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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. 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