationale of those truths which vegetal morphologists have established.
-Those homologies which Wolff indicated in their chief outlines and
-Goethe followed out in detail, have a new meaning given to them when we
-regard the phænogamic axis as having been evolved in the way described.
-Forming the modified conception which we are here led to do, respecting
-the units of which a flowering plant is composed, we are no longer
-left without an answer to the question--What is an axis? And we are
-helped to understand the naturalness of those correspondences which the
-successive members of each shoot display. Let us glance at the facts
-from our present standpoint.
-
-  [Illustration: Figs. 123–129.]
-
-The unit of composition of a Phænogam, is such portion of a shoot as
-answers to one of the primordial fronds. This portion is neither one
-of the foliar appendages nor one of the internodes; but it consists of
-a foliar appendage together with the preceding internode, including
-the axillary bud where this is developed. The parts intercepted by
-the dotted lines in Fig. 123, constitute such a segment; and the true
-homology is between this and any other foliar organ with the portion
-of the axis below it. And now observe how, when we take this for the
-unit of composition, the metamorphoses which the phænogamic axis
-displays, are inferable from known laws of development. Embryology
-teaches us that arrest of development shows itself first in the absence
-of those parts that have arisen latest in the course of evolution;
-that if defect of nutrition causes an earlier arrest, parts that are
-of more ancient origin abort; and that the part alone produced when
-the supply of materials fails near the outset, is the primordial
-part. We must infer, therefore, that in each segment of a Phænogam,
-the foliar organ, which answers to the primordial frond, will be the
-most constant element; and that the internode and the axillary bud,
-will be successively less constant. This we find. Along with a smaller
-size of foliar surface implying lower nutrition, it is usual to see a
-much-diminished internode and a less-pronounced axillary bud, as in
-Fig. 124. On approaching the flower, the axillary bud disappears; and
-the segment is reduced to a small foliar surface, with an internode
-which is in most cases very short if not absent, as in 125 and 126.
-In the flower itself, axillary buds and internodes are both wanting:
-there remains only a foliar surface (127), which, though often larger
-than the immediately-preceding foliar surface, shows failing nutrition
-by absence of chlorophyll. And then, in the quite terminal organs of
-fructification (129), we have the foliar part itself reduced to a
-mere rudiment. Though these progressive degenerations are by no means
-regular, being in many cases varied by adaptations to particular
-requirements, yet it cannot, I think, be questioned, that the general
-relations are as described, and that they are such as the hypothesis
-leads us to expect. Nor are we without a kindred explanation of certain
-remaining traits of foliar organs in their least-developed forms.
-Petals, stamens, pistils, &c., besides reminding us of the primordial
-fronds by their diminished sizes, and by the want of those several
-supplementary parts which the preceding segments possess, also remind
-us of them by their histological characters: they consist of simple
-cellular tissue, scarcely at all differentiated. The fructifying cells,
-too, which here make their appearance, are borne in ways like those in
-which the lower Acrogens bear them--at the edge of the frond, or at the
-end of a peduncle, or immersed in the general substance; as in Figs.
-128 and 129. Nay, it might even be said that the colours assumed by
-these terminal folia, call to mind the plants out of which we conclude
-that Phænogams have been evolved; for it is said of the fronds of the
-_Jungermanniaceæ_, that, “though under certain circumstances of a pure
-green, they are inclined to be shaded with red, purple, chocolate, or
-other tints.”
-
-As thus understood, then, the homologies among the parts of the
-phænogamic axis are interpretable, not as due to a needless adhesion
-to some typical form or fulfilment of a predetermined plan; but as
-the inevitable consequences of the mode in which the phænogamic axis
-originates.
-
-
-§ 197. And now it remains only to observe, in confirmation of
-the foregoing synthesis, that it at once explains for us various
-irregularities. When we see leaves sometimes producing leaflets from
-their edges or extremities, we recognize in the anomaly a resumption of
-an original mode of growth: fronds frequently do this. When we learn
-that a flowering plant, as the _Drosera intermedia_, has been known to
-develop a young plant from the surface of one of its leaves, we are
-at once reminded of the proliferous growths and fructifying organs in
-the Liverworts. The occasional production of bulbils by Phænogams,
-ceases to be so surprising when we find it to be habitual among the
-inferior Acrogens, and when we see that it is but a repetition,
-on a higher stage, of that self-detachment which is common among
-proliferously-produced fronds. Nor are we any longer without a solution
-of that transformation of foliar organs into axial organs, which not
-uncommonly takes place. How this last irregularity of development is to
-be accounted for, we will here pause a moment to consider. Let us first
-glance at our data.
-
-The form of every organism, we have seen, must depend on the structures
-of its physiological [or constitutional] units. Any group of such
-units will tend to arrange itself into the complete organism, if
-uncontrolled and placed in fit conditions. Hence the development of
-fertilized germs; and hence the development of those self-detached
-cells which characterize some plants. Conversely, physiological units
-which form a small group involved in a larger group, and are subject to
-all the forces of the larger group, will become subordinate in their
-structural arrangements to the larger group--will be co-ordinated into
-a part of the major whole, instead of co-ordinating themselves into a
-minor whole. This antithesis will be clearly understood on remembering
-how, on the one hand, a small detached part of a hydra soon moulds
-itself into the shape of an entire hydra; and how, on the other hand,
-the cellular mass that buds out in place of a lobster’s lost claw,
-gradually assumes the form of a claw--has its parts so moulded as to
-complete the structure of the organism: a result which we cannot but
-ascribe to the forces which the rest of the organism exerts upon it.
-Consequently, among plants, we may expect that whether any portion of
-protoplasm moulds itself into the typical form around an axis of its
-own, or is moulded into a part subordinate to another axis, will depend
-on the relative mass of its physiological units--the accumulation of
-them that has taken place before the assumption of any structural
-arrangement. A few illustrations will make clear the validity of this
-inference. In the compound leaf, Fig. 65, the several lateral growths
-_a_, _b_, _c_, _d_, are manifestly homologous; and on comparing a
-number of such leaves together, it will be seen that one of these
-lateral growths may assume any degree of complexity, according to the
-degree of its nutrition. Every fern-leaf exemplifies the same general
-truth still better. Whether each sub-frond remains an undeveloped
-wing of the main frond, or whether it organizes itself into a group
-of frondlets borne by a secondary rib, or whether, going further, as
-it often does, it gives rise to tertiary ribs bearing frondlets, is
-determined by the supply of materials for growth; since such higher
-developments are most marked at points where the nutrition is greatest;
-namely, next the stem. But the clearest evidence is afforded among the
-_Algæ_, which, not drawing nutriment from roots, have their parts much
-less mutually dependent; and are therefore capable of showing more
-clearly, how any part may remain an appendage or may become the parent
-of appendages, according to circumstances. In the annexed Fig. 130,
-representing a branch of _Ptilota plumosa_, we see how a wing grows
-into a wing-bearing branch if its nutrition passes a certain point.
-This form, so strikingly like that of the feathery crystallizations of
-many inorganic substances, implies that, as in such crystallizations,
-the simplicity or complexity of structure at any place depends on the
-quantity of matter that has to be arranged at that place in a given
-time.[14]
-
-  [Illustration: Fig. 130.]
-
-Hence, then, we are not without an interpretation of those
-over-developments which the phænogamic axis occasionally undergoes.
-Fig. 104, represents the phænogamic bud in its rudimentary state. The
-lateral process _b_, which ordinarily becomes a foliar appendage,
-differs very little from the terminal process _c_, which is to become
-an axis--differs mainly in having, at this period when its form is
-being determined, a smaller bulk. If while thus undifferentiated, its
-nutrition remains inferior to that of the terminal process, it becomes
-moulded into a part that is subordinate to the general axis. But if,
-as sometimes happens, there is supplied to it such an abundance of
-the materials needful for growth, that it becomes as large as the
-terminal process; then we may naturally expect it to begin moulding
-itself round an axis of its own: a foliar organ will be replaced by
-an axial organ. And this result will be especially liable to occur,
-when the growth of the axis has been previously undergoing that arrest
-which leads to the formation of a flower; that is when, from defect
-of materials, the terminal process has almost ceased to increase, and
-when some concurrence of favourable causes brings a sudden access of
-sap which reaches the lateral processes before it reaches the terminal
-process.[15]
-
-
-§198. The general conclusion to which these various lines of evidence
-converge, is, then, that the shoot of a flowering plant is an aggregate
-of the third degree of composition. Taking as aggregates of the first
-order, those small portions of protoplasm which ordinarily assume
-the forms under which they are known as cells; and considering as
-aggregates of the second order, those assemblages of such cells which,
-in the lower cryptogams, compose the various kinds of thallus; then
-that structure, common to the higher cryptogams and to phænogams,
-in which we find a series of such groups of cells bound up into a
-continuous whole, must be regarded as an aggregate of the third
-order. The inference drawn from analysis, and verified by a synthesis
-which corresponds in a remarkable manner with the facts, is that
-those compound parts which, in Monocotyledons and Dicotyledons are
-called axes, have really arisen by integration of such simple parts
-as in lower plants are called fronds. Here, on a higher level,
-appears to have taken place a repetition of the process already
-observed on lower levels. The formation of those small groups of
-physiological units which compose the lowest protophytes, is itself
-a process of integration; and the consolidation of such groups into
-definitely-circumscribed and coherent cells or morphological units, is
-a completing of the process. In those coalescences by which many such
-cells are joined into threads, and discs, and solid or flattened-out
-masses, we see these morphological units aggregating into units of a
-compound kind: the different phases of the transition being exemplified
-by groups of various sizes, various degrees of cohesion, and various
-degrees of definiteness. And now we find evidences of a like process
-on a larger scale: the compound groups are again compounded. Moreover,
-as before, there are not wanting types of organization by which the
-stages of this higher integration are shadowed forth. From fronds that
-occasionally produce other fronds from their surfaces, we pass to those
-that habitually produce them; from those that do so in an indefinite
-manner, to those that do so in a definite manner; and from those that
-do so singly, to those that do so doubly and triply through successive
-generations of fronds. Even within the limits of a sub-class, we find
-gradations between fronds irregularly proliferous, and groups of such
-fronds united into a regular series.
-
-Nor does the process end here. The flowering plant is rarely
-uniaxial--it is nearly always multiaxial. From its primary shoot there
-grow out secondary shoots of like kind. Though occasionally among
-Phænogams, and frequently among the higher Cryptogams, the germs of
-new axes detach themselves under the form of bulbils, and develop
-separately instead of in connexion with the parent axis; yet in most
-Phænogams the germ of each new axis maintains its connexion with the
-parent axis: whence results a group of axes--an aggregate of the fourth
-order. Every tree, by the production of branch out of branch, shows
-us this integration repeated over and over again; forming an aggregate
-having a degree of composition too complex to be any longer defined.
-
-       *       *       *       *       *
-
-[NOTE.--A criticism passed on the general argument set forth in the
-foregoing sections, runs as follows:--“I have already pointed out that
-the process of evolution by which you believe the Liverworts with a
-distinct axis and appendages to have been produced from the thalloid
-forms is not founded on sound evidence either in comparative morphology
-or development. But even if we admit that such an integration of a
-proliferously-produced colony might have given rise to the leafy
-_Jungermanniaceæ_, there are even more weighty objections to the
-supposition that the same process produced the shoot structures of
-the flowering plants. In the first place the flowering plant-body is
-_not homologous with the liverwort plant-body_, since they represent
-different generations. The liverwort plant-body or _gametophyte_,
-_i.e._, the generation bearing sexual organs, is homologous with the
-prothallus of ferns and other Pteridophytes, and in the Flowering
-Plants with reduced structures contained within the spores (embryo-sac
-and pollen-grain) but still giving rise to sexual cells. The liverwort
-spore-capsule and its accessory parts (in fact everything produced
-from the fertilized egg) is homologous with the sporogonium of the
-mosses, and, as most botanists think, with the leafy plant-body
-of Pteridophytes and Phanerogams. This generation is called the
-_sporophyte_ and from the spores which it produces are developed
-the gametophytes of the next generation. These generalizations were
-first established by Hofmeister, and all subsequent work has tended
-to establish them more firmly. The only doubtful question is (and the
-doubt is mainly, I think, peculiar to myself, certainly not being
-shared by the majority of botanists) whether the sporophyte of Mosses
-and Liverworts is really homologous with that of Pteridophytes and
-Phanerogams, whether it may not rather be regarded as a parallel
-development along another line of descent from the Green Algæ.
-
-“Hence we must look for the origin of the shoot-structure of flowering
-plants in the sporophytes of the Pteridophytes, from which group there
-is no reason to doubt that the phanerogams have arisen in descent.
-The various groups of Pteridophytes vary much in the organization of
-these shoot-systems, as a mental glance at the types exhibited by the
-Ferns, Horse-tails, Club-mosses, _Ophioglossaceæ_, and the isolated
-Isoetes will convince you at once. It may be that some of these groups
-are independent in descent, _i.e._, that the _Pteridophyta_ are
-polyphyletic, and the current hypothesis with regard to the phanerogams
-is that they have arisen by two, if not three, separate lines of
-descent from different groups of Pteridophytes (this is indicated in
-the classificatory diagram on p. 377 of vol. I). I should not, however,
-care to pin my faith to these or to any such lines of ancestry. Still
-I think we must look for the ancestors of the Flowering Plants among
-the Pteridophytes, and the latter always have a good distinction
-between axis and appendages. The problem of the evolution of these
-differentiated sporophytic shoots is undoubtedly the great outstanding
-problem of morphology. Various attempts have been made to solve it,
-of which probably the most important is the theory of Profs. Bown
-and Campbell, who derive the Pteridophytes from some Liverwort like
-_Anthoceros_, but the sporophyte of course from the sporophytic portion
-of the plant (not much more than a spore-capsule), the prothallus
-of the Fern representing the vegetative thallus of Anthoceros. I am
-not wholly convinced by these undoubtedly ingenious hypotheses, in
-support of which an immense amount of facts have been collected; but
-my position would, I know, simply ‘put us to ignorance again’ on this
-question.
-
-“I have discussed this at some length in order to bring out clearly
-the immense difficulty of constructing a wellgrounded theory of
-the origin of the differentiated shoot-system of the higher plant.
-I confess I don’t think it can be done at all with the materials at
-present at our disposal. Of course it is just possible to suppose that
-some ancestral sporophyte had the structure of a proliferous thalloid
-liverwort gametophyte, and that from it was evolved the phanerogamic
-shoot in the ways you suggest. This gives us absolutely no clue,
-however, to any Pteridophytic shoot, which ought to be intermediate
-(more or less) between the hypothetical ancestor and the Phanerogam,
-and is furthermore, as far as I can see, not supported by an atom of
-evidence of any kind. It is true that your theory fits in well with the
-phenomena exhibited by phanerogamic shoots themselves, but this fact
-you will see must lose much of its significance if the hypothesis lacks
-foundation.
-
-“With regard to your method of explaining the fundamental characters
-of ‘Exogens’ and ‘Endogens,’ this of course is part of the same
-hypothesis; but I may point out that since Von Mohl and Sanio, between
-1855 and 1865, showed (1) that the growth at the stem apex of a
-monocotyledon was _not_ endogenous, and (2) that the ‘thickening ring’
-near the apex of a dicotyledon was not to be confused, as had been
-done up till then, with the ring of _secondary meristem_ or _true
-cambium_, which arose lower down, and only in woody or practically
-woody stem, the terms ‘Exogen’ and ‘Endogen’ have necessarily fallen
-into disuse, since they imply a false conception of what happens. Both
-monocotyledons and dicotyledons have a ‘thickening ring,’ which gives
-rise to the primary vascular cylinder of the stem. When the stem is
-of considerable thickness, as in Palms, &c., it grows by the active
-cell-division of its outer layers, so that both classes are ‘exogenous’
-in this sense; while the addition of a centrifugal zone of secondary
-wood is confined to certain Dicotyledons (Trees, shrubs, &c.).
-
-“The distinction between the embryos, moreover, is not absolute. The
-single cotyledon is usually terminal in monocotyledons, but not always
-(_Dioscoraceæ_ have lateral cotyledons), but the plumule may push
-through it (Grasses) or make its exit sideways (Palms), or be formed at
-the side (_Alisma_); and Dicotyledons very similarly.
-
-“The occurrence of completely sheathing leaves in grasses is perhaps
-correlated with the absence of cambium, but grasses are an aberrant
-type among monocotyledons, and secondary thickening is only found in
-very few genera of this class, so that the correlation is, so to speak,
-negative and indirect.... It is clear that the greater part of the
-discussion will have to be re-written.”
-
-For the reasons assigned in the preface I cannot undertake to re-write
-the discussion, as suggested. It must stand for what it is worth. All I
-can do is here to include along with it the foregoing criticisms.
-
-I may, however, indicate the line of defence I should take were I to
-go again into the matter. The objections are based on the structure of
-existing Liverworts and Phænogams. But I have already referred to the
-probability--or, indeed, the certainty--that in conformity with the
-general principle set forth in the note to Chapter I, we must conclude
-that the early types of Liverworts out of which the Phænogams are
-supposed to have evolved, as well as the early types of Phænogams in
-which the stages of evolution were presented, no longer exist. We must
-infer that forms simpler than any now known, and more intermediate in
-their traits, were the forms concerned; and if so, it may be held that
-the incongruities with the hypothesis which are presented by existing
-forms, do not negative it. The scepticism my critic himself expresses
-respecting the current interpretation is a partial justification of
-this view. Moreover, his admission that the theory set forth “fits in
-well with the phenomena exhibited by phanerogamic shoots,” must, I
-think, be regarded as weighty evidence. On the Evolution hypothesis
-we are obliged to suppose that the Monocotyledons and Dicotyledons
-respectively arose by integration of fronds; and if to the question
-after what manner the integration took place, there is an hypothesis
-which renders it comprehensible, and agrees both with the structures of
-the two kinds of shoots and the structures of the two kinds of seeds,
-as well as with various of the other phenomena the two types present,
-it has strong claims for acceptance.
-
-Reconsideration suggests the following remarks.
-
-1. Alternation of generations is a means of furthering multiplication.
-To be effective each member of either generation must be a
-self-supporting centre of growth or diffusion or both. Hence if, as in
-the Liverworts, one of the so-called alternating generations is not
-independent, but a permanent growth on the other--a parasite--it is a
-misuse of words to call the arrangement Alternation of generations.
-(Since this was written I have found that Sir Edward Fry takes the
-same view. He approvingly quotes Professor Bower, who says that “the
-alternation of generations is not an accurate statement of facts or a
-useful analogy.”)
-
-2. The alternating of sexual and non-sexual processes is not
-fundamentally distinctive; for, as shown by sundry Archegoniates,
-it is an inconstant trait, and as shown by Klebs’ experiments on
-_Vaucheria_, the conditions may be varied so as to determine its
-occurrence or non-occurrence. Nay, the same individual may reproduce in
-either way.
-
-3. Still more significant is the fact that in some of the marine
-Thallophytes, there is a process like that which in a moss or a fern
-is considered an alternation of generations, whereas in others, as
-the Brown Wrack (_Fucus_), each generation is sexual. Thus the
-presence or absence of this mode of genesis cannot be a cardinal
-distinction.
-
-4. With these facts before us, it is not only a reasonable supposition
-but a highly probable supposition, that there have existed plants of
-the Liverwort type in which the so-called alternation of generations
-did not take place. If so, nearly all the foregoing objections to my
-hypothesis fall to the ground.]
-
-
-
-
-                              CHAPTER IV.
-
-               THE MORPHOLOGICAL COMPOSITION OF ANIMALS.
-
-
-§ 199. What was said in § 180, respecting the ultimate structure of
-organisms, holds more manifestly of animals than of plants. That
-throughout the vegetal kingdom the cell is the morphological unit, is
-a proposition admitting of a better defence, than the proposition that
-the cell is the morphological unit throughout the animal kingdom. The
-qualifications with which, as we saw, the cell-doctrine must be taken,
-are qualifications thrust upon us more especially by the facts which
-zoologists have brought to light. It is among the _Protozoa_ that
-there occur numerous cases of vital activity displayed by specks of
-protoplasm; and from the minute anatomy of all creatures above these,
-are drawn the numerous proofs that non-cellular tissues may arise by
-direct metamorphosis of mixed colloidal substances.[16]
-
-Our survey of morphological composition throughout the animal kingdom,
-must therefore begin with those undifferentiated aggregates of
-physiological units [or constitutional units], out of which are formed
-what we call, with considerable license, morphological units.
-
-
-§ 200. In that division of the _Protozoa_ distinguished as _Rhizopoda_,
-are presented, under various modifications, these minute portions of
-living organic matter, so little differentiated, if not positively
-undifferentiated, that animal individuality can scarcely be claimed
-for them. Figs. 131, 132, and 133, represent certain nearly-allied
-types of these--_Amœba_, _Actinophrys_, and _Lieberkühnia_. The
-viscid jelly or sarcode, comparable in its physical properties to
-white of egg, out of which one of these creatures is mainly formed,
-shows us in various ways, the feebleness with which the component
-physiological units are integrated--shows us this by its very slight
-cohesion, by the extreme indefiniteness and mutability of its form,
-and by the absence of a limiting membrane. It is no longer held even
-by unqualified adherents of the cell-doctrine that the _Amœba_ has an
-investment. Its outer surface, compared to the film which forms on
-the surface of paste, does not prevent the taking of solid particles
-into the mass of the body, and does not, in such kindred forms as Fig.
-133, prevent the pseudopodia from coalescing when they meet. Hence it
-cannot properly have the name of a cell-wall. A considerable portion
-of the body, however, in _Difflugia_, Fig. 134, has a denser coating
-formed of agglutinated foreign particles; so that the protrusion of
-the pseudopodia is limited to one part of it. And in the solitary
-_Foraminifera_, like _Gromia_, the sarcode is covered over most of its
-surface by a delicate calcareous shell, pierced with minute holes,
-through which the slender pseudopodia are thrust. The _Gregarina_
-exhibits an advance in integration, and a consequent greater
-definiteness. Figs. 135 and 136, exemplifying this type, show the
-complete membrane in which the substance of the creature is contained.
-Here there has arisen what may be properly called a cell: under its
-solitary form this animal is truly unicellular. Its embryology has
-considerable significance. After passing through a certain quiescent,
-“encysted” state, its interior breaks up into small portions, which,
-after their exit, assume forms like that of the _Amœba_; and from
-this young condition in which they are undifferentiated, they pass
-into that adult condition in which they have limiting membranes. If
-this development of the individual _Gregarina_ typifies the mode of
-evolution of the species, it yields further support to the belief, that
-fragments of sarcode existed earlier than any of the structures which
-are called cells. Among aggregates of the first order, there are some
-much more highly developed. These are the _Infusoria_, constituting
-the most numerous of the _Protozoa_, in species as in individuals.
-Figs. 137, 138, and 139, are examples. In them we find, along with
-greater definiteness, a considerable heterogeneity. The sarcode of
-which the body consists, has an indurated outer layer, bearing cilia
-and sometimes spines; there is an opening serving as mouth, a permanent
-œsophagus, and a cavity or cavities, temporarily formed in the interior
-of the sarcode, to serve as one or more stomachs; and there is a
-comparatively specific arrangement of these and various minor parts.
-
-  [Illustration: Figs. 131–139.]
-
-Thus in the animal kingdom, as in the vegetal kingdom, there exists
-a class of minute forms having this peculiarity, that no one of them
-is separable into a number of visible components homologous with one
-another--no one of them can be resolved into minor individualities.
-Its proximate units are those physiological units of which we conclude
-every organism consists. The aggregate is an aggregate of the first
-order.
-
-
-§ 201. Among plants are found types indicating a transition from
-aggregates of the first order to aggregates of the second order; and
-among animals we find analogous types. But the stages of progressing
-integration are not here so distinct. The reason probably is, that
-the simplest animals, having individualities much less marked than
-those of the simplest plants, do not afford us the same facilities for
-observation. In proportion as the limits of the minor individualities
-are indefinite, the formation of major individualities out of them,
-naturally leaves less conspicuous traces.
-
-  [Illustration: Figs. 140–145.]
-
-Be this as it may, however, in such types of _Protozoa_ as the compound
-_Radiolaria_, we find that though there is reason to regard the
-aggregate as an aggregate of the second order, yet its divisibility
-into minor individualities like those just described, is less manifest.
-Fig. 140 representing _Sphærozoum punctatum_, one of the group,
-illustrates this. The sceptically-minded may perhaps doubt whether we
-can regard the “cellæform bodies” contained in it, as the morphological
-units of the animal. The jelly-like mass in which they are imbedded, is
-but indefinitely divisible into portions having each a cell or nucleus
-for its centre.[17] Among the _Foraminifera_, we find only indefinite
-evidence of the coalescence of aggregates of the first order, into
-aggregates of the second order. There are solitary Foraminifers,
-allied to the creature represented in Fig. 134. Certain ideal types of
-combination among them, are shown in Fig. 141. And setting out from
-these, we may ascend in various directions to kinds compounded to
-an immense variety of degrees in an immense variety of ways. In all
-of them, however, the separability of the major individuality into
-minor individualities, is very incomplete. The portion of sarcode
-contained in one of these calcareous chambers, gives origin to an
-external bud; and this presently becomes covered, like its parent,
-with calcareous matter: the position in which each successive chamber
-is so produced, determining the form of the compound shell. But the
-portions of sarcode thus budded out one from another, do not become
-distinctly individualized. Fig. 142, representing the living network
-which remains when the shell of an Orbitolite has been dissolved,
-shows the continuity that exists among the occupants of its aggregated
-chambers.[18] In the compound _Infusoria_, the component units remain
-quite distinct. Being, as aggregates of the first order, much more
-definitely organized, their union into aggregates of the second
-order does not destroy their original individualities. Among the
-_Vorticellæ_, of which two kinds are delineated in Figs. 144 and 145,
-there are various illustrations of this: the members of the community
-being sometimes appended to a single stem; sometimes attached by long
-separate stems to a common base; and sometimes massed together.
-
-  [Illustration: Figs. 146–147.]
-
-Thus far, these aggregates of the second order exhibit but indefinite
-individualities. The integration is physical; but not physiological.
-Though, in the _Polycytharia_, there is a shape that has some symmetry;
-and though, in the _Foraminifera_, the formation of successive chambers
-proceeds in such methodic ways as to produce quite-regular and
-tolerably-specific shells; yet no more in these than in the Sponges or
-the compound _Vorticellæ_, do we find such co-ordination as gives the
-whole a life predominating over the lives of its parts. We have not
-yet reached an aggregate of the second order, so individuated as to be
-capable of serving as a unit in still higher combinations. But in the
-class _Cœlenterata_, this advance is displayed. The common _Hydra_,
-habitually taken as the type of the lowest division of this class,
-has specialized parts performing mutually-subservient functions, and
-thus exhibiting a total life distinct from the lives of the units.
-Fig. 146 represents one of these creatures in its contracted state
-and in its expanded state; while Fig. 147 is a diagram showing the
-wall of this creature’s sac-like body as seen in section under the
-microscope: _a_ and _b_ being the outer and inner cellular layers;
-while between them is the “mesoglœa” or “structureless lamella,” the
-supporting or skeletal layer. But this lowly-organized tissue of the
-Hydra, illustrates a phase of integration in which the lives of the
-minor aggregates are only partially-subordinated to the life of the
-major aggregate formed by them. For a _Hydra’s_ substance is separable
-into _Amœba_-like portions, capable of moving about independently.
-If we bear in mind how analogous are the extreme extensibility and
-contractility of a _Hydra’s_ body and tentacles, to the properties
-displayed by the sarcode among Rhizopods; we may infer that probably
-the movements and other actions of a _Hydra_, are due to the
-half-independent co-operation of the _Amœba_-like individuals composing
-it.
-
-
-§ 202. A truth which we before saw among plants, we here see repeated
-among animals--the truth that as soon as the integration of aggregates
-of the first order into aggregates of the second order, produces
-compound wholes so specific in their shapes and sizes, and so mutually
-dependent in their parts, as to have distinct individualities; there
-simultaneously arises the tendency in them to produce, by gemmation,
-other such aggregates of the second order. The approach towards
-definite limitation in an organism, is, by implication, an approach
-towards a state in which growth passing a certain point, results, not
-in the increase of the old individual, but in the formation of a new
-individual. Thus it happens that the common polype buds out other
-polypes, some of which very shortly do the like, as shown in Fig. 148:
-a process paralleled by the fronds of sundry _Algæ_, and by those of
-the lower _Jungermanniaceæ_. And just as, among these last plants,
-the proliferously-produced fronds, after growing to certain sizes
-and developing rootlets, detach themselves from their parent fronds;
-so among these animals, separation of the young ones from the bodies
-of their parents ensues when they have acquired tolerably complete
-organizations.
-
-  [Illustration: Figs. 148–150.]
-
-There is reason to think that the parallel holds still further.
-Within the limits of the _Jungermanniaceæ_, we found that while some
-genera exhibit this discontinuous development, other genera exhibit
-a development that is similar to it in all essential respects, save
-that it is continuous. And here within the limits of the _Hydrozoa_,
-we find, along with this genus in which the gemmiparous individuals
-are presently cast off, other genera in which they are not cast off,
-but form a permanent aggregate of the third order. Figs. 149 and 150,
-exemplify these compound _Hydrozoa_--one of them showing this mode
-of growth so carried out as to produce a single axis; and the other
-showing how, by repetitions of the process, lateral axes are produced.
-Integrations characterizing certain higher genera of the _Hydrozoa_
-which swim or float instead of being fixed, are indicated by Figs. 151
-and 152: the first of them representing the type of a group in which
-the polypes growing from an axis, or cœnosarc, are drawn through the
-water by the rhythmical contractions of the organs from which they
-hang; and the second of them representing a _Physalia_ the component
-polypes of which are united into a cluster, attached to an air-vessel.
-
-  [Illustration: Figs. 151–152.]
-
-A parallel series of illustrations might be drawn from that second
-division of the _Cœlenterata_, known as the _Actinozoa_. Here, too,
-we have a group of species--the Sea-anemones--the individuals of
-which are solitary. Here, too, we have agamogenetic multiplication:
-occasionally by gemmation, but more frequently by that modified process
-called spontaneous fission. And here, too, we have compound forms
-resulting from the arrest of this spontaneous fission before it is
-complete. To give examples is needless; since they would but show, in
-more varied ways, the truth already made sufficiently clear, that the
-compound _Cœlenterata_ are aggregates of the third order, produced by
-integration of aggregates of the second order such as we have in the
-_Hydra_. As before, it is manifest that on the hypothesis of evolution,
-these higher integrations will insensibly arise, if the separation
-of the gemmiparous polypes is longer and longer postponed; and that
-an increasing postponement will result by survival of the fittest,
-if it profits the group of individuals to remain united instead of
-dispersing.[19]
-
-
-§ 203. The like relations exist, and imply that the like processes
-have been gone through, among those more highly organized animals
-called _Polyzoa_ and _Tunicata_. We have solitary individuals, and we
-have variously-integrated groups of individuals: the chief difference
-between the evidence here furnished, and that furnished in the last
-case, being the absence of a type obviously linking the solitary state
-with the aggregated state.
-
-  [Illustration: Figs. 153–155.]
-
-This integration of aggregates of the second order, is carried on
-among the _Polyzoa_ in divers ways, and with different degrees of
-completeness. The little patches of minute cells, shown as magnified
-in Fig. 153, so common on the fronds of sea-weeds and the surfaces of
-rocks at low-water mark, display little beyond mechanical combination.
-The adjacent individuals, though severally originated by gemmation
-from the same germ, have but little physiological dependence. In
-kindred kinds, however, as shown in Figs. 154 and 155, one of which
-is a magnified portion of the other, the integration is somewhat
-greater: the co-operation of the united individuals being shown in the
-production of those tubular branches which form their common support,
-and establish among them a more decided community of nutrition.
-
-  [Illustration: Figs. 156–159.]
-
-Among the Ascidians this general law of morphological composition is
-once more displayed. Each of these creatures subsists on the nutritive
-particles contained in the water which it draws in through one orifice
-and sends out through another; and it may thus subsist either alone, or
-in connexion with others that are in some cases loosely aggregated and
-in other cases closely aggregated. Fig. 156, _Phallusia mentula_, is
-one of the solitary forms. A type in which the individuals are united
-by a stolon that gives origin to them by successive buds, is shown in
-_Perophora_, Fig. 157. Among the _Botryllidæ_, of which one kind is
-drawn on a small scale in Fig. 159, and a portion of the same on a
-larger scale in Fig. 158, there is a combination of the individuals
-into annular clusters, which are themselves imbedded in a common
-gelatinous matrix. And in this group there are integrations even a
-stage higher, in which several such clusters of clusters grow from
-a single base. Here the compounding and recompounding appears to be
-carried further than anywhere else in the animal kingdom.
-
-Thus far, however, among these aggregates of the third order, we see
-what we before saw among the simpler aggregates of the second order--we
-see that the component individualities are but to a very small extent
-subordinated to the individuality made up of them. In nearly all the
-forms indicated, the mutual dependence of the united animals is so
-slight, that they are more fitly comparable to societies, of which
-the members co-operate in securing certain common benefits. There is
-scarcely any specialization of functions among them. Only in the last
-type described do we see a number of individuals so completely combined
-as to simulate a single individual. And even here, though there appears
-to be an intimate community of nutrition, there is no physiological
-integration beyond that implied in several mouths and stomachs having a
-common vent.[20]
-
-
-§ 204. We come now to an extremely interesting question. Does there
-exist in other sub-kingdoms composition of the third degree, analogous
-to that which we have found so prevalent among the _Cœlenterata_ and
-the _Polyzoa_ and _Tunicata_? The question is not whether elsewhere
-there are tertiary aggregates produced by the branching or clustering
-of secondary aggregates, in ways like those above traced; but whether
-elsewhere there are aggregates which, though otherwise unlike in the
-arrangement of their parts, nevertheless consist of parts so similar to
-one another that we may suspect them to be united secondary aggregates.
-The various compound types above described, in which the united animals
-maintain their individualities so distinctly that the individuality
-of the aggregate remains vague, are constructed in such ways that the
-united animals carry on their several activities with scarcely any
-mutual hindrance. The members of a branched _Hydrozoon_, such as is
-shown in Fig. 149 or Fig. 150, are so placed that they can all spread
-their tentacles and catch their prey as well as though separately
-attached to stones or weeds. Packed side by side on a flat surface or
-forming a tree-like assemblage, the associated individuals among the
-_Polyzoa_ are not unequally conditioned: or if one has some advantage
-over another in a particular case, the mode of growth and the relations
-to surrounding objects are so irregular as to prevent this advantage
-re-appearing with constancy in successive generations. Similarly
-with the Ascidians growing from a stolon or those forming an annular
-cluster: each of them is as well placed as every other for drawing in
-the currents of sea-water from which it selects its food. In these
-cases the mode of aggregation does not expose the united individuals
-to multiform circumstances; and therefore is not calculated to produce
-among them any structural multiformity. For the same reason no marked
-physiological division of labour arises among them; and consequently no
-combination close enough to disguise their several individualities. But
-under converse conditions we may expect converse results. If there is a
-mode of integration which necessarily subjects the united individuals
-to unlike sets of incident forces, and does this with complete
-uniformity from generation to generation, it is to be inferred that
-the united individuals will become unlike. They will severally assume
-such different functions as their different positions enable them
-respectively to carry on with the greatest advantage to the assemblage.
-This heterogeneity of function arising among them, will be followed by
-heterogeneity of structure; as also by that closer combination which
-the better enables them to utilize one another’s functions. And hence,
-while the originally-like individuals are rendered unlike, they will
-have their homologies further obscured by their progressing fusion into
-an aggregate individual of a higher order.
-
-These converse conditions are in nearly all cases fulfilled where
-the successive individuals arising by continuous development are so
-budded-off as to form a linear series. I say in nearly all cases,
-because there are some types in which the associated individuals,
-though joined in single file, are not thereby rendered very unlike
-in their relations to the environment; and therefore do not become
-differentiated and integrated to any considerable extent. I refer
-to such Ascidians as the _Salpidæ_. These creatures float passively
-in the sea, attached together in strings. Being placed side by side
-and having mouths and vents that open laterally, each of them is as
-well circumstanced as its neighbours for absorbing and emitting the
-surrounding water; nor have the individuals at the two extremities any
-marked advantages over the rest in these respects. Hence in this type,
-and in the allied type _Pyrosoma_, which has its component individuals
-built into a hollow cylinder, linear aggregation may exist without the
-minor individualities becoming obscured and the major individuality
-marked: the conditions under which a differentiation and integration
-of the component individuals may be expected, are not fulfilled.
-But where the chain of individuals produced by gemmation, is either
-habitually fixed to some solid body by one of its extremities or moves
-actively through the water or over submerged stones and weeds, the
-several members of the chain become differently conditioned in the way
-above described; and may therefore be expected to become unlike while
-they become united. A clear idea of the contrast between these two
-linear arrangements and their two diverse results, will be obtained by
-considering what happens to a row of soldiers, when changed from the
-ordinary position of a single rank to the position of Indian file. So
-long as the men stand shoulder to shoulder, they are severally able
-to use their weapons in like ways with like efficiency; and could,
-if called on, similarly perform various manual processes directly
-or indirectly conducive to their welfare. But when, on the word of
-command “right face,” they so place themselves that each has one of
-his neighbours before him and another behind him, nearly all of them
-become incapacitated for fighting and for many other actions. They can
-walk or run one after another, so as to produce movement of the file
-in the direction of its length; but if the file has to oppose an enemy
-or remove an obstacle lying in the line of its march, the front man
-is the only one able to use his weapons or hands to much purpose. And
-manifestly such an arrangement could become advantageous only if the
-front man possessed powers peculiarly adapted to his position, while
-those behind him facilitated his actions by carrying supplies, &c. This
-simile, grotesque as it seems, serves to convey better perhaps than
-any other could do, a clear idea of the relations that must arise in a
-chain of individuals arising by gemmation, and continuing permanently
-united end to end. Such a chain can arise only on condition that
-combination is more advantageous than separation; and for it to be more
-advantageous, the anterior members of the series must become adapted to
-functions facilitated by their positions, while the posterior members
-become adapted to functions which their positions permit. Hence, direct
-or indirect equilibration or both, must tend continually to establish
-types in which the connected individuals are more and more unlike
-one another, at the same time that their several individualities are
-more and more disguised by the integration consequent on their mutual
-dependence.
-
-Such being the anticipations warranted by the general laws of
-evolution, we have now to inquire whether there are any animals which
-fulfil them. Very little search suffices; for structures of the kind
-to be expected are abundant. In that great division of the animal
-kingdom at one time called _Annulosa_, but now grouped into _Annelida_
-and _Arthropoda_, we find a variety of types having the looked-for
-characters. Let us contemplate some of them.
-
-
-§ 205. An adult Chætopod is composed of segments which repeat one
-another in their details as well as in their general shapes. Dissecting
-one of the lower orders, such as is shown in Fig. 160, proves that
-the successive segments, besides having like locomotive appendages,
-like branchiæ, and sometimes even like pairs of eyes, also have like
-internal organs. Each has its enlargement of the alimentary canal;
-each its contractile dilatation of the great blood-vessel; each its
-portion of the double nervous cord, with ganglia when these exist;
-each its branches from the nervous and vascular trunks answering to
-those of its neighbours; each its similarly answering set of muscles;
-each its pair of openings through the body-wall; and so on throughout,
-even to the organs of reproduction. That is to say, every segment is
-in great measure a physiological whole--every segment contains most
-of the organs essential to individual life and multiplication: such
-essential organs as it does not contain, being those which its position
-as one in the midst of a chain, prevents it from having or needing. If
-we ask what is the meaning of these homologies, no adequate answer is
-supplied by any current hypothesis. That this “vegetative repetition”
-is carried out to fulfil a predetermined plan, was shown to be quite
-an untenable notion (§§ 133, 134). On the one hand, we found nothing
-satisfactory in the conception of a Creator who prescribed to himself
-a certain unit of composition for all creatures of a particular class,
-and then displayed his ingenuity in building up a great variety of
-forms without departing from the “archetypal idea.” On the other hand,
-examination made it manifest that even were such a conception worthy
-of being entertained, it would have to be relinquished; since in each
-class there are numerous deviations from the supposed “archetypal
-idea.” Still less can these traits of structure be accounted for
-teleologically. That certain organs of nutrition and respiration and
-locomotion are repeated in each segment of a dorsibranchiate annelid,
-may be regarded as functionally advantageous for a creature following
-its mode of life. But why should there be a hundred or even two
-hundred pairs of ovaries? This is an arrangement at variance with that
-physiological division of labour which every organism profits by--is
-a less advantageous arrangement than might have been adopted. That
-is to say, the hypothesis of a designed adaptation fails to explain
-the facts. Contrariwise, these structural traits are just such as
-might naturally be looked for, if these annulose forms have arisen by
-the integration of simpler forms. Among the various compound animals
-already glanced at, it is very general for the united individuals to
-repeat one another in all their parts--reproductive organs included.
-Hence if, instead of a clustered or branched integration, such as
-the _Cœlenterata_, _Polyzoa_ and _Tunicata_ exhibit, there occurs a
-longitudinal integration; we may expect that the united individuals
-will habitually indicate their original independence by severally
-bearing germ-producing or sperm-producing organs.
-
-  [Illustration: Figs. 160–161.]
-
-The reasons for believing one of these creatures to be an aggregate of
-the third order, are greatly strengthened when we turn from the adult
-structure to the mode of development. Among the _Dorsibranchiata_
-and _Tubicolæ_, the embryo leaves the egg in the shape of a ciliated
-gemmule, not much more differentiated than that of a polype. As shown
-in Fig. 162, it is a nearly globular mass; and its interior consists
-of untransformed cells. The first appreciable change is an elongation
-and a simultaneous commencement of segmentation. The segments multiply
-by a modified gemmation, which takes place from the hinder end of the
-penultimate segment. And considerable progress in marking out these
-divisions is made before the internal organization begins. Figs. 163,
-164, 165, represent some of these early stages. In annelids of other
-orders, the embryo assumes the segmented form while still in the
-egg. But it does this in just the same manner as before. Indeed, the
-essential identity of the two modes of development is shown by the fact
-that the segmentation within the egg is only partially carried out: in
-all these types the segments continue to increase in number for some
-time after hatching. Now this process is as like that by which compound
-animals in general are formed, as the different conditions of the case
-permit. When new individuals are budded-out laterally, their unfolding
-is not hindered--there is nothing to disguise either the process or the
-product. But gemmæ produced one from another in the same straight line,
-and remaining connected, restrict one another’s developments; and that
-the resulting segments are so many gemmiparously-produced individuals,
-is necessarily less obvious.
-
-  [Illustration: Figs. 162–165.]
-
-
-§ 206. Evidence remains which adds very greatly to the weight of
-that already assigned. Thus far we have studied only the individual
-segmented animal; considering what may be inferred from its mode of
-evolution and final organization. We have now to study segmented
-animals in general. Comparison of different groups of them and of
-kinds within each group, will disclose various phases of progressive
-integration of the nature to be anticipated.
-
-  [Illustration: Figs. 166–169.]
-
-Among the simpler _Platyhelminthes_, as in some kinds of _Planaria_,
-transverse fission occurs. A portion of a _Planaria_ separated by
-spontaneous constriction, becomes an independent individual. Sir J.
-G. Dalyell found that in some cases numerous fragments artificially
-separated, grew into perfect animals.[21] In these creatures which thus
-remind us of the lowest _Hydrozoa_ in their powers of agamogenetic
-multiplication, the individuals produced one from another do not
-continue connected. As the young ones laterally budded-off by the
-_Hydra_ separate when complete, so do the young ones longitudinally
-budded-off by the _Planaria_. Fig. 166 indicates this. But there
-are allied types which show us a more or less persistent union of
-homologous parts, or individuals, similarly arising by longitudinal
-gemmation.[22] The cestoid _Entozoa_ furnish illustrations. Without
-dwelling on the fact that each segment of a _Tænia_, like each separate
-_Planaria_, is an independent hermaphrodite; and without specifying the
-sundry common structural traits which add probability to the suspicion
-that there is some kinship between the individuals of the one order
-and the segments of the other; it will suffice to point out that the
-two types are so far allied as to demand their union under the same
-sub-class title. And recognizing this kinship, we see significance
-in the fact that in the one case the longitudinally-produced gemmæ
-separate as complete individuals, and in the other continue united
-as segments in smaller or larger numbers and for shorter or longer
-periods. In _Tænia echinococcus_, represented in Fig. 167, we have
-a species in which the number of segments thus united does not
-exceed four. In _Echinobothrium typus_ there are eight or ten; and
-in cestoids generally they are numerous.[23] A considerable hiatus
-occurs between this phase of integration and the next higher phase
-which we meet with; but it is not greater than the hiatus between the
-types of the _Platyhelminthes_ and the _Chætopoda_, which present
-the two phases. Though it is doubtful whether separation of single
-segments occurs among the Annelida,[24] yet very often we find strings
-of segments, arising by repeated longitudinal budding, which after
-reaching certain lengths undergo spontaneous fission: in some cases
-doing this so as to form two or more similar strings of segments
-constituting independent individuals; and in other cases doing it so
-that the segments spontaneously separated are but a small part of the
-string. Thus a _Syllis_, Fig. 168, after reaching a certain length,
-begins to transform itself into two individuals: one of the posterior
-segments develops into an imperfect head, and simultaneously narrows
-its connexion with the preceding segments, from which it eventually
-separates. Still more remarkable is the extent to which this process
-is carried in certain kindred types; which exhibit to us several
-individuals thus being simultaneously formed out of groups of segments.
-Fig. 169, copied (omitting the appendages) from one contained in a
-memoir by M. Milne-Edwards, represents six worms of different ages
-in course of development: the terminal one being the eldest, the
-one having the greatest number of segments, and the one that will
-first detach itself; and the successively anterior ones, with their
-successively smaller numbers of segments, being successively less
-advanced towards fitness for separation and independence. Here among
-groups of segments we see repeated what in the previous cases occurs
-with single segments. And then in other annelids we find that the
-string of segments arising by gemmation from a single germ becomes a
-permanently united whole: the tendency to any more complete fission
-than that which marks out the segments, being lost; or, in other words,
-the integration having become relatively complete. Leaving out of sight
-the question of alliance among the types above grouped together, that
-which it here concerns us to notice is, that longitudinal gemmation
-does go on; that it is displayed in that primitive form in which the
-gemmæ separate as soon as produced; that we have types in which such
-gemmæ hang together in groups of four, or in groups of eight and ten,
-from which however the gemmæ successively separate as individuals;
-that among higher types we have long strings of similarly-formed
-gemmæ which do not become individually independent, but separate into
-organized groups; and that from these we advance to forms in which all
-the gemmæ remain parts of a single individual. One other significant
-fact must be added. There are cases in which annelids multiply by
-lateral gemmation.[25] That the longitudinally-produced gemmæ which
-compose an annelid, should thus have, one of them or several of them,
-the power of laterally budding-off gemmæ, from which other annelids
-arise, gives further support to the hypothesis that, primordially, the
-segments were independent individuals. And it suggests this belief
-the more strongly because, in certain types of _Cœlenterata_, we see
-that longitudinal and lateral gemmation _do_ occur together, where the
-longitudinally-united gemmæ are demonstrably independent individuals.
-
-
-§ 207. Though it seems next to impossible that we shall ever be able
-to find a type such as that which is here supposed to be the unit of
-composition of the annulose type, since we must assume such a type
-to have been long since extinct, yet the foregoing evidence goes
-far towards showing that an annulose animal is an aggregate of the
-third order. This repetition of segments, sometimes numbering several
-hundreds, like one another in all their organs even down to those of
-reproduction, while it is otherwise unaccountable, is fully accounted
-for if these segments are homologous with the separate individuals of
-some lower type. The gemmation by which these segments are produced, is
-as similar as the conditions allow, to the gemmation by which compound
-animals in general are produced. As among plants, and as among
-demonstrably-compound animals, we see that the only thing required
-for the formation of a permanent chain of gemmiparously-produced
-individuals, is that by remaining associated such individuals will
-have advantages greater than are to be gained by separation. Further,
-comparisons of the annuloid and lower annulose forms, disclose
-a number of those transitional phases of integration which the
-hypothesis leads us to expect. And, lastly, the differences among these
-united individuals or successive segments, are not greater than the
-differences in their positions and functions explain--not greater than
-such differences are known to produce among other united individuals:
-witness sundry compound _Hydrozoa_.
-
-Indirect evidence of much weight has still to be given. Thus far
-we have considered only the less developed _Annulosa_. The more
-integrated and more differentiated types of the class remain. If in
-them we find a carrying further of the processes by which the lower
-types are here supposed to have been evolved, we shall have additional
-reason for believing them to have been so evolved. If we find that in
-these superior orders, the individualities of the united segments are
-much less pronounced than in the inferior, we shall have grounds for
-suspecting that in the inferior the individualities of the segments are
-less pronounced than in those lost forms which initiated the annulose
-sub-kingdom.
-
-       *       *       *       *       *
-
-[NOTE.--Partly from the wish to incorporate further evidence, and
-partly from the wish to present the evidence, old and new, in a more
-effective order, I decide here to recast the foregoing exposition.
-
-Significant traits of development are exhibited in common by two groups
-otherwise unallied--certain of the _Platyhelminthes_ and certain of
-the lower _Annulosa_. Of the _Platyhelminthes_ the ordinary type is
-an unsegmented creature: a Planarian or a Trematode exemplifying it.
-Among the free forms, as in some Planarians, there occurs transverse
-fission, and prompt separation of the segments; while among some
-other free forms, as the _Microstomida_, the two segments first
-produced, themselves become segmented while still adherent, and this
-process is repeated until a string is formed. Another group of the
-_Platyhelminthes_, the Cestoid _Entozoa_, exhibit analogous processes.
-There are unsegmented forms, as the _Caryophyllæus_, and there are
-forms in which the segments, now few now many, adhere together in
-chains; the terminal members of which, however, eventually separate,
-and having before separation approached the trematode structure,
-become independent individuals which grow, creep about, and continue
-the race. In both of these types the condition under which the
-gemmiparously-produced members remain connected, is that they shall
-be able to feed individually: in the one case by lateral mouths, in
-the other case by absorption through the integument. It is further
-observable that in both cases separation of the component individuals
-occurs at sexual maturity, when advantage in nutrition has ceased
-to be the dominant need and dispersion of the species has taken its
-place in degree of importance. Among Annelids, higher though they are
-in type, we find parallelisms. Usually in its first stage an annelid
-is unsegmented, but as fast as it elongates lines of segmentation
-indent its surface. This segmentation proceeds in various ways, and
-the segments exhibit various degrees of dependence. In some low types,
-spontaneous fission goes on to the extent of producing single segments,
-each of which has such vitality that it buds out anterior and posterior
-parts at its two ends. Thus alike in the simple form which exists
-before segmentation and in the form exhibited by a detached segment, we
-have a unit analogous to each of the units which are joined together in
-certain free _Turbellaria_ and in the Cestoids: the difference being
-that in the Annelids the sexually mature units do not individually
-disunite. But though there does not take place separation of single
-completed segments, there takes place separation of groups of segments,
-which are either sexually mature at the time they drop off or presently
-become so. And the groups of segments which have become sexually mature
-before they drop off, have simultaneously acquired swimming organs
-and developed eyes, enabling them to spread and diffuse the species.
-Sundry biologists recognize a parallelism between that detachment of
-developed segments which goes on in the cestoid _Entozoa_, and that
-which goes on in the _Scyphomedusæ_. The successively detached members
-of the strobila are sexually matured or maturing individuals which,
-as medusæ, are fitted for swimming about, multiplying, and reaching
-other habitats; while each detached proglottis of the cestoid is, by
-the nature of its medium, limited to creeping about. Clearly this
-fissiparous process in such Annelids as the _Syllidæ_, which has
-similarly been compared to the strobilization of the _Scyphomedusæ_,
-differs simply in the respect that single segments are not adapted for
-locomotion, and it therefore profits the species to separate in groups.
-All these facts and analogies point to the conclusion that the remote
-ancestor of the Annelids was an unsegmented creature homologous with
-each of the segments of an existing Annelid.
-
-This conclusion is supported by other kinds of evidence here to
-be added. The larvæ of Annelids are very various; but amid their
-differences there is a recognizable type. “The Trochophore is the
-typical larval form of the Annelid stem”: a trochophore being a curious
-spheroidal ciliated structure suggestive of cœlenterate affinities. And
-this unsegmented larva, representing the remote ancestor from which the
-many Annelid types diverged, is similar to the larvæ of the _Rotifera_
-and the _Mollusca_: a trochophore is common to all these great classes.
-Moreover since, among the _Rhizota_ (a sub-class of the _Rotiferæ_),
-there is a species, _Trochosphæra_, solitary and free-swimming,
-resembling in form and structure a trochophore, though it is not a
-larva but an adult, we get further evidence that there was a primitive
-creature of this general character, of which the trochophores of
-_Mollusca_, _Rotifera_, and _Annelida_ are divergent modifications,
-and which was unsegmented: the implication being that the segmentation
-of the _Annelida_ was superinduced. That this segmentation resulted
-from gemmation is implied by what are called polytrochal larvæ. These
-“sometimes appear as a stage succeeding other larval types. Thus those
-of _Arenicola marina_ arise from larvæ which at first were monotrochal,
-later became telotrochal, and finally, by the appearance of new
-ciliated rings between those already present, assumed the stage of
-polytrochal larvæ.... This condition warrants the assumption that the
-segmented forms are to be looked upon as the younger, the unsegmented,
-on the other hand, as the phylogenetically older.” (Korschelt and
-Heider, i, 278.) And that the above-described rings of cilia mark
-off segments is shown by the case of _Ophryotrocha puerilis_, which
-“remains, as it were, in a larval condition, since the segments
-retain their ciliation throughout life.” (_Ib._, 277.) Yet one more
-significant fact must be named. In early stages of development each
-segment of an archiannelidan has cœlomic spaces separate from those
-of neighbouring segments, but in the adult the septa “generally break
-down either partially or completely, so that the peri-visceral cavity
-becomes a continuous space from end to end of the animal.” (Sedgwick,
-_Text Book_, 449.) While this fact is congruous with the hypothesis
-here maintained, it is incongruous with the hypothesis that the annelid
-was originally an elongated creature which afterwards became segmented;
-since in that case the implication would be that the cœlomic septa, not
-arising from recapitulation of an ancestral structure, but originated
-by the process of segmentation, were first superfluously formed and
-then destroyed.
-
-Various lines of evidence thus converge to the conclusion that an
-annulose animal is an aggregate of the third order.
-
-In June, 1865, when No. 14 of my serial containing the foregoing
-chapter was issued, I supposed myself to be alone in holding this
-belief respecting the annulose type, and long continued to suppose so.
-Over thirty years later, however, in M. Edmond Perrier’s work, _La
-Philosophie Zoologique avant Darwin_, I found mention of a lecture
-delivered by M. Lacaze-Duthiers at the École Normale Supérieure in
-Paris, and reported in the _Revue des Cours Scientifiques_ for January
-28, 1865, in which he enunciated a like belief. Judging, however, by
-the account of this lecture which M. Perrier gives (he was present), it
-appears that M. Lacaze-Duthiers simply contended that this view of the
-annulose structure as arising by union of once-independent units, is
-suggested by certain _à priori_ considerations. There is no indication
-that he assigned any of the classes of facts above given, which go to
-show that it _has_ thus arisen.
-
-For further facts and arguments concerning the genesis of the annulose
-type, see Appendix D 2.]
-
-
-
-
-                              CHAPTER V.
-
-               THE MORPHOLOGICAL COMPOSITION OF ANIMALS,
-                             _CONTINUED_.
-
-
-§ 208. Insects, Arachnids, Crustaceans, and Myriapods, are all members
-of that higher division of the _Annulosa_[26] called _Articulata_
-or now more generally _Arthropoda_. Though in these creatures the
-formation of segments may be interpreted as a disguised gemmation;
-and though, in some of them, the number of segments increases by this
-modified budding after leaving the egg, as it does among the Annelids;
-yet the process is not nearly so dominant: the segments are usually
-much less numerous than we find them in the types last considered. In
-most cases, too, the segments are in a greater degree differentiated
-one from another, at the same time that they are severally more
-differentiated within themselves. Nor is there any instance of
-spontaneous fission taking place in the series of segments composing
-an articulate animal. On the contrary, the integration, always great
-enough permanently to unite the segments, is frequently carried so
-far as to hide very completely the individualities of some or many of
-them; and occasionally, as among the Acari, the consolidation, or the
-arrest of segmentation, is so decided as to leave scarcely a trace
-of the articulate structure: the type being in these cases indicated
-chiefly by the presence of those characteristically-formed limbs, which
-give the alternative name _Arthropoda_ to all the higher _Annulosa_.
-Omitting the parasitic orders, which, as in other cases, are aberrant
-members of their sub-kingdom, comparisons between the different orders
-prove that the higher are strongly distinguished from the lower, by
-the much greater degree in which the individuality of the tertiary
-aggregate dominates over the individualities of those secondary
-aggregates called segments or “somites,” of which it is composed. The
-successive Figs. 170–176, representing (without their limbs) a Julus,
-a Scolopendra, an isopodous Crustacean, and four kinds of decapodous
-Crustaceans, ending with a Crab, will convey at a glance an idea of
-the way in which that greater size and heterogeneity reached by the
-higher types, is accompanied by an integration which, in the extreme
-cases, nearly obliterates all traces of composite structure. In the
-Crab the posterior segments, usually folded underneath the shell, alone
-preserve their primitive distinctness. So completely confluent are the
-rest, that it seems absurd to say that a Crab’s carapace is composed of
-as many segments as there are pairs of limbs, foot-jaws, and antennæ
-attached to it; and were it not that during early stages of the Crab’s
-development the segmentation is faintly marked, the assertion might be
-considered illegitimate.
-
-  [Illustration: Figs. 170–176.]
-
-That all articulate animals are thus composed from end to end
-of homologous segments, is, however, an accepted doctrine among
-naturalists. It is a doctrine that rests on careful observation of
-three classes of facts--the correspondences of parts in the successive
-“somites” of an adult articulate animal; the still more marked
-correspondences of such parts as they exist in the embryonic or larval
-articulate animal; and the maintenance of such correspondences in some
-types, which are absent in types otherwise near akin to them. The
-nature of the conclusion which these evidences unite in supporting,
-will best be shown by the annexed copies from the lecture-diagrams
-of Prof. Huxley; exhibiting the typical structures of a Myriapod, an
-Insect, a Spider, and a Crustacean, with their relations to a common
-plan, as interpreted by him.
-
-  [Illustration: Figs. 177–186.]
-
-Treating of these homologies, Prof. Huxley says “that a striking
-uniformity of composition is to be found in the heads of, at any rate,
-the more highly organized members of these four classes; and that,
-typically, the head of a Crustacean, an Arachnid, a Myriapod, or an
-Insect, is composed of six somites (or segments corresponding with
-those of the body) and their appendages, the latter being modified so
-as to serve the purpose of sensory and manducatory organs.”[27]
-
-Thus even in the higher _Arthropoda_, the much greater consolidation
-and much greater heterogeneity do not obliterate all evidence of the
-fact, that the organism is an aggregate of the third order. Comparisons
-show that it is divisible into a number of proximate units, each of
-which is akin in certain fundamental traits to its neighbours, and each
-of which is an aggregate of the second order, in so far as it is an
-organized combination of those aggregates of the first order which we
-call morphological units or cells. And that these segments or somites,
-which make up an annulose animal, were originally aggregates of the
-second order having independent individualities, is an hypothesis which
-gathers further support from the contrast between the higher and the
-lower Arthropods, as well as from the contrast between the Arthropods
-in general and the Annelids. For if that masking of the individualities
-of the segments which we find distinguishes the higher forms from the
-lower, has been going on from the beginning, as we may fairly assume;
-it is to be inferred that the individualities of the segments in the
-lower forms, were originally more marked than they now are. Reversing
-those processes of change by which the most developed _Annulosa_ have
-arisen from the least developed; and applying in thought this reversed
-process to the least developed, as they were described in the last
-Chapter; we are brought to the conception of attached segments that are
-all completely alike, and have their individualities in no appreciable
-degree subordinated to that of the chain they compose. From which there
-is but one step to the conception of gemmiparously-produced individuals
-which severally part one from another as soon as they are formed.
-
-
-§ 209. We must now return to a junction whence we diverged some time
-ago. As before explained under the head of Classification, organisms
-do not admit of uniserial arrangement, either in general or in detail;
-but everywhere form groups within groups. Hence, having traced the
-phases of morphological composition up to the highest forms in any
-sub-kingdom, we find ourselves at the extremity of a great branch, from
-which there is no access to another great branch, except by going back
-to some place of bifurcation low down in the tree.
-
-There exist such similarities of shape and structure between the larval
-forms of low Molluscs and those of Annelids and Rotifers, as to show
-that there was an early type common to them all; and its probable
-characters, suggested by comparison, seem to imply that it had arisen
-from some cœlenterate type, intermediate between the _Cnidaria_ and
-the _Ctenophora_. But there is this noteworthy difference between the
-molluscan larva and the allied larvæ, that it gives origin to only one
-animal and not to a group of animals, united or disunited. No true
-Mollusc multiplies by gemmation, either continuous or discontinuous;
-but the product of every fertilized germ is a single individual.
-
-It is a significant fact that here, where for the first time we have
-homogenesis holding throughout an entire sub-kingdom, we have also
-throughout an entire sub-kingdom no case in which the organism is
-divisible into two, three, or more, like parts. There is neither any
-such clustering or branching as a cœlenterate or molluscoid animal
-usually displays; nor is there any trace of that segmentation which
-characterizes the _Annulosa_. Among these animals in which no single
-egg produces several individuals, no individual is separable into
-several homologous divisions. This connexion will be seen to have
-a probable meaning, on remembering that it is the converse of the
-connexion which obtains among the _Annulosa_, considered as a group.
-
-A Mollusc, then, is an aggregate of the second order. Not only in the
-adult animal is there no sign of a multiplicity of like parts that
-have become obscured by integration; but there is no sign of such
-multiplicity in the embryo. And this unity is just as conspicuous in
-the lowest Lamellibranch as in the highest Cephalopod.
-
-  [Illustration: Figs. 188–190.]
-
-It may be well to note, however, more especially because it illustrates
-a danger of misinterpretation presently to be guarded against, that
-there are certain Molluscs which simulate the segmented structure.
-Externally a _Chiton_, Fig. 188, appears to be made up of divisions
-substantially like those of the creature Fig. 189; and one who judged
-only by externals, would say that the creature Fig. 190 differs as
-much from the creature Fig. 189, as this does from the preceding one.
-But the truth is, that while 190 and 189 are closely-allied types, 189
-differs from 188 much more widely than a man does from a fish. And the
-radical distinction between them is this:--Whereas in the Crustacean
-the segmentation is carried transversely through the whole mass of the
-body, so as to render the body more or less clearly divisible into
-a series of parts which are similarly composed; in the Mollusc the
-segmentation is limited to the shell carried on its upper surface,
-and leaves its body as completely undivided as is that of a common
-slug.[28] Were the body cut through at each of the divisions, the
-section of it attached to each portion of the shell would be unlike
-all the other sections. Here the segmentation has a purely functional
-derivation--is adaptive instead of genetic. The similarly-formed and
-similarly-placed parts, are not homologous in the same sense as are the
-appendages of a phænogamic axis or the limbs of an insect.
-
-
-§ 210. In studying the remaining and highest sub-kingdom of animals, it
-is important to recognize this radical difference in meaning between
-that likeness of parts which is produced by likeness of modifying
-forces, and that likeness of parts which is due to primordial identity
-of origin. On our recognition of this difference depends the view we
-take of certain doctrines that have long been dominant, and have still
-a wide currency.
-
-Among the _Vertebrata_, as among the _Mollusca_, homogenesis is
-universal. The two sub-kingdoms are like one another and unlike the
-remaining sub-kingdoms in this, that in all the types they severally
-include, a single fertilized ovum produces only a single individual. It
-is true that as the eggs of certain gasteropods occasionally exhibit
-spontaneous fission of the vitelline mass, which may or may not result
-in the formation of two individuals; so among vertebrate animals we
-now and then meet with double monsters, which appear to imply such a
-spontaneous fission imperfectly carried out. But these anomalies serve
-to render conspicuous the fact, that in both these sub-kingdoms the
-normal process is the integration of the whole germ-mass into a single
-organism, which at no phase of its development displays any tendency to
-separate into two or more parts.
-
-Equally as throughout the _Mollusca_, there holds throughout the
-_Vertebrata_ the correlative fact, that not even in its lowest any
-more than in its highest types, is the body divisible into homologous
-segments. The vertebrate animal, under its simplest as under its most
-complex form, is like the molluscous animal in this, that you cannot
-cut it into transverse slices, each of which contains a digestive
-organ, a respiratory organ, a reproductive organ, &c. The organs of the
-least-developed fish as well as those of the most developed mammal,
-form but a single physiological whole; and they show not the remotest
-trace of having ever been divisible into two or more physiological
-wholes. That segmentation which the vertebrate animal usually exhibits
-throughout part of its organization, is the same in origin and meaning
-as the segmentation of a _Chiton’s_ shell; and no more implies in the
-vertebrate animal a composite structure, than do the successive pairs
-of branchiæ of the _Doto_, or the transverse rows of branchiæ in the
-_Eolis_, imply composite structure in the molluscous animal. To some
-this will seem a very questionable proposition; and had we no evidence
-beyond that which adult vertebrate animals of developed types supply,
-it would be a proposition not easy to substantiate. But abundant
-support for it is to be found in the structure of the vertebrate
-embryo, and in the comparative morphology of the _Vertebrata_ in
-general.
-
-Embryologists teach us that the primordial relations of parts are most
-clearly displayed in the early stages of evolution; and that they
-generally become partially or completely disguised in its later stages.
-Hence, were the vertebrate animal on the same level as the annulose
-animal in degree of composition--did it similarly consist of segments
-which are homologous in the sense that they are the proximate units
-of composition; we ought to find this fundamental fact most strongly
-marked at the outset. As in the annelid-embryo the first conspicuous
-change is the elongation and division into segments, by constrictions
-that encircle the whole body; and as in the arthropod embryo the
-blastoderm becomes marked out transversely into pieces which extend
-themselves round the yelk before the internal organization has made
-any appreciable progress; so in the embryo of every vertebrate animal,
-had it an analogous composition, the first decided change should be
-a segmentation implicating the entire mass. But it is not so. Sundry
-important differentiations occur before any divisions begin to show
-themselves. There is the defining of that elongated, elevated area with
-its longitudinal groove, which becomes the seat of subsequent changes;
-there is the formation of the notochord lying beneath this groove;
-there is the growth upwards of the boundaries of the groove into the
-dorsal laminæ, which rapidly develop and fold over in the region of the
-head. Rathke, as quoted and indorsed by Prof. Huxley, describes the
-subsequent changes as follows:--“The gelatinous investing mass, which,
-at first, seems only to constitute a band to the right and to the left
-of the notochord forms around it, in the further course of development,
-a sheath, which ends in a point posteriorly. Anteriorly, it sends
-out two processes which underlie the lateral parts of the skull, but
-very soon coalesce for a longer or shorter distance. Posteriorly, the
-sheath projects but little beyond the notochord; but, anteriorly, for a
-considerable distance, as far as the infundibulum. It sends upwards two
-plates, which embrace the future central parts of the nervous system
-laterally, probably throughout their entire length.” That is to say, in
-the _Vertebrata_ the first step is the marking out on the blastoderm
-of an integrated structure, within which segments subsequently appear.
-When these do appear, they are for some time limited to the middle
-region of the spinal axis; and no more then than ever after, do they
-implicate the general mass of the body in their transverse divisions.
-On the contrary, before vertebral segmentation has made much progress,
-the rudiments of the vascular system are laid down in a manner
-showing no trace of any primordial correspondence of its parts with
-the divisions of the axis. Equally at variance with the belief that
-the vertebrate animal is essentially a series of homologous parts,
-is the heterogeneity which exists among these parts on their first
-appearance. Though in the head of an adult articulate animal there
-is little sign of divisibility into segments like those of the body;
-yet such segments, with their appropriate ganglia and appendages, are
-easily identifiable in the articulate embryo. But in the _Vertebrata_
-this antithesis is reversed. At the time when segmentation has become
-decided in the dorsal region of the spine, there is no trace of
-segments in the parts which are to form the skull--nothing whatever
-to suggest that the skull is being formed out of divisions homologous
-with vertebræ.[29] And minute observation no more discloses any such
-homology than does general appearance. “Remak,” says Prof. Huxley,
-“has more fully proved than any other observer, the segmentation into
-‘urwirbel,’ or proto-vertebræ, which is characteristic of the vertebral
-column, stops at the occipital margin of the skull--the base of which,
-before ossification, presents no trace of that segmentation which
-occurs throughout the vertebral column.”
-
-  [Illustration: Fig. 191.]
-
-Consider next the evidence supplied by comparative morphology. In
-preceding sections (§§ 206, 208) it has been shown that among annulose
-animals, the divisibility into homologous parts is most clearly
-demonstrable in the lowest types. Though in decapodous Crustaceans,
-in Insects, in Arachnids, there is difficulty in identifying some
-or many of the component somites; and though, when identified, they
-display only partial correspondences; yet on descending to Annelids,
-the composition of the entire body out of such somites becomes
-conspicuous, and the homology between each somite and its neighbours
-is shown by the repetition of one another’s structural details, as
-well as by their common gemmiparous origin: indeed, in some cases we
-have the homology directly demonstrated by seeing a somite of the body
-transformed into a head. If, then, a vertebrate animal had a segmental
-composition of kindred nature, we ought to find it most clearly
-marked in the lowest _Vertebrata_ and most disguised in the highest
-_Vertebrata_. But here, as before, the fact is just the reverse. Among
-the _Vertebrata_ of developed type, such segmentation as really exists
-remains conspicuous--is but little obscured even in parts of the spinal
-column formed out of integrated vertebræ. Whereas in the undeveloped
-vertebrate type, segmentation is scarcely at all traceable.[30] The
-_Amphioxus_, Fig. 191, is not only without ossified vertebræ; not
-only is it without cartilaginous representatives of them; but it is
-even without anything like distinct membranous divisions. The spinal
-column exists as a continuous notochord: the only signs of incipient
-segmentation being given by its membranous sheath, in the upper part
-of which “quadrate masses of somewhat denser tissue seem faintly to
-represent neural spines.” Moreover, throughout sundry groups of fishes
-and amphibians, the segmentation remains very imperfect: only certain
-peripheral appendages of the vertebræ becoming defined and solidified,
-while in place of the bodies of the vertebræ there still continues the
-undivided notochord. Thus, instead of being morphologically composed
-of vertebral segments, the vertebrate animal in its primitive form is
-entirely without vertebral segments; and vertebral segments begin to
-appear only as we advance towards developed forms. Once more, evidence
-equally adverse to the current hypothesis meets us on observing that
-the differences between the parts supposed to be homologous, are as
-great at first as at last. Did the vertebrate animal primordially
-consist of homologous segments from snout to tail; then the segments
-said to compose the skull ought, in the lowest _Vertebrata_, to show
-themselves much more like the remaining segments than they do in the
-highest _Vertebrata_. But they do not. Fishes have crania made up of
-bones that are no more clearly arrangeable into segments like vertebræ,
-than are the cranial bones of the highest mammal. Nay, indeed, the
-case is much stronger. The simplest fish possessing a skeleton, has a
-cranium composed of cartilage that is not segmented at all!
-
-Besides being inconsistent with the leading truths of Embryology
-and Comparative Morphology, the hypothesis of Goethe and Oken is
-inconsistent with itself. The facts brought forward to show that there
-exists an archetypal vertebra, and that the vertebrate animal is
-composed of archetypal vertebræ arranged in a series, and severally
-modified to fit their positions--these facts, I say, so far from
-proving as much, suffice, when impartially considered, to disprove it.
-No assigned, nor any conceivable, attribute of the supposed archetypal
-vertebra is uniformly maintained. The parts composing it are constant
-neither in their number, nor in their relative positions, nor in their
-modes of ossification, nor in the separateness of their several
-individualities when present. There is no fixity of any one element,
-or connexion, or mode of development, which justifies even a suspicion
-that vertebræ are modelled after an ideal pattern. To substantiate
-these assertions here would require too much space, and an amount of
-technical detail wearisome to the general reader. The warrant for them
-will be found in a criticism on the osteological works of Prof. Owen,
-originally published in the _British and Foreign Medico-Chirurgical
-Review_ for Oct. 1858. This criticism I add in the Appendices, for the
-convenience of those who may wish to study the question more fully.
-(See Appendix B.)
-
-Everything, then, goes to show that the segmental composition which
-characterises the apparatus of external relation in most _Vertebrata_,
-is not primordial or genetic, but functionally determined or adaptive.
-Our inference must be that the vertebrate animal is an aggregate of
-the second order, in which a relatively superficial segmentation has
-been produced by mechanical intercourse with the environment. We
-shall hereafter see that this conception leads us to a consistent
-interpretation of the facts--shows us why there has arisen such unity
-in variety as exists in every vertebral column, and why this unity
-in variety is displayed under countless modifications in different
-skeletons.[31]
-
-
-§ 211. On glancing back at the facts brought together in these two
-chapters, we see it to be probable that there has gone on among animals
-a process like that which we saw reason to think has gone on among
-plants. Minute aggregates of those physiological units which compose
-living protoplasm, exist as _Protozoa_: some of them incoherent,
-indefinite, and almost homogeneous, and others of them more coherent,
-definite, and heterogeneous. By union of these nucleated particles
-of sarcode, are produced various indefinite aggregates of the second
-order--Sponges, _Polycytharia_, Foraminifers, &c.; in which the
-compound individuality is scarcely enough marked to subordinate the
-primitive individualities. But in other types, as in _Hydra_, the lives
-of the morphological units are in a considerable degree, though not
-wholly, merged in the life of the integrated body they form. As the
-primary aggregate, when it passes a certain size, undergoes fission or
-gemmation; so does the secondary aggregate. And as on the lower stage
-so on the higher, we see cases in which the gemmiparously-produced
-individuals part as soon as formed, and other cases in which they
-continue united, though in great measure independent. This massing
-of secondary aggregates into tertiary aggregates, is variously
-carried on among the _Hydrozoa_, the _Actinozoa_, the _Polyzoa_,
-and the _Tunicata_. In most of the types so produced, the component
-individualities are very little subordinated to the individuality of
-the composite mass--there is only physical unity and not physiological
-unity; but in certain of the oceanic _Hydrozoa_, the individuals are
-so far differentiated and combined as very much to mask them. Forms
-showing us clearly the transition to well-developed individuals of
-the third order, are not to be found. Nevertheless, in the great
-sub-kingdom _Annulosa_, there are traits of structure, development,
-and mode of multiplication, which go far to show that its members are
-such individuals of the third order; and in the relations to external
-conditions involved by the mode of union, we find an adequate cause for
-that obscuration of the secondary individualities which we must suppose
-has taken place. The two other great subdivisions, _Mollusca_ and
-_Vertebrata_, between the lower members of which there are suggestive
-points of community, present us only with aggregates of the second
-order, that have in many cases become very large and very complex.
-We find in them no trace of the union of gemmiparously-produced
-individuals. Neither the molluscous nor the vertebrate animal shows
-the faintest trace of a segmentation affecting the totality of
-its structure; and we see good grounds for concluding that such
-segmentation as exceptionally occurs in the one and usually occurs in
-the other, is superinduced.
-
-       *       *       *       *       *
-
-[NOTE:--A critic calls in question the statement on p. 121 respecting
-the _Amphioxus_. At the outset, however, he admits that in the
-_Amphioxus_ “the central nervous system and the notochord are not
-segmented.” In the Annelid, however, the central nervous system
-is segmented, and there is segmentation of the part which, as a
-supporting structure, is analogous to the notochord in respect
-of function--the outer part which represents the exo-skeleton in
-contrast to the endo-skeleton. He goes on to say that “the gut is
-not involved [in the segmentation] and exhibits in _Amphioxus_ just
-as it does in worms differentiations entirely independent of the
-segmentation of the mesoblast.” Part of this statement is, I think,
-not congruous with all the facts. In _Protodrilus_, one of the lowest
-of the _Archiannelida_, “the intestine is moniliform, there being a
-constriction between each segment” and the next. (Shipley.) Complete
-segmentation of the intestine is obviously impossible, since, were
-the canal divided into portions by septa, no food could pass. But the
-fact that the gut has these successive expansions and constrictions,
-corresponding to the successive segments, and giving to each segment
-a partially-separate stomach, shows that segmentation has gone as
-far as consists with the carrying on of the lives of the segments.
-No such partial segmentation exists in the _Amphioxus_. Thus, then,
-three fundamental structures--the directive structure, the supporting
-structure, and the alimentary structure--are respectively simple in
-the lowest vertebrate and segmented, or partially segmented, in the
-lowest Annelid. Again, while it is said that the gill-clefts exhibit
-segmentation, it is admitted that this has no relevance to any
-constitutional segmentation: “they are segmented on a plan of their
-own” irrespective of other organs. Another allegation is that the
-ovaries of _Amphioxus_ are segmented. Their segmentation, however, like
-that of the gills, is isolated, and may be considered as illustrating
-those repetitions of like parts seen in supernumerary vertebræ in
-various creatures--a repetition which becomes habitual if the resulting
-structure is advantageous to the species. On the statement that while
-the _Amphioxus_ has no rudiments of a renal system the Elasmobranch
-embryo has such rudiments, which are as distinctly segmented as the
-nephridia of a worm, two comments may be made. The first is that if
-in these Vertebrates the nephridia bear a relation to the general
-structure like that which they do in Annelids, then one would expect
-to find the segmental arrangement shown in the lowest type, as in
-Annelids, rather than in a type considerably advanced in development.
-Should it be replied that in the _Amphioxus_ an excretory system had
-not yet arisen, though one is required for the higher organization
-of an Elasmobranch, then the answer may be that since the segmental
-arrangement in the Elasmobranch corresponds with that of the myotomes,
-it has no reference to any primordial segmentation, since the myotomes
-have been functionally generated. The second comment is that whereas
-the nephridia of the Annelid have independent external openings,
-the nephridia in the Elasmobranch have not. These discharge their
-secretions into certain general tubes of exit common to them all;
-showing that each of them, instead of being a member of a partially
-independent structure, is united with others in subordination to a
-general structure. That is to say, the segmentations are far from being
-parallel in their essential natures. The assertion accompanying these
-criticisms, that there is “no difference _in principle_ between the
-segmentation of _Amphioxus_ and Annelid” is difficult to reconcile
-with the visible contrast between the two. Whatever local segmentations
-there are in an _Amphioxus_ appear to me quite unlike “in principle”
-to those which an Annelid exhibits. Could its portion of gut be duly
-supplied with nutriment, the segment of a low Annelid could carry on
-its vital functions independently. In the parts of the _Amphioxus_ we
-see nothing approaching to this. Cut it into transverse sections and
-no one of them contains anything like the assemblage of structures
-required for living. The _Amphioxus_ is a physiological whole, and
-in that respect differs radically from the Annelid, each segment of
-which is in chief measure a physiological whole. No occurrence of
-local segmentation in the _Amphioxus_ can obliterate this fundamental
-contrast.
-
-An accompanying contrast tells the same story. On ascending from the
-lowest to the highest annulose types we see a progressing integration,
-morphological and physiological; so that whereas in a low annelid
-the successive parts are in large measure independent in their
-structures and in their lives, in a high arthropod, as a crab, most
-of the parts have lost their individualities and have become merged
-in a consolidated organism with a single life. Quite otherwise is it
-in the vertebrate series. Its lowest member is at the very outset a
-complete morphological and physiological whole, and the formation of
-those serial parts which some think analogous to the serial parts of an
-Annelid, begins at a later stage and becomes gradually pronounced. That
-is to say, the course of transformation is reversed.]
-
-
-
-
-                              CHAPTER VI.
-
-               MORPHOLOGICAL DIFFERENTIATION IN PLANTS.
-
-
-§ 212. While, in the course of their evolution, plants and animals
-have displayed progressive integrations, there have at the same time
-gone on progressive differentiations of the resulting aggregates,
-both as wholes and in their parts. These differentiations and the
-interpretations of them, form the second class of morphological
-problems.
-
-We commence as before with plants. We have to consider, first, the
-several kinds of modification in shape they have undergone; and,
-second, the relations between these kinds of modification and their
-factors. Let us glance at the leading questions that have to be
-answered.
-
-
-§ 213. Irrespective of their degrees of composition, plants may, and
-do, become changed in their general forms. Are their changes capable of
-being formulated? The inquiry which meets us at the outset is--does a
-plant’s shape admit of being expressed in any universal terms?--terms
-that remain the same for all genera, orders, and classes.
-
-After plants considered as wholes, have to be considered their
-proximate components, which vary with their degrees of composition,
-and in the highest plants are what we call branches. Is there any law
-traceable among the contrasted shapes of different branches in the same
-plant? Do the relative developments of parts in the same branch conform
-to any law? And are these laws, if they exist, allied with one another
-and with that to which the shape of the whole plant conforms?
-
-Descending to the components of these components, which in developed
-plants we distinguish as leaves, there meet us kindred questions
-respecting their relative sizes, their relative shapes, and their
-shapes as compared with those of foliar organs in general. Of their
-morphological differentiations, also, it has to be asked whether they
-exemplify any truth that is exemplified by the entire plant and by its
-larger parts.
-
-Then, a step lower, we come down to those morphological units of
-which leaves and fronds consist; and concerning these arise parallel
-inquiries touching their divergences from one another and from cells in
-general.
-
-The problems thus put together in several groups cannot of course be
-rigorously separated. Evolution presupposes transitions which make all
-such classings more or less conventional; and adherence to them must be
-subordinate to the needs of the occasion.
-
-
-§ 214. In studying the causes of the morphological differentiations
-thus divided out and prospectively generalized, we shall have to bear
-in mind several orders of forces which it will be well briefly to
-specify.
-
-Growth tends inevitably to initiate changes in the shape of any
-aggregate, by altering both the amounts of the incident forces and
-the forces which the parts exert on one another. With the mechanical
-actions this is obvious. Matter that is sensibly plastic cannot be
-increased in mass without undergoing a change in its proportions,
-consequent on the diminished ratio of its cohesive force to the force
-of gravitation. With the physiological actions it is equally obvious.
-Increase of size, other things equal, alters the relations of the parts
-to the material and dynamical factors of nutrition; and by so affecting
-differently the nutrition of different parts, initiates further changes
-of proportions.
-
-In plants of the third order it is thus with the proximate components:
-they are subject to mutual influences that are unlike one another and
-are continually changing. The earlier-formed units become mechanical
-supporters of the later-formed units, and so experience modifying
-forces from which the later-formed units are exempt. Further, these
-elder units simultaneously begin to serve as channels through which
-materials are carried to and from the younger units--another cause
-of differentiation that goes on increasing in intensity. Once more,
-there arise ever-strengthening contrasts between the amounts of light
-which fall upon the youngest or outermost units and the eldest or
-innermost units; whence result structural contrasts of yet another
-kind. Evidently, then, along with the progressive integration of cells
-into fronds, of fronds into axes, and of axes into plants still more
-composite, there come into play sundry causes of differentiation which
-act on the whole and on each of its parts, whatever their grade. The
-forces to be overcome, the forces to be utilized, and the matters to
-be appropriated, do not remain the same in their proportions and modes
-of action for any two members of the aggregate: be they members of the
-first, second, third, or any other order.
-
-
-§ 215. Nor are these the only kinds and causes of heterogeneity which
-we have to consider. Beyond the more general changes produced in the
-relative sizes and shapes of plants and their parts by progressive
-aggregation, there are the more particular changes determined by the
-more particular conditions.
-
-Plants as wholes assume unlike attitudes towards their environments;
-they have many ways of articulating their parts with one another; they
-have many ways of adjusting their parts towards surrounding agencies.
-These are causes of special differentiations additional to those
-general differentiations that result from increase of mass and increase
-of composition. In each part considered individually, there arises
-a characteristic shape consequent on that relative position towards
-external and internal forces, which the mode of growth entails. Every
-member of the aggregate presents itself in a more or less peculiar way
-towards the light, towards the air, and towards its point of support;
-and according to the relative homogeneity or heterogeneity in the
-incidence of the agencies thus brought to bear on it, will be the
-relative homogeneity or heterogeneity of its shape.
-
-
-§ 216. Before passing from this _à priori_ view of the morphological
-differentiations which necessarily accompany morphological
-integrations, to an _à posteriori_ view of them, it seems needful to
-specify the meanings of certain descriptive terms we shall have to
-employ.
-
-Taking for our broadest division among forms, the regular and the
-irregular, we may divide the latter into those which are wholly
-irregular and those which, being but partially irregular, suggest
-some regular form to which they approach. By slightly straining the
-difference between them, two current words may be conveniently used
-to describe these subdivisions. The entirely irregular forms we may
-class as _asymmetrical_--literally as forms without any equalities of
-dimensions. The forms which approximate towards regularity without
-reaching it, we may distinguish as _unsymmetrical_: a word which,
-though it asserts inequality of dimensions, has been associated by
-use rather with such slight inequality as constitutes an observable
-departure from equality.
-
-Of the regular forms there are several classes, differing in the number
-of directions in which equality of dimensions is repeated. Hence
-results the need for names by which symmetry of several kinds may be
-expressed.
-
-The most regular of figures is the sphere: its dimensions are the same
-from centre to surface in all directions; and if cut by any plane
-through the centre, the separated parts are equal and similar. This is
-a kind of symmetry which stands alone, and will be hereafter spoken of
-as _spherical symmetry_.
-
-When a sphere passes into a spheroid, either prolate or oblate, there
-remains but one set of planes that will divide it into halves, which
-are in all respects alike; namely, the planes in which its axis lies,
-or which have its axis for their line of intersection. Prolate and
-oblate spheroids may severally pass into various forms without losing
-this property. The prolate spheroid may become egg-shaped or pyriform,
-and it will still continue capable of being divided into two equal
-and similar parts by any plane cutting it down its axis; nor will the
-making of constrictions deprive it of this property. Similarly with the
-oblate spheroid. The transition from a slight oblateness, like that of
-an orange, to an oblateness reducing it nearly to a flat disc, does
-not alter its divisibility into like halves by every plane passing
-through its axis. And clearly the moulding of any such flattened oblate
-spheroid into the shape of a plate, leaves it as before, symmetrically
-divisible by all planes at right angles to its surface and passing
-through its centre. This species of symmetry is called _radial
-symmetry_. It is familiarly exemplified in such flowers as the daisy,
-the tulip, and the dahlia.
-
-From spherical symmetry, in which we have an infinite number of axes
-through each of which may pass an infinite number of planes severally
-dividing the aggregate into equal and similar parts; and from radial
-symmetry, in which we have a single axis through which may pass an
-infinite number of planes severally dividing the aggregate into equal
-and similar parts; we now turn to _bilateral symmetry_, in which the
-divisibility into equal and similar parts becomes much restricted.
-Noting, for the sake of completeness, that there is a sextuple
-bilateralness in the cube and its derivative forms which admit of
-division into equal and similar parts by planes passing through the
-three diagonal axes and by planes passing through the three axes
-that join the centres of the surfaces, let us limit our attention to
-the three kinds of bilateralness which here concern us. The first
-of these is _triple bilateral symmetry_. This is the symmetry of a
-figure having three axes at right angles to one another, through each
-of which there passes a single plane that divides the aggregate into
-corresponding halves. A common brick will serve as an example; and of
-objects not quite so simple, the most familiar is that modern kind of
-spectacle-case which is open at both ends. This may be divided into
-corresponding halves along its longitudinal axis by cutting it through
-in the direction of its thickness, or by cutting it through in the
-direction of its breadth; or it may be divided into corresponding
-halves by cutting it across the middle. Of objects which illustrate
-_double bilateral symmetry_, may be named one of those boats built for
-moving with equal facility in either direction, and therefore made
-alike at stem and stern. Obviously such a boat is separable into equal
-and similar parts by a vertical plane passing through stem and stern;
-and it is also separable into equal and similar parts by a vertical
-plane cutting it amidships. To exemplify _single bilateral symmetry_
-it needs but to turn to the ordinary boat of which the two ends are
-unlike. Here there remains but the one plane passing vertically
-through stem and stern, on the opposite sides of which the parts are
-symmetrically disposed.
-
-These several kinds of symmetry as placed in the foregoing order, imply
-increasing heterogeneity. The greatest uniformity in shape is shown
-by the divisibility into like parts in an infinite number of infinite
-series of ways; and the greatest degree of multiformity consistent
-with any regularity, is shown by the divisibility into like parts in
-only a single way. Hence, in tracing up organic evolution as displayed
-in morphological differentiations, we may expect to pass from the one
-extreme of spherical symmetry, to the other extreme of single bilateral
-symmetry. This expectation we shall find to be completely fulfilled.
-
-
-
-
-                             CHAPTER VII.
-
-                     THE GENERAL SHAPES OF PLANTS.
-
-
-§ 217. Among protophytes those exemplified by _Pleurococcus vulgaris_
-are by general consent considered the simplest. As shown in Fig.
-1, they are globular cells presenting no obvious differentiation
-save that between inner and outer parts. Their uniformity of figure
-co-exists with a mode of life involving the uniform exposure of all
-their sides to incident forces. For though each individual may have
-its external parts differently related to environing agencies, yet the
-new individuals produced by spontaneous fission, whether they part
-company or whether they form clusters and are made polyhedral by mutual
-pressure, have no means of maintaining parallel relations of position
-among their parts. On the contrary, the indefiniteness of the attitudes
-into which successive generations fall, must prevent the rise of any
-unlikeness between one portion of the surface and another. Spherical
-symmetry continues because, on the average of cases, incident forces
-are equal in all directions.
-
-  [Illustration: Figs. 1, 2, 3.]
-
-Other orders of _Protophyta_ have much more special forms, along with
-much more special attitudes: their homologous parts maintaining, from
-generation to generation, unlike relations to incident forces. The
-_Desmidiaceæ_ and _Diatomaceæ_, of which Figs. 2 and 3 show examples,
-severally include genera characterized by triple bilateral symmetry.
-A _Navicula_ is divisible into corresponding halves by a transverse
-plane and by two longitudinal planes--one cutting its valves at right
-angles and the other passing between its valves. The like is true
-of those numerous transversely-constricted forms of _Desmidiaceæ_,
-exemplified by the second of the individuals represented in Fig.
-2. If now we ask how a _Navicula_ is related to its environment,
-we see that its mode of life exposes it to three different sets of
-forces: each set being resolvable into two equal and opposite sets.
-A _Navicula_ moves in the direction of its length, with either
-end foremost. Hence, on the average, its ends are subject to like
-actions from the agencies to which its motions subject it. Further,
-either end while moving exposes its right and left sides to amounts
-of influence which in the long run must be equal. If, then, the two
-ends are not only like one another, but have corresponding right and
-left sides, the symmetrical distribution of parts answers to the
-symmetrical distribution of forces. Passing to the two edges and the
-two flat surfaces, we similarly find a clue to their likenesses and
-differences in their respective relations to the things around them.
-These locomotive protophytes move through the entangled masses of
-fragments and fibres produced by decaying organisms and confervoid
-growths. The interstices in such matted accumulations are nearly all
-of them much longer in one dimension than in the rest--form crevices
-rather than regular meshes. Hence, a small organism will have much
-greater facility of insinuating itself through this _débris_, in which
-it finds nutriment, if its transverse section is flattened instead of
-square or circular. And while we see how, by survival of the fittest, a
-flattened form is likely to be acquired by diatoms having this habit;
-we also see that likeness will be maintained between the two flat
-surfaces and between the two edges. For, on the average, the relations
-of the two flat surfaces to the sides of the openings through which
-the diatom passes, will be alike; and so, too, on the average, will be
-the relations of the two edges. In desmids of the type exemplified by
-the second individual in Fig. 2, a kindred equalization of dimensions
-is otherwise insured. There is nothing to keep one of the two surfaces
-uppermost rather than the other; and hence, in the long succession
-of individuals, the two surfaces are sure to be similarly exposed to
-light and agencies in general. When to this is added the fact that
-spontaneous fission occurs transversely in a constant way, it becomes
-manifest that the two ends, while they are maintained in conditions
-like one another, are maintained in conditions unlike those of the
-two edges. Here then, as before, triple bilateral symmetry in form,
-co-exists with a triple bilateral symmetry in the average distribution
-of actions.
-
-  [Illustration: Figs. 4, 5, 6.]
-
-Still confining our attention to aggregates of the first order, let us
-next note what results when the two ends are permanently subject to
-different conditions. The fixed unicellular plants, of which examples
-are given in Figs. 4, 5, and 6, severally illustrate the contrast
-in shape arising between the part that is applied to the supporting
-surface and the part that extends into the surrounding medium. These
-two parts which are the most unlike in their relations to incident
-forces, are the most unlike in the forms. Observe, next, that the part
-which lifts itself into the water or air, is more or less decidedly
-radial. Each outward-growing tubule of _Codium adhærens_, Fig. 4, has
-its parts disposed with some regularity around its axis; the upper stem
-and spore-vessel of _Botrydium_, Fig. 5, display a lateral growth
-that is approximately equal in every direction; and the stems of the
-_Mucor_, Fig. 6, shoot up with an approach to evenness on all sides.
-Plants of this low type are naturally very variable in their modes of
-growth: each individual being greatly modified in form by its special
-circumstances. But they nevertheless show us a general likeness between
-parts exposed to like forces, as well as a general unlikeness between
-parts exposed to unlike forces.
-
-Respecting the forms of these aggregates of the first order, it has
-only to be added that they are asymmetrical where there is total
-irregularity in the incidence of forces. We have an example in the
-indefinitely contorted and branched shape of a fungus-cell, growing as
-a mycelium among the particles of soil or through the interstices of
-organic tissue.
-
-
-§ 218. Re-illustrations of the general truths which the forms of
-these vegetal aggregates of the first order display, are furnished by
-vegetal aggregates of the second order. The equalities and inequalities
-of growth in different directions, prove to be similarly related to
-the equalities and inequalities of environing actions in different
-directions.
-
-Of spherical symmetry an instance occurs in _Eudorina elegans_.
-The ciliated cells are here so united as to produce a small,
-mulberry-shaped, hollow ball which, being similarly conditioned on all
-sides, shows no unlikenesses of structure. An allied form, however,
-_Volvox globator_, presents a highly instructive, though very trifling,
-modification. It is not absolutely homogeneous in its structure and
-is not absolutely homogeneous in its motions. The waving cilia of
-its component cells have fallen into such slight heterogeneities of
-action as to cause rotation in a constant direction; and along with a
-fixed axis of rotation there has arisen a fixed axis of progression. A
-concomitant fact is that the cells of the colony exhibit an appreciable
-differentiation in relation to the fixed axis. There is an incipient
-divergence from spherical uniformity along with this slight divergence
-from uniformity of conditions.
-
-Vegetal aggregates of the second order are usually fixed: locomotion
-is exceptional. Fixity implies that the surface of attachment is
-differently circumstanced from the free surface. Hence we may expect
-to find, as we do find, that among these rooted aggregates of the
-second order, as among those of the first order, the primary contrast
-of shape is between the adherent part and the loose part. Sea-weeds
-variously exemplify this. In some the fronds are very irregular and
-in some tolerably regular; in some the form is pseudo-foliar and in
-some pseud-axial; but differing though they do in these respects,
-they agree in having the end which is attached to a solid body unlike
-the other end. The same truth is seen in such secondary aggregates
-as the common Agarics, or rather in their immensely-developed organs
-of fructification. A puff-ball, Fig. 192, presents no other obvious
-unlikeness of parts than that between its under and upper surfaces. So
-too with the stalked kinds that frequent our woods and pastures. In the
-types which Figs. 193, 194, 195, delineate, the unlikenesses between
-the rooted ends and the expanded ends, as well as between the under
-and upper surfaces of the expanded ends, are obviously related to this
-fundamental contrast of conditions. Nor is this relation less clearly
-displayed in the sessile fungi which grow out from the sides of trees,
-as shown at _a_, _b_, Fig. 196. That which is common to this and the
-preceding types, is the contrast between the attached end and the free
-end.
-
-  [Illustration: Figs. 192–196.]
-
-From what these forms have in common, let us turn to that which they
-have not in common, and observe the causes of the want of community.
-A puff-ball shows us in the simplest way, the likeness of parts
-accompanying likeness of conditions, along with the unlikeness of parts
-accompanying unlikeness of conditions. For while, if we cut vertically
-through its centre, we find a difference between top and bottom, if
-we cut horizontally through its centre, we find no differences among
-its several sides. Being, on the average of cases, similarly related
-to the environment all round, it remains the same all round. The
-radial symmetry of the mushroom and other vertically-growing fungi,
-illustrates this connexion of cause and effect still better. But now
-mark what happens in the group of _Agaricus noli-tangere_, shown in
-Fig. 195. Radially-symmetrical as is the type, and radially symmetrical
-as are those centrally-placed individuals which are equally crowded
-all round, we see that the peripheral individuals, dissimilarly
-circumstanced on their outer sides and on their sides next the group,
-have partially changed their radial symmetry into bilateral symmetry.
-It is no longer possible to make two corresponding halves by _any_
-vertical plane cutting down through the pileus and the stem; but there
-is only _one_ vertical plane that will thus produce corresponding
-halves--the plane on the opposite sides of which the relations to the
-environment are alike. And then mark that the divergence from all-sided
-symmetry towards two-sided symmetry, here caused in the individual by
-special circumstances, is characteristic of the race where the habits
-of the race constantly involve two-sidedness of conditions. Besides
-being exemplified by such comparatively undifferentiated types as
-certain _Polypori_, Fig. 196, _a_, _b_, this truth is exemplified by
-members of the genus just named. In _Agaricus horizontalis_, Fig. 196,
-_c_, we have a departure from radial symmetry that is conspicuous only
-in the form of the stem. A more decided bilateralness exists in _A.
-subpalmatus_, shown in elevation at _d_ and in section at _d′ _. And
-_Lentinus flabelliformis_, of which _e_ and _e′_ are different views,
-exhibits complete bilateralness--a bilateralness in which there is the
-greatest likeness of the parts that are most similarly conditioned,
-and the greatest unlikeness of the parts that are most dissimilarly
-conditioned.
-
-Among plants of the second order of composition, it will suffice
-to note one further class of facts which are the converse of the
-foregoing and have the same implications. These are the facts showing
-that along with habitual irregularity in the relations to external
-forces, there is habitual irregularity in the mode of growth. Besides
-finding such facts among Thallophytes, as in the tubers of underground
-fungi and in the creeping films of sessile lichens, which severally
-show us variations of proportions obviously caused by variations in
-the amounts of the influences on their different sides, we also,
-among Archegoniates of inferior types, find irregularities of form
-along with irregularities in environing actions. The fronds of the
-_Marchantiaceæ_ or such _Jungermanniaceæ_ as are shown in
-Figs. 41, 42, 43, illustrate the way in which each lowly-organized
-aggregate of the second order, not individuated by the mutual
-dependence of its parts, has its form determined by the balance of
-facilities and resistances which each side of the frond meets with as
-it spreads.
-
-
-§ 219. Among plants displaying integration of the third degree, and
-among plants still further compounded, these same truths are equally
-manifest. In the forms of such plants we see primary contrasts and
-secondary contrasts which, no less clearly than the foregoing, are
-related to contrasts of conditions.
-
-That flowering plants from the daisy up to the oak, have in common the
-fundamental unlikeness between the upward growing part and the downward
-growing part; and that this most marked unlikeness corresponds with the
-most marked unlikeness between the two parts of their environment, soil
-and air; are facts too conspicuous to be named were they not important
-items in the argument. More instructive perhaps, because less familiar,
-is the fact that we miss this extreme contrast in flowering plants
-which have not their higher and lower portions exposed to conditions
-thus extremely contrasted. A parasite like the Dodder, growing in
-entangled masses upon other plants, from which it sucks the juices, is
-not thus divisible into two strongly-distinguished halves.
-
-Leaving out of consideration the difference between the supporting
-part and the supported part in phænogams, and looking at the supported
-part only, we observe between its form and the habitual incidence of
-forces, a relation like that which we observed in the simpler plants.
-Phænogams that are practically if not literally uniaxial, and those
-which develop their lateral axes only in the shape of axillary flowers,
-when uninterfered with commonly send up vertical stems round which the
-leaves and flowers are disposed with a more or less decided radial
-symmetry. Gardens and fields supply us with such instances as the Tulip
-and the Orchis; and, on a larger scale, the Palms and the Aloes are
-fertile in examples. The exceptions, too, are instructive. Besides the
-individual divergences arising from special interferences, there are
-to be traced general divergences where the habits of the plants expose
-them to general interferences in anything approaching to constant ways.
-Plants which, like the Foxglove, have spikes of flowers that are borne
-on flexible foot-stalks, have their flowers habitually bent round to
-one face of the stem: an unlikeness of distribution probably caused
-by unlikeness in the relation to the Sun’s rays. The wild Hyacinth,
-too, with stem so flexible that its upper part droops, shows us how a
-consequent difference in the action of gravity on the flowers, causes
-them to deviate from their typically-radial arrangement towards a
-bilateral arrangement.
-
-  [Illustration: Figs. 197–199.]
-
-Much more conspicuous are these general and special relations of form
-to general and special actions in the environment, among phænogams
-that are multiaxial. That when standing alone, and in places where
-the winds do not injure them nor adjacent things shade them, shrubs
-and trees develop with tolerable evenness on all sides, is an obvious
-truth. Equally obvious is the truth that, when growing together in
-a wood, and mutually interfered with on all sides, trees still show
-obscurely radial distributions of parts; though, under such conditions,
-they have tall taper stems with branches directed upwards--a difference
-of shape clearly due to the different incidence of forces. And almost
-equally obvious is the truth, that a tree of this same kind growing
-at the edge of the wood, has its outer branches well developed and
-its inner branches comparatively ill-developed. Fig. 197, which
-inaccurately represents this difference, will serve to make it manifest
-that while one of the peripheral trees can be cut into something like
-two similar halves by a vertical plane directed towards the centre of
-the wood--a plane on each side of which the conditions are alike--it
-cannot be cut into similar halves by any other plane. A like divergence
-from an indefinitely-radial symmetry towards an indefinitely-bilateral
-symmetry, occurs in trees that have their conditions made bilateral
-by growing on inclined surfaces. Two of the common forms observable
-in such cases are given in Fig. 198. Here there is divisibility into
-parts that are tolerably similar, by a vertical plane running directly
-down the hill; but not by any other plane. Then, further, there is the
-bilateralness, similar in general meaning though differently caused,
-often seen in trees exposed to strong prevailing winds. Almost every
-sea-coast has abundant examples of stunted trees which, like the one
-shown in Fig. 199, have been made to deviate from their ordinary equal
-growth on all sides of a vertical axis, to a growth that is equal only
-on the opposite sides of a vertical plane directed towards the wind’s
-eye.
-
-From among vegetal aggregates of the third order, we have now only to
-add examples of the entirely asymmetrical form which accompanies an
-entirely irregular distribution of incident forces. Creeping plants
-furnish such examples. They show, both when climbing up vertical or
-inclined surfaces and when trailing on the ground, that their branches
-grow hither and thither as the balance of forces aids or opposes;
-and the general outline is without symmetry of any kind, because the
-environing influences have no kind of regularity in their arrangement.
-
-
-§ 220. Along with some unfamiliar facts, I have here set down facts
-which are so familiar as to seem scarcely worth noting. It is because
-these facts have become meaningless to perceptions deadened by infinite
-repetitions of them, that it is needful here to point out their
-meanings. Not alone for its intrinsic importance has the unlikeness
-between the attached ends and the free ends been traced among plants
-of all degrees of integration. Nor is it simply because of the
-significance they have in themselves, that instances have been given
-of those varieties of symmetry and asymmetry which the free ends of
-plants equally display: be they plants of the first, second, third,
-or any higher order. Neither has the only other purpose been that of
-showing how, in the radial symmetry of some vegetal aggregates and
-the single bilateral symmetry of others, there are traceable the same
-ultimate principles as in the spherical symmetry and triple bilateral
-symmetry of certain minute plants first described. But the main object
-has been to present, under their simplest aspects, those general laws
-of morphological differentiation which are fulfilled by the component
-parts of each plant.
-
-If organic form is determined by the distribution of forces, and the
-approach in every case towards an equilibrium of inner actions with
-outer actions; then this relation between forms and forces must hold
-alike in the organism as a whole in its proximate units, and in its
-units of lower orders. Formulas which express the shapes of entire
-plants in terms of surrounding conditions, must be formulas which also
-express the shapes of their several parts in terms of surrounding
-conditions. If, therefore, we find that a plant as a whole is radially
-symmetrical or bilaterally symmetrical or asymmetrical, according as
-the incident forces affect it equally on all sides of an axis, or
-affect it equally only on the opposite sides of one plane, or affect
-it equally in no two directions; then, we may expect that, in like
-manner, each member of a plant will display radial symmetry where
-environing influences are alike along many radii, bilateral symmetry
-where there is bilateralness of environing influences, and unsymmetry
-or asymmetry where there is partial or entire departure from a balance
-of surrounding actions.
-
-To show that this expectation is borne out by the facts, will be the
-object of the following four chapters. Let us begin with the largest
-parts into which plants are divisible; and proceed to the successively
-smaller parts.
-
-
-
-
-                             CHAPTER VIII.
-
-                        THE SHAPES OF BRANCHES.
-
-
-§ 221. Aggregates of the first order supply a few examples of forms
-ramified in an approximately-regular manner, under conditions
-which subject their parts to approximately-regular distributions
-of forces. Some unicellular _Algæ_, becoming elaborately branched,
-assume very much the aspects of small trees; and show us in their
-branches analogous relations of forms to forces. _Bryopsis plumosa_
-may be instanced. Fig. 200 represents the end of one of its lateral
-ramifications, above and beneath which come others of like characters.
-Here it will be seen that the attached and free ends differ; that the
-two sides are much alike; and that they are unlike the upper and under
-surfaces, which resemble one another. The more highly developed members
-of the same group of _Algæ_, the _Siphoneæ_, show a marked radial
-symmetry co-existing with very elaborate branching, _e.g._, _Neomeris_,
-_Cymopolia_, and others.
-
-  [Illustration: Fig. 200.]
-
-
-§ 222. Fig. 201 shows us how, in an aggregate of the second order,
-each proximate component is modified by its relations to the rest;
-just as we before saw a whole fungus of the same type modified by
-its relations to environing objects. If a branch of the fungus here
-figured, be compared with one of the fungi clustered together in Fig.
-195, or, still better, with one of the laterally-growing fungi shown
-in Fig. 196, there will be perceived a kindred transition from radial
-to bilateral symmetry, occurring under kindred conditions. The portion
-of the pileus next to the side of attachment is undeveloped in this
-branched form as in the simpler form; and in the one case as in the
-other, the stem is modified towards the side of attachment. A division
-into similar halves, which, as shown in Fig. 196 _e_, might be made of
-the whole fungus by a vertical plane passing through the centre of the
-pileus and the axis of the supporting body, might here be made of the
-branch, by a vertical plane passing through the centre of its pileus
-and the axis of the main stem. Among aggregates of this order, the
-_Algæ_ furnish cases of kindred nature. In the branches of _Lessonia_,
-Fig. 37, may be observed a substantially-similar relationship. As their
-inner parts are less developed than their outer parts, while their two
-sides are developed in approximately equal degrees, they are rendered
-bilateral.
-
-  [Illustration: Fig. 201.]
-
-
-§ 223. These few cases introduce us to the more familiar but more
-complex cases which plants of the third degree of aggregation present.
-At _a_, _b_, _c_, Fig. 202, are sketched three homologous parts of the
-same tree: _a_ being the leading shoot; _b_ a lateral branch near
-the top, and _c_ a lateral branch lower down. There is here a double
-exemplification. While the branch _a_, as a whole, has its branchlets
-arranged with tolerable regularity all round, in correspondence with
-its equal exposure on all sides, each branchlet shows by its curve
-as much bilateral symmetry as its simple form permits. The branch
-_b_, dissimilarly circumstanced on the side next the main stem and
-on the side away from it, has an approximate bilateralness as a
-whole, while the bilateralness of its branchlets varies with their
-respective positions. And in the branch _c_, having its parts still
-more differently conditioned, these traits of structure are still more
-marked. Extremely strong contrasts of this kind occur in trees having
-very regular modes of growth. The uppermost branches of a Spruce-fir
-have radially-arranged branchlets: each of them, if growing vigorously,
-repeats the type of the leading shoot, as shown in Fig. 203, _a_,
-_b_. But if we examine branches lower and lower down the tree, we
-find the vertically-growing branchlets bear a less and less ratio to
-the horizontally-growing ones; until, towards the bottom, the radial
-arrangement has wholly merged into the bilateral. Shaded and confined
-by the branches above them, these eldest branches develop their
-offshoots in those directions where there is most space and light:
-becoming finally quite flattened and fan-shaped, as shown at Fig.
-203, _c_. And on remembering that each of these eldest branches, when
-first it diverged from the main stem, was radial, we see not only that
-between the upper and lower branches does this contrast in structure
-hold, but also that each branch is transformed from the radial to
-the bilateral by the progressive change in its environment. Other
-forces besides those which aid or hinder growth, conspire to produce
-this two-sided character in lateral branches. The annexed Fig. 204,
-sketched from an example of the _Pinus Coulterii_ at Kew, shows very
-clearly how, by mere gravitation, the once radially-arranged branchlets
-may be so bent as to produce in the branch as a whole a decided
-bilateralness. A full-grown _Araucaria_, too, exhibits in its lower
-branches modifications similarly caused; and in each of such branches
-there may be remarked the further fact, that its upward-bending
-termination has a partially-modified radialness, at the same time that
-its drooping lateral branchlets give to the part nearer the trunk a
-completely bilateral character.
-
-  [Illustration: Figs. 202–204.]
-
-Now in these few instances, typical of countless instances which might
-be given, we see, as we saw in the case of the fungi, that the same
-thing is true of the parts in their relations to the whole and to one
-another, which is true of the whole in its relations to the environment
-at large. Entire trees become bilateral instead of radial, when exposed
-to forces that are equal only on opposite sides of one plane; and in
-their branches, parallel changes of form occur under parallel changes
-of conditions.
-
-
-§ 224. There remains to be said something respecting the distribution
-of leaves. How a branch carries its leaves constitutes one of
-its characters as a branch, and is to be considered apart from
-the characters of the leaves themselves. The principles hitherto
-illustrated we shall here find illustrated still further.
-
-The leading shoot and all the upper twigs of a fir-tree, have their
-pin-shaped leaves evenly distributed all round, or placed radially;[32]
-but as we descend we find them beginning to assume a bilateral
-distribution; and on the lower, horizontally-growing branches, their
-distribution is quite bilateral.[33] Between the Irish and English
-kinds of Yew, there is a contrast of like significance. The branches
-of the one, shooting up as they do almost vertically, are clothed with
-leaves all round; while those of the other, which spread laterally,
-bear their leaves on the two sides. In trees with better-developed
-leaves, the same principle is more or less manifest in proportion as
-the leaves are more or less enabled by their structures to maintain
-fixed positions. Where the foot-stalks are long and slender, and where,
-consequently, each leaf, according to its weight, the flexibility and
-twist of its foot-stalk, and the direction of the branch it grows from,
-falls into some indefinite attitude, the relations are obscured. But
-where the foot-stalks are stiff, as in the Laurel, it will be found, as
-before, that from the topmost and upward growing branches the leaves
-diverge on all sides; while the undermost branches, growing out from
-the shade of those above, have their leaves so turned as to bring them
-into rows horizontally spread out on the two sides of each branch.
-
-  [Illustration: Fig. 205.]
-
-A kindred truth, having like implications, comes into view when we
-observe the relative sizes of leaves on the same branch, where their
-sizes differ. Fig. 205 represents a branch of a Horse-chestnut, taken
-from the lowermost fringe of the tree, where the light has been to
-a great extent intercepted from all but the most protruded parts.
-Beyond the fact that the leaves become by appropriate growths of their
-foot-stalks bilaterally distributed on this drooping branch, instead of
-being distributed symmetrically all round, as on one of the ascending
-shoots, we have here to note the fact that there is unequal development
-on the upper and lower sides. Each of the compound leaves acquires a
-foot-stalk and leaflets that are large in proportion to the supply
-of light; and hence, as we descend towards the bottom of the tree,
-the clusters of leaves display increasing contrasts. How marked these
-contrasts become will be seen on comparing _a_ and _b_, which form one
-pair of leaves that are normally equal, or _c_ and _d_, which form
-another pair normally equal.
-
-Let us not omit to note, while we have this case before us, the proof
-it affords that these differences of development are in a considerable
-degree determined by the different conditions of the parts after they
-have been unfolded. Though those inequalities of dimensions whence
-the differentiations of form result, may be in many cases largely due
-to the inequalities in the circumstances of the parts while in the
-bud (which are, however, representative of inequalities in ancestral
-circumstances); yet these are clearly not the sole causes of the
-unlikenesses which eventually arise. This bilateralness resulting from
-the unequal sizes of the leaves, must be considered as due to the
-differential actions that come into play after the leaves have assumed
-their typical structures.
-
-
-§ 225. How, in the arrangement of their twigs and leaves, branches tend
-to lapse from forms that are approximately symmetrical to forms that
-are quite asymmetrical, need not be demonstrated: it is sufficiently
-conspicuous. But it may be well to point out how the tendency to
-do this further enforces our argument. The comparatively regular
-budding out of secondary axes and tertiary axes, does not usually
-produce an aggregate which maintains its regularity, for the simple
-reason that many of the axes abort. Terminal buds are some of them
-destroyed by birds; others are burrowed into by insects; others are
-nipped by frost; others are broken off or injured during gales of wind.
-The environment of each branch and its branchlets is thus ever being
-varied on all sides: here, space being left vacant by the death of some
-shoot that would ordinarily have occupied it; and there, space being
-trenched on by the lateral growth of some adjacent branch that has had
-its main axis broken. Hence the asymmetry, or heterogeneity of form,
-assumed by the branch, is caused by the asymmetrical distribution of
-incident forces--a result and a cause which go on ever complicating.
-
-
-§ 226. One conspicuous trait in the shapes of branches has still to be
-named. Their proximal or attached ends differ from their distal or free
-ends, in the same way that the lower ends of trees differ from their
-upper ends. This fact, like the fact to which it is here paralleled,
-has had its significance obscured by its extreme familiarity. But
-it shows in a striking way how the most differently conditioned
-parts become the most strongly contrasted in their structures. A
-phænogamic axis is made up of homologous segments, marked off from
-one another by the nodes; and a compound branch consists of groups of
-such segments. The earliest-formed segments, alike of the tree and of
-each branch, serve as mechanical supports and channels for sap to the
-successive generations of segments that grow out of them; and become
-more and more shaded by their progeny as these increase. Hence the
-progressively-increasing contrasts which, while mainly due to the
-unlikenesses of bulk accompanying differences of age, are in part due
-to the unlikenesses of structure which differences of relation to the
-environment have caused.
-
-
-§ 227. Thus, then, it is with the proximate parts of plants as it is
-with plants as wholes. The radial symmetry, the bilateral symmetry, and
-the asymmetry, which branches display in different trees, in different
-parts of the same tree, and at different stages of their own growths,
-prove to be all consequent on the ways in which they stand towards the
-entire plexus of surrounding actions. The principle that the growths
-are unequal in proportion as the relations of parts to the environment
-are unequal, serves to explain all the leading traits of structure.
-
-
-
-
-                              CHAPTER IX.
-
-                         THE SHAPES OF LEAVES.
-
-
-§ 228. Next in the descending order of composition come compound
-leaves. The relative sizes and distributions of their leaflets, as
-affecting their forms as wholes, have to be considered in their
-relations to conditions. Figs. 206, 207, represent leaves of the
-common _Oxalis_ and of the _Marsilea_, in which radial symmetry is
-as completely displayed as the small number of leaflets permits.
-This equal development of the leaflets on all sides, occurs where
-the foot-stalks, growing up vertically from creeping or underground
-stems, are so long that the leaves either do not interfere with one
-another or do it in an inconstant way: the leaflets are not differently
-conditioned on different sides, as they are where the foot-stalks grow
-out in the ordinary manner. How unlikeness of position influences the
-leaflets is clearly shown in a Clover-leaf, Fig. 208, which deviates
-from the Oxalis-leaf but slightly towards bilateralness, as it deviates
-from it but slightly in the attitude of its petiole; which is a little
-inclined away from the others borne by the same procumbent axis.
-A familiar example of an almost radial symmetry along with almost
-equal relations to surrounding conditions, occurs in the root-leaves
-of the Lupin, Fig. 209 _b_. Here though we have lateral divergence
-from a vertical axis, yet the long foot-stalks preserve nearly erect
-positions, and carry their leaves to such distances from the axis,
-that the development of the leaflets on the side next it is not much
-hindered. Still the interference of the leaves with one another is, on
-the average, somewhat greater on the proximal side than on the distal
-side; and hence the interior leaflets are rather less than the exterior
-leaflets. In further proof of which influence, let it be added that, as
-shown in the figure, at _a_, the leaves growing out of the flowering
-stem deviate towards the two-sided form more decidedly. Two-sidedness
-is much greater where there is a greater relative proximity of the
-inner leaflets to the axis, or where the foot-stalk approaches towards
-a horizontal position. The Horse-chestnut, Fig. 205, already instanced
-as showing how the arrangements and sizes of leaflets are determined
-by the incidence of forces, serves also to show how the incidence of
-forces determines the relative sizes and arrangements of leaflets.
-Fig. 210, which shows a leaf of the _Bombax_, further illustrates this
-relation of structure to conditions.
-
-  [Illustration: Figs. 206–210.]
-
-  [Illustration: Figs. 211, 212.]
-
-Compound leaves that are completely bilateral, present us with
-modifications of form exemplifying the same general truth in another
-way. In them the proximal and distal parts have none of that
-resemblance which we see in those intermediate forms just described.
-The portion next the axis and the portion furthest from the axis
-are entirely different; and the only likeness is between the wings
-or leaflets on opposite sides of the main foot-stalk or mid-rib. On
-turning back to Fig. 65, it will be seen that the compound leaf there
-drawn to exemplify another truth, serves also to exemplify this truth:
-the homologous parts _a_, _b_, _c_, _d_, while they are unlike one
-another, are, in their main proportions, severally like the parts with
-which they are paired. And here let us not overlook a characteristic
-which is less conspicuous but not less significant. Each of the lateral
-wings has winglets that are larger on the one side than on the other;
-and in each case the two sides are dissimilarly conditioned. Even in
-the several components of each wing may be traced a like divergence
-from symmetry, along with a like inequality in the relations to the
-rest: the proximal half of each leaflet is habitually larger than the
-distal half. In the leaves of the Bramble, previously figured, kindred
-facts are presented. How far such differences of development are due to
-the positions of the parts in the bud; how far the respective spaces
-available for the parts when unfolded affect them; and how far the
-parts are rendered unlike by unlikenesses in their relations to light;
-it is difficult to say. Probably these several factors operate in all
-varieties of proportion. That the habitual shading of some parts by
-others largely aids in causing these divergences from symmetry, is very
-instructively shown by the compound leaves of the Cow-parsnip. Fig.
-211 represents one of these. While the leaf as a whole is bilaterally
-symmetrical, each of the wings has an unsymmetrical bilateralness: the
-side next the axis being larger than the remoter side. How does this
-happen? Fig. 212, which is a diagrammatic section down the mid-rib
-of the leaf, showing its inclined attitude and the positions of the
-wings _a_, _b_, _c_, will make the cause clear. As the wings overlap,
-like the bars of a Venetian blind, each intercepts some light from the
-one below it; and the one below it thus suffers more on its distal
-side than on its proximal side. Hence the smaller development of the
-distal side. That this is the cause is further shown by the proportion
-that is maintained between the degree of obscuration and the degree
-of non-development; for this unlikeness is greater between the two
-sides _a_ and _a′_, than between _b_ and _b′_ or _c_ and _c′_, at the
-same time that the interference is greater in the lower wings than in
-the upper. Of course in this case and in the kindred cases hereafter
-similarly interpreted, it is not meant that this differentiation
-is consequent solely, or even chiefly, on the differential actions
-experienced by the individual plant. Though there is good reason to
-believe that the rate of growth in each part of each leaf is affected
-by the incidence of light, yet contrasts so marked and so systematic as
-these are not explicable without taking into account the inheritance
-of modifications either functionally caused or caused by spontaneous
-variation. Clearly, the tendency will be towards the preservation of a
-plant which distributes its chlorophyll in the most advantageous way;
-and hence there will always be a gravitation towards a form in which
-shaded parts of leaves are undeveloped.
-
-
-§ 229. From compound leaves to simple ones, we find transitions in
-leaves of which the divisions are partial instead of total; and in
-these we see, with equal clearness, the relations between forms and
-positions that have been traced thus far. Fig. 213 is the leaf of a
-Winter-aconite in which, round a vertical petiole, there is a radial
-distribution of half-separated leaflets. The _Cecropia_-leaf, Fig. 214,
-shows us a two-sided development of the parts beginning to modify, but
-not obliterating, the all-sided arrangement; and this mixed symmetry
-occurs under conditions that are intermediate. A more marked degree of
-the same relation is presented in the leaf of the Lady’s Mantle, Fig.
-215. And then in the Sycamore and the Vine, we have a cleft type of
-leaf in which a decided bilateralness of form co-exists with a decided
-bilateralness of conditions.
-
-  [Illustration: Figs. 213–215.]
-
-  [Illustration: Figs. 216–218.]
-
-The quite simple leaves to which we now descend, exhibit, very
-distinctly, a parallel series of facts. Where they grow up on long and
-completely-independent foot-stalks, without definite subordination
-to some central vertical axis, the leaves of water-plants are
-symmetrically peltate. Of this the sacred Indian-bean, Fig. 216,
-furnishes an example. Here there is only a trace of bilateralness in
-the venation of the leaf, corresponding to the very small difference
-of the conditions on the proximal and distal sides. In the _Victoria
-regia_, Fig. 217, the foot-stalks, though radiating almost
-horizontally from a centre, are so long as to keep the leaves quite
-remote from one another; and in it each leaf is almost symmetrically
-peltate, with a bilateralness indicated only by a seam over the
-line of the foot-stalk. The leaves of the _Nymphæa_, Fig. 218,
-more closely clustered, and having less room transversely than
-longitudinally, exhibit a marked advance to the two-sided form; not
-only in the excess of the length over the breadth, but in the existence
-of a cleft, where in the _Victoria regia_ there is merely a seam.
-Among land-plants similar forms are found under analogous conditions.
-The common _Hydrocotyle_, Fig. 219, which sends up direct from its
-roots a few almost upright leaf-stalks, has these surmounted by peltate
-leaves; which leaves, however, diverge slightly from radial symmetry in
-correspondence with the slight contrast of circumstances which their
-grouping involves. Another case is supplied by the Nasturtium, Fig.
-220, which combines the characters--a creeping stem, long leaf-stalks
-growing up at right angles to it, and unsymmetrically peltate leaves,
-of which the least dimension is, on the average, towards the stem.
-But perhaps the most striking illustration is that furnished by the
-_Cotyledon umbilicus_, Fig. 221, in which different kinds of
-symmetry occur in the leaves of the same plant, along with differences
-in their relations to conditions. The root-leaves, _a_, growing up
-on vertical petioles before the flower-stalk makes its appearance, are
-symmetrically peltate; while the leaves which subsequently grow out of
-the flower-stalk, _b_, are at the bottom transitionally bilateral,
-and higher up completely bilateral.
-
-  [Illustration: Figs. 219–221.]
-
-That the bilateral form of leaf is the ordinary form, corresponds with
-the fact that, ordinarily, the circumstances of the leaf are different
-in the direction of the plant’s axis from what they are in the opposite
-direction, while transversely the circumstances are alike. It is
-needless to give diagrams to illustrate this extremely familiar truth.
-Whether they are broad or long, oval or heart-shaped, pointed or
-obtuse, the leaves of most trees and plants will be remembered by all
-as having the ends by which they are attached unlike the free ends,
-while the two sides are alike. And it will also be remembered that
-these equalities and inequalities of development correspond with the
-equalities and inequalities in the incidence of forces.
-
-
-§ 230. A confirmation that is interesting and important, is furnished
-by the cases in which leaves present unsymmetrical forms in positions
-where their parts are unsymmetrically related to the environment. A
-considerable deviation from bilateral symmetry may be seen in a leaf
-which habitually so carries itself, that the half on the one side of
-the mid-rib is more shaded than the other half. The drooping branches
-of the Lime, delineated in Fig. 222, show us leaves so arranged and
-so modified. On examining their attitudes and their relations one to
-another, it will be found that each leaf is so inclined that the half
-of it next to the shoot grows over the shoot and gets plenty of light;
-while the other half so hangs down that it comes a good deal into the
-shade of the preceding leaf. The result is that having leaves which
-fall into these positions, the species profits by a large development
-of the exposed halves; and by survival of the fittest, acting along
-with the direct effect of extra exposure, this modification becomes
-established. How unquestionable is the connexion between the relative
-positions of the halves and their relative developments, will be
-admitted on observing a converse case. Fig. 223 represents a shoot
-of _Strobilanthes glomeratus_. Here the leaves are so set on the stem
-that the inner half of each leaf is shaded by the subsequently-formed
-leaf, while its outer half is not thus shaded; and here we find the
-inner half less developed than the outer half. But the most conclusive
-evidence of this relation between unsymmetrical form and unsymmetrical
-distribution of surrounding forces, is supplied by the genus _Begonia_;
-for in it we have a manifest proportion between the degree of the
-alleged effect and the degree of the alleged cause. These plants
-produce their leaves in pairs, in such ways that the connate leaves
-interfere with one another, much or little according as the foot-stalks
-are short or long; and the result is a correlative divergence from
-symmetry. In _Begonia nelumbiifolia_, which has petioles so long that
-the connate leaves are not kept close together, there is but little
-deviation from a bilaterally-peltate form; whereas, accompanying the
-comparatively marked and constant proximity in _B. pruinata_, Fig. 224,
-we see a more decidedly unsymmetrical shape; and in _B. mahringii_,
-Fig. 225, the modification thus caused is pushed so far as to destroy
-the peltate structure.[34]
-
-  [Illustration: Figs. 222–225.]
-
-
-§ 231. Again, then, we are taught the same truth. Here, as before,
-we see that homologous units of any order become differentiated in
-proportion as their relations to incident forces become different. And
-here, as before, we see that in each unit, considered by itself, the
-differences of dimension are greatest in those directions in which the
-parts are most differently conditioned; while there are no differences
-between the dimensions of the parts that are not differently
-conditioned.[35]
-
-
-
-
-                              CHAPTER X.
-
-                        THE SHAPES OF FLOWERS.
-
-
-§ 232. Following an order like that of preceding chapters, let us first
-note a few typical facts respecting the forms of clusters of flowers,
-apart from the forms of the flowers themselves. Two kindred kinds of
-_Leguminosæ_ serve to show how the members of clusters are distributed
-in an all-sided manner or in a two-sided manner, according as the
-circumstances are alike on all sides or alike on only two sides. In
-_Hippocrepis_, represented in Fig. 226, the flowers growing at the
-end of a vertical stem, are arranged round it in radial symmetry.
-Contrariwise in _Melilotus_, Fig. 227, where the axillary stem bearing
-the flowers is so placed in relation to the main stem, that its outer
-and inner faces are differently conditioned, the flowers are all on the
-outer face: the cluster is bilaterally symmetrical, since it may be cut
-into approximately equal and similar groups by a vertical plane passing
-through the main axis.
-
-  [Illustration: Figs. 226, 227.]
-
-Plants of this same tribe furnish clusters of intermediate characters
-having intermediate conditions. Among these, as among the clusters
-which other types present, may be found some in which conformity
-to the general law is not obvious. The discussion of these apparent
-anomalies would carry us too much out of our course. A clue to the
-explanation of them will, I believe, be found in the explanation
-presently to be given of certain kindred anomalies in the forms of
-individual flowers.
-
-
-§ 233. The radially-symmetrical form is common to all individual
-flowers that have vertical axes. In plants which are practically if
-not literally uniaxial, and bear their flowers at the ends of upright
-stalks, so that the faces open horizontally, the petals are disposed in
-an all-sided way. Crocuses, Tulips, and Poppies are familiar examples
-of this structure occurring under these conditions. A Ranunculus
-flower, Fig. 228, will serve as a typical one. Similarly, flowers which
-have peduncles flexible enough to let them hang directly downwards, and
-are not laterally incommoded, are also radial; as in the _Fuchsia_,
-Fig. 229, as in _Cyclamen_, _Hyacinth_, &c. These relations of form
-to position are, I believe, uniform. Though some flowers carried at
-the ends of upright or downright stems have oblique shapes, it is only
-when they have inclined axes or are not equally conditioned all round.
-No solitary flower having an axis habitually vertical, presents a
-bilateral form. This is as we should expect; since flowers which open
-out their faces horizontally, whether facing upwards or downwards, are,
-on the average, similarly affected on all sides.
-
-  [Illustration: Figs. 228, 229.]
-
-  [Illustration: Figs. 230, 231.]
-
-At first it seems that flowers thus placed should alone be radial;
-but further consideration discloses conditions under which this
-type of symmetry may exist in flowers otherwise placed. Remembering
-that the radial form is the primitive form--that, morphologically
-speaking, it results from the contraction into a whorl, of parts that
-are originally arranged in the same spiral succession as the leaves;
-we must expect it to continue wherever there are no forces tending
-to change it. What now must be the forces tending to change it? They
-must be forces which do not simply affect differently the different
-parts of an individual flower. They must be forces which affect in
-like contrasted ways the homologous parts of other individual flowers,
-both on the same plant and on surrounding plants of the same species.
-A permanent modification can be expected only in cases where, by
-inheritance, the effects of the modifying causes accumulate. That
-they may accumulate the flowers must keep themselves so related to
-the environment, that the homologous parts may, generation after
-generation, be subjected to like differentiating forces. Hence, among
-a plant’s flowers which maintain no uniformity in the relations of
-their parts to surrounding influences, the radial form will continue.
-Let us glance at the several causes which entail this variability.
-When flowers are borne on many branches, which have all inclinations
-from the vertical to the horizontal--as are the flowers of the Apple,
-the Plum, the Hawthorn--they are placed in countless different
-attitudes. Consequently, any spontaneous variation in shape which
-might be advantageous were the attitude constant, is not likely to
-be advantageous; and any functionally-produced modification in one
-flower, is likely to be neutralized in offspring by some opposite
-functionally-produced modification in another flower. It is quite
-comprehensible, therefore, that irregularly-branched plants should
-thus preserve their laterally-borne flowers from undergoing permanent
-deviations from their primitive radial symmetry. Fig. 230, representing
-a blossoming twig of the Blackthorn, illustrates this. Again, upright
-panicles, such as those of the Saxifrage exemplified in Fig. 231,
-and irregular terminal groups of flowers otherwise named, furnish
-conditions under which there is similarly an absence of determinate
-relations between the parts of the flowers and the incident forces;
-and hence an absence of bilateralness. This inconstancy of relative
-position is produced in various other ways--by extreme flexibility of
-the stems, as in the Blue-bell; by the tendency of the peduncles to
-curl to a greater or less extent in diverse directions, as in _Pyrola_;
-by special twistings of the peduncles, differing in degree in different
-individuals, as in _Convolvulus_; by unusual laxity of the petals,
-as in _Lythrum_. Elsewhere the like general result arises from a
-progressive change of attitude, as in _Myosotis_, the stem of which as
-it unfolds causes each flower to undergo a transition from an upward
-position of the mouth to a lateral position; or as in most _Cruciferæ_,
-where the like effect follows from an altered direction of the peduncle.
-
-There are, however, certain seemingly-anomalous cases where radial
-symmetry is maintained by laterally-placed flowers, which keep
-their parts in relative positions that are tolerably constant. The
-explanation of these exceptions is not manifest. It is only when
-we take into account certain incident actions liable to be left
-unremembered, that we find a probable solution. It will be most
-convenient to postpone the consideration of these cases until we have
-reached the general rule to which they are exceptions.
-
-
-§ 234. Transitions varying in degree from the radial towards the
-bilateral, are common in flowers that are borne at the ends of branches
-or axes which are inclined in tolerably constant ways. We may see this
-in sundry garden flowers such as _Petunia_, or such as _Isoloma_ and
-_Achimenes_, shown in Figs. 232 and 233. If these plants be examined,
-it will be perceived that the mode of growth makes the flower unfold
-in a partially one-sided position; that its parts of attachment have
-rigidity sufficient to prevent this attitude from being very much
-interfered with; and that though the individual flowers vary somewhat
-in their attitudes, they do not vary to the extent of neutralizing the
-differentiating conditions--there remains an average divergence from a
-horizontal unfolding of the flower, to account for its divergence from
-radial symmetry.
-
-  [Illustration: Figs. 232, 233.]
-
-We pass insensibly from forms like these, to forms having bilateral
-symmetry strongly pronounced. Some such forms occur among flowers that
-grow at the ends of upright stems; as in _Pinguicula_, and in the
-Violet tribe. But this happens only where, in successive generations,
-the flower unfolds its parts sideways in constant relative positions.
-And in the immense majority of flowers having well-marked two-sided
-forms, the habitual exposure of the different parts to different
-sets of forces, is effectually secured by the mode of placing. As
-illustrations, I may name the genera--_Orchis_, _Utricularia_,
-_Salvia_, _Salix_, _Delphinium_, _Mentha_, _Teucrium_, _Ajuga_,
-_Ballota_, _Galeopsis_, _Lamium_, _Stachys_, _Nepeta_, _Marrubium_,
-_Calamintha_, _Melittis_, _Prunella_, _Scutellaria_, _Bartsia_,
-_Euphrasia_, _Rhinanthus_, _Melampyrum_, _Pedicularis_, _Linaria_,
-_Digitalis_, _Orobanche_, _Fumaria_, _&c._; to which may be added all
-the Grasses and all the _Papilionaceæ_. In most of these cases the
-flowers, being sessile on the sides of upright stems, are kept in quite
-fixed attitudes; and in the other cases the peduncles are very short,
-or else stiff enough to secure general uniformity in the positions. A
-few of the more marked types are shown in Figs. 234 to 241.
-
-  [Illustration: Figs. 234–241.]
-
-  [Illustration: Figs. 242–246.]
-
-  [Illustration: Figs. 247, 248.]
-
-Very instructive evidences here meet us. Sometimes within the limits
-of one genus we find radial flowers, bilateral flowers, and flowers of
-intermediate characters. The genus
-
-_Begonia_ may be instanced. In _B. rigida_ the flowers, various in
-their attitudes, are in their more conspicuous characters radial:
-though there is a certain bilateralness in the calyx, the five petals
-are symmetrically disposed all round. _B. Wageneriana_ furnishes
-two forms of flowers. On the same individual plant may be found
-radial flowers like Fig. 242, and others, like Fig. 243, which are
-merging into the bilateral. More decided is the bilateralness in _B.
-albo-coccinea_, Fig. 244; and still more in _B. nitida_, Fig. 245.
-While in _B. heracleifolia_, Fig. 246, the change reaches its extreme
-by the disappearance of the lateral petals. On examining the modes
-of growth in these several species, they will be seen to explain
-these changes in the manner alleged. Even more conclusive are the
-nearly-allied transformations occurring in artificially-produced
-varieties of the same species. _Gloxinia_ may be named in illustration.
-In Fig. 247 is represented one of the ordinary forms, which shows us
-bilateralness of shape along with a mode of growth that renders the
-conditions alike on the two sides while different above and below.
-But in _G. erecta_, Fig. 248, we have the flower assuming an upright
-attitude, and at the same time assuming the radial type. This is not
-to be interpreted as a production of radial symmetry out of bilateral
-symmetry, under the action of the appropriate conditions. It is rather
-to be taken as a case of what is termed “peloria”--a reversion to the
-primitive radial type, from which the bilateral modification had been
-derived. The significant inference to be drawn from it is, that this
-primitive radial type had an upright attitude; and that the derivation
-of a bilateral type from it, occurred along with the assumption of an
-inclined attitude.
-
-We come now to a group of cases above referred to, in which radial
-symmetry continues to co-exist with that constant lateral attitude
-ordinarily accompanied by the two-sided form. Two examples will
-suffice: one a very large flower, the Hollyhock, and the other a very
-small flower, the Agrimony. Why does the radial form here remain
-unchanged? and how does its continuance consist with the alleged
-general law?
-
-Until quite recently I have been unable to find any probable answers
-to these questions. When the difficulty first presented itself, I
-could think of no other possible cause for the anomaly, than that the
-parts of the Hollyhock-flower, unfolding spirally as they do, might
-have different degrees of spiral twist in different flowers, and might
-thus not be unfolded in sufficiently-constant positions. But this
-seemed a questionable interpretation; and one which did not obviously
-apply to the case of the Agrimony. It was only on inquiring what are
-the special causes of modifications in the forms of flowers, that a
-more feasible explanation suggested itself; and this would probably
-never have suggested itself, had not Mr. Darwin’s investigations into
-the fertilization of Orchids led me to take into account an unnoticed
-agency.
-
-  [Illustration: Fig. 249.]
-
-The actions which affect the forms of leaves, affect much less
-decidedly the forms of flowers; and the forms of flowers are influenced
-by actions which do not influence the forms of leaves. Partly through
-the direct action of incident forces and partly through the indirect
-action of natural selection, leaves get their parts distributed in
-ways that most facilitate their assimilative functions, under the
-circumstances in which they are placed; and their several types
-of symmetry are thus explicable. But in flowers, the petals and
-fructifying organs of which do not contain chlorophyll, the tendency to
-grow most where the supply of light is greatest, is less decided, if
-not absent; and a shape otherwise determined is hence less liable to
-alter in consequence of altered relations to sun and air. Gravity, too,
-must be comparatively ineffective in causing modifications: the smaller
-sizes of the parts, as well as their modes of attachment, giving them
-greater relative rigidity. Not, indeed, that these incident forces of
-the inorganic world are here quite inoperative. Fig. 249, representing
-a species of _Campanula_, shows that the developments of individual
-flowers are somewhat modified by the relations of their parts to
-general conditions. But the fact to be observed is, that the extreme
-transformations which flowers undergo are not likely to be thus caused:
-some further cause must be sought. And if we bear in mind the functions
-of flowers, we shall find in their adaptations to these functions,
-under conditions that are extremely varied, an adequate cause for the
-different types of symmetry, as well as for the exceptions to them.
-Flowers are parts in which fertilization is effected; and the active
-agents of this fertilization are insects--bees, moths, butterflies,
-&c. Mr. Darwin has shown in many cases, that the forms and positions
-of the essential organs of fructification, are such as to facilitate
-the actions of insects in transferring pollen from the anthers of one
-flower to the pistil of another--an arrangement produced by natural
-selection. And here we shall find reason for concluding, that the forms
-and positions of those subsidiary parts which give their shapes to
-flowers, similarly arise by the survival of individuals which have the
-subsidiary parts so adjusted as to aid this fertilizing process--the
-deviations from radial symmetry being among such adjustments. The
-reasoning is as follows. So long as the axis of a flower is vertical
-and the conditions are similar all round, a bee or butterfly alighting
-on it, will be as likely to come from one side as from another; and
-hence, hindrance rather than facilitation would result if the several
-sides of the flower did not afford it equally free access. In like
-manner, flowers which are distributed over a plant in such ways that
-their discs open out on planes of all directions and inclinations, will
-have no tendency to lose their radial symmetry; since, on the average,
-no part of the periphery is differently related to insect-agency
-from any other part. But flowers so fixed as to open out sideways in
-tolerably-constant attitudes, have their petals differently related
-to insect-agency. A bee or butterfly coming to a laterally-growing
-flower, does not settle on it in one way as readily as in another; but
-almost of necessity settles with the axis of its body inclined upwards
-towards the stem of the plant. Hence the side-petals of a flower so
-fixed, habitually stand to the alighting insect in relations different
-from those in which the upper and lower petals stand; and the upper
-and lower petals differ from one another in their relations to it. If,
-then, there so arises an habitual attitude of the insect towards the
-petals, there is likely to be some arrangement of the petals that will
-be most convenient to the insect--will most facilitate its entrance
-into the flower. Thus we see in many cases, that a long undermost
-petal or lip, by enabling the insect to settle in such way as to bring
-its head opposite to the opening of the tube, aids its fertilizing
-agency. But whatever be the special modifications of the corolla which
-facilitate the actions of the particular insects concerned, all of them
-will conduce to bilateral symmetry; since they will be alike for the
-two sides but unlike for the top and bottom. And now we are prepared
-for understanding the exceptions. Flowers growing sideways can become
-thus adapted by survival of the fittest, only if they are of such sizes
-and structures that insect-agency can affect them in the way described.
-But in the plants named above, this condition is not fulfilled. A
-Hollyhock-flower is so open, as well as so large, that its petals
-are not in any appreciable degree differently related to the insects
-which visit it. On the other hand, the flower of the Agrimony is so
-small, that unless visited by insects of a corresponding size which
-settle as bees and butterflies settle, its parts will not be affected
-in the alleged manner. That all anomalies of this kind can at once be
-satisfactorily explained, is scarcely to be expected: the circumstances
-of each case have to be studied. But it seems not improbable that they
-are due to causes of the kind indicated.[36]
-
-
-§ 235. We have already glanced at clusters of flowers for the purpose
-of considering their shapes as clusters. We must now return to them to
-observe the modifications undergone by their component flowers. Among
-these occur illustrations of great significance.
-
-An example of transition from the radial to the bilateral form in
-clustered flowers of the same species, is furnished by the cultivated
-_Geraniums_, called by florists _Pelargoniums_. Some of these, bearing
-somewhat small terminal clusters of flowers, which are closely packed
-together with their faces almost upwards, have radially-symmetrical
-flowers. But among other varieties having terminal clusters of which
-the members are mutually thrust on one side by crowding, the flowers
-depart very considerably from the radial shape towards the bilateral
-shape. A like result occurs under like conditions in Rhododendrons
-and Azaleas. The _Verbena_, too, furnishes an illustration of radial
-flowers rendered slightly two-sided by the slight two-sidedness
-of their relations to other flowers in the cluster. And among the
-_Cruciferæ_ a kindred case occurs in the cultivated Candytuft.
-
-Evidence of a somewhat different kind is offered us by clustered
-flowers in which the peripheral members of the clusters differ from
-the central members; and this evidence is especially significant
-where we find allied species that do not exhibit the deviation, at
-the same time that they do not fulfil the conditions under which it
-may be expected. Thus, in _Scabiosa succisa_, Fig. 250, which bears
-its numerous small flowers in a hemispherical knob, the component
-flowers, similarly circumstanced, are all equal and all radial; but in
-_Scabiosa arvensis_, Fig. 251, in which the numerous small flowers form
-a flattened disk only the confined central ones are radial: round the
-edge the flowers are much larger and conspicuously bilateral.
-
-  [Illustration: Figs. 250, 251.]
-
-  [Illustration: Fig. 252.]
-
-But the most remarkable and most conclusive proofs of these relations
-between forms and positions, are those given by the clustered flowers
-called _Umbelliferæ_. In some cases, as where the component flowers
-have all plenty of room, or where the surface of the umbel is more or
-less globular, the modifications are not conspicuous; but where, as
-in _Viburnum_, _Chærophyllum_, _Anthriscus_, _Torilis_, _Caucalis_,
-_Daucus_, _Tordylium_, &c., we have flowers clustered in such ways as
-to be differently conditioned, we find a number of modifications that
-are marked and varied in proportion as the differences of conditions
-are marked and varied. In _Chærophyllum_, where the flowers of each
-umbellule are closely placed so as to form a flat surface, but
-where the umbellules are wide apart and form a dispersed umbel, the
-umbellules do not differ from one another; though among the flowers
-of each umbellule there are decided differences: the central flowers
-being small and radial, while the peripheral ones are large and
-bilateral. But in other genera, where not only the flowers of each
-umbellule but also the umbellules themselves, are closely clustered
-into a flat surface, the umbellules themselves become contrasted;
-and many remarkable secondary modifications arise. In an umbel of
-_Heracleum_, for instance, there are to be noted the facts;--first,
-that the external umbellules are larger than the internal ones; second,
-that in each umbellule the central flowers are less developed than the
-peripheral ones; third, that this greater development of the peripheral
-flowers is most marked in the outer umbellules; fourth, that it is
-most marked on the outer sides of the outer umbellules; fifth, that
-while the interior flowers of each umbellule are radial, the exterior
-ones are bilateral; sixth, that this bilateralness is most marked in
-the peripheral flowers of the peripheral umbellules; seventh, that the
-flowers on the outer sides of these peripheral umbellules are those in
-which the bilateralness reaches a maximum; and eighth, that where the
-outer umbellules touch one another, the flowers, being unsymmetrically
-placed, are unsymmetrically bilateral.[37] The like modifications are
-displayed, though not in so clearly-traceable a way, in an umbel of
-_Tordylium_, Fig. 252. Considering how obviously these various forms
-are related to the various conditions, we should be scarcely able, even
-in the absence of all other facts, to resist the conclusion that the
-differences in the conditions are the causes of the differences in the
-forms.
-
-  [Illustration: Fig. 253.]
-
-Composite flowers furnish evidence so nearly allied to that which
-clustered flowers furnish, that we may fitly glance at them under
-the same head. Such a common type of this order as the Sun-flower,
-exemplifies the extremely marked difference which arises in many
-of these plants between the closely-packed internal florets, each
-similarly circumstanced on all sides, and the external florets, not
-similarly circumstanced on all sides. In Fig. 253, representing the
-inner and outer florets of a Daisy, the contrast is marked between the
-small radial corolla of the one and the larger bilateral corolla of the
-other. In many cases, however, this contrast is less marked: the inner
-florets also having their outward-growing prolongations--a difference
-possibly related to some difference in the habits of the insects that
-fertilize them. Nevertheless, these composite flowers which have
-inner florets with strap-shaped corollas outwardly directed, equally
-conform to the general principle; both in the radial arrangement of
-the assemblage of florets, and in the bilateral shape of each floret;
-which has its parts alike on the two sides of a line passing from the
-centre of the assemblage to the circumference. Certain other members
-of this order fulfil the law somewhat differently. In _Centaurea_,
-for instance, the inner florets are small and vertical in direction,
-while the outer florets are large and lateral in direction. And here
-may be remarked, in passing, a clear indication of the effect which
-great flexibility of the petals has in preventing a flower from
-losing its original radiate form; for while in _C. cyanus_, the large
-outward-growing florets, having short, stiff divisions of the corolla,
-are decidedly bilateral, in _C. scabiosa_, where the divisions of the
-corolla are long and flexible, the radial form is scarcely at all
-modified. On bearing in mind the probable relations of the forms to
-insect-agency, the meaning of this difference will not be difficult to
-understand.[38]
-
-
-§ 236. In extremely-varied ways there are thus re-illustrated among
-flowers, the general laws of form which leaves and branches and
-entire plants disclose to us. Composed as each cluster of flowers
-is of individuals that are originally similar; and composed as each
-flower is of homologous foliar organs; we see both that the like
-flowers become unlike and the like parts of each flower become unlike,
-where the positions involve unlike incidence of forces. The symmetry
-remains radial where the conditions are equal all round; shows
-deviation towards two-sidedness where there is slight two-sidedness
-of conditions; becomes decidedly bilateral where the conditions are
-decidedly bilateral; and passes into an unsymmetrical form where the
-relations to the environment are unsymmetrical.
-
-
-
-
-                              CHAPTER XI.
-
-                     THE SHAPES OF VEGETAL CELLS.
-
-
-§ 237. We come now to aggregates of the lowest order. Already something
-has been said (§ 217) concerning the forms of those morphological units
-which exist as independent plants. But it is here requisite briefly to
-note the modifications undergone by them where they become components
-of larger plants.
-
-  [Illustration: Fig. 254.]
-
-Of the numerous cell-forms which are found in the tissues of the higher
-plants, it will suffice to give, in Fig. 254, representing a section
-of a leaf, a single example. In this it will be seen that the cells
-forming the upper and lower surfaces, _a_ and _b_, have differences of
-shape related to differences in the incidence of forces: they are more
-or less flattened in relation to the environment. The underneath cells
-at _c_, form a class which, similarly exposed to light at their outer
-ends, and, as we may assume, largely developed in adjustment to their
-active assimilative functions, are, by mutual pressure, made to grow
-more in the direction of their lengths than in the direction of their
-breadths. Then on the other side we see that the cells _d_, next above
-the outer layer, while approximately similar, become more and more
-dissimilar as they diverge from the surface, and are quite irregular
-in the interior _e_, where there is no definiteness in the conditions
-to which they are exposed. Thus the divergences of these cells from
-primordial sphericity are such as correspond with unlikenesses in their
-circumstances. And throughout the more complex modifications which the
-cells of other tissues exhibit, the like correspondences hold.
-
-  [Illustration: Figs. 32–35.]
-
-Among plants of a lower order of aggregation, we have already seen
-how cells become metamorphosed as they become integrated into masses
-having definite organizations. The higher _Algæ_, exemplified in Figs.
-32, 34, 35, show this very clearly. Here the departure from the simple
-cell-form to the form of an elongated prism, is manifestly subordinated
-to the contrasts in the relations of the parts. And it is interesting
-to observe how, in one of the branches of Fig. 32, we pass from the
-small, almost-spherical cells which terminate the branchlets, to the
-large, much-modified cells which join the main stem, through gradations
-obviously related in their changed forms to the altered actions their
-positions expose them to.
-
-  [Illustration: Figs. 19–23.]
-
-More simply, but quite as conclusively, do the inferior _Algæ_, of
-which Figs. 19–23 are examples, show us how cells pass from their
-original spherical symmetry into radial symmetry, as they pass from a
-state in which they are similarly-conditioned on all sides, to a state
-in which two of their opposite sides or ends are conditioned in ways
-that are like one another, but unlike the ways in which all other sides
-are conditioned.
-
-Still more instructive are the morphological differentiations of
-those protophytes in which the first steps towards a higher degree of
-integration are shown. In Fig. 10, representing one of the transitional
-forms of _Desmidiaceæ_, it is to be noted that besides the difference
-between the transverse and longitudinal dimensions, which the component
-units display in common, the two end-units differ from the rest:
-they have appendages which the rest have not. Once more, where the
-integration is carried on in such ways as to produce not strings but
-clusters, there arise contrasts and correspondences just such as might
-be looked for. All the four members of the group shown in Fig. 12,
-are similarly conditioned; and each of them has a bilateral shape
-answering to its bilateral relations. In Fig. 14 we have a number of
-similarly-bilateral individuals on the circumference, including a
-central individual differing from the rest by having the bilateral
-character nearly obliterated. And then, in Fig. 15, we have two central
-components of the group, deviating more decidedly from those that
-surround them.[39]
-
-  [Illustration: Figs. 7–17.]
-
-
-
-
-                             CHAPTER XII.
-
-                  CHANGES OF SHAPE OTHERWISE CAUSED.
-
-
-§ 238. Besides the more special causes of modification in the shapes of
-plants and of their parts, certain more general causes must be briefly
-noticed. These may be described as consequences of variations in the
-total quantities of the matters and forces furnished to plants by their
-environments. Some of the changes of form so produced are displayed by
-plants as wholes, and others only by their parts. We will glance at
-them in this order.
-
-
-§ 239. It is a familiar fact that luxuriant shoots have relatively-long
-internodes; and, conversely, that a shoot dwarfed from lack of sap,
-has its nodes closely clustered: a concomitant result being that the
-lateral axes, where these are developed, become in the one case far
-apart and in the other case near together. Fig. 255 represents a branch
-to the parts of which the longer and shorter internodes so resulting
-give differential characters. A whole tree being in many cases
-simultaneously thus affected by states of the earth or the air, all
-parts of it may have such variations impressed on them; and, indeed,
-such variations, following more or less regularly the changes of the
-seasons, give to many trees manifest traits of structure. In Fig. 256,
-a shoot of _Phyllocactus crenatus_, we have an interesting example
-of a variation essentially of the same nature, little as it appears
-to be so. For each of the lateral indentations is here the seat of
-an axillary bud; and these we see are separated by internodes which,
-becoming broader as they become longer, and narrower as they become
-shorter, produce changes of form that correspond with changes in the
-luxuriance of growth.
-
-  [Illustration: Figs. 255, 256.]
-
-To complete the statement it must be added that these variations of
-nutrition often determine the development or non-development of lateral
-axes; and by so doing cause still more marked structural differences.
-The Foxglove may be named as a plant which illustrates this truth.[40]
-
-
-§ 240. From the morphological differentiations caused by unlikenesses
-of nutrition felt by the whole plant, we pass now to those which are
-thus caused in some of its parts and not in others. Among such are
-the contrasts between flowering axes, and the axes that bear leaves
-only. It has already been shown in § 78, that the belief expressed by
-Wolff in a direct connexion between fructification and innutrition,
-is justified inductively by many facts of many kinds. Deductively
-too, in § 79, we saw reason to conclude that such a relation would be
-established by survival of the fittest; seeing that it would profit
-a species for its members to begin sending off migrating germs from
-the ends of those axes which innutrition prevented from further
-agamogenetic multiplication. Once more, when considering the nature of
-the phænogamic axis, we found support for this belief in the fact that
-the components of a flower exhibit a reversion to that type from which
-the phænogamic type has probably arisen--a reversion which the laws of
-embryology would lead us to look for where innutrition had arrested
-development.
-
-Hence, then, we may properly count those deviations of structure
-which constitute inflorescence, as among the morphological
-differentiations produced by local innutrition. I do not mean that the
-detailed modifications which the essential and subservient organs of
-fructification display, are thus accounted for: we have seen reason
-to think them otherwise caused. But I mean that the morphological
-characters which distinguish gamogenetic axes in general from
-agamogenetic axes, such as non-development of the internodes and
-dwarfing of the foliar organs, are primarily results of failure in the
-supply of some material required for further growth.[41]
-
-
-§ 241. Another trait which has to be noticed under this head, is the
-spiral, or rather the helical, arrangement of parts. The successive
-nodes of a phænogam habitually bear their appendages in ways implying
-more or less twist in the substance of the axis; and in climbing plants
-the twist is such as to produce a corkscrew shape. This structure is
-ascribable to differences of interstitial nutrition. Take a shoot which
-is growing vertically. It is clear that if the molecules are added
-with perfect equality on all sides, there will be no tendency towards
-any kind of lateral deviation; and the successively-produced parts
-will be perpendicularly over one another. But any inequality in the
-rate of growth on the different sides of the shoot, will destroy this
-straightness in the lines of growth. If the greatest and least rates
-of molecular increase happen to be on opposite sides, the shoot must
-assume a curve of single curvature; but in every other case of unequal
-molecular increase, a curve of double curvature must result. Now it is
-a corollary from the instability of the homogeneous, that the rates of
-growth on all sides of a shoot can never be exactly alike; and it is
-also to be inferred from the same general law, that the greatest and
-least rates of growth will not occur on exactly opposite sides of the
-shoot, at the same time that equal rates of growth are preserved by the
-two other sides. Hence, there must almost inevitably arise more or less
-of twist; and the appendages of the internodes will so be prevented
-from occurring perpendicularly one over another.
-
-A deviation of this kind, necessarily initiated by physical causes
-in conformity with the general laws of evolution, is likely to be
-made regular and decided by natural selection. For under ordinary
-circumstances, a plant profits by having its axis so twisted as to
-bring the appended leaves into positions which prevent them from
-shading one another. And, manifestly, modifications in the forms,
-sizes, and insertions of the leaves, may, under the same agency, lead
-to adapted modifications of the twist. We must therefore ascribe this
-common characteristic of phænogams, primarily to local differences of
-nutrition, and secondarily to survival of the fittest.
-
-It is proper to add that there are some Monocotyledons, as _Ravenala
-madagascariensis_, in which this character does not occur. What
-conditions of existence they are that here hold this natural tendency
-in check, it is not easy to see.[42]
-
-
-
-
-                             CHAPTER XIII.
-
-               MORPHOLOGICAL DIFFERENTIATION IN ANIMALS.
-
-
-§ 242. The general considerations which preluded our inquiry into the
-shapes of plants and their parts, equally serve, so far as they go, to
-prelude an inquiry into the shapes of animals and their parts. Among
-animals, as among plants, the formation of aggregates greater in bulk
-or higher in degree of composition, or both, is accompanied by changes
-of form in the aggregates as wholes as well as by changes of form in
-their parts; and the processes of morphological differentiation conform
-to the same general laws in the one kingdom as in the other.
-
-It is needless to recapitulate the several kinds of modification to be
-explained, and the several factors that co-operate in working them.
-In so far as these are common to plants and animals, the preceding
-chapters have sufficiently familiarized them. Nor is it needful to
-specify afresh the several types of symmetry and their descriptive
-names; for what is true of them in the one case is true of them in the
-other. There is, however, one new and all-important factor which we
-shall have now to take into account; and about this a few preliminary
-remarks are requisite.
-
-
-§ 243. This new factor is motion--motion of the organism in relation
-to surrounding objects, or of the parts of the organism in relation
-to one another, or both. Though there are plants, especially of the
-simpler kinds, which move, and though a few of the simpler animals do
-not move; yet movements are so exceptional and unobtrusive in the one
-kingdom, while they are so general and conspicuous in the other, that
-the broad distinction commonly made is well warranted. What, among
-plants, is an inappreciable cause of morphological differentiation,
-becomes, among animals, the chief cause of morphological
-differentiation.
-
-Rooted animals or animals otherwise fixed, of course present traits
-of structure nearest akin to those we have lately been studying. The
-motions of parts in relation to one another and to the environment,
-being governed by the mode of aggregation and mode of fixing, we are
-presented with morphological differentiations similar in their general
-characters to those of plants, and showing us parallel kinds of
-symmetry under parallel conditions. But animals which move from place
-to place are subject to an additional class of actions and reactions.
-These actions and reactions affect them in various ways according to
-their various modes of movement. Let us glance at the several leading
-relations between shape and motion which we may expect to find.
-
-If an organism advances through a homogeneous medium with one end
-always foremost, that end, being exposed to forces unlike those to
-which the other end is exposed, may be expected to become unlike it;
-and supposing this to be the only constant contrast of conditions,
-we may expect an equal distribution of the parts round the axis
-of movement--a radial symmetry. If, in addition to this habitual
-attitude of the ends, one surface of the body is always uppermost
-and another always lowermost, there arise between the top and bottom
-dissimilarities of conditions, while the two sides remain similarly
-conditioned. Hence it is inferable that such an organism will be
-divisible into similar halves by a vertical plane passing through
-its axis of motion--will have a bilateral symmetry. We may presume
-that this symmetry will deviate but little from double bilateralness
-where the upper and under parts are not exposed to strongly-contrasted
-influences; while we may rationally look for single bilateral symmetry
-of a decided kind, in creatures having dorsal and ventral parts
-conversant with very unlike regions of the environment: as in all cases
-where the movement is over a solid surface. If the movement, though
-over a solid surface, is not constant in direction, but takes place as
-often on one side as on another, radial symmetry may be again looked
-for; and if the motions are still more variously directed--if they are
-not limited to approximately-plane surfaces, but extend to surfaces
-that are distributed all around with a regular irregularity--an
-approach of the radial towards the spherical symmetry is to be
-anticipated. Where the habits are such that the intercourse between the
-organism and its environment, does not involve an average equality of
-actions and reactions on any two or more sides, there may be expected
-either total irregularity or some divergence from regularity.
-
-The like general relations between forms and incident forces are
-inferable in the component parts of animals, as well as in the animals
-as wholes. It is needless, however, to occupy space by descriptions of
-these. Let us now pass to the facts, and see how they confirm, _à
-posteriori_, the conclusions here reached _à priori_.
-
-
-
-
-                             CHAPTER XIV.
-
-                    THE GENERAL SHAPES OF ANIMALS.
-
-
-§ 244. Certain of the _Protozoa_ are quite indefinite in their shapes,
-and quite inconstant in those indefinite shapes which they have--the
-relations of their parts are indeterminate both in space and time.
-In one of the simpler Rhizopods, at least during the active stage
-of its existence, no permanent distinction of inside and outside is
-established; and hence there can arise no established correspondence
-between the shape of the outside and the distribution of environing
-actions. But when the relation of inner and outer becomes fixed,
-either over part of the mass or over the whole of it, we have kinds
-of symmetry that correspond with the habitual incidence of forces. An
-_Amœba_ in becoming encysted, passes from an indefinite, ever-changing
-form into a spherical form; and the order of symmetry which it thus
-assumes, is in harmony with the average equality of the actions on all
-its sides. In _Difflugia_, Fig. 134, and still better in _Arcella_, we
-have an indefinitely-radial symmetry occurring where the conditions are
-different above and below but alike all around. Among the _Gregarinida_
-the spherical symmetry and symmetry passing from that into the radial,
-are such as appear to be congruous with the simple circumstances
-of these creatures in the intestines of insects. But the relations
-of these lowest types to their environments are comparatively so
-indeterminate, and our knowledge of their actions so scanty, that
-little beyond negative evidence can be expected from the study of them.
-
-  [Illustration: Figs. 131–139.]
-
-The like may be said of the _Infusoria_. These are more or less
-irregular. In some cases, where the line of movement through the
-water is tolerably definite and constant, we have a form that is
-approximately radial--externally at least. But usually, as shown
-in Figs. 137, 138, 139, there is either an unsymmetrical or an
-asymmetrical shape. And when one of these creatures is watched under
-the microscope, the congruity of this shape with the incidence of
-forces is manifest. For the movements are conspicuously varied and
-indeterminate--movements which do not expose any two or more sides of
-the mass to approximately equal sets of actions.[43]
-
-
-§ 245. Among aggregates of the second order, as among aggregates of the
-first order, we find that of those possessing any definite shapes the
-lowest are spherical or spheroidal. Such are some of the _Radiolaria_,
-as _Collozoum inerme_. These bodies which float passively in the sea,
-and present in turn all their sides to the same influences, have their
-parts disposed with approximate regularity round a centre--approximate,
-because in the absence of locomotion a slight irregularity of
-growth, almost certain to take place, may cause a fixed attitude and
-a resulting deviation from spherical symmetry. The best cases in
-illustration of the truth here named, are furnished by rotating and
-locomotive organisms respecting which there is a dispute whether they
-are animal or vegetal--the _Volvocineæ_. These, already instanced
-under the one head in § 218, may here be instanced afresh under the
-other. Further, among these secondary aggregates in which the units,
-only physically integrated, have not had their individualities merged
-into an individuality of a higher order, must be named the compound
-_Infusoria_. The cluster of _Vorticellæ_ in Fig. 144, will sufficiently
-exemplify them; and the striking resemblance borne by its individuals
-to those of a radially-arranged cluster of flowers, will show how,
-under analogous conditions, the general principles of morphological
-differentiation are similarly illustrated in the two kingdoms.
-
-
-§ 246. Radial symmetry is usual in low aggregates of the second order
-which have their parts sufficiently differentiated and integrated
-to give individualities to them as wholes. The _Cœlenterata_
-offer numerous examples of this. Solitary polypes--hydroid or
-helianthoid--mostly stationary, and when they move, moving with any
-side foremost, do not by locomotion subject their bodies to habitual
-contrasts of conditions. Seated with their mouths upwards or downwards,
-or else at all degrees of inclination, the individuals of a species
-taken together, are subject to no mechanical actions affecting some
-parts of their discs more than other parts. And this indeterminateness
-of attitude similarly prevents their relations to prey from being such
-as subject some of their prehensile organs to forces unlike those to
-which the rest are subject. The fixed end is differently conditioned
-from the free end, and the two are therefore different; but around the
-axis running from the fixed to the free end the conditions are alike
-in all directions, and the form therefore is radial. Again, among many
-of the simple free-swimming _Hydrozoa_, the same general truth is
-exemplified under other circumstances. In a common _Medusa_, advancing
-through the water by the rhythmical contractions of its disc, the
-mechanical reactions are the same on all sides; and as, from accidental
-causes, every part of the edge of the disc comes uppermost in its
-turn, no part is permanently affected in a different way from the rest.
-Hence the radial form continues.
-
-  [Illustration: Figs. 257, 258.]
-
-In others of this same group, however, there occur forms which show
-us an incipient bilateralness; and help us to see how a more decided
-bilateralness may arise. Sundry of the _Medusidæ_ are proliferous,
-giving origin to gemmæ from the body of the central polypite or from
-certain points on the edge of the disc; and this budding, unless it
-occurs equally on all sides, which it does not and is unlikely to do,
-must tend to destroy the balance of the disc, and to make its attitude
-less changeable. In other cases the growth of a large process [a
-much-developed tentacle] from the edge of the disc on one side, as in
-_Steenstrupia_, Fig. 257, constitutes a similar modification, and a
-cause of further modification. The animal is no longer divisible into
-any two quite similar halves, except those formed by a plane passing
-through the process; and unless the process is of the same specific
-gravity as the disc, it must tend towards either the lowest or the
-highest point, and must so serve to increase the bilateralness, by
-keeping the two sides of the disc similarly conditioned while the
-top and bottom are differently conditioned. Fig. 258 represents the
-underside of another _Medusa_, in which a more decided bilateralness
-is produced by the presence of two such processes. Among the simple
-free-swimming _Actinozoa_, occur like deviations from radial symmetry,
-along with like motions through the water in bilateral attitudes.
-Of this a _Cydippe_ is a familiar example. Though radial in some of
-its characters, as in the distribution of its meridional bands of
-locomotive paddles with their accompanying canals, this creature
-has a two-sided distribution of tentacles and various other parts,
-corresponding with its two-sided attitude in moving through the water.
-And in other genera of this group, as in _Cestum_, _Eurhamphæa_,
-and _Callianira_, that almost equal distribution of parts which
-characterizes the _Beroe_ is quite lost.
-
-Here seems a fit place to meet the objection which some may feel to
-this and other such illustrations, that they amount very much to
-physical truisms. If the parts of a _Medusa_ are disposed in radial
-symmetry round the axis of motion through the water, there will of
-course be no means of maintaining one part of its edge uppermost
-more than another; and the equality of conditions may be ascribed
-to the radiateness, as much as the radiateness to the equality of
-conditions. Conversely, when the parts are not radially arranged
-around the axis of motion, they must gravitate towards some one
-attitude, implying a balance on the two sides of a vertical plane--a
-bilateralness; and the two-sided conditions so necessitated, may be
-as much ascribed to the bilateralness as the bilateralness to the
-two-sided conditions. Doubtless the form and the conditions are, in
-the way alleged, necessary correlates; and in so far as it asserts
-this, the objection harmonizes with the argument. To the difficulty
-which it at the same time raises by the implied question--Why make
-the form the result of the conditions, rather than the conditions
-the result of the form? the reply is this:--The radial type, both as
-being the least differentiated type and as being the most obviously
-related to lower types, must be taken as antecedent to the bilateral
-type. The individual variations which incidental circumstances produce
-in the radial type, will not cause divergence of a species from the
-radial type, unless such variations give advantages to the individuals
-displaying them; which there is no reason to suppose they will always
-do. Those occasional deviations from the radial type, which the law of
-the instability of the homogeneous warrants us in expecting to take
-place, will, however, in some cases be beneficial; and will then be
-likely to establish themselves. Such deviations must tend to destroy
-the original indefiniteness and variability of attitude--must cause
-gravitation towards an habitual attitude. And gravitation towards an
-habitual attitude having once commenced, will continually increase,
-where increase of it is not negatived by adverse agencies: each further
-degree of bilateralness rendering more decided the actions that conduce
-to bilateralness. If this reply be thought insufficient, it may be
-enforced by the further one, that as, among plants, the incident forces
-are the antecedents and the forms the consequents (changes of forces
-being in many cases visibly followed by changes of forms) we are
-warranted in concluding that the like order of cause and effect holds
-among animals.[44]
-
-
-§ 247. Keeping to the same type but passing to a higher degree of
-composition, we meet more complex and varied illustrations of the same
-general laws. In the compound _Cœlenterata_, presenting clusters
-of individuals which are severally homologous with the solitary
-individuals last dealt with, we have to note both the shapes of the
-individuals thus united, and the shapes of the aggregates made up of
-them.
-
-  [Illustration: Figs. 149, 150.]
-
-Such of the fixed _Hydrozoa_ and _Actinozoa_ as form branched
-societies, continue radial; both because their varied attitudes do not
-expose them to appreciable differences in their relations to those
-surrounding actions which chiefly concern them (the actions of prey),
-and because such differences, even if they were appreciable, would
-be so averaged in their effects on the dissimilarly-placed members
-of each group as to be neutralized in the race. Among the tree-like
-coral-polypedoms, as well as in such ramified assemblages of simpler
-polypes as are shown in Figs. 149, 150, we have, indeed, cases in many
-respects parallel to the cases of scattered flowers (§ 233), which
-though placed laterally remain radial, because no differentiating
-agency can act uniformly on all of them. Meanwhile, in the groups which
-these united individuals compose, we see the shapes of plants further
-simulated under a further parallelism of conditions. The attached ends
-differ from the free ends as they do in plants; and the regular or
-irregular branches obviously stand to environing actions in relations
-analogous to those in which the branches of plants stand.
-
-The members of those compound _Cœlenterata_ which move through the
-water by their own actions, in attitudes that are approximately
-constant, show us a more or less distinct two-sidedness. _Diphyes_,
-Fig. 259, furnishes an example. Each of the largely-developed and
-modified polypites forming its swimming sacs is bilateral, in
-correspondence with the bilateralness of its conditions; and in each of
-the appended polypites the insertion of the solitary tentacle produces
-a kindred divergence from the primitive radial type. The aggregate,
-too, which here very much subordinates its members, exhibits the same
-conformity of structure to circumstances. It admits of symmetrical
-bisection by a plane passing through its two contractile sacs, or
-nectocalyces, but not by any other plane; and the plane which thus
-symmetrically bisects it, is the vertical plane on the two sides of
-which its parts are similarly conditioned as it propels itself through
-the water.
-
-  [Illustration: Fig. 259.]
-
-Another group of the oceanic _Hydrozoa_, the _Physophoridæ_, furnishes
-interesting evidence--not so much in respect of the forms of the united
-individuals, which we may pass over, as in respect of the forms of the
-aggregates. Some of these are without swimming organs, and have their
-parts suspended from air-vessels which habitually float on the surface
-of the water. Hence the distribution of their parts is asymmetrical.
-The _Physalia_, Fig. 152, is an example. Here the relations of the
-integrated group of individuals to the environment are indefinite; and
-there is thus no agency tending to change that comparatively irregular
-mode of growth which is probably derived from a primordial type of the
-branched _Hydrozoa_.
-
-  [Illustration: Fig. 152.]
-
-So various are the modes of union among the compound _Cœlenterata_,
-that it is out of the question to deal with them all. Even did
-space permit, it would be impracticable for any one but a professed
-naturalist, to trace throughout this group the relations between
-shapes and conditions of existence. The above must be taken simply as a
-few of the most significant and easily-interpretable cases.
-
-
-§ 248. In the sub-kingdoms _Polyzoa_ and _Tunicata_ we meet with
-examples not wholly unlike the foregoing. Among the types assembled
-under these names there are simple individuals or aggregates of the
-second order, and societies or tertiary aggregates produced by their
-union. The relations of forms to forces have to be traced in both.
-
-Solitary Ascidians, fixed or floating, carry on an inactive and
-indefinite converse with the actions in the environment. Without
-power to move about vivaciously, and unable to catch any prey but
-that contained in the currents of water they absorb and expel, these
-creatures are not exposed to sets of forces which are equal on two
-or more sides; and their shapes consequently remain vague. Though
-internally their parts have a partially-symmetrical arrangement, due
-to their derivation, yet they are substantially unsymmetrical in that
-part of the body which is concerned with the environment. Fig. 156 is
-an example.[45] Among the composite Ascidians, floating and fixed,
-the shape of the aggregate, partly determined by the habitual mode of
-gemmation and partly by the surrounding conditions in each case, is in
-great measure indefinite. We can say no more about it than that it is
-not obviously at variance with the laws alleged.
-
-Evidence of a more positive kind occurs among those compound
-_Molluscoida_ which are most like the compound _Cœlenterata_ in
-their modes of union--the _Polyzoa_. Many of these form groups that
-are more or less irregular--spreading as films over solid surfaces,
-combining into seaweed-like fronds, budding out from creeping stolons,
-or growing up into tree-shaped societies; and besides aggregating
-irregularly they are irregularly placed on surfaces inclined in all
-directions. Merely noting that this asymmetrical distribution of the
-united individuals is explained by the absence of definiteness in the
-relations of the aggregate to incident forces, it concerns us chiefly
-to observe that the united individuals severally exemplify the same
-truth as do similarly-united individuals among the _Cœlenterata_.
-Averaging the members of each society, the ciliated tentacles they
-protrude are similarly related to prey on all sides; and therefore
-remain the same on all sides. This distribution of tentacles is not,
-however, without exception. Among the fresh-water _Polyzoa_ there
-are some genera, as _Plumatella_ and _Crystatella_, in which the
-arrangement of these parts is very decidedly bilateral. Some species of
-them show us such relations of the individuals to one another and to
-their surface of attachment, as give a clue to the modification; but in
-other species the meaning of this deviation from the radial type is not
-obvious.
-
-
-§ 249. In the _Platyhelminthes_ good examples of the connexions between
-forms and forces occur. The _Planaria_ exemplifies the single bilateral
-symmetry which, even in very inferior forms, accompanies the habit of
-moving in one direction over a solid surface. Humbly organized as are
-these creatures and their allies the _Nemertidæ_, we see in them, just
-as clearly as in the highest animals, that where the movements subject
-the body to different forces at its two ends, different forces on its
-under and upper surfaces, and like forces along its two sides, there
-arises a corresponding form, unlike at its extremities, unlike above
-and below, but having its two sides alike.
-
-The _Echinodermata_ furnish us with instructive
-illustrations--instructive because among types that are nearly allied,
-we meet with wide deviations of form answering to marked contrasts in
-the relations to the environment. The facts fall into four groups.
-The _Crinoidea_, once so abundant and now so rare, present a radial
-symmetry answering to an incidence of forces that are equal on all
-sides. In the general attitudes of their parts towards surrounding
-actions, they are like uniaxial plants or like polypes; and show, as
-those do, marked differences between the attached ends and the free
-ends, along with even distributions of parts all round their axes.
-In the _Ophiuridea_, and in the Star-fishes, we have radial symmetry
-co-existing with very different habits; but habits which nevertheless
-account for the maintenance of the form. Holding on to rocks and weeds
-by its simple or branched arms, or by the suckers borne on the under
-surface of its rays, one of these creatures moves about not always
-with one side foremost, but with any side foremost. Consequently,
-averaging its movements, its arms or rays are equally affected, and
-therefore remain the same on all sides. On watching the ways of the
-common Sea-urchin, we are similarly furnished with an explanation of
-its spherical, or rather its spheroidal, figure. Here the habit is
-not to move over any one approximately-flat surface; but the habit is
-to hold on by several surfaces on different sides at the same time.
-Frequenting crevices and the interstices among stones and weeds, the
-Sea-urchin protrudes the suckers arranged in meridional bands over
-its shell, laying hold of objects now on this side and now on that,
-now above and now below: the result being that it does not move in
-all directions over one plane but in all directions through space.
-Hence the approach in general form towards spherical symmetry--an
-approach which is, however, restrained by the relations of the parts
-to the mouth and vent: the conditions not being exactly the same at
-the two poles as at other parts of the surface. Still more significant
-is that deviation from this shape which occurs among such of the
-_Echinidea_ as have habitats of a different kind, and consequently,
-different habits. The genera _Echinocyamus_, _Spatangus_, _Brissus_,
-and _Amphidotus_, diverge markedly towards a bilateral structure. These
-creatures are found not on rocky shores but on flat sea-bottoms, and
-some of them only on bottoms of sand or mud. Here, there is none of
-that distribution of surfaces on all sides which makes the spheroidal
-form congruous with the conditions. Having to move about over an
-approximately-horizontal plane, any deviation of structure arising
-accidentally which leads to one side being kept always foremost, will
-be an advantage: greater fitness to function becoming possible in
-proportion as function becomes fixed. Survival of the fittest will
-therefore tend to establish, under such conditions, a form that keeps
-the same part in advance--a form in which, consequently, the original
-radial symmetry diverges more and more towards bilateral symmetry.
-
-
-§ 250. Very definite and comparatively uniform, are the relations
-between shapes and circumstances among the _Annulosa_: including under
-that title the _Annelida_ and the _Arthropoda_. The agreements and the
-disagreements are equally instructive.
-
-At one time or other of its life, if not throughout its life, every
-annulose animal is locomotive; and its temporary or permanent
-locomotion, being carried on with one end habitually foremost and one
-surface habitually uppermost, it fulfils those conditions under which
-bilateral symmetry arises. Accordingly, bilateral symmetry is traceable
-throughout the whole of this sub-kingdom. Traceable, we must say,
-because, though it is extremely conspicuous in the immense majority
-of annulose types, it is to a considerable extent obscured where
-obscuration is to be expected. The embryos of the _Tubicolæ_, after
-swimming about a while, settle down and build themselves tubes, from
-which they protrude their heads; and in them, or in some of them, the
-bilateral symmetry is disguised by the development of head-appendages
-in an all-sided manner. The tentacles of _Terebella_ are distributed
-much in the same way as those of a polype. The breathing organs in
-_Sabella unispira_, Fig. 260, do not correspond on opposite sides of
-a median plane. Even here, however, the body retains its primitive
-bilateralness; and it is further to be remarked that this loss of
-bilateralness in the external appendages, does not occur where the
-relations to external conditions continue bilateral: witness the
-_Serpula_, Fig. 261, which has its respiratory tufts arranged in a
-two-sided way, under the two-sided conditions involved by the habitual
-position of its tube.
-
-  [Illustration: Figs. 260–261.]
-
-The community of symmetry among the higher _Annulosa_, has an
-unobserved significance. That Flies, Beetles, Lobsters, Centipedes,
-Spiders, Mites, have in common the characters, that the end which moves
-in advance differs from the hinder end, that the upper surface differs
-from the under surface, and that the two sides are alike, is a truth
-received as a matter of course. After all that has been said above,
-however, it will be seen to have a meaning not to be overlooked; since
-it supplies a million-fold illustration of the laws which have been set
-forth. It is needless to give diagrams. Every reader can call to mind
-the unity indicated.
-
-  [Illustration: Figs. 263–270.]
-
-While, however, annulose animals repeat so uniformly these traits of
-structure, there are certain other traits in which they are variously
-contrasted; and their contrasts have to be here noted, as serving
-further to build up the general argument. In them we see the stages
-through which bilateral symmetry becomes gradually more marked, as
-the conditions it responds to become more decided. A common Earth-worm
-may be instanced as a member of this sub-kingdom that is among the
-least-conspicuously bilateral. Though internally its parts have a
-two-sided arrangement; and though the positions of its orifices give
-it an external two-sidedness, at the same time that they establish
-a difference between the two ends; yet its two-sidedness is not
-strongly-marked. The form deviates but little from what we have
-distinguished as triple bilateral symmetry: if the creature is cut
-across the middle, the head and tail ends are very much alike; if
-cut in two along its axis by a horizontal plane, the under and upper
-halves are very much alike, externally if not internally; and if cut
-in two along its axis by a vertical plane, the two sides are quite
-alike. Figs. 263 and 264 will make this clear. Such creatures as
-the _Julus_ and the Centipede, may be taken as showing a transition
-to double bilateral symmetry. Besides being divisible into exactly
-similar halves by a vertical plane passing through its axis, one of
-these animals may be bisected transversely into parts that differ only
-slightly; but if cut in two by a horizontal plane passing through its
-axis, the under and upper halves are decidedly unlike. Figs. 265, 266,
-exhibit these traits. Among the isopodous crustaceans, the departure
-from these low types of symmetry is more marked. As shown in Figs. 267
-and 268, the contrast between the upper and under parts is greater,
-and the head and tail ends differ more obviously. In all the higher
-_Arthropoda_, the unlikeness between the front half and the hind half
-has become conspicuous. There is in them single bilateral symmetry of
-so pronounced a kind, that no other resemblance is suggested than that
-between the two sides. By Figs. 269 and 270, representing a decapodous
-crustacean divided longitudinally and transversely, this truth is made
-manifest. On calling to mind the habits of the creatures here drawn and
-described, it will be seen that they explain these forms. The incidence
-of forces is the same all around the Earth-worm as it burrows through
-the compact ground. The Centipede, creeping amid loose soil or _débris_
-or beneath stones, insinuates itself between solid surfaces--the
-interstices being mostly greater in one dimension than in others.
-And all the higher _Annulosa_, moving about as they do over exposed
-objects, have their dorsal and ventral parts as dissimilarly acted upon
-as are their two ends.
-
-  [Illustration: Fig. 271.]
-
-One other fact only respecting annulose animals needs to be noticed
-under this head--the fact, namely, that they become unsymmetrical
-where their parts are unsymmetrically related to the environment.
-The common Hermit-crab serves as an instance. Here, in addition
-to the unlikeness of the two sides implied by that curvature of
-the body which fits the creature to the shell it inhabits, there
-is an unlikeness due to the greater development of the limbs, and
-especially the claws, on the outer side. As in the embryo of the
-Hermit-crab the two sides are alike; and as both the embryo and
-the ancestor lived in such a way, being free, that the conditions
-were alike on the two sides; and as the embryo may be taken to
-represent the type from which the Hermit-crab has been derived; we
-have in this case evidence that a symmetrically-bilateral form has
-been moulded into an unsymmetrically-bilateral form, by the action
-of unsymmetrically-bilateral conditions. A further illustration
-is supplied by _Bopyrus_, Fig. 271: a parasite which lives in the
-branchial chamber of prawns, and whose habits similarly account for its
-distorted shape.
-
-
-§ 251. Among the _Mollusca_ we find more varied relations between
-shapes and circumstances. Some of these relations are highly
-instructive.
-
-Mollusks of one order, the _Pteropoda_, swim in the sea much in
-the same way that butterflies fly in the air, and have shapes not
-altogether unlike those of butterflies. Fig. 272 represents one of
-these creatures. That its bilaterally-symmetrical shape harmonizes with
-its bilaterally-symmetrical conditions is sufficiently obvious.
-
-  [Illustration: Fig. 272.]
-
-Among the _Lamellibranchiata_, we have diverse forms accompanying
-diverse modes of life. Such of them as frequently move about, like
-the fresh-water Mussel, have their two valves and the contained parts
-alike on the opposite sides of a vertical plane: they are bilaterally
-symmetrical in conformity with their mode of movement. The marine
-Mussel, too, though habitually fixed, and though not usually so fixed
-that its two valves are similarly conditioned, still retains that
-bilateral symmetry which is characteristic of the order; and it does
-this because in the species considered as a whole, the two valves
-are not dissimilarly conditioned. If the positions of the various
-individuals are averaged, it will be seen that the differentiating
-actions neutralize one another. In certain other fixed Lamellibranchs,
-however, there is a considerable deviation from bilateral symmetry;
-and it is a deviation of the kind to be anticipated under the
-circumstances. Where one valve is always downwards, or next to the
-surface of attachment, while the other valve is always upwards, or next
-to the environing water, we may expect to find the two valves become
-unlike. This we do find: witness the Oyster. In the Oyster, too, we see
-a further irregularity. There is a great indefiniteness of outline,
-both in the shell and in the animal--an indefiniteness made manifest by
-comparing different individuals. We have but to remember that growing
-clustered together, as Oysters do, they must interfere with one another
-in various ways and degrees, to see how the indeterminateness of form
-and the variety of form are accounted for.
-
-Among the Gasteropods modifications of a more definite kind occur. “In
-all Mollusks,” says Professor Huxley, “the axis of the body is at first
-straight, and its parts are arranged symmetrically with regard to a
-longitudinal vertical plane, just as in a vertebrate or an articulate
-embryo.” In some Gasteropods, as the _Chiton_, this bilateral
-symmetry is retained--the relations of the body to surrounding actions
-not being such as to disturb it. But in those more numerous types
-which have spiral shells, there is a marked deviation from bilateral
-symmetry, as might be expected. “This asymmetrical over-development
-never affects the head or foot of the mollusk”: only those parts
-which, by inclosure in a shell, are protected from environing actions,
-lose their bilateralness; while the external parts, subjected by the
-movements of the creatures to bilateral conditions, remain bilateral.
-Here, however, a difficulty meets us. Why is it that the naked
-Gasteropods, such as our common slugs, deviate from bilateral symmetry,
-though their modes of movement are those along with which complete
-bilateral symmetry usually occurs? The reply is that their deviations
-from bilateral symmetry are probably inherited, and that they are
-maintained in such parts of their organization as are not exposed to
-bilaterally-symmetrical conditions. There is reason to believe that the
-naked Gasteropods are descended from Gasteropods which had shells: the
-evidence being that the naked Gasteropods have shells during the early
-stages of their development, and that some of them retain rudimentary
-shells throughout life. Now the shelled Gasteropods deviate from
-bilateral symmetry in the disposition of both the alimentary system
-and the reproductive system. The naked Gasteropods, in losing their
-shells, have lost that immense one-sided development of the alimentary
-system which fitted them to their shells, and have acquired that
-bilateral symmetry of external figure which fits them for their habits
-of locomotion; but the reproductive system remains one-sided, because,
-in respect to it, the relations to external conditions remain one-sided.
-
-The Cephalopods show us bilaterally-symmetrical external forms along
-with habits of movement through the water in two-sided attitudes. At
-the same time, in the radial distribution of the arms, enabling one
-of these creatures to take an all-sided grasp of its prey, we see how
-readily upon one kind of symmetry there may be partially developed
-another kind of symmetry, where the relations to conditions favour it.
-
-
-§ 252. The _Vertebrata_ illustrate afresh the truths which we have
-already traced among the _Annulosa_. Flying through the air, swimming
-through the water, and running over the earth as vertebrate animals
-do, in common with annulose animals, they are, in common with annulose
-animals, different at their anterior and posterior ends, different
-at their dorsal and ventral surfaces, but alike along their two
-sides. This single bilateral symmetry remains constant under the
-extremest modifications of form. Among fish we see it alike in the
-horizontally-flattened Skate, in the vertically-flattened Bream, in the
-almost-spherical _Diodon_, and in the greatly-elongated _Syngnathus_.
-Among reptiles the Turtle, the Snake, and the Crocodile all display it.
-And under the countless modifications of structure displayed by birds
-and mammals, it remains conspicuous.
-
-  [Illustration: Figs. 273–280.]
-
-A less obvious fact which it concerns us to note among the
-_Vertebrata_, parallel to one which we noted among the _Annulosa_, is
-that whereas the lower vertebrate forms deviate but little from triple
-bilateral symmetry, the deviation becomes great as we ascend. Figs.
-273 and 274 show how, besides being divisible into similar halves by
-a vertical plane passing through its axis, a Fish is divisible into
-halves that are not very dissimilar by a horizontal plane passing
-through its axis, and also into other not very dissimilar halves by
-a plane cutting it transversely. If, as shown in Figs. 275 and 276,
-analogous sections be made of a superior Reptile, the divided parts
-differ more decidedly. When a Mammal and a Bird are treated in the
-same way, as shown in Figs. 277, 278, and Figs. 279, 280, the parts
-marked off by the dividing planes are unlike in far greater degrees. On
-considering the mechanical converse between organisms of these several
-types and their environments--on remembering that the fish habitually
-moves through a homogeneous medium of nearly the same specific gravity
-as itself, that the terrestrial reptile either crawls on the surface or
-raises itself very incompletely above it, that the more active mammal,
-having its supporting parts more fully developed, thereby has the under
-half of its body made more different from the upper half, and that the
-bird is subject by its mode of life to yet another set of actions and
-reactions; we shall see that these facts are quite congruous with the
-general doctrine, and furnish further support to it.
-
-One other significant piece of evidence must be named. Among the
-_Annulosa_ we found unsymmetrical bilateralness in creatures having
-habits exposing them to unlike conditions on their two sides; and
-among the _Vertebrata_ we find parallel cases. They are presented
-by the _Pleuronectidæ_--the order of distorted flat fishes to which
-the Sole and the Flounder belong. On the hypothesis of evolution, we
-must conclude that fishes of this order have arisen from an ordinary
-bilaterally-symmetrical type of fish, which, feeding at the bottom of
-the sea, gained some advantage by placing itself with one of its sides
-downwards, instead of maintaining the vertical attitude. Besides the
-general reason there are special reasons for concluding this. In the
-first place, the young Sole or Flounder is bilaterally symmetrical--has
-its eyes on opposite sides of its head and swims in the usual way. In
-the second place, the metamorphosis which produces the unsymmetrical
-structure sometimes does not take place--there are abnormal Flounders
-that swim vertically, like other fishes. In the third place, the
-transition from the symmetrical structure to the unsymmetrical
-structure may be traced. Almost incredible though it seems, one of the
-eyes is transferred from the underside of the head to the upper side:
-the transfer being effected by a distorted development of the cranial
-bones--atrophy of some and hypertrophy of others, along with a general
-twist. This metamorphosis furnishes several remarkable illustrations
-of the way in which forms become moulded into harmony with incident
-forces. For besides the divergence from bilateral symmetry involved
-by presence of both eyes upon the upper side, there is a further
-divergence from bilateral symmetry involved by differentiation of the
-two sides in respect to the contours of their surfaces and the sizes of
-their fins. And then, what is still more significant, there is a near
-approach to likeness between the halves that were originally unlike,
-but are, under the new circumstances, exposed to like conditions. The
-body is divisible into similarly-shaped parts by a plane cutting it
-along the side from head to tail: “the dorsal and ventral instead of
-the lateral halves become symmetrical in outline and are equipoised.”
-
-
-§ 253. Thus, little as there seems in common between the shapes of
-plants and the shapes of animals, we yet find, on analysis, that the
-same general truths are displayed by both. The one ultimate principle
-that in any organism equal amounts of growth take place in those
-directions in which the incident forces are equal, serves as a key to
-the phenomena of morphological differentiation. By it we are furnished
-with interpretations of those likenesses and unlikenesses of parts,
-which are exhibited in the several kinds of symmetry; and when we take
-into account inherited effects, wrought under ancestral conditions
-contrasted in various ways with present conditions, we are enabled to
-comprehend, in a general way, the actions by which animals have been
-moulded into the shapes they possess.
-
-To fill up the outline of the argument, so as to make it correspond
-throughout with the argument respecting vegetal forms, it would be
-proper here to devote a chapter to the differentiations of those
-homologous segments out of which animals of certain types are composed.
-Though, among most animals of the third degree of composition, such
-as the rooted _Hydrozoa_, the _Polyzoa_, and the _Ascidioida_, the
-united individuals are not reduced to the condition of segments of a
-composite individual, and do not display any marked differentiations;
-yet there are some animals in which such subordinations, and consequent
-heterogeneities, occur. The oceanic _Hydrozoa_ form one group of them;
-and we have seen reason to conclude that the _Annulosa_ form another
-group. It is not worth while, however, to occupy space in detailing
-these unlikenesses of homologous segments, and seeking specific
-explanations of them. Among the oceanic _Hydrozoa_ they are extremely
-varied; and the habits and derivations of these creatures are so little
-known, that there are no adequate data for interpreting the forms of
-the parts in terms of their relations to the environment. Conversely,
-among the _Annulosa_ those differentiations of the homologous segments
-which accompany their progressing integration, have so much in common,
-and have general causes which are so obvious, that it is needless to
-deal with them at any length. They are all explicable as due to the
-exposure of different parts of the chain of segments to different sets
-of actions and reactions: the most general contrast being that between
-the anterior segments and the posterior segments, answering to the most
-general contrast of conditions to which annulose animals subject their
-segments; and the more special contrasts answering to the contrasts of
-conditions entailed by their more special habits.
-
-Were an exhaustive treatment of the subject practicable, there should
-here, also, come a chapter devoted to the internal structures of
-animals--meaning, more especially, the shapes and arrangements of the
-viscera. The relations between forms and forces among these inclosed
-parts are, however, mostly too obscure to allow of interpretation.
-Protected as the viscera are in great measure from the incidence of
-external forces, we are not likely to find much correspondence between
-their distribution and the distribution of external forces. In this
-case the influences, partly mechanical, partly physiological, which
-the organs exercise on one another, become the chief causes of their
-changes of figure and arrangement; and these influences are complex and
-indefinite. One general fact may, indeed, be noted--the fact, namely,
-that the divergence towards asymmetry which generally characterizes
-the viscera, is marked among those of them which are most removed from
-mechanical converse with the environment, but not so marked among
-those of them which are less removed from such converse. Thus while,
-throughout the _Vertebrata_, the alimentary system, with the exception
-of its two extremities, is asymmetrically arranged, the respiratory
-system, which occupies one end of the body, generally deviates but
-little from bilateral symmetry, and the reproductive system, partly
-occupying the other end of the body, is in the main bilaterally
-symmetrical: such deviation from bilateral symmetry as occurs, being
-found in its most interiorly-placed parts, the ovaries. Just indicating
-these facts as having a certain significance, it will be best to leave
-this part of the subject as too involved for detailed treatment.
-
-Internal structures of one class, however, not included among
-the viscera, admit of general interpretation--structures which,
-though internal, are brought into tolerably-direct relations with
-environing forces, and are therefore subordinate in their forms to
-the distribution of those forces. These internal structures it will
-be desirable to deal with at some length; both because they furnish
-important illustrations enforcing the general argument, and because an
-interpretation of them which we have seen reason to reject, cannot be
-rejected without raising the demand for some other interpretation.
-
-
-
-
-                              CHAPTER XV.
-
-                  THE SHAPES OF VERTEBRATE SKELETONS.
-
-
-§ 254. When an elongated mass of any substance is transversely
-strained, different parts of the mass are exposed to forces of opposite
-kinds. If, for example, a bar of metal or wood is supported at its two
-ends, as shown in Fig. 281, and has to bear a weight on its centre, its
-lower part is thrown into a state of tension, while its upper part is
-thrown into a state of compression. As will be manifest to any one who
-observes what happens on breaking a stick across his knee, the greatest
-degree of tension falls on the fibres forming the convex surface, while
-the fibres forming the concave surface are subject to the greatest
-degree of compression. Between these extremes the fibres at different
-depths are subject to different forces. Progressing upwards from the
-under surface of the bar shown in Fig. 281, the tension of the fibres
-becomes less; and progressing downwards from the upper surface, the
-compression of the fibres becomes less; until, at a certain distance
-between the two surfaces, there is a place at which the fibres are
-neither extended nor compressed. This, shown by the dotted line in the
-figure, is called in mechanical language the “neutral axis.” It varies
-in position with the nature of the substance strained: being, in common
-pine-wood, at a distance of about five-eighths of the depth from the
-upper surface, or three-eighths from the under surface. Clearly, if
-such a piece of wood, instead of being subject to a downward force, is
-secured at its ends and subject to an upward force, the distribution
-of the compressions and tensions will be reversed, and the neutral
-axis will be nearest to the upper surface. Fig. 282 represents these
-opposite attitudes of the bar and the changed position of its neutral
-axis: the arrow indicating the direction of the force producing the
-upward bend, and the faint dotted line _a_, showing the previous
-position of the neutral axis. Between the two neutral axes will be seen
-a central space; and it is obvious that when the bar has its strain
-from time to time reversed, the repeated changes of its molecular
-condition must affect the central space in a way different from that
-in which they affect the two outer spaces. Fig. 283 is a diagram
-conveying some idea of these contrasts in molecular condition. If A B C
-D be the middle part of a bar thus treated, while G H and K L are the
-alternating neutral axes; then the forces to which the bar is in each
-case subject, may be readily shown. Supposing the deflecting force to
-be acting in the direction of the arrow E, then the tensions to which
-the fibres between G and F are exposed, will be represented by a series
-of lines increasing in length as the distance from G increases; so that
-the triangle G F M, will express the amount and distribution of all
-the molecular tensions. But the molecular compressions throughout the
-space from G to E, must balance the molecular tensions; and hence, if
-the triangle G E N be made equal to the triangle G F M, the parallel
-lines of which it is composed (here dotted for the sake of distinction)
-will express the amount and distribution of the compressions between
-E and G. Similarly, when the deflecting force is in the direction of
-the arrow F, the compressions and tensions will be quantitatively
-symbolized by the triangles K F O, and K E P. And thus the several
-spaces occupied by full lines and by dotted lines and by the two
-together, will represent the different actions to which different
-parts of the transverse section are subject by alternating transverse
-strains. Here, then, it is made manifest to the eye that the central
-space between G and K, is differently conditioned from the spaces above
-and below it; and that the difference of condition is sharply marked
-off. The fibres forming the outer surface C D, are subject to violent
-tensions and violent compressions. Progressing inwards the tensions and
-compressions decrease--the tensions the more rapidly. As we approach
-the point G, the tensions to which the fibres are alternately subject,
-bear smaller and smaller ratios to the compressions, and disappear
-at the point G. Thence to the centre occur compressions only, of
-alternating intensities, becoming at the centre small and equal; and
-from the centre we advance, through a reverse series of changes, to the
-other side.
-
-  [Illustration: Fig. 281.]
-
-  [Illustration: Fig. 282.]
-
-  [Illustration: Fig. 283.]
-
-Thus it is demonstrable that any substance in which the power of
-resisting compression is unequal to the power of resisting tension,
-cannot be subject to alternating transverse strains, without having
-a central portion differentiated in its conditions from the outer
-portions, and consequently differentiated in its structure. This
-conclusion may easily be verified by experiment. If something having
-a certain toughness but not difficult to break, as a thick piece of
-sheet lead, be bent from side to side till it is broken, the surface of
-fracture will exhibit an unlikeness of texture between the inner and
-outer parts.
-
-
-§ 255. And now for the application of this seemingly-irrelevant
-truth. Though it has no obvious connection with the interpretation of
-vertebral structure, we shall soon see that it fundamentally concerns
-us.
-
-  [Illustration: Fig. 284.]
-
-The simplest type of vertebrate animal, the fish, has a mode of
-locomotion which involves alternating transverse strains. It is not,
-indeed, subjected to alternating transverse strains by some outer
-agency, as in the case we have been investigating: it subjects itself
-to them. But though the strains are here internally produced instead
-of externally produced, the case is not therefore removed into a
-wholly different category. For supposing Fig. 284 to represent the
-outline of a fish when bent on one side (the dotted lines representing
-its outline when the bend is reversed), it is clear that part of the
-substance forming the convex half must be in a state of tension. This
-state of tension implies the existence in the other half of some
-counter-balancing compression. And between the two there must be a
-neutral axis. The way in which this conclusion is reconcilable with the
-fact that there is tension somewhere in the concave side of a fish,
-since the curve is caused by muscular contractions on the concave
-side, will be made clear by the rude illustration which a bow supplies.
-A bow may be bent by a thrust against its middle (the two ends being
-held back), or it may be bent by contracting a string that unites its
-ends; but the distributions of mechanical forces within the wood of the
-bow, though not quite alike in the two cases, will be very similar.
-Now while the muscular action on the concave side of a fish differs
-from that represented by the tightened string of a bow, the difference
-is not such as to destroy the applicability of the illustration: the
-parallel holds so far as this, that within that portion of the fish’s
-body which is passively bent by the contracting muscles, there must be,
-as in a strung bow, a part in compression, a part in tension, and an
-intermediate part which is neutral.
-
-After thus seeing that even in the developed fish with its complex
-locomotive apparatus, this law of the transverse strain holds in a
-qualified way, we shall understand how much more it must hold in any
-form that may be supposed to initiate the vertebrate type--a form
-devoid of that segmentation by which the vertebrate type is more or
-less characterized. We shall see that assuming a rudimentary animal,
-still simpler than the _Amphioxus_, to have a feeble power of moving
-itself through the water by the undulations of its body, or some part
-of its body, there will necessarily come into play certain reactions
-which must affect the median portion of the undulating mass in a way
-unlike that in which they affect its lateral portions. And if there
-exists in this median portion a tissue which keeps its place with any
-constancy, we may expect that the differential conditions produced in
-it by the transverse strain, will initiate a differentiation. It is
-true that the distribution of the viscera in the _Amphioxus_, Fig. 191,
-and in the type from which we may suppose it to have arisen, is such as
-to interfere with this process. It is also true that the actions and
-reactions described would not of themselves give to the median portion
-a cylindrical shape, like that of the cartilaginous rod running along
-the back of the _Amphioxus_. But what we have here to note in the
-first place is, that these habitual alternate flexions have a tendency
-to mark off from the outer parts an unlike inner part, which may be
-seized hold of, maintained, and further modified, by natural selection,
-should any advantage thereby result. And we have to note in the second
-place, that an advantage _is_ likely to result. The contractions
-cannot be effective in producing undulations, unless the general shape
-of the body is maintained. External muscular fibres unopposed by an
-internal resistant mass, would cause collapse of the body. To meet
-the requirements there must be a means of maintaining longitudinal
-rigidity without preventing bends from side to side; and such a means
-is presented by a structure initiated as described. In brief, whether
-we have or have not the actual cause, we have here at any rate “a true
-cause.” Though there are difficulties in tracing out the process in a
-definite way, it may at least be said that the mechanical genesis of
-this rudimentary vertebrate axis is quite conceivable. And even the
-difficulties may, I think, be more fully met than at first sight seems
-possible.
-
-  [Illustration: Fig. 191.]
-
-What is to be said of the other leading trait which the simplest
-vertebrate animal has in common with all higher vertebrate animals--the
-segmentation of its lateral muscular masses? Is this, too, explicable
-on the mechanical hypothesis? Have we, in the alternating transverse
-strains, a cause for the fact that while the rudimentary vertebrate
-axis is without any divisions, there are definite divisions of the
-substance forming the animal’s sides? I think we have. A glance at the
-distribution of forces under the transverse strain, as represented in
-the foregoing diagrams, will show how much more severe is the strain
-on the outer parts than on the inner parts; and how, consequently,
-any modifications of structure eventually necessitated, will arise
-peripherally before they arise centrally. The perception of this may be
-enforced by a simple experiment. Take a stick of sealing-wax and warm
-it slowly and moderately before the fire, so as to give it a little
-flexibility. Then bend it gently until it is curved into a semi-circle.
-On the convex surface small cracks will be seen, and on the concave
-surface wrinkles; while between the two the substance remains
-undistorted. If the bend be reversed and re-reversed, time after time,
-these cracks and wrinkles will become fissures which gradually deepen.
-But now, if changes of this class, entailed by alternating transverse
-strains, commence superficially, as they manifestly must; there arise
-the further questions--What will be the special modifications produced
-under these special conditions? and through what stages will these
-modifications progress? Every one has literally at hand an example
-of the way in which a flexible external layer that is now extended
-and now compressed, by the bending of the mass it covers, becomes
-creased; and a glance at the palms and the fingers will show that the
-creases are near one another where the skin is thin, and far apart
-where the skin is thick. Between this familiar case and the case of
-the rhinoceros-hide, in which there are but a few large folds, various
-gradations may be traced. Now the like must happen with the increasing
-layers of contractile fibres forming the sides of the muscular tunic
-in such a type as that supposed. The bendings will produce in them
-small wrinkles while they are thin, but more decided and comparatively
-distant fissures as they become thick. Fig. 289, which is a horizontal
-longitudinal section, shows how these thickening layers will adjust
-themselves on the convex and the concave surfaces, supposing the fibres
-of which they are composed to be oblique, as their function requires;
-and it is not difficult to see that when once definite divisions have
-been established, they will advance inwards as the layers develop;
-and will so produce a series of muscular bundles. Here then we have
-something like the _myocommata_ [or myotomes as now called] which are
-traceable in the _Amphioxus_, and are conspicuous in all superior
-fishes.
-
-  [Illustration: Fig. 289.]
-
-
-§ 256. These are highly speculative conceptions. I have ventured to
-present them with the view of implying that the hypothesis of the
-mechanical genesis of vertebrate structure is not wholly at fault when
-applied to the most rudimentary vertebrate animal. Lest it should be
-alleged that the question is begged if we set out with a type which,
-like the _Amphioxus_, already displays segmentation throughout
-its muscular system, it seemed needful to indicate conceivable modes
-in which there may have been mechanically produced those leading
-traits that distinguish the _Amphioxus_. All I intend to suggest
-is that mechanical actions have been at work, and that probably they
-have operated in the manner alleged: so preparing the way for natural
-selection.
-
-But now let us return to the region of established fact, and consider
-whether such actions and reactions as we actually witness, are adequate
-causes of those observed differentiations and integrations which
-distinguish the more-developed vertebrate animals. Let us see whether
-the theory of mechanical genesis affords us a deductive interpretation
-of the inductive generalizations.
-
-Before proceeding, we must note a process of functional adaptation
-which here co-operates with natural selection. I refer to the usual
-formation of denser tissues at those parts of an organism which are
-exposed to the greatest strains--either compressions or tensions.
-Instances of hardening under compression are made familiar to us by
-the skin. We have the general contrast between the soft skin covering
-the body at large, and the indurated skin covering the inner surfaces
-of the hands and the soles of the feet. We have the fact that even
-within these areas the parts on which the pressure is habitually
-greatest have the skin always thickest; and that in each person special
-points exposed to special pressures become specially dense--often as
-dense as horn. Further, we have the converse fact that the skin of
-little-used hands becomes abnormally thin--even losing, in places, that
-ribbed structure which distinguishes skin subject to rough usage. Of
-increased density directly following increased tension, the skeletons,
-whether of men or animals, furnish abundant evidence. Anatomists easily
-discriminate between the bones of a strong man and those of a weak
-man, by the greater development of those ridges and crests to which
-the muscles are attached; and naturalists, on comparing the remains of
-domesticated animals with those of wild animals of the same species,
-find kindred differences. The first of these facts shows unmistakably
-the immediate effect of function on structure, and by obvious alliance
-with it the second may be held to do the same: both implying that the
-deposit of dense substance capable of great resistance, constantly
-takes place at points where the tension is excessive.
-
-Taking into account, then, this adaptive process, continually aided by
-the survival of individuals in which it has taken place most rapidly,
-we may expect, on tracing up the evolution of the vertebrate axis, to
-find that as the muscular power becomes greater there arise larger and
-harder masses of tissue, serving the muscles as _points d’appui_; and
-that these arise first in those places where the strains are greatest.
-Now this is just what we _do_ find. The _myocommata_ are so placed
-that their actions are likely to affect first that upper coat of the
-notochord, where there are found “quadrate masses of somewhat denser
-tissue,” which “seem faintly to represent neural spines,” even in the
-_Amphioxus_. It is by the development of the neural spines, and after
-them of the hæmal spines, that the segments of the vertebral column are
-first marked out; and under the increasing strains of more-developed
-_myocommata_, it is just these peripheral appendages of the vertebral
-segments that must be most subject to the forces which cause the
-formation of denser tissue. It follows from the mechanical hypothesis
-that as the muscular segmentation must begin externally and progress
-inwards, so, too, must the vertebral segmentation. Besides thus
-finding reason for the fact that in fishes with wholly cartilaginous
-skeletons, the vertebral segments are indicated by these processes,
-while yet the notochord is unsegmented; we find a like reason for the
-fact that the transition from the less-dense cartilaginous skeleton
-to the more-dense osseous skeleton, pursues a parallel course. In the
-existing _Lepidosiren_, which by uniting certain piscine and amphibian
-characters betrays its close alliance with primitive types, the axial
-part of the vertebral column is unossified, while there is ossification
-of the peripheral parts. Similarly with numerous genera of fishes
-classed as palæozoic. The fossil remains of them show that while the
-neural and hæmal spines consisted of bone, the central parts of the
-vertebræ were not bony. It may in some cases be noted, too, both in
-extant and in fossil forms, that while the ossification is complete at
-the outer extremities of the spines it is incomplete at their inner
-extremities--thus similarly implying centripetal development.
-
-
-§ 257. After these explanations the process of eventual segmentation
-in the spinal axis itself, will be readily understood. The original
-cartilaginous rod has to maintain longitudinal rigidity while
-permitting lateral flexion. As fast as it becomes definitely marked
-out, it will begin to concentrate within itself a great part of those
-pressures and tensions caused by transverse strains. As already said,
-it must be acted upon much in the same manner as a bow, though it is
-bent by forces acting in a more indirect way; and like a bow, it must,
-at each bend, have the substance of its convex side extended and the
-substance of its concave side compressed. So long as the vertebrate
-animal is small or inert, such a cartilaginous rod may have sufficient
-strength to withstand the muscular strains; but, other things equal,
-the evolution of an animal that is large, or active, or both, implies
-muscular strains which must tend to cause modification in such a
-cartilaginous rod. The results of greater bulk and of greater vivacity
-may be best dealt with separately. As the animal increases in size,
-the rod will grow both longer and thicker. On looking back at the
-diagrams of forces caused by transverse strains, it will be seen that
-as the rod grows thicker, its outer parts must be exposed to more
-severe tensions and pressures if the degree of bend is the same. It is
-doubtless true that when the fish, advancing by lateral undulations,
-becomes longer, the curvature assumed by the body at each movement
-becomes less; and that from this cause the outer parts of the notochord
-are, other things equal, less strained--the two changes thus partially
-neutralizing one another. But other things are _not_ equal. For while,
-supposing the shape of the body to remain constant, the force exerted
-in moving the body increases as the cubes of its dimensions, the
-sectional area of the notochord, on which fall the reactions of this
-exerted force, increases only as the squares of the dimensions: whence
-results a greater stress upon its substance. This, however, will not
-be very decided where there is no considerable activity. It is clear
-that augmenting bulk, taken alone, involves but a moderate residuary
-increase of strain on each portion of the notochord; and this is
-probably the reason why it is possible for a large _sluggish_ fish like
-the Sturgeon, to retain the notochordal structure. But now, passing to
-the effects of greater activity, a like dynamical inquiry at once shows
-us how rapidly the violence of the actions and reactions rises as the
-movements become more vivacious. In the first place, the resistance
-of a medium such as water increases as the square of the velocity of
-the body moving through it; so that to _maintain_ double the speed, a
-fish has to expend four times the energy. But the fish has to do more
-than this--it has to _initiate_ this speed, or to impress on its mass
-the force implied by this speed. Now the _vis viva_ of a moving body
-varies as the square of the velocity; whence it follows that the energy
-required to generate that _vis viva_ is measured by the square of the
-velocity it produces. Consequently, did the fish put itself in motion
-_instantaneously_, the expenditure of energy in generating its own
-_vis viva_ and simultaneously overcoming the resistance of the water,
-would vary as the fourth power of the velocity. But the fish cannot
-put itself in motion instantaneously--it must do it by increments; and
-thus it results that the amounts of the forces expended to give itself
-different velocities must be represented by some series of numbers
-falling between the squares and the fourth powers of those velocities.
-Were the increments slowly accumulated, the ratios of increasing effort
-would but little exceed the ratios of the squares; but whoever observes
-the sudden, convulsive action with which an alarmed fish darts out
-of a shallow into deep water, will see that the velocity is rapidly
-generated, and that therefore the ratios of increasing effort probably
-exceed the ratios of the squares very considerably. At any rate it
-will be clear that the efforts made by fishes in rushing upon prey or
-escaping enemies (and it is these extreme efforts which here concern
-us) must, as fishes become more active, rapidly exalt the strains to
-be borne by their motor organs; and that of these strains, those which
-fall upon the notochord must be exalted in proportion to the rest. Thus
-the development of locomotive power, which survival of the fittest must
-tend in most cases to favour, involves such increase of stress on the
-primitive cartilaginous rod as will tend, other things equal, to cause
-its modification.
-
-  [Illustration: Figs. 291–293.]
-
-What must its modification be? Considering the complication of the
-influences at work, conspiring, as above indicated, in various ways and
-degrees, we cannot expect to do more than form an idea of its average
-character. The nature of the changes which the notochord is likely
-to undergo, where greater bulk is accompanied by higher activity, is
-rudely indicated by Figs. 291, 292, and 293. The successively thicker
-lines represent the successively greater strains to which the outer
-layers of tissue are exposed; and the widening interspaces represent
-the greater extensions which they have to bear when they become convex,
-or else the greater gaps that must be formed in them. Had these outer
-layers to undergo extension only, as on the convex side, continued
-natural selection might result in the formation of a tissue elastic
-enough to admit of the requisite stretching. But at each alternate
-bend these outer layers, becoming concave, are subject to increased
-compression--a compression which they cannot withstand if they have
-become simply more extensible. To withstand this greater compression
-they must become harder as well as more extensible. How are these two
-requirements to be reconciled? If, as facts warrant us in supposing, a
-formation of denser substance occurs at those parts of the notochord
-where the strain is greatest; it is clear that this formation cannot
-so go on as to produce a continuous mass: the perpetual flexions must
-prevent this. If matter that will not yield at each bend, is deposited
-while the bendings are continually taking place, the bendings will
-maintain certain places of discontinuity in the deposit--places at
-which the whole of the stretching consequent on each bend will be
-concentrated. And thus the tendency will be to form segments of hard
-tissue capable of great resistance to compression, with intervals
-filled by elastic tissue capable of great resistance to extension--a
-vertebral column.
-
-And now observe how the progress of ossification is just such as
-conforms to this view. That centripetal development of segments which
-holds of the vertebrate animal as a whole, as, if caused by transverse
-strains, it ought to do, and which holds of the vertebral column as
-a whole, as it ought to do, holds also of the central axis. On the
-mechanical hypothesis, the outer surface of the notochord should be
-the first part to undergo induration, and that division into segments
-which must accompany induration. And accordingly, in a vertebral column
-of which the axis is beginning to ossify, the centrums consist of bony
-rings inclosing a still-continuous rod of cartilage.
-
-
-§ 258. Sundry other general facts disclosed by the comparative
-morphology of the _Vertebrata_, supply further confirmation. Let
-us take first the structure of the skull.
-
-On considering the arrangement of the muscular flakes, or _myocommata_,
-in any ordinary fish which comes to table--an arrangement already
-sketched out in the _Amphioxus_--it is not difficult to see that that
-portion of the body out of which the head of the vertebrate animal
-becomes developed, is a portion which cannot subject itself to bendings
-in the same degree as the rest of the body. The muscles developed
-there must be comparatively short, and much interfered with by the
-pre-existing orifices. Hence the cephalic part will not partake in any
-considerable degree of the lateral undulations; and there will not tend
-to arise in it any such distinct segmentation as arises elsewhere. We
-have here, then, an explanation of the fact, that from the beginning
-the development of the head follows a course unlike that of the spinal
-column; and of the fact that the segmentation, so far as it can be
-traced in the head, is most readily to be traced in the occipital
-region and becomes lost in the region of the face. For if, as we have
-seen, the segmentation consequent on mechanical actions and reactions
-must progress from without inwards, affecting last of all the axis; and
-if, as we have seen, the region of the head is so circumstanced that
-the causes of segmentation act but feebly even on its periphery; then
-that terminal portion of the primitive notochord which is included in
-the head, having to undergo no lateral bendings, may ossify without
-division into segments.
-
-Of other incidental evidences supplied by comparative morphology, let
-me next refer to the supernumerary bones, which the theory of Goethe
-and Oken as elaborated by Prof. Owen, has to get rid of by gratuitous
-suppositions. In many fishes, for example, there are what have been
-called interneural spines and interhæmal spines. These cannot by any
-ingenuity be affiliated upon the archetypal vertebra, and they are
-therefore arbitrarily rejected as bones belonging to the exo-skeleton;
-though in shape and texture they are similar to the spines between
-which they are placed. On the hypothesis of evolution, however, these
-additional bones are accounted for as arising under actions like those
-that gave origin to the bones adjacent to them. And similarly with such
-bones as those called sesamoid; together with others too numerous to
-name.
-
-
-§ 259. Of course the foregoing synthesis is to be taken simply as an
-adumbration of the process by which the vertebrate structure may have
-arisen through the continued actions of known agencies. The motive
-for attempting it has been two-fold. Having, as before said, given
-reasons for concluding that the segments of a vertebrate animal are not
-homologous in the same sense as are those of an annulose animal, it
-seemed needful to do something towards showing how they are otherwise
-to be accounted for; and having here, for our general subject, the
-likenesses and differences among the parts of organisms, as determined
-by incident forces, it seemed out of the question to pass by the
-problem presented by the vertebrate skeleton.
-
-Leaving out all that is hypothetical, the general argument may be
-briefly presented thus:--The evolution from the simplest known
-vertebrate animal of a powerful and active vertebrate animal, implies
-the development of a stronger internal fulcrum. The internal fulcrum
-cannot be made stronger without becoming more dense. And it cannot
-become more dense while retaining its lateral flexibility, without
-becoming divided into segments. Further, in conformity with the general
-principles thus far traced, these segments must be alike in proportion
-as the forces to which they are exposed are alike, and unlike in
-proportion as these forces are unlike; and so there necessarily results
-that unity in variety by which the vertebral column is from the
-beginning characterized. Once more, we see that the explanation extends
-to those innumerable and more marked divergences from homogeneity,
-which vertebræ undergo in the various higher animals. Thus, the
-production of vertebræ, the production of likenesses among vertebræ,
-the production of unlikenesses among vertebræ, and the production of
-unlikenesses among vertebral columns, are interpretable as parts of one
-general process, and as harmonizing with one general principle.
-
-Whether sufficient or insufficient, the explanation here given assigns
-causes of known kinds producing effects such as they are known to
-produce. It does not, as a solution of one mystery, offer another
-mystery of which no solution is to be asked. It does not allege a
-Platonic ἰδέα, or fictitious entity, which explains the vertebrate
-skeleton by absorbing into itself all the inexplicability. On the
-contrary, it assumes nothing beyond agencies by which structures in
-general are moulded--agencies by which these particular structures
-are, indeed, notoriously modifiable. An ascertained cause of certain
-traits in vertebræ and other bones, it extends to all other traits of
-vertebræ; and at the same time assimilates the morphological phenomena
-they present to much wider classes of morphological phenomena.
-
-       *       *       *       *       *
-
-[NOTE.--The theory set forth in the foregoing chapter, is an
-elaboration of one suggested at the close of a criticism of Prof.
-Owen’s _Archetype and Homologies of the Vertebrate Skeleton_,
-already referred to in § 210 as having been published in the
-_Medico-Chirurgical Review_ for October, 1858. It is now reproduced in
-Appendix B. Since the issue of this elaborated exposition, in No. 15 of
-my serial in December, 1865, verifications of it have from time to time
-been published. In his work _The Primary Factors of Organic Evolution_,
-Prof. Cope of Philadelphia writes:--
-
-“Mr. Herbert Spencer has endeavoured to account for the origin of the
-segmentation of muscles into myotomes, and the division of the sheath
-of the notochord into vertebræ, by supposing it to be due to the
-lateral swimming movements of the fishes, which first exhibit these
-structures. With this view various later authors have agreed, and I
-have offered some additional evidence of the soundness of this position
-with respect to the vertebral axis of Batrachia, and the origin of
-limb articulations. It is true that the origin of segmentation in the
-vertebral column of the true fishes and the Batrachia turns out to have
-been less simple in its process than was suggested by Mr. Spencer, but
-his general principle holds good, now that paleontology has cleared up
-the subject” (pp. 367–8).
-
-An allusion in the foregoing extract is made by Prof. Cope to certain
-observations set forth in his work entitled _The Origin of the
-Fittest_. On pp. 305–6 of it will be found the following sentences:--
-
-“Now, all the Permian land-animals, reptiles and batrachians, retain
-this notochord with the elements of osseous vertebræ, in a greater or
-less degree of completeness. There are some in South Africa, I believe,
-in which the ossification has come clear through the notochord; but
-they are few.... There is something to be said as to the condition
-of the column from a mechanical standpoint, and it is this: that the
-chorda exists, with its osseous elements disposed about it; and in the
-Permian batrachians, equally related to salamanders and frogs, these
-osseous elements are arranged in the sheath or skin of the chorda;
-and they are in the form of regular concave segments, very much like
-such segments as you can take from the skin of an orange--but parts
-of a cylinder, and having greater or less dimensions according to the
-group or species. Now, the point of divergence of these segments is
-on the side of the column. The contacts are placed on the side of the
-column where the segments separate--the upper segments rising and the
-lower segments coming downward. To the upper segments are attached the
-arches and their articulations, and the lower segments are like the
-segments of a cylinder. If you take a flexible cylinder, and cover it
-with a more or less inflexible skin or sheath, and bend that cylinder
-sidewise, you of course will find that the wrinkles or fractures of
-that part of the surface will take place along the line of the shortest
-curve, which is on the side; and, as a matter of fact, you have breaks
-of very much the character of the segments of the Permian Batrachia....
-In the cylinder bending both ways, of course the shortest line of
-curve is right at the centre of the side of that cylinder, and the
-longest curve is of course at the summit and base, and the shortest
-curve will be the point of fracture. And that is exactly what I presume
-has happened in the case of the construction of the segments of the
-sheath of the vertebral column, by the lateral motion of the animal in
-swimming, and which has been the actual cause of the disposition of the
-osseous material in its form.... That is the state of the vertebral
-column of many of the Vertebrata of the Permian period.”
-
-In his essay on “The Mechanical Causes of the Development of the
-Hard Parts of the Mammalia,” published in the American _Journal of
-Morphology_ (Vol. III), Prof. Cope has carried the interpretation
-further, by showing that in kindred ways the genesis of articulations
-and limb-bones may be explained. On p. 163 he enunciates the general
-principle of his interpretation as follows:--
-
-“It cannot have been otherwise than that, since the motions of animals
-continued during the evolution of their hard parts, these hard parts
-grew in exact adaptation to these movements. Thus at the points of
-greatest flexure joints would be formed, and between these joints the
-deposit would be continuous.”
-
-Evidently if osseous structures are produced by deposits of calcareous
-matters in pre-existing cartilaginous structures, or other structures
-of flexible materials, the deposits must be so carried on that while
-dense resistant masses are produced these must admit of such free
-movements as the creature’s life necessitates, and must so form adapted
-joints.
-
-Let it be understood, however, that the hypothesis set forth in
-the foregoing chapter and extended by Prof. Cope, which serves to
-interpret a large part of the phenomena of osseous structures in the
-_Vertebrata_, does not serve to interpret them all. While the formation
-of hard parts has been in large measure initiated and regulated by
-tensions and pressures, there are hard parts the formation of which
-cannot be thus explained. The bones of the skull are the most obvious
-instances. These are apparently referable to no other cause than the
-survival of the fittest--the survival of individual animals in which
-greater density of the brain-covering yielded better protection against
-external injuries. Without enumerating other instances which might be
-given, it will suffice to recognize the truth that natural selection
-of favourable variations and the inheritance of functionally-produced
-changes have all along co-operated: each of them in some cases acting
-alone, but in other cases both acting together.]
-
-
-
-
-                             CHAPTER XVI.
-
-                      THE SHAPES OF ANIMAL CELLS.
-
-
-§ 260. Among animals as among plants, the laws of morphological
-differentiation must be conformed to by the morphological units, as
-well as by the larger parts and by the wholes formed of them. It
-remains here to point out that the conformity is traceable where the
-conditions are simple.
-
-  [Illustration: Fig. 294.]
-
-In the shapes assumed by those rapidly-multiplying cells out of which
-each animal is developed, there is a conspicuous subordination to
-the surrounding actions. Fig. 294 represents the cellular embryonic
-mass that arises by repeated spontaneous fissions. In it we see how
-the cells, originally spherical, are changed by pressure against one
-another and against the limiting membrane; and how their likenesses
-and unlikenesses are determined by the likenesses and unlikenesses
-of the forces to which they are exposed. This fact may be thought
-scarcely worth pointing out. But it is worth pointing out, because what
-is here so obvious a consequence of mechanical actions, is in other
-cases a consequence of actions composite in their kinds and involved
-in their distribution. Just as the equalities and inequalities of
-dimensions among aggregated cells, are here caused by the equalities
-and inequalities among their mutual pressures in different directions;
-so, though less manifestly, the equalities and inequalities of
-dimensions among other aggregated cells, are caused by the equalities
-and inequalities of the osmotic, chemical, thermal, and other forces
-besides the mechanical, to which their different positions subject them.
-
-
-§ 261. This we shall readily see on observing the ordinary structures
-of limiting membranes, internal and external. In Fig. 295, is shown
-a much-magnified section of a papilla from the gum. The cells of
-which it is composed originate in its deeper part; and are at first
-approximately spherical. Those of them which, as they develop, are
-thrust outwards by the new cells that continually take their places,
-have their shapes gradually changed. As they grow and successively
-advance to replace the superficial cells, when these exfoliate, they
-become exposed to forces which are more and more different in the
-direction of the surface from what they are in lateral directions; and
-their dimensions gradually assume corresponding differences.
-
-  [Illustration: Fig. 295.]
-
-  [Illustration: Fig. 296.]
-
-Another species of limiting membrane, called cylinder-epithelium,
-is represented in Fig. 296. Though its mode of development is
-such as to render the shapes of its cells quite unlike those of
-pavement-epithelium, as the above-described kind is sometimes called,
-its cells equally exemplify the same general truth. For the chief
-contrast which each of them presents, is the contrast between its
-dimension at right angles to the surface of the membrane, and its
-dimension parallel to that surface.
-
-It is needless for our present purpose to examine further the evidence
-furnished by Histology; nor, indeed, would further examination of this
-evidence be likely to yield definite results. In the cases given above
-we have marked differences among the incident forces; and therefore
-have a chance of finding, as we do find, relations between these and
-differences of form. But the cells composing masses of tissue are
-severally subject to forces which are indeterminate; and therefore
-the interpretation of their shapes is impracticable. It must suffice
-to observe that so far as the facts go they are congruous with the
-hypothesis.
-
-
-
-
-                             CHAPTER XVII.
-
-                 SUMMARY OF MORPHOLOGICAL DEVELOPMENT.
-
-
-§ 262. That any formula should be capable of expressing a common
-character in the shapes of things so unlike as a tree and a cow, a
-flower and a centipede, is a remarkable fact; and is a fact which
-affords strong _primâ facie_ evidence of truth. For in proportion to
-the diversity and multiplicity of the cases to which any statement
-applies, is the probability that it sets forth the essential relations.
-Those connexions which remain constant under all varieties of
-manifestation, are most likely to be the causal connexions.
-
-Still higher will appear the likelihood of an alleged law of organic
-form possessing so great a comprehensiveness, when we remember that on
-the hypothesis of Evolution, there must exist between all organisms
-and their environments, certain congruities expressible in terms of
-their actions and reactions. The forces being, on this hypothesis, the
-causes of the forms, it is inferable, _à priori_, that the forms
-must admit of generalization in terms of the forces; and hence, such
-a generalization arrived at _à posteriori_, gains the further
-probability due to fulfilment of anticipation.
-
-Nearer yet to certainty seems the conclusion thus reached, on finding
-that it does but assert in their special manifestations, the laws of
-Evolution in general--the laws of that universal re-distribution of
-matter and motion which hold throughout the totality of things, as
-well as in each of its parts.
-
-It will be useful to glance back over the various minor inferences
-arrived at, and contemplate them in their _ensemble_ from these
-higher points of view.
-
-
-§ 263. That process of integration which every plant displays during
-its life, we found reason to think has gone on during the life of the
-vegetal kingdom as a whole. Protoplasm into cells, cells into folia,
-folia into axes, axes into branched combinations--such, in brief, are
-the stages passed through by every shrub; and such appear to have been
-the stages through which plants of successively-higher kinds have been
-evolved from lower kinds. Even among certain groups of plants now
-existing, we find aggregates of the first order passing through various
-gradations into aggregates of the second order--here forming small,
-incoherent, indefinite assemblages, and there forming large, definite,
-coherent fronds. Similar transitions are traceable through which these
-integrated aggregates of the second order pass into aggregates of
-the third order: in one species the unions of parent-fronds with the
-fronds that bud out from them, being temporary, and in another species
-such unions being longer continued; until, in species still higher,
-by a gemmation which is habitual and regular, there is produced a
-definitely-integrated aggregate of the third order--an axis bearing
-fronds or leaves. And even between this type and a type further
-compounded, a link occurs in the plants which cast off, in the shape of
-bulbils, some of the young axes they produce. As among plants, so among
-animals. A like spontaneous fission of cells ends here in separation,
-there in partial aggregation, while elsewhere, by closer combination
-of the multiplying units, there arises a coherent and tolerably
-definite individual of the second order. By the budding of individuals
-of the second order, there are in some cases produced other separate
-individuals like them; in some cases temporary aggregates of such like
-individuals; and in other cases permanent aggregates of them: certain
-of which become so definitely integrated that the individualities of
-their component members are almost lost in a tertiary individuality.
-
-Along with this progressive integration there has gone on a progressive
-differentiation. Vegetal units of whatever order, originally
-homogeneous, have become heterogeneous while they have become united.
-Spherical cells aggregating into threads, into laminæ, into masses, and
-into special tissues, lose their sphericity; and instead of remaining
-all alike assume innumerable unlikenesses--from uniformity pass
-into multiformity. Fronds combining to form axes, severally acquire
-definite differences between their attached ends and their free ends;
-while they also diverge from one another in their shapes at different
-parts of the axes they compose. And axes, uniting into aggregates of
-a still higher order, become contrasted in their sizes, curvatures,
-and the arrangements of their appendages. Similarly among animals.
-Those components of them which, with a certain license, we class as
-morphological units, while losing their minor individualities in the
-major individualities formed of them, grow definitely unlike as they
-grow definitely combined. And where the aggregates so produced become,
-by coalescence, segments of aggregates of a still higher order, they,
-too, diverge from one another in their shapes.
-
-The morphological differentiation which thus goes hand in hand with
-morphological integration, is clearly what the perpetually-complicating
-conditions would lead us to anticipate. Every addition of a new unit
-to an aggregate of such units, must affect the circumstances of the
-other units in all varieties of ways and degrees, according to their
-relative positions--must alter the distribution of mechanical strains
-throughout the mass, must modify the process of nutrition, must affect
-the relations of neighbouring parts to surrounding diffused actions;
-that is, must initiate a changed incidence of forces tending ever to
-produce changed structural arrangements.
-
-
-§ 264. This broad statement of the correspondence between the general
-facts of Morphological Development and the principles of Evolution
-at large, may be reduced to statements of a much more specific kind.
-The phenomena of symmetry and unsymmetry and asymmetry, which we have
-traced out among organic forms, are demonstrably in harmony with those
-laws of the re-distribution of matter and motion to which Evolution
-conforms. Besides the myriad-fold illustrations of the instability of
-the homogeneous, afforded by these aggregates of units of each order,
-which, at first alike, lapse gradually into unlikeness; and besides
-the myriad-fold illustrations of the multiplication of effects, which
-these ever-complicating differentiations exhibit to us; we have also
-myriad-fold illustrations of the definite equalities and inequalities
-of structures, produced by definite equalities and inequalities of
-forces.
-
-The proposition arrived at when dealing with the causes of Evolution,
-“that in the actions and reactions of force and matter, an unlikeness
-in either of the factors necessitates an unlikeness in the effects; and
-that in the absence of unlikeness in either of the factors the effects
-must be alike” (_First Principles_, § 169), is a proposition which
-implies all these particular likenesses and unlikenesses of parts
-which we have been tracing. For have we not everywhere seen that the
-strongest contrasts are between the parts that are most contrasted in
-their conditions; while the most similar parts are those most similarly
-conditioned? In every plant the leading difference is between the
-attached end and the free end; in every branch it is the same; in every
-leaf it is the same. And in every plant the leading likenesses are
-those between the two sides of the branch, the two sides of the leaf,
-and the two sides of the flower, where these parts are two-sided in
-their conditions; or between all sides of the branch, all sides of
-the leaf, and all sides of the flower, where these parts are similarly
-conditioned on all sides. So, too, is it with animals which move about.
-The most marked contrasts they present are those between the part in
-advance and the part behind, and between the upper part and the under
-part; while there is complete correspondence between the two sides.
-Externally the likenesses and differences among limbs, and internally
-the likenesses and differences among vertebræ, are expressible in terms
-of this same law.
-
-And here, indeed, we may see clearly that these truths are corollaries
-from that ultimate truth to which all phenomena of Evolution are
-referable. It is an inevitable deduction from the persistence of force,
-that organic forms which have been progressively evolved, must present
-just those fundamental traits of form which we find them present. It
-cannot but be that during the intercourse between an organism and its
-environment, equal forces acting under equal conditions must produce
-equal effects; for to say otherwise is, by implication, to say that
-some force can produce more or less than its equivalent effect, which
-is to deny the persistence of force. Hence those parts of an organism
-which are, by its habits of life, exposed to like amounts and like
-combinations of actions and reactions, must develop alike; while
-unlikenesses of development must as unavoidably follow unlikenesses
-among these agencies. And this being so, all the specialities of
-symmetry and unsymmetry and asymmetry which we have traced, are
-necessary consequences.
-
-
-
-
-                                PART V.
-
-                      PHYSIOLOGICAL DEVELOPMENT.
-
-
-
-
-                              CHAPTER I.
-
-                      THE PROBLEMS OF PHYSIOLOGY.
-
-
-§ 265. The questions to be treated under the above title are widely
-different from those which it ordinarily expresses. We have no
-alternative, however, but to use Physiology in a sense co-extensive
-with that in which we have used Morphology. We must here consider
-the facts of function in a manner parallel to that in which we
-have, in the foregoing Part, considered the facts of structure.
-As, hitherto, we have concerned ourselves with those most general
-phenomena of organic form which, holding irrespective of class and
-order and sub-kingdom, illustrate the processes of integration and
-differentiation characterizing Evolution at large; so, now, we have
-to concern ourselves with the evidences of those differentiations and
-integrations of organic functions which have simultaneously arisen,
-and which similarly transcend the limits of zoological and botanical
-divisions. How heterogeneities of action have progressed along with
-heterogeneities of structure--that is the inquiry before us; and
-obviously, in pursuing it, all the specialities with which Physiology
-usually deals can serve us only as materials.
-
-Before entering on the study of Morphological Development, it
-was pointed out that while facts of structure may be empirically
-generalized apart from facts of function, they cannot be rationally
-interpreted apart; and throughout the foregoing pages this truth has
-been made abundantly manifest. Here we are obliged to recognize the
-interdependence still more distinctly; for the phenomena of function
-cannot even be conceived without direct and perpetual consciousness of
-the phenomena of structure. Though the subject-matter of Physiology
-is as broadly distinguished from the subject-matter of Morphology as
-motion is from matter; yet, just as the laws of motion cannot be known
-apart from some matter moved, so there can be no knowledge of function
-without a knowledge of some structure as performing function.
-
-Much more than this is obvious. The study of functions, considered from
-our present point of view as arising by Evolution, must be carried on
-_mainly_ by the study of the correlative structures. Doubtless, by
-experimenting on the organisms which are growing and moving around
-us, we may ascertain the connexions existing among certain of their
-actions, while we have little or no knowledge of the special parts
-concerned in those actions. In a living animal that can be conveniently
-kept under observation, we may learn the way in which conspicuous
-functions vary together--how the rate of a man’s pulse increases with
-the amount of muscular exertion he is undergoing; or how a horse’s
-rapidity of breathing is in part dependent on his speed. But though
-observations of this order are indispensable--though by accumulation
-and comparison of such observations we learn which parts perform which
-functions--though such observations, prosecuted so as to disclose
-the actions of all parts under all circumstances, constitute, when
-properly generalized and co-ordinated, what is commonly understood as
-Physiology; yet such observations help us but a little way towards
-learning how functions came to be established and specialized.
-We have next to no power of tracing up the genesis of a function
-considered purely as a function--no opportunity of observing the
-progressively-increasing quantities of a given action that have arisen
-in any order of organisms. In nearly all cases we are able only to
-show the greater growth of the part which we have found performs the
-action, and to infer that greater action of the part has accompanied
-greater growth of it. The tracing out of Physiological Development,
-then, becomes substantially a tracing out of the development of
-the organs by which the functions are known to be discharged--the
-differentiation and integration of the functions being presumed to have
-progressed hand in hand with the differentiation and integration of the
-organs. Between the inquiry pursued in Part IV, and the inquiry to be
-pursued in this Part, the contrast is that, in the first place, facts
-of structure are now to be used to interpret facts of function, instead
-of conversely; and, in the second place, the facts of structure to be
-so used are not those of conspicuous shape so much as those of minute
-texture and chemical composition.
-
-
-§ 266. The problems of Physiology, in the wide sense above
-described, are, like the problems of Morphology, to be considered
-as problems to which answers must be given in terms of incident
-forces. On the hypothesis of Evolution these specializations of
-tissues and accompanying concentrations of functions, must, like the
-specializations of shape in an organism and its component divisions,
-be due to the actions and reactions which its intercourse with the
-environment involves; and the task before us is to explain how they are
-wrought--how they are to be comprehended as results of such actions and
-reactions.
-
-Or, to define these problems still more specifically:--Those extremely
-unstable substances composing the protoplasm of which organisms are
-mainly built, have to be traced through the various modifications in
-their properties and powers, that are entailed on them by changes of
-relation to agencies of all kinds. Those organic colloids which pass
-from liquid to solid and from soluble to insoluble on the slightest
-molecular disturbance--those albuminoid matters which, as we see in
-clotted blood or the coagulable lymph poured out on abraded surfaces
-and causing adhesion between inflamed membranes, assume new forms
-with the greatest readiness--are to have their metamorphoses studied
-in connexion with the influences at work. Those compounds which, as
-we see in the quickly-acquired brownness of a bitten apple or in the
-dark stains produced by the milky juice of a Dandelion, immediately
-begin to alter when the surrounding actions alter, are to be everywhere
-considered as undergoing modifications by modified conditions. Organic
-bodies, consisting of substances that, as I here purposely remind the
-reader, are prone beyond all others to change when the incident forces
-are changed, we must contemplate as in all their parts differently
-changed in response to the different changes of the incident forces.
-And then we have to regard the concomitant differentiations of their
-reactions as being concomitant differentiations of their functions.
-
-Here, as before, we must take into account two classes of factors.
-We have to bear in mind the inherited results of actions to which
-antecedent organisms were exposed, and to join with these the results
-of present actions. Each organism is to be considered as presenting
-a moving equilibrium of functions, and a correlative arrangement
-of structures, produced by the aggregate of actions and reactions
-that have taken place between all ancestral organisms and their
-environments. The tendency in each organism to repeat this adjusted
-arrangement of functions and structures, must be regarded as from time
-to time interfered with by actions to which its inherited equilibrium
-is not adjusted--actions to which, therefore, its equilibrium has to be
-re-adjusted. And in studying physiological development we have in all
-cases to contemplate the progressing compromise between the old and the
-new, ending in a restored balance or adaptation.
-
-Manifestly our data are so scanty that nothing more than very general
-and approximate interpretations of this kind are possible. If the
-hypothesis of Evolution furnishes us with a rude conception of the
-way in which the more conspicuous and important differentiations of
-functions have arisen, it is as much as can be expected.
-
-
-§ 267. It will be best, for brevity and clearness, to deal with these
-physiological problems as we dealt with the morphological ones--to
-carry on the inductive statement and the deductive interpretation
-hand in hand: so disposing of each general truth before passing
-to the next. Treating separately vegetal organisms and animal
-organisms, we will in each kingdom consider:--first, the physiological
-differentiations and accompanying changes of structure which arise
-between outer tissues and inner tissues; next, those which arise
-between different parts of the outer tissues; and, finally, those which
-arise between different parts of the inner tissues. What little has
-to be said concerning physiological integration must come last. For
-though, in tracing up Morphological Evolution, we have to study those
-processes of integration by which organic aggregates are formed, before
-studying the differentiations that arise among their parts; we must,
-contrariwise, in tracing up Physiological Evolution, study the genesis
-of the different functions before we study the interdependence that
-eventually arises among them and constitutes physiological unity.
-
-
-
-
-                              CHAPTER II.
-
-    DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF PLANTS.
-
-
-§ 268. The simplest plant presents a contrast between its peripheral
-substance and its central substance. In each protophyte, be it a
-spherical cell or a branched tube, or such a more-specialized form as a
-Desmid, a marked unlikeness exists between the limiting layer and that
-which it limits. These vegetal aggregates of the first order may differ
-widely from one another in the natures of their outer coats and in the
-natures of their contents. As in the Palmella-form of one of the lower
-_Algæ_, there may exist a clothing of jelly; or, as in Diatom, the
-walls may take the form of silicious valves variously sculptured. The
-contained matter may be partly or wholly here green, there red, and in
-other cases brown. But amid all these diversities there is this one
-uniformity--a strong distinction between the parts in contact with the
-environment and the parts not in contact with the environment.
-
-When we remember that this trait is one which these simple living
-bodies have in common with bodies that are not living--when we
-remember that each inorganic mass eventually has its outer part more
-or less differentiated from its inner part, here by oxidation, there
-by drying, and elsewhere by the actions of light, of moisture, of
-frost; we can scarcely resist the conclusion that, in the one case
-as in the other, the contrast is due to the unlike actions to which
-the parts are subject. Given an originally-homogeneous portion of
-protoplasm, and it follows from the general laws of Evolution (First
-Principles, §§ 149–155), first, that it must lose its homogeneity, and,
-second, that the leading dissimilarities must arise between the parts
-most-dissimilarly conditioned--that is, between the outside and the
-inside. The exterior must bear amounts and kinds of force unlike the
-amounts and kinds which the interior bears; and from the persistence
-of force it follows inevitably that unlike effects must be wrought on
-them--they must be differentiated.
-
-What is the limit towards which the differentiation tends? We
-have seen that the re-distribution of matter and motion whence,
-under certain conditions, evolution results, can never cease until
-equilibrium is reached--proximately a moving equilibrium, and finally
-a complete equilibrium (_First Principles_, §§ 170–175). Hence, the
-differentiation must go on until it establishes such differences in the
-parts as shall balance the differences in the forces acting on them.
-When dealing with equilibration in general, we saw that this process
-is what is called adaptation (_First Principles_, § 173); and, in this
-work, we saw that by it the totality of functions of an organism is
-brought into correspondence with the totality of actions affecting it
-(§§ 159–163). Manifestly in this case, as in all others, either death
-or adjustment must eventually result. A force falling on one of these
-minute aggregates of protoplasm, must expend itself in working its
-equivalent of change. If this force is such that in expending itself
-it disturbs beyond rectification the balance of the organic processes,
-then the aggregate is disintegrated or decomposed. But if it does
-not overthrow that moving equilibrium constituting the life of the
-aggregate, then the aggregate continues in that modified form produced
-by the expenditure of the force. Thus, by direct equilibration,
-continually furthered by indirect equilibration, there must arise this
-distinction between the outer part adapted to meet outer forces, and
-the inner part adapted to meet inner forces. And their respective
-actions, as thus meeting outer and inner forces, must be what we call
-their respective functions.
-
-
-§ 269. Aggregates of the second order exhibit parallel traits,
-admitting of parallel interpretations. Integrated masses of cells or
-units homologous with protophytes, habitually show us contrasts between
-the characters of the superficial tissues and the central tissues. Such
-among these aggregates of the second order as have their component
-units arranged into threads or laminæ, single or double, cannot, of
-course, furnish contrasts of this kind; for all their units are as much
-external as internal. We must turn to the more or less massive forms.
-
-Of these, among _Fungi_, the common Puff-ball is a good example--good
-because it presents this fundamental differentiation but little
-complicated by others. In it we have a cortical layer of interwoven
-hyphæ obviously unlike the mass of spores which it incloses. So far as
-the unlikeness between external and internal parts is concerned, we see
-here a relation analogous to that existing in the simple cell; and we
-see in it a similar meaning: there is a physiological differentiation
-corresponding to the difference in the incidence of forces.
-
-Under various forms the _Algæ_ show just the same relation. Where,
-as in _Codium Bursa_, we have the ramified tubular branches of the
-thallus aggregated into a hollow globular mass, the outer and inner
-surfaces are contrasted both in colour and structure, though the
-tubules composing the two surfaces are continuous with one another. In
-_Rivularia_, again, we see the like, both in the radial arrangement
-of the imbedded threads and in the difference of colour between the
-exterior of the imbedding jelly and its interior. The more-developed
-_Algæ_ of all kinds repeat the antithesis. In branched stems, when
-they consist of more than single rows of cells, the outer cells become
-unlike the inner, as shown in Fig. 35. Such types as _Chrysymenia
-rosea_ show us this unlikeness very conspicuously. And it holds even
-with ribbon-shaped fronds. Wherever one of these is composed of three,
-four, or more layers, as in _Laminaria_ and _Punctaria_, the cells
-of the external layers are strongly distinguished from those of the
-internal layers, both by their comparative smallness and by their deep
-colour.
-
-
-§ 270. The higher plants variously display the like fundamental
-distinction between outer and inner tissues. Each leaf, thin as it is,
-exemplifies this differentiation of the parts immediately in contact
-with the environment from the parts not in immediate contact with
-the environment. Its epidermal cells, forming a protecting envelope,
-diverge physically and chemically from the mesophyll cells, which carry
-on the more active functions. And the contrast may be observed to
-establish itself in the course of development. At first the component
-cells of the leaf are all alike; and this unlikeness between the cells
-of the outer and inner layers, arises simultaneously with the rise of
-differences in their conditions--differences that have acted on all
-ancestral leaves as they act on the individual leaf.
-
-An unlikeness more marked in kind but similar in meaning, exists
-between the bark of every branch and the tissues it clothes. The
-phænogamic axis, especially when it undergoes what is known as
-secondary thickening, is commonly characterized by an outer zone of
-cells (the cork layer) differing from the inner layers in character and
-function, as it differs from them in position. Subject as this outer
-layer is to the unmitigated actions of forces around--to abrasions, to
-extremes of heat and cold, to evaporation and soaking with water--its
-units have to be brought into equilibrium with these more violent
-actions, and have acquired molecular constitutions more stable
-than those of the interior cells. That is to say, the forces which
-differentiate the cortical part from the rest are the forces which it
-has to resist, and from which it passively protects the parts within.
-How clearly this heterogeneity of structure and function is consequent
-upon intercourse with the environment, every tree and shrub shows. The
-young shoots, alike of annuals and perennials, are quite green and soft
-at their extremities. Among plants of short lives, there is usually
-but a slight development of bark or none at all: such traces of it as
-the surface of the axis acquires being seen only at its lowermost or
-oldest portion. In long-lived plants, however, this formation of a
-tough opaque coating takes place more rapidly; and shows us distinctly
-the connexion between the degree of differentiation and the length of
-exposure. For, in a growing twig, we see that the bark, invisible at
-the bud, thickens by insensible gradations as we go downwards to the
-junction of the twig with the branch; and we come to still thicker
-parts of it as we descend along the branch towards the main stem.
-Moreover, on examining main stems we find that while in some trees the
-bark, cracked by expansion of the wood, drops off in flakes, leaving
-exposed patches of the inner tissue which presently become green and
-finally develop new bark; in other trees the exfoliated flakes continue
-adherent, and in the course of years form a rugged fissured coat: so
-producing a still more marked contrast between outside and inside. Of
-course the establishment of this heterogeneity is furthered by natural
-selection, which, where a protective covering is needed, gives an
-advantage to those individuals in which it has become strongest. But
-that this divergence of structure commences as a direct adaptation, is
-clearly shown by other facts than the foregoing. There is the fact that
-many of the plants which in our gardens develop bark with considerable
-rapidity, do not develop it with the same rapidity in a greenhouse. And
-there is the fact that plants which, in some climates, have their stems
-covered only by thin semi-transparent layers, acquire thick opaque
-layers when taken to other climates.
-
-Just noting, for the sake of completeness, that in the roots of the
-higher plants there arises a contrast between outer and inner parts,
-parallel to the one we have traced in their branches, let me draw
-attention to another differentiation of the same ultimate nature, which
-the higher plants exhibit to us--a differentiation which, familiar
-though it is, gains a new meaning by association with those named
-above, and makes their meaning still more manifest. Each great plant
-shows it. When, by the budding of axes out of axes, there is produced
-one of those highly-compounded Phænogams which we call a tree, the
-central part of the aggregate becomes functionally and structurally
-unlike the peripheral part. On looking into a large tree, or even
-a small one which has thick foliage, like the Laurel, we see that
-the internal branches are almost or quite bare of leaves, while the
-leaf-clad branches form an external stratum; and all our experience
-unites in proving that this contrast arises by degrees, as fast as
-the growth of the tree entails a contrast between the conditions
-to which inner and outer branches are exposed. Now when, in these
-most-composite aggregates, we see a differentiation between peripheral
-and central parts demonstrably caused by a difference in the relations
-of these parts to environing forces, we get support for the conclusion
-otherwise reached, that there is a parallel cause for the parallel
-differentiations exhibited by all aggregates of lower orders--branches,
-leaves, cells.
-
-
-§ 271. Before leaving this most general physiological differentiation,
-it may be well to say something respecting certain secondary
-unlikenesses which usually arise between interior and exterior. For the
-contrast is not, as might be supposed from the foregoing descriptions,
-a simple contrast: it is a compound contrast. The outer structure
-itself is usually divisible into concentric structures. This is equally
-true of a protophyte and of a phænogamic axis. Between the centre of
-an independent vegetal cell and its surface, there are at least two
-layers; and the bark coating the substance of a shoot, besides being
-itself compound, includes another tissue lying between it and the
-wood. What is the physical interpretation of these facts?
-
-When a mass of something we distinguish as inert matter is exposed
-to external agencies capable of working changes in it--when it is
-chemically acted upon, or when, being dry, it is allowed to soak, or
-when, being wet, it is allowed to dry--the changes set up progress in
-an equable way from the surface towards the centre. At any time during
-the process (supposing no other action supervenes) the modification
-wrought, first completed at the outside, either gradually diminishes
-as we approach the centre, or ceases suddenly at a certain distance
-from the centre. But now suppose that the mass, instead of being
-inert, is the seat of active changes--suppose that it is a portion of
-complex colloidal substance, permeable by light and by fluids capable
-of affecting its unstable molecules--suppose that its interior is
-a source of forces continually liberated and diffusing themselves
-outwards. Is it not likely that while at the centre the action of the
-internally-liberated forces will dominate, and while at the surface
-the action of the environing forces will dominate, there will be
-between the two a certain place at which their actions balance? May
-we not expect that this will be the place where the most unstable
-matter exists--the place outside of which the matter becomes relatively
-stable in the face of external forces, and inside of which the matter
-becomes relatively stable in the face of internal forces? And must we
-not conclude that though part of the adjustment is due to indirect
-equilibration, the initiation of it is due to direct equilibration?
-
-But we are here chiefly concerned with the more general interpretation,
-which is independent of any such speculation as the foregoing. These
-contrasted tissues and the contrasted functions they severally perform
-are, beyond question, subordinated to the relations of outside and
-inside. And the evidence makes it tolerably clear that the unlike
-actions or forces involved by the relations of outside and inside,
-determine these contrasts--partly directly and partly indirectly.
-
-
-
-
-                             CHAPTER III.
-
-          DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS.
-
-
-§ 272. The motionless protococcoid forms of lower _Algæ_, which do
-not permanently expose any parts of their surfaces to actions unlike
-those which other parts are exposed to, have no parts of their
-surfaces unlike the rest in function and composition. This is what
-the hypothesis prepares us for. If physiological differentiations
-are determined by differences in the incidence of forces, then there
-will be no such differentiations where there are no such differences.
-Contrariwise, it is to be expected that the most conspicuous
-unlikeness of function and minute structure will arise between the
-most-dissimilarly circumstanced parts of the surface. We find that
-they do. The upper end and the lower end, or, more strictly speaking,
-the free end and the attached end, habitually present the strongest
-physiological contrasts.
-
-Even aggregates of the first order illustrate this truth. Such
-so-called unicellular plants as those delineated in Figs. 4, 5, and 6,
-show us, on comparing the contents of their fixed ends and their loose
-ends, that different processes are going on in them, and that different
-functions are being performed by their limiting membranes. _Caulerpa
-prolifera_, which “consists of a little creeping stem with roots below
-and leaves above,” originating “in the growth of a body which may be
-regarded as an individual cell,” supplies a still-better example. Among
-aggregates of the second order a like connexion is displayed in more
-various modes but with equal consistency. As before, the Puff-ball
-served to exemplify the primary physiological differentiation of outer
-parts from inner parts; so, here, it supplies a simple illustration of
-the way in which the differentiated outer part is re-differentiated,
-in correspondence with the chief contrast in its relations to the
-environment. The only marked unlikeness which the cortical layer of
-the Puff-ball presents, is that between the portion next the ground
-and the opposite portion. The better-developed _Fungi_ exhibit a
-more decided heterogeneity of parallel kind. Such incrusting _Algæ_
-as _Ralfsia verrucosa_ furnish a kindred contrast; and in the higher
-_Algæ_ it is uniformly repeated. Phænogams display this physiological
-differentiation very conspicuously. That earth and air are unlike
-portions of the environment, subjecting roots and leaves to unlike
-physical forces, which entail on them unlike reactions, and that the
-unlike functions and structures of their respective surfaces are fitted
-to these unlike physical forces, are familiar facts which it would be
-needless here to name, were it not that they must be counted as coming
-within a wider group of facts.
-
-Is this unlikeness between the outer tissues of the attached ends
-and those of the free ends in plants, determined by their converse
-with the unlike parts of the environment? That they result from an
-equilibration partly arising in the individual and partly arising by
-the survival of individuals in which it has been carried furthest, is
-inferable _à priori_; and this _à priori_ argument may be adequately
-enforced by arguments of the inductive order. A few typical ones must
-here suffice. The gemmules of the _Marchantia_ are little disc-shaped
-masses of cells composed of two or more layers. Their sides being
-alike, there is nothing to determine which side falls lowermost when
-one of them is detached. Whichever side falls lowermost, however,
-presently begins to send out rootlets, while the uppermost side
-begins to assume those characters which distinguish the face of the
-frond. When this differentiation has commenced, the tendency to its
-complete establishment becomes more and more decided; as is proved by
-the fact that if the positions of the surfaces be altered, the gemmule
-bends itself so as to re-adjust them: the change towards equilibrium
-with environing forces having been once set up, there is acquired,
-as it were, an increasing momentum which resists any counter-change.
-But the evidence shows that at the outset, the relations to earth
-and air alone determine the differentiation of the under surface
-from the upper. The experiences of the gardener, multiplying his
-plants by cuttings and layers, constitute another class of evidences
-not to be omitted: they are commonplace but instructive examples of
-physiological differentiation. While circumstanced as it usually is,
-the meristematic tissue of each branch in a Phænogam continues to
-perform its ordinary function--regularly producing on its outer side
-the cortical substances, and on its inner side the vascular and woody
-tissues. But change the conditions to those which the underground part
-of the plant is exposed to, and there begins another differentiation
-resulting in underground structures. Contact with water often suffices
-alone to produce this result, as in the branches of some trees when
-they droop into a pool, or as occasionally with a cutting placed in a
-bottle of water; and when the light is excluded by imbedding the end
-of the cutting, or the middle of the still-attached branch, in the
-earth, this production of tissues adapted to the function of absorbing
-moisture and mineral constituents proceeds still more readily.
-With such cases may be grouped those in which this development of
-underground organs by an above-ground tissue, is not exceptional but
-habitual. Creeping plants furnish good illustrations. From the shoots
-of the Ground-Ivy, rootlets are put out into the soil in a manner
-differing but little from that in which they are put out by an imbedded
-layer; save that the process follows naturally-induced conditions
-instead of following artificially-induced conditions. But in the common
-Ivy which, instead of running along the surface of the earth, runs
-up inclined or vertical surfaces, we see the process interestingly
-modified without being essentially changed. The rootlets, here
-differentiated by their conditions into organs of attachment much more
-than organs of absorption, still develop on that side of the shoot next
-the supporting surface, and do not develop where the shoot, growing
-away from the tree or wall, is surrounded equally on all sides by light
-and air: thus showing, undeniably, that the production of the rootlets
-is determined by the differential incidence of forces. Though survival
-of the fittest doubtless furthered this transition yet it clearly
-did not initiate it. That greenness which may be observed in these
-Ivy-branch rootlets while they are quite young, soft, and unshaded,
-introduces us to facts which are the converse of the foregoing facts;
-and proves that the parts ordinarily imbedded in the soil and adapted
-to its actions, acquire, often in very marked degrees, the superficial
-structures of the aërial parts, when they are exposed to light and air.
-This may be witnessed in Maize, which, when luxuriant, sends out from
-its nodes near the ground, clusters of roots that are thick, succulent,
-and of the same colour as the leaves. Examples more familiar to us
-in England occur in every field of turnips. On noting how green is
-the uncovered part of a turnip-root, and how manifestly the area over
-which the greenness extends varies with the area exposed to light, as
-well as with the degree of the exposure, it will be seen that beyond
-question, root-tissue assumes to a considerable extent the appearances
-and function of leaf tissue, when subject to the same agencies. Let
-us not forget, too, that where exposed roots do not approach in
-superficial character towards leaves, they approach in superficial
-character towards stems: becoming clothed with a thick, fissured bark,
-like that of the trunk and branches. But the most conclusive evidence
-is furnished by the actual substitutions of surface-structures and
-functions, that occur in aërial organs which have taken to growing
-permanently under ground, and in underground organs which have taken to
-growing permanently in the air. On the one hand, there is the rhizome
-exemplified by Ginger--a stem which, instead of shooting up vertically,
-runs horizontally below the surface of the soil, and assumes the
-character of a root, alike in colour, texture, and production of
-rootlets; and there is that kind of swollen underground axis, bearing
-axillary buds, which the Potato exemplifies--a structure which, though
-homologically an axis, simulates a tuberous root in surface-character,
-and when exposed to the air, manifests no greater readiness to develop
-chlorophyll than a tuberous root does. On the other hand, there are the
-aërial roots of certain Orchids which, habitually green at their tips,
-continue green throughout their whole lengths when kept moist; which
-have become leaf-like not only by this development of chlorophyll, but
-also by the acquirement of stomata; and which do not bury themselves in
-the soil when they have the opportunity.[46] Thus we have aërial organs
-so completely changed to fit underground actions, that they will not
-resume aërial functions; and underground organs so completely changed
-to fit aërial actions, that they will not resume underground functions.
-
-That the physiological differentiation between the part of a plant’s
-surface which is exposed to light and air and the part which is exposed
-to darkness and moisture and solid matter, is primarily due to the
-unlike actions of these unlike parts of the environment, is, then,
-clearly implied by observed facts--more clearly, indeed, than was to
-be expected. Considering how strong must be the inherited tendency
-of a plant to assume those special characters, physiological as well
-as morphological, which have resulted from an enormous accumulation
-of antecedent actions, it may be even thought surprising that this
-tendency can be counteracted to so great an extent by changed
-conditions. Such a degree of modifiability becomes comprehensible only
-when we remember how little a plant’s functions are integrated, and how
-much, therefore, the functions going on in each part may be altered
-without having to overcome the momentum of the functions throughout
-the whole plant. But this modifiability being as great as it is, we
-can have no difficulty in understanding how, by the cumulative aid
-of natural selection, this primary differentiation of the surface in
-plants has become what we see it.
-
-
-§ 273. We will leave now these contrasts between the free surfaces of
-plants and their attached or imbedded surfaces, and turn our attention
-to the secondary contrasts existing between different parts of their
-free surfaces. Were a full statement of the evidence practicable,
-it would be proper here to dwell on that which is furnished by the
-inferior classes. It might be pointed out in detail that where, as
-among the _Algæ_, the free surfaces are not dissimilarly conditioned,
-there is no systematic differentiation of them--that the frond of
-an _Ulva_, the ribbon-shaped divisions of a _Laminaria_, and the
-dichotomous expansions of the _Fuci_ which clothe the rocks between
-tide-marks, are alike on both sides; because, swayed about in all
-directions as they are by the waves and tides, their sides are equally
-affected. Conversely, from the _Fungi_ might be drawn abundant proof
-that even among Thallophytes, unlikenesses arise between different
-parts of the free surfaces when their circumstances are unlike.
-In such laterally-growing kinds as are shown in Fig. 196_b_, the
-honey-combed under surface and the smooth leathery upper surface,
-have their contrasts related to contrasted conditions; and in the
-adjacently-figured Agarics, and other stalked genera, the pileus
-exhibits a parallel difference, explicable in a parallel way. But
-passing over Cryptogams it must suffice if we examine more at length
-these traits as they are displayed by Phænogams. Let us first note the
-dissimilarities between the outer tissues of stems and leaves.
-
-That these dissimilarities arose by degrees, as fast as the units
-of which the phænogamic axis is composed became integrated, is a
-conclusion in harmony with the truth that in every shoot of every
-plant, they are at first slight and become gradually marked. Already,
-in briefly tracing the contrasts between the outer and inner tissues
-of plants, some facts have been named showing, by implication, how
-the cessation of the leaf-function in axes is due to that change of
-conditions entailed by the discharge of other functions. Here we have
-to consider more closely facts of this class, together with others
-immediately to the point. On pulling off from a stem of grass the
-successive sheaths of its leaves, the more-inclosed parts of which are
-of a fainter green than the outer parts, it will be found that the
-tubular axis eventually reached is of a still fainter green; but when
-the axis eventually shoots up into a flowering stem, its exposed part
-acquires as bright a green as the leaves. In other Monocotyledons, the
-leaf-sheaths of which are successively burst and exfoliated by the
-swelling axis, it may be observed that where the dead sheaths do not
-much obstruct the light and air, the surface of the axis underneath
-is full of chlorophyll. _Dendrobium_ is an example. But when the dead
-sheaths accumulate into an opaque envelope, the chlorophyll is absent,
-and also, we may infer, the function which its presence habitually
-implies. Carrying with us this evidence, we shall recognize a like
-relation in Dicotyledons. While its outer layer remains tolerably
-transparent, an exogenous stem or branch continues to show, by the
-formation of chlorophyll, that it shares in the duties of the leaves;
-but in proportion as a bark which the light cannot penetrate is
-produced by the adherent flakes of dead skin, or by the actual deposit
-of a protective substance, the differentiation of duties becomes
-more decided. Cactuses and Euphorbias supply us with converse facts
-having the same implication. The succulent axes so strangely combined
-in these plants, maintain for a long time the translucency of their
-outer layers and their greenness; and they so efficiently perform the
-offices of leaves that leaves are not produced. In some cases, axes
-that are not succulent participate largely in the leaf-function, or
-entirely usurp it--still, however, by fulfilling the same essential
-conditions. Occasionally, as in _Statice brassicæfolia_, stems become
-fringed; and the fringes they bear assume, along with the thinness of
-leaves, their darker green and general aspect. In the genus _Ruscus_,
-the flattened axis simulates so closely the leaf-structure, that were
-it not for the flower borne on its mid-rib, or edge, its axial nature
-would hardly be suspected. And let us not omit to note that where axes
-usurp the characters of leaves, in their attitudes as well as in their
-shapes and thicknesses, there are contrasts between their under and
-upper surfaces, answering to the contrasts between the relations of
-these surfaces to the light. Of this _Ruscus androgynus_ furnishes a
-striking example. In it the difference which the unaided eye perceives
-is much less conspicuous than that disclosed by the microscope; for
-I find that while the face of the pseudo-leaf has no stomata, the
-back is abundantly supplied with them. One more illustration must
-be added. Equally for the morphological and physiological truths
-which it enforces, the _Mühlenbeckia platyclada_ is one of the most
-instructive of plants. In it the simulation of forms and usurpation
-of functions, are carried out in a much more marvellous way than
-among the _Cactaceæ_. Imagine a growth resembling in outline a very
-long willow-leaf, but without a mid-rib, and having its two surfaces
-alike. Imagine that across this thin, green, semi-transparent
-structure, there are from ten to thirty divisions, which prove to be
-the successive nodes of an axis. Imagine that along the edges of this
-leaf-shaped aggregate of internodes, there arise axillary buds, some
-of which unfold into flowers, and others of which shoot up vertically
-into growths like the one which bears them. Imagine a whole plant thus
-seemingly composed of jointed willow-leaves growing from one another’s
-edges, and some conception will be formed of the _Mühlenbeckia_.
-The two facts which have meaning for us here are--first, that the
-performance of leaf-functions by these axes goes along with the
-assumption of a leaf-like translucency; and, second, that these
-flattened axes, retaining their upright attitudes, and therefore
-keeping their two faces similarly conditioned, have these two faces
-alike in colour and texture.
-
-That physiological differentiation of the surface which arises in
-Phænogams between axial organs and foliar organs, is thus traceable
-with tolerable clearness to those differences between their conditions
-which integration has entailed--partly in the way above described and
-partly in other ways still to be named. By its relative position, as
-being shaded by the leaves, the axis is less-favourably circumstanced
-for performing those assimilative actions effected by the aid of light.
-Further, that relatively-small ratio of surface to mass in the axis,
-which is necessitated by its functions as a support and a channel
-for circulation, prevents it from taking in, with the same facility
-as the leaves, those surrounding gases from which matter is to be
-assimilated. Both these special causes, however, in common with that
-previously assigned, fall within the general cause. And in the fact
-that where the differential conditions do not exist, the physiological
-differentiation does not arise, or is obliterated, we have clear proof
-that it is determined by unlikenesses in the relations of the parts to
-the environment.
-
-
-§ 274. From this most general contrast between aërial
-surface-tissues--those of axes and those of folia--we pass now to the
-more special contrasts of like kind existing in folia themselves.
-Leaves present us with superficial differentiations of structure and
-function; and we have to consider the relations between these and the
-environing forces.
-
-Over the whole surface of every phænogamic leaf, as over the fronds
-of the _Pteridophyta_, there extends a simple or compound epidermal
-layer, formed of cells that are closely united at their edges and
-devoid (in the Flowering Plants) of that granular colouring matter
-(chlorophyll) contained in the layers of cells they inclose: the result
-being that the membrane formed of them is comparatively transparent. On
-the submerged leaves of aquatic Phænogams, this outer layer is thin,
-delicate, and permeable by water; but on leaves exposed to the air, and
-especially on their upper surfaces, is comparatively strong, dense,
-often smooth and impermeable by water: being thus fitted to prevent the
-rapid escape of the contained juices by evaporation. Similarly, while
-the leaves of terrestrial plants which live in temperate climates,
-usually have comparatively thin coats thus composed, in climates that
-are both hot and dry, leaves are commonly clothed with a very thick
-cuticle. Nor is this all. The outside of an aërial leaf differs from
-that of a submerged leaf by containing a deposit of waxy substance.
-Whether this be exuded by the exposed surfaces of the cells, as some
-contend, or whether it is deposited within the cells, as thought by
-others, matters not in so far as the general result is concerned. In
-either case a waterproof coating is formed at the outermost sides of
-these outermost cells; and in many cases produces that polish by which
-the upper surface of the leaf is more or less distinguished from the
-under surface. This external pellicle presents us with another contrast
-of allied meaning. On the upper surfaces of leaves subject to the
-direct action of the sun’s rays, there are either few or none of those
-minute openings, or stomata, through which gases can enter or escape;
-but on the under surfaces these stomata are abundant: a distribution
-which, while permitting free absorption of the needful carbonic acid,
-puts a check on the exit of watery vapour. Two general exceptions to
-this arrangement may be noted. Leaves that float on the water have all
-their stomata on their upper sides, and leaves that are submerged have
-no stomata--modifications obviously appropriate to the conditions.
-What is to be said respecting the genesis of these differentiations?
-For the last there seems no direct cause: its cause must be indirect.
-The unlike actions to which the upper and under surfaces of leaves are
-subject, have no apparent tendency to produce unlikeness in the number
-of their breathing holes. Here the natural selection of spontaneous
-variations furnishes the only feasible explanation. For the first,
-however, there is a possible cause in the immediate actions of incident
-forces, which survival of the fittest continually furthers.
-
-The fluid exhaling through the walls of the cells next the air, will be
-likely to leave behind suspended substances on their outer surfaces.
-On remembering the pellicle which is apt to form on thick solutions or
-emulsions as they dry, and how this pellicle as it grows retards the
-further drying, it will be perceived that the deposit of waxy matter
-next to the outer surfaces of the cuticular cells in leaves, is not
-improbably initiated by the evaporation which it eventually checks.
-Should it be so, there results a very simple case of equilibration.
-Where the loss of water is too great, this waxy pellicle left behind by
-the escaping water will protect most those individuals of the species
-in which it is thickest or densest; and by inheritance and continual
-survival of the fittest, there will be established in the species that
-thickness of the layer which brings the evaporation to a balance with
-the supply of water.
-
-Another superficial differentiation, still more familiar, has to be
-noted. Every child soon learns to distinguish by its colour the upper
-side of a leaf from its under side, if the leaf is one that has grown
-in such way as to establish the relations of upper and under. The upper
-surfaces of leaves are habitually of a deeper green than the under.
-Microscopic examination shows that this deeper green results from the
-closer clustering of those parenchyma-cells full of chlorophyll that
-are in some way concerned with the assimilative actions; while beneath
-them are more numerous intercellular passages communicating with those
-openings or stomata through which is absorbed the needful air. Now when
-it is remembered that the formation of chlorophyll is clearly traceable
-to the action of light--when it is remembered that leaves are pale
-where they are much shaded and colourless when developed in the dark,
-as in the heart of a Cabbage--when it is remembered that succulent axes
-and petioles, like those of Sea-kale and Celery, remain white while
-the light is kept from them and become green when exposed; it cannot
-be questioned that this greater production of chlorophyll next to the
-upper surface of a leaf, is directly consequent on the greater amount
-of light received. Here, as in so many other cases, we must regard
-the differentiation as in part due to direct equilibration and in
-part to indirect equilibration. Familiar facts compel us to conclude
-that from the beginning, each individual foliar organ has undergone
-a certain immediate adaptation of its surfaces to the incidence of
-light; that when there arose a mode of growth which exposed the leaves
-of successive generations in similar ways, this immediately-produced
-adaptation, ever tending to be transmitted, was furthered by the
-survival of individuals inheriting it in the greatest degree; and that
-so there was gradually established that difference between the two
-surfaces which each leaf displays before it unfolds to the light, but
-which becomes more marked when it has unfolded.[47]
-
-From the ordinary cases let us now pass to the exceptional cases. We
-will look first at those in which the two faces of the leaves differ
-but little, or not at all--their circumstances being similar or equal.
-Leaves that grow in approximately-upright attitudes, and attitudes
-which do not maintain the relative positions of the two surfaces with
-constancy, may be expected to display an unusual likeness between the
-two surfaces; and among them we see it. The Grasses may be named as a
-group exemplifying this relation; and if, instead of comparing them as
-a group with other groups, we compare those dwarf kinds of them which
-spread out their leaves horizontally, with the large aspiring kinds,
-such as _Arundo_, we trace a like antithesis: in the one the contrast
-of upper and under is very obvious, while in the other it is scarcely
-to be detected. Leaves of various other Monocotyledons that grow in a
-similar way, similarly show us a near approach to uniformity of the
-two surfaces; as instance the genus _Clivia_ and the thinner-leaved
-kinds of _Yucca_. Where the contrast of upper and under is greatly
-diminished by the assumption of a rounded or cylindrical form, instead
-of a flattened form, the same thing happens. The genus _Kleinia_
-furnishes illustrations. It may be remarked, too, that even within the
-limits of this genus there are instructive variations; for while in
-_Kleinia ficoides_ the leaves, shaped like pea-pods, are broadest in a
-vertical direction, and have their lateral surfaces alike in conditions
-and structure, in other species the leaves, broader horizontally than
-vertically, exhibit unlikeness between the upper and under sides.
-Equally to the point is the evidence furnished by vertically-growing
-leaves that are cylindrical, as those of _Sanseviera cylindrica_, or as
-those of the Rush-tribe: the similarly-placed surface has all around
-a similar character. Of kindred meaning, and still more conclusive,
-are the cases in which the under side of the leaf, being more exposed
-to light than the upper side, usurps the character and function of
-the upper side. If a common Flag be pulled to pieces, it will be seen
-that what answers to the face in other leaves, forms merely the inside
-of the sheath including the younger leaves, and is obliterated higher
-up. The two surfaces of the blade answer to the two under halves of a
-leaf that has been, as it were, folded together lengthways, with the
-two halves of its upper surface in contact. And here, in default of an
-upper surface, the under surface acquires its character and discharges
-its function. A like substitution occurs in _Aristea corymbosa_; and
-there are some of the Orchids, as _Lockhartia_, which display it in a
-very obvious way.
-
-When joined with the foregoing evidence, the evidence which another
-kind of substitution supplies is of great weight. I refer to that which
-occurs in the Australian Acacias, already instanced as throwing light
-on morphological changes. In these plants the leaves properly so-called
-are undeveloped, and the foot-stalks, flattened out into foliaceous
-shapes, acquire veins and mid-ribs, and so far simulate leaves as
-ordinarily to be taken for them: a fact in itself of much physiological
-significance. But that which it concerns us especially to note, is
-the absence of distinction between the two faces of these phyllodes,
-as they are named, and the cause of its absence. These transformed
-petioles do not flatten themselves out horizontally, so as to acquire
-under and upper sides, as most true leaves do; but they flatten
-themselves out vertically: the result being that their two sides are
-similarly circumstanced with respect to light and other agencies; and
-there is consequently nothing to cause their differentiation. And then
-we find an analogous case where differential conditions arise, and
-where some differentiation results. In _Oxalis bupleurifolia_, Fig. 66,
-there is a similar flattening out of the petiole into a pseudo-leaf;
-but in it the flattening takes place in the same plane as the leaf, so
-as to produce an under and an upper surface; and here the two surfaces
-of the pseudo-leaf are slightly unlike--in contour if in nothing else.
-
-
-§ 275. We now come to such physiological differentiations among the
-outer tissues of plants, as are displayed in the contrasts between
-foliar organs on the same axis, or on different axes--contrasts between
-the seed-leaves and the leaves subsequently formed, between submerged
-and aërial leaves in certain aquatic plants, between leaves and bracts,
-and between bracts and sepals. To deal even briefly with these implies
-information which even a professed botanist would have to increase by
-special inquiries, before attempting interpretations. Here it must
-suffice to say something respecting those marked unlikenesses existing
-between the tissues of the more characteristic parts of flowers, and
-the tissues of the homologous foliar organs.
-
-It was pointed out in § 196, that the terminal folia of a phænogamic
-axis have sundry characters in common with such fronds as those
-out of which we concluded that the phænogamic axis has arisen by
-integration--common characters of a kind to be expected. In their
-simple cellular composition, comparative want of chlorophyll, and
-deficiency of vascular structures, the undeveloped ends of leaf-shoots
-and the developed ends of flower-shoots, approach to the fronds of the
-simpler Archegoniates. We also noted between them another resemblance.
-It is said of the _Jungermanniaceæ_, that “though under certain
-circumstances of a pure green, they are inclined to be shaded with
-red, purple, chocolate, or other tints;” and answering to this we have
-the facts that such colours commonly occur in the terminal folia of a
-phænogamic axis, when arrest of its development leads to the formation
-of a flower, and that very frequently they are visible at the ends
-of leaf-axes. In the unfolding parts of shoots, more or less of red,
-or copper-colour, or chocolate-colour, may generally be seen: often,
-indeed, it characterizes the leaves for some time after they are
-unfolded. Occasionally the traces of it are permanent; and, as in the
-scarlet terminal leaves of _Poinsettia pulcherrima_, we see that it may
-become, and continue, extremely conspicuous. The question, then, now
-to be asked is--has this colouring by which the immature part of the
-phænogamic axis is characterized, anything to do with the colouring of
-flowers? Has this difference between undeveloped folia and folia that
-are further developed, been increased by natural selection where an
-advantage accrued from it, until it has ended in the strong contrast we
-now see? I think we may not irrationally infer that this has happened.
-
-Facts, very numerous and varied, united to warrant us in concluding
-that gamogenesis commences where the forces which conduce to growth are
-nearly equilibrated by the forces which resist growth (§ 78); and the
-induction that in plants, fertilized germs are produced at places where
-there is an approach towards this balance, we found to be in harmony
-with the deduction that an advantage to the species must be gained by
-sending off migrating progeny from points where nutrition is failing.
-Other things equal, failure of nutrition may be expected in parts which
-have the most remote or most indirect access to the materials furnished
-by the roots--materials which have to be carried great distances by
-a very imperfect apparatus. The ends of lateral axes are therefore
-the probable points of fructification, in aggregates of the third
-order that have taken to growing vertically. But if these points at
-which nutrition is failing, are also the points at which the colours
-inherited from lower types are likely to recur in more marked degrees
-than elsewhere; then we may infer that the organs of fructification
-will not unfrequently co-exist with such colours at the ends of such
-axes. How may the resulting contrast between the older fronds and the
-fronds next the germ-producing organs be increased? If uninterfered
-with it would be likely to diminish. These traits inherited from remote
-ancestry might be expected slowly to fade away. How, then, is the
-intensification of them to be explained?
-
-If a contrast of the kind described favours the propagation of a race
-in which it exists, it will be maintained and increased; and if we
-take into account an agency of which Mr. Darwin has shown the great
-importance--the agency of insects--we shall have little difficulty
-in understanding how such a contrast may facilitate propagation. We
-cannot, of course, here assume the agency of insects so specialized
-in their habits as Bees and Butterflies; for their specialized habits
-imply the pre-existence of the contrast to be explained. But there is
-an insect-agency of a more general kind which may be fairly counted
-upon as coming into action. Various small Flies and Beetles wander
-over the surfaces of plants in search of food. It is a legitimate
-assumption that they will frequent most those parts in which they find
-most food, or food most to their liking--especially if at the same time
-they gain the advantage of concealment. Now the ends of axes, formed
-of young, soft, and closely-packed folia, are the parts which more
-than any others offer these several advantages. They afford shelter
-from enemies; they frequently contain exuded juices; and when they do
-not, their tissues are so tender as to be easily pierced in search
-of the sap. If, then, from the first, as at present, these ends of
-axes have been favourite haunts of small insects; and if, where the
-closely-clustered folia contained the generative organs, the insects
-frequenting them occasionally carried adherent fructifying cells from
-one plant to another, and so aided fertilization; it would follow
-that anything which made such terminal clusters more attractive to
-such insects, or more conspicuous to them, or both, would further the
-multiplication of the race, and would so be continually increased by
-the extra multiplication of individuals in which it was greatest.
-Here we find the clue. This contrast of colour between the folia next
-to the fructifying parts and all other folia, must constantly have
-facilitated insect-agency; supposing the insects to have had the power
-of distinguishing between colours. That Bees and Butterflies have this
-power is manifest. They may be watched flying from flower to flower,
-disregarding all other parts of the plants. And if the less-specialized
-insects possessed some degree of such discrimination, then the initial
-contrasts of colour above described would be maintained and increased.
-Let such a connexion be once established, and it must tend to become
-more decided. Insects most able to discern the parts of plants which
-afford what they seek, will be those most likely to survive and leave
-offspring. Plants presenting most of the desired food, and showing most
-clearly where it lies, will have their fertilization and multiplication
-furthered in the greatest degree. And so the mutual adaptation will
-become ever closer; while it is rendered at the same time more varied
-by the special requirements of the insects and of the plants in each
-locality, under each change of conditions. Of course, the genesis
-of the sweet secretions and the odours of flowers, has a parallel
-interpretation. The simultaneous production of honey, or some kindred
-substance, is implied above; since, unless a bait co-existed with
-the colour, the colour would not attract insects, and would not be
-maintained and intensified by natural selection. Gums, and resins, and
-balsams, are familiar products of plants; apparently, in many cases,
-excreted as useless matters from various parts of their surfaces. These
-substances, admitting of wide variations in quality, as they do, afford
-opportunities for the action of natural selection wherever any of
-them, attractive to insects, happen to be produced near the organs of
-fructification. And this action of natural selection once set up, may
-lead to the establishment of a local excretion, to the production of an
-excretion more and more attractive, and to the disposal of the organ
-containing it in such a way as most to facilitate the carrying away of
-pollen. Similarly and simultaneously with odours. Odours, like colours,
-draw insects to flowers. After observing how Bees come swarming into a
-house where honey is largely exposed, or how Wasps find their way into
-a shop containing much ripe fruit, it cannot be questioned that insects
-are to a considerable extent guided by scent. Being thus sensitive
-to the aromatic substances which flowers exhale, they may, when the
-flowers are in large masses, be attracted by them from distances
-at which the flowers themselves are invisible. And manifestly, the
-flowers which so attract them from the greatest distances, increasing
-thereby their chances of efficient fertilization, will be most likely
-to perpetuate themselves. That is to say, survival of the fittest
-must tend to produce perfumes that are both more powerful and more
-attractive.
-
-These physiological differentiations, then, which mark off the
-foliar organs constituting flowers from other foliar organs, are
-the consequences of indirect equilibration. They are not due to the
-immediate actions of unlike incident forces on the parts of the
-individual plant; but they are due to the actions of such unlike
-incident forces on the aggregate of individuals, generation after
-generation.[48]
-
-
-§ 276. The unity of interpretation which we here find for phenomena
-of such various orders, could hardly be found were the phenomena
-otherwise caused. That the stronger and the feebler contrasts among the
-different parts of the outer tissues in plants, should so constantly
-occur along with stronger and feebler contrasts among the incident
-forces, is in itself weighty evidence that unlike outer actions have
-caused unlike inner actions, and correspondingly-unlike structures;
-either by changing the functional equilibrium in the individual, or by
-changing it in the race, or by both.
-
-Even in the absence of more direct proof, there would be great
-significance in the marked differences that habitually exist between
-the exposed and imbedded parts of plants, between the stems and the
-leaves, and between the upper and under surfaces of the leaves. The
-significance of these differences is increased when we discover that
-they vary in degree as the differences in the conditions vary in
-degree. Still greater becomes the force of the evidence on finding
-that these strongly-contrasted parts may, when placed in one another’s
-conditions, and kept in them from generation to generation, permanently
-assume one another’s functions, and, in a great degree, one another’s
-structures. Even more conclusive yet is the argument rendered, by the
-discovery that, where these substitutions of function and structure
-take place, the superinduced modifications differ in different
-circumstances; just as the original modifications do. The fact that
-a flattened stem simulating a vertically-growing leaf has its two
-surfaces alike, while when it simulates a horizontally-growing leaf its
-upper and under surfaces differ, is a fact which, standing alone, might
-prove little, but proves much when joined with all the other evidence.
-And its profound meaning becomes the more obvious on discovering that
-the same thing happens with petioles when they usurp leaf-functions.
-
-Finally, when we remember how rapidly analogous modifications of
-function and structure arise in the superficial tissues of individual
-plants, the general inference can scarcely be resisted. When we meet
-with so striking a case as that of the _Begonia_-leaf, a fragment
-of which stuck in the ground produces roots from its under surface
-and leaves from its upper surface--when we see that though, in this
-case, the typical structure of the plant presently begins to control
-the organizing process, yet the initial differentiations are set
-up by the differential actions of the environment; the presumption
-becomes extremely strong that the heterogeneities of surface which
-we have considered, result, as alleged, directly or indirectly from
-heterogeneities in the incident forces.
-
-
-
-
-                              CHAPTER IV.
-
-        DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS.[49]
-
-
-§ 277. In passing from plants formed of threads or thin laminæ, to
-plants having some massiveness, we find that after the external and
-internal parts have become distinguished from one another, there arise
-distinctions among the internal parts themselves, as well as among the
-external parts themselves: the primarily-differentiated parts are both
-re-differentiated.
-
-From types of very low organisation illustrations of this may be
-drawn. In the thinner kinds of _Laminaria_ there exists but the
-single contrast between the outer layer of cells and an inner layer;
-but in larger species of the same genus, as _L. digitata_, there
-are three unlike layers on each side of a central layer differing
-from them--augmentation of bulk is accompanied by multiplication of
-concentric internal structures, having their unlikenesses obviously
-related to unlikenesses in their conditions. In _Furcellaria_ and
-various _Algæ_ of similarly swollen forms, the like relation may be
-traced.
-
-Just indicating the generality of this contrast, but not attempting to
-seek in these lower types for any more specific interpretation of it,
-let us pass to the higher types. The argument will be amply enforced by
-the evidence obtained from them. We will look first at the conditions
-which they have to fulfil; and then at the ways in which the functions
-and structures adapting them to these conditions arise.
-
-
-§ 278. A terrestrial plant that grows vertically needs no marked
-modification of its internal tissues, so long as the height it reaches
-is very small. As we before saw, the spiral or cylindrical rolling
-up of a simple cellular frond, or the more bulky growth of a simple
-cellular axis, may give the requisite strength; and the requisite
-circulation may be carried on through the unchanged cellular tissue.
-But in proportion as the height to be attained and the mass to be
-supported increase, the supporting part must acquire greater bulk or
-greater density, or both; and some modification that shall facilitate
-the transfer of nutritive liquids must take place. Hence, in the
-inner tissues of plants we may expect to find that structural changes
-answering to these requirements become marked, as the growth of the
-aërial part becomes great. Facts correspond with these expectations.
-
-Among the humbler Cormophytes, which creep over or raise themselves
-but little above, the surfaces they flourish upon, there is scarcely
-any internal differentiation: the vascular and woody structures, if
-not in all cases absolutely unrepresented, are rarely and very feebly
-indicated. But among the higher types--the Ferns and Lycopodiums--which
-raise their fronds to considerable heights, there are vascular bundles
-and hard tissues like wood; and by the Tree-Ferns massive axes are
-developed. That the relation which thus shows itself among Cryptogams
-is habitual among Phænogams, scarcely needs saying.
-
-Phænogams, however, are not universally thus characterized in a decided
-way. Besides the comparative want of woody tissue in flowering plants
-of humble growth, and besides the paucity of vessels in ordinary
-water-plants, there are cases of much more marked divergence from
-this typical internal structure. These exceptional cases occur under
-exceptional conditions, and are highly instructive. They are of two
-kinds. One group of them is furnished by certain plants which are
-parasitic on the exposed roots of trees--parasitic not partially,
-as the Mistletoe, but to the extent of subsisting wholly on the sap
-they absorb. Fungus-like in colour and texture, and having scales for
-leaves, these _Balanophoræ_ and _Rafflesiaceæ_ are recognizable as
-Phænogams by scarcely any other traits than their fructifications.
-Along with their aborted leaves and absence of chlorophyll, there is
-a great degradation of those internal tissues by which Phænogams are
-commonly distinguished. Though Dr. [now Sir J.] Hooker has shown that
-they are not, as some botanists thought, devoid of spiral vessels; yet,
-as shown by the mistake previously made in classifying them, their
-appliances for circulation are rudimentary. And this trait goes along
-with a greatly-simplified distribution of nutriment. In the absence
-of leaves there can be but little down-current of sap, such as leaves
-usually supply to roots: there cannot be much beyond an upward current
-of the absorbed juices. The other cases occur where circulation is
-arrested or checked in a different way; namely, in plants that are
-wholly submerged. These are the _Podostemaceæ_. Clothing as they do
-the submerged rocks, their roots play the part of rhizomes, being
-attached to the substratum by hairs and other processes, and having
-the leaf-bearing and flower-bearing shoots on their surfaces. The
-latter spread out more or less horizontally and are also fixed to the
-substratum in the same manner as the roots. Observe then the connexion
-of facts. One of these _Podostemaceæ_ needs no internal stiffening
-substance, for it exists in a medium of its own specific gravity; and
-being in a position to absorb water over its entire surface, it has no
-need for a circulation of crude sap--nor, indeed, in the absence of
-evaporation from any part of its surface, could any active circulation
-take place. Here, accordingly, the tracheal and mechanical elements are
-undeveloped. Though spiral vessels are not entirely absent, yet they
-are so rare as to do no more than verify the inference of phænogamic
-relationship drawn from the flowers.
-
-The method of agreement, the method of difference, and the method of
-concomitant variations, thus unite in proving a direct relation between
-the demand for support and circulation, and the existence of these
-vascular woody bundles which the higher plants habitually possess.
-The question which we have to consider is--Under what influences are
-these structures, answering to these requirements, developed? How are
-these internal differentiations caused? The inquiry may be conveniently
-divided. Though the supporting tissues and the tissues concerned in the
-circulation of liquids are closely connected, and indeed entangled,
-with one another, we may fitly deal with them apart. Let us take first
-the supporting tissue.
-
-
-§ 279. Many commonplace facts indicate that the mechanical strains to
-which upright-growing plants are exposed, themselves cause increase
-of the dense deposits by which such plants are enabled to resist
-such strains. There is the fact that the massiveness of a tree-trunk
-varies according to the stress habitually put upon it. If the contrast
-between the slender stem of a tree growing in a wood and the bulky stem
-of a kindred tree growing in the fields, be ascribed to difference
-of nutrition rather than difference of exposure to winds; there is
-still the fact that a tree trained against a wall has a less bulky
-stem than a tree of the same kind growing unsupported; and that
-between the long weak branches of the one and the stiff ones of the
-other there are decided contrasts. If it be objected that a tree so
-trained and branches so borne have relatively less foliage, and that
-therefore these unlikenesses also are due to unlikenesses of general
-nutrition, which may in part be true; there are still such cases as
-those of garden plants, which when held up by tying them to sticks
-have weaker stems than when they are unpropped, and sink down if their
-props are taken away. Again, there is the evidence supplied by roots.
-Though the contrast between the feeble roots of a sheltered tree
-and the strong roots of an exposed tree, may, like the contrast of
-their stems, be mainly due to difference of nutrition, and therefore
-supplies but doubtful evidence, we get tolerably clear evidence where
-trees growing on inclined rocky surfaces, send into crevices that
-afford little moisture or nutriment, roots which nevertheless become
-thick where they are so directed as to bear great strains. Suspicion
-thus raised is strengthened into conviction by special evidences
-occurring in the places where they are to be expected. The Cactuses,
-with their succulent growths that pass into woody growths slowly
-and irregularly, give us the opportunity of tracing the conditions
-under which the wood is formed. Good examples occur in the genus
-_Cereus_, and especially in forms like _C. crenulatus_. Here,
-from a massive vertically-growing rod of fleshy tissue, two inches
-or more in diameter, there grow at intervals lateral rods similarly
-bulky, which, quickly curving themselves, take vertical directions.
-One of these heavy branches puts great strains on its own substance
-and that of the stem at their point of junction; and here both of them
-become brown and hard, while they continue green and succulent all
-around. Such differentiations may be traced internally before they
-are visible on the surface. If a joint of an _Opuntia_ be sliced
-through longitudinally, the greater resistance to the knife all around
-the narrow neck, indicates there a larger deposit of lignin than
-elsewhere; and a section of the tissue placed under the microscope,
-exhibits at the narrowest part a concentration of the woody and
-vascular bundles. Clear evidence of another kind has been noted by
-Mr. Darwin, in the organs of attachment of climbing plants. Speaking
-of _Solanum jasminoides_ he says:--“When the flexible petiole
-of a half-or a quarter-grown leaf has clasped any object, in three
-or four days it increases much in thickness, and after several weeks
-becomes wonderfully hard and rigid; so that I could hardly remove one
-from its support. On comparing a thin transverse slice of this petiole
-with one from the next or older leaf beneath, which had not clasped
-anything, its diameter was found to be fully doubled, and its structure
-greatly changed.... This clasped petiole had actually become thicker
-than the stem close beneath; and this was chiefly due to the greater
-thickness of the ring of wood, which presented, both in transverse
-and longitudinal sections, a closely similar structure in the petiole
-and axis. The assumption by a petiole of this structure is a singular
-morphological fact; but it is a still more singular physiological fact
-that so great a change should have been induced by the mere act of
-clasping a support.”
-
-If there is a direct relation between mechanical stress and the
-formation of wood, it ought to explain for us the internal distribution
-of the wood. Let us see whether it does this.
-
-When seeking in mechanical actions and reactions the cause of that
-indurated structure which forms the vertebrate axis (§§ 254–7), it
-was pointed out that in a transversely-strained mass, the greatest
-pressures and tensions are thrown on the molecules of the concave and
-convex surfaces. Hence, supposing the transversely-strained mass to
-be a cylinder, bent backwards and forwards not in one plane but now
-in this plane and now in that, its peripheral layers will be those on
-which the greatest stress falls. An ordinary dicotyledonous axis is
-such a cylinder so strained. The maintenance of its attitude either as
-a lateral shoot or a vertical shoot, implies subjection to the bendings
-caused by its own weight and by the ever-varying wind. These bendings
-imply tensions and pressures falling most severely first on one side of
-its outer layers and then on another. And if the dense substance able
-to resist these tensions and pressures is deposited most where they are
-greatest, we ought to find it taking the shape of a cylindrical casing.
-This is just what we do find. On cutting across a shoot in course of
-formation, we see its central space either unoccupied or occupied only
-by soft tissue. That the layer of hard tissue surrounding this is not
-the outermost layer, is true: there lies beyond it the cambium layer,
-from which it is formed, the phloëm, and the cortex. But outside of the
-soft phloëm there is frequently another layer of dense tissue now known
-as the pericyclic fibres, having frequently a tenacity greater even
-than that of the wood--a layer which, while it protects the cambium
-and offers additional resistance to the transverse strain, admits of
-being fissured as fast as the cylinder of wood thickens. That is to
-say, the deposit of resisting substance is as completely peripheral as
-the exogenous mode of growth permits. So, too, in general arrangement
-is it with the ordinary monocotyledonous stem. Different as is here
-the internal structure, there yet holds the same general distribution
-of tissues, answering to the same mechanical conditions. The vascular
-woody bundles, more abundant towards the outside of the stem than
-near the centre, produce a harder casing surrounding a softer core.
-In the supporting structures of leaves we find significant deviations
-from this arrangement. While axes are on the average exposed to equal
-strains on all sides, most leaves, spreading out their surfaces
-horizontally, have their petioles subject to strains that are not alike
-in all directions; and in them the hard tissue is differently arranged.
-Its transverse section is not ring-shaped but crescent-shaped: the two
-horns being directed towards the upper surface of the petiole. That
-this arrangement is one which answers to the mechanical conditions,
-is not easy to demonstrate: we must satisfy ourselves by noting that
-here, where the distribution of forces is different, the distribution
-of resisting tissue is different. And then, showing conclusively the
-connexion between these differences, we have the fact that in petioles
-growing vertically and supporting peltate leaves--petioles which are
-therefore subject to equal transverse strains on all sides--the
-vascular bundles are arranged cylindrically, as in axes.
-
-Such, then, are some of the reasons for concluding that the development
-of the supporting tissue in plants, is caused by the incident forces
-which this tissue has to resist. The individuals in which this direct
-balancing of inner and outer actions progresses most favourably, are
-those which, other things equal, are most likely to prosper; and, by
-habitual survival of the fittest, there is established a systematic and
-constant distribution of a deposit adapted to the circumstances of each
-type.
-
-
-§ 280. The function of circulation may now be dealt with. We have to
-consider here by what structures this is discharged; and what connexion
-exists between the demand for them and the genesis of them.
-
-The contrast between the rates at which a dye passes through simple
-cellular tissue and cellular tissue of which the units have been
-elongated, indicates one of the structural changes required to
-facilitate circulation. If placed with its cut surface in a coloured
-liquid, the parenchyma of a potato or the medullary mass of a
-cabbage-stalk, will absorb the liquid with extreme slowness; but if
-the stalk of a fungus be similarly placed, the liquid runs up it, and
-especially up its loose central substance, very quickly. On comparing
-the tissues which thus behave so differently, we find that whereas in
-the one case the component cells, packed close together, have deviated
-from their primitive sphericity only as much as mutual pressure
-necessitates, in the other case they are drawn out into long tubules
-with narrow spaces among them--the greatest dimensions of the tubules
-and the spaces being in the direction which the dye takes so rapidly.
-That which we should infer, then, from the laws of capillary action,
-is experimentally shown: liquid moving through tissues follows the
-lines in which the elements of the tissues are most elongated. It does
-this for two reasons. That narrowing of the cells and intercellular
-spaces which accompanies their elongation, facilitates capillarity;
-and at the same time fewer of the septa formed by the joined ends of
-the cells have to be passed through in a given distance. Hence the
-general fact that the establishment of a rudimentary vascular system,
-is the formation of bundles of cells lengthened in the direction
-which the liquid is to take. This we see very obviously among the
-lower Cormophytes. In one of the lichen-like Liverworts, the veins
-which, branching through its frond, serve as communications with its
-scattered rootlets, are formed of cells longer than those composing the
-general tissue of the frond: the lengths of these cells corresponding
-in their directions with the lengths of the veins. So, too, is it
-with the mid-ribs of such fronds as assume more definite shapes; and
-so, too, is it with the creeping stems which unite many such fronds.
-That is to say, the current which sets towards the growing part from
-the part which supplies certain materials for growth, sets through
-a portion of the tissues composed of units that are longer in the
-line of the current than at right angles to that line. The like is
-true of Phænogams. Omitting all other characteristics of those parts
-of them through which chiefly the currents of sap flow, we find the
-uniform fact to be that they consist of cells and intercellular spaces
-distinguished from others by their lengths. It is thus with veins,
-and mid-ribs, and petioles; and if we wish proof that it is thus with
-stems, we have but to observe the course taken by a coloured solution
-into which a stem is inserted.
-
-What is the original cause of this differentiation? Is it possible
-that this modification of cell-structure which favours the transfer of
-liquid towards each place of demand, is itself caused by the current
-which the demand sets up? Does the stream make its own channel? There
-are various reasons for thinking that it does. In the first place,
-the simplest and earliest channels, such as we see in the Liverworts,
-do not develop in any systematic way, but branch out irregularly,
-following everywhere the irregular lobes of the fronds as these spread;
-and on examining under a magnifier the places at which the veins are
-lost in the cellular tissue, it will be seen that the cells are there
-slightly longer than those around: suggesting that the lengthening of
-them which produces an extension of the veins, takes place as fast
-as the growth of the tissue beyond causes a current to pass through
-them. In the second place, a disappearance of the granular contents of
-these cells accompanies their union into a vein--a result which the
-transmission of a current may not improbably bring about. But be the
-special causes of this differentiation what they may, the evidence
-favours very much the conclusion that the general cause is the setting
-up of a current towards a place where the sap is being consumed. In the
-histological development of the higher plants we find confirmation.
-The more finished distributing canals in Phænogams are formed of cells
-previously lengthened. At parts of which the typical structure is
-fixed, and the development direct, this fact is not easy to trace;
-the cells rapidly take their elongated structures in anticipation of
-their predetermined functions. But in places where new vessels are
-required in adaptation to a modifying growth, we may clearly trace
-this succession. The swelling root of a turnip, continually having its
-vascular system further developed, and the component vessels lengthened
-as well as multiplied, gives us an opportunity of watching the process.
-In it we see that the reticulated cells which unite to form ducts,
-arise in the midst of bundles of cells that have previously become
-elongated, and that they arise by transformation of such elongated
-cells; and we also see that these bundles of elongated cells have an
-arrangement suggestive of their formation by passing currents.
-
-Are there grounds for thinking that these further transformations by
-which strings of elongated cells pass into vessels lined with spiral,
-annular, reticulated, or other frameworks, are also in any way
-determined by the currents of sap carried? There are some such grounds.
-
-As just indicated, the only places where we may look for evidence
-with any rational hope of finding it, are places where some local
-requirement for vessels has arisen in consequence of some local
-development which the type does not involve. In these cases we
-find such evidence. Good illustrations occur in those genera of
-the _Cactaceæ_, which simulate leaves, like _Epiphyllum_ and
-_Phyllocactus_. A branch of one of these is outlined in Fig. 256. As
-before explained this is a flattened axis; and the notches along its
-edges are the seats of the axillary buds. Most of these axillary buds
-are arrested; but occasionally one of them grows. Now if, taking an
-_Epiphyllum_-shoot which bears a lateral shoot, we compare the parts
-of it that are near the aborted axillary buds with the part that is
-near the developed axillary bud, we find a conspicuous difference. In
-the neighbourhood of an aborted axillary bud there is no external sign
-of any internal differentiation; and on holding up the branch against
-the light, the uniform translucency shows that there is no greater
-amount of dense tissue near it than in other parts of the succulent
-mass. But where an axillary bud has developed, a prominent rounded
-ridge joins the mid-rib of the lateral branch with the mid-rib of the
-parent branch. In the midst of this rounded ridge an opaque core may be
-seen. And on cutting through it, this opaque core proves to be full of
-vascular bundles imbedded in woody deposits. Clearly, these clusters
-of vessels imply transformations of the tissues, caused by the passage
-of increased currents of sap. The vessels were not there when the
-axillary bud was formed; they would not have developed had the axillary
-bud proved abortive; but they arise as fast as growth of the axillary
-bud draws the sap along the lines in which they lie. Verification
-is obtained by examining the internal structures. If longitudinal
-sections be made through a growing bud of _Opuntia_ or _Cereus_, it
-will be found that the vessels in course of formation converge towards
-the point of growth, as they would do if the sap-currents determined
-their formation; that they are most developed near their place of
-convergence, which they would be if so produced; and that their
-terminations in the tissue of the parent shoot are partially-formed
-lines of irregular elongated cells, like those out of which the vessels
-of a leaf or bud are developed.
-
-Concluding, then, that sap-vessels arise along the lines of least
-resistance, through which currents are drawn or forced, the question
-to be asked is--What physical process produces them? Their component
-cells, united end to end more or less irregularly in ways determined
-by their original positions, form a channel much more permeable, both
-longitudinally and laterally, than the tissue around. How is this
-greater permeability caused? The idea, first propounded I believe by
-Wolff, that the adjoined ends of the cells are perforated or destroyed
-by the passing current, is one for which much is to be said. Whether
-these septa are dissolved by the liquids they transmit, or whether they
-are burst by those sudden gushes which, as we shall hereafter see, must
-frequently take place along these canals, need not be discussed: it is
-sufficient for us that the septa do, in many cases, disappear, leaving
-internal ridges showing their positions; and, in other cases, become
-extremely porous. Though it is manifest that this is not the process
-of vascular development in tissues that unfold after pr-determined
-types, since, in these, the dehiscences or perforations of septa occur
-before such direct actions can have come into play; yet it is still
-possible that the disappearances of septa which now arise by repetition
-of the type were established in the type by such direct actions. Be
-this as it may, however, a simultaneous change undergone by these
-longitudinally-united cells must be otherwise caused. Frame-works are
-formed in them--frameworks which, closely fitting their inner surfaces,
-may consist either of successive rings, or continuous spiral threads,
-or networks, or structures between spirals and networks, or networks
-with openings so far diminished that the cells containing them are
-distinguished as fenestrated. Their differences omitted, however, these
-structures have the common character that, while supporting the coats
-of the vessels, they also give special facilities for the passage of
-liquids, both through the sides of the transformed cells and through
-their united ends, where these are not destroyed.
-
-To attempt any physical interpretation of this change is scarcely
-safe: the conditions are so complex. There are reasons for suspecting,
-however, that it arises from a vacuolation of the substance deposited
-on the cell-wall. If rapidly deposited, as it is likely to be along
-lines where sap is freely supplied, this may, in passing from the
-state of a soluble colloid to that of an insoluble colloid, so
-contract as to leave uncovered spaces on the cell-membrane; and this
-change, originally consequent on a physico-chemical action, may be
-so maintained and utilized by natural selection, as to result in
-structures of definite kinds, regularly formed in growing parts in
-anticipation of functions to be afterwards discharged. But, without
-alleging any special cause for this metamorphosis, we may reasonably
-conclude that it is in some way consequent upon the carrying of sap.
-If we examine tissues such as that in the interior of a growing
-turnip that has not yet become stringy, we may, in the first place,
-find bundles of elongated cells not having yet developed in them
-those fenestrated or reticulated structures by which the ducts are
-eventually characterized. Along the centres of adjacent bundles we may
-find incomplete lines of such cells--some that are partially or wholly
-transformed, with some between them that are not transformed. In other
-bundles, completed chains of such transformed cells are visible. And
-then, in still older bundles, there are several complete chains running
-side by side. All which facts imply a metamorphosis of the elongated
-cells, indirectly caused by the continued action of the currents
-carried.
-
-
-§ 281. Here, however, presents itself a further problem. Taking
-it as manifest that there is a typical distribution of supporting
-tissue adapted to meet the mechanical strains a plant is exposed to
-by its typical mode of growth, and also that there goes on special
-adaptation of the supporting tissue to the special strains the
-individual plant has to bear; and taking it as tolerably evident that
-the sap-channels are originally determined by the passage of currents
-along lines of least resistance; there still remains the ultimate
-question--Through what physical actions are established these general
-and special adjustments of supporting tissue to the strains borne,
-and these distributions of nutritive liquid required to make possible
-such adjustments? Clearly, if the external actions produce internal
-reactions; and if this play of actions and reactions results in a
-balancing of the strains by the resistances; we may rationally suspect
-that the incident forces are directly conducive to the structural
-changes by which they are met. Let us consider how they must work.
-
-When any part of a plant is bent by the wind, the tissues on its convex
-surface are subject to longitudinal tension, and these extended outer
-layers compress the layers beneath them. Such of the vessels or canals
-in these subjacent layers as contain sap, must have some of this sap
-expelled. Part of it will be squeezed through the more or less porous
-walls of the canals into the surrounding tissue, thus supplying it with
-assimilable materials; while part of it, and probably the larger part,
-will be thrust along the canals longitudinally upwards and downwards.
-When the branch or twig or leaf-stalk recoils, these vessels, relieved
-from pressure, expand to their original diameters. As they expand, the
-sap rushes back into them from above and below. In whichever of these
-directions least has been expelled by the compression, from that
-direction most must return during the dilation; seeing that the force
-which more efficiently resisted the thrusting back of the sap is the
-same force which urges it into the expanded vessels again, when they
-are relieved from pressure. At the next bend of the part a further
-portion of sap will be squeezed out, and a further portion thrust
-forwards along the vessels. This rude pumping process thus serves for
-propelling the sap to heights which it could not reach by capillary
-action, at the same time that it incidentally serves to feed the parts
-in which it takes place. It strengthens them, too, just in proportion
-to the stress to be borne; since the more severe and the more repeated
-the strains, the greater must be the exudation of sap from the vessels
-or ducts into the surrounding tissue, and the greater the thickening
-of this tissue by secondary deposits. By this same action the movement
-of the sap is determined either upwards or downwards, according to the
-conditions. While the leaves are active and evaporation is going on
-from them, these oscillations of the branches and petioles urge forward
-the sap into them; because so long as the vessels of the leaves are
-being emptied, the sap in the compressed vessels of the oscillating
-parts will meet with less resistance in the direction of the leaves
-than in the opposite direction. But when evaporation ceases at night,
-this will no longer be the case. The sap drawn to the oscillating
-parts, to supply the place of the exuded sap, must come from the
-directions of least resistance. A slight breeze will bring it back from
-the leaves into the gently-swaying twigs, a stronger breeze into the
-bending branches, a gale into the strained stem and roots--roots in
-which longitudinal tension produces, in another way, the same effects
-that transverse tension does in the branches.
-
-Two possible misinterpretations must be guarded against. It is not to
-be supposed that this force-pump action causes movement of the sap
-towards one point rather than another: it is simply an aid to its
-movement. From the stock of sap distributed through the plant, more
-or less is everywhere being abstracted--here by evaporation, here by
-the unfolding of the parts into their typical shapes, here by both. The
-result is a tension on the contained liquid columns, which is greatest
-now in this direction and now in that. This tension it is which must be
-regarded as the force that determines the current upwards or downwards;
-and all which the mechanical actions do is to facilitate the transfer
-to the places of greatest demand. Hence it happens that in a plant
-prevented from oscillating, but having a typical tendency to assume
-a certain height and bulk, the demands set up by its unfolding parts
-will still cause currents; and there will still be alternate ascents
-and descents, according as the varying conditions change the direction
-of greatest demand--the only difference being that, in the absence of
-oscillations, the growth will be less vigorous. Similarly, it must not
-be supposed that mechanical actions are here alleged to be the sole
-causes of wood formation in the individual plant. The tendency of the
-individual plant to form wood at places where wood has been habitually
-formed by ancestral plants, is manifestly a cause, and, indeed, the
-chief cause. In this, as in all other cases, inherited structures
-repeat themselves irrespective of the circumstances of the individual:
-absence of the appropriate conditions resulting simply in imperfect
-repetition of the structures. Hence the fact that in trained trees and
-hothouse shrubs, dense substance is still largely deposited; though
-not so largely as where the normal mechanical strains have acted.
-Hence, too, the fact, that in such plants as the Elephant’s-foot or the
-_Welwitschia mirabilis_, which for untold generations can have
-undergone no oscillations, there is an extensive formation of wood
-(though not to any considerable height above the ground), in repetition
-of an ancestral type: natural selection having here maintained the
-habit as securing some other advantage than that of support.
-
-Still, it must be borne in mind that though intermittent mechanical
-strains cannot be assigned as the direct causes of these internal
-differentiations in plants that are artificially sheltered or
-supported, they are assignable as the indirect causes; since the
-inherited structures, repeated apart from such strains, are themselves
-interpretable as accumulated results of such strains acting on
-successive generations of ancestral plants. This will become clear on
-combining the several threads of the argument and bringing it to a
-close, which we may now do.
-
-
-§ 282. To put the co-operative actions in their actual order,
-would require us to consider them as working on individuals small
-modifications that become conspicuous and definite only by inheritance
-and gradual increase; but it will aid our comprehension without leading
-us into error, if we suppose the whole process resumed in a single
-continuously-existing plant.
-
-As the plant erects the integrated series of fronds whose united
-parts form its rudimentary axis, the increasing area of frond surface
-exposed to the sun’s rays entails an increasing draught upon the
-liquids contained in the rudimentary axis. The currents of sap so
-produced, once established along certain lines of cells that offer
-least resistance, render them by their continuous passage more and
-more permeable. This establishment of channels is aided by the wind.
-Each bend produced by it while yet the tissue is undifferentiated,
-squeezes towards the place of growth and evaporation the liquids
-that are passing by osmosis from cell to cell; and when the lines of
-movement become defined, each bend helps, by forcing the liquid along
-these lines, to remove obstructions and make continuous canals. As fast
-as this transfer of sap is facilitated, so fast is the plant enabled
-further to raise itself, and add to its assimilating surfaces; and so
-fast do the transverse strains, becoming greater, give more efficient
-aid. The canals thus formed can be neither in the centre of the
-rudimentary axis nor at its surface: for at neither of these places
-can the transverse strains produce any considerable compressions.
-They must arise along a tract between the outside of the axis and its
-core--a tract along which there occur the severest squeezes between the
-stretched outer layers and the internal mass. Just that distribution
-which we find, is the distribution which these mechanical actions tend
-to establish.
-
-As the plant gains in height, and as the mass of its foliage
-accumulates, the strains thrown upon its axis, and especially the
-lower part of its axis, rapidly increase. Supposing the forms to
-remain similar, the strains must increase in the ratio of the cubes of
-the dimensions; or even in a somewhat higher ratio. One consequence
-must be that the compressions to which the vessels at the lower
-part of the incipient stem are subject, become greater as fast as
-the height to which the sap has to be raised becomes greater; and
-another consequence must be that the local exudation of sap produced
-by the pressure is proportionately augmented. Hence the materials for
-interstitial nutrition being there supplied more abundantly, we may
-expect thickening of the surrounding tissues to show itself there
-first: in other words, wood will be formed round the vessels of the
-lower part of the incipient stem. The resulting greater ability of this
-lower part of the stem to bear strains, renders possible an increase of
-height; and while after an increase of height the lowest part becomes
-still further strained, and still further thickens, the part above it,
-exposed to like actions, undergoes a like thickening. This induration,
-while it spreads upwards, also spreads outwards. As fast as the rude
-cylinder of dense matter formed in this way, begins to inclose the
-original vessels, it begins to play the part of a resistant mass, which
-more and more prevents the contained vessels from being squeezed;
-while between it and the outer layers the greatest compression occurs
-at each bend. Thus at the same time that the original vessels become
-useless, the peripheral cells of the developing wood become those which
-have their liquid contents squeezed out longitudinally and laterally
-with increasing force; and, consequently, amid them are formed new
-sap-channels, from which there is the most active local exudation,
-producing the greatest deposit of dense matter.
-
-Thus fusing together, as it were, the individualities of successive
-generations of plants, and recognizing as all-important that
-facilitation of the process which natural selection has all along
-given, we are enabled to interpret the chief internal differentiations
-of plants as consequent on an equilibration between inner and outer
-forces. Here, indeed, we see illustrated in a way more than usually
-easy to follow, the eventual balancing of outer actions by inner
-reactions. The relation between the demand for liquid and the formation
-of channels that supply liquid, as well as that between the incidence
-of strains and the deposit of substance which resists strains, are
-among the clearest special examples of the general truth that the
-moving equilibrium of an organism, if not overthrown by an incident
-force, must eventually be adjusted to it.
-
-The processes here traced out are, of course, not to be taken as the
-only differentiating processes to which the inner tissues of plants
-have been subject. Besides the chief changes we have considered,
-various less conspicuous changes have taken place. These must be
-passed over as arising in ways too involved to admit of specific
-interpretations; even supposing them to have been produced by causes of
-the kind assigned. But the probability, or rather indeed the certainty,
-is that some of them have not been so produced. Here, as in nearly
-all other cases, indirect equilibration has worked in aid of direct
-equilibration; and in many cases indirect equilibration has been the
-sole agency. Besides ascribing to natural selection the rise of various
-internal modifications of other classes than those above treated, we
-must ascribe some even of these to natural selection. It is so with the
-dense deposits which form thorns and the shells of nuts: these cannot
-have resulted from any inner reactions immediately called forth by
-outer actions; but must have resulted immediately through the effects
-of such outer actions on the species. Let it be understood, therefore,
-that the differentiations to which the foregoing interpretation
-applies, are only those most conspicuous ones which are directly
-related to the most conspicuous incident forces. They must be taken as
-instances on the strength of which we may conclude that other internal
-differentiations have had a natural genesis, though in ways that we
-cannot trace.
-
-
-
-
-                              CHAPTER V.
-
-                 PHYSIOLOGICAL INTEGRATION IN PLANTS.
-
-
-§ 283. A good deal has been implied on this topic in the preceding
-chapters. Here, however, we must for a brief space turn our attention
-immediately to it.
-
-Plants do not display integration in such distinct and multiplied
-ways as do animals. But its advance may be traced both directly and
-indirectly--directly in the increasing co-ordination of actions, and
-indirectly in the effect of this upon the powers and habits.
-
-Let us group the facts under these heads: ascending in both cases from
-the lower to the higher types.
-
-
-§ 284. The inferior _Algæ_, along with little unlikeness of parts,
-show us little mutual dependence of parts. Having surfaces similarly
-circumstanced everywhere, much physiological division of labour
-cannot arise; and therefore there cannot be much physiological unity.
-Among the superior _Algæ_, however, the differentiation between the
-attached part and the free part is accompanied by some integration.
-There is evidently a certain transfer of materials, which is doubtless
-facilitated by the elongated forms of the cells in the stem, and
-probably leads to the formation of dense tissue at the places of
-greatest strain, in a way akin to that recently explained in other
-cases. And where there is this co-ordination of actions, the parts are
-so far mutually dependent that each dies if detached from the other.
-That though the organization is so low neither part can reproduce the
-other and survive by so doing, is probably due to the circumstance
-that neither part contains any considerable stock of untransformed
-protoplasm, out of which new tissues may be produced.
-
-Fungi and Lichens present no very significant advances of integration.
-We will therefore pass at once to the Archegoniates. In those of them
-which, either as single fronds or strings of fronds, spread over
-surfaces, and which, rooting themselves as they spread, do not need
-that each part should receive aid from remote parts, there is no
-developed vascular system serving to facilitate transfer of nutriment:
-the parts being little differentiated there is but little integration.
-But along with assumption of the upright attitude and the accompanying
-specializations, producing vessels for distributing sap and hard tissue
-for giving mechanical support, there arises a decided physiological
-division of labour; rendering the aërial part dependent on the embedded
-part and the embedded part dependent on the aërial part. Here, indeed,
-as elsewhere, these concomitant changes are but two aspects of the
-same change. Always the gain of power to discharge a special function
-involves a loss of power to perform other functions; and always,
-therefore, increased mutual dependence constituting physiological
-integration, must keep pace with that increased fitting of particular
-parts to particular duties which constitutes physiological
-differentiation.
-
-Making a great advance among the Archegoniates, this physiological
-integration reaches its climax among Phænogams. In them we see
-interdependence throughout masses that are immense. Along with
-specialized appliances for support and transfer, we find an exchange of
-aid at great distances. We see roots giving the vast aërial growth a
-hold tenacious enough to withstand violent winds, and supplying water
-enough even during periods of drought; we see a stem and branches of
-corresponding strength for upholding the assimilating organs under
-ordinary and extraordinary strains; and in these assimilating organs
-we see elaborate appliances for yielding to the stem and roots the
-materials enabling them to fulfil their offices. As a consequence of
-which greater integration accompanying the greater differentiation,
-there is ability to maintain life over an immense period under marked
-vicissitudes.
-
-Even more conspicuously exemplified in Phænogams, is that physiological
-integration which holds together the functions not of the individual
-only but of the species as a whole. The organs of reproduction, both
-in their relations to other parts of the individual bearing them and
-in their relations to corresponding parts of other individuals, show
-us a kind of integration conducing to the better preservation of the
-race; as those already specified conduce to the better preservation
-of the individual. In the first place, this greater co-ordination of
-functions just described, itself enables Phænogams to bequeath to
-the germs they cast off, stores of nutriment, protective envelopes,
-and more or less of organization: so giving them greater chances of
-rooting themselves. In the second place, certain differentiations
-among the parts of fructification, the meaning of which Mr. Darwin
-has so admirably explained, give to the individuals of the species a
-kind of integration that makes possible a mutual aid in the production
-of vigorous offspring. And it is interesting to observe how, in
-that dimorphism by which in some cases this mutual aid is made more
-efficient, the greater degree of integration is dependent on the
-greater degree of differentiation--not simply differentiation of
-the fructifying organs from other parts of the plant bearing them,
-but differentiation of these fructifying organs from the homologous
-organs of neighbouring individuals of the same race. Another form of
-this co-ordination of functions which conduces to the maintenance of
-the species, may be here named--partly for its intrinsic interest.
-I refer to the strange processes of multiplication occurring in the
-genus _Bryophyllum_. It is well known that the succulent leaves of
-_B. calycinum_, borne on foot-stalks so brittle that they are easily
-snapped by the wind, send forth from their edges when they fall to the
-ground, buds which root themselves and grow into independent plants.
-The correlation here obviously furthering the preservation of the race,
-is more definitely established in another species of the genus--_B.
-proliferum_. This plant, shooting up to a considerable height, and
-having a stem containing but little woody fibre, habitually breaks
-near the bottom while still in flower; and is thus generally prevented
-from ripening its seeds. The multiplication is, however, secured in
-another way. Before the stem is broken young plants have budded out
-from the pedicels of the flowers, and have grown to considerable
-lengths; and on the fall of the parent they forthwith commence their
-separate lives. Here natural selection has established a remarkable
-kind of co-ordination between a special habit of growth and decay, and
-a special habit of proliferation.
-
-
-§ 285. The advance of physiological integration among plants as we
-ascend to the higher types, is implied by their greater constancy of
-structure, as well as by the stricter limitations of their habitats and
-modes of life. “Complexity of structure is generally accompanied with a
-greater tendency to permanence in form,” says Dr. [now Sir J.] Hooker;
-or, conversely, “the least complex are also the most variable.” This is
-the second aspect under which we have to contemplate the facts.
-
-The differences between the simpler _Algæ_ and _Fungi_ are so feebly
-marked that botanists have had great difficulty in framing definitions
-of these classes. This structural indefiniteness is accompanied by
-functional indefiniteness. _Algæ_, which are mostly aquatic, include
-many small forms that frequent the damp places preferred by _Fungi_.
-Among _Fungi_, there are kinds which lead submerged lives like the
-_Algæ_. Besides this indistinctness of the classes, there is great
-variability in the shapes and modes of life of their species--a
-variability so great that what were at first taken to be different
-species, or different genera, or even different orders, have proved
-to be merely varieties of one species. So inconstant in structure are
-the _Algæ_ that Schleiden quotes with approval the opinion of Kutzing,
-that “there are no species but merely forms of _Algæ_:” an opinion
-which though now rejected sufficiently implies extreme indefiniteness.
-In all which facts we see that these lowest types of plants, little
-differentiated, are also but little integrated.
-
-Archegoniates present a like relation between the small specialization
-of functions which constitutes physiological differentiation, and
-the small combination of functions which constitutes physiological
-integration. “Mosses,” says Mr. Berkeley, “are no less variable than
-other cryptogams, and are therefore frequently very difficult to
-distinguish. Not only will the same species exhibit great diversity
-in the size, mode of branching, form and nervation of the leaves, but
-the characters of even the peristome itself are not constant.” And
-concerning the classification of the remaining group, _Filicales_,
-he says:--“Not only is there great difficulty in arranging ferns
-satisfactorily, but it is even more difficult to determine the limits
-of species.”
-
-After this vagueness of separation as well as inconstancy of structure
-and habit among the lower plants, the stability of structure and
-habit and divisibility of groups among the higher plants, appear
-relatively marked. Though Phænogams are much more variable than most
-botanists have until lately allowed, yet the definitions of species and
-genera may be made with far greater precision, and the forms are far
-less capable of change, than among Cryptogams. And this comparative
-fixity of type, implying, as it does, a closer combination of the
-component functions, we see to be the accompaniment of the greater
-differentiation of those functions and of the structures performing
-them. That these characters are correlatives is further shown by the
-fact that the higher plants are more restricted in their habitats
-than the lower plants, both in space and time. “The much narrower
-delimitation in area of animals than plants,” says Sir J. Hooker, “and
-greater restriction of Faunas than Floras, should lead us to anticipate
-that plant-types are, geologically speaking, more ancient and permanent
-than the higher animal types are, and so I believe them to be, and I
-would extend the doctrine even to plants of highly complex structure.”
-“Those classes and orders which are the least complex in organization
-are the most widely distributed.”
-
-
-§ 286. Thus that which the general doctrine of evolution leads us
-to anticipate, we find implied by the facts. The physiological
-division of labour among parts, can go on only in proportion to the
-mutual dependence of parts; and the mutual dependence of parts can
-progress only as fast as there arise structures by which the parts are
-efficiently combined, and the mutual utilization of their actions made
-easy.
-
-To say definitely by what process is brought about this co-ordination
-of functions which accompanies their specialization, is hardly
-practicable. Direct and indirect equilibration doubtless co-operate
-in establishing it. We may see, for example, that every increase of
-fitness for function produced in the aërial part of a plant by light,
-as well as every increase of fitness for function produced in its
-imbedded part by the direct action of the moist earth, must conduce
-to an increased current of the liquid evaporated from the one and
-supplied by the other--must serve, therefore, to aid the formation
-of sap-channels in the ways already described; that is--must serve
-to develop the structures through which mutual aid of the parts is
-given: the additional differentiation tends immediately to bring
-about the additional integration. Contrariwise, it is obvious that
-an interdependence such as we see between the secretion of honey and
-the fertilization of germs, or between the deposit of albumen in the
-cotyledons of an embryo-plant and its subsequent striking root, is
-a kind of integration in the actions of the individual or of the
-species, which no differentiation has a direct tendency to initiate.
-Hence we must regard the total results as due to a plexus of influences
-acting simultaneously on the individual and on the species: some
-chiefly affecting the one and some chiefly affecting the other.
-
-       *       *       *       *       *
-
-[NOTE.--In _Nature_ for June 11, 1896, Dr. Maxwell Masters, in an
-essay on “Plant Breeding,” names an instructive fact concerning the
-production of varieties by selection of slightly divergent forms. He
-says:--
-
-“To the untrained eye, the primordial differences noted are often very
-slight; even the botanist, unless his attention be specially directed
-to the matter, fails to see minute differences which are perceptible
-enough to the raiser or his workmen. Nor must it be thought that these
-variations, difficult as they are to recognise in the beginning, are
-unimportant. On the contrary, they are interesting, physiologically,
-as the potential origin of new species, and very often they are
-commercially valuable also. These apparently trifling morphological
-differences are often associated with physiological variations which
-render some varieties, say of wheat, much better enabled to resist
-mildew and disease generally than others. Some, again, prove to be
-better adapted for certain soils or for some climates than others; some
-are less liable to injury from predatory birds than others, and so on.”
-
-Thus we are shown that, to a much greater degree than might be
-supposed, minute changes of forms and functions in one part of a plant
-are correlated with changes of forms and functions throughout it. The
-interdependence--that is to say, the physiological integration--is very
-close at the same time that it is very complex.
-
-Here while naming these facts in illustration of physiological
-integration in plants I name them because they illustrate an important
-truth bearing upon the general question of heredity which I have dealt
-with in Appendix G, and to which I now especially draw attention.]
-
-
-
-
-                              CHAPTER VI.
-
-   DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF ANIMALS.
-
-
-§ 287. What was said respecting the primary physiological
-differentiation in plants, applies with little beyond change of terms
-to animals. Among _Protozoa_, as among _Protophyta_, the first definite
-contrast of parts is that between outside and inside. The speck of
-jelly or sarcode which appears to constitute the simplest animal,
-proves, on closer examination, to be a mass of substance containing a
-nucleus--a periplast in the midst of which there is a minute endoplast,
-consisting of a spherical membrane and its contents.
-
-This parallel, only just traceable among these Rhizopods, which are
-perpetually changing the distribution of their outer substance, becomes
-at once marked in those higher _Protozoa_ which have fixed shapes,
-and maintain constant relations between their surfaces and their
-environments. Indeed the Rhizopods themselves, on passing into a state
-of quiescence in which the relations of outer and inner parts are
-fixed, become encysted: there is formed a hardened outer coat different
-from the matter which it contains. And what is here a temporary
-character answering to a temporary definiteness of conditions, is
-in the _Infusoria_ a constant character, answering to definite
-conditions that are constant. Each of these minute creatures, though
-not coated by a distinct membrane, has an outer layer of excreted
-substance forming a delicate cuticle.
-
-
-§ 288. The early establishment of this primary contrast of tissues
-answering to this primary contrast of conditions, is no less
-conspicuous in aggregates of the second order. The feebly-integrated
-units of a Sponge, with individualities so little merged in that of
-the whole they form that most of them still retain their separate
-activities, nevertheless show us, in the unlikeness that arises between
-the outermost layer and the contained mass, the effect of converse with
-unlike conditions. This outermost layer is composed of units somewhat
-flattened and united into a continuous membrane--a kind of rudimentary
-skin.
-
-Secondary aggregates in which the lives of the units are more
-subordinate to the life of the whole, carry this distinction further.
-The leading physiological trait of every cœlenterate animal is the
-divisibility of its substance into endoderm and ectoderm--the part
-next the food and the part next the environment. Fig. 147 (§ 201),
-representing a portion of the body-wall of a _Hydra_ seen in section,
-gives some idea of this fundamental differentiation. The creature
-consists of a simple sac, the cavity of which is in communication with
-the surrounding water; and hence the unlikeness between the outer and
-inner layers has not become great. The essential contrast is that
-between the differentiated parts of what was originally the same
-part--a uniform membrane composed of juxtaposed cells.
-
-For here, indeed, we are shown unmistakably how the primary contrast
-of structures follows upon the primary contrast of conditions. The
-ordinary form from which low types of the _Metazoa_ set out, is a
-hollow sphere formed of cells packed side by side--a blastula, as it
-is called: all these cells being similarly exposed to the environment.
-The blastula presently changes into what is called a gastrula--a form
-resulting from the introversion of one of the sides of the blastula.
-If there be taken a small ball of vulcanized india-rubber, say an
-inch or more in diameter, and having a hole in it through which the
-air may escape, and if one side of it be thrust inwards so as to
-produce a cup, and if the wide opening of the cup be supposed to
-contract, thus becoming a narrow opening, there will result something
-like the gastrula form. Manifestly that part of the original layer
-which has become internal is differently conditioned from the rest
-which remains external: the one continuing to hold converse with the
-forces of the environment, while the other begins to hold converse
-with the nutritive matters taken into the sac-formed chamber--the
-archenteron or primitive stomach. Interesting evidence of the primitive
-externality of the digestive cavity is yielded by the fact that whereas
-the blastula consisted of ciliated cells, and whereas the ciliation
-persists throughout life on the outer layer, or parts of it, in sundry
-low types--even in some Chætopods--it persists also on the alimentary
-tract of sundry low types: not only in the _Hydra_ but commonly in
-Nemertines, in some _Platyhelminthes_, and even in some leeches.
-
-Besides being enabled thus to understand how an aggregate of
-_Amœba_-form units, originally consisting of a single layer, may pass
-into an aggregate consisting of a double layer; we may also understand
-under what influences the transition takes place. If the habit which
-some of the primary aggregates have, of wrapping themselves round
-masses of nutriment, is followed by a secondary aggregate, there will
-naturally arise just that re-differentiation which the _Hydra_ shows us.
-
-
-§ 289. This account of the primary differentiation carries us only
-half-way towards a true conception of the distinction between outer
-and inner tissues. Though, using words in their current senses, this
-introverted part of the primitive layer has become internal in contrast
-with the remainder, which continues external, yet this introverted
-part has not become internal in the strict physiological sense. For
-it remains subject to the actions of those environing matters which
-are taken in as food: such environing matters, when they happen to be
-moving prey, acting upon it much as they might act upon the exterior.
-So that this introverted part has a quasi-externality. It has not
-the same absolute internality as have those parts which never come
-in contact with products of the outer world. Here we must briefly
-recognize the distinction between these parts and the parts thus far
-considered.
-
-Reverting to our symbol, the india-rubber ball, it will be seen that
-the introversion may be so complete that the cavity is obliterated,
-with the result that the internal surfaces of the outer and inner
-layers come in contact. This is the state reached in the simplest
-cœlenterate animal, the _Hydra_: there being in it nothing more
-than a thin structureless lamella between the ectoderm and endoderm,
-as shown in Fig. 147. This lamella represents all that there is of
-strictly internal tissues. But the introversion, instead of bringing
-the inner surfaces of the ball into contact, may be so far incomplete
-as to leave a space, and in various creatures and embryos of others,
-symbolized by this arrangement, this space becomes occupied by a tissue
-formed from one or other or both of the two primary tissues--the
-mesoblast or mesoderm. This intermediate layer, sometimes, as in the
-_Medusa_, growing into a mass of jelly serving as a fulcrum for
-the creature’s contractions, or, as in the Sponge, giving a passive
-basis to the active tissues, becomes in higher animals the layer out
-of which the structures that support the body and move it about, as
-well as those that distribute prepared nutriment, are developed. From
-it arise the bones, the muscles, and the vascular system--the masses
-of differentiated tissue which are truly internal and occupy what is
-called the body-cavity or peri-visceral space.
-
-In the higher types of animals this space comes to be partially
-occupied by a structure that may be described as a cavity within
-a cavity--the cœlom. Most zoologists regard this as arising by a
-re-introversion of the _archenteron_ or primary alimentary sac. It
-is easily to be perceived that after the introversion which produces
-this digestive cavity, the wall of the cavity may be again introverted
-in such way as to intrude into the peri-visceral space. The cœlom thus
-formed is subsequently shut off. Becoming included among the more
-truly internal structures, and in part giving origin to certain lining
-membranes, it has for its chief function the formation of organs for
-the excretion and emission of nitrogenous waste and of the generative
-products: some portions of it retaining, as a consequence, indirect
-connexions with the environment and characters usually accompanying
-such connexions.
-
-Here we are not concerned with further details: the aim being simply to
-indicate the way in which out of the original layer, wholly external,
-there arise, by primary and secondary introversions, and the formation
-of intermediate membranes and spaces, the chief contrasts between
-outer and inner tissues, and how there simultaneously go on the
-differentiations accompanying different conditions.
-
-
-§ 289_a_. Another all-important differentiation between outer tissues
-and inner tissues has now to be set forth--that by which the nervous
-system becomes established and distinguished. Strangely enough, like
-the one above described, it is sequent upon an introversion: the
-nervous system is primarily a skin-structure and develops by the
-infolding of this skin-structure.
-
-In creatures possessing the earliest rudiments of nerves these exist
-in certain superficial cells. Each has a small tubular orifice from
-which projects a minute hair, and each has on its under side processes
-running into the tissue below, and serving, as it seems, to conduct
-impressions from the projecting hair when it is disturbed by contacts
-with foreign bodies. A plexus of fibres bringing the inner processes
-of such cells into communication arises, and forms something like
-a nervous layer capable of propagating impulses in all directions.
-At a subsequent stage some of the superficial cells, ceasing to be
-themselves the recipients of external stimuli, sink inwards and become
-ganglion-cells connected with the nervous plexus--agents, as we must
-suppose, for the reception, multiplication, and diffusion of the
-impulses received from the outer cells.
-
-As thus far developed, the nervous structure is one fitted only
-for a vague stimulation of dispersed contractile fibres, causing
-movements of an undirected kind. A concentration of these superficial
-nervous structures is a probable preliminary to the next change--an
-all-important change. For a part of the surface begins to sink inwards,
-forming, in the _Vertebrata_, a groove; and from the lining cells
-of this groove, which presently closes over, the central parts of
-the nervous system arise: definite nerves having meantime, as we may
-suppose, been developed out of the indefinite nervous plexus.
-
-Neglecting what there is in this of a speculative nature, it is
-sufficient for the present purpose to recognize the undoubted fact that
-the nervous system is developed from the ectoderm, and that, originally
-external, it is made internal by a process of sinking in or by a
-process of definite introversion.
-
-
-§ 290. Whether direct equilibration or indirect equilibration has had
-the greater share in producing these fundamental contrasts between
-the inner and outer tissues of animals, must be left undecided. The
-two causes have all along co-operated--modification of the individual
-accumulated by inheritance predominating in some cases, and in other
-cases modification of the race by survival of the incidentally fittest.
-On the other hand, the action of the medium on the organism cannot fail
-to change its surface more than its centre, and so differentiate the
-two; while, on the other hand, the surfaces of organisms inhabiting
-the same medium display extreme unlikenesses which cannot be due to
-the immediate actions of their medium. Let us dwell a moment on the
-antithesis.
-
-We have abundant evidence that animal protoplasm is rapidly modified by
-light, heat, air, water, and the salts contained in water--coagulated,
-turned from soluble into insoluble, partially changed into isomeric
-compounds, or otherwise chemically altered. Immediate metamorphoses
-of this kind are often obviously produced in ova by changes of their
-media. At the outset, therefore, before yet there existed any such
-differentiation as that which now usually arises by inheritance,
-these environing agencies must have tended to originate a protective
-envelope. For a modification produced by them on the superficial part
-of the protoplasm, must either have been a decomposition or else the
-formation of a compound which remained stable under their subsequent
-action. There would be generated an outer layer of substance that was
-so molecularly immobile as to be incapable of further metamorphoses,
-while it would shield the contained protoplasm from that too-great
-action of external forces which, by rapidly changing the unstable
-equilibrium of its molecules into a relatively stable equilibrium,
-would arrest development. Evidently organic evolution, whether
-individual or general, must always and everywhere have been subordinate
-to these physical necessities. Though natural selection, beginning with
-minute portions of protoplasm, must all along have tended to establish
-a molecular composition apt to undergo this differentiation of surface
-from centre to the most favourable extent, yet it must all along have
-done so while controlled by this process of direct equilibration.
-
-Contrariwise, the many and great unlikenesses among the dermal
-structures of creatures inhabiting the same element, cannot be ascribed
-to any such cause. The contrasts between naked and shelled Gastropods,
-between marine Worms and Crustaceans, between soft-skinned Fishes
-and Fishes in armour like the _Pterichthys_, must have been produced
-entirely by natural selection. Environing forces are, as before, the
-ultimate causes; but the forces are now not so much those exercised by
-the medium as those exercised by the other inhabitants of the medium;
-and they do not act by modifying the surface of the individual, but
-by killing off individuals whose surfaces are least fitted to the
-requirements: thus slowly affecting the species. Still the dermal
-skeleton bristling with spines, which protects the _Diodon_ or the
-_Cyclichthys_ from enemies it could not escape, comes within the
-general formula of an outer tissue differentiated from inner tissues by
-the outer actions to which the creature is exposed: the differentiation
-having gone on until there is equilibrium between the destructive
-forces to be met and the protective forces which meet them.
-
-If we venture to apportion the respective shares which mediate and
-immediate actions have had in differentiating outer from inner tissues,
-we shall probably not be far wrong in ascribing that part of the result
-which is alike in all animals, mainly to the direct actions of their
-media, while we ascribe the multitudinous unlikenesses of the results
-in various animals, partly to the indirect actions of the media, and
-partly to the indirect actions of other animals by which the media are
-inhabited. That is to say, while assigning the specialities of the
-differentiations to the specialities of converse with the agencies in
-the environment, most of them organic, we may assign to the constant
-and universal converse with its inorganic agencies, the universal
-characteristic of tegumentary structures--their growth outwards from a
-layer lying below the surface which continually produces new substance
-to replace the substance worn away or cast off.
-
-Here let me add a piece of evidence which strengthens the general
-argument, at the same time that it justifies this apportionment. When
-ulceration has gone deep enough to destroy the tegumentary structures,
-these are never reproduced. The puckered surface formed where an ulcer
-heals, or where a serious burn has destroyed the skin, consists of
-modified connective tissue, which, as the healing goes on, spreads
-inwards from the edges of the ulcer: some of it, perhaps, growing from
-the portions of connective tissue that dip down between the muscular
-bundles. This connective tissue is normally covered by the epidermis
-and thus sheltered from environing actions. What has happened to it?
-It has now become the outermost layer. And how does it comport itself
-under its new conditions? It produces a superficial substance which
-plays the part of the epidermis and grows outwardly. For since the
-surface, subject to friction and exfoliation, has to be continually
-renewed, there must be a continual reproduction of an outermost layer
-from a layer beneath. That is to say, the contact of this deep-seated
-tissue with outer agencies, produces in it some approach towards
-that character which we find universally characterizes outer tissue.
-But while we see under this exposure to the conditions common to
-all integument, a tendency to assume the structure common to all
-integument, we see no tendency to assume any of the specialities of
-tegumentary structure: no rudiments of glands or hair sacs make their
-appearance.
-
-Analogous conclusions may be drawn respecting the processes of
-differentiation by which from the outer layer nervous tissue and
-finally a nervous system are evolved. Here, also, both direct and
-indirect equilibration appear to have operated. Two reasons may be
-assigned for the belief that the transformation of certain superficial
-cells into sensitive cells was initiated by exposure to external
-stimuli. The first is that, extremely unstable as protoplasm is,
-disturbances received by the outer side of a specially-exposed cell
-could scarcely fail to cause changes passing through it towards the
-interior mass of the body, and that perpetual repetition of such
-changes would tend to generate channels of easy transmission through
-the protoplasm. The second reason is that, if we do not assume this
-process of initiation but assume that survival of the fittest was
-the sole agency, then no reason can be assigned why the nervous
-system should not have been at the outset formed internally instead
-of being initiated externally and then transferred to the interior:
-the roundabout process would be inexplicable. At the same time the
-production of a central nervous system by introversion of superficial
-sensitive cells cannot be ascribed to the differentiating effects
-of external stimuli, but must be ascribed to natural selection. No
-perpetual repetition of outer disturbances would cause the sinking
-inwards, and covering up, of the specially-sensitive area and
-the plexus below it. But it is manifest that since these nervous
-structures, at once all-important and easily injured, would be safer
-if removed from the surface, survival of the fittest, continually
-preserving those in which they were more deeply seated, would tend to
-produce an arrangement in which all parts but the actual receivers of
-external stimuli became internal.
-
-Hence, contemplating generally these two fundamental differentiations
-of inner from outer tissues, we may conclude that though their first
-stages resulted from direct equilibration, their subsequent and higher
-stages resulted from indirect equilibration.
-
-
-
-
-                             CHAPTER VII.
-
-         DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS.
-
-
-§ 291. The outer tissues of animals, originally homogeneous over their
-whole surfaces, pass into a heterogeneity which fits their respective
-parts to their respective conditions. So numerous and varied are the
-implied differentiations, that it is impracticable here to deal with
-them all even in outline. To trace them up through classes of animals
-of increasing degrees of aggregation, would carry us into undue detail.
-
-Did space permit, it would be possible to point out among the
-_Protozoa_, various cases analogous to that of the _Arcella_; which may
-be described as like a microscopic Limpet, having a sarcode body of
-which the upper surface has become horny, while the lower surface with
-its protruding pseudopodia, retains the primitive jelly-like character.
-That differentiations of this kind have been gradually established
-among these minute creatures through the unlike relations of their
-parts to the environment, is an inference supported by a form which,
-while the rest of the body has a scarcely distinguishable coating,
-“agrees with _Arcella_ and _Difflugia_ in having the pseudopodia
-protrusible from one extremity only of the body.”
-
-Many parallel specializations of surface among aggregates of the second
-order might be instanced from the _Cœlenterata_. In the _Hydra_, the
-ectoderm presents over its whole area no conspicuous unlikenesses;
-but there usually exist in the hydroid polypes of superior types,
-decided contrasts between the higher and lower parts. While the
-higher parts retain their original characters, the lower parts excrete
-hard outer layers yielding support and protection. Various stages
-of the differentiation might be followed. “In _Hydractinia_,” says
-Prof. Green, this horny layer “becomes elevated at intervals to form
-numerous rough processes or spines, while over the general surface
-of the ectoderm its presence is almost imperceptible.” In other
-types, as in _Cordylophora_, it spreads part way up the animal’s
-sides, ending indefinitely. In _Bimeria_ it “extends itself so as to
-enclose the entire body of each polypite, leaving bare only the mouth
-and tips of the tentacles.” While in _Campanularia_ it has become a
-partially-detached outer cell, into which the creature can retract its
-exposed parts.
-
-But it is as needless as it would be wearisome to trace through the
-several sub-kingdoms the rise of these multiform contrasts, with the
-view of seeking interpretations of them. It will suffice if we take a
-few groups of the illustrations furnished by the higher animals.
-
-
-§ 292. We may begin with those modifications of surface which subserve
-respiration. Though we ordinarily think of respiration as the quite
-special function of a quite special organ, yet originally it is not
-so. Little-developed animals part with their carbonic acid and absorb
-oxygen, through the general surface of the body. Even in the lower
-types of the higher classes, the general surface of the body aids
-largely in aërating the blood; and the parts which discharge the
-greater part of this function are substantially nothing more than
-slightly altered and extended portions of the skin.
-
-Such differentiations, marked in various degrees, are to be seen among
-_Mollusca_. In the _Pteropoda_ the only modification which appears to
-facilitate respiration, is the minute vascularity of one part of the
-skin. Higher types possess special skin-developments. The _Doris_ has
-appendages developed into elaborately-branched forms--small trees of
-blood-vessels covered by slightly-changed dermal tissues. And these
-arborescent branchiæ are gathered together into a single cluster. Thus
-there is evidence that large external respiratory organs have arisen
-by degrees from simple skin: as, indeed, they do arise during the
-development of each individual having them. Just as gradually as in
-the embryo a simple bud on the integument, with its contained vascular
-loop, passes by secondary buddings into a tree-like growth penetrated
-everywhere by dividing and subdividing blood-vessels; so gradually has
-there probably proceeded the differentiation which has turned part
-of the outer surface into an organ for excreting carbonic acid and
-absorbing oxygen.
-
-Certain inferior vertebrate animals present us with a like
-metamorphosis of tissues. These are the _Amphibia_. The branchiæ
-here developed from the skin, are covered with cellular epidermis, not
-much thinner than that covering the rest of the body. Like it they
-have their surfaces speckled with pigment-cells; and are not even
-conspicuous by their extra vascularity--where they are temporary at
-least. They facilitate the exchange of gases in scarcely any other
-way than by affording a larger area of contact with the water, and
-interposing a rather thinner layer of tissue between the water and the
-blood-vessels. Those very simple branchiæ of the larval _Amphibia_
-that have them but for a short time, graduate into the more complex
-ones of those that have them for a long time or permanently; showing,
-as before, the small stages by which this heterogeneity of surface
-accompanying heterogeneity of function may arise.
-
-In what way are such differentiations established? Mainly, no doubt, by
-natural selection; but also to some degree, I think, by the inheritance
-of direct adaptations. That a portion of the integument at which
-aëration is favoured by local conditions, should thereby be led to grow
-into a larger surface of aëration, appears improbable. Survival of
-those individuals which happen to have this portion of the integument
-somewhat more-developed, seems here the only likely cause.
-
-
-§ 293. Among the conspicuous modifications by which the
-originally-uniform outer layer is rendered multiform, are the
-protective structures. Let us look first at the few cases in which the
-formation of these is ascribable mainly to direct equilibration.
-
-Already reference has been more than once made to those thickenings
-that occur where the skin is exposed to unusual pressure and friction.
-Are these adaptations inheritable? and may they, by accumulation
-through many generations, produce permanent dermal structures fitted
-to permanent or frequently-recurring stress? Take, for instance, the
-callosities on the knuckles of the _Gorilla_, which are adapted to its
-habit of partially supporting itself on its closed hands when moving
-along the ground. Shall we suppose that these defensive thickenings are
-produced afresh in each individual by the direct actions; or that they
-are inherited modifications caused by such direct actions; or that they
-are wholly due to the natural selection of spontaneous variations? The
-last supposition does not seem a probable one. Such thickenings, if
-spontaneous, would be no more likely to occur on the knuckles than on
-any other of the hundred equal areas forming the skin-surface at large;
-and the chances against their simultaneous occurrence on all eight
-knuckles would be incalculable. Moreover, the implication would be that
-those slight extra thicknesses of skin on the knuckles, with which we
-must suppose the selection to have commenced, were so advantageous
-as to cause survivals of the individuals having them, in presence of
-other superiorities possessed by other individuals. Then that survivals
-so caused, if they ever occurred at all, should have occurred with
-the frequency requisite to establish and increase the variation, is
-hardly supposable. And if we reject, as also unlikely, the reproduction
-of these callosities _de novo_ in each individual (for this would
-imply that after a thousand generations each young gorilla began with
-knuckles having skin no thicker than elsewhere), there remains only the
-inference that they have arisen by the transmission and accumulation of
-functional adaptations. Another case which seems interpretable only in
-an analogous way, is that of the spurs that are developed on the wings
-of certain birds--on those of the Chaja screamer for example. These are
-weapons of offence and defence. It is a familiar fact that some birds
-strike with their wings, often giving severe blows; and in the birds
-named, the blows are made more formidable by the horny, dagger-shaped
-growths standing out from those points on the wings which deliver
-them. Are these spurs directly or indirectly adaptive? To conclude
-that natural selection of spontaneous variations has caused them, is
-to conclude that, without any local stimulus, thickenings of the skin
-occurred symmetrically on the two wings at the places required; that
-such thickenings, so localized, happened to arise in birds given to
-using their wings in fight; and that on their first appearance the
-thickenings were decided enough to give appreciable advantages to the
-individuals distinguished by them--advantages in bearing the reactions
-of the blows if not in inflicting the blows. But to conclude this
-is, I think, to conclude against probability. Contrariwise, if we
-assume that the thickening of the epidermis produced by habitual rough
-usage is inheritable, the development of these structures presents no
-difficulty. The points of impact would become indurated in wings used
-for striking with unusual frequency. The callosities of surface thus
-generated, rendering the parts less sensitive, would enable the bird in
-which they arose to give, without injury to itself, more violent blows
-and a greater number of them: so, in some cases, helping it to conquer
-and multiply. Among its descendants, inheriting the modification and
-the accompanying habit, the thickening would be further increased in
-the same way: survival of the fittest tending ever to accelerate the
-process. Presently the horny nodes so formed, hitherto defensive only
-in their effects, would, by their prominence, become offensive--would
-make the blows given more hurtful. And now natural selection, aiding
-more actively, would mould the nodes into spurs: the individuals
-in which the nodes were most pointed would be apt to survive and
-propagate; and the pointedness generation after generation thus
-increased, would end in the well-adapted shape we see.
-
-But if in these cases the differentiations which fit particular parts
-of the outer tissues to bear rough usage are caused mainly by the
-direct balancing of external actions by internal reactions, then we
-may suspect that the like is true of other modifications that occur
-where special strains and abrasions have to be met. Possibly it is
-true of sundry parts that are formed of hardened epidermis, such as
-the nails, claws, hoofs, and hollow horns of Mammals; “all of which,”
-says Prof. Huxley, “are constructed on essentially the same plan, being
-diverticula of the whole integument, the outer layer of whose ecderon
-has undergone horny metamorphosis.” Leaving open, however, the question
-what tegumentary structures are due to direct equilibration, furthered
-and controlled by indirect equilibration, it is tolerably clear that
-direct equilibration has been one of the factors.
-
-
-§ 294. Dermal structures of another class are developed mainly, if not
-wholly, by the actions of external causes on species rather than on
-individuals. These are the various kinds of clothing--hairs, feathers,
-quills, scales, scutes. Though it is no longer thought as at one time
-that all these various tegumentary structures are homologous with one
-another, yet it is unquestionable that sundry of the more conspicuous
-ones are. Those which are extremely unlike may be seen linked together
-by a long series of graduated forms. A retrograde metamorphosis from
-feathers to appendages that are almost scale-like, is well seen in
-the coat of the Penguin. There is manifest a transition from the
-bird-like covering to the fish-like covering--a transition so gradual
-that no place can be found where an appreciable break occurs; and if
-the scale-like appendages are not truly scales yet they exemplify
-an extreme metamorphosis. Less striking, perhaps, but scarcely less
-significant, are the modifications through which we pass from feathers
-to hairs, on the surfaces of the Ostrich and the Cassowary. The skin
-of the Porcupine shows us hairs and quills united by a series of
-intermediate structures, differing from one another inappreciably. Even
-more remarkable are certain other alliances of dermal structures. “It
-may be taken as certain, I think,” says Prof. Huxley, “that the scales,
-plates, and spines of all fishes are homologous organs; nor as less so
-that the tegumentary spines of the Plagiostomes are homologous with
-their teeth, and thence with the teeth of all vertebrata.”
-
-Further details concerning these tegumentary structures are not needful
-for present purposes, and are indeed but indirectly relevant to the
-subject of physiological development. Here they are of interest to us
-only by involving the general question--What physical influences have
-brought them into existence? Still with a view to definite presentation
-of the problem, it will be well to contemplate the mode of development
-common to the most familiar of them.
-
-Suppose a small pit to be formed on the previously flat skin; and
-suppose that the growth and casting off of horny cells which goes on
-over the skin in general, continues to go on at the usual rate over the
-depressed surface of this pit. Clearly the quantity of horny matter
-produced within this hollow, will be greater than that produced on a
-level portion of the skin subtending an equal area of the animal’s
-outside. Suppose such a pit to be deepened until it becomes a small
-sac. If the exfoliation goes on as before, the result will be that the
-horny matter, expelled, as it must be, through the mouth of the sac,
-which now bears a small proportion to the internal surface of the sac,
-will be large in quantity compared with that exfoliated from a portion
-of the skin equal in area to the mouth of the sac: there will be a
-conspicuous thrusting forth of horny matter. Suppose once more that
-the sac, instead of remaining simple, has its bottom pushed up into
-its interior, like the bottom of a wine-bottle--the introversion being
-carried so far that the introverted part reaches nearly to the external
-opening, and leaves scarcely any space between the introverted part and
-the walls of the sac. It is easy to see that the exfoliation continuing
-from the surface of the introverted part, as well as from the inside
-of the sac generally, the horny matter cast off will form a double
-layer; and will come out of the sac in the shape of a tube having
-within its lower end the introverted part, as the core on which it is
-moulded, and from the apex of which is cast off the substance filling,
-less densely, its interior. The structure resulting will be what we
-know as a hair. Manifestly by progressive enlargement of the sac, and
-further complication of that introverted part on which the excreted
-substance is moulded, the protruding growth may be rendered larger and
-more involved, as we see it in quills and feathers. So that insensible
-steps, thus indicated in principle, carry us from the exfoliation of
-epidermis by a flat surface, to the exfoliation of it by a hollow
-simple sac, an introverted sac, and a sac further complicated; each of
-which produces its modified kind of tegumentary appendage.
-
-But now, after contemplating this typical illustration, we return to
-the general question. What are the agencies which have been operative
-in developing these skin-structures? Indirect equilibration must have
-worked almost alone in producing them. No direct incidence of forces
-can have developed the enamelled armour of the _Lepidosteus_ or the
-tesselated plates of the _Glyptodon_ and its modern allies. Survival of
-the fittest must here and in multitudinous other cases be regarded as
-the sole cause.
-
-
-§ 295. Among many other differentiations of the outer tissues, the
-most worthy to be noticed in the space that remains, are those by which
-organs of sense are formed. We will begin with the simplest and most
-closely-allied to the foregoing.
-
-Every hair that is not too long or flexible to convey to its rooted end
-a strain put upon its free end, is a rudimentary tactual organ; as may
-be readily proved by touching one of those growing on the back of the
-hand. If, then, a creature has certain hairs so placed that they are
-habitually touched by the objects with which it deals, or amid which
-it moves, an advantage is likely to accrue if these hairs are modified
-in a way that enables them the better to transmit the impressions
-derived. Such modified hairs we have in the _vibrissæ_, or, as they are
-commonly called, the “whiskers” possessed by Cats and feline animals
-generally, as well as by Seals and many Rodents. These hairs are long
-enough to reach objects at considerable distances; they are so stiff
-that forces applied to their free ends, cause movements of their
-imbedded ends; and the sacs containing their imbedded ends being well
-covered with nerve-fibres, these developed hairs serve as instruments
-of exploration. By constant use of them the animal learns to judge of
-the relative positions of objects past which, or towards which, it
-is moving. When stealthily approaching prey or stealthily escaping
-enemies, such aids to perception are obviously important: indeed their
-importance has been proved by the diminished power of self-guidance in
-the dark, that results from cutting them off. These, then, are dermal
-appendages originally serving the purpose of clothing, but afterwards
-differentiated into sense-organs.
-
-That eyes are essentially dermal structures seems scarcely conceivable.
-Yet an examination of their rudimentary types, and of their genesis
-in creatures that have them well developed, shows us that they really
-arise by successive modifications of the double layer composing the
-integument. They make their first appearance among the simpler animals
-as specks of pigment, covered by portions of epidermis slightly
-convex and a little more transparent than that around it. Here their
-fundamental community of structure with the skin is easy to trace; and
-the formation of them by differentiation of it presents no difficulty.
-Not so far in advance of these as much to obscure the relationship,
-are the eyes which the Crustaceans possess. In every fishmonger’s shop
-we may see that the eyes of a Lobster are carried on pedicles; and
-when the Lobster casts its shell, the outer coat of each eye, being
-continuous with the epidermis of its pedicle, is thrown off along with
-the rest of the exo-skeleton. Beneath the transparent epidermic layer,
-there exists a group of eyes of the kind which we see in an insect;
-and these, according to a high authority, are inclosed in the dermal
-system. Describing the arrangement of the parts, M. Milne Edwards
-writes:--“But the most remarkable circumstance is, that the large
-cavity within which the whole of these parallel columns, every one of
-which is itself a perfect eye, are contained, is closed posteriorly by
-a membrane, which appears to be neither more nor less than the middle
-tegumentary membrane, pierced for the passage of the optic nerve; so
-that the ocular chamber at large results from the separation at a point
-of the two external layers of the general envelope.” Thus too is it,
-in the main, even with the highly developed eyes of the _Vertebrata_.
-“The three pairs of sensory organs appertaining to the higher senses,”
-says Prof. Huxley--“the nasal sacs, the eyes, and the ears--arise as
-simple cœcal involutions of the external integument of the head of
-the embryo. That such is the case, so far as the olfactory sacs are
-concerned, is obvious, and it is not difficult to observe that the
-lens and the anterior chamber of the eye are produced in a perfectly
-similar manner. It is not so easy to see that the labyrinth of the ear
-arises in this way, as the sac resulting from the involution of the
-integument is small, and remains open but a very short time. But I have
-so frequently verified Huschke’s and Remak’s statement that it does so
-arise, that I entertain no doubt whatever of the fact. The outer ends
-of the olfactory sacs remain open, but those of the ocular and auditory
-sacs rapidly close up, and shut off their contents from all direct
-communication with the exterior.” That is to say, the eye considered as
-an optical apparatus is produced by metamorphoses of the skin: the only
-parts of it not thus produced, being the membranes lying between the
-sclerotic and the vitreous humour, including those retinal structures
-formed in them. All is tegumentary save that which has to appreciate
-the impressions which the modified integument concentrates upon it.
-
-Thus, as Prof. Huxley has somewhere pointed out, there is a
-substantial parallelism between all the sensory organs in their modes
-of development; as there is, too, between their modes of action.
-A _vibrissa_ may be taken as their common type. Increased
-impressibility by an external stimulus, requires an increased
-peripheral expansion of the nervous system on which the stimulus may
-fall; and this is secured by an introversion of the integument, forming
-a sac on the walls of which a nerve may ramify. That the more extended
-sensory area thus constituted may be acted upon, there requires some
-apparatus conveying to it from without the appropriate stimulus; and
-in the case of the _vibrissa_, this apparatus is the epidermic
-growth which, under the form of a hair, protrudes from the sac. And
-that the greatest sensitiveness may be obtained, the external action
-must be exaggerated or multiplied by the apparatus which conveys it
-to the recipient nerve; as, in the case of the _vibrissa_, it is
-by the development of a hair into an elastic lever, that transforms
-the slight force acting through considerable space on its exposed end,
-into a greater force acting through a smaller space at its rooted end.
-Similarly with the organs of the higher senses. In a rudimentary eye,
-the slightly modified sense cell has but a rudimentary nerve to take
-cognizance of the impression; and to concentrate the impression upon
-it, there is nothing beyond a thickening of the epidermis into a
-lens-shape. But the developed eye shows us a termination of the nerve
-greatly expanded and divided to receive the external stimulus. It shows
-us an introverted portion of the integument containing the apparatus
-by which the external stimulus is conveyed to the recipient nerve. The
-structure developed in this sac not only conveys the stimulus, but
-also, like its homologue, concentrates it; and in the one case as in
-the other, the structure which does this is an epidermic growth from
-the bottom of the sac. Even with the ear it is the same. Again we have
-an introverted portion of the integument, on the walls of which the
-nerve is distributed in the primitive ear. The otolithes contained
-in the sac thus formed, are bodies which are set in motion by the
-vibrations of the surrounding water, and convey these vibrations in an
-exaggerated form to the nerves. And though it is not alleged that these
-otolithes are developed from the epidermic lining of the chamber, yet
-as, if not so developed, they are concretions from the contents of an
-epidermic sac, they must still be regarded as epidermic products.
-
-Whether these differentiations are due wholly to indirect
-equilibration, or whether direct equilibration has had a share in
-working them, are questions that must be left open. Possibly a short
-hair so placed on a mammal’s face as to be very often touched, may,
-by conveying excitations to the nerves and vessels at its root, cause
-extra growth of the bulb and its appendages, and so the development of
-a _vibrissa_ may be furthered. Possibly, too, the light itself, to
-which the tissues of some inferior animals are everywhere sensitive,
-may aid in setting up certain of the modifications by which the nervous
-parts of visual organs are formed: producing, as it must, the most
-powerful effects at those points on the surface which the movements of
-the animal expose to the greatest and most frequent contrasts of light
-and shade; and propagating from those points currents of molecular
-change through the organism. But it seems clear that the complexities
-of the sensory organs are not thus explicable. They must have arisen by
-the natural selection of favourable variations.
-
-
-§ 296. A group of facts, serving to elucidate those put together in
-the several foregoing sections, has to be added. I have reserved
-this group to the last, partly because it is transitional--links the
-differentiations of the literally outer tissues with those of the truly
-inner tissues. Though physically internal, the mucous coat of the
-alimentary canal has a _quasi_-externality from a physiological
-point of view. As was pointed out in the last chapter, the skin and
-the assimilating surface have this in common, that they come in direct
-contact with matters not belonging to the organism; and we saw that
-along with this community of relation to alien substances, there is a
-certain community of structure and development. The like holds with the
-linings of all internal cavities and canals that have external openings.
-
-The transition from the literally outer tissues to those tissues
-which are intermediate between them and the truly inner tissues,
-is visible at all the orifices of the body; where skin and mucous
-membrane are continuous, and the one passes insensibly into the
-other. This visible continuity is associated not simply with a great
-degree of morphological continuity, but also with a great degree of
-physiological continuity. That is to say, these literally outer and
-_quasi_-outer layers are capable of rapidly assuming one another’s
-structures and functions when subject to one another’s conditions.
-Mucous surfaces, normally kept covered, become skin-like if exposed
-to the air; but resume more or less fully their normal characters
-when restored to their normal positions. These are truths familiar to
-pathologists. They continually meet with proofs that permanent eversion
-of the mucous membrane, even where it is by prolapse of a part deeply
-seated within the body, is followed by an adaptation eventually almost
-complete: originally moist, tender to the touch, and irritated by the
-air, the surface gradually becomes covered with a thick, dry cuticle;
-and is then scarcely more sensitive than ordinary integument.
-
-Whether this equilibration between new outer forces and reactive inner
-forces, which is thus directly produced in individuals, is similarly
-produced in races, must remain as a question not to be answered in a
-positive way. On the one hand, we have the fact that among the higher
-animals there are cases of _quasi_-outer tissues which are in
-one species habitually ensheathed, while in another species they are
-not ensheathed; and that these two tissues, though unquestionably
-homologous, differ as much as skin and mucous membrane differ. On the
-other hand, there are certain analogous changes of surface, as on the
-abdomen of the Hermit-Crab, which give warrant to the supposition
-that survival of the fittest is the chief agent in establishing such
-differentiations; since the abdomen of a Hermit-Crab, bathed by water
-within the shell it occupies, is not exposed to physical conditions
-that directly tend to differentiate its surface from the surface of
-the thorax. But though in cases like this last, we must assign the
-result to the natural selection of variations arising incidentally; we
-may, I think, legitimately assign the result to the immediate action
-of changed conditions where, as in cases like the first, we see these
-producing in the individual, effects of the kinds observed in the race.
-
-However this may be, the force of the general argument remains the
-same. In these exchanges of structure and function between the outer
-and _quasi_-outer tissues, we get undeniable proof that they are
-easily differentiable. And seeing this, we are enabled the more clearly
-to see how there have, in course of time, arisen those extreme and
-multitudinous differentiations of the outer tissues which have been
-glanced at.
-
-
-
-
-                             CHAPTER VIII.
-
-         DIFFERENTIATIONS AMONG THE INNER TISSUES OF ANIMALS.
-
-
-§ 297. The change from the outside of the lips to their inside,
-introduces us to a new series of interesting and instructive facts,
-joining on to those with which the last chapter closed. They concern
-the differentiations of those coats of the alimentary canal which, as
-we have seen, are physiologically outer, though physically inner.
-
-These coats are greatly modified at different parts; and their
-modifications vary greatly in different animals. In the lower types,
-where they compose a simple tube running from end to end of the body,
-they are almost uniform in their histological characters; but on
-ascending from these types, we find them presenting an increasing
-variety of minute structures between their two ends. The argument
-will be adequately enforced if we limit ourselves to the leading
-modifications they display in some of the higher animals.
-
-The successive parts of the alimentary canal are so placed with respect
-to its contents, that the physical and chemical changes undergone by
-its contents while passing from one end to the other, inevitably tend
-to transform its originally homogeneous surface into a heterogeneous
-surface. Clearly, the effect produced on the food at any part of
-the canal by trituration, by adding a secretion, or by absorbing
-its nutritive matters, implies the delivery of the food into the
-next part of the canal in a state more or less unlike its previous
-states--implies that the surface with which it now comes in contact
-is differently affected by it from the preceding surfaces--implies,
-that is, a differentiating action. To use concrete language;--food
-that is broken down in the mouth acts on the œsophagus and stomach
-in a way unlike that which it would have done had it been swallowed
-whole; the masticated food, to which certain solvents or ferments
-are added, becomes to the intestine a different substance from that
-which it must have otherwise been; and the altered food, resolved
-by these additions into its proximate principles, cannot have those
-proximate principles absorbed in the next part of the intestine,
-without the remoter parts being affected as they would not have been
-in the absence of absorption. It is true that in developed alimentary
-canals, such as the reasoning here tacitly assumes, these marked
-successive differentiations of the food are themselves the results
-of pre-established differentiations in the successive parts of the
-canal. But it is also true that actions and reactions like those
-here so definitely marked, must go on indefinitely in an undeveloped
-alimentary canal. If the food is changed at all in the course of its
-transit, which it must be if the creature is to live by it, then it
-cannot but act dissimilarly on the successive tracts of the alimentary
-canal, and cannot but be dissimilarly reacted on by them. Inevitably,
-therefore, the uniformity of the surface must lapse into greater or
-less multiformity: the differentiation of each part tending ever to
-initiate differentiations of other parts.
-
-Not, indeed, that the implied process of direct equilibration can
-be regarded as the sole process. Indirect equilibration aids; and,
-doubtless, there are some of the modifications which only indirect
-equilibration can accomplish. But we have here one unquestionable
-cause--a cause that is known to work in individuals, changes of the
-kind alleged. Where, for instance, cancerous disease of the œsophagus
-so narrows the passage into the stomach as to prevent easy descent of
-the food, the œsophagus above the obstruction becomes enlarged into a
-kind of pouch; and the inner surface of this pouch begins to secrete
-juices that produce in the food a kind of rude digestion. Again,
-stricture of the intestine, when it arises gradually, is followed by
-hypertrophy of the muscular coat of the intestine above the constricted
-part: the ordinary peristaltic movements being insufficient to force
-the food forwards, and the lodged food serving as a constant stimulus
-to contraction, the muscular fibres, habitually more exercised,
-become more bulky. The deduction from general principles being thus
-inductively enforced, we cannot, I think, resist the conclusion that
-the direct actions and reactions between the food and the alimentary
-canal have been largely instrumental in establishing the contrasts
-among its parts. And we shall hold this view with the more confidence
-on observing how satisfactorily, in pursuance of it, we are enabled to
-explain one of the most striking of these differentiations, which we
-will take as a type of the class.
-
-The gizzard of a bird is an expanded portion of the alimentary canal,
-specially fitted to give the food that trituration which the toothless
-mouth of a bird cannot give. Besides having a greatly-developed
-muscular coat, this grinding-chamber is lined with a thick, hard
-cuticle, capable of bearing the friction of the pebbles swallowed to
-serve as grindstones. This differentiation of the mucous coat into a
-ridged and tubercled layer of horny matter--a differentiation which,
-in the analogous organs of certain _Mollusca_, is carried to the
-extent of producing from this membrane cartilaginous plates, and even
-teeth--varies in birds of different kinds, according to their food.
-It is moderate in birds that feed on flesh and fish, and extreme in
-granivorous birds and others that live on hard substances. How does
-this immense modification of the alimentary canal originate? In the
-stomach of a mammal, the macerating and solvent actions are united
-with that triturating action which finishes what the teeth have mainly
-done; but in the bird, unable to masticate, these internal functions
-are specialized, and while the crop is the macerating chamber, the
-gizzard becomes a chamber adapted to triturate more effectually. This
-adaptation requires simply an exaggeration of certain structures
-and actions which characterize stomachs in general, and, in a less
-degree, alimentary canals throughout their whole lengths. The massive
-muscles of the gizzard are simply extreme developments of the muscular
-tunic, which is already considerably developed over the stomach, and
-incloses also the œsophagus and the intestine. The indurated lining of
-the gizzard, thickened into horny buttons at the places of severest
-pressure, is nothing more than a greatly strengthened and modified
-epithelium. And the grinding action of the gizzard is but a specialized
-form of that rhythmical contraction by which an ordinary stomach kneads
-the contained food, and which in the œsophagus effects the act of
-swallowing, while in the intestine it becomes the peristaltic motion.
-Allied as the gizzard thus clearly is in structure and action to the
-stomach and alimentary canal in general; and capable of being gradually
-differentiated from a stomach where a growing habit of swallowing
-food unmasticated entails more trituration to be performed before
-the food passes the pylorus; the question is--Does this change of
-structure arise by direct adaptation? There is warrant for the belief
-that it does. Besides such collateral evidence as that mucous membrane
-becomes horny on the toothless gums of old people, when subject to
-continual rough usage, and that the muscular coat of the intestine
-thickens where unusual activity is demanded of it, we have the direct
-evidence of experiment. Hunter habituated a sea-gull to feed on grain,
-and found that the lining of its gizzard became hardened, while the
-gizzard-muscles doubled in thickness. A like change in the diet of a
-kite was followed by like results. Clearly, if differentiations so
-produced in the individuals of a race under changed habits, are in any
-degree inheritable, a structure like a gizzard will originate through
-the direct actions and reactions between the food and the alimentary
-canal.
-
-Another case--a very interesting one, somewhat allied to this--is
-presented by the ruminating animals. Here several dilatations of
-the alimentary canal precede the true stomach; and in them large
-quantities of unmasticated food are stored, to be afterwards returned
-to the mouth and masticated at leisure. What conditions have made
-this specialization advantageous? and by what process has it been
-established? To both these questions the facts indicate answers
-which are not unsatisfactory. [Creatures that obtain their food very
-irregularly--now having more than they can consume, and now being for
-long periods without any--must, in the first place, be apt, when very
-hungry, to eat to the extreme limits of their capacities; and must,
-in the second place, profit by peculiarities which enable them to
-compensate themselves for long fasts, past and future. A perch which,
-when its stomach is full of young frogs, goes on filling its œsophagus
-also; or a trout which, rising to the fisherman’s fly, proves when
-taken off the hook to be full of worms and insect-larvæ up to the very
-mouth, gains by its ability to take in such unusual supplies of food
-when it meets with them--obviously thrives better than it would do
-could it never eat more than a stomachful. That this ability to feed
-greatly in excess of immediate requirement, is one that varies in
-individuals of the same race, we see in the marked contrast between
-our own powers in this respect, and the powers of uncivilized men;
-whose fasting and gorging are to us so astonishing. Carrying with us
-these considerations, we shall not be surprised at finding dilatations
-of the œsophagus in vultures and eagles, which get their prey at long
-intervals in large masses; and we may naturally look for them, too, in
-birds like pigeons, which, coming in flocks upon occasional supplies
-of grain, individually profit by devouring the greatest quantity in
-a given time. Now where the trituration of the food is, as in these
-cases, carried on in a lower part of the alimentary canal, nothing
-further is required than the storing-chamber; but for a mammal,
-having its grinding apparatus in its mouth, to gain by the habit of
-hurriedly swallowing unmasticated food, it must also have the habit of
-regurgitating the food for subsequent mastication. This correlation of
-habits with their answering structures, may, as we shall see, arise in
-a very simple way. The starting point of the explanation is a familiar
-fact--the fact that indigestion, often resulting from excess of food,
-is apt to cause that reversed peristaltic action known as vomiting.
-From this we pass to the fact, also within the experience of most
-persons, that during slight indigestion the stomach sometimes quietly
-regurgitates a small part of its contents as far as the back of the
-mouth--giving an unpleasant acquaintance with the taste of the gastric
-juices. Exceptional facts of the same class help the argument a step
-further. “There are certain individuals who are capable of returning,
-at will, a greater or smaller portion of the contents of the digesting
-stomach into the cavity of the mouth.... In some of these cases, the
-expulsion of the food has required a violent effort. In the majority
-it has been easily evoked or suppressed. While in others, it has been
-almost uncontrollable; or its non-occurrence at the habitual time
-has been followed by a painful feeling of fulness, or by the act of
-vomiting.” Here we have a certain physiological action, occasionally
-happening in most persons and in some developed into a habit more or
-less pronounced: indigestion being the habitual antecedent. Suppose,
-then, that gregarious animals, living on innutritive food such as
-grass, are subject to a like physiological action, and are capable of
-like variations in the degree of it. What will naturally happen? They
-wander in herds, now over places where food is scarce and now coming
-to places where it is abundant. Some masticate their food completely
-before swallowing it, while some masticate it incompletely. If an
-oasis, presently bared by their grazing, has not supplied to the whole
-herd a full meal, then the individuals which masticate completely
-will have had less than those which masticate incompletely--will
-not have had enough. Those which masticate incompletely and distend
-their stomachs with food difficult to digest, will be liable to these
-regurgitations; but if they re-masticate what is thus returned to
-the mouth (and we know that animals often eat again what they have
-vomited), then the extra quantity of food taken, eventually made
-digestible, will yield them more nourishment than is obtained by those
-which masticate completely at first. The habit initiated in this
-natural way, and aiding survival when food is scarce, will be apt to
-cause modifications of the alimentary canal. We know that dilatations
-of canals readily arise under habitual distensions. We know that canals
-habitually distended become gradually more tolerant of the contained
-masses that at first irritated them. And we know that there commonly
-take place adaptive modifications of their surfaces. Hence if a habit
-of this kind and the structural changes resulting from it, are in
-any degree inheritable, it is clear that, increasing in successive
-generations, both immediately by the cumulative effect of repetitions
-and mediately by survival of the individuals in which they are most
-decided, they may go on until they end in the peculiarities which
-Ruminants display.
-
-
-§ 298. There are structures belonging to the same group which cannot,
-however, be accounted for in this way. They are the organs that
-secrete special products facilitating digestion--the liver, pancreas,
-and various smaller glands. All these appendages of the alimentary
-canal, large and independent as some of them seem, really arise by
-differentiations from its coats. The primordial liver consists of
-nothing more than bile-cells scattered along a tract of the intestinal
-surface. Accumulation of these bile-cells is accompanied by increased
-growth of the surface which bears them--a growth which at first takes
-the form of a _cul-de-sac_, having an outside that projects from the
-intestine into the peri-visceral cavity. As the mass of bile-cells
-becomes greater, there arise secondary lateral cavities opening into
-the primary one, and through it into the intestine; until, eventually,
-these cavities with their coatings of bile-cells, become ramifying
-ducts distributed through the solid mass we know as a liver. How is
-this differentiation caused?
-
-Before attempting any answer to this question, it is requisite to
-inquire the nature of bile. Is that which the liver throws into
-the intestines a waste product of the organic actions? or is it a
-secretion aiding digestion? or is it a mixture of these? Modern
-investigations imply that it is most likely the last. The liver
-is found to have a compound function. Bernard has proved to the
-satisfaction of physiologists, that there goes on in it a formation of
-glycogen--a substance which is transformed into sugar before it leaves
-the liver and is afterwards carried away by the blood to eventually
-disappear in the active organs, chiefly the muscles. It is also shown,
-experimentally, that there are generated in the liver certain biliary
-acids; and by the aid either of these or of some other compounds, it is
-clear that bile renders certain materials more absorbable. Its effect
-on fat is demonstrable out of the body; and the greatly diminished
-absorption of fat from the food when the discharge of bile into the
-intestine is prevented, is probably one of the causes of that pining
-away which results. But while recognizing the fact that the bile
-consists in part of a solvent, or solvents, aiding digestion, there
-is abundant evidence that one element of it is an effete product;
-and probably this is the primary element. The yellow-green substance
-called biliverdine in herbivora and bilirubin in man and carnivora,
-which gives its colour to bile, is a product the greater part of which
-is normally cast out from the system continually, as is shown by the
-contrast between the normal and abnormal colours of fæcal matters, and
-as is still more strikingly shown by the effects on the system when
-there is a stoppage of the excretion, and an attack of jaundice. Hence
-we are warranted in classing biliverdine as a waste product, and we
-may fairly infer that the excretion of it is the original function of
-the liver.
-
-One further preliminary is requisite. We must for a moment return to
-those physico-chemical data set down in the first chapter of this work
-(§§ 7–8). We there saw that the complex and large-atomed colloids which
-mainly compose living organic matter, have extremely little molecular
-mobility; and, consequently, extremely little power of diffusing
-themselves. Whereas we saw not only that those absorbed matters,
-gaseous and liquid, which further the decomposition of living organic
-matter, have very high diffusibilities, but also that the products
-of the decomposition are much more diffusible than the components of
-living organic matter. And we saw that, as a consequence of this, the
-tissues give ready entrance to the substances which decompose them,
-and ready exit to the substances into which they are decomposed. Hence
-it follows that, under its initial form, uncomplicated by nervous and
-other agencies, the escape of effete matters from the organism, is a
-physical action parallel to that which goes on among mixed colloids
-and crystalloids that are dead or even inorganic. Excretion is a
-specialized form of this spontaneous action; and we have to inquire how
-the specialization arises.
-
-Two causes conspire to establish it. The first is that these products
-of decomposition are diffusible in widely different degrees. While
-the carbonic acid and water permeate the tissues with ease in all
-directions, and escape more or less from the exposed surfaces, urea,
-and other waste substances incapable of being vaporized, cannot escape
-thus readily. The second is that the different parts of the body, being
-subject to different physical conditions, are from the outset sure
-severally to favour the exit of these various products of decomposition
-in various degrees. How these causes must have co-operated in
-localizing the excretions, we shall see on remembering how they now
-co-operate in localizing the separation of morbid materials. The
-characteristic substances of gout and rheumatism have their habitual
-places of deposit. Tuberculous matter, though it may be present in
-various organs, gravitates towards some much more than towards others.
-Certain products of disease are habitually got rid of by the skin,
-instead of collecting internally. Mostly, these have special parts of
-the skin which they affect rather than the rest; and there are those
-which, by breaking out symmetrically on the two sides of the body,
-show how definitely the places of their excretion are determined by
-certain favouring conditions, which corresponding parts may be presumed
-to furnish in equal degrees. Further, it is to be observed of these
-morbid substances circulating in the blood, that having once commenced
-segregating at particular places, they tend to continue segregating
-at those places. Assuming, then, as we may fairly do, that this
-localization of excretion, which we see continually commencing afresh
-with morbid matters, has always gone on with the matters produced by
-the waste of the tissues, let us take a further step, and ask how
-localizations become fixed. Other things equal, that which from its
-physical conditions is a place of least resistance to the exit of
-an effete product, will tend to become established as the place of
-excretion; since the rapid exit of an effete product will profit the
-organism. Other things equal, a place at which the excreted matter
-produces least detrimental effect will become the established place. If
-at any point the excreted matter produces a beneficial effect, then,
-other things equal, survival of the fittest will determine it to this
-point. And if facility of escape anywhere goes along with utilization
-of the escaping substance, then, other things equal, the excretion will
-be there localized still more decisively by survival of the fittest.
-
-Such being the conditions of the problem, let us ask what will happen
-with the lining membrane of the alimentary canal. This, physiologically
-considered, is an external surface; and matters thrown off from it
-make their way out of the body. It is also a surface along which is
-moving the food to be digested. Now, among the various waste products
-continually escaping from the living tissues, some of the more complex
-ones, not very stable in composition, are likely, if added to the food,
-to set up changes in it. Such changes may either aid or hinder the
-preparation of the food for absorption. If an effete matter, making its
-exit through the wall of the intestine, hinders the digestive process,
-the enfeeblement and disappearance of individuals in which this
-happens, will prevent the intestine from becoming the established place
-for its exit. While if it aids the digestive process, the intestine
-will, for converse reasons, become more and more the place to which
-its exit is limited. Equally manifest is it that if there is one part
-of this alimentary canal at which, more than at any other part, the
-favourable effect results, this will become the place of excretion.
-
-Thus, then, reverting to the case in question, we may understand how a
-product to be cast out, such as biliverdine, if it either directly or
-indirectly serves a useful purpose, when poured into a particular part
-of the intestine, may lead to the formation of a patch of excreting
-cells on its wall; and once this place of excretion having been
-established, the development of a liver is simply a question of time
-and natural selection.
-
-
-§ 299. A differentiation of another order occurring in the alimentary
-canal, is that by which a part of it is developed into a lateral
-chamber or chambers, through which carbonic acid exhales and oxygen
-is absorbed. Comparative anatomy and embryology unite in showing
-that a lung is formed, just as a liver or other appendage of the
-alimentary canal is formed, by the growth of a hollow bud into the
-peri-visceral cavity, or space between the alimentary canal and the
-wall of the body. The interior of this bud is simply a _cul-de-sac_ of
-the alimentary canal, with the mucous lining of which its own mucous
-lining is continuous. And the development of this _cul-de-sac_ into
-an air-chamber, simple or compound, is merely a great extension of
-area in the internal surface of the _cul-de-sac_, along with that
-specialization which fits it for excreting and absorbing substances
-different from those which other parts of the mucous surface excrete
-and absorb. These lateral air-chambers, universal among the higher
-_Vertebrata_ and very general among the lower, and everywhere attached
-to the alimentary canal between the mouth and the stomach, have not
-in all cases the respiratory function. In most fishes that have them
-they are what we know as swim-bladders. In some fishes the cavities of
-these swim-bladders are completely shut off from the alimentary canal:
-nevertheless showing, by the communications which they have with it
-during the embryonic stages, that they are originally _diverticula_
-from it. In other fishes there is a permanent _ductus pneumaticus_,
-uniting the cavity of the swim-bladder with that of the gullet: the
-function, however, being still not respiratory in an appreciable
-degree, if at all. But in certain still extant representatives of the
-sauroid fishes, as the _Lepidosteus_, the air-bladder is “divided into
-two sacs that possess a cellular structure,” and “the trachea which
-proceeds from it opens high up in the throat, and is surrounded with
-a glottis.” In the _Amphibia_ the corresponding organs are chambers
-over the surfaces of which there are saccular depressions, indicating a
-transition towards the air-cells characterizing lungs; and accompanying
-this advance we see, as in the common _Triton_, the habit of coming up
-to the surface and taking down a fresh supply of air in place of that
-discharged.
-
-How are the internal air-chambers, respiratory or nonrespiratory,
-developed? Upwards from the amphibian stage, in which they are
-partially refilled at long intervals, there is no difficulty in
-understanding how, by infinitesimal steps, they pass into complex
-and ever-moving lungs. But how is the differentiation that produces
-them initiated? How comes a portion of the internal surface to be
-specialized for converse with a medium to which it is not naturally
-exposed? The problem appears a difficult one; but there is a not
-unsatisfactory solution of it.
-
-When many gold-fish are kept in a small aquarium, as with thoughtless
-cruelty they frequently are, they swim close to the surface, so as to
-breathe that water which is from instant to instant absorbing fresh
-oxygen. In doing this they often put their mouths partly above the
-surface, so that in closing them they take in bubbles of air; and
-sometimes they may be seen to continue doing this--the relief due to
-the slight extra aëration of blood so secured, being the stimulus
-to continue. Air thus taken in may be detained. If a fish that has
-taken in a bubble turns its head downwards, the bubble will ascend
-to the back of its mouth, and there lodge; and coming within reach
-of the contractions of the œsophagus, it may be swallowed. If, then,
-among fish thus naturally led upon occasion to take in air-bubbles,
-there are any having slight differences in the alimentary canal that
-facilitate lodgment of the air, or slight nervous differences such as
-in human beings cause an accidental action to become “a trick,” it
-must happen that if an advantage accrues from the habitual detention
-of air-bubbles, those individuals most apt to detain them will,
-other things equal, be more likely than the rest to survive; and by
-the survival of descendants inheriting their peculiarities in the
-greatest degrees, and increasing them, an established structure and an
-established habit may arise. And that they do in some way arise we have
-proof. The common Loach swallows air, which it afterwards discharges
-loaded with carbonic acid.
-
-From air thus swallowed the advantages that may be derived are of two
-kinds. In the first place, the fish is made specifically lighter, and
-the muscular effort needed to keep it from sinking is diminished--or,
-indeed, if the bubble is of the right size, is altogether saved.
-The contrast between the movements of a Goby, which, after swimming
-up towards the surface, falls rapidly to the bottom on ceasing its
-exertions, and the movements of a Trout, which remains suspended just
-balancing itself by slight undulations of its fins, shows how great
-an economy results from an internal float, to fishes which seek their
-food in mid-water or at the surface. Hence the habit of swallowing
-air having been initiated in the way described, we see why natural
-selection will, in certain fishes, aid modifications of the alimentary
-canal favouring its lodgment--modifications constituting air-sacs.
-In the second place, while from air thus lodged in air-sacs thus
-developed, the advantage will be that of flotation only if the air
-is infrequently changed or never changed, the advantage will be that
-of supplementary respiration if the air-sacs are from time to time
-partially emptied and refilled. The requirements of the animal will
-determine which of the two functions predominates. Let us glance at the
-different sets of conditions under which these divergent modifications
-may be expected to arise.
-
-The respiratory development is not likely to take place in fishes
-that inhabit seas or rivers in which the supply of aërated water
-never fails: there is no obvious reason why the established branchial
-respiration should be replaced by a pulmonic respiration. Indeed, if
-a fish’s branchial respiration is adequate to its needs, a loss would
-result from the effort of coming to the surface for air; especially
-during those first stages of pulmonic development when the extra
-aëration achieved was but small. Hence in fishes so circumstanced,
-the air-chambers arising in the way described would naturally become
-specialized mainly or wholly into floats. Their contained air being
-infrequently changed, no advantage would arise from the development
-of vascular plexuses over their surfaces; nothing would be gained
-by keeping open the communication between them and the alimentary
-canal; and there might thus eventually result closed chambers the
-gaseous contents of which, instead of being obtained from without,
-were secreted from their walls, as gases often are from mucous
-membranes. Contrariwise, aquatic vertebrates in which the swallowing of
-air-bubbles, becoming habitual, had led to the formation of sacs that
-lodged the bubbles; and which continued to inhabit waters not always
-supplying them with sufficient oxygen, might be expected to have the
-sacs further developed, and the practice of changing the contained
-air made regular, if either of two advantages resulted--either the
-advantage of being able to live in old habitats that had become
-untenable without this modification, or the advantage of being able to
-occupy new habitats. Now it is just where these advantages are gained
-that we see the pulmonic respiration coming in aid of the branchial
-respiration, and in various degrees replacing it. Shallow waters are
-liable to three changes which conspire to make this supplementary
-respiration beneficial. The summer’s sun heats them, and raising the
-temperatures of the animals they contain, accelerates the circulation
-in these animals, exalts their functional activities, increases the
-production of carbonic acid, and thus makes aëration of the blood more
-needful than usual. Meanwhile the heated water, instead of yielding
-to the highly carbonized blood brought to the branchiæ the usual
-quantity of oxygen, yields less than usual; for as the heat of the
-water increases, the quantity of air it contains diminishes. And this
-greater demand for oxygen joined with smaller supply, pushed to an
-extreme where the water is nearly all evaporated, is at last still
-more intensely felt in consequence of the excess of carbonic acid
-discharged by the numerous creatures congregated in the muddy puddles
-that remain. Here, then, it is, that the habit of taking in air-bubbles
-is likely to become established, and the organs for utilizing them
-developed; and here it is, accordingly, that we find all stages of the
-transition to aërial respiration. The Loach before-mentioned, which
-swallows air, frequents small waters liable to be considerably warmed.
-The _Amphipnous Cuchia_, an anomalous eel-shaped fish, which has
-vascular air-sacs opening out at the back of the mouth, “is generally
-found lurking in holes and crevices, on the muddy banks of marshes or
-slow-moving rivers”; and though its air-sacs are not morphological
-equivalents of those above described, yet they equally well illustrate
-the relation between such organs and the environing condition. Still
-more significant is the fact that the _Lepidosiren_, or “mudfish” as it
-is called from its habits, though it is a true fish nevertheless has
-lungs. But it is among the _Amphibia_ that we see most conspicuously
-this relation between the development of air-breathing organs, and
-the peculiarities of the habitats. Pools, more or less dissipated
-annually, and so rendered uninhabitable by most fishes, are very
-generally peopled by these transitional types. Just as we see, too,
-that in various climates and in various kinds of shallow waters, the
-supplementary aërial respiration is needful in different degrees; so
-do we find among the _Amphibia_ many stages in the substitution of the
-one respiration for the other. The facts, then, are such as give to the
-hypothesis a _vraisemblance_ greater than could have been expected.
-
-The relative effects of direct and indirect equilibration in
-establishing this further heterogeneity, must, as in many other
-cases, remain undecided. The habit of taking in bubbles is scarcely
-interpretable as a result of spontaneous variation: we must regard it
-as arising accidentally during the effort to obtain the most aërated
-water; as being persevered in because of the relief obtained; and as
-growing by repetition into a tendency bequeathed to offspring, and
-by them, or some of them, increased and transmitted. The formation
-of the first slight modifications of the alimentary canal favouring
-the lodgment of bubbles, is not to be thus explained. Some favourable
-variation in the shape of the passage must here have been the initial
-step. But the gradual increase of this structural modification by the
-survival of individuals in which it is carried furthest, will, I think,
-be all along aided by immediate adaptation. The part of the alimentary
-canal previously kept from the air, but now habitually in contact with
-the air, must be in some degree modified by the action of the air;
-and the directly-produced modification, increasing in the individual
-and in successive individuals, cannot cease until there is a complete
-balance between the actions of the changed agency and the changed
-tissue.
-
-
-§ 300. We come now to differentiations among the truly inner
-tissues--the tissues which have direct converse neither with the
-environment nor with the foreign substances taken into the organism
-from the environment. These, speaking broadly, are the tissues
-which lie between the double layer forming the integument with its
-appendages, and the double layer forming the alimentary canal with
-its _diverticula_. We will take first the differentiation which
-produces the vascular system.
-
-Certain forces producing and aiding distribution of liquids in animals,
-come into play before any vascular system exists; and continue to
-further circulation after the development of a vascular system. The
-first of these is osmotic exchange, acting locally and having an
-indirect general action; the second is local variation of pressure,
-which movement of the body throws on the tissues and their contained
-liquids. A few words are needed in elucidation of each. If in any
-creature, however simple, different changes are going on in parts
-that are differently conditioned--if, as in a _Hydra_, one surface
-is exposed to the surrounding medium while the other surface is
-exposed to dissolved food; then between the unlike liquids which the
-dissimilarly-placed parts contain, osmotic currents must arise; and
-a movement of liquid through the intermediate tissue must go on as
-long as an unlikeness between the liquids is kept up. This primary
-cause of re-distribution remains one of the causes of re-distribution
-in every more-developed organism: the passage of matters into and
-out of the capillaries is everywhere thus set up. And obviously in
-producing these local currents, osmose must also indirectly produce
-general currents, or aid them if otherwise produced. In the absence
-of a pumping organ, this force is probably an important aid to that
-movement of the nutritive liquids which the functions set up. How
-the second cause--the changes of internal pressure which an animal’s
-movements produce--furthers circulation, will be sufficiently manifest.
-That parts which are bent or strained necessarily have their contained
-vessels squeezed, has been shown (§ 281); and whether the bend or
-strain is caused, as in a plant, by an external force, or, as usually
-in an animal, by an internal force, there must be a thrusting of
-liquids towards places of least resistance--commonly places of greatest
-consumption. This which in animals without hearts is a main agent of
-circulation, continues to further it very considerably even among the
-highest animals. In these the effect becomes as it were systematized.
-The valves in the veins necessitate perpetual propulsions towards the
-heart.
-
-Even in such simple types as the _Hydrozoa_, cavities in the tissues
-faintly indicate a structure which facilitates the transfer of
-nutritive matters. These cavities become reservoirs filled with the
-plasma that slowly oozes through the substance of the body; and every
-movement of the animal, accompanied as it must be by changed pressures
-and tensions on these reservoirs, tends here to fill them and there to
-squeeze out their contents in that or the other direction--possibly
-aiding to produce, by union of several cavities, those lacunæ or
-irregular canals which the body in some cases presents.
-
-Irregular canals of this kind, not lined with any membranes but being
-simply cavities running through the flesh, mainly constitute the
-vascular system in _Polyzoa_ and _Brachiopoda_ and some _Mollusca_.
-Though the central parts of a vascular system are rudely developed, yet
-its peripheral parts consist of sinuses permeating the tissues. The
-higher orders of _Mollusca_ have a more-developed system of vessels or
-arteries, which run into the substance of the body and end in lacunæ or
-simple fissures. This ending in lacunæ takes place at various distances
-from the vascular centre. In some genera the arterial structure is
-carried to the periphery of the blood-system, while in others it stops
-short midway. Throughout most orders of the _Mollusca_ the back
-current of blood continues to be carried by channels of the original
-kind: there are no true veins, but the blood having been delivered
-into the tissues, finds its way back to the peri-visceral cavity
-through inosculating sinuses. Among the Cephalopods, however, the
-afferent blood-canals, as well as the efferent ones, acquire distinct
-walls. On putting together these facts, we may conceive pretty clearly
-the stages of vascular development. From the original reservoir of
-nutritive liquid between the alimentary canal and the wall of the body,
-a portion partially shut off becomes a contractile vessel; and by its
-actions there is produced a more rapid transfer of the nutritive liquid
-than was originally produced by the motions of the animal. Clearly,
-the extension of this contractile tube and the development from it
-of branches running hither and thither into the tissues, must, by
-defining the channels of blood throughout a part of its course, render
-its distribution more regular and active. As fast as this centrifugal
-growth advances, so fast are the efferent currents of blood, prevented
-from escaping laterally, obliged to move from the centre towards
-the circumference; and so fast also does the less developed set of
-channels become, of necessity, occupied by afferent currents. When, by
-a parallel increase of definiteness, the lacunæ and irregular sinuses
-through which the afferent currents pass, become transformed into
-veins, the accompanying disappearance of all stagnant or slow-moving
-collections of blood, implies a further improvement in the circulation.
-
-By what agency is effected this differentiation of a definite vascular
-system? No sufficient reply is obvious. The genesis of the primordial
-heart is not comprehensible as a result of direct equilibration, and we
-cannot readily see our way to it as a result of indirect equilibration;
-for it is difficult to imagine what favourable variation natural
-selection could have seized hold of to produce such a structure. A
-contractile tube that aided the distribution of nutritive liquid,
-having been once established, survival of the fittest would suffice
-for its gradual extension and its successive modifications. But what
-were the early stages of the contractile tube, while it was yet not
-sufficiently formed to help circulation, and while it must nevertheless
-have had some advantage without which no selective process could go
-on? The question seems insoluble. To another part of the question,
-however, an answer may be ventured. If we ask the origin of these
-ramifying channels which, first appearing as simple lacunæ, eventually
-become vessels having definite walls, a reply admitting of considerable
-justification, is, that the currents of nutritive liquid forced and
-drawn hither and thither through the tissues, themselves initiate these
-channels. We know that streams running over and through solid and
-quasi-solid inorganic matter, tend to excavate definite courses. We saw
-reason for concluding that the development of sap-channels in plants
-conforms to this general principle. May we not then suspect that the
-nutritive liquid contained in the tissue of a simple animal, made to
-ooze now in this direction and now in that by the changes of pressure
-which the animal’s movements cause, comes to have certain lines along
-which it is thrust backwards and forwards more than along other lines;
-and must by repeated passings make these more and more permeable until
-they become lacunæ? Such actions will inevitably go on; and such
-actions appear competent to produce some, at least, of the observed
-effects. The leading facts which indicate that this is a part-cause of
-vascular development are these.
-
-Growths normally recurring in certain places at certain intervals,
-are accompanied by local formations of blood-vessels. The periodic
-maturation of ova among the _Mammalia_ supplies an instance. Through
-the stroma of an ovarium are distributed innumerable minute vesicles,
-which, in their early stages, are microscopic. Of these, severally
-contained in their minute ovi-sacs, any one may develop: the
-determining cause being probably some slight excess of nutrition. When
-the development is becoming rapid, the capillaries of the neighbouring
-stroma increase and form a plexus on the walls of the ovi-sac. Now
-since there is no typical distribution of the developing ova; and since
-the increase of an ovum to a certain size precedes the increase of
-vascularity round it; we can scarcely help concluding that the setting
-up of currents towards the point of growth determines the formation of
-the blood-vessels. It may be that having once commenced, this local
-vascular structure completes itself in a typical manner; but it seems
-clear that this greater development of blood-vessels around the growing
-ovum is initiated by the draught towards it. Abnormal growths show
-still better this relation of cause and effect. The false membranes
-sometimes found in the bronchial tubes in inflammatory diseases, may
-perhaps fairly be held abnormal in but a partial sense: it may be said
-that their vascular systems are formed after the type of the membranes
-to which they are akin. But this can scarcely be said of the morbid
-growths classed as malignant. The blood-vessels in an encephaloid
-cancer, are led to enlarge and ramify, often to an immense extent, by
-the unfolding of the morbid mass to which they carry blood. Alien as
-is the structure as a whole to the type of the organism; and alien in
-great measure as is its tissue to the tissue on which it is seated;
-it nevertheless happens that the growth of the alien tissue and
-accompanying abstraction of materials from the blood-vessels, determine
-a corresponding growth of these blood-vessels. Unless, then, we say
-that there is a providentially-created type of vascular structure for
-each kind of morbid growth (and even this would not much help us, since
-the vascular structure has no constancy within the limits of each
-kind), we are compelled to admit that in some way or other the currents
-of blood are here directly instrumental in forming their own channels.
-One more piece of evidence, before cited as exemplifying adaptation (§
-67), may be called to mind. When any main channel for blood, leading
-to or from a certain part of the body, has been rendered impervious,
-others among the channels leading to or from this same part, enlarge to
-the extent requisite for fulfilling the extra function that falls upon
-them: the enlargement being caused, as we must infer, by the increase
-of the currents carried.
-
-Here, then, are facts warranting inductively the deduction above drawn.
-It is true that we are left in the dark respecting the complexities
-of the process. How the channels for blood come to have limiting
-membranes, and many of them muscular coats, the hypothesis does not
-help us to say. But the evidence assigned goes far to warrant the
-belief that vascular development is initiated by direct equilibration;
-though indirect equilibration may have had the larger share in
-establishing the structures which distinguish finished vascular systems.
-
-
-§ 301. Of the inner tissues which remain let us next take bone. In what
-manner is differentiated this dense substance serving in most cases for
-internal support?
-
-When considering the vertebrate skeleton under its morphological aspect
-(§ 256), it was pointed out that the formation of dense tissues,
-internal as well as external, is, in some cases at least, brought
-about by the mechanical forces to be resisted. Through what process
-it is brought about we could not then stay to inquire: this question
-being not morphological but physiological. Answers to some kindred
-questions have since been attempted. Certain actions to which the
-internal dense tissues of plants may be ascribed, have been indicated;
-and more recently, analogous actions have been assigned as causes of
-some external dense tissues of animals. We have now to ask whether
-actions of the same nature have produced these internal dense tissues
-of animals.
-
-The problem is an involved one. Bones have more than one stage. They
-are membranous or cartilaginous before they become osseous; and their
-successive component substances so far differ that the effects
-of mechanical actions upon them differ. And having to deal with
-transitional states in which bone is formed of mixed tissues, having
-unlike physical properties and unlike minute structures, the effects of
-strains become too complicated to follow with precision. Anything in
-the way of interpretation must therefore be regarded as tentative. If
-analysis and comparison show that the phenomena are not inconsistent
-with the hypothesis of mechanical genesis, it is as much as can be
-expected. Let us first observe more nearly the mechanical conditions to
-which bones are subject.
-
-The endo-skeleton of a mammal with the muscles and ligaments holding
-it together, may be rudely compared to a structure built up of struts
-and ties; of which, speaking generally, the struts bear the pressures
-and the ties bear the tensions. The framework of an ordinary iron
-roof will give an idea of the functions of these two elements, and of
-the mechanical characters required by them. Such a framework consists
-partly of pieces which have each to bear a thrust in the direction of
-its length, and partly of pieces which have each to bear a pull in the
-direction of its length; and these struts and ties are differently
-formed to adapt them to these different strains. Further, it should
-be remarked that though the rigidity of the framework depends on the
-ties which are flexible, as much as on the struts which are stiff, yet
-the ties help to give the rigidity simply by so holding the struts
-in position that they cannot escape from the thrusts which fall on
-them. Now the like relation holds with a difference among the bones
-and muscles: the difference being that here the ties admit of being
-lengthened or shortened and the struts of being moved about upon their
-joints. The mechanical relations are not altered by this, however.
-The actions are of essentially the same kind in an animal that is
-standing, or keeping itself in a strained attitude, as in one that is
-changing its attitude--the same in so far that we have in each a set
-of flexible parts that are pulling and a set of rigid parts that are
-resisting. It needs but to remember the sudden collapse and fall which
-take place when the muscles are paralyzed, or to remember the inability
-of a bare skeleton to support itself, to see that the struts without
-the ties cannot suffice. And we have but to think of the formless mass
-into which a man would sink when deprived of his bones, to see that
-the ties without the struts cannot suffice. To trace the way in which
-a particular bone has its particular thrust thrown upon it, may not
-always be practicable. Though it is easy to perceive how a flexor or
-extensor of the arm causes by its tension a reactive pressure along the
-line of the humerus, and is enabled to produce its effect only by the
-rigidity of the humerus; yet it is not so easy to perceive how such
-bones as those of a horse’s pelvis are similarly acted upon. Still, as
-the weight of the hind quarters has to be transferred from the back to
-the feet, and must be so transferred through the bones, it is manifest
-that though these bones form a very crooked line, the weight must
-produce a pressure along the axis of each: the muscles and ligaments
-concerned serving here, as in other cases, so to hold the bones that
-they bear the pressure instead of being displaced by it. Not forgetting
-that many processes of the bones have to bear tensions, we may then
-say that generally, though by no means universally, bones are internal
-dense masses that have to bear pressures--pressures which in the
-cylindrical bones become longitudinal thrusts. Leaving out exceptional
-cases, let us consider bones as masses thus circumstanced.
-
-When giving reasons for the belief that the vertebrate skeleton is
-mechanically originated, one of the facts put in evidence was, that
-in the vertebrate series the transition from the cartilaginous to the
-osseous spine begins peripherally (§ 257): each vertebra being at first
-a ring of bone surrounding a mass of cartilage. And it was pointed
-out that this peripheral ossification is ossification at the region
-of greatest pressures. Now it is not vertebræ only that follow this
-course of development. In a cylindrical bone, though it is differently
-circumstanced, the places of commencing ossification are still the
-places on which the severest stress falls. Let us consider how such a
-bone that has to bear a longitudinal pressure is mechanically affected.
-If the end of a walking-cane be thrust with force against the ground,
-the cane bends; and partially resuming its straightness when relieved,
-again bends, usually towards the same side, when the thrust is renewed.
-A bend so caused acts on the fibres of the cane in nearly the same
-way as does a bend caused by supporting the cane horizontally at its
-two ends and suspending a weight from its middle. In either case the
-fibres on the convex side are extended and the fibres on the concave
-side compressed. Kindred actions occur in a rod that is so thick
-as not to yield visibly under the force applied. In the absence of
-complete homogeneity of its substance, complete symmetry in its form,
-and an application of a force exactly along its axis, there must be
-some lateral deflection; and therefore some distribution of tensions
-and pressures of the kind indicated. And then, as the fact which here
-specially concerns us, we have to note that the strongest tensions
-and pressures are borne by the outer layers of fibres. Now the shaft
-of a long bone, subject to mechanical actions of this kind, similarly
-has its outer layer most strained. In this layer, therefore, on the
-mechanical hypothesis, ossification should commence, and here it does
-commence--commences, too, midway between the ends, where the bends
-produce on the superficial parts their most intense effects. But we
-have not in this place simply to observe that ossification commences at
-the places of greatest stress, but to ask what causes it to do this.
-Can we trace the physical actions which set up this deposit of dense
-tissue? It is, I think, possible to indicate a “true cause” that is at
-work; though whether it is a sufficient cause may be questioned. We
-concluded that in certain other cases, the formation of dense tissue
-indirectly results from the alternate squeezing and relaxation of the
-vessels running through the part; and the inquiry now to be made is,
-whether, in developing bone, the same actions go on in such ways as to
-produce the observed effects. At the outset we are met by what seems a
-fatal difficulty--cartilage is a non-vascular tissue: this substance
-of which unossified bones consist is not permeated by minute canals
-carrying nutritive liquid, and cannot, therefore, be a seat of actions
-such as those assigned. This apparent difficulty, however, furnishes a
-confirmation. For cartilage that is wholly without permeating canals
-does not ossify: ossification takes place only at those parts of it
-into which the canals penetrate. Hence, we get additional reason for
-suspecting that bone-formation is due to the alleged cause; since it
-occurs where mechanical strains can produce the actions described, but
-does not occur where mechanical strains cannot produce them. Let us
-consider more closely what the several factors are. It will suffice
-for the argument if we commence with the external vascular layer as
-already existing, and consider what will take place in it. Cartilage
-is elastic--is somewhat extensible, and spreads out laterally under
-pressure, but resumes its form when relieved. How, then, will the
-minute channels traversing it in all directions be affected at the
-places where it is strained by a bend? Those on the convex side will
-be laterally squeezed, in the same way that we saw the sap-vessels on
-the convex side of a bent branch are squeezed; and as exudation of the
-sap into the adjacent prosenchyma will be caused in the one case, so,
-in the other, there will be caused exudation of serum into the adjacent
-cartilage: extra nutrition and increase of strength resulting in both
-cases. The parallel ceases here, however. In the shoot of a plant, bent
-in various directions by the wind, the side which was lately compressed
-is now extended; and hence that squeezing of the sap-vessels which
-results from extension, suffices to feed and harden the tissue on all
-sides of the shoot. But it is not so with a bone. Having yielded on
-one side under longitudinal pressure, and resumed as nearly as may be
-its previous shape when the pressure is taken off, the bone yields
-again towards the same side when again longitudinally pressed. Hence
-the substance of its concave side, never rendered convex by a bend
-in the opposite direction, would not receive any extra nutrition did
-no other action come into play. But if we consider how intermittent
-pressures must act on cartilage, we shall see that there will result
-extra nutrition of the concave side also. Squeeze between two pieces of
-glass a thin bit of caoutchouc which has a hole through it. While the
-caoutchouc spreads out away from the centre, it also spreads inwards,
-so as partially to close the hole. Everywhere its molecules move away
-in directions of least resistance; and for those near the hole, the
-direction of least resistance is towards the hole. Let this hole stand
-for the transverse section of one of the minute canals or channels
-passing through cartilage, and it will be manifest that on the side of
-the unossified bone made concave in the way described, the compressed
-cartilage will squeeze the canals traversing it; and, in the absence
-of perfect homogeneity in the cartilage, the squeeze will cause extra
-exudation from the canals into the cartilage. Thus every additional
-strain will give to the cartilage it falls upon, an additional supply
-of the materials for growth. So that presently the side which, by
-yielding more than any other, proves itself to be the weakest, will
-cease to be the weakest. What further will happen? Some other side
-will yield a little--the bends will take place in some other plane;
-and the portions of cartilage on which repeated tensions and pressures
-now fall will be strengthened. Thus the rate of nutrition, greatest at
-the place where the bending is greatest, and changing as the incidence
-of forces changes, will bring about at every point a balance between
-the resistances and the strains. Thus, too, there will be determined
-that peripheral induration which we see in bones so circumstanced.
-As in a shoot we saw that the woody deposit takes place towards the
-outside of the cylinder, where, according to the hypothesis, it ought
-to take place; so, here, we see that the excess of exudation and
-hardening, occurring where the strains are most intense, will form a
-cylinder having a dense outside and a porous or hollow inside. These
-processes will be essentially the same in bones subject to more complex
-mechanical actions, such as sundry of the flat bones and others that
-serve as internal fulcra. Be the strains transverse or longitudinal,
-be they torsion strains or mixed strains, the outer parts of the
-bone will be more affected by them than its inner parts. They will
-therefore tend everywhere to produce resisting masses having outer
-parts more dense than their inner parts. And by causing most growth
-where they are most intense, they will call out reactive forces
-adequate to balance them. There are doubtless obstacles in the way
-of this interpretation. It may be said that the forces acting on the
-outer layers in the manner described, would compress the canals too
-little to produce the alleged effects; and if evenly distributed along
-the whole lengths of the layers, they would probably do so. But it
-needs only to bend a flexible mass and observe the tendency to form
-creases on the concave surface, to feel assured that along the surface
-of an ossifying bone, the yielding of the tissue when bent will not
-be uniform. In the absence of complete homogeneity, the interstitial
-yielding will take place at some points more than others, and at one
-point above all others. When, at the weakest point--the centre of
-commencing ossification--an extra amount of deposit has been caused,
-it will cease to be the weakest; and adjacent points, now the weakest,
-will become the places of yielding and induration. It may be further
-objected that the hypothesis is incompatible with the persistence
-of cartilage for so long a time between the epiphysis of bones and
-the bony masses which they terminate. But there is the reply that
-the places occupied by this cartilage being places at which the bone
-lengthens, the non-ossification is in part apparent only--it is rather
-that new cartilage is formed as fast as the pre-existing cartilage
-ossifies; and there is the further reply that the slowness of the
-ultimate ossification of this part, is due to its non-vascularity, and
-to mechanical conditions which are unfavourable to its acquirement of
-vascularity. Once more, there is the demurrer that in the epiphyses
-ossification does not begin at the surface but within the mass of
-the cartilage. Explanation of this implies ability to follow out the
-mechanical actions in a resilient substance which, like india-rubber,
-admits of being distorted in all ways by pressure and recovering its
-form, and it seems impossible to say how the more superficial and more
-deep-seated canals traversing it will be respectively affected.
-
-Of course it is not meant that this osseous development by direct
-equilibration takes place in the individual. Though it is a corollary
-from the argument that in each individual the process must be furthered
-and modified by the particular actions to which the particular bones
-are exposed; yet the leading traits of structure assumed by the bones
-are assumed in conformity with the inherited type. This, however, is
-no difficulty. The type itself is to be regarded as the accumulated
-result of such modifications, transmitted and increased from generation
-to generation. The actions above described as taking place in the
-bone of an individual, must be understood as producing their total
-effect little by little in the corresponding bones of a long series of
-individuals. Even if but a small modification can be so wrought in the
-individual, yet if such modification, or a part of it, is inheritable,
-we may readily understand how, in the course of geologic epochs, the
-observed structures may arise in the assigned way.
-
-Here may fitly be added a strong confirmation. If we find cases
-where individual bones, subject in exceptional degrees to the
-actions described, present in exceptional amounts the modifications
-attributed to them, we are greatly helped in understanding how there
-may be produced in the race that aggregate of modifications which
-the hypothesis implies. Such cases occur in ricketty children. I
-am indebted to Mr. Busk for pointing out these abnormal formations
-of dense tissue, that are not apparently explicable as results of
-mechanical actions and reactions. It was only on tracing out the
-processes here at work, that there suggested itself the specific
-interpretation of the normal process, as above set forth. When, from
-constitutional defect, bones do not ossify with due rapidity, and are
-meanwhile subject to the ordinary strains, they become distorted.
-Remembering how a mass which has been made to yield in any direction
-by a force it cannot withstand, is some little time before it recovers
-completely its previous form, and usually, indeed, undergoes what
-is called a “permanent set;” it is inferable that when a bone is
-repeatedly bent at the same time that the liquid contained in its
-canals is poor in the materials for forming dense tissue, there
-will not take place a proportionate strengthening of the parts most
-strained; and these parts will give way. This happens in rickets. But
-this having happened, there goes on what, in teleological language,
-we call a remedial process. Supposing the bone to be one commonly
-affected--a femur; and supposing a permanent bend to have been caused
-in it by the weight of the body; the subsequent result is an unusual
-deposition of cartilaginous and osseous matter on the concave side of
-the bone. If the bone is represented by a strung bow, then the deposit
-occurs at the part represented by the space between the bow and the
-string. And thus occurring where its resistance is most effective,
-it increases until the approximately-straight piece of bone formed
-within the arc, has become strong enough to bear the pressure without
-appreciably yielding. Now this direct adaptation, seeming so like a
-special provision, and furnishing so remarkable an instance of what,
-in medical but unscientific language, is called the _vis medicatrix
-naturæ_, is simply a result of the above-described mechanical actions
-and reactions, going on under the exceptional conditions. Each time
-such a bent bone is subject to a force which again bends it, the
-severest compression falls on the substance of its concave side. Each
-time, then, the canals running through this part of its substance are
-violently squeezed--far more squeezed than they or any other of the
-canals would have been, had the bone remained straight. Hence, on every
-repetition of the strain, these canals near the concave surface have
-their contents forced out in more than normal abundance. The materials
-for the formation of tissue are supplied in quantity greater than can
-be assimilated by the tissue already formed; and from the excess of
-exuded plasma, new tissue arises.[50] A layer of organizable material
-accumulates between the concave surface and the periosteum; in this,
-according to the ordinary course of tissue-growth, new vessels appear;
-and the added layer presently assumes the histological character of the
-layer from which it has grown. What next happens? This added layer,
-further from the neutral axis than that which has thrown it out, is now
-the most severely compressed, and its vessels are the most severely
-squeezed. The place of greatest exudation and most rapid deposit
-of matter, is therefore transferred to this new layer; and at the
-same time that active nutrition increases its density, the excess of
-organizable material forms another layer external to it: the successive
-layers so added, encroaching on the space between the concave surface
-of the bone and the chord of its arc. What limits the encroachment on
-this space?--what stops the process of filling it up? The answer to
-this question will be manifest when observing that there comes into
-play a cause which gradually diminishes the forces falling on each new
-layer. For the transverse sectional area is step by step increased; and
-an increase of the area over which the weight borne is distributed,
-implies a relatively smaller pressure upon each part of it. Further, as
-the transverse dimensions of the bone increase, the materials composing
-its convex and concave layers, becoming further from the neutral axis,
-become better placed for resisting the strains to be borne. So that
-both by the increased quantity of dense matter and by its mechanically
-more-advantageous position, the bendings of the bone are progressively
-decreased. But as they are decreased, each new layer formed on the
-concave surface has its substance and its vessels less compressed; and
-the resulting growth and induration are rendered less rapid. Evidently,
-then, the additions, slowly diminishing, will eventually cease; and
-this will happen when the bone no longer bends. That is to say, the
-thickening of the bone will reach its limit when there is equilibrium
-between the incident forces and the forces which resist them. Here,
-indeed, we may trace with great clearness the process of direct
-equilibration--may see how an unusual force, falling on the moving
-equilibrium of an organism and not overthrowing it, goes on working
-modifications until the reaction balances the action.
-
-That, however, which now chiefly concerns us, is to note how this
-marked adaptation supports the general argument. Unquestionably bone
-is in this case formed under the influence of mechanical stress, and
-formed just where it most effectually meets the stress. This result,
-not otherwise explained, is explained by the hypothesis above set
-forth. And when we see that this special deposit of bone is accounted
-for by actions like those to which bone-formation in general is
-ascribed, the probability that these are the actions at work becomes
-very great.[51]
-
-Of course it is not alleged that osseous structures arise in this
-way alone. The bones of the skull and various dermal bones cannot be
-thus interpreted. Here the natural selection of favourable variations
-appears the only assignable cause--the equilibration is indirect. We
-know that ossific deposits now and then occur in tissues where they
-are not usually found; and such deposits, originally abnormal, if they
-occurred in places where advantages arose from them, might readily be
-established and increased by survival of the fittest. Especially might
-we expect this to happen when a constitutional tendency to form bone
-had been established by actions of the kind described; for it is a
-familiar fact that differentiated types of tissue, having once become
-elements of an organism, are apt occasionally to arise in unusual
-places, and there to repeat all their peculiar histological characters.
-And this may possibly be the reason why the bones of the skull, though
-not exposed to forces such as those which produce, in other bones,
-dense outer layers including less dense interiors, nevertheless repeat
-this general trait of bony structure. While, however, it is beyond
-doubt that some bones are not due to the direct influence of mechanical
-stress, we may, I think, conclude that mechanical stress initiates
-bone-formation.
-
-
-§ 302. What is the origin of nerve? In what way do its properties stand
-related to the properties of that protoplasm whence the tissues in
-general arise? and in what way is it differentiated from protoplasm
-simultaneously with the other tissues? These are profoundly interesting
-questions; but questions to which positive answers cannot be expected.
-All that can be done is to indicate answers which seem feasible.
-
-That the property specially displayed by nerve, is a property which
-protoplasm possesses in a lower degree, is manifest. The sarcode
-of a Rhizopod and the substance of an unimpregnated ovum, exhibit
-movements that imply a propagation of stimulus from one part of
-the mass to another. We have not far to seek for a probable origin
-of this phenomenon. There is good reason for ascribing it to the
-extreme instability of the organic colloids of which protoplasm
-consists. These, in common with colloids in general, assume
-different isomeric forms with great facility; and they display not
-simply isomerism but polymerism. Further, this readiness to undergo
-molecular re-arrangement, habitually shows itself in colloids by
-the rapid propagation of the re-arrangement from part to part. As
-Prof. Graham has shown, matter in this state often “pectizes” almost
-instantaneously--a touch will transform an entire mass. That is to say,
-the change of molecular state once set up at one end, spreads to the
-other end--there is a progress of a stimulus to change; and this is
-what we see in a nerve. So much being understood, let us re-state the
-case more completely.
-
-Molecular change, implying as it does motion of molecules, communicates
-motion to adjacent molecules; be they of the same kind or of a
-different kind. If the adjacent molecules, either of the same kind or
-of a different kind, be stable in composition, a temporary increase
-of oscillation in them as wholes, or in their parts, may be the only
-result; but if they are unstable there are apt to arise changes of
-arrangement among them, or among their parts, of more or less permanent
-kinds. Especially is this so with the complex molecules which form
-colloidal matter, and with the organic colloids above all. Hence it is
-to be inferred that a molecular disturbance in any part of a living
-animal, set up by either an external or internal agency, will almost
-certainly disturb and change some of the surrounding colloids not
-originally implicated--will diffuse a wave of change towards other
-parts of the organism: a wave which will, in the absence of perfect
-homogeneity, travel further in some directions than in others. Let us
-ask next what will determine the differences of distance travelled in
-different directions. Obviously any molecular agitation spreading from
-a centre, will go furthest along routes that offer least resistance.
-What routes will these be? Those along which there lie most molecules
-that are easily changed by the diffused molecular motion, and which
-yet do not take up much molecular motion in assuming their new states.
-Molecules which are tolerably stable will not readily propagate the
-agitation; for they will absorb it in the increase of their own
-oscillations, instead of passing it on. Molecules which are unstable
-but which, in assuming isomeric forms, absorb motion, will not readily
-propagate it; since it will disappear in working the changes in them.
-But unstable molecules which, in being isomerically transformed, do not
-absorb motion, and still more those which, in being so transformed,
-give out motion, will readily propagate any molecular agitation; since
-they will pass on the impulse either undiminished, or increased, to
-adjacent molecules. If then we assume, as we are not only warranted
-in doing but are obliged to do, that protoplasm contains two or more
-colloids, either mingled or feebly combined (since it cannot consist of
-simple albumen or fibrin or casein, or any allied proximate principle);
-it may be concluded that any molecular agitation set up by what we call
-a stimulus, will diffuse itself further along some lines than along
-others, if the components of the protoplasm are not quite homogeneously
-dispersed, and if some of them are isomerically transformed more
-easily, or with less expenditure of motion, than others; and it will
-especially travel along spaces occupied chiefly by those molecules
-which give out molecular motion during their metamorphoses, if there
-should be any such. But now let us ask what structural effects
-will be wrought along a tract traversed by this wave of molecular
-disturbance. As is shown by those transformations which so rapidly
-propagate themselves through colloids, molecules that have undergone a
-certain change of form, are apt to communicate a like change of form
-to adjacent molecules of the same kind--the impact of each overthrow
-is passed on and produces another overthrow. Probably the proneness
-towards isochronism of molecular movements necessitates this. If any
-molecule has had its components re-arranged, and their oscillations
-consequently altered, there result movements not concordant with the
-movements in adjacent untransformed molecules, but which, impressing
-themselves on the parts of such untransformed molecules, tend to
-generate in them concordant movements--tend, that is, to produce
-the re-arrangements involved by these concordant movements. Is this
-action limited to strictly isomeric substances? or may it extend to
-substances that are closely-allied? If along with the molecules of
-a compound colloid there are mingled those of some kindred colloid;
-or if with the molecules of this compound colloid there are mingled
-the components out of which other such molecules may be formed; then
-there arises the question--does the same influence which tends to
-propagate the isomeric transformations, tend also to form new molecules
-of the same kind out of the adjacent components? There is reason to
-suspect that it does. Already when treating of the nutrition of parts
-(§ 64), it was pointed out that we are obliged to recognize a power
-possessed by each tissue to build up, out of the materials brought
-to it, molecules of the same type as those of which it is formed.
-This building up of like molecules seems explicable as caused by the
-tendency of the new components which the blood supplies, to acquire
-movements isochronous with those of the like components in the tissue;
-which they can do only by uniting into like compound molecules.
-Necessarily they must gravitate towards a state of equilibrium; such
-state of equilibrium--moving equilibrium of course--must be one in
-which they oscillate in the same times with neighbouring molecules;
-and so to oscillate they must fall into groups identical with the
-groups around them. If this be a general principle of tissue-growth
-and repair, we may conclude that it will apply in the case before
-us. A wave of molecular disturbance passing along a tract of mingled
-colloids closely-allied in composition, and isomerically transforming
-the molecules of one of them, will be apt at the same time to form
-some new molecules of the same type, at any place where there exist
-the proximate components, either uncombined or feebly combined in some
-not very different way. And this will be most likely to occur where
-the molecules of the colloid that are undergoing the isomeric change,
-predominate, but have scattered through them the other molecules out of
-which they may be formed, either by composition or modification. That
-is to say, a wave of molecular disturbance diffused from a centre, and
-travelling furthest along a line where lie most molecules that can be
-isomerically transformed with facility, will be likely at the same time
-to further differentiate this line, and make it more characterized than
-before by the easy-transformability of its molecules. One additional
-step, and the interpretation is reached. Analogy shows it to be not
-improbable that these organic colloids, isomerically transformed by
-slight molecular impact or increase of molecular motion, will some of
-them resume their previous molecular structures after the disturbance
-has passed. We know that what are stable molecular arrangements under
-one degree of molecular agitation, are not stable under another degree;
-and there is evidence that re-arrangements of an inconspicuous kind
-are occasionally brought about by very slight changes of molecular
-agitation. Water supplies a clear case. Prof. Graham infers that water
-undergoes a molecular re-arrangement at about 32°--that ice has a
-colloid form as well as a crystalloid form, dependent on temperature.
-Send through it an extra wave of the molecular agitation we call heat,
-and its molecules aggregate in one way. Let the wave die away, and its
-molecules resume their previous mode of aggregation. And obviously
-such transformations may be repeated backwards and forwards within
-narrow limits of temperature. Now among the extremely unstable organic
-colloids, such a phenomenon is far more likely to happen. Suppose,
-then, that the nerve-colloid is one of which the molecules are changed
-in form by a passing wave of extra agitation, but resume their previous
-form when the wave has passed: the previous form being the most stable
-under the conditions which then recur. What follows? It follows that
-these molecules will be ready again to undergo isomeric transformation
-when there again occurs the stimulus; will, as before, propagate the
-transformation most along the tract where such molecules are most
-abundant; will, as before, tend to form new molecules of their own
-type; will, as before, make the line along which they lie one of easier
-transfer for the molecular agitation. Every repetition will help to
-increase, to integrate, to define more completely, the course of the
-escaping molecular motion--extending its remoter part while it makes
-its nearer part more permeable--will help, that is, to form a line of
-discharge, a line for conducting impressions, a nerve.
-
-Such seems to me a not unfair series of deductions from the known
-habitudes of colloids in general and the organic colloids in
-particular. And I think that the implied nature and properties of nerve
-correspond better with the observed phenomena than do the nature and
-properties implied by other hypotheses. Of course the speculation as
-it here stands is but tentative, and leaves much unexplained. It gives
-no obvious reply to the questions--what causes the formation of nerves
-in directions adapted to the needs? what determines their appropriate
-connexions?--questions, however, to which, when we come to deal with
-physiological integration, we may find not unsatisfactory answers.
-Moreover it says nothing about the genesis of ganglia. A ganglion,
-it is clear, must consist of a colloidal matter equally unstable, or
-still more unstable, which, when disturbed, falls into some different
-molecular arrangement, perhaps chemically simpler, and gives out in
-so doing a large amount of molecular motion--serves as a reservoir of
-molecular motion which may be suddenly discharged along an efferent
-nerve or nerves, when excitement of an afferent nerve has disengaged
-it. How such a structure as this results, the hypothesis does not show.
-But admitting these shortcomings it may still be held that we are, in
-the way pointed out, enabled to form some idea of the actions by which
-nervous tissue is differentiated.
-
-
-§ 303. A speculation akin to, and continuous with, the last,
-is suggested by an inquiry into the origin of muscular tissue.
-Contractility as well as irritability is a property of protoplasm
-or sarcode; and, as before suggested (§ 22), is not improbably
-due to isomeric change in one or more of its component colloids.
-It is a feasible supposition that of the several isomeric changes
-simultaneously set up among these component colloids, some may be
-accompanied by change of bulk and some not. Clearly the isomeric
-change undergone by the colloid which we suppose to form nerve, must
-be one not accompanied by appreciable change of bulk; since change of
-bulk implies “internal work,” as physicists term it, and therefore
-expenditure of force. Conversely, the colloid out of which muscle
-originates, may be one that readily passes into an isomeric state
-in which it occupies less space: the molecular disturbance causing
-this contraction being communicated to it from adjacent portions of
-nerve-substance that are molecularly disturbed; or being otherwise
-communicated to it by direct mechanical or chemical stimuli: as happens
-where nerves do not exist, or where their influence has been cut off.
-This interpretation seems, indeed, to be directly at variance with
-the fact that muscle does not diminish in bulk during contraction
-but merely changes its shape. That which we see take place with the
-muscle as a whole, is said also to take place with each fibre--while
-it shortens it also broadens. There is, however, a possible solution
-of this difficulty. A contracting colloid yields up its water; and
-the contracted colloid _plus_ the free water, may have the same bulk
-as before though the colloid has less. If it be replied that in this
-case the water should become visible between the substance of the
-fibre and its sarcolemma or sheath, it may be rejoined that this
-is not necessary--it may be deposited interstitially. Possibly the
-striated structure is one that facilitates its exudation and subsequent
-re-absorption; and to this may be due the superiority of striated
-muscle in rapidity of contraction. Granting the speculative character
-of this interpretation, let us see how far it agrees with the facts.
-If the actions are as here supposed, the contracted or more integrated
-state of the muscular colloid will be that which it tends continually
-to assume--that into which it has an increasing aptitude to pass when
-artificial paralysis has been produced, as shown by Dr. Norris--that
-into which it lapses completely in _rigor mortis_. The sensible motion
-generated by the contraction can arise only from the transformation
-of insensible motion. This insensible motion suddenly yielded up by
-a contracting mass, implies the fall of its component molecules into
-more stable arrangements. And there can be no such fall unless the
-previous arrangement is unstable. From this point of view, too, it is
-possible to see how the hydro-carbons and carbo-hydrates consumed in
-muscular action, may produce their effects. For these non-nitrogenous
-elements of food, when consumed in the tissues, give out large amounts
-of molecular motion. They do this in presence of the muscular colloids
-which have lost molecular motion during their fall in the stable or
-contracted state. From the molecular motion they give out, may be
-restored the molecular motion lost by the contracted colloids; and
-these contracted colloids may thus have their molecules raised to that
-unstable state from which, again falling, they can again generate
-mechanical motion.
-
-This conception of the nature and mode of action of muscle, while it
-is suggested by known properties of colloidal matter and conforms to
-the recent conclusions of organic chemistry and molecular physics,
-establishes a comprehensible relation between the vital actions of
-the lower and the higher animals. If we contemplate the movements
-of cilia, of a Rhizopod’s pseudopodia, of a Polype’s body, or of the
-long pendant tentacles of a _Medusa_, we shall see great congruity
-between them and this hypothesis. Bearing in mind that the contractile
-substance of developed muscle is affected not by nervous influence
-only, but, where nervous influence is destroyed, is made to contract
-by mechanical disturbance and chemical action, we may infer that
-it does not differ intrinsically from the primordial contractile
-substance which, in the lowest animals, changes its bulk under other
-stimuli than the nervous. We shall see significance in the fact
-ascertained by Dr. Ransom, that various agents which excite and arrest
-nervo-muscular movements in developed animals, excite and arrest the
-protoplasmic movements in ova. We shall understand how tissues not yet
-differentiated into muscle and nerve, have this joint irritability and
-contractility; how muscle and nerve may arise by the segregation of
-their mingled colloids, the one of which, not appreciably altering its
-bulk during isomeric change, readily propagates molecular disturbance,
-while the other, contracting when isomerically changed, less readily
-passes on the molecular disturbance; and how, by this differentiation
-and integration of the conducting and the contracting colloids, the one
-ramifying through the other, it becomes possible for a whole mass to
-contract suddenly, instead of contracting gradually, as it does when
-undifferentiated.
-
-The question remaining to be asked is--What causes the specialization
-of contractile substance?--What causes the growth of colloid masses
-which monopolize this contractility, and leave kindred colloids to
-monopolize other properties? Has natural selection gradually localized
-and increased the primordial muscular substance? or has the frequent
-recurrence of irritations and consequent contractions at particular
-parts done it? We have, I think, reason to conclude that direct
-equilibration rather than indirect equilibration has been chiefly
-operative. The reasoning that was used in the case of nerve applies
-equally in the case of muscle. A portion of undifferentiated tissue
-containing a predominance of the colloid that contracts in changing,
-will, during each change, tend to form new molecules of its own type
-from the other colloids diffused through it: the tendency of these
-entangled colloids to fall into unity with those around them, will
-be aided by every shock of isomeric transformation. Hence, repeated
-contractions will further the growth of the contracting mass, and
-advance its differentiation and integration. If, too, we remember that
-the muscular colloid is made to contract by mechanical disturbance, and
-that among mechanical disturbances one which will most readily affect
-it simultaneously throughout its mass is caused by stretching, we
-shall be considerably helped towards understanding how the contractile
-tissues are developed. If extension of a muscular colloid previously
-at rest, produces in it that molecular disturbance which leads to
-isomeric change and decrease of bulk, then there is no difficulty in
-explaining the movements of cilia; the formation of a contractile
-layer in the vascular system becomes comprehensible; each dilatation
-of a blood-vessel caused by a gush of blood, will be followed by a
-constriction; the heart will pulsate violently in proportion as it is
-violently distended; arteries will develop in power as the stress upon
-them becomes greater; and we shall similarly have an explanation of the
-increased muscularity of the alimentary canal which is brought about by
-increased distension of it.
-
-That the production of contractile tissue in certain localities,
-is due to the more frequent excitement in those localities of the
-contractility possessed by undifferentiated tissue in general, is a
-view harmonizing with traits which the differentiated contractile
-tissue exhibits. These are the relations between muscular exercise,
-muscular power, and muscular structure; and it is the more needful
-for us here to notice them because of certain anomalies they present,
-which, at first sight, seem inconsistent with the belief that the
-functionally-determined modifications of muscle are inheritable.
-
-Muscles disagree greatly in their tints: all gradations between white
-and deep red being observable. Contrasts are visible between the
-muscles of different animals, between the muscles of the same animal
-at different ages, and between different muscles of the same animal
-at the same age. We will glance at the facts under these heads:
-noting under each of them the connexion which here chiefly concerns
-us--that between the activity of muscle and its depth of colour. The
-cold-blooded _Vertebrata_ are, taken as a group, distinguished
-from the warmblooded by the whiteness of their flesh; and they are
-also distinguished by their comparative inertness. Though a fish
-or a reptile can exert considerable force for a short time, it is
-not capable of prolonged exertion. Birds and mammals show greater
-endurance along with the darker-coloured muscles. If among birds
-themselves or mammals themselves we make comparisons, we meet with
-kindred contrasts--especially between wild and domestic creatures of
-allied kinds. Barn-door fowls are lighter-fleshed than most untamed
-gallinaceous birds; and among these last the pheasant, moving about but
-little, is lighter-fleshed than the partridge and the grouse which are
-more nomadic. The muscles of the sheep are not on the average so dark
-as those of the deer; and it is said that the flesh of the wild-boar is
-darker than that of the pig. Perhaps, however, the contrast between the
-hare and the rabbit affords, among familiar animals, the best example
-of the alleged relation: the dark-fleshed hare having no retreat and
-making wide excursions, while the white-fleshed rabbit, passing a
-great part of its time in its burrow, rarely wanders far from home.
-The parallel contrast between young and old animals has a parallel
-meaning. Veal is much whiter than beef, and lamb is of lighter colour
-than mutton. Though at first sight these facts may not seem to furnish
-confirmatory evidence, since lambs in their play appear to expend
-more muscular force than their sedate dams; yet the meaning of the
-contrast is really as alleged. For in consequence of the law that the
-strains which animals have to overcome, increase as the cubes of the
-dimensions, while their powers of overcoming them increase only as
-the squares (§ 46), the movements of an adult animal cost much more
-in muscular effort than do those of a young animal: the result being
-that the sheep and the cow exercise their muscles more vigorously in
-their quiet movements, than the lamb and the calf in their lively
-movements. It may be added as significant, that the domestic animal
-in which no very marked darkening of the flesh takes place along with
-increasing age, namely the pig, is one which, ordinarily kept in a sty,
-leads so quiescent a life that the assigned cause of darkening does
-not come into action. But perhaps the most conclusive evidences are
-the contrasts which exist between the active and inactive muscles of
-the same animal. Between the leg-muscles of fowls and their pectoral
-muscles, the difference of colour is familiar; and we know that fowls
-exercise their leg-muscles much more than the muscles which move their
-wings. Similarly in the turkey, in the guinea fowl, in the pheasant.
-And then, adding much to the force of this evidence, we see that in
-partridges and grouse, which belong to the same order as our domestic
-fowls but use their wings as constantly as their legs, little or
-no difference is visible between the colour of these two groups of
-muscles. Special contrasts like these do not, however, exhaust the
-proofs; for there is a still more significant general contrast. The
-muscle of the heart, which is the most active of all muscles, is the
-darkest of all muscles.
-
-The connexion of phenomena thus shown in so many ways, implies that
-the bulk of a muscle is by no means the sole measure of the quantity
-of force it can evolve. It would seem that, other things equal, the
-depth of colour varies with the constancy of action; while, other
-things equal, the bulk varies with the amount of force that has to be
-put forth upon occasion. These of course are approximate relations.
-More correctly we may say that the actions of pale muscles are either
-relatively feeble though frequent (as in the massive flanks of a fish),
-or relatively infrequent though strong (as in the pectoral muscles of
-a common fowl); while the actions of dark muscles are both frequent
-and strong. Some such differentiation may be anticipated by inference
-from the respective physiological requirements. A muscle which has
-upon occasion to evolve considerable force, but which has thereafter a
-long period of rest during which repair may restore it to efficiency,
-requires neither a large reserve of the contractile substance that is
-in some way deteriorated by action, nor highly developed appliances for
-bringing it nutritive materials and removing effete products. Where,
-contrariwise, an exerted muscle which has undergone much molecular
-change in evolving much mechanical force, has soon again to evolve
-much mechanical force, and so on continually; it is clear that either
-the quantity of contractile substance present must be great, or the
-apparatus for nutrition and depuration must be very efficient, or both.
-Hence we may look for marked unlikenesses of minute structure between
-muscles which are markedly contrasted in activity. And we may suspect
-that these conspicuous contrasts of colour between active and inactive
-muscles, are due to these implied differences of minute structure:
-partly differences between the numbers of blood-vessels and partly
-differences between the quantities or qualities of sarcous matter.
-
-Here, then, we have a key to the apparent anomaly above hinted at--the
-maintenance of bulk by certain muscles which have been rendered
-comparatively inactive by changed habits of life. That the pectoral
-muscles of those domestic birds which fly but little, have not dwindled
-to any great extent, has been thought a fact at variance with the
-conclusion that functionally-produced adaptations are inheritable.
-It has been argued that if parts which are exercised increase, not
-only in the individual but in the race, while parts which become
-less active decrease; then a notable difference of size should exist
-between the muscles used for flight in birds that fly much, and those
-in birds of an allied kind that fly little. But, as we here see, this
-is not the true implication. The change in such cases must be chiefly
-in vascularity and abundance of contractile substance; and cannot be,
-to any great extent, in bulk. For a bird to fly at all, its pectoral
-muscles, bones of attachment, and all accompanying appliances, must be
-kept up to a certain level of power. If the parts dwindle much, the
-creature will be unable to lift itself from the ground. Bearing in mind
-that the force which a bird expends to sustain itself in the air during
-each successive instant of a short flight is, other things equal,
-the same as it expends in each successive instant of a long flight,
-we shall see that the muscles employed in the two cases must have
-something like equal intensities of contractile power; and that the
-structural differences between them must have relation mainly to the
-lengths of time during which they can continue to repeat contractions
-of like intensity. That is to say, while the power of flight is
-retained at all, the muscles and bones cannot greatly dwindle; but the
-dwindling, in birds whose flights are short or infrequent or both,
-will be in the reserve stock of the substance that is incapacitated by
-action, or in the appliances that keep the apparatus in repair, or in
-both. Only where, as in the struthious birds, the habit of flight is
-lost, can we expect atrophy of all the parts concerned in flight; and
-here we find it.
-
-Are such differentiations among the muscles functionally-produced? or
-are they produced by the natural selection of variations distinguished
-as spontaneous? We have, I think, good grounds for concluding that
-they are functionally-produced. We know that in individual men and
-animals, the power of sustained action in muscles is rapidly adaptable
-to the amount of sustained action required. We know that being “out of
-condition,” is usually less shown by the inability to put out a violent
-effort than by the inability to continue making violent efforts; and
-we know that the result of training for prize-fights and races, is more
-shown in the prolongation of energy than in the intensification of
-energy. At the same time, experience has taught us that the structural
-change which accompanies this functional change, is not so much a
-change in the bulk of the muscles as a change in their internal state:
-instead of being soft and flabby they become hard. We have inductive
-proof, then, that exercise of a muscle causes some interstitial
-growth along with the power of more sustained action; and there can
-be no doubt that the one is a condition to the other. What is this
-interstitial growth? There is reason to suspect that it is in part an
-increased deposit of the sarcous substance and in part a development of
-blood-vessels. Microscopic observation tends to confirm the conclusions
-before drawn, that repetition of contractions furthers the formation
-of the matter which contracts, and that greater draughts of blood
-determine greater vascularity. And if the contrasts of molecular
-structure and the contrasts of vascularity, directly caused in muscles
-by contrasts in their activities, are to any degree inheritable;
-there results an explanation of those constitutional differences in
-the colours and textures of muscles, which accompany constitutional
-differences in their degrees of activity.
-
-It may be added that if we are warranted in so ascribing the
-differentiations of muscles from one another to direct equilibration,
-then we have the more reason for thinking that the differentiation
-of muscles in general from other structures is also due to direct
-equilibration. That unlikenesses between parts of the contractile
-tissues having unlike functions, are caused by the unlikenesses of
-their functions, renders it the more probable that the unlikenesses
-between contractile tissue and other tissues, have been caused by
-analogous unlikenesses.
-
-
-§ 304. These interpretations, which have already occupied too
-large a space, must here be closed. Of course out of phenomena so
-multitudinous and varied, it has been impracticable to deal with any
-but the most important; and it has been practicable to deal with these
-only in a general way. Much, however, as remains to be explained, I
-think the possibility of tracing, in so many cases, the actions to
-which these internal differentiations may rationally be ascribed,
-makes it likely that the remaining internal differentiations are
-due to kindred actions. We find evidence that, in more cases than
-seemed probable, these actions produce their effects directly on the
-individual; and that the unlikenesses are produced by accumulation
-of such effects from generation to generation. While for all the
-other unlikenesses, we have, as an adequate cause, the indirect
-effects wrought by the survival, generation after generation, of the
-individuals in which favourable variations have occurred--variations
-such as those of which human anatomy furnishes endless instances.
-Thus accounting for so much, we may not unreasonably presume that
-these co-operative processes of direct and indirect equilibration will
-account for what remains.
-
-       *       *       *       *       *
-
-[NOTE.--After having dismissed this revised chapter as done
-with, and sent it to the printer, further thought concerning those
-differentiations which produce bone, has reminded me of a fact of
-extreme and varied significance named in the first volume. I refer
-to the formation of adaptive structures round the ends of dislocated
-bones, and to the formation of “false joints.”
-
-These are ontogenetic changes of which phylogeny yields no explanation.
-They do not repeat the traits of ancestral organisms, and they cannot
-be ascribed to either of the recognized evolutionary factors. If a
-humerus be broken across and, failing to set, presently comes to have
-its two loose ends so modified as in a measure to simulate the parts of
-a normal joint--the ends becoming smooth, covered with periosteum and
-supplied with fibrous tissue, and attached by ligaments in such ways
-as to allow of restrained movements--it is impossible to think that
-natural selection has had anything to do with the power of adjustment
-thus shown. No survival of individuals in which adaptations of this
-kind, now in one place and now in another, were better and better
-effected, could account for acquirement of the ability. Nor can it be
-supposed that the ability might result from a functionally-produced
-habit; since it is scarcely conceivable that the number of cases in
-which individuals profited by it (at first a little and gradually more)
-could be such (even did they survive) as to affect the constitution of
-the species. Both of the alleged causes of structural modifications
-are out of court. It is manifest, too, that the foregoing hypothesis
-respecting bone-formation yields us not the slightest help.
-
-But on carefully considering the facts, certain phenomena of profound
-meaning may strike us. Here, in a part of the body where no such
-tissues ordinarily exist and to which no such structures are ordinarily
-appropriate, there arise tissues and structures adapted to the physical
-circumstances imposed on that part. Out of what do these abnormal
-but appropriate tissues arise? The substances around--osseous,
-cartilaginous, membranous--consist of differentiated elements too
-far specialized to allow of transformation. These new tissues, then,
-must originate from the undifferentiated protoplasm pervading the
-part. The units of this protoplasm, subject to the actions proper
-to an articulation, begin to assume the appropriate histological
-traits--are determined by local stimuli to form tissues ordinarily
-associated with such stimuli. What is the inevitable implication? These
-units--physiological or constitutional, as we may call them--must
-have possessed latent potentialities of falling into these special
-arrangements under stress of such conditions. At one point there
-arises periosteum and at another ligamentous tissue, while for the
-shaping of the ends of the bones--here into a rude hinged form and
-there into a rude ball-and-socket form, according to the habitual
-movements--there goes on some appropriate deposit of bone. Hence we
-must conclude that in the units of protoplasm which have not yet been
-organized into special tissues, there resides the ability to take on
-one or other type of histological structure according to circumstances;
-and, further, that there resides in each of them the still more
-marvellous ability to co-operate with kindred units dispersed around
-in developing that arrangement of the parts required to constitute a
-“false joint.” So that while these units have a general proclivity
-towards the structure of the organism as a whole, they have also
-proclivities towards structures proper to the local conditions into
-which they fall. There is latent in each unit the constitution of the
-entire organism and by implication the constitution of every organ; and
-each unit while co-operating with the aggregate is ready to take part
-in that particular arrangement proper to the position it has fallen
-into. If the reader will refer back to §§ 97_d_, 97_e_, in which it is
-shown that each member of a human society possesses a combination of
-potentialities like these, he will be the better enabled to believe
-that this thing _may be_ so while he is unable to conceive how it _is_
-so.
-
-And here, indeed, let it be pointed out how completely irrelevant is
-the test of conceivableness as applied to these ultimate physiological
-actions. For as here, from the un-united ends of the broken bone, there
-presently arises a rude joint with fit membranes, ligaments, and even
-synovial fluid, though we are absolutely unable to imagine the process
-by which the adjacent tissues produce this structure; so there may
-be from an organ enlarged by function, such reactive effect upon the
-system at large as eventually to influence the reproductive cells,
-though we may be absolutely unable to imagine how this can be done.]
-
-
-
-
-                              CHAPTER IX.
-
-                 PHYSIOLOGICAL INTEGRATION IN ANIMALS.
-
-
-§ 305. Physiological differentiation and physiological integration,
-are correlatives that vary together. We have but to recollect the
-familiar parallel between the division of labour in a society and the
-physiological division of labour, to see that as fast as the kinds
-of work performed by the component parts of an organism become more
-numerous, and as fast as each part becomes more restricted to its
-own work, so fast must the parts have their actions combined in such
-ways that no one can go on without the rest and the rest cannot go on
-without each one.
-
-Here our inquiry must be, how the relationship of these two processes
-is established--what causes the integration to advance _pari passu_
-with the differentiation. Though it is manifest, _à priori_, that
-the mutual dependence of functions must be proportionate to the
-specialization of functions; yet it remains to find the mode in which
-the increasing co-ordination is determined.
-
-Already, among the Inductions of Biology, this relation between
-differentiation and integration has been specified and illustrated
-(§ 59). Before dealing with it deductively, a few further examples,
-grouped so as to exhibit its several aspects, will be advantageous.
-
-
-§ 306. If the lowly-organized _Planaria_ has its body broken up and
-its gullet detached, this will, for a while, continue to perform
-its function when called upon, just as though it were in its place:
-a fragment of the creature’s own body placed in the gullet, will
-be propelled through it, or swallowed by it. But, as the seeming
-strangeness of this fact implies, we find no such independent actions
-of analogous parts in the higher animals. Again, a piece cut out of the
-disc of a _Medusa_ continues with great persistence repeating those
-rhythmical contractions which we see in the disc as a whole; and thus
-proves to us that the contractile function in each portion of the disc,
-is in great measure independent. But it is not so with the locomotive
-organs of more differentiated types. When separated from the rest these
-lose their powers of movement. The only member of a vertebrate animal
-which continues to act after detachment, is the heart; and the heart
-has motor powers complete within itself.
-
-Where there is this small dependence of each part upon the whole,
-there is but small dependence of the whole upon each part. The longer
-time which it takes for the arrest of a function to produce death in
-a less-differentiated animal than in a more-differentiated animal,
-may be illustrated by the case of respiration. Suffocation in a man
-speedily causes resistance to the passage of the blood through the
-capillaries, followed by congestion and stoppage of the heart: great
-disturbance throughout the system results in a few seconds, and in
-a minute or two all the functions cease. But in a frog, with its
-undeveloped respiratory organ, and a skin through which a considerable
-aëration of the blood is carried on, breathing may be suspended for
-a long time without injury. Doubtless this difference is proximately
-due to the greater functional activity in the one case than in the
-other, and the more pressing need for discharging the produced carbon
-dioxide; but the greater functional activity being itself made possible
-by the higher specialization of functions, this remains the primary
-cause of the greater dependence of the other functions on respiration,
-where the respiratory apparatus has become highly specialized. Here
-indeed, we see the relation under another aspect. This more rapid
-rhythm of the functions which increased heterogeneity of structure
-makes possible, is itself a means of integrating the functions. Watch,
-when it is running down, a complicated machine of which the parts are
-not accurately adjusted, or are so worn as to be somewhat loose. There
-will be observed certain irregularities of movement just before it
-comes to rest--certain of the parts which stop first, are again made to
-move a little by the continued movement of the rest, and then become
-themselves, in turn, the causes of renewed motion in other parts which
-have ceased to move. That is to say, while the connected rhythmical
-changes of the machine are quick, their actions and reactions on one
-another are regular--all the motions are well integrated; but as the
-velocity diminishes irregularities arise--the motions become somewhat
-disintegrated. Similarly with organic functions: increase of their
-rapidity involves increase of a joint momentum which controls each
-and co-ordinates all. Thus if we compare a snake with a mammal, we
-see that its functions are not tied together so closely. The mammal,
-and especially the superior mammal, requires food with considerable
-regularity; keeps up a respiration which varies within but moderate
-limits; and has periods of activity and rest that alternate evenly and
-frequently. But the snake, taking food at long intervals, may have
-these intervals greatly extended without fatal results; its dormant and
-its active states recur less uniformly; and its rate of respiration
-varies within much wider limits--now being scarcely perceptible and
-now, as you may prove by exciting it, becoming conspicuous. So that
-here, where the rhythms are very slow, they are individually less
-regular, and are united into a less regular compound rhythm--are less
-integrated.
-
-Perhaps the clearest general idea of the co-ordination of functions
-that accompanies their specialization, is obtained by observing the
-slowness with which a little-differentiated animal responds to a
-stimulus applied to one of its parts, and the rapidity with which
-such a local stimulus is responded to by a more-differentiated animal.
-A sea-anemone and a fly will serve for the comparison. A tentacle of
-a sea-anemone, when touched, slowly contracts; and if the touch has
-been rude, the contraction presently extends to the other tentacles
-and eventually to the entire body: the stimulus to movement is
-gradually diffused throughout the organism. But if you touch a fly,
-or rather if you come near enough to threaten a touch, the entire
-apparatus of flight is instantly brought into combined action. Whence
-arises this contrast? The one creature has but faintly specialized
-contractile organs, and fibres for conveying impressions. The other
-has definite muscles and nerves and a co-ordinating centre. The parts
-of the little-differentiated sea-anemone have their functions so
-feebly co-ordinated, that one may be strongly affected for some time
-before any effect is felt by another at a distance from it; but in the
-much-differentiated fly, various remote parts instantly have changes
-propagated to them from the affected part, and by their united actions
-thus set up, the whole organism adjusts itself so as to avoid the
-danger.
-
-These few added illustrations will make the nature of this general
-relation sufficiently clear. Let us now pass to the interpretation of
-it.
-
-
-§ 307. If a _Hydra_ is cut in two, the nutritive liquids diffused
-through its substance cannot escape rapidly, since there are no
-open channels for them; and hence the conditions of the parts at
-a distance from the cut is but little affected. But where, as in
-the more-differentiated animals, the nutritive liquid is contained
-in vessels which have continuous communications, cutting the body
-in two, or cutting off any considerable portion of it, is followed
-by escape of the liquid from these vessels to a large extent; and
-this affects the nutrition and efficiency of organs remote from the
-place of injury. Then where, as in further-developed creatures,
-there exists an apparatus for propelling the blood through these
-ramifying channels, injury of a single one will cause a loss of blood
-that quickly prostrates the entire organism. Hence the rise of a
-completely-differentiated vascular system, is the rise of a system
-which integrates all members of the body, by making each dependent on
-the integrity of the vascular system, and therefore on the integrity
-of each member through which it ramifies. In another mode, too, the
-establishment of a distributing apparatus produces a physiological
-union that is great in proportion as this distributing apparatus is
-efficient. As fast as it assumes a function unlike the rest, each part
-of an animal modifies the blood in a way more or less unlike the rest,
-both by the materials it abstracts and by the products it adds; and
-hence the more differentiated the vascular system becomes, the more
-does it integrate all parts by making each of them feel the qualitative
-modification of the blood which every other has produced. This is
-simply and conspicuously exemplified by the lungs. In the absence of a
-vascular system, or in the absence of one that is well marked off from
-the imbedding tissues, the nutritive plasma or the crude blood, gets
-what small aëration it can, only by coming near the creature’s outer
-surface, or those inner surfaces which are bathed by water. But where
-there have been formed definite channels branching throughout the body,
-and particularly where there exist specialized organs for pumping the
-blood through these channels, it manifestly becomes possible for the
-aëration to be carried on in one part peculiarly modified to further
-it, while all other parts have the aërated blood brought to them. And
-how greatly the differentiation of the vascular system thus becomes a
-means of integrating the various organs, is shown by the fatal result
-that follows when the current of aërated blood is interrupted.
-
-Here, indeed, it becomes obvious both that certain physiological
-differentiations make possible certain physiological integrations;
-and that, conversely, these integrations make possible other
-differentiations. Besides the waste products which escape through the
-lungs, there are waste products which escape through the skin, the
-kidneys, the liver. The blood has separated from it in each of these
-structures, the particular product which this structure has become
-adapted to separate; leaving the other products to be separated by
-the other adapted structures. How have these special adaptations been
-made possible? By union of the organs as recipients of one circulating
-mass of blood. While there is no efficient apparatus for transfer of
-materials through the body, the waste products of each part have to
-make their escape locally; and the local channels of escape must be
-competent to take off indifferently all the waste products. But it
-becomes practicable and advantageous for the differently-localized
-excreting structures to become fitted to separate different waste
-products, as soon as the common circulation through them grows so
-efficient that the product left unexcreted by one is quickly carried to
-another better fitted to excrete it. So that the integration of them
-through a common vascular system, is the condition under which only
-they can become differentiated. Perhaps the clearest idea of the way in
-which differentiation leads to integration, and how, again, increased
-integration makes possible still further differentiation, will be
-obtained by contemplating the analogous dependence in the social
-organism. While it has no roads, a country cannot have its industries
-much specialized: each locality must produce, as best it can, the
-various commodities it consumes, so long as it has no facilities for
-barter with other localities. But the localities being unlike in their
-natural fitnesses for the various industries, there tends ever to
-arise some exchange of the commodities they can respectively produce
-with least labour. This exchange leads to the formation of channels
-of communication. The currents of commodities once set up, make
-their foot-paths and horse-tracks more permeable; and as fast as the
-resistance to exchange becomes less, the currents of commodities become
-greater. Each locality takes more of the products of adjacent ones,
-and each locality devotes itself more to the particular industry for
-which it is naturally best fitted: the functional integration makes
-possible a further functional differentiation. This further functional
-differentiation reacts. The greater demand for the special product of
-each locality, excites improvements in production--leads to the use of
-methods which both cheapen and perfect the commodity. Hence results a
-still more active exchange; a still clearer opening of the channels of
-communication; a still closer mutual dependence. Yet another influence
-comes into play. As fast as the intercourse, at first only between
-neighbouring localities, makes for itself better roads--as fast as
-rivers are bridged and marshes made easily passable, the resistance to
-distribution becomes so far diminished, that the things grown or made
-in each district can be profitably carried to a greater distance; and
-as the economical integration is thus extended over a wider area, the
-economical differentiation is again increased; since each district,
-having a larger market for its commodity, is led to devote itself more
-exclusively to producing this commodity. These actions and reactions
-continue until the various localities, becoming greatly developed
-and highly specialized in their industries, are at the same time
-functionally integrated by a network of roads, and finally railways,
-along which rapidly circulate the currents severally sent out and
-received by the localities. And it will be manifest that in individual
-organisms a like correlative progress must have been caused in an
-analogous way.
-
-
-§ 308. Another and higher form of physiological integration in animals,
-is that which the nervous system effects. Each part as it becomes
-specialized, begins to act upon the rest not only indirectly through
-the matters it takes from and adds to the blood, but also directly
-through the molecular disturbances it sets up and diffuses. Whether
-nerves themselves are differentiated by the molecular disturbances
-thus propagated in certain directions, or whether they are otherwise
-differentiated, it must equally happen that as fast as they become
-channels along which molecular disturbances travel, the parts they
-connect become physiologically integrated, in so far that a change
-in one initiates a change in the other. We may dimly perceive that
-if portions of what was originally a uniform mass having a common
-function, undertake subdivisions of the function, the molecular changes
-going on in them will be in some way complementary to one another:
-that peculiar form of molecular motion which the one has lost in
-becoming specialized, the other has gained in becoming specialized.
-And if the molecular motion that was common to the two portions while
-they were undifferentiated, becomes divided into two complementary
-kinds of molecular motion; then between these portions there will be
-a contrast of molecular motions such that whatever is _plus_ in the
-one will be _minus_ in the other; and hence there will be a special
-tendency towards a restoration of the molecular equilibrium between
-the two: the molecular motion continually propagated away from either
-will have its line of least resistance in the direction of the
-other. If, as argued in the last chapter, repeated restorations of
-molecular equilibrium, always following the line of least resistance,
-tend ever to make it a line of diminished resistance; then, in
-proportion as any parts become more physiologically integrated by
-the establishment of this channel for the easy transmission of
-molecular motion between them, they may become more physiologically
-differentiated. The contrast between their molecular motions leads to
-the line of discharge; the line of discharge, once formed, permits a
-greater contrast of their molecular motions to arise; thereupon the
-quantities of molecular motion transferred to restore equilibrium,
-being increased, the channel of transfer is made more permeable; and
-its further permeability, so caused, renders possible a still more
-marked unlikeness of action between the parts. Thus the differentiation
-and the integration progress hand in hand as before. How the same
-principle holds throughout the higher stages of nervous development,
-can be seen only still more vaguely. Nevertheless, it is comprehensible
-that as functions become further divided, there will arise the
-need for sub-connexions along which there may take place secondary
-equilibrations subordinate to the main ones. It is manifest, too, that
-whereas the differentiation of functions proceeds, not necessarily by
-division into two, but often by division into several, and usually in
-such ways as not to leave any two functions that are just complementary
-to one another, the restorations of equilibrium cannot be so simple
-as above supposed. And especially when we bear in mind that many
-differentiated functions, as those of the senses, cannot be held
-complementary to any other functions in particular; it becomes manifest
-that the equilibrations that have to be made in an organism of much
-heterogeneity, are extremely complex, and do not take place between
-each organ and some other, but between each organ and all the others.
-The peculiarity of the molecular motion propagated from each organ,
-has to be neutralized by some counter-peculiarity in the average of
-the molecular motions with which it is brought into relation. All the
-variously-modified molecular motions from the various parts, must have
-their pluses and minuses mutually cancelled: if not locally, then at
-some centre to which each unbalanced motion travels until it meets with
-some opposite unbalanced motion to destroy it. Still, involved as these
-actions must become, it is possible to see how the general principle
-illustrated by the simple case above supposed, will continue to hold.
-For always the molecular motion proceeding from any one differentiated
-part, will travel most readily towards that place where a molecular
-motion most complementary to it in kind exists--no matter whether this
-complementary molecular motion be that proceeding from any one other
-organ, or the _resultant_ of the molecular motions proceeding from
-many other organs. So that the tendency will be for each channel of
-communication or nerve, to unite itself with some centre or ganglion,
-where it comes into relation with other nerves. And if there be any
-parts of its peculiar molecular motion uncancelled by the molecular
-motions it meets at this centre; or if, as will probably happen, the
-average molecular motion which it there unites to produce, differs from
-the average molecular motion elsewhere; then, as before, there will
-arise a discharge along another channel or nerve to another centre
-or ganglion, where the residuary difference may be cancelled by the
-differences it meets; or whence it may be still further propagated till
-it is so cancelled. Thus there will be a tendency to a general nervous
-integration keeping pace with the differentiation.
-
-Of course this must be taken as nothing more than the indication of
-initial tendencies--not as an hypothesis sufficient to account for all
-the facts. It leaves out of sight the origin and functions of ganglia,
-considered as something more than nerve-junctions. Were there only
-these lines of easy transmission of molecular disturbance, a change
-set up in one organ could never do more than produce its equivalent of
-change in some other or others; and there could be none of that large
-amount of motion initiated by a small sensation, which we habitually
-see. The facts show, unmistakably, that the slight disturbance
-communicated to a ganglion, causes an overthrow of that highly-unstable
-nervous matter contained in it, and a discharge from it of the
-greatly-increased quantity of molecular motion so generated. This,
-however, is beyond our immediate topic. All we have here to note is the
-interdependence and unification of functions that naturally follow the
-differentiation of them.
-
-
-§ 309. Something might be added concerning the further class of
-integrations by which organisms are constituted mechanically-coherent
-wholes. Carrying further certain of the arguments contained in the
-last chapter, it might be not unreasonably inferred that the binding
-together of parts by bones, muscles, and ligaments, is a secondary
-result of those same actions by which bones, muscles, and ligaments
-are specialized. But adequate treatment of this division of the subject
-is at present scarcely possible.
-
-What little of fact and inference has been above set down, will,
-however, serve to make comprehensible the general truths respecting
-which, in their main outlines, there can be no question. Beginning with
-the feebly-differentiated sponge, of which the integration is also so
-feeble that cutting off a piece interferes in no appreciable degree
-with the activity and growth of the rest, it is undeniable that the
-advance is through stages in which the multiplication of unlike parts
-having unlike actions, is accompanied by an increasing interdependence
-of the parts and their actions; until we come to structures like our
-own, in which a slight change initiated in one part will instantly and
-powerfully affect all other parts--will convulse an immense number of
-muscles, send a wave of contraction through all the blood-vessels,
-awaken a crowd of ideas with an accompanying gush of emotions, affect
-the action of the lungs, of the stomach, and of all the secreting
-organs. And while it is a manifest necessity that along with this
-subdivision of functions which the higher organisms show us, there
-must be this close co-ordination of them, the foregoing paragraphs
-suggest how this necessary correlation is brought about. For a great
-part of the physiological union that accompanies the physiological
-specialization, there appears to be a sufficient cause in the process
-of direct equilibration; and indirect equilibration may be fairly
-presumed a sufficient cause for that which remains.
-
-
-
-
-                              CHAPTER X.
-
-                 SUMMARY OF PHYSIOLOGICAL DEVELOPMENT.
-
-
-§ 310. Intercourse between each part and the particular conditions
-to which it is exposed, either habitually in the individual
-or occasionally in the race, thus appears to be the origin of
-physiological development; as we found it to be the origin of
-morphological development. The unlikenesses of form that arise among
-members of an aggregate that were originally alike, we traced to
-unlikenesses in the incident forces. And in the foregoing chapters we
-have traced to unlikenesses in the incident forces, those unlikenesses
-of minute structure and chemical composition that simultaneously arise
-among the parts.
-
-In summing up the special truths illustrative of this general truth,
-it will be proper here to contemplate more especially their dependence
-on first principles. Dealing with biological phenomena as phenomena of
-evolution, we have to interpret not only the increasing morphological
-heterogeneity of organisms, but also their increasing physiological
-heterogeneity, in terms of the re-distribution of matter and motion.
-While we make our rapid re-survey of the facts, let us then more
-particularly observe how they are subordinate to the universal course
-of this re-distribution.
-
-
-§ 311. The instability of the homogeneous, or, strictly speaking, the
-inevitable lapse of the more homogeneous into the less homogeneous,
-which we before saw endlessly exemplified by the morphological
-differentiations of the parts of organisms, we have here seen
-afresh exemplified in ways also countless, by the physiological
-differentiations of their parts. And in the one case as in the other,
-this change from uniformity to multiformity in organic aggregates, is
-caused, as it is in all inorganic aggregates, by the necessary exposure
-of their component parts to actions unlike in kind or quantity or
-both. General proof of this is furnished by the order in which the
-differences appear. If parts are rendered physiologically heterogeneous
-by the heterogeneity of the incident forces, then the earliest
-contrasts should be between parts that are the most strongly contrasted
-in their relations to incident forces; the next earliest contrasts
-should occur where there are the next strongest contrasts in these
-relations; and so on. It turns out that they do so.
-
-Everywhere the differentiation of outside from inside comes first.
-In the simplest plants the unlikeness of the cell-wall to the
-cell-contents is the conspicuous trait of structure. The contrasts
-seen in the simplest animals are of the same kind: the film that
-covers a Rhizopod and the more indurated coat of an Infusorian, are
-more unlike the contained sarcode than the other parts of this are
-from one another; and the tendency during the life of the animal is
-for the unlikeness to become greater. What is true of _Protophyta_ and
-_Protozoa_, is true of the germs of all organisms up to the highest:
-the differentiation of outer from inner is the first step. When the
-protoplasm of an _Alga_-cell has broken up into the clusters of
-granules which are eventually to become spores, each of these quickly
-acquires a membranous coating, constituting an unlikeness between
-surface and centre. Similarly with the ovule of every higher plant:
-the mass of cells forming it, early exhibits an outside layer of cells
-distinguished from the cells within. With animal-germs it is the
-same. Be it in a ciliated gemmule, be it in the unfertilized ova of
-_Aphides_ and of the _Cecidomyia_, or be it in true ova, the primary
-differentiation conforms to the relations of exterior and interior.
-If we turn to adult organisms, vegetal or animal, we see that whether
-they do or do not display other contrasts of parts, they always display
-this contrast. Though otherwise almost homogeneous, such _Fungi_ as the
-puff-ball, or, among _Algæ_, all which have a thallus of any thickness,
-present marked differences between those of their cells which are in
-immediate contact with the environment and those which are not. Such
-differences they present in common with every higher plant; which,
-here in the shape of bark and there in the shape of cuticle, has an
-envelope inclosing it even up to its petals and stamens. In like manner
-among animals, there is always either a true skin or an outer coat
-analogous to one. Wherever aggregates of the first order have united
-into aggregates of the second and third orders--wherever they have
-become the morphological units of such higher aggregates--the outermost
-of them have grown unlike those lying within. Even the Sponge is not
-without a layer that may by analogy be called dermal.
-
-This lapse of the relatively homogeneous into the relatively
-heterogeneous, first showing itself, as on the hypothesis of evolution
-it must do, by the rise of an unlikeness between outside and inside,
-goes on next to show itself, as we infer that it must do, by the
-establishment of secondary contrasts among the outer parts answering to
-secondary contrasts among the forces falling on them. So long as the
-whole surface of a plant remains similarly related to the environment,
-as in a _Protococcus_, it remains uniform; but when there come to be
-an attached surface and a free surface, these, being subject to unlike
-actions, are rendered unlike. This is visible even in a unicellular
-_Alga_ when it becomes fixed; it is shown in the distinction between
-the under and upper parts of ordinary _Fungi_; and we see it in the
-universal difference between the imbedded ends and the exposed ends of
-the higher plants. And then among the less marked contrasts of surface
-answering to the less marked contrasts in the incident forces, come
-those between the upper and under sides of leaves; which, as we have
-seen, vary in degree as the contrasts of forces vary in degree, and
-disappear where these contrasts disappear. Equally clear proof is
-furnished by animals, that the original uniformity of surface lapses
-into multiformity, in proportion as the actions of the environment
-upon the surface become multiform. In a Worm, burrowing through damp
-soil which acts equally on all its sides, or in a _Tænia_, uniformly
-bathed by the contents of the intestine it inhabits, the parts of the
-integument do not appreciably differ from one another; but in creatures
-not surrounded by the same agencies, as those that crawl and those
-that have their bodies partially inclosed, there are unlikenesses of
-integument corresponding to unlikenesses of the conditions. A snail’s
-foot has an under surface not uniform with the exposed surface of
-its body, and this again is not uniform with the protected surface.
-Among articulate animals there is usually a distinction between the
-ventral and the dorsal aspects; and in those of the _Arthropoda_ which
-subject their anterior and posterior ends to different environing
-agencies, as do the ant-lion and the hermit-crab, these become
-superficially differentiated. Analogous general contrasts occur among
-the _Vertebrata_. Fishes, though their outsides are uniformly bathed
-by water, have their backs more exposed to light than their bellies,
-and the two are commonly distinct in colour. When it is not the back
-and belly which are thus dissimilarly conditioned, but the sides, as in
-the _Pleuronectidæ_, then it is the sides which become contrasted; and
-there may be significance in the fact that those abnormal individuals
-of this order which revert to the ancestral undistorted type, and
-swim vertically, have the two sides alike. In such higher vertebrates
-as reptiles, we see repeated this differentiation of the upper and
-under surfaces: especially in those of them which, like snakes, expose
-these surfaces to the most diverse actions. Even in birds and mammals
-which usually, by raising the under surface considerably above the
-ground, greatly diminish the contrast between its conditions and the
-conditions to which the upper surface is subject, there still remains
-some unlikeness of clothing answering to the remaining unlikeness
-between the conditions. Thus, without by any means saying that all such
-differentiations are directly caused by differences in the actions of
-incident forces, which, as before shown (§ 294), they cannot be, it is
-clear that many of them are so caused. It is clear that parts of the
-surface exposed to very unlike environing agencies, become very unlike;
-and this is all that needs to be shown.
-
-Complex as are the transformations of the inner parts of organisms
-from the relatively homogeneous into the relatively heterogeneous, we
-still see among them a conformity to the same general order. In both
-plants and animals the earlier internal differentiations answer to the
-stronger contrasts of conditions. Plants, absorbing all their nutriment
-through their outer surfaces, are internally modified mainly by the
-transfer of materials and by mechanical stress. Such of them as do not
-raise their fronds above the surface, have their inner tissues subject
-to no marked contrasts save those caused by currents of sap; and the
-lines of lengthened and otherwise changed cells which are formed where
-these currents run, and are most conspicuous where these currents must
-obviously be the strongest, are the only decided differentiations of
-the interior. But where, as in the higher Cryptogams and in Phænogams,
-the leaves are upheld, and the supporting stem is transversely bent by
-the wind, the inner tissues, subject to different amounts of mechanical
-strain, differentiate accordingly: the deposit of dense substance
-commences in that region where the sap-containing cells and canals
-suffer the greatest intermittent compressions. Animals, or at least
-such of them as take food into their interiors, are subject to forces
-of another class tending to destroy their original homogeneity. Food
-is a foreign substance which acts on the interior as an environing
-object which touches it acts on the exterior--is literally a portion
-of the environment which, when swallowed, becomes a cause of internal
-differentiations as the rest of the environment continues a cause
-of external differentiations. How essentially parallel are the two
-sets of actions and reactions, we have seen implied by the primordial
-identity of the endoderm and ectoderm in simple animals, and of the
-skin and mucous membrane in complex animals (§§ 288, 289). Here
-we have further to observe that as food is the original source of
-internal differentiations, these may be expected to show themselves
-first where the influence of the food is greatest; and to appear
-later in proportion as the parts are more removed from the influence
-of the food. They do this. In animals of low type, the coats of the
-alimentary cavity or canal are more differentiated than the tissue
-which lies between the alimentary canal and the wall of the body. This
-tissue in the higher _Cœlenterata_, is a feebly-organized parenchyma
-traversed by canals lined with simple ciliated cells; and in the lower
-_Mollusca_ the structures bounding the peri-visceral cavity and its
-ramifying sinuses, are similarly imperfect. Further, it is observable
-that the differentiation of this peri-visceral sac and its sinuses into
-a vascular system, proceeds centrifugally from the region where the
-absorbed nutriment enters the mass of circulating liquid, and where
-this liquid is qualitatively more unlike the tissues than it is at the
-remoter parts of the body.
-
-Physiological development, then, is initiated by that instability
-of the homogeneous which we have seen to be everywhere a cause of
-evolution (_First Principles_, §§ 149–155). That the passage
-from comparative uniformity of composition and minute structure to
-comparative multiformity, is set up in organic aggregates, as in all
-other aggregates, by the necessary unlikenesses of the actions to which
-the parts are subject, is shown by the universal rise of the primary
-differentiation into the parts that are universally most contrasted in
-their circumstances, and by the rise of secondary differentiations
-obviously related in their order to secondary contrasts of conditions.
-
-
-§ 312. How physiological development has all along been aided by the
-multiplication of effects--how each differentiation has ever tended to
-become the parent of new differentiations, we have had, incidentally,
-various illustrations. Let us here review the working of this cause.
-
-Among plants we see it in the production of progressively-multiplying
-heterogeneities of tissue by progressive increase of bulk. The
-integration of fronds into axes and of axes into groups of axes,
-sets up unlikenesses of action among the integrated units, followed
-by unlikenesses of minute structure. Each gust transversely strains
-the various parts of the stem in various degrees, and longitudinally
-strains in various degrees the roots; and while there is inequality of
-stress at every place in stem and branch, so, at every place in stem
-and branch, the outer layers and the successively inner layers are
-severally extended and compressed to unequal amounts, and have unequal
-modifications wrought in them. Let the tree add to its periphery
-another generation of the units composing it, and immediately the
-mechanical strains on the supporting parts are all changed in different
-degrees, initiating new differences internally. Externally, too, new
-differences are initiated. Shaded by the leaf-bearing outer stratum
-of shoots, the inner structures cease to bear leaves, or to put out
-shoots which bear leaves; and instead of that green covering which
-they originally had, become covered with bark of increasing thickness.
-Manifestly, then, the larger integration of units that are originally
-simple and uniform, entails physiological changes of various orders,
-varying in their degrees at all parts of the aggregate. Each branch
-which, favourably circumstanced, flourishes more than its neighbours,
-becomes a cause of physiological differentiations, not only in its
-neighbours from which it abstracts sap and presently turns from
-leaf-bearers into fruit-bearers, but also in the remoter parts.
-
-That among animals physiological development is furthered by the
-multiplication of effects, we have lately seen proved by the many
-changes in other organs, which the growth or modification of each
-excreting and secreting organ initiates. By the abstracted as well as
-by the added materials, it alters the quality of the blood passing
-through all members of the body; or by the liquid it pours into the
-alimentary canal, it acts on the food, and through it on the blood,
-and through it on the system as a whole: an additional differentiation
-in one part thus setting up additional differentiations in many other
-parts; from each of which, again, secondary differentiating forces
-reverberate through the organism. Or, to take an influence of another
-order, we have seen how the modified mechanical action of any member
-not only modifies that member, but becomes, by its reactions, a cause
-of secondary modifications--how, for example, the burrowing habits
-of the common mole, leading to an almost exclusive use of the fore
-limbs, have entailed a dwindling of the hind limbs, and a concomitant
-dwindling of the pelvis, which, becoming too small for the passage of
-the young, has initiated still more anomalous modifications.
-
-So that throughout physiological development, as in evolution at large,
-the multiplication of effects has been a factor constantly at work, and
-working more actively as the development has advanced. The secondary
-changes wrought by each primary change, have necessarily become more
-numerous in proportion as organisms have become more complex. And
-every increased multiplication of effects, further differentiating the
-organism and, by consequence, further integrating it, has prepared the
-way for still higher differentiations and integrations similarly caused.
-
-
-§ 313. The general truth next to be resumed, is that these processes
-have for their limit a state of equilibrium--proximately a moving
-equilibrium and ultimately a complete equilibrium. The changes we have
-contemplated are but the concomitants of a progressing equilibration.
-In every aggregate which we call living, as well as in all other
-aggregates, the instability of the homogeneous is but another name
-for the absence of balance between the incident forces and the forces
-which the aggregate opposes to them; and the passage into heterogeneity
-is the passage towards a state of balance. And to say that in every
-aggregate, organic or other, there goes on a multiplication of effects,
-is but to say that one part which has a fresh force impressed on it,
-must go on changing and communicating secondary changes, until the
-whole of the impressed force has been used up in generating equivalent
-reactive forces.
-
-The principle that whatever new action an organism is subject to, must
-either overthrow the moving equilibrium of its functions and cause the
-sudden equilibration called death, or else must progressively alter the
-organic rhythms until, by the establishment of a new reaction balancing
-the new action a new moving equilibrium is produced, applies as much
-to each member of an organism as to the organism in its totality. Any
-force falling on any part not adapted to bear it, must either cause
-local destruction of tissue, or must, without destroying the tissue,
-continue to change it until it can change it no further; that is--until
-the modified reaction of the part has become equal to the modified
-action. Whatever the nature of the force this must happen. If it is a
-mechanical force, then the immediate effect is some distortion of the
-part--a distortion having for its limit that attitude in which the
-resistance of the structures to further change of position, balances
-the force tending to produce the further change; and the ultimate
-effect, supposing the force to be continuous or recurrent, is such a
-permanent alteration of form, or alteration of structure, or both, as
-establishes a permanent balance. If the force is physico-chemical, or
-chemical, the general result is still the same: the component molecules
-of the tissue must have their molecular arrangements changed, and the
-change in their molecular arrangements must go on until their molecular
-motions are so re-adjusted as to equilibrate the molecular motions of
-the new physico-chemical or chemical agent. In other words, the organic
-matter composing the part, if it continues to be organic matter at all,
-must assume that molecular composition which enables it to bear, or as
-we say adapts it to, the incident forces.
-
-Nor is it less certain that throughout the organism as a whole,
-equilibration is alike the proximate limit of the changes wrought by
-each action, as well as the ultimate limit of the changes wrought by
-any recurrent actions or continuous action. The movements every instant
-going on, are movements towards a new state of equilibrium. Raising
-a limb causes a simultaneous shifting of the centre of gravity, and
-such altered tensions and pressures throughout the body as re-adjust
-the disturbed balance. Passage of liquid into or out of a tissue,
-implies some excess of force in one direction there at work; and ceases
-only when the force so diminishes or the counter-forces so increase
-that the excess disappears. A nervous discharge is reflected and
-re-reflected from part to part, until it has all been used up in the
-re-arrangements produced--equilibrated by the reactions called out. And
-what is thus obviously true of every normal change, is equally true of
-every abnormal change--every disturbance of the established rhythm of
-the functions. If such disturbance is a single one, the perturbations
-set up by it, reverberating throughout the system, leave its moving
-equilibrium slightly altered. If the disturbance is repeated or
-persistent, its successive effects accumulate until they have produced
-a new moving equilibrium adjusted to the new force.
-
-Each re-balancing of actions, having for its necessary concomitant a
-modification of tissues, it is an obvious corollary that organisms
-subjected to successive changes of conditions, must undergo successive
-differentiations and re-differentiations. Direct equilibration in
-organisms, with all its accompanying structural alterations, is as
-certain as is that universal progress towards equilibrium of which
-it forms part. And just as certain is that indirect equilibration
-in organisms to which the remaining large class of differentiations
-is due. The development of favourable variations by the killing of
-individuals in which they do not occur or are least marked, is, as
-before, a balancing between certain local structures and the forces
-they are exposed to; and is no less inevitable than the other.
-
-
-§ 314. In all which universal laws, we find ourselves again brought
-down to the persistence of force, as the deepest knowable cause of
-those modifications which constitute physiological development; as
-it is the deepest knowable cause of all other evolution. Here, as
-elsewhere, the perpetual lapse from less to greater heterogeneity,
-the perpetual begetting of secondary modifications by each primary
-modification, and the perpetual approach to a temporary balance on the
-way towards a final balance, are necessary implications of the ultimate
-fact that force cannot disappear but can only change its form.
-
-It is an unquestionable deduction from the persistence of force, that
-in every individual organism each new incident force must work its
-equivalent of change; and that where it is a constant or recurrent
-force, the limit of the change it works must be an adaptation of
-structure such as opposes to the new outer force an equal inner force.
-The only thing open to question is, whether such re-adjustment is
-inheritable; and further consideration will, I think, show, that to say
-it is not inheritable is indirectly to say that force does not persist.
-If all parts of an organism have their functions co-ordinated into a
-moving equilibrium, such that every part perpetually influences all
-other parts, and cannot be changed without initiating changes in all
-other parts--if the limit of change is the establishment of a complete
-harmony among the movements, molecular and other, of all parts; then
-among other parts that are modified, molecularly or otherwise, must
-be those which cast off the germs of new organisms. The molecules of
-their produced germs must tend ever to conform the motions of their
-components, and therefore the arrangements of their components, to the
-molecular forces of the organism as a whole; and if this aggregate of
-molecular forces be modified in its distribution by a local change
-of structure, the molecules of the germs must be gradually changed
-in the motions and arrangements of their components, until they are
-re-adjusted to the aggregate of molecular forces.
-
-
-
-
-                             CHAPTER X^A.
-
-                 THE INTEGRATION OF THE ORGANIC WORLD.
-
-
-§ 314_a_. That from the beginning of life there has been an
-ever-increasing heterogeneity in the Earth’s Flora and Fauna, is
-a truth recognized by all biologists who accept the doctrine of
-evolution. In discussing the origin of species Mr. Darwin and others
-have been mainly occupied in explaining the genesis of now this and
-now that form of organism, considered as a member of one or other
-series, and regarded as becoming differentiated from its allies. But by
-implication, if not avowedly, there has been simultaneously accepted
-the belief that the forms continually produced by divergences and
-re-divergences, have constituted an assemblage increasingly multiform
-in its included kinds. And this, which we are shown by the process of
-organic evolution as followed out in its details, is a corollary from
-the doctrine of evolution at large, as was pointed out in § 159 of
-_First Principles_.
-
-Meanwhile there has been little if any recognition of an accompanying
-change, no less fundamental. In the general transformation which
-constitutes Evolution, differentiation and integration advance hand
-in hand; so that along with the production of unlike parts there
-progresses the union of these unlike parts into a whole. Examples of
-various kinds before given will recur to the reader, and an addition
-to them has just been set forth in the chapter on “Physiological
-Integration.” One more example, world-wide in its reach, has still to
-be named.
-
-For here it remains to point out that along with the increasing
-multiplication of types of organisms covering the Earth’s surface,
-there has been ever going on an increasing mutual dependence of
-them--an increasing integration of the entire aggregate of living
-things.
-
-Many facts which are obvious and many which are quite familiar will
-be named as evidence. But I must be excused for reminding the reader
-of things that he knows and things that he may easily observe, since,
-unless the evidence, trite as it may be, is gathered together and
-properly marshalled, the generalization enunciated will not be thought
-valid.
-
-
-§ 314_b_. Respecting the physiological characters of the earliest
-forms there is an assumption from which no escape seems possible--the
-assumption that they united animal and vegetal characters. Even among
-existing microscopic types of the lowest classes, there is such
-community of plant-traits and animal-traits that doubts respecting
-their proper places in one or the other kingdom are continually
-raised--doubts, too, whether, if regarded as vegetal, they are to be
-grouped as algoid or fungoid.
-
-Here, however, without entering on moot questions, we may draw the
-_à priori_ conclusion that these earliest living things were
-double-natured, in so far that they must have had the ability to
-assimilate from the inorganic world all the materials of which
-protoplasm consists--must therefore, along with the power of
-appropriating carbon from its gaseous compound, also have had the power
-of appropriating nitrogen, either from one of its combined oxides
-or directly from the air with which water is more or less charged.
-For before organic substances existed there could have been none but
-inorganic sources from which nitrogen could be obtained.
-
-This conclusion concerns us only because it implies homogeneity of
-nature in these primordial forms of life. There could not at first
-have existed among these minutest of _Protozoa_ even such vague
-distinctions as are now presented in a shadowy way by their modern
-representatives. And the implication is that during the period
-throughout which these smallest, lowest, and simplest living things
-alone existed, there could have been, in the absence of kinds, no
-mutual dependence.
-
-Since, among various of the lowest types now known to us, the same
-individual exhibits a life which is now predominantly vegetal and now
-predominantly animal, we cannot err in assuming that there eventually
-took place differentiations of this original plant-animal type into
-types permanently unlike: some in which the traits were more markedly
-vegetal and others in which they were more markedly animal. As fast as
-this differentiation arose, there came the beginnings of co-operation
-between the predominantly vegetal types which by the aid of light
-formed organic matter from the inorganic world, and the predominantly
-animal types which, in chief measure, utilized the matter so formed.
-Evidently with the rise of such a differentiation came an incipient
-mutual dependence. If to the implied algoid type and the animal type
-there be added the fungoid type, somewhat intermediate in character,
-which in a large proportion of cases lives on the decaying remnants of
-the other two, we are furnished with a rude conception of the primary
-differentiations and the accompanying vague mutual dependences.
-
-Speculation aside, it suffices to say that early in the history of
-life there must have arisen the distinction between _Protozoa_ and
-_Protophyta_, and that this distinction foreshadowed that widest
-contrast which the higher organic world presents--the contrast
-between plants and animals. It is needless to do more than name the
-mutual dependence between these two great divisions. That, as being
-respectively decomposers of carbon dioxide and exhalers of carbon
-dioxide, they act reciprocally, as also in some measure by interchange
-of nitrogenous matters; and that the implied general co-operation
-serves in an indirect way to unite their lives, and in that sense to
-integrate the two kingdoms; needs not to be insisted upon. Further
-complications of the mutual dependence will be mentioned by and by. For
-the present it suffices to recognize this division of organic functions
-as the first which arose and as continuing to be that fundamental one
-which more than all others binds organisms at large together.
-
-
-§ 314_c_. It will be thought by many readers that in speaking of the
-contrasted vital activities of plants and animals as constituting a
-“division of organic functions,” I am straining words beyond their
-meanings; since the conception of organic functions postulates an
-organized whole in which they exist, and plants and animals constitute
-no such organized whole. But there is at hand an unexpected defence for
-this conception--a defence not forthcoming a generation ago, but which
-now all biologists will recognize as relevant. I refer to the phenomena
-of symbiosis. These present various cases in which the plant-function
-and the animal-function are carried on in the same body,--cases in
-which the co-operation is not between separate vegetal organisms which
-accumulate nutritive matters and separate animal organisms which
-consume them, but is a co-operation between vegetal elements and animal
-elements forming parts of the same organism.
-
-As introductory to examples of these must first, however, be named an
-example of such co-operation between the two great classes of vegetal
-organisms--the fungoid and the algoid. Incredible as the statement once
-seemed, it is a statement now accepted, that what we know as lichens,
-and used to consider as plants forming a certain low class, are now
-found to be not plants in the ordinary sense at all, but compound
-growths formed of minute algæ and minute fungi, carrying on their lives
-together: the algæ furnishing to the fungi certain constituents they
-need but cannot directly obtain, and the fungi profiting by certain
-materials they obtain from the algæ, either while living or while
-individually decaying. Whence it would seem that after the microscopic
-vegetal type had become in a large degree differentiated into two main
-types, in adaptation to different conditions of life, and had acquired
-appropriate specialities of nature, there grew up this communistic
-arrangement between certain of them, enabling each to benefit by the
-powers which the other had acquired: evidently an exchange of services,
-a physiological division of labour, a mutual dependence of functions
-analogous to that which exists between functions in an ordinary plant
-or animal.
-
-Not differing in principle but only in application, is that symbiosis
-above referred to as existing between _Protophyta_ and many _Protozoa_,
-as well as between such _Protophyta_ and the lowest kinds of _Metazoa_.
-A recent statement that certain amœbæ, made green by contained
-chlorophyll, continue to grow and multiply after they have consumed
-what nutritive matter may be at hand, is in harmony with various facts
-alleged of other _Protozoa_--various other kinds of _Rhizopods_,
-various _Heliozoa_, numerous ciliated and flagellated _Infusoria_.
-Among _Metazoa_ the like association occurs in one of the sponges,
-in the _Hydra viridis_, in various turbellarians, in a rotifer, and
-even in two molluscs. In these cases the partnership between the
-vegetal cells and the animal cells (existing either as units or as an
-organized group such as a polype), is a partnership which, as before,
-profits each of the partners--an inference supported by the fact that
-_Metazoa_ containing these algoid cells usually place themselves where
-the light falls upon them, and can therefore further the production of
-the carbo-hydrates which eventually become useful to the animal cells,
-while these in some way reciprocate the benefit.
-
-Here, then, we have exchange of services between associated
-plant-elements and animal-elements--a performance by them of different
-organic functions for the benefit of the aggregate which they unite
-to form. Hence, when these vegetal elements and animal elements
-are separately embodied in plants and animals, which profit by
-one another, we may still properly regard their respective lives
-as mutually-dependent organic functions, as said in the preceding
-section. We are enabled the better to see how the Earth’s Flora and
-Fauna, which are respectively accumulators of motion and expenders of
-motion, form mutually-dependent parts of a whole, and are in that sense
-integrated. And we shall be prepared to see how all other relations
-between organisms which make them subservient one to another, similarly
-constitute elements in a general integration of the organic world.
-
-
-§ 314_d_. Another form of mutual dependence and consequently of
-integration is conspicuous--that which accompanied the progressive
-increase of size in organisms of the higher classes. We have but to
-contemplate the possibilities to see that life must necessarily have
-commenced with minute forms, and that the progress to larger ones must
-have been by small steps.
-
-For had creatures of appreciable sizes been the first to exist they
-would inevitably have disappeared from lack of food. Having no resource
-but to devour one another, they would quickly have brought life to an
-end. There must have been smaller types serving as prey for larger
-ones before these could continue to exist and to multiply: microbes
-affording food to infusoria, infusoria affording food to such sized
-creatures as the _Entomostraca_, these again supplying food to
-small fishes, such as loch-trout, and these last yielding to larger
-fishes masses sufficiently great for their needs: each higher grade
-requiring lower grades of appropriate bulk. It needs but to ask what
-would become of tigers if there were no mammals larger than mice,
-to see that the animal world is a linked assemblage, of which the
-connected members stand within certain ratios of mass; and that during
-the evolution of higher and larger types the linking of grades has
-become closer.
-
-That among plants considered as an aggregate relations of like kind,
-though far less distinct ones, have all along been growing may be
-reasonably concluded. In a world peopled only by microscopic types
-there could not have existed the conditions needful for large trees.
-Gradual disintegration of rock-surfaces, partly effected by physical
-agencies and partly by low forms of plants, had to prepare the way for
-superior plants. The production of sufficient soil by mineralogical
-decay as well as by the decay of organisms, plant and animal, may be
-regarded as having been a preliminary to larger plant-growth; and
-though at present the dependence is far less close than that among
-animals, yet the benefits yielded to metaphytes by the decomposing
-actions carried on by protophytes, as well as those carried on by
-microbes permeating the soil, imply a continued general interdependence
-throughout the aggregate of plant-forms, apart from more special
-interdependences. And then along with this indebtedness of the greater
-plants to the smaller during the process of evolution, there must be
-named that indebtedness of plant-life to animal-life which Mr. Darwin
-has shown in his book on the agency of worms as producers of mould.
-
-
-§ 314_e_. Services of one to another, and consequent unions, of
-more special kinds are infinitely varied, alike within each kingdom
-and between the two kingdoms. I refer to those seen in parasitism,
-commensalism, and other forms of association. While they do not
-conduce to unions of the kind thus far considered, these nevertheless
-constitute innumerable links whereby the lives of organisms, plant and
-animal, are tied together; sometimes for the advantage of both but in
-most cases for the benefit of one to the injury of the other.
-
-Among plants the degrees of dependence are various. Unable to raise
-themselves into the air and light, some climb, like the ivy, by
-modified rootlets, or spirally coil themselves, or hang by tendrils.
-Others there are which gradually strangle the trees they embrace, or
-which, like lichens in damp climates, festooning the smaller trees,
-by and by cause their decay. Of higher types of epiphytes which
-use trees only to gain elevation, the orchids may be instanced.
-And then we have plants which, like the mistletoe, fix themselves
-on the bark of their hosts, utilizing them partly for purposes of
-elevation and partly by appropriation of their juices. After these
-may be named those extreme cases in which the parasitic plants,
-ceasing to have any chlorophyll-bearing leaves, live wholly on the
-juices of the invaded plants. At home the common dodder, and in the
-tropics the _Rafflesiaceæ_, belong to this group. There must be
-added the numerous forms of minute fungi which in like manner thrive
-at the expense of the plants they infest. In all these cases the
-interdependence is one-sided, though, as we shall presently see, while
-detrimental to one of the two concerned, it is not always detrimental
-to the organic world as a whole.
-
-That utilization of one by another among animals which causes immediate
-death, is familiar enough in the relations between carnivores and
-herbivores. Almost as familiar are those seen in parasitism. Less
-familiar are those seen in commensalism; and the least familiar are
-those which show us exchange of services. Among these last--the
-mutually-beneficial relations--that between the crocodile and the
-bird which picks parasites out of its teeth is a striking one; and no
-less so is that of the pique-gouffe, an African bird which pierces
-the tumour on a buffalo’s back that incloses a parasite. Then of
-another kind we have the connexion between aphides and ants: the
-one profiting by being carried to better pastures and the other by
-increased saccharine excretion. Next comes the class of messmates, the
-connexions between some of which are relatively innocent, as witness
-the Sea-anemone which settles itself on the shell occupied by a
-Hermit-crab, or as witness the _Remora_ fixed on a shark’s skin. Less
-innocent is the relation under which one of the two seizes a share
-of the food obtained by the other, like the annelid which insinuates
-itself between the Hermit-crab and the whelk-shell it inhabits, or like
-the small fishes inhabiting certain _Medusæ_, or those which nestle
-in the branchial sac of the _Lophius_. After these may be named the
-less injurious forms of parasites proper--those which, distinguished
-as _Epizoa_, fix themselves on the skins of their hosts, permanently
-or temporarily, such as, of the one kind, the _Lernæa_ on fishes, and
-of the other kind the Tick on mammals and birds. Then there come the
-other class of parasites, most of them highly injurious, distinguished
-as _Entozoa_, living within the bodies of their hosts, now in parts of
-their alimentary canals, now on other of their mucous surfaces, and now
-in various of their organs: these last two groups being so numerous in
-their kinds that there are commonly more species than one proper to
-each larger animal. One stage further in the complication meets us in
-the parasites upon parasites.
-
-But now the general fact, to which these brief indications are
-introductory, is that the use made of one organism by another has been
-ever widening and becoming more involved. Among plants utilization of
-the larger by the smaller--of trees by epiphytes and parasites--must
-have arisen since the times when the larger came into existence--times
-relatively late in the course of organic evolution. Moreover most
-of the plants which utilize others, either by climbing up them or
-settling themselves high up on their stems or sucking their juices,
-are phænogams, and the plants they utilize are also phænogams; so that
-these innumerable interdependences must have been established since
-the phænogamic type has become so predominant in respect of both size
-and kind. Similarly among animals. Though there are many parasites
-belonging, like the Trematodes, to very low classes, there are many
-which belong to the _Arthropoda_, and, being degraded forms of that
-class, must have come into existence after Arthropods of considerable
-structure had been evolved. Again, a large part of the animals infested
-by _Epizoa_ and _Entozoa_ are vertebrates--many of the highest types;
-and as these are relatively modern all this parasitism must be of
-late date. So, too, of much commensalism and many mutually-beneficial
-associations. The reciprocal services of ants and aphides must have
-originated since the _Hymenoptera_ and _Hemiptera_ became established
-types, and since the days when certain insects of the ant-type had
-become social, and since the days when aphides had become degraded
-members of their order: both dates being relatively recent. And still
-more recent must have been the commensalism between the ants and the
-many species of other insects which inhabit their nests.
-
-Leaving out relations of the kinds just named, it seems that down from
-those between carnivores and their prey to those between lice and their
-hosts, such relations profit one of the two species concerned and
-injure the other, and that there the matter ends. But it does not end
-there; for that multiplication of effects to which people are usually
-blind, brings about changes which, as hinted above, though injurious
-to the individual are beneficial to the species, and which, when not
-beneficial to the species, are often beneficial to the aggregate of
-species.
-
-Even where animals of one class live by devouring animals of another
-class, we see, on looking beyond the immediate results, certain remote
-results that are advantageous. In the first place the process is
-one by which inferior individuals--the least agile, swift, strong,
-or sagacious--are picked out and prevented from leaving posterity
-and lowering the average quality of their kind. At the same time
-individuals made feeble by injury or old age, are among those to be
-killed and saved from suffering prolonged pains: the evils of death by
-disease and starvation being thus limited to the predatory animals,
-relatively small in their numbers. Meanwhile a check is put on undue
-multiplication. Where a tract of country has been overrun by rabbits,
-weasels, thriving on the abundant supply of food, presently become
-numerous enough to bring the population of rabbits within moderate
-limits; and by doing this benefit not only all those kinds of plants
-which are being eaten down, and all those other animals which live on
-such plants, but also the rabbits themselves; since, increasing beyond
-the means of subsistence, a large part of them would, if not killed,
-die of hunger. Between aphides and lady-birds we see a connexion
-of like nature: great increase of the first yielding abundant food
-to larvæ of the second, ending after a season or so in swarms of
-lady-birds, and consequently of their larvæ, whereby the aphides,
-immensely diminished, cease so greatly to injure various plants and
-the animals dependent on them. Even minute parasites, by the evils
-they inflict on one species, profit others: instance the enormous
-destruction of flies which a microscopic fungus caused a few years
-ago--a destruction which relieved not only man but all the animals
-which flies irritate: often so much as to hinder them from feeding.
-Such instances remind us how numerous are the bonds by which the lives
-of organisms are tied together.
-
-
-§ 314_f_. I have reserved to the last the clearest and most striking
-illustration of this progressing integration throughout the organic
-world. I refer to the mutually-beneficial relations established between
-plants and animals through the agency of flowers and insects.
-
-Everyone nowadays has been made familiar with the process of
-plant-fertilization, and knows that (leaving out of consideration
-plants fertilized by wind-borne pollen) the ability to bear seed
-depends largely on the aid given by bees, butterflies, and moths. The
-exchange of services has been growing ever more various and complicated
-during long past periods. We have the acquirement by flowers of bright
-colours serving to guide these insects to places where honey is to be
-found; and we have their perfumes, also serving for guidance. Then we
-have the many different arrangements, often complicated, by which the
-visiting insects are obliged to carry away pollen and dust with it the
-stigmas of flowers on which they subsequently settle: thus effecting
-crossfertilization. _Pari passu_ have gone on insect-developments
-made possible by these arrangements and furthering them. Especially
-must be named the modification of certain _Hymenoptera_ into
-honey-storing bees: the implication being that the entire economy
-established by these social insects has been sequent on the growth
-of this system of reciprocal benefits. And then, just instancing the
-dependence between a particular flower having a long tubular corolla,
-and a particular moth having an appropriately long proboscis, it
-suffices to say that innumerable specialities of this general relation
-everywhere multiply the links by which the vegetal world and the animal
-world are here connected. That the effects of the connections tell
-largely on the prosperity of both, is suggested by some instances Mr.
-Darwin gives, and by a statement recently made in the United States,
-by Dr. L. O. Howard, that the greater fostering of bees would much
-increase certain of the crops.
-
-But now observe the broad fact to which these few details concerning
-plant-fertilization are introductory. All these general and special
-relations between plants and animals have arisen since the phænogamic
-type came into existence--have, indeed, arisen since the higher members
-of that type, the Angiosperms, have appeared; for the Gymnosperms do
-not play any part in this intercommunion. But so far as we can judge
-of present results of geologic explorations, there were no Angiosperms
-during the Eozoic and Paleozoic periods. So that this class of
-connexions between animals and vegetals must have been established
-since carboniferous times--a period long, indeed, but far shorter than
-that which organic evolution at large has occupied.
-
-
-§ 314_g_. I have but just touched on some salient parts of a subject,
-immense in extent and extremely involved, which it would take a volume
-to set forth adequately. Enough has been said, however, to indicate the
-truth which it is the purpose of the chapter to bring into view and
-emphasize--the truth that both of the two great laws of evolution are
-exemplified in the organic world as a whole, as they are exemplified in
-every organism, and in all other things.
-
-The reader has long since become familiar with the generalization that
-while Evolution is a change from the homogeneous to the heterogeneous,
-it is also a change from the incoherent to the coherent; and this
-change from the incoherent to the coherent has been above exhibited
-as going on even throughout that vast assemblage of organisms, plant
-and animal, which cover the Earth’s surface. In what we are obliged
-to conceive as the earliest stage, when the most minute types of life
-alone existed, the aggregate of living things was at once homogeneous
-and incoherent. In the course of epochs immeasurable in duration, this
-uniform aggregate of beings has been becoming more multiform. And now
-we see that instead of forms of life everywhere without the slightest
-union caused by mutual dependence, there have slowly arisen forms of
-life among which mutual dependences have entailed vital connexions
-correspondingly marked. Along with progressing differentiation there
-has ever been progressing integration. So that we may recognize
-something like a growing life of the entire aggregate of organisms in
-addition to the lives of individual organisms--an exchange of services
-among parts enhancing the life of the whole.
-
-In this final generalization the law of Evolution is manifested under
-its most transcendental form.
-
-
-
-
-                               PART VI.
-
-                        LAWS OF MULTIPLICATION.
-
-
-
-
-                              CHAPTER I.
-
-                           THE FACTORS.[52]
-
-
-§ 315. If organisms have been evolved, their respective powers of
-multiplication must have been determined by natural causes. Grant that
-the countless specialities of structure and function in plants and
-animals, have arisen from the actions and reactions between them and
-their environments, continued from generation to generation; and it
-follows that from these actions and reactions have also arisen those
-countless degrees of fertility which we see among them. As in all other
-respects an adaptation of each species to its conditions of existence
-is directly or indirectly brought about; so must there be directly or
-indirectly brought about an adaptation of its reproductive activity to
-its conditions of existence.
-
-We may expect to find, too, that permanent and temporary differences
-of fertility have the same general interpretation. If the small
-variations of structure and function that arise within the limits
-of each species, are due to actions like those which, by their
-long-accumulating effects, have produced the immense contrasts between
-the various types; we may conclude that, similarly, the actions to
-which changes in the rate of multiplication of each species are due,
-also produce, in great periods of time, the enormous differences
-between the rates of multiplication of different species.
-
-Before inquiring in what ways the rapidities of increase are adjusted
-to the requirements, both temporary and permanent, it will be needful
-to look at the factors. Let us set down first those which belong to the
-environment, and then those which belong to the organism.
-
-
-§ 316. Every living aggregate being one of which the inner actions are
-adjusted to balance outer actions, it follows that the maintenance of
-its moving equilibrium depends on its exposure to the right amounts of
-these actions. Its moving equilibrium may be overturned if one of these
-actions is either too great or too small in amount; and it may be so
-overturned either by excess or defect of some inorganic agency in its
-environment, or by excess or defect of some organic agency.
-
-Thus a plant, constitutionally fitted to a certain warmth and
-humidity, is killed by extremes of temperature, as well as by extremes
-of drought and moisture. It may dwindle away from want of soil, or
-die from the presence of too great or too small a quantity of some
-mineral substance which the soil supplies to it. In like manner, every
-animal can maintain the balance of its functions so long only as the
-environment adds to or deducts from its heat at rates not exceeding
-definite limits. Water, too, must be accessible in amount sufficient to
-compensate loss. If the parched air is rapidly abstracting its liquid
-which there is no pool or river to restore, its functions cease; and
-if it is an aquatic creature, drought may kill it either by drying up
-its medium or by giving it a medium inadequately aërated. Thus each
-organism, adjusted to a certain average in the actions of its inorganic
-environment, or rather, we should say, adjusted to certain moderate
-deviations from this average, is destroyed by extreme deviations. So,
-too, is it with the environing organic agencies. Among plants, only
-the parasitic kinds and those united by symbiosis (as well as a few
-innocent “lodgers”) depend for their individual preservation on the
-presence of certain other organisms (though the presence of certain
-other organisms is needful to most plants for the preservation of the
-race by aiding fertilization). Here, for the continuance of individual
-life, particular organisms must be absent or not very numerous--beasts
-that browse, caterpillars that devour leaves, aphides that suck juices.
-Among animals, however, the maintenance of the functional balance is
-both positively and negatively dependent on the amounts of surrounding
-organic agents. There must be an accessible sufficiency of the plants
-or animals serving for food; and of organisms that are predatory or
-parasitic or otherwise detrimental, the number must not pass a certain
-limit.
-
-This dependence of the moving equilibrium in every individual organism
-on an adjustment of its forces to the forces of the environment, and
-the overthrow of this equilibrium by failure of the adjustment, is
-comprehensive of all cases. At first sight it does not seem to include
-what we call natural death; but only death by violence, or starvation,
-or cold, or drought. But in reality natural death, no less than every
-other kind of death, is caused by the failure to meet some outer action
-by a proportionate inner action. The apparent difference is due to
-the fact that in old age, when the quantity of force evolved in the
-organism gradually diminishes, the momentum of the functions becomes
-step by step less, and the variations of the external forces relatively
-greater; until there finally comes an occasion when some quite moderate
-deviation from that average to which the feeble moving equilibrium is
-adjusted, produces in it a fatal perturbation.
-
-
-§ 317. The individuals of every species being thus dependent on certain
-environing actions; and severally having their moving equilibria sooner
-or later overthrown by one or other of these environing actions; we
-have next to consider in what ways the environing actions are so met
-as to prevent extinction of the species. There are two essentially
-different ways. There may be in each individual a small or great
-ability to adjust itself to variations of the agencies around it and to
-a small or great number of such varying agencies--there may be little
-or much power of preserving the balance of the functions. And there may
-be much or little power of producing new individuals to replace those
-whose moving equilibria have been overthrown. A few facts must be set
-down to enforce these abstract statements.
-
-There are both active and passive adaptations by which organisms
-are enabled to survive adverse influences. Plants show us but few
-active adaptations: that of the Pitcher-plant and those of the
-reproductive parts of some flowers (which do not, however, conduce to
-self-preservation) are exceptional instances. But plants have various
-passive adaptations; as thorns, stinging hairs, poisonous and acrid
-juices, repugnant odours, and the woolliness or toughness that makes
-their leaves uneatable. Animals exhibit far more numerous adjustments,
-both passive and active. In some cases they survive desiccation, they
-hybernate, they acquire thicker clothing, and so are fitted to bear
-unfavourable inorganic actions; and they are in many cases fitted
-passively to meet the adverse actions of other organisms, by bearing
-spines or armour or shells, by simulating neighbouring objects in
-colour or form or both, by emitting disagreeable odours, or by having
-disgusting tastes. In still more numerous ways they actively contend
-with unfavourable conditions. Against the seasons they guard by storing
-up food, by secreting themselves in crevices, or by forming burrows
-and nests. They save themselves from enemies by developed powers of
-locomotion, taking the shape of swiftness or agility or aptitude for
-changing their media; by their strength either alone or aided by
-weapons; lastly by their intelligence, without which, indeed, their
-other superiorities would avail them little. And then these various
-active powers serving for defence, become, in other cases, the powers
-that enable animals to aggress, and to preserve their lives by the
-success of their aggressions.
-
-The second process by which extinction is prevented--the formation of
-new individuals to replace the individuals destroyed--is carried on,
-as described in the chapter on “Genesis,” by two methods, the sexual
-and the asexual. Plants multiply by spontaneous fission, by gemmation,
-by proliferation, and by the evolution of young ones from detached
-cells and scales and leaves; and they also multiply by the casting
-off of spores and sporangia and seeds. In like manner among animals,
-there are varied kinds of agamogenesis, from spontaneous fission up to
-parthenogenesis, all of them conducing to rapid increase of numbers;
-and we have the more familiar process of gamogenesis, also carried
-on in a great variety of ways. This formation of new individuals to
-replace the old, is, however, inadequately conceived if we contemplate
-only the number born or detached on each occasion. There are four
-factors, all variable, on which the rate of multiplication depends.
-The first is the age at which reproduction commences; the second is
-the frequency with which broods are produced; the third is the number
-contained in each brood; and the fourth is the length of time during
-which the bringing forth of broods continues. There must be taken into
-account a further element--the amount of aid given by the parent to
-each germ in the shape of stored-up nutriment, continuous feeding,
-warmth, protection, &c.: on which amount of aid, varying between
-immensely wide limits, depends the number of the new individuals
-that survive long enough to replace the old, and perform the same
-reproductive process.
-
-Thus, regarding every living organism as having a moving equilibrium
-dependent on environing forces, but ever liable to be overthrown by
-irregularities in those forces, and always so overthrown sooner or
-later; we see that each species of organism can be maintained only
-by the generation of new individuals with a certain rapidity, and by
-helping them more or less fully to establish their moving equilibria.
-
-
-§ 318. Such are the factors with which we are here concerned. I have
-presented them in abstract shapes for the purpose of showing how they
-are expressible in general terms of force--how they stand related to
-the ultimate laws of re-distribution of matter and motion.
-
-For the purposes of the argument now to follow, we may, however,
-conveniently deal with these factors under a more familiar guise.
-Ignoring their other aspects, we may class the factors which affect
-each race of organisms as forming two conflicting sets. On the one
-hand, by what we call natural death, by enemies, by lack of food, by
-atmospheric changes, &c., the race is constantly being destroyed. On
-the other hand, partly by the endurance, the strength, the swiftness,
-and the sagacity of its members, and partly by their fertility, it
-is constantly being maintained. These conflicting sets of factors
-may be generalized as--the forces destructive of race and the forces
-preservative of race. So generalizing them, let us ask what are the
-necessary implications.
-
-
-
-
-                              CHAPTER II.
-
-                         _À PRIORI_ PRINCIPLE.
-
-
-§ 319. The number of a species must at any time be either decreasing
-or stationary or increasing. If, generation after generation, its
-members die faster than others are born, the species must dwindle
-and finally disappear. If its rate of multiplication is equal to its
-rate of mortality, there can be no numerical change in it. And if the
-deductions by death are fewer than the additions by birth, the species
-must become more abundant. These we may safely set down as necessities.
-The forces destructive of race must be either greater than the forces
-preservative of race, or equal to them, or less than them; and there
-cannot but result these effects on number.
-
-We are here concerned only with races that continue to exist; and may
-therefore leave out of consideration those in which the destructive
-forces, remaining permanently in excess of the preservative forces,
-cause extinction. Practically, too, we may exclude the stationary
-condition; for the chances are infinity to one against the maintenance
-of a permanent equality between the births and the deaths. Hence, our
-inquiry resolves itself into this:--In races that continue to exist,
-what laws of numerical variation result from these variable conflicting
-forces, which are respectively destructive of race and preservative of
-race?
-
-
-§ 320. Clearly if the forces destructive of race, when once in excess,
-had nothing to prevent them from remaining in excess, the race would
-disappear; and clearly if the forces preservative of race, when once in
-excess, had nothing to prevent them from remaining in excess, the race
-would go on increasing to infinity. In the absence of any compensating
-actions, the only possible avoidance of these opposite extremes would
-be an unstable equilibrium between the conflicting forces, resulting
-in a perfectly constant number of the species: a state which we know
-does not exist, and against the existence of which the probabilities
-are, as already said, infinite. It follows, then, that as in every
-continuously-existing species, neither of the two conflicting sets
-of forces remains permanently in excess; there must be some way of
-stopping that excess of the one or the other which is ever occurring.
-
-How is this done? Should any one allege, in conformity with the old
-method of interpretation, that there is in each case a providential
-interposition to rectify the disturbed balance, he commits himself to
-the supposition that of the millions of species inhabiting the Earth,
-each one is yearly regulated in its degree of fertility by a miracle;
-since in no two years do the forces which foster, or the forces which
-check, each species, remain the same; and therefore, in no two years
-is there required the same fertility to balance the mortality. Few if
-any will say that God continually alters the reproductive activity
-of every parasitic fungus and every Tape-worm or _Trichina_, so as
-to prevent its extinction or undue multiplication; which they must
-say if they adopt the hypothesis of supernatural adjustment. And in
-the absence of this hypothesis there remains only one other. The
-alternative possibility is, that the balance of the preservative and
-destructive forces is self-sustaining--is of the kind distinguished as
-a stable equilibrium: an equilibrium such that any excess of one of
-the forces at work, itself generates, by the deviation it produces,
-certain counter-forces which eventually out-balance it, and initiate an
-opposite deviation. Let us consider how, in the case before us, such a
-stable equilibrium must be constituted.
-
-
-§ 321. When a season favourable to it, or a diminution of creatures
-detrimental to it, causes any species to become more numerous than
-usual, an immediate increase of certain destructive influences takes
-place. If it be a plant, the supposed greater abundance itself implies
-fuller occupation of the places available for growth--an occupation
-which, leaving fewer such places as the multiplication goes on,
-becomes a check on further multiplication--itself causes a greater
-mortality of seeds that fail to root themselves. And afterwards, in
-addition to this passive resistance to continued increase, there comes
-an active resistance: the creatures which thrive at the expense of
-the species--the larvae, the birds, the herbivores--increase too. If
-it be an animal that has grown more numerous, then, unless by some
-exceptional coincidence a simultaneous and proportionate addition to
-the animals or plants serving for food has occurred, there must result
-a relative scarcity of food. Enemies, too, be they beasts of prey or
-be they parasites, must quickly begin to multiply. Hence, each kind
-of organism, previously existing in something like its normal number,
-cannot have its number raised without a rise of the destructive forces,
-negative and positive, quickly commencing. Both negative and positive
-destructive forces must augment until this increase of the species is
-arrested. The competition for places on which to grow, if the species
-be vegetal, or for food if it be animal, must become more intense as
-the over-peopling of the habitat progresses; until there is reached
-the limit at which the mortality equals the reproduction. And as, at
-the same time, enemies will multiply with a rapidity which soon brings
-them abreast of the augmented supply of prey, the positive restraint
-they exert will help to bring about an earlier arrest of the expansion
-than pressure of population alone would cause. One more inference may
-be drawn. Had the species to meet no repressing influence save that
-negative one of relatively-diminished space or relatively-diminished
-food-supply, the cause leading to its increase might carry it up
-to the limit set by this, and there leave it: its enlarged number
-might be permanent. But the positive repressing influence that has
-been called into existence, will prevent this. For the increase of
-enemies, commencing, as it must, after the increase of the species,
-and advancing in geometrical progression until it is itself checked
-in like manner, will end in an excess of enemies. Whereupon must
-result a mortality of the species greater than its multiplication--a
-decrease which will continue until its habitat is under-peopled, its
-unduly-numerous enemies decimated by starvation, and the destroying
-agencies reduced to a minimum. Whence will follow another increase.
-
-Thus, as before indicated (_First Prin._ §§ 85, 173), there is
-here, as wherever antagonistic forces are in action, an alternate
-predominance of each, causing a rhythmical movement--a rhythmical
-movement which constitutes a moving equilibrium in those cases where
-the forces are not dissipated with appreciable rapidity, or are
-re-supplied as fast as they are dissipated. While, therefore, on the
-one hand, we see that the continued existence of a species necessarily
-implies some action by which the destructive and preservative forces
-are self-adjusted; we see, on the other hand, that such an action is an
-inevitable consequence of the universal process of equilibration.
-
-
-§322. Is this the sole equilibration which must exist? Clearly not.
-The temporary compensating adjustments of multiplication to mortality
-in each species, are but introductory to the permanent compensating
-adjustments of multiplication to mortality among species in general.
-The above reasoning would hold just as it now does, were all species
-equally prolific and all equally short-lived. It yields no answer
-to the inquiries--why do their fertilities differ so enormously,
-or why do their mortalities differ so enormously? and how is the
-general fertility adapted to the general mortality in each? The
-balancing process we have contemplated can go on only within moderate
-limits--must fail entirely in the absence of a due proportion
-between the ordinary birth-rate and the ordinary death-rate. If the
-reproduction of mice proceeded as slowly as the reproduction of men,
-mice would be extinct before a new generation could arise: even did
-their natural lives extend to fifteen or sixteen years, it would still
-be extremely improbable that any would for so long survive all the
-dangers they are exposed to. Conversely, did oxen propagate as fast
-as infusoria, the race would die of starvation in a week. Hence, the
-minor adjustment of varying multiplication to varying mortality in each
-species, implies some major adjustment of average multiplication to
-average mortality. What must this adjustment be?
-
-We have already seen that the forces preservative of race are
-two--ability in each member of the race to preserve itself, and ability
-to produce other members--power to maintain individual life, and power
-to generate the species. These must vary inversely. When, from lowness
-of organization, the ability to contend with external dangers is
-small, there must be great fertility to compensate for the consequent
-mortality; otherwise the race must die out. When, on the contrary, high
-endowments give much capacity of self-preservation, a correspondingly
-low degree of fertility is requisite. Given the dangers to be met as a
-constant quantity; then as the ability to meet them must be a constant
-quantity too; and as this is made up of the two factors, power to
-maintain individual life and power to multiply, these cannot do other
-than vary inversely: one must decrease as the other increases.
-
-It needs but to conceive the results of nonconformity to this law,
-to see that every species must either conform to it or cease to
-exist. Suppose, first, a species whose individuals, having but small
-self-preservative powers, are rapidly destroyed, to be at the same
-time without reproductive powers proportionately great. The defect
-of fertility, if extreme, will result in the death of one generation
-before another has grown up. If less extreme, it will entail a
-scarcity such that in the next generation sexual congress will be too
-infrequent to maintain even the small number which remains; and the
-race will dwindle with increasing rapidity. If still less extreme, the
-consequent degree of sparseness, while not so great as to prevent an
-adequate number of procreative unions, will be so great as to render
-special food abundant and special enemies few--will thus diminish the
-destructive forces so much that the self-preservative forces will
-become _relatively_ great: so great, relatively, that when combined
-with the small ability to propagate the species, they will suffice to
-balance the small destructive forces. Suppose, next, a species whose
-individuals have high powers of self-preservation, while they have
-powers of multiplication much beyond what is needful. The excess of
-fertility, if extreme, will cause sudden extinction of the species by
-starvation. If less extreme, it must produce a permanent increase in
-the number of the species; and this, followed by intenser competition
-for food and augmented number of enemies, will involve such an increase
-of the dangers to individual life, that the great self-preserving
-powers of the individuals will not be more than sufficient to cope with
-them. That is to say, if the fertility is relatively too great, then
-the ability to maintain individual life inevitably becomes smaller,
-_relatively_ to the requirements; and the inverse proportion is thus
-established.
-
-So that when, from comparing the different states of the same species,
-we go on to compare the states of different species, we see that there
-is an analogous adjustment--analogous in the sense that great mortality
-is associated with great multiplication, and small mortality with small
-multiplication. And we see that the unlikeness of the cases consists
-merely in this, that what is a temporary relation in the one is a
-permanent relation in the other.
-
-
-§ 323. For the moment it does not concern us to inquire what is the
-origin of this permanent relation. That which we have now to note, is
-simply that in some way or other there must be established an inverse
-proportion between the power to sustain individual life and the power
-to produce new individuals. Whether or not this permanent relation is
-self-adjusting in long periods of time, as the temporary relation is
-self-adjusting in short periods of time, is a separate question. The
-purpose of this chapter is fulfilled by showing that such a permanent
-relation must exist.
-
-But having recognized the _à priori_ principle that in races which
-continuously survive, the forces destructive of race must be
-equilibrated by the forces preservative of race; and that, supposing
-these are constant, there must be an inverse proportion between
-self-preservation and race-preservation; we may go on to inquire how
-this relation, necessary in theory, arises in fact. Leaving out the
-untenable hypothesis of a supernatural pre-adjustment, we have to ask
-in what way an adjustment comes about as a result of Evolution. Is it
-due to the survival of varieties in which the proportion of fertility
-to mortality happens to be the best? Or is the fertility adapted to the
-mortality in a more direct way? To these questions let us now address
-ourselves.
-
-
-
-
-                             CHAPTER III.
-
-                     OBVERSE _À PRIORI_ PRINCIPLE.
-
-
-§ 324. When dealing with its phenomena inductively, we saw that however
-it may be carried on, Genesis “is a process of negative or positive
-disintegration; and is thus essentially opposed to that process of
-integration which is the primary process in individual evolution.”
-(§ 76.) Each new individual, whether separated as a germ or in some
-more-developed form, is a deduction from the mass of a pre-existing
-individual or of two pre-existing individuals. Whatever nutritive
-matter is stored-up along with the germ, if it be deposited in the
-shape of an egg, is so much nutritive matter lost to the parent.
-No drop of blood can be absorbed by the fœtus, nor any draught of
-milk sucked by the young when born, without taking from the mother
-tissue-forming and force-evolving materials to an equivalent amount.
-And all subsequent supplies given to progeny, if they are nurtured,
-involve, to a parent or parents, so much waste in exertion which does
-not bring its return in assimilated food.
-
-Conversely, the continued aggregation of materials into one organism,
-renders impossible the formation of other organisms out of those
-materials. As much assimilated food as is united into a single whole,
-is so much assimilated food withheld from a plurality of wholes which
-might else have been produced. Given the absorbed nutriment as a
-constant quantity, and the longer the building of it up into a concrete
-shape goes on, the longer must be postponed any building of it up into
-discrete shapes. And, similarly, the larger the proportion of matter
-consumed in the functional actions of parents, the smaller must be the
-proportion of matter which can remain to establish and support the
-functional actions of offspring.
-
-Though the necessity of these universal relations is tolerably obvious
-as thus stated generally, it will be useful to dwell for a brief space
-on their leading aspects.
-
-
-§ 325. That disintegration which constitutes genesis, may be such as
-to disperse entirely the aggregate which integration has previously
-produced--the parent may dissolve wholly into progeny. This dissolution
-of each aggregate into two or many aggregates, may occur at very short
-intervals, in which case the bulk attained can be but extremely small;
-or it may occur at longer intervals, in which case a larger bulk may be
-attained.
-
-Instead of quickly losing its own individuality in the individualities
-of its offspring, each member of the race may, after growing for a
-time, have portions of its substance begin to develop into the parental
-shape and presently detach themselves; and the parent, maintaining its
-own identity, may continue indefinitely so to produce young ones. But
-clearly, the earlier it commences doing this, and the more rapidly it
-does it, the sooner must the increase of its own bulk be stopped.
-
-Or again, growth and development continuing for a long period without
-any deduction of materials, an individual of considerable size and
-organization may result; and then the abstraction of substance for
-the formation of new individuals, or rather the eggs of them, may
-be so great that as soon as the eggs are laid the parent dies of
-exhaustion--dies, that is, from an excessive loss of the nutritive
-matters needed for its own activities.[53]
-
-Once more, the deduction of materials for the propagation of the
-species may be postponed long enough to allow of great bulk and complex
-structure being attained. The procreative subtraction then setting
-in, while it checks and presently stops growth, may be so moderate
-as to leave vital capital sufficient to carry on the activities of
-the parent; may go on as long as parental vigour suffices to furnish,
-without fatal result, the materials needed to produce young ones; and
-may cease when such a surplus cannot be supplied, leaving the parental
-life to continue.
-
-
-§ 326. The opposite side of this antagonism has also several aspects.
-Progress of organic evolution may be shown in increased bulk, in
-increased structure, in increased amount or variety of action, or
-in combinations of these; and under any of its forms this carrying
-higher of each individuality, implies a correlative retardation in the
-establishment of new individualities.
-
-Other things equal, every normal addition to the bulk of an organism
-is an augmentation of its life.[54] Besides being an advance in
-integration, it implies a greater total of activities gone through in
-the assimilation of materials; and it implies, thereafter, a greater
-total of the vital changes taking place from moment to moment in all
-parts of the enlarged mass. Moreover, while increased size is thus,
-in so far, the expression of increased life, it is also, where the
-organism is active, the expression of increased ability to maintain
-life--increased strength. Aggregation of substance is almost the only
-mode in which self-preserving power is shown among the lowest types;
-and even among the highest, sustaining the body in its integrity
-is that in which self-preservation fundamentally consists--is the
-end which the widest intelligence is indirectly made to subserve.
-While, on the one hand, the increase of tissue constituting growth is
-conservative both in essence and in result; on the other hand, decrease
-of tissue, either from injury, disease, or old age, is in both essence
-and result the reverse. And if so, every addition to individual life
-thus implied, necessarily delays or diminishes the casting off of
-matter to form new individuals.
-
-Other things equal, too, a greater degree of organization involves
-a smaller degree of that disorganization shown by the separation
-of reproductive gemmæ and germs. Detachment of a living portion or
-portions from what was previously a living whole, is a ceasing of
-co-ordination; and is therefore essentially at variance with that
-establishment of greater co-ordination which is achieved by structural
-development. In the extreme cases where a living mass is continually
-dividing and subdividing, it is manifest that there cannot arise
-much physiological division of labour; since progress towards mutual
-dependence of parts is prevented by the parts becoming independent.
-Contrariwise, it is equally clear that in proportion as the
-physiological division of labour is carried far, the separative process
-must be localized in some comparatively small portion of the organism,
-where it may go on without affecting the general structure--must
-become relatively subordinate. The advance that is shown by greater
-heterogeneity, must be a hindrance to multiplication in another way.
-For organization entails cost. That transfer and transformation
-of materials implied by differentiation, can be effected only by
-expenditure of force; and this supposes consumption of digested and
-absorbed food, which might otherwise have gone to make new organisms,
-or the germs of them. Hence, that individual evolution which consists
-in progressive differentiation, as well as that which consists in
-progressive integration, necessarily diminishes that species of
-dissolution, general or local, which propagation of the race exhibits.
-
-In active organisms we have yet a further opposition between
-self-maintenance and maintenance of the race. All motion, sensible and
-insensible, generated by an animal for the preservation of its life,
-is motion liberated from decomposed nutriment--nutriment which, if
-not thus decomposed, would have been available for reproduction; or
-rather--might have been replaced by nutriment fitted for reproductive
-purposes, absorbed from other kinds of food. Hence, in proportion as
-the activities increase--in proportion as, by its more varied, complex,
-rapid, and vigorous actions, an animal gains power to support itself
-and to cope with surrounding dangers, it must lose power to propagate.
-
-
-§ 327. How may this antagonism be best expressed in a brief way? If
-self-preservation displayed itself in the highest organisms, as it
-does in the lowest, in little else but continuous growth; and if
-race-preservation consisted always, as it does often, of nothing beyond
-detachment of portions from the parental mass; then the antagonism
-would be, throughout, the obviously-necessary one of integration and
-disintegration. Maintenance of the individual and propagation of the
-species, being respectively aggregative and separative, it would be
-as self-evident that they vary inversely, as it is self-evident that
-addition and subtraction undo one another. But though the simplest
-types show us the opposition of self-maintenance and race-maintenance
-almost wholly under this form; and though higher types, up to the
-most complex, exhibit it to a great extent under this form; yet, as
-we have just seen, this is not its only form. The total material
-monopolized by the individual and withheld from the race, must be
-stated as the quantity united to form its fabric, _plus_ the quantity
-expended in differentiating its fabric, _plus_ the quantity expended
-in its self-conserving actions. Similarly, the total material devoted
-to the race at the expense of the individual, includes that which is
-directly subtracted from the parent in the shape of egg or fœtus,
-_plus_ that which is directly subtracted in the shape of milk, _plus_
-that which is indirectly subtracted in the shape of matter consumed
-in exertions for fostering the young. Hence this inverse variation
-is not expressible in simple terms of aggregation and separation. As
-we advance to more highly-evolved organisms, the total cost of an
-individual becomes very much greater than is implied by the amount of
-tissue composing it. So, too, the total cost of producing each new
-individual becomes very much greater than that of its mere substance.
-And it is between these two total costs that the antagonism exists.
-
-We may, indeed, reduce the antagonism to a form comprehensive of all
-cases, if we consider it as existing between the sums of the forces,
-latent and active, used for the two purposes. The molecules which make
-up a plant or animal, have been formed by the absorption of forces
-directly or indirectly derived from the Sun; and hence the quantity
-of matter raised to the form called organic, which a plant or animal
-presents, is equivalent to a certain amount of force. Another amount of
-force is expressed by the totality of its differentiations. A further
-amount of force is that dissipated in its actions. And in these three
-amounts added together, we have the whole expense of the individual
-life. So, too, the whole expense of establishing each new individual
-includes--first the forces latent in the substance composing it when
-born or hatched; second the forces latent in the prepared nutriment
-afterwards supplied; and third the forces expended in feeding and
-protecting it. These two sets of forces being taken from a common fund,
-it is manifest that either set can increase only by decrease of the
-other. If, of the force which the parent obtains from the environment,
-much is consumed in its own life, little remains to be consumed in
-producing other lives; and, conversely, if there is a great consumption
-in producing other lives, it can only be where comparatively little is
-reserved for parental life.
-
-Hence, then, Individuation and Genesis are necessarily antagonistic.
-Grouping under the word Individuation all processes by which
-individual life is completed and maintained; and enlarging the
-meaning of the word Genesis so as to include all processes aiding
-the formation and perfecting of new individuals; we see that the
-two are fundamentally opposed. Assuming other things to remain the
-same--assuming that environing conditions as to climate, food,
-enemies, &c., continue constant; then, inevitably, every higher
-degree of individual evolution is followed by a lower degree of
-race-multiplication, and _vice versâ_. Progress in bulk, complexity, or
-activity, involves retrogress in fertility; and progress in fertility
-involves retrogress in bulk, complexity, or activity.
-
-This statement needs a slight qualification. For reasons to be
-hereafter assigned, the relation described is never completely
-maintained; and in the small departure from it, we shall find a
-remarkable self-acting tendency to further the supremacy of the most
-developed types. Here, however, this hint must suffice: explanation
-would carry us too far out of our line of argument. For the present
-it will not lead us astray if we regard this inverse variation of
-Individuation and Genesis as exact.
-
-
-§ 328. Thus, then, the condition which each race must fulfil if it is
-to survive, is a condition which, in the nature of things, it ever
-tends to fulfil. In the last chapter we saw that a species cannot be
-maintained unless the power to preserve individual life and the power
-to propagate other individuals vary inversely. And here we have seen
-that, irrespective of an end to be subserved, these powers cannot do
-other than vary inversely. On the one hand, given a certain totality
-of destroying forces with which the species has to contend; and in
-proportion as its members have severally but small ability to resist
-these forces, it is requisite that they should have great ability to
-form new individuals, and _vice versâ_. On the other hand, given the
-quantity of force, absorbed as food or otherwise, which the species
-can use to counterbalance these destroying forces; and in proportion
-as much of it is expended in preserving the individual, little of it
-can be reserved for producing new individuals, and _vice versâ_. There
-is thus complete accordance between the requirements considered under
-each aspect. The two necessities correspond.
-
-We might rest on these deductions and their several corollaries.
-Without going further we might with safety assert the general truths
-that, other things equal, advancing evolution must be accompanied
-by declining fertility; and that, in the highest types, fertility
-must still further decrease if evolution still further increases.
-We might be sure that if, other things equal, the relations between
-an organism and its environment become so changed as permanently
-to diminish the difficulties of self-preservation, there will be a
-permanent increase in the rate of multiplication; and, conversely, that
-a decrease of fertility will result where altered circumstances make
-self-preservation more laborious.
-
-But we need not content ourselves with these _à priori_ inferences. If
-they are true, there must be an agreement between them and the observed
-facts. Let us see how far such an agreement is traceable.
-
-
-
-
-                              CHAPTER IV.
-
-                DIFFICULTIES OF INDUCTIVE VERIFICATION.
-
-
-§ 329. Were all species subject to the same kinds and amounts of
-destructive forces, it would be easy, by comparing different species,
-to test the inverse variation of Individuation and Genesis. Or if
-either the power of self-preservation or the power of multiplication
-were constant, there would be little difficulty in seeing how the
-other changed as the destroying forces changed. But comparisons are
-nearly always partially vitiated by some want of parity. Each factor,
-besides being variable as a whole, is compounded of factors that are
-severally variable. Not simply is the sum of the forces destructive of
-race different in every case; and not simply are both sets of forces
-preservative of race unlike in their totalities in every case; but
-each is made up of actions that bear such changing proportions to one
-another as to prevent any positive estimation of its amount.
-
-Before dealing with the facts as well as we can, it will be best to
-glance at the chief difficulties; so that we may see the kind of
-verification which is alone possible.
-
-
-§ 330. Either absolutely, or relatively to any species, every
-environment differs more or less from every other.
-
-There are the unlikenesses of media--air, water, earth, organic matter;
-severally involving special resistances to movement, and special losses
-of heat. There are the contrasts of climate: here great expenditure
-for the maintenance of temperature is needed, and there very little;
-in one zone an organism is supplied with abundant light all the year
-round, and in another only for a few months; this region yields an
-almost unfailing supply of water, while that entails the exertion of
-travelling many miles every night for a draught.
-
-Permanent differences in the natures and distributions of aliment
-greatly interfere with the comparisons. The Swallow goes through more
-exertion than the Sparrow in securing a given weight of food; but then
-their foods are dissimilar in nutritive qualities. There is a want of
-parallelism between the circumstances of those herbivores which live
-where the plains are annually covered for a time with rich herbage, but
-afterwards become parched up, and of those inhabiting more temperate
-regions. Insects whose larvæ feed on an abundant plant, as do several
-of the genus _Vanessa_ on the Nettle, have practically an environment
-very unlike that of insects such as _Deilephila Euphorbiæ_, whose larvæ
-feed on a comparatively rare plant--the Sea-Spurge.
-
-Again, comparisons between creatures otherwise akin in their
-constitutions and circumstances, are hindered by inequalities in their
-relations to enemies. Two animals, of which one is predatory and has
-no foes but parasites while the other is much pursued, cannot properly
-be contrasted with a view to determining the influence of size or
-complexity.
-
-Without multiplying instances, it will be clear enough then that the
-aggregate of destructive actions, positive and negative, which each
-species has to contend with, is so undefinable in the amounts and kinds
-of its components, that nothing beyond a vague idea of its relative
-total can be formed.
-
-
-§ 331. Besides these immense variations in the outer actions to
-be counter-balanced, there are immense variations in the inner
-actions required to counterbalance them. Even were species similarly
-conditioned, self-preservation would require of them extremely unlike
-expenditures of force.
-
-The cost of locomotion increases in a greater ratio than the size. In
-virtue of the law that the weights of animals increase as the cubes
-of their dimensions, while their powers of bearing strains increase
-only as the squares of their dimensions (§ 46), preservation of its
-various attitudes requires a large animal to consume more substance in
-proportion to its weight, than it requires a small animal to consume;
-and there results, other things equal, a difficulty of self-maintenance
-which augments in a more rapid ratio than the bulk. Nor must we
-overlook the further complication, that among aquatic creatures the
-variation of resistance of the medium tends to produce an opposite
-effect.
-
-Again, the heat-consumption is a changing element in the total expense
-of self-preservation. Creatures which have temperatures scarcely
-above that of the air or water, may, other things equal, accumulate
-more surplus nutriment than creatures which have to keep their bodies
-warm spite of the continual loss by radiation and conduction. This
-difference of cost is modified by the presence or absence of natural
-clothing; and it is also modified by unlikenesses of size. Here the
-bulky animals have the advantage: small masses cooling more rapidly
-than large ones.
-
-Dissimilarities of attack and defence are also causes of variation
-in the outlay for self-maintenance. A creature that has to hunt, as
-compared with another that gets a sufficiency of prey by lying in wait,
-or a creature that escapes by speed as compared with another that
-escapes by concealment, obviously leads a life that is physiologically
-more costly. Animals which protect themselves passively, as the
-Hedge-hog by its spines or as the Skunk and the Musk-rat by their
-intolerable odours, are relatively economical; and have the more vital
-capital for other purposes.
-
-Amplification is needless. These instances will show that anything
-beyond very general conceptions of the individual expenditures in
-different cases, cannot be reached.
-
-
-§ 332. Still more entangled are we among qualifying considerations
-when we contrast species in their powers of multiplication. The total
-cost of Genesis admits of even less definite estimation than does the
-total cost of Individuation. I do not refer merely to the truth that
-the degree of fertility depends on four factors--the age of commencing
-reproduction, the number in each brood, the frequency of the broods,
-and the time during which broods continue to be repeated. There are
-many further obstacles in the way of comparisons.
-
-Were all multiplication carried on sexually, the problem would be
-less involved; but there are many kinds of asexual multiplication
-alternating with the sexual. This asexual multiplication is in some
-cases perpetual instead of occasional; and often has more forms than
-one in the same species. The result is that we have to compare what
-is here a periodic process with what is elsewhere a cyclical process
-partly continuous and partly periodic: the calculation of fertility in
-this last case being next to impossible.
-
-We have to avoid being misled by the assumption that the cost of
-Genesis is measured by the number of young produced, instead of being
-measured, as it is, by the weight of nutriment abstracted to form
-the young, _plus_ the weight consumed in caring for them. This total
-weight may be very diversely apportioned. In contrast to the Cod with
-its millions of small ova spawned without protection, we may put the
-_Hippocampus_, or the Pipe-fish, with its few relatively-large ova
-carried about by the male in a caudal pouch, or seated in hemispherical
-pits in its skin; or we may put the still more remarkable genus
-_Arius_, and especially _Arius Boakeii_--a fish some six or seven
-inches long, which produces ten or a dozen eggs 5–10 mm. in diameter,
-that are carried by the male in his mouth till they are hatched.
-Here though the degrees of fertility, if measured by the numbers of
-fertilized germs deposited, are extremely unlike, they are less unlike
-if measured by the numbers of young which are hatched and survive long
-enough to take care of themselves; nor will the tax on the parent-Cod
-seem so immensely different from that on the parent-_Arius_, if the
-masses of the ova, instead of their numbers, are compared. Again, while
-sometimes the parental loss is little else but the matter deducted to
-form eggs, &c., at other times it takes the shape of a small direct
-deduction joined with a large indirect outlay. The Mason-wasp furnishes
-a typical instance. In journeyings hither and thither to fetch bit
-by bit the materials for building a cell; in putting together these
-materials, as well as in secreting glutinous matter to act as cement;
-and then, afterwards, in the labour of seeking for, and carrying, the
-small caterpillars with which it fills up the cell to serve its larva
-with food when it emerges from the egg; the Mason-wasp expends more
-substance than is contained in the egg itself. And this supplementary
-expenditure is manifestly so great that but few eggs can be housed and
-provisioned.
-
-Estimates of the cost of Genesis are further complicated by variations
-in the ratio borne by the two sexes. Among Fishes the mass of milt
-approaches in size the mass of spawn; but among higher _Vertebrata_
-the substance lost by the one sex in the shape of sperm-cells is small
-compared with that lost by the other sex in the shape of albumen
-stored-up in the eggs, or blood supplied to the fœtus, or milk given
-to the young. Then there come the differences of indirect tax on males
-and females. While, frequently, the fostering of the young devolves
-entirely on the female, occasionally the male undertakes it wholly or
-in part. After building a nest, the male Stickleback guards the eggs
-till they are hatched; as does also the great _Silurus glanis_ for some
-forty days, during which he takes no food. And then, among most birds,
-we have the male occupied in feeding the female during incubation,
-and the young afterwards. Evidently all these differences affect the
-proportion between the total cost of reproduction and the total cost of
-individuation.
-
-Whether the species is monogamous or polygamous, and whether there are
-marked differences of size or of structure between males and females,
-are also questions not to be overlooked. If there are many females to
-one male, the total quantity of assimilated matter devoted by each
-generation to the production of a new generation, is greater than if
-there is a male to each female. Similarly, where the requirements
-are such that small males will suffice, the larger quantity of food
-left for the females makes possible a greater surplus available for
-reproduction. Another cause has a like effect. Where the habits of the
-race render it needless that both sexes should have developed powers of
-locomotion--where, as in the Glow-worm and sundry _Lepidoptera_, the
-female is wingless while the male has wings--the cost of Individuation
-not being so great for the species as a whole, there arises a greater
-reserve for Genesis: the matter which would otherwise have gone to the
-production of wings and the using of them, may go to the production of
-ova.
-
-Other complications, as those which we see in Bees and Ants, might be
-dwelt on; but the foregoing will amply serve the intended purpose.
-
-
-§ 333. To ascertain by comparison of cases whether Individuation
-and Genesis vary inversely, is thus an undertaking so beset with
-difficulties, that we might despair of any satisfactory results,
-were not the relation too marked a one to be hidden even by all
-these complexities. Species are so extremely contrasted in their
-degrees of evolution, and so extremely contrasted in their rates of
-multiplication, that the law of relation between these traits becomes
-unmistakable when the evidence is looked at in its _ensemble_. This we
-shall soon find on ranging in order a number of typical cases.
-
-In doing this it will be convenient to neglect, temporarily,
-all unlikenesses among the circumstances in which organisms are
-placed. At the outset, we will turn our attention wholly to the
-antagonism displayed between the integrative process which results in
-individual evolution and the disintegrative process which results in
-multiplication of individuals; and this we will consider first as we
-see it under the several forms of agamogenesis, and then as we see
-it under the several forms of gamogenesis. We will next look at the
-antagonism between propagation and that evolution which is shown by
-increased complexity. And then we will consider the remaining phase of
-the antagonism, as it exists between the degree of fertility and the
-degree of evolution expressed by activity.
-
-Afterwards, passing to the varying relations between organisms and
-their environments, we will note how relative increase in the supply
-of food, or relative decrease in the quantity of force expended by the
-individual, entails relative increase in the quantity of force devoted
-to multiplication, and _vice versâ_.
-
-Certain minor qualifications, together with sundry important
-corollaries, may then be entered upon.
-
-
-
-
-                              CHAPTER V.
-
-            ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS.
-
-
-§ 334. When illustrating, in Part IV, the morphological composition of
-plants and animals, there were set down in groups, numerous facts which
-we have here to look at from another point of view. Then we saw how,
-by union of small simple aggregates, there are produced large compound
-aggregates. Now we have to observe the reactive effect of this process
-on the relative numbers of the aggregates. Our present subject is the
-antagonism of Individuation and Genesis as seen under its simplest
-form, in the self-evident truth that the same quantity of matter may
-be divided into many small wholes or few large wholes; but that number
-negatives largeness and largeness negatives number.
-
-In setting down some examples we may conveniently adopt the same
-arrangement as before. We will look at the facts as they are presented
-by vegetal aggregates of the first order, of the second order, and of
-the third order; and then as they are presented by animal aggregates of
-the same three orders.
-
-
-§ 335. The ordinary unicellular plants are at once microscopic and
-enormously prolific. The often cited _Sphærella nivalis_, which shows
-its immense powers of multiplication by reddening wide tracts of snow
-in a single night, does this by developing in its cavity a brood of
-young cells, which, being presently set free by the bursting of the
-parent-cell, severally grow and quickly repeat the process. The like
-occurs among sundry of those kindred forms of minute _Algæ_ which,
-by their enormous numbers, sometimes suddenly change pools to an
-opaque green. So, too, the _Desmidiaceæ_ often multiply so greatly as
-to colour the water; and among the _Diatomaceæ_ the rate of genesis
-by self-division, “is something really extraordinary. So soon as a
-frustule is divided into two, each of the latter at once proceeds
-with the act of self-division; so that, to use Professor Smith’s
-approximative calculation of the possible rapidity of multiplication,
-supposing the process to occupy, in any single instance, twenty-four
-hours, ‘we should have, as the progeny of a single frustule, the
-amazing number of one thousand millions in a single month.’” In these
-cases the multiplication is so carried on that the parent is lost in
-the offspring--the old individuality disappears either in the swarms of
-zoospores it dissolves into, or in the two or four new individualities
-simultaneously produced by fission. Vegetal aggregates of the first
-order, have, however, a form of agamogenesis in which the parent
-individuality is not lost: the young cells arise from the old cells
-by external gemmation. This process, too, repeated as it is at short
-intervals, results in immense fertility. The Yeast-fungus, which in a
-few hours thus propagates itself throughout a large vat of wort, offers
-a familiar example.
-
-In certain compound forms that must be classed as plants of the second
-order of aggregation, though very minute ones, self-division similarly
-increases the numbers at high rates. The _Sarcina ventriculi_, a
-parasitic plant which infests the stomach and swarms afresh as fast as
-previous swarms are vomited, shows us a spontaneous fission of clusters
-of cells. An allied mode of increase occurs in _Gonium pectorale:_
-each cell of the cluster resolving itself into a secondary cluster,
-and the secondary clusters then separating. “Supposing, which is very
-probable, that a young _Gonium_ after twenty-four hours is capable of
-development by fission, it follows that under favourable conditions
-a single colony may on the second day develop 16, on the third 256,
-on the fourth 4,096, and at the end of a week 268,435,456 other
-organisms like itself.” In the _Volvocineæ_ this continual dissolution
-of a primary compound individual into secondary compound individuals,
-is carried on endogenously, and on a modified system: some only of
-the component cells giving origin to young colonies, and the parent
-bursting to liberate them. The numbers arising by this method also,
-are sometimes so great as to tint large bodies of water. More fully
-established and organized aggregates of the second order, such as the
-higher Thallophytes and the lower Archegoniates, do not sacrifice
-their individualities by fission; but nevertheless, by the kindred
-process of gemmation, are continually hindered in the increase of their
-individualities. The gemmæ called tetraspores are cast off in great
-numbers by the marine _Algæ_. Among those simple _Jungermanniaceæ_
-which consist of single fronds, the young ones that bud out grow for a
-time in connexion with their parents, send rootlets from their under
-sides into the soil, and presently separate themselves--a habit which
-augments the number of individuals in proportion as it checks their
-growths.
-
-Plants of the third order of composition, arising by arrest of this
-separation, exhibit a further corresponding decrease in the abundance
-of the aggregates formed. Archegoniates of inferior types, in which
-the axes produced by integration of fronds are but small and feeble,
-are characterized by the habit of throwing off bulbils--bud-shaped
-axes which, falling and taking root, add to the number of distinct
-individuals. This agamic multiplication, very general among the
-Mosses and their kindred, and not uncommon under a modified form
-in such higher types as the Ferns, many of which produce young
-ones from the surfaces of their fronds, becomes very unusual among
-Phænogams. The detachment of bulbils, though not unknown among them,
-is exceptional. And while it is true that some flowering plants, as
-the Strawberry, multiply by a process allied to gemmation, yet this
-is not characteristic of the class. A leading trait of these highest
-groups, to which the largest members of the vegetal kingdom belong, is
-that agamogenesis has so far ceased that it does not usually originate
-independent plants. Though the axes which, budding one out of another,
-compose a tree, are the equivalents of asexually-produced individuals;
-yet the asexual production of them stops short of separation. These
-vast integrations arise where spontaneous disintegration, and the
-multiplication effected by it, have come to an end.
-
-Thus, not forgetting that certain Phænogams, as _Begonia
-phyllomaniaca_, revert to quite primitive modes of increase, we may
-hold it as beyond question that while among the most minute plants
-asexual multiplication is universal, and produces enormous numbers in
-short periods, it becomes step by step more restricted in range and
-frequency as we advance to large and compound plants; and disappears so
-generally from the highest and largest, that its occurrence is regarded
-as anomalous.
-
-
-§ 336. Parallel examples furnished by animals make clear the purely
-quantitative nature of this relation under its original form. Among
-the _Protozoa_, as among the _Protophyta_, there occurs that process
-by which the individuality of the parent is wholly lost in producing
-offspring--the breaking up of the parental mass into a number of germs.
-Some of the _Infusoria_, as for instance those of the genus _Kolpoda_
-and several allied genera, become encysted and subsequently break
-up into young ones. The more familiar mode of increase among these
-animal-aggregates of the first order, by fission, though it sacrifices
-the parent individuality by merging it in the individualities of the
-two produced, sacrifices it less completely than does the dissolution
-into a great number of germs. Occurring, however, as this fission does,
-very frequently, and being completed, in some cases that have been
-observed, in the course of half-an-hour, it results in immensely-rapid
-multiplication. If all its offspring survive, and continue dividing
-themselves, a single _Paramœcium_ is said to be capable of thus
-originating 268 millions in the course of a month.[55] Nor is this
-the greatest known rate of increase. Another animalcule, visible only
-under a high magnifying power, “is calculated to generate 170 billions
-in four days.”[56] And these enormous powers of propagation are
-accompanied by a minuteness so extreme, that of some species one drop
-of water would contain as many individuals as there are human beings on
-the Earth! Even if we allow a large margin for exaggeration in these
-estimates, it is beyond question that among these smallest of animals
-the rate of asexual multiplication is immensely the greatest; and this
-suffices for the purposes of argument.
-
-Of animal aggregates belonging to the second order, that multiply
-asexually with rapidity, the familiar Polypes furnish conspicuous
-examples. By gemmation in most cases, in other cases by fission, and
-in some cases by both, the agamogenesis is carried on among these
-tribes. As shown in Fig. 148, the budding of young ones from the parent
-_Hydra_ is carried on so actively, that before the oldest of them is
-cast off half-a-dozen or more others have reached various stages of
-growth; and even while still attached, the first-formed of the group
-have commenced budding out from their sides a second generation of
-young ones. In the _Hydra tuba_ this gemmiparous multiplication is
-from time to time interrupted by a transverse splitting-up of the body
-into segments, which successively separate and swim away: the result
-of the two processes being that, in the course of a season, there are
-produced from a single germ great numbers of young _Medusæ_, which
-are the adult or sexual forms of the species. Respecting cœlenterate
-animals of this degree of composition, it may be added that when we
-ascend to the larger kinds we find asexual genesis far less active.
-Though comparisons are interfered with by differences of structure and
-mode of life, yet the contrasts are too striking to have their meanings
-much obscured. If, for instance, we take a solitary _Actinozoon_ and
-a solitary _Hydrozoon_, we see that the relatively-great bulk of the
-first, goes along with a relatively-slow agamogenesis. The common
-Sea-anemones are but occasionally observed to undergo self-division:
-multiplication by budding being in some cases largely followed, but
-their numbers are not rapidly increased by either process. A higher
-class of secondary aggregates exemplifies the same general truth with
-a difference. In the smaller members the agamogenesis is incomplete,
-and in the larger it disappears. The gemmation of the minute _Polyzoa_,
-though it does not end in the separation of the young individuals,
-habitually goes to the extent of producing families of partially
-independent individuals; but their near ally, the _Phoronis_, which
-immensely exceeds them in size, is solitary and not gemmiparous. So,
-too, is it with the _Ascidioida_. And then among the true _Mollusca_,
-which are relatively large, no such thing is known as fission or
-gemmation.
-
-Take next the _Annulosa_, including under this title the _Annelida_
-and _Arthropoda_. When treating of morphological composition, reasons
-were given for the belief that the annulose animal is an aggregate of
-the third order, the segments of which, produced one from another by
-gemmation, originally became separate; but by progressive integration,
-or arrested disintegration, there resulted a type in which many such
-segments were permanently united (§§ 205–7 and note to § 207). Part
-of the evidence there assigned, is evidence to be here repeated in
-illustration of the direct antagonism of Growth and Asexual Genesis.
-We saw how, among the lower Annelids, the string of segments produced
-by gemmation presently divides transversely into two strings; and how,
-in some cases, this resolution of the elongating string of segments
-into groups that are to form separate individuals, goes on so actively
-that as many as six groups are found in different stages of progress
-to ultimate independence--a fact implying a high rate of fissiparous
-multiplication.[57] Then we saw that, in the superior annulose types,
-distinguished in the mass by including the larger species, fission
-does not occur. The higher Annelids do not propagate in this way;
-there is no known case of new individuals being so formed among the
-_Myriapoda_; nor do the Crustaceans afford us a single instance of this
-primordial mode of increase. It is, indeed, true that while articulate
-animals never multiply asexually after this simplest method, and while
-they are characterized in the mass by the cessation of agamogenesis
-of every kind, there nevertheless occur in a few of their small
-species, those higher forms of agamogenesis known as parthenogenesis
-and pseudo-parthenogenesis; and that by these some of them multiply
-very rapidly. Hereafter we shall find, in the interpretation of these
-anomalies, further support for the general doctrine.
-
-To the above evidence has to be added that which the _Vertebrata_
-present. This may be very briefly summed up. On the one hand this
-class, whether looked at in the aggregate or in its particular species,
-immensely exceeds all other classes in the sizes of its individuals;
-and on the other hand, agamogenesis under any form is absolutely
-unknown in it. If it be said that budding occurs among the _Tunicata_
-which, under the common title of _Chordata_, are included in the same
-phylum with the _Vertebrata_, then it may firstly be replied that those
-types which have no vertebræ cannot properly be called _Vertebrata_,
-and secondly that if, as being _Chordata_, they must be recognized,
-then the exception which they present further illustrates the truth
-that agamogenetic multiplication occurs only in creatures small in
-size, or low in structure, or both.
-
-
-§ 337. Such are a few leading facts serving to show how deduction
-is inductively verified, in so far as the antagonism between Growth
-and Asexual Genesis is concerned. In whatever way we explain this
-opposition of the integrative and disintegrative processes, the facts
-and their implications remain the same. Indeed we need not commit
-ourselves to any hypothesis respecting the physical causation. It
-suffices to recognize the results under their most general aspects. We
-cannot help admitting there are at work these two antagonist tendencies
-to aggregation and separation; and we cannot help admitting that the
-proportion between the aggregative and separative tendencies, must
-in each case determine the relation between increase in bulk of the
-individual and increase of the race in number.
-
-The antithesis is as manifest _à posteriori_ as it is necessary _à
-priori_. While the minutest organisms multiply asexually in their
-billions; while the _Infusoria_ thus multiply in their millions;
-while the small compound types next above them thus multiply in their
-thousands; while larger and more compound types thus multiply in their
-hundreds and their tens; the largest types do not thus multiply at all.
-Conversely, those which do not multiply asexually at all, are a billion
-or a million times the size of those which thus multiply with greatest
-rapidity; and are a thousand times, or a hundred times, or ten times
-the size of those which thus multiply with less and less rapidity.
-Without saying that this inverse proportion is regular, which, as
-we shall hereafter see, it cannot be, we may unhesitatingly assert
-its average truth. That the smallest organisms habitually reproduce
-asexually with immense rapidity; that the largest organisms never
-reproduce at all in this manner; and that between these extremes there
-is a general decrease of asexual reproduction along with an increase of
-bulk; are propositions which admit of no dispute.
-
-
-
-
-                              CHAPTER VI.
-
-             ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS.
-
-
-§ 338. In so far as it is a process of separation, sexual genesis is
-like asexual genesis; and is therefore, equally with asexual genesis,
-opposed to that aggregation which results in growth. Whether deduction
-is made from one parent or from two, whether it is made from any part
-of the body indifferently or from a specialized part, or whether it is
-made directly or indirectly, it remains in any case a deduction; and in
-proportion as it is great, or frequent, or both, it must restrain the
-increase of the individual.
-
-Here we have to group together the leading illustrations of this truth.
-We will take them in the same order as before.
-
-
-§ 339. The lowest vegetal forms, or rather, we may say, those forms
-which we cannot class as either distinctly vegetal or distinctly
-animal, show us a process of sexual multiplication that differs much
-less from the asexual process than in the higher forms. The common
-character which distinguishes sexual from asexual genesis, is that
-the mass of protoplasm whence a new generation is to arise, has been
-produced by the union of two portions of matter which were before more
-widely separated. I use this general expression because, among the
-simplest _Algæ_, this is not invariably matter supplied by different
-individuals: certain _Diatomaceæ_ exhibit within a single cell, the
-formation of a sporangium by a drawing together of the opposite halves
-of the endochrome into a ball. Mostly, however, sporangia are products
-of conjugation. The protoplasmic contents of two cells unite to form
-the germ-mass or zygote; and these conjugating cells may be either
-entirely independent, as in many _Desmidiaceæ_ and in the gametes of
-many _Confervoideæ_; or they may be two of the adjacent cells forming
-a thread, as in some _Conjugateæ_ and the gametes of _Confervoideæ_;
-or they may be cells belonging to adjacent threads, as in other
-_Conjugateæ_. But whether it is originated by a single parent-cell, or
-by two parent-cells, the zygote, after remaining quiescent until there
-recur the fit conditions for growth, either breaks up into a multitude
-of spores, each of which produces an individual that usually multiplies
-asexually, or germinates directly to produce one new individual;
-and the fact here to be noted is, that as the entire contents of
-the parent-cells unite to form the zygote, their individualities
-are lost in the germs of a new generation. In these minute simple
-types, sexual propagation just as completely sacrifices the life of
-the parent or parents, as does that form of asexual propagation in
-which the protoplasm resolves itself directly into zoospores. And in
-the one case as in the other, this sacrifice is the concomitant of a
-prodigious fertility. Slightly in advance of this, but still showing
-us an almost equal loss of parental life in the lives of offspring,
-is the process seen in such unicellular _Algæ_ as _Botrydium_, and
-in minute _Fungi_ of the same degree of composition. These exhibit a
-relatively-enormous development of the spore-producing part, and an
-almost entire absorption of the parental substance into it. As evidence
-of the resulting powers of multiplication, we have but to remember that
-the spread of mould over stale food, the rapid destruction of crops
-by mildew, and other kindred occurrences, are made possible by the
-incalculably numerous spores thus generated and universally dispersed.
-
-Plants a degree higher in composition supply a parallel series of
-illustrations. We have among the larger _Fungi_, in which the
-reproductive apparatus is relatively so enormous as to constitute
-the ostensible plant, a similar subordination of the individual to
-the race, and a similarly-immense fertility. Thus, as quoted by Dr.
-Carpenter, Fries says--“in a single individual of _Reticularia maxima_,
-I have counted (calculated?) 10,000,000 sporules.” It needs but to
-note the clouds of particles, so minute as to look like smoke, which
-ripe puffballs give off when they are burst, and then to remember
-that each particle is a potential fungus, to be impressed with the
-almost inconceivable powers of propagation which these plants possess.
-The Lichens, too, furnish examples. Though they are nothing like so
-prolific as the _Fungi_ (the difference yielding, as we shall hereafter
-see, further support to the general argument), yet there is a great
-production of germs, and a proportionate sacrifice of the parental
-individuality. Considerable areas of the thallus develop into the
-fruit-bodies characteristic of the various fungi which, combined with
-algæ, form the different lichens (various members of the _Ascomycetes_
-and the _Basidiomycetes_). From these are produced great numbers of
-ascospores or basidiospores, as the case may be. Very many lichens also
-reproduce themselves by means of _Soredia_, _i.e._, little masses of
-algal cells closely wrapped in a weft of fungal hyphæ. Some contrasts
-presented by the higher _Algæ_ may also be named as exemplifying the
-inverse proportion between the size of the individual and the extent of
-the generative structures. While in the smaller kinds relatively large
-portions of the fronds are transformed into reproductive elements, in
-the larger kinds these portions are relatively small: instance the
-_Macrocystis pyrifera_, a gigantic seaweed which sometimes attains a
-length of 1,500 feet, of which Dr. Carpenter remarks--“This development
-of the nutritive surface takes place at the expense of the fructifying
-apparatus, which is here quite subordinate.”
-
-When we turn to vegetal aggregates of the third order of composition,
-facts having the same meaning are conspicuous. On the average
-these higher plants are far larger than plants of a lower degree of
-composition; and on the average their rates of sexual reproduction are
-far less. Similarly if, among Archegoniates and Phænogams, we compare
-the smaller types with the larger, we find them proportionately more
-prolific. This is not manifest if we simply calculate the number of
-seeds ripened by an individual in a single season; but it becomes
-manifest if we take into account the further factor which here
-complicates the result--the age at which sexual genesis commences. The
-smaller Phænogams are mostly either annuals, or perennials that die
-down annually; and seeding as they do annually before their deaths, or
-the deaths of their reproductive parts, it results that in the course
-of a year each gives origin to a multitude of potential plants, of
-which every one may the next year, if preserved, give origin to an
-equal multitude. Supposing but a hundred offspring to be produced the
-first year, ten thousand may be produced in the second year, a million
-in the third, a hundred millions in the fourth. Meanwhile, what has
-been the possible multiplication of a large Phænogam? While its small
-congener has been seeding and dying, and leaving multitudinous progeny
-to seed and die, it has simply been growing; and may so continue to
-grow for ten or a dozen years without bearing fruit. Before a Cocoa-nut
-tree has ripened its first cluster of nuts, the descendants of a wheat
-plant, supposing them all to survive and multiply, will have become
-numerous enough to occupy the whole surface of the Earth. So that
-though, when it begins to bear, a tree may annually shed as many seeds
-as an herb, yet in consequence of this delay in bearing, its fertility
-is incomparably less; and its relatively-small fertility becomes still
-further reduced where, as in _Lodoicea callipyge_, the seeds take two
-years from the date of fertilization to the date of germination.
-
-
-§ 340. Some observers state that in certain _Protozoa_ there occurs a
-process of conjugation akin to that which the _Protophyta_ exhibit--a
-coalescence of the substance of two individuals to form a germ-mass.
-This has been alleged more especially of _Actinophrys_. If this
-statement should be proved true,[58] then of the minute forms that
-appear to be more animal than vegetal in their characters, some have
-a mode of sexual multiplication by which the parents are sacrificed
-bodily in the production of a new generation.
-
-Among small animal aggregates of the second order, the first to be
-considered are of course the _Cœlenterata_. A _Hydra_ occasionally
-devotes a large part of its substance to sexual genesis. In the walls
-of its body groups of ova, or spermatozoa, or both, take their rise;
-and develop into masses greatly distorting the creature’s form, and
-leaving it much diminished when they escape. Here, however, gamogenesis
-is obviously supplementary to agamogenesis--the immensely rapid
-multiplication by budding continues as long as food is abundant and
-warmth sufficient, and is replaced by gamogenesis only at the close
-of the season. A better example of the relation between small size
-and active gamogenesis among low types of the _Metazoa_ is supplied
-by the _Rotifera_. Microscopic as these are, they have a great rate
-of sexual increase. According to Ehrenberg, _Hydatina senta_ “is
-capable of a four-fold propagation every twenty-four or thirty hours,
-bringing forth in this time four ova, which grow from the embryo to
-maturity, and exclude their fertile ova in the same period. The same
-individual, producing in ten days forty eggs, developed with the
-rapidity above cited, this rate, raised to the tenth power, gives
-one million of individuals from one parent, on the eleventh day
-four millions, and on the twelfth day sixteen millions, and so on.”
-Ehrenberg, however, characterized by Huxley as “the greatest looker
-and the worst observer,” is not a safe authority, and it is better to
-state the estimate of Ludwig Plate, who says that _Hydatina_ lays fifty
-eggs in two to three weeks--a number which, multiplying in the manner
-described, will yield in the time named a much smaller total though
-still an enormous total.
-
-The _Annulosa_, including among them the inferior types, have habits
-and conditions of life so various that only the broadest contrasts can
-be instanced in support of the proposition before us. The differences
-of organization and activity greatly complicate the inverse variation
-of fertility and bulk. Bearing in mind, however, that the rate of
-multiplication depends much less on the number of each brood than on
-the quickness with which maturity is reached and a new generation
-commenced, it will be obvious that though Annelids, relatively
-enormous in size, produce great numbers of ova, yet as they do this at
-comparatively long intervals, their rates of increase fall immensely
-below that just instanced in the Rotifers. And when at the other
-extreme we come to the large articulate animals, such as the Crab
-and the Lobster, the further diminution of fertility is seen in the
-still longer delay which occurs before each new generation begins to
-reproduce.
-
-Perhaps the best examples are supplied by vertebrate animals, and
-especially those that are most familiar to us. Comparisons between
-Fishes are unsatisfactory, because of our ignorance of their histories.
-In some cases Fishes equal in bulk produce widely different numbers of
-eggs; as the Cod which spawns millions at once, and the Salmon by which
-nothing like so great a number is spawned. But then the eggs are very
-unlike in size; and if the ovaria of the two fishes be compared, the
-difference between their masses is comparatively moderate. There are,
-indeed, contrasts which seem at variance with the alleged relation; as
-that between the Cod and the Stickleback which, though so much smaller,
-produces fewer ova. The Stickleback’s ova, however, are relatively
-large; and their total bulk bears as great a ratio to the bulk of the
-Stickleback as does the bulk of the Cod’s ova to that of the Cod.
-Moreover if, as is not improbable, the reproductive age is arrived
-at earlier by the Stickleback than by the Cod, the fertility of the
-species may be greater notwithstanding the smaller number produced
-by each individual. Evidence which admits of being tolerably well
-disentangled is furnished by Birds. They differ but little in their
-grades of organization; and the habits of life throughout extensive
-groups of them are so similar, that comparisons may be fairly made. It
-is true that, as hereafter to be shown, the differences of expenditure
-which differences of bulk entail, have doubtless much to do with the
-differences of fertility. But we may set down under the present head
-some of those cases in which the activity, being relatively slight,
-does not greatly interfere with the relation we are considering; and
-may note that among such birds having similarly slight activities, the
-small produce more eggs than the large, and eggs that bear in their
-total mass a greater ratio to the mass of the parent. Consider, for
-example, the gallinaceous birds; which are like one another and unlike
-birds of most other groups in flying comparatively little. Taking
-first the wild members of this order, which rarely breed more than
-once in a season, we find that the Pheasant has from 10 to 14 eggs,
-the Black-cock from 6 to 10, the Grouse 8 to 14, the Partridge 12 to
-20, the Quail still more, sometimes reaching two broods of 7 to 12 in
-each. Here the only exception to the relation between decreasing bulk
-and increasing number of eggs, occurs in the cases of the Pheasant
-and the Black-cock; and it is to be remembered, in explanation, that
-the Pheasant is constitutionally adapted to a warmer region, is
-better fed--often artificially--and leads a less active life. If we
-pass to domesticated genera of the same order, we meet with parallel
-differences. From the numbers of eggs laid, little can be inferred;
-for under the favourable conditions artificially maintained, the
-laying is carried on indefinitely. But though in the sizes of their
-broods the Turkey and the Fowl do not greatly differ, the Fowl begins
-breeding at a much earlier age than the Turkey, and produces broods
-more frequently: a considerably higher rate of multiplication being
-the result. Now these contrasts among domestic creatures which are
-similarly conditioned, and closely-allied by constitution, may be held
-to show, more clearly than most other contrasts, the inverse variation
-between bulk and sexual genesis; since here the cost of activity is
-diminished to a comparatively small amount. There is little expenditure
-in flight--sometimes almost none; and the expenditure in walking about
-is not great: there is more of standing than of actual movement. It
-is true that young Turkeys commence their existence as larger masses
-than chickens; but it is tolerably manifest that the total weight of
-the eggs laid by a Turkey during each season, bears a less ratio to
-the Turkey’s weight, than the total weight of the eggs which a Hen
-lays during each season, bears to the Hen’s weight; and this is the
-fairest way of making the comparison. The comparison so made shows
-a greater difference than appears likely to be due to the different
-costs of locomotion; considering the inertness of the creatures.
-Remembering that the assimilating surface increases only as the
-squares of the dimensions, while the mass of the fabric to be built up
-by the absorbed nutriment increases as the cubes of the dimensions,
-it will be seen that the expense of growth becomes relatively greater
-with each increment of size; and that hence, of two similar creatures
-commencing life with different sizes, the larger one in reaching its
-superior adult bulk, will do this at a more than proportionate expense;
-and so will either be delayed in commencing its reproduction, or will
-have a diminished reserve for reproduction, or both. Other orders
-of Birds, active in their habits, show more markedly the connexion
-between augmenting mass and declining fertility. But in them the
-increasing cost of locomotion becomes an important, and probably the
-most important, factor. The evidence they furnish will therefore
-come better under another head. Contrasts among Mammals, like those
-which Birds present, have their meanings obscured by inequalities of
-the expenditures for motion. The smaller fertility which habitually
-accompanies greater bulk, must in all cases be partly ascribed to this.
-Still, it may be well if we briefly note, for as much as they are
-worth, the broader contrasts. While a large Mammal bears but a single
-young one at a time, is several years before it commences doing this,
-and then repeats the reproduction at long intervals; we find, as we
-descend to the smaller members of the class, a very early commencement
-of breeding, an increasing number at a birth, reaching in small Rodents
-to 10 or even more, and a much more frequent recurrence of broods: the
-combined result being a relatively prodigious fertility. If a specific
-comparison be desired between Mammals that are similar in constitution,
-in food, in conditions of life, and all other things but size, the
-Deer-tribe supplies it. While the large Red-deer has but one at a
-birth, the small Roe-deer has frequently two at a birth.[59]
-
-
-§ 341. The antagonism between growth and sexual genesis, visible in
-these general contrasts, may also be traced in the history of each
-plant and animal. So familiar is the fact that sexual genesis does not
-occur early in life, and in all organisms which expend much begins only
-when the limit of size is nearly reached, that we do not sufficiently
-note its significance. It is a general physiological truth, however,
-that while the building-up of the individual is going on rapidly, the
-reproductive organs remain imperfectly developed and inactive; and that
-the commencement of reproduction at once indicates a declining rate
-of growth, and becomes a cause of arresting growth. As was shown in §
-78, the exceptions to this rule are found where the limit of growth is
-indefinite; either because the organism expends little or nothing in
-action, or expends in action so moderate an amount that the supply of
-nutriment is never equilibrated by its expenditure.
-
-We will pass over the inferior plants and, limiting ourselves to
-Phænogams, will not dwell on the less conspicuous evidence with the
-smaller types present. A few cases such as gardens supply will serve.
-All know that a Pear-tree increases in size for years before it begins
-to bear; and that, producing but few pears at first, it is long before
-it fruits abundantly. A young Mulberry-tree, branching out luxuriantly
-season after season, but covered with nothing but leaves, at length
-blossoms sparingly and sets some small and imperfect berries, which
-it drops while they are green; and it makes these futile attempts
-time after time before it succeeds in ripening any seeds. But these
-multiaxial plants, or aggregates of individuals some of which continue
-to grow while others become arrested and transformed into seed-bearers,
-show us the relation less definitely than certain plants that are
-substantially, if not literally, uniaxial. Of these the Cocoa-nut may
-be instanced. For some years it goes on shooting up without making
-any sign of becoming fertile. About the sixth year it flowers; but
-the flowers wither without result. In the seventh year it flowers and
-produces a few nuts; but these prove abortive and drop. In the eighth
-year it ripens a moderate number of nuts; and afterwards increases
-the number until, in the tenth year, it comes into full bearing.
-Meanwhile, from the time of its first flowering its growth begins
-to diminish, and goes on diminishing till the tenth year, when it
-ceases. Here we see the antagonism between growth and sexual genesis
-under both its aspects--see a struggle between self-evolution and
-race-evolution, in which the first for a time overcomes the last, and
-the last ultimately overcomes the first. The continued aggrandizement
-of the parent-individual makes abortive for two seasons the tendency to
-produce new individuals; and the tendency to produce new individuals,
-becoming more decided, stops any further aggrandizement of the parent
-individual.
-
-Parallel illustrations occur in the animal kingdom. The eggs laid by a
-pullet are relatively small and few. Similarly, it is alleged that, as
-a general rule, “a bitch has fewer puppies at first, than afterwards.”
-According to Burdach, as quoted by Dr. Duncan, “the elk, the bear, &c.,
-have at first only a single young one, then they come to have most
-frequently two, and at last again only one. The young hamster produces
-only from three to six young ones, while that of a more advanced age
-produces from eight to sixteen. The same is true of the pig.” It is
-remarked by Buffon that when a sow of less than a year old has young,
-the number of the litter is small, and its members are feeble and even
-imperfect. Here we have evidence that in animals growth checks sexual
-genesis. And then, on the other hand, we have evidence that sexual
-genesis checks growth. It is well known to breeders that if a filly
-is allowed to bear a foal, she is thereby prevented from reaching
-her proper size. And a like loss of perfection as an individual,
-is suffered by a cow which breeds too early. It may be added, as a
-converse fact, that castrated animals, as capons and notably cats,
-often become larger than their unmutilated associates.
-
-
-§ 342. Notwithstanding the way in which the inverse variation of
-growth and sexual genesis is complicated with other relations, its
-existence is, I think, sufficiently manifest. Individually, many of
-the foregoing instances are open to criticism, and have to be taken
-with qualifications; but when looked at in the mass their meaning is
-beyond doubt. Comparisons between the largest with the smallest types,
-whether vegetal or animal, yield results which are unmistakable. On
-the one hand, remembering the fact that during its centuries of life
-an Oak does not produce as many acorns as a Fungus does spores in a
-single night, we see that the Fungus has a fertility exceeding that
-of the Oak in a degree literally beyond our powers of calculation or
-imagination. On the other hand when, taking a microscopic protophyte
-which has billions of descendants in a few days, we ask how many such
-would be required to build up the forest tree which is years before
-it drops a seed, we are met by a parallel difficulty in conceiving
-the number, if not in setting it down. Similarly, if from the minute
-and prodigiously-fertile Rotifer we turn to the Elephant, which
-approaches thirty years before it bears a solitary young one, we find
-the connexions between small size and great fertility and between great
-size and small fertility, too intensely marked to be much disguised by
-the perturbing relations that have been indicated. Finally, as this
-induction, reached by a survey of organisms in general, is verified by
-observations on the relation between decreasing growth and commencing
-reproduction in individual organisms, we may, I think, consider the
-alleged antagonism as proved.[60]
-
-
-
-
-                             CHAPTER VII.
-
-  THE ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS, ASEXUAL AND SEXUAL.
-
-
-§ 343. By Development, as here to be dealt with apart from Growth, is
-meant increase of structure as distinguished from increase of mass. As
-was pointed out in § 50, this is the biological definition of the word.
-In the following sections, then, we have to note how complexity of
-organization is hindered by reproductive activity, and conversely.
-
-This relation partially coincides with that which we have just
-contemplated; for, as was shown in § 44, degree of growth is to a
-considerable extent dependent on degree of organization. But while the
-antagonism to be illustrated in this chapter is much entangled with
-that illustrated in the last chapter, it may be so far separated as to
-be identified as an additional antagonism.
-
-Besides the direct opposition between that continual disintegration
-which rapid genesis implies, and the fulfilment of that pre-requisite
-to extensive organization--the formation of an extensive aggregate,
-there is an indirect opposition which we may recognize under several
-aspects. The change from homogeneity to heterogeneity takes time;
-and time taken in transforming a relatively-structureless mass into
-a developed individual, delays the period of reproduction. Usually
-this time is merged in that taken for growth; but certain cases of
-metamorphosis show us the one separate from the other. An insect,
-passing from its lowly-organized caterpillar-stage into that of
-chrysalis, is afterwards a week, a fortnight, or a longer period in
-completing its structure: the re-commencement of genesis being by so
-much postponed, and the rate of multiplication therefore diminished.
-Further, that re-arrangement of substance which development implies,
-entails expenditure. The chrysalis loses weight in the course of its
-transformation; and that its loss is not loss of water only, may
-be inferred from the fact that it respires, and that respiration
-indicates consumption. Clearly the matter consumed is, other things
-equal, a deduction from the surplus which may go to reproduction.
-Yet again, the more widely and completely an organic mass becomes
-differentiated, the smaller is the portion of it which retains the
-relatively-undifferentiated state that admits of being moulded into
-new individuals, or the germs of them. Protoplasm which has become
-specialized tissue cannot be generalized afresh, and afterwards
-transformed into something else; and hence the progress of structure
-in an organism, by diminishing the unstructured part, diminishes the
-amount available for making offspring.
-
-It is true that higher structure, like greater growth, may insure to
-a species advantages which eventually further its multiplication--may
-give it access to larger supplies of food, or enable it to obtain food
-more economically; and we shall hereafter see how the inverse variation
-we are considering is thus qualified. But here we are concerned
-only with the necessary and direct effects; not with those that are
-contingent and remote. These necessary and direct effects we will now
-look at as exemplified.
-
-
-§ 344. Speaking generally, the simpler plants propagate both
-sexually and asexually; and, speaking comparatively, the complex
-plants propagate only sexually: their asexual propagation is usually
-incomplete--produces a united aggregate of individuals instead of
-numerous distinct individuals. The Protophytes that perpetually
-subdivide, the merely cellular _Algæ_ that shed their tetraspores, the
-Archegoniates that spontaneously separate their fronds or drop their
-gemmæ, show us an extra mode of multiplication which, among flowering
-plants, is exceptional. This extra mode of multiplication among these
-simpler plants, is made easy by their low development. Tetraspores
-arise only where the frond consists of untransformed cells; gemmæ bud
-out and drop off only where the tissue is comparatively homogeneous.
-
-Should it be said that this is but another aspect of the antagonism
-already set forth, since these undeveloped forms are also the smaller
-forms; the reply is that though in part true this is not wholly true.
-Various marine _Algæ_ which propagate asexually, are larger than some
-Phænogams which do not thus propagate. The objection that difference of
-medium vitiates this comparison, is met by the fact that it is the same
-among land-plants themselves. Sundry of the lowly-organized Liverworts
-which are habitually gemmiparous, exceed in size many flowering plants.
-And the Ferns show us agamic multiplication occurring in plants which,
-while they are inferior in complexity of structure, are superior in
-bulk to numbers of annual Monocotyledons and Dicotyledons.
-
-
-§ 345. In the ability of the lowly-organized substance of a Sponge to
-transform itself into multitudes of gemmules, we have an instance of
-this same direct relation in the animal kingdom. Moreover, the instance
-yields very distinct proof of an antagonism between development and
-genesis, independent of the antagonism between growth and genesis; for
-the Sponge which thus multiplies itself asexually, as well as sexually,
-is far larger than hosts of more complex animals which do not multiply
-asexually.
-
-Once again may be cited the creature so often brought in evidence, the
-_Hydra_, as showing us how rapidity of agamic propagation is associated
-with inferiority of structure. Its power to produce young ones from
-nearly all parts of its body, is due to the comparative homogeneity
-of its body. In kindred but more-organized types, the gemmiparity is
-greatly restricted, or disappears. Among the free-swimming _Hydrozoa_,
-multiplication by budding, when it occurs at all, occurs only at
-special places. That increase of structure apart from increase of size,
-is here a cause of declining agamogenesis, we may see in the contrast
-between the simple _Hydra_ and the compound Hydroids. These last, along
-with more-differentiated tissues, show us a gemmation which does not
-go on all over the body of each polype, and much of it does not end in
-separation.
-
-It is, however, among the _Annulosa_ that progressing organization
-is most conspicuously operative in diminishing agamogenesis. The
-segments or “somites” composing an animal belonging to this class, are
-primordially alike; and, as before argued (§§ 205–7), are probably
-the homologues of what were originally independent individuals. The
-progress from the lower to the higher types of the class, is at once
-a progress towards types in which the strings of segments cease to
-undergo subdivision, and towards types in which the segments, no longer
-alike in their structures and functions, have become physiologically
-integrated or mutually dependent. Already this group of cases has
-been named as illustrating the antagonism between growth and asexual
-genesis; but it is proper also to name it here, since, on the one hand,
-the greater size due to the ceasing of fission, is made possible only
-by the specialization of parts and the development of a co-ordinating
-apparatus to combine their actions, and since, on the other hand,
-specialization and co-ordination can advance only in proportion as
-fission ceases.
-
-
-§ 346. The inverse variation of development and sexual genesis is by no
-means easy to follow. One or two facts indicative of it may, however,
-be named.
-
-Phænogams that have but little supporting tissue may fairly be classed
-as structurally inferior to those having stems with a bulky and
-complex woody system; for these imply additional differentiations,
-and constitute wider departures from the primitive type of vegetal
-tissue. That the concomitant of this higher organization is a slower
-gamogenesis, scarcely needs pointing out. While the herbaceous annual
-is blossoming and ripening seed, the young tree is transforming its
-originally-succulent axis into dense fibrous substance; and year
-by year the young tree expends in doing the like, nutriment which
-successive generations of the annual expend in fruit. Here the inverse
-relation is between sexual reproduction and complexity, and not between
-sexual reproduction and bulk, seeing that besides seeding, the annual
-often grows to a size greater than that reached by the young infertile
-tree in several years.
-
-Proof of the antagonism between complexity and gamogenesis in animals,
-is still more difficult to disentangle. Perhaps the evidence most to
-the point is furnished by the contrast between Man and certain other
-Mammals approaching him in mass. To compare him with the domestic Sheep
-which, though not very unlike in size, is relatively prolific, is
-objectionable because of the relative inactivity of Sheep; and this,
-too, may be alleged as a reason why the Ox, though far more bulky, is
-also far more fertile, than Man. Further, against a comparison with
-the Horse which, while both larger and more prolific, is tolerably
-active, it may be urged that in his case, and the cases of herbivorous
-creatures generally, the small exertion required to procure food,
-joined with the great ratio borne by the alimentary organs to the
-organs they have to build up and repair, vitiates the result. We may,
-however, fairly draw a parallel between Man and a large carnivore. The
-Lion, superior in size, and perhaps equal in activity, has a digestive
-system not proportionately greater; and yet has a higher rate of
-multiplication than Man. Here the only decided want of parity, besides
-that of organization, is that of food. Possibly a carnivore gains an
-advantage in having a surplus nutriment consisting almost wholly of
-those nitrogenous materials from which the bodies of young ones are
-mainly formed. But, allowing for all other differences, it appears not
-improbable that the smallness of human fertility compared with the
-fertility of large feline animals, is due to the greater complexity of
-the human organization--more especially the organization of the nervous
-system. Taking degree of nervous organization as the chief correlative
-of mental capacity; and remembering the physiological cost of that
-slow evolution whereby high mental capacity is reached; we may suspect
-that nervous organization is very expensive: the inference being that
-bringing it up to the level it reaches in Man, whose digestive system,
-by no means large, has at the same time to supply materials for general
-growth and daily waste, involves a great retardation of maturity and
-sexual genesis.
-
-
-
-
-                             CHAPTER VIII.
-
-              ANTAGONISM BETWEEN EXPENDITURE AND GENESIS.
-
-
-§ 347. Under this head we have to set down no evidence derived from the
-vegetal kingdom. Plants are not expenders of force in such degrees as
-to affect the general relations with which we are dealing. They have
-not to maintain a heat above that of their environment, nor have they
-to generate motion; and hence consumption for these two purposes does
-not diminish the stock of material which serves on the one hand for
-growth and on the other hand for propagation.
-
-It will be well, too, if we pass over the lower animals: especially
-those aquatic ones which, being nearly of the same temperature as
-the water, and nearly of the same specific gravity, lose but little
-in evolving motion, sensible and insensible. A further reason for
-excluding from consideration these inferior types, is that we do not
-know enough of their rates of genesis to permit of our making, with any
-satisfaction, those involved comparisons here to be entered upon.
-
-The facts on which we must mainly depend are those to be gathered
-from terrestrial animals, and chiefly from those higher classes of
-them which are at the same time great expenders and have rates of
-multiplication about which our knowledge is tolerably definite. We
-will restrict ourselves, then, to the evidence which Birds and Mammals
-supply.
-
-
-§ 348. Satisfactory proof that loss of substance in the maintenance of
-heat diminishes the rapidity of propagation, is difficult to obtain.
-It is, indeed, obvious that the warmblooded _Vertebrata_ are less
-prolific than the cold-blooded; but then they are at the same time
-more vivacious. Similarly, between Mammals and Birds (which are the
-warmer-blooded of the two) there is, other things equal, a parallel,
-though much smaller, difference; but here, too, the unlikenesses of
-muscular action complicate the evidence. Again, the annual return of
-generative activity has an average correspondence with the annual
-return of a warmer season, which, did it stand alone, might be taken
-as evidence that a diminished cost of heat-maintenance leads to such a
-surplus as makes reproduction possible. But then, this periodic rise of
-temperature is habitually accompanied by an increase in the quantity
-of food--a factor of equal or greater importance. We must be content,
-therefore, with such few special facts as admit of being disentangled.
-
-Certain of these we are introduced to by the general relation last
-named--the habitual recurrence of genesis with the recurrence of
-spring. For in some cases a domesticated creature has its supplies
-of food almost equalized; and hence the effect of varying nutrition
-may be in great part eliminated from the comparison. The common
-Fowl yields an illustration. It is fed through the cold months, but
-nevertheless, in mid-winter, it either wholly leaves off laying or
-lays very sparingly. And then we have the further evidence that if it
-lays sparingly, it does so only on condition that the heat, as well as
-the food, is artificially maintained. Hens lay in cold weather only
-when they are kept warm. To which fact may be added the kindred one
-that “when pigeons receive artificial heat, they not only continue to
-hatch longer in autumn, but will recommence in spring sooner than they
-would otherwise do.” An analogous piece of evidence is that, in winter,
-inadequately sheltered Cows either cease to give milk or give it in
-diminished quantity. For though giving milk is not the same thing as
-bearing a young one, yet, as milk is part of the material from which
-a young one is built up, it is part of the outlay for reproductive
-purposes, and diminution of it is a loss of reproductive power. Indeed
-the case aptly illustrates, under another aspect, the struggle between
-self-preservation and race-preservation. Maintenance of the cow’s life
-depends on maintenance of its heat; and maintenance of its heat may
-entail such reduction in the supply of milk as to cause the death of
-the calf.
-
-Evidence derived from the habits of the same or allied genera in
-different climates, may naturally be looked for; but it is difficult
-to get, and it can scarcely be expected that the remaining conditions
-of existence will be so far similar as to allow of a fair comparison
-being made. The only illustrative facts I have met with which seem
-noteworthy, are some named by Mr. Gould in his work on _The Birds
-of Australia_. He says:--“I must not omit to mention, too, the
-extraordinary fecundity which prevails in Australia, many of its
-smaller birds breeding three or four times in a season; but laying
-fewer eggs in the early spring when insect life is less developed, and
-a greater number later in the season, when the supply of insect food
-has become more abundant. I have also some reason to believe that the
-young of many species breed during the first season, for among others,
-I frequently found one section of the Honey-eaters (the _Melithrepti_)
-sitting upon eggs while still clothed in the brown dress of immaturity;
-and we know that such is the case with the introduced _Gallinaceæ_ (or
-poultry) three or four generations of which have been often produced in
-the course of a year.” Though here Mr. Gould refers only to variation
-in the quantity of food as a cause of variation in the rate of
-multiplication, may we not suspect that warmth is a part-cause of the
-high rate which he describes as general?
-
-
-§ 349. Of the inverse variation between activity and genesis, we get
-clear proof. Let us begin with that which Birds furnish.
-
-First we have the average contrast, already hinted, between the
-fertility of Birds and the fertility of Mammals. Comparing the large
-with the large and the small with the small, we see that creatures
-which continually go through the muscular exertion of sustaining
-themselves in the air and propelling themselves rapidly through it,
-are less prolific than creatures of equal weights which go through the
-smaller exertion of moving about over solid surfaces. Predatory Birds
-have fewer young ones than predatory Mammals of approximately the same
-sizes. If we compare Rooks with Rats, or Finches with Mice, we find
-like differences. And these differences are greater than at first
-appears. For whereas among Mammals a mother is able, unaided, to bear
-and suckle and rear half-way to maturity, a brood that probably weighs
-more in proportion than does the brood of a Bird; a Bird, or at least
-a Bird that flies much, is unable to do this. Both parents have to
-help; and this indicates that the margin for reproduction in each adult
-individual is smaller.
-
-Among Birds themselves occur contrasts which may be next considered.
-In the Raptorial class, various species of which, differing in their
-sizes, are similarly active in their habits, we see that the small
-are more prolific than the large. The Golden Eagle has usually 2
-eggs: sometimes 3, sometimes only 1. As we descend to the Kites and
-Falcons, the number is 2 or 3, and 3 or 4. And when we come to the
-Sparrow-Hawk, 3 to 5 is the specified number. Similarly among the
-Owls: while the Great Eagle-Owl has 2 or 3 eggs, the comparatively
-small Common Owl has 4 or 5. As before hinted, it is impossible to say
-what proportions of these differences are due to unlikenesses of bulk
-merely, and what proportions are due to unlikenesses in the costs of
-locomotion. But we may fairly assume that the unlikenesses in the costs
-of locomotion are here the more important factors. Weights varying as
-the cubes of the dimensions, while the surfaces of digestive systems
-vary as the squares, the expense of flight increases more rapidly than
-does the ability to take in nutriment; and as motion through the
-air requires more effort than motion on the ground, this geometrical
-progression tells more rapidly on Birds than on Mammals. Be this as
-it may, however, these contrasts support the argument; as do various
-others which may be set down. The Finch-family, for example, have
-broods averaging about 5 in number, and have commonly 2 broods in
-the season; while in the Crow-family the number of the brood is on
-the average less, and there is but one brood in the season. And then
-on descending to such small birds as the Wrens and the Tits, we have
-8, 10, 12 to 15 eggs, and sometimes two broods in the year. One of
-the best illustrations is furnished by the Swallow-tribe, throughout
-which there is little or no difference in mode of life or in food. The
-Sand-Martin, much the least of them, has 4 to 6 eggs and two broods;
-the Swallow, somewhat larger, has 4 or 5; and the Swift (similar in
-habits though unrelated), larger still, has but 2. Here we see a lower
-fertility associated in part with greater size, but associated still
-more conspicuously with greater expenditure. For the difference of
-fertility is more than proportionate to the difference of bulk, as
-shown in other cases; and for this greater difference there is the
-reason, that the Swift has to support not only the cost of propelling
-its larger mass through the air, but also the cost of propelling it at
-a higher velocity.
-
-Omitting much evidence of like nature, let us note that disclosed by
-comparisons of certain groups of birds with other groups. “Skulkers”
-is the descriptive title applied to the Water-Rail, the Corn-Crake,
-and their allies, which evade enemies by concealment--consequently
-expending but little in locomotion. These birds have relatively large
-broods--6 to 11, 8 to 12, &c. Not less instructive are the contrasts
-between the Gallinaceous Birds and other Birds of like sizes but more
-active habits. The Partridge and the Wood-Pigeon are about equal in
-bulk and have much the same food. Yet while the one has from 12 to 20
-young ones, the other has but 2 young ones twice a-year: its annual
-reproduction is less than one-third. It may be said that the ability
-of the Partridge to bring up so large a brood, is due to that habit of
-its tribe which one of its names, “Scrapers,” describes; and to the
-accompanying habit of the young, which begin to get their own living as
-soon as they are hatched: so saving the parents’ labour. Conversely,
-it may be said that the inability of Pigeons to rear more than 2 at a
-time, is caused by the necessity of fetching everything they eat. But
-the alleged relation holds nevertheless. On the one hand, a great part
-of the food which the Partridge chicks pick up, is food which, in their
-absence, the mother would have picked up. Though each chick costs her
-far less than a young Pigeon costs its parents, yet the whole of her
-chicks cost her a great deal in the shape of abstinence--an abstinence
-she can bear because she has to fly but little. On the other hand,
-the Pigeon’s habit of laying and hatching but two eggs, must not be
-referred to any foreseen necessity of going through so much labour in
-supporting the young, but to a constitutional tendency established by
-such labour. This is proved by the curious fact that when domesticated,
-and saved from such labour by artificial feeding, Pigeons, says
-Macgillivray (quoting Aitkin), “are frequently seen sitting on eggs
-long before the former brood is able to leave the nest, so that the
-parent bird has at the same time young birds and eggs to take care of.”
-
-
-§ 350. Made to illustrate the effect of activity on fertility, most
-comparisons among Mammals are objectionable: other circumstances are
-not equal. A few, however, escape this criticism.
-
-One is that between the Hare and the Rabbit. These are closely-allied
-species of the same genus, similar in their diet but unlike in their
-expenditures for locomotion. The relatively-inert Rabbit has 6 young
-ones in a litter, and four litters a-year; while the relatively-active
-Hare has but 2 to 5 in a litter. This is not all. The Rabbit begins to
-breed at six months old; but a year elapses before the Hare begins
-to breed. These two factors compounded, result in a difference of
-fertility far greater than can be ascribed to unlikeness of the two
-creatures in size.
-
-Perhaps the most striking piece of evidence which Mammals furnish,
-is the extreme infertility of our common Bat. The _Cheiroptera_ and
-the _Rodentia_ are not very dissimilar in their internal structures.
-Diversity of constitution, therefore, cannot vitiate the comparison
-between Bats and Mice, which are about the same in size. Though their
-diets differ, the difference is in favour of the Bat: its food being
-exclusively animal while that of the Mouse is mainly vegetal. What now
-are their respective rates of genesis? The Mouse has several litters in
-a year of 5 to 7 in each; while the Bat produces only one at a time.
-Whether the Bat repeats its one more frequently than the Mouse repeats
-its 7 is not stated; but it is quite certain that even if it does so
-(an absurd supposition), the more frequent repetition cannot be such
-as to raise its fertility to anything like that of the Mouse. And this
-relatively-low rate of multiplication we may fairly ascribe to its
-relatively-high rate of expenditure.
-
-Here let us note, in passing, an interesting example of the
-way in which a species which has no specially-great power of
-self-preservation, while its power of multiplication is extremely
-small, nevertheless avoids extinction because it has to meet an
-unusually-small total of race-destroying forces. Leaving out parasites,
-the only enemy of the Bat is the Owl; and the Owl is sparingly
-distributed.
-
-
-§ 351. These general evidences may be enforced by some special
-evidences. We have few opportunities of observing how, within the
-same species, variations of expenditure are related to variations of
-fertility. But a fact or two showing the connexion may be named.
-
-Doctor Duncan quotes a statement to the point respecting the breeding
-of dogs. Already in §341 I have extracted a part of this statement,
-to the effect that before her growth is complete, a bitch bears at a
-birth fewer puppies than when she becomes full-grown. An accompanying
-allegation is, that her declining vigour is shown by a decrease in the
-number of puppies contained in a litter, “ending in one or two.” And
-then it is further alleged that, “as regards the amount of work a dog
-has to perform, so will the decline be rapid or gradual; and hence, if
-a bitch is worked hard year after year, she will fail rapidly, and the
-diminution of her puppies will be accordingly; but if worked moderately
-and well kept, she will fail gradually, and the diminution will be less
-rapid.”
-
-In this place, more fitly than elsewhere, may be added a fact of like
-implication, though of a different order. Of course whether excessive
-expenditure be in the continual repairs of nervo-muscular tissues or in
-replacing other tissues, the reactive effects, if not quite the same,
-will be similar--there will be a decrease of the surplus available for
-genesis. If, then, in any animals there from time to time occur unusual
-outlays for self-maintenance, we may expect the periods of such outlays
-to be periods of diminished or arrested reproduction. That they are so
-the moulting of birds shows us. When hens begin to moult they cease to
-lay. While they are expending so much in producing new clothing, they
-have nothing to expend for producing eggs.
-
-
-
-
-                              CHAPTER IX.
-
-            COINCIDENCE BETWEEN HIGH NUTRITION AND GENESIS.
-
-
-§ 352. Under this head may be grouped various facts which, in another
-way, tell the same tale as those contained in the last chapter. The
-evidence there put together went to show that increased cost of
-self-maintenance entailed decreased power of propagation. The evidence
-to be set down here, will go to show that power of propagation is
-augmented by making self-maintenance unusually easy. For into this may
-be translated the effect of abundant food.
-
-To put the proposition more specifically--we have seen that after
-individual growth, development, and daily consumption, have
-been provided for, the surplus nutriment measures the rate of
-multiplication. This surplus may be raised in amount by such changes in
-the environment as bring a larger supply of the materials or forces on
-which both parental life and the lives of offspring depend. Be there,
-or be there not, any expenditure, a higher nutrition will make possible
-a greater propagation. We may expect this to hold both of agamogenesis
-and of gamogenesis; and we shall find that it does so.
-
-
-§ 353. On multiaxial plants, the primary effect of surplus nutriment
-is a production of large and numerous leaf-shoots. How this asexual
-multiplication results from excessive nutrition, is well shown when the
-leading axis, or a chief branch, is broken off towards its extremity.
-The axillary buds below the breakage quickly swell and burst into
-lateral shoots, which often put forth secondary shoots: two generations
-of agamic individuals arise where there probably would have been none
-but for the local abundance of sap, no longer drawn off. In like manner
-the abnormal agamogenesis which we have in proliferous flowers, is
-habitually accompanied by a general luxuriance, implying an unusual
-plethora.
-
-No less conclusive is the evidence furnished by agamogenesis in
-animals. Sir John Dalyell, speaking of _Hydra tuba_, and of the period
-before strobilization commences, says--“It is singular how much
-propagation is promoted by abundant sustenance.” This Polype goes on
-budding out young polypes from its sides, with a rapidity proportionate
-to the supply of materials. So, too, is it with the agamic reproduction
-of the _Aphis_. As cited by Professor Huxley, Kyber “states that he
-raised viviparous broods of both this species (_Aphis Dianthi_) and
-_A. Rosæ_ for four consecutive years, without any intervention of
-males or oviparous females, and that the energy of the power of agamic
-reproduction was at the end of that period undiminished. The rapidity
-of the agamic proliferation throughout the whole period was directly
-proportional to the amount of warmth and food supplied.”
-
-In these cases the relation is not appreciably complicated by
-expenditure. The parent having reached its limit of growth, the
-absorbed food goes to asexual multiplication: scarcely any being
-deducted for the maintenance of parental life.
-
-
-§ 354. The sexual multiplication of organisms under changed conditions,
-undergoes variations conforming to a parallel law. Cultivated plants
-and domesticated animals yield us proof of this.
-
-Facts showing that in cultivated plants sexual genesis increases with
-nutrition, are obscured by facts showing that a less rapid asexual
-genesis, and an incipient sexual genesis, accompany the fall from a
-high to a moderate nutrition. The confounding of these two relations
-has led to mistaken inferences. When treating of Genesis inductively,
-we reached the generalization that “the products of a fertilized germ
-go on accumulating by simple growth, so long as the forces whence
-growth results are greatly in excess of the antagonist forces; but
-that when diminution of the one set of forces, or increase of the
-other, causes a considerable decline in this excess, and an approach
-towards equilibrium, fertilized germs are again produced.” (§ 78.)
-It was pointed out that this holds of organisms which multiply by
-heterogenesis, as well as those which multiply by homogenesis.
-And plants were referred to as illustrating, both generally and
-locally, the decline of agamic multiplication and commencement of
-gamic multiplication, along with a lessening rate of nutrition.
-Now the many cases which are given of fruitfulness caused in trees
-by depletion, are really cases of this change from agamogenesis
-to gamogenesis; and simply go to prove that what would naturally
-arise when decreased peripheral growth had followed increased size,
-may be brought about artificially by diminishing the supply of
-materials for growth. Cramping its roots in a pot, or cutting them,
-or ringing its branches, will make a tree bear very early: bringing
-about a premature establishment of that relative innutrition which
-would have spontaneously arisen in course of time. Such facts by
-no means show that in plants sexual genesis increases as nutrition
-diminishes. When it has once set in, sexual genesis is scanty or
-imperfect unless nutrition is good. Though the starved plant may
-blossom, yet many of its blossoms will fail; and such seeds as it
-produces will be ill-furnished with those enveloping structures and
-that store of albumen, &c., needed to give good chances of successful
-germination--the number of surviving offspring will be diminished. Were
-it otherwise, the manuring of fields which are to bear seed-crops,
-would be not simply useless but injurious. Were it otherwise, dunging
-the roots of a fruit-tree would in all cases be impolitic; instead
-of being impolitic only where the growth of sexless axes is still
-luxuriant. Were it otherwise, a tree which has borne a heavy crop
-should, by the consequent depletion, be led to bear a still heavier
-crop next year; whereas it is apt to be wholly or partially barren next
-year--has to recover a state of tolerably-high nutrition before its
-sexual genesis again becomes large.
-
-But the best illustrations are yielded by animals--those animals at
-least in which we have, besides an increased supply of nutriment, a
-diminished expenditure. Two classes of comparisons, alike in their
-implications, may be made--comparisons between tame and wild animals of
-the same species or genus, and comparisons between tame animals of the
-same species differently treated.
-
-To begin with Birds, let us first contrast the farm-yard _Gallinaceæ_
-with their kindred of the fields and woods. Notwithstanding their
-greater size, which, other things equal, should be accompanied by
-smaller fertility, the domesticated kinds have more numerous offspring
-than the wild kinds. A Turkey has a dozen in a brood, while a Pheasant
-has from 6 to 10. Twice or thrice in a season, a Hen rears as many
-chickens as a Partridge rears once in a season. Anserine birds show us
-parallel differences. The Tame Goose sits on 13 to 18 eggs and often
-sits a second time; but the Wild Goose sits on 5, 6, or 7, and these
-are noted as considerably smaller. It is the same with Ducks. The
-domesticated variety lays and hatches twice as many eggs as the wild
-variety. And the like holds of Pigeons. After remarking of the _Columba
-livia_ that “in spring when they have plenty of corn to pick from
-the newly-sown fields, they begin to get fat and pair; and again in
-harvest, when the corn is cut down,” Macgillivray goes on to say that
-“the same pair when tamed generally breed four times” in the year. That
-between different poultry-yards inequalities of fertility are caused by
-inequalities in the supplies of food, is a familiar truth. High feeding
-shows its effects not only in the continuous laying, but also in the
-sizes of the eggs. Among directions given for obtaining eggs from
-pullets late in the year, it is especially insisted on that they shall
-have a generous diet. Respecting Pigeons Macgillivray writes:--“that
-their breeding depends much on their having plenty of food to fatten
-them, seems, I think, evident from the circumstance that, when tamed,
-which they easily are, they are observed to breed in every month of
-the year. I do not mean that the same pair will breed every month; but
-some in the flock, if well fed, will breed at any season.” There may
-be added a fact of like meaning which partially-domesticated birds
-yield. The Sparrow is one of the Finch tribe that has taken to the
-neighbourhood of houses; and by its boldness secures food not available
-to its congeners. The result is that it has several broods in a season,
-while its field-haunting kindred have none of them more than two
-broods, and some have only one.
-
-Equally clear proof that abundant nutriment raises the rate of
-multiplication, occurs among Mammals. Compare the litters of the Dog
-with the litters of the Wolf and the Fox. Whereas those of the one
-range in number from 6 to 14, those of the others contain respectively
-5 or 6 or occasionally 7, and 4 or 5 or rarely 6. Again, the Wild Cat
-has 4 or 5 kittens; but the tame Cat has 5 or 6 kittens 2 or 3 times
-a-year. So, too, is it with the Weasel tribe. The Stoat has 5 young
-ones once a-year. The Ferret has 2 litters yearly, each containing
-from 6 to 9; and this notwithstanding that it is the larger of the
-two. Perhaps the most striking contrast is that between the wild and
-tame varieties of the Pig. While the one produces, according to its
-age, from 4 to 8 or 10 young ones once a year, the other produces
-sometimes as many as 17 in a litter; or, in other cases, will bring
-up 5 litters of 10 each in two years--a rate of reproduction which is
-unparalleled in animals of as large a size.[61] And let us not omit to
-note that this excessive fertility occurs where there is the greatest
-inactivity--where there is plenty to eat and nothing to do. There is no
-less distinct evidence that among domesticated Mammals themselves, the
-well-fed individuals are more prolific than the ill-fed individuals. On
-the high and comparatively-infertile Cotswolds, it is unusual for ewes
-to have twins; but they very commonly have twins in the adjacent rich
-valley of the Severn. Similarly, among the barren hills of the west of
-Scotland, two lambs will be borne by about one ewe in twenty; whereas
-in England, something like one ewe in three will bear two lambs. Nay,
-in rich pastures, twins are more frequent than single births; and it
-occasionally happens that, after a genial autumn and consequent good
-grazing, a flock of ewes will next spring yield double their number of
-lambs--the triplets balancing the uniparæ. So direct is this relation,
-that I have heard a farmer assert his ability to foretell, from the
-high, medium, or low, condition of an ewe in the autumn, whether she
-will next spring bear two, or one, or none.
-
-
-§ 355. An objection must here be met. Many facts may be brought to
-prove that fatness is not accompanied by fertility but by barrenness;
-and the inference drawn is that high feeding is unfavourable to
-genesis. The premiss may be admitted while the conclusion is denied.
-
-There is a distinction between what may be called normal plethora,
-and an abnormal plethora, liable to be confounded with it. The one
-is a mark of constitutional wealth; but the other is a mark of
-constitutional poverty. Normal plethora is a superfluity of materials
-both for the building up of tissue and the evolution of force; and
-this is the plethora which we have found to be associated with unusual
-fecundity. Abnormal plethora which, as truly alleged, is accompanied by
-infecundity, is a superfluity of force-evolving materials joined with
-either a positive or a relative deficiency of tissue-forming materials:
-the increased bulk indicating this state, being really the bulk of so
-much inert or dead matter. Note, first, a few of the facts which show
-us that obesity implies physiological impoverishment.
-
-Neither in brutes nor men does it ordinarily occur either in youth or
-in that early maturity during which the vigour is the greatest and the
-digestion the best: it does not habitually accompany the highest power
-of taking up nutritive materials. When fatness arises in the prime of
-life, whether from peculiarity of food or other circumstance, it is
-not the sign of an increased total vitality. On the contrary, if great
-muscular action has to be gone through, the fat must be got rid of;
-either, as in a man, by training, or as in a horse that has grown bulky
-while out at grass, by putting him on such more nutritive diet as oats.
-The frequency of senile fatness, both in domesticated creatures and in
-ourselves, has a similar implication. Whether we consider the smaller
-ability of those who display it to withstand large demands on their
-powers, or whether we consider the comparatively-inferior digestion
-common among them, we see that the increased size indicates, not an
-abundance of materials which the organism requires, but an abundance of
-materials which it does not require. Of like meaning is the fact that
-women who have had several children, and animals after they have gone
-on bearing young for some time, frequently become fat; and lose their
-fecundity as they do this. In such cases the fatness is not to be taken
-as the cause of the infecundity; but the constitutional exhaustion
-which the previous production of offspring has left, shows itself at
-once in the failing fecundity and the commencing fatness. There is yet
-another kind of evidence. Obesity not uncommonly sets in after the
-system has been subject to debilitating influences. Often a serious
-illness is followed by a corpulence to which there was previously no
-tendency. And the prolonged administration of mercury, constitutionally
-injurious as it is, sometimes produces a like effect.
-
-Closer inquiry verifies the conclusion to which these facts point.
-The microscope shows that along with the increase of bulk common in
-advanced life, there goes on what is called “fatty degeneration:”
-oil-globules are deposited where there should be particles of flesh--or
-rather, we may say, the hydrocarbonaceous molecules locally produced
-by decomposition of the nitrogenous molecules, have not been replaced
-by other nitrogenous molecules, as they should have been. This fatty
-degeneration is, indeed, a kind of local death. For so regarding it
-we have not simply the reason that an active substance has its place
-occupied by an inert substance; but we have the further reason that the
-flesh of dead bodies, under certain conditions, is transformed into a
-fatty matter called adipocere.
-
-The infertility that accompanies fatness in domestic animals has,
-however, other causes than that declining constitutional vigour which
-the fatness commonly indicates. Being artificially fed, these animals
-cannot always obtain what their systems need. That which is given to
-them is given expressly because of its fattening quality. And since the
-capacity of the digestive apparatus remains the same, the absorption
-of fat-producing materials in excess, implies defect in the absorption
-of materials from which the tissues are formed, and out of which young
-ones are built up. Moreover, this special feeding with a view to rapid
-and early fattening, continued as it is through generations, and
-accompanied as it is by a selection of individuals and varieties which
-fatten most readily, tends to establish a modified constitution, more
-fitted for producing fat and correspondingly-less fitted for producing
-flesh--a constitution which, from this relatively-deficient absorption
-of nitrogenous matters, is likely to become infertile; as, indeed,
-these varieties often do become. Hence, no conclusions respecting the
-effects of high nutrition, properly so-called, can be drawn from cases
-of this kind. The cases are, in truth, of a kind which could not exist
-but for human agency. Under natural conditions no animal would diet
-itself in the way required to produce such results. And if it did its
-race would quickly disappear.[62]
-
-There is yet another mode in which accumulation of fat diminishes
-fertility. Even supposing it unaccompanied by a smaller absorption of
-nitrogenous materials, it is still a cause of lessening the surplus
-of nitrogenous materials. For the repair of the motor tissues becomes
-more costly. Fat stored-up is weight to be carried. A creature loaded
-with inert matter must, other things equal, consume a greater amount
-of tissue-forming substances for keeping its locomotive apparatus
-in order; and thus expending more for self-maintenance can expend
-less for race-maintenance. Abnormal plethora is thus antagonistic
-to reproduction in a double way. It ordinarily implies a smaller
-absorption of tissue-forming matters, and an increased demand on
-the diminished supply. Hence fertility decreases in a geometrical
-progression.
-
-The counter-conclusion drawn from facts of this class is, then, due
-to a misconception of their nature--a misconception arising partly
-from the circumstance that the increase of bulk produced by fat is
-somewhat like the increase of bulk which growth of tissues causes,
-and partly from the circumstance that abundance of good food normally
-produces a certain quantity of fat, which, within narrow limits, is
-a valuable store of force-evolving material. When, however, we limit
-the phrase high nutrition to its proper meaning--an abundance of, and
-due proportion among, all the substances which the organism needs--we
-find that, other things equal, fertility always increases as nutrition
-increases. And we see that these apparently-exceptional cases, are
-cases which really show us the same thing; since they are cases of
-relative innutrition.
-
-       *       *       *       *       *
-
-[NOTE.--By a strange oversight when writing this chapter
-in the first edition--an oversight I was on the eve of repeating in
-this present edition--I omitted to bring forward the familiar and
-all-important evidence furnished by the variations of genesis which
-ordinarily accompany the alternations of the seasons. These variations,
-in multitudinous creatures of all types, show unmistakably that
-reproduction begins at those times of the year when greater warmth
-and larger supplies of food render maintenance of individual life
-relatively easy, and when there is therefore a surplus available for
-producing new individuals. Conversely, along with the decrease of
-heat and the relative deficiency of food which make it comparatively
-difficult in winter to maintain individual life, there ceases to be
-the power of producing other lives: the reproductive organs become
-quiescent and often dwindle. With this general fact is associated
-a special fact. Though among wild animals--birds, mammals, and
-others--breeding ceases when Nature no longer supplies abundant food
-and warmth; in domesticated mammals and birds, artificially supplied
-with food and warmth, the breeding season is greatly extended and often
-made continuous, as, under the same conditions, it is in Man himself.
-
-Evidence yielded by the vegetal world is less conspicuous, for the
-reason that the cold which arrests reproductive activity also arrests
-individual activity: growth of the individual and multiplication of
-the race vary simultaneously with variations in the seasons. Still
-there are some familiar facts showing that the external conditions
-which favour nutrition also bring about reproduction. Early in the
-year we are supplied with flowers from regions warmer than our own,
-and by and by there come to our markets fruits and vegetables from the
-south of France, the Channel Islands, and even from the Scilly Isles,
-which are much in advance of those furnished by the gardens of our own
-colder regions: reproduction commences earlier where the light and
-heat furthering nutrition are greater. And then there is a kindred
-meaning in the not unfrequent occurrence of a second flowering and even
-of a second fruiting in warm, bright and prolonged autumns. Here the
-abnormal re-commencement of reproduction is determined by an abnormal
-increase of nutrition.]
-
-
-
-
-                              CHAPTER X.
-
-                   SPECIALITIES OF THESE RELATIONS.
-
-
-§ 356. Tests of the general doctrines set forth in preceding chapters,
-are afforded by organisms having modes of life which diverge widely
-from ordinary modes. Here, as elsewhere, aberrant cases yield crucial
-proofs.
-
-If certain organisms are so circumstanced that highly-nutritive matter
-is supplied to them without stint, and they have nothing to do but
-absorb it, we may infer that their powers of propagation will be
-enormous.
-
-If there are classes of creatures which expend very little for
-self-support in comparison with allied creatures, a relatively-extreme
-prolificness may be expected of them.
-
-Or if, again, we find species presenting the peculiarity that while
-some of their individuals have much to do and little to eat, others of
-their individuals have much to eat and little to do, we may look for
-great fertility in these last and comparative infertility or barrenness
-in the first.
-
-These several anticipations we shall find completely verified.
-
-
-§ 357. Plants which, like the _Rafflesiaceæ_, carry their parasitism
-to the extent of living on the juices they absorb from other plants,
-exhibit one of these relations in the vegetal kingdom. In them the
-organs for self-support being needless, are rudimentary; and the parts
-directly or indirectly concerned in the production and distribution of
-germs, constitute the mass of the organism. That small ratio which the
-race-preserving structures bear to the self-preserving structures in
-ordinary Phænogams, is, in these Phænogams, inverted. A like relation
-occurs in the common Dodder.
-
-There may be added a kindred piece of evidence which the _Fungi_
-present. Those of them which grow on living plants, repeat the above
-connection completely; and those of them which, though not parasitic,
-nevertheless subsist on organized materials previously elaborated by
-other plants, substantially repeat it. The spore-producing part is
-relatively enormous; and the fertility is far greater than that of
-Cryptogams of like sizes, which have to form for themselves the organic
-compounds of which they and their germs consist.
-
-
-§ 358. The same lesson is taught us by animal-parasites. Along with the
-decreased cost of Individuation, they similarly show us an increased
-expenditure for Genesis; and they show us this in the most striking
-manner where the deviation from ordinary conditions of life is the
-greatest.
-
-Take, among the _Epizoa_, such an instance as _Chondracanthus
-gibbosus_. Belonging to the _Entomostraca_, both males and females
-of this species are, in their early days, similar to their allies;
-and the males, practically parasitic, though they become greatly
-degraded, continue throughout life to show by their segmentation and
-other external traits their original nature. The female, however,
-having fixed herself where she can suck the juices of her host, the
-_Lophius_, grows to twelve times the length of the male and probably
-a thousand times its bulk, and becomes utterly transformed by loss of
-the organs of animal life and enormous development of the organs of
-reproduction. “No heart is discoverable, and the nervous system and
-organs of sense (if any) are equally undistinguishable. The interspace
-between the alimentary canal and the walls of the body is almost wholly
-occupied by the ovarium.”[63] And then beyond this there are appended
-ovi-sacs twice the length of the body. So that the germ-producing
-organs and their contents, eventually acquire a total bulk many times
-that of all the other organs put together. Numerous species of this
-type and habit, repeat this relation between a life of inaction with
-high feeding, and an enormous rate of genesis. Parasites belonging to
-another great division of the animal kingdom, the _Platyhelminthes_,
-supply an example of an _epizoon_ in which the rate of multiplication
-is made great not so much by immense development of the egg-producing
-organs as by the rapidity with which generations succeed one another--a
-rapidity such that each generation partially develops the next before
-it is itself anything like ready for independent life. This is the
-_Gyrodactylus elegans_, of which it is said that “its most remarkable
-feature is that it is viviparous, and its embryos before they leave the
-body of their mother have already developed their embryos inside them;
-and the latter may contain their embryos, so that four generations may
-be included under the cuticle of the sexually mature animal.”[64]
-
-_Entozoa_ yield us many examples of this causal relation, raised to
-a still higher degree. The _Gordius_, or Hair-worm, is a creature
-which, finding its way when young into the body of an insect which is
-afterwards swallowed by a fish, there grows rapidly, and then emerging
-to breed, lays as many as 8,000,000 eggs in less than a day. Similarly
-with those larger types infesting the higher animals. It has been
-calculated by Dr. Eschricht, as quoted by Professor Owen, that there
-are “64,000,000 of ova in the mature female _Ascaris lumbricoides_.”
-Very many of the _Entozoa_ belong to the _Platyhelminthes_, and among
-them occur examples of fertility caused not only by great numbers of
-ova, but by rapid succession of partially-developed individuals and
-also examples of fertility caused by production of ova almost exceeding
-numeration. Among the first the Liver-fluke may be named. Of the
-half-million eggs it produces each yields a free-swimming ciliated
-embryo, and any one of these, which finds its way into a water-snail,
-becomes a sporocyst--a bag, presently occupied exclusively by masses
-of cells: each mass by and by becoming a _Redia_, which makes its way
-out. Like all its fellows which develop in succession, this, with
-the exception of a small space occupied by the stomach, devotes the
-whole of its interior partly to the formation of other _Rediæ_ (which
-presently escape and become similarly transformed), and partly to the
-development of _Cercariæ_, into which the internal substance of all
-the _Rediæ_ is eventually transformed: _Cercariæ_ which, escaping
-from the host, become agents for infecting other creatures. So that
-each ovum thus gives rise to a number of forms which severally
-subserve multiplication in different ways. Of the other division of
-_Platyhelminthes_ referred to as carrying on its multiplication by
-production of ova only, the commonest of the _Cestoidea_ furnishes the
-best example. Immersed as a Tape-worm is in nutritive liquid, which it
-absorbs through its integument, it requires no digestive apparatus. The
-room which one would occupy, and the materials it would use up, are
-therefore available for germ-producing organs, which nearly fill each
-segment: each segment, sexually complete in itself, is little else than
-an enormous reproductive system, with just enough of other structures
-to bind it together. Remembering that the Tape-worm, retaining its
-hold, continues to bud out such segments as fast as the fully-developed
-ones are cast off, and goes on doing this as long as the infested
-individual lives; we see that here, where there is no expenditure,
-where the cost of individuation is reduced to the greatest extent while
-the nutrition is the highest possible, the degree of fertility reaches
-its extreme. These _Entozoa_ yield us further interesting evidence. Of
-their various species, most if not all undergo passive migration from
-animal to animal before they become mature. Usually, the form assumed
-in the body of the first host is devoid of all that part in which the
-reproductive structures take their rise; and this part grows and
-develops reproductive structures, only in some predatory animal to
-which its first host falls a sacrifice. Occasionally, however, the egg
-gives origin to the sexual form in the animal that originally swallowed
-it, but the development remains incomplete--there is no sexual genesis,
-no formation of eggs in the rudimentary segments. That these may
-become fertile it is needful, as before, for the containing animal to
-be devoured; so that the imperfect Tape-worm may find its way into
-the intestine of a higher animal. Thus the _Bothriocephalus solidus_,
-found in the abdominal cavity of the Stickleback, is barren while it
-remains there; but if the Stickleback be eaten by a Water-fowl, the
-reproductive system of the transferred _Bothriocephalus_ (then known
-as _B. nodosus_) becomes developed and active. So, too, a kind of
-Tape-worm which remains infertile while in the intestine of a Mouse,
-becomes fertile in the intestine of a Cat that devours the mouse. May
-we not regard these facts as again showing the dependence of fertility
-on nutrition? Barrenness here accompanies conditions unfavourable
-to the absorption of nutriment; and it gives way to fecundity where
-nutriment is large in quantity and superior in quality.
-
-
-§ 359. Extremely significant are those cases of partial reversion to
-primitive forms of genesis, which occur under special conditions in
-some of the higher _Annulosa_. I refer to the pseudo-parthenogenesis
-and metagenesis in Insects.
-
-Under what conditions do the _Aphides_ exhibit this strange deviation
-from the habits of their order? Why among them should imperfect
-females produce, agamically, others like themselves, generation after
-generation, with great rapidity? There is the obvious explanation that
-they get plenty of easily-assimilated food without exertion. Piercing
-the tender coats of young shoots, they sit and suck--appropriating
-the nitrogenous elements of the sap and ejecting its saccharine
-matter as “honey dew.” Along with a sluggishness strongly contrasted
-with the activity of most insects--along with a very low rate of
-consumption and a correlative degradation of structure; we have here
-a retrogression to asexual genesis, and a greatly-increased rate of
-multiplication.
-
-The recently discovered instance of internal metagenesis in the
-maggots of certain Flies has a like meaning. Incredible as it at first
-seemed to naturalists, it is now proved that the _Cecydomia_-larva
-develops in its interior a brood of larvæ of like structure with
-itself. In this case, as in the last, abundant food is combined
-with low expenditure. These larvæ are found in such habitats as the
-refuse of beet-root-sugar factories--masses of nitrogenous _débris_
-remaining after the extraction of the saccharine matter. Each larva
-has a practically-unlimited supply of sustenance imbedding it on all
-sides.[65]
-
-It is true that some other maggots, as those of the Flesh-fly,
-are similarly, or still better, circumstanced; and, it may be
-said, ought therefore to have the same habit. But this does not
-necessarily follow. Survival of the fittest will determine whether
-such specially-favourable conditions result in aggrandizement of the
-individual or in multiplication of the race. And in the case of the
-Flesh-fly there is a reason why greater individuation rather than
-more rapid genesis will occur. For a decomposing animal body lasts
-so short a time, that were Flesh-fly larvae to multiply agamically,
-the second generation would die from the disappearance of their food.
-Hence individuals in which the excessive nutrition led to internal
-metagenesis, would leave no posterity, and natural selection would
-establish the variety in which greater growth resulted. All which the
-argument requires is that when such reversion to agamogenesis _does_
-take place, it shall be where the food is unusually abundant and the
-expenditure unusually small; and this the cases instanced go to show.
-
-
-§ 360. The physiological lesson taught us by Bees and Ants, not quite
-harmonizing with the moral lesson they are supposed to teach, is that
-highly-fed idleness is favourable to fertility, and that excessive
-industry has barrenness for its concomitant.
-
-The egg of a Bee develops into a small barren female or into a large
-fertile female, according to the supply of food given to the larva
-hatched from it. We here see that the germ-producing action is an
-overflow of the surplus remaining after completion of the individual;
-and that the lower feeding which the larva of a working Bee has,
-results in a dwarfing of the adult and an arrested development of
-the generative organs. Further, we have the fact that the condition
-under which the perfect female, or mother-Bee, goes on, unlike insects
-in general, laying eggs continuously, is that she has plenty of
-food brought to her, is kept warm, and goes through no considerable
-exertion. While, contrariwise, it is to be noted that the infertility
-of the workers is associated with the ceaseless labour of bringing
-materials for the combs and building them, as well as the labour of
-feeding the queen, the larvæ, and themselves.
-
-Ants also show us these relations, and they are shown in a greatly
-exaggerated form by what are called white ants--insects belonging to
-a quite different order. The contrast in bulk between the fecund and
-infecund females is here immensely greater. The mother-Ant has the
-reproductive system so enormously developed, that the remainder of her
-body is relatively insignificant. Entirely incapable of locomotion,
-she is unable to deposit her eggs in the places where they are to
-be hatched; so that they have to be carried away by the workers as
-fast as they are extruded. Her life is thus reduced substantially to
-that of a parasite--an absorption of abundant food supplied gratis,
-a total absence of expenditure, and a consequent excessive rate of
-genesis. “The queen-ant of the African _Termites_ lays 80,000 eggs in
-twenty-four hours.”
-
-
-§ 361. It may be needful to say that these exceptional relations cannot
-be ascribed to the assigned causes acting alone. The extreme fertility
-which, among parasites and social insects, accompanies extremely high
-feeding and an expenditure reduced nearly to zero, presupposes typical
-structures and tendencies of suitable kinds; and these are not directly
-accounted for. On creatures otherwise organized, unlimited supplies
-of food and total inactivity are not followed by such results. There
-of course requires a constitution fitted to the special conditions,
-and the evolution of this cannot be due simply to plethora joined with
-rest. These cases are given as illustrating the conditions under which
-extreme exaltations of fertility become possible. Their meanings, thus
-limited, are clear, and completely to the point. We see in them that
-the devotion of nutriment to race-preservation, is carried furthest
-where the cost of self-preservation is reduced to a minimum; and,
-conversely, that nothing is devoted directly to race-preservation
-by individuals on which falls an excessive expenditure for
-self-preservation and preservation of other’s offspring.
-
-       *       *       *       *       *
-
-[NOTE.--Among specialities of these relations may be fitly added here
-a very strange one, for a description of which I am indebted to M.
-Charles Julin, Professor of Comparative Anatomy in the University of
-Liège. In the _Revue Générale des Sciences_ for 30th August, 1894, in
-an account of certain investigations of M. Giard, he describes what
-he calls “la castration parasitaire”--a castration not of a literal
-kind but one effected by the arrest of development which follows from
-the depletion caused by a parasite. The _Sacculina_ is an amazingly
-transformed type belonging to the _Cirrhipedia_--a type without
-segments or appendages and without mouth and alimentary canal. Fixing
-itself, during its early locomotive stage, under the abdomen of a
-decapodous crustacean, and leaving behind its exo-skeleton, it makes
-its way into the interior, and there becoming a mere bag containing
-the reproductive organs, obtains the needful nutriment by developing
-what are practically roots and rootlets which run everywhere among the
-viscera and absorb nutriment from the surrounding tissues. Here we
-are concerned merely with the effect produced upon the host by this
-physiological robbery. This effect is to arrest the development not
-only of the primary sexual organs devoted to the production of germs,
-but also of those secondary sexual organs which characterize the male.
-M. Julin writes:--
-
-   “Il convient cependant de dire, pour être plus exact, que,
-   dans les cas des Crabes infesté par des Sacculines, il n’y a
-   pas, en réalité, apparition de caractères femelles chez le
-   sexe mâle, mais plutôt absence de développement des caractères
-   mâles. En fait, l’animal reste à un stade jeune, non différencié
-   sexuellement, tout en prenant une taille plus considérable. Cela
-   nous porte à attribuer les modifications dont nous avons parlé à
-   un simple arrêt de développement, qui est plus sensible chez le
-   mâle, parce que chez lui les caractères sexuels secondaires sont
-   à l’état normal plus développés que chez la femelle.
-
-   D’une manière générale, nous croyons, avec M. Giard, qu’il faut
-   assimiler les modifications dues à la castration parasitaire à
-   celles qui sont le résultat de la progenèse ou qui engendrent le
-   dimorphisme saisonnier.
-
-   Il y a _progenèse_ lorsque, chez un animal, la reproduction
-   sexuée s’opère d’une façon plus ou moins précoce, c’est-à-dire
-   lorsque les produits sexuels (œufs ou spermatozoïdes) se
-   forment et mûrissent avant que l’être n’ait atteint son complet
-   développement. On peut citer comme exemples les Axolotls et
-   les larves de Tritons qui, les uns normalement, les autres
-   accidentellement, pondent en ayant encore leurs branchies.
-
-   Très souvent la progenèse n’affecte qu’un seul sexe. Tantôt,
-   c’est le sexe femelle qui mûrit à l’état larvaire comme chez
-   les pucerons, les _Stylops_, etc.... Tantôt c’est le sexe mâle,
-   comme chez la Bonellie, les mâles complémentaires de Cirripèdes,
-   les mâles pygmées des Rotifères, le mâle de l’Anguille, etc.
-   D’autres fois, enfin, l’animal présente successivement les deux
-   sexes avec progenèse pour l’un d’entre eux. C’est ainsi qu’il
-   y a _progenèse protandrique_ chez les Crustacés cymothoadiens,
-   et, parmi les Vertébrés, chez les Myxines, qui, mâles dans le
-   jeune âge, deviennent femelles en vieillissant et en achevant
-   de prendre leur développement. Le cas des vieilles femelles de
-   Gallinacés à plumage et à instincts masculins semble être, au
-   contraire, un exemple imparfait de _progenèse protogynique_,
-   puisque ces femelles ont pondu lorsqu’elles avaient encore
-   la livrée des jeunes et qu’elles ont continué plus tard leur
-   développement, et présentent le caractère des mâles sans que,
-   cependant, l’on ait constaté la production de spermatozoïdes.
-
-   Dans les cas extrêmes de progenèse femelle, la reproduction se
-   fait même sans le concours de l’élément mâle, revenant ainsi
-   à la forme agamique primordiale. Ces cas sont connus depuis
-   longtemps sous le nom de _pédogenèse_. On les a observé chez les
-   larves de _Miastor_, de _Chironomus_ et chez certains pucerons.
-
-   Chaque fois qu’il y a progenèse dans un type déterminé, on
-   constate soit momentanément, soit d’une façon définitive, un
-   arrêt de croissance et de développement: l’animal progénétique
-   a, par suite, l’aspect d’une larve sexuée, lorsqu’on le compare
-   soit à l’autre sexe, soit aux formes voisines, qui ne présentent
-   pas le phénomène de la progenèse.
-
-   Cela est en parfaite harmonie avec le principe, si bien mis en
-   lumière par Herbert Spencer, de _l’antagonisme entre la genèse
-   et la croissance et entre la genèse et le développement_.
-   Cet antagonisme s’explique facilement si l’on songe que les
-   matériaux employés pour la reproduction ne peuvent servir à
-   l’accroissement de l’individu. S’il est avantageux pour un
-   organisme de se reproduire sans acquérir des organes inutiles,
-   la sélection naturelle déterminera bientôt une progenèse de plus
-   en plus complète. Les animaux parasites, outre qu’ils tirent de
-   leur hôte une nourriture abondante, n’ont guère besoin d’une
-   foule d’organes qui servent à leurs congénères libres dans la
-   vie de relation. Aussi voyons-nous qu’un très grand nombre
-   d’animaux parasites sont progénétiques. Les mâles progénétiques
-   de la Bonellie et des Cirripèdes vivent en parasites dans leurs
-   femelles. Chez certains types, les pucerons, la progenèse cesse
-   dès que, la nourriture devenant moins abondante, un déplacement
-   pourra être nécessaire.
-
-   En résumé, l’arrêt de développement dû à la progenèse résulte
-   d’une dérivation des principes nourriciers au détriment
-   de l’animal progénétique. Dans les exemples de castration
-   parasitaire que nous avons examinés, le parasite joue, par
-   rapport à son hôte, absolument le même rôle que la glande
-   génitale d’un type progénétique. Il détourne, pour sa propre
-   subsistance, une partie des principes qui auraient servi au
-   développement de l’animal. Aussi les effets produits sont-ils
-   tout à fait de même ordre.”
-
-A phenomenon so anomalous as this, explicable upon the hypothesis set
-forth but not otherwise explicable, furnishes striking verification.]
-
-
-
-
-                              CHAPTER XI.
-
-                   INTERPRETATION AND QUALIFICATION.
-
-
-§ 362. Considering the difficulties of inductive verification, we
-have, I think, as clear a correspondence between the _à priori_ and
-_à posteriori_ conclusions, as can be expected. The many factors
-co-operating to bring about the result in every case, are so
-variable in their absolute and relative amounts, that we can rarely
-disentangle the effect of each one, and have usually to be content
-with qualified inferences. Though in the mass organisms show us an
-unmistakable relation between great size and small fertility, yet
-special comparisons among them are nearly always partially vitiated
-by differences of structure, differences of nutrition, differences
-of expenditure. Though it is beyond question that the more complex
-organisms are the less prolific, yet as complexity has a certain
-general connexion with bulk, and in animals with expenditure, we cannot
-often identify its results as independent of these. And, similarly,
-though the creatures which waste much matter in producing motion,
-sensible and insensible, have lower rates of multiplication than those
-which waste less, yet, as the creatures which waste much are generally
-larger and more complex, we are again met by an obstacle which limits
-our comparisons, and compels us to accept conclusions less definite
-than are desirable.
-
-Such difficulties arise, however, only when we endeavour, as in
-foregoing chapters, to prove the inverse variation between Genesis
-and each separate element of Individuation--growth, development,
-activity. We are scarcely at all hampered by qualifications when,
-from contemplating these special relations, we return to the general
-relation. The antagonism between Individuation and Genesis is shown
-by all the facts which have been grouped under each head. We have
-seen that in ascending from the lowest to the highest types, there is
-a decrease of fertility so great as to be absolutely inconceivable,
-and even inexpressible by figures; and whether the superiority of
-type consists in relative largeness, in greater complexity, in
-higher activity, or in some or all of these combined, matters not
-to the ultimate inference. The broad fact, enough for us here, is
-that organisms in which the integration and differentiation of
-matter and motion have been carried furthest, are those in which the
-rate of multiplication has fallen lowest. How much of the decline
-of reproductive power is due to the greater integration of matter,
-how much to its greater differentiation, how much to the larger
-amounts of integrated and differentiated motions generated, it may
-be impossible to say; and it is not needful to say. These are all
-elements of a higher degree of life, an augmented ability to maintain
-the organic equilibrium amid environing actions, an increased power
-of self-preservation; and we find their invariable accompaniment
-to be, a diminished expenditure of matter, or motion, or both, in
-race-preservation.
-
-In brief, then, examination of the evidence shows that there _does_
-exist that relation which we inferred _must_ exist. Arguing from
-general data, we saw that for the maintenance of a species, the ability
-to produce offspring must be great, in proportion as the ability
-of the individuals to contend with destroying forces is small; and
-conversely. Arguing from other general data, we saw that, derived as
-the self-sustaining and race-sustaining forces are from a common stock
-of force, it necessarily happens that, other things equal, increase
-of one involves decrease of the other. And then, turning to special
-facts, we have found that this inverse variation is clearly traceable
-throughout both the animal and vegetal kingdoms. We may therefore set
-it down as a law, that every higher degree of organic evolution, has
-for its concomitant a lower degree of that peculiar organic dissolution
-which is seen in the production of new organisms.
-
-
-§ 363. Something remains to be said in reply to the inquiry--how is the
-ratio between Individuation and Genesis established in each case? This
-inquiry has been but partially answered in the course of the foregoing
-argument.
-
-Many specialities of the reproductive process are manifestly due to
-the natural selection of favourable variations. Whether a creature
-lays a few large eggs or many small ones equal in weight to the few
-large, is not determined by any physiological necessity: here the only
-assignable cause is the survival of varieties in which the matter
-devoted to reproduction happens to be divided into portions of such
-size and number as most to favour multiplication. Whether in any case
-there are frequent small broods or larger broods at longer intervals,
-depends wholly on the constitutional peculiarity that has arisen from
-the dying out of families in which the sizes and intervals of the
-broods were least suited to the conditions of life. Whether a species
-of animal produces many offspring of which it takes no care or a few
-of which it takes much care--that is, whether its reproductive surplus
-is laid out wholly in germs or partly in germs and partly in labour on
-their behalf--must have been decided by that moulding of constitution
-to conditions slowly effected through the more frequent preservation
-of descendants from those whose reproductive habits were best adapted
-to the circumstances of the species. Given a certain surplus available
-for race-preservation, and it is clear that by indirect equilibration
-only, can there be established the more or less peculiar distribution
-of this surplus which we see in each case. Obviously, too, survival
-of the fittest has a share in determining the proportion between the
-amount of matter that goes to Individuation and the amount that goes
-to Genesis. Whether the interests of the species are most subserved
-by a higher evolution of the individual joined with a diminished
-fertility, or by a lower evolution of the individual joined with an
-increased fertility, are questions ever being experimentally answered.
-If the more-developed and less-prolific variety has a greater number of
-survivors, it becomes established and predominant. If, contrariwise,
-the conditions of life being simple, the larger or more-organized
-individuals gain nothing by their greater size or better organization;
-then the greater fertility of the less evolved ones, will insure to
-their descendants an increasing predominance.
-
-But direct equilibration all along maintains the limits within which
-indirect equilibration thus works. The necessary antagonism we have
-traced, rigidly restricts the changes that natural selection can
-produce, under given conditions, in either direction. A greater demand
-for Individuation, be it a demand caused by some spontaneous variation
-or by an adaptive increase of structure and function, inevitably
-diminishes the supply for Genesis; and natural selection cannot, other
-things remaining the same, restore the rate of Genesis while the higher
-Individuation is maintained. Conversely, survival of the fittest,
-acting on a species that has, by spontaneous variation or otherwise,
-become more prolific, cannot again raise its lowered Individuation, so
-long as everything else continues constant.
-
-
-§ 364. Here, however, a qualification must be made. It was
-parenthetically remarked in § 327, that the inverse variation between
-Individuation and Genesis is not exact; and it was hinted that a slight
-modification of statement would be requisite at a more advanced stage
-of the argument. We have now reached the proper place for specifying
-this modification.
-
-Each increment of evolution entails a decrement of reproduction which
-is not accurately proportionate, but somewhat less than proportionate.
-The gain in the one direction is not wholly cancelled by a loss in
-the other direction, but only partially cancelled: leaving a margin
-of profit to the species. Though augmented power of self-maintenance
-habitually necessitates diminished power of race-propagation, yet the
-product of the two factors is greater than before; so that the forces
-preservative of race become, thereafter, in excess of the forces
-destructive of race, and the race spreads. We shall soon see why this
-happens.
-
-Every advance in evolution implies an economy. That any increase in
-bulk, or structure, or activity, may become established, the life of
-the organism must be to some extent facilitated by the change--the
-cost of self-support must be, on the average, reduced. If the greater
-complexity, or the larger size, or the more agile movement, entails
-on the individual an outlay that is not repaid in food more-easily
-obtained, or danger more-easily escaped; then the individual will
-be at a relative disadvantage, and its diminished posterity will
-disappear. If the extra outlay is but just made good by the extra
-advantage, the modified individual will not survive longer, or leave
-more descendants, than the unmodified individuals. Consequently, it
-is only when the expense of greater individuation is out-balanced by
-a subsequent saving, that it can tend to subserve the preservation of
-the individual, and, by implication, the preservation of the race.
-The vital capital invested in the alteration must bring a more than
-equivalent return. A few instances will show that, whether the change
-results from direct equilibration or from indirect equilibration, this
-must happen. Suppose a creature takes to performing some act in an
-unusual way--leaps where ordinarily its kindred crawl, eludes pursuit
-by diving instead of, like others of its kind, by swimming along the
-surface, escapes by doubling instead of by speed. Clearly, perseverance
-in the modified habit will, other things equal, imply that it takes
-less effort. The creature’s sensations will ever prompt desistance from
-the more laborious course; and hence a congenital habit is not likely
-to be diverged from unless an economy of force is achieved by the
-divergence. Assuming, then, that the new method has no advantage over
-the old in directly diminishing the chances of death, the establishment
-of it, and of the structural complications involved, nevertheless
-implies a physiological gain. Suppose, again, that an animal takes
-to some abundant food previously refused by its kind. It is likely
-to persist only if the comparative ease in obtaining this food, more
-than compensates for any want of adaptation to its digestive organs;
-so that superposed modifications of the digestive organs are likely
-to arise only when an average economy results. What now must be the
-influence on the creature’s system as a whole? Diminished expenditure
-in any direction, or increased nutrition however effected, will leave
-a greater surplus of materials. The animal will be physiological
-richer. Part of its augmented wealth will go towards its own greater
-individuation--its size, or its strength, or both, will increase; while
-another part will go towards more active genesis. Just as a state of
-plethora directly produced enhances fertility; so will such a state
-indirectly produced.
-
-In another way, the same thing must result from those additions
-to bulk or complexity or activity that are due to survival of the
-fittest. Any change which prolongs individual life will, other things
-remaining the same, further the production of offspring. Even when it
-is not, like the foregoing, a means of economizing the forces of the
-individual, still, if it increases the chances of escaping destruction,
-it increases the chances of leaving posterity. Any further degree of
-evolution, therefore, will be established only where the cost of it
-is more than repaid: part of the gain being shown in the lengthened
-life of the individual, and part in the greater production of other
-individuals.
-
-We have here the solution of various minor anomalies by which the
-inverse variation of Individuation and Genesis is obscured. Take as an
-instance the fertility of the Blackbird as compared with that of the
-Linnet. Both birds lay five eggs, and both usually have two broods. Yet
-the Blackbird is far the larger of the two, and ought, according to the
-general law, to be much less prolific. What causes this nonconformity?
-We shall find an answer in their respective foods and habits. Except
-during the time that it is rearing its young, the Linnet collects only
-vegetal food--lives during the winter on the seeds it finds in the
-fields, or, when hard pressed, picks up around farms; and to obtain
-this spare diet is continually flying about. The result, if it survives
-the frost and snow, is a considerable depletion; and it recovers its
-condition only after some length of spring weather. The Blackbird, on
-the other hand, is omnivorous. While it eats grain and fruit when they
-come in its way, it depends largely on animal food. It cuts to pieces
-and devours the dew-worms which, morning and evening, it finds on the
-surface of a lawn, and, even discovering where they are, unearths
-them; it swallows slugs, and breaking snail-shells, either with its
-beak or by hammering them against stones, tears out their tenants; and
-it eats beetles and larvæ. Thus the strength of the Blackbird opens
-to it a store of good food, much of which is inaccessible to so small
-and weak a bird as a Linnet--a store especially helpful to it during
-the cold months, when the hybernating snails in hedge-bottoms yield
-it abundant provision. The result is that the Blackbird is ready to
-breed very early in spring, and is able during the summer to rear
-a second, and sometimes even a third, brood. Here, then, a higher
-degree of Individuation secures advantages so great, as to much more
-than compensate its cost. It is not that the decline of Genesis is
-less than proportionate to the increase of Individuation, but there
-is no decline at all. Comparison of the Rat with the Mouse yields a
-parallel result. Though they differ greatly in size, yet the one is as
-prolific as the other. This absence of difference cannot be ascribed
-to their unlike degrees of activity. We must seek its cause in some
-facility of living secured to the Rat by its greater intelligence,
-greater power and courage, greater ability to utilize what it finds.
-The Rat is notoriously cunning; and its cunning gives success to its
-foraging expeditions. It is not, like the Mouse, limited mainly to
-vegetal food; but while it eats grain and beans like the Mouse, it also
-eats flesh and carrion, devours young poultry and eggs. The result is
-that, without a proportionate increase of expenditure, it gets a far
-larger supply of nourishment than the Mouse; and relative excess of
-nourishment makes possible a larger size without a smaller rate of
-multiplication. How clearly this is the cause, we see in the contrast
-between the common Rat and the Water-Rat. While the common Rat has
-ordinarily several broods a-year of from 10 to 12 each, the Water-Rat,
-though somewhat smaller, has but 5 or 6 in a brood, and but one brood,
-or sometimes two broods, a-year. But the Water-Rat lives on vegetal
-food, and it lacks all that its bold, sagacious, omnivorous congener
-gains from the warmth as well as the abundance which men’s habitations
-yield.
-
-The inverse variation of Individuation and Genesis is, therefore, but
-approximate. Recognizing the truth that every increment of evolution
-which is appropriate to the circumstances of an organism, brings an
-advantage somewhat in excess of its cost; we see the general law, as
-more strictly stated, to be that Genesis decreases not quite so fast
-as Individuation increases. Whether the greater Individuation takes
-the form of a larger bulk and accompanying access of strength; whether
-it be shown in higher speed or agility; whether it consists in a
-modification of structure which facilitates some habitual movement,
-or in a visceral change that helps to utilize better the absorbed
-aliment; the ultimate effect is identical. There is either a more
-economical performance of the same actions, internal or external, or
-there is a securing of greater advantages by modified actions, which
-cost no more, or have an increased cost less than the increased gain.
-In any case the result is a greater surplus of vital capital, part
-of which goes to the aggrandizement of the individual, and part to
-the formation of new individuals. While the higher tide of nutritive
-matters, everywhere filling the parent-organism, adds to its power of
-self-maintenance, it also causes a reproductive overflow larger than
-before.
-
-Hence every type which is best adapted to its conditions, (and this
-on the average means every higher type), has a rate of multiplication
-that insures a tendency to predominate. Survival of the fittest, acting
-alone, is ever replacing inferior species by superior species. But
-beyond the longer survival, and therefore greater chance of leaving
-offspring, which superiority gives, we see here another way in which
-the spread of the superior is insured. Though the more-evolved organism
-is the less fertile absolutely, it is the more fertile relatively.
-
-
-
-
-                             CHAPTER XII.
-
-                   MULTIPLICATION OF THE HUMAN RACE.
-
-
-§ 365. The relative fertility of Man considered as a species, and
-those changes in Man’s fertility which occur under changed conditions,
-must conform to the laws which we have traced thus far. As a matter
-of course, the inverse variation between Individuation and Genesis
-holds of him as of all other organized beings. His extremely low rate
-of multiplication--far below that of all terrestrial Mammals except
-the Elephant, (which though otherwise less evolved is, in extent
-of integration, more evolved)--we shall recognize as the necessary
-concomitant of his much higher evolution. And the causes of increase or
-decrease in his fertility, special or general, temporary or permanent,
-we shall expect to find in those changes of bulk, of structure, or of
-expenditure, which we have in all other cases seen associated with such
-effects.
-
-In the absence of detailed proof that these parallelisms exist, it
-might suffice to contemplate the several communities between the
-reproductive function in human beings and other beings. I do not refer
-simply to the fact that genesis proceeds in a similar manner; but I
-refer to the similarity of the relation between the generative function
-and the functions which have for their joint end the preservation
-of the individual. In Man, as in other creatures that expend much,
-genesis commences only when growth and development are declining in
-rapidity and approaching their termination. Among the higher organisms
-in general, the reproductive activity, continuing during the prime
-of life, ceases when the vigour declines, leaving a closing period of
-infertility; and in like manner among ourselves, barrenness supervenes
-when middle age brings the surplus vitality to an end. So, too, it is
-found that in Man, as in beings of lower orders, there is a period
-at which fecundity culminates. In § 341, facts were cited showing
-that at the commencement of the reproductive period, animals bear
-fewer offspring than afterwards; and that towards the close of the
-reproductive period, there is a decrease in the number produced. In
-like manner it is shown by the tables of Dr. Duncan’s recent work,
-that the fecundity of women increases up to the age of about 25
-years, and continuing high with but slight diminution till after 30,
-then gradually wanes. It is the same with the sizes and weights of
-offspring. Infants born of women from 25 to 29 years of age, are both
-longer and heavier than infants born of younger or older women; and
-this difference has the same implication as the greater total weight
-of the offspring produced at a birth, during the most fecund age of
-a pluriparous animal. Once more, there is the fact that a too-early
-bearing of young produces on a woman the same injurious effects as
-on an inferior creature--an arrest of growth and an enfeeblement of
-constitution.
-
-Considering these general and special parallelisms, we might safely
-infer that variations of human fertility conform to the same laws
-as do variations of fertility in general. But it is not needful to
-content ourselves with an implication. Evidence is assignable that
-what causes increase or decrease of genesis in other creatures, causes
-increase or decrease of genesis in Man. It is true that, even more
-than hitherto, our reasonings are beset by difficulties. So numerous
-are the inequalities in the conditions, that but few unobjectionable
-comparisons can be made. The human races differ considerably in their
-sizes, and notably in their degrees of cerebral development. The
-countries they inhabit entail on them widely different consumptions
-of matter for maintenance of temperature. Both in their qualities
-and quantities the foods they live on are unlike; and the supply is
-here regular and there very irregular. Their expenditures in bodily
-action are extremely unequal; and even still more unequal are their
-expenditures in mental action. Hence the factors, varying so much in
-their amounts and combinations, can scarcely ever have their respective
-effects identified. Nevertheless there are a few comparisons the
-results of which may withstand criticism.
-
-
-§ 366. The increase of fertility caused by a nutrition that is greatly
-in excess of the expenditure, is to be detected by contrasting
-populations of the same race, or allied races, one of which obtains
-good and abundant sustenance much more easily than the other. Three
-cases may here be set down.
-
-The traveller Barrow, describing the Cape-Boers, says:--“Unwilling
-to work and unable to think,” ... “indulging to excess in the
-gratification of every sensual appetite, the African peasant grows to
-an unwieldy size;” and respecting the other sex, he adds--“the women
-of the African peasantry lead a life of the most listless inactivity,”
-Then, after illustrating these statements, he goes on to note “the
-prolific tendency of all the African peasantry. Six or seven children
-in a family are considered as very few; from a dozen to twenty are
-not uncommon.” The native races of this region yield evidence to the
-same effect. Speaking of the cruelly-used Hottentots (he is writing a
-century ago), who, while they are poor and ill-fed, have to do all the
-work for the idle Boers, Barrow says that they “seldom have more than
-two or three children; and many of the women are barren.” This unusual
-infertility stands in remarkable contrast with the unusual fertility of
-the Kaffirs, of whom he afterwards gives an account. Rich in cattle,
-leading easy lives, and living almost exclusively on animal food
-(chiefly milk with occasional flesh), these people were then reputed
-to have a very high rate of multiplication. Barrow writes:--“They are
-said to be exceedingly prolific; that twins are almost as frequent as
-single births, and that it is no uncommon thing for a woman to have
-three at a time.” Probably both these statements are in excess of the
-truth; but there is room for large discounts without destroying the
-extreme difference. A third instance is that of the French-Canadians.
-“_Nous sommes terribles pour les enfants!_” observed one of them to
-Prof. Johnston, who tells us that the man who said this “was one of
-fourteen children--was himself the father of fourteen, and assured
-me that from eight to sixteen was the usual number of the farmers’
-families. He even named one or two women who had brought their husbands
-five-and-twenty, and threatened ‘_le vingt-sixième pour le prêtre_.’”
-From these large families, joined with the early marriages and low
-rate of mortality, it results that, by natural increase, “there are
-added to the French-Canadian population of Lower Canada four persons
-for every one that is added to the population of England.” Now these
-French-Canadians are described by Prof. Johnston as home-loving,
-contented, unenterprising; and as living in a region where “land and
-subsistence are easily obtained.” Very moderate industry brings to
-them liberal supplies of necessaries; and they pass a considerable
-portion of the year in idleness. Hence the cost of Individuation
-being much reduced, the rate of Genesis is much increased. That this
-uncommon fertility is not due to any direct influence of the locality,
-is implied by the fact that along with the “restless, discontented,
-striving, burning energy of their Saxon neighbours,” no such rate of
-multiplication is observed; while further south, where the physical
-circumstances are more favourable if anything, the Anglo-Saxons,
-leading lives of excessive activity, have a fertility below the
-average. And that the peculiarity is not a direct effect of race, is
-proved by the fact that in Europe, the rural French are certainly not
-more prolific than the rural English.
-
-To every reader there will probably occur the seemingly-adverse
-evidence furnished by the Irish; who, though not well fed, multiply
-fast. Part of this more rapid increase is due to the earlier marriages
-common among them, and consequent quicker succession of generations--a
-factor which, as we have seen, has a larger effect than any other on
-the rate of multiplication. Part of it is due to the greater generality
-of marriage--to the comparative smallness of the number who die without
-having had the opportunity of producing offspring. The effects of
-these causes having been deducted, we may doubt whether the Irish,
-individually considered, would be found more prolific than the English.
-Perhaps, however, it will be said that, considering their diet, they
-ought to be less prolific. This is by no means obvious. It is not
-simply a question of nutriment absorbed. It is a question of how much
-remains after the expenditure in self-maintenance. Now a notorious
-peculiarity in the life of the Irish peasant is, that he obtains a
-return of food which is large in proportion to his outlay in labour.
-The cultivation of his potatoe-ground occupies each cottager but a
-small part of the year; and the domestic economy of his wife is not of
-a kind to entail on her much daily exertion. Consequently the crop,
-tolerably abundant in quantity though innutritive in quality, possibly
-suffices to meet the comparatively-low expenditure, and to leave a good
-surplus for genesis--perhaps a greater surplus than remains to the
-males and females of the English peasantry, who, though fed on better
-food, are harder worked.
-
-We conclude, then, that in the human race, as in all other races, such
-absolute or relative abundance of nutriment as leaves a large excess
-after defraying the cost of carrying on parental life, is accompanied
-by a high rate of genesis.[66]
-
-
-§ 367. Evidence of the converse truth, that relative increase of
-expenditure, leaving a diminished surplus, reduces the degree of
-fertility, is not wanting. Some of it has been set down for the sake of
-antithesis in the foregoing section. Here may be grouped a few facts of
-a more special kind having the same implication.
-
-To prove that much bodily labour renders women less prolific, requires
-more evidence than has at present been collected. Nevertheless it may
-be noted that De Boismont in France and Dr. Szukits in Austria, have
-shown by extensive statistical comparisons, that the reproductive
-age is reached a year later by women of the labouring class than by
-middle-class women; and while ascribing this delay in part to inferior
-nutrition, we may suspect that it is in part due to greater muscular
-expenditure. A kindred fact, admitting of a kindred interpretation,
-may be added. Though the comparatively-low rate of increase in France
-is attributed to other causes, yet, very possibly, one of its causes
-is the greater proportion of hard work entailed on French women, by
-the excessive abstraction of men for non-productive occupations,
-military and civil. The higher rate of multiplication in England than
-in continental countries generally, is not improbably furthered by the
-easier lives which English women lead.
-
-That absolute or relative infertility is commonly produced in women
-by mental labour carried to excess, is more clearly shown. Though the
-regimen of upper-class girls is not what it should be, yet, considering
-that their feeding is better than that of girls belonging to the poorer
-classes, while, in most other respects, their physical treatment is
-not worse, the deficiency of reproductive power among them may be
-reasonably attributed to the overtaxing of their brains--an overtaxing
-which produces a serious reaction on the physique. This diminution
-of reproductive power is not shown only by the greater frequency of
-absolute sterility; nor is it shown only in the earlier cessation of
-child-bearing; but it is also shown in the very frequent inability of
-such women to suckle their infants. In its full sense, the reproductive
-power means the power to bear a well-developed infant and to supply
-that infant with the natural food for the natural period. Most of the
-flat-chested girls who survive their high-pressure education, are
-incompetent to do this. Were their fertility measured by the number
-of children they could rear without artificial aid, they would prove
-relatively very infertile.
-
-The cost of reproduction to males being so much less than it is to
-females, the antagonism between Genesis and Individuation is not often
-shown in men by suppression of generative power consequent on unusual
-expenditure in bodily action. Nevertheless, there are indications that
-this results in extreme cases. We read that the ancient _athletæ_
-rarely had children; and among such of their modern representatives as
-acrobats, an allied relation of cause and effect is alleged. Indirectly
-this truth, or rather its converse, appears to have been ascertained
-by those who train men for feats of strength--they find it needful to
-insist on continence.
-
-Special proofs that in men great cerebral expenditure diminishes or
-destroys generative power, are difficult to obtain. It is, indeed,
-asserted that intense application to mathematics, requiring as it does
-extreme concentration of thought, is apt to have this result; and it is
-asserted, too, that this result is produced by the excessive emotional
-excitement of gambling. Then, again, it is a matter of common remark
-how frequently men of unusual mental activity leave no offspring. But
-facts of this kind admit of another interpretation. The reaction of
-the brain on the body is so violent--the overtaxing of the nervous
-system is so apt to prostrate the heart and derange the digestion; that
-the incapacities caused in these cases, are probably often due more to
-constitutional disturbance than to the direct deduction which excessive
-action entails. Such instances harmonize with the hypothesis; but how
-far they yield it positive support we cannot say.
-
-
-§ 368. An objection must here be guarded against. It is likely to
-be urged that since the civilized races are, on the average, larger
-than many of the uncivilized races; and since they are also somewhat
-more complex as well as more active; they ought, in conformity with
-the alleged general law, to be less prolific. There is, however, no
-evidence to prove that they are so: on the whole, they seem rather the
-reverse.
-
-The reply is that were all other things equal, these superior varieties
-of men should have inferior rates of increase. But other things are not
-equal; and it is to the inequality of other things that this apparent
-anomaly is attributable. Already we have seen how much more fertile
-domesticated animals are than their wild kindred; and the causes of
-this greater fertility are also the causes of the greater fertility,
-relative or absolute, which civilized men exhibit when compared with
-savages.
-
-There is the difference in amount of food. Australians, Fuegians, and
-sundry races that might be named as having low rates of multiplication,
-are obviously underfed. The sketches of natives contained in the
-volumes of Livingstone, Baker, and others, yield clear proofs of the
-extreme depletion common among the uncivilized. In quality as well
-as in quantity, their feeding is bad. Wild fruits, insects, larvæ,
-vermin, &c., which we refuse with disgust, often enter largely into
-their dietary. Much of this inferior food they eat uncooked; and they
-have not our elaborate appliances for mechanically-preparing it, and
-rejecting its useless parts. So that they live on matters of less
-nutritive value, which cost more both to masticate and to digest.
-Further, to uncivilized men supplies of food come very irregularly.
-Long periods of scarcity are divided by short periods of abundance.
-And though by gorging when opportunity occurs, something is done
-towards compensating for previous fasting, yet the effects of prolonged
-starvation cannot be neutralized by occasional enormous meals. Bearing
-in mind, too, that improvident as they are, savages often bestir
-themselves only under pressure of hunger, we may fairly consider them
-as habitually ill-nourished--may see that even the poorer classes of
-civilized men, making regular meals on food separated from innutritive
-matters, easy to masticate and digest, tolerably good in quality and
-adequate if not abundant in quantity, are much better nourished.
-
-Then, again, though a greater consumption in muscular action appears
-to be undergone by civilized men than by savages; and though it is
-probably true that among our labouring people the daily repairs cost
-more; yet in many cases there does not exist so much difference as we
-are apt to suppose. The chase is very laborious; and great amounts of
-exertion are gone through by the lowest races in seeking and securing
-the odds and ends of wild food on which they largely depend. We
-naturally assume that because barbarians are averse to regular labour,
-their muscular action is less than our own. But this is not necessarily
-true. The monotonous toil is what they cannot tolerate; and they may
-be ready to go through as much or more exertion when it is joined with
-excitement. If we remember that the sportsman who gladly scrambles
-up and down rough hill-sides all day after grouse or deer, would
-think himself hardly used had he to spend as much effort and time in
-digging; we shall see that a savage who is the reverse of industrious,
-may nevertheless be subject to a muscular waste not very different in
-amount from that undergone by the industrious. When it is added that a
-larger physiological expenditure is entailed on the uncivilized than
-on the civilized by the absence of good appliances for shelter and
-protection--that in some cases they have to make good a greater loss
-of heat, and in other cases suffer much wear from irritating swarms of
-insects; we shall see that the total cost of self-maintenance among
-them is probably in many cases little less, and in some cases more,
-than it is among ourselves.
-
-So that though, on the average, the civilized are probably larger than
-the savage; and though they are, in their nervous systems at least,
-somewhat more complex; and though, other things equal, they ought to
-be the less prolific; yet other things are so unequal as to make it
-quite conformable to the general law that they should be more prolific.
-In § 365 we observed how, among inferior animals, higher evolution
-sometimes makes self-preservation far easier, by opening the way to
-resources previously unavailable: so involving an undiminished, or
-even an increased, rate of genesis. And similarly we may expect that
-among races of men, those whose slight further developments have been
-followed by habits and arts which immensely facilitate life, will not
-exhibit a lower degree of fertility, and may even exhibit a higher.
-
-
-§ 369. One more objection has to be met--a kindred objection to which
-there is a kindred reply. Cases may be named of men conspicuous
-for activity, bodily and mental, who were also noted, not for less
-generative power than usual, but for more. As their superiorities
-indicate higher degrees of evolution, it may be urged that such men
-should, according to the theory, have lower degrees of reproductive
-activity. The fact that here, along with increased powers of
-self-preservation, there go increased powers of race-propagation,
-seems irreconcilable with the general doctrine. Reconciliation is not
-difficult however.
-
-The cases are analogous to some before named, in which more abundant
-food simultaneously aggrandizes the individual and adds to the
-production of new individuals: the difference between the cases
-being, that instead of a better external supply of materials there
-is a better internal utilization of materials. Creatures of the same
-species notoriously differ in goodness of constitution. Here there
-is some visceral defect, showing itself in feebleness of all the
-functions; while here some peculiarity of organic balance, some high
-quality of tissue, some abundance or potency of the digestive juices,
-gives to the system a perpetual high tide of rich blood, which serves
-at once to enhance the vital activities and to raise the power of
-propagation. Such variations, however, are independent of changes in
-the _proportion_ between Individuation and Genesis. This remains the
-same, while both are increased or decreased by the increase or decrease
-of the common stock of materials.
-
-An illustration will best clear up any perplexity. Let us say that
-the fuel burnt in the furnace of a locomotive steam-engine, answers
-to the food which a man consumes. Let us say that the produced steam
-expended in working the engine, corresponds to that portion of absorbed
-nutriment which carries on the man’s functions and activities. And
-let us say that the steam blowing off at the safety-valve, answers to
-that portion of the absorbed nutriment which goes to the propagation
-of the race. Such being the conditions of the case, several kinds of
-variations are possible. All other circumstances remaining the same,
-there may be changes of proportion between the steam used for working
-the engine and the steam that escapes by the safety-valve. There may
-be a structural or organic change of proportion. By enlarging the
-safety-valve or weakening its spring, while the cylinders are reduced
-in size, there may be established a constitutionally-small power of
-locomotion and a constitutionally-large amount of escape-steam; and
-inverse variations so produced, will answer to the inverse variations
-between Individuation and Genesis which different types of organisms
-show us. Again, there may be a functional change of proportion. If the
-engine has to draw a considerable load, the abstraction of steam by
-the cylinders greatly reduces the discharge by the safety-valve; and
-if a high velocity is kept up, the discharge from the safety-valve
-entirely ceases. Conversely, if the velocity is low, the escape-steam
-bears a large ratio to the steam consumed by the motor apparatus;
-and if the engine becomes stationary the whole of the steam escapes
-by the safety-valve. This inverse variation answers to that which
-we have traced between Expenditure and Genesis, as displayed in the
-contrasts between species of the same type but unlike activities,
-and in the contrasts between active and inactive individuals of the
-same species. But now beyond these inverse variations between the
-quantities of consumed steam and escape-steam, which are structurally
-and functionally caused, there are coincident variations, producible
-in both by changes in the quantity of steam supplied--changes which
-may be caused in several ways. In the first place, the fuel thrown
-into the furnace may be increased or made better. Other things equal,
-there will result a more active locomotion as well as a greater escape;
-and this will answer to that simultaneous addition to its individual
-vigour and its reproductive activity, caused in an animal by a larger
-quantity, or a superior quality, of food. In the second place, the
-steam generated may be economized. Loss by radiation from the boiler
-may be lessened by a covering of non-conducting substances; and part
-of the steam thus prevented from condensing, will go to increase the
-working power of the engine, while part will be added to the quantity
-blowing off. This variation corresponds to that simultaneous addition
-to bodily vigour and propagative power, which results in animals
-that have to expend less in keeping up their temperatures. In the
-third place, by improvement of the steam-generating apparatus, more
-steam may be obtained from a given weight of fuel. A better-formed
-evaporating surface, or boiler tubes which conduct more rapidly, or
-an increased number of them may cause a larger absorption of heat
-from the burning mass or the hot gases it gives off; and the extra
-steam generated by this extra heat will, as before, augment both the
-motive force and the emission through the safety-valve. And this last
-case of coincident variation, is parallel to the case with which we
-are here concerned--the augmentation of individual expenditure and of
-reproductive energy, that may be caused by a superiority of some organ
-on which the utilizing or economizing of materials depends.
-
-Manifestly, therefore, an increased expenditure for Genesis, or an
-increased expenditure for Individuation, may arise in one of two quite
-different ways--either by diminution of the antagonistic expenditure,
-or by addition to the store which supplies both expenditures; and
-confusion results from not distinguishing between these. Given the
-ratio 4 to 20, as expressive of the relative costs of Genesis and
-Individuation; then the expenditure for Genesis may be raised to 5
-while the expenditure for Individuation is raised to 25, without any
-alteration of type, merely by favourable circumstances or superiority
-of constitution. On the other hand, circumstances remaining the same,
-the expenditure for Genesis may be raised from 4 to 5, by lowering the
-expenditure for Individuation from 20 to 19: which change of ratio may
-be either functional and temporary, or structural and permanent. And
-only when it is the last does it illustrate that inverse variation
-between degree of evolution and degree of procreative dissolution,
-which we have everywhere seen.
-
-
-§ 370. There is no reason to suppose, then, that the laws of
-multiplication which hold of other beings, do not hold of the human
-being. On the contrary, there are special facts which unite with
-general implications to show that these laws do hold of the human
-being. The absence of direct evidence in some cases where it might be
-looked for, we find fully explained when all the factors are taken into
-account. And certain seemingly-adverse facts prove, on examination, to
-be facts belonging to a different category from that in which they are
-placed, and harmonize with the rest when rightly interpreted.
-
-The conformity of human fertility to the laws of multiplication in
-general, being granted, it remains to inquire what effects must be
-caused by permanent changes in men’s natures and circumstances. Thus
-far we have observed how, by their exceptionally-high evolution and
-exceptionally-low fertility, mankind display the inverse variation
-between Individuation and Genesis, in one of its extremes. And we have
-also observed how mankind, like other kinds, are functionally changed
-in their rates of multiplication by changes of conditions. But we have
-not observed how alteration of structure in Man entails alteration
-of fertility. The influence of this factor is so entangled with the
-influences of other factors which are for the present more potent, that
-we cannot recognize it. Here, if we proceed at all, we must proceed
-deductively.
-
-       *       *       *       *       *
-
-[NOTE.--From among the publications of the American Academy of
-Political and Social Science, there was sent to me some years ago an
-essay entitled “The Significance of a Decreasing Birth Rate” by (Miss)
-J. L. Brownell, Fellow in Political Science, Bryn Mawr College. This
-essay contains a number of elaborate comparisons drawn from the vital
-statistics of the tenth United States Census. The results of these
-comparisons are thus summed up:--
-
-   “1. Whether or not it be true that the means spoken of by Dr.
-   Billings, M. Dumont, M. Levasseur, and Dr. Edson has become an
-   important factor in the diminishing birth-rate of civilized
-   countries, it is evident that it is not the only factor, and
-   that, quite apart from voluntary prevention, there is a distinct
-   problem to be investigated. This is shown by the fact that the
-   white and the colored birth-rate vary together.
-
-   “2. Mr. Spencer’s generalization that the birth-rate diminishes
-   as the rate of individual evolution increases is confirmed by a
-   comparison of the birth-rates with the death-rates from nervous
-   diseases, and also with the density of population, the values
-   of agricultural and manufactured products, and the mortgage
-   indebtedness.”
-
-Of course multitudinous differences of race, class, mode of living,
-occupation, locality, make it difficult to draw positive inferences
-from the data; but the inferences above drawn are held to remain
-outstanding after allowing for all the qualifying conditions.]
-
-
-
-
-                             CHAPTER XIII.
-
-                    HUMAN POPULATION IN THE FUTURE.
-
-
-§ 371. Any further evolution in the most highly-evolved of terrestrial
-beings, Man, must be of the same nature as evolution in general.
-Structurally considered, it may consist in greater integration,
-or greater differentiation, or both--augmented bulk, or increased
-heterogeneity and definiteness, or a combination of the two.
-Functionally considered, it may consist in a larger sum of actions,
-or more multiplied varieties of actions, or both--a larger amount of
-sensible and insensible motion generated, or motions more numerous in
-their kinds and more intricate and exact in their co-ordinations, or
-motions that are greater alike in quantity, complexity, and precision.
-
-Expressing the change in terms of that more special evolution displayed
-by organisms; we may say that it must be one which further adapts the
-moving equilibrium of organic actions. As was pointed out in _First
-Principles_, § 173, “the maintenance of such a moving equilibrium,
-requires the habitual genesis of internal forces corresponding in
-number, directions, and amounts to the external incident forces--as
-many inner functions, single or combined, as there are single or
-combined outer actions to be met.” And it was also pointed out that
-“the structural complexity accompanying functional equilibration, is
-definable as one in which there are as many specialized parts as are
-capable, separately and jointly, of counteracting the separate and
-joint forces amid which the organism exists.” Clearly, then, since
-all incompletenesses in Man as now constituted, are failures to meet
-certain of the outer actions (mostly involved, remote, irregular), to
-which he is exposed; every advance implies additional co-ordinations of
-actions and accompanying complexities of organization.
-
-Or, to specialize still further this conception of future progress, we
-may consider it as an advance towards completion of that continuous
-adjustment of internal to external relations, which Life shows us.
-In Part I. of this work, where it was shown that the correspondence
-between inner and outer actions which under its phenomenal aspect,
-we call Life, is a particular kind of what, in terms of Evolution,
-we called a moving equilibrium; it was shown that the degree of life
-varies as the degree of correspondence. Greater evolution or higher
-life implies, then, such modifications of human nature as shall make
-more exact the existing correspondences, or shall establish additional
-correspondences, or both. Connexions of phenomena of a rare, distant,
-unobtrusive, or intricate kind, which we either suffer from or do not
-take advantage of, have to be responded to by new connexions of ideas,
-and acts properly combined and proportioned: there must be increase
-of knowledge, or skill, or power, or of all these. And to effect this
-more extensive, more varied, and more accurate, co-ordination of
-actions, there must be organization of still greater heterogeneity and
-definiteness.
-
-
-§ 372. Let us, before proceeding, consider in what particular ways this
-further evolution, this higher life, this greater co-ordination of
-actions, may be expected to show itself.
-
-Will it be in strength? Probably not to any considerable degree.
-Mechanical appliances are fast supplanting brute force, and doubtless
-will continue doing this. Though at present civilized nations largely
-depend for self-preservation on vigour of limb, and are likely to do
-so while wars continue; yet that progressive adaptation to the social
-state which must at last bring wars to an end, will leave the amount
-of muscular power to adjust itself to the requirements of a peaceful
-_regime_. Though, taking all things into account, the muscular power
-then required may not be less than now, there seems no reason why more
-should be required.
-
-Will it be swiftness or agility? Probably not. In savages these are
-important elements of the ability to maintain life; but in civilized
-men they aid self-preservation in quite minor degrees, and there
-seems no circumstance likely to necessitate an increase of them.
-By games and gymnastic competitions, such attributes may indeed be
-artificially increased; but no artificial increase which does not bring
-a proportionate advantage can be permanent; since, other things equal,
-individuals and societies that devote the same amounts of energy in
-ways which subserve life more effectually, must by and by predominate.
-
-Will it be in mechanical skill, that is, in the better-co-ordination
-of complex movements? Most likely in some degree. Awkwardness is
-continually entailing injuries and deaths. Moreover the complicated
-tools which civilization brings into use, are constantly requiring
-greater delicacy of manipulation. All the arts, industrial and
-æsthetic, as they develop, imply a corresponding development of
-perceptive and executive faculties in men: the two act and react.
-
-Will it be in intelligence? Largely, no doubt. There is ample room
-for advance in this direction, and ample demand for it. Our lives are
-universally shortened by our ignorance. In attaining complete knowledge
-of our own natures and of the natures of surrounding things--in
-ascertaining the conditions of existence to which we must conform,
-and in discovering means of conforming to them under all variations
-of seasons and circumstances; we have abundant scope for intellectual
-progress.
-
-Will it be in morality, that is, in greater power of self-regulation?
-Largely also: perhaps most largely. Right conduct is usually come short
-of more from defect of will than defect of knowledge. For the right
-co-ordination of those complex actions which constitute human life in
-its civilized form, there goes not only the pre-requisite--recognition
-of the proper course; but the further pre-requisite--a due impulse to
-pursue that course. On calling to mind our daily failures to fulfil
-often-repeated resolutions, we shall perceive that lack of the needful
-desire, rather than lack of the needful insight, is the chief cause of
-faulty action. A further endowment of those feelings which civilization
-is developing in us--sentiments responding to the requirements of the
-social state--emotive faculties that find their gratifications in the
-duties devolving on us--must be acquired before the crimes, excesses,
-diseases, improvidences, dishonesties, and cruelties, that now so
-greatly diminish the duration of life, can cease.
-
-Thus, looking at the several possibilities, and asking what direction
-this further evolution, this more complete moving equilibrium, this
-better adjustment of inner to outer relations, this more perfect
-co-ordination of actions, is likely to take; we conclude that it must
-take mainly the direction of a higher intellectual and emotional
-development.
-
-
-§ 373. This conclusion we shall find equally forced on us if we
-inquire for the causes which are to bring about such results. No
-more in the case of Man than in the case of any other being, can
-we presume that evolution has taken place, or will hereafter take
-place, spontaneously. In the past, at present, and in the future, all
-modifications, functional and organic, have been, are, and must be,
-immediately or remotely consequent on surrounding conditions. What,
-then, are those changes in the environment to which, by direct or
-indirect equilibration, the human organism has been adjusting itself,
-is adjusting itself now, and will continue to adjust itself? And how
-do they necessitate a higher evolution of the organism?
-
-Civilization, everywhere having for its antecedent the increase
-of population, and everywhere having for one of its consequences
-a decrease of certain race-destroying forces, has for a further
-consequence an increase of certain other race-destroying forces. Danger
-of death from predatory animals lessens as men grow more numerous.
-Though, as they spread over the Earth and divide into tribes, men
-become wild beasts to one another, yet the danger of death from this
-cause also diminishes as tribes coalesce into nations. But the danger
-of death which does not diminish, is that produced by augmentation of
-numbers itself--the danger from deficiency of food. Supposing human
-nature to remain unchanged, the mortality hence resulting would, on
-the average, rise as human beings multiplied. If mortality, under
-such conditions, does not rise, it must be because the supply of food
-also augments; and this implies some change in human habits wrought
-by stress of human needs. Here, then, is the permanent cause of
-modification to which civilized men are exposed. Though the intensity
-of its action is ever being mitigated in one direction by greater
-production of food, it is, in the other direction, ever being added
-to by the greater production of individuals. Manifestly, the wants of
-their redundant numbers constitute the only stimulus mankind have to
-obtain more necessaries of life. Were not the demand beyond the supply,
-there would be no motive to increase the supply. And manifestly, this
-excess of demand over supply is perennial: this pressure of population,
-of which it is the index, cannot be eluded. Though by the emigration
-that takes place when the pressure arrives at a certain intensity,
-temporary relief is from time to time obtained; yet as, by this
-process, all habitable countries must become peopled, it follows that
-in the end the pressure, whatever it may then be, must be borne in full.
-
-This constant increase of people beyond the means of subsistence
-causes, then, a never-ceasing requirement for skill, intelligence, and
-self-control--involves, therefore, a constant exercise of these and
-gradual growth of them. Every industrial improvement is at once the
-product of a higher form of humanity, and demands that higher form of
-humanity to carry it into practice. The application of science to the
-arts, is the bringing to bear greater intelligence for satisfying our
-wants, and implies continued progress of that intelligence. To get more
-produce from the acre, the farmer must study chemistry, must adopt new
-mechanical appliances, and must, by the multiplication of processes,
-cultivate both his own powers and the powers of his labourers. To
-meet the requirements of the market, the manufacturer is perpetually
-improving his old machines and inventing new ones; and by the
-premium of high wages incites artizans to acquire greater skill. The
-daily-widening ramifications of commerce entail on the merchant a need
-for more knowledge and more complex calculations; while the lessening
-profits of the ship-owner force him to build more scientifically, to
-get captains of higher intelligence and better crews. In all cases
-pressure of population is the original cause. Were it not for the
-competition this entails, more thought and energy would not daily be
-spent on the business of life; and growth of mental power would not
-take place. Difficulty in getting a living is alike the incentive to a
-higher education of children, and to a more intense and long-continued
-application in adults. In the mother it prompts foresight, economy,
-and skilful house-keeping; in the father, laborious days and constant
-self-denial. Nothing but necessity could make men submit to this
-discipline; and nothing but this discipline could produce a continued
-progression.
-
-In this case, as in many others, Nature secures each step in advance by
-a succession of trials; which are perpetually repeated, and cannot fail
-to be repeated, until success is achieved. All mankind in turn subject
-themselves more or less to the discipline described; they either
-may or may not advance under it; but, in the nature of things, only
-those who _do_ advance under it eventually survive. For, necessarily,
-families and races whom this increasing difficulty of getting a living
-which excess of fertility entails, does not stimulate to improvements
-in production--that is, to greater mental activity--are on the high
-road to extinction; and must ultimately be supplanted by those whom
-the pressure does so stimulate. This truth we have recently seen
-exemplified in Ireland. And here, indeed, without further illustration,
-it will be seen that premature death, under all its forms and from all
-its causes, cannot fail to work in the same direction. For as those
-prematurely carried off must, in the average of cases, be those in whom
-the power of self-preservation is the least, it unavoidably follows
-that those left behind to continue the race, must be those in whom the
-power of self-preservation is the greatest--must be the select of their
-generation. So that, whether the dangers to existence be of the kind
-produced by excess of fertility, or of any other kind, it is clear that
-by the ceaseless exercise of the faculties needed to contend with them,
-and by the death of all men who fail to contend with them successfully,
-there is ensured a constant progress towards a higher degree of skill,
-intelligence, and self-regulation--a better co-ordination of actions--a
-more complete life.[67]
-
-
-§ 374. The proposition at which we have thus arrived is, then, that
-excess of fertility, through the changes it is ever working in Man’s
-environment, is itself the cause of Man’s further evolution; and the
-obvious corollary here to be drawn is, that Man’s further evolution so
-brought about, itself necessitates a decline in his fertility.
-
-All future progress in civilization which the never-ceasing pressure
-of population must produce, will be accompanied by an enhanced cost of
-Individuation, both in structure and function; and more especially in
-nervous structure and function. The peaceful struggle for existence in
-societies ever growing more crowded and more complicated, must have
-for its concomitant an increase of the great nervous centres in mass,
-in complexity, in activity. That larger body of emotion needed as a
-fountain of energy for men who have to hold their places and rear their
-families under the intensifying competition of social life, is, other
-things equal, the correlative of larger brain. Those higher feelings
-presupposed by the better self-regulation which, in a better society,
-can alone enable the individual to leave a persistent posterity, are,
-other things equal, the correlatives of a more complex brain; as are
-also those more numerous, more varied, more general, and more abstract
-ideas, which must also become increasingly requisite for successful
-life as society advances. And the genesis of this larger quantity of
-feeling and thought, in a brain thus augmented in size and developed
-in structure, is, other things equal, the correlative of a greater
-wear of nervous tissue and greater consumption of materials to repair
-it. So that both in original cost of construction and in subsequent
-cost of working, the nervous system must become a heavier tax on the
-organism. Already the brain of the civilized man is larger by nearly
-thirty per cent. than the brain of the savage. Already, too, it
-presents an increased heterogeneity--especially in the distribution
-of its convolutions. And further changes like these which have taken
-place under the discipline of civilized life, we infer will continue
-to take place. But everywhere and always, evolution is antagonistic
-to procreative dissolution. Whether it be in greater growth of the
-organs which subserve self-maintenance, whether it be in their added
-complexity of structure, or whether it be in their higher activity,
-the abstraction of the required materials implies a diminished reserve
-of materials for race-maintenance. And we have seen reason to believe
-that this antagonism between Individuation and Genesis, becomes
-unusually marked where the nervous system is concerned, because of the
-costliness of nervous structure and function. In § 346 was pointed out
-the apparent connexion between high cerebral development and prolonged
-delay of sexual maturity; and in §§ 366, 367, the evidence went to show
-that where exceptional fertility exists there is sluggishness of mind,
-and that where there has been during education excessive expenditure
-in mental action, there frequently follows a complete or partial
-infertility. Hence the particular kind of further evolution which Man
-is hereafter to undergo, is one which, more than any other, may be
-expected to cause a decline in his power of reproduction.
-
-The higher nervous development and greater expenditure in nervous
-action, here described as indirectly brought about by increase of
-numbers, and as thereafter becoming a check on the increase of numbers,
-must not be taken to imply an intenser strain--a mentally-laborious
-life. The greater emotional and intellectual power and activity above
-contemplated, must be understood as becoming, by small increments,
-organic, spontaneous, and pleasurable. As, even when relieved from
-the pressure of necessity, large-brained Europeans voluntarily enter
-on enterprises and activities which the savage could not keep up even
-to satisfy urgent wants; so, their still larger-brained descendants
-will, in a still higher degree, find their gratifications in careers
-entailing still greater mental expenditures. This enhanced demand for
-materials to establish and carry on the psychical functions, will be a
-constitutional demand. We must conceive the type gradually so modified,
-that the more-developed nervous system irresistibly draws off, for its
-normal and unforced activities, a larger proportion of the common stock
-of nutriment; and while so increasing the intensity, completeness, and
-length of the individual life, necessarily diminishing the reserve
-applicable to the setting up of new lives--no longer required to be so
-numerous.
-
-Though the working of this process will doubtless be interfered
-with and modified in the future, as it has been in the past, by the
-facilitations of living which civilization brings; yet nothing beyond
-temporary interruptions can so be caused. However much the industrial
-arts may be improved, there must be a limit to the improvement; while,
-with a rate of multiplication in excess of the rate of mortality,
-population must continually tread on the heels of production. So that
-though, during the earlier stages of civilization, an increased amount
-of food may accrue from a given amount of labour, there must come a
-time when this relation will be reversed, and when every additional
-increment of food will be obtained by a more than proportionate labour:
-the disproportion growing ever higher, and the diminution of the
-reproductive power becoming greater.
-
-
-§ 375. There now remains but to inquire towards what limit this
-progress tends. So long as the fertility of the race is more than
-sufficient to balance the diminution by deaths, population must
-continue to increase. So long as population continues to increase,
-there must be pressure on the means of subsistence. And so long
-as there is pressure on the means of subsistence, further mental
-development must go on, and further diminution of fertility must
-result; provided that the actions and reactions which have been
-described are not artificially interfered with. I append this
-qualifying clause advisedly, and especially emphasize it, because
-these actions and reactions have been hitherto, and are now,
-greatly interfered with by governments, and the continuance of the
-interferences may retard, if not stop, that further evolution which
-would else go on.
-
-I refer to those hindrances to the survival of the fittest which
-in earlier times resulted from the undiscriminating charities of
-monasteries and in later times from the operation of Poor Laws. Of
-course if the competition which increasing pressure of population
-entails, is prevented from acting on a considerable part of the
-community, such part, saved from the needed intellectual and moral
-stress, will not undergo any further mental development; and must
-ever tend to leave a posterity, and an increasing posterity, in which
-none of that higher individuation which checks genesis takes place.
-Such State-meddlings with the natural play of actions and reactions
-produce a further evil equally great or greater. For those who are
-not self-maintained, or but partially self-maintained, are supplied
-with the means they lack by the better members of the community; and
-these better members have thus not only to support themselves and
-their offspring, but also to support or aid the inferior members and
-their offspring. The under-working of one part is accompanied by the
-over-working of the other part--by a working which at each stage of
-progress exceeds that which the normal conditions necessitate, and
-results sometimes in illness, premature age, or death, or in lessened
-number of children, or in imperfect rearing of children: the bad are
-fostered and the good are repressed.
-
-It does not follow that the struggle for life and the survival of the
-fittest must be left to work out their effects without mitigation. It
-is contended only that there shall not be a forcible burdening of the
-superior for the support of the inferior. Such aid to the inferior as
-the superior voluntarily yield, kept as it will be within moderate
-limits, may be given with benefit to both--relief to the one, moral
-culture to the other. And aid willingly given (little to the least
-worthy and more to the most worthy) will usually be so given as not
-to further the increase of the unworthy. For in proportion as the
-emotional nature becomes more evolved, and there grows up a higher
-sense of parental responsibility, the begetting of children that cannot
-be properly reared will be universally held intolerable. If, as we
-see, public opinion in many places and times becomes coercive enough
-to force men to fight duels, we can scarcely doubt that at a higher
-stage of evolution it may become so coercive as to prevent men from
-marrying improvidently. If the frowns of their fellows can make men
-commit immoral acts, surely they may make men refrain from immoral
-acts--especially when the actors themselves feel that the threatened
-frowns would be justified. Hence with a higher moral nature will come a
-restriction on the multiplication of the inferior.
-
-In brief, the sole requirement is that there shall be no extensive
-suspension of that natural relation between merit and benefit which
-constitutes justice. Holding, then, that this all-essential condition
-will itself come to be recognized and enforced by a more evolved
-humanity, let us consider what is the goal towards which the restraint
-on genesis by individuation progresses.
-
-
-§ 375_a_. Supposing the Sun’s light and heat, on which all terrestrial
-life depends, to continue abundant for a period long enough to allow
-the entire evolution we are contemplating; there are still certain
-changes which must prevent such complete adjustment of human nature
-to surrounding conditions, as would permit the rate of multiplication
-to become equal to the rate of mortality. As before pointed out (§
-148), during an epoch of 21,000 years each hemisphere goes through
-a cycle of temperate seasons and seasons extreme in their heat and
-cold--variations which are themselves alternately exaggerated and
-mitigated in the course of far longer cycles; and we saw that these
-cause perpetual ebbings and flowings of species over different parts
-of the Earth’s surface. Further, by slow but inevitable geologic
-changes, especially those of elevation and subsidence, the climate and
-physical characters of every habitat are modified; while old habitats
-are destroyed and new are formed. This, too, we noted as a constant
-cause of migrations and of resulting alterations of environment. Now
-though the human race differs from other races in having a power of
-artificially counteracting external changes, yet there are limits to
-this power; and, even were there no limits, the changes could not
-fail to work their effects indirectly, if not directly. If, as is
-thought probable, these astronomic cycles entail recurrent glacial
-periods in each hemisphere, then parts of the Earth which are at one
-time thickly peopled, will at another time be almost deserted, and
-_vice versâ_. The geologically-caused alterations of climate and
-surface, must produce further slow re-distributions of population;
-and other currents of people, to and from different regions, will be
-necessitated by the rise of successive centres of higher civilization.
-Consequently, mankind cannot but continue to undergo changes of
-environment, physical and moral, analogous to those which they have
-thus far been undergoing. Such changes may eventually become slower
-and less marked; but they can never cease. And if they can never cease
-there can never arise a perfect adaptation of human nature to its
-conditions of existence. To establish that complete correspondence
-between inner and outer actions which constitutes the highest life
-and greatest power of self-preservation, there must be a prolonged
-converse between the organism and circumstances which remain the
-same. If the external relations are being altered while the internal
-relations are being adjusted to them, the adjustment can never become
-exact. And in the absence of exact adjustment, there cannot exist that
-theoretically-highest power of self-preservation with which there would
-co-exist the theoretically-lowest power of race-production.
-
-Hence though the number of premature deaths may ultimately become very
-small, it can never become so small as to allow the average number of
-offspring from each pair to fall so low as two. Some average number
-between two and three may be inferred as the limit--a number, however,
-which is not likely to be quite constant, but may be expected at
-one time to increase somewhat and afterwards to decrease somewhat,
-according as variations in physical and social conditions lower or
-raise the cost of self-preservation.
-
-To this qualification must be added a further qualification. The
-foregoing argument tacitly assumes that the causes described will
-continuously operate on all mankind; whereas a survey of the facts
-makes it clear that some parts only of the Earth’s surface are capable
-of bearing high types of civilization, and consequently high types
-of Man. There must remain hereafter, as there are now, considerable
-parts of its surface which can support only groups of nomads, or other
-groups obliged by their habitats to lead simple and inferior kinds of
-life. Only by subjection to the discipline we have been contemplating
-can there be produced the fully-developed Man; and evidently in many
-parts of the world this discipline will continue to be eluded. Not
-only must we conclude that the varieties of our race now living in
-desert regions and arctic climates will continue hereafter to do so,
-but we may conclude that always, as now, a certain proportion of men
-who are born in civilized societies, impatient of the stress which
-pressure of population puts on them, will escape into unoccupied or
-sparsely-peopled regions, where they can lead unrestrained lives
-though lives of hardship. Recognizing as we must the probability
-that in common with all other things, humanity will continue to
-differentiate and produce a more heterogeneous assemblage of types,
-we must infer that only in some of the highest of these will the
-antagonism of individuation and genesis have the anticipated effects.
-
-Restricting ourselves to these, then, we may conclude that in the
-end, pressure of population and its accompanying evils will almost
-disappear; and will leave a state of things requiring from each
-individual little more than a normal and pleasurable activity.
-Cessation in the decrease of fertility implies cessation in the
-development of the nervous system; and this implies a nervous system
-which has become equal to all that is demanded of it--has not to do
-more than is natural to it. But that exercise of faculties which does
-not exceed what is natural, constitutes gratification.
-
-The necessary antagonism of Individuation and Genesis, not only,
-then, fulfils the _à priori_ law of maintenance of race, from the
-monad up to Man, but ensures final attainment of the highest form of
-this maintenance--a form in which the amount of life shall be the
-greatest possible and the births and deaths the fewest possible. From
-the beginning pressure of population has been the proximate cause of
-progress. It produced the original diffusion of the race. It compelled
-men to abandon predatory habits and take to agriculture. It led to
-the clearing of the Earth’s surface. It forced men into the social
-state; made social organization inevitable; and has developed the
-social sentiments. It has stimulated to progressive improvements
-in production, and to increased skill and intelligence. It is
-daily thrusting us into closer contact and more mutually-dependent
-relationships. And after having caused, as it ultimately must, the due
-peopling of the globe, and the raising of its habitable parts into the
-highest state of culture--after having perfected all processes for the
-satisfaction of human wants--after having, at the same time, developed
-the intellect into competence for its work, and the feelings into
-fitness for social life--after having done all this, the pressure of
-population must gradually approach to an end--an end, however, which
-for the reasons given it cannot absolutely reach.
-
-
-§ 377. In closing the argument let us not overlook the
-self-sufficingness of those universal processes by which the results
-reached thus far have been wrought out, and which may be expected to
-work out these future results.
-
-Evolution under all its aspects, general and special, is an advance
-towards equilibrium. We have seen that the theoretical limit towards
-which the integration and differentiation of every aggregate advances,
-is a state of balance between all the forces to which its parts are
-subject, and the forces which its parts oppose to them (_First Prin._ §
-170). And we have seen that organic evolution is a progress towards a
-moving equilibrium completely adjusted to environing actions.
-
-It has been also pointed out that, in civilized Man, there is going
-on a new class of equilibrations--those between his actions and the
-actions of the societies he forms (_First Prin._ § 175). Social
-restraints and requirements are ever altering his activities and by
-consequence his nature; and as fast as his nature is altered, social
-restraints and requirements undergo more or less re-adjustment. Here
-the organism and the conditions are both modifiable; and by successive
-conciliations of the two, there is effected a progress towards
-equilibrium.
-
-More recently we have seen that in every species, there establishes
-itself an equilibrium of an involved kind between the total
-race-destroying forces and the total race-preserving forces--an
-equilibrium which implies that where the ability to maintain individual
-life is small, the ability ta propagate must be great, and _vice
-versâ_. Whence it follows that the evolution of a race more in
-equilibrium with the environment, is also the evolution of a race in
-which there is a correlative approach towards equilibrium between the
-number of new individuals produced and the number which survive and
-propagate.
-
-The final result to be observed is that in Man, all these
-equilibrations between constitution and conditions, between the
-structure of society and the nature of its members, between fertility
-and mortality, advance simultaneously towards a common climax. In
-approaching an equilibrium between his nature and the ever-varying
-circumstances of his inorganic environment, and in approaching an
-equilibrium between his nature and all the requirements of the social
-state, Man is at the same time approaching that lowest limit of
-fertility at which the equilibrium of population is maintained by the
-addition of as many infants as there are subtractions by death in old
-age. But in a universe of which all parts are in motion and every
-part is consequently subject to change of conditions, neither this
-equilibrium nor any other equilibrium can become complete.
-
-
-                               THE END.
-
-
-
-
-                              APPENDICES.
-
-
-
-
-                              APPENDIX A.
-
-          SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS.
-
-
-I append here the evidences referred to in § 190. The most numerous and
-striking I have met with among the _Umbelliferæ_. Monstrosities having
-the alleged implication, are frequent in the common Cow-Parsnep--so
-frequent that they must be familiar to botanists; and wild Angelica
-supplies many over-developments of like meaning. Omitting numerous
-cases of more or less significance, I will limit myself to two.
-
-  [Illustration: Fig. 69.]
-
-One of them is that of a terminal umbel, in which nine of the outer
-umbellules are variously transformed--here a single flower being
-made monstrous by the development of some of its members into buds;
-there several such malformed flowers being associated with rays that
-bear imperfect umbellules; and elsewhere, flowers being replaced by
-umbellules: some of which are perfect, and others imperfect only in
-the shortness of the flower-stalks. The annexed Fig. 69, representing
-in a somewhat conventionalized way, a part of the dried specimen,
-will give an idea of this Angelica. At _a_ is shown a single flower
-partially changed; in the umbellule marked _b_, one of the rays bears a
-secondary umbellule; and there may be seen at _c_ and _d_, several such
-over-developments.
-
-But the most conclusive instance is that of a Cow-Parsnep, in which a
-single terminal umbel, besides the transformations already mentioned,
-exhibits higher degrees of such transformations.[68] The components
-of this complex growth are;--three central umbellules, abnormal only
-in minor points; one umbellule, external to these, which is partially
-changed into an umbel; one rather more out of the centre, which is
-so far metamorphosed as to be more an umbel than an umbellule: nine
-peripheral clusters formed by the development of umbellules into
-umbels, some of which are partially compounded still further. Examined
-in detail, these structures present the following facts:--1. The
-innermost umbellule is normal, save in having a peripheral flower of
-which one member (apparently a petal) is transformed into a flower-bud.
-2. The next umbellule, not quite so central, has one of its peripheral
-flowers made monstrous by the growth of a bud from the base of the
-calyx. 3. The third of the central umbellules has two abnormal outer
-flowers. One of them carries a flower-bud on its edge, in place of
-a foliar member. The other is half flower and half umbellule: being
-composed of three petals, three stamens, and five flower-buds growing
-where the other petals and stamens should grow. 4. Outside of these
-umbellules comes one of the mixed clusters. Its five central flowers
-are normal. Surrounding these are several flowers transformed in
-different degrees: one having a stamen partially changed into a flower
-bud. And then, at the periphery of this mixed cluster, come three
-complete umbellules and an incomplete one in which some petals and
-stamens of the original flower remain. 5. A mixed cluster, in which
-the umbel-structure predominates, stands next. Its three central
-flowers are normal. Surrounding them are five flowers over-developed in
-various ways, like those already described. And on its periphery are
-seven complete umbellules in place of flowers; besides an incomplete
-umbellule that contains traces of the original flower, one of them
-being a petal imperfectly twisted up into a bud. 6. Of the nine
-external clusters, in which the development of simple into compound
-umbels is most decided, nearly all present anomalies. Three of them
-have each a central flower untransformed; and in others, the central
-umbellule is composed of two, three, or four flowers. 7. But the most
-remarkable fact is, that in sundry of these peripheral clusters,
-resulting from the metamorphosis of simple umbels into compound umbels,
-the like metamorphosis is carried a stage higher. Some of the component
-rays, are themselves the bearers of compound umbels instead of simple
-umbels. In Fig. 70, a portion of the dried specimen is represented.
-Two of the central umbellules are marked _a_ and _b_; those marked
-_c_ and _d_ are mixed clusters; at _e_ and _f_ are compound umbels
-replacing simple ones; and _g_ shows one of the rays on which the
-over-development goes still further.
-
-  [Illustration: Fig. 70.]
-
-Does not this evidence, enforced as it is by much more of like kind,
-go far to prove that foliar organs may be developed into axial organs?
-Even were not the transitional forms traceable, there would still,
-I think, be no other legitimate interpretation of the facts last
-detailed. The only way of eluding the conclusion here drawn, is by
-assuming that where a cluster of flowers replaces a single flower, it
-is because the axillary buds which hypothetically belong to the several
-foliar organs of the flower, become developed into axes; and assuming
-this, is basing an hypothesis on another hypothesis that is directly at
-variance with facts. The foliar organs of flowers do _not_ bear buds
-in their axils; and it would never have been supposed that such buds
-are typically present, had it not been for that mistaken conception of
-“type” which has led to many other errors in Biology. Goethe writes:
-“Now as we cannot realize the idea of a leaf apart from the node out of
-which it springs, or of a node without a bud, we may venture to infer,”
-&c. See here an example of a method of philosophizing not uncommon
-among the Germans. The method is this--Survey a portion of the
-facts, and draw from them a general conception; project this general
-conception back into the objective world, as a mould in which Nature
-casts her products; expect to find it everywhere fulfilled; and allege
-potential fulfilment where no actual fulfilment is visible.
-
-If instead of imposing our ideal forms on Nature, we are content to
-generalize the facts as Nature presents them, we shall find no warrant
-for the morphological doctrine above enunciated. The only conception
-of type justified by the logic of science, is--that correlation
-of parts which remains constant under all modifications of the
-structure to be defined. To ascertain this, we must compare all these
-modifications, and note what traits are common to them. On doing so
-with the successive segments of a phænogamic axis, we are brought to
-a conclusion widely different from that of Goethe. Axillary buds are
-almost universally absent from the cotyledons; they are habitually
-present in the axils of fully-developed leaves higher up the axis; they
-are often absent from leaves that are close to the flower; they are
-nearly always absent from the bracts; absent from the sepals; absent
-from the petals; absent from the stamens; absent from the carpels.
-Thus, out of eight leading forms which folia assume, one has the
-axillary bud and seven are without it. With these facts before us, it
-seems to me not difficult to “realize the idea” “of a node without a
-bud.” If we are not possessed by a foregone conclusion, the evidence
-will lead us to infer, that each node bears a foliar appendage and
-_may_ bear an axillary bud.
-
-Even, however, were it granted that the typical segment of a Phænogam
-includes an axillary bud, which must be regarded as always potentially
-present, no legitimate counter-interpretation of the monstrosities
-above described could thence be drawn. If when an umbellule is
-developed in place of a flower, the explanation is, that its component
-rays are axillary to the foliar organs of the flower superseded; we may
-fairly require that these foliar organs to which they are axillary,
-shall be shown. But there are none. In the last specimen figured,
-the inner rays of each such umbellule are without them; most of the
-outer rays are also without them; and in one cluster, only a single
-ray has a bract at its point of origin. There is a rejoinder ready,
-however: the foliar organs are said to be suppressed. Though Goethe
-could not “realize the idea” “of a node without a bud,” those who
-accept his typical form appear to find no difficulty in realizing the
-idea of an axillary bud without anything to which it is axillary. But
-letting this pass, suppose we ask what is the warrant for this assumed
-suppression. Axillary buds normally occur where the nutrition is high
-enough to produce fully-developed leaves; and when axillary buds are
-demonstrably present in flowers, they accompany foliar organs that
-are more leaf-like than usual--always greener if not always larger.
-That is to say, the normal and the abnormal axillary buds, are
-alike the concomitants of foliar organs coloured by that chlorophyll
-which habitually favours foliar development. How, then, can it be
-supposed that when, out of a flower there is developed a cluster
-of flower-bearing rays, the implied excess of nutrition causes the
-foliar organs to abort? It is true that very generally in a branched
-inflorescence, the bracts of the several flower-branches are very
-small (their smallness being probably due to that defective supply of
-certain chlorophyll-forming matters, which is the proximate cause of
-flowering); and it is true that, under these conditions, a flowering
-axis of considerable size, for the development of which chlorophyll is
-less needful, grows from the axil of a dwarfed leaf. But the inference
-that the foliar organ may therefore be entirely suppressed, seems
-to me irreconcilable with the fact, that the foliar organ is always
-developed to some extent _before_ the axillary bud appears. Until it
-has been shown that in some cases a lateral bud first appears, and a
-foliar organ _afterwards_ grows out beneath it, to form its axil, the
-conception of an axillary bud of which the foliar organ is suppressed,
-will remain at variance with the established truths of development.
-
-       *       *       *       *       *
-
-The above originally formed a portion of § 190. I have transferred it
-to the Appendix, partly because it contains too much detail to render
-it fit for the general argument, and partly because the interpretations
-being open to some question, it seemed undesirable to risk compromising
-that argument by including them. The criticisms passed upon these
-interpretations have not, however, sufficed to convince me of their
-incorrectness. Unfortunately, I have since had no opportunity of
-verifying the above statements by microscopic examinations, as I had
-intended.
-
-Though unable to enforce the inference drawn by further facts more
-minutely looked into, I may add some arguments based on facts that are
-well known. One of these is the fact that the so-called axillary bud
-is not universally axillary--is not universally seated in the angle
-made by the axis and an appended foliar organ. In certain plants the
-axillary bud is placed far above the node, half-way between it and the
-succeeding node. So that not only may a segment of a phæenogamic axis
-be without the axillary bud, but the axillary bud, when present, may be
-removed from that place in which, according to Goethe, it necessarily
-exists. Another fact not congruous with the current doctrine, is the
-common occurrence of “adventitious” buds--the buds that are put out
-from roots and from old stems or branches bare of leaves. The name
-under which they are thus classed, is meant to imply that they may be
-left out of consideration. Those, however, who have not got a theory to
-save by putting anomalies out of sight, may be inclined to think that
-the occurrence of buds where they are avowedly unconnected with nodes,
-and are axillary to nothing, tells very much against the assumption
-that every bud implies a node and a corresponding foliar organ. And
-they may also see that the development of these adventitious buds at
-places where there is excess of nutritive materials, favours the view
-above set forth. For if a bud thus arises at a place where it is not
-morphologically accounted for, simply because there happens to be at
-that place an abundance of unorganized protoplasm; then, clearly, it is
-likely that if the mass of protoplasm from which a leaf would usually
-arise, is greatly increased in mass by excess of nutrition, it may
-develop into an axis instead of a leaf.
-
-       *       *       *       *       *
-
-Many years after this work was published, I discovered among my papers
-a memorandum which unfortunately I had overlooked, containing further
-evidence in support of the foregoing conclusion. With the omission of
-an error concerning the species of plant, I reproduce this memorandum
-just as it stood:--
-
-“I found at Dieppe, July 1, 1860, in a garden near the sea a sample of
-cultivated wild flower (I thought it was grown as an ornamental flower)
-in which some of the single flowers of the umbel were developed into
-groups of flowers thus:--
-
-  [Illustration]
-
-“In the case where the transformation was fully effected the umbellule
-had _six_ flowers, answering to the _six petals_ of the original
-flowers. In other cases the transformation was incomplete. There were
-instances where but _two_ of the petals were developed into flowers;
-and the other petals remained unchanged. Others in which _three_ were
-developed; and others where four were developed. In some cases, too,
-the development of a petal into a flower was imperfect, in the absence
-of the flower-stalk--the flowers were sessile in the place where the
-petals would have been. In one case there was an _imperfect_ flower
-sessile; another _imperfect_ flower on a short stalk; and three perfect
-flowers on long stalks.
-
-“I was in some doubt whether the petals or the stamens were developed.
-In cases of imperfect transformation the petals at the base of the
-umbellule seemed to stand in the position of calyx or involucrum,
-giving the idea that the stamens were developed into flowers. But in
-the case where there were _six_ flowers developed there were no petals
-at the base.
-
-“That it was a matter of extra nutrition was shown by this:--
-
-“1. That they were cultivated as garden flowers.
-
-“2. That where there was one perfectly developed umbellule, it was the
-only one in the umbel.
-
-“3. That where there were three umbellules they were all imperfect.
-
-“4. That in this imperfect umbellule the perfect flowers were on long
-stalks and the imperfect ones sessile.
-
-“5. That the umbellules were on stalks both longer and thicker than
-those of single flowers.”
-
-       *       *       *       *       *
-
-[Concerning the foregoing argument at large an expert writes:--“The
-abnormalities you describe certainly show that an axis may arise
-abnormally in the place of a normal leaf-structure, and every modern
-botanist would be in agreement with you in your criticism of the
-older form of the doctrine of axillary buds. I think we are largely
-emancipated from the dextrous juggling with the arrangements and
-relations of organs which used to pass current as morphology.
-
-“You have quoted sufficient evidence in the text (§ 190) to establish
-the conclusion that no sharp line can be drawn between axes and
-leaf-structure; and a very great deal more could be added in the same
-sense. Petioles for instance, exist which the most highly trained
-histological observer could not distinguish from stems.
-
-“But I must demur to the suggestion that the replacement of one by the
-other is primarily a question of nutrition. We are as ignorant as ever
-of the proximate cause of the production of a leaf or a shoot at a
-certain spot in meristematic tissue.”
-
-To this last remark I had at first made only the reply that the plants
-exhibiting the abnormalities were in all cases excessively luxuriant in
-their growths; but to this I am now able to add a more definite reply.
-The expert from whom I have just quoted, had read this appendix before
-there had been made to it the above addition describing the flower
-from Dieppe; and I was not myself aware, until I came to read over
-this addition, what clear evidence it contains that extra nutrition
-was the cause of these transformations of foliar structures into axial
-structures; but the above paragraphs 1, 2, 3, 4, 5, contain different
-evidences conspiring to prove this.]
-
-
-
-
-                              APPENDIX B.
-
-    A CRITICISM ON PROF. OWEN’S THEORY OF THE VERTEBRATE SKELETON.
-
-[_From the_ BRITISH & FOREIGN MEDICO-CHIRURGICAL REVIEW FOR OCT., 1858.]
-
-
-   I. _On the Archetype and Homologies of the Vertebrate Skeleton._
-   _By_ RICHARD OWEN, _F.R.S._--_London_, 1848. _pp._ 172.
-
-   II. _Principes d’Ostéologie Comparée, ou Recherches sur
-   l’Archétype et les Homologies du Squelette Vertébré._ _Par_
-   RICHARD OWEN.--_Paris._
-
-   _Principles of Comparative Osteology; or, Researches on the
-   Archetype and the Homologies of the Vertebrate Skeleton._ _By_
-   RICHARD OWEN.
-
-   III. _On the Nature of Limbs. A Discourse delivered on Friday,
-   February 9, at an Evening Meeting of the Royal Institution of
-   Great Britain._ _By_ RICHARD OWEN, _F.R.S._--_London_, 1849.
-   _pp._ 119.
-
-Judging whether another proves his position is a widely different thing
-from proving your own. To establish a general law requires an extensive
-knowledge of the phenomena to be generalized; but to decide whether an
-alleged general law is established by the evidence assigned, requires
-merely an adequate reasoning faculty. Especially is such a decision
-easy where the premises do _not_ warrant the conclusion. It may be
-dangerous for one who has but little previous acquaintance with the
-facts, to say that a generalization is demonstrated; seeing that the
-argument may be one-sided: there may be many facts unknown to him which
-disprove it. But it is not dangerous to give a negative verdict when
-the alleged demonstration is manifestly insufficient. If the data put
-before him do not bear out the inference, it is competent for every
-logical reader to say so.
-
-From this standpoint, then, we venture to criticize some of Professor
-Owen’s osteological theories. For his knowledge of comparative
-osteology we have the highest respect. We believe that no living man
-has so wide and detailed an acquaintance with the bony structure of
-the _Vertebrata_. Indeed, there probably has never been any one whose
-information on the subject was so nearly exhaustive. Moreover, we
-confess that nearly all we know of this department of biology has been
-learnt from his lectures and writings. We pretend to no independent
-investigations, but merely to such knowledge of the phenomena as he has
-furnished us with. Our position, then, is such that, had Professor Owen
-simply enunciated his generalizations, we should have accepted them on
-his authority. But he has brought forward evidence to prove them. By
-so doing he has tacitly appealed to the judgments of his readers and
-hearers--has practically said, “Here are the facts; do they not warrant
-these conclusions?” And all we propose to do, is to consider whether
-the conclusions _are_ warranted by the facts brought forward.
-
-Let us first limit the scope of our criticisms. On that division
-of comparative osteology which deals with what Professor Owen
-distinguishes as “special homologies,” we do not propose to enter. That
-the wing of a bird is framed upon bones essentially parallel to those
-of a mammal’s fore-limb; that the cannon-bone of a horse’s leg answers
-to the middle metacarpal of the human hand; that various bones in the
-skull of a fish are homologous with bones in the skull of a man--these
-and countless similar facts, we take to be well established. It may
-be, indeed, that the doctrine of special homologies is at present
-carried too far. It may be that, just as the sweeping generalization
-at one time favoured, that the embryonic phases of the higher animals
-represent the adult forms of lower ones, has been found untrue in a
-literal sense, and is acceptable only in a qualified sense; so the
-sweeping generalization that the skeletons of all vertebrate animals
-consist of homologous parts, will have to undergo some modification.
-But that this generalization is substantially true, all comparative
-anatomists agree.
-
-The doctrine which we are here to consider, is quite a separate
-one--that of “general homologies.” The truth or falsity of this may
-be decided on quite apart from that of the other. Whether certain
-bones in one vertebrate animal’s skeleton correspond with certain
-bones in another’s, or in every other’s, is one question; and whether
-the skeleton of every vertebrate animal is divisible into a series of
-segments, each of which is modelled after the same type, is another
-question. While the first is answered in the affirmative, the last
-may be answered in the negative; and we propose to give reasons why it
-should be answered in the negative.
-
-       *       *       *       *       *
-
-In so far as his theory of the skeleton is concerned, Professor Owen
-is an avowed disciple of Plato. At the conclusion of his _Archetype
-and Homologies of the Vertebrate Skeleton_, he quotes approvingly
-the Platonic hypothesis of ἰδέαι, “a sort of models, or moulds in
-which matter is cast, and which regularly produce the same number and
-diversity of species.” The vertebrate form in general (see diagram of
-the _Archetypus_), or else the form of each kind of vertebrate animal
-(see p. 172, where this seems implied), Professor Owen conceives to
-exist as an “idea”--an “archetypal exemplar on which it has pleased the
-Creator to frame certain of his living creatures.” Whether Professor
-Owen holds that the typical vertebra also exists as an “idea,” is not
-so certain. From the title given to his figure of the “ideal typical
-vertebra,” it would seem that he does; and at p. 40 of his _Nature of
-Limbs_, and indeed throughout his general argument, this supposition is
-implied. But on the last two pages of the _Archetype and Homologies_,
-it is distinctly alleged that “the repetition of similar segments in
-a vertebral column, and of similar elements in a vertebral segment,
-is analogous to the repetition of similar crystals as the result of
-polarizing force in the growth of an inorganic body;” it is pointed
-out that, “as we descend the scale of animal life, the forms of the
-repeated parts of the skeleton approach more and more to geometrical
-figures;” and it is inferred that “the Platonic ἰδέα or specific
-organizing principle or force, would seem to be in antagonism with the
-general polarizing force, and to subdue and mould it in subserviency
-to the exigencies of the resulting specific form.” If Professor Owen’s
-doctrine is to be understood as expressed in these closing paragraphs
-of his _Archetype and Homologies_--if he considers that “the ἰδέα”
-“which produces the diversity of form belonging to living bodies of the
-same materials,” is met by the “counter-operation” of “the polarizing
-force pervading all space,” which produces “the similarity of forms,
-the repetition of parts, the signs of unity of organization,” and which
-is “_subdued_” as we ascend “in the scale of being;” then we may pass
-on with the remark that the hypothesis is too cumbrous and involved to
-have much _vraisemblance_. If, on the other hand, Professor Owen holds,
-as every reader would suppose from the general tenor of his reasonings,
-that not only does there exist an archetypal or ideal vertebrate
-skeleton, but that there also exists an archetypal or ideal vertebra;
-then he carries the Platonic hypothesis much further than Plato does.
-Plato’s argument, that before any species of object was created it
-must have existed as an idea of the Creative Intelligence, and that
-hence all objects of such species must be copies of this original
-idea, is tenable enough from the anthropomorphic point of view. But
-while those who, with Plato, think fit to base their theory of creation
-upon the analogy of a carpenter designing and making a table, must
-yield assent to Plato’s inference, they are by no means committed
-to Professor Owen’s expansion of it. To say that before creating a
-vertebrate animal, God must have had the conception of one, does not
-involve saying that God gratuitously bound himself to make a vertebrate
-animal out of segments all moulded after one pattern. As there is
-no conceivable advantage in this alleged adhesion to a fundamental
-pattern--as, for the fulfilment of the intended ends, it is not only
-needless, but often, as Professor Owen argues, less appropriate than
-some other construction would be (see _Nature of Limbs_, pp. 39, 40),
-to suppose the creative processes thus regulated, is not a little
-startling. Even those whose conceptions are so anthropomorphic as to
-think they honour the Creator by calling him “the Great Artificer,”
-will scarcely ascribe to him a proceeding which, in a human artificer,
-they would consider a not very worthy exercise of ingenuity.
-
-But whichever of these alternatives Professor Owen contends
-for--whether the typical vertebra is that more or less crystalline
-figure which osseous matter ever tends to assume in spite of “the ἰδέα
-or organizing principle,” or whether the typical vertebra is itself
-an “ἰδέα or organizing principle”--there is alike implied the belief
-that the typical vertebra has an abstract existence apart from actual
-vertebræ. It is a form which, in every endo-skeleton, strives to embody
-itself in matter--a form which is potentially present in each vertebra;
-which is manifested in each vertebra with more or less clearness; but
-which, in consequence of antagonizing forces, is nowhere completely
-realized. Apart from the philosophy of this hypothesis, let us here
-examine the evidence which is thought to justify it.
-
-       *       *       *       *       *
-
-And first as to the essential constituents of the “ideal typical
-vertebra.” Exclusive of “_diverging appendages_” which it “may also
-support,” “it consists in its typical completeness of the following
-elements and parts”:--A _centrum_ round which the rest are arranged in
-a somewhat radiate manner; above it two _neurapophyses_--converging
-as they ascend, and forming with the centrum a trianguloid space
-containing the neural axis; a _neural spine_, surmounting the two
-neurapophyses, and with them completing the neural arch; below the
-centrum two _hæmapophyses_ and a _hæmal spine_, forming a hæmal arch
-similar to the neural arch above, and enclosing the hæmal axis;
-two _pleurapophyses_ radiating horizontally from the sides of the
-centrum; and two _parapophyses_ diverging from the centrum below the
-pleurapophyses. “These,” says Professor Owen, “being usually developed
-from distinct and independent centres, I have termed ‘autogenous
-elements.’” The remaining elements, which he classes as “exogenous,”
-because they “shoot out as continuations from some of the preceding
-elements,” are the _diapophyses_ diverging from the upper part of the
-centrum as the parapophyses do below, and the _zygapophyses_ which grow
-out of the distal ends of the neurapophyses and hæmapophyses.
-
-If, now, these are the constituents of the vertebrate segment “in its
-typical completeness;” and if the vertebrate skeleton consists of a
-succession of such segments; we ought to have in these constituents,
-representatives of all the elements of the vertebrate skeleton--at
-any rate, all its essential elements. Are we then to conclude
-that the “diverging appendages,” which Professor Owen regards as
-rudimental limbs, and from certain of which he considers actual
-limbs to be developed, are typically less important than some of the
-above-specified exogenous parts--say the zygapophyses?
-
-That the meaning of this question may be understood, it will be needful
-briefly to state Professor Owen’s theory of _The Nature of Limbs_;
-and such criticisms as we have to make on it must be included in the
-parenthesis. In the first place, he aims to show that the scapular
-and pelvic arches, giving insertion to the fore and hind limbs
-respectively, are displaced and modified hæmal arches, originally
-belonging in the one case to the occipital vertebra, and in the
-other case to some trunk-vertebra not specified. In support of this
-assumption of displacement, carried in some cases to the extent of
-_twenty-seven_ vertebræ, Professor Owen cites certain acknowledged
-displacements which occur in the human skeleton to the extent of half
-a vertebra--a somewhat slender justification. But for proof that such
-a displacement _has_ taken place in the scapular arch, he chiefly
-relies on the fact that in fishes, the pectoral fins, which are the
-homologues of the fore-limbs, are directly articulated to certain bones
-at the back of the head, which he alleges are parts of the occipital
-vertebra. This appeal to the class of fishes is avowedly made on the
-principle that these lowest of the _Vertebrata_ approach closest to
-archetypal regularity, and may therefore be expected to show the
-original relations of the bones more nearly. Simply noting the facts
-that Professor Owen does not give us any transitional forms between
-the alleged normal position of the scapular arch in fishes, and its
-extraordinary displacement in the higher _Vertebrata_; and that he
-makes no reference to the embryonic phases of the higher _Vertebrata_,
-which might be expected to exhibit the progressive displacement; we
-go on to remark that, in the case of the pelvic arch, he abandons
-his principle of appealing to the lowest vertebrate forms for proof
-of the typical structure. In fishes, the rudimentary pelvis, widely
-removed from the spinal column, shows no signs of having belonged to
-any vertebra; and here Professor Owen instances the perennibranchiate
-_Batrachia_ as exhibiting the typical structure: remarking that
-“mammals, birds, and reptiles show the rule of connexion, and fishes
-the exception.” Thus in the case of the scapular arch, the evidence
-afforded by fishes is held of great weight, _because_ of their
-archetypal regularity; while in the case of the pelvic arch, their
-evidence is rejected as exceptional. But now, having, as he considers,
-shown that these bony frames to which the limbs are articulated are
-modified hæmal arches, Professor Owen points out that the hæmal
-arches habitually bear certain “diverging appendages;” and he aims
-to show that the “diverging appendages” of the scapular and pelvic
-arches respectively, are developed into the fore and hind limbs.
-There are several indirect ways in which we may test the probability
-of this conclusion. If these diverging appendages are “rudimental
-limbs”--“future possible or potential arms, legs, wings, or feet,” we
-may fairly expect them always to bear to the hæmal arches a relation
-such as the limbs do. But they by no means do this. “As the vertebræ
-approach the tail, these appendages are often transferred gradually
-from the pleurapophysis to the parapophysis, or even to the centrum
-and neural arch.” (_Arch. and Hom._, p. 93.) Again, it might naturally
-be assumed that in the lowest vertebrate forms, where the limbs are
-but little developed, they would most clearly display their alliance
-with the appendages, or “rudimental limbs,” by the similarity of their
-attachments. Instead of this, however, Professor Owen’s drawings
-show that whereas the appendages are habitually attached to the
-pleurapophyses, the limbs, in their earliest and lowest phase, alike in
-fishes and in the _Lepidosiren_, are articulated to the hæmapophyses.
-Most anomalous of all, however, is the process of development. When
-we speak of one thing as being developed out of another, we imply
-that the parts next to the germ are the first to appear, and the most
-constant. In the evolution of a tree out of a seed, there come at the
-outset the stem and the radicle; afterwards the branches and divergent
-roots; and still later the branchlets and rootlets; the remotest parts
-being the latest and most inconstant. If, then, a limb is developed out
-of a “diverging appendage” of the hæmal arch, the earliest and most
-constant bones should be the humerus and femur; next in order of time
-and constancy should come the coupled bones based on these; while the
-terminal groups of bones should be the last to make their appearance,
-and the most liable to be absent. Yet, as Professor Owen himself shows,
-the actual mode of development is the very reverse of this. At p. 16 of
-the _Archetype and Homologies_, he says:--
-
-   “The earlier stages in the development of all locomotive
-   extremities are permanently retained or represented in the
-   paired fins of fishes. First the essential part of the member,
-   the hand or foot, appears: then the fore-arm or leg, both much
-   shortened, flattened, and expanded, as in all fins and all
-   embryonic rudiments of limbs: finally come the humeral and
-   femoral segments; but this stage I have not found attained in
-   any fish.”
-
-That is to say, alike in ascending through the _Vertebrata_, generally,
-and in tracing up the successive phases of a mammalian embryo, the
-last-developed and least constant division of the limb, is that basic
-one by which it articulates with the hæmal arch. It seems to us that,
-so far from proving his hypothesis, Professor Owen’s own facts tend
-to show that limbs do not belong to the vertebræ at all: that they
-make their first appearance peripherally; that their development is
-centripetal; and that they become fixed to such parts of the vertebrate
-axis as the requirements of the case determine.
-
-But now, ending here this digressive exposition and criticism,
-and granting the position that limbs “are developments of costal
-appendages,” let us return to the question above put--Why are not
-these appendages included as elements of the “ideal typical vertebra?”
-It cannot be because of their comparative inconstancy; for judging
-from the illustrative figures, they seem to be as constant as the
-hæmal spine, which is one of the so-called autogenous elements: in
-the diagram of the _Archetypus_, the appendage is represented as
-attached to every vertebrate segment of the head and trunk, which the
-hæmal spine is not. It cannot be from their comparative unimportance;
-seeing that as potential limbs they are essential parts of nearly
-all the _Vertebrata_--much more obviously so than the diapophyses
-are. If, as Professor Owen argues, “the divine mind which planned the
-archetype also foreknew all its modifications;” and if, among these
-modifications, the development of limbs out of diverging appendages was
-one intended to characterize all the higher _Vertebrata_; then, surely,
-these diverging appendages must have been parts of the “ideal typical
-vertebra.” Or, if the “ideal typical vertebra” is to be understood as
-a crystalline form in antagonism with the organizing principle; then
-why should not the appendages be included among its various offshoots?
-We do not ask this question because of its intrinsic importance. We
-ask it for the purpose of ascertaining Professor Owen’s method of
-determining what are true vertebral constituents. He presents us with a
-diagram of the typical vertebra, in which are included certain bones,
-and from which are excluded certain others. If relative constancy
-is the criterion, then there arises the question--What degree of
-constancy entitles a bone to be included? If relative importance is the
-criterion, there comes not only the question--What degree of importance
-suffices? but the further question--How is importance to be measured?
-If neither of these is the criterion, then what is it? And if there is
-no criterion, does it not follow that the selection is arbitrary?
-
-       *       *       *       *       *
-
-This question serves to introduce a much wider one:--Has the “ideal
-typical vertebra” any essential constituents at all? It might
-naturally be supposed that though some bones are so rarely developed
-as not to seem worth including, and though some that are included are
-very apt to be absent, yet that certain others are invariable: forming,
-as it were, the basis of the ideal type. Let us see whether the facts
-bear out this supposition. In his “summary of modifications of corporal
-vertebræ” (p. 96), Professor Owen says--“The _hæmal spine_ is much
-less constant as to its existence, and is subject to a much greater
-range of variety, when present, than its vertical homotype above,
-which completes the neural arch.” Again he says--“The _hæmapophyses_,
-as osseous elements of a vertebra, are less constant than the
-pleurapophyses.” And again--“The _pleurapophyses_ are less constant
-elements than the neurapophyses.” And again--“Amongst air-breathing
-vertebrates the _pleurapophyses_ of the trunk segments are present
-only in those species in which the septum of the heart’s ventricle is
-complete and imperforate, and here they are exogenous and confined
-to the cervical and anterior thoracic vertebræ.” And once more, both
-the _neurapophyses_ and the _neural spine_ “are absent under both
-histological conditions, at the end of the tail in most air-breathing
-vertebrates, where the segments are reduced to their central elements.”
-That is to say, of all the peripheral elements of the “ideal typical
-vertebra,” there is not one which is always present. It will be
-expected, however, that at any rate the _centrum_ is constant: the bone
-which “forms the axis of the vertebral column, and commonly the central
-bond of union of the peripheral elements of the vertebrate (p. 97), is
-of course an invariable element. No: not even this is essential.
-
-   “The centrums do not pass beyond the primitive stage of the
-   notochord (undivided column) in the existing lepidosiren, and
-   they retained the like rudimental state in every fish whose
-   remains have been found in strata earlier than the permian
-   æra in Geology, though the number of vertebræ is frequently
-   indicated in Devonian and Silurian ichthyolites by the
-   fossilized neur-and hæmapophyses and their spines” (p. 96).
-
-Indeed, Professor Owen himself remarks that “the neurapophyses are
-more constant as osseous or cartilaginous elements of the vertebræ
-than the centrums” (p. 97). Thus, then, it appears that the several
-elements included in the “ideal typical vertebra” have various degrees
-of constancy, and that no one of them is essential. There is no one
-part of a vertebra which invariably answers to its exemplar in the
-pattern-group. How does this fact consist with the hypothesis? If
-the Creator saw fit to make the vertebrate skeleton out of a series
-of segments, all formed on essentially the same model--if, for the
-maintenance of the type, one of these bony segments is in many cases
-formed out of a coalesced group of pieces, where, as Professor Owen
-argues, a single piece would have served as well or better; then we
-ought to find this typical repetition of parts uniformly manifested.
-Without any change of shape, it would obviously have been quite
-possible for every actual vertebra to have contained all the parts
-of the ideal one--rudimentally where they were not wanted. Even one
-of the terminal bones of a mammal’s tail might have been formed
-out of the nine autogenous pieces, united by suture but admitting
-of identification. As, however, there is no such uniform typical
-repetition of parts, it seems to us that to account for the typical
-repetition which _does_ occur, by supposing the Creator to have fixed
-on a pattern-vertebra, is to ascribe to him the inconsistency of
-forming a plan and then abandoning it.
-
-If, on the other hand, Professor Owen means that the “ideal typical
-vertebra” is a crystalline form in antagonism with “the idea or
-organizing principle;” then we might fairly expect to find it most
-clearly displaying its crystalline character, and its full complement
-of parts, in those places where the organizing principle may be
-presumed to have “subdued” it to the smallest extent. Yet in the
-_Vertebrata_ generally, and even in Professor Owen’s _Archetypus_,
-the vertebræ of the tail, which must be considered as, if anything,
-less under the influence of the organizing principle than those
-of the trunk, do not manifest the ideal form more completely. On
-the contrary, as we approach the end of the tail, the successive
-segments not only lose their remaining typical elements, but become as
-uncrystalline-looking as can be conceived.
-
-       *       *       *       *       *
-
-Supposing, however, that the assumption of suppressed or undeveloped
-elements be granted--supposing it to be consistent with the hypothesis
-of an “ideal typical vertebra,” that the constituent parts may
-severally be absent in greater or less number, sometimes leaving only
-a single bone to represent them all; may it not be that such parts
-as _are_ present, show their respective typical natures by some
-constant character: say their mode of ossification?
-
-To this question some parts of the _Archetype and Homologies_ seem
-to reply, “Yes;” while others clearly answer, “No.” Criticising the
-opinions of Geoffrey St. Hilaire and Cuvier, who agreed in thinking
-that ossification from a separate centre was the test of a separate
-bone, and that thus there were as many elementary bones in the skeleton
-as there were centres of ossification, Professor Owen points out that,
-according to this test, the human femur, which is ossified from four
-centres, must be regarded as four bones; while the femur in birds and
-reptiles, which is ossified from a single centre, must be regarded as
-a single bone. Yet, on the other hand, he attaches weight to the fact
-that the skull of the human fœtus presents “the same ossific centres”
-as do those of the embryo kangaroo and the young bird. (_Nature of
-Limbs_, p. 40.) And at p. 104 of the _Homologies_, after giving
-a number of instances, he says--
-
-   “These and the like correspondences between the points of
-   ossification of the human fœtal skeleton, and the separate
-   bones of the adult skeletons of inferior animals, are pregnant
-   with interest, and rank among the most striking illustrations of
-   unity of plan in the vertebrate organization.”
-
-It is true that on the following page he seeks to explain this seeming
-contradiction by distinguishing
-
-   “between those centres of ossification that have homological
-   relations, and those that have teleological ones--_i.e._,
-   between the separate points of ossification of a human bone
-   which typify vertebral elements, often permanently distinct
-   bones in the lower animals; and the separate points which,
-   without such signification, facilitate the progress of
-   osteogeny, and have for their obvious final cause the well-being
-   of the growing animal.”
-
-But if there are thus centres of ossification which have homological
-meanings, and others which have not, there arises the question--How are
-they always to be distinguished? Evidently independent ossification
-ceases to be a homological test, if there are independent ossifications
-that have nothing to do with the homologies. And this becomes the
-more evident when we learn that there are cases where neither a
-homological nor a teleological meaning can be given. Among various
-modes of ossification of the centrum, Professor Owen points out that
-“the body of the human atlas is sometimes ossified from two, rarely
-from three, distinct centres placed side by side” (p. 89); while at
-p. 87 he says:--“In osseous fishes I find that the centrum is usually
-ossified from six points.” It is clear that this mode of ossification
-has here no homological signification; and it would be difficult to
-give any teleological reason why the small centrum of a fish should
-have more centres of ossification than the large centrum of a mammal.
-The truth is, that as a criterion of the identity or individuality of
-a bone, mode of ossification is quite untrustworthy. Though, in his
-“ideal typical vertebra,” Professor Owen delineates and classifies
-as separate “autogenous” elements, those parts which are “usually
-developed from distinct and independent centres;” and though by
-doing so he erects this characteristic into some sort of criterion;
-yet his own facts show it to be no criterion. The parapophyses are
-classed among the autogenous elements; yet they are autogenous in
-fishes alone, and in these only in the trunk vertebræ, while in all
-air-breathing vertebrates they are, when present at all, exogenous.
-The neurapophyses, again, “lose their primitive individuality by
-various kinds and degrees of confluence:” in the tails of the higher
-_Vertebrata_ they, in common with the neural spine, become exogenous.
-Nay, even the centrum may lose its autogenous character. Describing
-how, in some batrachians, “the ossification of the centrum is completed
-by an extension of bone from the bases of the neurapophyses, which
-effects also the coalescence of these with the centrum,” Professor Owen
-adds:--“In _Pelobates fuscus_ and _Pelobates cultripes_, Müller found
-the entire centrum ossified from this source, without any independent
-points of ossification” (p. 88). That is to say, the centrum is in
-these cases an exogenous process of the neurapophyses. We see, then,
-that these so-called typical elements of vertebræ have no constant
-developmental character by which they can be identified. Not only
-are they undistinguishable by any specific test from other bones not
-included as vertebral elements; not only do they fail to show their
-typical characters by their constant presence; but, when present, they
-exhibit no persistent marks of individuality. The central element may
-be ossified from six, four, three, or two points; or it may have no
-separate point of ossification at all: and similarly with various of
-the peripheral elements. The whole group of bones forming the “ideal
-typical vertebra” may severally have their one or more ossific centres;
-or they may, as in a mammal’s tail, lose their individualities in a
-single bone ossified from one or two points.
-
-       *       *       *       *       *
-
-Another fact which seems very difficult to reconcile with the
-hypothesis of an “ideal typical vertebra,” is the not infrequent
-presence of some of the typical elements in duplicate. Not only, as
-we have seen, may they severally be absent, but they may severally be
-present in greater number than they should be. When we see, in the
-ideal diagram, one centrum, two neurapophyses, two pleurapophyses,
-two hæmapophyses, one neural spine, and one hæmal spine, we naturally
-expect to find them always bearing to each other these numerical
-relations. Though we may not be greatly surprised by the absence
-of some of them, we are hardly prepared to find others multiplied.
-Yet such cases are common. Thus the neural spine “is double in the
-anterior vertebræ of some fishes” (p. 98). Again, in the abdominal
-region of extinct saurians, and in crocodiles, “the freely-suspended
-hæmapophyses are compounded of two or more overlapping bony pieces”
-(p. 100). Yet again, at p. 99, we read--“I have observed some of the
-expanded pleurapophyses in the great _Testudo elephantopus_ ossified
-from two centres, and the resulting divisions continuing distinct, but
-united by suture.” Once more “the neurapophyses, which do not advance
-beyond the cartilaginous stage in the sturgeon, consist in that fish
-of two distinct pieces of cartilage; and the anterior pleurapophyses
-also consist of two or more cartilages, set end on end” (p. 91). And
-elsewhere referring to this structure, he says:--
-
-   “Vegetative repetition of perivertebral parts not only manifests
-   itself in the composite neurapophyses and pleurapophyses, but
-   in a small accessory (interneural) cartilage, at the fore and
-   back part of the base of the neurapophysis; and by a similar
-   (interhæmal) one at the fore and back part of most of the
-   parapophyses” (p. 87).
-
-Thus the neural and hæmal spines, the neurapophyses, the
-pleurapophyses, the hæmapophyses, may severally consist of two or more
-pieces. This is not all: the like is true even of the centrums.
-
-   “In _Heptanchus_ (_Squalus cinereus_) the vertebral centres are
-   feebly and vegetatively marked out by numerous slender rings
-   of hard cartilage in the notochordal capsule, the number of
-   vertebræ being more definitely indicated by the neurapophyses
-   and parapophyses.... In the piked dog-fish (_Acanthias_) and the
-   spotted dog-fish (_Scyllium_) the vertebral centres coincide in
-   number with the neural arches” (p. 87).
-
-Is it not strange that the pattern-vertebra should be so little adhered
-to, that each of its single typical pieces may be transformed into two
-or three?
-
-But there are still more startling departures from the alleged type.
-The numerical relations of the elements vary not only in this way, but
-in the opposite way. A given part may be present not only in greater
-number than it should be, but also in less. In the tails of homocercal
-fishes, the centrums “are rendered by centripetal shortening and bony
-confluence fewer in number than the persistent, neural, and hæmal
-arches of that part”--that is, there is only a fraction of a centrum to
-each vertebra. Nay, even this is not the most heteroclite structure.
-Paradoxical as it may seem, there are cases in which the same vertebral
-element is, considered under different aspects, at once in excess and
-defect. Speaking of the hæmal spine, Professor Owen says:--
-
-   “The horizontal extension of this vertebral element is
-   sometimes accompanied by a median division, or in other
-   words, it is ossified from two lateral centres; this is seen
-   in the development of parts of the human sternum; the same
-   vegetative character is constant in the broader thoracic
-   hæmal spines of birds; though, sometimes, as _e.g._, in the
-   struthionidæ, _ossification extends from the same lateral centre
-   lengthwise--i.e., forwards and backwards, calcifying the connate
-   cartilaginous homologues of halves of four or five hæmal spines,
-   before these finally coalesce with their fellows at the median
-   line_” (p. 101).
-
-So that the sternum of the ostrich, which according to the hypothesis,
-should, in its cartilaginous stage, have consisted of _four or five
-transverse_ pieces, answering to the vertebral segments, and should
-have been ossified from four or five centres, one to each cartilaginous
-piece, shows not a trace of this structure; but instead, consists
-of _two longitudinal_ pieces of cartilage, each ossified from one
-centre, and finally coalescing on the median line. These four or five
-hæmal spines have at the same time doubled their individualities
-transversely, and entirely lost them longitudinally!
-
-       *       *       *       *       *
-
-There still remains to be considered the test of relative position.
-It might be held that, spite of all the foregoing anomalies, if the
-typical parts of the vertebræ always stood towards each other in the
-same relations--always preserved the same connexions, something like a
-case would be made out. Doubtless, relative position is an important
-point; and it is one on which Professor Owen manifestly places great
-dependence. In his discussion of “moot cases of special homology,” it
-is the general test to which he appeals. The typical natures of the
-alisphenoid, the mastoid, the orbito-sphenoid, the prefrontal, the
-malar, the squamosal, &c. he determines almost wholly by reference to
-the adjacent nerve-perforations and the articulations with neighbouring
-bones (see pp. 19 to 72): the general form of the argument being--This
-bone is to be classed as such or such, _because_ it is connected
-thus and thus with these others, which are so and so. Moreover, by
-putting forth an “ideal typical vertebra,” consisting of a number of
-elements standing towards each other in certain definite arrangement,
-this persistency of relative position is manifestly alleged. The
-essential attribute of this group of bones, considered as a typical
-group, is the constancy in the connexions of its parts: change the
-connexions, and the type is changed. But the constancy of relative
-position thus tacitly asserted, and appealed to as a conclusive test
-in “moot cases of special homology,” is clearly negatived by Professor
-Owen’s own facts. For instance, in the “ideal typical vertebra,” the
-hæmal arch is represented as formed by the two hæmapophyses and the
-hæmal spine; but at p. 91 we are told that
-
-   “The contracted hæmal arch in the caudal region of the body may
-   be formed by different elements of the typical vertebra: _e.g._,
-   by the parapophyses (fishes generally); by the pleurapophyses
-   (lepidosiren); by both parapophyses and pleurapophyses (_Sudis_,
-   _Lepidosteus_), and by hæmapophyses, shortened and directly
-   articulated with the centrums (reptiles and mammals).”
-
-And further, in the thorax of reptiles, birds, and mammals, “the
-hæmapophyses are removed from the centrum, and are articulated to
-the distal ends of the pleurapophyses; the bony hoop being completed
-by the intercalation of the hæmal spine” (p. 82). So that there are
-_five_ different ways in which the hæmal arch may be formed--four
-modes of attachment of the parts different from that shown in the
-typical diagram! Nor is this all. The pleurapophyses “may be quite
-detached from their proper segment, and suspended to the hæmal arch of
-another vertebra;” as we have already seen, the entire hæmal arch may
-be detached and removed to a distance, sometimes reaching the length
-of twenty-seven vertebræ; and, even more remarkable, the ventral fins
-of some fishes, which theoretically belong to the pelvic arch, are so
-much advanced forward as to be articulated to the scapular arch--“the
-ischium elongating to join the coracoid.” With these admissions it
-seems to us that relative position and connexions cannot be appealed to
-as tests of homology, nor as evidence of any original type of vertebra.
-
-In no class of facts, then, do we find a good foundation for the
-hypothesis of an “ideal typical vertebra.” There is no one conceivable
-attribute of this archetypal form which is habitually realised by
-actual vertebræ. The alleged group of true vertebral elements is not
-distinguished in any specified way from bones not included in it. Its
-members have various degrees of inconstancy; are rarely all present
-together; and no one of them is essential. They are severally developed
-in no uniform way: each of them may arise either out of a separate
-piece of cartilage, or out of a piece continuous with that of some
-other element; and each may be ossified from many independent points,
-from one, or from none. Not only may their respective individualities
-be lost by absence, or by confluence with others; but they may be
-doubled, or tripled, or halved, or may be multiplied in one direction
-and lost in another. The entire group of typical elements may coalesce
-into one simple bone representing the whole vertebra; and even, as in
-the terminal piece of a bird’s tail, half-a-dozen vertebræ, with all
-their many elements, may become entirely lost in a single mass. Lastly,
-the respective elements, when present, have no fixity of relative
-position: sundry of them are found articulated to various others than
-those with which they are typically connected; they are frequently
-displaced and attached to neighbouring vertebræ; and they are even
-removed to quite remote parts of the skeleton. It seems to us that if
-this want of congruity with the facts does not disprove the hypothesis,
-no such hypothesis admits of disproof.
-
-       *       *       *       *       *
-
-Unsatisfactory as is the evidence in the case of the trunk and tail
-vertebræ, to which we have hitherto confined ourselves, it is far worse
-in the case of the alleged cranial vertebræ. The mere fact that those
-who have contended for the vertebrate structure of the skull, have
-differed so astonishingly in their special interpretations of it, is
-enough to warrant great doubt as to the general truth of their theory.
-From Professor Owen’s history of the doctrine of general homology,
-we gather that Duméril wrote upon “la tête considérée comme _une_
-vertèbre;” that Kielmeyer, “instead of calling the skull a vertebra,
-said each vertebra might be called a skull;” that Oken recognized in
-the skull _three_ vertebræ and a rudiment; that Professor Owen himself
-makes out _four_ vertebræ; that Goethe’s idea, adopted and developed
-by Carus, was, that the skull is composed of _six_ vertebræ; and that
-Geoffrey St. Hilaire divided it into _seven_. Does not the fact that
-different comparative anatomists have arranged the same group of bones
-into _one_, _three_, _four_, _six_, and _seven_ vertebral segments,
-show that the mode of determination is arbitrary, and the conclusions
-arrived at fanciful? May we not properly entertain great doubts as to
-any one scheme being more valid than the others? And if out of these
-conflicting schemes we are asked to accept one, ought we not to accept
-it only on the production of some thoroughly conclusive proof--some
-rigorous test showing irrefragably that the others must be wrong and
-this alone right? Evidently where such contradictory opinions have been
-formed by so many competent judges, we ought, before deciding in favour
-of one of them, to have a clearness of demonstration much exceeding
-that required in any ordinary case. Let us see whether Professor Owen
-supplies us with any such clearness of demonstration.
-
-To bring the first or occipital segment of the skull into
-correspondence with the “ideal typical vertebra,” Professor Owen
-argues, in the case of the fish, that the parapophyses are _displaced_,
-and wedged between the neurapophyses and the neural spine--removed
-from the hæmal arch and built into the upper part of the neural arch.
-Further, he considers that the pleurapophyses are _teleologically
-compound_. And then, in all the higher vertebrata, he alleges that
-the hæmal arch is _separated_ from its centrum, taken to a distance,
-and transformed into the scapular arch. Add to which, he says that
-in mammals the displaced parapophyses are mere processes of the
-neurapophyses (p. 133): these vertebral elements, typically belonging
-to the lower part of the centrum, and in nearly all cases confluent
-with it, are not only removed to the far ends of elements placed above
-the centrum, but have become exogenous parts of them!
-
-Conformity of the second or parietal segment of the cranium with the
-pattern-vertebra, is produced thus:--The petrosals are _excluded_
-as being partially-ossified sense-capsules, not forming parts of the
-true vertebral system, but belonging to the “splanchno-skeleton.” A
-centrum is _artificially_ obtained by sawing in two the bone which
-serves in common as centrum to this and the preceding segment; and
-this though it is admitted that in fishes, where their individualities
-ought to be best seen, these two hypothetical centrums are not simply
-coalescent, but connate. Next, a similar _arbitrary bisection_ is
-made of certain elements of the hæmal arches. And then, “the principle
-of _vegetative repetition_ is still more manifest in this arch
-than in the occipital one:” each pleurapophysis is double; each
-hæmapophysis is double; and the hæmal spine consists of six pieces!
-
-The interpretation of the third and fourth segments being of the same
-general character, need not be detailed. The only point calling for
-remark being, that in addition to the above various modes of getting
-over anomalies, we find certain bones referred to the _dermo-skeleton_.
-
-Now it seems to us, that even supposing no antagonist interpretations
-had been given, an hypothesis reconcilable with the facts only by
-the aid of so many questionable devices, could not be considered
-satisfactory; and that when, as in this case, various comparative
-anatomists have contended for other interpretations, the character
-of this one is certainly not of a kind to warrant the rejection of
-the others in its favour; but rather of a kind to make us doubt
-the possibility of all such interpretations. The question which
-naturally arises is, whether by proceeding after this fashion,
-groups of bones might not be arranged into endless typical forms.
-If, when a given element was not in its place, we were at liberty
-to consider it as _suppressed_, or _connate_ with some neighbouring
-element, or _removed_ to some more or less distant position;--if,
-on finding a bone in excess, we might consider it, now as part of
-the _dermo-skeleton_, now as part of the _splanchno-skeleton_, now
-as _transplanted_ from its typical position, now as resulting from
-_vegetative repetition_, and now as a bone _teleologically compound_
-(for these last two are intrinsically different, though often used by
-Professor Owen as equivalents);--if, in other cases, a bone might be
-regarded as _spurious_ (p. 91), or again as having _usurped_ the place
-of another;--if, we say, these various liberties were allowed us, we
-should not despair of reconciling the facts with various diagrammatic
-types besides that adopted by Professor Owen.
-
-When, in 1851, we attended a course of Professor Owen’s lectures
-on Comparative Osteology, beginning though we did in the attitude
-of discipleship, our scepticism grew as we listened, and reached
-its climax when we came to the skull; the reduction of which to the
-vertebrate structure, reminded us very much of the interpretation of
-prophecy. The delivery, at the Royal Society, of the Croonian Lecture
-for 1858, in which Professor Huxley, confirming the statements of
-several German anatomists, has shown that the facts of embryology do
-not countenance Professor Owen’s views respecting the formation of the
-cranium, has induced us to reconsider the vertebral theory as a whole.
-Closer examination of Professor Owen’s doctrines, as set forth in his
-works, has certainly not removed the scepticism generated years ago
-by his lectures. On the contrary, that scepticism has deepened into
-disbelief. And we venture to think that the evidence above cited shows
-this disbelief to be warranted.
-
-       *       *       *       *       *
-
-There remains the question--What general views are we to take
-respecting the vertebrate structure? If the hypothesis of an “ideal
-typical vertebra” is not justified by the facts, how are we to
-understand that degree of similarity which vertebræ display?
-
-We believe the explanation is not far to seek. All that our space will
-here allow, is a brief indication of what seems to us the natural view
-of the matter.
-
-Professor Owen, in common with other comparative anatomists, regards
-the divergences of individual vertebræ from the average form, as due
-to adaptive modifications. If here one vertebral element is largely
-developed, while elsewhere it is small--if now the form, now the
-position, now the degree of coalescence, of a given part varies; it
-is that the local requirements have involved this change. The entire
-teaching of comparative osteology implies that differences in the
-conditions of the respective vertebræ necessitate differences in their
-structures.
-
-Now, it seems to us that the first step towards a right conception
-of the phenomena, is to recognize this general law in its converse
-application. If vertebræ are unlike in proportion to the unlikeness
-of their circumstances, then, by implication, they will be like in
-proportion to the likeness of their circumstances. While successive
-segments of the same skeleton, and of different skeletons, are all in
-some respects more or less differently acted on by incident forces,
-and are therefore required to be more or less different; they are all,
-in other respects, similarly acted on by incident forces, and are
-therefore required to be more or less similar. It is impossible to
-deny that if differences in the mechanical functions of the vertebræ
-involve differences in their forms; then, community in their mechanical
-functions, must involve community in their forms. And as we know that
-throughout the _Vertebrata_ generally, and in each vertebrate animal,
-the vertebræ, amid all their varying circumstances, _have_ a certain
-community of function, it follows necessarily that they will have a
-certain general resemblance--there will recur that average shape which
-has suggested the notion of a pattern-vertebra.
-
-A glance at the facts at once shows their harmony with this conclusion.
-In an eel or a snake, where the bodily actions are such as to involve
-great homogeneity in the mechanical conditions of the vertebræ, the
-series of them is comparatively homogeneous. On the contrary, in a
-mammal or a bird, where there is considerable heterogeneity in their
-circumstances, their similarity is no longer so great. And if, instead
-of comparing the vertebral columns of different animals, we compare
-the successive vertebræ of any one animal, we recognize the same law.
-In the segments of an individual spine, where is there the greatest
-divergence from the common mechanical conditions? and where may we
-therefore expect to find the widest departure from the average form?
-Obviously at the two extremities. And accordingly it is at the two
-extremities that the ordinary structure is lost.
-
-Still clearer becomes the truth of this view, when we consider the
-genesis of the vertebral column as displayed throughout the ascending
-grades of the _Vertebrata_. In its first embryonic stage, the spine
-is an undivided column of flexible substance. In the early fishes,
-while some of the peripheral elements of the vertebræ were marked out,
-the central axis was still a continuous unossified cord. And thus we
-have good reason for thinking that in the primitive vertebrate animal,
-as in the existing _Amphioxus_, the notochord was persistent. The
-production of a higher, more powerful, more active creature of the same
-type, by whatever method it is conceived to have taken place, involved
-a change in the notochordal structure. Greater muscular endowments
-presupposed a firmer internal fulcrum --a less yielding central axis.
-On the other hand, for the central axis to have become firmer while
-remaining continuous, would have entailed a stiffness incompatible
-with the creature’s movements. Hence, increasing density of the
-central axis necessarily went hand in hand with its segmentation: for
-strength, ossification was required; for flexibility, division into
-parts. The production of vertebræ resulting thus, there obviously
-would arise among them a general likeness, due to the similarity in
-their mechanical conditions, and more especially the muscular forces
-bearing on them. And then observe, lastly, that where, as in the head,
-the terminal position and the less space for development of muscles,
-entailed smaller lateral bendings, the segmentation would naturally
-be less decided, less regular, and would be lost as we approached the
-front of the head.
-
-But, it may be replied, this hypothesis does not explain all the
-facts. It does not tell us why a bone whose function in a given animal
-requires it to be solid, is formed not of a single piece, but by the
-coalescence of several pieces, which in other creatures are separate;
-it does not account for the frequent manifestations of unity of plan
-in defiance of teleological requirements. This is quite true. But it
-is not true, as Professor Owen argues respecting such cases, that
-“if the principle of special adaptation fails to explain them, and
-we reject the idea that these correspondences are manifestations of
-some archetypal exemplar, on which it has pleased the Creator to frame
-certain of his living creatures, there remains only the alternative
-that the organic atoms have concurred fortuitously to produce such
-harmony.” This is not the only alternative: there is another, which
-Professor Owen has overlooked. It is a perfectly tenable supposition
-that all higher vertebrate forms have arisen by _the superposing of
-adaptations upon adaptations_. Either of the two antagonist cosmogonies
-consists with this supposition. If, on the one hand, we conceive
-species to have resulted from acts of special creation; then it is
-quite a fair assumption that to produce a higher vertebrate animal,
-the Creator did not begin afresh, but took a lower vertebrate animal,
-and so far modified its pre-existing parts as to fit them for the
-new requirements; in which case the original structure would show
-itself through the superposed modifications. If, on the other hand, we
-conceive species to have resulted by gradual differentiations under the
-influence of changed conditions; then, it would manifestly follow that
-the higher, heterogeneous forms, would bear traces of the lower and
-more homogeneous forms from which they were evolved.
-
-Thus, besides finding that the hypothesis of an “ideal typical
-vertebra” is irreconcilable with the facts, we find that the facts are
-interpretable without gratuitous assumptions. The average community of
-form which vertebræ display, is explicable as resulting from natural
-causes. And those typical similarities which are traceable under
-adaptive modifications, must obviously exist if, throughout creation in
-general, there has gone on that continuous superposing of modifications
-upon modifications which goes on in every unfolding organism.
-
-       *       *       *       *       *
-
-[I might with propriety have added to the foregoing criticisms, the
-remark that Professor Owen has indirectly conferred a great benefit by
-the elaborate investigations he has made with the view of establishing
-his hypothesis. He has himself very conclusively proved that the
-teleological interpretation is quite irreconcilable with the facts.
-In gathering together evidence in support of his own conception of
-archetypal forms, he has disclosed adverse evidence which I think shows
-his conception to be untenable. The result is that the field is left
-clear for the hypothesis of Evolution as the only tenable one.]
-
-
-
-
-                              APPENDIX C.
-
-      [_From the_ TRANSACTIONS OF THE LINNEAN SOCIETY, VOL. XXV.]
-
-
-   XV. _On Circulation and the Formation of Wood in Plants. By_
-   HERBERT SPENCER, _Esq. Communicated by_ GEORGE BUSK, _Esq.,
-   F.R.S., Sec. L.S._
-
-                         Read March 1st, 1866.
-
-Opinions respecting the functions of the vascular tissues in plants
-appear to make but little progress towards agreement. The supposition
-that these vessels and strings of partially-united cells, lined with
-spiral, annular, reticulated, or other frameworks, are carriers of the
-plant-juices, is objected to on the ground that they often contain
-air: as the presence of air arrests the movement of blood through
-arteries and veins, its presence in the ducts of stems and petioles
-is assumed to unfit them as channels for sap. On the other hand, that
-these structures have a respiratory office, as some have thought, is
-certainly not more tenable, since, if the presence of air in them
-negatives the belief that their function is to distribute liquid, the
-presence of liquid in them equally negatives the belief that their
-function is to distribute air. Nor can any better defence be made for
-the hypothesis which I find propounded, that these parts serve “to give
-strength to the parenchyma.” Tubes with fenestrated and reticulated
-internal skeletons have, indeed, some power of supporting the tissue
-through which they pass; but tubes lined with spiral threads can yield
-extremely little support, while tubes lined with annuli, or spirals
-alternating with annuli, can yield no support whatever. Though all
-these types of internal framework are more or less efficient for
-preventing closure by lateral pressure, they are some of them quite
-useless for holding up the mass through which the vessels pass; and the
-best of them are for this purpose mechanically inferior to the simple
-cylinder. The same quantity of matter made into a continuous tube would
-be more effective in giving stiffness to the cellular tissue around it.
-
-In the absence of any feasible alternative, the hypothesis that these
-vessels are distributors of sap claims reconsideration. The objections
-are not, I think, so serious as they seem. The habitual presence of
-air in the ducts that traverse wood, can scarcely be held anomalous if
-when the wood is formed their function ceases. The canals which ramify
-through a Stag’s horn, contain air after the Stag’s horn is fully
-developed; but it is not thereby rendered doubtful whether it is the
-function of arteries to convey blood. Again, that air should frequently
-be found even in the vessels of petioles and leaves, will not appear
-remarkable when we call to mind the conditions to which a leaf is
-subject. Evaporation is going on from it. The thinner liquids pass
-by osmose out of the vessels into the tissues containing the liquids
-thickened by evaporation. And as the vessels are thus continually
-drained, a draught is made upon the liquid contained in the stem and
-roots. Suppose that this draught is unusually great, or suppose that
-around the roots there exists no adequate supply of moisture. A state
-of capillary tension must result--a tendency of the liquid to pass into
-the leaves resisted below by liquid cohesion. Now, had the vessels
-impermeable coats, only their upper extremities would under these
-conditions be slowly emptied. But their coats, in common with all the
-surrounding tissues, are permeable by air. Hence, under this state
-of capillary tension, air will enter; and as the upper ends of the
-tubes, being both smaller in diameter and less porous than the lower,
-will retain the liquids with greater tenacity, the air will enter the
-wider and more porous tubes below--the ducts of the stem and branches.
-Thus the entrance of air no more proves that these ducts are not
-sap-carriers, than does the emptiness of tropical river-beds in the dry
-season prove that they are not channels for water. There is, however, a
-difficulty which seems more serious. It is said that air, when present
-in these minute canals, must be a great obstacle to the movement of sap
-through them. The investigations of Jamin have shown that bubbles in a
-capillary tube resist the passage of liquid, and that their resistance
-becomes very great when the bubbles are numerous--reaching, in some
-experiments, as much as three atmospheres. Nevertheless the inference
-that any such resistance is offered by the air-bubbles in the vessels
-of a plant, is, I think, an erroneous one. What happens in a capillary
-tube having impervious sides, with which these experiments were made,
-will by no means happen in a capillary tube having pervious sides.
-Any pressure brought to bear on the column of liquid contained in the
-porous duct of a plant, must quickly cause the expulsion of a contained
-air-bubble through the minute openings in the coats of the duct. The
-greater molecular mobility of gases than liquids, implies that air will
-pass out far more readily than sap. Whilst, therefore, a slight tension
-on the column of sap will cause it to part and the air to enter, a
-slight pressure upon it will force out the air and reunite the divided
-parts of the column.
-
-To obtain data for an opinion on this vexed question, I have lately
-been experimenting on the absorption of dyes by plants. So far as I can
-learn, experiments of this kind have most, if not all of them, been
-made on stems, and, as it would seem from the results, on stems so far
-developed as to contain all their characteristic structures. The first
-experiments I made myself were on such parts, and yielded evidence that
-served but little to elucidate matters. It was only after trying like
-experiments with leaves of different ages and different characters,
-and with undeveloped axes, as well as with axes of special kinds, that
-comprehensible results were reached; and it then became manifest that
-the appearances presented by ordinary stems when thus tested, are in a
-great degree misleading. Let me briefly indicate the differences.
-
-If an adult shoot of a tree or shrub be cut off, and have its lower
-end placed in an alumed decoction of logwood or a dilute solution of
-magenta,[69] the dye will, in the course of a few hours, ascend to a
-distance varying according to the rate of evaporation from the leaves.
-On making longitudinal sections of the part traversed by it, the dye is
-found to have penetrated extensive tracts of the woody tissue; and on
-making transverse sections, the openings of the ducts appear as empty
-spaces in the midst of a deeply-coloured prosenchyma. It would thus
-seem that the liquid is carried up the denser parts of the vascular
-bundles; neglecting the cambium layer, neglecting the central pith,
-and neglecting the spiral vessels of the medullary sheath. Apparently
-the substance of the wood has afforded the readiest channel. When,
-however, we examine these appearances critically, we find reasons for
-doubting this conclusion. If a transverse section of the lower part,
-into which the dye passed first and has remained longest, be compared
-with a transverse section of the part which the dye has but just
-reached, a marked difference is visible. In the one case the whole of
-the dense tissue is stained; in the other case it is not. This uneven
-distribution of stain in the part which the dye has incompletely
-permeated is not at random; it admits of definite description. A
-tolerably regular continuous ring of colour distinguishes the outer
-part of the wood from the inner mass, implying a passage of liquid
-up the elongated cells next the cambium layer. And the inner mass is
-coloured more round the mouths of the pitted ducts than elsewhere: the
-dense tissue is darkest close to the edges of these ducts; the colour
-fades away gradually on receding from their edges; there is most colour
-where there are several ducts together; and the dense tissue which
-is fully dyed for some space, is that which lies between two or more
-ducts. These are indications that while the layer of pitted cells
-next the cambium has served as a channel for part of the liquid, the
-rest has ascended the pitted ducts, and oozed out of these into the
-prosenchyma around. And this conclusion is confirmed by the contrast
-between the appearances of the lowest part of a shoot under different
-conditions. For if, instead of allowing the dye time for oozing through
-the prosenchyma, the end of the shoot be just dipped into the dye and
-taken out again, we find, on making transverse sections of the part
-into which the dye has been rapidly taken up, that, though it has
-diffused to some distance round the ducts, it has left tracts of wood
-between the ducts uncoloured--a difference which would not exist had
-the ascent been through the substance of the wood. Even still stronger
-is the confirmation obtained by using one dye after another. If a shoot
-that has absorbed magenta for an hour be placed for five minutes in the
-logwood decoction, transverse sections of it taken at a short distance
-from its end show the mouths of the ducts surrounded by dark stains in
-the midst of the much wider red stains.
-
-Based on these comparisons only, the inference pointed out has little
-weight; but its weight is increased by the results of experiments on
-quite young shoots, and shoots that develope very little wood. The
-behaviour of these corresponds perfectly with the expectation that a
-liquid will ascend capillary tubes in preference to simple cellular
-tissue or tissue not differentiated into continuous canals. The
-vascular bundles of the medullary sheath are here the only channels
-which the coloured liquid takes. In sections of the parts up to which
-the dye has but just reached, the spiral, fenestrated, scalariform,
-or other vessels contained in these bundles are alone coloured, and
-lower down it is only after some hours that such an exudation of dye
-takes place as suffices partially to colour the other substances of the
-bundle. Further, it is to be noted that at the terminations of shoots,
-where the vessels are but incompletely formed out of irregularly-joined
-fibrous cells which still retain their original shapes, the dye runs up
-the incipient vessels and does not colour in the smallest degree the
-surrounding tissue.
-
-Experiments with leaves bring out parallel facts. On placing in a dye
-a petiole of an adult leaf of a tree, and putting it before the fire
-to accelerate evaporation, the dye will be found to ascend the mid-rib
-and veins at various rates, up even to a foot per hour. At first it
-is confined to the vessels; but by the time it has reached the point
-of the leaf, it will commonly be seen that at the lower part it has
-diffused itself into the sheaths of the vessels. In a quite young leaf
-from the same shoot, we find a much more rigorous restriction of the
-dye to the vessels. On making oblique sections of its petiole, mid-rib,
-and veins, the vessels have the appearance of groups of sharply
-defined coloured rods imbedded in the green prosenchyma; and this
-marked contrast continues with scarcely an appreciable change after
-plenty of time has been allowed for exudation.
-
-The facts thus grouped and thus contrasted seem, at first sight, to
-imply that while they are young the coats of these ramifying canals
-lined with spiral or allied structures are not readily permeable,
-but that, becoming porous as they grow old, they allow the liquids
-they carry to escape with increasing facility; and hence a possible
-interpretation of the fact that, in the older parts, the staining of
-the tissue around the vessels is so rapid as to suggest that the dye
-has ascended directly through this tissue, whereas in the younger
-parts the reverse appearance necessitates the reverse conclusion. But
-now, is this difference determined by difference of age, or is it
-otherwise determined? The evidence as presented in ordinary stems and
-leaves shows us that the parts of the vascular system at which there
-is a rapid escape of dye are not simply older parts, but are parts
-where a deposit of woody matter is taking place. Is it, then, that
-the increasing permeability of the ducts, instead of being directly
-associated with their increasing age, is directly associated with the
-increasing deposit of dense substance around them?
-
-To get proof that this last connexion is the true one, we have but to
-take a class of cases in which wood is formed only to a small extent.
-In such cases experiments show us a far more general and continued
-limitation of the dye to the vessels. Ordinary herbs and vegetables,
-when contrasted with shrubs and trees, illustrate this; as instance
-the petioles of Celery, or of the common Dock, and the leaves of
-Cabbages or Turnips. And then in very succulent plants, such as
-_Bryophyllum calycinum_, _Kalanchoë rotundifolia_, the various species
-of _Crassula_, _Cotyledon_, _Kleinia_, and others of like habit, the
-ducts of old and young leaves alike retain the dye very persistently:
-the concomitant in these cases being the small amount of prosenchyma
-around the ducts, or the small amount of deposit in it, or both. More
-conclusive yet is the evidence which meets us when we turn from very
-succulent leaves to very succulent axes. The tender young shoots of
-_Kleinia ante-euphorbium_, or _Euphorbia Mauritanica_, which for many
-inches of their lengths have scarcely any ligneous fibres, show us
-scarcely any escape of the coloured liquid from the vessels of the
-medullary sheath. So, too, is it with _Stapelia Buffonia_, a plant of
-another order, having soft swollen axes. And then we have a repetition
-of the like connexion of facts throughout the _Cactaceæ_: the most
-succulent showing us the smallest permeability of the vessels. In
-two species of _Rhipsalis_, in two species of _Cereus_, and in two
-species of _Mammillaria_, which I have tried, I have found this so.
-_Mammillaria gracilis_ may be named as exemplifying the relation under
-its extreme form. Into one of these small spheroidal masses, the dye
-ascends through the large bundles of spiral or annular ducts, or cells
-partially united into such ducts, colouring them deeply, and leaving
-the feebly-marked sheath of prosenchyma, together with the surrounding
-watery cellular tissue, perfectly uncoloured.
-
-The most conclusive evidence, however, is furnished by those _Cactaceæ_
-in which the transition from succulent to dense tissue takes place
-variably, according as local circumstances determine. _Opuntia_ yields
-good examples. If a piece of it including one of the joints at which
-wood is beginning to form, be allowed to absorb a coloured liquid,
-the liquid, running up the irregular bundles of vessels and into many
-of their minute ramifications, is restricted to these where they pass
-through the parenchyma forming the mass of the stem; but near the
-joints the hardened tissue around the vessels is coloured. In one of
-these fleshy growths we get clear evidence that the escape of the
-dye has no immediate dependence on the age of the vessels, since, in
-parts of the stem that are alike in age, some of the vessels retain
-their contents while others do not. Nay, we even find that the younger
-vessels are more pervious than the older ones, if round the younger
-ones there is a formation of wood.
-
-Thus, then, is confirmed the inference before drawn, that in ordinary
-stems the staining of the wood by an ascending coloured liquid is due,
-not to the passage of the coloured liquid up the substance of the wood,
-but to the permeability of its ducts and such of its pitted cells as
-are united into irregular canals. And the facts showing this, at the
-same time indicate with tolerable clearness the process by which wood
-is formed. What in these cases is seen to take place with a dye, may
-be fairly presumed to take place with sap. Where the dye exudes but
-slowly, we may infer that the sap exudes but slowly; and it is a fair
-inference that where the dye leaks rapidly out of the vessels, the sap
-does the same. Inferring, thus, that where-ever there is a considerable
-formation of wood there is a considerable escape of the sap, we see
-in the one the result of the other. The thickening of the prosenchyma
-is proportionate to the quantity of nutritive liquid passing into it;
-and this nutritive liquid passes into it from the vessels, ducts, and
-irregular canals it surrounds.
-
-But an objection is made to such experiments as the foregoing, and to
-all the inferences drawn from them. It is said that portions of plants
-cut off and thus treated, have their physiological actions arrested,
-or so changed as may render the results misleading; and it is said
-that when detached shoots and leaves have their cut ends placed in
-solutions, the open mouths of their vessels and ducts are directly
-presented with the liquids to be absorbed, which does not happen in
-their natural states. Further, making these objections look serious, it
-is alleged that when solutions are absorbed through the roots, quite
-different results are obtained: the absorbed matters are found in the
-tissues and not in the vessels. Clearly, were the experiments yielding
-these adverse results conducted in unobjectionable ways, the conclusion
-implied by them would negative the conclusions above drawn. But these
-experiments are no less objectionable than those to which they are
-opposed. Such mineral matters as salts of iron, solutions of which
-have in some cases been supplied to the roots for their absorption,
-are obviously so unlike the matters ordinarily absorbed, that they
-are likely to interfere fatally with the physiological actions. If
-experiments of this kind are made by immersing the roots in a dye,
-there is, besides the difficulty that the mineral mordant contained
-by the dye is injurious to the plant, the further difficulty that the
-colouring matter, being seized by the substances for which it has an
-affinity, is left behind in the first layers of root-tissues passed
-through, and that the decolorized water passing up into the plant is
-not traceable. To be conclusive, then, an experiment on absorption
-through roots must be made with some solution which will not seriously
-interfere with the plant’s vital processes, and which will not have
-its distinctive element left behind. To fulfil these requirements I
-adopted the following method. Having imbedded a well-soaked broad-bean
-in moist sand, contained in an inverted cone of cardboard with its
-apex cut off for the radicle to come through--having placed this in
-a wide-mouthed dwarf bottle, partly filled with water, so that the
-protruding radicle dipped into the water--and having waited until the
-young bean had a shoot some three or more inches high, and a cluster
-of secondary rootlets from an inch to an inch and a-half long--I
-supplied for its absorption a simple decoction of logwood, which,
-being a vegetal matter, was not likely to do it much harm, and which,
-being without a mordant, would not leave its suspended colour in the
-first tissues passed through. To avoid any possible injury, I did not
-remove the plant from the bottle, but slightly raising the cone out of
-its neck, I poured away the water through the crevice and then poured
-in the logwood decoction; so that there could have been no broken end
-or abraded surface of a rootlet through which the decoction might
-enter. Being prepared with some chloride of tin as a mordant, I cut
-off, after some three hours, one of the lowest leaves, expecting that
-the application of the mordant to the cut surface would bring out
-the characteristic colour if the logwood decoction had risen to that
-height. I got no reaction, however. But after eight hours I found, on
-cutting off another leaf, that the vessels of its petiole were made
-visible as dark streaks by the colour with which they were charged--a
-colour differing, as was to be expected, from that of the logwood
-decoction, which spontaneously changes even by simple exposure. It was
-then too late in the day to pursue the observations; but next morning
-the vessels of the whole plant, as far as the petioles of its highest
-unfolded leaves, were full of the colouring matter; and on applying
-chloride of tin to the cut surfaces, the vessels assumed that purplish
-red which this mordant produces when directly mixed with the logwood
-decoction. Subsequently, when one of the cotyledons was cut open
-by Prof. Oliver, to whom, in company with Dr. Hooker, I showed the
-specimen, we found that the whole of its vascular system was filled
-with the decoction, which everywhere gave the characteristic reaction.
-And it became manifest that the liquid absorbed through the rootlets,
-in the central vessels of which it was similarly traceable, had part
-of it passed directly up the vessels of the axis, while part of it
-had passed through other vessels into the cotyledon, out of which, no
-doubt, the liquid ordinarily so carried returns charged with a supply
-of the stored nutriment. I have since obtained a verification by
-varying the method. Digging up some young plants (Marigolds happened to
-afford the best choice) with large masses of soil round them, placing
-them in water, so as gradually to detach the soil without injuring the
-rootlets, planting them afresh in a flower-pot full of washed sand,
-and then, after a few days, watering them with a logwood decoction, I
-found, as before, that in less than twenty-four hours the colouring
-matter had run up into the vessels of the leaves. Though the reaction
-produced by the mordant was not so strong as before, it was marked
-enough to be quite unquestionable.
-
-As these experiments were so conducted that there was no access to
-the vessels except through the natural channels, and as the vital
-actions of the plants were so little interfered with that at the end
-of twenty-four hours they showed no traces of disturbance, I think the
-results must be held conclusive.
-
-Taking it, then, as a fact that in plants possessing them the vessels
-and ducts are the channels through which sap is distributed, we come
-now to the further question--What determines the varying permeability
-of the walls of the vessels and ducts, and the consequent varying
-formation of wood? To this question I believe the true reply is--The
-exposure of the parts to intermittent mechanical strains, actual or
-potential, or both. By actual strains I of course mean those which the
-plant experiences in the course of its individual life. By potential
-strains I mean those which the form, attitude, and circumstances
-common to its kind involve, and which its inherited structure is
-adapted to meet. In plants with stems, petioles, and leaves, having
-tolerably constant attitudes, the increasing porosity of the tubes and
-consequent deposit of dense tissue takes place in anticipation of the
-strains to which the parts of the individual are liable, but takes
-place at parts which have been habitually subject to such strains
-in ancestral individuals. But though in such plants the tendency to
-repeat that distribution of dense tissue caused by mechanical actions
-on past generations, goes on irrespective of the mechanical actions
-to which the developing individual is subject, these direct actions,
-while they greatly aid the assumption of the typical structure, are
-the sole causes of those deviations in the relative thickenings of
-parts which distinguish the individual from others of its kind. And
-then, in certain irregularly growing plants, such as Cactuses and
-Euphorbias, where the strains fall on parts that do not correspond in
-successive individuals, we distinctly trace a direct relation between
-the degrees of strain and the rates of these changes which result in
-dense tissue. I will not occupy space in detailing the evidence of this
-relation, which is conspicuous in the orders named, but will pass to
-the question--What are the physical processes by which intermittent
-mechanical strains produce this deposit of resistant substance at
-places where it is needed to meet the strains? We have not to seek far
-for an answer. If a trunk, a bough, a shoot, or a petiole, is bent
-by a gust of wind, the substance of its convex side is subject to
-longitudinal tension: the substance of its concave side being at the
-same time compressed. This is the primary mechanical effect. There is,
-however, a secondary mechanical effect, which here chiefly concerns
-us. That bend by which the tissues of the convex side are stretched,
-also produces lateral compression of them. Buttoning on a tight
-glove and then closing the hand, will make this necessity clear: the
-leather, while it is strained along the backs of the fingers, presses
-with considerable force on the knuckles. It is demonstrable that the
-tensions of the outer layer of a mass made convex by bending, must,
-by composition of forces, produce at every point a resultant at right
-angles to the layer beneath it; that, similarly, the joint tensions
-of these two layers must throw a pressure on the next deeper layer;
-and so on. Hence, if at some little distance beneath the surface of a
-stem, twig, or leaf-stalk, there exist longitudinal tubes, these tubes
-must be squeezed each time the side of the branch they are placed on
-becomes convex. Modifying the illustration just drawn from the clenched
-hand will make this clear. When, on forcibly grasping something, the
-skin is drawn tightly over the back of the hand, the whitening of
-the knuckles shows how the blood is expelled from the vessels below
-the surface by the pressure of the tightened skin. If, then, the
-sap-vessels must be thus compressed, what will happen to the liquid
-they contain? It will move away along the lines of least resistance.
-Part, and probably the greater part, will escape lengthways from the
-place of greatest pressure: some of it being expelled downwards, and
-some of it upwards. But, at the same time, part of it will be likely
-to ooze through the walls of the tubes. If these walls are so perfect
-as to permit the passage of liquid only by osmose, it may still be
-inferred that the osmose will increase under pressure; and probably,
-under recurrent pressure, the places at which the osmotic current
-passes most readily will become more and more permeable, until they
-eventually form pores. At any rate it is manifest that where pores
-and slits exist, whether thus formed or formed in any other way, the
-escape of sap into the adjacent tissue at each bend will become easy
-and rapid. What further must happen? When the branch or shoot recoils,
-the vessels on the side that was convex, being relieved from pressure,
-will tend to resume their previous diameters; and will be helped to do
-this by the elasticity of the surrounding tissue, as well as by those
-spiral, annular, and allied structures which they contain. But this
-resumption of their previous diameters must cause an immediate rush
-of sap back into them. Whence will it come? Not to any considerable
-extent from the surrounding tissues into which part of it has been
-squeezed, seeing that the resistance to the return of liquid through
-small pores will be greater than the resistance to its return along the
-vessels themselves. Manifestly the sap which was thrust up and down
-the vessels from the place of compression will return--the quantities
-returning from above and from below varying, as we shall hereafter
-see, according to circumstances. But this is not all. From some side a
-greater quantity must come back than was sent away; for the amount that
-has escaped out of the tube into the prosenchyma has to be replaced.
-Thus during the time when the side of the branch or twig becomes
-concave, more sap returns from above or below than was expelled upwards
-or downwards during the previous compression. The refilled vessels,
-when the next bend renders their side convex, again have part of their
-contents forced through their parietes, and are again refilled in the
-same way. There is thus set up a draught of sap to the place where
-these intermittent strains are going on, an exudation proportionate
-to the frequency and intensity of the strains, and a proportionate
-nutrition or thickening of the wood-cells, fitting them to resist the
-strains. A rude idea of this action may be obtained by grasping in one
-hand a damp sponge, having its lower end in water, while holding a
-piece of blotting-paper in contact with its upper end, and then giving
-the sponge repeated squeezes. At each squeeze some of the water will
-be sent into the blotting-paper; at each relaxation the sponge will
-refill from below, to give another portion of its contents to the
-blotting-paper when again squeezed.
-
-But how does this explanation apply to roots? If the formation of
-wood is due to intermittent transverse strains, such as are produced
-in the aërial parts of upright plants by the wind, how does it happen
-that woody matter is deposited in roots, where there are no lateral
-oscillations, no transverse strains? The answer is, that longitudinal
-strains also are capable of causing the effects described. It is
-true that perfectly straight fibres united into a bundle and pulled
-lengthways would not exert on one another any lateral pressure, and
-would not laterally compress any similarly-straight canals running
-along with them. But if the fibres united into a bundle are variously
-bent or twisted, they cannot be longitudinally strained without
-compressing one another and structures imbedded in them. It needs but
-to watch a wet rope drawn tight by a capstan, to see that an action
-like that which squeezes the water out of its strands, will squeeze the
-sap out of the vessels of a root into the surrounding tissue, as often
-as the root is pulled by the swaying of the plant it belongs to. Here,
-too, as before, the vessels will refill when the pull intermits; and
-so, in the roots as in the branches, this rude pumping process will
-produce a growth of hard tissue proportionate to the stress to be borne.
-
-These conclusions are supported by the evidence which exceptional
-cases supply. If intermittent mechanical strains thus cause the
-formation of wood where wood is found, then where it is not found,
-there should be an absence of intermittent mechanical strains. There
-is such an absence. Vascular plants characterized by little or no
-deposit of dense substance, are those having vessels so conditioned
-that no considerable pressures are borne by them. The more succulent a
-petiole or leaf becomes, the more do the effects of transverse strains
-fall on its outer layers of cells. Its mechanical support is chiefly
-derived from the ability of these minute vesicles, full of liquid, to
-resist bursting and tearing under the compressions and tensions they
-are exposed to. And just as fast as this change from a thin leaf or
-foot-stalk to a thick one entails increasing stress on the superficial
-tissue, so fast does it diminish the stress on the internally-seated
-vascular tissue. The succulent leaf cannot be swayed about by the wind
-as much as an ordinary leaf; and such small bends as can be given to
-it and its foot-stalk are prevented from affecting in any considerable
-degree the tubes running through its interior. Hence the retentiveness
-of the vessels in these fleshy leaves, as shown by the small exudation
-of dye; and hence the small thickening of their surrounding prosenchyma
-by woody deposit. Still more conspicuously is this connexion of facts
-shown when, from the soft thick leaves before named and such others as
-those of _Echeveria_, _Rochea_, _Pereskia_, we turn to the thick leaves
-that have strong exo-skeletons. _Gasteria_ serves as an illustration.
-The leathery or horny skin here evidently bears the entire weight of
-the leaf, and is so stiff as to prevent any oscillation. Here, then,
-the vessels running inside are protected from all mechanical stress;
-and accordingly we find that the cells surrounding them are not
-appreciably thickened.
-
-Equally clear, and more striking because more obviously exceptional,
-is the evidence given by succulent stems which are leafless. _Stapelia
-Buffonia_, having soft procumbent axes not liable to be bent backwards
-and forwards in any considerable degree by the wind, has, ramifying
-through its tissue, vessels that allow but an extremely slow escape
-of dye and have unthickened sheaths. Such of the Euphorbias as have
-acquired the fleshy character while retaining the arborescent growth,
-like _Euphorbia Canariensis_, teach us the same truth in another way.
-In them the formation of wood around the vessels is inconspicuous
-where the intermittent strains are but slight; but it is conspicuous
-at those joints on which lateral oscillations of the attached branches
-throw great extensions and compressions of tissue. Throughout
-the _Cactaceæ_ we find varied examples of the alleged relation.
-_Mammillaria_ furnishes a very marked one. The substance of one of
-these globular masses, resting on the ground, admits of no bending from
-side to side; and accordingly its large bundles of spiral and annular
-vessels, or partially-united cells, have very feebly-marked sheaths
-not at all thickened. In such types as _Cereus_ and _Opuntia_ we see,
-as in the Euphorbias, that where little stress falls on the vessels,
-little deposit takes place around them; while there is much deposit
-where there is much stress. Here let me add a confirmation obtained
-since writing the above. After observing among the Cactuses the very
-manifest relation between strain and the formation of wood, I inquired
-of Mr. Croucher, the intelligent foreman of the Cactus-house at Kew,
-whether he found this relation a constant one. He replied that he did,
-and that he had frequently tested it by artificially subjecting parts
-of them to strains. Neglecting at the time to inquire how he had done
-this, it afterwards occurred to me that if he had so done it as to
-cause constant strains, the observed result would not tell in favour of
-the foregoing interpretation. Subsequently, however, I learned that he
-had produced the strains by placing the plants in inclined attitudes--a
-method which, by permitting oscillations of the strained joints,
-allowed the strains to intermit. And then, making the proof conclusive,
-Mr. Croucher volunteered the statement that where he had produced
-constant strains by tying, no formation of wood took place.
-
-Aberrant growths of another class display the same relations of
-phenomena. Take first the underground stems, such as the Potato and
-the Artichoke. The vessels which run through these, slowly take up
-the dye without letting it pass to any considerable extent into the
-surrounding tissues.[70] Only after an interval of many hours does the
-prosenchyma become stained in some places. Here, as before, an absence
-of rapid exudation accompanies an absence of woody deposit; and both
-these go along with the absence of intermittent strains. Take again the
-fleshy roots. The Turnip, the Carrot, and the Beetroot, have vessels
-that retain very persistently the coloured liquids they take up. And
-differing in this, as these roots do, from ordinary roots, we see
-that they also differ from them in not being woody, and in not being
-appreciably subject to the usual mechanical actions. In these cases,
-as in the others, parts that ordinarily become dense, deviate from
-this typical character when they are not exposed to those forces which
-produce dense tissue by increasing the extravasation of sap.
-
-To complete the proof that such a relation exists, let me add the
-results of some experiments on equal and similarly-developed parts,
-kept respectively at rest and in motion. I have tested the effects on
-large petioles, on herbaceous shoots, and on woody shoots. If two such
-petioles as those of Rhubarb, with their leaves attached, have their
-cut ends inserted in bottles of dye, and the one be bent backwards
-and forwards while the other remains motionless, there arises, after
-the lapse of an hour, scarcely any difference in the states of their
-vessels: a certain proportion of these are in both cases charged with
-the dye, and little exudation has been produced by the motion. Here,
-however, it is to be observed that the causes of exudation are scarcely
-operative; the vascular bundles are distributed all through the mass
-of the petiole, which is formed of soft watery tissue; and they are,
-therefore, not so circumstanced as to be effectually compressed by the
-bends. In herbaceous stems, such as those of the Jerusalem Artichoke
-and of the Foxglove, an effect scarcely more decided is produced; and
-here, too, when we seek a reason, we find it in the non-fulfilment of
-the mechanical conditions; for the vascular bundles are not so seated
-between a tough layer of bark and a solid core as to be compressed at
-each bend. When, however, we come to experiment upon woody shoots, we
-meet with conspicuous effects, though by no means uniformly. In some
-cases oscillations produce immense amounts of exudation--parallel
-transverse sections of the compared shoots showing that where, in the
-one that has been at rest, there are spots of colour round but a few
-pitted ducts, in the one that has been kept in motion the substance of
-the wood is soaked almost uniformly through with dye. In other cases,
-especially where there is much undifferentiated tissue remaining, the
-exudation is not very marked. The difference appears to depend on
-the quantity of liquid contained in the shoot. If its substance is
-relatively dry, the exudation is great; but it is comparatively small
-if all the tissues are fully charged with sap. This contrast of results
-is one which contemplation of the mechanical actions will lead us to
-expect.
-
-And now, with these facts to aid our interpretation, let us return to
-ordinary stems. If the upper end of a growing shoot, the prosenchyma
-of which is but little thickened, be allowed to imbibe the dye, the
-vessels of its medullary sheath alone become charged; and from them
-there takes place but a slow oozing. If a like experiment be tried
-with a lower part of the shoot, where the wood in course of formation
-has its inner boundary marked but not its outer boundary, we find
-that the pitted ducts, and more especially the inner ones, come into
-play. And then lower still, where the wood has its periphery defined
-and its histological characters decided, the appearances show that
-the tissue forming its outer surface begins to take a leading part
-in the transmission of liquid. What now is the explanation of these
-changes, mechanically considered? In the young soft part of the shoot,
-as in all normal and abnormal growths that have not formed wood, the
-channels for the passage of sap are the spiral, annular, fenestrated,
-or reticulated vessels. These vessels, here included in the bundles
-of the medullary sheath, are, in common with the tissues around
-them, subject, by the bendings of the shoot, to slight intermittent
-compressions, and, especially the outermost of them, are thus forced
-to give the prosenchyma an extra supply of nutritive liquid. The
-thickening of the prosenchyma, spreading laterally as well as outwards
-from each bundle of the medullary sheath, goes on until it meets the
-thickenings that spread from the other bundles; and there is so formed
-an irregular cylinder of hardened tissue, surrounding the medulla and
-the vascular bundles of its sheath. As soon as this happens, these
-vascular bundles become, to a considerable extent, shielded from the
-effects of transverse strains, since the tensions and compressions
-chiefly fall on the developing wood outside of them. Clearly, too, the
-greatest stress must be felt by the outer layer of the developing wood:
-being further removed from the neutral axis, it must be subject to
-severer strains at each bend; and lying between the bark and the layer
-of wood first formed, it must be most exposed to lateral compressions.
-Among the elongated cells of this outer layer, some unite to form the
-pitted ducts. Being, as we see, better circumstanced mechanically,
-they become greater carriers of sap than the original vessels, and,
-in consequence of this, as well as in consequence of their relative
-proximity, become the sources of nutrition to the still more external
-layers of wood-cells. The same causes and the same effects hold with
-each new indurated coat deposited round the previously indurated coats.
-
-This description may be thought to go far towards justifying the
-current views respecting the course taken by the sap. But the
-justification is more apparent than real. In the first place, the
-implication here is that the sap-carrying function is at first
-discharged entirely by the vessels of the medullary sheath, and
-that they cease to discharge this function only as fast as they are
-relatively incapacitated by their mechanical circumstances. And the
-second implication is, that it is not the wood itself, but the more
-or less continuous canals formed in it, which are the subsequent
-sap-distributors. This, though readily made clear by microscopic
-examination of the large pitted ducts in a partially lignified shoot
-that has absorbed the dye, is less manifestly true of the peripheral
-layer of sap-carrying tissue finally formed. But it is really true
-here. For this layer, though nominally a layer of wood, is practically
-a layer of inosculating vessels. It is formed out of irregular lines
-and networks of elongated pitted cells, obliquely united by their
-ends. Examination of them after absorption of a dye, shows that it is
-only along the continuous channels they unite to form that the current
-has passed. But the essentially vascular character of this outer and
-latest-formed layer of the alburnum is best seen in the fact that
-the vascular systems of new axes take their rise from it, and form
-with it continuous canals. If a shoot of last year in which growth
-is recommencing, be cut lengthways after it has imbibed a dye, clear
-proof is obtained that the passage of the dye into a lateral bud takes
-place from this outermost layer of pitted cells, and that the channels
-taken by the dye through the new tissue are composed of cells that pass
-through modified forms into the spiral vessels of the new medullary
-sheath. This transition may be still more clearly traced in a terminal
-bud that continues the line of last year’s shoot. A longitudinal
-section of this shows that the vessels of the new medullary sheath do
-not obtain their sap from the vessels of last year’s sheath (which, as
-shown by the non-absorption of dye, have become inactive), but that
-their supplies are obtained from those inosculating canals formed out
-of last year’s outermost layer of prosenchyma, and that between the
-component cells of this and those of the new vascular system there are
-all gradations of structure.[71]
-
-It is not the aim of the foregoing reasoning to show that mechanical
-actions are the sole causes of the formation of dense tissue in
-plants. Dense tissue is in many cases formed where no such causes
-have come into play--as, for example, in thorns and in the shells
-of nuts. Here the natural selection of variations can alone have
-operated. It is manifest, too, that even those supporting structures
-the building up of which is above ascribed to intermittent strains,
-may, in the individual plant of a species that ordinarily has them, be
-developed to a great extent when intermittent strains are prevented.
-We see this in trees that are artificially supported by nailing to
-walls; and we also see a kindred fact in natural climbers. Though in
-these cases the formation of wood is obviously less than it would be
-were the stem and branches habitually moved about by the wind, it
-nevertheless goes on. Clearly the tendency of the plant to repeat the
-structure of its type (in the one case the structure of its species;
-and in the other case that of the order from which it has diverged in
-becoming a climber) is here almost the sole cause of wood formation.
-But though in plants so circumstanced intermittent mechanical strains
-have little or no direct share, it may still be true, and I believe
-is true, that intermittent mechanical strains are the original cause;
-for, as before hinted, the typical structure which the individual
-thus repeats irrespective of its own conditions, is interpretable
-as a typical structure that is itself the product of these actions
-and reactions between the plant and its environment. Grant the
-inheritance of functionally-produced modifications; grant that natural
-selection will always co-operate in such way as to favour those
-individuals and families in which functionally-produced modifications
-have progressed most advantageously; and it will follow that this
-mechanically-caused formation of dense substance, accumulating from
-generation to generation by the survival of the fittest, will result
-in an organic habit of forming dense tissue at the required places.
-The deposit arising from exudation at the places of greatest strain,
-recurring from generation to generation at the same places, will come
-to be reproduced in anticipation of strain, and will continue to
-be reproduced for a long time after a changed habit of the species
-prevents the strain--eventually, however, decreasing, both through
-functional inactivity and natural selection, to the point at which it
-is in equilibrium with the requirement.
-
-Another side of the general question may now be considered. We have
-seen how, by intermittent pressures on capillary vessels and ducts
-and inosculating canals, there must be produced a draught of sap
-towards the point of compression to replace the sap squeezed out. But
-we have still to inquire what will be the effect on the distribution
-of sap throughout the plant as a whole. It was concluded that out of
-the compressed vessels the greater part of the liquid would escape
-longitudinally--the longitudinal resistance to movement being least.
-In every case the probabilities are infinity to one against the
-resistances being equal upwards and downwards. Always, then, more sap
-will be expelled in one direction than in the other. But in whichever
-direction least sap is expelled, from that same direction most sap will
-return when the vessels are relieved from pressure--the force which is
-powerful in arresting the back current in that direction being the same
-force which is powerful in producing a forward current. Ordinarily, the
-more abundant supply of liquid being from below, there will result an
-upward current. At each bend a portion of the contents will be squeezed
-out through the sides of the vessels--a portion will be squeezed
-downwards, reversing the current ascending from the roots, but soon
-stopped by its resistance; while a larger portion will be squeezed
-upwards towards the extremities of the vessels, where consumption and
-loss are most rapid. At each recoil the vessels will be replenished,
-chiefly by the repressed upward current; and at the next bend more of
-it will be thrust onwards than backwards. Hence we have everywhere in
-action a kind of rude force-pump, worked by the wind; and we see how
-sap may thus be raised to a height far beyond that to which it could be
-raised by capillary action, aided by osmose and evaporation.
-
-Thus far, however, the argument proceeds on the assumption that there
-is liquid enough to replenish every time the vessels subject to
-this process. But suppose the supply fails--suppose the roots have
-exhausted the surrounding stock of moisture. Evidently the vessels thus
-repeatedly having their contents squeezed out into the surrounding
-tissue, cannot go on refilling themselves from other vessels without
-tending to empty the vascular system. On the one hand, evaporation from
-the leaves causing a draught on the capillary tubes that end in them,
-continually generates a capillary tension upwards; while, on the other
-hand, the vessels below, expanding after their sap has been squeezed
-out, produce a tension both upwards and downwards towards the point
-of loss. Were the limiting membranes of the vessels impermeable, the
-movement of sap would, under these conditions, soon be arrested. But
-these membranes are permeable; and the surrounding tissues readily
-permit the passage of air. This state of tension, then, will cause an
-entrance of air into the tubes; the columns of liquid they contain
-will be interrupted by bubbles. It seems, indeed, not improbable that
-this entrance of air may take place even when there is a good supply
-of liquid, if the mechanical strains are so violent and the exudation
-so rapid that the currents cannot refill the half-emptied vessels
-with sufficient rapidity. And in this case the intruding air may
-possibly play the same part as that contained in the air-chamber of a
-force-pump--tending, by moderating the violence of the jets, and by
-equalizing the strains, to prevent rupture of the apparatus. Of course
-when the supply of liquid becomes adequate, and the strains not too
-violent, these bubbles will be expelled as readily as they entered.
-
-Here, as before, let me add the conclusive proof furnished by a direct
-experiment. To ascertain the amount of this propulsive action, I took
-from the same tree, a Laurel, two equal shoots, and placing them in
-the same dye, subjected them to conditions that were alike in all
-respects save that of motion: while one remained at rest, the other
-was bent backwards and forwards, now by switching and now by straining
-with the fingers. After the lapse of an hour, I found that the dye had
-ascended the oscillating shoot three times as far as it had ascended
-the stationary shoot--this result being an average from several trials.
-Similar trials brought out similar effects in other structures. The
-various petioles and herbaceous shoots experimented upon for the
-purpose of ascertaining the amount of exudation produced by transverse
-strains, showed also the amount of longitudinal movement. It was
-observable that the height ascended by the dye was in all cases greater
-where there had been oscillation than where there had been rest--the
-difference, however, being much less marked in succulent structures
-than in woody ones.
-
-It need scarcely be said that this mechanical action is not here
-assigned as the sole cause of circulation, but as a cause co-operating
-with others, and helping others to produce effects that could not
-otherwise be produced. Trees growing in conservatories afford us
-abundant proof that sap is raised to considerable heights by other
-forces. Though it is notorious that trees so circumstanced do not
-thrive unless, through open sashes, they are frequently subject
-to breezes sufficient to make their parts oscillate, yet there is
-evidently a circulation that goes on without mechanical aid. The
-_causes_ of circulation are those actions only which disturb the liquid
-equilibrium in a plant, by permanently abstracting water or sap from
-some part of it; and of these the first is the absorption of materials
-for the formation of new tissue in growing parts; the second is the
-loss by evaporation, mainly through adult leaves; and the third is the
-loss by extravasation, through compressed vessels. Only so far as it
-produces this last, can mechanical strain be regarded as truly a cause
-of circulation. All the other actions concerned must be classed as
-_aids_ to circulation--as facilitating that re-distribution of liquid
-that continually restores the equilibrium continually disturbed; and
-of these capillary action may be named as the first, osmose as the
-second, and the propulsive effect of mechanical strains as the third.
-The first two of these aids are doubtless capable by themselves of
-producing a large part of the observed result--more of the observed
-result than is at first sight manifest; for there is an important
-indirect effect of osmotic action which appears to be overlooked.
-Osmose does not aid circulation only by setting up, within the plant,
-exchange currents between the more dense and the less dense solutions
-in different parts of it; but it aids circulation much more by
-producing distention of the plant as a whole. In consequence of the
-average contrast in density between the water outside of the plant and
-the sap inside of it, the constant tendency is for the plant to absorb
-a quantity in excess of its capacity, and so to produce distention
-and erection of its tissues. It is because of this that the drooping
-plant raises itself when watered; for capillary action alone could only
-refill its tissues without changing their attitudes. And it is because
-of this that juicy plants with collapsible structures bleed so rapidly
-when cut, not only from the cut surface of the rooted part, but from
-the cut surface of the detached part--the elastic tissues tending to
-press out the liquid which distends them. And manifestly if osmose
-serves thus to maintain a state of distention throughout a plant,
-it indirectly furthers circulation; since immediately evaporation
-or growth at any part, by abstracting liquid from the neighbouring
-tissues, begins to diminish the liquid pressure within such tissues,
-the distended structures throughout the rest of the plant thrust
-their liquid contents towards the place of diminished pressure. This,
-indeed, may very possibly be the most efficient of the agencies at
-work. Remembering how great is the distention producible by osmotic
-absorption--great enough to burst a bladder--it is clear that the
-force with which the distended tissues of a plant urge forward the sap
-to places of consumption, is probably very great. We must therefore
-regard the aid which mechanical strains give as being one of several.
-Oscillations help directly to restore any disturbed liquid equilibrium;
-and they also help indirectly, by facilitating the re-distribution
-caused by capillary action and the process just described; but in the
-absence of oscillations the equilibrium may still be restored, though
-less rapidly and within narrower limits of distance.
-
-One half of the problem of the circulation, however, has been left out
-of sight. Thus far our inquiry has been, how the ascending current
-of sap is produced. There remains the rationale of the descending
-current. What forces cause it, and through what tissues it takes place,
-are questions to which no satisfactory answers have been given. That
-the descent is due to gravitation, as some allege, is difficult to
-conceive, since, as gravitation acts equally on all liquid columns
-contained in the stem, it is not easy to see why it should produce
-downward movements in some while permitting upward movements in
-others--unless, indeed, there existed descending tubes too wide
-to admit of much capillary action, which there do not. Moreover,
-gravitation is clearly inadequate to cause currents towards the roots
-out of branches that droop to the ground. Here the gravitation of the
-contained liquid columns must nearly balance that of the connected
-columns in the stem, leaving no appreciable force to cause motion. Nor
-does there seem much probability in the assumption that the route of
-the descending sap is through the cambium layer, since experiments on
-the absorption of dyes prove that simple cellular tissue is a very bad
-conductor of liquids: their movement through it does not take place
-with one-fiftieth of the rapidity with which it takes place through
-vessels.[72]
-
-Of course the defence for these hypotheses is, that there must be a
-downward current, which must have a course and a cause; and the very
-natural assumption has been that the course and the cause must be other
-than those which produce the ascending current. Nevertheless there
-is an alternative supposition to which the foregoing considerations
-introduce us. It is quite possible for the same vascular system
-to serve as a channel for movement in opposite directions at
-different times. We have among animals well-known cases in which the
-blood-vessels carry a current first in one direction and then, after a
-brief pause, in the reverse direction. And there seems an _à priori_
-probability that, lowly-organized as they are, plants are more likely
-to have distributing appliances of this imperfect kind than to have
-two sets of channels for two simultaneous currents. If, led by this
-suspicion, we inquire whether among the forces which unite to produce
-movements of sap, there are any variations or intermissions capable of
-determining the currents in different directions, we quickly discover
-that there are such, and that the hypothesis of an alternating motion
-of the sap, now centrifugal and now centripetal, through the same
-vessels, has good warrant. What are the several forces at work? First
-may be set down that tendency existing in every part of a plant to
-expand into its typical form, and to absorb nutritive liquids in doing
-this. The resulting competition for sap will, other things being
-equal, cause currents towards the most rapidly-growing parts--towards
-unfolding shoots and leaves, but not towards adult leaves. Next we
-have evaporation, acting more on the adult leaves than on those which
-are in the bud, or but partially developed. This evaporation is both
-regularly and irregularly intermittent. Depending chiefly on the
-action of the sun, it is, in fine weather, greatly checked or wholly
-arrested every evening; and in cloudy weather must be much retarded
-during the day. Further, every hygrometric variation, as well as every
-variation in the movement of the air, must vary the evaporation.
-This chief action, therefore, which, by continually emptying the
-ends of the capillary tubes, makes upward currents possible, is one
-which intermits every night, and every day is strong or feeble as
-circumstances determine. Then, in the third place, we have this rude
-pumping process above described, going on with greater vigour when the
-wind is violent, and with less vigour when it is gentle--drawing liquid
-_towards_ different parts according to their degrees of oscillation,
-and _from_ different parts according as they can most readily furnish
-it. And now let us ask what must result under changing conditions
-from these variously-conflicting and conspiring forces. When a warm
-sunshine, causing rapid evaporation, is emptying the vessels of the
-leaves, the osmotic and capillary actions that refill them will be
-continually aided by the pumping action of the swaying petioles,
-twigs, and branches, provided their oscillations are moderate. Under
-these conditions the current of sap, moving in the direction of least
-resistance, will set towards the leaves. But what will happen when the
-sun sets? There is now nothing to determine currents either upwards
-or downwards, except the relative rates of growth in the parts and
-the relative demands set up by the oscillations; and the oscillations
-acting alone, will draw sap to the oscillating parts as much from above
-as from below. If the resistance to be overcome by a current setting
-back from the leaves is less than the resistance to be overcome by a
-current setting up from the roots, then a current will set back from
-the leaves. Now it is, I think, tolerably manifest that in the swaying
-twigs and minor branches, less force will be required to overcome the
-inertia of the short columns of liquid between them and the leaves than
-to overcome the inertia of the long columns between them and the roots.
-Hence during the night, as also at other times when evaporation is not
-going on, the sap will be drawn out of the leaves into the adjacent
-supporting parts; and their nutrition will be increased. If the wind is
-strong enough to produce a swaying of the thicker branches, the back
-current will extend to them also; and a further strengthening will
-result from their absorption of the elaborated sap. And when the great
-branches and the stem are bent backwards and forwards by a gale, they
-too will share in the nutrition. It may at first sight seem that these
-parts, being nearer to the roots than to the leaves, will draw their
-supplies from the roots only. But the quantity which the roots can
-furnish is insufficient to meet so great a demand. Under the conditions
-described, the exudation of sap from the vessels will be very great,
-and the draught of liquid required to refill them, not satisfied by
-that which the root-fibres can take in, will extend to the leaves.
-Thus sap will flow to the several parts according to their respective
-degrees of activity--to the leaves while light and heat enable them to
-discharge their functions, and back to the twigs, branches, stem, and
-roots when these become active and the leaves inactive, or when their
-activity dominates over that of the leaves. And this distribution of
-nutriment, varying with the varying activities of the parts, is just
-such a distribution as we know must be required to keep up the organic
-balance.
-
-To this explanation it may be objected that it does not account for
-the downward current of sap in plants that are sheltered. The stem and
-roots of a drawing-room Geranium display a thickening which implies
-that nutritive matters have descended from the leaves, although there
-are none of those oscillations by which the sap is said to be drawn
-downwards as well as upwards. The reply is, that the stem and roots
-tend to repeat their typical structures, and that the absorption of
-sap for the formation of their respective dense tissues, is here the
-force which determines the descent. Indeed it must be borne in mind
-that the mechanical strains and the pumping process which they keep
-up, as well as the distention caused by osmose, do not in themselves
-produce a current either upwards or downwards: they simply help to move
-the sap towards that place where there is the most rapid abstraction
-of it--the place towards which its motion is least resisted. Whether
-there is oscillation or whether there is not, the physiological demands
-of the different parts of the plant determine the direction of the
-current; and all which the oscillations and the distention do is to
-facilitate the supply of these demands. Just as much, therefore,
-in a plant at rest as in a plant in motion, the current will set
-downwards when the function of the leaves is arrested, and when there
-is nothing to resist that abstraction of sap caused by the tendency
-of the stem- and root-tissues to assume their typical structures. To
-which admission, however, it must be added that since this typical
-structure assumed, though imperfectly assumed, by the hothouse plant,
-is itself interpretable as the inherited effect of external mechanical
-actions on its ancestors, we may still consider the current set up by
-the assumption of the typical structure to be indirectly due to such
-actions.
-
-Interesting evidence of another order here demands notice. In the
-course of experiments on the absorption of dyes by leaves, it happened
-that in making sections parallel to the plane of a leaf, with the
-view of separating its middle layer containing the vessels, I came
-upon some structures that were new to me. These structures, where they
-are present, form the terminations of the vascular system. They are
-masses of irregular and imperfectly united fibrous cells, such as those
-out of which vessels are developed; and they are sometimes slender,
-sometimes bulky--usually, however, being more or less club-shaped. In
-transverse sections of leaves their distinctive characters are not
-shown: they are taken for the smaller veins. It is only by carefully
-slicing away the surface of a leaf until we come down to that part
-which contains them, that we get any idea of their nature. Fig. 1
-represents a specimen taken from a leaf of _Euphorbia neriifolia_.
-Occupying one of the interspaces of the ultimate venous network, it
-consists of a spirally-lined duct or set of ducts, which connects with
-the neighbouring vein a cluster of half-reticulated, half-scalariform
-cells. These cells have projections, many of them tapering, that insert
-themselves into the adjacent intercellular spaces, thus producing
-an extensive surface of contact between the organ and the imbedding
-tissues. A further trait is, that the ensheathing prosenchyma is
-either but little developed or wholly absent; and consequently this
-expanded vascular structure, especially at its end, comes immediately
-in contact with the tissues concerned in assimilation. The leaf of
-_Euphorbia neriifolia_ is a very fleshy one; and in it these organs
-are distributed through a compact, though watery, cellular mass. But
-in any leaf of the ordinary type which possesses them, they lie in
-the network-parenchyma composing its lower layer; and wherever they
-occur in this layer its cells unite to enclose them. This arrangement
-is shown in fig. 2, representing a sample from the Caoutchouc-leaf,
-as seen with the upper part of its envelope removed; and it is shown
-still more clearly in a sample from the leaf of _Panax Lessonii_,
-fig. 3. Figures 4 and 5 represent, without their sheaths, other such
-organs from the leaves of _Panax Lessonii_ and _Clusia flava_. Some
-relation seems to exist between their forms and the thicknesses of the
-layers in which they lie. Certain very thick leaves, such as those of
-_Clusia flava_, have them less abundantly distributed than is usual,
-but more massive. Where the parenchyma is developed not to so great an
-extreme, though still largely, as in the leaves of Holly, _Aucuba_,
-_Camellia_, they are not so bulky; and in thinner leaves, like those
-of Privet, Elder, &c., they become longer and less conspicuously
-club-shaped. Some adaptations to their respective positions seem
-implied by these modifications; and we may naturally expect that in
-many thin leaves these free ends, becoming still narrower, lose the
-distinctive and suggestive characters possessed by those shown in the
-diagrams. Relations of this kind are not regular, however. In various
-other genera, members of which I have examined, as _Rhus_, _Viburnum_,
-_Griselinia_, _Brexia_, _Botryodendron_, _Pereskia_, the variations in
-the bulk and form of these structures are not directly determined by
-the spaces which the leaves allow: obviously there are other modifying
-causes. It should be added that while these expanded free extremities
-graduate into tapering free extremities, not differing from ordinary
-vessels, they also pass insensibly into the ordinary inosculations.
-Occasionally, along with numerous free endings, there occur loops; and
-from such loops there are transitions to the ultimate meshes of the
-veins.
-
-These organs are by no means common to all leaves. In many that afford
-ample spaces for them they are not to be found. So far as I have
-observed, they are absent from the thick leaves of plants which form
-very little wood. In _Sempervivum_, in _Echeveria_, in _Bryophyllum_,
-they do not appear to exist; and I have been unable to discover them in
-_Kalanchoë rotundifolia_, in _Kleinia ante-euphorbium_ and _ficoides_,
-in the several species of _Crassula_, and in other succulent plants. It
-may be added that they are not absolutely confined to leaves, but occur
-in stems that have assumed the functions of leaves. At least I have
-found, in the green parenchyma of _Opuntia_, organs that are analogous
-though much more rudely and irregularly formed. In other parts, too,
-that have usurped the leaf-function, they occur, as in the phyllodes of
-the Australian Acacias. These have them abundantly developed; and it
-is interesting to observe that here, where the two vertically-placed
-surfaces of the flattened-out petiole are equally adapted to the
-assimilative function, there exist two layers of these expanded
-vascular terminations, one applied to the inner surface of each layer
-of parenchyma.
-
-Considering the structures and positions of these organs, as well as
-the natures of the plants possessing them, may we not form a shrewd
-suspicion respecting their function? Is it not probable that they
-facilitate absorption of the juices carried back from the leaf for
-the nutrition of the stem and roots? They are admirably adapted for
-performing this office. Their component fibrous cells, having angles
-insinuated between the cells of the parenchyma, are shaped just as they
-should be for taking up its contents; and the absence of sheathing
-tissue between them and the parenchyma facilitates the passage of the
-elaborated liquids. Moreover there is the fact that they are allied to
-organs which obviously have absorbent functions. I am indebted to Dr.
-Hooker for pointing out the figures of two such organs in the “Icones
-Anatomicæ” of Link. One of them is from the end of a dicotyledonous
-root-fibre, and the other is from the prothallus of a young Fern. In
-each case a cluster of fibrous cells, seated at a place from which
-liquid has to be drawn, is connected by vessels with the parts to which
-liquid has to be carried. There can scarcely be a doubt, then, that in
-both cases absorption is effected through them. I have met with another
-such organ, more elaborately constructed, but evidently adapted to the
-same office, in the common Turnip-root. As shown by the end view and
-longitudinal section in figs. 6 and 7, this organ consists of rings of
-fenestrated cells, arranged with varying degrees of regularity into a
-funnel, ordinarily having its apex directed towards the central mass
-of the Turnip, with which it has, in some cases at least, a traceable
-connexion by a canal. Presenting as it does an external porous surface
-terminating one of the branches of the vascular system, each of these
-organs is well fitted for taking up with rapidity the nutriment laid
-by in the Turnip-root, and used by the plant when it sends up its
-flower-stalk. Nor does even this exhaust the analogies. The cotyledons
-of the young bean, experimented upon as before described, furnished
-other examples of such structures, exactly in the places where, if
-they are absorbents, we might expect to find them. Amid the branchings
-and inosculations of the vascular layer running through the mass of
-nutriment deposited in each cotyledon, there are conspicuous free
-terminations that are club-shaped, and prove to be composed, like those
-in leaves, of irregularly formed and clustered fibrous cells; and some
-of them, diverging from the plane of the vascular layer, dip down into
-the mass of starch and albumen which the young plant has to utilize,
-and which these structures can have no other function but to take up.
-
-Besides being so well fitted for absorption, and besides being
-similar to organs which we cannot doubt are absorbents, these
-vascular terminations in leaves afford us yet another evidence of
-their functions. They are seated in a tissue so arranged as specially
-to facilitate the abstraction of liquid. The centripetal movement
-of the sap must be set up by a force that is comparatively feeble,
-since, the parietes of the ducts being porous, air will enter if the
-tension on the contained columns becomes considerable. Hence it is
-needful that the exit of sap from the leaves should meet with very
-little resistance. Now were it not for an adjustment presently to
-be described, it would meet with great resistance, notwithstanding
-the peculiar fitness of these organs to take it in. Liquid cannot be
-drawn out of any closed cavity without producing a collapse of the
-cavity’s sides; and if its sides are not readily collapsible, there
-must be a corresponding resistance to the abstraction of liquid from
-it. Clearly the like must happen if the liquid is to be drawn out of
-a tissue which cannot either diminish in bulk bodily or allow its
-components individually to diminish in bulk. In an ordinary leaf, the
-upper layer of parenchyma, formed as it is of closely-packed cells
-that are without interspaces, and are everywhere held fast within
-their framework of veins, can neither contract easily as a mass, nor
-allow its separate cells to do so. Quite otherwise is it with the
-network-parenchyma below. The long cells of this, united merely by
-their ends and having their flexible sides surrounded by air, may
-severally have their contents considerably increased and decreased
-without offering appreciable resistances: and the network-tissue
-which they form will, at the same time, be capable of undergoing
-slight expansions and contractions of its thickness. In this layer
-occur these organs that are so obviously fitted for absorption. Here
-we find them in direct communication with its system of collapsible
-cells. The probability appears to be, that when the current sets into
-the leaf, it passes through the vessels and their sheaths chiefly into
-the upper layer of cells (this upper layer having a larger surface
-of contact with the veins than the lower layer, and being the seat
-of more active processes); and that the juices of the upper layer,
-enriched by the assimilated matters, pass into the network-parenchyma,
-which serves as a reservoir from which they are from time to time
-drawn for the nutrition of the rest of the plant, when the actions
-determine the downward current. Should it be asked what happens where
-the absorbents, instead of being inserted in a network-parenchyma,
-are, as in the leaves of _Euphorbia neriifolia_, inserted in a solid
-parenchyma, the reply is, that such a parenchyma, though not furnished
-with systematically arranged air-chambers, nevertheless contains air
-in its intercellular spaces; and that when there occurs a draught upon
-its contents, the expansion of this air and the entrance of more from
-without, quickly supply the place of the abstracted liquid.
-
-If then, returning to the general argument, we conclude that these
-expanded terminations of the vascular system in leaves are absorbent
-organs, we find a further confirmation of the views set forth
-respecting the alternating movement of the sap along the same channels.
-These spongioles of the leaves, like the spongioles of the roots, being
-appliances by which liquid is taken up to be carried into the mass
-of the plant, we are obliged to regard the vessels that end in these
-spongioles of the leaves as being the channels of the down current
-whenever it is produced. If the elaborated sap is abstracted from the
-leaves by these absorbents, then we have no alternative but to suppose
-that, having entered the vascular system, the elaborated sap descends
-through it. And seeing how, by the help of these special terminations,
-it becomes possible for the same vessels to carry back a quality of sap
-unlike that which they bring up, we are enabled to understand tolerably
-well how this rhythmical movement produces a downward transfer of
-materials for growth.
-
-       *       *       *       *       *
-
-The several lines of argument may now be brought together; and along
-with them may be woven up such evidences as remain. Let me first point
-out the variety of questions to which the hypothesis supplies answers.
-
-It is required to account for the ascent of sap to a height beyond that
-to which capillary action can raise it. This ascent is accounted for
-by the propulsive action of transverse strains, joined with that of
-osmotic distention. A cause has to be assigned for that rise of sap
-which, in the spring, while yet there is no considerable evaporation to
-aid it, goes on with a power which capillarity does not explain. The
-co-operation of the same two agencies is assignable for this result
-also.[73] The circumstance that vessels and ducts here contain sap and
-there contain air, and at the same place contain at different seasons
-now air and now sap is a fact calling for explanation. An explanation
-is furnished by these mechanical actions which involve the entrance
-or expulsion of air according to the supply of liquid. That vessels
-and ducts which were originally active sap-carriers go completely out
-of use, and have their function discharged by other vessels or ducts,
-is an anomaly that has to be solved. Again, we are supplied with a
-solution: these deserted vessels and ducts are those which, by the
-formation of dense tissue outside of them, become so circumstanced that
-they cannot be compressed as they originally were. A channel has to be
-found for the downward current of sap, which, on any other hypothesis
-than the foregoing, must be a channel separate from that taken by the
-upward current; and yet no good evidence of a separate channel has
-been pointed out. Here, however, the difficulty disappears, since one
-channel suffices for the current alternating upwards and downwards
-according to the conditions. Moreover there has to be found a force
-producing or facilitating the downward current, capable even of drawing
-sap out of drooping branches; and no such force is forthcoming. The
-hypothesis set forth dispenses with this necessity; under the recurring
-change of conditions, the same distention and oscillation which before
-raised the sap to the places of consumption, now bring it down to the
-places of consumption. A physical process has to be pointed out by
-which the material that forms dense tissue is deposited at the places
-where it is wanted, rather than at other places. This physical process
-the hypothesis indicates. It is requisite to find an explanation of the
-fact that, when plants ordinarily swayed about by the wind are grown
-indoors, the formation of wood is so much diminished that they become
-abnormally slender. Of this an explanation is supplied. Yet a further
-fact to be interpreted is, that in the same individual plant homologous
-parts, which, according to the type of the plant, should be equally
-woody, become much thicker one than another if subject to greater
-mechanical stress. And of this too an interpretation is similarly
-afforded.
-
-Now the sufficiency of the assigned actions to account for so many
-phenomena not otherwise explained, would be strong evidence that the
-rationale is the true one, even were it of a purely hypothetical
-kind. How strong, then, becomes the reason for believing it the true
-one when we remember that the actions alleged demonstrably go on in
-the way asserted. They are ever operating before our eyes; and that
-they produce the effects in question is a conclusion deducible from
-mechanical principles, a conclusion established by induction, and a
-conclusion verified by experiment. These three orders of proof may be
-briefly summed up as follows.
-
-That plants which have to raise themselves above the earth’s surface,
-and to withstand the actions of the wind, must have a power of
-developing supporting structure, is an _à priori_ conclusion which may
-be safely drawn. It is an equally safe _à priori_ conclusion, that if
-the supporting structure, either as a whole or in any of its parts, has
-to adapt itself to the particular strains which the individual plant is
-subject to by its particular circumstances, there must be at work some
-process by which the strength of the supporting structure is everywhere
-brought into equilibrium with the forces it has to bear. Though the
-typical distribution of supporting structure in each kind of plant may
-be explained teleologically by those whom teleological explanations
-satisfy; and though otherwise this typical distribution may be ascribed
-to natural selection acting apart from any directly adaptive process;
-yet it is manifest that those departures from the typical distribution
-which fit the parts of each plant to their special conditions are
-explicable neither teleologically nor by natural selection. We are,
-therefore, compelled to admit that, if in each plant there goes on
-a balancing of the particular strains by the particular strengths,
-there must be a physical or physico-chemical process by which the
-adjustments of the two are effected. Meanwhile we are equally compelled
-to admit, _à priori_, that the mechanical actions to be resisted,
-themselves affect the internal tissues in such ways as to further the
-increase of that dense substance by which they are resisted. It is
-demonstrable that bending the petioles, shoots, and stems must compress
-the vessels beneath their surfaces, and increase the exudation of
-nutritive matters from them, and must do this actively in proportion
-as the bends are great and frequent; so that while, on the one hand,
-it is a necessary deduction that, if the parts of each plant are to be
-severally strengthened according to the several strains, there must
-be some direct connexion between strains and strengths, it is, on the
-other hand, a necessary deduction from mechanical principles that the
-strains do act in such ways as to aid the increase of the strengths.
-How a like correspondence between two _à priori_ arguments holds in
-the case of the circulation, needs not to be shown in detail. It will
-suffice to remind the reader that while the raising of sap to heights
-beyond the limit of capillarity implies some force to effect it, we
-have in the osmotic distention and the intermittent compressions caused
-by transverse strains, forces which, under the conditions, cannot but
-tend to effect it; and similarly with the requirement for a downward
-current, and the production of a downward current.
-
-Among the inductive proofs we find a kindred agreement. Different
-individuals of the same species, and different parts of the same
-individual, do strengthen in different degrees; and there is a clearly
-traceable connexion between their strengthenings and the intermittent
-strains they are exposed to. This evidence, derived from contrasts
-between growths on the same plant or on plants of the same type, is
-enforced by evidence derived from contrasts between plants of different
-types. The deficiency of woody tissue which we see in plants called
-succulent, is accompanied by a bulkiness of the parts which prevents
-any considerable oscillations; and this character is also habitually
-accompanied by a dwarfed growth. When, leaving these relations as
-displayed externally, we examine them internally, we find the facts
-uniting to show, by their agreements and differences, that between
-the compression of the sap-canals and the production of wood there
-is a direct relation. We have the facts, that in each plant, and in
-every new part of each plant, the formation of sap-canals precedes
-the formation of wood; that the deposit of woody matter, when it
-begins, takes place around these sap-canals, and afterwards around
-the new sap-canals successively developed; that this formation of
-wood around the sap-canals takes place where the coats of the canals
-are demonstrably permeable, and that the amount of wood formation is
-proportionate to the permeability. And then that the permeability and
-extravasation of sap occur wherever, in the individual or in the type,
-there are intermittent compressions, is proved alike by ordinary cases
-and by exceptional cases. In the one class of cases we see that the
-deposit of wood round the vessels begins to take place when they come
-into positions that subject them to intermittent compressions, while it
-ceases when they become shielded from compressions. And in the other
-class of cases, where, from the beginning, the vessels are shielded
-from compression by surrounding fleshy tissue, there is a permanent
-absence of wood formation.
-
-To which complete agreement between the deductive and inductive
-inferences has to be added the direct proof supplied by experiments. It
-is put beyond doubt by experiment that the liquids absorbed by plants
-are distributed to their different parts through their vessels--at
-first by the spiral or allied vessels originally developed, and then by
-the better-placed ducts formed later. By experiment it is demonstrated
-that the intermittent compressions caused by oscillations urge the
-sap along the vessels and ducts. And it is also experimentally proved
-that the same intermittent compressions produce exudation of sap from
-vessels and ducts into the surrounding tissue.
-
-That the processes here described, acting through all past time, have
-sufficed of themselves to develope the supporting and distributing
-structures of plants, is not alleged. What share the natural selection
-of variations distinguished as spontaneous, has had in establishing
-them, is a question which remains to be discussed. Whether acting
-alone natural selection would have sufficed to evolve these vascular
-and resisting tissues, I do not profess to say. That it has been a
-co-operating cause, I take to be self-evident: it must all along
-have furthered the action of any other cause, by preserving the
-individuals on which such other cause had acted most favourably.
-Seeing, however, the conclusive proof which we have that another cause
-has been in action--certainly on individuals, and, in all probability,
-by inheritance on races--we may most philosophically ascribe the
-genesis of these internal structures to this cause, and regard natural
-selection as having here played the part of an accelerator.
-
-
-                         EXPLANATION OF PLATE.
-
-Fig. 1. Absorbent organ from the leaf of _Euphorbia neriifolia_.
-The cluster of fibrous cells forming one of the terminations of the
-vascular system is here imbedded in a solid parenchyma.
-
-Fig. 2. A structure of analogous kind from the leaf of _Ficus
-elastica_. Here the expanded terminations of the vessels are imbedded
-in the network-parenchyma, the cells of which unite to form envelopes
-for them.
-
-Fig. 3. Shows on a larger scale one of these absorbents from the leaf
-of _Panax Lessonii_. In this figure is clearly seen the way in which
-the cells of the network-parenchyma unite into a closely-fitting case
-for the spiral cells.
-
-Fig. 4. Represents a much more massive absorbent from the same leaf,
-the surrounding tissues being omitted.
-
-Fig. 5. Similarly represents, without its sheath, an absorbent from the
-leaf of _Clusia flava_.
-
-Fig. 6. End view of an absorbent organ from the root of a Turnip. It is
-taken from the outermost layer of vessels. Its funnel-shaped interior
-is drawn as it presents itself when looked at from the outside of this
-layer, its narrow end being directed towards the centre of the Turnip.
-
-Fig. 7. A longitudinal section through the axis of another such organ,
-showing its annuli of reticulated cells when cut through. The cellular
-tissue which fills the interior is supposed to be removed.
-
-Fig. 8. A less developed absorbent, showing its approximate connexion
-with a duct. In their simplest forms, these structures consist of only
-two fenestrated cells, with their ends bent round so as to meet. Such
-types occur in the central mass of the Turnip, where the vascular
-system is relatively imperfect. Besides the comparatively regular forms
-of these absorbents, there are forms composed of amorphous masses
-of fenestrated cells. It should be added that both the regular and
-irregular kinds are very variable in their numbers: in some turnips
-they are abundant, and in others scarcely to be found. Possibly their
-presence depends on the age of the Turnip. Judging from the period
-during which my investigations were made, namely winter and early
-spring, I suspect that they are developed only in preparation for
-sending up the flower-stalk.
-
-  [Illustration: Figs.1–8.]
-
-Let me add that experiments on circulation in plants made during the
-state of inactivity, when it is to be presumed that the vessels and
-tissues contain but little gap, are much more successful than those
-made in the summer. It would seem that when the tissues are fully
-charged with sap the taking up of dyes is comparatively slow and the
-above-described effects are not so easily demonstrable.
-
-       *       *       *       *       *
-
-[An expert writes concerning this essay:--“I have not attempted to
-annotate critically this paper. There is no doubt that many of your
-conclusions are perfectly sound, particularly those relating to the
-passage of crude sap through the _cavities_ of the elements of the
-wood, though the opinion that the actual passage was through the walls
-very generally held till about 12 years ago.”]
-
-
-
-
-                              APPENDIX D.
-
-                 ON THE ORIGIN OF THE VERTEBRATE TYPE.
-
-
-   [_When studying the development of the vertebrate skeleton,
-   there occurred to me the following idea respecting the possible
-   origin of the notochord. I was eventually led to omit the few
-   pages of Appendix in which I had expressed this idea, because
-   it was unsupported by developmental evidence. The developmental
-   evidence recently discovered, however, has led Professor Haeckel
-   and others to analogous views respecting the affiliation of the_
-   Vertebrata _on the_ Molluscoida. _Having fortunately preserved a
-   proof of the suppressed pages, I am able now to add them. With
-   the omission of a superfluous paragraph, they are reprinted
-   verbatim from this proof, which dates back to the autumn of
-   1865, at which time the chapter on “The Shapes of Vertebrate
-   Skeletons” was written._--December, 1869.]
-
-The general argument contained in Chap. XVI. of Part IV., I have
-thought it undesirable to implicate with any conception more
-speculative than those essential to it; and to avoid so implicating it,
-I transfer to this place an hypothesis respecting the derivation of the
-rudimentary vertebrate structure, which appears to me worth considering.
-
-Among those molluscoid animals with which the lowest vertebrate animal
-has sundry traits in common, it very generally happens that while the
-adult is stationary the larva is locomotive. The locomotion of the
-larva is effected by the undulations of a tail. In shape and movement
-one of these young Ascidians is not altogether unlike a Tadpole.
-And as the tail of the Tadpole disappears when its function comes
-to be fulfilled by limbs; so the Ascidian larva’s tail disappears
-when fixation of the larva renders it useless. This disappearance of
-the tail, however, is not without exception. The _Appendicularia_
-is an Ascidian which retains its tail throughout life; and by its
-aid continues throughout life to swim about. Now this tail of the
-_Appendicularia_ has a very suggestive structure. It is long, tapering
-to a point, and flattened. From end to end there runs a mid-rib, which
-appears to be an imbedded gelatinous rod, not unlike a notochord.
-Extending along the two sides of this mid-rib, are bundles of muscular
-fibres; and its top bears a gangliated nervous thread, giving off, at
-intervals, branches to the muscular fibres. In the _Appendicularia_
-this tail, which is inserted at the lower part of the back, is bent
-forwards, so as not to be adapted for propelling the body of the animal
-head foremost; but the homologous tails of the larval Ascidians are
-directed backwards, so as to produce forward movement. If we suppose
-a type like the _Appendicularia_ in the structure and insertion of
-its permanent tail, but resembling the larval forms in the direction
-of its tail, it is, I think, not difficult to see that functional
-adaptation joined with natural selection, might readily produce a
-type approximating to that whose origin we are considering. It is a
-fair assumption that an habitually-locomotive creature would profit
-by increased power of locomotion. This granted, it follows that
-such further development of the tail-structures as might arise from
-enhanced function, and such better distribution of them as spontaneous
-variation might from time to time initiate, would be perpetuated. What
-must be the accompanying changes? The more vigorous action of such an
-appendage implies a firmer insertion into the body; and this would be
-effected by the prolongation forwards of the central axis of the tail
-into the creature’s back. As fast as there progressed this fusion of
-the increasingly-powerful tail with the body, the body would begin to
-partake of its oscillations; and at the same time that the resistant
-axis of the tail advanced along the dorsal region, its accompanying
-muscular fibres would spread over the sides of the body: gradually
-taking such modified directions and insertions as their new conditions
-rendered most advantageous. Without further explanation, those who
-examine drawings of the structures described, will, I think, see that
-in such a way a tail homologous with that of the _Appendicularia_,
-would be likely, in the course of that development required for its
-greater efficiency, gradually to encroach on the body, until its
-mid-rib became the dorsal axis, its gangliated nerve-thread the spinal
-chord, and its muscular fibres the myocommata. Such a development of an
-appendage into a dominant part of the organism, though at first sight
-a startling supposition, is not without plenty of parallels: instance
-the way in which the cerebral ganglia, originally mere adjuncts of the
-spinal chord, eventually become the great centres of the nervous system
-to which the spinal chord is quite subordinate; or instance the way in
-which the limbs, small and inconspicuous in fishes, become, in Man,
-masses which, taken together, outweigh the trunk. It may be added that
-these familiar cases have a further appropriateness; for they exhibit
-higher degrees of that same increasing dominance of the organs of
-external relation, which the hypothesis itself implies.
-
-Of course, if the rudimentary vertebrate apparatus thus grew into,
-and spread over, a molluscoid visceral system, the formation of the
-notochord under the action of alternating transverse strains, did not
-take place as suggested in § 255; but it does not therefore follow
-that its differentiation from surrounding tissues was not mechanically
-initiated in the way described. For what was said in that section
-respecting the effects of lateral bendings of the body, equally
-applies to lateral bendings of the tail; and as fast as the developing
-tail encroached on the body, the body would become implicated in the
-transverse strains, and the differentiation would advance forwards
-under the influences originally alleged. Obviously, too, though the
-lateral muscular masses would in this case have a different history;
-yet the segmentation of them would be eventually determined by the
-assigned causes. For as fast as the strata of contractile fibres,
-developing somewhat in advance of the dorsal axis, spread along the
-sides, they would come under the influence of the alternate flexions;
-and while, by survival of the fittest, their parts became adjusted
-in direction, their segmentation would, as before, accompany their
-increasing massiveness. The actions and reactions due to lateral
-undulations would still, therefore, be the causes of differentiation,
-with which natural selection would co-operate.
-
-
-
-
-                             APPENDIX D 2.
-
-                          THE ANNULOSE TYPE.
-
-
-The production of a segmental structure by undulatory movements,
-suggested in Appendix D, as also in B (first published in 1858) as
-explaining the vertebral column, has been recently suggested by Prof.
-Korschelt as the cause of that segmentation of the annulose type which
-gives the name to it. He espouses a--
-
-   “view which is based upon the assumption that at first an
-   unsegmented, elongated ancestral form was produced by terminal
-   growth, whereupon the entire body became separated at once into
-   a large number of segments by a re-arrangement of the individual
-   organs. This assumption is supported by the consideration that
-   with the lateral sinuous movement of the body, and with the
-   rigidity of the tissues caused by increasing differentiation,
-   the formation of alternating regions of greater and less
-   motility was of considerable advantage to the individual, and
-   rendered possible a further elongation of the body. The first
-   cause for the appearance of metameric segmentation would then be
-   sought in the manner of locomotion and in mechanical conditions.
-   However, this latter view is not supported in any way by
-   embryology.” (_Embryology of Invertebrates_, Part I, pp. 349–50.)
-
-I venture to think the confession that this view “is not supported in
-any way by embryology” should be joined with the confession that it
-is at variance with that abstract embryology which comprehends the
-process of development in general. The assumption that there took
-place “a re-arrangement of the individual organs” of “an unsegmented,
-elongated ancestral form,” in such wise that the organs, previously
-single, presently became multiple, so that instead of one organ of each
-kind there were substituted many organs of each kind, is inconsistent
-with the general law of evolution, organic and other--implies not
-integration but disintegration. Everywhere the advance is from many
-like parts performing like functions to relatively few unlike parts
-performing unlike functions. The higher forms of the annulose type
-itself show this. Compare a myriapod and a crab. In the one we have
-not only a great number of similar segments bearing similar limbs,
-but we have in each segment a dilatation of the main blood-vessel--a
-rudimentary heart--a swollen portion of the nerve cord--a small
-ganglion--and so on; whereas in the other, besides relatively few
-segments and few limbs (sundry of them extremely unlike the rest) we
-have a vascular system concentrated into a central heart with arteries
-and a concentrated nervous system, such that the great ganglia in the
-integrated carapace immensely subordinate the ganglia of the remaining
-segments; and similarly with the other organs. Now unless it be denied
-that these highest decapods have been evolved from low types akin to
-myriapods in composition, it must be admitted that the progress has
-been from a string of many like segments with similar sets of organs
-to a group of relatively-few unlike segments with dissimilar sets of
-organs. If so we cannot rationally deny that the progress has been
-of this nature up from the lowest annelid, instead of having been,
-as Prof. Korschelt’s hypothesis implies, of opposite nature at the
-beginning.
-
-In a preceding passage a clear recognition of the normal course of
-development occurs. In opposing the view set forth in §§ 205–7 of this
-work, Prof. Korschelt says:--
-
-   “It seems scarcely favourable to this theory that the degree
-   of independence which the individual segments present is
-   comparatively slight. The most important organs (nervous system,
-   body musculature, blood-vascular system) show themselves to be
-   single fundaments of the entire body, and are also developed as
-   such even though they also exhibit evidences of metamerism. Even
-   the excretory canals may give up their segmental isolation and
-   become united to one another by means of longitudinal canals.”
-   (_Ib._ p. 348.)
-
-On turning back to § 206, the reader will, I think, demur to the
-assertion that the independence is “comparatively slight”; seeing
-that, as in _Ctenodrilus_, a single segment sometimes becomes separate
-and reproduces other segments to form a new series. Instead of
-admitting that “the most important organs” “show themselves to be
-single fundaments of the entire body,” it may be held, contrariwise,
-that their original independence in each segment is masked only to
-the degree involved by their co-operation as parts of a compound
-organism. But chiefly I remark that when it is said that “the excretory
-canals may give up their segmental isolation and become united” by
-“longitudinal canals,” there is a clear confession that the isolation
-of these organs was original and their union superinduced--an
-implication that the course of evolution is as I have described it, and
-at variance with the course of evolution assumed by Prof. Korschelt.
-
-Yet another incongruity is involved in his interpretation. He writes:--
-
-   “Just as in the consideration of the tapeworm chain we were
-   induced by the comparison with unsegmented forms to refer the
-   entire chain to an unsegmented individual, and, on the other
-   hand, to see in the proglottis, not a complete individual, but
-   only the abstricted hinder portion of the body of the Cestode,
-   in the same manner, and with much more reason, we adhere to the
-   individuality of the Annelid body.” (P. 349.)
-
-And then on the preceding page, referring to the composition of the
-Annelid body, he says:--“The most natural comparisons are those
-with the tapeworm chain and with the strobila of the Scyphomedusæ.”
-Now since it is here assumed that the tapeworm and the strobila are
-analogous in composition, it is implied that the detached proglottis
-and the detached medusa are analogous; and hence if we are to regard
-the proglottis as “not a complete individual but only the abstricted
-hinder portion of the body of the Cestode,” then we must similarly
-regard the medusa as not a complete individual, but only the abstricted
-hinder portion of the strobila. This commits us to the strange
-conclusion that whereas individuality is ascribed to the original
-simple polyp, and by and by to the partially-segmented strobila, though
-these are without special senses and with only rudiments of muscular
-and nervous systems, individuality is denied to the detached medusa,
-which has organs of sense, a distinct nervo-muscular system and a
-considerable power of locomotion, as well as a generative system:
-traits which in other cases characterize developed individuals. Here
-also, then, there seems to be an inversion of the ordinary conception.
-
-This conception of the proglottis and the medusa is, I see, accepted by
-some as tenable. But if we accept it we must accept also an analogous
-conception, which will I think be regarded as untenable. It is that
-supplied by the _Aphides_. From an egg proceeds a series of sexless and
-wingless females, and at the end of the series there come winged males
-and females with resulting gamic reproduction. If instead of forming
-a discrete series the imperfect females formed a concrete series,
-the members of which could individually feed without being detached
-from one another, as the segments of a tapeworm can, the parallelism
-would be complete; and then, according to the view in question,
-we should have to regard the perfect males and females eventually
-arising, not as individuals but as terminal portions of the series,
-containing generative products and having wings for the dispersion of
-them--locomotive egg-bearing segments of the chain. Whoever espouses
-this view must hold either that the first imperfect female of the
-series was the individual or that the entire string of them constituted
-the individual (in conformity with a view once propounded by Prof.
-Huxley). But he must do more than this. Since the _Aphides_ have
-descended from some winged species of the order _Hemiptera_, he must
-hold that among those remote ancestors each particular fly, male or
-female, was an individual; but that when abundant food and inert life
-led to the partheno-genetic habit, and to chains of sexless forms, the
-males and females eventually produced at the end of each chain, though,
-like their remote ancestors, possessed of procreative organs and wings,
-are not individuals.
-
-[Some memoranda bearing on the question here discussed, mislaid at the
-time when the chapter dealing with it was revised, have been discovered
-in time for utilization in this appendix.]
-
-One of my critics says:--
-
-   “You have overstated the case in your favour: the alimentary
-   canal does not, as you suggest, show a segmentation
-   corresponding to that of the other organs in Annelids. Either
-   it is a simple uniform tube, or else its differentiations
-   (pharynx, œsophagus, crop, intestine) are quite independent of
-   the repetition of the somites.”
-
-In presence of statements made in works of authority, this objection
-greatly surprises me. I meet with the descriptive word “moniliform”
-applied to the intestine in some Annelids, and then in the Text Book of
-Claus, translated and edited by Sedgwick, it is said, concerning the
-alimentary canal in the _Annelida_:--
-
-   “This is followed by the gastric region of the gut, which
-   occupies the greatest portion of the length of the body, and
-   is either regularly constricted in correspondence with the
-   segments, or possesses lateral diverticula.” (P. 365.)
-
-And again on p. 369 it is said:--
-
-   “The intestine usually preserves the same structure in its
-   entire length and is divided by regular constrictions into
-   a number of divisions or chambers, which correspond to the
-   segments and dilate again into lateral diverticula and cæca.”
-
-The alimentary canal thus presents the segmental character as clearly
-as consists with fulfilment of its function. If the successive segments
-are co-operating units of a compound animal having but one mouth,
-then, necessarily, the gut cannot be completely cut into parts, each
-answering to a segment, for there could be, in that case, no passage
-for the food. If the portion of the intestine belonging to each segment
-has a conspicuous dilatation, or has a cæcum on each side, it exhibits
-the segmental character as much as the physical requirements permit. So
-far from being at variance with the hypothesis, its structure exhibits
-a verification of it.
-
-The next objection runs as follows:--
-
-   “Then, again, the ovaries and testes do not exhibit a
-   corresponding segmentation. When it is allowable to speak of
-   ovary or testis at all as in _Lumbricus_, we find that in
-   the case of both organs we have at most two pairs.”
-
-It seems to me that the distribution of the generative organs in
-a comparatively-developed member of the Annelid type, is not the
-question. We have to ask what it is in undeveloped members of that
-type. Among them the repetition of generative parts is in some cases
-just what the theory implies. Thus in Claus I read:--“In the marine
-_Chætopoda_, the ova or spermatozoa originate on the body-wall from
-cells of the peritoneal membrane, either in the anterior segments
-alone or along the whole length of the body.” So that in these last
-cases there are, in all the segments, parts from which arise generative
-products. The fact that these parts are not definite ovaries and testes
-is irrelevant. Ovaries and testes are developed generative structures,
-and in the order of evolution are preceded by undeveloped ones; and the
-fact that these undeveloped ones are found in little-developed members
-of the type conforms perfectly to the hypothesis. [I may remark in
-passing that here is a good illustration of that process of evolution
-which, in the above speculation of Prof. Korschelt, is supposed to be
-inverted: many dispersed, similar, and indefinite parts, are integrated
-into a few localized and definite parts.]
-
-In continuation the critic above quoted says:--“My position is that
-the repetition of segments in an Annelid is a phenomenon of the same
-nature as the repetition of hairs in a Mammal or of scutes in a
-Reptile”, and he proceeds to give instances of repetitions of organs
-in other types, as of the reproductive structures and excretory
-system in the young Dog-fish or of the ovaries in _Amphioxus_. These
-examples do not seem to me relevant. No parallelism exists between
-the repetition of a particular organ in an animal, and the repetition
-of an entire cluster of organs constituting a physiological whole.
-The repetitions of the ovaries in _Amphioxus_ and of the excretory
-system in a young Dog-fish, occur without threatening to divide into
-similar parts the entire organism. But the segmental repetitions in
-an annulose creature implicate the structures at large, and would, if
-pushed a little further, result in separate creatures. The segment
-of a low Annelid contains alimentary, vascular, nervous, excretory,
-reproductive, sensory and locomotive organs--all the organs required
-for carrying on life, save certain organs of external relation which
-its position excludes. When there is shown some vertebrate animal, or
-proto-vertebrate animal, that is divisible into parts each of which is
-in great measure physiologically independent, I shall feel obliged to
-abandon my position.
-
-       *       *       *       *       *
-
-While this appendix is in hand I have received from another expert,
-whose view is in general agreement with my own, a letter containing the
-following passage:--
-
-   “You will see that Dohrn’s theory was the antithesis of your
-   own view of vertebrate structure, namely that the vertebræ
-   were formed by the segmentation, from mechanical causes of a
-   body originally simple. This view of yours has been confirmed
-   by later researches, which have shown that the most primitive
-   forms allied to the Vertebrates, possessing the essential
-   organs, viz., gill-slits, notochord, and dorsal nerve cord,
-   are not segmented animals, like Annelids and _Crustacea_, but
-   simple animals, having at most three regions, not exactly
-   corresponding to segments. These primitive unsegmented forms are
-   Ascidian tadpoles, _Balanoglossus_, and certain other primitive
-   forms. The embryology of Vertebrates also proves that they are
-   originally simple and not segmented animals, especially the
-   fact that there is originally one pronephric duct or primitive
-   kidney.”
-
-Nevertheless there survives a leaning towards the notion of a segmental
-origin of the _Vertebrata_. But the repetitions of organs named in
-support of this notion have, I think, no more relation to the genesis
-of the vertebrate type than the multiplication of vertebræ in a snake
-has relation to the genesis of the vertebral column.
-
-
-
-
-                              APPENDIX E.
-
-                THE SHAPES AND ARRANGEMENTS OF FLOWERS.
-
-
-In Part IV., Chapter X., under the title of “The Shapes of Flowers,” I
-have, after describing their several kinds of symmetry, as habitually
-related to their positions, made some remarks by way of interpretation.
-The truth that flowers exhibit a radial symmetry when they are so
-placed as to be equally affected all round by incident forces, having
-been exemplified, and also the truth that they assume a bilateral
-symmetry when they are so placed that their two sides are conditioned
-in ways different from the ways in which their upper and lower
-parts are conditioned; I have gone on to inquire (in § 234) by what
-causes such modifications of form are produced. I have stated that,
-originally, I inclined to ascribe them entirely to differences in the
-relations of the parts to physical forces--light, heat, gravitation,
-etc.; but that I found sundry facts stood in the way of this
-interpretation. And I have said that “Mr. Darwin’s investigations into
-the fertilization of Orchids led me to take into account an unnoticed
-agency.” Continuing to recognize the physical forces as factors having
-some influence, I have concluded that the most important factor is
-the action of insects; which, aiding most the fertilization of those
-flowers which most facilitate their entrance, produce, in course of
-generations, a form of flower specially adapted to the special position.
-
-Though still adhering to this interpretation, I have since found reason
-to think that the original interpretation contains a larger portion
-of truth than I supposed at the time when I was led thus to revise
-it. While staying at Mürren, in Switzerland, in 1872, I observed
-some modifications in a species of Gentian, which proved to me that
-the action of incident physical forces on flowers is, in some cases,
-very rapid and decided. The species furnishing this evidence was the
-_Gentiana Asclepiadea_; which I found in a copse formed of bushes
-that were here wide apart and there close together. In some places not
-near to the bushes, the individuals of the species grew vertically; in
-other places, partially shaded, their inclined shoots curved in such
-directions as to get the most light; and in other cases their shoots
-were led to take directions almost or quite horizontal. That, along
-with these modifications in the directions of their shoots, there went
-adjustments in the attitudes of their leaves, was a fact not specially
-worthy of remark; for plants placed inside the windows of houses
-habitually show us that leaves quickly bend themselves into attitudes
-giving them the greatest amounts of light. But the fact which attracted
-my attention was, that the flowers changed their attitudes in an
-equally-marked manner. The radial distribution passed into a bilateral
-distribution with the greatest readiness. Comparison of the annexed
-figures will show the character of this change.
-
-Figure I. represents part of a vertically-growing shoot. This
-belonged to an individual growing unimpeded by bushes, and getting
-light on all sides. Here it is observable that the pairs of leaves,
-placed alternately in directions transverse to one another--one pair
-pointing, say, north and south, and the next pair pointing east and
-west--maintain, taking them in the aggregate, a radial distribution;
-and it is also observable that the alternate pairs of flowers are
-similarly arranged.
-
-Figure II. is a sketch from a shoot which leaned towards one side,
-and of which the higher part, as it bent more and more, got its upper
-side more and more differently conditioned from its lower side. Here
-we find that not only the leaves, but also the flowers, have adjusted
-themselves to the changed conditions. The leaves of the lowest pair
-hang out in the normal way, on the opposite sides of the axis, so that
-a plane passing through their surfaces will cut the axis transversely;
-and their two axillary flower-buds, _c_ and _d_, are similarly placed
-on opposite sides of the axis. But at the other part of the shoot, we
-see both that the leaves have adjusted themselves so that their planes,
-no longer cutting the axis transversely, keep a fit adjustment with
-respect to the light; and also that the flowers, no longer on opposite
-sides of the axis, have bent round to the upper side, as at _a_ and _b_.
-
-Figure III. shows us this re-arrangement carried still further. The
-shoot it represents was growing in a direction nearly horizontal, and
-therefore receiving the light only on one side. And here, besides
-seeing that the leaves have so adjusted themselves that they all lie in
-approximately the same plane, which is parallel to the axis instead of
-transverse to it, we see that the two pairs of flower-buds have both
-come round to the upper side of the axis. So that in this shoot, the
-original radial symmetry in the arrangement of leaves and flowers, is
-completely changed into a bilateral symmetry.
-
-  [Illustration: Figs. 1–3.]
-
-These facts do not, it is true, prove any modification in the forms
-of the flowers themselves: they only prove modification in the grouping
-of the flowers. But beyond showing, as they do conclusively, how
-readily a bilateral arrangement of flowers is producible out of an
-arrangement that was not bilateral, by the action of light, etc.;
-they give increased probability to the belief that changes in the
-shapes of flowers are producible by the same agencies. Doubtless this
-change in the attitudes of the flower-buds is due to the action of
-light on their calyces and peduncles more than to its action on their
-unfolding corollas. But along with an action so decided on the growth
-of these sheathing and supporting organs containing chlorophyll, it
-is scarcely probable that there is _no_ action on the growth of the
-petals, containing other colouring matter; considering that in both
-cases the development of the colouring matter depends on the action of
-light, and considering also the effect of light on petals, familiarly
-shown by their opening and closing. And if even but a small effect is
-producible on the growth of the corolla, then it is to be expected that
-light will be an agent in changing the form of the corolla, when the
-attitude of the flower causes its parts to be differently exposed. For
-a small effect on the individual flower will become a great effect in
-the flowers of remote descendants; provided the changed attitudes of
-the flowers preserve considerable constancy throughout the succession
-of individuals.
-
-Be this as it may, however, the facts I have here described, which
-I doubt not other observers have seen paralleled in other plants,
-are instructive, as showing how quickly certain metamorphoses are
-produced, and as implying the easy establishment of such metamorphoses
-as permanent characters in a species, if the modifying conditions
-become permanent. The changes of arrangement I have pointed out, do not
-become permanent in this species because its individuals are variously
-affected by the modifying forces: on some they do not act at all,
-on some a little, on some much; and even on the same individual the
-different shoots are quite differently affected. But if the habit of
-this plant were greatly changed--if, for instance, by spreading into
-habitats yielding abundant nutriment, the plant became very luxuriant,
-and, multiplying its branches, grew shrub-like; it is clear that, being
-shaded by one another, these branches would be habitually circumstanced
-in a way like that which we here see produces bilateralness in the
-distribution of the flowers, if not in the flowers themselves; and
-being thus permanently affected, would become permanently bilateral.
-Accumulating by inheritance, what is here only an individual
-peculiarity, would become a peculiarity of the species--a specific
-character.
-
-
-
-
-                              APPENDIX F.
-
-               PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS.
-
-
-There has recently come before me a fact which has a significant
-bearing on the hypothesis of Constitutional units: serving, indeed, to
-give an apparently conclusive proof of its truth. Before stating it,
-however, I may with advantage re-state the several evidences already
-assigned in support of it.
-
-       *       *       *       *       *
-
-1. First comes the _à priori_ reason. These units in the germ of an
-organism which cause development into a special structure, cannot be
-_chemical_ units--cannot be simply molecules of proteid substance in
-one or other of its forms; since these are not special to any type of
-creature but common to all creatures. Nor can they be what we may call
-_morphological_ units--the cells or protoplasts; because in the early
-stages of development the cells of one organism are indistinguishable
-from those of others, and because were cells the units of composition
-there could be no interpretation of what are called unicellular
-organisms--nothing to account for the innumerable varieties of them.
-Hence, of necessity, the structural elements of which each organism is
-built, being neither proteid molecules nor cells, must be something
-between them: probably some complex combination of different isomeric
-forms of proteids.
-
-       *       *       *       *       *
-
-2. That units of such natures are the essential components of each
-species of organism, is shown by the fact that in low types of
-creatures, little differentiated into special tissues, any considerable
-portion of the body will, when separated, begin to assume the structure
-proper to the species--a truth recently shown afresh by Prof. T.
-H. Morgan’s experiments on the regeneration of _Planaria maculata_
-(already referred to in § 206) showing that various fragments cut out
-develop into new individuals, and that when, being too small they die
-before doing this, there is always an abortive attempt to assume the
-specific structure.
-
-       *       *       *       *       *
-
-3. This truth that a portion of undifferentiated tissue, if adequate in
-quantity, assumes the structure of the type, illustrating as it does
-the proclivity of the constitutional units towards the structure of
-the species, allies itself with the phenomena of both agamogenesis and
-gamogenesis. The first of these shows us how a fissiparously-detached
-portion of the parental tissue takes on the same form as the parent;
-and the second shows how those small detached portions distinguished
-as sperm-cell and germ-cell also, when united and supplied with the
-needful materials, do the same thing.
-
-       *       *       *       *       *
-
-4. But the set of phenomena following the union of sperm-cell and
-germ-cell differ in a certain way from those which follow when a
-_gemma_ or other unfertilized portion of parental tissue is detached.
-The incomprehensibleness of this difference as otherwise contemplated,
-and the partial comprehensibleness of it when joined with the
-hypothesis of physiological units, furnish a further support for the
-hypothesis.
-
-The familiar truth learnt by the tyro in algebra that an apparent
-solution which contains the unknown quantity is no solution, is a
-truth apt to be overlooked in other spheres than the algebraic. An
-illustration is supplied by the answer once given in Parliament to the
-question “What is an Archdeacon?”--“One who discharges archidiaconal
-functions.” But science as well as daily life furnishes examples.
-When it is said by Engelmann, Hensen, Hertwig, and Maupas that “the
-essential end of sexuality is rejuvenescence, that is, the restoration
-of growth-energy,” we have another instance of an explanation which
-explains nothing. What is the phenomenon to be explained? That
-unfolding of an organism from a germ which displays growth-energy. And
-what is the explanation? The giving of fresh growth-energy. The unknown
-quantity “growth-energy” is contained in the explanation proposed.
-There exists no conception of “juvenescence” save that derived from
-observing developing plants and animals; and if “re” be prefixed, no
-interpretation is thereby given to the unexplained thing “juvenescence.”
-
-Coleridge somewhere comments on a source of fallacy which he calls the
-“hypostasis of a relation”--the changing of a relation into a thing.
-The plumber who tells you that water rises in a pump “by suction”
-supplies an instance. Having assumed suction to be an agent, he
-thinks that he understands how the piston does its work. Some of the
-explanations given of fertilization supply further instances. When it
-is said that sexual union has for its end “to give increased vigour
-to all the vital processes,” it is tacitly implied that vigour is a
-something--a something which can be given. But now, in the first place,
-it is only by the hypostasis of a relation that we are led to think
-of vigour as a thing. Vigour is a state--that state of a living body
-which enables it to give out much motion. What enables it to do this?
-The presence in it of abundant molecules containing much molecular
-motion which can be transformed into molar motion: the transformation
-being effected by the falling of these molecules into their simpler
-and relatively-inert components, which are thereupon excreted.
-Energy-containing matter is used up, and more energy or vigour can be
-given only by supplying more such matter. How then can the union of
-two nuclei--those of the sperm-cell and germ-cell--give vigour? Only
-an infinitesimal portion of vigour in the sense above explained exists
-in either, and the union of them leaves it still infinitesimal. And
-then, even supposing the vigour to be an entity and to be appreciable
-in quantity, how could it go on producing that immense combination
-of physiological actions seen in the unfolding of the germ into an
-organism? and how could it go on producing the physiological actions of
-an adult organism during a whole century?
-
-May we not then say that these proposed explanations leave the question
-where it was--are nominal solutions, not real solutions?
-
-       *       *       *       *       *
-
-5. But the hypothesis of constitutional units furnishes, if not a
-satisfactory answer yet, something in the nature of an answer--a true
-cause; that is to say, a cause actually known to us as operating in
-other cases. In § 92 it was pointed out that in proportion as units
-are similar, there may be built up from them an aggregate which is
-relatively stable, and that along with increasing dissimilarity the
-stability of the aggregate decreases. It was inferred that if a group
-of constitutional units belonging to one individual which have become
-moulded into relatively exact congruity with the organism and with
-one another by long co-operation, are mingled with some belonging
-to another individual which, differently circumstanced, has become
-somewhat different in itself and in its units, then the mass formed by
-the union of the two groups will be relatively unstable--relatively
-modifiable by incident forces. Whereas in either organism, no longer
-perpetually changed in the relations of its parts by growth, there is
-an approach towards equilibrium between the whole and its components,
-the components contributed by the two to form a germ, being slightly
-unlike one another, will not form a group in a state of equilibrium.
-The group they form will be capable of easy change by incident forces;
-and they will so be rendered free to follow their proclivities towards
-the typical form of the species. Inferring this we must also infer that
-so long as these two sets of slightly different units are not exposed
-to any constant forces tending to coerce them into the same form, there
-will continue to exist in the nuclei of all descendant cells this same
-relative instability and consequent plasticity.
-
-Such evidence as we have verifies this interpretation. There is
-first the universal fact that development of the germ begins when
-it is exposed to an incident force--heat--the undulations of which,
-increasing the oscillations of the mixed units, give them greater
-freedom to arrange themselves in conformity with their type. We see
-this alike when spring warmth makes a seed germinate and when the
-warmth of a sitting hen sets up organization in her eggs. Heat frees
-the molecules of inorganic matter from local restraints and, as we see
-in molten metal, lets them yield to other forces; and similarly in this
-organic matter, the units are made free to follow their proclivities.
-Then, secondly, there comes the evidence from comparisons between the
-effects of mixing constitutional units differing in various degrees.
-Let the cluster of mixed units be derived from animals that are
-ordinally distinct. Nothing happens. The units each contributes tend to
-arrange themselves after the parental type. Hence a conflict between
-the tendencies towards two markedly unlike structures, and no structure
-arises. Suppose the mixed units come from two kindred species--say
-horse and ass. The structures which they respectively tend to form,
-being in their main characters alike, there is such co-operation
-as produces a working organism but an organism in certain respects
-imperfect--a mule. Suppose, again, the units come from two varieties
-of the same species. A perfect organism results, and, as shown by Mr.
-Darwin when detailing the effects of crossing, an unusually vigorous
-organism. The units being more unlike than those belonging to the same
-variety, the instability of the germ-plasm is unusually great, and
-the transformations which constitute development and action become
-unusually active. When, as in ordinary cases, the units are supplied by
-members of the same variety who have not been made very much alike by
-their antecedents, there follows the usual amount of organic vigour.
-Coming now to the results of breeding in-and-in--breeding between
-individuals whose constitutions (_i.e._ constitutional units) have for
-generations been growing more alike in the absence of crossing with
-other stirps--we see that diminution of organic vigour is displayed:
-there is a decrease in the rate of physiological change. Finally, on
-coming to a closer relationship, as in marriages between cousins, in
-whom the constitutional units are more than commonly alike, we see
-there frequently follows either barrenness or the production of feeble
-offspring.
-
-All these facts, then, are congruous with the hypothesis that the
-use of fertilization is the mixing of unlike units, and consequent
-production of plasticity. Leaving out cases in which the unlikenesses
-are so great as wholly to prevent co-operation among the units, the
-degree of vigour, that is, the activity of physiological change, is
-great where the unlikeness is great and diminishes with the approach
-towards likeness.
-
-       *       *       *       *       *
-
-6. The existence of constitutional units seems otherwise necessarily
-implied. I refer to the fact that no organism is a homogeneous mean
-between its parents but consists of a mixture of parts, some following
-one parent and some the other. Among illustrations of this the most
-conspicuous are those yielded by the variously-mixed colours of
-hair or feathers. Horses, cattle, dogs, cats, hens, pigeons display
-these mixtures: colours in one place like the mother and in another
-place like the father. As the internal organs are invisible, and as
-visible organs have indefinite shapes and graduate indefinitely into
-adjacent ones, the mixture of traits is elsewhere less conspicuous; but
-occasional marked cases (especially in malformations) leave no doubt
-that it pervades the entire organism.
-
-This peculiarity of transmission seems necessarily to imply that there
-are distinct units derived from the two parents, and that in the course
-of development there is more or less segregation of them--those of
-the one origin predominating so far in some places as to give special
-likeness to one parent, and those derived from the other doing the like
-in other places. All which interpretation is impossible unless the
-hypothesis of constitutional units be admitted.
-
-       *       *       *       *       *
-
-7. I come at length to the special evidence referred to at the outset.
-It is evidence of the same nature as that just assigned, but carried
-to a higher stage. It is furnished not by the segregation of traits
-derived from two parents of the same variety, but is furnished by the
-segregation of traits derived from parents of different varieties. In
-articles on “Bud Variations or Sports” (_Gardener’s Chronicle_, 1891)
-Dr. Masters gives various examples of the separation or unmixing of
-ancestral constitutions. Mr. Noble formed a hybrid between _Clematis
-Jackmani_ and _C. patens_. One of these varieties flowers in the autumn
-on new wood, while the other flowers in the spring on old wood; and
-the result is that flowers of two kinds, quite unlike, are produced at
-different parts of the year, and that by pruning so as to cut away one
-or other set of shoots, the plant may be made to produce exclusively
-for the time being one or other sort of flower.
-
-   “Another very interesting case of unmixing, or, if it be
-   preferred, of partial mixture, is afforded by Neubert’s
-   Berberis. This is a hybrid between the evergreen pinnate-leaved
-   Mahonia and the deciduous simple-leaved Berberis vulgaris, and
-   it bears leaves some of which are intermediate in appearance,
-   while others are much like those of one or other of its parents.
-
-   “A not uncommon illustration of a similar kind, is the
-   production of a Peach and a Nectarine on the same branch, and we
-   have just learnt from Canon Ellacombe that some of the Berlin
-   Hellebores show evidence of their hybrid nature by occasionally
-   producing foliage [and flowers?] of the two parents separately
-   from the same root-stock.
-
-   “In addition to the cases given above, we may here cite a few
-   more which have come under our notice, such as a Chrysanthemum,
-   half the florets of which are of one colour, half of another.
-   A hybrid Calanthe, showing a similar piebald variation, is
-   shown in Fig. 14. A very curious case was that of the Narcissus
-   received from Mr. Walker, and in which flowers of two distinct
-   varieties sprang from the same bulb. Grapes not uncommonly show
-   their crossed origin by presenting a striped appearance, one
-   stripe being of one colour, one of another, as may also be seen
-   in the Orange, Apple, Lemon, and Currant.”
-
-Thus, however the germ-plasm is constituted its essential components
-cannot be all alike. Before there can be this dissociation of ancestral
-characters, there must be in the germ-plasm different elements capable
-of being dissociated. This single fact seems to compel us to assume
-constitutional units.
-
-
-
-
-                              APPENDIX G.
-
-         THE INHERITANCE OF FUNCTIONALLY-CAUSED MODIFICATIONS.
-
-
-In Part II, Chapter X^A, I have confessed that the process by which
-a structure changed by use or disuse affects the sperm-cells or
-germ-cells whence arise descendants, is unimaginable: without, however,
-inferring that therefore such a process does not exist. With others
-it seems different. Some three years ago the following expression of
-opinion came to me from a zoological expert:--
-
-   “Many zoologists--most of us here at Cambridge--are intensely
-   opposed to the doctrine of the inheritability of acquired
-   variations. Even assuming that the developmental power of a
-   germ is determined by its molecular structure (and I for one
-   would question this--Driesch and his school when they find that
-   they can squeeze a developing egg into all sorts of shapes
-   without altering the final result, that one blastomere in an
-   egg which has divided into 8 is still able to reproduce a whole
-   embryo--question it also), we still fail to conceive any means
-   by which, for instance, a change in the development of a muscle
-   or nerve can effect a corresponding change in that part of the
-   germ which is destined to produce a corresponding part in the
-   descendant.”
-
-Here it will be observed that belief in the inheritance of structural
-effects wrought by use and disuse, is rejected because of inability “to
-conceive any means” by which the modifications produced in an organ can
-effect a correlated modification in the germ of a descendant: failure
-to conceive is the test. The implication is that some alternative
-hypothesis is accepted because the correlating of a variation in an
-organ with a corresponding germ-variation is effected by a means which
-_is_ conceivable. This is the hypothesis of Weismann. Concerning its
-conceivability I have, in the chapter just named, already written as
-follows:--
-
-   “If we follow Prof. Weismann we are led into an astounding
-   supposition. He admits that every variable part must have a
-   special determinant, and that this results in the assumption of
-   over two hundred thousand for the four wings of a butterfly.
-   Let us ask what must happen in the case of a peacock’s feather.
-   On looking at the eye near its end, we see that the minute
-   processes on the edge of each lateral thread must have been in
-   some way exactly adjusted, in colour and position, so as to fall
-   into line with the processes on adjacent threads: otherwise the
-   symmetrical arrangement of coloured rings would be impossible.
-   Each of these processes, then, being an independent variable,
-   must have had its particular determinant. Now there are about
-   300 threads on the shaft of a large feather, and each of them
-   bears on the average 1,600 processes, making for the whole
-   feather 480,000 of these processes. For one feather alone there
-   must have been 480,000 determinants, and for the whole tail
-   many millions. And these, along with the determinants for the
-   detailed parts of all the other feathers, and for the variable
-   components of all organs forming the body at large, must have
-   been contained in the microscopic head of a spermatozoon!” [And
-   each of them must, throughout all the complex developmental
-   processes, have preserved the ability to find its way to the
-   exact place where it was wanted!]
-
-If my Cambridge correspondent is able to conceive this process implied
-by the hypothesis of Weismann, I can only say that he has an enviable
-power of imagination.
-
-       *       *       *       *       *
-
-But now comes the strange fact that an impossibility of thought implied
-by Weismann’s hypothesis does not cause rejection of it, but yet is
-urged as a reason for rejecting an alternative hypothesis which does
-not imply it. One objector cannot conceive that “a change in the
-development of a muscle or nerve can effect a corresponding change in
-that part of the germ which is destined to _produce a corresponding
-part_ in the descendant”; and another objector says it is “very hard to
-believe” that a functionally-changed organ will so affect spermatozoa
-and ova that “_one particular part of them_ will be so altered that
-the organisms which grow up from them will be able to present the
-same modification on the application of a different stimulus.” It is
-tacitly assumed by both that, as in the hypothesis of Weismann so in
-the counter-hypothesis, a particular part of the germ-plasm gives
-origin to a particular part of the developed organism. But nothing of
-the kind is implied. The nature of the counter-hypothesis (at any rate
-as held by me) is entirely misapprehended. Anyone who turns back to
-the chapters in the first volume where the conception of physiological
-units (or constitutional units) was set forth, or who re-reads the
-foregoing appendix, will see that there is altogether excluded any idea
-of correlation between certain parts of the germ and certain parts of
-the resulting organism. The units are supposed to be all alike, and
-during the progressive embryological changes local groups of them are
-supposed to take on different forms and structures under the combined
-forces, general and local, brought to bear on them. This conception
-is necessitated by all the evidence. The fact disclosed by the
-experiments of Driesch, Wilson, and Chabry, that from fractions of an
-ovum structures may be obtained like that obtained from the whole ovum,
-only smaller, necessitates it. The fact that any sufficiently large
-fragment of a polyp or planarian, no matter from what part of the body
-taken, will develop into a complete polyp or planarian necessitates
-it. The fact that from an undifferentiated portion of a plant, even
-so small as a scale, a complete plant may arise necessitates it. And
-it is necessitated by the fact that among plants, roots are produced
-by imbedded shoots and shoots by roots, as well as by the fact that
-low animals, such as hydroids, if deprived of both head and root,
-will develop a head from the root part and a root from the head
-part, if their respective conditions are inverted. All this evidence
-shows conclusively that the component units of each species, whether
-existing in the germ or in the developed organism, are, when not yet
-differentiated by local conditions, all alike, and that the notion of
-special parts of the germ-plasm correlated with special parts of the
-resulting organism, is entirely alien to the hypothesis.
-
-“But how do the units of a modified organ affect the units of the
-germ in such wise that these produce an inherited modification of
-the organ?” will be asked. This difficulty has been dealt with in
-§§ 97_d_, 97_e_, where the analogy between the social organism and
-the individual organism has been brought in aid: serving, if not to
-furnish a conception, yet to furnish an adumbration. Regarding citizens
-as the units of an unfolding society, say a colony, it was pointed
-out that the nature they inherit from a mother-society gives them a
-proclivity towards a society of like structure, the traits of which
-are progressively assumed as the colony grows sufficiently large to
-make them possible. At the same time it was pointed out that while the
-influence of the entire aggregate on the individuals is seen in this
-forming of them into a society of the inherited type, the influences of
-local circumstances, and of individuals on one another, in each group,
-make them differentiate into appropriate social structures, taking on
-fit occupations and industries: the implication being that in virtue
-of their inherited natures they all have partial capacities for the
-various activities they undertake; so that an immigrant clerk sets up
-a tavern, a compositor takes to carpentering, and a university man
-rides after cattle or is employed on a sheep farm. Evidence was given
-in that place, as in the above paragraph, that the constitutional units
-of an organism similarly have all of them potentialities for taking
-on this or that structure and mode of action which local conditions
-determine. It was further argued that as citizens are continually
-being remoulded by their society into congruity with it, and, if
-circumstances change them, tend to remould their society; so in the
-individual organism, there is this reciprocal action of the whole on
-the units and of the units on the whole. Hence it was inferred that the
-modified units in any modified part tend to diffuse modifications like
-their own through the units at large: being aided by the circulation of
-protoplasm, as suggested in §§ 54_d_ and 97_f_. And it was urged that,
-however inconceivably complex such a process may be, yet it seems not
-incredible when we recognise the probability that an organism is more
-or less permeable to undulations propagated by its molecules: Rontgen
-rays giving warrant. If such units throughout the tissues may take in
-and send out ethereal waves which bring it into rhythmical relations
-with others of its kind and tend to produce congruity, it becomes,
-if not conceivable still supposable, that throughout the circulating
-protoplasm there goes on a continual harmonization of its components--a
-moulding of each by all and of all by each. Should it be said that such
-a process is too marvellous to be reasonably assumed, the reply is that
-it is not more marvellous than heredity itself, which, were it not
-familiar to us, would be thought incredible.
-
-       *       *       *       *       *
-
-But as I have said in the place referred to--“At last then we are
-obliged to admit that the actual organizing process transcends
-conception. It is not enough to say that we cannot know it; we must say
-that we cannot even conceive it:” can only conceive the possibility of
-a suggested interpretation.
-
-Hence we have to rely upon evidences of other kinds. Among these, some
-which I think dispose absolutely of the fashionable hypothesis while
-they harmonize with the opposed hypothesis, have now to be named. That
-their implication should not have been generally recognized would have
-seemed to me incomprehensible were it not that I have myself only now
-observed this implication. The facts are these:--
-
-   “Verlot mentions a gardener who could distinguish 150 kinds
-   of camellia, when not in flower; and it has been positively
-   asserted that the famous old Dutch florist Voorhelm, who
-   kept above 1,200 varieties of the hyacinth, was hardly ever
-   deceived in knowing each variety by the bulb alone. Hence we
-   must conclude that the bulbs of the hyacinth and the branches
-   and leaves of the camellia, though appearing to an unpractised
-   eye absolutely undistinguishable, yet really differ.” (Darwin,
-   _Variation of Animals and Plants, &c._, vol. ii, p. 251.)
-
-More recently testimony to like effect has been given by Dr. Maxwell
-Masters, and has already been quoted by me in a note to § 286 in
-illustration of another truth. He says concerning such variations:--
-
-   “To the untrained eye, the primordial differences noted are
-   often very slight; even the botanist, unless his attention
-   be specially directed to the matter, fails to see minute
-   differences which are perceptible enough to the raiser or his
-   workmen.... These apparently trifling morphological differences
-   are often associated with physiological variations which render
-   some varieties, say of wheat, much better enabled to resist
-   mildew and disease generally than others. Some, again, prove to
-   be better adapted for certain soils or for some climates than
-   others; some are less liable to injury from predatory birds than
-   others, and so on.”
-
-In his _Vegetable Teratology_, p. 493, Dr. Masters names another
-fact having a like implication--the fact that among seedling stocks
-which have not yet flowered, those which will produce double flowers
-are distinguishable. He says:--
-
-   “This separation of the single from the double-flowered plants,
-   M. Chatié tells us is not so difficult as might be supposed. The
-   single stocks, he explains, have deep green leaves (glabrous in
-   certain species), rounded at the top, the heart being in the
-   form of a shuttlecock, and the plant stout and thick-set in its
-   general aspect, while the plants yielding double flowers have
-   very long leaves of a light green colour, hairy and curled at
-   the edges, the heart consisting of whitish leaves, curved so
-   that they enclose it completely.”
-
-What is the general truth implied? Clearly that there exists no such
-thing as an independent local variation. Some marked change in the form
-or colour of a flower or a fruit draws attention; and, being a change
-which interests the florist or gardener, pecuniarily or otherwise,
-not only draws attention but usually monopolizes attention: the
-natural impression produced being that this variation stands there by
-itself--is without relation to variations elsewhere. But now it turns
-out that there are concomitant variations all over the plant. Even in
-underground bulbs certain appreciable differences go along with certain
-conspicuous differences in the flowers. And if along with a striking
-change in a flower which the florist contemplates, there go changes
-all over the plant not obvious to careless observers but visible to
-him, we must infer that there are everywhere minute differences which
-even the florist cannot perceive: the whole constitution of the plant
-has diverged in some measure from the constitutions of kindred plants.
-Every local variation implies a change pervading the entire organism,
-manifested in concomitant variations everywhere else.
-
-If so, what becomes of the hypothesis of determinants--the hypothesis
-that there is a special element in the germ-plasm which results in a
-special local modification in the adult organism? That there are no
-facts supporting it has been all along manifest; but now it is manifest
-that the facts directly contradict it.
-
-At the same time it may be remarked that while the facts are wholly
-incongruous with the hypothesis of determinants and its accompanying
-elaborate speculation, they are not incongruous with the alternative
-hypothesis. Impossible though it may be to imagine the natures of
-those ultimate units peculiar to each species, which have proclivities
-towards the particular form of organization characterizing it, yet
-that a change of structure arising in one part of the organism is
-accompanied by multitudinous changes of structure in other parts of
-the organism, is not only congruous with the belief that there exist
-such constitutional units, but yields it distinct support. For if, as
-above argued, a conspicuous local variation is not the result of any
-modification of units special to the locality, but is the result of
-a modification of the units at large, then it must happen that such
-modification must have its effects on all other parts of the organism;
-so that there cannot fail to result all those small concomitant
-variations above indicated.
-
-May we not also say that it becomes less incomprehensible that
-structural changes caused by use and disuse are inherited? If, as
-we see, a local variation spontaneously arising is accompanied by
-multitudinous other local variations, implying a necessary correlation
-between each local variation and the general constitution of the
-organism; then it may be argued that if a marked change of function in
-an organ causes increase or decrease of it, this general correlation
-implies that there must be a reciprocal reaction between the part and
-the whole, tending to re-establish their congruity. The constitution
-at large will in so far be changed, and along with its change will go
-corresponding changes in the sperm-cells and germ-cells.
-
-       *       *       *       *       *
-
-Finally let me add, not another argument, but another fact of
-observation, of the kind which opponents demand, but which, when they
-are from time to time furnished, are severally pooh-poohed as not
-enough. Each of them is spoken of as a solitary fact and slighted as
-inadequate; and when by and by another is named, this is treated in
-the same way; so that the facts which if brought together would be
-recognized as sufficient are never brought together. That to which I
-refer is set forth in a pamphlet by M. Leo Errera, Professor at the
-University of Brussels, entitled “Hérédite d’un Caractère acquis chez
-un Champignon pluricellulaire;” being an account of experiments of
-Dr. Hunger, at the Botanical Institute in Brussels. First enumerating
-various instances of adaptations to climate, as those of plants which,
-fitted to northern regions, preserve their constitutional rapidity
-of growth and seeding when brought south, and do this for several
-generations, he goes on to detail the culture-experiments of M. Hunger,
-and sums up the results of these in the following words:--
-
-   “On déduit de là que:
-
-   “1^o Les conidies d’_Aspergillus niger_ sont adaptées à la
-   concentration du milieu où a vécu l’individu qui les porte; cet
-   effet est encore plus marqué après deux générations passées dans
-   un milieu donné (Expér. I et II);
-
-   “2^o II s’agit d’une véritable adaptation et non pas simplement
-   d’un accroissement de vigueur chez les conidies provenant des
-   liquides concentrés, car ces mêmes conidies germent moins
-   rapidement et donnent des plantes moins vigoureuses que
-   les conidies normales lorsqu’on les sème de nouveau sur le
-   milieu-type: en s’adaptant aux liquides concentrés, elles se
-   sont _désadaptées_ du liquide normal (Expér. III);
-
-   “3^o Une génération passée sur le liquide normal n’efface pas
-   l’influence d’une ou de deux générations antérieures passées sur
-   une liquide plus concentré (Expér. IV).
-
-   “Tous ces résultats concordent: _ils montrent une légère, mais
-   incontestable transmission héréditaire de l’adaptation au
-   milieu_.”
-
-
-
-
-                            SUBJECT-INDEX.
-
-   (For this Index as it appeared in previous editions the
-   Author is indebted to F. HOWARD COLLINS, Esq., of Edgbaston,
-   Birmingham. It has now been adjusted to suit the present revised
-   and enlarged edition.)
-
-
-    Acacia, foliar organs, =II=, 41, 264.
-
-    _Acalephæ_: environment, =I=, 105;
-      water in, =I=, 173.
-
-    _Acari_: special creation and effects of, =I=, 428;
-      direct transformations, =I=, 706;
-      segmentation, =II=, 111.
-
-    _Acorus calamus_, agamic propagation, =I=, 642.
-
-    Acquired characters, inheritance of: functionally-produced
-        modifications in plants and animals, =I=, 307–13, 318, 526,
-        541, 562, 692–5; =II=, 618–22;
-      conceivability of, on the hypothesis of physiological units, =I=,
-          368–71, 695; =II=, 618–22;
-      diminution of jaw, =I=, 541–2, 693;
-      current views on, =I=, 559–60;
-      cessation of selection, =I=, 560–3;
-      Eimer’s theory of orthogenesis, =I=, 560;
-      species differentiation, =I=, 573;
-      location of mammalian testes, =I=, 573;
-      tactual perceptiveness, =I=, 602–8, 633, 665, 666, 672–3, 692;
-      blindness of cave-animals, =I=, 612–3, 647–9;
-      co-adaptation of co-operative parts, =I=, 621, 663–5;
-      transmission of disease, =I=, 622–3;
-      hypothesis supported by telegony, =I=, 624–8, 644–6, 649–50;
-      views of Darwin and neo-Darwinists, =I=, 630, 685, 690;
-      why facts in support are meagre, =I=, 632;
-      degradation of little toe, =I=, 652–3, 673;
-      neuter forms of social insects, =I=, 658–9, 663–4, 670, 675;
-      degenerated instinct in ants, =I=, 660–2;
-      rudimentary limbs of whale, =I=, 669, 692;
-      importance of question, =I=, 672, 690;
-      monstrous development of honey-ants, =I=, 683–4;
-      osteology of Punjabis, =I=, 689;
-      summary of evidences in support, =I=, 692–5;
-      genesis of vertebrate skull, =II=, 227;
-      false joints, =II=, 371, 372;
-      conceivability of rival hypotheses, =II=, 618–22;
-      adaptation to environment in _Aspergillus_, =II=, 623.
-
-    Acrogens, the term, =II=, 55–6.
-      (See _Archegoniateæ_.)
-
-    _Actinophrys_: a primary aggregate, =II=, 76;
-      genesis, =II=, 452.
-
-    _Actinozoa_: multiaxial development, =I=, 166;
-      waste and repair, =I=, 213, 219;
-      differentiation, =I=, 391;
-      parasitism, =I=, 397;
-      integration, =II=, 92;
-      symmetry, =II=, 189, 192;
-      growth and genesis, =II=, 444.
-
-    Activity: the principle of, the essential element in Life, =I=,
-        113, 114, 122;
-      not inherent in living matter, =I=, 120;
-      nutrition and genesis, _résumé_, =II=, 497–9;
-      and evolution, =II=, 501–4.
-
-    Adaptation: general truths, =I=, 227–33, 233–5;
-      botanical, =I=, 227;
-      physiological, =I=, 228–33;
-      psychological, =I=, 229, 230–3;
-      structural, functional, and interdependence, =I=, 235–9, 240–1,
-          318;
-      social and organic stability, =I=, 240–2;
-      _résumé_, =I=, 242–3;
-      to varied media, =I=, 479–81, 489, 556;
-      multiplication of effects, =I=, 512–3, 550;
-      direct equilibration, =I=, 522–3;
-      natural selection and equilibration, =I=, 530–5;
-      non-adaptive specific characters, =I=, 565;
-      time required for effecting, =I=, 565–6;
-      an obstacle to re-adaptation, =II=, 11;
-      of skin and skeleton, =II=, 215, 217;
-      outer tissue, =II=, 312–4, 387;
-      skin and mucous membrane differentiation, =II=, 321–2, 389;
-      vascular system, =II=, 343–4;
-      osseous, =II=, 352;
-      muscular, =II=, 368–9, 391;
-      persistence of force and physiological, =II=, 394;
-      of reproductive activity to conditions, =II=, 411–6;
-      vertebræ development, =II=, 563–6.
-      (_See also_ Co-adaptation.)
-
-    Africa, effect of climate on inhabitants, =I=, 30.
-
-    Agamogenesis: alternation with gamogenesis, =I=, 266–7, 272–3,
-        284–94, 336, 592; =II=, 415;
-      parallelism in karyokinesis, =I=, 267–8;
-      a process of disintegration, =I=, 276–7;
-      conditions determining its continuance, =I=, 284–94, 295–7, 330;
-      physiological units, =I=, 351; =II=, 613;
-      spontaneous fission, =I=, 582, 584–7, 589–92, 595–6, 599;
-      remarkable extent of, under favourable conditions, =I=, 591–2,
-          640–1;
-      in _Actinozoa_, =II=, 92;
-      in _Hydrozoa_, =II=, 102;
-      in _Annelida_, =II=, 103;
-      innutrition, =II=, 179–80.
-
-    _Agaricinæ_, =II=, 139, 257.
-
-    Agassiz, L. J. R., zoological classification, =I=, 380.
-
-    Aggregates, Animal and Plant (_see_ Morphology).
-
-    Agility, a vital attribute, =I=, 578.
-
-    Agrimony, floral symmetry, =II=, 42, 167, 170.
-
-    Air, in vegetal tissues, =II=, 567–8, 583, 591, 593.
-
-    “Air plants,” =I=, 208.
-
-    Albumen: properties, =I=, 12;
-      Lieberkühn’s formula, =I=, 13;
-      diffusibility, =I=, 19;
-      in organic tissues, =I=, 41.
-
-    Alcohols, properties, =I=, 10–12.
-
-    _Algæ_: reproduction, and the dynamic element in life, =I=, 118–9;
-      multicentral development, =I=, 163, 164;
-      axial development, =I=, 165;
-      locomotive powers of minute forms, =I=, 196;
-      uniform tissue and function, =I=, 200, 586;
-      gamogenesis, =I=, 271, 279, 280, 283; =II=, 448, 449, 450;
-      fertility, =I=, 582; =II=, 440, 441;
-      fission, =I=, 584, 585;
-      unicellular forms, =II=, 22;
-      integration in _Confervoideæ_ and _Conjugateæ_, =II=, 25;
-      pseudo-foliar and axial development, =II=, 28–33, 57;
-      foliar development, =II=, 76, 91;
-      branch symmetry, =II=, 145;
-      cell metamorphoses, =II=, 176;
-      tissue differentiation, =II=, 244, 246, 251, 252, 256, 272,
-          385–6;
-      adaptation of reproductive activity to conditions, =II=, 289;
-      integration, =II=, 292;
-      indefiniteness, =II=, 295;
-      genesis and development, =II=, 463.
-
-    Alimentary canal: metabolic processes and agents, =I=, 68–9, 74;
-      structural traits, =I=, 192;
-      progressive development, =I=, 195;
-      relation to environment, =I=, 196;
-      function, =I=, 205;
-      segmentation in annelids, =II=, 125;
-      differentiation, =II=, 301, 302, 321–2, 323–5, 389;
-      specializations in birds, =II=, 325;
-      in ruminants, =II=, 327–9;
-      differentiation of liver, =II=, 329–33;
-      muscularity, =II=, 364.
-
-    Allotropism: of organic constituents, =I=, 4, 9;
-      muscular action, =I=, 59.
-
-    Alloys, melting point of, =I=, 339.
-
-    Alternation of generations, misleading application of term, =II=,
-        84.
-      (_See_ Agamogenesis _and_ Gamogenesis.)
-
-    Amitosis, occurrence of, in morbid tissues, =I=, 264.
-
-    Ammonia: properties, =I=, 7, 9;
-      nerve stimulation, =I=, 55.
-
-    _Amœba_: central development, =I=, 163;
-      a primary aggregate, =II=, 86;
-      symmetry of encysted, =II=, 186;
-      symbiosis, =II=, 400.
-
-    _Amphibia_: classification of, =I=, 392;
-      embryonic respiratory system, =I=, 457;
-      structure and media, =I=, 483;
-      limb locomotion, =II=, 15;
-      segmentation, =II=, 122, 225;
-      outer tissues, =II=, 311;
-      respiration, =II=, 334, 338;
-      Owen on skeleton, =II=, 552, 557, 558.
-
-    _Amphioxus_: separation of segmentation spheres of egg, =I=, 691;
-      embryogeny, =II=, 121;
-      local segmentation, =II=, 125–7, 605;
-      genesis of vertebrate axis, =II=, 213–6, 218, 222;
-      development, =II=, 564.
-
-    _Amphipnous cuchia_, vascular air-sacs, =II=, 337.
-
-    _Anabas scandens_, the climbing fish, =I=, 480, 483.
-
-    _Anacharis_ (see _Eloidea_).
-
-    Anæsthetics, diverse effects of, =I=, 55.
-
-    _Angræcum_, assimilative function of root, =II=, 255.
-
-    “Animal Spirits,” vitalism and, =I=, 115.
-
-    Animals: nutrition and molecular re-arrangement, =I=, 36–7;
-      nitrogenous character, =I=, 39–41;
-      sensible motion, =I=, 57;
-      metabolism, =I=, 62–77;
-      multiplication of energies, =I=, 75;
-      contrasted traits of plants and, =I=, 196;
-      what is an individual? =I=, 246–7;
-      solar influence, =I=, 500, 556;
-      geologic changes affecting, =I=, 501–4, 549, 550, 556;
-      interdependence with plants, =I=, 504–6, 514; =II=, 398–401;
-      complexity of influences affecting, =I=, 506;
-      geographical isolation and origin of species, =I=, 568–9;
-      vital attributes, =I=, 577–9;
-      distribution and antiquity of plant and animal types, =II=, 297;
-      mutual dependence of organisms at large, =II=, 397–408;
-      hypothetical plant-animal type, =II=, 397;
-      progressive increase of size, =II=, 401;
-      laws of multiplication, =II=, 411–6;
-      rhythm in numbers, =II=, 419;
-      law of weights and dimensions, =II=, 434.
-
-    Animals, domesticated: variation, =I=, 324, 326, 560, 563, 693;
-      interbreeding, =I=, 345–7, 354; =II=, 615;
-      pure and mixed breeds, =I=, 354, 625.
-
-    _Annelida_: phosphorescence, =I=, 50;
-      axial development, =I=, 165, 166;
-      integration, =I=, 363;
-      larval forms and phylogeny, =I=, 447; =II=, 115;
-      segmental fission, =I=, 588–9;
-      segmentation, =I=, 98–101, 103–4, 602–5; =II=, 107–9, 125–7;
-      lateral gemmation, =II=, 105;
-      embryogeny, =II=, 119;
-      bilateral symmetry, =II=, 197–200;
-      genesis, =II=, 444, 453.
-
-    _Annulosa_: regeneration, =I=, 361–2;
-      distinctive traits, =I=, 392;
-      origin of type, =II=, 98–110, 602–6;
-      unit of composition, =II=, 105;
-      application of term, =II=, 111;
-      vertebrate symmetry compared, =II=, 203–6;
-      segmental differentiation, =II=, 207–9;
-      unintegrated function in _Planaria_, =II=, 373;
-      development and genesis, =II=, 464;
-      nutrition and genesis, =II=, 490.
-      (See also _Annelida_ and _Arthropoda_.)
-
-    Anthropomorphism, former prevalence of, =I=, 419.
-
-    Ants: utilization of aphids, =I=, 660–1; =II=, 403, 405;
-      nest-mates, =II=, 405;
-      castes in social species, =I=, 658–9, 670, 675;
-      loss of self-feeding instinct in Amazons, =I=, 660–1, 663–4;
-      monstrous development of Honey-ants, =I=, 683;
-      bulk and fecundity, =II=, 492.
-      (See also _Termites_.)
-
-    _Aphis_: individuality, =I=, 249, 250; =II=, 603;
-      parthenogenesis, =I=, 274–5, 289;
-      fertility, =I=, 582, 640–1; =II=, 476, 490;
-      utilized by ants, =I=, 660–1; =II=, 403, 405;
-      over-multiplication checked by lady-bird, =II=, 406.
-
-    Aquatic animals, large size attained by, =I=, 156.
-
-    _Arachnida_: avoidance of danger, =I=, 92;
-      oviparous homogenesis, =I=, 271;
-      segmentation, =I=, 469; =II=, 113, 114;
-      integration and homology, =II=, 111, 121;
-      bilateral symmetry, =II=, 198.
-
-    _Arcella_: symmetry, =II=, 186;
-      outer tissue differentiation, =II=, 309.
-
-    _Archegoniateæ_: morphological composition, =II=, 32–5;
-      growth and development, =II=, 50–6;
-      tubular structure, =II=, 58, 62;
-      alternating generation not distinctive, =II=, 84;
-      asymmetry and environment, =II=, 140;
-      integration, =II=, 293, 296;
-      individuation and genesis, =II=, 441, 451, 463.
-
-    Archenteron: primitive externality, =II=, 301;
-      formation of cœlom, =II=, 302.
-
-    _Archiannelida_: segmentation, =II=, 125.
-
-    _Arenicola marina_: polytrochal larvæ, =II=, 109.
-
-    Arm: embryogeny of human, =I=, 169;
-      vicarious use of, =I=, 209.
-
-    Army, morphological analogy, =II=, 6.
-
-    Arteries (_see_ Vascular System).
-
-    _Arthropoda_: uniaxial development, =I=, 165;
-      protoplasmic continuity, =I=, 190, 629;
-      excursiveness, =I=, 481;
-      limb locomotion, =II=, 15;
-      integration and homology, =II=, 111–4, 121;
-      bilateral symmetry, =II=, 197–200;
-      genesis, =II=, 445, 453.
-
-    Ascidians: multiaxial development, =I=, 165, 166;
-      functional differentiation, =I=, 202;
-      composite individuality of _Doliolum_, =I=, 247;
-      self-fertilization, =I=, 342;
-      integration, =II=, 94, 96, 97;
-      symmetry, =II=, 194;
-      origin of vertebrate type, =II=, 194, 598, 605.
-
-    _Ascomycetes_, reproduction, =II=, 450.
-
-    Assimilation: compared with reasoning, =I=, 81–7;
-      a trait of vitality, =I=, 577.
-
-    _Asteroidea_, radial symmetry, =II=, 196.
-
-    Astronomy: growth of celestial bodies, =I=, 135;
-      Schleiden on individuality, =I=, 245;
-      evolution, =I=, 432, 435;
-      classification of stars, =I=, 444;
-      rhythm of, and organic change, =I=, 499–501, 557;
-      law of equilibration, =I=, 519–20;
-      co-operation of structure and function, =II=, 3.
-
-    Atavism: occurrence of, =I=, 305–6, 314;
-      digital variation, =I=, 321–3.
-
-    Atoms: use of term, =I=, 6, 31;
-      ethereal undulations and oscillations, =I=, 31–5.
-
-    Australia: settler’s usages, =I=, 364;
-      ratio of jaw to skull in natives, =I=, 541.
-
-    Axillary buds, origin and development, =II=, 65–8.
-
-    Axis: “neutral” of mechanics, =II=, 210;
-      genesis of vertebrate, =II=, 212–6, 224–7.
-
-
-    _Bacteria_: fission, =I=, 270;
-      non-nucleated, =II=, 20;
-      rate of increase, =II=, 443.
-
-    Baer, K. E. von: embryological formula, =I=, 171, 172, 451, 453,
-        461, 466;
-      zoological classification, =I=, 383;
-      on animal transitions, =I=, 480.
-
-    _Balanophoræ_, inner tissue, =II=, 274.
-
-    Bark: varied development, =II=, 247–9;
-      physiological differentiation, =II=, 249–50, 258, 386.
-
-    _Basidiomycetes_, reproduction, =II=, 450.
-
-    Bat, infertility of, =II=, 473.
-
-    Bates, H. W., protective mimicry of butterflies, =I=, 398.
-
-    _Batrachia_ (see _Amphibia_).
-
-    Bean, vascular system, =II=, 573, 591.
-
-    Beaver, tail and co-adapted structures, =I=, 616.
-
-    Bees (_see_ Insects).
-
-    _Begoniaceæ_: multiplication =I=, 224, 317, 442;
-      individuality, =I=, 251;
-      development from scales, =I=, 282;
-      symmetry, =II=, 159, 166;
-      development, =II=, 271.
-
-    Berkeley, M. J., indefiniteness of mosses and ferns, =II=, 296.
-
-    Bile, arrest of excretion, =I=, 209.
-
-    Bilirubin and biliverdine, function of, =II=, 330, 333.
-
-    Biology: definition and divisions, =I=, 124–5;
-      organic structural phenomena, =I=, 125–7;
-      also functional, =I=, 127–9;
-      actions and reactions of function and structure, =I=, 129–30;
-      genesis, =I=, 130–1;
-      limited knowledge of, =I=, 131;
-      evolution, =I=, 432, 434;
-      sociological analogies (_see_ Sociology).
-
-    Biophors, Weismann’s germ-plasm units (_see_ Weismann).
-
-    Birds: flesh-eating and grain-eating contrasted, =I=, 68;
-      growth and expenditure of force, =I=, 142;
-      size of egg and adult, =I=, 144;
-      limitations on flight, =I=, 155;
-      self-mobility, =I=, 175;
-      temperature, =I=, 176;
-      functional and structural differentiation, =I=, 201;
-      food of starving pigeon, =I=, 215;
-      viviparousness, =I=, 271;
-      heredity and pigeon breeding, =I=, 305;
-      atavism in pigeon, =I=, 314;
-      osseous variation in pigeon, =I=, 321;
-      classification, =I=, 392;
-      migrations and change of habits, =I=, 399, 402, 500;
-      distribution in time, =I=, 410;
-      Darwin on petrels, =I=, 455;
-      rudimentary teeth, =I=, 457;
-      vertebræ, =I=, 471; =II=, 564;
-      feather development, =I=, 473;
-      habits of water ouzel, =I=, 485;
-      egg shells and direct equilibration, =I=, 526;
-      bones of waders and direct equilibration, =I=, 527;
-      fertility and nervous development, =I=, 598;
-      cellular continuity, =I=, 629;
-      adaptation of structure to environment, =II=, 12;
-      sexual selection, =II=, 269;
-      wing spurs, =II=, 313;
-      outer tissue differentiation, =II=, 314–5, 387;
-      alimentary canal development, =II=, 325, 327;
-      muscular colour and activity, =II=, 365–9;
-      nutrition, =II=, 433;
-      cost of genesis, =II=, 436;
-      growth and genesis, =II=, 454, 458;
-      heat expenditure and genesis, =II=, 468–9, 474;
-      activity and genesis, =II=, 470–2, 474;
-      contrasted mammalian fertility, =II=, 470;
-      eggs of wild and tame, =II=, 478;
-      fertility of blackbird and linnet compared, =II=, 503;
-      Owen on skeleton of, =II=, 559, 560, 561.
-
-    Bischoff, embryogeny of human arm, =I=, 169.
-
-    Bison, modifications entailed by increased weight of head, =I=,
-        512.
-
-    Blackbird, contrasted with linnet in development, =II=, 503.
-
-    Blainville, de, definition of life, =I=, 79, 93.
-
-    Blastosphere, independence of cells in Echinoderm larvæ, =I=, 185.
-
-    Blastula, definition of life and formation of, =I=, 112.
-
-    Blood: similarity of iron peroxide, =I=, 17;
-      metabolic processes, =I=, 69;
-      segregation of abnormal constituents, =I=, 180;
-      protozoon life of corpuscles, =I=, 186–7;
-      morbid changes, =I=, 221, 701;
-      assimilative power and organic repair, =I=, 221–2;
-      respiratory tissue differentiation, =II=, 310–1;
-      pressure in mammals, =II=, 340.
-      (_See also_ Vascular System.)
-
-    Blow-fly, Weismann on nutrition and genesis in, =I=, 678–9.
-
-    Boers, Cape, habits and fertility, =II=, 508.
-
-    Boismont, A. B. de, on human fertility, =II=, 511.
-
-    Bone: growth and function, =I=, 151;
-      adaptability, =I=, 230; =II=, 217–8;
-      function and weight, =I=, 308, 693;
-      mammalian cervical vertebræ, =I=, 394;
-      evolution and vertebral column, =I=, 470–1;
-      partial development, =I=, 473;
-      size of head as influencing, =I=, 512, 536–9;
-      direct equilibration and strength, =I=, 527;
-      natural selection and co-adaptations, =I=, 614–21, 674, 677;
-      rudimentary limbs of whale, =I=, 668, 685, 692;
-      inheritance of acquired modifications in Punjabis, =I=, 689;
-      skull development, =II=, 222;
-      theory of supernumerary, =II=, 223;
-      Cope on origin of vertebrate osseous system, =II=, 225–7;
-      differentiation, =II=, 344–56;
-      false joints, =II=, 370–2;
-      Owen’s theory of vertebrate skeleton, =II=, 548–66.
-
-    Book-worm, food of, =I=, 77.
-
-    Born, G., experiments on frog larvæ, =I=, 365.
-
-    Botany, biological classification, =I=, 124, 125.
-      (_See_ Plants.)
-
-    _Bothriocephalus_, development, =II=, 490.
-
-    _Botryllidæ_: development, =I=, 166;
-      independence of components, =I=, 247;
-      agamogenesis, =I=, 641.
-
-    Bower, Prof., on alternation of generations, =II=, 84.
-
-    _Brachiopoda_, rude vascular system, =II=, 340.
-
-    Bradbury, J. B., on vaso-dilators, =I=, 55.
-
-    Brain: natural selection and mental evolution, =I=, 553;
-      analysis of substance, =I=, 596;
-      weight in higher animals, =I=, 598–9;
-      size in civilized and uncivilized, =II=, 530.
-
-    Branches (_see_ Morphology).
-
-    _Branchiæ_ (_see_ Respiratory System).
-
-    Brass, effect of antimony on, =I=, 121.
-
-    Bread, diamagnetism, =I=, 370.
-
-    Breeding: heredity, =I=, 304–5;
-      in-and-in, =I=, 344–7, 353; =II=, 615;
-      pure and mixed, =I=, 354, 625.
-
-    Bricks, changed equilibrium shown by, =I=, 38, 42.
-
-    Brodie, T. G., cell chemistry, =I=, 260.
-
-    Brownell, Miss J. L., on birth-rate in United States, =II=, 520.
-
-    Brown-Séquard, on inherited epilepsy, =I=, 312, 624.
-
-    _Bryophyllum_, peculiar proliferation, =II=, 295.
-
-    _Bryophyta_, large size attained by some, =I=, 138.
-
-    _Bryozoa_, gemmation, =I=, 588.
-
-    Budding (_see_ Gemmation).
-
-    Buds: development, =I=, 167–8;
-      theories of heredity and cauline, =I=, 358–9, 360;
-      axillary, =II=, 65–9;
-      effects of nutrition, =II=, 73–4.
-
-    Butterfly: protective mimicry, =I=, 398;
-      instance of tame, =I=, 684.
-
-
-    Cabbage, varieties of, =I=, 302.
-
-    _Cactaceæ_: foliar and axial development, =II=, 47–9;
-      differentiation in, =II=, 258, 276, 282;
-      vascular system, =II=, 282;
-      dye permeability and circulation, =II=, 571, 572;
-      wood formation, =II=, 575, 577, 578, 580.
-
-    “Callus,” budding from, =I=, 358, 359.
-
-    Camel, natural selection and hump of, =I=, 534.
-
-    Canadians, French, fertility of, =II=, 509.
-
-    Cancer, the definition of life, =I=, 111;
-      œsophageal, =II=, 324;
-      and vascular system, =II=, 343.
-
-    Caoutchouc, leaf-structure, =II=, 589.
-
-    Capillaries (_see_ Vascular System).
-
-    Capillarity, and vegetal vascular system, =II=, 279–80, 286, 568,
-        570, 585, 587, 592–6.
-
-    Carbohydrates: instability, =I=, 10;
-      the term “hydro-carbon,” _ib._;
-      molecular changes in, =I=, 42–3;
-      organic transformation, =I=, 43, 48;
-      metabolic processes, =I=, 63–77, 262–3; =II=, 362.
-
-    Carbon: properties, =I=, 3–5, 20;
-      compounds, =I=, 6, 7, 9, 10–12, 13, 24–5.
-
-    Carbonic acid (carbon dioxide): properties, =I=, 6, 7, 9;
-      in animal and plant functions, =I=, 62, 214; =II=, 398;
-      diffusibility, =II=, 331.
-
-    Carbonic oxide, properties, =I=, 6.
-
-    Carnivores: nitrogenous food, =I=, 47, 68;
-      katabolic process, =I=, 71;
-      restricted environment, =I=, 396;
-      their beneficial effects on animal life, =II=, 405–6.
-
-    Carpenter, W. B.: on functional specialization, =I=, 208;
-      reproduction of seaweed, =I=, 582;
-      vegetal cell multiplication, =I=, 585;
-      structure and multiplication of compound organisms, =I=, 586–9;
-      on fundamental traits of sex, =I=, 595;
-      nutritive system of invertebrates, =I=, 595;
-      _Macrocystis_, =II=, 450;
-      nutrition and reproductive function, =II=, 460.
-
-    Cartilage (_see_ Bone).
-
-    Castration, effect of, on growth, =II=, 459.
-
-    “Castration parasitaire,” Julin on, =II=, 493–6.
-
-    Catalysis, and vital metamorphosis, =I=, 39, 43.
-
-    Cattell, McKeen, on tactual perceptiveness, =I=, 666.
-
-    _Caulerpa_, simulation of higher plant-forms, =II=, 22.
-
-    Cave-animals, degeneration of eyes, =I=, 309, 612–3, 614, 647–9,
-        693.
-
-    Cell, the: incomprehensibility of forces at work in, =I=, 118;
-      protoplasts and their traits, =I=, 181;
-      the cell-theory, =I=, 184, 252; =II=, 17–21, 85;
-      differentiation, =I=, 188–9, 194;
-      the continuity of protoplasm, =I=, 190–2, 194, 628–30; =II=, 21;
-      its structure, =I=, 253–5;
-      function of centrosome, =I=, 254–5, 257;
-      structure and function of nucleus, =I=, 255–6, 258–9;
-      karyokinesis, =I=, 257–8;
-      function of chromatin, =I=, 259–65;
-      fertilization and function of polar bodies, =I=, 266–8;
-      theories of heredity based on theory, =I=, 356;
-      Weismann’s differentiation into reproductive and somatic, =I=,
-          622, 628–30, 633–44;
-      nucleus absent or dispersed, =II=, 20, 85;
-      morphological differentiation, =II=, 175–7;
-      animal morphology, =II=, 228–30;
-      morphological summary, =II=, 233;
-      vegetal tissue differentiation, =II=, 249–50, 386;
-      vascular development, =II=, 279–84, 389.
-
-    Centipede, bilateral symmetry, =II=, 198–200.
-
-    _Cephalopoda_: bilateral symmetry, =II=, 203;
-      vascular system, =II=, 341.
-
-    _Cercariæ_ (see _Distoma_).
-
-    _Cereus_, tissue differentiation, =II=, 276, 283.
-
-    Cesalpino, =I=, 377.
-
-    _Cestoda_ (see _Entozoa_).
-
-    _Chætopoda_, segmentation, =II=, 98, 103, 605.
-
-    _Chaja_, wing spurs, =II=, 313.
-
-    Change, and definition of life, =I=, 81–90, 113.
-
-    Charles, R. H., on inheritance of acquired modifications in
-        leg-bones of Punjabis, =I=, 689.
-
-    Chatié, on single and double stocks, =II=, 622.
-
-    Chemistry: properties of organic elements, =I=, 3–5, 20, 22;
-      of diatomic compounds, =I=, 7–10;
-      tri-atomic, =I=, 10–12;
-      poly-atomic, =I=, 12–13, 25;
-      traits of evolution, =I=, 23–4;
-      ethereal undulations and atomic oscillation, =I=, 31–6;
-      chemical affinity and organic change, =I=, 36–7, 38–43;
-      oxidation and generation of heat, =I=, 46–9, 60;
-      generation of nerve force, =I=, 52, 60;
-      metabolism, =I=, 62–77;
-      physiology and organic, =I=, 127;
-      flesh constituents, =I=, 154;
-      composition of organisms and environment, =I=, 173;
-      organic development and differential assimilation, =I=, 179–80;
-      chemical units, =I=, 225; =II=, 612;
-      primitive ideas of elements, =I=, 417;
-      evolution of organic compounds, =I=, 696–701, 703.
-
-    Chestnut, leaf symmetry, =II=, 149, 153.
-
-    _Chiton_: simulation of segmentation, =II=, 116, 118;
-      symmetry, =II=, 202.
-
-    Chlorophyll: function, =I=, 65; =II=, 263;
-      nutrition and absence of, =II=, 74;
-      constitution, =II=, 262;
-      symbiotic presence in animals, =II=, 400.
-
-    _Chondracanthus gibbosus_, enormous development of reproductive
-        system, =II=, 487.
-
-    _Chordata_, affinities, =I=, 466.
-
-    Chromatin (_see_ Cell).
-
-    Circle, the, and evolution hypothesis, =I=, 433.
-
-    Circulation (_see_ Vascular System).
-
-    _Cirrhipedia_: Darwin on retrograde development, =I=, 458;
-      remarkable transformation in _Sacculina_, =II=, 494–5.
-
-    Civilization, human evolution and genesis, =II=, 529–31.
-
-    _Cladophora_: integration, =II=, 25;
-      axial development, =II=, 28.
-
-    Classification: subjective conception, =I=, 78;
-      two purposes of, =I=, 374;
-      a gradual process, =I=, 375;
-      botanical, =I=, 377–80, 389–90;
-      zoological, =I=, 380–9;
-      incomplete equivalence of groups, =I=, 389, 445–6, 448, 555, 572;
-      group attributes, =I=, 390–3;
-      the truths interpreted, =I=, 393–4;
-      ethnologic and linguistic evolution, =I=, 441–6;
-      organic evolution, =I=, 443, 447, 555;
-      differences in kind and degree, =I=, 444–6;
-      antecedent structural similarity, =I=, 447, 448–9;
-      Von Baer’s formula, =I=, 451–4, 555;
-      organic, not uniserial, =II=, 115.
-
-    _Classification of the Sciences, The_, and evolution and
-        dissolution, =II=, 5.
-
-    Claus, C, on segmentation in Annelids and Chætopods, =II=, 605.
-
-    Clover: flower and axial development, =II=, 45;
-      symmetry, =II=, 152.
-
-    Co-adaptation of co-operative parts: principles underlying, =I=,
-        234–5, 511–3, 514–5;
-      slow operation of the process, =I=, 236;
-      sociological analogy, =I=, 237–40;
-      reversion under original conditions, =I=, 240;
-      the analogy continued, _ib._;
-      the case of bison’s head, =I=, 512;
-      natural selection an inadequate explanation, =I=, 535, 614–21,
-          692;
-      Romanes on “cessation of selection” as effecting, =I=, 560,
-          561–2;
-      Weismann’s theories, =I=, 560–3, 663–5, 670, 674–5;
-      natural selection and economy of growth, =I=, 562;
-      physiological processes involved, =I=, 566–7;
-      Wallace’s argument from artificial selection, =I=, 615;
-      what are co-operative parts? =I=, 616–7;
-      “intra-selection” examined, =I=, 676–8.
-
-    Coal, social effects of supply, =I=, 238–9, 241.
-
-    Cocoa-nut, growth and fertility, =II=, 457.
-
-    Coccospheres: vital problem presented by protective structures,
-        =I=, 119;
-      imbricated plates, =I=, 182.
-
-    Cockroach, ousting of European species, =I=, 399.
-
-    Cod: ova of, =II=, 435;
-      growth and fertility, =II=, 454.
-
-    _Codium_: symmetry, =II=, 136;
-      tissue differentiation, =II=, 246.
-
-    _Cœlenterata_: rudimentary contractile organs, =I=, 58;
-      vital changes in polyp, =I=, 95;
-      axial development, =I=, 165, 166;
-      environment and structure, =I=, 173;
-      self-mobility, =I=, 175; =II=, 14, 15;
-      functional differentiation, =I=, 201, 391;
-      inactivity and waste, =I=, 213;
-      reparative power, =I=, 219, 224;
-      individuality, =I=, 246, 247, 250;
-      heterogenesis, =I=, 273, 277, 296;
-      negative disintegration in _Hydrozoa_, =I=, 276, 587;
-      reproductive tissue, =I=, 281;
-      differentiation in _Hydrozoa_, =I=, 391;
-      classificatory value, =I=, 446;
-      regeneration of fragments, =II=, 90;
-      integration, =II=, 90, 102, 105, 124;
-      gemmation, =II=, 91;
-      tertiary aggregation, =II=, 92, 95, 124;
-      molluscan affinities, =II=, 115;
-      radial symmetry, =II=, 188;
-      symmetry of compound, =II=, 192–3;
-      segmental differentiation, =II=, 207;
-      physiological differentiation in _Hydra_ and analogy, =II=, 300;
-      ciliation of blastula, =II=, 301;
-      tissue reduplication, =II=, 301–2, 389;
-      outer tissue differentiation, =II=, 309;
-      osmosis in _Hydra_, =II=, 339;
-      vascular system in _Hydra_, =II=, 340, 376;
-      functional co-ordination, =II=, 376;
-      symbiosis, =II=, 400;
-      asexual genesis, =II=, 443–4;
-      growth and sexual genesis, =II=, 452;
-      development and genesis, =II=, 462;
-      nutrition and genesis, =II=, 476.
-
-    Cœlom, origin and function, =II=, 302–3.
-
-    Collins, F. Howard, jaws and teeth of savages and civilized, =I=,
-        541.
-
-    Colloids: T. Graham on, =I=, 15–8;
-      diffusibility, =I=, 18–21;
-      organic, =I=, 21, 25, 26;
-      pliability and elasticity, =I=, 27;
-      capillary affinity, =I=, 28;
-      isomerism, =I=, 59;
-      instability, =I=, 350;
-      molecular mobility and diffusibility, =II=, 331;
-      instability of, and nerve differentiation, =II=, 356–61;
-      and muscular tissue, =II=, 361–4.
-
-    Colonies, autogenous development and parallel in heredity, =I=,
-        366–8; =II=, 620.
-
-    Colour: sensation of, =I=, 54;
-      phœnogamic, =II=, 75, 265–6;
-      light and vegetal, =II=, 261–2;
-      floral fertilization, =II=, 267–9;
-      sexual selection, =II=, 269;
-      activity and muscular, =II=, 365–9;
-      physiological units and mixture of, in offspring, =II=, 616, 617.
-
-    Commensalism, organic Integration as displayed in, =II=, 402–4.
-
-    _Compositæ_: floral symmetry, =II=, 173.
-
-    Condor, weight of, =I=, 155.
-
-    _Confervoideæ_, =I=, 279, 280; =II=, 25, 28, 449.
-      (See _Algæ_.)
-
-    _Conjugateæ_, =II=, 449.
-      (See _Algæ_.)
-
-    Conjugation, in _Algæ_, =I=, 279;
-      in _Protozoa_, =I=, 280; =II=, 452;
-      can fission persist without? =I=, 637;
-      relation to growth, =II=, 449.
-
-    Connective tissue, Hertwig’s classification, =I=, 189.
-
-    Constitutional units, =I=, 369.
-      (_See_ Physiological Units.)
-
-    Consumption, hereditary transmission, =I=, 307.
-
-    Co-ordination of actions (_see_ Life).
-
-    Cope, E. D., on origin of vertebrate structure, =II=, 225–7.
-
-    _Cormophyta_: slight internal differentiation, =II=, 273;
-      vascular system, =II=, 280.
-
-    _Corpuscula tactus_, their function, =I=, 75.
-
-    Correspondence, use of word, =I=, 97.
-      (_See_ Life.)
-
-    Cousin-marriages, =I=, 345; =II=, 615.
-
-    Cow: what prompts her to mumble a bone? =I=, 120.
-
-    Cow-parsnip (see _Heracleum_).
-
-    Crab (see _Crustacea_).
-
-    Creation (_see_ Special creation).
-
-    _Crinoidea_, symmetry, =II=, 195–6.
-
-    Crocodile, continuous growth, =I=, 154, 292.
-
-    Crookes, Sir W., hypothetical chemical unit “protyle,” =I=, 22, 23.
-
-    _Cruciferæ_, floral symmetry, =II=, 164, 171.
-
-    _Crustacea_: locomotion of lobster, =I=, 175;
-      regeneration of limbs, =I=, 224, 360, 589; =II=, 76;
-      homogenesis, =I=, 271;
-      genesis and nutrition in _Daphnidæ_, =I=, 290–1;
-      growth and genesis, =I=, 292;
-      degeneration of eye in cave-inhabiting, =I=, 309, 614, 648;
-      hermit-crab parasite, =I=, 397;
-      changes of media, =I=, 401, 481–2;
-      retrograde development in cirripedes, =I=, 458;
-      segmentation, =I=, 468–9; =II=, 114;
-      Darwin on jaws and legs, =I=, 471;
-      survival of cirripedes, =I=, 517;
-      integration and homology, =II=, 111–4, 121, 603;
-      bilateral symmetry, =II=, 198–201;
-      eyes, =II=, 318;
-      dermal structure of hermit-crab, =II=, 322, 387;
-      fertility, =II=, 453;
-      nutrition and genesis in parasitic species, =II=, 487;
-      “castration parasitaire,” =II=, 493–6.
-
-    Crystalloids: Prof. Graham on, =I=, 15–8;
-      diffusibility, =I=, 18–21;
-      organic, =I=, 21–2, 26.
-
-    Crystals: simulation of life in “storm glass,” =I=, 96;
-      growth, =I=, 135–7, 577;
-      segregation, =I=, 179, 221, 223;
-      equilibration, =I=, 337;
-      physiological units and polarity, =I=, 701–6;
-      time and formation, =II=, 77.
-
-    _Ctenodrilus_, segmental individuality, =II=, 103, 603, 604.
-
-    Cube, bilateral symmetry, =II=, 132.
-
-    Cunningham, J. T., =I=, vi; =II=, vi;
-      on non-adaptive specific characters, =I=, 565;
-      food of blow-fly larvæ, =I=, 678;
-      arthropod segmentation, =II=, 114;
-      egg-production of Conger, =II=, 425.
-
-    Cuttle-fish, Individuality of _Hectocotylus_, =I=, 250.
-
-    Cuvier, zoological classification, =I=, 381.
-
-    Cyanogen, properties, =I=, 7, 9.
-
-    _Cyclichthys_, dermal structure, =II=, 306.
-
-
-    Dalyell, Sir J., regeneration in _Dasychone_, =I=, 361;
-      propagation of _Hydra_, =II=, 476.
-
-    _Daphnidæ_, heterogenesis and nutrition, =I=, 290–1.
-
-    Darwin, C: _Origin of Species_, =I=, 129; =II=, 528;
-      natural selection and function, =I=, 308–9, 693;
-      atavism, =I=, 314;
-      osseous variations in pigeons, =I=, 321;
-      plant variation and domestication, =I=, 325;
-      “spontaneous variation,” =I=, 328, 697;
-      floral fertilization, =I=, 340; =II=, 168, 267, 407, 608;
-      intercrossing and self-fertilization, =I=, 344, 345;
-      intercrossing =I=, 347, 611, 669;
-      his theory of pangenesis examined, =I=, 356–62, 370, 372;
-      plant-fertilization and distribution, =I=, 397;
-      habits of birds, =I=, 400;
-      distribution and natural barriers, =I=, 402, 476;
-      disappearance and non-reappearance of species, =I=, 406;
-      distribution in time and space, =I=, 410;
-      linguistic classification, =I=, 442;
-      classification of organisms, =I=, 443;
-      classification and descent, =I=, 448;
-      on petrels, =I=, 455;
-      suppression of organs, =I=, 457;
-      development of _Cirrhipedia_, =I=, 438;
-      jaws and legs of _Crustacea_, =I=, 471;
-      aborted organs, =I=, 474, 563;
-      relations of species in Galapagos archipelago, =I=, 478;
-      opinions of E. Darwin and Lamarck, =I=, 491;
-      the term “survival of the fittest,” =I=, 530;
-      Indirect equilibration by natural selection, =I=, 530–5;
-      inheritance of acquired characters, =I=, 535–42, 560, 630, 685,
-          690;
-      Wallace on natural selection in man, =I=, 553;
-      misleading connotations of term “natural selection,” =I=, 609,
-          695;
-      caste gradations and jaws of driver ants, =I=, 658;
-      attachment of climbing plants, =II=, 276–7;
-      vegetal fructification, =II=, 294;
-      earth-worm, =II=, 402;
-      animal sterility and domestication, =II=, 480, 483;
-      variation in hyacinth and camellia, =II=, 621.
-
-    Darwin, Dr. E., modifiability of organisms, =I=, 490, 492–7.
-
-    Death: an arrest of vital correspondence, =I=, 102;
-      only limit to vegetal growth, =I=, 153;
-      cessation of co-ordination of actions, =I=, 578, 579;
-      Weismann’s hypothesis, =I=, 636–8;
-      physiological integration, =II=, 374, 392;
-      cause of natural, =II=, 413;
-      relation to births, =II=, 417.
-
-    Definiteness: of vital change, =I=, 87–90, 106, 109;
-      developmental, =I=, 178;
-      functional, =I=, 212;
-      segregation of evolution, =I=, 514–6.
-
-    Definition, difficulties of, =I=, 78; =II=, 17.
-
-    Degeneracy, morphological obscurations due to, =II=, 12, 13.
-
-    _Dendrobium_ (_see_ Orchids).
-
-    _Desmidiaceæ_: unicellular, =II=, 21;
-      linear and central aggregation, =II=, 23;
-      natural selection and symmetry, =II=, 134, 133;
-      morphological differentiation, =II=, 177;
-      tissue, =II=, 244;
-      genesis, =II=, 440, 449.
-
-    Determinants, Weismann’s germ-plasm units (_see_ Germ-plasm).
-
-    Development: an increase of structure, =I=, 162; =II=, 461;
-      primarily central, =I=, 162, 166;
-      uni- and multicentral, =I=, 163–4, 166–7;
-      axial, =I=, 164, 167;
-      uni- and multiaxial, =I=, 165–6;
-      a change to coherent definite heterogeneity, =I=, 167–70, 179;
-      Von Baer’s formula, =I=, 171–2;
-      individual differentiation from environment, =I=, 172–8;
-      cell-formation, =I=, 225;
-      discontinuous, and agamogenesis, =I=, 275;
-      Prof. Huxley’s classification, =I=, 276;
-      sociological parallel to autogenous, =I=, 364–8; =II=, 620;
-      retrograde, =I=, 457–8;
-      inequalities among co-operative parts, =I=, 617;
-      “heterochrony,” =I=, 655;
-      continuous and discontinuous vegetal, =II=, 52;
-      summary of physiological, =II=, 384–90;
-      nutrition and genesis, _résumé_, =II=, 497–9;
-      evolution, =II=, 501–5;
-      commencement of genesis, =II=, 506;
-      of vertebrate limbs, =II=, 553.
-      (_See also_ Multiplication.)
-
-    _Development Hypothesis, The_, =I=, 417.
-
-    Dialects (_see_ Language).
-
-    Dialysis, and diffusibillty. =I=, 19, 20.
-
-    Diastase, decomposition of, =I=, 38, 40.
-
-    _Diatomaceæ_: tissue, =II=, 244;
-      genesis, =II=, 440, 448.
-
-    Diatomic compounds (_see_ Chemistry).
-
-    Dicotyledons: growth, =I=, 139, 143; =II=, 63–4, 69–72, 78, 82–3;
-      uniaxial development, =I=, 165;
-      stem and leaf functions, =II=, 257;
-      mechanical stress and wood formation, =II=, 277;
-      growth and genesis, =II=, 451.
-
-    Differentiation (_see_ Morphology _and_ Physiology).
-
-    _Difflugia_: primary aggregate, =II=, 86–7;
-      symmetry, =II=, 186;
-      outer tissue differentiation, =II=, 309.
-
-    Diffusion, of colloids and crystalloids, =I=, 18–20; =II=, 331.
-
-    Digestion: action of nitrogenous compounds, =I=, 69;
-      obesity, =II=, 480–4;
-      fertility, =II=, 514.
-
-    Dimorphism: floral, =I=, 534;
-      sexual, in parasites, =I=, 315;
-      social insects (_see_ Insects).
-
-    Dinosaurs, size of, =I=, 139.
-
-    _Diphyes_: individuality, =I=, 246;
-      symmetry, =II=, 192.
-
-    Disease: segregation of blood constituents, =I=, 179;
-      changes in blood from, =I=, 221, 701;
-      heredity, =I=, 306–7, 312–3, 622–3;
-      belief in supernatural origin, =I=, 419;
-      parasitism and special creation, =I=, 427;
-      morbid products as specific characters, =I=, 567;
-      telegony, =I=, 646;
-      dermal structure, =II=, 306;
-      intestinal muscular hypertrophy, =II=, 325;
-      indigestion and alimentary canal development, =II=, 328;
-      jaundice and bilirubin, =II=, 330;
-      localization of excretion, =II=, 331;
-      membranes in inflammatory, =II=, 343;
-      osseous differentiation in rickets, =II=, 352;
-      fatty degeneration, =II=, 482.
-
-    Disintegration, physiological (_see_ Physiology).
-
-    _Distoma_: metagenesis, =I=, 273–4;
-      disintegration of genesis, =I=, 276;
-      cycle of generations, =II=, 489.
-
-    Distribution: physical limits, =I=, 396;
-      organic environment, =I=, 396–8;
-      parasitic conditions, =I=, 397–8;
-      simultaneity of agencies affecting, =I=, 398;
-      mutual encroachments of species, =I=, 398–401, 477, 489;
-      facts disproving pre-adaptation to habitats, =I=, 401–3, 411–2;
-      of animals and plants in time, =I=, 404–11, 412;
-      ousting of native species in New Zealand, =I=, 477;
-      local influences, =I=, 477–9, 489;
-      through varied media, =I=, 479–85, 489, 556;
-      past and present organic forms, =I=, 485–9, 556;
-      complex organization and, =II=, 296–7.
-
-    Division of labour, physiological (_see_ Labour).
-
-    Dog: contrasted lives of tortoise and, =I=, 103, 104;
-      inherited habits, =I=, 309, 573;
-      abnormal digits, =I=, 324;
-      interbreeding of divergent varieties, =I=, 565;
-      decrease of jaw, =I=, 615, 693;
-      telegony, =I=, 645;
-      conditions affecting fertility, =II=, 474, 479.
-
-    Dohrn, theory of vertebrate structure, =II=, 606.
-
-    _Doliolum_, combination of individualities, =I=, 247.
-
-    Domestication (_see_ Animals).
-
-    Doubleday, E., on nutrition of genesis, =II=, 510–2.
-
-    Driesch, separation of segmentation spheres of _Echinus_ ovum, =I=,
-        691; =II=, 618.
-
-    Dropsy (_see_ Disease).
-
-    _Drosera_: individuality, =I=, 251;
-      proliferous growth, =II=, 75.
-
-    Du Bois-Reymond, E. H., electricity from muscles and nerves, =I=,
-        50.
-
-    Dumas, antithesis of animals and plants, =I=, 62.
-
-    Dwarfs, Hindu family of, =I=, 316.
-
-
-    Ear, development of vertebrate, =II=, 318, 320.
-
-    Earth, climatic rhythm and organic change, =I=, 499–501, 557.
-
-    Earth-worm: bilateral symmetry, =II=, 199, 200;
-      mould production, =II=, 402.
-
-    _Echinococcus_ (see _Entozoa_).
-
-    _Echinodermata_: independence of blastosphere cells, =I=, 185;
-      protoplasmic continuity in embryos, =I=, 190;
-      separation of segmentation spheres of ovum, =I=, 691; =II=, 618;
-      symmetry, =II=, 191, 195–6.
-
-    Economy: of growth in natural selection, =I=, 536, 562;
-      a trait of organic evolution, =II=, 501, 504.
-
-    Ectoderm: functional differentiation, =I=, 202, 203;
-      functional vicariousness, =I=, 209;
-      reproductive function, =I=, 281.
-
-    Effects, Multiplication of: variation, =I=, 329–30, 333;
-      organic evolution, =I=, 511–4, 515, 517, 549, 557; =II=, 405–6;
-      morphological development, =II=, 7–9, 234;
-      physiological differentiation, =II=, 390–1, 392.
-
-    Eggs (_see_ Embryology).
-
-    Eimer, T., theory of orthogenesis, =I=, 563–4.
-
-    _Elasmobranchii_: protoplasmic continuity, =I=, 629;
-      segmentation, =II=, 126.
-
-    Electricity: genesis in organic matter, =I=, 50–2, 60;
-      muscular action, =I=, 59;
-      incomprehensibility, =I=, 121.
-
-    Elephant: fertility, =I=, 583, 599; =II=, 459, 506;
-      cerebro-spinal system, =I=, 598, 599.
-
-    Elk, Irish, horns and correlated parts, =I=, 537, 674.
-
-    _Eloidea canadensis_: individuality, =I=, 248;
-      enormous agamic multiplication, =I=, 642.
-
-    Elongation, and locomotion in animals, =II=, 15.
-
-    Embryology: as aiding biology, =I=, 125–6;
-      simulated growth, =I=, 136;
-      initial and final organic bulks, =I=, 143, 158, 161;
-      fœtal flesh constituents, =I=, 154;
-      human arm development, =I=, 169;
-      Von Baer’s formula, =I=, 170–2, 451–4, 466;
-      embryonic heat, =I=, 177;
-      spherical organic form, =I=, 177;
-      unit-life in multicellular organisms, =I=, 185–6;
-      functional differentiation, =I=, 203;
-      individuality, =I=, 246–7;
-      unspecialized reproductive tissue, =I=, 279–83, 317;
-      changes following impregnation, =I=, 283–4;
-      nutrition and vegetal growth, =I=, 285–8, 295–7;
-      and animal growth, =I=, 289–94, 295–7;
-      physiological units and heredity, =I=, 317–9;
-      variation and parental functional condition, =I=, 324;
-      uterine environment, =I=, 327–8;
-      physiological units and variation, =I=, 330–4, 458;
-      fertilized and unfertilized ova, =I=, 340–1;
-      hermaphrodism, =I=, 341–2, 344;
-      sociological parallel, =I=, 366–8;
-      evolution hypothesis, =I=, 434, 436, 453, 454, 555;
-      petrel development, =I=, 455;
-      substitution and suppression of organs, =I=, 456–8, 466, 472–3;
-      structural proclivities of physiological units, =I=, 458;
-      abridgment of stages, =I=, 458–9, 464;
-      disappearance of intermediate forms, =I=, 459–60, 463;
-      pre-adaptation, =I=, 461–3;
-      discrimination of species in early stages, =I=, 461;
-      anomalous persistence of ancestral traits, =I=, 463–5;
-      phylogeny, =I=, 486;
-      egg-shell function, =I=, 527;
-      genesis of grades in social insects, =I=, 654–6, 658–9, 679–80;
-      determination of sex, =I=, 657;
-      order of development qualified by needs, =I=, 679;
-      osteology of Punjabis, =I=, 689;
-      direct transformations and physiological units, =I=, 706;
-      transformation of blastema, =II=, 20;
-      arrest of growth and innutrition, =II=, 73;
-      development of segmented animals, =II=, 100–2, 602;
-      adaptive vertebrate segmentation, =II=, 118–23, 124, 223–4,
-          605–6;
-      animal cell morphology, =II=, 228;
-      primary differentiations of germinal layers, =II=, 300–2;
-      lung development, =II=, 333–4;
-      mammalian ova-maturation, =II=, 342–3;
-      movements of ova, =II=, 356, 363;
-      modifications in mole, =II=, 391;
-      genesis and nutrition, =II=, 424, 425;
-      fish ova, =II=, 435, 454;
-      cost of genesis, =II=, 435–6;
-      number of birds’ eggs, =II=, 454–6, 478;
-      heat and genesis, =II=, 468, 474;
-      activity and genesis in birds, =II=, 470–2, 474;
-      vertebrate limb development, =II=, 553;
-      ossification in vertebrates, =II=, 556;
-      Owen’s vertebrate theory, =II=, 563;
-      development of vertebræ, =II=, 564.
-      (_See also_ Multiplication.)
-
-    Embryology of conceptions, =I=, 451.
-
-    Emigrants, type of organization among, =I=, 364; =II=, 620.
-
-    Endoderm: functional differentiation, =I=, 202, 203;
-      functional vicariousness, =I=, 209.
-
-    Endogen, application of term, =II=, 62, 78, 82.
-      (_See_ Monocotyledons.)
-
-    Energy: evolution of, in animals, =I=, 71–7;
-      organic growth and expenditure, =I=, 141;
-      functional transfer, =I=, 201–6;
-      chromatin as the source of, in karyokinesis, =II=, 261–5.
-      (_See also_ Force.)
-
-    _Entozoa_: metagenesis, =I=, 273, 641;
-      self-fertilization, =I=, 342;
-      distribution, =I=, 398;
-      and special creation, =I=, 428;
-      fission in simple types, =I=, 584;
-      nutrition and genesis, =I=, 641; =II=, 488;
-      direct transformation, =I=, 706;
-      integration, =II=, 102;
-      segmentation, =II=, 107, 108;
-      interdependence and organic integration, =II=, 404.
-
-    Environment: degree of life and complexity of, =I=, 104–7;
-      relation to organic structure and function, =I=, 172–8; =II=,
-          12–5;
-      adaptation to varied media an evidence of evolution, =I=, 479–81,
-          556;
-      influence of solar system, =I=, 500, 556;
-      inherited adaptation to, =II=, 623.
-
-    _Eolis_, branchiæ, =II=, 118.
-
-    Epidermis (_see_ Skin).
-
-    Epilepsy: definition of life and movements in, =I=, 112;
-      heredity, =I=, 312.
-
-    Epithelium: ciliated, =I=, 187;
-      Hertwig’s classification, =I=, 189;
-      reproductive function, =I=, 280;
-      “pavement” and “cylinder,” =II=, 229.
-
-    _Epizoa_: distribution, =I=, 398;
-      special creation and effects of, =I=, 428;
-      interdependence and organic integration, =II=, 404;
-      nutrition and genesis, =II=, 487.
-
-    Equilibration: variation and law of, =I=, 326, 334;
-      molecular arrangement, =I=, 337–45;
-      of organic change, =I=, 348, 347, 557;
-      direct and indirect, =I=, 519–22, 573;
-      adaptation by direct, =I=, 522–3, 551, 557;
-      nutrition, defence, and fertilization of plants, =I=, 523–5;
-      direct of animals, =I=, 525–8, 551, 557;
-      natural selection and indirect, =I=, 530–4, 552, 557;
-      of natural selection, =I=, 543–7, 552–3, 557;
-      increasing importance of direct, =I=, 553;
-      of forces acting on species, =I=, 571–2; =II=, 417–20;
-      phenomena not accounted for by, =I=, 573;
-      tissue differentiation, =II=, 245;
-      genesis of nervous system, =II=, 307–8;
-      functional, =II=, 391–4;
-      laws of multiplication, =II=, 411–6;
-      in human and social evolution, =II=, 537.
-      (_See also_ Acquired characters _and_ Natural selection.)
-
-    Errera, L., on inherited adaptation to environment in Aspergillus,
-        =II=, 623.
-
-    Ethnology: heredity, =I=, 303–4, 310;
-      plasticity of mixed races, =I=, 354;
-      primitive ideas, =I=, 417;
-      evolution and classification, =I=, 441–3, 446;
-      natural selection, =I=, 553.
-
-    _Euphorbiaceæ_: foliar and axial development, =II=, 47–8;
-      physiological differentiation, =II=, 258;
-      dye permeability and circulation, =II=, 571;
-      wood formation, =II=, 575, 577, 578;
-      foliar vascular system, =II=, 589–92, 596.
-
-    Evaporation: organic change, =I=, 28;
-      vegetal circulation, =II=, 587.
-
-    Evolution: chemical elements and compounds, =I=, 22–4, 67;
-      primordial form of living matter, =I=, 63–4, 181; =II=, 21–2;
-      definitions of life, =I=, 107–10;
-      growth the primary trait of, =I=, 135;
-      comprehends growth and development, =I=, 162;
-      illustrations in development, =I=, 167–70, 178–9;
-      progressive structural differentiation, =I=, 181–4, 192–6, 211–2;
-      life before organization, =I=, 210;
-      heterogeneity of function, =I=, 211;
-      stability of species, =I=, 242, 515, 518;
-      individuality, =I=, 247;
-      cell-organization, =I=, 262;
-      genesis, heredity, and variation resulting from, =I=, 354–5;
-      period required for organic, =I=, 407, 565–6;
-      contrasted with special creation hypothesis, =I=, 415, 431–40;
-      derivation of hypothesis, =I=, 431, 439, 554;
-      increasing belief in, =I=, 431–3, 439;
-      experiences supporting conceivability, =I=, 433–5, 439;
-      direct evidence, =I=, 435–7, 439;
-      malevolence not implied by, =I=, 437–9;
-      evidence from classification, =I=, 443, 444, 449, 466, 555;
-      embryology, =I=, 451–3, 466;
-      substitution and suppression of organs, =I=, 456–8, 466, 472–3;
-      insect segmentation, =I=, 468–9;
-      vertebral column development, =I=, 470–2;
-      rudimentary organs, =I=, 472–5;
-      adaptation to varied media, =I=, 479–85, 556;
-      growth of the theory of organic, =I=, 490–8;
-      instability of the homogeneous, a cause, =I=, 509–11, 516, 550;
-      multiplication of effects, =I=, 511–14, 517–8, 550; =II=, 405;
-      segregation, and heterogeneity and definiteness of, =I=, 514–8,
-          550;
-      natural selection and general doctrine of, =I=, 543–8, 557;
-      factors tabulated, =I=, 551;
-      inductive evidences summarized, =I=, 555–6;
-      surviving disbelief in France, =I=, 559;
-      current theories of organic, =I=, 559–74;
-      Eimer’s theory of orthogenesis, =I=, 563–4;
-      Gulick on monotypic and polytypic, =I=, 569;
-      phenomena unexplained by theories, =I=, 573–4;
-      inorganic and the _System of Philosophy_, =I=, 696;
-      “spontaneous generation,” =I=, 696–701, 702;
-      dissolution and problems of morphology, =II=, 4–6;
-      morphology and formula, =II=, 7–9, 231–5;
-      difficulties of definition, =II=, 17;
-      cell-doctrine, =II=, 17–21, 85;
-      unicellular origin of plants, =II=, 21–2;
-      _résumé_ of plant-morphology, =II=, 78–80;
-      origin and differentiation of phænogamic type, =II=, 83;
-      physiological problems, =II=, 239–43;
-      tissue differentiation, =II=, 244–6, 385;
-      integration of organic world, =II=, 396, 406;
-      race and individual multiplication, =II=, 428–30;
-      declining fertility and human, =II=, 431, 529–30;
-      individuation, genesis, and, =II=, 501–5;
-      human life, prospective, =II=, 522–5;
-      forces influencing human, =II=, 525–8;
-      future of population, =II=, 532–7;
-      self-sufficingness of, =II=, 537;
-      vertebral, =II=, 563–6.
-
-    Excretion: genesis of organs of, =II=, 303;
-      localization of, =II=, 331–3.
-
-    Exogen, application of term, =II=, 82.
-      (_See_ Dicotyledons.)
-
-    Expenditure (_see_ Multiplication).
-
-    Eye, the: molecular transformations in visual process, =I=, 75–6;
-      progressive development, =I=, 195; =II=, 317–9;
-      waste and repair, =I=, 218;
-      transmitted defects, =I=, 306, 311, 694;
-      degeneration in cave-animals, =I=, 309, 612–3, 614, 647–9, 693;
-      late development in insects, =I=, 658;
-      migration in flat fishes, =II=, 205.
-
-
-    Fabre, J. H., nutrition and sex in _Osmia tricornis_, =I=, 657.
-
-    False joints, =I=, 230;
-      theories of heredity and, =I=, 362, 364; =II=, 371–2.
-
-    Fats, the: physical and chemical properties, =I=, 10–12;
-      non-nitrogenous, =I=, 41;
-      action of bile, =II=, 330.
-
-    Fatty degeneration, and failing vitality, =I=, 41.
-
-    Feathers, development, =I=, 474; =II=, 314–6.
-
-    Feet, heredity and size, =I=, 311.
-
-    Ferments, changes and nitrogenous character of, =I=, 38.
-
-    Ferns: foliar development and nutrition, =II=, 76;
-      inner tissue differentiation, =II=, 273;
-      indefiniteness, =II=, 296;
-      genesis, =II=, 441, 463.
-
-    Fertility, the General Law of Animal, =I=, 577–601.
-      (_See_ Multiplication.)
-
-    Fertilization: unit-life of generative elements, =I=, 185–6;
-      the function of chromatin, =I=, 260, 263–5;
-      extrusion of polar bodies, =I=, 266–8;
-      nature and functions of generative elements, =I=, 279–83, 317,
-          334, 342, 593–7;
-      differentiation and variation effected by, =I=, 330–2;
-      the essential object of, =I=, 340–1; =II=, 614–6;
-      hermaphrodism and self-, =I=, 341–2;
-      crossing and its effects, =I=, 343–7;
-      isolation of species in respect of, =I=, 570;
-      floral (_see_ Flowers).
-
-    _Ficus_, foliar structure, =II=, 589, 596.
-
-    Fingers: embryogeny of human, =I=, 169;
-      heredity and abnormal, =I=, 305, 314, 321–3;
-      autogenous development of supernumerary, =I=, 363;
-      rudimentary, =I=, 473.
-
-    Fishes: sizes of ova and adult, =I=, 143–4;
-      growth of pike, =I=, 154, 292;
-      size and environment, =I=, 156;
-      temperature, =I=, 174;
-      self-mobility, =I=, 175;
-      continuity of blastomeres, =I=, 214; =II=, 327;
-      genesis, =I=, 271; =II=, 435, 436;
-      conditions affecting genesis, =I=, 292–3, 583, 598, 599; =II=,
-          454;
-      classification, =I=, 392;
-      change of media, =I=, 401, 480;
-      distribution in time, =I=, 408–9;
-      climbing species, =I=, 480, 482;
-      migrations, =I=, 500;
-      dermal structure, =I=, 526; =II=, 305–6, 315, 387;
-      Cunningham on non-adaptive specific characters, =I=, 565;
-      elongation and locomotion, =II=, 15;
-      segmentation, =II=, 122, 225;
-      bilateral symmetry, =II=, 203–5;
-      eyes of _Pleuronectidæ_, =II=, 205;
-      genesis of vertebrate axis, =II=, 212–6, 218–21, 225;
-      ossification of paleozoic, =II=, 218;
-      respiratory organs, =II=, 334–8;
-      activity and muscular colour, =II=, 365–9;
-      Owen on skeleton, =II=, 552, 557, 558–60, 562, 564.
-
-    Fission (_see_ Agamogenesis).
-
-    Flint, Austin, on telegony, =I=, 644.
-
-    Flounder, symmetry and eyes, =II=, 205.
-
-    Flower, Sir W., on ferret, =II=, 480.
-
-    Flowers: pollen propulsion in orchids, =I=, 57;
-      nature of reproductive elements, =I=, 283;
-      insect fertilization, =I=, 340, 525; =II=, 168, 174, 267, 407;
-      self- and mutual fertilization, =I=, 342–5, 570;
-      Darwin on homologies, =I=, 472;
-      direct equilibration and fertilization, =I=, 524–5;
-      dimorphism, =I=, 534;
-      foliar homology of petals, =II=, 43–6;
-      symmetry, =II=, 132, 161, 162–4, 170, 174, 608;
-      fertilization and symmetry, =II=, 164–70;
-      clusters and components, =II=, 170–4;
-      nutrition and inflorescence, =II=, 179–80, 541–2, 546–7;
-      tissue differentiation, =II=, 265–9;
-      separation of ancestral traits in hybrids, =II=, 616–7.
-
-    Fly, beneficial parasitism, =II=, 406.
-
-    Food (_see_ Nutrition).
-
-    Food-cavity, genesis and development of, =I=, 188, 195.
-
-    _Foraminifera_: form, =I=, 173;
-      primary aggregate, =II=, 87, 124;
-      progressing integration, =II=, 89–90, 124.
-
-    Force: action on like and unlike units, =I=, 5;
-      expenditure and organic growth, =I=, 149–54, 161;
-      functional accumulation, transfer, and expenditure, =I=, 198–9,
-          201–3, 391;
-      waste and expenditure, =I=, 214–5;
-      distribution during strain, =II=, 209–12.
-      (_See also_ Energy, _and_ Persistence of Force.)
-
-    Fossils (_see_ Palæontology).
-
-    Foster, Sir M., on storage of glycogen, =I=, 70, 74;
-      Increase of weight in hybernating dormouse, =I=, 214.
-
-    Fowls (see _Gallinaceæ_).
-
-    Foxglove: abnormal development, =I=, 287; =II=, 46;
-      floral distribution, =II=, 141;
-      nutrition and growth, =II=, 179.
-
-    France: surviving disbelief in organic evolution, =I=, 559;
-      rate of multiplication, =II=, 509, 512.
-
-    Frankland, Sir E., on isomerism of protein, =I=, 700.
-
-    Fraser, Col. A. T., on family of Hindu dwarfs, =I=, 316.
-
-    Fries, E., multiplication of _Reticularia_, =I=, 582; =II=, 450.
-
-    Frog: vitality of detached heart, =I=, 111;
-      of larval fragments, =I=, 365.
-
-    Fry, Sir E., on alternation of generations, =II=, 84.
-
-    _Fuci_: cell multiplication, =II=, 27;
-      undifferentiated outer tissue, =II=, 256.
-
-    Function: as a basis of classification, =I=, 124–9, 129–31;
-      simultaneous progress of structure and, =I=, 197, 211;
-      divisions of, =I=, 198–200, 391;
-      correlative complexity of structure, =I=, 200, 210–1;
-      progressive differentiations, =I=, 201–4;
-      concomitant integration, =I=, 205–8;
-      specialization and vicariousness, =I=, 208–10;
-      formula of evolution, =I=, 211;
-      diminished ability and overwork, =I=, 215–6;
-      growth and increased, =I=, 228–33, 234–5;
-      interdependence of social and organic, =I=, 237–9, 240–2;
-      structure and heredity, =I=, 306–13, 318–9 (_see_ Acquired
-          Characters);
-      aids natural selection, =I=, 308;
-      organic interdependence, =I=, 318–9;
-      parental condition and variation, =I=, 324, 326;
-      variation and altered, =I=, 325–6, 333–4;
-      as causing variation, =I=, 334–5;
-      effect on physiological units, =I=, 353–4; =II=, 620;
-      zoological classification, =I=, 391–3;
-      multiplication of effects, =I=, 512;
-      law of equilibration, =I=, 519–22, 557;
-      correlation of changes in, =I=, 529;
-      structural effects of changing, =I=, 541–2;
-      structural co-operation, =II=, 3, 217;
-      vicarious vegetal, =II=, 270;
-      vicariousness and specialization, =II=, 293;
-      epidermic structure, =II=, 312–4, 387;
-      structure and muscular, =II=, 369, 391;
-      adaptive bone-structures, =II=, 370–1;
-      equilibration and adaptation, =II=, 392;
-      persistence of force and adaptation, =II=, 394.
-      (_See also_ Physiology.)
-
-    _Fungi_: nitrogenous character, =I=, 40;
-      development, =I=, 163, 164, 165;
-      conjugation, =I=, 279; =II=, 449;
-      fission, =I=, 584, 585;
-      integration, =II=, 24–5, 293;
-      symmetry, =II=, 137–40, 146;
-      puff-ball tissue, =II=, 246, 252, 386;
-      tissue differentiation, =II=, 256;
-      inner tissue, =II=, 279;
-      indefiniteness, =II=, 295;
-      growth and genesis, =II=, 459;
-      nutrition and genesis, =II=, 487.
-
-
-    _Gallinaceæ_: conditions affecting fertility, =II=, 454–5, 469,
-        471;
-      masculine traits of old hens, =II=, 495.
-
-    Galls: definition of life and, =I=, 111;
-      Hertwig on, =I=, 690.
-
-    Galton, F., on variation outside the mean, =I=, 669.
-
-    Gamogenesis: homogenesis, =I=, 270, 271, 336;
-      heterogenesis, =I=, 270, 336;
-      independence of offspring, =I=, 278;
-      reproductive tissue, =I=, 279–84;
-      vegetal nutrition, =I=, 285–8, 293; =II=, 39;
-      animal nutrition, =I=, 289–94, 297;
-      when and why does it recur? =I=, 294–7, 336–40;
-      effect on species, =I=, 347–9;
-      leaf formation, =II=, 39;
-      alternating generation in liverworts, =II=, 80–4;
-      molluscan homogenesis, =II=, 116, 117–8;
-      vertebrate, =II=, 118;
-      growth, =II=, 266.
-      (_See also_ Fertilization, _and_ Multiplication.)
-
-    _Gasteropoda_ (see _Mollusca_).
-
-    Geddes and Thompson, on the determination of sex, =I=, 657.
-
-    Gelatine, nutritive value of, =I=, 77.
-
-    Gemmation: and genesis, =I=, 272–6;
-      theories of heredity and, =I=, 361;
-      annulose, =II=, 100–5, 106.
-
-    Generalization, impossibility of perfect, =I=, 450.
-
-    Generation, and genesis: the words, =I=, 269.
-
-    Genesis (_see_ Multiplication).
-
-    _Gentiana_: floral arrangement, =II=, 608–11.
-
-    Genus: indefinite value, =I=, 389, 446;
-      instability of homogeneous and heterogeneity of, =I=, 509–11,
-          515, 517–8, 550, 557.
-
-    Geology: growth displayed in, =I=, 135, 136;
-      distribution in time, =I=, 404–11, 412;
-      special creation, =I=, 419, 426;
-      evolution, =I=, 432, 437;
-      record congruous with evolution, =I=, 485–9, 556;
-      organic influence of changes, =I=, 501–3, 549, 550, 557;
-      climatic influence of changes, =I=, 503;
-      time required for organic evolution, =I=, 565–6;
-      rise of insect and plant relations, =II=, 407;
-      human evolution and changes, =II=, 534.
-
-    Geometry, evolution illustrated by, =I=, 433–4.
-
-    Germ-cell: unspecialized nature, =I=, 279–83, 317;
-      dissimilarity, =I=, 330, 332, 334, 342;
-      equilibrium, =I=, 340.
-      (_See also_ Fertilization.)
-
-    Germ-plasm, Weismann’s theory of, =I=, 357–8;
-      inconsistent with plant embryogeny, =I=, 359;
-      regeneration of lost limbs, =I=, 362;
-      variations in peacock’s tail feather, =I=, 372, 695; =II=, 618–9;
-      alleged differentiation of reproductive and somatic cells, =I=,
-          622, 628–30, 633–44, 646;
-      origin of variations in neuter insects, =I=, 659, 663–5, 671,
-          675;
-      correlated variations in stag, =I=, 677;
-      insuperable difficulties, =I=, 682;
-      conceivability of hypothesis, =I=, 695; =II=, 619;
-      correlated variations in cultivated plants, =II=, 621–2.
-
-    Ghost-theory, Vitalism and, =I=, 114.
-
-    Giraffe, co-adaptation of structures, =I=, 615.
-
-    Gizzard, development of birds, =II=, 320.
-
-    Glass, molecular re-arrangement, =I=, 337, 352, 704.
-
-    Glove, strain analogy, =II=, 575.
-
-    Glycogen, In animal metabolism, =I=, 70, 72.
-
-    Goethe, J. W. von: foliar homology, =II=, 43–4, 543, 544;
-      archetypal hypothesis, =II=, 122;
-      vegetal fructification and nutrition, =II=, 180;
-      theory of supernumerary bones, =II=, 223;
-      on the skull, =II=, 561.
-
-    Gold, effect of bismuth on, =I=, 121.
-
-    Gorilla, callosities, =II=, 312.
-
-    Gould, J., _Birds of Australia_, =II=, 469.
-
-    Gout (_see_ Disease).
-
-    Grafting, Born’s experiments with frog larvæ, =I=, 365.
-
-    Graham, T., properties of water, =I=, 9; =II=, 359;
-      colloids and crystalloids, =I=, 15–8; =II=, 356;
-      their diffusibility, =I=, 18–20;
-      sapid and insipid substances, =I=, 53.
-
-    _Graminæ_: foliar surfaces, =II=, 61, 263;
-      floral symmetry, =II=, 165;
-      physiological differentiation, =II=, 257.
-
-    Graminivores, food contrasted with that of carnivores, =I=, 68.
-
-    Grassi, on food habits of _Termites_, =I=, 686.
-
-    Gravity: its ultimate incomprehensibility, =I=, 121;
-      vegetal circulation, =II=, 586.
-      (_See also_ Specific Gravity.)
-
-    _Gregarina_: central development, =I=, 163;
-      primary aggregate, =II=, 87;
-      symmetry, =II=, 186.
-
-    Grimaux, on artificial proteids, =I=, 39.
-
-    Growth: organic and inorganic, =I=, 135–7;
-      simulation of, =I=, 136;
-      limits to, =I=, 137, 155–7;
-      structural complexity, =I=, 138–40, 145–7, 161;
-      nutrition, =I=, 140, 147–9, 161;
-      expenditure of energy, =I=, 141–3, 161;
-      initial and final bulks, =I=, 143–4, 157–60, 161;
-      final arrest of, =I=, 149–55, 639;
-      where unceasing, =I=, 154;
-      _résumé_ with generalizations, =I=, 161;
-      defined, =I=, 162; =II=, 461;
-      increased function, =I=, 228–33, 234–5;
-      functional interdependence, =I=, 235–9, 240;
-      nutrition and vegetal, =I=, 293, 294–7, 336; =II=, 39;
-      heterogenesis and animal nutrition, =I=, 289–93, 296, 336;
-      homo- and heterogenesis and natural selection, =I=, 294–8;
-      of acrogens, =II=, 56;
-      cylindrical form of vegetal, =II=, 56–64;
-      endogenous, =II=, 60–2, 78;
-      exogenous, =II=, 63–4, 78;
-      plant differentiation, =II=, 129–131;
-      tissue differentiation, =II=, 370;
-      formation of adaptive bone-structures, =II=, 370–2;
-      progressive increase of size with evolution, =II=, 401–2;
-      vegetal, and asexual genesis, =II=, 439–42;
-      animal, and asexual genesis, =II=, 442–5;
-      antagonistic to asexual genesis, =II=, 446;
-      vegetal and sexual genesis, =II=, 448–51;
-      animal and sexual genesis, =II=, 452–6, 495;
-      antagonistic to sexual genesis, =II=, 457–8;
-      nutrition and genesis, _résumé_, =II=, 497–9;
-      evolution and, =II=, 501–5;
-      commencement of genesis, =II=, 506;
-      fertilization and restoration of growth-energy, =II=, 613.
-
-    Gulick, T.: on monotypic and polytypic evolution, =I=, 569;
-      physiological selection, =I=, 569–71.
-
-    Gunpowder, nitrogenous instability, =I=, 8, 43.
-
-    _Gymnotus_, electricity of, =I=, 51.
-
-    _Gyrodactylus elegans_, rapid succession of generations, =I=, 641;
-        =II=, 488.
-
-
-    Habit, change of, in plants, =I=, 308.
-
-    Hæmal, term applied to female element, =I=, 594–5.
-
-    Hairs: non-conductors of heat, =I=, 526;
-      vegetal, and natural selection, =I=, 532;
-      development, =II=, 314–6;
-      tactual organs, =II=, 317.
-
-    Hand: embryogeny, =I=, 169;
-      heredity and size of, =I=, 311;
-      distribution of veins, =I=, 364.
-
-    Hardy, W. B., =I=, vii; =II=, vi.
-
-    Hare: activity and muscular colour, =II=, 365;
-      expenditure and genesis, =II=, 472.
-
-    Hart, J. A., on “Parasol” ants, =I=, 687–8.
-
-    Havilland, G. D., collection of Termites, =I=, 687.
-
-    Haystack, chemical action in, =I=, 74.
-
-    Head, structural influence of size, =I=, 512, 537.
-
-    Hearing: the sense of, =I=, 54;
-      multiplying agencies, =I=, 75.
-
-    Heart (_see_ Vascular System).
-
-    Heat: action on di- and tri-atomic compounds, =I=, 7–8, 10–12, 23,
-        24;
-      on colloids and crystalloids, =I=, 26;
-      organic changes from evaporation, =I=, 29;
-      chemical decomposition by, =I=, 33;
-      organic oxidation, =I=, 46–9, 60;
-      growth and organic, =I=, 152–3;
-      animal, vegetal, and environment, =I=, 174–5, 177;
-      alloy melting points, =I=, 339;
-      organic effects of rhythm in terrestrial, =I=, 498, 557;
-      effect on physiological units, =I=, 705;
-      respiration in fishes, =II=, 337;
-      animal preservation, =II=, 434;
-      vertebrate expenditure and genesis, =II=, 468–9, 474;
-      insect genesis, =II=, 476;
-      seasonal variations and genesis, =II=, 484–5;
-      in germination, =II=, 615.
-
-    Hebrew idea of creation, =I=, 421.
-
-    _Hectocotylus_, individuality, =I=, 250.
-
-    Hellin, D., on multiparity and twin-births, =II=, 457.
-
-    Hen, what prompts her to pick up egg-shell fragments? =I=, 120.
-
-    Henslow, Rev. G., inheritance of functionally-produced changes,
-        =I=, 560.
-
-    _Hepaticæ_: Schleiden on, =II=, 51, 52;
-      continuous and discontinuous development, =II=, 52;
-      phyletic homologies, =II=, 80–4;
-      meaning of so-called alternating generation, =II=, 84;
-      vascular system, =II=, 280;
-      genesis and development, =II=, 463.
-
-    Heredity: structural modification, =I=, 232;
-      function of cell-nucleus in, =I=, 258–59;
-      general truths, =I=, 301–4;
-      transmission of congenital peculiarities, =I=, 304–7;
-      structure and altered function, =I=, 307–13, 318–9 (_see also_
-          Acquired Characters);
-      atavism, or recurrence of ancestral traits, =I=, 314;
-      sex limitation, =I=, 314–6;
-      physiological units, _résumé_, =I=, 350–5; =II=, 612–6;
-      Darwin’s and Weismann’s theories examined, =I=, 356 _et seq._,
-          559–61; =II=, 622;
-      true theory must include plants, =I=, 358;
-      inadequacy of theory of physiological units, =I=, 360–1;
-      sociological parallel, =I=, 366–8;
-      natural selection (_q. v._), =I=, 545–7, 553, 557;
-      ethnology and natural selection, =I=, 553;
-      unsolved problems, =I=, 573–4;
-      mutilations, =I=, 631;
-      ultimate process incomprehensible, =I=, 695;
-      cell-doctrine, =II=, 19;
-      physiological development, =II=, 242;
-      wood formation, =II=, 287;
-      tissue differentiation, =II=, 304, 312–4;
-      respiratory system, =II=, 311;
-      osseous differentiation, =II=, 351;
-      muscular adaptation, =II=, 367;
-      persistence of force and physiological adaptation, =II=, 394;
-      vegetal vascular system, =II=, 574, 582, 588, 596.
-
-    Hermaphrodism, =I=, 340–3.
-
-    Hertwig, O.: on Weismann’s germ-plasm theory, =I=, 690;
-      cell characters, =I=, 691;
-      meaning of fertilization, =II=, 613.
-
-    Hertwig, R., classification of tissues, =I=, 189.
-
-    Heterochrony of development, =I=, 655.
-
-    Heterogeneity: in chemical evolution, =I=, 23–4;
-      of vital changes, =I=, 84–90;
-      of development, =I=, 170, 178;
-      functional, =I=, 204–8, 211–2;
-      of organic matter, =I=, 350–5;
-      organic and instability of homogeneous, =I=, 509–11, 517, 549,
-          557;
-      segregation accompanying, =I=, 514–6, 517–8, 550.
-
-    Heterogenesis: occurrence, =I=, 270, 272–5, 336;
-      animal nutrition, =I=, 289–91, 295–7;
-      natural selection, =I=, 295–8;
-      heredity, =I=, 301.
-
-    Hindus: food, =I=, 68;
-      dwarf family, =I=, 316.
-
-    Histology (_see_ Physiology).
-
-    Hofmeister, sporophytic generation of Archegoniates, =II=, 80.
-
-    Hollyhock, floral symmetry, =II=, 167, 169, 170.
-
-    Homogeneous, instability of the: variation, =I=, 330, 334, 342;
-      evolution, =I=, 509–11, 517, 549, 557;
-      morphological development, =II=, 7–9, 234;
-      direction of vegetal growth, =II=, 181;
-      radial symmetry, =II=, 190;
-      physiological differentiation, =II=, 384, 392.
-
-    Homogenesis (_see_ Gamogenesis).
-
-    Homology, simulation of, by analogy, =II=, 14.
-
-    Hooker, Sir J. D., =I=, ix;
-      European plants in New Zealand, =I=, 477;
-      plant distribution, =I=, 479;
-      adaptation of plants to varied media, =I=, 484;
-      plant growth, =II=, 56;
-      _Balanophoræ_ and _Raffiesiaceæ_, =II=, 274;
-      structural complexity, =II=, 295, 297;
-      relative antiquity and distribution of plants and animals, =II=,
-          297;
-      bean vascular system, =II=, 574.
-
-    Hooker, Sir W., on fructification in _Jungermanniaceæ_, =II=, 52.
-
-    Horns, natural selection and correlated variation, =I=, 537, 567,
-        674, 677.
-
-    Horse: ancestral types, =I=, 409;
-      fertility, =I=, 598;
-      weight of brain, =I=, 599;
-      quagga markings, =I=, 624, 627.
-
-    Husbandry, co-ordination of actions in, =I=, 96, 579.
-
-    Hutchinson, Sir J., hereditary syphilis, =I=, 623.
-
-    Huxley, T. H., =I=, ix;
-      “continuous” and “discontinuous” development, =I=, 164;
-      classification of development, =I=, 276;
-      hermaphrodism, =I=, 344;
-      zoological classification, =I=, 383;
-      on “Persistent Types,” =I=, 408–9;
-      ancestral equine types, _ib._;
-      segmentation of articulates, =I=, 468–9; =II=, 113;
-      agamic multiplication of _Aphis_ and _Entozoa_, =I=, 640–1; =II=,
-          476;
-      cell-doctrine, =II=, 21;
-      vertebrate embryo, =II=, 119, 120;
-      molluscan symmetry, =II=, 202;
-      tegumentary organs, =II=, 314, 315;
-      vertebrate sensory organs, =II=, 318, 319;
-      _Chondracanthus_, =II=, 487;
-      Owen’s vertebrate theory, =II=, 563.
-
-    Hyacinth: lateral spike, =II=, 42;
-      symmetry, =II=, 141, 162.
-
-    Hybernation, waste and repair in, =I=, 214–5.
-
-    Hybrids, separation of ancestral traits in, =II=, 616–7.
-
-    Hydro-carbons: properties, =I=, 6–9;
-      the term carbo-hydrates (_q. v._), =I=, 10.
-
-    Hydrochloric acid, in gastric juice, =I=, 69.
-
-    Hydrogen: chemical and physical properties, =I=, 3–5;
-      compounds, =I=, 6, 8, 9, 10–12, 12–13.
-
-    _Hydrozoa_ (see _Cœlenterata_).
-
-    _Hymenoptera_ (_see_ Insects).
-
-    Hypertrophy (_see_ Disease).
-
-    Hypospadias, telegonic transmission, =I=, 646.
-
-    Hypostasis of a relation, exemplified in explanations of
-        fertilization, =II=, 613.
-
-
-    Ideas (_see_ Psychology).
-
-    Individuality: the botanical, =I=, 244–6;
-      the zoological, =I=, 246–7;
-      the fertilized germ product, =I=, 248–9;
-      definition of life, =I=, 250.
-
-    Individuation: and genesis, =I=, 583–4; =II=, 428–30, 499;
-      total cost, =II=, 435–7;
-      genesis and evolution, =II=, 501–5, 529, 530.
-
-    _Infusoria_: functional specialization, =I=, 391;
-      primary aggregate, =II=, 87;
-      asymmetry, =II=, 187, 188;
-      differentiation, =II=, 299, 385;
-      genesis, =II=, 442, 446, 452.
-
-    Injuries, repair of animal, =I=, 219, 222–4, 316; =II=, 102, 611.
-
-    Insanity, inherited, =I=, 314.
-
-    Insects: temperature, =I=, 47, 174;
-      phosphorescence, =I=, 49;
-      self-mobility, =I=, 175;
-      parthenogenesis, =I=, 274–5, 277, 294, 592, 640;
-      growth and reproduction, =I=, 292;
-      species distribution determined by presence of, =I=, 396–7;
-      eyes of cave-inhabiting, =I=, 309, 612–3, 614, 647–9, 693;
-      persistent types, =I=, 408;
-      retrograde development, =I=, 458;
-      segmentation, =I=, 468–9; =II=, 114;
-      aborted organs, =I=, 474;
-      East Indian distribution, =I=, 478;
-      floral fertilization, =I=, 525; =II=, 168–9, 406–7, 608;
-      appliances for cleaning antennæ, =I=, 651;
-      eyes, =I=, 658; =II=, 318;
-      integration and homology, =II=, 111–3, 121;
-      bilateral symmetry, =II=, 198;
-      sexual selection, =II=, 269;
-      eyes, =II=, 318;
-      environment, =II=, 433;
-      cost of genesis, =II=, 436, 437;
-      development and genesis, =II=, 461;
-      nutrition and genesis, =II=, 476, 490–2.
-
-    Insects, Social, origin of caste gradations in, =I=, 654–65, 670,
-        674, 675, 678–84, 686–8.
-
-    Instability of the homogeneous (_see_ Homogeneous).
-
-    Instinct: organic evolution and co-ordination of, in mason-wasp,
-        =I=, 574;
-      a vital attribute, =I=, 578;
-      loss of self-feeding, in Amazon ants, =I=, 660–1, 663–4.
-
-    Integration: in chemical evolution, =I=, 23;
-      morphological composition, =II=, 4–6;
-      arthropod, =II=, 111–4, 121;
-      physiological, in plants, =II=, 292–5, 295–8, 390;
-      of organic world, =II=, 396–408;
-      genesis, =II=, 424, 426–9.
-
-    Intelligence, a vital attribute, =I=, 579.
-
-    Internodes: varied development, =II=, 45;
-      nutrition and length, =II=, 178–9.
-
-    Intestine (_see_ Alimentary Canal).
-
-    Intra-selection, Roux’s theory of, =I=, 562, 676–8.
-
-    Irish, nutrition and genesis, =II=, 510.
-
-    Iron: colloidal form of peroxide, =I=, 17, 20;
-      molecular re-arrangement, =I=, 337, 704;
-      vegetal absorption, =II=, 573.
-
-    Iron industry, interdependence of social function, =I=, 237–41.
-
-    Isolation, and species differentiation, =I=, 568–9.
-
-    Isomerism: of organic constituents, =I=, 4, 9, 25;
-      tri- and poly-atomic compounds, =I=, 11, 13, 25;
-      muscular action, =I=, 59;
-      organic evolution, =I=, 700, 703;
-      differentiation of nerve tissue, =II=, 356–60, 361;
-      of muscular tissue, =II=, 361–4.
-
-
-    Jackson, J. Hughlings, on inheritance of nervous peculiarities,
-        =I=, 313, 694.
-
-    Jaundice (_see_ Disease).
-
-    Jaws, of uncivilized and civilized, =I=, 541–2, 612, 693.
-
-    Johnson, G. Lindsey, on inherited myopia, =I=, 694.
-
-    Jones, T. Rymer, on fission, =I=, 585, 590.
-
-    Julin, C., on “castration parasitaire” in Crustaceans, =II=, 493–6.
-
-    _Jungermanniaceæ_: morphology, =II=, 33–4;
-      relations of high and low types, =II=, 35, 55;
-      continuous and discontinuous development, =II=, 52–5, 92;
-      tubular structure, =II=, 58, 62;
-      proliferous growth, =II=, 67, 91;
-      colour, =II=, 75, 265;
-      symmetry, =II=, 140;
-      fertility and growth, =II=, 441.
-
-    Jussieu, A. de, plant classification, =I=, 378.
-
-
-    Karyokinesis, =I=, 257, 259, 263–5.
-
-    Kerner, A., on cauline buds, =I=, 358;
-      plant classification in _Natural History of Plants_, =I=, 378–9.
-
-    Kidd, Benj., his acceptance of Weismannism, =I=, 690.
-
-    Kitto, Dr., his visual memory and deafness, =I=, 230.
-
-    Klebs, on _Hydrodictyon_, =I=, 288;
-      _Vaucheria_, =II=, 84.
-
-    Klein, E., multiplication of _Bacteria_, =II=, 443.
-
-    Korschelt, E., annulose segmentation, =II=, 103, 601–3, 605;
-      _Arenicola_ larvæ, =II=, 109.
-
-
-    Labour, physiological division of, =I=, 204, 207, 591; =II=, 373;
-      its meaning and Weismann’s fallacious interpretation, =I=, 634–5.
-
-    Lacaze-Duthiers, on origin of annulose type, =II=, 110.
-
-    Lamarck: zoological classification, =I=, 382;
-      opinions of E. Darwin and, =I=, 491, 493–7;
-      neo-Darwinists and, =I=, 630–1.
-
-    _Laminariaceæ_: pseudo-foliar and axial development, =II=, 30;
-      tissue, =II=, 247, 256, 272.
-
-    Language: and evolution, =I=, 442, 444, 446;
-      perceptiveness of tongue-tip, =I=, 607.
-
-    Lankester, Sir E. Ray, absence of nucleus in _Archerina_, =I=, 183;
-      diversity of _Protozoa_, _ib._;
-      zoological classification, =I=, 387;
-      blindness of cave-animals, =I=, 647–8, 649.
-
-    Laugh, definition of life and, =I=, 112.
-
-    Laurel, leaves of, =II=, 149, 249.
-
-    Leaves: growth of shoot, =I=, 168;
-      development and aggregation, =II=, 37–42, 76;
-      stem-like stalks, =II=, 41;
-      homologies, =II=, 42, 75–7, 83;
-      nutrition and compound, =II=, 42;
-      foliar and axial development, =II=, 46–50, 541–7;
-      “adnate,” =II=, 58;
-      proliferous growth, =II=, 67, 91;
-      nutrition and development, =II=, 76–8;
-      symmetry, and of branches, =II=, 148–50, 151;
-      size and distribution of leaflets, =II=, 152–5;
-      transition from compound to simple, =II=, 155–8;
-      unsymmetrical form, =II=, 158–9;
-      natural selection and distribution, =II=, 179;
-      morphological summary, =II=, 234–5;
-      tissue differentiation, =II=, 247;
-      distribution, =II=, 249;
-      outer tissues of stem and, =II=, 256–9, 270, 386;
-      distribution of stomata, =II=, 260–1;
-      wax deposit on, =II=, 260, 261;
-      light and colour, =II=, 261–2;
-      superficial differentiation, =II=, 263–5, 270, 387;
-      abortive in parasitic plants, =II=, 274;
-      submerged, in aquatic plants, =II=, 274–5;
-      inner tissue differentiation, =II=, 278, 388;
-      vascular tissue differentiation, =II=, 286, 288, 388;
-      dye absorption and circulation, =II=, 570–4, 577;
-      vascular system, =II=, 588–92, 596;
-      arrangement, =II=, 608–11.
-
-    _Lepidoptera_ (_see_ Insects).
-
-    _Lepidosiren_: ossification, =II=, 218;
-      respiration, =II=, 338;
-      skeleton, =II=, 553, 555, 560.
-
-    _Lepidosteus_: armour, =I=, 526;
-      air-bladder, =II=, 334.
-
-    Leroy-Beaulieu, Pierre, on Australian miners’ usages, =I=, 364.
-
-    _Lessonia_: Hooker on growth, =II=, 56;
-      branch symmetry, =II=, 146.
-
-    Lewes, G. H., definition of life, =I=, 80.
-
-    Lichens: tissue, =I=, 586;
-      cell multiplication, =II=, 27;
-      Hooker on growth, =II=, 56;
-      tubular structure, =II=, 57;
-      integration, =II=, 293;
-      dual nature, =II=, 399;
-      reproduction, =II=, 450.
-
-    Liebig, Baron, nitrogenous food stuffs, =I=, 47–8.
-
-    Life: co-ordination of actions, =I=, 79, 89, 577–80;
-      defined by Schelling, =I=, 78, 178;
-      Richeraud, =I=, 79;
-      De Blainville, =I=, 79, 93;
-      Lewes, =I=, 80;
-      definition yielded by contrasting most unlike kinds, =I=, 81–8;
-      changes showing, =I=, 91;
-      vital actions and environment, =I=, 92–3;
-      resulting addition to conception, =I=, 93, 326;
-      Comte’s definition, =I=, 93;
-      correspondence of external and internal relations, =I=, 93–6,
-          100; =II=, 523;
-      continuous adjustment of such relations, =I=, 99;
-      completeness proportionate to correspondence, =I=, 101–4, 109,
-          349;
-      length and complexity, =I=, 103;
-      complexity of environment and degree of, =I=, 104–6;
-      definitions of evolution and, =I=, 107–10;
-      deficiencies of formula, =I=, 112–3;
-      activity the essential element, =I=, 113;
-      hypothesis of independent vital principle examined, =I=, 114–7;
-      difficulties of physico-chemical theory, =I=, 117–20;
-      ultimate incomprehensibility, =I=, 120–3, 373;
-      validity of conclusions reached, =I=, 123;
-      is organization produced by? =I=, 197;
-      precedes organization, =I=, 210;
-      definitions of individuality and, =I=, 250;
-      effect of incident forces on, =I=, 348–9, 355;
-      length in individuals and species, =I=, 422;
-      equilibration of, =I=, 547, 557;
-      final formulation of definition, =I=, 580;
-      co-ordination of actions and sexual differentiation, =I=, 593;
-      “absolute” commencement of, =I=, 699, 702;
-      integration and augmentation, =II=, 426;
-      prospective human, =II=, 522–5.
-
-    Light: influence on organisms, =I=, 30–6; =II=, 433;
-      nitrogenous plants, =I=, 40;
-      organic phosphorescence, =I=, 49;
-      heliotropism, =I=, 92; =II=, 160;
-      effects on organic matter, =I=, 149;
-      plant adaptation, =I=, 227;
-      rhythmical variation of, and organic life, =I=, 499, 557;
-      vegetal influences, =II=, 130, 131, 147, 149, 158;
-      influence on flowers, =II=, 167–8, 608–11;
-      vegetal tissue differentiation, =II=, 253–5, 258, 259;
-      action on leaves, =II=, 260–4;
-      on plant vascular system, =II=, 288, 297, 586;
-      development of sensory organs, =II=, 320.
-
-    _Liliaceæ_, floral symmetry, =II=, 170.
-
-    Lime, leaf forms, =II=, 158, 159.
-
-    Lindley, J., plant classification, =I=, 377.
-
-    Linnæus, C., classificatory system, =I=, 377, 380.
-
-    Linnet, contrasted with blackbird in development, =II=, 503.
-
-    Liver: metabolic processes, =I=, 69, 70;
-      vitality of excised, =I=, 111;
-      development, =II=, 329–33.
-
-    Liver-fluke (see _Distoma_).
-
-    Liverworts (see _Hepaticæ_).
-
-    Lizard, regeneration of lost tail, =I=, 360.
-
-    Locomotion (_see_ Motion).
-
-    Logic, reasoning and definition of life, =I=, 81–6.
-
-    Logwood, vegetal staining, =II=, 569–74, 577–81, 584.
-
-    Longevity, and complexity of life, =I=, 102–3.
-
-    Lubbock, Sir J.: on growth and genesis in insects and crustaceans,
-        =I=, 292;
-      aquatic flies, =I=, 400.
-
-    Lungs (_see_ Respiratory System).
-
-    Lymphatic system: amœboid cells, =I=, 187;
-      structural traits, =I=, 192, 193.
-
-
-    MacBride, E. W., =I=, vi; =II=, vi;
-      zoological phyla, =I=, 386–7;
-      arthropod segmentation, =II=, 114;
-      ctenidia of slug, =II=, 117;
-      conjugation of _Paramæcium_, =II=, 452.
-
-    _Macrocystis pyrifera_, gigantic seaweed, =I=, 121.
-
-    Magenta, vegetal staining, =II=, 569–74, 577–81, 584.
-
-    Magnetism: muscular action, =I=, 59;
-      incomprehensibility, =I=, 121.
-
-    Maillet, B. de, modifiability of organisms, =I=, 490, 496.
-
-    _Mammalia_: temperature and molecular change, =I=, 30;
-      nutrition and growth, =I=, 141;
-      expenditure of force, =I=, 142, 156;
-      flesh constituents, =I=, 154;
-      temperature, =I=, 174, 177;
-      self-mobility, =I=, 175;
-      functional and structural differentiation, =I=, 201;
-      heart-function, =I=, 206;
-      viviparous homogenesis, =I=, 271;
-      variation and uterine environment, =I=, 327;
-      classification, =I=, 392;
-      cervical vertebræ, =I=, 394; =II=, 564;
-      aquatic types, =I=, 400;
-      fossil remains and rate of evolution, =I=, 407;
-      ancient and modern forms contrasted, =I=, 408–10;
-      embryonic respiratory system, =I=, 456;
-      suppression of teeth, =I=, 457;
-      arrested development, =I=, 473–4;
-      simulated homologies, =I=, 485;
-      natural selection and inactive parts, =I=, 534;
-      re-development of rudimentary organs, =I=, 563;
-      location of testes and current theories, =I=, 573;
-      fertility and development, =I=, 583; =II=, 465;
-      fertility and nervous development, =I=, 598–9;
-      locomotion and elongated form, =II=, 15;
-      symmetry, =II=, 204;
-      tegumentary structure, =II=, 314;
-      circulation, =II=, 340;
-      vascularity and ova-maturation, =II=, 342–3;
-      activity and muscular colour, =II=, 365–9;
-      functional integration, =II=, 375;
-      outer tissue differentiation, =II=, 387;
-      growth and genesis, =II=, 456, 459;
-      comparative fertility, =II=, 465, 470;
-      heat expenditure and genesis, =II=, 467–9;
-      activity and fertility, =II=, 472;
-      nutrition and genesis, =II=, 479–80.
-
-    Man: effect of climate on vigour, =I=, 30;
-      flesh and grain eaters compared, =I=, 68;
-      longevity and life, =I=, 103;
-      complex environment, =I=, 105;
-      embryogeny of arm, =I=, 169;
-      fertility and conditions affecting it, =I=, 300, 570, 583; =II=,
-          484, 506–21;
-      inheritance of functionally-produced changes, =I=, 310–3, 541,
-          605, 608, 612, 652, 673, 689, 693–4;
-      heredity and sex, =I=, 315–6;
-      function of bilirubin, =I=, 330;
-      cousin-marriages, =I=, 346; =II=, 615;
-      primitive notions, =I=, 417–9;
-      inutility of _Appendix vermiformis_, =I=, 474;
-      diminution of jaw, =I=, 541, 612, 693;
-      co-ordination of actions greatest in, =I=, 579;
-      fundamental traits of sex, =I=, 594–7;
-      obesity, =I=, 594;
-      substance and weight of brain, =I=, 596, 599;
-      distribution of tactual perceptiveness, =I=, 602–8, 665–6, 672–3,
-          692;
-      telegony, =I=, 625, 644–5;
-      degradation of little toe, =I=, 652, 673;
-      transmitted osteological peculiarities of Punjabis, =I=, 689;
-      traits of twin-bearing women, =II=, 457;
-      comparative mammalian fertility, =II=, 465;
-      future evolution, =II=, 522–37.
-      (_See also_ Language _and_ Sociology.)
-
-    Manatee, nailless paddles, =I=, 473.
-
-    Manx cats, =I=, 303.
-
-    _Marchantiaceæ_: symmetry, =II=, 140;
-      outer tissue differentiation, =II=, 252.
-
-    Marmot, hybernation and waste, =I=, 214–5.
-
-    Marriage (_see_ Multiplication).
-
-    Marsh, O. C., on telegony, =I=, 644.
-
-    Masters, M. T., on foliar homology, =II=, 46–7;
-      selection of inconspicuous variations in plants, =II=, 298, 621;
-      separation of ancestral constitutions in plant hybrids, =II=,
-          616;
-      single and double stocks, =II=, 622.
-
-    Matter, incomprehensibility of interactions, =I=, 121–2.
-
-    Mechanics: transverse strains, =II=, 209–12;
-      genesis of vertebrate axis, =II=, 212–6, 216–8, 224, 225–7;
-      osseous differentiation, =II=, 345–51;
-      disintegrated motion, =II=, 375;
-      analogy from locomotive, =II=, 517–9;
-      future human evolution, =II=, 524;
-      strain and vegetal structure, =II=, 574–88, 592–6.
-
-    _Medusæ_: contractile functions, =I=, 58; =II=, 374;
-      individuality, =I=, 248;
-      heterogenesis, =I=, 273;
-      fertility, =I=, 582;
-      strobilization, =I=, 592;
-      symmetry, =II=, 188–91.
-
-    Mehnert, E., on feet of pentadactyle vertebrates, =I=, 461.
-
-    Mensel’s salt, temperature and isomerism, =I=, 77.
-
-    Metabolism: antithesis between plants and animals, =I=, 62–3;
-      evolution hypothesis and primordial, =I=, 63–4;
-      in plants, =I=, 64–7;
-      animals, =I=, 67–77;
-      nervo-muscular activities, =I=, 71–7;
-      summary, =I=, 77;
-      cell processes, =I=, 261.
-
-    Metals: remarkable interactions of some, =I=, 121;
-      melting of alloys, =I=, 339;
-      atomic re-arrangement, =I=, 352.
-
-    Metamerism (_see_ Segmentation).
-
-    _Metazoa_: cellular structure, =I=, 184, 194; =II=, 21;
-      subordination of units, =I=, 185–7;
-      general characters of tissues, =I=, 188–9;
-      protoplasmic continuity, =I=, 190–2, 194, 628;
-      genesis of food-cavity and visual organ, =I=, 195;
-      Weismann’s differentiation theory, =I=, 637–43.
-
-    Meteorology: non-vital changes shown in, =I=, 82, 84;
-      crystallization of “storm glass,” =I=, 96;
-      special creation, =I=, 419;
-      rhythm in, and organic change, =I=, 499–501, 557;
-      variations due to geologic change, =I=, 503.
-
-    _Microstomida_, segmental reproduction, =II=, 102.
-
-    Migration: of animal species, =I=, 396–401, 411;
-      solar influences, =I=, 500;
-      part played by, in organic evolution, =I=, 568;
-      causes of, =II=, 533–4.
-
-    Milk, heat and supply of, =II=, 468.
-
-    Milne-Edwards, H., “physiological division of labour,” =I=, 204;
-      Weismann’s erroneous application of it, =I=, 634;
-      on ocular structure, =II=, 318.
-
-    Mind (_see_ Psychology).
-
-    Mitosis (_see_ Karyokinesis).
-
-    Mobility, molar and molecular, =I=, 14;
-      environment and self-mobility, =I=, 177.
-
-    Mohl, on phænogamic growth, =II=, 82.
-
-    Mole, modifications due to habits, =II=, 391.
-
-    Molecules: mechanically considered, =I=, 14;
-      stability, =I=, 337–40;
-      nerve differentiation, =II=, 355–61, 379–82.
-
-    _Mollusca_: axial development, =I=, 165;
-      genesis, =I=, 271; =II=, 444;
-      hermaphrodism, =I=, 341;
-      classificatory traits, =I=, 392;
-      distribution in time, =I=, 405, 408, 410, 446–7;
-      trochophore and its relationships, =I=, 447; =II=, 108, 109, 115;
-      development, =I=, 460;
-      amphibious and terrestrial, =I=, 481;
-      indirect equilibration, =I=, 534;
-      secondary aggregation, =II=, 115–7;
-      symmetry, =II=, 201–3;
-      outer tissue, =II=, 310, 387;
-      alimentary system, =II=, 325;
-      vascular system, =II=, 340–1.
-
-    _Molluscoida_, =II=, 598.
-      (See _Polyzoa_ and _Tunicata_.)
-
-    Monocotyledons: growth, =I=, 138, 139, 143;
-      uniaxial development, =I=, 165;
-      cotyledonous germination and endogenous growth, =II=, 59–62,
-          69–72, 82–3, 181–2;
-      absence of helical phyllotaxy in _Ravenala_, =II=, 182;
-      surface contrasts, =II=, 257;
-      outer leaf tissue, =II=, 263;
-      wood formation, =II=, 278;
-      growth and genesis, =II=, 451.
-
-    Monstrosities, in plants, =II=, 78, 541, 546;
-      vertebrate, =II=, 118.
-
-    Morgan, T. H., on regeneration of _Planaria_, =II=, 102, 611.
-
-    Morphology: facts comprised in, =I=, 125–6;
-      morphological units, =I=, 190–2, 225;
-      rudimentary organs, =I=, 472–5, 556;
-      structural and functional co-operation, =II=, 3, 239;
-      integration, =II=, 4–6, 181–96;
-      change of shape, =II=, 6;
-      formula of evolution, =II=, 7–9;
-      as interpreted by phylogeny, =II=, 10–6;
-      evolution and cell-doctrine, =II=, 17–21.
-
-    Morphology, Animal: evolution and segmentation of _Articulata_,
-        =I=, 468–9;
-      vertebral column development, =I=, 470;
-      simulated homologies, =II=, 14–5;
-      primary aggregates, =II=, 85–8, 123–4;
-      secondary, =II=, 88–91, 124;
-      tertiary, =II=, 91–3;
-      integration and independence of individuality, =II=, 93–9, 124;
-      annulose segmentation, =II=, 98–101, 106–10, 125–7, 602–7;
-      progressive annulose integration, =II=, 100–5, 111–5, 121, 124,
-          223;
-      unintegrated molluscan form, =II=, 115–7;
-      vertebrate segmentation and integration, =II=, 117–23, 124–7,
-          223–4, 602, 606–7;
-      motion and symmetry, =II=, 183–5;
-      symmetry of primary and secondary aggregates, =II=, 186, 187–91;
-      of compound _Cœlenterata_, =II=, 192–4;
-      simulation of plant shapes, =II=, 192;
-      symmetry of _Polyzoa_ and _Tunicata_, =II=, 194;
-      of _Platyhelminthes_ and _Echinoderms_, =II=, 195–7;
-      of _Annulosa_, =II=, 197–201;
-      of molluscs, =II=, 201–3;
-      of vertebrates, =II=, 203–6, 208;
-      similarity of animal and plant, =II=, 206;
-      cell-shapes, =II=, 228–30;
-      evolution and generalizations summarized, =II=, 231–5.
-      (_See also_ Structure.)
-
-    Morphology, Vegetal: simulated homologies, =II=, 13–4;
-      unicellular plants, =II=, 21;
-      aggregation and integration, =II=, 22–6, 78–9;
-      pseudo-foliar development, =II=, 26–8;
-      pseudo-axial, =II=, 28–9;
-      pseudo-foliar and axial, =II=, 30–2;
-      composition of Archegoniates, =II=, 33–5;
-      leaf development and aggregation, =II=, 37–42, 75–8;
-      foliar homologies, =II=, 42–6, 75–8;
-      foliar and axial development, =II=, 46–50, 541–7;
-      growth and development of Archegoniates, =II=, 50–6;
-      of Phænogams, =II=, 56–64, 78–80;
-      axillary bud development, =II=, 65–9;
-      phænogamic modes of growth, =II=, 69–72;
-      homologies, =II=, 73–5, 80–4;
-      development of foliar into axial organs, =II=, 75–8;
-      _résumé_, =II=, 78–80;
-      criticisms and replies, =II=, 80–4;
-      can plant shapes be formulated? =II=, 128;
-      growth and differentiation, =II=, 129–31;
-      kinds of symmetry, =II=, 131–3;
-      symmetry of primary aggregates, =II=, 134–7;
-      of secondary, =II=, 137–40;
-      tertiary, =II=, 140–3;
-      symmetry and environing influences, =II=, 143–4;
-      symmetry of branches, =II=, 145–8;
-      leaf and branch symmetry, =II=, 148–50;
-      phænogamic unit homology, =II=, 151;
-      size and distribution of leaflets, =II=, 152–5;
-      transition from compound to simple leaves, =II=, 155–8;
-      unsymmetrical leaf development, =II=, 158–9;
-      differentiation of homologous units, =II=, 159–60;
-      floral symmetry, =II=, 161–74;
-      cell-differentiation and metamorphosis, =II=, 175–7;
-      nutrition and differentiation, =II=, 178;
-      and inflorescence, =II=, 179;
-      helical growth of phænogams, =II=, 180–1;
-      summary of symmetry, =II=, 234;
-      stress and structure, =II=, 275–9, 388.
-      (_See also_ Structure.)
-
-    Morton, Lord, quagga-marked foal, =I=, 624.
-
-    Moser, impressions produced by light on metals, =I=, 352.
-
-    Mosses: varied development, =II=, 501, 52;
-      homologies, =II=, 80, 81;
-      indefiniteness, =II=, 296;
-      multiplication, =II=, 441.
-
-    Moth, clothes, food of larva, =I=, 77.
-
-    Motion: organic, and environment, =I=, 75–7, 175–8, 196;
-      of animals and waste, =I=, 214, 220;
-      simulation of locomotive structures, =II=, 15.
-
-    Motor organs, differentiation of, =I=, 262.
-
-    Mountains: climatic effects, =I=, 504;
-      growth of trees on, =II=, 142.
-
-    Mouse: fertility of, =II=, 421, 473;
-      tapeworm parasitism, =II=, 490;
-      compared with rat, =II=, 503–4.
-
-    _Mucor_, =II=, 22, 123.
-
-    Mucous membrane, differentiation, =II=, 321–2, 389.
-
-    Multiplication: decline of fertility with evolution, =I=, 103;
-        =II=, 431;
-      vitalism, =I=, 116;
-      phenomena classified, =I=, 130;
-      the term “genesis,” =I=, 269;
-      processes classified, =I=, 270–6, 336, 583;
-      a process of disintegration, =I=, 276;
-      reproductive tissue in gamogenesis, =I=, 278–84;
-      nutrition and growth, =I=, 285–94, 295–7, 299;
-      natural selection, =I=, 295–8;
-      hermaphrodism, =I=, 340–4;
-      in-and-in breeding, =I=, 344–7;
-      physiological units, =I=, 350–5;
-      law of race-maintenance, =I=, 581; =II=, 420–3, 430;
-      effect of mental application, =I=, 597; =II=, 511–4, 516–9, 530;
-      individuation antagonistic to, =I=, 598–600; =II=, 428–30, 435–7,
-          499, 501–5;
-      checks put by carnivores on, =II=, 405;
-      four factors in rate of, =II=, 416, 435;
-      destructive and preservative forces, =II=, 417–20;
-      rhythm of species, =II=, 419;
-      nutrition and disintegration of, =II=, 424, 425, 430;
-      integration and genesis, =II=, 426–8;
-      influence of environment, =II=, 432–3;
-      and variations of expenditure, =II=, 433–5;
-      growth and asexual genesis, =II=, 439–46;
-      asexual and sexual distinguished, =II=, 448;
-      sexual genesis and growth, =II=, 448–58, 495;
-      and development, =II=, 461–5;
-      plant expenditure, =II=, 467;
-      animal expenditure, =II=, 468–72;
-      nutrition in plants, =II=, 475, 511;
-      in animals, =II=, 476–84, 511;
-      seasonal variations, =II=, 484–5;
-      nutrition, _résumé_, =II=, 486, 497–9;
-      nutrition and parasitic, =II=, 486–90;
-      reversion to agamogenesis, =II=, 490–2;
-      human fertility, =II=, 506–10;
-      Doubleday on, =II=, 510–2;
-      civilized and uncivilized, =II=, 514–6;
-      human evolution and decline in, =II=, 529–31;
-      the future of population, =II=, 532–7;
-      equilibration and evolution, =II=, 537.
-
-    Muscle: electrical contrasts, =I=, 50;
-      action of, =I=, 59;
-      metabolism, =I=, 70, 71–4;
-      definition of life and actions of, =I=, 112–3;
-      growth and function, =I=, 151, 155;
-      development, =I=, 170;
-      Hertwig’s classification of tissues, =I=, 189;
-      functional differentiation, =I=, 203–4;
-      waste and repair, =I=, 215–7;
-      modifiability and adaptability, =I=, 228–9, 230, 232;
-      correlated variations, =I=, 536–9, 614–21, 676, 693;
-      resistance to strains, =I=, 639;
-      action on bones in Punjabis, =I=, 689;
-      differentiation, =II=, 361–9;
-      activity and colour, =II=, 365–9;
-      integration, =II=, 376, 382;
-      equilibration in action, =II=, 393;
-      activity and fertility in birds, =II=, 470–2;
-      future human evolution, =II=, 523;
-      origin of vertebrate type, =II=, 598–600.
-
-    Music: limited adaptability of voice and ear, =I=, 231;
-      inheritance of faculty, =I=, 311–2, 694.
-
-    Mutilations, the question of their inheritance, =I=, 631.
-
-    _Mycetozoa_, growth and reproduction, =I=, 298–9.
-
-    Myocommata (myotomes), and vertebrate skeleton, =II=, 216, 217–8,
-        222.
-
-    Myopia, inheritance of, =I=, 306, 694.
-
-    _Myrianida fasciata_, =I=, 361; =II=, 445.
-
-    _Myriapoda_: gemmation, =I=, 589;
-      segmentation, =I=, 590; =II=, 113, 114, 601;
-      degenerated eyes of cave-inhabiting, =I=, 649;
-      integration and homology, =II=, 111–4;
-      genesis, =II=, 445.
-
-    _Myxothallophyta_, =I=, 378.
-
-
-    Nails, mammalian, =I=, 473.
-
-    _Nais_: regeneration of detached parts, =I=, 219, 361.
-
-    Narcissus, separation of ancestral traits in hybrids, =II=, 617.
-
-    Natural selection: structural modification, =I=, 211;
-      in cell processes, =I=, 263–4;
-      multiplication, =I=, 295–8;
-      aided by function, =I=, 308–10;
-      special creation, =I=, 426–7;
-      the term “survival of the fittest,” =I=, 530;
-      indirect equilibration, =I=, 530–5, 552–3, 557, 571;
-      changes unexplained by, =I=, 535–42, 571; =II=, 371;
-      tendency to economy, =I=, 536, 562;
-      decrease of jaw, =I=, 541, 693;
-      general doctrine of evolution, =I=, 543–8, 557;
-      unceasing operation, =I=, 552;
-      human races, =I=, 553;
-      current views, =I=, 559–60;
-      panmixia and cessation of selection, =I=, 560–3;
-      intra-selection, =I=, 562, 676–8;
-      Eimer’s theory of orthogenesis, =I=, 564;
-      Mr. Cunningham’s criticism, =I=, 565–6;
-      location of mammalian testes, =I=, 573;
-      co-ordinated instincts of mason-wasp, =I=, 574;
-      tactual perceptiveness, =I=, 603–8, 633, 646, 665, 671, 672, 692;
-      erroneously identified with artificial selection, =I=, 609, 695;
-      reversed selection, =I=, 611;
-      blindness of cave-animals, =I=, 613, 614, 647–8, 693;
-      co-adaptation of co-operative parts, =I=, 614, 621, 663–5, 670,
-          674, 675, 689, 692;
-      where operative, =I=, 632;
-      Weismann on conceivability of process, =I=, 651;
-      degeneration of little toe, =I=, 652–3, 673;
-      genesis of caste gradations In social insects, =I=, 654–60, 663,
-          670, 675, 684;
-      self-feeding instinct in ants, =I=, 660–2, 670;
-      rudimentary organs, =I=, 667–9, 671, 692;
-      horns of stag, =I=, 676–8, 692;
-      musical faculty, =I=, 694;
-      the neo-Darwinian position reviewed, =I=, 694–5;
-      vegetal nutrition, =II=, 51–2;
-      upright vegetal growth, =II=, 56–7;
-      endogenous growth, =II=, 57–8;
-      exogenous, =II=, 64;
-      _Navicula_ symmetry, =II=, 135;
-      foliar, =II=, 158;
-      foliar distribution, =II=, 167, 179;
-      floral fertilization and symmetry, =II=, 168–70, 608–11;
-      helical phænogamic growth, =II=, 181;
-      _Echinodermata_ and bilateral symmetry, =II=, 197;
-      vertebrate structure, =II=, 214–20, 227;
-      phænogamic tissue differentiation, =II=, 248;
-      physiological differentiation, =II=, 252, 256;
-      rootlets of ivy, =II=, 254;
-      stomata and foliar surfaces, =II=, 261, 262;
-      floral fertilization, =II=, 268–9;
-      sexual selection, =II=, 269;
-      vegetal tissue differentiation, =II=, 279;
-      wood formation, =II=, 287–8, 290;
-      animal tissue differentiation, =II=, 304–8;
-      evolution of nervous system, =II=, 307–8;
-      respiratory system, =II=, 311;
-      dermal callosities, =II=, 312–4;
-      sensory organ complexities, =II=, 321;
-      skin and mucous membrane differentiation, =II=, 322;
-      localization of excretion, =II=, 333;
-      respiratory organs of fishes, =II=, 335–8;
-      heart and vascular system, =II=, 341, 344;
-      osseous differentiation, =II=, 355;
-      also muscular, =II=, 363, 368–9;
-      “false joints,” =II=, 371;
-      insect nutrition and genesis, =II=, 499;
-      economics of evolution, =II=, 501–5;
-      author’s enunciation of survival of the fittest in 1852, =II=,
-          528–9;
-      evils of interference with, in man, =II=, 532–3;
-      vegetal tissue formation, =II=, 582, 594–6;
-      origin of vertebrate type, =II=, 599.
-
-    Nature, more complex than supposed, =I=, 252, 450.
-
-    _Navicula_, symmetry, =II=, 134–5.
-
-    “Nebular Hypothesis,” =I=, 23.
-
-    Negation, inconceivability of, the ultimate test of truth, =I=,
-        675.
-
-    Negroes, telegony In United States, =I=, 644–5.
-
-    _Nemertidæ_: continuing vitality of pilidium, =I=, 250;
-      bilateral symmetry, =II=, 195.
-
-    Neo-Darwinists, and Lamarck, =I=, 630;
-      their position reviewed, =I=, 694–5.
-
-    Nerves: electrical phenomena, =I=, 51;
-      generation of nerve force, =I=, 52–6, 60;
-      _corpuscula tactus_, =I=, 75;
-      Hertwig’s classification of tissues, =I=, 189;
-      structural traits, =I=, 192, 193;
-      environment and structure, =I=, 196;
-      differentiation, =I=, 203; =II=, 355–61;
-      vasomotor system, =I=, 206;
-      vicarious function, =I=, 209;
-      activity and waste, =I=, 216;
-      adaptability, =I=, 229, 232, 236;
-      parallelism in cell processes, =I=, 260–2;
-      heredity, =I=, 313;
-      effects of severance, =I=, 349;
-      relative development in men and women, =I=, 594;
-      analysis of brain substance, =I=, 596;
-      individuation and development of, =I=, 598, 599, 600;
-      distribution of tactual perceptiveness, =I=, 603–8, 633, 646,
-          665–6, 671, 672, 692;
-      alleged costliness of tissue, =I=, 662;
-      instinct degeneration in ants, _ib._;
-      “sensation areas,” =I=, 666;
-      segmentation in Annelids, =II=, 125;
-      ectodermal derivation, =II=, 303–4;
-      co-operating factors in evolution of, =II=, 307–8;
-      differentiation from muscle, =II=, 363.
-      (_See also_ Psychology.)
-
-    Nervousness, hereditary transmission, =I=, 307.
-
-    Neurine, =I=, 594, 597.
-
-    Neuter insects (_see_ Insects).
-
-    New Zealand: invasion of alien species, =I=, 477;
-      kinship of past and present forms, =I=, 489.
-
-    Nitrogen: properties, =I=, 3–5, 20, 24;
-      compounds and their properties, =I=, 6, 8, 9, 12–14, 25–6, 39,
-          41, 42–3; =II=, 250;
-      organic importance, =I=, 42–3;
-      evolution of heat and oxidation, =I=, 47;
-      violent organic effects of compounds, =I=, 54–5;
-      function in metabolism, =I=, 63–4, 66, 68–76;
-      presence in protoplasm, =I=, 66;
-      action in digestion, =I=, 69;
-      fat accumulation and fertility, =II=, 483.
-
-    Nitro-glycerine, violent effects of, =I=, 55, 122.
-
-    Notochord: segmentation, =II=, 125, 218–22;
-      formation, =II=, 217–8, 600;
-      in Permian vertebrates, =II=, 225.
-
-    Noumenon, life not manifested as, =I=, 580.
-
-    Nuclein, =II=, 21.
-
-    Nucleus: central development, =I=, 163;
-      in simple organisms, =I=, 183;
-      phenomena exhibited by, =I=, 255–8;
-      current hypotheses of function, =I=, 258–9;
-      properties and function of chromatin, =I=, 259–65;
-      fusion in fertilization, =I=, 283–4;
-      function in unicellular reproduction, =I=, 595–6;
-      absence of, =II=, 20–1;
-      diffused form, =II=, 85;
-      macro- and micronucleus in conjugation, =II=, 452.
-
-    Nutrition: organic molecular re-arrangement, =I=, 36;
-      nitrogenous and non-nitrogenous, =I=, 47–8, 68, 71–4, 77; =II=,
-          362;
-      food assimilation and reasoning, =I=, 81;
-      needful for vital change, =I=, 94;
-      relation to growth, =I=, 140, 143, 144, 147–9, 150, 157, 161;
-      expenditure of energy, =I=, 157, 391;
-      fluid, =I=, 208;
-      vegetal fructification, =I=, 267; =II=, 266;
-      vegetal growth and genesis, =I=, 293, 294–7, 336;
-      animal growth and genesis, =I=, 289–93, 295–7, 336;
-      conditions qualifying antagonism of growth and genesis, =I=, 299;
-      competition among parts of an organism for, =I=, 562, 566, 676;
-      sex differentiation, =I=, 594–5;
-      cell multiplication, =I=, 638;
-      differentiation of neuter insects, =I=, 655–60, 670, 674, 686–8;
-      monstrous ant forms, =I=, 683–4;
-      leaf development, =II=, 39, 42, 73–8;
-      vegetal development, =II=, 51–2, 178, 276;
-      axillary buds, =II=, 65–9, 73–4;
-      effect on animal aggregation, =II=, 93;
-      internodes and inflorescence, =II=, 178–80;
-      helical phænogamic growth, =II=, 181;
-      action of bile, =II=, 330;
-      osseous development, =II=, 349, 353;
-      genesis, =II=, 419, 422, 427, 435–7, 452;
-      parental loss in feeding young, =II=, 424, 429;
-      diverse sources, =II=, 433;
-      Carpenter on reproduction and, =II=, 460;
-      animal development and genesis, =II=, 465;
-      expenditure and genesis, =II=, 468;
-      variations of genesis, =II=, 475–80, 511;
-      obesity and genesis, =II=, 480–4, 511;
-      general doctrine of genesis, =II=, 486;
-      genesis and vegetal parasitism, =II=, 486;
-      also animal, =II=, 487–90, 495;
-      insect genesis, =II=, 490–2;
-      genesis, _résumé_, =II=, 497–9;
-      and evolution, =II=, 501–4;
-      of blackbird and linnet, =II=, 503;
-      genesis in human race, =II=, 508–10, 514–6;
-      Doubleday on, =II=, 510–2;
-      future human evolution, =II=, 526, 531;
-      floral monstrosities, =II=, 542, 546, 547.
-
-
-    Obesity, nutrition and genesis, =II=, 480–4, 511.
-
-    Odoriferous glands, natural selection and, =I=, 534.
-
-    Odours: floral fertilization, =II=, 268–9;
-      animal protection, =II=, 434.
-
-    Offspring: parental loss entailed by nurture, =II=, 424, 429;
-      influence of age on, =II=, 507.
-
-    Oken, L., archetypal hypothesis, =II=, 122;
-      theory of supernumerary bones, =II=, 223;
-      on the skull, =II=, 561.
-
-    Oliver, F. W., classification of plants, =I=, 378–9.
-
-    _Ophryotrocha puerilis_, ciliation of segments, =II=, 109.
-
-    Orchids: pollen propulsion, =I=, 57;
-      leaf formation in _Dendrobium_, =II=, 60–1;
-      aërial roots and physiological differentiation, =II=, 255, 257;
-      foliar surface, =II=, 264.
-
-    Organic matter: properties of elements, =I=, 3–5, 22;
-      of compounds, =I=, 5–13, 25;
-      molar and molecular mobility, =I=, 12–14;
-      colloid and crystalloid form, =I=, 15–8, 25;
-      their diffusibility, =I=, 18–21, 26;
-      extreme complexity, =I=, 21;
-      laws of evolution and genesis of, =I=, 22–4;
-      modifiability, =I=, 27, 44;
-      capillarity and osmosis, =I=, 28;
-      effects of heat, =I=, 29;
-      of light, =I=, 30–4;
-      nitrogenous, =I=, 39–43;
-      oxidation and evolution of heat, =I=, 46, 60;
-      genesis of electricity, =I=, 50–2, 60;
-      sensible motions in, =I=, 59;
-      transformations and persistence of force, =I=, 61;
-      metabolism, =I=, 62–77;
-      artificial production of compounds, =I=, 64;
-      contrasted with inorganic matter, =I=, 107–8;
-      incomprehensibility of vital changes in, =I=, 122;
-      instability, =I=, 149, 508;
-      phosphorus in cell-organization, =I=, 260–1;
-      heterogeneity, =I=, 350–5;
-      “spontaneous generation” and evolution of, =I=, 696–701;
-      cell-doctrine and evolution of, =II=, 17–21.
-
-    Organization (_see_ Structure).
-
-    Ormerod, Dr., on sex and nutrition in wasps, =I=, 656.
-
-    Orthogenesis, Eimer’s theory of, =I=, 563–4.
-
-    Osmosis: organic effects, =I=, 28, 29;
-      in animals, =I=, 58;
-      in vascular system, =II=, 339;
-      in vegetal tissue, =II=, 568, 575, 577, 585, 592–6.
-
-    Osteology (_see_ Bone).
-
-    Ovum (_see_ Egg _and_ Fertilization).
-
-    Owen, Sir R.: metagenesis and parthenogenesis, =I=, 273–4;
-      fossil mammals, =I=, 410;
-      human parasites, =I=, 427;
-      continuous operation of creative power, =I=, 492;
-      fission in _Infusoria_, =I=, 584, 585, 595–6;
-      parthenogenesis, =I=, 592;
-      theory of vertebrate skeleton, =II=, 123, 548–66;
-      theory of supernumerary bones, =II=, 223;
-      Eschricht on _Ascaris_, =II=, 488.
-
-    _Oxalis_: radial symmetry, =II=, 152;
-      foliar surface, =II=, 264.
-
-    Oxen: comparison with sheep, =I=, 158, 160;
-      cerebro-spinal system, =I=, 508.
-
-    Oxidation (_see_ Oxygen).
-
-    Oxygen: properties, =I=, 3–5, 20, 22;
-      compounds, =I=, 6–7, 10–13, 22, 24–5;
-      a crystalloid, =I=, 21;
-      combining power and atomic weight, =I=, 33;
-      organic change from, =I=, 37;
-      heat generation, =I=, 46–9;
-      phosphorescence, =I=, 49;
-      nerve force dependent on, =I=, 53;
-      animal metabolism, =I=, 72, 73;
-      necessary to animal life, =I=, 94–5, 577;
-      activity and amount inhaled, =I=, 214.
-
-
-    Packard, A. S., on eyes of cave-animals, =I=, 648–9, 693.
-
-    Paget, Sir J., blood changes in small-pox and scarlatina, =I=, 221,
-        701.
-
-    Palæontology: distribution in time, =I=, 404–11, 412;
-      special creation, =I=, 425;
-      congruity with evolution hypothesis, =I=, 485–9, 556;
-      relations of present to extinct species, =II=, 10–11;
-      scarcity of remains, =II=, 34–5;
-      secondary thickening in plants, =II=, 56;
-      Cope on osteology of Permian Vertebrates, =II=, 225–6.
-
-    Pangenesis, Darwin’s theory of, =I=, 356, 357, 359, 360, 362, 372.
-
-    Panmixia, Weismann’s hypothesis of: Its relation to Romanes’
-        “cessation of selection,” =I=, 560;
-      alleged selective process denied, =I=, 561–3, 667, 685;
-      distribution of tactual perceptiveness, =I=, 608;
-      rudimentary eyes of cave fauna, =I=, 612–3, 647;
-      Romanes on process, =I=, 649, 667;
-      degeneration of self-feeding instinct In Amazon ants, =I=, 660–2,
-          670;
-      rudimentary limbs of whale, =I=, 668–9, 685;
-      a pure speculation, =I=, 671;
-      markings on leg-bones of Punjabis, =I=, 689.
-
-    _Paramœcium_: parasite infesting, =I=, 427;
-      reproduction, =II=, 443, 452.
-
-    Parasites: sexual dimorphism, =I=, 315;
-      limits to distribution, =I=, 397;
-      special creation and, =I=, 427–9, 438;
-      retrograde development, =I=, 457; =II=, 12;
-      aphis and ant, =I=, 660–1; =II=, 403, 405;
-      as an integrating agency, =II=, 402–4;
-      its comparative recency, =II=, 404;
-      nutrition and genesis in vegetal, =II=, 486;
-      in animal, =II=, 487–90, 493;
-      “castration parasitaire” in crustaceans, =II=, 493–6.
-
-    Parasol Ants, origin of classes, =I=, 687–8.
-
-    Parthenogenesis: occurrence, =I=, 274–5;
-      alternating with gamogenesis, =I=, 289–91;
-      Owen on, =I=, 592;
-      laws of multiplication, =II=, 415;
-      in articulate animals, =II=, 445.
-
-    Pasteur, L., silkworm diseases, =I=, 622–3.
-
-    Peacock: theories of heredity and structure of tail feather, =I=,
-        372–3, 695; =II=, 618–9.
-
-    Pear, foreright shoots, =I=, 287.
-
-    Peloria: in _gloxinia_, =II=, 166;
-      phænogams, =II=, 180.
-
-    Penguin, dermal structure, =II=, 314.
-
-    Pepsin, =I=, 69.
-
-    Pericyclic fibres of monocotyledons, =II=, 278.
-
-    _Peripatus capensis_, protoplasmic continuity, =I=, 629.
-
-    Peri-visceral sac, function and differentiation, =I=, 391.
-
-    Perkin, W. H., =I=, vi.
-
-    Persistence of force, corollaries from: properties of compounds,
-        =I=, 3;
-      organic transformation, =I=, 60;
-      growth, =I=, 150;
-      organic energy, =I=, 220;
-      variation, =I=, 335;
-      genesis, heredity, and variation, =I=, 354–5;
-      morphological summary, =II=, 235;
-      vegetal tissue differentiation, =II=, 245;
-      physiological development, =II=, 394.
-
-    Petals: foliar homology, =II=, 43–6;
-      “adnate,” =II=, 58.
-
-    Petrels, Darwin on, =I=, 455.
-
-    Phænogams: production of spermatozoids, =I=, 186;
-      morphological composition, =II=, 37–79;
-      leaf transitions, =II=, 37–42;
-      foliar homologies, =II=, 42–9;
-      origin of type, =II=, 49–84;
-      vertical growth, =II=, 56–64;
-      axillary buds, =II=, 66;
-      cotyledonous germination and endogenous growth, =II=, 69–72;
-      axial homologies, =II=, 73–5;
-      irregular development, =II=, 75–8;
-      degree of composition, =II=, 78;
-      reproductive homology, =II=, 80–4;
-      uni- and multiaxial symmetry, =II=, 141–3;
-      unit of composition, =II=, 151;
-      helical growth, =II=, 181;
-      secondary thickening, =II=, 247;
-      tissue and leaf differentiation, =II=, 247–9, 387;
-      also bark and cambium, =II=, 249–50, 386;
-      also outer tissue, =II=, 252, 256–9, 270, 386–7;
-      wax deposit on leaves, =II=, 260–2;
-      differentiation of inner tissues, =II=, 273–5, 388;
-      vascular system development, =II=, 280–4, 388;
-      integration, =II=, 293–5, 296, 390;
-      insect fertilization, =II=, 407;
-      multiplication, =II=, 441, 442;
-      genesis and growth, =II=, 451, 457;
-      and development, =II=, 464;
-      and nutrition, =II=, 476, 477, 511;
-      substitution of axial for foliar organs, =II=, 541–7.
-
-    Phenomenon, life manifested as, =I=, 580.
-
-    Philology (_see_ Language).
-
-    _Phoronis_, individuality, =II=, 444.
-
-    Phosphorescence, organic, =I=, 49.
-
-    Phosphorus: allotropic, =I=, 4;
-      in cell physiology, =I=, 259–62;
-      cerebral activity, =I=, 596–7;
-      organic evolution, =I=, 703.
-
-    Photogenes, visibility of, =I=, 218.
-
-    Phylogeny: as interpreting morphology, =II=, 10–12;
-      difficulties of affiliation, =II=, 34–5.
-      (_See_ Embryology _and_ Evolution.)
-
-    Physiological Units: definition, =I=, 226;
-      genesis, =I=, 280–1, 316;
-      heredity, =I=, 315–9;
-      variation, =I=, 330, 331–2, 333; =II=, 619;
-      stability, =I=, 340; =II=, 614;
-      self-fertilization, =I=, 342–4, 353;
-      interbreeding, =I=, 345, 353; =II=, 615;
-      recapitulation of hypothesis, =I=, 350–5; =II=, 612–7;
-      structural proclivities, =I=, 362, 364, 369–71; =II=, 613, 622;
-      sociological analogy, =I=, 364; =II=, 620;
-      complexity in organized types, =I=, 368–70;
-      re-named “constitutional units,” =I=, 369;
-      telegony, =I=, 650;
-      “mechanical theory,” =I=, 701–6;
-      morphological development, =II=, 7–9;
-      cell-doctrine, =II=, 17–21;
-      development, =II=, 76;
-      “false joints,” =II=, 371–2;
-      dissociation of ancestral traits in hybrids, =II=, 616–7;
-      inheritance of acquired characters, =II=, 618–23.
-
-    Physiological division of labour (_see_ Labour).
-
-    Physiological Selection, =I=, 569–71.
-
-    Physiology: and psychology, =I=, 127;
-      subdivisions, =I=, 128;
-      vicarious function, =I=, 208;
-      primitive interpretations, =I=, 417;
-      multiplication of effects exemplified, =I=, 512; =II=, 390;
-      relations to morphology, =II=, 3, 239–41;
-      evolutionary interpretation of phenomena, =II=, 241–5, 384–95;
-      ultimate inconceivability of processes, =II=, 372;
-      correlated integration and differentiation, =II=, 373.
-
-    Physiology, Animal: metabolism, =I=, 67–77;
-      vertebrate internal symmetry, =II=, 108;
-      tissue differentiation in _Protozoa_, =II=, 299, 385;
-      primary tissue differentiation, =II=, 300–2, 382, 389;
-      natural selection and tissue differentiation, =II=, 304–8;
-      outer tissue in _Cœlenterata_, =II=, 309–10;
-      respiratory organs, =II=, 310–1, 333–8;
-      differentiation of animal epidermic tissue, =II=, 312–4, 387;
-      development of tegumentary organs, =II=, 314–6;
-      of sensory, =II=, 317–20;
-      inner and outer tissue transition, =II=, 321–2, 389;
-      alimentary canal differentiation, =II=, 323–5;
-      gizzard development in birds, =II=, 325;
-      alimentary canal of ruminants, =II=, 327–9;
-      differentiation of liver, =II=, 329–33;
-      of animal vascular system, =II=, 339–44;
-      of osseous system, =II=, 344–55;
-      of nerve tissue, =II=, 355–61;
-      of muscle, =II=, 361–9;
-      differentiation and integration, =II=, 373–6;
-      in vascular system, =II=, 376–9, 383;
-      in nerves, =II=, 379–82;
-      origin of development, =II=, 384;
-      differentiation and instability of homogeneous, =II=, 384–9, 392;
-      summary of development, =II=, 384–94;
-      multiplication of effects, =II=, 390–1, 392;
-      equilibration, =II=, 391–4.
-      (_See also_ Function.)
-
-    Physiology, Plant: metabolism, =I=, 62–7;
-      tissue differentiation in secondary aggregates, =II=, 246, 385;
-      in phænogams, =II=, 247–9, 386;
-      in bark and cambium, =II=, 249–50, 386;
-      in free and fixed surfaces, =II=, 251–6, 270, 386;
-      outer stem and leaf tissue, =II=, 256–9, 270, 386;
-      superficial differentiation in leaves, =II=, 260–4, 270, 387;
-      floral tissue differentiation, =II=, 265–9;
-      outer tissue, _résumé_, =II=, 270;
-      inner tissue differentiation, =II=, 273–5, 388;
-      supporting tissue, =II=, 275–9, 285–8, 388;
-      vascular system development, =II=, 273–5, 279–84, 285–8, 388;
-      inner tissue, summary, =II=, 288–91, 388;
-      integration, =II=, 292–8;
-      differentiation and instability of homogeneous, =II=, 384–9, 392;
-      multiplication of effects, =II=, 390–1, 392;
-      equilibration, =II=, 391–4;
-      circulation and wood formation, =II=, 564–97;
-      dye permeability, =II=, 569–74, 577–81, 584, 586.
-      (_See also_ Function.)
-
-    Pickering, J. W., on artificial proteids, =I=, 39.
-
-    Pig: colour of muscles, =I=, 365–6;
-      telegony, =I=, 627;
-      fertility of domestic and wild sow, =II=, 479–80.
-
-    Pigeons: food of starving, =I=, 215;
-      heredity and variation, =I=, 305, 321, 615;
-      atavism, =I=, 314;
-      fertility, =II=, 471–2, 478.
-
-    Pike, unceasing growth, =I=, 154, 292.
-
-    Pique-gouffe, commensal relations with buffalo, =II=, 403.
-
-    _Plagiochila_, evolution of stem, =II=, 62.
-
-    _Planaria_: integration, =II=, 101–2;
-      Morgan on regeneration, =II=, 102, 611;
-      segmentation, =II=, 107;
-      symmetry, =II=, 195;
-      unintegrated function, =II=, 373.
-
-    Plants: influence of heat, =I=, 29;
-      effect of solar rays, =I=, 31–6, 500, 557;
-      chemical composition, =I=, 40–1;
-      heat generation, =I=, 47;
-      phosphorescence, =I=, 49;
-      electricity, =I=, 51;
-      sensible motion, =I=, 56–7, 58;
-      metabolism, =I=, 62–7, 70;
-      vital changes, =I=, 86, 87, 91, 94;
-      simulation by crystals, =I=, 96;
-      vital adjustments, =I=, 102;
-      length and complexity of life, =I=, 103–4;
-      biological classification, =I=, 125;
-      growth, =I=, 136, 138, 140, 143, 145–9, 153, 160–1; =II=, 401–2;
-      development, =I=, 163–5, 167–70, 272;
-      weight, temperature, and self-mobility, =I=, 174;
-      function, =I=, 174–8;
-      structure, =I=, 194–6; =II=, 21;
-      animal structure contrasted, =I=, 196;
-      function and structure, =I=, 200;
-      vicarious function, =I=, 208–9;
-      waste and repair, =I=, 213, 220;
-      physiological units, =I=, 225–6, 317, 360;
-      adaptation, =I=, 227;
-      what is an individual? =I=, 244–6, 250–1;
-      genesis, =I=, 270, 271, 272–3, 274, 276–8, 279–85;
-      relation of nutrition to growth and genesis, =I=, 284–9, 294,
-          295–300, 642; =II=, 39;
-      ovule homologues, =I=, 288;
-      natural selection, =I=, 294–8, 532, 533; =II=, 51;
-      heredity, =I=, 301–4, 308, 358–60;
-      variation, =I=, 320, 323–4, 325–6;
-      fertilization, =I=, 340–5;
-      classification, =I=, 377–80, 389–90;
-      distribution, =I=, 396–400, 401–3, 404–12, 478–9, 556;
-      special creation and parasitism, =I=, 428;
-      evolution hypothesis, =I=, 434, 443, 449–50;
-      rudimentary organs, =I=, 474, 475, 556;
-      varied media, =I=, 484; =II=, 32;
-      alien and native species in New Zealand, =I=, 477;
-      E. Darwin and Lamarck on evolution of, =I=, 490–8;
-      geologic changes affecting, =I=, 501–3, 557;
-      interdependence of animals and, =I=, 504–6, 514; =II=, 398;
-      complexity of influences affecting, =I=, 506;
-      direct equilibration, =I=, 523–5;
-      indirect, =I=, 532, 533;
-      seed distribution, =I=, 546;
-      wood development, =II=, 285–7, 289, 567–97;
-      interdependence, =II=, 402–3, 404;
-      insect relations, =II=, 406–7;
-      adaptation and multiplication, =II=, 411–6;
-      rhythm in numbers, =II=, 419;
-      growth and asexual genesis, =II=, 439–42;
-      growth and sexual genesis, =II=, 448–51;
-      expenditure, =II=, 467;
-      horticulture, nutrition, and genesis, =II=, 477;
-      tree development, =II=, 553;
-      circulation and wood formation, =II=, 567–92;
-      dye permeability and circulation, =II=, 569–74, 577–81, 584, 586;
-      _résumé_ on circulation and wood formation, =II=, 592–7.
-      (_See also_ Multiplication, Morphology, _and_ Physiology.)
-
-    Plasmodium, dissolution of, =I=, 185.
-
-    Plato, ἰδέα of, =II=, 550.
-
-    _Platyhelminthes_: transverse fission, =II=, 101;
-      segmented and non-segmented types, =II=, 102, 107;
-      symmetry, =II=, 195, 197;
-      multiplication and growth, =II=, 488–9.
-
-    Plethora, fertility and, =II=, 480–4, 511.
-
-    _Pleurococcaceæ_, unicellular form, =II=, 21, 134.
-
-    _Pleuronectidæ_: symmetry and location of eyes, =II=, 205;
-      outer tissue, =II=, 387.
-
-    _Plumatella_: metagenesis, =I=, 277;
-      symmetry, =II=, 195.
-
-    _Podostemaceæ_, undeveloped circulatory system, =II=, 274.
-
-    Polar bodies, hypothesis concerning extrusion of, =I=, 266–8.
-
-    Polarity, organic, of physiological units, =I=, 226, 315, 317, 332,
-        350–1, 701–6.
-
-    Polyatomic compounds (_see_ Chemistry).
-
-    _Polychætæ_, anomalous development in _Myrianida_, =I=, 361.
-
-    _Polycytharia_, integration, =II=, 90, 124.
-
-    _Polygastrica_, aggregation, =I=, 586.
-
-    Polymerism: of compounds, =I=, 9, 11, 25;
-      nerve tissue, =II=, 356.
-
-    _Polypori_, symmetry and environment, =II=, 139.
-
-    Polyps (see _Cœlenterata_).
-
-    _Polyzoa_: size, =I=, 140;
-      multiaxial development, =I=, 165;
-      structural indefiniteness, =I=, 173;
-      functional differentiation, =I=, 202;
-      trochophoral kinship, =I=, 447;
-      integration, =II=, 93–4, 96, 124;
-      symmetry, =II=, 194, 207;
-      vascular system, =II=, 340;
-      gemmation, =II=, 444.
-
-    Poor Laws, and natural selection, =II=, 532.
-
-    _Population, A Theory of_, =I=, 265, 577–601; =II=, 411.
-
-    Potato: simulated growth, =I=, 136;
-      vicarious function of tuber, =I=, 209; =II=, 255;
-      sub-species, =I=, 302;
-      dye absorption, =II=, 279.
-
-    Preservation: fertility and self-, =I=, 581; =II=, 423, 430;
-      nutrition, =II=, 493.
-
-    “Progress; its Law and Cause,” theory of species differentiation,
-        =I=, 568.
-
-    Projectiles, factors in flight of, =I=, 450–1.
-
-    Proteids: metabolic function, =I=, 67, 68, 69, 72, 76;
-      complexity of molecule, =I=, 122.
-
-    Protein: evolution, =I=, 23, 24;
-      isomerism, =I=, 700, 703, 704.
-
-    _Proteus_, degeneration of eye, =I=, 613.
-
-    _Protodrilus_, intestine segmentation, =II=, 125.
-
-    _Protophyta_: internal movements, =I=, 56;
-      limit of growth, =I=, 138;
-      development, =I=, 164;
-      structure, =I=, 173, 181–3;
-      self-mobility, =I=, 175;
-      individuality, =I=, 245;
-      multiplication, =I=, 270, 276, 279, 581, 584–5; =II=, 439, 462;
-      genesis and nutrition, =I=, 295;
-      unicellular, =II=, 21;
-      central aggregation, =II=, 24;
-      symmetry, =II=, 134;
-      tissues, =II=, 244, 249;
-      primary differentiation, =II=, 385;
-      primordial type, =II=, 398;
-      symbiosis, =II=, 400.
-
-    Protoplasm: self-increasing function of primordial, =I=, 63–4;
-      plant metabolism, =I=, 65–7;
-      complexity, =I=, 122, 253–5;
-      differentiation in simple organisms, =I=, 182–3;
-      continuity and inter-circulation, =I=, 190–2, 371, 629; =II=, 21,
-          620;
-      “streaming,” =I=, 253;
-      structure, =I=, 253–5.
-      (_See also_ Cell.)
-
-    _Protozoa_: inorganic components, =I=, 17;
-      locomotion, =I=, 58, 175; =II=, 14;
-      vital changes shown by, =I=, 94;
-      limitation of growth, =I=, 138;
-      development, =I=, 164;
-      structure, =I=, 173, 181–3;
-      incipient differentiation, =I=, 198, 391; =II=, 299, 309;
-      multiplication, =I=, 270, 276, 279, 280, 582, 584; =II=, 442,
-          451–2;
-      genesis and nutrition, =I=, 295;
-      distribution, =I=, 396;
-      parasites infesting, =I=, 427;
-      Weismann’s hypothesis of immortality, =I=, 637;
-      “spontaneous generation,” =I=, 697–701;
-      non-nucleated, =II=, 20;
-      primary aggregate, =II=, 86–7, 124;
-      progressing integration, =II=, 89–91, 124;
-      symmetry, =II=, 186;
-      primordial plant-animal type, =II=, 397–8;
-      symbiosis, =II=, 400.
-
-    Protyle, hypothetical chemical unit, =I=, 22, 23.
-
-    Pseud-axial development, vegetal, =II=, 28–9, 30.
-
-    Pseudo-foliar development, vegetal, =II=, 26–8, 30.
-
-    _Psychidæ_: parthenogenesis, =I=, 275;
-      sexual dimorphism, =I=, 683.
-
-    Psychology: reasoning and definition of life, =I=, 81–8;
-      correspondence shown by _recognition_, =I=, 95;
-      contrasted with physiology, =I=, 127;
-      departments of, =I=, 127–8;
-      vicarious function, =I=, 209;
-      waste and repair in sensory organs, =I=, 217;
-      sensory adaptability, =I=, 229, 231, 232;
-      inheritance of sensory defects, =I=, 306;
-      musical talent, =I=, 311–2;
-      intellectual progress and special creation hypothesis, =I=, 417;
-      special creation a pseud-idea, =I=, 420, 429, 433, 554;
-      legitimacy of evolution hypothesis, =I=, 433–5, 439, 554;
-      embryology of ideas, =I=, 450, 457;
-      persistent formative power unrepresentable, =I=, 492;
-      E. Darwin’s and Lamarck’s theory of desires, =I=, 494;
-      natural selection and brain evolution, =I=, 553;
-      genesis and cerebral activity, =I=, 594; =II=, 512–4, 516–9, 530;
-      heredity and distribution of tactual perceptiveness, =I=, 602–8,
-          646, 665–6, 672, 692;
-      inconceivability of the negation, =I=, 675;
-      vitiation of evidence, =II=, 88;
-      repetition and perception, =II=, 143;
-      differentiation of sensory organs, =II=, 317–20;
-      differentiation of nerve tissue, =II=, 355–61;
-      functional integration, =II=, 376;
-      also integration, =II=, 380–2;
-      equilibration of nerve discharge, =II=, 393;
-      human fertility and nerve development, =II=, 466, 532;
-      future human evolution, =II=, 523–5, 527;
-      human evolution and genesis, =II=, 529–31;
-      future mental development, =II=, 535;
-      origin of vertebrate type, =II=, 598–600.
-
-    _Pteridophyta_: size attained by, =I=, 138, 139;
-      homologies, =II=, 80–1, 82;
-      frond surface differentiation, =II=, 260.
-
-    _Pteropoda_: bilateral symmetry, =II=, 201;
-      dermal respiration, =II=, 310.
-
-    Ptyaline, metabolic function, =I=, 69.
-
-    Punjabis, inheritance of acquired osteological peculiarities, =I=,
-        689.
-
-    _Pyrosomidæ_: phosphorescence, =I=, 47;
-      integration, =I=, 588; =II=, 97.
-
-
-    Quagga, telegonic transmission of markings to offspring of mare,
-        =I=, 624, 627, 646.
-
-    Quills, development, =II=, 314–6.
-
-
-    Rabbit: activity and muscle colour, =II=, 365;
-      over-running checked by weasels, =II=, 405;
-      expenditure and genesis, =II=, 472.
-
-    Radial, definition, =II=, 148.
-
-    _Radiolaria_: unicentral development, =I=, 163;
-      secondary aggregation, =II=, 88;
-      symmetry, =II=, 187.
-
-    _Radula_, development of roots from leaflets, =II=, 34.
-
-    _Rafflesiaceæ_: homogenesis, =I=, 272;
-      tissue differentiation, =II=, 274;
-      nutrition and genesis, =II=, 486.
-
-    Rat (see _Rodentia_).
-
-    Rathke, H., on vertebrate embryo, =II=, 119.
-
-    Ray, J., plant classification, =I=, 378.
-
-    Reasoning, compared with assimilation, =I=, 81–7.
-
-    Recapitulation, embryological, =I=, 453.
-
-    Regeneration (_see_ Repair).
-
-    Rejuvenescence, and sexual fertilization, =I=, 637; =II=, 613.
-
-    Remak, R., vertebrate embryo, =II=, 120.
-
-    Repair: continuity of, =I=, 216–9;
-      animal injuries, =I=, 219, 222–4; =II=, 102, 611;
-      deductive interpretation, =I=, 221–2;
-      theories of heredity and regenerative phenomena, =I=, 360–1.
-
-    Repetition of like parts, =II=, 126.
-
-    Reproduction (_see_ Multiplication).
-
-    _Reptilia_: growth and expenditure of force, =I=, 142;
-      sizes of ova and adult, =I=, 144;
-      longevity of crocodile, =I=, 154;
-      temperature, =I=, 174;
-      waste, =I=, 214;
-      distinctive characters, =I=, 392;
-      distribution in time, =I=, 409, 412;
-      vertebral segmentation, =I=, 470;
-      rudimentary limbs of snakes, =I=, 473;
-      fertility and development, =I=, 583, 598, 599;
-      regeneration, =I=, 589;
-      elongated form, =II=, 15;
-      supernumerary vertebræ, =II=, 123, 564;
-      bilateral symmetry, =II=, 203, 204;
-      Cope on segmentation in extinct, =II=, 225, 226;
-      activity and muscular colour, =II=, 365;
-      functional integration, =II=, 375;
-      outer tissue differentiation, =II=, 387;
-      Owen on skeleton, =II=, 560.
-
-    Resistance of media to locomotion, =II=, 15.
-
-    Respiratory System: effect of light, =I=, 31;
-      organic re-arrangement, =I=, 37;
-      cutaneous, =I=, 209;
-      air-cells of lungs, =I=, 254;
-      embryonic branchiæ of salamander, =I=, 457;
-      differentiation, =II=, 310–1, 333–8;
-      physiological integration, =II=, 374–5, 382;
-      vascular differentiation and integration, =II=, 377.
-
-    Retrograde metamorphoses, in animals, =II=, 12.
-
-    Retzius, G., superficial nerve-endings, =I=, 666.
-
-    Reversed Selection, =I=, 611, 612.
-
-    Rhabdospheres, calcareous armour and dynamic element in life, =I=,
-        119.
-
-    Rhizoids, foliar expansions, =II=, 50.
-
-    _Rhizopoda_: structure, =I=, 173;
-      undifferentiated function, =I=, 200;
-      a primary aggregate, =II=, 86;
-      symmetry, =II=, 186;
-      tissue differentiated, =II=, 299, 385;
-      motion of sarcode, =II=, 356;
-      symbiosis, =II=, 400.
-
-    Rhythm: astronomic and organic, =I=, 499, 557;
-      law of equilibration, =I=, 520–1;
-      in multiplication, =II=, 419.
-
-    Richeraud, Baron A., definition of life, =I=, 79.
-
-    Riley, C. V., on telegony, =I=, 645;
-      _Termites_, =I=, 680, 681;
-      pouch of Honey-ants, =I=, 684.
-
-    _Rodentia_: incursions, =I=, 399;
-      American types, =I=, 403;
-      fertility and development, =I=, 583, 599.
-
-    Rivinus, plant classification, =I=, 377.
-
-    Rokitansky, on false joints, =I=, 230.
-
-    Romanes, G. J.: on “cessation of selection,” =I=, 560–2;
-      isolation and species differentiation, =I=, 569;
-      “physiological selection,” =I=, 569–71;
-      panmixia, =I=, 649, 667;
-      influence of a previous sire on progeny, =I=, 649.
-
-    Röntgen rays, =I=, 121; =II=, 621.
-
-    Roots: developed from leaflets, =II=, 34;
-      physiological differentiation, =II=, 253–5, 270;
-      nutrition from leaves, =II=, 274;
-      size and function, =II=, 276.
-
-    _Rotiferæ_: latent vitality of desiccated, =I=, 117;
-      trochopore, =II=, 108, 109;
-      molluscan relationship, =II=, 115;
-      fertility and size, =II=, 453, 459.
-
-    Roux, W.: “intra-selection,” =I=, 676;
-      functional adaptation, =II=, 354.
-
-    Rudimentary organs: the definition of life and, =I=, 112;
-      natural selection and eyes of cave fauna, =I=, 309, 612–4, 647–9,
-          693;
-      evolution hypothesis, =I=, 472–5, 556;
-      limbs of whale, =I=, 668–9, 685, 693.
-
-    Ruminants, alimentary canal development, =II=, 327–9.
-
-
-    Salamander, embryonic branchiæ, =I=, 457.
-
-    _Salmonidæ_, reproduction and growth, =I=, 291–3; =II=, 454.
-
-    _Salpidæ_: heterogenesis, =I=, 272, 277;
-      integration, =I=, 588; =II=, 97.
-
-    Sap (_see_ Vascular system).
-
-    _Sarcina_: central aggregation, =II=, 24;
-      fertility, =II=, 440.
-
-    Savage, Dr., on “Heredity and Neurosis,” =I=, 313.
-
-    _Scenedesmus_, individuation, =II=, 24.
-
-    Scent: natural selection and keenness of, =I=, 610;
-      floral fertilization, =II=, 268–9;
-      animal protection, =II=, 434.
-
-    Schelling, E. W. J. von, definition of life, =I=, 78, 178.
-
-    Schleiden, J. M., on individuality, =I=, 245;
-      on liverworts, =II=, 50, 52;
-      algal indefiniteness, =II=, 296.
-
-    Science, complex revelations of, =I=, 252, 369, 450.
-
-    _Scyphomedusæ_, strobilization, =II=, 108.
-
-    Sea: changes and movements in, =I=, 83;
-      life in, lower than terrestrial, =I=, 104;
-      distribution, =I=, 396, 517;
-      change of media caused by, =I=, 481;
-      geologic influence, =I=, 502.
-
-    Seals: nail-bearing toes, =I=, 473;
-      _vibrissæ_, =II=, 317.
-
-    Seasons: reproductive periodicity, =I=, 299;
-      variations of genesis with, =II=, 484–5.
-
-    Sedgwick, Adam: on continuity of protoplasm in animals, =I=, 190,
-        629; =II=, 21;
-      zoological classification, =I=, 387;
-      discrimination of species in embryonic stages, =I=, 461;
-      persistence of ancestral traits, =I=, 463–4;
-      Archiannelidan segmentation, =II=, 109.
-
-    Sedgwick, Wm.: heredity and sex, =I=, 305, 314;
-      telegonic transmission of hypospadias, =I=, 646.
-
-    Seeds: nitrogenous, =I=, 40;
-      temperature of germinating, =I=, 47; =II=, 615;
-      vitalism and latent vitality of, =I=, 116–7;
-      variation in environment, =I=, 327;
-      natural selection among, =I=, 532.
-
-    Segmentation (metameric): special creation hypothesis, =I=, 468–9;
-      Huxley on number of somites in higher articulates, _ib._;
-      in annulose animals, =II=, 98–110, 111–5, 601–5;
-      simulated molluscan, =II=, 116;
-      in vertebrates, =II=, 125–7, 225–7, 606–7;
-      in elasmobranchs, =II=, 126.
-
-    Segregation: of growth, =I=, 136;
-      of like units, =I=, 179;
-      organic repair, =I=, 221;
-      variation, =I=, 331, 334;
-      heterogeneity, and definiteness of evolution, =I=, 514–6, 517–8;
-      morphological development, =II=, 7–9;
-      physiological units, =II=, 616.
-
-    Self-fertilization, animal and vegetal, =I=, 341–4, 353.
-
-    Senses, the (_see_ Psychology).
-
-    Sex: in Ascidian colonies, =I=, 247;
-      limitation of heredity by, =I=, 314–6;
-      correlated traits, =I=, 371–2, 513;
-      nutrition and determination of, in social insects, =I=, 655–60,
-          678–84, 686–9;
-      neural and hæmal traits, =I=, 683;
-      differentiation of organs, =II=, 303;
-      castration and growth, =II=, 459;
-      Julin on “castration parasitaire” in crustaceans, =II=, 493–6;
-      the object of fertilization, =II=, 613.
-      (_See also_ Fertilization.)
-
-    Sexual Selection (_see_ Natural Selection).
-
-    Sharp, D.: on insect somites, =I=, 469;
-      food habits of _Termites_, =I=, 686–7.
-
-    Sheep: contrasted with oxen, =I=, 158, 160;
-      crossing of English and French breeds, =I=, 625;
-      nutrition and genesis, =II=, 480.
-
-    Sherrington, Prof., on effects of nerve severance, =I=, 349.
-
-    Ship-building, interdependence of social functions, =I=, 237–9,
-        241.
-
-    Shipley, A. E.: segmentation of _Microstomida_, =II=, 102;
-      _Protodrilus_, =II=, 125.
-
-    Silica, colloid and crystalloid, =I=, 16.
-
-    Silicic acid: properties, =I=, 16;
-      isomerism, =I=, 59.
-
-    Silicon, allotropic, =I=, 4.
-
-    Silkworm disease, =I=, 622–3.
-
-    Simulation: of homology by analogy, =II=, 14, 485;
-      of segmented structure by molluscs, =II=, 116.
-
-    _Siphonophora_, specialization of component polyps, =II=, 95.
-
-    _Sirenia_, simulated fish form, =I=, 485.
-
-    Size (_see_ Growth).
-
-    Skeleton, vertebrate (see _Vertebrata_).
-
-    Skin: respiratory function, =I=, 209;
-      adaptability, =I=, 228; =II=, 312–4, 387;
-      transmitted peculiarities, =I=, 306;
-      Wallace on distribution of sensitiveness, =I=, 646–7;
-      differentiation, =II=, 215, 217, 304–7;
-      tegumentary development, =II=, 314–6, 387;
-      differentiation of sensory organs, =II=, 317–20;
-      and mucous membrane, =II=, 303–4, 321–2, 389.
-
-    “Skin friction,” and locomotion of aquatic animals, =I=, 156.
-
-    Skull (see _Vertebrata_).
-
-    Sleep, repair favoured by, =I=, 216.
-
-    Small-pox, blood changes from, =I=, 221.
-
-    Smith, Prof. W., on fertility of _diatomaceæ_, =II=, 440.
-
-    Smith, W. P., on telegony in calves and foals, =I=, 645.
-
-    Smith, W. W., on habits of Tetramorium, =I=, 660.
-
-    Snakes (see _Reptilia_).
-
-    “Social organism,” author’s essay on, =I=, 363, 676.
-
-    Sociology: environment and degree of life, =I=, 105–6;
-      functional differentiation, =I=, 204;
-      division of labour, =I=, 207, 363–4, 367;
-      functional interdependence, =I=, 237–9, 240–2;
-      autogenous development of units in colonies, =I=, 364, 367–8;
-          =II=, 620;
-      belief in social evolution, =I=, 432;
-      natural selection, =I=, 553; =II=, 532;
-      integration and differentiation, =II=, 378–9;
-      effects of population, =II=, 535–6;
-      equilibration, =II=, 537.
-
-    Soil, dependence of plant evolution on, =II=, 402.
-
-    _Solanum jasminoides_, organs of attachment, =II=, 276.
-
-    Solar system, autogenous development illustrated by distribution
-        of forces in, =I=, 366.
-
-    Sole, symmetry and location of eyes, =II=, 205.
-
-    Soma-plasm, Weismann’s theory of differentiation from germ-plasm,
-        =I=, 357, 622, 628–30, 633–44.
-
-    Somites (_see_ Segmentation).
-
-    Special creation: and evolution, =I=, 412, 415, 431;
-      improbabilities, =I=, 418–9, 430, 439, 554;
-      inconceivability, =I=, 420, 429, 431, 554;
-      of individuals and species, =I=, 421–4;
-      the implication of beneficence, =I=, 425–9;
-      summary, =I=, 429, 554;
-      Von Baer’s formula, =I=, 451–6;
-      vertebrate skeleton, =II=, 551, 556, 565.
-
-    Species: adaptation and stability, =I=, 242;
-      hereditary transmission, =I=, 301–4;
-      variation in wild and cultivated, =I=, 323–5, 326, 693;
-      gamogenesis and life of, =I=, 347–9;
-      physiological units, =I=, 362, 364, 369–71, 458; =II=, 613;
-      indefiniteness, =I=, 389, 445, 572;
-      special creation, =I=, 422–4;
-      instability of homogeneous, and differentiation of, =I=, 509–11,
-          515, 517–8, 550, 557;
-      persistence of, =I=, 516, 518; =II=, 10–11;
-      natural selection and equilibration, =I=, 543–8, 553, 557;
-      non-adaptive characters, =I=, 565;
-      morbid products as marks of, =I=, 567;
-      migration and isolation as causes of differentiation, =I=, 568–9;
-      increasing multiformity of aggregate, =II=, 396.
-
-    Specific gravity, of organisms and environment, =I=, 174, 177.
-
-    Spermatozoa}
-    Sperm-cell } (_see_ Fertilization).
-
-    Sphere: tendency of units to form, =I=, 15;
-      the embryonic form, =I=, 177;
-      symmetry, =II=, 131.
-
-    Spheroid, symmetry, =II=, 132.
-
-    Spiders (see _Arachnida_).
-
-    Spine (see _Vertebrata_).
-
-    Sponge: structure and dynamic element in life, =I=, 119;
-      multicentral development, =I=, 164;
-      units and aggregate, =I=, 185;
-      reproductive tissue, =I=, 283;
-      integration, =I=, 586; =II=, 90, 383;
-      physiological differentiation, =II=, 300, 386;
-      development and genesis, =II=, 463;
-      analogy from, =II=, 576.
-
-    Spontaneous generation: and heterogenesis, =I=, 270;
-      and evolution, =I=, 696–701, 703.
-
-    Stag, horns and correlated structures, =I=, 567, 670, 676–7, 692.
-
-    Stamens, and foliar homology, =II=, 44.
-
-    Starches: properties, =I=, 11;
-      transformations, =I=, 66, 68, 69, 70; =II=, 593.
-
-    Star-fishes (see _Asteroidea_).
-
-    Statoblasts, of _Plumatella_, =I=, 277.
-
-    Steenstrup, on “Alternate Generation,” =I=, 592.
-
-    Sterility (_see_ Multiplication).
-
-    Stickleback: ova, =II=, 454;
-      _bothriocephalus_ in, =II=, 490.
-
-    Stomach (_see_ Alimentary canal).
-
-    Stomata, distribution, =II=, 260–1.
-
-    Straight line, and evolution hypothesis, =I=, 433.
-
-    Strain: compression and tension of, =I=, 151; =II=, 209–12;
-      relation to mass, =I=, 155–7;
-      vegetal structure, =II=, 574–88, 592–6;
-      origin of vertebrate type, =II=, 600.
-
-    Strawberry: multiaxial development, =I=, 166;
-      multiplication, =II=, 441.
-
-    Strength, a vital attribute, =I=, 578.
-
-    Structure: appliances for generating motion, =I=, 75–7;
-      biological classification, =I=, 125–7, 129;
-      size and organic, =I=, 137;
-      growth and complexity, =I=, 138, 145, 161;
-      relation to environment, =I=, 172–8, 195–6;
-      of unicellular organisms, =I=, 181–3;
-      multicellular, =I=, 183–96;
-      Hertwig’s classification of tissues, =I=, 189;
-      continuity of units, =I=, 190–2;
-      systems of organs, =I=, 192;
-      division into universal and particular, =I=, 193–4;
-      general truths, =I=, 194–5;
-      plant and animal, contrasted, =I=, 195–6;
-      precedence of function or, =I=, 197, 211;
-      correlative complexity of function and, =I=, 200, 211;
-      progressive concomitant differentiation, =I=, 201–4;
-      physiological units, =I=, 225–6, 362, 364, 369–71; =II=, 613;
-      social and organic interdependence, =I=, 235–42;
-      varied by function, =I=, 334, 535; =II=, 217 (_see_ Acquired
-          Characters);
-      zoological classification, =I=, 390–2;
-      equilibration, =I=, 521, 557;
-      progress of, and genesis, =I=, 590–1; =II=, 462;
-      co-operation with function, =II=, 3;
-      evolution and increased, =II=, 4;
-      retrograde metamorphosis, =II=, 12;
-      simulated homologies, =II=, 13–14;
-      earliest organic forms, =II=, 19;
-      cylindrical vegetal, =II=, 57–62;
-      permanence and complexity, =II=, 295, 296;
-      function and epidermic, =II=, 312–4, 387;
-      and muscular, =II=, 369, 391;
-      adaptation and equilibration, =II=, 392;
-      persistence of force and physiological adaptation, =II=, 394;
-      evolution, =II=, 501–4.
-      (_See also_ Morphology.)
-
-    Struggle, for nutriment among components of an organism, =I=, 562,
-        676;
-      for existence (_see_ Natural Selection).
-
-    Struthers, Sir J.: on heredity, =I=, 305, 314;
-      digital variation, =I=, 321;
-      rudimentary limbs of whale, =I=, 668.
-
-    Strychnine, effects of, =I=, 54, 55.
-
-    Sturgeon, size of ova and adult, =I=, 144.
-
-    Sugars: properties, =I=, 10–11;
-      transformations, =I=, 38, 40, 66, 69, 70; =II=, 593.
-
-    Suicide, hereditary tendency to, =I=, 307.
-
-    Sulphur: allotropic, =I=, 4, 59;
-      organic evolution, =I=, 703.
-
-    Sun (_see_ Light).
-
-    Survival of the Fittest, the expression, =I=, 530, 610.
-      (_See_ Natural Selection.)
-
-    Swan, vertebræ of neck, =II=, 123.
-
-    Swiftness, a vital attribute, =I=, 578.
-
-    _Syllis ramosa_, lateral branching, =I=, 166, 361; =II=, 105, 108.
-
-    Symbiosis, =II=, 399, 400.
-
-    Symmetry (_see_ Morphology).
-
-    Syphilis, hereditary transmission, =I=, 623.
-
-
-    Tactual, Perceptiveness, heredity and the distribution of, =I=,
-        602–8, 633, 665, 666, 672, 692.
-
-    _Tænia_ (see _Entozoa_).
-
-    Tansley, A. G., =I=, vi; =II=, vi;
-      adaptation of reproductive activity to conditions in _Algæ_, =I=,
-          288–9;
-      shapes of _Caulerpa_, =II=, 22;
-      stem-thickening in extinct Thallophytes, =II=, 56;
-      natural selection and leaf-distribution, =II=, 179.
-
-    Tape-worm (see _Entozoa_).
-
-    Taste, dependent on chemical action, =I=, 54.
-
-    Teeth: hereditary transmission, =I=, 306;
-      suppression of mammalian, =I=, 457;
-      of uncivilized and civilized, =I=, 541, 693.
-
-    Tegumentary organs, origin of, =I=, 314–6.
-
-    Telegony, or the Influence of a previous sire on offspring, =I=,
-        624–7, 644–6, 649–50.
-
-    Temperature (_see_ Heat).
-
-    Tension (_see_ Strain).
-
-    _Termites_: fertility, =I=, 583; =II=, 493;
-      late development of sexual organs, =I=, 680;
-      nutrition and differentiation of forms, =I=, 681.
-
-    _Tetramorium_, utilization of aphides by, =I=, 660–1.
-
-    _Thallophyta_: size, =I=, 138, 139;
-      low co-ordination of parts, =I=, 164;
-      pseudo-foliar, =II=, 28;
-      “transition place,” =II=, 30;
-      simulation of higher types, =II=, 32;
-      secondary thickening in extinct species, =II=, 56;
-      sexual and asexual genesis, =II=, 84.
-      (See also _Algæ_.)
-
-    Tickling, physiology of, =I=, 76.
-
-    Tide (_see_ Sea).
-
-    Time, as a factor in growth, =II=, 77.
-
-    Tissue, Hertwig’s classification, =I=, 189.
-      (_See_ Physiology.)
-
-    Tongue, perceptiveness of tip, =I=, 606–8, 665, 672–3.
-
-    Tortoise: contrasted life of dog and, =I=, 103–4;
-      natural selection and carapace, =I=, 534.
-
-    “Transcendental Physiology,” =I=, 176.
-
-    Tree, as symbolizing phylogeny, =I=, 428, 452–3.
-      (_See_ Plants.)
-
-    _Trematoda_: agamogenesis, =I=, 277;
-      parasitism, =I=, 428;
-      alternate generation, =I=, 592.
-
-    Trembley, A., on the polyp, =I=, 223.
-
-    Trichinosis, in Germany, =I=, 428.
-
-    Trochophore, phyletic relationships shown by, =I=, 447; =II=,
-        108–9.
-
-    _Tubicolæ_: development, =II=, 100;
-      bilateral symmetry, =II=, 197.
-
-    _Tunicata_: gemmation, =I=, 588; =II=, 445;
-      alternate generation, =I=, 592;
-      integration, =II=, 93–4;
-      tertiary aggregation, =II=, 124;
-      symmetry, =II=, 194–5.
-
-    Tunny, size of ova and adult, =I=, 144.
-
-    _Turbellaria_: segmentation, =II=, 102;
-      symbiosis, =II=, 400.
-
-    Turnip: chlorophyll in roots, =I=, 209; =II=, 254;
-      vascular system, =II=, 281, 284, 578, 591, 596.
-
-    Twins: similarity of, =I=, 324;
-      traits of women bearing, =II=, 457.
-
-    “Types, persistent,” Huxley on, =I=, 408.
-
-
-    Ulcer, dermal structure, =II=, 306.
-
-    Ultimate Reality, incomprehensibility of, =I=, 120.
-
-    _Ulva_: cell multiplication, =II=, 26;
-      outer tissue, =II=, 256.
-
-    _Umbelliferæ_: floral symmetry, =II=, 171;
-      axial and foliar organs, =II=, 541–6.
-
-    United States: cases of telegony, =I=, 644–5;
-      birth-rate, =II=, 520.
-
-    Units: differentiation and dissimilarity, =I=, 20;
-      “protyle,” =I=, 22–3;
-      shapes in higher types, =I=, 164;
-      differential assimilation, =I=, 180;
-      primordial organic, =I=, 181;
-      morphological composition, =I=, 184–7, 194, 252; =II=, 5, 7–9,
-          21, 79, 85–6;
-      segregation and organic repair, =I=, 221–2, 222–6;
-      chemical, morphological, and physiological, =I=, 225–6; =II=,
-          612;
-      stability, =I=, 339;
-      instability and heterogeneity of organic, =I=, 350;
-      Darwin’s gemmules, =I=, 356–60, 362, 372;
-      Weissmann’s germ-plasm (_q. v._) _ib._;
-      sociological comparison, =I=, 363–8;
-      specific proclivities in embryogeny, =I=, 458;
-      phænogamic, =II=, 73, 151;
-      annulose, =II=, 105;
-      incident force and homologous, =II=, 159;
-      morphological summary, =II=, 233.
-      (_See also_ Physiological Units.)
-
-    “Universal Postulate,” =I=, 675.
-
-    Unsymmetrical, definition, =II=, 131.
-
-    Urea, muscular energy and excretion, =I=, 72.
-
-
-    Van Beneden, P. J., on _Tænia_, =II=, 103.
-
-    Variation: digital, =I=, 331;
-      effects of parental conditions, =I=, 324;
-      of altered function, =I=, 325, 334, 693;
-      dissimilarity of initial conditions, =I=, 327–32, 333;
-      “spontaneous,” =I=, 328, 513, 697; =II=, 529;
-      persistence of force, =I=, 335;
-      physiological units, =I=, 348–54, 360, 369, 371–3; =II=, 614–7,
-          622–3;
-      Weismann’s germ-plasm theory, =I=, 357–8, 372–3, 671, 677; =II=,
-          622;
-      equilibration and vegetal, =I=, 523–5;
-      Weismann’s panmixia theory, =I=, 561–3, 649, 667–9, 671, 685;
-      reproductive organs, =I=, 570;
-      natural selection and concomitant, =I=, 614–21, 653, 664, 674,
-          692;
-      and disused organs, =I=, 648, 668;
-      plus and minus. =I=, 667, 685;
-      Masters on correlated, in plants, =II=, 298, 621–2;
-      equilibration of favourable, =II=, 394.
-
-    Vascular System: effects of vegeto-alkalies, =I=, 55;
-      nutrition, =I=, 146, 148;
-      embryonic development, =I=, 169;
-      structural traits, =I=, 192, 193;
-      function, =I=, 199;
-      of Ascidians. =I=, 202;
-      functional differentiation and integration, =I=, 205–6;
-      organic repair, =I=, 217, 221–2;
-      effect of function, =I=, 229, 234–5, 236;
-      equilibration, =I=, 535;
-      community in compound organisms, =I=, 588;
-      development of vegetal, =II=, 273–5, 279–84, 285–8, 388;
-      differentiation of, summary, =II=, 288–90, 388;
-      differentiation of animal, =II=, 339–44;
-      osseous development, =II=, 347–51;
-      muscularity, =II=, 364;
-      muscular colour, =II=, 365–9;
-      heart-motor apparatus, =II=, 374;
-      differentiation and integration in animal, =II=, 376–9, 383;
-      wood formation, =II=, 567–92;
-      _résumé_ of wood formation, =II=, 592–7.
-
-    _Vaucheria_, reproduction, =I=, 279, 289.
-
-    Vegetative System, co-ordination of actions in, =I=, 578.
-
-    Vegeto-alkalies, physiological effects of, =I=, 54–5.
-
-    Velocity, of moving bodies, =II=, 219–20.
-
-    _Vertebrata_: size, =I=, 139;
-      size at birth and maturity, =I=, 144;
-      axial structure, =I=, 165;
-      embryonic development and self-mobility, =I=, 175;
-      functional differentiation, =I=, 206, 591;
-      reparative power, =I=, 219, 223, 589;
-      homogenesis universal, =I=, 271;
-      distinctive traits, =I=, 392; =II=, 35;
-      distribution in time, =I=, 408;
-      classificatory value, =I=, 446;
-      embryonic mammalian respiratory system, =I=, 456;
-      embryological pre-adaptation, =I=, 461;
-      evolution and vertebral column, =I=, 470;
-      rudimentary organs, =I=, 473;
-      evolution and varied media, =I=, 479–85;
-      size of head and vertebræ, =I=, 512, 537;
-      segregation and evolution of vertebræ, =I=, 515;
-      fertility and development, =I=, 583, 598–9;
-      Weismann on reproductive cells, =I=, 635;
-      limb locomotion, =II=, 15;
-      adaptive segmentation, =II=, 117–23, 125–7, 223, 602, 605–7;
-      supernumerary vertebræ, =II=, 123;
-      bilateral symmetry, =II=, 203–6;
-      internal organic symmetry, =II=, 208;
-      genesis of rudimentary axis, =II=, 212–6;
-      natural selection and genesis of structure, =II=, 216, 227;
-      origin of notochord, =II=, 216–8;
-      spinal segmentation, =II=, 218–22, 224;
-      skull development, =II=, 222, 227;
-      _résumé_ of axis development, =II=, 224;
-      Cope on author’s theory, =II=, 225–7;
-      nerve differentiation, =II=, 304;
-      sensory organs, =II=, 318;
-      air-chambers, =II=, 334;
-      osseous differentiation, =II=, 344–55;
-      activity and muscular colour, =II=, 365–9;
-      heart-motor apparatus, =II=, 374;
-      cost of genesis, =II=, 436;
-      agamogenesis unknown, =II=, 445;
-      growth and genesis, =II=, 454;
-      heat expenditure and genesis, =II=, 468–9, 474;
-      Owen, theory of skeleton, =II=, 548–66;
-      evolution of vertebræ, =II=, 563–6;
-      origin of type, =II=, 598–600.
-
-    _Vestiges of Creation_, =I=, 491.
-
-    Vibrissæ, function of, =I=, 75.
-
-    Vitalism, hypothesis examined, =I=, 114–7.
-
-    Vittadini, C., on silkworm disease, =I=, 622–3.
-
-    Viviparons genesis, =I=, 271, 274–5, 278.
-
-    Voice, correlated sexual traits, =I=, 371–2.
-
-    Volcano, definition of life and, =I=, 85, 89.
-
-    _Volvocineæ_: unicentral development, =I=, 163;
-      individuality, =I=, 245;
-      disintegration of genesis, =I=, 276, 587;
-      spherical aggregation, =II=, 24;
-      symmetry, =II=, 137, 187;
-      fertility, =II=, 441.
-
-    Vomiting, alimentary canal development, =II=, 328.
-
-    _Vorticella_: secondary aggregate, =II=, 90;
-      symmetry, =II=, 188.
-
-
-    Wallace, A. R.: “The Origin of the Human Races,” =I=, 553;
-      the expression “Survival of the Fittest,” =I=, 530;
-      his association of natural with artificial selection, =I=, 609;
-      co-adaptation in giraffe, =I=, 615;
-      skin sensitiveness, =I=, 646.
-
-    Wasp: co-ordination of instincts in Mason-, =I=, 574, 679–80;
-      genesis of worker, =I=, 654–7.
-
-    Waste, animal, =I=, 69, 213–5, 228;
-      relation to activity, =I=, 196, 220–1;
-      in plants, =I=, 213, 220.
-
-    Water: properties, =I=, 7, 9;
-      colloidal affinity for, =I=, 28;
-      organic change from, =I=, 29;
-      organic need for, =I=, 147;
-      proportion in mammalian adult and fœtus, =I=, 154;
-      motion through, =I=, 156;
-      organic development and environment, =I=, 173, 177, 479;
-      terrestrial organisms inhabiting, =I=, 400;
-      adaptation of organisms to change of media, =I=, 479–85;
-      vegetal tissue differentiation, =II=, 253;
-      molecular re-arrangement, =II=, 359;
-      colloidal contraction, =II=, 361–2.
-
-    Water-weed, American, invasion of, =I=, 399.
-
-    Watts, Dr., on _The Principles of Biology_, =I=, ix.
-
-    Wax, foliar deposit, =II=, 260–1.
-
-    Weber, on tactual discriminativeness, =I=, 602.
-
-    Weight: relation to environment of organic, =I=, 174, 177;
-      varying as cube of dimensions, =I=, 151; =II=, 434, 470.
-
-    Weismann, Aug.: reproductive tissue in _Medusæ_, =I=, 281;
-      in _Daphnidæ_, =I=, 290;
-      his theory of the differentiated germ-plasm and its fundamental
-          units, =I=, 357, 622–3, 628–30, 633–44, 646; =II=, 618–9,
-          622;
-      the alleged differentiation and plant-phenomena, =I=, 359–60;
-      and regenerative processes, =I=, 360;
-      false joints, =I=, 362;
-      implied complexity of determinants, =I=, 370;
-      theory inadequate to explain correlation of sexual traits, =I=,
-          372;
-      and variations in peacock’s tail feather, =I=, 372–3, 695; =II=,
-          618;
-      his view of natural selection as sole factor in organic evolution,
-          =I=, 559;
-      the doctrine of panmixia, =I=, 561–3, 612, 632, 649, 667–9, 671,
-          685, 689;
-      arguments against inheritance of acquired characters, =I=, 612–3,
-          651–65, 669–71;
-      blindness of cave-animals, =I=, 613;
-      current acceptance of his views, =I=, 631, 690;
-      cannot explain the process of natural selection, =I=, 651;
-      the degradation of the little toe in man, =I=, 652, 669, 673;
-      caste gradations of social insects, =I=, 654, 658–65, 670, 675,
-          678–84, 685;
-      food-seeking instinct in Amazon ants, =I=, 660, 670;
-      the co-adaptation of co-operative parts, =I=, 663–4, 670, 674,
-          675, 676;
-      tactual discriminativeness, =I=, 665, 672;
-      intra-selection, =I=, 676–8;
-      effect of nutrition on fertility of blow-fly, =I=, 678–9.
-
-    Whale: weight of brain, =I=, 599;
-      rudimentary limbs, =I=, 668–9, 685, 693.
-
-    Wheat, adaptive variations, =II=, 298.
-
-    Whistling, definition of life and, =I=, 112.
-
-    White-Cooper, Mr., on inheritance of abnormal vision, =I=, 306.
-
-    Willow, nutrition and growth, =I=, 294.
-
-    Wilson, E. B.: composition of chromatin, =I=, 260;
-      separation of segmentation spheres of _Amphioxus_ ovum, =I=, 691.
-
-    Wind: and vegetal bilateral symmetry, =II=, 142;
-      and inner vegetal tissue differentiation, =II=, 275–9, 285, 288,
-          388;
-      and proliferation of _Bryophyllum_, =II=, 295;
-      and vegetal sap movement, =II=, 583, 584, 587;
-      _résumé_, 592–6.
-
-    Wolff, C.: vegetal fructification and nutrition, =I=, 283; =II=,
-        179–80;
-      vegetal vascular system, =II=, 283.
-
-    Women (_see_ Man).
-
-    Wood (_see_ Plants).
-
-
-    Yeast: fermentation, =I=, 38;
-      fertility, =I=, 581; =II=, 440;
-      linear aggregation, =I=, 587; =II=, 23.
-
-
-    Zebra marks in horses, =I=, 314.
-
-    Zoology, classification, =I=, 124–5, 380–9.
-
-    Zoophytes, structural indefiniteness, =I=, 173.
-
-    Zoospores, unit-life of, =I=, 185.
-
-    Zygote, of conjugating _Algæ_, =I=, 283.
-
-
-                               THE END.
-
-
-FOOTNOTES:
-
-[1] It seems needful here to say, that allusion is made in this
-paragraph to a proposition respecting the ultimate natures of Evolution
-and Dissolution, which is contained in an essay on _The Classification
-of the Sciences_, published in March, 1864. When the opportunity
-comes, I hope to make the definition there arrived at, the basis of a
-re-organization of the second part of _First Principles_: giving to
-that work a higher development, and a greater cohesion, than it at
-present possesses. [The intention here indicated was duly carried out
-in 1867.]
-
-[2] Let me here refer those who are interested in this question,
-to Prof. Huxley’s criticism on the cell-doctrine, published in the
-_Medico-Chirurgical Review_ in 1853.
-
-A critic who thinks the above statements are “rather misleading”
-admits that the lowest types of organisms yield them support, saying
-that “there are certainly masses of protoplasm containing many nuclei,
-but no trace of cellular structure, in both animals and plants. Such
-non-cellular masses may exist during development and later become
-separated up into cells, but there are certain low organisms in which
-such masses exist in the adult state. They are called by some botanists
-non-cellular, by others multi-nucleate cells. Clearly the difference
-lies in the criteria of a cell. There are also some _Protozoa_, and
-the _Bacteria_, in which no nucleus has certainly been demonstrated.
-But it is usual to consider the bodies of such organisms as cells
-nevertheless, and it is supposed that such cells represent a stage of
-development in which the nucleus has not yet been evolved, though the
-chemical substance ‘nuclein’ has been formed in some of them.”
-
-Perhaps it will be most correct to say that, excluding the minute,
-non-nucleated organisms, all the higher organisms--_Metazoa_ and
-_Metaphyta_--are composed throughout of cells, or of tissues originally
-cellular, or of materials which have in the course of development been
-derived from cells. It must, however, be borne in mind that, according
-to sundry leading biologists, cells in the strict sense are not the
-immediate products either of the primitive fissions or of subsequent
-fissions; but that the multiplying so-called cells are nucleated masses
-of protoplasm which remain connected by strands of protoplasm, and
-which acquire limiting membranes by a secondary process. So that, in
-the view of Mr. Adam Sedgwick and others, the substance of an organism
-is in fact a continuous mass of vacuolated protoplasm.
-
-[3] In further illustration, Mr. Tansley names the fact that in
-the genus _Caulerpa_ we have extremely complicated forms often of
-considerable size produced in the same way. The various species
-simulate very perfectly the members of different groups among the
-higher plants, such as Horse-tails, Mosses, Cactuses, Conifers and the
-like.
-
-[4] It may be objected that in _Cladophora_ the separate compartments
-of the thallus severally contain many nuclei, making it doubtful
-whether they descend from uni-nucleate cells. If, however, they do not
-they simply illustrate another form of integration.
-
-[5] The great mass of early ancestral types--plant and
-animal--consisting of soft tissues, have left no remains whatever, and
-we have no reason to suppose that those which left remains fell within
-the direct ancestral lines of any existing forms. Contrariwise, we have
-reason to suppose that they fell within lines of evolution out of which
-the lines ending in existing forms diverged. We must therefore infer
-that the difficulties of affiliation which arise if we contemplate
-divergent types now existing, would not arise if we had before us all
-the early intermediate types. The Mammalia differ in sundry respects
-from all other kinds of Vertebrata--Fishes, Reptiles, Birds; and if
-the absence of hair, mammæ, and two occipital condyles, in these other
-vertebrates were taken to imply a fundamental distinction, it might, in
-the absence of any known fossil links, be inferred that the Mammalia
-belonged to a separate phylum. But these differences are not held to
-negative the assumed relationship. Similarly among plants. We must
-not reject an hypothesis respecting a certain supposed type, because
-the existing types it must have been akin to present traits which it
-could not have had. We are justified in assuming, within limits, a
-hypothetical type, unlike existing types in traits of some importance.
-Hence results the answer to a criticism passed on the above argument,
-that it implies relations between the undeveloped and developed forms
-of the _Jungermanniaceæ_ such as the facts do not show us. This
-objection is met on remembering that the types in which the supposed
-transition took place disappeared myriads of years ago.
-
-[6] There is much force in the criticism passed on the above paragraph,
-and by implication on some preceding paragraphs, that though in plants
-which tend to produce compound leaves the production is largely
-dependent on the supply of nutriment, yet the unqualified statement of
-this relation as a general one, is negatived by the existence of plants
-which bear only simple leaves, however much high nutrition causes
-growth. But mostly valid though this objection is, it is probably not
-universally valid. I am led to say this by what occasionally occurs
-in flowers. The flowering stem of the Hyacinth is single; but I have
-seen a cultivated Hyacinth in which one of the flowers had developed
-into a lateral spike. Still more striking evidence was once supplied
-to me by Agrimony. All samples of this plant previously seen had
-single flowering spikes, but some years ago I met with one, extremely
-luxuriant, in which some flowers of the primitive spike were replaced
-by lateral spikes; and I am not sure that some of these, again, did
-not bear lateral spikes. Now if in plants which, in probably millions
-of cases, have their flowering stems single, excessive nutrition
-changes certain of their flowers into new spikes, it is a reasonable
-supposition that in like manner plants which are thought invariably to
-bear only single leaves, will, under kindred conditions, bear compound
-leaves.
-
-[7] See _British and Foreign Medico-Chirurgical Review_ for January,
-1862.
-
-[8] Schleiden, who chooses to regard as an axis that which Mr.
-Berkeley, with more obvious truth, calls a mid-rib, says:--“The flat
-stem of the Liverworts presents many varieties, consisting frequently
-of one simple layer of thin-walled cells, or it exhibits in its
-axis the elements of the ordinary stem.” This passage exemplifies
-the wholly gratuitous hypotheses which men will sometimes espouse,
-to escape hypotheses they dislike. Schleiden, with the positiveness
-characteristic of him, asserts the primordial distinction between
-axial organs and foliar organs. In the higher Archegoniates he sees an
-undeniable stem. In the lower Archegoniates, clearly allied to them
-by their fructification, there is no structure having the remotest
-resemblance to a stem. But to save his hypothesis, Schleiden calls
-that “a flat stem,” which is obviously a structure in which stem and
-leaf are not differentiated. He is the more to be blamed for this
-unphilosophical assumption, since he is merciless in his strictures on
-the unphilosophical assumptions of other botanists.
-
-[9] To this interpretation it is objected that “the more-developed
-_Jungermanniaceæ_” do not appear to have arisen from the lower forms
-of _Jungermanniaceæ_--that is to say, from such lower forms as are now
-existing. It may, however, be contended that this fact does not exclude
-the interpretation given; since the higher forms may well have been
-evolved, not from any of the lower forms we now know, but from lower
-forms which have become extinct. This, indeed, is the implication of
-the evolutionary process as pointed out in the note to Chap. I. If then
-we assume some early type of intermediate structure, the explanation
-may not improbably hold.
-
-[10] I am indebted to Dr. Hooker for pointing out further facts
-supporting this view. In his _Flora Antarctica_, he describes the genus
-_Lessonia_ (see Fig. 37), and especially _L. ovata_, as having a mode
-of growth simulating that of the dicotyledonous trees, not only in
-general form but in internal structure. The tall vertical stem thickens
-as it grows, by the periodical addition of layers to its periphery.
-That even Thallophytes should thus, under certain conditions, present
-a transversely-increasing axis, shows that there is nothing absolutely
-characteristic of Phanerogams in their habit of stem-thickening. Mr.
-Tansley gives me further verification by the statement that “it is
-also now certain that members of the _Equisetineæ_ and _Lycopodineæ_,
-as well as some Ferns which flourished in Carboniferous times, had
-secondary thickening in their stems quite comparable to that of modern
-Dicotyledonous trees.”
-
-[11] See note at the end of the chapter.
-
-[12] Since this paragraph was put in type [this refers to the first
-edition], I have observed that in some varieties of _Cineraria_, as
-probably in other plants, a single individual furnishes all these forms
-of leaves--all gradations between unstipulated leaves on long petioles,
-and leaves that embrace the axis. It may be added that the distribution
-of these various forms is quite in harmony with the rationale above
-given.
-
-[13] Since these figures were put on the block, it has occurred to
-me that the relations would be still clearer, were the primary frond
-represented as not taking part in these processes of modification,
-which have been described as giving rise to the erect form; as,
-indeed, the rooting of its under surface will prevent it from doing
-in any considerable degree. In such case, each of the Figs. 111 to
-117, should have a horizontal rooted frond at its base, homologous
-with the pro-embryo among Acrogens. This primary frond would then more
-manifestly stand in the same relation to the rest, as the cotyledon
-does to the plumule--both by position, and as a supplier of nutriment.
-Fig. 117_a_, which I am enabled to add, shows that this would complete
-the interpretation. Of the dicotyledonous series, it is needful to add
-no further explanation than that the difference in habit of growth,
-will permit the second frond to root itself as well as the first; and
-so to become an additional source of nutriment, similarly circumstanced
-to the first and equal with it.
-
-[14] How the element of time modifies the result, is shown by the
-familiar fact that crystals rapidly formed are small, and become
-relatively large when left to form more slowly. If the quantity of
-molecules contained in a solution is relatively great, so that the
-mutual polarities of the molecules crowded together in every place
-throughout the solution are intense, there arises a crystalline
-aggregation around local axes; whereas, in proportion as the local
-action of molecules on one another is rendered less intense by their
-wider dispersion, they become relatively more subordinate to the forces
-exerted on them by the larger aggregates of molecules that are at
-greater distances, and thus are left to arrange themselves round fewer
-axes into larger crystals.
-
-[15] It is objected that these transformations should be much
-commoner than they are, were they caused solely by the variations of
-nutrition described. The reply is that they are comparatively rare
-in uncultivated plants, where such variations are not frequent. The
-occurrence of them is chiefly among cultivated plants which, being
-artificially manured, are specially liable to immense accessions of
-nutriment, caused now by sudden supplies of fertilizing matters, and
-now by sudden arrival of the roots at such matters already deposited in
-the soil. It is to these great _changes_ of nutrition, especially apt
-to take place in gardens, that these monstrosities are ascribed; and it
-seems to me that they are as frequent as may be expected.
-
-[16] Since this paragraph was published in 1865, much has been learned
-concerning cell-structure, as is shown in Chapter VI^A of Part I.
-While some assert that there exist portions of living protoplasm
-without nuclei, others assert that a nucleus is in every case present,
-and that where it does not exist in a definite aggregated form it
-exists in a dispersed form. As remarked in the chapter named, “the
-evidence is somewhat strained to justify this dogma.” Words are taken
-in their non-natural senses, if one which connotes an individualized
-body is applied to the widely-diffused components of such a body; and
-this perverting of proper meanings leads to obscuration of what may
-perhaps be an essential truth. As argued in the chapter named (§§
-74_e_, 74_f_), nuclear matter is, as shown by its chemical character,
-an extremely unstable substance, the molecular changes of which,
-perpetually going on, initiate shocks, producing changes all around.
-In the earlier stages of cell-evolution this unstable substance is
-dispersed throughout the cytoplasm; whereas in the more advanced stages
-it is gathered together in one mass. If so, instead of saying there is
-a dispersed nucleus we should say there are the materials of a nucleus
-not yet integrated.
-
-[17] This statement seems at variance with the figure; but the figure
-is very inaccurate. Its inaccuracy curiously illustrates the vitiation
-of evidence. When I saw the drawing on the block, I pointed out to
-the draughtsman, that he had made the surrounding curves much more
-obviously related to the contained bodies, than they were in the
-original (in Dr. Carpenter’s _Foraminifera_); and having looked on
-while he in great measure remedied this defect, thought no further
-care was needed. Now, however, on seeing the figure in the printer’s
-proof, I find that the engraver, swayed by the same supposition as the
-draughtsman that such a relation was meant to be shown, has made his
-lines represent it still more decidedly than those of the draughtsman
-before they were corrected. Thus, vague linear representations, like
-vague verbal ones, are apt to grow more definite when repeated.
-Hypothesis warps perceptions as it warps thoughts.
-
-[18] Though the subdivision into chambers of the shell does not
-correspond to the subdivision into cell-units it may still be held
-that since in the solitary types the subdivision of the nucleus is
-followed by formation of new individuals which separate, and since in
-the compound types the subdivision of the nucleus is followed by growth
-and formation of new chambers, the compound type must be regarded as an
-aggregate of the second order.
-
-[19] A critic says the question is “what are the forces internal
-or external which produce union or separation.” A proximate reply
-is--degree of nutrition. As in a plant new individuals or rudiments of
-them are cast off where nutrition is failing, so in a compound animal.
-The connecting part dwindles if it ceases to carry nutriment.
-
-[20] It has been pointed out that I have here understated the evidence
-of physiological integration. An instance of it among _Hydrozoa_ is
-shown in Fig. 151, but by a strange oversight I have forgotten to
-name the various cases furnished by the _Siphonophora_ in which the
-individual polypes of a compound aggregate are greatly specialized in
-adaptation to different functions.
-
-[21] Recently Mr. T. H. Morgan has made elaborate experiments which
-show that _Planaria Maculata_ may be cut into many pieces from various
-parts and of various shapes--even a slice out of the side--and each, if
-not too small, will produce a perfect animal.
-
-[22] Since this was written in 1865 there has come to light evidence
-more completely to the point than any at that time known. In the
-subdivision of _Platyhelminthes_ known as _Turbellaria_, there are
-some, the _Microstomida_ which, by a process of segmentation form
-“chains of 4, then 8, then 16, and sometimes even 32 individuals.”
-“Each forms a mouth [lateral] and for some time the chain persists, but
-the individuals ultimately become sexually matured and then separate.”
-(Shipley, _Zoology of the Invertebrata_, p. 92.) Here it should be
-remarked that the lateral mouths enable the members of a string to feed
-separately, and that nutrition not being interfered with they doubtless
-gain some advantage by temporary maintenance of their union--probably
-in creeping.
-
-[23] I find that the reasons for regarding the segment of a _Tænia_
-as answering to an individual of the second order of aggregation, are
-much stronger than I supposed when writing the above. Van Beneden
-says:--“Le Proglottis (segment) ayant acquis tout son développement,
-se détache ordinairement de la colonie et continue encore à croître
-dans l’intestin du même animal; il change même souvent de forme et
-semble doué d’une nouvelle vie; ses angles s’effacent, tout le corps
-s’arrondit, et il nage comme une Planaire au milieu des muscosités
-intestinales.”
-
-[24] Though this was doubtful in 1865 it is no longer doubtful. In
-an individual _Ctenodrilus monostylus_, which multiplies by dividing
-and subdividing itself, “parts arise which are destitute of both head
-and anus and at times consist of only a single segment.” In another
-species, _C. pardalis_, there is separation into many segments; and
-each segment before separating forms a budding zone out of which other
-segments are afterwards produced, completing the animal (Korschelt and
-Heider, _Embryology_, i, 301–2).
-
-[25] In place of those originally here instanced about which there
-are disputes, I may give an undoubted one described by McIntosh, the
-_Syllis ramosa_, a species of chætopod living in hexactinellid sponges
-from the Arafura Sea, which branches laterally repeatedly so as to
-extend in all directions through the canals of the sponge. In most
-cases the buds terminate in oval segments with two long cirri each.
-But male and female buds were found, provided each with a head, and
-containing ovaries and testes. Sometimes these sexual buds had become
-separate from the branched stock.
-
-[26] The name _Annulosa_, once used to embrace the _Annelida_ and
-_Arthropoda_, has of late ceased to be used. It seems to me better than
-_Appendiculata_, both as being more obviously descriptive and as being
-more exclusive.
-
-[27] The fusion of the segments forming the Arthropod head and the
-extreme changes, or perhaps in some cases disappearances, of their
-appendages, put great difficulties in the way of identification;
-so that there are differences of opinion respecting the number of
-included segments. Prof. MacBride writes:--“It is highly probable that
-a primary head (præoral lobe or præstomium) has been derived from
-annelid ancestors, but the secondary fusion of body-segments with this
-head, in other words the formation of a secondary head, has gone on
-independently in the different classes of the phylum _Arthropoda_,
-viz., _Arachnida_, _Crustacea_, and _Tracheata_ (including Insects and
-Myriapods). Judged by the number of appendages (which gives an inferior
-limit) the head of a malacostracous Crustacean consists of præstomium
-and 8 segments; the head of an insect of præstomium and 4 segments; the
-head of a Myriapod of præstomium and 3 segments; and the head of an
-Arachnid of præstomium and 3 segments.” Again, the comment of Mr. J. T.
-Cunningham is:--“According to Claus and most modern authorities there
-are only 5 segments in the head of an Arthropod, the eyes not counting
-as appendages; and further it should be noted that the second pair of
-antennæ are wanting in Insects.”
-
-Of course difference of opinion respecting the number of somites in the
-head involves difference of opinion respecting the number constituting
-the entire body, which, in the higher Arthropods, is said by some to
-be 19 and by others 20. But those who thus differ in detail, agree in
-regarding all the segments of head and body as homologous, and this is
-the essential point with which we are here concerned.
-
-[28] Prof. MacBride corrects this statement by saying that “The
-ctenidia or gills (which in _Mollusca_ generally are represented only
-by a single pair) are here represented by a large number of pairs;
-they do not, however, correspond in either number or position to the
-shell plates.” It may, I think, be contended that if these had any
-morphological significance, they would not differ in arrangement from
-the shell plates, and would not be limited to this special type of
-Mollusc.
-
-[29] Though it is alleged that at a later stage the posterior part
-of the skull is formed by fusion of divisions which are assumed to
-represent vertebræ, yet it is admitted that the anterior part of the
-skull never shows any signs of such division. Moreover in both parts
-the bones show no trace of primitive segmentation.
-
-[30] See note at the end of the chapter.
-
-[31] A qualifying fact should be named. When the production of
-vertebral segments has become constitutionally established, so that
-there is an innate tendency to form them, there arises a liability to
-form supernumerary ones; and this, from time to time recurring, may
-lengthen the series, as in the body of a snake or the neck of a swan.
-This qualification, however, affects equally the hypothesis of an ideal
-type and the hypothesis of mechanical genesis.
-
-[32] Here and throughout, the word _radial_ is applied equally to the
-spiral and the whorled structures. These, as being alike on all sides,
-are similarly distinguished from arrangements that are alike on two
-sides only.
-
-[33] It should be added that this change of distribution is not due to
-change in the relative positions of the insertions of the leaves but to
-their twistings.
-
-[34] We may note that some of these leaves, as those of the Lime,
-furnish indications of the ratio which exists between the effects of
-individual circumstances and those of typical tendencies. On the one
-hand, the leaves borne by these drooping branches of the Lime are
-with hardly an exception unsymmetrical more or less decidedly, even
-in positions where the causes of unsymmetry are not in action: a fact
-showing us the repetition of the type irrespective of the conditions.
-On the other hand, the degree of deviation from symmetry is extremely
-variable, even on the same shoot: a fact proving that the circumstances
-of the individual leaf are influential in modifying its form. But the
-most striking evidence of this direct modification is afforded by the
-suckers of the Lime. Growing, as these do, in approximately upright
-attitudes, the leaves they bear do not stand to one another in the way
-above described, and the causes of unsymmetry are not in action; and
-here, though there is a general leaning to the unsymmetrical form, a
-large proportion of the leaves become quite symmetrical.
-
-[35] It was by an observation on the forms of leaves, that I was first
-led to the views set forth in the preceding and succeeding chapters
-on the morphological differentiation of plants and animals. In the
-year 1851, during a country ramble in which the structures of plants
-had been a topic of conversation with a friend--Mr. G. H. Lewes--I
-happened to pick up the leaf of a buttercup, and, drawing it by its
-foot-stalk through my fingers so as to thrust together its deeply-cleft
-divisions, observed that its palmate and almost radial form was changed
-into a bilateral one; and that were the divisions to grow together in
-this new position, an ordinary bilateral leaf would result. Joining
-this observation with the familiar fact that leaves, in common with
-the larger members of plants, habitually turn themselves to the
-light, it occurred to me that a natural change in the circumstances
-of the leaf might readily cause such a modification of form as that
-which I had produced artificially. If, as they often do with plants,
-soil and climate were greatly to change the habit of the buttercup,
-making it branched and shrub-like; and if these palmate leaves were
-thus much overshadowed by one another; would not the inner segments
-of the leaves grow towards the periphery of the plant where the light
-was greatest, and so change the palmate form into a more decidedly
-bilateral form? Immediately I began to look round for evidence of the
-relation between the forms of leaves and the general characters of the
-plants they belong to; and soon found some signs of connexion. Certain
-anomalies, or seeming anomalies, however, prevented me from then
-pursuing the inquiry much further. But consideration cleared up these
-difficulties; and the idea afterwards widened into the general doctrine
-here elaborated. Occupation with other things prevented me from giving
-expression to this general doctrine until Jan. 1859; when I published
-an outline of it in the _Medico-Chirugical Review_.
-
-[36] It is objected to the above interpretation that “many flowers of
-sizes intermediate between the Hollyhock and the Agrimony are radially
-symmetrical and yet grow sideways. I may mention various _Liliaceæ_,
-e.g. _Chlorophytum_, _Eucomis_, _Muscari_, _Anthericum_. _Sagittaria_,
-also, has many of its flowers in this position. Further, if the higher
-insects alight on flowers in a definite way, as they do, the parts
-of the flower must bear different relations to the visiting insect,
-however large, so that flowers unvisited ought all to be zygomorphic.”
-My reply is that in the sense which here concerns us, the different
-petals of the Hollyhock-flower do not bear different relations to the
-visiting insect; since, practically, the upper and lateral petals
-bear no physical relations at all: in so far as the visiting bee is
-concerned they are non-existent. The argument implies that change in
-the form of a flower from the radial to the bilateral is likely to take
-place only when the contact-relations of the petals to the visiting
-insect, are such as to make some forms facilitate its action more than
-others; and the large petals of the Hollyhock cannot facilitate its
-action at all. In respect of the _Liliaceæ_ instanced, it is needful
-to inquire whether the structures are such that this alleged cause of
-bilateral symmetry can come into play.
-
-[37] I had intended here to insert a figure exhibiting these
-differences; but as the Cow-parsnip does not flower till July, and as
-I can find no drawing of the umbel which adequately represents its
-details, I am obliged to take another instance.
-
-[38] It has been pointed out to me that “the extreme development of the
-corolla so often found in the outer flowers or on the outer side of the
-outer flowers in closely-packed inflorescences, associated as it often
-is with disappearance of stamens or carpels or both, is usually put
-down to specialization of these outer flowers for attractive purposes.
-Since the whole inflorescence is increased in conspicuousness by such
-a modification, it is supposed that natural selection favoured those
-plants which sacrificed a portion of their seed-bearing capacity for
-the supposed greater advantage of securing more insect visits.” But
-granting this interpretation, it may still be held that increase of
-attractiveness due to increase of area must be achieved by florets at
-the periphery, and that their ability to achieve it depends on their
-having an outer, unoccupied, space which the inner florets have not;
-so that, though in a more indirect way, their different development is
-determined by different exposure to conditions.
-
-[39] One of my critics writes:--“This chapter might of course be
-enormously extended, not only as in the preceding ones by citation of
-quite similar cases, but by the introduction of fresh groups of cases.”
-
-[40] Natural selection may have operated in establishing a
-constitutional tendency to other sudden abridgments. Mr. Tansley
-alleges that this is a part-cause of the varying distribution of
-leaves. He says:--“I have myself made some observations on the length
-of internodes in the Beech, and am satisfied that it follows quite
-other laws, connected with the suitable disposition of the leaves on
-the branch. Although I have not had the opportunity of following up
-this line of work so as in any way to generalize the results, I suspect
-that ‘indirect equilibration’ is a widespread cause of such variation.”
-
-[41] It is but just to the memory of Wolff, here to point out that
-he was immensely in advance of Goethe in his rationale of these
-metamorphoses. Whatever greater elaboration Goethe gave to the theory
-considered as an induction, seems to me more than counter-balanced
-by the irrationality of his deductive interpretation; which unites
-mediæval physiology with Platonic philosophy. A dominant idea with him
-is that leaves exist for the purpose of carrying off crude juices--that
-“as long as there are crude juices to be carried off, the plant must
-be provided with organs competent to effect the task”; that while “the
-less pure fluids are got rid of, purer ones are introduced” and that
-“if nourishment is withheld, that operation of nature (flowering) is
-facilitated and hastened; the organs of the nodes (leaves) become
-more refined in texture, the action of the purified juices becomes
-stronger, and the transformation of parts having now become possible,
-takes place without delay.” This being the proximate explanation, the
-ultimate explanation is, that Nature wishes to form flowers--that when
-a plant flowers it “attains the end prescribed to it by nature”; and
-that so “Nature at length attains her object.” Instead of vitiating
-his induction by a teleology that is as unwarranted in its assigned
-object as in its assigned means, Wolff ascribes the phenomena to a
-cause which, whether sufficient or not, is strictly scientific in its
-character. Variation of nutrition is unquestionably a “true cause” of
-variation in plant-structure. We have here no imaginary action of a
-fictitious agency; but an ascertained action of a known agency.
-
-[42] The _Natural History Review_ for July, 1865, contained an article
-on the doctrine of morphological composition set forth in the foregoing
-Chaps. I. to III. In this article, which unites exposition and
-criticism in a way that is unhappily not common with reviewers, it is
-suggested that the spiral structure may be caused by natural selection.
-When this article appeared, the foregoing five pages were standing over
-in type, as surplus from No. 14, issued in June, 1865.
-
-[43] A verifying comment on this paragraph runs as follows:--“In the
-Hypotricha Infusoria, which creep over solid surfaces, there is a
-differentiation between ventral and dorsal surface and an approach to
-bilateral symmetry. The ventral surface is provided with movable cilia,
-the dorsal with immobile setæ.”
-
-[44] Criticisms on the above passage have shown the need for naming
-sundry complications. These complications chiefly, if not wholly,
-arise from changes in modes of life--changes from the locomotive to
-the stationary, and from the stationary to the locomotive. Referring
-to my statement that (ignoring the spherical) the radial type is the
-lowest and must be taken as antecedent to the bilateral type, it is
-alleged that all existing “radial animals above Protozoa are probably
-derived from free-swimming, bilaterally-symmetrical animals.” If this
-is intended to include the planulæ of the hydroid polyps, then it seems
-rather a straining of the evidence. These locomotive embryos, described
-as severally having the structure of a gastrula with a closed mouth,
-can be said to show bilateralness only because the first two tentacles
-make their appearance on opposite sides of the mouth--a bilateralness
-which lasts only till two other tentacles make their appearance in a
-plane at right angles, so giving the radial structure. I think the
-criticism applies only to cases furnished by Echinoderms. The larvæ
-of these creatures have bilaterally-symmetrical structures, which
-they retain as long as they swim about and which such of them as fix
-themselves lose by becoming similarly related to conditions all round:
-the radial structure being retained by those types which, becoming
-subsequently detached, move about miscellaneously. But, as happens
-in some of the Sea-urchins and still more among the Holothurians,
-the structure is again made bilaterally-symmetrical by a locomotive
-life pursued with one end foremost. Should it be contended that the
-conditions and the forms are reciprocally influential--that either may
-initiate the other, it still remains unquestionable that ordinarily the
-conditions are the antecedents, as is so abundantly shown by plants.
-
-[45] Should it be proved that the Ascidian is a degraded vertebrate,
-then the argument will be strengthened; since loss of bilateral
-symmetry has gone along with change to asymmetrical conditions.
-
-[46] A critical comment made on this sentence runs as follows:--“The
-aërial roots of most epiphytic orchids contain chlorophyll in their
-cortex throughout their length, but the cortex being covered by a
-‘velamen’ of air-containing cells which break up and reflect incident
-light, the green colour is not visible through this opaque coat. When
-moistened the cells of the velamen take up water and the green colour
-immediately shows through. Such roots do not however possess stomata.
-The roots of certain species of _Angræcum_, however, contain the whole
-of the assimilating tissue of the plant.”
-
-[47] The current doctrine that chlorophyll is _the_ special substance
-concerned in vegetal assimilation, either as an agent or as an
-incidental product, must be taken with considerable qualification.
-Besides the fact that among the _Algæ_ there are many red and brown
-kinds which thrive; and besides the fact that among the lower
-Archegoniates there are species which are purple or chocolate-coloured;
-there is the fact that Phænogams are not all green. We have the
-Copper-Beech, we have the black-purple _Coleus Verschaffeltii_, and we
-have the red variety of Cabbage, which seems to flourish as well as
-the other varieties. Chlorophyll, then, must be regarded simply as the
-most general of the colouring matters found in those parts of plants in
-which assimilation is being effected by the agency of light. Though it
-is always present _along with_ the red and brown pigments, yet there is
-much evidence to show that these are the actual assimilative pigments.
-
-[48] This seems as fit a place as any for noting the fact, that the
-greater part of what we call beauty in the organic world, is in some
-way dependent on the sexual relation. It is not only so with the
-colours and odours of flowers. It is so, too, with the brilliant
-plumage of birds; and it is probable that the colours of the more
-conspicuous insects are in part similarly determined. The remarkable
-circumstance is, that these characteristics, which have originated
-by furthering the production of the best offspring, while they are
-naturally those which render the organisms possessing them attractive
-to one another, directly or indirectly, should also be those which are
-so generally attractive to us--those without which the fields and woods
-would lose half their charm. It is interesting, too, to observe how the
-conception of human beauty is in a considerable degree thus originated.
-And the trite observation that the element of beauty which grows out of
-the sexual relation is so predominant in æsthetic products--in music,
-in the drama, in fiction, in poetry--gains a new meaning when we see
-how deep down in organic nature this connexion extends.
-
-[49] Students of vegetal physiology, familiar with the controversies
-respecting sundry points dealt with in this chapter, will probably be
-surprised to find taken for granted in it, propositions which they have
-habitually regarded as open to doubt. Hence it seems needful to say
-that the conclusions here set forth, have resulted from investigations
-undertaken for the purpose of forming opinions on several unsettled
-questions which I had to treat, but which I could find in books no
-adequate data for treating. The details of these investigations, and
-the entire argument of which this chapter is partly an abstract, will
-be found in Appendix C.
-
-[50] To this implied inference it is objected that “excess of nutritive
-material does not necessarily lead to correspondingly increased
-growth.” My reply is that a concomitant factor is activity of the
-tissue, and that in its absence growth is not to be expected.
-
-[51] In recent years (since 1890) Prof. Wilhelm Roux, in essays on
-functional adaptation, has set forth some views akin to the foregoing
-in respect to the general belief they imply, though differing in
-respect of the physiological processes he indicates. The following
-relevant passage has been translated for me from an article of his in
-the _Real-Encyclopädie der gesammten Heilkunde_:--“A more complete
-theory of functional adaptation by the author is founded on the
-assumption that the ‘functional’ stimulus, or ‘the act of exercising
-the function’ (in muscles and glands), and especially, in the case of
-bones, the concussion and tension caused by stress and strain, exert
-a ‘trophic’ stimulus on the cells, in consequence of which, and along
-with an increased absorption of nutriment, they grow and eventually
-increase (or the osteoblasts at the point of greater stimulus form more
-bone); while, conversely, with continued inactivity, by absence of
-these stimuli the nourishment of the cell declines so that the waste is
-insufficiently replaced (or otherwise that the bone-substance gradually
-loses its power of resistance to the osteoblasts formed as a result of
-inactivity”).
-
-[52] An outline of the doctrine set forth in the following chapters,
-was originally published in the _Westminster Review_ for April, 1852,
-under the title--_A Theory of Population deduced from the General Law
-of Animal Fertility_; and was shortly afterwards republished with a
-prefatory note stating that it must be accepted as a sketch which I
-hoped at some future time to elaborate. In now revising and completing
-it, I have omitted a non-essential part of the argument, while I
-have expanded the remainder by adding to the number of facts put in
-evidence, by meeting objections which want of space before obliged
-me to pass over, and by drawing various secondary conclusions. The
-original paper, with omissions, will be found in Appendix A to Volume I
-of this work.
-
-[53] I was here thinking only of the cases which are general among
-insects, but it seems that vertebrate animals, too, furnish cases. Mr.
-Cunningham writes:--“There is a curious instance of this in the Conger:
-the female grows to 6 or 7 feet long and a weight of 60 lbs. and
-upwards and then ceases to feed for 6 months while the eggs develop,
-and when the eggs are shed dies.”
-
-[54] I say “normal” for the purpose of excluding not only morbid
-growths but excess of fat.
-
-[55] To meet a possible criticism it should be remarked that this
-calculation assumes that the power of asexual reproduction is not
-exhausted by the end of the month. It has been found that “the
-successive fissions of _Paramœcium_ cannot continue indefinitely. After
-some hundreds of generations the products of fission are small, have no
-mouth, and die unless before this they have been allowed to conjugate
-with individuals of another brood.” It may, however, be fairly taken
-for granted that “some hundreds of generations” would take longer than
-a month.
-
-[56] Even this number is far exceeded. Dr. Edward Klein, in a lecture
-he gave at the Royal Institution on June 2, 1898, asserted that 246
-bacteria in a cubic centimetre of nutritive liquid would multiply to
-20,000,000 in the course of twenty-four hours: a rate which, at the end
-of the _third_ day, would give, as the offspring of one individual,
-537,367,797,000,000.
-
-[57] It has since been shown that in _Myrianida fasciata_ as many as
-29 attached groups exist. See _Cambridge Natural History_, Vol. II,
-_Worms, Rotifers and Polyzoa_, p. 280.
-
-[58] To this passage Prof. MacBride appends the remark:--“This is quite
-proven now, and the statement as it stands is quite correct; but far
-better and more minutely worked out cases are to be found amongst the
-_Infusoria_. In _Paramœcium_ for example, there are normally present a
-large macronucleus and a small micronucleus lying alongside of it. When
-two individuals adhere preparatory to conjugation, the macronucleus
-breaks up into fragments which are absorbed: the micronucleus--which
-has some time previously divided into two--begins to break up further
-and eventually forms eight bodies; all of these except one disappear;
-this last piece then divides into two; of these two one represents
-a male genital cell, for it passes over into the body of the other
-_Paramœcium_ and fuses with one of the two corresponding nuclei there;
-thus each of the two individuals which adhere fertilizes the other.
-The two individuals then separate and the nucleus (result of fusion of
-male and female nuclei) in each divides into four. Of these, two move
-to one end of the animal and two to the other. The animal then divides
-into two transversely--each of the products thus having two nuclei
-which form the micro-and macronucleus of it. Thus it appears that the
-function of sexual union is simply to give increased vigour to all the
-vital processes _including fission_. Since as mentioned above (p. 443)
-if it is prevented, the products of fission are eventually unable to
-feed themselves.”
-
-[59] A passage translated for me from the German may be here given
-in verification. Dr. Dionys Hellin in an essay on the origin of
-Multiparity and Twin-births, refers to the thesis above set forth, and
-says that “the fact that it is generally women of small growth who bear
-twins is in complete agreement with it.” He adds that “Puech is right
-in his opinion that twin pregnancies are a direct result of relatively
-large ovaries (_i.e._, in comparison with the whole body). He has
-observed that for the same size of body the ovarium of a pluriparous
-animal is always of greater volume than that of a uniparous animal ...
-a sow has ovaries as large as a cow’s; but while the latter bears only
-one calf [at a time], the sow brings forth 6–15 at each litter. Even
-in animals of the same species but belonging to different races these
-relations may be verified,” _e.g._, Barbary sheep and ordinary sheep.
-
-[60] When, after having held for some years the general doctrine
-elaborated in these chapters, I agreed, early in 1852, to prepare an
-outline of it for the _Westminster Review_, I consulted, among other
-works, the just-issued third edition of Dr. Carpenter’s _Principles
-of Physiology, General and Comparative_--seeking in it for facts
-illustrating the different degrees of fertility of different organisms,
-I met with a passage, quoted above in § 339, which seemed tacitly
-to assert that individual aggrandizement is at variance with the
-propagation of the race; but nowhere found a distinct enunciation of
-this truth. I did not then read the Chapter entitled “General View
-of the Functions,” which held out no promise of such evidence as I
-was looking for. But on since referring to this chapter, I discovered
-in it the definite statement that--“there is a certain degree of
-antagonism between the Nutritive and Reproductive functions, the
-one being executed at the expense of the other. The reproductive
-apparatus derives the materials of its operations through the nutritive
-system, and is entirely dependent upon it for the continuance of its
-function. If, therefore, it be in a state of excessive activity, it
-will necessarily draw off from the individual fabric some portion
-of the aliment destined for its maintenance. It may be universally
-observed that, when the nutritive functions are particularly active
-in supporting the _individual_, the reproductive system is in a
-corresponding degree undeveloped,--and _vice versâ_.” P. 592.
-
-[61] The climate, the locality, and the kind of food, are of course all
-factors; and hence, probably, the differences between the statements of
-different authorities concerning these several cases. Prof. MacBride
-writes:--
-
-“According to Flower (_Mammals, Living and Extinct_) the Ferret is a
-domesticated variety of the common polecat, which has 3 to 8 young.
-Darwin (_Animals and Plants_) says that the wild sow often breeds twice
-a year and produces a litter of 4 to 8, and sometimes even 12. The
-domestic sow breeds twice and would breed oftener if permitted, and if
-any good at all produces 8 in litter.”
-
-[62] It is worth while inquiring whether unfitness of the food given
-to them, is not the chief cause of that sterility which, as Mr.
-Darwin says, “is the great bar to the domestication of animals.” He
-remarks that “when animals and plants are removed from their natural
-conditions, they are extremely liable to have their reproductive
-systems seriously affected.” Possibly the relative or absolute arrest
-of genesis, is less due to a direct effect on the reproductive
-system, than to a changed nutrition of which the reproductive system
-most clearly shows the results. The matters required for forming an
-embryo are in a greater proportion nitrogenous than are the matters
-required for maintaining an adult. Hence, an animal forced to live on
-insufficiently-nitrogenized food, may have its surplus for reproduction
-cut off, but still have a sufficiency to keep its own tissues in
-repair, and appear to be in good health--meanwhile increasing in bulk
-from excess of the non-nitrogenous matters it eats.
-
-[63] Huxley, _Anatomy of Invertebrated Animals_, p. 274.
-
-[64] Shipley, _Zoology of Invertebrata_, p. 112.
-
-[65] I am told that “Wagner, who described the larva, found that it
-bored into the bark of trees. It attacks also the wheat plant, and is
-a most destructive parasite.” Apparently this statement is at variance
-with the foregoing inference. It is clear, however, that since these
-heaps of nitrogenous refuse in which it has been found are artificial
-and recent, they cannot be its natural habitats; and it seems not
-improbable that these larvæ, suddenly supplied with a more nutritive
-food in unlimited amount, may have as a consequence acquired this habit
-of agamogenetic multiplication which did not characterize the species
-under its natural conditions and relatively low nutrition.
-
-[66] This is exactly the reverse of Mr. Doubleday’s doctrine; which is
-that throughout both the animal and vegetable kingdoms, “over-feeding
-checks increase; whilst, on the other hand, a limited or deficient
-nutriment stimulates and adds to it.” Or, as he elsewhere says--“Be the
-range of the natural power to increase in any species what it may, the
-_plethoric_ state invariably checks it, and the _deplethoric_ state
-invariably develops it; and this happens in the exact ratio of the
-intensity and completeness of each state, until each state be carried
-so far as to bring about the actual death of the animal or plant
-itself.”
-
-I have space here only to indicate the misinterpretations on which Mr.
-Doubleday has based his argument.
-
-In the first place, he has confounded normal plethora with what I have,
-in § 355, distinguished as abnormal plethora. The cases of infertility
-accompanying fatness, which he cites in proof that over-feeding checks
-increase, are not cases of high nutrition properly so-called; but
-cases of such defective absorption or assimilation as constitutes
-low nutrition. In Chap. IX, abundant proof was given that a truly
-plethoric state is an unusually fertile state. It may be added that
-much of the evidence by which Mr. Doubleday seeks to show that among
-men, highly-fed classes are infertile classes, may be out-balanced by
-counter-evidence. Many years ago Mr. G. H. Lewes pointed this out:
-extracting from a book on the peerage, the names of 16 peers who had,
-at that time, 186 children; giving an average of 11·6 in a family.
-
-Mr. Doubleday insists much on the support given to his theory by the
-barrenness of very luxuriant plants, and the fruitfulness produced
-in plants by depletion. Had he been aware that the change from
-barrenness to fruitfulness in plants, is a change from agamogenesis to
-gamogenesis--had it been as well known at the time when he wrote as it
-is now, that a tree which goes on putting out sexless shoots, is thus
-producing new individuals; and that when it begins to bear fruit, it
-simply begins to produce new individuals after another manner--he would
-have perceived that facts of this class do not tell in his favour.
-
-In the law which Mr. Doubleday alleges, he sees a guarantee for the
-maintenance of species. He argues that the plethoric state of the
-individuals constituting any race of organisms, presupposes conditions
-so favourable to life that the race can be in no danger; and that
-rapidity of multiplication becomes needless. Conversely, he argues
-that a deplethoric state implies unfavourable conditions--implies,
-consequently, unusual mortality; that is--implies a necessity for
-increased fertility to prevent the race from dying out. It may be
-readily shown, however, that such an arrangement would be the reverse
-of self-adjusting. Suppose a species, too numerous for its food, to
-be in the resulting deplethoric state. It will, according to Mr.
-Doubleday, become unusually fertile; and the next generation will be
-more numerous rather than less numerous. For, by the hypothesis, the
-unusual fertility due to the deplethoric state, is the cause of undue
-increase of population. But if the next generation is more numerous
-while the supply of food has not increased in proportion, then this
-next generation will be in a still more deplethoric state, and will be
-still more fertile. Thus there will go on an ever-increasing rate of
-multiplication, and an ever-decreasing share of food, for each person,
-until the species disappears. Suppose, on the other hand, the members
-of a species to be in an unusually plethoric state. Their rate of
-multiplication, ordinarily sufficient to maintain their numbers, will
-become insufficient to maintain their numbers. In the next generation,
-therefore, there will be fewer to eat the already abundant food, which
-becoming relatively still more abundant, will render the fewer members
-of the species still more plethoric, and still less fertile, than their
-parents. And the actions and reactions continuing, the species will
-presently die out from absolute barrenness.
-
-[67] A good deal of this chapter retains its original form; and the
-above paragraph is reprinted verbatim from the _Westminster Review_
-for April, 1852, in which the views developed in the foregoing hundred
-pages were first sketched out. This paragraph shows how near one may
-be to a great generalization without seeing it. Though the struggle
-for life is the alleged motive force; though the process of natural
-selection is recognized; and though to it is ascribed a share in the
-evolution of a higher type; yet the conception is not that which Mr.
-Darwin has worked out with such wonderful skill and knowledge. In the
-first place, natural selection is here described only as furthering
-direct adaptation--only as aiding progress by the preservation of
-individuals in whom functionally-produced modifications have gone on
-most favourably. In the second place, there is no trace of the idea
-that natural selection may by co-operation with the cause assigned, or
-with other causes, produce _divergences_ of structure; and of course,
-in the absence of this idea, there is no implication that natural
-selection has anything to do with the origin of species. And in the
-third place, the all-important factor of variation--“spontaneous,”
-or incidental as we may otherwise call it--is wholly ignored. Though
-use and disuse are, I think, much more potent causes of organic
-modification than Mr. Darwin supposes--though, while pursuing the
-inquiry in detail, I have been led to believe that direct equilibration
-has played a more active part even than I had myself at one time
-thought; yet I hold Mr. Darwin to have shown beyond question, that a
-great part of the facts--perhaps the greater part--are explicable only
-as resulting from the survival of individuals which have deviated in
-some indirectly-caused way from the ancestral type. Thus, the above
-paragraph contains merely a passing recognition of the selective
-process; and indicates no suspicion of the enormous range of its
-effects, or of the conditions under which a large part of its effects
-are produced.
-
-[68] For the information of those who may wish to examine metamorphoses
-of these kinds, I may here state that I have found nearly all the
-examples described, in the neighbourhood of the sea--the last-named,
-on the shore of Locheil, near Fort William. Whether it is that I have
-sought more diligently for cases when in such localities, or whether
-it is that the sea-air favours that excessive nutrition whence these
-transformations result, I am unable to say.
-
-[69] These two dyes have affinities for different components of the
-tissues, and may be advantageously used in different cases. Magenta
-is rapidly taken up by woody matter and other secondary deposits;
-while logwood colours the cell-membranes, and takes but reluctantly
-to the substances seized by magenta. By trying both of them on the
-same structure, we may guard ourselves against any error arising from
-selective combination.
-
-[70] Those who repeat these experiments must be prepared for great
-irregularities in the rates of absorption. Succulent structures in
-general absorb much more slowly than others, and sometimes will
-scarcely take up the dye at all. The differences between different
-structures, and the same structure at different times, probably depend
-on the degrees in which the tissues are charged with liquid and the
-rates at which they are losing it by evaporation.
-
-[71] It may be added here that, on considering the mechanical actions
-that must go on, we are enabled in some measure to understand both
-how such inosculating channels are initiated, and how the structures
-of their component cells are explicable. What must happen to one of
-these elongated prosenchyma-cells if, in the course of its development,
-it is subject to intermittent compressions? Its squeezed-out liquid
-while partially escaping laterally, will more largely escape upwards
-and downwards; and while repeated lateral escape will tend to form
-lateral channels communicating with laterally-adjacent cells, repeated
-longitudinal escape will tend to form channels communicating with
-longitudinally-adjacent cells--so producing continuous though irregular
-longitudinal canals. Meanwhile each cell into and out of which the
-nutritive liquid is from time to time squeezed through small openings
-in its walls, cannot thicken internally in an even manner: deposition
-will be interfered with by the passage of the currents through
-the pores. The rush to or from each pore will tend to maintain a
-funnel-shaped depression in the deposit around; and the opening from
-cell to cell will so acquire just that shape which the microscope
-shows up--two hollow cones with their apices meeting at the point
-where the cell-membranes are in contact. Moreover, as confirming this
-interpretation, it may be remarked that we are thus supplied with a
-reason for the differences of shape between these passages from one
-pitted cell to another, and the analogous passages that exist between
-cells otherwise formed and otherwise conditioned. In the cells of the
-medulla, and others which are but little exposed to compression, the
-passages are severally formed more like a tube with two trumpet-mouths,
-one in each cell. This is just the form which might be expected where
-the nutritive fluid passes from cell to cell in moderate currents,
-and not by the violent rushes caused by intermittent pressures. Of
-course it is not meant that in each individual cell these structures
-are determined by these mechanical actions. The facts clearly negative
-any such conclusion, showing us, as they in many cases do, that these
-structures are assumed in advance of these mechanical actions. The
-implication is, that such mechanical actions initiated modifications
-that have, with the aid of natural selection, been accumulated
-from generation to generation; until, in conformity with ordinary
-embryological laws, the cells of the parts exposed to such actions
-assume these special structures irrespective of the actions--the
-actions, however, still serving to aid and complete the assumption of
-the inherited type.
-
-[72] Some exceptions to this occur in plants that have retrograded
-in the character of their tissues towards the simpler vegetal types.
-Certain very succulent leaves, such as those of _Sempervivum_, in which
-the cellular tissue is immensely developed in comparison with the
-vascular tissue, seem to have resumed to a considerable extent what
-we must regard as the primitive form of vegetal circulation--simple
-absorption from cell to cell. These, when they have lost much of their
-water, will take up the dye to some distance through their general
-substance, or rather through its interstices, even neglecting the
-vessels. At other times, in the same leaves, the vessels will become
-charged while comparatively little absorption takes place through the
-cellular tissue. Even in these exceptional cases, however, the movement
-through cellular tissue is nothing like as fast as the movement through
-vessels.
-
-[73] It seems probable, however, that osmotic distention is here,
-especially, the more important of the two factors. The rising of the
-sap in spring may indirectly result, like the sprouting of the seed,
-from the transformation of starch into sugar. During germination, this
-change of an oxy-hydro-carbon from an insoluble into a soluble form,
-leads to rapid endosmose; consequently to great distention of the seed;
-and therefore to a force which thrusts the contained liquids into the
-plumule and radicle, and gives them power to displace the soil in their
-way: it sets up an active internal movement when neither evaporation
-nor the change which light produces can be operative. And similarly,
-if, in the spring, the starch stored-up in the roots of a tree passes
-into the form of sugar, the unusual osmotic absorption that arises will
-cause an unusual distention--a distention which, being resisted by the
-tough bark of the roots and stem, will result in a powerful upward
-thrust of the contained liquid.
-
-
-Transcriber’s Note:
-
-1. Obvious printers’, spelling and punctuation errors have been
-silently corrected.
-
-2. Where appropriate, original spelling has been retained.
-
-3. Both hyphenated and non-hyphenated versions of the same words have been
-retained where deemed appropriate.
-
-4. Superscripts are represented using the caret character, e.g. D^r.
-
-5. Italics are shown as _xxx_, bold print is shown as =xxx=.
-
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-<p style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of The Principles of Biology, Volume 2 (of 2), by Herbert Spencer</p>
-<div style='display:block; margin:1em 0'>
-This eBook is for the use of anyone anywhere in the United States and
-most other parts of the world at no cost and with almost no restrictions
-whatsoever. You may copy it, give it away or re-use it under the terms
-of the Project Gutenberg License included with this eBook or online
-at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you
-are not located in the United States, you will have to check the laws of the
-country where you are located before using this eBook.
-</div>
-
-<p style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: The Principles of Biology, Volume 2 (of 2)</p>
-<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Author: Herbert Spencer</p>
-<p style='display:block; text-indent:0; margin:1em 0'>Release Date: January 30, 2022 [eBook #67282]</p>
-<p style='display:block; text-indent:0; margin:1em 0'>Language: English</p>
-  <p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em; text-align:left'>Produced by: Karin Spence, Tim Lindell, Adrian Mastronardi, Josep Cols Canals and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive/American Libraries.)</p>
-<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK THE PRINCIPLES OF BIOLOGY, VOLUME 2 (OF 2) ***</div>
-
-
-
-
-<h1>THE PRINCIPLES OF<br />
-BIOLOGY</h1>
-
-
-<p class="center p-left sm p4 ">BY</p>
-
-<p class="center p-left">HERBERT SPENCER</p>
-
-
-<p class="center p-left sm p4"><i>IN TWO VOLUMES</i><br />
-VOL. II</p>
-
-<p class="center p-left p2 xs">REVISED AND ENLARGED EDITION<br />
-1899</p>
-
-<p class="center p-left">NEW YORK<br />
-D. APPLETON AND COMPANY<br />
-1900</p>
-
-<div class="chapter">
-
-<p class="center p-left xs p6"><span class="smcap">Copyright</span>, 1867, 1899,<br />
-<span class="smcap">By</span> D. APPLETON AND COMPANY.</p></div>
-
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_v">[v]</span></p>
-
-<h2 class="smaller">PREFACE<br />
-<span class="subhed">TO THE REVISED AND ENLARGED EDITION OF VOL. II.</span></h2></div>
-
-<p>To the statements made in the preface to the first volume of this
-revised edition, there must here be added a few having special
-reference to this second volume.</p>
-
-<p>One of them is that the revision has not been carried out in quite the
-same way, but in a way somewhat less complete. When reviewing the first
-volume a friendly critic, Prof. Lloyd Morgan, said:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“But though the intellectual weight has also been augmented,
-it is an open question whether it would not have been wiser
-to leave intact a treatise, &amp;c... relegating corrections and
-additions to notes and appendices.”</p>
-</div>
-
-<p>I think that Prof. Morgan is right. Though at the close of the preface
-to volume I, I wrote:&mdash;“in all sections not marked as new, the
-essential ideas set forth are the same as they were in the original
-edition of 1864,” yet the reader who has not read this statement,
-or does not bear it in mind, will suppose that all or most of the
-enunciated conceptions are of recent date, whereas only a small part of
-them are. I have therefore decided to follow, in this second volume, a
-course somewhat like that suggested by Prof. Morgan&mdash;somewhat like, I
-say, because in sundry cases the amendments could not be satisfactorily
-made by appended notes.</p>
-
-<p><span class="pagenum" id="Page_vi">[vi]</span></p>
-
-<p>But there has been a further reason for this change of method. An
-invalid who is nearly eighty cannot with prudence enter upon work which
-will take long to complete. Hence I have thought it better to make the
-needful alterations and additions in ways requiring relatively moderate
-time and labour.</p>
-
-<p>The additions made to this volume are less numerous and less important
-than those made to the first volume. A new chapter ending Part V, on
-“The Integration of the Organic World,” serves to round off the general
-theory of Evolution in its application to living things. Beyond a new
-section (<a href="#sect289">§ 289</a><i>a</i>) and the various foot-notes, serving chiefly the
-purpose of elucidation, there are notes of some significance appended
-to Chapters I, III, IV, and V, in Part IV, Chapters V and VIII, in Part
-V, and Chapters IX, X, and XII in Part VI. Moreover there are three
-further appendices, D<sup>2</sup>, F, and G, which have, I think, considerable
-significance as serving to make clearer some of the views expressed in
-the body of the work.</p>
-
-<p>Turning from the additions to the revisions, I have to say that the
-aid needed for bringing up to date the contents of this volume, has
-been given me by the gentlemen who gave me like aid in revising the
-first volume: omitting Prof. Perkin, within whose province none of the
-contents of this volume fall. Plant-Morphology and Plant-Physiology
-have been overseen by Mr. A. G. Tansley. Criticisms upon parts dealing
-with Animal Morphology I owe to Mr. J. T. Cunningham and Prof. E. W.
-MacBride. And the statements included under Animal Physiology have been
-checked by Mr. W. B. Hardy.</p>
-
-<p><span class="pagenum" id="Page_vii">[vii]</span></p>
-
-<p>For reasons like those named in the preface to the first volume, I
-have not submitted the proofs of this revised second volume to these
-gentlemen: a fact which it is needful to name, since one or other of
-them might else be held responsible for some error which is not his but
-mine. It is the more requisite to say this because while, in respect
-of matters of fact, I have, save in one or two cases, accepted their
-corrections as not to be questioned, I have not always done this in
-respect of matters of inference, but in sundry places have adhered to
-my own interpretations.</p>
-
-<p>Perhaps I may be excused for expressing some satisfaction that I
-have not been obliged to relinquish the views set forth in 1864–7.
-The hypothesis of physiological units&mdash;or, as I would now call them,
-constitutional units&mdash;has been adopted by several zoologists under
-modified forms. So far as I am aware, the alleged general law of
-organic symmetry has not called forth any manifestations of dissent.
-The suggested theory of vertebrate structure appears to have become
-current; and from the investigations of the late Prof. Cope, has
-received verification. The conclusions drawn in Part VI on “The Laws
-of Multiplication,” have not, I believe, been controverted. And though
-only some works on botany have given currency to the doctrine set forth
-in Appendix C, “On Circulation and the Formation of Wood in Plants,”
-yet I have met with no attempt to disprove it. The only views contested
-by certain of the gentlemen above named, are those concerning the
-origin of the two great phænogamic types of plants, and the origin of
-the annulose type of animals. I have not, however,&mdash;perhaps because
-of natural bias&mdash;found myself compelled<span class="pagenum" id="Page_viii">[viii]</span> to surrender these views. My
-reasons for adhering to them will be found in notes to the ends of
-Chapters III and IV in Part IV, and in Appendix D<sup>2</sup>.</p>
-
-<p>On now finally leaving biological studies, it remains only to say that
-I am glad I have survived long enough to give this work its finished
-form.</p>
-
-<p><span class="smcap">Brighton</span>,<br />
-
-<span class="left2"><i>October, 1899</i>.</span></p>
-
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_ix">[ix]</span></p>
-
-<h2 class="smaller">PREFACE TO VOL. II.</h2>
-</div>
-
-
-<p>The proof sheets of this volume, like those of the last volume, have
-been looked through by Dr. Hooker and Prof. Huxley; and I have, as
-before, to thank them for their valuable criticisms, and for the
-trouble they have taken in checking the numerous statements of fact on
-which the arguments proceed. The consciousness that their many duties
-render time extremely precious to them, makes me feel how heavy is my
-obligation.</p>
-
-<p>Part IV., with which this volume commences, contains numerous figures.
-Nearly one half of them are repetitions, mostly altered in scale and
-simplified in execution, of figures, or parts of figures, contained
-in the works of various Botanists and Zoologists. Among the authors
-whom I have laid under contribution, I may name Berkeley, Carpenter,
-Cuvier, Green, Harvey, Hooker, Huxley, Milne-Edwards, Ralfs, Smith.
-The remaining figures, numbering 150, are from original sketches and
-diagrams.</p>
-
-<p>The successive instalments which compose this volume, were issued
-to the Subscribers at the following dates:&mdash;No. 13 (pp. 1–80) in
-January, 1865; No. 14 (pp. 81–160) in June, 1865; No. 15 (pp. 161–240)
-in December, 1865; No. 16 (pp. 241–320) in June, 1866; No. 17 (pp.
-321–400) in November, 1866; and No. 18 (pp. 401–566) in March, 1867.</p>
-
-<p><span class="smcap">London</span>, <i>March 23rd, 1867</i>.</p>
-
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_xi">[xi]</span></p>
-
-<h2>CONTENTS OF VOL. II.</h2>
-</div>
-
-<table summary="contents" class="smaller">
-  <tr>
-    <td class="header" colspan="3">PART IV.&mdash;MORPHOLOGICAL DEVELOPMENT.</td>
-  </tr>
-
-  <tr>
-    <th class="chap">CHAP.</th>
-    <th></th>
-    <th class="pag">PAGE</th>
-  </tr>
-
-  <tr>
-    <td class="chn">I.&mdash;</td>
-    <td class="cht">The problems of morphology</td>
-    <td class="pag"><a href="#Page_3">3</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">II.&mdash;</td>
-    <td class="cht">The morphological composition of plants</td>
-    <td class="pag"><a href="#Page_17">17</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">III.&mdash;</td>
-    <td class="cht1"><span class="smcap">The morphological composition of plants</span>&mdash;<i>Continued</i></td>
-    <td class="pag"><a href="#Page_37">37</a></td>
-  </tr>
-
-
-  <tr>
-    <td class="chn">IV.&mdash;</td>
-    <td class="cht">The morphological composition of animals</td>
-    <td class="pag"><a href="#Page_85">85</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">V.&mdash;</td>
-    <td class="cht1"><span class="smcap">The morphological composition of animals</span>&mdash;<i>Continued</i></td>
-    <td class="pag"><a href="#Page_111">111</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">VI.&mdash;</td>
-    <td class="cht">Morphological differentiation in plants</td>
-    <td class="pag"><a href="#Page_128">128</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">VII.&mdash;</td>
-    <td class="cht">The general shapes of plants</td>
-    <td class="pag"><a href="#Page_134">134</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">VIII.&mdash;</td>
-    <td class="cht">The shapes of branches</td>
-    <td class="pag"><a href="#Page_145">145</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">IX.&mdash;</td>
-    <td class="cht">The shapes of leaves</td>
-    <td class="pag"><a href="#Page_152">152</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">X.&mdash;</td>
-    <td class="cht">The shapes of flowers</td>
-    <td class="pag"><a href="#Page_161">161</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">XI.&mdash;</td>
-    <td class="cht">The shapes of vegetal cells</td>
-    <td class="pag"><a href="#Page_175">175</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">XII.&mdash;</td>
-    <td class="cht">Changes of shape otherwise caused</td>
-    <td class="pag"><a href="#Page_178">178</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">XIII.&mdash;</td>
-    <td class="cht">Morphological differentiation in animals</td>
-    <td class="pag"><a href="#Page_183">183</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">XIV.&mdash;</td>
-    <td class="cht">The general shapes of animals</td>
-    <td class="pag"><a href="#Page_186">186</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">XV.&mdash;</td>
-    <td class="cht">The shapes of vertebrate skeletons</td>
-    <td class="pag"><a href="#Page_209">209</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">XVI.&mdash;</td>
-    <td class="cht">The shapes of animal cells</td>
-    <td class="pag"><a href="#Page_228">228</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">XVII.&mdash;</td>
-    <td class="cht">Summary of morphological development</td>
-    <td class="pag"><a href="#Page_231">231</a></td>
-  </tr>
-
-  <tr>
-    <td class="header" colspan="3">PART V.&mdash;PHYSIOLOGICAL DEVELOPMENT.</td>
-  </tr>
-
-  <tr>
-    <td class="chn">I.&mdash;</td>
-    <td class="cht">The problems of physiology</td>
-    <td class="pag"><a href="#Page_239">239</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">II.&mdash;</td>
-    <td class="cht">Differentiations between the outer and inner tissues of plants</td>
-    <td class="pag"><a href="#Page_244">244</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">III.&mdash;</td>
-    <td class="cht">Differentiations among the outer tissues of plants</td>
-    <td class="pag"><a href="#Page_251">251</a><span class="pagenum" id="Page_xii">[xii]</span></td>
-  </tr>
-
-  <tr>
-    <td class="chn">IV.&mdash;</td>
-    <td class="cht">Differentiations among the inner tissues of plants</td>
-    <td class="pag"><a href="#Page_272">272</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">V.&mdash;</td>
-    <td class="cht">Physiological integration in Plants</td>
-    <td class="pag"><a href="#Page_292">292</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">VI.&mdash;</td>
-    <td class="cht">Differentiations between the outer and inner tissues of animals</td>
-    <td class="pag"><a href="#Page_299">299</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">VII.&mdash;</td>
-    <td class="cht">Differentiations among the outer tissues of animals</td>
-    <td class="pag"><a href="#Page_309">309</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">VIII.&mdash;</td>
-    <td class="cht">Differentiations among the inner tissues of animals</td>
-    <td class="pag"><a href="#Page_323">323</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">IX.&mdash;</td>
-    <td class="cht">Physiological integration in animals</td>
-    <td class="pag"><a href="#Page_373">373</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">X.&mdash;</td>
-    <td class="cht">Summary of physiological development</td>
-    <td class="pag"><a href="#Page_384">384</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">X<sup>A</sup>.&mdash;</td>
-    <td class="cht">The integration of the organic world</td>
-    <td class="pag"><a href="#Page_396">396</a></td>
-  </tr>
-
-  <tr>
-    <td class="header" colspan="3">PART VI.&mdash;LAWS OF MULTIPLICATION.</td>
-  </tr>
-
-  <tr>
-    <td class="chn">I.&mdash;</td>
-    <td class="cht">The factors</td>
-    <td class="pag"><a href="#Page_411">411</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">II.&mdash;</td>
-    <td class="cht"><i>À PRIORI</i> principle</td>
-    <td class="pag"><a href="#Page_417">417</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">III.&mdash;</td>
-    <td class="cht">Obverse <i>À PRIORI</i> principle</td>
-    <td class="pag"><a href="#Page_424">424</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">IV.&mdash;</td>
-    <td class="cht">Difficulties of inductive verification</td>
-    <td class="pag"><a href="#Page_432">432</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">V.&mdash;</td>
-    <td class="cht">Antagonism between growth and asexual genesis</td>
-    <td class="pag"><a href="#Page_439">439</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">VI.&mdash;</td>
-    <td class="cht">Antagonism between growth and sexual genesis</td>
-    <td class="pag"><a href="#Page_448">448</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">VII.&mdash;</td>
-    <td class="cht">The antagonism between development and genesis, asexual and sexual</td>
-    <td class="pag"><a href="#Page_461">461</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">VIII.&mdash;</td>
-    <td class="cht">Antagonism between expenditure and genesis</td>
-    <td class="pag"><a href="#Page_467">467</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">IX.&mdash;</td>
-    <td class="cht">Coincidence between high nutrition and genesis</td>
-    <td class="pag"><a href="#Page_475">475</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">X.&mdash;</td>
-    <td class="cht">Specialities of these relations</td>
-    <td class="pag"><a href="#Page_486">486</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">XI.&mdash;</td>
-    <td class="cht">Interpretation and qualification</td>
-    <td class="pag"><a href="#Page_497">497</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">XII.&mdash;</td>
-    <td class="cht">Multiplication of the human race</td>
-    <td class="pag"><a href="#Page_506">506</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn">XIII.&mdash;</td>
-    <td class="cht">Human population in the future</td>
-    <td class="pag"><a href="#Page_522">522</a></td>
-  </tr>
-
-  <tr>
-    <td class="header" colspan="3">APPENDICES.</td>
-  </tr>
-
-  <tr>
-    <td class="chn1">A.&mdash;</td>
-    <td class="cht">Substitution of axial for foliar organs in plants</td>
-    <td class="pag"><a href="#Page_541">541</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn1">B.&mdash;</td>
-    <td class="cht">A criticism on Prof. Owen’s theory of the vertebrate skeleton</td>
-    <td class="pag"><a href="#Page_548">548</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn1">C.&mdash;</td>
-    <td class="cht">On circulation and the formation of wood in plants</td>
-    <td class="pag"><a href="#Page_567">567</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn1">D.&mdash;</td>
-    <td class="cht">On the origin of the vertebrate type</td>
-    <td class="pag"><a href="#Page_599">599</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn1">D<sup>2</sup>.&mdash;</td>
-    <td class="cht">The annulose type</td>
-    <td class="pag"><a href="#Page_602">602</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn1">E.&mdash;</td>
-    <td class="cht">The shapes and arrangements of flowers</td>
-    <td class="pag"><a href="#Page_608">608</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn1">F.&mdash;</td>
-    <td class="cht">Physiological (or constitutional) units</td>
-    <td class="pag"><a href="#Page_612">612</a></td>
-  </tr>
-
-  <tr>
-    <td class="chn1">G.&mdash;</td>
-    <td class="cht">The inheritance of functionally-caused modifications</td>
-    <td class="pag"><a href="#Page_618">618</a></td>
-  </tr>
-</table>
-
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-
-<h2>PART IV.<br />
-<span class="subhed">MORPHOLOGICAL DEVELOPMENT.</span></h2></div>
-
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_3">[3]</span></p>
-
-<h3>CHAPTER I.<br />
-<span class="subhed1">THE PROBLEMS OF MORPHOLOGY.</span></h3></div>
-
-<p class="p2">§ 175. The division of Morphology from Physiology, is one which may
-be tolerably-well preserved so long as we do not carry our inquiries
-beyond the empirical generalizations of their respective phenomena; but
-it is one which becomes in great measure nominal, when the phenomena
-are to be rationally interpreted. It would be possible, after analyzing
-our Solar System, to set down certain general truths respecting the
-sizes and distances of its primary and secondary members, omitting
-all mention of their motions; and it would be possible to set down
-certain other general truths respecting their motions, without
-specifying their dimensions or positions, further than as greater
-or less, nearer or more remote. But on seeking to account for these
-general truths, arrived at by induction, we find ourselves obliged to
-consider simultaneously the relative sizes and places of the masses,
-and the relative amounts and directions of their motions. Similarly
-with organisms. Though we may frame sundry comprehensive propositions
-respecting the arrangements of their organs, considered as so many
-inert parts; and though we may establish several wide conclusions
-respecting the separate and combined actions of their organs, without
-knowing anything definite respecting the forms and positions of these
-organs; yet we cannot reach such a rationale of the facts as<span class="pagenum" id="Page_4">[4]</span> the
-hypothesis of Evolution aims at, without contemplating structures and
-functions in their mutual relations. Everywhere structures in great
-measure determine functions; and everywhere functions are incessantly
-modifying structures. In Nature the two are inseparable co-operators;
-and Science can give no true interpretation of Nature without keeping
-their co-operation constantly in view. An account of organic evolution,
-in its more special aspects, must be essentially an account of the
-interactions of structures and functions, as perpetually altered by
-changes of conditions.</p>
-
-<p>Hence, when treating apart Morphological Development and Physiological
-Development, all we can do is to direct our attention mainly to the
-one or to the other, as the case may be. In dealing with the facts of
-structure, we must consider the facts of function only in such general
-way as is needful to explain the facts of structure; and conversely
-when dealing with the facts of function.</p>
-
-
-<p class="p2">§ 176. The problems of Morphology fall into two distinct classes,
-answering respectively to the two leading aspects of Evolution. In
-things which evolve there go on two processes&mdash;increase of mass
-and increase of structure. Increase of mass is primary, and in
-simple evolution takes place almost alone. Increase of structure is
-secondary, accompanying or following increase of mass with more or
-less regularity, wherever evolution rises above that form which small
-inorganic bodies, such as crystals, present to us. As the fundamental
-antagonism between Dissolution and Evolution consists in this, that
-while the one is an integration of motion and disintegration of matter,
-the other is an integration of matter and disintegration of motion; and
-as this integration of matter accompanying disintegration of motion,
-is a necessary antecedent to the differentiation of the matter so
-integrated; it follows that questions concerning the mode in which the
-parts are united into a whole, must be dealt with before<span class="pagenum" id="Page_5">[5]</span> questions
-concerning the mode in which these parts become modified.<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">[1]</a></p>
-
-<p>This is not obviously a morphological question. But an illustration or
-two will make it manifest that fundamental differences may be produced
-between aggregates by differences in the degrees of composition of the
-increments: the ultimate units of the increments being the same. Thus
-an accumulation of things of a given kind may be made by adding one
-at a time. Or the things may be tied up into bundles of ten, and the
-tens placed together. Or the tens may be united into hundreds, and a
-pile of hundreds formed. Such unlikenesses in the structures of masses
-are habitually seen in our mercantile transactions. Articles which
-the consumer recognizes as single, the retailer keeps wrapped up in
-dozens, the wholesaler sends in gross, and the manufacturer supplies
-in packages of a hundred gross. That is, they severally increase
-their stocks by units of simple, of compound, and of doubly-compound
-kinds. Similarly result those differences of morphological composition
-which we have first to consider. An organism consists of units.
-These units may be aggregated into a mass by the addition of unit
-to unit. Or they may be united into groups, and the groups joined
-together. Or these groups of groups may be so combined as to form a
-doubly-compound aggregate. Hence there arises respecting each organic
-form the question&mdash;is its composition of the first, second, third,
-or fourth order?&mdash;does it exhibit units of a singly-compounded kind
-only, or are these consolidated into units of a doubly-compounded
-kind, or a triply-compounded kind? And if it displays double or triple
-composition,<span class="pagenum" id="Page_6">[6]</span> the homologies of its different parts become problems.
-Under the disguises induced by the consolidation of primary, secondary,
-and tertiary units, it has to be ascertained which answer to which, in
-their degrees of composition.</p>
-
-<p>Such questions are more intricate than they at first appear; since,
-besides the obscurities caused by progressive integration, and those
-due to accompanying modifications of form, further obscurities
-result from the variable growths of units of the different orders.
-Just as an army may be augmented by recruiting each company, without
-increasing the number of companies; or may be augmented by making up
-the full complement of companies in each regiment, while the number
-of regiments remains the same; or may be augmented by putting more
-regiments into each division, other things being unchanged; or may be
-augmented by adding to the number of its divisions without altering
-the components of each division; or may be augmented by two or three
-of these processes at once; so, in organisms, increase of mass may
-result from additions of units of the first order, or those of the
-second order, or those of still higher orders; or it may be due to
-simultaneous additions to units of several orders. And this last mode
-of integration being the general mode, puts difficulties in the way of
-analysis. Just as the structure of an army would be made less easy to
-understand if companies often outgrew regiments, or regiments became
-larger than brigades; so these questions of morphological composition
-are complicated by the indeterminate sizes of the units of each
-kind: relatively-simple units frequently becoming more bulky than
-relatively-compound units.</p>
-
-
-<p class="p2">§ 177. The morphological problems of the second class are those having
-for their subject-matter the changes of shape which accompany changes
-of aggregation. The most general questions respecting the structure
-of an organism, having been answered when it is ascertained of what
-units it is composed as a whole, and in its several parts; there come
-the more<span class="pagenum" id="Page_7">[7]</span> special questions concerning its form&mdash;form in the ordinary
-sense. After the contrasts caused by variations in the process of
-integration, we have to consider the contrasts caused by variations
-in the process of differentiation. To speak specifically&mdash;the shape
-of the organism as a whole, irrespective of its composition, has to
-be accounted for. Reasons have to be found for the unlikeness between
-its general outlines and the general outlines of allied organisms. And
-there have to be answered kindred inquiries respecting the proportions
-of its component parts:&mdash;Why, among such of these as are homologous
-with one another, have there arisen the differences that exist? And how
-have there been produced the contrasts between them and the homologous
-parts of organisms of the same type?</p>
-
-<p>Very numerous are the heterogeneities of form presenting themselves
-for interpretation under these heads. The ultimate morphological
-units combined in any group, may be differentiated individually, or
-collectively, or both: each of them may undergo changes of shape;
-or some of them may be changed and others not; or the group may be
-rendered multiform by the greater growth of some of its units than of
-others. Similarly with the compound units arising by union of these
-simple units. Aggregates of the second order may be made relatively
-complex in form, by inequalities in the rates of multiplication of
-their component units in diverse directions; and among a number of such
-aggregates, numerous unlikenesses may be constituted by differences in
-their degrees of growth, and by differences in their modes of growth.
-Manifestly, at each higher stage of composition the possible sources of
-divergence are multiplied still further.</p>
-
-<p>That facts of this order can be accounted for in detail is not to
-be expected&mdash;the data are wanting. All that we may hope to do is to
-ascertain their general laws. How this is to be attempted we will now
-consider.</p>
-
-
-<p class="p2">§ 178. The task before us is to trace throughout these<span class="pagenum" id="Page_8">[8]</span> phenomena
-the process of evolution; and to show how, as displayed in them, it
-conforms to those first principles which evolution in general conforms
-to. Two sets of factors have to be taken into account. Let us look at
-them.</p>
-
-<p>The factors of the first class are those which tend directly to
-change an organic aggregate, in common with every other aggregate,
-from that more simple form which is not in equilibrium with incident
-forces, to that more complex form which is in equilibrium with them.
-We have to mark how, in correspondence with the universal law that
-the uniform lapses into the multiform, and the less multiform into
-the more multiform, the parts of each organism are ever becoming
-further differentiated; and we have to trace the varying relations
-to incident forces by which further differentiations are entailed.
-We have to observe, too, how each primary modification of structure,
-induced by an altered distribution of forces, becomes a parent of
-secondary modifications&mdash;how, through the necessary multiplication of
-effects, change of form in one part brings about changes of form in
-other parts. And then we have also to note the metamorphoses constantly
-being induced by the process of segregation&mdash;by the gradual union
-of like parts exposed to like forces, and the gradual separation of
-like parts exposed to unlike forces. The factors of the second class
-which we have to keep in view throughout our interpretations, are
-the formative tendencies of organisms themselves&mdash;the proclivities
-inherited by them from antecedent organisms, and which past processes
-of evolution have bequeathed. We have seen it to be inferable from
-various orders of facts (§§ <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect65">65</a>, <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect84">84</a>, <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect97">97–97<i>g</i></a>), that organisms are
-built up of certain highly-complex molecules, which we distinguished as
-physiological units [or constitutional units as they might otherwise
-be called]&mdash;each kind of organism being built up of units peculiar to
-itself. We recognized in these units, powers of arranging themselves
-into the forms of the organisms to which they belong; analogous
-to the powers which the molecules of inorganic substances have of
-aggregating into specific crystalline<span class="pagenum" id="Page_9">[9]</span> forms. We have consequently
-to regard this proclivity of the physiological units, as producing,
-during the development of any organism, a combination of internal
-forces that expend themselves in working out a structure in equilibrium
-with the forces to which ancestral organisms were exposed; but not in
-equilibrium with the forces to which the existing organism is exposed,
-if the environment has been changed. Hence the problem in all cases is,
-to ascertain the resultant of internal organizing forces, tending to
-reproduce the ancestral form, and external modifying forces, tending
-to cause deviations from that form. Moreover, we have to take into
-account, not only the characters of immediately-preceding ancestors,
-but also those of their ancestors, and ancestors of all degrees of
-remoteness. Setting out with rudimentary types, we have to consider
-how, in each successive stage of evolution, the structures acquired
-during previous stages have been obscured by further integrations
-and further differentiations; or, conversely, how the lineaments of
-primitive organisms have all along continued to manifest themselves
-under the superposed modifications.</p>
-
-
-<p class="p2">§179. Two ways of carrying on the inquiry suggest themselves. We may
-go through the several great groups of organisms, with the view of
-reaching, by comparison of parts, certain general truths respecting
-the homologies, the forms, and the relations of their parts; and then,
-having dealt with the phenomena inductively, may retrace our steps with
-the view of deductively interpreting the general truths reached. Or,
-instead of thus separating the two investigations, we may carry them on
-hand in hand&mdash;first establishing each general truth empirically, and
-then proceeding to the rationale of it. This last method will, I think,
-conduce to both brevity and clearness. Let us now thus deal with the
-first class of morphological problems.</p>
-
-
-<p>[<span class="smcap">Note.</span>&mdash;In preparation for treating of morphological
-development,<span class="pagenum" id="Page_10">[10]</span> sundry other general considerations should have been
-included in the foregoing chapter when originally published. This seems
-the most appropriate place for now naming them. Some were implicitly
-contained in the first volume, but it will be well definitely to state
-these, as well as the others not yet implied.</p>
-
-<p>Interpretation of the forms of organisms and the forms of their parts,
-must depend mainly on the conclusions previously drawn respecting
-their phylogeny; and the drawing of such conclusions must be guided
-by recognition of the various factors of Evolution, as well as by
-recognition of certain extremely general results of Evolution and
-certain concomitants of Evolution.</p>
-
-<p>A primary one among these is that no existing species can exhibit
-more than approximately the ancestral structure of any other existing
-species. As all ancestors have disappeared, so, in a greater or less
-degree, the traits, specific, generic, or ordinal, which distinguished
-the earlier of them have disappeared. Setting out with the familiar
-symbol, a tree, let us regard its peripheral twigs as representing
-extant species; let us assume that the interior of the tree is filled
-up with some supporting substance, leaving only the ends of the
-living twigs projecting; and let us suppose the trunk, main branches,
-secondary branches, tertiary branches, &amp;c., have decayed away.
-Then if we take these decayed parts to stand for the divergent and
-re-divergent lines of evolution which are represented by fossils in the
-Earth’s crust, it will be manifest, first, that no one of the living
-superficial twigs (or species) exhibits the ancestral organization
-whence any other of the living superficial twigs (or species) has been
-developed; it will be manifest, second, that the generic structure
-inherited by any existing species must be a structure out of which
-came sundry allied species&mdash;the fork, as it were, at which adjacent
-twigs diverged; and third, that the ancestor of an order must, in like
-manner, be sought at some point deeper down in the symbolic tree&mdash;a
-place of divergence of<span class="pagenum" id="Page_11">[11]</span> the sub-branches representing allied genera.
-Similarly with the ancestral types of classes, still deeper down in
-the tree or further back in time. So that phylogeny becomes more and
-more speculative as its questions become more and more radical. And
-the difficulty is made greater by the deficiency of palæontological
-evidence.</p>
-
-<p>One obvious corollary is that an ancestral type from which sundry
-allied types now existing diverged, was, speaking generally, simpler
-than these; since the divergent types became different by the
-superposing of modifications, adding to their complexities. There is
-a further reason for inferring that the least specialized member of
-any group is more like the remote ancestor than any of the others; for
-every adaptation stands in the way of subsequent re-adaptations: it
-presents a greater amount of structure to be undone. To get some idea
-of the ancestral type where no extant member of the group is manifestly
-simpler than the rest, the method must be to take all its extant
-members and, after letting their differences mutually cancel, observe
-what remains common to them all.</p>
-
-<p>But there are difficulties standing in the way of phylogeny, and
-consequently of morphology, much greater than these. Returning to
-our symbolic tree, it is clear that it would be far from easy to say
-of any one twig which extinct sub-branch, branch, and main branch it
-belonged to, even supposing that the growths of all parts had been
-uniformly outwards. Immensely more perplexing, then, must be the
-affiliation if various of the branches, sub-branches, &amp;c., have sent
-out backward-growing shoots which have come to the surface only after
-prolonged retrograde courses, and if other branches have sent shoots
-into regions occupied by alien branches&mdash;shoots bearing twigs which
-come to the surface along with those to which they are but remotely
-allied. The problems of origin and of structure which organisms
-present, are met by both of the difficulties thus symbolized.</p>
-
-<p>One of them arises from the prevalence of retrograde<span class="pagenum" id="Page_12">[12]</span> metamorphoses.
-Throughout the animal world these are variously displayed by parasites,
-multitudinous in their kinds; for most of them belong to types
-much higher in organization. Changed habits and consequent changed
-structures have so transferred them that only by study of their
-embryonic stages can their kinships be made out. And these retrograde
-metamorphoses, conspicuous among parasites, have, in the course of
-evolution, affected some members of all groups; for in all groups the
-struggle for existence has compelled some to adopt careers less trying
-but less profitable.</p>
-
-<p>Not only by forcing on many kinds of organisms simpler ways of living,
-and consequent degeneracy, has the universal competition caused
-obscuring transformations. It has done this also by tempting many other
-kinds of organisms to adopt ways of life not simpler than before but
-merely different. Pressure continually prompts every type to intrude on
-other types’ spheres of activity; and so causes it to assume certain
-structural characters of the types whose spheres it invades, masking
-its previous characters. Modifications hence arising have, in the great
-mass of cases, been superposed one on another time after time. The
-aquatic animal becomes through several transitions a land-animal, and
-then the land-animal through other transitions becomes now an aërial
-animal like the bat and now an aquatic animal like the whale. Certain
-kinds of birds furnish extreme illustrations. There was the change from
-the fish to the water-breathing amphibian and then to the air-breathing
-amphibian; thence to the reptile living on the Earth’s surface; thence
-to the flying reptile and the bird; then came the diving birds, joining
-with their aërial life a life passed partly in the water; and finally
-came a type like the penguin, in which the power of flight has been
-lost and the water has again become the almost exclusive medium, except
-for breathing. Of course the mouldings and re-mouldings of structure
-resulting from these successive unlike modes of life, in many cases put
-great difficulties in the way of ascertaining<span class="pagenum" id="Page_13">[13]</span> which are the original
-corresponding parts. Some parts have become abnormally large; others
-have dwindled or disappeared; and the relative positions of parts
-have often been greatly changed. A bat’s wing and a bird’s wing are
-analogous organs, but their frameworks are but partially homologous.
-While in the bird the terminal parts of the fore-limb do little towards
-supporting the wing, in the bat the wing is mainly supported by
-enormously-developed terminal parts.</p>
-
-<p>The effects of the struggle to survive, which here prompts a simpler
-life with resulting degeneracy and there a different life with
-resulting new developments, are far from being the only causes of
-morphological obscurations. Fulfilment of certain highly general
-requirements gives certain common traits to plants of widely divergent
-classes; and fulfilment of certain other highly general requirements
-gives certain common traits to animals of widely divergent classes.
-It was remarked in the first volume (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect54">§ 54<i>f</i></a>) that the cardinal
-distinction between the characters of plants and animals arises from
-the fact that while the chief food of plants is universally present the
-food of animals is scattered. Here it has to be added that to utilize
-the universally distributed food the ordinary plant needs the aid of
-light, and has to acquire structures enabling it to get that aid;
-while the ordinary animal, to utilize the scattered food, must acquire
-the structures needful for locomotion. Let us contemplate separately
-the traits hence resulting in the vegetal world and the traits hence
-resulting in the animal world.</p>
-
-<p>The familiar plantain meets the requirement by growing stiff leaves
-enabling it to press down the competing grasses around which would
-else shade it; but the great majority of ordinary plants meet the
-requirement by raising themselves into the air. Hence the need for a
-stem, and hence the fact that plants of widely unlike natures similarly
-form stems which, in achieving strength enough to support the foliage
-and resist the wind, acquire certain adaptive structures having<span class="pagenum" id="Page_14">[14]</span> a
-general similarity. Here from the edge of a pool is a reed, and here
-from the adjacent copse is a hemlock: the one having grown tall in
-escaping the shade of its companions and the other in escaping the
-shade of the surrounding brushwood. On being cut across each discloses
-a tube, and each exhibits septa dividing this tube into chambers. In
-either case by the tubular structure is gained the greatest strength
-with the least material; but there is no morphological kinship between
-the tubes nor between the septa. Still more marked is the simulation
-of homology by analogy in another plant which the adjacent ditch may
-furnish&mdash;the common Horsetail. In this, again, we see an elongated
-vertical-growing part, raising the foliage into the air; and, as
-before, this is tubular and divided by septa. A type utterly alien from
-the other two has, by survival of the fittest, been similarly moulded
-to meet mechanical needs.</p>
-
-<p>Passing now to the obscurations in the animal world caused
-by alterations favouring locomotion, we note first that the
-locomotive power is at the outset very slight. Among many orders of
-<i>Protozoa</i>, as also among many low types of <i>Metazoa</i>,
-vibratile cilia are the most general agents of locomotion&mdash;necessarily
-feeble locomotion. Regarded in the mass, the <i>Cœlenterata</i>, when
-not stationary like the <i>Hydra</i> or higher types in the hydroid
-stage, usually possess only such small self-mobility as the slow
-rhythmical contractions of their umbrella-disks effect, or else such
-as is effected by bands of cilia or of vibratile plates, as in the
-<i>Beroe</i>. Even among these low tpes of <i>Metazoa</i>, however, in
-which ordinarily the radial structure is conspicuous, or but slightly
-obscured by an ovoid form as in the <i>Ctenophora</i>, we find, in the
-<i>Cestus veneris</i>, extreme obscuration caused by an elongation
-which facilitates movement through the water; alike by the actions of
-its vibratile plates and by its undulations, which simulate those of
-sundry higher animals.</p>
-
-<p>And here we come upon the essential fact to be recognized. Elongation
-favours locomotion in various ways that are<span class="pagenum" id="Page_15">[15]</span> severally taken advantage
-of by different types of creatures. (1) To a given mass of moving
-matter the resistance of the medium decreases along with decrease
-in the area of its transverse section, and this implies increase
-of length: a given force will move the lengthened mass along with
-greater facility. (2) Reaching a certain point the elongated form
-enables an animal to progress by undulations, as in the water fish
-do, and even some cœlenterates and turbellarians do, and as on land
-snakes do: lateral resistances serving in either case as fulcra. (3)
-Lengthening of the body serves otherwise to aid locomotion in the
-creeping or burrowing worm, which, utilizing the statical resistance
-of its hinder part thrusts onwards its fore part, and then, holding
-fast its fore part by the aid of minute <i>setæ</i>, draws the hinder
-part after it. But elongation, doubly advantageous at first, while
-the body is itself the chief instrument of locomotion, gradually
-loses its advantageousness as special instruments of locomotion are
-developed. (4) This we see in that locomotive action effected by limbs,
-which, many and small in the lower <i>Arthropoda</i> and becoming
-few and larger in the higher, at length give great activity to a
-shortened and consolidated body: a stage reached only through stages
-of decreasing elongation accompanying increase of limb-power. (5) In
-the <i>Vertebrata</i> locomotion by undulations comes, along certain
-lines of evolution, to be replaced by that limb locomotion which
-accompanies the rise from water-life to land-life: the evolution of
-Amphibians exhibiting the transition. (6) Further, we see among mammals
-that as limbs become efficient the elongated body ceases to be itself
-instrumental in locomotion, but that still some elongation remains a
-characteristic. (7) Finally, where limb locomotion reaches its highest
-degree, as in birds, elongation disappears.</p>
-
-<p>These classes of familiar facts I have recalled to show that, in
-the course of evolution, achievement by plants of the all-essential
-elevation into the air and by animals of the all-essential power of
-movement have developed this trait<span class="pagenum" id="Page_16">[16]</span> of elongation in various types;
-and that in each kingdom acquisition of the common trait has had a
-tendency now to obscure morphological equivalence, and now to give the
-appearance of kinship where there is none. A further purpose has been
-to prepare the way for a question hereafter to be discussed&mdash;whether,
-in the various types of either kingdom, the elongation is effected
-in the same ways or in different ways. We shall have to ask whether
-the vertically-growing part is always, like that of <i>Lessonia</i>,
-a simple individual, or whether, as possibly in Phænogams, it is a
-united series of individuals; and similarly whether the elongated body
-is always single, like that of a mollusc, or whether, as possibly in
-annulose animals, it is a series of united individuals.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_17">[17]</span></p>
-
-<h3>CHAPTER II.<br />
-<span class="subhed1">THE MORPHOLOGICAL COMPOSITION OF PLANTS.</span></h3></div>
-
-<p id="sect180">§ 180. Evolution implies insensible modifications and gradual
-transitions, which render definition difficult&mdash;which make it
-impossible to separate absolutely the phases of organization from
-one another. And this indefiniteness of distinction, to be expected
-<i>à priori</i>, we are compelled to recognize <i>à posteriori</i>,
-the moment we begin to group morphological phenomena into general
-propositions. Thus, on inquiring what is the morphological unit,
-whether of plants or of animals, we find that the facts refuse to be
-included in any rigid formula. The doctrine that all organisms are
-built up of cells, or that cells are the elements out of which every
-tissue is developed, is but approximately true. There are living
-forms of which cellular structure cannot be asserted; and in living
-forms that are for the most part cellular, there are nevertheless
-certain portions which are not produced by the metamorphosis of cells.
-Supposing that clay were the only material available for building,
-the proposition that all houses are built of bricks, would bear about
-the same relation to the truth, as does the proposition that all
-organisms are composed of cells. This generalization respecting houses
-would be open to two criticisms:&mdash;first, that certain houses of a
-primitive kind are formed, not of bricks, but out of unmoulded clay;
-and second, that though other houses consist mainly of bricks, yet
-their chimney-pots, drain-pipes, and<span class="pagenum" id="Page_18">[18]</span> ridge-tiles, do not result from
-combination or metamorphosis of bricks, but are made directly out of
-the original clay. And of like natures are the criticisms which must be
-passed on the generalization, that cells are the morphological units
-of organisms. To continue the simile, the truth turns out to be, that
-the primitive clay or protoplasm out of which organisms are built, may
-be moulded either directly, or with various degrees of indirectness,
-into organic structures. The physiological units which we are obliged
-to assume as the components of this protoplasm, must, as we have seen,
-be the possessors of those proclivities which result in the structural
-arrangements of the organism. The assumption of such structural
-arrangements may go on, and in many cases does go on, by the shortest
-route; without the passage through what we call metamorphoses. But
-where such structural arrangements are reached by a circuitous route,
-the first stage is the formation of these small aggregates which, under
-the name of cells, are currently regarded as morphological units.</p>
-
-<p>The rationale of these truths appears to be furnished by the hypothesis
-of evolution. We set out with molecules some degrees higher in
-complexity than those molecules of nitrogenous colloidal substance
-into which organic matter is resolvable; and we regard these very much
-more complex molecules as having the implied greater instability,
-greater sensitiveness to surrounding influences, and consequent
-greater mobility of form. Such being the primitive physiological
-units, organic evolution must begin with the formation of a minute
-aggregate of them&mdash;an aggregate showing vitality by a higher degree
-of that readiness to change its form of aggregation which colloidal
-matter in general displays; and by its ability to unite the nitrogenous
-molecules it meets with, into complex molecules like those of which
-it is composed. Obviously, the earliest forms must have been minute;
-since, in the absence of any but diffused organic matter, no form
-but a minute one could find nutriment. Obviously, too,<span class="pagenum" id="Page_19">[19]</span> it must have
-been structureless; since, as differentiations are producible only
-by the unlike actions of incident forces, there could have been no
-differentiations before such forces had had time to work. Hence,
-distinctions of parts like those required to constitute a cell were
-necessarily absent at first. And we need not therefore be surprised
-to find, as we do find, specks of protoplasm manifesting life, and
-yet showing no signs of organization. A further stage of evolution
-is reached when the imperfectly integrated molecules forming one of
-these minute aggregates, become more coherent; at the same time as
-they pass into a state of heterogeneity, gradually increasing in its
-definiteness. That is to say, we may look for the assumption by them,
-of some distinctions of parts, such as we find in cells and in what
-are called unicellular organisms. They cannot retain their primordial
-uniformity; and while in a few cases they may depart from it but
-slightly, they will, in the great majority of cases, acquire a decided
-multiformity: there will result the comparatively integrated and
-comparatively differentiated <i>Protophyta</i> and <i>Protozoa</i>.
-The production of minute aggregates of physiological units being
-the first step, and the passage of such minute aggregates into more
-consolidated and more complex forms being the second step, it must
-naturally happen that all higher organic types, subsequently arising
-by further integrations and differentiations, will everywhere bear the
-impress of this earliest phase of evolution. From the law of heredity,
-considered as extending to the entire succession of living things
-during the Earth’s past history, it follows that since the formation
-of these small, simple organisms must have preceded the formation
-of larger and more complex organisms, the larger and more complex
-organisms must inherit their essential characters. We may anticipate
-that the multiplication and combination of these minute aggregates or
-cells, will be conspicuous in the early developmental stages of plants
-and animals; and that throughout all subsequent stages, cell-production
-and cell-differentiation<span class="pagenum" id="Page_20">[20]</span> will be dominant characteristics. The
-physiological units peculiar to each higher species will, speaking
-generally, pass through this form of aggregation on their way towards
-the final arrangement they are to assume; because those primordial
-physiological units from which they are remotely descended, aggregated
-into this form. And yet, just as in other cases we found reasons for
-inferring (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect">§ 131</a>) that the traits of ancestral organization may,
-under certain conditions, be partially or wholly obliterated, and the
-ultimate structure assumed without passing through them; so, here,
-it is to be inferred that the process of cell-formation may, in some
-cases, be passed over. Thus the hypothesis of evolution prepares us for
-those two radical modifications of the cell-doctrine which the facts
-oblige us to make. It leads us to expect that as structureless portions
-of protoplasm must have preceded cells in the process of general
-evolution; so, in the special evolution of each higher organism, there
-will be an habitual production of cells out of structureless blastema.
-And it leads us to expect that though, generally, the physiological
-units composing a structureless blastema, will display their inherited
-proclivities by cell-development and metamorphosis; there will
-nevertheless occur cases in which the tissue to be formed, is formed by
-direct transformation of the blastema.<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">[2]</a></p>
-
-<p><span class="pagenum" id="Page_21">[21]</span></p>
-
-<p>Interpreting the facts in this manner, we may recognize that large
-amount of truth which the cell-doctrine contains, without committing
-ourselves to the errors involved by a sweeping assertion of it. We
-are enabled to understand how it happens that organic structures are
-usually cellular in their composition, at the same time that they are
-not universally so. We are shown that while we may properly continue to
-regard the cell as the morphological unit, we must constantly bear in
-mind that it is such only in a qualified sense.</p>
-
-
-<p class="p2">§ 181. These aggregates of the lowest order, each formed of
-physiological units united into a group that is structurally single
-and cannot be divided without destruction of its individuality, may,
-as above implied, exist as independent organisms. The assumption to
-which we are committed by the hypothesis of evolution, that such
-so called unicellular plants were at first the only kinds of plants,
-is in harmony with the fact that habitats not occupied by plants of
-higher orders, commonly contain these protophytes in great abundance
-and great variety. The various species of <i>Pleurococcaceæ</i>,
-of <i>Desmidiaceæ</i>, and <i>Diatomaceæ</i>, supply examples of
-morphological units living and propagating separately, under numerous
-modifications of form and structure. Figures <a href="#i_022a">1, 2, and 3</a>, represent a
-few of the commonest types.</p>
-
-  <div class="figcenter" id="i_022a">
-     <img
-    class="p2"
-    src="images/i_022a.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 1, 2, 3.</p>
-  </div>
-
-  <div class="figcenter" id="i_022b">
-     <img
-    class="p2"
-    src="images/i_022b.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 4, 5, 6.</p>
-  </div>
-
-<p class="p2">Mostly, simple plants are too small to be individually<span class="pagenum" id="Page_22">[22]</span> visible
-without the microscope. But, in some cases, these vegetal aggregates
-of the first order grow to appreciable sizes. In the mycelium of
-some fungi, we have single cells developed into long branched
-filaments, or ramified tubules, that are of considerable lengths. An
-analogous structure characterizes certain tribes of <i>Algæ</i>, of
-which <i>Codium adhærens</i>, Fig. <a href="#i_022b">4</a>, may serve as an example. In
-<i>Botrydium</i>, another alga, Fig. <a href="#i_022b">5</a>, we have a structure which is
-described as simulating a higher plant, with root, stem, bud, and
-fruit, all produced by the branching of a single cell. And among
-fungi the genus <i>Mucor</i>, Fig. <a href="#i_022b">6</a>, furnishes an example of allied
-kind.<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">[3]</a> Here, though the size attained is much greater than that of
-many organisms which are morphologically compound, we are compelled to
-consider the morphological composition as simple; since the whole can
-no more be separated into minor wholes, than can the branched vascular<span class="pagenum" id="Page_23">[23]</span>
-system of an animal. In these cases we have considerable bulk attained,
-not by a number of aggregates of the first order being united into
-an aggregate of the second order, but by the continuous growth of an
-aggregate of the first order.</p>
-
-
-<p class="p2" id="sect182">§ 182. The transition to higher forms begins in a very unobtrusive
-manner. Among these aggregates of the first order, an approach towards
-that union by which aggregates of the second order are produced, is
-indicated by mere juxtaposition. Protophytes multiply rapidly; and
-their rapid multiplication sometimes causes crowding. When, instead
-of floating free in the water, they form a thin film on a moist
-surface, or are imbedded in a common matrix of mucilage; the mechanical
-obstacles to dispersion result in a kind of feeble integration, vaguely
-shadowing forth a combined group. Somewhat more definite combination
-is shown us by such plants as <i>Palmella botryoides</i>. Here the
-members of a family of cells, arising by the spontaneous fission of
-a parent-cell, remain united by slender threads of that jelly-like
-substance which envelops their surfaces. In some <i>Diatomaceæ</i>
-several individuals, instead of completely separating, hold together by
-their angles; and in other <i>Diatomaceæ</i>, as the <i>Bacillaria</i>,
-a variable number of units cohere so slightly, that they are
-continually moving in relation to one another.</p>
-
-<p>This formation of aggregates of the second order, faintly indicated
-in feeble and variable unions like the above, may be traced through
-phases of increasing permanence and definiteness, as well as
-increasing extent. In the yeast-plant, Fig. <a href="#i_024">7</a>, we have cells which
-may exist singly, or joined into groups of several; and which have
-their shapes scarcely at all modified by their connexion. Among the
-<i>Desmidiaceæ</i>, it happens in many cases that the two individuals
-produced by division of a parent-individual, part as soon as they are
-fully formed; but in other cases, instead of parting they compose a
-group of two. Allied kinds show us how, by subsequent fissions of the
-adherent individuals and their progeny, there<span class="pagenum" id="Page_24">[24]</span> result longer groups;
-and in some species, a continuous thread of them is thus produced.
-Figs. <a href="#i_024">8, 9, 11</a>, exhibit these several stages. Fig. 10 represents a
-<i>Scenedesmus</i> in which the individuation of the group is manifest.
-Instead of linear aggregation, many protophytes illustrate central
-aggregation; as shown in Figs. <a href="#i_024">12, 13, 14, 15</a>. Other instances are
-furnished by such forms as the <i>Gonium pectorale</i>, Fig. <a href="#i_024">16</a>
-(<i>a</i> being the front view, and <i>b</i> the edge view), and the
-<i>Sarcina ventriculi</i>, Fig. <a href="#i_024">17</a>. Further, we have that spherical
-mode of aggregation of which the <i>Volvox globator</i> furnishes a
-familiar instance.</p>
-
-  <div class="figcenter" id="i_024">
-     <img
-    class="p2"
-    src="images/i_024.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 7–17.</p>
-  </div>
-
-  <div class="figcenter" id="i_025">
-     <img
-    class="p2"
-    src="images/i_025.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 18–23.</p>
-  </div>
-
-<p class="p2">Thus far, however, the individuality of the secondary aggregate is
-feebly pronounced: not simply in the sense that it is small; but also
-in the sense that the individualities of the primary aggregates are
-very little subordinated. But on seeking further, we find transitions
-towards forms in which the compound individuality is more dominant,
-while the simple individualities are more obscured. Obscuration of one
-kind accompanies mere increase of size in the secondary aggregate.
-In proportion to the greater number of the morphological units held
-together in one mass, becomes their relative insignificance as
-individuals. We see this in the irregularly-spreading lichens that
-form patches on rocks; and in such creeping fungi as grow in films
-or laminæ on decaying wood and the bark of trees. In these cases,
-however, the integration of the component cells is of an almost<span class="pagenum" id="Page_25">[25]</span>
-mechanical kind. The aggregate of them is scarcely more individuated
-than a lump of inorganic matter: as witness the way in which the lichen
-extends its curved edges in this or that direction, as the surface
-favours; or the way in which the fungus grows round and imbeds the
-shoots and leaves that lie in its way, just as so much plastic clay
-might do. Though here, in the augmentation of mass, we see a progress
-towards the evolution of a higher type, we have as yet none of that
-definiteness required to constitute a compound unit, or true aggregate
-of the second order. Another kind of obscuration of the morphological
-units, is brought about by their more complete coalescence into the
-form of some structure made by their union. This is well exemplified
-among the <i>Confervoideæ</i> and <i>Conjugatæ</i>. In Fig. <a href="#i_025">18</a>, there
-are represented the stages of a growing <i>Mougeotia genuflexa</i>,
-in which this merging of the simple individualities into the compound
-individuality, is shown in the history of a single plant; and in Figs.
-<a href="#i_025">19, 20, 21, 22, 23</a>, are represented a series of species from this
-group, and that of <i>Cladophora</i>,<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">[4]</a> in which we see a progressing
-integration. While, in the lower types, the primitive spheroidal forms
-of the cells are scarcely altered, in the higher types the cells are
-so fused together as to constitute cylinders divided by septa. Here,
-however,<span class="pagenum" id="Page_26">[26]</span> the indefiniteness is still great. There are no specific
-limits to the length of any thread thus produced, and there is none of
-that differentiation of parts required to give a decided individuality
-to the whole.</p>
-
-<p>To constitute something like a true aggregate of the second order,
-capable of serving as a compound unit that may be combined with others
-like itself into still higher aggregates, there must exist both mass
-and definiteness.</p>
-
-
-<p class="p2">§ 183. An approach towards plants which unite these characters, may
-be traced in such forms as <i>Bangia ciliaris</i>, Fig. <a href="#i_026">24</a>. The
-multiplication of cells here takes place, not in a longitudinal
-direction only, but also in a transverse direction; and the transverse
-multiplication being greater towards the middle of the frond, there
-results a difference between the middle and the two extremities&mdash;a
-character which, in a feeble way, unites all the parts into a whole.
-Even this slight individuation is, however, very indefinitely marked;
-since, as shown by the figures, the lateral multiplication of cells
-does not go on in a precise manner.</p>
-
-  <div class="figcenter" id="i_026">
-     <img
-    class="p2"
-    src="images/i_026.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 24.</p>
-  </div>
-
-<p class="p2">From some such type as this there appear to arise, through slight
-differences in the modes of growth, two closely-allied groups of
-plants, having individualities somewhat more pronounced. If, while
-the cells multiply longitudinally, their lateral multiplication
-goes on in one direction only, there results a flat surface, as in
-the genus <i>Ulva</i> (Sea-lettuce) or in the upper part of the
-thallus of <i>Enteromorpha Linza</i>, Fig. <a href="#i_027">25</a>; or where the lateral
-multiplication is less uniform in its rate, in types like Fig. <a href="#i_027">26</a>. But
-where the lateral multiplication occurs in two directions transverse
-to one another, a hollow frond may be produced&mdash;sometimes irregularly
-spheroidal, and sometimes<span class="pagenum" id="Page_27">[27]</span> irregularly tubular; as in <i>Enteromorpha
-intestinalis</i>, Fig. <a href="#i_027">27</a>. And often, as in <i>Enteromorpha
-compressa</i>, Fig. <a href="#i_027">28</a>, and other species, this tubular frond becomes
-branched. Figs. <a href="#i_027">29</a> and <a href="#i_027">30</a> are magnified portions of such fronds,
-showing the simple cellular aggregation which allies them with the
-preceding forms.</p>
-
-  <div class="figcenter" id="i_027">
-     <img
-    class="p2"
-    src="images/i_027.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 25–30.</p>
-  </div>
-
-<p class="p2">In the common <i>Fuci</i> of our coasts, other and somewhat higher
-stages of this integration are displayed. We have fronds preserving
-something like constant breadths and dividing dichotomously with
-approximate regularity. Though the subdivisions so produced are not
-to be regarded as separate fronds, but only as extensions of one
-frond, they foreshadow a higher degree of composition; and by the
-comparatively methodic way in which they are united, give to the
-aggregate a more definite, as well as a more complex, individuality.
-Many of the higher lichens exhibit an analogous advance. While in the
-lowest lichens, the different parts of the thallus are held together
-only by being all attached to the supporting surface, in the higher
-lichens the thallus is so far integrated that it can support itself by
-attachment to such surface at one point only. And then, in still more
-developed kinds, we find the thallus assuming a<span class="pagenum" id="Page_28">[28]</span> dichotomously-branched
-form, and so gaining a more specific character as well as greater size.</p>
-
-<p>Where, as in types like these, the morphological units show an inherent
-tendency to arrange themselves in a manner which is so far constant
-as to give characteristic proportions, we may say that there is a
-recognizable compound individuality. Considering the Thallophytes which
-grow in this way apart from their kinships, and wholly with reference
-to their morphological composition, we might not inaptly describe them
-as pseudo-foliar.</p>
-
-
-<p class="p2">§ 184. Another mode in which aggregation is so carried on as to produce
-a compound individuality of considerable definiteness, is variously
-displayed among other families of <i>Algæ</i>. When the cells, instead
-of multiplying longitudinally alone, and instead of all multiplying
-laterally as well as longitudinally, multiply laterally only at
-particular places, they produce branched structures.</p>
-
-<p>Indications of this mode of aggregation occur among the
-<i>Confervoideæ</i>, as shown in Figs. <a href="#i_025">22, 23</a>. Though, in some of the
-more-developed <i>Algæ</i> which exhibit the ramified arrangement in
-a higher degree, the component cells are, like those of the lower
-<i>Algæ</i>, united together end to end, in such way as but little
-to obscure their separate forms, as in <i>Cladophora Hutchinsiæ</i>,
-Fig. <a href="#i_029">31</a>; they nevertheless evince greater subordination to the whole
-of which they are parts, by arranging themselves more methodically.
-Still further pronounced becomes the compound individuality when, while
-the component cells of the branches unite completely into jointed
-cylinders, the component cells of the stem form an axis distinguished
-by its relative thickness and complexity. Such types of structures
-are indicated by Figs. <a href="#i_029">32, 33</a>&mdash;figures representing small portions of
-plants which are quite tree-like in their entire outlines. On examining
-Figs. <a href="#i_029">34, 35, 36</a>, which show the structures of the stems in these
-types, it will be seen, too, that the component cells in becoming
-more<span class="pagenum" id="Page_29">[29]</span> coherent, have undergone changes of form which obscure their
-individualities more than before. Not only are they much elongated, but
-they are so compressed as to be prismatic rather than cylindrical. This
-structure, besides displaying integration of the morphological units
-carried on in two directions instead of one; and besides displaying
-this higher integration in the greater merging of the individualities
-of the morphological units in the general individuality; also displays
-it in the more pronounced subordination of the branches and branchlets
-to the main stem. This differentiation and consolidation of the stem,
-brings all the secondary growths into more marked dependence; and so
-renders the individuality of the aggregate more decided.</p>
-
-  <div class="figcenter" id="i_029">
-     <img
-    class="p2"
-    src="images/i_029.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 31–36.</p>
-  </div>
-
-<p class="p2">We might not inappropriately call this type of structure pseud-axial.
-It simulates that of the higher plants in certain superficial
-characters. We see in it a primary axis along which development may
-continue indefinitely, and from which there bud out, laterally,
-secondary axes of like nature, bearing like tertiary axes; and this is
-a mode of growth with which Phænogams make us familiar.</p>
-
-
-<p class="p2">§ 185. Some of the larger <i>Algæ</i> supply examples of an
-integration still more advanced; not simply inasmuch as they unite
-much greater numbers of morphological units<span class="pagenum" id="Page_30">[30]</span> into continuous masses,
-but also inasmuch as they combine the pseudo-foliar structure with
-the pseud-axial structure. Our own shores furnish an instance
-of this in the common <i>Laminaria</i>; and certain gigantic
-<i>Laminariaceæ</i> of the Antarctic seas, furnish yet better
-instances. In <i>Necrocystis</i> the germ develops a very long
-slender stem, which eventually expands into a large bladder-like
-or cylindrical air-vessel; and the surface of this bears numerous
-leaf-shaped expansions. Another kind, <i>Lessonia fuscescens</i>,
-Fig. <a href="#i_030">37</a>, shows us a massive stem growing up through water many feet
-deep&mdash;a stem which, bifurcating as it approaches the surface, flattens
-out the ends of its subdivisions into fronds like ribands. These,
-however, are not true foliar appendages, since they are merely expanded
-continuations of the stem. In <i>Egregia</i> branches of the thallus
-not only take the form of leaves, but these are differentiated into
-several categories in accordance with a division of labour. In any of
-these <i>Laminariaceæ</i> the whole plant, great as may be its size,
-and made up though it seems to be of many groups of morphological
-units, united into a compound group by their marked subordination to a
-connecting mass, is nevertheless a single thallus, which is added to by
-intercalary growth at the “transition place,” at the junction of the
-stem-like and leaf-like portions. The aggregate is still an aggregate
-of the second order.</p>
-
-  <div class="figcenter" id="i_030">
-     <img
-    class="p2"
-    src="images/i_030.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 37.</p>
-  </div>
-
-  <div class="figcenter" id="i_031">
-     <img
-    class="p2"
-    src="images/i_031.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 38–40.</p>
-  </div>
-
-<p class="p2">But among certain of the highest <i>Algæ</i>, we <i>do</i> find
-something more than this union of the pseud-axial with the
-pseudo-foliar structure. In addition to pseud-axes of comparative
-complexity; and in addition to pseudo-folia that are like leaves, not
-only in their general shapes but in having mid-ribs and even veins;
-there are the beginnings of a higher stage of integration. Figs. <a href="#i_031">38,
-39, and 40</a>, show some of the steps. In <i>Rhodymenia palmata</i>,
-Fig. <a href="#i_031">38</a>, the<span class="pagenum" id="Page_31">[31]</span> parent-frond is comparatively irregular in form, and
-without a mid-rib; and along with this very imperfect integration, we
-see that the secondary fronds growing from the edges are distributed
-very much at random, and are by no means specific in their shapes. A
-considerable advance is displayed by <i>Phyllophora rubens</i>, Fig.
-<a href="#i_031">39</a>. Here the frond, primary, secondary, or tertiary, betrays some
-approach towards regularity in both form and size; by which, as also
-by its partially-developed mid-rib, there is established a more marked
-individuality; and at the same time, the growth of the secondary
-fronds no longer occurs anywhere on the edge, in the same plane as the
-parent-frond, but from the surface at specific places. <i>Delesseria
-sanguinea</i>, Fig. <a href="#i_031">40</a>, illustrates a much more definite arrangement
-of the same kind. The fronds of this plant, quite regularly shaped,
-have their parts decidedly subordinated to the whole; and from their
-mid-ribs grow other fronds which are just like them. Each of these
-fronds is an organized group of those morphological units which we
-distinguish as aggregates of the first order. And in this case, two or
-more such aggregates of the second<span class="pagenum" id="Page_32">[32]</span> order, well individuated by their
-forms and structures, are united together; and the plant composed of
-them is thus rendered, in so far, an aggregate of the third order.</p>
-
-<p>Just noting that in certain of the most developed <i>Algæ</i>, as
-the <i>Sargassum</i>, or common gulf-weed, this tertiary degree of
-composition is far more completely displayed, so as to produce among
-Thallophytes a type of structure closely simulating that of the higher
-plants, let us now pass to the consideration of these higher plants.</p>
-
-
-<p class="p2">§ 186. Having the surface of the soil for a support and the air for a
-medium, terrestrial plants are mechanically circumstanced in a manner
-widely different from that in which aquatic plants are circumstanced.
-Instead of being buoyed up by a surrounding fluid of specific gravity
-equal to their own, they have to erect themselves into a rare fluid
-which yields no appreciable support. Further, they are dissimilarly
-conditioned in having two sources of nutriment in place of one. Unlike
-the <i>Algæ</i>, which derive all the materials for their tissues from
-the water bathing their entire surfaces, and use their roots only
-for attachment, most of the plants which cover the Earth’s surface,
-absorb part of their food through their imbedded roots and part through
-their exposed leaves. These two marked unlikenesses in the relations
-to surrounding conditions, profoundly affect the respective modes of
-growth. We must duly bear them in mind while studying the further
-advance of composition.</p>
-
-  <div class="figcenter" id="i_033">
-     <img
-    class="p2"
-    src="images/i_033.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 41–44.</p>
-  </div>
-
-  <div class="figcenter" id="i_034">
-     <img
-    class="p2"
-    src="images/i_034.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 45–49.</p>
-  </div>
-
-<p class="p2">The class of plants to which we now turn&mdash;that of the
-<i>Archegoniatæ</i>&mdash;is nearly related by its lower members to the
-classes above dealt with: so much so, that some of the inferior
-liverworts are quite licheniform, and are often mistaken for lichens.
-Passing over these, let us recommence our synthesis with such members
-of the class as repeat those indications of progress towards a higher
-composition, which we have just observed among the more-developed
-<i>Algæ</i>. The <i>Jungermanniaceæ</i> furnish us with a series of
-types, clearly indicating the<span class="pagenum" id="Page_33">[33]</span> transition from an aggregate of the
-second order to an aggregate of the third order. Figs. <a href="#i_033">41</a> and <a href="#i_033">42</a>,
-indicate the structure among the lowest of this group. Here there is
-but an incomplete development of the second order of aggregate. The
-frond grows as irregularly as the thallus of a lichen: it is indefinite
-in size and outline, spreading hither or thither as the conditions
-favour. Moreover, it lacks the differentiations required to subordinate
-its parts to the whole: it is uniformly cellular, having neither
-mid-rib nor veins; and it puts out rootlets indifferently from all
-parts of its under surface. In Fig. <a href="#i_033">43</a>, <i>Pellia epiphylla</i>, we
-have an advance on this type. There is here, as shown in the transverse
-section, Fig. <a href="#i_033">44</a>, a thickening of the frond along its central portion,
-producing something like an approach towards a mid-rib; and from this
-the rootlets are chiefly given off. The outline, too, is much less
-irregular; whence results greater distinctness of the individuality.
-A further step is displayed in <i>Metzgeria furcata</i>, Fig. <a href="#i_034">45</a>.
-The frond of this plant, comparatively well integrated by the
-distribution of its substance around a decided mid-rib, and by its
-comparatively-definite outlines, produces secondary fronds. There is
-what is called proliferous growth; and occasionally, as shown in Fig.
-<a href="#i_034">46</a>, representing an enlarged portion, the growth is doubly-proliferous.
-In these cases, however, the tertiary aggregate, so far as it is
-formed, is but very feebly integrated; and its integration is but
-temporary. For not only do these younger fronds that bud out from the
-mid-ribs of older fronds, develop rootlets of their own; but as soon
-as they are well grown and adequately rooted, they dissolve their
-connexions with the parent-fronds,<span class="pagenum" id="Page_34">[34]</span> and become quite independent. From
-these transitional forms we pass, in the higher <i>Jungermanniaceæ</i>,
-to forms composed of many fronds that are permanently united by a
-continuous stem. A more-developed aggregate of the third order is
-thus produced. But though, along with increased definiteness in
-the secondary aggregates, there is here an integration of them so
-extensive and so regular, that they are visibly subordinated to the
-whole they form; yet the subordination is really very incomplete. In
-some instances, as in <i>Radula complanata</i>, Fig. <a href="#i_034">47</a>, the leaflets
-develop roots from their under surfaces, just as the primitive frond
-does; and in the majority of the group, as in <i>J. capitata</i>, Fig.
-<a href="#i_034">48</a>, roots are given off all along the connecting stem, at the spots
-where the leaflets or frondlets join it: the result being that though
-the connected frondlets form a physical whole, they do not form, in any
-decided manner, a physiological whole; since successive portions of
-the united series, carry on their functions independently of the rest.
-Finally, the most developed members of the group, whether lineally
-descended from the less developed or from an early type common to the
-two, present us with tertiary aggregates which are physiologically as
-well as physically integrated.<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">[5]</a> Not lying<span class="pagenum" id="Page_35">[35]</span> prone like the kinds thus
-far described, but growing erect, the stem and attached leaflets become
-dependent upon a single root or group of roots; and being so prevented
-from carrying on their functions separately, are made members of a
-compound individual: there arises a definitely-established aggregate of
-the third degree of composition.</p>
-
-<p>The facts as arranged in the above order are suggestive. Minute
-aggregates, or cells, the grouping of which we traced in <a href="#sect182">§ 182</a>, showed
-us analogous phases of indefinite union, which appeared to lead the
-way towards definite union. We see here among compound aggregates, as
-we saw there among simple aggregates, the establishment of a specific
-form, and a size that falls within moderate limits of variation. This
-passage from less definite extension to more definite extension,
-seems in the one case, as the other, to be accompanied by the result,
-that growth exceeding a certain rate, ends in the formation of a new
-aggregate, rather than an enlargement of the old. And on the higher
-stage, as on the lower, this process, irregularly carried out in the
-simpler types, produces in them unions that are but temporary; while in
-the more-developed types, it proceeds in a systematic way, and ends in
-the production of a permanent aggregate that is doubly compound.</p>
-
-<p><span class="pagenum" id="Page_36">[36]</span></p>
-
-<p>Must we then conclude that as cells, or morphological units, are
-integrated into a unit of a higher order, which we call a thallus
-or frond; so, by the integration of fronds, there is evolved a
-structure such as the above-delineated species possess? Whether this
-is the interpretation to be given of these plants, we shall best see
-when considering whether it is the interpretation to be given of
-plants which rank above them. Thus far we have dealt only with the
-Cryptogamia. We have now to deal with the Phanerogamia or Phænogamia.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_37">[37]</span></p>
-
-<h3>CHAPTER III.<br />
-<span class="subhed1">THE MORPHOLOGICAL COMPOSITION OF PLANTS, <i>CONTINUED</i>.</span></h3></div>
-
-<p class="p2">§ 187. That advanced composition arrived at in the <i>Archegoniatæ</i>,
-is carried still further in the Flowering Plants. In these
-most-elevated vegetal forms, aggregation of the third order is always
-distinctly displayed; and aggregates of the fourth, fifth, sixth, &amp;c.,
-orders are very common.</p>
-
-<p>Our inquiry into the morphology of these flowering plants, may be
-advantageously commenced by studying the development of simple leaves
-into compound leaves. It is easy to trace the transition, as well as
-the conditions under which it occurs; and tracing it will prepare us
-for understanding how, and when, metamorphoses still greater in degree
-take place.</p>
-
-
-<p class="p2" id="sect188">§ 188. If we examine a branch of the common bramble, when in flower
-or afterwards, we shall not unfrequently find a simple or undivided
-leaf, at the insertion of one of the lateral flower-bearing axes,
-composing the terminal cluster of flowers. Sometimes this leaf is
-partially lobed; sometimes cleft into three small leaflets. Lower down
-on the shoot, if it be a lateral one, occur larger leaves, composed
-of three leaflets; and in some of these, two of the leaflets may be
-lobed more or less deeply. On the main stem the leaves, usually still
-larger, will be found to have five leaflets. Supposing the plant to be
-a well-grown one, it will furnish all<span class="pagenum" id="Page_38">[38]</span> gradations between the simple,
-very small leaf, and the large composite leaf, containing sometimes
-even seven leaflets. Figs. <a href="#i_038">50 to 64</a>, represent leading stages of the
-transition. What determines this transition? Observation shows that
-the quintuple leaves occur where the materials for growth are supplied
-in greatest abundance; that the leaves become<span class="pagenum" id="Page_39">[39]</span> less and less compound,
-in proportion to their remoteness from the main currents of sap; and
-that where an entire absence of divisions or lobes is observed, it is
-on leaves within the flower-bunch: at the place, that is, where the
-forces which cause growth are nearly equilibrated by the forces which
-oppose growth; and where, as a consequence, gamogenesis is about to
-be set in (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect78">§ 78</a>). Additional evidence that the degree of nutrition
-determines the degree of composition of the leaf, is furnished by
-the relative sizes of the leaves. Not only, on the average, is the
-quintuple leaf much larger in its total area than the triple leaf;
-but the component leaflets of the one, are usually much larger than
-those of the other. The like contrasts are still more marked between
-triple leaves and simple leaves. This connection of decreasing size
-with decreasing composition, is conspicuous in the series of figures:
-the differences shown being not nearly so great as may be frequently
-observed. Confirmation may be drawn from the fact that when the leading
-shoot is broken or arrested in its growth, the shoots it gives off
-(provided they are given off after the injury), and into which its
-checked currents of sap are thrown, produce leaves of five leaflets
-where ordinarily leaves of three leaflets occur. Of course incidental
-circumstances, as variations in the amounts of sunshine, or of rain,
-or of matter supplied to the roots, are ever producing changes in the
-state of the plant as a whole; and by thus affecting the nutrition of
-its leaf-buds at the times of their formation, cause irregularities
-in the relations of size and composition above described. But taking
-these causes into account, it is abundantly manifest that a leaf-bud of
-the bramble will develop into a simple leaf or into a leaf compounded
-in different degrees, according to the quantity of assimilable matter
-brought to it at the time when the rudiments of its structure are
-being fixed. And on studying the habits of other plants&mdash;on observing
-how annuals that have compound leaves usually bear simple leaves at
-the outset, when the assimilating surface is but small; and how, when
-compound-leaved<span class="pagenum" id="Page_40">[40]</span> plants in full growth bear simple leaves in the midst
-of compound ones, the relative smallness of such simple leaves shows
-that the buds from which they arose were ill-supplied with sap; it
-will cease to be doubted that a foliar organ may be metamorphosed into
-a group of foliar organs, if furnished, at the right time, with a
-quantity of matter greater than can be readily organized round a single
-centre of growth. An examination of the transitions through which
-a compound leaf passes into a doubly-compound leaf, as seen in the
-various intermediate forms of leaflets in Fig. <a href="#i_040">65</a>, will further enforce
-this conclusion.</p>
-
-  <div class="figcenter" id="i_038">
-     <img
-    class="p2"
-    src="images/i_038.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 50–64.</p>
-  </div>
-
-  <div class="figcenter" id="i_040">
-     <img
-    class="p2"
-    src="images/i_040.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 65.</p>
-  </div>
-
-<p class="p2">Here we may advantageously note, too, how in such cases the leaf-stalk
-undergoes concomitant changes of structure. In the bramble-leaves above
-described, it becomes compound simultaneously with the leaf&mdash;the veins
-become mid-ribs while the mid-ribs become petioles. Moreover, the
-secondary stalks, and still more the main stalks, bear thorns similar
-in their<span class="pagenum" id="Page_41">[41]</span> shapes, and approaching in their sizes, to those on the stem;
-besides simulating the stem in colour and texture. In the petioles of
-large compound leaves, like those of the common <i>Heracleum</i>, we
-see still more distinctly both internal and external approximations
-in character to axes. Nor are there wanting plants whose large,
-though simple, leaves, are held out far from the stems by foot-stalks
-that are, near the ends, sometimes so like axes that the transverse
-sections of the two are indistinguishable; as instance the <i>Calla
-palustris</i>.</p>
-
-<p>One other fact respecting the modifications which leaves undergo,
-should be set down. Not only may leaf-stalks assume to a great degree
-the characters of stems, when they have to discharge the functions
-of stems, by supporting many leaves or very large leaves; but they
-may assume the characters of leaves, when they have to undertake the
-functions of leaves. The Australian Acacias furnish a remarkable
-illustration of this. Acacias elsewhere found bear pinnate leaves;
-but the majority of those found in Australia bear what appear to be
-simple leaves. It turns out, however, that these are merely leaf-stalks
-flattened out into foliar shapes: the laminæ of the leaves being
-undeveloped. And the proof is that in young plants, showing their
-kinships by their embryonic characters, these leaf-like petioles bear
-true leaflets at their ends. A metamorphosis of like kind occurs in
-<i>Oxalis bupleurifolia</i>, Fig. <a href="#i_041">66</a>. The fact most deserving of
-notice, however, is that these leaf-stalks, in usurping the general
-aspects and functions of leaf-blades, have, to some also usurped their
-structures: though their venation is not like that of the leaf-blades
-they replace, yet they have veins, and in some cases mid-ribs.</p>
-
-  <div class="figcenter" id="i_041">
-     <img
-    class="p2"
-    src="images/i_041.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 66.</p>
-  </div>
-
-<p class="p2">Reduced to their most general expression, the truths above shadowed
-forth are these:&mdash;That group of morphological units, or cells, which
-we see integrated into the compound<span class="pagenum" id="Page_42">[42]</span> unit called a leaf, has, in
-each higher plant, a typical form, due to the special arrangement
-of these cells around a mid-rib and veins. If the multiplication of
-morphological units, at the time when the leaf-bud is taking on its
-main outlines, exceeds a certain limit, these units begin to arrange
-themselves round secondary centres, or lines of growth, in such ways as
-to repeat, in part or wholly, the typical form: the larger veins become
-transformed into imperfect mid-ribs of partially independent leaves;
-or into complete mid-ribs of quite separate leaves. And as there goes
-on this transition from a single aggregate of cells to a group of
-such aggregates, there simultaneously arises, by similarly insensible
-steps, a distinct structure which supports the several aggregates thus
-produced, and unites them into a compound aggregate. These phenomena
-should be carefully studied; since they give us a key to more involved
-phenomena.<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">[6]</a></p>
-
-
-<p class="p2">§ 189. Thus far we have dealt with leaves ordinarily so-called: briefly
-indicating the homologies between the parts of the simple and the
-compound. Let us now turn to the homologies among foliar organs in
-general. These have been<span class="pagenum" id="Page_43">[43]</span> made familiar to readers of natural history
-by popularized outlines of <i>The Metamorphosis of Plants</i>&mdash;a title,
-by the way, which is far too extensive; since the phenomena treated of
-under it, form but a small portion of those it properly includes.</p>
-
-<p>Passing over certain vague anticipations which have been quoted from
-ancient writers, and noting only that some clearer recognitions
-were reached by Joachim Jung, a Hamburg professor, in the middle of
-the 17th century; we come to the <i>Theoria Generationis</i>, which
-Wolff published in 1759, and in which he gives definite forms to the
-conceptions that have since become current. Specifying the views of
-Wolff, Dr. Masters writes:&mdash;“After speaking of the homologous nature
-of the leaves, the sepals and petals, an homology consequent on their
-similarity of structure and identity of origin, he goes on to state
-that the ‘pericarp is manifestly composed of several leaves, as in the
-calyx, with this difference only, that the leaves which are merely
-placed in close contact in the calyx, are here united together’; a
-view which he corroborates by referring to the manner in which many
-capsules open and separate ‘into their leaves.’ The seeds, too, he
-looks upon as consisting of leaves in close combination. His reasons
-for considering the petals and stamens as homologous with leaves, are
-based upon the same facts as those which led Linnæus, and, many years
-afterwards, Goethe, to the same conclusion. ‘In a word,’ says Wolff,
-‘we see nothing in the whole plant, whose parts at first sight differ
-so remarkably from each other, but leaves and stem, to which latter
-the root is referrible.’” It appears that Wolff, too, enunciated the
-now-accepted interpretation of compound fruits: basing it on the same
-evidence as that since assigned. In the essay of Goethe, published
-thirty years after, these relations among the parts of flowering plants
-were traced out in greater detail, but not in so radical a way; for
-Goethe did not, as did Wolff, verify his hypothesis by dissecting buds
-in their early stages of development. Goethe appears to have<span class="pagenum" id="Page_44">[44]</span> arrived
-at his conclusions independently. But that they were original with
-him, and that he gave a more variously-illustrated exposition of them
-than had been given by Wolff, does not entitle him to anything beyond
-a secondary place, among those who have established this important
-generalization.</p>
-
-<p>Were it not that these pages may be read by some to whom Biology, in
-all its divisions, is a new subject of study, it would be needless to
-name the evidence on which this now-familiar generalization rests. For
-the information of such it will suffice to say, that the fundamental
-kinship existing among all the foliar organs of a flowering plant, is
-shown by the transitional forms which may be traced between them, and
-by the occasional assumption of one another’s forms. “Floral leaves, or
-bracts, are frequently only to be distinguished from ordinary leaves
-by their position at the base of the flower; at other times the bracts
-gradually assume more and more of the appearance of the sepals.” The
-sepals, or divisions of the calyx, are not unlike undeveloped leaves:
-sometimes assuming quite the structure of leaves. In other cases, they
-acquire partially or wholly the colours of the petals&mdash;as, indeed, the
-bracts and uppermost stem-leaves occasionally do. Similarly, the petals
-show their alliances to the foliar organs lower down on the axis, and
-to those higher up on the axis. On the one hand, they may develop into
-ordinary leaves that are green and veined; and, on the other hand, as
-so commonly seen in double flowers, they may bear anthers on their
-edges. All varieties of gradation into neighbouring foliar organs may
-be witnessed in stamens. Flattened and tinted in various degrees, they
-pass insensibly into petals, and through them prove their homology with
-leaves; into which, indeed, they are transformed in flowers that become
-wholly foliaceous. The style, too, is occasionally changed into petals
-or into green leaflets; and even the ovules are now and then seen to
-take on leaf-like forms. Thus we have clear evidence that in Phænogams,
-all the<span class="pagenum" id="Page_45">[45]</span> appendages of the axis are homologues: they are all modified
-leaves.</p>
-
-<p>Wolff established, and Goethe further illustrated, another general law
-of structure in flowering plants. Each leaf commonly contains in its
-axil a bud, similar in structure to the terminal bud. This axillary bud
-may remain undeveloped; or it may develop into a lateral shoot like
-the main shoot; or it may develop into a flower. If a shoot bearing
-lateral flowers be examined, it will be found that the internode, or
-space which separates each leaf with its axillary flower from the leaf
-and axillary flower above it, becomes gradually less towards the upper
-end of the shoot. In some plants, as in the fox-glove, the internodes
-constitute a regularly-diminishing series. In other plants, the series
-they form suddenly begins to diminish so rapidly, as to bring the
-flowers into a short spike: instance the common orchis. And again, by
-still more sudden dwarfing of the internodes, the flowers are brought
-into a cluster; as they are in the cowslip. On contemplating a clover
-flower, in which this clustering has been carried so far as to produce
-a compact head; and on considering what must happen if, by a further
-arrest of axial development, the foot-stalks of the florets disappear;
-it will be seen that there must result a crowd of flowers, seated
-close together on the end of the axis. And if, at the same time, the
-internodes of the upper stem-leaves also remain undeveloped, these
-stem-leaves will be grouped into a common involucre: we shall have
-a composite flower, such as the thistle. Hence, to modifications in
-the developments of foliar organs, have to be added modifications in
-the developments of axial organs. Comparisons disclose the gradations
-through which axes, like their appendages, pass into all varieties
-of size, proportion, and structure. And we learn that the occurrence
-of these two kinds of metamorphosis, in all conceivable degrees
-and combinations, furnishes us with a proximate interpretation of
-morphological composition in Phænogams.</p>
-
-<p><span class="pagenum" id="Page_46">[46]</span></p>
-
-<p>I say a proximate interpretation, because there remain to be solved
-certain deeper problems; one of which at once presents itself to be
-dealt with under the present head. Leaves, petals, stamens, &amp;c., being
-shown to be homologous foliar organs; and the part to which they are
-attached, proving to be an indefinitely-extended axis of growth, or
-axial organ; we are met by the questions,&mdash;What is a foliar organ? and
-What is an axial organ? The morphological composition of a Phænogam is
-undetermined, so long as we cannot say to what lower structures leaves
-and shoots are homologous; and how this integration of them originates.
-To these questions let us now address ourselves.</p>
-
-
-<p class="p2" id="sect190">§ 190–1. Already, in <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect78">§ 78</a>, reference has been made to the occasional
-development of foliar organs into axial organs: the special case there
-described being that of a fox-glove, in which some of the sepals were
-replaced by flower-buds. The observation of these and some analogous
-monstrosities, raising the suspicion that the distinction between
-foliar organs and axial organs is not absolute, led me to examine into
-the matter; and the result has been the deepening of this suspicion
-into a conviction. Part of the evidence is given in Appendix A.</p>
-
-<p>Some time after having reached this conviction, I found on looking
-into the literature of the subject, that analogous irregularities had
-suggested to other observers, beliefs similarly at variance with the
-current morphological creed. Difficulties in satisfactorily defining
-these two elements, have served to shake this creed in some minds.
-To others, the strange leaf-like developments which axes undergo in
-certain plants, have afforded reasons for doubting the constancy of
-this distinction which vegetal morphologists usually draw. And those
-not otherwise rendered sceptical, have been made to hesitate by such
-cases as that of the Nepaul-barley, in which the glume, a foliar
-organ, becomes developed into an axis and bears flowers. In his
-essay&mdash;“Vegetable Morphology:<span class="pagenum" id="Page_47">[47]</span> its History and Present Condition,”<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">[7]</a>
-whence I have already quoted, Dr. Masters indicates sundry of
-the grounds for thinking that there is no impassable demarcation
-between leaf and stem. Among other difficulties which meet us if
-we assume that the distinction is absolute, one is implied by this
-question:&mdash;“What shall we say to cases such as those afforded by the
-leaves of <i>Guarea</i> and <i>Trichilia</i>, where the leaves after a
-time assume the condition of branches and develop young leaflets from
-their free extremities, a process less perfectly seen in some of the
-pinnate-leaved kinds of <i>Berberis</i> or <i>Mahonia</i>, to be found
-in almost every shrubbery?”</p>
-
-<p>A class of facts on which it will be desirable for us here to dwell
-a moment, before proceeding to deal with the matter deductively, is
-presented by the <i>Cactaceæ</i>. In this remarkable group of plants,
-deviating in such varied ways from the ordinary phænogamic type, we
-find many highly instructive modifications of form and structure. By
-contemplating the changes here displayed within the limits of a single
-order, we shall greatly widen our conception of the possibilities of
-metamorphosis in the vegetal kingdom, taken as a whole. Two different,
-but similarly-significant, truths are illustrated. First, we are shown
-how, of these two components of a flowering plant, commonly regarded
-as primordially distinguished, one may assume, throughout numerous
-species, the functions, and to a great degree the appearance, of the
-other. Second, we are shown how, in the same individual, there may
-occur a re-metamorphosis: the usurped function and appearance being
-maintained in one part of the plant, while in another part there is
-a return to the ordinary appearance and function. We will consider
-these two truths separately. Some of the <i>Euphorbiaceæ</i>, which
-simulate Cactuses, show us the stages through which such abnormal
-structures are arrived at. In <i>Euphorbia splendens</i>, the lateral
-axes are considerably swollen at their distal ends, so as often to
-be club-shaped: still, however, being covered with bark<span class="pagenum" id="Page_48">[48]</span> of the
-ordinary colour, and still bearing leaves. But in kindred plants,
-as <i>Euphorbia neriifolia</i>, this swelling of the lateral axes
-is carried to a far greater extent; and, at the same time, a green
-colour and a fleshy consistence have been acquired: the typical
-relations nevertheless being still shown by the few leaves that grow
-out of these soft and swollen axes. In the <i>Cactaceæ</i>, which
-are thus resembled by plants not otherwise allied to them, we have
-indications of a parallel transformation. Some kinds, not commonly
-brought to England, bear leaves; but in the species most familiar to
-us, the leaves are undeveloped and the axes assume their functions.
-Passing over the many varieties of form and combination which these
-green succulent growths display, we have to note that in some genera,
-as in <i>Phyllocactus</i>, they become flattened out into foliaceous
-shapes, having mid-ribs and something approaching to veins. So that
-here, and in the genus <i>Epiphyllum</i>, which has this character
-still more marked, the plant appears to be composed of fleshy leaves
-growing one upon another. And then, in <i>Rhipsalis</i>, the same
-parts are so leaf-like, that an uncritical observer would regard them
-as leaves. These which are axial organs in their homologies, have
-become foliar organs in their analogies. When, instead of comparing
-these strangely-modified axes in different genera of Cactuses, we
-compare them in the same individual, we meet with transformations no
-less striking. Where a tree-like form is produced by the growth of
-these foliaceous shoots, one on another; and where, as a consequence,
-the first-formed of them become the main stem that acts as support
-to secondary and tertiary stems; they lose their green, succulent
-character, acquire bark, and become woody. In resuming the functions of
-axes they resume the structures of axes, from which they had deviated.
-In Fig. <a href="#i_049">71</a> are shown some of the leaf-like axes of <i>Rhipsalis
-rhombea</i> in their young state; while Fig. <a href="#i_049">72</a> represents the oldest
-portion of the same plant, in which the foliaceous characters are
-quite obliterated, and there has resulted<span class="pagenum" id="Page_49">[49]</span> an ordinary stem-structure.
-One further fact is to be noted. At the same time that their
-leaf-like appearances are lost, the axes also lose their separate
-individualities. As they become stem-like, they also become integrated;
-and they do this so effectually that their original points of junction,
-at first so strongly marked, are effaced, and a consolidated trunk is
-produced.</p>
-
-  <div class="figcenter" id="i_049">
-     <img
-    class="p2"
-    src="images/i_049.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 71–72.</p>
-  </div>
-
-<p class="p2">Joined with the facts previously specified, these facts help us to
-conceive how, in the evolution of flowering plants in general, the
-morphological components that were once distinct, may become extremely
-disguised. We may rationally expect that during so long a course of
-modification, much greater changes of form, and much more decided
-fusions of parts, have taken place. Seeing how, in an individual
-plant, the single leaves pass into compound leaves, by the development
-of their veins into mid-ribs while their petioles begin to simulate
-axes; and seeing that leaves ordinarily exhibiting definitely-limited
-developments, occasionally produce other leaves from their edges; we
-are led to suspect the possibility of still greater changes in foliar
-organs. When, further, we find that within the limits of one natural
-order, petioles usurp the functions and appearances of leaves, at the
-same time that in other orders, as in <i>Ruscus</i>, lateral axes so
-simulate leaves that their axial nature would by most not be suspected,
-did they not bear flowers on their mid-ribs or edges; and when, among
-Cactuses, we perceive that such metamorphoses and re-metamorphoses
-take place with great facility; our suspicion that the morphological
-elements of Phænogams admit of profound transformations, is deepened.
-And then, on discovering how frequent are the monstrosities which do
-not seem satisfactorily explicable without admitting the development
-of foliar organs into axial organs; we become<span class="pagenum" id="Page_50">[50]</span> ready to entertain
-the hypothesis that during the evolution of the phænogamic type, the
-distinction between leaves and axes has arisen by degrees.</p>
-
-<p>With our preconceptions loosened by such facts, and carrying with us
-the general idea which such facts suggest, let us now consider in what
-way the typical structure of a flowering plant may be interpreted.</p>
-
-
-<p class="p2">§ 192. To proceed methodically, we must seek a clue to the structures
-of Phanerogams, in the structures of those inferior plants that
-approach to them&mdash;<i>Archegoniatæ</i>. The various divisions of this
-class present, along with sundry characters which ally them with
-Thallophytes, other characters by which the phænogamic structure
-is shadowed forth. While some of the inferior <i>Hepaticæ</i> or
-Liverworts, severally consist of little more than a thallus-like
-frond, among the higher members of this group, and still more among
-the Mosses and Ferns, we find a distinctly marked stem.<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">[8]</a> Some
-Archegoniates (or rather Rhizoids) have foliar expansions that are
-indefinite in their forms; and some have quite definitely-shaped
-leaves. Roots are possessed by all the more-developed genera of the
-class; but there are other genera, as <i>Sphagnum</i>, which have no
-roots. Here the fronds are formed of only a single layer of cells;
-and there a double layer gives them a higher character&mdash;a difference<span class="pagenum" id="Page_51">[51]</span>
-exhibited between closely-allied genera of one group, the Mosses.
-Equally varied are the developments of the foliar organs in their
-detailed structures: now being without mid-ribs or veins; now having
-mid-ribs but no veins; now having both mid-ribs and veins. Nor must
-we omit the similarly-significant circumstance, that whereas in the
-lower Archegoniates the reproductive elements are immersed here and
-there in the thallus-like frond, they are, in the higher orders, seated
-in well-specialized and quite distinct fructifying organs, having
-analogies with the flowers of Phænogams. Thus, many facts imply that if
-the Phænogamic type is to be analyzed at all, we must look among the
-Archegoniates for its morphological components, and the manner of their
-integration.</p>
-
-<p>Already we have seen among the lower <i>Cryptogamia</i>, how, as they
-became integrated and definitely limited, aggregates acquire the habit
-of budding out other aggregates, on reaching certain stages of growth.
-Cells produce other cells endogenously or exogenously; and fronds
-give origin to other fronds from their edges or surfaces. We have
-seen, too, that the new aggregates so produced, whether of the first
-order or the second order, may either separate or remain connected.
-Fissiparously-multiplying cells in some cases part company, while in
-other cases they unite into threads or laminæ or masses; and fronds
-originating proliferously from other fronds, sometimes when mature
-disconnect themselves from their parents, and sometimes continue
-attached to them. Whether they do or do not part, is clearly determined
-by their nutrition. If the conditions are such that they can severally
-thrive better by separating after a certain development is reached,
-it will become their habit then to separate; since natural selection
-will favour the propagation of those which separate most nearly at
-that time. If, conversely, it profits the species for the cells or
-fronds to continue longer attached, which it can only do if their
-growths and subsequent powers of multiplication are thereby increased,
-it must happen, through the continual survival of the fittest, that<span class="pagenum" id="Page_52">[52]</span>
-longer attachment will become an established characteristic; and,
-by persistence in this process, permanent attachment will result
-when permanent attachment is advantageous. That disunion is really
-a consequence of relative innutrition, and union a consequence of
-relative nutrition, is clear <i>à posteriori</i>. On the one hand, the
-separation of the new individuals, whether in germs or as developed
-aggregates, is a dissolving away of the connecting substance; and
-this implies that the connecting substance has ceased to perform its
-function as a channel of nutriment. On the other hand, where, as
-we see among Phænogams, there is about to take place a separation
-of new individuals in the shape of germs, at the point where the
-nutrition is the lowest, a sudden increase of nutrition will cause the
-impending separation to be arrested; and the fructifying elements,
-reverting towards the ordinary form, thereupon develop in connexion
-with the parent. Turning to the Archegoniates, we find among them
-many indications of this transition from discontinuous development
-to continuous development. Thus the Liverworts give origin to new
-plants by cells which they throw off from their surfaces; as, indeed,
-we have seen that much higher plants do. “According to Bischoff,”
-says Schleiden, “both the cells of the stem (<i>Jungermannia</i> [now
-<i>Lophocolea</i>] <i>bidentata</i>) and those of the leaves (<i>J.
-exsecta</i>) separate themselves as propagative cells from the plant,
-and isolated cells shoot out and develop while still connected with the
-parent plant into small cellular bodies (<i>Metzgeria furcata</i>),
-which separate from the plant, and grow into new plants, as in <i>Mnium
-androgynum</i> among the Mosses.” Now in the way above explained,
-these propagative cells and proliferous buds, may continue developing
-in connexion with the parent to various degrees before separating; or
-the buds which are about to become fructifying organs may similarly,
-under increased nutrition, develop into young fronds. As Sir W. Hooker
-says of the male fructification in <i>Metzgeria furcata</i>,&mdash;“It
-has the appearance of being a young shoot or innovation (for in
-colour<span class="pagenum" id="Page_53">[53]</span> and texture I can perceive no difference) rolled up into a
-spherical figure.” On finding in this same plant, that sometimes the
-proliferously-produced frond buds out from itself another frond before
-separating from the parent, as shown in Fig. <a href="#i_034">46</a>, it becomes clear
-that this long-continued connexion may readily pass into permanent
-connexion. And when we see how, even among Phænogams, buds may either
-detach themselves as bulbils, or remain attached and become shoots; we
-can scarcely doubt that among inferior plants, less definite in their
-modes of organization, such transitions must continually occur.</p>
-
-  <div class="figcenter" id="i_053">
-     <img
-    class="p2"
-    src="images/i_053.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 73–76.</p>
-  </div>
-
-<p class="p2">Let us suppose, then, that Fig. <a href="#i_053">73</a> is the frond of some primitive
-Archegoniate, similar in general characters to <i>Pellia epiphylla</i>,
-Fig. <a href="#i_033">43</a>; bearing, like it, the fructifying buds on its upper surface,
-and having a slightly-marked mid-rib and rootlets. And suppose
-that, as shown, a secondary frond is proliferously produced from
-the mid-rib, and continues attached to it. Evidently the ordinary
-discontinuous development, can thus become a continuous development,
-only on condition that there is an adequate supply, to the secondary
-frond, of such materials as are furnished by the rootlets: the
-remaining materials being obtainable by itself from the air. Hence,
-that portion of the mid-rib lying between the secondary frond and
-the chief rootlets, having its function increased, will increase in
-bulk. An additional consequence will be a greater concentration of
-the rootlets&mdash;there will be extra growth of those which are most
-serviceably placed. Observe,<span class="pagenum" id="Page_54">[54]</span> next, that the structure so arising
-is likely to be maintained. Such a variation implying, as it does,
-circumstances especially favourable to the growth of the plant, will
-give to the plant extra chances of leaving descendants; since the area
-of frond supported by a given area of the soil, being greater than
-in other individuals, there may be a greater production of spores.
-And then, among the more numerous descendants thus secured by it, the
-variation will give advantages to those in which it recurs. Such a
-mode of growth having, in this manner, become established, let us ask
-what is next likely to result. If it becomes the habit of the primary
-frond to bear a secondary frond from its mid-rib, this secondary frond,
-composed of physiological units of the same kind, will inherit the
-habit; and supposing that the supply of mineral matters obtained by
-the rootlets suffices for the full development of the secondary frond,
-there is a likelihood that the growth from it of a tertiary frond,
-will become an habitual characteristic of the variety. Along with the
-establishment of such a tertiary frond, as shown in Fig. <a href="#i_053">74</a>, there
-must arise a further development of mid-rib in the primary frond, as
-well as in the secondary frond&mdash;a development which must bring with it
-a greater integration of the two; while, simultaneously, extra growth
-will take place in such of the rootlets as are most directly connected
-with this main channel of circulation. Without further explanation it
-will be seen, on inspecting Figs. <a href="#i_053">75 and 76</a>, that there may in this
-manner result an integrated series of fronds, placed alternately on
-opposite sides of a connecting vascular structure. That this connecting
-vascular structure will, as shown in the figures, become more distinct
-from the foliar surfaces as these multiply, is no unwarranted
-assumption; for we have seen in compound-leaved plants, how, under
-analogous conditions, mid-ribs become developed into separate
-supporting parts, which acquire some of the characters of axes while
-assuming their functions. And now mark how clearly the structure thus
-built up by integration<span class="pagenum" id="Page_55">[55]</span> of proliferously-growing fronds, corresponds
-with the structure of the more-developed <i>Jungermanniaceæ</i>. Each
-of the fronds successively produced, repeating the characters of its
-parent, will bear roots; and will bear them in homologous places, as
-shown. Further, the united mid-ribs having but very little rigidity,
-will be unable to maintain an erect position. Hence there will result
-the recumbent, continuously-rooted stem, which these types exhibit: an
-embryo phænogam having the weakness of an embryo.<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">[9]</a></p>
-
-<p>A natural concomitant of the mode of growth here described, is that
-the stem, while it increases longitudinally, increases scarcely at
-all transversely: hence the old name Acrogens. Clearly the transverse
-development of a stem is the correlative, partly of its function as
-a channel of circulation, and partly of its function as a mechanical
-support. That an axis may lift its attached leaves into the air,
-implies thickness and solidity proportionate to the mass of such
-leaves; and an increase of its sap-vessels, also proportionate to the
-mass of such leaves, is necessitated when the roots are all at one end
-and the leaves at the other. But in the generality of Acrogens, these
-conditions, under which arises the necessity for transverse growth
-of the axis, are absent wholly or in great part. The stem habitually
-creeps below the surface, or lies prone upon the surface; and where
-it grows in a vertical or inclined direction, does this by attaching
-itself to a vertical or inclined object. Moreover, throwing out
-rootlets, as it mostly does, at intervals throughout its length, it
-is not called upon in any considerable degree, to transfer nutritive
-materials from one of its ends to the other.<span class="pagenum" id="Page_56">[56]</span> Hence this peculiarity
-which gives their name to the Acrogens, now called Archegoniates, is a
-natural accompaniment of the low degree of specialization reached in
-them. And that it is an incidental and not a necessary peculiarity, is
-demonstrated by two converse facts. On the one hand, in those higher
-Acrogens which, like the tree-ferns, lift large masses of foliage into
-the air, there is just as decided a transverse expansion of the axis
-as in dicotyledonous trees. On the other hand, in those Dicotyledons
-which, like the common Dodder, gain support and nutriment from the
-surfaces over which they creep, there is no more lateral expansion of
-the axis than is habitual among Acrogens or Archegoniates. Concluding,
-as we are thus fully justified in doing, that the lateral expansion
-accompanying longitudinal extension, which is a general characteristic
-of Phanerogams as distinguished from Archegoniates, is nothing more
-than a concomitant of their usually-vertical growth;<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">[10]</a> let us now
-go on to consider how vertical growth originates, and what are the
-structural changes it involves.</p>
-
-
-<p class="p2">§ 193. Plants depend for their prosperity mainly on air and light: they
-dwindle where they are smothered, and thrive where they can expand
-their leaves into free space and sunshine. Those kinds which assume
-prone positions, consequently labour under disadvantages in being
-habitually interfered with by one another&mdash;they are mutually shaded
-and<span class="pagenum" id="Page_57">[57]</span> mutually injured. Such of them, however, as happen, by variations
-in mode of growth, to rise higher than others, are more likely to
-flourish and leave offspring than others. That is to say, natural
-selection will favour the more upright-growing forms. Individuals
-with structures which lift them above the rest, are the fittest for
-the conditions; and by the continual survival of the fittest, such
-structures must become established. There are two essentially-different
-ways in which the integrated series of fronds above described, may
-be modified so as to acquire the stiffness needful for maintaining
-perpendicularity. We will consider them separately.</p>
-
-  <div class="figcenter" id="i_057">
-     <img
-    class="p2"
-    src="images/i_057.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 77, 78.</p>
-  </div>
-
-<p class="p2">A thin layer of substance gains greatly in power of resisting a
-transverse strain, if it is bent round so as to form a tube: witness
-the difference between the pliability of a sheet of paper when
-outspread, and the rigidity of the same sheet of paper when rolled up.
-Engineers constantly recognize this truth, in devising appliances by
-which the greatest strength shall be obtained at the smallest cost of
-material; and among organisms, we see that natural selection habitually
-establishes structures conforming to the same principle, wherever
-lightness and stiffness are to be combined. The cylindrical bones of
-mammals and birds, and the hollow shafts of feathers, are examples.
-The lower plants, too, furnish cases where the strength needful for
-maintaining an upright position, is acquired by this rolling up of
-a flat thallus or frond. In Fig. <a href="#i_057">77</a> we have an <i>Alga</i> which
-approaches towards a tubular distribution of substance; and which has a
-consequent rigidity. Sundry common forms of lichen, having the thallus
-folded into a branched tube, still more decidedly display the connexion
-between this structural arrangement<span class="pagenum" id="Page_58">[58]</span> and this mechanical advantage.
-And from the particular class of plants we are here dealing with&mdash;the
-Archegoniates&mdash;a type is shown in Fig. <a href="#i_057">78</a>, <i>Riella helicophylla</i>,
-similarly characterized by a thin frond that is made stiff enough to
-stand, by an incurving which, though it does not produce a hollow
-cylinder, produces a kindred form. If, then, as we have seen, natural
-selection or survival of the fittest will favour such among these
-recumbent Archegoniates as are enabled, by variations in their
-structures, to maintain raised postures; it will favour the formation
-of fronds that curve round upon themselves, and curve round upon the
-fronds growing out of them. What, now, will be the result should such a
-modification take place in the group of proliferous fronds represented
-in Fig. <a href="#i_053">76</a>? Clearly, the result will be a structure like that shown
-in Fig. <a href="#i_058">79</a>. And if this inrolling becomes more complete, a form like
-<i>Jungermannia cordifolia</i>, represented in Fig. <a href="#i_058">80</a>, will be
-produced.</p>
-
-  <div class="figcenter" id="i_058">
-     <img
-    class="p2"
-    src="images/i_058.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 79, 80.</p>
-  </div>
-
-  <div class="figcenter" id="i_059">
-     <img
-    class="p2"
-    src="images/i_059.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 81–89.</p>
-  </div>
-
-  <div class="figcenter" id="i_060">
-     <img
-    class="p2"
-    src="images/i_060.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 90, 91.</p>
-  </div>
-
-<p class="p2">When the successive fronds are thus folded round so completely that
-their opposite edges meet, these opposite edges will be apt to unite:
-not that they will grow together after being formed, but that they will
-develop in connexion; or, in botanical language, will become “adnate.”
-That foliar surfaces which, in their embryonic state, are in close
-contact, often join into one, is a familiar fact. It is habitually so
-with sepals or divisions of the calyx. In all campanulate flowers it is
-so with petals. And in some tribes of plants it is so with stamens. We
-are therefore well warranted in inferring that, under the conditions
-above described, the successive fronds or leaflets will, by union of
-their remote edges, first at their points of origin and afterwards
-higher up,<span class="pagenum" id="Page_59">[59]</span> form sheaths inserted one within another, and including the
-axis. This incurving of the successive fronds, ending in the formation
-of sheaths, may be accompanied by different sets of modifications.
-Supposing Fig. <a href="#i_059">81</a> to be a transverse section of such type (<i>a</i>
-being the mid-rib, and <i>b</i> the expansion of an older frond; while
-<i>c</i> is a younger frond proliferously developed within it), there
-may begin two divergent kinds of changes, leading to two contrasted
-structures. If, while frond continues to grow out of frond, the series
-of united mid-ribs continues to be the channel of circulation between
-the uppermost fronds and the roots&mdash;if, as a consequence, the compound
-mid-rib, or rudimentary axis, continues to increase in size laterally;
-there will arise the series of transitional forms represented by the
-transverse sections 82, 83, 84, 85; ending in the production of a solid
-axis, everywhere wrapped round by the foliar surface of the frond, as
-an outer layer or sheath. But if, on the other hand, circumstances
-favour a form of plant which maintains its uprightness at the smallest
-cost of substance&mdash;if the vascular bundles of each succeeding mid-rib,
-instead of remaining concentrated, become distributed all round the
-tube<span class="pagenum" id="Page_60">[60]</span> formed by the infolded frond; then the structure eventually
-reached, through the transitional forms 86, 87, 88, 89, will be a
-hollow cylinder.<a id="FNanchor_11" href="#Footnote_11" class="fnanchor">[11]</a> And now observe how the two structures thus
-produced, correspond with two kinds of Monocotyledons. Fig. <a href="#i_060">90</a>
-represents a species of <i>Dendrobium</i>, in which we see clearly
-how each leaf is but a continuation of the external layer of a solid
-axis&mdash;a sheath such as would result from the infolded edges of a frond
-becoming adnate; and on examining how the sheath of each leaf includes
-the one above it, and how the successive sheaths include the axis, it
-will be manifest that the relations of parts are just such as exist
-in the united series of fronds shown in Fig. <a href="#i_058">79</a>&mdash;the successive nodes
-answering to the successive points of origin of the fronds. Conversely,
-the stem of a grass, Fig. <a href="#i_060">91</a>, displays just such relations of parts,
-as would result from the development of the type shown in Fig. <a href="#i_058">79</a>,
-if instead of the mid-ribs thickening into a solid axis, the matter
-composing them became evenly distributed round the foliar surfaces,
-at the same time that the incurved edges of the foliar surfaces
-united. The arrangements of the tubular axis and its appendages,
-thus resulting, are still more instructive than those<span class="pagenum" id="Page_61">[61]</span> of the solid
-axis. For while, even more clearly than in the <i>Dendrobium</i>,
-we see at the point <i>b</i>, a continuity of structure between the
-substance of the axis below the node, and the substance of the sheath
-above the node: we see that this sheath, instead of having its edges
-united as in <i>Dendrobium</i>, has them simply overlapping, so as
-to form an incomplete hollow cylinder which may be taken off and
-unrolled; and we see that were the overlapping edges of this sheath
-united all the way from the node <i>a</i> to the node <i>b</i>, it
-would constitute a tubular axis, like that which precedes it or like
-that which it includes. And then, giving an unexpected conclusiveness
-to the argument, it turns out that in one family of grasses, the
-overlapping edges of the sheaths <i>do</i> unite: thus furnishing us
-with a demonstration that tubular structures <i>are</i> produced by
-the incurving and joining of foliar surfaces; and that so, hollow axes
-may be interpreted as above, without making any assumption unwarranted
-by fact. One further correspondence between the type thus ideally
-constructed, and the monocotyledonous type, must be noted. If, as
-already pointed out, the transverse growth of an axis arises when the
-axis comes to be a channel of circulation between all the roots at one
-of its extremities and all the leaves at the other; and if this lateral
-bulging must increase as fast as the quantity of foliage to be brought
-in communication with the roots increases&mdash;especially if such foliage
-has at the same time to be raised high above the earth’s surface; what
-must happen to a plant constructed in the manner just described? The
-elder fronds or foliar organs, ensheathing the younger ones, as well
-as the incipient axis serving as a bond of union, are at first of such
-circumference only as suffices to inclose these undeveloped parts.
-What, then, will take place when the inclosed parts grow&mdash;when the axis
-thickens while it elongates? Evidently the earliest-formed sheaths, not
-being large enough for the swelling axis, must burst; and evidently
-each of the later-formed sheaths must, in its turn, do the like. There
-must<span class="pagenum" id="Page_62">[62]</span> result a gradual exfoliation of the successive sheaths, like that
-indicated as beginning in the above figure of <i>Dendrobium</i>; which,
-at <i>a</i>, shows the bud of the undeveloped parts just visible above
-the enwrapping sheaths, while at <i>b</i>, and <i>c</i>, it shows the
-older sheaths in process of being split open. That is to say, there
-must result the mode of growth which helped to give the name Endogens
-to this class.</p>
-
-  <div class="figcenter" id="i_063">
-     <img
-    class="p2"
-    src="images/i_063.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 92–94.</p>
-  </div>
-
-  <div class="figcenter" id="i_064">
-     <img
-    class="p2"
-    src="images/i_064.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 95–99.</p>
-  </div>
-
-<p class="p2">The other way in which an integrated series of fronds may acquire
-the rigidity needful for maintaining an erect position, has next to
-be considered. If the successive fronds do not acquire such habit
-of curling as may be taken advantage of by natural selection, so as
-to produce the requisite stiffness; then, the only way in which the
-requisite stiffness appears producible, is by the thickening and
-hardening of the fused series of mid-ribs. The incipient axis will not,
-in this case, be inclosed by the rolled-up fronds; but will continue
-exposed. Survival of the fittest will favour the genesis of a type, in
-which those portions of the successive mid-ribs that enter into the
-continuous bond, become more bulky than the disengaged portions of the
-mid-ribs: the individuals which thrive and have the best chances of
-leaving offspring, being, by the hypothesis, individuals having axes
-stiff enough to raise their foliage above that of their fellows. At
-the same time, under the same influences, there will tend to result
-an elongation of those portions of the mid-ribs, which become parts
-of the incipient axis; seeing that it will profit the plant to have
-its leaves so far removed from one another, as to prevent mutual
-interferences. Hence, from the recumbent type there will evolve, by
-indirect equilibration (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect167">§ 167</a>), such modifications as are shown in
-Figs. <a href="#i_063">92, 93, 94</a>; the first of which is a slight advance on the ideal
-type represented in Fig. <a href="#i_053">76</a>, arising in the way described; and the
-others of which are actual plants&mdash;<i>Haplomitrium Hookeri</i>, and
-<i>Plagiochila decipiens</i>. Thus the higher Archegoniates show us
-how, along with an assumption of the upright attitude, there does go
-on, as we see there must go on, a separation<span class="pagenum" id="Page_63">[63]</span> of the leaf-producing
-parts from the root-producing parts; a greater development of that
-connecting portion of the successive fronds, by which they are kept
-in communication with the roots, and raised above the ground; and
-a consequent increased differentiation of such connecting portion
-from the parts attached to it. And this lateral bulging of the axis,
-directly or indirectly consequent on its functions as a support and
-a channel, being here unrestrained by the early-formed fronds folded
-round it, goes on without the bursting of these. Hence arises a
-leading character of what is called exogenous growth&mdash;a growth which
-is, however, still habitually accompanied by exfoliation, in flasks,
-of the outermost layers, continually being cracked and split by the
-accumulation of layers within them. And now if we examine plants of
-the exogenous type, we find among them many displaying the stages of
-this metamorphosis. In Fig. <a href="#i_064">95</a>, is shown a form in which the continuity
-of the axis with the mid-rib of the leaf, is manifest&mdash;a continuity
-that is conspicuous in the common thistle. Here the foliar expansion,
-running some distance down the axis, makes the included portion of the
-axis a part of its mid-rib; just as in the ideal types above drawn.
-By the greater growth of the internodes,<span class="pagenum" id="Page_64">[64]</span> which are very variable,
-not only in different plants but in the same plant, there results a
-modification like that delineated in Fig. <a href="#i_064">96</a>. And then, in such forms
-as Fig. <a href="#i_064">97</a>, there is shown the arrangement that arises when, by more
-rapid development of the proximal end of the mid-rib, the distal part
-of the foliar surface is separated from the part which embraces the
-axis: the wings of the mid-rib still serving, however, to connect the
-two portions of the foliar surface. Such a separation is, as pointed
-out in <a href="#sect188">§ 188</a>, an habitual occurrence; and in some compound leaves, an
-actual tearing of the inter-venous tissue is caused by extra growth
-of the mid-rib. Modifications like this, and the further one in Fig.
-<a href="#i_064">98</a>, we may expect to be established by survival of the fittest, among
-those plants which produce considerable masses of leaves; since the
-development of mid-ribs into foot-stalks, by throwing the leaves
-further away from the axes, will diminish the shading of the leaves,
-one by another. And then, among plants of bushy growth, in which
-the assimilating surfaces become still more liable to intercept one
-another’s light, natural selection will continue to give an advantage
-to those which carry their assimilating surfaces at the ends of the
-petioles, and do not develop assimilating surfaces close to the
-axis, where they are most shaded. Whence will result a disappearance
-of the stipules and the foliar fringes of the mid-ribs; ending in
-the production of the ordinary stalked leaf, Fig. <a href="#i_064">99</a>, which is
-characteristic of trees. Meanwhile, the axis thickens in proportion to
-the number of leaves it has to carry, and to put in communication with
-the roots; and so<span class="pagenum" id="Page_65">[65]</span> there comes to be a more marked contrast between it
-and the petioles, severally carrying a leaf each.<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">[12]</a></p>
-
-
-<p class="p2">§ 194. When, in the course of the process above sketched out, there has
-arisen such community of nutrition among the fronds thus integrated
-into a series, that the younger ones are aided by materials which the
-older ones have elaborated; the younger fronds will begin to show, at
-earlier and earlier periods of development, the structures about to
-originate from them. Abundant nutrition will abbreviate the intervals
-between the successive prolifications; so that eventually, while each
-frond is yet imperfectly formed, the rudiment of the next will begin to
-show itself. All embryology justifies this inference. The analogies it
-furnishes lead us to expect that when this serial arrangement becomes
-organic, the growing part of the series will show the general relations
-of the forthcoming parts, while they are very small and unspecialized.
-What will in such case be the appearances they assume? We shall have
-no difficulty in perceiving what it will be, if we take a form like
-that shown in Fig. <a href="#i_063">92</a>, and dwarf its several parts at the same time
-that we generalize them. Figs. <a href="#i_065">100, 101, 102, and 103</a>, will show the
-result; and in Fig. <a href="#i_065">104</a>, which is the bud of a dicotyledon, we see how
-clear is the morphological correspondence: <i>a</i> being the rudiment
-of a foliar organ beginning to take shape; <i>b</i> being the almost
-formless rudiment of the next foliar organ; and<span class="pagenum" id="Page_66">[66]</span> <i>c</i> being the
-quite-undifferentiated part whence the rudiments of subsequent foliar
-organs are to arise.</p>
-
-  <div class="figcenter" id="i_065">
-     <img
-    class="p2"
-    src="images/i_065.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 100–104.</p>
-  </div>
-
-  <div class="figcenter" id="i_066">
-     <img
-    class="p2"
-    src="images/i_066.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 105–106.</p>
-  </div>
-
-<p class="p2">And now we are prepared for entering on a still-remaining question
-respecting the structure of Phænogams&mdash;what is the origin of axillary
-buds? As the synthesis at present stands, it does not account for
-these; but on looking a little more closely into the matter, we shall
-find that the axillary buds are interpretable in the same manner as the
-terminal buds. So to interpret them, however, we must return to that
-process of proliferous growth with which we set out, for the purpose of
-observing some facts not before named. <i>Delesseria hypoglossum</i>,
-Fig. <a href="#i_066">105</a>, represents a seaweed of the same genus as one outlined in
-Fig. <a href="#i_031">40</a>; but of a species in which proliferous growth is carried much
-further. Here, not only does the primary frond bud out many secondary
-fronds from its mid-rib; but most of the secondary fronds similarly
-bud out several tertiary fronds; and even by some of the tertiary
-fronds, this prolification is repeated. Besides being shown that the
-budding out of several fronds from one frond, may become habitual; we
-are also shown that it may become a<span class="pagenum" id="Page_67">[67]</span> habit inherited by the fronds
-so produced, and also by the fronds they produce: the manifestation
-of the tendency being probably limited only by failure of nutrition.
-That under fit conditions an analogous mode of growth will occur in
-fronds of the acrogenic type, like those we set out with, is shown
-by the case of <i>Metzgeria furcata</i>, Figs. <a href="#i_034">45, 46,</a> in which such
-compound prolification is partially displayed. Let us suppose, then,
-that the frond <i>a</i>, Fig. <a href="#i_066">106</a>, produces not only a single secondary
-frond <i>b</i>, but also another such secondary frond <i>b’</i>. Let
-us suppose, further, that the frond <i>b</i> is in like manner doubly
-proliferous: producing both <i>c</i> and <i>c’</i>. Lastly, let us
-suppose that in the second frond <i>b’</i> which <i>a</i> produces,
-as well as in the second frond <i>c’</i> which <i>b</i> produces, the
-doubly-proliferous habit is manifested. If, now, this habit grows
-organic&mdash;if it becomes, as it naturally will become, the characteristic
-of a plant of luxuriant growth, the unfolding parts of which can be
-fed by the unfolded parts; it will happen with each lateral series,
-as with the main series, that its successive components will begin to
-show themselves at earlier and earlier stages of development. And in
-the same way that, by dwarfing and generalizing the original series, we
-arrive at a structure like that of the terminal bud; by dwarfing and
-generalizing a lateral series, as shown in Figs. <a href="#i_067">107–110</a>, we arrive at
-a structure answering in nature and position to the axillary bud.</p>
-
-  <div class="figcenter" id="i_067">
-     <img
-    class="p2"
-    src="images/i_067.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 107–110.</p>
-  </div>
-
-<p class="p2">Facts confirming these interpretations are afforded by the structure
-and distribution of buds. The phænogamic axis in its primordial form,
-being an integrated series of folia; and the development of that part
-by which these folia are held together at considerable distances
-from one another, taking place afterwards; it is inferable from the
-general<span class="pagenum" id="Page_68">[68]</span> principles of embryology, that in its rudimentary stages, the
-phænogamic shoot will have its foliar parts more clearly marked out
-than its axial parts. This we see in every bud. Every bud consists
-of the rudiments of leaves packed together without any appreciable
-internodal spaces; and the internodal spaces begin to increase with
-rapidity, only when the foliar organs have been considerably developed.
-Moreover, where nutrition falls short, and arrest of development
-takes place&mdash;that is, where a flower is formed&mdash;the internodes remain
-undeveloped: the unfolding ceases before the later-acquired characters
-of the phænogamic shoot are assumed. Lastly, as the hypothesis leads us
-to expect, axillary buds make their appearances later than the foliar
-organs which they accompany; and where, as at the ends of shoots, these
-foliar organs show failure of chlorophyll, the axillary buds are not
-produced at all. That these are inferable traits of structure, will be
-manifest on inspecting Figs. <a href="#i_066">106</a>–<a href="#i_067">110</a>; and on observing, first, that
-the doubly-proliferous tendency of which the axillary bud is a result,
-implies abundant nutrition; and on observing, next, that the original
-place of secondary prolification, is such that the foliar surface on
-which it occurs, must grow to some extent before the bud appears.</p>
-
-<p>On thus looking at the matter&mdash;on contemplating afresh the ideal type
-shown in Fig. <a href="#i_066">106</a>, and noting how, by the conditions of the case, the
-secondary prolifications must cease before that primary prolification
-which produces the main axis; we are enabled to reconcile all the
-phenomena of axillary gemmation. We see harmony among the several
-facts&mdash;first, that the axillary bud becomes a lateral, leaf-bearing
-axis if there is abundant material for growth; second, that its
-development is arrested, or it becomes a flower-bearing axis, if the
-supply of sap is but moderate; third, that it is absent when the
-nutrition is failing. We are no longer committed to the gratuitous
-assumption that, in the phænogamic type, there must exist an axillary
-bud to each foliar<span class="pagenum" id="Page_69">[69]</span> organ; but we are led to conclude, <i>à priori</i>,
-that which we find, <i>à posteriori</i>, that axillary buds are as
-normally absent in flowers as they are normally present lower down
-the axis. And then, to complete the argument, we are prepared for the
-corollary that axillary prolification may naturally arise even at the
-ends of axes, should the failing nutrition which causes the dwarfing of
-the foliar organs to form a flower, be suddenly changed into such high
-nutrition as to transform the components of the flower into appendages
-that are green, if not otherwise leaf-like&mdash;a condition under which
-only, this phenomenon is proved to occur.</p>
-
-
-<p class="p2">§ 195. One more question presents itself, when we contrast the
-early stages of development in the two classes of Phænogams; and a
-further answer, supplied by the hypothesis, gives to the hypothesis
-a further probability. It is characteristic of a monocotyledon, to
-have a single seed-leaf or cotyledon; and it is characteristic of a
-dicotyledon, to have at least two cotyledons, if not more than two.
-That is to say, the monocotyledonous mode of germination everywhere
-co-exists with the endogenous mode of growth; and along with the
-exogenous mode of growth, there always goes either a dicotyledonous or
-polycotyledonous germination. Why is this? Such correlations cannot
-be accidental&mdash;cannot be meaningless. A true theory of the phænogamic
-types in their origin and divergence, should account for the connexion
-of these traits. Let us see whether the foregoing theory does this.</p>
-
-<p>The higher plants, like the higher animals, bequeath to their offspring
-more or less of nutriment and structure. Superior organisms of either
-kingdom do not, as do all inferior organisms, cast off their progeny
-in the shape of minute portions of protoplasm, unorganized and without
-stocks of material for them to organize; but they either deposit
-along with the germs they cast off, certain quantities of albuminoid
-substance to be appropriated by them while they<span class="pagenum" id="Page_70">[70]</span> develop themselves,
-or else they continue to supply such substance while the germs
-partially develop themselves before their detachment. Among plants
-this constitutes one distinction between seeds and spores. Every seed
-contains a store of food to serve the young plant during the first
-stages of its independent life; and usually, too, before the seed is
-detached, the young plant is so far advanced in structure, that it
-bears to the attached stock of nutriment much the same relation that
-the young fish bears to the appended yelk-bag at the time of leaving
-the egg. Sometimes, indeed, the development of chlorophyll gives the
-seed-leaves a bright green, while the seed is still contained in the
-parent-pod. This early organization of the phænogam must be supposed
-rudely to indicate the type out of which the phænogamic type arose.
-On the foregoing hypothesis, the seed-leaves therefore represent the
-primordial fronds; which, indeed, they simulate in their simple,
-cellular, unveined structures. And the question here to be asked is&mdash;do
-the different relations of the parts in young monocotyledons and
-dicotyledons correspond with the different relations of the primordial
-fronds, implied by the endogenous and the exogenous modes of growth? We
-shall find that they do.</p>
-
-  <div class="figcenter" id="i_071">
-     <img
-    class="p2"
-    src="images/i_071.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 111–122.</p>
-  </div>
-
-<p class="p2">Starting, as before, with the proliferous form shown in Fig. <a href="#i_071">111</a>, it
-is clear that if the strength required for maintaining the vertical
-attitude, is obtained by the rolling up of the fronds, the primary
-frond will more and more conceal the secondary frond within it. At the
-same time, the secondary frond must continue to be dependent on the
-first for its nutrition; and, being produced within the first, must
-be prevented by defective supply of light and air, from ever becoming
-synchronous in its development with the first. Hence, this infolding
-which leads to the endogenous mode of growth, implies that there must
-always continue such pre-eminence of the first-formed frond or its
-representative, as to make the germination monocotyledonous. Figs.
-<a href="#i_071">111 to 115</a>, show the transitional forms that would result from the
-infolding of<span class="pagenum" id="Page_71">[71]</span> the fronds. In Fig. <a href="#i_071">116</a> (a vertical section of the form
-represented in Fig. <a href="#i_071">115</a>) are exhibited the relations of the successive
-fronds to each other. The modified relations that would result, if
-the nutrition of the embryo admitted of anticipatory development of
-the successive fronds, are shown in Fig. <a href="#i_071">117</a>. And how readily the
-structure may pass into that of the monocotyledonous germ, will be
-seen on inspecting Fig. <a href="#i_071">118</a>; which is a vertical section of an actual
-monocotyledon at an early stage&mdash;the incomplete lines at the left of
-its root, indicating its connexion with the seed.<a id="FNanchor_13" href="#Footnote_13" class="fnanchor">[13]</a> Contrariwise,<span class="pagenum" id="Page_72">[72]</span>
-where the strength required for maintaining an upright attitude is not
-obtained by the rolling up of the fronds, but by the strengthening
-of the continuous mid-rib, the second frond, so far from being less
-favourably circumstanced than the first, becomes in some respects even
-more favourably circumstanced: being above the other, it gets a greater
-share of light, and it is less restricted by surrounding obstacles.
-There is nothing, therefore, to prevent it from rapidly gaining an
-equality with the first. And if we assume, as the truths of embryology
-entitle us to do, an increasing tendency towards anticipation in the
-development of subsequent fronds&mdash;if we assume that here, as in other
-cases, structures which were originally produced in succession will,
-if the nutrition allows and no mechanical dependence hinders, come to
-be produced simultaneously; there is nothing to prevent the passage of
-the type represented in Fig. <a href="#i_071">111</a>, into that represented in Fig. <a href="#i_071">122</a>.
-Or rather, there is everything to facilitate it; seeing that natural
-selection will continually favour the production of a form in which the
-second frond grows in such way as not to shade the first, and in such
-way as allows the axis readily to assume a vertical position.</p>
-
-<p>Thus, then, is interpretable the universal connexion between
-monocotyledonous germination and endogenous growth; as well as
-the similarly-universal connexion between exogenous growth and
-the development of two or more cotyledons. That it explains these
-fundamental relations, adds very greatly to the probability of the
-hypothesis.</p>
-
-
-<p class="p2" id="sect196">§ 196. While we are in this manner enabled to discern the kinship that
-exists between the higher vegetal types themselves, as well as between
-them and the lower types; we<span class="pagenum" id="Page_73">[73]</span> are at the same time supplied with a
-rationale of those truths which vegetal morphologists have established.
-Those homologies which Wolff indicated in their chief outlines and
-Goethe followed out in detail, have a new meaning given to them when we
-regard the phænogamic axis as having been evolved in the way described.
-Forming the modified conception which we are here led to do, respecting
-the units of which a flowering plant is composed, we are no longer
-left without an answer to the question&mdash;What is an axis? And we are
-helped to understand the naturalness of those correspondences which the
-successive members of each shoot display. Let us glance at the facts
-from our present standpoint.</p>
-
-  <div class="figcenter" id="i_074">
-     <img
-    class="p2"
-    src="images/i_074.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 123–129.</p>
-  </div>
-
-<p class="p2">The unit of composition of a Phænogam, is such portion of a shoot as
-answers to one of the primordial fronds. This portion is neither one
-of the foliar appendages nor one of the internodes; but it consists of
-a foliar appendage together with the preceding internode, including
-the axillary bud where this is developed. The parts intercepted by
-the dotted lines in Fig. <a href="#i_074">123</a>, constitute such a segment; and the true
-homology is between this and any other foliar organ with the portion
-of the axis below it. And now observe how, when we take this for the
-unit of composition, the metamorphoses which the phænogamic axis
-displays, are inferable from known laws of development. Embryology
-teaches us that arrest of development shows itself first in the absence
-of those parts that have arisen latest in the course of evolution;
-that if defect of nutrition causes an earlier arrest, parts that are
-of more ancient origin abort; and that the part alone produced when
-the supply of materials fails near the outset, is the primordial
-part. We must infer, therefore, that in each segment of a Phænogam,
-the foliar organ, which answers to the primordial frond, will be the
-most constant element; and that the internode and the axillary bud,
-will be successively less constant. This we find. Along with a smaller
-size of foliar surface implying lower nutrition, it is usual to see a<span class="pagenum" id="Page_74">[74]</span>
-much-diminished internode and a less-pronounced axillary bud, as in
-Fig. <a href="#i_074">124</a>. On approaching the flower, the axillary bud disappears; and
-the segment is reduced to a small foliar surface, with an internode
-which is in most cases very short if not absent, as in 125 and 126.
-In the flower itself, axillary buds and internodes are both wanting:
-there remains only a foliar surface (127), which, though often larger
-than the immediately-preceding foliar surface, shows failing nutrition
-by absence of chlorophyll. And then, in the quite terminal organs of
-fructification (129), we have the foliar part itself reduced to a
-mere rudiment. Though these progressive degenerations are by no means
-regular, being in many cases varied by adaptations to particular
-requirements, yet it cannot, I think, be questioned, that the general
-relations are as described, and that they are such as the hypothesis
-leads us to expect. Nor are we without a kindred explanation of certain
-remaining traits of foliar organs in their least-developed forms.
-Petals, stamens, pistils, &amp;c., besides reminding us of the primordial
-fronds by their diminished sizes, and by the want of those several
-supplementary parts which the preceding segments possess, also remind
-us of them by their histological characters: they consist of simple
-cellular tissue, scarcely at all differentiated. The fructifying cells,
-too, which here make their appearance, are borne in ways like those in
-which the lower Acrogens bear them&mdash;at the edge of the frond, or at the
-end of a peduncle, or immersed in the general substance; as in Figs.
-<a href="#i_074">128 and 129</a>. Nay, it might even be said that<span class="pagenum" id="Page_75">[75]</span> the colours assumed by
-these terminal folia, call to mind the plants out of which we conclude
-that Phænogams have been evolved; for it is said of the fronds of the
-<i>Jungermanniaceæ</i>, that, “though under certain circumstances of a
-pure green, they are inclined to be shaded with red, purple, chocolate,
-or other tints.”</p>
-
-<p>As thus understood, then, the homologies among the parts of the
-phænogamic axis are interpretable, not as due to a needless adhesion
-to some typical form or fulfilment of a predetermined plan; but as
-the inevitable consequences of the mode in which the phænogamic axis
-originates.</p>
-
-
-<p class="p2">§ 197. And now it remains only to observe, in confirmation of
-the foregoing synthesis, that it at once explains for us various
-irregularities. When we see leaves sometimes producing leaflets from
-their edges or extremities, we recognize in the anomaly a resumption
-of an original mode of growth: fronds frequently do this. When we
-learn that a flowering plant, as the <i>Drosera intermedia</i>,
-has been known to develop a young plant from the surface of one of
-its leaves, we are at once reminded of the proliferous growths and
-fructifying organs in the Liverworts. The occasional production of
-bulbils by Phænogams, ceases to be so surprising when we find it to
-be habitual among the inferior Acrogens, and when we see that it is
-but a repetition, on a higher stage, of that self-detachment which
-is common among proliferously-produced fronds. Nor are we any longer
-without a solution of that transformation of foliar organs into axial
-organs, which not uncommonly takes place. How this last irregularity
-of development is to be accounted for, we will here pause a moment to
-consider. Let us first glance at our data.</p>
-
-<p>The form of every organism, we have seen, must depend on the structures
-of its physiological [or constitutional] units. Any group of such
-units will tend to arrange itself into the complete organism, if
-uncontrolled and placed in fit conditions. Hence the development of
-fertilized germs; and<span class="pagenum" id="Page_76">[76]</span> hence the development of those self-detached
-cells which characterize some plants. Conversely, physiological units
-which form a small group involved in a larger group, and are subject to
-all the forces of the larger group, will become subordinate in their
-structural arrangements to the larger group&mdash;will be co-ordinated into
-a part of the major whole, instead of co-ordinating themselves into a
-minor whole. This antithesis will be clearly understood on remembering
-how, on the one hand, a small detached part of a hydra soon moulds
-itself into the shape of an entire hydra; and how, on the other hand,
-the cellular mass that buds out in place of a lobster’s lost claw,
-gradually assumes the form of a claw&mdash;has its parts so moulded as to
-complete the structure of the organism: a result which we cannot but
-ascribe to the forces which the rest of the organism exerts upon it.
-Consequently, among plants, we may expect that whether any portion of
-protoplasm moulds itself into the typical form around an axis of its
-own, or is moulded into a part subordinate to another axis, will depend
-on the relative mass of its physiological units&mdash;the accumulation of
-them that has taken place before the assumption of any structural
-arrangement. A few illustrations will make clear the validity of this
-inference. In the compound leaf, Fig. <a href="#i_040">65</a>, the several lateral growths
-<i>a</i>, <i>b</i>, <i>c</i>, <i>d</i>, are manifestly homologous;
-and on comparing a number of such leaves together, it will be seen
-that one of these lateral growths may assume any degree of complexity,
-according to the degree of its nutrition. Every fern-leaf exemplifies
-the same general truth still better. Whether each sub-frond remains
-an undeveloped wing of the main frond, or whether it organizes itself
-into a group of frondlets borne by a secondary rib, or whether, going
-further, as it often does, it gives rise to tertiary ribs bearing
-frondlets, is determined by the supply of materials for growth; since
-such higher developments are most marked at points where the nutrition
-is greatest; namely, next the stem. But the clearest evidence is
-afforded among the <i>Algæ</i>,<span class="pagenum" id="Page_77">[77]</span> which, not drawing nutriment from
-roots, have their parts much less mutually dependent; and are therefore
-capable of showing more clearly, how any part may remain an appendage
-or may become the parent of appendages, according to circumstances. In
-the annexed Fig. <a href="#i_077">130</a>, representing a branch of <i>Ptilota plumosa</i>,
-we see how a wing grows into a wing-bearing branch if its nutrition
-passes a certain point. This form, so strikingly like that of the
-feathery crystallizations of many inorganic substances, implies that,
-as in such crystallizations, the simplicity or complexity of structure
-at any place depends on the quantity of matter that has to be arranged
-at that place in a given time.<a id="FNanchor_14" href="#Footnote_14" class="fnanchor">[14]</a></p>
-
-  <div class="figcenter" id="i_077">
-     <img
-    class="p2"
-    src="images/i_077.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 130.</p>
-  </div>
-
-<p class="p2">Hence, then, we are not without an interpretation of those
-over-developments which the phænogamic axis occasionally undergoes.
-Fig. <a href="#i_065">104</a>, represents the phænogamic bud in its rudimentary state. The
-lateral process <i>b</i>, which ordinarily becomes a foliar appendage,
-differs very little from the terminal process <i>c</i>, which is to
-become an axis&mdash;differs mainly in having, at this period when its form
-is being determined, a smaller bulk. If while thus undifferentiated,
-its nutrition remains inferior to that of the terminal process, it
-becomes moulded into a part that is subordinate to the general axis.
-But if, as sometimes happens, there is supplied to it such an abundance
-of the materials needful for growth, that it becomes as large as the
-terminal process; then we<span class="pagenum" id="Page_78">[78]</span> may naturally expect it to begin moulding
-itself round an axis of its own: a foliar organ will be replaced by
-an axial organ. And this result will be especially liable to occur,
-when the growth of the axis has been previously undergoing that arrest
-which leads to the formation of a flower; that is when, from defect
-of materials, the terminal process has almost ceased to increase, and
-when some concurrence of favourable causes brings a sudden access of
-sap which reaches the lateral processes before it reaches the terminal
-process.<a id="FNanchor_15" href="#Footnote_15" class="fnanchor">[15]</a></p>
-
-
-<p class="p2">§198. The general conclusion to which these various lines of evidence
-converge, is, then, that the shoot of a flowering plant is an aggregate
-of the third degree of composition. Taking as aggregates of the first
-order, those small portions of protoplasm which ordinarily assume
-the forms under which they are known as cells; and considering as
-aggregates of the second order, those assemblages of such cells which,
-in the lower cryptogams, compose the various kinds of thallus; then
-that structure, common to the higher cryptogams and to phænogams,
-in which we find a series of such groups of cells bound up into a
-continuous whole, must be regarded as an aggregate of the third
-order. The inference drawn from analysis, and verified by a synthesis
-which corresponds in a remarkable manner with the facts, is that
-those compound parts which, in Monocotyledons and Dicotyledons are
-called axes, have really arisen by integration of such simple parts
-as in lower plants are called fronds. Here, on a higher<span class="pagenum" id="Page_79">[79]</span> level,
-appears to have taken place a repetition of the process already
-observed on lower levels. The formation of those small groups of
-physiological units which compose the lowest protophytes, is itself
-a process of integration; and the consolidation of such groups into
-definitely-circumscribed and coherent cells or morphological units, is
-a completing of the process. In those coalescences by which many such
-cells are joined into threads, and discs, and solid or flattened-out
-masses, we see these morphological units aggregating into units of a
-compound kind: the different phases of the transition being exemplified
-by groups of various sizes, various degrees of cohesion, and various
-degrees of definiteness. And now we find evidences of a like process
-on a larger scale: the compound groups are again compounded. Moreover,
-as before, there are not wanting types of organization by which the
-stages of this higher integration are shadowed forth. From fronds that
-occasionally produce other fronds from their surfaces, we pass to those
-that habitually produce them; from those that do so in an indefinite
-manner, to those that do so in a definite manner; and from those that
-do so singly, to those that do so doubly and triply through successive
-generations of fronds. Even within the limits of a sub-class, we find
-gradations between fronds irregularly proliferous, and groups of such
-fronds united into a regular series.</p>
-
-<p>Nor does the process end here. The flowering plant is rarely
-uniaxial&mdash;it is nearly always multiaxial. From its primary shoot there
-grow out secondary shoots of like kind. Though occasionally among
-Phænogams, and frequently among the higher Cryptogams, the germs of
-new axes detach themselves under the form of bulbils, and develop
-separately instead of in connexion with the parent axis; yet in most
-Phænogams the germ of each new axis maintains its connexion with the
-parent axis: whence results a group of axes&mdash;an aggregate of the fourth
-order. Every tree, by the production<span class="pagenum" id="Page_80">[80]</span> of branch out of branch, shows
-us this integration repeated over and over again; forming an aggregate
-having a degree of composition too complex to be any longer defined.</p>
-
-<hr class="tb" />
-
-<p>[<span class="smcap">Note.</span>&mdash;A criticism passed on the general argument set
-forth in the foregoing sections, runs as follows:&mdash;“I have already
-pointed out that the process of evolution by which you believe the
-Liverworts with a distinct axis and appendages to have been produced
-from the thalloid forms is not founded on sound evidence either in
-comparative morphology or development. But even if we admit that such
-an integration of a proliferously-produced colony might have given
-rise to the leafy <i>Jungermanniaceæ</i>, there are even more weighty
-objections to the supposition that the same process produced the shoot
-structures of the flowering plants. In the first place the flowering
-plant-body is <i>not homologous with the liverwort plant-body</i>,
-since they represent different generations. The liverwort plant-body or
-<i>gametophyte</i>, <i>i.e.</i>, the generation bearing sexual organs,
-is homologous with the prothallus of ferns and other Pteridophytes,
-and in the Flowering Plants with reduced structures contained within
-the spores (embryo-sac and pollen-grain) but still giving rise to
-sexual cells. The liverwort spore-capsule and its accessory parts
-(in fact everything produced from the fertilized egg) is homologous
-with the sporogonium of the mosses, and, as most botanists think,
-with the leafy plant-body of Pteridophytes and Phanerogams. This
-generation is called the <i>sporophyte</i> and from the spores which
-it produces are developed the gametophytes of the next generation.
-These generalizations were first established by Hofmeister, and all
-subsequent work has tended to establish them more firmly. The only
-doubtful question is (and the doubt is mainly, I think, peculiar to
-myself, certainly not being shared by the majority of botanists)
-whether the sporophyte of Mosses and Liverworts is really homologous
-with that of Pteridophytes and<span class="pagenum" id="Page_81">[81]</span> Phanerogams, whether it may not rather
-be regarded as a parallel development along another line of descent
-from the Green Algæ.</p>
-
-<p>“Hence we must look for the origin of the shoot-structure of flowering
-plants in the sporophytes of the Pteridophytes, from which group there
-is no reason to doubt that the phanerogams have arisen in descent.
-The various groups of Pteridophytes vary much in the organization
-of these shoot-systems, as a mental glance at the types exhibited
-by the Ferns, Horse-tails, Club-mosses, <i>Ophioglossaceæ</i>, and
-the isolated Isoetes will convince you at once. It may be that some
-of these groups are independent in descent, <i>i.e.</i>, that the
-<i>Pteridophyta</i> are polyphyletic, and the current hypothesis with
-regard to the phanerogams is that they have arisen by two, if not
-three, separate lines of descent from different groups of Pteridophytes
-(this is indicated in the classificatory diagram on p. 377 of vol. I).
-I should not, however, care to pin my faith to these or to any such
-lines of ancestry. Still I think we must look for the ancestors of the
-Flowering Plants among the Pteridophytes, and the latter always have
-a good distinction between axis and appendages. The problem of the
-evolution of these differentiated sporophytic shoots is undoubtedly the
-great outstanding problem of morphology. Various attempts have been
-made to solve it, of which probably the most important is the theory
-of Profs. Bown and Campbell, who derive the Pteridophytes from some
-Liverwort like <i>Anthoceros</i>, but the sporophyte of course from the
-sporophytic portion of the plant (not much more than a spore-capsule),
-the prothallus of the Fern representing the vegetative thallus of
-Anthoceros. I am not wholly convinced by these undoubtedly ingenious
-hypotheses, in support of which an immense amount of facts have been
-collected; but my position would, I know, simply ‘put us to ignorance
-again’ on this question.</p>
-
-<p>“I have discussed this at some length in order to bring out clearly
-the immense difficulty of constructing a wellgrounded<span class="pagenum" id="Page_82">[82]</span> theory of
-the origin of the differentiated shoot-system of the higher plant.
-I confess I don’t think it can be done at all with the materials at
-present at our disposal. Of course it is just possible to suppose that
-some ancestral sporophyte had the structure of a proliferous thalloid
-liverwort gametophyte, and that from it was evolved the phanerogamic
-shoot in the ways you suggest. This gives us absolutely no clue,
-however, to any Pteridophytic shoot, which ought to be intermediate
-(more or less) between the hypothetical ancestor and the Phanerogam,
-and is furthermore, as far as I can see, not supported by an atom of
-evidence of any kind. It is true that your theory fits in well with the
-phenomena exhibited by phanerogamic shoots themselves, but this fact
-you will see must lose much of its significance if the hypothesis lacks
-foundation.</p>
-
-<p>“With regard to your method of explaining the fundamental characters
-of ‘Exogens’ and ‘Endogens,’ this of course is part of the same
-hypothesis; but I may point out that since Von Mohl and Sanio, between
-1855 and 1865, showed (1) that the growth at the stem apex of a
-monocotyledon was <i>not</i> endogenous, and (2) that the ‘thickening
-ring’ near the apex of a dicotyledon was not to be confused, as had
-been done up till then, with the ring of <i>secondary meristem</i>
-or <i>true cambium</i>, which arose lower down, and only in woody
-or practically woody stem, the terms ‘Exogen’ and ‘Endogen’ have
-necessarily fallen into disuse, since they imply a false conception of
-what happens. Both monocotyledons and dicotyledons have a ‘thickening
-ring,’ which gives rise to the primary vascular cylinder of the stem.
-When the stem is of considerable thickness, as in Palms, &amp;c., it grows
-by the active cell-division of its outer layers, so that both classes
-are ‘exogenous’ in this sense; while the addition of a centrifugal zone
-of secondary wood is confined to certain Dicotyledons (Trees, shrubs,
-&amp;c.).</p>
-
-<p>“The distinction between the embryos, moreover, is not absolute. The
-single cotyledon is usually terminal in monocotyledons,<span class="pagenum" id="Page_83">[83]</span> but not always
-(<i>Dioscoraceæ</i> have lateral cotyledons), but the plumule may push
-through it (Grasses) or make its exit sideways (Palms), or be formed at
-the side (<i>Alisma</i>); and Dicotyledons very similarly.</p>
-
-<p>“The occurrence of completely sheathing leaves in grasses is perhaps
-correlated with the absence of cambium, but grasses are an aberrant
-type among monocotyledons, and secondary thickening is only found in
-very few genera of this class, so that the correlation is, so to speak,
-negative and indirect.... It is clear that the greater part of the
-discussion will have to be re-written.”</p>
-
-<p>For the reasons assigned in the preface I cannot undertake to re-write
-the discussion, as suggested. It must stand for what it is worth. All I
-can do is here to include along with it the foregoing criticisms.</p>
-
-<p>I may, however, indicate the line of defence I should take were I to
-go again into the matter. The objections are based on the structure of
-existing Liverworts and Phænogams. But I have already referred to the
-probability&mdash;or, indeed, the certainty&mdash;that in conformity with the
-general principle set forth in the note to Chapter I, we must conclude
-that the early types of Liverworts out of which the Phænogams are
-supposed to have evolved, as well as the early types of Phænogams in
-which the stages of evolution were presented, no longer exist. We must
-infer that forms simpler than any now known, and more intermediate in
-their traits, were the forms concerned; and if so, it may be held that
-the incongruities with the hypothesis which are presented by existing
-forms, do not negative it. The scepticism my critic himself expresses
-respecting the current interpretation is a partial justification of
-this view. Moreover, his admission that the theory set forth “fits in
-well with the phenomena exhibited by phanerogamic shoots,” must, I
-think, be regarded as weighty evidence. On the Evolution hypothesis
-we are obliged to suppose that the Monocotyledons and Dicotyledons
-respectively arose by integration of fronds; and if to the<span class="pagenum" id="Page_84">[84]</span> question
-after what manner the integration took place, there is an hypothesis
-which renders it comprehensible, and agrees both with the structures of
-the two kinds of shoots and the structures of the two kinds of seeds,
-as well as with various of the other phenomena the two types present,
-it has strong claims for acceptance.</p>
-
-<p>Reconsideration suggests the following remarks.</p>
-
-<p>1. Alternation of generations is a means of furthering multiplication.
-To be effective each member of either generation must be a
-self-supporting centre of growth or diffusion or both. Hence if, as in
-the Liverworts, one of the so-called alternating generations is not
-independent, but a permanent growth on the other&mdash;a parasite&mdash;it is a
-misuse of words to call the arrangement Alternation of generations.
-(Since this was written I have found that Sir Edward Fry takes the
-same view. He approvingly quotes Professor Bower, who says that “the
-alternation of generations is not an accurate statement of facts or a
-useful analogy.”)</p>
-
-<p>2. The alternating of sexual and non-sexual processes is not
-fundamentally distinctive; for, as shown by sundry Archegoniates,
-it is an inconstant trait, and as shown by Klebs’ experiments on
-<i>Vaucheria</i>, the conditions may be varied so as to determine its
-occurrence or non-occurrence. Nay, the same individual may reproduce in
-either way.</p>
-
-<p>3. Still more significant is the fact that in some of the marine
-Thallophytes, there is a process like that which in a moss or a fern
-is considered an alternation of generations, whereas in others, as
-the Brown Wrack (<i>Fucus</i>), each generation is sexual. Thus the
-presence or absence of this mode of genesis cannot be a cardinal
-distinction.</p>
-
-<p>4. With these facts before us, it is not only a reasonable supposition
-but a highly probable supposition, that there have existed plants of
-the Liverwort type in which the so-called alternation of generations
-did not take place. If so, nearly all the foregoing objections to my
-hypothesis fall to the ground.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_85">[85]</span></p>
-
-<h3>CHAPTER IV.<br />
-<span class="subhed1">THE MORPHOLOGICAL COMPOSITION OF ANIMALS.</span></h3></div>
-
-<p>§ 199. What was said in <a href="#sect180">§ 180</a>, respecting the ultimate structure of
-organisms, holds more manifestly of animals than of plants. That
-throughout the vegetal kingdom the cell is the morphological unit, is
-a proposition admitting of a better defence, than the proposition that
-the cell is the morphological unit throughout the animal kingdom. The
-qualifications with which, as we saw, the cell-doctrine must be taken,
-are qualifications thrust upon us more especially by the facts which
-zoologists have brought to light. It is among the <i>Protozoa</i> that
-there occur numerous cases of vital activity displayed by specks of
-protoplasm; and from the minute anatomy of all creatures above these,
-are drawn the numerous proofs that non-cellular tissues may arise by
-direct metamorphosis of mixed colloidal substances.<a id="FNanchor_16" href="#Footnote_16" class="fnanchor">[16]</a></p>
-
-<p><span class="pagenum" id="Page_86">[86]</span></p>
-
-<p>Our survey of morphological composition throughout the animal kingdom,
-must therefore begin with those undifferentiated aggregates of
-physiological units [or constitutional units], out of which are formed
-what we call, with considerable license, morphological units.</p>
-
-
-<p class="p2">§ 200. In that division of the <i>Protozoa</i> distinguished as
-<i>Rhizopoda</i>, are presented, under various modifications, these
-minute portions of living organic matter, so little differentiated,
-if not positively undifferentiated, that animal individuality can
-scarcely be claimed for them. Figs. <a href="#i_086">131, 132, and 133</a>, represent
-certain nearly-allied types of these&mdash;<i>Amœba</i>, <i>Actinophrys</i>,
-and <i>Lieberkühnia</i>. The viscid jelly or sarcode, comparable in
-its physical properties to white of egg, out of which one of these
-creatures is mainly formed, shows us in various ways, the feebleness
-with which the component physiological units are integrated&mdash;shows us
-this by its very slight cohesion, by the extreme indefiniteness and
-mutability of its form, and by the absence of a limiting membrane. It
-is no longer held even by unqualified adherents of the cell-doctrine
-that the <i>Amœba</i> has an investment. Its outer surface, compared
-to the film which forms on the surface of paste, does not prevent the
-taking of solid particles into the mass of the body, and does not, in
-such kindred forms as Fig. <a href="#i_086">133</a>, prevent the pseudopodia from coalescing
-when they meet. Hence it cannot properly have the name of a cell-wall.
-A considerable portion of the body, however, in <i>Difflugia</i>,<span class="pagenum" id="Page_87">[87]</span> Fig.
-<a href="#i_086">134</a>, has a denser coating formed of agglutinated foreign particles;
-so that the protrusion of the pseudopodia is limited to one part of
-it. And in the solitary <i>Foraminifera</i>, like <i>Gromia</i>,
-the sarcode is covered over most of its surface by a delicate
-calcareous shell, pierced with minute holes, through which the slender
-pseudopodia are thrust. The <i>Gregarina</i> exhibits an advance in
-integration, and a consequent greater definiteness. Figs. <a href="#i_086">135 and
-136</a>, exemplifying this type, show the complete membrane in which the
-substance of the creature is contained. Here there has arisen what
-may be properly called a cell: under its solitary form this animal is
-truly unicellular. Its embryology has considerable significance. After
-passing through a certain quiescent, “encysted” state, its interior
-breaks up into small portions, which, after their exit, assume forms
-like that of the <i>Amœba</i>; and from this young condition in
-which they are undifferentiated, they pass into that adult condition
-in which they have limiting membranes. If this development of the
-individual <i>Gregarina</i> typifies the mode of evolution of the
-species, it yields further support to the belief, that fragments of
-sarcode existed earlier than any of the structures which are called
-cells. Among aggregates of the first order, there are some much more
-highly developed. These are the <i>Infusoria</i>, constituting the
-most numerous of the <i>Protozoa</i>, in species as in individuals.
-Figs. <a href="#i_086">137, 138, and 139</a>, are examples. In them we find, along with
-greater definiteness, a considerable heterogeneity. The sarcode of
-which the body consists, has an indurated outer layer, bearing cilia
-and sometimes spines; there is an opening serving as mouth, a permanent
-œsophagus, and a cavity or cavities, temporarily formed in the interior
-of the sarcode, to serve as one or more stomachs; and there is a
-comparatively specific arrangement of these and various minor parts.</p>
-
-  <div class="figcenter" id="i_086">
-     <img
-    class="p2"
-    src="images/i_086.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 131–139.</p>
-  </div>
-
-<p class="p2">Thus in the animal kingdom, as in the vegetal kingdom, there exists
-a class of minute forms having this peculiarity, that no one of them
-is separable into a number of visible<span class="pagenum" id="Page_88">[88]</span> components homologous with one
-another&mdash;no one of them can be resolved into minor individualities.
-Its proximate units are those physiological units of which we conclude
-every organism consists. The aggregate is an aggregate of the first
-order.</p>
-
-
-<p class="p2" id="sect201">§ 201. Among plants are found types indicating a transition from
-aggregates of the first order to aggregates of the second order; and
-among animals we find analogous types. But the stages of progressing
-integration are not here so distinct. The reason probably is, that
-the simplest animals, having individualities much less marked than
-those of the simplest plants, do not afford us the same facilities for
-observation. In proportion as the limits of the minor individualities
-are indefinite, the formation of major individualities out of them,
-naturally leaves less conspicuous traces.</p>
-
-  <div class="figcenter" id="i_088">
-     <img
-    class="p2"
-    src="images/i_088.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 140–145.</p>
-  </div>
-
-<p class="p2">Be this as it may, however, in such types of <i>Protozoa</i> as the
-compound <i>Radiolaria</i>, we find that though there is reason to
-regard the aggregate as an aggregate of the second order, yet its
-divisibility into minor individualities like those just described,
-is less manifest. Fig. <a href="#i_088">140</a> representing <i>Sphærozoum punctatum</i>,
-one of the group, illustrates this. The sceptically-minded may
-perhaps doubt whether we can regard the “cellæform bodies” contained
-in it, as the morphological units of the animal. The jelly-like
-mass in which they are imbedded, is but indefinitely divisible into
-portions having each a cell or nucleus for its centre.<a id="FNanchor_17" href="#Footnote_17" class="fnanchor">[17]</a> Among
-the<span class="pagenum" id="Page_89">[89]</span> <i>Foraminifera</i>, we find only indefinite evidence of the
-coalescence of aggregates of the first order, into aggregates of the
-second order. There are solitary Foraminifers, allied to the creature
-represented in Fig. <a href="#i_086">134</a>. Certain ideal types of combination among
-them, are shown in Fig. <a href="#i_088">141</a>. And setting out from these, we may ascend
-in various directions to kinds compounded to an immense variety of
-degrees in an immense variety of ways. In all of them, however, the
-separability of the major individuality into minor individualities,
-is very incomplete. The portion of sarcode contained in one of these
-calcareous chambers, gives origin to an external bud; and this
-presently becomes covered, like its parent, with calcareous matter: the
-position in which each successive chamber is so produced, determining
-the form of the compound shell. But the portions of sarcode thus budded
-out one from another, do not become distinctly individualized. Fig.
-<a href="#i_088">142</a>, representing the living network which remains when the shell of
-an Orbitolite has been dissolved, shows the continuity that exists
-among the occupants of its aggregated chambers.<a id="FNanchor_18" href="#Footnote_18" class="fnanchor">[18]</a> In the compound
-<i>Infusoria</i>, the component units remain quite distinct. Being,
-as aggregates of the first order, much more definitely organized,
-their union into aggregates of the second order does<span class="pagenum" id="Page_90">[90]</span> not destroy
-their original individualities. Among the <i>Vorticellæ</i>, of which
-two kinds are delineated in Figs. <a href="#i_088">144 and 145</a>, there are various
-illustrations of this: the members of the community being sometimes
-appended to a single stem; sometimes attached by long separate stems to
-a common base; and sometimes massed together.</p>
-
-  <div class="figcenter" id="i_090">
-     <img
-    class="p2"
-    src="images/i_090.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 146–147.</p>
-  </div>
-
-<p class="p2">Thus far, these aggregates of the second order exhibit but indefinite
-individualities. The integration is physical; but not physiological.
-Though, in the <i>Polycytharia</i>, there is a shape that has some
-symmetry; and though, in the <i>Foraminifera</i>, the formation of
-successive chambers proceeds in such methodic ways as to produce
-quite-regular and tolerably-specific shells; yet no more in these than
-in the Sponges or the compound <i>Vorticellæ</i>, do we find such
-co-ordination as gives the whole a life predominating over the lives of
-its parts. We have not yet reached an aggregate of the second order,
-so individuated as to be capable of serving as a unit in still higher
-combinations. But in the class <i>Cœlenterata</i>, this advance is
-displayed. The common <i>Hydra</i>, habitually taken as the type of
-the lowest division of this class, has specialized parts performing
-mutually-subservient functions, and thus exhibiting a total life
-distinct from the lives of the units. Fig. <a href="#i_090">146</a> represents one of these
-creatures in its contracted state and in its expanded state; while
-Fig. <a href="#i_090">147</a> is a diagram showing the wall of this creature’s sac-like
-body as seen in section under the microscope: <i>a</i> and <i>b</i>
-being the outer and inner cellular layers; while between them is the
-“mesoglœa” or “structureless lamella,” the supporting or skeletal
-layer. But this lowly-organized tissue of the Hydra, illustrates
-a phase of integration in which the lives of the minor aggregates
-are only partially-subordinated to the life of the major aggregate
-formed by them. For a <i>Hydra’s</i> substance is separable<span class="pagenum" id="Page_91">[91]</span> into
-<i>Amœba</i>-like portions, capable of moving about independently.
-If we bear in mind how analogous are the extreme extensibility and
-contractility of a <i>Hydra’s</i> body and tentacles, to the properties
-displayed by the sarcode among Rhizopods; we may infer that probably
-the movements and other actions of a <i>Hydra</i>, are due to the
-half-independent co-operation of the <i>Amœba</i>-like individuals
-composing it.</p>
-
-
-<p class="p2">§ 202. A truth which we before saw among plants, we here see repeated
-among animals&mdash;the truth that as soon as the integration of aggregates
-of the first order into aggregates of the second order, produces
-compound wholes so specific in their shapes and sizes, and so mutually
-dependent in their parts, as to have distinct individualities; there
-simultaneously arises the tendency in them to produce, by gemmation,
-other such aggregates of the second order. The approach towards
-definite limitation in an organism, is, by implication, an approach
-towards a state in which growth passing a certain point, results, not
-in the increase of the old individual, but in the formation of a new
-individual. Thus it happens that the common polype buds out other
-polypes, some of which very shortly do the like, as shown in Fig. <a href="#i_091">148</a>:
-a process paralleled by the fronds of sundry <i>Algæ</i>, and by those
-of the lower <i>Jungermanniaceæ</i>. And just as, among these last
-plants, the proliferously-produced fronds, after growing to certain
-sizes and developing rootlets, detach themselves from their parent
-fronds; so among these animals, separation of the young ones from
-the bodies of their parents ensues when they have acquired tolerably
-complete organizations.</p>
-
-  <div class="figcenter" id="i_091">
-     <img
-    class="p2"
-    src="images/i_091.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 148–150.</p>
-  </div>
-
-<p class="p2">There is reason to think that the parallel holds still further.<span class="pagenum" id="Page_92">[92]</span> Within
-the limits of the <i>Jungermanniaceæ</i>, we found that while some
-genera exhibit this discontinuous development, other genera exhibit a
-development that is similar to it in all essential respects, save that
-it is continuous. And here within the limits of the <i>Hydrozoa</i>,
-we find, along with this genus in which the gemmiparous individuals
-are presently cast off, other genera in which they are not cast off,
-but form a permanent aggregate of the third order. Figs. <a href="#i_091">149 and
-150</a>, exemplify these compound <i>Hydrozoa</i>&mdash;one of them showing
-this mode of growth so carried out as to produce a single axis; and
-the other showing how, by repetitions of the process, lateral axes
-are produced. Integrations characterizing certain higher genera of
-the <i>Hydrozoa</i> which swim or float instead of being fixed, are
-indicated by Figs. <a href="#i_092">151 and 152</a>: the first of them representing the type
-of a group in which the polypes growing from an axis, or cœnosarc,
-are drawn through the water by the rhythmical contractions of the
-organs from which they hang; and the second of them representing a
-<i>Physalia</i> the component polypes of which are united into a
-cluster, attached to an air-vessel.</p>
-
-  <div class="figcenter" id="i_092">
-     <img
-    class="p2"
-    src="images/i_092.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 151–152.</p>
-  </div>
-
-<p class="p2">A parallel series of illustrations might be drawn from that second
-division of the <i>Cœlenterata</i>, known as the <i>Actinozoa</i>.
-Here, too, we have a group of species&mdash;the Sea-anemones&mdash;the
-individuals of which are solitary. Here, too, we have agamogenetic
-multiplication: occasionally by gemmation, but more frequently by that
-modified process called spontaneous fission. And here, too, we have
-compound forms resulting from the arrest of this spontaneous fission
-before it is complete. To give examples is needless; since they would
-but show, in more varied ways, the truth already made sufficiently<span class="pagenum" id="Page_93">[93]</span>
-clear, that the compound <i>Cœlenterata</i> are aggregates of the third
-order, produced by integration of aggregates of the second order such
-as we have in the <i>Hydra</i>. As before, it is manifest that on the
-hypothesis of evolution, these higher integrations will insensibly
-arise, if the separation of the gemmiparous polypes is longer and
-longer postponed; and that an increasing postponement will result by
-survival of the fittest, if it profits the group of individuals to
-remain united instead of dispersing.<a id="FNanchor_19" href="#Footnote_19" class="fnanchor">[19]</a></p>
-
-
-<p class="p2">§ 203. The like relations exist, and imply that the like processes have
-been gone through, among those more highly organized animals called
-<i>Polyzoa</i> and <i>Tunicata</i>. We have solitary individuals,
-and we have variously-integrated groups of individuals: the chief
-difference between the evidence here furnished, and that furnished
-in the last case, being the absence of a type obviously linking the
-solitary state with the aggregated state.</p>
-
-  <div class="figcenter" id="i_093">
-     <img
-    class="p2"
-    src="images/i_093.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 153–155.</p>
-  </div>
-
-<p class="p2">This integration of aggregates of the second order, is carried on
-among the <i>Polyzoa</i> in divers ways, and with different degrees of
-completeness. The little patches of minute cells, shown as magnified
-in Fig. <a href="#i_093">153</a>, so common on the fronds of sea-weeds and the surfaces of
-rocks at low-water mark, display little beyond mechanical combination.
-The adjacent individuals,<span class="pagenum" id="Page_94">[94]</span> though severally originated by gemmation
-from the same germ, have but little physiological dependence. In
-kindred kinds, however, as shown in Figs. <a href="#i_093">154 and 155</a>, one of which
-is a magnified portion of the other, the integration is somewhat
-greater: the co-operation of the united individuals being shown in the
-production of those tubular branches which form their common support,
-and establish among them a more decided community of nutrition.</p>
-
-  <div class="figcenter" id="i_094">
-     <img
-    class="p2"
-    src="images/i_094.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 156–159.</p>
-  </div>
-
-<p class="p2">Among the Ascidians this general law of morphological composition is
-once more displayed. Each of these creatures subsists on the nutritive
-particles contained in the water which it draws in through one orifice
-and sends out through another; and it may thus subsist either alone, or
-in connexion with others that are in some cases loosely aggregated and
-in other cases closely aggregated. Fig. <a href="#i_094">156</a>, <i>Phallusia mentula</i>,
-is one of the solitary forms. A type in which the individuals are
-united by a stolon that gives origin to them by successive buds, is
-shown in <i>Perophora</i>, Fig. <a href="#i_094">157</a>. Among the <i>Botryllidæ</i>, of
-which one kind is drawn on a small scale in Fig. <a href="#i_094">159</a>, and a portion of
-the same on a larger scale in Fig. <a href="#i_094">158</a>, there is a combination of the
-individuals into annular clusters, which are themselves imbedded in a
-common gelatinous matrix. And in this group there are integrations even
-a stage higher, in which several such clusters of clusters grow from
-a single base. Here the compounding and recompounding appears to be
-carried further than anywhere else in the animal kingdom.</p>
-
-<p>Thus far, however, among these aggregates of the third order, we see
-what we before saw among the simpler aggregates of the second order&mdash;we
-see that the component individualities are but to a very small extent
-subordinated to<span class="pagenum" id="Page_95">[95]</span> the individuality made up of them. In nearly all the
-forms indicated, the mutual dependence of the united animals is so
-slight, that they are more fitly comparable to societies, of which
-the members co-operate in securing certain common benefits. There is
-scarcely any specialization of functions among them. Only in the last
-type described do we see a number of individuals so completely combined
-as to simulate a single individual. And even here, though there appears
-to be an intimate community of nutrition, there is no physiological
-integration beyond that implied in several mouths and stomachs having a
-common vent.<a id="FNanchor_20" href="#Footnote_20" class="fnanchor">[20]</a></p>
-
-
-<p class="p2">§ 204. We come now to an extremely interesting question. Does there
-exist in other sub-kingdoms composition of the third degree, analogous
-to that which we have found so prevalent among the <i>Cœlenterata</i>
-and the <i>Polyzoa</i> and <i>Tunicata</i>? The question is not
-whether elsewhere there are tertiary aggregates produced by the
-branching or clustering of secondary aggregates, in ways like those
-above traced; but whether elsewhere there are aggregates which, though
-otherwise unlike in the arrangement of their parts, nevertheless
-consist of parts so similar to one another that we may suspect them
-to be united secondary aggregates. The various compound types above
-described, in which the united animals maintain their individualities
-so distinctly that the individuality of the aggregate remains vague,
-are constructed in such ways that the united animals carry on their
-several activities with scarcely any mutual hindrance. The members of
-a branched <i>Hydrozoon</i>, such as is shown in Fig. <a href="#i_091">149</a> or Fig. <a href="#i_091">150</a>,
-are so placed that they can all spread their tentacles and catch their
-prey as well as though separately attached to stones or weeds. Packed
-side by side on a flat surface or<span class="pagenum" id="Page_96">[96]</span> forming a tree-like assemblage,
-the associated individuals among the <i>Polyzoa</i> are not unequally
-conditioned: or if one has some advantage over another in a particular
-case, the mode of growth and the relations to surrounding objects are
-so irregular as to prevent this advantage re-appearing with constancy
-in successive generations. Similarly with the Ascidians growing from
-a stolon or those forming an annular cluster: each of them is as well
-placed as every other for drawing in the currents of sea-water from
-which it selects its food. In these cases the mode of aggregation
-does not expose the united individuals to multiform circumstances;
-and therefore is not calculated to produce among them any structural
-multiformity. For the same reason no marked physiological division
-of labour arises among them; and consequently no combination close
-enough to disguise their several individualities. But under converse
-conditions we may expect converse results. If there is a mode of
-integration which necessarily subjects the united individuals
-to unlike sets of incident forces, and does this with complete
-uniformity from generation to generation, it is to be inferred that
-the united individuals will become unlike. They will severally assume
-such different functions as their different positions enable them
-respectively to carry on with the greatest advantage to the assemblage.
-This heterogeneity of function arising among them, will be followed by
-heterogeneity of structure; as also by that closer combination which
-the better enables them to utilize one another’s functions. And hence,
-while the originally-like individuals are rendered unlike, they will
-have their homologies further obscured by their progressing fusion into
-an aggregate individual of a higher order.</p>
-
-<p>These converse conditions are in nearly all cases fulfilled where
-the successive individuals arising by continuous development are so
-budded-off as to form a linear series. I say in nearly all cases,
-because there are some types in which the associated individuals,
-though joined in single file, are<span class="pagenum" id="Page_97">[97]</span> not thereby rendered very unlike
-in their relations to the environment; and therefore do not become
-differentiated and integrated to any considerable extent. I refer to
-such Ascidians as the <i>Salpidæ</i>. These creatures float passively
-in the sea, attached together in strings. Being placed side by side
-and having mouths and vents that open laterally, each of them is as
-well circumstanced as its neighbours for absorbing and emitting the
-surrounding water; nor have the individuals at the two extremities
-any marked advantages over the rest in these respects. Hence in this
-type, and in the allied type <i>Pyrosoma</i>, which has its component
-individuals built into a hollow cylinder, linear aggregation may exist
-without the minor individualities becoming obscured and the major
-individuality marked: the conditions under which a differentiation
-and integration of the component individuals may be expected, are not
-fulfilled. But where the chain of individuals produced by gemmation, is
-either habitually fixed to some solid body by one of its extremities or
-moves actively through the water or over submerged stones and weeds,
-the several members of the chain become differently conditioned in the
-way above described; and may therefore be expected to become unlike
-while they become united. A clear idea of the contrast between these
-two linear arrangements and their two diverse results, will be obtained
-by considering what happens to a row of soldiers, when changed from
-the ordinary position of a single rank to the position of Indian file.
-So long as the men stand shoulder to shoulder, they are severally able
-to use their weapons in like ways with like efficiency; and could,
-if called on, similarly perform various manual processes directly
-or indirectly conducive to their welfare. But when, on the word of
-command “right face,” they so place themselves that each has one of
-his neighbours before him and another behind him, nearly all of them
-become incapacitated for fighting and for many other actions. They can
-walk or run one after another, so as to produce movement of the file
-in<span class="pagenum" id="Page_98">[98]</span> the direction of its length; but if the file has to oppose an enemy
-or remove an obstacle lying in the line of its march, the front man
-is the only one able to use his weapons or hands to much purpose. And
-manifestly such an arrangement could become advantageous only if the
-front man possessed powers peculiarly adapted to his position, while
-those behind him facilitated his actions by carrying supplies, &amp;c. This
-simile, grotesque as it seems, serves to convey better perhaps than
-any other could do, a clear idea of the relations that must arise in a
-chain of individuals arising by gemmation, and continuing permanently
-united end to end. Such a chain can arise only on condition that
-combination is more advantageous than separation; and for it to be more
-advantageous, the anterior members of the series must become adapted to
-functions facilitated by their positions, while the posterior members
-become adapted to functions which their positions permit. Hence, direct
-or indirect equilibration or both, must tend continually to establish
-types in which the connected individuals are more and more unlike
-one another, at the same time that their several individualities are
-more and more disguised by the integration consequent on their mutual
-dependence.</p>
-
-<p>Such being the anticipations warranted by the general laws of
-evolution, we have now to inquire whether there are any animals which
-fulfil them. Very little search suffices; for structures of the kind
-to be expected are abundant. In that great division of the animal
-kingdom at one time called <i>Annulosa</i>, but now grouped into
-<i>Annelida</i> and <i>Arthropoda</i>, we find a variety of types
-having the looked-for characters. Let us contemplate some of them.</p>
-
-
-<p class="p2" id="sect205">§ 205. An adult Chætopod is composed of segments which repeat one
-another in their details as well as in their general shapes. Dissecting
-one of the lower orders, such as is shown in Fig. <a href="#i_099">160</a>, proves that
-the successive segments, besides having like locomotive appendages,
-like branchiæ, and<span class="pagenum" id="Page_99">[99]</span> sometimes even like pairs of eyes, also have like
-internal organs. Each has its enlargement of the alimentary canal;
-each its contractile dilatation of the great blood-vessel; each its
-portion of the double nervous cord, with ganglia when these exist;
-each its branches from the nervous and vascular trunks answering to
-those of its neighbours; each its similarly answering set of muscles;
-each its pair of openings through the body-wall; and so on throughout,
-even to the organs of reproduction. That is to say, every segment is
-in great measure a physiological whole&mdash;every segment contains most
-of the organs essential to individual life and multiplication: such
-essential organs as it does not contain, being those which its position
-as one in the midst of a chain, prevents it from having or needing. If
-we ask what is the meaning of these homologies, no adequate answer is
-supplied by any current hypothesis. That this “vegetative repetition”
-is carried out to fulfil a predetermined plan, was shown to be quite
-an untenable notion (§§ <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect133">133</a>, <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect134">134</a>). On the one hand, we found nothing
-satisfactory in the conception of a Creator who prescribed to himself
-a certain unit of composition for all creatures of a particular class,
-and then displayed his ingenuity in building up a great variety of
-forms without departing from the “archetypal idea.” On the other hand,
-examination made it manifest that even were such a conception worthy
-of being entertained, it would have to be relinquished; since in each
-class there are numerous deviations from the supposed “archetypal
-idea.” Still less can these traits of structure be accounted<span class="pagenum" id="Page_100">[100]</span> for
-teleologically. That certain organs of nutrition and respiration and
-locomotion are repeated in each segment of a dorsibranchiate annelid,
-may be regarded as functionally advantageous for a creature following
-its mode of life. But why should there be a hundred or even two
-hundred pairs of ovaries? This is an arrangement at variance with that
-physiological division of labour which every organism profits by&mdash;is
-a less advantageous arrangement than might have been adopted. That
-is to say, the hypothesis of a designed adaptation fails to explain
-the facts. Contrariwise, these structural traits are just such as
-might naturally be looked for, if these annulose forms have arisen by
-the integration of simpler forms. Among the various compound animals
-already glanced at, it is very general for the united individuals to
-repeat one another in all their parts&mdash;reproductive organs included.
-Hence if, instead of a clustered or branched integration, such as
-the <i>Cœlenterata</i>, <i>Polyzoa</i> and <i>Tunicata</i> exhibit,
-there occurs a longitudinal integration; we may expect that the united
-individuals will habitually indicate their original independence by
-severally bearing germ-producing or sperm-producing organs.</p>
-
-  <div class="figcenter" id="i_099">
-     <img
-    class="p2"
-    src="images/i_099.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 160–161.</p>
-  </div>
-
-<p class="p2">The reasons for believing one of these creatures to be an aggregate of
-the third order, are greatly strengthened when we turn from the adult
-structure to the mode of development. Among the <i>Dorsibranchiata</i>
-and <i>Tubicolæ</i>, the embryo leaves the egg in the shape of a
-ciliated gemmule, not much more differentiated than that of a polype.
-As shown in Fig. <a href="#i_101">162</a>, it is a nearly globular mass; and its interior
-consists of untransformed cells. The first appreciable change is
-an elongation and a simultaneous commencement of segmentation. The
-segments multiply by a modified gemmation, which takes place from the
-hinder end of the penultimate segment. And considerable progress in
-marking out these divisions is made before the internal organization
-begins. Figs. <a href="#i_101">163, 164, 165</a>, represent some of these early stages. In
-annelids of other orders, the embryo assumes the segmented<span class="pagenum" id="Page_101">[101]</span> form while
-still in the egg. But it does this in just the same manner as before.
-Indeed, the essential identity of the two modes of development is shown
-by the fact that the segmentation within the egg is only partially
-carried out: in all these types the segments continue to increase in
-number for some time after hatching. Now this process is as like that
-by which compound animals in general are formed, as the different
-conditions of the case permit. When new individuals are budded-out
-laterally, their unfolding is not hindered&mdash;there is nothing to
-disguise either the process or the product. But gemmæ produced one from
-another in the same straight line, and remaining connected, restrict
-one another’s developments; and that the resulting segments are so many
-gemmiparously-produced individuals, is necessarily less obvious.</p>
-
-  <div class="figcenter" id="i_101">
-     <img
-    class="p2"
-    src="images/i_101.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 162–165.</p>
-  </div>
-
-<p class="p2" id="sect206">§ 206. Evidence remains which adds very greatly to the weight of
-that already assigned. Thus far we have studied only the individual
-segmented animal; considering what may be inferred from its mode of
-evolution and final organization. We have now to study segmented
-animals in general. Comparison of different groups of them and of
-kinds within each group, will disclose various phases of progressive
-integration of the nature to be anticipated.</p>
-
-  <div class="figcenter" id="i_103">
-     <img
-    class="p2"
-    src="images/i_103.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 166–169.</p>
-  </div>
-
-<p class="p2">Among the simpler <i>Platyhelminthes</i>, as in some kinds of
-<i>Planaria</i>, transverse fission occurs. A portion of a
-<i>Planaria</i> separated by spontaneous constriction, becomes
-an independent<span class="pagenum" id="Page_102">[102]</span> individual. Sir J. G. Dalyell found that in some
-cases numerous fragments artificially separated, grew into perfect
-animals.<a id="FNanchor_21" href="#Footnote_21" class="fnanchor">[21]</a> In these creatures which thus remind us of the lowest
-<i>Hydrozoa</i> in their powers of agamogenetic multiplication, the
-individuals produced one from another do not continue connected. As
-the young ones laterally budded-off by the <i>Hydra</i> separate
-when complete, so do the young ones longitudinally budded-off by the
-<i>Planaria</i>. Fig. <a href="#i_103">166</a> indicates this. But there are allied types
-which show us a more or less persistent union of homologous parts,
-or individuals, similarly arising by longitudinal gemmation.<a id="FNanchor_22" href="#Footnote_22" class="fnanchor">[22]</a>
-The cestoid <i>Entozoa</i> furnish illustrations. Without dwelling
-on the fact that each segment of a <i>Tænia</i>, like each separate
-<i>Planaria</i>, is an independent hermaphrodite; and without
-specifying the sundry common structural traits which add probability to
-the suspicion that there is some kinship between the individuals of the
-one order and the segments of the other; it will suffice to point out
-that the two types are so far allied as to demand their union under the
-same sub-class title. And recognizing this kinship, we see significance
-in the fact that in the one case the longitudinally-produced gemmæ
-separate as complete individuals, and in the other continue united
-as segments in smaller or larger numbers and for shorter or longer
-periods. In <i>Tænia echinococcus</i>,<span class="pagenum" id="Page_103">[103]</span> represented in Fig. <a href="#i_103">167</a>, we
-have a species in which the number of segments thus united does not
-exceed four. In <i>Echinobothrium typus</i> there are eight or ten;
-and in cestoids generally they are numerous.<a id="FNanchor_23" href="#Footnote_23" class="fnanchor">[23]</a> A considerable hiatus
-occurs between this phase of integration and the next higher phase
-which we meet with; but it is not greater than the hiatus between the
-types of the <i>Platyhelminthes</i> and the <i>Chætopoda</i>, which
-present the two phases. Though it is doubtful whether separation of
-single segments occurs among the Annelida,<a id="FNanchor_24" href="#Footnote_24" class="fnanchor">[24]</a> yet very often we
-find strings of segments,<span class="pagenum" id="Page_104">[104]</span> arising by repeated longitudinal budding,
-which after reaching certain lengths undergo spontaneous fission: in
-some cases doing this so as to form two or more similar strings of
-segments constituting independent individuals; and in other cases
-doing it so that the segments spontaneously separated are but a small
-part of the string. Thus a <i>Syllis</i>, Fig. <a href="#i_103">168</a>, after reaching
-a certain length, begins to transform itself into two individuals:
-one of the posterior segments develops into an imperfect head, and
-simultaneously narrows its connexion with the preceding segments, from
-which it eventually separates. Still more remarkable is the extent to
-which this process is carried in certain kindred types; which exhibit
-to us several individuals thus being simultaneously formed out of
-groups of segments. Fig. <a href="#i_103">169</a>, copied (omitting the appendages) from
-one contained in a memoir by M. Milne-Edwards, represents six worms of
-different ages in course of development: the terminal one being the
-eldest, the one having the greatest number of segments, and the one
-that will first detach itself; and the successively anterior ones, with
-their successively smaller numbers of segments, being successively
-less advanced towards fitness for separation and independence. Here
-among groups of segments we see repeated what in the previous cases
-occurs with single segments. And then in other annelids we find that
-the string of segments arising by gemmation from a single germ becomes
-a permanently united whole: the tendency to any more complete fission
-than that which marks out the segments, being lost; or, in other words,
-the integration having become relatively complete. Leaving out of sight
-the question of alliance among the types above grouped together, that
-which it here concerns us to notice is, that longitudinal gemmation
-does go on; that it is displayed in that primitive form in which the
-gemmæ separate as soon as produced; that we have types in which such
-gemmæ hang together in groups of four, or in groups of eight and ten,
-from which however the gemmæ successively separate as individuals;<span class="pagenum" id="Page_105">[105]</span>
-that among higher types we have long strings of similarly-formed
-gemmæ which do not become individually independent, but separate into
-organized groups; and that from these we advance to forms in which all
-the gemmæ remain parts of a single individual. One other significant
-fact must be added. There are cases in which annelids multiply by
-lateral gemmation.<a id="FNanchor_25" href="#Footnote_25" class="fnanchor">[25]</a> That the longitudinally-produced gemmæ which
-compose an annelid, should thus have, one of them or several of them,
-the power of laterally budding-off gemmæ, from which other annelids
-arise, gives further support to the hypothesis that, primordially, the
-segments were independent individuals. And it suggests this belief
-the more strongly because, in certain types of <i>Cœlenterata</i>, we
-see that longitudinal and lateral gemmation <i>do</i> occur together,
-where the longitudinally-united gemmæ are demonstrably independent
-individuals.</p>
-
-
-<p class="p2" id="sect207">§ 207. Though it seems next to impossible that we shall ever be able
-to find a type such as that which is here supposed to be the unit of
-composition of the annulose type, since we must assume such a type
-to have been long since extinct, yet the foregoing evidence goes
-far towards showing that an annulose animal is an aggregate of the
-third order. This repetition of segments, sometimes numbering several
-hundreds, like one another in all their organs even down to those of
-reproduction, while it is otherwise unaccountable, is fully accounted
-for if these segments are homologous with the separate individuals of
-some lower type. The gemmation by which these segments are produced, is
-as similar as the conditions allow, to the gemmation by which compound<span class="pagenum" id="Page_106">[106]</span>
-animals in general are produced. As among plants, and as among
-demonstrably-compound animals, we see that the only thing required
-for the formation of a permanent chain of gemmiparously-produced
-individuals, is that by remaining associated such individuals will
-have advantages greater than are to be gained by separation. Further,
-comparisons of the annuloid and lower annulose forms, disclose
-a number of those transitional phases of integration which the
-hypothesis leads us to expect. And, lastly, the differences among these
-united individuals or successive segments, are not greater than the
-differences in their positions and functions explain&mdash;not greater than
-such differences are known to produce among other united individuals:
-witness sundry compound <i>Hydrozoa</i>.</p>
-
-<p>Indirect evidence of much weight has still to be given. Thus far we
-have considered only the less developed <i>Annulosa</i>. The more
-integrated and more differentiated types of the class remain. If in
-them we find a carrying further of the processes by which the lower
-types are here supposed to have been evolved, we shall have additional
-reason for believing them to have been so evolved. If we find that in
-these superior orders, the individualities of the united segments are
-much less pronounced than in the inferior, we shall have grounds for
-suspecting that in the inferior the individualities of the segments are
-less pronounced than in those lost forms which initiated the annulose
-sub-kingdom.</p>
-
-<hr class="tb" />
-
-<p>[<span class="smcap">Note.</span>&mdash;Partly from the wish to incorporate further evidence,
-and partly from the wish to present the evidence, old and new, in a
-more effective order, I decide here to recast the foregoing exposition.</p>
-
-<p>Significant traits of development are exhibited in common by two
-groups otherwise unallied&mdash;certain of the <i>Platyhelminthes</i> and
-certain of the lower <i>Annulosa</i>. Of the <i>Platyhelminthes</i> the
-ordinary type is an unsegmented creature: a<span class="pagenum" id="Page_107">[107]</span> Planarian or a Trematode
-exemplifying it. Among the free forms, as in some Planarians, there
-occurs transverse fission, and prompt separation of the segments;
-while among some other free forms, as the <i>Microstomida</i>, the
-two segments first produced, themselves become segmented while
-still adherent, and this process is repeated until a string is
-formed. Another group of the <i>Platyhelminthes</i>, the Cestoid
-<i>Entozoa</i>, exhibit analogous processes. There are unsegmented
-forms, as the <i>Caryophyllæus</i>, and there are forms in which the
-segments, now few now many, adhere together in chains; the terminal
-members of which, however, eventually separate, and having before
-separation approached the trematode structure, become independent
-individuals which grow, creep about, and continue the race. In both
-of these types the condition under which the gemmiparously-produced
-members remain connected, is that they shall be able to feed
-individually: in the one case by lateral mouths, in the other case by
-absorption through the integument. It is further observable that in
-both cases separation of the component individuals occurs at sexual
-maturity, when advantage in nutrition has ceased to be the dominant
-need and dispersion of the species has taken its place in degree of
-importance. Among Annelids, higher though they are in type, we find
-parallelisms. Usually in its first stage an annelid is unsegmented,
-but as fast as it elongates lines of segmentation indent its surface.
-This segmentation proceeds in various ways, and the segments exhibit
-various degrees of dependence. In some low types, spontaneous fission
-goes on to the extent of producing single segments, each of which has
-such vitality that it buds out anterior and posterior parts at its two
-ends. Thus alike in the simple form which exists before segmentation
-and in the form exhibited by a detached segment, we have a unit
-analogous to each of the units which are joined together in certain
-free <i>Turbellaria</i> and in the Cestoids: the difference being that
-in the Annelids the sexually mature units do not individually disunite.
-But though there does<span class="pagenum" id="Page_108">[108]</span> not take place separation of single completed
-segments, there takes place separation of groups of segments, which
-are either sexually mature at the time they drop off or presently
-become so. And the groups of segments which have become sexually mature
-before they drop off, have simultaneously acquired swimming organs and
-developed eyes, enabling them to spread and diffuse the species. Sundry
-biologists recognize a parallelism between that detachment of developed
-segments which goes on in the cestoid <i>Entozoa</i>, and that which
-goes on in the <i>Scyphomedusæ</i>. The successively detached members
-of the strobila are sexually matured or maturing individuals which,
-as medusæ, are fitted for swimming about, multiplying, and reaching
-other habitats; while each detached proglottis of the cestoid is,
-by the nature of its medium, limited to creeping about. Clearly
-this fissiparous process in such Annelids as the <i>Syllidæ</i>,
-which has similarly been compared to the strobilization of the
-<i>Scyphomedusæ</i>, differs simply in the respect that single segments
-are not adapted for locomotion, and it therefore profits the species
-to separate in groups. All these facts and analogies point to the
-conclusion that the remote ancestor of the Annelids was an unsegmented
-creature homologous with each of the segments of an existing Annelid.</p>
-
-<p>This conclusion is supported by other kinds of evidence here to
-be added. The larvæ of Annelids are very various; but amid their
-differences there is a recognizable type. “The Trochophore is the
-typical larval form of the Annelid stem”: a trochophore being a
-curious spheroidal ciliated structure suggestive of cœlenterate
-affinities. And this unsegmented larva, representing the remote
-ancestor from which the many Annelid types diverged, is similar to the
-larvæ of the <i>Rotifera</i> and the <i>Mollusca</i>: a trochophore
-is common to all these great classes. Moreover since, among the
-<i>Rhizota</i> (a sub-class of the <i>Rotiferæ</i>), there is a
-species, <i>Trochosphæra</i>, solitary and free-swimming, resembling
-in form and structure a trochophore, though it is not a larva but an
-adult, we get further<span class="pagenum" id="Page_109">[109]</span> evidence that there was a primitive creature of
-this general character, of which the trochophores of <i>Mollusca</i>,
-<i>Rotifera</i>, and <i>Annelida</i> are divergent modifications, and
-which was unsegmented: the implication being that the segmentation of
-the <i>Annelida</i> was superinduced. That this segmentation resulted
-from gemmation is implied by what are called polytrochal larvæ. These
-“sometimes appear as a stage succeeding other larval types. Thus
-those of <i>Arenicola marina</i> arise from larvæ which at first were
-monotrochal, later became telotrochal, and finally, by the appearance
-of new ciliated rings between those already present, assumed the
-stage of polytrochal larvæ.... This condition warrants the assumption
-that the segmented forms are to be looked upon as the younger, the
-unsegmented, on the other hand, as the phylogenetically older.”
-(Korschelt and Heider, i, 278.) And that the above-described rings
-of cilia mark off segments is shown by the case of <i>Ophryotrocha
-puerilis</i>, which “remains, as it were, in a larval condition, since
-the segments retain their ciliation throughout life.” (<i>Ib.</i>,
-277.) Yet one more significant fact must be named. In early stages
-of development each segment of an archiannelidan has cœlomic spaces
-separate from those of neighbouring segments, but in the adult the
-septa “generally break down either partially or completely, so that
-the peri-visceral cavity becomes a continuous space from end to end
-of the animal.” (Sedgwick, <i>Text Book</i>, 449.) While this fact is
-congruous with the hypothesis here maintained, it is incongruous with
-the hypothesis that the annelid was originally an elongated creature
-which afterwards became segmented; since in that case the implication
-would be that the cœlomic septa, not arising from recapitulation of an
-ancestral structure, but originated by the process of segmentation,
-were first superfluously formed and then destroyed.</p>
-
-<p>Various lines of evidence thus converge to the conclusion that an
-annulose animal is an aggregate of the third order.</p>
-
-<p>In June, 1865, when No. 14 of my serial containing the<span class="pagenum" id="Page_110">[110]</span> foregoing
-chapter was issued, I supposed myself to be alone in holding this
-belief respecting the annulose type, and long continued to suppose
-so. Over thirty years later, however, in M. Edmond Perrier’s work,
-<i>La Philosophie Zoologique avant Darwin</i>, I found mention of a
-lecture delivered by M. Lacaze-Duthiers at the École Normale Supérieure
-in Paris, and reported in the <i>Revue des Cours Scientifiques</i>
-for January 28, 1865, in which he enunciated a like belief. Judging,
-however, by the account of this lecture which M. Perrier gives (he was
-present), it appears that M. Lacaze-Duthiers simply contended that this
-view of the annulose structure as arising by union of once-independent
-units, is suggested by certain <i>à priori</i> considerations. There is
-no indication that he assigned any of the classes of facts above given,
-which go to show that it <i>has</i> thus arisen.</p>
-
-<p>For further facts and arguments concerning the genesis of the annulose
-type, see <a href="#Page_602">Appendix D 2.</a>]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_111">[111]</span></p>
-
-<h3>CHAPTER V.<br />
-<span class="subhed1">THE MORPHOLOGICAL COMPOSITION OF ANIMALS, <i>CONTINUED</i>.</span></h3></div>
-
-<p id="sect208">§ 208. Insects, Arachnids, Crustaceans, and Myriapods, are all
-members of that higher division of the <i>Annulosa</i><a id="FNanchor_26" href="#Footnote_26" class="fnanchor">[26]</a> called
-<i>Articulata</i> or now more generally <i>Arthropoda</i>. Though
-in these creatures the formation of segments may be interpreted as
-a disguised gemmation; and though, in some of them, the number of
-segments increases by this modified budding after leaving the egg,
-as it does among the Annelids; yet the process is not nearly so
-dominant: the segments are usually much less numerous than we find
-them in the types last considered. In most cases, too, the segments
-are in a greater degree differentiated one from another, at the same
-time that they are severally more differentiated within themselves.
-Nor is there any instance of spontaneous fission taking place in the
-series of segments composing an articulate animal. On the contrary,
-the integration, always great enough permanently to unite the
-segments, is frequently carried so far as to hide very completely the
-individualities of some or many of them; and occasionally, as among
-the Acari, the consolidation, or the arrest of segmentation, is so<span class="pagenum" id="Page_112">[112]</span>
-decided as to leave scarcely a trace of the articulate structure:
-the type being in these cases indicated chiefly by the presence of
-those characteristically-formed limbs, which give the alternative
-name <i>Arthropoda</i> to all the higher <i>Annulosa</i>. Omitting
-the parasitic orders, which, as in other cases, are aberrant members
-of their sub-kingdom, comparisons between the different orders prove
-that the higher are strongly distinguished from the lower, by the much
-greater degree in which the individuality of the tertiary aggregate
-dominates over the individualities of those secondary aggregates called
-segments or “somites,” of which it is composed. The successive Figs.
-<a href="#i_112">170–176</a>, representing (without their limbs) a Julus, a Scolopendra,
-an isopodous Crustacean, and four kinds of decapodous Crustaceans,
-ending with a Crab, will convey at a glance an idea of the way in
-which that greater size and heterogeneity reached by the higher
-types, is accompanied by an integration which, in the extreme cases,
-nearly obliterates all traces of composite structure. In the Crab the
-posterior segments, usually folded underneath the shell, alone preserve
-their primitive distinctness. So completely confluent are the rest,
-that it seems absurd to say that a Crab’s carapace is composed of as
-many segments as there are pairs of limbs, foot-jaws, and antennæ
-attached to it; and were it not that during early stages of the Crab’s
-development<span class="pagenum" id="Page_113">[113]</span> the segmentation is faintly marked, the assertion might be
-considered illegitimate.</p>
-
-  <div class="figcenter" id="i_112">
-     <img
-    class="p2"
-    src="images/i_112.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 170–176.</p>
-  </div>
-
-<p class="p2">That all articulate animals are thus composed from end to end
-of homologous segments, is, however, an accepted doctrine among
-naturalists. It is a doctrine that rests on careful observation of
-three classes of facts&mdash;the correspondences of parts in the successive
-“somites” of an adult articulate animal; the still more marked
-correspondences of such parts as they exist in the embryonic or larval
-articulate animal; and the maintenance of such correspondences in some
-types, which are absent in types otherwise near akin to them. The
-nature of the conclusion which these evidences unite in supporting,
-will best be shown by the annexed copies from the lecture-diagrams
-of Prof. Huxley; exhibiting the typical structures of a Myriapod, an
-Insect, a Spider, and a Crustacean, with their relations to a common
-plan, as interpreted by him.</p>
-
-  <div class="figcenter" id="i_113">
-     <img
-    class="p2"
-    src="images/i_113.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 177–186.</p>
-  </div>
-
-<p class="p2">Treating of these homologies, Prof. Huxley says “that a striking
-uniformity of composition is to be found in the heads of, at any rate,
-the more highly organized members of these four classes; and that,
-typically, the head of a Crustacean, an Arachnid, a Myriapod, or an
-Insect, is composed of six<span class="pagenum" id="Page_114">[114]</span> somites (or segments corresponding with
-those of the body) and their appendages, the latter being modified so
-as to serve the purpose of sensory and manducatory organs.”<a id="FNanchor_27" href="#Footnote_27" class="fnanchor">[27]</a></p>
-
-<p>Thus even in the higher <i>Arthropoda</i>, the much greater
-consolidation and much greater heterogeneity do not obliterate all
-evidence of the fact, that the organism is an aggregate of the third
-order. Comparisons show that it is divisible into a number of proximate
-units, each of which is akin in certain fundamental traits to its
-neighbours, and each of which is an aggregate of the second order, in
-so far as it is an organized combination of those aggregates of the
-first order which we call morphological units or cells. And that these
-segments or somites, which make up an annulose animal, were originally
-aggregates of the second order having independent individualities,
-is an hypothesis which gathers further support from the contrast
-between the higher and the lower Arthropods, as well as from the
-contrast between the Arthropods<span class="pagenum" id="Page_115">[115]</span> in general and the Annelids. For if
-that masking of the individualities of the segments which we find
-distinguishes the higher forms from the lower, has been going on from
-the beginning, as we may fairly assume; it is to be inferred that the
-individualities of the segments in the lower forms, were originally
-more marked than they now are. Reversing those processes of change by
-which the most developed <i>Annulosa</i> have arisen from the least
-developed; and applying in thought this reversed process to the least
-developed, as they were described in the last Chapter; we are brought
-to the conception of attached segments that are all completely alike,
-and have their individualities in no appreciable degree subordinated to
-that of the chain they compose. From which there is but one step to the
-conception of gemmiparously-produced individuals which severally part
-one from another as soon as they are formed.</p>
-
-
-<p class="p2">§ 209. We must now return to a junction whence we diverged some time
-ago. As before explained under the head of Classification, organisms
-do not admit of uniserial arrangement, either in general or in detail;
-but everywhere form groups within groups. Hence, having traced the
-phases of morphological composition up to the highest forms in any
-sub-kingdom, we find ourselves at the extremity of a great branch, from
-which there is no access to another great branch, except by going back
-to some place of bifurcation low down in the tree.</p>
-
-<p>There exist such similarities of shape and structure between the larval
-forms of low Molluscs and those of Annelids and Rotifers, as to show
-that there was an early type common to them all; and its probable
-characters, suggested by comparison, seem to imply that it had arisen
-from some cœlenterate type, intermediate between the <i>Cnidaria</i>
-and the <i>Ctenophora</i>. But there is this noteworthy difference
-between the molluscan larva and the allied larvæ, that it gives
-origin to only one animal and not to a group of animals, united or<span class="pagenum" id="Page_116">[116]</span>
-disunited. No true Mollusc multiplies by gemmation, either continuous
-or discontinuous; but the product of every fertilized germ is a single
-individual.</p>
-
-<p>It is a significant fact that here, where for the first time we have
-homogenesis holding throughout an entire sub-kingdom, we have also
-throughout an entire sub-kingdom no case in which the organism is
-divisible into two, three, or more, like parts. There is neither any
-such clustering or branching as a cœlenterate or molluscoid animal
-usually displays; nor is there any trace of that segmentation which
-characterizes the <i>Annulosa</i>. Among these animals in which no
-single egg produces several individuals, no individual is separable
-into several homologous divisions. This connexion will be seen to have
-a probable meaning, on remembering that it is the converse of the
-connexion which obtains among the <i>Annulosa</i>, considered as a
-group.</p>
-
-<p>A Mollusc, then, is an aggregate of the second order. Not only in the
-adult animal is there no sign of a multiplicity of like parts that
-have become obscured by integration; but there is no sign of such
-multiplicity in the embryo. And this unity is just as conspicuous in
-the lowest Lamellibranch as in the highest Cephalopod.</p>
-
-  <div class="figcenter" id="i_116">
-     <img
-    class="p2"
-    src="images/i_116.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 188–190.</p>
-  </div>
-
-<p class="p2">It may be well to note, however, more especially because it illustrates
-a danger of misinterpretation presently to be guarded against, that
-there are certain Molluscs which simulate the segmented structure.
-Externally a <i>Chiton</i>, Fig. <a href="#i_116">188</a>, appears to be made up of
-divisions substantially like those of the creature Fig. <a href="#i_116">189</a>; and
-one who judged only by externals, would say that the creature Fig.
-<a href="#i_116">190</a> differs as much from the creature Fig. 189, as this does from
-the preceding one. But the truth is, that while <a href="#i_116">190</a> and <a href="#i_116">189</a> are
-closely-allied types, <a href="#i_116">189</a> differs from <a href="#i_116">188</a><span class="pagenum" id="Page_117">[117]</span> much more widely than
-a man does from a fish. And the radical distinction between them
-is this:&mdash;Whereas in the Crustacean the segmentation is carried
-transversely through the whole mass of the body, so as to render the
-body more or less clearly divisible into a series of parts which are
-similarly composed; in the Mollusc the segmentation is limited to the
-shell carried on its upper surface, and leaves its body as completely
-undivided as is that of a common slug.<a id="FNanchor_28" href="#Footnote_28" class="fnanchor">[28]</a> Were the body cut through at
-each of the divisions, the section of it attached to each portion of
-the shell would be unlike all the other sections. Here the segmentation
-has a purely functional derivation&mdash;is adaptive instead of genetic. The
-similarly-formed and similarly-placed parts, are not homologous in the
-same sense as are the appendages of a phænogamic axis or the limbs of
-an insect.</p>
-
-
-<p class="p2" id="sect210">§ 210. In studying the remaining and highest sub-kingdom of animals, it
-is important to recognize this radical difference in meaning between
-that likeness of parts which is produced by likeness of modifying
-forces, and that likeness of parts which is due to primordial identity
-of origin. On our recognition of this difference depends the view we
-take of certain doctrines that have long been dominant, and have still
-a wide currency.</p>
-
-<p>Among the <i>Vertebrata</i>, as among the <i>Mollusca</i>, homogenesis
-is universal. The two sub-kingdoms are like one another and unlike the
-remaining sub-kingdoms in this, that in all the types they severally
-include, a single fertilized ovum produces only a single individual. It
-is true that as the eggs of certain gasteropods occasionally exhibit
-spontaneous fission<span class="pagenum" id="Page_118">[118]</span> of the vitelline mass, which may or may not result
-in the formation of two individuals; so among vertebrate animals we
-now and then meet with double monsters, which appear to imply such a
-spontaneous fission imperfectly carried out. But these anomalies serve
-to render conspicuous the fact, that in both these sub-kingdoms the
-normal process is the integration of the whole germ-mass into a single
-organism, which at no phase of its development displays any tendency to
-separate into two or more parts.</p>
-
-<p>Equally as throughout the <i>Mollusca</i>, there holds throughout the
-<i>Vertebrata</i> the correlative fact, that not even in its lowest any
-more than in its highest types, is the body divisible into homologous
-segments. The vertebrate animal, under its simplest as under its most
-complex form, is like the molluscous animal in this, that you cannot
-cut it into transverse slices, each of which contains a digestive
-organ, a respiratory organ, a reproductive organ, &amp;c. The organs of the
-least-developed fish as well as those of the most developed mammal,
-form but a single physiological whole; and they show not the remotest
-trace of having ever been divisible into two or more physiological
-wholes. That segmentation which the vertebrate animal usually exhibits
-throughout part of its organization, is the same in origin and
-meaning as the segmentation of a <i>Chiton’s</i> shell; and no more
-implies in the vertebrate animal a composite structure, than do the
-successive pairs of branchiæ of the <i>Doto</i>, or the transverse
-rows of branchiæ in the <i>Eolis</i>, imply composite structure in
-the molluscous animal. To some this will seem a very questionable
-proposition; and had we no evidence beyond that which adult vertebrate
-animals of developed types supply, it would be a proposition not easy
-to substantiate. But abundant support for it is to be found in the
-structure of the vertebrate embryo, and in the comparative morphology
-of the <i>Vertebrata</i> in general.</p>
-
-<p>Embryologists teach us that the primordial relations of parts are most
-clearly displayed in the early stages of evolution;<span class="pagenum" id="Page_119">[119]</span> and that they
-generally become partially or completely disguised in its later stages.
-Hence, were the vertebrate animal on the same level as the annulose
-animal in degree of composition&mdash;did it similarly consist of segments
-which are homologous in the sense that they are the proximate units
-of composition; we ought to find this fundamental fact most strongly
-marked at the outset. As in the annelid-embryo the first conspicuous
-change is the elongation and division into segments, by constrictions
-that encircle the whole body; and as in the arthropod embryo the
-blastoderm becomes marked out transversely into pieces which extend
-themselves round the yelk before the internal organization has made
-any appreciable progress; so in the embryo of every vertebrate animal,
-had it an analogous composition, the first decided change should be
-a segmentation implicating the entire mass. But it is not so. Sundry
-important differentiations occur before any divisions begin to show
-themselves. There is the defining of that elongated, elevated area with
-its longitudinal groove, which becomes the seat of subsequent changes;
-there is the formation of the notochord lying beneath this groove;
-there is the growth upwards of the boundaries of the groove into the
-dorsal laminæ, which rapidly develop and fold over in the region of
-the head. Rathke, as quoted and indorsed by Prof. Huxley, describes
-the subsequent changes as follows:&mdash;“The gelatinous investing mass,
-which, at first, seems only to constitute a band to the right and to
-the left of the notochord forms around it, in the further course of
-development, a sheath, which ends in a point posteriorly. Anteriorly,
-it sends out two processes which underlie the lateral parts of the
-skull, but very soon coalesce for a longer or shorter distance.
-Posteriorly, the sheath projects but little beyond the notochord; but,
-anteriorly, for a considerable distance, as far as the infundibulum.
-It sends upwards two plates, which embrace the future central parts
-of the nervous system laterally, probably throughout their entire
-length.” That is to say, in the<span class="pagenum" id="Page_120">[120]</span> <i>Vertebrata</i> the first step is
-the marking out on the blastoderm of an integrated structure, within
-which segments subsequently appear. When these do appear, they are
-for some time limited to the middle region of the spinal axis; and no
-more then than ever after, do they implicate the general mass of the
-body in their transverse divisions. On the contrary, before vertebral
-segmentation has made much progress, the rudiments of the vascular
-system are laid down in a manner showing no trace of any primordial
-correspondence of its parts with the divisions of the axis. Equally
-at variance with the belief that the vertebrate animal is essentially
-a series of homologous parts, is the heterogeneity which exists among
-these parts on their first appearance. Though in the head of an adult
-articulate animal there is little sign of divisibility into segments
-like those of the body; yet such segments, with their appropriate
-ganglia and appendages, are easily identifiable in the articulate
-embryo. But in the <i>Vertebrata</i> this antithesis is reversed. At
-the time when segmentation has become decided in the dorsal region of
-the spine, there is no trace of segments in the parts which are to form
-the skull&mdash;nothing whatever to suggest that the skull is being formed
-out of divisions homologous with vertebræ.<a id="FNanchor_29" href="#Footnote_29" class="fnanchor">[29]</a> And minute observation
-no more discloses any such homology than does general appearance.
-“Remak,” says Prof. Huxley, “has more fully proved than any other
-observer, the segmentation into ‘urwirbel,’ or proto-vertebræ, which is
-characteristic of the vertebral column, stops at the occipital margin
-of the skull&mdash;the base of which, before ossification, presents no trace
-of that segmentation which occurs throughout the vertebral column.”</p>
-
-  <div class="figcenter" id="i_121">
-     <img
-    class="p2"
-    src="images/i_121.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 191.</p>
-  </div>
-
-<p class="p2">Consider next the evidence supplied by comparative morphology. In
-preceding sections (<a href="#sect206">§§ 206</a>, <a href="#sect208">208</a>) it has been<span class="pagenum" id="Page_121">[121]</span> shown that among annulose
-animals, the divisibility into homologous parts is most clearly
-demonstrable in the lowest types. Though in decapodous Crustaceans, in
-Insects, in Arachnids, there is difficulty in identifying some or many
-of the component somites; and though, when identified, they display
-only partial correspondences; yet on descending to Annelids, the
-composition of the entire body out of such somites becomes conspicuous,
-and the homology between each somite and its neighbours is shown by the
-repetition of one another’s structural details, as well as by their
-common gemmiparous origin: indeed, in some cases we have the homology
-directly demonstrated by seeing a somite of the body transformed into
-a head. If, then, a vertebrate animal had a segmental composition of
-kindred nature, we ought to find it most clearly marked in the lowest
-<i>Vertebrata</i> and most disguised in the highest <i>Vertebrata</i>.
-But here, as before, the fact is just the reverse. Among the
-<i>Vertebrata</i> of developed type, such segmentation as really exists
-remains conspicuous&mdash;is but little obscured even in parts of the spinal
-column formed out of integrated vertebræ. Whereas in the undeveloped
-vertebrate type, segmentation is scarcely at all traceable.<a id="FNanchor_30" href="#Footnote_30" class="fnanchor">[30]</a> The
-<i>Amphioxus</i>, Fig. <a href="#i_121">191</a>, is not only without ossified vertebræ; not
-only is it without cartilaginous representatives of them; but it is
-even without anything like distinct membranous divisions. The spinal
-column exists as a continuous notochord: the only signs of incipient
-segmentation being given by its membranous sheath, in the upper part
-of which “quadrate masses of somewhat denser<span class="pagenum" id="Page_122">[122]</span> tissue seem faintly to
-represent neural spines.” Moreover, throughout sundry groups of fishes
-and amphibians, the segmentation remains very imperfect: only certain
-peripheral appendages of the vertebræ becoming defined and solidified,
-while in place of the bodies of the vertebræ there still continues the
-undivided notochord. Thus, instead of being morphologically composed
-of vertebral segments, the vertebrate animal in its primitive form is
-entirely without vertebral segments; and vertebral segments begin to
-appear only as we advance towards developed forms. Once more, evidence
-equally adverse to the current hypothesis meets us on observing that
-the differences between the parts supposed to be homologous, are as
-great at first as at last. Did the vertebrate animal primordially
-consist of homologous segments from snout to tail; then the segments
-said to compose the skull ought, in the lowest <i>Vertebrata</i>, to
-show themselves much more like the remaining segments than they do in
-the highest <i>Vertebrata</i>. But they do not. Fishes have crania
-made up of bones that are no more clearly arrangeable into segments
-like vertebræ, than are the cranial bones of the highest mammal. Nay,
-indeed, the case is much stronger. The simplest fish possessing a
-skeleton, has a cranium composed of cartilage that is not segmented at
-all!</p>
-
-<p>Besides being inconsistent with the leading truths of Embryology
-and Comparative Morphology, the hypothesis of Goethe and Oken is
-inconsistent with itself. The facts brought forward to show that there
-exists an archetypal vertebra, and that the vertebrate animal is
-composed of archetypal vertebræ arranged in a series, and severally
-modified to fit their positions&mdash;these facts, I say, so far from
-proving as much, suffice, when impartially considered, to disprove it.
-No assigned, nor any conceivable, attribute of the supposed archetypal
-vertebra is uniformly maintained. The parts composing it are constant
-neither in their number, nor in their relative positions, nor in their
-modes of ossification,<span class="pagenum" id="Page_123">[123]</span> nor in the separateness of their several
-individualities when present. There is no fixity of any one element,
-or connexion, or mode of development, which justifies even a suspicion
-that vertebræ are modelled after an ideal pattern. To substantiate
-these assertions here would require too much space, and an amount of
-technical detail wearisome to the general reader. The warrant for them
-will be found in a criticism on the osteological works of Prof. Owen,
-originally published in the <i>British and Foreign Medico-Chirurgical
-Review</i> for Oct. 1858. This criticism I add in the Appendices, for
-the convenience of those who may wish to study the question more fully.
-(See Appendix B.)</p>
-
-<p>Everything, then, goes to show that the segmental composition
-which characterises the apparatus of external relation in most
-<i>Vertebrata</i>, is not primordial or genetic, but functionally
-determined or adaptive. Our inference must be that the vertebrate
-animal is an aggregate of the second order, in which a relatively
-superficial segmentation has been produced by mechanical intercourse
-with the environment. We shall hereafter see that this conception leads
-us to a consistent interpretation of the facts&mdash;shows us why there has
-arisen such unity in variety as exists in every vertebral column, and
-why this unity in variety is displayed under countless modifications in
-different skeletons.<a id="FNanchor_31" href="#Footnote_31" class="fnanchor">[31]</a></p>
-
-
-<p class="p2">§ 211. On glancing back at the facts brought together in these two
-chapters, we see it to be probable that there has gone on among animals
-a process like that which we saw reason to think has gone on among
-plants. Minute aggregates of those physiological units which compose
-living protoplasm,<span class="pagenum" id="Page_124">[124]</span> exist as <i>Protozoa</i>: some of them incoherent,
-indefinite, and almost homogeneous, and others of them more coherent,
-definite, and heterogeneous. By union of these nucleated particles
-of sarcode, are produced various indefinite aggregates of the second
-order&mdash;Sponges, <i>Polycytharia</i>, Foraminifers, &amp;c.; in which the
-compound individuality is scarcely enough marked to subordinate the
-primitive individualities. But in other types, as in <i>Hydra</i>, the
-lives of the morphological units are in a considerable degree, though
-not wholly, merged in the life of the integrated body they form. As the
-primary aggregate, when it passes a certain size, undergoes fission or
-gemmation; so does the secondary aggregate. And as on the lower stage
-so on the higher, we see cases in which the gemmiparously-produced
-individuals part as soon as formed, and other cases in which they
-continue united, though in great measure independent. This massing
-of secondary aggregates into tertiary aggregates, is variously
-carried on among the <i>Hydrozoa</i>, the <i>Actinozoa</i>, the
-<i>Polyzoa</i>, and the <i>Tunicata</i>. In most of the types so
-produced, the component individualities are very little subordinated
-to the individuality of the composite mass&mdash;there is only physical
-unity and not physiological unity; but in certain of the oceanic
-<i>Hydrozoa</i>, the individuals are so far differentiated and combined
-as very much to mask them. Forms showing us clearly the transition to
-well-developed individuals of the third order, are not to be found.
-Nevertheless, in the great sub-kingdom <i>Annulosa</i>, there are
-traits of structure, development, and mode of multiplication, which go
-far to show that its members are such individuals of the third order;
-and in the relations to external conditions involved by the mode of
-union, we find an adequate cause for that obscuration of the secondary
-individualities which we must suppose has taken place. The two other
-great subdivisions, <i>Mollusca</i> and <i>Vertebrata</i>, between
-the lower members of which there are suggestive points of community,
-present us only with aggregates of the second order, that have in many
-cases become very large and<span class="pagenum" id="Page_125">[125]</span> very complex. We find in them no trace of
-the union of gemmiparously-produced individuals. Neither the molluscous
-nor the vertebrate animal shows the faintest trace of a segmentation
-affecting the totality of its structure; and we see good grounds for
-concluding that such segmentation as exceptionally occurs in the one
-and usually occurs in the other, is superinduced.</p>
-
-<hr class="tb" />
-
-<p>[<span class="smcap">Note</span>:&mdash;A critic calls in question the statement on p. 121
-respecting the <i>Amphioxus</i>. At the outset, however, he admits
-that in the <i>Amphioxus</i> “the central nervous system and the
-notochord are not segmented.” In the Annelid, however, the central
-nervous system is segmented, and there is segmentation of the part
-which, as a supporting structure, is analogous to the notochord in
-respect of function&mdash;the outer part which represents the exo-skeleton
-in contrast to the endo-skeleton. He goes on to say that “the gut is
-not involved [in the segmentation] and exhibits in <i>Amphioxus</i>
-just as it does in worms differentiations entirely independent of the
-segmentation of the mesoblast.” Part of this statement is, I think,
-not congruous with all the facts. In <i>Protodrilus</i>, one of the
-lowest of the <i>Archiannelida</i>, “the intestine is moniliform, there
-being a constriction between each segment” and the next. (Shipley.)
-Complete segmentation of the intestine is obviously impossible,
-since, were the canal divided into portions by septa, no food could
-pass. But the fact that the gut has these successive expansions and
-constrictions, corresponding to the successive segments, and giving
-to each segment a partially-separate stomach, shows that segmentation
-has gone as far as consists with the carrying on of the lives of the
-segments. No such partial segmentation exists in the <i>Amphioxus</i>.
-Thus, then, three fundamental structures&mdash;the directive structure, the
-supporting structure, and the alimentary structure&mdash;are respectively
-simple in the lowest vertebrate and segmented, or<span class="pagenum" id="Page_126">[126]</span> partially segmented,
-in the lowest Annelid. Again, while it is said that the gill-clefts
-exhibit segmentation, it is admitted that this has no relevance to
-any constitutional segmentation: “they are segmented on a plan of
-their own” irrespective of other organs. Another allegation is that
-the ovaries of <i>Amphioxus</i> are segmented. Their segmentation,
-however, like that of the gills, is isolated, and may be considered
-as illustrating those repetitions of like parts seen in supernumerary
-vertebræ in various creatures&mdash;a repetition which becomes habitual
-if the resulting structure is advantageous to the species. On the
-statement that while the <i>Amphioxus</i> has no rudiments of a
-renal system the Elasmobranch embryo has such rudiments, which are
-as distinctly segmented as the nephridia of a worm, two comments may
-be made. The first is that if in these Vertebrates the nephridia
-bear a relation to the general structure like that which they do in
-Annelids, then one would expect to find the segmental arrangement
-shown in the lowest type, as in Annelids, rather than in a type
-considerably advanced in development. Should it be replied that in the
-<i>Amphioxus</i> an excretory system had not yet arisen, though one
-is required for the higher organization of an Elasmobranch, then the
-answer may be that since the segmental arrangement in the Elasmobranch
-corresponds with that of the myotomes, it has no reference to any
-primordial segmentation, since the myotomes have been functionally
-generated. The second comment is that whereas the nephridia of the
-Annelid have independent external openings, the nephridia in the
-Elasmobranch have not. These discharge their secretions into certain
-general tubes of exit common to them all; showing that each of them,
-instead of being a member of a partially independent structure, is
-united with others in subordination to a general structure. That
-is to say, the segmentations are far from being parallel in their
-essential natures. The assertion accompanying these criticisms, that
-there is “no difference <i>in principle</i> between the segmentation
-of <i>Amphioxus</i><span class="pagenum" id="Page_127">[127]</span> and Annelid” is difficult to reconcile with the
-visible contrast between the two. Whatever local segmentations there
-are in an <i>Amphioxus</i> appear to me quite unlike “in principle”
-to those which an Annelid exhibits. Could its portion of gut be duly
-supplied with nutriment, the segment of a low Annelid could carry on
-its vital functions independently. In the parts of the <i>Amphioxus</i>
-we see nothing approaching to this. Cut it into transverse sections
-and no one of them contains anything like the assemblage of structures
-required for living. The <i>Amphioxus</i> is a physiological whole, and
-in that respect differs radically from the Annelid, each segment of
-which is in chief measure a physiological whole. No occurrence of local
-segmentation in the <i>Amphioxus</i> can obliterate this fundamental
-contrast.</p>
-
-<p>An accompanying contrast tells the same story. On ascending from the
-lowest to the highest annulose types we see a progressing integration,
-morphological and physiological; so that whereas in a low annelid
-the successive parts are in large measure independent in their
-structures and in their lives, in a high arthropod, as a crab, most
-of the parts have lost their individualities and have become merged
-in a consolidated organism with a single life. Quite otherwise is it
-in the vertebrate series. Its lowest member is at the very outset a
-complete morphological and physiological whole, and the formation of
-those serial parts which some think analogous to the serial parts of an
-Annelid, begins at a later stage and becomes gradually pronounced. That
-is to say, the course of transformation is reversed.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_128">[128]</span></p>
-
-<h3>CHAPTER VI.<br />
-<span class="subhed1">MORPHOLOGICAL DIFFERENTIATION IN PLANTS.</span></h3></div>
-
-<p>§ 212. While, in the course of their evolution, plants and animals
-have displayed progressive integrations, there have at the same time
-gone on progressive differentiations of the resulting aggregates,
-both as wholes and in their parts. These differentiations and the
-interpretations of them, form the second class of morphological
-problems.</p>
-
-<p>We commence as before with plants. We have to consider, first, the
-several kinds of modification in shape they have undergone; and,
-second, the relations between these kinds of modification and their
-factors. Let us glance at the leading questions that have to be
-answered.</p>
-
-
-<p class="p2">§ 213. Irrespective of their degrees of composition, plants may, and
-do, become changed in their general forms. Are their changes capable of
-being formulated? The inquiry which meets us at the outset is&mdash;does a
-plant’s shape admit of being expressed in any universal terms?&mdash;terms
-that remain the same for all genera, orders, and classes.</p>
-
-<p>After plants considered as wholes, have to be considered their
-proximate components, which vary with their degrees of composition,
-and in the highest plants are what we call branches. Is there any law
-traceable among the contrasted shapes of different branches in the same
-plant? Do the relative developments of parts in the same branch conform
-to any law? And are these laws, if they exist, allied with one<span class="pagenum" id="Page_129">[129]</span> another
-and with that to which the shape of the whole plant conforms?</p>
-
-<p>Descending to the components of these components, which in developed
-plants we distinguish as leaves, there meet us kindred questions
-respecting their relative sizes, their relative shapes, and their
-shapes as compared with those of foliar organs in general. Of their
-morphological differentiations, also, it has to be asked whether they
-exemplify any truth that is exemplified by the entire plant and by its
-larger parts.</p>
-
-<p>Then, a step lower, we come down to those morphological units of
-which leaves and fronds consist; and concerning these arise parallel
-inquiries touching their divergences from one another and from cells in
-general.</p>
-
-<p>The problems thus put together in several groups cannot of course be
-rigorously separated. Evolution presupposes transitions which make all
-such classings more or less conventional; and adherence to them must be
-subordinate to the needs of the occasion.</p>
-
-
-<p class="p2">§ 214. In studying the causes of the morphological differentiations
-thus divided out and prospectively generalized, we shall have to bear
-in mind several orders of forces which it will be well briefly to
-specify.</p>
-
-<p>Growth tends inevitably to initiate changes in the shape of any
-aggregate, by altering both the amounts of the incident forces and
-the forces which the parts exert on one another. With the mechanical
-actions this is obvious. Matter that is sensibly plastic cannot be
-increased in mass without undergoing a change in its proportions,
-consequent on the diminished ratio of its cohesive force to the force
-of gravitation. With the physiological actions it is equally obvious.
-Increase of size, other things equal, alters the relations of the parts
-to the material and dynamical factors of nutrition; and by so affecting
-differently the nutrition of different parts, initiates further changes
-of proportions.</p>
-
-<p><span class="pagenum" id="Page_130">[130]</span></p>
-
-<p>In plants of the third order it is thus with the proximate components:
-they are subject to mutual influences that are unlike one another and
-are continually changing. The earlier-formed units become mechanical
-supporters of the later-formed units, and so experience modifying
-forces from which the later-formed units are exempt. Further, these
-elder units simultaneously begin to serve as channels through which
-materials are carried to and from the younger units&mdash;another cause
-of differentiation that goes on increasing in intensity. Once more,
-there arise ever-strengthening contrasts between the amounts of light
-which fall upon the youngest or outermost units and the eldest or
-innermost units; whence result structural contrasts of yet another
-kind. Evidently, then, along with the progressive integration of cells
-into fronds, of fronds into axes, and of axes into plants still more
-composite, there come into play sundry causes of differentiation which
-act on the whole and on each of its parts, whatever their grade. The
-forces to be overcome, the forces to be utilized, and the matters to
-be appropriated, do not remain the same in their proportions and modes
-of action for any two members of the aggregate: be they members of the
-first, second, third, or any other order.</p>
-
-
-<p class="p2">§ 215. Nor are these the only kinds and causes of heterogeneity which
-we have to consider. Beyond the more general changes produced in the
-relative sizes and shapes of plants and their parts by progressive
-aggregation, there are the more particular changes determined by the
-more particular conditions.</p>
-
-<p>Plants as wholes assume unlike attitudes towards their environments;
-they have many ways of articulating their parts with one another; they
-have many ways of adjusting their parts towards surrounding agencies.
-These are causes of special differentiations additional to those
-general differentiations that result from increase of mass and increase
-of composition. In each part considered individually, there arises<span class="pagenum" id="Page_131">[131]</span>
-a characteristic shape consequent on that relative position towards
-external and internal forces, which the mode of growth entails. Every
-member of the aggregate presents itself in a more or less peculiar way
-towards the light, towards the air, and towards its point of support;
-and according to the relative homogeneity or heterogeneity in the
-incidence of the agencies thus brought to bear on it, will be the
-relative homogeneity or heterogeneity of its shape.</p>
-
-
-<p class="p2">§ 216. Before passing from this <i>à priori</i> view of the
-morphological differentiations which necessarily accompany
-morphological integrations, to an <i>à posteriori</i> view of them, it
-seems needful to specify the meanings of certain descriptive terms we
-shall have to employ.</p>
-
-<p>Taking for our broadest division among forms, the regular and the
-irregular, we may divide the latter into those which are wholly
-irregular and those which, being but partially irregular, suggest
-some regular form to which they approach. By slightly straining the
-difference between them, two current words may be conveniently used to
-describe these subdivisions. The entirely irregular forms we may class
-as <i>asymmetrical</i>&mdash;literally as forms without any equalities of
-dimensions. The forms which approximate towards regularity without
-reaching it, we may distinguish as <i>unsymmetrical</i>: a word which,
-though it asserts inequality of dimensions, has been associated by
-use rather with such slight inequality as constitutes an observable
-departure from equality.</p>
-
-<p>Of the regular forms there are several classes, differing in the number
-of directions in which equality of dimensions is repeated. Hence
-results the need for names by which symmetry of several kinds may be
-expressed.</p>
-
-<p>The most regular of figures is the sphere: its dimensions are the same
-from centre to surface in all directions; and if cut by any plane
-through the centre, the separated parts are equal and similar. This is
-a kind of symmetry which stands alone, and will be hereafter spoken of
-as <i>spherical symmetry</i>.</p>
-
-<p><span class="pagenum" id="Page_132">[132]</span></p>
-
-<p>When a sphere passes into a spheroid, either prolate or oblate, there
-remains but one set of planes that will divide it into halves, which
-are in all respects alike; namely, the planes in which its axis lies,
-or which have its axis for their line of intersection. Prolate and
-oblate spheroids may severally pass into various forms without losing
-this property. The prolate spheroid may become egg-shaped or pyriform,
-and it will still continue capable of being divided into two equal
-and similar parts by any plane cutting it down its axis; nor will the
-making of constrictions deprive it of this property. Similarly with the
-oblate spheroid. The transition from a slight oblateness, like that of
-an orange, to an oblateness reducing it nearly to a flat disc, does
-not alter its divisibility into like halves by every plane passing
-through its axis. And clearly the moulding of any such flattened oblate
-spheroid into the shape of a plate, leaves it as before, symmetrically
-divisible by all planes at right angles to its surface and passing
-through its centre. This species of symmetry is called <i>radial
-symmetry</i>. It is familiarly exemplified in such flowers as the
-daisy, the tulip, and the dahlia.</p>
-
-<p>From spherical symmetry, in which we have an infinite number of axes
-through each of which may pass an infinite number of planes severally
-dividing the aggregate into equal and similar parts; and from radial
-symmetry, in which we have a single axis through which may pass an
-infinite number of planes severally dividing the aggregate into
-equal and similar parts; we now turn to <i>bilateral symmetry</i>,
-in which the divisibility into equal and similar parts becomes much
-restricted. Noting, for the sake of completeness, that there is a
-sextuple bilateralness in the cube and its derivative forms which admit
-of division into equal and similar parts by planes passing through
-the three diagonal axes and by planes passing through the three axes
-that join the centres of the surfaces, let us limit our attention to
-the three kinds of bilateralness which here concern us. The first of
-these is <i>triple<span class="pagenum" id="Page_133">[133]</span> bilateral symmetry</i>. This is the symmetry of
-a figure having three axes at right angles to one another, through
-each of which there passes a single plane that divides the aggregate
-into corresponding halves. A common brick will serve as an example;
-and of objects not quite so simple, the most familiar is that modern
-kind of spectacle-case which is open at both ends. This may be divided
-into corresponding halves along its longitudinal axis by cutting it
-through in the direction of its thickness, or by cutting it through in
-the direction of its breadth; or it may be divided into corresponding
-halves by cutting it across the middle. Of objects which illustrate
-<i>double bilateral symmetry</i>, may be named one of those boats built
-for moving with equal facility in either direction, and therefore
-made alike at stem and stern. Obviously such a boat is separable into
-equal and similar parts by a vertical plane passing through stem and
-stern; and it is also separable into equal and similar parts by a
-vertical plane cutting it amidships. To exemplify <i>single bilateral
-symmetry</i> it needs but to turn to the ordinary boat of which the
-two ends are unlike. Here there remains but the one plane passing
-vertically through stem and stern, on the opposite sides of which the
-parts are symmetrically disposed.</p>
-
-<p>These several kinds of symmetry as placed in the foregoing order, imply
-increasing heterogeneity. The greatest uniformity in shape is shown
-by the divisibility into like parts in an infinite number of infinite
-series of ways; and the greatest degree of multiformity consistent
-with any regularity, is shown by the divisibility into like parts in
-only a single way. Hence, in tracing up organic evolution as displayed
-in morphological differentiations, we may expect to pass from the one
-extreme of spherical symmetry, to the other extreme of single bilateral
-symmetry. This expectation we shall find to be completely fulfilled.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_134">[134]</span></p>
-
-<h3>CHAPTER VII.<br />
-<span class="subhed1">THE GENERAL SHAPES OF PLANTS.</span></h3></div>
-
-<p id="sect217">§ 217. Among protophytes those exemplified by <i>Pleurococcus
-vulgaris</i> are by general consent considered the simplest. As
-shown in Fig. <a href="#i_022a">1</a>, they are globular cells presenting no obvious
-differentiation save that between inner and outer parts. Their
-uniformity of figure co-exists with a mode of life involving the
-uniform exposure of all their sides to incident forces. For though
-each individual may have its external parts differently related to
-environing agencies, yet the new individuals produced by spontaneous
-fission, whether they part company or whether they form clusters and
-are made polyhedral by mutual pressure, have no means of maintaining
-parallel relations of position among their parts. On the contrary,
-the indefiniteness of the attitudes into which successive generations
-fall, must prevent the rise of any unlikeness between one portion of
-the surface and another. Spherical symmetry continues because, on the
-average of cases, incident forces are equal in all directions.</p>
-
-  <div class="figcenter" id="i_135">
-     <img
-    class="p2"
-    src="images/i_135.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 1, 2, 3.</p>
-  </div>
-
-<p class="p2">Other orders of <i>Protophyta</i> have much more special forms, along
-with much more special attitudes: their homologous parts maintaining,
-from generation to generation, unlike relations to incident forces. The
-<i>Desmidiaceæ</i> and <i>Diatomaceæ</i>, of which Figs.<a href="#i_135"> 2 and 3</a> show
-examples, severally include genera characterized by triple bilateral
-symmetry. A <i>Navicula</i> is divisible into corresponding halves
-by a transverse<span class="pagenum" id="Page_135">[135]</span> plane and by two longitudinal planes&mdash;one cutting
-its valves at right angles and the other passing between its valves.
-The like is true of those numerous transversely-constricted forms
-of <i>Desmidiaceæ</i>, exemplified by the second of the individuals
-represented in Fig. <a href="#i_135">2</a>. If now we ask how a <i>Navicula</i> is related
-to its environment, we see that its mode of life exposes it to three
-different sets of forces: each set being resolvable into two equal and
-opposite sets. A <i>Navicula</i> moves in the direction of its length,
-with either end foremost. Hence, on the average, its ends are subject
-to like actions from the agencies to which its motions subject it.
-Further, either end while moving exposes its right and left sides to
-amounts of influence which in the long run must be equal. If, then,
-the two ends are not only like one another, but have corresponding
-right and left sides, the symmetrical distribution of parts answers to
-the symmetrical distribution of forces. Passing to the two edges and
-the two flat surfaces, we similarly find a clue to their likenesses
-and differences in their respective relations to the things around
-them. These locomotive protophytes move through the entangled masses
-of fragments and fibres produced by decaying organisms and confervoid
-growths. The interstices in such matted accumulations are nearly all
-of them much longer in one dimension than in the rest&mdash;form crevices
-rather than regular meshes. Hence, a small organism will have much
-greater facility of insinuating itself through this <i>débris</i>,
-in which it finds nutriment, if its transverse section is flattened
-instead of square or circular. And while we see how, by survival of the
-fittest, a flattened form is likely to be acquired by diatoms having
-this habit; we also see that likeness will be maintained between the
-two flat surfaces and between the two edges. For, on the average,
-the relations of the two flat surfaces to the sides of the openings
-through which the<span class="pagenum" id="Page_136">[136]</span> diatom passes, will be alike; and so, too, on the
-average, will be the relations of the two edges. In desmids of the type
-exemplified by the second individual in Fig. <a href="#i_135">2</a>, a kindred equalization
-of dimensions is otherwise insured. There is nothing to keep one of the
-two surfaces uppermost rather than the other; and hence, in the long
-succession of individuals, the two surfaces are sure to be similarly
-exposed to light and agencies in general. When to this is added the
-fact that spontaneous fission occurs transversely in a constant way,
-it becomes manifest that the two ends, while they are maintained in
-conditions like one another, are maintained in conditions unlike those
-of the two edges. Here then, as before, triple bilateral symmetry
-in form, co-exists with a triple bilateral symmetry in the average
-distribution of actions.</p>
-
-  <div class="figcenter" id="i_136">
-     <img
-    class="p2"
-    src="images/i_136.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 4, 5, 6.</p>
-  </div>
-
-<p class="p2">Still confining our attention to aggregates of the first order, let us
-next note what results when the two ends are permanently subject to
-different conditions. The fixed unicellular plants, of which examples
-are given in Figs. <a href="#i_136">4, 5, and 6</a>, severally illustrate the contrast
-in shape arising between the part that is applied to the supporting
-surface and the part that extends into the surrounding medium. These
-two parts which are the most unlike in their relations to incident
-forces, are the most unlike in the forms. Observe, next, that the part
-which lifts itself into the water or air, is more or less decidedly
-radial. Each outward-growing tubule of <i>Codium adhærens</i>, Fig. <a href="#i_136">4</a>,
-has its parts disposed with some regularity around its axis; the upper
-stem and spore-vessel<span class="pagenum" id="Page_137">[137]</span> of <i>Botrydium</i>, Fig. <a href="#i_136">5</a>, display a lateral
-growth that is approximately equal in every direction; and the stems
-of the <i>Mucor</i>, Fig. <a href="#i_136">6</a>, shoot up with an approach to evenness on
-all sides. Plants of this low type are naturally very variable in their
-modes of growth: each individual being greatly modified in form by its
-special circumstances. But they nevertheless show us a general likeness
-between parts exposed to like forces, as well as a general unlikeness
-between parts exposed to unlike forces.</p>
-
-<p>Respecting the forms of these aggregates of the first order, it has
-only to be added that they are asymmetrical where there is total
-irregularity in the incidence of forces. We have an example in the
-indefinitely contorted and branched shape of a fungus-cell, growing as
-a mycelium among the particles of soil or through the interstices of
-organic tissue.</p>
-
-
-<p class="p2" id="sect218">§ 218. Re-illustrations of the general truths which the forms of
-these vegetal aggregates of the first order display, are furnished by
-vegetal aggregates of the second order. The equalities and inequalities
-of growth in different directions, prove to be similarly related to
-the equalities and inequalities of environing actions in different
-directions.</p>
-
-<p>Of spherical symmetry an instance occurs in <i>Eudorina elegans</i>.
-The ciliated cells are here so united as to produce a small,
-mulberry-shaped, hollow ball which, being similarly conditioned
-on all sides, shows no unlikenesses of structure. An allied form,
-however, <i>Volvox globator</i>, presents a highly instructive,
-though very trifling, modification. It is not absolutely homogeneous
-in its structure and is not absolutely homogeneous in its motions.
-The waving cilia of its component cells have fallen into such slight
-heterogeneities of action as to cause rotation in a constant direction;
-and along with a fixed axis of rotation there has arisen a fixed axis
-of progression. A concomitant fact is that the cells of the colony
-exhibit an appreciable differentiation in relation to the fixed axis.
-There is an incipient divergence from<span class="pagenum" id="Page_138">[138]</span> spherical uniformity along with
-this slight divergence from uniformity of conditions.</p>
-
-<p>Vegetal aggregates of the second order are usually fixed: locomotion
-is exceptional. Fixity implies that the surface of attachment is
-differently circumstanced from the free surface. Hence we may expect
-to find, as we do find, that among these rooted aggregates of the
-second order, as among those of the first order, the primary contrast
-of shape is between the adherent part and the loose part. Sea-weeds
-variously exemplify this. In some the fronds are very irregular and
-in some tolerably regular; in some the form is pseudo-foliar and in
-some pseud-axial; but differing though they do in these respects,
-they agree in having the end which is attached to a solid body unlike
-the other end. The same truth is seen in such secondary aggregates
-as the common Agarics, or rather in their immensely-developed organs
-of fructification. A puff-ball, Fig. <a href="#i_138">192</a>, presents no other obvious
-unlikeness of parts than that between its under and upper surfaces. So
-too with the stalked kinds that frequent our woods and pastures. In the
-types which Figs. <a href="#i_138">193, 194, 195</a>, delineate, the unlikenesses between
-the rooted ends and the expanded ends, as well as between the under
-and upper surfaces of the expanded ends, are obviously related to this
-fundamental contrast of conditions. Nor is this relation less clearly
-displayed in the sessile fungi which grow out from the sides of trees,
-as shown at <i>a</i>, <i>b</i>, Fig. <a href="#i_138">196</a>. That which is common to this
-and the preceding types, is the contrast between the attached end and
-the free end.</p>
-
-  <div class="figcenter" id="i_138">
-     <img
-    class="p2"
-    src="images/i_138.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 192–196.</p>
-  </div>
-
-<p><span class="pagenum" id="Page_139">[139]</span></p>
-
-<p class="p2">From what these forms have in common, let us turn to that which they
-have not in common, and observe the causes of the want of community.
-A puff-ball shows us in the simplest way, the likeness of parts
-accompanying likeness of conditions, along with the unlikeness of parts
-accompanying unlikeness of conditions. For while, if we cut vertically
-through its centre, we find a difference between top and bottom, if
-we cut horizontally through its centre, we find no differences among
-its several sides. Being, on the average of cases, similarly related
-to the environment all round, it remains the same all round. The
-radial symmetry of the mushroom and other vertically-growing fungi,
-illustrates this connexion of cause and effect still better. But
-now mark what happens in the group of <i>Agaricus noli-tangere</i>,
-shown in Fig. <a href="#i_138">195</a>. Radially-symmetrical as is the type, and radially
-symmetrical as are those centrally-placed individuals which are equally
-crowded all round, we see that the peripheral individuals, dissimilarly
-circumstanced on their outer sides and on their sides next the group,
-have partially changed their radial symmetry into bilateral symmetry.
-It is no longer possible to make two corresponding halves by <i>any</i>
-vertical plane cutting down through the pileus and the stem; but there
-is only <i>one</i> vertical plane that will thus produce corresponding
-halves&mdash;the plane on the opposite sides of which the relations to the
-environment are alike. And then mark that the divergence from all-sided
-symmetry towards two-sided symmetry, here caused in the individual by
-special circumstances, is characteristic of the race where the habits
-of the race constantly involve two-sidedness of conditions. Besides
-being exemplified by such comparatively undifferentiated types as
-certain <i>Polypori</i>, Fig. <a href="#i_138">196</a>, <i>a</i>, <i>b</i>, this truth
-is exemplified by members of the genus just named. In <i>Agaricus
-horizontalis</i>, Fig. <a href="#i_138">196</a>, <i>c</i>, we have a departure from radial
-symmetry that is conspicuous only in the form of the stem. A more
-decided bilateralness exists in <i>A. subpalmatus</i>, shown in
-elevation at <i>d</i> and in section at <i>d´</i>. And<span class="pagenum" id="Page_140">[140]</span> <i>Lentinus
-flabelliformis</i>, of which <i>e</i> and <i>e´</i> are different
-views, exhibits complete bilateralness&mdash;a bilateralness in which
-there is the greatest likeness of the parts that are most similarly
-conditioned, and the greatest unlikeness of the parts that are most
-dissimilarly conditioned.</p>
-
-<p>Among plants of the second order of composition, it will suffice
-to note one further class of facts which are the converse of the
-foregoing and have the same implications. These are the facts showing
-that along with habitual irregularity in the relations to external
-forces, there is habitual irregularity in the mode of growth. Besides
-finding such facts among Thallophytes, as in the tubers of underground
-fungi and in the creeping films of sessile lichens, which severally
-show us variations of proportions obviously caused by variations in
-the amounts of the influences on their different sides, we also,
-among Archegoniates of inferior types, find irregularities of form
-along with irregularities in environing actions. The fronds of the
-<i>Marchantiaceæ</i> or such <i>Jungermanniaceæ</i> as are shown in
-Figs. <a href="#i_033">41, 42, 43</a>, illustrate the way in which each lowly-organized
-aggregate of the second order, not individuated by the mutual
-dependence of its parts, has its form determined by the balance of
-facilities and resistances which each side of the frond meets with as
-it spreads.</p>
-
-
-<p class="p2">§ 219. Among plants displaying integration of the third degree, and
-among plants still further compounded, these same truths are equally
-manifest. In the forms of such plants we see primary contrasts and
-secondary contrasts which, no less clearly than the foregoing, are
-related to contrasts of conditions.</p>
-
-<p>That flowering plants from the daisy up to the oak, have in common the
-fundamental unlikeness between the upward growing part and the downward
-growing part; and that this most marked unlikeness corresponds with the
-most marked unlikeness between the two parts of their environment, soil
-and air; are facts too conspicuous to be named<span class="pagenum" id="Page_141">[141]</span> were they not important
-items in the argument. More instructive perhaps, because less familiar,
-is the fact that we miss this extreme contrast in flowering plants
-which have not their higher and lower portions exposed to conditions
-thus extremely contrasted. A parasite like the Dodder, growing in
-entangled masses upon other plants, from which it sucks the juices, is
-not thus divisible into two strongly-distinguished halves.</p>
-
-<p>Leaving out of consideration the difference between the supporting
-part and the supported part in phænogams, and looking at the supported
-part only, we observe between its form and the habitual incidence of
-forces, a relation like that which we observed in the simpler plants.
-Phænogams that are practically if not literally uniaxial, and those
-which develop their lateral axes only in the shape of axillary flowers,
-when uninterfered with commonly send up vertical stems round which the
-leaves and flowers are disposed with a more or less decided radial
-symmetry. Gardens and fields supply us with such instances as the Tulip
-and the Orchis; and, on a larger scale, the Palms and the Aloes are
-fertile in examples. The exceptions, too, are instructive. Besides the
-individual divergences arising from special interferences, there are
-to be traced general divergences where the habits of the plants expose
-them to general interferences in anything approaching to constant ways.
-Plants which, like the Foxglove, have spikes of flowers that are borne
-on flexible foot-stalks, have their flowers habitually bent round to
-one face of the stem: an unlikeness of distribution probably caused
-by unlikeness in the relation to the Sun’s rays. The wild Hyacinth,
-too, with stem so flexible that its upper part droops, shows us how a
-consequent difference in the action of gravity on the flowers, causes
-them to deviate from their typically-radial arrangement towards a
-bilateral arrangement.</p>
-
-  <div class="figcenter" id="i_142">
-     <img
-    class="p2"
-    src="images/i_142.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 197–199.</p>
-  </div>
-
-<p class="p2">Much more conspicuous are these general and special relations of form
-to general and special actions in the environment, among phænogams
-that are multiaxial. That when<span class="pagenum" id="Page_142">[142]</span> standing alone, and in places where
-the winds do not injure them nor adjacent things shade them, shrubs
-and trees develop with tolerable evenness on all sides, is an obvious
-truth. Equally obvious is the truth that, when growing together in
-a wood, and mutually interfered with on all sides, trees still show
-obscurely radial distributions of parts; though, under such conditions,
-they have tall taper stems with branches directed upwards&mdash;a difference
-of shape clearly due to the different incidence of forces. And almost
-equally obvious is the truth, that a tree of this same kind growing
-at the edge of the wood, has its outer branches well developed and
-its inner branches comparatively ill-developed. Fig. <a href="#i_142">197</a>, which
-inaccurately represents this difference, will serve to make it manifest
-that while one of the peripheral trees can be cut into something like
-two similar halves by a vertical plane directed towards the centre of
-the wood&mdash;a plane on each side of which the conditions are alike&mdash;it
-cannot be cut into similar halves by any other plane. A like divergence
-from an indefinitely-radial symmetry towards an indefinitely-bilateral
-symmetry, occurs in trees that have their conditions made bilateral
-by growing on inclined surfaces. Two of the common forms observable
-in such cases are given in Fig. <a href="#i_142">198</a>. Here there is divisibility into
-parts that are tolerably similar, by a vertical plane running directly
-down the hill; but not by any other plane. Then, further, there is the
-bilateralness, similar in general meaning though differently caused,
-often seen in trees exposed to strong prevailing winds. Almost<span class="pagenum" id="Page_143">[143]</span> every
-sea-coast has abundant examples of stunted trees which, like the one
-shown in Fig. <a href="#i_142">199</a>, have been made to deviate from their ordinary equal
-growth on all sides of a vertical axis, to a growth that is equal only
-on the opposite sides of a vertical plane directed towards the wind’s
-eye.</p>
-
-<p>From among vegetal aggregates of the third order, we have now only to
-add examples of the entirely asymmetrical form which accompanies an
-entirely irregular distribution of incident forces. Creeping plants
-furnish such examples. They show, both when climbing up vertical or
-inclined surfaces and when trailing on the ground, that their branches
-grow hither and thither as the balance of forces aids or opposes;
-and the general outline is without symmetry of any kind, because the
-environing influences have no kind of regularity in their arrangement.</p>
-
-
-<p class="p2">§ 220. Along with some unfamiliar facts, I have here set down facts
-which are so familiar as to seem scarcely worth noting. It is because
-these facts have become meaningless to perceptions deadened by infinite
-repetitions of them, that it is needful here to point out their
-meanings. Not alone for its intrinsic importance has the unlikeness
-between the attached ends and the free ends been traced among plants
-of all degrees of integration. Nor is it simply because of the
-significance they have in themselves, that instances have been given
-of those varieties of symmetry and asymmetry which the free ends of
-plants equally display: be they plants of the first, second, third,
-or any higher order. Neither has the only other purpose been that of
-showing how, in the radial symmetry of some vegetal aggregates and
-the single bilateral symmetry of others, there are traceable the same
-ultimate principles as in the spherical symmetry and triple bilateral
-symmetry of certain minute plants first described. But the main object
-has been to present, under their simplest aspects, those general laws
-of morphological differentiation which are fulfilled by the component
-parts of each plant.</p>
-
-<p><span class="pagenum" id="Page_144">[144]</span></p>
-
-<p>If organic form is determined by the distribution of forces, and the
-approach in every case towards an equilibrium of inner actions with
-outer actions; then this relation between forms and forces must hold
-alike in the organism as a whole in its proximate units, and in its
-units of lower orders. Formulas which express the shapes of entire
-plants in terms of surrounding conditions, must be formulas which also
-express the shapes of their several parts in terms of surrounding
-conditions. If, therefore, we find that a plant as a whole is radially
-symmetrical or bilaterally symmetrical or asymmetrical, according as
-the incident forces affect it equally on all sides of an axis, or
-affect it equally only on the opposite sides of one plane, or affect
-it equally in no two directions; then, we may expect that, in like
-manner, each member of a plant will display radial symmetry where
-environing influences are alike along many radii, bilateral symmetry
-where there is bilateralness of environing influences, and unsymmetry
-or asymmetry where there is partial or entire departure from a balance
-of surrounding actions.</p>
-
-<p>To show that this expectation is borne out by the facts, will be the
-object of the following four chapters. Let us begin with the largest
-parts into which plants are divisible; and proceed to the successively
-smaller parts.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_145">[145]</span></p>
-
-<h3>CHAPTER VIII.<br />
-<span class="subhed1">THE SHAPES OF BRANCHES.</span></h3></div>
-
-
-<p>§ 221. Aggregates of the first order supply a few examples of forms
-ramified in an approximately-regular manner, under conditions which
-subject their parts to approximately-regular distributions of forces.
-Some unicellular <i>Algæ</i>, becoming elaborately branched, assume
-very much the aspects of small trees; and show us in their branches
-analogous relations of forms to forces. <i>Bryopsis plumosa</i> may
-be instanced. Fig. <a href="#i_145a">200</a> represents the end of one of its lateral
-ramifications, above and beneath which come others of like characters.
-Here it will be seen that the attached and free ends differ; that the
-two sides are much alike; and that they are unlike the upper and under
-surfaces, which resemble one another. The more highly developed members
-of the same group of <i>Algæ</i>, the <i>Siphoneæ</i>, show a marked
-radial symmetry co-existing with very elaborate branching, <i>e.g.</i>,
-<i>Neomeris</i>, <i>Cymopolia</i>, and others.</p>
-
-  <div class="figcenter" id="i_145a">
-     <img
-    class="p2"
-    src="images/i_145a.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 200.</p>
-  </div>
-
-
-<p class="p2">§ 222. Fig. <a href="#i_145b">201</a> shows us how, in an aggregate of the second order,
-each proximate component is modified by its relations to the rest;
-just as we before saw a whole fungus of the same type modified<span class="pagenum" id="Page_146">[146]</span> by
-its relations to environing objects. If a branch of the fungus here
-figured, be compared with one of the fungi clustered together in Fig.
-195, or, still better, with one of the laterally-growing fungi shown
-in Fig. <a href="#i_138">196</a>, there will be perceived a kindred transition from radial
-to bilateral symmetry, occurring under kindred conditions. The portion
-of the pileus next to the side of attachment is undeveloped in this
-branched form as in the simpler form; and in the one case as in the
-other, the stem is modified towards the side of attachment. A division
-into similar halves, which, as shown in Fig. <a href="#i_138">196</a> <i>e</i>, might be
-made of the whole fungus by a vertical plane passing through the centre
-of the pileus and the axis of the supporting body, might here be made
-of the branch, by a vertical plane passing through the centre of its
-pileus and the axis of the main stem. Among aggregates of this order,
-the <i>Algæ</i> furnish cases of kindred nature. In the branches of
-<i>Lessonia</i>, Fig. <a href="#i_030">37</a>, may be observed a substantially-similar
-relationship. As their inner parts are less developed than their outer
-parts, while their two sides are developed in approximately equal
-degrees, they are rendered bilateral.</p>
-
-  <div class="figcenter" id="i_145b">
-     <img
-    class="p2"
-    src="images/i_145b.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 201.</p>
-  </div>
-
-
-<p class="p2">§ 223. These few cases introduce us to the more familiar but more
-complex cases which plants of the third degree of aggregation
-present. At <i>a</i>, <i>b</i>, <i>c</i>, Fig. <a href="#i_146">202</a>, are sketched
-three homologous parts of the same tree: <i>a</i> being the leading<span class="pagenum" id="Page_147">[147]</span>
-shoot; <i>b</i> a lateral branch near the top, and <i>c</i> a lateral
-branch lower down. There is here a double exemplification. While
-the branch <i>a</i>, as a whole, has its branchlets arranged with
-tolerable regularity all round, in correspondence with its equal
-exposure on all sides, each branchlet shows by its curve as much
-bilateral symmetry as its simple form permits. The branch <i>b</i>,
-dissimilarly circumstanced on the side next the main stem and on the
-side away from it, has an approximate bilateralness as a whole, while
-the bilateralness of its branchlets varies with their respective
-positions. And in the branch <i>c</i>, having its parts still more
-differently conditioned, these traits of structure are still more
-marked. Extremely strong contrasts of this kind occur in trees having
-very regular modes of growth. The uppermost branches of a Spruce-fir
-have radially-arranged branchlets: each of them, if growing vigorously,
-repeats the type of the leading shoot, as shown in Fig. <a href="#i_146">203</a>, <i>a</i>,
-<i>b</i>. But if we examine branches lower and lower down the tree, we
-find the vertically-growing branchlets bear a less and less ratio to
-the horizontally-growing ones; until, towards the bottom, the radial
-arrangement has wholly merged into the bilateral. Shaded and confined
-by the branches above them, these eldest branches develop their
-offshoots in those directions where there is most space and light:
-becoming finally quite flattened and fan-shaped, as shown at Fig. <a href="#i_146">203</a>,
-<i>c</i>. And on remembering that each of these eldest branches, when
-first it diverged from the main stem, was radial, we see not only that
-between the upper and lower branches does this contrast in structure
-hold, but also that each branch is transformed from the radial to
-the bilateral by the progressive change in its environment. Other
-forces besides those which aid or hinder growth, conspire to produce
-this two-sided character in lateral branches. The annexed Fig. <a href="#i_146">204</a>,
-sketched from an example of the <i>Pinus Coulterii</i> at Kew, shows
-very clearly how, by mere gravitation, the once radially-arranged
-branchlets may be so bent as to produce in the branch as a whole a<span class="pagenum" id="Page_148">[148]</span>
-decided bilateralness. A full-grown <i>Araucaria</i>, too, exhibits
-in its lower branches modifications similarly caused; and in each
-of such branches there may be remarked the further fact, that its
-upward-bending termination has a partially-modified radialness, at the
-same time that its drooping lateral branchlets give to the part nearer
-the trunk a completely bilateral character.</p>
-
-  <div class="figcenter" id="i_146">
-     <img
-    class="p2"
-    src="images/i_146.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 202–204.</p>
-  </div>
-
-<p class="p2">Now in these few instances, typical of countless instances which might
-be given, we see, as we saw in the case of the fungi, that the same
-thing is true of the parts in their relations to the whole and to one
-another, which is true of the whole in its relations to the environment
-at large. Entire trees become bilateral instead of radial, when exposed
-to forces that are equal only on opposite sides of one plane; and in
-their branches, parallel changes of form occur under parallel changes
-of conditions.</p>
-
-
-<p class="p2">§ 224. There remains to be said something respecting the distribution
-of leaves. How a branch carries its leaves constitutes one of
-its characters as a branch, and is to be considered apart from
-the characters of the leaves themselves. The principles hitherto
-illustrated we shall here find illustrated still further.</p>
-
-<p>The leading shoot and all the upper twigs of a fir-tree, have their
-pin-shaped leaves evenly distributed all round, or placed radially;<a id="FNanchor_32" href="#Footnote_32" class="fnanchor">[32]</a>
-but as we descend we find them beginning to assume a bilateral
-distribution; and on the lower, horizontally-growing branches, their
-distribution is quite bilateral.<a id="FNanchor_33" href="#Footnote_33" class="fnanchor">[33]</a> Between the Irish and English
-kinds of Yew, there is a contrast of like significance. The branches
-of the one, shooting up as they do almost vertically, are clothed with
-leaves<span class="pagenum" id="Page_149">[149]</span> all round; while those of the other, which spread laterally,
-bear their leaves on the two sides. In trees with better-developed
-leaves, the same principle is more or less manifest in proportion as
-the leaves are more or less enabled by their structures to maintain
-fixed positions. Where the foot-stalks are long and slender, and where,
-consequently, each leaf, according to its weight, the flexibility and
-twist of its foot-stalk, and the direction of the branch it grows from,
-falls into some indefinite attitude, the relations are obscured. But
-where the foot-stalks are stiff, as in the Laurel, it will be found, as
-before, that from the topmost and upward growing branches the leaves
-diverge on all sides; while the undermost branches, growing out from
-the shade of those above, have their leaves so turned as to bring them
-into rows horizontally spread out on the two sides of each branch.</p>
-
-  <div class="figcenter" id="i_149">
-     <img
-    class="p2"
-    src="images/i_149.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 205.</p>
-  </div>
-
-<p class="p2">A kindred truth, having like implications, comes into view when we
-observe the relative sizes of leaves on the same branch, where their
-sizes differ. Fig. <a href="#i_149">205</a> represents a branch of a Horse-chestnut, taken
-from the lowermost fringe of the tree, where the light has been to
-a great extent intercepted from all but the most protruded parts.
-Beyond the fact that the leaves become by appropriate growths of their
-foot-stalks bilaterally distributed on this drooping branch, instead of
-being distributed symmetrically all round, as on one of the ascending
-shoots, we have here to note the fact that there is unequal development
-on the upper and lower sides. Each of the compound leaves acquires a
-foot-stalk and leaflets that are large in proportion to the supply
-of light; and hence, as we descend towards the bottom of the tree,
-the clusters of leaves display increasing contrasts. How marked these
-contrasts become will be seen on comparing <i>a</i> and <i>b</i>, which
-form one pair of leaves that<span class="pagenum" id="Page_150">[150]</span> are normally equal, or <i>c</i> and
-<i>d</i>, which form another pair normally equal.</p>
-
-<p>Let us not omit to note, while we have this case before us, the proof
-it affords that these differences of development are in a considerable
-degree determined by the different conditions of the parts after they
-have been unfolded. Though those inequalities of dimensions whence
-the differentiations of form result, may be in many cases largely due
-to the inequalities in the circumstances of the parts while in the
-bud (which are, however, representative of inequalities in ancestral
-circumstances); yet these are clearly not the sole causes of the
-unlikenesses which eventually arise. This bilateralness resulting from
-the unequal sizes of the leaves, must be considered as due to the
-differential actions that come into play after the leaves have assumed
-their typical structures.</p>
-
-
-<p class="p2">§ 225. How, in the arrangement of their twigs and leaves, branches tend
-to lapse from forms that are approximately symmetrical to forms that
-are quite asymmetrical, need not be demonstrated: it is sufficiently
-conspicuous. But it may be well to point out how the tendency to
-do this further enforces our argument. The comparatively regular
-budding out of secondary axes and tertiary axes, does not usually
-produce an aggregate which maintains its regularity, for the simple
-reason that many of the axes abort. Terminal buds are some of them
-destroyed by birds; others are burrowed into by insects; others are
-nipped by frost; others are broken off or injured during gales of wind.
-The environment of each branch and its branchlets is thus ever being
-varied on all sides: here, space being left vacant by the death of some
-shoot that would ordinarily have occupied it; and there, space being
-trenched on by the lateral growth of some adjacent branch that has had
-its main axis broken. Hence the asymmetry, or heterogeneity of form,
-assumed by the branch, is caused by the asymmetrical distribution<span class="pagenum" id="Page_151">[151]</span> of
-incident forces&mdash;a result and a cause which go on ever complicating.</p>
-
-
-<p class="p2">§ 226. One conspicuous trait in the shapes of branches has still to be
-named. Their proximal or attached ends differ from their distal or free
-ends, in the same way that the lower ends of trees differ from their
-upper ends. This fact, like the fact to which it is here paralleled,
-has had its significance obscured by its extreme familiarity. But
-it shows in a striking way how the most differently conditioned
-parts become the most strongly contrasted in their structures. A
-phænogamic axis is made up of homologous segments, marked off from
-one another by the nodes; and a compound branch consists of groups of
-such segments. The earliest-formed segments, alike of the tree and of
-each branch, serve as mechanical supports and channels for sap to the
-successive generations of segments that grow out of them; and become
-more and more shaded by their progeny as these increase. Hence the
-progressively-increasing contrasts which, while mainly due to the
-unlikenesses of bulk accompanying differences of age, are in part due
-to the unlikenesses of structure which differences of relation to the
-environment have caused.</p>
-
-
-<p class="p2">§ 227. Thus, then, it is with the proximate parts of plants as it is
-with plants as wholes. The radial symmetry, the bilateral symmetry, and
-the asymmetry, which branches display in different trees, in different
-parts of the same tree, and at different stages of their own growths,
-prove to be all consequent on the ways in which they stand towards the
-entire plexus of surrounding actions. The principle that the growths
-are unequal in proportion as the relations of parts to the environment
-are unequal, serves to explain all the leading traits of structure.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_152">[152]</span></p>
-
-<h3>CHAPTER IX.<br />
-<span class="subhed1">THE SHAPES OF LEAVES.</span></h3></div>
-
-
-<p>§ 228. Next in the descending order of composition come compound
-leaves. The relative sizes and distributions of their leaflets, as
-affecting their forms as wholes, have to be considered in their
-relations to conditions. Figs. <a href="#i_153">206, 207</a>, represent leaves of the common
-<i>Oxalis</i> and of the <i>Marsilea</i>, in which radial symmetry
-is as completely displayed as the small number of leaflets permits.
-This equal development of the leaflets on all sides, occurs where
-the foot-stalks, growing up vertically from creeping or underground
-stems, are so long that the leaves either do not interfere with one
-another or do it in an inconstant way: the leaflets are not differently
-conditioned on different sides, as they are where the foot-stalks grow
-out in the ordinary manner. How unlikeness of position influences the
-leaflets is clearly shown in a Clover-leaf, Fig. <a href="#i_153">208</a>, which deviates
-from the Oxalis-leaf but slightly towards bilateralness, as it deviates
-from it but slightly in the attitude of its petiole; which is a little
-inclined away from the others borne by the same procumbent axis. A
-familiar example of an almost radial symmetry along with almost equal
-relations to surrounding conditions, occurs in the root-leaves of
-the Lupin, Fig. <a href="#i_153">209</a> <i>b</i>. Here though we have lateral divergence
-from a vertical axis, yet the long foot-stalks preserve nearly erect
-positions, and carry their leaves to such distances from the axis,
-that the development of the leaflets on the side next it is not much<span class="pagenum" id="Page_153">[153]</span>
-hindered. Still the interference of the leaves with one another is,
-on the average, somewhat greater on the proximal side than on the
-distal side; and hence the interior leaflets are rather less than the
-exterior leaflets. In further proof of which influence, let it be added
-that, as shown in the figure, at <i>a</i>, the leaves growing out of
-the flowering stem deviate towards the two-sided form more decidedly.
-Two-sidedness is much greater where there is a greater relative
-proximity of the inner leaflets to the axis, or where the foot-stalk
-approaches towards a horizontal position. The Horse-chestnut, Fig. <a href="#i_149">205</a>,
-already instanced as showing how the arrangements and sizes of leaflets
-are determined by the incidence of forces, serves also to show how the
-incidence of forces determines the relative sizes and arrangements of
-leaflets. Fig. <a href="#i_153">210</a>, which shows a leaf of the <i>Bombax</i>, further
-illustrates this relation of structure to conditions.</p>
-
-  <div class="figcenter" id="i_153">
-     <img
-    class="p2"
-    src="images/i_153.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 206–210.</p>
-  </div>
-
-  <div class="figcenter" id="i_154">
-     <img
-    class="p2"
-    src="images/i_154.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 211, 212.</p>
-  </div>
-
-<p class="p2">Compound leaves that are completely bilateral, present us with
-modifications of form exemplifying the same general truth in another
-way. In them the proximal and distal parts have none of that
-resemblance which we see in those intermediate forms just described.
-The portion next the axis and the portion furthest from the axis
-are entirely different; and the only likeness is between the wings
-or leaflets on opposite sides of the main foot-stalk or mid-rib.
-On turning back to Fig. <a href="#i_040">65</a>, it will be seen that the compound leaf
-there<span class="pagenum" id="Page_154">[154]</span> drawn to exemplify another truth, serves also to exemplify this
-truth: the homologous parts <i>a</i>, <i>b</i>, <i>c</i>, <i>d</i>,
-while they are unlike one another, are, in their main proportions,
-severally like the parts with which they are paired. And here let us
-not overlook a characteristic which is less conspicuous but not less
-significant. Each of the lateral wings has winglets that are larger
-on the one side than on the other; and in each case the two sides
-are dissimilarly conditioned. Even in the several components of each
-wing may be traced a like divergence from symmetry, along with a like
-inequality in the relations to the rest: the proximal half of each
-leaflet is habitually larger than the distal half. In the leaves of
-the Bramble, previously figured, kindred facts are presented. How far
-such differences of development are due to the positions of the parts
-in the bud; how far the respective spaces available for the parts
-when unfolded affect them; and how far the parts are rendered unlike
-by unlikenesses in their relations to light; it is difficult to say.
-Probably these several factors operate in all varieties of proportion.
-That the habitual shading of some parts by others largely aids in
-causing these divergences from symmetry, is very instructively shown
-by the compound leaves of the Cow-parsnip. Fig. <a href="#i_154">211</a> represents one
-of these. While the leaf as a whole is bilaterally symmetrical, each
-of the wings has an unsymmetrical bilateralness: the side next the
-axis being larger than the remoter side. How does this happen? Fig.
-<a href="#i_154">212</a>, which is a diagrammatic section down the mid-rib of the leaf,
-showing its inclined attitude and the positions of the<span class="pagenum" id="Page_155">[155]</span> wings <i>a</i>,
-<i>b</i>, <i>c</i>, will make the cause clear. As the wings overlap,
-like the bars of a Venetian blind, each intercepts some light from the
-one below it; and the one below it thus suffers more on its distal
-side than on its proximal side. Hence the smaller development of the
-distal side. That this is the cause is further shown by the proportion
-that is maintained between the degree of obscuration and the degree of
-non-development; for this unlikeness is greater between the two sides
-<i>a</i> and <i>a´</i>, than between <i>b</i> and <i>b´</i> or <i>c</i>
-and <i>c´</i>, at the same time that the interference is greater in
-the lower wings than in the upper. Of course in this case and in the
-kindred cases hereafter similarly interpreted, it is not meant that
-this differentiation is consequent solely, or even chiefly, on the
-differential actions experienced by the individual plant. Though there
-is good reason to believe that the rate of growth in each part of each
-leaf is affected by the incidence of light, yet contrasts so marked and
-so systematic as these are not explicable without taking into account
-the inheritance of modifications either functionally caused or caused
-by spontaneous variation. Clearly, the tendency will be towards the
-preservation of a plant which distributes its chlorophyll in the most
-advantageous way; and hence there will always be a gravitation towards
-a form in which shaded parts of leaves are undeveloped.</p>
-
-
-<p class="p2">§ 229. From compound leaves to simple ones, we find transitions in
-leaves of which the divisions are partial instead of total; and in
-these we see, with equal clearness, the relations between forms and
-positions that have been traced thus far. Fig. <a href="#i_156a">213</a> is the leaf of a
-Winter-aconite in which, round a vertical petiole, there is a radial
-distribution of half-separated leaflets. The <i>Cecropia</i>-leaf,
-Fig. <a href="#i_156a">214</a>, shows us a two-sided development of the parts beginning to
-modify, but not obliterating, the all-sided arrangement; and this mixed
-symmetry occurs under conditions that are intermediate. A more marked
-degree of the same relation is presented<span class="pagenum" id="Page_156">[156]</span> in the leaf of the Lady’s
-Mantle, Fig. <a href="#i_156a">215</a>. And then in the Sycamore and the Vine, we have a
-cleft type of leaf in which a decided bilateralness of form co-exists
-with a decided bilateralness of conditions.</p>
-
-  <div class="figcenter" id="i_156a">
-     <img
-    class="p2"
-    src="images/i_156a.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 213–215.</p>
-  </div>
-
-  <div class="figcenter" id="i_156b">
-     <img
-    class="p2"
-    src="images/i_156b.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 216–218.</p>
-  </div>
-
-<p class="p2">The quite simple leaves to which we now descend, exhibit, very
-distinctly, a parallel series of facts. Where they grow up on long and
-completely-independent foot-stalks, without definite subordination
-to some central vertical axis, the leaves of water-plants are
-symmetrically peltate. Of this the sacred Indian-bean, Fig. <a href="#i_156b">216</a>,
-furnishes an example. Here there is only a trace of bilateralness in
-the venation of the leaf, corresponding to the very small difference
-of the conditions on the proximal and distal sides. In the <i>Victoria
-regia</i>, Fig. <a href="#i_156b">217</a>, the foot-stalks, though radiating almost
-horizontally from a centre, are so long as to keep the leaves quite
-remote from one another; and in it each leaf is almost symmetrically
-peltate, with a bilateralness indicated only by a seam over the
-line of the foot-stalk. The leaves of the <i>Nymphæa</i>, Fig. <a href="#i_156b">218</a>,
-more closely clustered, and having less room transversely than
-longitudinally, exhibit a marked advance to the two-sided form; not
-only in the excess of the length over the breadth, but in the existence
-of a cleft,<span class="pagenum" id="Page_157">[157]</span> where in the <i>Victoria regia</i> there is merely a seam.
-Among land-plants similar forms are found under analogous conditions.
-The common <i>Hydrocotyle</i>, Fig. <a href="#i_157">219</a>, which sends up direct from its
-roots a few almost upright leaf-stalks, has these surmounted by peltate
-leaves; which leaves, however, diverge slightly from radial symmetry in
-correspondence with the slight contrast of circumstances which their
-grouping involves. Another case is supplied by the Nasturtium, Fig.
-<a href="#i_157">220</a>, which combines the characters&mdash;a creeping stem, long leaf-stalks
-growing up at right angles to it, and unsymmetrically peltate leaves,
-of which the least dimension is, on the average, towards the stem.
-But perhaps the most striking illustration is that furnished by the
-<i>Cotyledon umbilicus</i>, Fig. <a href="#i_157">221</a>, in which different kinds of
-symmetry occur in the leaves of the same plant, along with differences
-in their relations to conditions. The root-leaves, <i>a</i>, growing up
-on vertical petioles before the flower-stalk makes its appearance, are
-symmetrically peltate; while the leaves which subsequently grow out of
-the flower-stalk, <i>b</i>, are at the bottom transitionally bilateral,
-and higher up completely bilateral.</p>
-
-  <div class="figcenter" id="i_157">
-     <img
-    class="p2"
-    src="images/i_157.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 219–221.</p>
-  </div>
-
-<p class="p2">That the bilateral form of leaf is the ordinary form, corresponds with
-the fact that, ordinarily, the circumstances of the leaf are different
-in the direction of the plant’s axis from what they are in the opposite
-direction, while<span class="pagenum" id="Page_158">[158]</span> transversely the circumstances are alike. It is
-needless to give diagrams to illustrate this extremely familiar truth.
-Whether they are broad or long, oval or heart-shaped, pointed or
-obtuse, the leaves of most trees and plants will be remembered by all
-as having the ends by which they are attached unlike the free ends,
-while the two sides are alike. And it will also be remembered that
-these equalities and inequalities of development correspond with the
-equalities and inequalities in the incidence of forces.</p>
-
-
-<p class="p2">§ 230. A confirmation that is interesting and important, is furnished
-by the cases in which leaves present unsymmetrical forms in positions
-where their parts are unsymmetrically related to the environment. A
-considerable deviation from bilateral symmetry may be seen in a leaf
-which habitually so carries itself, that the half on the one side of
-the mid-rib is more shaded than the other half. The drooping branches
-of the Lime, delineated in Fig. <a href="#i_158">222</a>, show us leaves so arranged and
-so modified. On examining their attitudes and their relations one to
-another, it will be found that each leaf is so inclined that the half
-of it next to the shoot grows over the shoot and gets plenty of light;
-while the other half so hangs down that it comes a good deal into the
-shade of the preceding leaf. The result is that having leaves which
-fall into these positions, the species profits by a large development
-of the exposed halves; and by survival of the fittest, acting along
-with the direct effect of extra exposure, this modification becomes
-established. How unquestionable is the connexion between the relative
-positions of the halves and their relative developments, will be
-admitted on observing a<span class="pagenum" id="Page_159">[159]</span> converse case. Fig. <a href="#i_158">223</a> represents a shoot of
-<i>Strobilanthes glomeratus</i>. Here the leaves are so set on the stem
-that the inner half of each leaf is shaded by the subsequently-formed
-leaf, while its outer half is not thus shaded; and here we find
-the inner half less developed than the outer half. But the most
-conclusive evidence of this relation between unsymmetrical form and
-unsymmetrical distribution of surrounding forces, is supplied by the
-genus <i>Begonia</i>; for in it we have a manifest proportion between
-the degree of the alleged effect and the degree of the alleged cause.
-These plants produce their leaves in pairs, in such ways that the
-connate leaves interfere with one another, much or little according
-as the foot-stalks are short or long; and the result is a correlative
-divergence from symmetry. In <i>Begonia nelumbiifolia</i>, which has
-petioles so long that the connate leaves are not kept close together,
-there is but little deviation from a bilaterally-peltate form; whereas,
-accompanying the comparatively marked and constant proximity in <i>B.
-pruinata</i>, Fig. <a href="#i_158">224</a>, we see a more decidedly unsymmetrical shape;
-and in <i>B. mahringii</i>, Fig. <a href="#i_158">225</a>, the modification thus caused is
-pushed so far as to destroy the peltate structure.<a id="FNanchor_34" href="#Footnote_34" class="fnanchor">[34]</a></p>
-
-  <div class="figcenter" id="i_158">
-     <img
-    class="p2"
-    src="images/i_158.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 222–225.</p>
-  </div>
-
-
-<p class="p2">§ 231. Again, then, we are taught the same truth. Here, as before,
-we see that homologous units of any order become<span class="pagenum" id="Page_160">[160]</span> differentiated in
-proportion as their relations to incident forces become different. And
-here, as before, we see that in each unit, considered by itself, the
-differences of dimension are greatest in those directions in which the
-parts are most differently conditioned; while there are no differences
-between the dimensions of the parts that are not differently
-conditioned.<a id="FNanchor_35" href="#Footnote_35" class="fnanchor">[35]</a></p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_161">[161]</span></p>
-
-<h3>CHAPTER X.<br />
-<span class="subhed1">THE SHAPES OF FLOWERS.</span></h3></div>
-
-
-<p class="p2">§ 232. Following an order like that of preceding chapters, let us first
-note a few typical facts respecting the forms of clusters of flowers,
-apart from the forms of the flowers themselves. Two kindred kinds
-of <i>Leguminosæ</i> serve to show how the members of clusters are
-distributed in an all-sided manner or in a two-sided manner, according
-as the circumstances are alike on all sides or alike on only two sides.
-In <i>Hippocrepis</i>, represented in Fig. <a href="#i_161">226</a>, the flowers growing at
-the end of a vertical stem, are arranged round it in radial symmetry.
-Contrariwise in <i>Melilotus</i>, Fig. <a href="#i_161">227</a>, where the axillary stem
-bearing the flowers is so placed in relation to the main stem, that its
-outer and inner faces are differently conditioned, the flowers are all
-on the outer face: the cluster is bilaterally symmetrical, since it may
-be cut into approximately equal and similar groups by a vertical plane
-passing through the main axis.</p>
-
-  <div class="figcenter" id="i_161">
-     <img
-    class="p2"
-    src="images/i_161.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 226, 227.</p>
-  </div>
-
-<p class="p2">Plants of this same tribe furnish clusters of intermediate characters
-having intermediate conditions. Among these, as among the clusters
-which other types present, may be<span class="pagenum" id="Page_162">[162]</span> found some in which conformity
-to the general law is not obvious. The discussion of these apparent
-anomalies would carry us too much out of our course. A clue to the
-explanation of them will, I believe, be found in the explanation
-presently to be given of certain kindred anomalies in the forms of
-individual flowers.</p>
-
-
-<p class="p2" id="sect233">§ 233. The radially-symmetrical form is common to all individual
-flowers that have vertical axes. In plants which are practically if
-not literally uniaxial, and bear their flowers at the ends of upright
-stalks, so that the faces open horizontally, the petals are disposed
-in an all-sided way. Crocuses, Tulips, and Poppies are familiar
-examples of this structure occurring under these conditions. A
-Ranunculus flower, Fig. <a href="#i_162">228</a>, will serve as a typical one. Similarly,
-flowers which have peduncles flexible enough to let them hang directly
-downwards, and are not laterally incommoded, are also radial; as in
-the <i>Fuchsia</i>, Fig. <a href="#i_162">229</a>, as in <i>Cyclamen</i>, <i>Hyacinth</i>,
-&amp;c. These relations of form to position are, I believe, uniform.
-Though some flowers carried at the ends of upright or downright stems
-have oblique shapes, it is only when they have inclined axes or are
-not equally conditioned all round. No solitary flower having an axis
-habitually vertical, presents a bilateral form. This is as we should
-expect; since flowers which open out their faces horizontally, whether
-facing upwards or downwards, are, on the average, similarly affected on
-all sides.</p>
-
-  <div class="figcenter" id="i_162">
-     <img
-    class="p2"
-    src="images/i_162.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 228, 229.</p>
-  </div>
-
-  <div class="figcenter" id="i_163">
-     <img
-    class="p2"
-    src="images/i_163.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 230, 231.</p>
-  </div>
-
-<p class="p2">At first it seems that flowers thus placed should alone be radial;
-but further consideration discloses conditions under which this
-type of symmetry may exist in flowers otherwise placed. Remembering
-that the radial form is the primitive form&mdash;that, morphologically
-speaking, it results from the contraction into a whorl, of parts that
-are originally arranged in the same spiral succession as the leaves;
-we must expect<span class="pagenum" id="Page_163">[163]</span> it to continue wherever there are no forces tending
-to change it. What now must be the forces tending to change it? They
-must be forces which do not simply affect differently the different
-parts of an individual flower. They must be forces which affect in
-like contrasted ways the homologous parts of other individual flowers,
-both on the same plant and on surrounding plants of the same species.
-A permanent modification can be expected only in cases where, by
-inheritance, the effects of the modifying causes accumulate. That
-they may accumulate the flowers must keep themselves so related to
-the environment, that the homologous parts may, generation after
-generation, be subjected to like differentiating forces. Hence, among
-a plant’s flowers which maintain no uniformity in the relations of
-their parts to surrounding influences, the radial form will continue.
-Let us glance at the several causes which entail this variability.
-When flowers are borne on many branches, which have all inclinations
-from the vertical to the horizontal&mdash;as are the flowers of the Apple,
-the Plum, the Hawthorn&mdash;they are placed in countless different
-attitudes. Consequently, any spontaneous variation in shape which
-might be advantageous were the attitude constant, is not likely to
-be advantageous; and any functionally-produced modification in one
-flower, is likely to be neutralized in offspring by some opposite
-functionally-produced modification in another flower. It is quite
-comprehensible, therefore, that irregularly-branched plants should
-thus preserve their laterally-borne flowers from undergoing permanent
-deviations from their primitive radial symmetry. Fig. <a href="#i_163">230</a>, representing
-a blossoming twig of the Blackthorn, illustrates this. Again, upright
-panicles, such as those of the Saxifrage exemplified in Fig. <a href="#i_163">231</a>,
-and irregular terminal groups of flowers otherwise<span class="pagenum" id="Page_164">[164]</span> named, furnish
-conditions under which there is similarly an absence of determinate
-relations between the parts of the flowers and the incident forces;
-and hence an absence of bilateralness. This inconstancy of relative
-position is produced in various other ways&mdash;by extreme flexibility
-of the stems, as in the Blue-bell; by the tendency of the peduncles
-to curl to a greater or less extent in diverse directions, as in
-<i>Pyrola</i>; by special twistings of the peduncles, differing in
-degree in different individuals, as in <i>Convolvulus</i>; by unusual
-laxity of the petals, as in <i>Lythrum</i>. Elsewhere the like
-general result arises from a progressive change of attitude, as in
-<i>Myosotis</i>, the stem of which as it unfolds causes each flower to
-undergo a transition from an upward position of the mouth to a lateral
-position; or as in most <i>Cruciferæ</i>, where the like effect follows
-from an altered direction of the peduncle.</p>
-
-<p>There are, however, certain seemingly-anomalous cases where radial
-symmetry is maintained by laterally-placed flowers, which keep
-their parts in relative positions that are tolerably constant. The
-explanation of these exceptions is not manifest. It is only when
-we take into account certain incident actions liable to be left
-unremembered, that we find a probable solution. It will be most
-convenient to postpone the consideration of these cases until we have
-reached the general rule to which they are exceptions.</p>
-
-
-<p class="p2" id="sect234">§ 234. Transitions varying in degree from the radial towards the
-bilateral, are common in flowers that are borne at the ends of branches
-or axes which are inclined in tolerably constant ways. We may see
-this in sundry garden flowers such as <i>Petunia</i>, or such as
-<i>Isoloma</i> and <i>Achimenes</i>, shown in Figs. <a href="#i_164">232 and 233</a>. If
-these plants be examined, it will be perceived that the mode of growth
-makes the flower unfold in a partially one-sided<span class="pagenum" id="Page_165">[165]</span> position; that its
-parts of attachment have rigidity sufficient to prevent this attitude
-from being very much interfered with; and that though the individual
-flowers vary somewhat in their attitudes, they do not vary to the
-extent of neutralizing the differentiating conditions&mdash;there remains
-an average divergence from a horizontal unfolding of the flower, to
-account for its divergence from radial symmetry.</p>
-
-  <div class="figcenter" id="i_164">
-     <img
-    class="p2"
-    src="images/i_164.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 232, 233.</p>
-  </div>
-
-<p class="p2">We pass insensibly from forms like these, to forms having bilateral
-symmetry strongly pronounced. Some such forms occur among flowers
-that grow at the ends of upright stems; as in <i>Pinguicula</i>,
-and in the Violet tribe. But this happens only where, in successive
-generations, the flower unfolds its parts sideways in constant
-relative positions. And in the immense majority of flowers
-having well-marked two-sided forms, the habitual exposure of
-the different parts to different sets of forces, is effectually
-secured by the mode of placing. As illustrations, I may name the
-genera&mdash;<i>Orchis</i>, <i>Utricularia</i>, <i>Salvia</i>, <i>Salix</i>,
-<i>Delphinium</i>, <i>Mentha</i>, <i>Teucrium</i>, <i>Ajuga</i>,
-<i>Ballota</i>, <i>Galeopsis</i>, <i>Lamium</i>, <i>Stachys</i>,
-<i>Nepeta</i>, <i>Marrubium</i>, <i>Calamintha</i>, <i>Melittis</i>,
-<i>Prunella</i>, <i>Scutellaria</i>, <i>Bartsia</i>, <i>Euphrasia</i>,
-<i>Rhinanthus</i>, <i>Melampyrum</i>, <i>Pedicularis</i>,
-<i>Linaria</i>, <i>Digitalis</i>, <i>Orobanche</i>, <i>Fumaria</i>,
-<i>&amp;c.</i>; to which may be added all the Grasses and all the
-<i>Papilionaceæ</i>. In most of these cases the flowers, being sessile
-on the sides of upright stems, are kept in quite fixed attitudes; and
-in the other cases the peduncles are very short, or else stiff enough
-to secure general uniformity in the positions. A few of the more marked
-types are shown in Figs. <a href="#i_165">234 to 241</a>.</p>
-
-  <div class="figcenter" id="i_165">
-     <img
-    class="p2"
-    src="images/i_165.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 234–241.</p>
-  </div>
-
-  <div class="figcenter" id="i_166a">
-     <img
-    class="p2"
-    src="images/i_166a.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 242–246.</p>
-  </div>
-
-  <div class="figcenter" id="i_166b">
-     <img
-    class="p2"
-    src="images/i_166b.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 247, 248.</p>
-  </div>
-
-<p class="p2">Very instructive evidences here meet us. Sometimes within the limits
-of one genus we find radial flowers, bilateral flowers, and flowers of
-intermediate characters. The genus</p>
-
-<p><span class="pagenum" id="Page_166">[166]</span></p>
-
-<p><i>Begonia</i> may be instanced. In <i>B. rigida</i> the flowers,
-various in their attitudes, are in their more conspicuous characters
-radial: though there is a certain bilateralness in the calyx, the five
-petals are symmetrically disposed all round. <i>B. Wageneriana</i>
-furnishes two forms of flowers. On the same individual plant may be
-found radial flowers like Fig. <a href="#i_166a">242</a>, and others, like Fig. <a href="#i_166a">243</a>, which
-are merging into the bilateral. More decided is the bilateralness in
-<i>B. albo-coccinea</i>, Fig. <a href="#i_166a">244</a>; and still more in <i>B. nitida</i>,
-Fig. <a href="#i_166a">245</a>. While in <i>B. heracleifolia</i>, Fig. <a href="#i_166a">246</a>, the change
-reaches its extreme by the disappearance of the lateral petals.
-On examining the modes of growth in these several species, they
-will be seen to explain these changes in the manner alleged. Even
-more conclusive are the nearly-allied transformations occurring in
-artificially-produced varieties of the same species. <i>Gloxinia</i>
-may be named in illustration. In Fig. <a href="#i_166b">247</a> is represented one of the
-ordinary forms, which shows us bilateralness of shape along with a
-mode of growth that renders the conditions alike on the two sides
-while different above and below. But in <i>G. erecta</i>, Fig. <a href="#i_166b">248</a>,
-we have the flower assuming an upright attitude, and at the same time
-assuming the radial type. This is not to be interpreted as a production
-of radial symmetry out of bilateral symmetry, under the action of the
-appropriate conditions. It is rather to be taken as a case of what is
-termed “peloria”&mdash;a reversion to the primitive radial type, from which
-the bilateral modification had been derived. The significant inference
-to be drawn from it is,<span class="pagenum" id="Page_167">[167]</span> that this primitive radial type had an upright
-attitude; and that the derivation of a bilateral type from it, occurred
-along with the assumption of an inclined attitude.</p>
-
-<p>We come now to a group of cases above referred to, in which radial
-symmetry continues to co-exist with that constant lateral attitude
-ordinarily accompanied by the two-sided form. Two examples will
-suffice: one a very large flower, the Hollyhock, and the other a very
-small flower, the Agrimony. Why does the radial form here remain
-unchanged? and how does its continuance consist with the alleged
-general law?</p>
-
-<p>Until quite recently I have been unable to find any probable answers
-to these questions. When the difficulty first presented itself, I
-could think of no other possible cause for the anomaly, than that the
-parts of the Hollyhock-flower, unfolding spirally as they do, might
-have different degrees of spiral twist in different flowers, and might
-thus not be unfolded in sufficiently-constant positions. But this
-seemed a questionable interpretation; and one which did not obviously
-apply to the case of the Agrimony. It was only on inquiring what are
-the special causes of modifications in the forms of flowers, that a
-more feasible explanation suggested itself; and this would probably
-never have suggested itself, had not Mr. Darwin’s investigations into
-the fertilization of Orchids led me to take into account an unnoticed
-agency.</p>
-
-  <div class="figcenter" id="i_168">
-     <img
-    class="p2"
-    src="images/i_168.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 249.</p>
-  </div>
-
-<p class="p2">The actions which affect the forms of leaves, affect much less
-decidedly the forms of flowers; and the forms of flowers are influenced
-by actions which do not influence the forms of leaves. Partly through
-the direct action of incident forces and partly through the indirect
-action of natural selection, leaves get their parts distributed in
-ways that most facilitate their assimilative functions, under the
-circumstances in which they are placed; and their several types
-of symmetry are thus explicable. But in flowers, the petals and
-fructifying organs of which do not contain chlorophyll, the tendency
-to grow most where the supply of light is greatest, is less decided,
-if<span class="pagenum" id="Page_168">[168]</span> not absent; and a shape otherwise determined is hence less liable
-to alter in consequence of altered relations to sun and air. Gravity,
-too, must be comparatively ineffective in causing modifications: the
-smaller sizes of the parts, as well as their modes of attachment,
-giving them greater relative rigidity. Not, indeed, that these
-incident forces of the inorganic world are here quite inoperative.
-Fig. <a href="#i_168">249</a>, representing a species of <i>Campanula</i>, shows that
-the developments of individual flowers are somewhat modified by the
-relations of their parts to general conditions. But the fact to be
-observed is, that the extreme transformations which flowers undergo are
-not likely to be thus caused: some further cause must be sought. And
-if we bear in mind the functions of flowers, we shall find in their
-adaptations to these functions, under conditions that are extremely
-varied, an adequate cause for the different types of symmetry, as
-well as for the exceptions to them. Flowers are parts in which
-fertilization is effected; and the active agents of this fertilization
-are insects&mdash;bees, moths, butterflies, &amp;c. Mr. Darwin has shown in
-many cases, that the forms and positions of the essential organs of
-fructification, are such as to facilitate the actions of insects in
-transferring pollen from the anthers of one flower to the pistil of
-another&mdash;an arrangement produced by natural selection. And here we
-shall find reason for concluding, that the forms and positions of
-those subsidiary parts which give their shapes to flowers, similarly
-arise by the survival of individuals which have the subsidiary parts
-so adjusted as to aid this fertilizing process&mdash;the deviations from
-radial symmetry being among such adjustments. The reasoning is as
-follows. So long as the axis of a flower is vertical and the conditions
-are similar all round, a bee or butterfly alighting on it, will be as
-likely to come from one side as from another; and hence, hindrance
-rather than facilitation would<span class="pagenum" id="Page_169">[169]</span> result if the several sides of the
-flower did not afford it equally free access. In like manner, flowers
-which are distributed over a plant in such ways that their discs open
-out on planes of all directions and inclinations, will have no tendency
-to lose their radial symmetry; since, on the average, no part of the
-periphery is differently related to insect-agency from any other part.
-But flowers so fixed as to open out sideways in tolerably-constant
-attitudes, have their petals differently related to insect-agency. A
-bee or butterfly coming to a laterally-growing flower, does not settle
-on it in one way as readily as in another; but almost of necessity
-settles with the axis of its body inclined upwards towards the stem of
-the plant. Hence the side-petals of a flower so fixed, habitually stand
-to the alighting insect in relations different from those in which the
-upper and lower petals stand; and the upper and lower petals differ
-from one another in their relations to it. If, then, there so arises
-an habitual attitude of the insect towards the petals, there is likely
-to be some arrangement of the petals that will be most convenient to
-the insect&mdash;will most facilitate its entrance into the flower. Thus
-we see in many cases, that a long undermost petal or lip, by enabling
-the insect to settle in such way as to bring its head opposite to the
-opening of the tube, aids its fertilizing agency. But whatever be the
-special modifications of the corolla which facilitate the actions of
-the particular insects concerned, all of them will conduce to bilateral
-symmetry; since they will be alike for the two sides but unlike for
-the top and bottom. And now we are prepared for understanding the
-exceptions. Flowers growing sideways can become thus adapted by
-survival of the fittest, only if they are of such sizes and structures
-that insect-agency can affect them in the way described. But in the
-plants named above, this condition is not fulfilled. A Hollyhock-flower
-is so open, as well as so large, that its petals are not in any
-appreciable degree differently related to the insects which visit
-it. On the other hand, the flower of the<span class="pagenum" id="Page_170">[170]</span> Agrimony is so small, that
-unless visited by insects of a corresponding size which settle as bees
-and butterflies settle, its parts will not be affected in the alleged
-manner. That all anomalies of this kind can at once be satisfactorily
-explained, is scarcely to be expected: the circumstances of each case
-have to be studied. But it seems not improbable that they are due to
-causes of the kind indicated.<a id="FNanchor_36" href="#Footnote_36" class="fnanchor">[36]</a></p>
-
-
-<p class="p2">§ 235. We have already glanced at clusters of flowers for the purpose
-of considering their shapes as clusters. We must now return to them to
-observe the modifications undergone by their component flowers. Among
-these occur illustrations of great significance.</p>
-
-<p>An example of transition from the radial to the bilateral form in
-clustered flowers of the same species, is furnished by the cultivated
-<i>Geraniums</i>, called by florists <i>Pelargoniums</i>. Some of
-these, bearing somewhat small terminal clusters of flowers, which
-are closely packed together with their faces almost upwards, have
-radially-symmetrical flowers. But among other varieties having terminal
-clusters of which the members are mutually thrust on one side by
-crowding, the flowers depart very considerably from the radial shape<span class="pagenum" id="Page_171">[171]</span>
-towards the bilateral shape. A like result occurs under like conditions
-in Rhododendrons and Azaleas. The <i>Verbena</i>, too, furnishes
-an illustration of radial flowers rendered slightly two-sided by
-the slight two-sidedness of their relations to other flowers in the
-cluster. And among the <i>Cruciferæ</i> a kindred case occurs in the
-cultivated Candytuft.</p>
-
-<p>Evidence of a somewhat different kind is offered us by clustered
-flowers in which the peripheral members of the clusters differ from the
-central members; and this evidence is especially significant where we
-find allied species that do not exhibit the deviation, at the same time
-that they do not fulfil the conditions under which it may be expected.
-Thus, in <i>Scabiosa succisa</i>, Fig. <a href="#i_171">250</a>, which bears its numerous
-small flowers in a hemispherical knob, the component flowers, similarly
-circumstanced, are all equal and all radial; but in <i>Scabiosa
-arvensis</i>, Fig. <a href="#i_171">251</a>, in which the numerous small flowers form a
-flattened disk only the confined central ones are radial: round the
-edge the flowers are much larger and conspicuously bilateral.</p>
-
-  <div class="figcenter" id="i_171">
-     <img
-    class="p2"
-    src="images/i_171.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 250, 251.</p>
-  </div>
-
-  <div class="figcenter" id="i_172">
-     <img
-    class="p2"
-    src="images/i_172.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 252.</p>
-  </div>
-
-<p class="p2">But the most remarkable and most conclusive proofs of these relations
-between forms and positions, are those given by the clustered flowers
-called <i>Umbelliferæ</i>. In some cases, as where the component
-flowers have all plenty of room, or where the surface of the umbel
-is more or less globular, the modifications are not conspicuous; but
-where, as in <i>Viburnum</i>, <i>Chærophyllum</i>, <i>Anthriscus</i>,
-<i>Torilis</i>, <i>Caucalis</i>, <i>Daucus</i>, <i>Tordylium</i>,
-&amp;c., we have flowers clustered in such ways as to be differently
-conditioned, we find a number of modifications that are marked and
-varied in proportion as the differences of conditions are marked and
-varied. In <i>Chærophyllum</i>, where the flowers of each umbellule are
-closely placed so as to form a flat surface, but where the umbellules
-are wide apart and form a dispersed umbel, the umbellules do not
-differ from one another; though among the<span class="pagenum" id="Page_172">[172]</span> flowers of each umbellule
-there are decided differences: the central flowers being small and
-radial, while the peripheral ones are large and bilateral. But in
-other genera, where not only the flowers of each umbellule but also
-the umbellules themselves, are closely clustered into a flat surface,
-the umbellules themselves become contrasted; and many remarkable
-secondary modifications arise. In an umbel of <i>Heracleum</i>, for
-instance, there are to be noted the facts;&mdash;first, that the external
-umbellules are larger than the internal ones; second, that in each
-umbellule the central flowers are less developed than the peripheral
-ones; third, that this greater development of the peripheral flowers
-is most marked in the outer umbellules; fourth, that it is most
-marked on the outer sides of the outer umbellules; fifth, that while
-the interior flowers of each umbellule are radial, the exterior ones
-are bilateral; sixth, that this bilateralness is most marked in the
-peripheral flowers of the peripheral umbellules; seventh, that the
-flowers on the outer sides of these peripheral umbellules are those in
-which the bilateralness reaches a maximum; and eighth, that where the
-outer umbellules touch one another, the flowers, being unsymmetrically
-placed, are unsymmetrically bilateral.<a id="FNanchor_37" href="#Footnote_37" class="fnanchor">[37]</a> The like modifications
-are displayed, though not in so clearly-traceable a way, in an umbel
-of <i>Tordylium</i>, Fig. <a href="#i_172">252</a>. Considering how obviously these
-various forms are related to the various conditions, we should be
-scarcely able, even in the<span class="pagenum" id="Page_173">[173]</span> absence of all other facts, to resist the
-conclusion that the differences in the conditions are the causes of the
-differences in the forms.</p>
-
-  <div class="figcenter" id="i_173">
-     <img
-    class="p2"
-    src="images/i_173.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 253.</p>
-  </div>
-
-<p class="p2">Composite flowers furnish evidence so nearly allied to that which
-clustered flowers furnish, that we may fitly glance at them under
-the same head. Such a common type of this order as the Sun-flower,
-exemplifies the extremely marked difference which arises in many
-of these plants between the closely-packed internal florets, each
-similarly circumstanced on all sides, and the external florets, not
-similarly circumstanced on all sides. In Fig. <a href="#i_173">253</a>, representing the
-inner and outer florets of a Daisy, the contrast is marked between the
-small radial corolla of the one and the larger bilateral corolla of the
-other. In many cases, however, this contrast is less marked: the inner
-florets also having their outward-growing prolongations&mdash;a difference
-possibly related to some difference in the habits of the insects that
-fertilize them. Nevertheless, these composite flowers which have
-inner florets with strap-shaped corollas outwardly directed, equally
-conform to the general principle; both in the radial arrangement of
-the assemblage of florets, and in the bilateral shape of each floret;
-which has its parts alike on the two sides of a line passing from the
-centre of the assemblage to the circumference. Certain other members of
-this order fulfil the law somewhat differently. In <i>Centaurea</i>,
-for instance, the inner florets are small and vertical in direction,
-while the outer florets are large and lateral in direction. And here
-may be remarked, in passing, a clear indication of the effect which
-great flexibility of the petals has in preventing a flower from losing
-its original radiate form; for while in <i>C. cyanus</i>, the large
-outward-growing florets, having short, stiff divisions of the corolla,
-are decidedly bilateral, in <i>C. scabiosa</i>, where the divisions of
-the<span class="pagenum" id="Page_174">[174]</span> corolla are long and flexible, the radial form is scarcely at all
-modified. On bearing in mind the probable relations of the forms to
-insect-agency, the meaning of this difference will not be difficult to
-understand.<a id="FNanchor_38" href="#Footnote_38" class="fnanchor">[38]</a></p>
-
-
-<p class="p2">§ 236. In extremely-varied ways there are thus re-illustrated among
-flowers, the general laws of form which leaves and branches and
-entire plants disclose to us. Composed as each cluster of flowers
-is of individuals that are originally similar; and composed as each
-flower is of homologous foliar organs; we see both that the like
-flowers become unlike and the like parts of each flower become unlike,
-where the positions involve unlike incidence of forces. The symmetry
-remains radial where the conditions are equal all round; shows
-deviation towards two-sidedness where there is slight two-sidedness
-of conditions; becomes decidedly bilateral where the conditions are
-decidedly bilateral; and passes into an unsymmetrical form where the
-relations to the environment are unsymmetrical.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_175">[175]</span></p>
-
-<h3>CHAPTER XI.<br />
-<span class="subhed1">THE SHAPES OF VEGETAL CELLS.</span></h3></div>
-
-
-<p>§ 237. We come now to aggregates of the lowest order. Already something
-has been said (<a href="#sect217">§ 217</a>) concerning the forms of those morphological units
-which exist as independent plants. But it is here requisite briefly to
-note the modifications undergone by them where they become components
-of larger plants.</p>
-
-  <div class="figcenter" id="i_175">
-     <img
-    class="p2"
-    src="images/i_175.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 254.</p>
-  </div>
-
-<p class="p2">Of the numerous cell-forms which are found in the tissues of the higher
-plants, it will suffice to give, in Fig. <a href="#i_175">254</a>, representing a section
-of a leaf, a single example. In this it will be seen that the cells
-forming the upper and lower surfaces, <i>a</i> and <i>b</i>, have
-differences of shape related to differences in the incidence of forces:
-they are more or less flattened in relation to the environment. The
-underneath cells at <i>c</i>, form a class which, similarly exposed to
-light at their outer ends, and, as we may assume, largely developed
-in adjustment to their active assimilative functions, are, by mutual
-pressure, made to grow more in the direction of their lengths than in
-the direction of their breadths. Then on the other side we see that
-the cells <i>d</i>, next above the outer layer, while approximately
-similar, become more and more dissimilar as they diverge from the
-surface, and are quite<span class="pagenum" id="Page_176">[176]</span> irregular in the interior <i>e</i>, where there
-is no definiteness in the conditions to which they are exposed. Thus
-the divergences of these cells from primordial sphericity are such as
-correspond with unlikenesses in their circumstances. And throughout the
-more complex modifications which the cells of other tissues exhibit,
-the like correspondences hold.</p>
-
-  <div class="figcenter" id="i_176a">
-     <img
-    class="p2"
-    src="images/i_176a.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 32–35.</p>
-  </div>
-
-<p class="p2">Among plants of a lower order of aggregation, we have already seen how
-cells become metamorphosed as they become integrated into masses having
-definite organizations. The higher <i>Algæ</i>, exemplified in Figs.
-<a href="#i_176a">32, 34, 35</a>, show this very clearly. Here the departure from the simple
-cell-form to the form of an elongated prism, is manifestly subordinated
-to the contrasts in the relations of the parts. And it is interesting
-to observe how, in one of the branches of Fig. <a href="#i_176a">32</a>, we pass from the
-small, almost-spherical cells which terminate the branchlets, to the
-large, much-modified cells which join the main stem, through gradations
-obviously related in their changed forms to the altered actions their
-positions expose them to.</p>
-
-  <div class="figcenter" id="i_176b">
-     <img
-    class="p2"
-    src="images/i_176b.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 19–23.</p>
-  </div>
-
-<p class="p2">More simply, but quite as conclusively, do the inferior <i>Algæ</i>,
-of which Figs. <a href="#i_176b">19–23</a> are examples, show us how cells pass from their
-original spherical symmetry into radial symmetry, as they pass from a
-state in which they are similarly-conditioned<span class="pagenum" id="Page_177">[177]</span> on all sides, to a state
-in which two of their opposite sides or ends are conditioned in ways
-that are like one another, but unlike the ways in which all other sides
-are conditioned.</p>
-
-<p>Still more instructive are the morphological differentiations of
-those protophytes in which the first steps towards a higher degree of
-integration are shown. In Fig. <a href="#i_177">10</a>, representing one of the transitional
-forms of <i>Desmidiaceæ</i>, it is to be noted that besides the
-difference between the transverse and longitudinal dimensions, which
-the component units display in common, the two end-units differ from
-the rest: they have appendages which the rest have not. Once more,
-where the integration is carried on in such ways as to produce not
-strings but clusters, there arise contrasts and correspondences just
-such as might be looked for. All the four members of the group shown in
-Fig. <a href="#i_177">12</a>, are similarly conditioned; and each of them has a bilateral
-shape answering to its bilateral relations. In Fig. <a href="#i_177">14</a> we have a number
-of similarly-bilateral individuals on the circumference, including a
-central individual differing from the rest by having the bilateral
-character nearly obliterated. And then, in Fig. <a href="#i_177">15</a>, we have two central
-components of the group, deviating more decidedly from those that
-surround them.<a id="FNanchor_39" href="#Footnote_39" class="fnanchor">[39]</a></p>
-
-  <div class="figcenter" id="i_177">
-     <img
-    class="p2"
-    src="images/i_177.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 7–17.</p>
-  </div>
-
-
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_178">[178]</span></p>
-
-<h3>CHAPTER XII.<br />
-<span class="subhed1">CHANGES OF SHAPE OTHERWISE CAUSED.</span></h3></div>
-
-
-<p>§ 238. Besides the more special causes of modification in the shapes of
-plants and of their parts, certain more general causes must be briefly
-noticed. These may be described as consequences of variations in the
-total quantities of the matters and forces furnished to plants by their
-environments. Some of the changes of form so produced are displayed by
-plants as wholes, and others only by their parts. We will glance at
-them in this order.</p>
-
-
-<p class="p2">§ 239. It is a familiar fact that luxuriant shoots have relatively-long
-internodes; and, conversely, that a shoot dwarfed from lack of sap,
-has its nodes closely clustered: a concomitant result being that the
-lateral axes, where these are developed, become in the one case far
-apart and in the other case near together. Fig. <a href="#i_178">255</a> represents a branch
-to the parts of which the longer and shorter internodes so resulting
-give differential characters. A whole tree being in many cases
-simultaneously thus affected by states of the earth or the air, all
-parts of it may have such variations impressed on them; and, indeed,
-such variations, following more or less regularly the changes of the
-seasons, give to many trees manifest<span class="pagenum" id="Page_179">[179]</span> traits of structure. In Fig. <a href="#i_178">256</a>,
-a shoot of <i>Phyllocactus crenatus</i>, we have an interesting example
-of a variation essentially of the same nature, little as it appears
-to be so. For each of the lateral indentations is here the seat of
-an axillary bud; and these we see are separated by internodes which,
-becoming broader as they become longer, and narrower as they become
-shorter, produce changes of form that correspond with changes in the
-luxuriance of growth.</p>
-
-  <div class="figcenter" id="i_178">
-     <img
-    class="p2"
-    src="images/i_178.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 255, 256.</p>
-  </div>
-
-<p class="p2">To complete the statement it must be added that these variations of
-nutrition often determine the development or non-development of lateral
-axes; and by so doing cause still more marked structural differences.
-The Foxglove may be named as a plant which illustrates this truth.<a id="FNanchor_40" href="#Footnote_40" class="fnanchor">[40]</a></p>
-
-
-<p class="p2">§ 240. From the morphological differentiations caused by unlikenesses
-of nutrition felt by the whole plant, we pass now to those which are
-thus caused in some of its parts and not in others. Among such are
-the contrasts between flowering axes, and the axes that bear leaves
-only. It has already been shown in <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect78">§ 78</a>, that the belief expressed by
-Wolff in a direct connexion between fructification and innutrition,
-is justified inductively by many facts of many kinds. Deductively
-too, in <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect79">§ 79</a>, we saw reason to conclude that such a relation would be
-established by survival of the fittest; seeing that it would profit
-a species for its members to begin sending off migrating germs from
-the ends of those axes which innutrition prevented from further
-agamogenetic multiplication. Once more, when considering the nature of
-the phænogamic axis, we found support for this belief in the fact<span class="pagenum" id="Page_180">[180]</span> that
-the components of a flower exhibit a reversion to that type from which
-the phænogamic type has probably arisen&mdash;a reversion which the laws of
-embryology would lead us to look for where innutrition had arrested
-development.</p>
-
-<p>Hence, then, we may properly count those deviations of structure
-which constitute inflorescence, as among the morphological
-differentiations produced by local innutrition. I do not mean that the
-detailed modifications which the essential and subservient organs of
-fructification display, are thus accounted for: we have seen reason
-to think them otherwise caused. But I mean that the morphological
-characters which distinguish gamogenetic axes in general from
-agamogenetic axes, such as non-development of the internodes and
-dwarfing of the foliar organs, are primarily results of failure in the
-supply of some material required for further growth.<a id="FNanchor_41" href="#Footnote_41" class="fnanchor">[41]</a></p>
-
-
-<p class="p2">§ 241. Another trait which has to be noticed under this head, is the
-spiral, or rather the helical, arrangement of parts. The successive
-nodes of a phænogam habitually bear their appendages in ways implying
-more or less twist in the substance of the axis; and in climbing plants
-the twist is such<span class="pagenum" id="Page_181">[181]</span> as to produce a corkscrew shape. This structure is
-ascribable to differences of interstitial nutrition. Take a shoot which
-is growing vertically. It is clear that if the molecules are added
-with perfect equality on all sides, there will be no tendency towards
-any kind of lateral deviation; and the successively-produced parts
-will be perpendicularly over one another. But any inequality in the
-rate of growth on the different sides of the shoot, will destroy this
-straightness in the lines of growth. If the greatest and least rates
-of molecular increase happen to be on opposite sides, the shoot must
-assume a curve of single curvature; but in every other case of unequal
-molecular increase, a curve of double curvature must result. Now it is
-a corollary from the instability of the homogeneous, that the rates of
-growth on all sides of a shoot can never be exactly alike; and it is
-also to be inferred from the same general law, that the greatest and
-least rates of growth will not occur on exactly opposite sides of the
-shoot, at the same time that equal rates of growth are preserved by the
-two other sides. Hence, there must almost inevitably arise more or less
-of twist; and the appendages of the internodes will so be prevented
-from occurring perpendicularly one over another.</p>
-
-<p>A deviation of this kind, necessarily initiated by physical causes
-in conformity with the general laws of evolution, is likely to be
-made regular and decided by natural selection. For under ordinary
-circumstances, a plant profits by having its axis so twisted as to
-bring the appended leaves into positions which prevent them from
-shading one another. And, manifestly, modifications in the forms,
-sizes, and insertions of the leaves, may, under the same agency, lead
-to adapted modifications of the twist. We must therefore ascribe this
-common characteristic of phænogams, primarily to local differences of
-nutrition, and secondarily to survival of the fittest.</p>
-
-<p><span class="pagenum" id="Page_182">[182]</span></p>
-
-<p>It is proper to add that there are some Monocotyledons, as <i>Ravenala
-madagascariensis</i>, in which this character does not occur. What
-conditions of existence they are that here hold this natural tendency
-in check, it is not easy to see.<a id="FNanchor_42" href="#Footnote_42" class="fnanchor">[42]</a></p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_183">[183]</span></p>
-
-<h3>CHAPTER XIII.<br />
-<span class="subhed1">MORPHOLOGICAL DIFFERENTIATION IN ANIMALS.</span></h3></div>
-
-
-<p>§ 242. The general considerations which preluded our inquiry into the
-shapes of plants and their parts, equally serve, so far as they go, to
-prelude an inquiry into the shapes of animals and their parts. Among
-animals, as among plants, the formation of aggregates greater in bulk
-or higher in degree of composition, or both, is accompanied by changes
-of form in the aggregates as wholes as well as by changes of form in
-their parts; and the processes of morphological differentiation conform
-to the same general laws in the one kingdom as in the other.</p>
-
-<p>It is needless to recapitulate the several kinds of modification to be
-explained, and the several factors that co-operate in working them.
-In so far as these are common to plants and animals, the preceding
-chapters have sufficiently familiarized them. Nor is it needful to
-specify afresh the several types of symmetry and their descriptive
-names; for what is true of them in the one case is true of them in the
-other. There is, however, one new and all-important factor which we
-shall have now to take into account; and about this a few preliminary
-remarks are requisite.</p>
-
-
-<p class="p2">§ 243. This new factor is motion&mdash;motion of the organism in relation
-to surrounding objects, or of the parts of the<span class="pagenum" id="Page_184">[184]</span> organism in relation
-to one another, or both. Though there are plants, especially of the
-simpler kinds, which move, and though a few of the simpler animals do
-not move; yet movements are so exceptional and unobtrusive in the one
-kingdom, while they are so general and conspicuous in the other, that
-the broad distinction commonly made is well warranted. What, among
-plants, is an inappreciable cause of morphological differentiation,
-becomes, among animals, the chief cause of morphological
-differentiation.</p>
-
-<p>Rooted animals or animals otherwise fixed, of course present traits
-of structure nearest akin to those we have lately been studying. The
-motions of parts in relation to one another and to the environment,
-being governed by the mode of aggregation and mode of fixing, we are
-presented with morphological differentiations similar in their general
-characters to those of plants, and showing us parallel kinds of
-symmetry under parallel conditions. But animals which move from place
-to place are subject to an additional class of actions and reactions.
-These actions and reactions affect them in various ways according to
-their various modes of movement. Let us glance at the several leading
-relations between shape and motion which we may expect to find.</p>
-
-<p>If an organism advances through a homogeneous medium with one end
-always foremost, that end, being exposed to forces unlike those to
-which the other end is exposed, may be expected to become unlike it;
-and supposing this to be the only constant contrast of conditions,
-we may expect an equal distribution of the parts round the axis
-of movement&mdash;a radial symmetry. If, in addition to this habitual
-attitude of the ends, one surface of the body is always uppermost
-and another always lowermost, there arise between the top and bottom
-dissimilarities of conditions, while the two sides remain similarly
-conditioned. Hence it is inferable that such an organism will be
-divisible into similar halves by a vertical plane passing through
-its axis of motion&mdash;will have a bilateral symmetry. We may presume
-that this<span class="pagenum" id="Page_185">[185]</span> symmetry will deviate but little from double bilateralness
-where the upper and under parts are not exposed to strongly-contrasted
-influences; while we may rationally look for single bilateral symmetry
-of a decided kind, in creatures having dorsal and ventral parts
-conversant with very unlike regions of the environment: as in all cases
-where the movement is over a solid surface. If the movement, though
-over a solid surface, is not constant in direction, but takes place as
-often on one side as on another, radial symmetry may be again looked
-for; and if the motions are still more variously directed&mdash;if they are
-not limited to approximately-plane surfaces, but extend to surfaces
-that are distributed all around with a regular irregularity&mdash;an
-approach of the radial towards the spherical symmetry is to be
-anticipated. Where the habits are such that the intercourse between the
-organism and its environment, does not involve an average equality of
-actions and reactions on any two or more sides, there may be expected
-either total irregularity or some divergence from regularity.</p>
-
-<p>The like general relations between forms and incident forces are
-inferable in the component parts of animals, as well as in the animals
-as wholes. It is needless, however, to occupy space by descriptions of
-these. Let us now pass to the facts, and see how they confirm, <i>à
-posteriori</i>, the conclusions here reached <i>à priori</i>.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_186">[186]</span></p>
-
-<h3>CHAPTER XIV.<br />
-<span class="subhed1">THE GENERAL SHAPES OF ANIMALS.</span></h3></div>
-
-
-<p>§ 244. Certain of the <i>Protozoa</i> are quite indefinite in their
-shapes, and quite inconstant in those indefinite shapes which they
-have&mdash;the relations of their parts are indeterminate both in space and
-time. In one of the simpler Rhizopods, at least during the active stage
-of its existence, no permanent distinction of inside and outside is
-established; and hence there can arise no established correspondence
-between the shape of the outside and the distribution of environing
-actions. But when the relation of inner and outer becomes fixed,
-either over part of the mass or over the whole of it, we have kinds
-of symmetry that correspond with the habitual incidence of forces.
-An <i>Amœba</i> in becoming encysted, passes from an indefinite,
-ever-changing form into a spherical form; and the order of symmetry
-which it thus assumes, is in harmony with the average equality of the
-actions on all its sides. In <i>Difflugia</i>, Fig. <a href="#i_187">134</a>, and still
-better in <i>Arcella</i>, we have an indefinitely-radial symmetry
-occurring where the conditions are different above and below but alike
-all around. Among the <i>Gregarinida</i> the spherical symmetry and
-symmetry passing from that into the radial, are such as appear to be
-congruous with the simple circumstances of these creatures in the
-intestines of insects. But the relations of these lowest types to their
-environments are comparatively so indeterminate, and our knowledge of
-their actions so<span class="pagenum" id="Page_187">[187]</span> scanty, that little beyond negative evidence can be
-expected from the study of them.</p>
-
-  <div class="figcenter" id="i_187">
-     <img
-    class="p2"
-    src="images/i_187.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 131–139.</p>
-  </div>
-
-<p class="p2">The like may be said of the <i>Infusoria</i>. These are more or
-less irregular. In some cases, where the line of movement through
-the water is tolerably definite and constant, we have a form that
-is approximately radial&mdash;externally at least. But usually, as shown
-in Figs. <a href="#i_187">137, 138, 139</a>, there is either an unsymmetrical or an
-asymmetrical shape. And when one of these creatures is watched under
-the microscope, the congruity of this shape with the incidence of
-forces is manifest. For the movements are conspicuously varied and
-indeterminate&mdash;movements which do not expose any two or more sides of
-the mass to approximately equal sets of actions.<a id="FNanchor_43" href="#Footnote_43" class="fnanchor">[43]</a></p>
-
-
-<p class="p2">§ 245. Among aggregates of the second order, as among aggregates
-of the first order, we find that of those possessing any definite
-shapes the lowest are spherical or spheroidal. Such are some of the
-<i>Radiolaria</i>, as <i>Collozoum inerme</i>. These bodies which float
-passively in the sea, and present in turn all their sides to the same
-influences, have their parts disposed with approximate regularity round
-a centre&mdash;approximate, because in the absence of locomotion a slight
-irregularity of growth, almost certain to take place, may cause a
-fixed attitude and a resulting deviation from spherical symmetry. The
-best cases in illustration of the truth here named, are furnished by
-rotating and locomotive organisms respecting which there is a dispute
-whether they are animal or vegetal&mdash;the <i>Volvocineæ</i>. These,
-already instanced under<span class="pagenum" id="Page_188">[188]</span> the one head in <a href="#sect218">§ 218</a>, may here be instanced
-afresh under the other. Further, among these secondary aggregates
-in which the units, only physically integrated, have not had their
-individualities merged into an individuality of a higher order, must be
-named the compound <i>Infusoria</i>. The cluster of <i>Vorticellæ</i>
-in Fig. <a href="#i_088">144</a>, will sufficiently exemplify them; and the striking
-resemblance borne by its individuals to those of a radially-arranged
-cluster of flowers, will show how, under analogous conditions, the
-general principles of morphological differentiation are similarly
-illustrated in the two kingdoms.</p>
-
-
-<p class="p2">§ 246. Radial symmetry is usual in low aggregates of the second order
-which have their parts sufficiently differentiated and integrated
-to give individualities to them as wholes. The <i>Cœlenterata</i>
-offer numerous examples of this. Solitary polypes&mdash;hydroid or
-helianthoid&mdash;mostly stationary, and when they move, moving with any
-side foremost, do not by locomotion subject their bodies to habitual
-contrasts of conditions. Seated with their mouths upwards or downwards,
-or else at all degrees of inclination, the individuals of a species
-taken together, are subject to no mechanical actions affecting some
-parts of their discs more than other parts. And this indeterminateness
-of attitude similarly prevents their relations to prey from being such
-as subject some of their prehensile organs to forces unlike those to
-which the rest are subject. The fixed end is differently conditioned
-from the free end, and the two are therefore different; but around the
-axis running from the fixed to the free end the conditions are alike
-in all directions, and the form therefore is radial. Again, among many
-of the simple free-swimming <i>Hydrozoa</i>, the same general truth
-is exemplified under other circumstances. In a common <i>Medusa</i>,
-advancing through the water by the rhythmical contractions of its
-disc, the mechanical reactions are the same on all sides; and as, from
-accidental causes, every part of the edge of the disc<span class="pagenum" id="Page_189">[189]</span> comes uppermost
-in its turn, no part is permanently affected in a different way from
-the rest. Hence the radial form continues.</p>
-
-  <div class="figcenter" id="i_189">
-     <img
-    class="p2"
-    src="images/i_189.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 257, 258.</p>
-  </div>
-
-<p class="p2">In others of this same group, however, there occur forms which show
-us an incipient bilateralness; and help us to see how a more decided
-bilateralness may arise. Sundry of the <i>Medusidæ</i> are proliferous,
-giving origin to gemmæ from the body of the central polypite or from
-certain points on the edge of the disc; and this budding, unless it
-occurs equally on all sides, which it does not and is unlikely to
-do, must tend to destroy the balance of the disc, and to make its
-attitude less changeable. In other cases the growth of a large process
-[a much-developed tentacle] from the edge of the disc on one side, as
-in <i>Steenstrupia</i>, Fig. <a href="#i_189">257</a>, constitutes a similar modification,
-and a cause of further modification. The animal is no longer divisible
-into any two quite similar halves, except those formed by a plane
-passing through the process; and unless the process is of the same
-specific gravity as the disc, it must tend towards either the lowest
-or the highest point, and must so serve to increase the bilateralness,
-by keeping the two sides of the disc similarly conditioned while
-the top and bottom are differently conditioned. Fig. <a href="#i_189">258</a> represents
-the underside of another <i>Medusa</i>, in which a more decided
-bilateralness is produced by the presence of two such processes. Among
-the simple free-swimming <i>Actinozoa</i>, occur like deviations
-from radial symmetry, along with like motions through the water in
-bilateral attitudes. Of this a <i>Cydippe</i> is a familiar example.
-Though radial in some of its characters, as in the distribution of
-its meridional bands of locomotive paddles with their accompanying
-canals, this creature has a two-sided distribution of tentacles<span class="pagenum" id="Page_190">[190]</span> and
-various other parts, corresponding with its two-sided attitude in
-moving through the water. And in other genera of this group, as in
-<i>Cestum</i>, <i>Eurhamphæa</i>, and <i>Callianira</i>, that almost
-equal distribution of parts which characterizes the <i>Beroe</i> is
-quite lost.</p>
-
-<p>Here seems a fit place to meet the objection which some may feel to
-this and other such illustrations, that they amount very much to
-physical truisms. If the parts of a <i>Medusa</i> are disposed in
-radial symmetry round the axis of motion through the water, there will
-of course be no means of maintaining one part of its edge uppermost
-more than another; and the equality of conditions may be ascribed
-to the radiateness, as much as the radiateness to the equality of
-conditions. Conversely, when the parts are not radially arranged
-around the axis of motion, they must gravitate towards some one
-attitude, implying a balance on the two sides of a vertical plane&mdash;a
-bilateralness; and the two-sided conditions so necessitated, may be
-as much ascribed to the bilateralness as the bilateralness to the
-two-sided conditions. Doubtless the form and the conditions are, in
-the way alleged, necessary correlates; and in so far as it asserts
-this, the objection harmonizes with the argument. To the difficulty
-which it at the same time raises by the implied question&mdash;Why make
-the form the result of the conditions, rather than the conditions
-the result of the form? the reply is this:&mdash;The radial type, both as
-being the least differentiated type and as being the most obviously
-related to lower types, must be taken as antecedent to the bilateral
-type. The individual variations which incidental circumstances produce
-in the radial type, will not cause divergence of a species from the
-radial type, unless such variations give advantages to the individuals
-displaying them; which there is no reason to suppose they will always
-do. Those occasional deviations from the radial type, which the law of
-the instability of the homogeneous warrants us in expecting to take
-place, will, however, in some cases be beneficial; and will then be
-likely to establish<span class="pagenum" id="Page_191">[191]</span> themselves. Such deviations must tend to destroy
-the original indefiniteness and variability of attitude&mdash;must cause
-gravitation towards an habitual attitude. And gravitation towards an
-habitual attitude having once commenced, will continually increase,
-where increase of it is not negatived by adverse agencies: each further
-degree of bilateralness rendering more decided the actions that conduce
-to bilateralness. If this reply be thought insufficient, it may be
-enforced by the further one, that as, among plants, the incident forces
-are the antecedents and the forms the consequents (changes of forces
-being in many cases visibly followed by changes of forms) we are
-warranted in concluding that the like order of cause and effect holds
-among animals.<a id="FNanchor_44" href="#Footnote_44" class="fnanchor">[44]</a></p>
-
-
-<p class="p2">§ 247. Keeping to the same type but passing to a higher degree of
-composition, we meet more complex and varied illustrations of the same
-general laws. In the compound<span class="pagenum" id="Page_192">[192]</span> <i>Cœlenterata</i>, presenting clusters
-of individuals which are severally homologous with the solitary
-individuals last dealt with, we have to note both the shapes of the
-individuals thus united, and the shapes of the aggregates made up of
-them.</p>
-
-  <div class="figcenter" id="i_192">
-     <img
-    class="p2"
-    src="images/i_192.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 149, 150.</p>
-  </div>
-
-<p class="p2">Such of the fixed <i>Hydrozoa</i> and <i>Actinozoa</i> as form branched
-societies, continue radial; both because their varied attitudes do not
-expose them to appreciable differences in their relations to those
-surrounding actions which chiefly concern them (the actions of prey),
-and because such differences, even if they were appreciable, would
-be so averaged in their effects on the dissimilarly-placed members
-of each group as to be neutralized in the race. Among the tree-like
-coral-polypedoms, as well as in such ramified assemblages of simpler
-polypes as are shown in Figs. <a href="#i_192">149, 150</a>, we have, indeed, cases in many
-respects parallel to the cases of scattered flowers (<a href="#sect233">§ 233</a>), which
-though placed laterally remain radial, because no differentiating
-agency can act uniformly on all of them. Meanwhile, in the groups which
-these united individuals compose, we see the shapes of plants further
-simulated under a further parallelism of conditions. The attached ends
-differ from the free ends as they do in plants; and the regular or
-irregular branches obviously stand to environing actions in relations
-analogous to those in which the branches of plants stand.</p>
-
-<p>The members of those compound <i>Cœlenterata</i> which move
-through the water by their own actions, in attitudes that are
-approximately constant, show us a more or less distinct two-sidedness.
-<i>Diphyes</i>, Fig. <a href="#i_193a">259</a>, furnishes an example. Each of the
-largely-developed and modified polypites forming its swimming sacs is
-bilateral, in correspondence with the bilateralness of its conditions;
-and in each of the appended polypites the insertion of the solitary
-tentacle produces a kindred divergence from the primitive radial type.
-The aggregate, too, which here very much subordinates its members,<span class="pagenum" id="Page_193">[193]</span>
-exhibits the same conformity of structure to circumstances. It admits
-of symmetrical bisection by a plane passing through its two contractile
-sacs, or nectocalyces, but not by any other plane; and the plane which
-thus symmetrically bisects it, is the vertical plane on the two sides
-of which its parts are similarly conditioned as it propels itself
-through the water.</p>
-
-  <div class="figcenter" id="i_193a">
-     <img
-    class="p2"
-    src="images/i_193a.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 259.</p>
-  </div>
-
-<p class="p2">Another group of the oceanic <i>Hydrozoa</i>, the <i>Physophoridæ</i>,
-furnishes interesting evidence&mdash;not so much in respect of the forms of
-the united individuals, which we may pass over, as in respect of the
-forms of the aggregates. Some of these are without swimming organs,
-and have their parts suspended from air-vessels which habitually float
-on the surface of the water. Hence the distribution of their parts is
-asymmetrical. The <i>Physalia</i>, Fig. <a href="#i_193b">152</a>, is an example. Here the
-relations of the integrated group of individuals to the environment
-are indefinite; and there is thus no agency tending to change that
-comparatively irregular mode of growth which is probably derived from a
-primordial type of the branched <i>Hydrozoa</i>.</p>
-
-  <div class="figcenter" id="i_193b">
-     <img
-    class="p2"
-    src="images/i_193b.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 152.</p>
-  </div>
-
-<p class="p2">So various are the modes of union among the compound
-<i>Cœlenterata</i>, that it is out of the question to deal with them
-all. Even did space permit, it would be impracticable for any one but
-a professed naturalist, to trace throughout<span class="pagenum" id="Page_194">[194]</span> this group the relations
-between shapes and conditions of existence. The above must be taken
-simply as a few of the most significant and easily-interpretable cases.</p>
-
-
-<p class="p2">§ 248. In the sub-kingdoms <i>Polyzoa</i> and <i>Tunicata</i> we
-meet with examples not wholly unlike the foregoing. Among the types
-assembled under these names there are simple individuals or aggregates
-of the second order, and societies or tertiary aggregates produced by
-their union. The relations of forms to forces have to be traced in both.</p>
-
-<p>Solitary Ascidians, fixed or floating, carry on an inactive and
-indefinite converse with the actions in the environment. Without
-power to move about vivaciously, and unable to catch any prey but
-that contained in the currents of water they absorb and expel, these
-creatures are not exposed to sets of forces which are equal on two
-or more sides; and their shapes consequently remain vague. Though
-internally their parts have a partially-symmetrical arrangement, due
-to their derivation, yet they are substantially unsymmetrical in that
-part of the body which is concerned with the environment. Fig. <a href="#i_094">156</a> is
-an example.<a id="FNanchor_45" href="#Footnote_45" class="fnanchor">[45]</a> Among the composite Ascidians, floating and fixed,
-the shape of the aggregate, partly determined by the habitual mode of
-gemmation and partly by the surrounding conditions in each case, is in
-great measure indefinite. We can say no more about it than that it is
-not obviously at variance with the laws alleged.</p>
-
-<p>Evidence of a more positive kind occurs among those compound
-<i>Molluscoida</i> which are most like the compound <i>Cœlenterata</i>
-in their modes of union&mdash;the <i>Polyzoa</i>. Many of these form
-groups that are more or less irregular&mdash;spreading as films over
-solid surfaces, combining into seaweed-like fronds, budding out from
-creeping stolons, or growing up into tree-shaped societies; and besides
-aggregating<span class="pagenum" id="Page_195">[195]</span> irregularly they are irregularly placed on surfaces
-inclined in all directions. Merely noting that this asymmetrical
-distribution of the united individuals is explained by the absence of
-definiteness in the relations of the aggregate to incident forces, it
-concerns us chiefly to observe that the united individuals severally
-exemplify the same truth as do similarly-united individuals among
-the <i>Cœlenterata</i>. Averaging the members of each society, the
-ciliated tentacles they protrude are similarly related to prey on all
-sides; and therefore remain the same on all sides. This distribution
-of tentacles is not, however, without exception. Among the fresh-water
-<i>Polyzoa</i> there are some genera, as <i>Plumatella</i> and
-<i>Crystatella</i>, in which the arrangement of these parts is very
-decidedly bilateral. Some species of them show us such relations of the
-individuals to one another and to their surface of attachment, as give
-a clue to the modification; but in other species the meaning of this
-deviation from the radial type is not obvious.</p>
-
-
-<p class="p2">§ 249. In the <i>Platyhelminthes</i> good examples of the connexions
-between forms and forces occur. The <i>Planaria</i> exemplifies
-the single bilateral symmetry which, even in very inferior forms,
-accompanies the habit of moving in one direction over a solid
-surface. Humbly organized as are these creatures and their allies the
-<i>Nemertidæ</i>, we see in them, just as clearly as in the highest
-animals, that where the movements subject the body to different forces
-at its two ends, different forces on its under and upper surfaces, and
-like forces along its two sides, there arises a corresponding form,
-unlike at its extremities, unlike above and below, but having its two
-sides alike.</p>
-
-<p>The <i>Echinodermata</i> furnish us with instructive
-illustrations&mdash;instructive because among types that are nearly allied,
-we meet with wide deviations of form answering to marked contrasts in
-the relations to the environment. The facts fall into four groups. The
-<i>Crinoidea</i>, once so abundant<span class="pagenum" id="Page_196">[196]</span> and now so rare, present a radial
-symmetry answering to an incidence of forces that are equal on all
-sides. In the general attitudes of their parts towards surrounding
-actions, they are like uniaxial plants or like polypes; and show, as
-those do, marked differences between the attached ends and the free
-ends, along with even distributions of parts all round their axes. In
-the <i>Ophiuridea</i>, and in the Star-fishes, we have radial symmetry
-co-existing with very different habits; but habits which nevertheless
-account for the maintenance of the form. Holding on to rocks and weeds
-by its simple or branched arms, or by the suckers borne on the under
-surface of its rays, one of these creatures moves about not always with
-one side foremost, but with any side foremost. Consequently, averaging
-its movements, its arms or rays are equally affected, and therefore
-remain the same on all sides. On watching the ways of the common
-Sea-urchin, we are similarly furnished with an explanation of its
-spherical, or rather its spheroidal, figure. Here the habit is not to
-move over any one approximately-flat surface; but the habit is to hold
-on by several surfaces on different sides at the same time. Frequenting
-crevices and the interstices among stones and weeds, the Sea-urchin
-protrudes the suckers arranged in meridional bands over its shell,
-laying hold of objects now on this side and now on that, now above and
-now below: the result being that it does not move in all directions
-over one plane but in all directions through space. Hence the approach
-in general form towards spherical symmetry&mdash;an approach which is,
-however, restrained by the relations of the parts to the mouth and
-vent: the conditions not being exactly the same at the two poles as at
-other parts of the surface. Still more significant is that deviation
-from this shape which occurs among such of the <i>Echinidea</i> as
-have habitats of a different kind, and consequently, different habits.
-The genera <i>Echinocyamus</i>, <i>Spatangus</i>, <i>Brissus</i>, and
-<i>Amphidotus</i>, diverge markedly towards a bilateral structure.
-These creatures are found not on rocky<span class="pagenum" id="Page_197">[197]</span> shores but on flat sea-bottoms,
-and some of them only on bottoms of sand or mud. Here, there is none of
-that distribution of surfaces on all sides which makes the spheroidal
-form congruous with the conditions. Having to move about over an
-approximately-horizontal plane, any deviation of structure arising
-accidentally which leads to one side being kept always foremost, will
-be an advantage: greater fitness to function becoming possible in
-proportion as function becomes fixed. Survival of the fittest will
-therefore tend to establish, under such conditions, a form that keeps
-the same part in advance&mdash;a form in which, consequently, the original
-radial symmetry diverges more and more towards bilateral symmetry.</p>
-
-
-<p class="p2">§ 250. Very definite and comparatively uniform, are the relations
-between shapes and circumstances among the <i>Annulosa</i>: including
-under that title the <i>Annelida</i> and the <i>Arthropoda</i>. The
-agreements and the disagreements are equally instructive.</p>
-
-<p>At one time or other of its life, if not throughout its life, every
-annulose animal is locomotive; and its temporary or permanent
-locomotion, being carried on with one end habitually foremost and
-one surface habitually uppermost, it fulfils those conditions under
-which bilateral symmetry arises. Accordingly, bilateral symmetry
-is traceable throughout the whole of this sub-kingdom. Traceable,
-we must say, because, though it is extremely conspicuous in the
-immense majority of annulose types, it is to a considerable extent
-obscured where obscuration is to be expected. The embryos of the
-<i>Tubicolæ</i>, after swimming about a while, settle down and build
-themselves tubes, from which they protrude their heads; and in them,
-or in some of them, the bilateral symmetry is disguised by the
-development of head-appendages in an all-sided manner. The tentacles
-of <i>Terebella</i> are distributed much in the same way as those of
-a polype. The breathing organs in <i>Sabella unispira</i>, Fig. <a href="#i_198">260</a>,
-do not correspond<span class="pagenum" id="Page_198">[198]</span> on opposite sides of a median plane. Even here,
-however, the body retains its primitive bilateralness; and it is
-further to be remarked that this loss of bilateralness in the external
-appendages, does not occur where the relations to external conditions
-continue bilateral: witness the <i>Serpula</i>, Fig. <a href="#i_198">261</a>, which has
-its respiratory tufts arranged in a two-sided way, under the two-sided
-conditions involved by the habitual position of its tube.</p>
-
-  <div class="figcenter" id="i_198">
-     <img
-    class="p2"
-    src="images/i_198.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 260–261.</p>
-  </div>
-
-<p class="p2">The community of symmetry among the higher <i>Annulosa</i>, has an
-unobserved significance. That Flies, Beetles, Lobsters, Centipedes,
-Spiders, Mites, have in common the characters, that the end which moves
-in advance differs from the hinder end, that the upper surface differs
-from the under surface, and that the two sides are alike, is a truth
-received as a matter of course. After all that has been said above,
-however, it will be seen to have a meaning not to be overlooked; since
-it supplies a million-fold illustration of the laws which have been set
-forth. It is needless to give diagrams. Every reader can call to mind
-the unity indicated.</p>
-
-  <div class="figcenter" id="i_199">
-     <img
-    class="p2"
-    src="images/i_199.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 263–270.</p>
-  </div>
-
-<p class="p2">While, however, annulose animals repeat so uniformly these traits of
-structure, there are certain other traits in which they are variously
-contrasted; and their contrasts have to be here noted, as serving
-further to build up the general argument. In them we see the stages
-through which<span class="pagenum" id="Page_199">[199]</span> bilateral symmetry becomes gradually more marked, as
-the conditions it responds to become more decided. A common Earth-worm
-may be instanced as a member of this sub-kingdom that is among the
-least-conspicuously bilateral. Though internally its parts have a
-two-sided arrangement; and though the positions of its orifices give
-it an external two-sidedness, at the same time that they establish
-a difference between the two ends; yet its two-sidedness is not
-strongly-marked. The form deviates but little from what we have
-distinguished as triple bilateral symmetry: if the creature is cut
-across the middle, the head and tail ends are very much alike; if
-cut in two along its axis by a horizontal plane, the under and upper
-halves are very much alike, externally if not internally; and if cut
-in two along its axis by a vertical plane, the two sides are quite
-alike. Figs. <a href="#i_199">263 and 264</a> will make this clear. Such creatures as the
-<i>Julus</i> and the Centipede, may be taken as showing a transition
-to double bilateral symmetry. Besides being divisible into exactly
-similar halves by a vertical plane passing through its axis, one of
-these animals may be bisected transversely into parts that differ only
-slightly; but if cut in two by a horizontal plane passing through its
-axis, the under and upper halves are decidedly unlike. Figs. <a href="#i_199">265, 266</a>,
-exhibit these traits. Among the isopodous crustaceans, the departure
-from these low types of symmetry is more<span class="pagenum" id="Page_200">[200]</span> marked. As shown in Figs.<a href="#i_199"> 267</a>
-and 268, the contrast between the upper and under parts is greater,
-and the head and tail ends differ more obviously. In all the higher
-<i>Arthropoda</i>, the unlikeness between the front half and the hind
-half has become conspicuous. There is in them single bilateral symmetry
-of so pronounced a kind, that no other resemblance is suggested than
-that between the two sides. By Figs. <a href="#i_199">269 and 270</a>, representing a
-decapodous crustacean divided longitudinally and transversely, this
-truth is made manifest. On calling to mind the habits of the creatures
-here drawn and described, it will be seen that they explain these
-forms. The incidence of forces is the same all around the Earth-worm
-as it burrows through the compact ground. The Centipede, creeping
-amid loose soil or <i>débris</i> or beneath stones, insinuates itself
-between solid surfaces&mdash;the interstices being mostly greater in one
-dimension than in others. And all the higher <i>Annulosa</i>, moving
-about as they do over exposed objects, have their dorsal and ventral
-parts as dissimilarly acted upon as are their two ends.</p>
-
-  <div class="figcenter" id="i_200">
-     <img
-    class="p2"
-    src="images/i_200.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 271.</p>
-  </div>
-
-<p class="p2">One other fact only respecting annulose animals needs to be noticed
-under this head&mdash;the fact, namely, that they become unsymmetrical
-where their parts are unsymmetrically related to the environment.
-The common Hermit-crab serves as an instance. Here, in addition
-to the unlikeness of the two sides implied by that curvature of
-the body which fits the creature to the shell it inhabits, there
-is an unlikeness due to the greater development of the limbs, and
-especially the claws, on the outer side. As in the embryo of the
-Hermit-crab the two sides are alike; and as both the embryo and the
-ancestor lived in such a way, being free, that the conditions were
-alike on the two sides; and as the embryo may be taken to represent
-the type from which the Hermit-crab has been derived; we have in
-this case evidence that a symmetrically-bilateral form has been
-moulded into an unsymmetrically-bilateral<span class="pagenum" id="Page_201">[201]</span> form, by the action of
-unsymmetrically-bilateral conditions. A further illustration is
-supplied by <i>Bopyrus</i>, Fig. <a href="#i_200">271</a>: a parasite which lives in the
-branchial chamber of prawns, and whose habits similarly account for its
-distorted shape.</p>
-
-
-<p class="p2">§ 251. Among the <i>Mollusca</i> we find more varied relations
-between shapes and circumstances. Some of these relations are highly
-instructive.</p>
-
-<p>Mollusks of one order, the <i>Pteropoda</i>, swim in the sea much in
-the same way that butterflies fly in the air, and have shapes not
-altogether unlike those of butterflies. Fig. <a href="#i_201">272</a> represents one of
-these creatures. That its bilaterally-symmetrical shape harmonizes with
-its bilaterally-symmetrical conditions is sufficiently obvious.</p>
-
-
-  <div class="figcenter" id="i_201">
-     <img
-    class="p2"
-    src="images/i_201.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 272.</p>
-  </div>
-
-<p class="p2">Among the <i>Lamellibranchiata</i>, we have diverse forms accompanying
-diverse modes of life. Such of them as frequently move about, like
-the fresh-water Mussel, have their two valves and the contained parts
-alike on the opposite sides of a vertical plane: they are bilaterally
-symmetrical in conformity with their mode of movement. The marine
-Mussel, too, though habitually fixed, and though not usually so fixed
-that its two valves are similarly conditioned, still retains that
-bilateral symmetry which is characteristic of the order; and it does
-this because in the species considered as a whole, the two valves
-are not dissimilarly conditioned. If the positions of the various
-individuals are averaged, it will be seen that the differentiating
-actions neutralize one another. In certain other fixed Lamellibranchs,
-however, there is a considerable deviation from bilateral symmetry;
-and it is a deviation of the kind to be anticipated under the
-circumstances. Where one valve is always downwards, or next to the
-surface of attachment, while the other valve is always upwards, or next
-to the environing water, we may expect to find the two<span class="pagenum" id="Page_202">[202]</span> valves become
-unlike. This we do find: witness the Oyster. In the Oyster, too, we see
-a further irregularity. There is a great indefiniteness of outline,
-both in the shell and in the animal&mdash;an indefiniteness made manifest by
-comparing different individuals. We have but to remember that growing
-clustered together, as Oysters do, they must interfere with one another
-in various ways and degrees, to see how the indeterminateness of form
-and the variety of form are accounted for.</p>
-
-<p>Among the Gasteropods modifications of a more definite kind occur. “In
-all Mollusks,” says Professor Huxley, “the axis of the body is at first
-straight, and its parts are arranged symmetrically with regard to a
-longitudinal vertical plane, just as in a vertebrate or an articulate
-embryo.” In some Gasteropods, as the <i>Chiton</i>, this bilateral
-symmetry is retained&mdash;the relations of the body to surrounding actions
-not being such as to disturb it. But in those more numerous types
-which have spiral shells, there is a marked deviation from bilateral
-symmetry, as might be expected. “This asymmetrical over-development
-never affects the head or foot of the mollusk”: only those parts
-which, by inclosure in a shell, are protected from environing actions,
-lose their bilateralness; while the external parts, subjected by the
-movements of the creatures to bilateral conditions, remain bilateral.
-Here, however, a difficulty meets us. Why is it that the naked
-Gasteropods, such as our common slugs, deviate from bilateral symmetry,
-though their modes of movement are those along with which complete
-bilateral symmetry usually occurs? The reply is that their deviations
-from bilateral symmetry are probably inherited, and that they are
-maintained in such parts of their organization as are not exposed to
-bilaterally-symmetrical conditions. There is reason to believe that the
-naked Gasteropods are descended from Gasteropods which had shells: the
-evidence being that the naked Gasteropods have shells during the early
-stages of their development, and that some of them<span class="pagenum" id="Page_203">[203]</span> retain rudimentary
-shells throughout life. Now the shelled Gasteropods deviate from
-bilateral symmetry in the disposition of both the alimentary system
-and the reproductive system. The naked Gasteropods, in losing their
-shells, have lost that immense one-sided development of the alimentary
-system which fitted them to their shells, and have acquired that
-bilateral symmetry of external figure which fits them for their habits
-of locomotion; but the reproductive system remains one-sided, because,
-in respect to it, the relations to external conditions remain one-sided.</p>
-
-<p>The Cephalopods show us bilaterally-symmetrical external forms along
-with habits of movement through the water in two-sided attitudes. At
-the same time, in the radial distribution of the arms, enabling one
-of these creatures to take an all-sided grasp of its prey, we see how
-readily upon one kind of symmetry there may be partially developed
-another kind of symmetry, where the relations to conditions favour it.</p>
-
-
-<p class="p2">§ 252. The <i>Vertebrata</i> illustrate afresh the truths which we
-have already traced among the <i>Annulosa</i>. Flying through the air,
-swimming through the water, and running over the earth as vertebrate
-animals do, in common with annulose animals, they are, in common with
-annulose animals, different at their anterior and posterior ends,
-different at their dorsal and ventral surfaces, but alike along their
-two sides. This single bilateral symmetry remains constant under the
-extremest modifications of form. Among fish we see it alike in the
-horizontally-flattened Skate, in the vertically-flattened Bream, in
-the almost-spherical <i>Diodon</i>, and in the greatly-elongated
-<i>Syngnathus</i>. Among reptiles the Turtle, the Snake, and the
-Crocodile all display it. And under the countless modifications of
-structure displayed by birds and mammals, it remains conspicuous.</p>
-
-
-  <div class="figcenter" id="i_204">
-     <img
-    class="p2"
-    src="images/i_204.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 273–280.</p>
-  </div>
-
-<p class="p2">A less obvious fact which it concerns us to note among the
-<i>Vertebrata</i>, parallel to one which we noted among the
-<i>Annulosa</i>, is that whereas the lower vertebrate forms deviate<span class="pagenum" id="Page_204">[204]</span>
-but little from triple bilateral symmetry, the deviation becomes great
-as we ascend. Figs. <a href="#i_204">273 and 274</a> show how, besides being divisible into
-similar halves by a vertical plane passing through its axis, a Fish
-is divisible into halves that are not very dissimilar by a horizontal
-plane passing through its axis, and also into other not very dissimilar
-halves by a plane cutting it transversely. If, as shown in Figs. <a href="#i_204">275
-and 276</a>, analogous sections be made of a superior Reptile, the divided
-parts differ more decidedly. When a Mammal and a Bird are treated in
-the same way, as shown in Figs. <a href="#i_204">277, 278</a>, and Figs. <a href="#i_204">279, 280</a>, the parts
-marked off by the dividing planes are unlike in far greater degrees. On
-considering<span class="pagenum" id="Page_205">[205]</span> the mechanical converse between organisms of these several
-types and their environments&mdash;on remembering that the fish habitually
-moves through a homogeneous medium of nearly the same specific gravity
-as itself, that the terrestrial reptile either crawls on the surface or
-raises itself very incompletely above it, that the more active mammal,
-having its supporting parts more fully developed, thereby has the under
-half of its body made more different from the upper half, and that the
-bird is subject by its mode of life to yet another set of actions and
-reactions; we shall see that these facts are quite congruous with the
-general doctrine, and furnish further support to it.</p>
-
-<p>One other significant piece of evidence must be named. Among the
-<i>Annulosa</i> we found unsymmetrical bilateralness in creatures
-having habits exposing them to unlike conditions on their two sides;
-and among the <i>Vertebrata</i> we find parallel cases. They are
-presented by the <i>Pleuronectidæ</i>&mdash;the order of distorted flat
-fishes to which the Sole and the Flounder belong. On the hypothesis of
-evolution, we must conclude that fishes of this order have arisen from
-an ordinary bilaterally-symmetrical type of fish, which, feeding at the
-bottom of the sea, gained some advantage by placing itself with one
-of its sides downwards, instead of maintaining the vertical attitude.
-Besides the general reason there are special reasons for concluding
-this. In the first place, the young Sole or Flounder is bilaterally
-symmetrical&mdash;has its eyes on opposite sides of its head and swims in
-the usual way. In the second place, the metamorphosis which produces
-the unsymmetrical structure sometimes does not take place&mdash;there are
-abnormal Flounders that swim vertically, like other fishes. In the
-third place, the transition from the symmetrical structure to the
-unsymmetrical structure may be traced. Almost incredible though it
-seems, one of the eyes is transferred from the underside of the head to
-the upper side: the transfer being effected by a distorted development
-of the cranial bones&mdash;atrophy of some and hypertrophy of others,<span class="pagenum" id="Page_206">[206]</span>
-along with a general twist. This metamorphosis furnishes several
-remarkable illustrations of the way in which forms become moulded
-into harmony with incident forces. For besides the divergence from
-bilateral symmetry involved by presence of both eyes upon the upper
-side, there is a further divergence from bilateral symmetry involved by
-differentiation of the two sides in respect to the contours of their
-surfaces and the sizes of their fins. And then, what is still more
-significant, there is a near approach to likeness between the halves
-that were originally unlike, but are, under the new circumstances,
-exposed to like conditions. The body is divisible into similarly-shaped
-parts by a plane cutting it along the side from head to tail: “the
-dorsal and ventral instead of the lateral halves become symmetrical in
-outline and are equipoised.”</p>
-
-
-<p class="p2">§ 253. Thus, little as there seems in common between the shapes of
-plants and the shapes of animals, we yet find, on analysis, that the
-same general truths are displayed by both. The one ultimate principle
-that in any organism equal amounts of growth take place in those
-directions in which the incident forces are equal, serves as a key to
-the phenomena of morphological differentiation. By it we are furnished
-with interpretations of those likenesses and unlikenesses of parts,
-which are exhibited in the several kinds of symmetry; and when we take
-into account inherited effects, wrought under ancestral conditions
-contrasted in various ways with present conditions, we are enabled to
-comprehend, in a general way, the actions by which animals have been
-moulded into the shapes they possess.</p>
-
-<p>To fill up the outline of the argument, so as to make it correspond
-throughout with the argument respecting vegetal forms, it would be
-proper here to devote a chapter to the differentiations of those
-homologous segments out of which animals of certain types are composed.
-Though, among most animals of the third degree of composition,
-such as the<span class="pagenum" id="Page_207">[207]</span> rooted <i>Hydrozoa</i>, the <i>Polyzoa</i>, and the
-<i>Ascidioida</i>, the united individuals are not reduced to the
-condition of segments of a composite individual, and do not display
-any marked differentiations; yet there are some animals in which such
-subordinations, and consequent heterogeneities, occur. The oceanic
-<i>Hydrozoa</i> form one group of them; and we have seen reason to
-conclude that the <i>Annulosa</i> form another group. It is not worth
-while, however, to occupy space in detailing these unlikenesses of
-homologous segments, and seeking specific explanations of them.
-Among the oceanic <i>Hydrozoa</i> they are extremely varied; and the
-habits and derivations of these creatures are so little known, that
-there are no adequate data for interpreting the forms of the parts in
-terms of their relations to the environment. Conversely, among the
-<i>Annulosa</i> those differentiations of the homologous segments
-which accompany their progressing integration, have so much in common,
-and have general causes which are so obvious, that it is needless to
-deal with them at any length. They are all explicable as due to the
-exposure of different parts of the chain of segments to different sets
-of actions and reactions: the most general contrast being that between
-the anterior segments and the posterior segments, answering to the most
-general contrast of conditions to which annulose animals subject their
-segments; and the more special contrasts answering to the contrasts of
-conditions entailed by their more special habits.</p>
-
-<p>Were an exhaustive treatment of the subject practicable, there should
-here, also, come a chapter devoted to the internal structures of
-animals&mdash;meaning, more especially, the shapes and arrangements of the
-viscera. The relations between forms and forces among these inclosed
-parts are, however, mostly too obscure to allow of interpretation.
-Protected as the viscera are in great measure from the incidence of
-external forces, we are not likely to find much correspondence between
-their distribution and the distribution of external forces. In this
-case the influences, partly<span class="pagenum" id="Page_208">[208]</span> mechanical, partly physiological, which
-the organs exercise on one another, become the chief causes of their
-changes of figure and arrangement; and these influences are complex and
-indefinite. One general fact may, indeed, be noted&mdash;the fact, namely,
-that the divergence towards asymmetry which generally characterizes
-the viscera, is marked among those of them which are most removed
-from mechanical converse with the environment, but not so marked
-among those of them which are less removed from such converse. Thus
-while, throughout the <i>Vertebrata</i>, the alimentary system, with
-the exception of its two extremities, is asymmetrically arranged, the
-respiratory system, which occupies one end of the body, generally
-deviates but little from bilateral symmetry, and the reproductive
-system, partly occupying the other end of the body, is in the main
-bilaterally symmetrical: such deviation from bilateral symmetry as
-occurs, being found in its most interiorly-placed parts, the ovaries.
-Just indicating these facts as having a certain significance, it will
-be best to leave this part of the subject as too involved for detailed
-treatment.</p>
-
-<p>Internal structures of one class, however, not included among
-the viscera, admit of general interpretation&mdash;structures which,
-though internal, are brought into tolerably-direct relations with
-environing forces, and are therefore subordinate in their forms to
-the distribution of those forces. These internal structures it will
-be desirable to deal with at some length; both because they furnish
-important illustrations enforcing the general argument, and because an
-interpretation of them which we have seen reason to reject, cannot be
-rejected without raising the demand for some other interpretation.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_209">[209]</span></p>
-
-<h3>CHAPTER XV.<br />
-<span class="subhed1">THE SHAPES OF VERTEBRATE SKELETONS.</span></h3></div>
-
-
-<p id="sect254">§ 254. When an elongated mass of any substance is transversely
-strained, different parts of the mass are exposed to forces of opposite
-kinds. If, for example, a bar of metal or wood is supported at its two
-ends, as shown in Fig. <a href="#i_209">281</a>, and has to bear a weight on its centre, its
-lower part is thrown into a state of tension, while its upper part is
-thrown into a state of compression. As will be manifest to any one who
-observes what happens on breaking a stick across his knee, the greatest
-degree of tension falls on the fibres forming the convex surface, while
-the fibres forming the concave surface are subject to the greatest
-degree of compression. Between these extremes the fibres at different
-depths are subject to different forces. Progressing upwards from the
-under surface of the bar shown in Fig. <a href="#i_209">281</a>, the tension of the fibres
-becomes less; and progressing downwards from the upper surface, the
-compression of the fibres becomes less; until, at a certain distance
-between the two surfaces, there is a place at which the fibres are
-neither extended nor compressed. This, shown by the dotted line in<span class="pagenum" id="Page_210">[210]</span> the
-figure, is called in mechanical language the “neutral axis.” It varies
-in position with the nature of the substance strained: being, in common
-pine-wood, at a distance of about five-eighths of the depth from the
-upper surface, or three-eighths from the under surface. Clearly, if
-such a piece of wood, instead of being subject to a downward force, is
-secured at its ends and subject to an upward force, the distribution
-of the compressions and tensions will be reversed, and the neutral
-axis will be nearest to the upper surface. Fig. <a href="#i_210">282</a> represents these
-opposite attitudes of the bar and the changed position of its neutral
-axis: the arrow indicating the direction of the force producing the
-upward bend, and the faint dotted line <i>a</i>, showing the previous
-position of the neutral axis. Between the two neutral axes will be seen
-a central space; and it is obvious that when the bar has its strain
-from time to time reversed, the repeated changes of its molecular
-condition must affect the central space in a way different from that
-in which they affect the two outer spaces. Fig. <a href="#i_211">283</a> is a diagram
-conveying some idea of these contrasts in molecular condition. If A B C
-D be the middle part of a bar thus treated, while G H and K L are the
-alternating neutral axes; then the forces to which the bar is in each
-case subject, may be readily shown. Supposing the deflecting force to
-be acting in the direction of the arrow E, then the tensions to which
-the fibres between G and F are exposed, will be represented by a series
-of lines increasing in length as the distance from G increases; so that
-the triangle G F M, will express the amount and distribution of all
-the molecular tensions. But the molecular compressions throughout the
-space from G to E, must balance the molecular tensions; and hence, if
-the triangle G E N be made equal to the triangle<span class="pagenum" id="Page_211">[211]</span> G F M, the parallel
-lines of which it is composed (here dotted for the sake of distinction)
-will express the amount and distribution of the compressions between
-E and G. Similarly, when the deflecting force is in the direction of
-the arrow F, the compressions and tensions will be quantitatively
-symbolized by the triangles K F O, and K E P. And thus the several
-spaces occupied by full lines and by dotted lines and by the two
-together, will represent the different actions to which different
-parts of the transverse section are subject by alternating transverse
-strains. Here, then, it is made manifest to the eye that the central
-space between G and K, is differently conditioned from the spaces above
-and below it; and that the difference of condition is sharply marked
-off. The fibres forming the outer surface C D, are subject to violent
-tensions and violent compressions. Progressing inwards the tensions and
-compressions decrease&mdash;the tensions the more rapidly. As we approach
-the point G, the tensions to which the fibres are alternately subject,
-bear smaller and smaller ratios to the compressions, and disappear
-at the point G. Thence to the centre occur compressions only, of
-alternating intensities, becoming at the centre small<span class="pagenum" id="Page_212">[212]</span> and equal; and
-from the centre we advance, through a reverse series of changes, to the
-other side.</p>
-
-  <div class="figcenter" id="i_209">
-     <img
-    class="p2"
-    src="images/i_209.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 281.</p>
-  </div>
-
-  <div class="figcenter" id="i_210">
-     <img
-    class="p2"
-    src="images/i_210.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 282.</p>
-  </div>
-
-  <div class="figcenter" id="i_211">
-     <img
-    class="p2"
-    src="images/i_211.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 283.</p>
-  </div>
-
-<p class="p2">Thus it is demonstrable that any substance in which the power of
-resisting compression is unequal to the power of resisting tension,
-cannot be subject to alternating transverse strains, without having
-a central portion differentiated in its conditions from the outer
-portions, and consequently differentiated in its structure. This
-conclusion may easily be verified by experiment. If something having
-a certain toughness but not difficult to break, as a thick piece of
-sheet lead, be bent from side to side till it is broken, the surface of
-fracture will exhibit an unlikeness of texture between the inner and
-outer parts.</p>
-
-
-<p class="p2" id="sect255">§ 255. And now for the application of this seemingly-irrelevant
-truth. Though it has no obvious connection with the interpretation of
-vertebral structure, we shall soon see that it fundamentally concerns
-us.</p>
-
-  <div class="figcenter" id="i_212">
-     <img
-    class="p2"
-    src="images/i_212.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 284.</p>
-  </div>
-
-<p class="p2">The simplest type of vertebrate animal, the fish, has a mode of
-locomotion which involves alternating transverse strains. It is not,
-indeed, subjected to alternating transverse strains by some outer
-agency, as in the case we have been investigating: it subjects itself
-to them. But though the strains are here internally produced instead
-of externally produced, the case is not therefore removed into a
-wholly different category. For supposing Fig. <a href="#i_212">284</a> to represent the
-outline of a fish when bent on one side (the dotted lines representing
-its outline when the bend is reversed), it is clear that part of the
-substance forming the convex half must be in a state of tension. This
-state of tension implies the existence in the other half of some
-counter-balancing compression. And between the two there must be a
-neutral axis. The way in which this conclusion is reconcilable with the
-fact that there is tension somewhere in the concave side of a fish,
-since the curve is<span class="pagenum" id="Page_213">[213]</span> caused by muscular contractions on the concave
-side, will be made clear by the rude illustration which a bow supplies.
-A bow may be bent by a thrust against its middle (the two ends being
-held back), or it may be bent by contracting a string that unites its
-ends; but the distributions of mechanical forces within the wood of the
-bow, though not quite alike in the two cases, will be very similar.
-Now while the muscular action on the concave side of a fish differs
-from that represented by the tightened string of a bow, the difference
-is not such as to destroy the applicability of the illustration: the
-parallel holds so far as this, that within that portion of the fish’s
-body which is passively bent by the contracting muscles, there must be,
-as in a strung bow, a part in compression, a part in tension, and an
-intermediate part which is neutral.</p>
-
-<p>After thus seeing that even in the developed fish with its complex
-locomotive apparatus, this law of the transverse strain holds in a
-qualified way, we shall understand how much more it must hold in any
-form that may be supposed to initiate the vertebrate type&mdash;a form
-devoid of that segmentation by which the vertebrate type is more or
-less characterized. We shall see that assuming a rudimentary animal,
-still simpler than the <i>Amphioxus</i>, to have a feeble power of
-moving itself through the water by the undulations of its body, or
-some part of its body, there will necessarily come into play certain
-reactions which must affect the median portion of the undulating mass
-in a way unlike that in which they affect its lateral portions. And if
-there exists in this median portion a tissue which keeps its place with
-any constancy, we may expect that the differential conditions produced
-in it by the transverse strain, will initiate a differentiation. It
-is true that the distribution of the viscera in the <i>Amphioxus</i>,
-Fig. <a href="#i_214">191</a>, and in the type from which we may suppose it to have arisen,
-is such as to interfere with this process. It is also true that the
-actions and reactions described would not of themselves give to the
-median portion<span class="pagenum" id="Page_214">[214]</span> a cylindrical shape, like that of the cartilaginous rod
-running along the back of the <i>Amphioxus</i>. But what we have here
-to note in the first place is, that these habitual alternate flexions
-have a tendency to mark off from the outer parts an unlike inner part,
-which may be seized hold of, maintained, and further modified, by
-natural selection, should any advantage thereby result. And we have to
-note in the second place, that an advantage <i>is</i> likely to result.
-The contractions cannot be effective in producing undulations, unless
-the general shape of the body is maintained. External muscular fibres
-unopposed by an internal resistant mass, would cause collapse of the
-body. To meet the requirements there must be a means of maintaining
-longitudinal rigidity without preventing bends from side to side;
-and such a means is presented by a structure initiated as described.
-In brief, whether we have or have not the actual cause, we have here
-at any rate “a true cause.” Though there are difficulties in tracing
-out the process in a definite way, it may at least be said that the
-mechanical genesis of this rudimentary vertebrate axis is quite
-conceivable. And even the difficulties may, I think, be more fully met
-than at first sight seems possible.</p>
-
-  <div class="figcenter" id="i_214">
-     <img
-    class="p2"
-    src="images/i_214.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 191.</p>
-  </div>
-
-<p class="p2">What is to be said of the other leading trait which the simplest
-vertebrate animal has in common with all higher vertebrate animals&mdash;the
-segmentation of its lateral muscular masses? Is this, too, explicable
-on the mechanical hypothesis? Have we, in the alternating transverse
-strains, a cause for the fact that while the rudimentary vertebrate
-axis<span class="pagenum" id="Page_215">[215]</span> is without any divisions, there are definite divisions of the
-substance forming the animal’s sides? I think we have. A glance at the
-distribution of forces under the transverse strain, as represented in
-the foregoing diagrams, will show how much more severe is the strain
-on the outer parts than on the inner parts; and how, consequently,
-any modifications of structure eventually necessitated, will arise
-peripherally before they arise centrally. The perception of this may be
-enforced by a simple experiment. Take a stick of sealing-wax and warm
-it slowly and moderately before the fire, so as to give it a little
-flexibility. Then bend it gently until it is curved into a semi-circle.
-On the convex surface small cracks will be seen, and on the concave
-surface wrinkles; while between the two the substance remains
-undistorted. If the bend be reversed and re-reversed, time after time,
-these cracks and wrinkles will become fissures which gradually deepen.
-But now, if changes of this class, entailed by alternating transverse
-strains, commence superficially, as they manifestly must; there arise
-the further questions&mdash;What will be the special modifications produced
-under these special conditions? and through what stages will these
-modifications progress? Every one has literally at hand an example
-of the way in which a flexible external layer that is now extended
-and now compressed, by the bending of the mass it covers, becomes
-creased; and a glance at the palms and the fingers will show that the
-creases are near one another where the skin is thin, and far apart
-where the skin is thick. Between this familiar case and the case of
-the rhinoceros-hide, in which there are but a few large folds, various
-gradations may be traced. Now the like must happen with the increasing
-layers of contractile fibres forming the sides of the muscular tunic
-in such a type as that supposed. The bendings will produce in them
-small wrinkles while they are thin, but more decided and comparatively
-distant fissures as they become thick. Fig. <a href="#i_216">289</a>, which is a horizontal
-longitudinal section, shows how these thickening layers will<span class="pagenum" id="Page_216">[216]</span> adjust
-themselves on the convex and the concave surfaces, supposing the fibres
-of which they are composed to be oblique, as their function requires;
-and it is not difficult to see that when once definite divisions have
-been established, they will advance inwards as the layers develop;
-and will so produce a series of muscular bundles. Here then we have
-something like the <i>myocommata</i> [or myotomes as now called] which
-are traceable in the <i>Amphioxus</i>, and are conspicuous in all
-superior fishes.</p>
-
-  <div class="figcenter" id="i_216">
-     <img
-    class="p2"
-    src="images/i_216.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 289.</p>
-  </div>
-
-
-<p class="p2" id="sect256">§ 256. These are highly speculative conceptions. I have ventured to
-present them with the view of implying that the hypothesis of the
-mechanical genesis of vertebrate structure is not wholly at fault when
-applied to the most rudimentary vertebrate animal. Lest it should be
-alleged that the question is begged if we set out with a type which,
-like the <i>Amphioxus</i>, already displays segmentation throughout
-its muscular system, it seemed needful to indicate conceivable modes
-in which there may have been mechanically produced those leading
-traits that distinguish the <i>Amphioxus</i>. All I intend to suggest
-is that mechanical actions have been at work, and that probably they
-have operated in the manner alleged: so preparing the way for natural
-selection.</p>
-
-<p>But now let us return to the region of established fact, and consider
-whether such actions and reactions as we actually witness, are adequate
-causes of those observed differentiations and integrations which
-distinguish the more-developed vertebrate animals. Let us see whether
-the theory of mechanical genesis affords us a deductive interpretation
-of the inductive generalizations.</p>
-
-<p>Before proceeding, we must note a process of functional adaptation
-which here co-operates with natural selection. I refer to the usual
-formation of denser tissues at those parts of an organism which are
-exposed to the greatest<span class="pagenum" id="Page_217">[217]</span> strains&mdash;either compressions or tensions.
-Instances of hardening under compression are made familiar to us by
-the skin. We have the general contrast between the soft skin covering
-the body at large, and the indurated skin covering the inner surfaces
-of the hands and the soles of the feet. We have the fact that even
-within these areas the parts on which the pressure is habitually
-greatest have the skin always thickest; and that in each person special
-points exposed to special pressures become specially dense&mdash;often as
-dense as horn. Further, we have the converse fact that the skin of
-little-used hands becomes abnormally thin&mdash;even losing, in places, that
-ribbed structure which distinguishes skin subject to rough usage. Of
-increased density directly following increased tension, the skeletons,
-whether of men or animals, furnish abundant evidence. Anatomists easily
-discriminate between the bones of a strong man and those of a weak
-man, by the greater development of those ridges and crests to which
-the muscles are attached; and naturalists, on comparing the remains of
-domesticated animals with those of wild animals of the same species,
-find kindred differences. The first of these facts shows unmistakably
-the immediate effect of function on structure, and by obvious alliance
-with it the second may be held to do the same: both implying that the
-deposit of dense substance capable of great resistance, constantly
-takes place at points where the tension is excessive.</p>
-
-<p>Taking into account, then, this adaptive process, continually aided
-by the survival of individuals in which it has taken place most
-rapidly, we may expect, on tracing up the evolution of the vertebrate
-axis, to find that as the muscular power becomes greater there arise
-larger and harder masses of tissue, serving the muscles as <i>points
-d’appui</i>; and that these arise first in those places where the
-strains are greatest. Now this is just what we <i>do</i> find. The
-<i>myocommata</i> are so placed that their actions are likely to
-affect first that upper coat of the notochord, where there are found<span class="pagenum" id="Page_218">[218]</span>
-“quadrate masses of somewhat denser tissue,” which “seem faintly to
-represent neural spines,” even in the <i>Amphioxus</i>. It is by the
-development of the neural spines, and after them of the hæmal spines,
-that the segments of the vertebral column are first marked out; and
-under the increasing strains of more-developed <i>myocommata</i>,
-it is just these peripheral appendages of the vertebral segments
-that must be most subject to the forces which cause the formation of
-denser tissue. It follows from the mechanical hypothesis that as the
-muscular segmentation must begin externally and progress inwards, so,
-too, must the vertebral segmentation. Besides thus finding reason
-for the fact that in fishes with wholly cartilaginous skeletons, the
-vertebral segments are indicated by these processes, while yet the
-notochord is unsegmented; we find a like reason for the fact that
-the transition from the less-dense cartilaginous skeleton to the
-more-dense osseous skeleton, pursues a parallel course. In the existing
-<i>Lepidosiren</i>, which by uniting certain piscine and amphibian
-characters betrays its close alliance with primitive types, the axial
-part of the vertebral column is unossified, while there is ossification
-of the peripheral parts. Similarly with numerous genera of fishes
-classed as palæozoic. The fossil remains of them show that while the
-neural and hæmal spines consisted of bone, the central parts of the
-vertebræ were not bony. It may in some cases be noted, too, both in
-extant and in fossil forms, that while the ossification is complete at
-the outer extremities of the spines it is incomplete at their inner
-extremities&mdash;thus similarly implying centripetal development.</p>
-
-
-<p class="p2" id="sect257">§ 257. After these explanations the process of eventual segmentation
-in the spinal axis itself, will be readily understood. The original
-cartilaginous rod has to maintain longitudinal rigidity while
-permitting lateral flexion. As fast as it becomes definitely marked
-out, it will begin to concentrate within itself a great part of those
-pressures and tensions<span class="pagenum" id="Page_219">[219]</span> caused by transverse strains. As already said,
-it must be acted upon much in the same manner as a bow, though it is
-bent by forces acting in a more indirect way; and like a bow, it must,
-at each bend, have the substance of its convex side extended and the
-substance of its concave side compressed. So long as the vertebrate
-animal is small or inert, such a cartilaginous rod may have sufficient
-strength to withstand the muscular strains; but, other things equal,
-the evolution of an animal that is large, or active, or both, implies
-muscular strains which must tend to cause modification in such a
-cartilaginous rod. The results of greater bulk and of greater vivacity
-may be best dealt with separately. As the animal increases in size,
-the rod will grow both longer and thicker. On looking back at the
-diagrams of forces caused by transverse strains, it will be seen that
-as the rod grows thicker, its outer parts must be exposed to more
-severe tensions and pressures if the degree of bend is the same. It is
-doubtless true that when the fish, advancing by lateral undulations,
-becomes longer, the curvature assumed by the body at each movement
-becomes less; and that from this cause the outer parts of the notochord
-are, other things equal, less strained&mdash;the two changes thus partially
-neutralizing one another. But other things are <i>not</i> equal. For
-while, supposing the shape of the body to remain constant, the force
-exerted in moving the body increases as the cubes of its dimensions,
-the sectional area of the notochord, on which fall the reactions of
-this exerted force, increases only as the squares of the dimensions:
-whence results a greater stress upon its substance. This, however,
-will not be very decided where there is no considerable activity. It
-is clear that augmenting bulk, taken alone, involves but a moderate
-residuary increase of strain on each portion of the notochord; and this
-is probably the reason why it is possible for a large <i>sluggish</i>
-fish like the Sturgeon, to retain the notochordal structure. But now,
-passing to the effects of greater activity, a like dynamical inquiry at
-once shows us how rapidly<span class="pagenum" id="Page_220">[220]</span> the violence of the actions and reactions
-rises as the movements become more vivacious. In the first place, the
-resistance of a medium such as water increases as the square of the
-velocity of the body moving through it; so that to <i>maintain</i>
-double the speed, a fish has to expend four times the energy. But the
-fish has to do more than this&mdash;it has to <i>initiate</i> this speed, or
-to impress on its mass the force implied by this speed. Now the <i>vis
-viva</i> of a moving body varies as the square of the velocity; whence
-it follows that the energy required to generate that <i>vis viva</i> is
-measured by the square of the velocity it produces. Consequently, did
-the fish put itself in motion <i>instantaneously</i>, the expenditure
-of energy in generating its own <i>vis viva</i> and simultaneously
-overcoming the resistance of the water, would vary as the fourth
-power of the velocity. But the fish cannot put itself in motion
-instantaneously&mdash;it must do it by increments; and thus it results that
-the amounts of the forces expended to give itself different velocities
-must be represented by some series of numbers falling between the
-squares and the fourth powers of those velocities. Were the increments
-slowly accumulated, the ratios of increasing effort would but little
-exceed the ratios of the squares; but whoever observes the sudden,
-convulsive action with which an alarmed fish darts out of a shallow
-into deep water, will see that the velocity is rapidly generated,
-and that therefore the ratios of increasing effort probably exceed
-the ratios of the squares very considerably. At any rate it will be
-clear that the efforts made by fishes in rushing upon prey or escaping
-enemies (and it is these extreme efforts which here concern us) must,
-as fishes become more active, rapidly exalt the strains to be borne
-by their motor organs; and that of these strains, those which fall
-upon the notochord must be exalted in proportion to the rest. Thus the
-development of locomotive power, which survival of the fittest must
-tend in most cases to favour, involves such increase of stress on the
-primitive cartilaginous rod as will tend, other things equal, to cause
-its modification.</p>
-
-<p><span class="pagenum" id="Page_221">[221]</span></p>
-
-  <div class="figcenter" id="i_221">
-     <img
-    class="p2"
-    src="images/i_221.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 291–293.</p>
-  </div>
-
-<p class="p2">What must its modification be? Considering the complication of the
-influences at work, conspiring, as above indicated, in various ways and
-degrees, we cannot expect to do more than form an idea of its average
-character. The nature of the changes which the notochord is likely
-to undergo, where greater bulk is accompanied by higher activity, is
-rudely indicated by Figs. <a href="#i_221">291, 292, and 293</a>. The successively thicker
-lines represent the successively greater strains to which the outer
-layers of tissue are exposed; and the widening interspaces represent
-the greater extensions which they have to bear when they become convex,
-or else the greater gaps that must be formed in them. Had these outer
-layers to undergo extension only, as on the convex side, continued
-natural selection might result in the formation of a tissue elastic
-enough to admit of the requisite stretching. But at each alternate
-bend these outer layers, becoming concave, are subject to increased
-compression&mdash;a compression which they cannot withstand if they have
-become simply more extensible. To withstand this greater compression
-they must become harder as well as more extensible. How are these two
-requirements to be reconciled? If, as facts warrant us in supposing, a
-formation of denser substance occurs at those parts of the notochord
-where the strain is greatest; it is clear that this formation cannot
-so go on as to produce a continuous mass: the perpetual flexions must
-prevent this. If matter that will not yield at each bend, is deposited
-while the bendings are continually taking place, the bendings will
-maintain certain places of discontinuity in the deposit&mdash;places at
-which the whole of the stretching consequent on each bend will be
-concentrated. And thus the tendency will be to form segments of hard
-tissue capable of great resistance<span class="pagenum" id="Page_222">[222]</span> to compression, with intervals
-filled by elastic tissue capable of great resistance to extension&mdash;a
-vertebral column.</p>
-
-<p>And now observe how the progress of ossification is just such as
-conforms to this view. That centripetal development of segments which
-holds of the vertebrate animal as a whole, as, if caused by transverse
-strains, it ought to do, and which holds of the vertebral column as
-a whole, as it ought to do, holds also of the central axis. On the
-mechanical hypothesis, the outer surface of the notochord should be
-the first part to undergo induration, and that division into segments
-which must accompany induration. And accordingly, in a vertebral column
-of which the axis is beginning to ossify, the centrums consist of bony
-rings inclosing a still-continuous rod of cartilage.</p>
-
-
-<p class="p2">§ 258. Sundry other general facts disclosed by the comparative
-morphology of the <i>Vertebrata</i>, supply further confirmation. Let
-us take first the structure of the skull.</p>
-
-<p>On considering the arrangement of the muscular flakes, or
-<i>myocommata</i>, in any ordinary fish which comes to table&mdash;an
-arrangement already sketched out in the <i>Amphioxus</i>&mdash;it is not
-difficult to see that that portion of the body out of which the head
-of the vertebrate animal becomes developed, is a portion which cannot
-subject itself to bendings in the same degree as the rest of the
-body. The muscles developed there must be comparatively short, and
-much interfered with by the pre-existing orifices. Hence the cephalic
-part will not partake in any considerable degree of the lateral
-undulations; and there will not tend to arise in it any such distinct
-segmentation as arises elsewhere. We have here, then, an explanation of
-the fact, that from the beginning the development of the head follows
-a course unlike that of the spinal column; and of the fact that the
-segmentation, so far as it can be traced in the head, is most readily
-to be traced in the occipital region and becomes lost in the region
-of the face. For if, as we have seen, the segmentation consequent on<span class="pagenum" id="Page_223">[223]</span>
-mechanical actions and reactions must progress from without inwards,
-affecting last of all the axis; and if, as we have seen, the region
-of the head is so circumstanced that the causes of segmentation act
-but feebly even on its periphery; then that terminal portion of the
-primitive notochord which is included in the head, having to undergo no
-lateral bendings, may ossify without division into segments.</p>
-
-<p>Of other incidental evidences supplied by comparative morphology, let
-me next refer to the supernumerary bones, which the theory of Goethe
-and Oken as elaborated by Prof. Owen, has to get rid of by gratuitous
-suppositions. In many fishes, for example, there are what have been
-called interneural spines and interhæmal spines. These cannot by any
-ingenuity be affiliated upon the archetypal vertebra, and they are
-therefore arbitrarily rejected as bones belonging to the exo-skeleton;
-though in shape and texture they are similar to the spines between
-which they are placed. On the hypothesis of evolution, however, these
-additional bones are accounted for as arising under actions like those
-that gave origin to the bones adjacent to them. And similarly with such
-bones as those called sesamoid; together with others too numerous to
-name.</p>
-
-
-<p class="p2">§ 259. Of course the foregoing synthesis is to be taken simply as an
-adumbration of the process by which the vertebrate structure may have
-arisen through the continued actions of known agencies. The motive
-for attempting it has been two-fold. Having, as before said, given
-reasons for concluding that the segments of a vertebrate animal are not
-homologous in the same sense as are those of an annulose animal, it
-seemed needful to do something towards showing how they are otherwise
-to be accounted for; and having here, for our general subject, the
-likenesses and differences among the parts of organisms, as determined
-by incident forces, it seemed out of the question to pass by the
-problem presented by the vertebrate skeleton.</p>
-
-<p><span class="pagenum" id="Page_224">[224]</span></p>
-
-<p>Leaving out all that is hypothetical, the general argument may be
-briefly presented thus:&mdash;The evolution from the simplest known
-vertebrate animal of a powerful and active vertebrate animal, implies
-the development of a stronger internal fulcrum. The internal fulcrum
-cannot be made stronger without becoming more dense. And it cannot
-become more dense while retaining its lateral flexibility, without
-becoming divided into segments. Further, in conformity with the general
-principles thus far traced, these segments must be alike in proportion
-as the forces to which they are exposed are alike, and unlike in
-proportion as these forces are unlike; and so there necessarily results
-that unity in variety by which the vertebral column is from the
-beginning characterized. Once more, we see that the explanation extends
-to those innumerable and more marked divergences from homogeneity,
-which vertebræ undergo in the various higher animals. Thus, the
-production of vertebræ, the production of likenesses among vertebræ,
-the production of unlikenesses among vertebræ, and the production of
-unlikenesses among vertebral columns, are interpretable as parts of one
-general process, and as harmonizing with one general principle.</p>
-
-<p>Whether sufficient or insufficient, the explanation here given assigns
-causes of known kinds producing effects such as they are known to
-produce. It does not, as a solution of one mystery, offer another
-mystery of which no solution is to be asked. It does not allege a
-Platonic ἰδέα, or fictitious entity, which explains the vertebrate
-skeleton by absorbing into itself all the inexplicability. On the
-contrary, it assumes nothing beyond agencies by which structures in
-general are moulded&mdash;agencies by which these particular structures
-are, indeed, notoriously modifiable. An ascertained cause of certain
-traits in vertebræ and other bones, it extends to all other traits of
-vertebræ; and at the same time assimilates the morphological phenomena
-they present to much wider classes of morphological phenomena.</p>
-
-<p><span class="pagenum" id="Page_225">[225]</span></p>
-
-<hr class="tb" />
-
-<p>[<span class="smcap">Note.</span>&mdash;The theory set forth in the foregoing chapter,
-is an elaboration of one suggested at the close of a criticism
-of Prof. Owen’s <i>Archetype and Homologies of the Vertebrate
-Skeleton</i>, already referred to in <a href="#sect210">§ 210</a> as having been published
-in the <i>Medico-Chirurgical Review</i> for October, 1858. It is
-now reproduced in Appendix B. Since the issue of this elaborated
-exposition, in No. 15 of my serial in December, 1865, verifications of
-it have from time to time been published. In his work <i>The Primary
-Factors of Organic Evolution</i>, Prof. Cope of Philadelphia writes:&mdash;</p>
-
-<p>“Mr. Herbert Spencer has endeavoured to account for the origin of the
-segmentation of muscles into myotomes, and the division of the sheath
-of the notochord into vertebræ, by supposing it to be due to the
-lateral swimming movements of the fishes, which first exhibit these
-structures. With this view various later authors have agreed, and I
-have offered some additional evidence of the soundness of this position
-with respect to the vertebral axis of Batrachia, and the origin of
-limb articulations. It is true that the origin of segmentation in the
-vertebral column of the true fishes and the Batrachia turns out to have
-been less simple in its process than was suggested by Mr. Spencer, but
-his general principle holds good, now that paleontology has cleared up
-the subject” (pp. 367–8).</p>
-
-<p>An allusion in the foregoing extract is made by Prof. Cope to certain
-observations set forth in his work entitled <i>The Origin of the
-Fittest</i>. On pp. 305–6 of it will be found the following sentences:&mdash;</p>
-
-<p>“Now, all the Permian land-animals, reptiles and batrachians, retain
-this notochord with the elements of osseous vertebræ, in a greater or
-less degree of completeness. There are some in South Africa, I believe,
-in which the ossification has come clear through the notochord; but
-they are few.... There is something to be said as to the condition
-of the column from a mechanical standpoint, and it is this: that the
-chorda exists, with its osseous elements disposed<span class="pagenum" id="Page_226">[226]</span> about it; and in the
-Permian batrachians, equally related to salamanders and frogs, these
-osseous elements are arranged in the sheath or skin of the chorda;
-and they are in the form of regular concave segments, very much like
-such segments as you can take from the skin of an orange&mdash;but parts
-of a cylinder, and having greater or less dimensions according to the
-group or species. Now, the point of divergence of these segments is
-on the side of the column. The contacts are placed on the side of the
-column where the segments separate&mdash;the upper segments rising and the
-lower segments coming downward. To the upper segments are attached the
-arches and their articulations, and the lower segments are like the
-segments of a cylinder. If you take a flexible cylinder, and cover it
-with a more or less inflexible skin or sheath, and bend that cylinder
-sidewise, you of course will find that the wrinkles or fractures of
-that part of the surface will take place along the line of the shortest
-curve, which is on the side; and, as a matter of fact, you have breaks
-of very much the character of the segments of the Permian Batrachia....
-In the cylinder bending both ways, of course the shortest line of
-curve is right at the centre of the side of that cylinder, and the
-longest curve is of course at the summit and base, and the shortest
-curve will be the point of fracture. And that is exactly what I presume
-has happened in the case of the construction of the segments of the
-sheath of the vertebral column, by the lateral motion of the animal in
-swimming, and which has been the actual cause of the disposition of the
-osseous material in its form.... That is the state of the vertebral
-column of many of the Vertebrata of the Permian period.”</p>
-
-<p>In his essay on “The Mechanical Causes of the Development of the
-Hard Parts of the Mammalia,” published in the American <i>Journal of
-Morphology</i> (Vol. III), Prof. Cope has carried the interpretation
-further, by showing that in kindred ways the genesis of articulations
-and limb-bones may be explained.<span class="pagenum" id="Page_227">[227]</span> On p. 163 he enunciates the general
-principle of his interpretation as follows:&mdash;</p>
-
-<p>“It cannot have been otherwise than that, since the motions of animals
-continued during the evolution of their hard parts, these hard parts
-grew in exact adaptation to these movements. Thus at the points of
-greatest flexure joints would be formed, and between these joints the
-deposit would be continuous.”</p>
-
-<p>Evidently if osseous structures are produced by deposits of calcareous
-matters in pre-existing cartilaginous structures, or other structures
-of flexible materials, the deposits must be so carried on that while
-dense resistant masses are produced these must admit of such free
-movements as the creature’s life necessitates, and must so form adapted
-joints.</p>
-
-<p>Let it be understood, however, that the hypothesis set forth in
-the foregoing chapter and extended by Prof. Cope, which serves to
-interpret a large part of the phenomena of osseous structures in
-the <i>Vertebrata</i>, does not serve to interpret them all. While
-the formation of hard parts has been in large measure initiated and
-regulated by tensions and pressures, there are hard parts the formation
-of which cannot be thus explained. The bones of the skull are the
-most obvious instances. These are apparently referable to no other
-cause than the survival of the fittest&mdash;the survival of individual
-animals in which greater density of the brain-covering yielded better
-protection against external injuries. Without enumerating other
-instances which might be given, it will suffice to recognize the truth
-that natural selection of favourable variations and the inheritance of
-functionally-produced changes have all along co-operated: each of them
-in some cases acting alone, but in other cases both acting together.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_228">[228]</span></p>
-
-<h3>CHAPTER XVI.<br />
-<span class="subhed1">THE SHAPES OF ANIMAL CELLS.</span></h3></div>
-
-
-<p>§ 260. Among animals as among plants, the laws of morphological
-differentiation must be conformed to by the morphological units, as
-well as by the larger parts and by the wholes formed of them. It
-remains here to point out that the conformity is traceable where the
-conditions are simple.</p>
-
-  <div class="figcenter" id="i_228">
-     <img
-    class="p2"
-    src="images/i_228.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 294.</p>
-  </div>
-
-<p class="p2">In the shapes assumed by those rapidly-multiplying cells out of which
-each animal is developed, there is a conspicuous subordination to
-the surrounding actions. Fig. <a href="#i_228">294</a> represents the cellular embryonic
-mass that arises by repeated spontaneous fissions. In it we see how
-the cells, originally spherical, are changed by pressure against one
-another and against the limiting membrane; and how their likenesses
-and unlikenesses are determined by the likenesses and unlikenesses
-of the forces to which they are exposed. This fact may be thought
-scarcely worth pointing out. But it is worth pointing out, because what
-is here so obvious a consequence of mechanical actions, is in other
-cases a consequence of actions composite in their kinds and involved
-in their distribution. Just as the equalities and inequalities of
-dimensions among aggregated cells, are here caused by the equalities
-and inequalities among their mutual pressures in different directions;
-so, though less manifestly, the equalities<span class="pagenum" id="Page_229">[229]</span> and inequalities of
-dimensions among other aggregated cells, are caused by the equalities
-and inequalities of the osmotic, chemical, thermal, and other forces
-besides the mechanical, to which their different positions subject them.</p>
-
-
-<p class="p2">§ 261. This we shall readily see on observing the ordinary structures
-of limiting membranes, internal and external. In Fig. <a href="#i_229a">295</a>, is shown
-a much-magnified section of a papilla from the gum. The cells of
-which it is composed originate in its deeper part; and are at first
-approximately spherical. Those of them which, as they develop, are
-thrust outwards by the new cells that continually take their places,
-have their shapes gradually changed. As they grow and successively
-advance to replace the superficial cells, when these exfoliate, they
-become exposed to forces which are more and more different in the
-direction of the surface from what they are in lateral directions; and
-their dimensions gradually assume corresponding differences.</p>
-
-  <div class="figcenter" id="i_229a">
-     <img
-    class="p2"
-    src="images/i_229a.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 295.</p>
-  </div>
-
-  <div class="figcenter" id="i_229b">
-     <img
-    class="p2"
-    src="images/i_229b.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 296.</p>
-  </div>
-
-<p class="p2">Another species of limiting membrane, called cylinder-epithelium,
-is represented in Fig. <a href="#i_229b">296</a>. Though its mode of development is
-such as to render the shapes of its cells quite unlike those of
-pavement-epithelium, as the above-described kind is sometimes called,
-its cells equally exemplify the same general truth. For the chief
-contrast which each of them presents, is the contrast between its
-dimension at right angles to the surface of the membrane, and its
-dimension parallel to that surface.</p>
-
-<p>It is needless for our present purpose to examine further<span class="pagenum" id="Page_230">[230]</span> the evidence
-furnished by Histology; nor, indeed, would further examination of this
-evidence be likely to yield definite results. In the cases given above
-we have marked differences among the incident forces; and therefore
-have a chance of finding, as we do find, relations between these and
-differences of form. But the cells composing masses of tissue are
-severally subject to forces which are indeterminate; and therefore
-the interpretation of their shapes is impracticable. It must suffice
-to observe that so far as the facts go they are congruous with the
-hypothesis.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_231">[231]</span></p>
-
-<h3>CHAPTER XVII.<br />
-<span class="subhed1">SUMMARY OF MORPHOLOGICAL DEVELOPMENT.</span></h3></div>
-
-
-<p class="p2">§ 262. That any formula should be capable of expressing a common
-character in the shapes of things so unlike as a tree and a cow, a
-flower and a centipede, is a remarkable fact; and is a fact which
-affords strong <i>primâ facie</i> evidence of truth. For in proportion
-to the diversity and multiplicity of the cases to which any statement
-applies, is the probability that it sets forth the essential relations.
-Those connexions which remain constant under all varieties of
-manifestation, are most likely to be the causal connexions.</p>
-
-<p>Still higher will appear the likelihood of an alleged law of organic
-form possessing so great a comprehensiveness, when we remember that on
-the hypothesis of Evolution, there must exist between all organisms
-and their environments, certain congruities expressible in terms of
-their actions and reactions. The forces being, on this hypothesis, the
-causes of the forms, it is inferable, <i>à priori</i>, that the forms
-must admit of generalization in terms of the forces; and hence, such
-a generalization arrived at <i>à posteriori</i>, gains the further
-probability due to fulfilment of anticipation.</p>
-
-<p>Nearer yet to certainty seems the conclusion thus reached, on finding
-that it does but assert in their special manifestations, the laws of
-Evolution in general&mdash;the laws of that universal re-distribution of
-matter and motion which hold<span class="pagenum" id="Page_232">[232]</span> throughout the totality of things, as
-well as in each of its parts.</p>
-
-<p>It will be useful to glance back over the various minor inferences
-arrived at, and contemplate them in their <i>ensemble</i> from these
-higher points of view.</p>
-
-
-<p class="p2">§ 263. That process of integration which every plant displays during
-its life, we found reason to think has gone on during the life of the
-vegetal kingdom as a whole. Protoplasm into cells, cells into folia,
-folia into axes, axes into branched combinations&mdash;such, in brief, are
-the stages passed through by every shrub; and such appear to have been
-the stages through which plants of successively-higher kinds have been
-evolved from lower kinds. Even among certain groups of plants now
-existing, we find aggregates of the first order passing through various
-gradations into aggregates of the second order&mdash;here forming small,
-incoherent, indefinite assemblages, and there forming large, definite,
-coherent fronds. Similar transitions are traceable through which these
-integrated aggregates of the second order pass into aggregates of
-the third order: in one species the unions of parent-fronds with the
-fronds that bud out from them, being temporary, and in another species
-such unions being longer continued; until, in species still higher,
-by a gemmation which is habitual and regular, there is produced a
-definitely-integrated aggregate of the third order&mdash;an axis bearing
-fronds or leaves. And even between this type and a type further
-compounded, a link occurs in the plants which cast off, in the shape of
-bulbils, some of the young axes they produce. As among plants, so among
-animals. A like spontaneous fission of cells ends here in separation,
-there in partial aggregation, while elsewhere, by closer combination
-of the multiplying units, there arises a coherent and tolerably
-definite individual of the second order. By the budding of individuals
-of the second order, there are in some cases produced other separate
-individuals like them; in some<span class="pagenum" id="Page_233">[233]</span> cases temporary aggregates of such like
-individuals; and in other cases permanent aggregates of them: certain
-of which become so definitely integrated that the individualities of
-their component members are almost lost in a tertiary individuality.</p>
-
-<p>Along with this progressive integration there has gone on a progressive
-differentiation. Vegetal units of whatever order, originally
-homogeneous, have become heterogeneous while they have become united.
-Spherical cells aggregating into threads, into laminæ, into masses, and
-into special tissues, lose their sphericity; and instead of remaining
-all alike assume innumerable unlikenesses&mdash;from uniformity pass
-into multiformity. Fronds combining to form axes, severally acquire
-definite differences between their attached ends and their free ends;
-while they also diverge from one another in their shapes at different
-parts of the axes they compose. And axes, uniting into aggregates of
-a still higher order, become contrasted in their sizes, curvatures,
-and the arrangements of their appendages. Similarly among animals.
-Those components of them which, with a certain license, we class as
-morphological units, while losing their minor individualities in the
-major individualities formed of them, grow definitely unlike as they
-grow definitely combined. And where the aggregates so produced become,
-by coalescence, segments of aggregates of a still higher order, they,
-too, diverge from one another in their shapes.</p>
-
-<p>The morphological differentiation which thus goes hand in hand with
-morphological integration, is clearly what the perpetually-complicating
-conditions would lead us to anticipate. Every addition of a new unit
-to an aggregate of such units, must affect the circumstances of the
-other units in all varieties of ways and degrees, according to their
-relative positions&mdash;must alter the distribution of mechanical strains
-throughout the mass, must modify the process of nutrition, must affect
-the relations of neighbouring parts to surrounding diffused actions;
-that is, must initiate a changed incidence<span class="pagenum" id="Page_234">[234]</span> of forces tending ever to
-produce changed structural arrangements.</p>
-
-
-<p class="p2">§ 264. This broad statement of the correspondence between the general
-facts of Morphological Development and the principles of Evolution
-at large, may be reduced to statements of a much more specific kind.
-The phenomena of symmetry and unsymmetry and asymmetry, which we have
-traced out among organic forms, are demonstrably in harmony with those
-laws of the re-distribution of matter and motion to which Evolution
-conforms. Besides the myriad-fold illustrations of the instability of
-the homogeneous, afforded by these aggregates of units of each order,
-which, at first alike, lapse gradually into unlikeness; and besides
-the myriad-fold illustrations of the multiplication of effects, which
-these ever-complicating differentiations exhibit to us; we have also
-myriad-fold illustrations of the definite equalities and inequalities
-of structures, produced by definite equalities and inequalities of
-forces.</p>
-
-<p>The proposition arrived at when dealing with the causes of Evolution,
-“that in the actions and reactions of force and matter, an unlikeness
-in either of the factors necessitates an unlikeness in the effects; and
-that in the absence of unlikeness in either of the factors the effects
-must be alike” (<i>First Principles</i>, § 169), is a proposition which
-implies all these particular likenesses and unlikenesses of parts
-which we have been tracing. For have we not everywhere seen that the
-strongest contrasts are between the parts that are most contrasted in
-their conditions; while the most similar parts are those most similarly
-conditioned? In every plant the leading difference is between the
-attached end and the free end; in every branch it is the same; in every
-leaf it is the same. And in every plant the leading likenesses are
-those between the two sides of the branch, the two sides of the leaf,
-and the two sides of the flower, where these parts are two-sided in
-their conditions; or between all sides of the<span class="pagenum" id="Page_235">[235]</span> branch, all sides of
-the leaf, and all sides of the flower, where these parts are similarly
-conditioned on all sides. So, too, is it with animals which move about.
-The most marked contrasts they present are those between the part in
-advance and the part behind, and between the upper part and the under
-part; while there is complete correspondence between the two sides.
-Externally the likenesses and differences among limbs, and internally
-the likenesses and differences among vertebræ, are expressible in terms
-of this same law.</p>
-
-<p>And here, indeed, we may see clearly that these truths are corollaries
-from that ultimate truth to which all phenomena of Evolution are
-referable. It is an inevitable deduction from the persistence of force,
-that organic forms which have been progressively evolved, must present
-just those fundamental traits of form which we find them present. It
-cannot but be that during the intercourse between an organism and its
-environment, equal forces acting under equal conditions must produce
-equal effects; for to say otherwise is, by implication, to say that
-some force can produce more or less than its equivalent effect, which
-is to deny the persistence of force. Hence those parts of an organism
-which are, by its habits of life, exposed to like amounts and like
-combinations of actions and reactions, must develop alike; while
-unlikenesses of development must as unavoidably follow unlikenesses
-among these agencies. And this being so, all the specialities of
-symmetry and unsymmetry and asymmetry which we have traced, are
-necessary consequences.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_237">[237]</span></p>
-
-<h2>PART V.<br />
-<span class="subhed">PHYSIOLOGICAL DEVELOPMENT.</span></h2></div>
-
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_239">[239]</span></p>
-
-<h3>CHAPTER I.<br />
-<span class="subhed1">THE PROBLEMS OF PHYSIOLOGY.</span></h3></div>
-
-
-<p>§ 265. The questions to be treated under the above title are widely
-different from those which it ordinarily expresses. We have no
-alternative, however, but to use Physiology in a sense co-extensive
-with that in which we have used Morphology. We must here consider
-the facts of function in a manner parallel to that in which we
-have, in the foregoing Part, considered the facts of structure.
-As, hitherto, we have concerned ourselves with those most general
-phenomena of organic form which, holding irrespective of class and
-order and sub-kingdom, illustrate the processes of integration and
-differentiation characterizing Evolution at large; so, now, we have
-to concern ourselves with the evidences of those differentiations and
-integrations of organic functions which have simultaneously arisen,
-and which similarly transcend the limits of zoological and botanical
-divisions. How heterogeneities of action have progressed along with
-heterogeneities of structure&mdash;that is the inquiry before us; and
-obviously, in pursuing it, all the specialities with which Physiology
-usually deals can serve us only as materials.</p>
-
-<p>Before entering on the study of Morphological Development, it
-was pointed out that while facts of structure may be empirically
-generalized apart from facts of function, they cannot be rationally
-interpreted apart; and throughout the<span class="pagenum" id="Page_240">[240]</span> foregoing pages this truth has
-been made abundantly manifest. Here we are obliged to recognize the
-interdependence still more distinctly; for the phenomena of function
-cannot even be conceived without direct and perpetual consciousness of
-the phenomena of structure. Though the subject-matter of Physiology
-is as broadly distinguished from the subject-matter of Morphology as
-motion is from matter; yet, just as the laws of motion cannot be known
-apart from some matter moved, so there can be no knowledge of function
-without a knowledge of some structure as performing function.</p>
-
-<p>Much more than this is obvious. The study of functions, considered from
-our present point of view as arising by Evolution, must be carried on
-<i>mainly</i> by the study of the correlative structures. Doubtless,
-by experimenting on the organisms which are growing and moving around
-us, we may ascertain the connexions existing among certain of their
-actions, while we have little or no knowledge of the special parts
-concerned in those actions. In a living animal that can be conveniently
-kept under observation, we may learn the way in which conspicuous
-functions vary together&mdash;how the rate of a man’s pulse increases with
-the amount of muscular exertion he is undergoing; or how a horse’s
-rapidity of breathing is in part dependent on his speed. But though
-observations of this order are indispensable&mdash;though by accumulation
-and comparison of such observations we learn which parts perform which
-functions&mdash;though such observations, prosecuted so as to disclose
-the actions of all parts under all circumstances, constitute, when
-properly generalized and co-ordinated, what is commonly understood as
-Physiology; yet such observations help us but a little way towards
-learning how functions came to be established and specialized.
-We have next to no power of tracing up the genesis of a function
-considered purely as a function&mdash;no opportunity of observing the
-progressively-increasing quantities of a given action that have arisen
-in any order of organisms. In nearly all cases we are able only to
-show<span class="pagenum" id="Page_241">[241]</span> the greater growth of the part which we have found performs the
-action, and to infer that greater action of the part has accompanied
-greater growth of it. The tracing out of Physiological Development,
-then, becomes substantially a tracing out of the development of
-the organs by which the functions are known to be discharged&mdash;the
-differentiation and integration of the functions being presumed to have
-progressed hand in hand with the differentiation and integration of the
-organs. Between the inquiry pursued in Part IV, and the inquiry to be
-pursued in this Part, the contrast is that, in the first place, facts
-of structure are now to be used to interpret facts of function, instead
-of conversely; and, in the second place, the facts of structure to be
-so used are not those of conspicuous shape so much as those of minute
-texture and chemical composition.</p>
-
-
-<p class="p2">§ 266. The problems of Physiology, in the wide sense above
-described, are, like the problems of Morphology, to be considered
-as problems to which answers must be given in terms of incident
-forces. On the hypothesis of Evolution these specializations of
-tissues and accompanying concentrations of functions, must, like the
-specializations of shape in an organism and its component divisions,
-be due to the actions and reactions which its intercourse with the
-environment involves; and the task before us is to explain how they are
-wrought&mdash;how they are to be comprehended as results of such actions and
-reactions.</p>
-
-<p>Or, to define these problems still more specifically:&mdash;Those extremely
-unstable substances composing the protoplasm of which organisms are
-mainly built, have to be traced through the various modifications in
-their properties and powers, that are entailed on them by changes of
-relation to agencies of all kinds. Those organic colloids which pass
-from liquid to solid and from soluble to insoluble on the slightest
-molecular disturbance&mdash;those albuminoid matters which, as we see in
-clotted blood or the coagulable lymph poured<span class="pagenum" id="Page_242">[242]</span> out on abraded surfaces
-and causing adhesion between inflamed membranes, assume new forms
-with the greatest readiness&mdash;are to have their metamorphoses studied
-in connexion with the influences at work. Those compounds which, as
-we see in the quickly-acquired brownness of a bitten apple or in the
-dark stains produced by the milky juice of a Dandelion, immediately
-begin to alter when the surrounding actions alter, are to be everywhere
-considered as undergoing modifications by modified conditions. Organic
-bodies, consisting of substances that, as I here purposely remind the
-reader, are prone beyond all others to change when the incident forces
-are changed, we must contemplate as in all their parts differently
-changed in response to the different changes of the incident forces.
-And then we have to regard the concomitant differentiations of their
-reactions as being concomitant differentiations of their functions.</p>
-
-<p>Here, as before, we must take into account two classes of factors.
-We have to bear in mind the inherited results of actions to which
-antecedent organisms were exposed, and to join with these the results
-of present actions. Each organism is to be considered as presenting
-a moving equilibrium of functions, and a correlative arrangement
-of structures, produced by the aggregate of actions and reactions
-that have taken place between all ancestral organisms and their
-environments. The tendency in each organism to repeat this adjusted
-arrangement of functions and structures, must be regarded as from time
-to time interfered with by actions to which its inherited equilibrium
-is not adjusted&mdash;actions to which, therefore, its equilibrium has to be
-re-adjusted. And in studying physiological development we have in all
-cases to contemplate the progressing compromise between the old and the
-new, ending in a restored balance or adaptation.</p>
-
-<p>Manifestly our data are so scanty that nothing more than very general
-and approximate interpretations of this kind are possible. If the
-hypothesis of Evolution furnishes us with a rude conception of the
-way in which the<span class="pagenum" id="Page_243">[243]</span> more conspicuous and important differentiations of
-functions have arisen, it is as much as can be expected.</p>
-
-
-<p class="p2">§ 267. It will be best, for brevity and clearness, to deal with these
-physiological problems as we dealt with the morphological ones&mdash;to
-carry on the inductive statement and the deductive interpretation
-hand in hand: so disposing of each general truth before passing
-to the next. Treating separately vegetal organisms and animal
-organisms, we will in each kingdom consider:&mdash;first, the physiological
-differentiations and accompanying changes of structure which arise
-between outer tissues and inner tissues; next, those which arise
-between different parts of the outer tissues; and, finally, those which
-arise between different parts of the inner tissues. What little has
-to be said concerning physiological integration must come last. For
-though, in tracing up Morphological Evolution, we have to study those
-processes of integration by which organic aggregates are formed, before
-studying the differentiations that arise among their parts; we must,
-contrariwise, in tracing up Physiological Evolution, study the genesis
-of the different functions before we study the interdependence that
-eventually arises among them and constitutes physiological unity.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_244">[244]</span></p>
-
-<h3>CHAPTER II.<br />
-<span class="subhed1">DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF PLANTS.</span></h3></div>
-
-
-<p>§ 268. The simplest plant presents a contrast between its peripheral
-substance and its central substance. In each protophyte, be it a
-spherical cell or a branched tube, or such a more-specialized form as a
-Desmid, a marked unlikeness exists between the limiting layer and that
-which it limits. These vegetal aggregates of the first order may differ
-widely from one another in the natures of their outer coats and in the
-natures of their contents. As in the Palmella-form of one of the lower
-<i>Algæ</i>, there may exist a clothing of jelly; or, as in Diatom, the
-walls may take the form of silicious valves variously sculptured. The
-contained matter may be partly or wholly here green, there red, and in
-other cases brown. But amid all these diversities there is this one
-uniformity&mdash;a strong distinction between the parts in contact with the
-environment and the parts not in contact with the environment.</p>
-
-<p>When we remember that this trait is one which these simple living
-bodies have in common with bodies that are not living&mdash;when we
-remember that each inorganic mass eventually has its outer part more
-or less differentiated from its inner part, here by oxidation, there
-by drying, and elsewhere by the actions of light, of moisture, of
-frost; we can scarcely resist the conclusion that, in the one case
-as in the other, the contrast is due to the unlike actions to which
-the<span class="pagenum" id="Page_245">[245]</span> parts are subject. Given an originally-homogeneous portion of
-protoplasm, and it follows from the general laws of Evolution (First
-Principles, §§ 149–155), first, that it must lose its homogeneity, and,
-second, that the leading dissimilarities must arise between the parts
-most-dissimilarly conditioned&mdash;that is, between the outside and the
-inside. The exterior must bear amounts and kinds of force unlike the
-amounts and kinds which the interior bears; and from the persistence
-of force it follows inevitably that unlike effects must be wrought on
-them&mdash;they must be differentiated.</p>
-
-<p>What is the limit towards which the differentiation tends? We have seen
-that the re-distribution of matter and motion whence, under certain
-conditions, evolution results, can never cease until equilibrium is
-reached&mdash;proximately a moving equilibrium, and finally a complete
-equilibrium (<i>First Principles</i>, §§ 170–175). Hence, the
-differentiation must go on until it establishes such differences in the
-parts as shall balance the differences in the forces acting on them.
-When dealing with equilibration in general, we saw that this process
-is what is called adaptation (<i>First Principles</i>, § 173); and, in
-this work, we saw that by it the totality of functions of an organism
-is brought into correspondence with the totality of actions affecting
-it (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect159">§§ 159–163</a>). Manifestly in this case, as in all others, either
-death or adjustment must eventually result. A force falling on one of
-these minute aggregates of protoplasm, must expend itself in working
-its equivalent of change. If this force is such that in expending
-itself it disturbs beyond rectification the balance of the organic
-processes, then the aggregate is disintegrated or decomposed. But if it
-does not overthrow that moving equilibrium constituting the life of the
-aggregate, then the aggregate continues in that modified form produced
-by the expenditure of the force. Thus, by direct equilibration,
-continually furthered by indirect equilibration, there must arise this
-distinction between the outer part adapted to meet outer forces, and
-the inner part adapted to meet inner forces. And<span class="pagenum" id="Page_246">[246]</span> their respective
-actions, as thus meeting outer and inner forces, must be what we call
-their respective functions.</p>
-
-
-<p class="p2">§ 269. Aggregates of the second order exhibit parallel traits,
-admitting of parallel interpretations. Integrated masses of cells or
-units homologous with protophytes, habitually show us contrasts between
-the characters of the superficial tissues and the central tissues. Such
-among these aggregates of the second order as have their component
-units arranged into threads or laminæ, single or double, cannot, of
-course, furnish contrasts of this kind; for all their units are as much
-external as internal. We must turn to the more or less massive forms.</p>
-
-<p>Of these, among <i>Fungi</i>, the common Puff-ball is a good
-example&mdash;good because it presents this fundamental differentiation
-but little complicated by others. In it we have a cortical layer
-of interwoven hyphæ obviously unlike the mass of spores which it
-incloses. So far as the unlikeness between external and internal
-parts is concerned, we see here a relation analogous to that existing
-in the simple cell; and we see in it a similar meaning: there is a
-physiological differentiation corresponding to the difference in the
-incidence of forces.</p>
-
-<p>Under various forms the <i>Algæ</i> show just the same relation.
-Where, as in <i>Codium Bursa</i>, we have the ramified tubular
-branches of the thallus aggregated into a hollow globular mass, the
-outer and inner surfaces are contrasted both in colour and structure,
-though the tubules composing the two surfaces are continuous with one
-another. In <i>Rivularia</i>, again, we see the like, both in the
-radial arrangement of the imbedded threads and in the difference of
-colour between the exterior of the imbedding jelly and its interior.
-The more-developed <i>Algæ</i> of all kinds repeat the antithesis.
-In branched stems, when they consist of more than single rows of
-cells, the outer cells become unlike the inner, as shown in Fig. <a href="#i_029">35</a>.
-Such types as <i>Chrysymenia rosea</i> show us this<span class="pagenum" id="Page_247">[247]</span> unlikeness very
-conspicuously. And it holds even with ribbon-shaped fronds. Wherever
-one of these is composed of three, four, or more layers, as in
-<i>Laminaria</i> and <i>Punctaria</i>, the cells of the external layers
-are strongly distinguished from those of the internal layers, both by
-their comparative smallness and by their deep colour.</p>
-
-
-<p class="p2">§ 270. The higher plants variously display the like fundamental
-distinction between outer and inner tissues. Each leaf, thin as it is,
-exemplifies this differentiation of the parts immediately in contact
-with the environment from the parts not in immediate contact with
-the environment. Its epidermal cells, forming a protecting envelope,
-diverge physically and chemically from the mesophyll cells, which carry
-on the more active functions. And the contrast may be observed to
-establish itself in the course of development. At first the component
-cells of the leaf are all alike; and this unlikeness between the cells
-of the outer and inner layers, arises simultaneously with the rise of
-differences in their conditions&mdash;differences that have acted on all
-ancestral leaves as they act on the individual leaf.</p>
-
-<p>An unlikeness more marked in kind but similar in meaning, exists
-between the bark of every branch and the tissues it clothes. The
-phænogamic axis, especially when it undergoes what is known as
-secondary thickening, is commonly characterized by an outer zone of
-cells (the cork layer) differing from the inner layers in character and
-function, as it differs from them in position. Subject as this outer
-layer is to the unmitigated actions of forces around&mdash;to abrasions, to
-extremes of heat and cold, to evaporation and soaking with water&mdash;its
-units have to be brought into equilibrium with these more violent
-actions, and have acquired molecular constitutions more stable
-than those of the interior cells. That is to say, the forces which
-differentiate the cortical part from the rest are the forces which it
-has to resist, and from which it passively protects the parts within.
-How<span class="pagenum" id="Page_248">[248]</span> clearly this heterogeneity of structure and function is consequent
-upon intercourse with the environment, every tree and shrub shows. The
-young shoots, alike of annuals and perennials, are quite green and soft
-at their extremities. Among plants of short lives, there is usually
-but a slight development of bark or none at all: such traces of it as
-the surface of the axis acquires being seen only at its lowermost or
-oldest portion. In long-lived plants, however, this formation of a
-tough opaque coating takes place more rapidly; and shows us distinctly
-the connexion between the degree of differentiation and the length of
-exposure. For, in a growing twig, we see that the bark, invisible at
-the bud, thickens by insensible gradations as we go downwards to the
-junction of the twig with the branch; and we come to still thicker
-parts of it as we descend along the branch towards the main stem.
-Moreover, on examining main stems we find that while in some trees the
-bark, cracked by expansion of the wood, drops off in flakes, leaving
-exposed patches of the inner tissue which presently become green and
-finally develop new bark; in other trees the exfoliated flakes continue
-adherent, and in the course of years form a rugged fissured coat: so
-producing a still more marked contrast between outside and inside. Of
-course the establishment of this heterogeneity is furthered by natural
-selection, which, where a protective covering is needed, gives an
-advantage to those individuals in which it has become strongest. But
-that this divergence of structure commences as a direct adaptation, is
-clearly shown by other facts than the foregoing. There is the fact that
-many of the plants which in our gardens develop bark with considerable
-rapidity, do not develop it with the same rapidity in a greenhouse. And
-there is the fact that plants which, in some climates, have their stems
-covered only by thin semi-transparent layers, acquire thick opaque
-layers when taken to other climates.</p>
-
-<p>Just noting, for the sake of completeness, that in the roots of the
-higher plants there arises a contrast between<span class="pagenum" id="Page_249">[249]</span> outer and inner parts,
-parallel to the one we have traced in their branches, let me draw
-attention to another differentiation of the same ultimate nature, which
-the higher plants exhibit to us&mdash;a differentiation which, familiar
-though it is, gains a new meaning by association with those named
-above, and makes their meaning still more manifest. Each great plant
-shows it. When, by the budding of axes out of axes, there is produced
-one of those highly-compounded Phænogams which we call a tree, the
-central part of the aggregate becomes functionally and structurally
-unlike the peripheral part. On looking into a large tree, or even
-a small one which has thick foliage, like the Laurel, we see that
-the internal branches are almost or quite bare of leaves, while the
-leaf-clad branches form an external stratum; and all our experience
-unites in proving that this contrast arises by degrees, as fast as
-the growth of the tree entails a contrast between the conditions
-to which inner and outer branches are exposed. Now when, in these
-most-composite aggregates, we see a differentiation between peripheral
-and central parts demonstrably caused by a difference in the relations
-of these parts to environing forces, we get support for the conclusion
-otherwise reached, that there is a parallel cause for the parallel
-differentiations exhibited by all aggregates of lower orders&mdash;branches,
-leaves, cells.</p>
-
-
-<p class="p2">§ 271. Before leaving this most general physiological differentiation,
-it may be well to say something respecting certain secondary
-unlikenesses which usually arise between interior and exterior. For the
-contrast is not, as might be supposed from the foregoing descriptions,
-a simple contrast: it is a compound contrast. The outer structure
-itself is usually divisible into concentric structures. This is equally
-true of a protophyte and of a phænogamic axis. Between the centre of
-an independent vegetal cell and its surface, there are at least two
-layers; and the bark coating the substance of a shoot, besides being
-itself compound, includes<span class="pagenum" id="Page_250">[250]</span> another tissue lying between it and the
-wood. What is the physical interpretation of these facts?</p>
-
-<p>When a mass of something we distinguish as inert matter is exposed
-to external agencies capable of working changes in it&mdash;when it is
-chemically acted upon, or when, being dry, it is allowed to soak, or
-when, being wet, it is allowed to dry&mdash;the changes set up progress in
-an equable way from the surface towards the centre. At any time during
-the process (supposing no other action supervenes) the modification
-wrought, first completed at the outside, either gradually diminishes
-as we approach the centre, or ceases suddenly at a certain distance
-from the centre. But now suppose that the mass, instead of being
-inert, is the seat of active changes&mdash;suppose that it is a portion of
-complex colloidal substance, permeable by light and by fluids capable
-of affecting its unstable molecules&mdash;suppose that its interior is
-a source of forces continually liberated and diffusing themselves
-outwards. Is it not likely that while at the centre the action of the
-internally-liberated forces will dominate, and while at the surface
-the action of the environing forces will dominate, there will be
-between the two a certain place at which their actions balance? May
-we not expect that this will be the place where the most unstable
-matter exists&mdash;the place outside of which the matter becomes relatively
-stable in the face of external forces, and inside of which the matter
-becomes relatively stable in the face of internal forces? And must we
-not conclude that though part of the adjustment is due to indirect
-equilibration, the initiation of it is due to direct equilibration?</p>
-
-<p>But we are here chiefly concerned with the more general interpretation,
-which is independent of any such speculation as the foregoing. These
-contrasted tissues and the contrasted functions they severally perform
-are, beyond question, subordinated to the relations of outside and
-inside. And the evidence makes it tolerably clear that the unlike
-actions or forces involved by the relations of outside and inside,
-determine these contrasts&mdash;partly directly and partly indirectly.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_251">[251]</span></p>
-
-<h3>CHAPTER III.<br />
-<span class="subhed1">DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS.</span></h3></div>
-
-
-<p>§ 272. The motionless protococcoid forms of lower <i>Algæ</i>, which
-do not permanently expose any parts of their surfaces to actions
-unlike those which other parts are exposed to, have no parts of their
-surfaces unlike the rest in function and composition. This is what
-the hypothesis prepares us for. If physiological differentiations
-are determined by differences in the incidence of forces, then there
-will be no such differentiations where there are no such differences.
-Contrariwise, it is to be expected that the most conspicuous
-unlikeness of function and minute structure will arise between the
-most-dissimilarly circumstanced parts of the surface. We find that
-they do. The upper end and the lower end, or, more strictly speaking,
-the free end and the attached end, habitually present the strongest
-physiological contrasts.</p>
-
-<p>Even aggregates of the first order illustrate this truth. Such
-so-called unicellular plants as those delineated in Figs. <a href="#i_022b">4, 5, and 6</a>,
-show us, on comparing the contents of their fixed ends and their loose
-ends, that different processes are going on in them, and that different
-functions are being performed by their limiting membranes. <i>Caulerpa
-prolifera</i>, which “consists of a little creeping stem with roots
-below and leaves above,” originating “in the growth of a body which may
-be regarded as an individual cell,” supplies a still-better example.
-Among aggregates<span class="pagenum" id="Page_252">[252]</span> of the second order a like connexion is displayed in
-more various modes but with equal consistency. As before, the Puff-ball
-served to exemplify the primary physiological differentiation of outer
-parts from inner parts; so, here, it supplies a simple illustration of
-the way in which the differentiated outer part is re-differentiated,
-in correspondence with the chief contrast in its relations to the
-environment. The only marked unlikeness which the cortical layer of the
-Puff-ball presents, is that between the portion next the ground and
-the opposite portion. The better-developed <i>Fungi</i> exhibit a more
-decided heterogeneity of parallel kind. Such incrusting <i>Algæ</i>
-as <i>Ralfsia verrucosa</i> furnish a kindred contrast; and in the
-higher <i>Algæ</i> it is uniformly repeated. Phænogams display this
-physiological differentiation very conspicuously. That earth and air
-are unlike portions of the environment, subjecting roots and leaves
-to unlike physical forces, which entail on them unlike reactions, and
-that the unlike functions and structures of their respective surfaces
-are fitted to these unlike physical forces, are familiar facts which it
-would be needless here to name, were it not that they must be counted
-as coming within a wider group of facts.</p>
-
-<p>Is this unlikeness between the outer tissues of the attached ends
-and those of the free ends in plants, determined by their converse
-with the unlike parts of the environment? That they result from an
-equilibration partly arising in the individual and partly arising by
-the survival of individuals in which it has been carried furthest, is
-inferable <i>à priori</i>; and this <i>à priori</i> argument may be
-adequately enforced by arguments of the inductive order. A few typical
-ones must here suffice. The gemmules of the <i>Marchantia</i> are
-little disc-shaped masses of cells composed of two or more layers.
-Their sides being alike, there is nothing to determine which side falls
-lowermost when one of them is detached. Whichever side falls lowermost,
-however, presently begins to send out rootlets, while<span class="pagenum" id="Page_253">[253]</span> the uppermost
-side begins to assume those characters which distinguish the face of
-the frond. When this differentiation has commenced, the tendency to its
-complete establishment becomes more and more decided; as is proved by
-the fact that if the positions of the surfaces be altered, the gemmule
-bends itself so as to re-adjust them: the change towards equilibrium
-with environing forces having been once set up, there is acquired,
-as it were, an increasing momentum which resists any counter-change.
-But the evidence shows that at the outset, the relations to earth
-and air alone determine the differentiation of the under surface
-from the upper. The experiences of the gardener, multiplying his
-plants by cuttings and layers, constitute another class of evidences
-not to be omitted: they are commonplace but instructive examples of
-physiological differentiation. While circumstanced as it usually is,
-the meristematic tissue of each branch in a Phænogam continues to
-perform its ordinary function&mdash;regularly producing on its outer side
-the cortical substances, and on its inner side the vascular and woody
-tissues. But change the conditions to those which the underground part
-of the plant is exposed to, and there begins another differentiation
-resulting in underground structures. Contact with water often suffices
-alone to produce this result, as in the branches of some trees when
-they droop into a pool, or as occasionally with a cutting placed in a
-bottle of water; and when the light is excluded by imbedding the end
-of the cutting, or the middle of the still-attached branch, in the
-earth, this production of tissues adapted to the function of absorbing
-moisture and mineral constituents proceeds still more readily.
-With such cases may be grouped those in which this development of
-underground organs by an above-ground tissue, is not exceptional but
-habitual. Creeping plants furnish good illustrations. From the shoots
-of the Ground-Ivy, rootlets are put out into the soil in a manner
-differing but little from that in which they are put out by an imbedded
-layer; save<span class="pagenum" id="Page_254">[254]</span> that the process follows naturally-induced conditions
-instead of following artificially-induced conditions. But in the common
-Ivy which, instead of running along the surface of the earth, runs
-up inclined or vertical surfaces, we see the process interestingly
-modified without being essentially changed. The rootlets, here
-differentiated by their conditions into organs of attachment much more
-than organs of absorption, still develop on that side of the shoot next
-the supporting surface, and do not develop where the shoot, growing
-away from the tree or wall, is surrounded equally on all sides by light
-and air: thus showing, undeniably, that the production of the rootlets
-is determined by the differential incidence of forces. Though survival
-of the fittest doubtless furthered this transition yet it clearly
-did not initiate it. That greenness which may be observed in these
-Ivy-branch rootlets while they are quite young, soft, and unshaded,
-introduces us to facts which are the converse of the foregoing facts;
-and proves that the parts ordinarily imbedded in the soil and adapted
-to its actions, acquire, often in very marked degrees, the superficial
-structures of the aërial parts, when they are exposed to light and air.
-This may be witnessed in Maize, which, when luxuriant, sends out from
-its nodes near the ground, clusters of roots that are thick, succulent,
-and of the same colour as the leaves. Examples more familiar to us
-in England occur in every field of turnips. On noting how green is
-the uncovered part of a turnip-root, and how manifestly the area over
-which the greenness extends varies with the area exposed to light, as
-well as with the degree of the exposure, it will be seen that beyond
-question, root-tissue assumes to a considerable extent the appearances
-and function of leaf tissue, when subject to the same agencies. Let
-us not forget, too, that where exposed roots do not approach in
-superficial character towards leaves, they approach in superficial
-character towards stems: becoming clothed with a thick, fissured bark,
-like that of the trunk and<span class="pagenum" id="Page_255">[255]</span> branches. But the most conclusive evidence
-is furnished by the actual substitutions of surface-structures and
-functions, that occur in aërial organs which have taken to growing
-permanently under ground, and in underground organs which have taken to
-growing permanently in the air. On the one hand, there is the rhizome
-exemplified by Ginger&mdash;a stem which, instead of shooting up vertically,
-runs horizontally below the surface of the soil, and assumes the
-character of a root, alike in colour, texture, and production of
-rootlets; and there is that kind of swollen underground axis, bearing
-axillary buds, which the Potato exemplifies&mdash;a structure which, though
-homologically an axis, simulates a tuberous root in surface-character,
-and when exposed to the air, manifests no greater readiness to develop
-chlorophyll than a tuberous root does. On the other hand, there are the
-aërial roots of certain Orchids which, habitually green at their tips,
-continue green throughout their whole lengths when kept moist; which
-have become leaf-like not only by this development of chlorophyll, but
-also by the acquirement of stomata; and which do not bury themselves in
-the soil when they have the opportunity.<a id="FNanchor_46" href="#Footnote_46" class="fnanchor">[46]</a> Thus we have aërial organs
-so completely changed to fit underground actions, that they will not
-resume aërial functions; and underground organs so completely changed
-to fit aërial actions, that they will not resume underground functions.</p>
-
-<p>That the physiological differentiation between the part of a plant’s
-surface which is exposed to light and air and the part which is exposed
-to darkness and moisture and solid<span class="pagenum" id="Page_256">[256]</span> matter, is primarily due to the
-unlike actions of these unlike parts of the environment, is, then,
-clearly implied by observed facts&mdash;more clearly, indeed, than was to
-be expected. Considering how strong must be the inherited tendency
-of a plant to assume those special characters, physiological as well
-as morphological, which have resulted from an enormous accumulation
-of antecedent actions, it may be even thought surprising that this
-tendency can be counteracted to so great an extent by changed
-conditions. Such a degree of modifiability becomes comprehensible only
-when we remember how little a plant’s functions are integrated, and how
-much, therefore, the functions going on in each part may be altered
-without having to overcome the momentum of the functions throughout
-the whole plant. But this modifiability being as great as it is, we
-can have no difficulty in understanding how, by the cumulative aid
-of natural selection, this primary differentiation of the surface in
-plants has become what we see it.</p>
-
-
-<p class="p2">§ 273. We will leave now these contrasts between the free surfaces
-of plants and their attached or imbedded surfaces, and turn our
-attention to the secondary contrasts existing between different
-parts of their free surfaces. Were a full statement of the evidence
-practicable, it would be proper here to dwell on that which is
-furnished by the inferior classes. It might be pointed out in detail
-that where, as among the <i>Algæ</i>, the free surfaces are not
-dissimilarly conditioned, there is no systematic differentiation
-of them&mdash;that the frond of an <i>Ulva</i>, the ribbon-shaped
-divisions of a <i>Laminaria</i>, and the dichotomous expansions
-of the <i>Fuci</i> which clothe the rocks between tide-marks, are
-alike on both sides; because, swayed about in all directions as
-they are by the waves and tides, their sides are equally affected.
-Conversely, from the <i>Fungi</i> might be drawn abundant proof
-that even among Thallophytes, unlikenesses arise between different
-parts of the free surfaces when their circumstances are unlike.<span class="pagenum" id="Page_257">[257]</span> In
-such laterally-growing kinds as are shown in Fig. <a href="#i_138">196<i>b</i></a>, the
-honey-combed under surface and the smooth leathery upper surface,
-have their contrasts related to contrasted conditions; and in the
-adjacently-figured Agarics, and other stalked genera, the pileus
-exhibits a parallel difference, explicable in a parallel way. But
-passing over Cryptogams it must suffice if we examine more at length
-these traits as they are displayed by Phænogams. Let us first note the
-dissimilarities between the outer tissues of stems and leaves.</p>
-
-<p>That these dissimilarities arose by degrees, as fast as the units
-of which the phænogamic axis is composed became integrated, is a
-conclusion in harmony with the truth that in every shoot of every
-plant, they are at first slight and become gradually marked. Already,
-in briefly tracing the contrasts between the outer and inner tissues
-of plants, some facts have been named showing, by implication, how
-the cessation of the leaf-function in axes is due to that change of
-conditions entailed by the discharge of other functions. Here we have
-to consider more closely facts of this class, together with others
-immediately to the point. On pulling off from a stem of grass the
-successive sheaths of its leaves, the more-inclosed parts of which are
-of a fainter green than the outer parts, it will be found that the
-tubular axis eventually reached is of a still fainter green; but when
-the axis eventually shoots up into a flowering stem, its exposed part
-acquires as bright a green as the leaves. In other Monocotyledons, the
-leaf-sheaths of which are successively burst and exfoliated by the
-swelling axis, it may be observed that where the dead sheaths do not
-much obstruct the light and air, the surface of the axis underneath is
-full of chlorophyll. <i>Dendrobium</i> is an example. But when the dead
-sheaths accumulate into an opaque envelope, the chlorophyll is absent,
-and also, we may infer, the function which its presence habitually
-implies. Carrying with us this evidence, we shall recognize a like
-relation in Dicotyledons. While its outer layer remains tolerably
-transparent, an exogenous stem or<span class="pagenum" id="Page_258">[258]</span> branch continues to show, by the
-formation of chlorophyll, that it shares in the duties of the leaves;
-but in proportion as a bark which the light cannot penetrate is
-produced by the adherent flakes of dead skin, or by the actual deposit
-of a protective substance, the differentiation of duties becomes more
-decided. Cactuses and Euphorbias supply us with converse facts having
-the same implication. The succulent axes so strangely combined in
-these plants, maintain for a long time the translucency of their outer
-layers and their greenness; and they so efficiently perform the offices
-of leaves that leaves are not produced. In some cases, axes that are
-not succulent participate largely in the leaf-function, or entirely
-usurp it&mdash;still, however, by fulfilling the same essential conditions.
-Occasionally, as in <i>Statice brassicæfolia</i>, stems become fringed;
-and the fringes they bear assume, along with the thinness of leaves,
-their darker green and general aspect. In the genus <i>Ruscus</i>,
-the flattened axis simulates so closely the leaf-structure, that
-were it not for the flower borne on its mid-rib, or edge, its axial
-nature would hardly be suspected. And let us not omit to note that
-where axes usurp the characters of leaves, in their attitudes as
-well as in their shapes and thicknesses, there are contrasts between
-their under and upper surfaces, answering to the contrasts between
-the relations of these surfaces to the light. Of this <i>Ruscus
-androgynus</i> furnishes a striking example. In it the difference
-which the unaided eye perceives is much less conspicuous than that
-disclosed by the microscope; for I find that while the face of the
-pseudo-leaf has no stomata, the back is abundantly supplied with them.
-One more illustration must be added. Equally for the morphological
-and physiological truths which it enforces, the <i>Mühlenbeckia
-platyclada</i> is one of the most instructive of plants. In it the
-simulation of forms and usurpation of functions, are carried out in
-a much more marvellous way than among the <i>Cactaceæ</i>. Imagine a
-growth resembling in outline a very long willow-leaf, but without a
-mid-rib, and having its two surfaces alike.<span class="pagenum" id="Page_259">[259]</span> Imagine that across this
-thin, green, semi-transparent structure, there are from ten to thirty
-divisions, which prove to be the successive nodes of an axis. Imagine
-that along the edges of this leaf-shaped aggregate of internodes, there
-arise axillary buds, some of which unfold into flowers, and others of
-which shoot up vertically into growths like the one which bears them.
-Imagine a whole plant thus seemingly composed of jointed willow-leaves
-growing from one another’s edges, and some conception will be formed of
-the <i>Mühlenbeckia</i>. The two facts which have meaning for us here
-are&mdash;first, that the performance of leaf-functions by these axes goes
-along with the assumption of a leaf-like translucency; and, second,
-that these flattened axes, retaining their upright attitudes, and
-therefore keeping their two faces similarly conditioned, have these two
-faces alike in colour and texture.</p>
-
-<p>That physiological differentiation of the surface which arises in
-Phænogams between axial organs and foliar organs, is thus traceable
-with tolerable clearness to those differences between their conditions
-which integration has entailed&mdash;partly in the way above described and
-partly in other ways still to be named. By its relative position, as
-being shaded by the leaves, the axis is less-favourably circumstanced
-for performing those assimilative actions effected by the aid of light.
-Further, that relatively-small ratio of surface to mass in the axis,
-which is necessitated by its functions as a support and a channel
-for circulation, prevents it from taking in, with the same facility
-as the leaves, those surrounding gases from which matter is to be
-assimilated. Both these special causes, however, in common with that
-previously assigned, fall within the general cause. And in the fact
-that where the differential conditions do not exist, the physiological
-differentiation does not arise, or is obliterated, we have clear proof
-that it is determined by unlikenesses in the relations of the parts to
-the environment.</p>
-
-
-<p class="p2">§ 274. From this most general contrast between aërial<span class="pagenum" id="Page_260">[260]</span>
-surface-tissues&mdash;those of axes and those of folia&mdash;we pass now to the
-more special contrasts of like kind existing in folia themselves.
-Leaves present us with superficial differentiations of structure and
-function; and we have to consider the relations between these and the
-environing forces.</p>
-
-<p>Over the whole surface of every phænogamic leaf, as over the fronds of
-the <i>Pteridophyta</i>, there extends a simple or compound epidermal
-layer, formed of cells that are closely united at their edges and
-devoid (in the Flowering Plants) of that granular colouring matter
-(chlorophyll) contained in the layers of cells they inclose: the result
-being that the membrane formed of them is comparatively transparent. On
-the submerged leaves of aquatic Phænogams, this outer layer is thin,
-delicate, and permeable by water; but on leaves exposed to the air, and
-especially on their upper surfaces, is comparatively strong, dense,
-often smooth and impermeable by water: being thus fitted to prevent the
-rapid escape of the contained juices by evaporation. Similarly, while
-the leaves of terrestrial plants which live in temperate climates,
-usually have comparatively thin coats thus composed, in climates that
-are both hot and dry, leaves are commonly clothed with a very thick
-cuticle. Nor is this all. The outside of an aërial leaf differs from
-that of a submerged leaf by containing a deposit of waxy substance.
-Whether this be exuded by the exposed surfaces of the cells, as some
-contend, or whether it is deposited within the cells, as thought by
-others, matters not in so far as the general result is concerned. In
-either case a waterproof coating is formed at the outermost sides of
-these outermost cells; and in many cases produces that polish by which
-the upper surface of the leaf is more or less distinguished from the
-under surface. This external pellicle presents us with another contrast
-of allied meaning. On the upper surfaces of leaves subject to the
-direct action of the sun’s rays, there are either few or none of those
-minute openings, or stomata, through which gases can enter or escape;
-but on the under surfaces these stomata<span class="pagenum" id="Page_261">[261]</span> are abundant: a distribution
-which, while permitting free absorption of the needful carbonic acid,
-puts a check on the exit of watery vapour. Two general exceptions to
-this arrangement may be noted. Leaves that float on the water have all
-their stomata on their upper sides, and leaves that are submerged have
-no stomata&mdash;modifications obviously appropriate to the conditions.
-What is to be said respecting the genesis of these differentiations?
-For the last there seems no direct cause: its cause must be indirect.
-The unlike actions to which the upper and under surfaces of leaves are
-subject, have no apparent tendency to produce unlikeness in the number
-of their breathing holes. Here the natural selection of spontaneous
-variations furnishes the only feasible explanation. For the first,
-however, there is a possible cause in the immediate actions of incident
-forces, which survival of the fittest continually furthers.</p>
-
-<p>The fluid exhaling through the walls of the cells next the air, will be
-likely to leave behind suspended substances on their outer surfaces.
-On remembering the pellicle which is apt to form on thick solutions or
-emulsions as they dry, and how this pellicle as it grows retards the
-further drying, it will be perceived that the deposit of waxy matter
-next to the outer surfaces of the cuticular cells in leaves, is not
-improbably initiated by the evaporation which it eventually checks.
-Should it be so, there results a very simple case of equilibration.
-Where the loss of water is too great, this waxy pellicle left behind by
-the escaping water will protect most those individuals of the species
-in which it is thickest or densest; and by inheritance and continual
-survival of the fittest, there will be established in the species that
-thickness of the layer which brings the evaporation to a balance with
-the supply of water.</p>
-
-<p>Another superficial differentiation, still more familiar, has to be
-noted. Every child soon learns to distinguish by its colour the upper
-side of a leaf from its under side, if the leaf is one that has grown
-in such way as to establish the relations of upper and under. The upper
-surfaces of leaves are<span class="pagenum" id="Page_262">[262]</span> habitually of a deeper green than the under.
-Microscopic examination shows that this deeper green results from the
-closer clustering of those parenchyma-cells full of chlorophyll that
-are in some way concerned with the assimilative actions; while beneath
-them are more numerous intercellular passages communicating with those
-openings or stomata through which is absorbed the needful air. Now when
-it is remembered that the formation of chlorophyll is clearly traceable
-to the action of light&mdash;when it is remembered that leaves are pale
-where they are much shaded and colourless when developed in the dark,
-as in the heart of a Cabbage&mdash;when it is remembered that succulent axes
-and petioles, like those of Sea-kale and Celery, remain white while
-the light is kept from them and become green when exposed; it cannot
-be questioned that this greater production of chlorophyll next to the
-upper surface of a leaf, is directly consequent on the greater amount
-of light received. Here, as in so many other cases, we must regard
-the differentiation as in part due to direct equilibration and in
-part to indirect equilibration. Familiar facts compel us to conclude
-that from the beginning, each individual foliar organ has undergone
-a certain immediate adaptation of its surfaces to the incidence of
-light; that when there arose a mode of growth which exposed the leaves
-of successive generations in similar ways, this immediately-produced
-adaptation, ever tending to be transmitted, was furthered by the
-survival of individuals inheriting it in the greatest degree; and that
-so there was gradually established that difference between the two
-surfaces which each leaf displays before it unfolds to the light, but
-which becomes more marked when it has unfolded.<a id="FNanchor_47" href="#Footnote_47" class="fnanchor">[47]</a></p>
-
-<p><span class="pagenum" id="Page_263">[263]</span></p>
-
-<p>From the ordinary cases let us now pass to the exceptional cases. We
-will look first at those in which the two faces of the leaves differ
-but little, or not at all&mdash;their circumstances being similar or equal.
-Leaves that grow in approximately-upright attitudes, and attitudes
-which do not maintain the relative positions of the two surfaces with
-constancy, may be expected to display an unusual likeness between the
-two surfaces; and among them we see it. The Grasses may be named as a
-group exemplifying this relation; and if, instead of comparing them
-as a group with other groups, we compare those dwarf kinds of them
-which spread out their leaves horizontally, with the large aspiring
-kinds, such as <i>Arundo</i>, we trace a like antithesis: in the one
-the contrast of upper and under is very obvious, while in the other
-it is scarcely to be detected. Leaves of various other Monocotyledons
-that grow in a similar way, similarly show us a near approach to
-uniformity of the two surfaces; as instance the genus <i>Clivia</i>
-and the thinner-leaved kinds of <i>Yucca</i>. Where the contrast of
-upper and under is greatly diminished by the assumption of a rounded or
-cylindrical form, instead of a flattened form, the same thing happens.
-The genus <i>Kleinia</i> furnishes illustrations. It may be remarked,
-too, that even within the limits of this genus there are instructive
-variations; for while in <i>Kleinia ficoides</i> the leaves, shaped
-like pea-pods, are broadest in a vertical direction, and have their
-lateral surfaces alike in conditions and structure, in other species
-the leaves, broader horizontally than vertically, exhibit unlikeness
-between the upper and under sides. Equally to the point is the evidence
-furnished by vertically-growing leaves that are cylindrical, as those
-of <i>Sanseviera cylindrica</i>, or as those of the Rush-tribe: the
-similarly-placed surface has all around a similar character. Of kindred
-meaning,<span class="pagenum" id="Page_264">[264]</span> and still more conclusive, are the cases in which the under
-side of the leaf, being more exposed to light than the upper side,
-usurps the character and function of the upper side. If a common Flag
-be pulled to pieces, it will be seen that what answers to the face
-in other leaves, forms merely the inside of the sheath including the
-younger leaves, and is obliterated higher up. The two surfaces of the
-blade answer to the two under halves of a leaf that has been, as it
-were, folded together lengthways, with the two halves of its upper
-surface in contact. And here, in default of an upper surface, the under
-surface acquires its character and discharges its function. A like
-substitution occurs in <i>Aristea corymbosa</i>; and there are some of
-the Orchids, as <i>Lockhartia</i>, which display it in a very obvious
-way.</p>
-
-<p>When joined with the foregoing evidence, the evidence which another
-kind of substitution supplies is of great weight. I refer to that which
-occurs in the Australian Acacias, already instanced as throwing light
-on morphological changes. In these plants the leaves properly so-called
-are undeveloped, and the foot-stalks, flattened out into foliaceous
-shapes, acquire veins and mid-ribs, and so far simulate leaves as
-ordinarily to be taken for them: a fact in itself of much physiological
-significance. But that which it concerns us especially to note, is
-the absence of distinction between the two faces of these phyllodes,
-as they are named, and the cause of its absence. These transformed
-petioles do not flatten themselves out horizontally, so as to acquire
-under and upper sides, as most true leaves do; but they flatten
-themselves out vertically: the result being that their two sides are
-similarly circumstanced with respect to light and other agencies; and
-there is consequently nothing to cause their differentiation. And then
-we find an analogous case where differential conditions arise, and
-where some differentiation results. In <i>Oxalis bupleurifolia</i>,
-Fig. <a href="#i_041">66</a>, there is a similar flattening out of the petiole into a
-pseudo-leaf; but in it the flattening takes place in the same plane as
-the leaf,<span class="pagenum" id="Page_265">[265]</span> so as to produce an under and an upper surface; and here the
-two surfaces of the pseudo-leaf are slightly unlike&mdash;in contour if in
-nothing else.</p>
-
-
-<p class="p2">§ 275. We now come to such physiological differentiations among the
-outer tissues of plants, as are displayed in the contrasts between
-foliar organs on the same axis, or on different axes&mdash;contrasts between
-the seed-leaves and the leaves subsequently formed, between submerged
-and aërial leaves in certain aquatic plants, between leaves and bracts,
-and between bracts and sepals. To deal even briefly with these implies
-information which even a professed botanist would have to increase by
-special inquiries, before attempting interpretations. Here it must
-suffice to say something respecting those marked unlikenesses existing
-between the tissues of the more characteristic parts of flowers, and
-the tissues of the homologous foliar organs.</p>
-
-<p>It was pointed out in <a href="#sect196">§ 196</a>, that the terminal folia of a phænogamic
-axis have sundry characters in common with such fronds as those
-out of which we concluded that the phænogamic axis has arisen by
-integration&mdash;common characters of a kind to be expected. In their
-simple cellular composition, comparative want of chlorophyll, and
-deficiency of vascular structures, the undeveloped ends of leaf-shoots
-and the developed ends of flower-shoots, approach to the fronds of the
-simpler Archegoniates. We also noted between them another resemblance.
-It is said of the <i>Jungermanniaceæ</i>, that “though under certain
-circumstances of a pure green, they are inclined to be shaded with
-red, purple, chocolate, or other tints;” and answering to this we
-have the facts that such colours commonly occur in the terminal folia
-of a phænogamic axis, when arrest of its development leads to the
-formation of a flower, and that very frequently they are visible at
-the ends of leaf-axes. In the unfolding parts of shoots, more or less
-of red, or copper-colour, or chocolate-colour, may generally be seen:
-often, indeed, it characterizes<span class="pagenum" id="Page_266">[266]</span> the leaves for some time after they
-are unfolded. Occasionally the traces of it are permanent; and, as in
-the scarlet terminal leaves of <i>Poinsettia pulcherrima</i>, we see
-that it may become, and continue, extremely conspicuous. The question,
-then, now to be asked is&mdash;has this colouring by which the immature
-part of the phænogamic axis is characterized, anything to do with the
-colouring of flowers? Has this difference between undeveloped folia and
-folia that are further developed, been increased by natural selection
-where an advantage accrued from it, until it has ended in the strong
-contrast we now see? I think we may not irrationally infer that this
-has happened.</p>
-
-<p>Facts, very numerous and varied, united to warrant us in concluding
-that gamogenesis commences where the forces which conduce to growth are
-nearly equilibrated by the forces which resist growth (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect78">§ 78</a>); and the
-induction that in plants, fertilized germs are produced at places where
-there is an approach towards this balance, we found to be in harmony
-with the deduction that an advantage to the species must be gained by
-sending off migrating progeny from points where nutrition is failing.
-Other things equal, failure of nutrition may be expected in parts which
-have the most remote or most indirect access to the materials furnished
-by the roots&mdash;materials which have to be carried great distances by
-a very imperfect apparatus. The ends of lateral axes are therefore
-the probable points of fructification, in aggregates of the third
-order that have taken to growing vertically. But if these points at
-which nutrition is failing, are also the points at which the colours
-inherited from lower types are likely to recur in more marked degrees
-than elsewhere; then we may infer that the organs of fructification
-will not unfrequently co-exist with such colours at the ends of such
-axes. How may the resulting contrast between the older fronds and the
-fronds next the germ-producing organs be increased? If uninterfered
-with it would be likely to diminish. These traits inherited from remote
-ancestry might be expected<span class="pagenum" id="Page_267">[267]</span> slowly to fade away. How, then, is the
-intensification of them to be explained?</p>
-
-<p>If a contrast of the kind described favours the propagation of a race
-in which it exists, it will be maintained and increased; and if we
-take into account an agency of which Mr. Darwin has shown the great
-importance&mdash;the agency of insects&mdash;we shall have little difficulty
-in understanding how such a contrast may facilitate propagation. We
-cannot, of course, here assume the agency of insects so specialized
-in their habits as Bees and Butterflies; for their specialized habits
-imply the pre-existence of the contrast to be explained. But there is
-an insect-agency of a more general kind which may be fairly counted
-upon as coming into action. Various small Flies and Beetles wander
-over the surfaces of plants in search of food. It is a legitimate
-assumption that they will frequent most those parts in which they find
-most food, or food most to their liking&mdash;especially if at the same time
-they gain the advantage of concealment. Now the ends of axes, formed
-of young, soft, and closely-packed folia, are the parts which more
-than any others offer these several advantages. They afford shelter
-from enemies; they frequently contain exuded juices; and when they do
-not, their tissues are so tender as to be easily pierced in search
-of the sap. If, then, from the first, as at present, these ends of
-axes have been favourite haunts of small insects; and if, where the
-closely-clustered folia contained the generative organs, the insects
-frequenting them occasionally carried adherent fructifying cells from
-one plant to another, and so aided fertilization; it would follow
-that anything which made such terminal clusters more attractive to
-such insects, or more conspicuous to them, or both, would further the
-multiplication of the race, and would so be continually increased by
-the extra multiplication of individuals in which it was greatest.
-Here we find the clue. This contrast of colour between the folia next
-to the fructifying parts and all other folia, must constantly have
-facilitated insect-agency; supposing<span class="pagenum" id="Page_268">[268]</span> the insects to have had the power
-of distinguishing between colours. That Bees and Butterflies have this
-power is manifest. They may be watched flying from flower to flower,
-disregarding all other parts of the plants. And if the less-specialized
-insects possessed some degree of such discrimination, then the initial
-contrasts of colour above described would be maintained and increased.
-Let such a connexion be once established, and it must tend to become
-more decided. Insects most able to discern the parts of plants which
-afford what they seek, will be those most likely to survive and leave
-offspring. Plants presenting most of the desired food, and showing most
-clearly where it lies, will have their fertilization and multiplication
-furthered in the greatest degree. And so the mutual adaptation will
-become ever closer; while it is rendered at the same time more varied
-by the special requirements of the insects and of the plants in each
-locality, under each change of conditions. Of course, the genesis
-of the sweet secretions and the odours of flowers, has a parallel
-interpretation. The simultaneous production of honey, or some kindred
-substance, is implied above; since, unless a bait co-existed with
-the colour, the colour would not attract insects, and would not be
-maintained and intensified by natural selection. Gums, and resins, and
-balsams, are familiar products of plants; apparently, in many cases,
-excreted as useless matters from various parts of their surfaces. These
-substances, admitting of wide variations in quality, as they do, afford
-opportunities for the action of natural selection wherever any of
-them, attractive to insects, happen to be produced near the organs of
-fructification. And this action of natural selection once set up, may
-lead to the establishment of a local excretion, to the production of an
-excretion more and more attractive, and to the disposal of the organ
-containing it in such a way as most to facilitate the carrying away of
-pollen. Similarly and simultaneously with odours. Odours, like colours,
-draw insects to flowers. After<span class="pagenum" id="Page_269">[269]</span> observing how Bees come swarming into a
-house where honey is largely exposed, or how Wasps find their way into
-a shop containing much ripe fruit, it cannot be questioned that insects
-are to a considerable extent guided by scent. Being thus sensitive
-to the aromatic substances which flowers exhale, they may, when the
-flowers are in large masses, be attracted by them from distances
-at which the flowers themselves are invisible. And manifestly, the
-flowers which so attract them from the greatest distances, increasing
-thereby their chances of efficient fertilization, will be most likely
-to perpetuate themselves. That is to say, survival of the fittest
-must tend to produce perfumes that are both more powerful and more
-attractive.</p>
-
-<p>These physiological differentiations, then, which mark off the
-foliar organs constituting flowers from other foliar organs, are
-the consequences of indirect equilibration. They are not due to the
-immediate actions of unlike incident forces on the parts of the
-individual plant; but they are due to the actions of such unlike
-incident forces on the aggregate of individuals, generation after
-generation.<a id="FNanchor_48" href="#Footnote_48" class="fnanchor">[48]</a></p>
-
-
-<p class="p2">§ 276. The unity of interpretation which we here find for phenomena
-of such various orders, could hardly be found<span class="pagenum" id="Page_270">[270]</span> were the phenomena
-otherwise caused. That the stronger and the feebler contrasts among the
-different parts of the outer tissues in plants, should so constantly
-occur along with stronger and feebler contrasts among the incident
-forces, is in itself weighty evidence that unlike outer actions have
-caused unlike inner actions, and correspondingly-unlike structures;
-either by changing the functional equilibrium in the individual, or by
-changing it in the race, or by both.</p>
-
-<p>Even in the absence of more direct proof, there would be great
-significance in the marked differences that habitually exist between
-the exposed and imbedded parts of plants, between the stems and the
-leaves, and between the upper and under surfaces of the leaves. The
-significance of these differences is increased when we discover that
-they vary in degree as the differences in the conditions vary in
-degree. Still greater becomes the force of the evidence on finding
-that these strongly-contrasted parts may, when placed in one another’s
-conditions, and kept in them from generation to generation, permanently
-assume one another’s functions, and, in a great degree, one another’s
-structures. Even more conclusive yet is the argument rendered, by the
-discovery that, where these substitutions of function and structure
-take place, the superinduced modifications differ in different
-circumstances; just as the original modifications do. The fact that
-a flattened stem simulating a vertically-growing leaf has its two
-surfaces alike, while when it simulates a horizontally-growing leaf its
-upper and under surfaces differ, is a fact which, standing alone, might
-prove little, but proves much when joined with all the other evidence.
-And its profound meaning becomes the more obvious on discovering that
-the same thing happens with petioles when they usurp leaf-functions.</p>
-
-<p>Finally, when we remember how rapidly analogous modifications of
-function and structure arise in the superficial tissues of individual
-plants, the general inference can scarcely be resisted. When we
-meet with so striking a case as that<span class="pagenum" id="Page_271">[271]</span> of the <i>Begonia</i>-leaf, a
-fragment of which stuck in the ground produces roots from its under
-surface and leaves from its upper surface&mdash;when we see that though,
-in this case, the typical structure of the plant presently begins to
-control the organizing process, yet the initial differentiations are
-set up by the differential actions of the environment; the presumption
-becomes extremely strong that the heterogeneities of surface which
-we have considered, result, as alleged, directly or indirectly from
-heterogeneities in the incident forces.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_272">[272]</span></p>
-
-<h3>CHAPTER IV.<br />
-<span class="subhed1">DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS.<a id="FNanchor_49" href="#Footnote_49" class="fnanchor">[49]</a></span></h3></div>
-
-
-<p>§ 277. In passing from plants formed of threads or thin laminæ, to
-plants having some massiveness, we find that after the external and
-internal parts have become distinguished from one another, there arise
-distinctions among the internal parts themselves, as well as among the
-external parts themselves: the primarily-differentiated parts are both
-re-differentiated.</p>
-
-<p>From types of very low organisation illustrations of this may be
-drawn. In the thinner kinds of <i>Laminaria</i> there exists but the
-single contrast between the outer layer of cells and an inner layer;
-but in larger species of the same genus, as <i>L. digitata</i>, there
-are three unlike layers on each side of a central layer differing
-from them&mdash;augmentation of bulk is accompanied by multiplication of
-concentric internal structures, having their unlikenesses obviously
-related to unlikenesses in their conditions. In <i>Furcellaria</i> and
-various <i>Algæ</i> of similarly swollen forms, the like relation may
-be traced.</p>
-
-<p>Just indicating the generality of this contrast, but not<span class="pagenum" id="Page_273">[273]</span> attempting to
-seek in these lower types for any more specific interpretation of it,
-let us pass to the higher types. The argument will be amply enforced by
-the evidence obtained from them. We will look first at the conditions
-which they have to fulfil; and then at the ways in which the functions
-and structures adapting them to these conditions arise.</p>
-
-
-<p class="p2">§ 278. A terrestrial plant that grows vertically needs no marked
-modification of its internal tissues, so long as the height it reaches
-is very small. As we before saw, the spiral or cylindrical rolling
-up of a simple cellular frond, or the more bulky growth of a simple
-cellular axis, may give the requisite strength; and the requisite
-circulation may be carried on through the unchanged cellular tissue.
-But in proportion as the height to be attained and the mass to be
-supported increase, the supporting part must acquire greater bulk or
-greater density, or both; and some modification that shall facilitate
-the transfer of nutritive liquids must take place. Hence, in the
-inner tissues of plants we may expect to find that structural changes
-answering to these requirements become marked, as the growth of the
-aërial part becomes great. Facts correspond with these expectations.</p>
-
-<p>Among the humbler Cormophytes, which creep over or raise themselves
-but little above, the surfaces they flourish upon, there is scarcely
-any internal differentiation: the vascular and woody structures, if
-not in all cases absolutely unrepresented, are rarely and very feebly
-indicated. But among the higher types&mdash;the Ferns and Lycopodiums&mdash;which
-raise their fronds to considerable heights, there are vascular bundles
-and hard tissues like wood; and by the Tree-Ferns massive axes are
-developed. That the relation which thus shows itself among Cryptogams
-is habitual among Phænogams, scarcely needs saying.</p>
-
-<p>Phænogams, however, are not universally thus characterized in a decided
-way. Besides the comparative want of woody tissue in flowering plants
-of humble growth, and<span class="pagenum" id="Page_274">[274]</span> besides the paucity of vessels in ordinary
-water-plants, there are cases of much more marked divergence from
-this typical internal structure. These exceptional cases occur under
-exceptional conditions, and are highly instructive. They are of two
-kinds. One group of them is furnished by certain plants which are
-parasitic on the exposed roots of trees&mdash;parasitic not partially,
-as the Mistletoe, but to the extent of subsisting wholly on the sap
-they absorb. Fungus-like in colour and texture, and having scales
-for leaves, these <i>Balanophoræ</i> and <i>Rafflesiaceæ</i> are
-recognizable as Phænogams by scarcely any other traits than their
-fructifications. Along with their aborted leaves and absence of
-chlorophyll, there is a great degradation of those internal tissues
-by which Phænogams are commonly distinguished. Though Dr. [now Sir
-J.] Hooker has shown that they are not, as some botanists thought,
-devoid of spiral vessels; yet, as shown by the mistake previously
-made in classifying them, their appliances for circulation are
-rudimentary. And this trait goes along with a greatly-simplified
-distribution of nutriment. In the absence of leaves there can be but
-little down-current of sap, such as leaves usually supply to roots:
-there cannot be much beyond an upward current of the absorbed juices.
-The other cases occur where circulation is arrested or checked in a
-different way; namely, in plants that are wholly submerged. These
-are the <i>Podostemaceæ</i>. Clothing as they do the submerged
-rocks, their roots play the part of rhizomes, being attached to the
-substratum by hairs and other processes, and having the leaf-bearing
-and flower-bearing shoots on their surfaces. The latter spread out
-more or less horizontally and are also fixed to the substratum in the
-same manner as the roots. Observe then the connexion of facts. One
-of these <i>Podostemaceæ</i> needs no internal stiffening substance,
-for it exists in a medium of its own specific gravity; and being in a
-position to absorb water over its entire surface, it has no need for a
-circulation of crude sap&mdash;nor, indeed, in the absence of evaporation
-from any part of its surface, could<span class="pagenum" id="Page_275">[275]</span> any active circulation take
-place. Here, accordingly, the tracheal and mechanical elements are
-undeveloped. Though spiral vessels are not entirely absent, yet they
-are so rare as to do no more than verify the inference of phænogamic
-relationship drawn from the flowers.</p>
-
-<p>The method of agreement, the method of difference, and the method of
-concomitant variations, thus unite in proving a direct relation between
-the demand for support and circulation, and the existence of these
-vascular woody bundles which the higher plants habitually possess.
-The question which we have to consider is&mdash;Under what influences are
-these structures, answering to these requirements, developed? How are
-these internal differentiations caused? The inquiry may be conveniently
-divided. Though the supporting tissues and the tissues concerned in the
-circulation of liquids are closely connected, and indeed entangled,
-with one another, we may fitly deal with them apart. Let us take first
-the supporting tissue.</p>
-
-
-<p class="p2">§ 279. Many commonplace facts indicate that the mechanical strains to
-which upright-growing plants are exposed, themselves cause increase
-of the dense deposits by which such plants are enabled to resist
-such strains. There is the fact that the massiveness of a tree-trunk
-varies according to the stress habitually put upon it. If the contrast
-between the slender stem of a tree growing in a wood and the bulky stem
-of a kindred tree growing in the fields, be ascribed to difference
-of nutrition rather than difference of exposure to winds; there is
-still the fact that a tree trained against a wall has a less bulky
-stem than a tree of the same kind growing unsupported; and that
-between the long weak branches of the one and the stiff ones of the
-other there are decided contrasts. If it be objected that a tree so
-trained and branches so borne have relatively less foliage, and that
-therefore these unlikenesses also are due to unlikenesses of general
-nutrition, which may in part be true; there are still such cases as
-those of<span class="pagenum" id="Page_276">[276]</span> garden plants, which when held up by tying them to sticks
-have weaker stems than when they are unpropped, and sink down if their
-props are taken away. Again, there is the evidence supplied by roots.
-Though the contrast between the feeble roots of a sheltered tree
-and the strong roots of an exposed tree, may, like the contrast of
-their stems, be mainly due to difference of nutrition, and therefore
-supplies but doubtful evidence, we get tolerably clear evidence where
-trees growing on inclined rocky surfaces, send into crevices that
-afford little moisture or nutriment, roots which nevertheless become
-thick where they are so directed as to bear great strains. Suspicion
-thus raised is strengthened into conviction by special evidences
-occurring in the places where they are to be expected. The Cactuses,
-with their succulent growths that pass into woody growths slowly
-and irregularly, give us the opportunity of tracing the conditions
-under which the wood is formed. Good examples occur in the genus
-<i>Cereus</i>, and especially in forms like <i>C. crenulatus</i>. Here,
-from a massive vertically-growing rod of fleshy tissue, two inches
-or more in diameter, there grow at intervals lateral rods similarly
-bulky, which, quickly curving themselves, take vertical directions.
-One of these heavy branches puts great strains on its own substance
-and that of the stem at their point of junction; and here both of them
-become brown and hard, while they continue green and succulent all
-around. Such differentiations may be traced internally before they
-are visible on the surface. If a joint of an <i>Opuntia</i> be sliced
-through longitudinally, the greater resistance to the knife all around
-the narrow neck, indicates there a larger deposit of lignin than
-elsewhere; and a section of the tissue placed under the microscope,
-exhibits at the narrowest part a concentration of the woody and
-vascular bundles. Clear evidence of another kind has been noted by
-Mr. Darwin, in the organs of attachment of climbing plants. Speaking
-of <i>Solanum jasminoides</i> he says:&mdash;“When the flexible petiole
-of a half-or a quarter-grown leaf has clasped any object, in three<span class="pagenum" id="Page_277">[277]</span>
-or four days it increases much in thickness, and after several weeks
-becomes wonderfully hard and rigid; so that I could hardly remove one
-from its support. On comparing a thin transverse slice of this petiole
-with one from the next or older leaf beneath, which had not clasped
-anything, its diameter was found to be fully doubled, and its structure
-greatly changed.... This clasped petiole had actually become thicker
-than the stem close beneath; and this was chiefly due to the greater
-thickness of the ring of wood, which presented, both in transverse
-and longitudinal sections, a closely similar structure in the petiole
-and axis. The assumption by a petiole of this structure is a singular
-morphological fact; but it is a still more singular physiological fact
-that so great a change should have been induced by the mere act of
-clasping a support.”</p>
-
-<p>If there is a direct relation between mechanical stress and the
-formation of wood, it ought to explain for us the internal distribution
-of the wood. Let us see whether it does this.</p>
-
-<p>When seeking in mechanical actions and reactions the cause of that
-indurated structure which forms the vertebrate axis (<a href="#sect254">§§ 254–7</a>), it
-was pointed out that in a transversely-strained mass, the greatest
-pressures and tensions are thrown on the molecules of the concave and
-convex surfaces. Hence, supposing the transversely-strained mass to
-be a cylinder, bent backwards and forwards not in one plane but now
-in this plane and now in that, its peripheral layers will be those on
-which the greatest stress falls. An ordinary dicotyledonous axis is
-such a cylinder so strained. The maintenance of its attitude either as
-a lateral shoot or a vertical shoot, implies subjection to the bendings
-caused by its own weight and by the ever-varying wind. These bendings
-imply tensions and pressures falling most severely first on one side of
-its outer layers and then on another. And if the dense substance able
-to resist these tensions and pressures is deposited most where they are
-greatest, we ought to find it taking the shape of a cylindrical casing.
-This is just what<span class="pagenum" id="Page_278">[278]</span> we do find. On cutting across a shoot in course of
-formation, we see its central space either unoccupied or occupied only
-by soft tissue. That the layer of hard tissue surrounding this is not
-the outermost layer, is true: there lies beyond it the cambium layer,
-from which it is formed, the phloëm, and the cortex. But outside of the
-soft phloëm there is frequently another layer of dense tissue now known
-as the pericyclic fibres, having frequently a tenacity greater even
-than that of the wood&mdash;a layer which, while it protects the cambium
-and offers additional resistance to the transverse strain, admits of
-being fissured as fast as the cylinder of wood thickens. That is to
-say, the deposit of resisting substance is as completely peripheral as
-the exogenous mode of growth permits. So, too, in general arrangement
-is it with the ordinary monocotyledonous stem. Different as is here
-the internal structure, there yet holds the same general distribution
-of tissues, answering to the same mechanical conditions. The vascular
-woody bundles, more abundant towards the outside of the stem than
-near the centre, produce a harder casing surrounding a softer core.
-In the supporting structures of leaves we find significant deviations
-from this arrangement. While axes are on the average exposed to equal
-strains on all sides, most leaves, spreading out their surfaces
-horizontally, have their petioles subject to strains that are not alike
-in all directions; and in them the hard tissue is differently arranged.
-Its transverse section is not ring-shaped but crescent-shaped: the two
-horns being directed towards the upper surface of the petiole. That
-this arrangement is one which answers to the mechanical conditions,
-is not easy to demonstrate: we must satisfy ourselves by noting that
-here, where the distribution of forces is different, the distribution
-of resisting tissue is different. And then, showing conclusively the
-connexion between these differences, we have the fact that in petioles
-growing vertically and supporting peltate leaves&mdash;petioles which are
-therefore<span class="pagenum" id="Page_279">[279]</span> subject to equal transverse strains on all sides&mdash;the
-vascular bundles are arranged cylindrically, as in axes.</p>
-
-<p>Such, then, are some of the reasons for concluding that the development
-of the supporting tissue in plants, is caused by the incident forces
-which this tissue has to resist. The individuals in which this direct
-balancing of inner and outer actions progresses most favourably, are
-those which, other things equal, are most likely to prosper; and, by
-habitual survival of the fittest, there is established a systematic and
-constant distribution of a deposit adapted to the circumstances of each
-type.</p>
-
-
-<p class="p2">§ 280. The function of circulation may now be dealt with. We have to
-consider here by what structures this is discharged; and what connexion
-exists between the demand for them and the genesis of them.</p>
-
-<p>The contrast between the rates at which a dye passes through simple
-cellular tissue and cellular tissue of which the units have been
-elongated, indicates one of the structural changes required to
-facilitate circulation. If placed with its cut surface in a coloured
-liquid, the parenchyma of a potato or the medullary mass of a
-cabbage-stalk, will absorb the liquid with extreme slowness; but if
-the stalk of a fungus be similarly placed, the liquid runs up it, and
-especially up its loose central substance, very quickly. On comparing
-the tissues which thus behave so differently, we find that whereas in
-the one case the component cells, packed close together, have deviated
-from their primitive sphericity only as much as mutual pressure
-necessitates, in the other case they are drawn out into long tubules
-with narrow spaces among them&mdash;the greatest dimensions of the tubules
-and the spaces being in the direction which the dye takes so rapidly.
-That which we should infer, then, from the laws of capillary action,
-is experimentally shown: liquid moving through tissues follows the
-lines in which the elements of the tissues are most<span class="pagenum" id="Page_280">[280]</span> elongated. It does
-this for two reasons. That narrowing of the cells and intercellular
-spaces which accompanies their elongation, facilitates capillarity;
-and at the same time fewer of the septa formed by the joined ends of
-the cells have to be passed through in a given distance. Hence the
-general fact that the establishment of a rudimentary vascular system,
-is the formation of bundles of cells lengthened in the direction
-which the liquid is to take. This we see very obviously among the
-lower Cormophytes. In one of the lichen-like Liverworts, the veins
-which, branching through its frond, serve as communications with its
-scattered rootlets, are formed of cells longer than those composing the
-general tissue of the frond: the lengths of these cells corresponding
-in their directions with the lengths of the veins. So, too, is it
-with the mid-ribs of such fronds as assume more definite shapes; and
-so, too, is it with the creeping stems which unite many such fronds.
-That is to say, the current which sets towards the growing part from
-the part which supplies certain materials for growth, sets through
-a portion of the tissues composed of units that are longer in the
-line of the current than at right angles to that line. The like is
-true of Phænogams. Omitting all other characteristics of those parts
-of them through which chiefly the currents of sap flow, we find the
-uniform fact to be that they consist of cells and intercellular spaces
-distinguished from others by their lengths. It is thus with veins,
-and mid-ribs, and petioles; and if we wish proof that it is thus with
-stems, we have but to observe the course taken by a coloured solution
-into which a stem is inserted.</p>
-
-<p>What is the original cause of this differentiation? Is it possible
-that this modification of cell-structure which favours the transfer of
-liquid towards each place of demand, is itself caused by the current
-which the demand sets up? Does the stream make its own channel? There
-are various reasons for thinking that it does. In the first place,
-the simplest and earliest channels, such as we see in the Liverworts,
-do not<span class="pagenum" id="Page_281">[281]</span> develop in any systematic way, but branch out irregularly,
-following everywhere the irregular lobes of the fronds as these spread;
-and on examining under a magnifier the places at which the veins are
-lost in the cellular tissue, it will be seen that the cells are there
-slightly longer than those around: suggesting that the lengthening of
-them which produces an extension of the veins, takes place as fast
-as the growth of the tissue beyond causes a current to pass through
-them. In the second place, a disappearance of the granular contents of
-these cells accompanies their union into a vein&mdash;a result which the
-transmission of a current may not improbably bring about. But be the
-special causes of this differentiation what they may, the evidence
-favours very much the conclusion that the general cause is the setting
-up of a current towards a place where the sap is being consumed. In the
-histological development of the higher plants we find confirmation.
-The more finished distributing canals in Phænogams are formed of cells
-previously lengthened. At parts of which the typical structure is
-fixed, and the development direct, this fact is not easy to trace;
-the cells rapidly take their elongated structures in anticipation of
-their predetermined functions. But in places where new vessels are
-required in adaptation to a modifying growth, we may clearly trace
-this succession. The swelling root of a turnip, continually having its
-vascular system further developed, and the component vessels lengthened
-as well as multiplied, gives us an opportunity of watching the process.
-In it we see that the reticulated cells which unite to form ducts,
-arise in the midst of bundles of cells that have previously become
-elongated, and that they arise by transformation of such elongated
-cells; and we also see that these bundles of elongated cells have an
-arrangement suggestive of their formation by passing currents.</p>
-
-<p>Are there grounds for thinking that these further transformations by
-which strings of elongated cells pass into vessels lined with spiral,
-annular, reticulated, or other<span class="pagenum" id="Page_282">[282]</span> frameworks, are also in any way
-determined by the currents of sap carried? There are some such grounds.</p>
-
-<p>As just indicated, the only places where we may look for evidence
-with any rational hope of finding it, are places where some local
-requirement for vessels has arisen in consequence of some local
-development which the type does not involve. In these cases we find
-such evidence. Good illustrations occur in those genera of the
-<i>Cactaceæ</i>, which simulate leaves, like <i>Epiphyllum</i> and
-<i>Phyllocactus</i>. A branch of one of these is outlined in Fig. <a href="#i_178">256</a>.
-As before explained this is a flattened axis; and the notches along
-its edges are the seats of the axillary buds. Most of these axillary
-buds are arrested; but occasionally one of them grows. Now if, taking
-an <i>Epiphyllum</i>-shoot which bears a lateral shoot, we compare the
-parts of it that are near the aborted axillary buds with the part that
-is near the developed axillary bud, we find a conspicuous difference.
-In the neighbourhood of an aborted axillary bud there is no external
-sign of any internal differentiation; and on holding up the branch
-against the light, the uniform translucency shows that there is no
-greater amount of dense tissue near it than in other parts of the
-succulent mass. But where an axillary bud has developed, a prominent
-rounded ridge joins the mid-rib of the lateral branch with the mid-rib
-of the parent branch. In the midst of this rounded ridge an opaque core
-may be seen. And on cutting through it, this opaque core proves to be
-full of vascular bundles imbedded in woody deposits. Clearly, these
-clusters of vessels imply transformations of the tissues, caused by
-the passage of increased currents of sap. The vessels were not there
-when the axillary bud was formed; they would not have developed had
-the axillary bud proved abortive; but they arise as fast as growth
-of the axillary bud draws the sap along the lines in which they lie.
-Verification is obtained by examining the internal structures. If
-longitudinal sections be made through a growing bud of <i>Opuntia</i>
-or<span class="pagenum" id="Page_283">[283]</span> <i>Cereus</i>, it will be found that the vessels in course of
-formation converge towards the point of growth, as they would do if the
-sap-currents determined their formation; that they are most developed
-near their place of convergence, which they would be if so produced;
-and that their terminations in the tissue of the parent shoot are
-partially-formed lines of irregular elongated cells, like those out of
-which the vessels of a leaf or bud are developed.</p>
-
-<p>Concluding, then, that sap-vessels arise along the lines of least
-resistance, through which currents are drawn or forced, the question
-to be asked is&mdash;What physical process produces them? Their component
-cells, united end to end more or less irregularly in ways determined
-by their original positions, form a channel much more permeable, both
-longitudinally and laterally, than the tissue around. How is this
-greater permeability caused? The idea, first propounded I believe by
-Wolff, that the adjoined ends of the cells are perforated or destroyed
-by the passing current, is one for which much is to be said. Whether
-these septa are dissolved by the liquids they transmit, or whether they
-are burst by those sudden gushes which, as we shall hereafter see, must
-frequently take place along these canals, need not be discussed: it is
-sufficient for us that the septa do, in many cases, disappear, leaving
-internal ridges showing their positions; and, in other cases, become
-extremely porous. Though it is manifest that this is not the process
-of vascular development in tissues that unfold after pr-determined
-types, since, in these, the dehiscences or perforations of septa occur
-before such direct actions can have come into play; yet it is still
-possible that the disappearances of septa which now arise by repetition
-of the type were established in the type by such direct actions. Be
-this as it may, however, a simultaneous change undergone by these
-longitudinally-united cells must be otherwise caused. Frame-works are
-formed in them&mdash;frameworks which, closely fitting their inner surfaces,
-may consist either of successive rings, or continuous<span class="pagenum" id="Page_284">[284]</span> spiral threads,
-or networks, or structures between spirals and networks, or networks
-with openings so far diminished that the cells containing them are
-distinguished as fenestrated. Their differences omitted, however, these
-structures have the common character that, while supporting the coats
-of the vessels, they also give special facilities for the passage of
-liquids, both through the sides of the transformed cells and through
-their united ends, where these are not destroyed.</p>
-
-<p>To attempt any physical interpretation of this change is scarcely
-safe: the conditions are so complex. There are reasons for suspecting,
-however, that it arises from a vacuolation of the substance deposited
-on the cell-wall. If rapidly deposited, as it is likely to be along
-lines where sap is freely supplied, this may, in passing from the
-state of a soluble colloid to that of an insoluble colloid, so
-contract as to leave uncovered spaces on the cell-membrane; and this
-change, originally consequent on a physico-chemical action, may be
-so maintained and utilized by natural selection, as to result in
-structures of definite kinds, regularly formed in growing parts in
-anticipation of functions to be afterwards discharged. But, without
-alleging any special cause for this metamorphosis, we may reasonably
-conclude that it is in some way consequent upon the carrying of sap.
-If we examine tissues such as that in the interior of a growing
-turnip that has not yet become stringy, we may, in the first place,
-find bundles of elongated cells not having yet developed in them
-those fenestrated or reticulated structures by which the ducts are
-eventually characterized. Along the centres of adjacent bundles we may
-find incomplete lines of such cells&mdash;some that are partially or wholly
-transformed, with some between them that are not transformed. In other
-bundles, completed chains of such transformed cells are visible. And
-then, in still older bundles, there are several complete chains running
-side by side. All which facts imply a metamorphosis of the<span class="pagenum" id="Page_285">[285]</span> elongated
-cells, indirectly caused by the continued action of the currents
-carried.</p>
-
-
-<p class="p2" id="sect281">§ 281. Here, however, presents itself a further problem. Taking
-it as manifest that there is a typical distribution of supporting
-tissue adapted to meet the mechanical strains a plant is exposed to
-by its typical mode of growth, and also that there goes on special
-adaptation of the supporting tissue to the special strains the
-individual plant has to bear; and taking it as tolerably evident that
-the sap-channels are originally determined by the passage of currents
-along lines of least resistance; there still remains the ultimate
-question&mdash;Through what physical actions are established these general
-and special adjustments of supporting tissue to the strains borne,
-and these distributions of nutritive liquid required to make possible
-such adjustments? Clearly, if the external actions produce internal
-reactions; and if this play of actions and reactions results in a
-balancing of the strains by the resistances; we may rationally suspect
-that the incident forces are directly conducive to the structural
-changes by which they are met. Let us consider how they must work.</p>
-
-<p>When any part of a plant is bent by the wind, the tissues on its convex
-surface are subject to longitudinal tension, and these extended outer
-layers compress the layers beneath them. Such of the vessels or canals
-in these subjacent layers as contain sap, must have some of this sap
-expelled. Part of it will be squeezed through the more or less porous
-walls of the canals into the surrounding tissue, thus supplying it with
-assimilable materials; while part of it, and probably the larger part,
-will be thrust along the canals longitudinally upwards and downwards.
-When the branch or twig or leaf-stalk recoils, these vessels, relieved
-from pressure, expand to their original diameters. As they expand, the
-sap rushes back into them from above and below. In whichever of these
-directions least has been expelled by the compression,<span class="pagenum" id="Page_286">[286]</span> from that
-direction most must return during the dilation; seeing that the force
-which more efficiently resisted the thrusting back of the sap is the
-same force which urges it into the expanded vessels again, when they
-are relieved from pressure. At the next bend of the part a further
-portion of sap will be squeezed out, and a further portion thrust
-forwards along the vessels. This rude pumping process thus serves for
-propelling the sap to heights which it could not reach by capillary
-action, at the same time that it incidentally serves to feed the parts
-in which it takes place. It strengthens them, too, just in proportion
-to the stress to be borne; since the more severe and the more repeated
-the strains, the greater must be the exudation of sap from the vessels
-or ducts into the surrounding tissue, and the greater the thickening
-of this tissue by secondary deposits. By this same action the movement
-of the sap is determined either upwards or downwards, according to the
-conditions. While the leaves are active and evaporation is going on
-from them, these oscillations of the branches and petioles urge forward
-the sap into them; because so long as the vessels of the leaves are
-being emptied, the sap in the compressed vessels of the oscillating
-parts will meet with less resistance in the direction of the leaves
-than in the opposite direction. But when evaporation ceases at night,
-this will no longer be the case. The sap drawn to the oscillating
-parts, to supply the place of the exuded sap, must come from the
-directions of least resistance. A slight breeze will bring it back from
-the leaves into the gently-swaying twigs, a stronger breeze into the
-bending branches, a gale into the strained stem and roots&mdash;roots in
-which longitudinal tension produces, in another way, the same effects
-that transverse tension does in the branches.</p>
-
-<p>Two possible misinterpretations must be guarded against. It is not to
-be supposed that this force-pump action causes movement of the sap
-towards one point rather than another: it is simply an aid to its
-movement. From the stock of sap<span class="pagenum" id="Page_287">[287]</span> distributed through the plant, more
-or less is everywhere being abstracted&mdash;here by evaporation, here by
-the unfolding of the parts into their typical shapes, here by both. The
-result is a tension on the contained liquid columns, which is greatest
-now in this direction and now in that. This tension it is which must be
-regarded as the force that determines the current upwards or downwards;
-and all which the mechanical actions do is to facilitate the transfer
-to the places of greatest demand. Hence it happens that in a plant
-prevented from oscillating, but having a typical tendency to assume
-a certain height and bulk, the demands set up by its unfolding parts
-will still cause currents; and there will still be alternate ascents
-and descents, according as the varying conditions change the direction
-of greatest demand&mdash;the only difference being that, in the absence of
-oscillations, the growth will be less vigorous. Similarly, it must not
-be supposed that mechanical actions are here alleged to be the sole
-causes of wood formation in the individual plant. The tendency of the
-individual plant to form wood at places where wood has been habitually
-formed by ancestral plants, is manifestly a cause, and, indeed, the
-chief cause. In this, as in all other cases, inherited structures
-repeat themselves irrespective of the circumstances of the individual:
-absence of the appropriate conditions resulting simply in imperfect
-repetition of the structures. Hence the fact that in trained trees and
-hothouse shrubs, dense substance is still largely deposited; though
-not so largely as where the normal mechanical strains have acted.
-Hence, too, the fact, that in such plants as the Elephant’s-foot or the
-<i>Welwitschia mirabilis</i>, which for untold generations can have
-undergone no oscillations, there is an extensive formation of wood
-(though not to any considerable height above the ground), in repetition
-of an ancestral type: natural selection having here maintained the
-habit as securing some other advantage than that of support.</p>
-
-<p>Still, it must be borne in mind that though intermittent<span class="pagenum" id="Page_288">[288]</span> mechanical
-strains cannot be assigned as the direct causes of these internal
-differentiations in plants that are artificially sheltered or
-supported, they are assignable as the indirect causes; since the
-inherited structures, repeated apart from such strains, are themselves
-interpretable as accumulated results of such strains acting on
-successive generations of ancestral plants. This will become clear on
-combining the several threads of the argument and bringing it to a
-close, which we may now do.</p>
-
-
-<p class="p2">§ 282. To put the co-operative actions in their actual order,
-would require us to consider them as working on individuals small
-modifications that become conspicuous and definite only by inheritance
-and gradual increase; but it will aid our comprehension without leading
-us into error, if we suppose the whole process resumed in a single
-continuously-existing plant.</p>
-
-<p>As the plant erects the integrated series of fronds whose united
-parts form its rudimentary axis, the increasing area of frond surface
-exposed to the sun’s rays entails an increasing draught upon the
-liquids contained in the rudimentary axis. The currents of sap so
-produced, once established along certain lines of cells that offer
-least resistance, render them by their continuous passage more and
-more permeable. This establishment of channels is aided by the wind.
-Each bend produced by it while yet the tissue is undifferentiated,
-squeezes towards the place of growth and evaporation the liquids
-that are passing by osmosis from cell to cell; and when the lines of
-movement become defined, each bend helps, by forcing the liquid along
-these lines, to remove obstructions and make continuous canals. As fast
-as this transfer of sap is facilitated, so fast is the plant enabled
-further to raise itself, and add to its assimilating surfaces; and so
-fast do the transverse strains, becoming greater, give more efficient
-aid. The canals thus formed can be neither in the centre of the
-rudimentary axis nor at its surface: for at neither of these<span class="pagenum" id="Page_289">[289]</span> places
-can the transverse strains produce any considerable compressions.
-They must arise along a tract between the outside of the axis and its
-core&mdash;a tract along which there occur the severest squeezes between the
-stretched outer layers and the internal mass. Just that distribution
-which we find, is the distribution which these mechanical actions tend
-to establish.</p>
-
-<p>As the plant gains in height, and as the mass of its foliage
-accumulates, the strains thrown upon its axis, and especially the
-lower part of its axis, rapidly increase. Supposing the forms to
-remain similar, the strains must increase in the ratio of the cubes of
-the dimensions; or even in a somewhat higher ratio. One consequence
-must be that the compressions to which the vessels at the lower
-part of the incipient stem are subject, become greater as fast as
-the height to which the sap has to be raised becomes greater; and
-another consequence must be that the local exudation of sap produced
-by the pressure is proportionately augmented. Hence the materials for
-interstitial nutrition being there supplied more abundantly, we may
-expect thickening of the surrounding tissues to show itself there
-first: in other words, wood will be formed round the vessels of the
-lower part of the incipient stem. The resulting greater ability of this
-lower part of the stem to bear strains, renders possible an increase of
-height; and while after an increase of height the lowest part becomes
-still further strained, and still further thickens, the part above it,
-exposed to like actions, undergoes a like thickening. This induration,
-while it spreads upwards, also spreads outwards. As fast as the rude
-cylinder of dense matter formed in this way, begins to inclose the
-original vessels, it begins to play the part of a resistant mass, which
-more and more prevents the contained vessels from being squeezed;
-while between it and the outer layers the greatest compression occurs
-at each bend. Thus at the same time that the original vessels become
-useless, the peripheral cells of the developing wood become those which
-have their liquid<span class="pagenum" id="Page_290">[290]</span> contents squeezed out longitudinally and laterally
-with increasing force; and, consequently, amid them are formed new
-sap-channels, from which there is the most active local exudation,
-producing the greatest deposit of dense matter.</p>
-
-<p>Thus fusing together, as it were, the individualities of successive
-generations of plants, and recognizing as all-important that
-facilitation of the process which natural selection has all along
-given, we are enabled to interpret the chief internal differentiations
-of plants as consequent on an equilibration between inner and outer
-forces. Here, indeed, we see illustrated in a way more than usually
-easy to follow, the eventual balancing of outer actions by inner
-reactions. The relation between the demand for liquid and the formation
-of channels that supply liquid, as well as that between the incidence
-of strains and the deposit of substance which resists strains, are
-among the clearest special examples of the general truth that the
-moving equilibrium of an organism, if not overthrown by an incident
-force, must eventually be adjusted to it.</p>
-
-<p>The processes here traced out are, of course, not to be taken as the
-only differentiating processes to which the inner tissues of plants
-have been subject. Besides the chief changes we have considered,
-various less conspicuous changes have taken place. These must be
-passed over as arising in ways too involved to admit of specific
-interpretations; even supposing them to have been produced by causes of
-the kind assigned. But the probability, or rather indeed the certainty,
-is that some of them have not been so produced. Here, as in nearly
-all other cases, indirect equilibration has worked in aid of direct
-equilibration; and in many cases indirect equilibration has been the
-sole agency. Besides ascribing to natural selection the rise of various
-internal modifications of other classes than those above treated, we
-must ascribe some even of these to natural selection. It is so with the
-dense deposits which form thorns and the shells of nuts: these cannot
-have resulted from any inner reactions immediately<span class="pagenum" id="Page_291">[291]</span> called forth by
-outer actions; but must have resulted immediately through the effects
-of such outer actions on the species. Let it be understood, therefore,
-that the differentiations to which the foregoing interpretation
-applies, are only those most conspicuous ones which are directly
-related to the most conspicuous incident forces. They must be taken as
-instances on the strength of which we may conclude that other internal
-differentiations have had a natural genesis, though in ways that we
-cannot trace.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_292">[292]</span></p>
-
-<h3>CHAPTER V.<br />
-<span class="subhed1">PHYSIOLOGICAL INTEGRATION IN PLANTS.</span></h3></div>
-
-
-<p>§ 283. A good deal has been implied on this topic in the preceding
-chapters. Here, however, we must for a brief space turn our attention
-immediately to it.</p>
-
-<p>Plants do not display integration in such distinct and multiplied
-ways as do animals. But its advance may be traced both directly and
-indirectly&mdash;directly in the increasing co-ordination of actions, and
-indirectly in the effect of this upon the powers and habits.</p>
-
-<p>Let us group the facts under these heads: ascending in both cases from
-the lower to the higher types.</p>
-
-
-<p class="p2">§ 284. The inferior <i>Algæ</i>, along with little unlikeness of
-parts, show us little mutual dependence of parts. Having surfaces
-similarly circumstanced everywhere, much physiological division of
-labour cannot arise; and therefore there cannot be much physiological
-unity. Among the superior <i>Algæ</i>, however, the differentiation
-between the attached part and the free part is accompanied by some
-integration. There is evidently a certain transfer of materials, which
-is doubtless facilitated by the elongated forms of the cells in the
-stem, and probably leads to the formation of dense tissue at the places
-of greatest strain, in a way akin to that recently explained in other
-cases. And where there is this co-ordination of actions, the parts are
-so far mutually dependent that<span class="pagenum" id="Page_293">[293]</span> each dies if detached from the other.
-That though the organization is so low neither part can reproduce the
-other and survive by so doing, is probably due to the circumstance
-that neither part contains any considerable stock of untransformed
-protoplasm, out of which new tissues may be produced.</p>
-
-<p>Fungi and Lichens present no very significant advances of integration.
-We will therefore pass at once to the Archegoniates. In those of them
-which, either as single fronds or strings of fronds, spread over
-surfaces, and which, rooting themselves as they spread, do not need
-that each part should receive aid from remote parts, there is no
-developed vascular system serving to facilitate transfer of nutriment:
-the parts being little differentiated there is but little integration.
-But along with assumption of the upright attitude and the accompanying
-specializations, producing vessels for distributing sap and hard tissue
-for giving mechanical support, there arises a decided physiological
-division of labour; rendering the aërial part dependent on the embedded
-part and the embedded part dependent on the aërial part. Here, indeed,
-as elsewhere, these concomitant changes are but two aspects of the
-same change. Always the gain of power to discharge a special function
-involves a loss of power to perform other functions; and always,
-therefore, increased mutual dependence constituting physiological
-integration, must keep pace with that increased fitting of particular
-parts to particular duties which constitutes physiological
-differentiation.</p>
-
-<p>Making a great advance among the Archegoniates, this physiological
-integration reaches its climax among Phænogams. In them we see
-interdependence throughout masses that are immense. Along with
-specialized appliances for support and transfer, we find an exchange of
-aid at great distances. We see roots giving the vast aërial growth a
-hold tenacious enough to withstand violent winds, and supplying water
-enough even during periods of drought; we see a stem and branches of
-corresponding strength for upholding<span class="pagenum" id="Page_294">[294]</span> the assimilating organs under
-ordinary and extraordinary strains; and in these assimilating organs
-we see elaborate appliances for yielding to the stem and roots the
-materials enabling them to fulfil their offices. As a consequence of
-which greater integration accompanying the greater differentiation,
-there is ability to maintain life over an immense period under marked
-vicissitudes.</p>
-
-<p>Even more conspicuously exemplified in Phænogams, is that physiological
-integration which holds together the functions not of the individual
-only but of the species as a whole. The organs of reproduction, both in
-their relations to other parts of the individual bearing them and in
-their relations to corresponding parts of other individuals, show us a
-kind of integration conducing to the better preservation of the race;
-as those already specified conduce to the better preservation of the
-individual. In the first place, this greater co-ordination of functions
-just described, itself enables Phænogams to bequeath to the germs they
-cast off, stores of nutriment, protective envelopes, and more or less
-of organization: so giving them greater chances of rooting themselves.
-In the second place, certain differentiations among the parts of
-fructification, the meaning of which Mr. Darwin has so admirably
-explained, give to the individuals of the species a kind of integration
-that makes possible a mutual aid in the production of vigorous
-offspring. And it is interesting to observe how, in that dimorphism
-by which in some cases this mutual aid is made more efficient, the
-greater degree of integration is dependent on the greater degree of
-differentiation&mdash;not simply differentiation of the fructifying organs
-from other parts of the plant bearing them, but differentiation of
-these fructifying organs from the homologous organs of neighbouring
-individuals of the same race. Another form of this co-ordination of
-functions which conduces to the maintenance of the species, may be
-here named&mdash;partly for its intrinsic interest. I refer to the strange
-processes of multiplication occurring in the genus <i>Bryophyllum</i>.
-It is well known that<span class="pagenum" id="Page_295">[295]</span> the succulent leaves of <i>B. calycinum</i>,
-borne on foot-stalks so brittle that they are easily snapped by the
-wind, send forth from their edges when they fall to the ground,
-buds which root themselves and grow into independent plants. The
-correlation here obviously furthering the preservation of the race,
-is more definitely established in another species of the genus&mdash;<i>B.
-proliferum</i>. This plant, shooting up to a considerable height, and
-having a stem containing but little woody fibre, habitually breaks
-near the bottom while still in flower; and is thus generally prevented
-from ripening its seeds. The multiplication is, however, secured in
-another way. Before the stem is broken young plants have budded out
-from the pedicels of the flowers, and have grown to considerable
-lengths; and on the fall of the parent they forthwith commence their
-separate lives. Here natural selection has established a remarkable
-kind of co-ordination between a special habit of growth and decay, and
-a special habit of proliferation.</p>
-
-
-<p class="p2">§ 285. The advance of physiological integration among plants as we
-ascend to the higher types, is implied by their greater constancy of
-structure, as well as by the stricter limitations of their habitats and
-modes of life. “Complexity of structure is generally accompanied with a
-greater tendency to permanence in form,” says Dr. [now Sir J.] Hooker;
-or, conversely, “the least complex are also the most variable.” This is
-the second aspect under which we have to contemplate the facts.</p>
-
-<p>The differences between the simpler <i>Algæ</i> and <i>Fungi</i>
-are so feebly marked that botanists have had great difficulty in
-framing definitions of these classes. This structural indefiniteness
-is accompanied by functional indefiniteness. <i>Algæ</i>, which are
-mostly aquatic, include many small forms that frequent the damp places
-preferred by <i>Fungi</i>. Among <i>Fungi</i>, there are kinds which
-lead submerged lives like the <i>Algæ</i>. Besides this indistinctness
-of the classes, there is great variability in the shapes and modes
-of life of their species&mdash;a variability<span class="pagenum" id="Page_296">[296]</span> so great that what were at
-first taken to be different species, or different genera, or even
-different orders, have proved to be merely varieties of one species.
-So inconstant in structure are the <i>Algæ</i> that Schleiden quotes
-with approval the opinion of Kutzing, that “there are no species but
-merely forms of <i>Algæ</i>:” an opinion which though now rejected
-sufficiently implies extreme indefiniteness. In all which facts we see
-that these lowest types of plants, little differentiated, are also but
-little integrated.</p>
-
-<p>Archegoniates present a like relation between the small specialization
-of functions which constitutes physiological differentiation, and
-the small combination of functions which constitutes physiological
-integration. “Mosses,” says Mr. Berkeley, “are no less variable than
-other cryptogams, and are therefore frequently very difficult to
-distinguish. Not only will the same species exhibit great diversity
-in the size, mode of branching, form and nervation of the leaves, but
-the characters of even the peristome itself are not constant.” And
-concerning the classification of the remaining group, <i>Filicales</i>,
-he says:&mdash;“Not only is there great difficulty in arranging ferns
-satisfactorily, but it is even more difficult to determine the limits
-of species.”</p>
-
-<p>After this vagueness of separation as well as inconstancy of structure
-and habit among the lower plants, the stability of structure and
-habit and divisibility of groups among the higher plants, appear
-relatively marked. Though Phænogams are much more variable than most
-botanists have until lately allowed, yet the definitions of species and
-genera may be made with far greater precision, and the forms are far
-less capable of change, than among Cryptogams. And this comparative
-fixity of type, implying, as it does, a closer combination of the
-component functions, we see to be the accompaniment of the greater
-differentiation of those functions and of the structures performing
-them. That these characters are correlatives is further shown by the
-fact that the higher plants are more restricted in their habitats
-than<span class="pagenum" id="Page_297">[297]</span> the lower plants, both in space and time. “The much narrower
-delimitation in area of animals than plants,” says Sir J. Hooker, “and
-greater restriction of Faunas than Floras, should lead us to anticipate
-that plant-types are, geologically speaking, more ancient and permanent
-than the higher animal types are, and so I believe them to be, and I
-would extend the doctrine even to plants of highly complex structure.”
-“Those classes and orders which are the least complex in organization
-are the most widely distributed.”</p>
-
-
-<p class="p2" id="sect286">§ 286. Thus that which the general doctrine of evolution leads us
-to anticipate, we find implied by the facts. The physiological
-division of labour among parts, can go on only in proportion to the
-mutual dependence of parts; and the mutual dependence of parts can
-progress only as fast as there arise structures by which the parts are
-efficiently combined, and the mutual utilization of their actions made
-easy.</p>
-
-<p>To say definitely by what process is brought about this co-ordination
-of functions which accompanies their specialization, is hardly
-practicable. Direct and indirect equilibration doubtless co-operate
-in establishing it. We may see, for example, that every increase of
-fitness for function produced in the aërial part of a plant by light,
-as well as every increase of fitness for function produced in its
-imbedded part by the direct action of the moist earth, must conduce
-to an increased current of the liquid evaporated from the one and
-supplied by the other&mdash;must serve, therefore, to aid the formation
-of sap-channels in the ways already described; that is&mdash;must serve
-to develop the structures through which mutual aid of the parts is
-given: the additional differentiation tends immediately to bring
-about the additional integration. Contrariwise, it is obvious that
-an interdependence such as we see between the secretion of honey and
-the fertilization of germs, or between the deposit of albumen in the
-cotyledons of an embryo-plant and its subsequent striking root, is
-a kind of integration in the actions of the individual or of the<span class="pagenum" id="Page_298">[298]</span>
-species, which no differentiation has a direct tendency to initiate.
-Hence we must regard the total results as due to a plexus of influences
-acting simultaneously on the individual and on the species: some
-chiefly affecting the one and some chiefly affecting the other.</p>
-
-<hr class="tb" />
-
-<p>[<span class="smcap">Note.</span>&mdash;In <i>Nature</i> for June 11, 1896, Dr. Maxwell
-Masters, in an essay on “Plant Breeding,” names an instructive fact
-concerning the production of varieties by selection of slightly
-divergent forms. He says:&mdash;</p>
-
-<p>“To the untrained eye, the primordial differences noted are often very
-slight; even the botanist, unless his attention be specially directed
-to the matter, fails to see minute differences which are perceptible
-enough to the raiser or his workmen. Nor must it be thought that these
-variations, difficult as they are to recognise in the beginning, are
-unimportant. On the contrary, they are interesting, physiologically,
-as the potential origin of new species, and very often they are
-commercially valuable also. These apparently trifling morphological
-differences are often associated with physiological variations which
-render some varieties, say of wheat, much better enabled to resist
-mildew and disease generally than others. Some, again, prove to be
-better adapted for certain soils or for some climates than others; some
-are less liable to injury from predatory birds than others, and so on.”</p>
-
-<p>Thus we are shown that, to a much greater degree than might be
-supposed, minute changes of forms and functions in one part of a plant
-are correlated with changes of forms and functions throughout it. The
-interdependence&mdash;that is to say, the physiological integration&mdash;is very
-close at the same time that it is very complex.</p>
-
-<p>Here while naming these facts in illustration of physiological
-integration in plants I name them because they illustrate an important
-truth bearing upon the general question of heredity which I have dealt
-with in Appendix G, and to which I now especially draw attention.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_299">[299]</span></p>
-
-<h3>CHAPTER VI.<br />
-<span class="subhed1">DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF ANIMALS.</span></h3></div>
-
-
-<p>§ 287. What was said respecting the primary physiological
-differentiation in plants, applies with little beyond change of terms
-to animals. Among <i>Protozoa</i>, as among <i>Protophyta</i>, the
-first definite contrast of parts is that between outside and inside.
-The speck of jelly or sarcode which appears to constitute the simplest
-animal, proves, on closer examination, to be a mass of substance
-containing a nucleus&mdash;a periplast in the midst of which there is a
-minute endoplast, consisting of a spherical membrane and its contents.</p>
-
-<p>This parallel, only just traceable among these Rhizopods, which are
-perpetually changing the distribution of their outer substance, becomes
-at once marked in those higher <i>Protozoa</i> which have fixed shapes,
-and maintain constant relations between their surfaces and their
-environments. Indeed the Rhizopods themselves, on passing into a state
-of quiescence in which the relations of outer and inner parts are
-fixed, become encysted: there is formed a hardened outer coat different
-from the matter which it contains. And what is here a temporary
-character answering to a temporary definiteness of conditions, is
-in the <i>Infusoria</i> a constant character, answering to definite
-conditions that are constant. Each of these minute creatures, though
-not coated by a distinct membrane, has an outer layer of excreted
-substance forming a delicate cuticle.</p>
-
-
-<p class="p2" id="sect288">§ 288. The early establishment of this primary contrast of<span class="pagenum" id="Page_300">[300]</span> tissues
-answering to this primary contrast of conditions, is no less
-conspicuous in aggregates of the second order. The feebly-integrated
-units of a Sponge, with individualities so little merged in that of
-the whole they form that most of them still retain their separate
-activities, nevertheless show us, in the unlikeness that arises between
-the outermost layer and the contained mass, the effect of converse with
-unlike conditions. This outermost layer is composed of units somewhat
-flattened and united into a continuous membrane&mdash;a kind of rudimentary
-skin.</p>
-
-<p>Secondary aggregates in which the lives of the units are more
-subordinate to the life of the whole, carry this distinction further.
-The leading physiological trait of every cœlenterate animal is the
-divisibility of its substance into endoderm and ectoderm&mdash;the part
-next the food and the part next the environment. Fig. <a href="#i_090">147</a> (<a href="#sect201">§ 201</a>),
-representing a portion of the body-wall of a <i>Hydra</i> seen
-in section, gives some idea of this fundamental differentiation.
-The creature consists of a simple sac, the cavity of which is in
-communication with the surrounding water; and hence the unlikeness
-between the outer and inner layers has not become great. The essential
-contrast is that between the differentiated parts of what was
-originally the same part&mdash;a uniform membrane composed of juxtaposed
-cells.</p>
-
-<p>For here, indeed, we are shown unmistakably how the primary contrast
-of structures follows upon the primary contrast of conditions. The
-ordinary form from which low types of the <i>Metazoa</i> set out, is a
-hollow sphere formed of cells packed side by side&mdash;a blastula, as it
-is called: all these cells being similarly exposed to the environment.
-The blastula presently changes into what is called a gastrula&mdash;a form
-resulting from the introversion of one of the sides of the blastula.
-If there be taken a small ball of vulcanized india-rubber, say an
-inch or more in diameter, and having a hole in it through which the
-air may escape, and if one side of it be thrust inwards so as to
-produce a cup, and if the<span class="pagenum" id="Page_301">[301]</span> wide opening of the cup be supposed to
-contract, thus becoming a narrow opening, there will result something
-like the gastrula form. Manifestly that part of the original layer
-which has become internal is differently conditioned from the rest
-which remains external: the one continuing to hold converse with the
-forces of the environment, while the other begins to hold converse
-with the nutritive matters taken into the sac-formed chamber&mdash;the
-archenteron or primitive stomach. Interesting evidence of the primitive
-externality of the digestive cavity is yielded by the fact that whereas
-the blastula consisted of ciliated cells, and whereas the ciliation
-persists throughout life on the outer layer, or parts of it, in sundry
-low types&mdash;even in some Chætopods&mdash;it persists also on the alimentary
-tract of sundry low types: not only in the <i>Hydra</i> but commonly in
-Nemertines, in some <i>Platyhelminthes</i>, and even in some leeches.</p>
-
-<p>Besides being enabled thus to understand how an aggregate of
-<i>Amœba</i>-form units, originally consisting of a single layer,
-may pass into an aggregate consisting of a double layer; we may also
-understand under what influences the transition takes place. If the
-habit which some of the primary aggregates have, of wrapping themselves
-round masses of nutriment, is followed by a secondary aggregate,
-there will naturally arise just that re-differentiation which the
-<i>Hydra</i> shows us.</p>
-
-
-<p class="p2" id="sect289">§ 289. This account of the primary differentiation carries us only
-half-way towards a true conception of the distinction between outer
-and inner tissues. Though, using words in their current senses, this
-introverted part of the primitive layer has become internal in contrast
-with the remainder, which continues external, yet this introverted
-part has not become internal in the strict physiological sense. For
-it remains subject to the actions of those environing matters which
-are taken in as food: such environing matters, when they happen to be
-moving prey, acting upon it much as they<span class="pagenum" id="Page_302">[302]</span> might act upon the exterior.
-So that this introverted part has a quasi-externality. It has not
-the same absolute internality as have those parts which never come
-in contact with products of the outer world. Here we must briefly
-recognize the distinction between these parts and the parts thus far
-considered.</p>
-
-<p>Reverting to our symbol, the india-rubber ball, it will be seen that
-the introversion may be so complete that the cavity is obliterated,
-with the result that the internal surfaces of the outer and inner
-layers come in contact. This is the state reached in the simplest
-cœlenterate animal, the <i>Hydra</i>: there being in it nothing more
-than a thin structureless lamella between the ectoderm and endoderm,
-as shown in Fig. <a href="#i_090">147</a>. This lamella represents all that there is of
-strictly internal tissues. But the introversion, instead of bringing
-the inner surfaces of the ball into contact, may be so far incomplete
-as to leave a space, and in various creatures and embryos of others,
-symbolized by this arrangement, this space becomes occupied by a tissue
-formed from one or other or both of the two primary tissues&mdash;the
-mesoblast or mesoderm. This intermediate layer, sometimes, as in the
-<i>Medusa</i>, growing into a mass of jelly serving as a fulcrum for
-the creature’s contractions, or, as in the Sponge, giving a passive
-basis to the active tissues, becomes in higher animals the layer out
-of which the structures that support the body and move it about, as
-well as those that distribute prepared nutriment, are developed. From
-it arise the bones, the muscles, and the vascular system&mdash;the masses
-of differentiated tissue which are truly internal and occupy what is
-called the body-cavity or peri-visceral space.</p>
-
-<p>In the higher types of animals this space comes to be partially
-occupied by a structure that may be described as a cavity within
-a cavity&mdash;the cœlom. Most zoologists regard this as arising by a
-re-introversion of the <i>archenteron</i> or primary alimentary sac. It
-is easily to be perceived that after the introversion which produces
-this digestive cavity, the<span class="pagenum" id="Page_303">[303]</span> wall of the cavity may be again introverted
-in such way as to intrude into the peri-visceral space. The cœlom thus
-formed is subsequently shut off. Becoming included among the more
-truly internal structures, and in part giving origin to certain lining
-membranes, it has for its chief function the formation of organs for
-the excretion and emission of nitrogenous waste and of the generative
-products: some portions of it retaining, as a consequence, indirect
-connexions with the environment and characters usually accompanying
-such connexions.</p>
-
-<p>Here we are not concerned with further details: the aim being simply to
-indicate the way in which out of the original layer, wholly external,
-there arise, by primary and secondary introversions, and the formation
-of intermediate membranes and spaces, the chief contrasts between
-outer and inner tissues, and how there simultaneously go on the
-differentiations accompanying different conditions.</p>
-
-
-<p class="p2">§ 289<i>a</i>. Another all-important differentiation between outer
-tissues and inner tissues has now to be set forth&mdash;that by which the
-nervous system becomes established and distinguished. Strangely enough,
-like the one above described, it is sequent upon an introversion:
-the nervous system is primarily a skin-structure and develops by the
-infolding of this skin-structure.</p>
-
-<p>In creatures possessing the earliest rudiments of nerves these exist
-in certain superficial cells. Each has a small tubular orifice from
-which projects a minute hair, and each has on its under side processes
-running into the tissue below, and serving, as it seems, to conduct
-impressions from the projecting hair when it is disturbed by contacts
-with foreign bodies. A plexus of fibres bringing the inner processes
-of such cells into communication arises, and forms something like
-a nervous layer capable of propagating impulses in all directions.
-At a subsequent stage some of the superficial cells, ceasing to be
-themselves the recipients of external stimuli, sink inwards and become
-ganglion-cells connected<span class="pagenum" id="Page_304">[304]</span> with the nervous plexus&mdash;agents, as we must
-suppose, for the reception, multiplication, and diffusion of the
-impulses received from the outer cells.</p>
-
-<p>As thus far developed, the nervous structure is one fitted only
-for a vague stimulation of dispersed contractile fibres, causing
-movements of an undirected kind. A concentration of these superficial
-nervous structures is a probable preliminary to the next change&mdash;an
-all-important change. For a part of the surface begins to sink inwards,
-forming, in the <i>Vertebrata</i>, a groove; and from the lining cells
-of this groove, which presently closes over, the central parts of
-the nervous system arise: definite nerves having meantime, as we may
-suppose, been developed out of the indefinite nervous plexus.</p>
-
-<p>Neglecting what there is in this of a speculative nature, it is
-sufficient for the present purpose to recognize the undoubted fact that
-the nervous system is developed from the ectoderm, and that, originally
-external, it is made internal by a process of sinking in or by a
-process of definite introversion.</p>
-
-
-<p class="p2">§ 290. Whether direct equilibration or indirect equilibration has had
-the greater share in producing these fundamental contrasts between
-the inner and outer tissues of animals, must be left undecided. The
-two causes have all along co-operated&mdash;modification of the individual
-accumulated by inheritance predominating in some cases, and in other
-cases modification of the race by survival of the incidentally fittest.
-On the other hand, the action of the medium on the organism cannot fail
-to change its surface more than its centre, and so differentiate the
-two; while, on the other hand, the surfaces of organisms inhabiting
-the same medium display extreme unlikenesses which cannot be due to
-the immediate actions of their medium. Let us dwell a moment on the
-antithesis.</p>
-
-<p>We have abundant evidence that animal protoplasm is rapidly modified by
-light, heat, air, water, and the salts contained in water&mdash;coagulated,
-turned from soluble into insoluble, partially changed into isomeric
-compounds, or otherwise<span class="pagenum" id="Page_305">[305]</span> chemically altered. Immediate metamorphoses
-of this kind are often obviously produced in ova by changes of their
-media. At the outset, therefore, before yet there existed any such
-differentiation as that which now usually arises by inheritance,
-these environing agencies must have tended to originate a protective
-envelope. For a modification produced by them on the superficial part
-of the protoplasm, must either have been a decomposition or else the
-formation of a compound which remained stable under their subsequent
-action. There would be generated an outer layer of substance that was
-so molecularly immobile as to be incapable of further metamorphoses,
-while it would shield the contained protoplasm from that too-great
-action of external forces which, by rapidly changing the unstable
-equilibrium of its molecules into a relatively stable equilibrium,
-would arrest development. Evidently organic evolution, whether
-individual or general, must always and everywhere have been subordinate
-to these physical necessities. Though natural selection, beginning with
-minute portions of protoplasm, must all along have tended to establish
-a molecular composition apt to undergo this differentiation of surface
-from centre to the most favourable extent, yet it must all along have
-done so while controlled by this process of direct equilibration.</p>
-
-<p>Contrariwise, the many and great unlikenesses among the dermal
-structures of creatures inhabiting the same element, cannot be ascribed
-to any such cause. The contrasts between naked and shelled Gastropods,
-between marine Worms and Crustaceans, between soft-skinned Fishes and
-Fishes in armour like the <i>Pterichthys</i>, must have been produced
-entirely by natural selection. Environing forces are, as before, the
-ultimate causes; but the forces are now not so much those exercised by
-the medium as those exercised by the other inhabitants of the medium;
-and they do not act by modifying the surface of the individual, but
-by killing off individuals whose surfaces are least fitted to the
-requirements: thus slowly affecting the species. Still the dermal
-skeleton bristling<span class="pagenum" id="Page_306">[306]</span> with spines, which protects the <i>Diodon</i> or
-the <i>Cyclichthys</i> from enemies it could not escape, comes within
-the general formula of an outer tissue differentiated from inner
-tissues by the outer actions to which the creature is exposed: the
-differentiation having gone on until there is equilibrium between the
-destructive forces to be met and the protective forces which meet them.</p>
-
-<p>If we venture to apportion the respective shares which mediate and
-immediate actions have had in differentiating outer from inner tissues,
-we shall probably not be far wrong in ascribing that part of the result
-which is alike in all animals, mainly to the direct actions of their
-media, while we ascribe the multitudinous unlikenesses of the results
-in various animals, partly to the indirect actions of the media, and
-partly to the indirect actions of other animals by which the media are
-inhabited. That is to say, while assigning the specialities of the
-differentiations to the specialities of converse with the agencies in
-the environment, most of them organic, we may assign to the constant
-and universal converse with its inorganic agencies, the universal
-characteristic of tegumentary structures&mdash;their growth outwards from a
-layer lying below the surface which continually produces new substance
-to replace the substance worn away or cast off.</p>
-
-<p>Here let me add a piece of evidence which strengthens the general
-argument, at the same time that it justifies this apportionment. When
-ulceration has gone deep enough to destroy the tegumentary structures,
-these are never reproduced. The puckered surface formed where an ulcer
-heals, or where a serious burn has destroyed the skin, consists of
-modified connective tissue, which, as the healing goes on, spreads
-inwards from the edges of the ulcer: some of it, perhaps, growing from
-the portions of connective tissue that dip down between the muscular
-bundles. This connective tissue is normally covered by the epidermis
-and thus sheltered from environing actions. What has happened to it?
-It has now become the outermost layer. And how does it comport<span class="pagenum" id="Page_307">[307]</span> itself
-under its new conditions? It produces a superficial substance which
-plays the part of the epidermis and grows outwardly. For since the
-surface, subject to friction and exfoliation, has to be continually
-renewed, there must be a continual reproduction of an outermost layer
-from a layer beneath. That is to say, the contact of this deep-seated
-tissue with outer agencies, produces in it some approach towards
-that character which we find universally characterizes outer tissue.
-But while we see under this exposure to the conditions common to
-all integument, a tendency to assume the structure common to all
-integument, we see no tendency to assume any of the specialities of
-tegumentary structure: no rudiments of glands or hair sacs make their
-appearance.</p>
-
-<p>Analogous conclusions may be drawn respecting the processes of
-differentiation by which from the outer layer nervous tissue and
-finally a nervous system are evolved. Here, also, both direct and
-indirect equilibration appear to have operated. Two reasons may be
-assigned for the belief that the transformation of certain superficial
-cells into sensitive cells was initiated by exposure to external
-stimuli. The first is that, extremely unstable as protoplasm is,
-disturbances received by the outer side of a specially-exposed cell
-could scarcely fail to cause changes passing through it towards the
-interior mass of the body, and that perpetual repetition of such
-changes would tend to generate channels of easy transmission through
-the protoplasm. The second reason is that, if we do not assume this
-process of initiation but assume that survival of the fittest was
-the sole agency, then no reason can be assigned why the nervous
-system should not have been at the outset formed internally instead
-of being initiated externally and then transferred to the interior:
-the roundabout process would be inexplicable. At the same time the
-production of a central nervous system by introversion of superficial
-sensitive cells cannot be ascribed to the differentiating effects
-of external stimuli, but must be ascribed to natural selection. No
-perpetual repetition of<span class="pagenum" id="Page_308">[308]</span> outer disturbances would cause the sinking
-inwards, and covering up, of the specially-sensitive area and
-the plexus below it. But it is manifest that since these nervous
-structures, at once all-important and easily injured, would be safer
-if removed from the surface, survival of the fittest, continually
-preserving those in which they were more deeply seated, would tend to
-produce an arrangement in which all parts but the actual receivers of
-external stimuli became internal.</p>
-
-<p>Hence, contemplating generally these two fundamental differentiations
-of inner from outer tissues, we may conclude that though their first
-stages resulted from direct equilibration, their subsequent and higher
-stages resulted from indirect equilibration.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_309">[309]</span></p>
-
-<h3>CHAPTER VII.<br />
-<span class="subhed1">DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS.</span></h3></div>
-
-
-<p>§ 291. The outer tissues of animals, originally homogeneous over their
-whole surfaces, pass into a heterogeneity which fits their respective
-parts to their respective conditions. So numerous and varied are the
-implied differentiations, that it is impracticable here to deal with
-them all even in outline. To trace them up through classes of animals
-of increasing degrees of aggregation, would carry us into undue detail.</p>
-
-<p>Did space permit, it would be possible to point out among the
-<i>Protozoa</i>, various cases analogous to that of the <i>Arcella</i>;
-which may be described as like a microscopic Limpet, having a
-sarcode body of which the upper surface has become horny, while
-the lower surface with its protruding pseudopodia, retains the
-primitive jelly-like character. That differentiations of this
-kind have been gradually established among these minute creatures
-through the unlike relations of their parts to the environment, is
-an inference supported by a form which, while the rest of the body
-has a scarcely distinguishable coating, “agrees with <i>Arcella</i>
-and <i>Difflugia</i> in having the pseudopodia protrusible from one
-extremity only of the body.”</p>
-
-<p>Many parallel specializations of surface among aggregates of the
-second order might be instanced from the <i>Cœlenterata</i>. In
-the <i>Hydra</i>, the ectoderm presents over its whole area no
-conspicuous unlikenesses; but there usually exist in the hydroid
-polypes of superior types, decided contrasts between<span class="pagenum" id="Page_310">[310]</span> the higher and
-lower parts. While the higher parts retain their original characters,
-the lower parts excrete hard outer layers yielding support and
-protection. Various stages of the differentiation might be followed.
-“In <i>Hydractinia</i>,” says Prof. Green, this horny layer “becomes
-elevated at intervals to form numerous rough processes or spines,
-while over the general surface of the ectoderm its presence is almost
-imperceptible.” In other types, as in <i>Cordylophora</i>, it spreads
-part way up the animal’s sides, ending indefinitely. In <i>Bimeria</i>
-it “extends itself so as to enclose the entire body of each polypite,
-leaving bare only the mouth and tips of the tentacles.” While in
-<i>Campanularia</i> it has become a partially-detached outer cell, into
-which the creature can retract its exposed parts.</p>
-
-<p>But it is as needless as it would be wearisome to trace through the
-several sub-kingdoms the rise of these multiform contrasts, with the
-view of seeking interpretations of them. It will suffice if we take a
-few groups of the illustrations furnished by the higher animals.</p>
-
-
-<p class="p2">§ 292. We may begin with those modifications of surface which subserve
-respiration. Though we ordinarily think of respiration as the quite
-special function of a quite special organ, yet originally it is not
-so. Little-developed animals part with their carbonic acid and absorb
-oxygen, through the general surface of the body. Even in the lower
-types of the higher classes, the general surface of the body aids
-largely in aërating the blood; and the parts which discharge the
-greater part of this function are substantially nothing more than
-slightly altered and extended portions of the skin.</p>
-
-<p>Such differentiations, marked in various degrees, are to be seen among
-<i>Mollusca</i>. In the <i>Pteropoda</i> the only modification which
-appears to facilitate respiration, is the minute vascularity of one
-part of the skin. Higher types possess special skin-developments.
-The <i>Doris</i> has appendages developed into elaborately-branched
-forms&mdash;small trees of blood-vessels<span class="pagenum" id="Page_311">[311]</span> covered by slightly-changed dermal
-tissues. And these arborescent branchiæ are gathered together into a
-single cluster. Thus there is evidence that large external respiratory
-organs have arisen by degrees from simple skin: as, indeed, they do
-arise during the development of each individual having them. Just as
-gradually as in the embryo a simple bud on the integument, with its
-contained vascular loop, passes by secondary buddings into a tree-like
-growth penetrated everywhere by dividing and subdividing blood-vessels;
-so gradually has there probably proceeded the differentiation which has
-turned part of the outer surface into an organ for excreting carbonic
-acid and absorbing oxygen.</p>
-
-<p>Certain inferior vertebrate animals present us with a like
-metamorphosis of tissues. These are the <i>Amphibia</i>. The branchiæ
-here developed from the skin, are covered with cellular epidermis, not
-much thinner than that covering the rest of the body. Like it they
-have their surfaces speckled with pigment-cells; and are not even
-conspicuous by their extra vascularity&mdash;where they are temporary at
-least. They facilitate the exchange of gases in scarcely any other
-way than by affording a larger area of contact with the water, and
-interposing a rather thinner layer of tissue between the water and the
-blood-vessels. Those very simple branchiæ of the larval <i>Amphibia</i>
-that have them but for a short time, graduate into the more complex
-ones of those that have them for a long time or permanently; showing,
-as before, the small stages by which this heterogeneity of surface
-accompanying heterogeneity of function may arise.</p>
-
-<p>In what way are such differentiations established? Mainly, no doubt, by
-natural selection; but also to some degree, I think, by the inheritance
-of direct adaptations. That a portion of the integument at which
-aëration is favoured by local conditions, should thereby be led to grow
-into a larger surface of aëration, appears improbable. Survival of
-those individuals which happen to have this portion of the integument<span class="pagenum" id="Page_312">[312]</span>
-somewhat more-developed, seems here the only likely cause.</p>
-
-
-<p class="p2">§ 293. Among the conspicuous modifications by which the
-originally-uniform outer layer is rendered multiform, are the
-protective structures. Let us look first at the few cases in which the
-formation of these is ascribable mainly to direct equilibration.</p>
-
-<p>Already reference has been more than once made to those thickenings
-that occur where the skin is exposed to unusual pressure and friction.
-Are these adaptations inheritable? and may they, by accumulation
-through many generations, produce permanent dermal structures fitted
-to permanent or frequently-recurring stress? Take, for instance, the
-callosities on the knuckles of the <i>Gorilla</i>, which are adapted
-to its habit of partially supporting itself on its closed hands
-when moving along the ground. Shall we suppose that these defensive
-thickenings are produced afresh in each individual by the direct
-actions; or that they are inherited modifications caused by such direct
-actions; or that they are wholly due to the natural selection of
-spontaneous variations? The last supposition does not seem a probable
-one. Such thickenings, if spontaneous, would be no more likely to occur
-on the knuckles than on any other of the hundred equal areas forming
-the skin-surface at large; and the chances against their simultaneous
-occurrence on all eight knuckles would be incalculable. Moreover, the
-implication would be that those slight extra thicknesses of skin on the
-knuckles, with which we must suppose the selection to have commenced,
-were so advantageous as to cause survivals of the individuals
-having them, in presence of other superiorities possessed by other
-individuals. Then that survivals so caused, if they ever occurred at
-all, should have occurred with the frequency requisite to establish and
-increase the variation, is hardly supposable. And if we reject, as also
-unlikely, the reproduction of these callosities <i>de novo</i> in each
-individual (for<span class="pagenum" id="Page_313">[313]</span> this would imply that after a thousand generations
-each young gorilla began with knuckles having skin no thicker than
-elsewhere), there remains only the inference that they have arisen by
-the transmission and accumulation of functional adaptations. Another
-case which seems interpretable only in an analogous way, is that of
-the spurs that are developed on the wings of certain birds&mdash;on those
-of the Chaja screamer for example. These are weapons of offence and
-defence. It is a familiar fact that some birds strike with their
-wings, often giving severe blows; and in the birds named, the blows
-are made more formidable by the horny, dagger-shaped growths standing
-out from those points on the wings which deliver them. Are these spurs
-directly or indirectly adaptive? To conclude that natural selection of
-spontaneous variations has caused them, is to conclude that, without
-any local stimulus, thickenings of the skin occurred symmetrically
-on the two wings at the places required; that such thickenings, so
-localized, happened to arise in birds given to using their wings in
-fight; and that on their first appearance the thickenings were decided
-enough to give appreciable advantages to the individuals distinguished
-by them&mdash;advantages in bearing the reactions of the blows if not in
-inflicting the blows. But to conclude this is, I think, to conclude
-against probability. Contrariwise, if we assume that the thickening
-of the epidermis produced by habitual rough usage is inheritable, the
-development of these structures presents no difficulty. The points of
-impact would become indurated in wings used for striking with unusual
-frequency. The callosities of surface thus generated, rendering the
-parts less sensitive, would enable the bird in which they arose to
-give, without injury to itself, more violent blows and a greater number
-of them: so, in some cases, helping it to conquer and multiply. Among
-its descendants, inheriting the modification and the accompanying
-habit, the thickening would be further increased in the same way:
-survival of the fittest tending ever to accelerate the process.<span class="pagenum" id="Page_314">[314]</span>
-Presently the horny nodes so formed, hitherto defensive only in their
-effects, would, by their prominence, become offensive&mdash;would make
-the blows given more hurtful. And now natural selection, aiding more
-actively, would mould the nodes into spurs: the individuals in which
-the nodes were most pointed would be apt to survive and propagate; and
-the pointedness generation after generation thus increased, would end
-in the well-adapted shape we see.</p>
-
-<p>But if in these cases the differentiations which fit particular parts
-of the outer tissues to bear rough usage are caused mainly by the
-direct balancing of external actions by internal reactions, then we
-may suspect that the like is true of other modifications that occur
-where special strains and abrasions have to be met. Possibly it is
-true of sundry parts that are formed of hardened epidermis, such as
-the nails, claws, hoofs, and hollow horns of Mammals; “all of which,”
-says Prof. Huxley, “are constructed on essentially the same plan, being
-diverticula of the whole integument, the outer layer of whose ecderon
-has undergone horny metamorphosis.” Leaving open, however, the question
-what tegumentary structures are due to direct equilibration, furthered
-and controlled by indirect equilibration, it is tolerably clear that
-direct equilibration has been one of the factors.</p>
-
-
-<p class="p2" id="sect294">§ 294. Dermal structures of another class are developed mainly, if not
-wholly, by the actions of external causes on species rather than on
-individuals. These are the various kinds of clothing&mdash;hairs, feathers,
-quills, scales, scutes. Though it is no longer thought as at one time
-that all these various tegumentary structures are homologous with one
-another, yet it is unquestionable that sundry of the more conspicuous
-ones are. Those which are extremely unlike may be seen linked together
-by a long series of graduated forms. A retrograde metamorphosis from
-feathers to appendages that are almost scale-like, is well seen in
-the coat of the Penguin. There is manifest a transition from the<span class="pagenum" id="Page_315">[315]</span>
-bird-like covering to the fish-like covering&mdash;a transition so gradual
-that no place can be found where an appreciable break occurs; and if
-the scale-like appendages are not truly scales yet they exemplify
-an extreme metamorphosis. Less striking, perhaps, but scarcely less
-significant, are the modifications through which we pass from feathers
-to hairs, on the surfaces of the Ostrich and the Cassowary. The skin
-of the Porcupine shows us hairs and quills united by a series of
-intermediate structures, differing from one another inappreciably. Even
-more remarkable are certain other alliances of dermal structures. “It
-may be taken as certain, I think,” says Prof. Huxley, “that the scales,
-plates, and spines of all fishes are homologous organs; nor as less so
-that the tegumentary spines of the Plagiostomes are homologous with
-their teeth, and thence with the teeth of all vertebrata.”</p>
-
-<p>Further details concerning these tegumentary structures are not needful
-for present purposes, and are indeed but indirectly relevant to the
-subject of physiological development. Here they are of interest to us
-only by involving the general question&mdash;What physical influences have
-brought them into existence? Still with a view to definite presentation
-of the problem, it will be well to contemplate the mode of development
-common to the most familiar of them.</p>
-
-<p>Suppose a small pit to be formed on the previously flat skin; and
-suppose that the growth and casting off of horny cells which goes on
-over the skin in general, continues to go on at the usual rate over the
-depressed surface of this pit. Clearly the quantity of horny matter
-produced within this hollow, will be greater than that produced on a
-level portion of the skin subtending an equal area of the animal’s
-outside. Suppose such a pit to be deepened until it becomes a small
-sac. If the exfoliation goes on as before, the result will be that the
-horny matter, expelled, as it must be, through the mouth of the sac,
-which now bears a small proportion to the internal surface of the sac,
-will be<span class="pagenum" id="Page_316">[316]</span> large in quantity compared with that exfoliated from a portion
-of the skin equal in area to the mouth of the sac: there will be a
-conspicuous thrusting forth of horny matter. Suppose once more that
-the sac, instead of remaining simple, has its bottom pushed up into
-its interior, like the bottom of a wine-bottle&mdash;the introversion being
-carried so far that the introverted part reaches nearly to the external
-opening, and leaves scarcely any space between the introverted part and
-the walls of the sac. It is easy to see that the exfoliation continuing
-from the surface of the introverted part, as well as from the inside
-of the sac generally, the horny matter cast off will form a double
-layer; and will come out of the sac in the shape of a tube having
-within its lower end the introverted part, as the core on which it is
-moulded, and from the apex of which is cast off the substance filling,
-less densely, its interior. The structure resulting will be what we
-know as a hair. Manifestly by progressive enlargement of the sac, and
-further complication of that introverted part on which the excreted
-substance is moulded, the protruding growth may be rendered larger and
-more involved, as we see it in quills and feathers. So that insensible
-steps, thus indicated in principle, carry us from the exfoliation of
-epidermis by a flat surface, to the exfoliation of it by a hollow
-simple sac, an introverted sac, and a sac further complicated; each of
-which produces its modified kind of tegumentary appendage.</p>
-
-<p>But now, after contemplating this typical illustration, we return to
-the general question. What are the agencies which have been operative
-in developing these skin-structures? Indirect equilibration must have
-worked almost alone in producing them. No direct incidence of forces
-can have developed the enamelled armour of the <i>Lepidosteus</i> or
-the tesselated plates of the <i>Glyptodon</i> and its modern allies.
-Survival of the fittest must here and in multitudinous other cases be
-regarded as the sole cause.</p>
-
-
-<p class="p2">§ 295. Among many other differentiations of the outer<span class="pagenum" id="Page_317">[317]</span> tissues, the
-most worthy to be noticed in the space that remains, are those by which
-organs of sense are formed. We will begin with the simplest and most
-closely-allied to the foregoing.</p>
-
-<p>Every hair that is not too long or flexible to convey to its rooted end
-a strain put upon its free end, is a rudimentary tactual organ; as may
-be readily proved by touching one of those growing on the back of the
-hand. If, then, a creature has certain hairs so placed that they are
-habitually touched by the objects with which it deals, or amid which
-it moves, an advantage is likely to accrue if these hairs are modified
-in a way that enables them the better to transmit the impressions
-derived. Such modified hairs we have in the <i>vibrissæ</i>, or, as
-they are commonly called, the “whiskers” possessed by Cats and feline
-animals generally, as well as by Seals and many Rodents. These hairs
-are long enough to reach objects at considerable distances; they are so
-stiff that forces applied to their free ends, cause movements of their
-imbedded ends; and the sacs containing their imbedded ends being well
-covered with nerve-fibres, these developed hairs serve as instruments
-of exploration. By constant use of them the animal learns to judge of
-the relative positions of objects past which, or towards which, it
-is moving. When stealthily approaching prey or stealthily escaping
-enemies, such aids to perception are obviously important: indeed their
-importance has been proved by the diminished power of self-guidance in
-the dark, that results from cutting them off. These, then, are dermal
-appendages originally serving the purpose of clothing, but afterwards
-differentiated into sense-organs.</p>
-
-<p>That eyes are essentially dermal structures seems scarcely conceivable.
-Yet an examination of their rudimentary types, and of their genesis
-in creatures that have them well developed, shows us that they really
-arise by successive modifications of the double layer composing the
-integument. They make their first appearance among the simpler animals
-as specks of pigment, covered by portions of epidermis slightly<span class="pagenum" id="Page_318">[318]</span>
-convex and a little more transparent than that around it. Here their
-fundamental community of structure with the skin is easy to trace; and
-the formation of them by differentiation of it presents no difficulty.
-Not so far in advance of these as much to obscure the relationship,
-are the eyes which the Crustaceans possess. In every fishmonger’s shop
-we may see that the eyes of a Lobster are carried on pedicles; and
-when the Lobster casts its shell, the outer coat of each eye, being
-continuous with the epidermis of its pedicle, is thrown off along with
-the rest of the exo-skeleton. Beneath the transparent epidermic layer,
-there exists a group of eyes of the kind which we see in an insect;
-and these, according to a high authority, are inclosed in the dermal
-system. Describing the arrangement of the parts, M. Milne Edwards
-writes:&mdash;“But the most remarkable circumstance is, that the large
-cavity within which the whole of these parallel columns, every one of
-which is itself a perfect eye, are contained, is closed posteriorly by
-a membrane, which appears to be neither more nor less than the middle
-tegumentary membrane, pierced for the passage of the optic nerve; so
-that the ocular chamber at large results from the separation at a point
-of the two external layers of the general envelope.” Thus too is it, in
-the main, even with the highly developed eyes of the <i>Vertebrata</i>.
-“The three pairs of sensory organs appertaining to the higher senses,”
-says Prof. Huxley&mdash;“the nasal sacs, the eyes, and the ears&mdash;arise as
-simple cœcal involutions of the external integument of the head of
-the embryo. That such is the case, so far as the olfactory sacs are
-concerned, is obvious, and it is not difficult to observe that the
-lens and the anterior chamber of the eye are produced in a perfectly
-similar manner. It is not so easy to see that the labyrinth of the ear
-arises in this way, as the sac resulting from the involution of the
-integument is small, and remains open but a very short time. But I have
-so frequently verified Huschke’s and Remak’s statement that it does so
-arise, that<span class="pagenum" id="Page_319">[319]</span> I entertain no doubt whatever of the fact. The outer ends
-of the olfactory sacs remain open, but those of the ocular and auditory
-sacs rapidly close up, and shut off their contents from all direct
-communication with the exterior.” That is to say, the eye considered as
-an optical apparatus is produced by metamorphoses of the skin: the only
-parts of it not thus produced, being the membranes lying between the
-sclerotic and the vitreous humour, including those retinal structures
-formed in them. All is tegumentary save that which has to appreciate
-the impressions which the modified integument concentrates upon it.</p>
-
-<p>Thus, as Prof. Huxley has somewhere pointed out, there is a
-substantial parallelism between all the sensory organs in their modes
-of development; as there is, too, between their modes of action.
-A <i>vibrissa</i> may be taken as their common type. Increased
-impressibility by an external stimulus, requires an increased
-peripheral expansion of the nervous system on which the stimulus may
-fall; and this is secured by an introversion of the integument, forming
-a sac on the walls of which a nerve may ramify. That the more extended
-sensory area thus constituted may be acted upon, there requires some
-apparatus conveying to it from without the appropriate stimulus; and
-in the case of the <i>vibrissa</i>, this apparatus is the epidermic
-growth which, under the form of a hair, protrudes from the sac. And
-that the greatest sensitiveness may be obtained, the external action
-must be exaggerated or multiplied by the apparatus which conveys it
-to the recipient nerve; as, in the case of the <i>vibrissa</i>, it is
-by the development of a hair into an elastic lever, that transforms
-the slight force acting through considerable space on its exposed end,
-into a greater force acting through a smaller space at its rooted end.
-Similarly with the organs of the higher senses. In a rudimentary eye,
-the slightly modified sense cell has but a rudimentary nerve to take
-cognizance of the impression; and to concentrate the impression upon
-it, there is nothing beyond a thickening of the epidermis into a<span class="pagenum" id="Page_320">[320]</span>
-lens-shape. But the developed eye shows us a termination of the nerve
-greatly expanded and divided to receive the external stimulus. It shows
-us an introverted portion of the integument containing the apparatus
-by which the external stimulus is conveyed to the recipient nerve. The
-structure developed in this sac not only conveys the stimulus, but
-also, like its homologue, concentrates it; and in the one case as in
-the other, the structure which does this is an epidermic growth from
-the bottom of the sac. Even with the ear it is the same. Again we have
-an introverted portion of the integument, on the walls of which the
-nerve is distributed in the primitive ear. The otolithes contained
-in the sac thus formed, are bodies which are set in motion by the
-vibrations of the surrounding water, and convey these vibrations in an
-exaggerated form to the nerves. And though it is not alleged that these
-otolithes are developed from the epidermic lining of the chamber, yet
-as, if not so developed, they are concretions from the contents of an
-epidermic sac, they must still be regarded as epidermic products.</p>
-
-<p>Whether these differentiations are due wholly to indirect
-equilibration, or whether direct equilibration has had a share in
-working them, are questions that must be left open. Possibly a short
-hair so placed on a mammal’s face as to be very often touched, may,
-by conveying excitations to the nerves and vessels at its root, cause
-extra growth of the bulb and its appendages, and so the development of
-a <i>vibrissa</i> may be furthered. Possibly, too, the light itself, to
-which the tissues of some inferior animals are everywhere sensitive,
-may aid in setting up certain of the modifications by which the nervous
-parts of visual organs are formed: producing, as it must, the most
-powerful effects at those points on the surface which the movements of
-the animal expose to the greatest and most frequent contrasts of light
-and shade; and propagating from those points currents of molecular
-change through the organism. But it seems clear that the complexities<span class="pagenum" id="Page_321">[321]</span>
-of the sensory organs are not thus explicable. They must have arisen by
-the natural selection of favourable variations.</p>
-
-
-<p class="p2">§ 296. A group of facts, serving to elucidate those put together in
-the several foregoing sections, has to be added. I have reserved
-this group to the last, partly because it is transitional&mdash;links the
-differentiations of the literally outer tissues with those of the truly
-inner tissues. Though physically internal, the mucous coat of the
-alimentary canal has a <i>quasi</i>-externality from a physiological
-point of view. As was pointed out in the last chapter, the skin and
-the assimilating surface have this in common, that they come in direct
-contact with matters not belonging to the organism; and we saw that
-along with this community of relation to alien substances, there is a
-certain community of structure and development. The like holds with the
-linings of all internal cavities and canals that have external openings.</p>
-
-<p>The transition from the literally outer tissues to those tissues
-which are intermediate between them and the truly inner tissues,
-is visible at all the orifices of the body; where skin and mucous
-membrane are continuous, and the one passes insensibly into the
-other. This visible continuity is associated not simply with a great
-degree of morphological continuity, but also with a great degree of
-physiological continuity. That is to say, these literally outer and
-<i>quasi</i>-outer layers are capable of rapidly assuming one another’s
-structures and functions when subject to one another’s conditions.
-Mucous surfaces, normally kept covered, become skin-like if exposed
-to the air; but resume more or less fully their normal characters
-when restored to their normal positions. These are truths familiar to
-pathologists. They continually meet with proofs that permanent eversion
-of the mucous membrane, even where it is by prolapse of a part deeply
-seated within the body, is followed by an adaptation eventually almost
-complete: originally moist, tender to the touch, and irritated by the
-air, the surface gradually becomes covered<span class="pagenum" id="Page_322">[322]</span> with a thick, dry cuticle;
-and is then scarcely more sensitive than ordinary integument.</p>
-
-<p>Whether this equilibration between new outer forces and reactive inner
-forces, which is thus directly produced in individuals, is similarly
-produced in races, must remain as a question not to be answered in a
-positive way. On the one hand, we have the fact that among the higher
-animals there are cases of <i>quasi</i>-outer tissues which are in
-one species habitually ensheathed, while in another species they are
-not ensheathed; and that these two tissues, though unquestionably
-homologous, differ as much as skin and mucous membrane differ. On the
-other hand, there are certain analogous changes of surface, as on the
-abdomen of the Hermit-Crab, which give warrant to the supposition
-that survival of the fittest is the chief agent in establishing such
-differentiations; since the abdomen of a Hermit-Crab, bathed by water
-within the shell it occupies, is not exposed to physical conditions
-that directly tend to differentiate its surface from the surface of
-the thorax. But though in cases like this last, we must assign the
-result to the natural selection of variations arising incidentally; we
-may, I think, legitimately assign the result to the immediate action
-of changed conditions where, as in cases like the first, we see these
-producing in the individual, effects of the kinds observed in the race.</p>
-
-<p>However this may be, the force of the general argument remains the
-same. In these exchanges of structure and function between the outer
-and <i>quasi</i>-outer tissues, we get undeniable proof that they are
-easily differentiable. And seeing this, we are enabled the more clearly
-to see how there have, in course of time, arisen those extreme and
-multitudinous differentiations of the outer tissues which have been
-glanced at.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_323">[323]</span></p>
-
-<h3>CHAPTER VIII.<br />
-<span class="subhed1">DIFFERENTIATIONS AMONG THE INNER TISSUES OF ANIMALS.</span></h3></div>
-
-
-<p>§ 297. The change from the outside of the lips to their inside,
-introduces us to a new series of interesting and instructive facts,
-joining on to those with which the last chapter closed. They concern
-the differentiations of those coats of the alimentary canal which, as
-we have seen, are physiologically outer, though physically inner.</p>
-
-<p>These coats are greatly modified at different parts; and their
-modifications vary greatly in different animals. In the lower types,
-where they compose a simple tube running from end to end of the body,
-they are almost uniform in their histological characters; but on
-ascending from these types, we find them presenting an increasing
-variety of minute structures between their two ends. The argument
-will be adequately enforced if we limit ourselves to the leading
-modifications they display in some of the higher animals.</p>
-
-<p>The successive parts of the alimentary canal are so placed with respect
-to its contents, that the physical and chemical changes undergone by
-its contents while passing from one end to the other, inevitably tend
-to transform its originally homogeneous surface into a heterogeneous
-surface. Clearly, the effect produced on the food at any part of
-the canal by trituration, by adding a secretion, or by absorbing
-its nutritive matters, implies the delivery of the food into the
-next part of the canal in a state more or less unlike its previous<span class="pagenum" id="Page_324">[324]</span>
-states&mdash;implies that the surface with which it now comes in contact
-is differently affected by it from the preceding surfaces&mdash;implies,
-that is, a differentiating action. To use concrete language;&mdash;food
-that is broken down in the mouth acts on the œsophagus and stomach
-in a way unlike that which it would have done had it been swallowed
-whole; the masticated food, to which certain solvents or ferments
-are added, becomes to the intestine a different substance from that
-which it must have otherwise been; and the altered food, resolved
-by these additions into its proximate principles, cannot have those
-proximate principles absorbed in the next part of the intestine,
-without the remoter parts being affected as they would not have been
-in the absence of absorption. It is true that in developed alimentary
-canals, such as the reasoning here tacitly assumes, these marked
-successive differentiations of the food are themselves the results
-of pre-established differentiations in the successive parts of the
-canal. But it is also true that actions and reactions like those
-here so definitely marked, must go on indefinitely in an undeveloped
-alimentary canal. If the food is changed at all in the course of its
-transit, which it must be if the creature is to live by it, then it
-cannot but act dissimilarly on the successive tracts of the alimentary
-canal, and cannot but be dissimilarly reacted on by them. Inevitably,
-therefore, the uniformity of the surface must lapse into greater or
-less multiformity: the differentiation of each part tending ever to
-initiate differentiations of other parts.</p>
-
-<p>Not, indeed, that the implied process of direct equilibration can
-be regarded as the sole process. Indirect equilibration aids; and,
-doubtless, there are some of the modifications which only indirect
-equilibration can accomplish. But we have here one unquestionable
-cause&mdash;a cause that is known to work in individuals, changes of the
-kind alleged. Where, for instance, cancerous disease of the œsophagus
-so narrows the passage into the stomach as to prevent easy descent of
-the food, the œsophagus above the obstruction becomes<span class="pagenum" id="Page_325">[325]</span> enlarged into a
-kind of pouch; and the inner surface of this pouch begins to secrete
-juices that produce in the food a kind of rude digestion. Again,
-stricture of the intestine, when it arises gradually, is followed by
-hypertrophy of the muscular coat of the intestine above the constricted
-part: the ordinary peristaltic movements being insufficient to force
-the food forwards, and the lodged food serving as a constant stimulus
-to contraction, the muscular fibres, habitually more exercised,
-become more bulky. The deduction from general principles being thus
-inductively enforced, we cannot, I think, resist the conclusion that
-the direct actions and reactions between the food and the alimentary
-canal have been largely instrumental in establishing the contrasts
-among its parts. And we shall hold this view with the more confidence
-on observing how satisfactorily, in pursuance of it, we are enabled to
-explain one of the most striking of these differentiations, which we
-will take as a type of the class.</p>
-
-<p>The gizzard of a bird is an expanded portion of the alimentary canal,
-specially fitted to give the food that trituration which the toothless
-mouth of a bird cannot give. Besides having a greatly-developed
-muscular coat, this grinding-chamber is lined with a thick, hard
-cuticle, capable of bearing the friction of the pebbles swallowed to
-serve as grindstones. This differentiation of the mucous coat into a
-ridged and tubercled layer of horny matter&mdash;a differentiation which,
-in the analogous organs of certain <i>Mollusca</i>, is carried to the
-extent of producing from this membrane cartilaginous plates, and even
-teeth&mdash;varies in birds of different kinds, according to their food.
-It is moderate in birds that feed on flesh and fish, and extreme in
-granivorous birds and others that live on hard substances. How does
-this immense modification of the alimentary canal originate? In the
-stomach of a mammal, the macerating and solvent actions are united
-with that triturating action which finishes what the teeth have mainly
-done; but in the bird, unable to masticate, these internal functions
-are specialized, and while the crop is the<span class="pagenum" id="Page_326">[326]</span> macerating chamber, the
-gizzard becomes a chamber adapted to triturate more effectually. This
-adaptation requires simply an exaggeration of certain structures
-and actions which characterize stomachs in general, and, in a less
-degree, alimentary canals throughout their whole lengths. The massive
-muscles of the gizzard are simply extreme developments of the muscular
-tunic, which is already considerably developed over the stomach, and
-incloses also the œsophagus and the intestine. The indurated lining of
-the gizzard, thickened into horny buttons at the places of severest
-pressure, is nothing more than a greatly strengthened and modified
-epithelium. And the grinding action of the gizzard is but a specialized
-form of that rhythmical contraction by which an ordinary stomach kneads
-the contained food, and which in the œsophagus effects the act of
-swallowing, while in the intestine it becomes the peristaltic motion.
-Allied as the gizzard thus clearly is in structure and action to the
-stomach and alimentary canal in general; and capable of being gradually
-differentiated from a stomach where a growing habit of swallowing
-food unmasticated entails more trituration to be performed before
-the food passes the pylorus; the question is&mdash;Does this change of
-structure arise by direct adaptation? There is warrant for the belief
-that it does. Besides such collateral evidence as that mucous membrane
-becomes horny on the toothless gums of old people, when subject to
-continual rough usage, and that the muscular coat of the intestine
-thickens where unusual activity is demanded of it, we have the direct
-evidence of experiment. Hunter habituated a sea-gull to feed on grain,
-and found that the lining of its gizzard became hardened, while the
-gizzard-muscles doubled in thickness. A like change in the diet of a
-kite was followed by like results. Clearly, if differentiations so
-produced in the individuals of a race under changed habits, are in any
-degree inheritable, a structure like a gizzard will originate through
-the direct actions and reactions between the food and the alimentary
-canal.</p>
-
-<p><span class="pagenum" id="Page_327">[327]</span></p>
-
-<p>Another case&mdash;a very interesting one, somewhat allied to this&mdash;is
-presented by the ruminating animals. Here several dilatations of
-the alimentary canal precede the true stomach; and in them large
-quantities of unmasticated food are stored, to be afterwards returned
-to the mouth and masticated at leisure. What conditions have made
-this specialization advantageous? and by what process has it been
-established? To both these questions the facts indicate answers
-which are not unsatisfactory. [Creatures that obtain their food very
-irregularly&mdash;now having more than they can consume, and now being for
-long periods without any&mdash;must, in the first place, be apt, when very
-hungry, to eat to the extreme limits of their capacities; and must,
-in the second place, profit by peculiarities which enable them to
-compensate themselves for long fasts, past and future. A perch which,
-when its stomach is full of young frogs, goes on filling its œsophagus
-also; or a trout which, rising to the fisherman’s fly, proves when
-taken off the hook to be full of worms and insect-larvæ up to the very
-mouth, gains by its ability to take in such unusual supplies of food
-when it meets with them&mdash;obviously thrives better than it would do
-could it never eat more than a stomachful. That this ability to feed
-greatly in excess of immediate requirement, is one that varies in
-individuals of the same race, we see in the marked contrast between
-our own powers in this respect, and the powers of uncivilized men;
-whose fasting and gorging are to us so astonishing. Carrying with us
-these considerations, we shall not be surprised at finding dilatations
-of the œsophagus in vultures and eagles, which get their prey at long
-intervals in large masses; and we may naturally look for them, too, in
-birds like pigeons, which, coming in flocks upon occasional supplies
-of grain, individually profit by devouring the greatest quantity in
-a given time. Now where the trituration of the food is, as in these
-cases, carried on in a lower part of the alimentary canal, nothing
-further is required than the storing-chamber; but for a mammal,
-having its grinding<span class="pagenum" id="Page_328">[328]</span> apparatus in its mouth, to gain by the habit of
-hurriedly swallowing unmasticated food, it must also have the habit of
-regurgitating the food for subsequent mastication. This correlation of
-habits with their answering structures, may, as we shall see, arise in
-a very simple way. The starting point of the explanation is a familiar
-fact&mdash;the fact that indigestion, often resulting from excess of food,
-is apt to cause that reversed peristaltic action known as vomiting.
-From this we pass to the fact, also within the experience of most
-persons, that during slight indigestion the stomach sometimes quietly
-regurgitates a small part of its contents as far as the back of the
-mouth&mdash;giving an unpleasant acquaintance with the taste of the gastric
-juices. Exceptional facts of the same class help the argument a step
-further. “There are certain individuals who are capable of returning,
-at will, a greater or smaller portion of the contents of the digesting
-stomach into the cavity of the mouth.... In some of these cases, the
-expulsion of the food has required a violent effort. In the majority
-it has been easily evoked or suppressed. While in others, it has been
-almost uncontrollable; or its non-occurrence at the habitual time
-has been followed by a painful feeling of fulness, or by the act of
-vomiting.” Here we have a certain physiological action, occasionally
-happening in most persons and in some developed into a habit more or
-less pronounced: indigestion being the habitual antecedent. Suppose,
-then, that gregarious animals, living on innutritive food such as
-grass, are subject to a like physiological action, and are capable of
-like variations in the degree of it. What will naturally happen? They
-wander in herds, now over places where food is scarce and now coming
-to places where it is abundant. Some masticate their food completely
-before swallowing it, while some masticate it incompletely. If an
-oasis, presently bared by their grazing, has not supplied to the whole
-herd a full meal, then the individuals which masticate completely
-will have had less than those which masticate incompletely&mdash;will
-not<span class="pagenum" id="Page_329">[329]</span> have had enough. Those which masticate incompletely and distend
-their stomachs with food difficult to digest, will be liable to these
-regurgitations; but if they re-masticate what is thus returned to
-the mouth (and we know that animals often eat again what they have
-vomited), then the extra quantity of food taken, eventually made
-digestible, will yield them more nourishment than is obtained by those
-which masticate completely at first. The habit initiated in this
-natural way, and aiding survival when food is scarce, will be apt to
-cause modifications of the alimentary canal. We know that dilatations
-of canals readily arise under habitual distensions. We know that canals
-habitually distended become gradually more tolerant of the contained
-masses that at first irritated them. And we know that there commonly
-take place adaptive modifications of their surfaces. Hence if a habit
-of this kind and the structural changes resulting from it, are in
-any degree inheritable, it is clear that, increasing in successive
-generations, both immediately by the cumulative effect of repetitions
-and mediately by survival of the individuals in which they are most
-decided, they may go on until they end in the peculiarities which
-Ruminants display.</p>
-
-
-<p class="p2">§ 298. There are structures belonging to the same group which cannot,
-however, be accounted for in this way. They are the organs that
-secrete special products facilitating digestion&mdash;the liver, pancreas,
-and various smaller glands. All these appendages of the alimentary
-canal, large and independent as some of them seem, really arise by
-differentiations from its coats. The primordial liver consists of
-nothing more than bile-cells scattered along a tract of the intestinal
-surface. Accumulation of these bile-cells is accompanied by increased
-growth of the surface which bears them&mdash;a growth which at first takes
-the form of a <i>cul-de-sac</i>, having an outside that projects from
-the intestine into the peri-visceral cavity. As the mass of bile-cells
-becomes greater, there arise secondary<span class="pagenum" id="Page_330">[330]</span> lateral cavities opening into
-the primary one, and through it into the intestine; until, eventually,
-these cavities with their coatings of bile-cells, become ramifying
-ducts distributed through the solid mass we know as a liver. How is
-this differentiation caused?</p>
-
-<p>Before attempting any answer to this question, it is requisite to
-inquire the nature of bile. Is that which the liver throws into
-the intestines a waste product of the organic actions? or is it a
-secretion aiding digestion? or is it a mixture of these? Modern
-investigations imply that it is most likely the last. The liver
-is found to have a compound function. Bernard has proved to the
-satisfaction of physiologists, that there goes on in it a formation of
-glycogen&mdash;a substance which is transformed into sugar before it leaves
-the liver and is afterwards carried away by the blood to eventually
-disappear in the active organs, chiefly the muscles. It is also shown,
-experimentally, that there are generated in the liver certain biliary
-acids; and by the aid either of these or of some other compounds, it is
-clear that bile renders certain materials more absorbable. Its effect
-on fat is demonstrable out of the body; and the greatly diminished
-absorption of fat from the food when the discharge of bile into the
-intestine is prevented, is probably one of the causes of that pining
-away which results. But while recognizing the fact that the bile
-consists in part of a solvent, or solvents, aiding digestion, there
-is abundant evidence that one element of it is an effete product;
-and probably this is the primary element. The yellow-green substance
-called biliverdine in herbivora and bilirubin in man and carnivora,
-which gives its colour to bile, is a product the greater part of which
-is normally cast out from the system continually, as is shown by the
-contrast between the normal and abnormal colours of fæcal matters, and
-as is still more strikingly shown by the effects on the system when
-there is a stoppage of the excretion, and an attack of jaundice. Hence
-we are warranted in classing biliverdine as a waste product, and we<span class="pagenum" id="Page_331">[331]</span>
-may fairly infer that the excretion of it is the original function of
-the liver.</p>
-
-<p>One further preliminary is requisite. We must for a moment return to
-those physico-chemical data set down in the first chapter of this work
-(<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect7">§§ 7–8</a>). We there saw that the complex and large-atomed colloids which
-mainly compose living organic matter, have extremely little molecular
-mobility; and, consequently, extremely little power of diffusing
-themselves. Whereas we saw not only that those absorbed matters,
-gaseous and liquid, which further the decomposition of living organic
-matter, have very high diffusibilities, but also that the products
-of the decomposition are much more diffusible than the components of
-living organic matter. And we saw that, as a consequence of this, the
-tissues give ready entrance to the substances which decompose them,
-and ready exit to the substances into which they are decomposed. Hence
-it follows that, under its initial form, uncomplicated by nervous and
-other agencies, the escape of effete matters from the organism, is a
-physical action parallel to that which goes on among mixed colloids
-and crystalloids that are dead or even inorganic. Excretion is a
-specialized form of this spontaneous action; and we have to inquire how
-the specialization arises.</p>
-
-<p>Two causes conspire to establish it. The first is that these products
-of decomposition are diffusible in widely different degrees. While
-the carbonic acid and water permeate the tissues with ease in all
-directions, and escape more or less from the exposed surfaces, urea,
-and other waste substances incapable of being vaporized, cannot escape
-thus readily. The second is that the different parts of the body, being
-subject to different physical conditions, are from the outset sure
-severally to favour the exit of these various products of decomposition
-in various degrees. How these causes must have co-operated in
-localizing the excretions, we shall see on remembering how they now
-co-operate in localizing the separation of morbid materials. The
-characteristic substances of<span class="pagenum" id="Page_332">[332]</span> gout and rheumatism have their habitual
-places of deposit. Tuberculous matter, though it may be present in
-various organs, gravitates towards some much more than towards others.
-Certain products of disease are habitually got rid of by the skin,
-instead of collecting internally. Mostly, these have special parts of
-the skin which they affect rather than the rest; and there are those
-which, by breaking out symmetrically on the two sides of the body,
-show how definitely the places of their excretion are determined by
-certain favouring conditions, which corresponding parts may be presumed
-to furnish in equal degrees. Further, it is to be observed of these
-morbid substances circulating in the blood, that having once commenced
-segregating at particular places, they tend to continue segregating
-at those places. Assuming, then, as we may fairly do, that this
-localization of excretion, which we see continually commencing afresh
-with morbid matters, has always gone on with the matters produced by
-the waste of the tissues, let us take a further step, and ask how
-localizations become fixed. Other things equal, that which from its
-physical conditions is a place of least resistance to the exit of
-an effete product, will tend to become established as the place of
-excretion; since the rapid exit of an effete product will profit the
-organism. Other things equal, a place at which the excreted matter
-produces least detrimental effect will become the established place. If
-at any point the excreted matter produces a beneficial effect, then,
-other things equal, survival of the fittest will determine it to this
-point. And if facility of escape anywhere goes along with utilization
-of the escaping substance, then, other things equal, the excretion will
-be there localized still more decisively by survival of the fittest.</p>
-
-<p>Such being the conditions of the problem, let us ask what will happen
-with the lining membrane of the alimentary canal. This, physiologically
-considered, is an external surface; and matters thrown off from it
-make their way out of the body. It is also a surface along which is
-moving the food<span class="pagenum" id="Page_333">[333]</span> to be digested. Now, among the various waste products
-continually escaping from the living tissues, some of the more complex
-ones, not very stable in composition, are likely, if added to the food,
-to set up changes in it. Such changes may either aid or hinder the
-preparation of the food for absorption. If an effete matter, making its
-exit through the wall of the intestine, hinders the digestive process,
-the enfeeblement and disappearance of individuals in which this
-happens, will prevent the intestine from becoming the established place
-for its exit. While if it aids the digestive process, the intestine
-will, for converse reasons, become more and more the place to which
-its exit is limited. Equally manifest is it that if there is one part
-of this alimentary canal at which, more than at any other part, the
-favourable effect results, this will become the place of excretion.</p>
-
-<p>Thus, then, reverting to the case in question, we may understand how a
-product to be cast out, such as biliverdine, if it either directly or
-indirectly serves a useful purpose, when poured into a particular part
-of the intestine, may lead to the formation of a patch of excreting
-cells on its wall; and once this place of excretion having been
-established, the development of a liver is simply a question of time
-and natural selection.</p>
-
-
-<p class="p2">§ 299. A differentiation of another order occurring in the alimentary
-canal, is that by which a part of it is developed into a lateral
-chamber or chambers, through which carbonic acid exhales and oxygen is
-absorbed. Comparative anatomy and embryology unite in showing that a
-lung is formed, just as a liver or other appendage of the alimentary
-canal is formed, by the growth of a hollow bud into the peri-visceral
-cavity, or space between the alimentary canal and the wall of the
-body. The interior of this bud is simply a <i>cul-de-sac</i> of the
-alimentary canal, with the mucous lining of which its own mucous lining
-is continuous. And the development of this <i>cul-de-sac</i> into an
-air-chamber, simple or compound, is<span class="pagenum" id="Page_334">[334]</span> merely a great extension of area
-in the internal surface of the <i>cul-de-sac</i>, along with that
-specialization which fits it for excreting and absorbing substances
-different from those which other parts of the mucous surface excrete
-and absorb. These lateral air-chambers, universal among the higher
-<i>Vertebrata</i> and very general among the lower, and everywhere
-attached to the alimentary canal between the mouth and the stomach,
-have not in all cases the respiratory function. In most fishes that
-have them they are what we know as swim-bladders. In some fishes the
-cavities of these swim-bladders are completely shut off from the
-alimentary canal: nevertheless showing, by the communications which
-they have with it during the embryonic stages, that they are originally
-<i>diverticula</i> from it. In other fishes there is a permanent
-<i>ductus pneumaticus</i>, uniting the cavity of the swim-bladder with
-that of the gullet: the function, however, being still not respiratory
-in an appreciable degree, if at all. But in certain still extant
-representatives of the sauroid fishes, as the <i>Lepidosteus</i>,
-the air-bladder is “divided into two sacs that possess a cellular
-structure,” and “the trachea which proceeds from it opens high up in
-the throat, and is surrounded with a glottis.” In the <i>Amphibia</i>
-the corresponding organs are chambers over the surfaces of which there
-are saccular depressions, indicating a transition towards the air-cells
-characterizing lungs; and accompanying this advance we see, as in the
-common <i>Triton</i>, the habit of coming up to the surface and taking
-down a fresh supply of air in place of that discharged.</p>
-
-<p>How are the internal air-chambers, respiratory or nonrespiratory,
-developed? Upwards from the amphibian stage, in which they are
-partially refilled at long intervals, there is no difficulty in
-understanding how, by infinitesimal steps, they pass into complex
-and ever-moving lungs. But how is the differentiation that produces
-them initiated? How comes a portion of the internal surface to be
-specialized for converse with a medium to which it is not naturally<span class="pagenum" id="Page_335">[335]</span>
-exposed? The problem appears a difficult one; but there is a not
-unsatisfactory solution of it.</p>
-
-<p>When many gold-fish are kept in a small aquarium, as with thoughtless
-cruelty they frequently are, they swim close to the surface, so as to
-breathe that water which is from instant to instant absorbing fresh
-oxygen. In doing this they often put their mouths partly above the
-surface, so that in closing them they take in bubbles of air; and
-sometimes they may be seen to continue doing this&mdash;the relief due to
-the slight extra aëration of blood so secured, being the stimulus
-to continue. Air thus taken in may be detained. If a fish that has
-taken in a bubble turns its head downwards, the bubble will ascend
-to the back of its mouth, and there lodge; and coming within reach
-of the contractions of the œsophagus, it may be swallowed. If, then,
-among fish thus naturally led upon occasion to take in air-bubbles,
-there are any having slight differences in the alimentary canal that
-facilitate lodgment of the air, or slight nervous differences such as
-in human beings cause an accidental action to become “a trick,” it
-must happen that if an advantage accrues from the habitual detention
-of air-bubbles, those individuals most apt to detain them will,
-other things equal, be more likely than the rest to survive; and by
-the survival of descendants inheriting their peculiarities in the
-greatest degrees, and increasing them, an established structure and an
-established habit may arise. And that they do in some way arise we have
-proof. The common Loach swallows air, which it afterwards discharges
-loaded with carbonic acid.</p>
-
-<p>From air thus swallowed the advantages that may be derived are of two
-kinds. In the first place, the fish is made specifically lighter, and
-the muscular effort needed to keep it from sinking is diminished&mdash;or,
-indeed, if the bubble is of the right size, is altogether saved.
-The contrast between the movements of a Goby, which, after swimming
-up towards the surface, falls rapidly to the bottom on ceasing its
-exertions, and the movements of a Trout, which remains suspended just<span class="pagenum" id="Page_336">[336]</span>
-balancing itself by slight undulations of its fins, shows how great
-an economy results from an internal float, to fishes which seek their
-food in mid-water or at the surface. Hence the habit of swallowing
-air having been initiated in the way described, we see why natural
-selection will, in certain fishes, aid modifications of the alimentary
-canal favouring its lodgment&mdash;modifications constituting air-sacs.
-In the second place, while from air thus lodged in air-sacs thus
-developed, the advantage will be that of flotation only if the air
-is infrequently changed or never changed, the advantage will be that
-of supplementary respiration if the air-sacs are from time to time
-partially emptied and refilled. The requirements of the animal will
-determine which of the two functions predominates. Let us glance at the
-different sets of conditions under which these divergent modifications
-may be expected to arise.</p>
-
-<p>The respiratory development is not likely to take place in fishes
-that inhabit seas or rivers in which the supply of aërated water
-never fails: there is no obvious reason why the established branchial
-respiration should be replaced by a pulmonic respiration. Indeed, if
-a fish’s branchial respiration is adequate to its needs, a loss would
-result from the effort of coming to the surface for air; especially
-during those first stages of pulmonic development when the extra
-aëration achieved was but small. Hence in fishes so circumstanced,
-the air-chambers arising in the way described would naturally become
-specialized mainly or wholly into floats. Their contained air being
-infrequently changed, no advantage would arise from the development
-of vascular plexuses over their surfaces; nothing would be gained
-by keeping open the communication between them and the alimentary
-canal; and there might thus eventually result closed chambers the
-gaseous contents of which, instead of being obtained from without,
-were secreted from their walls, as gases often are from mucous
-membranes. Contrariwise, aquatic vertebrates in which the swallowing of
-air-bubbles, becoming<span class="pagenum" id="Page_337">[337]</span> habitual, had led to the formation of sacs that
-lodged the bubbles; and which continued to inhabit waters not always
-supplying them with sufficient oxygen, might be expected to have the
-sacs further developed, and the practice of changing the contained
-air made regular, if either of two advantages resulted&mdash;either the
-advantage of being able to live in old habitats that had become
-untenable without this modification, or the advantage of being able to
-occupy new habitats. Now it is just where these advantages are gained
-that we see the pulmonic respiration coming in aid of the branchial
-respiration, and in various degrees replacing it. Shallow waters are
-liable to three changes which conspire to make this supplementary
-respiration beneficial. The summer’s sun heats them, and raising the
-temperatures of the animals they contain, accelerates the circulation
-in these animals, exalts their functional activities, increases the
-production of carbonic acid, and thus makes aëration of the blood more
-needful than usual. Meanwhile the heated water, instead of yielding
-to the highly carbonized blood brought to the branchiæ the usual
-quantity of oxygen, yields less than usual; for as the heat of the
-water increases, the quantity of air it contains diminishes. And this
-greater demand for oxygen joined with smaller supply, pushed to an
-extreme where the water is nearly all evaporated, is at last still
-more intensely felt in consequence of the excess of carbonic acid
-discharged by the numerous creatures congregated in the muddy puddles
-that remain. Here, then, it is, that the habit of taking in air-bubbles
-is likely to become established, and the organs for utilizing them
-developed; and here it is, accordingly, that we find all stages of the
-transition to aërial respiration. The Loach before-mentioned, which
-swallows air, frequents small waters liable to be considerably warmed.
-The <i>Amphipnous Cuchia</i>, an anomalous eel-shaped fish, which has
-vascular air-sacs opening out at the back of the mouth, “is generally
-found lurking in holes and crevices, on the muddy banks of marshes or
-slow-moving rivers”; and<span class="pagenum" id="Page_338">[338]</span> though its air-sacs are not morphological
-equivalents of those above described, yet they equally well illustrate
-the relation between such organs and the environing condition. Still
-more significant is the fact that the <i>Lepidosiren</i>, or “mudfish”
-as it is called from its habits, though it is a true fish nevertheless
-has lungs. But it is among the <i>Amphibia</i> that we see most
-conspicuously this relation between the development of air-breathing
-organs, and the peculiarities of the habitats. Pools, more or less
-dissipated annually, and so rendered uninhabitable by most fishes, are
-very generally peopled by these transitional types. Just as we see,
-too, that in various climates and in various kinds of shallow waters,
-the supplementary aërial respiration is needful in different degrees;
-so do we find among the <i>Amphibia</i> many stages in the substitution
-of the one respiration for the other. The facts, then, are such as give
-to the hypothesis a <i>vraisemblance</i> greater than could have been
-expected.</p>
-
-<p>The relative effects of direct and indirect equilibration in
-establishing this further heterogeneity, must, as in many other
-cases, remain undecided. The habit of taking in bubbles is scarcely
-interpretable as a result of spontaneous variation: we must regard it
-as arising accidentally during the effort to obtain the most aërated
-water; as being persevered in because of the relief obtained; and as
-growing by repetition into a tendency bequeathed to offspring, and
-by them, or some of them, increased and transmitted. The formation
-of the first slight modifications of the alimentary canal favouring
-the lodgment of bubbles, is not to be thus explained. Some favourable
-variation in the shape of the passage must here have been the initial
-step. But the gradual increase of this structural modification by the
-survival of individuals in which it is carried furthest, will, I think,
-be all along aided by immediate adaptation. The part of the alimentary
-canal previously kept from the air, but now habitually in contact with
-the air, must be in some degree modified by the action of the air;
-and the directly-produced modification, increasing<span class="pagenum" id="Page_339">[339]</span> in the individual
-and in successive individuals, cannot cease until there is a complete
-balance between the actions of the changed agency and the changed
-tissue.</p>
-
-
-<p class="p2">§ 300. We come now to differentiations among the truly inner
-tissues&mdash;the tissues which have direct converse neither with the
-environment nor with the foreign substances taken into the organism
-from the environment. These, speaking broadly, are the tissues
-which lie between the double layer forming the integument with its
-appendages, and the double layer forming the alimentary canal with
-its <i>diverticula</i>. We will take first the differentiation which
-produces the vascular system.</p>
-
-<p>Certain forces producing and aiding distribution of liquids in animals,
-come into play before any vascular system exists; and continue to
-further circulation after the development of a vascular system. The
-first of these is osmotic exchange, acting locally and having an
-indirect general action; the second is local variation of pressure,
-which movement of the body throws on the tissues and their contained
-liquids. A few words are needed in elucidation of each. If in any
-creature, however simple, different changes are going on in parts
-that are differently conditioned&mdash;if, as in a <i>Hydra</i>, one
-surface is exposed to the surrounding medium while the other surface
-is exposed to dissolved food; then between the unlike liquids which
-the dissimilarly-placed parts contain, osmotic currents must arise;
-and a movement of liquid through the intermediate tissue must go on
-as long as an unlikeness between the liquids is kept up. This primary
-cause of re-distribution remains one of the causes of re-distribution
-in every more-developed organism: the passage of matters into and
-out of the capillaries is everywhere thus set up. And obviously in
-producing these local currents, osmose must also indirectly produce
-general currents, or aid them if otherwise produced. In the absence
-of a pumping organ, this force is probably an important aid to that<span class="pagenum" id="Page_340">[340]</span>
-movement of the nutritive liquids which the functions set up. How
-the second cause&mdash;the changes of internal pressure which an animal’s
-movements produce&mdash;furthers circulation, will be sufficiently manifest.
-That parts which are bent or strained necessarily have their contained
-vessels squeezed, has been shown (<a href="#sect281">§ 281</a>); and whether the bend or
-strain is caused, as in a plant, by an external force, or, as usually
-in an animal, by an internal force, there must be a thrusting of
-liquids towards places of least resistance&mdash;commonly places of greatest
-consumption. This which in animals without hearts is a main agent of
-circulation, continues to further it very considerably even among the
-highest animals. In these the effect becomes as it were systematized.
-The valves in the veins necessitate perpetual propulsions towards the
-heart.</p>
-
-<p>Even in such simple types as the <i>Hydrozoa</i>, cavities in the
-tissues faintly indicate a structure which facilitates the transfer of
-nutritive matters. These cavities become reservoirs filled with the
-plasma that slowly oozes through the substance of the body; and every
-movement of the animal, accompanied as it must be by changed pressures
-and tensions on these reservoirs, tends here to fill them and there to
-squeeze out their contents in that or the other direction&mdash;possibly
-aiding to produce, by union of several cavities, those lacunæ or
-irregular canals which the body in some cases presents.</p>
-
-<p>Irregular canals of this kind, not lined with any membranes but
-being simply cavities running through the flesh, mainly constitute
-the vascular system in <i>Polyzoa</i> and <i>Brachiopoda</i> and
-some <i>Mollusca</i>. Though the central parts of a vascular system
-are rudely developed, yet its peripheral parts consist of sinuses
-permeating the tissues. The higher orders of <i>Mollusca</i> have
-a more-developed system of vessels or arteries, which run into the
-substance of the body and end in lacunæ or simple fissures. This ending
-in lacunæ takes place at various distances from the vascular centre. In
-some genera the arterial structure is carried to the periphery of the
-blood-system, while in others it stops short midway.<span class="pagenum" id="Page_341">[341]</span> Throughout most
-orders of the <i>Mollusca</i> the back current of blood continues to
-be carried by channels of the original kind: there are no true veins,
-but the blood having been delivered into the tissues, finds its way
-back to the peri-visceral cavity through inosculating sinuses. Among
-the Cephalopods, however, the afferent blood-canals, as well as the
-efferent ones, acquire distinct walls. On putting together these facts,
-we may conceive pretty clearly the stages of vascular development.
-From the original reservoir of nutritive liquid between the alimentary
-canal and the wall of the body, a portion partially shut off becomes a
-contractile vessel; and by its actions there is produced a more rapid
-transfer of the nutritive liquid than was originally produced by the
-motions of the animal. Clearly, the extension of this contractile tube
-and the development from it of branches running hither and thither
-into the tissues, must, by defining the channels of blood throughout a
-part of its course, render its distribution more regular and active.
-As fast as this centrifugal growth advances, so fast are the efferent
-currents of blood, prevented from escaping laterally, obliged to move
-from the centre towards the circumference; and so fast also does
-the less developed set of channels become, of necessity, occupied
-by afferent currents. When, by a parallel increase of definiteness,
-the lacunæ and irregular sinuses through which the afferent currents
-pass, become transformed into veins, the accompanying disappearance of
-all stagnant or slow-moving collections of blood, implies a further
-improvement in the circulation.</p>
-
-<p>By what agency is effected this differentiation of a definite vascular
-system? No sufficient reply is obvious. The genesis of the primordial
-heart is not comprehensible as a result of direct equilibration, and we
-cannot readily see our way to it as a result of indirect equilibration;
-for it is difficult to imagine what favourable variation natural
-selection could have seized hold of to produce such a structure. A
-contractile tube that aided the distribution of nutritive<span class="pagenum" id="Page_342">[342]</span> liquid,
-having been once established, survival of the fittest would suffice
-for its gradual extension and its successive modifications. But what
-were the early stages of the contractile tube, while it was yet not
-sufficiently formed to help circulation, and while it must nevertheless
-have had some advantage without which no selective process could go
-on? The question seems insoluble. To another part of the question,
-however, an answer may be ventured. If we ask the origin of these
-ramifying channels which, first appearing as simple lacunæ, eventually
-become vessels having definite walls, a reply admitting of considerable
-justification, is, that the currents of nutritive liquid forced and
-drawn hither and thither through the tissues, themselves initiate these
-channels. We know that streams running over and through solid and
-quasi-solid inorganic matter, tend to excavate definite courses. We saw
-reason for concluding that the development of sap-channels in plants
-conforms to this general principle. May we not then suspect that the
-nutritive liquid contained in the tissue of a simple animal, made to
-ooze now in this direction and now in that by the changes of pressure
-which the animal’s movements cause, comes to have certain lines along
-which it is thrust backwards and forwards more than along other lines;
-and must by repeated passings make these more and more permeable until
-they become lacunæ? Such actions will inevitably go on; and such
-actions appear competent to produce some, at least, of the observed
-effects. The leading facts which indicate that this is a part-cause of
-vascular development are these.</p>
-
-<p>Growths normally recurring in certain places at certain intervals,
-are accompanied by local formations of blood-vessels. The periodic
-maturation of ova among the <i>Mammalia</i> supplies an instance.
-Through the stroma of an ovarium are distributed innumerable minute
-vesicles, which, in their early stages, are microscopic. Of these,
-severally contained in their minute ovi-sacs, any one may develop: the
-determining<span class="pagenum" id="Page_343">[343]</span> cause being probably some slight excess of nutrition. When
-the development is becoming rapid, the capillaries of the neighbouring
-stroma increase and form a plexus on the walls of the ovi-sac. Now
-since there is no typical distribution of the developing ova; and since
-the increase of an ovum to a certain size precedes the increase of
-vascularity round it; we can scarcely help concluding that the setting
-up of currents towards the point of growth determines the formation of
-the blood-vessels. It may be that having once commenced, this local
-vascular structure completes itself in a typical manner; but it seems
-clear that this greater development of blood-vessels around the growing
-ovum is initiated by the draught towards it. Abnormal growths show
-still better this relation of cause and effect. The false membranes
-sometimes found in the bronchial tubes in inflammatory diseases, may
-perhaps fairly be held abnormal in but a partial sense: it may be said
-that their vascular systems are formed after the type of the membranes
-to which they are akin. But this can scarcely be said of the morbid
-growths classed as malignant. The blood-vessels in an encephaloid
-cancer, are led to enlarge and ramify, often to an immense extent, by
-the unfolding of the morbid mass to which they carry blood. Alien as
-is the structure as a whole to the type of the organism; and alien in
-great measure as is its tissue to the tissue on which it is seated;
-it nevertheless happens that the growth of the alien tissue and
-accompanying abstraction of materials from the blood-vessels, determine
-a corresponding growth of these blood-vessels. Unless, then, we say
-that there is a providentially-created type of vascular structure for
-each kind of morbid growth (and even this would not much help us, since
-the vascular structure has no constancy within the limits of each
-kind), we are compelled to admit that in some way or other the currents
-of blood are here directly instrumental in forming their own channels.
-One more piece of evidence, before cited as exemplifying adaptation (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect67">§ 67</a>),
-may be called to mind.<span class="pagenum" id="Page_344">[344]</span> When any main channel for blood, leading
-to or from a certain part of the body, has been rendered impervious,
-others among the channels leading to or from this same part, enlarge to
-the extent requisite for fulfilling the extra function that falls upon
-them: the enlargement being caused, as we must infer, by the increase
-of the currents carried.</p>
-
-<p>Here, then, are facts warranting inductively the deduction above drawn.
-It is true that we are left in the dark respecting the complexities
-of the process. How the channels for blood come to have limiting
-membranes, and many of them muscular coats, the hypothesis does not
-help us to say. But the evidence assigned goes far to warrant the
-belief that vascular development is initiated by direct equilibration;
-though indirect equilibration may have had the larger share in
-establishing the structures which distinguish finished vascular systems.</p>
-
-
-<p class="p2">§ 301. Of the inner tissues which remain let us next take bone. In what
-manner is differentiated this dense substance serving in most cases for
-internal support?</p>
-
-<p>When considering the vertebrate skeleton under its morphological aspect
-(<a href="#sect256">§ 256</a>), it was pointed out that the formation of dense tissues,
-internal as well as external, is, in some cases at least, brought
-about by the mechanical forces to be resisted. Through what process
-it is brought about we could not then stay to inquire: this question
-being not morphological but physiological. Answers to some kindred
-questions have since been attempted. Certain actions to which the
-internal dense tissues of plants may be ascribed, have been indicated;
-and more recently, analogous actions have been assigned as causes of
-some external dense tissues of animals. We have now to ask whether
-actions of the same nature have produced these internal dense tissues
-of animals.</p>
-
-<p>The problem is an involved one. Bones have more than one stage. They
-are membranous or cartilaginous before they become osseous; and their
-successive component substances<span class="pagenum" id="Page_345">[345]</span> so far differ that the effects
-of mechanical actions upon them differ. And having to deal with
-transitional states in which bone is formed of mixed tissues, having
-unlike physical properties and unlike minute structures, the effects of
-strains become too complicated to follow with precision. Anything in
-the way of interpretation must therefore be regarded as tentative. If
-analysis and comparison show that the phenomena are not inconsistent
-with the hypothesis of mechanical genesis, it is as much as can be
-expected. Let us first observe more nearly the mechanical conditions to
-which bones are subject.</p>
-
-<p>The endo-skeleton of a mammal with the muscles and ligaments holding
-it together, may be rudely compared to a structure built up of struts
-and ties; of which, speaking generally, the struts bear the pressures
-and the ties bear the tensions. The framework of an ordinary iron
-roof will give an idea of the functions of these two elements, and of
-the mechanical characters required by them. Such a framework consists
-partly of pieces which have each to bear a thrust in the direction of
-its length, and partly of pieces which have each to bear a pull in the
-direction of its length; and these struts and ties are differently
-formed to adapt them to these different strains. Further, it should
-be remarked that though the rigidity of the framework depends on the
-ties which are flexible, as much as on the struts which are stiff, yet
-the ties help to give the rigidity simply by so holding the struts
-in position that they cannot escape from the thrusts which fall on
-them. Now the like relation holds with a difference among the bones
-and muscles: the difference being that here the ties admit of being
-lengthened or shortened and the struts of being moved about upon their
-joints. The mechanical relations are not altered by this, however.
-The actions are of essentially the same kind in an animal that is
-standing, or keeping itself in a strained attitude, as in one that is
-changing its attitude&mdash;the same in so far that we have in each a set
-of flexible parts that are pulling and a set of<span class="pagenum" id="Page_346">[346]</span> rigid parts that are
-resisting. It needs but to remember the sudden collapse and fall which
-take place when the muscles are paralyzed, or to remember the inability
-of a bare skeleton to support itself, to see that the struts without
-the ties cannot suffice. And we have but to think of the formless mass
-into which a man would sink when deprived of his bones, to see that
-the ties without the struts cannot suffice. To trace the way in which
-a particular bone has its particular thrust thrown upon it, may not
-always be practicable. Though it is easy to perceive how a flexor or
-extensor of the arm causes by its tension a reactive pressure along the
-line of the humerus, and is enabled to produce its effect only by the
-rigidity of the humerus; yet it is not so easy to perceive how such
-bones as those of a horse’s pelvis are similarly acted upon. Still, as
-the weight of the hind quarters has to be transferred from the back to
-the feet, and must be so transferred through the bones, it is manifest
-that though these bones form a very crooked line, the weight must
-produce a pressure along the axis of each: the muscles and ligaments
-concerned serving here, as in other cases, so to hold the bones that
-they bear the pressure instead of being displaced by it. Not forgetting
-that many processes of the bones have to bear tensions, we may then
-say that generally, though by no means universally, bones are internal
-dense masses that have to bear pressures&mdash;pressures which in the
-cylindrical bones become longitudinal thrusts. Leaving out exceptional
-cases, let us consider bones as masses thus circumstanced.</p>
-
-<p>When giving reasons for the belief that the vertebrate skeleton is
-mechanically originated, one of the facts put in evidence was, that
-in the vertebrate series the transition from the cartilaginous to the
-osseous spine begins peripherally (<a href="#sect257">§ 257</a>): each vertebra being at first
-a ring of bone surrounding a mass of cartilage. And it was pointed
-out that this peripheral ossification is ossification at the region
-of greatest pressures. Now it is not vertebræ only that follow this
-course of development. In a cylindrical bone, though<span class="pagenum" id="Page_347">[347]</span> it is differently
-circumstanced, the places of commencing ossification are still the
-places on which the severest stress falls. Let us consider how such a
-bone that has to bear a longitudinal pressure is mechanically affected.
-If the end of a walking-cane be thrust with force against the ground,
-the cane bends; and partially resuming its straightness when relieved,
-again bends, usually towards the same side, when the thrust is renewed.
-A bend so caused acts on the fibres of the cane in nearly the same
-way as does a bend caused by supporting the cane horizontally at its
-two ends and suspending a weight from its middle. In either case the
-fibres on the convex side are extended and the fibres on the concave
-side compressed. Kindred actions occur in a rod that is so thick
-as not to yield visibly under the force applied. In the absence of
-complete homogeneity of its substance, complete symmetry in its form,
-and an application of a force exactly along its axis, there must be
-some lateral deflection; and therefore some distribution of tensions
-and pressures of the kind indicated. And then, as the fact which here
-specially concerns us, we have to note that the strongest tensions
-and pressures are borne by the outer layers of fibres. Now the shaft
-of a long bone, subject to mechanical actions of this kind, similarly
-has its outer layer most strained. In this layer, therefore, on the
-mechanical hypothesis, ossification should commence, and here it does
-commence&mdash;commences, too, midway between the ends, where the bends
-produce on the superficial parts their most intense effects. But we
-have not in this place simply to observe that ossification commences at
-the places of greatest stress, but to ask what causes it to do this.
-Can we trace the physical actions which set up this deposit of dense
-tissue? It is, I think, possible to indicate a “true cause” that is at
-work; though whether it is a sufficient cause may be questioned. We
-concluded that in certain other cases, the formation of dense tissue
-indirectly results from the alternate squeezing and relaxation of the
-vessels running through the part; and the inquiry now to be made is,
-whether, in developing bone,<span class="pagenum" id="Page_348">[348]</span> the same actions go on in such ways as to
-produce the observed effects. At the outset we are met by what seems a
-fatal difficulty&mdash;cartilage is a non-vascular tissue: this substance
-of which unossified bones consist is not permeated by minute canals
-carrying nutritive liquid, and cannot, therefore, be a seat of actions
-such as those assigned. This apparent difficulty, however, furnishes a
-confirmation. For cartilage that is wholly without permeating canals
-does not ossify: ossification takes place only at those parts of it
-into which the canals penetrate. Hence, we get additional reason for
-suspecting that bone-formation is due to the alleged cause; since it
-occurs where mechanical strains can produce the actions described, but
-does not occur where mechanical strains cannot produce them. Let us
-consider more closely what the several factors are. It will suffice
-for the argument if we commence with the external vascular layer as
-already existing, and consider what will take place in it. Cartilage
-is elastic&mdash;is somewhat extensible, and spreads out laterally under
-pressure, but resumes its form when relieved. How, then, will the
-minute channels traversing it in all directions be affected at the
-places where it is strained by a bend? Those on the convex side will
-be laterally squeezed, in the same way that we saw the sap-vessels on
-the convex side of a bent branch are squeezed; and as exudation of the
-sap into the adjacent prosenchyma will be caused in the one case, so,
-in the other, there will be caused exudation of serum into the adjacent
-cartilage: extra nutrition and increase of strength resulting in both
-cases. The parallel ceases here, however. In the shoot of a plant, bent
-in various directions by the wind, the side which was lately compressed
-is now extended; and hence that squeezing of the sap-vessels which
-results from extension, suffices to feed and harden the tissue on all
-sides of the shoot. But it is not so with a bone. Having yielded on
-one side under longitudinal pressure, and resumed as nearly as may be
-its previous shape when the pressure is<span class="pagenum" id="Page_349">[349]</span> taken off, the bone yields
-again towards the same side when again longitudinally pressed. Hence
-the substance of its concave side, never rendered convex by a bend
-in the opposite direction, would not receive any extra nutrition did
-no other action come into play. But if we consider how intermittent
-pressures must act on cartilage, we shall see that there will result
-extra nutrition of the concave side also. Squeeze between two pieces of
-glass a thin bit of caoutchouc which has a hole through it. While the
-caoutchouc spreads out away from the centre, it also spreads inwards,
-so as partially to close the hole. Everywhere its molecules move away
-in directions of least resistance; and for those near the hole, the
-direction of least resistance is towards the hole. Let this hole stand
-for the transverse section of one of the minute canals or channels
-passing through cartilage, and it will be manifest that on the side of
-the unossified bone made concave in the way described, the compressed
-cartilage will squeeze the canals traversing it; and, in the absence
-of perfect homogeneity in the cartilage, the squeeze will cause extra
-exudation from the canals into the cartilage. Thus every additional
-strain will give to the cartilage it falls upon, an additional supply
-of the materials for growth. So that presently the side which, by
-yielding more than any other, proves itself to be the weakest, will
-cease to be the weakest. What further will happen? Some other side
-will yield a little&mdash;the bends will take place in some other plane;
-and the portions of cartilage on which repeated tensions and pressures
-now fall will be strengthened. Thus the rate of nutrition, greatest at
-the place where the bending is greatest, and changing as the incidence
-of forces changes, will bring about at every point a balance between
-the resistances and the strains. Thus, too, there will be determined
-that peripheral induration which we see in bones so circumstanced.
-As in a shoot we saw that the woody deposit takes place towards the
-outside of the cylinder, where, according to the hypothesis, it ought
-to take place; so, here, we see that the<span class="pagenum" id="Page_350">[350]</span> excess of exudation and
-hardening, occurring where the strains are most intense, will form a
-cylinder having a dense outside and a porous or hollow inside. These
-processes will be essentially the same in bones subject to more complex
-mechanical actions, such as sundry of the flat bones and others that
-serve as internal fulcra. Be the strains transverse or longitudinal,
-be they torsion strains or mixed strains, the outer parts of the
-bone will be more affected by them than its inner parts. They will
-therefore tend everywhere to produce resisting masses having outer
-parts more dense than their inner parts. And by causing most growth
-where they are most intense, they will call out reactive forces
-adequate to balance them. There are doubtless obstacles in the way
-of this interpretation. It may be said that the forces acting on the
-outer layers in the manner described, would compress the canals too
-little to produce the alleged effects; and if evenly distributed along
-the whole lengths of the layers, they would probably do so. But it
-needs only to bend a flexible mass and observe the tendency to form
-creases on the concave surface, to feel assured that along the surface
-of an ossifying bone, the yielding of the tissue when bent will not
-be uniform. In the absence of complete homogeneity, the interstitial
-yielding will take place at some points more than others, and at one
-point above all others. When, at the weakest point&mdash;the centre of
-commencing ossification&mdash;an extra amount of deposit has been caused,
-it will cease to be the weakest; and adjacent points, now the weakest,
-will become the places of yielding and induration. It may be further
-objected that the hypothesis is incompatible with the persistence
-of cartilage for so long a time between the epiphysis of bones and
-the bony masses which they terminate. But there is the reply that
-the places occupied by this cartilage being places at which the bone
-lengthens, the non-ossification is in part apparent only&mdash;it is rather
-that new cartilage is formed as fast as the pre-existing cartilage
-ossifies; and there is the further reply<span class="pagenum" id="Page_351">[351]</span> that the slowness of the
-ultimate ossification of this part, is due to its non-vascularity, and
-to mechanical conditions which are unfavourable to its acquirement of
-vascularity. Once more, there is the demurrer that in the epiphyses
-ossification does not begin at the surface but within the mass of
-the cartilage. Explanation of this implies ability to follow out the
-mechanical actions in a resilient substance which, like india-rubber,
-admits of being distorted in all ways by pressure and recovering its
-form, and it seems impossible to say how the more superficial and more
-deep-seated canals traversing it will be respectively affected.</p>
-
-<p>Of course it is not meant that this osseous development by direct
-equilibration takes place in the individual. Though it is a corollary
-from the argument that in each individual the process must be furthered
-and modified by the particular actions to which the particular bones
-are exposed; yet the leading traits of structure assumed by the bones
-are assumed in conformity with the inherited type. This, however, is
-no difficulty. The type itself is to be regarded as the accumulated
-result of such modifications, transmitted and increased from generation
-to generation. The actions above described as taking place in the
-bone of an individual, must be understood as producing their total
-effect little by little in the corresponding bones of a long series of
-individuals. Even if but a small modification can be so wrought in the
-individual, yet if such modification, or a part of it, is inheritable,
-we may readily understand how, in the course of geologic epochs, the
-observed structures may arise in the assigned way.</p>
-
-<p>Here may fitly be added a strong confirmation. If we find cases
-where individual bones, subject in exceptional degrees to the
-actions described, present in exceptional amounts the modifications
-attributed to them, we are greatly helped in understanding how there
-may be produced in the race that aggregate of modifications which
-the hypothesis implies. Such cases occur in ricketty children. I
-am indebted to Mr. Busk for pointing out these abnormal formations
-of dense tissue, that are not apparently explicable as results of<span class="pagenum" id="Page_352">[352]</span>
-mechanical actions and reactions. It was only on tracing out the
-processes here at work, that there suggested itself the specific
-interpretation of the normal process, as above set forth. When, from
-constitutional defect, bones do not ossify with due rapidity, and are
-meanwhile subject to the ordinary strains, they become distorted.
-Remembering how a mass which has been made to yield in any direction
-by a force it cannot withstand, is some little time before it recovers
-completely its previous form, and usually, indeed, undergoes what
-is called a “permanent set;” it is inferable that when a bone is
-repeatedly bent at the same time that the liquid contained in its
-canals is poor in the materials for forming dense tissue, there
-will not take place a proportionate strengthening of the parts most
-strained; and these parts will give way. This happens in rickets. But
-this having happened, there goes on what, in teleological language,
-we call a remedial process. Supposing the bone to be one commonly
-affected&mdash;a femur; and supposing a permanent bend to have been caused
-in it by the weight of the body; the subsequent result is an unusual
-deposition of cartilaginous and osseous matter on the concave side of
-the bone. If the bone is represented by a strung bow, then the deposit
-occurs at the part represented by the space between the bow and the
-string. And thus occurring where its resistance is most effective,
-it increases until the approximately-straight piece of bone formed
-within the arc, has become strong enough to bear the pressure without
-appreciably yielding. Now this direct adaptation, seeming so like a
-special provision, and furnishing so remarkable an instance of what,
-in medical but unscientific language, is called the <i>vis medicatrix
-naturæ</i>, is simply a result of the above-described mechanical
-actions and reactions, going on under the exceptional conditions. Each
-time such a bent bone is subject to a force which again bends it, the
-severest compression falls on the substance of its concave side. Each
-time, then, the canals running through this part of its substance are
-violently squeezed&mdash;far<span class="pagenum" id="Page_353">[353]</span> more squeezed than they or any other of the
-canals would have been, had the bone remained straight. Hence, on every
-repetition of the strain, these canals near the concave surface have
-their contents forced out in more than normal abundance. The materials
-for the formation of tissue are supplied in quantity greater than can
-be assimilated by the tissue already formed; and from the excess of
-exuded plasma, new tissue arises.<a id="FNanchor_50" href="#Footnote_50" class="fnanchor">[50]</a> A layer of organizable material
-accumulates between the concave surface and the periosteum; in this,
-according to the ordinary course of tissue-growth, new vessels appear;
-and the added layer presently assumes the histological character of the
-layer from which it has grown. What next happens? This added layer,
-further from the neutral axis than that which has thrown it out, is now
-the most severely compressed, and its vessels are the most severely
-squeezed. The place of greatest exudation and most rapid deposit
-of matter, is therefore transferred to this new layer; and at the
-same time that active nutrition increases its density, the excess of
-organizable material forms another layer external to it: the successive
-layers so added, encroaching on the space between the concave surface
-of the bone and the chord of its arc. What limits the encroachment on
-this space?&mdash;what stops the process of filling it up? The answer to
-this question will be manifest when observing that there comes into
-play a cause which gradually diminishes the forces falling on each new
-layer. For the transverse sectional area is step by step increased; and
-an increase of the area over which the weight borne is distributed,
-implies a relatively smaller pressure upon each part of it. Further, as
-the transverse dimensions of the bone increase, the materials composing
-its convex and concave layers, becoming further from the neutral axis,
-become better<span class="pagenum" id="Page_354">[354]</span> placed for resisting the strains to be borne. So that
-both by the increased quantity of dense matter and by its mechanically
-more-advantageous position, the bendings of the bone are progressively
-decreased. But as they are decreased, each new layer formed on the
-concave surface has its substance and its vessels less compressed; and
-the resulting growth and induration are rendered less rapid. Evidently,
-then, the additions, slowly diminishing, will eventually cease; and
-this will happen when the bone no longer bends. That is to say, the
-thickening of the bone will reach its limit when there is equilibrium
-between the incident forces and the forces which resist them. Here,
-indeed, we may trace with great clearness the process of direct
-equilibration&mdash;may see how an unusual force, falling on the moving
-equilibrium of an organism and not overthrowing it, goes on working
-modifications until the reaction balances the action.</p>
-
-<p>That, however, which now chiefly concerns us, is to note how this
-marked adaptation supports the general argument. Unquestionably bone
-is in this case formed under the influence of mechanical stress, and
-formed just where it most effectually meets the stress. This result,
-not otherwise explained, is explained by the hypothesis above set
-forth. And when we see that this special deposit of bone is accounted
-for by actions like those to which bone-formation in general is
-ascribed, the probability that these are the actions at work becomes
-very great.<a id="FNanchor_51" href="#Footnote_51" class="fnanchor">[51]</a></p>
-
-<p><span class="pagenum" id="Page_355">[355]</span></p>
-
-<p>Of course it is not alleged that osseous structures arise in this
-way alone. The bones of the skull and various dermal bones cannot be
-thus interpreted. Here the natural selection of favourable variations
-appears the only assignable cause&mdash;the equilibration is indirect. We
-know that ossific deposits now and then occur in tissues where they
-are not usually found; and such deposits, originally abnormal, if they
-occurred in places where advantages arose from them, might readily be
-established and increased by survival of the fittest. Especially might
-we expect this to happen when a constitutional tendency to form bone
-had been established by actions of the kind described; for it is a
-familiar fact that differentiated types of tissue, having once become
-elements of an organism, are apt occasionally to arise in unusual
-places, and there to repeat all their peculiar histological characters.
-And this may possibly be the reason why the bones of the skull, though
-not exposed to forces such as those which produce, in other bones,
-dense outer layers including less dense interiors, nevertheless repeat
-this general trait of bony structure. While, however, it is beyond
-doubt that some bones are not due to the direct influence of mechanical
-stress, we may, I think, conclude that mechanical stress initiates
-bone-formation.</p>
-
-
-<p class="p2">§ 302. What is the origin of nerve? In what way do its properties stand
-related to the properties of that protoplasm whence the tissues in
-general arise? and in what way is it differentiated from protoplasm
-simultaneously with the other tissues? These are profoundly interesting
-questions; but questions to which positive answers cannot be expected.
-All that can be done is to indicate answers which seem feasible.</p>
-
-<p>That the property specially displayed by nerve, is a property<span class="pagenum" id="Page_356">[356]</span> which
-protoplasm possesses in a lower degree, is manifest. The sarcode
-of a Rhizopod and the substance of an unimpregnated ovum, exhibit
-movements that imply a propagation of stimulus from one part of
-the mass to another. We have not far to seek for a probable origin
-of this phenomenon. There is good reason for ascribing it to the
-extreme instability of the organic colloids of which protoplasm
-consists. These, in common with colloids in general, assume
-different isomeric forms with great facility; and they display not
-simply isomerism but polymerism. Further, this readiness to undergo
-molecular re-arrangement, habitually shows itself in colloids by
-the rapid propagation of the re-arrangement from part to part. As
-Prof. Graham has shown, matter in this state often “pectizes” almost
-instantaneously&mdash;a touch will transform an entire mass. That is to say,
-the change of molecular state once set up at one end, spreads to the
-other end&mdash;there is a progress of a stimulus to change; and this is
-what we see in a nerve. So much being understood, let us re-state the
-case more completely.</p>
-
-<p>Molecular change, implying as it does motion of molecules, communicates
-motion to adjacent molecules; be they of the same kind or of a
-different kind. If the adjacent molecules, either of the same kind or
-of a different kind, be stable in composition, a temporary increase
-of oscillation in them as wholes, or in their parts, may be the only
-result; but if they are unstable there are apt to arise changes of
-arrangement among them, or among their parts, of more or less permanent
-kinds. Especially is this so with the complex molecules which form
-colloidal matter, and with the organic colloids above all. Hence it is
-to be inferred that a molecular disturbance in any part of a living
-animal, set up by either an external or internal agency, will almost
-certainly disturb and change some of the surrounding colloids not
-originally implicated&mdash;will diffuse a wave of change towards other
-parts of the organism: a wave which will, in the absence of perfect
-homogeneity, travel further in some directions than in<span class="pagenum" id="Page_357">[357]</span> others. Let us
-ask next what will determine the differences of distance travelled in
-different directions. Obviously any molecular agitation spreading from
-a centre, will go furthest along routes that offer least resistance.
-What routes will these be? Those along which there lie most molecules
-that are easily changed by the diffused molecular motion, and which
-yet do not take up much molecular motion in assuming their new states.
-Molecules which are tolerably stable will not readily propagate the
-agitation; for they will absorb it in the increase of their own
-oscillations, instead of passing it on. Molecules which are unstable
-but which, in assuming isomeric forms, absorb motion, will not readily
-propagate it; since it will disappear in working the changes in them.
-But unstable molecules which, in being isomerically transformed, do not
-absorb motion, and still more those which, in being so transformed,
-give out motion, will readily propagate any molecular agitation; since
-they will pass on the impulse either undiminished, or increased, to
-adjacent molecules. If then we assume, as we are not only warranted
-in doing but are obliged to do, that protoplasm contains two or more
-colloids, either mingled or feebly combined (since it cannot consist of
-simple albumen or fibrin or casein, or any allied proximate principle);
-it may be concluded that any molecular agitation set up by what we call
-a stimulus, will diffuse itself further along some lines than along
-others, if the components of the protoplasm are not quite homogeneously
-dispersed, and if some of them are isomerically transformed more
-easily, or with less expenditure of motion, than others; and it will
-especially travel along spaces occupied chiefly by those molecules
-which give out molecular motion during their metamorphoses, if there
-should be any such. But now let us ask what structural effects
-will be wrought along a tract traversed by this wave of molecular
-disturbance. As is shown by those transformations which so rapidly
-propagate themselves through colloids, molecules that have undergone a
-certain change of form,<span class="pagenum" id="Page_358">[358]</span> are apt to communicate a like change of form
-to adjacent molecules of the same kind&mdash;the impact of each overthrow
-is passed on and produces another overthrow. Probably the proneness
-towards isochronism of molecular movements necessitates this. If any
-molecule has had its components re-arranged, and their oscillations
-consequently altered, there result movements not concordant with the
-movements in adjacent untransformed molecules, but which, impressing
-themselves on the parts of such untransformed molecules, tend to
-generate in them concordant movements&mdash;tend, that is, to produce
-the re-arrangements involved by these concordant movements. Is this
-action limited to strictly isomeric substances? or may it extend to
-substances that are closely-allied? If along with the molecules of
-a compound colloid there are mingled those of some kindred colloid;
-or if with the molecules of this compound colloid there are mingled
-the components out of which other such molecules may be formed; then
-there arises the question&mdash;does the same influence which tends to
-propagate the isomeric transformations, tend also to form new molecules
-of the same kind out of the adjacent components? There is reason to
-suspect that it does. Already when treating of the nutrition of parts
-(<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect64">§ 64</a>), it was pointed out that we are obliged to recognize a power
-possessed by each tissue to build up, out of the materials brought
-to it, molecules of the same type as those of which it is formed.
-This building up of like molecules seems explicable as caused by the
-tendency of the new components which the blood supplies, to acquire
-movements isochronous with those of the like components in the tissue;
-which they can do only by uniting into like compound molecules.
-Necessarily they must gravitate towards a state of equilibrium; such
-state of equilibrium&mdash;moving equilibrium of course&mdash;must be one in
-which they oscillate in the same times with neighbouring molecules;
-and so to oscillate they must fall into groups identical with the
-groups around them. If this be a general principle of<span class="pagenum" id="Page_359">[359]</span> tissue-growth
-and repair, we may conclude that it will apply in the case before
-us. A wave of molecular disturbance passing along a tract of mingled
-colloids closely-allied in composition, and isomerically transforming
-the molecules of one of them, will be apt at the same time to form
-some new molecules of the same type, at any place where there exist
-the proximate components, either uncombined or feebly combined in some
-not very different way. And this will be most likely to occur where
-the molecules of the colloid that are undergoing the isomeric change,
-predominate, but have scattered through them the other molecules out of
-which they may be formed, either by composition or modification. That
-is to say, a wave of molecular disturbance diffused from a centre, and
-travelling furthest along a line where lie most molecules that can be
-isomerically transformed with facility, will be likely at the same time
-to further differentiate this line, and make it more characterized than
-before by the easy-transformability of its molecules. One additional
-step, and the interpretation is reached. Analogy shows it to be not
-improbable that these organic colloids, isomerically transformed by
-slight molecular impact or increase of molecular motion, will some of
-them resume their previous molecular structures after the disturbance
-has passed. We know that what are stable molecular arrangements under
-one degree of molecular agitation, are not stable under another degree;
-and there is evidence that re-arrangements of an inconspicuous kind
-are occasionally brought about by very slight changes of molecular
-agitation. Water supplies a clear case. Prof. Graham infers that water
-undergoes a molecular re-arrangement at about 32°&mdash;that ice has a
-colloid form as well as a crystalloid form, dependent on temperature.
-Send through it an extra wave of the molecular agitation we call heat,
-and its molecules aggregate in one way. Let the wave die away, and its
-molecules resume their previous mode of aggregation. And obviously
-such transformations may be repeated backwards and forwards within
-narrow limits of temperature.<span class="pagenum" id="Page_360">[360]</span> Now among the extremely unstable organic
-colloids, such a phenomenon is far more likely to happen. Suppose,
-then, that the nerve-colloid is one of which the molecules are changed
-in form by a passing wave of extra agitation, but resume their previous
-form when the wave has passed: the previous form being the most stable
-under the conditions which then recur. What follows? It follows that
-these molecules will be ready again to undergo isomeric transformation
-when there again occurs the stimulus; will, as before, propagate the
-transformation most along the tract where such molecules are most
-abundant; will, as before, tend to form new molecules of their own
-type; will, as before, make the line along which they lie one of easier
-transfer for the molecular agitation. Every repetition will help to
-increase, to integrate, to define more completely, the course of the
-escaping molecular motion&mdash;extending its remoter part while it makes
-its nearer part more permeable&mdash;will help, that is, to form a line of
-discharge, a line for conducting impressions, a nerve.</p>
-
-<p>Such seems to me a not unfair series of deductions from the known
-habitudes of colloids in general and the organic colloids in
-particular. And I think that the implied nature and properties of nerve
-correspond better with the observed phenomena than do the nature and
-properties implied by other hypotheses. Of course the speculation as
-it here stands is but tentative, and leaves much unexplained. It gives
-no obvious reply to the questions&mdash;what causes the formation of nerves
-in directions adapted to the needs? what determines their appropriate
-connexions?&mdash;questions, however, to which, when we come to deal with
-physiological integration, we may find not unsatisfactory answers.
-Moreover it says nothing about the genesis of ganglia. A ganglion,
-it is clear, must consist of a colloidal matter equally unstable, or
-still more unstable, which, when disturbed, falls into some different
-molecular arrangement, perhaps chemically simpler, and gives out in
-so doing a large amount of molecular motion&mdash;serves as a reservoir of
-molecular motion which may be suddenly<span class="pagenum" id="Page_361">[361]</span> discharged along an efferent
-nerve or nerves, when excitement of an afferent nerve has disengaged
-it. How such a structure as this results, the hypothesis does not show.
-But admitting these shortcomings it may still be held that we are, in
-the way pointed out, enabled to form some idea of the actions by which
-nervous tissue is differentiated.</p>
-
-
-<p class="p2">§ 303. A speculation akin to, and continuous with, the last,
-is suggested by an inquiry into the origin of muscular tissue.
-Contractility as well as irritability is a property of protoplasm
-or sarcode; and, as before suggested (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect22">§ 22</a>), is not improbably
-due to isomeric change in one or more of its component colloids.
-It is a feasible supposition that of the several isomeric changes
-simultaneously set up among these component colloids, some may be
-accompanied by change of bulk and some not. Clearly the isomeric
-change undergone by the colloid which we suppose to form nerve, must
-be one not accompanied by appreciable change of bulk; since change of
-bulk implies “internal work,” as physicists term it, and therefore
-expenditure of force. Conversely, the colloid out of which muscle
-originates, may be one that readily passes into an isomeric state
-in which it occupies less space: the molecular disturbance causing
-this contraction being communicated to it from adjacent portions of
-nerve-substance that are molecularly disturbed; or being otherwise
-communicated to it by direct mechanical or chemical stimuli: as happens
-where nerves do not exist, or where their influence has been cut off.
-This interpretation seems, indeed, to be directly at variance with
-the fact that muscle does not diminish in bulk during contraction
-but merely changes its shape. That which we see take place with the
-muscle as a whole, is said also to take place with each fibre&mdash;while
-it shortens it also broadens. There is, however, a possible solution
-of this difficulty. A contracting colloid yields up its water; and
-the contracted colloid <i>plus</i> the free water, may have the same
-bulk as before though the colloid has less. If it be replied that in
-this<span class="pagenum" id="Page_362">[362]</span> case the water should become visible between the substance of
-the fibre and its sarcolemma or sheath, it may be rejoined that this
-is not necessary&mdash;it may be deposited interstitially. Possibly the
-striated structure is one that facilitates its exudation and subsequent
-re-absorption; and to this may be due the superiority of striated
-muscle in rapidity of contraction. Granting the speculative character
-of this interpretation, let us see how far it agrees with the facts.
-If the actions are as here supposed, the contracted or more integrated
-state of the muscular colloid will be that which it tends continually
-to assume&mdash;that into which it has an increasing aptitude to pass when
-artificial paralysis has been produced, as shown by Dr. Norris&mdash;that
-into which it lapses completely in <i>rigor mortis</i>. The sensible
-motion generated by the contraction can arise only from the
-transformation of insensible motion. This insensible motion suddenly
-yielded up by a contracting mass, implies the fall of its component
-molecules into more stable arrangements. And there can be no such fall
-unless the previous arrangement is unstable. From this point of view,
-too, it is possible to see how the hydro-carbons and carbo-hydrates
-consumed in muscular action, may produce their effects. For these
-non-nitrogenous elements of food, when consumed in the tissues, give
-out large amounts of molecular motion. They do this in presence of
-the muscular colloids which have lost molecular motion during their
-fall in the stable or contracted state. From the molecular motion they
-give out, may be restored the molecular motion lost by the contracted
-colloids; and these contracted colloids may thus have their molecules
-raised to that unstable state from which, again falling, they can again
-generate mechanical motion.</p>
-
-<p>This conception of the nature and mode of action of muscle, while it
-is suggested by known properties of colloidal matter and conforms to
-the recent conclusions of organic chemistry and molecular physics,
-establishes a comprehensible relation between the vital actions of
-the lower and the higher<span class="pagenum" id="Page_363">[363]</span> animals. If we contemplate the movements
-of cilia, of a Rhizopod’s pseudopodia, of a Polype’s body, or of
-the long pendant tentacles of a <i>Medusa</i>, we shall see great
-congruity between them and this hypothesis. Bearing in mind that the
-contractile substance of developed muscle is affected not by nervous
-influence only, but, where nervous influence is destroyed, is made to
-contract by mechanical disturbance and chemical action, we may infer
-that it does not differ intrinsically from the primordial contractile
-substance which, in the lowest animals, changes its bulk under other
-stimuli than the nervous. We shall see significance in the fact
-ascertained by Dr. Ransom, that various agents which excite and arrest
-nervo-muscular movements in developed animals, excite and arrest the
-protoplasmic movements in ova. We shall understand how tissues not yet
-differentiated into muscle and nerve, have this joint irritability and
-contractility; how muscle and nerve may arise by the segregation of
-their mingled colloids, the one of which, not appreciably altering its
-bulk during isomeric change, readily propagates molecular disturbance,
-while the other, contracting when isomerically changed, less readily
-passes on the molecular disturbance; and how, by this differentiation
-and integration of the conducting and the contracting colloids, the one
-ramifying through the other, it becomes possible for a whole mass to
-contract suddenly, instead of contracting gradually, as it does when
-undifferentiated.</p>
-
-<p>The question remaining to be asked is&mdash;What causes the specialization
-of contractile substance?&mdash;What causes the growth of colloid masses
-which monopolize this contractility, and leave kindred colloids to
-monopolize other properties? Has natural selection gradually localized
-and increased the primordial muscular substance? or has the frequent
-recurrence of irritations and consequent contractions at particular
-parts done it? We have, I think, reason to conclude that direct
-equilibration rather than indirect equilibration has been chiefly
-operative. The reasoning that was used in the case<span class="pagenum" id="Page_364">[364]</span> of nerve applies
-equally in the case of muscle. A portion of undifferentiated tissue
-containing a predominance of the colloid that contracts in changing,
-will, during each change, tend to form new molecules of its own type
-from the other colloids diffused through it: the tendency of these
-entangled colloids to fall into unity with those around them, will
-be aided by every shock of isomeric transformation. Hence, repeated
-contractions will further the growth of the contracting mass, and
-advance its differentiation and integration. If, too, we remember that
-the muscular colloid is made to contract by mechanical disturbance, and
-that among mechanical disturbances one which will most readily affect
-it simultaneously throughout its mass is caused by stretching, we
-shall be considerably helped towards understanding how the contractile
-tissues are developed. If extension of a muscular colloid previously
-at rest, produces in it that molecular disturbance which leads to
-isomeric change and decrease of bulk, then there is no difficulty in
-explaining the movements of cilia; the formation of a contractile
-layer in the vascular system becomes comprehensible; each dilatation
-of a blood-vessel caused by a gush of blood, will be followed by a
-constriction; the heart will pulsate violently in proportion as it is
-violently distended; arteries will develop in power as the stress upon
-them becomes greater; and we shall similarly have an explanation of the
-increased muscularity of the alimentary canal which is brought about by
-increased distension of it.</p>
-
-<p>That the production of contractile tissue in certain localities,
-is due to the more frequent excitement in those localities of the
-contractility possessed by undifferentiated tissue in general, is a
-view harmonizing with traits which the differentiated contractile
-tissue exhibits. These are the relations between muscular exercise,
-muscular power, and muscular structure; and it is the more needful
-for us here to notice them because of certain anomalies they present,
-which, at first sight, seem inconsistent with the belief that<span class="pagenum" id="Page_365">[365]</span> the
-functionally-determined modifications of muscle are inheritable.</p>
-
-<p>Muscles disagree greatly in their tints: all gradations between white
-and deep red being observable. Contrasts are visible between the
-muscles of different animals, between the muscles of the same animal
-at different ages, and between different muscles of the same animal
-at the same age. We will glance at the facts under these heads:
-noting under each of them the connexion which here chiefly concerns
-us&mdash;that between the activity of muscle and its depth of colour. The
-cold-blooded <i>Vertebrata</i> are, taken as a group, distinguished
-from the warmblooded by the whiteness of their flesh; and they are
-also distinguished by their comparative inertness. Though a fish
-or a reptile can exert considerable force for a short time, it is
-not capable of prolonged exertion. Birds and mammals show greater
-endurance along with the darker-coloured muscles. If among birds
-themselves or mammals themselves we make comparisons, we meet with
-kindred contrasts&mdash;especially between wild and domestic creatures of
-allied kinds. Barn-door fowls are lighter-fleshed than most untamed
-gallinaceous birds; and among these last the pheasant, moving about but
-little, is lighter-fleshed than the partridge and the grouse which are
-more nomadic. The muscles of the sheep are not on the average so dark
-as those of the deer; and it is said that the flesh of the wild-boar is
-darker than that of the pig. Perhaps, however, the contrast between the
-hare and the rabbit affords, among familiar animals, the best example
-of the alleged relation: the dark-fleshed hare having no retreat and
-making wide excursions, while the white-fleshed rabbit, passing a
-great part of its time in its burrow, rarely wanders far from home.
-The parallel contrast between young and old animals has a parallel
-meaning. Veal is much whiter than beef, and lamb is of lighter colour
-than mutton. Though at first sight these facts may not seem to furnish
-confirmatory evidence, since lambs in their play appear to<span class="pagenum" id="Page_366">[366]</span> expend
-more muscular force than their sedate dams; yet the meaning of the
-contrast is really as alleged. For in consequence of the law that the
-strains which animals have to overcome, increase as the cubes of the
-dimensions, while their powers of overcoming them increase only as
-the squares (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect46">§ 46</a>), the movements of an adult animal cost much more
-in muscular effort than do those of a young animal: the result being
-that the sheep and the cow exercise their muscles more vigorously in
-their quiet movements, than the lamb and the calf in their lively
-movements. It may be added as significant, that the domestic animal
-in which no very marked darkening of the flesh takes place along with
-increasing age, namely the pig, is one which, ordinarily kept in a sty,
-leads so quiescent a life that the assigned cause of darkening does
-not come into action. But perhaps the most conclusive evidences are
-the contrasts which exist between the active and inactive muscles of
-the same animal. Between the leg-muscles of fowls and their pectoral
-muscles, the difference of colour is familiar; and we know that fowls
-exercise their leg-muscles much more than the muscles which move their
-wings. Similarly in the turkey, in the guinea fowl, in the pheasant.
-And then, adding much to the force of this evidence, we see that in
-partridges and grouse, which belong to the same order as our domestic
-fowls but use their wings as constantly as their legs, little or
-no difference is visible between the colour of these two groups of
-muscles. Special contrasts like these do not, however, exhaust the
-proofs; for there is a still more significant general contrast. The
-muscle of the heart, which is the most active of all muscles, is the
-darkest of all muscles.</p>
-
-<p>The connexion of phenomena thus shown in so many ways, implies that
-the bulk of a muscle is by no means the sole measure of the quantity
-of force it can evolve. It would seem that, other things equal, the
-depth of colour varies with the constancy of action; while, other
-things equal, the bulk varies with the amount of force that has to be
-put forth upon occasion.<span class="pagenum" id="Page_367">[367]</span> These of course are approximate relations.
-More correctly we may say that the actions of pale muscles are either
-relatively feeble though frequent (as in the massive flanks of a fish),
-or relatively infrequent though strong (as in the pectoral muscles of
-a common fowl); while the actions of dark muscles are both frequent
-and strong. Some such differentiation may be anticipated by inference
-from the respective physiological requirements. A muscle which has
-upon occasion to evolve considerable force, but which has thereafter a
-long period of rest during which repair may restore it to efficiency,
-requires neither a large reserve of the contractile substance that is
-in some way deteriorated by action, nor highly developed appliances for
-bringing it nutritive materials and removing effete products. Where,
-contrariwise, an exerted muscle which has undergone much molecular
-change in evolving much mechanical force, has soon again to evolve
-much mechanical force, and so on continually; it is clear that either
-the quantity of contractile substance present must be great, or the
-apparatus for nutrition and depuration must be very efficient, or both.
-Hence we may look for marked unlikenesses of minute structure between
-muscles which are markedly contrasted in activity. And we may suspect
-that these conspicuous contrasts of colour between active and inactive
-muscles, are due to these implied differences of minute structure:
-partly differences between the numbers of blood-vessels and partly
-differences between the quantities or qualities of sarcous matter.</p>
-
-<p>Here, then, we have a key to the apparent anomaly above hinted at&mdash;the
-maintenance of bulk by certain muscles which have been rendered
-comparatively inactive by changed habits of life. That the pectoral
-muscles of those domestic birds which fly but little, have not dwindled
-to any great extent, has been thought a fact at variance with the
-conclusion that functionally-produced adaptations are inheritable.
-It has been argued that if parts which are exercised increase, not
-only in the individual but in the race, while parts which<span class="pagenum" id="Page_368">[368]</span> become
-less active decrease; then a notable difference of size should exist
-between the muscles used for flight in birds that fly much, and those
-in birds of an allied kind that fly little. But, as we here see, this
-is not the true implication. The change in such cases must be chiefly
-in vascularity and abundance of contractile substance; and cannot be,
-to any great extent, in bulk. For a bird to fly at all, its pectoral
-muscles, bones of attachment, and all accompanying appliances, must be
-kept up to a certain level of power. If the parts dwindle much, the
-creature will be unable to lift itself from the ground. Bearing in mind
-that the force which a bird expends to sustain itself in the air during
-each successive instant of a short flight is, other things equal,
-the same as it expends in each successive instant of a long flight,
-we shall see that the muscles employed in the two cases must have
-something like equal intensities of contractile power; and that the
-structural differences between them must have relation mainly to the
-lengths of time during which they can continue to repeat contractions
-of like intensity. That is to say, while the power of flight is
-retained at all, the muscles and bones cannot greatly dwindle; but the
-dwindling, in birds whose flights are short or infrequent or both,
-will be in the reserve stock of the substance that is incapacitated by
-action, or in the appliances that keep the apparatus in repair, or in
-both. Only where, as in the struthious birds, the habit of flight is
-lost, can we expect atrophy of all the parts concerned in flight; and
-here we find it.</p>
-
-<p>Are such differentiations among the muscles functionally-produced? or
-are they produced by the natural selection of variations distinguished
-as spontaneous? We have, I think, good grounds for concluding that
-they are functionally-produced. We know that in individual men and
-animals, the power of sustained action in muscles is rapidly adaptable
-to the amount of sustained action required. We know that being “out of
-condition,” is usually less shown by the inability to put out a violent
-effort than by the inability to continue<span class="pagenum" id="Page_369">[369]</span> making violent efforts; and
-we know that the result of training for prize-fights and races, is more
-shown in the prolongation of energy than in the intensification of
-energy. At the same time, experience has taught us that the structural
-change which accompanies this functional change, is not so much a
-change in the bulk of the muscles as a change in their internal state:
-instead of being soft and flabby they become hard. We have inductive
-proof, then, that exercise of a muscle causes some interstitial
-growth along with the power of more sustained action; and there can
-be no doubt that the one is a condition to the other. What is this
-interstitial growth? There is reason to suspect that it is in part an
-increased deposit of the sarcous substance and in part a development of
-blood-vessels. Microscopic observation tends to confirm the conclusions
-before drawn, that repetition of contractions furthers the formation
-of the matter which contracts, and that greater draughts of blood
-determine greater vascularity. And if the contrasts of molecular
-structure and the contrasts of vascularity, directly caused in muscles
-by contrasts in their activities, are to any degree inheritable;
-there results an explanation of those constitutional differences in
-the colours and textures of muscles, which accompany constitutional
-differences in their degrees of activity.</p>
-
-<p>It may be added that if we are warranted in so ascribing the
-differentiations of muscles from one another to direct equilibration,
-then we have the more reason for thinking that the differentiation
-of muscles in general from other structures is also due to direct
-equilibration. That unlikenesses between parts of the contractile
-tissues having unlike functions, are caused by the unlikenesses of
-their functions, renders it the more probable that the unlikenesses
-between contractile tissue and other tissues, have been caused by
-analogous unlikenesses.</p>
-
-
-<p class="p2">§ 304. These interpretations, which have already occupied too
-large a space, must here be closed. Of course out of<span class="pagenum" id="Page_370">[370]</span> phenomena so
-multitudinous and varied, it has been impracticable to deal with any
-but the most important; and it has been practicable to deal with these
-only in a general way. Much, however, as remains to be explained, I
-think the possibility of tracing, in so many cases, the actions to
-which these internal differentiations may rationally be ascribed,
-makes it likely that the remaining internal differentiations are
-due to kindred actions. We find evidence that, in more cases than
-seemed probable, these actions produce their effects directly on the
-individual; and that the unlikenesses are produced by accumulation
-of such effects from generation to generation. While for all the
-other unlikenesses, we have, as an adequate cause, the indirect
-effects wrought by the survival, generation after generation, of the
-individuals in which favourable variations have occurred&mdash;variations
-such as those of which human anatomy furnishes endless instances.
-Thus accounting for so much, we may not unreasonably presume that
-these co-operative processes of direct and indirect equilibration will
-account for what remains.</p>
-
-<hr class="tb" />
-
-<p>[<span class="smcap">Note.</span>&mdash;After having dismissed this revised chapter as done
-with, and sent it to the printer, further thought concerning those
-differentiations which produce bone, has reminded me of a fact of
-extreme and varied significance named in the first volume. I refer
-to the formation of adaptive structures round the ends of dislocated
-bones, and to the formation of “false joints.”</p>
-
-<p>These are ontogenetic changes of which phylogeny yields no explanation.
-They do not repeat the traits of ancestral organisms, and they cannot
-be ascribed to either of the recognized evolutionary factors. If a
-humerus be broken across and, failing to set, presently comes to have
-its two loose ends so modified as in a measure to simulate the parts of
-a normal joint&mdash;the ends becoming smooth, covered with periosteum and
-supplied with fibrous tissue, and attached by ligaments in such ways
-as to allow of restrained movements&mdash;it<span class="pagenum" id="Page_371">[371]</span> is impossible to think that
-natural selection has had anything to do with the power of adjustment
-thus shown. No survival of individuals in which adaptations of this
-kind, now in one place and now in another, were better and better
-effected, could account for acquirement of the ability. Nor can it be
-supposed that the ability might result from a functionally-produced
-habit; since it is scarcely conceivable that the number of cases in
-which individuals profited by it (at first a little and gradually more)
-could be such (even did they survive) as to affect the constitution of
-the species. Both of the alleged causes of structural modifications
-are out of court. It is manifest, too, that the foregoing hypothesis
-respecting bone-formation yields us not the slightest help.</p>
-
-<p>But on carefully considering the facts, certain phenomena of profound
-meaning may strike us. Here, in a part of the body where no such
-tissues ordinarily exist and to which no such structures are ordinarily
-appropriate, there arise tissues and structures adapted to the physical
-circumstances imposed on that part. Out of what do these abnormal
-but appropriate tissues arise? The substances around&mdash;osseous,
-cartilaginous, membranous&mdash;consist of differentiated elements too
-far specialized to allow of transformation. These new tissues, then,
-must originate from the undifferentiated protoplasm pervading the
-part. The units of this protoplasm, subject to the actions proper
-to an articulation, begin to assume the appropriate histological
-traits&mdash;are determined by local stimuli to form tissues ordinarily
-associated with such stimuli. What is the inevitable implication? These
-units&mdash;physiological or constitutional, as we may call them&mdash;must
-have possessed latent potentialities of falling into these special
-arrangements under stress of such conditions. At one point there arises
-periosteum and at another ligamentous tissue, while for the shaping of
-the ends of the bones&mdash;here into a rude hinged form and there into a
-rude ball-and-socket form, according to the habitual movements&mdash;there
-goes on some appropriate deposit<span class="pagenum" id="Page_372">[372]</span> of bone. Hence we must conclude that
-in the units of protoplasm which have not yet been organized into
-special tissues, there resides the ability to take on one or other type
-of histological structure according to circumstances; and, further,
-that there resides in each of them the still more marvellous ability
-to co-operate with kindred units dispersed around in developing that
-arrangement of the parts required to constitute a “false joint.” So
-that while these units have a general proclivity towards the structure
-of the organism as a whole, they have also proclivities towards
-structures proper to the local conditions into which they fall.
-There is latent in each unit the constitution of the entire organism
-and by implication the constitution of every organ; and each unit
-while co-operating with the aggregate is ready to take part in that
-particular arrangement proper to the position it has fallen into. If
-the reader will refer back to <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect97">§§ 97<i>d</i>, 97<i>e</i></a>, in which it
-is shown that each member of a human society possesses a combination
-of potentialities like these, he will be the better enabled to believe
-that this thing <i>may be</i> so while he is unable to conceive how it
-<i>is</i> so.</p>
-
-<p>And here, indeed, let it be pointed out how completely irrelevant is
-the test of conceivableness as applied to these ultimate physiological
-actions. For as here, from the un-united ends of the broken bone, there
-presently arises a rude joint with fit membranes, ligaments, and even
-synovial fluid, though we are absolutely unable to imagine the process
-by which the adjacent tissues produce this structure; so there may
-be from an organ enlarged by function, such reactive effect upon the
-system at large as eventually to influence the reproductive cells,
-though we may be absolutely unable to imagine how this can be done.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_373">[373]</span></p>
-
-<h3>CHAPTER IX.<br />
-<span class="subhed1">PHYSIOLOGICAL INTEGRATION IN ANIMALS.</span></h3></div>
-
-
-<p>§ 305. Physiological differentiation and physiological integration,
-are correlatives that vary together. We have but to recollect the
-familiar parallel between the division of labour in a society and the
-physiological division of labour, to see that as fast as the kinds
-of work performed by the component parts of an organism become more
-numerous, and as fast as each part becomes more restricted to its
-own work, so fast must the parts have their actions combined in such
-ways that no one can go on without the rest and the rest cannot go on
-without each one.</p>
-
-<p>Here our inquiry must be, how the relationship of these two
-processes is established&mdash;what causes the integration to advance
-<i>pari passu</i> with the differentiation. Though it is manifest,
-<i>à priori</i>, that the mutual dependence of functions must be
-proportionate to the specialization of functions; yet it remains to
-find the mode in which the increasing co-ordination is determined.</p>
-
-<p>Already, among the Inductions of Biology, this relation between
-differentiation and integration has been specified and illustrated
-(<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect59">§ 59</a>). Before dealing with it deductively, a few further examples,
-grouped so as to exhibit its several aspects, will be advantageous.</p>
-
-
-<p class="p2">§ 306. If the lowly-organized <i>Planaria</i> has its body broken
-up and its gullet detached, this will, for a while, continue<span class="pagenum" id="Page_374">[374]</span> to
-perform its function when called upon, just as though it were in its
-place: a fragment of the creature’s own body placed in the gullet,
-will be propelled through it, or swallowed by it. But, as the seeming
-strangeness of this fact implies, we find no such independent actions
-of analogous parts in the higher animals. Again, a piece cut out of
-the disc of a <i>Medusa</i> continues with great persistence repeating
-those rhythmical contractions which we see in the disc as a whole;
-and thus proves to us that the contractile function in each portion
-of the disc, is in great measure independent. But it is not so with
-the locomotive organs of more differentiated types. When separated
-from the rest these lose their powers of movement. The only member of
-a vertebrate animal which continues to act after detachment, is the
-heart; and the heart has motor powers complete within itself.</p>
-
-<p>Where there is this small dependence of each part upon the whole,
-there is but small dependence of the whole upon each part. The longer
-time which it takes for the arrest of a function to produce death in
-a less-differentiated animal than in a more-differentiated animal,
-may be illustrated by the case of respiration. Suffocation in a man
-speedily causes resistance to the passage of the blood through the
-capillaries, followed by congestion and stoppage of the heart: great
-disturbance throughout the system results in a few seconds, and in
-a minute or two all the functions cease. But in a frog, with its
-undeveloped respiratory organ, and a skin through which a considerable
-aëration of the blood is carried on, breathing may be suspended for
-a long time without injury. Doubtless this difference is proximately
-due to the greater functional activity in the one case than in the
-other, and the more pressing need for discharging the produced carbon
-dioxide; but the greater functional activity being itself made possible
-by the higher specialization of functions, this remains the primary
-cause of the greater dependence of the other functions on respiration,
-where the respiratory apparatus has become highly specialized. Here<span class="pagenum" id="Page_375">[375]</span>
-indeed, we see the relation under another aspect. This more rapid
-rhythm of the functions which increased heterogeneity of structure
-makes possible, is itself a means of integrating the functions. Watch,
-when it is running down, a complicated machine of which the parts are
-not accurately adjusted, or are so worn as to be somewhat loose. There
-will be observed certain irregularities of movement just before it
-comes to rest&mdash;certain of the parts which stop first, are again made to
-move a little by the continued movement of the rest, and then become
-themselves, in turn, the causes of renewed motion in other parts which
-have ceased to move. That is to say, while the connected rhythmical
-changes of the machine are quick, their actions and reactions on one
-another are regular&mdash;all the motions are well integrated; but as the
-velocity diminishes irregularities arise&mdash;the motions become somewhat
-disintegrated. Similarly with organic functions: increase of their
-rapidity involves increase of a joint momentum which controls each
-and co-ordinates all. Thus if we compare a snake with a mammal, we
-see that its functions are not tied together so closely. The mammal,
-and especially the superior mammal, requires food with considerable
-regularity; keeps up a respiration which varies within but moderate
-limits; and has periods of activity and rest that alternate evenly and
-frequently. But the snake, taking food at long intervals, may have
-these intervals greatly extended without fatal results; its dormant and
-its active states recur less uniformly; and its rate of respiration
-varies within much wider limits&mdash;now being scarcely perceptible and
-now, as you may prove by exciting it, becoming conspicuous. So that
-here, where the rhythms are very slow, they are individually less
-regular, and are united into a less regular compound rhythm&mdash;are less
-integrated.</p>
-
-<p>Perhaps the clearest general idea of the co-ordination of functions
-that accompanies their specialization, is obtained by observing the
-slowness with which a little-differentiated animal responds to a
-stimulus applied to one of its parts,<span class="pagenum" id="Page_376">[376]</span> and the rapidity with which
-such a local stimulus is responded to by a more-differentiated animal.
-A sea-anemone and a fly will serve for the comparison. A tentacle of
-a sea-anemone, when touched, slowly contracts; and if the touch has
-been rude, the contraction presently extends to the other tentacles
-and eventually to the entire body: the stimulus to movement is
-gradually diffused throughout the organism. But if you touch a fly,
-or rather if you come near enough to threaten a touch, the entire
-apparatus of flight is instantly brought into combined action. Whence
-arises this contrast? The one creature has but faintly specialized
-contractile organs, and fibres for conveying impressions. The other
-has definite muscles and nerves and a co-ordinating centre. The parts
-of the little-differentiated sea-anemone have their functions so
-feebly co-ordinated, that one may be strongly affected for some time
-before any effect is felt by another at a distance from it; but in the
-much-differentiated fly, various remote parts instantly have changes
-propagated to them from the affected part, and by their united actions
-thus set up, the whole organism adjusts itself so as to avoid the
-danger.</p>
-
-<p>These few added illustrations will make the nature of this general
-relation sufficiently clear. Let us now pass to the interpretation of
-it.</p>
-
-
-<p class="p2">§ 307. If a <i>Hydra</i> is cut in two, the nutritive liquids
-diffused through its substance cannot escape rapidly, since there
-are no open channels for them; and hence the conditions of the parts
-at a distance from the cut is but little affected. But where, as in
-the more-differentiated animals, the nutritive liquid is contained
-in vessels which have continuous communications, cutting the body
-in two, or cutting off any considerable portion of it, is followed
-by escape of the liquid from these vessels to a large extent; and
-this affects the nutrition and efficiency of organs remote from the
-place of injury. Then where, as in further-developed creatures,
-there exists an apparatus for propelling the blood<span class="pagenum" id="Page_377">[377]</span> through these
-ramifying channels, injury of a single one will cause a loss of blood
-that quickly prostrates the entire organism. Hence the rise of a
-completely-differentiated vascular system, is the rise of a system
-which integrates all members of the body, by making each dependent on
-the integrity of the vascular system, and therefore on the integrity
-of each member through which it ramifies. In another mode, too, the
-establishment of a distributing apparatus produces a physiological
-union that is great in proportion as this distributing apparatus is
-efficient. As fast as it assumes a function unlike the rest, each part
-of an animal modifies the blood in a way more or less unlike the rest,
-both by the materials it abstracts and by the products it adds; and
-hence the more differentiated the vascular system becomes, the more
-does it integrate all parts by making each of them feel the qualitative
-modification of the blood which every other has produced. This is
-simply and conspicuously exemplified by the lungs. In the absence of a
-vascular system, or in the absence of one that is well marked off from
-the imbedding tissues, the nutritive plasma or the crude blood, gets
-what small aëration it can, only by coming near the creature’s outer
-surface, or those inner surfaces which are bathed by water. But where
-there have been formed definite channels branching throughout the body,
-and particularly where there exist specialized organs for pumping the
-blood through these channels, it manifestly becomes possible for the
-aëration to be carried on in one part peculiarly modified to further
-it, while all other parts have the aërated blood brought to them. And
-how greatly the differentiation of the vascular system thus becomes a
-means of integrating the various organs, is shown by the fatal result
-that follows when the current of aërated blood is interrupted.</p>
-
-<p>Here, indeed, it becomes obvious both that certain physiological
-differentiations make possible certain physiological integrations;
-and that, conversely, these integrations make possible other
-differentiations. Besides the waste products<span class="pagenum" id="Page_378">[378]</span> which escape through the
-lungs, there are waste products which escape through the skin, the
-kidneys, the liver. The blood has separated from it in each of these
-structures, the particular product which this structure has become
-adapted to separate; leaving the other products to be separated by
-the other adapted structures. How have these special adaptations been
-made possible? By union of the organs as recipients of one circulating
-mass of blood. While there is no efficient apparatus for transfer of
-materials through the body, the waste products of each part have to
-make their escape locally; and the local channels of escape must be
-competent to take off indifferently all the waste products. But it
-becomes practicable and advantageous for the differently-localized
-excreting structures to become fitted to separate different waste
-products, as soon as the common circulation through them grows so
-efficient that the product left unexcreted by one is quickly carried to
-another better fitted to excrete it. So that the integration of them
-through a common vascular system, is the condition under which only
-they can become differentiated. Perhaps the clearest idea of the way in
-which differentiation leads to integration, and how, again, increased
-integration makes possible still further differentiation, will be
-obtained by contemplating the analogous dependence in the social
-organism. While it has no roads, a country cannot have its industries
-much specialized: each locality must produce, as best it can, the
-various commodities it consumes, so long as it has no facilities for
-barter with other localities. But the localities being unlike in their
-natural fitnesses for the various industries, there tends ever to
-arise some exchange of the commodities they can respectively produce
-with least labour. This exchange leads to the formation of channels
-of communication. The currents of commodities once set up, make
-their foot-paths and horse-tracks more permeable; and as fast as the
-resistance to exchange becomes less, the currents of commodities become
-greater. Each locality takes more of the products<span class="pagenum" id="Page_379">[379]</span> of adjacent ones,
-and each locality devotes itself more to the particular industry for
-which it is naturally best fitted: the functional integration makes
-possible a further functional differentiation. This further functional
-differentiation reacts. The greater demand for the special product of
-each locality, excites improvements in production&mdash;leads to the use of
-methods which both cheapen and perfect the commodity. Hence results a
-still more active exchange; a still clearer opening of the channels of
-communication; a still closer mutual dependence. Yet another influence
-comes into play. As fast as the intercourse, at first only between
-neighbouring localities, makes for itself better roads&mdash;as fast as
-rivers are bridged and marshes made easily passable, the resistance to
-distribution becomes so far diminished, that the things grown or made
-in each district can be profitably carried to a greater distance; and
-as the economical integration is thus extended over a wider area, the
-economical differentiation is again increased; since each district,
-having a larger market for its commodity, is led to devote itself more
-exclusively to producing this commodity. These actions and reactions
-continue until the various localities, becoming greatly developed
-and highly specialized in their industries, are at the same time
-functionally integrated by a network of roads, and finally railways,
-along which rapidly circulate the currents severally sent out and
-received by the localities. And it will be manifest that in individual
-organisms a like correlative progress must have been caused in an
-analogous way.</p>
-
-
-<p class="p2">§ 308. Another and higher form of physiological integration in animals,
-is that which the nervous system effects. Each part as it becomes
-specialized, begins to act upon the rest not only indirectly through
-the matters it takes from and adds to the blood, but also directly
-through the molecular disturbances it sets up and diffuses. Whether
-nerves themselves are differentiated by the molecular disturbances
-thus propagated in certain directions, or whether they are otherwise<span class="pagenum" id="Page_380">[380]</span>
-differentiated, it must equally happen that as fast as they become
-channels along which molecular disturbances travel, the parts they
-connect become physiologically integrated, in so far that a change
-in one initiates a change in the other. We may dimly perceive that
-if portions of what was originally a uniform mass having a common
-function, undertake subdivisions of the function, the molecular changes
-going on in them will be in some way complementary to one another:
-that peculiar form of molecular motion which the one has lost in
-becoming specialized, the other has gained in becoming specialized.
-And if the molecular motion that was common to the two portions while
-they were undifferentiated, becomes divided into two complementary
-kinds of molecular motion; then between these portions there will be
-a contrast of molecular motions such that whatever is <i>plus</i> in
-the one will be <i>minus</i> in the other; and hence there will be a
-special tendency towards a restoration of the molecular equilibrium
-between the two: the molecular motion continually propagated away from
-either will have its line of least resistance in the direction of the
-other. If, as argued in the last chapter, repeated restorations of
-molecular equilibrium, always following the line of least resistance,
-tend ever to make it a line of diminished resistance; then, in
-proportion as any parts become more physiologically integrated by
-the establishment of this channel for the easy transmission of
-molecular motion between them, they may become more physiologically
-differentiated. The contrast between their molecular motions leads to
-the line of discharge; the line of discharge, once formed, permits a
-greater contrast of their molecular motions to arise; thereupon the
-quantities of molecular motion transferred to restore equilibrium,
-being increased, the channel of transfer is made more permeable; and
-its further permeability, so caused, renders possible a still more
-marked unlikeness of action between the parts. Thus the differentiation
-and the integration progress hand in hand as before. How the same
-principle holds throughout<span class="pagenum" id="Page_381">[381]</span> the higher stages of nervous development,
-can be seen only still more vaguely. Nevertheless, it is comprehensible
-that as functions become further divided, there will arise the
-need for sub-connexions along which there may take place secondary
-equilibrations subordinate to the main ones. It is manifest, too, that
-whereas the differentiation of functions proceeds, not necessarily by
-division into two, but often by division into several, and usually in
-such ways as not to leave any two functions that are just complementary
-to one another, the restorations of equilibrium cannot be so simple
-as above supposed. And especially when we bear in mind that many
-differentiated functions, as those of the senses, cannot be held
-complementary to any other functions in particular; it becomes manifest
-that the equilibrations that have to be made in an organism of much
-heterogeneity, are extremely complex, and do not take place between
-each organ and some other, but between each organ and all the others.
-The peculiarity of the molecular motion propagated from each organ,
-has to be neutralized by some counter-peculiarity in the average of
-the molecular motions with which it is brought into relation. All the
-variously-modified molecular motions from the various parts, must have
-their pluses and minuses mutually cancelled: if not locally, then at
-some centre to which each unbalanced motion travels until it meets with
-some opposite unbalanced motion to destroy it. Still, involved as these
-actions must become, it is possible to see how the general principle
-illustrated by the simple case above supposed, will continue to hold.
-For always the molecular motion proceeding from any one differentiated
-part, will travel most readily towards that place where a molecular
-motion most complementary to it in kind exists&mdash;no matter whether this
-complementary molecular motion be that proceeding from any one other
-organ, or the <i>resultant</i> of the molecular motions proceeding from
-many other organs. So that the tendency will be for each channel of
-communication or nerve, to unite itself with some centre or ganglion,
-where it<span class="pagenum" id="Page_382">[382]</span> comes into relation with other nerves. And if there be any
-parts of its peculiar molecular motion uncancelled by the molecular
-motions it meets at this centre; or if, as will probably happen, the
-average molecular motion which it there unites to produce, differs from
-the average molecular motion elsewhere; then, as before, there will
-arise a discharge along another channel or nerve to another centre
-or ganglion, where the residuary difference may be cancelled by the
-differences it meets; or whence it may be still further propagated till
-it is so cancelled. Thus there will be a tendency to a general nervous
-integration keeping pace with the differentiation.</p>
-
-<p>Of course this must be taken as nothing more than the indication of
-initial tendencies&mdash;not as an hypothesis sufficient to account for all
-the facts. It leaves out of sight the origin and functions of ganglia,
-considered as something more than nerve-junctions. Were there only
-these lines of easy transmission of molecular disturbance, a change
-set up in one organ could never do more than produce its equivalent of
-change in some other or others; and there could be none of that large
-amount of motion initiated by a small sensation, which we habitually
-see. The facts show, unmistakably, that the slight disturbance
-communicated to a ganglion, causes an overthrow of that highly-unstable
-nervous matter contained in it, and a discharge from it of the
-greatly-increased quantity of molecular motion so generated. This,
-however, is beyond our immediate topic. All we have here to note is the
-interdependence and unification of functions that naturally follow the
-differentiation of them.</p>
-
-
-<p class="p2">§ 309. Something might be added concerning the further class of
-integrations by which organisms are constituted mechanically-coherent
-wholes. Carrying further certain of the arguments contained in the
-last chapter, it might be not unreasonably inferred that the binding
-together of parts by bones, muscles, and ligaments, is a secondary
-result of<span class="pagenum" id="Page_383">[383]</span> those same actions by which bones, muscles, and ligaments
-are specialized. But adequate treatment of this division of the subject
-is at present scarcely possible.</p>
-
-<p>What little of fact and inference has been above set down, will,
-however, serve to make comprehensible the general truths respecting
-which, in their main outlines, there can be no question. Beginning with
-the feebly-differentiated sponge, of which the integration is also so
-feeble that cutting off a piece interferes in no appreciable degree
-with the activity and growth of the rest, it is undeniable that the
-advance is through stages in which the multiplication of unlike parts
-having unlike actions, is accompanied by an increasing interdependence
-of the parts and their actions; until we come to structures like our
-own, in which a slight change initiated in one part will instantly and
-powerfully affect all other parts&mdash;will convulse an immense number of
-muscles, send a wave of contraction through all the blood-vessels,
-awaken a crowd of ideas with an accompanying gush of emotions, affect
-the action of the lungs, of the stomach, and of all the secreting
-organs. And while it is a manifest necessity that along with this
-subdivision of functions which the higher organisms show us, there
-must be this close co-ordination of them, the foregoing paragraphs
-suggest how this necessary correlation is brought about. For a great
-part of the physiological union that accompanies the physiological
-specialization, there appears to be a sufficient cause in the process
-of direct equilibration; and indirect equilibration may be fairly
-presumed a sufficient cause for that which remains.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_384">[384]</span></p>
-
-<h3>CHAPTER X.<br />
-<span class="subhed1">SUMMARY OF PHYSIOLOGICAL DEVELOPMENT.</span></h3></div>
-
-
-<p>§ 310. Intercourse between each part and the particular conditions
-to which it is exposed, either habitually in the individual
-or occasionally in the race, thus appears to be the origin of
-physiological development; as we found it to be the origin of
-morphological development. The unlikenesses of form that arise among
-members of an aggregate that were originally alike, we traced to
-unlikenesses in the incident forces. And in the foregoing chapters we
-have traced to unlikenesses in the incident forces, those unlikenesses
-of minute structure and chemical composition that simultaneously arise
-among the parts.</p>
-
-<p>In summing up the special truths illustrative of this general truth,
-it will be proper here to contemplate more especially their dependence
-on first principles. Dealing with biological phenomena as phenomena of
-evolution, we have to interpret not only the increasing morphological
-heterogeneity of organisms, but also their increasing physiological
-heterogeneity, in terms of the re-distribution of matter and motion.
-While we make our rapid re-survey of the facts, let us then more
-particularly observe how they are subordinate to the universal course
-of this re-distribution.</p>
-
-
-<p class="p2">§ 311. The instability of the homogeneous, or, strictly speaking, the
-inevitable lapse of the more homogeneous into the less homogeneous,
-which we before saw endlessly exemplified<span class="pagenum" id="Page_385">[385]</span> by the morphological
-differentiations of the parts of organisms, we have here seen
-afresh exemplified in ways also countless, by the physiological
-differentiations of their parts. And in the one case as in the other,
-this change from uniformity to multiformity in organic aggregates, is
-caused, as it is in all inorganic aggregates, by the necessary exposure
-of their component parts to actions unlike in kind or quantity or
-both. General proof of this is furnished by the order in which the
-differences appear. If parts are rendered physiologically heterogeneous
-by the heterogeneity of the incident forces, then the earliest
-contrasts should be between parts that are the most strongly contrasted
-in their relations to incident forces; the next earliest contrasts
-should occur where there are the next strongest contrasts in these
-relations; and so on. It turns out that they do so.</p>
-
-<p>Everywhere the differentiation of outside from inside comes first.
-In the simplest plants the unlikeness of the cell-wall to the
-cell-contents is the conspicuous trait of structure. The contrasts seen
-in the simplest animals are of the same kind: the film that covers
-a Rhizopod and the more indurated coat of an Infusorian, are more
-unlike the contained sarcode than the other parts of this are from
-one another; and the tendency during the life of the animal is for
-the unlikeness to become greater. What is true of <i>Protophyta</i>
-and <i>Protozoa</i>, is true of the germs of all organisms up to the
-highest: the differentiation of outer from inner is the first step.
-When the protoplasm of an <i>Alga</i>-cell has broken up into the
-clusters of granules which are eventually to become spores, each of
-these quickly acquires a membranous coating, constituting an unlikeness
-between surface and centre. Similarly with the ovule of every higher
-plant: the mass of cells forming it, early exhibits an outside layer
-of cells distinguished from the cells within. With animal-germs it is
-the same. Be it in a ciliated gemmule, be it in the unfertilized ova
-of <i>Aphides</i> and of the <i>Cecidomyia</i>, or be it in true ova,
-the primary differentiation conforms to the relations<span class="pagenum" id="Page_386">[386]</span> of exterior and
-interior. If we turn to adult organisms, vegetal or animal, we see
-that whether they do or do not display other contrasts of parts, they
-always display this contrast. Though otherwise almost homogeneous, such
-<i>Fungi</i> as the puff-ball, or, among <i>Algæ</i>, all which have
-a thallus of any thickness, present marked differences between those
-of their cells which are in immediate contact with the environment
-and those which are not. Such differences they present in common with
-every higher plant; which, here in the shape of bark and there in the
-shape of cuticle, has an envelope inclosing it even up to its petals
-and stamens. In like manner among animals, there is always either a
-true skin or an outer coat analogous to one. Wherever aggregates of
-the first order have united into aggregates of the second and third
-orders&mdash;wherever they have become the morphological units of such
-higher aggregates&mdash;the outermost of them have grown unlike those lying
-within. Even the Sponge is not without a layer that may by analogy be
-called dermal.</p>
-
-<p>This lapse of the relatively homogeneous into the relatively
-heterogeneous, first showing itself, as on the hypothesis of evolution
-it must do, by the rise of an unlikeness between outside and inside,
-goes on next to show itself, as we infer that it must do, by the
-establishment of secondary contrasts among the outer parts answering
-to secondary contrasts among the forces falling on them. So long
-as the whole surface of a plant remains similarly related to the
-environment, as in a <i>Protococcus</i>, it remains uniform; but
-when there come to be an attached surface and a free surface, these,
-being subject to unlike actions, are rendered unlike. This is visible
-even in a unicellular <i>Alga</i> when it becomes fixed; it is shown
-in the distinction between the under and upper parts of ordinary
-<i>Fungi</i>; and we see it in the universal difference between the
-imbedded ends and the exposed ends of the higher plants. And then among
-the less marked contrasts of surface answering to the less marked
-contrasts in the incident<span class="pagenum" id="Page_387">[387]</span> forces, come those between the upper and
-under sides of leaves; which, as we have seen, vary in degree as the
-contrasts of forces vary in degree, and disappear where these contrasts
-disappear. Equally clear proof is furnished by animals, that the
-original uniformity of surface lapses into multiformity, in proportion
-as the actions of the environment upon the surface become multiform.
-In a Worm, burrowing through damp soil which acts equally on all its
-sides, or in a <i>Tænia</i>, uniformly bathed by the contents of the
-intestine it inhabits, the parts of the integument do not appreciably
-differ from one another; but in creatures not surrounded by the
-same agencies, as those that crawl and those that have their bodies
-partially inclosed, there are unlikenesses of integument corresponding
-to unlikenesses of the conditions. A snail’s foot has an under surface
-not uniform with the exposed surface of its body, and this again is
-not uniform with the protected surface. Among articulate animals there
-is usually a distinction between the ventral and the dorsal aspects;
-and in those of the <i>Arthropoda</i> which subject their anterior and
-posterior ends to different environing agencies, as do the ant-lion
-and the hermit-crab, these become superficially differentiated.
-Analogous general contrasts occur among the <i>Vertebrata</i>.
-Fishes, though their outsides are uniformly bathed by water, have
-their backs more exposed to light than their bellies, and the two
-are commonly distinct in colour. When it is not the back and belly
-which are thus dissimilarly conditioned, but the sides, as in the
-<i>Pleuronectidæ</i>, then it is the sides which become contrasted; and
-there may be significance in the fact that those abnormal individuals
-of this order which revert to the ancestral undistorted type, and
-swim vertically, have the two sides alike. In such higher vertebrates
-as reptiles, we see repeated this differentiation of the upper and
-under surfaces: especially in those of them which, like snakes, expose
-these surfaces to the most diverse actions. Even in birds and mammals
-which usually, by raising the under surface considerably above the
-ground,<span class="pagenum" id="Page_388">[388]</span> greatly diminish the contrast between its conditions and the
-conditions to which the upper surface is subject, there still remains
-some unlikeness of clothing answering to the remaining unlikeness
-between the conditions. Thus, without by any means saying that all such
-differentiations are directly caused by differences in the actions of
-incident forces, which, as before shown (<a href="#sect294">§ 294</a>), they cannot be, it is
-clear that many of them are so caused. It is clear that parts of the
-surface exposed to very unlike environing agencies, become very unlike;
-and this is all that needs to be shown.</p>
-
-<p>Complex as are the transformations of the inner parts of organisms
-from the relatively homogeneous into the relatively heterogeneous, we
-still see among them a conformity to the same general order. In both
-plants and animals the earlier internal differentiations answer to the
-stronger contrasts of conditions. Plants, absorbing all their nutriment
-through their outer surfaces, are internally modified mainly by the
-transfer of materials and by mechanical stress. Such of them as do not
-raise their fronds above the surface, have their inner tissues subject
-to no marked contrasts save those caused by currents of sap; and the
-lines of lengthened and otherwise changed cells which are formed where
-these currents run, and are most conspicuous where these currents must
-obviously be the strongest, are the only decided differentiations of
-the interior. But where, as in the higher Cryptogams and in Phænogams,
-the leaves are upheld, and the supporting stem is transversely bent by
-the wind, the inner tissues, subject to different amounts of mechanical
-strain, differentiate accordingly: the deposit of dense substance
-commences in that region where the sap-containing cells and canals
-suffer the greatest intermittent compressions. Animals, or at least
-such of them as take food into their interiors, are subject to forces
-of another class tending to destroy their original homogeneity. Food
-is a foreign substance which acts on the interior as an environing
-object which touches it acts on the exterior&mdash;is<span class="pagenum" id="Page_389">[389]</span> literally a portion
-of the environment which, when swallowed, becomes a cause of internal
-differentiations as the rest of the environment continues a cause
-of external differentiations. How essentially parallel are the two
-sets of actions and reactions, we have seen implied by the primordial
-identity of the endoderm and ectoderm in simple animals, and of the
-skin and mucous membrane in complex animals (<a href="#sect288">§§ 288</a>, <a href="#sect289">289</a>). Here we have
-further to observe that as food is the original source of internal
-differentiations, these may be expected to show themselves first
-where the influence of the food is greatest; and to appear later in
-proportion as the parts are more removed from the influence of the
-food. They do this. In animals of low type, the coats of the alimentary
-cavity or canal are more differentiated than the tissue which lies
-between the alimentary canal and the wall of the body. This tissue
-in the higher <i>Cœlenterata</i>, is a feebly-organized parenchyma
-traversed by canals lined with simple ciliated cells; and in the lower
-<i>Mollusca</i> the structures bounding the peri-visceral cavity
-and its ramifying sinuses, are similarly imperfect. Further, it is
-observable that the differentiation of this peri-visceral sac and its
-sinuses into a vascular system, proceeds centrifugally from the region
-where the absorbed nutriment enters the mass of circulating liquid, and
-where this liquid is qualitatively more unlike the tissues than it is
-at the remoter parts of the body.</p>
-
-<p>Physiological development, then, is initiated by that instability
-of the homogeneous which we have seen to be everywhere a cause of
-evolution (<i>First Principles</i>, §§ 149–155). That the passage
-from comparative uniformity of composition and minute structure to
-comparative multiformity, is set up in organic aggregates, as in all
-other aggregates, by the necessary unlikenesses of the actions to which
-the parts are subject, is shown by the universal rise of the primary
-differentiation into the parts that are universally most contrasted in
-their circumstances, and by the rise of secondary differentiations<span class="pagenum" id="Page_390">[390]</span>
-obviously related in their order to secondary contrasts of conditions.</p>
-
-
-<p class="p2">§ 312. How physiological development has all along been aided by the
-multiplication of effects&mdash;how each differentiation has ever tended to
-become the parent of new differentiations, we have had, incidentally,
-various illustrations. Let us here review the working of this cause.</p>
-
-<p>Among plants we see it in the production of progressively-multiplying
-heterogeneities of tissue by progressive increase of bulk. The
-integration of fronds into axes and of axes into groups of axes,
-sets up unlikenesses of action among the integrated units, followed
-by unlikenesses of minute structure. Each gust transversely strains
-the various parts of the stem in various degrees, and longitudinally
-strains in various degrees the roots; and while there is inequality of
-stress at every place in stem and branch, so, at every place in stem
-and branch, the outer layers and the successively inner layers are
-severally extended and compressed to unequal amounts, and have unequal
-modifications wrought in them. Let the tree add to its periphery
-another generation of the units composing it, and immediately the
-mechanical strains on the supporting parts are all changed in different
-degrees, initiating new differences internally. Externally, too, new
-differences are initiated. Shaded by the leaf-bearing outer stratum
-of shoots, the inner structures cease to bear leaves, or to put out
-shoots which bear leaves; and instead of that green covering which
-they originally had, become covered with bark of increasing thickness.
-Manifestly, then, the larger integration of units that are originally
-simple and uniform, entails physiological changes of various orders,
-varying in their degrees at all parts of the aggregate. Each branch
-which, favourably circumstanced, flourishes more than its neighbours,
-becomes a cause of physiological differentiations, not only in its
-neighbours from which it abstracts sap and presently<span class="pagenum" id="Page_391">[391]</span> turns from
-leaf-bearers into fruit-bearers, but also in the remoter parts.</p>
-
-<p>That among animals physiological development is furthered by the
-multiplication of effects, we have lately seen proved by the many
-changes in other organs, which the growth or modification of each
-excreting and secreting organ initiates. By the abstracted as well as
-by the added materials, it alters the quality of the blood passing
-through all members of the body; or by the liquid it pours into the
-alimentary canal, it acts on the food, and through it on the blood,
-and through it on the system as a whole: an additional differentiation
-in one part thus setting up additional differentiations in many other
-parts; from each of which, again, secondary differentiating forces
-reverberate through the organism. Or, to take an influence of another
-order, we have seen how the modified mechanical action of any member
-not only modifies that member, but becomes, by its reactions, a cause
-of secondary modifications&mdash;how, for example, the burrowing habits
-of the common mole, leading to an almost exclusive use of the fore
-limbs, have entailed a dwindling of the hind limbs, and a concomitant
-dwindling of the pelvis, which, becoming too small for the passage of
-the young, has initiated still more anomalous modifications.</p>
-
-<p>So that throughout physiological development, as in evolution at large,
-the multiplication of effects has been a factor constantly at work, and
-working more actively as the development has advanced. The secondary
-changes wrought by each primary change, have necessarily become more
-numerous in proportion as organisms have become more complex. And
-every increased multiplication of effects, further differentiating the
-organism and, by consequence, further integrating it, has prepared the
-way for still higher differentiations and integrations similarly caused.</p>
-
-
-<p class="p2">§ 313. The general truth next to be resumed, is that these processes
-have for their limit a state of equilibrium&mdash;proximately<span class="pagenum" id="Page_392">[392]</span> a moving
-equilibrium and ultimately a complete equilibrium. The changes we have
-contemplated are but the concomitants of a progressing equilibration.
-In every aggregate which we call living, as well as in all other
-aggregates, the instability of the homogeneous is but another name
-for the absence of balance between the incident forces and the forces
-which the aggregate opposes to them; and the passage into heterogeneity
-is the passage towards a state of balance. And to say that in every
-aggregate, organic or other, there goes on a multiplication of effects,
-is but to say that one part which has a fresh force impressed on it,
-must go on changing and communicating secondary changes, until the
-whole of the impressed force has been used up in generating equivalent
-reactive forces.</p>
-
-<p>The principle that whatever new action an organism is subject to, must
-either overthrow the moving equilibrium of its functions and cause the
-sudden equilibration called death, or else must progressively alter the
-organic rhythms until, by the establishment of a new reaction balancing
-the new action a new moving equilibrium is produced, applies as much
-to each member of an organism as to the organism in its totality. Any
-force falling on any part not adapted to bear it, must either cause
-local destruction of tissue, or must, without destroying the tissue,
-continue to change it until it can change it no further; that is&mdash;until
-the modified reaction of the part has become equal to the modified
-action. Whatever the nature of the force this must happen. If it is a
-mechanical force, then the immediate effect is some distortion of the
-part&mdash;a distortion having for its limit that attitude in which the
-resistance of the structures to further change of position, balances
-the force tending to produce the further change; and the ultimate
-effect, supposing the force to be continuous or recurrent, is such a
-permanent alteration of form, or alteration of structure, or both, as
-establishes a permanent balance. If the force is physico-chemical, or
-chemical, the general result is still the same: the component molecules
-of<span class="pagenum" id="Page_393">[393]</span> the tissue must have their molecular arrangements changed, and the
-change in their molecular arrangements must go on until their molecular
-motions are so re-adjusted as to equilibrate the molecular motions of
-the new physico-chemical or chemical agent. In other words, the organic
-matter composing the part, if it continues to be organic matter at all,
-must assume that molecular composition which enables it to bear, or as
-we say adapts it to, the incident forces.</p>
-
-<p>Nor is it less certain that throughout the organism as a whole,
-equilibration is alike the proximate limit of the changes wrought by
-each action, as well as the ultimate limit of the changes wrought by
-any recurrent actions or continuous action. The movements every instant
-going on, are movements towards a new state of equilibrium. Raising
-a limb causes a simultaneous shifting of the centre of gravity, and
-such altered tensions and pressures throughout the body as re-adjust
-the disturbed balance. Passage of liquid into or out of a tissue,
-implies some excess of force in one direction there at work; and ceases
-only when the force so diminishes or the counter-forces so increase
-that the excess disappears. A nervous discharge is reflected and
-re-reflected from part to part, until it has all been used up in the
-re-arrangements produced&mdash;equilibrated by the reactions called out. And
-what is thus obviously true of every normal change, is equally true of
-every abnormal change&mdash;every disturbance of the established rhythm of
-the functions. If such disturbance is a single one, the perturbations
-set up by it, reverberating throughout the system, leave its moving
-equilibrium slightly altered. If the disturbance is repeated or
-persistent, its successive effects accumulate until they have produced
-a new moving equilibrium adjusted to the new force.</p>
-
-<p>Each re-balancing of actions, having for its necessary concomitant a
-modification of tissues, it is an obvious corollary that organisms
-subjected to successive changes of conditions, must undergo successive
-differentiations and re-differentiations. Direct equilibration in
-organisms, with all its accompanying<span class="pagenum" id="Page_394">[394]</span> structural alterations, is as
-certain as is that universal progress towards equilibrium of which
-it forms part. And just as certain is that indirect equilibration
-in organisms to which the remaining large class of differentiations
-is due. The development of favourable variations by the killing of
-individuals in which they do not occur or are least marked, is, as
-before, a balancing between certain local structures and the forces
-they are exposed to; and is no less inevitable than the other.</p>
-
-
-<p class="p2">§ 314. In all which universal laws, we find ourselves again brought
-down to the persistence of force, as the deepest knowable cause of
-those modifications which constitute physiological development; as
-it is the deepest knowable cause of all other evolution. Here, as
-elsewhere, the perpetual lapse from less to greater heterogeneity,
-the perpetual begetting of secondary modifications by each primary
-modification, and the perpetual approach to a temporary balance on the
-way towards a final balance, are necessary implications of the ultimate
-fact that force cannot disappear but can only change its form.</p>
-
-<p>It is an unquestionable deduction from the persistence of force, that
-in every individual organism each new incident force must work its
-equivalent of change; and that where it is a constant or recurrent
-force, the limit of the change it works must be an adaptation of
-structure such as opposes to the new outer force an equal inner force.
-The only thing open to question is, whether such re-adjustment is
-inheritable; and further consideration will, I think, show, that to say
-it is not inheritable is indirectly to say that force does not persist.
-If all parts of an organism have their functions co-ordinated into a
-moving equilibrium, such that every part perpetually influences all
-other parts, and cannot be changed without initiating changes in all
-other parts&mdash;if the limit of change is the establishment of a complete
-harmony among the movements, molecular and other, of all parts; then<span class="pagenum" id="Page_395">[395]</span>
-among other parts that are modified, molecularly or otherwise, must
-be those which cast off the germs of new organisms. The molecules of
-their produced germs must tend ever to conform the motions of their
-components, and therefore the arrangements of their components, to the
-molecular forces of the organism as a whole; and if this aggregate of
-molecular forces be modified in its distribution by a local change
-of structure, the molecules of the germs must be gradually changed
-in the motions and arrangements of their components, until they are
-re-adjusted to the aggregate of molecular forces.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_396">[396]</span></p>
-
-<h3>CHAPTER X<sup>A</sup>.<br />
-<span class="subhed1">THE INTEGRATION OF THE ORGANIC WORLD.</span></h3></div>
-
-
-<p>§ 314<i>a</i>. That from the beginning of life there has been an
-ever-increasing heterogeneity in the Earth’s Flora and Fauna, is
-a truth recognized by all biologists who accept the doctrine of
-evolution. In discussing the origin of species Mr. Darwin and others
-have been mainly occupied in explaining the genesis of now this and
-now that form of organism, considered as a member of one or other
-series, and regarded as becoming differentiated from its allies. But by
-implication, if not avowedly, there has been simultaneously accepted
-the belief that the forms continually produced by divergences and
-re-divergences, have constituted an assemblage increasingly multiform
-in its included kinds. And this, which we are shown by the process of
-organic evolution as followed out in its details, is a corollary from
-the doctrine of evolution at large, as was pointed out in § 159 of
-<i>First Principles</i>.</p>
-
-<p>Meanwhile there has been little if any recognition of an accompanying
-change, no less fundamental. In the general transformation which
-constitutes Evolution, differentiation and integration advance hand
-in hand; so that along with the production of unlike parts there
-progresses the union of these unlike parts into a whole. Examples of
-various kinds before given will recur to the reader, and an addition
-to them has just been set forth in the chapter on “Physiological
-Integration.” One more example, world-wide in its reach, has still to
-be named.</p>
-
-<p><span class="pagenum" id="Page_397">[397]</span></p>
-
-<p>For here it remains to point out that along with the increasing
-multiplication of types of organisms covering the Earth’s surface,
-there has been ever going on an increasing mutual dependence of
-them&mdash;an increasing integration of the entire aggregate of living
-things.</p>
-
-<p>Many facts which are obvious and many which are quite familiar will
-be named as evidence. But I must be excused for reminding the reader
-of things that he knows and things that he may easily observe, since,
-unless the evidence, trite as it may be, is gathered together and
-properly marshalled, the generalization enunciated will not be thought
-valid.</p>
-
-
-<p class="p2">§ 314<i>b</i>. Respecting the physiological characters of the earliest
-forms there is an assumption from which no escape seems possible&mdash;the
-assumption that they united animal and vegetal characters. Even among
-existing microscopic types of the lowest classes, there is such
-community of plant-traits and animal-traits that doubts respecting
-their proper places in one or the other kingdom are continually
-raised&mdash;doubts, too, whether, if regarded as vegetal, they are to be
-grouped as algoid or fungoid.</p>
-
-<p>Here, however, without entering on moot questions, we may draw the
-<i>à priori</i> conclusion that these earliest living things were
-double-natured, in so far that they must have had the ability to
-assimilate from the inorganic world all the materials of which
-protoplasm consists&mdash;must therefore, along with the power of
-appropriating carbon from its gaseous compound, also have had the power
-of appropriating nitrogen, either from one of its combined oxides
-or directly from the air with which water is more or less charged.
-For before organic substances existed there could have been none but
-inorganic sources from which nitrogen could be obtained.</p>
-
-<p>This conclusion concerns us only because it implies homogeneity of
-nature in these primordial forms of life. There could not at first
-have existed among these minutest of <i>Protozoa</i> even such vague
-distinctions as are now presented<span class="pagenum" id="Page_398">[398]</span> in a shadowy way by their modern
-representatives. And the implication is that during the period
-throughout which these smallest, lowest, and simplest living things
-alone existed, there could have been, in the absence of kinds, no
-mutual dependence.</p>
-
-<p>Since, among various of the lowest types now known to us, the same
-individual exhibits a life which is now predominantly vegetal and now
-predominantly animal, we cannot err in assuming that there eventually
-took place differentiations of this original plant-animal type into
-types permanently unlike: some in which the traits were more markedly
-vegetal and others in which they were more markedly animal. As fast as
-this differentiation arose, there came the beginnings of co-operation
-between the predominantly vegetal types which by the aid of light
-formed organic matter from the inorganic world, and the predominantly
-animal types which, in chief measure, utilized the matter so formed.
-Evidently with the rise of such a differentiation came an incipient
-mutual dependence. If to the implied algoid type and the animal type
-there be added the fungoid type, somewhat intermediate in character,
-which in a large proportion of cases lives on the decaying remnants of
-the other two, we are furnished with a rude conception of the primary
-differentiations and the accompanying vague mutual dependences.</p>
-
-<p>Speculation aside, it suffices to say that early in the history of
-life there must have arisen the distinction between <i>Protozoa</i>
-and <i>Protophyta</i>, and that this distinction foreshadowed that
-widest contrast which the higher organic world presents&mdash;the contrast
-between plants and animals. It is needless to do more than name the
-mutual dependence between these two great divisions. That, as being
-respectively decomposers of carbon dioxide and exhalers of carbon
-dioxide, they act reciprocally, as also in some measure by interchange
-of nitrogenous matters; and that the implied general co-operation
-serves in an indirect way to unite their lives, and in that<span class="pagenum" id="Page_399">[399]</span> sense to
-integrate the two kingdoms; needs not to be insisted upon. Further
-complications of the mutual dependence will be mentioned by and by. For
-the present it suffices to recognize this division of organic functions
-as the first which arose and as continuing to be that fundamental one
-which more than all others binds organisms at large together.</p>
-
-
-<p class="p2">§ 314<i>c</i>. It will be thought by many readers that in speaking of
-the contrasted vital activities of plants and animals as constituting
-a “division of organic functions,” I am straining words beyond their
-meanings; since the conception of organic functions postulates an
-organized whole in which they exist, and plants and animals constitute
-no such organized whole. But there is at hand an unexpected defence for
-this conception&mdash;a defence not forthcoming a generation ago, but which
-now all biologists will recognize as relevant. I refer to the phenomena
-of symbiosis. These present various cases in which the plant-function
-and the animal-function are carried on in the same body,&mdash;cases in
-which the co-operation is not between separate vegetal organisms which
-accumulate nutritive matters and separate animal organisms which
-consume them, but is a co-operation between vegetal elements and animal
-elements forming parts of the same organism.</p>
-
-<p>As introductory to examples of these must first, however, be named an
-example of such co-operation between the two great classes of vegetal
-organisms&mdash;the fungoid and the algoid. Incredible as the statement once
-seemed, it is a statement now accepted, that what we know as lichens,
-and used to consider as plants forming a certain low class, are now
-found to be not plants in the ordinary sense at all, but compound
-growths formed of minute algæ and minute fungi, carrying on their lives
-together: the algæ furnishing to the fungi certain constituents they
-need but cannot directly obtain, and the fungi profiting by certain
-materials they obtain from the algæ, either while living or while
-individually decaying. Whence it would seem that after the microscopic
-vegetal<span class="pagenum" id="Page_400">[400]</span> type had become in a large degree differentiated into two main
-types, in adaptation to different conditions of life, and had acquired
-appropriate specialities of nature, there grew up this communistic
-arrangement between certain of them, enabling each to benefit by the
-powers which the other had acquired: evidently an exchange of services,
-a physiological division of labour, a mutual dependence of functions
-analogous to that which exists between functions in an ordinary plant
-or animal.</p>
-
-<p>Not differing in principle but only in application, is that symbiosis
-above referred to as existing between <i>Protophyta</i> and many
-<i>Protozoa</i>, as well as between such <i>Protophyta</i> and the
-lowest kinds of <i>Metazoa</i>. A recent statement that certain amœbæ,
-made green by contained chlorophyll, continue to grow and multiply
-after they have consumed what nutritive matter may be at hand, is in
-harmony with various facts alleged of other <i>Protozoa</i>&mdash;various
-other kinds of <i>Rhizopods</i>, various <i>Heliozoa</i>, numerous
-ciliated and flagellated <i>Infusoria</i>. Among <i>Metazoa</i>
-the like association occurs in one of the sponges, in the <i>Hydra
-viridis</i>, in various turbellarians, in a rotifer, and even in two
-molluscs. In these cases the partnership between the vegetal cells and
-the animal cells (existing either as units or as an organized group
-such as a polype), is a partnership which, as before, profits each of
-the partners&mdash;an inference supported by the fact that <i>Metazoa</i>
-containing these algoid cells usually place themselves where the light
-falls upon them, and can therefore further the production of the
-carbo-hydrates which eventually become useful to the animal cells,
-while these in some way reciprocate the benefit.</p>
-
-<p>Here, then, we have exchange of services between associated
-plant-elements and animal-elements&mdash;a performance by them of different
-organic functions for the benefit of the aggregate which they unite
-to form. Hence, when these vegetal elements and animal elements
-are separately embodied in plants and animals, which profit by
-one another,<span class="pagenum" id="Page_401">[401]</span> we may still properly regard their respective lives
-as mutually-dependent organic functions, as said in the preceding
-section. We are enabled the better to see how the Earth’s Flora and
-Fauna, which are respectively accumulators of motion and expenders of
-motion, form mutually-dependent parts of a whole, and are in that sense
-integrated. And we shall be prepared to see how all other relations
-between organisms which make them subservient one to another, similarly
-constitute elements in a general integration of the organic world.</p>
-
-
-<p class="p2">§ 314<i>d</i>. Another form of mutual dependence and consequently of
-integration is conspicuous&mdash;that which accompanied the progressive
-increase of size in organisms of the higher classes. We have but to
-contemplate the possibilities to see that life must necessarily have
-commenced with minute forms, and that the progress to larger ones must
-have been by small steps.</p>
-
-<p>For had creatures of appreciable sizes been the first to exist they
-would inevitably have disappeared from lack of food. Having no resource
-but to devour one another, they would quickly have brought life to an
-end. There must have been smaller types serving as prey for larger
-ones before these could continue to exist and to multiply: microbes
-affording food to infusoria, infusoria affording food to such sized
-creatures as the <i>Entomostraca</i>, these again supplying food to
-small fishes, such as loch-trout, and these last yielding to larger
-fishes masses sufficiently great for their needs: each higher grade
-requiring lower grades of appropriate bulk. It needs but to ask what
-would become of tigers if there were no mammals larger than mice,
-to see that the animal world is a linked assemblage, of which the
-connected members stand within certain ratios of mass; and that during
-the evolution of higher and larger types the linking of grades has
-become closer.</p>
-
-<p>That among plants considered as an aggregate relations of like kind,
-though far less distinct ones, have all along<span class="pagenum" id="Page_402">[402]</span> been growing may be
-reasonably concluded. In a world peopled only by microscopic types
-there could not have existed the conditions needful for large trees.
-Gradual disintegration of rock-surfaces, partly effected by physical
-agencies and partly by low forms of plants, had to prepare the way for
-superior plants. The production of sufficient soil by mineralogical
-decay as well as by the decay of organisms, plant and animal, may be
-regarded as having been a preliminary to larger plant-growth; and
-though at present the dependence is far less close than that among
-animals, yet the benefits yielded to metaphytes by the decomposing
-actions carried on by protophytes, as well as those carried on by
-microbes permeating the soil, imply a continued general interdependence
-throughout the aggregate of plant-forms, apart from more special
-interdependences. And then along with this indebtedness of the greater
-plants to the smaller during the process of evolution, there must be
-named that indebtedness of plant-life to animal-life which Mr. Darwin
-has shown in his book on the agency of worms as producers of mould.</p>
-
-
-<p class="p2">§ 314<i>e</i>. Services of one to another, and consequent unions, of
-more special kinds are infinitely varied, alike within each kingdom
-and between the two kingdoms. I refer to those seen in parasitism,
-commensalism, and other forms of association. While they do not
-conduce to unions of the kind thus far considered, these nevertheless
-constitute innumerable links whereby the lives of organisms, plant and
-animal, are tied together; sometimes for the advantage of both but in
-most cases for the benefit of one to the injury of the other.</p>
-
-<p>Among plants the degrees of dependence are various. Unable to raise
-themselves into the air and light, some climb, like the ivy, by
-modified rootlets, or spirally coil themselves, or hang by tendrils.
-Others there are which gradually strangle the trees they embrace, or
-which, like lichens in damp climates, festooning the smaller trees,
-by and by cause<span class="pagenum" id="Page_403">[403]</span> their decay. Of higher types of epiphytes which
-use trees only to gain elevation, the orchids may be instanced.
-And then we have plants which, like the mistletoe, fix themselves
-on the bark of their hosts, utilizing them partly for purposes of
-elevation and partly by appropriation of their juices. After these
-may be named those extreme cases in which the parasitic plants,
-ceasing to have any chlorophyll-bearing leaves, live wholly on the
-juices of the invaded plants. At home the common dodder, and in the
-tropics the <i>Rafflesiaceæ</i>, belong to this group. There must be
-added the numerous forms of minute fungi which in like manner thrive
-at the expense of the plants they infest. In all these cases the
-interdependence is one-sided, though, as we shall presently see, while
-detrimental to one of the two concerned, it is not always detrimental
-to the organic world as a whole.</p>
-
-<p>That utilization of one by another among animals which causes immediate
-death, is familiar enough in the relations between carnivores and
-herbivores. Almost as familiar are those seen in parasitism. Less
-familiar are those seen in commensalism; and the least familiar are
-those which show us exchange of services. Among these last&mdash;the
-mutually-beneficial relations&mdash;that between the crocodile and the
-bird which picks parasites out of its teeth is a striking one; and no
-less so is that of the pique-gouffe, an African bird which pierces
-the tumour on a buffalo’s back that incloses a parasite. Then of
-another kind we have the connexion between aphides and ants: the
-one profiting by being carried to better pastures and the other by
-increased saccharine excretion. Next comes the class of messmates,
-the connexions between some of which are relatively innocent, as
-witness the Sea-anemone which settles itself on the shell occupied
-by a Hermit-crab, or as witness the <i>Remora</i> fixed on a shark’s
-skin. Less innocent is the relation under which one of the two seizes
-a share of the food obtained by the other, like the annelid which
-insinuates itself between the Hermit-crab and the whelk-shell it
-inhabits, or like the small fishes<span class="pagenum" id="Page_404">[404]</span> inhabiting certain <i>Medusæ</i>,
-or those which nestle in the branchial sac of the <i>Lophius</i>. After
-these may be named the less injurious forms of parasites proper&mdash;those
-which, distinguished as <i>Epizoa</i>, fix themselves on the skins of
-their hosts, permanently or temporarily, such as, of the one kind, the
-<i>Lernæa</i> on fishes, and of the other kind the Tick on mammals
-and birds. Then there come the other class of parasites, most of them
-highly injurious, distinguished as <i>Entozoa</i>, living within the
-bodies of their hosts, now in parts of their alimentary canals, now on
-other of their mucous surfaces, and now in various of their organs:
-these last two groups being so numerous in their kinds that there are
-commonly more species than one proper to each larger animal. One stage
-further in the complication meets us in the parasites upon parasites.</p>
-
-<p>But now the general fact, to which these brief indications are
-introductory, is that the use made of one organism by another has been
-ever widening and becoming more involved. Among plants utilization of
-the larger by the smaller&mdash;of trees by epiphytes and parasites&mdash;must
-have arisen since the times when the larger came into existence&mdash;times
-relatively late in the course of organic evolution. Moreover most
-of the plants which utilize others, either by climbing up them or
-settling themselves high up on their stems or sucking their juices,
-are phænogams, and the plants they utilize are also phænogams; so
-that these innumerable interdependences must have been established
-since the phænogamic type has become so predominant in respect of
-both size and kind. Similarly among animals. Though there are many
-parasites belonging, like the Trematodes, to very low classes, there
-are many which belong to the <i>Arthropoda</i>, and, being degraded
-forms of that class, must have come into existence after Arthropods
-of considerable structure had been evolved. Again, a large part
-of the animals infested by <i>Epizoa</i> and <i>Entozoa</i> are
-vertebrates&mdash;many of the highest types; and as these are relatively
-modern all this parasitism must be of<span class="pagenum" id="Page_405">[405]</span> late date. So, too, of
-much commensalism and many mutually-beneficial associations. The
-reciprocal services of ants and aphides must have originated since the
-<i>Hymenoptera</i> and <i>Hemiptera</i> became established types, and
-since the days when certain insects of the ant-type had become social,
-and since the days when aphides had become degraded members of their
-order: both dates being relatively recent. And still more recent must
-have been the commensalism between the ants and the many species of
-other insects which inhabit their nests.</p>
-
-<p>Leaving out relations of the kinds just named, it seems that down from
-those between carnivores and their prey to those between lice and their
-hosts, such relations profit one of the two species concerned and
-injure the other, and that there the matter ends. But it does not end
-there; for that multiplication of effects to which people are usually
-blind, brings about changes which, as hinted above, though injurious
-to the individual are beneficial to the species, and which, when not
-beneficial to the species, are often beneficial to the aggregate of
-species.</p>
-
-<p>Even where animals of one class live by devouring animals of another
-class, we see, on looking beyond the immediate results, certain remote
-results that are advantageous. In the first place the process is
-one by which inferior individuals&mdash;the least agile, swift, strong,
-or sagacious&mdash;are picked out and prevented from leaving posterity
-and lowering the average quality of their kind. At the same time
-individuals made feeble by injury or old age, are among those to be
-killed and saved from suffering prolonged pains: the evils of death by
-disease and starvation being thus limited to the predatory animals,
-relatively small in their numbers. Meanwhile a check is put on undue
-multiplication. Where a tract of country has been overrun by rabbits,
-weasels, thriving on the abundant supply of food, presently become
-numerous enough to bring the population of rabbits within moderate
-limits; and by doing this benefit not only all those kinds of<span class="pagenum" id="Page_406">[406]</span> plants
-which are being eaten down, and all those other animals which live on
-such plants, but also the rabbits themselves; since, increasing beyond
-the means of subsistence, a large part of them would, if not killed,
-die of hunger. Between aphides and lady-birds we see a connexion
-of like nature: great increase of the first yielding abundant food
-to larvæ of the second, ending after a season or so in swarms of
-lady-birds, and consequently of their larvæ, whereby the aphides,
-immensely diminished, cease so greatly to injure various plants and
-the animals dependent on them. Even minute parasites, by the evils
-they inflict on one species, profit others: instance the enormous
-destruction of flies which a microscopic fungus caused a few years
-ago&mdash;a destruction which relieved not only man but all the animals
-which flies irritate: often so much as to hinder them from feeding.
-Such instances remind us how numerous are the bonds by which the lives
-of organisms are tied together.</p>
-
-
-<p class="p2">§ 314<i>f</i>. I have reserved to the last the clearest and most
-striking illustration of this progressing integration throughout the
-organic world. I refer to the mutually-beneficial relations established
-between plants and animals through the agency of flowers and insects.</p>
-
-<p>Everyone nowadays has been made familiar with the process of
-plant-fertilization, and knows that (leaving out of consideration
-plants fertilized by wind-borne pollen) the ability to bear seed
-depends largely on the aid given by bees, butterflies, and moths. The
-exchange of services has been growing ever more various and complicated
-during long past periods. We have the acquirement by flowers of bright
-colours serving to guide these insects to places where honey is to be
-found; and we have their perfumes, also serving for guidance. Then we
-have the many different arrangements, often complicated, by which the
-visiting insects are obliged to carry away pollen and dust with it the
-stigmas of flowers on which they subsequently settle: thus effecting
-crossfertilization.<span class="pagenum" id="Page_407">[407]</span> <i>Pari passu</i> have gone on insect-developments
-made possible by these arrangements and furthering them. Especially
-must be named the modification of certain <i>Hymenoptera</i> into
-honey-storing bees: the implication being that the entire economy
-established by these social insects has been sequent on the growth
-of this system of reciprocal benefits. And then, just instancing the
-dependence between a particular flower having a long tubular corolla,
-and a particular moth having an appropriately long proboscis, it
-suffices to say that innumerable specialities of this general relation
-everywhere multiply the links by which the vegetal world and the animal
-world are here connected. That the effects of the connections tell
-largely on the prosperity of both, is suggested by some instances Mr.
-Darwin gives, and by a statement recently made in the United States,
-by Dr. L. O. Howard, that the greater fostering of bees would much
-increase certain of the crops.</p>
-
-<p>But now observe the broad fact to which these few details concerning
-plant-fertilization are introductory. All these general and special
-relations between plants and animals have arisen since the phænogamic
-type came into existence&mdash;have, indeed, arisen since the higher members
-of that type, the Angiosperms, have appeared; for the Gymnosperms do
-not play any part in this intercommunion. But so far as we can judge
-of present results of geologic explorations, there were no Angiosperms
-during the Eozoic and Paleozoic periods. So that this class of
-connexions between animals and vegetals must have been established
-since carboniferous times&mdash;a period long, indeed, but far shorter than
-that which organic evolution at large has occupied.</p>
-
-
-<p class="p2">§ 314<i>g</i>. I have but just touched on some salient parts of a
-subject, immense in extent and extremely involved, which it would take
-a volume to set forth adequately. Enough has been said, however, to
-indicate the truth which it is the purpose of the chapter to bring into
-view and emphasize&mdash;<span class="pagenum" id="Page_408">[408]</span> the truth that both of the two great laws of
-evolution are exemplified in the organic world as a whole, as they are
-exemplified in every organism, and in all other things.</p>
-
-<p>The reader has long since become familiar with the generalization that
-while Evolution is a change from the homogeneous to the heterogeneous,
-it is also a change from the incoherent to the coherent; and this
-change from the incoherent to the coherent has been above exhibited
-as going on even throughout that vast assemblage of organisms, plant
-and animal, which cover the Earth’s surface. In what we are obliged
-to conceive as the earliest stage, when the most minute types of life
-alone existed, the aggregate of living things was at once homogeneous
-and incoherent. In the course of epochs immeasurable in duration, this
-uniform aggregate of beings has been becoming more multiform. And now
-we see that instead of forms of life everywhere without the slightest
-union caused by mutual dependence, there have slowly arisen forms of
-life among which mutual dependences have entailed vital connexions
-correspondingly marked. Along with progressing differentiation there
-has ever been progressing integration. So that we may recognize
-something like a growing life of the entire aggregate of organisms in
-addition to the lives of individual organisms&mdash;an exchange of services
-among parts enhancing the life of the whole.</p>
-
-<p>In this final generalization the law of Evolution is manifested under
-its most transcendental form.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_409">[409]</span></p>
-
-<h2>PART VI.<br />
-<span class="subhed">LAWS OF MULTIPLICATION.</span></h2></div>
-
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_411">[411]</span></p>
-
-<h3>CHAPTER I.<br />
-<span class="subhed1">THE FACTORS.<a id="FNanchor_52" href="#Footnote_52" class="fnanchor">[52]</a></span></h3></div>
-
-
-<p>§ 315. If organisms have been evolved, their respective powers of
-multiplication must have been determined by natural causes. Grant that
-the countless specialities of structure and function in plants and
-animals, have arisen from the actions and reactions between them and
-their environments, continued from generation to generation; and it
-follows that from these actions and reactions have also arisen those
-countless degrees of fertility which we see among them. As in all other
-respects an adaptation of each species to its conditions of existence
-is directly or indirectly brought about; so must there be directly or
-indirectly brought about an adaptation of its reproductive activity to
-its conditions of existence.</p>
-
-<p>We may expect to find, too, that permanent and temporary<span class="pagenum" id="Page_412">[412]</span> differences
-of fertility have the same general interpretation. If the small
-variations of structure and function that arise within the limits
-of each species, are due to actions like those which, by their
-long-accumulating effects, have produced the immense contrasts between
-the various types; we may conclude that, similarly, the actions to
-which changes in the rate of multiplication of each species are due,
-also produce, in great periods of time, the enormous differences
-between the rates of multiplication of different species.</p>
-
-<p>Before inquiring in what ways the rapidities of increase are adjusted
-to the requirements, both temporary and permanent, it will be needful
-to look at the factors. Let us set down first those which belong to the
-environment, and then those which belong to the organism.</p>
-
-
-<p class="p2">§ 316. Every living aggregate being one of which the inner actions are
-adjusted to balance outer actions, it follows that the maintenance of
-its moving equilibrium depends on its exposure to the right amounts of
-these actions. Its moving equilibrium may be overturned if one of these
-actions is either too great or too small in amount; and it may be so
-overturned either by excess or defect of some inorganic agency in its
-environment, or by excess or defect of some organic agency.</p>
-
-<p>Thus a plant, constitutionally fitted to a certain warmth and
-humidity, is killed by extremes of temperature, as well as by extremes
-of drought and moisture. It may dwindle away from want of soil, or
-die from the presence of too great or too small a quantity of some
-mineral substance which the soil supplies to it. In like manner, every
-animal can maintain the balance of its functions so long only as the
-environment adds to or deducts from its heat at rates not exceeding
-definite limits. Water, too, must be accessible in amount sufficient to
-compensate loss. If the parched air is rapidly abstracting its liquid
-which there is no pool or river to restore, its functions cease; and
-if it is an aquatic creature,<span class="pagenum" id="Page_413">[413]</span> drought may kill it either by drying up
-its medium or by giving it a medium inadequately aërated. Thus each
-organism, adjusted to a certain average in the actions of its inorganic
-environment, or rather, we should say, adjusted to certain moderate
-deviations from this average, is destroyed by extreme deviations. So,
-too, is it with the environing organic agencies. Among plants, only
-the parasitic kinds and those united by symbiosis (as well as a few
-innocent “lodgers”) depend for their individual preservation on the
-presence of certain other organisms (though the presence of certain
-other organisms is needful to most plants for the preservation of the
-race by aiding fertilization). Here, for the continuance of individual
-life, particular organisms must be absent or not very numerous&mdash;beasts
-that browse, caterpillars that devour leaves, aphides that suck juices.
-Among animals, however, the maintenance of the functional balance is
-both positively and negatively dependent on the amounts of surrounding
-organic agents. There must be an accessible sufficiency of the plants
-or animals serving for food; and of organisms that are predatory or
-parasitic or otherwise detrimental, the number must not pass a certain
-limit.</p>
-
-<p>This dependence of the moving equilibrium in every individual organism
-on an adjustment of its forces to the forces of the environment, and
-the overthrow of this equilibrium by failure of the adjustment, is
-comprehensive of all cases. At first sight it does not seem to include
-what we call natural death; but only death by violence, or starvation,
-or cold, or drought. But in reality natural death, no less than every
-other kind of death, is caused by the failure to meet some outer action
-by a proportionate inner action. The apparent difference is due to
-the fact that in old age, when the quantity of force evolved in the
-organism gradually diminishes, the momentum of the functions becomes
-step by step less, and the variations of the external forces relatively
-greater; until there finally comes an occasion when some quite moderate
-deviation from that average to which the<span class="pagenum" id="Page_414">[414]</span> feeble moving equilibrium is
-adjusted, produces in it a fatal perturbation.</p>
-
-
-<p class="p2">§ 317. The individuals of every species being thus dependent on certain
-environing actions; and severally having their moving equilibria sooner
-or later overthrown by one or other of these environing actions; we
-have next to consider in what ways the environing actions are so met
-as to prevent extinction of the species. There are two essentially
-different ways. There may be in each individual a small or great
-ability to adjust itself to variations of the agencies around it and to
-a small or great number of such varying agencies&mdash;there may be little
-or much power of preserving the balance of the functions. And there may
-be much or little power of producing new individuals to replace those
-whose moving equilibria have been overthrown. A few facts must be set
-down to enforce these abstract statements.</p>
-
-<p>There are both active and passive adaptations by which organisms
-are enabled to survive adverse influences. Plants show us but few
-active adaptations: that of the Pitcher-plant and those of the
-reproductive parts of some flowers (which do not, however, conduce to
-self-preservation) are exceptional instances. But plants have various
-passive adaptations; as thorns, stinging hairs, poisonous and acrid
-juices, repugnant odours, and the woolliness or toughness that makes
-their leaves uneatable. Animals exhibit far more numerous adjustments,
-both passive and active. In some cases they survive desiccation, they
-hybernate, they acquire thicker clothing, and so are fitted to bear
-unfavourable inorganic actions; and they are in many cases fitted
-passively to meet the adverse actions of other organisms, by bearing
-spines or armour or shells, by simulating neighbouring objects in
-colour or form or both, by emitting disagreeable odours, or by having
-disgusting tastes. In still more numerous ways they actively contend
-with unfavourable conditions. Against the seasons they guard by storing
-up food, by secreting themselves in<span class="pagenum" id="Page_415">[415]</span> crevices, or by forming burrows
-and nests. They save themselves from enemies by developed powers of
-locomotion, taking the shape of swiftness or agility or aptitude for
-changing their media; by their strength either alone or aided by
-weapons; lastly by their intelligence, without which, indeed, their
-other superiorities would avail them little. And then these various
-active powers serving for defence, become, in other cases, the powers
-that enable animals to aggress, and to preserve their lives by the
-success of their aggressions.</p>
-
-<p>The second process by which extinction is prevented&mdash;the formation of
-new individuals to replace the individuals destroyed&mdash;is carried on,
-as described in the chapter on “Genesis,” by two methods, the sexual
-and the asexual. Plants multiply by spontaneous fission, by gemmation,
-by proliferation, and by the evolution of young ones from detached
-cells and scales and leaves; and they also multiply by the casting
-off of spores and sporangia and seeds. In like manner among animals,
-there are varied kinds of agamogenesis, from spontaneous fission up to
-parthenogenesis, all of them conducing to rapid increase of numbers;
-and we have the more familiar process of gamogenesis, also carried
-on in a great variety of ways. This formation of new individuals to
-replace the old, is, however, inadequately conceived if we contemplate
-only the number born or detached on each occasion. There are four
-factors, all variable, on which the rate of multiplication depends.
-The first is the age at which reproduction commences; the second is
-the frequency with which broods are produced; the third is the number
-contained in each brood; and the fourth is the length of time during
-which the bringing forth of broods continues. There must be taken into
-account a further element&mdash;the amount of aid given by the parent to
-each germ in the shape of stored-up nutriment, continuous feeding,
-warmth, protection, &amp;c.: on which amount of aid, varying between
-immensely wide limits, depends the number of<span class="pagenum" id="Page_416">[416]</span> the new individuals
-that survive long enough to replace the old, and perform the same
-reproductive process.</p>
-
-<p>Thus, regarding every living organism as having a moving equilibrium
-dependent on environing forces, but ever liable to be overthrown by
-irregularities in those forces, and always so overthrown sooner or
-later; we see that each species of organism can be maintained only
-by the generation of new individuals with a certain rapidity, and by
-helping them more or less fully to establish their moving equilibria.</p>
-
-
-<p class="p2">§ 318. Such are the factors with which we are here concerned. I have
-presented them in abstract shapes for the purpose of showing how they
-are expressible in general terms of force&mdash;how they stand related to
-the ultimate laws of re-distribution of matter and motion.</p>
-
-<p>For the purposes of the argument now to follow, we may, however,
-conveniently deal with these factors under a more familiar guise.
-Ignoring their other aspects, we may class the factors which affect
-each race of organisms as forming two conflicting sets. On the one
-hand, by what we call natural death, by enemies, by lack of food, by
-atmospheric changes, &amp;c., the race is constantly being destroyed. On
-the other hand, partly by the endurance, the strength, the swiftness,
-and the sagacity of its members, and partly by their fertility, it
-is constantly being maintained. These conflicting sets of factors
-may be generalized as&mdash;the forces destructive of race and the forces
-preservative of race. So generalizing them, let us ask what are the
-necessary implications.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_417">[417]</span></p>
-
-<h3>CHAPTER II.<br />
-<span class="subhed1"><i>À PRIORI</i> PRINCIPLE.</span></h3></div>
-
-
-<p>§ 319. The number of a species must at any time be either decreasing
-or stationary or increasing. If, generation after generation, its
-members die faster than others are born, the species must dwindle
-and finally disappear. If its rate of multiplication is equal to its
-rate of mortality, there can be no numerical change in it. And if the
-deductions by death are fewer than the additions by birth, the species
-must become more abundant. These we may safely set down as necessities.
-The forces destructive of race must be either greater than the forces
-preservative of race, or equal to them, or less than them; and there
-cannot but result these effects on number.</p>
-
-<p>We are here concerned only with races that continue to exist; and may
-therefore leave out of consideration those in which the destructive
-forces, remaining permanently in excess of the preservative forces,
-cause extinction. Practically, too, we may exclude the stationary
-condition; for the chances are infinity to one against the maintenance
-of a permanent equality between the births and the deaths. Hence, our
-inquiry resolves itself into this:&mdash;In races that continue to exist,
-what laws of numerical variation result from these variable conflicting
-forces, which are respectively destructive of race and preservative of
-race?</p>
-
-
-<p class="p2">§ 320. Clearly if the forces destructive of race, when once<span class="pagenum" id="Page_418">[418]</span> in excess,
-had nothing to prevent them from remaining in excess, the race would
-disappear; and clearly if the forces preservative of race, when once in
-excess, had nothing to prevent them from remaining in excess, the race
-would go on increasing to infinity. In the absence of any compensating
-actions, the only possible avoidance of these opposite extremes would
-be an unstable equilibrium between the conflicting forces, resulting
-in a perfectly constant number of the species: a state which we know
-does not exist, and against the existence of which the probabilities
-are, as already said, infinite. It follows, then, that as in every
-continuously-existing species, neither of the two conflicting sets
-of forces remains permanently in excess; there must be some way of
-stopping that excess of the one or the other which is ever occurring.</p>
-
-<p>How is this done? Should any one allege, in conformity with the old
-method of interpretation, that there is in each case a providential
-interposition to rectify the disturbed balance, he commits himself to
-the supposition that of the millions of species inhabiting the Earth,
-each one is yearly regulated in its degree of fertility by a miracle;
-since in no two years do the forces which foster, or the forces which
-check, each species, remain the same; and therefore, in no two years
-is there required the same fertility to balance the mortality. Few if
-any will say that God continually alters the reproductive activity
-of every parasitic fungus and every Tape-worm or <i>Trichina</i>,
-so as to prevent its extinction or undue multiplication; which they
-must say if they adopt the hypothesis of supernatural adjustment. And
-in the absence of this hypothesis there remains only one other. The
-alternative possibility is, that the balance of the preservative and
-destructive forces is self-sustaining&mdash;is of the kind distinguished as
-a stable equilibrium: an equilibrium such that any excess of one of
-the forces at work, itself generates, by the deviation it produces,
-certain counter-forces which eventually out-balance it, and initiate an
-opposite deviation.<span class="pagenum" id="Page_419">[419]</span> Let us consider how, in the case before us, such a
-stable equilibrium must be constituted.</p>
-
-
-<p class="p2">§ 321. When a season favourable to it, or a diminution of creatures
-detrimental to it, causes any species to become more numerous than
-usual, an immediate increase of certain destructive influences takes
-place. If it be a plant, the supposed greater abundance itself implies
-fuller occupation of the places available for growth&mdash;an occupation
-which, leaving fewer such places as the multiplication goes on,
-becomes a check on further multiplication&mdash;itself causes a greater
-mortality of seeds that fail to root themselves. And afterwards, in
-addition to this passive resistance to continued increase, there comes
-an active resistance: the creatures which thrive at the expense of
-the species&mdash;the larvae, the birds, the herbivores&mdash;increase too. If
-it be an animal that has grown more numerous, then, unless by some
-exceptional coincidence a simultaneous and proportionate addition to
-the animals or plants serving for food has occurred, there must result
-a relative scarcity of food. Enemies, too, be they beasts of prey or
-be they parasites, must quickly begin to multiply. Hence, each kind
-of organism, previously existing in something like its normal number,
-cannot have its number raised without a rise of the destructive forces,
-negative and positive, quickly commencing. Both negative and positive
-destructive forces must augment until this increase of the species is
-arrested. The competition for places on which to grow, if the species
-be vegetal, or for food if it be animal, must become more intense as
-the over-peopling of the habitat progresses; until there is reached
-the limit at which the mortality equals the reproduction. And as, at
-the same time, enemies will multiply with a rapidity which soon brings
-them abreast of the augmented supply of prey, the positive restraint
-they exert will help to bring about an earlier arrest of the expansion
-than pressure of population alone would cause. One more inference may
-be<span class="pagenum" id="Page_420">[420]</span> drawn. Had the species to meet no repressing influence save that
-negative one of relatively-diminished space or relatively-diminished
-food-supply, the cause leading to its increase might carry it up
-to the limit set by this, and there leave it: its enlarged number
-might be permanent. But the positive repressing influence that has
-been called into existence, will prevent this. For the increase of
-enemies, commencing, as it must, after the increase of the species,
-and advancing in geometrical progression until it is itself checked
-in like manner, will end in an excess of enemies. Whereupon must
-result a mortality of the species greater than its multiplication&mdash;a
-decrease which will continue until its habitat is under-peopled, its
-unduly-numerous enemies decimated by starvation, and the destroying
-agencies reduced to a minimum. Whence will follow another increase.</p>
-
-<p>Thus, as before indicated (<i>First Prin.</i> §§ 85, 173), there is
-here, as wherever antagonistic forces are in action, an alternate
-predominance of each, causing a rhythmical movement&mdash;a rhythmical
-movement which constitutes a moving equilibrium in those cases where
-the forces are not dissipated with appreciable rapidity, or are
-re-supplied as fast as they are dissipated. While, therefore, on the
-one hand, we see that the continued existence of a species necessarily
-implies some action by which the destructive and preservative forces
-are self-adjusted; we see, on the other hand, that such an action is an
-inevitable consequence of the universal process of equilibration.</p>
-
-
-<p class="p2">§322. Is this the sole equilibration which must exist? Clearly not.
-The temporary compensating adjustments of multiplication to mortality
-in each species, are but introductory to the permanent compensating
-adjustments of multiplication to mortality among species in general.
-The above reasoning would hold just as it now does, were all species
-equally prolific and all equally short-lived. It yields no<span class="pagenum" id="Page_421">[421]</span> answer
-to the inquiries&mdash;why do their fertilities differ so enormously,
-or why do their mortalities differ so enormously? and how is the
-general fertility adapted to the general mortality in each? The
-balancing process we have contemplated can go on only within moderate
-limits&mdash;must fail entirely in the absence of a due proportion
-between the ordinary birth-rate and the ordinary death-rate. If the
-reproduction of mice proceeded as slowly as the reproduction of men,
-mice would be extinct before a new generation could arise: even did
-their natural lives extend to fifteen or sixteen years, it would still
-be extremely improbable that any would for so long survive all the
-dangers they are exposed to. Conversely, did oxen propagate as fast
-as infusoria, the race would die of starvation in a week. Hence, the
-minor adjustment of varying multiplication to varying mortality in each
-species, implies some major adjustment of average multiplication to
-average mortality. What must this adjustment be?</p>
-
-<p>We have already seen that the forces preservative of race are
-two&mdash;ability in each member of the race to preserve itself, and ability
-to produce other members&mdash;power to maintain individual life, and power
-to generate the species. These must vary inversely. When, from lowness
-of organization, the ability to contend with external dangers is
-small, there must be great fertility to compensate for the consequent
-mortality; otherwise the race must die out. When, on the contrary, high
-endowments give much capacity of self-preservation, a correspondingly
-low degree of fertility is requisite. Given the dangers to be met as a
-constant quantity; then as the ability to meet them must be a constant
-quantity too; and as this is made up of the two factors, power to
-maintain individual life and power to multiply, these cannot do other
-than vary inversely: one must decrease as the other increases.</p>
-
-<p>It needs but to conceive the results of nonconformity to this law,
-to see that every species must either conform to it or cease to
-exist. Suppose, first, a species whose individuals,<span class="pagenum" id="Page_422">[422]</span> having but small
-self-preservative powers, are rapidly destroyed, to be at the same
-time without reproductive powers proportionately great. The defect
-of fertility, if extreme, will result in the death of one generation
-before another has grown up. If less extreme, it will entail a
-scarcity such that in the next generation sexual congress will be too
-infrequent to maintain even the small number which remains; and the
-race will dwindle with increasing rapidity. If still less extreme, the
-consequent degree of sparseness, while not so great as to prevent an
-adequate number of procreative unions, will be so great as to render
-special food abundant and special enemies few&mdash;will thus diminish
-the destructive forces so much that the self-preservative forces
-will become <i>relatively</i> great: so great, relatively, that when
-combined with the small ability to propagate the species, they will
-suffice to balance the small destructive forces. Suppose, next, a
-species whose individuals have high powers of self-preservation, while
-they have powers of multiplication much beyond what is needful. The
-excess of fertility, if extreme, will cause sudden extinction of the
-species by starvation. If less extreme, it must produce a permanent
-increase in the number of the species; and this, followed by intenser
-competition for food and augmented number of enemies, will involve
-such an increase of the dangers to individual life, that the great
-self-preserving powers of the individuals will not be more than
-sufficient to cope with them. That is to say, if the fertility is
-relatively too great, then the ability to maintain individual life
-inevitably becomes smaller, <i>relatively</i> to the requirements; and
-the inverse proportion is thus established.</p>
-
-<p>So that when, from comparing the different states of the same species,
-we go on to compare the states of different species, we see that there
-is an analogous adjustment&mdash;analogous in the sense that great mortality
-is associated with great multiplication, and small mortality with small
-multiplication. And we see that the unlikeness of the cases consists<span class="pagenum" id="Page_423">[423]</span>
-merely in this, that what is a temporary relation in the one is a
-permanent relation in the other.</p>
-
-
-<p class="p2">§ 323. For the moment it does not concern us to inquire what is the
-origin of this permanent relation. That which we have now to note, is
-simply that in some way or other there must be established an inverse
-proportion between the power to sustain individual life and the power
-to produce new individuals. Whether or not this permanent relation is
-self-adjusting in long periods of time, as the temporary relation is
-self-adjusting in short periods of time, is a separate question. The
-purpose of this chapter is fulfilled by showing that such a permanent
-relation must exist.</p>
-
-<p>But having recognized the <i>à priori</i> principle that in races
-which continuously survive, the forces destructive of race must be
-equilibrated by the forces preservative of race; and that, supposing
-these are constant, there must be an inverse proportion between
-self-preservation and race-preservation; we may go on to inquire how
-this relation, necessary in theory, arises in fact. Leaving out the
-untenable hypothesis of a supernatural pre-adjustment, we have to ask
-in what way an adjustment comes about as a result of Evolution. Is it
-due to the survival of varieties in which the proportion of fertility
-to mortality happens to be the best? Or is the fertility adapted to the
-mortality in a more direct way? To these questions let us now address
-ourselves.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_424">[424]</span></p>
-
-<h3>CHAPTER III.<br />
-<span class="subhed1">OBVERSE <i>À PRIORI</i> PRINCIPLE.</span></h3></div>
-
-
-<p>§ 324. When dealing with its phenomena inductively, we saw that however
-it may be carried on, Genesis “is a process of negative or positive
-disintegration; and is thus essentially opposed to that process of
-integration which is the primary process in individual evolution.”
-(<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect76">§ 76</a>.) Each new individual, whether separated as a germ or in some
-more-developed form, is a deduction from the mass of a pre-existing
-individual or of two pre-existing individuals. Whatever nutritive
-matter is stored-up along with the germ, if it be deposited in the
-shape of an egg, is so much nutritive matter lost to the parent.
-No drop of blood can be absorbed by the fœtus, nor any draught of
-milk sucked by the young when born, without taking from the mother
-tissue-forming and force-evolving materials to an equivalent amount.
-And all subsequent supplies given to progeny, if they are nurtured,
-involve, to a parent or parents, so much waste in exertion which does
-not bring its return in assimilated food.</p>
-
-<p>Conversely, the continued aggregation of materials into one organism,
-renders impossible the formation of other organisms out of those
-materials. As much assimilated food as is united into a single whole,
-is so much assimilated food withheld from a plurality of wholes which
-might else have been produced. Given the absorbed nutriment as a
-constant quantity, and the longer the building of it up into a concrete
-shape goes on, the longer must be postponed any building<span class="pagenum" id="Page_425">[425]</span> of it up into
-discrete shapes. And, similarly, the larger the proportion of matter
-consumed in the functional actions of parents, the smaller must be the
-proportion of matter which can remain to establish and support the
-functional actions of offspring.</p>
-
-<p>Though the necessity of these universal relations is tolerably obvious
-as thus stated generally, it will be useful to dwell for a brief space
-on their leading aspects.</p>
-
-
-<p class="p2">§ 325. That disintegration which constitutes genesis, may be such as
-to disperse entirely the aggregate which integration has previously
-produced&mdash;the parent may dissolve wholly into progeny. This dissolution
-of each aggregate into two or many aggregates, may occur at very short
-intervals, in which case the bulk attained can be but extremely small;
-or it may occur at longer intervals, in which case a larger bulk may be
-attained.</p>
-
-<p>Instead of quickly losing its own individuality in the individualities
-of its offspring, each member of the race may, after growing for a
-time, have portions of its substance begin to develop into the parental
-shape and presently detach themselves; and the parent, maintaining its
-own identity, may continue indefinitely so to produce young ones. But
-clearly, the earlier it commences doing this, and the more rapidly it
-does it, the sooner must the increase of its own bulk be stopped.</p>
-
-<p>Or again, growth and development continuing for a long period without
-any deduction of materials, an individual of considerable size and
-organization may result; and then the abstraction of substance for
-the formation of new individuals, or rather the eggs of them, may
-be so great that as soon as the eggs are laid the parent dies of
-exhaustion&mdash;dies, that is, from an excessive loss of the nutritive
-matters needed for its own activities.<a id="FNanchor_53" href="#Footnote_53" class="fnanchor">[53]</a></p>
-
-<p><span class="pagenum" id="Page_426">[426]</span></p>
-
-<p>Once more, the deduction of materials for the propagation of the
-species may be postponed long enough to allow of great bulk and complex
-structure being attained. The procreative subtraction then setting
-in, while it checks and presently stops growth, may be so moderate
-as to leave vital capital sufficient to carry on the activities of
-the parent; may go on as long as parental vigour suffices to furnish,
-without fatal result, the materials needed to produce young ones; and
-may cease when such a surplus cannot be supplied, leaving the parental
-life to continue.</p>
-
-
-<p class="p2">§ 326. The opposite side of this antagonism has also several aspects.
-Progress of organic evolution may be shown in increased bulk, in
-increased structure, in increased amount or variety of action, or
-in combinations of these; and under any of its forms this carrying
-higher of each individuality, implies a correlative retardation in the
-establishment of new individualities.</p>
-
-<p>Other things equal, every normal addition to the bulk of an organism
-is an augmentation of its life.<a id="FNanchor_54" href="#Footnote_54" class="fnanchor">[54]</a> Besides being an advance in
-integration, it implies a greater total of activities gone through in
-the assimilation of materials; and it implies, thereafter, a greater
-total of the vital changes taking place from moment to moment in all
-parts of the enlarged mass. Moreover, while increased size is thus,
-in so far, the expression of increased life, it is also, where the
-organism is active, the expression of increased ability to maintain
-life&mdash;increased strength. Aggregation of substance is almost the only
-mode in which self-preserving power is shown among the lowest types;
-and even among the highest, sustaining the body in its integrity
-is that in which self-preservation fundamentally consists&mdash;is the
-end which the widest intelligence is indirectly<span class="pagenum" id="Page_427">[427]</span> made to subserve.
-While, on the one hand, the increase of tissue constituting growth is
-conservative both in essence and in result; on the other hand, decrease
-of tissue, either from injury, disease, or old age, is in both essence
-and result the reverse. And if so, every addition to individual life
-thus implied, necessarily delays or diminishes the casting off of
-matter to form new individuals.</p>
-
-<p>Other things equal, too, a greater degree of organization involves
-a smaller degree of that disorganization shown by the separation
-of reproductive gemmæ and germs. Detachment of a living portion or
-portions from what was previously a living whole, is a ceasing of
-co-ordination; and is therefore essentially at variance with that
-establishment of greater co-ordination which is achieved by structural
-development. In the extreme cases where a living mass is continually
-dividing and subdividing, it is manifest that there cannot arise
-much physiological division of labour; since progress towards mutual
-dependence of parts is prevented by the parts becoming independent.
-Contrariwise, it is equally clear that in proportion as the
-physiological division of labour is carried far, the separative process
-must be localized in some comparatively small portion of the organism,
-where it may go on without affecting the general structure&mdash;must
-become relatively subordinate. The advance that is shown by greater
-heterogeneity, must be a hindrance to multiplication in another way.
-For organization entails cost. That transfer and transformation
-of materials implied by differentiation, can be effected only by
-expenditure of force; and this supposes consumption of digested and
-absorbed food, which might otherwise have gone to make new organisms,
-or the germs of them. Hence, that individual evolution which consists
-in progressive differentiation, as well as that which consists in
-progressive integration, necessarily diminishes that species of
-dissolution, general or local, which propagation of the race exhibits.</p>
-
-<p>In active organisms we have yet a further opposition<span class="pagenum" id="Page_428">[428]</span> between
-self-maintenance and maintenance of the race. All motion, sensible and
-insensible, generated by an animal for the preservation of its life,
-is motion liberated from decomposed nutriment&mdash;nutriment which, if
-not thus decomposed, would have been available for reproduction; or
-rather&mdash;might have been replaced by nutriment fitted for reproductive
-purposes, absorbed from other kinds of food. Hence, in proportion as
-the activities increase&mdash;in proportion as, by its more varied, complex,
-rapid, and vigorous actions, an animal gains power to support itself
-and to cope with surrounding dangers, it must lose power to propagate.</p>
-
-
-<p class="p2" id="sect327">§ 327. How may this antagonism be best expressed in a brief way? If
-self-preservation displayed itself in the highest organisms, as it
-does in the lowest, in little else but continuous growth; and if
-race-preservation consisted always, as it does often, of nothing beyond
-detachment of portions from the parental mass; then the antagonism
-would be, throughout, the obviously-necessary one of integration and
-disintegration. Maintenance of the individual and propagation of the
-species, being respectively aggregative and separative, it would be
-as self-evident that they vary inversely, as it is self-evident that
-addition and subtraction undo one another. But though the simplest
-types show us the opposition of self-maintenance and race-maintenance
-almost wholly under this form; and though higher types, up to the most
-complex, exhibit it to a great extent under this form; yet, as we have
-just seen, this is not its only form. The total material monopolized
-by the individual and withheld from the race, must be stated as the
-quantity united to form its fabric, <i>plus</i> the quantity expended
-in differentiating its fabric, <i>plus</i> the quantity expended in its
-self-conserving actions. Similarly, the total material devoted to the
-race at the expense of the individual, includes that which is directly
-subtracted from the parent in the shape of egg or fœtus, <i>plus</i>
-that which is directly subtracted in the shape of milk, <i>plus</i><span class="pagenum" id="Page_429">[429]</span>
-that which is indirectly subtracted in the shape of matter consumed
-in exertions for fostering the young. Hence this inverse variation
-is not expressible in simple terms of aggregation and separation. As
-we advance to more highly-evolved organisms, the total cost of an
-individual becomes very much greater than is implied by the amount of
-tissue composing it. So, too, the total cost of producing each new
-individual becomes very much greater than that of its mere substance.
-And it is between these two total costs that the antagonism exists.</p>
-
-<p>We may, indeed, reduce the antagonism to a form comprehensive of all
-cases, if we consider it as existing between the sums of the forces,
-latent and active, used for the two purposes. The molecules which make
-up a plant or animal, have been formed by the absorption of forces
-directly or indirectly derived from the Sun; and hence the quantity
-of matter raised to the form called organic, which a plant or animal
-presents, is equivalent to a certain amount of force. Another amount of
-force is expressed by the totality of its differentiations. A further
-amount of force is that dissipated in its actions. And in these three
-amounts added together, we have the whole expense of the individual
-life. So, too, the whole expense of establishing each new individual
-includes&mdash;first the forces latent in the substance composing it when
-born or hatched; second the forces latent in the prepared nutriment
-afterwards supplied; and third the forces expended in feeding and
-protecting it. These two sets of forces being taken from a common fund,
-it is manifest that either set can increase only by decrease of the
-other. If, of the force which the parent obtains from the environment,
-much is consumed in its own life, little remains to be consumed in
-producing other lives; and, conversely, if there is a great consumption
-in producing other lives, it can only be where comparatively little is
-reserved for parental life.</p>
-
-<p>Hence, then, Individuation and Genesis are necessarily antagonistic.
-Grouping under the word Individuation all<span class="pagenum" id="Page_430">[430]</span> processes by which
-individual life is completed and maintained; and enlarging the
-meaning of the word Genesis so as to include all processes aiding
-the formation and perfecting of new individuals; we see that the
-two are fundamentally opposed. Assuming other things to remain the
-same&mdash;assuming that environing conditions as to climate, food,
-enemies, &amp;c., continue constant; then, inevitably, every higher
-degree of individual evolution is followed by a lower degree of
-race-multiplication, and <i>vice versâ</i>. Progress in bulk,
-complexity, or activity, involves retrogress in fertility; and progress
-in fertility involves retrogress in bulk, complexity, or activity.</p>
-
-<p>This statement needs a slight qualification. For reasons to be
-hereafter assigned, the relation described is never completely
-maintained; and in the small departure from it, we shall find a
-remarkable self-acting tendency to further the supremacy of the
-most developed types. Here, however, this hint must suffice:
-explanation would carry us too far out of our line of argument. For the
-present it will not lead us astray if we regard this inverse variation
-of Individuation and Genesis as exact.</p>
-
-
-<p class="p2">§ 328. Thus, then, the condition which each race must fulfil if it is
-to survive, is a condition which, in the nature of things, it ever
-tends to fulfil. In the last chapter we saw that a species cannot be
-maintained unless the power to preserve individual life and the power
-to propagate other individuals vary inversely. And here we have seen
-that, irrespective of an end to be subserved, these powers cannot do
-other than vary inversely. On the one hand, given a certain totality
-of destroying forces with which the species has to contend; and in
-proportion as its members have severally but small ability to resist
-these forces, it is requisite that they should have great ability to
-form new individuals, and <i>vice versâ</i>. On the other hand, given
-the quantity of force, absorbed as food or otherwise, which the species
-can<span class="pagenum" id="Page_431">[431]</span> use to counterbalance these destroying forces; and in proportion
-as much of it is expended in preserving the individual, little of it
-can be reserved for producing new individuals, and <i>vice versâ</i>.
-There is thus complete accordance between the requirements considered
-under each aspect. The two necessities correspond.</p>
-
-<p>We might rest on these deductions and their several corollaries.
-Without going further we might with safety assert the general truths
-that, other things equal, advancing evolution must be accompanied
-by declining fertility; and that, in the highest types, fertility
-must still further decrease if evolution still further increases.
-We might be sure that if, other things equal, the relations between
-an organism and its environment become so changed as permanently
-to diminish the difficulties of self-preservation, there will be a
-permanent increase in the rate of multiplication; and, conversely, that
-a decrease of fertility will result where altered circumstances make
-self-preservation more laborious.</p>
-
-<p>But we need not content ourselves with these <i>à priori</i>
-inferences. If they are true, there must be an agreement between
-them and the observed facts. Let us see how far such an agreement is
-traceable.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_432">[432]</span></p>
-
-<h3>CHAPTER IV.<br />
-<span class="subhed1">DIFFICULTIES OF INDUCTIVE VERIFICATION.</span></h3></div>
-
-
-<p>§ 329. Were all species subject to the same kinds and amounts of
-destructive forces, it would be easy, by comparing different species,
-to test the inverse variation of Individuation and Genesis. Or if
-either the power of self-preservation or the power of multiplication
-were constant, there would be little difficulty in seeing how the
-other changed as the destroying forces changed. But comparisons are
-nearly always partially vitiated by some want of parity. Each factor,
-besides being variable as a whole, is compounded of factors that are
-severally variable. Not simply is the sum of the forces destructive of
-race different in every case; and not simply are both sets of forces
-preservative of race unlike in their totalities in every case; but
-each is made up of actions that bear such changing proportions to one
-another as to prevent any positive estimation of its amount.</p>
-
-<p>Before dealing with the facts as well as we can, it will be best to
-glance at the chief difficulties; so that we may see the kind of
-verification which is alone possible.</p>
-
-
-<p class="p2">§ 330. Either absolutely, or relatively to any species, every
-environment differs more or less from every other.</p>
-
-<p>There are the unlikenesses of media&mdash;air, water, earth, organic matter;
-severally involving special resistances to movement, and special losses
-of heat. There are the contrasts<span class="pagenum" id="Page_433">[433]</span> of climate: here great expenditure
-for the maintenance of temperature is needed, and there very little;
-in one zone an organism is supplied with abundant light all the year
-round, and in another only for a few months; this region yields an
-almost unfailing supply of water, while that entails the exertion of
-travelling many miles every night for a draught.</p>
-
-<p>Permanent differences in the natures and distributions of aliment
-greatly interfere with the comparisons. The Swallow goes through
-more exertion than the Sparrow in securing a given weight of food;
-but then their foods are dissimilar in nutritive qualities. There is
-a want of parallelism between the circumstances of those herbivores
-which live where the plains are annually covered for a time with rich
-herbage, but afterwards become parched up, and of those inhabiting more
-temperate regions. Insects whose larvæ feed on an abundant plant, as
-do several of the genus <i>Vanessa</i> on the Nettle, have practically
-an environment very unlike that of insects such as <i>Deilephila
-Euphorbiæ</i>, whose larvæ feed on a comparatively rare plant&mdash;the
-Sea-Spurge.</p>
-
-<p>Again, comparisons between creatures otherwise akin in their
-constitutions and circumstances, are hindered by inequalities in their
-relations to enemies. Two animals, of which one is predatory and has
-no foes but parasites while the other is much pursued, cannot properly
-be contrasted with a view to determining the influence of size or
-complexity.</p>
-
-<p>Without multiplying instances, it will be clear enough then that the
-aggregate of destructive actions, positive and negative, which each
-species has to contend with, is so undefinable in the amounts and kinds
-of its components, that nothing beyond a vague idea of its relative
-total can be formed.</p>
-
-
-<p class="p2">§ 331. Besides these immense variations in the outer actions to
-be counter-balanced, there are immense variations<span class="pagenum" id="Page_434">[434]</span> in the inner
-actions required to counterbalance them. Even were species similarly
-conditioned, self-preservation would require of them extremely unlike
-expenditures of force.</p>
-
-<p>The cost of locomotion increases in a greater ratio than the size. In
-virtue of the law that the weights of animals increase as the cubes
-of their dimensions, while their powers of bearing strains increase
-only as the squares of their dimensions (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect46">§ 46</a>), preservation of its
-various attitudes requires a large animal to consume more substance in
-proportion to its weight, than it requires a small animal to consume;
-and there results, other things equal, a difficulty of self-maintenance
-which augments in a more rapid ratio than the bulk. Nor must we
-overlook the further complication, that among aquatic creatures the
-variation of resistance of the medium tends to produce an opposite
-effect.</p>
-
-<p>Again, the heat-consumption is a changing element in the total expense
-of self-preservation. Creatures which have temperatures scarcely
-above that of the air or water, may, other things equal, accumulate
-more surplus nutriment than creatures which have to keep their bodies
-warm spite of the continual loss by radiation and conduction. This
-difference of cost is modified by the presence or absence of natural
-clothing; and it is also modified by unlikenesses of size. Here the
-bulky animals have the advantage: small masses cooling more rapidly
-than large ones.</p>
-
-<p>Dissimilarities of attack and defence are also causes of variation
-in the outlay for self-maintenance. A creature that has to hunt, as
-compared with another that gets a sufficiency of prey by lying in wait,
-or a creature that escapes by speed as compared with another that
-escapes by concealment, obviously leads a life that is physiologically
-more costly. Animals which protect themselves passively, as the
-Hedge-hog by its spines or as the Skunk and the Musk-rat by their
-intolerable odours, are relatively economical; and have the more vital
-capital for other purposes.</p>
-
-<p>Amplification is needless. These instances will show that<span class="pagenum" id="Page_435">[435]</span> anything
-beyond very general conceptions of the individual expenditures in
-different cases, cannot be reached.</p>
-
-
-<p class="p2">§ 332. Still more entangled are we among qualifying considerations
-when we contrast species in their powers of multiplication. The total
-cost of Genesis admits of even less definite estimation than does the
-total cost of Individuation. I do not refer merely to the truth that
-the degree of fertility depends on four factors&mdash;the age of commencing
-reproduction, the number in each brood, the frequency of the broods,
-and the time during which broods continue to be repeated. There are
-many further obstacles in the way of comparisons.</p>
-
-<p>Were all multiplication carried on sexually, the problem would be
-less involved; but there are many kinds of asexual multiplication
-alternating with the sexual. This asexual multiplication is in some
-cases perpetual instead of occasional; and often has more forms than
-one in the same species. The result is that we have to compare what
-is here a periodic process with what is elsewhere a cyclical process
-partly continuous and partly periodic: the calculation of fertility in
-this last case being next to impossible.</p>
-
-<p>We have to avoid being misled by the assumption that the cost of
-Genesis is measured by the number of young produced, instead of
-being measured, as it is, by the weight of nutriment abstracted to
-form the young, <i>plus</i> the weight consumed in caring for them.
-This total weight may be very diversely apportioned. In contrast to
-the Cod with its millions of small ova spawned without protection,
-we may put the <i>Hippocampus</i>, or the Pipe-fish, with its few
-relatively-large ova carried about by the male in a caudal pouch, or
-seated in hemispherical pits in its skin; or we may put the still more
-remarkable genus <i>Arius</i>, and especially <i>Arius Boakeii</i>&mdash;a
-fish some six or seven inches long, which produces ten or a dozen eggs
-5–10 mm. in diameter, that are carried by the male in his mouth till
-they are hatched. Here though<span class="pagenum" id="Page_436">[436]</span> the degrees of fertility, if measured
-by the numbers of fertilized germs deposited, are extremely unlike,
-they are less unlike if measured by the numbers of young which are
-hatched and survive long enough to take care of themselves; nor will
-the tax on the parent-Cod seem so immensely different from that on
-the parent-<i>Arius</i>, if the masses of the ova, instead of their
-numbers, are compared. Again, while sometimes the parental loss is
-little else but the matter deducted to form eggs, &amp;c., at other times
-it takes the shape of a small direct deduction joined with a large
-indirect outlay. The Mason-wasp furnishes a typical instance. In
-journeyings hither and thither to fetch bit by bit the materials for
-building a cell; in putting together these materials, as well as in
-secreting glutinous matter to act as cement; and then, afterwards, in
-the labour of seeking for, and carrying, the small caterpillars with
-which it fills up the cell to serve its larva with food when it emerges
-from the egg; the Mason-wasp expends more substance than is contained
-in the egg itself. And this supplementary expenditure is manifestly so
-great that but few eggs can be housed and provisioned.</p>
-
-<p>Estimates of the cost of Genesis are further complicated by
-variations in the ratio borne by the two sexes. Among Fishes the
-mass of milt approaches in size the mass of spawn; but among higher
-<i>Vertebrata</i> the substance lost by the one sex in the shape of
-sperm-cells is small compared with that lost by the other sex in the
-shape of albumen stored-up in the eggs, or blood supplied to the fœtus,
-or milk given to the young. Then there come the differences of indirect
-tax on males and females. While, frequently, the fostering of the young
-devolves entirely on the female, occasionally the male undertakes it
-wholly or in part. After building a nest, the male Stickleback guards
-the eggs till they are hatched; as does also the great <i>Silurus
-glanis</i> for some forty days, during which he takes no food. And
-then, among most birds, we have the male occupied in feeding the
-female during<span class="pagenum" id="Page_437">[437]</span> incubation, and the young afterwards. Evidently all
-these differences affect the proportion between the total cost of
-reproduction and the total cost of individuation.</p>
-
-<p>Whether the species is monogamous or polygamous, and whether there are
-marked differences of size or of structure between males and females,
-are also questions not to be overlooked. If there are many females to
-one male, the total quantity of assimilated matter devoted by each
-generation to the production of a new generation, is greater than if
-there is a male to each female. Similarly, where the requirements
-are such that small males will suffice, the larger quantity of food
-left for the females makes possible a greater surplus available for
-reproduction. Another cause has a like effect. Where the habits of the
-race render it needless that both sexes should have developed powers of
-locomotion&mdash;where, as in the Glow-worm and sundry <i>Lepidoptera</i>,
-the female is wingless while the male has wings&mdash;the cost of
-Individuation not being so great for the species as a whole, there
-arises a greater reserve for Genesis: the matter which would otherwise
-have gone to the production of wings and the using of them, may go to
-the production of ova.</p>
-
-<p>Other complications, as those which we see in Bees and Ants, might be
-dwelt on; but the foregoing will amply serve the intended purpose.</p>
-
-
-<p class="p2">§ 333. To ascertain by comparison of cases whether Individuation
-and Genesis vary inversely, is thus an undertaking so beset with
-difficulties, that we might despair of any satisfactory results,
-were not the relation too marked a one to be hidden even by all
-these complexities. Species are so extremely contrasted in their
-degrees of evolution, and so extremely contrasted in their rates of
-multiplication, that the law of relation between these traits becomes
-unmistakable when the evidence is looked at in its <i>ensemble</i>.
-This we shall soon find on ranging in order a number of typical cases.</p>
-
-<p>In doing this it will be convenient to neglect, temporarily,<span class="pagenum" id="Page_438">[438]</span>
-all unlikenesses among the circumstances in which organisms are
-placed. At the outset, we will turn our attention wholly to the
-antagonism displayed between the integrative process which results in
-individual evolution and the disintegrative process which results in
-multiplication of individuals; and this we will consider first as we
-see it under the several forms of agamogenesis, and then as we see
-it under the several forms of gamogenesis. We will next look at the
-antagonism between propagation and that evolution which is shown by
-increased complexity. And then we will consider the remaining phase of
-the antagonism, as it exists between the degree of fertility and the
-degree of evolution expressed by activity.</p>
-
-<p>Afterwards, passing to the varying relations between organisms and
-their environments, we will note how relative increase in the supply
-of food, or relative decrease in the quantity of force expended by the
-individual, entails relative increase in the quantity of force devoted
-to multiplication, and <i>vice versâ</i>.</p>
-
-<p>Certain minor qualifications, together with sundry important
-corollaries, may then be entered upon.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_439">[439]</span></p>
-
-<h3>CHAPTER V.<br />
-<span class="subhed1">ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS.</span></h3></div>
-
-
-<p>§ 334. When illustrating, in Part IV, the morphological composition of
-plants and animals, there were set down in groups, numerous facts which
-we have here to look at from another point of view. Then we saw how,
-by union of small simple aggregates, there are produced large compound
-aggregates. Now we have to observe the reactive effect of this process
-on the relative numbers of the aggregates. Our present subject is the
-antagonism of Individuation and Genesis as seen under its simplest
-form, in the self-evident truth that the same quantity of matter may
-be divided into many small wholes or few large wholes; but that number
-negatives largeness and largeness negatives number.</p>
-
-<p>In setting down some examples we may conveniently adopt the same
-arrangement as before. We will look at the facts as they are presented
-by vegetal aggregates of the first order, of the second order, and of
-the third order; and then as they are presented by animal aggregates of
-the same three orders.</p>
-
-
-<p class="p2">§ 335. The ordinary unicellular plants are at once microscopic and
-enormously prolific. The often cited <i>Sphærella nivalis</i>, which
-shows its immense powers of multiplication by reddening wide tracts of
-snow in a single night, does this by developing in its cavity a brood
-of young cells, which, being<span class="pagenum" id="Page_440">[440]</span> presently set free by the bursting of
-the parent-cell, severally grow and quickly repeat the process. The
-like occurs among sundry of those kindred forms of minute <i>Algæ</i>
-which, by their enormous numbers, sometimes suddenly change pools to an
-opaque green. So, too, the <i>Desmidiaceæ</i> often multiply so greatly
-as to colour the water; and among the <i>Diatomaceæ</i> the rate of
-genesis by self-division, “is something really extraordinary. So soon
-as a frustule is divided into two, each of the latter at once proceeds
-with the act of self-division; so that, to use Professor Smith’s
-approximative calculation of the possible rapidity of multiplication,
-supposing the process to occupy, in any single instance, twenty-four
-hours, ‘we should have, as the progeny of a single frustule, the
-amazing number of one thousand millions in a single month.’” In these
-cases the multiplication is so carried on that the parent is lost in
-the offspring&mdash;the old individuality disappears either in the swarms of
-zoospores it dissolves into, or in the two or four new individualities
-simultaneously produced by fission. Vegetal aggregates of the first
-order, have, however, a form of agamogenesis in which the parent
-individuality is not lost: the young cells arise from the old cells
-by external gemmation. This process, too, repeated as it is at short
-intervals, results in immense fertility. The Yeast-fungus, which in a
-few hours thus propagates itself throughout a large vat of wort, offers
-a familiar example.</p>
-
-<p>In certain compound forms that must be classed as plants of the second
-order of aggregation, though very minute ones, self-division similarly
-increases the numbers at high rates. The <i>Sarcina ventriculi</i>, a
-parasitic plant which infests the stomach and swarms afresh as fast as
-previous swarms are vomited, shows us a spontaneous fission of clusters
-of cells. An allied mode of increase occurs in <i>Gonium pectorale:</i>
-each cell of the cluster resolving itself into a secondary cluster,
-and the secondary clusters then separating. “Supposing, which is
-very probable, that a young <i>Gonium</i> after twenty-four hours is
-capable of development by fission, it follows<span class="pagenum" id="Page_441">[441]</span> that under favourable
-conditions a single colony may on the second day develop 16, on the
-third 256, on the fourth 4,096, and at the end of a week 268,435,456
-other organisms like itself.” In the <i>Volvocineæ</i> this continual
-dissolution of a primary compound individual into secondary compound
-individuals, is carried on endogenously, and on a modified system:
-some only of the component cells giving origin to young colonies,
-and the parent bursting to liberate them. The numbers arising by
-this method also, are sometimes so great as to tint large bodies of
-water. More fully established and organized aggregates of the second
-order, such as the higher Thallophytes and the lower Archegoniates,
-do not sacrifice their individualities by fission; but nevertheless,
-by the kindred process of gemmation, are continually hindered in the
-increase of their individualities. The gemmæ called tetraspores are
-cast off in great numbers by the marine <i>Algæ</i>. Among those simple
-<i>Jungermanniaceæ</i> which consist of single fronds, the young ones
-that bud out grow for a time in connexion with their parents, send
-rootlets from their under sides into the soil, and presently separate
-themselves&mdash;a habit which augments the number of individuals in
-proportion as it checks their growths.</p>
-
-<p>Plants of the third order of composition, arising by arrest of this
-separation, exhibit a further corresponding decrease in the abundance
-of the aggregates formed. Archegoniates of inferior types, in which
-the axes produced by integration of fronds are but small and feeble,
-are characterized by the habit of throwing off bulbils&mdash;bud-shaped
-axes which, falling and taking root, add to the number of distinct
-individuals. This agamic multiplication, very general among the
-Mosses and their kindred, and not uncommon under a modified form
-in such higher types as the Ferns, many of which produce young
-ones from the surfaces of their fronds, becomes very unusual among
-Phænogams. The detachment of bulbils, though not unknown among them,
-is exceptional. And while it is true that some flowering plants, as
-the Strawberry,<span class="pagenum" id="Page_442">[442]</span> multiply by a process allied to gemmation, yet this
-is not characteristic of the class. A leading trait of these highest
-groups, to which the largest members of the vegetal kingdom belong, is
-that agamogenesis has so far ceased that it does not usually originate
-independent plants. Though the axes which, budding one out of another,
-compose a tree, are the equivalents of asexually-produced individuals;
-yet the asexual production of them stops short of separation. These
-vast integrations arise where spontaneous disintegration, and the
-multiplication effected by it, have come to an end.</p>
-
-<p>Thus, not forgetting that certain Phænogams, as <i>Begonia
-phyllomaniaca</i>, revert to quite primitive modes of increase, we may
-hold it as beyond question that while among the most minute plants
-asexual multiplication is universal, and produces enormous numbers in
-short periods, it becomes step by step more restricted in range and
-frequency as we advance to large and compound plants; and disappears so
-generally from the highest and largest, that its occurrence is regarded
-as anomalous.</p>
-
-
-<p class="p2">§ 336. Parallel examples furnished by animals make clear the purely
-quantitative nature of this relation under its original form. Among
-the <i>Protozoa</i>, as among the <i>Protophyta</i>, there occurs
-that process by which the individuality of the parent is wholly lost
-in producing offspring&mdash;the breaking up of the parental mass into a
-number of germs. Some of the <i>Infusoria</i>, as for instance those
-of the genus <i>Kolpoda</i> and several allied genera, become encysted
-and subsequently break up into young ones. The more familiar mode of
-increase among these animal-aggregates of the first order, by fission,
-though it sacrifices the parent individuality by merging it in the
-individualities of the two produced, sacrifices it less completely
-than does the dissolution into a great number of germs. Occurring,
-however, as this fission does, very frequently, and being completed,
-in some cases that<span class="pagenum" id="Page_443">[443]</span> have been observed, in the course of half-an-hour,
-it results in immensely-rapid multiplication. If all its offspring
-survive, and continue dividing themselves, a single <i>Paramœcium</i>
-is said to be capable of thus originating 268 millions in the course of
-a month.<a id="FNanchor_55" href="#Footnote_55" class="fnanchor">[55]</a> Nor is this the greatest known rate of increase. Another
-animalcule, visible only under a high magnifying power, “is calculated
-to generate 170 billions in four days.”<a id="FNanchor_56" href="#Footnote_56" class="fnanchor">[56]</a> And these enormous powers
-of propagation are accompanied by a minuteness so extreme, that of some
-species one drop of water would contain as many individuals as there
-are human beings on the Earth! Even if we allow a large margin for
-exaggeration in these estimates, it is beyond question that among these
-smallest of animals the rate of asexual multiplication is immensely the
-greatest; and this suffices for the purposes of argument.</p>
-
-<p>Of animal aggregates belonging to the second order, that multiply
-asexually with rapidity, the familiar Polypes furnish conspicuous
-examples. By gemmation in most cases, in other cases by fission, and
-in some cases by both, the agamogenesis is carried on among these
-tribes. As shown in Fig. <a href="#i_091">148</a>, the budding of young ones from the
-parent <i>Hydra</i> is carried on so actively, that before the oldest
-of them is cast off half-a-dozen or more others have reached various
-stages of growth; and even while still attached, the first-formed
-of the group have commenced budding out from their sides a second
-generation of young ones. In the <i>Hydra tuba</i><span class="pagenum" id="Page_444">[444]</span> this gemmiparous
-multiplication is from time to time interrupted by a transverse
-splitting-up of the body into segments, which successively separate
-and swim away: the result of the two processes being that, in the
-course of a season, there are produced from a single germ great numbers
-of young <i>Medusæ</i>, which are the adult or sexual forms of the
-species. Respecting cœlenterate animals of this degree of composition,
-it may be added that when we ascend to the larger kinds we find asexual
-genesis far less active. Though comparisons are interfered with by
-differences of structure and mode of life, yet the contrasts are too
-striking to have their meanings much obscured. If, for instance, we
-take a solitary <i>Actinozoon</i> and a solitary <i>Hydrozoon</i>,
-we see that the relatively-great bulk of the first, goes along with
-a relatively-slow agamogenesis. The common Sea-anemones are but
-occasionally observed to undergo self-division: multiplication by
-budding being in some cases largely followed, but their numbers are
-not rapidly increased by either process. A higher class of secondary
-aggregates exemplifies the same general truth with a difference. In the
-smaller members the agamogenesis is incomplete, and in the larger it
-disappears. The gemmation of the minute <i>Polyzoa</i>, though it does
-not end in the separation of the young individuals, habitually goes to
-the extent of producing families of partially independent individuals;
-but their near ally, the <i>Phoronis</i>, which immensely exceeds them
-in size, is solitary and not gemmiparous. So, too, is it with the
-<i>Ascidioida</i>. And then among the true <i>Mollusca</i>, which are
-relatively large, no such thing is known as fission or gemmation.</p>
-
-<p>Take next the <i>Annulosa</i>, including under this title the
-<i>Annelida</i> and <i>Arthropoda</i>. When treating of morphological
-composition, reasons were given for the belief that the annulose animal
-is an aggregate of the third order, the segments of which, produced
-one from another by gemmation, originally became separate; but by
-progressive integration, or arrested disintegration, there resulted a
-type in which many<span class="pagenum" id="Page_445">[445]</span> such segments were permanently united (<a href="#sect205">§§ 205–7</a>
-and note to <a href="#sect207">§ 207</a>). Part of the evidence there assigned, is evidence
-to be here repeated in illustration of the direct antagonism of Growth
-and Asexual Genesis. We saw how, among the lower Annelids, the string
-of segments produced by gemmation presently divides transversely into
-two strings; and how, in some cases, this resolution of the elongating
-string of segments into groups that are to form separate individuals,
-goes on so actively that as many as six groups are found in different
-stages of progress to ultimate independence&mdash;a fact implying a high
-rate of fissiparous multiplication.<a id="FNanchor_57" href="#Footnote_57" class="fnanchor">[57]</a> Then we saw that, in the
-superior annulose types, distinguished in the mass by including the
-larger species, fission does not occur. The higher Annelids do not
-propagate in this way; there is no known case of new individuals being
-so formed among the <i>Myriapoda</i>; nor do the Crustaceans afford us
-a single instance of this primordial mode of increase. It is, indeed,
-true that while articulate animals never multiply asexually after
-this simplest method, and while they are characterized in the mass by
-the cessation of agamogenesis of every kind, there nevertheless occur
-in a few of their small species, those higher forms of agamogenesis
-known as parthenogenesis and pseudo-parthenogenesis; and that by these
-some of them multiply very rapidly. Hereafter we shall find, in the
-interpretation of these anomalies, further support for the general
-doctrine.</p>
-
-<p>To the above evidence has to be added that which the <i>Vertebrata</i>
-present. This may be very briefly summed up. On the one hand this
-class, whether looked at in the aggregate or in its particular
-species, immensely exceeds all other classes in the sizes of its
-individuals; and on the other hand, agamogenesis under any form is
-absolutely unknown in it. If it be said that budding occurs among the
-<i>Tunicata</i> which,<span class="pagenum" id="Page_446">[446]</span> under the common title of <i>Chordata</i>,
-are included in the same phylum with the <i>Vertebrata</i>, then it
-may firstly be replied that those types which have no vertebræ cannot
-properly be called <i>Vertebrata</i>, and secondly that if, as being
-<i>Chordata</i>, they must be recognized, then the exception which they
-present further illustrates the truth that agamogenetic multiplication
-occurs only in creatures small in size, or low in structure, or both.</p>
-
-
-<p class="p2">§ 337. Such are a few leading facts serving to show how deduction
-is inductively verified, in so far as the antagonism between Growth
-and Asexual Genesis is concerned. In whatever way we explain this
-opposition of the integrative and disintegrative processes, the facts
-and their implications remain the same. Indeed we need not commit
-ourselves to any hypothesis respecting the physical causation. It
-suffices to recognize the results under their most general aspects. We
-cannot help admitting there are at work these two antagonist tendencies
-to aggregation and separation; and we cannot help admitting that the
-proportion between the aggregative and separative tendencies, must
-in each case determine the relation between increase in bulk of the
-individual and increase of the race in number.</p>
-
-<p>The antithesis is as manifest <i>à posteriori</i> as it is necessary
-<i>à priori</i>. While the minutest organisms multiply asexually in
-their billions; while the <i>Infusoria</i> thus multiply in their
-millions; while the small compound types next above them thus multiply
-in their thousands; while larger and more compound types thus multiply
-in their hundreds and their tens; the largest types do not thus
-multiply at all. Conversely, those which do not multiply asexually at
-all, are a billion or a million times the size of those which thus
-multiply with greatest rapidity; and are a thousand times, or a hundred
-times, or ten times the size of those which thus multiply with less and
-less rapidity. Without saying that this inverse proportion is regular,
-which, as we shall hereafter<span class="pagenum" id="Page_447">[447]</span> see, it cannot be, we may unhesitatingly
-assert its average truth. That the smallest organisms habitually
-reproduce asexually with immense rapidity; that the largest organisms
-never reproduce at all in this manner; and that between these extremes
-there is a general decrease of asexual reproduction along with an
-increase of bulk; are propositions which admit of no dispute.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_448">[448]</span></p>
-
-<h3>CHAPTER VI.<br />
-<span class="subhed1">ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS.</span></h3></div>
-
-
-<p>§ 338. In so far as it is a process of separation, sexual genesis is
-like asexual genesis; and is therefore, equally with asexual genesis,
-opposed to that aggregation which results in growth. Whether deduction
-is made from one parent or from two, whether it is made from any part
-of the body indifferently or from a specialized part, or whether it is
-made directly or indirectly, it remains in any case a deduction; and in
-proportion as it is great, or frequent, or both, it must restrain the
-increase of the individual.</p>
-
-<p>Here we have to group together the leading illustrations of this truth.
-We will take them in the same order as before.</p>
-
-
-<p class="p2" id="sect339">§ 339. The lowest vegetal forms, or rather, we may say, those forms
-which we cannot class as either distinctly vegetal or distinctly
-animal, show us a process of sexual multiplication that differs much
-less from the asexual process than in the higher forms. The common
-character which distinguishes sexual from asexual genesis, is that
-the mass of protoplasm whence a new generation is to arise, has been
-produced by the union of two portions of matter which were before
-more widely separated. I use this general expression because, among
-the simplest <i>Algæ</i>, this is not invariably matter supplied by
-different individuals: certain <i>Diatomaceæ</i> exhibit within a
-single cell, the formation of a sporangium by a drawing<span class="pagenum" id="Page_449">[449]</span> together of
-the opposite halves of the endochrome into a ball. Mostly, however,
-sporangia are products of conjugation. The protoplasmic contents of
-two cells unite to form the germ-mass or zygote; and these conjugating
-cells may be either entirely independent, as in many <i>Desmidiaceæ</i>
-and in the gametes of many <i>Confervoideæ</i>; or they may be two of
-the adjacent cells forming a thread, as in some <i>Conjugateæ</i> and
-the gametes of <i>Confervoideæ</i>; or they may be cells belonging
-to adjacent threads, as in other <i>Conjugateæ</i>. But whether it
-is originated by a single parent-cell, or by two parent-cells, the
-zygote, after remaining quiescent until there recur the fit conditions
-for growth, either breaks up into a multitude of spores, each of
-which produces an individual that usually multiplies asexually, or
-germinates directly to produce one new individual; and the fact here to
-be noted is, that as the entire contents of the parent-cells unite to
-form the zygote, their individualities are lost in the germs of a new
-generation. In these minute simple types, sexual propagation just as
-completely sacrifices the life of the parent or parents, as does that
-form of asexual propagation in which the protoplasm resolves itself
-directly into zoospores. And in the one case as in the other, this
-sacrifice is the concomitant of a prodigious fertility. Slightly in
-advance of this, but still showing us an almost equal loss of parental
-life in the lives of offspring, is the process seen in such unicellular
-<i>Algæ</i> as <i>Botrydium</i>, and in minute <i>Fungi</i> of the same
-degree of composition. These exhibit a relatively-enormous development
-of the spore-producing part, and an almost entire absorption of the
-parental substance into it. As evidence of the resulting powers of
-multiplication, we have but to remember that the spread of mould over
-stale food, the rapid destruction of crops by mildew, and other kindred
-occurrences, are made possible by the incalculably numerous spores thus
-generated and universally dispersed.</p>
-
-<p>Plants a degree higher in composition supply a parallel series of
-illustrations. We have among the larger <i>Fungi</i>, in<span class="pagenum" id="Page_450">[450]</span> which the
-reproductive apparatus is relatively so enormous as to constitute
-the ostensible plant, a similar subordination of the individual to
-the race, and a similarly-immense fertility. Thus, as quoted by Dr.
-Carpenter, Fries says&mdash;“in a single individual of <i>Reticularia
-maxima</i>, I have counted (calculated?) 10,000,000 sporules.” It
-needs but to note the clouds of particles, so minute as to look like
-smoke, which ripe puffballs give off when they are burst, and then to
-remember that each particle is a potential fungus, to be impressed
-with the almost inconceivable powers of propagation which these plants
-possess. The Lichens, too, furnish examples. Though they are nothing
-like so prolific as the <i>Fungi</i> (the difference yielding, as we
-shall hereafter see, further support to the general argument), yet
-there is a great production of germs, and a proportionate sacrifice
-of the parental individuality. Considerable areas of the thallus
-develop into the fruit-bodies characteristic of the various fungi
-which, combined with algæ, form the different lichens (various members
-of the <i>Ascomycetes</i> and the <i>Basidiomycetes</i>). From these
-are produced great numbers of ascospores or basidiospores, as the
-case may be. Very many lichens also reproduce themselves by means of
-<i>Soredia</i>, <i>i.e.</i>, little masses of algal cells closely
-wrapped in a weft of fungal hyphæ. Some contrasts presented by the
-higher <i>Algæ</i> may also be named as exemplifying the inverse
-proportion between the size of the individual and the extent of the
-generative structures. While in the smaller kinds relatively large
-portions of the fronds are transformed into reproductive elements,
-in the larger kinds these portions are relatively small: instance
-the <i>Macrocystis pyrifera</i>, a gigantic seaweed which sometimes
-attains a length of 1,500 feet, of which Dr. Carpenter remarks&mdash;“This
-development of the nutritive surface takes place at the expense of the
-fructifying apparatus, which is here quite subordinate.”</p>
-
-<p>When we turn to vegetal aggregates of the third order of composition,
-facts having the same meaning are conspicuous.<span class="pagenum" id="Page_451">[451]</span> On the average
-these higher plants are far larger than plants of a lower degree of
-composition; and on the average their rates of sexual reproduction are
-far less. Similarly if, among Archegoniates and Phænogams, we compare
-the smaller types with the larger, we find them proportionately more
-prolific. This is not manifest if we simply calculate the number of
-seeds ripened by an individual in a single season; but it becomes
-manifest if we take into account the further factor which here
-complicates the result&mdash;the age at which sexual genesis commences. The
-smaller Phænogams are mostly either annuals, or perennials that die
-down annually; and seeding as they do annually before their deaths, or
-the deaths of their reproductive parts, it results that in the course
-of a year each gives origin to a multitude of potential plants, of
-which every one may the next year, if preserved, give origin to an
-equal multitude. Supposing but a hundred offspring to be produced the
-first year, ten thousand may be produced in the second year, a million
-in the third, a hundred millions in the fourth. Meanwhile, what has
-been the possible multiplication of a large Phænogam? While its small
-congener has been seeding and dying, and leaving multitudinous progeny
-to seed and die, it has simply been growing; and may so continue to
-grow for ten or a dozen years without bearing fruit. Before a Cocoa-nut
-tree has ripened its first cluster of nuts, the descendants of a wheat
-plant, supposing them all to survive and multiply, will have become
-numerous enough to occupy the whole surface of the Earth. So that
-though, when it begins to bear, a tree may annually shed as many seeds
-as an herb, yet in consequence of this delay in bearing, its fertility
-is incomparably less; and its relatively-small fertility becomes still
-further reduced where, as in <i>Lodoicea callipyge</i>, the seeds take
-two years from the date of fertilization to the date of germination.</p>
-
-
-<p class="p2">§ 340. Some observers state that in certain <i>Protozoa</i>
-there<span class="pagenum" id="Page_452">[452]</span> occurs a process of conjugation akin to that which the
-<i>Protophyta</i> exhibit&mdash;a coalescence of the substance of two
-individuals to form a germ-mass. This has been alleged more especially
-of <i>Actinophrys</i>. If this statement should be proved true,<a id="FNanchor_58" href="#Footnote_58" class="fnanchor">[58]</a>
-then of the minute forms that appear to be more animal than vegetal in
-their characters, some have a mode of sexual multiplication by which
-the parents are sacrificed bodily in the production of a new generation.</p>
-
-<p>Among small animal aggregates of the second order, the first to be
-considered are of course the <i>Cœlenterata</i>. A <i>Hydra</i>
-occasionally devotes a large part of its substance to sexual genesis.
-In the walls of its body groups of ova, or spermatozoa, or both, take
-their rise; and develop into masses greatly distorting the creature’s
-form, and leaving it much diminished when they escape. Here, however,
-gamogenesis is obviously supplementary to agamogenesis&mdash;the immensely
-rapid multiplication by budding continues as long as food is abundant
-and warmth sufficient, and is replaced by gamogenesis only at the
-close of the season. A better example<span class="pagenum" id="Page_453">[453]</span> of the relation between small
-size and active gamogenesis among low types of the <i>Metazoa</i> is
-supplied by the <i>Rotifera</i>. Microscopic as these are, they have
-a great rate of sexual increase. According to Ehrenberg, <i>Hydatina
-senta</i> “is capable of a four-fold propagation every twenty-four or
-thirty hours, bringing forth in this time four ova, which grow from the
-embryo to maturity, and exclude their fertile ova in the same period.
-The same individual, producing in ten days forty eggs, developed
-with the rapidity above cited, this rate, raised to the tenth power,
-gives one million of individuals from one parent, on the eleventh day
-four millions, and on the twelfth day sixteen millions, and so on.”
-Ehrenberg, however, characterized by Huxley as “the greatest looker and
-the worst observer,” is not a safe authority, and it is better to state
-the estimate of Ludwig Plate, who says that <i>Hydatina</i> lays fifty
-eggs in two to three weeks&mdash;a number which, multiplying in the manner
-described, will yield in the time named a much smaller total though
-still an enormous total.</p>
-
-<p>The <i>Annulosa</i>, including among them the inferior types, have
-habits and conditions of life so various that only the broadest
-contrasts can be instanced in support of the proposition before us.
-The differences of organization and activity greatly complicate the
-inverse variation of fertility and bulk. Bearing in mind, however,
-that the rate of multiplication depends much less on the number of
-each brood than on the quickness with which maturity is reached and
-a new generation commenced, it will be obvious that though Annelids,
-relatively enormous in size, produce great numbers of ova, yet as they
-do this at comparatively long intervals, their rates of increase fall
-immensely below that just instanced in the Rotifers. And when at the
-other extreme we come to the large articulate animals, such as the Crab
-and the Lobster, the further diminution of fertility is seen in the
-still longer delay which occurs before each new generation begins to
-reproduce.</p>
-
-<p><span class="pagenum" id="Page_454">[454]</span></p>
-
-<p>Perhaps the best examples are supplied by vertebrate animals, and
-especially those that are most familiar to us. Comparisons between
-Fishes are unsatisfactory, because of our ignorance of their histories.
-In some cases Fishes equal in bulk produce widely different numbers of
-eggs; as the Cod which spawns millions at once, and the Salmon by which
-nothing like so great a number is spawned. But then the eggs are very
-unlike in size; and if the ovaria of the two fishes be compared, the
-difference between their masses is comparatively moderate. There are,
-indeed, contrasts which seem at variance with the alleged relation; as
-that between the Cod and the Stickleback which, though so much smaller,
-produces fewer ova. The Stickleback’s ova, however, are relatively
-large; and their total bulk bears as great a ratio to the bulk of the
-Stickleback as does the bulk of the Cod’s ova to that of the Cod.
-Moreover if, as is not improbable, the reproductive age is arrived
-at earlier by the Stickleback than by the Cod, the fertility of the
-species may be greater notwithstanding the smaller number produced
-by each individual. Evidence which admits of being tolerably well
-disentangled is furnished by Birds. They differ but little in their
-grades of organization; and the habits of life throughout extensive
-groups of them are so similar, that comparisons may be fairly made. It
-is true that, as hereafter to be shown, the differences of expenditure
-which differences of bulk entail, have doubtless much to do with the
-differences of fertility. But we may set down under the present head
-some of those cases in which the activity, being relatively slight,
-does not greatly interfere with the relation we are considering; and
-may note that among such birds having similarly slight activities, the
-small produce more eggs than the large, and eggs that bear in their
-total mass a greater ratio to the mass of the parent. Consider, for
-example, the gallinaceous birds; which are like one another and unlike
-birds of most other groups in flying comparatively little. Taking
-first the wild members of this order, which<span class="pagenum" id="Page_455">[455]</span> rarely breed more than
-once in a season, we find that the Pheasant has from 10 to 14 eggs,
-the Black-cock from 6 to 10, the Grouse 8 to 14, the Partridge 12 to
-20, the Quail still more, sometimes reaching two broods of 7 to 12 in
-each. Here the only exception to the relation between decreasing bulk
-and increasing number of eggs, occurs in the cases of the Pheasant
-and the Black-cock; and it is to be remembered, in explanation, that
-the Pheasant is constitutionally adapted to a warmer region, is
-better fed&mdash;often artificially&mdash;and leads a less active life. If we
-pass to domesticated genera of the same order, we meet with parallel
-differences. From the numbers of eggs laid, little can be inferred;
-for under the favourable conditions artificially maintained, the
-laying is carried on indefinitely. But though in the sizes of their
-broods the Turkey and the Fowl do not greatly differ, the Fowl begins
-breeding at a much earlier age than the Turkey, and produces broods
-more frequently: a considerably higher rate of multiplication being
-the result. Now these contrasts among domestic creatures which are
-similarly conditioned, and closely-allied by constitution, may be held
-to show, more clearly than most other contrasts, the inverse variation
-between bulk and sexual genesis; since here the cost of activity is
-diminished to a comparatively small amount. There is little expenditure
-in flight&mdash;sometimes almost none; and the expenditure in walking about
-is not great: there is more of standing than of actual movement. It
-is true that young Turkeys commence their existence as larger masses
-than chickens; but it is tolerably manifest that the total weight of
-the eggs laid by a Turkey during each season, bears a less ratio to
-the Turkey’s weight, than the total weight of the eggs which a Hen
-lays during each season, bears to the Hen’s weight; and this is the
-fairest way of making the comparison. The comparison so made shows
-a greater difference than appears likely to be due to the different
-costs of locomotion; considering the inertness of the creatures.
-Remembering that<span class="pagenum" id="Page_456">[456]</span> the assimilating surface increases only as the
-squares of the dimensions, while the mass of the fabric to be built up
-by the absorbed nutriment increases as the cubes of the dimensions,
-it will be seen that the expense of growth becomes relatively greater
-with each increment of size; and that hence, of two similar creatures
-commencing life with different sizes, the larger one in reaching its
-superior adult bulk, will do this at a more than proportionate expense;
-and so will either be delayed in commencing its reproduction, or will
-have a diminished reserve for reproduction, or both. Other orders
-of Birds, active in their habits, show more markedly the connexion
-between augmenting mass and declining fertility. But in them the
-increasing cost of locomotion becomes an important, and probably the
-most important, factor. The evidence they furnish will therefore
-come better under another head. Contrasts among Mammals, like those
-which Birds present, have their meanings obscured by inequalities of
-the expenditures for motion. The smaller fertility which habitually
-accompanies greater bulk, must in all cases be partly ascribed to this.
-Still, it may be well if we briefly note, for as much as they are
-worth, the broader contrasts. While a large Mammal bears but a single
-young one at a time, is several years before it commences doing this,
-and then repeats the reproduction at long intervals; we find, as we
-descend to the smaller members of the class, a very early commencement
-of breeding, an increasing number at a birth, reaching in small Rodents
-to 10 or even more, and a much more frequent recurrence of broods: the
-combined result being a relatively prodigious fertility. If a specific
-comparison be desired between Mammals that are similar in constitution,
-in food, in conditions of life, and all other things but size, the
-Deer-tribe supplies it. While the large Red-deer has but one at a
-birth, the small Roe-deer has frequently two at a birth.<a id="FNanchor_59" href="#Footnote_59" class="fnanchor">[59]</a></p>
-
-
-
-<p class="p2" id="sect341">§ 341. The antagonism between growth and sexual genesis,<span class="pagenum" id="Page_457">[457]</span> visible in
-these general contrasts, may also be traced in the history of each
-plant and animal. So familiar is the fact that sexual genesis does not
-occur early in life, and in all organisms which expend much begins only
-when the limit of size is nearly reached, that we do not sufficiently
-note its significance. It is a general physiological truth, however,
-that while the building-up of the individual is going on rapidly, the
-reproductive organs remain imperfectly developed and inactive; and that
-the commencement of reproduction at once indicates a declining rate
-of growth, and becomes a cause of arresting growth. As was shown in <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect78">§ 78</a>,
-the exceptions to this rule are found where the limit of growth is
-indefinite; either because the organism expends little or nothing in
-action, or expends in action so moderate an amount that the supply of
-nutriment is never equilibrated by its expenditure.</p>
-
-<p>We will pass over the inferior plants and, limiting ourselves to
-Phænogams, will not dwell on the less conspicuous evidence with the
-smaller types present. A few cases such as gardens supply will serve.
-All know that a Pear-tree increases in size for years before it begins
-to bear; and that, producing but few pears at first, it is long before
-it fruits abundantly. A young Mulberry-tree, branching out luxuriantly
-season after season, but covered with nothing but leaves, at length
-blossoms sparingly and sets some small and imperfect berries, which
-it drops while they are green; and it makes these futile attempts
-time after<span class="pagenum" id="Page_458">[458]</span> time before it succeeds in ripening any seeds. But these
-multiaxial plants, or aggregates of individuals some of which continue
-to grow while others become arrested and transformed into seed-bearers,
-show us the relation less definitely than certain plants that are
-substantially, if not literally, uniaxial. Of these the Cocoa-nut may
-be instanced. For some years it goes on shooting up without making
-any sign of becoming fertile. About the sixth year it flowers; but
-the flowers wither without result. In the seventh year it flowers and
-produces a few nuts; but these prove abortive and drop. In the eighth
-year it ripens a moderate number of nuts; and afterwards increases
-the number until, in the tenth year, it comes into full bearing.
-Meanwhile, from the time of its first flowering its growth begins
-to diminish, and goes on diminishing till the tenth year, when it
-ceases. Here we see the antagonism between growth and sexual genesis
-under both its aspects&mdash;see a struggle between self-evolution and
-race-evolution, in which the first for a time overcomes the last, and
-the last ultimately overcomes the first. The continued aggrandizement
-of the parent-individual makes abortive for two seasons the tendency to
-produce new individuals; and the tendency to produce new individuals,
-becoming more decided, stops any further aggrandizement of the parent
-individual.</p>
-
-<p>Parallel illustrations occur in the animal kingdom. The eggs laid by a
-pullet are relatively small and few. Similarly, it is alleged that, as
-a general rule, “a bitch has fewer puppies at first, than afterwards.”
-According to Burdach, as quoted by Dr. Duncan, “the elk, the bear, &amp;c.,
-have at first only a single young one, then they come to have most
-frequently two, and at last again only one. The young hamster produces
-only from three to six young ones, while that of a more advanced age
-produces from eight to sixteen. The same is true of the pig.” It is
-remarked by Buffon that when a sow of less than a year old has young,
-the number of the litter is small, and its members are feeble and even
-imperfect.<span class="pagenum" id="Page_459">[459]</span> Here we have evidence that in animals growth checks sexual
-genesis. And then, on the other hand, we have evidence that sexual
-genesis checks growth. It is well known to breeders that if a filly
-is allowed to bear a foal, she is thereby prevented from reaching
-her proper size. And a like loss of perfection as an individual,
-is suffered by a cow which breeds too early. It may be added, as a
-converse fact, that castrated animals, as capons and notably cats,
-often become larger than their unmutilated associates.</p>
-
-
-<p class="p2">§ 342. Notwithstanding the way in which the inverse variation of
-growth and sexual genesis is complicated with other relations, its
-existence is, I think, sufficiently manifest. Individually, many of
-the foregoing instances are open to criticism, and have to be taken
-with qualifications; but when looked at in the mass their meaning is
-beyond doubt. Comparisons between the largest with the smallest types,
-whether vegetal or animal, yield results which are unmistakable. On
-the one hand, remembering the fact that during its centuries of life
-an Oak does not produce as many acorns as a Fungus does spores in a
-single night, we see that the Fungus has a fertility exceeding that
-of the Oak in a degree literally beyond our powers of calculation or
-imagination. On the other hand when, taking a microscopic protophyte
-which has billions of descendants in a few days, we ask how many such
-would be required to build up the forest tree which is years before
-it drops a seed, we are met by a parallel difficulty in conceiving
-the number, if not in setting it down. Similarly, if from the minute
-and prodigiously-fertile Rotifer we turn to the Elephant, which
-approaches thirty years before it bears a solitary young one, we find
-the connexions between small size and great fertility and between great
-size and small fertility, too intensely marked to be much disguised by
-the perturbing relations that have been indicated. Finally, as this
-induction, reached by a survey of organisms in general, is verified by
-observations on the relation<span class="pagenum" id="Page_460">[460]</span> between decreasing growth and commencing
-reproduction in individual organisms, we may, I think, consider the
-alleged antagonism as proved.<a id="FNanchor_60" href="#Footnote_60" class="fnanchor">[60]</a></p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_461">[461]</span></p>
-
-<h3>CHAPTER VII.<br />
-<span class="subhed1">THE ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS, ASEXUAL AND SEXUAL.</span></h3></div>
-
-
-<p>§ 343. By Development, as here to be dealt with apart from Growth, is
-meant increase of structure as distinguished from increase of mass. As
-was pointed out in <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect50">§ 50</a>, this is the biological definition of the word.
-In the following sections, then, we have to note how complexity of
-organization is hindered by reproductive activity, and conversely.</p>
-
-<p>This relation partially coincides with that which we have just
-contemplated; for, as was shown in <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect44">§ 44</a>, degree of growth is to a
-considerable extent dependent on degree of organization. But while the
-antagonism to be illustrated in this chapter is much entangled with
-that illustrated in the last chapter, it may be so far separated as to
-be identified as an additional antagonism.</p>
-
-<p>Besides the direct opposition between that continual disintegration
-which rapid genesis implies, and the fulfilment of that pre-requisite
-to extensive organization&mdash;the formation of an extensive aggregate,
-there is an indirect opposition which we may recognize under several
-aspects. The change from homogeneity to heterogeneity takes time;
-and time taken in transforming a relatively-structureless mass into
-a developed individual, delays the period of reproduction. Usually
-this time is merged in that taken for growth; but certain cases of
-metamorphosis show us the one separate from the other. An insect,
-passing from its lowly-organized caterpillar-stage<span class="pagenum" id="Page_462">[462]</span> into that of
-chrysalis, is afterwards a week, a fortnight, or a longer period in
-completing its structure: the re-commencement of genesis being by so
-much postponed, and the rate of multiplication therefore diminished.
-Further, that re-arrangement of substance which development implies,
-entails expenditure. The chrysalis loses weight in the course of its
-transformation; and that its loss is not loss of water only, may
-be inferred from the fact that it respires, and that respiration
-indicates consumption. Clearly the matter consumed is, other things
-equal, a deduction from the surplus which may go to reproduction.
-Yet again, the more widely and completely an organic mass becomes
-differentiated, the smaller is the portion of it which retains the
-relatively-undifferentiated state that admits of being moulded into
-new individuals, or the germs of them. Protoplasm which has become
-specialized tissue cannot be generalized afresh, and afterwards
-transformed into something else; and hence the progress of structure
-in an organism, by diminishing the unstructured part, diminishes the
-amount available for making offspring.</p>
-
-<p>It is true that higher structure, like greater growth, may insure to
-a species advantages which eventually further its multiplication&mdash;may
-give it access to larger supplies of food, or enable it to obtain food
-more economically; and we shall hereafter see how the inverse variation
-we are considering is thus qualified. But here we are concerned
-only with the necessary and direct effects; not with those that are
-contingent and remote. These necessary and direct effects we will now
-look at as exemplified.</p>
-
-
-<p class="p2">§ 344. Speaking generally, the simpler plants propagate both
-sexually and asexually; and, speaking comparatively, the complex
-plants propagate only sexually: their asexual propagation is usually
-incomplete&mdash;produces a united aggregate of individuals instead of
-numerous distinct individuals. The Protophytes that perpetually
-subdivide, the merely<span class="pagenum" id="Page_463">[463]</span> cellular <i>Algæ</i> that shed their
-tetraspores, the Archegoniates that spontaneously separate their
-fronds or drop their gemmæ, show us an extra mode of multiplication
-which, among flowering plants, is exceptional. This extra mode of
-multiplication among these simpler plants, is made easy by their
-low development. Tetraspores arise only where the frond consists of
-untransformed cells; gemmæ bud out and drop off only where the tissue
-is comparatively homogeneous.</p>
-
-<p>Should it be said that this is but another aspect of the antagonism
-already set forth, since these undeveloped forms are also the smaller
-forms; the reply is that though in part true this is not wholly true.
-Various marine <i>Algæ</i> which propagate asexually, are larger
-than some Phænogams which do not thus propagate. The objection that
-difference of medium vitiates this comparison, is met by the fact
-that it is the same among land-plants themselves. Sundry of the
-lowly-organized Liverworts which are habitually gemmiparous, exceed in
-size many flowering plants. And the Ferns show us agamic multiplication
-occurring in plants which, while they are inferior in complexity of
-structure, are superior in bulk to numbers of annual Monocotyledons and
-Dicotyledons.</p>
-
-
-<p class="p2">§ 345. In the ability of the lowly-organized substance of a Sponge to
-transform itself into multitudes of gemmules, we have an instance of
-this same direct relation in the animal kingdom. Moreover, the instance
-yields very distinct proof of an antagonism between development and
-genesis, independent of the antagonism between growth and genesis; for
-the Sponge which thus multiplies itself asexually, as well as sexually,
-is far larger than hosts of more complex animals which do not multiply
-asexually.</p>
-
-<p>Once again may be cited the creature so often brought in evidence,
-the <i>Hydra</i>, as showing us how rapidity of agamic propagation
-is associated with inferiority of structure. Its power to produce
-young ones from nearly all parts of its body, is due to the
-comparative homogeneity of its body. In<span class="pagenum" id="Page_464">[464]</span> kindred but more-organized
-types, the gemmiparity is greatly restricted, or disappears. Among
-the free-swimming <i>Hydrozoa</i>, multiplication by budding, when
-it occurs at all, occurs only at special places. That increase of
-structure apart from increase of size, is here a cause of declining
-agamogenesis, we may see in the contrast between the simple
-<i>Hydra</i> and the compound Hydroids. These last, along with
-more-differentiated tissues, show us a gemmation which does not go
-on all over the body of each polype, and much of it does not end in
-separation.</p>
-
-<p>It is, however, among the <i>Annulosa</i> that progressing organization
-is most conspicuously operative in diminishing agamogenesis. The
-segments or “somites” composing an animal belonging to this class, are
-primordially alike; and, as before argued (<a href="#sect205">§§ 205–7</a>), are probably
-the homologues of what were originally independent individuals. The
-progress from the lower to the higher types of the class, is at once
-a progress towards types in which the strings of segments cease to
-undergo subdivision, and towards types in which the segments, no longer
-alike in their structures and functions, have become physiologically
-integrated or mutually dependent. Already this group of cases has
-been named as illustrating the antagonism between growth and asexual
-genesis; but it is proper also to name it here, since, on the one hand,
-the greater size due to the ceasing of fission, is made possible only
-by the specialization of parts and the development of a co-ordinating
-apparatus to combine their actions, and since, on the other hand,
-specialization and co-ordination can advance only in proportion as
-fission ceases.</p>
-
-
-<p class="p2" id="sect346">§ 346. The inverse variation of development and sexual genesis is by no
-means easy to follow. One or two facts indicative of it may, however,
-be named.</p>
-
-<p>Phænogams that have but little supporting tissue may fairly be classed
-as structurally inferior to those having stems with a bulky and
-complex woody system; for these imply<span class="pagenum" id="Page_465">[465]</span> additional differentiations,
-and constitute wider departures from the primitive type of vegetal
-tissue. That the concomitant of this higher organization is a slower
-gamogenesis, scarcely needs pointing out. While the herbaceous annual
-is blossoming and ripening seed, the young tree is transforming its
-originally-succulent axis into dense fibrous substance; and year
-by year the young tree expends in doing the like, nutriment which
-successive generations of the annual expend in fruit. Here the inverse
-relation is between sexual reproduction and complexity, and not between
-sexual reproduction and bulk, seeing that besides seeding, the annual
-often grows to a size greater than that reached by the young infertile
-tree in several years.</p>
-
-<p>Proof of the antagonism between complexity and gamogenesis in animals,
-is still more difficult to disentangle. Perhaps the evidence most to
-the point is furnished by the contrast between Man and certain other
-Mammals approaching him in mass. To compare him with the domestic Sheep
-which, though not very unlike in size, is relatively prolific, is
-objectionable because of the relative inactivity of Sheep; and this,
-too, may be alleged as a reason why the Ox, though far more bulky, is
-also far more fertile, than Man. Further, against a comparison with
-the Horse which, while both larger and more prolific, is tolerably
-active, it may be urged that in his case, and the cases of herbivorous
-creatures generally, the small exertion required to procure food,
-joined with the great ratio borne by the alimentary organs to the
-organs they have to build up and repair, vitiates the result. We may,
-however, fairly draw a parallel between Man and a large carnivore. The
-Lion, superior in size, and perhaps equal in activity, has a digestive
-system not proportionately greater; and yet has a higher rate of
-multiplication than Man. Here the only decided want of parity, besides
-that of organization, is that of food. Possibly a carnivore gains an
-advantage in having a surplus nutriment consisting almost wholly of
-those nitrogenous materials from which the bodies<span class="pagenum" id="Page_466">[466]</span> of young ones are
-mainly formed. But, allowing for all other differences, it appears not
-improbable that the smallness of human fertility compared with the
-fertility of large feline animals, is due to the greater complexity of
-the human organization&mdash;more especially the organization of the nervous
-system. Taking degree of nervous organization as the chief correlative
-of mental capacity; and remembering the physiological cost of that
-slow evolution whereby high mental capacity is reached; we may suspect
-that nervous organization is very expensive: the inference being that
-bringing it up to the level it reaches in Man, whose digestive system,
-by no means large, has at the same time to supply materials for general
-growth and daily waste, involves a great retardation of maturity and
-sexual genesis.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_467">[467]</span></p>
-
-<h3>CHAPTER VIII.<br />
-<span class="subhed1">ANTAGONISM BETWEEN EXPENDITURE AND GENESIS.</span></h3></div>
-
-
-<p>§ 347. Under this head we have to set down no evidence derived from the
-vegetal kingdom. Plants are not expenders of force in such degrees as
-to affect the general relations with which we are dealing. They have
-not to maintain a heat above that of their environment, nor have they
-to generate motion; and hence consumption for these two purposes does
-not diminish the stock of material which serves on the one hand for
-growth and on the other hand for propagation.</p>
-
-<p>It will be well, too, if we pass over the lower animals: especially
-those aquatic ones which, being nearly of the same temperature as
-the water, and nearly of the same specific gravity, lose but little
-in evolving motion, sensible and insensible. A further reason for
-excluding from consideration these inferior types, is that we do not
-know enough of their rates of genesis to permit of our making, with any
-satisfaction, those involved comparisons here to be entered upon.</p>
-
-<p>The facts on which we must mainly depend are those to be gathered
-from terrestrial animals, and chiefly from those higher classes of
-them which are at the same time great expenders and have rates of
-multiplication about which our knowledge is tolerably definite. We
-will restrict ourselves, then, to the evidence which Birds and Mammals
-supply.</p>
-
-
-<p class="p2">§ 348. Satisfactory proof that loss of substance in the<span class="pagenum" id="Page_468">[468]</span> maintenance of
-heat diminishes the rapidity of propagation, is difficult to obtain.
-It is, indeed, obvious that the warmblooded <i>Vertebrata</i> are less
-prolific than the cold-blooded; but then they are at the same time
-more vivacious. Similarly, between Mammals and Birds (which are the
-warmer-blooded of the two) there is, other things equal, a parallel,
-though much smaller, difference; but here, too, the unlikenesses of
-muscular action complicate the evidence. Again, the annual return of
-generative activity has an average correspondence with the annual
-return of a warmer season, which, did it stand alone, might be taken
-as evidence that a diminished cost of heat-maintenance leads to such a
-surplus as makes reproduction possible. But then, this periodic rise of
-temperature is habitually accompanied by an increase in the quantity
-of food&mdash;a factor of equal or greater importance. We must be content,
-therefore, with such few special facts as admit of being disentangled.</p>
-
-<p>Certain of these we are introduced to by the general relation last
-named&mdash;the habitual recurrence of genesis with the recurrence of
-spring. For in some cases a domesticated creature has its supplies
-of food almost equalized; and hence the effect of varying nutrition
-may be in great part eliminated from the comparison. The common
-Fowl yields an illustration. It is fed through the cold months, but
-nevertheless, in mid-winter, it either wholly leaves off laying or
-lays very sparingly. And then we have the further evidence that if it
-lays sparingly, it does so only on condition that the heat, as well as
-the food, is artificially maintained. Hens lay in cold weather only
-when they are kept warm. To which fact may be added the kindred one
-that “when pigeons receive artificial heat, they not only continue to
-hatch longer in autumn, but will recommence in spring sooner than they
-would otherwise do.” An analogous piece of evidence is that, in winter,
-inadequately sheltered Cows either cease to give milk or give it in
-diminished quantity. For though giving milk is not the same thing as
-bearing a young one, yet, as milk<span class="pagenum" id="Page_469">[469]</span> is part of the material from which
-a young one is built up, it is part of the outlay for reproductive
-purposes, and diminution of it is a loss of reproductive power. Indeed
-the case aptly illustrates, under another aspect, the struggle between
-self-preservation and race-preservation. Maintenance of the cow’s life
-depends on maintenance of its heat; and maintenance of its heat may
-entail such reduction in the supply of milk as to cause the death of
-the calf.</p>
-
-<p>Evidence derived from the habits of the same or allied genera in
-different climates, may naturally be looked for; but it is difficult
-to get, and it can scarcely be expected that the remaining conditions
-of existence will be so far similar as to allow of a fair comparison
-being made. The only illustrative facts I have met with which seem
-noteworthy, are some named by Mr. Gould in his work on <i>The Birds
-of Australia</i>. He says:&mdash;“I must not omit to mention, too, the
-extraordinary fecundity which prevails in Australia, many of its
-smaller birds breeding three or four times in a season; but laying
-fewer eggs in the early spring when insect life is less developed,
-and a greater number later in the season, when the supply of insect
-food has become more abundant. I have also some reason to believe
-that the young of many species breed during the first season, for
-among others, I frequently found one section of the Honey-eaters (the
-<i>Melithrepti</i>) sitting upon eggs while still clothed in the
-brown dress of immaturity; and we know that such is the case with the
-introduced <i>Gallinaceæ</i> (or poultry) three or four generations of
-which have been often produced in the course of a year.” Though here
-Mr. Gould refers only to variation in the quantity of food as a cause
-of variation in the rate of multiplication, may we not suspect that
-warmth is a part-cause of the high rate which he describes as general?</p>
-
-
-<p class="p2">§ 349. Of the inverse variation between activity and genesis, we get
-clear proof. Let us begin with that which Birds furnish.</p>
-
-<p><span class="pagenum" id="Page_470">[470]</span></p>
-
-<p>First we have the average contrast, already hinted, between the
-fertility of Birds and the fertility of Mammals. Comparing the large
-with the large and the small with the small, we see that creatures
-which continually go through the muscular exertion of sustaining
-themselves in the air and propelling themselves rapidly through it,
-are less prolific than creatures of equal weights which go through the
-smaller exertion of moving about over solid surfaces. Predatory Birds
-have fewer young ones than predatory Mammals of approximately the same
-sizes. If we compare Rooks with Rats, or Finches with Mice, we find
-like differences. And these differences are greater than at first
-appears. For whereas among Mammals a mother is able, unaided, to bear
-and suckle and rear half-way to maturity, a brood that probably weighs
-more in proportion than does the brood of a Bird; a Bird, or at least
-a Bird that flies much, is unable to do this. Both parents have to
-help; and this indicates that the margin for reproduction in each adult
-individual is smaller.</p>
-
-<p>Among Birds themselves occur contrasts which may be next considered.
-In the Raptorial class, various species of which, differing in their
-sizes, are similarly active in their habits, we see that the small
-are more prolific than the large. The Golden Eagle has usually 2
-eggs: sometimes 3, sometimes only 1. As we descend to the Kites and
-Falcons, the number is 2 or 3, and 3 or 4. And when we come to the
-Sparrow-Hawk, 3 to 5 is the specified number. Similarly among the
-Owls: while the Great Eagle-Owl has 2 or 3 eggs, the comparatively
-small Common Owl has 4 or 5. As before hinted, it is impossible to say
-what proportions of these differences are due to unlikenesses of bulk
-merely, and what proportions are due to unlikenesses in the costs of
-locomotion. But we may fairly assume that the unlikenesses in the costs
-of locomotion are here the more important factors. Weights varying as
-the cubes of the dimensions, while the surfaces of digestive systems
-vary as the squares, the expense of flight increases more rapidly than
-does the ability to take<span class="pagenum" id="Page_471">[471]</span> in nutriment; and as motion through the
-air requires more effort than motion on the ground, this geometrical
-progression tells more rapidly on Birds than on Mammals. Be this as
-it may, however, these contrasts support the argument; as do various
-others which may be set down. The Finch-family, for example, have
-broods averaging about 5 in number, and have commonly 2 broods in
-the season; while in the Crow-family the number of the brood is on
-the average less, and there is but one brood in the season. And then
-on descending to such small birds as the Wrens and the Tits, we have
-8, 10, 12 to 15 eggs, and sometimes two broods in the year. One of
-the best illustrations is furnished by the Swallow-tribe, throughout
-which there is little or no difference in mode of life or in food. The
-Sand-Martin, much the least of them, has 4 to 6 eggs and two broods;
-the Swallow, somewhat larger, has 4 or 5; and the Swift (similar in
-habits though unrelated), larger still, has but 2. Here we see a lower
-fertility associated in part with greater size, but associated still
-more conspicuously with greater expenditure. For the difference of
-fertility is more than proportionate to the difference of bulk, as
-shown in other cases; and for this greater difference there is the
-reason, that the Swift has to support not only the cost of propelling
-its larger mass through the air, but also the cost of propelling it at
-a higher velocity.</p>
-
-<p>Omitting much evidence of like nature, let us note that disclosed by
-comparisons of certain groups of birds with other groups. “Skulkers”
-is the descriptive title applied to the Water-Rail, the Corn-Crake,
-and their allies, which evade enemies by concealment&mdash;consequently
-expending but little in locomotion. These birds have relatively large
-broods&mdash;6 to 11, 8 to 12, &amp;c. Not less instructive are the contrasts
-between the Gallinaceous Birds and other Birds of like sizes but more
-active habits. The Partridge and the Wood-Pigeon are about equal in
-bulk and have much the same food. Yet while the one has from 12 to 20
-young ones, the other has but 2 young ones twice a-year: its annual
-reproduction is less than<span class="pagenum" id="Page_472">[472]</span> one-third. It may be said that the ability
-of the Partridge to bring up so large a brood, is due to that habit of
-its tribe which one of its names, “Scrapers,” describes; and to the
-accompanying habit of the young, which begin to get their own living as
-soon as they are hatched: so saving the parents’ labour. Conversely,
-it may be said that the inability of Pigeons to rear more than 2 at a
-time, is caused by the necessity of fetching everything they eat. But
-the alleged relation holds nevertheless. On the one hand, a great part
-of the food which the Partridge chicks pick up, is food which, in their
-absence, the mother would have picked up. Though each chick costs her
-far less than a young Pigeon costs its parents, yet the whole of her
-chicks cost her a great deal in the shape of abstinence&mdash;an abstinence
-she can bear because she has to fly but little. On the other hand,
-the Pigeon’s habit of laying and hatching but two eggs, must not be
-referred to any foreseen necessity of going through so much labour in
-supporting the young, but to a constitutional tendency established by
-such labour. This is proved by the curious fact that when domesticated,
-and saved from such labour by artificial feeding, Pigeons, says
-Macgillivray (quoting Aitkin), “are frequently seen sitting on eggs
-long before the former brood is able to leave the nest, so that the
-parent bird has at the same time young birds and eggs to take care of.”</p>
-
-
-<p class="p2">§ 350. Made to illustrate the effect of activity on fertility, most
-comparisons among Mammals are objectionable: other circumstances are
-not equal. A few, however, escape this criticism.</p>
-
-<p>One is that between the Hare and the Rabbit. These are closely-allied
-species of the same genus, similar in their diet but unlike in their
-expenditures for locomotion. The relatively-inert Rabbit has 6 young
-ones in a litter, and four litters a-year; while the relatively-active
-Hare has but 2 to 5 in a litter. This is not all. The Rabbit begins to
-breed at six months old; but a year elapses before the Hare begins<span class="pagenum" id="Page_473">[473]</span>
-to breed. These two factors compounded, result in a difference of
-fertility far greater than can be ascribed to unlikeness of the two
-creatures in size.</p>
-
-<p>Perhaps the most striking piece of evidence which Mammals furnish,
-is the extreme infertility of our common Bat. The <i>Cheiroptera</i>
-and the <i>Rodentia</i> are not very dissimilar in their internal
-structures. Diversity of constitution, therefore, cannot vitiate the
-comparison between Bats and Mice, which are about the same in size.
-Though their diets differ, the difference is in favour of the Bat:
-its food being exclusively animal while that of the Mouse is mainly
-vegetal. What now are their respective rates of genesis? The Mouse has
-several litters in a year of 5 to 7 in each; while the Bat produces
-only one at a time. Whether the Bat repeats its one more frequently
-than the Mouse repeats its 7 is not stated; but it is quite certain
-that even if it does so (an absurd supposition), the more frequent
-repetition cannot be such as to raise its fertility to anything like
-that of the Mouse. And this relatively-low rate of multiplication we
-may fairly ascribe to its relatively-high rate of expenditure.</p>
-
-<p>Here let us note, in passing, an interesting example of the
-way in which a species which has no specially-great power of
-self-preservation, while its power of multiplication is extremely
-small, nevertheless avoids extinction because it has to meet an
-unusually-small total of race-destroying forces. Leaving out parasites,
-the only enemy of the Bat is the Owl; and the Owl is sparingly
-distributed.</p>
-
-
-<p class="p2">§ 351. These general evidences may be enforced by some special
-evidences. We have few opportunities of observing how, within the
-same species, variations of expenditure are related to variations of
-fertility. But a fact or two showing the connexion may be named.</p>
-
-<p>Doctor Duncan quotes a statement to the point respecting the breeding
-of dogs. Already in <a href="#sect341">§341</a> I have extracted a part of this statement,
-to the effect that before her growth is complete,<span class="pagenum" id="Page_474">[474]</span> a bitch bears at a
-birth fewer puppies than when she becomes full-grown. An accompanying
-allegation is, that her declining vigour is shown by a decrease in the
-number of puppies contained in a litter, “ending in one or two.” And
-then it is further alleged that, “as regards the amount of work a dog
-has to perform, so will the decline be rapid or gradual; and hence, if
-a bitch is worked hard year after year, she will fail rapidly, and the
-diminution of her puppies will be accordingly; but if worked moderately
-and well kept, she will fail gradually, and the diminution will be less
-rapid.”</p>
-
-<p>In this place, more fitly than elsewhere, may be added a fact of like
-implication, though of a different order. Of course whether excessive
-expenditure be in the continual repairs of nervo-muscular tissues or in
-replacing other tissues, the reactive effects, if not quite the same,
-will be similar&mdash;there will be a decrease of the surplus available for
-genesis. If, then, in any animals there from time to time occur unusual
-outlays for self-maintenance, we may expect the periods of such outlays
-to be periods of diminished or arrested reproduction. That they are so
-the moulting of birds shows us. When hens begin to moult they cease to
-lay. While they are expending so much in producing new clothing, they
-have nothing to expend for producing eggs.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_475">[475]</span></p>
-
-<h3>CHAPTER IX.<br />
-<span class="subhed1">COINCIDENCE BETWEEN HIGH NUTRITION AND GENESIS.</span></h3></div>
-
-
-<p>§ 352. Under this head may be grouped various facts which, in another
-way, tell the same tale as those contained in the last chapter. The
-evidence there put together went to show that increased cost of
-self-maintenance entailed decreased power of propagation. The evidence
-to be set down here, will go to show that power of propagation is
-augmented by making self-maintenance unusually easy. For into this may
-be translated the effect of abundant food.</p>
-
-<p>To put the proposition more specifically&mdash;we have seen that after
-individual growth, development, and daily consumption, have
-been provided for, the surplus nutriment measures the rate of
-multiplication. This surplus may be raised in amount by such changes in
-the environment as bring a larger supply of the materials or forces on
-which both parental life and the lives of offspring depend. Be there,
-or be there not, any expenditure, a higher nutrition will make possible
-a greater propagation. We may expect this to hold both of agamogenesis
-and of gamogenesis; and we shall find that it does so.</p>
-
-
-<p class="p2">§ 353. On multiaxial plants, the primary effect of surplus nutriment
-is a production of large and numerous leaf-shoots. How this asexual
-multiplication results from excessive nutrition, is well shown when the
-leading axis, or a chief branch, is broken off towards its extremity.
-The axillary buds below<span class="pagenum" id="Page_476">[476]</span> the breakage quickly swell and burst into
-lateral shoots, which often put forth secondary shoots: two generations
-of agamic individuals arise where there probably would have been none
-but for the local abundance of sap, no longer drawn off. In like manner
-the abnormal agamogenesis which we have in proliferous flowers, is
-habitually accompanied by a general luxuriance, implying an unusual
-plethora.</p>
-
-<p>No less conclusive is the evidence furnished by agamogenesis in
-animals. Sir John Dalyell, speaking of <i>Hydra tuba</i>, and of the
-period before strobilization commences, says&mdash;“It is singular how much
-propagation is promoted by abundant sustenance.” This Polype goes on
-budding out young polypes from its sides, with a rapidity proportionate
-to the supply of materials. So, too, is it with the agamic reproduction
-of the <i>Aphis</i>. As cited by Professor Huxley, Kyber “states that
-he raised viviparous broods of both this species (<i>Aphis Dianthi</i>)
-and <i>A. Rosæ</i> for four consecutive years, without any intervention
-of males or oviparous females, and that the energy of the power of
-agamic reproduction was at the end of that period undiminished. The
-rapidity of the agamic proliferation throughout the whole period was
-directly proportional to the amount of warmth and food supplied.”</p>
-
-<p>In these cases the relation is not appreciably complicated by
-expenditure. The parent having reached its limit of growth, the
-absorbed food goes to asexual multiplication: scarcely any being
-deducted for the maintenance of parental life.</p>
-
-
-<p class="p2">§ 354. The sexual multiplication of organisms under changed conditions,
-undergoes variations conforming to a parallel law. Cultivated plants
-and domesticated animals yield us proof of this.</p>
-
-<p>Facts showing that in cultivated plants sexual genesis increases with
-nutrition, are obscured by facts showing that a less rapid asexual
-genesis, and an incipient sexual genesis,<span class="pagenum" id="Page_477">[477]</span> accompany the fall from a
-high to a moderate nutrition. The confounding of these two relations
-has led to mistaken inferences. When treating of Genesis inductively,
-we reached the generalization that “the products of a fertilized germ
-go on accumulating by simple growth, so long as the forces whence
-growth results are greatly in excess of the antagonist forces; but
-that when diminution of the one set of forces, or increase of the
-other, causes a considerable decline in this excess, and an approach
-towards equilibrium, fertilized germs are again produced.” (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect78">§ 78</a>.)
-It was pointed out that this holds of organisms which multiply by
-heterogenesis, as well as those which multiply by homogenesis.
-And plants were referred to as illustrating, both generally and
-locally, the decline of agamic multiplication and commencement of
-gamic multiplication, along with a lessening rate of nutrition.
-Now the many cases which are given of fruitfulness caused in trees
-by depletion, are really cases of this change from agamogenesis
-to gamogenesis; and simply go to prove that what would naturally
-arise when decreased peripheral growth had followed increased size,
-may be brought about artificially by diminishing the supply of
-materials for growth. Cramping its roots in a pot, or cutting them,
-or ringing its branches, will make a tree bear very early: bringing
-about a premature establishment of that relative innutrition which
-would have spontaneously arisen in course of time. Such facts by
-no means show that in plants sexual genesis increases as nutrition
-diminishes. When it has once set in, sexual genesis is scanty or
-imperfect unless nutrition is good. Though the starved plant may
-blossom, yet many of its blossoms will fail; and such seeds as it
-produces will be ill-furnished with those enveloping structures and
-that store of albumen, &amp;c., needed to give good chances of successful
-germination&mdash;the number of surviving offspring will be diminished. Were
-it otherwise, the manuring of fields which are to bear seed-crops,
-would be not simply useless but injurious. Were it otherwise, dunging
-the roots of a fruit-tree would in<span class="pagenum" id="Page_478">[478]</span> all cases be impolitic; instead
-of being impolitic only where the growth of sexless axes is still
-luxuriant. Were it otherwise, a tree which has borne a heavy crop
-should, by the consequent depletion, be led to bear a still heavier
-crop next year; whereas it is apt to be wholly or partially barren next
-year&mdash;has to recover a state of tolerably-high nutrition before its
-sexual genesis again becomes large.</p>
-
-<p>But the best illustrations are yielded by animals&mdash;those animals at
-least in which we have, besides an increased supply of nutriment, a
-diminished expenditure. Two classes of comparisons, alike in their
-implications, may be made&mdash;comparisons between tame and wild animals of
-the same species or genus, and comparisons between tame animals of the
-same species differently treated.</p>
-
-<p>To begin with Birds, let us first contrast the farm-yard
-<i>Gallinaceæ</i> with their kindred of the fields and woods.
-Notwithstanding their greater size, which, other things equal, should
-be accompanied by smaller fertility, the domesticated kinds have more
-numerous offspring than the wild kinds. A Turkey has a dozen in a
-brood, while a Pheasant has from 6 to 10. Twice or thrice in a season,
-a Hen rears as many chickens as a Partridge rears once in a season.
-Anserine birds show us parallel differences. The Tame Goose sits on
-13 to 18 eggs and often sits a second time; but the Wild Goose sits
-on 5, 6, or 7, and these are noted as considerably smaller. It is the
-same with Ducks. The domesticated variety lays and hatches twice as
-many eggs as the wild variety. And the like holds of Pigeons. After
-remarking of the <i>Columba livia</i> that “in spring when they have
-plenty of corn to pick from the newly-sown fields, they begin to
-get fat and pair; and again in harvest, when the corn is cut down,”
-Macgillivray goes on to say that “the same pair when tamed generally
-breed four times” in the year. That between different poultry-yards
-inequalities of fertility are caused by inequalities in the supplies
-of food, is a familiar truth. High feeding shows its effects not only
-in the continuous<span class="pagenum" id="Page_479">[479]</span> laying, but also in the sizes of the eggs. Among
-directions given for obtaining eggs from pullets late in the year,
-it is especially insisted on that they shall have a generous diet.
-Respecting Pigeons Macgillivray writes:&mdash;“that their breeding depends
-much on their having plenty of food to fatten them, seems, I think,
-evident from the circumstance that, when tamed, which they easily are,
-they are observed to breed in every month of the year. I do not mean
-that the same pair will breed every month; but some in the flock, if
-well fed, will breed at any season.” There may be added a fact of like
-meaning which partially-domesticated birds yield. The Sparrow is one of
-the Finch tribe that has taken to the neighbourhood of houses; and by
-its boldness secures food not available to its congeners. The result
-is that it has several broods in a season, while its field-haunting
-kindred have none of them more than two broods, and some have only one.</p>
-
-<p>Equally clear proof that abundant nutriment raises the rate of
-multiplication, occurs among Mammals. Compare the litters of the Dog
-with the litters of the Wolf and the Fox. Whereas those of the one
-range in number from 6 to 14, those of the others contain respectively
-5 or 6 or occasionally 7, and 4 or 5 or rarely 6. Again, the Wild Cat
-has 4 or 5 kittens; but the tame Cat has 5 or 6 kittens 2 or 3 times
-a-year. So, too, is it with the Weasel tribe. The Stoat has 5 young
-ones once a-year. The Ferret has 2 litters yearly, each containing
-from 6 to 9; and this notwithstanding that it is the larger of the
-two. Perhaps the most striking contrast is that between the wild and
-tame varieties of the Pig. While the one produces, according to its
-age, from 4 to 8 or 10 young ones once a year, the other produces
-sometimes as many as 17 in a litter; or, in other cases, will bring
-up 5 litters of 10 each in two years&mdash;a rate of reproduction which is
-unparalleled in animals of as large a size.<a id="FNanchor_61" href="#Footnote_61" class="fnanchor">[61]</a> And let us not omit to
-note that this excessive fertility occurs where there is the<span class="pagenum" id="Page_480">[480]</span> greatest
-inactivity&mdash;where there is plenty to eat and nothing to do. There is no
-less distinct evidence that among domesticated Mammals themselves, the
-well-fed individuals are more prolific than the ill-fed individuals. On
-the high and comparatively-infertile Cotswolds, it is unusual for ewes
-to have twins; but they very commonly have twins in the adjacent rich
-valley of the Severn. Similarly, among the barren hills of the west of
-Scotland, two lambs will be borne by about one ewe in twenty; whereas
-in England, something like one ewe in three will bear two lambs. Nay,
-in rich pastures, twins are more frequent than single births; and it
-occasionally happens that, after a genial autumn and consequent good
-grazing, a flock of ewes will next spring yield double their number of
-lambs&mdash;the triplets balancing the uniparæ. So direct is this relation,
-that I have heard a farmer assert his ability to foretell, from the
-high, medium, or low, condition of an ewe in the autumn, whether she
-will next spring bear two, or one, or none.</p>
-
-
-
-<p class="p2" id="sect355">§ 355. An objection must here be met. Many facts may be brought to
-prove that fatness is not accompanied by fertility but by barrenness;
-and the inference drawn is that high feeding is unfavourable to
-genesis. The premiss may be admitted while the conclusion is denied.</p>
-
-<p>There is a distinction between what may be called normal plethora,
-and an abnormal plethora, liable to be confounded with it. The one
-is a mark of constitutional wealth; but the other is a mark of
-constitutional poverty. Normal plethora is a superfluity of materials
-both for the building up of<span class="pagenum" id="Page_481">[481]</span> tissue and the evolution of force; and
-this is the plethora which we have found to be associated with unusual
-fecundity. Abnormal plethora which, as truly alleged, is accompanied by
-infecundity, is a superfluity of force-evolving materials joined with
-either a positive or a relative deficiency of tissue-forming materials:
-the increased bulk indicating this state, being really the bulk of so
-much inert or dead matter. Note, first, a few of the facts which show
-us that obesity implies physiological impoverishment.</p>
-
-<p>Neither in brutes nor men does it ordinarily occur either in youth or
-in that early maturity during which the vigour is the greatest and the
-digestion the best: it does not habitually accompany the highest power
-of taking up nutritive materials. When fatness arises in the prime of
-life, whether from peculiarity of food or other circumstance, it is
-not the sign of an increased total vitality. On the contrary, if great
-muscular action has to be gone through, the fat must be got rid of;
-either, as in a man, by training, or as in a horse that has grown bulky
-while out at grass, by putting him on such more nutritive diet as oats.
-The frequency of senile fatness, both in domesticated creatures and in
-ourselves, has a similar implication. Whether we consider the smaller
-ability of those who display it to withstand large demands on their
-powers, or whether we consider the comparatively-inferior digestion
-common among them, we see that the increased size indicates, not an
-abundance of materials which the organism requires, but an abundance of
-materials which it does not require. Of like meaning is the fact that
-women who have had several children, and animals after they have gone
-on bearing young for some time, frequently become fat; and lose their
-fecundity as they do this. In such cases the fatness is not to be taken
-as the cause of the infecundity; but the constitutional exhaustion
-which the previous production of offspring has left, shows itself at
-once in the failing fecundity and the commencing fatness. There is yet
-another kind of evidence.<span class="pagenum" id="Page_482">[482]</span> Obesity not uncommonly sets in after the
-system has been subject to debilitating influences. Often a serious
-illness is followed by a corpulence to which there was previously no
-tendency. And the prolonged administration of mercury, constitutionally
-injurious as it is, sometimes produces a like effect.</p>
-
-<p>Closer inquiry verifies the conclusion to which these facts point.
-The microscope shows that along with the increase of bulk common in
-advanced life, there goes on what is called “fatty degeneration:”
-oil-globules are deposited where there should be particles of flesh&mdash;or
-rather, we may say, the hydrocarbonaceous molecules locally produced
-by decomposition of the nitrogenous molecules, have not been replaced
-by other nitrogenous molecules, as they should have been. This fatty
-degeneration is, indeed, a kind of local death. For so regarding it
-we have not simply the reason that an active substance has its place
-occupied by an inert substance; but we have the further reason that the
-flesh of dead bodies, under certain conditions, is transformed into a
-fatty matter called adipocere.</p>
-
-<p>The infertility that accompanies fatness in domestic animals has,
-however, other causes than that declining constitutional vigour which
-the fatness commonly indicates. Being artificially fed, these animals
-cannot always obtain what their systems need. That which is given to
-them is given expressly because of its fattening quality. And since the
-capacity of the digestive apparatus remains the same, the absorption
-of fat-producing materials in excess, implies defect in the absorption
-of materials from which the tissues are formed, and out of which young
-ones are built up. Moreover, this special feeding with a view to rapid
-and early fattening, continued as it is through generations, and
-accompanied as it is by a selection of individuals and varieties which
-fatten most readily, tends to establish a modified constitution, more
-fitted for producing fat and correspondingly-less fitted for producing
-flesh&mdash;a<span class="pagenum" id="Page_483">[483]</span> constitution which, from this relatively-deficient absorption
-of nitrogenous matters, is likely to become infertile; as, indeed,
-these varieties often do become. Hence, no conclusions respecting the
-effects of high nutrition, properly so-called, can be drawn from cases
-of this kind. The cases are, in truth, of a kind which could not exist
-but for human agency. Under natural conditions no animal would diet
-itself in the way required to produce such results. And if it did its
-race would quickly disappear.<a id="FNanchor_62" href="#Footnote_62" class="fnanchor">[62]</a></p>
-
-<p>There is yet another mode in which accumulation of fat diminishes
-fertility. Even supposing it unaccompanied by a smaller absorption of
-nitrogenous materials, it is still a cause of lessening the surplus
-of nitrogenous materials. For the repair of the motor tissues becomes
-more costly. Fat stored-up is weight to be carried. A creature loaded
-with inert matter must, other things equal, consume a greater amount
-of tissue-forming substances for keeping its locomotive apparatus
-in order; and thus expending more for self-maintenance can expend
-less for race-maintenance. Abnormal plethora is thus antagonistic
-to reproduction in a double way. It ordinarily implies a smaller
-absorption of tissue-forming matters, and an increased demand on
-the diminished supply. Hence fertility decreases in a geometrical
-progression.</p>
-
-<p>The counter-conclusion drawn from facts of this class is, then, due
-to a misconception of their nature&mdash;a misconception<span class="pagenum" id="Page_484">[484]</span> arising partly
-from the circumstance that the increase of bulk produced by fat is
-somewhat like the increase of bulk which growth of tissues causes,
-and partly from the circumstance that abundance of good food normally
-produces a certain quantity of fat, which, within narrow limits, is
-a valuable store of force-evolving material. When, however, we limit
-the phrase high nutrition to its proper meaning&mdash;an abundance of, and
-due proportion among, all the substances which the organism needs&mdash;we
-find that, other things equal, fertility always increases as nutrition
-increases. And we see that these apparently-exceptional cases, are
-cases which really show us the same thing; since they are cases of
-relative innutrition.</p>
-
-<hr class="tb" />
-
-<p>[<span class="smcap">Note.</span>&mdash;By a strange oversight when writing this chapter
-in the first edition&mdash;an oversight I was on the eve of repeating in
-this present edition&mdash;I omitted to bring forward the familiar and
-all-important evidence furnished by the variations of genesis which
-ordinarily accompany the alternations of the seasons. These variations,
-in multitudinous creatures of all types, show unmistakably that
-reproduction begins at those times of the year when greater warmth
-and larger supplies of food render maintenance of individual life
-relatively easy, and when there is therefore a surplus available for
-producing new individuals. Conversely, along with the decrease of
-heat and the relative deficiency of food which make it comparatively
-difficult in winter to maintain individual life, there ceases to be
-the power of producing other lives: the reproductive organs become
-quiescent and often dwindle. With this general fact is associated
-a special fact. Though among wild animals&mdash;birds, mammals, and
-others&mdash;breeding ceases when Nature no longer supplies abundant food
-and warmth; in domesticated mammals and birds, artificially supplied
-with food and warmth, the breeding season is greatly extended and often
-made continuous, as, under the same conditions, it is in Man himself.</p>
-
-<p>Evidence yielded by the vegetal world is less conspicuous,<span class="pagenum" id="Page_485">[485]</span> for the
-reason that the cold which arrests reproductive activity also arrests
-individual activity: growth of the individual and multiplication of
-the race vary simultaneously with variations in the seasons. Still
-there are some familiar facts showing that the external conditions
-which favour nutrition also bring about reproduction. Early in the
-year we are supplied with flowers from regions warmer than our own,
-and by and by there come to our markets fruits and vegetables from the
-south of France, the Channel Islands, and even from the Scilly Isles,
-which are much in advance of those furnished by the gardens of our own
-colder regions: reproduction commences earlier where the light and
-heat furthering nutrition are greater. And then there is a kindred
-meaning in the not unfrequent occurrence of a second flowering and even
-of a second fruiting in warm, bright and prolonged autumns. Here the
-abnormal re-commencement of reproduction is determined by an abnormal
-increase of nutrition.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_486">[486]</span></p>
-
-<h3>CHAPTER X.<br />
-<span class="subhed1">SPECIALITIES OF THESE RELATIONS.</span></h3></div>
-
-
-<p>§ 356. Tests of the general doctrines set forth in preceding chapters,
-are afforded by organisms having modes of life which diverge widely
-from ordinary modes. Here, as elsewhere, aberrant cases yield crucial
-proofs.</p>
-
-<p>If certain organisms are so circumstanced that highly-nutritive matter
-is supplied to them without stint, and they have nothing to do but
-absorb it, we may infer that their powers of propagation will be
-enormous.</p>
-
-<p>If there are classes of creatures which expend very little for
-self-support in comparison with allied creatures, a relatively-extreme
-prolificness may be expected of them.</p>
-
-<p>Or if, again, we find species presenting the peculiarity that while
-some of their individuals have much to do and little to eat, others of
-their individuals have much to eat and little to do, we may look for
-great fertility in these last and comparative infertility or barrenness
-in the first.</p>
-
-<p>These several anticipations we shall find completely verified.</p>
-
-
-<p class="p2">§ 357. Plants which, like the <i>Rafflesiaceæ</i>, carry their
-parasitism to the extent of living on the juices they absorb from
-other plants, exhibit one of these relations in the vegetal kingdom.
-In them the organs for self-support being needless, are rudimentary;
-and the parts directly or indirectly concerned in the production and
-distribution of germs, constitute the mass of the organism. That small
-ratio which<span class="pagenum" id="Page_487">[487]</span> the race-preserving structures bear to the self-preserving
-structures in ordinary Phænogams, is, in these Phænogams, inverted. A
-like relation occurs in the common Dodder.</p>
-
-<p>There may be added a kindred piece of evidence which the <i>Fungi</i>
-present. Those of them which grow on living plants, repeat the above
-connection completely; and those of them which, though not parasitic,
-nevertheless subsist on organized materials previously elaborated by
-other plants, substantially repeat it. The spore-producing part is
-relatively enormous; and the fertility is far greater than that of
-Cryptogams of like sizes, which have to form for themselves the organic
-compounds of which they and their germs consist.</p>
-
-
-<p class="p2">§ 358. The same lesson is taught us by animal-parasites. Along with the
-decreased cost of Individuation, they similarly show us an increased
-expenditure for Genesis; and they show us this in the most striking
-manner where the deviation from ordinary conditions of life is the
-greatest.</p>
-
-<p>Take, among the <i>Epizoa</i>, such an instance as <i>Chondracanthus
-gibbosus</i>. Belonging to the <i>Entomostraca</i>, both males
-and females of this species are, in their early days, similar to
-their allies; and the males, practically parasitic, though they
-become greatly degraded, continue throughout life to show by their
-segmentation and other external traits their original nature. The
-female, however, having fixed herself where she can suck the juices
-of her host, the <i>Lophius</i>, grows to twelve times the length
-of the male and probably a thousand times its bulk, and becomes
-utterly transformed by loss of the organs of animal life and enormous
-development of the organs of reproduction. “No heart is discoverable,
-and the nervous system and organs of sense (if any) are equally
-undistinguishable. The interspace between the alimentary canal and
-the walls of the body is almost wholly occupied by the ovarium.”<a id="FNanchor_63" href="#Footnote_63" class="fnanchor">[63]</a>
-And then beyond this there are appended ovi-sacs twice the length
-of the body. So that the<span class="pagenum" id="Page_488">[488]</span> germ-producing organs and their contents,
-eventually acquire a total bulk many times that of all the other organs
-put together. Numerous species of this type and habit, repeat this
-relation between a life of inaction with high feeding, and an enormous
-rate of genesis. Parasites belonging to another great division of the
-animal kingdom, the <i>Platyhelminthes</i>, supply an example of an
-<i>epizoon</i> in which the rate of multiplication is made great not
-so much by immense development of the egg-producing organs as by the
-rapidity with which generations succeed one another&mdash;a rapidity such
-that each generation partially develops the next before it is itself
-anything like ready for independent life. This is the <i>Gyrodactylus
-elegans</i>, of which it is said that “its most remarkable feature is
-that it is viviparous, and its embryos before they leave the body of
-their mother have already developed their embryos inside them; and
-the latter may contain their embryos, so that four generations may be
-included under the cuticle of the sexually mature animal.”<a id="FNanchor_64" href="#Footnote_64" class="fnanchor">[64]</a></p>
-
-<p><i>Entozoa</i> yield us many examples of this causal relation,
-raised to a still higher degree. The <i>Gordius</i>, or Hair-worm,
-is a creature which, finding its way when young into the body of
-an insect which is afterwards swallowed by a fish, there grows
-rapidly, and then emerging to breed, lays as many as 8,000,000 eggs
-in less than a day. Similarly with those larger types infesting the
-higher animals. It has been calculated by Dr. Eschricht, as quoted
-by Professor Owen, that there are “64,000,000 of ova in the mature
-female <i>Ascaris lumbricoides</i>.” Very many of the <i>Entozoa</i>
-belong to the <i>Platyhelminthes</i>, and among them occur examples
-of fertility caused not only by great numbers of ova, but by rapid
-succession of partially-developed individuals and also examples of
-fertility caused by production of ova almost exceeding numeration.
-Among the first the Liver-fluke may be named. Of the half-million
-eggs it produces each yields a free-swimming<span class="pagenum" id="Page_489">[489]</span> ciliated embryo, and
-any one of these, which finds its way into a water-snail, becomes a
-sporocyst&mdash;a bag, presently occupied exclusively by masses of cells:
-each mass by and by becoming a <i>Redia</i>, which makes its way out.
-Like all its fellows which develop in succession, this, with the
-exception of a small space occupied by the stomach, devotes the whole
-of its interior partly to the formation of other <i>Rediæ</i> (which
-presently escape and become similarly transformed), and partly to the
-development of <i>Cercariæ</i>, into which the internal substance of
-all the <i>Rediæ</i> is eventually transformed: <i>Cercariæ</i> which,
-escaping from the host, become agents for infecting other creatures.
-So that each ovum thus gives rise to a number of forms which severally
-subserve multiplication in different ways. Of the other division of
-<i>Platyhelminthes</i> referred to as carrying on its multiplication
-by production of ova only, the commonest of the <i>Cestoidea</i>
-furnishes the best example. Immersed as a Tape-worm is in nutritive
-liquid, which it absorbs through its integument, it requires no
-digestive apparatus. The room which one would occupy, and the materials
-it would use up, are therefore available for germ-producing organs,
-which nearly fill each segment: each segment, sexually complete in
-itself, is little else than an enormous reproductive system, with just
-enough of other structures to bind it together. Remembering that the
-Tape-worm, retaining its hold, continues to bud out such segments as
-fast as the fully-developed ones are cast off, and goes on doing this
-as long as the infested individual lives; we see that here, where
-there is no expenditure, where the cost of individuation is reduced to
-the greatest extent while the nutrition is the highest possible, the
-degree of fertility reaches its extreme. These <i>Entozoa</i> yield us
-further interesting evidence. Of their various species, most if not
-all undergo passive migration from animal to animal before they become
-mature. Usually, the form assumed in the body of the first host is
-devoid of all that part in which the reproductive structures take their
-rise; and this part grows and<span class="pagenum" id="Page_490">[490]</span> develops reproductive structures, only
-in some predatory animal to which its first host falls a sacrifice.
-Occasionally, however, the egg gives origin to the sexual form in
-the animal that originally swallowed it, but the development remains
-incomplete&mdash;there is no sexual genesis, no formation of eggs in the
-rudimentary segments. That these may become fertile it is needful, as
-before, for the containing animal to be devoured; so that the imperfect
-Tape-worm may find its way into the intestine of a higher animal. Thus
-the <i>Bothriocephalus solidus</i>, found in the abdominal cavity
-of the Stickleback, is barren while it remains there; but if the
-Stickleback be eaten by a Water-fowl, the reproductive system of the
-transferred <i>Bothriocephalus</i> (then known as <i>B. nodosus</i>)
-becomes developed and active. So, too, a kind of Tape-worm which
-remains infertile while in the intestine of a Mouse, becomes fertile
-in the intestine of a Cat that devours the mouse. May we not regard
-these facts as again showing the dependence of fertility on nutrition?
-Barrenness here accompanies conditions unfavourable to the absorption
-of nutriment; and it gives way to fecundity where nutriment is large in
-quantity and superior in quality.</p>
-
-
-<p class="p2">§ 359. Extremely significant are those cases of partial reversion
-to primitive forms of genesis, which occur under special
-conditions in some of the higher <i>Annulosa</i>. I refer to the
-pseudo-parthenogenesis and metagenesis in Insects.</p>
-
-<p>Under what conditions do the <i>Aphides</i> exhibit this strange
-deviation from the habits of their order? Why among them should
-imperfect females produce, agamically, others like themselves,
-generation after generation, with great rapidity? There is the obvious
-explanation that they get plenty of easily-assimilated food without
-exertion. Piercing the tender coats of young shoots, they sit and
-suck&mdash;appropriating the nitrogenous elements of the sap and ejecting
-its saccharine matter as “honey dew.” Along with a sluggishness
-strongly contrasted with the activity of most insects&mdash;along<span class="pagenum" id="Page_491">[491]</span> with
-a very low rate of consumption and a correlative degradation of
-structure; we have here a retrogression to asexual genesis, and a
-greatly-increased rate of multiplication.</p>
-
-<p>The recently discovered instance of internal metagenesis in the maggots
-of certain Flies has a like meaning. Incredible as it at first seemed
-to naturalists, it is now proved that the <i>Cecydomia</i>-larva
-develops in its interior a brood of larvæ of like structure with
-itself. In this case, as in the last, abundant food is combined with
-low expenditure. These larvæ are found in such habitats as the refuse
-of beet-root-sugar factories&mdash;masses of nitrogenous <i>débris</i>
-remaining after the extraction of the saccharine matter. Each larva
-has a practically-unlimited supply of sustenance imbedding it on all
-sides.<a id="FNanchor_65" href="#Footnote_65" class="fnanchor">[65]</a></p>
-
-<p>It is true that some other maggots, as those of the Flesh-fly,
-are similarly, or still better, circumstanced; and, it may be
-said, ought therefore to have the same habit. But this does not
-necessarily follow. Survival of the fittest will determine whether
-such specially-favourable conditions result in aggrandizement of the
-individual or in multiplication of the race. And in the case of the
-Flesh-fly there is a reason why greater individuation rather than
-more rapid genesis will occur. For a decomposing animal body lasts
-so short a time, that were Flesh-fly larvae to multiply agamically,
-the second generation would die from the disappearance of their food.
-Hence individuals in which the excessive nutrition led to internal
-metagenesis, would leave no posterity, and natural selection would
-establish the variety in<span class="pagenum" id="Page_492">[492]</span> which greater growth resulted. All which
-the argument requires is that when such reversion to agamogenesis
-<i>does</i> take place, it shall be where the food is unusually
-abundant and the expenditure unusually small; and this the cases
-instanced go to show.</p>
-
-
-<p class="p2">§ 360. The physiological lesson taught us by Bees and Ants, not quite
-harmonizing with the moral lesson they are supposed to teach, is that
-highly-fed idleness is favourable to fertility, and that excessive
-industry has barrenness for its concomitant.</p>
-
-<p>The egg of a Bee develops into a small barren female or into a large
-fertile female, according to the supply of food given to the larva
-hatched from it. We here see that the germ-producing action is an
-overflow of the surplus remaining after completion of the individual;
-and that the lower feeding which the larva of a working Bee has,
-results in a dwarfing of the adult and an arrested development of
-the generative organs. Further, we have the fact that the condition
-under which the perfect female, or mother-Bee, goes on, unlike insects
-in general, laying eggs continuously, is that she has plenty of
-food brought to her, is kept warm, and goes through no considerable
-exertion. While, contrariwise, it is to be noted that the infertility
-of the workers is associated with the ceaseless labour of bringing
-materials for the combs and building them, as well as the labour of
-feeding the queen, the larvæ, and themselves.</p>
-
-<p>Ants also show us these relations, and they are shown in a greatly
-exaggerated form by what are called white ants&mdash;insects belonging to
-a quite different order. The contrast in bulk between the fecund and
-infecund females is here immensely greater. The mother-Ant has the
-reproductive system so enormously developed, that the remainder of her
-body is relatively insignificant. Entirely incapable of locomotion,
-she is unable to deposit her eggs in the places where they are to be
-hatched; so that they have to be carried away<span class="pagenum" id="Page_493">[493]</span> by the workers as fast
-as they are extruded. Her life is thus reduced substantially to that
-of a parasite&mdash;an absorption of abundant food supplied gratis, a total
-absence of expenditure, and a consequent excessive rate of genesis.
-“The queen-ant of the African <i>Termites</i> lays 80,000 eggs in
-twenty-four hours.”</p>
-
-
-<p class="p2">§ 361. It may be needful to say that these exceptional relations cannot
-be ascribed to the assigned causes acting alone. The extreme fertility
-which, among parasites and social insects, accompanies extremely high
-feeding and an expenditure reduced nearly to zero, presupposes typical
-structures and tendencies of suitable kinds; and these are not directly
-accounted for. On creatures otherwise organized, unlimited supplies
-of food and total inactivity are not followed by such results. There
-of course requires a constitution fitted to the special conditions,
-and the evolution of this cannot be due simply to plethora joined with
-rest. These cases are given as illustrating the conditions under which
-extreme exaltations of fertility become possible. Their meanings, thus
-limited, are clear, and completely to the point. We see in them that
-the devotion of nutriment to race-preservation, is carried furthest
-where the cost of self-preservation is reduced to a minimum; and,
-conversely, that nothing is devoted directly to race-preservation
-by individuals on which falls an excessive expenditure for
-self-preservation and preservation of other’s offspring.</p>
-
-<hr class="tb" />
-
-<p>[<span class="smcap">Note.</span>&mdash;Among specialities of these relations may be fitly
-added here a very strange one, for a description of which I am
-indebted to M. Charles Julin, Professor of Comparative Anatomy in
-the University of Liège. In the <i>Revue Générale des Sciences</i>
-for 30th August, 1894, in an account of certain investigations of
-M. Giard, he describes what he calls “la castration parasitaire”&mdash;a
-castration not of a literal kind<span class="pagenum" id="Page_494">[494]</span> but one effected by the arrest of
-development which follows from the depletion caused by a parasite. The
-<i>Sacculina</i> is an amazingly transformed type belonging to the
-<i>Cirrhipedia</i>&mdash;a type without segments or appendages and without
-mouth and alimentary canal. Fixing itself, during its early locomotive
-stage, under the abdomen of a decapodous crustacean, and leaving
-behind its exo-skeleton, it makes its way into the interior, and there
-becoming a mere bag containing the reproductive organs, obtains the
-needful nutriment by developing what are practically roots and rootlets
-which run everywhere among the viscera and absorb nutriment from the
-surrounding tissues. Here we are concerned merely with the effect
-produced upon the host by this physiological robbery. This effect is to
-arrest the development not only of the primary sexual organs devoted
-to the production of germs, but also of those secondary sexual organs
-which characterize the male. M. Julin writes:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“Il convient cependant de dire, pour être plus exact, que,
-dans les cas des Crabes infesté par des Sacculines, il n’y a
-pas, en réalité, apparition de caractères femelles chez le
-sexe mâle, mais plutôt absence de développement des caractères
-mâles. En fait, l’animal reste à un stade jeune, non différencié
-sexuellement, tout en prenant une taille plus considérable. Cela
-nous porte à attribuer les modifications dont nous avons parlé à
-un simple arrêt de développement, qui est plus sensible chez le
-mâle, parce que chez lui les caractères sexuels secondaires sont
-à l’état normal plus développés que chez la femelle.</p>
-
-<p>D’une manière générale, nous croyons, avec M. Giard, qu’il faut
-assimiler les modifications dues à la castration parasitaire à
-celles qui sont le résultat de la progenèse ou qui engendrent le
-dimorphisme saisonnier.</p>
-
-<p>Il y a <i>progenèse</i> lorsque, chez un animal, la reproduction
-sexuée s’opère d’une façon plus ou moins précoce, c’est-à-dire
-lorsque les produits sexuels (œufs ou spermatozoïdes) se
-forment et mûrissent avant que l’être n’ait atteint son complet
-développement. On peut citer comme exemples les Axolotls et
-les larves de Tritons qui, les uns normalement, les autres
-accidentellement, pondent en ayant encore leurs branchies.</p>
-
-<p>Très souvent la progenèse n’affecte qu’un seul sexe. Tantôt,
-c’est<span class="pagenum" id="Page_495">[495]</span> le sexe femelle qui mûrit à l’état larvaire comme chez
-les pucerons, les <i>Stylops</i>, etc.... Tantôt c’est le
-sexe mâle, comme chez la Bonellie, les mâles complémentaires
-de Cirripèdes, les mâles pygmées des Rotifères, le mâle de
-l’Anguille, etc. D’autres fois, enfin, l’animal présente
-successivement les deux sexes avec progenèse pour l’un d’entre
-eux. C’est ainsi qu’il y a <i>progenèse protandrique</i> chez
-les Crustacés cymothoadiens, et, parmi les Vertébrés, chez les
-Myxines, qui, mâles dans le jeune âge, deviennent femelles en
-vieillissant et en achevant de prendre leur développement. Le
-cas des vieilles femelles de Gallinacés à plumage et à instincts
-masculins semble être, au contraire, un exemple imparfait de
-<i>progenèse protogynique</i>, puisque ces femelles ont pondu
-lorsqu’elles avaient encore la livrée des jeunes et qu’elles
-ont continué plus tard leur développement, et présentent le
-caractère des mâles sans que, cependant, l’on ait constaté la
-production de spermatozoïdes.</p>
-
-<p>Dans les cas extrêmes de progenèse femelle, la reproduction se
-fait même sans le concours de l’élément mâle, revenant ainsi
-à la forme agamique primordiale. Ces cas sont connus depuis
-longtemps sous le nom de <i>pédogenèse</i>. On les a observé
-chez les larves de <i>Miastor</i>, de <i>Chironomus</i> et chez
-certains pucerons.</p>
-
-<p>Chaque fois qu’il y a progenèse dans un type déterminé, on
-constate soit momentanément, soit d’une façon définitive, un
-arrêt de croissance et de développement: l’animal progénétique
-a, par suite, l’aspect d’une larve sexuée, lorsqu’on le compare
-soit à l’autre sexe, soit aux formes voisines, qui ne présentent
-pas le phénomène de la progenèse.</p>
-
-<p>Cela est en parfaite harmonie avec le principe, si bien mis en
-lumière par Herbert Spencer, de <i>l’antagonisme entre la genèse
-et la croissance et entre la genèse et le développement</i>.
-Cet antagonisme s’explique facilement si l’on songe que les
-matériaux employés pour la reproduction ne peuvent servir à
-l’accroissement de l’individu. S’il est avantageux pour un
-organisme de se reproduire sans acquérir des organes inutiles,
-la sélection naturelle déterminera bientôt une progenèse de plus
-en plus complète. Les animaux parasites, outre qu’ils tirent de
-leur hôte une nourriture abondante, n’ont guère besoin d’une
-foule d’organes qui servent à leurs congénères libres dans la
-vie de relation. Aussi voyons-nous qu’un très grand nombre
-d’animaux parasites sont progénétiques. Les mâles progénétiques
-de la Bonellie et des Cirripèdes vivent en parasites dans leurs
-femelles. Chez certains types, les pucerons, la progenèse cesse
-dès que, la nourriture devenant moins abondante, un déplacement
-pourra être nécessaire.</p>
-
-<p><span class="pagenum" id="Page_496">[496]</span></p>
-
-<p>En résumé, l’arrêt de développement dû à la progenèse résulte
-d’une dérivation des principes nourriciers au détriment
-de l’animal progénétique. Dans les exemples de castration
-parasitaire que nous avons examinés, le parasite joue, par
-rapport à son hôte, absolument le même rôle que la glande
-génitale d’un type progénétique. Il détourne, pour sa propre
-subsistance, une partie des principes qui auraient servi au
-développement de l’animal. Aussi les effets produits sont-ils
-tout à fait de même ordre.”</p>
-</div>
-
-<p>A phenomenon so anomalous as this, explicable upon the hypothesis set
-forth but not otherwise explicable, furnishes striking verification.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_497">[497]</span></p>
-
-<h3>CHAPTER XI.<br />
-<span class="subhed1">INTERPRETATION AND QUALIFICATION.</span></h3></div>
-
-
-<p>§ 362. Considering the difficulties of inductive verification, we
-have, I think, as clear a correspondence between the <i>à priori</i>
-and <i>à posteriori</i> conclusions, as can be expected. The many
-factors co-operating to bring about the result in every case, are so
-variable in their absolute and relative amounts, that we can rarely
-disentangle the effect of each one, and have usually to be content
-with qualified inferences. Though in the mass organisms show us an
-unmistakable relation between great size and small fertility, yet
-special comparisons among them are nearly always partially vitiated
-by differences of structure, differences of nutrition, differences
-of expenditure. Though it is beyond question that the more complex
-organisms are the less prolific, yet as complexity has a certain
-general connexion with bulk, and in animals with expenditure, we cannot
-often identify its results as independent of these. And, similarly,
-though the creatures which waste much matter in producing motion,
-sensible and insensible, have lower rates of multiplication than those
-which waste less, yet, as the creatures which waste much are generally
-larger and more complex, we are again met by an obstacle which limits
-our comparisons, and compels us to accept conclusions less definite
-than are desirable.</p>
-
-<p>Such difficulties arise, however, only when we endeavour, as in
-foregoing chapters, to prove the inverse variation<span class="pagenum" id="Page_498">[498]</span> between Genesis
-and each separate element of Individuation&mdash;growth, development,
-activity. We are scarcely at all hampered by qualifications when,
-from contemplating these special relations, we return to the general
-relation. The antagonism between Individuation and Genesis is shown
-by all the facts which have been grouped under each head. We have
-seen that in ascending from the lowest to the highest types, there is
-a decrease of fertility so great as to be absolutely inconceivable,
-and even inexpressible by figures; and whether the superiority of
-type consists in relative largeness, in greater complexity, in
-higher activity, or in some or all of these combined, matters not
-to the ultimate inference. The broad fact, enough for us here, is
-that organisms in which the integration and differentiation of
-matter and motion have been carried furthest, are those in which the
-rate of multiplication has fallen lowest. How much of the decline
-of reproductive power is due to the greater integration of matter,
-how much to its greater differentiation, how much to the larger
-amounts of integrated and differentiated motions generated, it may
-be impossible to say; and it is not needful to say. These are all
-elements of a higher degree of life, an augmented ability to maintain
-the organic equilibrium amid environing actions, an increased power
-of self-preservation; and we find their invariable accompaniment
-to be, a diminished expenditure of matter, or motion, or both, in
-race-preservation.</p>
-
-<p>In brief, then, examination of the evidence shows that there
-<i>does</i> exist that relation which we inferred <i>must</i> exist.
-Arguing from general data, we saw that for the maintenance of a
-species, the ability to produce offspring must be great, in proportion
-as the ability of the individuals to contend with destroying forces is
-small; and conversely. Arguing from other general data, we saw that,
-derived as the self-sustaining and race-sustaining forces are from a
-common stock of force, it necessarily happens that, other things equal,
-increase of one involves decrease of the other. And then, turning<span class="pagenum" id="Page_499">[499]</span> to
-special facts, we have found that this inverse variation is clearly
-traceable throughout both the animal and vegetal kingdoms. We may
-therefore set it down as a law, that every higher degree of organic
-evolution, has for its concomitant a lower degree of that peculiar
-organic dissolution which is seen in the production of new organisms.</p>
-
-
-<p class="p2">§ 363. Something remains to be said in reply to the inquiry&mdash;how is the
-ratio between Individuation and Genesis established in each case? This
-inquiry has been but partially answered in the course of the foregoing
-argument.</p>
-
-<p>Many specialities of the reproductive process are manifestly due to
-the natural selection of favourable variations. Whether a creature
-lays a few large eggs or many small ones equal in weight to the few
-large, is not determined by any physiological necessity: here the only
-assignable cause is the survival of varieties in which the matter
-devoted to reproduction happens to be divided into portions of such
-size and number as most to favour multiplication. Whether in any case
-there are frequent small broods or larger broods at longer intervals,
-depends wholly on the constitutional peculiarity that has arisen from
-the dying out of families in which the sizes and intervals of the
-broods were least suited to the conditions of life. Whether a species
-of animal produces many offspring of which it takes no care or a few
-of which it takes much care&mdash;that is, whether its reproductive surplus
-is laid out wholly in germs or partly in germs and partly in labour on
-their behalf&mdash;must have been decided by that moulding of constitution
-to conditions slowly effected through the more frequent preservation
-of descendants from those whose reproductive habits were best adapted
-to the circumstances of the species. Given a certain surplus available
-for race-preservation, and it is clear that by indirect equilibration
-only, can there be established the more or less peculiar distribution
-of this surplus which we see in each case. Obviously, too, survival
-of the fittest<span class="pagenum" id="Page_500">[500]</span> has a share in determining the proportion between the
-amount of matter that goes to Individuation and the amount that goes
-to Genesis. Whether the interests of the species are most subserved
-by a higher evolution of the individual joined with a diminished
-fertility, or by a lower evolution of the individual joined with an
-increased fertility, are questions ever being experimentally answered.
-If the more-developed and less-prolific variety has a greater number of
-survivors, it becomes established and predominant. If, contrariwise,
-the conditions of life being simple, the larger or more-organized
-individuals gain nothing by their greater size or better organization;
-then the greater fertility of the less evolved ones, will insure to
-their descendants an increasing predominance.</p>
-
-<p>But direct equilibration all along maintains the limits within which
-indirect equilibration thus works. The necessary antagonism we have
-traced, rigidly restricts the changes that natural selection can
-produce, under given conditions, in either direction. A greater demand
-for Individuation, be it a demand caused by some spontaneous variation
-or by an adaptive increase of structure and function, inevitably
-diminishes the supply for Genesis; and natural selection cannot, other
-things remaining the same, restore the rate of Genesis while the higher
-Individuation is maintained. Conversely, survival of the fittest,
-acting on a species that has, by spontaneous variation or otherwise,
-become more prolific, cannot again raise its lowered Individuation, so
-long as everything else continues constant.</p>
-
-
-<p class="p2">§ 364. Here, however, a qualification must be made. It was
-parenthetically remarked in <a href="#sect327">§ 327</a>, that the inverse variation between
-Individuation and Genesis is not exact; and it was hinted that a slight
-modification of statement would be requisite at a more advanced stage
-of the argument. We have now reached the proper place for specifying
-this modification.</p>
-
-<p><span class="pagenum" id="Page_501">[501]</span></p>
-
-<p>Each increment of evolution entails a decrement of reproduction which
-is not accurately proportionate, but somewhat less than proportionate.
-The gain in the one direction is not wholly cancelled by a loss in
-the other direction, but only partially cancelled: leaving a margin
-of profit to the species. Though augmented power of self-maintenance
-habitually necessitates diminished power of race-propagation, yet the
-product of the two factors is greater than before; so that the forces
-preservative of race become, thereafter, in excess of the forces
-destructive of race, and the race spreads. We shall soon see why this
-happens.</p>
-
-<p>Every advance in evolution implies an economy. That any increase in
-bulk, or structure, or activity, may become established, the life of
-the organism must be to some extent facilitated by the change&mdash;the
-cost of self-support must be, on the average, reduced. If the greater
-complexity, or the larger size, or the more agile movement, entails
-on the individual an outlay that is not repaid in food more-easily
-obtained, or danger more-easily escaped; then the individual will
-be at a relative disadvantage, and its diminished posterity will
-disappear. If the extra outlay is but just made good by the extra
-advantage, the modified individual will not survive longer, or leave
-more descendants, than the unmodified individuals. Consequently, it
-is only when the expense of greater individuation is out-balanced by
-a subsequent saving, that it can tend to subserve the preservation of
-the individual, and, by implication, the preservation of the race.
-The vital capital invested in the alteration must bring a more than
-equivalent return. A few instances will show that, whether the change
-results from direct equilibration or from indirect equilibration, this
-must happen. Suppose a creature takes to performing some act in an
-unusual way&mdash;leaps where ordinarily its kindred crawl, eludes pursuit
-by diving instead of, like others of its kind, by swimming along the
-surface, escapes by doubling instead of by speed. Clearly, perseverance
-in the modified habit will, other<span class="pagenum" id="Page_502">[502]</span> things equal, imply that it takes
-less effort. The creature’s sensations will ever prompt desistance from
-the more laborious course; and hence a congenital habit is not likely
-to be diverged from unless an economy of force is achieved by the
-divergence. Assuming, then, that the new method has no advantage over
-the old in directly diminishing the chances of death, the establishment
-of it, and of the structural complications involved, nevertheless
-implies a physiological gain. Suppose, again, that an animal takes
-to some abundant food previously refused by its kind. It is likely
-to persist only if the comparative ease in obtaining this food, more
-than compensates for any want of adaptation to its digestive organs;
-so that superposed modifications of the digestive organs are likely
-to arise only when an average economy results. What now must be the
-influence on the creature’s system as a whole? Diminished expenditure
-in any direction, or increased nutrition however effected, will leave
-a greater surplus of materials. The animal will be physiological
-richer. Part of its augmented wealth will go towards its own greater
-individuation&mdash;its size, or its strength, or both, will increase; while
-another part will go towards more active genesis. Just as a state of
-plethora directly produced enhances fertility; so will such a state
-indirectly produced.</p>
-
-<p>In another way, the same thing must result from those additions
-to bulk or complexity or activity that are due to survival of the
-fittest. Any change which prolongs individual life will, other things
-remaining the same, further the production of offspring. Even when it
-is not, like the foregoing, a means of economizing the forces of the
-individual, still, if it increases the chances of escaping destruction,
-it increases the chances of leaving posterity. Any further degree of
-evolution, therefore, will be established only where the cost of it
-is more than repaid: part of the gain being shown in the lengthened
-life of the individual, and part in the greater production of other
-individuals.</p>
-
-<p><span class="pagenum" id="Page_503">[503]</span></p>
-
-<p>We have here the solution of various minor anomalies by which the
-inverse variation of Individuation and Genesis is obscured. Take as an
-instance the fertility of the Blackbird as compared with that of the
-Linnet. Both birds lay five eggs, and both usually have two broods. Yet
-the Blackbird is far the larger of the two, and ought, according to the
-general law, to be much less prolific. What causes this nonconformity?
-We shall find an answer in their respective foods and habits. Except
-during the time that it is rearing its young, the Linnet collects only
-vegetal food&mdash;lives during the winter on the seeds it finds in the
-fields, or, when hard pressed, picks up around farms; and to obtain
-this spare diet is continually flying about. The result, if it survives
-the frost and snow, is a considerable depletion; and it recovers its
-condition only after some length of spring weather. The Blackbird, on
-the other hand, is omnivorous. While it eats grain and fruit when they
-come in its way, it depends largely on animal food. It cuts to pieces
-and devours the dew-worms which, morning and evening, it finds on the
-surface of a lawn, and, even discovering where they are, unearths
-them; it swallows slugs, and breaking snail-shells, either with its
-beak or by hammering them against stones, tears out their tenants; and
-it eats beetles and larvæ. Thus the strength of the Blackbird opens
-to it a store of good food, much of which is inaccessible to so small
-and weak a bird as a Linnet&mdash;a store especially helpful to it during
-the cold months, when the hybernating snails in hedge-bottoms yield
-it abundant provision. The result is that the Blackbird is ready to
-breed very early in spring, and is able during the summer to rear
-a second, and sometimes even a third, brood. Here, then, a higher
-degree of Individuation secures advantages so great, as to much more
-than compensate its cost. It is not that the decline of Genesis is
-less than proportionate to the increase of Individuation, but there
-is no decline at all. Comparison of the Rat with the Mouse yields a
-parallel result. Though they differ greatly in size, yet the one is as
-prolific<span class="pagenum" id="Page_504">[504]</span> as the other. This absence of difference cannot be ascribed
-to their unlike degrees of activity. We must seek its cause in some
-facility of living secured to the Rat by its greater intelligence,
-greater power and courage, greater ability to utilize what it finds.
-The Rat is notoriously cunning; and its cunning gives success to its
-foraging expeditions. It is not, like the Mouse, limited mainly to
-vegetal food; but while it eats grain and beans like the Mouse, it also
-eats flesh and carrion, devours young poultry and eggs. The result is
-that, without a proportionate increase of expenditure, it gets a far
-larger supply of nourishment than the Mouse; and relative excess of
-nourishment makes possible a larger size without a smaller rate of
-multiplication. How clearly this is the cause, we see in the contrast
-between the common Rat and the Water-Rat. While the common Rat has
-ordinarily several broods a-year of from 10 to 12 each, the Water-Rat,
-though somewhat smaller, has but 5 or 6 in a brood, and but one brood,
-or sometimes two broods, a-year. But the Water-Rat lives on vegetal
-food, and it lacks all that its bold, sagacious, omnivorous congener
-gains from the warmth as well as the abundance which men’s habitations
-yield.</p>
-
-<p>The inverse variation of Individuation and Genesis is, therefore, but
-approximate. Recognizing the truth that every increment of evolution
-which is appropriate to the circumstances of an organism, brings an
-advantage somewhat in excess of its cost; we see the general law, as
-more strictly stated, to be that Genesis decreases not quite so fast
-as Individuation increases. Whether the greater Individuation takes
-the form of a larger bulk and accompanying access of strength; whether
-it be shown in higher speed or agility; whether it consists in a
-modification of structure which facilitates some habitual movement,
-or in a visceral change that helps to utilize better the absorbed
-aliment; the ultimate effect is identical. There is either a more
-economical performance of the same actions, internal or external, or
-there is a securing of greater advantages by modified actions, which<span class="pagenum" id="Page_505">[505]</span>
-cost no more, or have an increased cost less than the increased gain.
-In any case the result is a greater surplus of vital capital, part
-of which goes to the aggrandizement of the individual, and part to
-the formation of new individuals. While the higher tide of nutritive
-matters, everywhere filling the parent-organism, adds to its power of
-self-maintenance, it also causes a reproductive overflow larger than
-before.</p>
-
-<p>Hence every type which is best adapted to its conditions, (and this
-on the average means every higher type), has a rate of multiplication
-that insures a tendency to predominate. Survival of the fittest, acting
-alone, is ever replacing inferior species by superior species. But
-beyond the longer survival, and therefore greater chance of leaving
-offspring, which superiority gives, we see here another way in which
-the spread of the superior is insured. Though the more-evolved organism
-is the less fertile absolutely, it is the more fertile relatively.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_506">[506]</span></p>
-
-<h3>CHAPTER XII.<br />
-<span class="subhed1">MULTIPLICATION OF THE HUMAN RACE.</span></h3></div>
-
-
-<p id="sect365">§ 365. The relative fertility of Man considered as a species, and
-those changes in Man’s fertility which occur under changed conditions,
-must conform to the laws which we have traced thus far. As a matter
-of course, the inverse variation between Individuation and Genesis
-holds of him as of all other organized beings. His extremely low rate
-of multiplication&mdash;far below that of all terrestrial Mammals except
-the Elephant, (which though otherwise less evolved is, in extent
-of integration, more evolved)&mdash;we shall recognize as the necessary
-concomitant of his much higher evolution. And the causes of increase or
-decrease in his fertility, special or general, temporary or permanent,
-we shall expect to find in those changes of bulk, of structure, or of
-expenditure, which we have in all other cases seen associated with such
-effects.</p>
-
-<p>In the absence of detailed proof that these parallelisms exist, it
-might suffice to contemplate the several communities between the
-reproductive function in human beings and other beings. I do not refer
-simply to the fact that genesis proceeds in a similar manner; but I
-refer to the similarity of the relation between the generative function
-and the functions which have for their joint end the preservation
-of the individual. In Man, as in other creatures that expend much,
-genesis commences only when growth and development are declining in
-rapidity and approaching their termination. Among the higher organisms
-in general, the reproductive<span class="pagenum" id="Page_507">[507]</span> activity, continuing during the prime
-of life, ceases when the vigour declines, leaving a closing period of
-infertility; and in like manner among ourselves, barrenness supervenes
-when middle age brings the surplus vitality to an end. So, too, it is
-found that in Man, as in beings of lower orders, there is a period
-at which fecundity culminates. In <a href="#sect341">§ 341</a>, facts were cited showing
-that at the commencement of the reproductive period, animals bear
-fewer offspring than afterwards; and that towards the close of the
-reproductive period, there is a decrease in the number produced. In
-like manner it is shown by the tables of Dr. Duncan’s recent work,
-that the fecundity of women increases up to the age of about 25
-years, and continuing high with but slight diminution till after 30,
-then gradually wanes. It is the same with the sizes and weights of
-offspring. Infants born of women from 25 to 29 years of age, are both
-longer and heavier than infants born of younger or older women; and
-this difference has the same implication as the greater total weight
-of the offspring produced at a birth, during the most fecund age of
-a pluriparous animal. Once more, there is the fact that a too-early
-bearing of young produces on a woman the same injurious effects as
-on an inferior creature&mdash;an arrest of growth and an enfeeblement of
-constitution.</p>
-
-<p>Considering these general and special parallelisms, we might safely
-infer that variations of human fertility conform to the same laws
-as do variations of fertility in general. But it is not needful to
-content ourselves with an implication. Evidence is assignable that
-what causes increase or decrease of genesis in other creatures, causes
-increase or decrease of genesis in Man. It is true that, even more
-than hitherto, our reasonings are beset by difficulties. So numerous
-are the inequalities in the conditions, that but few unobjectionable
-comparisons can be made. The human races differ considerably in their
-sizes, and notably in their degrees of cerebral development. The
-countries they inhabit entail on them widely different consumptions
-of matter for maintenance of<span class="pagenum" id="Page_508">[508]</span> temperature. Both in their qualities
-and quantities the foods they live on are unlike; and the supply is
-here regular and there very irregular. Their expenditures in bodily
-action are extremely unequal; and even still more unequal are their
-expenditures in mental action. Hence the factors, varying so much in
-their amounts and combinations, can scarcely ever have their respective
-effects identified. Nevertheless there are a few comparisons the
-results of which may withstand criticism.</p>
-
-
-<p class="p2" id="sect366">§ 366. The increase of fertility caused by a nutrition that is greatly
-in excess of the expenditure, is to be detected by contrasting
-populations of the same race, or allied races, one of which obtains
-good and abundant sustenance much more easily than the other. Three
-cases may here be set down.</p>
-
-<p>The traveller Barrow, describing the Cape-Boers, says:&mdash;“Unwilling
-to work and unable to think,” ... “indulging to excess in the
-gratification of every sensual appetite, the African peasant grows to
-an unwieldy size;” and respecting the other sex, he adds&mdash;“the women
-of the African peasantry lead a life of the most listless inactivity,”
-Then, after illustrating these statements, he goes on to note “the
-prolific tendency of all the African peasantry. Six or seven children
-in a family are considered as very few; from a dozen to twenty are
-not uncommon.” The native races of this region yield evidence to the
-same effect. Speaking of the cruelly-used Hottentots (he is writing
-a century ago), who, while they are poor and ill-fed, have to do all
-the work for the idle Boers, Barrow says that they “seldom have more
-than two or three children; and many of the women are barren.” This
-unusual infertility stands in remarkable contrast with the unusual
-fertility of the Kaffirs, of whom he afterwards gives an account.
-Rich in cattle, leading easy lives, and living almost exclusively on
-animal food (chiefly milk with occasional flesh), these people were
-then reputed<span class="pagenum" id="Page_509">[509]</span> to have a very high rate of multiplication. Barrow
-writes:&mdash;“They are said to be exceedingly prolific; that twins are
-almost as frequent as single births, and that it is no uncommon
-thing for a woman to have three at a time.” Probably both these
-statements are in excess of the truth; but there is room for large
-discounts without destroying the extreme difference. A third instance
-is that of the French-Canadians. “<i>Nous sommes terribles pour les
-enfants!</i>” observed one of them to Prof. Johnston, who tells us
-that the man who said this “was one of fourteen children&mdash;was himself
-the father of fourteen, and assured me that from eight to sixteen was
-the usual number of the farmers’ families. He even named one or two
-women who had brought their husbands five-and-twenty, and threatened
-‘<i>le vingt-sixième pour le prêtre</i>.’” From these large families,
-joined with the early marriages and low rate of mortality, it results
-that, by natural increase, “there are added to the French-Canadian
-population of Lower Canada four persons for every one that is added to
-the population of England.” Now these French-Canadians are described
-by Prof. Johnston as home-loving, contented, unenterprising; and as
-living in a region where “land and subsistence are easily obtained.”
-Very moderate industry brings to them liberal supplies of necessaries;
-and they pass a considerable portion of the year in idleness. Hence
-the cost of Individuation being much reduced, the rate of Genesis
-is much increased. That this uncommon fertility is not due to any
-direct influence of the locality, is implied by the fact that along
-with the “restless, discontented, striving, burning energy of their
-Saxon neighbours,” no such rate of multiplication is observed; while
-further south, where the physical circumstances are more favourable if
-anything, the Anglo-Saxons, leading lives of excessive activity, have a
-fertility below the average. And that the peculiarity is not a direct
-effect of race, is proved by the fact that in Europe, the rural French
-are certainly not more prolific than the rural English.</p>
-
-<p><span class="pagenum" id="Page_510">[510]</span></p>
-
-<p>To every reader there will probably occur the seemingly-adverse
-evidence furnished by the Irish; who, though not well fed, multiply
-fast. Part of this more rapid increase is due to the earlier marriages
-common among them, and consequent quicker succession of generations&mdash;a
-factor which, as we have seen, has a larger effect than any other on
-the rate of multiplication. Part of it is due to the greater generality
-of marriage&mdash;to the comparative smallness of the number who die without
-having had the opportunity of producing offspring. The effects of
-these causes having been deducted, we may doubt whether the Irish,
-individually considered, would be found more prolific than the English.
-Perhaps, however, it will be said that, considering their diet, they
-ought to be less prolific. This is by no means obvious. It is not
-simply a question of nutriment absorbed. It is a question of how much
-remains after the expenditure in self-maintenance. Now a notorious
-peculiarity in the life of the Irish peasant is, that he obtains a
-return of food which is large in proportion to his outlay in labour.
-The cultivation of his potatoe-ground occupies each cottager but a
-small part of the year; and the domestic economy of his wife is not of
-a kind to entail on her much daily exertion. Consequently the crop,
-tolerably abundant in quantity though innutritive in quality, possibly
-suffices to meet the comparatively-low expenditure, and to leave a good
-surplus for genesis&mdash;perhaps a greater surplus than remains to the
-males and females of the English peasantry, who, though fed on better
-food, are harder worked.</p>
-
-<p>We conclude, then, that in the human race, as in all other races, such
-absolute or relative abundance of nutriment as leaves a large excess
-after defraying the cost of carrying on parental life, is accompanied
-by a high rate of genesis.<a id="FNanchor_66" href="#Footnote_66" class="fnanchor">[66]</a></p>
-
-
-<p class="p2" id="sect367">§ 367. Evidence of the converse truth, that relative increase<span class="pagenum" id="Page_511">[511]</span> of
-expenditure, leaving a diminished surplus, reduces the degree of
-fertility, is not wanting. Some of it has been set down for the sake of
-antithesis in the foregoing section. Here may be grouped a few facts of
-a more special kind having the same implication.</p>
-
-<p>To prove that much bodily labour renders women less prolific, requires
-more evidence than has at present been collected. Nevertheless it may
-be noted that De Boismont in France and Dr. Szukits in Austria, have
-shown by extensive statistical comparisons, that the reproductive
-age is reached a year later by women of the labouring class than by
-middle-class women; and while ascribing this delay in part to inferior<span class="pagenum" id="Page_512">[512]</span>
-nutrition, we may suspect that it is in part due to greater muscular
-expenditure. A kindred fact, admitting of a kindred interpretation,
-may be added. Though the comparatively-low rate of increase in France
-is attributed to other causes, yet, very possibly, one of its causes
-is the greater proportion of hard work entailed on French women, by
-the excessive abstraction of men for non-productive occupations,
-military and civil. The higher rate of multiplication in England than
-in continental countries generally, is not improbably furthered by the
-easier lives which English women lead.</p>
-
-<p>That absolute or relative infertility is commonly produced in women
-by mental labour carried to excess, is more clearly shown. Though the
-regimen of upper-class girls is not what it should be, yet, considering
-that their feeding is better than that of girls belonging to the poorer
-classes, while, in most other respects, their physical treatment is
-not worse, the<span class="pagenum" id="Page_513">[513]</span> deficiency of reproductive power among them may be
-reasonably attributed to the overtaxing of their brains&mdash;an overtaxing
-which produces a serious reaction on the physique. This diminution
-of reproductive power is not shown only by the greater frequency of
-absolute sterility; nor is it shown only in the earlier cessation of
-child-bearing; but it is also shown in the very frequent inability of
-such women to suckle their infants. In its full sense, the reproductive
-power means the power to bear a well-developed infant and to supply
-that infant with the natural food for the natural period. Most of the
-flat-chested girls who survive their high-pressure education, are
-incompetent to do this. Were their fertility measured by the number
-of children they could rear without artificial aid, they would prove
-relatively very infertile.</p>
-
-<p>The cost of reproduction to males being so much less than it is to
-females, the antagonism between Genesis and Individuation is not often
-shown in men by suppression of generative power consequent on unusual
-expenditure in bodily action. Nevertheless, there are indications that
-this results in extreme cases. We read that the ancient <i>athletæ</i>
-rarely had children; and among such of their modern representatives as
-acrobats, an allied relation of cause and effect is alleged. Indirectly
-this truth, or rather its converse, appears to have been ascertained
-by those who train men for feats of strength&mdash;they find it needful to
-insist on continence.</p>
-
-<p>Special proofs that in men great cerebral expenditure diminishes or
-destroys generative power, are difficult to obtain. It is, indeed,
-asserted that intense application to mathematics, requiring as it does
-extreme concentration of thought, is apt to have this result; and it is
-asserted, too, that this result is produced by the excessive emotional
-excitement of gambling. Then, again, it is a matter of common remark
-how frequently men of unusual mental activity leave no offspring. But
-facts of this kind admit of another interpretation. The reaction of
-the brain on the body is so violent&mdash;the<span class="pagenum" id="Page_514">[514]</span> overtaxing of the nervous
-system is so apt to prostrate the heart and derange the digestion; that
-the incapacities caused in these cases, are probably often due more to
-constitutional disturbance than to the direct deduction which excessive
-action entails. Such instances harmonize with the hypothesis; but how
-far they yield it positive support we cannot say.</p>
-
-
-<p class="p2">§ 368. An objection must here be guarded against. It is likely to
-be urged that since the civilized races are, on the average, larger
-than many of the uncivilized races; and since they are also somewhat
-more complex as well as more active; they ought, in conformity with
-the alleged general law, to be less prolific. There is, however, no
-evidence to prove that they are so: on the whole, they seem rather the
-reverse.</p>
-
-<p>The reply is that were all other things equal, these superior varieties
-of men should have inferior rates of increase. But other things are not
-equal; and it is to the inequality of other things that this apparent
-anomaly is attributable. Already we have seen how much more fertile
-domesticated animals are than their wild kindred; and the causes of
-this greater fertility are also the causes of the greater fertility,
-relative or absolute, which civilized men exhibit when compared with
-savages.</p>
-
-<p>There is the difference in amount of food. Australians, Fuegians, and
-sundry races that might be named as having low rates of multiplication,
-are obviously underfed. The sketches of natives contained in the
-volumes of Livingstone, Baker, and others, yield clear proofs of the
-extreme depletion common among the uncivilized. In quality as well
-as in quantity, their feeding is bad. Wild fruits, insects, larvæ,
-vermin, &amp;c., which we refuse with disgust, often enter largely into
-their dietary. Much of this inferior food they eat uncooked; and they
-have not our elaborate appliances for mechanically-preparing it, and
-rejecting its useless parts. So that they live on matters of less
-nutritive value, which<span class="pagenum" id="Page_515">[515]</span> cost more both to masticate and to digest.
-Further, to uncivilized men supplies of food come very irregularly.
-Long periods of scarcity are divided by short periods of abundance.
-And though by gorging when opportunity occurs, something is done
-towards compensating for previous fasting, yet the effects of prolonged
-starvation cannot be neutralized by occasional enormous meals. Bearing
-in mind, too, that improvident as they are, savages often bestir
-themselves only under pressure of hunger, we may fairly consider them
-as habitually ill-nourished&mdash;may see that even the poorer classes of
-civilized men, making regular meals on food separated from innutritive
-matters, easy to masticate and digest, tolerably good in quality and
-adequate if not abundant in quantity, are much better nourished.</p>
-
-<p>Then, again, though a greater consumption in muscular action appears
-to be undergone by civilized men than by savages; and though it is
-probably true that among our labouring people the daily repairs cost
-more; yet in many cases there does not exist so much difference as we
-are apt to suppose. The chase is very laborious; and great amounts of
-exertion are gone through by the lowest races in seeking and securing
-the odds and ends of wild food on which they largely depend. We
-naturally assume that because barbarians are averse to regular labour,
-their muscular action is less than our own. But this is not necessarily
-true. The monotonous toil is what they cannot tolerate; and they may
-be ready to go through as much or more exertion when it is joined with
-excitement. If we remember that the sportsman who gladly scrambles
-up and down rough hill-sides all day after grouse or deer, would
-think himself hardly used had he to spend as much effort and time in
-digging; we shall see that a savage who is the reverse of industrious,
-may nevertheless be subject to a muscular waste not very different in
-amount from that undergone by the industrious. When it is added that a
-larger physiological expenditure is entailed on the uncivilized than
-on the<span class="pagenum" id="Page_516">[516]</span> civilized by the absence of good appliances for shelter and
-protection&mdash;that in some cases they have to make good a greater loss
-of heat, and in other cases suffer much wear from irritating swarms of
-insects; we shall see that the total cost of self-maintenance among
-them is probably in many cases little less, and in some cases more,
-than it is among ourselves.</p>
-
-<p>So that though, on the average, the civilized are probably larger than
-the savage; and though they are, in their nervous systems at least,
-somewhat more complex; and though, other things equal, they ought to
-be the less prolific; yet other things are so unequal as to make it
-quite conformable to the general law that they should be more prolific.
-In <a href="#sect365">§ 365</a> we observed how, among inferior animals, higher evolution
-sometimes makes self-preservation far easier, by opening the way to
-resources previously unavailable: so involving an undiminished, or
-even an increased, rate of genesis. And similarly we may expect that
-among races of men, those whose slight further developments have been
-followed by habits and arts which immensely facilitate life, will not
-exhibit a lower degree of fertility, and may even exhibit a higher.</p>
-
-
-<p class="p2">§ 369. One more objection has to be met&mdash;a kindred objection to which
-there is a kindred reply. Cases may be named of men conspicuous
-for activity, bodily and mental, who were also noted, not for less
-generative power than usual, but for more. As their superiorities
-indicate higher degrees of evolution, it may be urged that such men
-should, according to the theory, have lower degrees of reproductive
-activity. The fact that here, along with increased powers of
-self-preservation, there go increased powers of race-propagation,
-seems irreconcilable with the general doctrine. Reconciliation is not
-difficult however.</p>
-
-<p>The cases are analogous to some before named, in which more abundant
-food simultaneously aggrandizes the individual<span class="pagenum" id="Page_517">[517]</span> and adds to the
-production of new individuals: the difference between the cases
-being, that instead of a better external supply of materials there
-is a better internal utilization of materials. Creatures of the same
-species notoriously differ in goodness of constitution. Here there
-is some visceral defect, showing itself in feebleness of all the
-functions; while here some peculiarity of organic balance, some high
-quality of tissue, some abundance or potency of the digestive juices,
-gives to the system a perpetual high tide of rich blood, which serves
-at once to enhance the vital activities and to raise the power of
-propagation. Such variations, however, are independent of changes in
-the <i>proportion</i> between Individuation and Genesis. This remains
-the same, while both are increased or decreased by the increase or
-decrease of the common stock of materials.</p>
-
-<p>An illustration will best clear up any perplexity. Let us say that
-the fuel burnt in the furnace of a locomotive steam-engine, answers
-to the food which a man consumes. Let us say that the produced steam
-expended in working the engine, corresponds to that portion of absorbed
-nutriment which carries on the man’s functions and activities. And
-let us say that the steam blowing off at the safety-valve, answers to
-that portion of the absorbed nutriment which goes to the propagation
-of the race. Such being the conditions of the case, several kinds of
-variations are possible. All other circumstances remaining the same,
-there may be changes of proportion between the steam used for working
-the engine and the steam that escapes by the safety-valve. There may
-be a structural or organic change of proportion. By enlarging the
-safety-valve or weakening its spring, while the cylinders are reduced
-in size, there may be established a constitutionally-small power of
-locomotion and a constitutionally-large amount of escape-steam; and
-inverse variations so produced, will answer to the inverse variations
-between Individuation and Genesis which different types of organisms<span class="pagenum" id="Page_518">[518]</span>
-show us. Again, there may be a functional change of proportion. If the
-engine has to draw a considerable load, the abstraction of steam by
-the cylinders greatly reduces the discharge by the safety-valve; and
-if a high velocity is kept up, the discharge from the safety-valve
-entirely ceases. Conversely, if the velocity is low, the escape-steam
-bears a large ratio to the steam consumed by the motor apparatus;
-and if the engine becomes stationary the whole of the steam escapes
-by the safety-valve. This inverse variation answers to that which
-we have traced between Expenditure and Genesis, as displayed in the
-contrasts between species of the same type but unlike activities,
-and in the contrasts between active and inactive individuals of the
-same species. But now beyond these inverse variations between the
-quantities of consumed steam and escape-steam, which are structurally
-and functionally caused, there are coincident variations, producible
-in both by changes in the quantity of steam supplied&mdash;changes which
-may be caused in several ways. In the first place, the fuel thrown
-into the furnace may be increased or made better. Other things equal,
-there will result a more active locomotion as well as a greater escape;
-and this will answer to that simultaneous addition to its individual
-vigour and its reproductive activity, caused in an animal by a larger
-quantity, or a superior quality, of food. In the second place, the
-steam generated may be economized. Loss by radiation from the boiler
-may be lessened by a covering of non-conducting substances; and part
-of the steam thus prevented from condensing, will go to increase the
-working power of the engine, while part will be added to the quantity
-blowing off. This variation corresponds to that simultaneous addition
-to bodily vigour and propagative power, which results in animals
-that have to expend less in keeping up their temperatures. In the
-third place, by improvement of the steam-generating apparatus, more
-steam may be obtained from a given weight of fuel. A better-formed
-evaporating surface, or boiler tubes which conduct more rapidly, or
-an increased number of them<span class="pagenum" id="Page_519">[519]</span> may cause a larger absorption of heat
-from the burning mass or the hot gases it gives off; and the extra
-steam generated by this extra heat will, as before, augment both the
-motive force and the emission through the safety-valve. And this last
-case of coincident variation, is parallel to the case with which we
-are here concerned&mdash;the augmentation of individual expenditure and of
-reproductive energy, that may be caused by a superiority of some organ
-on which the utilizing or economizing of materials depends.</p>
-
-<p>Manifestly, therefore, an increased expenditure for Genesis, or an
-increased expenditure for Individuation, may arise in one of two quite
-different ways&mdash;either by diminution of the antagonistic expenditure,
-or by addition to the store which supplies both expenditures; and
-confusion results from not distinguishing between these. Given the
-ratio 4 to 20, as expressive of the relative costs of Genesis and
-Individuation; then the expenditure for Genesis may be raised to 5
-while the expenditure for Individuation is raised to 25, without any
-alteration of type, merely by favourable circumstances or superiority
-of constitution. On the other hand, circumstances remaining the same,
-the expenditure for Genesis may be raised from 4 to 5, by lowering the
-expenditure for Individuation from 20 to 19: which change of ratio may
-be either functional and temporary, or structural and permanent. And
-only when it is the last does it illustrate that inverse variation
-between degree of evolution and degree of procreative dissolution,
-which we have everywhere seen.</p>
-
-
-<p class="p2">§ 370. There is no reason to suppose, then, that the laws of
-multiplication which hold of other beings, do not hold of the human
-being. On the contrary, there are special facts which unite with
-general implications to show that these laws do hold of the human
-being. The absence of direct evidence in some cases where it might be
-looked for, we find fully explained when all the factors are taken into
-account. And certain seemingly-adverse facts prove, on examination,<span class="pagenum" id="Page_520">[520]</span> to
-be facts belonging to a different category from that in which they are
-placed, and harmonize with the rest when rightly interpreted.</p>
-
-<p>The conformity of human fertility to the laws of multiplication in
-general, being granted, it remains to inquire what effects must be
-caused by permanent changes in men’s natures and circumstances. Thus
-far we have observed how, by their exceptionally-high evolution and
-exceptionally-low fertility, mankind display the inverse variation
-between Individuation and Genesis, in one of its extremes. And we have
-also observed how mankind, like other kinds, are functionally changed
-in their rates of multiplication by changes of conditions. But we have
-not observed how alteration of structure in Man entails alteration
-of fertility. The influence of this factor is so entangled with the
-influences of other factors which are for the present more potent, that
-we cannot recognize it. Here, if we proceed at all, we must proceed
-deductively.</p>
-
-<hr class="tb" />
-
-<p>[<span class="smcap">Note.</span>&mdash;From among the publications of the American Academy of
-Political and Social Science, there was sent to me some years ago an
-essay entitled “The Significance of a Decreasing Birth Rate” by (Miss)
-J. L. Brownell, Fellow in Political Science, Bryn Mawr College. This
-essay contains a number of elaborate comparisons drawn from the vital
-statistics of the tenth United States Census. The results of these
-comparisons are thus summed up:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“1. Whether or not it be true that the means spoken of by Dr.
-Billings, M. Dumont, M. Levasseur, and Dr. Edson has become an
-important factor in the diminishing birth-rate of civilized
-countries, it is evident that it is not the only factor, and
-that, quite apart from voluntary prevention, there is a distinct
-problem to be investigated. This is shown by the fact that the
-white and the colored birth-rate vary together.</p>
-
-<p>“2. Mr. Spencer’s generalization that the birth-rate diminishes
-as the rate of individual evolution increases is confirmed by a
-comparison of the birth-rates with the death-rates from nervous
-diseases, and also<span class="pagenum" id="Page_521">[521]</span> with the density of population, the values
-of agricultural and manufactured products, and the mortgage
-indebtedness.”</p>
-</div>
-
-<p>Of course multitudinous differences of race, class, mode of living,
-occupation, locality, make it difficult to draw positive inferences
-from the data; but the inferences above drawn are held to remain
-outstanding after allowing for all the qualifying conditions.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_522">[522]</span></p>
-
-<h3>CHAPTER XIII.<br />
-<span class="subhed1">HUMAN POPULATION IN THE FUTURE.</span></h3></div>
-
-
-<p>§ 371. Any further evolution in the most highly-evolved of terrestrial
-beings, Man, must be of the same nature as evolution in general.
-Structurally considered, it may consist in greater integration,
-or greater differentiation, or both&mdash;augmented bulk, or increased
-heterogeneity and definiteness, or a combination of the two.
-Functionally considered, it may consist in a larger sum of actions,
-or more multiplied varieties of actions, or both&mdash;a larger amount of
-sensible and insensible motion generated, or motions more numerous in
-their kinds and more intricate and exact in their co-ordinations, or
-motions that are greater alike in quantity, complexity, and precision.</p>
-
-<p>Expressing the change in terms of that more special evolution displayed
-by organisms; we may say that it must be one which further adapts the
-moving equilibrium of organic actions. As was pointed out in <i>First
-Principles</i>, § 173, “the maintenance of such a moving equilibrium,
-requires the habitual genesis of internal forces corresponding in
-number, directions, and amounts to the external incident forces&mdash;as
-many inner functions, single or combined, as there are single or
-combined outer actions to be met.” And it was also pointed out that
-“the structural complexity accompanying functional equilibration, is
-definable as one in which there are as many specialized parts as are
-capable, separately and jointly, of counteracting the separate and
-joint forces<span class="pagenum" id="Page_523">[523]</span> amid which the organism exists.” Clearly, then, since
-all incompletenesses in Man as now constituted, are failures to meet
-certain of the outer actions (mostly involved, remote, irregular), to
-which he is exposed; every advance implies additional co-ordinations of
-actions and accompanying complexities of organization.</p>
-
-<p>Or, to specialize still further this conception of future progress, we
-may consider it as an advance towards completion of that continuous
-adjustment of internal to external relations, which Life shows us.
-In Part I. of this work, where it was shown that the correspondence
-between inner and outer actions which under its phenomenal aspect,
-we call Life, is a particular kind of what, in terms of Evolution,
-we called a moving equilibrium; it was shown that the degree of life
-varies as the degree of correspondence. Greater evolution or higher
-life implies, then, such modifications of human nature as shall make
-more exact the existing correspondences, or shall establish additional
-correspondences, or both. Connexions of phenomena of a rare, distant,
-unobtrusive, or intricate kind, which we either suffer from or do not
-take advantage of, have to be responded to by new connexions of ideas,
-and acts properly combined and proportioned: there must be increase
-of knowledge, or skill, or power, or of all these. And to effect this
-more extensive, more varied, and more accurate, co-ordination of
-actions, there must be organization of still greater heterogeneity and
-definiteness.</p>
-
-
-<p class="p2">§ 372. Let us, before proceeding, consider in what particular ways this
-further evolution, this higher life, this greater co-ordination of
-actions, may be expected to show itself.</p>
-
-<p>Will it be in strength? Probably not to any considerable degree.
-Mechanical appliances are fast supplanting brute force, and doubtless
-will continue doing this. Though at present civilized nations largely
-depend for self-preservation on vigour of limb, and are likely to do
-so while wars continue;<span class="pagenum" id="Page_524">[524]</span> yet that progressive adaptation to the social
-state which must at last bring wars to an end, will leave the amount
-of muscular power to adjust itself to the requirements of a peaceful
-<i>regime</i>. Though, taking all things into account, the muscular
-power then required may not be less than now, there seems no reason why
-more should be required.</p>
-
-<p>Will it be swiftness or agility? Probably not. In savages these are
-important elements of the ability to maintain life; but in civilized
-men they aid self-preservation in quite minor degrees, and there
-seems no circumstance likely to necessitate an increase of them.
-By games and gymnastic competitions, such attributes may indeed be
-artificially increased; but no artificial increase which does not bring
-a proportionate advantage can be permanent; since, other things equal,
-individuals and societies that devote the same amounts of energy in
-ways which subserve life more effectually, must by and by predominate.</p>
-
-<p>Will it be in mechanical skill, that is, in the better-co-ordination
-of complex movements? Most likely in some degree. Awkwardness is
-continually entailing injuries and deaths. Moreover the complicated
-tools which civilization brings into use, are constantly requiring
-greater delicacy of manipulation. All the arts, industrial and
-æsthetic, as they develop, imply a corresponding development of
-perceptive and executive faculties in men: the two act and react.</p>
-
-<p>Will it be in intelligence? Largely, no doubt. There is ample room
-for advance in this direction, and ample demand for it. Our lives are
-universally shortened by our ignorance. In attaining complete knowledge
-of our own natures and of the natures of surrounding things&mdash;in
-ascertaining the conditions of existence to which we must conform,
-and in discovering means of conforming to them under all variations
-of seasons and circumstances; we have abundant scope for intellectual
-progress.</p>
-
-<p>Will it be in morality, that is, in greater power of self-regulation?<span class="pagenum" id="Page_525">[525]</span>
-Largely also: perhaps most largely. Right conduct is usually come short
-of more from defect of will than defect of knowledge. For the right
-co-ordination of those complex actions which constitute human life in
-its civilized form, there goes not only the pre-requisite&mdash;recognition
-of the proper course; but the further pre-requisite&mdash;a due impulse to
-pursue that course. On calling to mind our daily failures to fulfil
-often-repeated resolutions, we shall perceive that lack of the needful
-desire, rather than lack of the needful insight, is the chief cause of
-faulty action. A further endowment of those feelings which civilization
-is developing in us&mdash;sentiments responding to the requirements of the
-social state&mdash;emotive faculties that find their gratifications in the
-duties devolving on us&mdash;must be acquired before the crimes, excesses,
-diseases, improvidences, dishonesties, and cruelties, that now so
-greatly diminish the duration of life, can cease.</p>
-
-<p>Thus, looking at the several possibilities, and asking what direction
-this further evolution, this more complete moving equilibrium, this
-better adjustment of inner to outer relations, this more perfect
-co-ordination of actions, is likely to take; we conclude that it must
-take mainly the direction of a higher intellectual and emotional
-development.</p>
-
-
-<p class="p2">§ 373. This conclusion we shall find equally forced on us if we
-inquire for the causes which are to bring about such results. No
-more in the case of Man than in the case of any other being, can
-we presume that evolution has taken place, or will hereafter take
-place, spontaneously. In the past, at present, and in the future, all
-modifications, functional and organic, have been, are, and must be,
-immediately or remotely consequent on surrounding conditions. What,
-then, are those changes in the environment to which, by direct or
-indirect equilibration, the human organism has been adjusting itself,
-is adjusting itself now, and will continue to<span class="pagenum" id="Page_526">[526]</span> adjust itself? And how
-do they necessitate a higher evolution of the organism?</p>
-
-<p>Civilization, everywhere having for its antecedent the increase
-of population, and everywhere having for one of its consequences
-a decrease of certain race-destroying forces, has for a further
-consequence an increase of certain other race-destroying forces. Danger
-of death from predatory animals lessens as men grow more numerous.
-Though, as they spread over the Earth and divide into tribes, men
-become wild beasts to one another, yet the danger of death from this
-cause also diminishes as tribes coalesce into nations. But the danger
-of death which does not diminish, is that produced by augmentation of
-numbers itself&mdash;the danger from deficiency of food. Supposing human
-nature to remain unchanged, the mortality hence resulting would, on
-the average, rise as human beings multiplied. If mortality, under
-such conditions, does not rise, it must be because the supply of food
-also augments; and this implies some change in human habits wrought
-by stress of human needs. Here, then, is the permanent cause of
-modification to which civilized men are exposed. Though the intensity
-of its action is ever being mitigated in one direction by greater
-production of food, it is, in the other direction, ever being added
-to by the greater production of individuals. Manifestly, the wants of
-their redundant numbers constitute the only stimulus mankind have to
-obtain more necessaries of life. Were not the demand beyond the supply,
-there would be no motive to increase the supply. And manifestly, this
-excess of demand over supply is perennial: this pressure of population,
-of which it is the index, cannot be eluded. Though by the emigration
-that takes place when the pressure arrives at a certain intensity,
-temporary relief is from time to time obtained; yet as, by this
-process, all habitable countries must become peopled, it follows that
-in the end the pressure, whatever it may then be, must be borne in full.</p>
-
-<p>This constant increase of people beyond the means of subsistence<span class="pagenum" id="Page_527">[527]</span>
-causes, then, a never-ceasing requirement for skill, intelligence, and
-self-control&mdash;involves, therefore, a constant exercise of these and
-gradual growth of them. Every industrial improvement is at once the
-product of a higher form of humanity, and demands that higher form of
-humanity to carry it into practice. The application of science to the
-arts, is the bringing to bear greater intelligence for satisfying our
-wants, and implies continued progress of that intelligence. To get more
-produce from the acre, the farmer must study chemistry, must adopt new
-mechanical appliances, and must, by the multiplication of processes,
-cultivate both his own powers and the powers of his labourers. To
-meet the requirements of the market, the manufacturer is perpetually
-improving his old machines and inventing new ones; and by the
-premium of high wages incites artizans to acquire greater skill. The
-daily-widening ramifications of commerce entail on the merchant a need
-for more knowledge and more complex calculations; while the lessening
-profits of the ship-owner force him to build more scientifically, to
-get captains of higher intelligence and better crews. In all cases
-pressure of population is the original cause. Were it not for the
-competition this entails, more thought and energy would not daily be
-spent on the business of life; and growth of mental power would not
-take place. Difficulty in getting a living is alike the incentive to a
-higher education of children, and to a more intense and long-continued
-application in adults. In the mother it prompts foresight, economy,
-and skilful house-keeping; in the father, laborious days and constant
-self-denial. Nothing but necessity could make men submit to this
-discipline; and nothing but this discipline could produce a continued
-progression.</p>
-
-<p>In this case, as in many others, Nature secures each step in advance by
-a succession of trials; which are perpetually repeated, and cannot fail
-to be repeated, until success is achieved. All mankind in turn subject
-themselves more or<span class="pagenum" id="Page_528">[528]</span> less to the discipline described; they either may
-or may not advance under it; but, in the nature of things, only those
-who <i>do</i> advance under it eventually survive. For, necessarily,
-families and races whom this increasing difficulty of getting a living
-which excess of fertility entails, does not stimulate to improvements
-in production&mdash;that is, to greater mental activity&mdash;are on the high
-road to extinction; and must ultimately be supplanted by those whom
-the pressure does so stimulate. This truth we have recently seen
-exemplified in Ireland. And here, indeed, without further illustration,
-it will be seen that premature death, under all its forms and from all
-its causes, cannot fail to work in the same direction. For as those
-prematurely carried off must, in the average of cases, be those in whom
-the power of self-preservation is the least, it unavoidably follows
-that those left behind to continue the race, must be those in whom the
-power of self-preservation is the greatest&mdash;must be the select of their
-generation. So that, whether the dangers to existence be of the kind
-produced by excess of fertility, or of any other kind, it is clear that
-by the ceaseless exercise of the faculties needed to contend with them,
-and by the death of all men who fail to contend with them successfully,
-there is ensured a constant progress towards a higher degree of skill,
-intelligence, and self-regulation&mdash;a better co-ordination of actions&mdash;a
-more complete life.<a id="FNanchor_67" href="#Footnote_67" class="fnanchor">[67]</a></p>
-
-
-<p class="p2">§ 374. The proposition at which we have thus arrived is, then, that
-excess of fertility, through the changes it is ever<span class="pagenum" id="Page_529">[529]</span> working in Man’s
-environment, is itself the cause of Man’s further evolution; and the
-obvious corollary here to be drawn is, that Man’s further evolution so
-brought about, itself necessitates a decline in his fertility.</p>
-
-<p>All future progress in civilization which the never-ceasing pressure
-of population must produce, will be accompanied by an enhanced cost of
-Individuation, both in structure and function; and more especially in
-nervous structure and function. The peaceful struggle for existence in
-societies ever growing more crowded and more complicated, must have
-for its concomitant an increase of the great nervous centres in mass,
-in complexity, in activity. That larger body of emotion needed as a
-fountain of energy for men who have to hold their places and rear their
-families under the intensifying competition of social life, is, other
-things equal, the correlative of larger brain. Those higher feelings
-presupposed by the better self-regulation which, in a better society,
-can alone enable the individual to leave a persistent posterity, are,
-other things equal, the correlatives of a more complex brain; as are
-also those more numerous, more varied, more general, and more abstract
-ideas, which must also become increasingly requisite for successful
-life as society advances. And the genesis of this larger quantity of<span class="pagenum" id="Page_530">[530]</span>
-feeling and thought, in a brain thus augmented in size and developed
-in structure, is, other things equal, the correlative of a greater
-wear of nervous tissue and greater consumption of materials to repair
-it. So that both in original cost of construction and in subsequent
-cost of working, the nervous system must become a heavier tax on the
-organism. Already the brain of the civilized man is larger by nearly
-thirty per cent. than the brain of the savage. Already, too, it
-presents an increased heterogeneity&mdash;especially in the distribution
-of its convolutions. And further changes like these which have taken
-place under the discipline of civilized life, we infer will continue
-to take place. But everywhere and always, evolution is antagonistic
-to procreative dissolution. Whether it be in greater growth of the
-organs which subserve self-maintenance, whether it be in their added
-complexity of structure, or whether it be in their higher activity,
-the abstraction of the required materials implies a diminished reserve
-of materials for race-maintenance. And we have seen reason to believe
-that this antagonism between Individuation and Genesis, becomes
-unusually marked where the nervous system is concerned, because of the
-costliness of nervous structure and function. In <a href="#sect346">§ 346</a> was pointed out
-the apparent connexion between high cerebral development and prolonged
-delay of sexual maturity; and in <a href="#sect366">§§ 366</a>, <a href="#sect367">367</a>, the evidence went to show
-that where exceptional fertility exists there is sluggishness of mind,
-and that where there has been during education excessive expenditure
-in mental action, there frequently follows a complete or partial
-infertility. Hence the particular kind of further evolution which Man
-is hereafter to undergo, is one which, more than any other, may be
-expected to cause a decline in his power of reproduction.</p>
-
-<p>The higher nervous development and greater expenditure in nervous
-action, here described as indirectly brought about by increase of
-numbers, and as thereafter becoming a check on the increase of numbers,
-must not be taken to imply<span class="pagenum" id="Page_531">[531]</span> an intenser strain&mdash;a mentally-laborious
-life. The greater emotional and intellectual power and activity above
-contemplated, must be understood as becoming, by small increments,
-organic, spontaneous, and pleasurable. As, even when relieved from
-the pressure of necessity, large-brained Europeans voluntarily enter
-on enterprises and activities which the savage could not keep up even
-to satisfy urgent wants; so, their still larger-brained descendants
-will, in a still higher degree, find their gratifications in careers
-entailing still greater mental expenditures. This enhanced demand for
-materials to establish and carry on the psychical functions, will be a
-constitutional demand. We must conceive the type gradually so modified,
-that the more-developed nervous system irresistibly draws off, for its
-normal and unforced activities, a larger proportion of the common stock
-of nutriment; and while so increasing the intensity, completeness, and
-length of the individual life, necessarily diminishing the reserve
-applicable to the setting up of new lives&mdash;no longer required to be so
-numerous.</p>
-
-<p>Though the working of this process will doubtless be interfered
-with and modified in the future, as it has been in the past, by the
-facilitations of living which civilization brings; yet nothing beyond
-temporary interruptions can so be caused. However much the industrial
-arts may be improved, there must be a limit to the improvement; while,
-with a rate of multiplication in excess of the rate of mortality,
-population must continually tread on the heels of production. So that
-though, during the earlier stages of civilization, an increased amount
-of food may accrue from a given amount of labour, there must come a
-time when this relation will be reversed, and when every additional
-increment of food will be obtained by a more than proportionate labour:
-the disproportion growing ever higher, and the diminution of the
-reproductive power becoming greater.</p>
-
-
-<p class="p2">§ 375. There now remains but to inquire towards what<span class="pagenum" id="Page_532">[532]</span> limit this
-progress tends. So long as the fertility of the race is more than
-sufficient to balance the diminution by deaths, population must
-continue to increase. So long as population continues to increase,
-there must be pressure on the means of subsistence. And so long
-as there is pressure on the means of subsistence, further mental
-development must go on, and further diminution of fertility must
-result; provided that the actions and reactions which have been
-described are not artificially interfered with. I append this
-qualifying clause advisedly, and especially emphasize it, because
-these actions and reactions have been hitherto, and are now,
-greatly interfered with by governments, and the continuance of the
-interferences may retard, if not stop, that further evolution which
-would else go on.</p>
-
-<p>I refer to those hindrances to the survival of the fittest which
-in earlier times resulted from the undiscriminating charities of
-monasteries and in later times from the operation of Poor Laws. Of
-course if the competition which increasing pressure of population
-entails, is prevented from acting on a considerable part of the
-community, such part, saved from the needed intellectual and moral
-stress, will not undergo any further mental development; and must
-ever tend to leave a posterity, and an increasing posterity, in which
-none of that higher individuation which checks genesis takes place.
-Such State-meddlings with the natural play of actions and reactions
-produce a further evil equally great or greater. For those who are
-not self-maintained, or but partially self-maintained, are supplied
-with the means they lack by the better members of the community; and
-these better members have thus not only to support themselves and
-their offspring, but also to support or aid the inferior members and
-their offspring. The under-working of one part is accompanied by the
-over-working of the other part&mdash;by a working which at each stage of
-progress exceeds that which the normal conditions necessitate, and
-results sometimes in illness, premature age, or death, or in lessened<span class="pagenum" id="Page_533">[533]</span>
-number of children, or in imperfect rearing of children: the bad are
-fostered and the good are repressed.</p>
-
-<p>It does not follow that the struggle for life and the survival of the
-fittest must be left to work out their effects without mitigation. It
-is contended only that there shall not be a forcible burdening of the
-superior for the support of the inferior. Such aid to the inferior as
-the superior voluntarily yield, kept as it will be within moderate
-limits, may be given with benefit to both&mdash;relief to the one, moral
-culture to the other. And aid willingly given (little to the least
-worthy and more to the most worthy) will usually be so given as not
-to further the increase of the unworthy. For in proportion as the
-emotional nature becomes more evolved, and there grows up a higher
-sense of parental responsibility, the begetting of children that cannot
-be properly reared will be universally held intolerable. If, as we
-see, public opinion in many places and times becomes coercive enough
-to force men to fight duels, we can scarcely doubt that at a higher
-stage of evolution it may become so coercive as to prevent men from
-marrying improvidently. If the frowns of their fellows can make men
-commit immoral acts, surely they may make men refrain from immoral
-acts&mdash;especially when the actors themselves feel that the threatened
-frowns would be justified. Hence with a higher moral nature will come a
-restriction on the multiplication of the inferior.</p>
-
-<p>In brief, the sole requirement is that there shall be no extensive
-suspension of that natural relation between merit and benefit which
-constitutes justice. Holding, then, that this all-essential condition
-will itself come to be recognized and enforced by a more evolved
-humanity, let us consider what is the goal towards which the restraint
-on genesis by individuation progresses.</p>
-
-
-<p class="p2">§ 375<i>a</i>. Supposing the Sun’s light and heat, on which all
-terrestrial life depends, to continue abundant for a period long
-enough to allow the entire evolution we are contemplating;<span class="pagenum" id="Page_534">[534]</span> there are
-still certain changes which must prevent such complete adjustment of
-human nature to surrounding conditions, as would permit the rate of
-multiplication to become equal to the rate of mortality. As before
-pointed out (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect148">§ 148</a>), during an epoch of 21,000 years each hemisphere
-goes through a cycle of temperate seasons and seasons extreme in their
-heat and cold&mdash;variations which are themselves alternately exaggerated
-and mitigated in the course of far longer cycles; and we saw that
-these cause perpetual ebbings and flowings of species over different
-parts of the Earth’s surface. Further, by slow but inevitable geologic
-changes, especially those of elevation and subsidence, the climate and
-physical characters of every habitat are modified; while old habitats
-are destroyed and new are formed. This, too, we noted as a constant
-cause of migrations and of resulting alterations of environment. Now
-though the human race differs from other races in having a power of
-artificially counteracting external changes, yet there are limits to
-this power; and, even were there no limits, the changes could not
-fail to work their effects indirectly, if not directly. If, as is
-thought probable, these astronomic cycles entail recurrent glacial
-periods in each hemisphere, then parts of the Earth which are at one
-time thickly peopled, will at another time be almost deserted, and
-<i>vice versâ</i>. The geologically-caused alterations of climate and
-surface, must produce further slow re-distributions of population;
-and other currents of people, to and from different regions, will be
-necessitated by the rise of successive centres of higher civilization.
-Consequently, mankind cannot but continue to undergo changes of
-environment, physical and moral, analogous to those which they have
-thus far been undergoing. Such changes may eventually become slower
-and less marked; but they can never cease. And if they can never cease
-there can never arise a perfect adaptation of human nature to its
-conditions of existence. To establish that complete correspondence
-between inner and outer actions which constitutes the highest life
-and<span class="pagenum" id="Page_535">[535]</span> greatest power of self-preservation, there must be a prolonged
-converse between the organism and circumstances which remain the
-same. If the external relations are being altered while the internal
-relations are being adjusted to them, the adjustment can never become
-exact. And in the absence of exact adjustment, there cannot exist that
-theoretically-highest power of self-preservation with which there would
-co-exist the theoretically-lowest power of race-production.</p>
-
-<p>Hence though the number of premature deaths may ultimately become very
-small, it can never become so small as to allow the average number of
-offspring from each pair to fall so low as two. Some average number
-between two and three may be inferred as the limit&mdash;a number, however,
-which is not likely to be quite constant, but may be expected at
-one time to increase somewhat and afterwards to decrease somewhat,
-according as variations in physical and social conditions lower or
-raise the cost of self-preservation.</p>
-
-<p>To this qualification must be added a further qualification. The
-foregoing argument tacitly assumes that the causes described will
-continuously operate on all mankind; whereas a survey of the facts
-makes it clear that some parts only of the Earth’s surface are capable
-of bearing high types of civilization, and consequently high types
-of Man. There must remain hereafter, as there are now, considerable
-parts of its surface which can support only groups of nomads, or other
-groups obliged by their habitats to lead simple and inferior kinds of
-life. Only by subjection to the discipline we have been contemplating
-can there be produced the fully-developed Man; and evidently in many
-parts of the world this discipline will continue to be eluded. Not
-only must we conclude that the varieties of our race now living in
-desert regions and arctic climates will continue hereafter to do so,
-but we may conclude that always, as now, a certain proportion of men
-who are born in civilized societies, impatient of the stress which
-pressure of population puts on them, will escape into unoccupied or
-sparsely-peopled<span class="pagenum" id="Page_536">[536]</span> regions, where they can lead unrestrained lives
-though lives of hardship. Recognizing as we must the probability
-that in common with all other things, humanity will continue to
-differentiate and produce a more heterogeneous assemblage of types,
-we must infer that only in some of the highest of these will the
-antagonism of individuation and genesis have the anticipated effects.</p>
-
-<p>Restricting ourselves to these, then, we may conclude that in the
-end, pressure of population and its accompanying evils will almost
-disappear; and will leave a state of things requiring from each
-individual little more than a normal and pleasurable activity.
-Cessation in the decrease of fertility implies cessation in the
-development of the nervous system; and this implies a nervous system
-which has become equal to all that is demanded of it&mdash;has not to do
-more than is natural to it. But that exercise of faculties which does
-not exceed what is natural, constitutes gratification.</p>
-
-<p>The necessary antagonism of Individuation and Genesis, not only,
-then, fulfils the <i>à priori</i> law of maintenance of race, from
-the monad up to Man, but ensures final attainment of the highest form
-of this maintenance&mdash;a form in which the amount of life shall be the
-greatest possible and the births and deaths the fewest possible. From
-the beginning pressure of population has been the proximate cause of
-progress. It produced the original diffusion of the race. It compelled
-men to abandon predatory habits and take to agriculture. It led to
-the clearing of the Earth’s surface. It forced men into the social
-state; made social organization inevitable; and has developed the
-social sentiments. It has stimulated to progressive improvements
-in production, and to increased skill and intelligence. It is
-daily thrusting us into closer contact and more mutually-dependent
-relationships. And after having caused, as it ultimately must, the due
-peopling of the globe, and the raising of its habitable parts into the
-highest state of culture&mdash;after having perfected all processes for the
-satisfaction of human wants&mdash;after<span class="pagenum" id="Page_537">[537]</span> having, at the same time, developed
-the intellect into competence for its work, and the feelings into
-fitness for social life&mdash;after having done all this, the pressure of
-population must gradually approach to an end&mdash;an end, however, which
-for the reasons given it cannot absolutely reach.</p>
-
-
-<p class="p2">§ 377. In closing the argument let us not overlook the
-self-sufficingness of those universal processes by which the results
-reached thus far have been wrought out, and which may be expected to
-work out these future results.</p>
-
-<p>Evolution under all its aspects, general and special, is an advance
-towards equilibrium. We have seen that the theoretical limit towards
-which the integration and differentiation of every aggregate advances,
-is a state of balance between all the forces to which its parts are
-subject, and the forces which its parts oppose to them (<i>First
-Prin.</i> § 170). And we have seen that organic evolution is a progress
-towards a moving equilibrium completely adjusted to environing actions.</p>
-
-<p>It has been also pointed out that, in civilized Man, there is going
-on a new class of equilibrations&mdash;those between his actions and the
-actions of the societies he forms (<i>First Prin.</i> § 175). Social
-restraints and requirements are ever altering his activities and by
-consequence his nature; and as fast as his nature is altered, social
-restraints and requirements undergo more or less re-adjustment. Here
-the organism and the conditions are both modifiable; and by successive
-conciliations of the two, there is effected a progress towards
-equilibrium.</p>
-
-<p>More recently we have seen that in every species, there establishes
-itself an equilibrium of an involved kind between the total
-race-destroying forces and the total race-preserving forces&mdash;an
-equilibrium which implies that where the ability to maintain individual
-life is small, the ability ta propagate must be great, and <i>vice
-versâ</i>. Whence it follows that the evolution of a race more in
-equilibrium with the environment, is also the evolution of a race in
-which there is a correlative<span class="pagenum" id="Page_538">[538]</span> approach towards equilibrium between the
-number of new individuals produced and the number which survive and
-propagate.</p>
-
-<p>The final result to be observed is that in Man, all these
-equilibrations between constitution and conditions, between the
-structure of society and the nature of its members, between fertility
-and mortality, advance simultaneously towards a common climax. In
-approaching an equilibrium between his nature and the ever-varying
-circumstances of his inorganic environment, and in approaching an
-equilibrium between his nature and all the requirements of the social
-state, Man is at the same time approaching that lowest limit of
-fertility at which the equilibrium of population is maintained by the
-addition of as many infants as there are subtractions by death in old
-age. But in a universe of which all parts are in motion and every
-part is consequently subject to change of conditions, neither this
-equilibrium nor any other equilibrium can become complete.</p>
-
-
-<p class="center p-left sm p4">THE END.</p>
-
-<p><span class="pagenum" id="Page_539">[539]</span></p>
-
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_541">[541]</span></p>
-
-<h2>APPENDICES.</h2>
-</div>
-
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-
-<h3>APPENDIX A.<br />
-<span class="subhed1">SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS.</span></h3></div>
-
-
-<p>I append here the evidences referred to in <a href="#sect190">§ 190</a>. The most numerous and
-striking I have met with among the <i>Umbelliferæ</i>. Monstrosities
-having the alleged implication, are frequent in the common
-Cow-Parsnep&mdash;so frequent that they must be familiar to botanists; and
-wild Angelica supplies many over-developments of like meaning. Omitting
-numerous cases of more or less significance, I will limit myself to two.</p>
-
-  <div class="figcenter" id="i_541">
-     <img
-    class="p2"
-    src="images/i_541.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 69.</p>
-  </div>
-
-<p class="p2">One of them is that of a terminal umbel, in which nine of the outer
-umbellules are variously transformed&mdash;here a single flower being
-made monstrous by the development of some of its members into buds;
-there several such malformed flowers being associated with rays that
-bear imperfect umbellules; and elsewhere, flowers being replaced by
-umbellules: some of which are perfect, and others imperfect only in the
-shortness of the flower-stalks. The annexed Fig. <a href="#i_541">69</a>, representing in
-a somewhat conventionalized way, a part of<span class="pagenum" id="Page_542">[542]</span> the dried specimen, will
-give an idea of this Angelica. At <i>a</i> is shown a single flower
-partially changed; in the umbellule marked <i>b</i>, one of the rays
-bears a secondary umbellule; and there may be seen at <i>c</i> and
-<i>d</i>, several such over-developments.</p>
-
-<p>But the most conclusive instance is that of a Cow-Parsnep, in which a
-single terminal umbel, besides the transformations already mentioned,
-exhibits higher degrees of such transformations.<a id="FNanchor_68" href="#Footnote_68" class="fnanchor">[68]</a> The components
-of this complex growth are;&mdash;three central umbellules, abnormal only
-in minor points; one umbellule, external to these, which is partially
-changed into an umbel; one rather more out of the centre, which is
-so far metamorphosed as to be more an umbel than an umbellule: nine
-peripheral clusters formed by the development of umbellules into
-umbels, some of which are partially compounded still further. Examined
-in detail, these structures present the following facts:&mdash;1. The
-innermost umbellule is normal, save in having a peripheral flower of
-which one member (apparently a petal) is transformed into a flower-bud.
-2. The next umbellule, not quite so central, has one of its peripheral
-flowers made monstrous by the growth of a bud from the base of the
-calyx. 3. The third of the central umbellules has two abnormal outer
-flowers. One of them carries a flower-bud on its edge, in place of
-a foliar member. The other is half flower and half umbellule: being
-composed of three petals, three stamens, and five flower-buds growing
-where the other petals and stamens should grow. 4. Outside of these
-umbellules comes one of the mixed clusters. Its five central flowers
-are normal. Surrounding these are several flowers transformed in
-different degrees: one having a stamen partially changed into a flower
-bud. And then, at the periphery of this mixed cluster, come three
-complete umbellules and an incomplete one in which some petals and
-stamens of the original flower remain. 5. A mixed cluster, in which
-the umbel-structure predominates, stands next. Its three central
-flowers are normal. Surrounding them are five flowers over-developed in
-various ways, like those already described. And on its periphery are
-seven complete umbellules in place of flowers; besides an incomplete
-umbellule that contains traces of the original flower, one of them
-being a petal imperfectly twisted up into a bud. 6. Of the nine
-external clusters, in which the development of simple into compound
-umbels is most decided, nearly all present anomalies. Three of them
-have each a central flower untransformed; and in others, the central<span class="pagenum" id="Page_543">[543]</span>
-umbellule is composed of two, three, or four flowers. 7. But the most
-remarkable fact is, that in sundry of these peripheral clusters,
-resulting from the metamorphosis of simple umbels into compound umbels,
-the like metamorphosis is carried a stage higher. Some of the component
-rays, are themselves the bearers of compound umbels instead of simple
-umbels. In Fig. <a href="#i_543">70</a>, a portion of the dried specimen is represented.
-Two of the central umbellules are marked <i>a</i> and <i>b</i>; those
-marked <i>c</i> and <i>d</i> are mixed clusters; at <i>e</i> and
-<i>f</i> are compound umbels replacing simple ones; and <i>g</i> shows
-one of the rays on which the over-development goes still further.</p>
-
-  <div class="figcenter" id="i_543">
-     <img
-    class="p2"
-    src="images/i_543.jpg"
-     alt="" />
-     <p class="p0 center p-left">Fig. 70.</p>
-  </div>
-
-<p class="p2">Does not this evidence, enforced as it is by much more of like kind,
-go far to prove that foliar organs may be developed into axial organs?
-Even were not the transitional forms traceable, there would still,
-I think, be no other legitimate interpretation of the facts last
-detailed. The only way of eluding the conclusion here drawn, is by
-assuming that where a cluster of flowers replaces a single flower,
-it is because the axillary buds which hypothetically belong to the
-several foliar organs of the flower, become developed into axes; and
-assuming this, is basing an hypothesis on another hypothesis that
-is directly at variance with facts. The foliar organs of flowers do
-<i>not</i> bear buds in their axils; and it would never have been
-supposed that such buds are typically present, had it not been for
-that mistaken conception of “type” which has led to many other errors
-in Biology. Goethe writes: “Now as we cannot realize the idea of a
-leaf apart from the node out of which it springs, or of a node without
-a bud, we may venture to infer,” &amp;c. See here an example of a method
-of philosophizing not uncommon among the Germans.<span class="pagenum" id="Page_544">[544]</span> The method is
-this&mdash;Survey a portion of the facts, and draw from them a general
-conception; project this general conception back into the objective
-world, as a mould in which Nature casts her products; expect to find it
-everywhere fulfilled; and allege potential fulfilment where no actual
-fulfilment is visible.</p>
-
-<p>If instead of imposing our ideal forms on Nature, we are content to
-generalize the facts as Nature presents them, we shall find no warrant
-for the morphological doctrine above enunciated. The only conception
-of type justified by the logic of science, is&mdash;that correlation
-of parts which remains constant under all modifications of the
-structure to be defined. To ascertain this, we must compare all these
-modifications, and note what traits are common to them. On doing so
-with the successive segments of a phænogamic axis, we are brought to
-a conclusion widely different from that of Goethe. Axillary buds are
-almost universally absent from the cotyledons; they are habitually
-present in the axils of fully-developed leaves higher up the axis; they
-are often absent from leaves that are close to the flower; they are
-nearly always absent from the bracts; absent from the sepals; absent
-from the petals; absent from the stamens; absent from the carpels.
-Thus, out of eight leading forms which folia assume, one has the
-axillary bud and seven are without it. With these facts before us, it
-seems to me not difficult to “realize the idea” “of a node without a
-bud.” If we are not possessed by a foregone conclusion, the evidence
-will lead us to infer, that each node bears a foliar appendage and
-<i>may</i> bear an axillary bud.</p>
-
-<p>Even, however, were it granted that the typical segment of a Phænogam
-includes an axillary bud, which must be regarded as always potentially
-present, no legitimate counter-interpretation of the monstrosities
-above described could thence be drawn. If when an umbellule is
-developed in place of a flower, the explanation is, that its component
-rays are axillary to the foliar organs of the flower superseded; we may
-fairly require that these foliar organs to which they are axillary,
-shall be shown. But there are none. In the last specimen figured,
-the inner rays of each such umbellule are without them; most of the
-outer rays are also without them; and in one cluster, only a single
-ray has a bract at its point of origin. There is a rejoinder ready,
-however: the foliar organs are said to be suppressed. Though Goethe
-could not “realize the idea” “of a node without a bud,” those who
-accept his typical form appear to find no difficulty in realizing the
-idea of an axillary bud without anything to which it is axillary. But
-letting this pass, suppose we ask what is the warrant for this assumed
-suppression. Axillary buds normally occur where the nutrition is high
-enough to produce fully-developed leaves; and when axillary buds are
-demonstrably present in flowers, they accompany foliar organs that
-are more leaf-like than usual&mdash;always greener if not always larger.
-That is to<span class="pagenum" id="Page_545">[545]</span> say, the normal and the abnormal axillary buds, are
-alike the concomitants of foliar organs coloured by that chlorophyll
-which habitually favours foliar development. How, then, can it be
-supposed that when, out of a flower there is developed a cluster
-of flower-bearing rays, the implied excess of nutrition causes the
-foliar organs to abort? It is true that very generally in a branched
-inflorescence, the bracts of the several flower-branches are very
-small (their smallness being probably due to that defective supply of
-certain chlorophyll-forming matters, which is the proximate cause of
-flowering); and it is true that, under these conditions, a flowering
-axis of considerable size, for the development of which chlorophyll is
-less needful, grows from the axil of a dwarfed leaf. But the inference
-that the foliar organ may therefore be entirely suppressed, seems to me
-irreconcilable with the fact, that the foliar organ is always developed
-to some extent <i>before</i> the axillary bud appears. Until it has
-been shown that in some cases a lateral bud first appears, and a foliar
-organ <i>afterwards</i> grows out beneath it, to form its axil, the
-conception of an axillary bud of which the foliar organ is suppressed,
-will remain at variance with the established truths of development.</p>
-
-<hr class="tb" />
-
-<p>The above originally formed a portion of <a href="#sect190">§ 190</a>. I have transferred it
-to the Appendix, partly because it contains too much detail to render
-it fit for the general argument, and partly because the interpretations
-being open to some question, it seemed undesirable to risk compromising
-that argument by including them. The criticisms passed upon these
-interpretations have not, however, sufficed to convince me of their
-incorrectness. Unfortunately, I have since had no opportunity of
-verifying the above statements by microscopic examinations, as I had
-intended.</p>
-
-<p>Though unable to enforce the inference drawn by further facts more
-minutely looked into, I may add some arguments based on facts that are
-well known. One of these is the fact that the so-called axillary bud
-is not universally axillary&mdash;is not universally seated in the angle
-made by the axis and an appended foliar organ. In certain plants the
-axillary bud is placed far above the node, half-way between it and the
-succeeding node. So that not only may a segment of a phæenogamic axis
-be without the axillary bud, but the axillary bud, when present, may be
-removed from that place in which, according to Goethe, it necessarily
-exists. Another fact not congruous with the current doctrine, is the
-common occurrence of “adventitious” buds&mdash;the buds that are put out
-from roots and from old stems or branches bare of leaves. The name
-under which they are thus classed, is meant to imply that they may be
-left out of consideration. Those, however, who have not got a theory to
-save by<span class="pagenum" id="Page_546">[546]</span> putting anomalies out of sight, may be inclined to think that
-the occurrence of buds where they are avowedly unconnected with nodes,
-and are axillary to nothing, tells very much against the assumption
-that every bud implies a node and a corresponding foliar organ. And
-they may also see that the development of these adventitious buds at
-places where there is excess of nutritive materials, favours the view
-above set forth. For if a bud thus arises at a place where it is not
-morphologically accounted for, simply because there happens to be at
-that place an abundance of unorganized protoplasm; then, clearly, it is
-likely that if the mass of protoplasm from which a leaf would usually
-arise, is greatly increased in mass by excess of nutrition, it may
-develop into an axis instead of a leaf.</p>
-
-<hr class="tb" />
-
-<p>Many years after this work was published, I discovered among my papers
-a memorandum which unfortunately I had overlooked, containing further
-evidence in support of the foregoing conclusion. With the omission of
-an error concerning the species of plant, I reproduce this memorandum
-just as it stood:&mdash;</p>
-
-<p>“I found at Dieppe, July 1, 1860, in a garden near the sea a sample of
-cultivated wild flower (I thought it was grown as an ornamental flower)
-in which some of the single flowers of the umbel were developed into
-groups of flowers thus:&mdash;</p>
-
-  <div class="figcenter" id="i_546">
-     <img
-    class="p2"
-    src="images/i_546.jpg"
-     alt="" />
-     <p class="p0 center p-left"></p>
-  </div>
-
-<p class="p2">“In the case where the transformation was fully effected the umbellule
-had <i>six</i> flowers, answering to the <i>six petals</i> of the
-original flowers. In other cases the transformation was incomplete.
-There were instances where but <i>two</i> of the petals were developed
-into flowers; and the other petals remained unchanged. Others in which
-<i>three</i> were developed; and others where four were developed.
-In some cases, too, the development of a petal into a flower was
-imperfect, in the absence of the flower-stalk&mdash;the flowers were sessile
-in the place where the petals would have been. In one case there was an
-<i>imperfect</i> flower sessile; another <i>imperfect</i> flower on a
-short stalk; and three perfect flowers on long stalks.</p>
-
-<p><span class="pagenum" id="Page_547">[547]</span></p>
-
-<p>“I was in some doubt whether the petals or the stamens were developed.
-In cases of imperfect transformation the petals at the base of the
-umbellule seemed to stand in the position of calyx or involucrum,
-giving the idea that the stamens were developed into flowers. But in
-the case where there were <i>six</i> flowers developed there were no
-petals at the base.</p>
-
-<p>“That it was a matter of extra nutrition was shown by this:&mdash;</p>
-
-<p>“1. That they were cultivated as garden flowers.</p>
-
-<p>“2. That where there was one perfectly developed umbellule, it was the
-only one in the umbel.</p>
-
-<p>“3. That where there were three umbellules they were all imperfect.</p>
-
-<p>“4. That in this imperfect umbellule the perfect flowers were on long
-stalks and the imperfect ones sessile.</p>
-
-<p>“5. That the umbellules were on stalks both longer and thicker than
-those of single flowers.”</p>
-
-<hr class="tb" />
-
-<p>[Concerning the foregoing argument at large an expert writes:&mdash;“The
-abnormalities you describe certainly show that an axis may arise
-abnormally in the place of a normal leaf-structure, and every modern
-botanist would be in agreement with you in your criticism of the
-older form of the doctrine of axillary buds. I think we are largely
-emancipated from the dextrous juggling with the arrangements and
-relations of organs which used to pass current as morphology.</p>
-
-<p>“You have quoted sufficient evidence in the text (<a href="#sect190">§ 190</a>) to establish
-the conclusion that no sharp line can be drawn between axes and
-leaf-structure; and a very great deal more could be added in the same
-sense. Petioles for instance, exist which the most highly trained
-histological observer could not distinguish from stems.</p>
-
-<p>“But I must demur to the suggestion that the replacement of one by the
-other is primarily a question of nutrition. We are as ignorant as ever
-of the proximate cause of the production of a leaf or a shoot at a
-certain spot in meristematic tissue.”</p>
-
-<p>To this last remark I had at first made only the reply that the plants
-exhibiting the abnormalities were in all cases excessively luxuriant in
-their growths; but to this I am now able to add a more definite reply.
-The expert from whom I have just quoted, had read this appendix before
-there had been made to it the above addition describing the flower
-from Dieppe; and I was not myself aware, until I came to read over
-this addition, what clear evidence it contains that extra nutrition
-was the cause of these transformations of foliar structures into axial
-structures; but the above paragraphs 1, 2, 3, 4, 5, contain different
-evidences conspiring to prove this.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_548">[548]</span></p>
-
-<h3>APPENDIX B.<br />
-<span class="subhed1">A CRITICISM ON PROF. OWEN’S THEORY OF THE VERTEBRATE SKELETON.</span></h3></div>
-
-<p>[<i>From the</i> <span class="smcap">British &amp; Foreign Medico-Chirurgical Review for
-Oct., 1858.</span>]</p>
-
-<hr class="full" />
-
-<div class="blockquot">
-
-<p class="hangingindent">I. <i>On the Archetype and Homologies of the Vertebrate
-Skeleton.</i> <i>By</i> <span class="smcap">Richard Owen</span>,
-<i>F.R.S.</i>&mdash;<i>London</i>, 1848. <i>pp.</i> 172.</p>
-
-<p class="hangingindent">II. <i>Principes d’Ostéologie Comparée, ou Recherches sur
-l’Archétype et les Homologies du Squelette Vertébré.</i>
-<i>Par</i> <span class="smcap">Richard Owen</span>.&mdash;<i>Paris.</i></p>
-
-<p class="hangingindent"><i>Principles of Comparative Osteology; or, Researches on the
-Archetype and the Homologies of the Vertebrate Skeleton.</i>
-<i>By</i> <span class="smcap">Richard Owen</span>.</p>
-
-<p class="hangingindent">III. <i>On the Nature of Limbs. A Discourse delivered on Friday,
-February 9, at an Evening Meeting of the Royal Institution
-of Great Britain.</i> <i>By</i> <span class="smcap">Richard Owen</span>,
-<i>F.R.S.</i>&mdash;<i>London</i>, 1849. <i>pp.</i> 119.</p>
-</div>
-
-<p>Judging whether another proves his position is a widely different thing
-from proving your own. To establish a general law requires an extensive
-knowledge of the phenomena to be generalized; but to decide whether an
-alleged general law is established by the evidence assigned, requires
-merely an adequate reasoning faculty. Especially is such a decision
-easy where the premises do <i>not</i> warrant the conclusion. It may
-be dangerous for one who has but little previous acquaintance with the
-facts, to say that a generalization is demonstrated; seeing that the
-argument may be one-sided: there may be many facts unknown to him which
-disprove it. But it is not dangerous to give a negative verdict when
-the alleged demonstration<span class="pagenum" id="Page_549">[549]</span> is manifestly insufficient. If the data put
-before him do not bear out the inference, it is competent for every
-logical reader to say so.</p>
-
-<p>From this standpoint, then, we venture to criticize some of Professor
-Owen’s osteological theories. For his knowledge of comparative
-osteology we have the highest respect. We believe that no living man
-has so wide and detailed an acquaintance with the bony structure of
-the <i>Vertebrata</i>. Indeed, there probably has never been any one
-whose information on the subject was so nearly exhaustive. Moreover, we
-confess that nearly all we know of this department of biology has been
-learnt from his lectures and writings. We pretend to no independent
-investigations, but merely to such knowledge of the phenomena as he has
-furnished us with. Our position, then, is such that, had Professor Owen
-simply enunciated his generalizations, we should have accepted them on
-his authority. But he has brought forward evidence to prove them. By
-so doing he has tacitly appealed to the judgments of his readers and
-hearers&mdash;has practically said, “Here are the facts; do they not warrant
-these conclusions?” And all we propose to do, is to consider whether
-the conclusions <i>are</i> warranted by the facts brought forward.</p>
-
-<p>Let us first limit the scope of our criticisms. On that division
-of comparative osteology which deals with what Professor Owen
-distinguishes as “special homologies,” we do not propose to enter. That
-the wing of a bird is framed upon bones essentially parallel to those
-of a mammal’s fore-limb; that the cannon-bone of a horse’s leg answers
-to the middle metacarpal of the human hand; that various bones in the
-skull of a fish are homologous with bones in the skull of a man&mdash;these
-and countless similar facts, we take to be well established. It may
-be, indeed, that the doctrine of special homologies is at present
-carried too far. It may be that, just as the sweeping generalization
-at one time favoured, that the embryonic phases of the higher animals
-represent the adult forms of lower ones, has been found untrue in a
-literal sense, and is acceptable only in a qualified sense; so the
-sweeping generalization that the skeletons of all vertebrate animals
-consist of homologous parts, will have to undergo some modification.
-But that this generalization is substantially true, all comparative
-anatomists agree.</p>
-
-<p>The doctrine which we are here to consider, is quite a separate
-one&mdash;that of “general homologies.” The truth or falsity of this may
-be decided on quite apart from that of the other. Whether certain
-bones in one vertebrate animal’s skeleton correspond with certain
-bones in another’s, or in every other’s, is one question; and whether
-the skeleton of every vertebrate animal is divisible into a series of
-segments, each of which is modelled after the same type, is another
-question. While the first is answered in the affirmative,<span class="pagenum" id="Page_550">[550]</span> the last
-may be answered in the negative; and we propose to give reasons why it
-should be answered in the negative.</p>
-
-<hr class="tb" />
-
-<p>In so far as his theory of the skeleton is concerned, Professor Owen
-is an avowed disciple of Plato. At the conclusion of his <i>Archetype
-and Homologies of the Vertebrate Skeleton</i>, he quotes approvingly
-the Platonic hypothesis of ἰδέαι, “a sort of models, or moulds in
-which matter is cast, and which regularly produce the same number
-and diversity of species.” The vertebrate form in general (see
-diagram of the <i>Archetypus</i>), or else the form of each kind of
-vertebrate animal (see p. 172, where this seems implied), Professor
-Owen conceives to exist as an “idea”&mdash;an “archetypal exemplar on which
-it has pleased the Creator to frame certain of his living creatures.”
-Whether Professor Owen holds that the typical vertebra also exists as
-an “idea,” is not so certain. From the title given to his figure of
-the “ideal typical vertebra,” it would seem that he does; and at p.
-40 of his <i>Nature of Limbs</i>, and indeed throughout his general
-argument, this supposition is implied. But on the last two pages of the
-<i>Archetype and Homologies</i>, it is distinctly alleged that “the
-repetition of similar segments in a vertebral column, and of similar
-elements in a vertebral segment, is analogous to the repetition of
-similar crystals as the result of polarizing force in the growth of an
-inorganic body;” it is pointed out that, “as we descend the scale of
-animal life, the forms of the repeated parts of the skeleton approach
-more and more to geometrical figures;” and it is inferred that “the
-Platonic ἰδέα or specific organizing principle or force, would seem to
-be in antagonism with the general polarizing force, and to subdue and
-mould it in subserviency to the exigencies of the resulting specific
-form.” If Professor Owen’s doctrine is to be understood as expressed
-in these closing paragraphs of his <i>Archetype and Homologies</i>&mdash;if
-he considers that “the ἰδέα” “which produces the diversity of form
-belonging to living bodies of the same materials,” is met by the
-“counter-operation” of “the polarizing force pervading all space,”
-which produces “the similarity of forms, the repetition of parts,
-the signs of unity of organization,” and which is “<i>subdued</i>”
-as we ascend “in the scale of being;” then we may pass on with the
-remark that the hypothesis is too cumbrous and involved to have much
-<i>vraisemblance</i>. If, on the other hand, Professor Owen holds, as
-every reader would suppose from the general tenor of his reasonings,
-that not only does there exist an archetypal or ideal vertebrate
-skeleton, but that there also exists an archetypal or ideal vertebra;
-then he carries the Platonic hypothesis much further than Plato does.
-Plato’s argument, that before any species of object was created it
-must have existed as an idea of the Creative Intelligence, and that
-hence all objects of such species must be<span class="pagenum" id="Page_551">[551]</span> copies of this original
-idea, is tenable enough from the anthropomorphic point of view. But
-while those who, with Plato, think fit to base their theory of creation
-upon the analogy of a carpenter designing and making a table, must
-yield assent to Plato’s inference, they are by no means committed
-to Professor Owen’s expansion of it. To say that before creating a
-vertebrate animal, God must have had the conception of one, does not
-involve saying that God gratuitously bound himself to make a vertebrate
-animal out of segments all moulded after one pattern. As there is
-no conceivable advantage in this alleged adhesion to a fundamental
-pattern&mdash;as, for the fulfilment of the intended ends, it is not only
-needless, but often, as Professor Owen argues, less appropriate than
-some other construction would be (see <i>Nature of Limbs</i>, pp. 39,
-40), to suppose the creative processes thus regulated, is not a little
-startling. Even those whose conceptions are so anthropomorphic as to
-think they honour the Creator by calling him “the Great Artificer,”
-will scarcely ascribe to him a proceeding which, in a human artificer,
-they would consider a not very worthy exercise of ingenuity.</p>
-
-<p>But whichever of these alternatives Professor Owen contends
-for&mdash;whether the typical vertebra is that more or less crystalline
-figure which osseous matter ever tends to assume in spite of “the ἰδέα
-or organizing principle,” or whether the typical vertebra is itself
-an “ἰδέα or organizing principle”&mdash;there is alike implied the belief
-that the typical vertebra has an abstract existence apart from actual
-vertebræ. It is a form which, in every endo-skeleton, strives to embody
-itself in matter&mdash;a form which is potentially present in each vertebra;
-which is manifested in each vertebra with more or less clearness; but
-which, in consequence of antagonizing forces, is nowhere completely
-realized. Apart from the philosophy of this hypothesis, let us here
-examine the evidence which is thought to justify it.</p>
-
-<hr class="tb" />
-
-<p>And first as to the essential constituents of the “ideal typical
-vertebra.” Exclusive of “<i>diverging appendages</i>” which it
-“may also support,” “it consists in its typical completeness of
-the following elements and parts”:&mdash;A <i>centrum</i> round which
-the rest are arranged in a somewhat radiate manner; above it two
-<i>neurapophyses</i>&mdash;converging as they ascend, and forming with the
-centrum a trianguloid space containing the neural axis; a <i>neural
-spine</i>, surmounting the two neurapophyses, and with them completing
-the neural arch; below the centrum two <i>hæmapophyses</i> and a
-<i>hæmal spine</i>, forming a hæmal arch similar to the neural arch
-above, and enclosing the hæmal axis; two <i>pleurapophyses</i>
-radiating horizontally from the sides of the centrum; and
-two <i>parapophyses</i> diverging from the centrum below the
-pleurapophyses. “These,” says Professor Owen, “being usually developed
-from distinct and independent<span class="pagenum" id="Page_552">[552]</span> centres, I have termed ‘autogenous
-elements.’” The remaining elements, which he classes as “exogenous,”
-because they “shoot out as continuations from some of the preceding
-elements,” are the <i>diapophyses</i> diverging from the upper part of
-the centrum as the parapophyses do below, and the <i>zygapophyses</i>
-which grow out of the distal ends of the neurapophyses and hæmapophyses.</p>
-
-<p>If, now, these are the constituents of the vertebrate segment “in its
-typical completeness;” and if the vertebrate skeleton consists of a
-succession of such segments; we ought to have in these constituents,
-representatives of all the elements of the vertebrate skeleton&mdash;at
-any rate, all its essential elements. Are we then to conclude
-that the “diverging appendages,” which Professor Owen regards as
-rudimental limbs, and from certain of which he considers actual
-limbs to be developed, are typically less important than some of the
-above-specified exogenous parts&mdash;say the zygapophyses?</p>
-
-<p>That the meaning of this question may be understood, it will be
-needful briefly to state Professor Owen’s theory of <i>The Nature
-of Limbs</i>; and such criticisms as we have to make on it must be
-included in the parenthesis. In the first place, he aims to show that
-the scapular and pelvic arches, giving insertion to the fore and
-hind limbs respectively, are displaced and modified hæmal arches,
-originally belonging in the one case to the occipital vertebra, and
-in the other case to some trunk-vertebra not specified. In support of
-this assumption of displacement, carried in some cases to the extent of
-<i>twenty-seven</i> vertebræ, Professor Owen cites certain acknowledged
-displacements which occur in the human skeleton to the extent of half
-a vertebra&mdash;a somewhat slender justification. But for proof that such
-a displacement <i>has</i> taken place in the scapular arch, he chiefly
-relies on the fact that in fishes, the pectoral fins, which are the
-homologues of the fore-limbs, are directly articulated to certain bones
-at the back of the head, which he alleges are parts of the occipital
-vertebra. This appeal to the class of fishes is avowedly made on
-the principle that these lowest of the <i>Vertebrata</i> approach
-closest to archetypal regularity, and may therefore be expected to
-show the original relations of the bones more nearly. Simply noting
-the facts that Professor Owen does not give us any transitional forms
-between the alleged normal position of the scapular arch in fishes,
-and its extraordinary displacement in the higher <i>Vertebrata</i>;
-and that he makes no reference to the embryonic phases of the higher
-<i>Vertebrata</i>, which might be expected to exhibit the progressive
-displacement; we go on to remark that, in the case of the pelvic
-arch, he abandons his principle of appealing to the lowest vertebrate
-forms for proof of the typical structure. In fishes, the rudimentary
-pelvis, widely removed from the spinal column, shows no signs of
-having belonged to any vertebra; and here Professor Owen instances
-the perennibranchiate <i>Batrachia</i> as<span class="pagenum" id="Page_553">[553]</span> exhibiting the typical
-structure: remarking that “mammals, birds, and reptiles show the
-rule of connexion, and fishes the exception.” Thus in the case of
-the scapular arch, the evidence afforded by fishes is held of great
-weight, <i>because</i> of their archetypal regularity; while in the
-case of the pelvic arch, their evidence is rejected as exceptional. But
-now, having, as he considers, shown that these bony frames to which
-the limbs are articulated are modified hæmal arches, Professor Owen
-points out that the hæmal arches habitually bear certain “diverging
-appendages;” and he aims to show that the “diverging appendages” of the
-scapular and pelvic arches respectively, are developed into the fore
-and hind limbs. There are several indirect ways in which we may test
-the probability of this conclusion. If these diverging appendages are
-“rudimental limbs”&mdash;“future possible or potential arms, legs, wings,
-or feet,” we may fairly expect them always to bear to the hæmal arches
-a relation such as the limbs do. But they by no means do this. “As the
-vertebræ approach the tail, these appendages are often transferred
-gradually from the pleurapophysis to the parapophysis, or even to the
-centrum and neural arch.” (<i>Arch. and Hom.</i>, p. 93.) Again, it
-might naturally be assumed that in the lowest vertebrate forms, where
-the limbs are but little developed, they would most clearly display
-their alliance with the appendages, or “rudimental limbs,” by the
-similarity of their attachments. Instead of this, however, Professor
-Owen’s drawings show that whereas the appendages are habitually
-attached to the pleurapophyses, the limbs, in their earliest and lowest
-phase, alike in fishes and in the <i>Lepidosiren</i>, are articulated
-to the hæmapophyses. Most anomalous of all, however, is the process
-of development. When we speak of one thing as being developed out of
-another, we imply that the parts next to the germ are the first to
-appear, and the most constant. In the evolution of a tree out of a
-seed, there come at the outset the stem and the radicle; afterwards
-the branches and divergent roots; and still later the branchlets and
-rootlets; the remotest parts being the latest and most inconstant. If,
-then, a limb is developed out of a “diverging appendage” of the hæmal
-arch, the earliest and most constant bones should be the humerus and
-femur; next in order of time and constancy should come the coupled
-bones based on these; while the terminal groups of bones should be the
-last to make their appearance, and the most liable to be absent. Yet,
-as Professor Owen himself shows, the actual mode of development is the
-very reverse of this. At p. 16 of the <i>Archetype and Homologies</i>,
-he says:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“The earlier stages in the development of all locomotive
-extremities are permanently retained or represented in the
-paired fins of fishes. First the essential part of the member,
-the hand or foot, appears: then the fore-arm or leg, both much
-shortened, flattened, and expanded, as in all fins and all
-embryonic rudiments of limbs: finally come the humeral and
-femoral segments; but this stage I have not found attained in
-any fish.”</p>
-</div>
-
-<p><span class="pagenum" id="Page_554">[554]</span></p>
-
-<p>That is to say, alike in ascending through the <i>Vertebrata</i>,
-generally, and in tracing up the successive phases of a mammalian
-embryo, the last-developed and least constant division of the limb, is
-that basic one by which it articulates with the hæmal arch. It seems to
-us that, so far from proving his hypothesis, Professor Owen’s own facts
-tend to show that limbs do not belong to the vertebræ at all: that they
-make their first appearance peripherally; that their development is
-centripetal; and that they become fixed to such parts of the vertebrate
-axis as the requirements of the case determine.</p>
-
-<p>But now, ending here this digressive exposition and criticism,
-and granting the position that limbs “are developments of costal
-appendages,” let us return to the question above put&mdash;Why are not
-these appendages included as elements of the “ideal typical vertebra?”
-It cannot be because of their comparative inconstancy; for judging
-from the illustrative figures, they seem to be as constant as the
-hæmal spine, which is one of the so-called autogenous elements: in
-the diagram of the <i>Archetypus</i>, the appendage is represented as
-attached to every vertebrate segment of the head and trunk, which the
-hæmal spine is not. It cannot be from their comparative unimportance;
-seeing that as potential limbs they are essential parts of nearly all
-the <i>Vertebrata</i>&mdash;much more obviously so than the diapophyses
-are. If, as Professor Owen argues, “the divine mind which planned the
-archetype also foreknew all its modifications;” and if, among these
-modifications, the development of limbs out of diverging appendages
-was one intended to characterize all the higher <i>Vertebrata</i>;
-then, surely, these diverging appendages must have been parts of the
-“ideal typical vertebra.” Or, if the “ideal typical vertebra” is to
-be understood as a crystalline form in antagonism with the organizing
-principle; then why should not the appendages be included among its
-various offshoots? We do not ask this question because of its intrinsic
-importance. We ask it for the purpose of ascertaining Professor Owen’s
-method of determining what are true vertebral constituents. He presents
-us with a diagram of the typical vertebra, in which are included
-certain bones, and from which are excluded certain others. If relative
-constancy is the criterion, then there arises the question&mdash;What degree
-of constancy entitles a bone to be included? If relative importance
-is the criterion, there comes not only the question&mdash;What degree of
-importance suffices? but the further question&mdash;How is importance to
-be measured? If neither of these is the criterion, then what is it?
-And if there is no criterion, does it not follow that the selection is
-arbitrary?</p>
-
-<hr class="tb" />
-
-<p>This question serves to introduce a much wider one:&mdash;Has the “ideal
-typical vertebra” any essential constituents at all? It might<span class="pagenum" id="Page_555">[555]</span>
-naturally be supposed that though some bones are so rarely developed
-as not to seem worth including, and though some that are included are
-very apt to be absent, yet that certain others are invariable: forming,
-as it were, the basis of the ideal type. Let us see whether the facts
-bear out this supposition. In his “summary of modifications of corporal
-vertebræ” (p. 96), Professor Owen says&mdash;“The <i>hæmal spine</i> is much
-less constant as to its existence, and is subject to a much greater
-range of variety, when present, than its vertical homotype above, which
-completes the neural arch.” Again he says&mdash;“The <i>hæmapophyses</i>,
-as osseous elements of a vertebra, are less constant than the
-pleurapophyses.” And again&mdash;“The <i>pleurapophyses</i> are less
-constant elements than the neurapophyses.” And again&mdash;“Amongst
-air-breathing vertebrates the <i>pleurapophyses</i> of the trunk
-segments are present only in those species in which the septum of
-the heart’s ventricle is complete and imperforate, and here they
-are exogenous and confined to the cervical and anterior thoracic
-vertebræ.” And once more, both the <i>neurapophyses</i> and the
-<i>neural spine</i> “are absent under both histological conditions,
-at the end of the tail in most air-breathing vertebrates, where the
-segments are reduced to their central elements.” That is to say, of
-all the peripheral elements of the “ideal typical vertebra,” there is
-not one which is always present. It will be expected, however, that
-at any rate the <i>centrum</i> is constant: the bone which “forms the
-axis of the vertebral column, and commonly the central bond of union
-of the peripheral elements of the vertebrate (p. 97), is of course an
-invariable element. No: not even this is essential.</p>
-
-<div class="blockquot">
-
-<p>“The centrums do not pass beyond the primitive stage of the
-notochord (undivided column) in the existing lepidosiren, and
-they retained the like rudimental state in every fish whose
-remains have been found in strata earlier than the permian
-æra in Geology, though the number of vertebræ is frequently
-indicated in Devonian and Silurian ichthyolites by the
-fossilized neur-and hæmapophyses and their spines” (p. 96).</p>
-</div>
-
-<p>Indeed, Professor Owen himself remarks that “the neurapophyses are
-more constant as osseous or cartilaginous elements of the vertebræ
-than the centrums” (p. 97). Thus, then, it appears that the several
-elements included in the “ideal typical vertebra” have various degrees
-of constancy, and that no one of them is essential. There is no one
-part of a vertebra which invariably answers to its exemplar in the
-pattern-group. How does this fact consist with the hypothesis? If
-the Creator saw fit to make the vertebrate skeleton out of a series
-of segments, all formed on essentially the same model&mdash;if, for the
-maintenance of the type, one of these bony segments is in many cases
-formed out of a coalesced group of pieces, where, as Professor Owen
-argues, a single piece would have served as well or better; then we
-ought to find this typical repetition of parts<span class="pagenum" id="Page_556">[556]</span> uniformly manifested.
-Without any change of shape, it would obviously have been quite
-possible for every actual vertebra to have contained all the parts
-of the ideal one&mdash;rudimentally where they were not wanted. Even one
-of the terminal bones of a mammal’s tail might have been formed
-out of the nine autogenous pieces, united by suture but admitting
-of identification. As, however, there is no such uniform typical
-repetition of parts, it seems to us that to account for the typical
-repetition which <i>does</i> occur, by supposing the Creator to have
-fixed on a pattern-vertebra, is to ascribe to him the inconsistency of
-forming a plan and then abandoning it.</p>
-
-<p>If, on the other hand, Professor Owen means that the “ideal typical
-vertebra” is a crystalline form in antagonism with “the idea or
-organizing principle;” then we might fairly expect to find it
-most clearly displaying its crystalline character, and its full
-complement of parts, in those places where the organizing principle
-may be presumed to have “subdued” it to the smallest extent. Yet
-in the <i>Vertebrata</i> generally, and even in Professor Owen’s
-<i>Archetypus</i>, the vertebræ of the tail, which must be considered
-as, if anything, less under the influence of the organizing principle
-than those of the trunk, do not manifest the ideal form more
-completely. On the contrary, as we approach the end of the tail, the
-successive segments not only lose their remaining typical elements, but
-become as uncrystalline-looking as can be conceived.</p>
-
-<hr class="tb" />
-
-<p>Supposing, however, that the assumption of suppressed or undeveloped
-elements be granted&mdash;supposing it to be consistent with the hypothesis
-of an “ideal typical vertebra,” that the constituent parts may
-severally be absent in greater or less number, sometimes leaving only
-a single bone to represent them all; may it not be that such parts
-as <i>are</i> present, show their respective typical natures by some
-constant character: say their mode of ossification?</p>
-
-<p>To this question some parts of the <i>Archetype and Homologies</i> seem
-to reply, “Yes;” while others clearly answer, “No.” Criticising the
-opinions of Geoffrey St. Hilaire and Cuvier, who agreed in thinking
-that ossification from a separate centre was the test of a separate
-bone, and that thus there were as many elementary bones in the skeleton
-as there were centres of ossification, Professor Owen points out that,
-according to this test, the human femur, which is ossified from four
-centres, must be regarded as four bones; while the femur in birds and
-reptiles, which is ossified from a single centre, must be regarded as
-a single bone. Yet, on the other hand, he attaches weight to the fact
-that the skull of the human fœtus presents “the same ossific centres”
-as do those of the embryo kangaroo and the young bird. (<i>Nature of
-Limbs</i>, p. 40.) And at p. 104 of the <i>Homologies</i>, after giving
-a number of instances, he says&mdash;</p>
-
-<div class="blockquot">
-
-<p>“These and the like correspondences between the points of
-ossification of<span class="pagenum" id="Page_557">[557]</span> the human fœtal skeleton, and the separate
-bones of the adult skeletons of inferior animals, are pregnant
-with interest, and rank among the most striking illustrations of
-unity of plan in the vertebrate organization.”</p>
-</div>
-
-<p>It is true that on the following page he seeks to explain this seeming
-contradiction by distinguishing</p>
-
-<div class="blockquot">
-
-<p>“between those centres of ossification that have homological
-relations, and those that have teleological ones&mdash;<i>i.e.</i>,
-between the separate points of ossification of a human bone
-which typify vertebral elements, often permanently distinct
-bones in the lower animals; and the separate points which,
-without such signification, facilitate the progress of
-osteogeny, and have for their obvious final cause the well-being
-of the growing animal.”</p>
-</div>
-
-<p>But if there are thus centres of ossification which have homological
-meanings, and others which have not, there arises the question&mdash;How are
-they always to be distinguished? Evidently independent ossification
-ceases to be a homological test, if there are independent ossifications
-that have nothing to do with the homologies. And this becomes the
-more evident when we learn that there are cases where neither a
-homological nor a teleological meaning can be given. Among various
-modes of ossification of the centrum, Professor Owen points out
-that “the body of the human atlas is sometimes ossified from two,
-rarely from three, distinct centres placed side by side” (p. 89);
-while at p. 87 he says:&mdash;“In osseous fishes I find that the centrum
-is usually ossified from six points.” It is clear that this mode of
-ossification has here no homological signification; and it would be
-difficult to give any teleological reason why the small centrum of a
-fish should have more centres of ossification than the large centrum
-of a mammal. The truth is, that as a criterion of the identity or
-individuality of a bone, mode of ossification is quite untrustworthy.
-Though, in his “ideal typical vertebra,” Professor Owen delineates
-and classifies as separate “autogenous” elements, those parts which
-are “usually developed from distinct and independent centres;” and
-though by doing so he erects this characteristic into some sort
-of criterion; yet his own facts show it to be no criterion. The
-parapophyses are classed among the autogenous elements; yet they are
-autogenous in fishes alone, and in these only in the trunk vertebræ,
-while in all air-breathing vertebrates they are, when present at
-all, exogenous. The neurapophyses, again, “lose their primitive
-individuality by various kinds and degrees of confluence:” in the
-tails of the higher <i>Vertebrata</i> they, in common with the neural
-spine, become exogenous. Nay, even the centrum may lose its autogenous
-character. Describing how, in some batrachians, “the ossification of
-the centrum is completed by an extension of bone from the bases of
-the neurapophyses, which effects also the coalescence of these with
-the centrum,” Professor Owen adds:&mdash;“In <i>Pelobates fuscus</i> and
-<i>Pelobates cultripes</i>, Müller found the entire<span class="pagenum" id="Page_558">[558]</span> centrum ossified
-from this source, without any independent points of ossification” (p.
-88). That is to say, the centrum is in these cases an exogenous process
-of the neurapophyses. We see, then, that these so-called typical
-elements of vertebræ have no constant developmental character by which
-they can be identified. Not only are they undistinguishable by any
-specific test from other bones not included as vertebral elements; not
-only do they fail to show their typical characters by their constant
-presence; but, when present, they exhibit no persistent marks of
-individuality. The central element may be ossified from six, four,
-three, or two points; or it may have no separate point of ossification
-at all: and similarly with various of the peripheral elements. The
-whole group of bones forming the “ideal typical vertebra” may severally
-have their one or more ossific centres; or they may, as in a mammal’s
-tail, lose their individualities in a single bone ossified from one or
-two points.</p>
-
-<hr class="tb" />
-
-<p>Another fact which seems very difficult to reconcile with the
-hypothesis of an “ideal typical vertebra,” is the not infrequent
-presence of some of the typical elements in duplicate. Not only, as
-we have seen, may they severally be absent, but they may severally be
-present in greater number than they should be. When we see, in the
-ideal diagram, one centrum, two neurapophyses, two pleurapophyses,
-two hæmapophyses, one neural spine, and one hæmal spine, we naturally
-expect to find them always bearing to each other these numerical
-relations. Though we may not be greatly surprised by the absence of
-some of them, we are hardly prepared to find others multiplied. Yet
-such cases are common. Thus the neural spine “is double in the anterior
-vertebræ of some fishes” (p. 98). Again, in the abdominal region of
-extinct saurians, and in crocodiles, “the freely-suspended hæmapophyses
-are compounded of two or more overlapping bony pieces” (p. 100).
-Yet again, at p. 99, we read&mdash;“I have observed some of the expanded
-pleurapophyses in the great <i>Testudo elephantopus</i> ossified from
-two centres, and the resulting divisions continuing distinct, but
-united by suture.” Once more “the neurapophyses, which do not advance
-beyond the cartilaginous stage in the sturgeon, consist in that fish
-of two distinct pieces of cartilage; and the anterior pleurapophyses
-also consist of two or more cartilages, set end on end” (p. 91). And
-elsewhere referring to this structure, he says:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“Vegetative repetition of perivertebral parts not only manifests
-itself in the composite neurapophyses and pleurapophyses, but
-in a small accessory (interneural) cartilage, at the fore and
-back part of the base of the neurapophysis; and by a similar
-(interhæmal) one at the fore and back part of most of the
-parapophyses” (p. 87).</p>
-</div>
-
-<p>Thus the neural and hæmal spines, the neurapophyses, the
-pleurapophyses,<span class="pagenum" id="Page_559">[559]</span> the hæmapophyses, may severally consist of two or more
-pieces. This is not all: the like is true even of the centrums.</p>
-
-<div class="blockquot">
-
-<p>“In <i>Heptanchus</i> (<i>Squalus cinereus</i>) the vertebral
-centres are feebly and vegetatively marked out by numerous
-slender rings of hard cartilage in the notochordal capsule,
-the number of vertebræ being more definitely indicated by
-the neurapophyses and parapophyses.... In the piked dog-fish
-(<i>Acanthias</i>) and the spotted dog-fish (<i>Scyllium</i>)
-the vertebral centres coincide in number with the neural arches”
-(p. 87).</p>
-</div>
-
-<p>Is it not strange that the pattern-vertebra should be so little adhered
-to, that each of its single typical pieces may be transformed into two
-or three?</p>
-
-<p>But there are still more startling departures from the alleged type.
-The numerical relations of the elements vary not only in this way, but
-in the opposite way. A given part may be present not only in greater
-number than it should be, but also in less. In the tails of homocercal
-fishes, the centrums “are rendered by centripetal shortening and bony
-confluence fewer in number than the persistent, neural, and hæmal
-arches of that part”&mdash;that is, there is only a fraction of a centrum to
-each vertebra. Nay, even this is not the most heteroclite structure.
-Paradoxical as it may seem, there are cases in which the same vertebral
-element is, considered under different aspects, at once in excess and
-defect. Speaking of the hæmal spine, Professor Owen says:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“The horizontal extension of this vertebral element is
-sometimes accompanied by a median division, or in other words,
-it is ossified from two lateral centres; this is seen in the
-development of parts of the human sternum; the same vegetative
-character is constant in the broader thoracic hæmal spines of
-birds; though, sometimes, as <i>e.g.</i>, in the struthionidæ,
-<i>ossification extends from the same lateral centre
-lengthwise&mdash;i.e., forwards and backwards, calcifying the connate
-cartilaginous homologues of halves of four or five hæmal spines,
-before these finally coalesce with their fellows at the median
-line</i>” (p. 101).</p>
-</div>
-
-<p>So that the sternum of the ostrich, which according to the hypothesis,
-should, in its cartilaginous stage, have consisted of <i>four or five
-transverse</i> pieces, answering to the vertebral segments, and should
-have been ossified from four or five centres, one to each cartilaginous
-piece, shows not a trace of this structure; but instead, consists
-of <i>two longitudinal</i> pieces of cartilage, each ossified from
-one centre, and finally coalescing on the median line. These four or
-five hæmal spines have at the same time doubled their individualities
-transversely, and entirely lost them longitudinally!</p>
-
-<hr class="tb" />
-
-<p>There still remains to be considered the test of relative position.
-It might be held that, spite of all the foregoing anomalies, if the
-typical parts of the vertebræ always stood towards each other in the
-same relations&mdash;always preserved the same connexions, something like a
-case would be made out. Doubtless, relative position<span class="pagenum" id="Page_560">[560]</span> is an important
-point; and it is one on which Professor Owen manifestly places great
-dependence. In his discussion of “moot cases of special homology,” it
-is the general test to which he appeals. The typical natures of the
-alisphenoid, the mastoid, the orbito-sphenoid, the prefrontal, the
-malar, the squamosal, &amp;c. he determines almost wholly by reference to
-the adjacent nerve-perforations and the articulations with neighbouring
-bones (see pp. 19 to 72): the general form of the argument being&mdash;This
-bone is to be classed as such or such, <i>because</i> it is connected
-thus and thus with these others, which are so and so. Moreover, by
-putting forth an “ideal typical vertebra,” consisting of a number of
-elements standing towards each other in certain definite arrangement,
-this persistency of relative position is manifestly alleged. The
-essential attribute of this group of bones, considered as a typical
-group, is the constancy in the connexions of its parts: change the
-connexions, and the type is changed. But the constancy of relative
-position thus tacitly asserted, and appealed to as a conclusive test
-in “moot cases of special homology,” is clearly negatived by Professor
-Owen’s own facts. For instance, in the “ideal typical vertebra,” the
-hæmal arch is represented as formed by the two hæmapophyses and the
-hæmal spine; but at p. 91 we are told that</p>
-
-<div class="blockquot">
-
-<p>“The contracted hæmal arch in the caudal region of the body
-may be formed by different elements of the typical vertebra:
-<i>e.g.</i>, by the parapophyses (fishes generally); by
-the pleurapophyses (lepidosiren); by both parapophyses and
-pleurapophyses (<i>Sudis</i>, <i>Lepidosteus</i>), and by
-hæmapophyses, shortened and directly articulated with the
-centrums (reptiles and mammals).”</p>
-</div>
-
-<p>And further, in the thorax of reptiles, birds, and mammals, “the
-hæmapophyses are removed from the centrum, and are articulated to
-the distal ends of the pleurapophyses; the bony hoop being completed
-by the intercalation of the hæmal spine” (p. 82). So that there are
-<i>five</i> different ways in which the hæmal arch may be formed&mdash;four
-modes of attachment of the parts different from that shown in the
-typical diagram! Nor is this all. The pleurapophyses “may be quite
-detached from their proper segment, and suspended to the hæmal arch of
-another vertebra;” as we have already seen, the entire hæmal arch may
-be detached and removed to a distance, sometimes reaching the length
-of twenty-seven vertebræ; and, even more remarkable, the ventral fins
-of some fishes, which theoretically belong to the pelvic arch, are so
-much advanced forward as to be articulated to the scapular arch&mdash;“the
-ischium elongating to join the coracoid.” With these admissions it
-seems to us that relative position and connexions cannot be appealed to
-as tests of homology, nor as evidence of any original type of vertebra.</p>
-
-<p>In no class of facts, then, do we find a good foundation for the
-hypothesis of an “ideal typical vertebra.” There is no one conceivable<span class="pagenum" id="Page_561">[561]</span>
-attribute of this archetypal form which is habitually realised by
-actual vertebræ. The alleged group of true vertebral elements is not
-distinguished in any specified way from bones not included in it. Its
-members have various degrees of inconstancy; are rarely all present
-together; and no one of them is essential. They are severally developed
-in no uniform way: each of them may arise either out of a separate
-piece of cartilage, or out of a piece continuous with that of some
-other element; and each may be ossified from many independent points,
-from one, or from none. Not only may their respective individualities
-be lost by absence, or by confluence with others; but they may be
-doubled, or tripled, or halved, or may be multiplied in one direction
-and lost in another. The entire group of typical elements may coalesce
-into one simple bone representing the whole vertebra; and even, as in
-the terminal piece of a bird’s tail, half-a-dozen vertebræ, with all
-their many elements, may become entirely lost in a single mass. Lastly,
-the respective elements, when present, have no fixity of relative
-position: sundry of them are found articulated to various others than
-those with which they are typically connected; they are frequently
-displaced and attached to neighbouring vertebræ; and they are even
-removed to quite remote parts of the skeleton. It seems to us that if
-this want of congruity with the facts does not disprove the hypothesis,
-no such hypothesis admits of disproof.</p>
-
-<hr class="tb" />
-
-<p>Unsatisfactory as is the evidence in the case of the trunk and tail
-vertebræ, to which we have hitherto confined ourselves, it is far worse
-in the case of the alleged cranial vertebræ. The mere fact that those
-who have contended for the vertebrate structure of the skull, have
-differed so astonishingly in their special interpretations of it, is
-enough to warrant great doubt as to the general truth of their theory.
-From Professor Owen’s history of the doctrine of general homology, we
-gather that Duméril wrote upon “la tête considérée comme <i>une</i>
-vertèbre;” that Kielmeyer, “instead of calling the skull a vertebra,
-said each vertebra might be called a skull;” that Oken recognized in
-the skull <i>three</i> vertebræ and a rudiment; that Professor Owen
-himself makes out <i>four</i> vertebræ; that Goethe’s idea, adopted
-and developed by Carus, was, that the skull is composed of <i>six</i>
-vertebræ; and that Geoffrey St. Hilaire divided it into <i>seven</i>.
-Does not the fact that different comparative anatomists have arranged
-the same group of bones into <i>one</i>, <i>three</i>, <i>four</i>,
-<i>six</i>, and <i>seven</i> vertebral segments, show that the mode of
-determination is arbitrary, and the conclusions arrived at fanciful?
-May we not properly entertain great doubts as to any one scheme being
-more valid than the others? And if out of these conflicting schemes
-we are asked to accept one, ought we not to accept it only on the
-production of some thoroughly conclusive proof&mdash;some<span class="pagenum" id="Page_562">[562]</span> rigorous test
-showing irrefragably that the others must be wrong and this alone
-right? Evidently where such contradictory opinions have been formed by
-so many competent judges, we ought, before deciding in favour of one of
-them, to have a clearness of demonstration much exceeding that required
-in any ordinary case. Let us see whether Professor Owen supplies us
-with any such clearness of demonstration.</p>
-
-<p>To bring the first or occipital segment of the skull into
-correspondence with the “ideal typical vertebra,” Professor
-Owen argues, in the case of the fish, that the parapophyses are
-<i>displaced</i>, and wedged between the neurapophyses and the neural
-spine&mdash;removed from the hæmal arch and built into the upper part of
-the neural arch. Further, he considers that the pleurapophyses are
-<i>teleologically compound</i>. And then, in all the higher vertebrata,
-he alleges that the hæmal arch is <i>separated</i> from its centrum,
-taken to a distance, and transformed into the scapular arch. Add to
-which, he says that in mammals the displaced parapophyses are mere
-processes of the neurapophyses (p. 133): these vertebral elements,
-typically belonging to the lower part of the centrum, and in nearly
-all cases confluent with it, are not only removed to the far ends of
-elements placed above the centrum, but have become exogenous parts of
-them!</p>
-
-<p>Conformity of the second or parietal segment of the cranium with the
-pattern-vertebra, is produced thus:&mdash;The petrosals are <i>excluded</i>
-as being partially-ossified sense-capsules, not forming parts of the
-true vertebral system, but belonging to the “splanchno-skeleton.” A
-centrum is <i>artificially</i> obtained by sawing in two the bone which
-serves in common as centrum to this and the preceding segment; and
-this though it is admitted that in fishes, where their individualities
-ought to be best seen, these two hypothetical centrums are not simply
-coalescent, but connate. Next, a similar <i>arbitrary bisection</i> is
-made of certain elements of the hæmal arches. And then, “the principle
-of <i>vegetative repetition</i> is still more manifest in this arch
-than in the occipital one:” each pleurapophysis is double; each
-hæmapophysis is double; and the hæmal spine consists of six pieces!</p>
-
-<p>The interpretation of the third and fourth segments being of the
-same general character, need not be detailed. The only point calling
-for remark being, that in addition to the above various modes of
-getting over anomalies, we find certain bones referred to the
-<i>dermo-skeleton</i>.</p>
-
-<p>Now it seems to us, that even supposing no antagonist interpretations
-had been given, an hypothesis reconcilable with the facts only by
-the aid of so many questionable devices, could not be considered
-satisfactory; and that when, as in this case, various comparative
-anatomists have contended for other interpretations, the character
-of this one is certainly not of a kind to warrant the rejection of
-the others in its favour; but rather of a kind to make<span class="pagenum" id="Page_563">[563]</span> us doubt the
-possibility of all such interpretations. The question which naturally
-arises is, whether by proceeding after this fashion, groups of bones
-might not be arranged into endless typical forms. If, when a given
-element was not in its place, we were at liberty to consider it as
-<i>suppressed</i>, or <i>connate</i> with some neighbouring element,
-or <i>removed</i> to some more or less distant position;&mdash;if, on
-finding a bone in excess, we might consider it, now as part of the
-<i>dermo-skeleton</i>, now as part of the <i>splanchno-skeleton</i>,
-now as <i>transplanted</i> from its typical position, now as resulting
-from <i>vegetative repetition</i>, and now as a bone <i>teleologically
-compound</i> (for these last two are intrinsically different, though
-often used by Professor Owen as equivalents);&mdash;if, in other cases,
-a bone might be regarded as <i>spurious</i> (p. 91), or again as
-having <i>usurped</i> the place of another;&mdash;if, we say, these various
-liberties were allowed us, we should not despair of reconciling the
-facts with various diagrammatic types besides that adopted by Professor
-Owen.</p>
-
-<p>When, in 1851, we attended a course of Professor Owen’s lectures
-on Comparative Osteology, beginning though we did in the attitude
-of discipleship, our scepticism grew as we listened, and reached
-its climax when we came to the skull; the reduction of which to the
-vertebrate structure, reminded us very much of the interpretation of
-prophecy. The delivery, at the Royal Society, of the Croonian Lecture
-for 1858, in which Professor Huxley, confirming the statements of
-several German anatomists, has shown that the facts of embryology do
-not countenance Professor Owen’s views respecting the formation of the
-cranium, has induced us to reconsider the vertebral theory as a whole.
-Closer examination of Professor Owen’s doctrines, as set forth in his
-works, has certainly not removed the scepticism generated years ago
-by his lectures. On the contrary, that scepticism has deepened into
-disbelief. And we venture to think that the evidence above cited shows
-this disbelief to be warranted.</p>
-
-<hr class="tb" />
-
-<p>There remains the question&mdash;What general views are we to take
-respecting the vertebrate structure? If the hypothesis of an “ideal
-typical vertebra” is not justified by the facts, how are we to
-understand that degree of similarity which vertebræ display?</p>
-
-<p>We believe the explanation is not far to seek. All that our space will
-here allow, is a brief indication of what seems to us the natural view
-of the matter.</p>
-
-<p>Professor Owen, in common with other comparative anatomists, regards
-the divergences of individual vertebræ from the average form, as due
-to adaptive modifications. If here one vertebral element is largely
-developed, while elsewhere it is small&mdash;if now the form, now the
-position, now the degree of coalescence, of a given part varies; it
-is that the local requirements have involved this change. The entire
-teaching of comparative osteology implies that<span class="pagenum" id="Page_564">[564]</span> differences in the
-conditions of the respective vertebræ necessitate differences in their
-structures.</p>
-
-<p>Now, it seems to us that the first step towards a right conception
-of the phenomena, is to recognize this general law in its converse
-application. If vertebræ are unlike in proportion to the unlikeness
-of their circumstances, then, by implication, they will be like in
-proportion to the likeness of their circumstances. While successive
-segments of the same skeleton, and of different skeletons, are all in
-some respects more or less differently acted on by incident forces,
-and are therefore required to be more or less different; they are all,
-in other respects, similarly acted on by incident forces, and are
-therefore required to be more or less similar. It is impossible to
-deny that if differences in the mechanical functions of the vertebræ
-involve differences in their forms; then, community in their mechanical
-functions, must involve community in their forms. And as we know that
-throughout the <i>Vertebrata</i> generally, and in each vertebrate
-animal, the vertebræ, amid all their varying circumstances, <i>have</i>
-a certain community of function, it follows necessarily that they will
-have a certain general resemblance&mdash;there will recur that average shape
-which has suggested the notion of a pattern-vertebra.</p>
-
-<p>A glance at the facts at once shows their harmony with this conclusion.
-In an eel or a snake, where the bodily actions are such as to involve
-great homogeneity in the mechanical conditions of the vertebræ, the
-series of them is comparatively homogeneous. On the contrary, in a
-mammal or a bird, where there is considerable heterogeneity in their
-circumstances, their similarity is no longer so great. And if, instead
-of comparing the vertebral columns of different animals, we compare
-the successive vertebræ of any one animal, we recognize the same law.
-In the segments of an individual spine, where is there the greatest
-divergence from the common mechanical conditions? and where may we
-therefore expect to find the widest departure from the average form?
-Obviously at the two extremities. And accordingly it is at the two
-extremities that the ordinary structure is lost.</p>
-
-<p>Still clearer becomes the truth of this view, when we consider the
-genesis of the vertebral column as displayed throughout the ascending
-grades of the <i>Vertebrata</i>. In its first embryonic stage, the
-spine is an undivided column of flexible substance. In the early
-fishes, while some of the peripheral elements of the vertebræ were
-marked out, the central axis was still a continuous unossified cord.
-And thus we have good reason for thinking that in the primitive
-vertebrate animal, as in the existing <i>Amphioxus</i>, the notochord
-was persistent. The production of a higher, more powerful, more
-active creature of the same type, by whatever method it is conceived
-to have taken place, involved a change in the notochordal structure.
-Greater muscular endowments presupposed a firmer internal fulcrum<span class="pagenum" id="Page_565">[565]</span>
-&mdash;a less yielding central axis. On the other hand, for the central
-axis to have become firmer while remaining continuous, would have
-entailed a stiffness incompatible with the creature’s movements. Hence,
-increasing density of the central axis necessarily went hand in hand
-with its segmentation: for strength, ossification was required; for
-flexibility, division into parts. The production of vertebræ resulting
-thus, there obviously would arise among them a general likeness, due
-to the similarity in their mechanical conditions, and more especially
-the muscular forces bearing on them. And then observe, lastly, that
-where, as in the head, the terminal position and the less space
-for development of muscles, entailed smaller lateral bendings, the
-segmentation would naturally be less decided, less regular, and would
-be lost as we approached the front of the head.</p>
-
-<p>But, it may be replied, this hypothesis does not explain all the
-facts. It does not tell us why a bone whose function in a given animal
-requires it to be solid, is formed not of a single piece, but by the
-coalescence of several pieces, which in other creatures are separate;
-it does not account for the frequent manifestations of unity of plan
-in defiance of teleological requirements. This is quite true. But it
-is not true, as Professor Owen argues respecting such cases, that
-“if the principle of special adaptation fails to explain them, and
-we reject the idea that these correspondences are manifestations of
-some archetypal exemplar, on which it has pleased the Creator to frame
-certain of his living creatures, there remains only the alternative
-that the organic atoms have concurred fortuitously to produce such
-harmony.” This is not the only alternative: there is another, which
-Professor Owen has overlooked. It is a perfectly tenable supposition
-that all higher vertebrate forms have arisen by <i>the superposing
-of adaptations upon adaptations</i>. Either of the two antagonist
-cosmogonies consists with this supposition. If, on the one hand, we
-conceive species to have resulted from acts of special creation; then
-it is quite a fair assumption that to produce a higher vertebrate
-animal, the Creator did not begin afresh, but took a lower vertebrate
-animal, and so far modified its pre-existing parts as to fit them for
-the new requirements; in which case the original structure would show
-itself through the superposed modifications. If, on the other hand, we
-conceive species to have resulted by gradual differentiations under the
-influence of changed conditions; then, it would manifestly follow that
-the higher, heterogeneous forms, would bear traces of the lower and
-more homogeneous forms from which they were evolved.</p>
-
-<p>Thus, besides finding that the hypothesis of an “ideal typical
-vertebra” is irreconcilable with the facts, we find that the facts are
-interpretable without gratuitous assumptions. The average community of
-form which vertebræ display, is explicable as resulting<span class="pagenum" id="Page_566">[566]</span> from natural
-causes. And those typical similarities which are traceable under
-adaptive modifications, must obviously exist if, throughout creation in
-general, there has gone on that continuous superposing of modifications
-upon modifications which goes on in every unfolding organism.</p>
-
-<hr class="tb" />
-
-<p>[I might with propriety have added to the foregoing criticisms, the
-remark that Professor Owen has indirectly conferred a great benefit by
-the elaborate investigations he has made with the view of establishing
-his hypothesis. He has himself very conclusively proved that the
-teleological interpretation is quite irreconcilable with the facts.
-In gathering together evidence in support of his own conception of
-archetypal forms, he has disclosed adverse evidence which I think shows
-his conception to be untenable. The result is that the field is left
-clear for the hypothesis of Evolution as the only tenable one.]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_567">[567]</span></p>
-
-<h3>APPENDIX C.</h3>
-</div>
-
-<p>[<i>From the</i> <span class="smcap">Transactions of the Linnean Society, vol.
-xxv</span>.]</p>
-
-<hr class="full" />
-
-<div class="blockquot">
-
-<p>XV. <i>On Circulation and the Formation of Wood in Plants.
-By</i> <span class="smcap">Herbert Spencer</span>, <i>Esq. Communicated by</i>
-<span class="smcap">George Busk</span>, <i>Esq., F.R.S., Sec. L.S.</i></p>
-</div>
-
-<p class="center p-left p2">Read March 1st, 1866.</p>
-
-<p>Opinions respecting the functions of the vascular tissues in plants
-appear to make but little progress towards agreement. The supposition
-that these vessels and strings of partially-united cells, lined with
-spiral, annular, reticulated, or other frameworks, are carriers of the
-plant-juices, is objected to on the ground that they often contain
-air: as the presence of air arrests the movement of blood through
-arteries and veins, its presence in the ducts of stems and petioles
-is assumed to unfit them as channels for sap. On the other hand, that
-these structures have a respiratory office, as some have thought, is
-certainly not more tenable, since, if the presence of air in them
-negatives the belief that their function is to distribute liquid, the
-presence of liquid in them equally negatives the belief that their
-function is to distribute air. Nor can any better defence be made for
-the hypothesis which I find propounded, that these parts serve “to give
-strength to the parenchyma.” Tubes with fenestrated and reticulated
-internal skeletons have, indeed, some power of supporting the tissue
-through which they pass; but tubes lined with spiral threads can yield
-extremely little support, while tubes lined with annuli, or spirals
-alternating with annuli, can yield no support whatever. Though all
-these types of internal framework are more or less efficient for
-preventing closure by lateral pressure, they are some of them quite
-useless for holding up the mass through which the vessels pass; and the
-best of them are for this purpose mechanically inferior to the simple
-cylinder. The same quantity of matter made into a continuous tube would
-be more effective in giving stiffness to the cellular tissue around it.</p>
-
-<p>In the absence of any feasible alternative, the hypothesis that these
-vessels are distributors of sap claims reconsideration. The objections
-are not, I think, so serious as they seem. The habitual<span class="pagenum" id="Page_568">[568]</span> presence of
-air in the ducts that traverse wood, can scarcely be held anomalous if
-when the wood is formed their function ceases. The canals which ramify
-through a Stag’s horn, contain air after the Stag’s horn is fully
-developed; but it is not thereby rendered doubtful whether it is the
-function of arteries to convey blood. Again, that air should frequently
-be found even in the vessels of petioles and leaves, will not appear
-remarkable when we call to mind the conditions to which a leaf is
-subject. Evaporation is going on from it. The thinner liquids pass
-by osmose out of the vessels into the tissues containing the liquids
-thickened by evaporation. And as the vessels are thus continually
-drained, a draught is made upon the liquid contained in the stem and
-roots. Suppose that this draught is unusually great, or suppose that
-around the roots there exists no adequate supply of moisture. A state
-of capillary tension must result&mdash;a tendency of the liquid to pass into
-the leaves resisted below by liquid cohesion. Now, had the vessels
-impermeable coats, only their upper extremities would under these
-conditions be slowly emptied. But their coats, in common with all the
-surrounding tissues, are permeable by air. Hence, under this state
-of capillary tension, air will enter; and as the upper ends of the
-tubes, being both smaller in diameter and less porous than the lower,
-will retain the liquids with greater tenacity, the air will enter the
-wider and more porous tubes below&mdash;the ducts of the stem and branches.
-Thus the entrance of air no more proves that these ducts are not
-sap-carriers, than does the emptiness of tropical river-beds in the dry
-season prove that they are not channels for water. There is, however, a
-difficulty which seems more serious. It is said that air, when present
-in these minute canals, must be a great obstacle to the movement of sap
-through them. The investigations of Jamin have shown that bubbles in a
-capillary tube resist the passage of liquid, and that their resistance
-becomes very great when the bubbles are numerous&mdash;reaching, in some
-experiments, as much as three atmospheres. Nevertheless the inference
-that any such resistance is offered by the air-bubbles in the vessels
-of a plant, is, I think, an erroneous one. What happens in a capillary
-tube having impervious sides, with which these experiments were made,
-will by no means happen in a capillary tube having pervious sides.
-Any pressure brought to bear on the column of liquid contained in the
-porous duct of a plant, must quickly cause the expulsion of a contained
-air-bubble through the minute openings in the coats of the duct. The
-greater molecular mobility of gases than liquids, implies that air will
-pass out far more readily than sap. Whilst, therefore, a slight tension
-on the column of sap will cause it to part and the air to enter, a
-slight pressure upon it will force out the air and reunite the divided
-parts of the column.</p>
-
-<p>To obtain data for an opinion on this vexed question, I have<span class="pagenum" id="Page_569">[569]</span> lately
-been experimenting on the absorption of dyes by plants. So far as I can
-learn, experiments of this kind have most, if not all of them, been
-made on stems, and, as it would seem from the results, on stems so far
-developed as to contain all their characteristic structures. The first
-experiments I made myself were on such parts, and yielded evidence that
-served but little to elucidate matters. It was only after trying like
-experiments with leaves of different ages and different characters,
-and with undeveloped axes, as well as with axes of special kinds, that
-comprehensible results were reached; and it then became manifest that
-the appearances presented by ordinary stems when thus tested, are in a
-great degree misleading. Let me briefly indicate the differences.</p>
-
-<p>If an adult shoot of a tree or shrub be cut off, and have its lower
-end placed in an alumed decoction of logwood or a dilute solution of
-magenta,<a id="FNanchor_69" href="#Footnote_69" class="fnanchor">[69]</a> the dye will, in the course of a few hours, ascend to a
-distance varying according to the rate of evaporation from the leaves.
-On making longitudinal sections of the part traversed by it, the dye is
-found to have penetrated extensive tracts of the woody tissue; and on
-making transverse sections, the openings of the ducts appear as empty
-spaces in the midst of a deeply-coloured prosenchyma. It would thus
-seem that the liquid is carried up the denser parts of the vascular
-bundles; neglecting the cambium layer, neglecting the central pith,
-and neglecting the spiral vessels of the medullary sheath. Apparently
-the substance of the wood has afforded the readiest channel. When,
-however, we examine these appearances critically, we find reasons for
-doubting this conclusion. If a transverse section of the lower part,
-into which the dye passed first and has remained longest, be compared
-with a transverse section of the part which the dye has but just
-reached, a marked difference is visible. In the one case the whole of
-the dense tissue is stained; in the other case it is not. This uneven
-distribution of stain in the part which the dye has incompletely
-permeated is not at random; it admits of definite description. A
-tolerably regular continuous ring of colour distinguishes the outer
-part of the wood from the inner mass, implying a passage of liquid
-up the elongated cells next the cambium layer. And the inner mass is
-coloured more round the mouths of the pitted ducts than elsewhere: the
-dense tissue is darkest close to the edges of these ducts; the colour
-fades away gradually on receding from their edges; there is most colour
-where there are several ducts together; and the dense tissue which<span class="pagenum" id="Page_570">[570]</span>
-is fully dyed for some space, is that which lies between two or more
-ducts. These are indications that while the layer of pitted cells
-next the cambium has served as a channel for part of the liquid, the
-rest has ascended the pitted ducts, and oozed out of these into the
-prosenchyma around. And this conclusion is confirmed by the contrast
-between the appearances of the lowest part of a shoot under different
-conditions. For if, instead of allowing the dye time for oozing through
-the prosenchyma, the end of the shoot be just dipped into the dye and
-taken out again, we find, on making transverse sections of the part
-into which the dye has been rapidly taken up, that, though it has
-diffused to some distance round the ducts, it has left tracts of wood
-between the ducts uncoloured&mdash;a difference which would not exist had
-the ascent been through the substance of the wood. Even still stronger
-is the confirmation obtained by using one dye after another. If a shoot
-that has absorbed magenta for an hour be placed for five minutes in the
-logwood decoction, transverse sections of it taken at a short distance
-from its end show the mouths of the ducts surrounded by dark stains in
-the midst of the much wider red stains.</p>
-
-<p>Based on these comparisons only, the inference pointed out has little
-weight; but its weight is increased by the results of experiments on
-quite young shoots, and shoots that develope very little wood. The
-behaviour of these corresponds perfectly with the expectation that a
-liquid will ascend capillary tubes in preference to simple cellular
-tissue or tissue not differentiated into continuous canals. The
-vascular bundles of the medullary sheath are here the only channels
-which the coloured liquid takes. In sections of the parts up to which
-the dye has but just reached, the spiral, fenestrated, scalariform,
-or other vessels contained in these bundles are alone coloured, and
-lower down it is only after some hours that such an exudation of dye
-takes place as suffices partially to colour the other substances of the
-bundle. Further, it is to be noted that at the terminations of shoots,
-where the vessels are but incompletely formed out of irregularly-joined
-fibrous cells which still retain their original shapes, the dye runs up
-the incipient vessels and does not colour in the smallest degree the
-surrounding tissue.</p>
-
-<p>Experiments with leaves bring out parallel facts. On placing in a dye
-a petiole of an adult leaf of a tree, and putting it before the fire
-to accelerate evaporation, the dye will be found to ascend the mid-rib
-and veins at various rates, up even to a foot per hour. At first it
-is confined to the vessels; but by the time it has reached the point
-of the leaf, it will commonly be seen that at the lower part it has
-diffused itself into the sheaths of the vessels. In a quite young leaf
-from the same shoot, we find a much more rigorous restriction of the
-dye to the vessels. On making oblique sections of its petiole, mid-rib,
-and veins, the vessels have the appearance of groups of<span class="pagenum" id="Page_571">[571]</span> sharply
-defined coloured rods imbedded in the green prosenchyma; and this
-marked contrast continues with scarcely an appreciable change after
-plenty of time has been allowed for exudation.</p>
-
-<p>The facts thus grouped and thus contrasted seem, at first sight, to
-imply that while they are young the coats of these ramifying canals
-lined with spiral or allied structures are not readily permeable,
-but that, becoming porous as they grow old, they allow the liquids
-they carry to escape with increasing facility; and hence a possible
-interpretation of the fact that, in the older parts, the staining of
-the tissue around the vessels is so rapid as to suggest that the dye
-has ascended directly through this tissue, whereas in the younger
-parts the reverse appearance necessitates the reverse conclusion. But
-now, is this difference determined by difference of age, or is it
-otherwise determined? The evidence as presented in ordinary stems and
-leaves shows us that the parts of the vascular system at which there
-is a rapid escape of dye are not simply older parts, but are parts
-where a deposit of woody matter is taking place. Is it, then, that
-the increasing permeability of the ducts, instead of being directly
-associated with their increasing age, is directly associated with the
-increasing deposit of dense substance around them?</p>
-
-<p>To get proof that this last connexion is the true one, we have but
-to take a class of cases in which wood is formed only to a small
-extent. In such cases experiments show us a far more general and
-continued limitation of the dye to the vessels. Ordinary herbs and
-vegetables, when contrasted with shrubs and trees, illustrate this;
-as instance the petioles of Celery, or of the common Dock, and the
-leaves of Cabbages or Turnips. And then in very succulent plants, such
-as <i>Bryophyllum calycinum</i>, <i>Kalanchoë rotundifolia</i>, the
-various species of <i>Crassula</i>, <i>Cotyledon</i>, <i>Kleinia</i>,
-and others of like habit, the ducts of old and young leaves alike
-retain the dye very persistently: the concomitant in these cases being
-the small amount of prosenchyma around the ducts, or the small amount
-of deposit in it, or both. More conclusive yet is the evidence which
-meets us when we turn from very succulent leaves to very succulent
-axes. The tender young shoots of <i>Kleinia ante-euphorbium</i>, or
-<i>Euphorbia Mauritanica</i>, which for many inches of their lengths
-have scarcely any ligneous fibres, show us scarcely any escape of
-the coloured liquid from the vessels of the medullary sheath. So,
-too, is it with <i>Stapelia Buffonia</i>, a plant of another order,
-having soft swollen axes. And then we have a repetition of the like
-connexion of facts throughout the <i>Cactaceæ</i>: the most succulent
-showing us the smallest permeability of the vessels. In two species of
-<i>Rhipsalis</i>, in two species of <i>Cereus</i>, and in two species
-of <i>Mammillaria</i>, which I have tried, I have found this so.
-<i>Mammillaria gracilis</i> may be named as exemplifying the relation
-under its extreme form. Into one of these small spheroidal masses, the
-dye ascends through the large bundles<span class="pagenum" id="Page_572">[572]</span> of spiral or annular ducts, or
-cells partially united into such ducts, colouring them deeply, and
-leaving the feebly-marked sheath of prosenchyma, together with the
-surrounding watery cellular tissue, perfectly uncoloured.</p>
-
-<p>The most conclusive evidence, however, is furnished by those
-<i>Cactaceæ</i> in which the transition from succulent to dense tissue
-takes place variably, according as local circumstances determine.
-<i>Opuntia</i> yields good examples. If a piece of it including one of
-the joints at which wood is beginning to form, be allowed to absorb
-a coloured liquid, the liquid, running up the irregular bundles of
-vessels and into many of their minute ramifications, is restricted to
-these where they pass through the parenchyma forming the mass of the
-stem; but near the joints the hardened tissue around the vessels is
-coloured. In one of these fleshy growths we get clear evidence that
-the escape of the dye has no immediate dependence on the age of the
-vessels, since, in parts of the stem that are alike in age, some of
-the vessels retain their contents while others do not. Nay, we even
-find that the younger vessels are more pervious than the older ones, if
-round the younger ones there is a formation of wood.</p>
-
-<p>Thus, then, is confirmed the inference before drawn, that in ordinary
-stems the staining of the wood by an ascending coloured liquid is due,
-not to the passage of the coloured liquid up the substance of the wood,
-but to the permeability of its ducts and such of its pitted cells as
-are united into irregular canals. And the facts showing this, at the
-same time indicate with tolerable clearness the process by which wood
-is formed. What in these cases is seen to take place with a dye, may
-be fairly presumed to take place with sap. Where the dye exudes but
-slowly, we may infer that the sap exudes but slowly; and it is a fair
-inference that where the dye leaks rapidly out of the vessels, the sap
-does the same. Inferring, thus, that where-ever there is a considerable
-formation of wood there is a considerable escape of the sap, we see
-in the one the result of the other. The thickening of the prosenchyma
-is proportionate to the quantity of nutritive liquid passing into it;
-and this nutritive liquid passes into it from the vessels, ducts, and
-irregular canals it surrounds.</p>
-
-<p>But an objection is made to such experiments as the foregoing, and to
-all the inferences drawn from them. It is said that portions of plants
-cut off and thus treated, have their physiological actions arrested,
-or so changed as may render the results misleading; and it is said
-that when detached shoots and leaves have their cut ends placed in
-solutions, the open mouths of their vessels and ducts are directly
-presented with the liquids to be absorbed, which does not happen in
-their natural states. Further, making these objections look serious, it
-is alleged that when solutions are absorbed through the roots, quite
-different results are obtained: the absorbed matters are found in the
-tissues and not in the vessels. Clearly, were the experiments<span class="pagenum" id="Page_573">[573]</span> yielding
-these adverse results conducted in unobjectionable ways, the conclusion
-implied by them would negative the conclusions above drawn. But these
-experiments are no less objectionable than those to which they are
-opposed. Such mineral matters as salts of iron, solutions of which
-have in some cases been supplied to the roots for their absorption,
-are obviously so unlike the matters ordinarily absorbed, that they
-are likely to interfere fatally with the physiological actions. If
-experiments of this kind are made by immersing the roots in a dye,
-there is, besides the difficulty that the mineral mordant contained
-by the dye is injurious to the plant, the further difficulty that the
-colouring matter, being seized by the substances for which it has an
-affinity, is left behind in the first layers of root-tissues passed
-through, and that the decolorized water passing up into the plant is
-not traceable. To be conclusive, then, an experiment on absorption
-through roots must be made with some solution which will not seriously
-interfere with the plant’s vital processes, and which will not have
-its distinctive element left behind. To fulfil these requirements I
-adopted the following method. Having imbedded a well-soaked broad-bean
-in moist sand, contained in an inverted cone of cardboard with its
-apex cut off for the radicle to come through&mdash;having placed this in
-a wide-mouthed dwarf bottle, partly filled with water, so that the
-protruding radicle dipped into the water&mdash;and having waited until the
-young bean had a shoot some three or more inches high, and a cluster
-of secondary rootlets from an inch to an inch and a-half long&mdash;I
-supplied for its absorption a simple decoction of logwood, which,
-being a vegetal matter, was not likely to do it much harm, and which,
-being without a mordant, would not leave its suspended colour in the
-first tissues passed through. To avoid any possible injury, I did not
-remove the plant from the bottle, but slightly raising the cone out of
-its neck, I poured away the water through the crevice and then poured
-in the logwood decoction; so that there could have been no broken end
-or abraded surface of a rootlet through which the decoction might
-enter. Being prepared with some chloride of tin as a mordant, I cut
-off, after some three hours, one of the lowest leaves, expecting that
-the application of the mordant to the cut surface would bring out
-the characteristic colour if the logwood decoction had risen to that
-height. I got no reaction, however. But after eight hours I found, on
-cutting off another leaf, that the vessels of its petiole were made
-visible as dark streaks by the colour with which they were charged&mdash;a
-colour differing, as was to be expected, from that of the logwood
-decoction, which spontaneously changes even by simple exposure. It was
-then too late in the day to pursue the observations; but next morning
-the vessels of the whole plant, as far as the petioles of its highest
-unfolded leaves, were full of the colouring matter; and on applying
-chloride of tin to the cut surfaces, the vessels assumed that purplish<span class="pagenum" id="Page_574">[574]</span>
-red which this mordant produces when directly mixed with the logwood
-decoction. Subsequently, when one of the cotyledons was cut open
-by Prof. Oliver, to whom, in company with Dr. Hooker, I showed the
-specimen, we found that the whole of its vascular system was filled
-with the decoction, which everywhere gave the characteristic reaction.
-And it became manifest that the liquid absorbed through the rootlets,
-in the central vessels of which it was similarly traceable, had part
-of it passed directly up the vessels of the axis, while part of it
-had passed through other vessels into the cotyledon, out of which, no
-doubt, the liquid ordinarily so carried returns charged with a supply
-of the stored nutriment. I have since obtained a verification by
-varying the method. Digging up some young plants (Marigolds happened
-to afford the best choice) with large masses of soil round them,
-placing them in water, so as gradually to detach the soil without
-injuring the rootlets, planting them afresh in a flower-pot full of
-washed sand, and then, after a few days, watering them with a logwood
-decoction, I found, as before, that in less than twenty-four hours the
-colouring matter had run up into the vessels of the leaves. Though the
-reaction produced by the mordant was not so strong as before, it was
-marked enough to be quite unquestionable.</p>
-
-<p>As these experiments were so conducted that there was no access to
-the vessels except through the natural channels, and as the vital
-actions of the plants were so little interfered with that at the end
-of twenty-four hours they showed no traces of disturbance, I think the
-results must be held conclusive.</p>
-
-<p>Taking it, then, as a fact that in plants possessing them the vessels
-and ducts are the channels through which sap is distributed, we come
-now to the further question&mdash;What determines the varying permeability
-of the walls of the vessels and ducts, and the consequent varying
-formation of wood? To this question I believe the true reply is&mdash;The
-exposure of the parts to intermittent mechanical strains, actual or
-potential, or both. By actual strains I of course mean those which the
-plant experiences in the course of its individual life. By potential
-strains I mean those which the form, attitude, and circumstances
-common to its kind involve, and which its inherited structure is
-adapted to meet. In plants with stems, petioles, and leaves, having
-tolerably constant attitudes, the increasing porosity of the tubes and
-consequent deposit of dense tissue takes place in anticipation of the
-strains to which the parts of the individual are liable, but takes
-place at parts which have been habitually subject to such strains
-in ancestral individuals. But though in such plants the tendency to
-repeat that distribution of dense tissue caused by mechanical actions
-on past generations, goes on irrespective of the mechanical actions
-to which the developing individual is subject, these direct actions,
-while they greatly aid the assumption of the typical structure, are
-the sole causes of those deviations in the relative<span class="pagenum" id="Page_575">[575]</span> thickenings of
-parts which distinguish the individual from others of its kind. And
-then, in certain irregularly growing plants, such as Cactuses and
-Euphorbias, where the strains fall on parts that do not correspond in
-successive individuals, we distinctly trace a direct relation between
-the degrees of strain and the rates of these changes which result in
-dense tissue. I will not occupy space in detailing the evidence of this
-relation, which is conspicuous in the orders named, but will pass to
-the question&mdash;What are the physical processes by which intermittent
-mechanical strains produce this deposit of resistant substance at
-places where it is needed to meet the strains? We have not to seek far
-for an answer. If a trunk, a bough, a shoot, or a petiole, is bent
-by a gust of wind, the substance of its convex side is subject to
-longitudinal tension: the substance of its concave side being at the
-same time compressed. This is the primary mechanical effect. There is,
-however, a secondary mechanical effect, which here chiefly concerns
-us. That bend by which the tissues of the convex side are stretched,
-also produces lateral compression of them. Buttoning on a tight
-glove and then closing the hand, will make this necessity clear: the
-leather, while it is strained along the backs of the fingers, presses
-with considerable force on the knuckles. It is demonstrable that the
-tensions of the outer layer of a mass made convex by bending, must,
-by composition of forces, produce at every point a resultant at right
-angles to the layer beneath it; that, similarly, the joint tensions
-of these two layers must throw a pressure on the next deeper layer;
-and so on. Hence, if at some little distance beneath the surface of a
-stem, twig, or leaf-stalk, there exist longitudinal tubes, these tubes
-must be squeezed each time the side of the branch they are placed on
-becomes convex. Modifying the illustration just drawn from the clenched
-hand will make this clear. When, on forcibly grasping something, the
-skin is drawn tightly over the back of the hand, the whitening of
-the knuckles shows how the blood is expelled from the vessels below
-the surface by the pressure of the tightened skin. If, then, the
-sap-vessels must be thus compressed, what will happen to the liquid
-they contain? It will move away along the lines of least resistance.
-Part, and probably the greater part, will escape lengthways from the
-place of greatest pressure: some of it being expelled downwards, and
-some of it upwards. But, at the same time, part of it will be likely
-to ooze through the walls of the tubes. If these walls are so perfect
-as to permit the passage of liquid only by osmose, it may still be
-inferred that the osmose will increase under pressure; and probably,
-under recurrent pressure, the places at which the osmotic current
-passes most readily will become more and more permeable, until they
-eventually form pores. At any rate it is manifest that where pores
-and slits exist, whether thus formed or formed in any other way, the
-escape of sap into the adjacent tissue at each bend<span class="pagenum" id="Page_576">[576]</span> will become easy
-and rapid. What further must happen? When the branch or shoot recoils,
-the vessels on the side that was convex, being relieved from pressure,
-will tend to resume their previous diameters; and will be helped to do
-this by the elasticity of the surrounding tissue, as well as by those
-spiral, annular, and allied structures which they contain. But this
-resumption of their previous diameters must cause an immediate rush
-of sap back into them. Whence will it come? Not to any considerable
-extent from the surrounding tissues into which part of it has been
-squeezed, seeing that the resistance to the return of liquid through
-small pores will be greater than the resistance to its return along the
-vessels themselves. Manifestly the sap which was thrust up and down
-the vessels from the place of compression will return&mdash;the quantities
-returning from above and from below varying, as we shall hereafter
-see, according to circumstances. But this is not all. From some side a
-greater quantity must come back than was sent away; for the amount that
-has escaped out of the tube into the prosenchyma has to be replaced.
-Thus during the time when the side of the branch or twig becomes
-concave, more sap returns from above or below than was expelled upwards
-or downwards during the previous compression. The refilled vessels,
-when the next bend renders their side convex, again have part of their
-contents forced through their parietes, and are again refilled in the
-same way. There is thus set up a draught of sap to the place where
-these intermittent strains are going on, an exudation proportionate
-to the frequency and intensity of the strains, and a proportionate
-nutrition or thickening of the wood-cells, fitting them to resist the
-strains. A rude idea of this action may be obtained by grasping in one
-hand a damp sponge, having its lower end in water, while holding a
-piece of blotting-paper in contact with its upper end, and then giving
-the sponge repeated squeezes. At each squeeze some of the water will
-be sent into the blotting-paper; at each relaxation the sponge will
-refill from below, to give another portion of its contents to the
-blotting-paper when again squeezed.</p>
-
-<p>But how does this explanation apply to roots? If the formation of
-wood is due to intermittent transverse strains, such as are produced
-in the aërial parts of upright plants by the wind, how does it happen
-that woody matter is deposited in roots, where there are no lateral
-oscillations, no transverse strains? The answer is, that longitudinal
-strains also are capable of causing the effects described. It is
-true that perfectly straight fibres united into a bundle and pulled
-lengthways would not exert on one another any lateral pressure, and
-would not laterally compress any similarly-straight canals running
-along with them. But if the fibres united into a bundle are variously
-bent or twisted, they cannot be longitudinally strained without
-compressing one another and structures imbedded in them. It needs<span class="pagenum" id="Page_577">[577]</span> but
-to watch a wet rope drawn tight by a capstan, to see that an action
-like that which squeezes the water out of its strands, will squeeze the
-sap out of the vessels of a root into the surrounding tissue, as often
-as the root is pulled by the swaying of the plant it belongs to. Here,
-too, as before, the vessels will refill when the pull intermits; and
-so, in the roots as in the branches, this rude pumping process will
-produce a growth of hard tissue proportionate to the stress to be borne.</p>
-
-<p>These conclusions are supported by the evidence which exceptional
-cases supply. If intermittent mechanical strains thus cause the
-formation of wood where wood is found, then where it is not found,
-there should be an absence of intermittent mechanical strains. There
-is such an absence. Vascular plants characterized by little or no
-deposit of dense substance, are those having vessels so conditioned
-that no considerable pressures are borne by them. The more succulent a
-petiole or leaf becomes, the more do the effects of transverse strains
-fall on its outer layers of cells. Its mechanical support is chiefly
-derived from the ability of these minute vesicles, full of liquid, to
-resist bursting and tearing under the compressions and tensions they
-are exposed to. And just as fast as this change from a thin leaf or
-foot-stalk to a thick one entails increasing stress on the superficial
-tissue, so fast does it diminish the stress on the internally-seated
-vascular tissue. The succulent leaf cannot be swayed about by the wind
-as much as an ordinary leaf; and such small bends as can be given to
-it and its foot-stalk are prevented from affecting in any considerable
-degree the tubes running through its interior. Hence the retentiveness
-of the vessels in these fleshy leaves, as shown by the small exudation
-of dye; and hence the small thickening of their surrounding prosenchyma
-by woody deposit. Still more conspicuously is this connexion of facts
-shown when, from the soft thick leaves before named and such others
-as those of <i>Echeveria</i>, <i>Rochea</i>, <i>Pereskia</i>, we turn
-to the thick leaves that have strong exo-skeletons. <i>Gasteria</i>
-serves as an illustration. The leathery or horny skin here evidently
-bears the entire weight of the leaf, and is so stiff as to prevent
-any oscillation. Here, then, the vessels running inside are protected
-from all mechanical stress; and accordingly we find that the cells
-surrounding them are not appreciably thickened.</p>
-
-<p>Equally clear, and more striking because more obviously exceptional, is
-the evidence given by succulent stems which are leafless. <i>Stapelia
-Buffonia</i>, having soft procumbent axes not liable to be bent
-backwards and forwards in any considerable degree by the wind, has,
-ramifying through its tissue, vessels that allow but an extremely slow
-escape of dye and have unthickened sheaths. Such of the Euphorbias as
-have acquired the fleshy character while retaining the arborescent
-growth, like <i>Euphorbia Canariensis</i>, teach us the same truth
-in another way. In them the formation of wood around the<span class="pagenum" id="Page_578">[578]</span> vessels is
-inconspicuous where the intermittent strains are but slight; but it
-is conspicuous at those joints on which lateral oscillations of the
-attached branches throw great extensions and compressions of tissue.
-Throughout the <i>Cactaceæ</i> we find varied examples of the alleged
-relation. <i>Mammillaria</i> furnishes a very marked one. The substance
-of one of these globular masses, resting on the ground, admits of no
-bending from side to side; and accordingly its large bundles of spiral
-and annular vessels, or partially-united cells, have very feebly-marked
-sheaths not at all thickened. In such types as <i>Cereus</i> and
-<i>Opuntia</i> we see, as in the Euphorbias, that where little stress
-falls on the vessels, little deposit takes place around them; while
-there is much deposit where there is much stress. Here let me add
-a confirmation obtained since writing the above. After observing
-among the Cactuses the very manifest relation between strain and the
-formation of wood, I inquired of Mr. Croucher, the intelligent foreman
-of the Cactus-house at Kew, whether he found this relation a constant
-one. He replied that he did, and that he had frequently tested it by
-artificially subjecting parts of them to strains. Neglecting at the
-time to inquire how he had done this, it afterwards occurred to me
-that if he had so done it as to cause constant strains, the observed
-result would not tell in favour of the foregoing interpretation.
-Subsequently, however, I learned that he had produced the strains by
-placing the plants in inclined attitudes&mdash;a method which, by permitting
-oscillations of the strained joints, allowed the strains to intermit.
-And then, making the proof conclusive, Mr. Croucher volunteered the
-statement that where he had produced constant strains by tying, no
-formation of wood took place.</p>
-
-<p>Aberrant growths of another class display the same relations of
-phenomena. Take first the underground stems, such as the Potato and
-the Artichoke. The vessels which run through these, slowly take up
-the dye without letting it pass to any considerable extent into the
-surrounding tissues.<a id="FNanchor_70" href="#Footnote_70" class="fnanchor">[70]</a> Only after an interval of many hours does the
-prosenchyma become stained in some places. Here, as before, an absence
-of rapid exudation accompanies an absence of woody deposit; and both
-these go along with the absence of intermittent strains. Take again the
-fleshy roots. The Turnip, the Carrot, and the Beetroot, have vessels
-that retain very persistently the coloured liquids they take up. And
-differing in this, as these roots do, from ordinary roots, we see
-that they also differ from them in not being woody, and in not being
-appreciably subject<span class="pagenum" id="Page_579">[579]</span> to the usual mechanical actions. In these cases,
-as in the others, parts that ordinarily become dense, deviate from
-this typical character when they are not exposed to those forces which
-produce dense tissue by increasing the extravasation of sap.</p>
-
-<p>To complete the proof that such a relation exists, let me add the
-results of some experiments on equal and similarly-developed parts,
-kept respectively at rest and in motion. I have tested the effects on
-large petioles, on herbaceous shoots, and on woody shoots. If two such
-petioles as those of Rhubarb, with their leaves attached, have their
-cut ends inserted in bottles of dye, and the one be bent backwards
-and forwards while the other remains motionless, there arises, after
-the lapse of an hour, scarcely any difference in the states of their
-vessels: a certain proportion of these are in both cases charged with
-the dye, and little exudation has been produced by the motion. Here,
-however, it is to be observed that the causes of exudation are scarcely
-operative; the vascular bundles are distributed all through the mass
-of the petiole, which is formed of soft watery tissue; and they are,
-therefore, not so circumstanced as to be effectually compressed by the
-bends. In herbaceous stems, such as those of the Jerusalem Artichoke
-and of the Foxglove, an effect scarcely more decided is produced; and
-here, too, when we seek a reason, we find it in the non-fulfilment of
-the mechanical conditions; for the vascular bundles are not so seated
-between a tough layer of bark and a solid core as to be compressed at
-each bend. When, however, we come to experiment upon woody shoots, we
-meet with conspicuous effects, though by no means uniformly. In some
-cases oscillations produce immense amounts of exudation&mdash;parallel
-transverse sections of the compared shoots showing that where, in the
-one that has been at rest, there are spots of colour round but a few
-pitted ducts, in the one that has been kept in motion the substance of
-the wood is soaked almost uniformly through with dye. In other cases,
-especially where there is much undifferentiated tissue remaining, the
-exudation is not very marked. The difference appears to depend on
-the quantity of liquid contained in the shoot. If its substance is
-relatively dry, the exudation is great; but it is comparatively small
-if all the tissues are fully charged with sap. This contrast of results
-is one which contemplation of the mechanical actions will lead us to
-expect.</p>
-
-<p>And now, with these facts to aid our interpretation, let us return to
-ordinary stems. If the upper end of a growing shoot, the prosenchyma
-of which is but little thickened, be allowed to imbibe the dye, the
-vessels of its medullary sheath alone become charged; and from them
-there takes place but a slow oozing. If a like experiment be tried
-with a lower part of the shoot, where the wood in course of formation
-has its inner boundary marked but not its outer boundary, we find
-that the pitted ducts, and more especially the inner ones, come into
-play. And then lower still, where the wood<span class="pagenum" id="Page_580">[580]</span> has its periphery defined
-and its histological characters decided, the appearances show that
-the tissue forming its outer surface begins to take a leading part
-in the transmission of liquid. What now is the explanation of these
-changes, mechanically considered? In the young soft part of the shoot,
-as in all normal and abnormal growths that have not formed wood, the
-channels for the passage of sap are the spiral, annular, fenestrated,
-or reticulated vessels. These vessels, here included in the bundles
-of the medullary sheath, are, in common with the tissues around
-them, subject, by the bendings of the shoot, to slight intermittent
-compressions, and, especially the outermost of them, are thus forced
-to give the prosenchyma an extra supply of nutritive liquid. The
-thickening of the prosenchyma, spreading laterally as well as outwards
-from each bundle of the medullary sheath, goes on until it meets the
-thickenings that spread from the other bundles; and there is so formed
-an irregular cylinder of hardened tissue, surrounding the medulla and
-the vascular bundles of its sheath. As soon as this happens, these
-vascular bundles become, to a considerable extent, shielded from the
-effects of transverse strains, since the tensions and compressions
-chiefly fall on the developing wood outside of them. Clearly, too, the
-greatest stress must be felt by the outer layer of the developing wood:
-being further removed from the neutral axis, it must be subject to
-severer strains at each bend; and lying between the bark and the layer
-of wood first formed, it must be most exposed to lateral compressions.
-Among the elongated cells of this outer layer, some unite to form the
-pitted ducts. Being, as we see, better circumstanced mechanically,
-they become greater carriers of sap than the original vessels, and,
-in consequence of this, as well as in consequence of their relative
-proximity, become the sources of nutrition to the still more external
-layers of wood-cells. The same causes and the same effects hold with
-each new indurated coat deposited round the previously indurated coats.</p>
-
-<p>This description may be thought to go far towards justifying the
-current views respecting the course taken by the sap. But the
-justification is more apparent than real. In the first place, the
-implication here is that the sap-carrying function is at first
-discharged entirely by the vessels of the medullary sheath, and
-that they cease to discharge this function only as fast as they are
-relatively incapacitated by their mechanical circumstances. And the
-second implication is, that it is not the wood itself, but the more
-or less continuous canals formed in it, which are the subsequent
-sap-distributors. This, though readily made clear by microscopic
-examination of the large pitted ducts in a partially lignified shoot
-that has absorbed the dye, is less manifestly true of the peripheral
-layer of sap-carrying tissue finally formed. But it is really true
-here. For this layer, though nominally a layer of wood, is practically
-a layer of inosculating<span class="pagenum" id="Page_581">[581]</span> vessels. It is formed out of irregular lines
-and networks of elongated pitted cells, obliquely united by their
-ends. Examination of them after absorption of a dye, shows that it is
-only along the continuous channels they unite to form that the current
-has passed. But the essentially vascular character of this outer and
-latest-formed layer of the alburnum is best seen in the fact that
-the vascular systems of new axes take their rise from it, and form
-with it continuous canals. If a shoot of last year in which growth
-is recommencing, be cut lengthways after it has imbibed a dye, clear
-proof is obtained that the passage of the dye into a lateral bud takes
-place from this outermost layer of pitted cells, and that the channels
-taken by the dye through the new tissue are composed of cells that pass
-through modified forms into the spiral vessels of the new medullary
-sheath. This transition may be still more clearly traced in a terminal
-bud that continues the line of last year’s shoot. A longitudinal
-section of this shows that the vessels of the new medullary sheath do
-not obtain their sap from the vessels of last year’s sheath (which, as
-shown by the non-absorption of dye, have become inactive), but that
-their supplies are obtained from those inosculating canals formed out
-of last year’s outermost layer of prosenchyma, and that between the
-component cells of this and those of the new vascular system there are
-all gradations of structure.<a id="FNanchor_71" href="#Footnote_71" class="fnanchor">[71]</a></p>
-
-<p><span class="pagenum" id="Page_582">[582]</span></p>
-
-<p>It is not the aim of the foregoing reasoning to show that mechanical
-actions are the sole causes of the formation of dense tissue in
-plants. Dense tissue is in many cases formed where no such causes
-have come into play&mdash;as, for example, in thorns and in the shells
-of nuts. Here the natural selection of variations can alone have
-operated. It is manifest, too, that even those supporting structures
-the building up of which is above ascribed to intermittent strains,
-may, in the individual plant of a species that ordinarily has them, be
-developed to a great extent when intermittent strains are prevented.
-We see this in trees that are artificially supported by nailing to
-walls; and we also see a kindred fact in natural climbers. Though in
-these cases the formation of wood is obviously less than it would be
-were the stem and branches habitually moved about by the wind, it
-nevertheless goes on. Clearly the tendency of the plant to repeat the
-structure of its type (in the one case the structure of its species;
-and in the other case that of the order from which it has diverged in
-becoming a climber) is here almost the sole cause of wood formation.
-But though in plants so circumstanced intermittent mechanical strains
-have little or no direct share, it may still be true, and I believe
-is true, that intermittent mechanical strains are the original cause;
-for, as before hinted, the typical structure which the individual
-thus repeats irrespective of its own conditions, is interpretable
-as a typical structure that is itself the product of these actions
-and reactions between the plant and its environment. Grant the
-inheritance of functionally-produced modifications; grant that natural
-selection will always co-operate in such way as to favour those
-individuals and families in which functionally-produced modifications
-have progressed most advantageously; and it will follow that this
-mechanically-caused formation of dense substance, accumulating from
-generation to generation by the survival of the fittest, will result
-in an organic habit of forming dense tissue at the required places.
-The deposit arising from exudation at the places of greatest strain,
-recurring from generation to generation at the same places, will come
-to be reproduced in anticipation of strain, and will continue to
-be reproduced for a long time after a changed habit of the species
-prevents the strain&mdash;eventually, however, decreasing, both through
-functional inactivity and natural selection, to the point at which it
-is in equilibrium with the requirement.</p>
-
-<p><span class="pagenum" id="Page_583">[583]</span></p>
-
-<p>Another side of the general question may now be considered. We have
-seen how, by intermittent pressures on capillary vessels and ducts
-and inosculating canals, there must be produced a draught of sap
-towards the point of compression to replace the sap squeezed out. But
-we have still to inquire what will be the effect on the distribution
-of sap throughout the plant as a whole. It was concluded that out of
-the compressed vessels the greater part of the liquid would escape
-longitudinally&mdash;the longitudinal resistance to movement being least.
-In every case the probabilities are infinity to one against the
-resistances being equal upwards and downwards. Always, then, more sap
-will be expelled in one direction than in the other. But in whichever
-direction least sap is expelled, from that same direction most sap will
-return when the vessels are relieved from pressure&mdash;the force which is
-powerful in arresting the back current in that direction being the same
-force which is powerful in producing a forward current. Ordinarily, the
-more abundant supply of liquid being from below, there will result an
-upward current. At each bend a portion of the contents will be squeezed
-out through the sides of the vessels&mdash;a portion will be squeezed
-downwards, reversing the current ascending from the roots, but soon
-stopped by its resistance; while a larger portion will be squeezed
-upwards towards the extremities of the vessels, where consumption and
-loss are most rapid. At each recoil the vessels will be replenished,
-chiefly by the repressed upward current; and at the next bend more of
-it will be thrust onwards than backwards. Hence we have everywhere in
-action a kind of rude force-pump, worked by the wind; and we see how
-sap may thus be raised to a height far beyond that to which it could be
-raised by capillary action, aided by osmose and evaporation.</p>
-
-<p>Thus far, however, the argument proceeds on the assumption that there
-is liquid enough to replenish every time the vessels subject to
-this process. But suppose the supply fails&mdash;suppose the roots have
-exhausted the surrounding stock of moisture. Evidently the vessels thus
-repeatedly having their contents squeezed out into the surrounding
-tissue, cannot go on refilling themselves from other vessels without
-tending to empty the vascular system. On the one hand, evaporation from
-the leaves causing a draught on the capillary tubes that end in them,
-continually generates a capillary tension upwards; while, on the other
-hand, the vessels below, expanding after their sap has been squeezed
-out, produce a tension both upwards and downwards towards the point
-of loss. Were the limiting membranes of the vessels impermeable, the
-movement of sap would, under these conditions, soon be arrested. But
-these membranes are permeable; and the surrounding tissues readily
-permit the passage of air. This state of tension, then, will cause an
-entrance of air into the tubes; the columns of liquid they contain
-will be interrupted by bubbles. It seems, indeed, not improbable that
-this entrance of air may take<span class="pagenum" id="Page_584">[584]</span> place even when there is a good supply
-of liquid, if the mechanical strains are so violent and the exudation
-so rapid that the currents cannot refill the half-emptied vessels
-with sufficient rapidity. And in this case the intruding air may
-possibly play the same part as that contained in the air-chamber of a
-force-pump&mdash;tending, by moderating the violence of the jets, and by
-equalizing the strains, to prevent rupture of the apparatus. Of course
-when the supply of liquid becomes adequate, and the strains not too
-violent, these bubbles will be expelled as readily as they entered.</p>
-
-<p>Here, as before, let me add the conclusive proof furnished by a direct
-experiment. To ascertain the amount of this propulsive action, I took
-from the same tree, a Laurel, two equal shoots, and placing them in
-the same dye, subjected them to conditions that were alike in all
-respects save that of motion: while one remained at rest, the other
-was bent backwards and forwards, now by switching and now by straining
-with the fingers. After the lapse of an hour, I found that the dye had
-ascended the oscillating shoot three times as far as it had ascended
-the stationary shoot&mdash;this result being an average from several trials.
-Similar trials brought out similar effects in other structures. The
-various petioles and herbaceous shoots experimented upon for the
-purpose of ascertaining the amount of exudation produced by transverse
-strains, showed also the amount of longitudinal movement. It was
-observable that the height ascended by the dye was in all cases greater
-where there had been oscillation than where there had been rest&mdash;the
-difference, however, being much less marked in succulent structures
-than in woody ones.</p>
-
-<p>It need scarcely be said that this mechanical action is not here
-assigned as the sole cause of circulation, but as a cause co-operating
-with others, and helping others to produce effects that could not
-otherwise be produced. Trees growing in conservatories afford us
-abundant proof that sap is raised to considerable heights by other
-forces. Though it is notorious that trees so circumstanced do not
-thrive unless, through open sashes, they are frequently subject
-to breezes sufficient to make their parts oscillate, yet there is
-evidently a circulation that goes on without mechanical aid. The
-<i>causes</i> of circulation are those actions only which disturb the
-liquid equilibrium in a plant, by permanently abstracting water or
-sap from some part of it; and of these the first is the absorption of
-materials for the formation of new tissue in growing parts; the second
-is the loss by evaporation, mainly through adult leaves; and the third
-is the loss by extravasation, through compressed vessels. Only so far
-as it produces this last, can mechanical strain be regarded as truly a
-cause of circulation. All the other actions concerned must be classed
-as <i>aids</i> to circulation&mdash;as facilitating that re-distribution
-of liquid that continually restores the equilibrium continually
-disturbed; and of these<span class="pagenum" id="Page_585">[585]</span> capillary action may be named as the first,
-osmose as the second, and the propulsive effect of mechanical strains
-as the third. The first two of these aids are doubtless capable by
-themselves of producing a large part of the observed result&mdash;more of
-the observed result than is at first sight manifest; for there is
-an important indirect effect of osmotic action which appears to be
-overlooked. Osmose does not aid circulation only by setting up, within
-the plant, exchange currents between the more dense and the less dense
-solutions in different parts of it; but it aids circulation much more
-by producing distention of the plant as a whole. In consequence of the
-average contrast in density between the water outside of the plant and
-the sap inside of it, the constant tendency is for the plant to absorb
-a quantity in excess of its capacity, and so to produce distention
-and erection of its tissues. It is because of this that the drooping
-plant raises itself when watered; for capillary action alone could only
-refill its tissues without changing their attitudes. And it is because
-of this that juicy plants with collapsible structures bleed so rapidly
-when cut, not only from the cut surface of the rooted part, but from
-the cut surface of the detached part&mdash;the elastic tissues tending to
-press out the liquid which distends them. And manifestly if osmose
-serves thus to maintain a state of distention throughout a plant,
-it indirectly furthers circulation; since immediately evaporation
-or growth at any part, by abstracting liquid from the neighbouring
-tissues, begins to diminish the liquid pressure within such tissues,
-the distended structures throughout the rest of the plant thrust
-their liquid contents towards the place of diminished pressure. This,
-indeed, may very possibly be the most efficient of the agencies at
-work. Remembering how great is the distention producible by osmotic
-absorption&mdash;great enough to burst a bladder&mdash;it is clear that the
-force with which the distended tissues of a plant urge forward the sap
-to places of consumption, is probably very great. We must therefore
-regard the aid which mechanical strains give as being one of several.
-Oscillations help directly to restore any disturbed liquid equilibrium;
-and they also help indirectly, by facilitating the re-distribution
-caused by capillary action and the process just described; but in the
-absence of oscillations the equilibrium may still be restored, though
-less rapidly and within narrower limits of distance.</p>
-
-<p>One half of the problem of the circulation, however, has been left out
-of sight. Thus far our inquiry has been, how the ascending current
-of sap is produced. There remains the rationale of the descending
-current. What forces cause it, and through what tissues it takes place,
-are questions to which no satisfactory answers have been given. That
-the descent is due to gravitation, as some allege, is difficult to
-conceive, since, as gravitation acts equally on all liquid columns
-contained in the stem, it is not easy to see why it should produce
-downward movements in some while permitting<span class="pagenum" id="Page_586">[586]</span> upward movements in
-others&mdash;unless, indeed, there existed descending tubes too wide
-to admit of much capillary action, which there do not. Moreover,
-gravitation is clearly inadequate to cause currents towards the roots
-out of branches that droop to the ground. Here the gravitation of the
-contained liquid columns must nearly balance that of the connected
-columns in the stem, leaving no appreciable force to cause motion. Nor
-does there seem much probability in the assumption that the route of
-the descending sap is through the cambium layer, since experiments on
-the absorption of dyes prove that simple cellular tissue is a very bad
-conductor of liquids: their movement through it does not take place
-with one-fiftieth of the rapidity with which it takes place through
-vessels.<a id="FNanchor_72" href="#Footnote_72" class="fnanchor">[72]</a></p>
-
-<p>Of course the defence for these hypotheses is, that there must be a
-downward current, which must have a course and a cause; and the very
-natural assumption has been that the course and the cause must be other
-than those which produce the ascending current. Nevertheless there
-is an alternative supposition to which the foregoing considerations
-introduce us. It is quite possible for the same vascular system
-to serve as a channel for movement in opposite directions at
-different times. We have among animals well-known cases in which the
-blood-vessels carry a current first in one direction and then, after
-a brief pause, in the reverse direction. And there seems an <i>à
-priori</i> probability that, lowly-organized as they are, plants are
-more likely to have distributing appliances of this imperfect kind than
-to have two sets of channels for two simultaneous currents. If, led
-by this suspicion, we inquire whether among the forces which unite to
-produce movements of sap, there are any variations or intermissions
-capable of determining the currents in different directions, we quickly
-discover that there are such, and that the hypothesis of an alternating
-motion of the sap, now centrifugal and now centripetal, through the
-same vessels, has good warrant. What are the several forces at work?
-First may be set down that tendency existing in every part of a plant
-to expand into its typical form, and to absorb nutritive liquids in
-doing this. The resulting competition<span class="pagenum" id="Page_587">[587]</span> for sap will, other things being
-equal, cause currents towards the most rapidly-growing parts&mdash;towards
-unfolding shoots and leaves, but not towards adult leaves. Next we
-have evaporation, acting more on the adult leaves than on those which
-are in the bud, or but partially developed. This evaporation is both
-regularly and irregularly intermittent. Depending chiefly on the
-action of the sun, it is, in fine weather, greatly checked or wholly
-arrested every evening; and in cloudy weather must be much retarded
-during the day. Further, every hygrometric variation, as well as every
-variation in the movement of the air, must vary the evaporation.
-This chief action, therefore, which, by continually emptying the
-ends of the capillary tubes, makes upward currents possible, is one
-which intermits every night, and every day is strong or feeble as
-circumstances determine. Then, in the third place, we have this rude
-pumping process above described, going on with greater vigour when
-the wind is violent, and with less vigour when it is gentle&mdash;drawing
-liquid <i>towards</i> different parts according to their degrees of
-oscillation, and <i>from</i> different parts according as they can most
-readily furnish it. And now let us ask what must result under changing
-conditions from these variously-conflicting and conspiring forces. When
-a warm sunshine, causing rapid evaporation, is emptying the vessels of
-the leaves, the osmotic and capillary actions that refill them will
-be continually aided by the pumping action of the swaying petioles,
-twigs, and branches, provided their oscillations are moderate. Under
-these conditions the current of sap, moving in the direction of least
-resistance, will set towards the leaves. But what will happen when the
-sun sets? There is now nothing to determine currents either upwards
-or downwards, except the relative rates of growth in the parts and
-the relative demands set up by the oscillations; and the oscillations
-acting alone, will draw sap to the oscillating parts as much from above
-as from below. If the resistance to be overcome by a current setting
-back from the leaves is less than the resistance to be overcome by a
-current setting up from the roots, then a current will set back from
-the leaves. Now it is, I think, tolerably manifest that in the swaying
-twigs and minor branches, less force will be required to overcome the
-inertia of the short columns of liquid between them and the leaves than
-to overcome the inertia of the long columns between them and the roots.
-Hence during the night, as also at other times when evaporation is not
-going on, the sap will be drawn out of the leaves into the adjacent
-supporting parts; and their nutrition will be increased. If the wind is
-strong enough to produce a swaying of the thicker branches, the back
-current will extend to them also; and a further strengthening will
-result from their absorption of the elaborated sap. And when the great
-branches and the stem are bent backwards and forwards by a<span class="pagenum" id="Page_588">[588]</span> gale, they
-too will share in the nutrition. It may at first sight seem that these
-parts, being nearer to the roots than to the leaves, will draw their
-supplies from the roots only. But the quantity which the roots can
-furnish is insufficient to meet so great a demand. Under the conditions
-described, the exudation of sap from the vessels will be very great,
-and the draught of liquid required to refill them, not satisfied by
-that which the root-fibres can take in, will extend to the leaves.
-Thus sap will flow to the several parts according to their respective
-degrees of activity&mdash;to the leaves while light and heat enable them to
-discharge their functions, and back to the twigs, branches, stem, and
-roots when these become active and the leaves inactive, or when their
-activity dominates over that of the leaves. And this distribution of
-nutriment, varying with the varying activities of the parts, is just
-such a distribution as we know must be required to keep up the organic
-balance.</p>
-
-<p>To this explanation it may be objected that it does not account for
-the downward current of sap in plants that are sheltered. The stem and
-roots of a drawing-room Geranium display a thickening which implies
-that nutritive matters have descended from the leaves, although there
-are none of those oscillations by which the sap is said to be drawn
-downwards as well as upwards. The reply is, that the stem and roots
-tend to repeat their typical structures, and that the absorption of
-sap for the formation of their respective dense tissues, is here the
-force which determines the descent. Indeed it must be borne in mind
-that the mechanical strains and the pumping process which they keep
-up, as well as the distention caused by osmose, do not in themselves
-produce a current either upwards or downwards: they simply help to move
-the sap towards that place where there is the most rapid abstraction
-of it&mdash;the place towards which its motion is least resisted. Whether
-there is oscillation or whether there is not, the physiological demands
-of the different parts of the plant determine the direction of the
-current; and all which the oscillations and the distention do is to
-facilitate the supply of these demands. Just as much, therefore,
-in a plant at rest as in a plant in motion, the current will set
-downwards when the function of the leaves is arrested, and when there
-is nothing to resist that abstraction of sap caused by the tendency
-of the stem- and root-tissues to assume their typical structures. To
-which admission, however, it must be added that since this typical
-structure assumed, though imperfectly assumed, by the hothouse plant,
-is itself interpretable as the inherited effect of external mechanical
-actions on its ancestors, we may still consider the current set up by
-the assumption of the typical structure to be indirectly due to such
-actions.</p>
-
-<p>Interesting evidence of another order here demands notice. In the
-course of experiments on the absorption of dyes by leaves, it happened
-that in making sections parallel to the plane of a leaf, with<span class="pagenum" id="Page_589">[589]</span> the
-view of separating its middle layer containing the vessels, I came
-upon some structures that were new to me. These structures, where they
-are present, form the terminations of the vascular system. They are
-masses of irregular and imperfectly united fibrous cells, such as those
-out of which vessels are developed; and they are sometimes slender,
-sometimes bulky&mdash;usually, however, being more or less club-shaped. In
-transverse sections of leaves their distinctive characters are not
-shown: they are taken for the smaller veins. It is only by carefully
-slicing away the surface of a leaf until we come down to that part
-which contains them, that we get any idea of their nature. Fig. 1
-represents a specimen taken from a leaf of <i>Euphorbia neriifolia</i>.
-Occupying one of the interspaces of the ultimate venous network, it
-consists of a spirally-lined duct or set of ducts, which connects with
-the neighbouring vein a cluster of half-reticulated, half-scalariform
-cells. These cells have projections, many of them tapering, that insert
-themselves into the adjacent intercellular spaces, thus producing
-an extensive surface of contact between the organ and the imbedding
-tissues. A further trait is, that the ensheathing prosenchyma is
-either but little developed or wholly absent; and consequently this
-expanded vascular structure, especially at its end, comes immediately
-in contact with the tissues concerned in assimilation. The leaf of
-<i>Euphorbia neriifolia</i> is a very fleshy one; and in it these
-organs are distributed through a compact, though watery, cellular mass.
-But in any leaf of the ordinary type which possesses them, they lie in
-the network-parenchyma composing its lower layer; and wherever they
-occur in this layer its cells unite to enclose them. This arrangement
-is shown in fig. 2, representing a sample from the Caoutchouc-leaf, as
-seen with the upper part of its envelope removed; and it is shown still
-more clearly in a sample from the leaf of <i>Panax Lessonii</i>, fig.
-3. Figures 4 and 5 represent, without their sheaths, other such organs
-from the leaves of <i>Panax Lessonii</i> and <i>Clusia flava</i>. Some
-relation seems to exist between their forms and the thicknesses of the
-layers in which they lie. Certain very thick leaves, such as those of
-<i>Clusia flava</i>, have them less abundantly distributed than is
-usual, but more massive. Where the parenchyma is developed not to so
-great an extreme, though still largely, as in the leaves of Holly,
-<i>Aucuba</i>, <i>Camellia</i>, they are not so bulky; and in thinner
-leaves, like those of Privet, Elder, &amp;c., they become longer and
-less conspicuously club-shaped. Some adaptations to their respective
-positions seem implied by these modifications; and we may naturally
-expect that in many thin leaves these free ends, becoming still
-narrower, lose the distinctive and suggestive characters possessed by
-those shown in the diagrams. Relations of this kind are not regular,
-however. In various other genera, members of which I have examined,
-as <i>Rhus</i>, <i>Viburnum</i>, <i>Griselinia</i>, <i>Brexia</i>,
-<i>Botryodendron</i>, <i>Pereskia</i>, the variations in the<span class="pagenum" id="Page_590">[590]</span> bulk
-and form of these structures are not directly determined by the
-spaces which the leaves allow: obviously there are other modifying
-causes. It should be added that while these expanded free extremities
-graduate into tapering free extremities, not differing from ordinary
-vessels, they also pass insensibly into the ordinary inosculations.
-Occasionally, along with numerous free endings, there occur loops; and
-from such loops there are transitions to the ultimate meshes of the
-veins.</p>
-
-<p>These organs are by no means common to all leaves. In many that
-afford ample spaces for them they are not to be found. So far as I
-have observed, they are absent from the thick leaves of plants which
-form very little wood. In <i>Sempervivum</i>, in <i>Echeveria</i>,
-in <i>Bryophyllum</i>, they do not appear to exist; and I have been
-unable to discover them in <i>Kalanchoë rotundifolia</i>, in <i>Kleinia
-ante-euphorbium</i> and <i>ficoides</i>, in the several species of
-<i>Crassula</i>, and in other succulent plants. It may be added that
-they are not absolutely confined to leaves, but occur in stems that
-have assumed the functions of leaves. At least I have found, in the
-green parenchyma of <i>Opuntia</i>, organs that are analogous though
-much more rudely and irregularly formed. In other parts, too, that
-have usurped the leaf-function, they occur, as in the phyllodes of
-the Australian Acacias. These have them abundantly developed; and it
-is interesting to observe that here, where the two vertically-placed
-surfaces of the flattened-out petiole are equally adapted to the
-assimilative function, there exist two layers of these expanded
-vascular terminations, one applied to the inner surface of each layer
-of parenchyma.</p>
-
-<p>Considering the structures and positions of these organs, as well as
-the natures of the plants possessing them, may we not form a shrewd
-suspicion respecting their function? Is it not probable that they
-facilitate absorption of the juices carried back from the leaf for
-the nutrition of the stem and roots? They are admirably adapted for
-performing this office. Their component fibrous cells, having angles
-insinuated between the cells of the parenchyma, are shaped just as they
-should be for taking up its contents; and the absence of sheathing
-tissue between them and the parenchyma facilitates the passage of the
-elaborated liquids. Moreover there is the fact that they are allied to
-organs which obviously have absorbent functions. I am indebted to Dr.
-Hooker for pointing out the figures of two such organs in the “Icones
-Anatomicæ” of Link. One of them is from the end of a dicotyledonous
-root-fibre, and the other is from the prothallus of a young Fern. In
-each case a cluster of fibrous cells, seated at a place from which
-liquid has to be drawn, is connected by vessels with the parts to which
-liquid has to be carried. There can scarcely be a doubt, then, that in
-both cases absorption is effected through them. I have met with another
-such organ, more elaborately constructed, but evidently adapted to the
-same<span class="pagenum" id="Page_591">[591]</span> office, in the common Turnip-root. As shown by the end view and
-longitudinal section in figs. 6 and 7, this organ consists of rings of
-fenestrated cells, arranged with varying degrees of regularity into a
-funnel, ordinarily having its apex directed towards the central mass
-of the Turnip, with which it has, in some cases at least, a traceable
-connexion by a canal. Presenting as it does an external porous surface
-terminating one of the branches of the vascular system, each of these
-organs is well fitted for taking up with rapidity the nutriment laid
-by in the Turnip-root, and used by the plant when it sends up its
-flower-stalk. Nor does even this exhaust the analogies. The cotyledons
-of the young bean, experimented upon as before described, furnished
-other examples of such structures, exactly in the places where, if
-they are absorbents, we might expect to find them. Amid the branchings
-and inosculations of the vascular layer running through the mass of
-nutriment deposited in each cotyledon, there are conspicuous free
-terminations that are club-shaped, and prove to be composed, like those
-in leaves, of irregularly formed and clustered fibrous cells; and some
-of them, diverging from the plane of the vascular layer, dip down into
-the mass of starch and albumen which the young plant has to utilize,
-and which these structures can have no other function but to take up.</p>
-
-<p>Besides being so well fitted for absorption, and besides being
-similar to organs which we cannot doubt are absorbents, these
-vascular terminations in leaves afford us yet another evidence of
-their functions. They are seated in a tissue so arranged as specially
-to facilitate the abstraction of liquid. The centripetal movement
-of the sap must be set up by a force that is comparatively feeble,
-since, the parietes of the ducts being porous, air will enter if the
-tension on the contained columns becomes considerable. Hence it is
-needful that the exit of sap from the leaves should meet with very
-little resistance. Now were it not for an adjustment presently to
-be described, it would meet with great resistance, notwithstanding
-the peculiar fitness of these organs to take it in. Liquid cannot be
-drawn out of any closed cavity without producing a collapse of the
-cavity’s sides; and if its sides are not readily collapsible, there
-must be a corresponding resistance to the abstraction of liquid from
-it. Clearly the like must happen if the liquid is to be drawn out of
-a tissue which cannot either diminish in bulk bodily or allow its
-components individually to diminish in bulk. In an ordinary leaf, the
-upper layer of parenchyma, formed as it is of closely-packed cells
-that are without interspaces, and are everywhere held fast within
-their framework of veins, can neither contract easily as a mass, nor
-allow its separate cells to do so. Quite otherwise is it with the
-network-parenchyma below. The long cells of this, united merely by
-their ends and having their flexible sides surrounded by air, may
-severally have their contents considerably increased and decreased
-without offering<span class="pagenum" id="Page_592">[592]</span> appreciable resistances: and the network-tissue
-which they form will, at the same time, be capable of undergoing
-slight expansions and contractions of its thickness. In this layer
-occur these organs that are so obviously fitted for absorption. Here
-we find them in direct communication with its system of collapsible
-cells. The probability appears to be, that when the current sets into
-the leaf, it passes through the vessels and their sheaths chiefly into
-the upper layer of cells (this upper layer having a larger surface
-of contact with the veins than the lower layer, and being the seat
-of more active processes); and that the juices of the upper layer,
-enriched by the assimilated matters, pass into the network-parenchyma,
-which serves as a reservoir from which they are from time to time
-drawn for the nutrition of the rest of the plant, when the actions
-determine the downward current. Should it be asked what happens where
-the absorbents, instead of being inserted in a network-parenchyma, are,
-as in the leaves of <i>Euphorbia neriifolia</i>, inserted in a solid
-parenchyma, the reply is, that such a parenchyma, though not furnished
-with systematically arranged air-chambers, nevertheless contains air
-in its intercellular spaces; and that when there occurs a draught upon
-its contents, the expansion of this air and the entrance of more from
-without, quickly supply the place of the abstracted liquid.</p>
-
-<p>If then, returning to the general argument, we conclude that these
-expanded terminations of the vascular system in leaves are absorbent
-organs, we find a further confirmation of the views set forth
-respecting the alternating movement of the sap along the same channels.
-These spongioles of the leaves, like the spongioles of the roots, being
-appliances by which liquid is taken up to be carried into the mass
-of the plant, we are obliged to regard the vessels that end in these
-spongioles of the leaves as being the channels of the down current
-whenever it is produced. If the elaborated sap is abstracted from the
-leaves by these absorbents, then we have no alternative but to suppose
-that, having entered the vascular system, the elaborated sap descends
-through it. And seeing how, by the help of these special terminations,
-it becomes possible for the same vessels to carry back a quality of sap
-unlike that which they bring up, we are enabled to understand tolerably
-well how this rhythmical movement produces a downward transfer of
-materials for growth.</p>
-
-<hr class="tb" />
-
-<p>The several lines of argument may now be brought together; and along
-with them may be woven up such evidences as remain. Let me first point
-out the variety of questions to which the hypothesis supplies answers.</p>
-
-<p>It is required to account for the ascent of sap to a height beyond that
-to which capillary action can raise it. This ascent is accounted for
-by the propulsive action of transverse strains, joined with that of
-osmotic distention. A cause has to be assigned for that rise of sap<span class="pagenum" id="Page_593">[593]</span>
-which, in the spring, while yet there is no considerable evaporation to
-aid it, goes on with a power which capillarity does not explain. The
-co-operation of the same two agencies is assignable for this result
-also.<a id="FNanchor_73" href="#Footnote_73" class="fnanchor">[73]</a> The circumstance that vessels and ducts here contain sap and
-there contain air, and at the same place contain at different seasons
-now air and now sap is a fact calling for explanation. An explanation
-is furnished by these mechanical actions which involve the entrance
-or expulsion of air according to the supply of liquid. That vessels
-and ducts which were originally active sap-carriers go completely out
-of use, and have their function discharged by other vessels or ducts,
-is an anomaly that has to be solved. Again, we are supplied with a
-solution: these deserted vessels and ducts are those which, by the
-formation of dense tissue outside of them, become so circumstanced that
-they cannot be compressed as they originally were. A channel has to be
-found for the downward current of sap, which, on any other hypothesis
-than the foregoing, must be a channel separate from that taken by the
-upward current; and yet no good evidence of a separate channel has
-been pointed out. Here, however, the difficulty disappears, since one
-channel suffices for the current alternating upwards and downwards
-according to the conditions. Moreover there has to be found a force
-producing or facilitating the downward current, capable even of drawing
-sap out of drooping branches; and no such force is forthcoming. The
-hypothesis set forth dispenses with this necessity; under the recurring
-change of conditions, the same distention and oscillation which before
-raised the sap to the places of consumption, now bring it down to the
-places of consumption. A physical process has to be pointed out by
-which the material that forms dense tissue is deposited at the places
-where it is wanted, rather than at other places. This physical process
-the hypothesis indicates. It is requisite to find an explanation of the
-fact that, when plants ordinarily swayed about by the wind are grown
-indoors, the formation of wood is so much diminished that they become
-abnormally slender. Of this an explanation is supplied. Yet a further<span class="pagenum" id="Page_594">[594]</span>
-fact to be interpreted is, that in the same individual plant homologous
-parts, which, according to the type of the plant, should be equally
-woody, become much thicker one than another if subject to greater
-mechanical stress. And of this too an interpretation is similarly
-afforded.</p>
-
-<p>Now the sufficiency of the assigned actions to account for so many
-phenomena not otherwise explained, would be strong evidence that the
-rationale is the true one, even were it of a purely hypothetical
-kind. How strong, then, becomes the reason for believing it the true
-one when we remember that the actions alleged demonstrably go on in
-the way asserted. They are ever operating before our eyes; and that
-they produce the effects in question is a conclusion deducible from
-mechanical principles, a conclusion established by induction, and a
-conclusion verified by experiment. These three orders of proof may be
-briefly summed up as follows.</p>
-
-<p>That plants which have to raise themselves above the earth’s surface,
-and to withstand the actions of the wind, must have a power of
-developing supporting structure, is an <i>à priori</i> conclusion which
-may be safely drawn. It is an equally safe <i>à priori</i> conclusion,
-that if the supporting structure, either as a whole or in any of
-its parts, has to adapt itself to the particular strains which the
-individual plant is subject to by its particular circumstances, there
-must be at work some process by which the strength of the supporting
-structure is everywhere brought into equilibrium with the forces it
-has to bear. Though the typical distribution of supporting structure
-in each kind of plant may be explained teleologically by those whom
-teleological explanations satisfy; and though otherwise this typical
-distribution may be ascribed to natural selection acting apart from any
-directly adaptive process; yet it is manifest that those departures
-from the typical distribution which fit the parts of each plant to
-their special conditions are explicable neither teleologically nor by
-natural selection. We are, therefore, compelled to admit that, if in
-each plant there goes on a balancing of the particular strains by the
-particular strengths, there must be a physical or physico-chemical
-process by which the adjustments of the two are effected. Meanwhile we
-are equally compelled to admit, <i>à priori</i>, that the mechanical
-actions to be resisted, themselves affect the internal tissues in
-such ways as to further the increase of that dense substance by which
-they are resisted. It is demonstrable that bending the petioles,
-shoots, and stems must compress the vessels beneath their surfaces,
-and increase the exudation of nutritive matters from them, and must do
-this actively in proportion as the bends are great and frequent; so
-that while, on the one hand, it is a necessary deduction that, if the
-parts of each plant are to be severally strengthened according to the
-several strains, there must be some direct connexion between strains
-and strengths, it is, on the other hand, a necessary deduction from<span class="pagenum" id="Page_595">[595]</span>
-mechanical principles that the strains do act in such ways as to aid
-the increase of the strengths. How a like correspondence between two
-<i>à priori</i> arguments holds in the case of the circulation, needs
-not to be shown in detail. It will suffice to remind the reader that
-while the raising of sap to heights beyond the limit of capillarity
-implies some force to effect it, we have in the osmotic distention
-and the intermittent compressions caused by transverse strains,
-forces which, under the conditions, cannot but tend to effect it;
-and similarly with the requirement for a downward current, and the
-production of a downward current.</p>
-
-<p>Among the inductive proofs we find a kindred agreement. Different
-individuals of the same species, and different parts of the same
-individual, do strengthen in different degrees; and there is a clearly
-traceable connexion between their strengthenings and the intermittent
-strains they are exposed to. This evidence, derived from contrasts
-between growths on the same plant or on plants of the same type, is
-enforced by evidence derived from contrasts between plants of different
-types. The deficiency of woody tissue which we see in plants called
-succulent, is accompanied by a bulkiness of the parts which prevents
-any considerable oscillations; and this character is also habitually
-accompanied by a dwarfed growth. When, leaving these relations as
-displayed externally, we examine them internally, we find the facts
-uniting to show, by their agreements and differences, that between
-the compression of the sap-canals and the production of wood there
-is a direct relation. We have the facts, that in each plant, and in
-every new part of each plant, the formation of sap-canals precedes
-the formation of wood; that the deposit of woody matter, when it
-begins, takes place around these sap-canals, and afterwards around
-the new sap-canals successively developed; that this formation of
-wood around the sap-canals takes place where the coats of the canals
-are demonstrably permeable, and that the amount of wood formation is
-proportionate to the permeability. And then that the permeability and
-extravasation of sap occur wherever, in the individual or in the type,
-there are intermittent compressions, is proved alike by ordinary cases
-and by exceptional cases. In the one class of cases we see that the
-deposit of wood round the vessels begins to take place when they come
-into positions that subject them to intermittent compressions, while it
-ceases when they become shielded from compressions. And in the other
-class of cases, where, from the beginning, the vessels are shielded
-from compression by surrounding fleshy tissue, there is a permanent
-absence of wood formation.</p>
-
-<p>To which complete agreement between the deductive and inductive
-inferences has to be added the direct proof supplied by experiments. It
-is put beyond doubt by experiment that the liquids absorbed by plants
-are distributed to their different parts through their<span class="pagenum" id="Page_596">[596]</span> vessels&mdash;at
-first by the spiral or allied vessels originally developed, and then by
-the better-placed ducts formed later. By experiment it is demonstrated
-that the intermittent compressions caused by oscillations urge the
-sap along the vessels and ducts. And it is also experimentally proved
-that the same intermittent compressions produce exudation of sap from
-vessels and ducts into the surrounding tissue.</p>
-
-<p>That the processes here described, acting through all past time, have
-sufficed of themselves to develope the supporting and distributing
-structures of plants, is not alleged. What share the natural selection
-of variations distinguished as spontaneous, has had in establishing
-them, is a question which remains to be discussed. Whether acting
-alone natural selection would have sufficed to evolve these vascular
-and resisting tissues, I do not profess to say. That it has been a
-co-operating cause, I take to be self-evident: it must all along
-have furthered the action of any other cause, by preserving the
-individuals on which such other cause had acted most favourably.
-Seeing, however, the conclusive proof which we have that another cause
-has been in action&mdash;certainly on individuals, and, in all probability,
-by inheritance on races&mdash;we may most philosophically ascribe the
-genesis of these internal structures to this cause, and regard natural
-selection as having here played the part of an accelerator.</p>
-
-
-<h4 class="smaller">EXPLANATION OF PLATE.</h4>
-
-<p>Fig. 1. Absorbent organ from the leaf of <i>Euphorbia neriifolia</i>.
-The cluster of fibrous cells forming one of the terminations of the
-vascular system is here imbedded in a solid parenchyma.</p>
-
-<p>Fig. 2. A structure of analogous kind from the leaf of <i>Ficus
-elastica</i>. Here the expanded terminations of the vessels are
-imbedded in the network-parenchyma, the cells of which unite to form
-envelopes for them.</p>
-
-<p>Fig. 3. Shows on a larger scale one of these absorbents from the leaf
-of <i>Panax Lessonii</i>. In this figure is clearly seen the way in
-which the cells of the network-parenchyma unite into a closely-fitting
-case for the spiral cells.</p>
-
-<p>Fig. 4. Represents a much more massive absorbent from the same leaf,
-the surrounding tissues being omitted.</p>
-
-<p>Fig. 5. Similarly represents, without its sheath, an absorbent from the
-leaf of <i>Clusia flava</i>.</p>
-
-<p>Fig. 6. End view of an absorbent organ from the root of a Turnip. It is
-taken from the outermost layer of vessels. Its funnel-shaped interior
-is drawn as it presents itself when looked at from the outside of this
-layer, its narrow end being directed towards the centre of the Turnip.</p>
-
-<p>Fig. 7. A longitudinal section through the axis of another such organ,
-showing its annuli of reticulated cells when cut through. The cellular
-tissue which fills the interior is supposed to be removed.</p>
-
-<p><span class="pagenum" id="Page_597">[597]</span></p>
-
-<p>Fig. 8. A less developed absorbent, showing its approximate connexion
-with a duct. In their simplest forms, these structures consist of only
-two fenestrated cells, with their ends bent round so as to meet. Such
-types occur in the central mass of the Turnip, where the vascular
-system is relatively imperfect. Besides the comparatively regular forms
-of these absorbents, there are forms composed of amorphous masses
-of fenestrated cells. It should be added that both the regular and
-irregular kinds are very variable in their numbers: in some turnips
-they are abundant, and in others scarcely to be found. Possibly their
-presence depends on<span class="pagenum" id="Page_598">[598]</span> the age of the Turnip. Judging from the period
-during which my investigations were made, namely winter and early
-spring, I suspect that they are developed only in preparation for
-sending up the flower-stalk.</p>
-
-  <div class="figcenter" id="i_597">
-     <img
-    class="p2"
-    src="images/i_597.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs.1–8.</p>
-  </div>
-
-<p class="p2">Let me add that experiments on circulation in plants made during the
-state of inactivity, when it is to be presumed that the vessels and
-tissues contain but little gap, are much more successful than those
-made in the summer. It would seem that when the tissues are fully
-charged with sap the taking up of dyes is comparatively slow and the
-above-described effects are not so easily demonstrable.</p>
-
-<hr class="tb" />
-
-<p>[An expert writes concerning this essay:&mdash;“I have not attempted to
-annotate critically this paper. There is no doubt that many of your
-conclusions are perfectly sound, particularly those relating to the
-passage of crude sap through the <i>cavities</i> of the elements of the
-wood, though the opinion that the actual passage was through the walls
-very generally held till about 12 years ago.”]</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_599">[599]</span></p>
-
-<h3>APPENDIX D.<br />
-<span class="subhed1">ON THE ORIGIN OF THE VERTEBRATE TYPE.</span></h3></div>
-
-
-<div class="blockquot">
-
-<p>[<i>When studying the development of the vertebrate skeleton,
-there occurred to me the following idea respecting the possible
-origin of the notochord. I was eventually led to omit the few
-pages of Appendix in which I had expressed this idea, because
-it was unsupported by developmental evidence. The developmental
-evidence recently discovered, however, has led Professor Haeckel
-and others to analogous views respecting the affiliation
-of the</i> Vertebrata <i>on the</i> Molluscoida. <i>Having
-fortunately preserved a proof of the suppressed pages, I am able
-now to add them. With the omission of a superfluous paragraph,
-they are reprinted verbatim from this proof, which dates back to
-the autumn of 1865, at which time the chapter on “The Shapes of
-Vertebrate Skeletons” was written.</i>&mdash;December, 1869.]</p>
-</div>
-
-<p>The general argument contained in Chap. XVI. of Part IV., I have
-thought it undesirable to implicate with any conception more
-speculative than those essential to it; and to avoid so implicating it,
-I transfer to this place an hypothesis respecting the derivation of the
-rudimentary vertebrate structure, which appears to me worth considering.</p>
-
-<p>Among those molluscoid animals with which the lowest vertebrate animal
-has sundry traits in common, it very generally happens that while the
-adult is stationary the larva is locomotive. The locomotion of the
-larva is effected by the undulations of a tail. In shape and movement
-one of these young Ascidians is not altogether unlike a Tadpole. And
-as the tail of the Tadpole disappears when its function comes to be
-fulfilled by limbs; so the Ascidian larva’s tail disappears when
-fixation of the larva renders it useless. This disappearance of the
-tail, however, is not without exception. The <i>Appendicularia</i>
-is an Ascidian which retains its tail throughout life; and by its
-aid continues throughout life to swim about. Now this tail of the
-<i>Appendicularia</i> has a very suggestive structure. It is long,
-tapering to a point, and flattened. From end to end there runs a
-mid-rib, which appears to be an imbedded gelatinous rod, not unlike
-a notochord. Extending along the two sides of<span class="pagenum" id="Page_600">[600]</span> this mid-rib, are
-bundles of muscular fibres; and its top bears a gangliated nervous
-thread, giving off, at intervals, branches to the muscular fibres.
-In the <i>Appendicularia</i> this tail, which is inserted at the
-lower part of the back, is bent forwards, so as not to be adapted for
-propelling the body of the animal head foremost; but the homologous
-tails of the larval Ascidians are directed backwards, so as to produce
-forward movement. If we suppose a type like the <i>Appendicularia</i>
-in the structure and insertion of its permanent tail, but resembling
-the larval forms in the direction of its tail, it is, I think, not
-difficult to see that functional adaptation joined with natural
-selection, might readily produce a type approximating to that
-whose origin we are considering. It is a fair assumption that an
-habitually-locomotive creature would profit by increased power of
-locomotion. This granted, it follows that such further development of
-the tail-structures as might arise from enhanced function, and such
-better distribution of them as spontaneous variation might from time
-to time initiate, would be perpetuated. What must be the accompanying
-changes? The more vigorous action of such an appendage implies a firmer
-insertion into the body; and this would be effected by the prolongation
-forwards of the central axis of the tail into the creature’s back. As
-fast as there progressed this fusion of the increasingly-powerful tail
-with the body, the body would begin to partake of its oscillations; and
-at the same time that the resistant axis of the tail advanced along
-the dorsal region, its accompanying muscular fibres would spread over
-the sides of the body: gradually taking such modified directions and
-insertions as their new conditions rendered most advantageous. Without
-further explanation, those who examine drawings of the structures
-described, will, I think, see that in such a way a tail homologous
-with that of the <i>Appendicularia</i>, would be likely, in the course
-of that development required for its greater efficiency, gradually to
-encroach on the body, until its mid-rib became the dorsal axis, its
-gangliated nerve-thread the spinal chord, and its muscular fibres the
-myocommata. Such a development of an appendage into a dominant part of
-the organism, though at first sight a startling supposition, is not
-without plenty of parallels: instance the way in which the cerebral
-ganglia, originally mere adjuncts of the spinal chord, eventually
-become the great centres of the nervous system to which the spinal
-chord is quite subordinate; or instance the way in which the limbs,
-small and inconspicuous in fishes, become, in Man, masses which, taken
-together, outweigh the trunk. It may be added that these familiar cases
-have a further appropriateness; for they exhibit higher degrees of that
-same increasing dominance of the organs of external relation, which the
-hypothesis itself implies.</p>
-
-<p><span class="pagenum" id="Page_601">[601]</span></p>
-
-<p>Of course, if the rudimentary vertebrate apparatus thus grew into,
-and spread over, a molluscoid visceral system, the formation of the
-notochord under the action of alternating transverse strains, did not
-take place as suggested in <a href="#sect255">§ 255</a>; but it does not therefore follow
-that its differentiation from surrounding tissues was not mechanically
-initiated in the way described. For what was said in that section
-respecting the effects of lateral bendings of the body, equally
-applies to lateral bendings of the tail; and as fast as the developing
-tail encroached on the body, the body would become implicated in the
-transverse strains, and the differentiation would advance forwards
-under the influences originally alleged. Obviously, too, though the
-lateral muscular masses would in this case have a different history;
-yet the segmentation of them would be eventually determined by the
-assigned causes. For as fast as the strata of contractile fibres,
-developing somewhat in advance of the dorsal axis, spread along the
-sides, they would come under the influence of the alternate flexions;
-and while, by survival of the fittest, their parts became adjusted
-in direction, their segmentation would, as before, accompany their
-increasing massiveness. The actions and reactions due to lateral
-undulations would still, therefore, be the causes of differentiation,
-with which natural selection would co-operate.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_602">[602]</span></p>
-
-<h3>APPENDIX D 2.<br />
-<span class="subhed1">THE ANNULOSE TYPE.</span></h3></div>
-
-
-<p>The production of a segmental structure by undulatory movements,
-suggested in Appendix D, as also in B (first published in 1858) as
-explaining the vertebral column, has been recently suggested by Prof.
-Korschelt as the cause of that segmentation of the annulose type which
-gives the name to it. He espouses a&mdash;</p>
-
-<div class="blockquot">
-
-<p>“view which is based upon the assumption that at first an
-unsegmented, elongated ancestral form was produced by terminal
-growth, whereupon the entire body became separated at once into
-a large number of segments by a re-arrangement of the individual
-organs. This assumption is supported by the consideration that
-with the lateral sinuous movement of the body, and with the
-rigidity of the tissues caused by increasing differentiation,
-the formation of alternating regions of greater and less
-motility was of considerable advantage to the individual, and
-rendered possible a further elongation of the body. The first
-cause for the appearance of metameric segmentation would then be
-sought in the manner of locomotion and in mechanical conditions.
-However, this latter view is not supported in any way by
-embryology.” (<i>Embryology of Invertebrates</i>, Part I, pp.
-349–50.)</p>
-</div>
-
-<p>I venture to think the confession that this view “is not supported in
-any way by embryology” should be joined with the confession that it
-is at variance with that abstract embryology which comprehends the
-process of development in general. The assumption that there took
-place “a re-arrangement of the individual organs” of “an unsegmented,
-elongated ancestral form,” in such wise that the organs, previously
-single, presently became multiple, so that instead of one organ of each
-kind there were substituted many organs of each kind, is inconsistent
-with the general law of evolution, organic and other&mdash;implies not
-integration but disintegration. Everywhere the advance is from many
-like parts performing like functions to relatively few unlike parts
-performing unlike functions. The higher forms of the annulose type
-itself show this. Compare a myriapod and a crab. In the one we have
-not only a great number of similar segments bearing similar limbs,
-but we have in each segment a dilatation of the main blood-vessel&mdash;a
-rudimentary heart&mdash;a swollen portion of the nerve cord&mdash;a small
-ganglion&mdash;and so on; whereas in the other,<span class="pagenum" id="Page_603">[603]</span> besides relatively few
-segments and few limbs (sundry of them extremely unlike the rest) we
-have a vascular system concentrated into a central heart with arteries
-and a concentrated nervous system, such that the great ganglia in the
-integrated carapace immensely subordinate the ganglia of the remaining
-segments; and similarly with the other organs. Now unless it be denied
-that these highest decapods have been evolved from low types akin to
-myriapods in composition, it must be admitted that the progress has
-been from a string of many like segments with similar sets of organs
-to a group of relatively-few unlike segments with dissimilar sets of
-organs. If so we cannot rationally deny that the progress has been
-of this nature up from the lowest annelid, instead of having been,
-as Prof. Korschelt’s hypothesis implies, of opposite nature at the
-beginning.</p>
-
-<p>In a preceding passage a clear recognition of the normal course of
-development occurs. In opposing the view set forth in <a href="#sect205">§§ 205–7</a> of this
-work, Prof. Korschelt says:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“It seems scarcely favourable to this theory that the degree
-of independence which the individual segments present is
-comparatively slight. The most important organs (nervous system,
-body musculature, blood-vascular system) show themselves to be
-single fundaments of the entire body, and are also developed as
-such even though they also exhibit evidences of metamerism. Even
-the excretory canals may give up their segmental isolation and
-become united to one another by means of longitudinal canals.”
-(<i>Ib.</i> p. 348.)</p>
-</div>
-
-<p>On turning back to <a href="#sect206">§ 206</a>, the reader will, I think, demur to the
-assertion that the independence is “comparatively slight”; seeing
-that, as in <i>Ctenodrilus</i>, a single segment sometimes becomes
-separate and reproduces other segments to form a new series. Instead
-of admitting that “the most important organs” “show themselves to be
-single fundaments of the entire body,” it may be held, contrariwise,
-that their original independence in each segment is masked only to
-the degree involved by their co-operation as parts of a compound
-organism. But chiefly I remark that when it is said that “the excretory
-canals may give up their segmental isolation and become united” by
-“longitudinal canals,” there is a clear confession that the isolation
-of these organs was original and their union superinduced&mdash;an
-implication that the course of evolution is as I have described it, and
-at variance with the course of evolution assumed by Prof. Korschelt.</p>
-
-<p>Yet another incongruity is involved in his interpretation. He writes:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“Just as in the consideration of the tapeworm chain we were
-induced by the comparison with unsegmented forms to refer the
-entire chain to an unsegmented individual, and, on the other
-hand, to see in the proglottis, not a complete individual, but
-only the abstricted hinder portion of the body of the Cestode,
-in the same manner, and with much more reason, we adhere to the
-individuality of the Annelid body.” (P. 349.)</p>
-</div>
-
-<p><span class="pagenum" id="Page_604">[604]</span></p>
-
-<p>And then on the preceding page, referring to the composition of the
-Annelid body, he says:&mdash;“The most natural comparisons are those
-with the tapeworm chain and with the strobila of the Scyphomedusæ.”
-Now since it is here assumed that the tapeworm and the strobila are
-analogous in composition, it is implied that the detached proglottis
-and the detached medusa are analogous; and hence if we are to regard
-the proglottis as “not a complete individual but only the abstricted
-hinder portion of the body of the Cestode,” then we must similarly
-regard the medusa as not a complete individual, but only the abstricted
-hinder portion of the strobila. This commits us to the strange
-conclusion that whereas individuality is ascribed to the original
-simple polyp, and by and by to the partially-segmented strobila, though
-these are without special senses and with only rudiments of muscular
-and nervous systems, individuality is denied to the detached medusa,
-which has organs of sense, a distinct nervo-muscular system and a
-considerable power of locomotion, as well as a generative system:
-traits which in other cases characterize developed individuals. Here
-also, then, there seems to be an inversion of the ordinary conception.</p>
-
-<p>This conception of the proglottis and the medusa is, I see, accepted
-by some as tenable. But if we accept it we must accept also an
-analogous conception, which will I think be regarded as untenable. It
-is that supplied by the <i>Aphides</i>. From an egg proceeds a series
-of sexless and wingless females, and at the end of the series there
-come winged males and females with resulting gamic reproduction. If
-instead of forming a discrete series the imperfect females formed a
-concrete series, the members of which could individually feed without
-being detached from one another, as the segments of a tapeworm can,
-the parallelism would be complete; and then, according to the view
-in question, we should have to regard the perfect males and females
-eventually arising, not as individuals but as terminal portions of
-the series, containing generative products and having wings for the
-dispersion of them&mdash;locomotive egg-bearing segments of the chain.
-Whoever espouses this view must hold either that the first imperfect
-female of the series was the individual or that the entire string
-of them constituted the individual (in conformity with a view once
-propounded by Prof. Huxley). But he must do more than this. Since the
-<i>Aphides</i> have descended from some winged species of the order
-<i>Hemiptera</i>, he must hold that among those remote ancestors each
-particular fly, male or female, was an individual; but that when
-abundant food and inert life led to the partheno-genetic habit, and to
-chains of sexless forms, the males and females eventually produced at
-the end of each chain, though, like their remote ancestors, possessed
-of procreative organs and wings, are not individuals.</p>
-
-<p><span class="pagenum" id="Page_605">[605]</span></p>
-
-<p>[Some memoranda bearing on the question here discussed, mislaid at the
-time when the chapter dealing with it was revised, have been discovered
-in time for utilization in this appendix.]</p>
-
-<p>One of my critics says:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“You have overstated the case in your favour: the alimentary
-canal does not, as you suggest, show a segmentation
-corresponding to that of the other organs in Annelids. Either
-it is a simple uniform tube, or else its differentiations
-(pharynx, œsophagus, crop, intestine) are quite independent of
-the repetition of the somites.”</p>
-</div>
-
-<p>In presence of statements made in works of authority, this objection
-greatly surprises me. I meet with the descriptive word “moniliform”
-applied to the intestine in some Annelids, and then in the Text Book of
-Claus, translated and edited by Sedgwick, it is said, concerning the
-alimentary canal in the <i>Annelida</i>:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“This is followed by the gastric region of the gut, which
-occupies the greatest portion of the length of the body, and
-is either regularly constricted in correspondence with the
-segments, or possesses lateral diverticula.” (P. 365.)</p>
-</div>
-
-<p>And again on p. 369 it is said:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“The intestine usually preserves the same structure in its
-entire length and is divided by regular constrictions into
-a number of divisions or chambers, which correspond to the
-segments and dilate again into lateral diverticula and cæca.”</p>
-</div>
-
-<p>The alimentary canal thus presents the segmental character as clearly
-as consists with fulfilment of its function. If the successive segments
-are co-operating units of a compound animal having but one mouth,
-then, necessarily, the gut cannot be completely cut into parts, each
-answering to a segment, for there could be, in that case, no passage
-for the food. If the portion of the intestine belonging to each segment
-has a conspicuous dilatation, or has a cæcum on each side, it exhibits
-the segmental character as much as the physical requirements permit. So
-far from being at variance with the hypothesis, its structure exhibits
-a verification of it.</p>
-
-<p>The next objection runs as follows:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“Then, again, the ovaries and testes do not exhibit a
-corresponding segmentation. When it is allowable to speak of
-ovary or testis at all as in <i>Lumbricus</i>, we find that in
-the case of both organs we have at most two pairs.”</p>
-</div>
-
-<p>It seems to me that the distribution of the generative organs in
-a comparatively-developed member of the Annelid type, is not the
-question. We have to ask what it is in undeveloped members of that
-type. Among them the repetition of generative parts is in some cases
-just what the theory implies. Thus in Claus I read:&mdash;“In the marine
-<i>Chætopoda</i>, the ova or spermatozoa originate on the body-wall
-from cells of the peritoneal membrane, either in<span class="pagenum" id="Page_606">[606]</span> the anterior segments
-alone or along the whole length of the body.” So that in these last
-cases there are, in all the segments, parts from which arise generative
-products. The fact that these parts are not definite ovaries and testes
-is irrelevant. Ovaries and testes are developed generative structures,
-and in the order of evolution are preceded by undeveloped ones; and the
-fact that these undeveloped ones are found in little-developed members
-of the type conforms perfectly to the hypothesis. [I may remark in
-passing that here is a good illustration of that process of evolution
-which, in the above speculation of Prof. Korschelt, is supposed to be
-inverted: many dispersed, similar, and indefinite parts, are integrated
-into a few localized and definite parts.]</p>
-
-<p>In continuation the critic above quoted says:&mdash;“My position is that
-the repetition of segments in an Annelid is a phenomenon of the same
-nature as the repetition of hairs in a Mammal or of scutes in a
-Reptile”, and he proceeds to give instances of repetitions of organs
-in other types, as of the reproductive structures and excretory system
-in the young Dog-fish or of the ovaries in <i>Amphioxus</i>. These
-examples do not seem to me relevant. No parallelism exists between the
-repetition of a particular organ in an animal, and the repetition of
-an entire cluster of organs constituting a physiological whole. The
-repetitions of the ovaries in <i>Amphioxus</i> and of the excretory
-system in a young Dog-fish, occur without threatening to divide into
-similar parts the entire organism. But the segmental repetitions in
-an annulose creature implicate the structures at large, and would, if
-pushed a little further, result in separate creatures. The segment
-of a low Annelid contains alimentary, vascular, nervous, excretory,
-reproductive, sensory and locomotive organs&mdash;all the organs required
-for carrying on life, save certain organs of external relation which
-its position excludes. When there is shown some vertebrate animal, or
-proto-vertebrate animal, that is divisible into parts each of which is
-in great measure physiologically independent, I shall feel obliged to
-abandon my position.</p>
-
-<hr class="tb" />
-
-<p>While this appendix is in hand I have received from another expert,
-whose view is in general agreement with my own, a letter containing the
-following passage:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“You will see that Dohrn’s theory was the antithesis of your
-own view of vertebrate structure, namely that the vertebræ
-were formed by the segmentation, from mechanical causes of a
-body originally simple. This view of yours has been confirmed
-by later researches, which have shown that the most primitive
-forms allied to the Vertebrates, possessing the essential
-organs, viz., gill-slits, notochord, and dorsal nerve cord,
-are not segmented animals, like Annelids and <i>Crustacea</i>,
-but simple animals, having at most three regions, not exactly
-corresponding to segments. These primitive unsegmented forms
-are Ascidian tadpoles, <i>Balanoglossus</i>, and certain other
-primitive forms. The embryology of Vertebrates also proves
-that they are<span class="pagenum" id="Page_607">[607]</span> originally simple and not segmented animals,
-especially the fact that there is originally one pronephric duct
-or primitive kidney.”</p>
-</div>
-
-<p>Nevertheless there survives a leaning towards the notion of a segmental
-origin of the <i>Vertebrata</i>. But the repetitions of organs named in
-support of this notion have, I think, no more relation to the genesis
-of the vertebrate type than the multiplication of vertebræ in a snake
-has relation to the genesis of the vertebral column.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_608">[608]</span></p>
-
-<h3>APPENDIX E.<br />
-<span class="subhed1">THE SHAPES AND ARRANGEMENTS OF FLOWERS.</span></h3></div>
-
-
-<p>In Part IV., Chapter X., under the title of “The Shapes of Flowers,” I
-have, after describing their several kinds of symmetry, as habitually
-related to their positions, made some remarks by way of interpretation.
-The truth that flowers exhibit a radial symmetry when they are so
-placed as to be equally affected all round by incident forces, having
-been exemplified, and also the truth that they assume a bilateral
-symmetry when they are so placed that their two sides are conditioned
-in ways different from the ways in which their upper and lower
-parts are conditioned; I have gone on to inquire (in <a href="#sect234">§ 234</a>) by what
-causes such modifications of form are produced. I have stated that,
-originally, I inclined to ascribe them entirely to differences in the
-relations of the parts to physical forces&mdash;light, heat, gravitation,
-etc.; but that I found sundry facts stood in the way of this
-interpretation. And I have said that “Mr. Darwin’s investigations into
-the fertilization of Orchids led me to take into account an unnoticed
-agency.” Continuing to recognize the physical forces as factors having
-some influence, I have concluded that the most important factor is
-the action of insects; which, aiding most the fertilization of those
-flowers which most facilitate their entrance, produce, in course of
-generations, a form of flower specially adapted to the special position.</p>
-
-<p>Though still adhering to this interpretation, I have since found reason
-to think that the original interpretation contains a larger portion
-of truth than I supposed at the time when I was led thus to revise
-it. While staying at Mürren, in Switzerland, in 1872, I observed
-some modifications in a species of Gentian, which proved to me that
-the action of incident physical forces on flowers is, in some cases,
-very rapid and decided. The species furnishing this evidence was the
-<i>Gentiana Asclepiadea</i>; which I found in a copse formed of bushes
-that were here wide apart and there close together. In some places not
-near to the bushes, the individuals of the species grew vertically; in
-other places, partially shaded, their inclined shoots curved in such<span class="pagenum" id="Page_609">[609]</span>
-directions as to get the most light; and in other cases their shoots
-were led to take directions almost or quite horizontal. That, along
-with these modifications in the directions of their shoots, there went
-adjustments in the attitudes of their leaves, was a fact not specially
-worthy of remark; for plants placed inside the windows of houses
-habitually show us that leaves quickly bend themselves into attitudes
-giving them the greatest amounts of light. But the fact which attracted
-my attention was, that the flowers changed their attitudes in an
-equally-marked manner. The radial distribution passed into a bilateral
-distribution with the greatest readiness. Comparison of the annexed
-figures will show the character of this change.</p>
-
-<p>Figure I. represents part of a vertically-growing shoot. This
-belonged to an individual growing unimpeded by bushes, and getting
-light on all sides. Here it is observable that the pairs of leaves,
-placed alternately in directions transverse to one another&mdash;one pair
-pointing, say, north and south, and the next pair pointing east and
-west&mdash;maintain, taking them in the aggregate, a radial distribution;
-and it is also observable that the alternate pairs of flowers are
-similarly arranged.</p>
-
-<p>Figure II. is a sketch from a shoot which leaned towards one side,
-and of which the higher part, as it bent more and more, got its upper
-side more and more differently conditioned from its lower side. Here
-we find that not only the leaves, but also the flowers, have adjusted
-themselves to the changed conditions. The leaves of the lowest pair
-hang out in the normal way, on the opposite sides of the axis, so that
-a plane passing through their surfaces will cut the axis transversely;
-and their two axillary flower-buds, <i>c</i> and <i>d</i>, are
-similarly placed on opposite sides of the axis. But at the other part
-of the shoot, we see both that the leaves have adjusted themselves so
-that their planes, no longer cutting the axis transversely, keep a
-fit adjustment with respect to the light; and also that the flowers,
-no longer on opposite sides of the axis, have bent round to the upper
-side, as at <i>a</i> and <i>b</i>.</p>
-
-<p>Figure III. shows us this re-arrangement carried still further. The
-shoot it represents was growing in a direction nearly horizontal, and
-therefore receiving the light only on one side. And here, besides
-seeing that the leaves have so adjusted themselves that they all lie in
-approximately the same plane, which is parallel to the axis instead of
-transverse to it, we see that the two pairs of flower-buds have both
-come round to the upper side of the axis. So that in this shoot, the
-original radial symmetry in the arrangement of leaves and flowers, is
-completely changed into a bilateral symmetry.</p>
-
-  <div class="figcenter" id="i_610">
-     <img
-    class="p2"
-    src="images/i_610.jpg"
-     alt="" />
-     <p class="p0 center p-left">Figs. 1–3.</p>
-  </div>
-
-<p class="p2">These facts do not, it is true, prove any modification in the forms<span class="pagenum" id="Page_611">[611]</span>
-of the flowers themselves: they only prove modification in the grouping
-of the flowers. But beyond showing, as they do conclusively, how
-readily a bilateral arrangement of flowers is producible out of an
-arrangement that was not bilateral, by the action of light, etc.; they
-give increased probability to the belief that changes in the shapes
-of flowers are producible by the same agencies. Doubtless this change
-in the attitudes of the flower-buds is due to the action of light on
-their calyces and peduncles more than to its action on their unfolding
-corollas. But along with an action so decided on the growth of these
-sheathing and supporting organs containing chlorophyll, it is scarcely
-probable that there is <i>no</i> action on the growth of the petals,
-containing other colouring matter; considering that in both cases the
-development of the colouring matter depends on the action of light, and
-considering also the effect of light on petals, familiarly shown by
-their opening and closing. And if even but a small effect is producible
-on the growth of the corolla, then it is to be expected that light will
-be an agent in changing the form of the corolla, when the attitude of
-the flower causes its parts to be differently exposed. For a small
-effect on the individual flower will become a great effect in the
-flowers of remote descendants; provided the changed attitudes of the
-flowers preserve considerable constancy throughout the succession of
-individuals.</p>
-
-<p>Be this as it may, however, the facts I have here described, which
-I doubt not other observers have seen paralleled in other plants,
-are instructive, as showing how quickly certain metamorphoses are
-produced, and as implying the easy establishment of such metamorphoses
-as permanent characters in a species, if the modifying conditions
-become permanent. The changes of arrangement I have pointed out, do not
-become permanent in this species because its individuals are variously
-affected by the modifying forces: on some they do not act at all,
-on some a little, on some much; and even on the same individual the
-different shoots are quite differently affected. But if the habit of
-this plant were greatly changed&mdash;if, for instance, by spreading into
-habitats yielding abundant nutriment, the plant became very luxuriant,
-and, multiplying its branches, grew shrub-like; it is clear that, being
-shaded by one another, these branches would be habitually circumstanced
-in a way like that which we here see produces bilateralness in the
-distribution of the flowers, if not in the flowers themselves; and
-being thus permanently affected, would become permanently bilateral.
-Accumulating by inheritance, what is here only an individual
-peculiarity, would become a peculiarity of the species&mdash;a specific
-character.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_612">[612]</span></p>
-
-<h3>APPENDIX F.<br />
-<span class="subhed1">PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS.</span></h3></div>
-
-
-<p>There has recently come before me a fact which has a significant
-bearing on the hypothesis of Constitutional units: serving, indeed, to
-give an apparently conclusive proof of its truth. Before stating it,
-however, I may with advantage re-state the several evidences already
-assigned in support of it.</p>
-
-<hr class="tb" />
-
-<p>1. First comes the <i>à priori</i> reason. These units in the germ
-of an organism which cause development into a special structure,
-cannot be <i>chemical</i> units&mdash;cannot be simply molecules of proteid
-substance in one or other of its forms; since these are not special
-to any type of creature but common to all creatures. Nor can they be
-what we may call <i>morphological</i> units&mdash;the cells or protoplasts;
-because in the early stages of development the cells of one organism
-are indistinguishable from those of others, and because were cells
-the units of composition there could be no interpretation of what are
-called unicellular organisms&mdash;nothing to account for the innumerable
-varieties of them. Hence, of necessity, the structural elements
-of which each organism is built, being neither proteid molecules
-nor cells, must be something between them: probably some complex
-combination of different isomeric forms of proteids.</p>
-
-<hr class="tb" />
-
-<p>2. That units of such natures are the essential components of each
-species of organism, is shown by the fact that in low types of
-creatures, little differentiated into special tissues, any considerable
-portion of the body will, when separated, begin to assume the structure
-proper to the species&mdash;a truth recently shown afresh by Prof. T. H.
-Morgan’s experiments on the regeneration of <i>Planaria maculata</i>
-(already referred to in <a href="#sect206">§ 206</a>) showing that various fragments cut out
-develop into new individuals, and that when, being too small they die
-before doing this, there is always an abortive attempt to assume the
-specific structure.</p>
-
-<hr class="tb" />
-
-<p>3. This truth that a portion of undifferentiated tissue, if adequate in
-quantity, assumes the structure of the type, illustrating<span class="pagenum" id="Page_613">[613]</span> as it does
-the proclivity of the constitutional units towards the structure of
-the species, allies itself with the phenomena of both agamogenesis and
-gamogenesis. The first of these shows us how a fissiparously-detached
-portion of the parental tissue takes on the same form as the parent;
-and the second shows how those small detached portions distinguished
-as sperm-cell and germ-cell also, when united and supplied with the
-needful materials, do the same thing.</p>
-
-<hr class="tb" />
-
-<p>4. But the set of phenomena following the union of sperm-cell and
-germ-cell differ in a certain way from those which follow when a
-<i>gemma</i> or other unfertilized portion of parental tissue is
-detached. The incomprehensibleness of this difference as otherwise
-contemplated, and the partial comprehensibleness of it when joined with
-the hypothesis of physiological units, furnish a further support for
-the hypothesis.</p>
-
-<p>The familiar truth learnt by the tyro in algebra that an apparent
-solution which contains the unknown quantity is no solution, is a
-truth apt to be overlooked in other spheres than the algebraic. An
-illustration is supplied by the answer once given in Parliament to the
-question “What is an Archdeacon?”&mdash;“One who discharges archidiaconal
-functions.” But science as well as daily life furnishes examples.
-When it is said by Engelmann, Hensen, Hertwig, and Maupas that “the
-essential end of sexuality is rejuvenescence, that is, the restoration
-of growth-energy,” we have another instance of an explanation which
-explains nothing. What is the phenomenon to be explained? That
-unfolding of an organism from a germ which displays growth-energy. And
-what is the explanation? The giving of fresh growth-energy. The unknown
-quantity “growth-energy” is contained in the explanation proposed.
-There exists no conception of “juvenescence” save that derived from
-observing developing plants and animals; and if “re” be prefixed, no
-interpretation is thereby given to the unexplained thing “juvenescence.”</p>
-
-<p>Coleridge somewhere comments on a source of fallacy which he calls the
-“hypostasis of a relation”&mdash;the changing of a relation into a thing.
-The plumber who tells you that water rises in a pump “by suction”
-supplies an instance. Having assumed suction to be an agent, he
-thinks that he understands how the piston does its work. Some of the
-explanations given of fertilization supply further instances. When it
-is said that sexual union has for its end “to give increased vigour
-to all the vital processes,” it is tacitly implied that vigour is a
-something&mdash;a something which can be given. But now, in the first place,
-it is only by the hypostasis of a relation that we are led to think
-of vigour as a thing. Vigour is a state&mdash;that state of a living body
-which enables it to<span class="pagenum" id="Page_614">[614]</span> give out much motion. What enables it to do this?
-The presence in it of abundant molecules containing much molecular
-motion which can be transformed into molar motion: the transformation
-being effected by the falling of these molecules into their simpler
-and relatively-inert components, which are thereupon excreted.
-Energy-containing matter is used up, and more energy or vigour can be
-given only by supplying more such matter. How then can the union of
-two nuclei&mdash;those of the sperm-cell and germ-cell&mdash;give vigour? Only
-an infinitesimal portion of vigour in the sense above explained exists
-in either, and the union of them leaves it still infinitesimal. And
-then, even supposing the vigour to be an entity and to be appreciable
-in quantity, how could it go on producing that immense combination
-of physiological actions seen in the unfolding of the germ into an
-organism? and how could it go on producing the physiological actions of
-an adult organism during a whole century?</p>
-
-<p>May we not then say that these proposed explanations leave the question
-where it was&mdash;are nominal solutions, not real solutions?</p>
-
-<hr class="tb" />
-
-<p>5. But the hypothesis of constitutional units furnishes, if not a
-satisfactory answer yet, something in the nature of an answer&mdash;a true
-cause; that is to say, a cause actually known to us as operating in
-other cases. In <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect92">§ 92</a> it was pointed out that in proportion as units
-are similar, there may be built up from them an aggregate which is
-relatively stable, and that along with increasing dissimilarity the
-stability of the aggregate decreases. It was inferred that if a group
-of constitutional units belonging to one individual which have become
-moulded into relatively exact congruity with the organism and with
-one another by long co-operation, are mingled with some belonging
-to another individual which, differently circumstanced, has become
-somewhat different in itself and in its units, then the mass formed by
-the union of the two groups will be relatively unstable&mdash;relatively
-modifiable by incident forces. Whereas in either organism, no longer
-perpetually changed in the relations of its parts by growth, there is
-an approach towards equilibrium between the whole and its components,
-the components contributed by the two to form a germ, being slightly
-unlike one another, will not form a group in a state of equilibrium.
-The group they form will be capable of easy change by incident forces;
-and they will so be rendered free to follow their proclivities towards
-the typical form of the species. Inferring this we must also infer that
-so long as these two sets of slightly different units are not exposed
-to any constant forces tending to coerce them into the same form, there
-will continue to exist in the nuclei of all descendant cells this same
-relative instability and consequent plasticity.</p>
-
-<p><span class="pagenum" id="Page_615">[615]</span></p>
-
-<p>Such evidence as we have verifies this interpretation. There is
-first the universal fact that development of the germ begins when
-it is exposed to an incident force&mdash;heat&mdash;the undulations of which,
-increasing the oscillations of the mixed units, give them greater
-freedom to arrange themselves in conformity with their type. We see
-this alike when spring warmth makes a seed germinate and when the
-warmth of a sitting hen sets up organization in her eggs. Heat frees
-the molecules of inorganic matter from local restraints and, as we see
-in molten metal, lets them yield to other forces; and similarly in this
-organic matter, the units are made free to follow their proclivities.
-Then, secondly, there comes the evidence from comparisons between the
-effects of mixing constitutional units differing in various degrees.
-Let the cluster of mixed units be derived from animals that are
-ordinally distinct. Nothing happens. The units each contributes tend to
-arrange themselves after the parental type. Hence a conflict between
-the tendencies towards two markedly unlike structures, and no structure
-arises. Suppose the mixed units come from two kindred species&mdash;say
-horse and ass. The structures which they respectively tend to form,
-being in their main characters alike, there is such co-operation
-as produces a working organism but an organism in certain respects
-imperfect&mdash;a mule. Suppose, again, the units come from two varieties
-of the same species. A perfect organism results, and, as shown by Mr.
-Darwin when detailing the effects of crossing, an unusually vigorous
-organism. The units being more unlike than those belonging to the same
-variety, the instability of the germ-plasm is unusually great, and
-the transformations which constitute development and action become
-unusually active. When, as in ordinary cases, the units are supplied by
-members of the same variety who have not been made very much alike by
-their antecedents, there follows the usual amount of organic vigour.
-Coming now to the results of breeding in-and-in&mdash;breeding between
-individuals whose constitutions (<i>i.e.</i> constitutional units) have
-for generations been growing more alike in the absence of crossing with
-other stirps&mdash;we see that diminution of organic vigour is displayed:
-there is a decrease in the rate of physiological change. Finally, on
-coming to a closer relationship, as in marriages between cousins, in
-whom the constitutional units are more than commonly alike, we see
-there frequently follows either barrenness or the production of feeble
-offspring.</p>
-
-<p>All these facts, then, are congruous with the hypothesis that the
-use of fertilization is the mixing of unlike units, and consequent
-production of plasticity. Leaving out cases in which the unlikenesses
-are so great as wholly to prevent co-operation among the units, the
-degree of vigour, that is, the activity of physiological<span class="pagenum" id="Page_616">[616]</span> change, is
-great where the unlikeness is great and diminishes with the approach
-towards likeness.</p>
-
-<hr class="tb" />
-
-<p>6. The existence of constitutional units seems otherwise necessarily
-implied. I refer to the fact that no organism is a homogeneous mean
-between its parents but consists of a mixture of parts, some following
-one parent and some the other. Among illustrations of this the most
-conspicuous are those yielded by the variously-mixed colours of
-hair or feathers. Horses, cattle, dogs, cats, hens, pigeons display
-these mixtures: colours in one place like the mother and in another
-place like the father. As the internal organs are invisible, and as
-visible organs have indefinite shapes and graduate indefinitely into
-adjacent ones, the mixture of traits is elsewhere less conspicuous; but
-occasional marked cases (especially in malformations) leave no doubt
-that it pervades the entire organism.</p>
-
-<p>This peculiarity of transmission seems necessarily to imply that there
-are distinct units derived from the two parents, and that in the course
-of development there is more or less segregation of them&mdash;those of
-the one origin predominating so far in some places as to give special
-likeness to one parent, and those derived from the other doing the like
-in other places. All which interpretation is impossible unless the
-hypothesis of constitutional units be admitted.</p>
-
-<hr class="tb" />
-
-<p>7. I come at length to the special evidence referred to at the outset.
-It is evidence of the same nature as that just assigned, but carried
-to a higher stage. It is furnished not by the segregation of traits
-derived from two parents of the same variety, but is furnished by the
-segregation of traits derived from parents of different varieties. In
-articles on “Bud Variations or Sports” (<i>Gardener’s Chronicle</i>,
-1891) Dr. Masters gives various examples of the separation or unmixing
-of ancestral constitutions. Mr. Noble formed a hybrid between
-<i>Clematis Jackmani</i> and <i>C. patens</i>. One of these varieties
-flowers in the autumn on new wood, while the other flowers in the
-spring on old wood; and the result is that flowers of two kinds, quite
-unlike, are produced at different parts of the year, and that by
-pruning so as to cut away one or other set of shoots, the plant may be
-made to produce exclusively for the time being one or other sort of
-flower.</p>
-
-<div class="blockquot">
-
-<p>“Another very interesting case of unmixing, or, if it be
-preferred, of partial mixture, is afforded by Neubert’s
-Berberis. This is a hybrid between the evergreen pinnate-leaved
-Mahonia and the deciduous simple-leaved Berberis vulgaris, and
-it bears leaves some of which are intermediate in appearance,
-while others are much like those of one or other of its parents.</p>
-
-<p>“A not uncommon illustration of a similar kind, is the
-production of a Peach and a Nectarine on the same branch, and we
-have just learnt from<span class="pagenum" id="Page_617">[617]</span> Canon Ellacombe that some of the Berlin
-Hellebores show evidence of their hybrid nature by occasionally
-producing foliage [and flowers?] of the two parents separately
-from the same root-stock.</p>
-
-<p>“In addition to the cases given above, we may here cite a few
-more which have come under our notice, such as a Chrysanthemum,
-half the florets of which are of one colour, half of another.
-A hybrid Calanthe, showing a similar piebald variation, is
-shown in Fig. 14. A very curious case was that of the Narcissus
-received from Mr. Walker, and in which flowers of two distinct
-varieties sprang from the same bulb. Grapes not uncommonly show
-their crossed origin by presenting a striped appearance, one
-stripe being of one colour, one of another, as may also be seen
-in the Orange, Apple, Lemon, and Currant.”</p>
-</div>
-
-<p>Thus, however the germ-plasm is constituted its essential components
-cannot be all alike. Before there can be this dissociation of ancestral
-characters, there must be in the germ-plasm different elements capable
-of being dissociated. This single fact seems to compel us to assume
-constitutional units.</p>
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_618">[618]</span></p>
-
-<h3>APPENDIX G.<br />
-<span class="subhed1">THE INHERITANCE OF FUNCTIONALLY-CAUSED MODIFICATIONS.</span></h3></div>
-
-
-<p>In Part II, Chapter X<sup>A</sup>, I have confessed that the process by which
-a structure changed by use or disuse affects the sperm-cells or
-germ-cells whence arise descendants, is unimaginable: without, however,
-inferring that therefore such a process does not exist. With others
-it seems different. Some three years ago the following expression of
-opinion came to me from a zoological expert:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“Many zoologists&mdash;most of us here at Cambridge&mdash;are intensely
-opposed to the doctrine of the inheritability of acquired
-variations. Even assuming that the developmental power of a
-germ is determined by its molecular structure (and I for one
-would question this&mdash;Driesch and his school when they find that
-they can squeeze a developing egg into all sorts of shapes
-without altering the final result, that one blastomere in an
-egg which has divided into 8 is still able to reproduce a whole
-embryo&mdash;question it also), we still fail to conceive any means
-by which, for instance, a change in the development of a muscle
-or nerve can effect a corresponding change in that part of the
-germ which is destined to produce a corresponding part in the
-descendant.”</p>
-</div>
-
-<p>Here it will be observed that belief in the inheritance of structural
-effects wrought by use and disuse, is rejected because of inability “to
-conceive any means” by which the modifications produced in an organ can
-effect a correlated modification in the germ of a descendant: failure
-to conceive is the test. The implication is that some alternative
-hypothesis is accepted because the correlating of a variation in an
-organ with a corresponding germ-variation is effected by a means which
-<i>is</i> conceivable. This is the hypothesis of Weismann. Concerning
-its conceivability I have, in the chapter just named, already written
-as follows:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“If we follow Prof. Weismann we are led into an astounding
-supposition. He admits that every variable part must have a
-special determinant, and that this results in the assumption of
-over two hundred thousand for the four wings of a butterfly.
-Let us ask what must happen in the case of a peacock’s feather.
-On looking at the eye near its end, we see that the minute
-processes on the edge of each lateral thread must have been in
-some way exactly adjusted, in colour and position, so as to fall
-into line with the processes on adjacent threads: otherwise the
-symmetrical arrangement of coloured rings would be impossible.
-Each of these processes, then, being an<span class="pagenum" id="Page_619">[619]</span> independent variable,
-must have had its particular determinant. Now there are about
-300 threads on the shaft of a large feather, and each of them
-bears on the average 1,600 processes, making for the whole
-feather 480,000 of these processes. For one feather alone there
-must have been 480,000 determinants, and for the whole tail
-many millions. And these, along with the determinants for the
-detailed parts of all the other feathers, and for the variable
-components of all organs forming the body at large, must have
-been contained in the microscopic head of a spermatozoon!” [And
-each of them must, throughout all the complex developmental
-processes, have preserved the ability to find its way to the
-exact place where it was wanted!]</p>
-</div>
-
-<p>If my Cambridge correspondent is able to conceive this process implied
-by the hypothesis of Weismann, I can only say that he has an enviable
-power of imagination.</p>
-
-<hr class="tb" />
-
-<p>But now comes the strange fact that an impossibility of thought
-implied by Weismann’s hypothesis does not cause rejection of it, but
-yet is urged as a reason for rejecting an alternative hypothesis
-which does not imply it. One objector cannot conceive that “a change
-in the development of a muscle or nerve can effect a corresponding
-change in that part of the germ which is destined to <i>produce a
-corresponding part</i> in the descendant”; and another objector says
-it is “very hard to believe” that a functionally-changed organ will so
-affect spermatozoa and ova that “<i>one particular part of them</i>
-will be so altered that the organisms which grow up from them will
-be able to present the same modification on the application of a
-different stimulus.” It is tacitly assumed by both that, as in the
-hypothesis of Weismann so in the counter-hypothesis, a particular
-part of the germ-plasm gives origin to a particular part of the
-developed organism. But nothing of the kind is implied. The nature
-of the counter-hypothesis (at any rate as held by me) is entirely
-misapprehended. Anyone who turns back to the chapters in the first
-volume where the conception of physiological units (or constitutional
-units) was set forth, or who re-reads the foregoing appendix, will
-see that there is altogether excluded any idea of correlation between
-certain parts of the germ and certain parts of the resulting organism.
-The units are supposed to be all alike, and during the progressive
-embryological changes local groups of them are supposed to take on
-different forms and structures under the combined forces, general and
-local, brought to bear on them. This conception is necessitated by all
-the evidence. The fact disclosed by the experiments of Driesch, Wilson,
-and Chabry, that from fractions of an ovum structures may be obtained
-like that obtained from the whole ovum, only smaller, necessitates
-it. The fact that any sufficiently large fragment of a polyp or
-planarian, no matter from what part of the body taken, will develop
-into a complete polyp or planarian necessitates it. The fact that from
-an undifferentiated portion<span class="pagenum" id="Page_620">[620]</span> of a plant, even so small as a scale,
-a complete plant may arise necessitates it. And it is necessitated
-by the fact that among plants, roots are produced by imbedded shoots
-and shoots by roots, as well as by the fact that low animals, such
-as hydroids, if deprived of both head and root, will develop a head
-from the root part and a root from the head part, if their respective
-conditions are inverted. All this evidence shows conclusively that
-the component units of each species, whether existing in the germ or
-in the developed organism, are, when not yet differentiated by local
-conditions, all alike, and that the notion of special parts of the
-germ-plasm correlated with special parts of the resulting organism, is
-entirely alien to the hypothesis.</p>
-
-<p>“But how do the units of a modified organ affect the units of the
-germ in such wise that these produce an inherited modification of
-the organ?” will be asked. This difficulty has been dealt with in <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect97">§§ 97<i>d</i>, 97<i>e</i></a>,
- where the analogy between the social organism
-and the individual organism has been brought in aid: serving, if not to
-furnish a conception, yet to furnish an adumbration. Regarding citizens
-as the units of an unfolding society, say a colony, it was pointed
-out that the nature they inherit from a mother-society gives them a
-proclivity towards a society of like structure, the traits of which
-are progressively assumed as the colony grows sufficiently large to
-make them possible. At the same time it was pointed out that while the
-influence of the entire aggregate on the individuals is seen in this
-forming of them into a society of the inherited type, the influences of
-local circumstances, and of individuals on one another, in each group,
-make them differentiate into appropriate social structures, taking on
-fit occupations and industries: the implication being that in virtue
-of their inherited natures they all have partial capacities for the
-various activities they undertake; so that an immigrant clerk sets up a
-tavern, a compositor takes to carpentering, and a university man rides
-after cattle or is employed on a sheep farm. Evidence was given in that
-place, as in the above paragraph, that the constitutional units of an
-organism similarly have all of them potentialities for taking on this
-or that structure and mode of action which local conditions determine.
-It was further argued that as citizens are continually being remoulded
-by their society into congruity with it, and, if circumstances change
-them, tend to remould their society; so in the individual organism,
-there is this reciprocal action of the whole on the units and of the
-units on the whole. Hence it was inferred that the modified units in
-any modified part tend to diffuse modifications like their own through
-the units at large: being aided by the circulation of protoplasm, as
-suggested in <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect54">§§ 54<i>d</i></a> and <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect97">97<i>f</i></a>. And it was urged that,
-however inconceivably complex such a process may be, yet it seems not<span class="pagenum" id="Page_621">[621]</span>
-incredible when we recognise the probability that an organism is more
-or less permeable to undulations propagated by its molecules: Rontgen
-rays giving warrant. If such units throughout the tissues may take in
-and send out ethereal waves which bring it into rhythmical relations
-with others of its kind and tend to produce congruity, it becomes,
-if not conceivable still supposable, that throughout the circulating
-protoplasm there goes on a continual harmonization of its components&mdash;a
-moulding of each by all and of all by each. Should it be said that such
-a process is too marvellous to be reasonably assumed, the reply is that
-it is not more marvellous than heredity itself, which, were it not
-familiar to us, would be thought incredible.</p>
-
-<hr class="tb" />
-
-<p>But as I have said in the place referred to&mdash;“At last then we are
-obliged to admit that the actual organizing process transcends
-conception. It is not enough to say that we cannot know it; we must say
-that we cannot even conceive it:” can only conceive the possibility of
-a suggested interpretation.</p>
-
-<p>Hence we have to rely upon evidences of other kinds. Among these, some
-which I think dispose absolutely of the fashionable hypothesis while
-they harmonize with the opposed hypothesis, have now to be named. That
-their implication should not have been generally recognized would have
-seemed to me incomprehensible were it not that I have myself only now
-observed this implication. The facts are these:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“Verlot mentions a gardener who could distinguish 150 kinds
-of camellia, when not in flower; and it has been positively
-asserted that the famous old Dutch florist Voorhelm, who
-kept above 1,200 varieties of the hyacinth, was hardly ever
-deceived in knowing each variety by the bulb alone. Hence we
-must conclude that the bulbs of the hyacinth and the branches
-and leaves of the camellia, though appearing to an unpractised
-eye absolutely undistinguishable, yet really differ.” (Darwin,
-<i>Variation of Animals and Plants, &amp;c.</i>, vol. ii, p. 251.)</p>
-</div>
-
-<p>More recently testimony to like effect has been given by Dr. Maxwell
-Masters, and has already been quoted by me in a note to <a href="#sect286">§ 286</a> in
-illustration of another truth. He says concerning such variations:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“To the untrained eye, the primordial differences noted are
-often very slight; even the botanist, unless his attention
-be specially directed to the matter, fails to see minute
-differences which are perceptible enough to the raiser or his
-workmen.... These apparently trifling morphological differences
-are often associated with physiological variations which render
-some varieties, say of wheat, much better enabled to resist
-mildew and disease generally than others. Some, again, prove to
-be better adapted for certain soils or for some climates than
-others; some are less liable to injury from predatory birds than
-others, and so on.”</p>
-</div>
-
-<p>In his <i>Vegetable Teratology</i>, p. 493, Dr. Masters names another
-fact having a like implication&mdash;the fact that among seedling<span class="pagenum" id="Page_622">[622]</span> stocks
-which have not yet flowered, those which will produce double flowers
-are distinguishable. He says:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“This separation of the single from the double-flowered plants,
-M. Chatié tells us is not so difficult as might be supposed. The
-single stocks, he explains, have deep green leaves (glabrous in
-certain species), rounded at the top, the heart being in the
-form of a shuttlecock, and the plant stout and thick-set in its
-general aspect, while the plants yielding double flowers have
-very long leaves of a light green colour, hairy and curled at
-the edges, the heart consisting of whitish leaves, curved so
-that they enclose it completely.”</p>
-</div>
-
-<p>What is the general truth implied? Clearly that there exists no such
-thing as an independent local variation. Some marked change in the form
-or colour of a flower or a fruit draws attention; and, being a change
-which interests the florist or gardener, pecuniarily or otherwise,
-not only draws attention but usually monopolizes attention: the
-natural impression produced being that this variation stands there by
-itself&mdash;is without relation to variations elsewhere. But now it turns
-out that there are concomitant variations all over the plant. Even in
-underground bulbs certain appreciable differences go along with certain
-conspicuous differences in the flowers. And if along with a striking
-change in a flower which the florist contemplates, there go changes
-all over the plant not obvious to careless observers but visible to
-him, we must infer that there are everywhere minute differences which
-even the florist cannot perceive: the whole constitution of the plant
-has diverged in some measure from the constitutions of kindred plants.
-Every local variation implies a change pervading the entire organism,
-manifested in concomitant variations everywhere else.</p>
-
-<p>If so, what becomes of the hypothesis of determinants&mdash;the hypothesis
-that there is a special element in the germ-plasm which results in a
-special local modification in the adult organism? That there are no
-facts supporting it has been all along manifest; but now it is manifest
-that the facts directly contradict it.</p>
-
-<p>At the same time it may be remarked that while the facts are wholly
-incongruous with the hypothesis of determinants and its accompanying
-elaborate speculation, they are not incongruous with the alternative
-hypothesis. Impossible though it may be to imagine the natures of
-those ultimate units peculiar to each species, which have proclivities
-towards the particular form of organization characterizing it, yet
-that a change of structure arising in one part of the organism is
-accompanied by multitudinous changes of structure in other parts of
-the organism, is not only congruous with the belief that there exist
-such constitutional units, but yields it distinct support. For if, as
-above argued, a conspicuous local variation is not the result of any
-modification of units special to the locality, but is the result of
-a modification of<span class="pagenum" id="Page_623">[623]</span> the units at large, then it must happen that such
-modification must have its effects on all other parts of the organism;
-so that there cannot fail to result all those small concomitant
-variations above indicated.</p>
-
-<p>May we not also say that it becomes less incomprehensible that
-structural changes caused by use and disuse are inherited? If, as
-we see, a local variation spontaneously arising is accompanied by
-multitudinous other local variations, implying a necessary correlation
-between each local variation and the general constitution of the
-organism; then it may be argued that if a marked change of function in
-an organ causes increase or decrease of it, this general correlation
-implies that there must be a reciprocal reaction between the part and
-the whole, tending to re-establish their congruity. The constitution
-at large will in so far be changed, and along with its change will go
-corresponding changes in the sperm-cells and germ-cells.</p>
-
-<hr class="tb" />
-
-<p>Finally let me add, not another argument, but another fact of
-observation, of the kind which opponents demand, but which, when they
-are from time to time furnished, are severally pooh-poohed as not
-enough. Each of them is spoken of as a solitary fact and slighted as
-inadequate; and when by and by another is named, this is treated in
-the same way; so that the facts which if brought together would be
-recognized as sufficient are never brought together. That to which I
-refer is set forth in a pamphlet by M. Leo Errera, Professor at the
-University of Brussels, entitled “Hérédite d’un Caractère acquis chez
-un Champignon pluricellulaire;” being an account of experiments of
-Dr. Hunger, at the Botanical Institute in Brussels. First enumerating
-various instances of adaptations to climate, as those of plants which,
-fitted to northern regions, preserve their constitutional rapidity
-of growth and seeding when brought south, and do this for several
-generations, he goes on to detail the culture-experiments of M. Hunger,
-and sums up the results of these in the following words:&mdash;</p>
-
-<div class="blockquot">
-
-<p>“On déduit de là que:</p>
-
-<p>“1<sup>o</sup> Les conidies d’<i>Aspergillus niger</i> sont adaptées à la
-concentration du milieu où a vécu l’individu qui les porte; cet
-effet est encore plus marqué après deux générations passées dans
-un milieu donné (Expér. I et II);</p>
-
-<p>“2<sup>o</sup> II s’agit d’une véritable adaptation et non pas simplement
-d’un accroissement de vigueur chez les conidies provenant des
-liquides concentrés, car ces mêmes conidies germent moins
-rapidement et donnent des plantes moins vigoureuses que
-les conidies normales lorsqu’on les sème de nouveau sur le
-milieu-type: en s’adaptant aux liquides concentrés, elles se
-sont <i>désadaptées</i> du liquide normal (Expér. III);</p>
-
-<p>“3<sup>o</sup> Une génération passée sur le liquide normal n’efface pas
-l’influence d’une ou de deux générations antérieures passées sur
-une liquide plus concentré (Expér. IV).</p>
-
-<p>“Tous ces résultats concordent: <i>ils montrent une légère,
-mais incontestable transmission héréditaire de l’adaptation au
-milieu</i>.”</p>
-</div>
-
-<hr class="chap x-ebookmaker-drop" />
-
-<div class="chapter">
-<p><span class="pagenum" id="Page_625">[625]</span></p>
-
-<h2 class="smaller">SUBJECT-INDEX.</h2>
-</div>
-
-<div class="blockquot">
-
-<p>(For this Index as it appeared in previous editions the Author
-is indebted to <span class="smcap">F. Howard Collins</span>, Esq., of Edgbaston,
-Birmingham. It has now been adjusted to suit the present revised
-and enlarged edition.)</p>
-</div>
-
-<ul>
-  <li>Acacia, foliar organs, <b>II</b>,
-    <a href="#Page_41">41</a>,
-    <a href="#Page_264">264</a>.</li>
-
-  <li><i>Acalephæ</i>: environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page105">105</a>;</li>
-  <li class="i1">water in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page173">173</a>.</li>
-
-  <li><i>Acari</i>: special creation and effects of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page_428">428</a>;</li>
-  <li class="i1">direct transformations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page706">706</a>;</li>
-  <li class="i1">segmentation, <b>II</b>,
-    <a href="#Page_111">111</a>.</li>
-
-  <li><i>Acorus calamus</i>, agamic propagation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page642">642</a>.</li>
-
-  <li class="hangingindent" id="Acquired_characters">Acquired characters, inheritance of: functionally-produced modifications in plants and animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page307">307–13</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page318">318</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page526">526</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page541">541</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page562">562</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692–5</a>; <b>II</b>,
-    <a href="#Page_618">618–22</a>;</li>
-  <li class="hangingindent1">conceivability of, on the hypothesis of physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page368">368–71</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page695">695</a>; <b>II</b>,
-    <a href="#Page_618">618–22</a>;</li>
-  <li class="i1">diminution of jaw, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page541">541–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">current views on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page559">559–60</a>;</li>
-  <li class="i1">cessation of selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page560">560–3</a>;</li>
-  <li class="i1">Eimer’s theory of orthogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page560">560</a>;</li>
-  <li class="i1">species differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page573">573</a>;</li>
-  <li class="i1">location of mammalian testes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page573">573</a>;</li>
-  <li class="hangingindent1">tactual perceptiveness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page602">602–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page633">633</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page665">665</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page666">666</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page672">672–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="i1">blindness of cave-animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page647">647–9</a>;</li>
-  <li class="i1">co-adaptation of co-operative parts, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page621">621</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page663">663–5</a>;</li>
-  <li class="i1">transmission of disease, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page622">622–3</a>;</li>
-  <li class="hangingindent1">hypothesis supported by telegony, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page624">624–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page644">644–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page649">649–50</a>;</li>
-  <li class="hangingindent1">views of Darwin and neo-Darwinists, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page630">630</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page690">690</a>;</li>
-  <li class="i1">why facts in support are meagre, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page632">632</a>;</li>
-  <li class="i1">degradation of little toe, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page652">652–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page673">673</a>;</li>
-  <li class="hangingindent1">neuter forms of social insects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page658">658–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page663">663–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>;</li>
-  <li class="i1">degenerated instinct in ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660–2</a>;</li>
-  <li class="i1">rudimentary limbs of whale, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page669">669</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="i1">importance of question, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page672">672</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page690">690</a>;</li>
-  <li class="i1">monstrous development of honey-ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page683">683–4</a>;</li>
-  <li class="i1">osteology of Punjabis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>;</li>
-  <li class="i1">summary of evidences in support, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692–5</a>;</li>
-  <li class="i1">genesis of vertebrate skull, <b>II</b>,
-    <a href="#Page_227">227</a>;</li>
-  <li class="i1">false joints, <b>II</b>,
-    <a href="#Page_371">371</a>,
-    <a href="#Page_372">372</a>;</li>
-  <li class="i1">conceivability of rival hypotheses, <b>II</b>,
-    <a href="#Page_618">618–22</a>;</li>
-  <li class="i1">adaptation to environment in <i>Aspergillus</i>, <b>II</b>,
-    <a href="#Page_623">623</a>.</li>
-
-  <li>Acrogens, the term, <b>II</b>,
-    <a href="#Page_55">55–6</a>.</li>
-  <li class="i1">(See <a href="#Archegoniateae"><i>Archegoniateæ</i></a>.)</li>
-
-  <li><i>Actinophrys</i>: a primary aggregate, <b>II</b>,
-    <a href="#Page_76">76</a>;</li>
-  <li class="i1">genesis, <b>II</b>,
-    <a href="#Page_452">452</a>.</li>
-
-  <li><i>Actinozoa</i>: multiaxial development, <b>I</b>,
-    <a href="#Page_166">166</a>;</li>
-  <li class="i1">waste and repair, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page213">213</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page219">219</a>;</li>
-  <li class="i1">differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page391">391</a>;</li>
-  <li class="i1">parasitism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page397">397</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_92">92</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_189">189</a>,
-    <a href="#Page_192">192</a>;</li>
-  <li class="i1">growth and genesis, <b>II</b>,
-    <a href="#Page_444">444</a>.</li>
-
-  <li class="hangingindent">Activity: the principle of, the essential element in Life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page113">113</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page114">114</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page122">122</a>;</li>
-  <li class="i1">not inherent in living matter, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page120">120</a>;</li>
-  <li class="i1">nutrition and genesis, <i>résumé</i>, <b>II</b>,
-    <a href="#Page_497">497–9</a>;</li>
-  <li class="i1">and evolution, <b>II</b>,
-    <a href="#Page_501">501–4</a>.</li>
-
-  <li>Adaptation: general truths, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page227">227–33</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page233">233–5</a>;</li>
-  <li class="i1">botanical, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page227">227</a>;</li>
-  <li class="i1">physiological, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page228">228–33</a>;</li>
-  <li class="i1">psychological, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page229">229</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page230">230–3</a>;</li>
-  <li class="hangingindent1">structural, functional, and interdependence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page235">235–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page240">240–1</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page318">318</a>;</li>
-  <li class="i1">social and organic stability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page240">240–2</a>;</li>
-  <li class="i1"><i>résumé</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page242">242–3</a>;</li>
-  <li class="i1">to varied media, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page479">479–81</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page489">489</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">multiplication of effects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page512">512–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page550">550</a>;</li>
-  <li class="i1">direct equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page522">522–3</a>;</li>
-  <li class="i1">natural selection and equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page530">530–5</a>;</li>
-  <li class="i1">non-adaptive specific characters, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page565">565</a>;</li>
-  <li class="i1">time required for effecting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page565">565–6</a>;</li>
-  <li class="i1">an obstacle to re-adaptation, <b>II</b>,
-    <a href="#Page_11">11</a>;</li>
-  <li class="i1">of skin and skeleton, <b>II</b>,
-    <a href="#Page_215">215</a>,
-    <a href="#Page_217">217</a>;</li>
-  <li class="i1">outer tissue, <b>II</b>,
-    <a href="#Page_312">312–4</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="hangingindent1">skin and mucous membrane differentiation, <b>II</b>,
-    <a href="#Page_321">321–2</a>,
-    <a href="#Page_389">389</a>;</li>
-  <li class="i1">vascular system, <b>II</b>,
-    <a href="#Page_343">343–4</a>;</li>
-  <li class="i1">osseous, <b>II</b>,
-    <a href="#Page_352">352</a>;</li>
-  <li class="i1">muscular, <b>II</b>,
-    <a href="#Page_368">368–9</a>,
-    <a href="#Page_391">391</a>;</li>
-  <li class="i1">persistence of force and physiological, <b>II</b>,
-    <a href="#Page_394">394</a>;</li>
-  <li class="i1">of reproductive activity to conditions, <b>II</b>,
-    <a href="#Page_411">411–6</a>;</li>
-  <li class="i1">vertebræ development, <b>II</b>,
-    <a href="#Page_563">563–6</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#adaptation">Co-adaptation</a>.)</li>
-
-  <li>Africa, effect of climate on inhabitants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page30">30</a>.</li>
-
-  <li class="hangingindent" id="Agamogenesis">Agamogenesis: alternation with gamogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page266">266–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page272">272–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page284">284–94</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page336">336</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page592">592</a>; <b>II</b>,
-    <a href="#Page_415">415</a>;</li>
-  <li class="i1">parallelism in karyokinesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page267">267–8</a>;</li>
-  <li class="i1">a process of disintegration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page276">276–7</a>;</li>
-  <li class="hangingindent1">conditions determining its continuance, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page284">284–94</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page330">330</a>;</li>
-  <li class="i1">physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page351">351</a>; <b>II</b>,
-    <a href="#Page_613">613</a>;</li>
-  <li class="hangingindent1">spontaneous fission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page582">582</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page584">584–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page589">589–92</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page595">595–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page599">599</a>;</li>
-  <li class="hangingindent1">remarkable extent of, under favourable conditions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page591">591–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page640">640–1</a>;</li>
-  <li class="i1">in <i>Actinozoa</i>, <b>II</b>,
-    <a href="#Page_92">92</a>;</li>
-  <li class="i1">in <i>Hydrozoa</i>, <b>II</b>,
-    <a href="#Page_102">102</a>;</li>
-  <li class="i1">in <i>Annelida</i>, <b>II</b>,
-    <a href="#Page_103">103</a>;</li>
-  <li class="i1">innutrition, <b>II</b>,
-    <a href="#Page_179">179–80</a>.</li>
-
-  <li><i>Agaricinæ</i>, <b>II</b>,
-    <a href="#Page_139">139</a>,
-    <a href="#Page_257">257</a>.</li>
-
-  <li>Agassiz, L. J. R., zoological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page380">380</a>.</li>
-
-  <li>Aggregates, Animal and Plant (<i>see</i> <a href="#Morphology">Morphology</a>).</li>
-
-  <li>Agility, a vital attribute, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page578">578</a>.</li>
-
-  <li>Agrimony, floral symmetry, <b>II</b>,
-    <a href="#Page_42">42</a>,
-    <a href="#Page_167">167</a>,
-    <a href="#Page_170">170</a>.</li>
-
-  <li>Air, in vegetal tissues, <b>II</b>,
-    <a href="#Page_567">567–8</a>,
-    <a href="#Page_583">583</a>,
-    <a href="#Page_591">591</a>,
-    <a href="#Page_593">593</a>.</li>
-
-  <li>“Air plants,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page208">208</a>.</li>
-
-  <li>Albumen: properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page12">12</a>;</li>
-  <li class="i1">Lieberkühn’s formula, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page13">13</a>;</li>
-  <li class="i1">diffusibility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page19">19</a>;</li>
-  <li class="i1">in organic tissues, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page41">41</a>.</li>
-
-  <li>Alcohols, properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page10">10–12</a>.</li>
-
-  <li class="hangingindent" id="Algae"><i>Algæ</i>: reproduction, and the dynamic element in life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page118">118–9</a>;</li>
-  <li class="i1">multicentral development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page163">163</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page164">164</a>;</li>
-  <li class="i1">axial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>;</li>
-  <li class="i1">locomotive powers of minute forms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page196">196</a>;</li>
-  <li class="i1">uniform tissue and function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page200">200</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page586">586</a>;</li>
-  <li class="hangingindent1">gamogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page280">280</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page283">283</a>; <b>II</b>,
-    <a href="#Page_448">448</a>,
-    <a href="#Page_449">449</a>,
-    <a href="#Page_450">450</a>;</li>
-  <li class="i1">fertility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page582">582</a>; <b>II</b>,
-    <a href="#Page_440">440</a>,
-    <a href="#Page_441">441</a>;</li>
-  <li class="i1">fission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page584">584</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page585">585</a>;</li>
-  <li class="i1">unicellular forms, <b>II</b>,
-    <a href="#Page_22">22</a>;</li>
-  <li class="hangingindent1">integration in <i>Confervoideæ</i> and <i>Conjugateæ</i>, <b>II</b>,
-    <a href="#Page_25">25</a>;</li>
-  <li class="hangingindent1">pseudo-foliar and axial development, <b>II</b>,
-    <a href="#Page_28">28–33</a>,
-    <a href="#Page_57">57</a>;</li>
-  <li class="i1">foliar development, <b>II</b>,
-    <a href="#Page_76">76</a>,
-    <a href="#Page_91">91</a>;</li>
-  <li class="i1">branch symmetry, <b>II</b>,
-    <a href="#Page_145">145</a>;</li>
-  <li class="i1">cell metamorphoses, <b>II</b>,
-    <a href="#Page_176">176</a>;</li>
-  <li class="hangingindent1">tissue differentiation, <b>II</b>,
-    <a href="#Page_244">244</a>,
-    <a href="#Page_246">246</a>,
-    <a href="#Page_251">251</a>,
-    <a href="#Page_252">252</a>,
-    <a href="#Page_256">256</a>,
-    <a href="#Page_272">272</a>,
-    <a href="#Page_385">385–6</a>;</li>
-  <li class="hangingindent1">adaptation of reproductive activity to conditions, <b>II</b>,
-    <a href="#Page_289">289</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_292">292</a>;</li>
-  <li class="i1">indefiniteness, <b>II</b>,
-    <a href="#Page_295">295</a>;</li>
-  <li class="i1">genesis and development, <b>II</b>,
-    <a href="#Page_463">463</a>.</li>
-
-  <li class="hangingindent" id="Alimentary_canal">Alimentary canal: metabolic processes and agents, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page68">68–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page74">74</a>;</li>
-  <li class="i1">structural traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page192">192</a>;</li>
-  <li class="i1">progressive development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page195">195</a>;</li>
-  <li class="i1">relation to environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page196">196</a>;</li>
-  <li class="i1">function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page205">205</a>;</li>
-  <li class="i1">segmentation in annelids, <b>II</b>,
-    <a href="#Page_125">125</a>;</li>
-  <li class="i1">differentiation, <b>II</b>,
-    <a href="#Page_301">301</a>,
-    <a href="#Page_302">302</a>,
-    <a href="#Page_321">321–2</a>,
-    <a href="#Page_323">323–5</a>,
-    <a href="#Page_389">389</a>;</li>
-  <li class="i1">specializations in birds, <b>II</b>,
-    <a href="#Page_325">325</a>;</li>
-  <li class="i1">in ruminants, <b>II</b>,
-    <a href="#Page_327">327–9</a>;</li>
-  <li class="i1">differentiation of liver, <b>II</b>,
-    <a href="#Page_329">329–33</a>;</li>
-  <li class="i1">muscularity, <b>II</b>,
-    <a href="#Page_364">364</a>.</li>
-
-  <li>Allotropism: of organic constituents, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page4">4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>;</li>
-  <li class="i1">muscular action, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page59">59</a>.</li>
-
-  <li>Alloys, melting point of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page339">339</a>.</li>
-
-  <li class="hangingindent">Alternation of generations, misleading application of term, <b>II</b>,
-    <a href="#Page_84">84</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Agamogenesis">Agamogenesis</a> <i>and</i> <a href="#Gamogenesis">Gamogenesis</a>.)</li>
-
-  <li>Amitosis, occurrence of, in morbid tissues, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page264">264</a>.</li>
-
-  <li>Ammonia: properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page7">7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>;</li>
-  <li class="i1">nerve stimulation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page55">55</a>.</li>
-
-  <li><i>Amœba</i>: central development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page163">163</a>;</li>
-  <li class="i1">a primary aggregate, <b>II</b>,
-    <a href="#Page_86">86</a>;</li>
-  <li class="i1">symmetry of encysted, <b>II</b>,
-    <a href="#Page_186">186</a>;</li>
-  <li class="i1">symbiosis, <b>II</b>,
-    <a href="#Page_400">400</a>.</li>
-
-  <li id="Amphibia"><i>Amphibia</i>: classification of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page392">392</a>;</li>
-  <li class="i1">embryonic respiratory system, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page457">457</a>;</li>
-  <li class="i1">structure and media, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page483">483</a>;</li>
-  <li class="i1">limb locomotion, <b>II</b>,
-    <a href="#Page_15">15</a>;</li>
-  <li class="i1">segmentation, <b>II</b>,
-    <a href="#Page_122">122</a>,
-    <a href="#Page_225">225</a>;</li>
-  <li class="i1">outer tissues, <b>II</b>,
-    <a href="#Page_311">311</a>;</li>
-  <li class="i1">respiration, <b>II</b>,
-    <a href="#Page_334">334</a>,
-    <a href="#Page_338">338</a>;</li>
-  <li class="i1">Owen on skeleton, <b>II</b>,
-    <a href="#Page_552">552</a>,
-    <a href="#Page_557">557</a>,
-    <a href="#Page_558">558</a>.</li>
-
-  <li class="hangingindent"><i>Amphioxus</i>: separation of segmentation spheres of egg, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page691">691</a>;</li>
-  <li class="i1">embryogeny, <b>II</b>,
-    <a href="#Page_121">121</a>;</li>
-  <li class="i1">local segmentation, <b>II</b>,
-    <a href="#Page_125">125–7</a>,
-    <a href="#Page_605">605</a>;</li>
-  <li class="hangingindent1">genesis of vertebrate axis, <b>II</b>,
-    <a href="#Page_213">213–6</a>,
-    <a href="#Page_218">218</a>,
-    <a href="#Page_222">222</a>;</li>
-  <li class="i1">development, <b>II</b>,
-    <a href="#Page_564">564</a>.</li>
-
-  <li class="hangingindent"><i>Amphipnous cuchia</i>, vascular air-sacs, <b>II</b>,
-    <a href="#Page_337">337</a>.</li>
-
-  <li class="hangingindent"><i>Anabas scandens</i>, the climbing fish, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page480">480</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page483">483</a>.</li>
-
-  <li><i>Anacharis</i> (see <a href="#Eloidea"><i>Eloidea</i></a>).</li>
-
-  <li>Anæsthetics, diverse effects of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page55">55</a>.</li>
-
-  <li class="hangingindent"><i>Angræcum</i>, assimilative function of root, <b>II</b>,
-    <a href="#Page_255">255</a>.</li>
-
-  <li>“Animal Spirits,” vitalism and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page115">115</a>.</li>
-
-  <li class="hangingindent" id="Animals">Animals: nutrition and molecular re-arrangement, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page36">36–7</a>;</li>
-  <li class="i1">nitrogenous character, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page39">39–41</a>;</li>
-  <li class="i1">sensible motion, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page57">57</a>;</li>
-  <li class="i1">metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page62">62–77</a>;</li>
-  <li class="i1">multiplication of energies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page75">75</a>;</li>
-  <li class="i1">contrasted traits of plants and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page196">196</a>;</li>
-  <li class="i1">what is an individual? <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page246">246–7</a>;</li>
-  <li class="i1">solar influence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page500">500</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="hangingindent1">geologic changes affecting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page501">501–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page549">549</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page550">550</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="hangingindent1">interdependence with plants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page504">504–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page514">514</a>; <b>II</b>,
-    <a href="#Page_398">398–401</a>;</li>
-  <li class="i1">complexity of influences affecting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page506">506</a>;</li>
-  <li class="hangingindent1">geographical isolation and origin of species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page568">568–9</a>;</li>
-  <li class="i1">vital attributes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page577">577–9</a>;</li>
-  <li class="hangingindent1">distribution and antiquity of plant and animal types, <b>II</b>,
-    <a href="#Page_297">297</a>;</li>
-  <li class="hangingindent1">mutual dependence of organisms at large, <b>II</b>,
-    <a href="#Page_397">397–408</a>;</li>
-  <li class="i1">hypothetical plant-animal type, <b>II</b>,
-    <a href="#Page_397">397</a>;</li>
-  <li class="i1">progressive increase of size, <b>II</b>,
-    <a href="#Page_401">401</a>;</li>
-  <li class="i1">laws of multiplication, <b>II</b>,
-    <a href="#Page_411">411–6</a>;</li>
-  <li class="i1">rhythm in numbers, <b>II</b>,
-    <a href="#Page_419">419</a>;</li>
-  <li class="i1">law of weights and dimensions, <b>II</b>,
-    <a href="#Page_434">434</a>.</li>
-
-  <li class="hangingindent">Animals, domesticated: variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page324">324</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page326">326</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page560">560</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page563">563</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">interbreeding, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page345">345–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page354">354</a>; <b>II</b>,
-    <a href="#Page_615">615</a>;</li>
-  <li class="i1">pure and mixed breeds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page354">354</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page625">625</a>.</li>
-
-  <li id="Annelida"><i>Annelida</i>: phosphorescence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page50">50</a>;</li>
-  <li class="i1">axial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page166">166</a>;</li>
-  <li class="i1">integration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page363">363</a>;</li>
-  <li class="hangingindent1">larval forms and phylogeny, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page447">447</a>; <b>II</b>,
-    <a href="#Page_115">115</a>;</li>
-  <li class="i1">segmental fission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page588">588–9</a>;</li>
-  <li class="hangingindent1">segmentation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page98">98–101</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page103">103–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page602">602–5</a>; <b>II</b>,
-    <a href="#Page_107">107–9</a>,
-    <a href="#Page_125">125–7</a>;</li>
-  <li class="i1">lateral gemmation, <b>II</b>,
-    <a href="#Page_105">105</a>;</li>
-  <li class="i1">embryogeny, <b>II</b>,
-    <a href="#Page_119">119</a>;</li>
-  <li class="i1">bilateral symmetry, <b>II</b>,
-    <a href="#Page_197">197–200</a>;</li>
-  <li class="i1">genesis, <b>II</b>,
-    <a href="#Page_444">444</a>,
-    <a href="#Page_453">453</a>.</li>
-
-  <li><i>Annulosa</i>: regeneration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page361">361–2</a>;</li>
-  <li class="i1">distinctive traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page392">392</a>;</li>
-  <li class="i1">origin of type, <b>II</b>,
-    <a href="#Page_98">98–110</a>,
-    <a href="#Page_602">602–6</a>;</li>
-  <li class="i1">unit of composition, <b>II</b>,
-    <a href="#Page_105">105</a>;</li>
-  <li class="i1">application of term, <b>II</b>,
-    <a href="#Page_111">111</a>;</li>
-  <li class="i1">vertebrate symmetry compared, <b>II</b>,
-    <a href="#Page_203">203–6</a>;</li>
-  <li class="i1">segmental differentiation, <b>II</b>,
-    <a href="#Page_207">207–9</a>;</li>
-  <li class="i1">unintegrated function in <i>Planaria</i>, <b>II</b>,
-    <a href="#Page_373">373</a>;</li>
-  <li class="i1">development and genesis, <b>II</b>,
-    <a href="#Page_464">464</a>;</li>
-  <li class="i1">nutrition and genesis, <b>II</b>,
-    <a href="#Page_490">490</a>.</li>
-  <li class="i1">(See also <a href="#Annelida"><i>Annelida</i></a> and <a href="#Arthropoda"><i>Arthropoda</i></a>.)</li>
-
-  <li>Anthropomorphism, former prevalence of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page419">419</a>.</li>
-
-  <li>Ants: utilization of aphids, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660–1</a>; <b>II</b>,
-    <a href="#Page_403">403</a>,
-    <a href="#Page_405">405</a>;</li>
-  <li class="i1">nest-mates, <b>II</b>,
-    <a href="#Page_405">405</a>;</li>
-  <li class="i1">castes in social species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page658">658–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>;</li>
-  <li class="hangingindent1">loss of self-feeding instinct in Amazons, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660–1</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page663">663–4</a>;</li>
-  <li class="i1">monstrous development of Honey-ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page683">683</a>;</li>
-  <li class="i1">bulk and fecundity, <b>II</b>,
-    <a href="#Page_492">492</a>.</li>
-  <li class="i1">(See also <a href="#Termites"><i>Termites</i></a>.)</li>
-
-  <li><i>Aphis</i>: individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page249">249</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page250">250</a>; <b>II</b>,
-    <a href="#Page_603">603</a>;</li>
-  <li class="i1">parthenogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page274">274–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page289">289</a>;</li>
-  <li class="i1">fertility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page582">582</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page640">640–1</a>; <b>II</b>,
-    <a href="#Page_476">476</a>,
-    <a href="#Page_490">490</a>;</li>
-  <li class="i1">utilized by ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660–1</a>; <b>II</b>,
-    <a href="#Page_403">403</a>,
-    <a href="#Page_405">405</a>;</li>
-  <li class="hangingindent1">over-multiplication checked by lady-bird, <b>II</b>,
-    <a href="#Page_406">406</a>.</li>
-
-  <li>Aquatic animals, large size attained by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page156">156</a>.</li>
-
-  <li id="Arachnida"><i>Arachnida</i>: avoidance of danger, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page92">92</a>;</li>
-  <li class="i1">oviparous homogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>;</li>
-  <li class="i1">segmentation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page469">469</a>; <b>II</b>,
-    <a href="#Page_113">113</a>,
-    <a href="#Page_114">114</a>;</li>
-  <li class="i1">integration and homology, <b>II</b>,
-    <a href="#Page_111">111</a>,
-    <a href="#Page_121">121</a>;</li>
-  <li class="i1">bilateral symmetry, <b>II</b>,
-    <a href="#Page_198">198</a>.</li>
-
-  <li><i>Arcella</i>: symmetry, <b>II</b>,
-    <a href="#Page_186">186</a>;</li>
-  <li class="i1">outer tissue differentiation, <b>II</b>,
-    <a href="#Page_309">309</a>.</li>
-
-  <li id="Archegoniateae"><i>Archegoniateæ</i>: morphological composition, <b>II</b>,
-    <a href="#Page_32">32–5</a>;</li>
-  <li class="i1">growth and development, <b>II</b>,
-    <a href="#Page_50">50–6</a>;</li>
-  <li class="i1">tubular structure, <b>II</b>,
-    <a href="#Page_58">58</a>,
-    <a href="#Page_62">62</a>;</li>
-  <li class="i1">alternating generation not distinctive, <b>II</b>,
-    <a href="#Page_84">84</a>;</li>
-  <li class="i1">asymmetry and environment, <b>II</b>,
-    <a href="#Page_140">140</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_293">293</a>,
-    <a href="#Page_296">296</a>;</li>
-  <li class="i1">individuation and genesis, <b>II</b>,
-    <a href="#Page_441">441</a>,
-    <a href="#Page_451">451</a>,
-    <a href="#Page_463">463</a>.</li>
-
-  <li>Archenteron: primitive externality, <b>II</b>,
-    <a href="#Page_301">301</a>;</li>
-  <li class="i1">formation of cœlom, <b>II</b>,
-    <a href="#Page_302">302</a>.</li>
-
-  <li><i>Archiannelida</i>: segmentation, <b>II</b>,
-    <a href="#Page_125">125</a>.</li>
-
-  <li><i>Arenicola marina</i>: polytrochal larvæ, <b>II</b>,
-    <a href="#Page_109">109</a>.</li>
-
-  <li>Arm: embryogeny of human, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page169">169</a>;</li>
-  <li class="i1">vicarious use of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page209">209</a>.</li>
-
-  <li>Army, morphological analogy, <b>II</b>,
-    <a href="#Page_6">6</a>.</li>
-
-  <li>Arteries (<i>see</i> <a href="#Vascular_System">Vascular System</a>).</li>
-
-  <li id="Arthropoda"><i>Arthropoda</i>: uniaxial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>;</li>
-  <li class="i1">protoplasmic continuity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page190">190</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page629">629</a>;</li>
-  <li class="i1">excursiveness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page481">481</a>;</li>
-  <li class="i1">limb locomotion, <b>II</b>,
-    <a href="#Page_15">15</a>;</li>
-  <li class="i1">integration and homology, <b>II</b>,
-    <a href="#Page_111">111–4</a>,
-    <a href="#Page_121">121</a>;</li>
-  <li class="i1">bilateral symmetry, <b>II</b>,
-    <a href="#Page_197">197–200</a>;</li>
-  <li class="i1">genesis, <b>II</b>,
-    <a href="#Page_445">445</a>,
-    <a href="#Page_453">453</a>.</li>
-
-  <li>Ascidians: multiaxial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page166">166</a>;</li>
-  <li class="i1">functional differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page202">202</a>;</li>
-  <li class="i1">composite individuality of <i>Doliolum</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page247">247</a>;</li>
-  <li class="i1">self-fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page342">342</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_94">94</a>,
-    <a href="#Page_96">96</a>,
-    <a href="#Page_97">97</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_194">194</a>;</li>
-  <li class="i1">origin of vertebrate type, <b>II</b>,
-    <a href="#Page_194">194</a>,
-    <a href="#Page_598">598</a>,
-    <a href="#Page_605">605</a>.</li>
-
-  <li><i>Ascomycetes</i>, reproduction, <b>II</b>,
-    <a href="#Page_450">450</a>.</li>
-
-  <li>Assimilation: compared with reasoning, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page81">81–7</a>;</li>
-  <li class="i1">a trait of vitality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page577">577</a>.</li>
-
-  <li id="Asteroidea"><i>Asteroidea</i>, radial symmetry, <b>II</b>,
-    <a href="#Page_196">196</a>.</li>
-
-  <li>Astronomy: growth of celestial bodies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page135">135</a>;</li>
-  <li class="i1">Schleiden on individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page245">245</a>;</li>
-  <li class="i1">evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page432">432</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page435">435</a>;</li>
-  <li class="i1">classification of stars, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page444">444</a>;</li>
-  <li class="i1">rhythm of, and organic change, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page499">499–501</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">law of equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page519">519–20</a>;</li>
-  <li class="i1">co-operation of structure and function, <b>II</b>,
-    <a href="#Page_3">3</a>.</li>
-
-  <li>Atavism: occurrence of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page305">305–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314</a>;</li>
-  <li class="i1">digital variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page321">321–3</a>.</li>
-
-  <li>Atoms: use of term, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page6">6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page31">31</a>;</li>
-  <li class="i1">ethereal undulations and oscillations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page31">31–5</a>.</li>
-
-  <li>Australia: settler’s usages, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364</a>;</li>
-  <li class="i1">ratio of jaw to skull in natives, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page541">541</a>.</li>
-
-  <li>Axillary buds, origin and development, <b>II</b>,
-    <a href="#Page_65">65–8</a>.</li>
-
-  <li>Axis: “neutral” of mechanics, <b>II</b>,
-    <a href="#Page_210">210</a>;</li>
-  <li class="i1">genesis of vertebrate, <b>II</b>,
-    <a href="#Page_212">212–6</a>,
-    <a href="#Page_224">224–7</a>.</li>
-</ul>
-
-<ul>
-  <li><i>Bacteria</i>: fission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page270">270</a>;</li>
-  <li class="i1">non-nucleated, <b>II</b>,
-    <a href="#Page_20">20</a>;</li>
-  <li class="i1">rate of increase, <b>II</b>,
-    <a href="#Page_443">443</a>.</li>
-
-  <li class="hangingindent">Baer, K. E. von: embryological formula, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page171">171</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page172">172</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page451">451</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page453">453</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page461">461</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page466">466</a>;</li>
-  <li class="i1">zoological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page383">383</a>;</li>
-  <li class="i1">on animal transitions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page480">480</a>.</li>
-
-  <li><i>Balanophoræ</i>, inner tissue, <b>II</b>,
-    <a href="#Page_274">274</a>.</li>
-
-  <li>Bark: varied development, <b>II</b>,
-    <a href="#Page_247">247–9</a>;</li>
-  <li class="hangingindent1">physiological differentiation, <b>II</b>,
-    <a href="#Page_249">249–50</a>,
-    <a href="#Page_258">258</a>,
-    <a href="#Page_386">386</a>.</li>
-
-  <li><i>Basidiomycetes</i>, reproduction, <b>II</b>,
-    <a href="#Page_450">450</a>.</li>
-
-  <li>Bat, infertility of, <b>II</b>,
-    <a href="#Page_473">473</a>.</li>
-
-  <li class="hangingindent">Bates, H. W., protective mimicry of butterflies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page398">398</a>.</li>
-
-  <li><i>Batrachia</i> (see <a href="#Amphibia"><i>Amphibia</i></a>).</li>
-
-  <li>Bean, vascular system, <b>II</b>,
-    <a href="#Page_573">573</a>,
-    <a href="#Page_591">591</a>.</li>
-
-  <li>Beaver, tail and co-adapted structures, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page616">616</a>.</li>
-
-  <li>Bees (<i>see</i> <a href="#Insects">Insects</a>).</li>
-
-  <li><i>Begoniaceæ</i>: multiplication <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page224">224</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page317">317</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page442">442</a>;</li>
-  <li class="i1">individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page251">251</a>;</li>
-  <li class="i1">development from scales, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page282">282</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_159">159</a>,
-    <a href="#Page_166">166</a>;</li>
-  <li class="i1">development, <b>II</b>,
-    <a href="#Page_271">271</a>.</li>
-
-  <li class="hangingindent">Berkeley, M. J., indefiniteness of mosses and ferns, <b>II</b>,
-    <a href="#Page_296">296</a>.</li>
-
-  <li>Bile, arrest of excretion, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page209">209</a>.</li>
-
-  <li>Bilirubin and biliverdine, function of, <b>II</b>,
-    <a href="#Page_330">330</a>,
-    <a href="#Page_333">333</a>.</li>
-
-  <li>Biology: definition and divisions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page124">124–5</a>;</li>
-  <li class="i1">organic structural phenomena, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page125">125–7</a>;</li>
-  <li class="i1">also functional, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page127">127–9</a>;</li>
-  <li class="hangingindent1">actions and reactions of function and structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page129">129–30</a>;</li>
-  <li class="i1">genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page130">130–1</a>;</li>
-  <li class="i1">limited knowledge of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page131">131</a>;</li>
-  <li class="i1">evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page432">432</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page434">434</a>;</li>
-  <li class="i1">sociological analogies (<i>see</i> <a href="#Sociology">Sociology</a>).</li>
-
-  <li class="hangingindent">Biophors, Weismann’s germ-plasm units (<i>see</i> <a href="#Weismann">Weismann</a>).</li>
-
-  <li class="hangingindent">Birds: flesh-eating and grain-eating contrasted, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page68">68</a>;</li>
-  <li class="i1">growth and expenditure of force, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page142">142</a>;</li>
-  <li class="i1">size of egg and adult, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page144">144</a>;</li>
-  <li class="i1">limitations on flight, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page155">155</a>;</li>
-  <li class="i1">self-mobility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page175">175</a>;</li>
-  <li class="i1">temperature, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page176">176</a>;</li>
-  <li class="hangingindent1">functional and structural differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page201">201</a>;</li>
-  <li class="i1">food of starving pigeon, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page215">215</a>;</li>
-  <li class="i1">viviparousness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>;</li>
-  <li class="i1">heredity and pigeon breeding, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page305">305</a>;</li>
-  <li class="i1">atavism in pigeon, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314</a>;</li>
-  <li class="i1">osseous variation in pigeon, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page321">321</a>;</li>
-  <li class="i1">classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page392">392</a>;</li>
-  <li class="hangingindent1">migrations and change of habits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page399">399</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page402">402</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page500">500</a>;</li>
-  <li class="i1">distribution in time, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page410">410</a>;</li>
-  <li class="i1">Darwin on petrels, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page455">455</a>;</li>
-  <li class="i1">rudimentary teeth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page457">457</a>;</li>
-  <li class="i1">vertebræ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page471">471</a>; <b>II</b>,
-    <a href="#Page_564">564</a>;</li>
-  <li class="i1">feather development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page473">473</a>;</li>
-  <li class="i1">habits of water ouzel, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page485">485</a>;</li>
-  <li class="i1">egg shells and direct equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page526">526</a>;</li>
-  <li class="i1">bones of waders and direct equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page527">527</a>;</li>
-  <li class="i1">fertility and nervous development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page598">598</a>;</li>
-  <li class="i1">cellular continuity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page629">629</a>;</li>
-  <li class="i1">adaptation of structure to environment, <b>II</b>,
-    <a href="#Page_12">12</a>;</li>
-  <li class="i1">sexual selection, <b>II</b>,
-    <a href="#Page_269">269</a>;</li>
-  <li class="i1">wing spurs, <b>II</b>,
-    <a href="#Page_313">313</a>;</li>
-  <li class="i1">outer tissue differentiation, <b>II</b>,
-    <a href="#Page_314">314–5</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">alimentary canal development, <b>II</b>,
-    <a href="#Page_325">325</a>,
-    <a href="#Page_327">327</a>;</li>
-  <li class="i1">muscular colour and activity, <b>II</b>,
-    <a href="#Page_365">365–9</a>;</li>
-  <li class="i1">nutrition, <b>II</b>,
-    <a href="#Page_433">433</a>;</li>
-  <li class="i1">cost of genesis, <b>II</b>,
-    <a href="#Page_436">436</a>;</li>
-  <li class="i1">growth and genesis, <b>II</b>,
-    <a href="#Page_454">454</a>,
-    <a href="#Page_458">458</a>;</li>
-  <li class="i1">heat expenditure and genesis, <b>II</b>,
-    <a href="#Page_468">468–9</a>,
-    <a href="#Page_474">474</a>;</li>
-  <li class="i1">activity and genesis, <b>II</b>,
-    <a href="#Page_470">470–2</a>,
-    <a href="#Page_474">474</a>;</li>
-  <li class="i1">contrasted mammalian fertility, <b>II</b>,
-    <a href="#Page_470">470</a>;</li>
-  <li class="i1">eggs of wild and tame, <b>II</b>,
-    <a href="#Page_478">478</a>;</li>
-  <li class="i1">fertility of blackbird and linnet compared, <b>II</b>,
-    <a href="#Page_503">503</a>;</li>
-  <li class="i1">Owen on skeleton of, <b>II</b>,
-    <a href="#Page_559">559</a>,
-    <a href="#Page_560">560</a>,
-    <a href="#Page_561">561</a>.</li>
-
-  <li>Bischoff, embryogeny of human arm, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page169">169</a>.</li>
-
-  <li class="hangingindent">Bison, modifications entailed by increased weight of head, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page512">512</a>.</li>
-
-  <li class="hangingindent">Blackbird, contrasted with linnet in development, <b>II</b>,
-    <a href="#Page_503">503</a>.</li>
-
-  <li>Blainville, de, definition of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page79">79</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page93">93</a>.</li>
-
-  <li class="hangingindent">Blastosphere, independence of cells in Echinoderm larvæ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page185">185</a>.</li>
-
-  <li class="hangingindent">Blastula, definition of life and formation of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page112">112</a>.</li>
-
-  <li>Blood: similarity of iron peroxide, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page17">17</a>;</li>
-  <li class="i1">metabolic processes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>;</li>
-  <li class="i1">segregation of abnormal constituents, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page180">180</a>;</li>
-  <li class="i1">protozoon life of corpuscles, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page186">186–7</a>;</li>
-  <li class="i1">morbid changes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page221">221</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page701">701</a>;</li>
-  <li class="i1">assimilative power and organic repair, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page221">221–2</a>;</li>
-  <li class="i1">respiratory tissue differentiation, <b>II</b>,
-    <a href="#Page_310">310–1</a>;</li>
-  <li class="i1">pressure in mammals, <b>II</b>,
-    <a href="#Page_340">340</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Vascular_System">Vascular System</a>.)</li>
-
-  <li class="hangingindent">Blow-fly, Weismann on nutrition and genesis in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page678">678–9</a>.</li>
-
-  <li>Boers, Cape, habits and fertility, <b>II</b>,
-    <a href="#Page_508">508</a>.</li>
-
-  <li>Boismont, A. B. de, on human fertility, <b>II</b>,
-    <a href="#Page_511">511</a>.</li>
-
-  <li id="Bone">Bone: growth and function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page151">151</a>;</li>
-  <li class="i1">adaptability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page230">230</a>; <b>II</b>,
-    <a href="#Page_217">217–8</a>;</li>
-  <li class="i1">function and weight, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page308">308</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">mammalian cervical vertebræ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page394">394</a>;</li>
-  <li class="i1">evolution and vertebral column, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page470">470–1</a>;</li>
-  <li class="i1">partial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page473">473</a>;</li>
-  <li class="i1">size of head as influencing, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page512">512</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page536">536–9</a>;</li>
-  <li class="i1">direct equilibration and strength, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page527">527</a>;</li>
-  <li class="hangingindent1">natural selection and co-adaptations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page614">614–21</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page674">674</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page677">677</a>;</li>
-  <li class="i1">rudimentary limbs of whale, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page668">668</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="hangingindent1">inheritance of acquired modifications in Punjabis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>;</li>
-  <li class="i1">skull development, <b>II</b>,
-    <a href="#Page_222">222</a>;</li>
-  <li class="i1">theory of supernumerary, <b>II</b>,
-    <a href="#Page_223">223</a>;</li>
-  <li class="hangingindent1">Cope on origin of vertebrate osseous system, <b>II</b>,
-    <a href="#Page_225">225–7</a>;</li>
-  <li class="i1">differentiation, <b>II</b>,
-    <a href="#Page_344">344–56</a>;</li>
-  <li class="i1">false joints, <b>II</b>,
-    <a href="#Page_370">370–2</a>;</li>
-  <li class="i1">Owen’s theory of vertebrate skeleton, <b>II</b>,
-    <a href="#Page_548">548–66</a>.</li>
-
-  <li>Book-worm, food of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page77">77</a>.</li>
-
-  <li>Born, G., experiments on frog larvæ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page365">365</a>.</li>
-
-  <li>Botany, biological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page124">124</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page125">125</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Plants">Plants</a>.)</li>
-
-  <li><i>Bothriocephalus</i>, development, <b>II</b>,
-    <a href="#Page_490">490</a>.</li>
-
-  <li><i>Botryllidæ</i>: development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page166">166</a>;</li>
-  <li class="i1">independence of components, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page247">247</a>;</li>
-  <li class="i1">agamogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page641">641</a>.</li>
-
-  <li>Bower, Prof., on alternation of generations, <b>II</b>,
-    <a href="#Page_84">84</a>.</li>
-
-  <li><i>Brachiopoda</i>, rude vascular system, <b>II</b>,
-    <a href="#Page_340">340</a>.</li>
-
-  <li>Bradbury, J. B., on vaso-dilators, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page55">55</a>.</li>
-
-  <li class="hangingindent">Brain: natural selection and mental evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>;</li>
-  <li class="i1">analysis of substance, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page596">596</a>;</li>
-  <li class="i1">weight in higher animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page598">598–9</a>;</li>
-  <li class="i1">size in civilized and uncivilized, <b>II</b>,
-    <a href="#Page_530">530</a>.</li>
-
-  <li>Branches (<i>see</i> <a href="#Morphology">Morphology</a>).</li>
-
-  <li><i>Branchiæ</i> (<i>see</i> <a href="#Respiratory_System">Respiratory System</a>).</li>
-
-  <li>Brass, effect of antimony on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page121">121</a>.</li>
-
-  <li>Bread, diamagnetism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page370">370</a>.</li>
-
-  <li>Breeding: heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page304">304–5</a>;</li>
-  <li class="i1">in-and-in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page344">344–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page353">353</a>; <b>II</b>,
-    <a href="#Page_615">615</a>;</li>
-  <li class="i1">pure and mixed, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page354">354</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page625">625</a>.</li>
-
-  <li>Bricks, changed equilibrium shown by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page38">38</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page42">42</a>.</li>
-
-  <li>Brodie, T. G., cell chemistry, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page260">260</a>.</li>
-
-  <li class="hangingindent">Brownell, Miss J. L., on birth-rate in United States, <b>II</b>,
-    <a href="#Page_520">520</a>.</li>
-
-  <li>Brown-Séquard, on inherited epilepsy, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page312">312</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page624">624</a>.</li>
-
-  <li><i>Bryophyllum</i>, peculiar proliferation, <b>II</b>,
-    <a href="#Page_295">295</a>.</li>
-
-  <li><i>Bryophyta</i>, large size attained by some, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page138">138</a>.</li>
-
-  <li><i>Bryozoa</i>, gemmation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page588">588</a>.</li>
-
-  <li>Budding (<i>see</i> <a href="#Gemmation">Gemmation</a>).</li>
-
-  <li>Buds: development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page167">167–8</a>;</li>
-  <li class="i1">theories of heredity and cauline, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page358">358–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page360">360</a>;</li>
-  <li class="i1">axillary, <b>II</b>,
-    <a href="#Page_65">65–9</a>;</li>
-  <li class="i1">effects of nutrition, <b>II</b>,
-    <a href="#Page_73">73–4</a>.</li>
-
-  <li>Butterfly: protective mimicry, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page398">398</a>;</li>
-  <li class="i1">instance of tame, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page684">684</a>.</li>
-</ul>
-
-<ul>
-
-  <li>Cabbage, varieties of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page302">302</a>.</li>
-
-  <li class="hangingindent"><i>Cactaceæ</i>: foliar and axial development, <b>II</b>,
-    <a href="#Page_47">47–9</a>;</li>
-  <li class="i1">differentiation in, <b>II</b>,
-    <a href="#Page_258">258</a>,
-    <a href="#Page_276">276</a>,
-    <a href="#Page_282">282</a>;</li>
-  <li class="i1">vascular system, <b>II</b>,
-    <a href="#Page_282">282</a>;</li>
-  <li class="i1">dye permeability and circulation, <b>II</b>,
-    <a href="#Page_571">571</a>,
-    <a href="#Page_572">572</a>;</li>
-  <li class="i1">wood formation, <b>II</b>,
-    <a href="#Page_575">575</a>,
-    <a href="#Page_577">577</a>,
-    <a href="#Page_578">578</a>,
-    <a href="#Page_580">580</a>.</li>
-
-  <li>“Callus,” budding from, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page358">358</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page359">359</a>.</li>
-
-  <li>Camel, natural selection and hump of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page534">534</a>.</li>
-
-  <li>Canadians, French, fertility of, <b>II</b>,
-    <a href="#Page_509">509</a>.</li>
-
-  <li>Cancer, the definition of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page111">111</a>;</li>
-  <li class="i1">œsophageal, <b>II</b>,
-    <a href="#Page_324">324</a>;</li>
-  <li class="i1">and vascular system, <b>II</b>,
-    <a href="#Page_343">343</a>.</li>
-
-  <li>Caoutchouc, leaf-structure, <b>II</b>,
-    <a href="#Page_589">589</a>.</li>
-
-  <li>Capillaries (<i>see</i> <a href="#Vascular_System">Vascular System</a>).</li>
-
-  <li class="hangingindent">Capillarity, and vegetal vascular system, <b>II</b>,
-    <a href="#Page_279">279–80</a>,
-    <a href="#Page_286">286</a>,
-    <a href="#Page_568">568</a>,
-    <a href="#Page_570">570</a>,
-    <a href="#Page_585">585</a>,
-    <a href="#Page_587">587</a>,
-    <a href="#Page_592">592–6</a>.</li>
-
-  <li>Carbohydrates: instability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page10">10</a>;</li>
-  <li class="i1">the term “hydro-carbon,” <i>ib.</i>;</li>
-  <li class="i1">molecular changes in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page42">42–3</a>;</li>
-  <li class="i1">organic transformation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page43">43</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page48">48</a>;</li>
-  <li class="i1">metabolic processes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page63">63–77</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page262">262–3</a>; <b>II</b>,
-    <a href="#Page_362">362</a>.</li>
-
-  <li>Carbon: properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page3">3–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page20">20</a>;</li>
-  <li class="i1">compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page6">6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page7">7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page10">10–12</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page13">13</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page24">24–5</a>.</li>
-
-  <li class="hangingindent">Carbonic acid (carbon dioxide): properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page6">6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page7">7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>;</li>
-  <li class="hangingindent1">in animal and plant functions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page62">62</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page214">214</a>; <b>II</b>,
-    <a href="#Page_398">398</a>;</li>
-  <li class="i1">diffusibility, <b>II</b>,
-    <a href="#Page_331">331</a>.</li>
-
-  <li>Carbonic oxide, properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page6">6</a>.</li>
-
-  <li>Carnivores: nitrogenous food, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page47">47</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page68">68</a>;</li>
-  <li class="i1">katabolic process, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page71">71</a>;</li>
-  <li class="i1">restricted environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page396">396</a>;</li>
-  <li class="hangingindent1">their beneficial effects on animal life, <b>II</b>,
-    <a href="#Page_405">405–6</a>.</li>
-
-  <li class="hangingindent">Carpenter, W. B.: on functional specialization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page208">208</a>;</li>
-  <li class="i1">reproduction of seaweed, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page582">582</a>;</li>
-  <li class="i1">vegetal cell multiplication, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page585">585</a>;</li>
-  <li class="hangingindent1">structure and multiplication of compound organisms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page586">586–9</a>;</li>
-  <li class="i1">on fundamental traits of sex, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page595">595</a>;</li>
-  <li class="i1">nutritive system of invertebrates, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page595">595</a>;</li>
-  <li class="i1"><i>Macrocystis</i>, <b>II</b>,
-    <a href="#Page_450">450</a>;</li>
-  <li class="i1">nutrition and reproductive function, <b>II</b>,
-    <a href="#Page_460">460</a>.</li>
-
-  <li>Cartilage (<i>see</i> <a href="#Bone">Bone</a>).</li>
-
-  <li>Castration, effect of, on growth, <b>II</b>,
-    <a href="#Page_459">459</a>.</li>
-
-  <li>“Castration parasitaire,” Julin on, <b>II</b>,
-    <a href="#Page_493">493–6</a>.</li>
-
-  <li>Catalysis, and vital metamorphosis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page39">39</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page43">43</a>.</li>
-
-  <li>Cattell, McKeen, on tactual perceptiveness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page666">666</a>.</li>
-
-  <li class="hangingindent"><i>Caulerpa</i>, simulation of higher plant-forms, <b>II</b>,
-    <a href="#Page_22">22</a>.</li>
-
-  <li class="hangingindent">Cave-animals, degeneration of eyes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page309">309</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page614">614</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page647">647–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>.</li>
-
-  <li class="hangingindent" id="Cell">Cell, the: incomprehensibility of forces at work in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page118">118</a>;</li>
-  <li class="i1">protoplasts and their traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page181">181</a>;</li>
-  <li class="hangingindent1">the cell-theory, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page184">184</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page252">252</a>; <b>II</b>,
-    <a href="#Page_17">17–21</a>,
-    <a href="#Page_85">85</a>;</li>
-  <li class="i1">differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page188">188–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page194">194</a>;</li>
-  <li class="hangingindent1">the continuity of protoplasm, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page190">190–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page194">194</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page628">628–30</a>; <b>II</b>,
-    <a href="#Page_21">21</a>;</li>
-  <li class="i1">its structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page253">253–5</a>;</li>
-  <li class="i1">function of centrosome, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page254">254–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page257">257</a>;</li>
-  <li class="hangingindent1">structure and function of nucleus, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page255">255–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page258">258–9</a>;</li>
-  <li class="i1">karyokinesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page257">257–8</a>;</li>
-  <li class="i1">function of chromatin, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page259">259–65</a>;</li>
-  <li class="hangingindent1">fertilization and function of polar bodies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page266">266–8</a>;</li>
-  <li class="i1">theories of heredity based on theory, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page356">356</a>;</li>
-  <li class="hangingindent1">Weismann’s differentiation into reproductive and somatic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page622">622</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page628">628–30</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page633">633–44</a>;</li>
-  <li class="i1">nucleus absent or dispersed, <b>II</b>,
-    <a href="#Page_20">20</a>,
-    <a href="#Page_85">85</a>;</li>
-  <li class="i1">morphological differentiation, <b>II</b>,
-    <a href="#Page_175">175–7</a>;</li>
-  <li class="i1">animal morphology, <b>II</b>,
-    <a href="#Page_228">228–30</a>;</li>
-  <li class="i1">morphological summary, <b>II</b>,
-    <a href="#Page_233">233</a>;</li>
-  <li class="i1">vegetal tissue differentiation, <b>II</b>,
-    <a href="#Page_249">249–50</a>,
-    <a href="#Page_386">386</a>;</li>
-  <li class="i1">vascular development, <b>II</b>,
-    <a href="#Page_279">279–84</a>,
-    <a href="#Page_389">389</a>.</li>
-
-  <li>Centipede, bilateral symmetry, <b>II</b>,
-    <a href="#Page_198">198–200</a>.</li>
-
-  <li><i>Cephalopoda</i>: bilateral symmetry, <b>II</b>,
-    <a href="#Page_203">203</a>;</li>
-  <li class="i1">vascular system, <b>II</b>,
-    <a href="#Page_341">341</a>.</li>
-
-  <li><i>Cercariæ</i> (see <a href="#Distoma"><i>Distoma</i></a>).</li>
-
-  <li><i>Cereus</i>, tissue differentiation, <b>II</b>,
-    <a href="#Page_276">276</a>,
-    <a href="#Page_283">283</a>.</li>
-
-  <li>Cesalpino, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page377">377</a>.</li>
-
-  <li><i>Cestoda</i> (see <a href="#Entozoa"><i>Entozoa</i></a>).</li>
-
-  <li><i>Chætopoda</i>, segmentation, <b>II</b>,
-    <a href="#Page_98">98</a>,
-    <a href="#Page_103">103</a>,
-    <a href="#Page_605">605</a>.</li>
-
-  <li><i>Chaja</i>, wing spurs, <b>II</b>,
-    <a href="#Page_313">313</a>.</li>
-
-  <li>Change, and definition of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page81">81–90</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page113">113</a>.</li>
-
-  <li class="hangingindent">Charles, R. H., on inheritance of acquired modifications in leg-bones of Punjabis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>.</li>
-
-  <li>Chatié, on single and double stocks, <b>II</b>,
-    <a href="#Page_622">622</a>.</li>
-
-  <li class="hangingindent" id="Chemistry">Chemistry: properties of organic elements, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page3">3–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page20">20</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page22">22</a>;</li>
-  <li class="i1">of diatomic compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page7">7–10</a>;</li>
-  <li class="i1">tri-atomic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page10">10–12</a>;</li>
-  <li class="i1">poly-atomic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page12">12–13</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page25">25</a>;</li>
-  <li class="i1">traits of evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page23">23–4</a>;</li>
-  <li class="hangingindent1">ethereal undulations and atomic oscillation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page31">31–6</a>;</li>
-  <li class="hangingindent1">chemical affinity and organic change, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page36">36–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page38">38–43</a>;</li>
-  <li class="i1">oxidation and generation of heat, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page46">46–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page60">60</a>;</li>
-  <li class="i1">generation of nerve force, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page52">52</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page60">60</a>;</li>
-  <li class="i1">metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page62">62–77</a>;</li>
-  <li class="i1">physiology and organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page127">127</a>;</li>
-  <li class="i1">flesh constituents, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page154">154</a>;</li>
-  <li class="hangingindent1">composition of organisms and environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page173">173</a>;</li>
-  <li class="hangingindent1">organic development and differential assimilation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page179">179–80</a>;</li>
-  <li class="i1">chemical units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page225">225</a>; <b>II</b>,
-    <a href="#Page_612">612</a>;</li>
-  <li class="i1">primitive ideas of elements, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page417">417</a>;</li>
-  <li class="i1">evolution of organic compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page696">696–701</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page703">703</a>.</li>
-
-  <li>Chestnut, leaf symmetry, <b>II</b>,
-    <a href="#Page_149">149</a>,
-    <a href="#Page_153">153</a>.</li>
-
-  <li class="hangingindent"><i>Chiton</i>: simulation of segmentation, <b>II</b>,
-    <a href="#Page_116">116</a>,
-    <a href="#Page_118">118</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_202">202</a>.</li>
-
-  <li>Chlorophyll: function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page65">65</a>; <b>II</b>,
-    <a href="#Page_263">263</a>;</li>
-  <li class="i1">nutrition and absence of, <b>II</b>,
-    <a href="#Page_74">74</a>;</li>
-  <li class="i1">constitution, <b>II</b>,
-    <a href="#Page_262">262</a>;</li>
-  <li class="i1">symbiotic presence in animals, <b>II</b>,
-    <a href="#Page_400">400</a>.</li>
-
-  <li class="hangingindent"><i>Chondracanthus gibbosus</i>, enormous development of reproductive system, <b>II</b>,
-    <a href="#Page_487">487</a>.</li>
-
-  <li><i>Chordata</i>, affinities, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page466">466</a>.</li>
-
-  <li>Chromatin (<i>see</i> <a href="#Cell">Cell</a>).</li>
-
-  <li>Circle, the, and evolution hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page433">433</a>.</li>
-
-  <li>Circulation (<i>see</i> <a href="#Vascular_System">Vascular System</a>).</li>
-
-  <li class="hangingindent"><i>Cirrhipedia</i>: Darwin on retrograde development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page458">458</a>;</li>
-  <li class="hangingindent1">remarkable transformation in <i>Sacculina</i>, <b>II</b>,
-    <a href="#Page_494">494–5</a>.</li>
-
-  <li class="hangingindent">Civilization, human evolution and genesis, <b>II</b>,
-    <a href="#Page_529">529–31</a>.</li>
-
-  <li><i>Cladophora</i>: integration, <b>II</b>,
-    <a href="#Page_25">25</a>;</li>
-  <li class="i1">axial development, <b>II</b>,
-    <a href="#Page_28">28</a>.</li>
-
-  <li>Classification: subjective conception, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page78">78</a>;</li>
-  <li class="i1">two purposes of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page374">374</a>;</li>
-  <li class="i1">a gradual process, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page375">375</a>;</li>
-  <li class="i1">botanical, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page377">377–80</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page389">389–90</a>;</li>
-  <li class="i1">zoological, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page380">380–9</a>;</li>
-  <li class="hangingindent1">incomplete equivalence of groups, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page389">389</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page445">445–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page448">448</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page555">555</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page572">572</a>;</li>
-  <li class="i1">group attributes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page390">390–3</a>;</li>
-  <li class="i1">the truths interpreted, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page393">393–4</a>;</li>
-  <li class="i1">ethnologic and linguistic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page441">441–6</a>;</li>
-  <li class="i1">organic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page443">443</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page447">447</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page555">555</a>;</li>
-  <li class="i1">differences in kind and degree, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page444">444–6</a>;</li>
-  <li class="i1">antecedent structural similarity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page447">447</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page448">448–9</a>;</li>
-  <li class="i1">Von Baer’s formula, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page451">451–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page555">555</a>;</li>
-  <li class="i1">organic, not uniserial, <b>II</b>,
-    <a href="#Page_115">115</a>.</li>
-
-  <li class="hangingindent"><i>Classification of the Sciences, The</i>, and evolution and dissolution, <b>II</b>,
-    <a href="#Page_5">5</a>.</li>
-
-  <li class="hangingindent">Claus, C, on segmentation in Annelids and Chætopods, <b>II</b>,
-    <a href="#Page_605">605</a>.</li>
-
-  <li>Clover: flower and axial development, <b>II</b>,
-    <a href="#Page_45">45</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_152">152</a>.</li>
-
-  <li class="hangingindent" id="adaptation">Co-adaptation of co-operative parts: principles underlying, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page234">234–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page511">511–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page514">514–5</a>;</li>
-  <li class="i1">slow operation of the process, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page236">236</a>;</li>
-  <li class="i1">sociological analogy, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page237">237–40</a>;</li>
-  <li class="i1">reversion under original conditions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page240">240</a>;</li>
-  <li class="i1">the analogy continued, <i>ib.</i>;</li>
-  <li class="i1">the case of bison’s head, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page512">512</a>;</li>
-  <li class="hangingindent1">natural selection an inadequate explanation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page535">535</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page614">614–21</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="hangingindent1">Romanes on “cessation of selection” as effecting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page560">560</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page561">561–2</a>;</li>
-  <li class="hangingindent1">Weismann’s theories, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page560">560–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page663">663–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page674">674–5</a>;</li>
-  <li class="i1">natural selection and economy of growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page562">562</a>;</li>
-  <li class="i1">physiological processes involved, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page566">566–7</a>;</li>
-  <li class="hangingindent1">Wallace’s argument from artificial selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page615">615</a>;</li>
-  <li class="i1">what are co-operative parts? <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page616">616–7</a>;</li>
-  <li class="i1">“intra-selection” examined, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676–8</a>.</li>
-
-  <li>Coal, social effects of supply, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page238">238–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page241">241</a>.</li>
-
-  <li>Cocoa-nut, growth and fertility, <b>II</b>,
-    <a href="#Page_457">457</a>.</li>
-
-  <li class="hangingindent">Coccospheres: vital problem presented by protective structures, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page119">119</a>;</li>
-  <li class="i1">imbricated plates, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page182">182</a>.</li>
-
-  <li>Cockroach, ousting of European species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page399">399</a>.</li>
-
-  <li>Cod: ova of, <b>II</b>,
-    <a href="#Page_435">435</a>;</li>
-  <li class="i1">growth and fertility, <b>II</b>,
-    <a href="#Page_454">454</a>.</li>
-
-  <li><i>Codium</i>: symmetry, <b>II</b>,
-    <a href="#Page_136">136</a>;</li>
-  <li class="i1">tissue differentiation, <b>II</b>,
-    <a href="#Page_246">246</a>.</li>
-
-  <li class="hangingindent" id="Coelenterata"><i>Cœlenterata</i>: rudimentary contractile organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page58">58</a>;</li>
-  <li class="i1">vital changes in polyp, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page95">95</a>;</li>
-  <li class="i1">axial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page166">166</a>;</li>
-  <li class="i1">environment and structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page173">173</a>;</li>
-  <li class="i1">self-mobility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page175">175</a>; <b>II</b>,
-    <a href="#Page_14">14</a>,
-    <a href="#Page_15">15</a>;</li>
-  <li class="i1">functional differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page201">201</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page391">391</a>;</li>
-  <li class="i1">inactivity and waste, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page213">213</a>;</li>
-  <li class="i1">reparative power, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page219">219</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page224">224</a>;</li>
-  <li class="i1">individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page246">246</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page247">247</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page250">250</a>;</li>
-  <li class="i1">heterogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page273">273</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page277">277</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page296">296</a>;</li>
-  <li class="hangingindent1">negative disintegration in <i>Hydrozoa</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page276">276</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page587">587</a>;</li>
-  <li class="i1">reproductive tissue, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page281">281</a>;</li>
-  <li class="i1">differentiation in <i>Hydrozoa</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page391">391</a>;</li>
-  <li class="i1">classificatory value, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page446">446</a>;</li>
-  <li class="i1">regeneration of fragments, <b>II</b>,
-    <a href="#Page_90">90</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_90">90</a>,
-    <a href="#Page_102">102</a>,
-    <a href="#Page_105">105</a>,
-    <a href="#Page_124">124</a>;</li>
-  <li class="i1">gemmation, <b>II</b>,
-    <a href="#Page_91">91</a>;</li>
-  <li class="i1">tertiary aggregation, <b>II</b>,
-    <a href="#Page_92">92</a>,
-    <a href="#Page_95">95</a>,
-    <a href="#Page_124">124</a>;</li>
-  <li class="i1">molluscan affinities, <b>II</b>,
-    <a href="#Page_115">115</a>;</li>
-  <li class="i1">radial symmetry, <b>II</b>,
-    <a href="#Page_188">188</a>;</li>
-  <li class="i1">symmetry of compound, <b>II</b>,
-    <a href="#Page_192">192–3</a>;</li>
-  <li class="i1">segmental differentiation, <b>II</b>,
-    <a href="#Page_207">207</a>;</li>
-  <li class="hangingindent1">physiological differentiation in <i>Hydra</i> and analogy, <b>II</b>,
-    <a href="#Page_300">300</a>;</li>
-  <li class="i1">ciliation of blastula, <b>II</b>,
-    <a href="#Page_301">301</a>;</li>
-  <li class="i1">tissue reduplication, <b>II</b>,
-    <a href="#Page_301">301–2</a>,
-    <a href="#Page_389">389</a>;</li>
-  <li class="i1">outer tissue differentiation, <b>II</b>,
-    <a href="#Page_309">309</a>;</li>
-  <li class="i1">osmosis in <i>Hydra</i>, <b>II</b>,
-    <a href="#Page_339">339</a>;</li>
-  <li class="i1">vascular system in <i>Hydra</i>, <b>II</b>,
-    <a href="#Page_340">340</a>,
-    <a href="#Page_376">376</a>;</li>
-  <li class="i1">functional co-ordination, <b>II</b>,
-    <a href="#Page_376">376</a>;</li>
-  <li class="i1">symbiosis, <b>II</b>,
-    <a href="#Page_400">400</a>;</li>
-  <li class="i1">asexual genesis, <b>II</b>,
-    <a href="#Page_443">443–4</a>;</li>
-  <li class="i1">growth and sexual genesis, <b>II</b>,
-    <a href="#Page_452">452</a>;</li>
-  <li class="i1">development and genesis, <b>II</b>,
-    <a href="#Page_462">462</a>;</li>
-  <li class="i1">nutrition and genesis, <b>II</b>,
-    <a href="#Page_476">476</a>.</li>
-
-  <li>Cœlom, origin and function, <b>II</b>,
-    <a href="#Page_302">302–3</a>.</li>
-
-  <li class="hangingindent">Collins, F. Howard, jaws and teeth of savages and civilized, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page541">541</a>.</li>
-
-  <li>Colloids: T. Graham on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page15">15–8</a>;</li>
-  <li class="i1">diffusibility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page18">18–21</a>;</li>
-  <li class="i1">organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page21">21</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page25">25</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page26">26</a>;</li>
-  <li class="i1">pliability and elasticity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page27">27</a>;</li>
-  <li class="i1">capillary affinity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page28">28</a>;</li>
-  <li class="i1">isomerism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page59">59</a>;</li>
-  <li class="i1">instability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page350">350</a>;</li>
-  <li class="i1">molecular mobility and diffusibility, <b>II</b>,
-    <a href="#Page_331">331</a>;</li>
-  <li class="hangingindent1">instability of, and nerve differentiation, <b>II</b>,
-    <a href="#Page_356">356–61</a>;</li>
-  <li class="i1">and muscular tissue, <b>II</b>,
-    <a href="#Page_361">361–4</a>.</li>
-
-  <li class="hangingindent">Colonies, autogenous development and parallel in heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page366">366–8</a>; <b>II</b>,
-    <a href="#Page_620">620</a>.</li>
-
-  <li>Colour: sensation of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page54">54</a>;</li>
-  <li class="i1">phœnogamic, <b>II</b>,
-    <a href="#Page_75">75</a>,
-    <a href="#Page_265">265–6</a>;</li>
-  <li class="i1">light and vegetal, <b>II</b>,
-    <a href="#Page_261">261–2</a>;</li>
-  <li class="i1">floral fertilization, <b>II</b>,
-    <a href="#Page_267">267–9</a>;</li>
-  <li class="i1">sexual selection, <b>II</b>,
-    <a href="#Page_269">269</a>;</li>
-  <li class="i1">activity and muscular, <b>II</b>,
-    <a href="#Page_365">365–9</a>;</li>
-  <li class="hangingindent1">physiological units and mixture of, in offspring, <b>II</b>,
-    <a href="#Page_616">616</a>,
-    <a href="#Page_617">617</a>.</li>
-
-  <li class="hangingindent">Commensalism, organic Integration as displayed in, <b>II</b>,
-    <a href="#Page_402">402–4</a>.</li>
-
-  <li><i>Compositæ</i>: floral symmetry, <b>II</b>,
-    <a href="#Page_173">173</a>.</li>
-
-  <li>Condor, weight of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page155">155</a>.</li>
-
-  <li><i>Confervoideæ</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page280">280</a>; <b>II</b>,
-    <a href="#Page_25">25</a>,
-    <a href="#Page_28">28</a>,
-    <a href="#Page_449">449</a>.</li>
-  <li class="i1">(See <a href="#Algae"><i>Algæ</i></a>.)</li>
-
-  <li><i>Conjugateæ</i>, <b>II</b>,
-    <a href="#Page_449">449</a>.</li>
-  <li class="i1">(See <a href="#Algae"><i>Algæ</i></a>.)</li>
-
-  <li>Conjugation, in <i>Algæ</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279</a>;</li>
-  <li class="i1">in <i>Protozoa</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page280">280</a>; <b>II</b>,
-    <a href="#Page_452">452</a>;</li>
-  <li class="i1">can fission persist without? <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page637">637</a>;</li>
-  <li class="i1">relation to growth, <b>II</b>,
-    <a href="#Page_449">449</a>.</li>
-
-  <li>Connective tissue, Hertwig’s classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page189">189</a>.</li>
-
-  <li>Constitutional units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page369">369</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Physiological_Units">Physiological Units</a>.)</li>
-
-  <li>Consumption, hereditary transmission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page307">307</a>.</li>
-
-  <li>Co-ordination of actions (<i>see</i> <a href="#Life">Life</a>).</li>
-
-  <li class="hangingindent">Cope, E. D., on origin of vertebrate structure, <b>II</b>,
-    <a href="#Page_225">225–7</a>.</li>
-
-  <li class="hangingindent"><i>Cormophyta</i>: slight internal differentiation, <b>II</b>,
-    <a href="#Page_273">273</a>;</li>
-  <li class="i1">vascular system, <b>II</b>,
-    <a href="#Page_280">280</a>.</li>
-
-  <li><i>Corpuscula tactus</i>, their function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page75">75</a>.</li>
-
-  <li>Correspondence, use of word, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page97">97</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Life">Life</a>.)</li>
-
-  <li>Cousin-marriages, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page345">345</a>; <b>II</b>,
-    <a href="#Page_615">615</a>.</li>
-
-  <li>Cow: what prompts her to mumble a bone? <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page120">120</a>.</li>
-
-  <li>Cow-parsnip (see <i>Heracleum</i>).</li>
-
-  <li>Crab (see <a href="#Crustacea"><i>Crustacea</i></a>).</li>
-
-  <li>Creation (<i>see</i> <a href="#Special_creation">Special creation</a>).</li>
-
-  <li><i>Crinoidea</i>, symmetry, <b>II</b>,
-    <a href="#Page_195">195–6</a>.</li>
-
-  <li>Crocodile, continuous growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page154">154</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page292">292</a>.</li>
-
-  <li class="hangingindent">Crookes, Sir W., hypothetical chemical unit “protyle,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page22">22</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page23">23</a>.</li>
-
-  <li><i>Cruciferæ</i>, floral symmetry, <b>II</b>,
-    <a href="#Page_164">164</a>,
-    <a href="#Page_171">171</a>.</li>
-
-  <li id="Crustacea"><i>Crustacea</i>: locomotion of lobster, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page175">175</a>;</li>
-  <li class="i1">regeneration of limbs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page224">224</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page360">360</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page589">589</a>; <b>II</b>,
-    <a href="#Page_76">76</a>;</li>
-  <li class="i1">homogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>;</li>
-  <li class="hangingindent1">genesis and nutrition in <i>Daphnidæ</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page290">290–1</a>;</li>
-  <li class="i1">growth and genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page292">292</a>;</li>
-  <li class="hangingindent1">degeneration of eye in cave-inhabiting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page309">309</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page614">614</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page648">648</a>;</li>
-  <li class="i1">hermit-crab parasite, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page397">397</a>;</li>
-  <li class="i1">changes of media, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page401">401</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page481">481–2</a>;</li>
-  <li class="i1">retrograde development in cirripedes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page458">458</a>;</li>
-  <li class="i1">segmentation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page468">468–9</a>; <b>II</b>,
-    <a href="#Page_114">114</a>;</li>
-  <li class="i1">Darwin on jaws and legs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page471">471</a>;</li>
-  <li class="i1">survival of cirripedes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page517">517</a>;</li>
-  <li class="i1">integration and homology, <b>II</b>,
-    <a href="#Page_111">111–4</a>,
-    <a href="#Page_121">121</a>,
-    <a href="#Page_603">603</a>;</li>
-  <li class="i1">bilateral symmetry, <b>II</b>,
-    <a href="#Page_198">198–201</a>;</li>
-  <li class="i1">eyes, <b>II</b>,
-    <a href="#Page_318">318</a>;</li>
-  <li class="i1">dermal structure of hermit-crab, <b>II</b>,
-    <a href="#Page_322">322</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">fertility, <b>II</b>,
-    <a href="#Page_453">453</a>;</li>
-  <li class="hangingindent1">nutrition and genesis in parasitic species, <b>II</b>,
-    <a href="#Page_487">487</a>;</li>
-  <li class="i1">“castration parasitaire,” <b>II</b>,
-    <a href="#Page_493">493–6</a>.</li>
-
-  <li>Crystalloids: Prof. Graham on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page15">15–8</a>;</li>
-  <li class="i1">diffusibility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page18">18–21</a>;</li>
-  <li class="i1">organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page21">21–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page26">26</a>.</li>
-
-  <li class="hangingindent">Crystals: simulation of life in “storm glass,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page96">96</a>;</li>
-  <li class="i1">growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page135">135–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page577">577</a>;</li>
-  <li class="i1">segregation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page179">179</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page221">221</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page223">223</a>;</li>
-  <li class="i1">equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page337">337</a>;</li>
-  <li class="i1">physiological units and polarity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page701">701–6</a>;</li>
-  <li class="i1">time and formation, <b>II</b>,
-    <a href="#Page_77">77</a>.</li>
-
-  <li class="hangingindent"><i>Ctenodrilus</i>, segmental individuality, <b>II</b>,
-    <a href="#Page_103">103</a>,
-    <a href="#Page_603">603</a>,
-    <a href="#Page_604">604</a>.</li>
-
-  <li>Cube, bilateral symmetry, <b>II</b>,
-    <a href="#Page_132">132</a>.</li>
-
-  <li>Cunningham, J. T., <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#pagevi">vi</a>; <b>II</b>,
-    <a href="#Page_vi">vi</a>;</li>
-  <li class="i1">on non-adaptive specific characters, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page565">565</a>;</li>
-  <li class="i1">food of blow-fly larvæ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page678">678</a>;</li>
-  <li class="i1">arthropod segmentation, <b>II</b>,
-    <a href="#Page_114">114</a>;</li>
-  <li class="i1">egg-production of Conger, <b>II</b>,
-    <a href="#Page_425">425</a>.</li>
-
-  <li>Cuttle-fish, Individuality of <i>Hectocotylus</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page250">250</a>.</li>
-
-  <li>Cuvier, zoological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page381">381</a>.</li>
-
-  <li>Cyanogen, properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page7">7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>.</li>
-
-  <li><i>Cyclichthys</i>, dermal structure, <b>II</b>,
-    <a href="#Page_306">306</a>.</li>
-</ul>
-
-<ul>
-  <li>Dalyell, Sir J., regeneration in <i>Dasychone</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page361">361</a>;</li>
-  <li class="i1">propagation of <i>Hydra</i>, <b>II</b>,
-    <a href="#Page_476">476</a>.</li>
-
-  <li><i>Daphnidæ</i>, heterogenesis and nutrition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page290">290–1</a>.</li>
-
-  <li>Darwin, C: <i>Origin of Species</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page129">129</a>; <b>II</b>,
-    <a href="#Page_528">528</a>;</li>
-  <li class="i1">natural selection and function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page308">308–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">atavism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314</a>;</li>
-  <li class="i1">osseous variations in pigeons, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page321">321</a>;</li>
-  <li class="i1">plant variation and domestication, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page325">325</a>;</li>
-  <li class="i1">“spontaneous variation,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page328">328</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page697">697</a>;</li>
-  <li class="hangingindent1">floral fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page340">340</a>; <b>II</b>,
-    <a href="#Page_168">168</a>,
-    <a href="#Page_267">267</a>,
-    <a href="#Page_407">407</a>,
-    <a href="#Page_608">608</a>;</li>
-  <li class="hangingindent1">intercrossing and self-fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page344">344</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page345">345</a>;</li>
-  <li class="i1">intercrossing <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page347">347</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page611">611</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page669">669</a>;</li>
-  <li class="hangingindent1">his theory of pangenesis examined, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page356">356–62</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page370">370</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page372">372</a>;</li>
-  <li class="i1">plant-fertilization and distribution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page397">397</a>;</li>
-  <li class="i1">habits of birds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page400">400</a>;</li>
-  <li class="i1">distribution and natural barriers, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page402">402</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page476">476</a>;</li>
-  <li class="hangingindent1">disappearance and non-reappearance of species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page406">406</a>;</li>
-  <li class="i1">distribution in time and space, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page410">410</a>;</li>
-  <li class="i1">linguistic classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page442">442</a>;</li>
-  <li class="i1">classification of organisms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page443">443</a>;</li>
-  <li class="i1">classification and descent, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page448">448</a>;</li>
-  <li class="i1">on petrels, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page455">455</a>;</li>
-  <li class="i1">suppression of organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page457">457</a>;</li>
-  <li class="i1">development of <i>Cirrhipedia</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page438">438</a>;</li>
-  <li class="i1">jaws and legs of <i>Crustacea</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page471">471</a>;</li>
-  <li class="i1">aborted organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page474">474</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page563">563</a>;</li>
-  <li class="hangingindent1">relations of species in Galapagos archipelago, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page478">478</a>;</li>
-  <li class="i1">opinions of E. Darwin and Lamarck, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page491">491</a>;</li>
-  <li class="i1">the term “survival of the fittest,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page530">530</a>;</li>
-  <li class="hangingindent1">Indirect equilibration by natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page530">530–5</a>;</li>
-  <li class="hangingindent1">inheritance of acquired characters, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page535">535–42</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page560">560</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page630">630</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page690">690</a>;</li>
-  <li class="i1">Wallace on natural selection in man, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>;</li>
-  <li class="hangingindent1">misleading connotations of term “natural selection,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page609">609</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page695">695</a>;</li>
-  <li class="i1">caste gradations and jaws of driver ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page658">658</a>;</li>
-  <li class="i1">attachment of climbing plants, <b>II</b>,
-    <a href="#Page_276">276–7</a>;</li>
-  <li class="i1">vegetal fructification, <b>II</b>,
-    <a href="#Page_294">294</a>;</li>
-  <li class="i1">earth-worm, <b>II</b>,
-    <a href="#Page_402">402</a>;</li>
-  <li class="hangingindent1">animal sterility and domestication, <b>II</b>,
-    <a href="#Page_480">480</a>,
-    <a href="#Page_483">483</a>;</li>
-  <li class="i1">variation in hyacinth and camellia, <b>II</b>,
-    <a href="#Page_621">621</a>.</li>
-
-  <li class="hangingindent">Darwin, Dr. E., modifiability of organisms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page490">490</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page492">492–7</a>.</li>
-
-  <li>Death: an arrest of vital correspondence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page102">102</a>;</li>
-  <li class="i1">only limit to vegetal growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page153">153</a>;</li>
-  <li class="hangingindent1">cessation of co-ordination of actions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page578">578</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page579">579</a>;</li>
-  <li class="i1">Weismann’s hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page636">636–8</a>;</li>
-  <li class="i1">physiological integration, <b>II</b>,
-    <a href="#Page_374">374</a>,
-    <a href="#Page_392">392</a>;</li>
-  <li class="i1">cause of natural, <b>II</b>,
-    <a href="#Page_413">413</a>;</li>
-  <li class="i1">relation to births, <b>II</b>,
-    <a href="#Page_417">417</a>.</li>
-
-  <li class="hangingindent">Definiteness: of vital change, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page87">87–90</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page106">106</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page109">109</a>;</li>
-  <li class="i1">developmental, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page178">178</a>;</li>
-  <li class="i1">functional, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page212">212</a>;</li>
-  <li class="i1">segregation of evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page514">514–6</a>.</li>
-
-  <li>Definition, difficulties of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page78">78</a>; <b>II</b>,
-    <a href="#Page_17">17</a>.</li>
-
-  <li class="hangingindent">Degeneracy, morphological obscurations due to, <b>II</b>,
-    <a href="#Page_12">12</a>,
-    <a href="#Page_13">13</a>.</li>
-
-  <li><i>Dendrobium</i> (<i>see</i> <a href="#Orchids">Orchids</a>).</li>
-
-  <li><i>Desmidiaceæ</i>: unicellular, <b>II</b>,
-    <a href="#Page_21">21</a>;</li>
-  <li class="i1">linear and central aggregation, <b>II</b>,
-    <a href="#Page_23">23</a>;</li>
-  <li class="i1">natural selection and symmetry, <b>II</b>,
-    <a href="#Page_134">134</a>,
-    <a href="#Page_133">133</a>;</li>
-  <li class="i1">morphological differentiation, <b>II</b>,
-    <a href="#Page_177">177</a>;</li>
-  <li class="i1">tissue, <b>II</b>,
-    <a href="#Page_244">244</a>;</li>
-  <li class="i1">genesis, <b>II</b>,
-    <a href="#Page_440">440</a>,
-    <a href="#Page_449">449</a>.</li>
-
-  <li class="hangingindent">Determinants, Weismann’s germ-plasm units (<i>see</i> <a href="#Germ_plasm">Germ-plasm</a>).</li>
-
-  <li class="hangingindent">Development: an increase of structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page162">162</a>; <b>II</b>,
-    <a href="#Page_461">461</a>;</li>
-  <li class="i1">primarily central, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page162">162</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page166">166</a>;</li>
-  <li class="i1">uni- and multicentral, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page163">163–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page166">166–7</a>;</li>
-  <li class="i1">axial, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page164">164</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page167">167</a>;</li>
-  <li class="i1">uni- and multiaxial, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165–6</a>;</li>
-  <li class="hangingindent1">a change to coherent definite heterogeneity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page167">167–70</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page179">179</a>;</li>
-  <li class="i1">Von Baer’s formula, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page171">171–2</a>;</li>
-  <li class="hangingindent1">individual differentiation from environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page172">172–8</a>;</li>
-  <li class="i1">cell-formation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page225">225</a>;</li>
-  <li class="i1">discontinuous, and agamogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page275">275</a>;</li>
-  <li class="i1">Prof. Huxley’s classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page276">276</a>;</li>
-  <li class="hangingindent1">sociological parallel to autogenous, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364–8</a>; <b>II</b>,
-    <a href="#Page_620">620</a>;</li>
-  <li class="i1">retrograde, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page457">457–8</a>;</li>
-  <li class="i1">inequalities among co-operative parts, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page617">617</a>;</li>
-  <li class="i1">“heterochrony,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page655">655</a>;</li>
-  <li class="i1">continuous and discontinuous vegetal, <b>II</b>,
-    <a href="#Page_52">52</a>;</li>
-  <li class="i1">summary of physiological, <b>II</b>,
-    <a href="#Page_384">384–90</a>;</li>
-  <li class="i1">nutrition and genesis, <i>résumé</i>, <b>II</b>,
-    <a href="#Page_497">497–9</a>;</li>
-  <li class="i1">evolution, <b>II</b>,
-    <a href="#Page_501">501–5</a>;</li>
-  <li class="i1">commencement of genesis, <b>II</b>,
-    <a href="#Page_506">506</a>;</li>
-  <li class="i1">of vertebrate limbs, <b>II</b>,
-    <a href="#Page_553">553</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Multiplication">Multiplication</a>.)</li>
-
-  <li><i>Development Hypothesis, The</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page417">417</a>.</li>
-
-  <li>Dialects (<i>see</i> <a href="#Language">Language</a>).</li>
-
-  <li>Dialysis, and diffusibillty. <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page19">19</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page20">20</a>.</li>
-
-  <li>Diastase, decomposition of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page38">38</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page40">40</a>.</li>
-
-  <li><i>Diatomaceæ</i>: tissue, <b>II</b>,
-    <a href="#Page_244">244</a>;</li>
-  <li class="i1">genesis, <b>II</b>,
-    <a href="#Page_440">440</a>,
-    <a href="#Page_448">448</a>.</li>
-
-  <li>Diatomic compounds (<i>see</i> <a href="#Chemistry">Chemistry</a>).</li>
-
-  <li class="hangingindent" id="Dicotyledons">Dicotyledons: growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page139">139</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page143">143</a>; <b>II</b>,
-    <a href="#Page_63">63–4</a>,
-    <a href="#Page_69">69–72</a>,
-    <a href="#Page_78">78</a>,
-    <a href="#Page_82">82–3</a>;</li>
-  <li class="i1">uniaxial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>;</li>
-  <li class="i1">stem and leaf functions, <b>II</b>,
-    <a href="#Page_257">257</a>;</li>
-  <li class="i1">mechanical stress and wood formation, <b>II</b>,
-    <a href="#Page_277">277</a>;</li>
-  <li class="i1">growth and genesis, <b>II</b>,
-    <a href="#Page_451">451</a>.</li>
-
-  <li>Differentiation (<i>see</i> <a href="#Morphology">Morphology</a> <i>and</i> <a href="#Physiology">Physiology</a>).</li>
-
-  <li><i>Difflugia</i>: primary aggregate, <b>II</b>,
-    <a href="#Page_86">86–7</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_186">186</a>;</li>
-  <li class="i1">outer tissue differentiation, <b>II</b>,
-    <a href="#Page_309">309</a>.</li>
-
-  <li class="hangingindent">Diffusion, of colloids and crystalloids, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page18">18–20</a>; <b>II</b>,
-    <a href="#Page_331">331</a>.</li>
-
-  <li>Digestion: action of nitrogenous compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>;</li>
-  <li class="i1">obesity, <b>II</b>,
-    <a href="#Page_480">480–4</a>;</li>
-  <li class="i1">fertility, <b>II</b>,
-    <a href="#Page_514">514</a>.</li>
-
-  <li>Dimorphism: floral, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page534">534</a>;</li>
-  <li class="i1">sexual, in parasites, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page315">315</a>;</li>
-  <li class="i1">social insects (<i>see</i> <a href="#Insects">Insects</a>).</li>
-
-  <li>Dinosaurs, size of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page139">139</a>.</li>
-
-  <li><i>Diphyes</i>: individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page246">246</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_192">192</a>.</li>
-
-  <li id="Disease">Disease: segregation of blood constituents, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page179">179</a>;</li>
-  <li class="i1">changes in blood from, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page221">221</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page701">701</a>;</li>
-  <li class="i1">heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page306">306–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page312">312–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page622">622–3</a>;</li>
-  <li class="i1">belief in supernatural origin, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page419">419</a>;</li>
-  <li class="i1">parasitism and special creation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page427">427</a>;</li>
-  <li class="i1">morbid products as specific characters, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page567">567</a>;</li>
-  <li class="i1">telegony, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646</a>;</li>
-  <li class="i1">dermal structure, <b>II</b>,
-    <a href="#Page_306">306</a>;</li>
-  <li class="i1">intestinal muscular hypertrophy, <b>II</b>,
-    <a href="#Page_325">325</a>;</li>
-  <li class="hangingindent1">indigestion and alimentary canal development, <b>II</b>,
-    <a href="#Page_328">328</a>;</li>
-  <li class="i1">jaundice and bilirubin, <b>II</b>,
-    <a href="#Page_330">330</a>;</li>
-  <li class="i1">localization of excretion, <b>II</b>,
-    <a href="#Page_331">331</a>;</li>
-  <li class="i1">membranes in inflammatory, <b>II</b>,
-    <a href="#Page_343">343</a>;</li>
-  <li class="i1">osseous differentiation in rickets, <b>II</b>,
-    <a href="#Page_352">352</a>;</li>
-  <li class="i1">fatty degeneration, <b>II</b>,
-    <a href="#Page_482">482</a>.</li>
-
-  <li>Disintegration, physiological (<i>see</i> <a href="#Physiology">Physiology</a>).</li>
-
-  <li id="Distoma"><i>Distoma</i>: metagenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page273">273–4</a>;</li>
-  <li class="i1">disintegration of genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page276">276</a>;</li>
-  <li class="i1">cycle of generations, <b>II</b>,
-    <a href="#Page_489">489</a>.</li>
-
-  <li>Distribution: physical limits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page396">396</a>;</li>
-  <li class="i1">organic environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page396">396–8</a>;</li>
-  <li class="i1">parasitic conditions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page397">397–8</a>;</li>
-  <li class="i1">simultaneity of agencies affecting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page398">398</a>;</li>
-  <li class="hangingindent1">mutual encroachments of species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page398">398–401</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page477">477</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page489">489</a>;</li>
-  <li class="hangingindent1">facts disproving pre-adaptation to habitats, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page401">401–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page411">411–2</a>;</li>
-  <li class="i1">of animals and plants in time, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page404">404–11</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page412">412</a>;</li>
-  <li class="hangingindent1">ousting of native species in New Zealand, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page477">477</a>;</li>
-  <li class="i1">local influences, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page477">477–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page489">489</a>;</li>
-  <li class="i1">through varied media, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page479">479–85</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page489">489</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">past and present organic forms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page485">485–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">complex organization and, <b>II</b>,
-    <a href="#Page_296">296–7</a>.</li>
-
-  <li>Division of labour, physiological (<i>see</i> <a href="#Labour">Labour</a>).</li>
-
-  <li>Dog: contrasted lives of tortoise and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page103">103</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page104">104</a>;</li>
-  <li class="i1">inherited habits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page309">309</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page573">573</a>;</li>
-  <li class="i1">abnormal digits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page324">324</a>;</li>
-  <li class="i1">interbreeding of divergent varieties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page565">565</a>;</li>
-  <li class="i1">decrease of jaw, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page615">615</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">telegony, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page645">645</a>;</li>
-  <li class="i1">conditions affecting fertility, <b>II</b>,
-    <a href="#Page_474">474</a>,
-    <a href="#Page_479">479</a>.</li>
-
-  <li>Dohrn, theory of vertebrate structure, <b>II</b>,
-    <a href="#Page_606">606</a>.</li>
-
-  <li class="hangingindent"><i>Doliolum</i>, combination of individualities, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page247">247</a>.</li>
-
-  <li>Domestication (<i>see</i> <a href="#Animals">Animals</a>).</li>
-
-  <li>Doubleday, E., on nutrition of genesis, <b>II</b>,
-    <a href="#Page_510">510–2</a>.</li>
-
-  <li class="hangingindent">Driesch, separation of segmentation spheres of <i>Echinus</i> ovum, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page691">691</a>; <b>II</b>,
-    <a href="#Page_618">618</a>.</li>
-
-  <li>Dropsy (<i>see</i> <a href="#Disease">Disease</a>).</li>
-
-  <li><i>Drosera</i>: individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page251">251</a>;</li>
-  <li class="i1">proliferous growth, <b>II</b>,
-    <a href="#Page_75">75</a>.</li>
-
-  <li class="hangingindent">Du Bois-Reymond, E. H., electricity from muscles and nerves, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page50">50</a>.</li>
-
-  <li>Dumas, antithesis of animals and plants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page62">62</a>.</li>
-
-  <li>Dwarfs, Hindu family of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page316">316</a>.</li>
-</ul>
-
-<ul>
-  <li>Ear, development of vertebrate, <b>II</b>,
-    <a href="#Page_318">318</a>,
-    <a href="#Page_320">320</a>.</li>
-
-  <li class="hangingindent">Earth, climatic rhythm and organic change, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page499">499–501</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>.</li>
-
-  <li>Earth-worm: bilateral symmetry, <b>II</b>,
-    <a href="#Page_199">199</a>,
-    <a href="#Page_200">200</a>;</li>
-  <li class="i1">mould production, <b>II</b>,
-    <a href="#Page_402">402</a>.</li>
-
-  <li><i>Echinococcus</i> (see <a href="#Entozoa"><i>Entozoa</i></a>).</li>
-
-  <li class="hangingindent"><i>Echinodermata</i>: independence of blastosphere cells, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page185">185</a>;</li>
-  <li class="i1">protoplasmic continuity in embryos, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page190">190</a>;</li>
-  <li class="hangingindent1">separation of segmentation spheres of ovum, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page691">691</a>; <b>II</b>,
-    <a href="#Page_618">618</a>;</li>
-  <li class="i1">symmetry, <b>II</b>, 191,
-    <a href="#Page_195">195–6</a>.</li>
-
-  <li class="hangingindent">Economy: of growth in natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page536">536</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page562">562</a>;</li>
-  <li class="i1">a trait of organic evolution, <b>II</b>,
-    <a href="#Page_501">501</a>,
-    <a href="#Page_504">504</a>.</li>
-
-  <li>Ectoderm: functional differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page202">202</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page203">203</a>;</li>
-  <li class="i1">functional vicariousness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page209">209</a>;</li>
-  <li class="i1">reproductive function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page281">281</a>.</li>
-
-  <li class="hangingindent">Effects, Multiplication of: variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page329">329–30</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page333">333</a>;</li>
-  <li class="hangingindent1">organic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page511">511–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page515">515</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page517">517</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page549">549</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>; <b>II</b>,
-    <a href="#Page_405">405–6</a>;</li>
-  <li class="i1">morphological development, <b>II</b>,
-    <a href="#Page_7">7–9</a>,
-    <a href="#Page_234">234</a>;</li>
-  <li class="i1">physiological differentiation, <b>II</b>,
-    <a href="#Page_390">390–1</a>,
-    <a href="#Page_392">392</a>.</li>
-
-  <li id="Egg">Eggs (<i>see</i> <a href="#Embryology">Embryology</a>).</li>
-
-  <li>Eimer, T., theory of orthogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page563">563–4</a>.</li>
-
-  <li class="hangingindent"><i>Elasmobranchii</i>: protoplasmic continuity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page629">629</a>;</li>
-  <li class="i1">segmentation, <b>II</b>,
-    <a href="#Page_126">126</a>.</li>
-
-  <li>Electricity: genesis in organic matter, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page50">50–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page60">60</a>;</li>
-  <li class="i1">muscular action, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page59">59</a>;</li>
-  <li class="i1">incomprehensibility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page121">121</a>.</li>
-
-  <li class="hangingindent">Elephant: fertility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page583">583</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page599">599</a>; <b>II</b>,
-    <a href="#Page_459">459</a>,
-    <a href="#Page_506">506</a>;</li>
-  <li class="i1">cerebro-spinal system, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page598">598</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page599">599</a>.</li>
-
-  <li>Elk, Irish, horns and correlated parts, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page537">537</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page674">674</a>.</li>
-
-  <li id="Eloidea"><i>Eloidea canadensis</i>: individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page248">248</a>;</li>
-  <li class="i1">enormous agamic multiplication, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page642">642</a>.</li>
-
-  <li>Elongation, and locomotion in animals, <b>II</b>,
-    <a href="#Page_15">15</a>.</li>
-
-  <li id="Embryology">Embryology: as aiding biology, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page125">125–6</a>;</li>
-  <li class="i1">simulated growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page136">136</a>;</li>
-  <li class="hangingindent1">initial and final organic bulks, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page143">143</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page158">158</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page161">161</a>;</li>
-  <li class="i1">fœtal flesh constituents, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page154">154</a>;</li>
-  <li class="i1">human arm development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page169">169</a>;</li>
-  <li class="i1">Von Baer’s formula, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page170">170–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page451">451–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page466">466</a>;</li>
-  <li class="i1">embryonic heat, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page177">177</a>;</li>
-  <li class="i1">spherical organic form, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page177">177</a>;</li>
-  <li class="i1">unit-life in multicellular organisms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page185">185–6</a>;</li>
-  <li class="i1">functional differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page203">203</a>;</li>
-  <li class="i1">individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page246">246–7</a>;</li>
-  <li class="i1">unspecialized reproductive tissue, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279–83</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page317">317</a>;</li>
-  <li class="i1">changes following impregnation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page283">283–4</a>;</li>
-  <li class="i1">nutrition and vegetal growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page285">285–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295–7</a>;</li>
-  <li class="i1">and animal growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page289">289–94</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295–7</a>;</li>
-  <li class="i1">physiological units and heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page317">317–9</a>;</li>
-  <li class="hangingindent1">variation and parental functional condition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page324">324</a>;</li>
-  <li class="i1">uterine environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page327">327–8</a>;</li>
-  <li class="hangingindent1">physiological units and variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page330">330–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page458">458</a>;</li>
-  <li class="i1">fertilized and unfertilized ova, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page340">340–1</a>;</li>
-  <li class="i1">hermaphrodism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page341">341–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page344">344</a>;</li>
-  <li class="i1">sociological parallel, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page366">366–8</a>;</li>
-  <li class="hangingindent1">evolution hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page434">434</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page436">436</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page453">453</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page454">454</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page555">555</a>;</li>
-  <li class="i1">petrel development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page455">455</a>;</li>
-  <li class="hangingindent1">substitution and suppression of organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page456">456–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page466">466</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page472">472–3</a>;</li>
-  <li class="hangingindent1">structural proclivities of physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page458">458</a>;</li>
-  <li class="i1">abridgment of stages, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page458">458–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page464">464</a>;</li>
-  <li class="hangingindent1">disappearance of intermediate forms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page459">459–60</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page463">463</a>;</li>
-  <li class="i1">pre-adaptation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page461">461–3</a>;</li>
-  <li class="hangingindent1">discrimination of species in early stages, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page461">461</a>;</li>
-  <li class="hangingindent1">anomalous persistence of ancestral traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page463">463–5</a>;</li>
-  <li class="i1">phylogeny, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page486">486</a>;</li>
-  <li class="i1">egg-shell function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page527">527</a>;</li>
-  <li class="hangingindent1">genesis of grades in social insects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page654">654–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page658">658–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page679">679–80</a>;</li>
-  <li class="i1">determination of sex, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page657">657</a>;</li>
-  <li class="i1">order of development qualified by needs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page679">679</a>;</li>
-  <li class="i1">osteology of Punjabis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>;</li>
-  <li class="hangingindent1">direct transformations and physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page706">706</a>;</li>
-  <li class="i1">transformation of blastema, <b>II</b>,
-    <a href="#Page_20">20</a>;</li>
-  <li class="i1">arrest of growth and innutrition, <b>II</b>,
-    <a href="#Page_73">73</a>;</li>
-  <li class="hangingindent1">development of segmented animals, <b>II</b>,
-    <a href="#Page_100">100–2</a>,
-    <a href="#Page_602">602</a>;</li>
-  <li class="hangingindent1">adaptive vertebrate segmentation, <b>II</b>,
-    <a href="#Page_118">118–23</a>,
-    <a href="#Page_124">124</a>,
-    <a href="#Page_223">223–4</a>,
-    <a href="#Page_605">605–6</a>;</li>
-  <li class="i1">animal cell morphology, <b>II</b>,
-    <a href="#Page_228">228</a>;</li>
-  <li class="hangingindent1">primary differentiations of germinal layers, <b>II</b>,
-    <a href="#Page_300">300–2</a>;</li>
-  <li class="i1">lung development, <b>II</b>,
-    <a href="#Page_333">333–4</a>;</li>
-  <li class="i1">mammalian ova-maturation, <b>II</b>,
-    <a href="#Page_342">342–3</a>;</li>
-  <li class="i1">movements of ova, <b>II</b>,
-    <a href="#Page_356">356</a>,
-    <a href="#Page_363">363</a>;</li>
-  <li class="i1">modifications in mole, <b>II</b>,
-    <a href="#Page_391">391</a>;</li>
-  <li class="i1">genesis and nutrition, <b>II</b>,
-    <a href="#Page_424">424</a>,
-    <a href="#Page_425">425</a>;</li>
-  <li class="i1">fish ova, <b>II</b>,
-    <a href="#Page_435">435</a>,
-    <a href="#Page_454">454</a>;</li>
-  <li class="i1">cost of genesis, <b>II</b>,
-    <a href="#Page_435">435–6</a>;</li>
-  <li class="i1">number of birds’ eggs, <b>II</b>,
-    <a href="#Page_454">454–6</a>,
-    <a href="#Page_478">478</a>;</li>
-  <li class="i1">heat and genesis, <b>II</b>,
-    <a href="#Page_468">468</a>,
-    <a href="#Page_474">474</a>;</li>
-  <li class="i1">activity and genesis in birds, <b>II</b>,
-    <a href="#Page_470">470–2</a>,
-    <a href="#Page_474">474</a>;</li>
-  <li class="i1">vertebrate limb development, <b>II</b>,
-    <a href="#Page_553">553</a>;</li>
-  <li class="i1">ossification in vertebrates, <b>II</b>,
-    <a href="#Page_556">556</a>;</li>
-  <li class="i1">Owen’s vertebrate theory, <b>II</b>,
-    <a href="#Page_563">563</a>;</li>
-  <li class="i1">development of vertebræ, <b>II</b>,
-    <a href="#Page_564">564</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Multiplication">Multiplication</a>.)</li>
-
-  <li>Embryology of conceptions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page451">451</a>.</li>
-
-  <li class="hangingindent">Emigrants, type of organization among, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364</a>; <b>II</b>,
-    <a href="#Page_620">620</a>.</li>
-
-  <li>Endoderm: functional differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page202">202</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page203">203</a>;</li>
-  <li class="i1">functional vicariousness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page209">209</a>.</li>
-
-  <li>Endogen, application of term, <b>II</b>,
-    <a href="#Page_62">62</a>,
-    <a href="#Page_78">78</a>,
-    <a href="#Page_82">82</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Monocotyledons">Monocotyledons</a>.)</li>
-
-  <li id="Energy">Energy: evolution of, in animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page71">71–7</a>;</li>
-  <li class="i1">organic growth and expenditure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page141">141</a>;</li>
-  <li class="i1">functional transfer, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page201">201–6</a>;</li>
-  <li class="hangingindent1">chromatin as the source of, in karyokinesis, <b>II</b>,
-    <a href="#Page_261">261–5</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Force">Force</a>.)</li>
-
-  <li id="Entozoa"><i>Entozoa</i>: metagenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page273">273</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page641">641</a>;</li>
-  <li class="i1">self-fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page342">342</a>;</li>
-  <li class="i1">distribution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page398">398</a>;</li>
-  <li class="i1">and special creation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page428">428</a>;</li>
-  <li class="i1">fission in simple types, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page584">584</a>;</li>
-  <li class="i1">nutrition and genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page641">641</a>; <b>II</b>,
-    <a href="#Page_488">488</a>;</li>
-  <li class="i1">direct transformation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page706">706</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_102">102</a>;</li>
-  <li class="i1">segmentation, <b>II</b>,
-    <a href="#Page_107">107</a>,
-    <a href="#Page_108">108</a>;</li>
-  <li class="hangingindent1">interdependence and organic integration, <b>II</b>,
-    <a href="#Page_404">404</a>.</li>
-
-  <li class="hangingindent">Environment: degree of life and complexity of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page104">104–7</a>;</li>
-  <li class="hangingindent1">relation to organic structure and function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page172">172–8</a>; <b>II</b>,
-    <a href="#Page_12">12–5</a>;</li>
-  <li class="hangingindent1">adaptation to varied media an evidence of evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page479">479–81</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">influence of solar system, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page500">500</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">inherited adaptation to, <b>II</b>,
-    <a href="#Page_623">623</a>.</li>
-
-  <li><i>Eolis</i>, branchiæ, <b>II</b>,
-    <a href="#Page_118">118</a>.</li>
-
-  <li>Epidermis (<i>see</i> <a href="#Skin">Skin</a>).</li>
-
-  <li class="hangingindent">Epilepsy: definition of life and movements in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page112">112</a>;</li>
-  <li class="i1">heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page312">312</a>.</li>
-
-  <li>Epithelium: ciliated, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page187">187</a>;</li>
-  <li class="i1">Hertwig’s classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page189">189</a>;</li>
-  <li class="i1">reproductive function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page280">280</a>;</li>
-  <li class="i1">“pavement” and “cylinder,” <b>II</b>,
-    <a href="#Page_229">229</a>.</li>
-
-  <li><i>Epizoa</i>: distribution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page398">398</a>;</li>
-  <li class="i1">special creation and effects of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page428">428</a>;</li>
-  <li class="hangingindent1">interdependence and organic integration, <b>II</b>,
-    <a href="#Page_404">404</a>;</li>
-  <li class="i1">nutrition and genesis, <b>II</b>,
-    <a href="#Page_487">487</a>.</li>
-
-  <li>Equilibration: variation and law of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page326">326</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page334">334</a>;</li>
-  <li class="i1">molecular arrangement, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page337">337–45</a>;</li>
-  <li class="i1">of organic change, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page348">348</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page347">347</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">direct and indirect, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page519">519–22</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page573">573</a>;</li>
-  <li class="i1">adaptation by direct, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page522">522–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page551">551</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="hangingindent1">nutrition, defence, and fertilization of plants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page523">523–5</a>;</li>
-  <li class="i1">direct of animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page525">525–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page551">551</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="hangingindent1">natural selection and indirect, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page530">530–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page552">552</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">of natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page543">543–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page552">552–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">increasing importance of direct, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>;</li>
-  <li class="hangingindent1">of forces acting on species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page571">571–2</a>; <b>II</b>,
-    <a href="#Page_417">417–20</a>;</li>
-  <li class="i1">phenomena not accounted for by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page573">573</a>;</li>
-  <li class="i1">tissue differentiation, <b>II</b>,
-    <a href="#Page_245">245</a>;</li>
-  <li class="i1">genesis of nervous system, <b>II</b>,
-    <a href="#Page_307">307–8</a>;</li>
-  <li class="i1">functional, <b>II</b>,
-    <a href="#Page_391">391–4</a>;</li>
-  <li class="i1">laws of multiplication, <b>II</b>,
-    <a href="#Page_411">411–6</a>;</li>
-  <li class="i1">in human and social evolution, <b>II</b>,
-    <a href="#Page_537">537</a>.</li>
-  <li class="hangingindent1">(<i>See also</i> <a href="#Acquired_characters">Acquired characters</a> <i>and</i> <a href="#Natural_selection">Natural selection</a>.)</li>
-
-  <li class="hangingindent">Errera, L., on inherited adaptation to environment in Aspergillus, <b>II</b>,
-    <a href="#Page_623">623</a>.</li>
-
-  <li>Ethnology: heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page303">303–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page310">310</a>;</li>
-  <li class="i1">plasticity of mixed races, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page354">354</a>;</li>
-  <li class="i1">primitive ideas, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page417">417</a>;</li>
-  <li class="i1">evolution and classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page441">441–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page446">446</a>;</li>
-  <li class="i1">natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>.</li>
-
-  <li class="hangingindent"><i>Euphorbiaceæ</i>: foliar and axial development, <b>II</b>,
-    <a href="#Page_47">47–8</a>;</li>
-  <li class="i1">physiological differentiation, <b>II</b>,
-    <a href="#Page_258">258</a>;</li>
-  <li class="i1">dye permeability and circulation, <b>II</b>,
-    <a href="#Page_571">571</a>;</li>
-  <li class="i1">wood formation, <b>II</b>,
-    <a href="#Page_575">575</a>,
-    <a href="#Page_577">577</a>,
-    <a href="#Page_578">578</a>;</li>
-  <li class="i1">foliar vascular system, <b>II</b>,
-    <a href="#Page_589">589–92</a>,
-    <a href="#Page_596">596</a>.</li>
-
-  <li>Evaporation: organic change, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page28">28</a>;</li>
-  <li class="i1">vegetal circulation, <b>II</b>,
-    <a href="#Page_587">587</a>.</li>
-
-  <li class="hangingindent" id="Evolution">Evolution: chemical elements and compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page22">22–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page67">67</a>;</li>
-  <li class="hangingindent1">primordial form of living matter, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page63">63–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page181">181</a>; <b>II</b>,
-    <a href="#Page_21">21–2</a>;</li>
-  <li class="i1">definitions of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page107">107–10</a>;</li>
-  <li class="i1">growth the primary trait of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page135">135</a>;</li>
-  <li class="i1">comprehends growth and development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page162">162</a>;</li>
-  <li class="i1">illustrations in development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page167">167–70</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page178">178–9</a>;</li>
-  <li class="hangingindent1">progressive structural differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page181">181–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page192">192–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page211">211–2</a>;</li>
-  <li class="i1">life before organization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page210">210</a>;</li>
-  <li class="i1">heterogeneity of function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page211">211</a>;</li>
-  <li class="i1">stability of species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page242">242</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page515">515</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page518">518</a>;</li>
-  <li class="i1">individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page247">247</a>;</li>
-  <li class="i1">cell-organization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page262">262</a>;</li>
-  <li class="hangingindent1">genesis, heredity, and variation resulting from, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page354">354–5</a>;</li>
-  <li class="i1">period required for organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page407">407</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page565">565–6</a>;</li>
-  <li class="hangingindent1">contrasted with special creation hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page415">415</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page431">431–40</a>;</li>
-  <li class="i1">derivation of hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page431">431</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page439">439</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page554">554</a>;</li>
-  <li class="i1">increasing belief in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page431">431–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page439">439</a>;</li>
-  <li class="hangingindent1">experiences supporting conceivability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page433">433–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page439">439</a>;</li>
-  <li class="i1">direct evidence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page435">435–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page439">439</a>;</li>
-  <li class="i1">malevolence not implied by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page437">437–9</a>;</li>
-  <li class="hangingindent1">evidence from classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page443">443</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page444">444</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page449">449</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page466">466</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page555">555</a>;</li>
-  <li class="i1">embryology, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page451">451–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page466">466</a>;</li>
-  <li class="hangingindent1">substitution and suppression of organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page456">456–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page466">466</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page472">472–3</a>;</li>
-  <li class="i1">insect segmentation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page468">468–9</a>;</li>
-  <li class="i1">vertebral column development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page470">470–2</a>;</li>
-  <li class="i1">rudimentary organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page472">472–5</a>;</li>
-  <li class="i1">adaptation to varied media, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page479">479–85</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">growth of the theory of organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page490">490–8</a>;</li>
-  <li class="hangingindent1">instability of the homogeneous, a cause, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page509">509–11</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page516">516</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page550">550</a>;</li>
-  <li class="hangingindent1">multiplication of effects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page511">511–14</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page517">517–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page550">550</a>; <b>II</b>,
-    <a href="#Page_405">405</a>;</li>
-  <li class="hangingindent1">segregation, and heterogeneity and definiteness of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page514">514–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page550">550</a>;</li>
-  <li class="hangingindent1">natural selection and general doctrine of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page543">543–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">factors tabulated, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page551">551</a>;</li>
-  <li class="i1">inductive evidences summarized, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page555">555–6</a>;</li>
-  <li class="i1">surviving disbelief in France, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page559">559</a>;</li>
-  <li class="i1">current theories of organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page559">559–74</a>;</li>
-  <li class="i1">Eimer’s theory of orthogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page563">563–4</a>;</li>
-  <li class="i1">Gulick on monotypic and polytypic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page569">569</a>;</li>
-  <li class="i1">phenomena unexplained by theories, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page573">573–4</a>;</li>
-  <li class="hangingindent1">inorganic and the <i>System of Philosophy</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page696">696</a>;</li>
-  <li class="i1">“spontaneous generation,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page696">696–701</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page702">702</a>;</li>
-  <li class="i1">dissolution and problems of morphology, <b>II</b>,
-    <a href="#Page_4">4–6</a>;</li>
-  <li class="i1">morphology and formula, <b>II</b>,
-    <a href="#Page_7">7–9</a>,
-    <a href="#Page_231">231–5</a>;</li>
-  <li class="i1">difficulties of definition, <b>II</b>,
-    <a href="#Page_17">17</a>;</li>
-  <li class="i1">cell-doctrine, <b>II</b>,
-    <a href="#Page_17">17–21</a>,
-    <a href="#Page_85">85</a>;</li>
-  <li class="i1">unicellular origin of plants, <b>II</b>,
-    <a href="#Page_21">21–2</a>;</li>
-  <li class="i1"><i>résumé</i> of plant-morphology, <b>II</b>,
-    <a href="#Page_78">78–80</a>;</li>
-  <li class="hangingindent1">origin and differentiation of phænogamic type, <b>II</b>,
-    <a href="#Page_83">83</a>;</li>
-  <li class="i1">physiological problems, <b>II</b>,
-    <a href="#Page_239">239–43</a>;</li>
-  <li class="i1">tissue differentiation, <b>II</b>,
-    <a href="#Page_244">244–6</a>,
-    <a href="#Page_385">385</a>;</li>
-  <li class="i1">integration of organic world, <b>II</b>,
-    <a href="#Page_396">396</a>,
-    <a href="#Page_406">406</a>;</li>
-  <li class="i1">race and individual multiplication, <b>II</b>,
-    <a href="#Page_428">428–30</a>;</li>
-  <li class="i1">declining fertility and human, <b>II</b>,
-    <a href="#Page_431">431</a>,
-    <a href="#Page_529">529–30</a>;</li>
-  <li class="i1">individuation, genesis, and, <b>II</b>,
-    <a href="#Page_501">501–5</a>;</li>
-  <li class="i1">human life, prospective, <b>II</b>,
-    <a href="#Page_522">522–5</a>;</li>
-  <li class="i1">forces influencing human, <b>II</b>,
-    <a href="#Page_525">525–8</a>;</li>
-  <li class="i1">future of population, <b>II</b>,
-    <a href="#Page_532">532–7</a>;</li>
-  <li class="i1">self-sufficingness of, <b>II</b>,
-    <a href="#Page_537">537</a>;</li>
-  <li class="i1">vertebral, <b>II</b>,
-    <a href="#Page_563">563–6</a>.</li>
-
-  <li>Excretion: genesis of organs of, <b>II</b>,
-    <a href="#Page_303">303</a>;</li>
-  <li class="i1">localization of, <b>II</b>,
-    <a href="#Page_331">331–3</a>.</li>
-
-  <li>Exogen, application of term, <b>II</b>,
-    <a href="#Page_82">82</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Dicotyledons">Dicotyledons</a>.)</li>
-
-  <li>Expenditure (<i>see</i> <a href="#Multiplication">Multiplication</a>).</li>
-
-  <li class="hangingindent">Eye, the: molecular transformations in visual process, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page75">75–6</a>;</li>
-  <li class="hangingindent1">progressive development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page195">195</a>; <b>II</b>,
-    <a href="#Page_317">317–9</a>;</li>
-  <li class="i1">waste and repair, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page218">218</a>;</li>
-  <li class="i1">transmitted defects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page306">306</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page311">311</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page694">694</a>;</li>
-  <li class="hangingindent1">degeneration in cave-animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page309">309</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page614">614</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page647">647–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">late development in insects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page658">658</a>;</li>
-  <li class="i1">migration in flat fishes, <b>II</b>,
-    <a href="#Page_205">205</a>.</li>
-</ul>
-
-<ul>
-  <li class="hangingindent">Fabre, J. H., nutrition and sex in <i>Osmia tricornis</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page657">657</a>.</li>
-
-  <li>False joints, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page230">230</a>;</li>
-  <li class="hangingindent1">theories of heredity and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page362">362</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364</a>; <b>II</b>,
-    <a href="#Page_371">371–2</a>.</li>
-
-  <li class="hangingindent">Fats, the: physical and chemical properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page10">10–12</a>;</li>
-  <li class="i1">non-nitrogenous, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page41">41</a>;</li>
-  <li class="i1">action of bile, <b>II</b>,
-    <a href="#Page_330">330</a>.</li>
-
-  <li>Fatty degeneration, and failing vitality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page41">41</a>.</li>
-
-  <li>Feathers, development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page474">474</a>; <b>II</b>,
-    <a href="#Page_314">314–6</a>.</li>
-
-  <li>Feet, heredity and size, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page311">311</a>.</li>
-
-  <li class="hangingindent">Ferments, changes and nitrogenous character of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page38">38</a>.</li>
-
-  <li>Ferns: foliar development and nutrition, <b>II</b>,
-    <a href="#Page_76">76</a>;</li>
-  <li class="i1">inner tissue differentiation, <b>II</b>,
-    <a href="#Page_273">273</a>;</li>
-  <li class="i1">indefiniteness, <b>II</b>,
-    <a href="#Page_296">296</a>;</li>
-  <li class="i1">genesis, <b>II</b>,
-    <a href="#Page_441">441</a>,
-    <a href="#Page_463">463</a>.</li>
-
-  <li>Fertility, the General Law of Animal, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page577">577–601</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Multiplication">Multiplication</a>.)</li>
-
-  <li class="hangingindent" id="Fertilization">Fertilization: unit-life of generative elements, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page185">185–6</a>;</li>
-  <li class="i1">the function of chromatin, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page260">260</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page263">263–5</a>;</li>
-  <li class="i1">extrusion of polar bodies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page266">266–8</a>;</li>
-  <li class="hangingindent1">nature and functions of generative elements, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279–83</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page317">317</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page334">334</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page342">342</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page593">593–7</a>;</li>
-  <li class="hangingindent1">differentiation and variation effected by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page330">330–2</a>;</li>
-  <li class="hangingindent1">the essential object of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page340">340–1</a>; <b>II</b>,
-    <a href="#Page_614">614–6</a>;</li>
-  <li class="i1">hermaphrodism and self-, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page341">341–2</a>;</li>
-  <li class="i1">crossing and its effects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page343">343–7</a>;</li>
-  <li class="i1">isolation of species in respect of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page570">570</a>;</li>
-  <li class="i1">floral (<i>see</i> <a href="#Flowers">Flowers</a>).</li>
-
-  <li><i>Ficus</i>, foliar structure, <b>II</b>,
-    <a href="#Page_589">589</a>,
-    <a href="#Page_596">596</a>.</li>
-
-  <li>Fingers: embryogeny of human, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page169">169</a>;</li>
-  <li class="i1">heredity and abnormal, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page305">305</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page321">321–3</a>;</li>
-  <li class="i1">autogenous development of supernumerary, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page363">363</a>;</li>
-  <li class="i1">rudimentary, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page473">473</a>.</li>
-
-  <li>Fishes: sizes of ova and adult, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page143">143–4</a>;</li>
-  <li class="i1">growth of pike, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page154">154</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page292">292</a>;</li>
-  <li class="i1">size and environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page156">156</a>;</li>
-  <li class="i1">temperature, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page174">174</a>;</li>
-  <li class="i1">self-mobility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page175">175</a>;</li>
-  <li class="i1">continuity of blastomeres, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page214">214</a>; <b>II</b>,
-    <a href="#Page_327">327</a>;</li>
-  <li class="i1">genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>; <b>II</b>,
-    <a href="#Page_435">435</a>,
-    <a href="#Page_436">436</a>;</li>
-  <li class="hangingindent1">conditions affecting genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page292">292–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page583">583</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page598">598</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page599">599</a>; <b>II</b>,
-    <a href="#Page_454">454</a>;</li>
-  <li class="i1">classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page392">392</a>;</li>
-  <li class="i1">change of media, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page401">401</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page480">480</a>;</li>
-  <li class="i1">distribution in time, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page408">408–9</a>;</li>
-  <li class="i1">climbing species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page480">480</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page482">482</a>;</li>
-  <li class="i1">migrations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page500">500</a>;</li>
-  <li class="i1">dermal structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page526">526</a>; <b>II</b>,
-    <a href="#Page_305">305–6</a>,
-    <a href="#Page_315">315</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="hangingindent1">Cunningham on non-adaptive specific characters, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page565">565</a>;</li>
-  <li class="i1">elongation and locomotion, <b>II</b>,
-    <a href="#Page_15">15</a>;</li>
-  <li class="i1">segmentation, <b>II</b>,
-    <a href="#Page_122">122</a>,
-    <a href="#Page_225">225</a>;</li>
-  <li class="i1">bilateral symmetry, <b>II</b>,
-    <a href="#Page_203">203–5</a>;</li>
-  <li class="i1">eyes of <i>Pleuronectidæ</i>, <b>II</b>,
-    <a href="#Page_205">205</a>;</li>
-  <li class="hangingindent1">genesis of vertebrate axis, <b>II</b>,
-    <a href="#Page_212">212–6</a>,
-    <a href="#Page_218">218–21</a>,
-    <a href="#Page_225">225</a>;</li>
-  <li class="i1">ossification of paleozoic, <b>II</b>,
-    <a href="#Page_218">218</a>;</li>
-  <li class="i1">respiratory organs, <b>II</b>,
-    <a href="#Page_334">334–8</a>;</li>
-  <li class="i1">activity and muscular colour, <b>II</b>,
-    <a href="#Page_365">365–9</a>;</li>
-  <li class="hangingindent1">Owen on skeleton, <b>II</b>,
-    <a href="#Page_552">552</a>,
-    <a href="#Page_557">557</a>,
-    <a href="#Page_558">558–60</a>,
-    <a href="#Page_562">562</a>,
-    <a href="#Page_564">564</a>.</li>
-
-  <li>Fission (<i>see</i> <a href="#Agamogenesis">Agamogenesis</a>).</li>
-
-  <li>Flint, Austin, on telegony, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page644">644</a>.</li>
-
-  <li>Flounder, symmetry and eyes, <b>II</b>,
-    <a href="#Page_205">205</a>.</li>
-
-  <li>Flower, Sir W., on ferret, <b>II</b>,
-    <a href="#Page_480">480</a>.</li>
-
-  <li id="Flowers">Flowers: pollen propulsion in orchids, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page57">57</a>;</li>
-  <li class="i1">nature of reproductive elements, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page283">283</a>;</li>
-  <li class="hangingindent1">insect fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page340">340</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page525">525</a>; <b>II</b>,
-    <a href="#Page_168">168</a>,
-    <a href="#Page_174">174</a>,
-    <a href="#Page_267">267</a>,
-    <a href="#Page_407">407</a>;</li>
-  <li class="i1">self- and mutual fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page342">342–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page570">570</a>;</li>
-  <li class="i1">Darwin on homologies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page472">472</a>;</li>
-  <li class="hangingindent1">direct equilibration and fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page525">524–5</a>;</li>
-  <li class="i1">dimorphism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page534">534</a>;</li>
-  <li class="i1">foliar homology of petals, <b>II</b>,
-    <a href="#Page_43">43–6</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_132">132</a>,
-    <a href="#Page_161">161</a>,
-    <a href="#Page_162">162–4</a>,
-    <a href="#Page_170">170</a>,
-    <a href="#Page_174">174</a>,
-    <a href="#Page_608">608</a>;</li>
-  <li class="i1">fertilization and symmetry, <b>II</b>,
-    <a href="#Page_164">164–70</a>;</li>
-  <li class="i1">clusters and components, <b>II</b>,
-    <a href="#Page_170">170–4</a>;</li>
-  <li class="hangingindent1">nutrition and inflorescence, <b>II</b>,
-    <a href="#Page_179">179–80</a>,
-    <a href="#Page_541">541–2</a>,
-    <a href="#Page_546">546–7</a>;</li>
-  <li class="i1">tissue differentiation, <b>II</b>,
-    <a href="#Page_265">265–9</a>;</li>
-  <li class="hangingindent1">separation of ancestral traits in hybrids, <b>II</b>,
-    <a href="#Page_616">616–7</a>.</li>
-
-  <li>Fly, beneficial parasitism, <b>II</b>,
-    <a href="#Page_406">406</a>.</li>
-
-  <li>Food (<i>see</i> <a href="#Nutrition">Nutrition</a>).</li>
-
-  <li class="hangingindent">Food-cavity, genesis and development of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page188">188</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page195">195</a>.</li>
-
-  <li><i>Foraminifera</i>: form, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page173">173</a>;</li>
-  <li class="i1">primary aggregate, <b>II</b>,
-    <a href="#Page_87">87</a>,
-    <a href="#Page_124">124</a>;</li>
-  <li class="i1">progressing integration, <b>II</b>,
-    <a href="#Page_89">89–90</a>,
-    <a href="#Page_124">124</a>.</li>
-
-  <li id="Force">Force: action on like and unlike units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page5">5</a>;</li>
-  <li class="i1">expenditure and organic growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page149">149–54</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page161">161</a>;</li>
-  <li class="hangingindent1">functional accumulation, transfer, and expenditure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page198">198–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page201">201–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page391">391</a>;</li>
-  <li class="i1">waste and expenditure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page214">214–5</a>;</li>
-  <li class="i1">distribution during strain, <b>II</b>,
-    <a href="#Page_209">209–12</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Energy">Energy</a>, <i>and</i> <a href="#Persistence_of_force">Persistence of Force</a>.)</li>
-
-  <li>Fossils (<i>see</i> <a href="#Palaeontology">Palæontology</a>).</li>
-
-  <li>Foster, Sir M., on storage of glycogen, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page70">70</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page74">74</a>;</li>
-  <li class="hangingindent1">Increase of weight in hybernating dormouse, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page214">214</a>.</li>
-
-  <li>Fowls (see <a href="#Gallinaceae"><i>Gallinaceæ</i></a>).</li>
-
-  <li class="hangingindent">Foxglove: abnormal development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page287">287</a>; <b>II</b>,
-    <a href="#Page_46">46</a>;</li>
-  <li class="i1">floral distribution, <b>II</b>,
-    <a href="#Page_141">141</a>;</li>
-  <li class="i1">nutrition and growth, <b>II</b>,
-    <a href="#Page_179">179</a>.</li>
-
-  <li class="hangingindent">France: surviving disbelief in organic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page559">559</a>;</li>
-  <li class="i1">rate of multiplication, <b>II</b>,
-    <a href="#Page_509">509</a>,
-    <a href="#Page_512">512</a>.</li>
-
-  <li>Frankland, Sir E., on isomerism of protein, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page700">700</a>.</li>
-
-  <li class="hangingindent">Fraser, Col. A. T., on family of Hindu dwarfs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page316">316</a>.</li>
-
-  <li class="hangingindent">Fries, E., multiplication of <i>Reticularia</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page582">582</a>; <b>II</b>,
-    <a href="#Page_450">450</a>.</li>
-
-  <li>Frog: vitality of detached heart, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page111">111</a>;</li>
-  <li class="i1">of larval fragments, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page365">365</a>.</li>
-
-  <li>Fry, Sir E., on alternation of generations, <b>II</b>,
-    <a href="#Page_84">84</a>.</li>
-
-  <li><i>Fuci</i>: cell multiplication, <b>II</b>,
-    <a href="#Page_27">27</a>;</li>
-  <li class="i1">undifferentiated outer tissue, <b>II</b>,
-    <a href="#Page_256">256</a>.</li>
-
-  <li class="hangingindent" id="Function">Function: as a basis of classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page124">124–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page129">129–31</a>;</li>
-  <li class="hangingindent1">simultaneous progress of structure and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page197">197</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page211">211</a>;</li>
-  <li class="i1">divisions of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page198">198–200</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page391">391</a>;</li>
-  <li class="hangingindent1">correlative complexity of structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page200">200</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page210">210–1</a>;</li>
-  <li class="i1">progressive differentiations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page201">201–4</a>;</li>
-  <li class="i1">concomitant integration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page205">205–8</a>;</li>
-  <li class="i1">specialization and vicariousness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page208">208–10</a>;</li>
-  <li class="i1">formula of evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page211">211</a>;</li>
-  <li class="i1">diminished ability and overwork, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page215">215–6</a>;</li>
-  <li class="i1">growth and increased, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page228">228–33</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page234">234–5</a>;</li>
-  <li class="hangingindent1">interdependence of social and organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page237">237–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page240">240–2</a>;</li>
-  <li class="hangingindent1">structure and heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page306">306–13</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page318">318–9</a>
-    (<i>see</i> <a href="#Acquired_characters">Acquired Characters</a>);</li>
-  <li class="i1">aids natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page308">308</a>;</li>
-  <li class="i1">organic interdependence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page318">318–9</a>;</li>
-  <li class="i1">parental condition and variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page324">324</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page326">326</a>;</li>
-  <li class="i1">variation and altered, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page325">325–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page333">333–4</a>;</li>
-  <li class="i1">as causing variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page334">334–5</a>;</li>
-  <li class="hangingindent1">effect on physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page353">353–4</a>; <b>II</b>,
-    <a href="#Page_620">620</a>;</li>
-  <li class="i1">zoological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page391">391–3</a>;</li>
-  <li class="i1">multiplication of effects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page512">512</a>;</li>
-  <li class="i1">law of equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page519">519–22</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">correlation of changes in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page529">529</a>;</li>
-  <li class="i1">structural effects of changing, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page541">541–2</a>;</li>
-  <li class="i1">structural co-operation, <b>II</b>,
-    <a href="#Page_3">3</a>,
-    <a href="#Page_217">217</a>;</li>
-  <li class="i1">vicarious vegetal, <b>II</b>,
-    <a href="#Page_270">270</a>;</li>
-  <li class="i1">vicariousness and specialization, <b>II</b>,
-    <a href="#Page_293">293</a>;</li>
-  <li class="i1">epidermic structure, <b>II</b>,
-    <a href="#Page_312">312–4</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">structure and muscular, <b>II</b>,
-    <a href="#Page_369">369</a>,
-    <a href="#Page_391">391</a>;</li>
-  <li class="i1">adaptive bone-structures, <b>II</b>,
-    <a href="#Page_370">370–1</a>;</li>
-  <li class="i1">equilibration and adaptation, <b>II</b>,
-    <a href="#Page_392">392</a>;</li>
-  <li class="i1">persistence of force and adaptation, <b>II</b>,
-    <a href="#Page_394">394</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Physiology">Physiology</a>.)</li>
-
-  <li><i>Fungi</i>: nitrogenous character, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page40">40</a>;</li>
-  <li class="i1">development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page163">163</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page164">164</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>;</li>
-  <li class="i1">conjugation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279</a>; <b>II</b>,
-    <a href="#Page_449">449</a>;</li>
-  <li class="i1">fission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page584">584</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page585">585</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_24">24–5</a>,
-    <a href="#Page_293">293</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_137">137–40</a>,
-    <a href="#Page_146">146</a>;</li>
-  <li class="i1">puff-ball tissue, <b>II</b>,
-    <a href="#Page_246">246</a>,
-    <a href="#Page_252">252</a>,
-    <a href="#Page_386">386</a>;</li>
-  <li class="i1">tissue differentiation, <b>II</b>,
-    <a href="#Page_256">256</a>;</li>
-  <li class="i1">inner tissue, <b>II</b>,
-    <a href="#Page_279">279</a>;</li>
-  <li class="i1">indefiniteness, <b>II</b>,
-    <a href="#Page_295">295</a>;</li>
-  <li class="i1">growth and genesis, <b>II</b>,
-    <a href="#Page_459">459</a>;</li>
-  <li class="i1">nutrition and genesis, <b>II</b>,
-    <a href="#Page_487">487</a>.</li>
-</ul>
-
-<ul>
-  <li class="hangingindent" id="Gallinaceae"><i>Gallinaceæ</i>: conditions affecting fertility, <b>II</b>,
-    <a href="#Page_454">454–5</a>,
-    <a href="#Page_469">469</a>,
-    <a href="#Page_471">471</a>;</li>
-  <li class="i1">masculine traits of old hens, <b>II</b>,
-    <a href="#Page_495">495</a>.</li>
-
-  <li>Galls: definition of life and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page111">111</a>;</li>
-  <li class="i1">Hertwig on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page690">690</a>.</li>
-
-  <li>Galton, F., on variation outside the mean, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page669">669</a>.</li>
-
-  <li id="Gamogenesis">Gamogenesis: homogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page270">270</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page336">336</a>;</li>
-  <li class="i1">heterogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page270">270</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page336">336</a>;</li>
-  <li class="i1">independence of offspring, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page278">278</a>;</li>
-  <li class="i1">reproductive tissue, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279–84</a>;</li>
-  <li class="i1">vegetal nutrition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page285">285–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page293">293</a>; <b>II</b>,
-    <a href="#Page_39">39</a>;</li>
-  <li class="i1">animal nutrition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page289">289–94</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page297">297</a>;</li>
-  <li class="i1">when and why does it recur? <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page294">294–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page336">336–40</a>;</li>
-  <li class="i1">effect on species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page347">347–9</a>;</li>
-  <li class="i1">leaf formation, <b>II</b>,
-    <a href="#Page_39">39</a>;</li>
-  <li class="i1">alternating generation in liverworts, <b>II</b>,
-    <a href="#Page_80">80–4</a>;</li>
-  <li class="i1">molluscan homogenesis, <b>II</b>,
-    <a href="#Page_116">116</a>,
-    <a href="#Page_117">117–8</a>;</li>
-  <li class="i1">vertebrate, <b>II</b>,
-    <a href="#Page_118">118</a>;</li>
-  <li class="i1">growth, <b>II</b>,
-    <a href="#Page_266">266</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Fertilization">Fertilization</a>, <i>and</i> <a href="#Multiplication">Multiplication</a>.)</li>
-
-  <li><i>Gasteropoda</i> (see <a href="#Mollusca"><i>Mollusca</i></a>).</li>
-
-  <li class="hangingindent">Geddes and Thompson, on the determination of sex, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page657">657</a>.</li>
-
-  <li>Gelatine, nutritive value of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page77">77</a>.</li>
-
-  <li id="Gemmation">Gemmation: and genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page272">272–6</a>;</li>
-  <li class="i1">theories of heredity and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page361">361</a>;</li>
-  <li class="i1">annulose, <b>II</b>,
-    <a href="#Page_100">100–5</a>,
-    <a href="#Page_106">106</a>.</li>
-
-  <li>Generalization, impossibility of perfect, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page450">450</a>.</li>
-
-  <li>Generation, and genesis: the words, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page269">269</a>.</li>
-
-  <li>Genesis (<i>see</i> <a href="#Multiplication">Multiplication</a>).</li>
-
-  <li><i>Gentiana</i>: floral arrangement, <b>II</b>,
-    <a href="#Page_608">608–11</a>.</li>
-
-  <li>Genus: indefinite value, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page389">389</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page446">446</a>;</li>
-  <li class="hangingindent1">instability of homogeneous and heterogeneity of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page509">509–11</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page515">515</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page517">517–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page550">550</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>.</li>
-
-  <li>Geology: growth displayed in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page135">135</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page136">136</a>;</li>
-  <li class="i1">distribution in time, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page404">404–11</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page412">412</a>;</li>
-  <li class="i1">special creation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page419">419</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page426">426</a>;</li>
-  <li class="i1">evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page432">432</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page437">437</a>;</li>
-  <li class="i1">record congruous with evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page485">485–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="hangingindent1">organic influence of changes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page501">501–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page549">549</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page550">550</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">climatic influence of changes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page503">503</a>;</li>
-  <li class="i1">time required for organic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page555">565–6</a>;</li>
-  <li class="i1">rise of insect and plant relations, <b>II</b>,
-    <a href="#Page_407">407</a>;</li>
-  <li class="i1">human evolution and changes, <b>II</b>,
-    <a href="#Page_534">534</a>.</li>
-
-  <li>Geometry, evolution illustrated by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page433">433–4</a>.</li>
-
-  <li>Germ-cell: unspecialized nature, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279–83</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page317">317</a>;</li>
-  <li class="i1">dissimilarity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page330">330</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page332">332</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page334">334</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page342">342</a>;</li>
-  <li class="i1">equilibrium, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page340">340</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Fertilization">Fertilization</a>.)</li>
-
-  <li id="Germ_plasm">Germ-plasm, Weismann’s theory of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page357">357–8</a>;</li>
-  <li class="i1">inconsistent with plant embryogeny, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page359">359</a>;</li>
-  <li class="i1">regeneration of lost limbs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page362">362</a>;</li>
-  <li class="hangingindent1">variations in peacock’s tail feather, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page372">372</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page695">695</a>; <b>II</b>,
-    <a href="#Page_618">618–9</a>;</li>
-  <li class="hangingindent1">alleged differentiation of reproductive and somatic cells, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page622">622</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page628">628–30</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page633">633–44</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646</a>;</li>
-  <li class="hangingindent1">origin of variations in neuter insects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page659">659</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page663">663–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page671">671</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>;</li>
-  <li class="i1">correlated variations in stag, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page677">677</a>;</li>
-  <li class="i1">insuperable difficulties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page682">682</a>;</li>
-  <li class="hangingindent1">conceivability of hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page695">695</a>; <b>II</b>,
-    <a href="#Page_619">619</a>;</li>
-  <li class="hangingindent1">correlated variations in cultivated plants, <b>II</b>,
-    <a href="#Page_621">621–2</a>.</li>
-
-  <li>Ghost-theory, Vitalism and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page114">114</a>.</li>
-
-  <li>Giraffe, co-adaptation of structures, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page615">615</a>.</li>
-
-  <li>Gizzard, development of birds, <b>II</b>,
-    <a href="#Page_320">320</a>.</li>
-
-  <li>Glass, molecular re-arrangement, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page337">337</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page352">352</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page704">704</a>.</li>
-
-  <li>Glove, strain analogy, <b>II</b>,
-    <a href="#Page_575">575</a>.</li>
-
-  <li>Glycogen, In animal metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page70">70</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page72">72</a>.</li>
-
-  <li class="hangingindent">Goethe, J. W. von: foliar homology, <b>II</b>,
-    <a href="#Page_43">43–4</a>,
-    <a href="#Page_543">543</a>,
-    <a href="#Page_544">544</a>;</li>
-  <li class="i1">archetypal hypothesis, <b>II</b>,
-    <a href="#Page_122">122</a>;</li>
-  <li class="i1">vegetal fructification and nutrition, <b>II</b>,
-    <a href="#Page_180">180</a>;</li>
-  <li class="i1">theory of supernumerary bones, <b>II</b>,
-    <a href="#Page_223">223</a>;</li>
-  <li class="i1">on the skull, <b>II</b>,
-    <a href="#Page_561">561</a>.</li>
-
-  <li>Gold, effect of bismuth on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page121">121</a>.</li>
-
-  <li>Gorilla, callosities, <b>II</b>,
-    <a href="#Page_312">312</a>.</li>
-
-  <li>Gould, J., <i>Birds of Australia</i>, <b>II</b>,
-    <a href="#Page_469">469</a>.</li>
-
-  <li>Gout (<i>see</i> <a href="#Disease">Disease</a>).</li>
-
-  <li class="hangingindent">Grafting, Born’s experiments with frog larvæ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page365">365</a>.</li>
-
-  <li class="hangingindent">Graham, T., properties of water, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>; <b>II</b>,
-    <a href="#Page_359">359</a>;</li>
-  <li class="hangingindent1">colloids and crystalloids, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page15">15–8</a>; <b>II</b>,
-    <a href="#Page_356">356</a>;</li>
-  <li class="i1">their diffusibility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page18">18–20</a>;</li>
-  <li class="i1">sapid and insipid substances, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page53">53</a>.</li>
-
-  <li><i>Graminæ</i>: foliar surfaces, <b>II</b>,
-    <a href="#Page_61">61</a>,
-    <a href="#Page_263">263</a>;</li>
-  <li class="i1">floral symmetry, <b>II</b>,
-    <a href="#Page_165">165</a>;</li>
-  <li class="i1">physiological differentiation, <b>II</b>,
-    <a href="#Page_257">257</a>.</li>
-
-  <li class="hangingindent">Graminivores, food contrasted with that of carnivores, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page68">68</a>.</li>
-
-  <li>Grassi, on food habits of <i>Termites</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page686">686</a>.</li>
-
-  <li>Gravity: its ultimate incomprehensibility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page121">121</a>;</li>
-  <li class="i1">vegetal circulation, <b>II</b>,
-    <a href="#Page_586">586</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Specific_Gravity">Specific Gravity</a>.)</li>
-
-  <li><i>Gregarina</i>: central development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page163">163</a>;</li>
-  <li class="i1">primary aggregate, <b>II</b>,
-    <a href="#Page_87">87</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_186">186</a>.</li>
-
-  <li>Grimaux, on artificial proteids, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page39">39</a>.</li>
-
-  <li id="Growth">Growth: organic and inorganic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page135">135–7</a>;</li>
-  <li class="i1">simulation of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page136">136</a>;</li>
-  <li class="i1">limits to, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page137">137</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page155">155–7</a>;</li>
-  <li class="i1">structural complexity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page138">138–40</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page145">145–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page161">161</a>;</li>
-  <li class="i1">nutrition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page140">140</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page147">147–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page161">161</a>;</li>
-  <li class="i1">expenditure of energy, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page141">141–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page161">161</a>;</li>
-  <li class="i1">initial and final bulks, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page143">143–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page157">157–60</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page161">161</a>;</li>
-  <li class="i1">final arrest of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page149">149–55</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page639">639</a>;</li>
-  <li class="i1">where unceasing, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page154">154</a>;</li>
-  <li class="i1"><i>résumé</i> with generalizations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page161">161</a>;</li>
-  <li class="i1">defined, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page162">162</a>; <b>II</b>,
-    <a href="#Page_461">461</a>;</li>
-  <li class="i1">increased function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page228">228–33</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page234">234–5</a>;</li>
-  <li class="i1">functional interdependence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page235">235–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page240">240</a>;</li>
-  <li class="hangingindent1">nutrition and vegetal, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page293">293</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page294">294–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page336">336</a>; <b>II</b>,
-    <a href="#Page_39">39</a>;</li>
-  <li class="hangingindent1">heterogenesis and animal nutrition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page289">289–93</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page296">296</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page336">336</a>;</li>
-  <li class="hangingindent1">homo- and heterogenesis and natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page294">294–8</a>;</li>
-  <li class="i1">of acrogens, <b>II</b>,
-    <a href="#Page_56">56</a>;</li>
-  <li class="i1">cylindrical form of vegetal, <b>II</b>,
-    <a href="#Page_56">56–64</a>;</li>
-  <li class="i1">endogenous, <b>II</b>,
-    <a href="#Page_60">60–2</a>,
-    <a href="#Page_78">78</a>;</li>
-  <li class="i1">exogenous, <b>II</b>,
-    <a href="#Page_63">63–4</a>,
-    <a href="#Page_78">78</a>;</li>
-  <li class="i1">plant differentiation, <b>II</b>,
-    <a href="#Page_129">129–131</a>;</li>
-  <li class="i1">tissue differentiation, <b>II</b>,
-    <a href="#Page_370">370</a>;</li>
-  <li class="hangingindent1">formation of adaptive bone-structures, <b>II</b>,
-    <a href="#Page_370">370–2</a>;</li>
-  <li class="hangingindent1">progressive increase of size with evolution, <b>II</b>,
-    <a href="#Page_401">401–2</a>;</li>
-  <li class="i1">vegetal, and asexual genesis, <b>II</b>,
-    <a href="#Page_439">439–42</a>;</li>
-  <li class="i1">animal, and asexual genesis, <b>II</b>,
-    <a href="#Page_442">442–5</a>;</li>
-  <li class="i1">antagonistic to asexual genesis, <b>II</b>,
-    <a href="#Page_446">446</a>;</li>
-  <li class="i1">vegetal and sexual genesis, <b>II</b>,
-    <a href="#Page_448">448–51</a>;</li>
-  <li class="i1">animal and sexual genesis, <b>II</b>,
-    <a href="#Page_452">452–6</a>,
-    <a href="#Page_495">495</a>;</li>
-  <li class="i1">antagonistic to sexual genesis, <b>II</b>,
-    <a href="#Page_457">457–8</a>;</li>
-  <li class="i1">nutrition and genesis, <i>résumé</i>, <b>II</b>,
-    <a href="#Page_497">497–9</a>;</li>
-  <li class="i1">evolution and, <b>II</b>,
-    <a href="#Page_501">501–5</a>;</li>
-  <li class="i1">commencement of genesis, <b>II</b>,
-    <a href="#Page_506">506</a>;</li>
-  <li class="hangingindent1">fertilization and restoration of growth-energy, <b>II</b>,
-    <a href="#Page_613">613</a>.</li>
-
-  <li class="hangingindent">Gulick, T.: on monotypic and polytypic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page569">569</a>;</li>
-  <li class="i1">physiological selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page569">569–71</a>.</li>
-
-  <li>Gunpowder, nitrogenous instability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page8">8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page43">43</a>.</li>
-
-  <li><i>Gymnotus</i>, electricity of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page51">51</a>.</li>
-
-  <li class="hangingindent"><i>Gyrodactylus elegans</i>, rapid succession of generations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page641">641</a>; <b>II</b>,
-    <a href="#Page_488">488</a>.</li>
-</ul>
-
-<ul>
-  <li>Habit, change of, in plants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page308">308</a>.</li>
-
-  <li>Hæmal, term applied to female element, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page594">594–5</a>.</li>
-
-  <li>Hairs: non-conductors of heat, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page526">526</a>;</li>
-  <li class="i1">vegetal, and natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page532">532</a>;</li>
-  <li class="i1">development, <b>II</b>,
-    <a href="#Page_314">314–6</a>;</li>
-  <li class="i1">tactual organs, <b>II</b>,
-    <a href="#Page_317">317</a>.</li>
-
-  <li>Hand: embryogeny, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page169">169</a>;</li>
-  <li class="i1">heredity and size of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page311">311</a>;</li>
-  <li class="i1">distribution of veins, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364</a>.</li>
-
-  <li>Hardy, W. B., <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#pagevii">vii</a>; <b>II</b>,
-    <a href="#Page_vi">vi</a>.</li>
-
-  <li>Hare: activity and muscular colour, <b>II</b>,
-    <a href="#Page_365">365</a>;</li>
-  <li class="i1">expenditure and genesis, <b>II</b>,
-    <a href="#Page_472">472</a>.</li>
-
-  <li>Hart, J. A., on “Parasol” ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page687">687–8</a>.</li>
-
-  <li>Havilland, G. D., collection of Termites, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page687">687</a>.</li>
-
-  <li>Haystack, chemical action in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page74">74</a>.</li>
-
-  <li>Head, structural influence of size, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page512">512</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page537">537</a>.</li>
-
-  <li>Hearing: the sense of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page54">54</a>;</li>
-  <li class="i1">multiplying agencies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page75">75</a>.</li>
-
-  <li>Heart (<i>see</i> <a href="#Vascular_System">Vascular System</a>).</li>
-
-  <li class="hangingindent" id="Heat">Heat: action on di- and tri-atomic compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page7">7–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page10">10–12</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page23">23</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page24">24</a>;</li>
-  <li class="i1">on colloids and crystalloids, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page26">26</a>;</li>
-  <li class="i1">organic changes from evaporation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page29">29</a>;</li>
-  <li class="i1">chemical decomposition by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page33">33</a>;</li>
-  <li class="i1">organic oxidation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page46">46–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page60">60</a>;</li>
-  <li class="i1">growth and organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page152">152–3</a>;</li>
-  <li class="i1">animal, vegetal, and environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page174">174–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page177">177</a>;</li>
-  <li class="i1">alloy melting points, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page339">339</a>;</li>
-  <li class="hangingindent1">organic effects of rhythm in terrestrial, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page498">498</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">effect on physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page705">705</a>;</li>
-  <li class="i1">respiration in fishes, <b>II</b>,
-    <a href="#Page_337">337</a>;</li>
-  <li class="i1">animal preservation, <b>II</b>,
-    <a href="#Page_434">434</a>;</li>
-  <li class="hangingindent1">vertebrate expenditure and genesis, <b>II</b>,
-    <a href="#Page_468">468–9</a>,
-    <a href="#Page_474">474</a>;</li>
-  <li class="i1">insect genesis, <b>II</b>,
-    <a href="#Page_476">476</a>;</li>
-  <li class="i1">seasonal variations and genesis, <b>II</b>,
-    <a href="#Page_484">484–5</a>;</li>
-  <li class="i1">in germination, <b>II</b>,
-    <a href="#Page_615">615</a>.</li>
-
-  <li>Hebrew idea of creation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page421">421</a>.</li>
-
-  <li><i>Hectocotylus</i>, individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page250">250</a>.</li>
-
-  <li>Hellin, D., on multiparity and twin-births, <b>II</b>,
-    <a href="#Page_457">457</a>.</li>
-
-  <li class="hangingindent">Hen, what prompts her to pick up egg-shell fragments? <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page120">120</a>.</li>
-
-  <li class="hangingindent">Henslow, Rev. G., inheritance of functionally-produced changes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page560">560</a>.</li>
-
-  <li id="Hepaticae"><i>Hepaticæ</i>: Schleiden on, <b>II</b>,
-    <a href="#Page_51">51</a>,
-    <a href="#Page_52">52</a>;</li>
-  <li class="i1">continuous and discontinuous development, <b>II</b>,
-    <a href="#Page_52">52</a>;</li>
-  <li class="i1">phyletic homologies, <b>II</b>,
-    <a href="#Page_80">80–4</a>;</li>
-  <li class="hangingindent1">meaning of so-called alternating generation, <b>II</b>,
-    <a href="#Page_84">84</a>;</li>
-  <li class="i1">vascular system, <b>II</b>,
-    <a href="#Page_280">280</a>;</li>
-  <li class="i1">genesis and development, <b>II</b>,
-    <a href="#Page_463">463</a>.</li>
-
-  <li>Heredity: structural modification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page232">232</a>;</li>
-  <li class="i1">function of cell-nucleus in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page258">258–59</a>;</li>
-  <li class="i1">general truths, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page301">301–4</a>;</li>
-  <li class="hangingindent1">transmission of congenital peculiarities, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page304">304–7</a>;</li>
-  <li class="hangingindent1">structure and altered function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page307">307–13</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page318">318–9</a>
-     (<i>see also</i> <a href="#Acquired_characters">Acquired Characters</a>);</li>
-  <li class="hangingindent1">atavism, or recurrence of ancestral traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314</a>;</li>
-  <li class="i1">sex limitation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314–6</a>;</li>
-  <li class="hangingindent1">physiological units, <i>résumé</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page350">350–5</a>; <b>II</b>,
-    <a href="#Page_612">612–6</a>;</li>
-  <li class="hangingindent1">Darwin’s and Weismann’s theories examined, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page356">356</a> <i>et seq.</i>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page559">559–61</a>; <b>II</b>,
-    <a href="#Page_622">622</a>;</li>
-  <li class="i1">true theory must include plants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page358">358</a>;</li>
-  <li class="hangingindent1">inadequacy of theory of physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page360">360–1</a>;</li>
-  <li class="i1">sociological parallel, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page366">366–8</a>;</li>
-  <li class="i1">natural selection (<i>q. v.</i>), <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page545">545–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">ethnology and natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>;</li>
-  <li class="i1">unsolved problems, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page573">573–4</a>;</li>
-  <li class="i1">mutilations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page631">631</a>;</li>
-  <li class="i1">ultimate process incomprehensible, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page695">695</a>;</li>
-  <li class="i1">cell-doctrine, <b>II</b>,
-    <a href="#Page_19">19</a>;</li>
-  <li class="i1">physiological development, <b>II</b>,
-    <a href="#Page_242">242</a>;</li>
-  <li class="i1">wood formation, <b>II</b>,
-    <a href="#Page_287">287</a>;</li>
-  <li class="i1">tissue differentiation, <b>II</b>,
-    <a href="#Page_304">304</a>,
-    <a href="#Page_312">312–4</a>;</li>
-  <li class="i1">respiratory system, <b>II</b>,
-    <a href="#Page_311">311</a>;</li>
-  <li class="i1">osseous differentiation, <b>II</b>,
-    <a href="#Page_351">351</a>;</li>
-  <li class="i1">muscular adaptation, <b>II</b>,
-    <a href="#Page_367">367</a>;</li>
-  <li class="hangingindent1">persistence of force and physiological adaptation, <b>II</b>,
-    <a href="#Page_394">394</a>;</li>
-  <li class="i1">vegetal vascular system, <b>II</b>,
-    <a href="#Page_574">574</a>,
-    <a href="#Page_582">582</a>,
-    <a href="#Page_588">588</a>,
-    <a href="#Page_596">596</a>.</li>
-
-  <li>Hermaphrodism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page340">340–3</a>.</li>
-
-  <li class="hangingindent">Hertwig, O.: on Weismann’s germ-plasm theory, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page690">690</a>;</li>
-  <li class="i1">cell characters, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page691">691</a>;</li>
-  <li class="i1">meaning of fertilization, <b>II</b>,
-    <a href="#Page_613">613</a>.</li>
-
-  <li>Hertwig, R., classification of tissues, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page189">189</a>.</li>
-
-  <li>Heterochrony of development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page655">655</a>.</li>
-
-  <li>Heterogeneity: in chemical evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page23">23–4</a>;</li>
-  <li class="i1">of vital changes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page84">84–90</a>;</li>
-  <li class="i1">of development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page170">170</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page178">178</a>;</li>
-  <li class="i1">functional, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page204">204–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page211">211–2</a>;</li>
-  <li class="i1">of organic matter, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page350">350–5</a>;</li>
-  <li class="hangingindent1">organic and instability of homogeneous, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page509">509–11</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page517">517</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page549">549</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">segregation accompanying, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page514">514–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page517">517–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page550">550</a>.</li>
-
-  <li>Heterogenesis: occurrence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page270">270</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page272">272–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page336">336</a>;</li>
-  <li class="i1">animal nutrition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page289">289–91</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295–7</a>;</li>
-  <li class="i1">natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295–8</a>;</li>
-  <li class="i1">heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page301">301</a>.</li>
-
-  <li>Hindus: food, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page68">68</a>;</li>
-  <li class="i1">dwarf family, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page316">316</a>.</li>
-
-  <li>Histology (<i>see</i> <a href="#Physiology">Physiology</a>).</li>
-
-  <li class="hangingindent">Hofmeister, sporophytic generation of Archegoniates, <b>II</b>,
-    <a href="#Page_80">80</a>.</li>
-
-  <li>Hollyhock, floral symmetry, <b>II</b>,
-    <a href="#Page_167">167</a>,
-    <a href="#Page_169">169</a>,
-    <a href="#Page_170">170</a>.</li>
-
-  <li class="hangingindent">Homogeneous, instability of the: variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page330">330</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page334">334</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page342">342</a>;</li>
-  <li class="i1">evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page509">509–11</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page517">517</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page549">549</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">morphological development, <b>II</b>,
-    <a href="#Page_7">7–9</a>,
-    <a href="#Page_234">234</a>;</li>
-  <li class="i1">direction of vegetal growth, <b>II</b>,
-    <a href="#Page_181">181</a>;</li>
-  <li class="i1">radial symmetry, <b>II</b>,
-    <a href="#Page_190">190</a>;</li>
-  <li class="i1">physiological differentiation, <b>II</b>,
-    <a href="#Page_384">384</a>,
-    <a href="#Page_392">392</a>.</li>
-
-  <li>Homogenesis (<i>see</i> <a href="#Gamogenesis">Gamogenesis</a>).</li>
-
-  <li>Homology, simulation of, by analogy, <b>II</b>,
-    <a href="#Page_14">14</a>.</li>
-
-  <li>Hooker, Sir J. D., <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#pageix">ix</a>;</li>
-  <li class="i1">European plants in New Zealand, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page477">477</a>;</li>
-  <li class="i1">plant distribution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page479">479</a>;</li>
-  <li class="i1">adaptation of plants to varied media, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page484">484</a>;</li>
-  <li class="i1">plant growth, <b>II</b>,
-    <a href="#Page_56">56</a>;</li>
-  <li class="i1"><i>Balanophoræ</i> and <i>Raffiesiaceæ</i>, <b>II</b>,
-    <a href="#Page_274">274</a>;</li>
-  <li class="i1">structural complexity, <b>II</b>,
-    <a href="#Page_295">295</a>,
-    <a href="#Page_297">297</a>;</li>
-  <li class="hangingindent1">relative antiquity and distribution of plants and animals, <b>II</b>,
-    <a href="#Page_297">297</a>;</li>
-  <li class="i1">bean vascular system, <b>II</b>,
-    <a href="#Page_574">574</a>.</li>
-
-  <li class="hangingindent">Hooker, Sir W., on fructification in <i>Jungermanniaceæ</i>, <b>II</b>,
-    <a href="#Page_52">52</a>.</li>
-
-  <li class="hangingindent">Horns, natural selection and correlated variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page537">537</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page567">567</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page674">674</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page677">677</a>.</li>
-
-  <li>Horse: ancestral types, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page409">409</a>;</li>
-  <li class="i1">fertility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page598">598</a>;</li>
-  <li class="i1">weight of brain, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page599">599</a>;</li>
-  <li class="i1">quagga markings, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page624">624</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page627">627</a>.</li>
-
-  <li>Husbandry, co-ordination of actions in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page96">96</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page579">579</a>.</li>
-
-  <li>Hutchinson, Sir J., hereditary syphilis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page623">623</a>.</li>
-
-  <li>Huxley, T. H., <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#pageix">ix</a>;</li>
-  <li class="hangingindent1">“continuous” and “discontinuous” development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page164">164</a>;</li>
-  <li class="i1">classification of development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page276">276</a>;</li>
-  <li class="i1">hermaphrodism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page344">344</a>;</li>
-  <li class="i1">zoological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page383">383</a>;</li>
-  <li class="i1">on “Persistent Types,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page408">408–9</a>;</li>
-  <li class="i1">ancestral equine types, <i>ib.</i>;</li>
-  <li class="hangingindent1">segmentation of articulates, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page468">468–9</a>; <b>II</b>,
-    <a href="#Page_113">113</a>;</li>
-  <li class="hangingindent1">agamic multiplication of <i>Aphis</i> and <i>Entozoa</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page640">640–1</a>; <b>II</b>,
-    <a href="#Page_476">476</a>;</li>
-  <li class="i1">cell-doctrine, <b>II</b>,
-    <a href="#Page_21">21</a>;</li>
-  <li class="i1">vertebrate embryo, <b>II</b>,
-    <a href="#Page_119">119</a>,
-    <a href="#Page_120">120</a>;</li>
-  <li class="i1">molluscan symmetry, <b>II</b>,
-    <a href="#Page_202">202</a>;</li>
-  <li class="i1">tegumentary organs, <b>II</b>,
-    <a href="#Page_314">314</a>,
-    <a href="#Page_315">315</a>;</li>
-  <li class="i1">vertebrate sensory organs, <b>II</b>,
-    <a href="#Page_318">318</a>,
-    <a href="#Page_319">319</a>;</li>
-  <li class="i1"><i>Chondracanthus</i>, <b>II</b>,
-    <a href="#Page_487">487</a>;</li>
-  <li class="i1">Owen’s vertebrate theory, <b>II</b>,
-    <a href="#Page_563">563</a>.</li>
-
-  <li>Hyacinth: lateral spike, <b>II</b>,
-    <a href="#Page_42">42</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_141">141</a>,
-    <a href="#Page_162">162</a>.</li>
-
-  <li>Hybernation, waste and repair in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page214">214–5</a>.</li>
-
-  <li class="hangingindent">Hybrids, separation of ancestral traits in, <b>II</b>,
-    <a href="#Page_616">616–7</a>.</li>
-
-  <li>Hydro-carbons: properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page6">6–9</a>;</li>
-  <li class="i1">the term carbo-hydrates (<i>q. v.</i>), <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page10">10</a>.</li>
-
-  <li>Hydrochloric acid, in gastric juice, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>.</li>
-
-  <li>Hydrogen: chemical and physical properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page3">3–5</a>;</li>
-  <li class="i1">compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page6">6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page8">8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page10">10–12</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page12">12–13</a>.</li>
-
-  <li><i>Hydrozoa</i> (see <a href="#Coelenterata"><i>Cœlenterata</i></a>).</li>
-
-  <li><i>Hymenoptera</i> (<i>see</i> <a href="#Insects">Insects</a>).</li>
-
-  <li>Hypertrophy (<i>see</i> <a href="#Disease">Disease</a>).</li>
-
-  <li>Hypospadias, telegonic transmission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646</a>.</li>
-
-  <li class="hangingindent">Hypostasis of a relation, exemplified in explanations of fertilization, <b>II</b>,
-    <a href="#Page_613">613</a>.</li>
-</ul>
-
-<ul>
-  <li>Ideas (<i>see</i> <a href="#Psychology">Psychology</a>).</li>
-
-  <li>Individuality: the botanical, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page244">244–6</a>;</li>
-  <li class="i1">the zoological, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page246">246–7</a>;</li>
-  <li class="i1">the fertilized germ product, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page248">248–9</a>;</li>
-  <li class="i1">definition of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page250">250</a>.</li>
-
-  <li class="hangingindent">Individuation: and genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page583">583–4</a>; <b>II</b>,
-    <a href="#Page_428">428–30</a>,
-    <a href="#Page_499">499</a>;</li>
-  <li class="i1">total cost, <b>II</b>,
-    <a href="#Page_435">435–7</a>;</li>
-  <li class="i1">genesis and evolution, <b>II</b>,
-    <a href="#Page_501">501–5</a>,
-    <a href="#Page_529">529</a>,
-    <a href="#Page_530">530</a>.</li>
-
-  <li><i>Infusoria</i>: functional specialization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page391">391</a>;</li>
-  <li class="i1">primary aggregate, <b>II</b>,
-    <a href="#Page_87">87</a>;</li>
-  <li class="i1">asymmetry, <b>II</b>,
-    <a href="#Page_187">187</a>,
-    <a href="#Page_188">188</a>;</li>
-  <li class="i1">differentiation, <b>II</b>,
-    <a href="#Page_299">299</a>,
-    <a href="#Page_385">385</a>;</li>
-  <li class="i1">genesis, <b>II</b>,
-    <a href="#Page_442">442</a>,
-    <a href="#Page_446">446</a>,
-    <a href="#Page_452">452</a>.</li>
-
-  <li class="hangingindent">Injuries, repair of animal, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page219">219</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page222">222–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page316">316</a>; <b>II</b>,
-    <a href="#Page_102">102</a>,
-    <a href="#Page_611">611</a>.</li>
-
-  <li>Insanity, inherited, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314</a>.</li>
-
-  <li id="Insects">Insects: temperature, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page47">47</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page174">174</a>;</li>
-  <li class="i1">phosphorescence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page49">49</a>;</li>
-  <li class="i1">self-mobility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page175">175</a>;</li>
-  <li class="i1">parthenogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page274">274–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page277">277</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page294">294</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page592">592</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page640">640</a>;</li>
-  <li class="i1">growth and reproduction, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page292">292</a>;</li>
-  <li class="hangingindent1">species distribution determined by presence of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page396">396–7</a>;</li>
-  <li class="hangingindent1">eyes of cave-inhabiting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page309">309</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page614">614</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page647">647–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">persistent types, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page408">408</a>;</li>
-  <li class="i1">retrograde development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page458">458</a>;</li>
-  <li class="i1">segmentation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page468">468–9</a>; <b>II</b>,
-    <a href="#Page_114">114</a>;</li>
-  <li class="i1">aborted organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page474">474</a>;</li>
-  <li class="i1">East Indian distribution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page478">478</a>;</li>
-  <li class="hangingindent1">floral fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page525">525</a>; <b>II</b>,
-    <a href="#Page_168">168–9</a>,
-    <a href="#Page_406">406–7</a>,
-    <a href="#Page_608">608</a>;</li>
-  <li class="i1">appliances for cleaning antennæ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page651">651</a>;</li>
-  <li class="i1">eyes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page658">658</a>; <b>II</b>,
-    <a href="#Page_318">318</a>;</li>
-  <li class="i1">integration and homology, <b>II</b>,
-    <a href="#Page_111">111–3</a>,
-    <a href="#Page_121">121</a>;</li>
-  <li class="i1">bilateral symmetry, <b>II</b>,
-    <a href="#Page_198">198</a>;</li>
-  <li class="i1">sexual selection, <b>II</b>,
-    <a href="#Page_269">269</a>;</li>
-  <li class="i1">eyes, <b>II</b>,
-    <a href="#Page_318">318</a>;</li>
-  <li class="i1">environment, <b>II</b>,
-    <a href="#Page_433">433</a>;</li>
-  <li class="i1">cost of genesis, <b>II</b>,
-    <a href="#Page_436">436</a>,
-    <a href="#Page_437">437</a>;</li>
-  <li class="i1">development and genesis, <b>II</b>,
-    <a href="#Page_461">461</a>;</li>
-  <li class="i1">nutrition and genesis, <b>II</b>,
-    <a href="#Page_476">476</a>,
-    <a href="#Page_490">490–2</a>.</li>
-
-  <li class="hangingindent">Insects, Social, origin of caste gradations in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page654">654–65</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page674">674</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page678">678–84</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page686">686–8</a>.</li>
-
-  <li>Instability of the homogeneous (<i>see</i> Homogeneous).</li>
-
-  <li class="hangingindent">Instinct: organic evolution and co-ordination of, in mason-wasp, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page574">574</a>;</li>
-  <li class="i1">a vital attribute, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page578">578</a>;</li>
-  <li class="hangingindent1">loss of self-feeding, in Amazon ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660–1</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page663">663–4</a>.</li>
-
-  <li>Integration: in chemical evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page23">23</a>;</li>
-  <li class="i1">morphological composition, <b>II</b>,
-    <a href="#Page_4">4–6</a>;</li>
-  <li class="i1">arthropod, <b>II</b>,
-    <a href="#Page_111">111–4</a>,
-    <a href="#Page_121">121</a>;</li>
-  <li class="i1">physiological, in plants, <b>II</b>,
-    <a href="#Page_292">292–5</a>,
-    <a href="#Page_295">295–8</a>,
-    <a href="#Page_390">390</a>;</li>
-  <li class="i1">of organic world, <b>II</b>,
-    <a href="#Page_396">396–408</a>;</li>
-  <li class="i1">genesis, <b>II</b>,
-    <a href="#Page_424">424</a>,
-    <a href="#Page_426">426–9</a>.</li>
-
-  <li>Intelligence, a vital attribute, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page579">579</a>.</li>
-
-  <li>Internodes: varied development, <b>II</b>,
-    <a href="#Page_45">45</a>;</li>
-  <li class="i1">nutrition and length, <b>II</b>,
-    <a href="#Page_178">178–9</a>.</li>
-
-  <li>Intestine (<i>see</i> <a href="#Alimentary_canal">Alimentary Canal</a>).</li>
-
-  <li>Intra-selection, Roux’s theory of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page562">562</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676–8</a>.</li>
-
-  <li>Irish, nutrition and genesis, <b>II</b>,
-    <a href="#Page_510">510</a>.</li>
-
-  <li>Iron: colloidal form of peroxide, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page17">17</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page20">20</a>;</li>
-  <li class="i1">molecular re-arrangement, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page337">337</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page704">704</a>;</li>
-  <li class="i1">vegetal absorption, <b>II</b>,
-    <a href="#Page_573">573</a>.</li>
-
-  <li class="hangingindent">Iron industry, interdependence of social function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page237">237–41</a>.</li>
-
-  <li>Isolation, and species differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page568">568–9</a>.</li>
-
-  <li>Isomerism: of organic constituents, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page4">4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page25">25</a>;</li>
-  <li class="i1">tri- and poly-atomic compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page11">11</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page13">13</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page25">25</a>;</li>
-  <li class="i1">muscular action, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page59">59</a>;</li>
-  <li class="i1">organic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page700">700</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page703">703</a>;</li>
-  <li class="i1">differentiation of nerve tissue, <b>II</b>,
-    <a href="#Page_356">356–60</a>,
-    <a href="#Page_361">361</a>;</li>
-  <li class="i1">of muscular tissue, <b>II</b>,
-    <a href="#Page_361">361–4</a>.</li>
-</ul>
-
-<ul>
-  <li class="hangingindent">Jackson, J. Hughlings, on inheritance of nervous peculiarities, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page313">313</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page694">694</a>.</li>
-
-  <li>Jaundice (<i>see</i> <a href="#Disease">Disease</a>).</li>
-
-  <li>Jaws, of uncivilized and civilized, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page541">541–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>.</li>
-
-  <li>Johnson, G. Lindsey, on inherited myopia, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page694">694</a>.</li>
-
-  <li>Jones, T. Rymer, on fission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page585">585</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page590">590</a>.</li>
-
-  <li class="hangingindent">Julin, C., on “castration parasitaire” in Crustaceans, <b>II</b>,
-    <a href="#Page_493">493–6</a>.</li>
-
-  <li><i>Jungermanniaceæ</i>: morphology, <b>II</b>,
-    <a href="#Page_33">33–4</a>;</li>
-  <li class="i1">relations of high and low types, <b>II</b>,
-    <a href="#Page_35">35</a>,
-    <a href="#Page_55">55</a>;</li>
-  <li class="hangingindent1">continuous and discontinuous development, <b>II</b>,
-    <a href="#Page_52">52–5</a>,
-    <a href="#Page_92">92</a>;</li>
-  <li class="i1">tubular structure, <b>II</b>,
-    <a href="#Page_58">58</a>,
-    <a href="#Page_62">62</a>;</li>
-  <li class="i1">proliferous growth, <b>II</b>,
-    <a href="#Page_67">67</a>,
-    <a href="#Page_91">91</a>;</li>
-  <li class="i1">colour, <b>II</b>,
-    <a href="#Page_75">75</a>,
-    <a href="#Page_265">265</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_140">140</a>;</li>
-  <li class="i1">fertility and growth, <b>II</b>,
-    <a href="#Page_441">441</a>.</li>
-
-  <li>Jussieu, A. de, plant classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page378">378</a>.</li>
-</ul>
-
-<ul>
-  <li id="Karyokinesis">Karyokinesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page257">257</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page259">259</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page263">263–5</a>.</li>
-
-  <li>Kerner, A., on cauline buds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page358">358</a>;</li>
-  <li class="hangingindent1">plant classification in <i>Natural History of Plants</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page378">378–9</a>.</li>
-
-  <li>Kidd, Benj., his acceptance of Weismannism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page690">690</a>.</li>
-
-  <li>Kitto, Dr., his visual memory and deafness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page230">230</a>.</li>
-
-  <li>Klebs, on <i>Hydrodictyon</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page288">288</a>;</li>
-  <li class="i1"><i>Vaucheria</i>, <b>II</b>,
-    <a href="#Page_84">84</a>.</li>
-
-  <li class="hangingindent">Klein, E., multiplication of <i>Bacteria</i>, <b>II</b>,
-    <a href="#Page_443">443</a>.</li>
-
-  <li class="hangingindent">Korschelt, E., annulose segmentation, <b>II</b>,
-    <a href="#Page_103">103</a>,
-    <a href="#Page_601">601–3</a>,
-    <a href="#Page_605">605</a>;</li>
-  <li class="i1"><i>Arenicola</i> larvæ, <b>II</b>,
-    <a href="#Page_109">109</a>.</li>
-</ul>
-
-<ul>
-  <li class="hangingindent" id="Labour">Labour, physiological division of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page204">204</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page207">207</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page591">591</a>; <b>II</b>,
-    <a href="#Page_373">373</a>;</li>
-  <li class="hangingindent1">its meaning and Weismann’s fallacious interpretation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page634">634–5</a>.</li>
-
-  <li class="hangingindent">Lacaze-Duthiers, on origin of annulose type, <b>II</b>,
-    <a href="#Page_110">110</a>.</li>
-
-  <li>Lamarck: zoological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page382">382</a>;</li>
-  <li class="i1">opinions of E. Darwin and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page491">491</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page493">493–7</a>;</li>
-  <li class="i1">neo-Darwinists and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page630">630–1</a>.</li>
-
-  <li class="hangingindent"><i>Laminariaceæ</i>: pseudo-foliar and axial development, <b>II</b>,
-    <a href="#Page_30">30</a>;</li>
-  <li class="i1">tissue, <b>II</b>,
-    <a href="#Page_247">247</a>,
-    <a href="#Page_256">256</a>,
-    <a href="#Page_272">272</a>.</li>
-
-  <li id="Language">Language: and evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page442">442</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page444">444</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page446">446</a>;</li>
-  <li class="i1">perceptiveness of tongue-tip, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page607">607</a>.</li>
-
-  <li class="hangingindent">Lankester, Sir E. Ray, absence of nucleus in <i>Archerina</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page183">183</a>;</li>
-  <li class="i1">diversity of <i>Protozoa</i>, <i>ib.</i>;</li>
-  <li class="i1">zoological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page387">387</a>;</li>
-  <li class="i1">blindness of cave-animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page647">647–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page649">649</a>.</li>
-
-  <li>Laugh, definition of life and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page112">112</a>.</li>
-
-  <li>Laurel, leaves of, <b>II</b>,
-    <a href="#Page_149">149</a>,
-    <a href="#Page_249">249</a>.</li>
-
-  <li>Leaves: growth of shoot, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page168">168</a>;</li>
-  <li class="i1">development and aggregation, <b>II</b>,
-    <a href="#Page_37">37–42</a>,
-    <a href="#Page_76">76</a>;</li>
-  <li class="i1">stem-like stalks, <b>II</b>,
-    <a href="#Page_41">41</a>;</li>
-  <li class="i1">homologies, <b>II</b>,
-    <a href="#Page_42">42</a>,
-    <a href="#Page_75">75–7</a>,
-    <a href="#Page_83">83</a>;</li>
-  <li class="i1">nutrition and compound, <b>II</b>,
-    <a href="#Page_42">42</a>;</li>
-  <li class="i1">foliar and axial development, <b>II</b>,
-    <a href="#Page_46">46–50</a>,
-    <a href="#Page_541">541–7</a>;</li>
-  <li class="i1">“adnate,” <b>II</b>,
-    <a href="#Page_58">58</a>;</li>
-  <li class="i1">proliferous growth, <b>II</b>,
-    <a href="#Page_67">67</a>,
-    <a href="#Page_91">91</a>;</li>
-  <li class="i1">nutrition and development, <b>II</b>,
-    <a href="#Page_76">76–8</a>;</li>
-  <li class="i1">symmetry, and of branches, <b>II</b>,
-    <a href="#Page_148">148–50</a>,
-    <a href="#Page_151">151</a>;</li>
-  <li class="i1">size and distribution of leaflets, <b>II</b>,
-    <a href="#Page_152">152–5</a>;</li>
-  <li class="i1">transition from compound to simple, <b>II</b>,
-    <a href="#Page_155">155–8</a>;</li>
-  <li class="i1">unsymmetrical form, <b>II</b>,
-    <a href="#Page_158">158–9</a>;</li>
-  <li class="i1">natural selection and distribution, <b>II</b>,
-    <a href="#Page_179">179</a>;</li>
-  <li class="i1">morphological summary, <b>II</b>,
-    <a href="#Page_234">234–5</a>;</li>
-  <li class="i1">tissue differentiation, <b>II</b>,
-    <a href="#Page_247">247</a>;</li>
-  <li class="i1">distribution, <b>II</b>,
-    <a href="#Page_249">249</a>;</li>
-  <li class="i1">outer tissues of stem and, <b>II</b>,
-    <a href="#Page_256">256–9</a>,
-    <a href="#Page_270">270</a>,
-    <a href="#Page_386">386</a>;</li>
-  <li class="i1">distribution of stomata, <b>II</b>,
-    <a href="#Page_260">260–1</a>;</li>
-  <li class="i1">wax deposit on, <b>II</b>,
-    <a href="#Page_260">260</a>,
-    <a href="#Page_261">261</a>;</li>
-  <li class="i1">light and colour, <b>II</b>,
-    <a href="#Page_261">261–2</a>;</li>
-  <li class="i1">superficial differentiation, <b>II</b>,
-    <a href="#Page_263">263–5</a>,
-    <a href="#Page_270">270</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">abortive in parasitic plants, <b>II</b>,
-    <a href="#Page_274">274</a>;</li>
-  <li class="i1">submerged, in aquatic plants, <b>II</b>,
-    <a href="#Page_274">274–5</a>;</li>
-  <li class="i1">inner tissue differentiation, <b>II</b>,
-    <a href="#Page_278">278</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="i1">vascular tissue differentiation, <b>II</b>,
-    <a href="#Page_286">286</a>,
-    <a href="#Page_288">288</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="i1">dye absorption and circulation, <b>II</b>,
-    <a href="#Page_570">570–4</a>,
-    <a href="#Page_577">577</a>;</li>
-  <li class="i1">vascular system, <b>II</b>,
-    <a href="#Page_588">588–92</a>,
-    <a href="#Page_596">596</a>;</li>
-  <li class="i1">arrangement, <b>II</b>,
-    <a href="#Page_608">608–11</a>.</li>
-
-  <li><i>Lepidoptera</i> (<i>see</i> <a href="#Insects">Insects</a>).</li>
-
-  <li><i>Lepidosiren</i>: ossification, <b>II</b>,
-    <a href="#Page_218">218</a>;</li>
-  <li class="i1">respiration, <b>II</b>,
-    <a href="#Page_338">338</a>;</li>
-  <li class="i1">skeleton, <b>II</b>,
-    <a href="#Page_553">553</a>,
-    <a href="#Page_555">555</a>,
-    <a href="#Page_560">560</a>.</li>
-
-  <li><i>Lepidosteus</i>: armour, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page526">526</a>;</li>
-  <li class="i1">air-bladder, <b>II</b>,
-    <a href="#Page_334">334</a>.</li>
-
-  <li class="hangingindent">Leroy-Beaulieu, Pierre, on Australian miners’ usages, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364</a>.</li>
-
-  <li><i>Lessonia</i>: Hooker on growth, <b>II</b>,
-    <a href="#Page_56">56</a>;</li>
-  <li class="i1">branch symmetry, <b>II</b>,
-    <a href="#Page_146">146</a>.</li>
-
-  <li>Lewes, G. H., definition of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page80">80</a>.</li>
-
-  <li>Lichens: tissue, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page586">586</a>;</li>
-  <li class="i1">cell multiplication, <b>II</b>,
-    <a href="#Page_27">27</a>;</li>
-  <li class="i1">Hooker on growth, <b>II</b>,
-    <a href="#Page_56">56</a>;</li>
-  <li class="i1">tubular structure, <b>II</b>,
-    <a href="#Page_57">57</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_293">293</a>;</li>
-  <li class="i1">dual nature, <b>II</b>,
-    <a href="#Page_399">399</a>;</li>
-  <li class="i1">reproduction, <b>II</b>,
-    <a href="#Page_450">450</a>.</li>
-
-  <li>Liebig, Baron, nitrogenous food stuffs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page47">47–8</a>.</li>
-
-  <li id="Life">Life: co-ordination of actions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page79">79</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page89">89</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page577">577–80</a>;</li>
-  <li class="i1">defined by Schelling, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page78">78</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page178">178</a>;</li>
-  <li class="i1">Richeraud, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page79">79</a>;</li>
-  <li class="i1">De Blainville, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page79">79</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page93">93</a>;</li>
-  <li class="i1">Lewes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page80">80</a>;</li>
-  <li class="hangingindent1">definition yielded by contrasting most unlike kinds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page81">81–8</a>;</li>
-  <li class="i1">changes showing, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page91">91</a>;</li>
-  <li class="i1">vital actions and environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page92">92–3</a>;</li>
-  <li class="i1">resulting addition to conception, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page93">93</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page326">326</a>;</li>
-  <li class="i1">Comte’s definition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page93">93</a>;</li>
-  <li class="hangingindent1">correspondence of external and internal relations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page93">93–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page100">100</a>; <b>II</b>,
-    <a href="#Page_523">523</a>;</li>
-  <li class="i1">continuous adjustment of such relations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page99">99</a>;</li>
-  <li class="hangingindent1">completeness proportionate to correspondence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page101">101–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page109">109</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page349">349</a>;</li>
-  <li class="i1">length and complexity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page103">103</a>;</li>
-  <li class="hangingindent1">complexity of environment and degree of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page104">104–6</a>;</li>
-  <li class="i1">definitions of evolution and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page107">107–10</a>;</li>
-  <li class="i1">deficiencies of formula, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page112">112–3</a>;</li>
-  <li class="i1">activity the essential element, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page113">113</a>;</li>
-  <li class="hangingindent1">hypothesis of independent vital principle examined, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page114">114–7</a>;</li>
-  <li class="hangingindent1">difficulties of physico-chemical theory, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page117">117–20</a>;</li>
-  <li class="i1">ultimate incomprehensibility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page120">120–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page373">373</a>;</li>
-  <li class="i1">validity of conclusions reached, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page123">123</a>;</li>
-  <li class="i1">is organization produced by? <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page197">197</a>;</li>
-  <li class="i1">precedes organization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page210">210</a>;</li>
-  <li class="i1">definitions of individuality and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page250">250</a>;</li>
-  <li class="i1">effect of incident forces on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page348">348–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page355">355</a>;</li>
-  <li class="i1">length in individuals and species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page422">422</a>;</li>
-  <li class="i1">equilibration of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page547">547</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">final formulation of definition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page580">580</a>;</li>
-  <li class="hangingindent1">co-ordination of actions and sexual differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page593">593</a>;</li>
-  <li class="i1">“absolute” commencement of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page699">699</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page702">702</a>;</li>
-  <li class="i1">integration and augmentation, <b>II</b>,
-    <a href="#Page_426">426</a>;</li>
-  <li class="i1">prospective human, <b>II</b>,
-    <a href="#Page_522">522–5</a>.</li>
-
-  <li class="hangingindent" id="Light">Light: influence on organisms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page30">30–6</a>; <b>II</b>,
-    <a href="#Page_433">433</a>;</li>
-  <li class="i1">nitrogenous plants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page40">40</a>;</li>
-  <li class="i1">organic phosphorescence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page49">49</a>;</li>
-  <li class="i1">heliotropism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page92">92</a>; <b>II</b>,
-    <a href="#Page_160">160</a>;</li>
-  <li class="i1">effects on organic matter, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page149">149</a>;</li>
-  <li class="i1">plant adaptation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page227">227</a>;</li>
-  <li class="hangingindent1">rhythmical variation of, and organic life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page499">499</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">vegetal influences, <b>II</b>,
-    <a href="#Page_130">130</a>,
-    <a href="#Page_131">131</a>,
-    <a href="#Page_147">147</a>,
-    <a href="#Page_149">149</a>,
-    <a href="#Page_158">158</a>;</li>
-  <li class="i1">influence on flowers, <b>II</b>,
-    <a href="#Page_167">167–8</a>,
-    <a href="#Page_608">608–11</a>;</li>
-  <li class="i1">vegetal tissue differentiation, <b>II</b>,
-    <a href="#Page_253">253–5</a>,
-    <a href="#Page_258">258</a>,
-    <a href="#Page_259">259</a>;</li>
-  <li class="i1">action on leaves, <b>II</b>,
-    <a href="#Page_260">260–4</a>;</li>
-  <li class="i1">on plant vascular system, <b>II</b>,
-    <a href="#Page_288">288</a>,
-    <a href="#Page_297">297</a>,
-    <a href="#Page_586">586</a>;</li>
-  <li class="i1">development of sensory organs, <b>II</b>,
-    <a href="#Page_320">320</a>.</li>
-
-  <li><i>Liliaceæ</i>, floral symmetry, <b>II</b>,
-    <a href="#Page_170">170</a>.</li>
-
-  <li>Lime, leaf forms, <b>II</b>,
-    <a href="#Page_158">158</a>,
-    <a href="#Page_159">159</a>.</li>
-
-  <li>Lindley, J., plant classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page377">377</a>.</li>
-
-  <li>Linnæus, C., classificatory system, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page377">377</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page380">380</a>.</li>
-
-  <li class="hangingindent">Linnet, contrasted with blackbird in development, <b>II</b>,
-    <a href="#Page_503">503</a>.</li>
-
-  <li>Liver: metabolic processes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page70">70</a>;</li>
-  <li class="i1">vitality of excised, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page111">111</a>;</li>
-  <li class="i1">development, <b>II</b>,
-    <a href="#Page_329">329–33</a>.</li>
-
-  <li>Liver-fluke (see <a href="#Distoma"><i>Distoma</i></a>).</li>
-
-  <li>Liverworts (see <a href="#Hepaticae"><i>Hepaticæ</i></a>).</li>
-
-  <li>Lizard, regeneration of lost tail, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page360">360</a>.</li>
-
-  <li>Locomotion (<i>see</i> <a href="#Motion">Motion</a>).</li>
-
-  <li>Logic, reasoning and definition of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page81">81–6</a>.</li>
-
-  <li>Logwood, vegetal staining, <b>II</b>,
-    <a href="#Page_569">569–74</a>,
-    <a href="#Page_577">577–81</a>,
-    <a href="#Page_584">584</a>.</li>
-
-  <li>Longevity, and complexity of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page102">102–3</a>.</li>
-
-  <li class="hangingindent">Lubbock, Sir J.: on growth and genesis in insects and crustaceans, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page292">292</a>;</li>
-  <li class="i1">aquatic flies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page400">400</a>.</li>
-
-  <li>Lungs (<i>see</i> <a href="#Respiratory_System">Respiratory System</a>).</li>
-
-  <li>Lymphatic system: amœboid cells, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page187">187</a>;</li>
-  <li class="i1">structural traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page192">192</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page193">193</a>.</li>
-</ul>
-
-<ul>
-  <li>MacBride, E. W., <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#pagevi">vi</a>; <b>II</b>,
-    <a href="#Page_vi">vi</a>;</li>
-  <li class="i1">zoological phyla, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page396">386–7</a>;</li>
-  <li class="i1">arthropod segmentation, <b>II</b>,
-    <a href="#Page_114">114</a>;</li>
-  <li class="i1">ctenidia of slug, <b>II</b>,
-    <a href="#Page_117">117</a>;</li>
-  <li class="i1">conjugation of <i>Paramæcium</i>, <b>II</b>,
-    <a href="#Page_452">452</a>.</li>
-
-  <li class="hangingindent"><i>Macrocystis pyrifera</i>, gigantic seaweed, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page121">121</a>.</li>
-
-  <li>Magenta, vegetal staining, <b>II</b>,
-    <a href="#Page_569">569–74</a>,
-    <a href="#Page_577">577–81</a>,
-    <a href="#Page_584">584</a>.</li>
-
-  <li>Magnetism: muscular action, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page59">59</a>;</li>
-  <li>incomprehensibility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page121">121</a>.</li>
-
-  <li class="hangingindent">Maillet, B. de, modifiability of organisms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page490">490</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page496">496</a>.</li>
-
-  <li class="hangingindent"><i>Mammalia</i>: temperature and molecular change, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page30">30</a>;</li>
-  <li class="i1">nutrition and growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page141">141</a>;</li>
-  <li class="i1">expenditure of force, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page142">142</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page156">156</a>;</li>
-  <li class="i1">flesh constituents, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page154">154</a>;</li>
-  <li class="i1">temperature, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page174">174</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page177">177</a>;</li>
-  <li class="i1">self-mobility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page175">175</a>;</li>
-  <li class="hangingindent1">functional and structural differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page201">201</a>;</li>
-  <li class="i1">heart-function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page206">206</a>;</li>
-  <li class="i1">viviparous homogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>;</li>
-  <li class="i1">variation and uterine environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page327">327</a>;</li>
-  <li class="i1">classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page392">392</a>;</li>
-  <li class="i1">cervical vertebræ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page394">394</a>; <b>II</b>,
-    <a href="#Page_564">564</a>;</li>
-  <li class="i1">aquatic types, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page400">400</a>;</li>
-  <li class="i1">fossil remains and rate of evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page407">407</a>;</li>
-  <li class="i1">ancient and modern forms contrasted, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page408">408–10</a>;</li>
-  <li class="i1">embryonic respiratory system, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page456">456</a>;</li>
-  <li class="i1">suppression of teeth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page457">457</a>;</li>
-  <li class="i1">arrested development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page473">473–4</a>;</li>
-  <li class="i1">simulated homologies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page485">485</a>;</li>
-  <li class="i1">natural selection and inactive parts, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page534">534</a>;</li>
-  <li class="i1">re-development of rudimentary organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page563">563</a>;</li>
-  <li class="i1">location of testes and current theories, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page573">573</a>;</li>
-  <li class="i1">fertility and development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page583">583</a>; <b>II</b>,
-    <a href="#Page_465">465</a>;</li>
-  <li class="i1">fertility and nervous development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page598">598–9</a>;</li>
-  <li class="i1">locomotion and elongated form, <b>II</b>,
-    <a href="#Page_15">15</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_204">204</a>;</li>
-  <li class="i1">tegumentary structure, <b>II</b>,
-    <a href="#Page_314">314</a>;</li>
-  <li class="i1">circulation, <b>II</b>,
-    <a href="#Page_340">340</a>;</li>
-  <li class="i1">vascularity and ova-maturation, <b>II</b>,
-    <a href="#Page_342">342–3</a>;</li>
-  <li class="i1">activity and muscular colour, <b>II</b>,
-    <a href="#Page_365">365–9</a>;</li>
-  <li class="i1">functional integration, <b>II</b>,
-    <a href="#Page_375">375</a>;</li>
-  <li class="i1">outer tissue differentiation, <b>II</b>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">growth and genesis, <b>II</b>,
-    <a href="#Page_456">456</a>,
-    <a href="#Page_459">459</a>;</li>
-  <li class="i1">comparative fertility, <b>II</b>,
-    <a href="#Page_465">465</a>,
-    <a href="#Page_470">470</a>;</li>
-  <li class="i1">heat expenditure and genesis, <b>II</b>,
-    <a href="#Page_467">467–9</a>;</li>
-  <li class="i1">activity and fertility, <b>II</b>,
-    <a href="#Page_472">472</a>;</li>
-  <li class="i1">nutrition and genesis, <b>II</b>,
-    <a href="#Page_479">479–80</a>.</li>
-
-  <li id="Man">Man: effect of climate on vigour, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page30">30</a>;</li>
-  <li class="i1">flesh and grain eaters compared, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page68">68</a>;</li>
-  <li class="i1">longevity and life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page103">103</a>;</li>
-  <li class="i1">complex environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page105">105</a>;</li>
-  <li class="i1">embryogeny of arm, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page169">169</a>;</li>
-  <li class="hangingindent1">fertility and conditions affecting it, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page300">300</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page570">570</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page583">583</a>; <b>II</b>,
-    <a href="#Page_484">484</a>,
-    <a href="#Page_506">506–21</a>;</li>
-  <li class="hangingindent1">inheritance of functionally-produced changes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page310">310–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page541">541</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page605">605</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page608">608</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page652">652</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page673">673</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693–4</a>;</li>
-  <li class="i1">heredity and sex, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page315">315–6</a>;</li>
-  <li class="i1">function of bilirubin, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page330">330</a>;</li>
-  <li class="i1">cousin-marriages, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page346">346</a>; <b>II</b>,
-    <a href="#Page_615">615</a>;</li>
-  <li class="i1">primitive notions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page417">417–9</a>;</li>
-  <li class="i1">inutility of <i>Appendix vermiformis</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page474">474</a>;</li>
-  <li class="i1">diminution of jaw, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page541">541</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">co-ordination of actions greatest in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page579">579</a>;</li>
-  <li class="i1">fundamental traits of sex, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page594">594–7</a>;</li>
-  <li class="i1">obesity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page594">594</a>;</li>
-  <li class="i1">substance and weight of brain, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page596">596</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page599">599</a>;</li>
-  <li class="hangingindent1">distribution of tactual perceptiveness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page602">602–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page665">665–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page672">672–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="i1">telegony, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page625">625</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page644">644–5</a>;</li>
-  <li class="i1">degradation of little toe, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page652">652</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page673">673</a>;</li>
-  <li class="hangingindent1">transmitted osteological peculiarities of Punjabis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>;</li>
-  <li class="i1">traits of twin-bearing women, <b>II</b>,
-    <a href="#Page_457">457</a>;</li>
-  <li class="i1">comparative mammalian fertility, <b>II</b>,
-    <a href="#Page_465">465</a>;</li>
-  <li class="i1">future evolution, <b>II</b>,
-    <a href="#Page_522">522–37</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Language">Language</a> <i>and</i> <a href="#Sociology">Sociology</a>.)</li>
-
-  <li>Manatee, nailless paddles, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page473">473</a>.</li>
-
-  <li>Manx cats, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page303">303</a>.</li>
-
-  <li><i>Marchantiaceæ</i>: symmetry, <b>II</b>,
-    <a href="#Page_140">140</a>;</li>
-  <li class="i1">outer tissue differentiation, <b>II</b>,
-    <a href="#Page_252">252</a>.</li>
-
-  <li>Marmot, hybernation and waste, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page214">214–5</a>.</li>
-
-  <li>Marriage (<i>see</i> <a href="#Multiplication">Multiplication</a>).</li>
-
-  <li>Marsh, O. C., on telegony, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page644">644</a>.</li>
-
-  <li>Masters, M. T., on foliar homology, <b>II</b>,
-    <a href="#Page_46">46–7</a>;</li>
-  <li class="hangingindent1">selection of inconspicuous variations in plants, <b>II</b>,
-    <a href="#Page_298">298</a>,
-    <a href="#Page_621">621</a>;</li>
-  <li class="hangingindent1">separation of ancestral constitutions in plant hybrids, <b>II</b>,
-    <a href="#Page_616">616</a>;</li>
-  <li class="i1">single and double stocks, <b>II</b>,
-    <a href="#Page_622">622</a>.</li>
-
-  <li class="hangingindent">Matter, incomprehensibility of interactions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page121">121–2</a>.</li>
-
-  <li>Mechanics: transverse strains, <b>II</b>,
-    <a href="#Page_209">209–12</a>;</li>
-  <li class="hangingindent1">genesis of vertebrate axis, <b>II</b>,
-    <a href="#Page_212">212–6</a>,
-    <a href="#Page_216">216–8</a>,
-    <a href="#Page_224">224</a>,
-    <a href="#Page_225">225–7</a>;</li>
-  <li class="i1">osseous differentiation, <b>II</b>,
-    <a href="#Page_345">345–51</a>;</li>
-  <li class="i1">disintegrated motion, <b>II</b>,
-    <a href="#Page_375">375</a>;</li>
-  <li class="i1">analogy from locomotive, <b>II</b>,
-    <a href="#Page_517">517–9</a>;</li>
-  <li class="i1">future human evolution, <b>II</b>,
-    <a href="#Page_524">524</a>;</li>
-  <li class="i1">strain and vegetal structure, <b>II</b>,
-    <a href="#Page_574">574–88</a>,
-    <a href="#Page_592">592–6</a>.</li>
-
-  <li><i>Medusæ</i>: contractile functions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page58">58</a>; <b>II</b>,
-    <a href="#Page_374">374</a>;</li>
-  <li class="i1">individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page248">248</a>;</li>
-  <li class="i1">heterogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page273">273</a>;</li>
-  <li class="i1">fertility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page582">582</a>;</li>
-  <li class="i1">strobilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page592">592</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_188">188–91</a>.</li>
-
-  <li class="hangingindent">Mehnert, E., on feet of pentadactyle vertebrates, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page461">461</a>.</li>
-
-  <li>Mensel’s salt, temperature and isomerism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page77">77</a>.</li>
-
-  <li class="hangingindent">Metabolism: antithesis between plants and animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page62">62–3</a>;</li>
-  <li class="i1">evolution hypothesis and primordial, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page63">63–4</a>;</li>
-  <li class="i1">in plants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page64">64–7</a>;</li>
-  <li class="i1">animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page67">67–77</a>;</li>
-  <li class="i1">nervo-muscular activities, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page71">71–7</a>;</li>
-  <li class="i1">summary, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page77">77</a>;</li>
-  <li class="i1">cell processes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page261">261</a>.</li>
-
-  <li>Metals: remarkable interactions of some, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page121">121</a>;</li>
-  <li class="i1">melting of alloys, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page339">339</a>;</li>
-  <li class="i1">atomic re-arrangement, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page352">352</a>.</li>
-
-  <li>Metamerism (<i>see</i> <a href="#Segmentation">Segmentation</a>).</li>
-
-  <li class="hangingindent"><i>Metazoa</i>: cellular structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page184">184</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page194">194</a>; <b>II</b>,
-    <a href="#Page_21">21</a>;</li>
-  <li class="i1">subordination of units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page185">185–7</a>;</li>
-  <li class="i1">general characters of tissues, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page188">188–9</a>;</li>
-  <li class="i1">protoplasmic continuity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page190">190–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page194">194</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page628">628</a>;</li>
-  <li class="i1">genesis of food-cavity and visual organ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page195">195</a>;</li>
-  <li class="i1">Weismann’s differentiation theory, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page637">637–43</a>.</li>
-
-  <li>Meteorology: non-vital changes shown in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page82">82</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page84">84</a>;</li>
-  <li class="i1">crystallization of “storm glass,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page96">96</a>;</li>
-  <li class="i1">special creation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page419">419</a>;</li>
-  <li class="i1">rhythm in, and organic change, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page499">499–501</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">variations due to geologic change, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page503">503</a>.</li>
-
-  <li><i>Microstomida</i>, segmental reproduction, <b>II</b>,
-    <a href="#Page_102">102</a>.</li>
-
-  <li>Migration: of animal species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page396">396–401</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page411">411</a>;</li>
-  <li class="i1">solar influences, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page500">500</a>;</li>
-  <li class="i1">part played by, in organic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page568">568</a>;</li>
-  <li class="i1">causes of, <b>II</b>,
-    <a href="#Page_533">533–4</a>.</li>
-
-  <li>Milk, heat and supply of, <b>II</b>,
-    <a href="#Page_468">468</a>.</li>
-
-  <li class="hangingindent">Milne-Edwards, H., “physiological division of labour,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page204">204</a>;</li>
-  <li class="i1">Weismann’s erroneous application of it, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page634">634</a>;</li>
-  <li class="i1">on ocular structure, <b>II</b>,
-    <a href="#Page_318">318</a>.</li>
-
-  <li>Mind (<i>see</i> <a href="#Psychology">Psychology</a>).</li>
-
-  <li>Mitosis (<i>see</i> <a href="#Karyokinesis">Karyokinesis</a>).</li>
-
-  <li>Mobility, molar and molecular, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page14">14</a>;</li>
-  <li class="i1">environment and self-mobility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page177">177</a>.</li>
-
-  <li>Mohl, on phænogamic growth, <b>II</b>,
-    <a href="#Page_82">82</a>.</li>
-
-  <li>Mole, modifications due to habits, <b>II</b>,
-    <a href="#Page_391">391</a>.</li>
-
-  <li>Molecules: mechanically considered, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page14">14</a>;</li>
-  <li class="i1">stability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page337">337–40</a>;</li>
-  <li class="i1">nerve differentiation, <b>II</b>,
-    <a href="#Page_355">355–61</a>,
-    <a href="#Page_379">379–82</a>.</li>
-
-  <li id="Mollusca"><i>Mollusca</i>: axial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>;</li>
-  <li class="i1">genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>; <b>II</b>,
-    <a href="#Page_444">444</a>;</li>
-  <li class="i1">hermaphrodism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page341">341</a>;</li>
-  <li class="i1">classificatory traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page392">392</a>;</li>
-  <li class="i1">distribution in time, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page405">405</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page408">408</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page410">410</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page446">446–7</a>;</li>
-  <li class="hangingindent1">trochophore and its relationships, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page447">447</a>; <b>II</b>,
-    <a href="#Page_108">108</a>,
-    <a href="#Page_109">109</a>,
-    <a href="#Page_115">115</a>;</li>
-  <li class="i1">development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page460">460</a>;</li>
-  <li class="i1">amphibious and terrestrial, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page481">481</a>;</li>
-  <li class="i1">indirect equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page534">534</a>;</li>
-  <li class="i1">secondary aggregation, <b>II</b>,
-    <a href="#Page_115">115–7</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_201">201–3</a>;</li>
-  <li class="i1">outer tissue, <b>II</b>,
-    <a href="#Page_310">310</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">alimentary system, <b>II</b>,
-    <a href="#Page_325">325</a>;</li>
-  <li class="i1">vascular system, <b>II</b>,
-    <a href="#Page_340">340–1</a>.</li>
-
-  <li><i>Molluscoida</i>, <b>II</b>,
-    <a href="#Page_598">598</a>.</li>
-  <li class="i1">(See <a href="#Polyzoa"><i>Polyzoa</i></a> and <a href="#Tunicata"><i>Tunicata</i></a>.)</li>
-
-  <li id="Monocotyledons">Monocotyledons: growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page138">138</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page139">139</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page143">143</a>;</li>
-  <li class="i1">uniaxial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>;</li>
-  <li class="hangingindent1">cotyledonous germination and endogenous growth, <b>II</b>,
-    <a href="#Page_59">59–62</a>,
-    <a href="#Page_69">69–72</a>,
-    <a href="#Page_82">82–3</a>,
-    <a href="#Page_181">181–2</a>;</li>
-  <li class="hangingindent1">absence of helical phyllotaxy in <i>Ravenala</i>, <b>II</b>,
-    <a href="#Page_182">182</a>;</li>
-  <li class="i1">surface contrasts, <b>II</b>,
-    <a href="#Page_257">257</a>;</li>
-  <li class="i1">outer leaf tissue, <b>II</b>,
-    <a href="#Page_263">263</a>;</li>
-  <li class="i1">wood formation, <b>II</b>,
-    <a href="#Page_278">278</a>;</li>
-  <li class="i1">growth and genesis, <b>II</b>,
-    <a href="#Page_451">451</a>.</li>
-
-  <li>Monstrosities, in plants, <b>II</b>,
-    <a href="#Page_78">78</a>,
-    <a href="#Page_541">541</a>,
-    <a href="#Page_546">546</a>;</li>
-  <li class="i1">vertebrate, <b>II</b>,
-    <a href="#Page_118">118</a>.</li>
-
-  <li class="hangingindent">Morgan, T. H., on regeneration of <i>Planaria</i>, <b>II</b>,
-    <a href="#Page_102">102</a>,
-    <a href="#Page_611">611</a>.</li>
-
-  <li id="Morphology">Morphology: facts comprised in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page125">125–6</a>;</li>
-  <li class="i1">morphological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page190">190–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page225">225</a>;</li>
-  <li class="i1">rudimentary organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page472">472–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">structural and functional co-operation, <b>II</b>,
-    <a href="#Page_3">3</a>,
-    <a href="#Page_239">239</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_4">4–6</a>,
-    <a href="#Page_181">181–96</a>;</li>
-  <li class="i1">change of shape, <b>II</b>,
-    <a href="#Page_6">6</a>;</li>
-  <li class="i1">formula of evolution, <b>II</b>,
-    <a href="#Page_7">7–9</a>;</li>
-  <li class="i1">as interpreted by phylogeny, <b>II</b>,
-    <a href="#Page_10">10–6</a>;</li>
-  <li class="i1">evolution and cell-doctrine, <b>II</b>,
-    <a href="#Page_17">17–21</a>.</li>
-
-  <li class="hangingindent">Morphology, Animal: evolution and segmentation of <i>Articulata</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page468">468–9</a>;</li>
-  <li class="i1">vertebral column development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page470">470</a>;</li>
-  <li class="i1">simulated homologies, <b>II</b>,
-    <a href="#Page_14">14–5</a>;</li>
-  <li class="i1">primary aggregates, <b>II</b>,
-    <a href="#Page_85">85–8</a>,
-    <a href="#Page_123">123–4</a>;</li>
-  <li class="i1">secondary, <b>II</b>,
-    <a href="#Page_88">88–91</a>,
-    <a href="#Page_124">124</a>;</li>
-  <li class="i1">tertiary, <b>II</b>,
-    <a href="#Page_91">91–3</a>;</li>
-  <li class="hangingindent1">integration and independence of individuality, <b>II</b>,
-    <a href="#Page_93">93–9</a>,
-    <a href="#Page_124">124</a>;</li>
-  <li class="hangingindent1">annulose segmentation, <b>II</b>,
-    <a href="#Page_98">98–101</a>,
-    <a href="#Page_106">106–10</a>,
-    <a href="#Page_125">125–7</a>,
-    <a href="#Page_602">602–7</a>;</li>
-  <li class="hangingindent1">progressive annulose integration, <b>II</b>,
-    <a href="#Page_100">100–5</a>,
-    <a href="#Page_111">111–5</a>,
-    <a href="#Page_121">121</a>,
-    <a href="#Page_124">124</a>,
-    <a href="#Page_223">223</a>;</li>
-  <li class="i1">unintegrated molluscan form, <b>II</b>,
-    <a href="#Page_115">115–7</a>;</li>
-  <li class="hangingindent1">vertebrate segmentation and integration, <b>II</b>,
-    <a href="#Page_117">117–23</a>,
-    <a href="#Page_124">124–7</a>,
-    <a href="#Page_223">223–4</a>,
-    <a href="#Page_602">602</a>,
-    <a href="#Page_606">606–7</a>;</li>
-  <li class="i1">motion and symmetry, <b>II</b>,
-    <a href="#Page_183">183–5</a>;</li>
-  <li class="hangingindent1">symmetry of primary and secondary aggregates, <b>II</b>,
-    <a href="#Page_186">186</a>,
-    <a href="#Page_187">187–91</a>;</li>
-  <li class="i1">of compound <i>Cœlenterata</i>, <b>II</b>,
-    <a href="#Page_192">192–4</a>;</li>
-  <li class="i1">simulation of plant shapes, <b>II</b>,
-    <a href="#Page_192">192</a>;</li>
-  <li class="hangingindent1">symmetry of <i>Polyzoa</i> and <i>Tunicata</i>, <b>II</b>,
-    <a href="#Page_194">194</a>;</li>
-  <li class="hangingindent1">of <i>Platyhelminthes</i> and <i>Echinoderms</i>, <b>II</b>,
-    <a href="#Page_195">195–7</a>;</li>
-  <li class="i1">of <i>Annulosa</i>, <b>II</b>,
-    <a href="#Page_197">197–201</a>;</li>
-  <li class="i1">of molluscs, <b>II</b>,
-    <a href="#Page_201">201–3</a>;</li>
-  <li class="i1">of vertebrates, <b>II</b>,
-    <a href="#Page_203">203–6</a>,
-    <a href="#Page_208">208</a>;</li>
-  <li class="i1">similarity of animal and plant, <b>II</b>,
-    <a href="#Page_206">206</a>;</li>
-  <li class="i1">cell-shapes, <b>II</b>,
-    <a href="#Page_228">228–30</a>;</li>
-  <li class="hangingindent1">evolution and generalizations summarized, <b>II</b>,
-    <a href="#Page_231">231–5</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Structure">Structure</a>.)</li>
-
-  <li class="hangingindent">Morphology, Vegetal: simulated homologies, <b>II</b>,
-    <a href="#Page_13">13–4</a>;</li>
-  <li class="i1">unicellular plants, <b>II</b>,
-    <a href="#Page_21">21</a>;</li>
-  <li class="i1">aggregation and integration, <b>II</b>,
-    <a href="#Page_22">22–6</a>,
-    <a href="#Page_78">78–9</a>;</li>
-  <li class="i1">pseudo-foliar development, <b>II</b>,
-    <a href="#Page_26">26–8</a>;</li>
-  <li class="i1">pseudo-axial, <b>II</b>,
-    <a href="#Page_28">28–9</a>;</li>
-  <li class="i1">pseudo-foliar and axial, <b>II</b>,
-    <a href="#Page_30">30–2</a>;</li>
-  <li class="i1">composition of Archegoniates, <b>II</b>,
-    <a href="#Page_33">33–5</a>;</li>
-  <li class="hangingindent1">leaf development and aggregation, <b>II</b>,
-    <a href="#Page_37">37–42</a>,
-    <a href="#Page_75">75–8</a>;</li>
-  <li class="i1">foliar homologies, <b>II</b>,
-    <a href="#Page_42">42–6</a>,
-    <a href="#Page_75">75–8</a>;</li>
-  <li class="i1">foliar and axial development, <b>II</b>,
-    <a href="#Page_46">46–50</a>,
-    <a href="#Page_541">541–7</a>;</li>
-  <li class="hangingindent1">growth and development of Archegoniates, <b>II</b>,
-    <a href="#Page_50">50–6</a>;</li>
-  <li class="i1">of Phænogams, <b>II</b>,
-    <a href="#Page_56">56–64</a>,
-    <a href="#Page_78">78–80</a>;</li>
-  <li class="i1">axillary bud development, <b>II</b>,
-    <a href="#Page_65">65–9</a>;</li>
-  <li class="i1">phænogamic modes of growth, <b>II</b>,
-    <a href="#Page_69">69–72</a>;</li>
-  <li class="i1">homologies, <b>II</b>,
-    <a href="#Page_73">73–5</a>,
-    <a href="#Page_80">80–4</a>;</li>
-  <li class="hangingindent1">development of foliar into axial organs, <b>II</b>,
-    <a href="#Page_75">75–8</a>;</li>
-  <li class="i1"><i>résumé</i>, <b>II</b>,
-    <a href="#Page_78">78–80</a>;</li>
-  <li class="i1">criticisms and replies, <b>II</b>,
-    <a href="#Page_80">80–4</a>;</li>
-  <li class="i1">can plant shapes be formulated? <b>II</b>,
-    <a href="#Page_128">128</a>;</li>
-  <li class="i1">growth and differentiation, <b>II</b>,
-    <a href="#Page_129">129–31</a>;</li>
-  <li class="i1">kinds of symmetry, <b>II</b>,
-    <a href="#Page_131">131–3</a>;</li>
-  <li class="i1">symmetry of primary aggregates, <b>II</b>,
-    <a href="#Page_134">134–7</a>;</li>
-  <li class="i1">of secondary, <b>II</b>,
-    <a href="#Page_137">137–40</a>;</li>
-  <li class="i1">tertiary, <b>II</b>,
-    <a href="#Page_140">140–3</a>;</li>
-  <li class="i1">symmetry and environing influences, <b>II</b>,
-    <a href="#Page_143">143–4</a>;</li>
-  <li class="i1">symmetry of branches, <b>II</b>,
-    <a href="#Page_145">145–8</a>;</li>
-  <li class="i1">leaf and branch symmetry, <b>II</b>,
-    <a href="#Page_148">148–50</a>;</li>
-  <li class="i1">phænogamic unit homology, <b>II</b>,
-    <a href="#Page_151">151</a>;</li>
-  <li class="i1">size and distribution of leaflets, <b>II</b>,
-    <a href="#Page_152">152–5</a>;</li>
-  <li class="hangingindent1">transition from compound to simple leaves, <b>II</b>,
-    <a href="#Page_155">155–8</a>;</li>
-  <li class="i1">unsymmetrical leaf development, <b>II</b>,
-    <a href="#Page_158">158–9</a>;</li>
-  <li class="i1">differentiation of homologous units, <b>II</b>,
-    <a href="#Page_159">159–60</a>;</li>
-  <li class="i1">floral symmetry, <b>II</b>,
-    <a href="#Page_161">161–74</a>;</li>
-  <li class="hangingindent1">cell-differentiation and metamorphosis, <b>II</b>,
-    <a href="#Page_175">175–7</a>;</li>
-  <li class="i1">nutrition and differentiation, <b>II</b>,
-    <a href="#Page_178">178</a>;</li>
-  <li class="i1">and inflorescence, <b>II</b>,
-    <a href="#Page_179">179</a>;</li>
-  <li class="i1">helical growth of phænogams, <b>II</b>,
-    <a href="#Page_180">180–1</a>;</li>
-  <li class="i1">summary of symmetry, <b>II</b>,
-    <a href="#Page_234">234</a>;</li>
-  <li class="i1">stress and structure, <b>II</b>,
-    <a href="#Page_275">275–9</a>,
-    <a href="#Page_388">388</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Structure">Structure</a>.)</li>
-
-  <li>Morton, Lord, quagga-marked foal, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page624">624</a>.</li>
-
-  <li class="hangingindent">Moser, impressions produced by light on metals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page352">352</a>.</li>
-
-  <li>Mosses: varied development, <b>II</b>,
-    <a href="#Page_501">501</a>,
-    <a href="#Page_52">52</a>;</li>
-  <li class="i1">homologies, <b>II</b>,
-    <a href="#Page_80">80</a>,
-    <a href="#Page_81">81</a>;</li>
-  <li class="i1">indefiniteness, <b>II</b>,
-    <a href="#Page_296">296</a>;</li>
-  <li class="i1">multiplication, <b>II</b>,
-    <a href="#Page_441">441</a>.</li>
-
-  <li>Moth, clothes, food of larva, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page77">77</a>.</li>
-
-  <li class="hangingindent" id="Motion">Motion: organic, and environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page75">75–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page175">175–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page196">196</a>;</li>
-  <li class="i1">of animals and waste, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page214">214</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page220">220</a>;</li>
-  <li class="i1">simulation of locomotive structures, <b>II</b>,
-    <a href="#Page_15">15</a>.</li>
-
-  <li>Motor organs, differentiation of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page262">262</a>.</li>
-
-  <li>Mountains: climatic effects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page504">504</a>;</li>
-  <li class="i1">growth of trees on, <b>II</b>,
-    <a href="#Page_142">142</a>.</li>
-
-  <li>Mouse: fertility of, <b>II</b>,
-    <a href="#Page_421">421</a>,
-    <a href="#Page_473">473</a>;</li>
-  <li class="i1">tapeworm parasitism, <b>II</b>,
-    <a href="#Page_490">490</a>;</li>
-  <li class="i1">compared with rat, <b>II</b>,
-    <a href="#Page_503">503–4</a>.</li>
-
-  <li><i>Mucor</i>, <b>II</b>,
-    <a href="#Page_22">22</a>,
-    <a href="#Page_123">123</a>.</li>
-
-  <li>Mucous membrane, differentiation, <b>II</b>,
-    <a href="#Page_321">321–2</a>,
-    <a href="#Page_389">389</a>.</li>
-
-  <li class="hangingindent" id="Multiplication">Multiplication: decline of fertility with evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page103">103</a>; <b>II</b>,
-    <a href="#Page_431">431</a>;</li>
-  <li class="i1">vitalism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page116">116</a>;</li>
-  <li class="i1">phenomena classified, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page130">130</a>;</li>
-  <li class="i1">the term “genesis,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page269">269</a>;</li>
-  <li class="i1">processes classified, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page270">270–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page336">336</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page583">583</a>;</li>
-  <li class="i1">a process of disintegration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page276">276</a>;</li>
-  <li class="i1">reproductive tissue in gamogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page278">278–84</a>;</li>
-  <li class="i1">nutrition and growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page285">285–94</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page299">299</a>;</li>
-  <li class="i1">natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295–8</a>;</li>
-  <li class="i1">hermaphrodism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page340">340–4</a>;</li>
-  <li class="i1">in-and-in breeding, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page344">344–7</a>;</li>
-  <li class="i1">physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page350">350–5</a>;</li>
-  <li class="hangingindent1">law of race-maintenance, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page581">581</a>; <b>II</b>,
-    <a href="#Page_420">420–3</a>,
-    <a href="#Page_430">430</a>;</li>
-  <li class="hangingindent1">effect of mental application, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page597">597</a>; <b>II</b>,
-    <a href="#Page_511">511–4</a>,
-    <a href="#Page_516">516–9</a>,
-    <a href="#Page_530">530</a>;</li>
-  <li class="hangingindent1">individuation antagonistic to, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page598">598–600</a>; <b>II</b>,
-    <a href="#Page_428">428–30</a>,
-    <a href="#Page_435">435–7</a>,
-    <a href="#Page_499">499</a>,
-    <a href="#Page_501">501–5</a>;</li>
-  <li class="i1">checks put by carnivores on, <b>II</b>,
-    <a href="#Page_405">405</a>;</li>
-  <li class="i1">four factors in rate of, <b>II</b>,
-    <a href="#Page_416">416</a>,
-    <a href="#Page_435">435</a>;</li>
-  <li class="i1">destructive and preservative forces, <b>II</b>,
-    <a href="#Page_417">417–20</a>;</li>
-  <li class="i1">rhythm of species, <b>II</b>,
-    <a href="#Page_419">419</a>;</li>
-  <li class="hangingindent1">nutrition and disintegration of, <b>II</b>,
-    <a href="#Page_424">424</a>,
-    <a href="#Page_425">425</a>,
-    <a href="#Page_430">430</a>;</li>
-  <li class="i1">integration and genesis, <b>II</b>,
-    <a href="#Page_426">426–8</a>;</li>
-  <li class="i1">influence of environment, <b>II</b>,
-    <a href="#Page_432">432–3</a>;</li>
-  <li class="i1">and variations of expenditure, <b>II</b>,
-    <a href="#Page_433">433–5</a>;</li>
-  <li class="i1">growth and asexual genesis, <b>II</b>,
-    <a href="#Page_439">439–46</a>;</li>
-  <li class="i1">asexual and sexual distinguished, <b>II</b>,
-    <a href="#Page_448">448</a>;</li>
-  <li class="i1">sexual genesis and growth, <b>II</b>,
-    <a href="#Page_448">448–58</a>,
-    <a href="#Page_495">495</a>;</li>
-  <li class="i1">and development, <b>II</b>,
-    <a href="#Page_461">461–5</a>;</li>
-  <li class="i1">plant expenditure, <b>II</b>,
-    <a href="#Page_467">467</a>;</li>
-  <li class="i1">animal expenditure, <b>II</b>,
-    <a href="#Page_468">468–72</a>;</li>
-  <li class="i1">nutrition in plants, <b>II</b>,
-    <a href="#Page_475">475</a>,
-    <a href="#Page_511">511</a>;</li>
-  <li class="i1">in animals, <b>II</b>,
-    <a href="#Page_476">476–84</a>,
-    <a href="#Page_511">511</a>;</li>
-  <li class="i1">seasonal variations, <b>II</b>,
-    <a href="#Page_484">484–5</a>;</li>
-  <li class="i1">nutrition, <i>résumé</i>, <b>II</b>,
-    <a href="#Page_486">486</a>,
-    <a href="#Page_497">497–9</a>;</li>
-  <li class="i1">nutrition and parasitic, <b>II</b>,
-    <a href="#Page_486">486–90</a>;</li>
-  <li class="i1">reversion to agamogenesis, <b>II</b>,
-    <a href="#Page_490">490–2</a>;</li>
-  <li class="i1">human fertility, <b>II</b>,
-    <a href="#Page_506">506–10</a>;</li>
-  <li class="i1">Doubleday on, <b>II</b>,
-    <a href="#Page_510">510–2</a>;</li>
-  <li class="i1">civilized and uncivilized, <b>II</b>,
-    <a href="#Page_514">514–6</a>;</li>
-  <li class="i1">human evolution and decline in, <b>II</b>,
-    <a href="#Page_529">529–31</a>;</li>
-  <li class="i1">the future of population, <b>II</b>,
-    <a href="#Page_532">532–7</a>;</li>
-  <li class="i1">equilibration and evolution, <b>II</b>,
-    <a href="#Page_537">537</a>.</li>
-
-  <li>Muscle: electrical contrasts, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page50">50</a>;</li>
-  <li class="i1">action of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page59">59</a>;</li>
-  <li class="i1">metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page70">70</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page71">71–4</a>;</li>
-  <li class="i1">definition of life and actions of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page112">112–3</a>;</li>
-  <li class="i1">growth and function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page151">151</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page155">155</a>;</li>
-  <li class="i1">development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page170">170</a>;</li>
-  <li class="i1">Hertwig’s classification of tissues, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page189">189</a>;</li>
-  <li class="i1">functional differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page203">203–4</a>;</li>
-  <li class="i1">waste and repair, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page215">215–7</a>;</li>
-  <li class="hangingindent1">modifiability and adaptability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page228">228–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page230">230</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page232">232</a>;</li>
-  <li class="hangingindent1">correlated variations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page536">536–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page614">614–21</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">resistance to strains, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page639">639</a>;</li>
-  <li class="i1">action on bones in Punjabis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>;</li>
-  <li class="i1">differentiation, <b>II</b>,
-    <a href="#Page_361">361–9</a>;</li>
-  <li class="i1">activity and colour, <b>II</b>,
-    <a href="#Page_365">365–9</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_376">376</a>,
-    <a href="#Page_382">382</a>;</li>
-  <li class="i1">equilibration in action, <b>II</b>,
-    <a href="#Page_393">393</a>;</li>
-  <li class="i1">activity and fertility in birds, <b>II</b>,
-    <a href="#Page_470">470–2</a>;</li>
-  <li class="i1">future human evolution, <b>II</b>,
-    <a href="#Page_523">523</a>;</li>
-  <li class="i1">origin of vertebrate type, <b>II</b>,
-    <a href="#Page_598">598–600</a>.</li>
-
-  <li>Music: limited adaptability of voice and ear, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page231">231</a>;</li>
-  <li class="i1">inheritance of faculty, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page311">311–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page694">694</a>.</li>
-
-  <li class="hangingindent">Mutilations, the question of their inheritance, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page631">631</a>.</li>
-
-  <li><i>Mycetozoa</i>, growth and reproduction, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page298">298–9</a>.</li>
-
-  <li class="hangingindent">Myocommata (myotomes), and vertebrate skeleton, <b>II</b>,
-    <a href="#Page_216">216</a>,
-    <a href="#Page_217">217–8</a>,
-    <a href="#Page_222">222</a>.</li>
-
-  <li>Myopia, inheritance of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page306">306</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page694">694</a>.</li>
-
-  <li><i>Myrianida fasciata</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page361">361</a>; <b>II</b>,
-    <a href="#Page_445">445</a>.</li>
-
-  <li><i>Myriapoda</i>: gemmation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page589">589</a>;</li>
-  <li class="i1">segmentation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page590">590</a>; <b>II</b>,
-    <a href="#Page_113">113</a>,
-    <a href="#Page_114">114</a>,
-    <a href="#Page_601">601</a>;</li>
-  <li class="i1">degenerated eyes of cave-inhabiting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page649">649</a>;</li>
-  <li class="i1">integration and homology, <b>II</b>,
-    <a href="#Page_111">111–4</a>;</li>
-  <li class="i1">genesis, <b>II</b>,
-    <a href="#Page_445">445</a>.</li>
-
-  <li><i>Myxothallophyta</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page378">378</a>.</li>
-</ul>
-
-<ul>
-  <li>Nails, mammalian, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page473">473</a>.</li>
-
-  <li class="hangingindent"><i>Nais</i>: regeneration of detached parts, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page219">219</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page361">361</a>.</li>
-
-  <li class="hangingindent">Narcissus, separation of ancestral traits in hybrids, <b>II</b>,
-    <a href="#Page_617">617</a>.</li>
-
-  <li id="Natural_selection">Natural selection: structural modification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page211">211</a>;</li>
-  <li class="i1">in cell processes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page263">263–4</a>;</li>
-  <li class="i1">multiplication, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295–8</a>;</li>
-  <li class="i1">aided by function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page308">308–10</a>;</li>
-  <li class="i1">special creation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page426">426–7</a>;</li>
-  <li class="i1">the term “survival of the fittest,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page530">530</a>;</li>
-  <li class="hangingindent1">indirect equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page530">530–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page552">552–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page571">571</a>;</li>
-  <li class="hangingindent1">changes unexplained by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page535">535–42</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page571">571</a>; <b>II</b>,
-    <a href="#Page_371">371</a>;</li>
-  <li class="i1">tendency to economy, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page536">536</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page562">562</a>;</li>
-  <li class="i1">decrease of jaw, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page541">541</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">general doctrine of evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page543">543–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">unceasing operation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page552">552</a>;</li>
-  <li class="i1">human races, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>;</li>
-  <li class="i1">current views, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page559">559–60</a>;</li>
-  <li class="i1">panmixia and cessation of selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page560">560–3</a>;</li>
-  <li class="i1">intra-selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page562">562</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676–8</a>;</li>
-  <li class="i1">Eimer’s theory of orthogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page564">564</a>;</li>
-  <li class="i1">Mr. Cunningham’s criticism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page565">565–6</a>;</li>
-  <li class="i1">location of mammalian testes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page573">573</a>;</li>
-  <li class="i1">co-ordinated instincts of mason-wasp, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page574">574</a>;</li>
-  <li class="hangingindent1">tactual perceptiveness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page603">603–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page633">633</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page665">665</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page671">671</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page672">672</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="hangingindent1">erroneously identified with artificial selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page609">609</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page695">695</a>;</li>
-  <li class="i1">reversed selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page611">611</a>;</li>
-  <li class="hangingindent1">blindness of cave-animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page613">613</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page614">614</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page647">647–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="hangingindent1">co-adaptation of co-operative parts, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page614">614</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page621">621</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page663">663–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page674">674</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="i1">where operative, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page632">632</a>;</li>
-  <li class="i1">Weismann on conceivability of process, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page651">651</a>;</li>
-  <li class="i1">degeneration of little toe, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page652">652–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page673">673</a>;</li>
-  <li class="hangingindent1">genesis of caste gradations In social insects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page654">654–60</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page663">663</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page684">684</a>;</li>
-  <li class="i1">self-feeding instinct in ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>;</li>
-  <li class="i1">rudimentary organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page667">667–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page671">671</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="i1">horns of stag, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="i1">musical faculty, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page694">694</a>;</li>
-  <li class="i1">the neo-Darwinian position reviewed, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page694">694–5</a>;</li>
-  <li class="i1">vegetal nutrition, <b>II</b>,
-    <a href="#Page_51">51–2</a>;</li>
-  <li class="i1">upright vegetal growth, <b>II</b>,
-    <a href="#Page_56">56–7</a>;</li>
-  <li class="i1">endogenous growth, <b>II</b>,
-    <a href="#Page_57">57–8</a>;</li>
-  <li class="i1">exogenous, <b>II</b>,
-    <a href="#Page_64">64</a>;</li>
-  <li class="i1"><i>Navicula</i> symmetry, <b>II</b>,
-    <a href="#Page_135">135</a>;</li>
-  <li class="i1">foliar, <b>II</b>,
-    <a href="#Page_158">158</a>;</li>
-  <li class="i1">foliar distribution, <b>II</b>,
-    <a href="#Page_167">167</a>,
-    <a href="#Page_179">179</a>;</li>
-  <li class="hangingindent1">floral fertilization and symmetry, <b>II</b>,
-    <a href="#Page_168">168–70</a>,
-    <a href="#Page_608">608–11</a>;</li>
-  <li class="i1">helical phænogamic growth, <b>II</b>,
-    <a href="#Page_181">181</a>;</li>
-  <li class="hangingindent1"><i>Echinodermata</i> and bilateral symmetry, <b>II</b>,
-    <a href="#Page_197">197</a>;</li>
-  <li class="i1">vertebrate structure, <b>II</b>,
-    <a href="#Page_214">214–20</a>,
-    <a href="#Page_227">227</a>;</li>
-  <li class="i1">phænogamic tissue differentiation, <b>II</b>,
-    <a href="#Page_248">248</a>;</li>
-  <li class="i1">physiological differentiation, <b>II</b>,
-    <a href="#Page_252">252</a>,
-    <a href="#Page_256">256</a>;</li>
-  <li class="i1">rootlets of ivy, <b>II</b>,
-    <a href="#Page_254">254</a>;</li>
-  <li class="i1">stomata and foliar surfaces, <b>II</b>,
-    <a href="#Page_261">261</a>,
-    <a href="#Page_262">262</a>;</li>
-  <li class="i1">floral fertilization, <b>II</b>,
-    <a href="#Page_268">268–9</a>;</li>
-  <li class="i1">sexual selection, <b>II</b>,
-    <a href="#Page_269">269</a>;</li>
-  <li class="i1">vegetal tissue differentiation, <b>II</b>,
-    <a href="#Page_279">279</a>;</li>
-  <li class="i1">wood formation, <b>II</b>,
-    <a href="#Page_287">287–8</a>,
-    <a href="#Page_290">290</a>;</li>
-  <li class="i1">animal tissue differentiation, <b>II</b>,
-    <a href="#Page_304">304–8</a>;</li>
-  <li class="i1">evolution of nervous system, <b>II</b>,
-    <a href="#Page_307">307–8</a>;</li>
-  <li class="i1">respiratory system, <b>II</b>,
-    <a href="#Page_311">311</a>;</li>
-  <li class="i1">dermal callosities, <b>II</b>,
-    <a href="#Page_312">312–4</a>;</li>
-  <li class="i1">sensory organ complexities, <b>II</b>,
-    <a href="#Page_321">321</a>;</li>
-  <li class="hangingindent1">skin and mucous membrane differentiation, <b>II</b>,
-    <a href="#Page_322">322</a>;</li>
-  <li class="i1">localization of excretion, <b>II</b>,
-    <a href="#Page_333">333</a>;</li>
-  <li class="i1">respiratory organs of fishes, <b>II</b>,
-    <a href="#Page_335">335–8</a>;</li>
-  <li class="i1">heart and vascular system, <b>II</b>,
-    <a href="#Page_341">341</a>,
-    <a href="#Page_344">344</a>;</li>
-  <li class="i1">osseous differentiation, <b>II</b>,
-    <a href="#Page_355">355</a>;</li>
-  <li class="i1">also muscular, <b>II</b>,
-    <a href="#Page_363">363</a>,
-    <a href="#Page_368">368–9</a>;</li>
-  <li class="i1">“false joints,” <b>II</b>,
-    <a href="#Page_371">371</a>;</li>
-  <li class="i1">insect nutrition and genesis, <b>II</b>,
-    <a href="#Page_499">499</a>;</li>
-  <li class="i1">economics of evolution, <b>II</b>,
-    <a href="#Page_501">501–5</a>;</li>
-  <li class="hangingindent1">author’s enunciation of survival of the fittest in 1852, <b>II</b>,
-    <a href="#Page_528">528–9</a>;</li>
-  <li class="i1">evils of interference with, in man, <b>II</b>,
-    <a href="#Page_532">532–3</a>;</li>
-  <li class="i1">vegetal tissue formation, <b>II</b>,
-    <a href="#Page_582">582</a>,
-    <a href="#Page_594">594–6</a>;</li>
-  <li class="i1">origin of vertebrate type, <b>II</b>,
-    <a href="#Page_599">599</a>.</li>
-
-  <li>Nature, more complex than supposed, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page252">252</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page450">450</a>.</li>
-
-  <li><i>Navicula</i>, symmetry, <b>II</b>,
-    <a href="#Page_134">134–5</a>.</li>
-
-  <li>“Nebular Hypothesis,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page23">23</a>.</li>
-
-  <li class="hangingindent">Negation, inconceivability of, the ultimate test of truth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>.</li>
-
-  <li>Negroes, telegony In United States, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page644">644–5</a>.</li>
-
-  <li class="hangingindent"><i>Nemertidæ</i>: continuing vitality of pilidium, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page250">250</a>;</li>
-  <li class="i1">bilateral symmetry, <b>II</b>,
-    <a href="#Page_195">195</a>.</li>
-
-  <li>Neo-Darwinists, and Lamarck, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page630">630</a>;</li>
-  <li class="i1">their position reviewed, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page694">694–5</a>.</li>
-
-  <li>Nerves: electrical phenomena, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page51">51</a>;</li>
-  <li class="i1">generation of nerve force, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page52">52–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page60">60</a>;</li>
-  <li class="i1"><i>corpuscula tactus</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page75">75</a>;</li>
-  <li class="i1">Hertwig’s classification of tissues, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page189">189</a>;</li>
-  <li class="i1">structural traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page192">192</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page193">193</a>;</li>
-  <li class="i1">environment and structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page196">196</a>;</li>
-  <li class="i1">differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page203">203</a>; <b>II</b>,
-    <a href="#Page_355">355–61</a>;</li>
-  <li class="i1">vasomotor system, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page206">206</a>;</li>
-  <li class="i1">vicarious function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page209">209</a>;</li>
-  <li class="i1">activity and waste, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page216">216</a>;</li>
-  <li class="i1">adaptability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page229">229</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page232">232</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page236">236</a>;</li>
-  <li class="i1">parallelism in cell processes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page260">260–2</a>;</li>
-  <li class="i1">heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page313">313</a>;</li>
-  <li class="i1">effects of severance, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page349">349</a>;</li>
-  <li class="i1">relative development in men and women, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page594">594</a>;</li>
-  <li class="i1">analysis of brain substance, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page596">596</a>;</li>
-  <li class="hangingindent1">individuation and development of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page598">598</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page599">599</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page600">600</a>;</li>
-  <li class="hangingindent1">distribution of tactual perceptiveness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page603">603–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page633">633</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page665">665–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page671">671</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page672">672</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="i1">alleged costliness of tissue, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page662">662</a>;</li>
-  <li class="i1">instinct degeneration in ants, <i>ib.</i>;</li>
-  <li class="i1">“sensation areas,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page666">666</a>;</li>
-  <li class="i1">segmentation in Annelids, <b>II</b>,
-    <a href="#Page_125">125</a>;</li>
-  <li class="i1">ectodermal derivation, <b>II</b>,
-    <a href="#Page_303">303–4</a>;</li>
-  <li class="i1">co-operating factors in evolution of, <b>II</b>,
-    <a href="#Page_307">307–8</a>;</li>
-  <li class="i1">differentiation from muscle, <b>II</b>,
-    <a href="#Page_363">363</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Psychology">Psychology</a>.)</li>
-
-  <li>Nervousness, hereditary transmission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page307">307</a>.</li>
-
-  <li>Neurine, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page594">594</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page597">597</a>.</li>
-
-  <li>Neuter insects (<i>see</i> <a href="#Insects">Insects</a>).</li>
-
-  <li>New Zealand: invasion of alien species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page477">477</a>;</li>
-  <li class="i1">kinship of past and present forms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page489">489</a>.</li>
-
-  <li>Nitrogen: properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page3">3–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page20">20</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page24">24</a>;</li>
-  <li class="hangingindent1">compounds and their properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page6">6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page8">8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page12">12–14</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page25">25–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page39">39</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page41">41</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page42">42–3</a>; <b>II</b>,
-    <a href="#Page_250">250</a>;</li>
-  <li class="i1">organic importance, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page42">42–3</a>;</li>
-  <li class="i1">evolution of heat and oxidation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page47">47</a>;</li>
-  <li class="i1">violent organic effects of compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page54">54–5</a>;</li>
-  <li class="i1">function in metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page63">63–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page66">66</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page68">68–76</a>;</li>
-  <li class="i1">presence in protoplasm, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page66">66</a>;</li>
-  <li class="i1">action in digestion, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>;</li>
-  <li class="i1">fat accumulation and fertility, <b>II</b>,
-    <a href="#Page_483">483</a>.</li>
-
-  <li>Nitro-glycerine, violent effects of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page55">55</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page122">122</a>.</li>
-
-  <li>Notochord: segmentation, <b>II</b>,
-    <a href="#Page_125">125</a>,
-    <a href="#Page_218">218–22</a>;</li>
-  <li class="i1">formation, <b>II</b>,
-    <a href="#Page_217">217–8</a>,
-    <a href="#Page_600">600</a>;</li>
-  <li class="i1">in Permian vertebrates, <b>II</b>,
-    <a href="#Page_225">225</a>.</li>
-
-  <li>Noumenon, life not manifested as, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page580">580</a>.</li>
-
-  <li>Nuclein, <b>II</b>,
-    <a href="#Page_21">21</a>.</li>
-
-  <li>Nucleus: central development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page163">163</a>;</li>
-  <li class="i1">in simple organisms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page183">183</a>;</li>
-  <li class="i1">phenomena exhibited by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page255">255–8</a>;</li>
-  <li class="i1">current hypotheses of function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page258">258–9</a>;</li>
-  <li class="i1">properties and function of chromatin, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page259">259–65</a>;</li>
-  <li class="i1">fusion in fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page283">283–4</a>;</li>
-  <li class="i1">function in unicellular reproduction, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page595">595–6</a>;</li>
-  <li class="i1">absence of, <b>II</b>,
-    <a href="#Page_20">20–1</a>;</li>
-  <li class="i1">diffused form, <b>II</b>,
-    <a href="#Page_85">85</a>;</li>
-  <li class="i1">macro- and micronucleus in conjugation, <b>II</b>,
-    <a href="#Page_452">452</a>.</li>
-
-  <li id="Nutrition">Nutrition: organic molecular re-arrangement, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page36">36</a>;</li>
-  <li class="hangingindent1">nitrogenous and non-nitrogenous, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page47">47–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page68">68</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page71">71–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page77">77</a>; <b>II</b>,
-    <a href="#Page_362">362</a>;</li>
-  <li class="i1">food assimilation and reasoning, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page81">81</a>;</li>
-  <li class="i1">needful for vital change, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page94">94</a>;</li>
-  <li class="hangingindent1">relation to growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page140">140</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page143">143</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page144">144</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page147">147–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page150">150</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page157">157</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page161">161</a>;</li>
-  <li class="i1">expenditure of energy, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page157">157</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page391">391</a>;</li>
-  <li class="i1">fluid, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page208">208</a>;</li>
-  <li class="i1">vegetal fructification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page267">267</a>; <b>II</b>,
-    <a href="#Page_266">266</a>;</li>
-  <li class="i1">vegetal growth and genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page293">293</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page294">294–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page336">336</a>;</li>
-  <li class="i1">animal growth and genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page289">289–93</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page336">336</a>;</li>
-  <li class="hangingindent1">conditions qualifying antagonism of growth and genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page299">299</a>;</li>
-  <li class="hangingindent1">competition among parts of an organism for, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page562">562</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page566">566</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676</a>;</li>
-  <li class="i1">sex differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page594">594–5</a>;</li>
-  <li class="i1">cell multiplication, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page638">638</a>;</li>
-  <li class="hangingindent1">differentiation of neuter insects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page655">655–60</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page674">674</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page686">686–8</a>;</li>
-  <li class="i1">monstrous ant forms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page683">683–4</a>;</li>
-  <li class="i1">leaf development, <b>II</b>,
-    <a href="#Page_39">39</a>,
-    <a href="#Page_42">42</a>,
-    <a href="#Page_73">73–8</a>;</li>
-  <li class="i1">vegetal development, <b>II</b>,
-    <a href="#Page_51">51–2</a>,
-    <a href="#Page_178">178</a>,
-    <a href="#Page_276">276</a>;</li>
-  <li class="i1">axillary buds, <b>II</b>,
-    <a href="#Page_65">65–9</a>,
-    <a href="#Page_73">73–4</a>;</li>
-  <li class="i1">effect on animal aggregation, <b>II</b>,
-    <a href="#Page_93">93</a>;</li>
-  <li class="i1">internodes and inflorescence, <b>II</b>,
-    <a href="#Page_178">178–80</a>;</li>
-  <li class="i1">helical phænogamic growth, <b>II</b>,
-    <a href="#Page_181">181</a>;</li>
-  <li class="i1">action of bile, <b>II</b>,
-    <a href="#Page_330">330</a>;</li>
-  <li class="i1">osseous development, <b>II</b>,
-    <a href="#Page_349">349</a>,
-    <a href="#Page_353">353</a>;</li>
-  <li class="i1">genesis, <b>II</b>,
-    <a href="#Page_419">419</a>,
-    <a href="#Page_422">422</a>,
-    <a href="#Page_427">427</a>,
-    <a href="#Page_435">435–7</a>,
-    <a href="#Page_452">452</a>;</li>
-  <li class="i1">parental loss in feeding young, <b>II</b>,
-    <a href="#Page_424">424</a>,
-    <a href="#Page_429">429</a>;</li>
-  <li class="i1">diverse sources, <b>II</b>,
-    <a href="#Page_433">433</a>;</li>
-  <li class="i1">Carpenter on reproduction and, <b>II</b>,
-    <a href="#Page_460">460</a>;</li>
-  <li class="i1">animal development and genesis, <b>II</b>,
-    <a href="#Page_465">465</a>;</li>
-  <li class="i1">expenditure and genesis, <b>II</b>,
-    <a href="#Page_468">468</a>;</li>
-  <li class="i1">variations of genesis, <b>II</b>,
-    <a href="#Page_475">475–80</a>,
-    <a href="#Page_511">511</a>;</li>
-  <li class="i1">obesity and genesis, <b>II</b>,
-    <a href="#Page_480">480–4</a>,
-    <a href="#Page_511">511</a>;</li>
-  <li class="i1">general doctrine of genesis, <b>II</b>,
-    <a href="#Page_486">486</a>;</li>
-  <li class="i1">genesis and vegetal parasitism, <b>II</b>,
-    <a href="#Page_486">486</a>;</li>
-  <li class="i1">also animal, <b>II</b>,
-    <a href="#Page_487">487–90</a>,
-    <a href="#Page_495">495</a>;</li>
-  <li class="i1">insect genesis, <b>II</b>,
-    <a href="#Page_490">490–2</a>;</li>
-  <li class="i1">genesis, <i>résumé</i>, <b>II</b>,
-    <a href="#Page_497">497–9</a>;</li>
-  <li class="i1">and evolution, <b>II</b>,
-    <a href="#Page_501">501–4</a>;</li>
-  <li class="i1">of blackbird and linnet, <b>II</b>,
-    <a href="#Page_503">503</a>;</li>
-  <li class="i1">genesis in human race, <b>II</b>,
-    <a href="#Page_508">508–10</a>,
-    <a href="#Page_514">514–6</a>;</li>
-  <li class="i1">Doubleday on, <b>II</b>,
-    <a href="#Page_510">510–2</a>;</li>
-  <li class="i1">future human evolution, <b>II</b>,
-    <a href="#Page_526">526</a>,
-    <a href="#Page_531">531</a>;</li>
-  <li class="i1">floral monstrosities, <b>II</b>,
-    <a href="#Page_542">542</a>,
-    <a href="#Page_546">546</a>,
-    <a href="#Page_547">547</a>.</li>
-</ul>
-
-<ul>
-  <li>Obesity, nutrition and genesis, <b>II</b>,
-    <a href="#Page_480">480–4</a>,
-    <a href="#Page_511">511</a>.</li>
-
-  <li>Odoriferous glands, natural selection and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page534">534</a>.</li>
-
-  <li>Odours: floral fertilization, <b>II</b>,
-    <a href="#Page_268">268–9</a>;</li>
-  <li class="i1">animal protection, <b>II</b>,
-    <a href="#Page_434">434</a>.</li>
-
-  <li class="hangingindent">Offspring: parental loss entailed by nurture, <b>II</b>,
-    <a href="#Page_424">424</a>,
-    <a href="#Page_429">429</a>;</li>
-  <li class="i1">influence of age on, <b>II</b>,
-    <a href="#Page_507">507</a>.</li>
-
-  <li>Oken, L., archetypal hypothesis, <b>II</b>,
-    <a href="#Page_122">122</a>;</li>
-  <li class="i1">theory of supernumerary bones, <b>II</b>,
-    <a href="#Page_223">223</a>;</li>
-  <li class="i1">on the skull, <b>II</b>,
-    <a href="#Page_561">561</a>.</li>
-
-  <li>Oliver, F. W., classification of plants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page378">378–9</a>.</li>
-
-  <li class="hangingindent"><i>Ophryotrocha puerilis</i>, ciliation of segments, <b>II</b>,
-    <a href="#Page_109">109</a>.</li>
-
-  <li id="Orchids">Orchids: pollen propulsion, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page57">57</a>;</li>
-  <li class="i1">leaf formation in <i>Dendrobium</i>, <b>II</b>,
-    <a href="#Page_60">60–1</a>;</li>
-  <li class="hangingindent1">aërial roots and physiological differentiation, <b>II</b>,
-    <a href="#Page_255">255</a>,
-    <a href="#Page_257">257</a>;</li>
-  <li class="i1">foliar surface, <b>II</b>,
-    <a href="#Page_264">264</a>.</li>
-
-  <li>Organic matter: properties of elements, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page3">3–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page22">22</a>;</li>
-  <li class="i1">of compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page5">5–13</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page25">25</a>;</li>
-  <li class="i1">molar and molecular mobility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page12">12–14</a>;</li>
-  <li class="i1">colloid and crystalloid form, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page15">15–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page25">25</a>;</li>
-  <li class="i1">their diffusibility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page18">18–21</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page26">26</a>;</li>
-  <li class="i1">extreme complexity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page21">21</a>;</li>
-  <li class="i1">laws of evolution and genesis of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page22">22–4</a>;</li>
-  <li class="i1">modifiability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page27">27</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page44">44</a>;</li>
-  <li class="i1">capillarity and osmosis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page28">28</a>;</li>
-  <li class="i1">effects of heat, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page29">29</a>;</li>
-  <li class="i1">of light, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page30">30–4</a>;</li>
-  <li class="i1">nitrogenous, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page39">39–43</a>;</li>
-  <li class="i1">oxidation and evolution of heat, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page46">46</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page60">60</a>;</li>
-  <li class="i1">genesis of electricity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page50">50–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page60">60</a>;</li>
-  <li class="i1">sensible motions in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page59">59</a>;</li>
-  <li class="i1">transformations and persistence of force, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page61">61</a>;</li>
-  <li class="i1">metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page62">62–77</a>;</li>
-  <li class="i1">artificial production of compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page64">64</a>;</li>
-  <li class="i1">contrasted with inorganic matter, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page107">107–8</a>;</li>
-  <li class="i1">incomprehensibility of vital changes in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page122">122</a>;</li>
-  <li class="i1">instability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page149">149</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page508">508</a>;</li>
-  <li class="i1">phosphorus in cell-organization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page260">260–1</a>;</li>
-  <li class="i1">heterogeneity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page350">350–5</a>;</li>
-  <li class="hangingindent1">“spontaneous generation” and evolution of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page696">696–701</a>;</li>
-  <li class="i1">cell-doctrine and evolution of, <b>II</b>,
-    <a href="#Page_17">17–21</a>.</li>
-
-  <li>Organization (<i>see</i> <a href="#Structure">Structure</a>).</li>
-
-  <li>Ormerod, Dr., on sex and nutrition in wasps, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page656">656</a>.</li>
-
-  <li>Orthogenesis, Eimer’s theory of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page563">563–4</a>.</li>
-
-  <li>Osmosis: organic effects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page28">28</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page29">29</a>;</li>
-  <li class="i1">in animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page58">58</a>;</li>
-  <li class="i1">in vascular system, <b>II</b>,
-    <a href="#Page_339">339</a>;</li>
-  <li class="i1">in vegetal tissue, <b>II</b>,
-    <a href="#Page_568">568</a>,
-    <a href="#Page_575">575</a>,
-    <a href="#Page_577">577</a>,
-    <a href="#Page_585">585</a>,
-    <a href="#Page_592">592–6</a>.</li>
-
-  <li>Osteology (<i>see</i> <a href="#Bone">Bone</a>).</li>
-
-  <li>Ovum (<i>see</i> <a href="#Egg">Egg</a> <i>and</i> <a href="#Fertilization">Fertilization</a>).</li>
-
-  <li class="hangingindent">Owen, Sir R.: metagenesis and parthenogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page273">273–4</a>;</li>
-  <li class="i1">fossil mammals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page410">410</a>;</li>
-  <li class="i1">human parasites, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page427">427</a>;</li>
-  <li class="i1">continuous operation of creative power, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page492">492</a>;</li>
-  <li class="i1">fission in <i>Infusoria</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page584">584</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page585">585</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page595">595–6</a>;</li>
-  <li class="i1">parthenogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page592">592</a>;</li>
-  <li class="i1">theory of vertebrate skeleton, <b>II</b>,
-    <a href="#Page_123">123</a>,
-    <a href="#Page_548">548–66</a>;</li>
-  <li class="i1">theory of supernumerary bones, <b>II</b>,
-    <a href="#Page_223">223</a>;</li>
-  <li class="i1">Eschricht on <i>Ascaris</i>, <b>II</b>,
-    <a href="#Page_488">488</a>.</li>
-
-  <li><i>Oxalis</i>: radial symmetry, <b>II</b>,
-    <a href="#Page_152">152</a>;</li>
-  <li class="i1">foliar surface, <b>II</b>,
-    <a href="#Page_264">264</a>.</li>
-
-  <li>Oxen: comparison with sheep, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page158">158</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page160">160</a>;</li>
-  <li class="i1">cerebro-spinal system, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page508">508</a>.</li>
-
-  <li>Oxidation (<i>see</i> <a href="#Oxygen">Oxygen</a>).</li>
-
-  <li id="Oxygen">Oxygen: properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page3">3–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page20">20</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page22">22</a>;</li>
-  <li class="i1">compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page6">6–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page10">10–13</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page22">22</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page24">24–5</a>;</li>
-  <li class="i1">a crystalloid, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page21">21</a>;</li>
-  <li class="i1">combining power and atomic weight, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page33">33</a>;</li>
-  <li class="i1">organic change from, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page37">37</a>;</li>
-  <li class="i1">heat generation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page46">46–9</a>;</li>
-  <li class="i1">phosphorescence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page49">49</a>;</li>
-  <li class="i1">nerve force dependent on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page53">53</a>;</li>
-  <li class="i1">animal metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page72">72</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page73">73</a>;</li>
-  <li class="i1">necessary to animal life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page94">94–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page577">577</a>;</li>
-  <li class="i1">activity and amount inhaled, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page214">214</a>.</li>
-</ul>
-
-<ul>
-  <li class="hangingindent">Packard, A. S., on eyes of cave-animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page648">648–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>.</li>
-
-  <li class="hangingindent">Paget, Sir J., blood changes in small-pox and scarlatina, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page221">221</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page701">701</a>.</li>
-
-  <li id="Palaeontology">Palæontology: distribution in time, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page404">404–11</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page412">412</a>;</li>
-  <li class="i1">special creation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page425">425</a>;</li>
-  <li class="i1">congruity with evolution hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page485">485–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">relations of present to extinct species, <b>II</b>,
-    <a href="#Page_10">10–11</a>;</li>
-  <li class="i1">scarcity of remains, <b>II</b>,
-    <a href="#Page_34">34–5</a>;</li>
-  <li class="i1">secondary thickening in plants, <b>II</b>,
-    <a href="#Page_56">56</a>;</li>
-  <li class="hangingindent1">Cope on osteology of Permian Vertebrates, <b>II</b>,
-    <a href="#Page_225">225–6</a>.</li>
-
-  <li class="hangingindent">Pangenesis, Darwin’s theory of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page356">356</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page357">357</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page359">359</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page360">360</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page362">362</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page372">372</a>.</li>
-
-  <li class="hangingindent">Panmixia, Weismann’s hypothesis of: Its relation to Romanes’ “cessation of selection,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page560">560</a>;</li>
-  <li class="hangingindent1">alleged selective process denied, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page561">561–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page667">667</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>;</li>
-  <li class="i1">distribution of tactual perceptiveness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page608">608</a>;</li>
-  <li class="i1">rudimentary eyes of cave fauna, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page647">647</a>;</li>
-  <li class="i1">Romanes on process, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page649">649</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page667">667</a>;</li>
-  <li class="hangingindent1">degeneration of self-feeding instinct In Amazon ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>;</li>
-  <li class="i1">rudimentary limbs of whale, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page668">668–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>;</li>
-  <li class="i1">a pure speculation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page671">671</a>;</li>
-  <li class="i1">markings on leg-bones of Punjabis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>.</li>
-
-  <li><i>Paramœcium</i>: parasite infesting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page427">427</a>;</li>
-  <li class="i1">reproduction, <b>II</b>,
-    <a href="#Page_443">443</a>,
-    <a href="#Page_452">452</a>.</li>
-
-  <li>Parasites: sexual dimorphism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page315">315</a>;</li>
-  <li class="i1">limits to distribution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page397">397</a>;</li>
-  <li class="i1">special creation and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page427">427–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page438">438</a>;</li>
-  <li class="i1">retrograde development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page457">457</a>; <b>II</b>,
-    <a href="#Page_12">12</a>;</li>
-  <li class="i1">aphis and ant, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660–1</a>; <b>II</b>,
-    <a href="#Page_403">403</a>,
-    <a href="#Page_405">405</a>;</li>
-  <li class="i1">as an integrating agency, <b>II</b>,
-    <a href="#Page_402">402–4</a>;</li>
-  <li class="i1">its comparative recency, <b>II</b>,
-    <a href="#Page_404">404</a>;</li>
-  <li class="i1">nutrition and genesis in vegetal, <b>II</b>,
-    <a href="#Page_486">486</a>;</li>
-  <li class="i1">in animal, <b>II</b>,
-    <a href="#Page_487">487–90</a>,
-    <a href="#Page_493">493</a>;</li>
-  <li class="hangingindent1">“castration parasitaire” in crustaceans, <b>II</b>,
-    <a href="#Page_493">493–6</a>.</li>
-
-  <li>Parasol Ants, origin of classes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page687">687–8</a>.</li>
-
-  <li>Parthenogenesis: occurrence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page274">274–5</a>;</li>
-  <li class="i1">alternating with gamogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page289">289–91</a>;</li>
-  <li class="i1">Owen on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page592">592</a>;</li>
-  <li class="i1">laws of multiplication, <b>II</b>,
-    <a href="#Page_415">415</a>;</li>
-  <li class="i1">in articulate animals, <b>II</b>,
-    <a href="#Page_445">445</a>.</li>
-
-  <li>Pasteur, L., silkworm diseases, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page622">622–3</a>.</li>
-
-  <li class="hangingindent">Peacock: theories of heredity and structure of tail feather, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page372">372–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page695">695</a>; <b>II</b>,
-    <a href="#Page_618">618–9</a>.</li>
-
-  <li>Pear, foreright shoots, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page287">287</a>.</li>
-
-  <li>Peloria: in <i>gloxinia</i>, <b>II</b>,
-    <a href="#Page_166">166</a>;</li>
-  <li class="i1">phænogams, <b>II</b>,
-    <a href="#Page_180">180</a>.</li>
-
-  <li>Penguin, dermal structure, <b>II</b>,
-    <a href="#Page_314">314</a>.</li>
-
-  <li>Pepsin, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>.</li>
-
-  <li>Pericyclic fibres of monocotyledons, <b>II</b>,
-    <a href="#Page_278">278</a>.</li>
-
-  <li class="hangingindent"><i>Peripatus capensis</i>, protoplasmic continuity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page629">629</a>.</li>
-
-  <li class="hangingindent">Peri-visceral sac, function and differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page391">391</a>.</li>
-
-  <li>Perkin, W. H., <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#pagevi">vi</a>.</li>
-
-  <li class="hangingindent" id="Persistence_of_force">Persistence of force, corollaries from: properties of compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page3">3</a>;</li>
-  <li class="i1">organic transformation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page60">60</a>;</li>
-  <li class="i1">growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page150">150</a>;</li>
-  <li class="i1">organic energy, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page220">220</a>;</li>
-  <li class="i1">variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page335">335</a>;</li>
-  <li class="i1">genesis, heredity, and variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page354">354–5</a>;</li>
-  <li class="i1">morphological summary, <b>II</b>,
-    <a href="#Page_235">235</a>;</li>
-  <li class="i1">vegetal tissue differentiation, <b>II</b>,
-    <a href="#Page_245">245</a>;</li>
-  <li class="i1">physiological development, <b>II</b>,
-    <a href="#Page_394">394</a>.</li>
-
-  <li>Petals: foliar homology, <b>II</b>,
-    <a href="#Page_43">43–6</a>;</li>
-  <li class="i1">“adnate,” <b>II</b>,
-    <a href="#Page_58">58</a>.</li>
-
-  <li>Petrels, Darwin on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page455">455</a>.</li>
-
-  <li>Phænogams: production of spermatozoids, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page186">186</a>;</li>
-  <li class="i1">morphological composition, <b>II</b>,
-    <a href="#Page_37">37–79</a>;</li>
-  <li class="i1">leaf transitions, <b>II</b>,
-    <a href="#Page_37">37–42</a>;</li>
-  <li class="i1">foliar homologies, <b>II</b>,
-    <a href="#Page_42">42–9</a>;</li>
-  <li class="i1">origin of type, <b>II</b>,
-    <a href="#Page_49">49–84</a>;</li>
-  <li class="i1">vertical growth, <b>II</b>,
-    <a href="#Page_56">56–64</a>;</li>
-  <li class="i1">axillary buds, <b>II</b>,
-    <a href="#Page_66">66</a>;</li>
-  <li class="hangingindent1">cotyledonous germination and endogenous growth, <b>II</b>,
-    <a href="#Page_69">69–72</a>;</li>
-  <li class="i1">axial homologies, <b>II</b>,
-    <a href="#Page_73">73–5</a>;</li>
-  <li class="i1">irregular development, <b>II</b>,
-    <a href="#Page_75">75–8</a>;</li>
-  <li class="i1">degree of composition, <b>II</b>,
-    <a href="#Page_78">78</a>;</li>
-  <li class="i1">reproductive homology, <b>II</b>,
-    <a href="#Page_80">80–4</a>;</li>
-  <li class="i1">uni- and multiaxial symmetry, <b>II</b>,
-    <a href="#Page_141">141–3</a>;</li>
-  <li class="i1">unit of composition, <b>II</b>,
-    <a href="#Page_151">151</a>;</li>
-  <li class="i1">helical growth, <b>II</b>,
-    <a href="#Page_181">181</a>;</li>
-  <li class="i1">secondary thickening, <b>II</b>,
-    <a href="#Page_247">247</a>;</li>
-  <li class="hangingindent1">tissue and leaf differentiation, <b>II</b>,
-    <a href="#Page_247">247–9</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">also bark and cambium, <b>II</b>,
-    <a href="#Page_249">249–50</a>,
-    <a href="#Page_386">386</a>;</li>
-  <li class="i1">also outer tissue, <b>II</b>,
-    <a href="#Page_252">252</a>,
-    <a href="#Page_256">256–9</a>,
-    <a href="#Page_270">270</a>,
-    <a href="#Page_386">386–7</a>;</li>
-  <li class="i1">wax deposit on leaves, <b>II</b>,
-    <a href="#Page_260">260–2</a>;</li>
-  <li class="hangingindent1">differentiation of inner tissues, <b>II</b>,
-    <a href="#Page_273">273–5</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="i1">vascular system development, <b>II</b>,
-    <a href="#Page_280">280–4</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_293">293–5</a>,
-    <a href="#Page_296">296</a>,
-    <a href="#Page_390">390</a>;</li>
-  <li class="i1">insect fertilization, <b>II</b>,
-    <a href="#Page_407">407</a>;</li>
-  <li class="i1">multiplication, <b>II</b>,
-    <a href="#Page_441">441</a>,
-    <a href="#Page_442">442</a>;</li>
-  <li class="i1">genesis and growth, <b>II</b>,
-    <a href="#Page_451">451</a>,
-    <a href="#Page_457">457</a>;</li>
-  <li class="i1">and development, <b>II</b>,
-    <a href="#Page_464">464</a>;</li>
-  <li class="i1">and nutrition, <b>II</b>,
-    <a href="#Page_476">476</a>,
-    <a href="#Page_477">477</a>,
-    <a href="#Page_511">511</a>;</li>
-  <li class="hangingindent1">substitution of axial for foliar organs, <b>II</b>,
-    <a href="#Page_541">541–7</a>.</li>
-
-  <li>Phenomenon, life manifested as, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page580">580</a>.</li>
-
-  <li>Philology (<i>see</i> <a href="#Language">Language</a>).</li>
-
-  <li><i>Phoronis</i>, individuality, <b>II</b>,
-    <a href="#Page_444">444</a>.</li>
-
-  <li>Phosphorescence, organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page49">49</a>.</li>
-
-  <li>Phosphorus: allotropic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page4">4</a>;</li>
-  <li class="i1">in cell physiology, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page259">259–62</a>;</li>
-  <li class="i1">cerebral activity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page596">596–7</a>;</li>
-  <li class="i1">organic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page703">703</a>.</li>
-
-  <li>Photogenes, visibility of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page218">218</a>.</li>
-
-  <li>Phylogeny: as interpreting morphology, <b>II</b>,
-    <a href="#Page_10">10–12</a>;</li>
-  <li class="i1">difficulties of affiliation, <b>II</b>,
-    <a href="#Page_34">34–5</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Embryology">Embryology</a> <i>and</i> <a href="#Evolution">Evolution</a>.)</li>
-
-  <li id="Physiological_Units">Physiological Units: definition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page226">226</a>;</li>
-  <li class="i1">genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page280">280–1</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page316">316</a>;</li>
-  <li class="i1">heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page315">315–9</a>;</li>
-  <li class="i1">variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page330">330</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page331">331–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page333">333</a>; <b>II</b>,
-    <a href="#Page_619">619</a>;</li>
-  <li class="i1">stability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page340">340</a>; <b>II</b>,
-    <a href="#Page_614">614</a>;</li>
-  <li class="i1">self-fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page342">342–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page353">353</a>;</li>
-  <li class="i1">interbreeding, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page345">345</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page353">353</a>; <b>II</b>,
-    <a href="#Page_615">615</a>;</li>
-  <li class="hangingindent1">recapitulation of hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page350">350–5</a>; <b>II</b>,
-    <a href="#Page_612">612–7</a>;</li>
-  <li class="hangingindent1">structural proclivities, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page362">362</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page369">369–71</a>; <b>II</b>,
-    <a href="#Page_613">613</a>,
-    <a href="#Page_622">622</a>;</li>
-  <li class="i1">sociological analogy, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364</a>; <b>II</b>,
-    <a href="#Page_620">620</a>;</li>
-  <li class="i1">complexity in organized types, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page368">368–70</a>;</li>
-  <li class="i1">re-named “constitutional units,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page369">369</a>;</li>
-  <li class="i1">telegony, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page650">650</a>;</li>
-  <li class="i1">“mechanical theory,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page701">701–6</a>;</li>
-  <li class="i1">morphological development, <b>II</b>,
-    <a href="#Page_7">7–9</a>;</li>
-  <li class="i1">cell-doctrine, <b>II</b>,
-    <a href="#Page_17">17–21</a>;</li>
-  <li class="i1">development, <b>II</b>,
-    <a href="#Page_76">76</a>;</li>
-  <li class="i1">“false joints,” <b>II</b>,
-    <a href="#Page_371">371–2</a>;</li>
-  <li class="hangingindent1">dissociation of ancestral traits in hybrids, <b>II</b>,
-    <a href="#Page_616">616–7</a>;</li>
-  <li class="i1">inheritance of acquired characters, <b>II</b>,
-    <a href="#Page_618">618–23</a>.</li>
-
-  <li>Physiological division of labour (<i>see</i> <a href="#Labour">Labour</a>).</li>
-
-  <li>Physiological Selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page569">569–71</a>.</li>
-
-  <li id="Physiology">Physiology: and psychology, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page127">127</a>;</li>
-  <li class="i1">subdivisions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page128">128</a>;</li>
-  <li class="i1">vicarious function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page208">208</a>;</li>
-  <li class="i1">primitive interpretations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page417">417</a>;</li>
-  <li class="hangingindent1">multiplication of effects exemplified, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page512">512</a>; <b>II</b>,
-    <a href="#Page_390">390</a>;</li>
-  <li class="i1">relations to morphology, <b>II</b>,
-    <a href="#Page_3">3</a>,
-    <a href="#Page_239">239–41</a>;</li>
-  <li class="hangingindent1">evolutionary interpretation of phenomena, <b>II</b>,
-    <a href="#Page_241">241–5</a>,
-    <a href="#Page_384">384–95</a>;</li>
-  <li class="i1">ultimate inconceivability of processes, <b>II</b>,
-    <a href="#Page_372">372</a>;</li>
-  <li class="hangingindent1">correlated integration and differentiation, <b>II</b>,
-    <a href="#Page_373">373</a>.</li>
-
-  <li>Physiology, Animal: metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page67">67–77</a>;</li>
-  <li class="i1">vertebrate internal symmetry, <b>II</b>,
-    <a href="#Page_108">108</a>;</li>
-  <li class="hangingindent1">tissue differentiation in <i>Protozoa</i>, <b>II</b>,
-    <a href="#Page_299">299</a>,
-    <a href="#Page_385">385</a>;</li>
-  <li class="hangingindent1">primary tissue differentiation, <b>II</b>,
-    <a href="#Page_300">300–2</a>,
-    <a href="#Page_382">382</a>,
-    <a href="#Page_389">389</a>;</li>
-  <li class="hangingindent1">natural selection and tissue differentiation, <b>II</b>,
-    <a href="#Page_304">304–8</a>;</li>
-  <li class="i1">outer tissue in <i>Cœlenterata</i>, <b>II</b>,
-    <a href="#Page_309">309–10</a>;</li>
-  <li class="i1">respiratory organs, <b>II</b>,
-    <a href="#Page_310">310–1</a>,
-    <a href="#Page_333">333–8</a>;</li>
-  <li class="hangingindent1">differentiation of animal epidermic tissue, <b>II</b>,
-    <a href="#Page_312">312–4</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">development of tegumentary organs, <b>II</b>,
-    <a href="#Page_314">314–6</a>;</li>
-  <li class="i1">of sensory, <b>II</b>,
-    <a href="#Page_317">317–20</a>;</li>
-  <li class="hangingindent1">inner and outer tissue transition, <b>II</b>,
-    <a href="#Page_321">321–2</a>,
-    <a href="#Page_389">389</a>;</li>
-  <li class="i1">alimentary canal differentiation, <b>II</b>,
-    <a href="#Page_323">323–5</a>;</li>
-  <li class="i1">gizzard development in birds, <b>II</b>,
-    <a href="#Page_325">325</a>;</li>
-  <li class="i1">alimentary canal of ruminants, <b>II</b>,
-    <a href="#Page_327">327–9</a>;</li>
-  <li class="i1">differentiation of liver, <b>II</b>,
-    <a href="#Page_329">329–33</a>;</li>
-  <li class="i1">of animal vascular system, <b>II</b>,
-    <a href="#Page_339">339–44</a>;</li>
-  <li class="i1">of osseous system, <b>II</b>,
-    <a href="#Page_344">344–55</a>;</li>
-  <li class="i1">of nerve tissue, <b>II</b>,
-    <a href="#Page_355">355–61</a>;</li>
-  <li class="i1">of muscle, <b>II</b>,
-    <a href="#Page_361">361–9</a>;</li>
-  <li class="i1">differentiation and integration, <b>II</b>,
-    <a href="#Page_373">373–6</a>;</li>
-  <li class="i1">in vascular system, <b>II</b>,
-    <a href="#Page_376">376–9</a>,
-    <a href="#Page_383">383</a>;</li>
-  <li class="i1">in nerves, <b>II</b>,
-    <a href="#Page_379">379–82</a>;</li>
-  <li class="i1">origin of development, <b>II</b>,
-    <a href="#Page_384">384</a>;</li>
-  <li class="hangingindent1">differentiation and instability of homogeneous, <b>II</b>,
-    <a href="#Page_384">384–9</a>,
-    <a href="#Page_392">392</a>;</li>
-  <li class="i1">summary of development, <b>II</b>,
-    <a href="#Page_384">384–94</a>;</li>
-  <li class="i1">multiplication of effects, <b>II</b>,
-    <a href="#Page_390">390–1</a>,
-    <a href="#Page_392">392</a>;</li>
-  <li class="i1">equilibration, <b>II</b>,
-    <a href="#Page_391">391–4</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Function">Function</a>.)</li>
-
-  <li>Physiology, Plant: metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page62">62–7</a>;</li>
-  <li class="hangingindent1">tissue differentiation in secondary aggregates, <b>II</b>,
-    <a href="#Page_246">246</a>,
-    <a href="#Page_385">385</a>;</li>
-  <li class="i1">in phænogams, <b>II</b>,
-    <a href="#Page_247">247–9</a>,
-    <a href="#Page_386">386</a>;</li>
-  <li class="i1">in bark and cambium, <b>II</b>,
-    <a href="#Page_249">249–50</a>,
-    <a href="#Page_386">386</a>;</li>
-  <li class="i1">in free and fixed surfaces, <b>II</b>,
-    <a href="#Page_251">251–6</a>,
-    <a href="#Page_270">270</a>,
-    <a href="#Page_386">386</a>;</li>
-  <li class="i1">outer stem and leaf tissue, <b>II</b>,
-    <a href="#Page_256">256–9</a>,
-    <a href="#Page_270">270</a>,
-    <a href="#Page_386">386</a>;</li>
-  <li class="hangingindent1">superficial differentiation in leaves, <b>II</b>,
-    <a href="#Page_260">260–4</a>,
-    <a href="#Page_270">270</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">floral tissue differentiation, <b>II</b>,
-    <a href="#Page_265">265–9</a>;</li>
-  <li class="i1">outer tissue, <i>résumé</i>, <b>II</b>,
-    <a href="#Page_270">270</a>;</li>
-  <li class="i1">inner tissue differentiation, <b>II</b>,
-    <a href="#Page_273">273–5</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="i1">supporting tissue, <b>II</b>,
-    <a href="#Page_275">275–9</a>,
-    <a href="#Page_285">285–8</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="hangingindent1">vascular system development, <b>II</b>,
-    <a href="#Page_273">273–5</a>,
-    <a href="#Page_279">279–84</a>,
-    <a href="#Page_285">285–8</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="i1">inner tissue, summary, <b>II</b>,
-    <a href="#Page_288">288–91</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_292">292–8</a>;</li>
-  <li class="hangingindent1">differentiation and instability of homogeneous, <b>II</b>,
-    <a href="#Page_384">384–9</a>,
-    <a href="#Page_392">392</a>;</li>
-  <li class="i1">multiplication of effects, <b>II</b>,
-    <a href="#Page_390">390–1</a>,
-    <a href="#Page_392">392</a>;</li>
-  <li class="i1">equilibration, <b>II</b>,
-    <a href="#Page_391">391–4</a>;</li>
-  <li class="i1">circulation and wood formation, <b>II</b>,
-    <a href="#Page_564">564–97</a>;</li>
-  <li class="i1">dye permeability, <b>II</b>,
-    <a href="#Page_569">569–74</a>,
-    <a href="#Page_577">577–81</a>,
-    <a href="#Page_584">584</a>,
-    <a href="#Page_586">586</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Function">Function</a>.)</li>
-
-  <li>Pickering, J. W., on artificial proteids, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page39">39</a>.</li>
-
-  <li>Pig: colour of muscles, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page365">365–6</a>;</li>
-  <li class="i1">telegony, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page627">627</a>;</li>
-  <li class="i1">fertility of domestic and wild sow, <b>II</b>,
-    <a href="#Page_479">479–80</a>.</li>
-
-  <li>Pigeons: food of starving, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page215">215</a>;</li>
-  <li class="i1">heredity and variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page305">305</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page321">321</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page615">615</a>;</li>
-  <li class="i1">atavism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314</a>;</li>
-  <li class="i1">fertility, <b>II</b>,
-    <a href="#Page_471">471–2</a>,
-    <a href="#Page_478">478</a>.</li>
-
-  <li>Pike, unceasing growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page154">154</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page292">292</a>.</li>
-
-  <li class="hangingindent">Pique-gouffe, commensal relations with buffalo, <b>II</b>,
-    <a href="#Page_403">403</a>.</li>
-
-  <li><i>Plagiochila</i>, evolution of stem, <b>II</b>,
-    <a href="#Page_62">62</a>.</li>
-
-  <li><i>Planaria</i>: integration, <b>II</b>,
-    <a href="#Page_101">101–2</a>;</li>
-  <li class="i1">Morgan on regeneration, <b>II</b>,
-    <a href="#Page_102">102</a>,
-    <a href="#Page_611">611</a>;</li>
-  <li class="i1">segmentation, <b>II</b>,
-    <a href="#Page_107">107</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_195">195</a>;</li>
-  <li class="i1">unintegrated function, <b>II</b>,
-    <a href="#Page_373">373</a>.</li>
-
-  <li id="Plants">Plants: influence of heat, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page29">29</a>;</li>
-  <li class="i1">effect of solar rays, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page31">31–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page500">500</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">chemical composition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page40">40–1</a>;</li>
-  <li class="i1">heat generation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page47">47</a>;</li>
-  <li class="i1">phosphorescence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page49">49</a>;</li>
-  <li class="i1">electricity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page51">51</a>;</li>
-  <li class="i1">sensible motion, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page56">56–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page58">58</a>;</li>
-  <li class="i1">metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page62">62–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page70">70</a>;</li>
-  <li class="i1">vital changes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page86">86</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page87">87</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page91">91</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page94">94</a>;</li>
-  <li class="i1">simulation by crystals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page96">96</a>;</li>
-  <li class="i1">vital adjustments, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page102">102</a>;</li>
-  <li class="i1">length and complexity of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page103">103–4</a>;</li>
-  <li class="i1">biological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page125">125</a>;</li>
-  <li class="hangingindent1">growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page136">136</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page138">138</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page140">140</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page143">143</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page145">145–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page153">153</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page160">160–1</a>; <b>II</b>,
-    <a href="#Page_401">401–2</a>;</li>
-  <li class="i1">development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page163">163–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page167">167–70</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page272">272</a>;</li>
-  <li class="i1">weight, temperature, and self-mobility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page174">174</a>;</li>
-  <li class="i1">function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page174">174–8</a>;</li>
-  <li class="i1">structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page194">194–6</a>; <b>II</b>,
-    <a href="#Page_21">21</a>;</li>
-  <li class="i1">animal structure contrasted, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page196">196</a>;</li>
-  <li class="i1">function and structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page200">200</a>;</li>
-  <li class="i1">vicarious function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page208">208–9</a>;</li>
-  <li class="i1">waste and repair, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page213">213</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page220">220</a>;</li>
-  <li class="i1">physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page225">225–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page317">317</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page360">360</a>;</li>
-  <li class="i1">adaptation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page227">227</a>;</li>
-  <li class="i1">what is an individual? <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page244">244–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page250">250–1</a>;</li>
-  <li class="i1">genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page270">270</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page272">272–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page274">274</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page276">276–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279–85</a>;</li>
-  <li class="hangingindent1">relation of nutrition to growth and genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page284">284–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page294">294</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295–300</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page642">642</a>; <b>II</b>,
-    <a href="#Page_39">39</a>;</li>
-  <li class="i1">ovule homologues, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page288">288</a>;</li>
-  <li class="hangingindent1">natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page294">294–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page532">532</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page533">533</a>; <b>II</b>,
-    <a href="#Page_51">51</a>;</li>
-  <li class="i1">heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page301">301–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page308">308</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page358">358–60</a>;</li>
-  <li class="i1">variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page320">320</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page323">323–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page325">325–6</a>;</li>
-  <li class="i1">fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page340">340–5</a>;</li>
-  <li class="i1">classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page377">377–80</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page389">389–90</a>;</li>
-  <li class="hangingindent1">distribution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page396">396–400</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page401">401–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page404">404–12</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page478">478–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">special creation and parasitism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page428">428</a>;</li>
-  <li class="i1">evolution hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page434">434</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page443">443</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page449">449–50</a>;</li>
-  <li class="i1">rudimentary organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page474">474</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page475">475</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">varied media, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page484">484</a>; <b>II</b>,
-    <a href="#Page_32">32</a>;</li>
-  <li class="i1">alien and native species in New Zealand, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page477">477</a>;</li>
-  <li class="i1">E. Darwin and Lamarck on evolution of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page490">490–8</a>;</li>
-  <li class="i1">geologic changes affecting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page501">501–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="hangingindent1">interdependence of animals and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page504">504–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page514">514</a>; <b>II</b>,
-    <a href="#Page_398">398</a>;</li>
-  <li class="i1">complexity of influences affecting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page506">506</a>;</li>
-  <li class="i1">direct equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page523">523–5</a>;</li>
-  <li class="i1">indirect, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page532">532</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page533">533</a>;</li>
-  <li class="i1">seed distribution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page546">546</a>;</li>
-  <li class="i1">wood development, <b>II</b>,
-    <a href="#Page_285">285–7</a>,
-    <a href="#Page_289">289</a>,
-    <a href="#Page_567">567–97</a>;</li>
-  <li class="i1">interdependence, <b>II</b>,
-    <a href="#Page_402">402–3</a>,
-    <a href="#Page_404">404</a>;</li>
-  <li class="i1">insect relations, <b>II</b>,
-    <a href="#Page_406">406–7</a>;</li>
-  <li class="i1">adaptation and multiplication, <b>II</b>,
-    <a href="#Page_411">411–6</a>;</li>
-  <li class="i1">rhythm in numbers, <b>II</b>,
-    <a href="#Page_419">419</a>;</li>
-  <li class="i1">growth and asexual genesis, <b>II</b>,
-    <a href="#Page_439">439–42</a>;</li>
-  <li class="i1">growth and sexual genesis, <b>II</b>,
-    <a href="#Page_448">448–51</a>;</li>
-  <li class="i1">expenditure, <b>II</b>,
-    <a href="#Page_467">467</a>;</li>
-  <li class="i1">horticulture, nutrition, and genesis, <b>II</b>,
-    <a href="#Page_477">477</a>;</li>
-  <li class="i1">tree development, <b>II</b>,
-    <a href="#Page_553">553</a>;</li>
-  <li class="i1">circulation and wood formation, <b>II</b>,
-    <a href="#Page_567">567–92</a>;</li>
-  <li class="hangingindent1">dye permeability and circulation, <b>II</b>,
-    <a href="#Page_569">569–74</a>,
-    <a href="#Page_577">577–81</a>,
-    <a href="#Page_584">584</a>,
-    <a href="#Page_586">586</a>;</li>
-  <li class="hangingindent1"><i>résumé</i> on circulation and wood formation, <b>II</b>,
-    <a href="#Page_592">592–7</a>.</li>
-  <li class="hangingindent1">(<i>See also</i> <a href="#Multiplication">Multiplication</a>, <a href="#Morphology">Morphology</a>, <i>and</i> <a href="#Physiology">Physiology</a>.)</li>
-
-  <li>Plasmodium, dissolution of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page185">185</a>.</li>
-
-  <li>Plato, ἰδέα of, <b>II</b>,
-    <a href="#Page_550">550</a>.</li>
-
-  <li><i>Platyhelminthes</i>: transverse fission, <b>II</b>,
-    <a href="#Page_101">101</a>;</li>
-  <li class="i1">segmented and non-segmented types, <b>II</b>,
-    <a href="#Page_102">102</a>,
-    <a href="#Page_107">107</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_195">195</a>,
-    <a href="#Page_197">197</a>;</li>
-  <li class="i1">multiplication and growth, <b>II</b>,
-    <a href="#Page_488">488–9</a>.</li>
-
-  <li>Plethora, fertility and, <b>II</b>,
-    <a href="#Page_480">480–4</a>,
-    <a href="#Page_511">511</a>.</li>
-
-  <li><i>Pleurococcaceæ</i>, unicellular form, <b>II</b>,
-    <a href="#Page_21">21</a>,
-    <a href="#Page_134">134</a>.</li>
-
-  <li class="hangingindent"><i>Pleuronectidæ</i>: symmetry and location of eyes, <b>II</b>,
-    <a href="#Page_205">205</a>;</li>
-  <li class="i1">outer tissue, <b>II</b>,
-    <a href="#Page_387">387</a>.</li>
-
-  <li><i>Plumatella</i>: metagenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page277">277</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_195">195</a>.</li>
-
-  <li class="hangingindent"><i>Podostemaceæ</i>, undeveloped circulatory system, <b>II</b>,
-    <a href="#Page_274">274</a>.</li>
-
-  <li class="hangingindent">Polar bodies, hypothesis concerning extrusion of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page266">266–8</a>.</li>
-
-  <li class="hangingindent">Polarity, organic, of physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page226">226</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page315">315</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page317">317</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page332">332</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page350">350–1</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page701">701–6</a>.</li>
-
-  <li>Polyatomic compounds (<i>see</i> <a href="#Chemistry">Chemistry</a>).</li>
-
-  <li class="hangingindent"><i>Polychætæ</i>, anomalous development in <i>Myrianida</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page361">361</a>.</li>
-
-  <li><i>Polycytharia</i>, integration, <b>II</b>,
-    <a href="#Page_90">90</a>,
-    <a href="#Page_124">124</a>.</li>
-
-  <li><i>Polygastrica</i>, aggregation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page586">586</a>.</li>
-
-  <li>Polymerism: of compounds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page11">11</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page25">25</a>;</li>
-  <li class="i1">nerve tissue, <b>II</b>,
-    <a href="#Page_356">356</a>.</li>
-
-  <li><i>Polypori</i>, symmetry and environment, <b>II</b>,
-    <a href="#Page_139">139</a>.</li>
-
-  <li>Polyps (see <a href="#Coelenterata"><i>Cœlenterata</i></a>).</li>
-
-  <li id="Polyzoa"><i>Polyzoa</i>: size, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page140">140</a>;</li>
-  <li class="i1">multiaxial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>;</li>
-  <li class="i1">structural indefiniteness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page173">173</a>;</li>
-  <li class="i1">functional differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page202">202</a>;</li>
-  <li class="i1">trochophoral kinship, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page447">447</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_93">93–4</a>,
-    <a href="#Page_96">96</a>,
-    <a href="#Page_124">124</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_194">194</a>,
-    <a href="#Page_207">207</a>;</li>
-  <li class="i1">vascular system, <b>II</b>,
-    <a href="#Page_340">340</a>;</li>
-  <li class="i1">gemmation, <b>II</b>,
-    <a href="#Page_444">444</a>.</li>
-
-  <li>Poor Laws, and natural selection, <b>II</b>,
-    <a href="#Page_532">532</a>.</li>
-
-  <li class="hangingindent"><i>Population, A Theory of</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page265">265</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page577">577–601</a>; <b>II</b>,
-    <a href="#Page_411">411</a>.</li>
-
-  <li>Potato: simulated growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page136">136</a>;</li>
-  <li class="hangingindent1">vicarious function of tuber, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page209">209</a>; <b>II</b>,
-    <a href="#Page_255">255</a>;</li>
-  <li class="i1">sub-species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page302">302</a>;</li>
-  <li class="i1">dye absorption, <b>II</b>,
-    <a href="#Page_279">279</a>.</li>
-
-  <li class="hangingindent">Preservation: fertility and self-, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page581">581</a>; <b>II</b>,
-    <a href="#Page_423">423</a>,
-    <a href="#Page_430">430</a>;</li>
-  <li class="i1">nutrition, <b>II</b>,
-    <a href="#Page_493">493</a>.</li>
-
-  <li class="hangingindent">“Progress; its Law and Cause,” theory of species differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page568">568</a>.</li>
-
-  <li>Projectiles, factors in flight of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page450">450–1</a>.</li>
-
-  <li>Proteids: metabolic function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page67">67</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page68">68</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page72">72</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page76">76</a>;</li>
-  <li class="i1">complexity of molecule, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page122">122</a>.</li>
-
-  <li>Protein: evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page23">23</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page24">24</a>;</li>
-  <li class="i1">isomerism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page700">700</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page703">703</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page704">704</a>.</li>
-
-  <li><i>Proteus</i>, degeneration of eye, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page613">613</a>.</li>
-
-  <li><i>Protodrilus</i>, intestine segmentation, <b>II</b>,
-    <a href="#Page_125">125</a>.</li>
-
-  <li><i>Protophyta</i>: internal movements, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page56">56</a>;</li>
-  <li class="i1">limit of growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page138">138</a>;</li>
-  <li class="i1">development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page164">164</a>;</li>
-  <li class="i1">structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page173">173</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page181">181–3</a>;</li>
-  <li class="i1">self-mobility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page175">175</a>;</li>
-  <li class="i1">individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page245">245</a>;</li>
-  <li class="hangingindent1">multiplication, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page270">270</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page276">276</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page581">581</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page584">584–5</a>; <b>II</b>,
-    <a href="#Page_439">439</a>,
-    <a href="#Page_462">462</a>;</li>
-  <li class="i1">genesis and nutrition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295</a>;</li>
-  <li class="i1">unicellular, <b>II</b>,
-    <a href="#Page_21">21</a>;</li>
-  <li class="i1">central aggregation, <b>II</b>,
-    <a href="#Page_24">24</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_134">134</a>;</li>
-  <li class="i1">tissues, <b>II</b>,
-    <a href="#Page_244">244</a>,
-    <a href="#Page_249">249</a>;</li>
-  <li class="i1">primary differentiation, <b>II</b>,
-    <a href="#Page_385">385</a>;</li>
-  <li class="i1">primordial type, <b>II</b>,
-    <a href="#Page_398">398</a>;</li>
-  <li class="i1">symbiosis, <b>II</b>,
-    <a href="#Page_400">400</a>.</li>
-
-  <li class="hangingindent">Protoplasm: self-increasing function of primordial, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page63">63–4</a>;</li>
-  <li class="i1">plant metabolism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page65">65–7</a>;</li>
-  <li class="i1">complexity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page122">122</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page253">253–5</a>;</li>
-  <li class="i1">differentiation in simple organisms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page182">182–3</a>;</li>
-  <li class="hangingindent1">continuity and inter-circulation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page190">190–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page371">371</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page629">629</a>; <b>II</b>,
-    <a href="#Page_21">21</a>,
-    <a href="#Page_620">620</a>;</li>
-  <li class="i1">“streaming,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page253">253</a>;</li>
-  <li class="i1">structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page253">253–5</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Cell">Cell</a>.)</li>
-
-  <li><i>Protozoa</i>: inorganic components, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page17">17</a>;</li>
-  <li class="i1">locomotion, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page58">58</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page175">175</a>; <b>II</b>,
-    <a href="#Page_14">14</a>;</li>
-  <li class="i1">vital changes shown by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page94">94</a>;</li>
-  <li class="i1">limitation of growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page138">138</a>;</li>
-  <li class="i1">development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page164">164</a>;</li>
-  <li class="i1">structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page173">173</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page181">181–3</a>;</li>
-  <li class="hangingindent1">incipient differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page198">198</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page391">391</a>; <b>II</b>,
-    <a href="#Page_299">299</a>,
-    <a href="#Page_309">309</a>;</li>
-  <li class="hangingindent1">multiplication, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page270">270</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page276">276</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page280">280</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page582">582</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page584">584</a>; <b>II</b>,
-    <a href="#Page_442">442</a>,
-    <a href="#Page_451">451–2</a>;</li>
-  <li class="i1">genesis and nutrition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page295">295</a>;</li>
-  <li class="i1">distribution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page396">396</a>;</li>
-  <li class="i1">parasites infesting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page427">427</a>;</li>
-  <li class="i1">Weismann’s hypothesis of immortality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page637">637</a>;</li>
-  <li class="i1">“spontaneous generation,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page697">697–701</a>;</li>
-  <li class="i1">non-nucleated, <b>II</b>,
-    <a href="#Page_20">20</a>;</li>
-  <li class="i1">primary aggregate, <b>II</b>,
-    <a href="#Page_86">86–7</a>,
-    <a href="#Page_124">124</a>;</li>
-  <li class="i1">progressing integration, <b>II</b>,
-    <a href="#Page_89">89–91</a>,
-    <a href="#Page_124">124</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_186">186</a>;</li>
-  <li class="i1">primordial plant-animal type, <b>II</b>,
-    <a href="#Page_397">397–8</a>;</li>
-  <li class="i1">symbiosis, <b>II</b>,
-    <a href="#Page_400">400</a>.</li>
-
-  <li>Protyle, hypothetical chemical unit, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page22">22</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page23">23</a>.</li>
-
-  <li>Pseud-axial development, vegetal, <b>II</b>,
-    <a href="#Page_28">28–9</a>,
-    <a href="#Page_30">30</a>.</li>
-
-  <li class="hangingindent">Pseudo-foliar development, vegetal, <b>II</b>,
-    <a href="#Page_26">26–8</a>,
-    <a href="#Page_30">30</a>.</li>
-
-  <li><i>Psychidæ</i>: parthenogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page275">275</a>;</li>
-  <li class="i1">sexual dimorphism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page683">683</a>.</li>
-
-  <li class="hangingindent" id="Psychology">Psychology: reasoning and definition of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page81">81–8</a>;</li>
-  <li class="hangingindent1">correspondence shown by <i>recognition</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page95">95</a>;</li>
-  <li class="i1">contrasted with physiology, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page127">127</a>;</li>
-  <li class="i1">departments of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page127">127–8</a>;</li>
-  <li class="i1">vicarious function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page209">209</a>;</li>
-  <li class="i1">waste and repair in sensory organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page217">217</a>;</li>
-  <li class="i1">sensory adaptability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page229">229</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page231">231</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page232">232</a>;</li>
-  <li class="i1">inheritance of sensory defects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page306">306</a>;</li>
-  <li class="i1">musical talent, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page311">311–2</a>;</li>
-  <li class="hangingindent1">intellectual progress and special creation hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page417">417</a>;</li>
-  <li class="hangingindent1">special creation a pseud-idea, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page420">420</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page429">429</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page433">433</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page554">554</a>;</li>
-  <li class="hangingindent1">legitimacy of evolution hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page433">433–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page439">439</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page554">554</a>;</li>
-  <li class="i1">embryology of ideas, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page450">450</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page457">457</a>;</li>
-  <li class="hangingindent1">persistent formative power unrepresentable, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page492">492</a>;</li>
-  <li class="hangingindent1">E. Darwin’s and Lamarck’s theory of desires, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page494">494</a>;</li>
-  <li class="i1">natural selection and brain evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>;</li>
-  <li class="hangingindent1">genesis and cerebral activity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page594">594</a>; <b>II</b>,
-    <a href="#Page_512">512–4</a>,
-    <a href="#Page_516">516–9</a>,
-    <a href="#Page_530">530</a>;</li>
-  <li class="hangingindent1">heredity and distribution of tactual perceptiveness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page602">602–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page665">665–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page672">672</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="i1">inconceivability of the negation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>;</li>
-  <li class="i1">vitiation of evidence, <b>II</b>,
-    <a href="#Page_88">88</a>;</li>
-  <li class="i1">repetition and perception, <b>II</b>,
-    <a href="#Page_143">143</a>;</li>
-  <li class="i1">differentiation of sensory organs, <b>II</b>,
-    <a href="#Page_317">317–20</a>;</li>
-  <li class="i1">differentiation of nerve tissue, <b>II</b>,
-    <a href="#Page_355">355–61</a>;</li>
-  <li class="i1">functional integration, <b>II</b>,
-    <a href="#Page_376">376</a>;</li>
-  <li class="i1">also integration, <b>II</b>,
-    <a href="#Page_380">380–2</a>;</li>
-  <li class="i1">equilibration of nerve discharge, <b>II</b>,
-    <a href="#Page_393">393</a>;</li>
-  <li class="hangingindent1">human fertility and nerve development, <b>II</b>,
-    <a href="#Page_466">466</a>,
-    <a href="#Page_532">532</a>;</li>
-  <li class="i1">future human evolution, <b>II</b>,
-    <a href="#Page_523">523–5</a>,
-    <a href="#Page_527">527</a>;</li>
-  <li class="i1">human evolution and genesis, <b>II</b>,
-    <a href="#Page_529">529–31</a>;</li>
-  <li class="i1">future mental development, <b>II</b>,
-    <a href="#Page_535">535</a>;</li>
-  <li class="i1">origin of vertebrate type, <b>II</b>,
-    <a href="#Page_598">598–600</a>.</li>
-
-  <li><i>Pteridophyta</i>: size attained by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page138">138</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page139">139</a>;</li>
-  <li class="i1">homologies, <b>II</b>,
-    <a href="#Page_80">80–1</a>,
-    <a href="#Page_82">82</a>;</li>
-  <li class="i1">frond surface differentiation, <b>II</b>,
-    <a href="#Page_260">260</a>.</li>
-
-  <li><i>Pteropoda</i>: bilateral symmetry, <b>II</b>,
-    <a href="#Page_201">201</a>;</li>
-  <li class="i1">dermal respiration, <b>II</b>,
-    <a href="#Page_310">310</a>.</li>
-
-  <li>Ptyaline, metabolic function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>.</li>
-
-  <li class="hangingindent">Punjabis, inheritance of acquired osteological peculiarities, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>.</li>
-
-  <li><i>Pyrosomidæ</i>: phosphorescence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page47">47</a>;</li>
-  <li class="i1">integration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page588">588</a>; <b>II</b>,
-    <a href="#Page_97">97</a>.</li>
-</ul>
-
-<ul>
-  <li class="hangingindent">Quagga, telegonic transmission of markings to offspring of mare, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page624">624</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page627">627</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646</a>.</li>
-
-  <li>Quills, development, <b>II</b>,
-    <a href="#Page_314">314–6</a>.</li>
-</ul>
-
-<ul>
-  <li>Rabbit: activity and muscle colour, <b>II</b>,
-    <a href="#Page_365">365</a>;</li>
-  <li class="i1">over-running checked by weasels, <b>II</b>,
-    <a href="#Page_405">405</a>;</li>
-  <li class="i1">expenditure and genesis, <b>II</b>,
-    <a href="#Page_472">472</a>.</li>
-
-  <li>Radial, definition, <b>II</b>,
-    <a href="#Page_148">148</a>.</li>
-
-  <li><i>Radiolaria</i>: unicentral development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page163">163</a>;</li>
-  <li class="i1">secondary aggregation, <b>II</b>,
-    <a href="#Page_88">88</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_187">187</a>.</li>
-
-  <li class="hangingindent"><i>Radula</i>, development of roots from leaflets, <b>II</b>,
-    <a href="#Page_34">34</a>.</li>
-
-  <li><i>Rafflesiaceæ</i>: homogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page272">272</a>;</li>
-  <li class="i1">tissue differentiation, <b>II</b>,
-    <a href="#Page_274">274</a>;</li>
-  <li class="i1">nutrition and genesis, <b>II</b>,
-    <a href="#Page_486">486</a>.</li>
-
-  <li>Rat (see <a href="#Rodentia"><i>Rodentia</i></a>).</li>
-
-  <li>Rathke, H., on vertebrate embryo, <b>II</b>,
-    <a href="#Page_119">119</a>.</li>
-
-  <li>Ray, J., plant classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page378">378</a>.</li>
-
-  <li>Reasoning, compared with assimilation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page81">81–7</a>.</li>
-
-  <li>Recapitulation, embryological, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page453">453</a>.</li>
-
-  <li>Regeneration (<i>see</i> <a href="#Repair">Repair</a>).</li>
-
-  <li class="hangingindent">Rejuvenescence, and sexual fertilization, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page637">637</a>; <b>II</b>,
-    <a href="#Page_613">613</a>.</li>
-
-  <li>Remak, R., vertebrate embryo, <b>II</b>,
-    <a href="#Page_120">120</a>.</li>
-
-  <li id="Repair">Repair: continuity of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page216">216–9</a>;</li>
-  <li class="hangingindent1">animal injuries, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page219">219</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page222">222–4</a>; <b>II</b>,
-    <a href="#Page_102">102</a>,
-    <a href="#Page_611">611</a>;</li>
-  <li class="i1">deductive interpretation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page221">221–2</a>;</li>
-  <li class="hangingindent1">theories of heredity and regenerative phenomena, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page360">360–1</a>.</li>
-
-  <li>Repetition of like parts, <b>II</b>,
-    <a href="#Page_126">126</a>.</li>
-
-  <li>Reproduction (<i>see</i> <a href="#Multiplication">Multiplication</a>).</li>
-
-  <li id="Reptilia"><i>Reptilia</i>: growth and expenditure of force, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page142">142</a>;</li>
-  <li class="i1">sizes of ova and adult, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page144">144</a>;</li>
-  <li class="i1">longevity of crocodile, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page154">154</a>;</li>
-  <li class="i1">temperature, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page174">174</a>;</li>
-  <li class="i1">waste, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page214">214</a>;</li>
-  <li class="i1">distinctive characters, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page392">392</a>;</li>
-  <li class="i1">distribution in time, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page409">409</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page412">412</a>;</li>
-  <li class="i1">vertebral segmentation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page470">470</a>;</li>
-  <li class="i1">rudimentary limbs of snakes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page473">473</a>;</li>
-  <li class="i1">fertility and development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page583">583</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page598">598</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page599">599</a>;</li>
-  <li class="i1">regeneration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page589">589</a>;</li>
-  <li class="i1">elongated form, <b>II</b>,
-    <a href="#Page_15">15</a>;</li>
-  <li class="i1">supernumerary vertebræ, <b>II</b>,
-    <a href="#Page_123">123</a>,
-    <a href="#Page_564">564</a>;</li>
-  <li class="i1">bilateral symmetry, <b>II</b>,
-    <a href="#Page_203">203</a>,
-    <a href="#Page_204">204</a>;</li>
-  <li class="i1">Cope on segmentation in extinct, <b>II</b>,
-    <a href="#Page_225">225</a>,
-    <a href="#Page_226">226</a>;</li>
-  <li class="i1">activity and muscular colour, <b>II</b>,
-    <a href="#Page_365">365</a>;</li>
-  <li class="i1">functional integration, <b>II</b>,
-    <a href="#Page_375">375</a>;</li>
-  <li class="i1">outer tissue differentiation, <b>II</b>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">Owen on skeleton, <b>II</b>,
-    <a href="#Page_560">560</a>.</li>
-
-  <li>Resistance of media to locomotion, <b>II</b>,
-    <a href="#Page_15">15</a>.</li>
-
-  <li id="Respiratory_System">Respiratory System: effect of light, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page31">31</a>;</li>
-  <li class="i1">organic re-arrangement, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page37">37</a>;</li>
-  <li class="i1">cutaneous, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page209">209</a>;</li>
-  <li class="i1">air-cells of lungs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page254">254</a>;</li>
-  <li class="i1">embryonic branchiæ of salamander, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page457">457</a>;</li>
-  <li class="i1">differentiation, <b>II</b>,
-    <a href="#Page_310">310–1</a>,
-    <a href="#Page_333">333–8</a>;</li>
-  <li class="i1">physiological integration, <b>II</b>,
-    <a href="#Page_374">374–5</a>,
-    <a href="#Page_382">382</a>;</li>
-  <li class="hangingindent1">vascular differentiation and integration, <b>II</b>,
-    <a href="#Page_377">377</a>.</li>
-
-  <li>Retrograde metamorphoses, in animals, <b>II</b>,
-    <a href="#Page_12">12</a>.</li>
-
-  <li>Retzius, G., superficial nerve-endings, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page666">666</a>.</li>
-
-  <li>Reversed Selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page611">611</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612</a>.</li>
-
-  <li class="hangingindent">Rhabdospheres, calcareous armour and dynamic element in life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page119">119</a>.</li>
-
-  <li>Rhizoids, foliar expansions, <b>II</b>,
-    <a href="#Page_50">50</a>.</li>
-
-  <li><i>Rhizopoda</i>: structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page173">173</a>;</li>
-  <li class="i1">undifferentiated function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page200">200</a>;</li>
-  <li class="i1">a primary aggregate, <b>II</b>,
-    <a href="#Page_86">86</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_186">186</a>;</li>
-  <li class="i1">tissue differentiated, <b>II</b>,
-    <a href="#Page_299">299</a>,
-    <a href="#Page_385">385</a>;</li>
-  <li class="i1">motion of sarcode, <b>II</b>,
-    <a href="#Page_356">356</a>;</li>
-  <li class="i1">symbiosis, <b>II</b>,
-    <a href="#Page_400">400</a>.</li>
-
-  <li>Rhythm: astronomic and organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page499">499</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">law of equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page520">520–1</a>;</li>
-  <li class="i1">in multiplication, <b>II</b>,
-    <a href="#Page_419">419</a>.</li>
-
-  <li>Richeraud, Baron A., definition of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page79">79</a>.</li>
-
-  <li>Riley, C. V., on telegony, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page645">645</a>;</li>
-  <li class="i1"><i>Termites</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page680">680</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page681">681</a>;</li>
-  <li class="i1">pouch of Honey-ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page684">684</a>.</li>
-
-  <li id="Rodentia"><i>Rodentia</i>: incursions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page399">399</a>;</li>
-  <li class="i1">American types, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page403">403</a>;</li>
-  <li class="i1">fertility and development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page583">583</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page599">599</a>.</li>
-
-  <li>Rivinus, plant classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page377">377</a>.</li>
-
-  <li>Rokitansky, on false joints, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page230">230</a>.</li>
-
-  <li class="hangingindent">Romanes, G. J.: on “cessation of selection,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page560">560–2</a>;</li>
-  <li class="i1">isolation and species differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page569">569</a>;</li>
-  <li class="i1">“physiological selection,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page569">569–71</a>;</li>
-  <li class="i1">panmixia, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page649">649</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page667">667</a>;</li>
-  <li class="i1">influence of a previous sire on progeny, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page649">649</a>.</li>
-
-  <li>Röntgen rays, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page121">121</a>; <b>II</b>,
-    <a href="#Page_621">621</a>.</li>
-
-  <li>Roots: developed from leaflets, <b>II</b>,
-    <a href="#Page_34">34</a>;</li>
-  <li class="i1">physiological differentiation, <b>II</b>,
-    <a href="#Page_253">253–5</a>,
-    <a href="#Page_270">270</a>;</li>
-  <li class="i1">nutrition from leaves, <b>II</b>,
-    <a href="#Page_274">274</a>;</li>
-  <li class="i1">size and function, <b>II</b>,
-    <a href="#Page_276">276</a>.</li>
-
-  <li><i>Rotiferæ</i>: latent vitality of desiccated, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page117">117</a>;</li>
-  <li class="i1">trochopore, <b>II</b>,
-    <a href="#Page_108">108</a>,
-    <a href="#Page_109">109</a>;</li>
-  <li class="i1">molluscan relationship, <b>II</b>,
-    <a href="#Page_115">115</a>;</li>
-  <li class="i1">fertility and size, <b>II</b>,
-    <a href="#Page_453">453</a>,
-    <a href="#Page_459">459</a>.</li>
-
-  <li>Roux, W.: “intra-selection,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676</a>;</li>
-  <li class="i1">functional adaptation, <b>II</b>,
-    <a href="#Page_354">354</a>.</li>
-
-  <li class="hangingindent">Rudimentary organs: the definition of life and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page112">112</a>;</li>
-  <li class="hangingindent1">natural selection and eyes of cave fauna, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page309">309</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page647">647–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">evolution hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page472">472–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page556">556</a>;</li>
-  <li class="i1">limbs of whale, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page668">668–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>.</li>
-
-  <li>Ruminants, alimentary canal development, <b>II</b>,
-    <a href="#Page_327">327–9</a>.</li>
-</ul>
-
-<ul>
-  <li>Salamander, embryonic branchiæ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page457">457</a>.</li>
-
-  <li class="hangingindent"><i>Salmonidæ</i>, reproduction and growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page291">291–3</a>; <b>II</b>,
-    <a href="#Page_454">454</a>.</li>
-
-  <li><i>Salpidæ</i>: heterogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page272">272</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page277">277</a>;</li>
-  <li class="i1">integration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page588">588</a>; <b>II</b>,
-    <a href="#Page_97">97</a>.</li>
-
-  <li>Sap (<i>see</i> <a href="#Vascular_System">Vascular system</a>).</li>
-
-  <li><i>Sarcina</i>: central aggregation, <b>II</b>,
-    <a href="#Page_24">24</a>;</li>
-  <li class="i1">fertility, <b>II</b>,
-    <a href="#Page_440">440</a>.</li>
-
-  <li>Savage, Dr., on “Heredity and Neurosis,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page313">313</a>.</li>
-
-  <li><i>Scenedesmus</i>, individuation, <b>II</b>,
-    <a href="#Page_24">24</a>.</li>
-
-  <li>Scent: natural selection and keenness of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page610">610</a>;</li>
-  <li class="i1">floral fertilization, <b>II</b>,
-    <a href="#Page_268">268–9</a>;</li>
-  <li class="i1">animal protection, <b>II</b>,
-    <a href="#Page_434">434</a>.</li>
-
-  <li class="hangingindent">Schelling, E. W. J. von, definition of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page78">78</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page178">178</a>.</li>
-
-  <li>Schleiden, J. M., on individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page245">245</a>;</li>
-  <li class="i1">on liverworts, <b>II</b>,
-    <a href="#Page_50">50</a>,
-    <a href="#Page_52">52</a>;</li>
-  <li class="i1">algal indefiniteness, <b>II</b>,
-    <a href="#Page_296">296</a>.</li>
-
-  <li>Science, complex revelations of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page252">252</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page369">369</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page450">450</a>.</li>
-
-  <li><i>Scyphomedusæ</i>, strobilization, <b>II</b>,
-    <a href="#Page_108">108</a>.</li>
-
-  <li id="Sea">Sea: changes and movements in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page83">83</a>;</li>
-  <li class="i1">life in, lower than terrestrial, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page104">104</a>;</li>
-  <li class="i1">distribution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page396">396</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page517">517</a>;</li>
-  <li class="i1">change of media caused by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page481">481</a>;</li>
-  <li class="i1">geologic influence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page502">502</a>.</li>
-
-  <li>Seals: nail-bearing toes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page473">473</a>;</li>
-  <li class="i1"><i>vibrissæ</i>, <b>II</b>,
-    <a href="#Page_317">317</a>.</li>
-
-  <li>Seasons: reproductive periodicity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page299">299</a>;</li>
-  <li class="i1">variations of genesis with, <b>II</b>,
-    <a href="#Page_484">484–5</a>.</li>
-
-  <li class="hangingindent">Sedgwick, Adam: on continuity of protoplasm in animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page190">190</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page629">629</a>; <b>II</b>,
-    <a href="#Page_21">21</a>;</li>
-  <li class="i1">zoological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page387">387</a>;</li>
-  <li class="hangingindent1">discrimination of species in embryonic stages, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page461">461</a>;</li>
-  <li class="i1">persistence of ancestral traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page463">463–4</a>;</li>
-  <li class="i1">Archiannelidan segmentation, <b>II</b>,
-    <a href="#Page_109">109</a>.</li>
-
-  <li>Sedgwick, Wm.: heredity and sex, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page305">305</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314</a>;</li>
-  <li class="i1">telegonic transmission of hypospadias, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646</a>.</li>
-
-  <li>Seeds: nitrogenous, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page40">40</a>;</li>
-  <li class="hangingindent1">temperature of germinating, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page47">47</a>; <b>II</b>,
-    <a href="#Page_615">615</a>;</li>
-  <li class="i1">vitalism and latent vitality of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page116">116–7</a>;</li>
-  <li class="i1">variation in environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page327">327</a>;</li>
-  <li class="i1">natural selection among, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page532">532</a>.</li>
-
-  <li class="hangingindent" id="Segmentation">Segmentation (metameric): special creation hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page468">468–9</a>;</li>
-  <li class="hangingindent1">Huxley on number of somites in higher articulates, <i>ib.</i>;</li>
-  <li class="i1">in annulose animals, <b>II</b>,
-    <a href="#Page_98">98–110</a>,
-    <a href="#Page_111">111–5</a>,
-    <a href="#Page_601">601–5</a>;</li>
-  <li class="i1">simulated molluscan, <b>II</b>,
-    <a href="#Page_116">116</a>;</li>
-  <li class="i1">in vertebrates, <b>II</b>,
-    <a href="#Page_125">125–7</a>,
-    <a href="#Page_225">225–7</a>,
-    <a href="#Page_606">606–7</a>;</li>
-  <li class="i1">in elasmobranchs, <b>II</b>,
-    <a href="#Page_126">126</a>.</li>
-
-  <li>Segregation: of growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page136">136</a>;</li>
-  <li class="i1">of like units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page179">179</a>;</li>
-  <li class="i1">organic repair, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page221">221</a>;</li>
-  <li class="i1">variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page331">331</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page334">334</a>;</li>
-  <li class="hangingindent1">heterogeneity, and definiteness of evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page514">514–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page517">517–8</a>;</li>
-  <li class="i1">morphological development, <b>II</b>,
-    <a href="#Page_7">7–9</a>;</li>
-  <li class="i1">physiological units, <b>II</b>,
-    <a href="#Page_616">616</a>.</li>
-
-  <li class="hangingindent">Self-fertilization, animal and vegetal, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page341">341–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page353">353</a>.</li>
-
-  <li>Senses, the (<i>see</i> <a href="#Psychology">Psychology</a>).</li>
-
-  <li>Sex: in Ascidian colonies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page247">247</a>;</li>
-  <li class="i1">limitation of heredity by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314–6</a>;</li>
-  <li class="i1">correlated traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page371">371–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page513">513</a>;</li>
-  <li class="hangingindent1">nutrition and determination of, in social insects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page655">655–60</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page678">678–84</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page686">686–9</a>;</li>
-  <li class="i1">neural and hæmal traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page683">683</a>;</li>
-  <li class="i1">differentiation of organs, <b>II</b>,
-    <a href="#Page_303">303</a>;</li>
-  <li class="i1">castration and growth, <b>II</b>,
-    <a href="#Page_459">459</a>;</li>
-  <li class="hangingindent1">Julin on “castration parasitaire” in crustaceans, <b>II</b>,
-    <a href="#Page_493">493–6</a>;</li>
-  <li class="i1">the object of fertilization, <b>II</b>,
-    <a href="#Page_613">613</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Fertilization">Fertilization</a>.)</li>
-
-  <li>Sexual Selection (<i>see</i> <a href="#Natural_selection">Natural Selection</a>).</li>
-
-  <li>Sharp, D.: on insect somites, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page469">469</a>;</li>
-  <li class="i1">food habits of <i>Termites</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page686">686–7</a>.</li>
-
-  <li>Sheep: contrasted with oxen, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page158">158</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page160">160</a>;</li>
-  <li class="i1">crossing of English and French breeds, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page625">625</a>;</li>
-  <li class="i1">nutrition and genesis, <b>II</b>,
-    <a href="#Page_480">480</a>.</li>
-
-  <li class="hangingindent">Sherrington, Prof., on effects of nerve severance, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page349">349</a>.</li>
-
-  <li class="hangingindent">Ship-building, interdependence of social functions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page237">237–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page241">241</a>.</li>
-
-  <li class="hangingindent">Shipley, A. E.: segmentation of <i>Microstomida</i>, <b>II</b>,
-    <a href="#Page_102">102</a>;</li>
-  <li class="i1"><i>Protodrilus</i>, <b>II</b>,
-    <a href="#Page_125">125</a>.</li>
-
-  <li>Silica, colloid and crystalloid, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page16">16</a>.</li>
-
-  <li>Silicic acid: properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page16">16</a>;</li>
-  <li class="i1">isomerism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page59">59</a>.</li>
-
-  <li>Silicon, allotropic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page4">4</a>.</li>
-
-  <li>Silkworm disease, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page622">622–3</a>.</li>
-
-  <li>Simulation: of homology by analogy, <b>II</b>,
-    <a href="#Page_14">14</a>,
-    <a href="#Page_485">485</a>;</li>
-  <li class="i1">of segmented structure by molluscs, <b>II</b>,
-    <a href="#Page_116">116</a>.</li>
-
-  <li class="hangingindent"><i>Siphonophora</i>, specialization of component polyps, <b>II</b>,
-    <a href="#Page_95">95</a>.</li>
-
-  <li><i>Sirenia</i>, simulated fish form, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page485">485</a>.</li>
-
-  <li>Size (<i>see</i> <a href="#Growth">Growth</a>).</li>
-
-  <li>Skeleton, vertebrate (see <a href="#Vertebrata"><i>Vertebrata</i></a>).</li>
-
-  <li id="Skin">Skin: respiratory function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page209">209</a>;</li>
-  <li class="i1">adaptability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page228">228</a>; <b>II</b>,
-    <a href="#Page_312">312–4</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">transmitted peculiarities, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page306">306</a>;</li>
-  <li class="hangingindent1">Wallace on distribution of sensitiveness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646–7</a>;</li>
-  <li class="i1">differentiation, <b>II</b>,
-    <a href="#Page_215">215</a>,
-    <a href="#Page_217">217</a>,
-    <a href="#Page_304">304–7</a>;</li>
-  <li class="i1">tegumentary development, <b>II</b>,
-    <a href="#Page_314">314–6</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">differentiation of sensory organs, <b>II</b>,
-    <a href="#Page_317">317–20</a>;</li>
-  <li class="i1">and mucous membrane, <b>II</b>,
-    <a href="#Page_303">303–4</a>,
-    <a href="#Page_321">321–2</a>,
-    <a href="#Page_389">389</a>.</li>
-
-  <li class="hangingindent">“Skin friction,” and locomotion of aquatic animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page156">156</a>.</li>
-
-  <li>Skull (see <a href="#Vertebrata"><i>Vertebrata</i></a>).</li>
-
-  <li>Sleep, repair favoured by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page216">216</a>.</li>
-
-  <li>Small-pox, blood changes from, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page221">221</a>.</li>
-
-  <li class="hangingindent">Smith, Prof. W., on fertility of <i>diatomaceæ</i>, <b>II</b>,
-    <a href="#Page_440">440</a>.</li>
-
-  <li class="hangingindent">Smith, W. P., on telegony in calves and foals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page645">645</a>.</li>
-
-  <li>Smith, W. W., on habits of Tetramorium, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660</a>.</li>
-
-  <li>Snakes (see <a href="#Reptilia"><i>Reptilia</i></a>).</li>
-
-  <li>“Social organism,” author’s essay on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page363">363</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676</a>.</li>
-
-  <li id="Sociology">Sociology: environment and degree of life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page105">105–6</a>;</li>
-  <li class="i1">functional differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page204">204</a>;</li>
-  <li class="i1">division of labour, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page207">207</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page363">363–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page367">367</a>;</li>
-  <li class="i1">functional interdependence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page237">237–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page240">240–2</a>;</li>
-  <li class="hangingindent1">autogenous development of units in colonies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page367">367–8</a>; <b>II</b>,
-    <a href="#Page_620">620</a>;</li>
-  <li class="i1">belief in social evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page432">432</a>;</li>
-  <li class="i1">natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>; <b>II</b>,
-    <a href="#Page_532">532</a>;</li>
-  <li class="i1">integration and differentiation, <b>II</b>,
-    <a href="#Page_378">378–9</a>;</li>
-  <li class="i1">effects of population, <b>II</b>,
-    <a href="#Page_535">535–6</a>;</li>
-  <li class="i1">equilibration, <b>II</b>,
-    <a href="#Page_537">537</a>.</li>
-
-  <li>Soil, dependence of plant evolution on, <b>II</b>,
-    <a href="#Page_402">402</a>.</li>
-
-  <li class="hangingindent"><i>Solanum jasminoides</i>, organs of attachment, <b>II</b>,
-    <a href="#Page_276">276</a>.</li>
-
-  <li class="hangingindent">Solar system, autogenous development illustrated by distribution of forces in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page366">366</a>.</li>
-
-  <li>Sole, symmetry and location of eyes, <b>II</b>,
-    <a href="#Page_205">205</a>.</li>
-
-  <li class="hangingindent">Soma-plasm, Weismann’s theory of differentiation from germ-plasm, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page357">357</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page622">622</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page628">628–30</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page633">633–44</a>.</li>
-
-  <li>Somites (<i>see</i> <a href="#Segmentation">Segmentation</a>).</li>
-
-  <li id="Special_creation">Special creation: and evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page412">412</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page415">415</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page431">431</a>;</li>
-  <li class="i1">improbabilities, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page418">418–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page430">430</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page439">439</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page554">554</a>;</li>
-  <li class="i1">inconceivability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page420">420</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page429">429</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page431">431</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page554">554</a>;</li>
-  <li class="i1">of individuals and species, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page421">421–4</a>;</li>
-  <li class="i1">the implication of beneficence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page425">425–9</a>;</li>
-  <li class="i1">summary, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page429">429</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page554">554</a>;</li>
-  <li class="i1">Von Baer’s formula, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page451">451–6</a>;</li>
-  <li class="i1">vertebrate skeleton, <b>II</b>,
-    <a href="#Page_551">551</a>,
-    <a href="#Page_556">556</a>,
-    <a href="#Page_565">565</a>.</li>
-
-  <li>Species: adaptation and stability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page242">242</a>;</li>
-  <li class="i1">hereditary transmission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page301">301–4</a>;</li>
-  <li class="hangingindent1">variation in wild and cultivated, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page323">323–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page326">326</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="i1">gamogenesis and life of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page347">347–9</a>;</li>
-  <li class="hangingindent1">physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page362">362</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page369">369–71</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page458">458</a>; <b>II</b>,
-    <a href="#Page_613">613</a>;</li>
-  <li class="i1">indefiniteness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page389">389</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page445">445</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page572">572</a>;</li>
-  <li class="i1">special creation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page422">422–4</a>;</li>
-  <li class="hangingindent1">instability of homogeneous, and differentiation of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page509">509–11</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page515">515</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page517">517–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page550">550</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">persistence of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page516">516</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page518">518</a>; <b>II</b>,
-    <a href="#Page_10">10–11</a>;</li>
-  <li class="hangingindent1">natural selection and equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page543">543–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="i1">non-adaptive characters, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page565">565</a>;</li>
-  <li class="i1">morbid products as marks of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page567">567</a>;</li>
-  <li class="hangingindent1">migration and isolation as causes of differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page568">568–9</a>;</li>
-  <li class="i1">increasing multiformity of aggregate, <b>II</b>,
-    <a href="#Page_396">396</a>.</li>
-
-  <li class="hangingindent" id="Specific_Gravity">Specific gravity, of organisms and environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page174">174</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page177">177</a>.</li>
-
-  <li>Spermatozoa&nbsp;|</li>
-  <li>Sperm-cell&emsp;| (<i>see</i> <a href="#Fertilization">Fertilization</a>).</li>
-
-  <li>Sphere: tendency of units to form, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page15">15</a>;</li>
-  <li class="i1">the embryonic form, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page177">177</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_131">131</a>.</li>
-
-  <li>Spheroid, symmetry, <b>II</b>,
-    <a href="#Page_132">132</a>.</li>
-
-  <li>Spiders (see <a href="#Arachnida"><i>Arachnida</i></a>).</li>
-
-  <li>Spine (see <a href="#Vertebrata"><i>Vertebrata</i></a>).</li>
-
-  <li class="hangingindent">Sponge: structure and dynamic element in life, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page119">119</a>;</li>
-  <li class="i1">multicentral development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page164">164</a>;</li>
-  <li class="i1">units and aggregate, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page185">185</a>;</li>
-  <li class="i1">reproductive tissue, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page283">283</a>;</li>
-  <li class="i1">integration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page586">586</a>; <b>II</b>,
-    <a href="#Page_90">90</a>,
-    <a href="#Page_383">383</a>;</li>
-  <li class="i1">physiological differentiation, <b>II</b>,
-    <a href="#Page_300">300</a>,
-    <a href="#Page_386">386</a>;</li>
-  <li class="i1">development and genesis, <b>II</b>,
-    <a href="#Page_463">463</a>;</li>
-  <li class="i1">analogy from, <b>II</b>,
-    <a href="#Page_576">576</a>.</li>
-
-  <li>Spontaneous generation: and heterogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page270">270</a>;</li>
-  <li class="i1">and evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page696">696–701</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page703">703</a>.</li>
-
-  <li class="hangingindent">Stag, horns and correlated structures, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page567">567</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>.</li>
-
-  <li>Stamens, and foliar homology, <b>II</b>,
-    <a href="#Page_44">44</a>.</li>
-
-  <li>Starches: properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page11">11</a>;</li>
-  <li class="hangingindent1">transformations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page66">66</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page68">68</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page70">70</a>; <b>II</b>,
-    <a href="#Page_593">593</a>.</li>
-
-  <li>Star-fishes (see <a href="#Asteroidea"><i>Asteroidea</i></a>).</li>
-
-  <li>Statoblasts, of <i>Plumatella</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page277">277</a>.</li>
-
-  <li>Steenstrup, on “Alternate Generation,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page592">592</a>.</li>
-
-  <li>Sterility (<i>see</i> <a href="#Multiplication">Multiplication</a>).</li>
-
-  <li>Stickleback: ova, <b>II</b>,
-    <a href="#Page_454">454</a>;</li>
-  <li class="i1"><i>bothriocephalus</i> in, <b>II</b>,
-    <a href="#Page_490">490</a>.</li>
-
-  <li>Stomach (<i>see</i> <a href="#Alimentary_canal">Alimentary Canal</a>).</li>
-
-  <li>Stomata, distribution, <b>II</b>,
-    <a href="#Page_260">260–1</a>.</li>
-
-  <li>Straight line, and evolution hypothesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page433">433</a>.</li>
-
-  <li class="hangingindent" id="Strain">Strain: compression and tension of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page151">151</a>; <b>II</b>,
-    <a href="#Page_209">209–12</a>;</li>
-  <li class="i1">relation to mass, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page155">155–7</a>;</li>
-  <li class="i1">vegetal structure, <b>II</b>,
-    <a href="#Page_574">574–88</a>,
-    <a href="#Page_592">592–6</a>;</li>
-  <li class="i1">origin of vertebrate type, <b>II</b>,
-    <a href="#Page_600">600</a>.</li>
-
-  <li>Strawberry: multiaxial development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page166">166</a>;</li>
-  <li class="i1">multiplication, <b>II</b>,
-    <a href="#Page_441">441</a>.</li>
-
-  <li>Strength, a vital attribute, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page578">578</a>.</li>
-
-  <li class="hangingindent" id="Structure">Structure: appliances for generating motion, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page75">75–7</a>;</li>
-  <li class="i1">biological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page125">125–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page129">129</a>;</li>
-  <li class="i1">size and organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page137">137</a>;</li>
-  <li class="i1">growth and complexity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page138">138</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page145">145</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page161">161</a>;</li>
-  <li class="i1">relation to environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page172">172–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page195">195–6</a>;</li>
-  <li class="i1">of unicellular organisms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page181">181–3</a>;</li>
-  <li class="i1">multicellular, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page183">183–96</a>;</li>
-  <li class="i1">Hertwig’s classification of tissues, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page189">189</a>;</li>
-  <li class="i1">continuity of units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page190">190–2</a>;</li>
-  <li class="i1">systems of organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page192">192</a>;</li>
-  <li class="i1">division into universal and particular, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page193">193–4</a>;</li>
-  <li class="i1">general truths, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page194">194–5</a>;</li>
-  <li class="i1">plant and animal, contrasted, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page195">195–6</a>;</li>
-  <li class="i1">precedence of function or, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page197">197</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page211">211</a>;</li>
-  <li class="hangingindent1">correlative complexity of function and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page200">200</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page211">211</a>;</li>
-  <li class="hangingindent1">progressive concomitant differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page201">201–4</a>;</li>
-  <li class="hangingindent1">physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page225">225–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page362">362</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page364">364</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page369">369–71</a>; <b>II</b>,
-    <a href="#Page_613">613</a>;</li>
-  <li class="i1">social and organic interdependence, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page235">235–42</a>;</li>
-  <li class="hangingindent1">varied by function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page334">334</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page535">535</a>; <b>II</b>,
-    <a href="#Page_217">217</a> (<i>see</i> <a href="#Acquired_characters">Acquired Characters</a>);</li>
-  <li class="i1">zoological classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page390">390–2</a>;</li>
-  <li class="i1">equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page521">521</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page557">557</a>;</li>
-  <li class="hangingindent1">progress of, and genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page590">590–1</a>; <b>II</b>,
-    <a href="#Page_462">462</a>;</li>
-  <li class="i1">co-operation with function, <b>II</b>,
-    <a href="#Page_3">3</a>;</li>
-  <li class="i1">evolution and increased, <b>II</b>,
-    <a href="#Page_4">4</a>;</li>
-  <li class="i1">retrograde metamorphosis, <b>II</b>,
-    <a href="#Page_12">12</a>;</li>
-  <li class="i1">simulated homologies, <b>II</b>,
-    <a href="#Page_13">13–14</a>;</li>
-  <li class="i1">earliest organic forms, <b>II</b>,
-    <a href="#Page_19">19</a>;</li>
-  <li class="i1">cylindrical vegetal, <b>II</b>,
-    <a href="#Page_57">57–62</a>;</li>
-  <li class="i1">permanence and complexity, <b>II</b>,
-    <a href="#Page_295">295</a>,
-    <a href="#Page_296">296</a>;</li>
-  <li class="i1">function and epidermic, <b>II</b>,
-    <a href="#Page_312">312–4</a>,
-    <a href="#Page_387">387</a>;</li>
-  <li class="i1">and muscular, <b>II</b>,
-    <a href="#Page_369">369</a>,
-    <a href="#Page_391">391</a>;</li>
-  <li class="i1">adaptation and equilibration, <b>II</b>,
-    <a href="#Page_392">392</a>;</li>
-  <li class="hangingindent1">persistence of force and physiological adaptation, <b>II</b>,
-    <a href="#Page_394">394</a>;</li>
-  <li class="i1">evolution, <b>II</b>,
-    <a href="#Page_501">501–4</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Morphology">Morphology</a>.)</li>
-
-  <li class="hangingindent">Struggle, for nutriment among components of an organism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page562">562</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676</a>;</li>
-  <li class="i1">for existence (<i>see</i> <a href="#Natural_selection">Natural Selection</a>).</li>
-
-  <li>Struthers, Sir J.: on heredity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page305">305</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314</a>;</li>
-  <li class="i1">digital variation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page321">321</a>;</li>
-  <li class="i1">rudimentary limbs of whale, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page668">668</a>.</li>
-
-  <li>Strychnine, effects of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page54">54</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page55">55</a>.</li>
-
-  <li>Sturgeon, size of ova and adult, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page144">144</a>.</li>
-
-  <li>Sugars: properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page10">10–11</a>;</li>
-  <li class="hangingindent1">transformations, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page38">38</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page40">40</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page66">66</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page70">70</a>; <b>II</b>,
-    <a href="#Page_593">593</a>.</li>
-
-  <li>Suicide, hereditary tendency to, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page307">307</a>.</li>
-
-  <li>Sulphur: allotropic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page4">4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page59">59</a>;</li>
-  <li class="i1">organic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page703">703</a>.</li>
-
-  <li>Sun (<i>see</i> <a href="#Light">Light</a>).</li>
-
-  <li class="hangingindent">Survival of the Fittest, the expression, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page530">530</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page610">610</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Natural_selection">Natural Selection</a>.)</li>
-
-  <li>Swan, vertebræ of neck, <b>II</b>,
-    <a href="#Page_123">123</a>.</li>
-
-  <li>Swiftness, a vital attribute, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page578">578</a>.</li>
-
-  <li class="hangingindent"><i>Syllis ramosa</i>, lateral branching, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page166">166</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page361">361</a>; <b>II</b>,
-    <a href="#Page_105">105</a>,
-    <a href="#Page_108">108</a>.</li>
-
-  <li>Symbiosis, <b>II</b>,
-    <a href="#Page_399">399</a>,
-    <a href="#Page_400">400</a>.</li>
-
-  <li>Symmetry (<i>see</i> <a href="#Morphology">Morphology</a>).</li>
-
-  <li>Syphilis, hereditary transmission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page623">623</a>.</li>
-</ul>
-
-<ul>
-  <li class="hangingindent">Tactual, Perceptiveness, heredity and the distribution of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page602">602–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page633">633</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page665">665</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page666">666</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page672">672</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>.</li>
-
-  <li><i>Tænia</i> (see <a href="#Entozoa"><i>Entozoa</i></a>).</li>
-
-  <li>Tansley, A. G., <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#pagevi">vi</a>; <b>II</b>,
-    <a href="#Page_vi">vi</a>;</li>
-  <li class="hangingindent1">adaptation of reproductive activity to conditions in <i>Algæ</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page288">288–9</a>;</li>
-  <li class="i1">shapes of <i>Caulerpa</i>, <b>II</b>,
-    <a href="#Page_22">22</a>;</li>
-  <li class="i1">stem-thickening in extinct Thallophytes, <b>II</b>,
-    <a href="#Page_56">56</a>;</li>
-  <li class="i1">natural selection and leaf-distribution, <b>II</b>,
-    <a href="#Page_179">179</a>.</li>
-
-  <li>Tape-worm (see <a href="#Entozoa"><i>Entozoa</i></a>).</li>
-
-  <li>Taste, dependent on chemical action, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page54">54</a>.</li>
-
-  <li>Teeth: hereditary transmission, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page306">306</a>;</li>
-  <li class="i1">suppression of mammalian, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page457">457</a>;</li>
-  <li class="i1">of uncivilized and civilized, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page541">541</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>.</li>
-
-  <li>Tegumentary organs, origin of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314–6</a>.</li>
-
-  <li class="hangingindent">Telegony, or the Influence of a previous sire on offspring, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page624">624–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page644">644–6</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page649">649–50</a>.</li>
-
-  <li>Temperature (<i>see</i> <a href="#Heat">Heat</a>).</li>
-
-  <li>Tension (<i>see</i> <a href="#Strain">Strain</a>).</li>
-
-  <li id="Termites"><i>Termites</i>: fertility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page583">583</a>; <b>II</b>,
-    <a href="#Page_493">493</a>;</li>
-  <li class="i1">late development of sexual organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page680">680</a>;</li>
-  <li class="i1">nutrition and differentiation of forms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page681">681</a>.</li>
-
-  <li><i>Tetramorium</i>, utilization of aphides by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660–1</a>.</li>
-
-  <li><i>Thallophyta</i>: size, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page138">138</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page139">139</a>;</li>
-  <li class="i1">low co-ordination of parts, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page164">164</a>;</li>
-  <li class="i1">pseudo-foliar, <b>II</b>,
-    <a href="#Page_28">28</a>;</li>
-  <li class="i1">“transition place,” <b>II</b>,
-    <a href="#Page_30">30</a>;</li>
-  <li class="i1">simulation of higher types, <b>II</b>,
-    <a href="#Page_32">32</a>;</li>
-  <li class="i1">secondary thickening in extinct species, <b>II</b>,
-    <a href="#Page_56">56</a>;</li>
-  <li class="i1">sexual and asexual genesis, <b>II</b>,
-    <a href="#Page_84">84</a>.</li>
-  <li class="i1">(See also <a href="#Algae"><i>Algæ</i></a>.)</li>
-
-  <li>Tickling, physiology of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page76">76</a>.</li>
-
-  <li>Tide (<i>see</i> <a href="#Sea">Sea</a>).</li>
-
-  <li>Time, as a factor in growth, <b>II</b>,
-    <a href="#Page_77">77</a>.</li>
-
-  <li>Tissue, Hertwig’s classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page189">189</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Physiology">Physiology</a>.)</li>
-
-  <li>Tongue, perceptiveness of tip, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page606">606–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page665">665</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page672">672–3</a>.</li>
-
-  <li>Tortoise: contrasted life of dog and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page103">103–4</a>;</li>
-  <li class="i1">natural selection and carapace, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page534">534</a>.</li>
-
-  <li>“Transcendental Physiology,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page176">176</a>.</li>
-
-  <li>Tree, as symbolizing phylogeny, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page428">428</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page452">452–3</a>.</li>
-  <li class="i1">(<i>See</i> <a href="#Plants">Plants</a>.)</li>
-
-  <li><i>Trematoda</i>: agamogenesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page277">277</a>;</li>
-  <li class="i1">parasitism, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page428">428</a>;</li>
-  <li class="i1">alternate generation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page592">592</a>.</li>
-
-  <li>Trembley, A., on the polyp, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page223">223</a>.</li>
-
-  <li>Trichinosis, in Germany, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page428">428</a>.</li>
-
-  <li class="hangingindent">Trochophore, phyletic relationships shown by, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page447">447</a>; <b>II</b>,
-    <a href="#Page_108">108–9</a>.</li>
-
-  <li><i>Tubicolæ</i>: development, <b>II</b>,
-    <a href="#Page_100">100</a>;</li>
-  <li class="i1">bilateral symmetry, <b>II</b>,
-    <a href="#Page_197">197</a>.</li>
-
-  <li id="Tunicata"><i>Tunicata</i>: gemmation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page588">588</a>; <b>II</b>,
-    <a href="#Page_445">445</a>;</li>
-  <li class="i1">alternate generation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page592">592</a>;</li>
-  <li class="i1">integration, <b>II</b>,
-    <a href="#Page_93">93–4</a>;</li>
-  <li class="i1">tertiary aggregation, <b>II</b>,
-    <a href="#Page_124">124</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_194">194–5</a>.</li>
-
-  <li>Tunny, size of ova and adult, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page144">144</a>.</li>
-
-  <li><i>Turbellaria</i>: segmentation, <b>II</b>,
-    <a href="#Page_102">102</a>;</li>
-  <li class="i1">symbiosis, <b>II</b>,
-    <a href="#Page_400">400</a>.</li>
-
-  <li>Turnip: chlorophyll in roots, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page209">209</a>; <b>II</b>,
-    <a href="#Page_254">254</a>;</li>
-  <li class="i1">vascular system, <b>II</b>,
-    <a href="#Page_281">281</a>,
-    <a href="#Page_284">284</a>,
-    <a href="#Page_578">578</a>,
-    <a href="#Page_591">591</a>,
-    <a href="#Page_596">596</a>.</li>
-
-  <li>Twins: similarity of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page324">324</a>;</li>
-  <li class="i1">traits of women bearing, <b>II</b>,
-    <a href="#Page_457">457</a>.</li>
-
-  <li>“Types, persistent,” Huxley on, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page408">408</a>.</li>
-</ul>
-
-<ul>
-  <li>Ulcer, dermal structure, <b>II</b>,
-    <a href="#Page_306">306</a>.</li>
-
-  <li>Ultimate Reality, incomprehensibility of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page120">120</a>.</li>
-
-  <li><i>Ulva</i>: cell multiplication, <b>II</b>,
-    <a href="#Page_26">26</a>;</li>
-  <li class="i1">outer tissue, <b>II</b>,
-    <a href="#Page_256">256</a>.</li>
-
-  <li><i>Umbelliferæ</i>: floral symmetry, <b>II</b>,
-    <a href="#Page_171">171</a>;</li>
-  <li class="i1">axial and foliar organs, <b>II</b>,
-    <a href="#Page_541">541–6</a>.</li>
-
-  <li>United States: cases of telegony, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page644">644–5</a>;</li>
-  <li class="i1">birth-rate, <b>II</b>,
-    <a href="#Page_520">520</a>.</li>
-
-  <li>Units: differentiation and dissimilarity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page20">20</a>;</li>
-  <li class="i1">“protyle,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page22">22–3</a>;</li>
-  <li class="i1">shapes in higher types, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page164">164</a>;</li>
-  <li class="i1">differential assimilation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page180">180</a>;</li>
-  <li class="i1">primordial organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page181">181</a>;</li>
-  <li class="hangingindent1">morphological composition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page184">184–7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page194">194</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page252">252</a>; <b>II</b>,
-    <a href="#Page_5">5</a>,
-    <a href="#Page_7">7–9</a>,
-    <a href="#Page_21">21</a>,
-    <a href="#Page_79">79</a>,
-    <a href="#Page_85">85–6</a>;</li>
-  <li class="i1">segregation and organic repair, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page221">221–2</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page222">222–6</a>;</li>
-  <li class="hangingindent1">chemical, morphological, and physiological, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page225">225–6</a>; <b>II</b>,
-    <a href="#Page_612">612</a>;</li>
-  <li class="i1">stability, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page339">339</a>;</li>
-  <li class="i1">instability and heterogeneity of organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page350">350</a>;</li>
-  <li class="i1">Darwin’s gemmules, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page356">356–60</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page362">362</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page372">372</a>;</li>
-  <li class="i1">Weissmann’s germ-plasm (<i>q. v.</i>) <i>ib.</i>;</li>
-  <li class="i1">sociological comparison, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page363">363–8</a>;</li>
-  <li class="i1">specific proclivities in embryogeny, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page458">458</a>;</li>
-  <li class="i1">phænogamic, <b>II</b>,
-    <a href="#Page_73">73</a>,
-    <a href="#Page_151">151</a>;</li>
-  <li class="i1">annulose, <b>II</b>,
-    <a href="#Page_105">105</a>;</li>
-  <li class="i1">incident force and homologous, <b>II</b>,
-    <a href="#Page_159">159</a>;</li>
-  <li class="i1">morphological summary, <b>II</b>,
-    <a href="#Page_233">233</a>.</li>
-  <li class="i1">(<i>See also</i> <a href="#Physiological_Units">Physiological Units</a>.)</li>
-
-  <li>“Universal Postulate,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>.</li>
-
-  <li>Unsymmetrical, definition, <b>II</b>,
-    <a href="#Page_131">131</a>.</li>
-
-  <li>Urea, muscular energy and excretion, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page72">72</a>.</li>
-</ul>
-
-<ul>
-  <li>Van Beneden, P. J., on <i>Tænia</i>, <b>II</b>,
-    <a href="#Page_103">103</a>.</li>
-
-  <li>Variation: digital, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page331">331</a>;</li>
-  <li class="i1">effects of parental conditions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page324">324</a>;</li>
-  <li class="i1">of altered function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page325">325</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page334">334</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>;</li>
-  <li class="hangingindent1">dissimilarity of initial conditions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page327">327–32</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page333">333</a>;</li>
-  <li class="i1">“spontaneous,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page328">328</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page513">513</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page697">697</a>; <b>II</b>,
-    <a href="#Page_529">529</a>;</li>
-  <li class="i1">persistence of force, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page335">335</a>;</li>
-  <li class="hangingindent1">physiological units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page348">348–54</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page360">360</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page369">369</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page371">371–3</a>; <b>II</b>,
-    <a href="#Page_614">614–7</a>,
-    <a href="#Page_622">622–3</a>;</li>
-  <li class="hangingindent1">Weismann’s germ-plasm theory, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page357">357–8</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page372">372–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page671">671</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page677">677</a>; <b>II</b>,
-    <a href="#Page_622">622</a>;</li>
-  <li class="i1">equilibration and vegetal, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page523">523–5</a>;</li>
-  <li class="hangingindent1">Weismann’s panmixia theory, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page561">561–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page649">649</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page667">667–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page671">671</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>;</li>
-  <li class="i1">reproductive organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page570">570</a>;</li>
-  <li class="hangingindent1">natural selection and concomitant, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page614">614–21</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page653">653</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page664">664</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page674">674</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page692">692</a>;</li>
-  <li class="i1">and disused organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page648">648</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page668">668</a>;</li>
-  <li class="i1">plus and minus. <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page667">667</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>;</li>
-  <li class="hangingindent1">Masters on correlated, in plants, <b>II</b>,
-    <a href="#Page_298">298</a>,
-    <a href="#Page_621">621–2</a>;</li>
-  <li class="i1">equilibration of favourable, <b>II</b>,
-    <a href="#Page_394">394</a>.</li>
-
-  <li class="hangingindent" id="Vascular_System">Vascular System: effects of vegeto-alkalies, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page55">55</a>;</li>
-  <li class="i1">nutrition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page146">146</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page148">148</a>;</li>
-  <li class="i1">embryonic development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page169">169</a>;</li>
-  <li class="i1">structural traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page192">192</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page193">193</a>;</li>
-  <li class="i1">function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page199">199</a>;</li>
-  <li class="i1">of Ascidians. <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page202">202</a>;</li>
-  <li class="hangingindent1">functional differentiation and integration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page205">205–6</a>;</li>
-  <li class="i1">organic repair, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page217">217</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page221">221–2</a>;</li>
-  <li class="i1">effect of function, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page229">229</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page234">234–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page236">236</a>;</li>
-  <li class="i1">equilibration, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page535">535</a>;</li>
-  <li class="i1">community in compound organisms, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page588">588</a>;</li>
-  <li class="hangingindent1">development of vegetal, <b>II</b>,
-    <a href="#Page_273">273–5</a>,
-    <a href="#Page_279">279–84</a>,
-    <a href="#Page_285">285–8</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="i1">differentiation of, summary, <b>II</b>,
-    <a href="#Page_288">288–90</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="i1">differentiation of animal, <b>II</b>,
-    <a href="#Page_339">339–44</a>;</li>
-  <li class="i1">osseous development, <b>II</b>,
-    <a href="#Page_347">347–51</a>;</li>
-  <li class="i1">muscularity, <b>II</b>,
-    <a href="#Page_364">364</a>;</li>
-  <li class="i1">muscular colour, <b>II</b>,
-    <a href="#Page_365">365–9</a>;</li>
-  <li class="i1">heart-motor apparatus, <b>II</b>,
-    <a href="#Page_374">374</a>;</li>
-  <li class="hangingindent1">differentiation and integration in animal, <b>II</b>,
-    <a href="#Page_376">376–9</a>,
-    <a href="#Page_383">383</a>;</li>
-  <li class="i1">wood formation, <b>II</b>,
-    <a href="#Page_567">567–92</a>;</li>
-  <li class="i1"><i>résumé</i> of wood formation, <b>II</b>,
-    <a href="#Page_592">592–7</a>.</li>
-
-  <li><i>Vaucheria</i>, reproduction, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page279">279</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page289">289</a>.</li>
-
-  <li class="hangingindent">Vegetative System, co-ordination of actions in, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page578">578</a>.</li>
-
-  <li>Vegeto-alkalies, physiological effects of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page54">54–5</a>.</li>
-
-  <li>Velocity, of moving bodies, <b>II</b>,
-    <a href="#Page_219">219–20</a>.</li>
-
-  <li id="Vertebrata"><i>Vertebrata</i>: size, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page139">139</a>;</li>
-  <li class="i1">size at birth and maturity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page144">144</a>;</li>
-  <li class="i1">axial structure, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page165">165</a>;</li>
-  <li class="i1">embryonic development and self-mobility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page175">175</a>;</li>
-  <li class="i1">functional differentiation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page206">206</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page591">591</a>;</li>
-  <li class="i1">reparative power, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page219">219</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page223">223</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page589">589</a>;</li>
-  <li class="i1">homogenesis universal, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>;</li>
-  <li class="i1">distinctive traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page392">392</a>; <b>II</b>,
-    <a href="#Page_35">35</a>;</li>
-  <li class="i1">distribution in time, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page408">408</a>;</li>
-  <li class="i1">classificatory value, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page446">446</a>;</li>
-  <li class="i1">embryonic mammalian respiratory system, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page456">456</a>;</li>
-  <li class="i1">embryological pre-adaptation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page461">461</a>;</li>
-  <li class="i1">evolution and vertebral column, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page470">470</a>;</li>
-  <li class="i1">rudimentary organs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page473">473</a>;</li>
-  <li class="i1">evolution and varied media, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page479">479–85</a>;</li>
-  <li class="i1">size of head and vertebræ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page512">512</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page537">537</a>;</li>
-  <li class="i1">segregation and evolution of vertebræ, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page515">515</a>;</li>
-  <li class="i1">fertility and development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page583">583</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page598">598–9</a>;</li>
-  <li class="i1">Weismann on reproductive cells, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page635">635</a>;</li>
-  <li class="i1">limb locomotion, <b>II</b>,
-    <a href="#Page_15">15</a>;</li>
-  <li class="hangingindent1">adaptive segmentation, <b>II</b>,
-    <a href="#Page_117">117–23</a>,
-    <a href="#Page_125">125–7</a>,
-    <a href="#Page_223">223</a>,
-    <a href="#Page_602">602</a>,
-    <a href="#Page_605">605–7</a>;</li>
-  <li class="i1">supernumerary vertebræ, <b>II</b>,
-    <a href="#Page_123">123</a>;</li>
-  <li class="i1">bilateral symmetry, <b>II</b>,
-    <a href="#Page_203">203–6</a>;</li>
-  <li class="i1">internal organic symmetry, <b>II</b>,
-    <a href="#Page_208">208</a>;</li>
-  <li class="i1">genesis of rudimentary axis, <b>II</b>,
-    <a href="#Page_212">212–6</a>;</li>
-  <li class="hangingindent1">natural selection and genesis of structure, <b>II</b>,
-    <a href="#Page_216">216</a>,
-    <a href="#Page_227">227</a>;</li>
-  <li class="i1">origin of notochord, <b>II</b>,
-    <a href="#Page_216">216–8</a>;</li>
-  <li class="i1">spinal segmentation, <b>II</b>,
-    <a href="#Page_218">218–22</a>,
-    <a href="#Page_224">224</a>;</li>
-  <li class="i1">skull development, <b>II</b>,
-    <a href="#Page_222">222</a>,
-    <a href="#Page_227">227</a>;</li>
-  <li class="i1"><i>résumé</i> of axis development, <b>II</b>,
-    <a href="#Page_224">224</a>;</li>
-  <li class="i1">Cope on author’s theory, <b>II</b>,
-    <a href="#Page_225">225–7</a>;</li>
-  <li class="i1">nerve differentiation, <b>II</b>,
-    <a href="#Page_304">304</a>;</li>
-  <li class="i1">sensory organs, <b>II</b>,
-    <a href="#Page_318">318</a>;</li>
-  <li class="i1">air-chambers, <b>II</b>,
-    <a href="#Page_334">334</a>;</li>
-  <li class="i1">osseous differentiation, <b>II</b>,
-    <a href="#Page_344">344–55</a>;</li>
-  <li class="i1">activity and muscular colour, <b>II</b>,
-    <a href="#Page_365">365–9</a>;</li>
-  <li class="i1">heart-motor apparatus, <b>II</b>,
-    <a href="#Page_374">374</a>;</li>
-  <li class="i1">cost of genesis, <b>II</b>,
-    <a href="#Page_436">436</a>;</li>
-  <li class="i1">agamogenesis unknown, <b>II</b>,
-    <a href="#Page_445">445</a>;</li>
-  <li class="i1">growth and genesis, <b>II</b>,
-    <a href="#Page_454">454</a>;</li>
-  <li class="i1">heat expenditure and genesis, <b>II</b>,
-    <a href="#Page_468">468–9</a>,
-    <a href="#Page_474">474</a>;</li>
-  <li class="i1">Owen, theory of skeleton, <b>II</b>,
-    <a href="#Page_548">548–66</a>;</li>
-  <li class="i1">evolution of vertebræ, <b>II</b>,
-    <a href="#Page_563">563–6</a>;</li>
-  <li class="i1">origin of type, <b>II</b>,
-    <a href="#Page_598">598–600</a>.</li>
-
-  <li><i>Vestiges of Creation</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page491">491</a>.</li>
-
-  <li>Vibrissæ, function of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page75">75</a>.</li>
-
-  <li>Vitalism, hypothesis examined, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page114">114–7</a>.</li>
-
-  <li>Vittadini, C., on silkworm disease, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page622">622–3</a>.</li>
-
-  <li>Viviparons genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page271">271</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page274">274–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page278">278</a>.</li>
-
-  <li>Voice, correlated sexual traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page371">371–2</a>.</li>
-
-  <li>Volcano, definition of life and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page85">85</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page89">89</a>.</li>
-
-  <li><i>Volvocineæ</i>: unicentral development, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page163">163</a>;</li>
-  <li class="i1">individuality, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page245">245</a>;</li>
-  <li class="i1">disintegration of genesis, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page276">276</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page587">587</a>;</li>
-  <li class="i1">spherical aggregation, <b>II</b>,
-    <a href="#Page_24">24</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_137">137</a>,
-    <a href="#Page_187">187</a>;</li>
-  <li class="i1">fertility, <b>II</b>,
-    <a href="#Page_441">441</a>.</li>
-
-  <li>Vomiting, alimentary canal development, <b>II</b>,
-    <a href="#Page_328">328</a>.</li>
-
-  <li><i>Vorticella</i>: secondary aggregate, <b>II</b>,
-    <a href="#Page_90">90</a>;</li>
-  <li class="i1">symmetry, <b>II</b>,
-    <a href="#Page_188">188</a>.</li>
-</ul>
-
-<ul>
-  <li class="hangingindent">Wallace, A. R.: “The Origin of the Human Races,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page553">553</a>;</li>
-  <li class="hangingindent1">the expression “Survival of the Fittest,” <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page530">530</a>;</li>
-  <li class="hangingindent1">his association of natural with artificial selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page609">609</a>;</li>
-  <li class="i1">co-adaptation in giraffe, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page615">615</a>;</li>
-  <li class="i1">skin sensitiveness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646</a>.</li>
-
-  <li class="hangingindent">Wasp: co-ordination of instincts in Mason-, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page574">574</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page679">679–80</a>;</li>
-  <li class="i1">genesis of worker, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page654">654–7</a>.</li>
-
-  <li>Waste, animal, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page69">69</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page213">213–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page228">228</a>;</li>
-  <li class="i1">relation to activity, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page196">196</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page220">220–1</a>;</li>
-  <li class="i1">in plants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page213">213</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page220">220</a>.</li>
-
-  <li>Water: properties, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page7">7</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page9">9</a>;</li>
-  <li class="i1">colloidal affinity for, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page28">28</a>;</li>
-  <li class="i1">organic change from, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page29">29</a>;</li>
-  <li class="i1">organic need for, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page147">147</a>;</li>
-  <li class="hangingindent1">proportion in mammalian adult and fœtus, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page154">154</a>;</li>
-  <li class="i1">motion through, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page156">156</a>;</li>
-  <li class="hangingindent1">organic development and environment, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page173">173</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page177">177</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page479">479</a>;</li>
-  <li class="i1">terrestrial organisms inhabiting, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page400">400</a>;</li>
-  <li class="hangingindent1">adaptation of organisms to change of media, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page479">479–85</a>;</li>
-  <li class="i1">vegetal tissue differentiation, <b>II</b>,
-    <a href="#Page_253">253</a>;</li>
-  <li class="i1">molecular re-arrangement, <b>II</b>,
-    <a href="#Page_359">359</a>;</li>
-  <li class="i1">colloidal contraction, <b>II</b>,
-    <a href="#Page_361">361–2</a>.</li>
-
-  <li>Water-weed, American, invasion of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page399">399</a>.</li>
-
-  <li class="hangingindent">Watts, Dr., on <i>The Principles of Biology</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#pageix">ix</a>.</li>
-
-  <li>Wax, foliar deposit, <b>II</b>,
-    <a href="#Page_260">260–1</a>.</li>
-
-  <li>Weber, on tactual discriminativeness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page602">602</a>.</li>
-
-  <li class="hangingindent">Weight: relation to environment of organic, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page174">174</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page177">177</a>;</li>
-  <li class="hangingindent1">varying as cube of dimensions, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page151">151</a>; <b>II</b>,
-    <a href="#Page_434">434</a>,
-    <a href="#Page_470">470</a>.</li>
-
-  <li class="hangingindent" id="Weismann">Weismann, Aug.: reproductive tissue in <i>Medusæ</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page281">281</a>;</li>
-  <li class="i1">in <i>Daphnidæ</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page290">290</a>;</li>
-  <li class="hangingindent1">his theory of the differentiated germ-plasm and its fundamental units, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page357">357</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page622">622–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page628">628–30</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page633">633–44</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page646">646</a>; <b>II</b>,
-    <a href="#Page_618">618–9</a>,
-    <a href="#Page_622">622</a>;</li>
-  <li class="hangingindent1">the alleged differentiation and plant-phenomena, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page359">359–60</a>;</li>
-  <li class="i1">and regenerative processes, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page360">360</a>;</li>
-  <li class="i1">false joints, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page362">362</a>;</li>
-  <li class="i1">implied complexity of determinants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page370">370</a>;</li>
-  <li class="hangingindent1">theory inadequate to explain correlation of sexual traits, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page372">372</a>;</li>
-  <li class="hangingindent1">and variations in peacock’s tail feather, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page372">372–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page695">695</a>; <b>II</b>,
-    <a href="#Page_618">618</a>;</li>
-  <li class="hangingindent1">his view of natural selection as sole factor in organic evolution, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page559">559</a>;</li>
-  <li class="hangingindent1">the doctrine of panmixia, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page561">561–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page632">632</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page649">649</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page667">667–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page671">671</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page689">689</a>;</li>
-  <li class="hangingindent1">arguments against inheritance of acquired characters, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page612">612–3</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page651">651–65</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page669">669–71</a>;</li>
-  <li class="i1">blindness of cave-animals, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page613">613</a>;</li>
-  <li class="i1">current acceptance of his views, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page631">631</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page690">690</a>;</li>
-  <li class="hangingindent1">cannot explain the process of natural selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page651">651</a>;</li>
-  <li class="hangingindent1">the degradation of the little toe in man, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page652">652</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page669">669</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page673">673</a>;</li>
-  <li class="hangingindent1">caste gradations of social insects, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page654">654</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page658">658–65</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page678">678–84</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>;</li>
-  <li class="hangingindent1">food-seeking instinct in Amazon ants, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page660">660</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>;</li>
-  <li class="hangingindent1">the co-adaptation of co-operative parts, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page663">663–4</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page670">670</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page674">674</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page675">675</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676</a>;</li>
-  <li class="i1">tactual discriminativeness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page665">665</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page672">672</a>;</li>
-  <li class="i1">intra-selection, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page676">676–8</a>;</li>
-  <li class="hangingindent1">effect of nutrition on fertility of blow-fly, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page678">678–9</a>.</li>
-
-  <li>Whale: weight of brain, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page599">599</a>;</li>
-  <li class="i1">rudimentary limbs, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page668">668–9</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page685">685</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page693">693</a>.</li>
-
-  <li>Wheat, adaptive variations, <b>II</b>,
-    <a href="#Page_298">298</a>.</li>
-
-  <li>Whistling, definition of life and, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page112">112</a>.</li>
-
-  <li class="hangingindent">White-Cooper, Mr., on inheritance of abnormal vision, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page306">306</a>.</li>
-
-  <li>Willow, nutrition and growth, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page294">294</a>.</li>
-
-  <li>Wilson, E. B.: composition of chromatin, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page260">260</a>;</li>
-  <li class="hangingindent1">separation of segmentation spheres of <i>Amphioxus</i> ovum, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page691">691</a>.</li>
-
-  <li>Wind: and vegetal bilateral symmetry, <b>II</b>,
-    <a href="#Page_142">142</a>;</li>
-  <li class="hangingindent1">and inner vegetal tissue differentiation, <b>II</b>,
-    <a href="#Page_275">275–9</a>,
-    <a href="#Page_285">285</a>,
-    <a href="#Page_288">288</a>,
-    <a href="#Page_388">388</a>;</li>
-  <li class="i1">and proliferation of <i>Bryophyllum</i>, <b>II</b>,
-    <a href="#Page_295">295</a>;</li>
-  <li class="i1">and vegetal sap movement, <b>II</b>,
-    <a href="#Page_583">583</a>,
-    <a href="#Page_584">584</a>,
-    <a href="#Page_587">587</a>;</li>
-  <li class="i1"><i>résumé</i>,
-    <a href="#Page_592">592–6</a>.</li>
-
-  <li class="hangingindent">Wolff, C.: vegetal fructification and nutrition, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page283">283</a>; <b>II</b>,
-    <a href="#Page_179">179–80</a>;</li>
-  <li class="i1">vegetal vascular system, <b>II</b>,
-    <a href="#Page_283">283</a>.</li>
-
-  <li>Women (<i>see</i> <a href="#Man">Man</a>).</li>
-
-  <li>Wood (<i>see</i> <a href="#Plants">Plants</a>).</li>
-</ul>
-
-<ul>
-  <li>Yeast: fermentation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page38">38</a>;</li>
-  <li class="i1">fertility, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page581">581</a>; <b>II</b>,
-    <a href="#Page_440">440</a>;</li>
-  <li class="i1">linear aggregation, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page587">587</a>; <b>II</b>,
-    <a href="#Page_23">23</a>.</li>
-</ul>
-
-<ul>
-  <li>Zebra marks in horses, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page314">314</a>.</li>
-
-  <li>Zoology, classification, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page124">124–5</a>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page380">380–9</a>.</li>
-
-  <li>Zoophytes, structural indefiniteness, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page173">173</a>.</li>
-
-  <li>Zoospores, unit-life of, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page185">185</a>.</li>
-
-  <li>Zygote, of conjugating <i>Algæ</i>, <b>I</b>,
-    <a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#page283">283</a>.</li>
-</ul>
-
-
-<p class="center p-left sm">THE END.</p>
-
-
-<div class="footnotes"><h2>FOOTNOTES:</h2>
-
-<div class="footnote">
-
-<p><a id="Footnote_1" href="#FNanchor_1" class="label">[1]</a> It seems needful here to say, that allusion is made in
-this paragraph to a proposition respecting the ultimate natures of
-Evolution and Dissolution, which is contained in an essay on <i>The
-Classification of the Sciences</i>, published in March, 1864. When
-the opportunity comes, I hope to make the definition there arrived
-at, the basis of a re-organization of the second part of <i>First
-Principles</i>: giving to that work a higher development, and a greater
-cohesion, than it at present possesses. [The intention here indicated
-was duly carried out in 1867.]</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_2" href="#FNanchor_2" class="label">[2]</a> Let me here refer those who are interested in this
-question, to Prof. Huxley’s criticism on the cell-doctrine, published
-in the <i>Medico-Chirurgical Review</i> in 1853.</p>
-
-<p>A critic who thinks the above statements are “rather misleading”
-admits that the lowest types of organisms yield them support, saying
-that “there are certainly masses of protoplasm containing many nuclei,
-but no trace of cellular structure, in both animals and plants. Such
-non-cellular masses may exist during development and later become
-separated up into cells, but there are certain low organisms in which
-such masses exist in the adult state. They are called by some botanists
-non-cellular, by others multi-nucleate cells. Clearly the difference
-lies in the criteria of a cell. There are also some <i>Protozoa</i>,
-and the <i>Bacteria</i>, in which no nucleus has certainly been
-demonstrated. But it is usual to consider the bodies of such organisms
-as cells nevertheless, and it is supposed that such cells represent a
-stage of development in which the nucleus has not yet been evolved,
-though the chemical substance ‘nuclein’ has been formed in some of
-them.”</p>
-
-<p>Perhaps it will be most correct to say that, excluding the minute,
-non-nucleated organisms, all the higher organisms&mdash;<i>Metazoa</i> and
-<i>Metaphyta</i>&mdash;are composed throughout of cells, or of tissues
-originally cellular, or of materials which have in the course of
-development been derived from cells. It must, however, be borne in
-mind that, according to sundry leading biologists, cells in the strict
-sense are not the immediate products either of the primitive fissions
-or of subsequent fissions; but that the multiplying so-called cells
-are nucleated masses of protoplasm which remain connected by strands
-of protoplasm, and which acquire limiting membranes by a secondary
-process. So that, in the view of Mr. Adam Sedgwick and others, the
-substance of an organism is in fact a continuous mass of vacuolated
-protoplasm.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_3" href="#FNanchor_3" class="label">[3]</a> In further illustration, Mr. Tansley names the fact that
-in the genus <i>Caulerpa</i> we have extremely complicated forms often
-of considerable size produced in the same way. The various species
-simulate very perfectly the members of different groups among the
-higher plants, such as Horse-tails, Mosses, Cactuses, Conifers and the
-like.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_4" href="#FNanchor_4" class="label">[4]</a> It may be objected that in <i>Cladophora</i> the separate
-compartments of the thallus severally contain many nuclei, making it
-doubtful whether they descend from uni-nucleate cells. If, however,
-they do not they simply illustrate another form of integration.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_5" href="#FNanchor_5" class="label">[5]</a> The great mass of early ancestral types&mdash;plant and
-animal&mdash;consisting of soft tissues, have left no remains whatever, and
-we have no reason to suppose that those which left remains fell within
-the direct ancestral lines of any existing forms. Contrariwise, we have
-reason to suppose that they fell within lines of evolution out of which
-the lines ending in existing forms diverged. We must therefore infer
-that the difficulties of affiliation which arise if we contemplate
-divergent types now existing, would not arise if we had before us all
-the early intermediate types. The Mammalia differ in sundry respects
-from all other kinds of Vertebrata&mdash;Fishes, Reptiles, Birds; and if
-the absence of hair, mammæ, and two occipital condyles, in these other
-vertebrates were taken to imply a fundamental distinction, it might, in
-the absence of any known fossil links, be inferred that the Mammalia
-belonged to a separate phylum. But these differences are not held to
-negative the assumed relationship. Similarly among plants. We must
-not reject an hypothesis respecting a certain supposed type, because
-the existing types it must have been akin to present traits which it
-could not have had. We are justified in assuming, within limits, a
-hypothetical type, unlike existing types in traits of some importance.
-Hence results the answer to a criticism passed on the above argument,
-that it implies relations between the undeveloped and developed forms
-of the <i>Jungermanniaceæ</i> such as the facts do not show us. This
-objection is met on remembering that the types in which the supposed
-transition took place disappeared myriads of years ago.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_6" href="#FNanchor_6" class="label">[6]</a> There is much force in the criticism passed on the above
-paragraph, and by implication on some preceding paragraphs, that
-though in plants which tend to produce compound leaves the production
-is largely dependent on the supply of nutriment, yet the unqualified
-statement of this relation as a general one, is negatived by the
-existence of plants which bear only simple leaves, however much high
-nutrition causes growth. But mostly valid though this objection is,
-it is probably not universally valid. I am led to say this by what
-occasionally occurs in flowers. The flowering stem of the Hyacinth
-is single; but I have seen a cultivated Hyacinth in which one of
-the flowers had developed into a lateral spike. Still more striking
-evidence was once supplied to me by Agrimony. All samples of this plant
-previously seen had single flowering spikes, but some years ago I met
-with one, extremely luxuriant, in which some flowers of the primitive
-spike were replaced by lateral spikes; and I am not sure that some of
-these, again, did not bear lateral spikes. Now if in plants which,
-in probably millions of cases, have their flowering stems single,
-excessive nutrition changes certain of their flowers into new spikes,
-it is a reasonable supposition that in like manner plants which are
-thought invariably to bear only single leaves, will, under kindred
-conditions, bear compound leaves.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_7" href="#FNanchor_7" class="label">[7]</a> See <i>British and Foreign Medico-Chirurgical Review</i>
-for January, 1862.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_8" href="#FNanchor_8" class="label">[8]</a> Schleiden, who chooses to regard as an axis that which
-Mr. Berkeley, with more obvious truth, calls a mid-rib, says:&mdash;“The
-flat stem of the Liverworts presents many varieties, consisting
-frequently of one simple layer of thin-walled cells, or it exhibits in
-its axis the elements of the ordinary stem.” This passage exemplifies
-the wholly gratuitous hypotheses which men will sometimes espouse,
-to escape hypotheses they dislike. Schleiden, with the positiveness
-characteristic of him, asserts the primordial distinction between
-axial organs and foliar organs. In the higher Archegoniates he sees an
-undeniable stem. In the lower Archegoniates, clearly allied to them
-by their fructification, there is no structure having the remotest
-resemblance to a stem. But to save his hypothesis, Schleiden calls
-that “a flat stem,” which is obviously a structure in which stem and
-leaf are not differentiated. He is the more to be blamed for this
-unphilosophical assumption, since he is merciless in his strictures on
-the unphilosophical assumptions of other botanists.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_9" href="#FNanchor_9" class="label">[9]</a> To this interpretation it is objected that “the
-more-developed <i>Jungermanniaceæ</i>” do not appear to have arisen from the
-lower forms of <i>Jungermanniaceæ</i>&mdash;that is to say, from such lower
-forms as are now existing. It may, however, be contended that this fact
-does not exclude the interpretation given; since the higher forms may
-well have been evolved, not from any of the lower forms we now know,
-but from lower forms which have become extinct. This, indeed, is the
-implication of the evolutionary process as pointed out in the note to
-Chap. I. If then we assume some early type of intermediate structure,
-the explanation may not improbably hold.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_10" href="#FNanchor_10" class="label">[10]</a> I am indebted to Dr. Hooker for pointing out further
-facts supporting this view. In his <i>Flora Antarctica</i>, he
-describes the genus <i>Lessonia</i> (see Fig. <a href="#i_030">37</a>), and especially
-<i>L. ovata</i>, as having a mode of growth simulating that
-of the dicotyledonous trees, not only in general form but in
-internal structure. The tall vertical stem thickens as it grows,
-by the periodical addition of layers to its periphery. That even
-Thallophytes should thus, under certain conditions, present a
-transversely-increasing axis, shows that there is nothing absolutely
-characteristic of Phanerogams in their habit of stem-thickening.
-Mr. Tansley gives me further verification by the statement that
-“it is also now certain that members of the <i>Equisetineæ</i>
-and <i>Lycopodineæ</i>, as well as some Ferns which flourished in
-Carboniferous times, had secondary thickening in their stems quite
-comparable to that of modern Dicotyledonous trees.”</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_11" href="#FNanchor_11" class="label">[11]</a> See note at the end of the chapter.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_12" href="#FNanchor_12" class="label">[12]</a> Since this paragraph was put in type [this refers
-to the first edition], I have observed that in some varieties of
-<i>Cineraria</i>, as probably in other plants, a single individual
-furnishes all these forms of leaves&mdash;all gradations between
-unstipulated leaves on long petioles, and leaves that embrace the axis.
-It may be added that the distribution of these various forms is quite
-in harmony with the rationale above given.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_13" href="#FNanchor_13" class="label">[13]</a> Since these figures were put on the block, it has
-occurred to me that the relations would be still clearer, were the
-primary frond represented as not taking part in these processes of
-modification, which have been described as giving rise to the erect
-form; as, indeed, the rooting of its under surface will prevent it from
-doing in any considerable degree. In such case, each of the Figs. <a href="#i_071">111
-to 117</a>, should have a horizontal rooted frond at its base, homologous
-with the pro-embryo among Acrogens. This primary frond would then more
-manifestly stand in the same relation to the rest, as the cotyledon
-does to the plumule&mdash;both by position, and as a supplier of nutriment.
-Fig. <a href="#i_071">117<i>a</i></a>, which I am enabled to add, shows that this would
-complete the interpretation. Of the dicotyledonous series, it is
-needful to add no further explanation than that the difference in habit
-of growth, will permit the second frond to root itself as well as the
-first; and so to become an additional source of nutriment, similarly
-circumstanced to the first and equal with it.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_14" href="#FNanchor_14" class="label">[14]</a> How the element of time modifies the result, is shown by
-the familiar fact that crystals rapidly formed are small, and become
-relatively large when left to form more slowly. If the quantity of
-molecules contained in a solution is relatively great, so that the
-mutual polarities of the molecules crowded together in every place
-throughout the solution are intense, there arises a crystalline
-aggregation around local axes; whereas, in proportion as the local
-action of molecules on one another is rendered less intense by their
-wider dispersion, they become relatively more subordinate to the forces
-exerted on them by the larger aggregates of molecules that are at
-greater distances, and thus are left to arrange themselves round fewer
-axes into larger crystals.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_15" href="#FNanchor_15" class="label">[15]</a> It is objected that these transformations should be much
-commoner than they are, were they caused solely by the variations of
-nutrition described. The reply is that they are comparatively rare
-in uncultivated plants, where such variations are not frequent. The
-occurrence of them is chiefly among cultivated plants which, being
-artificially manured, are specially liable to immense accessions of
-nutriment, caused now by sudden supplies of fertilizing matters, and
-now by sudden arrival of the roots at such matters already deposited in
-the soil. It is to these great <i>changes</i> of nutrition, especially
-apt to take place in gardens, that these monstrosities are ascribed;
-and it seems to me that they are as frequent as may be expected.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_16" href="#FNanchor_16" class="label">[16]</a> Since this paragraph was published in 1865, much has been
-learned concerning cell-structure, as is shown in Chapter VI<sup>A</sup> of Part
-I. While some assert that there exist portions of living protoplasm
-without nuclei, others assert that a nucleus is in every case present,
-and that where it does not exist in a definite aggregated form it
-exists in a dispersed form. As remarked in the chapter named, “the
-evidence is somewhat strained to justify this dogma.” Words are taken
-in their non-natural senses, if one which connotes an individualized
-body is applied to the widely-diffused components of such a body; and
-this perverting of proper meanings leads to obscuration of what may
-perhaps be an essential truth. As argued in the chapter named (<a href="http://www.gutenberg.org/files/54612/54612-h/54612-h.htm#sect74">§§ 74<i>e</i>, 74<i>f</i></a>),
-nuclear matter is, as shown by its chemical
-character, an extremely unstable substance, the molecular changes of
-which, perpetually going on, initiate shocks, producing changes all
-around. In the earlier stages of cell-evolution this unstable substance
-is dispersed throughout the cytoplasm; whereas in the more advanced
-stages it is gathered together in one mass. If so, instead of saying
-there is a dispersed nucleus we should say there are the materials of a
-nucleus not yet integrated.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_17" href="#FNanchor_17" class="label">[17]</a> This statement seems at variance with the figure; but the
-figure is very inaccurate. Its inaccuracy curiously illustrates the
-vitiation of evidence. When I saw the drawing on the block, I pointed
-out to the draughtsman, that he had made the surrounding curves much
-more obviously related to the contained bodies, than they were in the
-original (in Dr. Carpenter’s <i>Foraminifera</i>); and having looked
-on while he in great measure remedied this defect, thought no further
-care was needed. Now, however, on seeing the figure in the printer’s
-proof, I find that the engraver, swayed by the same supposition as the
-draughtsman that such a relation was meant to be shown, has made his
-lines represent it still more decidedly than those of the draughtsman
-before they were corrected. Thus, vague linear representations, like
-vague verbal ones, are apt to grow more definite when repeated.
-Hypothesis warps perceptions as it warps thoughts.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_18" href="#FNanchor_18" class="label">[18]</a> Though the subdivision into chambers of the shell does
-not correspond to the subdivision into cell-units it may still be held
-that since in the solitary types the subdivision of the nucleus is
-followed by formation of new individuals which separate, and since in
-the compound types the subdivision of the nucleus is followed by growth
-and formation of new chambers, the compound type must be regarded as an
-aggregate of the second order.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_19" href="#FNanchor_19" class="label">[19]</a> A critic says the question is “what are the forces
-internal or external which produce union or separation.” A proximate
-reply is&mdash;degree of nutrition. As in a plant new individuals or
-rudiments of them are cast off where nutrition is failing, so in a
-compound animal. The connecting part dwindles if it ceases to carry
-nutriment.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_20" href="#FNanchor_20" class="label">[20]</a> It has been pointed out that I have here understated
-the evidence of physiological integration. An instance of it among
-<i>Hydrozoa</i> is shown in Fig. <a href="#i_092">151</a>, but by a strange oversight
-I have forgotten to name the various cases furnished by the
-<i>Siphonophora</i> in which the individual polypes of a compound
-aggregate are greatly specialized in adaptation to different functions.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_21" href="#FNanchor_21" class="label">[21]</a> Recently Mr. T. H. Morgan has made elaborate experiments
-which show that <i>Planaria Maculata</i> may be cut into many pieces
-from various parts and of various shapes&mdash;even a slice out of the
-side&mdash;and each, if not too small, will produce a perfect animal.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_22" href="#FNanchor_22" class="label">[22]</a> Since this was written in 1865 there has come to light
-evidence more completely to the point than any at that time known. In
-the subdivision of <i>Platyhelminthes</i> known as <i>Turbellaria</i>,
-there are some, the <i>Microstomida</i> which, by a process of
-segmentation form “chains of 4, then 8, then 16, and sometimes even 32
-individuals.” “Each forms a mouth [lateral] and for some time the chain
-persists, but the individuals ultimately become sexually matured and
-then separate.” (Shipley, <i>Zoology of the Invertebrata</i>, p. 92.)
-Here it should be remarked that the lateral mouths enable the members
-of a string to feed separately, and that nutrition not being interfered
-with they doubtless gain some advantage by temporary maintenance of
-their union&mdash;probably in creeping.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_23" href="#FNanchor_23" class="label">[23]</a> I find that the reasons for regarding the segment of
-a <i>Tænia</i> as answering to an individual of the second order
-of aggregation, are much stronger than I supposed when writing the
-above. Van Beneden says:&mdash;“Le Proglottis (segment) ayant acquis tout
-son développement, se détache ordinairement de la colonie et continue
-encore à croître dans l’intestin du même animal; il change même souvent
-de forme et semble doué d’une nouvelle vie; ses angles s’effacent,
-tout le corps s’arrondit, et il nage comme une Planaire au milieu des
-muscosités intestinales.”</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_24" href="#FNanchor_24" class="label">[24]</a> Though this was doubtful in 1865 it is no longer
-doubtful. In an individual <i>Ctenodrilus monostylus</i>, which
-multiplies by dividing and subdividing itself, “parts arise which are
-destitute of both head and anus and at times consist of only a single
-segment.” In another species, <i>C. pardalis</i>, there is separation
-into many segments; and each segment before separating forms a budding
-zone out of which other segments are afterwards produced, completing
-the animal (Korschelt and Heider, <i>Embryology</i>, i, 301–2).</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_25" href="#FNanchor_25" class="label">[25]</a> In place of those originally here instanced about which
-there are disputes, I may give an undoubted one described by McIntosh,
-the <i>Syllis ramosa</i>, a species of chætopod living in hexactinellid
-sponges from the Arafura Sea, which branches laterally repeatedly so as
-to extend in all directions through the canals of the sponge. In most
-cases the buds terminate in oval segments with two long cirri each.
-But male and female buds were found, provided each with a head, and
-containing ovaries and testes. Sometimes these sexual buds had become
-separate from the branched stock.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_26" href="#FNanchor_26" class="label">[26]</a> The name <i>Annulosa</i>, once used to embrace the
-<i>Annelida</i> and <i>Arthropoda</i>, has of late ceased to be used.
-It seems to me better than <i>Appendiculata</i>, both as being more
-obviously descriptive and as being more exclusive.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_27" href="#FNanchor_27" class="label">[27]</a> The fusion of the segments forming the Arthropod head
-and the extreme changes, or perhaps in some cases disappearances, of
-their appendages, put great difficulties in the way of identification;
-so that there are differences of opinion respecting the number of
-included segments. Prof. MacBride writes:&mdash;“It is highly probable that
-a primary head (præoral lobe or præstomium) has been derived from
-annelid ancestors, but the secondary fusion of body-segments with this
-head, in other words the formation of a secondary head, has gone on
-independently in the different classes of the phylum <i>Arthropoda</i>,
-viz., <i>Arachnida</i>, <i>Crustacea</i>, and <i>Tracheata</i>
-(including Insects and Myriapods). Judged by the number of appendages
-(which gives an inferior limit) the head of a malacostracous Crustacean
-consists of præstomium and 8 segments; the head of an insect of
-præstomium and 4 segments; the head of a Myriapod of præstomium and 3
-segments; and the head of an Arachnid of præstomium and 3 segments.”
-Again, the comment of Mr. J. T. Cunningham is:&mdash;“According to Claus and
-most modern authorities there are only 5 segments in the head of an
-Arthropod, the eyes not counting as appendages; and further it should
-be noted that the second pair of antennæ are wanting in Insects.”</p>
-
-<p>Of course difference of opinion respecting the number of somites in the
-head involves difference of opinion respecting the number constituting
-the entire body, which, in the higher Arthropods, is said by some to
-be 19 and by others 20. But those who thus differ in detail, agree in
-regarding all the segments of head and body as homologous, and this is
-the essential point with which we are here concerned.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_28" href="#FNanchor_28" class="label">[28]</a> Prof. MacBride corrects this statement by saying that
-“The ctenidia or gills (which in <i>Mollusca</i> generally are
-represented only by a single pair) are here represented by a large
-number of pairs; they do not, however, correspond in either number or
-position to the shell plates.” It may, I think, be contended that if
-these had any morphological significance, they would not differ in
-arrangement from the shell plates, and would not be limited to this
-special type of Mollusc.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_29" href="#FNanchor_29" class="label">[29]</a> Though it is alleged that at a later stage the posterior
-part of the skull is formed by fusion of divisions which are assumed to
-represent vertebræ, yet it is admitted that the anterior part of the
-skull never shows any signs of such division. Moreover in both parts
-the bones show no trace of primitive segmentation.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_30" href="#FNanchor_30" class="label">[30]</a> See note at the end of the chapter.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_31" href="#FNanchor_31" class="label">[31]</a> A qualifying fact should be named. When the production
-of vertebral segments has become constitutionally established, so that
-there is an innate tendency to form them, there arises a liability to
-form supernumerary ones; and this, from time to time recurring, may
-lengthen the series, as in the body of a snake or the neck of a swan.
-This qualification, however, affects equally the hypothesis of an ideal
-type and the hypothesis of mechanical genesis.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_32" href="#FNanchor_32" class="label">[32]</a> Here and throughout, the word <i>radial</i> is applied
-equally to the spiral and the whorled structures. These, as being alike
-on all sides, are similarly distinguished from arrangements that are
-alike on two sides only.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_33" href="#FNanchor_33" class="label">[33]</a> It should be added that this change of distribution is
-not due to change in the relative positions of the insertions of the
-leaves but to their twistings.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_34" href="#FNanchor_34" class="label">[34]</a> We may note that some of these leaves, as those of the
-Lime, furnish indications of the ratio which exists between the effects
-of individual circumstances and those of typical tendencies. On the
-one hand, the leaves borne by these drooping branches of the Lime are
-with hardly an exception unsymmetrical more or less decidedly, even
-in positions where the causes of unsymmetry are not in action: a fact
-showing us the repetition of the type irrespective of the conditions.
-On the other hand, the degree of deviation from symmetry is extremely
-variable, even on the same shoot: a fact proving that the circumstances
-of the individual leaf are influential in modifying its form. But the
-most striking evidence of this direct modification is afforded by the
-suckers of the Lime. Growing, as these do, in approximately upright
-attitudes, the leaves they bear do not stand to one another in the way
-above described, and the causes of unsymmetry are not in action; and
-here, though there is a general leaning to the unsymmetrical form, a
-large proportion of the leaves become quite symmetrical.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_35" href="#FNanchor_35" class="label">[35]</a> It was by an observation on the forms of leaves, that I
-was first led to the views set forth in the preceding and succeeding
-chapters on the morphological differentiation of plants and animals.
-In the year 1851, during a country ramble in which the structures
-of plants had been a topic of conversation with a friend&mdash;Mr. G. H.
-Lewes&mdash;I happened to pick up the leaf of a buttercup, and, drawing
-it by its foot-stalk through my fingers so as to thrust together its
-deeply-cleft divisions, observed that its palmate and almost radial
-form was changed into a bilateral one; and that were the divisions to
-grow together in this new position, an ordinary bilateral leaf would
-result. Joining this observation with the familiar fact that leaves, in
-common with the larger members of plants, habitually turn themselves to
-the light, it occurred to me that a natural change in the circumstances
-of the leaf might readily cause such a modification of form as that
-which I had produced artificially. If, as they often do with plants,
-soil and climate were greatly to change the habit of the buttercup,
-making it branched and shrub-like; and if these palmate leaves were
-thus much overshadowed by one another; would not the inner segments
-of the leaves grow towards the periphery of the plant where the light
-was greatest, and so change the palmate form into a more decidedly
-bilateral form? Immediately I began to look round for evidence of the
-relation between the forms of leaves and the general characters of the
-plants they belong to; and soon found some signs of connexion. Certain
-anomalies, or seeming anomalies, however, prevented me from then
-pursuing the inquiry much further. But consideration cleared up these
-difficulties; and the idea afterwards widened into the general doctrine
-here elaborated. Occupation with other things prevented me from giving
-expression to this general doctrine until Jan. 1859; when I published
-an outline of it in the <i>Medico-Chirugical Review</i>.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_36" href="#FNanchor_36" class="label">[36]</a> It is objected to the above interpretation that “many
-flowers of sizes intermediate between the Hollyhock and the Agrimony
-are radially symmetrical and yet grow sideways. I may mention
-various <i>Liliaceæ</i>, e.g. <i>Chlorophytum</i>, <i>Eucomis</i>,
-<i>Muscari</i>, <i>Anthericum</i>. <i>Sagittaria</i>, also, has many
-of its flowers in this position. Further, if the higher insects alight
-on flowers in a definite way, as they do, the parts of the flower
-must bear different relations to the visiting insect, however large,
-so that flowers unvisited ought all to be zygomorphic.” My reply is
-that in the sense which here concerns us, the different petals of
-the Hollyhock-flower do not bear different relations to the visiting
-insect; since, practically, the upper and lateral petals bear no
-physical relations at all: in so far as the visiting bee is concerned
-they are non-existent. The argument implies that change in the form of
-a flower from the radial to the bilateral is likely to take place only
-when the contact-relations of the petals to the visiting insect, are
-such as to make some forms facilitate its action more than others; and
-the large petals of the Hollyhock cannot facilitate its action at all.
-In respect of the <i>Liliaceæ</i> instanced, it is needful to inquire
-whether the structures are such that this alleged cause of bilateral
-symmetry can come into play.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_37" href="#FNanchor_37" class="label">[37]</a> I had intended here to insert a figure exhibiting these
-differences; but as the Cow-parsnip does not flower till July, and as
-I can find no drawing of the umbel which adequately represents its
-details, I am obliged to take another instance.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_38" href="#FNanchor_38" class="label">[38]</a> It has been pointed out to me that “the extreme
-development of the corolla so often found in the outer flowers or on
-the outer side of the outer flowers in closely-packed inflorescences,
-associated as it often is with disappearance of stamens or carpels or
-both, is usually put down to specialization of these outer flowers for
-attractive purposes. Since the whole inflorescence is increased in
-conspicuousness by such a modification, it is supposed that natural
-selection favoured those plants which sacrificed a portion of their
-seed-bearing capacity for the supposed greater advantage of securing
-more insect visits.” But granting this interpretation, it may still
-be held that increase of attractiveness due to increase of area must
-be achieved by florets at the periphery, and that their ability to
-achieve it depends on their having an outer, unoccupied, space which
-the inner florets have not; so that, though in a more indirect way,
-their different development is determined by different exposure to
-conditions.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_39" href="#FNanchor_39" class="label">[39]</a> One of my critics writes:&mdash;“This chapter might of course
-be enormously extended, not only as in the preceding ones by citation
-of quite similar cases, but by the introduction of fresh groups of
-cases.”</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_40" href="#FNanchor_40" class="label">[40]</a> Natural selection may have operated in establishing
-a constitutional tendency to other sudden abridgments. Mr. Tansley
-alleges that this is a part-cause of the varying distribution of
-leaves. He says:&mdash;“I have myself made some observations on the length
-of internodes in the Beech, and am satisfied that it follows quite
-other laws, connected with the suitable disposition of the leaves on
-the branch. Although I have not had the opportunity of following up
-this line of work so as in any way to generalize the results, I suspect
-that ‘indirect equilibration’ is a widespread cause of such variation.”</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_41" href="#FNanchor_41" class="label">[41]</a> It is but just to the memory of Wolff, here to point out
-that he was immensely in advance of Goethe in his rationale of these
-metamorphoses. Whatever greater elaboration Goethe gave to the theory
-considered as an induction, seems to me more than counter-balanced
-by the irrationality of his deductive interpretation; which unites
-mediæval physiology with Platonic philosophy. A dominant idea with him
-is that leaves exist for the purpose of carrying off crude juices&mdash;that
-“as long as there are crude juices to be carried off, the plant must
-be provided with organs competent to effect the task”; that while “the
-less pure fluids are got rid of, purer ones are introduced” and that
-“if nourishment is withheld, that operation of nature (flowering) is
-facilitated and hastened; the organs of the nodes (leaves) become
-more refined in texture, the action of the purified juices becomes
-stronger, and the transformation of parts having now become possible,
-takes place without delay.” This being the proximate explanation, the
-ultimate explanation is, that Nature wishes to form flowers&mdash;that when
-a plant flowers it “attains the end prescribed to it by nature”; and
-that so “Nature at length attains her object.” Instead of vitiating
-his induction by a teleology that is as unwarranted in its assigned
-object as in its assigned means, Wolff ascribes the phenomena to a
-cause which, whether sufficient or not, is strictly scientific in its
-character. Variation of nutrition is unquestionably a “true cause” of
-variation in plant-structure. We have here no imaginary action of a
-fictitious agency; but an ascertained action of a known agency.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_42" href="#FNanchor_42" class="label">[42]</a> The <i>Natural History Review</i> for July, 1865,
-contained an article on the doctrine of morphological composition set
-forth in the foregoing Chaps. I. to III. In this article, which unites
-exposition and criticism in a way that is unhappily not common with
-reviewers, it is suggested that the spiral structure may be caused by
-natural selection. When this article appeared, the foregoing five pages
-were standing over in type, as surplus from No. 14, issued in June,
-1865.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_43" href="#FNanchor_43" class="label">[43]</a> A verifying comment on this paragraph runs as
-follows:&mdash;“In the Hypotricha Infusoria, which creep over solid
-surfaces, there is a differentiation between ventral and dorsal surface
-and an approach to bilateral symmetry. The ventral surface is provided
-with movable cilia, the dorsal with immobile setæ.”</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_44" href="#FNanchor_44" class="label">[44]</a> Criticisms on the above passage have shown the need
-for naming sundry complications. These complications chiefly, if
-not wholly, arise from changes in modes of life&mdash;changes from
-the locomotive to the stationary, and from the stationary to the
-locomotive. Referring to my statement that (ignoring the spherical)
-the radial type is the lowest and must be taken as antecedent
-to the bilateral type, it is alleged that all existing “radial
-animals above Protozoa are probably derived from free-swimming,
-bilaterally-symmetrical animals.” If this is intended to include
-the planulæ of the hydroid polyps, then it seems rather a straining
-of the evidence. These locomotive embryos, described as severally
-having the structure of a gastrula with a closed mouth, can be said
-to show bilateralness only because the first two tentacles make their
-appearance on opposite sides of the mouth&mdash;a bilateralness which lasts
-only till two other tentacles make their appearance in a plane at
-right angles, so giving the radial structure. I think the criticism
-applies only to cases furnished by Echinoderms. The larvæ of these
-creatures have bilaterally-symmetrical structures, which they retain
-as long as they swim about and which such of them as fix themselves
-lose by becoming similarly related to conditions all round: the radial
-structure being retained by those types which, becoming subsequently
-detached, move about miscellaneously. But, as happens in some of the
-Sea-urchins and still more among the Holothurians, the structure is
-again made bilaterally-symmetrical by a locomotive life pursued with
-one end foremost. Should it be contended that the conditions and the
-forms are reciprocally influential&mdash;that either may initiate the other,
-it still remains unquestionable that ordinarily the conditions are the
-antecedents, as is so abundantly shown by plants.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_45" href="#FNanchor_45" class="label">[45]</a> Should it be proved that the Ascidian is a degraded
-vertebrate, then the argument will be strengthened; since loss
-of bilateral symmetry has gone along with change to asymmetrical
-conditions.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_46" href="#FNanchor_46" class="label">[46]</a> A critical comment made on this sentence runs as
-follows:&mdash;“The aërial roots of most epiphytic orchids contain
-chlorophyll in their cortex throughout their length, but the cortex
-being covered by a ‘velamen’ of air-containing cells which break up and
-reflect incident light, the green colour is not visible through this
-opaque coat. When moistened the cells of the velamen take up water and
-the green colour immediately shows through. Such roots do not however
-possess stomata. The roots of certain species of <i>Angræcum</i>,
-however, contain the whole of the assimilating tissue of the plant.”</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_47" href="#FNanchor_47" class="label">[47]</a> The current doctrine that chlorophyll is <i>the</i>
-special substance concerned in vegetal assimilation, either as an
-agent or as an incidental product, must be taken with considerable
-qualification. Besides the fact that among the <i>Algæ</i> there are
-many red and brown kinds which thrive; and besides the fact that
-among the lower Archegoniates there are species which are purple or
-chocolate-coloured; there is the fact that Phænogams are not all
-green. We have the Copper-Beech, we have the black-purple <i>Coleus
-Verschaffeltii</i>, and we have the red variety of Cabbage, which seems
-to flourish as well as the other varieties. Chlorophyll, then, must be
-regarded simply as the most general of the colouring matters found in
-those parts of plants in which assimilation is being effected by the
-agency of light. Though it is always present <i>along with</i> the red
-and brown pigments, yet there is much evidence to show that these are
-the actual assimilative pigments.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_48" href="#FNanchor_48" class="label">[48]</a> This seems as fit a place as any for noting the fact,
-that the greater part of what we call beauty in the organic world, is
-in some way dependent on the sexual relation. It is not only so with
-the colours and odours of flowers. It is so, too, with the brilliant
-plumage of birds; and it is probable that the colours of the more
-conspicuous insects are in part similarly determined. The remarkable
-circumstance is, that these characteristics, which have originated
-by furthering the production of the best offspring, while they are
-naturally those which render the organisms possessing them attractive
-to one another, directly or indirectly, should also be those which are
-so generally attractive to us&mdash;those without which the fields and woods
-would lose half their charm. It is interesting, too, to observe how the
-conception of human beauty is in a considerable degree thus originated.
-And the trite observation that the element of beauty which grows out of
-the sexual relation is so predominant in æsthetic products&mdash;in music,
-in the drama, in fiction, in poetry&mdash;gains a new meaning when we see
-how deep down in organic nature this connexion extends.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_49" href="#FNanchor_49" class="label">[49]</a> Students of vegetal physiology, familiar with the
-controversies respecting sundry points dealt with in this chapter, will
-probably be surprised to find taken for granted in it, propositions
-which they have habitually regarded as open to doubt. Hence it seems
-needful to say that the conclusions here set forth, have resulted
-from investigations undertaken for the purpose of forming opinions on
-several unsettled questions which I had to treat, but which I could
-find in books no adequate data for treating. The details of these
-investigations, and the entire argument of which this chapter is partly
-an abstract, will be found in Appendix C.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_50" href="#FNanchor_50" class="label">[50]</a> To this implied inference it is objected that “excess
-of nutritive material does not necessarily lead to correspondingly
-increased growth.” My reply is that a concomitant factor is activity of
-the tissue, and that in its absence growth is not to be expected.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_51" href="#FNanchor_51" class="label">[51]</a> In recent years (since 1890) Prof. Wilhelm Roux, in
-essays on functional adaptation, has set forth some views akin to the
-foregoing in respect to the general belief they imply, though differing
-in respect of the physiological processes he indicates. The following
-relevant passage has been translated for me from an article of his in
-the <i>Real-Encyclopädie der gesammten Heilkunde</i>:&mdash;“A more complete
-theory of functional adaptation by the author is founded on the
-assumption that the ‘functional’ stimulus, or ‘the act of exercising
-the function’ (in muscles and glands), and especially, in the case of
-bones, the concussion and tension caused by stress and strain, exert
-a ‘trophic’ stimulus on the cells, in consequence of which, and along
-with an increased absorption of nutriment, they grow and eventually
-increase (or the osteoblasts at the point of greater stimulus form more
-bone); while, conversely, with continued inactivity, by absence of
-these stimuli the nourishment of the cell declines so that the waste is
-insufficiently replaced (or otherwise that the bone-substance gradually
-loses its power of resistance to the osteoblasts formed as a result of
-inactivity”).</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_52" href="#FNanchor_52" class="label">[52]</a> An outline of the doctrine set forth in the following
-chapters, was originally published in the <i>Westminster Review</i> for
-April, 1852, under the title&mdash;<i>A Theory of Population deduced from
-the General Law of Animal Fertility</i>; and was shortly afterwards
-republished with a prefatory note stating that it must be accepted as a
-sketch which I hoped at some future time to elaborate. In now revising
-and completing it, I have omitted a non-essential part of the argument,
-while I have expanded the remainder by adding to the number of facts
-put in evidence, by meeting objections which want of space before
-obliged me to pass over, and by drawing various secondary conclusions.
-The original paper, with omissions, will be found in Appendix A to
-Volume I of this work.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_53" href="#FNanchor_53" class="label">[53]</a> I was here thinking only of the cases which are general
-among insects, but it seems that vertebrate animals, too, furnish
-cases. Mr. Cunningham writes:&mdash;“There is a curious instance of this in
-the Conger: the female grows to 6 or 7 feet long and a weight of 60
-lbs. and upwards and then ceases to feed for 6 months while the eggs
-develop, and when the eggs are shed dies.”</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_54" href="#FNanchor_54" class="label">[54]</a> I say “normal” for the purpose of excluding not only
-morbid growths but excess of fat.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_55" href="#FNanchor_55" class="label">[55]</a> To meet a possible criticism it should be remarked that
-this calculation assumes that the power of asexual reproduction is
-not exhausted by the end of the month. It has been found that “the
-successive fissions of <i>Paramœcium</i> cannot continue indefinitely.
-After some hundreds of generations the products of fission are small,
-have no mouth, and die unless before this they have been allowed to
-conjugate with individuals of another brood.” It may, however, be
-fairly taken for granted that “some hundreds of generations” would take
-longer than a month.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_56" href="#FNanchor_56" class="label">[56]</a> Even this number is far exceeded. Dr. Edward Klein, in a
-lecture he gave at the Royal Institution on June 2, 1898, asserted that
-246 bacteria in a cubic centimetre of nutritive liquid would multiply
-to 20,000,000 in the course of twenty-four hours: a rate which, at
-the end of the <i>third</i> day, would give, as the offspring of one
-individual, 537,367,797,000,000.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_57" href="#FNanchor_57" class="label">[57]</a> It has since been shown that in <i>Myrianida fasciata</i>
-as many as 29 attached groups exist. See <i>Cambridge Natural
-History</i>, Vol. II, <i>Worms, Rotifers and Polyzoa</i>, p. 280.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_58" href="#FNanchor_58" class="label">[58]</a> To this passage Prof. MacBride appends the remark:&mdash;“This
-is quite proven now, and the statement as it stands is quite correct;
-but far better and more minutely worked out cases are to be found
-amongst the <i>Infusoria</i>. In <i>Paramœcium</i> for example, there
-are normally present a large macronucleus and a small micronucleus
-lying alongside of it. When two individuals adhere preparatory to
-conjugation, the macronucleus breaks up into fragments which are
-absorbed: the micronucleus&mdash;which has some time previously divided into
-two&mdash;begins to break up further and eventually forms eight bodies; all
-of these except one disappear; this last piece then divides into two;
-of these two one represents a male genital cell, for it passes over
-into the body of the other <i>Paramœcium</i> and fuses with one of the
-two corresponding nuclei there; thus each of the two individuals which
-adhere fertilizes the other. The two individuals then separate and the
-nucleus (result of fusion of male and female nuclei) in each divides
-into four. Of these, two move to one end of the animal and two to the
-other. The animal then divides into two transversely&mdash;each of the
-products thus having two nuclei which form the micro-and macronucleus
-of it. Thus it appears that the function of sexual union is simply
-to give increased vigour to all the vital processes <i>including
-fission</i>. Since as mentioned above (p. 443) if it is prevented, the
-products of fission are eventually unable to feed themselves.”</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_59" href="#FNanchor_59" class="label">[59]</a> A passage translated for me from the German may be here
-given in verification. Dr. Dionys Hellin in an essay on the origin of
-Multiparity and Twin-births, refers to the thesis above set forth, and
-says that “the fact that it is generally women of small growth who bear
-twins is in complete agreement with it.” He adds that “Puech is right
-in his opinion that twin pregnancies are a direct result of relatively
-large ovaries (<i>i.e.</i>, in comparison with the whole body). He has
-observed that for the same size of body the ovarium of a pluriparous
-animal is always of greater volume than that of a uniparous animal ...
-a sow has ovaries as large as a cow’s; but while the latter bears only
-one calf [at a time], the sow brings forth 6–15 at each litter. Even
-in animals of the same species but belonging to different races these
-relations may be verified,” <i>e.g.</i>, Barbary sheep and ordinary
-sheep.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_60" href="#FNanchor_60" class="label">[60]</a> When, after having held for some years the general
-doctrine elaborated in these chapters, I agreed, early in 1852,
-to prepare an outline of it for the <i>Westminster Review</i>,
-I consulted, among other works, the just-issued third edition
-of Dr. Carpenter’s <i>Principles of Physiology, General and
-Comparative</i>&mdash;seeking in it for facts illustrating the different
-degrees of fertility of different organisms, I met with a passage,
-quoted above in <a href="#sect339">§ 339</a>, which seemed tacitly to assert that individual
-aggrandizement is at variance with the propagation of the race; but
-nowhere found a distinct enunciation of this truth. I did not then
-read the Chapter entitled “General View of the Functions,” which held
-out no promise of such evidence as I was looking for. But on since
-referring to this chapter, I discovered in it the definite statement
-that&mdash;“there is a certain degree of antagonism between the Nutritive
-and Reproductive functions, the one being executed at the expense of
-the other. The reproductive apparatus derives the materials of its
-operations through the nutritive system, and is entirely dependent
-upon it for the continuance of its function. If, therefore, it be
-in a state of excessive activity, it will necessarily draw off from
-the individual fabric some portion of the aliment destined for its
-maintenance. It may be universally observed that, when the nutritive
-functions are particularly active in supporting the <i>individual</i>,
-the reproductive system is in a corresponding degree undeveloped,&mdash;and
-<i>vice versâ</i>.” P. 592.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_61" href="#FNanchor_61" class="label">[61]</a> The climate, the locality, and the kind of food, are of
-course all factors; and hence, probably, the differences between the
-statements of different authorities concerning these several cases.
-Prof. MacBride writes:&mdash;</p>
-
-<p>“According to Flower (<i>Mammals, Living and Extinct</i>) the Ferret is
-a domesticated variety of the common polecat, which has 3 to 8 young.
-Darwin (<i>Animals and Plants</i>) says that the wild sow often breeds
-twice a year and produces a litter of 4 to 8, and sometimes even 12.
-The domestic sow breeds twice and would breed oftener if permitted, and
-if any good at all produces 8 in litter.”</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_62" href="#FNanchor_62" class="label">[62]</a> It is worth while inquiring whether unfitness of the food
-given to them, is not the chief cause of that sterility which, as Mr.
-Darwin says, “is the great bar to the domestication of animals.” He
-remarks that “when animals and plants are removed from their natural
-conditions, they are extremely liable to have their reproductive
-systems seriously affected.” Possibly the relative or absolute arrest
-of genesis, is less due to a direct effect on the reproductive
-system, than to a changed nutrition of which the reproductive system
-most clearly shows the results. The matters required for forming an
-embryo are in a greater proportion nitrogenous than are the matters
-required for maintaining an adult. Hence, an animal forced to live on
-insufficiently-nitrogenized food, may have its surplus for reproduction
-cut off, but still have a sufficiency to keep its own tissues in
-repair, and appear to be in good health&mdash;meanwhile increasing in bulk
-from excess of the non-nitrogenous matters it eats.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_63" href="#FNanchor_63" class="label">[63]</a> Huxley, <i>Anatomy of Invertebrated Animals</i>, p. 274.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_64" href="#FNanchor_64" class="label">[64]</a> Shipley, <i>Zoology of Invertebrata</i>, p. 112.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_65" href="#FNanchor_65" class="label">[65]</a> I am told that “Wagner, who described the larva, found
-that it bored into the bark of trees. It attacks also the wheat plant,
-and is a most destructive parasite.” Apparently this statement is at
-variance with the foregoing inference. It is clear, however, that
-since these heaps of nitrogenous refuse in which it has been found are
-artificial and recent, they cannot be its natural habitats; and it
-seems not improbable that these larvæ, suddenly supplied with a more
-nutritive food in unlimited amount, may have as a consequence acquired
-this habit of agamogenetic multiplication which did not characterize
-the species under its natural conditions and relatively low nutrition.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_66" href="#FNanchor_66" class="label">[66]</a> This is exactly the reverse of Mr. Doubleday’s doctrine;
-which is that throughout both the animal and vegetable kingdoms,
-“over-feeding checks increase; whilst, on the other hand, a limited or
-deficient nutriment stimulates and adds to it.” Or, as he elsewhere
-says&mdash;“Be the range of the natural power to increase in any species
-what it may, the <i>plethoric</i> state invariably checks it, and the
-<i>deplethoric</i> state invariably develops it; and this happens in
-the exact ratio of the intensity and completeness of each state, until
-each state be carried so far as to bring about the actual death of the
-animal or plant itself.”</p>
-
-<p>I have space here only to indicate the misinterpretations on which Mr.
-Doubleday has based his argument.</p>
-
-<p>In the first place, he has confounded normal plethora with what I have,
-in <a href="#sect355">§ 355</a>, distinguished as abnormal plethora. The cases of infertility
-accompanying fatness, which he cites in proof that over-feeding checks
-increase, are not cases of high nutrition properly so-called; but
-cases of such defective absorption or assimilation as constitutes
-low nutrition. In Chap. IX, abundant proof was given that a truly
-plethoric state is an unusually fertile state. It may be added that
-much of the evidence by which Mr. Doubleday seeks to show that among
-men, highly-fed classes are infertile classes, may be out-balanced by
-counter-evidence. Many years ago Mr. G. H. Lewes pointed this out:
-extracting from a book on the peerage, the names of 16 peers who had,
-at that time, 186 children; giving an average of 11·6 in a family.</p>
-
-<p>Mr. Doubleday insists much on the support given to his theory by the
-barrenness of very luxuriant plants, and the fruitfulness produced
-in plants by depletion. Had he been aware that the change from
-barrenness to fruitfulness in plants, is a change from agamogenesis to
-gamogenesis&mdash;had it been as well known at the time when he wrote as it
-is now, that a tree which goes on putting out sexless shoots, is thus
-producing new individuals; and that when it begins to bear fruit, it
-simply begins to produce new individuals after another manner&mdash;he would
-have perceived that facts of this class do not tell in his favour.</p>
-
-<p>In the law which Mr. Doubleday alleges, he sees a guarantee for the
-maintenance of species. He argues that the plethoric state of the
-individuals constituting any race of organisms, presupposes conditions
-so favourable to life that the race can be in no danger; and that
-rapidity of multiplication becomes needless. Conversely, he argues
-that a deplethoric state implies unfavourable conditions&mdash;implies,
-consequently, unusual mortality; that is&mdash;implies a necessity for
-increased fertility to prevent the race from dying out. It may be
-readily shown, however, that such an arrangement would be the reverse
-of self-adjusting. Suppose a species, too numerous for its food, to
-be in the resulting deplethoric state. It will, according to Mr.
-Doubleday, become unusually fertile; and the next generation will be
-more numerous rather than less numerous. For, by the hypothesis, the
-unusual fertility due to the deplethoric state, is the cause of undue
-increase of population. But if the next generation is more numerous
-while the supply of food has not increased in proportion, then this
-next generation will be in a still more deplethoric state, and will be
-still more fertile. Thus there will go on an ever-increasing rate of
-multiplication, and an ever-decreasing share of food, for each person,
-until the species disappears. Suppose, on the other hand, the members
-of a species to be in an unusually plethoric state. Their rate of
-multiplication, ordinarily sufficient to maintain their numbers, will
-become insufficient to maintain their numbers. In the next generation,
-therefore, there will be fewer to eat the already abundant food, which
-becoming relatively still more abundant, will render the fewer members
-of the species still more plethoric, and still less fertile, than their
-parents. And the actions and reactions continuing, the species will
-presently die out from absolute barrenness.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_67" href="#FNanchor_67" class="label">[67]</a> A good deal of this chapter retains its original form;
-and the above paragraph is reprinted verbatim from the <i>Westminster
-Review</i> for April, 1852, in which the views developed in the
-foregoing hundred pages were first sketched out. This paragraph shows
-how near one may be to a great generalization without seeing it. Though
-the struggle for life is the alleged motive force; though the process
-of natural selection is recognized; and though to it is ascribed a
-share in the evolution of a higher type; yet the conception is not
-that which Mr. Darwin has worked out with such wonderful skill and
-knowledge. In the first place, natural selection is here described
-only as furthering direct adaptation&mdash;only as aiding progress by the
-preservation of individuals in whom functionally-produced modifications
-have gone on most favourably. In the second place, there is no trace
-of the idea that natural selection may by co-operation with the
-cause assigned, or with other causes, produce <i>divergences</i>
-of structure; and of course, in the absence of this idea, there is
-no implication that natural selection has anything to do with the
-origin of species. And in the third place, the all-important factor
-of variation&mdash;“spontaneous,” or incidental as we may otherwise call
-it&mdash;is wholly ignored. Though use and disuse are, I think, much more
-potent causes of organic modification than Mr. Darwin supposes&mdash;though,
-while pursuing the inquiry in detail, I have been led to believe that
-direct equilibration has played a more active part even than I had
-myself at one time thought; yet I hold Mr. Darwin to have shown beyond
-question, that a great part of the facts&mdash;perhaps the greater part&mdash;are
-explicable only as resulting from the survival of individuals which
-have deviated in some indirectly-caused way from the ancestral type.
-Thus, the above paragraph contains merely a passing recognition of the
-selective process; and indicates no suspicion of the enormous range
-of its effects, or of the conditions under which a large part of its
-effects are produced.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_68" href="#FNanchor_68" class="label">[68]</a> For the information of those who may wish to examine
-metamorphoses of these kinds, I may here state that I have found nearly
-all the examples described, in the neighbourhood of the sea&mdash;the
-last-named, on the shore of Locheil, near Fort William. Whether it is
-that I have sought more diligently for cases when in such localities,
-or whether it is that the sea-air favours that excessive nutrition
-whence these transformations result, I am unable to say.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_69" href="#FNanchor_69" class="label">[69]</a> These two dyes have affinities for different components
-of the tissues, and may be advantageously used in different cases.
-Magenta is rapidly taken up by woody matter and other secondary
-deposits; while logwood colours the cell-membranes, and takes but
-reluctantly to the substances seized by magenta. By trying both of them
-on the same structure, we may guard ourselves against any error arising
-from selective combination.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_70" href="#FNanchor_70" class="label">[70]</a> Those who repeat these experiments must be prepared for
-great irregularities in the rates of absorption. Succulent structures
-in general absorb much more slowly than others, and sometimes will
-scarcely take up the dye at all. The differences between different
-structures, and the same structure at different times, probably depend
-on the degrees in which the tissues are charged with liquid and the
-rates at which they are losing it by evaporation.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_71" href="#FNanchor_71" class="label">[71]</a> It may be added here that, on considering the mechanical
-actions that must go on, we are enabled in some measure to understand
-both how such inosculating channels are initiated, and how the
-structures of their component cells are explicable. What must happen
-to one of these elongated prosenchyma-cells if, in the course of
-its development, it is subject to intermittent compressions? Its
-squeezed-out liquid while partially escaping laterally, will more
-largely escape upwards and downwards; and while repeated lateral
-escape will tend to form lateral channels communicating with
-laterally-adjacent cells, repeated longitudinal escape will tend to
-form channels communicating with longitudinally-adjacent cells&mdash;so
-producing continuous though irregular longitudinal canals. Meanwhile
-each cell into and out of which the nutritive liquid is from time to
-time squeezed through small openings in its walls, cannot thicken
-internally in an even manner: deposition will be interfered with by the
-passage of the currents through the pores. The rush to or from each
-pore will tend to maintain a funnel-shaped depression in the deposit
-around; and the opening from cell to cell will so acquire just that
-shape which the microscope shows up&mdash;two hollow cones with their apices
-meeting at the point where the cell-membranes are in contact. Moreover,
-as confirming this interpretation, it may be remarked that we are thus
-supplied with a reason for the differences of shape between these
-passages from one pitted cell to another, and the analogous passages
-that exist between cells otherwise formed and otherwise conditioned.
-In the cells of the medulla, and others which are but little exposed
-to compression, the passages are severally formed more like a tube
-with two trumpet-mouths, one in each cell. This is just the form which
-might be expected where the nutritive fluid passes from cell to cell in
-moderate currents, and not by the violent rushes caused by intermittent
-pressures. Of course it is not meant that in each individual cell
-these structures are determined by these mechanical actions. The
-facts clearly negative any such conclusion, showing us, as they in
-many cases do, that these structures are assumed in advance of these
-mechanical actions. The implication is, that such mechanical actions
-initiated modifications that have, with the aid of natural selection,
-been accumulated from generation to generation; until, in conformity
-with ordinary embryological laws, the cells of the parts exposed to
-such actions assume these special structures irrespective of the
-actions&mdash;the actions, however, still serving to aid and complete the
-assumption of the inherited type.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_72" href="#FNanchor_72" class="label">[72]</a> Some exceptions to this occur in plants that have
-retrograded in the character of their tissues towards the simpler
-vegetal types. Certain very succulent leaves, such as those of
-<i>Sempervivum</i>, in which the cellular tissue is immensely developed
-in comparison with the vascular tissue, seem to have resumed to a
-considerable extent what we must regard as the primitive form of
-vegetal circulation&mdash;simple absorption from cell to cell. These,
-when they have lost much of their water, will take up the dye to
-some distance through their general substance, or rather through its
-interstices, even neglecting the vessels. At other times, in the same
-leaves, the vessels will become charged while comparatively little
-absorption takes place through the cellular tissue. Even in these
-exceptional cases, however, the movement through cellular tissue is
-nothing like as fast as the movement through vessels.</p>
-
-</div>
-
-<div class="footnote">
-
-<p><a id="Footnote_73" href="#FNanchor_73" class="label">[73]</a> It seems probable, however, that osmotic distention is
-here, especially, the more important of the two factors. The rising
-of the sap in spring may indirectly result, like the sprouting of the
-seed, from the transformation of starch into sugar. During germination,
-this change of an oxy-hydro-carbon from an insoluble into a soluble
-form, leads to rapid endosmose; consequently to great distention of
-the seed; and therefore to a force which thrusts the contained liquids
-into the plumule and radicle, and gives them power to displace the
-soil in their way: it sets up an active internal movement when neither
-evaporation nor the change which light produces can be operative. And
-similarly, if, in the spring, the starch stored-up in the roots of a
-tree passes into the form of sugar, the unusual osmotic absorption that
-arises will cause an unusual distention&mdash;a distention which, being
-resisted by the tough bark of the roots and stem, will result in a
-powerful upward thrust of the contained liquid.</p>
-
-</div>
-</div>
-
-
-<p class="transnote">Transcriber’s Note:<br />
-
-1. Obvious printers’, spelling and punctuation errors have been
-silently corrected.<br />
-
-2. Where appropriate, original spelling has been retained.<br />
-
-3. Both hyphenated and non-hyphenated versions of the same words have been
-retained where deemed appropriate.</p>
-
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