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+*** START OF THE PROJECT GUTENBERG EBOOK 44582 ***
+
+                    YALE UNIVERSITY
+
+        MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES
+
+
+                  PROBLEMS OF GENETICS
+
+              SILLIMAN MEMORIAL LECTURES
+          PUBLISHED BY YALE UNIVERSITY PRESS
+
+  ELECTRICITY AND MATTER. _By_ JOSEPH JOHN THOMSON,
+    D.SC., LL.D., PH.D., F.R.S., _Fellow of Trinity College,
+    Cambridge, Cavendish Professor of Experimental Physics, Cambridge_.
+  _Price $1.25 net; postage 10 cents extra._
+
+  THE INTEGRATIVE ACTION OF THE NERVOUS SYSTEM.
+    _By_ CHARLES S. SHERRINGTON,
+     D.SC., M.D., HON. LL.D., TOR., F.R.S.,
+    _Holt Professor of Physiology in the University of Liverpool_.
+  _Price $3.50 net; postage 25 cents extra._
+
+  RADIOACTIVE TRANSFORMATIONS. _By_ ERNEST RUTHERFORD,
+    D.SC., LL.D., F.R.S., _Macdonald Professor of Physics,
+    McGill University_.
+  _Price $3.50 net; postage 22 cents extra._
+
+  EXPERIMENTAL AND THEORETICAL APPLICATIONS OF
+      THERMODYNAMICS TO CHEMISTRY.
+  _By_ DR. WALTHER NERNST, _Professor and Director of the
+    Institute of Physical Chemistry in the University of Berlin_.
+  _Price $1.25 net; postage 10 cents extra._
+
+  THE PROBLEMS OF GENETICS. _By_ WILLIAM BATESON, M.A.,
+    F.R.S., _Director of the John Innes Horticultural Institution,
+    Merton Park, Surrey, England_.
+  _Price $4.00 net; postage 25 cents extra._
+
+  STELLAR MOTIONS.
+    WITH SPECIAL REFERENCE TO MOTIONS DETERMINED BY MEANS OF
+    THE SPECTROGRAPH. _By_ WILLIAM WALLACE CAMPBELL, SC.D., LL.D.,
+   _Director of the Lick Observatory, University of California_.
+  _Price $4.00 net; postage 30 cents extra._
+
+  THEORIES OF SOLUTIONS. _By_ SVANTE AUGUST ARRHENIUS,
+    PH.D., SC.D., M.D., _Director of the Physico-Chemical
+    Department of the Nobel Institute, Stockholm, Sweden_.
+  _Price $2.25 net; postage 15 cents extra._
+
+  IRRITABILITY.
+       A PHYSIOLOGICAL ANALYSIS OF THE GENERAL EFFECT OF
+       STIMULI IN LIVING SUBSTANCES.
+    _By_ MAX VERWORN,
+    _Professor at Bonn Physiological Institute_.
+  _Price $3.50 net; postage 20 cents extra._
+
+  THE EVOLUTION OF MODERN MEDICINE.
+    _By_ SIR WILLIAM OSLER, BART., M.D., LL.D., SC.D.,
+    _Regius Professor of Medicine, Oxford University_.
+  _Price $3.00 net; postage 40 cents extra._
+
+
+
+
+                     PROBLEMS OF GENETICS
+
+                              BY
+                 WILLIAM BATESON, M.A., F.R.S.
+
+        DIRECTOR OF THE JOHN INNES HORTICULTURAL INSTITUTION,
+            HON. FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE,
+        AND FORMERLY PROFESSOR OF BIOLOGY IN THE UNIVERSITY
+
+                     _WITH ILLUSTRATIONS_
+
+                         [Illustration]
+
+                NEW HAVEN: YALE UNIVERSITY PRESS
+                    LONDON: HUMPHREY MILFORD
+                     OXFORD UNIVERSITY PRESS
+
+                             MCMXIII
+
+
+                         Copyright, 1913
+                       By YALE UNIVERSITY
+
+                First printed August, 1913, 1000 copies
+
+
+[** Transcriber's Note:
+     Underscores "_" before and after a word or phrase indicate ITALICS
+       in the original text.
+     Hyphenation was used inconsistently by the author and has been
+        left as in the original text. ]
+
+
+
+
+                    THE SILLIMAN FOUNDATION
+
+In the year 1883 a legacy of about eighty-five thousand dollars was left
+to the President and Fellows of Yale College in the city of New Haven,
+to be held in trust, as a gift from her children, in memory of their
+beloved and honored mother, Mrs. Hepsa Ely Silliman.
+
+On this foundation Yale College was requested and directed to establish
+an annual course of lectures designed to illustrate the presence and
+providence, the wisdom and goodness of God, as manifested in the natural
+and moral world. These were to be designated as the Mrs. Hepsa Ely
+Silliman Memorial Lectures. It was the belief of the testator that any
+orderly presentation of the facts of nature or history contributed
+to the end of this foundation more effectively than any attempt to
+emphasize the elements of doctrine or of creed; and he therefore
+provided that lectures on dogmatic or polemical theology should be
+excluded from the scope of this foundation, and that the subjects should
+be selected rather from the domains of natural science and history,
+giving special prominence to astronomy, chemistry, geology, and anatomy.
+
+It was further directed that each annual course should be made the basis
+of a volume to form part of a series constituting a memorial to Mrs.
+Silliman. The memorial fund came into the possession of the Corporation
+of Yale University in the year 1901; and the present volume constitutes
+the fifth of the series of memorial lectures.
+
+
+
+
+                           PREFACE
+
+This book gives the substance of a series of lectures delivered in Yale
+University, where I had the privilege of holding the office of Silliman
+Lecturer in 1907.
+
+The delay in publication was brought about by a variety of causes.
+
+Inasmuch as the purpose of the lectures is to discuss some of the wider
+problems of biology in the light of knowledge acquired by Mendelian
+methods of analysis, it was essential that a fairly full account of
+the conclusions established by them should first be undertaken and I
+therefore postponed the present work till a book on Mendel's Principles
+had been completed.
+
+On attempting a more general discussion of the bearing of the phenomena
+on the theory of Evolution, I found myself continually hindered by the
+consciousness that such treatment is premature, and by doubt whether
+it were not better that the debate should for the present stand
+indefinitely adjourned. That species have come into existence by an
+evolutionary process no one seriously doubts; but few who are familiar
+with the facts that genetic research has revealed are now inclined to
+speculate as to the manner by which the process has been accomplished.
+Our knowledge of the nature and properties of living things is far too
+meagre to justify any such attempts. Suggestions of course can be made:
+though, however, these ideas may have a stimulating value in the lecture
+room, they look weak and thin when set out in print. The work which may
+one day give them a body has yet to be done.
+
+The development of negations is always an ungrateful task apt to be
+postponed for the positive business of experiment. Such work is happily
+now going forward in most of the centers of scientific life. Of many
+of the subjects here treated we already know more than we did in 1907.
+The delay in production has made it possible to incorporate these new
+contributions.
+
+The book makes no pretence at being a treatise and the number of
+illustrative cases has been kept within a moderate compass. A good many
+of the examples have been chosen from American natural history, as being
+appropriate to a book intended primarily for American readers. The facts
+are largely given on the authority of others, and I wish to express my
+gratitude for the abundant assistance received from American colleagues,
+especially from the staffs of the American Museum in New York, and of
+the Boston Museum of Natural History. In connexion with the particular
+subjects personal acknowledgments are made.
+
+Dr. F. M. Chapman was so good as to supervise the preparation of the
+coloured Plate of _Colaptes_, and to authorize the loan of the Plate
+representing the various forms of _Helminthophila_, which is taken from
+his _North American Warblers_.
+
+I am under obligation to Messrs. Macmillan & Co., for permission to
+reproduce several figures from _Materials for the Study of Variation_,
+illustrating subjects which I wished to treat in new associations, and
+to M. Leduc for leave to use Fig. 9.
+
+In conclusion I thank my friends in Yale for the high honour they did me
+by their invitation to contribute to the series of Silliman Lectures,
+and for much kindness received during a delightful sojourn in that
+genial home of learning.
+
+
+
+
+                    TABLE OF CONTENTS.
+
+  CHAPTER                                                  PAGE
+
+     I. INTRODUCTORY. THE PROBLEM OF SPECIES AND VARIETY     1
+    II. MERISTIC PHENOMENA                                  31
+   III. SEGMENTATION, ORGANIC AND MECHANICAL                60
+    IV. THE CLASSIFICATION OF VARIATION AND THE NATURE
+                OF SUBSTANTIVE VARIATION                    83
+        NOTE TO CHAPTER IV                                  94
+     V. THE MUTATION THEORY                                 97
+        NOTE TO CHAPTER V                                  116
+    VI. VARIATION AND LOCALITY                             118
+   VII. LOCAL DIFFERENTIATION--_continued_.
+              OVERLAPPING FORMS                            146
+  VIII. LOCALLY DIFFERENTIATED FORMS--_continued_.
+              CLIMATIC VARIETIES                           164
+    IX. THE EFFECTS OF CHANGED CONDITIONS                  187
+     X. THE EFFECTS OF CHANGED CONDITIONS--_continued_.
+              THE CAUSES OF GENETIC VARIATION              212
+    XI. THE STERILITY OF HYBRIDS. CONCLUDING REMARKS       233
+        APPENDIX TO CHAPTER X                              250
+        INDEX                                              251
+
+
+
+
+
+
+                        PROBLEMS OF GENETICS
+
+
+
+
+                              CHAPTER I
+
+                             INTRODUCTORY
+
+The purpose of these lectures is to discuss some of the familiar
+phenomena of biology in the light of modern discoveries. In the last
+decade of the nineteenth century many of us perceived that if any
+serious advance was to be made with the group of problems generally
+spoken of as the Theory of Evolution, methods of investigation must be
+devised and applied of a kind more direct and more penetrating than
+those which after the general acceptance of the Darwinian views had been
+deemed adequate. Such methods obviously were to be found in a critical
+and exhaustive study of the facts of variation and heredity, upon which
+all conceptions of evolution are based. To construct a true synthetic
+theory of Evolution it was necessary that variation and heredity instead
+of being merely postulated as axioms should be minutely examined as
+phenomena. Such a study Darwin himself had indeed tentatively begun, but
+work of a more thorough and comprehensive quality was required. In the
+conventional view which the orthodoxy of the day prescribed, the terms
+variation and heredity stood for processes so vague and indefinite that
+no analytical investigation of them could be contemplated. So soon,
+however, as systematic inquiry into the natural facts was begun it was
+at once found that the accepted ideas of variation were unfounded.
+Variation was seen very frequently to be a definite and specific
+phenomenon, affecting different forms of life in different ways, but
+in all its diversity showing manifold and often obvious indications
+of regularity. This observation was not in its essence novel. Several
+examples of definite variation had been well known to Darwin and
+others, but many, especially Darwin himself in his later years, had
+nevertheless been disposed to depreciate the significance of such
+facts. They consequently then lapsed into general disparagement. Upon
+more careful inquiry the abundance of such phenomena proved to be far
+greater than was currently supposed, and a discussion of their nature
+brought into prominence a consideration of greater weight, namely that
+the differences by which these definite or discontinuous variations are
+constituted again and again approximate to and are comparable with the
+class of differences by which species are distinguished from each other.
+
+The interest of such observations could no longer be denied. The
+more they were examined the more apparent it became that by means of
+the facts of variation a new light was obtained on the physiological
+composition and capabilities of living things. Genetics thus cease to be
+merely a method of investigating theories of evolution or of the origin
+of species but provide a novel and hitherto untried instrument by which
+the nature of the living organism may be explored. Just as in the study
+of non-living matter science began by regarding the external properties
+of weight, opacity, colour, hardness, mode of occurrence, etc., noting
+only such evidences of chemical attributes and powers as chance
+spontaneously revealed; and much later proceeded to the discovery that
+these casual manifestations of chemical properties, rightly interpreted,
+afford a key to the intrinsic nature of the diversity of matter, so in
+biology, having examined those features of living things which ordinary
+observations can perceive, we come at last to realize that when studied
+for their own sake the properties of living organisms in respect of
+heredity and variation are indications of their inner nature and provide
+evidences of that nature which can be obtained from no other source.
+
+While such ideas were gradually forming in our minds, came the
+rediscovery of Mendel's work. Investigations which before had only
+been imagined as desirable now became easy to pursue, and questions
+as to the genetic inter-relations and compositions of varieties can
+now be definitely answered. Without prejudice to what the future may
+disclose whether by way of limitation or extension of Mendelian method,
+it can be declared with confidence and certainty that we have now the
+means of beginning an analysis of living organisms, and distinguishing
+many of the units or factors which essentially determine and cause the
+development of their several attributes.
+
+Briefly put, the essence of Mendelism lies in the discovery of the
+existence of unit characters or factors. For an account of the Mendelian
+method, how it is applied and what it has already accomplished,
+reference must be made to other works.[1] With this part of the subject
+I shall assume a sufficient acquaintance. In these lectures I have
+rather set myself the task of considering how certain problems appear
+when viewed from the standpoint to which the application of these
+methods has led us. It is indeed somewhat premature to discuss such
+questions. The work of Mendelian analysis is progressing with great
+rapidity and anything I can say may very soon be superseded as out
+of date. Nevertheless a discussion of this kind may be of at least
+temporary service in directing inquiry to the points of special interest.
+
+
+             THE PROBLEM OF SPECIES AND VARIETY
+
+Nowhere does our new knowledge of heredity and variation apply more
+directly than to the problem what is a species and what is a variety? I
+cannot assert that we are already in a position to answer this important
+question, but as will presently appear, our mode of attack and the
+answers we expect to receive are not those that were contemplated by our
+predecessors. If we glance at the history of the scientific conception
+of Species we find many signs that it was not till comparatively recent
+times that the definiteness of species became a strict canon of the
+scientific faith and that attempts were made to give precise limits
+to that conception. When the diversity of living things began to be
+accurately studied in the sixteenth and seventeenth centuries names
+were applied in the loosest fashion, and in giving a name to an animal
+or a plant the naturalists of those times had no ulterior intention.
+Names were bestowed on those creatures about which the writer proposed
+to speak. When Gesner or Aldrovandi refer to all the kinds of horses,
+unicorns, dogs, mermaids, etc., which they had seen or read of, giving
+to each a descriptive name, they do not mean to "elevate" each named
+kind to "specific rank"; and if anyone had asked them what they meant by
+a species, it is practically certain that they would have had not the
+slightest idea what the question might imply, or any suspicion that it
+raised a fundamental problem of nature.
+
+Spontaneous generation being a matter of daily observation, then
+unquestioned, and supernatural events of all kinds being commonly
+reported by many witnesses, transmutation of species had no inherent
+improbability. Matthioli,[2] for instance, did not expect to be charged
+with heresy when he declared _Stirpium mutatio_ to be of ordinary
+occurrence. After giving instances of induced modifications he wrote,
+"Tantum enim in plantis naturae germanitas potest, ut non solum saepe
+praedictos praestet effectus, sed etiam ut alteram in alteram stirpem
+facile vertat, ut cassiam in cinnamomum, sisymbrium in mentham, triticum
+in lolium, hordeum in avenam, et ocymum in serpyllum."
+
+I do not know who first emphasized the need for a clear understanding
+of the sense in which the term species is to be applied. In the second
+half of the seventeenth century Ray shows some degree of concern on
+this matter. In the introduction to the _Historia Plantarum_, 1686, he
+discusses some of the difficulties and lays down the principle that
+varieties which can be produced from the seed of the same plant are to
+be regarded as belonging to one species, being, I believe, the first
+to suggest this definition. That new species can come into existence
+he denies as inconsistent with Genesis 2, in which it is declared that
+God finished the work of Creation in six days. Nevertheless he does not
+wholly discredit the possibility of a "transmutation" of species, such
+that one species may as an exceptional occurrence give rise by seed
+to another and nearly allied species. Of such a phenomenon he gives
+illustrations the authenticity of which he says he is, against his will,
+compelled to admit. He adds that some might doubt whether in the cases
+quoted the two forms concerned are really distinct species, but the
+passage is none the less of value for it shews that the conception of
+species as being distinct unchangeable entities was not to Ray the dogma
+sacrosanct and unquestionable which it afterwards became.[3]
+
+In the beginning of the eighteenth century Marchant,[4] having observed
+the sudden appearance of a lacinated variety of _Mercurialis_, makes the
+suggestion that species in general may have arisen by similar mutations.
+Indeed from various passages it is manifest that to the authors of the
+seventeenth and early eighteenth centuries species appeared simply as
+groups more or less definite, the boundaries of which it was unnecessary
+to determine with great exactitude. Such views were in accord with the
+general scientific conception of the time. The mutability of species is
+for example sometimes likened (see for instance Sharrock, loc. cit.) to
+the metamorphoses of insects, and it is to be remembered that the search
+for the Philosopher's Stone by which the transmutation of metals was to
+be effected had only recently fallen into discredit as a pursuit.
+
+The notion indeed of a peculiar, fixed meaning to be attached to species
+as distinct from variety is I think but rarely to be found categorically
+expressed in prae-Linnaean writings.
+
+But with the appearance of the _Systema Naturae_ a great change
+supervened. Linnaeus was before all a man of order. Foreseeing the
+immense practical gain to science that must come from a codification of
+nomenclature, he invented such a system.
+
+It is not in question that Linnaeus did great things for us and made
+Natural History a manageable and accessible collection of facts instead
+of a disorderly heap; but orderliness of mind has another side, and
+inventors and interpreters of systems soon attribute to them a force and
+a precision which in fact they have not.
+
+The systematist is primarily a giver of names, as Ray with his broader
+views perceived. Linnaeus too in the exordium to the _Systema Naturae_
+naively remarks, that he is setting out to continue the work which
+Adam began in the Golden Age, to give names to the living creatures.
+Naming however involves very delicate processes of mind and of logic.
+Carried out by the light of meagre and imperfect knowledge it entails
+all the mischievous consequences of premature definition, and promotes
+facile illusions of finality. So was it with the Linnaean system. An
+interesting piece of biological history might be written respecting the
+growth and gradual hardening of the conception of Species. To readers
+of Linnaeus's own writings it is well known that his views cannot be
+summarized in a few words. Expressed as they were at various times
+during a long life and in various connexions, they present those divers
+inconsistencies which commonly reflect a mind retaining the power of
+development. Nothing certainly could be clearer than the often quoted
+declaration of the _Philosophia Botanica_, "Species tot numeramus quot
+diversae formae in principio sunt creatae," with the associated passage
+"Varietates sunt plantae ejusdem speciei mutatae a caussa quacunque
+occasionali." Those sayings however do not stand alone. In several
+places, notably in the famous dissertation on the peloric _Linaria_
+he explicitly contemplates the possibility that new species may arise
+by crossing, declaring nevertheless that he thinks such an event to
+be improbable. In that essay he refers to Marchant's observation on a
+laciniate _Mercurialis_, but though he states clearly that that plant
+should only be regarded as a variety of the normal, he does not express
+any opinion that the contemporary genesis of new species must be an
+impossibility. In the later dissertation on Hybrid Plants he returns to
+the same topic. Again though he states the belief that species cannot
+be generated by cross-breedings, he treats the subject not as heretical
+absurdity but as one deserving respectful consideration.
+
+The significance of the aphorisms that precede the lectures on the
+Natural Orders is not easy to apprehend. These are expressed with the
+utmost formality, and we cannot doubt that in them we have Linnaeus's
+own words, though for the record we are dependent on the transcripts of
+his pupils.
+
+   The text of the first five is as follows:
+
+   1. Creator T. O. in primordio vestiit Vegetabile _Medullare_
+   principiis constitutivis diversi _Corticalis_ unde tot difformia
+   individua, quot _Ordines_ Naturales prognata.
+
+   2. _Classicas_ has (1) plantas Omnipotens miscuit inter se,
+   unde tot _Genera_ ordinum, quot inde plantae.
+
+   3. _Genericas_ has (2) miscuit Natura, unde tot _Species_
+   congeneres quot hodie existunt.
+
+   4. _Species_ has miscuit Casus, unde totidem quot passim
+   occurrunt, _Varietates_.
+
+   5. Suadent haec (1-4) Creatoris leges a simplicibus ad
+   Composita.
+
+   Naturae leges generationis in hybridis.
+
+   Hominis leges ex observatis a posteriori.
+
+I am not clear as to the parts assigned in the first sentence
+respectively to the "_Medulla_" and the "_Cortex_," beyond that Linnaeus
+conceived that multiformity was first brought about by diversity in
+the "_Cortex_." The passage is rendered still more obscure if read in
+connection with the essay on "_Generatio Ambigena_," where he expresses
+the conviction that the _Medulla_ is contributed by the mother, and the
+_Cortex_ by the father, both in plants and animals.[5]
+
+But however that may be, he regards this original diversity as resulting
+in the constitution of the Natural Orders, each represented by one
+individual.
+
+In the second aphorism the Omnipotent is represented as creating the
+genera by intermixing the individual _plantae classicae_, or prototypes
+of the Natural Orders.
+
+The third statement is the most remarkable, for in it he declares that
+Species were formed by the act of Nature, who by inter-mixing the genera
+produced _Species congeneres_, namely species inside each genus, to the
+number which now exist. Lastly, Chance or Accident, intermixing the
+species, produced as many varieties as there are about us.
+
+Linnaeus thus evidently regarded the intermixing of an originally
+limited number of types as the sufficient cause of all subsequent
+diversity, and it is clear that he draws an antithesis between
+_Creator_, _Natura_, and _Casus_, assigning to each a special part
+in the operations. The acts resulting in the formation of genera are
+obviously regarded as completed within the days of the Creation, but the
+words do not definitely show that the parts played by Nature and Chance
+were so limited.
+
+Recently also E. L. Greene[6] has called attention to some curious
+utterances buried in the _Species Plantarum_, in which Linnaeus refers
+to intermediate and transitional species, using language that even
+suggests evolutionary proclivities of a modern kind, and it is not easy
+to interpret them otherwise.
+
+Whatever Linnaeus himself believed to be the truth, the effect of his
+writings was to induce a conviction that the species of animals and
+plants were immutably fixed. Linnaeus had reduced the whole mass of
+names to order and the old fantastical transformations with the growth
+of knowledge had lapsed into discredit; the fixity of species was
+taken for granted, but not till the overt proclamation of evolutionary
+doctrine by Lamarck do we find the strenuous and passionate assertions
+of immutability characteristic of the first half of the nineteenth
+century.
+
+It is not to be supposed that the champions of fixity were unacquainted
+with varietal differences and with the problem thus created, but in
+their view these difficulties were apparent merely, and by sufficiently
+careful observation they supposed that the critical and permanent
+distinctions of the true species could be discovered, and the
+impermanent variations detected and set aside.
+
+This at all events was the opinion formed by the great body of
+naturalists at the end of the eighteenth and beginning of the nineteenth
+centuries, and to all intents and purposes in spite of the growth of
+evolutionary ideas, it remains the guiding principle of systematists
+to the present day. There are 'good species' and 'bad species' and the
+systematists of Europe and America spend most of their time in making
+and debating them.
+
+In some of its aspects the problem of course confronted earlier
+naturalists. Parkinson for instance (1640) in introducing his treatment
+of _Hieracium_ wrote, "To set forth the whole family of the Hawkeweedes
+in due forme and order is such a world of worke that I am in much doubt
+of mine own abilitie, it having lyen heavie on his shoudiers that
+hath already waded through them ... for such a multitude of varieties
+in forme pertaining to one herbe is not to be found againe in _rerum
+natura_ as I thinke," and the same idea, that the difficulty lay rather
+in man's imperfect powers of discrimination than in the nature of the
+materials to be discriminated, is reflected in many treatises early and
+late.
+
+It was however with the great ouburst of scientific activity which
+followed Linnaeus that the difficulty became acute. Simultaneously
+vast masses of new material were being collected from all parts of the
+world into the museums, and the products of the older countries were
+re-examined with a fresh zeal and on a scale of quantity previously
+unattempted. But the problem how to name the forms and where to draw
+lines, how much should be included under one name and where a new name
+was required, all this was felt, rather as a cataloguer's difficulty
+than as a physiological problem. And so we still hear on the one hand
+of the confusion caused by excessive "splitting" and subdivisions, and
+on the other of the uncritical "lumpers" who associate together under
+one name forms which another collector or observer would like to see
+distinguished.
+
+In spite of Darwin's hopes, the acceptance of his views has led to no
+real improvement--scarcely indeed to any change at all in either the
+practice or aims of systematists. In a famous passage in the _Origin_ he
+confidently declares that when his interpretation is generally adopted
+"Systematists will be able to pursue their labours as at present; but
+they will not be incessantly haunted by the shadowy doubt whether this
+or that form be a true species. This, I feel sure, and I speak after
+experience, will be no slight relief. The endless disputes whether or
+not some fifty species of British brambles are good species will cease."
+Those disputes nevertheless proceed almost exactly as before. It is
+true that biologists in general do not, as formerly, participate in
+these discussions because they have abandoned systematics altogether;
+but those who are engaged in the actual work of naming and cataloguing
+animals and plants usually debate the old questions in the old way.
+There is still the same divergence of opinion and of practice, some
+inclining to make much of small differences, others to neglect them.
+
+Not only does the work of the systematists as a whole proceed as if
+Darwin had never written but their attitude towards these problems is
+but little changed. In support of this statement I may refer to several
+British Museum Catalogues, much of the _Biologia Centrali-Americana_,
+Ridgway's _Birds of North America_, the _Fauna Hawaiensis_, indeed to
+almost any of the most important systematic publications of England,
+America, or any other country. These works are compiled by the most
+proficient systematists of all countries in the several groups, but
+with rare exceptions they show little misgiving as to the fundamental
+reality of specific differences. That the systematists consider the
+species-unit as of primary importance is shown by the fact that the
+whole business of collection and distribution of specimens is arranged
+with regard to it.
+
+Almost always the collections are arranged in such a way that the
+phenomena of variation are masked. Forms intermediate between two
+species are, if possible, sorted into separate boxes under a third
+specific name. If a species is liable to be constantly associated
+with a mutational form, the mutants are picked out, regardless of the
+circumstances of their origin, from the samples among which they were
+captured, and put apart under a special name. Only by a minute study
+of the original labels of the specimens and by redistributing them
+according to locality and dates, can their natural relations be traced.
+The published accounts of these collections often take no notice of
+variations, others make them the subject of casual reference. Very few
+indeed treat them as of much importance. From such indications it is
+surely evident that the systematists attach to the conception of species
+a significance altogether different from that which Darwin contemplated.
+
+I am well aware that some very eminent systematists regard the whole
+problem as solved. They hold as Darwin did that specific diversity
+has no physiological foundation or causation apart from fitness, and
+that species are impermanent groups, the delimitations of which are
+ultimately determined by environmental exigency or "fitness." The
+specific diversity of living things is thus regarded as being something
+quite different in nature from the specific diversity of inorganic
+substances. In practice those who share these opinions are, as might
+be anticipated, to be found among the 'lumpers' rather than among the
+'splitters.' In their work, certainly, the Darwinian theory is actually
+followed as a guiding principle; unanalysed inter-gradations of all
+kinds are accepted as impugning the integrity of species; the underlying
+physiological problem is forgotten, and while the product is almost
+valueless as a contribution to biological research, I can scarcely
+suppose that it aids greatly in the advances of other branches of our
+science.
+
+But why is it that, with these exceptions, the consequences of the
+admittedly general acceptance of a theory of evolution are so little
+reflected in the systematic treatment of living things? Surely the
+reason is that though the systematist may be convinced of the general
+truth of the evolution theory at large, he is still of opinion that
+species are really distinct things. For him there are still 'good'
+species and 'bad' species and his experience tells him that the
+distinction between the two is not simply a question of degree or a
+matter of opinion.
+
+To some it may seem that this is mere perversity, a refusal to see
+obvious truth, a manifestation of the spirit of the collector rather
+than of the naturalist. But while recognising that from a magnification
+of the conception of species the systematists are occasionally led
+into absurdity I do not think the grounds for their belief have in
+recent times been examined with the consideration they deserve. The
+phenomenon of specific diversity is manifested to a similar degree by
+living things belonging to all the great groups, from the highest to the
+lowest, Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae,
+and Bacteria, all present diversities of such a kind that among them the
+existence of specific differences can on the whole be recognised with
+a similar degree of success and with very similar limitations. In all
+these groups there are many species quite definite and unmistakable, and
+others practically indefinite. The universal presence of specificity,
+as we may call it, similarly limited and characterised, is one of its
+most remarkable features. Not only is this specificity thus universally
+present among the different forms of life, but it manifests itself
+in respect of the most diverse characteristics which living things
+display. Species may thus be distinguished by peculiarities of form,
+of number, of geometrical arrangement, of chemical constitution and
+properties, of sexual differentiation, of development, and of many
+other properties. In any one or in several of these features together,
+species may be found distinguished from other species. It is also to be
+observed that the definiteness of these distinctions has no essential
+dependence on the nature of the characteristic which manifests them.
+It is for example sometimes said that colour-distinctions are of small
+systematic importance, but every systematist is familiar with examples
+(like that of the wild species of _Gallus_) in which colours though
+complex, show very little variation. On the other hand features of
+structure, sexual differentiation, and other attributes which by our
+standards are estimated as essential, may be declared to show much
+variation or little, not according to any principle which can be
+detected, but simply as the attention happens to be applied to one
+species or group of species, or to another. In many groups of animals
+and plants observers have hit upon characters which were for a time
+thought to be finally diagnostic of species. The Lepidoptera and Diptera
+for instance, have been re-classified according to their neuration.
+Through a considerable range of forms determinations may be easily made
+on these characters, but as is now well known, neuration is no more
+immune from variation than any other feature of organisation, and in
+some species great variability is the rule. Again it was once believed
+by some that the genitalia of the Lepidoptera provided a basis of final
+determination--with a similar sequel. In some groups, for example
+the Lycaenidae, or the Hesperidae, there are forms almost or quite
+indistinguishable on external examination, but a glance at the genitalia
+suffices to distinguish numerous species, while on the contrary among
+Pieridae a great range of species show scarcely any difference in these
+respects: and again in occasional species the genitalia show very
+considerable variations.
+
+The proposition that animals and plants are on the whole divisible into
+definite and recognisable species is an approximation to the truth.
+Such a statement is readily defensible, whereas to assert the contrary
+would be palpably absurd. For example, a very competent authority
+lately wrote: "In the whole Lepidopterous fauna of England there is no
+species of really uncertain limits."[7] Others may be disposed to make
+certain reservations, but such exceptions would be so few as scarcely to
+impair the validity of the general statement. The declaration might be
+extended to other orders and other lands.
+
+We know, of course, that the phenomenon of specific diversity is
+complicated by local differentiation: that, in general, forms which
+cannot disperse themselves freely exhibit a multitude of local races,
+and that of these some are obviously adaptative, and that a few even owe
+their peculiarity to direct environmental effects. Every systematist
+also is perfectly aware that in dealing with collections from little
+explored countries the occurrence of polymorphism or even of sporadic
+variation may make the practical business of distinguishing the species
+difficult and perhaps for the time impossible; still, conceding that a
+great part of the diversity is due to geographical differentiation, and
+that some is sporadic variation, our experience of our own floras and
+faunas encourages the belief that if we were thoroughly familiar with
+these exotic productions it would usually be possible to assign their
+specific limitations with an approach to certainty.
+
+For apart from any question of the justice of these wider inferences,
+if we examine the phenomenon of specificity as it appears in those
+examples which are nearest to hand, surely we find signs in plenty that
+specific distinction is no mere consequence of Natural Selection. The
+strength of this proposition has lain mainly in the appeal to ignorance.
+Steadily with the growth of knowledge has its cogency diminished, and
+such a belief could only have been formulated at a time when the facts
+of variation were unknown.
+
+In Darwin's time no serious attempt had been made to examine the
+manifestations of variability. A vast assemblage of miscellaneous facts
+could formerly be adduced as seemingly comparable illustrations of the
+phenomenon "Variation." Time has shown this mass of evidence to be
+capable of analysis. When first promulgated it produced the impression
+that variability was a phenomenon generally distributed amongst living
+things in such a way that the specific divisions must be arbitrary.
+When this variability is sorted out, and is seen to be in part a result
+of hybridisation, in part a consequence of the persistence of hybrids
+by parthenogenetic reproduction, a polymorphism due to the continued
+presence of individuals representing various combinations of Mendelian
+allelomorphs, partly also the transient effect of alteration in external
+circumstances, we see how cautious we must be in drawing inferences as
+to the indefiniteness of specific limits from a bare knowledge that
+intermediates exist. Conversely, from the accident of collocation or
+from a misleading resemblance in features we deem essential, forms
+genetically distinct are often confounded together, and thus the
+divergence of such forms in their other features, which we declare
+to be non-essential, passes as an example of variation. Lastly, and
+this is perhaps the most fertile of all the sources of confusion, the
+impression of the indefiniteness of species is created by the existence
+of numerous local forms, isolated geographically from each other, forms
+whose differences may be referable to any one of the categories I have
+enumerated.
+
+The advance has been from many sides. Something has come from the work
+of systematists, something from cultural experiments, something from
+the direct study of variation as it appears in nature, but progress
+is especially due to experimental investigation of heredity. From all
+these lines of inquiry we get the same answer; that what the naturalists
+of fifty years ago regarded as variation is not one phenomenon but
+many, and that what they would have adduced as evidence against the
+definiteness of species may not in fact be capable of this construction
+at all.
+
+If we may once more introduce a physical analogy, the distinctions with
+which the systematic naturalist is concerned in the study of living
+things are as multifarious as those by which chemists were confronted in
+the early days of their science. Diversities due to mechanical mixtures,
+to allotropy, to differences of temperature and pressure, or to degree
+of hydration, had all to be severally distinguished before the essential
+diversity due to variety of chemical constitution stood out clearly,
+and I surmise that not till a stricter analysis of the diversities of
+animals and plants has been made on a comprehensive scale, shall we be
+in a position to declare with any confidence whether there is or is not
+a natural and physiological distinction between species and variety.
+
+As I have said above, it is in the cases nearest to hand that the
+problem may be most effectively studied. Comparison between forms from
+dissimilar situations contributes something; but it is by a close
+examination of the behaviour, especially the genetic behaviour, of
+familiar species when living in the presence of their nearest allies
+that the most direct light on the problem is to be obtained. I cannot
+understand the attitude of those who, contemplating such facts as this
+examination elicits, can complacently declare that specific difference
+is a mere question of degree. With the spread of evolutionary ideas to
+speak much of the fixity of species has become unfashionable, and yet
+how striking and inscrutable are the manifestations of that fixity!
+
+Consider the group of species composing the _agrestis_ section of the
+genus _Veronica_, namely _Tournefortii_, _agrestis_, and _polita_.
+
+These three grow side by side in my garden, as they do in suitable
+situations over a vast area of the temperate regions. I have for years
+noticed them with some care and become familiar with their distinctions
+and resemblances. Never is there any real doubt as to the identity of
+any plant. The species show some variability, but I have never seen
+one which assumed any of the distinguishing features of the others. A
+glance at the fruits decides at once to which species a plant belongs. I
+find it impossible to believe that the fixity of these distinctions is
+directly dependent on their value as aids in the struggle for existence.
+The mode of existence of the three forms in so far as we can tell is
+closely similar. By whatever standard we reckon systematic affinity I
+suppose we shall agree that these species come very near indeed to each
+other. Bentham even takes the view that _polita_ is a mere variety of
+_agrestis_.
+
+Now in such cases as this it has been argued that the specific
+features of the several types have been separately developed in as
+many distinct localities, and that their present association is due
+to subsequent redistribution. Of these Veronicas indeed we know that
+one, _Tournefortii_ (= _Buxbaumii_) is as a matter of fact a recent
+introduction from the east.[8] But this course of argument leads to
+still further difficulties. For if it is true that the peculiarities
+of the several species have been perfected and preserved on account
+of their survival-value to their possessors, it follows that there
+must be many ways of attaining the same result. But since sufficient
+adaptation may be ensured in so many ways, the disappearance of the
+common parent of these forms is difficult to understand. Obviously it
+must have been a plant very similar in general construction to its
+modern representatives. Like them it must have been an annual weed, with
+an organisation conformable to that mode of life. Why then, after having
+been duly perfected for that existence should it have been entirely
+superseded in favour of a number of other distinct contrivances for
+doing the same thing, and--if a gradual transition be predicated--not
+only by them, but by each intermediate stage between them and the
+original progenitor? Surely the obvious inference from such facts is
+that the burden cast upon the theory of gradual selection is far greater
+than it can bear; that adaptation is not in practice a very close fit,
+and that the distinctions between these several species of Veronica have
+not arisen on account of their survival-value but rather because none
+of their diversities was so damaging as to lead to the extermination
+of its possessor. When we see these various Veronicas each rigidly
+reproducing its parental type, all comfortably surviving in competition
+with each other, are we not forced to the conclusion that _tolerance_
+has as much to do with the diversity of species as the stringency of
+Selection? Certainly these species owe their continued existence to the
+fact that they are each good enough to live, but how shall we refer the
+distinctions between them directly or indirectly to the determination of
+Natural Selection?
+
+The control of Selection is loose while the conformity to specific
+distinction is often very strict and precise, and no less so even when
+several closely related species co-exist in the same area and in the
+same circumstances.
+
+The theory of Selection fails at exactly the point where it was devised
+to help: _Specific_ distinction.
+
+Let us examine a somewhat different set of facts in the case of another
+pair of nearly allied species _Lychnis diurna_ and _vespertina_. The
+two plants have much in common. Both are dioecious perennials, with
+somewhat similar flowers, the one crimson, the other white. Each however
+has its peculiarities which are discernible in almost any part of its
+structure, whether flower, leaf, fruit or seed, distinctions which
+would enable a person thoroughly familiar with the plants to determine
+at once from which species even a small piece had been taken. There is
+so much resemblance however as readily to support the surmise that the
+two were mere varieties of one species. Bentham, following Linnaeus,
+in fact actually makes this suggestion, with what propriety we will
+afterwards consider. Now this case is typical of many. The two forms
+have a wide distribution, occurring sometimes separately, sometimes
+in juxtaposition. _L. diurna_ is a plant of hedgerows and sheltered
+situations. _L. vespertina_ is common in fields and open spaces, where
+_diurna_ is hardly ever found; but not rarely _vespertina_ occurs in
+association with _diurna_ in the places which that plant frequents. In
+this case I do not doubt that we have to do with organisms of somewhat
+different aptitudes. That _L. vespertina_ has powers which _diurna_
+has not is shown very clearly by the fact that _diurna_ is sometimes
+entirely absent from areas where _vespertina_ can abound.[9] But in
+order to understand the true genetic relations of the two plants to
+each other it is necessary to observe their behaviour when they meet
+as they not unfrequently do. If the _Lychnis_ population of such a
+locality be examined it will be found to consist of many undoubted
+and unmodified _diurna_, a number--sometimes few, sometimes many--of
+similarly unmodified _vespertina_, and an uncertain but usually rather
+small proportion of plants obviously hybrids between the two. How
+is it possible to reconcile these facts with the view that specific
+distinction has no natural basis apart from environmental exigency?
+
+Darwinian orthodoxy suggests that by a gradual process of Natural
+Selection either one of these two types was evolved from the other,
+or both from a third type. I cannot imagine that anyone familiar with
+the facts would propose the first hypothesis in the case of _Lychnis_,
+nor can I conceive of any process, whether gradual or sudden, by
+which _diurna_ could have come out of _vespertina_, or _vespertina_
+out of _diurna_. Both however may no doubt have been derived from
+some original third type. It is conceivable that _Lychnis macrocarpa_
+of Boissier, a native of Southern Spain and Morocco, may be this
+original form. This species is said to combine a white flower (like
+that of _L. vespertina_), with capsule-teeth rolled back (like those
+of _diurna_).[10] But whatever the common progenitor may have been, if
+we are to believe that these two species have been evolved from it by
+a gradual process of Natural Selection based on adaptation, enormous
+assumptions must be made regarding the special fitness of these two
+forms and the special unfitness of the common parent, and these
+assumptions must be specially invoked and repeated for each several
+feature of structure or habits distinguishing the three forms.
+
+Why, if the common parent was strong enough to live to give rise to
+these two species, is it either altogether lost now, or at least absent
+from the whole of Northern Europe? Its two putative descendants, though
+so distinct from each other, are, as we have seen, able often to
+occupy the same ground. If they were gradually derived from a common
+progenitor--necessarily very like themselves--can we believe that
+this original form should always, in all the diversities of soil and
+situation which they inhabit, be unable to exist? Some one may fancy
+that the hybrids which are found in the situations occupied by both
+forms are this original parental species. But nothing can be more
+certain than that these plants are simply heterozygous combinations
+made by the union of gametes bearing the characters of _diurna_ and
+_vespertina_.[11] For they may be reproduced exactly in F_{1} or in
+later generations of that cross when it is artificially made; when bred
+from their families exhibit palpable phenomena of segregation more or
+less complex; and usually, if perhaps not always, they are partially
+sterile.[12] In a locality on the Norfolk coast that I know well,
+there is a strip of rough ground chiefly sand-bank, which runs along
+the shore. This ground is full of _vespertina_. Not a hundred yards
+inland is a lane containing _diurna_, and among the _vespertina_ on
+the sand-bank are always some of the hybrid form, doubtless the result
+of fertilisation from the neighbouring _diurna_ population. Seed saved
+from these hybrids gave _vespertina_ and hybrids again, having obviously
+been fertilised by other _vespertina_ or by other hybrids, and I have no
+doubt that such hybrid plants if fertilised by _diurna_ would have shown
+some _diurna_ offspring. The absence of _diurna_ in such localities may
+fairly be construed as an indication that _diurna_ is there at a real
+disadvantage in the competition for life.
+
+But if, admitting this, we proceed to consider how the special aptitude
+of _vespertina_ is constituted, or what it is that puts _diurna_ at
+a disadvantage, we find ourselves quite unable to show the slightest
+connexion between the success of one or the failure of the other on the
+one hand, and _the specific characteristics_ which distinguish the two
+forms on the other. The orthodox Selectionist would, as usual, appeal to
+ignorance. We ask what can _vespertina_ gain by its white flowers, its
+more lanceolate leaves, its grey seeds, its almost erect capsule-teeth,
+its longer fruits, which _diurna_ loses by reason of its red flowers,
+more ovate leaves, dark seeds, capsule-teeth rolled back, and shorter
+fruits? We are told that each of these things _may_ affect the viability
+of their possessors. We cannot assert that this is untrue, but we should
+like to have evidence that it is true. The same problem confronts us
+in thousands upon thousands of examples, and as time goes on we begin
+to feel that speculative appeals to ignorance, though dialectically
+admissible, provide an insufficient basis for a proposition which,
+if granted, is to become the foundation of a vast scheme of positive
+construction.
+
+One thing must be abundantly clear to all, that to treat two forms so
+profoundly different as one, because intermediates of unknown nature can
+be shown to exist between them, is a mere shirking of the difficulties,
+and this course indeed creates artificial obstacles in the way of those
+who are seeking to discover the origin of organic diversity.
+
+In the enthusiasm with which evolutionary ideas were received the
+specificity of living things was almost forgotten. The exactitude with
+which the members of a species so often conform in the diagnostic,
+specific features passed out of account; and the scientific world by
+dwelling with a constant emphasis on the fact of variability, persuaded
+itself readily that species had after all been a mere figment of
+the human mind. Without presuming to declare what future research
+only can reveal, I anticipate that, when variation has been properly
+examined and the several kinds of variability have been successfully
+distinguished according to their respective natures, the result will
+render the natural definiteness of species increasingly apparent.
+Formerly in such a case as that of the two _Lychnis_ species, the series
+of "intermediates" was taken to be a palpable proof that _vespertina_
+"graded" to _diurna_. It is this fact, doubtless, upon which Bentham
+would have relied in suggesting that both may be one species.[13]
+Genetic tests, though as yet imperfectly applied, make it almost certain
+that these inter-grading forms are not in any true sense variations
+from either species in the direction of the other, but combinations of
+elements derived from both.
+
+The points in which very closely allied species are distinguished
+from each other may be found in the most diverse features of their
+organisation. Sometimes specific difference is to be seen in a
+character which we can believe to be important in the struggle, but at
+least as often it is some little detail that we cannot but regard as
+trivial which suffices to differentiate the two species. Even when the
+diagnostic point is of such a nature that we can imagine it to make a
+serious difference in the economy we are absolutely at a loss to suggest
+why this feature should be a necessity to species A and unnecessary to
+species B its nearest ally. The house sparrow (_Passer domesticus_) is
+in general structure very like the tree sparrow (_P. montanus_). They
+differ in small points of colour. For instance _montanus_ has a black
+patch on the cheek which is absent in _domesticus_. The presence in the
+one species and the absence in the other are equally definite, and in
+both cases we are equally unable to suggest any consideration of utility
+in relation to these features. The two species are distinguished also by
+a characteristic that may well be supposed to be of great significance.
+In _domesticus_ the two sexes are strongly differentiated, the cock
+being more ornate than the hen. On the other hand the two sexes in
+_montanus_ are alike, and, if we take a standard from _domesticus_,
+we may fairly say that in _montanus_ the hen has the colouration of
+the male. It is not unreasonable to suppose that such a distinction
+may betoken some great difference in physiological economy, but the
+economical significance of this perhaps important distinction is just as
+unaccountable as that of the seemingly trivial but equally diagnostic
+colour-point.
+
+I have spoken of the fixed characteristics of the two species. If
+we turn to a very different feature, their respective liability to
+albinistic variation, we find ourselves in precisely similar difficulty.
+_Passer domesticus_ is a species in which individuals more or less pied
+occur with especial frequency, but in _P. montanus_ such variation is
+extremely rare if it occurs at all. The writer of the section on Birds
+in the _Royal Natural History_ (III., 1894-5, p. 393) calls attention to
+this fact and remarks that in that species he knows no such instance.
+
+The two species therefore, apart from any differences that we can
+suppose to be related to their respective habits, are characterised by
+small fixed distinctions in colour-markings, by a striking difference
+in secondary sexual characters, and by a difference in variability. In
+all these respects we can form no surmise as to any economic reason why
+the one species should be differentiated in the one way and the other in
+the other way, and I believe it is mere self-deception which suggests
+the hope that with fuller knowledge reasons of this nature would be
+discovered.
+
+The two common British wasps, _Vespa vulgaris_ and _Vespa germanica_,
+are another pair of species closely allied although sharply
+distinguished, which suggest similar reflexions. Both usually make
+subterranean nests but of somewhat different materials. _V. vulgaris_
+uses rotten wood from which the nest derives a characteristic yellow
+colour, while _V. germanica_ scrapes off the weathered surfaces of
+palings and other exposed timber, material which is converted into
+the grey walls of the nest. The stalk by which the nest is suspended
+(usually to a root) in the case of _germanica_ passes freely through
+a hole in the external envelope, but _vulgaris_ unites this external
+wall solidly to the stalk. In bodily appearance and structure the
+two species are so much alike that they have often been confounded
+even by naturalists, and to the untrained observer they are quite
+indistinguishable. There are nevertheless small points of difference
+which almost though not quite always suffice to distinguish the
+two forms. For example the yellow part of the sinus of the eyes is
+emarginate in _vulgaris_ but not emarginate in _germanica_. _V.
+vulgaris_ often has black spots on the tibiae while in _germanica_ the
+tibiae are usually plain yellow. In both species there is a horizontal
+yellow stripe on the thorax, but whereas in _vulgaris_ this is a plain
+narrow stripe, it is in _germanica_ enlarged downwards in the middle.
+These and other apparently trivial details of colouration, though not
+absolutely constant, are yet so nearly constant that irregularities
+in these respects are quite exceptional. Lastly the genitalia of the
+males, though not very different, present small structural points
+of distinction which are enough to distinguish the two species at a
+glance.[14]
+
+In considering the meaning of the distinctions between these two wasps
+we meet the old problem illustrated by the Sparrows. The two species
+have somewhat different habits of life and we should readily expect
+to find differences of bodily organisation corresponding with the
+differences of habits. But is that what we do find? Surely not. To
+suppose that there is a correspondence between the little points of
+colour and structure which we see and the respective modes of life of
+the two species is perfectly gratuitous. We have no inkling of the
+nature of such a correspondence, how it can be constituted, or in what
+it may consist.
+
+Is it not time to abandon these fanciful expectations which are never
+realised? Everywhere both among animals and plants does the problem of
+specific difference reiterate itself in the same form. In view of such
+facts as I have related and might indefinitely multiply, the fixity of
+specific characters cannot readily be held to be a measure of their
+economic importance to their possessors. The incidence of specific
+fixity is arbitrary and capricious, sometimes lighting on a feature or
+a property which can be supposed to matter much, but as often is it
+attached to the most trifling of superficial peculiarities.
+
+The incidence of _variability_ is no less paradoxical, and without
+investigation of the particular case no one can say what will be found
+to show much or little variability. The very characteristic which in
+one species may exhibit extreme variability may in an allied species
+show extreme constancy. Illustrations will occur to any naturalist, but
+nowhere is this truth more strikingly presented than in the British
+Noctuid Moths. Many are so variable that, in the common phrase,
+"scarcely two can be found alike," while others show comparatively
+slight variation. It need scarcely be remarked that, in the instances
+I have in mind, the evidence of great variability is in no way due to
+the abundance with which the particular species occurs, for common
+species may show constancy, and less abundant species may show great
+variability. The polymorphism seems to be now at least a general
+property of the variable species, as the fixity is a property of the
+fixed species. In illustration I may refer to the following examples.
+
+_Dianthoecia capsincola_ is a common and widely distributed moth which
+feeds on _Lychnis_. It shows little variation. _Dianthoecia carpophaga_
+is another species which feeds chiefly on _Silene_. Its habits are
+very similar to those of _capsincola_. Like that species it has a wide
+geographical range and is abundant in its localities, but in contrast
+to the fixity of _capsincola_, _carpophaga_ exhibits a complex series
+of varieties. _Agrotis suffusa_ (= _ypsilon_) is a moth widely spread
+through the southern half of England. It is very constant in colour and
+markings. _Agrotis segetum_ and _tritici_ are excessively variable both
+in ground colour and markings, being found in an immense profusion of
+dissimilar forms throughout their distribution. Of these and several
+other species of _Agrotis_ there are many named varieties, some of
+which have by various writers been regarded as specifically distinct.
+Of the genus _Noctua_ many species (e. g. _festiva_) show a similar
+polymorphism, but _N. triangulum_, though showing some variation in
+certain respects, is usually very constant to its type, and the same is
+true of _N. umbrosa_.
+
+In several species of _Taeniocampa_, especially _instabilis_, the
+multiplicity of forms is extreme, while _cruda_ (= _pulverulenta_) is
+a comparatively constant species. The genus _Plusia_ contains a number
+of constant species, but in _Plusia interrogationis_ we meet the fact
+that the central silvery mark undergoes endless variation. "Truly no
+two are alike," says Mr. Tutt, "and to look down a long series of
+_interrogationis_ is something like looking at a series of Chinese
+characters." In contrast to this we have the fact that in _Plusia gamma_
+the very similar silvery mark is by no means variable.
+
+I have taken this series of cases from the Noctuid moths, but it would
+be as easy to illustrate the same proposition from the Geometridae or
+the Micro-Lepidoptera.[15] I have a long series of _Peronea cristana_,
+for example, which was given to me by Mr. W. H. B. Fletcher, of Bognor.
+All were beaten out of the same hedge, and their polymorphism is such
+that no one unaccustomed to such examples could suppose that they
+belonged to a single species. Another common form, _P. schalleriana_,
+which lives in similar circumstances, exhibits comparatively slight
+variability.
+
+It should be expressly noted that the variation of which I am speaking
+is a genuine polymorphism. Several of the species enumerated exhibit
+also geographical variation, possessing definite and often strikingly
+distinct races peculiar to certain localities; but apart from the
+existence of such local differentiation, stands out the fact upon which
+I would lay stress, that some species are excessively variable while
+others are by comparison constant, in circumstances that we may fairly
+regard as comparable.
+
+This fact is difficult to reconcile with the conventional view that
+specific type is directly determined by Natural Selection and that
+the precision with which a species conforms to its pattern is an
+indication of the closeness of that control. Anyone familiar with the
+characteristics of Moths will agree that the Noctuids, Geometrids and
+Tortricids are creatures whose existence depends in some degree on the
+success with which they can escape detection by their enemies in the
+imaginal state. We are therefore not surprised to find that some species
+of these orders exhibit definite geographical variation in conformity
+with the character of the ground, which may reasonably be supposed to
+aid in their protection. If this were all, there would be nothing to
+cause surprise. We might even be disposed to allow that variability
+might contribute to the perpetuation of animals so situated, on the
+principle that among a variety of surroundings some would probably be
+in harmony with the objects on which they rest. But we cannot admit
+the plausibility of an argument which demands on the one hand that
+the extreme precision with which species A adheres in the minutest
+details of its colour and pattern to a certain type shall be ascribed
+to the protective fitness of those details, and on the other hand that
+the abundant variability of species B shall be ascribed to the same
+determination. If it is absolutely necessary for A to conform to one
+type how comes it that B may range through some twenty distinct forms,
+any two of which differ more from each other than the regular species
+of many other genera? The only reply I can conceive is a suggestion
+that there _may_ be some circumstance which differentiates the various
+classes of cases, that the exigencies of the fixed species _may_ be
+different from those of the variable. Those who make such appeals to
+ignorance do not always perhaps realise whither this course of reasoning
+may lead. If admissible here the same argument would lead us to suggest
+that because albino moles have for an indefinite period occurred on a
+certain land near Bath there may be something in the soil or in the
+conditions of life near Bath which requires a proportion of albinos in
+its mole population. Or again, because the butterfly _Thais rumina_
+in one locality, Digne in the south of France, has a percentage of
+individuals of the variety _Honoratii_ (with certain normally yellow
+spots on the hind wing coloured bright red) and nowhere else throughout
+its distribution, that therefore we may suggest that there is some
+difference in the condition of life at Digne which makes the continuance
+of _Honoratii_ there possible and beneficial.
+
+A polymorphism offering a parallel to that of the variable moths is
+afforded by the breeding plumage of the Ruff, the male of _Machetes
+pugnax_. The variety of plumage which these cocks exhibit is such
+that the statement that no two can be found alike is only a venial
+exaggeration. Newton remarks[16] "that all this wonderful 'show' is
+the consequence of the polygamous habit of the Ruff can scarcely
+be doubtful"; but even if it be conceded that the great external
+differentiation of the cocks may be a result of sexual selection, the
+problem of their _polymorphism_ remains unsolved, for, as we are well
+aware, polygamy is not usually associated with polymorphism of the male.
+The Black Cock (_Tetrao tetrix_), for example, is as polygamous as the
+Ruff, but in that and countless other cases, both sexes are constant to
+one type of plumage.
+
+When we thus compare the polymorphism of one species with the fixity
+of another, and attempt to determine the causes which have led to
+these extraordinary contrasts, two distinct lines of argument are
+open to us. We may ascribe the difference either to causes external
+to the organisms, primarily, that is to say, to a difference in the
+exigencies of Adaptation under Natural Selection; or on the other hand
+we may conceive the difference as due to innate distinctions in the
+chemical and physiological constitutions of the fixed and the variable
+respectively. There is truth undoubtedly in both conceptions. If the
+mole were physiologically incapable of producing an albino that variety
+would not have come into being, and if the albino were totally incapable
+of getting its living it would not be able to hold its own. Were
+_Plotheia frontalis_ constructed on a chemical plan which admitted of no
+variation, the countless varieties would not have been produced; and if
+one of its varieties had an overwhelming success out of all proportion
+to that of the rest, then the species would soon become monomorphic
+again. We cannot declare that Natural Selection has no part in the
+determination of fixity or variability; nevertheless looking at the
+whole mass of fact which a study of the incidence of variation provides,
+I incline to the view that the variability of polymorphic forms should
+be regarded rather as a thing tolerated than as an element contributing
+directly to their chances of life; and on the other hand that the fixity
+of the monomorphic forms should be looked upon not so much as a proof
+that Natural Selection controls them with a greater stringency, but
+rather as evidence of a natural and intrinsic stability of chemical
+constitution.
+
+Compare the condition of a variable form like the male Ruff (or in
+a less degree the Red Grouse in both its sexes) with that of the
+common Pheasant which is comparatively constant. In the Pheasant no
+doubt variations do occur as in other wild birds, but apart from the
+effects of mongrelisation the species is unquestionably uniform. Could
+it seriously be proposed that we should regard the constancy of the
+pheasant's plumage in this country as depending on the special fitness
+of that type of colouration? Even if the pheasant be not an alien in
+Western Europe, it has certainly been protected for centuries, and for a
+considerable period has existed in a state of semi-domestication. Such
+conditions should give good opportunity for polymorphism to be produced.
+In some coverts various aberrations do of course occur and persist,
+yet there is nothing indicative of a general relaxation of the fixity
+of the specific type, and the pheasant remains substantially a fixed
+species.[17] The common pheasant (_Phasianus colchicus_) even shows
+little of that disposition to form local races which appears in the
+species of Further India. Are we not then on safer ground in regarding
+the fixity of our species as a property inherent in its own nature
+and constitution? Just as in ages of domestication no rose has ever
+given off a blue variety so has the pheasant never broken out into the
+polymorphism of the Ruff.
+
+As soon as it is realised how largely the phenomena of variation and
+stability must be an index of the internal constitution of organisms,
+and not mere consequences of their relations to the outer world, such
+phenomena acquire a new and more profound significance.
+
+FOOTNOTES:
+
+[1] In _Mendel's Principles of Heredity_ (Cambridge University Press,
+1909) I have dealt with this subject, giving an account of the principal
+facts discovered up to the beginning of 1909.
+
+[2] Matthioli Opera, Ed. 1598, p. 8, originally published 1565.
+
+[3] Ray's instances relate to Kales, and in most of these examples
+we can see that there was no question of mutation or transmutation
+at all, but that the occurrence was due either to mistake or to
+cross-fertilisation. Sharrock, to whom Ray refers, was inclined to
+discredit stories of transmutation, but he has also this passage
+(_History of the Propagation and Improvement of Vegetables by the
+Concurrence of Art and Nature_, Oxford, 1660, p. 29):
+
+"It is indeed growen to be a great question, whether the transmutation
+of a species be possible either in the vegetable, Animal, or Minerall
+Kingdome. For the possibility of it in the vegetable; I have heard _Mr.
+Bobart_ and his _Son_ often report it, and proffer to make oath that the
+Crocus and Gladiolus, as likewise the Leucoium, and Hyacinths by a long
+standing without replanting have in his garden changed from one kind to
+the other: and for satisfaction about the curiosity in the presence of
+_Mr. Boyle_ I tooke up some bulbs of the very numericall roots whereof
+the relation was made, though the alteration was perfected before, where
+we saw the diverse bulbs growing as it were on the same stoole, close
+together, but no bulb half of the one kind, and the other half of the
+other: But the changetime being past it was reason we should believe the
+report of good artists in matters of their own faculty."
+
+Robert Sharrock was a fellow of New College, Oxford. Both the Bobarts
+were professional botanists, the father was author of a Catalogue of
+the plants in the Hortus Medicus at Oxford, and the son was afterwards
+Curator of the Oxford Garden.
+
+[4] _Mém. Ac. roy. des Sci._ for 1719 (1721), p. 59.
+
+[5] _Amoen. Acad._, 1789, vol. 6. I do not know whether attention has
+been called to the curious mistake which Linnaeus makes in the course of
+this argument. He cites the differences between the Mule and the Hinny
+in illustration of his thesis, pointing out that the Mule is externally
+more like a horse and the Hinny more like an ass. This, he says, is
+because the Mule has the horse for a father, and the Hinny the ass, thus
+inverting the actual facts!
+
+[6] _Proc. Washington Ac. Sci._, 1909, XI, pp. 17-26.
+
+[7] J. W. Tutt, in _Ent. Rec._, 1909, XXI, p. 185.
+
+[8] E. Lehmann (_Bull. l'Herb. Boissier_, Ser. 2, VIII, 1908, p. 229)
+has published an admirable paper on the interrelationships of these
+species and has instituted cultural experiments which will probably
+much elucidate the nature of their specific distinctness. As regards
+the existence of intermediate forms he comes to the conclusion that
+two only can be so regarded. The first was described by Kuntze from
+specimens found on a flower-pot on board a Caspian steamer, from which
+Lehmann proposes the new specific name _Siaretensis_. This comes between
+_polita_ and _filiformis_, a close ally of _Tournefortii_. The other,
+which combines some of the features of both _polita_ and _Tournefortii_,
+was found in the province of Asterabad.
+
+[9] In Cambridgeshire for example _vespertina_ is common but _diurna_ is
+absent. Whether this absence is connected with the general presence of
+chalk I cannot say. When introduced artificially _diurna_ establishes
+itself, for a time at least, without any apparent difficulty and
+occasionally escapes from the garden on to the neighbouring roadside.
+
+[10] Conceivably however it may be a segregated combination. For an
+account of this plant see Boissier, _Voy. Bot. Midi de l'Espagne_, 1839,
+II, 722.
+
+[11] A discussion of this subject with references to literature is
+given by Rolfe, in an excellent paper on "Hybridisation viewed from the
+standpoint of Systematic Botany" (_Jour. R. Hort. Soc._, XXIV, 1900, p.
+197). He concludes: "The simple fact is that the two plants (_L. diurna_
+and _vespertina_) are thoroughly distinct in numerous particulars, and
+affect such different habitats that in some localities one or the other
+of them is completely wanting. But when their stations are adjacent they
+hybridise together very readily, and it is here that these intermediate
+forms occur which have puzzled botanists so much." The same paper
+contains valuable information concerning several cognate illustrations.
+
+[12] In only two cases have I seen such plants (both females) completely
+sterile.
+
+[13] As is well known, in an even more notorious example, he proposed
+to unite _Primula vulgaris_, _P. elatior_, and _P. acaulis_, similarly
+relying on the existence of "intermediates," which we now well know to
+be mongrels between the species.
+
+[14] For an account of the distinctions between _Vespa vulgaris_ and
+_germanica_ see Ch. Janet, _Études sur les Fourmis, les Guêpes et les
+Abeilles_, 11^e, Note. Sur _Vespa germanica_ et _V. vulgaris_. Limoges
+(Ducourtieux), 1895; and R. du Buysson, Monographie des Guêpes, _Ann.
+Soc. Ent. France_, 1903, Vol. LXXII, p. 603, Pl. VIII.
+
+[15] The statements made above are for the most part taken from Barrett,
+C. G., _Lepidoptera of the British Islands_, and from Tutt, J. W., _The
+British Noctuae and their Varieties_. The reader who is unfamiliar
+with the amazing polymorphism exhibited by some of these moths should
+if possible take an opportunity of looking over a long series in a
+collection, or, if that be impossible, refer to the admirable coloured
+plates published by Barrett. It may not be superfluous to observe that
+plenty of similar examples are known in other countries. For instance
+_Plotheia frontalis_, a Noctuid which often abounds in Ceylon, shows
+an equally bewildering wealth of forms. If a dozen specimens of such a
+species were to be brought home from some little known country, each
+individual would almost certainly be described as the type of a distinct
+species. (See the coloured plate published by Sir G. Hampson, Cat. Brit.
+Mus., Heterocera, Vol. IX.)
+
+[16] _Dict. of Birds_, p. 800. It would be interesting and profitable
+to attempt in a long series of Ruffs to determine the Mendelian factors
+which by their combinations give rise to this complex assemblage
+of varietal forms. A few such factors both of colour and pattern
+can be at once distinguished, and it is noticeable that some of the
+resulting types of barring, spangling and penciling show a perceptible
+correspondence with some of the types of colouration found in the breeds
+of domestic fowls.
+
+[17] Howard Saunders (_Illust. Manual of British Birds_, 1899, p. 499)
+states that there is evidence that the pheasant had become naturalized
+in the south of England before the Norman invasion. He adds, "little, if
+any, deviation from the typical _P. colchicus_ took place up to the end
+of last century, when the introduction of the Chinese Ring-necked _P.
+torquatus_ commenced, which has left almost indelible marks, especially
+with regard to the characteristic white collar."
+
+
+
+
+                         CHAPTER II
+
+                     MERISTIC PHENOMENA
+
+Twenty years ago in describing the facts of Variation, argument was
+necessary to show that these phenomena had a special value in the
+sciences of Zoology and Botany. This value is now universally understood
+and appreciated. In spite however of the general attention devoted to
+the study of Variation, and the accumulation of material bearing on the
+problem, no satisfactory or searching classification of the phenomena is
+possible. The reason for this failure is that a real classification must
+presuppose knowledge of the chemistry and physics of living things which
+at present is quite beyond our reach.
+
+It is however becoming probable that if more knowledge of the chemical
+and physical structure of organisms is to be attained, the clue will
+be found through Genetics, and thus that even in the uncoordinated
+accumulation of facts of Variation we are providing the means of
+analysis applicable not only to them, but to the problems of normality
+also.
+
+The only classification that we can yet institute with any confidence
+among the phenomena of Variation is that which distinguishes on the one
+hand variations in the processes of division from variations in the
+nature of the substances divided.
+
+Variations in the processes of division are most often made apparent by
+a change in the number of the parts, and are therefore called _Meristic_
+Variations, while the changes in actual composition of material are
+spoken of as _Substantive_ Variations. The Meristic Variations form on
+the whole a natural and fairly well defined group, but the Substantive
+Variations are obviously a heterogeneous assemblage.
+
+Though this distinction does not go very far, it is useful, and in
+all probability fundamental. It is of value inasmuch as it brings
+into prominence the distinct and peculiar part which the process
+of division, or, more generally, repetition of parts, plays in the
+constitution of the forms of living things.
+
+That there may be a real independence between the Meristic and the
+Substantive phenomena is evident from the fact both that Meristic
+changes may occur without Substantive Variation, and that the substances
+composing an organism may change without any perceptible alteration
+in its meristic structure. When the distinction between these two
+classes of phenomena is perceived it will be realised that the study
+of genetics has on the one hand a physical, or perhaps more strictly
+a mechanical aspect, which relates to the manner in which material is
+divided and distributed; and also a chemical aspect, which relates
+to the constitution of the materials themselves. Somewhat as the
+philosophers of the seventeenth and eighteenth centuries were awaiting
+both a chemical and a mechanical discovery which should serve as a
+key to the problems of unorganised matter, so have biologists been
+awaiting two several clues. In Mendelian analysis we have now, it is
+true, something comparable with the clue of chemistry, but there is
+still little prospect of penetrating the obscurity which envelops the
+mechanical aspect of our phenomena. To make clear the application of
+the terms chemical and mechanical to the problem of Genetics the nature
+of that problem must be more fully described. In its most concrete form
+this problem is expressed in the question, how does a cell divide? If
+the organism is unicellular, and the single cell is the whole body,
+then the process of heredity is accomplished in the single operation of
+cell-division. Similarly in animals and plants whose bodies are made
+up of many cells, the whole process of heredity is accomplished in the
+cell-divisions by which the germ-cells are formed. When therefore we see
+a cell dividing, we are witnessing the process by which the form and the
+properties of the daughter-cells are determined.
+
+Now this process has the two aspects which I have called mechanical
+and chemical. The term "_Entwicklungsmechanik_" has familiarised us
+with the application of the word mechanics to these processes, but on
+reflexion it will be seen that this comprehensive term includes two
+sorts of events which are sometimes readily distinguishable. There
+is the event by which the cell _divides_, and the event by which the
+two halves or their descendants are or may be _differentiated_. It
+is common knowledge that in some cell-divisions two similar halves,
+indistinguishable in appearance, properties, and subsequent fate, may
+be produced, while in other divisions daughter-cells with distinct
+properties and powers are formed. We cannot imagine but that in the
+first case, when the resulting cells are identical, the division is a
+mechanical process by which the mother-cell is simply cut in two; while
+in order that two differentiated halves may be produced, some event must
+have taken place by which a chemical distinction between the two halves
+is effected.[1] In any ordinary Mendelian case we have a clear proof
+that such a chemical difference may be established between germ-cells.
+The facts of colour-inheritance for instance prove that germ-cells,
+otherwise identical, may be formed _possessing_ the chromogen-factor
+which is necessary to the formation of colour in the flowers, or
+_destitute_ of that factor. Similarly the germ-cells may possess the
+ferment which, by its action on the chromogenic substance, produces the
+colour, or they may be without that ferment. The same line of argument
+applied to a great range of cases. Nevertheless, though differences
+in chemical properties are often thus constituted by cell-divisions,
+and though we are thus able to make a quasi-chemical analysis of the
+individual by determining and enumerating these properties, yet it is
+evident that the distribution of these factors is not itself a chemical
+process. This is proved by the fact that similar divisions may be
+effected between halves which are exactly alike, and also by the fact
+that the numbers in which the various types of germ-cells are formed
+negative any suggestion of valency between them. The recognition of the
+unit-factors may lead--indeed must lead--to great advances in chemical
+physiology which without that clue would have been impossible, but
+in causation the chemical phenomena of heredity must be regarded as
+secondary to the physical or mechanical phenomena by which the cells
+and their constituents are divided and separated. When therefore we
+speak of the _essential_ phenomena of heredity we mean the mechanics
+of division, especially, though not, as we shall see, exclusively, of
+_cell_-division; and in the relation between the two halves of the
+dividing cell we have the problem presented in what seems to be its
+simplest form.
+
+In attempting to form some conception of the processes by which bodily
+characteristics are transmitted, or--to avoid that confusing metaphor
+of "transmission"--how it comes about that the offspring can grow to
+resemble its parent, continuity of the germ-substance which in some
+animals is a visible phenomenon,[2] gives at least apparent help. An egg
+for example on becoming adult develops in certain parts a particular
+pigment. The eggs of that adult when they reach the appropriate age
+develop the same pigment. We have no clear picture of the mechanism by
+which this process is effected, but when we realise that the pigment
+results from the interaction of certain substances, and that since
+all the eggs are in reality pieces of the same material, it seems,
+unless we inquire closely, not unnatural that the several pieces of the
+material should exhibit the same colours at the same periods of their
+development. The continuity of the material of the germs suggests that
+there is a continuity of the materials from which the pigment is formed,
+and that thus an actual bit of those substances passes into each egg
+ready at the appropriate moment to generate the pigment. The argument
+thus outlined applies to all _substantive_ characteristics. In each case
+we can imagine, if we will, the appearance of that characteristic as due
+to the contribution of its rudiment from the germ tissues.
+
+When we consider more critically it becomes evident that the aid given
+by this mental picture is of very doubtful reality, for even if it were
+true that any predestined particle actually corresponding with the
+pigment-forming materials is definitely passed on from germ to germ,
+yet the power of increase which must be attributed to it remains so
+incomprehensible that the mystery is hardly at all illuminated.
+
+When however we pass from the substantive to the meristic characters,
+the conception that the character depends on the possession by the germ
+of a particle of a specific material becomes even less plausible. Hardly
+by any effort of imagination can we see any way by which the division
+of the vertebral column into _x_ segments or into _y_ segments, or of a
+Medusa into 4 segments or into 6, can be determined by the possession
+or by the want of a material particle. The distinction must surely be
+of a different order. If we are to look for a physical analogy at all
+we should rather be led to suppose that these differences in segmental
+numbers corresponded with changes in the amplitude or number of dividing
+waves than with any change in the substance or material divided.
+
+
+                     PHENOMENA OF DIVISION
+
+I have said that in the division of a cell we seem to see the problem in
+its simplest form, but it is important to observe that the problem of
+division may be presented by the bodies of animals and plants in forms
+which are independent of the divisions between cells. The existence of
+pattern implies a repetition of parts, and repetition of parts when
+developed in a material originally homogeneous can only be created by
+division. Cell-division is probably only a special case of a process
+similar to that by which the pattern of the skeleton is laid down in
+a unicellular body such as that of a Radiolarian or Foraminiferan.
+Attempts have lately been made to apply mathematical treatment to
+problems of biology. It has sometimes seemed to me that it is in the
+geometrical phenomena of life that the most hopeful field for the
+introduction of mathematics will be found. If anyone will compare one
+of our animal patterns, say that of a zebra's hide, with patterns known
+to be of purely mechanical production, he will need no argument to
+convince him that there must be an essential similarity between the
+processes by which the two kinds of patterns were made and that parts
+at least of the analysis applicable to the mechanical patterns are
+applicable to the zebra stripes also. Patterns mechanically produced are
+of many and very diverse kinds. One of the most familiar examples, and
+one presenting some especially striking analogies to organic patterns,
+is that provided by the ripples of a mackerel sky, or those made in a
+flat sandy beach by the wind or the ebbing tide. With a little search
+we can find among the ripple-marks, and in other patterns produced by
+simple physical means, the closest parallels to all the phenomena of
+striping as we see them in our animals. The forking of the stripes, the
+differentiation of two "faces," the deflections round the limbs and
+so forth, which in the body we know to be phenomena of division, are
+common both to the mechanical and the animal patterns. We cannot tell
+what in the zebra corresponds to the wind or the flow of the current,
+but we can perceive that in the distribution of the pigments, that
+is to say, of the chromogen-substances or of the ferments which act
+upon them, a rhythmical disturbance has been set up which has produced
+the pattern we see; and I think we are entitled to the inference that
+in the formation of patterns in animals and plants mechanical forces
+are operating which ought to be, and will prove to be, capable of
+mathematical analysis. The comparison between the striping of a living
+organism and the sand-ripples will serve us yet a little farther,
+for a pattern may either be formed by actual cell-divisions, and
+the distribution of differentiation coincidently determined, or--as
+visibly in the pigmentation of many animal and plant tissues--the
+pattern may be laid down and the pigment (for example) distributed
+through a tissue across or independently of the cell-divisions of the
+tissue. Our tissues therefore are like a beach composed of sands of
+different kinds, and different kinds of sands may show distinct and
+interpenetrating ripples. When the essential analogy between these
+various classes of phenomena is perceived, no one will be astonished at,
+or reluctant to admit, the reality of discontinuity in Variation, and if
+we are as far as ever from knowing the actual causation of pattern we
+ought not to feel surprised that it may arise suddenly or be suddenly
+modified in descent. Biologists have felt it easier to conceive the
+evolution of a striped animal like a zebra from a self-coloured type
+like a horse (or of the self-coloured from the striped) as a process
+involving many intergradational steps; but so far as the _pattern_ is
+concerned, the change may have been decided by a single event, just as
+the multitudinous and ordered rippling of a beach may be created or
+obliterated at one tide.
+
+[Illustration: FIG. 1. Tusk of Indian elephant, showing an abnormal
+segmentation.]
+
+This point is well illustrated by the tusk of an Indian elephant which
+I lately found in a London sale-room. This tusk is by some unknown
+cause, presumably a chronic inflammation, thrown up into thirteen
+well-marked ridges which closely simulate a series of segments (Fig. 1).
+Whatever the cause the condition shows how easily a normally unsegmented
+structure may be converted into a series of repeated parts.
+
+The spread of segmentation through tissues normally unsegmented is very
+clearly exemplified in the skates' jaws shown in Fig. 2. The right
+side of the upper figure shows the normal arrangement in the species
+_Rhinoptera jussieui_, but the structure on the left side is very
+different. The probable relations of the several rows of teeth to the
+normal rows is indicated by the lettering, but it is evident that by
+the appearance of new planes of division constituting separate centers
+of growth, the series has been recast. The pattern of the left side is
+so definite that had the variation affected the right side also, no
+systematist would have hesitated to give the specimen a new specific
+name. The other two drawings show similar variations of a less extensive
+kind, the nature of which is explained by the lettering of the rows of
+teeth.
+
+[Illustration: FIG. 2. Jaws of Skates (_Rhinoptera_) showing meristic
+variation. (For a detailed discussion see _Materials for the Study of
+Variation_, p. 259.)]
+
+This power to divide is a fundamental attribute of life, and of that
+power cell-division is a special example. In regard to almost all the
+chief vital phenomena we can say with truth that science has made some
+progress. If I mention respiration, metabolism, digestion, each of these
+words calls to mind something more than a bare statement that such
+acts are performed by an animal or a plant. Each stands for volumes of
+successful experiment and research, But the expression cell-division,
+the fundamental act which typifies the rest, and on which they all
+depend, remains a bare name. We can see with the microscope the outward
+symptoms of division, but we have no surmise as to the nature of the
+process by which the division is begun or accomplished. I know nothing
+which to a man well trained in scientific knowledge and method brings
+so vivid a realisation of our ignorance of the nature of life as the
+mystery of cell-division. What is a living thing? The best answer in
+few words that I know is one which my old teacher, Michael Foster, used
+to give in his lectures introductory to biology. "A living thing is a
+vortex of chemical and molecular change." This description gives much,
+if not all, that is of the essence of life. The living thing is unlike
+ordinary matter in the fact that, through it, matter is always passing.
+Matter is essential to it; but, provided that the flow in and out is
+unimpeded, the life-process can go on so far as we know indefinitely.
+Yet the living "vortex" differs from all others in the fact that it
+can divide and throw off other "vortices," through which again matter
+continually swirls.
+
+We may perhaps take the parallel a stage further. A simple vortex, like
+a smoke-ring, if projected in a suitable way will twist and form two
+rings. If each loop as it is formed could grow and then twist again to
+form more loops, we should have a model representing several of the
+essential features of living things.
+
+It is this power of spontaneous division which most sharply
+distinguishes the living from the non-living. In the excellent book
+dealing with the problems of development, lately published by Mr.
+Jenkinson a special emphasis is very properly laid on the distinction
+between the processes of division, and those of differentiation. Too
+often in discussions of the developmental processes the distinction
+is obscured. He regards differentiation as the "central difficulty."
+"Growth and division of the nucleus and the cells," he tells us,
+are side-issues. This view is quite defensible, but I suspect that
+the division _is_ the central difficulty, and that if we could get
+a rationale of what is happening in cell-division we should not be
+long before we had a clue to the nature of differentiation. It may be
+self-deception, but I do not feel it impossible to form some hypothesis
+as to the mode of differentiation, but in no mood of freest speculation
+are we ever able to form a guess as to the nature of the division. We
+see differentiations occurring in the course of chemical action, in
+some phenomena of vibration and so forth: but where do we see anything
+like the spontaneous division of the living cell? Excite a gold-leaf
+electroscope, and the leaves separate, but we know that is because they
+were double before. In electrolysis various substances separate out at
+the positive and negative poles respectively. Now if in cell-division
+the two daughter-cells were always dissimilar--that is to say, if
+differentiation always occurred--we could conceive some rough comparison
+with such dissociations. But we know the dissimilarity between
+daughter-cells is not essential. In the reproduction of unicellular
+organisms and many other cases, the products formed at the two poles
+are, so far as we can tell, identical. Any assumption to the contrary,
+if we were disposed to make it, would involve us in difficulties still
+more serious. At any rate, therefore, if differentiation be really the
+central difficulty in development, it is division which is the essential
+problem of heredity.
+
+Sir George Darwin and Professor Jeans tell us that "gravitational
+instability" consequent on the condensation of gases is "the primary
+agent at work in the actual evolution of the universe," which has led to
+the division of the heavenly bodies. The greatest advance I can conceive
+in biology would be the discovery of the nature of the instability which
+leads to the continual division of the cell. When I look at a dividing
+cell I feel as an astronomer might do if he beheld the formation of a
+double star: that an original act of creation is taking place before me.
+Enigmatical as the phenomenon seems, I am not without hope that, if it
+were studied for its own sake, dissociated from the complications which
+obscure it when regarded as a mere incident in development, some hint as
+to the nature of division could be found. It is I fear a problem rather
+for the physicist than for the biologist. The sentiment may not be a
+popular one to utter before an assembly of biologists, but looking at
+the truth impersonally I suspect that when at length minds of first rate
+analytical power are attracted to biological problems, some advance will
+be made of the kind which we are awaiting.
+
+The study of the phenomena of bodily symmetry offers perhaps the
+most hopeful point of attack. The essential fact in reproduction is
+cell-division, and the essential basis of hereditary resemblance is
+the symmetry of cell-division. The phenomena of twinning provide a
+convincing demonstration that this is so. By twinning we mean the
+production of equivalent structures by division. The process is one
+which may affect the whole body of an animal or plant, or certain of
+its parts. The term twin as ordinarily used refers to the simultaneous
+birth of two individuals. Those who are naturalists know that such twins
+are of two kinds, (1) twins that are not more alike than any other
+two members of the same family, and (2) twins that are so much alike
+that even intimate friends mistake them. These latter twins, except in
+imaginative literature, are always of the same sex.
+
+It is scarcely necessary for me to repeat the evidence from which it has
+been concluded that without doubt such twins arise by division of the
+same fertilised ovum. There is a perfect series of gradations connecting
+them with the various forms of double monsters united by homologous
+parts. They have been shown several times to be enclosed in the same
+chorion, and the proofs of experimental embryology show that in several
+animals by the separation of the two first hemispheres of a dividing egg
+twins can be produced. Lastly we have recently had the extraordinarily
+interesting demonstration of Loeb, to which I may specially refer.
+Herbst some years ago found that in sea water, from which all lime salts
+had been removed, the segments of the living egg fall apart as they are
+formed. Using this method Loeb has shown that a temporary immersion in
+lime-free sea water may result in the production of 90 per cent. of
+twins. We are therefore safe in regarding the homologous or "identical"
+twins as resulting from the divisions of one fertilised egg, while the
+non-identical or "fraternal" twins, as they are called, arise by the
+fertilisation of two separate ova.[3]
+
+In the resemblance of identical twins we have an extreme case of
+hereditary likeness[4] and a proof, if any were needed, that the
+cause of individual variation is to be sought in the differentiation
+of germ-cells. The resemblance of identical twins depends on two
+circumstances, First, since only two germ-cells take part in their
+production, difference between the germ cells of the same individual
+cannot affect them. Secondly the division of the fertilised ovum,
+the process by which they became two instead of one, must have been
+a symmetrical division. The structure of twins raises however one
+extremely significant difficulty, which as yet we cannot in any way
+explain. The resemblance between twins is a phenomenon of symmetry,
+like the resemblance between the two sides of a bilaterally symmetrical
+body. Not only is the general resemblance readily so interpreted, but
+we know also that in double monsters, namely unseparated twins, various
+anatomical abnormalities shown by the one half-body are frequently shown
+by the other half-also.[5] The two belong to one system of symmetry.
+How then does it happen that the body of one of a pair of twins does
+not show a transposition of viscera? We know that the relation of right
+and left implies that the one should be the mirror-image of the other.
+Such a relation of images may be maintained even in minute details.
+For example if the same pattern of finger-print is given by the fingers
+of the two hands, one is the reverse of the other. In double monsters,
+namely unseparated twins, there is evidence that an inversion of viscera
+does occur with some frequency. Evidence from such cases is not so clear
+and simple as might be expected, because as a matter of fact, the heart
+and stomach, upon which the asymmetry of the viscera chiefly depend, are
+usually common to the two bodies. Duplicity generally affects either
+the anterior end alone, or the posterior end alone. The division is
+generally _from the heart forwards_, giving two heads and two pairs of
+anterior limbs on a common trunk, or _from the heart backwards_, giving
+two pairs of posterior limbs with the anterior body common. In either
+case, though the bodies may be grouped in a common system of symmetry,
+neither can be proved to show definite reversal of the parts. To see
+that reversal recourse must be had to more extreme duplications, such as
+the famous Siamese Twins. They, as a matter of fact, were an excellent
+instance of the proposition that twins are related as mirror-images,
+for both of them had eleven pairs of ribs instead of the normal twelve,
+and one of them had a partial reversal of viscera.[6] (Küchenmeister,
+_Verlagerung_, etc., p. 204.)
+
+If anyone could show how it is that neither of a pair of twins has
+transposition of viscera the whole mystery of division would, I expect,
+be greatly illuminated.[7] At present we have simply to accept the fact
+that twins, by virtue of their detachment from each other, have the
+power of resuming the polarity which is proper to any normal individual.
+It was nevertheless with great interest that I read Wilder's recent
+observation[8] that occasionally in identical twins the finger-print of
+one or both the index-fingers may be reversed, showing that there is
+after all some truth in the notion that reversal should occur in them.
+
+There is another phenomenon by twinning which, if we could understand
+it, might help. I refer to the free-martin, the subject of one of John
+Hunter's masterpieces of anatomical description. In horned cattle twin
+births are rare, and when twins of opposite sexes are born, the male
+is perfect and normal, but the reproductive organs of the female are
+deformed and sterile, being known as a free-martin. The same thing
+occasionally happens in sheep, suggesting that in sheep also twins may
+be formed by the division of one ovum; for it is impossible to suppose
+that mere development in juxtaposition can produce a change of this
+character. I mention the free-martin because it raises a question of
+absorbing interest. It is conceivable that we should interpret it by
+reference to the phenomenon of gynandromorphism, seen occasionally in
+insects, and also in birds as a great rarity. In the gynandromorph one
+side of the body is male, the other female. A bullfinch for instance has
+been described with a sharp line of division down the breast between the
+red feathers of the cock on one side and the brown feathers of the hen
+on the other. (Poll, H., _SB. Ges. Nat. Fr._, Berlin, 1909, p. 338.) In
+such cases neither side is sexually perfect. If the halves of such a
+gynandromorph came apart, perhaps one would be a free-martin.
+
+The behaviour of homologous twinning in heredity has been little
+studied. It does not exist as a normal feature in any animal which
+is amenable to experiment, and we cannot positively assert that a
+comparable phenomenon exists in plants; for in them--the Orange, for
+example--polyembryony may evidently be produced by a parthenogenetic
+development of nucellar tissue. It is possible that in Man twinning
+is due to a peculiarity of the mother, not of the father. It may and
+not rarely does descend from mother to daughter, but whether it can be
+passed on through a male generation to a daughter again, there is not
+sufficient evidence to show. The facts as far as they go are consistent
+with the inference which may be drawn from Loeb's experiment, that the
+twinning of a fertilized ovum may be determined not by the germ-cells
+which united to form it, but by the environment in which it begins to
+develop. The opinion that twinning may descend through the male directly
+has been lately expressed by Dr. J. Oliver in the _Eugenics Review_
+(1912), on the evidence of cases in which twins had occurred among the
+relations of fathers of twins, but I do not know of any comprehensive
+collection of evidence bearing on the subject.
+
+Besides twinning of the whole body a comparable duplicity of various
+parts of the same body may occur. Such divisions affect especially those
+organs which have an axis of bilateral symmetry, such as the thumb, a
+cotyledon, a median petal, the frond of a fern or the anal fin of a
+fish. From the little yet known it is clear that the genetic analysis
+of these conditions must be very difficult, but evidence of any kind
+regarding them will be valuable. We want especially to know whether
+these divisions are due to the _addition_ of some factor or power which
+enables the part to divide, or whether the division results from the
+_absence_ of something which in the normal body prevents the part from
+dividing. Breeding experiments, so far as they go, suggest that the less
+divided state is usually dominant to the more divided.[9] The two-celled
+Tomato fruit is dominant to the many-celled type. The Manx Cat's tail,
+with its suppression of caudal segmentation is a partial dominant over
+the normal tail. The tail of the Fowl in what is called the "Rumpless"
+condition is at least superficially comparable with that of the Manx
+Cat, and though the evidence is not wholly consistent, Davenport
+obtained facts indicating that this suppressed condition of the caudal
+vertebrae is an imperfect dominant.[10]
+
+Some evidence may also be derived from other examples of differences
+which at first sight appear to be substantive though they are more
+probably meristic in ultimate nature. The distinction between the
+normal and the "Angora" hair of the Rabbit is a case in point. We can
+scarcely doubt that one of the essential differences between these two
+types is that in the Angora coat the hair-follicles are more finely
+divided than they are in the normal coat, and we know that the normal,
+or less-divided condition, is dominant to the Angora, or more finely
+divided.
+
+[Illustration: FIG. 3. _I_, _II_, _III_, various degrees of syndactyly
+affecting the medius and annularis in the hand; _IV_, syndactyly
+affecting the index and medius in the foot. (After Annandale.)]
+
+In the case of the solid-hoofed or "mule-footed" swine, the evidence
+shows, as Spillman has lately pointed out,[11] that the condition
+behaves as a dominant. The essential feature of this abnormality is
+that the digits III and IV are partially united. The union is greatest
+peripherally. Sometimes the third phalanges only are joined to form one
+bone, but the second and even the first phalanges may also be compounded
+together. Here the variation is obviously meristic and consists in a
+failure to divide, the normal separation of the median digits of the
+foot being suppressed.
+
+[Illustration: FIG. 4. Case of complete syndactyly in the foot. _II_ and
+_III_, digit apparently representing the index and medius. _c_^{2} +
+_c_^{3}, bone apparently representing the middle and external cuneiform;
+_cb_, cuboid; _c_^{1}, internal cuneiform. (After Gruber.)]
+
+Webbing between the digits, in at least some of its manifestations, is
+a variation of similar nature. The family recorded by Newsholme[12]
+very clearly shows the dominance of this condition. The case is
+morphologically of great interest and must undoubtedly have a bearing on
+the problems of the mechanics of Division. In discussing the phenomena
+of syndactylism I pointed out some years ago that the digits most
+frequently united in the human hand are III and IV, while in the foot,
+union most frequently takes place between II and III.[13] In Newsholme's
+family the union was always between II and III of the foot, except in
+the case of one male who had the digits III and IV of the right _hand_
+alone webbed together. There can be little doubt that the geometrical
+system on which the foot is planned has an axis of symmetry passing
+between the digits II and III, while the corresponding axis in the hand
+passes between III and IV. Union between such digits may therefore be
+regarded as comparable with any non-division or "coalescence" of lateral
+structures in a middle line, and when as in these examples such a
+condition is shown to be a dominant we cannot avoid the inference that
+some concrete factor has the power of suppressing or inhibiting this
+division. Figs. 3 and 4 illustrate degrees of union between digits in
+the human hand and foot.
+
+It is not in question that various other forms of irregular webbing
+and coalescence of digits exist, and respecting the genetic behaviour
+of these practically nothing is as yet known. Such a case is described
+by Walker,[14] in which the first and second metacarpals of both
+feet were fused in mother and daughter, and several more are found
+in literature. Contrasted with these phenomena we have the curious
+fact that in the Pigeon, Staples-Browne found webbing of the toes a
+_recessive_ character. The question thus arises whether this webbing is
+of the same nature as that shown to be a dominant in Man, and indeed
+whether the phenomenon in pigeons is really meristic at all. There is
+some difference perceptible between the two conditions; for in Man
+there is not so much a development of a special web-like skin uniting
+the digits as a want of proper division between the digits themselves,
+and in extreme cases two digits may be represented by a single one. In
+the Pigeon I am not aware that a real union of this kind has ever been
+observed, and though the web-like skin may extend the whole length of
+the digits and be so narrow as to prevent the spread of the toes, it
+may, I think, be maintained that the unity of the digits is unimpaired.
+For the present the nature of this variation in the pigeon's feet must
+be regarded as doubtful, and we should note that if it is actually an
+example of a more perfect division being dominant to a less perfect
+division, the case is a marked exception to the general rule that
+non-division is dominant to division.
+
+Reference must also be made to the phenomenon of fasciation in the stems
+of plants. As Mendel showed in the case of _Pisum_ this condition is
+often a recessive. The appearances suggest that the difference between a
+normal and a fasciated plant consists in the inability of the fasciated
+plant to separate its lateral branches. The nature of the condition is
+however very obscure and it is equally likely that some multiplication
+of the growing point is the essential phenomenon.[15]
+
+Stockard's interesting experiments[16] illustrate this question. He
+showed that by treating the embryos of a fish (_Fundulus heteroclitus_)
+with a dilute solution of magnesium salts, various cyclopian
+monstrosities were frequently produced. These have been called cases
+of _fusion_ of the optic vesicles. I would prefer to regard them as
+cases of a division suppressed or restricted by the control of the
+environment. Conversely, the splendid discovery of Loeb, that an
+unfertilised egg will divide and develop parthenogenetically without
+fertilisation, as a consequence of exposure to various media, may be
+interpreted as suggesting that the action of those media releases
+the strains already present in the ovum, though I admit that an
+interpretation based on the converse hypothesis, that the medium acts as
+a stimulus, is as yet by no means excluded.
+
+In these cases we come nearest to the direct causation or the direct
+inhibition of a division, but the meaning of the evidence is still
+ambiguous. I incline to compare Loeb's parthenogenesis with the
+development (and of course accompanying cell-division) of dormant buds
+on stems which have been cut back.
+
+It is interesting to note that sometimes as an abnormality, the
+faculty of division gets out of hand and runs a course apparently
+uncontrolled. A remarkable instance of this condition is seen in
+_Begonia_ "_phyllomaniaca_", which breaks out into buds at any point on
+the stem, petioles, or leaves, each bud having, like other buds, the
+power of becoming a new plant if removed. We would give much to know the
+genetic properties of _B. phyllomaniaca_, and in conjunction with Mr. W.
+O. Backhouse I have for some time been experimenting with this plant.
+It proved totally sterile. Its own anthers produce no pollen, and all
+attempts to fertilise it with other species failed though the pollen of
+a great number of forms was tried.
+
+Recently however we have succeeded in making plants which are in every
+respect _Begonia phyllomaniaca_, so far as the characters of stems and
+leaves are concerned. These plants, of which we have sixteen, were made
+by fertilising _B. heracleifolia_ with _B. polyantha_. They are all
+beginning to break out in "phyllomania." As yet they have not flowered,
+but as they agree in all details with _phyllomaniaca_ there can be
+little doubt that the original plant bearing that name was a hybrid
+similarly produced. The production of "phyllomania" on a hybrid Begonia
+has also been previously recorded by Duchartre.[17] In this case the
+cross was made between _B. incarnata_ and _lucida_. The synonymy of
+the last species is unfortunately obscure, and I have not succeeded in
+repeating the experiment.
+
+[Illustration: FIG. 5. Piece of petiole of _Begonia phyllomaniaca_. The
+proximal end is to the right of the figure.]
+
+From these facts it seems practically certain that the condition is one
+which is due to the meeting of complementary factors. At first sight
+we may incline to think that the phyllomania is in some way due to the
+sterility. This however cannot be seriously maintained; for not only is
+sterility in plants not usually associated with such manifestations, but
+we know a Begonia called "Wilhelma" which is exactly _phyllomaniaca_ and
+equally sterile, though it has no trace of phyllomania. This plant arose
+in the nurseries of MM. P. Bruant of Poitiers, and has generally been
+described as a seedling of _phyllomaniaca_, but from the total sterility
+of that form this account of its origin must be set aside.
+
+[Illustration: FIG. 6. Two right hind feet of polydactyle cats. _II_
+shows the lowest development of the condition yet recorded. The digit,
+_d_^{1}, which stands as hallux is fully formed and has three phalanges.
+Both it and the digit marked _d_^{2} are formed as _left_ digits. In the
+normal hind foot of the cat the hallux is represented by a rudiment only.
+
+_I_ shows a further development of the condition. In this foot there are
+_six_ digits. _d_^{1} has two phalanges, but both it and _d_^{2} and
+_d_^{3} are shaped as left digits. Thus _d_^{3}, which in the normal
+foot would be shaped as a right digit, is transformed so as to look like
+a _left_ digit.]
+
+The phenomenon in this case can hardly be regarded as due to the
+excitation of dormant buds, for it is apparent on examination that
+the new growths are not placed in any fixed geometrical relation to
+the original plant. They arise on the petiole, for example, as small
+green outgrowths each of which gradually becomes a tiny leaf. The
+attitude of these leaves is quite indeterminate, and they may point
+in any direction, some having their apices turned peripherally, some
+centrally, and others in various oblique or transverse positions (Fig.
+5). These little leaves are thus comparable with seedlings, in that
+their polarity is not related to, or consequent upon that of the parent
+plant. They have in fact that "individuality," which we associate with
+germinal reproduction.
+
+There are many curious phenomena seen in the behaviour of parts normally
+repeated in bilateral symmetry which may some day guide us towards
+an understanding of the mechanics of division. A part like a hand,
+which needs the other hand to complete its symmetry, cannot twin by
+mere division, yet by proliferation and special modifications on the
+radial side of the same limb, even a hand may be twinned. In the well
+known polydactyle cats a change of this kind is very common and indeed
+almost the rule. When extra digits appear at the inner (tibial) side
+of the limb, they are shaped as digits of the other side, and even the
+normal digit II (index) is usually converted into the mirror-image
+of its normal self. The limb then develops a new symmetry in itself.
+Nevertheless it is not easy to interpret these facts as meaning that
+there has been some interruption in the control which one side of the
+body exercises over the other. The heredity of polydactylism is complex
+but there is little doubt that the condition familiar in the Cat is a
+dominant. In some human cases also the descent is that of a dominant,
+but irregularities are so frequent that no general rule can yet be
+perceived. The dominance of such a condition is an exception to the
+principle that the less-divided is usually dominant to the more-divided,
+a fact which probably should be interpreted as meaning that divisions
+are of more than one kind.
+
+Among ordinary somatic divisions, whether of organs, cells, or patterns
+of differentiation, the control of symmetry is usually manifested. There
+is however one class of somatic differentiations which are exceptionally
+interesting from the fact that they may show a complete independence
+of such geometrical control. The most familiar examples of these
+geometrically uncontrolled Variations are to be seen in bud-sports.
+The normal differentiation of the organs of a plant is arranged on
+a definite geometrical system, which to those who have never given
+special attention to such things before, will often seem surprisingly
+precise. The arrangement of the leaves on uninjured, free-growing shoots
+can generally be seen to follow a very definite order, just as do the
+flowers or the parts of the flowers. If however bud sports occur,
+then though the parts included in the sports show all the geometrical
+peculiarities proper to the sport-variety, yet the sporting-buds
+themselves are not related to each other according to any geometrical
+plan.
+
+A very familiar illustration is provided by the distribution of colour
+in those Carnations that are not self-coloured. The pigment may, as in
+Picotees, be distributed peripherally with great regularity to the edges
+of the petals; or, as in Bizarres and Flakes, it may be scattered in
+radial sectors which show no geometrical regularity. Now in this case
+the pigments are the same in both types of flower, and the chemical
+factors concerned in their production must surely be the same. The
+difference must lie in the mechanical processes of distribution of
+the pigment. In the Picotee we see the orderly differentiation which
+we associate with normality; in the Bizarre we see the disorderly
+differentiation characteristic of bud-sports. The distribution of colour
+in this case lies outside the scheme of symmetry of the plant.
+
+Such a distribution is characteristic of bud-sports, and of certain
+other differentiations in both plants and animals, which I cannot on
+this occasion discuss. Now reflexion will show that these facts have
+an intimate bearing on the mechanical problems of heredity. For first
+in the bud-sports we are witnessing the distribution of factors which
+distinguish genetic varieties. We do not know the physical nature of
+those factors, but if we must give them a name, I suppose we should call
+them "ferments" exactly as Boyle did in 1666. He is discussing how it
+comes about that a bud, budded on a stock, becomes a branch bearing the
+fruit of its special kind. He notes that though the bud inserted be "not
+so big oftentimes as a Pea," yet "whether by the help of some peculiar
+kind of Strainer or by the Operation of some powerful Ferment lodged in
+it, or by both these, or some other cause," the sap is "so far changed
+as to constitute a Fruit quite otherwise qualify'd."[18] We can add
+nothing to his speculation, and we believe still that by a differential
+distribution of "ferments" the sports are produced. All the factors are
+together present in the normal parts; some are left out in the sport. In
+an analogous case however, that of a variegated _Pelargonium_ which has
+green and also albino shoots, Baur proved that the shoots pure in colour
+are also pure in their posterity. There can be no doubt that the sports
+of Carnations, Azaleas, Chrysanthemums, etc., would behave in the same
+way.
+
+The well-known Azaleas Perle de Ledeburg, President Kerchove,
+and _Vervaeana_ are familiar illustrations. Perle de Ledeburg is
+predominantly white, but it has red streaks in some of its flowers. It
+not very rarely gives off a self-red sport. This is evidently due to
+the development of a bud in a red-bearing area of the stem. The red in
+this plant is not under "geometrical control." Many plants have white
+flowers with no markings, but if the red markings are geometrically
+ordered differentiations, no self-coloured sports are formed. The case
+of _Vervaeana_ is a good illustration of this proposition. It has white
+flowers with red markings arranged in an orderly manner on the lower
+parts of the petals, especially on the dorsal petals. This is one of the
+Azaleas most liable to have red sports, and at first sight it might seem
+that the sport represented the red of the central marks. Examination
+however of a good many flowers shows that irregular red streaks like
+those of Perle de Ledeburg occur, about as commonly as in that variety.
+_Vervaeana_ in fact is Perle de Ledeburg with _definite_ red markings
+added, and its red sports obviously are those branches the germs of
+which came in a patch of the stem bearing these red elements. That this
+is the true account is rendered quite obvious by the fact that the red
+of the sport is a colour somewhat different from that of the definite
+marks, and that these marks are still present on the red ground of the
+sporting flowers.
+
+It will be understood that these remarks apply to those cases in which
+the production of sports is habitual or frequent, and I imagine in
+all such examples it will be found that there are indications of
+irregularity in the distribution of the differentiations such as to
+justify the view that they are not under that geometrical control which
+governs the normal differentiation of the parts. The question next
+arises whether these considerations apply also to the production of a
+bud-sport as a rare exception, but by the nature of the case it is not
+possible to say positively whether the appearance of an exceptional
+sport is due to the unsuspected presence of a pre-existing fragment of
+material having a special constitution, or to the origin, _de novo_, of
+such a material. For instance one of the garden forms of _Pelargonium_
+known as _altum_ is liable perhaps once in some hundreds of flowers to
+have one or two magenta petals. The normal colour is a brilliant red;
+and as we may be fairly sure that this red is recessive to magenta the
+interpretation would be quite different according as the appearance of
+the magenta is regarded as due to the presence of small areas endowed
+with magentaness, or to the spontaneous generation of the factor for
+that pigment. Either interpretation is possible on the facts, but the
+view that the whole plant has in it scarce mosaic particles of magenta
+seems on the whole more consistent with present knowledge.
+
+In _Pelargonium altum_ the enzyme causing the magenta colours must
+be distributed in very small areas, but a case in which the magenta
+is similarly arranged in a much coarser patchwork may be seen in the
+_Pelargonium_ "Don Juan," which often bears whole trusses or branches of
+red flowers upon plants having the normal dominant magenta trusses. In
+most cases there is little doubt that though the magenta flowered parts
+can "sport" to red, the red parts could not produce the magenta flowers.
+
+The asymmetrical, or to speak more precisely, the disorderly, mingling
+of the colours in the somatic parts is thus an indication of a similarly
+disorderly mixing of the factors for those colours in the germ-tissues,
+so that some of the gametes bear enough of the colour-factors to make a
+self-coloured plant, while others bear so little that the plant to which
+they give rise is a patchwork. If this view is correct we may extend it
+so far as to consider whether the fineness or coarseness of the mixture
+visible in the flowers or leaves may not give an indication of the
+degree to which the factors are subdivided among the germ-cells. We know
+very little about the genetic properties of striped varieties. In both
+_Antirrhinum_ and _Mirabilis_ it has been found that the striped may
+occasionally and irregularly throw self-coloured plants, and therefore
+the striping cannot be regarded simply as a recessive character. On the
+other hand in _Primula Sinensis_ there are well-known flaked varieties
+which ordinarily at least breed true. Whether these ever throw selfs I
+do not know, but if they do it must be quite exceptionally. The power
+of these flaked plants to breed true is, I suspect, connected with the
+fact that in their flowers the coloured and white parts are _intimately_
+mixed, this intimate mixture thus being an indication of a similarly
+intimate mixture in the germ-cells. It would be important to ascertain
+whether self-fertilised seed from the occasional flowers in which the
+colour has run together to join a large patch gives more self-coloured
+plants than the intimately flaked flowers do.
+
+The next fact may eventually prove of great importance. We have seen
+that in bud-sports the differentiation is of the same nature as
+that between pure types, and also that in the sporting plant this
+differentiation is distributed without any reference to the plant's
+axis, or any other consideration of symmetry. Now among the germ-cells
+of a Mendelian hybrid exactly such characters are being distributed
+allelomorphically, and there again we have strong evidence for believing
+that the distribution obeys no pattern. For example, we can in the case
+of seeds still _in situ_ perceive how the characters were distributed
+among the germ-cells, and there is certainly no obvious pattern
+connecting them, nor can we suppose that there is an actual pattern
+obscured.
+
+Of this one illustration is especially curious. Individual plants of
+the same species are, as regards the decussations of their leaves and
+in other respects, _either rights or lefts_. The fact is not emphasized
+in modern botany and is in some danger of being forgotten. When, as
+in the flowers of Arum, some _Gladioli_, _Exacum_, _St. Paulia_, or
+the fruits of _Loasa_, rights and lefts occur on the same stem,
+they come off alternately. But if, as in the seedlings of Barley the
+twist of the first leaf be examined, it will be seen to be either a
+right-or left-handed screw. An ear of barley, say a two-row barley,
+is a definitely symmetrical structure. The seeds stand in their
+envelopes back to back in definite positions. Each has its organs
+placed in perfectly definite places. _If these seeds were buds_ their
+differentiations would be grouped into a common plan. One might expect
+that the differentiations of these embryos would still fall into the
+pattern; but they do not, and so far as I have tested them, any one
+may be a right or a left, just as each may carry any of the Mendelian
+allelomorphs possessed by the parent plant, without reference to the
+differentiation of any other seed. The fertilisation may be responsible,
+but our experience of the allelomorphic characters suggest that the
+irregularity is in the egg-cells themselves.[19]
+
+_Germ cells thus differ from somatic cells in the fact that their
+differentiations are outside the geometrical order which governs
+the differentiation of the somatic cells._ I can think of possible
+exceptions, but I have confidence that the rule is true and I regard it
+as of great significance.
+
+The old riddle, what is an individual, finds at least a partial solution
+in the reply that an individual is a group of parts differentiated in a
+geometrically interdependent order. With the germ-cell a new geometrical
+order, with independent polarity is almost if not quite always, begun,
+and with this geometrical independence the power of rejuvenescence may
+possibly be associated.
+
+The problems thus raised are unsolved, but they do not look insoluble.
+The solution may be nearer than we have thought. In a study of the
+geometry of differentiation, germinal and somatic, there is a way
+of watching and perhaps analyzing what may be distinguished as the
+mechanical phenomena of heredity. If any one could in the cases of
+the Picotee and the Bizarre Carnation, respectively, detect the real
+distinction between the two types of distribution, he would make a
+most notable advance. Any one acquainted with mechanical devices can
+construct a model which will reproduce some of these distinctions more
+or less faithfully. The point I would not lose sight of is that the
+analogy with such models must for a long way be a true and valuable
+guide. I trust that some one with the right intellectual equipment will
+endeavor to follow this guide; and I am sanguine enough to think that a
+comprehensive study of the geometrical phenomena of differentiation will
+suggest to a penetrative mind that critical experiment which may one day
+reveal the meaning of spontaneous division, the mystery through which
+lies the road, perhaps the most hopeful, to a knowledge of the nature of
+life.
+
+FOOTNOTES:
+
+[1] In saying this we make no assumption as to the particular
+cell-division at which differentiation occurs. This may be one of the
+maturation-divisions, or it may perhaps be much earlier.
+
+[2] From the recent discoveries of Erwin Baur we are led to surmise
+that in the flowering plants the sub-epidermal layer, or some of its
+elements, may legitimately be regarded as a similar germ-substance,
+continuous in Weismann's sense.
+
+[3] These fraternal twins, which show no special resemblance to each
+other, are like the multiple births of other animals, and there is no
+disposition for them to be of the same sex. In the sheep, for example,
+statistics show that the frequency of pairs of twins, male and female,
+is approximately double that of the frequency of pairs, both male or
+both female, as it should be if the sex-distribution were fortuitous.
+For instance Bernadin (_La Bergerie de Rambouillet_, 1890, p. 100)
+gives the following figures for twin-lambs in Merinos: both male, 87;
+both female, 83; sexes mixed, 187. The 9-banded Armadillo (_Dasypus
+novemcinctus_), in which the young born in one litter are said to be
+always of one sex, is the only known exception in Vertebrates, and
+is presumably a genuine case of normal polyembryony (see especially,
+Rosner, _Bull. Ac. Soc. Cracovie_, 1901, p. 443, and Newman and
+Patterson, _Biol. Bull._, XVII, 1909, p. 181), and an important paper
+lately published by H. H. Newman and J. T. Patterson, _Jour. Morph._,
+1911, XXII, p. 855.
+
+[4] A good collection of evidence as to disease in homologous twins was
+lately published by E. A. Cockayne, _Brit. Jour. Child. Diseases_, Nov.,
+1911.
+
+[5] Cp. Windle, B. C. A., _Jour. Anal. Phys._, XXVI, p. 295.
+
+[6] Mr. E. Nettleship tells me that in the course of collecting
+pedigrees of families containing colour-blind members he has discovered
+two cases (shortly to be published) of pairs of twins, which on account
+of their very close resemblances must be deemed homologous, one of each
+pair being colour-blind and the other normal. Such a distinction between
+closely similar twins is most curious and unexpected.
+
+[7] Another paradoxical phenomenon of the same nature occurs in the
+Narwhal The males normally have the _left_ tusk alone developed, the
+corresponding right tusk remaining as an undeveloped rudiment in its
+socket. The left tusk is a left-handed screw. Occasionally the right
+tusk is also developed and grows to the same length as that of the
+left side, but in such specimens the right tusk is also a left-hand
+screw like the tusk of the other side, instead of being reversed as we
+should certainly have expected. It need scarcely be remarked that in the
+case of the horns of antelopes, and in other examples of spiral organs
+arranged in pairs, that of one side of the body is the mirror image of
+that on the other side. The Narwhal's tusks in being both twisted in the
+same direction are thus highly anomalous, and are comparable with pairs
+of twins.
+
+[8] Wilder, H. H., _Amer. Jour. Anat._, 1904, III, p. 452.
+
+[9] Polydactylism which is often a dominant and the web-foot of Pigeons
+which is recessive should be remembered as possible exceptions (see p.
+49).
+
+[10] Davenport inclined at first to regard rumplessness as a recessive,
+but in his latest publication on the subject he definitely concludes
+that it is an imperfect dominant. This conclusion accords well with
+evidence quoted by Darwin (_An. and Plts._, II, ed. 2, p. 4) that
+rumpless fowls may throw tailed offspring. (_Amer. Nat._, 1910, XLIV, p.
+134.)
+
+[11] Spillman, W. J., _Amer. Breeders Mag._, 1910, I, p. 178.
+
+[12] Newsholme, _Lancet_, December 10, 1910, p. 1690.
+
+[13] _Materials for the Study of Variation_, 1894, p. 358.
+
+[14] Walker, G., _Johns Hopkins Hospital Bulletin_, XII, 1901, p. 129.
+
+[15] Cp. R. H. Compton, _New Phytologist_, 1911, p. 249.
+
+[16] _Arch. f. Entwickelungsmech._, 1907, XXIII, p. 249.
+
+[17] Bull. Soc. Bot. de France, xxxiv, 1887, p. 182.
+
+[18] R. Boyle, _The Origine of Formes and Qualities_, Oxford, 1666.
+
+[19] Remarkable experiments on this question have lately been carried
+out by R. H. Compton (_Camb. Phil. Soc._, XV, 1910, p. 495), showing
+that in a certain Barley, "Plumage Corn," the average ratio of left to
+right is about 1.5. A fuller paper has since been published by Compton,
+_Jour. Genetics_, 1912, II, I, p. 53.
+
+
+
+
+                                CHAPTER III
+
+                    SEGMENTATION, ORGANIC AND MECHANICAL
+
+Models may be and often have been devised imitating some of the
+phenomena of division, but none of them have reproduced the peculiarity
+which characterises divisions of living tissues, that _the position
+of chemical differentiation_ is _determined by those divisions_. For
+example, models of segmentation, whether radial or linear, may be made
+by the vibration of plates as in the familiar Chladni figures of the
+physical laboratory, or by the bowing of a tube dusted on the inside
+with lycopodium powder, and in various other ways. The sand or the
+powder will be heaped up in the nodes or regions of least movement, and
+the patterns thus formed reproduce many of the geometrical features
+of segmentation. But in the segmentations of living things the nodes
+and internodes, once determined by the dividing forces, would each
+become the seat of appropriate and distinct chemical processes leading
+to the differentiation of the parts, and the deposition of the bones,
+petals, spines, hairs, and other organs in relation to the meristic
+ground-plan. The "ripples" of meristic division not merely divide but
+differentiate, and when a "ripple" forks the result is not merely a
+division but a reduplication of the organ through which the fork runs.
+An example illustrating such a consequence is that of the half-vertebrae
+of the Python. On the left side the vertebra is single (Fig. 7) and
+bears a single rib, but on the right side a division has occurred with
+the result that two half-vertebrae, each bearing a rib, are formed, one
+standing in succession to the other. We cannot, indeed, imagine any
+operation of physiological division carried out in such an organ as a
+vertebra, passing through a plane at right angles to the long axis of
+the body, which does not necessarily involve the further process of
+reduplication.
+
+As the meristic system of distribution spreads through the body,
+chemical differentiations follow in its track, with segmentation and
+pattern as the visible result. Could we analyse these simultaneous
+phenomena and show how it is that the places of chemical differentiation
+are determined by the system of division, progress would then be rapid.
+It is here that all speculation fails.
+
+[Illustration: FIGS. 7 and 8. Two examples of imperfect division in the
+vertebræ of a python. _I_, the vertebræ 147-150 from the right side,
+showing imperfect division between the 148th and 149th. The condition on
+the left side of this vertebra was the same. _II_, the dorsal surface of
+vertebræ 165-167. On the right side the 166th is double and bears two
+ribs, but on the left side it is normal and has one rib only.]
+
+Many attempts have been made to interpret the processes of division
+and repetition, in terms of mechanics, or at least to refer them to
+their nearest mechanical analogies, so far with little success. The
+problem is beset with difficulties as yet insurmountable and of these
+one must be especially noticed. In the living thing the process by which
+repetition and patterns come into being consists partly in division but
+partly also in growth. We have no means of studying the phenomena of
+pattern-formation except in association with that of growth. Growth soon
+ceases unless division takes place, and if growth is impossible division
+soon ceases also. In consequence of this fact that the final pattern
+is partly a product of growth, it can never be used as unimpeachable
+evidence of the primary geometrical relations of the members as laid
+down in the divisions.
+
+In the last chapter in referring to the problem of repetition I
+introduced an analogy, comparing the patterns of the organic world
+with those produced in unorganised materials by wave-motion. In the
+preliminary stage of ignorance, having no more trustworthy clue, I do
+not think it wholly unprofitable to consider the applicability of this
+analogy somewhat more fully. It possesses, as I hope to show, at least
+so much validity as to encourage the belief that morphology may safely
+discard one source of long-standing error and confusion.
+
+Those who have studied the structure of parts repeated in series will
+have encountered the old morphological problem of "Serial Homology,"
+which has absorbed so much of the attention of naturalists and
+especially of zoologists at various periods. This problem includes
+two separate questions. The first of these is the origin in evolution
+of the resemblance between two organs occurring in a repeated series,
+of which the fore and hind limbs of Vertebrates are the prerogative
+instance. From the fact that these resemblances can be traced very far,
+often into minute details of structure, many anatomists have inclined to
+the opinion that the resemblance must originally have been still more
+complete, and that the two limbs, for instance, must have acquired their
+present forms by the differentiation of two identical groups of parts.
+
+Similar questions arise whenever parts are repeated in series, whether
+the series be linear or radial, and, though less obviously, even when
+the repetition is bilateral only. In each such example the question
+arises, is the resemblance between the parts the remains of a still
+closer resemblance, or is differentiation original? Sometimes the
+view that these parts have arisen by the differentiation of a series
+of identical parts is plausible enough, as for example when the
+peculiarities of various appendages of a Decapod Crustacean are referred
+to modifications of the Phyllopod series. In application to other cases
+however we soon meet with difficulty, and the suggestion that the
+segments of a vertebrate were originally all alike is seen at once to be
+absurd, for the reason that a creature so constituted could not exist,
+and that, differentiation of at least one anterior and one posterior
+segment, is an essential condition of a viable organism consisting of
+parts repeated in a linear series. Between these two terminal segments
+it is possible to imagine the addition of one segment, or of a series
+of approximately similar segments; but when once it is realised that
+the terminals must have been differentiated from the beginning, it
+will be seen that the problem of the origin of the resemblance between
+segments is not rendered more comprehensible by the suggestion that
+even the intervening members were originally alike. Seeing indeed that
+some differentiation must have existed primordially it is as easy to
+imagine that the original body was composed of a series grading from the
+condition of the anterior segment to that of the posterior, as any other
+arrangement. The existence of a linear or successive series in fact
+postulates a polarity of the whole, and in such a system the conception
+of an ideal segment containing all the parts represented in the others
+has manifestly no place. The introduction of that conception though
+sanctioned by the great masters of comparative anatomy, has, as I think,
+really delayed the progress of a rational study of the phenomena of
+division. The same notion has been applied to every class of repetition
+both in animals and plants, generally with the same unhappy results. In
+the cruder forms in which this doctrine was taught thirty years ago it
+is now seldom expressed, but modified presentations of it still survive
+and confuse our judgments.
+
+The process of repetition of parts in the bodies of organisms is
+however a periodic phenomenon. This much, provided we remain free from
+prejudice as to the nature and causation of the period or rhythm, we
+may safely declare, and a comparison may thus be instituted between the
+consequences of meristic repetition in the bodies of living things and
+those repetitions which in the inorganic world are due to rhythmical
+processes. Of such processes there is a practically unlimited diversity
+and we have nothing to indicate with which of them our repetitions
+should rather be compared.
+
+[Illustration: FIG. 9. Osmotic growths simulating segmentation. (After
+Leduc.)]
+
+In some respects perhaps the best models of living organisms yet made
+are the "osmotic growths" produced by Leduc.[1] These curious structures
+were formed by placing a fragment of a salt, for instance calcium
+chloride, in a solution of some colloidal substance. As the solid takes
+up water from the solution a permeable pellicle or membrane is formed
+around it. The vesicle thus enclosed grows by further absorption of
+water, often extending in a linear direction, and in many examples this
+growth occurs by a series of rhythmically interrupted extensions. Some
+of the growths thus formed are remarkably like organic structures,
+and might pass for a series of antennary segments or many other
+organs consisting of a linear series of repeated parts. In admitting
+the essential resemblance between these "osmotic growths" and living
+bodies or their organs I lay less stress on the general conformation
+of the growths, which often as Leduc points out, recall the forms
+of fungi or hydroids, but rather on the fact that the interruptions
+in the development of these systems are so closely analogous to the
+segmentations or repetitions of parts characteristic of living things
+(Fig. 9). In the same way I am less impressed by Leduc's models of
+Karyokinesis, wonderful as they nevertheless are, for the division is
+here imitated by putting separate drops on the gelatine film. What we
+most want to know is how in the living creature one drop becomes two.
+The models of linear segmentation have the remarkable merit that they do
+in some measure imitate the process of actual division or repetition.
+So in a somewhat modified method Leduc, by causing the diffusion
+of a solution in a gelatine film, produced rhythmical or periodic
+precipitations strikingly reminiscent of various organic tissues, for
+here also the process of periodic repetition is imitated with success.
+
+It is a feature common to these and to all other rhythmical repetitions
+produced by purely mechanical forces that there is resemblance between
+the members of the series, and that this similarity of conformation may
+be maintained in most complex detail. When however in the mechanical
+series some of the members differ from the rest we have no difficulty
+in recognising that these differences--which correspond with the
+differentiations of the organic series--are due to special heterogeneity
+in the conditions or in the materials, and it never occurs to us to
+suppose that all the members must have been primordially alike. For
+example, in the case of ripple-marks on the sand, which I choose as one
+of the most familiar and obvious illustrations of a repeated series
+due to mechanical agencies, if we notice one ripple different in form
+from those adjacent to it, we do not suppose that this variation must
+have been brought about by deformation of a ripple which was at first
+formed like the others, but we ascribe it to a difference in the sand at
+that point, or to a difference in the way in which the wind or the tide
+dealt with it. We may press the analogy further by observing that in as
+much as such a series of waves has a beginning and an end, it possesses
+polarity like that of the various linear series of parts in organisms,
+and even the formation of each member must influence the shape of its
+successor. Since in an organism the beginning and end of the series
+are always included, some differentiation among the repetitions must
+be inevitable. If therefore it be conceded, as I think it must, that
+segmentation and pattern are the consequence of a periodic process we
+realize that it is at least as easy to imagine the formation of such a
+series of parts having family likeness combined with differentiation
+as it would be to conceive of their arising primordially as a series
+of identical repetitions. The suggestion that the likenesses which we
+now perceive are the remains of a still more complete resemblance is a
+substitution of a more complex conception for a simpler one.
+
+The other question raised by the problem of Serial Homology is how far
+there is a correspondence between individual members of series when
+the series differ from each other either in the number of parts, or in
+the mode of distribution of differentiation among them. Students, for
+example, of vertebrate morphology debate whether the _n_th vertebra
+which carries the pelvic girdle in Lizard A is individually homologous
+with the _n_ + _x_th vertebra which fulfils this function in Lizard B,
+or whether it is not more truly homologous with the vertebra standing in
+the _n_th ordinal position, though that vertebra in Lizard B is free.
+
+In various and more complex aspects the same question is debated in
+regard to the cranial and spinal nerves, the branches of the aorta,
+the appendages of Arthropoda, and indeed in regard to all such series
+of differentiated parts in linear or successive repetition. Persons
+exercised with these problems should before making up their minds
+consider how similar questions would be answered in the case of any
+series of rhythmical repetitions formed by mechanical agencies. In the
+case of our illustration of the ripples in the sand, given the same
+forces acting on the same materials in the same area, the number of
+ripples produced will be the same, and the _n_th ripple counting from
+the end of the series will stand in the same place whenever the series
+is evoked. If any of the conditions be changed, the number and shapes
+can be changed too, and a fresh "distribution of differentiation"
+created. Stated in this form it is evident that the considerations
+which would guide the judgment in the case of the sand ripples are not
+essentially different from those which govern the problem of individual
+homology in its application to vertebrae, nerves, or digits.
+
+The fact that the unit of repetition is also the unit of growth is the
+source of the obscurity which veils the process. When we compare the
+skeleton of a long-tailed monkey with that of a short-tailed or tailless
+ape we see at once how readily the additional series of caudal segments
+may be described as a consequence of the propagation of the "waves" of
+segmentation beyond the point where they die out in the shorter column,
+and we see that with an extension of the series of repetitions there is
+growth and extension of material.
+
+The considerations which apply to this example will be found operating
+in many cases of the variation of terminal members of linear series.
+Some of these series, like the teeth of the dog, end in a terminal
+member of a size greatly reduced below that of the next to it. Even when
+there is thus a definite specialisation of the last member of the series
+it not infrequently happens that the addition, by variation, of a member
+beyond the normal terminal, is accompanied by a very palpable increase
+in size of the member which stands numerically in the place of the
+normal terminal.[2] So also with variation in the number of ribs, when
+a lumbar vertebra varies homoeotically into the likeness of the last
+dorsal and bears a rib, the rib placed next in front of this, which in
+the normal trunk is the last, shows a definite increase in development.
+
+The consequences of such homoeoses are sometimes very extensive,
+involving readjustments of differentiation affecting a long series of
+members, as may easily be seen by comparing the vertebral columns of
+several individual Sloths[3] (whether _Bradypus_ or _Choloepus_) to take
+a specially striking example.
+
+It may be urged that no feature as yet enables us to perceive wherein
+lies the primary distinction which determines such variation, whether it
+is due to a difference in the dividing forces or in the material to be
+divided. If for instance we were to imitate such a series of segments
+by pressing hanging drops of a viscous fluid out of a paint-tube by
+successive squeezes, the number of times the tube is contracted before
+it is empty will give the number of the segments, but their size may
+depend either on the force of the contractions or on the capacity of
+the tube, or on various other factors. Nevertheless in the case of the
+variation of terminal members, whatever be the nature of the rhythmical
+impulse which produces the series of organs, the elevation of the
+normally terminal member in correspondence with the addition of another
+is what we should expect.
+
+If the organism acquired its full size first and the delimitation of
+the parts took place afterwards, there might be some hope that the
+resemblance between living patterns and those mechanically caused by
+wave-motion might be shown to be a consequence of some real similarity
+of causation, but in view of the part played by growth, appeal to these
+mechanical phenomena cannot be declared to have more than illustrative
+value. Similarly in as much as living patterns appear, and almost
+certainly do in reality come into existence by a rhythmical process,
+comparisons of these patterns with those developed in crystalline
+structures, and in the various fields of force are, as it seems to me,
+inadmissible, or at least inappropriate.
+
+However their intermittence be determined, the rhythms of division must
+be looked upon as the immediate source of those geometrically ordered
+repetitions universally characteristic of organic life. In the same
+category we may thus group the segmentation of the Vertebrates and of
+the Arthropods, the concentric growth of the Lamellibranch shells or of
+Fishes' scales, the ripples on the horns of a goat, or the skeletons of
+the Foraminifera or of the Heliozoa. In the case of plant-structures
+Church[4] has admirably shown, with an abundance of detail, how on
+analysis the definiteness of phyllotaxis is an expression of such
+rhythm in the division of the apical tissues, and how the spirals
+and "orthostichies" displayed in the grown plant are its ultimate
+consequences. The problem thus narrows itself down to the question of
+the mode whereby these rhythms are determined.
+
+It is natural that we should incline to refer them to a chemical source.
+If we think of the illustration just given, of the segmentation of a
+viscous fluid into drops by successive contractions of a soft-walled
+tube we can, I think, conceive of such rhythmic contractions as due to
+summations of chemical stimuli, somewhat as are the beats of the heart.
+But when we recognize the vast diversity of materials the distribution
+of which is determined by an ostensibly similar rhythmic process it
+seems hopeless to look forward to a directly chemical solution. That the
+chemical degradation of protoplasm or of materials which it contains is
+the source of the energy used in the divisions cannot be in dispute, but
+that these divisions can be themselves the manifestations of chemical
+action seems in the highest degree improbable.
+
+We may therefore insist with some confidence on the distinction between
+the Meristic and the substantive constitution of organisms, between,
+that is to say, the system according to which the materials are divided
+and the essential composition of the materials, conscious of the fact
+that the energy of division is supplied from the materials, and that
+in the ontogeny the manner in which the divisions are effected must
+depend secondarily on the nature of the substances to be divided.
+The mechanical processes of division remain a distinguishable group
+of phenomena, and variations in the substances to be distributed in
+division may be independent of variations in the system by which the
+distribution is effected.
+
+Modern genetic analysis supplies many remarkable examples of this
+distinction. When formerly we compared the leaves of a normal palmatifid
+Chinese Primula with the pinnatifid leaves[5] of its fern-leaved variety
+we were quite unable to say whether the difference between the two types
+of leaf was due to a difference in the material cut up in the process
+of division or to a difference in that process itself. Knowledge that
+the distinction is determined by a single segregable factor tends to
+prove that the critical difference is one of substance. So also in the
+Silky fowl we know that the condition of its feathers is due to the
+absence of some one factor present in the normal form. We may conceive
+such differences as due to change of form in the successive "waves" of
+division, but we cannot yet imagine segregation otherwise than as acting
+by the removal or retention of a material element. Future observation
+by some novel method may suggest some other possibility, but such cases
+bring before us very clearly the difficulties by which the problem is
+beset.
+
+[Illustration: FIG. 10. The palm-and fern type of leaf in _Primula
+Sinensis_. The palm is dominant and the fern is recessive.]
+
+In another region of observation phenomena occur which as it seems to
+me put it beyond question that the meristic forces are essentially
+independent of the materials upon which they act, save, in the remoter
+sense, in so far as these materials are the sources of energy. The
+physiology of those regenerations and repetitions which follow upon
+mutilation supplies a group of facts which both stimulate and limit
+speculation. No satisfactory interpretations of these extraordinary
+occurrences has ever been found, but we already know enough to feel
+sure that in them we are witnessing indications which should lead
+to the discovery of the true mechanics of repetition and pattern.
+The consequences of mutilation in causing new growth or perhaps more
+strictly in enabling new growth to take place, are such that they cannot
+be interpreted as responses to chemical stimuli in any sense which
+the word chemical at present connotes. Powers are released by mutilation
+of which in the normal conditions of life no sign can be detected. All
+who have tried to analyse the phenomena of regeneration are compelled
+to have recourse to the metaphor of equilibrium, speaking of the normal
+body as in a state of strain or tension (Morgan) which when disturbed by
+mutilation results in new division and growth. The forces of division
+are inacessible to ordinary means of stimulation. Applications, for
+example, of heat or of electricity excite no responses of a positive
+kind unless the stimuli are so violent as to bring about actual
+destruction.[6] These agents do not, to use a loose expression, come
+into touch with the meristic forces. Changes in the chemical environment
+of cells may, as in the experiments of Loeb and of Stockard produce
+definite effects, but the facts suggest that these effects are due
+rather to alterations in the living material than to influence exerted
+directly on the forces of division themselves.
+
+By destruction of tissue however the forces both of growth and of
+division also may often be called into action with a resulting
+regeneration. Interruption of the solid connexion between the parts may
+produce the same effects, as for example when the new heads or tails
+grow on the divided edges of Planarians (Morgan), or when from each half
+embryo partially separated from its normally corresponding half, a new
+half is formed with a twin monster as the result.
+
+Often classed with regenerations but in reality quite distinct from
+them are those special and most interesting examples where the growth
+of a _paired_ structure is excited by a simple wound. Some of the best
+known of these instances are presented by the paired extra appendages
+of Insects and Crustacea. Some years ago I made an examination of all
+the examples of such monstrosities to which access was to be obtained,
+and it was with no ordinary feeling of excitement that I found that
+these supernumerary structures were commonly disposed on a recognizable
+geometrical plan, having definite spatial relations both to each other
+and to the normal limb from which they grew. The more recent researches
+of Tornier[7] and especially his experiments on the Frog have shown
+that a cut into the posterior limb-bud induces the outgrowth of such a
+_pair_ of limbs at the wounded place. Few observations can compare with
+this in novelty or significance; and though we cannot yet interpret
+these phenomena or place them in their proper relations with normal
+occurrences, we feel convinced that here is an observation which is no
+mere isolated curiosity but a discovery destined to throw a new light on
+biological mechanics. The supernumerary legs of the Frog are evidently
+grouped in a system of symmetry similar to that which those of the
+Arthropods exhibit, and though in Arthropods paired repetitions have not
+been actually produced by injury under experimental conditions we need
+now have no hesitation in referring them to these causes as Przibram has
+done.
+
+At this point some of the special features of the supernumerary
+appendages become important. First they may arise at any point on the
+normal limb, being found in all situations from the base to the apex.
+Nor are they limited as to the surface from which they spring, arising
+sometimes from the dorsal, anterior, ventral, or posterior surfaces, or
+at points intermediate between these principal surfaces.
+
+With rare and dubious exceptions, the parts which are contained in these
+extra appendages are only those which lie _peripheral to their point of
+origin_. Thus when the point of origin is in the apical joint of the
+tarsus, the extra growth if completely developed consists of a double
+tarsal apex bearing two pairs of claws. If they arise from the tibia,
+two complete tarsi are added. If they spring from the actual base of
+the appendage then two complete appendages may be developed in addition
+to the normal one. We must therefore conclude that in any point on a
+normal appendage the power exists which, if released, may produce a bud
+containing in it a paired set of the parts peripheral to this point.
+
+[Illustration: FIG. 11. Diagrams of the geometrical relations which are
+generally exhibited by extra pairs of appendages in Arthropoda. The
+sections are supposed to be those of the apex of a tibia in a beetle.
+_A_, anterior, _P_, posterior, _D_, dorsal, _V_, ventral. _M_^{1},
+_M_^{2} are the imaginary planes of reflexion. The shaded figure is in
+each case a limb formed like that of the other side of the body, and the
+outer unshaded figures are shaped like the normal for the side on which
+the appendages are. On the several radii are shown the extra pairs in
+their several possible relations to the normal from which they arise.
+The normal is drawn in thick lines in the center.]
+
+Next the geometrical relations of the halves of the supernumerary pair
+are determined by the position in which they stand in regard to the
+original appendage. These relations are best explained by the diagram
+(Fig. 11), from which it will be seen that the two supernumerary
+appendages stand as images of each other; and, of them, that which is
+adjacent to the normal appendage forms an image of it. Thus if the
+supernumerary pair arise from a point on the dorsal surface of the
+normal appendage, the two _ventral_ surfaces of the extra pair will
+face each other. If they arise on the anterior surface of the normal
+appendage, their morphologically posterior surfaces will be adjacent,
+and so on.
+
+These facts give us a view of the relations of the two halves of a
+dividing bud very different from that which is to be derived from the
+exclusive study of normal structures. Ordinary morphological conceptions
+no longer apply. The distribution of the parts shows that the bud or
+rudiment which becomes the supernumerary pair may break or open out in
+various ways according to its relations to the normal limb. Its planes
+of division are decided by its geometrical relations to the normal body.
+
+Especially curious are some of the cases in which the extra pair are
+imperfectly formed. The appearance produced is then that of two limbs
+in various stages of coalescence, though in reality of course they are
+stages of imperfect separation. The plane of "coalescence" may fall
+anywhere, and the two appendages may thus be compounded with each other
+much as an object partially immersed in mercury "compounds" with its
+optical image reflected from the surface.
+
+Supernumerary paired structures are not usually, if ever, formed when
+an appendage is simply amputated. Cases occasionally are seen which
+nevertheless seem to be of this nature. Borradaile,[8] for example,
+described a crab (_Cancer pagurus_) having in place of the right chela
+three _small_ chelae arising from a common base, where the appearances
+suggested that the three reduced limbs replaced a single normal limb.
+From the details reported however it seems still possible that one of
+the chelae (that lettered F. I in Borradaile's figure) may be the normal
+one, and the other two an extra pair. The chela which I suspect to be
+the normal is in several respects deformed as well as being reduced in
+size, and this deformity may perhaps have ensued as a consequence of the
+same wound which excited the growth of the extra pair. Its reduced size
+may be due to the same injury, which may quite well have checked its
+growth to full proportions.
+
+Admitting doubt in these ambiguous cases it seems to be a general rule
+that for the production of the extra pair the normal limb should persist
+in connexion with the body. Moreover it is practically certain that
+in no case can a _single_, viz. an unpaired, duplicate of the normal
+appendage grow from it. Many examples have been described as of this
+nature, but all of them may be with confidence regarded as instances of
+a supernumerary pair in which only the two morphologically anterior or
+the two morphologically posterior surfaces are developed. We have thus
+the paradox that a limb of one side of the body, say the right, has in
+it the power to form a pair of limbs, right and left, as an outgrowth of
+itself, but cannot form a second left limb alone.
+
+A very interesting question arises whether it is strictly correct to
+describe the extra pair as a right and a left, or whether they are not
+rather two lefts or two rights of which one is reversed. This question
+did not occur to me when in former years I studied these subjects. It
+was suggested to me by Dr. Przibram. The answer might have an important
+bearing on biological mechanics, but I know no evidence from which the
+point can be determined with certainty. In order to decide this question
+it would be necessary to have cases in which the paired repetition
+affected a limb markedly differentiated on the two sides of the body,
+and of course the development of the extra parts in order to be decisive
+must be fairly complete. One example only is known to me which at all
+satisfies these requirements, that of the lobster's chela figured (after
+Van Beneden) in _Materials for the Study of Variation_, p. 531, Fig.
+184, III.
+
+Here the drawing distinctly suggests that one of the extra
+dactylopodites, namely that lettered R, is differentiated as a left
+and not merely a reversed right. For the teeth on this dactylopodite
+are those of a cutting claw, not of a crushing claw, whereas the
+dactylopodites R' and L' bear crushing teeth. The figure makes it fairly
+certain also that the limb affected was a crushing claw. Accepting this
+interpretation, we reach the remarkable conclusion that the bud of new
+growth consisted of halves differentiated into cutter and crusher as the
+normal claws are, and that the extra crusher is geometrically a left
+but physiologically a right. Though shaped as a left in respect of the
+direction in which it points, the extra crusher is really an optically
+reversed right, while the dactylopodite R, which is placed pointing like
+a right, is really a reversed left (Fig. 12).
+
+[Illustration: FIG. 12. Right claw of lobster bearing a pair of extra
+dactylopodites (after van Beneden). The fine toothing on R suggests that
+this is part of a cutting claw, though the limb bearing it is a crusher.]
+
+If these indications are reliable[9] and are established by further
+observation we shall be led to the conclusion that the bud which
+becomes an extra pair of limbs does not merely contain the parts proper
+to the side on which it grows, but is comparable with the original
+zygotic cell, and consists not simply of two halves, but of two halves
+differentiated as a right and a left like the two halves of the normal
+body.
+
+Phenomena of this kind, evoked by mutilation or injury, together with
+the cognate observations on regeneration throw very curious lights
+on the nature of living things. To an understanding of the nature of
+the mechanics of living matter and its relation to matter at large
+they offer the most hopeful line of approach. I allude especially to
+the examples in which it has been established that the part which is
+produced after mutilation is a structure different from that which was
+removed. The term "regeneration" was introduced before such phenomena
+were discovered, and though every one recognizes its inapplicability
+to these remarkable cases, the word still misleads us by presenting
+a wrong picture to the mind. The expression "heteromorphosis" (Loeb)
+has been appropriately applied to various phenomena of this kind, and
+Morgan has given the name "morphallaxis" to another group of cases in
+which the renewal occurs by the transformation of a previously existing
+part.[10] But we must continually remember that all these occurrences
+which we know only as abnormalities and curiosities must in reality be
+exemplifications of the normal mechanics of division and growth. The
+conditions needed to call them forth are abnormal, but the responses
+which the system makes are evidences of its normal constitution. When
+therefore, for example, the posterior end of a worm produces a reversed
+tail from its cut end we have a proof that there must be in the normal
+body forces ready to cause this outgrowth. The new structure is not
+an ill-shaped head-end, for, as Morgan shows, the nephridial ducts
+have their funnels perforating the segments in a reversed direction.
+The "tension" of growth is actually reversed.[11] So also when in a
+Planarian amputation of the body immediately behind the head leads to
+the formation of a new reversed head at the back of the normal head,
+while amputation further back leads to the regeneration of a new tail,
+these responses give indications of forces normally present in the
+body of the Planarian. Such facts open up a great field of speculation
+and research. Especially important it would be to determine where the
+critical region may be at which the one response is replaced by the
+other. I suppose it is even possible that there is some neutral zone in
+which neither kind of response is made.
+
+Physical parallels to the phenomena of regeneration are not easy
+to find and we still cannot penetrate beyond the empirical facts.
+Przibram has laid stress on the general resemblance between the new
+growth of an amputated part in an animal and the way in which a broken
+crystal repairs itself when placed in the mother-solution. That the
+two processes have interesting points of likeness cannot be denied.
+It must however never be forgotten that there is one feature strongly
+distinguishing the two; for I believe it is universally recognized
+by physicists that all the phenomena of geometrical regularity which
+crystals display are ultimately dependent on the forms of the particles
+of the crystalline body. This cannot in any sense be supposed to hold
+in regard to protoplasm or its constituents. The definiteness of
+crystals is also an unlikely guide for the reason that it is absolute
+and perfect, or in other words because this kind of regularity cannot
+be disturbed at all without a change so great that the substance itself
+is altered; whereas we know that the forms of living things are capable
+of such changes, great and small, that we must regard perfection of
+form, whether manifested in symmetry or in number, as an ideal which
+will only be produced in the absence of disturbance. The symmetry of the
+living things is like the symmetry of the concentric waves in a pool
+caused by a splash. Perfect circles are made only in the imaginary case
+of mathematical uniformity, but the system maintains an approximate
+symmetry though liable to manifold deformation.
+
+Since the geometrical order of the living body cannot be a direct
+function of the materials it must be referred to some more proximate
+control. In renewing a part the body must possess the power of seizing
+particles of many dissimilar kinds, and whirl them into their several
+and proper places. The action in renewal, like that of original growth,
+may be compared--very crudely--with the action of a separator which
+simultaneously distributes a variety of heterogeneous materials in an
+orderly fashion; but in the living body the thing distributed must
+rather be the _appetency_ for special materials, not the materials
+themselves.
+
+If the analogy of crystals be set aside and we seek for other parallels
+to regeneration there are none very obvious. I have sometimes wondered
+whether it might not be possible to institute a fruitful comparison
+between the renewal of parts and the reformation of waves of certain
+classes after obliteration. In several respects, as I have already said,
+some curious resemblances with the repetitions formed by wave-motion
+are to be traced in our organic phenomena, and though admitting that
+I cannot develop these comparisons, I think nevertheless they may be
+worth bearing in mind. When, after obliteration, an eddy in a stream,
+or a ripple-mark (a more complex case of eddy-formation) in blown sand
+is re-formed, we have an example in which pattern is reconstituted and
+growth takes place not by virtue of the composition of the materials--in
+this case the water or the sand--but by the way in which they are acted
+upon by extraneous forces.
+
+A feature in the actual mode by which ripple-marks are reconstituted may
+not be without interest in connexion with our phenomena of regeneration.
+When, for example, the wind is blowing steadily over a surface of fine,
+dry sand, the familiar ripple-marks are formed by a heaping of the sand
+in lines transverse to the direction of the wind. The heaping is due to
+the formation of eddies corresponding with positions of instability.
+When the wind is steady and the sand homogeneous, the distances between
+the ripples, or wave-lengths, are sensibly equal. If while the wind
+continues to blow, the ripples are obliterated with a soft brush they
+will quickly be re-formed over the whole area, but I have noticed that
+at first their wave-length is approximately half that of the ripples
+in the undisturbed parts of the system.[12] The normal wave-length is
+restored by the gradual accentuation of alternate ripples. Of course
+the sand-ripples are in reality slowly travelling forward in the
+direction towards which the wind is blowing, and for this our living
+segmentations afford no obvious parallel, but the appearances in the
+area of reformation, and especially the forking of the old ridges where
+they join the new ones, are curiously reminiscent of the irregularities
+of segmentation seen in regenerated structures. The value of the
+considerations adduced in the chapter is, I admit, very small. The
+utmost that can be claimed for them is that mechanical segmentations,
+like those seen in ripple-mark, or in Leduc's osmotic growths, show
+how by the action of a continuous force in one direction, repeated
+and serially homologous divisions can be produced having features of
+similarity common to those repetitions by which organic forms and
+patterns are characterised. The analogy supplies a vicarious picture of
+the phenomena which in default of one more true may in a slight degree
+assist our thoughts. It suggests that the rhythms of segmentation may be
+the consequence of a single force definite in direction and continuously
+acting during the time of growth. The polarity of the organism would
+thus be the expression of the fact that this meristic force is
+definitely directed after it has once been excited, and the reversal
+seen in some products of regeneration suggest further that it is capable
+of being reflected. This polarity cannot be a property of the material,
+as such, but is determined by a force acting on that material, just as
+the polarity of a magnet is not determined by the arrangement of its
+particles, but by the direction in which the current flows.
+
+To some it may appear that even to embark on such discussions as this is
+to enter into a perilous flirtation with vitalistic theories. How, they
+may ask, can any force competent to produce chemical and geometrical
+differentiation in the body be distinguished from the "Entelechy" of
+Driesch? Let me admit that in this reflexion there is one element of
+truth. If those who proclaim a vitalistic faith intend thereby to
+affirm that in the processes by which growth and division are effected
+in the body, a part is played by an orderly force which we cannot
+_now_ translate into terms of any known mechanics, what observant man
+is not a vitalist? Driesch's first volume, putting as it does into
+intelligible language that positive deduction from the facts--especially
+of regeneration--should carry a vivid realisation of this truth to any
+mind. If after their existence is realised, it is desired that these
+unknown forces of order should have a name, and the word entelechy is
+proposed, the only objection I have to make is that the adoption of a
+term from Aristotelian philosophy carries a plain hint that we propose
+to relegate the future study of the problem to metaphysic.
+
+From this implication the vitalist does not shrink. But I cannot find in
+the facts yet known to us any justification of so hopeless a course. It
+was but yesterday that the study of _Entwicklungsmechanik_ was begun,
+and if in our slight survey we have not yet seen how the living machine
+is to be expressed in terms of natural knowledge that is poor cause for
+despair. Driesch sums up his argument thus:[13]
+
+"It seems to me that there is only one conclusion possible. If we are
+going to explain what happens in our harmonious-equipotential systems
+by the aid of causality based upon the constellation of single chemical
+factors and events, there _must_ be some such thing as a machine. Now
+the assumption of the existence of a machine proves to be absolutely
+absurd in the light of the experimental facts. _Therefore there can
+be neither any sort of a machine nor any sort of causality based upon
+constellation underlying the differentiation of harmonious-equipotential
+systems._"
+
+"For a machine, typical with regard to the three chief dimensions
+of space, cannot remain itself if you remove parts of it or if you
+rearrange its parts at will."
+
+To the last clause a note is added as follows:
+
+"The pressure experiments and the dislocation experiments come into
+account here; for the sake of simplicity they have not been alluded to
+in the main line of our argument."
+
+I doubt whether any man has sufficient knowledge of all possible
+machines to give reality to this statement. In spite also of the
+astonishing results of experiments in dislocation, doubt may further be
+expressed as to whether they have been tried in such variety or on such
+a scale as to justify the suggestion that the living organism remains
+itself if its parts are rearranged at will. All we know is that it can
+"remain itself" when much is removed, and when much rearrangement has
+been affected, which is a different thing altogether.
+
+I scarcely like to venture into a region of which my ignorance is so
+profound, but remembering the powers of eddies to re-form after partial
+obliteration or disturbance, I almost wonder whether they are not
+essentially machines which remain themselves when parts of them are
+removed.
+
+Real progress in this most obscure province is not likely to be made
+till it attracts the attention of physicists; and though they for long
+may have to forego the application of exact quantitative methods, I
+confidently anticipate that careful comparison between the phenomena
+of repetition formed in living organisms and the various kinds of
+segmentation produced by mechanical agencies would be productive of
+illuminating discoveries.
+
+FOOTNOTES:
+
+[1] Stéphane Leduc, _Théorie Physico-Chymique de la Vie_, Paris, 1910.
+
+[2] _Materials for the Study of Variation_, No. 249, p. 217; and p. 272.
+
+[3] _Materials_, p. 118.
+
+[4] Church, A. H., _On the Relation of Phyllotaxis to Mechanical Laws_,
+London, 1904.
+
+[5] It is a question whether the dominance of the palmatifid leaf over
+the pinnatifid is not really an example of the dominance of a lower
+number of segmentations over a higher. From the uncertainty whether two
+given leaves of two separate plants are actually comparable one cannot
+institute quite satisfactory numerical comparisons, but I think the view
+that the "Fern" leaf has more lobes than an otherwise similar "Palm"
+leaf may be fairly maintained. If this be admitted, the "Palm" leaf
+represents the dominant low number and its round shape is a consequence
+of the greater powers of growth which are so often possessed by the
+members of a shorter series.
+
+[6] It is perhaps of importance to remember that in certain species
+of bacteria (e. g. _Bacillus Anthracis_) division may cease where the
+organism is cultivated under certain artificial conditions though growth
+continues. In this way very long unsegmented threads are produced.
+
+[7] _Arch. f. Entwm._, XX, 1905, p. 76; _Sitzungsb. d. Ges. Naturf._,
+Berlin, 1907, p. 41, etc.
+
+[8] Borradaile, L. A., _Jour. Marine Zool._, 1897, No. 8.
+
+[9] Dr. Przibram, I should mention, concludes that on the whole the
+facts are against this interpretation, but as more evidence is certainly
+required, I call attention to the possibility.
+
+[10] Morgan, T. H., _Regeneration_, 1901.
+
+[11] It would be interesting to know whether growth continues at the
+original posterior end after the new "posterior" end has been formed in
+front.
+
+[12] In the actual case observed, the ripples unsmoothed had a
+wave-length of about 2-1/2 inches; and when the new ones were first
+formed, there were about 30 ridges in the length originally traversed by
+15 or 16.
+
+[13] _The Science and Philosophy of the Organism_; Gifford Lectures,
+1907. London, 1908, p. 141.
+
+
+
+
+                       CHAPTER IV
+
+       THE CLASSIFICATION OF VARIATION AND THE NATURE OF
+                   SUBSTANTIVE FACTORS
+
+We have now seen that among the normal physiological processes the
+phenomena of division form a recognisable, and in all likelihood a
+naturally distinct group. Variations in these respects may thus be
+regarded as constituting a special class among variations in general.
+
+The substantive variations have only one property in common--the
+negative one that they are not Meristic. The work of classifying them
+and distinguishing them according to their several types demands a
+knowledge of the chemistry of life far higher than that to which science
+has yet attained. In reference to some of the simplest variations Garrod
+has introduced the appropriate term "Chemical sports." The condition in
+man known as Alkaptonuria in which the urine is red is due especially
+to the absence of the enzyme which decomposes the excretory substance,
+alkapton. The "chemical sport" here consists in the inability to break
+up the benzene ring. The chemical feature which distinguishes and is the
+proximate cause of several colour-varieties can now in a few cases be
+declared. The work of Miss Wheldale has shown that colour-varieties may
+be produced by the absence of the chromogen compound the oxidation of
+which gives rise to sap-colours, by differences in the completeness of
+this process of oxidation, and by a process of reduction supervening on
+or perhaps suppressing the oxidation. Some of these processes moreover
+may be brought about by the combined action of two bodies, the one an
+enzyme, for example an oxygenase, and the other a substance regarded as
+a peroxide, contributing the oxygen necessary for the oxidation to take
+place. Variation in colour may thus be brought about by the addition or
+omission of any one of the bodies concerned in the action.
+
+Similar variations, or rather similar series of variations will
+undoubtedly hereafter be identified in reference to all the various
+kinds of chemical processes upon which the structure and functions of
+living things depend. The identification of these processes and of
+the bodies concerned in them will lead to a real classification of
+Substantive Variations.
+
+To forecast the lines on which such classification will proceed is to
+look too far ahead. We may nevertheless anticipate with some confidence
+that future analysis will recognise among the contributing elements,
+some which are intrinsic and inalienable, and others which are extrinsic
+and superadded.
+
+We already know that there may be such interdependence among the
+substantive characters that to disentangle them will be a work of
+extreme difficulty. The mere fact that in our estimation characters
+belong to distinct physiological systems is no proof of their actual
+independence. In illustration may be mentioned the sap-colour in Stocks
+and the development of hoariness on the leaves and stems, which Miss
+Saunders's experiments have shown to be intimately connected, so that
+in certain varieties no hoariness is produced unless the elements for
+sap-colour are already present in the individual plant.
+
+The first step in the classification of substantive variations is
+therefore to determine which are due to the addition of new elements
+or factors, and which are produced by the omission of old ones. _A
+priori_ there is no valid criterion by which this can be known, and
+actual experiments in analytical breeding can alone provide the
+knowledge required. Some very curious results have by this method been
+obtained, which throw an altogether unexpected light on these problems.
+For example, in order that the remarkable development of mesoblastic
+black pigment characteristic of the Silky Fowl should be developed, it
+is practically certain that two distinct variations from such a type
+as _Gallus bankiva_ must have occurred. I assume, as is reasonable,
+that _G. bankiva_ has genetic properties similar to those of the Brown
+Leghorn breed which has been used in the experiments which Mr. Punnett
+and I have conducted. _Gallus bankiva_ was not available but the Brown
+Leghorn agrees with it very closely in colouration, and probably in
+the general physiology of its pigmentation. Setting aside the various
+structural differences between the two breeds, the Silky is immediately
+distinguished from the Leghorn by the fact that the skin of the whole
+body including that of the face and comb appears to be of a deep
+purplish colour. The face and comb of the Leghorn are red and the skin
+of the body is whitish yellow. On examination it is found that the
+purple colour of the Silky is in reality due to the distribution of a
+deep black pigment in the mesoblastic membranes throughout the body. The
+somatopleura, the pleura, _pia mater_, the dermis, and in most organs
+the connective tissue and the sheaths of the blood-vessels, are thus
+impregnated with black. No such pigmentation exists in the Leghorn.
+As the result of an elaborate series of experimental matings we have
+proved that the distinction between the Leghorn and the Silky consists
+primarily in the fact that the Silky possesses a pigment-producing
+factor, _P_, which is not present in the Leghorn.
+
+This variation must undoubtedly have been one of _addition_. But besides
+this there is another difference of an altogether dissimilar nature; for
+the Brown Leghorn possesses a factor which has the power of partially or
+completely restricting the operation of the pigment-producing factor,
+_P_. Moreover in respect of this pigment-restricting factor which we
+may call _D_, the sexes of the Brown Leghorn differ, for the male is
+homozygous or _DD_, but the female is heterozygous, _Dd_. Thus in order
+that the black-skinned breed could be evolved from such a type as a
+Brown Leghorn it must be necessary _both_ that _P_ should be added and
+that _D_ should drop out. We have not the faintest conception of the
+process by which either of these events have come to pass, but there is
+no reasonable doubt that in the evolution of the Silky fowl they did
+actually happen.
+
+We may anticipate that numerous interdependences of this kind will be
+discovered.
+
+Before any indisputable progress can be made with the problem of
+evolution it is necessary that we should acquire some real knowledge
+of the genesis of that class of phenomena which formed the subject of
+the last chapter. So long as the process of division remains entirely
+mysterious we can form no conception even of the haziest sort as
+to the nature of living organisms, or of the proximate causes which
+determine their forms, still less can we attempt any answer to those
+remoter questions of origin and destiny which form the subject of the
+philosopher's contemplation. It is in no spirit of dogmatism that I
+have ventured to indicate the direction in which I look for a solution,
+though I have none to offer. It may well be that before any solution is
+attained, our knowledge of the nature of unorganised matter must first
+be increased. For a long time yet we may have to halt, but we none
+the less do well to prepare ourselves to utilise any means of advance
+that may be offered, by carefully reconnoitering the ground we have to
+traverse. The real difficulty which blocks our progress is ignorance of
+the nature of division, or to use the more general term, of repetition.
+
+Let us turn to the more familiar problem of the causes of variation. Now
+since variation consists as much in meristic change as in alteration in
+substance or material, there is one great range of problems of causation
+from which we are as yet entirely cut off. We know nothing of the
+causation of division, and we have scarcely an observation, experiment
+or surmise touching the causes by which the meristic processes may be
+altered.
+
+Of the way in which variations in the substantive composition of
+organisms are caused we have almost as little real evidence, but we are
+beginning to know in what such variations must consist. These changes
+must occur either by the addition or loss of factors.
+
+We must not lose sight of the fact that though the factors operate by
+the production of enzymes, of bodies on which these enzymes can act, and
+of intermediary substances necessary to complete the enzyme-action, yet
+these bodies themselves can scarcely be themselves genetic factors, but
+consequences of their existence. What then are the factors themselves?
+Whence do they come? How do they become integral parts of the organism?
+Whence, for example, came the power which is present in a White Leghorn
+of destroying--probably reducing--the pigment in its feathers? That
+power is now a definite possession of the breed, present in all its
+germ-cells, male and female, taking part in their symmetrical divisions,
+and passed on equally to all as much as is the protoplasm or any other
+attribute of the breed. From the body of the bird the critical and
+efficient substance could in all likelihood be isolated by suitable
+means, just as the glycogen of the liver can be. But even when this
+extraction has been accomplished and the reducing body isolated, we
+shall know no more than we did before respecting the mode by which
+the power to produce it was conferred on the fowl, any more than we
+know how the walls of its blood-vessels acquired the power to form a
+fibrin-ferment.
+
+It is when the scope of such considerations as this are fully grasped
+that we realise the fatuousness of the conventional treatment which the
+problem of the causes of variation commonly receives. Environmental
+change, chemical injury, differences in food supply, in temperature,
+in moisture, or the like have been proposed as "causes." Admitting
+as we must do, that changes may be produced--usually inhibitions of
+development--by subjecting living things to changes in these respects,
+how can we suppose it in the smallest degree likely that very precise,
+new, and adaptative powers can be conferred on the germs by such
+treatment? Reports of positive genetic consequences observed comparable
+with those I have mentioned, become from time to time current. We
+should I think regard them with the gravest doubt. Few, so far as I am
+aware, have ever been confirmed, though clear and repeated confirmation
+should be demanded before we suffer ourselves at all to build upon such
+evidence. In a subsequent chapter some of these cases will be considered
+in detail.
+
+In no class of cases would the transmission of an acquired character
+superficially appear so probable as in those where power of resisting
+the attack of a pathogenic organism is acquired in the lifetime of
+the zygote. The possession of such a power is moreover a distinction
+comparable with those which differentiate varieties and species. It
+is due to the development in the blood of specific substances which
+pervade the whole fluid. This development is exactly one of those
+"appropriate responses to stimuli" which naturalists who incline to
+regard adaptation as a direct consequence of an environmental influence
+might most readily invoke as an illustration of their views. And yet all
+evidence is definitely unfavourable to the suggestion of an inheritance
+of the acquired power of resistance. Such change as can be perceived
+in the virulence of the attacks on successive generations may be most
+easily regarded as due to the extermination of the more susceptible
+strains, and perhaps in some measure to variation in the invading
+organisms themselves, an "acquired character" of quite different import.
+
+The specific "anti-body" may have been produced in response to the
+stimulus of disease, but the power to produce it without this special
+stimulus is not included in the germ-cells any more than a pigment.
+All that they bear is the _power to produce_ the anti-bodies when the
+stimulus is applied.
+
+If we could conceive of an organism like one of those to which disease
+may be due becoming actually incorporated with the system of its host,
+so as to form a constituent of its germ-cells and to take part in the
+symmetry of their divisions, we should have something analogous to the
+case of a species which acquires a new factor and emits a dominant
+variety. When we see the phenomenon in this light we realise the
+obscurity of the problem. The appearance of recessive varieties is
+comparatively easy to understand. All that is implied is the omission of
+a constituent. How precisely the omission is effected we cannot suggest,
+but it is not very difficult to suppose that by some mechanical fault
+of cell-division a power may be lost. Such variation by unpacking,
+or analysis of a previously existing complex, though unaccountable,
+is not inconceivable. But whence come the new dominants? Whether we
+imagine that they are created by some rearrangement or other change
+internal to the organism, or whether we try to conceive them as due to
+the assumption of something from without we are confronted by equally
+hopeless difficulty.
+
+The mystery of the origin of a dominant increases when it is realised
+that there is scarcely any recent and authentic account of such an event
+occurring under critical observation, which can be taken as a basis for
+discussion. The literature of horticulture for example abounds in cases
+alleged, but I do not think anyone can produce an illustration quite
+free from doubt. Such evidence is usually open to the suspicion that the
+plant was either introduced by some accident, or that it arose from a
+cross with a pre-existing dominant, or that it owed its origin to the
+meeting of complementary factors. In medical literature almost alone
+however, there are numerous records of the spontaneous origin of various
+abnormal conditions in man which habitually behave as dominants, and of
+the authenticity of some of these there can be no doubt.
+
+When we know that such conditions as hereditary cataract or various
+deformities of the fingers behave as dominants, we recognize that
+those conditions must be due to the addition of some element to the
+constitution of the normal man. In the collections of pedigrees relating
+to such pathological dominants there are usually to be found alleged
+instances of the origin of the condition _de novo_. Not only do these
+records occur with such frequency that they cannot be readily set aside
+as errors, but from general considerations it must be obvious that as
+these malformations are not common to normal humanity they must at
+some moment of time have been introduced. The lay reader may not be
+so much impressed with the difficulty as we are. He is accustomed to
+regard the origin of _any_ new character as equally mysterious, but when
+once dominants are distinguished from recessives the problem wears a
+new aspect. Thus the appearance of high artistic gifts, whether as an
+attribute of a race or as a sporadic event among the children of parents
+destitute of such faculties, is not very surprising, for we feel fairly
+sure that the faculty is a recessive, due to the loss of a controlling
+or inhibiting factor; but the _de novo_ origin of brachydactylous
+fingers in a child of normal parents is of quite a different nature, and
+must indicate the action of some new specific cause.
+
+Whether such evidence is applicable to the general problem of evolution
+may with some plausibility be questioned; but there is an obvious
+significance in the fact that it is among these pathological occurrences
+that we meet with phenomena most nearly resembling the spontaneous
+origin of dominant factors, and I cannot see such pedigrees as these
+without recalling Virchow's aphorism that every variation owes its
+origin to some pathological accident. In the evolution of domestic
+poultry, if _Gallus bankiva_ be indeed the parent form of all our
+breeds, at least some half dozen new factors must have been added during
+the process. In _bankiva_ there is, for example, no factor for rose
+comb, pea comb, barring on the feathers, or for the various dominant
+types of dark plumage. Whence came all these? It is, I think, by no
+means impossible that some other wild species now extinct did take
+part in the constitution of domestic poultry. It seems indeed to me
+improbable that the heavy breeds descend from _bankiva_. Both in regard
+to domestic races of fowls, pigeons, and some other forms, the belief in
+origin within the period of human civilization from one simple primitive
+wild type seems on a balance of probabilities insecurely founded, but
+allowing something for multiplicity of origin we still fall far short
+of the requisite total of factors. Elements exist in our domesticated
+breeds which we may feel with confidence have come in since their
+captivity began. Such elements in fowls are dominant whiteness, extra
+toe, feathered leg, frizzling, etc., so that even hypothetical extension
+of the range of origin is only a slight alleviation of the difficulty.
+
+Somehow or other, therefore, we must recognize that dominant factors
+do arise. Whether they are created by internal change, or whether, as
+seems to me not wholly beyond possibility, they obtain entrance from
+without, there is no evidence to show. If they were proved to enter from
+without, like pathogenic organisms, we should have to account for the
+extraordinary fact that they are distributed with fair constancy to half
+the gametes of the heterozygote.
+
+In proportion as the nature of dominants grows more clear so does it
+become increasingly difficult to make any plausible suggestion as to
+their possible derivation. On the other hand the origin of a recessive
+variety by the loss of a factor is a process so readily imagined that
+our wonder is rather that the phenomenon is not observed far more
+often. Some slip in the accurate working of the mechanical process of
+division, and a factor gets left out, the loss being attested by the
+appearance of a recessive variety in some subsequent generation.
+
+Consistently with this presentation of the facts we find that, as in our
+domesticated animals and plants, a diversity of recessives may appear
+within a moderately short period, and that when variations come they
+often do not come alone. Witness the cultural history of the Sweet Pea,
+_Primula Sinensis_, _Primula obconica_, _Nemesia strumosa_ and many such
+examples in which variation when it did come was abundant. The fact
+cannot be too often emphasized that in the vast proportion of these
+examples of substantive variation under domestication, as well as of
+substantive variation in the natural state, the change has come about by
+omission, not by addition. To take, for example, the case of the Potato,
+in which so many spontaneous bud-variations have been recorded, East
+after a careful study of the evidence has lately declared his belief
+that all are of this nature, and the opinion might be extended to many
+other groups of cases whether of bud or seminal variation. Morgan draws
+the same conclusion in reference to the many varieties he has studied in
+_Drosophila_.
+
+In the Sweet Pea, a form which is beyond suspicion of having been
+crossed with anything else, and has certainly produced all the multitude
+of types which we now possess by variations from one wild species,
+there is only one character of the modern types which could, with any
+plausibility, be referred to a factor not originally forming part
+of the constituents of the wild species. This is the waved edge, so
+characteristic of the "Spencer" varieties; for the cross between a
+smooth-edged and a waved type gives an intermediate not unfrequently.
+Nevertheless there is practically no doubt that this is merely an
+imperfection in the dominance of the smooth edge, and we may feel sure
+that any plant homozygous for smooth edge would show no wave at all.
+Hence it is quite possible that even the appearance of the original
+waved type, Countess Spencer, was due to the loss of one of the factors
+for smooth edge at some time in the history of the Sweet Pea.
+
+In the case of the Chinese Primrose (_Primula Sinensis_) one dominant
+factor has been introduced in modern times, probably within the last six
+years at most. This is the factor which causes suppression of the yellow
+eye, giving rise to the curious type known as "Queen Alexandra." Mr. R.
+P. Gregory's experiments proved that this was a very definite dominant,
+and the element responsible for this development is undoubtedly an
+addition to the original ingredient-properties, with which the species
+was endowed. Unfortunately, as happens in almost every case of the kind,
+the origin of this important novelty appears to be lost. Its behaviour,
+however, when crossed with various other types is that of a simple
+dominant giving an ordinary 3:1 ratio. There is therefore no real doubt
+that it came into existence by the definite addition of a new factor,
+for if it was simply a case of the appearance of a new character made by
+combination of two previously existing complementary factors we should
+expect that when Queen Alexandra was self-fertilised a 9:7 ratio would
+be a fairly common result, which is not in practice found.
+
+In _Oenothera_ Gates[1] has observed the appearance, in a large
+sowing of about 1,000 _Oenothera rubrinervis_, of a single individual
+having considerably more red pigment in the calyx than is usual in
+_rubrinervis_. The whole of the hypanthium in the flowers of this
+plant was red instead of green as in _rubrinervis_, and the whole of
+the sepals were red in the bud-stage, except for small green areas at
+the base. This type behaved as a dominant over _rubrinervis_, but so
+far a pure-breeding individual was not found. Admittedly the variation
+of this plant from the type of _rubrinervis_ can be represented as
+one of degree, though there is a very sensible gap in the series
+between the new form which Gates names "_rubricalyx_" and the reddest
+_rubrinervis_ seen in his cultures. It must certainly be recognised as
+a new dominant. Gates, rightly as I consider, regards the distinction
+between _rubrinervis_ and _rubricalyx_ as a quantitative one, and the
+same remark applies to certain other types differing in the amount
+of anthocyanin which they produce. I do not understand the argument
+which Gates introduces to the effect that the difference between
+such quantitative types cannot be represented in terms of presence
+and absence. We are quite accustomed to the fact that in the rabbit
+self-colour segregates from the Dutch-marked type. These two types
+differ in a manner which we may reasonably regard as quantitative. It
+is no doubt possible that the self-coloured type contains an ingredient
+which enables the colour to spread over the whole body, but it is, I
+think, perhaps more easy to regard the Dutch type as a form from which a
+part of the colour is absent. It may be spoken of in terms I have used,
+as a _subtraction-stage_ in colour. Following a similar method we may
+regard _rubricalyx_ as an addition-stage in colour-variation. The fact
+that crosses between _rubrinervis_, or _rubricalyx_ and _Lamarckiana_
+give a mixture of types in F_{1}, does not I think show, as Gates
+declares, that there is any system here at work to which a factorial or
+Mendelian analysis does not apply; but that question may be more fitly
+discussed in connexion with the other problems raised by the behaviour
+of _Oenothera_ species in their crosses.
+
+I do, however, feel that, interesting as this case must be admitted to
+be, we cannot quite satisfactorily discuss it as an illustration of
+the _de novo_ origin of a dominant factor. The difference between the
+novelty and the type is quantitative, and it is not unreasonable to
+think of such a difference being brought about by some "pathological
+accident" in a cell-division.
+
+Recognition of the distinction between dominant and recessive characters
+has, it must be conceded, created a very serious obstacle in the way
+of any rational and concrete theory of evolution. While variations
+of all kinds could be regarded as manifestations of some mysterious
+instability of organisms this difficulty did not occur to the mind of
+evolutionists. To most of those who have taken part in genetic analysis
+it has become a permanent and continual obsession. With regard to the
+origin of recessive variations, there is, as we have seen, no special
+difficulty. They are negative and are due to absences, but as soon as it
+is understood that dominants are caused by an addition we are completely
+at a loss to account for their origin, for we cannot surmise any source
+from which they may have been derived. Just as when typhoid fever breaks
+out in his district the medical officer of health knows for certain that
+the bacillus of typhoid fever has by some means been brought into that
+district so do we know that when first dominant white fowls arose in the
+evolution of the domestic breeds, by some means the factor for dominant
+whiteness got into a bird, or into at least one of its germ-cells.
+Whence it came we cannot surmise.
+
+Whether we look to the outer world or to some rearrangement within the
+organism itself, the prospect of finding a source of such new elements
+is equally hopeless.
+
+Leaving this fundamental question aside as one which it is as yet quite
+unprofitable to discuss, we are on safe ground in foreseeing that the
+future classification of substantive variations, which genetic research
+must before long make possible, will be based on a reference to the
+modes of action of the several factors. Some will be seen to produce
+their effects by oxidation, some by reduction, some by generating
+substances of various types, sugars, enzymes, activators, and so forth.
+It may thus be anticipated that the relation of varieties to each other
+and to types from which they are derived will be expressible in terms
+of definite synthetical formulae. Clearly it will not for an indefinite
+time be possible to do this in practice for more than a few species
+and for characters especially amenable to experimental tests, but as
+soon as the applicability of such treatment is generally understood the
+influence on systematics must be immediate and profound, for the nature
+of the problem will at length be clear and, though the ideal may be
+unattainable, its significance cannot be gainsaid.
+
+                 *       *       *       *       *
+
+_Note._--With hesitation I allow this chapter to appear in the form in
+which it was printed a year ago, but in passing it for the press after
+that interval I feel it necessary to call attention to a possible line
+of argument not hitherto introduced.
+
+In all our discussions we have felt justified in declaring that the
+dominance of any character indicates that some factor is present which
+is responsible for the production of that character. Where there is no
+definite dominance and the heterozygote is of an intermediate nature
+we should be unable to declare on which side the factor concerned was
+present and from which side it was absent. The degree of dominance
+becomes thus the deciding criterion by which we distinguish the
+existence of factors. But it should be clearly realized that in any
+given case the argument can with perfect logic be inverted. We already
+recognize cases in which by the presence of an inhibiting factor a
+character may be suppressed and purely as a matter of symbolical
+expression we might apply the same conception of inhibition to any
+example of factorial influence whatever. For instance we say that in
+as much as two normal persons do not have brachydactylous children,
+there must be some factor in these abnormal persons which causes the
+modification. Our conclusion is based on the observed fact that the
+modification is a dominant. But it may be that normal persons are
+homozygous in respect of some factor _N_, which prevents the appearance
+of brachydactyly, and that in any one heterozygous, _Nn_, for this
+inhibiting factor, brachydactyly can appear. Similarly the round pea we
+say contains _R_, a factor which confers this property of roundness,
+without which its seeds would be wrinkled. But here we know that the
+wrinkled seed is in reality one having compound starch-grains, and
+that the heterozygote, though outwardly round enough, is intermediate
+in that starch-character. If we chose to say that the compoundness of
+the grains is due to a factor _C_ and that two doses of it are needed
+to make the seed wrinkled, I know no evidence by which such a thesis
+could be actually refuted. That such reasoning is seemingly perverse
+must be conceded; but when we consider the extraordinary difficulties
+which beset any attempt to conceive the mode of origin of a new dominant
+factor, we are bound to remember that there is this other line of
+argument which avoids that difficulty altogether. In the case of the
+"Alexandra"-eye in _Primula_, or the red calyx in Gates's _Oenothera_,
+inverting the reasoning adopted in the text, we may see that only the
+_Primula_ homozygous for the yellow eye can develop it and that two
+doses of the factor for the _rubrinervis_ calyx are required to prevent
+that part of the plant from being red.
+
+We may proceed further and extend this mode of reasoning to all cases
+of genetic variation, and thus conceive of all alike as due to loss of
+factors present in the original complex. Until we can recognize factors
+by means more direct than are provided by a perception of their effects,
+this doubt cannot be positively removed. For all practical purposes of
+symbolic expression we may still continue to use in our analyses the
+modes of representation hitherto adopted, but we must not, merely on
+the ground of its apparent perversity, refuse to admit that the line of
+argument here indicated may some day prove sound.
+
+FOOTNOTES:
+
+[1] Gates, R. R., _Zts. f. Abstammungslehre_, 1911, IV, pp. 341 and 361.
+
+
+
+
+                        CHAPTER V
+
+                    THE MUTATION THEORY
+
+When with the thoughts suggested in the last chapter we contemplate
+the problem of Evolution at large the hope at the present time of
+constructing even a mental picture of that process grows weak almost
+to the point of vanishing. We are left wondering that so lately men
+in general, whether scientific or lay, were so easily satisfied. Our
+satisfaction, as we now see, was chiefly founded on ignorance.
+
+Every specific evolutionary change must represent a definite event in
+the construction of the living complex. That event may be a disturbance
+in the meristic system, showing itself in a change in the frequency of
+the repetitions or in the distribution of differentiation among them, or
+again it may be a chemical change, adding or removing some factor from
+the sum total.
+
+If an attempt be made to apply these conceptions to an actual series
+of allied species the complexity of the problem is such that the mind
+is appalled. Ideas which in the abstract are apprehended and accepted
+with facility fade away before the concrete case. It is easy to imagine
+how Man was evolved from an _Amoeba_, but we cannot form a plausible
+guess as to how _Veronica agrestis_ and _Veronica polita_ were evolved,
+either one from the other, or both from a common form. We have not even
+an inkling of the steps by which a Silver Wyandotte fowl descended from
+_Gallus Bankiva_, and we can scarcely even believe that it did. The
+Wyandotte has its enormous size, its rose comb, its silver lacing, its
+tame spirit, and its high egg production. The tameness and the high egg
+production are probably enough both recessives, and though we cannot
+guess how the corresponding dominant factors have got lost, it is not
+very difficult to imagine that they were lost somehow. But the rose comb
+and the silver colour are _dominants_. The heavy weight also appears in
+the crosses with Leghorns, but we need not at once conclude that it
+depends on a simple dominant factor, because the big size of the crosses
+may be a consequence of the cross and may depend on other elements.
+
+Now no wild fowl known to us has these qualities. May we suppose that
+some extinct wild species had them? If so, may we again make the same
+supposition in all similar cases? To do so is little gain, for we are
+left with the further problem, whence did those lost wild species
+acquire those dominants? Suppositions of this kind help no more than
+did the once famous conjecture as to the origin of living things--that
+perhaps they came to earth on a meteorite. The unpacking of an
+original complex, the loss of various elements, and the recombination
+of pre-existing materials may all be invoked as sources of specific
+diversity. Undoubtedly the range of possibilities thus opened up is
+large. It will even cover an immense number of actual examples which
+in practice pass as illustrations of specific distinction. The Indian
+Rock pigeon which has a blue rump may quite reasonably be regarded as
+a geographically separated recessive form of our own _Columba livia_,
+for as Staples-Browne has shown the white rump of _livia_ is due to
+a dominant factor. The various degrees to which the leaves of Indian
+Cottons are incised have, as Leake says, been freely used as a means
+of classification. The diversities thus caused are very remarkable,
+and when taken together with diversities in habit, whether sympodial
+or monopodial, the various combinations of points of difference
+are sufficiently distinctive to justify any botanist in making a
+considerable number of species by reference to them alone. Nevertheless
+Leake's work goes far to prove that all of these forms represent the
+re-combinations of a very small number of factors. The classical example
+of _Primula Sinensis_ and its multiform races is in fact for a long
+way a true guide as to the actual interrelations of the species which
+systematists have made. That they did make them was due to no mistake
+in judgment or in principle, but simply to the want of that extended
+knowledge of the physiological nature of the specific cases which we now
+know to be a prime necessity.
+
+But will such analysis cover all or even most of the ordinary cases
+of specific diversity between near allies? Postponing the problem of
+the interrelations of the larger divisions as altogether beyond present
+comprehension, can we suppose, that in general, closely allied species
+and varieties represent the various consequences of the presence or
+absence of allelomorphic factors in their several combinations? The
+difficulty in making a positive answer lies in the fact that in most
+of the examples in which it has been possible to institute breeding
+experiments with a view to testing the question, a greater or less
+sterility is encountered. Where, however, no such sterility is met
+with, as for instance in the crosses made by E. Baur among the species
+of _Antirrhinum_ there is every reason to think that the whole mass of
+differences can and will eventually be expressed in terms of ordinary
+Mendelian factors. Baur has for example crossed species so unlike as
+_Antirrhinum majus_ and _molle_, forms differing from each other in
+almost every feature of organisation.[1] The F_{2} generation from this
+cross presents an amazingly motley array of types which might easily if
+met with in nature be described as many distinct species. Yet all are
+fertile and there is not the slightest difficulty in believing that they
+can all be reduced to terms of factorial analysis.
+
+If allowance be made for the complicating effects of sterility, is there
+anything which prevents us from supposing that such good species as
+those of _Veronica_ or of any other genus comprising well-defined forms
+may not be similarly related? I do not know any reason which can be
+pointed to as finally excluding such a possibility. Nevertheless it has
+been urged with some plausibility that good species are distinguished
+by _groups_ of differentiating characters, whereas if they were really
+related as the terms of a Mendelian F_{2} family are, we should expect
+to find not groups of characters in association, but rather series of
+forms corresponding to the presence and absence of the integral factors
+composing the groups of characters. I am not well enough versed in
+systematic work to be able to decide with confidence how much weight
+should be attached to this consideration. Some weight it certainly
+has, but I cannot yet regard it as forming a fatal objection to the
+application of factorial conceptions on the grand scale. It may be
+recalled that we are no longer under any difficulty in supposing that
+differences of all classes may be caused by the presence or absence of
+factors. It seemed at first for example that such characters as those of
+leaf shape might be too subtle and complex to be reducible to a limited
+number of factors. But first the work of Gregory on _Primula Sinensis_
+showed that several very distinct types of leaves were related to each
+other in the simplest way. In that particular example, intermediates are
+so rare as to be negligible, but subsequently Shull dealing with such
+a complicated example as _Capsella_, and Leake in regard to Cottons,
+both forms in which intergrades occur in abundance, have shown that a
+simple factorial scheme is applicable. We need not therefore, to take
+an extreme case, doubt that if it were possible to examine the various
+forms of fruit seen in the Squashes by really comprehensive breeding
+tests, even this excessive polymorphism in respect of structural
+features would be similarly reducible to factorial order.
+
+It must always be remembered also that in a vast number of cases, nearly
+allied forms which are distinct, occupy distinct ground. Moreover, by
+whatever of the many available mechanisms that end be attained, it is
+clear that nature very often does succeed in preventing intercrossing
+between distinct forms so far that the occurrence of that phenomenon is
+a rarity under natural conditions. The facts may, I think, fairly be
+summarized in the statement that species are on the whole distinct and
+not intergrading, and that the distinctions between them are usually
+such as might be caused by the presence, absence, or inter-combination
+of groups of Mendelian factors; but that they are so caused the evidence
+is not yet sufficient to prove in more than a very few instances.
+
+The alternative, be it explicitly stated, is not to return to the view
+formerly so widely held, that the distinctions between species have
+arisen by the accumulation of minute or insensible differences. The
+further we proceed with our analyses the more inadequate and untenable
+does that conception of evolutionary change become. If the differences
+between species have not come about by the addition or loss of factors
+one at a time, then we must suppose that the changes have been effected
+by even larger steps, and variations including groups of characters,
+must be invoked.
+
+That changes of this latter order are really those by which species
+arise, is the view with which de Vries has now made us familiar by his
+writings on the Mutation Theory. In so far as mutations may consist
+in meristic changes of many kinds and in the loss of factors it is
+unnecessary to repeat that we have abundant evidence of their frequent
+occurrence. That they may also more rarely occur by the addition of a
+factor we are, I think, compelled to believe, though as yet the evidence
+is almost entirely circumstantial rather than direct. The evidence for
+the occurrence of those mutations of higher order, by which new species
+characterized by several distinct features are created, is far less
+strong, and after the best study of the records which I have been able
+to make, I find myself unconvinced. The facts alleged appear capable of
+other interpretations.
+
+The most famous and best studied examples are of course the forms
+of _Oenothera_ raised by de Vries from _Oenothera Lamarckiana_ in
+circumstances well known to all readers of genetic literature. Whatever
+be the true significance of these extraordinary "mutations" there can
+be no question about the great interest which attaches to them, and the
+historical importance which they will long preserve. Apart also from
+these considerations it is becoming more and more evident that in their
+peculiarities they provide illustrations of physiological phenomena of
+the highest consequence in the study of genetics at large.
+
+De Vries found, as is well known, that _Oenothera Lamarckiana_ gives off
+plants unlike itself. These mutational forms are of several distinct
+and recognizable types which recur, and several of them breed true from
+their first appearance. The obvious difficulty, which in my judgment
+should make us unwilling at present to accept these occurrences as proof
+of the genesis of new species by mutation, is that we have as yet no
+certainty that the appearance of the new forms is not an effect of the
+recombination of factors, such as is to be seen in so many generations
+of plants derived from a cross involving many genetic elements. The
+first question is what is _Oenothera Lamarckiana_? Is it itself a plant
+of hybrid origin? To this fundamental question no satisfactory answer
+has yet been given. All attempts to find it as a wild plant in America
+have failed. It existed in Europe in the latter half of the eighteenth
+century. Whence it came is still uncertain, but the view that it came
+into existence in Europe and perhaps in Paris, seems on the whole the
+most probable. The question has been debated by Macdougal, Gates, and
+Davis. From historical sources there is little expectation of further
+light. Those who favour the notion of a hybrid origin look on _Oenothera
+biennis_ as one of the putative parents. It has been conjectured that
+a species called _grandiflora_ lately re-discovered on the Alabama
+river was the other parent. Experiments have been instituted by Davis
+to discover whether _Lamarckiana_ can be made artificially by crossing
+these two species. The results so far have shown that while plants
+approximating in various respects to _Lamarckiana_ have thus been
+produced, none agree exactly with that form. Davis, to whom reference
+should be made for a full account of the present state of the enquiry,
+points out that there are many strains of _biennis_ in existence and
+that it is by no means impossible that by using others of these strains
+a still closer approximation can be made. None of Davis's artificial
+productions as yet breed at all true, as _Lamarckiana_ on the whole
+does. In such a case, however, where several characters are involved,
+this is perhaps hardly to be expected.
+
+One feature of the _Oenotheras_ is very curious. Not only _Lamarckiana_,
+but all the allied species so far as I am aware, have a considerable
+proportion of bad and shrivelled pollen grains. This is undoubtedly true
+of species living in the wild state as well as of those in cultivation.
+I have had opportunities of verifying this for myself in the United
+States. No one looking at the pollen of an _Oenothera_ would doubt that
+it was taken from some hybrid plant exhibiting partial sterility. On the
+other hand, it is difficult to suppose that numbers, perhaps all, of
+the "species" of the genus are really hybrids, and many of them breed
+substantially true. I regard this constant presence of bad pollen grains
+as an indication that the genetic physiology of _Oenothera_ is in some
+way abnormal, and as we shall presently see, there are several other
+signs which point in the same direction.
+
+Discussion of the whole series of phenomena is rendered exceedingly
+difficult first, by reason of the actual nature of the material. The
+characteristics of many of the types which de Vries has named are
+evasive. A few of these types, for instance, _gigas_, _nanella_,
+_albida_, _brevistylis_, and perhaps a few more are evidently clear
+enough, but we have as yet no figures and descriptions precise enough
+to enable a reader to appreciate exactly the peculiarities of the vast
+number of forms which have now to be considered in any attempt to gain a
+comprehensive view of the whole mass of facts. It is also not in dispute
+that the forms are susceptible of great variations due simply to soil
+and cultural influences.
+
+The fact that no Mendelian analysis has yet been found applicable to
+this group of _Oenotheras_ as a whole is perhaps largely due to the fact
+that until recently such analysis has not been seriously attempted.
+Following the system which he had adopted before the rediscovery of
+Mendelism, or at all events, before the development of that method of
+analysis, de Vries has freely applied _names_ to special combinations of
+characters and has scarcely ever instituted a factorial analysis. Before
+we can get much further this must be attempted. It may fail, but we must
+know exactly where and how this failure comes about. There are several
+indications that such a recognition of factorial characters, could be
+carried some way. For example, the height, the size of the flowers, the
+crinkling of the leaves, the brittleness of the stems, perhaps even the
+red stripes on stems and fruits, and many more, are all characters which
+may or may not depend on distinct factors, but if such characters are
+really transmitted in unresolved groups, the limitations of those groups
+should be carefully determined. The free use of names for the several
+forms, rather than for the characters, has greatly contributed to deepen
+the obscurity which veils the whole subject.
+
+I do not mean to suggest that these _Oenotheras_ follow a simple
+Mendelian system. All that we know of them goes to show that there
+are curious complications involved. One of these, probably the most
+important of all, has lately been recognized by de Vries himself,
+namely, that in certain types the characters borne by the female and
+the male germ-cells of the same plant are demonstrably different. There
+can be little doubt that further research will reveal cognate phenomena
+in many unsuspected places. The first example in which such a state of
+things was proved to exist is that of the Stocks investigated by Miss
+Saunders.[2] By a long course of analysis she succeeded in establishing
+in 1908 the fact that if a plant of _Matthiola_ is of that eversporting
+kind which gives a large proportion of double-flowered plants among
+its offspring (produced by self-fertilisation), then the egg-cells of
+such a plant are mixed in type, but the pollen of the same plant is
+homogeneous. Some of the egg-cells have in them the two factors for
+singleness, but some of them are short of one or both of these factors.
+The pollen-grains, however, are all recessives, containing neither of
+these factors. The egg-cells, in other words, are mixed, "singles"
+and "doubles," while the pollen-grains are all "doubles." The same is
+true of the factor differentiating "white," or colourless plastids
+from cream-coloured plastids in _Matthiola_, the egg-cells being mixed
+"whites" and "creams," while the pollen-grains are all "creams," viz:
+recessives. Later in the same year (1908) de Vries[3] announced a
+remarkable case which will be discussed in detail subsequently. It
+relates to certain _Oenotheras_ heterozygous for dwarfness, in which (p.
+113) the ovules were mixed, tails and dwarfs, while the pollen is all
+dwarf.
+
+Again in _Petunia_ Miss Saunders's[4] work has shown that a somewhat
+similar state of things exists, but with this remarkable difference,
+that though the egg-cells are mixed, singles and doubles, the
+pollen-grains are all _singles_, viz: dominants. All the _Petunias_ yet
+examined have been in this condition, including some which in botanic
+gardens pass for original species. Whether actual wild plants from
+their native habitats are in the same state, is not yet known, but it
+is by no means improbable. The case may be compared with that of the
+moth _Abraxas grossulariata_ studied by Doncaster and Raynor, in which
+the females are all heterozygous, or we may almost say "hybrids" of
+_grossulariata_ and the variety _lacticolor_. Similarly we may say that
+at least garden Petunias are heterozygous in respect of singleness.
+The proof of this is of course that when fertilised with the pollen of
+doubles they throw a mixture of doubles and singles. The statements
+which de Vries has published regarding the behaviour of several of the
+_Oenotheras_ go far to show that they must have a somewhat similar
+organisation. On the present evidence it is still quite impossible to
+construct a coherent scheme which will represent all the phenomena in
+their interrelations, and among the facts are several which, as will
+appear, seem mutually incompatible. The first indication that the
+_Oenotheras_ may have either mixed ovules or mixed pollen appears in the
+fact that _Lamarckiana_ and several of its "mutants" used as males, with
+several other forms as females, give a mixed offspring. For example, de
+Vries (1907) found that
+
+          _biennis_ [F] × _Lamarckiana_ [M]
+          _biennis cruciata_ [F] × _Lamarckiana_ [M]
+          _muricata_ [F] × _Lamarckiana_ [M]
+          _biennis_ [F] × _rubrinervis_ [M]
+          _biennis cruciata_ [F] × _rubrinervis_ [M]
+
+all give a mixture of two distinct types which he names _laeta_ and
+_velutina_, consisting of about equal numbers of each. On account of the
+fact that the two forms are produced in association de Vries has called
+these forms "twin hybrids," a designation which is not fortunate, seeing
+that it is impossible to imagine that any kind of twinning is concerned
+in their production. The distinction between these two seems to be
+considerable, _laeta_ having leaves broader, bright green in colour, and
+flat, with pollen scanty, while _velutina_ has leaves narrower, grayish
+green, more hairy, and furrow-shaped, with pollen abundant.
+
+We next meet the remarkable fact that these two forms, _laeta_
+and _velutina_ breed true to their respective types, and do not
+reproduce the parent-types among their offspring resulting from
+self-fertilisation. This statement must be qualified in two respects.
+When _muricata_ [M] is fertilised by _brevistylis_ the forms _laeta_
+and _velutina_ are produced, but each of them subsequently throws the
+short-styled form as a recessive (de Vries, 1907, p. 406). It may be
+remembered that de Vries's previous publications had already shown that
+the short style of _brevistylis_, one of the _Lamarckiana_ "mutants,"
+behaves as a recessive habitually (_Mutationstheorie_, II, p. 178, etc.).
+
+Also when _nanella_, the dwarf "mutant" of _Lamarckiana_ is used as
+male on _muricata_ as female, _laeta_ and _velutina_ are produced, but
+one only of these, namely, _velutina_, subsequently throws dwarfs on
+self-fertilisation. The dwarfs thus thrown are said to form about 50 per
+cent. of the families in which they occur (de Vries, 1908, p. 668). The
+fact that the two forms, _laeta_ and _velutina_, are produced by many
+matings in which _Lamarckiana_ and its mutant _rubrinervis_ are used as
+males is confirmed abundantly by Honing, who has carried out extensive
+researches on the subject. After carefully reading his paper, I have
+failed to understand the main purport of the argument respecting the
+"double nature" of _Lamarckiana_ which he founds on these results, but
+I gather that in some way _laeta_ is shown to partake especially of the
+nature of _Lamarckiana_, while _velutina_ is a form of _rubrinervis_.
+The paper contains many records which will be of value in subsequent
+analysis of these forms.
+
+Before considering the possible meaning of these facts we must have
+in our minds the next and most novel of the recent extensions of
+knowledge as to the genetic properties of the _Oenotheras_. In the
+previous statement we have been concerned with the results of using
+either _Lamarckiana_ itself or one of its "mutants" _rubrinervis_,
+_brevistylis_, or _nanella_ as male, on one of the species _biennis_
+or _muricata_. The new experiments relate to crosses between the two
+species _biennis_ and _muricata_ themselves.
+
+De Vries found:
+
+1. That the reciprocal hybrids from these two species differed,
+_biennis_ × _muricata_ producing one type of F_{1} and _muricata_ ×
+_biennis_ producing another. Each F_{1} resembled the father more than
+the mother.
+
+2. That each of the hybrids so produced breeds true on
+self-fertilisation.
+
+3. That if we speak of the hybrid from _biennis_ × _muricata_ as _BM_
+and of the reciprocal as _MB_, then
+
+          _BM_ × _MB_
+
+gives exclusively offspring of _biennis_ type but that
+
+          _MB_ × _BM_
+
+gives exclusively offspring of _muricata_ type. Evidently, apart
+from all controversy as to the significance of the "mutants" of
+_Lamarckiana_, we have here a series of observations of the first
+importance.
+
+The fact that reciprocal crossings give constantly distinct results
+must be taken to indicate that the male and female sides of one, if not
+of both, of the parents are different in respect of characters which
+they bear. This is de Vries's view, and he concludes rightly, I think,
+that the evidence from all the experiments shows that both _biennis_
+and _muricata_ are in this condition, having one set of characters
+represented in their pollen-grains and another in their ovules. The
+plants breed true, but their somatic structures are compounded of the
+two sets of elements which pass into them from their maternal and
+paternal sides respectively. This possibility that species may exist of
+which the males really belong to one form and the females to another, is
+one which it was evident from the first announcement of the discovery of
+Mendelian segregation might be found realised in nature.[5]
+
+_Oe. biennis_ and _muricata_ were crossed reciprocally with each other
+and with a number of other species, and the behaviour of each, when
+used as mother, was consistently different from its behaviour when
+used as father. De Vries is evidently justified by the results of
+this series of experiments in stating that the "Bild," as he terms
+it, or composition of the male and female sides of these two species,
+_biennis_ and _muricata_, are distinct. On the evidence before us it
+is not, however, possible to form a perfectly clear idea of each, and
+until details are published, a reader without personal knowledge of the
+material cannot do more than follow the general course of the argument.
+For fuller comprehension a proper analysis of the characters with a
+clear statement of how they are distributed among the several types and
+crosses is absolutely necessary. According to de Vries the female of
+_biennis_ possesses a group of characters which he defines as "_conica_"
+in allusion to the shape of the flower-buds. Besides the conical buds,
+this group of features includes imperfect development of wood, rendering
+the plant very liable to attacks of _Botrytis_, and comparatively narrow
+leaves.
+
+The female of _muricata_ carries a group of features which he
+calls "_frigida_," and, though this is not quite explicitly stated
+in a definition of that type, it is to be inferred[6] that its
+characteristics are regarded as greater height, strong development of
+wood with comparative resistance to _Botrytis_, and broad leaves.
+
+The characters borne by the male parts of the two species are in general
+those by which they are outwardly distinguished. For example, the
+leaves of _Oe. biennis_ are comparatively broad and are bright green,
+while those of _muricata_ are much narrower and of a glaucous green,
+and I understand that de Vries regards these properties as contributed
+by the male side in each case and to be carried by the male cells of
+each species. The suggestion as regards _biennis_ and _muricata_ comes
+near the conception often expressed by naturalists in former times (_e.
+g._, Linnaeus) and not rarely entertained by breeders at the present
+day, that the internal structure is contributed by the mother and the
+external by the father.
+
+On the other hand, the offspring of each species when used as mother is
+regarded as possessing in the main the features of the maternal "Bild,"
+but the matter is naturally complicated by the introduction of features
+from the father's side, and it is here especially that the account
+provided is at present unsatisfactory and inconclusive. There seems,
+however, to be no serious doubt that _biennis_ and _muricata_ each in
+their outward appearance exhibit on the whole the features which their
+pollens respectively carry, and that the features borne by their ovules
+are in many respects distinct.
+
+The _types_ are thus "hybrids" which breed true. The results of
+intercrossing them each way are again "hybrids" which breed true. It
+will be remembered that on former occasions de Vries has formulated a
+general rule that _species_-hybrids breed true, but that the cross-breds
+raised by interbreeding _varieties_ do not. One of these very cases
+was quoted[7] as an illustration of this principle, viz: _muricata_ ×
+_biennis_. The grounds for this general statement have always appeared
+to me insufficient, and with the further knowledge which the new
+evidence provides we are encouraged to hope that when a proper factorial
+analysis of the types is instituted we shall find that the phenomenon
+of a constant hybrid will be readily brought into line with the systems
+of descent already worked out for such cases as that of the Stocks, and
+others already mentioned.
+
+In further discussion of these facts de Vries makes a suggestion which
+seems to me improbable. Since the egg-cells of _muricata_, for instance,
+bear a certain group of features which are missing on the male side,
+and conversely the pollen bears features absent from the female side,
+he is inclined to regard the _bad pollen grains_ as the bearers of the
+missing elements of the male side and to infer that there must similarly
+be defective ovules representing the missing elements of the female
+side. No consideration is adduced in support of this view beyond the
+simple fact that the characters borne by male and female are dissimilar,
+whereas it would be more in accord with preconception if the same sets
+of combinations were represented in each--as in a normal Mendelian case.
+There is as yet no instance in which the absence of any particular
+class of gametes has been shown with any plausibility to be due to
+defective viability, though there are, of course, cases in which certain
+classes of zygotes do not survive owing to defective constitution (_e.
+g._, the albinos of _Antirrhinum_ studied by Baur, and the homozygous
+yellow mice). I am rather inclined to suppose that in these examples of
+hybrids breeding true we shall find a state of things comparable with
+that to which we formerly applied the terms "coupling" and "repulsion."
+In these cases certain of the possible combinations of factors occur
+in the gametic series with special frequency, being in excess, while
+the gametes representing other combinations are comparatively few.
+In a recent paper on these cases Professor Punnett and I have shown
+that these curious results vary according to the manner in which the
+factors are grouped in the parents. If _A_ and _B_ are two factors which
+exhibit these phenomena we find that the gametic series of the double
+heterozygote differs according as the combination is made by crossing
+_AB × ab_, or by crossing _AB × aB_. In a normal Mendelian case the
+F_{1} form, _AaBb_, produces gametes _AB_, _Ab_, _aB_, _ab_, in equal
+numbers; but in these peculiar cases those gametes which contain
+
+                          Gametic series      Number of   Number of
+                      ---------------------    gametes     zygotes
+                       AB    Ab    aB    ab   in series    formed
+
+  Partial repulsion {   1  (n-1) (n-1)    1      2n         4n^{2}
+     from zygote    {   1    31    31     1      64         4096
+       of form      {   1    15    15     1      32         1024
+        Ab×aB       {   1     7     7     1      16          256
+                    {   1     3     3     1       8           64
+                        1     1     1     1       4           16
+  Partial coupling  {   3     1     1     3       8           64
+    from zygote     {   7     1     1     7      16          256
+      of form       {  15     1     1    15      32         1024
+       AB×ab        {  31     1     1    31      64         4096
+                    {  63     1     1    63     128        16384
+                    { (n-1)   1     1   (n-1)    2n         4n^{2}
+
+
+                            Nature of zygotic series
+                      ---------------------------------------
+                             AB        Ab     aB        ab
+  Partial repulsion {     2n^{2}+1  n^{2}-1  n^{2}-1     1
+     from zygote    {       2049      1023    1023       1
+       of form      {        513       255     255       1
+        Ab×aB       {        129        63      63       1
+                    {         33        15      15       1
+                               9         3       3       1
+                    {         41         7       7       9
+  Partial coupling  {        177        15      15      49
+     from zygote    {        737        31      31     225
+       of form      {       3009        63      63     961
+        AB×ab       {      12161       127     127    3969
+                    { 3n^{2}-(2n - 1) 2n-1  2n-1  n^{2}-(2n-1)
+
+
+the _parental combinations_ are in excess. This excess almost certainly
+follows the system indicated by the accompanying table. In the general
+expressions _n_ is half the number of gametes required to express the
+whole system. Now if we imagine that sex-factors are involved with the
+others concerned in such a relationship as this we have a system of
+distribution approximating to that found in _biennis_ and _muricata_.
+The difference in reciprocals is represented in a not improbable way.
+It cannot yet be said that the rarer terms in the series are formed at
+all, and perhaps they are not. As we pointed out in our discussion of
+these phenomena, the peculiar distribution of factors in these cases
+must be taken to mean that the planes of division at some critical stage
+in the segregation are determined with reference to the parental groups
+of factors, or in other words, that the whole system has a polarity,
+and that the distribution of factors with reference to this polarity
+differs according to the grouping of factors in the gametes which united
+in fertilization to produce the plant. Subsequent proliferation of
+cells representing certain combinations would then lead to excess of
+the gametes bearing them. It is on similar lines that I anticipate we
+shall hereafter find the interpretation of the curious facts discovered
+by de Vries, though it is evident that a long course of experiment and
+analysis must be carried through before any certainty is reached. The
+work must be begun by a careful study of the descent of some single
+factor, for example, that causing the broader leaf of _biennis_, and we
+may hope that the study of _Oenothera_ by proper analytical methods will
+no longer be deferred.
+
+We have now to return to the relations of _laeta_ and _velutina_.
+These two forms, it will be remembered are frequently produced when
+_Lamarckiana_ or one of its derivatives is used as male, and the most
+unexpected feature in their behaviour is that _both breed true as
+regards their essential characteristics, on self-fertilisation_. If one
+only bred true the case might, in view of the approximate numerical
+equality of the two types, be difficult to interpret on ordinary lines,
+but as both breed true it must be clear that some quite special system
+of segregation is at work. What this may be cannot be detected on the
+evidence, but with the results from the _biennis-muricata_ experiments
+before us, it is natural to suspect that we may here again have to
+recognise a process of allocation of different factors to the male and
+female sides in _laeta_ and _velutina_. That some such system is in
+operation becomes the more probable from the new fact which de Vries
+states in describing the group of characters which he calls _conica_,
+namely that this type is the same as that of _velutina_.
+
+There are many collateral observations recorded both by de Vries and
+others which have a bearing on the problems, but they do not yet fall
+into a coherent scheme. For example, we cannot yet represent the
+formation of _laeta_ and _velutina_ from the various species fertilised
+by _Lamarckiana_ [M]. That this is not due to any special property
+associated with the pollen of _Lamarckiana_ is shown by the fact that
+a species called _Hookeri_ gives _laeta_ and _velutina_ in both its
+reciprocal crosses with _Lamarckiana_ (de Vries, 1909, p. 3), and also
+by the similar fact that _Lamarckiana_ [F] fertilised by the pollen of
+a peculiar race of _biennis_ named _biennis Chicago_ throws the same
+types. Before these very complicated phenomena can be usefully discussed
+particulars must be provided as to the individuality of the various
+plants used. This criticism applies to much of the work which de Vries
+has lately published, for, as we now know familiarly, plants to which
+the same name applies can be quite different in genetic composition.
+
+Attention should also be called to one curiously paradoxical series of
+results. When the dwarf "mutant" of _Lamarckiana_ which de Vries names
+"_nanella_" is used as father on _muricata_, F_{1} consists of _laeta_
+and _velutina_ in approximately equal numbers. Both forms breed true to
+their special characteristics, but _velutina_ throws dwarfs of its own
+type, while _laeta_ does not throw dwarfs. Subsequent investigation of
+the properties of these types has led to some remarkable conclusions,
+and it was in a study of these plants that de Vries first came upon the
+phenomena of dissimilarity between the factors borne by the male and
+female cells of the same plant, a condition which had been recently
+detected in the Stocks as a result of Miss Saunders's investigations.
+The details are very remarkable. We have first the fact that _muricata_
+[F] × dwarf _nanella_ [M] gives about 50 per cent. _laeta_ and about 50
+per cent. of _velutina_.
+
+As regards _Velutina_ it was shown that:
+
+                                                 Talls,     Dwarfs,
+                                                per cent.  per cent.
+
+  1. Velutina selfed gave                          38         62
+
+    {Velutina [F] × dwarf nanella [M] gave         39         61
+  2.{    do.      × do.               gave         49         51
+    {    do.      × dwarf [M] derived from
+                                 velutina gave     43         57
+  3. Dwarfs × velutina [M] gave                    --     all dwarfs
+
+The three experiments taken together prove, as de Vries says, that the
+ovules of _velutina_ are mixed, talls and dwarfs, and that the pollen is
+all dwarf. The condition is almost the same as that of the Stocks. It
+may be noted also that in the Stocks the egg-cells of the "double" type
+are in excess, being approximately 9 to 7 of the "single" type, but de
+Vries regards the two types in _velutina_ as probably equal in number.
+The figures (169:231) rather suggest some excess of the recessives,
+perhaps 9:7, and the point would be worth a further investigation.
+
+As regards _laeta_, by self-fertilisation _no dwarfs were produced_, but
+in all other respects it behaved almost exactly like _velutina_. The
+ovules are evidently mixed talls and dwarfs, and whether fertilised by
+dwarfs or by the pollen of _velutina_, which is already proved to be all
+dwarf, the result was a steady 50 per cent. of talls and 50 per cent. of
+dwarfs. The pollen of _laeta_ used on dwarfs gives nothing but dwarfs,
+and in three series of such experiments 226 dwarfs were produced.
+
+We are thus faced with this difficulty. Since the egg-cells of _laeta_
+are evidently mixed, talls and dwarfs, and the pollen used on dwarfs
+gives all dwarfs, why does not self-fertilisation give a mixed result,
+talls and dwarfs, instead of _all talls_? De Vries regards the result
+of self-fertilisation as showing the real nature of the pollen, and
+declares it to be all talls, while he represents the behaviour of
+the same pollen used on dwarfs by stating that in these combinations
+the dwarf character dominates. This does not seem to me a natural
+interpretation. I should regard the pollen of _laeta_ as identical
+with that of _velutina_, namely dwarf, and I suspect the difficulty
+is really created by the behaviour of _laeta_ on self-fertilisation.
+Until a proper analysis is made in which the identity of the different
+individuals used is recorded, no further discussion is possible.[8]
+
+Other results of a complicated kind involving production of _laeta_ and
+_velutina_ together with a third form have been published by de Vries in
+his paper on "Triple Hybrids." To these also the same criticism applies.
+Some of the observations seem capable of simple factorial representation
+and others are conflicting.
+
+Taking the work on _Oenothera_ as a whole we see in it continually
+glimpses of order which further on are still blocked by difficulties
+and apparent inconsistencies. Through such a stage all the successful
+researches in complicated factorial analysis have passed and I see
+no reason for supposing that with the application of more stringent
+methods this more difficult set of problems will be found incapable
+of similar solutions. To return to the original question whether in
+_Oenothera_ we can claim to see a special contemporaneous output of new
+species in actual process of creation, it will be obvious that while
+the interrelation of the several types is still so little understood,
+such a claim has no adequate support. It is true that many of the
+"mutants" of _Lamarckiana_ can well pass for species, but this is
+equally true of many new combinations of pre-existing factors as we
+have seen in _Primula Sinensis_ and other cases. Still less can it be
+admitted that these facts of uncertain import supply a justification for
+the conception which has played a prominent part in the scheme of the
+_Mutationstheorie_, namely that there are special periods of Mutation,
+when the parent-species has peculiar genetic properties. To conclude:
+The impression which the evidence leaves most definitely on the mind is
+that further discussion of the bearing which the _Oenotheras_ may have
+on the problem of evolution should be postponed until we have before
+us the results of a searching analysis applied to a limited part of
+the field. In such an analysis it is to be especially remembered that
+we have now a new clue in the well-ascertained fact that the genetic
+composition of the male and female germ-cells of the same individual
+may be quite different. When with this possibility in view the behaviour
+of the types is re-examined I anticipate that many of the difficulties
+will be removed.
+
+Outside the evidence from _Oenothera_, which, as we have seen, is
+still ambiguous, I know no considerable body of facts favourable to
+that special view of Mutation which de Vries has promulgated. Of
+variation, or if we will, Mutation, in respect of some one character,
+or resulting from recombination, there is proof in abundance; but of
+that simultaneous variation in several independent respects to which de
+Vries especially attributes the origin of new specific types I know only
+casual records which have yet to undergo the process of criticism.
+
+                *       *       *       *       *
+
+Besides de Vries's "_Mutationstheorie_" and "Species and Varieties"
+the chief publications relating to the subject of the behaviour of
+_Oenothera_ are the following: (Many other papers relating especially to
+the cytology of the forms have appeared.)
+
+   Davis, B. M. Genetical Studies on _Oenothera_,
+     I. _Amer. Nat._, XLIV, 1910, p. 108. Genetical Studies on
+     _Oenothera_, II. _Ibid._, XLV, 1911, p. 193.
+
+   Gates, R. R. An Analytical Key to some of the Segregates of
+     Oenothera. _Twentieth Annual Report of the Missouri Botanical
+     Garden_, 1909.
+
+   Studies on the Variability and Heritability of Pigmentation in
+     _Oenothera_. _Ztsch. f. Abstammungslehre_, 1911, IV, p. 337.
+
+   Honing, J. A. Die Doppelnatur der _Oenothera Lamarckiana_.
+     _Ztsch. f. Abstammungslehre_, 1911, IV, p. 227.
+
+   Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K.
+     Small). Mutants and Hybrids of the _Oenotheras_. _Carnegie
+     Institution's Publication_, No. 24, 1905.
+
+   Macdougal, D. T., Vail, A. M., Shull, J. H. Mutations,
+     Variations and Relationships of the _Oenotheras_. _Carnegie
+     Institution's Publication_, No. 81, 1907.
+
+   de Vries, H. On Atavistic Variation in _Oenothera cruciata_.
+     _Bull. Torrey Club_, 1903, Vol. 30, p. 75.
+
+   On Twin Hybrids, _Bot. Gaz._, Vol. 44, 1907, p. 401.
+
+   Ueber die Zwillingsbastarde von _Oenothera nanella_. _Ber.
+     Deut. Bot. Ges._, 1908, XXVI, _a_, p. 667.
+
+   Bastarde von _Oenothera gigas_. _Ibid._, p. 754.
+
+   On Triple Hybrids. _Bot. Gaz._, 1909, Vol. 47, p. 1.
+
+   Ueb. doppeltreziproke Bastarde von _Oenothera biennis_ L. und
+     _Oenothera muricata_ L. _Biol. Cbltt._, 1911, XXXI, p. 97.
+
+   Zeijlstra, H. H. _Oenothera nanella_ de Vries, eine krankhafte
+     Pflanzenart. _Biol. Cbltt._, 1911, XXXI, p. 129.
+
+
+                              NOTE.
+
+Since this chapter was written two contributions of special importance
+have been made to the study of the _Oenothera_ problems. The first is
+that of Heribert-Nilsson.[9] The author begins by giving a critical
+account of the evidence for de Vries's interpretation of the nature of
+the mutants. In general this criticism pursues lines similar to those
+sketched in the foregoing chapter, concluding, as I have done, that the
+chief reason why factorial analysis has been declared to be inapplicable
+to the _Oenothera_ mutants is because no one has hitherto set about this
+analysis in the right way. He has also himself made a valuable beginning
+of such an analysis and gives good evidential reasons for the belief
+that at least the red veining depends on a definite factor which also
+influences the size of certain parts of the plant. He argues further
+that many of the distinctions between the mutants are quantitative
+in nature. With great plausibility he suggests that the system of
+cumulative factors which Nilsson-Ehle discovered in the case of wheat
+(subsequently traced by East in regard to maize) may be operating also
+in these _Oenotheras_. According to this system several factors having
+similar powers may coexist in the same individual, and together produce
+a cumulative effect. Scope would thus be given for the production of
+the curious and seemingly irregular numbers so often recorded in the
+"mutating" families.
+
+Another remarkable observation relating to the crosses of _muricata_
+and _biennis_ has been published by Goldschmidt.[10] He finds that in
+the formation of this cross the female pronucleus takes no part in the
+development of the zygotic cell, but that when the male pronucleus
+enters, the female pronucleus is pushed aside and degenerates. As de
+Vries observed, the reciprocal hybrids are in each case very like the
+father ("_stark patroklin_"), a consequence which finds a natural
+explanation in the phenomenon witnessed by Goldschmidt. The results
+of the subsequent matings can also be readily interpreted on the same
+lines. Indications of maternal characters are nevertheless mentioned
+by de Vries, and if Goldschmidt's account of the cytology is confirmed,
+these must presumably be referred to the influence of the maternal
+cytoplasm. Clearly this new work opens up lines of exceptional interest.
+The interpretation I have offered above must probably be reconsidered.
+The distinction between the male and female cells of the types may no
+doubt be ultimately factorial, but it is difficult to regard such a
+distinction as created by a differential distribution of the ordinary
+factors.
+
+FOOTNOTES:
+
+[1] See Lotsy and Baur, Rep. Genetics Conf., Paris, 1911, pp. 416-426.
+Compare Lecoq on _Mirabilis jalapa_ × _longiflora_, Fécondation des
+Végétaux, 1862, p. 311.
+
+[2] _Rep. Evol. Ctee. R. S._, IV, 1908, p. 38.
+
+[3] _Ber. Deut. Bot. Ges._, 1908, XXVI, _a_, p. 672.
+
+[4] _Jour. Genetics_, 1, 1910, p. 57.
+
+[5] In Rep. 1 to Evol. Committee, 1902, p. 132, attention was called to
+this possibility, though of course at that date it was in sexual animals
+alone that it was supposed to exist. It had not occurred to me that even
+a hermaphrodite plant might be in this condition.
+
+[6] From the description of the offspring of _muricata_ used as mother.
+
+[7] de Vries, _Species and Varieties_, 1905, p. 259.
+
+[8] Zeijlstra in a recent paper announces that many _nanella_ plants
+are the subject of a bacterial disease to which he attributes their
+dwarfness. I gather that this does not apply to all _nanella_ plants
+and that some are dwarfs apart from disease. The matter may no doubt be
+further complicated from this cause.
+
+[9] _Zts. f. Abstamm._, 1912, VIII.
+
+[10] _Arch. f. Zellforschung_, 1912, IX, p. 331.
+
+
+
+
+                       CHAPTER VI
+
+                  VARIATION AND LOCALITY
+
+In all discussions of the modes of Evolution the phenomena of
+Geographical Distribution have been admitted to be of paramount
+importance. First came the broad question, were the facts of
+distribution consistent with the Doctrine of Descent? I suppose all
+naturalists are now agreed that they are thus consistent, and that
+though some very curious and as yet inexplicable cases remain to be
+accounted for, the distribution of animal and plant life on the face
+of the earth is much what we might expect as a result of a process of
+descent with modification. Passing from this general admission to the
+more particular question whether the facts of distribution favour one
+special conception of the mode of progress of evolution rather than
+another, no agreement has yet been reached. One outstanding feature
+is hardly in dispute, namely that prolonged isolation is generally
+followed by greater or less change in the population isolated. Groups
+of individuals which from various causes are debarred from free
+intermixture with other groups almost always exhibit peculiarities,
+but on the other hand, cosmopolitan types which range over wide areas
+are on the whole uniform, or nearly so throughout their distribution.
+Examples of these two categories will be familiar to all naturalists.
+The barriers to intercourse may be seas, deserts, prairies,
+mountain-chains, or circumstances of a much less obvious character
+which isolate quite as effectually. The local unit is not necessarily
+an island, a district, or an area of special geological formation,
+but may, as every collector knows, be a valley, a pond, a creek, a
+"bank" in the sea, a clump of trees, a group of rocks in a bay, or a
+particular patch of ground on a mountain side. All the great groups
+provide examples of such specially isolated forms. The botanist knows
+them well; the conchologist, the entomologist, the ornithologist and the
+student of marine life are all equally aware that special varieties or
+special species come from special places and from nowhere else. In one
+remarkable case the season of appearance plainly acts as the isolating
+barrier. _Tephrosia bistortata_ is a small Geometrid moth which has two
+broods, appearing in _March_ and _July_ respectively. It is closely
+allied to _T. crepuscularia_ which emerges in _May_ and _June_. From the
+fact that occasional specimens cannot be quite certainly referred to
+one or other of the two, many have held that the two are one species.
+Nevertheless, in general they present distinctions which are plain
+enough. Some localities have one form only, but in several woods they
+co-exist. Experiment has shown that the two can be crossed, and that the
+cross-breds can breed _inter se_ and with at least one of the parent
+stocks.[1] Some diminution in fertility was observed, but perhaps not
+more than is commonly encountered when wild forms are bred in captivity.
+In such a case it can scarcely be doubted that the distinctness of
+the two forms in the places where they co-exist is maintained by the
+seasonal isolation.
+
+Just as the consequences of isolation are to be seen in the most
+different forms of life so may they also affect the most diverse
+features of organisation, such as size, colour, sculpture, shape, or
+number of parts. In the Sloth (_Choloepus_) the geographical races
+differ in the number of cervical vertebrae--or in other words, in the
+distribution of vertebral differentiation. The geographical races of
+_Cistudo_ differ in the number of claws and phalanges.[2]
+
+In Shetland, the males of _Hepialus humuli_ (the Ghost Moth) are
+not sharply differentiated in colour from the females, as they are
+elsewhere, but in varying degrees resemble them.[3] No such males are
+found in other localities, and even in the other Scottish islands they
+are normal. In the island of Waigiu the converse phenomenon has been
+observed in _Phalanger maculatus_. Generally the male is spotted with
+white, and the female is without spots, but in Waigiu the females are
+spotted like the males.[4]
+
+The following striking illustration was pointed out to me by Dr. W. D.
+Miller. _Euphonia elegantissima_ as it occurs in Mexico and Central
+America has the two sexes very distinct from each other. The male has
+the lower parts orange and the upper parts a dark indigo blue, with a
+bright turquoise-blue head and neck. The female, except for the head,
+is of a bright olive green. A form in which the sexes are similarly
+differentiated exists in Porto Rico and is known as _E. Sclateri_. But
+in many of the other West Indian islands the representative "species"
+(_E. flavifrons_) has the two sexes closely resembling the _female_
+of _E. elegantissima_. This form is found in Antigua, Barbados, St.
+Vincent, and Guadeloupe, from which localities the British Museum has
+specimens. All three so-called species are very much alike otherwise.
+
+In the genus _Pyrrhulagra_ (_Loxigilla_) to which Mr. Outram Bangs
+called my attention, several distinct and alternative possibilities
+occur. The genus has many local species occurring on the various West
+Indian islands. These species are characterized by differences in size,
+colour, and the shape of the bill. The colours have a narrow range,
+being black or greyish, with or without chestnut marks about the head
+and throat. In most of the islands the males are in general colour a
+full black, and the females are distinctly grey. They are thus found
+in San Domingo, Jamaica, Bahama, and most of the Lesser Antilles. In
+Porto Rico we meet the peculiarity that the hens are almost as black
+as the males (Ridgway describes the black of the hens as slightly less
+intense). This form is called _portoricensis_. A larger type, known
+as _grandis_, similarly coloured, inhabits St. Kitt's. Then, on the
+contrary, in Barbados, _both sexes_ are a dull blackish grey, like the
+hens of the Lesser Antilles in general.
+
+The local species of _Agelaius_ show similarly capricious distinctions.
+_A. phoeniceus_ is a widely spread species, found over a great part of
+North America. The male is black with red-orange bars on the wings, but
+the female is somewhat thrush-like in colour. In the island of Porto
+Rico there is a form called _xanthomus_, in which _both sexes_ are like
+the males of the mainland. A similar species called _humeralis_, also
+with both sexes male-like, lives in Cuba. The island of Cuba, curiously
+enough, has also a distinct species named _assimilis_, in which the
+female is a dull black all over, though the male is like the mainland
+type.
+
+So also may local races differ in respect of variability. _Argynnis
+paphia_, the Silver Washed Fritillary, through a great part of its
+distribution has only one female form. In the English New Forest a
+second female form, _valesina_, co-exists with the ordinary _paphia_
+female. But in the southern valleys of the Alps the _valesina_ female
+is much the commoner of the two, and indeed in some localities where
+the species is abundant, I have seen no _paphia_ females in many days
+collecting.
+
+The beetle _Gonioctena variabilis_ furnishes an illustration of a
+comparable phenomenon affecting the male sex. In 1894 and 1895 I
+studied the curious colour variations of this species especially in the
+neighbourhood of Granada, and Mr. Doncaster ten years later repeated the
+observations on the same ground, and also collected the insect in other
+places in the south of Spain. The distinctions are not easy to give in
+words and the reader is referred to the colour plate accompanying my
+paper.[5] The essential fact is that the males commonly have the elytra
+_red with black spots_ and the females for the most part have greenish
+grey elytra with black stripes. In some localities a large minority of
+males closely resemble the female type, being identical in colour and
+then only distinguishable by structural differences. In two Granada
+localities I found the proportion of such males quite different. In the
+Darro valley about 38 per cent. (in 718) were of this feminine type,
+but on the hills some 300 feet above only 19 per cent. (in 3,230) were
+like the females. At Castillejo, not far from Toledo I found no such
+male in 75 specimens.
+
+Mr. Doncaster collected from several localities, especially from two
+areas near Malaga, about 5 miles apart. In one of these the female-like
+males were, as usual, in a minority, but in the other these were
+actually in great excess, amounting to about 81 per cent. in the 173
+taken. Doncaster found a doubtful indication that the composition of the
+population varies with the season, which is quite possible, but it is
+most interesting to note that in my chief locality after the lapse of
+ten years he found the proportions very much the same as I had done at
+the same season, for where I had 19 per cent. of the female-like males
+his collecting gave 16 per cent. In other respects also, his statistics
+corresponded very closely with mine.[6]
+
+The various forms of _Heliconius erato_ are well known to entomologists.
+They are strikingly distinguished by the colours of the strong comb-like
+marking on the hind wing, which may be red, yellow, green or blue. In
+various parts of the distribution in South America sometimes two and
+sometimes three of these distinct types co-exist.[7]
+
+The distribution of the varieties of _Noctua castanea_ typifies a large
+range of cases. The form which is reckoned the normal of the species
+has red fore-wings. It is practically restricted to Great Britain and
+Germany, according to Tutt. The other common form, _neglecta_, has grey
+fore-wings, and in this pattern it ranges through West Central Europe
+from North Italy to Germany. In the British Isles it extends up to
+Orkney. In Britain this grey form is by far the commoner, occurring
+wherever the species is found. The red form is much scarcer in England,
+and does not occur at all in many localities where the grey form is
+common. Mr. Woodforde, from whom this account is taken,[8] states that
+in August, 1899, he saw considerably over a hundred of the grey in the
+New Forest at sugar, but only two red ones. In Staffordshire however the
+red is proportionately more numerous and he estimates them as 40 per
+cent. of the population. Lastly a form has been taken in Staffordshire
+as a rarity in which the red is replaced by yellow, and this has
+hitherto been seen nowhere else. It is beyond our immediate purposes
+to discuss the genetic relationships of such forms, but the details
+of this case are interesting as making fairly clear the fact that the
+distinctions between _castanea_ and _neglecta_ are due to combinations
+of the presence of and absence of two pairs of factors, of which one
+produces a red pigment in the ground colour of the forewing and the
+other irrorates the same region with black scales. Mr. Woodforde states
+that all intermediates exist, and that in Staffordshire the greys always
+have a pinkish tinge. The yellow is doubtless another recessive to the
+red.
+
+Species which are uniform in some localities may be polymorphic in
+others. Such a phenomenon is well exemplified by the orchid _Aceras
+hircina_. Of this species distinct varieties had previously been known
+in Germany, but Gallé[9] has lately given a detailed account of a
+number of most diverse forms found growing in a district of Eastern
+France. Without reference to his plates it is impossible to give any
+adequate conception of the profusion of types which the flowers of
+the species there assume. In some the lip is elongated to many times
+its usual length, twisting and dividing in a fashion suggesting some
+of the strangest of the Tropical Orchids. In others the labellum
+and the lateral petals are all comparatively short and wide (Fig.
+13). Intermediates, combining these qualities in various degrees,
+were abundant, and the condition of the species, which was the only
+representative of the genus in the locality, recalls the extreme
+polymorphism of many of the Noctuid Moths.
+
+[Illustration: FIG. 13. Various forms of _Aceras hircina_. (After
+Gallé.) This figure only shows a few of the more striking forms
+illustrated in Gallé's plates.]
+
+Somewhat comparable variability has been seen in another Orchid genus
+_Ophrys_. In Great Britain the species _apifera_, _aranifera_ and
+_muscifera_ though variable are fairly distinct, but Moggridge has
+published two series of plates[10] showing a very different state of
+things as regards the _Ophrys_ population of the Riviera. Here the
+outward diversity is such that the ordinary specific names cannot be
+applied with any confidence and the limits of the species are quite
+uncertain. It may well be supposed that these Riviera plants are
+interbreeding, and indeed we may safely assume that they are. It is,
+however, to be remembered that Darwin showed _apifera_ in this country
+to be habitually self-fertilised, so that the different behaviour on
+the Riviera may itself constitute a local peculiarity. Moreover it is
+to be gathered from Moggridge's account that in the districts which he
+examined the condition was not to be described by the statement that
+our three types were there co-existing and hybridising, but rather we
+should say that the population was polymorphic, containing these three
+types amongst others. Conchologists are aware that on the Dogger Bank
+_Modiola_ attains a size unparalleled elsewhere. The same is true of
+the sponges _Grantia compressa_ and _Grantia ciliata_ in the estuary
+of the Orwell.[11] Conversely, as we know so well in the case of Man,
+dwarf races occur in several special localities. Such examples may be
+multiplied indefinitely.
+
+The relation of local forms to species has often been discussed from
+many points of view, but I know no treatment of the subject clearer or
+more comprehensive than an excellent account of some of the various
+manifestations of local differentiation as they appear in Helicidæ
+published by Coutagne[12] and a reader interested in the problem which
+they raise would do well to make himself acquainted with the original
+from which the following notes are taken. He speaks for example of
+_Helix lapicida_. This is on the whole a constant form ranging up to the
+altitude of 1,300 m., common all over France except at great heights
+and in the Olive regions where it is restricted to moist places. Though
+subjected to such diverse conditions it shows only trivial variations in
+colour and other respects throughout its distribution, excepting that
+on both sides of the Pyrenees it has a very distinct sporadic variety
+called _Andorrica_ or _microporus_. This variety occurs here and there,
+together with the type-form sometimes in colonies (pp. 26-30 and 86).
+
+_Bulimus detritus_ though more restricted in geographical range is a
+much more variable form. It exhibits great variations in colour, form,
+and size, and as Coutagne well insists, these are independent of each
+other. Foreshadowing the methods of factorial analysis he suggests that
+distinctions in each respect, the "modes" as he calls them, should
+be denoted by a letter, or if desired, by a name, and the several
+combinations of differences might thus be most logically and usefully
+expressed. Of such combinations he says there are at least 18, all of
+which can be found. The whole possible series does not necessarily
+occur in the same place, and various localities are characterised by
+the presence or absence of certain of the combinations as Coutagne
+calls them, and by the relative frequency with which they occur. The
+ideas thus enunciated are much in advance of the ordinary practice of
+systematists, who give names to forms which are nothing but accidental
+combinations of factors, just as the horticulturists for practical
+reasons give names to similar combinations, which as we now know are
+merely specially noticeable terms in a long series of possibilities.
+In each case it is rather the _factors_ which should be named than the
+forms which are constituted by their casual collocation. In this special
+example of _Bulimus detritus_ the 18 forms are made by the combinations
+of three pairs of independent factors. Besides these combinations which
+may occur anywhere or almost anywhere in the distribution there are
+two more distinct local forms, each of which is regarded by Coutagne
+as probably constituting a fresh "mode," perhaps compatible with the
+others.
+
+_Helix striata_ (Draparnauld)[13] is truly polymorphic; and its various
+forms have been described under various specific names. It abounds in
+the calcareous hills of Provence and Languedoc, disappearing in the
+alluvial lowlands and equally in the upper levels at about 800-1,000 m.
+From this district it extends through regions of similar altitude over a
+great part of France (details given).
+
+Locard in his monograph of this group, which he calls collectively the
+group of _Helix Heripensis_, tabulates 27 distinct named forms. The
+characteristics in which these forms differ have been reckoned as 17,
+and as several of these vary in degree of development, the number of
+modes may be increased to 109. For practical purposes however Coutagne
+considers that the various developments of 7 characteristics in their
+several combinations are enough to express the various forms, and he
+gives examples of this method of definition. As he observes, though
+names may be required to define the modes, no one need be alarmed at
+that, for the same names of modes will be applicable to a great range of
+distinct species, and the formulae expressing their combinations will
+replace the varietal names.
+
+This particular example of polymorphism is but little limited by
+locality. Occasional colonies present some special physiognomy which may
+in a given place seem almost invariable, though in this very respect the
+colonies found elsewhere may be highly variable, but such limitations
+are exceptional for _H. striata_.
+
+Some distinct and obvious susceptibilities to the influence of soil
+and climate are however noticeable. For example on siliceous ground
+the shells are thinner, while on calcareous soils they are thicker;
+similarly those from the Northern districts attain a larger size
+than those from further South. Moreover those subjected to curtailed
+development, whether from drought, heat or cold often show a shortening
+of the spire. In contrast with this case Coutagne describes the
+varieties of _Helix caespitum_, which he says are for the most part
+localised, quoting many illustrative cases.
+
+Another remarkable case in which locality plays a curious part is
+provided by the two species _Helix trochoides_ and _pyramidata_. In
+France generally they are distinct enough from each other, _trochoides_
+being smaller and having a characteristic keel. Coutagne says that after
+having collected these species from more than a score of localities he
+came upon a colony of _trochoides_ on the island of Pomègues in which
+the shells were relatively enormous, most of them having only a slight
+keel, and a few none at all. On the other hand he received a consignment
+of _pyramidata_ from four localities in Sicily, all small, and one of
+them exactly like the _trochoides_ from Pomègues. Judging by the samples
+received from Sicily, _trochoides_ is there not more variable than it is
+in Provence, while the Sicilian _pyramidata_ is protean.
+
+The relations of the two species _Helix nemoralis_ and _hortensis_
+provide an illustration of another kind of manifestation of local
+peculiarity. _H. hortensis_ and _nemoralis_ as usually met with, are
+two very distinct forms. _H. hortensis_ is smaller and duller, and its
+peristome is white. _H. nemoralis_ is larger and more shiny, and its
+peristome is brown. In several anatomical points, moreover, especially
+in the shape of the dart, there are great differences. For a full
+account of these peculiarities of the two forms and a discussion of
+their inter-relations the reader is referred to the elaborate work of
+A. Lang[14] who has studied them extensively and has also succeeded in
+experimentally raising hybrids between them. These hybrids were in a
+slight degree fertile with both the parent species, but up to the time
+of publication no young had been reared from hybrids _inter se_.
+
+Coutagne describes the result of collections made in 62 French
+localities. Some had exclusively _hortensis_, some exclusively
+_nemoralis_, and in some the two were found in association. He gives
+details of five of these collections from which I take the following
+summary of the more essential facts, omitting much that is almost
+equally significant.
+
+_Locality A_, near Honfleur. Both forms present, each sharply and
+normally distinguished, without any intermediates. They are thus found
+in many places. Coutagne instances Müller's observations in Denmark, his
+own series from the Jura, etc.
+
+_Locality B._ Vonges (Côte d'Or), 242 _hortensis_ taken at random,
+showed 128 with light peristomes (either more or less pinkish or
+quite white) and 114 with dark _brown_ peristomes; together with 26
+_nemoralis_ all with the usual brown peristomes.
+
+Of the _hortensis_ 50 were in ground-colour _opalescens_ and 1 _roseus_;
+and in shape 5 were _umbilicatus_.
+
+_Locality C_, about 3 kilometres from _B_. There were found 35
+_hortensis_, of which 20 had light peristomes and 15 brown; together
+with 7 _nemoralis_.
+
+Of the _hortensis_ none were _opalescens_; 18 were _roseus_ and none has
+the shape of _umbilicatus_.
+
+_Locality D_, about 1,200 metres from _B_. 147 _hortensis_, of which 4
+had light peristomes and 143 had brown. No _nemoralis_ were found.
+
+None of the _hortensis_ were _opalescens_ or _roseus_, but 30 were
+_umbilicatus_.
+
+In these localities intermediates of every grade existed between the
+well-characterised _opalescens_, _roseus_, or _umbilicatus_, and the
+other forms, but there were no intergrades between the other _nemoralis_
+and the smaller _hortensis_, about which there was no hesitation. In the
+next locality a very different state of things was found.
+
+_Locality E._ Banks of the Yvette at Orsay (Seine-et-Oise). The
+actual numbers are not given, but we are told that 58 per cent. were
+_hortensis_, 33 per cent. _nemoralis_, and 9 per cent. intermediate. As
+at Honfleur, the _hortensis_ had white peristomes, and the _nemoralis_
+brown. Coutagne's visits to this locality were in 1878 and 1880, and he
+calls attention to the fact that Pascal found similar intermediates in
+the same neighbourhood in 1873.
+
+The two species, in Coutagne's view, when they occur together, can
+generally be sorted from each other with perfect confidence, and it is
+only in exceptional localities that these intermediates occur. Whether
+they are hybrids, or whether sometimes the species in their variations
+transgress their usual limitations is regarded both by Coutagne and
+by Lang as a question not yet answerable with certainty. Coutagne
+moreover lays stress on the fact that although each species may be
+easily known from the other _in its own district_, yet when shells from
+different districts are brought together it is sometimes impossible to
+sort them. He mentions an example of such casual intermixture occurring
+under natural conditions on an island in the Rhone, to which it may
+well be supposed that floods had brought immigrants from miscellaneous
+localities. This population contained a very large number of uncertain
+specimens, and as he says, it was much as if he were to mix the shells
+from his 62 localities, after which it would certainly be impossible to
+separate the two species again.[15]
+
+Further evidence is given in the same treatise as to other examples
+of polymorphism, especially in the genus _Anodonta_, of which Locard
+made 251 species for France alone. Here again are cases like those
+already given, and many forms or "modes" are found restricted to special
+localities, while occasionally in the same locality dissimilar forms are
+found, collectively forming a colony, without intermediates.
+
+Taken as a whole the evidence shows the following conclusions to be
+true. Local races, whether of animals or plants, may be distinguished
+by characters which we are compelled to regard as trivial, or again by
+features of such magnitude that if they were known to us only as the
+characteristics of a uniform species they would certainly be assumed
+without hesitation to be essential for its maintenance. Local forms
+may be sharply differentiated from the corresponding populations of
+other localities or they may be connected with them by numbers of
+intermediates. Not rarely also we find a fact which has always seemed to
+me of special significance, that the peculiarity of the local population
+or colony may show itself in a special liability to variation, and
+this variability may show itself in one of many degrees, either in the
+constant possession of a definite aberration, in a dimorphism, or in an
+extreme polymorphism.
+
+At this stage attention should be called to two points. First, that
+when the details of the geographical distribution of any variable
+species are studied in that thorough and minute fashion which is
+necessary for any true knowledge of the interrelations of the several
+forms, the conception of a species invented by the popular expositions
+of Evolution under Selection is found to be rarely if ever realised in
+nature.
+
+A species in this generalised sense is an aggregate of individuals,
+none exactly alike, but varying round a normal type, the characters of
+which are fixed in so far as they are adapted to environmental exigency.
+In nature, however, the occurrence of the varieties, and even the
+occurrence of the variability is sporadic. In one place a population may
+be perfectly uniform. In another it may be again uniform but distinct.
+In others the two forms may occur together, sometimes with and sometimes
+without intergrades. In some localities a sporadic variety may be an
+element of the population, persisting through long periods of time.
+In other localities there may be several such aberrations occurring
+together which are absent elsewhere.
+
+Secondly, I would remind the reader that in the light of genetic
+analysis we know that intergrades, when they do occur, cannot be assumed
+to represent conditions through which the species must pass or has
+passed on its way to the extreme and definite forms.
+
+Often, perhaps generally, they are nothing but heterozygous forms,
+and often also they are conditions corresponding with the presence of
+factors in their reduction-stages.
+
+A broad survey of the facts shows beyond question that it is impossible
+to reconcile the mode of distribution of local forms with any belief
+that they are on the whole adaptational. Their peculiarities are
+occasionally the result of direct environmental influence, as we shall
+hereafter notice in certain cases, but none can attribute such sporadic
+and irregular phenomena to causes uniformly acting.
+
+Writers on systematics, especially those of former generations often
+conjecture or assert that local distinctions are caused by "differences
+of climate, soil, food, etc.," in vague general terms. It is usually
+safe to assume that these remarks do not represent conclusions drawn
+from actual evidence, for only rarely can they be translated into more
+precise language. So thoroughly have the biological sciences become
+permeated with the belief that all distinctions are dependent upon
+adaptation, that the mere existence of definite distinctions is felt
+by many to be sufficient ground to warrant an assumption that these
+distinctions are directly or indirectly due to special local conditions.
+For example, Dr. J. A. Allen, who has done so much careful and valuable
+work in delimiting the local forms of the United States fauna, writes of
+the Ground Squirrels (Tamias)[16] as follows:--
+
+   "From the extreme susceptibility of this plastic
+   group to the influences of environment, it is one of the most
+   instructive and fascinating groups among North American mammals.
+   No one can doubt its comparatively recent differentiation from
+   a common stock, and its dispersion from some common centre.
+   Whether the type originated at some point in North America, or in
+   the Northern part of Eurasia, it is perhaps idle to speculate,
+   but that it has increased, multiplied, spread, and become
+   differentiated to a wonderful degree in North America is beyond
+   question; as it is found from the Arctic regions to the high
+   mountain ranges of Central Mexico, and has developed some twenty
+   to thirty very palpable local phases."
+
+   "Some of them easily take rank as species, others as
+   subspecies. Probably a more striking illustration of evolution by
+   environment cannot be cited."
+
+He proceeds to point out that the habits of these creatures are such as
+lead to isolation. This may well be admitted, and indeed no exception
+can possibly be taken to the passage as a whole, save in the one
+respect that there is no real proof that the local diversity is due to
+"evolution by environment" or an indication of "susceptibility to the
+influences of environment."
+
+Dr. Allen does indeed adduce the fact that California "extending through
+800 miles of latitude, with numerous sharply contrasted physiographic
+regions, has apparently no less than six strongly differentiated forms,
+while the region east of the Rocky Mountains from a little below the
+northern boundary of the United States northward to the limit of
+trees--a slightly diversified region of at least ten times the area of
+California--has only one"! But when one comes to ask how the various
+forms are adaptational, and how the influences of environment have led
+to their production, only conjectures of a preliminary and tentative
+character could be expected in reply. Desert forms are no doubt pallid
+as in so many instances, and forest forms are more fully coloured, and
+we may readily enough accept such facts as indications of a connection
+between bodily features and the conditions of life, but further than
+that no one can go; so that when we find size, length of ears or of
+tail, the number of dorsal stripes, the pattern of the colours, not to
+speak of differences in the pigments themselves, all exhibiting large
+modifications, we cannot refer these peculiarities to the causation
+of environmental difference, save as a simple expression of faith. I
+incline far more to agree with Gulick who, after years of study of the
+local variations of the Achatinellidae, came to the conclusion that it
+was useless to expect that such local differentiation can be referred to
+adaptation in any sense.[17] Even the most convinced Selectionist must
+hesitate before such facts as those related by A. G. Mayer regarding
+the distribution of _Partula otaheitana_, one of these Achatinellidae.
+The island of Tahiti has been scored by erosion so that a series of
+separated valleys radiate to the coast. From four successive valleys
+Mayer collected the species, and found that in the first (Tipaerui)
+valley all the shells were dextral (115, containing 73 young); in the
+second valley (Fautaua) 54 per cent. of adults and 55.5 per cent. of
+the young contained were sinistral; in the third valley (Hamuta) 69
+per cent. of adults and 73 per cent. of young contained in them were
+sinistral; and lastly, in the fourth valley (Pirae) all the shells
+(131, containing 62 young) were sinistral.[18] In connection with
+these observations I may mention the fact that in a certain pond in
+the North of England[19] the sinistral form of _Limnaea peregra_ has
+been known to occur for about fifty years. Visiting it lately I found
+the left-handed shells to be about 3 per cent. of the population. The
+species is the commonest British freshwater shell, but left-handed
+specimens are exceedingly rare. Will anyone ask us to suppose that the
+persistence of a percentage of this rarity in the same place is an
+indication of some specially favouring circumstance in the waters of
+that pond? It is a horse-pond to all appearances exactly like any other
+horse-pond; and I believe that in perfect confidence we may accept the
+suggestion of common sense, which teaches us that there is nothing
+particular in the circumstances which either calls such varieties into
+existence or contributes in any direct way to their survival. Had the
+phenomenon of local variation been studied in detail before Darwin
+wrote, the attempt to make selection responsible for fixity wherever
+found, could never have been made. The proposition that not only the
+definiteness of local forms but their variability also is sporadic,
+can be established by countless illustrations taken from any group of
+either the animal or the vegetable kingdoms. Only exceptionally can the
+fixed differences be even suspected of contributing to adaptation, and
+sporadic variability, which is a no less positive fact, must manifestly
+lie outside the range of such suspicions. It is open to any one to
+suggest speculatively that the persistence of special varieties or of
+special variability in special places is an indication that in those
+places the conditions of life are such that the forms in question are
+tolerated though elsewhere the same types are exterminated; but that
+consideration, even if it could be proved to be well founded, is not
+one which lends much force to the thesis that definiteness of type is
+a consequence of Natural Selection. On the contrary, recourse to such
+reasoning implies the inevitable but very damaging admission that the
+stringency of Selection is frequently so far relaxed that two or more
+equally definite forms of the same species can persist side by side.
+There is no doubt that this is the simple truth, but when once that
+truth is perceived it is useless to invoke the control of Selection as
+the factor to which definiteness of type in general must be referred.
+
+The genetic relations of local forms to each other cannot in the
+absence of actual breeding experiments be often ascertained. Standfuss
+formerly enunciated as a general principle that when two forms
+co-exist in the same locality and are able to interbreed, they do not
+produce intermediates; but that when the forms are geographically
+separated as local races, crosses between them result in a series of
+intermediates.[20] In this aphorism there is a good deal of truth, but
+if in the light of Mendelian principles we examine the two statements
+we see now that the first is in reality only another way of saying
+that the distinctness of an aberrational form co-existing with another
+is due to segregation, accompanied by some degree of dominance of one
+type. Whether, however, one geographically isolated race will give
+intermediates when bred with another must depend entirely on the genetic
+physiology of the special case, and no general rule can be laid down.
+It may well be that, inasmuch as the distinctness of the variety is
+maintained by isolation, the difference in factorial composition between
+it and the representative form in another area is neither simple nor
+sharp; but when two varieties co-exist, though interbreeding, it is now
+clear that their differences must depend on the segregation of simple
+factors. Plainly such aberrations may in one place co-exist with another
+type, and elsewhere be separated from it as local races.
+
+Excellent illustrations of these two stages in evolution are provided
+by the melanic varieties of British Lepidoptera. The fact that black or
+blackish varieties of many species especially of Geometridae have come
+into existence in recent years is well known to British collectors,
+and it is not in dispute that they have in several instances replaced
+the older type more or less completely in certain districts. In the
+year 1900 the Evolution Committee of the Royal Society instituted a
+collective inquiry as to the contemporary distribution of these dark
+varieties. As the change had happened within living memory and had
+greatly progressed in recent years it was hoped that a record of the
+existing distribution would serve as a point of departure for future
+comparison. The records thus obtained were tabulated by Mr. L.
+Doncaster.[21] From that account and from the statements in Barrett's
+British Lepidoptera[22] this description of some of the more notable
+cases is taken.
+
+The most striking and familiar case is that of _Amphidasys betularia_,
+of which only the ordinary type was known in any locality until about
+1848-1850, when the totally black var. _doubledayaria_ first appeared
+in the neighbourhood of Manchester. This black form was subsequently
+recorded in Huddersfield between 1860 and 1870; Kendal about 1870;
+Cannock Chase, 1878; Berkshire, 1885; Norfolk, Essex and Cambridge about
+1892; Suffolk, 1894; London, 1897. For the Southern Counties of England,
+except in the London district, there are still very few records. It
+cannot of course be asserted positively that the variety spread from its
+place of first appearance into the other localities, and that it did not
+arise _de novo_ in them, but there can be little doubt that the process
+was one of colonisation. On the European Continent the first records are
+from Hanover in 1884, Belgium 1886 and 1894, Crefeld 188-, Berlin 1903,
+Dresden about the same date.
+
+As regards the increase of the variety we have the fact that in
+Lancashire, Cheshire and the West Riding of Yorkshire the black is now
+the prevalent form; and in some places, as for example, Huddersfield,
+the black alone is now found, though it was unknown there till between
+1860 and 1870. About 1870 at Newport, Monmouth, the two forms were in
+about equal numbers, but a few years later the type had almost vanished.
+Similarly in Crefeld, where the black form was still very rare in the
+eighties, it now forms about 50 per cent. of the population. In the
+London district the black remains scarce and at the date of the report
+it was still very scarce. From Ireland there is only one record and
+there are hardly any from Scotland.
+
+_Boarmia repandata_ is another species which is behaving in a somewhat
+similar way. Unlike _betularia_, however, the species is a variable
+one, and has several colour-forms, amongst them the banded var.
+_conversaria_, and many others. In addition to these there is a
+black form in the North of England which seems to be spreading. In
+Huddersfield the black was first recorded in 1888, and in 1900 20-25 per
+cent. were black. At Rotherham the black or very dark are now prevalent
+and have increased in the last 15 years. From the Midlands, East Anglia
+and Southern Counties the returns show only the light and medium forms.
+
+Of _Odontoptera bidentata_ several intergrading dark forms exist, and
+these are found exclusively in the North and the Midlands. Unicolorous
+blacks have been found recently in the Lancashire mosses and at
+Wakefield. At Huddersfield 50 years ago the light forms were prevalent,
+but now a rather dark brown, not infrequently suffused with black, is
+the commonest. In Southern Counties only light forms are known.
+
+_Phigalia pilosaria_ in South England is always light, but in the North
+the prevalent form is darker. About 35 years ago a form with unicolorous
+sooty fore-wings and dull grey hind wings was first seen in Yorkshire
+and a similar form is now taken regularly in South Wales.
+
+In the following cases the dark varieties were found originally only in
+the South.
+
+_Boarmia rhomboidaria_ gave rise about 40 years ago to a unicolorous
+smoky variety called _perfumaria_. This was at first peculiar to the
+London district, but it has since been taken in Birmingham and other
+large cities. More lately coal-black specimens have been found at
+Norwich, and others similar but hardly so dark were taken in the South
+of Scotland and at Cannock Chase.
+
+_Eupithecia rectangulata_ is a similar case. Formerly the light forms
+were prevalent but within sixty years they have almost entirely been
+replaced in the South of London by a nearly black form.
+
+_Tephrosia_ (_Boarmia_) _consortaria_ and _Tephrosia consonaria_ are
+exceptionally interesting, for they have both given off dark forms in
+the same wood near Maidstone, which is far from the usual "centres of
+melanism." They were discovered in this locality by Mr. E. Goodwin. That
+of _consortaria_ is a dark grey, but that of _consonaria_ is a full
+black, and nothing like either has been found anywhere else.
+
+These examples are all taken from the Geometridae but others, though
+of a less conspicuous kind, could be given from the Noctuidae or the
+Micro-Lepidoptera. _Acronycta psi_, for instance, has a suffused form
+which is believed to be becoming more frequent in the London district.
+_Polia chi_ has two dark forms, _olivacea_, a yellowish grey with dark
+markings, and _suffusa_ which is a darker, blackish-slate colour. Both
+occur in the North of England, sometimes together, sometimes separately,
+or mixed with the type and many intermediates. The distribution is
+peculiarly irregular. At Huddersfield, where the very dark form appeared
+suddenly about 1890, some 30 per cent. are said to be now dark and about
+6-7 per cent. very dark, but at Saddleworth, 12 miles away, only the
+pale forms occur.
+
+Several questions of interest arise in regard to this evidence. This
+progressive Melanism has arisen in certain families only, and may be
+confined to certain species only, within those families. As in almost
+all other examples in which variation has been much observed, its
+incidence is capricious and specific. A collateral line of inquiry
+relates to the degree of discontinuity which the variation manifests.
+Here again there is no rule. Generally speaking, in _A. betularia_,
+to take the case most fully studied, the variation is discontinuous.
+Real intermediates between _betularia_ and _doubledayaria_ are in most
+localities absent or rare. The black spots of _betularia_ may often
+be larger or more numerous than in the normal, but this variation has
+nothing to do with _doubledayaria_, and is not an intermediate stage
+towards it, though sometimes wrongly so described. _Doubledayaria_ owes
+its characteristic appearance to a factor which blurs the surface of the
+wings with a layer of black. Sometimes this blurring is slighter than
+in the real _doubledayaria_, and these forms are real intermediates.
+Occasionally the fore-wings alone are thus blurred. These intermediates
+are clearly due to reduction-stages of the _doubledayaria_ factor,
+and are related to it as a blue mouse is to a black, or a dutch
+rabbit to a self-colour. It cannot positively be asserted that the
+full _doubledayaria_ existed before the intermediate, but it almost
+certainly did. In certain places as for instance in Belgium, there is
+evidence that intermediates have at various times been fairly abundant,
+but they have never become common, nor are they known to exist in the
+absence of _doubledayaria_. When the black variety and the light type
+breed together they do not usually have intermediates among their
+offspring, and the evidence is consistent with the view that the
+black is a complete dominant. The same is probably true of _Tephrosia
+consonaria_.
+
+In some of the other species we know that the darkest forms did
+not appear first. For example in _Phigalia pilosaria_ and _Boarmia
+rhomboidaria_ dark forms existed and are believed to have increased in
+number before the darkest made its appearance. _Hybernia progemmaria_ is
+said to have become darker gradually both in Cheshire and in the West
+Riding, and a uniformly smoky variety appeared in South Yorkshire less
+than 45 years ago which has spread to neighbouring counties. The dark
+medium has become the commonest form in Huddersfield district, where the
+very dark variety is now about 20 per cent. of the population, though
+the light form is still common.
+
+Taking the evidence together we find it consistent with the view that
+dark forms have appeared sporadically, in some species the very dark
+appearing first and intermediates later, in others the moderately dark
+came first and the darkest later in time. It is practically certain that
+the change has in general come about not by a gradual change supervening
+on the population at large, but by the sporadic appearance of dark
+specimens as a new element in the population, and strains derived from
+these dark individuals have gradually superseded the normal type more or
+less completely.
+
+If it could be shown that these melanic novelties had a definite
+advantage in the struggle for existence they would provide an instance
+of evolution proceeding much in the way which Darwin contemplated. The
+whole process would differ from that conceived by him as the normal
+method of evolution only in so far as the change has come about with
+great rapidity and in some instances largely by the appearance and
+success of discontinuous varieties. The question, however, must be
+asked whether the dark form can reasonably be supposed to have an
+advantage by reason of their darkness. Some naturalists believe that
+the darkness of the colours does thus definitely contribute to their
+protection by making the insects less conspicuous and thus more likely
+to escape the search of birds. In support of this view it may be
+pointed out that it is in the manufacturing districts of Lancashire and
+Yorkshire, and again in the London area that the melanics have attained
+their greatest development. Consistently with this argument also, it is
+in the neighbourhood of Crefeld and Essen, the black country of Germany,
+that they have chiefly established themselves on the Continent, and
+_Phigalia pilosaria_ in the black form is now at home in South Wales.
+Thus superficially regarded, the evidence looks rather strong, but it is
+difficult to apply the reasoning in detail. We have first the difficulty
+that the black form of _betularia_ for instance has established itself
+in thoroughly rural districts, notably near King's Lynn in Norfolk,
+and in the neighbourhood of Kendal and Windermere. The black form
+of _consonaria_ and the dark _consortaria_ appeared in a wood near
+Maidstone, far from town smoke, and the black _rhomboidaria_ was first
+found at Norwich, which, as towns go, is clean. Then again the spread of
+the melanics is very irregular and unaccountable. The black _pilosaria_
+is found both in the West Riding and in the Swansea district, but
+not yet elsewhere. It rapidly increased at Huddersfield, but made no
+noticeable progress at Sheffield though recorded there for ten years.
+It is also a remarkable fact that no similar melanic development has
+been observed in America, and, so far as I am aware, comparable melanic
+varieties have not appeared on the European continent except in the case
+of the few sorts which possibly may have come from England.
+
+The whole subject is beset with complications. It must not be forgotten
+that in a few species of moths there is an obvious and recognised
+conformity between the colours of the perfect insect and that of the
+soil on which they live, comparable with that which is so striking in
+the case of some Oedipodidae and other grasshoppers. Of this phenomenon
+the clearest example is _Gnophos obscurata_, which is a most variable
+species with many local forms. Of these a well-known dark variety lives
+on the peaty heaths of the New Forest and other districts, but on the
+chalk hills of Kent, Sussex and Surrey various light varieties are
+found, of which one is a bright silvery white, very near in colour to
+the colour of a chalky bank. This case does not seem to be one of direct
+environmental action,[23] for Poulton found no change induced by rearing
+larvae among either white or black surrounding objects. No one however
+can doubt that there is some indirect connection between the colour of
+the ground and that of the moths.
+
+To my mind there is a serious objection to the theory of protective
+resemblance in application to such a case as that of the _betularia_
+forms, which arises from the fact that the black _doubledayaria_ is a
+fairly conspicuous insect anywhere except perhaps on actually black
+materials, which are not common in any locality. Tree trunks and walls
+are dirty in smoky districts but they are not often black, and I doubt
+whether in the neighbourhood of Rotherham, for instance, which is one
+of the great melanic centres, _doubledayaria_ can be harder for a bird
+to find than _betularia_ would be. After all, too, many of the species
+much affected are not urban insects. They live in country places between
+the towns, and the general tone of these places even in Lancashire
+and the West Riding is not very different from that of similar places
+elsewhere. As against the objection that the black varieties are much
+blacker than the case requires it may be replied that we know nothing
+of the senses of birds, and that perhaps to their eyes blackness does
+constitute a disguise even though the surroundings are much less dark.
+This is undeniable, but recourse to such an argument is dangerous; for
+if the sight of the insect-eating birds is so dull that it does not
+distinguish dark things from dingy grey, we cannot subsequently regard
+the keen sight of birds as the sufficient control which has led to the
+minute and detailed resemblance of many insects to their surroundings.
+Those who see in such cases examples of the omnipotence of Selection
+must frequently find themselves in this dilemma.
+
+Taking the evidence as a whole, we may say that it fairly suggests the
+existence of some connection between modern urban developments and the
+appearance and rise of the melanic varieties. More than that we cannot
+yet affirm. It is a subject in which problems open up on every side,
+and all of them are profitable subjects for investigation. Unhappily
+such animals are difficult to rear successfully in captivity for many
+generations, owing to their extreme liability to disease. Not the least
+interesting feature of the melanics is the fact that the black varieties
+provide about the best and clearest example of a new dominant factor
+attaching itself to a wild species in recent times. None of the cases
+are satisfactorily recorded or analysed as yet, but the evidence is
+clear that _doubledayaria_ is a dominant to its type, and in several
+other dark varieties, though the pigment deposited is not black, the
+records show that the increased amount of the pigment almost certainly
+is due to a positive factor. Of this, _Hemerophila abruptaria_ is a
+good example.[24] There are some irregularities in the results, but
+taken together they leave little doubt that the dark brown variety is a
+dominant and the light, yellowish brown a recessive.
+
+A curious parallel to the rise of the melanic moths in England
+is provided by the case of the Honey-creepers or Sugar-birds, in
+certain West Indian islands.[25] These birds of the genus _Coereba_
+(_Certhiola_) range from Southern Mexico to the Northern parts of
+South America and through the whole chain of the West Indian islands
+and Bahamas except Cuba. There are numerous local forms, and many of
+the islands have types peculiar to themselves, as is usual in such
+cases. Some of the types or species range through several islands, but
+according to Austin Clark[26] no island has more than one of them.
+Cory[27] reckoned twelve such species within the Antillean region. They
+are small birds about the size of a nuthatch with a general colouring of
+black, yellow, and white. From the island of St. Vincent the Smithsonian
+Institution received in the late seventies of last century several
+completely black specimens in addition to two of the usual type of
+colouring. The black were described by W. N. Lawrence as _atrata_, and
+those marked with the usual yellow and white were called _saccharina_.
+The collector (Mr. F. A. Ober) reported that the black form was common,
+and that the _saccharina_ form was rarer. Lawrence remarks, "Had there
+been only a single example (of the black form) I should have considered
+it as probably a case of abnormal colouring, but it seems to be a
+representative form of the genus in this island."[28] There is of course
+no doubt of the correctness of the view taken by Austin Clark that
+"_atrata_" is a black variety. The black bird is in every respect, other
+than colour, identical with _saccharina_, and it is even possible to
+detect a greenish colour in the areas which would normally be yellow,
+showing plainly enough the yellow pigment obscured by the black.
+
+We have next the interesting fact that like our melanic moths the dark
+form is replacing the "type." At the time of Ober's visit the type was
+already in a minority, but now it is nearly or perhaps actually extinct,
+though the black form is one of the commonest birds on the island.
+Austin Clark found no specimen when he collected there in 1903-4, though
+formerly it was not uncommon in the vicinity of Kingston and in the
+immediate windward district of St. Vincent.
+
+The Grenadines are geographically just south of St. Vincent, though
+separated by a deep channel. In these islands no black forms have
+yet been taken, but Grenada, the next island to the south, has both
+normals and blacks. There are trifling differences of size between the
+Grenada birds and those from St. Vincent, the Grenada specimens being
+slightly smaller and for this reason they have received distinct names,
+the form marked with yellow and white being called _Godmani_ (Cory)
+and the black, _Wellsi_ (Cory), but this merely introduces a useless
+complication. There is evidence that in Grenada, as in St. Vincent,
+the black is gradually ousting the original type, but the process has
+not gone so far as in St. Vincent. Austin Clark very properly compares
+this case of the Sugar-birds with that of _Papilio turnus_, which
+as is well-known, has a black female in the southern parts of its
+distribution, in addition to a female of the yellow type, but in the
+Northern States the black female does not occur.
+
+During the present year P. R. Lowe, who lately studied _Coerebas_ on
+a large scale in the West Indies, has published an important paper on
+the subject.[29] He calls attention to the fact that Cory recently
+found a black form of _Coereba_ on Los Roques Islands, and he himself
+discovered another on the Testigos Islands. Both localities are on the
+coast of Venezuela, far from St. Vincent and Grenada. The whole problem
+is thus further complicated by the fact that the black varieties have,
+as we are almost driven to admit, arisen independently in remote places.
+Improbable as this conclusion may be, it is still more difficult to
+regard all the black forms as derived from one source. For first, they
+present definite small differences from each other; and secondly we have
+to remember a consideration of greater importance, that the very fact
+that each island has its own type must be accepted as proving that the
+localities are effectively isolated from each other, and that migration
+must be a very rare event.
+
+The rarity of such illustrative cases is, I believe, more apparent than
+real. It is probably due to the extreme reluctance of systematists to
+admit that such things can be, and of course to the almost complete
+absence of knowledge as to the genetic behaviour of wild animals and
+plants. Only in such examples as this of the _Coereba_, where colour
+constitutes the sole difference, or that of the moths which have been
+minutely studied by many collectors, does the significance of the facts
+appear. The arrangement of catalogues and collections is such that much
+practical difficulty of a quite unnecessary kind is introduced. For
+example, in this very case of _Coereba_, I find the British Museum has
+a fine series from Grenada including 3 normals and 11 black, and also
+16 blacks from St. Vincent. If the black specimens from Grenada were
+put with the normals which are almost certainly nothing but a recessive
+form of the same bird, the variation would strike the eye on even a
+superficial glance at the drawer. But following the notions so naively
+expressed in the passage quoted above from W. N. Lawrence, the blacks
+from Grenada are put apart together with the other blacks from St.
+Vincent, though two of them were shot on the same date as one of the
+normals.
+
+FOOTNOTES:
+
+[1] For the evidence see Tutt, J. W., _Trans. Ent. Soc._, 1898, p. 17.
+Compare the remarkable case given by Gulick (_Evolution Racial and
+Habitudinal_, p. 123) of the two races of _Cicada_, which are separated
+by reason of their life-cycles, one having a period of 13, the other 17
+years.
+
+[2] For references see _Materials_, p. 396, and also G. Baur, _Amer.
+Nat._, 1893, July, p. 677.
+
+[3] Jenner Weir, _Entomologist_, 1880, XIII, p. 251.
+
+[4] Jentink, _Notes Leyden Mus._, 1885, VII, p. 111. Specimens
+illustrating this peculiarity are in the British Museum.
+
+[5] _Proc. Zool. Soc._, 1895, p. 850. Plate. Many points beyond that
+mentioned above are involved in this remarkable case. For example, not
+only are there males like females, but a small proportion of females
+resemble the ordinary male type. The stripes are not merely the spots
+produced, for they occupy different anatomical positions. The spots
+almost always go with a black ventral surface, but the striped forms
+nearly always have that region testaceous. _Spartium retama_, the
+food-plant, will not grow in England, but if it could be naturalised in
+America the whole problem might be investigated there and results of
+exceptional interest would almost certainly be attained.
+
+[6] Doncaster, L., _Proc. Zool. Soc._, 1905, II, p. 528.
+
+[7] I am not aware that the details of this striking case have ever
+been worked out. It should be noted that the green and blue forms are
+not due to simple modification of the red pigment; for these colours,
+due to interference, fork over the area occupied by the red lines. The
+distinctions between these forms cannot therefore be simply chemical,
+as we may suppose them to be, for instance, in the case of many red
+and yellow forms, and the genetic relationships of the _Heliconid_
+varieties would raise many novel problems and be well worth studying
+experimentally.
+
+[8] Woodeforde, F. C., _Trans. North Staffordshire Field Club_, XXXV,
+1901, Plate.
+
+[9] E. Gallé, _Compte Rendus du Congres Internat. de Bot. a l'Expos.
+Univ._, 1900, p. 112.
+
+[10] Flora of Mentone, 1864-8, _Nova Acta Acad. Caes._, XXXV, 1869.
+
+[11] I owe these facts to Canon A. M. Norman, who showed me illustrative
+specimens. They were originally described by Bowerbank (_Monogr. Brit.
+Spongiadae_, vol. II, pp. 18 and XX; vol. III, Pls. I and III). A
+specimen of _G. compressa_ measured 5 inches, with a greatest width of
+3-1/4 in. _G. ciliata_ was found measuring 3 in. long and 3/4 in. wide.
+These dimensions are many times those of normal specimens.
+
+[12] Coutagne, G., _Recherches sur le Polymorphisme des Mollusques de
+France_, _Annales Soc. d'Agric. Sci. et Industr. Lyon_, 1895.
+
+[13] As to the synonymy and references see Coutagne, p. 45.
+
+[14] A. Lang, _Die Bastarde von H. hortensis Muller H. nemoralis L._
+Jena, G. Fischer, 1908; with a fine coloured plate showing the varieties
+of the species and their hybrids.
+
+[15] With this evidence compare that given by A. Delcourt in his
+valuable papers lately published relating to the variations of
+_Notonecta_. See especially _Bull. Sci. Fr. Belg._, 1909, XLIII, p. 443;
+and _C. R. Soc. Biol._, 1909, LXVI, p. 589.
+
+[16] Allen, J. A., _Bull. Amer. Mus. N. H._, III, 1891, pp. 51-54.
+
+[17] J. T. Gulick, _Evolution, Racial and Habitudinal_, Carnegie
+Institution, Publication No. 25, 1905.
+
+[18] A. G. Mayer, _Mem. Mus. Comp. Anat. Harvard_, Vol. XXVI, 1902, p.
+117. From the tables given I cannot ascertain the actual numbers from
+the two intermediate valleys, but they were considerable.
+
+[19] To which I was very kindly guided by Mr. C. T. Trechmann.
+
+[20] Standfuss, _Handbuch d. paläarkt Gross-schmet_, 1896, p. 321.
+
+[21] _Ent. Rec._, XVIII, No. 7, 1906.
+
+[22] This evidence was largely collected by Mr. G. T. Porritt, who has
+given much attention to the subject.
+
+[23] Such direct action has of course been proved to occur in the case
+of several dimorphic larvae (_e. g._, _A. betularia_, itself) and pupae.
+
+[24] See Harris, _Proc. Ent. Soc. London_, 1904, p. lxxii, and 1905, p.
+lxiii; also Hamling, _Trans. City of London Ent. Soc._, 1905, p. 5.
+
+[25] I am indebted to Mr. Outram Bangs of the Harvard Museum for calling
+my attention to this remarkable case.
+
+[26] _Auk_, 1889, VI, p. 219.
+
+[27] _Ann. N. Y. Acad. Sci._, 1878, I, p. 149.
+
+[28] _Ann. N. Y. Acad. Sci._, 1878, I, p. 149.
+
+[29] _Ibid_, 1912, pp. 523-8.
+
+
+
+
+                        CHAPTER VII
+
+                 LOCAL DIFFERENTIATION. _Continued_
+
+
+                       OVERLAPPING FORMS
+
+The facts of the distribution of local forms on the whole are consistent
+with the view that these forms come into existence by the sporadic
+appearance of varieties in a population, rather than by transformation
+of the population as a whole. Of such sporadically occurring varieties
+there are examples in great abundance, though by the nature of the
+case it can be but rarely that we are able to produce evidence of a
+previous type being actually superseded by the variety. When the two
+forms are found co-existing in the same area they are usually recorded
+as one species if intergrades are observed, and as two species if the
+intergrades are absent. On the other hand when two forms are found
+occupying separate areas, when, that is, the process of replacement is
+completed in one of the areas, then forthwith each is named separately
+either as species or subspecies. Successive observations carried out
+through considerable periods of time would be necessary to establish
+beyond question that the history proceeds in one way rather than
+another. Such continuity of observation has for the most part never
+been attempted. The kind of information wanted has indeed only been
+lately recognized, and really critical collecting is a thing of only
+the last few decades. The methods of the older collectors, who aimed at
+bringing together a few typical specimens of all distinct forms, are of
+little service in this class of inquiry, which is better promoted by the
+indiscriminate collection of large numbers of common forms from many
+localities. When this has been done on a comprehensive scale we shall be
+in a position to form much more confident judgments as to the general
+theory of evolution.
+
+Some little work of the kind has however been done and the results are
+already of great value. Seeing that the differentiation of local forms
+is only made possible by isolation, it necessarily happens that the
+collector finds one form in one locality and another in a distinct
+locality, and there is no evidence as to the behaviour which the two
+representative species might exhibit if they came into touch with each
+other. In the most familiar examples of such distinction each inhabits
+an island, completely occupying it to the exclusion of any other similar
+form. It can only be when the two representative species occupy parts
+of a continental area connected with each other by regions habitable
+for the organism in question, that there is a chance of seeing the two
+forms in contact. Often also, even where this condition is satisfied,
+the habits, social organisation, or some other special cause may
+act as a barrier which prevents the distinguishable forms from ever
+coming into such complete contact as to interbreed or to behave as a
+genetically continuous race. When genetic continuity is ensured by a
+constant diffusion of the population over the whole area which they
+inhabit there will manifestly be no formation of local races. The
+practical uniformity, for example, of so many species of birds which
+inhabit widely extended ranges of Western Europe is doubtless maintained
+by such constant diffusion. When, as in the case of the Falcons, many
+localities have peculiar forms, the fact may be taken as conclusive
+evidence that there is little or no diffusion; and when we find in such
+a species as the Goldfinch that in spite of migratory fluctuations there
+are nevertheless geographical races fairly well differentiated, it may
+similarly be inferred that these fluctuations habitually move up and
+down on paths which do not intermingle. There are however a few examples
+of animals, not given to much irregular wandering, which occupy a wide
+and continuous range of diversified country and are differentiated as
+local races in two or more districts, though the distinct races meet
+in intervening areas. Of these the most notorious illustration which
+has been investigated with any thoroughness is that of the species of
+_Colaptes_ (Woodpeckers) known in the United States as Flickers. The
+study of the variations of these forms, made by J. A. Allen[1] is an
+admirable piece of work, with which every student of variation and
+evolutionary problems should make himself familiar. The two forms with
+which we are most concerned are known as _C. auratus_ and _C. cafer_,
+and are very strikingly different in appearance. In size, proportions,
+general pattern of colouration, habits, and notes, the two are alike,
+but they differ in the following seven respects as stated by Allen.
+
+          _Auratus_                          _Cafer_
+
+  1. Quills _yellow_.                   1. Quills _red_.
+  2. Male with a _black_ malar          2. Male with a _red_
+       stripe.                               malar stripe.
+  3. Adult female with _no_             3. Adult female with usually a
+       malar stripe.                         brown malar stripe.
+  4. _A scarlet nuchal crescent         4. No nuchal crescent in
+       in both sexes._                       either sex.
+  5. Throat and fore neck               5. Throat and fore neck
+       _brown_.                               _grey_.
+  6. Whole top of head and hind neck    6. Whole top of neck and hind
+       _grey_.                                neck _brown_.
+  7. General plumage with an            7. General plumage with a
+       _olivaceous_ cast.                     _rufescent_ cast.
+
+These differences are illustrated in the accompanying coloured plate,
+which has been most kindly prepared for me under the instructions of
+Dr. F. M. Chapman of the American Museum of Natural History. Before
+going further it is worth considering the nature of these differences
+a little more closely. All but the last are large differences which no
+one would overlook even in a hasty glance at the birds. If the only
+distinction lay in the colour of the quills we might feel fairly sure
+that _auratus_ was a recessive form of _cafer_, and so probably it is in
+this respect. Similarly the black malar stripe of _auratus_ is in all
+probability recessive to the red malar stripe of _cafer_ and I imagine
+the pigments concerned are comparable with those in the Gouldian Finch
+(_Poephila gouldiae_) of Australia. Both sexes in that species may have
+the head black, red, or, less often, yellow, and though it is not any
+longer in question that birds may breed in either plumage, I believe
+that the young are always black-headed and I imagine that those which
+become red-headed possess a dominant factor absent from the permanently
+black-headed birds.[2] Yellow as a recessive form of a red is certainly
+very common, but red and black as variants of the same pigment are less
+usual. In the Gouldian Finch we seem to have a case where a pigment can
+assume all three forms. It would be interesting to know whether the red
+of the malar stripes in _Colaptes_ is a pigment of the same nature as
+the red of the quills. Both in _Colaptes_ and in _Poephila gouldiae_ I
+have seen specimens intermediate between the black and the red, and the
+appearance of the part affected was exactly alike in the two cases, red
+feathers coming up among the black ones, and many feathers containing
+both red and black pigments mixed together. The development of the
+scarlet nuchal crescent in _auratus_ and the absence of this conspicuous
+mark in _cafer_ constitute from the physiological point of view the most
+remarkable pair of differences. When the red crescent is not formed, the
+feathers which would bear it are exactly like the rest, and no special
+pigment is visible in them which one can regard as ready to be modified
+into red. If the crescent is due to a factor it must therefore be
+supposed that this factor has the power of modifying the pigment of the
+neck in one special place alone. Dr. W. D. Miller called my attention to
+the fact that a similar variation occurs in another American woodpecker,
+the Sapsucker, _Sphyropicus varius_.[3]
+
+I do not suggest that such variations are without parallel: indeed in
+_P. gouldiae_ the factor which turns the black of the head into scarlet
+affects one special region of the black only, being sharply distinct
+from the unmodified black of the throat. These regions of the head are
+however often the seat of special colours in birds.[4] So also may be
+instanced the variety of the Common Guillemot (_Uria troile_) which
+has a white line round the eyes and at the sides of the head where the
+normal has no such mark; but this line is formed in a very special
+place, the groove joining the eye to the ear, whereas the feathers
+of the nuchal crescent are not ostensibly distinguished from those
+adjacent.[5]
+
+The transposition of the brown and the grey on the back and front of the
+neck also constitutes a very remarkable difference. If either grey or
+brown depends on a factor then it must be supposed that _auratus_ has
+one of these factors and _cafer_ the other.
+
+From these several considerations it is quite clear that if _auratus_
+and _cafer_ are modifications of the same type produced by presence or
+absence of factors, several independent elements must be concerned, and
+to unravel their inter-relations would be most difficult even if it were
+possible to breed the types under observation, which is of course quite
+beyond present possibilities.
+
+The distribution of the two is as follows. On the east side of the
+Continent _C. auratus_, relatively pure, occupies the whole of Canada
+and the States from the North to Galveston. Westward it extends across
+the whole continent in the more northern region to Alaska, but in its
+pure form it only reaches down the Pacific coast to about the northern
+border of British Columbia. Its southern and western limit is thus
+roughly a line drawn from north of Vancouver, southeast to North Dakota
+and then south to Galveston. _C. cafer_ in the comparatively pure
+form inhabits Mexico, Arizona, California (except Lower California
+and the opposite coast), central and western Nevada, Utah, Oregon,
+and is bounded on the east by a line drawn from the Pacific south of
+Washington, south and eastward through Colorado to the mouth of the
+Rio Grande or the Gulf of Mexico. Between the two lines thus roughly
+defined is a band of country about 1,200-1,300 miles long and 300-400
+miles wide, which contains some normal birds of each type, but chiefly
+birds exhibiting the characters of both, mixed together in various and
+irregular ways. Even in the areas occupied by the pure forms occasional
+birds are recorded with more or less indication of characteristics
+of the other form, but within the area in which the two forms are
+conterminous, the mixed birds are in the majority. The condition of
+these birds of mixed character is described by Allen as follows:
+
+   "As has been long known--indeed, as shown by Baird
+   in 1858--the 'intermediates' or 'hybrids' present ever-varying
+   combinations of the characters of the two birds, from individuals
+   of _C. auratus_ presenting only the slightest traces of the
+   characters of _C. cafer_, or, conversely--individuals of _C.
+   cafer_ presenting only the slightest traces of the characters
+   of _C. auratus_--to birds in which the characters of the two
+   are about equally blended. Thus we may have _C. auratus_ with
+   merely a few red feathers in the black malar stripe, or with the
+   quills merely slightly flushed with orange, or _C. cafer_ with
+   either merely a few black feathers in the red malar stripe, or
+   a few red feathers at the sides of the nape, or an incipient,
+   barely traceable scarlet nuchal crescent. Where the blending
+   of the characters is more strongly marked, the quills may be
+   orange-yellow or orange-red, or of any shade between yellow and
+   red, with the other features of the two birds about equally
+   blended. But such examples are exceptional, an unsymmetrical
+   blending being the rule, the two sides of the same bird being
+   often unlike. The quills of the tail, for example, may be part
+   red and part yellow, the number of yellow or red feathers varying
+   in different individuals, and very often in the opposite sides of
+   the tail in the same bird. The same irregularity occurs also, but
+   apparently less frequently, in the quills of the wings. In such
+   cases the quills may be mostly yellow with a few red or orange
+   quills intermixed, or red with a similar mixture of yellow. A
+   bird may have the general colouration of true _cafer_ combined
+   with a well-developed nuchal crescent, or nearly pure _auratus_
+   with the red malar stripes of a _cafer_. Sometimes the body
+   plumage is that of _C. auratus_ with the head nearly as in pure
+   _cafer_, or exactly the reverse may occur. Or we may have the
+   general plumage as in _cafer_ with the throat and crown as in
+   _auratus_, and the malar stripe either red or black, or mixed
+   red and black, and so on in almost endless variations, it being
+   rare to find, even in birds of the same nest, two individuals
+   alike in all their features of colouration. Usually the first
+   trace of _cafer_ seen in _auratus_ manifests itself as a mixture
+   of red in the black malar stripe, either as a few red feathers,
+   or as a tipping of the black feathers with red, or with merely
+   the basal portion of the feathers red. Sometimes, however, there
+   is a mixture of orange or reddish quills, while the malar stripe
+   remains normal. In _C. cafer_ the traces of _auratus_ are usually
+   shown by a tendency to an incipient nuchal crescent, represented
+   often by merely a few red-tipped feathers on the sides of the
+   nape; at other times by a slight mixture of black in the red
+   malar stripe."
+
+Such a state of things accords very imperfectly with expectations under
+any received theory of Evolution. As in some of the instances discussed
+in the first chapter we have here two fairly definite forms, nearly
+allied, which on any evolutionary hypothesis must have been evolved
+either the one from the other, or both from a third form at a time not
+very remote from the present, as time must be measured in evolution. Yet
+though intermediates exist in some quantity, no one can for a moment
+suggest that they are that definite intermediate from which _auratus_
+and _cafer_ descend in common. One cannot imagine that the immediate
+ancestor of these birds was a mosaic, made up of asymmetrical patches
+of each sort: but that is what many of the intermediates are. It is not
+much easier to suppose the ancestor to have been a nondescript, with a
+compromise between the developed characters of each, with quills buff,
+malar stripes neither black nor red, with a trace of nuchal crescent,
+and so on. Such Frankenstein-monsters have played, a considerable part
+in the imaginations of evolutionary philosophers, but if it were true
+that there was once a population of these monsters capable of successful
+existence, surely they should now be found as a population occupying the
+neutral zone between the two modern forms. Yet, though much remains to
+be done in clearing up the facts, one thing is certain, namely that the
+neutral zone has not a definite and normally intermediate population,
+but on the contrary it is peopled by fragments of the two definite types
+and miscellaneous mongrels between them.
+
+On the other hand, one cannot readily suppose that either form was
+the parent of the other. The process must have involved both addition
+and loss of factors, for whatever hypothesis be adopted, such changes
+must be supposed to have occurred. A careful statistical tabulation of
+the way in which the characters are distributed in the population of
+the mixed zone would be of great value, and till that has been done
+there is little that can be said with certainty as to the genetics of
+these characters. In the collection of Dr. Bishop of New Haven I was
+very kindly allowed to examine a sample, all taken at random, near
+together, in Saskatchewan. There were females 4 adult, 2 young; males
+4 adult and 5 young. This number, though of course insufficient, is
+enough to give some guide as to the degree of definiteness which the
+characters generally show in their variations. Of the 15 birds, 8 had
+simply yellow quills; 2 had red; 1 was almost red but had one yellow
+tail-quill; 3 were intermediate and 1 was buff. As regards the malar
+patch, which can only be determined properly in the adult males, 1 was
+red, 1 was approximately red, 2 intermediate. As to nuchal crescent 4
+females had none, 2 females very slight; 7 males had it, 1 had only a
+slight crescent, and 1 had none. In point of quills therefore 10 were
+definite out of 15; in point of crescent, 11 were definite out of 15;
+and in point of malar patch 1 only was definite out of 4. The last is a
+feature directly dependent on age and so counts for less, but as regards
+the other two features there is some indication that the factors show
+definiteness in their behaviour. It must be remembered that we have no
+knowledge what the heterozygous form may be, and in the case of red
+and yellow it is probably a reddish buff. The patch-works are no doubt
+to be compared with other well-known pied forms, and in these we must
+suppose the active factor broken up, which it probably can be very
+easily. The asymmetry, which Allen notices as so marked a feature, in
+the distribution of the red and yellow quills of the tail especially,
+recalls that of the black markings in the pied Canaries. As is well
+known to students of variations _some_ pigment-factors in _some_ animals
+are apparently uncontrolled by symmetry, while in other specific cases
+symmetry is the rule. On the other hand the blackness or redness of
+the malar patches is, I think, as a rule nearly symmetrical. It should
+be mentioned that two of Dr. Bishop's young birds belonged to the same
+nest, one a female with _red_ quills, the other a male with _yellow_.
+Both are without crescent.
+
+As to the question whether certain combinations of characters occur
+with special frequency, the evidence is insufficient to give a definite
+answer. Among all the birds I have seen in America or in England I
+have not yet found one having the malar patches black without any
+nuchal crescent. Of Dr. Bishop's 8 adults not one, however, showed the
+combination of the three chief features normal for _auratus_ or for
+_cafer_.
+
+Besides the two forms that we have hitherto considered, several other
+local types exist, and these throw some further light on the problem.
+Of these the most important in this connexion is _chrysoides_, which
+inhabits the whole of southern California and the mainland opposite.
+This remarkable form is as Allen says, very different from _auratus_
+except that it has the quills yellow like _auratus_, not red like
+_cafer_. So that we find here in the extreme west of the whole
+distribution a type agreeing in one of its chief features with the
+eastern type. Between this and _cafer_ intergrades have, according to
+Allen, not been found. The relations of this _chrysoides_ are, Allen
+thinks, rather with _mexicanoides_, a southern, smaller race with
+colours more intense, which inhabits Guatemala, but however that may
+be, it must be regarded as a _cafer_ which has lost its red quills. The
+island of Guadeloupe off Lower California has an island form. Beyond the
+other side of the continent there is also an island form of _auratus_,
+inhabiting Cuba, so that clearly the yellow quills can extend into the
+tropics.
+
+The above account is in many respects incomplete, but it suffices to
+give an outline of the chief facts. The whole problem is complicated by
+the undoubted effects of an uncertain amount of migration, and in many,
+perhaps all, districts, the winter population differs from the summer
+population of the same localities. The existence of these seasonal ebbs
+and flows is now well known to ornithologists, and most of the bird
+species of temperate regions are subject to them.
+
+Difficult as it may be to conceive the actual process of origin of the
+two types _auratus_ and _cafer_, it is I think still harder to suggest
+any possible circumstance which can have determined their development as
+distinct races, or which can maintain that distinctness when created.
+Some will no doubt be disposed to appeal once more to our ignorance
+and suggest that if we only knew more we should see that the yellow
+quills, the black "moustache" and the red crescent, specially qualify
+_auratus_ for the north and eastern region, and the red quills, red
+"moustache" and absence of crescent fit _cafer_ to the conditions of its
+homes. Each can judge for himself, but my own view is that this is a
+vain delusion, and that to cherish it merely blunts the receptivity of
+the mind, which if unoccupied with such fancies would be more ready to
+perceive the truth when at last it shall appear. Think of the range of
+conditions prevailing in the country occupied by _auratus_--a triangle
+with its apex in Florida and its base the whole Arctic region of North
+America. Is it seriously suggested that there is some element common to
+the "conditions" of such an area which demands a nuchal crescent in the
+Flickers, though the birds of the _cafer_ area, almost equally varied,
+can dispense with the same character? Curiously enough, the geographical
+variation of _Sphyropicus varius_, another though a very different
+Woodpecker[6] shows that conversely the nuchal crescent can be dispensed
+with in the Eastern form though it is assumed by the Western.[7]
+
+Allen points out the interesting additional fact that superposed
+upon each of the two distinct forms, _auratus_ and _cafer_, are
+many geographical variations which can very naturally be regarded
+as climatic. Each decreases in size from the North southward, as so
+many species do.[8] They become paler in the arid plains, and show
+the ordinary phases which are seen in other birds having the same
+distribution. Such differences we may well suppose to be determined
+directly or indirectly, by environment, and we may anticipate with
+fuller knowledge it will be possible to distinguish variations of this
+nature as in the broad sense environmental, from the larger differences
+separating the two main types of _Colaptes_, which I surmise are
+altogether independent of such influences.
+
+It is generally supposed that phenomena like those now so well
+established in the case of _Colaptes_ are very exceptional, and as has
+already been stated a number of circumstances must combine in order
+that they may be produced. I suspect however that the examples are
+more numerous than is commonly thought. In all likelihood the three
+forms _Sphyropicus varius_, _nuchalis_ and _ruber_ are in a very
+similar condition though the details have not, so far as I know, been
+worked out. A complex example which is closely parallel to the case of
+_Colaptes_ was described by F. M. Chapman[9] at the same date as Allen's
+work. This is the case of _Quiscalus_, the Grackles, which in the North
+American Continent have three fairly distinct forms which Chapman speaks
+of as _Q. aeneus_, _Q. quiscula_, and _Q. quiscula aglaeus_. The birds
+are all, so far as pigment is concerned, dark blackish brown, but the
+head and mantle have superposed a metallic sheen of interference-colours
+which in the various forms take different tints, bluish green, bronze
+green, or bronze purple. The details are complicated and difficult
+to appreciate without actual specimens, but the two common types are
+sufficiently distinct. The birds inhabit the whole area east of the
+Rockies, _quiscula aglaeus_ occupying Florida and the Southern States
+southwest of a band of country about a hundred miles broad extending
+roughly from Connecticut to the mouth of the Mississippi; and _aeneus_
+taking the area north and west of this band. In discussing this case
+Chapman expresses the same view as Allen does in the _Colaptes_ case,
+that there are two distinct populations, substantially fixed, and
+that the band of country in which they meet each other has a mongrel
+population, with no consistent type, but showing miscellaneous
+combinations of the character of the two chief types.
+
+The warblers of the genus _Helminthophila_ provide another illustration
+which has points of special interest. The two chief species are _H.
+pinus_, which has a yellow mantle and lower parts, white bars on the
+wings, a black patch behind the eyes and a broad black mark on the
+throat; and _H. chrysoptera_ with dark grey mantle and pale whitish grey
+lower parts, yellow bars on the wings, and grey marks on cheeks and
+throat where _pinus_ has black. These two birds are exceeding distinct,
+and in addition their songs are quite unlike. _H. pinus_ ranges through
+the eastern United States up to Connecticut and Iowa. _H. chrysoptera_
+is a northern form extending down to Connecticut and New Jersey. Both
+are migrants.
+
+In these two States, where the two types overlap, certain forms have
+been repeatedly found which have been described as two distinct species,
+_Lawrencei_ and _leucobronchialis_. Dr. L. B. Bishop and Mr. Brewster
+showed me two long series of _Helminthophila_ containing various
+intergrades between the four named kinds, and details regarding these
+may be found in Chapman's _North American Warblers_ and in Dr. Bishop's
+paper in Auk, 1905, XXII. Though the characters evidently break up to
+some extent, the series can be represented as due to recombinations of
+definite factors more easily than the others which I have described. The
+differentiating characters are:
+
+               _Pinus_
+
+  1.  Mantle and lower parts _yellow_ (Y^1).
+  2.  Wing-bars _white_ (y^2).
+  3.  Cheek and throat _not black_ (b).
+
+
+             _Chrysoptera_
+
+  1. Mantle and lower parts _grey_ (y^1).
+  2. Wing-bars _yellow_ (Y^2).
+  3. Cheek and throat _black_ (B).
+
+The grey pigment of the mantle is common to both, but is masked by the
+yellow in _pinus_, the net result being an olive-green.[10]
+
+I am much indebted to Dr. F. M. Chapman for the loan of the coloured
+plate in which these distinctions are shown. It first appeared in his
+book, _North American Warblers_.
+
+We cannot tell whether _yellow_ or _not-yellow_ is due to the presence
+of a factor, but we may suppose that one or other gives the special
+colour to the parts. The black of character 3 is no doubt a dominant.
+Thus _pinus_ becomes Y^{1}y^{2}b and _chrysoptera_ in y^{1}Y^{2}B. The
+_Lawrencei_ which has the underparts _yellow_, wing-bars _white_, and
+_black_ patches is Y^{1}y^{2}B and _leucobronchialis_ which has mantle
+and underparts _not-yellow_, wing-bars _yellow_ and _no black patches_
+is y^{1}Y^{2}b. This representation, it should be clearly understood,
+is tentative and approximate only. The characters are not really sharp,
+for there is much grading; but allowing for the effects of heterozygosis
+and for some actual breaking-up of factors I believe it gives a fairly
+correct view of the case. In particular we can see how it meets the
+difficulty which Chapman felt in accepting _leucobronchialis_ as in any
+sense derived from _pinus_ which has a yellow breast, and _chrysoptera_
+which has a black throat, seeing that _leucobronchialis_ has neither.
+We now recognize at once that this form could be produced by ordinary
+re-combination of the absence of Y^{1} with the absence of B.
+
+I note also with great interest that the modern observers agree that the
+so-called hybrids may have the song either of the one species, or of the
+other, or a song intermediate between the two. It may also be added that
+these two types have several times been seen, in the breeding season,
+paired with each other or with one of the other combinations.
+
+[Illustration: FIG. 1. _Helminthophila pinus_, male.
+
+FIG. 2. _Helminthophila pinus_, female.
+
+FIG. 3. "Lawrence's Warbler," male; one of the integrading forms.
+
+FIG. 4. "Brewster's Warbler," male; another of the integrading forms.
+
+FIG. 5. _Helminthophila chrysoptera_, male.
+
+FIG. 6. _Helminthophila chrysoptera_, female.]
+
+Allen[11] has described another excellent American example, the Tits of
+the group _Baeolophus bicolor-atricristatus_. The form _bicolor_ belongs
+to the eastern States and ranges from the Atlantic coast to the Great
+Plains, and _atricristatus_, of east Mexico, extends from Vera Cruz to
+central Texas. In southern and central Texas the breeding ranges adjoin,
+and in this country various intermediates occur. The chief types differ
+in two main points.
+
+         _B. bicolor_
+
+  Forehead varies from deep _black_
+    to dull black, suffused with
+    rusty brown.
+
+  Crown and crest _grey_,
+    slightly darker than the back.
+
+
+        _B. atricristatus_
+
+  Forehead _white_ to buffish white.
+
+  Crown and crest _black_, abruptly
+    contrasting with the back.
+
+The intergrades between the two have, as usual, received specific names.
+A detailed description is given by Allen, from which it appears that
+the gradation is very complete. In one case a series of 16 adults were
+all intermediates. It is not stated whether the collector took these at
+random, but from the local lists it is clear that the types are found
+not far away from the place where the intergrades were shot.
+
+Another very striking case is that of the Tanagers, of the genus
+_Rhamphocoelus_. In this group there are several local forms which
+are related to each other in remarkable ways. The forms known as
+_passerinii_ and _icteronotus_ exhibit the clearest phenomena of
+intergradation. The species _passerinii_ has a brilliant scarlet
+and black male, and it inhabits Honduras and Nicaragua. Proceeding
+southwards along the isthmus we find next _costaricensis_ which has
+a male like that of _passerinii_ (but a female with more orange than
+the olive-grey female of _passerinii_). Next we come to Panama which
+is occupied by _icteronotus_, sharply distinguished from _passerinii_
+by the fact that the _scarlet is replaced by lemon-yellow_. This same
+_icteronotus_ occurs again as a pure type in Ecuador and many other
+parts of South America; but Colombia, _between Panama and Ecuador_,
+contains scarlets like _passerinii_, yellows like _icteronotus_, and
+various intergrades of several shades of orange. The _passerinii_ males
+from Nicaragua are indistinguishable from those of Colombia, and the
+_icteronotus_ of Ecuador are the same as those in Panama. The orange
+intergrades, doubtless heterozygous forms, though collected at the
+same locality (Medellin in Colombia) as several pure yellows and pure
+scarlets, are in the British Museum series sorted out as a separate
+species under the name _chrysonotus_! Complications are introduced by
+the relations of these forms to another named type, _flammigerus_, but
+we may for our purpose leave that out of consideration, and say that the
+order of geographical sequence from Honduras to Ecuador is (1) scarlet,
+(2) yellow, (3) mixture of types, scarlet, yellow, orange, (4)yellow.
+
+Similar examples exist in the birds of the old world, but I do not know
+of any that have been studied so fully as those of America. The best
+known is that of the two Rollers, _Coracias indicus_ which spreads from
+Asia Minor through Persia, Baluchistan, the Indian Peninsula and Ceylon,
+and _affinis_ which ranges from Nepal, through Assam, Tenasserim and
+the Indo-Chinese countries. The two types are very different and may be
+distinguished as follows:
+
+        _C. indicus_
+
+  _Mantle_ drab brown-chestnut.
+  _Breast_ chestnut.
+  _Throat_ purplish, streaked with white.
+  _Upper tail-coverts_ indigo.
+
+
+        _C. affinis_
+
+  Dark olive-green.
+  Dull purple brown.
+  Purple, streaked with blue.
+  Turquoise.
+
+The wings are the same in both. In the provinces of Nepal, Sikhim, and
+Darjiling the two species coexist, with the result that intergrades
+have been frequently recorded. The line of intergradation extends to
+the coast, and birds showing various combinations of the two types from
+the Calcutta district exist in collections.[12] The case is interesting
+inasmuch as like that of _Quiscalus_ it shows a series of combinations
+of various metallic colours. Some of these are probably evoked by
+the development of pigment behind striations or other interferences
+already existing, but in the present state of knowledge it would be
+quite impossible to suggest what the actual factors producing these
+appearances may be.
+
+There are, naturally, many other cases among birds which are suspected
+of being in reality comparable, but in most of them the evidence is
+still inadequate. Among Lepidoptera also there are a few of these;
+perhaps the most striking is that of _Basilarchia "proserpina."_[13] The
+genus is well known to European collectors under the name _Limenitis_,
+of which we in England have one species, _L. sibylla_, the "White
+Admiral." A species very like _sibylla_ in general appearance is common
+in the northern parts of the United States, ranging through Canada and
+Northern New England, but rarely south of Boston. This species has the
+conspicuous white bands across both wings like our _sibylla_.
+
+There is also a more Southern type known as _astyanax_, which is very
+different in its appearance, being without the white bands and having
+a broad irroration of blue scales on the posterior border of the hind
+wings. The two are so distinct that one would not be tempted to suspect
+any very close relation between them. In its distribution _astyanax_ is
+described by Field as replacing arthemis south of latitude 42°. About
+Boston it is much more common than _arthemis_.
+
+The two forms encroach but little on each other's territory, but where
+they do coexist, a third form, known as _proserpina_, is found which is
+almost intermediate, with the white bands much reduced. There is now
+no doubt that this _proserpina_ is a heterozygous form, resulting from
+a combination of the characters of _arthemis_ and _astyanax_. Field
+succeeded in rearing a brood of 16 from a _proserpina_ mother caught
+wild which laid 31 eggs, and of these, nine (five males, four females)
+resembled the mother, being _proserpina_, and seven (four males, three
+females) were _arthemis_. There can be no question therefore that the
+mother had been fertilised by a male _arthemis_ and that _no-white-band_
+is a factor partially dominant over the _white band_. Another point of
+interest which Field observed was that the _proserpina_ female refused
+to lay on birch, poplar or willow, but accepted wild cherry (_Prunus
+serotina_) a species on which _astyanax_ can live, though that tree is
+not known to be eaten by _arthemis_. Incidentally also the observations
+show that sterility cannot be supposed to be the bar which maintains the
+distinctness of _arthemis_ and _astyanax_.
+
+In this connection _Papilio oregonia_ and _bairdii_ should be
+mentioned.[14] _P. oregonia_ is one of the numerous forms like
+_machaon_, but rather paler. It is a northern insect, inhabiting British
+Colombia east of the Cascade Range, and reaching to Colorado. _P.
+bairdii_ is a much darker butterfly, representing the _asterias_ group
+of the genus _Papilio_. Like _asterias_ it has the abdomen spotted at
+the sides, not banded as in the _machaon_ group. It belongs to Arizona
+and Utah extending into Colorado. From Colorado the form _brucei_ is
+described, more or less intermediate, like _bairdii_ but with the
+abdomen banded as in _oregonia_. W. H. Edwards records the results of
+rearing the offspring of the _bairdii_-like and of the _oregonia_-like
+mothers. Each was found able to have offspring of both kinds, that is to
+say, _bairdii_ females gave both forms, and _oregonia_ females gave both
+forms. It is not possible to say which is dominant, since the fathers
+were unknown. On general grounds one may expect that the _bairdii_ form
+will be found to dominate, but this is quite doubtful.
+
+From this particular discussion I omit reference to those examples in
+which the permanently established types are obviously associated with
+special conditions of life. Where considerable climatic differences
+exist between localities, or when we pass from South to North, or from
+the plains into Alpine levels we often find that in correspondence
+with the change of climate there is a change in the characteristics of
+a species common to both. When I say "species" in such a connection I
+am obviously using the term in the inclusive sense. Some would prefer
+to say that in the two sets of conditions two _representative species_
+exist. Whichever expression be preferred it is plain that such examples
+present another phase of the problem we have been just considering, and
+in them also we have an opportunity of observing the consequences of
+the overlap of two closely related types, but there are advantages in
+considering them separately. In the examples hitherto given, with the
+possible exception of the Papilios,[15] the two fixed types severally
+range over so extensive a region that it may fairly be supposed that
+in the different parts they are subject to considerable diversities of
+climate. There is no outstanding difference that we know distinguishing
+the habitats of the two forms; but in comparing Alpine with Lowland
+forms, or essentially northern with essentially southern forms we do
+know an external circumstance, temperature, that may reasonably be
+supposed to have an influence, direct or indirect, on the population.
+
+FOOTNOTES:
+
+[1] J. A. Allen, _The North American Species of the Genus Colaptes,
+Considered with Special Reference to the Relationships of C. auratus and
+C. cafer_. Bull. Am. Mus. Nat. Hist., IV, 1892.
+
+[2] For a case in which a red-headed female × a black-headed male gave a
+black-headed female and a red-headed male, see _Avian Mag._, N. S., IV,
+pp. 49 and 329
+
+[3] The other variations of this bird are also interesting and
+important. The normal male has a red head and a red throat. The female
+has a red head and a white throat, but varieties of the female are known
+with a black head, thus again illustrating the change from black to
+red. It should be noted that this is not a mere retention of a juvenile
+character, but, as the birds mature, the red feathers come up, or as an
+exception, the black. There is also a western species, _ruber_, in which
+both sexes have a great extension of red, and are alike. The male of
+_nuchalis_ intergrades with this type, but the female does not.
+
+[4] Dr. W. Brewster, for example, has a remarkable specimen of the Teal
+(_Nettion carolinense_) with a white collar strongly developed at the
+front and sides of the neck, in a place where the normal has no such
+mark.
+
+[5] This variety is spoken of as the Ringed Guillemot and is sometimes
+regarded as a distinct species to which the name _ringvia_ was given by
+Brünnich. In support of this view Dr. William Brewster, to whom I am
+indebted for much assistance in regard to the variation of birds, called
+my attention to observations of his own and also of Maynard's, that the
+ringed birds were sometimes mated together, though in a small minority
+(see Brewster, _Proc. Boston Soc. N. H._, XXII, 1883, p. 410). It
+would however be possible to produce many instances of varieties mated
+together though surrounded by a typical population (_e. g._, two varying
+Blackbirds, _Zoologist_, p. 2765; two varying Nightjars, _ibid._, p.
+5278). I am inclined to believe that in nature matings between brothers
+and sisters are frequent in many species of animals, and that the
+production of sporadically varying colonies is thus greatly assisted.
+
+[6] The Sap-suckers feed on trees and somewhat resemble our Spotted
+Woodpeckers in general appearance. _Colaptes_ feeds on the ground and
+corresponds perhaps rather with the European Green Woodpecker.
+
+[7] For an introduction to this example I am indebted to Mr. W. D.
+Miller of the American Museum of Natural History. Some account of the
+facts is given by Baird, Brewer, and Ridgway (_A Hist. of N. Amer.
+Birds_. 1874, II, pp. 540, 544, etc.). _S. varius_ occupies the whole
+country in suitable places from the Atlantic to the eastern slopes of
+the Rockies, and all Mexico to Guatemala. _S. nuchalis_ was first known
+from the Southern Rockies only, but many were afterwards taken in Utah.
+_S. ruber_ is restricted to the Pacific coast. In Ridgway's opinion all
+three are geographical forms of one species. In _ruber_ the sexes are
+alike having both a great extension of the red in the throat, and a red
+crescent. The male of _nuchalis_ grades to the _ruber_ form, but the
+female does not. This female has some red in the throat like the male of
+_varius_, whereas the female of _varius_ has a whitish throat.
+
+[8] Not only vertebrates but the marine Crustacea and Mollusca
+illustrate this curious "principle" of variation, as Canon Norman
+formerly pointed out to me with abundant illustrations. There are of
+course cases to the contrary also.
+
+[9] Chapman, F. M., _Bull. Amer. Mus._, IV, 1892, p. 1; see also
+Ridgway, _Birds of North and Middle America_, 1902, Part II, p. 214.
+
+[10] It would aid greatly in factorial analysis if the descriptive term
+"green" could be avoided in application to cases where the green effect
+is due only to a mixture of black and yellow pigments. The absence of
+yellow is the sole difference between the mantle and underparts of
+_pinus_ and _chrysoptera_.
+
+[11] _Bull. Amer. Mus. Nat. Hist._, XXIII, 1907, p. 467.
+
+[12] References on this subject will be found in _Brit. Mus. Cat.
+Birds_, XVII, p. 13.
+
+[13] For these facts I am indebted to Mr. W. L. W. Field, who has
+lately published an account of his observations and experiments. See
+especially, _Psyche_, 1910, XVII, No. 3, where full references to
+previous publications are given.
+
+[14] For the facts and further references see W. H. Edwards,
+_Butterflies of N. America_, 2d series, Papilio VII and X; 3d series,
+1897, Papilio IV, _Can. Entom._, 1895, XXVII, p. 239.
+
+[15] I think this case is fairly included because the _machaon_ type
+is so widespread that it cannot be regarded as a product of a Northern
+climate, nor can _asterias_ be claimed as especially a warm country
+form, seeing that _brevicauda_, which is scarcely distinguishable from
+_asterias_, inhabits Newfoundland (having a curious phase there in which
+the yellow is largely replaced by red).
+
+
+
+
+                        CHAPTER VIII
+
+              LOCALLY DIFFERENTIATED FORMS. _Continued._
+
+
+                    CLIMATIC VARIETIES
+
+In this chapter we will examine certain cases which illustrate phenomena
+comparable with those just considered, though as I have already
+indicated, they form to some extent a special group. The outstanding
+fact that emerges prominently from the study of the local forms is that
+when two definite types, nearly allied, and capable of interbreeding
+with production of fertile offspring, meet together in the region
+where their distributions overlap, though intergrades are habitually
+found, there is no normally or uniformly intermediate population
+occupying the area of intergradation. Such phenomena as these must, I
+think, be admitted to have great weight in any attempt to construct a
+theory of evolution. True we must hesitate in asserting their positive
+significance, but I see no escape from the conclusion that they throw
+grave doubt on conventional views. Again and again the same question
+presents itself. If _A_ and _B_ lately emerged from a common form why is
+that common form so utterly lost that it does not even maintain itself
+in the region of overlapping? Almost equally difficult is it, in the
+cases which I have numerated, to apply concrete suggestions based on
+any factorial scheme. We may see that in _Heliconius erato_ the type
+with the red mark on the hind wing probably contains a dominant factor,
+and that where the red mark is absent the metallic colours are exposed;
+and that similarly the green metallic colour may have another factor
+which distinguishes it from the blue. In this way we can fairly easily
+represent the various types of _erato_ on a factorial system as the
+result of the various possible combinations of two pairs of factors. But
+there we stop, and we are quite unable to suggest any reason why one
+area should have the red and the green type while another should have
+the blue also. So again with _Colaptes_ or the Warblers. By application
+of a factorial system, admittedly in a somewhat lax fashion, the
+genetic interrelations of the types can be represented; but how it comes
+about that each type maintains a high degree of integrity in its own
+region we can only imagine. Each has in actual fact a stability which
+the intermediate forms have not, but we cannot yet analyse the nature
+of that stability. Mendelian conceptions show us how by segregation the
+integrity of the factors can be in some degree maintained, but not why
+certain combinations of factors should be exceptionally stable. All that
+is left us to fall back on is the old unsatisfying suggestions that some
+combinations _may_ have greater viability than others, that there _may_
+be a tendency for like to mate with like, and so forth.
+
+These difficulties acquire more than ordinary force in those cases in
+which the two fixed types inhabit regions differing in some respect
+so obvious and definite that we are compelled to regard each type as
+climatic and as specially adapted to the conditions. When for example
+an animal has a distinct type never met with except in Arctic or Alpine
+conditions, and another type proper to the plains and temperate regions,
+what are the characteristics of the population of intermediate latitudes
+or at intermediate levels? Some of the examples discussed in the last
+chapter may be instances of this very nature, but even if they are not,
+others are forthcoming which certainly are. The evidence of these cases
+leads to the suspicion that with further knowledge they will be found
+to consist of two classes, some in which the observer as he passes from
+the one climate to the other will find the intermediate area actually
+occupied by a population of intermediate character, and others in which,
+though we may presume the maintenance of intermediate conditions in
+the transitional area, there is no definite transitional population.
+This interrupted or discontinuous distribution seems, so far as I have
+means of judging, to be by far the more common of the two. I do not
+doubt that by sufficient search individuals representing every or almost
+every transitional form can be found, but it is apparently rare that
+_populations_ corresponding to these several grades can be seen. The
+question has in few if any cases been studied with precision sufficient
+to provide a positive answer; but I suspect that real and complete
+continuity, in the sense thus defined, will only be found where the
+character of the local populations depends _directly_ on the conditions
+of life, and shows an immediate response to changes in them apart from
+that postponed response which we suppose to be achieved by selection.
+Obviously the character must be one, like size for instance, capable of
+sensibly complete gradation.
+
+The only example I have met with of the phenomenon of anything like a
+complete intergradation between local types really distinct in kind is
+that provided by the butterfly _Pararge egeria_. It is well known to
+entomologists that this insect exists in two very different types, a
+northern one, the "Speckled Wood" of England, in which the spots are
+a pale whitish yellow, and a southern type having the full fulvous
+colour that we know as characteristic of _megaera_, the "Gatekeeper."
+It appears that Linnaeus gave the name _egeria_ to the southern
+type,[1] and our own is now called _egerides_. Broadly speaking, so
+far as Great Britain, France, and the Spanish Peninsula are concerned,
+the tawny-coloured _egeria_ occupies Spain and western France up to
+the latitude of Poitiers and the pale yellow _egerides_ extends from
+Scotland, where it has a scanty distribution, through southern England,
+where in suitable localities it is common, and the north of France
+to Paris.[2] The two types when placed side by side are strikingly
+different from each other, and are an excellent illustration of what is
+meant by climatic variation. The insect is not a great traveller and
+probably scarcely ever wanders far from its home. It should therefore
+be possible by collecting from north to south to find out how the
+transition is effected, whether suddenly or gradually. This at various
+times I have endeavoured to do, but I am still without exact information
+as to the population in certain critical areas. In addition to the
+information derived from specimens which I have collected or seen
+in the collections of others there is a good account of the general
+distribution in Europe given by the Speyers,[3] who evidently paid
+more attention to the subject than most lepidopterists have done, and
+many more recent records. In particular Oberthür[4] has published many
+details as to the distribution in western France and I am especially
+indebted to Mr. H. Rowland-Brown for a long series of notes as to the
+distribution in France generally, and to Mr. H. E. Page and Dr. T. A.
+Chapman, Mr. Oberthür Prof. Arrigoni degli Oddi, Mr. H. Williams and
+other correspondents, for showing me forms from many localities. The
+butterfly is attached for the most part to woods of deciduous trees and
+to country abounding in tall hedges or rough scrub. It is not usually to
+be found in highly cultivated districts or in very dry regions. Hence
+there is necessarily some want of continuity in the distribution at the
+present time and I should think a mile or two of arable land without big
+hedges would constitute a barrier hardly ever passed. The larva feeds
+on several coarse grasses, especially _Dactylis glomerata_. Barrett
+mentions also _Triticum repens_. In this country the winter is usually
+passed in the larval stage, but I have found that in captivity, at
+least, there is much irregularity. The larvæ feed whenever the weather
+is not very cold and may pupate, but if sharp cold comes on when they
+are pupating or nearly full-grown they often get killed unless protected.
+
+Some writers speak of a difference between the early and later broods,
+but I have never noticed this, and I do not think that the general tone
+of the yellow is affected by the seasons (see Tutt, _Ent. Rec._, IX,
+1897, p. 37).[5]
+
+Beginning at the south of Spain the thoroughly fulvous type _egeria_
+is common at Gibraltar in the Cork woods, at Granada, and doubtless
+generally. Lederer is said to have found only this type in Spain
+(Speyer), and though I have no precise information as to other places
+in the Peninsula north of Jaen I feel tolerably sure that there is no
+change from south to north.[6] Immediately north of the Pyrenees we
+still meet _egeria_ exclusively, and up to Poitiers at least there is no
+noticeable change. But somewhere between Poitiers and the bottom of the
+Loire valley at Tours, the genuine southern type comes to an end, and
+the whole population begins at the Loire to be of an intermediate type,
+easy to distinguish both from _egeria_ and from _egerides_. As to the
+exact condition of the species in the fifty miles separating St. Savin
+on the Vienne from places on the Loire I have no adequate information. I
+have only one small sample from there, but it does contain insects both
+of the southern and intermediate types taken on the same day, in a wood
+near Preuilly. Oberthür also states that at Nantes the true southern
+form exists in company with the northern. From this I infer that the
+southern form extends up the coast further than it does inland, but
+I imagine the representative spoken of as northern would be of usual
+Brittany or intermediate type.
+
+The Vienne river joins the Loire, so the true southern type reaches over
+into the basin of the Loire. From the Loire (Tours, Corméry) north to
+Calvados (Balleroy) only the intermediate is found, so far as I know,
+and the same type extends over Brittany.[7] In general, however, the
+woods near Paris have the thoroughly northern type _egerides_, but at
+St. Germain-en-Laye and at Etampes (Oberthür) the population approaches
+the intermediate type.
+
+On the whole the intermediate type is certainly less homogeneous than
+either of the extremes, and females with the two central spots either
+paler or more fulvous than the rest are not uncommon, but I have never
+taken one on the Loire or in Brittany which I should class with either
+of the extreme types.
+
+Before speaking of the distribution in other parts of France and in
+Europe generally I will briefly state the results of my breeding
+experiments. The work was done many years ago before we had the
+Mendelian clue, and it is greatly to be hoped that some one will
+find opportunities of repeating it. Crossing the English and the
+thoroughly southern type the families produced agree entirely with
+the intermediates of Brittany and the Loire. Reciprocals are alike.
+Of F_{2} I only succeeded in raising very few and of those that I had
+(about 30) nearly all were intermediate in character, though perhaps
+rather less uniform than F_{1}. One family alone, containing only 4
+specimens, had one _egerides_, and three fulvous intermediates. As the
+case stands alone I hesitate whether or not to suppose it due to some
+mistake. Moreover from F_{1} crossed back with the respective parental
+types I had fairly long series, especially from F_{1} × the southern
+type, and looking at these families I cannot see any clear evidence
+of segregation. On the contrary, I think that though there are slight
+irregularities, they would, taken as a whole, be classed as coming
+between the intermediate type and the extreme form used as the second
+parent. This at least is true when the second parent was of the southern
+type.
+
+On this evidence I have regarded the case as one in which there is
+no good evidence of segregation and as conforming most nearly with
+the conventional view of gradual transition in response to climatic
+influences. Such influence must however be indirect; for I reared five
+generations of the northern type in England, and these, though they
+included several abnormal-looking specimens in the last generation and
+then died out, did not show any noticeable change from the fulvous
+colour of the wild type. Merrifield[8] also found that heat applied to
+pupae of the northern type produced no approach to the southern type.
+
+Looking at the facts now in the light of more experience it seems to me
+just possible that the case may be one in which, as in Nilson-Ehle's
+Wheats, the dominant differs from the recessive in having two pairs of
+factors with similar effects. The fulvous type for example may have
+two or more elements in separate pairs which together produce the full
+effect, and the intermediate may have one of these. If this were so,
+some segregation should of course eventually be observable, but the
+proportion of the various fulvous and fulvous-intermediate individuals
+would be large, and the reappearance of actual representatives of the
+northern type might be rare. I admit that this is a somewhat strained
+interpretation of the facts, and as yet it is not entitled to serious
+consideration. Nevertheless I am led to form some such expectation
+partly from the great difficulty in the way of any other, partly from
+the evidence of the small mixed sample found at Preuilly and partly from
+the statements given by Oberthür. There are moreover other features in
+the general distribution of the species which make it improbable that
+the dependence on climate can after all be so close. Published lists
+are unfortunately of little use in deciding which form occurs at a
+particular place, because, since the name _Meone_ has ceased to be used
+for the southern form, there is no complete unanimity among authors as
+to the application of the names _egeria_ and _egerides_, and unless
+more particulars are given, either name may be used for either form.
+Besides this, difficulty arises from the fact that the intermediate
+type is not generally distinguished at all, and English collectors
+finding it, may easily record it as the southern type. From Staudinger's
+note on the distribution, I gather that he, on the contrary, reckoned
+the intermediate with the northern type, as do the Speyers also. The
+late Mr. J. W. Tutt was careful to distinguish the three forms and has
+left several useful records. Easy therefore as it might seem to be to
+make out the distribution of such a familiar insect in its various
+modifications, there are serious practical difficulties, and until
+long series are brought together with this special object in view many
+obscurities will remain.
+
+With only the series from England, the west of France, and Spain before
+one it would be easy to regard the successive series of tones as a fair
+measure of climate; the brighter the colour, the hotter might one expect
+the locality to be. Such rough correspondence is often to be observed
+in butterflies and birds. It becomes impossible to take these simple
+views in the light of more complete knowledge. Beginning with France
+the fulvous _egeria_ occupies the lower valley of the Rhone, probably
+from well above Lyon, though I have no exact information respecting the
+country above Avignon. According to Speyer it also takes the department
+of Lozère. The same authority says that Puy-de-Dôme has "_egeria_,"
+meaning perhaps the intermediate form, with the fulvous form much less
+commonly. Next comes the curious fact that though the Lower Rhone
+(Avignon, Tarascon, Nîmes) has the true fulvous form, Hyères, Cannes,
+Grasse, Nice, Digne, and Alassio have _the intermediate_. Savoy has the
+intermediate (Chambéry) and even _egerides_ perhaps, though in the same
+latitude on the west of France there is nothing but the fulvous type.
+At Chalseul and Besançon (Doubs) the ordinary northern type is found.
+Switzerland generally, I believe, has the northern type, but Staudinger
+gives _egeria_ for Valais and the intermediate occurs in Vaud.[9] The
+south side of the Alps has probably colonies of the pale _egerides_, and
+of intermediates. Orta, with a very hot summer, has the English type
+(Tutt, _Ent. Rec._, XII, 1900, p. 328). Locarno has the intermediate
+(_ibid._, XV, 1903, p. 321). North Italy in general and western
+Piedmont have the intermediate; but further south _egeria_ begins,
+at what region I do not know. Speyer gives on his own authority the
+remarkable statement that at Florence both extremes occur, but chiefly
+intermediates between the two. Mr. R. Verity however kindly informs
+me that in his experience this is not so, and that neither the real
+southern type nor the northern occur there. Sardinia, Sicily, Crete all
+have the southern type. Greece probably has various types. Staudinger
+(_Hor. Ross._, VII, 1870, p. 78) says intermediates resembling Nice
+types common everywhere, but from "Greece" the British Museum has a
+series that would pass for English specimens; and the same type occurs
+near Constantinople. The island of Corfu has a pale intermediate,
+distinct from _egerides_ but approaching it. In Roumania all three
+forms are recorded from various places: _egeria_ in the Dobrutscha; not
+quite typical (presumably an intermediate) at Bukharest; intermediate
+in various mountainous localities as well as in Macedonia and Dalmatia;
+but _egerides_ in Azuga at about 3,000 feet.[10] Hungary has the true
+_egerides_ also. (Cf. Caradja, _Deut. Ent. Zt._, IX, p. 58.) Mathew
+records the same from Gallipoli (_E. M. M._, 1881, p. 95). Staudinger
+does not distinguish the intermediates from the northern, but he gives
+"_egerides_" for Armenia and Fergana (Central Asia). As against the mere
+proximity of a great mountain chain being the influence which keeps
+the Riviera population intermediate may be mentioned the fact that the
+northern foothills of the Pyrenees have the pure southern type, and the
+climate of Cambo must surely be far cooler than that of Nice. The exact
+locality of the Greek specimens is not given, but there can be no part
+of Greece which is not much hotter in summer than Brittany, or Calvados,
+which have the intermediate, not the English type.
+
+In face of these facts it can scarcely be maintained that average
+temperature is the efficient cause of the particular tone of colour
+which the butterfly shows in a given region. Nevertheless it is clear
+that climate counts for much in determining the distribution. It is
+noticeable that though the pale _egerides_ can be established in a
+warm climate we never find _egeria_ in cold climates, and even the
+intermediate is not found in places that have a hard winter. I suspect
+that the distribution of the broods through the year and the condition
+of the animal at the onset of hard frost are features which really
+determine whether a strain can live in a particular place or not.
+Though the truth of the suggestion cannot be tested by experiments in
+captivity, which at once introduce disturbances, I incline to the idea
+that _egeria_ has not got the right periodicity for northern climates.
+If it could arrange its life so that the population consisted either
+of young larvae, or perhaps of thoroughly formed pupae[11] at the
+onset of winter, it might, for any obvious reason to the contrary,
+be able to live in England. It is irregularly "polyvoltine," as the
+silk-worm breeders say, and as soon as a little warmth encourages it,
+a new generation starts into being, which if the frost comes at an
+untimely moment, is immediately destroyed. Many species are continually
+throwing off individuals which feed up fast[12] and emerge at once if
+the temperature permits, and I imagine a species of Satyrid wholly
+or largely represented by such individuals could scarcely survive in
+a country which had a hard winter. For such a climate some definite
+periodicity in the appearance of the broods may well be indispensable.
+But assuming that _egeria_ is cut off from cold climates for such a
+reason, there is nothing yet to connect these habits with the fulvous
+colour, and until breeding can be carried out on a satisfactory scale
+there is no more to be said.
+
+From time to time records appear of individual specimens more or
+less fulvous being caught in southern England, especially in the
+New Forest.[13] It would be interesting to know what offspring such
+individuals might produce. From the evidence now given some notion
+both of the strength and the weakness of the case considered as one of
+continuous climatic variation can be formed. I know no other equally
+satisfactory. Whether or not definite mixture of the intermediates
+with either of the extremes will be proved to occur, the case differs
+materially from those considered in the last chapter in the fact that
+at all events there is no general overlapping of forms. In a species so
+little given to wandering, overlapping could indeed scarcely be expected
+to occur. It is this circumstance which makes the species preeminently
+suitable as a subject for the study of climatic influences, and I trust
+that entomologists with the right opportunities may be disposed to
+explore the facts further.
+
+Just as many species, like _egeria_, have varieties which can be
+regarded as adapted to northern and southern regions, so there are
+also several which have lowland and Alpine forms quite distinct from
+each other. Every such case presents an example of the problem we
+have been considering. As the collector passes from the plains to the
+Alpine region, how will he find the transition from one form to the
+other effected? Does the lowland form give place to the Alpine form
+suddenly, with a region in which the two are mixed, or will he find a
+zone inhabited by an intermediate population? I have spent a good deal
+of time examining the facts in the case of _Pieris napi_ and its Alpine
+female variety _bryoniae_, and though there are many complications which
+still have to be cleared up, no doubt is possible as to the main lines
+of the answer. If in any valley in the Alps inhabited by both _napi_ and
+_bryoniae_ the collector catches every specimen he can, beginning at
+the bottom and working up to 7,000 feet, he will at first get nothing
+but _napi_. At about 2,500 feet, he may catch an occasional _bryoniae_
+flying with the _napi_. After 3,000 feet _napi_ usually ceases, and
+only _bryoniae_ are found. As an exception a colony of _napi_ may be
+met with at much greater heights. I once found them in numbers at about
+6,000 feet.[14] Not only were they free from any trace of modification
+in the direction of _bryoniae_, but they were of the thoroughly southern
+type of _napi_, being a late brood of that large and very pale kind
+(_meridionalis_) almost destitute both of dark veining above and of
+green veining below, which are common on the shores of Lago Maggiore
+and in other hot southern localities. Not far off at the same level
+were typical _bryoniae_ in fair abundance. Occasionally an intermediate
+may be met with. I have taken a few, for example, at Macugnaga and at
+Fobello. These, however, in my experience are rarities in the Alps.
+Fleck[15] gives notes on the distribution in Roumania which shows the
+same state of things. The lowland form is not transformed though found
+at great heights, and at Azuga (nearly 3,000 feet) _bryoniae_ occurs
+with only occasional "_flavescens_," viz., intermediates of the second
+brood.
+
+If this were all the evidence we should be satisfied that the lowland
+and Alpine types keep practically distinct, overlapping occasionally,
+but rarely interbreeding. The problem would remain, how is the
+distinctness of the two types maintained in the region of overlapping?
+Nowadays, I suppose, we should incline to answer this question by
+reference to segregation, and perhaps by an appeal to selective mating.
+The suggestion that segregation does take place is certainly true to
+some extent. There are, however, difficulties in the way, and the whole
+subject is one of great complexity. My own experiments were made in
+pre-Mendelian times and were not arranged with the simplicity which we
+now know to be essential. The results are neither extensive enough nor
+clear enough to settle the many collateral questions which have to be
+considered, and the work ought to be done again. Nevertheless, some
+notes of the observations may have a suggestive value.
+
+When I began, I did not sufficiently appreciate that the "_napi_" group,
+omitting the North American forms, and the Asiatic representatives, has
+at least three chief types in western Europe. The differences we have
+to deal with are manifested by the females only, so in this account
+particulars as to the males are omitted for the most part. These are (1)
+our own British _napi_; (2) the form found in the south, from the Loire
+downwards, and in the Italian Alps, which I think may be spoken of as
+_meridionalis_; (3) _bryoniae_, which is a form clearly recognizable in
+the _female_ only, and is found only in the arctic regions and in the
+Alps above 2,500 feet. The first two have several broods, two, three,
+or more, according to opportunity, and the first brood is different
+from the later ones. In _napi_ the markings on the upper surface are
+a dark grey but in _meridionalis_ they are a pale silvery grey and
+much less extensive. In the later broods of _napi_ there is much less
+general irroration of the veins, and the spots stand out as more defined
+and blacker. These differences vary greatly in degree of emphasis. In
+_meridionalis_ the later broods are entirely different from the first.
+Instead of having silvery markings they have the ground colour quite
+white, with the spots large and a full black. On the under side of the
+hind wings the usual green veins are almost absent, and I have seen
+individuals which could scarcely be distinguished from _rapae_. To these
+later broods the term _napaeae_ is sometimes applied, but I here use
+_meridionalis_ for the southern race in general as applicable to all
+broods.
+
+The female _bryoniae_ is totally unlike the others. The ground colour
+is a full yellow, and each nervure is thickly irrorated with a brown
+pigment often spreading so far as to hide the ground almost entirely
+in the fore-wings. The males corresponding with these females are not
+certainly distinguishable from those of our own _napi_. Both sexes have
+the green veining of the underside of the hind wing fully developed,
+rather more than is usual in the lowland races, but this is not really
+diagnostic of the variety. The first serious difficulty arises in regard
+to the second brood of _bryoniae_. It is stated that there is only one
+brood,[16] but I feel fairly sure that a second brood is sometimes
+produced, and that the females with a yellow ground and diminished
+irroration of the veins, not very uncommon in the Italian Alps in July
+to August, are generally representatives of it. Such insects would of
+course be classed with _bryoniae_ in collections.
+
+My experiments began with eggs of true _bryoniae_ females caught at
+about 2,500 feet early in July. These emerged in August-September as
+intermediates with yellow ground and about half as much black on the
+upper surface as _bryoniae_. They are exactly like the intermediates
+usually found in nature and in the light of later experience I regard
+them as natural F_{1} forms, and I think the mothers had been fertilised
+by _napi_ males, though I admit that in view of the rarity of natural
+intermediates there is a difficulty in this suggestion. Three of these
+females were mated with males raised from thorough _meridionalis_
+females, and three families were produced. Two of them showed distinct
+evidence of segregation, some being yellow and some white with various
+intergrades, some being no blacker than _meridionalis_ and some ranging
+up to a dark intermediate type. Part emerged in the same autumn; and
+part overwintered, emerging as the spring _meridionalis_ or as the
+peculiar type which I afterwards learnt to know as the spring F_{1}
+form. The distinctions were fairly sharp between the several forms. But
+the offspring of the third female gave a series practically continuous
+from _meridionalis_ to the F_{1} type. The work of subsequent years
+gave results similarly irregular which could only be described
+adequately at great length. The outcome may however be summed up in
+the statement that there is evidence that both the yellow ground and
+the dark veining are due to factors, but that there are several of
+these and that imperfect segregation is not uncommon, producing various
+reduction-stages. The yellow ground may be due to one factor, and the
+several shades may be the result of irregularities in dominance, but
+the black markings when fully developed cannot I think be the result of
+less than three factors, one for the basal darkening, one for general
+irroration, and one for the margins. Probably also the enlargement of
+the spots is produced by a fourth factor.
+
+There was not, in my experience any great difficulty in getting the
+various forms to pair in captivity. Some attempts were made to see
+whether individuals of either type selected mates of their own type in
+preference to those of the other, but the results were inconclusive.
+There were some indications of such a preference; though, from
+the impossibility of judging how much of this may be due to other
+circumstances, I could not come to a positive conclusion on the rather
+meagre evidence.
+
+Recently Schima[17] has given a careful and detailed account of all
+the forms found in Lower Austria which he enumerates under 14 distinct
+varietal names. He gives full references to previous accounts,
+especially to the beautiful plates lately published by Roger Verity.[18]
+Examination of these and of my own specimens strongly suggests that
+the several forms are due to the recombination of the factors I have
+named. Among those which I have bred are representatives of most if not
+all the types enumerated by Schima in addition to other curious forms.
+For example I have _bryoniae_ markings on a ground practically white;
+the dark veins with spots almost obsolete; _meridionalis_ on a yellow
+ground; the intermediate amount of black on a white ground, etc. The
+last-named may occur wild and I have one from Macugnaga as well as one
+given me by Mr. F. Gayner from Lulea (Lapmark).
+
+To obtain really exact knowledge of the number of factors and
+their properties it would be necessary to repeat the work. After
+the beginning, I made a mistake in using British _napi_ instead of
+_meridionalis_ and the results were much confused thereby. The contrast
+between _meridionalis_ and the various dark forms is much greater and
+classification of the types would have been therefore easier. The
+British form is presumably _meridionalis_ plus the factor for the basal
+pigmentation. The problem is greatly complicated by the differentiation
+of the seasonal forms. The first point to be determined is whether
+_bryoniae_ is capable of producing a second brood when it is thoroughly
+pure-bred, and whether such a second brood is, as I suspect, normally
+intermediate in character.
+
+In the Alps generally there is no definitely intermediate population;
+nor I believe, is any such population met with in the north where
+the arctic _bryoniae_ meets _napi_, but as to this I have no precise
+information. One curious fact, however, must be mentioned, namely that
+there is a population that can probably be so described with fairness
+established at Mödling near Vienna. This is not in any sense an Alpine
+locality, and does not, as I am told, differ in any obvious way from
+the other suburbs of Vienna. Dr. H. Przibram was so good as to send me
+a set taken at this place, representing a second brood, and they were
+decidedly heterogeneous, ranging from an intermediate form such as
+_bryoniae_ fertilised by _napi_ usually produces, to a light yellowish
+second-brood type with little dark pigment. There are also two actual
+_bryoniae_. Whether true _napi_ also occur there I do not know, but I
+have no doubt they do. It would be well worth while to investigate the
+Mödling population statistically, and to breed from the intermediates
+which might not impossibly prove to be heterozygotes. There are also
+records of such intermediates being occasionally found in some parts
+of Ireland, in the north of Scotland, and in south Wales,[19] but I do
+not know of any regular colony of these forms. We can scarcely avoid
+the inference that one or more of the factors which make up _bryoniae_
+may be carried by these intermediates. It is not clear why their
+interbreeding does not produce actual _bryoniae_ occasionally. If this
+occurred, the probability is that the fact would be known to collectors,
+at least in the British localities. The absence of true _bryoniae_ must,
+I think, be taken to mean that some essential factor is absent from
+these intermediates.
+
+To sum up the evidence, the facts that are clear may be thus enumerated:
+
+   1. _Napi_ and _bryoniae_, or in the Italian Alps,
+      _napaeae_ and _bryoniae_ frequently meet each other.
+
+   2. They cross without difficulty, producing fertile offspring.
+
+   3. But in the levels at which they overlap there is no
+      intermediate population, and only occasional intermediate
+      individuals.
+
+   4. In certain parts of the distribution of _napi_ similar
+      intermediates sometimes occur, and at one place (Mödling) they
+      are so frequent as apparently to constitute a colony.
+
+   5. As to the genetic relations of the two forms there is no
+      complete certainty. Indications of segregation have been observed
+      in some cases, but there are several factors concerned and they
+      are liable to some disintegration.
+
+Another form in which I tried to investigate the same problem is
+_Coenonympha arcania_, which has one Alpine form known as _darwiniana_,
+and another, _satyrion_. In calling _satyrion_ a form of _arcania_ I
+follow Staudinger and other authorities, but I have never been quite
+satisfied that it should be so regarded. The differences between
+_arcania_ and _darwiniana_ are essentially differences of degree; _C.
+arcania_ occurs in places where there is cover, and reaches up the
+valleys usually as high as the mixed woods of deciduous trees, which
+is about 2,500 feet. The variety _darwiniana_, on the contrary, is an
+insect of treeless hillsides, and I regard it as a dwarf and possibly
+a stunted form. It would not greatly surprise me to find that with
+the application of good conditions _arcania_ could be raised from
+_darwiniana_ eggs, or that if _arcania_ larvae were starved they might
+give rise to _darwiniana_ butterflies. I have been unsuccessful in
+trying to rear the species, having lost the larvae by disease. Usually
+one does not catch _arcania_ and _darwiniana_ on the same ground, and
+as _Festuca ovina_--a typically hill-side grass--is a common food-plant
+of _darwiniana_ there can be little doubt that _arcania_ feeds on
+some other grass, probably woodland species. Colonies of _arcania_ of
+varying size and brightness are commonly found, and though a sample of
+_arcania_, finely grown, from a warm Italian wood, presents a striking
+contrast with _darwiniana_ from an Alpine pasture, one certainly may get
+samples which fill all the gradations. Generally the sample from a given
+locality is fairly homogeneous.
+
+Of _satyrion_ I have little personal experience. I only twice found
+it, namely at Zinal, and at Hallstatt in Austria, but it occurs at
+Zermatt, Arolla, and in several Swiss localities above 5,000 feet, and
+I understand that it is the typical Alpine form in the Engadine. With
+its darkened colour and reduced size it might well be expected to be a
+still further stunted form of _darwiniana_. Yet I have never found the
+one succeed to the other at the higher levels. If _darwiniana_ appears
+when Alpine conditions are reached in a valley it will be met with
+up to the highest level at which such butterflies live. Tutt was of
+opinion that _satyrion_ is a distinct species.[20] I once, at the top
+of the Vorderrheinthal caught a sample of _darwiniana_ a few of which
+(males) were so dark and had the eye spots so poorly developed that they
+looked like transitions to _satyrion_. Otherwise I never found any such
+transitional forms and they are certainly exceptional. There is further
+a record[21] of _satyrion_ having been taken flying with _arcania_.
+This was near Susa, at about 2,000 feet I infer. Mr. H. E. Page has
+similar specimens from Caud and from St. Anton (Arlberg). The females,
+however, both of mine and of Mr. Page's samples are a pale brown, quite
+unlike the females both of _arcania_ and of the dark Zinal _satyrion_.
+The difficulty thus raised has not I think yet been considered by the
+authorities, and it is possible that the Alpine forms of _arcania_ are
+in reality three, not two.
+
+The evidence taken together suggests, I think, that _darwiniana_ is
+related to _arcania_ much as so many of the Alpine varieties of plants
+are to the well-developed individuals of the lower levels. I do not
+anticipate that factorial differences will be found in these insects,
+and it is by no means impossible that the distinctions between them
+are the direct consequences of altered conditions. The relations of
+_arcania_ to _satyrion_ are more doubtful, and in that case a factorial
+difference may at least be suspected.
+
+The species of the genus _Setina_ have Alpine forms which agree in
+possessing a characteristic extension of the black pigment to form
+radiating junctions between the spots on the wings. Speyer, who
+discussed the interrelations of these forms in detail,[22] lays stress
+on the absence of genuine transitional forms between _aurita_ and the
+variety _ramosa_. Both are mountain insects but _ramosa_ extends to
+levels higher than that at which _aurita_ ceases, which is about 4,000
+feet. The two forms are often found flying together. Speyer says that
+his brother searched diligently for transitional forms at the level of
+overlapping, but found none, so that at least they may be regarded as
+rare. The variety _ramosa_ is not infrequent at much lower levels (_e.
+g._, Chiavenna, 1,020 feet; Reussthal, 1,500 feet) and extends as high
+as the permanent snows. In the British Museum collection, however, I
+have seen several that I should regard as transitional. Speyer perhaps
+would have classed as _ramosa_ all in which the spots of the central
+field were united, and it is by no means unlikely that breeding would
+prove such individuals to be heterozygous.[23]
+
+There can scarcely be a doubt that the distinction between _aurita_ and
+_ramosa_ is factorial, the radiate _ramosa_ probably having the factor
+for striping. In support of this view may be mentioned the observation
+of Boisduval,[24] respecting a gynandromorphous individual, which was
+_aurita_ male on one side, and _ramosa_ female on the other. Speyer
+makes another excellent comment. He points out that the simple notion
+that the radiation is a mere extension of pigmentation consequent on
+the climate of the higher levels, will not fit the facts very easily,
+because the size of the spots varies greatly in _aurita_ itself at any
+level, and lowland specimens may actually have more black confined
+to the spots alone than some _ramosa_ possess on spots and lines
+combined.[25]
+
+The two Salamanders, _S. maculosa_ and its Alpine form _atra_, might
+not improbably furnish evidence bearing on the same problem. The two
+are of course very distinct, not merely in colour (_maculosa_ being
+spotted with yellow or orange while _atra_ is entirely black) but also
+in the mode of reproduction, a feature to which reference will be made
+in the next chapter. I cannot, however, find any evidence as to the
+overlapping of the two forms. _S. atra_ occurs from about 3,000 feet
+or somewhat less, and reaches great elevations in the Eastern Alps,
+but I do not know if the two forms ever occur in the same localities.
+Leydig,[26] Boulenger,[27] and most modern authorities regard the two
+types as distinct species, but they are in any case closely allied, and
+it would be of interest to have exact knowledge of their geographical
+delimitations.
+
+The reader who has considered the cases adduced will appreciate the
+difficulties which must be faced in any attempt to account for the
+facts in a rational way. As always in a problem of Evolution, two
+separate questions have to be answered. First how did the form under
+consideration come into existence, and secondly, how did it succeed
+in maintaining itself so as to become a race? The evidence from the
+local forms, though very far from giving complete answers to either of
+these questions definitely refutes the popular notion that a new race
+comes into existence by transformation of an older race. If a gradual
+mass-transformation of this kind took place we should certainly expect
+that when two types, nearly allied and capable of interbreeding, overlap
+each other in their geographical distribution, a normally intermediate
+population would exist. If each type can maintain itself, and if
+each came into existence by gradual transformation, then there must
+have been an intermediate capable of existing and maintaining itself
+as a population; and if this had ever been, surely in the region of
+overlapping, that intermediate population should continue. Especially
+should such a population be found when the two extreme types are
+adaptational forms and the region of overlap is a region of intermediate
+conditions. But of the examples we have examined there is only one, that
+of _Pararge egeria_ and _egerides_, which can at all be so interpreted,
+and even in that case it is not impossible that more minute observation
+would reveal discontinuity between the extremes and the admittedly
+normal intermediate population. Granting provisionally however that
+this example, as it stands, is consistent with the conventional theory
+of evolution, I know not where we should look for another case equally
+good. When the distinctions are produced by direct influence of
+conditions operating during the lifetime of the individuals, examples of
+intermediate populations occupying the areas of intermediate conditions
+can no doubt be produced. Many turf-like Alpine plants, for instance,
+if protected from exposure and properly nourished can grow as large as
+those of the same species found in the valleys, and in the case of such
+quantitative effects, intermediate conditions can doubtless produce
+intermediate characters.
+
+Even these examples however are not very abundant, and often the
+intermediate locality has not a form intermediate between those of
+the two extreme localities, but some third form distinct from either.
+This is the case for instance in the fauna of brackish waters. We are
+taught to believe that the fresh water fauna was evolved from the
+marine fauna, which it well may have been; but as students of Crustacea
+and Mollusca know familiarly, the brackish water forms are not as a
+rule intermediates between fresh water species and sea species, but
+more usually they are special forms belonging to the brackish waters,
+with the peculiar property that they can tolerate a great range of
+conditions, and live without ostensible variation in waters of most
+various compositions and densities, which very few marine or fresh water
+species are able to do.
+
+Sometimes the distinction between local races, as in _Rhamphocoelus
+passerinii_ and _icteronotus_ may be regarded with confidence as due to
+one simple Mendelian factor possessed by one race and absent from the
+other, but I think, more often, as in _Colaptes_ or in the varieties
+of _Pieris napi_, the existence of several distinct factors is to
+be inferred. As we have seen, the races of _Colaptes_ show almost
+beyond doubt that in different areas at least three distinct factorial
+combinations can be perpetuated as races.
+
+In the distribution of variability we find, I think, some hint as to
+the steps by which the phenomena under consideration have come to their
+present stage, and I am disposed to regard the facts so well attested
+in the case of our own melanic moths as a true indication of the
+process. Following this indication we should regard the change in the
+character of a population as beginning sporadically, by the appearance
+of varying individuals, possibly only one varying individual, in, it may
+be, one place only. As to _why_ a variety should increase in numbers
+we have nothing but mere speculation to offer, and for the present we
+must simply recognise the fact that it may. That such survival and
+replacement may reasonably be taken as an indication that the replacing
+race has some superior power of holding its own I am quite disposed to
+admit. Nevertheless it seems in the highest degree unlikely that the
+outward and perceptible character or characters which we recognise as
+differentiating the race should be the actual features which contribute
+effectively to that result.
+
+In discussions of geographical distribution in relation to problems of
+origin it is generally said that very nearly allied species usually
+occupy distinct areas, while other competent observers state the exact
+contrary. Lately, for example, Dr. R. G. Leavitt[28] has published an
+important collection of evidence upholding the latter proposition, taken
+chiefly from the botanical side, showing how in numerous genera two or
+more closely allied species coexist, frequently without intermediates,
+in the same localities, and may even be thus found in company throughout
+their distribution. The difference of opinion evidently arises from a
+confusion as to the sense in which the term "species" is understood and
+applied. Leavitt, for example, is avowedly following Jordan and, among
+moderns, Sargent, in applying a close analysis, and denoting as species
+all forms which are distinct and breed true. Against this use of the
+term I know no valid objection[29] but it must be obvious that if others
+follow a different practice confusion may result when observations are
+summarised in general statements. We will consider this subject again
+in another place, but here it may be sufficient to say that there can
+scarcely now be a doubt that numbers of these associated species, such
+as Jordan discriminated, represent various combinations of the presence
+and absence of Mendelian factors. This does not in any way weaken the
+argument which Leavitt founds upon the facts, namely, that the observed
+distribution of these forms is consistent with the supposition of an
+evolution largely discontinuous.
+
+On the other hand, those who have come to the opinion that nearly
+allied species generally occupy distinct ground are presumably more
+impressed by the characters differentiating the geographically distinct
+or adaptational races, seeing that genuine intermediates between
+them are less commonly found. Those geographical races may no doubt
+contain various differentiated forms; but when all live together,
+occasional intermediates are usually to be found even in the case of
+characters habitually segregating. These segregating forms Jordan would
+certainly have determined as species, and it must be conceded that no
+physiological definition has yet been drawn which consistently excludes
+them.
+
+FOOTNOTES:
+
+[1] Often referred to by older writers as _Meone_, Esper's name.
+
+[2] There are also two distinct island forms, unlike the European,
+_Xiphia_ of Madeira, and a smaller variety, _Xiphioides_ of Canary. See
+especially, Baker, G. T., _Trans. Ent. Soc. London_, 1891, p. 292.
+
+[3] Speyer, Adolf, and August. _Verbreitung der Schmetterlinge_, 1858,
+I, p. 217.
+
+[4] _Lepid. Comparée_, fsc. III, p. 372.
+
+[5] Mr. Rowland-Brown has called my attention to a statement by Dr.
+Vaillantin (_Petites Nouv. Ent._, II, 235) that in Indre-et-Cher the
+first brood is of the northern type and the second of the southern. My
+experience is that in captivity these distinctions do not occur, and I
+have true _egeria_ as first brood from Vienne and as the late brood from
+the Landes. I never collected in Indre-et-Cher.
+
+[6] I have since seen true _egeria_ from Ferrol in the extreme
+northwest, which was in Mr. Tutt's collection.
+
+[7] Mr. G. Wheeler kindly showed me a series identical with this type,
+from Guernsey, and others from near Laon.
+
+[8] _Ent. Rec._, V, 1894, p. 134.
+
+[9] Mr. Wheeler has some pale but rather worn specimens from the Rhone
+Valley at Vernayaz.
+
+[10] See Fleck, E., Die Macrolep. Rumäniens, _Bul. Soc. Sciinte_, VIII,
+1899, p. 720.
+
+[11] My experience agrees with that of Mr. H. Williams (_Ent. Rec._,
+VIII, 1896, p. 181) that pupae, well-formed, can stand considerable
+frost; but I used to find that half-grown larvae usually died if
+unprotected, and I believe that larvae which attempted to pupate in warm
+autumn weather and then got caught by frosts, always died. Small larvae
+which can creep into shelter at the bottom of the plants survived, and I
+expect that in the north the winter is usually passed in that state (see
+also Merrifield, F., _Ent. Rec._, VIII, 1896, p. 168, and Carpenter, J.
+H., _ibid._).
+
+[12] Some most unlikely species do this. I once had a larva of
+_Parnassius delius_, found at about 5,500 feet, which emerged late
+in the autumn (in October I believe), a season at which it must have
+perished in its own country.
+
+[13] See, for examples, Barrett, G. C., _Lepidoptera of the Brit.
+Islands_, I, 1893, p. 229; also Grover, W., _Ent. Rec._, IX, 1897, p.
+314; Williams, H., _Proc. Ent. Soc._, 1898, who reared several specimens
+from the New Forest which would pass for Bretons, though the rest of the
+family were true _egerides_.
+
+[14] Above the Tosa falls.
+
+[15] _Bul. Soc. Sciinte_, VIII, 1899, p. 691.
+
+[16] The fact that Weismann by heating pupæ obtained only one autumn
+specimen seems to me to show rather that a second brood can be produced
+than that it cannot, which is the inference usually drawn.
+
+[17] Schima, K., _Verh. Zool. bot. Ges. Wien_, LX, 1910, p. 268.
+
+[18] _Rhopalocera Palaearctica_, Florence, 1905-11, especially Pl. XXXII.
+
+[19] See figures in Barrett, G. C., _Lepidoptera of Brit. Islands_, I,
+pt. 3, p. 25.
+
+[20] Tutt, J. W., _Ent. Rec._, XVIII, 1905, p. 5. In the same place he
+states that on the Mendel Pass _arcania_ "runs into" _darwiniana_ and
+that in the Tyrolean localities the transition is especially evident.
+Wheeler (_ibid._, XIII, 1901, p. 121) expresses the contrary opinion,
+that _satyrion_ does grade to _arcania_.
+
+[21] H. Rowland-Brown, _Ent. Rec._, XI, 1899, p. 293.
+
+[22] Speyer, Stettiner, _Ent. Ztg._, XXXI, 1870, p. 63.
+
+[23] In regard to the closely analogous case of _Spilosoma lubricipeda_,
+Standfuss makes a similar statement. He bred the type on a large scale
+with the radiate form which he calls _intermedia_, and says that in four
+years of miscellaneous crossing he never obtained really transitional
+forms. Nevertheless after examining large series, especially those of
+Mr. W. H. B. Fletcher, I came to the conclusion that several might be
+so classed, but I am quite prepared to find that such specimens are
+heterozygous. (See Standfuss, _Handb. d. Gross-Schmet._, 1896, p. 307.)
+It is by no means unlikely that various dark forms of _lubricipeda_
+correspond with a progressive series of factorial additions. Many of
+the stages have been named, and of these the most definite are the
+_intermedia_ of Standfuss (probably = _eboraci_ of Tugwell) and the very
+dark _Zatima_ of Heligoland, in which only the thorax, the nervures
+and a small field in the fore-wings remain yellow. A form was bred
+by Deschange from _Zatima_ in which even the field in the forewing
+is obliterated. The exact circumstances in which _Zatima_ occurs in
+Heligoland would be worthy of special investigation, for the normal
+_lubricipeda_ is also found on the island. For references as to the
+British occurrences see especially, Hewett, W., _Naturalist_, 1894, p.
+353. As to _Zatima_ see especially Krancher, _Soc. Ent._, II, 1887-8, p.
+26. I am indebted to Dr. Hartlaub for information as to the Heligoland
+types.
+
+[24] Boisduval, _Bull. Soc. Ent. Fr._, III, 1834, p. 5.
+
+[25] The systematics of _Setina_ have been much controverted, but no one
+I believe doubts that _aurita_ and _ramosa_ are forms of one species.
+See also Chapman, A. T., _Ent. Rec._, XIII, 1901, p. 139.
+
+[26] _Arch. Naturg._, 33, 1867, p. 116.
+
+[27] _Brit. Mus. Cat., Batrachia Gradientia_, 1882.
+
+[28] The Geographical Distribution of nearly related Species. _Amer.
+Nat._, XLI. 1907, p. 207.
+
+[29] See later, p. 242.
+
+
+
+
+                        CHAPTER IX
+
+         THE EFFECTS OF CHANGED CONDITIONS: ADAPTATION
+
+In the attempt to conceive a process by which Evolution may have come
+about, the first phenomenon to be recognized and accounted for is
+specific difference. With that recognition the outline of the problem is
+defined. The second prerogative fact is adaptation. Forms of life are
+_on the whole_ divided into species, and these species _on the whole_
+are adapted and fit the places in which they live. To many students of
+Evolution, adaptation has proved so much more interesting and impressive
+than specific diversity that they have preferred it to the first place
+in their considerations.
+
+Whether this is, as I believe, an inversion of the logical order or
+not, there is one most serious practical objection to such preference,
+that whereas specific diversity is a subject which can be investigated
+both by the study of variation and by the analytical apparatus which
+modern genetic science has developed, we have no very effectual means of
+directly attacking the problems of Adaptation.
+
+The absence of any definite progress in genetics in the last century was
+in great measure due to the exclusive prominence given to the problem
+of Adaptation. Almost all debates on heredity centered in that part of
+the subject. No one disputes that the adaptation of organisms to their
+surroundings is one of the great problems of nature, but it is not the
+primary problem of descent. Moreover, until the normal and undisturbed
+course of descent under uniform conditions is ascertained with some
+exactness, it is useless to attempt a survey of the consequences of
+external interference; nor as a rule can it be even possible to decide
+with much confidence whether such interferences have or have not
+definite consequences. Those, for example, who debated with enthusiasm
+whether acquired characters are or are not transmitted were constantly
+engaged in discussing occurrences which we now know to be ordinary
+features of descent under uniform conditions, and the origin of
+variations which were certainly not caused directly by circumstances at
+all. In the absence of any factorial analysis, or of any conception of
+what factorial composition means and implies, no one knew what varieties
+might be expected from given parents. The appearance of any recessive
+variety was claimed as a consequence of some treatment which might have
+been applied to the parents. There was no possible standard of evidence
+or means of controlling it, and thus the discussion was singularly
+unfruitful. Before we can tell how the course of descent has departed
+from the normal, we must know what the normal would have been if we
+had let alone. We are still far from having such knowledge in adequate
+measure, but it does now exist in some degree, and we are steadily
+approaching a position from which we shall be able to form fairly
+sound estimates of the true significance of evidence for or against
+the proposition that environmental treatment can produce positive
+disturbances in the physiological course of descent.
+
+Thus described, the field for consideration is very wide. Though the
+effects of changed conditions were especially studied in the hope of
+solving the problem of adaptation by direct observation, that, as all
+are now agreed, is but a part of a more general question. We must ask
+not only do changed conditions produce an _adaptative_ response on the
+part of the offspring, but whether they produce any response on the
+part of the offspring at all. It is not in doubt that by violent means,
+such as starvation or poisoning of the reproductive cells, effects of a
+kind, stunting and deformity for instance, can be made evident, just as
+similar effects may follow similar treatment during embryonic or larval
+life. Apart from interferences of this class, are there any that may be
+reasonably invoked as modifying the course of inheritance?
+
+No epitome of the older evidence for the inheritance of adaptative
+changes is here required. That has often been collected, especially
+by Weismann, who exposed its weaknesses so thoroughly as to carry
+conviction to most minds, and showed that whether the phenomenon
+occurs or not, no one can yet prove that it does. Belief in these
+transmissions, after being almost universally held, was with singular
+unanimity abandoned. This change in opinion, though doing credit to the
+faith of the scientific community in evidential reasoning, is the more
+remarkable inasmuch as the strength of the idea was not derived from the
+minute amounts of supposed facts now demolished. On the contrary, it was
+really an instinctive deduction from a wide superficial acquaintance
+with the properties of animals and plants. They _can_ accommodate
+themselves to circumstances. They _do_ make responses sometimes
+marvellously appropriate to demands for which they can scarcely have
+been prepared. What more natural than to suppose that the permanent
+adaptations have been achieved by inherited summation of such responses?
+No one had actually been driven to believe in the inheritance of
+adaptative changes because bitches which had been docked had been known
+to give birth to tailless puppies, or because certain wheat in Norway
+was alleged to have become acclimatized in a few generations. Evidence
+of this kind was collected and produced rather as an ornamental appendix
+to a proposition already accepted, and held to be plainly demonstrated
+by the facts of nature. Looked at indeed in that preliminary and
+uncritical way, the case is simply overwhelming. Those who desire to see
+how strong it is should turn to Samuel Butler's _Life and Habit_, and
+even if in reading they reiterate to themselves that no experimental
+evidence exists in support of the propositions advanced, the misgiving
+that none the less they may be true is likely to remain. Making every
+deduction for the fact that the wonders of adaptation have been grossly
+exaggerated, and that marvels of fitness and correspondence between
+means and ends have grown out of mere anthropomorphic speculations,
+there is much more left to be accounted for than can at all comfortably
+be accepted as the product of happy accidents. So oppressive are these
+difficulties that we can scarcely blame those who imagine that the study
+of heredity is primarily directed to the problem of the transmission of
+acquired characters, a preconception still almost universal among the
+laity.
+
+But since the belief in transmission of acquired adaptations arose
+from preconception rather than from evidence, it is worth observing
+that, rightly considered, the probability should surely be the other
+way. For the adaptations relate to every variety of exigency. To supply
+themselves with food, to find it, to seize and digest it, to protect
+themselves from predatory enemies whether by offence or defence, to
+counter-balance the changes of temperature, or pressure, to provide for
+mechanical strains, to obtain immunity from poison and from invading
+organisms, to bring the sexual elements into contact, to ensure the
+distribution of the type; all these and many more are accomplished by
+organisms in a thousand most diverse and alternative methods. Those are
+the things that are hard to imagine as produced by any concatenation
+of natural events; but the suggestions that organisms had had from the
+beginning innate in them a power of modifying themselves, their organs
+and their instincts so as to meet these multifarious requirements does
+not materially differ from the more overt appeals to supernatural
+intervention.
+
+The conception, originally introduced by Hering and independently by
+S. Butler, that adaptation is a consequence or product of accumulated
+_memory_ was of late revived by Semon and has been received with some
+approval, especially by F. Darwin. I see nothing fantastic in the notion
+that memory may be unconsciously preserved with the same continuity
+that the protoplasmic basis of life possesses. That idea, though purely
+speculative and, as yet, incapable of proof or disproof contains
+nothing which our experience of matter or of life at all refutes. On
+the contrary, we probably do well to retain the suggestion as a clue
+that may some day be of service. But if adaptation is to be the product
+of these accumulated experiences, _they must in some way be translated
+into terms of physiological and structural change_, a process frankly
+inconceivable.
+
+To attempt any representation of heredity as a product of memory is,
+moreover, to substitute the more obscure for the less. Both are now
+inscrutable; but while we may not unreasonably aspire to analyse
+heredity into simpler components by ordinary methods of research, the
+case of memory is altogether different. Memory is a mystery as deep
+as any that even psychology can propound. Philosophers might perhaps
+encourage themselves to attack the problem of the nature of memory by
+reflecting that after all the process may in some of its aspects be
+comparable with that of inheritance, but the student of genetics, as
+long as he can keep in close touch with a profitable basis of material
+fact, will scarcely be tempted to look for inspiration in psychical
+analogies.
+
+For a summary of the recent evidence I may refer the reader to Semon's
+paper[1] where he will find a collection of these observations described
+from the standpoint of a convinced believer. At the outset one cannot
+help being struck by the fact that of the instances alleged, very few,
+even if authentic, show the transmission of acquired modifications
+which can in any sense be regarded as adaptative, and many are examples
+not so much of a transmission of characters produced in the parents as
+of variation induced in the offspring as a consequence of treatment
+to which the parents were submitted, the parents themselves remaining
+apparently unmodified. No one questions the great importance of evidence
+of this latter class as touching the problem of the causes of variation,
+but it is not obvious why it is introduced in support of the thesis that
+acquired characters are inherited.
+
+It is most difficult to form a clear judgment of the value of the
+evidence as a whole. To doubt the validity of testimony put forward
+by reputable authors is to incur a charge of obstinacy or caprice;
+nevertheless in matters of this kind, where the alleged phenomena are,
+if genuine, of such exceptional significance, belief should only be
+extended to evidence after every possible source of doubt has been
+excluded. We believe such things when we must, but not before. At the
+very least we are entitled to require that confirmatory evidence should
+be forthcoming from independent witnesses. So far as I have seen, this
+requirement is satisfied in scarcely any of the examples that have been
+lately published, and until it is, judgment may reasonably be suspended.
+
+In some cases, however, the facts are not doubtful. Standfuss, by
+subjecting pupae of _Vanessa urticae_ to cold, produced the now
+well-known temperature-aberrations in which the dark pigment is greatly
+extended. He put together in a breeding-cage 32 males and 10 females
+showing this modification in various degrees. Two of these females died
+without leaving young. Seven produced exclusively normal offspring. From
+the eighth female 43 butterflies were bred, and of these there were four
+(all males) which to a greater or less extent exhibited the aberrational
+form.[2] The mother of this family was the most abnormal of the 10
+females originally put in.
+
+Fischer's experiment with _Aretia caja_ was on similar lines. From
+pupae which had been frozen almost all the moths which emerged showed
+aberrational markings. A pair of these mated and produced 173 young
+which pupated. Those which emerged early were all normal, but of those
+which emerged late, 17 had in various degrees abnormal markings like
+those of the parents.[3] In neither of these examples is there any
+question as to the facts. Both observers have great experience and give
+full details of their work.
+
+As regards _Vanessa urticae_, however, it must be recalled that Fischer
+himself showed that in Nymphalids somewhat similar aberrations could be
+produced both by heat and by cold, and even by centrifuging the pupae.
+Frl. von Linden produced a transitional form of the same aberration
+in _V. urticae_ by the action of carbonic acid gas.[4] It is highly
+probable that the appearance is due to a morbid change, perhaps an
+arrest of development, which may be brought about by a great diversity
+of causes. In the experiments the cause probably was a diseased
+condition of the tissues of the mother herself. She had been subjected
+to freezing sufficiently severe to prevent the proper development of
+the pigments and some of the ovarian cells presumably suffered also.
+It will be observed that the only specimens which were affected were
+the offspring of the most abnormal female, and of them only four out of
+forty-three showed any change.
+
+The same interpretation probably applies to the cases in _Arctia caja_.
+In this species the markings are well known to be liable to great
+variation. As Barrett says, even in nature individuals are rarely quite
+alike, and an immense number of strange forms occur in collections.[5]
+These are greatly sought after by some collectors, especially in
+England, where they fetch high prices at auctions, and it is notorious
+that most of them come from Lancashire and the West Riding of Yorkshire.
+It is commonly supposed that the breeders of that district subject them
+to abnormal conditions, and especially to unnatural feeding, but I know
+no clear evidence that this is true. From whatever cause it is certain
+that the natural pattern is, in some strains at all events, very easily
+disturbed.
+
+The elaborate experiments of Schröder with _Abraxas grossulariata_ are
+difficult to follow and are complicated by the fact that the series
+which was submitted to abnormal temperatures was derived from an
+abnormal original pair. From the evidence given it is not clear to me
+whether the temperature had a distinct effect. This insect, like _Arctia
+caja_, produces an immense number of variations (especially in the
+amount of the black pigment) and as most of these are, I believe, reared
+in domestication for sale, it is highly probable that the species is
+easily influenced by cultural conditions.
+
+Schröder describes two other experiments which have been accepted
+by Semon and other supporters of the view that acquired characters
+are transmitted. In the first, _Phratora vitellinae_, a phytophagous
+beetle living on the undersides of leaves, was used. It naturally
+feeds on _Salix fragilis_, a species without a felt, or tomentum, on
+the underside of the leaves. Larvae were transferred to another willow
+(near _S. viminalis_) which has the undersides of the leaves felted. The
+larvae took readily to the new food, pushing the tomentum before them as
+they gnawed the leaves. They came to maturity and when they were about
+to lay their eggs they were given a free choice between _S. fragilis_
+and the tomentose species. The greater number of ovipositions, 219,
+took place on _fragilis_, and there were 127 on the tomentose bush,
+which we are told was six times as large as the _fragilis_. The larvae
+from _fragilis_ were next put on the tomentose species and reared on
+it. When they became imagines they were similarly given their choice,
+with the result that there were 104 ovipositions on the tomentose
+species and only 83 on _fragilis_. In the next generations there were 48
+ovipositions on the tomentose and 11 on _fragilis_. Finally the fourth
+generation made 15 ovipositions on the tomentose and none on _fragilis_.
+
+The difficulty about such experiments is obviously that one has no
+assurance that the change of instinct, in so far as there is any,
+may not be a mere consequence of the captivity. It must, besides, be
+extremely difficult to arrange the experiment so that there is really
+an equal choice between the two bushes, when one stands beside the
+other. Przibram, in quoting this case, considers that as the tomentose
+bush was about six times as large as the _fragilis_, some indication of
+the relative attractiveness of the two may be obtained by dividing the
+ovipositions on the larger bush by six, but I imagine the matter must be
+much more complex.
+
+Schröder's second example is not more convincing, in my opinion, though
+Semon regards it as one of the most important pieces of evidence. It
+concerns a leaf-rolling moth, _Gracilaria stigmatella_, the larva of
+which is said normally to make its house by bending over the _tips_ of
+the sallow leaves on which it feeds. Schröder placed larvae on leaves
+from which the tips had been cut, and these larvae made their houses by
+rolling over the _sides_ of the leaves. Their offspring were again fed
+on leaves without tips, and as before, they rolled in the leaf-margins
+either on one side or both. The offspring of this second generation were
+then fed on entire leaves. There were 19 houses made by these (?19)
+larvae, and of them 15 were normal, made by folding down the tips of
+the leaves, while 4 were abnormal, made by rolling in the leaf-margins.
+Schröder says that in nature he has only twice seen abnormal houses; but
+it is clearly essential not only that the frequency of such variability
+in nature should be thoroughly examined, but also that we should know
+whether when the species is bred in captivity these irregularities of
+behaviour do or do not occur when the larvae are fed on uninjured leaves.
+
+The famous case of Schübeler's wheat is revived by Semon. The story will
+be familiar to most readers of the literature of the subject. Briefly
+it is that annuals, especially wheat and maize, raised from seed in
+Central Europe take more time in coming to maturity and ripening than
+similar plants raised in Norway, where the summer days are much longer.
+The received account is that he imported seed especially of maize and
+of wheat from Central Europe to Norway and found that in successive
+years the period of growth and ripening was increasingly reduced. After
+two generations seed of the accelerated wheat was sent back to Breslau
+where it was grown, and was found to ripen rather more slowly than in
+Norway, but much more quickly than the original stock had done. The
+facts recorded by Schübeler[6] are that he received seed from Eldena,
+which is on the Baltic near Greifswald. The variety is described as
+"_100 tägiger Sommer Weizen_," but no more exact record of its behaviour
+in Germany is given. This wheat, grown at Christiania in 1857, took
+103 days to harvest. Its seed was again grown in Christiania in 1858,
+and took 93 days, and sown again in 1859 it took only 75 days, 28 days
+less than in the first year of cultivation in Norway. Seed of the 1858
+crop was sent to Breslau, and grown there by Roedelius in 1859; it
+took 80 days. Evidently before such a record can be used as proving an
+inheritance of acquired characters numbers of particulars should be
+forthcoming. The view that Johannsen has taken is that the result was
+probably due to unconscious selection of the earlier individuals among a
+population consisting of many types of various compositions. Some effect
+may no doubt be ascribed to that cause, but I cannot think that alone
+it would account for the results. My impression is rather that they
+were produced by differences in the cultivation and especially in the
+seasons. Research of an elaborate character would be necessary in order
+to eliminate the various sources of error, and nothing of the kind has
+been done; nor does Semon allude to these difficulties in prominently
+adducing Schübeler's evidence. A difference of even three weeks in time
+of harvesting may easily be due to variation in the season. It would in
+any case be difficult to analyse the meteorological conditions, and to
+decide how much effect in postponing or accelerating the harvest might
+be due to cold days, to cloudy days, to wet weather, to fluctuations in
+average temperature, to hot days, and other such incidents occurring at
+the different periods of growth, even if they were specially watched
+while the experiments were in progress, and at this distance of time
+such analysis is practically impossible. Without careful simultaneous
+control-experiments this evidence is almost worthless. The director of
+the Meteorological Office[7] has, however, kindly sent me some details
+of the weather at Breslau from 1857 to 1860, and I notice that as a
+matter of fact July, 1859, was an exceptionally hot month, _having an
+average of 2.67° C. above the mean_ for the twenty years 1848-1867. June
+in that year was slightly (0.31° C.) below the mean and May slightly
+above it (0.18° C.). August was also abnormally hot, 2.35° C. above the
+average. The Breslau wheat was sown on _May 19_ and harvested on August
+6. There was a cold spell from May 11 to 14, which this wheat escaped,
+as it was sown on May 19. In the other years the cold spell came much
+later. These elements of the weather may possibly have done something to
+hurry the ripening in 1859. It unfortunate that we are not told how long
+similar wheat from Breslau seed took to ripen in that year.
+
+As regards the Norway cultivations we have the average monthly
+temperatures recorded by Schübeler, though he does not discuss them
+in connection with this special problem. It is quite clear that 1857,
+in which the period was 103 days, was an exceptionally cold summer,
+especially as regards the months of June and July, but though there
+was, so far as the temperature records go, no great difference between
+1858 and 1859, the year 1859, in which the period of ripening was the
+shortest, was somewhat colder in Norway than 1858. But we have the
+further difficulty that there were ten days difference in sowing, for
+in 1858 the sowing was made on May 14, and in 1859 on May 24. With
+all these possibilities uncontrolled, and indeed unconsidered, I am
+surprised that Semon should claim these experiments as one of the chief
+supports for his views.
+
+Schübeler's other allegations respecting the influence of climate on
+plants grown in various places and especially at different elevations
+in Norway have been destructively criticised by Wille[8] to whose paper
+readers interested in the subject should refer.
+
+Before the appearance of Wille's criticisms Wettstein[9] made a
+favourable reference to Schübeler's work, accepting his conclusion. He
+states also that he has himself made analogous experiments with flax,
+finding that the length of the period of development and a series of
+morphological characters show an adaptation to local conditions, and
+that on transference of seed to other conditions the previous effects
+are maintained. No details, however, are given, and I do not know if
+anything more on the subject has appeared since. The other examples
+cited by Wettstein, such as the observations of Cieslar on forest-trees
+and those of Jakowatz on gentians seem to me open to all the usual
+objections applicable to evidence of this kind. Such work, to be of any
+value for the purpose to which it is applied, must be preceded by a
+study of the normal heredity and of the variations of the species.
+
+Most of the recent writers (Semon, Przibram, etc.) on the inheritance
+of acquired characters accept the story of Brown-Séquard's guinea pigs,
+which are said to have inherited a liability to peculiar epileptiform
+attacks induced in their parents by various nervous lesions.
+
+The question has been often debated and several observers have repeated
+the experiments with varying results, some failing to confirm
+Brown-Séquard, others finding evidence which in various degrees
+supported his conclusions. Recently a new and especially valuable paper
+has been published by Mr. T. Graham Brown[10] which goes far towards
+settling this outstanding question. He states that "the Brown-Séquard
+phenomenon is nothing more or less than a specific instance of the
+scratch-reflex," and it is due to a raised excitability of the mechanism
+of this reflex. This raised excitability is the character acquired as a
+consequence, for instance, of the removal of part of one great sciatic
+nerve. The nature of this raised excitability and its causation are
+discussed and elucidated, but this part of the work is not essential
+to the present consideration. Mr. Graham Brown in his summary of
+conclusions remarks that it is very difficult to see how this condition
+of raised excitability can be transmitted to the offspring, and this
+comment which might be made in reference to any of the alleged cases
+certainly applies with special cogency to the present example.
+
+He then calls special attention to three observations:
+
+1. That guinea pigs which had a "trophic" change in the foot, as a
+result of division of the great sciatic nerve, have repeatedly been seen
+to nibble the feet of other guinea pigs which had this change in the
+foot from the same causes.
+
+2. That accidental injury to the toes may be followed by the
+Brown-Séquard phenomenon in an otherwise normal animal.
+
+3. That in several instances the young of guinea pigs which exhibited
+the phenomenon have been noticed to have one or more toes eaten off by
+the mother.
+
+Brown-Séquard noticed that almost all his animals in which the great
+sciatic was divided acquired the "epilepsy" and nibbled those parts of
+their feet in which sensation had been lost. Of the offspring of such
+animals he found that a very small proportion exhibited a malformation
+of the feet, and of these some showed the "epilepsy." The proportion
+which showed the "epilepsy" was one to two per cent. of the offspring.
+
+Morgan[11] is quoted by Graham Brown as having suggested that the
+loss of toes in the offspring may have been due to mutilation by the
+mother, following his experience in a case in which the tails of mice
+in succeeding litters were thus devoured, and there can be little doubt
+that in this suggestion lies the clue to the explanation of the whole
+mystery. Graham Brown concludes that it may be supposed with every
+degree of probability that the "transmission" was due to injuries
+inflicted upon the young by their parents. With this conclusion most
+people will now be disposed to agree, and we may hope that we shall
+hear the last of this curious myth--to the elucidation of which a vast
+quantity of research has been devoted.
+
+The series of experiments made by Kammerer with various Amphibia have
+attracted much attention and have been acclaimed by Semon and other
+believers in the transmission of acquired characters as giving proof of
+the truth of their views. With respect to these observations the chief
+comment to be made is that they are as yet unconfirmed. Many of the
+results that are described, it is scarcely necessary to say, will strike
+most readers as very improbable; but coming from a man of Dr. Kammerer's
+wide experience, and accepted as they are by Dr. Przibram, under whose
+auspices the work was done in the Biologische Vesuchsanstalt at Vienna,
+the published accounts are worthy of the most respectful attention.
+
+The evidence relates chiefly to three distinct groups of occurrences:
+
+  1. Modification in _Alytes obstetricans_, the Midwife Toad, affecting
+     both the structure and the mode of reproduction, induced by compulsory
+     change of habits.
+
+  2. Modification in the mode of reproduction of _Salamandra atra_ and
+     _maculosa_ induced by compulsory change of habits.
+
+  3. Modification in the colour of _Salamandra maculosa_ induced by change
+     in the colour of the soil on which the animals were kept.
+
+1. I will take first the case of _Alytes_,[12] because it is the most
+definite example, and because it is the case which most readily admits
+of repetition and verification.
+
+The habits of _Alytes obstetricans_ are well known. The animals copulate
+on land. As the strings of eggs leave the female they are entangled by
+the hind legs of the male, and being adhesive they stick to him and
+undergo their development attached to his back and legs. The number of
+eggs varies from 18 to 86, a number much smaller than is usual in toads
+and frogs which lay their eggs in water. The eggs are large and full of
+yolk.
+
+There are two breeding seasons, one about April and the other about
+September, and a winter hibernation. Not only animals brought in from
+outside, but their offspring reared in domestication maintain these
+normal habits in confinement, if the temperature does not exceed 17° C.
+(pp. 499 and 534).
+
+If, however, the temperature be artificially raised and kept at 25-30°
+C., the males do not attach the eggs to themselves when spawning occurs
+on land but let them lie. The adhesion of the eggs is said to be
+hindered by the comparatively rapid drying of their surfaces.
+
+More usually in the high temperatures the animals _take to the water_
+and copulate there. The eggs are ejected into the water, and as their
+gelatinous coverings immediately swell up, they do not stick to the
+males.
+
+The offspring thus derived from the parents subjected to heat for one
+breeding-period only, whether they were laid in water or on land, did
+not show departures from the normal type.
+
+Kammerer states next, however, that in subsequent breeding-periods the
+same parents frequently take to the water to breed, though they have
+become quite accustomed to the heated chamber; and furthermore that if
+such animals, having thus lost their instinct to brood their young, be
+transferred to ordinary temperatures they do not readily reassume their
+normal habits, but for several breeding seasons--at least four--will
+take to the water. These parents lay from 90 to 115 eggs, which are
+small and contain little yolk, and the larvae, on hatching, breathe with
+their embryonic gills until they are absorbed instead of being broken
+off as normally.
+
+The offspring thus abnormally developed when they mature are said never
+to brood their eggs. If they are derived from the earlier spawnings of
+their parents, before, that is to say, the parents had been submitted
+to the changed conditions long enough to transmit their effects, they
+lay on land; but if they are derived from the later spawnings, they
+lay in the water. These changes of habit are manifested without the
+continued application of the abnormal experimental conditions, and, as I
+understand the account, in normal conditions of temperature.
+
+If the abnormal experimental conditions are continued, the toads always
+lay in water, and their eggs become progressively smaller and more
+numerous. The larvae in the fourth generation acquire three pairs of
+gills instead of one pair, and are in other respects also different from
+the normal form.
+
+Respecting the _Alytes_ bred in this way Kammerer makes the very
+striking statement that _the males in the third generation_ (p. 535)
+_have roughened swellings on their thumbs and that in the fourth
+generation_ (pp. 516 and 535) _these swellings develop black pigment_.
+Together with the appearance of this secondary sexual character there
+is hypertrophy of the muscles of the fore-arm. To my mind this is
+the critical observation. If it can be substantiated it would go far
+towards proving Kammerer's case. _Alytes_, among toads and frogs, is
+peculiar in that the males do not develop these lumps in the breeding
+season, and the fact may no doubt be taken to be correlated with the
+breeding habits, copulation occurring on land and not in water as is
+usual with Batrachians. It is to be expressly noticed that these lumps
+on the thumbs or arms of male toads and frogs are not merely pigmented
+swellings, but are pads bearing numerous minute horny black spines,
+which are used in holding the females in the water. The figures which
+Kammerer gives (Taf. XVI, figs. 26 and 26a) are quite inadequate, and as
+they merely indicate a dark patch on the thumbs it is not possible to
+form any opinion as to the nature of the structure they represent.
+
+The systematists who have made a special study of Batrachia appear to
+be agreed that _Alytes_ in nature does not have these structures; and
+when individuals possessing them can be produced for inspection it
+will, I think be time to examine the evidence for the inheritance of
+acquired characters more seriously. I wrote to Dr. Kammerer in July,
+1910, asking him for the loan of such a specimen[13] and on visiting
+the Biologische Versuchsanstalt in September of the same year I made
+the same request, but hitherto none has been produced. In matters of
+this kind much generally depends on interpretations made at the time
+of observation; here, however, is an example which could readily be
+attested by preserved material. I notice with some surprise that in a
+later publication on the same subject no reference to the development of
+these structures is made (see below).
+
+The statements here given represent but a small part of Kammerer's
+papers on the subject. He gives much further information as to the
+course of the experiments, especially in regard to the fate of the eggs
+laid on land and the aberrations induced in them by treatment. The
+ramifications of the experiments are, however, very difficult to follow,
+and as I am not sure that I have always understood them I must refer the
+reader to the original.
+
+More recently Kammerer has published[14] a most curious account of
+experiments in crossing his modified and abnormal _Alytes_, derived from
+the water-eggs, with normal individuals.
+
+In the first case the cross was made between a _normal female_ and an
+_abnormal male_. The offspring were normal in their habits. In the next
+generation bred from these almost exactly a quarter showed the abnormal
+instinct.
+
+The reciprocal cross was made between an _abnormal female_ and a _normal
+male_. In this case the offspring were abnormal in their behaviour; but
+the second generation bred from them showed three quarters abnormal and
+one quarter normal.
+
+Certain details as to numbers and sexes of the various families bred
+in the course of this amazing experiment are given in a subsequent
+publication.[15] This later paper goes somewhat fully into the question
+of the difference in behaviour between the normal and modified
+individuals, describing the ways in which the males and females
+possessing the acquired character could be recognised from the males and
+females which were normal, but in this account I find no reference to
+the development of the "_Brunftschwielen_"--the horny pads on the hands
+of the males. As these structures would be of special value in such a
+diagnosis the omission of any allusion to them calls for explanation.
+Kammerer claims the evidence as proof of Mendelian segregation in
+regard to an acquired character, the first example recorded. Pending a
+repetition of the experiments there is no more to be said.
+
+2. _The Mode of Reproduction of Salamandra atra and
+maculosa._[16]--_Salamandra maculosa_, the common lowland form, with
+yellow bands or spots, deposits its young in water, generally as
+gill-bearing tadpoles, with a wide, swimming tail, though occasionally
+they are born still enclosed in the egg-capsule out of which they soon
+hatch. Spawning extends over a considerable period, often many weeks,
+and during the season one female may bear more than 50 young.
+
+_S. atra_, the black Alpine form, produces its young on land. They are
+born without gills, ready to breathe air, and with the rounded tail
+of the adult. These differences may, as Kammerer says, naturally be
+regarded as adaptations to the Alpine conditions. Moreover, the female
+bears _only two_ young in a season, and this reduction in the number
+must be taken to be a consequence or condition of viviparity. There are
+many eggs in the ovary, but all except the two which are destined to
+develop degenerate and form a yolk-material on which these two survivors
+feed.
+
+Kammerer gives a long account of the various conditions to which he
+subjected both species. The treatment was complicated in many ways, but
+the essential statements are, as regards _S. maculosa_, that when no
+water was provided in which the young might be born, they were dropped
+on land, larger and in a later stage of development and of a darker
+colour than is normal; that the larvae so born gradually diminished
+in number until only two were deposited in each breeding-period;
+that dissection showed that the other ova degenerated to form a
+yolk-material. The larvae so produced reached maturity. The summary of
+results describes their behaviour, stating that they produced:
+
+(_a_) _In water_, either (1) _very_ advanced, large-headed larvae 45
+mm. long (instead of 25-30 mm.) with gills already reduced, which had
+awkward, embryo-like movements, and in some few days metamorphosed
+into small perfect salamanders; or (2) moderately advanced, properly
+proportioned larvae, 40-41 mm. long, provided with large gills of (at
+first) intrauterine character, which were reduced during aquatic life.
+
+(_b_) _On land_, small (26 mm. long) larvae with rudimentary gills,
+having the body rounded instead of being flattened from above downwards,
+and an elongated narrow head, which were unable to live in deep water.
+These larvae changed to the salamander colour in 10-12 days, and after
+four weeks metamorphosed into salamanders 29 mm. long.
+
+(_c_) In the foregoing cases the experimental conditions were not
+continued, or in other words, basins of water were provided in which
+they could spawn. But if the experimental conditions are continued,
+these _Salamandra maculosa_ which were born newt-like (viz., not in a
+larval condition), are themselves newt-bearing from the first time they
+give birth, using the dry land, and bringing forth only two young, the
+normal number for the births of _S. atra_. These young are 40-41 mm.
+long, and are dark-coloured, resembling greatly the normal new-born _S.
+atra_.
+
+This epitome of the observations illustrating the inheritance of
+acquired characters has been very widely quoted, and may not unnaturally
+be taken to summarize a wide experience of the modified animals.
+Reference to the details given in the same paper shows that, as alleged,
+each of the four types of behaviour enumerated was witnessed _once_ only
+in the case of each of four females, no two agreeing with each other.
+As to the number of the males or their habits nothing is said. The first
+female, _a_ (1), bore five young; the second, _a_ (2), bore two, of
+which one was a partial albino; the third, _b_, produced four young; and
+the fourth, _c_, two as already stated.
+
+In the case of _c_ the details show that the female gave birth
+immediately after being transferred from the open-air terrarium to
+one indoors, which contained no basin of water. This is the example
+of the consequences which follow on a continuance of the experimental
+conditions.[17]
+
+As regards _S. atra_ the converse is reported. Various means were
+used to induce them to eject their young prematurely in water, such
+as massaging the sides of the mothers, or raising the temperature to
+25° or 30° C., with various degrees of success. But afterwards it was
+found that specimens collected wild at an elevation of about 1,000
+metres responded to much simpler treatment, and gave birth prematurely
+in water when they were kept in a large shallow basin of water not so
+deep but that they could everywhere touch the bottom with their feet and
+keep their heads above the surface. With specimens collected at higher
+elevations this treatment was inoperative, and the suggestion is made
+that _S. atra_ at the lower confines of its habitat partakes more of the
+nature of _maculosa_ than do the individuals from greater heights; for
+Kammerer argues that pools suitable for breeding must be more uncommon
+at those elevations than they are lower down.
+
+In the earlier paper[18] Kammerer states that newly caught females
+of _S. atra_ often give birth in the water, and show an undoubted
+preference for doing so. He describes also how he once saw several
+females, wild in their natural habitat, lay their young in a rain-puddle
+at 1,800 metres elevation, but the larvae thus born were fully formed.
+
+When the deposition of the young as larvae has become "habitual"[19]
+with _S. atra_, three to nine larvae may be produced at one spawning
+period, from 35 to 45 mm. long, with gills at most 8 mm. long, and a
+tail-fin 2-3 mm. broad. Such larvae are generally coffee-brown, or grey
+(instead of black), and show other minor differences.
+
+The summary states that when grown to maturity they become in their turn
+larva-bearing, and go into the water to bring forth. Their young are
+more than two (3 to 5 being the numbers observed) with a length of 33-40
+mm. or of 21-23 mm. at birth. They are light grey, spotted (mottled with
+lighter and darker colour), have relatively short gills (8 to 9 mm.
+at most) and a broad tail-fin (3 mm. wide). At metamorphosis they are
+relatively long (44 mm.) and one of them had some yellow pigment.
+
+Here again this summary is, as a matter of fact, describing the
+behaviour of two mothers, of which one produced three, and the other
+five young.
+
+To my mind these experiments suggest that the reproductive habits
+of both species, if closely observed, will be found to be subject
+to considerable variation, and I think it not impossible that each
+species is, especially in confinement, capable of being a good deal
+deflected from its normal behaviour. Moreover, there seems to me no
+great improbability in the idea that there is an interdependence
+between the number of young and the stage of maturity in which they
+are born. But, at the same time, the case as told by Kammerer strikes
+me as proving too much. If each species is so sensitive to conditions
+that the normal procedure is gravely modified in one generation, and
+if that modification can reappear in a pronounced form in the next
+generation without a renewal of the disturbing conditions, it becomes
+extremely difficult to understand how the regularity which each species
+is believed to display in nature can be maintained. Surely both species
+might be expected to be in confusion. From a passage in Kammerer's
+earlier paper (1904, p. 55) on the subject, I infer that he also would
+expect considerable irregularity in the natural behaviour, but that he
+has not investigated the point.[20]
+
+3. _Modification of the Colour of Salamandra maculosa induced by Change
+in the Colour of the Soil on which the Animals were kept._--Kammerer
+speaks of this as the most convincing of all his experiments on the
+transmission of acquired characters. So far, however, no full account of
+them has been published.[21] The statement is that when salamanders are
+kept in yellow surroundings the yellow markings gradually in the course
+of years increase in amount relatively to the black ground colour.
+Conversely by keeping the animals on black garden soil, the yellow may
+be greatly diminished in quantity until it largely disappears. (The
+account in _Natur_ adds that very moist conditions also favour the
+increase of yellow, and that with less moist conditions the yellow
+diminishes.) From each kind, the (induced) yellower and the (induced)
+blacker, a second generation was raised, on soil of neutral colour, and
+each family was later divided into two parts, half being put on black
+and half on yellow ground.
+
+As regards the offspring of those which had lived on _black_ soil no
+positive result had been reached up to the date of publication, but it
+is stated that these young resembled their parents in having the yellow
+distributed in _irregular spots_.
+
+As regards the offspring of those which had lived on yellow soil the
+account follows up the story of that part of the offspring which were
+put on yellow soil again. It is stated that these, though derived from
+parents with irregular spots, _developed the yellow as longitudinal
+bands_.
+
+This account is given with slight differences of expression in the
+three places to which I have referred. On returning from Vienna in
+1910 I consulted Mr. G. A. Boulenger in reference to the subject, and
+he very kindly showed me the fine series from many localities in the
+British Museum, and pointed out that in nature the colour-varieties
+can be grouped into two distinct types, one in which the yellow of
+the body is irregularly distributed in spots and one in which this
+yellow is arranged for the most part in two longitudinal bands which
+may be continuous or interrupted. _The spotted form is, as he showed
+me, an eastern variety, and the striped form belongs to western
+Europe._ Mr. E. G. Boulenger[22] has since published a careful account
+of the distribution of the two forms. The spotted he regards as the
+typical form, var. _typica_, and for the striped he uses the name
+var. _taeniata_. The typical form occupies eastern Europe in general,
+including Austria and Italy, extending as far west as parts of eastern
+France. The var. _taeniata_ is found all over France, excepting parts of
+the eastern border, Belgium and western Germany, Spain and Portugal. Of
+the very large series examined there was only one specimen (Lausanne)
+which could not with confidence be referred to one or other of the
+two varieties. Mr. E. G. Boulenger points out that both varieties
+inhabit very large areas, and live on soils of most different colours
+and compositions. Both are liable to variations in the amount and the
+shade of the yellow, but that any suggestion that _taeniata_ belongs
+especially to yellow soils and _typica_ to black soils is altogether
+inadmissible. He expresses surprise that Kammerer should not allude to
+these peculiarities in the geographical distribution of the two forms.
+He suggests further that it is more likely that some mistake occurred in
+Kammerer's observations than that the east European _typica_ should, in
+the course of a generation, have been transformed into the west European
+_taeniata_ by the influence of yellow clay soil.
+
+In his last paper on the subject Kammerer states incidentally[23] that
+he has found the _striped form recessive to the spotted_. No evidence
+for this statement is given, and I have not found any other reference
+to crosses effected between the two natural types. If, however, this
+representation is correct, it is conceivable that the production of
+_taeniata_ from _typica_ was in fact the re-appearance of a recessive
+form. The plate which Kammerer gives in illustration of his modified
+parent figures a single animal at four stages, and though it is
+certainly more like the spotted than the striped form, it has a certain
+suggestion of the striped arrangement, such as I can well imagine being
+produced in the heterozygote.[24]
+
+In continuation[25] of the experiments on the colour of _S. maculosa_
+Kammerer publishes an account of elaborate experiments in grafting
+ovaries of the various forms, modified and unmodified, into each other,
+and describes the offspring which followed. Before pursuing this part of
+the inquiry I am disposed to wait until the earlier steps have been made
+much more secure than they yet are.
+
+More recently Kammerer has published similar statements in regard to the
+inheritance of characters induced in various lizards by keeping them in
+abnormal temperatures, high and low. The changes induced affected in
+some species the colours, in others the reproductive habits. Respecting
+these examples I feel the same scepticism that I have indicated in
+regard to the others, somewhat heightened by the fact that insufficient
+evidence is given both regarding the behaviour of these various species
+in captivity when not subjected to abnormal temperatures, and in the
+wild state.
+
+Respecting this part of the evidence Mr. G. A. Boulenger has lately
+published a criticism[26] from which I extract the following passages.
+Referring to a previous note[27] on the question of the melanism of the
+various insular forms of _Lacerta muralis_ he writes: "I also alluded
+(_l. c._) to the theories that have been propounded to explain the
+melanism of various insular forms. This is a subject which has been
+lately taken up by Dr. Kammerer at the Biologische Versuchsanstalt in
+Vienna, and he claims to have produced nigrinos artificially by a very
+strong elevation of the temperature, accompanied by extreme dryness. Dr.
+Werner[28] has already opposed his own experiments to those of Kammerer,
+artificial melanism having been produced by him in _Lacerta oxycephala_
+by keeping two very light specimens from Ragusa for a whole summer
+in very damp conditions. Neither is Kammerer's theory in accordance
+with the distribution of the black lizards, as pointed out by Werner.
+Kammerer also finds that those forms which are known to produce melanic
+races in a state of nature, lend themselves more readily than the others
+to the success of his experiments. But he shows himself misinformed
+when he states that the variety called _Lacerta fiumana_ belongs to the
+category of those of which black forms are not known. He overlooks the
+fact, first pointed out by Scherer in 1904, and which I can confirm,
+that the black lizard from Melisello near Lissa in the Adriatic is
+unquestionably derived from the lizard from Lissa, which he correctly
+regards as not separable from _L. fiumana_...."
+
+"Another colour modification which Dr. Kammerer states that he obtained
+by raising the temperature is the assumption by the female of the
+typical _Lacerta muralis_ of the bright red colour of the lower parts
+which often distinguishes the male from the female, and which was not
+shown by the individuals of the latter sex kept by him under normal
+conditions. He quotes various authorities to show that the lower parts
+are never red in the females, but he has omitted to consult others who
+say the contrary. Thus Bedriaga (1878 and 1879) remarks that a so-called
+var. _rubriventris_ of the typical wall lizard has the lower parts red
+in both sexes."[29]
+
+In reading such papers as those of Semon or Kammerer the thought
+uppermost in my mind is that to multiply illustrations of supposed
+transmission of acquired characters is of little use until some one
+example has been thoroughly investigated. If we had certain assurance
+that even a single unimpeachable case could be repeated at will, the
+whole matter would assume a more serious aspect. If, for instance,
+Kammerer were able to show us _Alytes_ males with horny pads on their
+hands, it would be something tangible; still more, if the experiment
+were repeated by others until no doubt remained that the offspring
+of _Alytes_ which had bred in water for some three generations did
+acquire these pads and that they could transmit these novelties to
+descendants raised in normal conditions. Till evidence of this kind is
+published by at least two independent observers investigating similar
+material, I find it easier to believe that mistakes of observation or
+of interpretation have been made than that any genuine transmission of
+acquired characters has been witnessed.
+
+Meanwhile there is no denying that the origin of adaptational features
+is a very grave difficulty. With the lapse of time since evolutionary
+conceptions have become a universal subject of study that difficulty
+has, so far as I see, been in nowise diminished. But I find nothing in
+the evidence recently put forward which justifies departure from the
+agnostic position which most of us have felt obliged to assume.[30]
+
+
+                 APPENDIX TO CHAPTER IX.
+
+Professor G. Klebs, as is well known to students of evolutionary
+phenomena, has for several years been engaged in investigations relating
+to the inheritance of acquired characters. In his many publications
+on the subject the issue has always been represented as more or less
+uncertain.
+
+Desiring to know how the matter now stands according to Professor Klebs'
+present judgment I wrote to him asking him to favour me with a brief
+general statement. This he most kindly sent in a letter dated 8th July,
+1912.
+
+As such a statement will be read with the greatest interest by all who
+are watching the progress of these studies I obtained permission to
+publish it as follows:
+
+                                                    8. Juli 1912
+
+   Ihre liebenswurdige Anfrage will ich sehr gern
+   beantworten, obwohl ich sie nicht so beantworten kann wie ich
+   erwünschte. Ihr Skepticismus in der Frage der Uebertragung
+   erworbener Charactere auf die Nachkommen ist nur zu berechtigt.
+   Meine Versuche mit Veronica sind _nicht_ beweisend, da es mir
+   bisher nicht gelungen ist eine einigermasse konstante Varietät
+   mit verlaubten Inflorescenze zu erzeugen. In Bezug auf mein
+   Semper vivum bin ich allerdings noch heute der Meinung dass
+   die starke künstliche Veränderung der Blüte einen Einfluss auf
+   einzelnen Nachkommen gehabt hat. Ich habe seither nichts darüber
+   veröffentlicht: die Mehrzahl der anormalen gefüllten Blüten war
+   leider steril. Von einem weniger veränderten Exemplar erhielt
+   ich einige Sämlinge, aber sie haben noch nicht geblüht. Es kann
+   sich in diesem Falle nur um eine _Nachwirkung in der ersten
+   Generation_ handeln, vergleichbar jenen Fällen in denen Samen von
+   Bäumen aus den hohen Alpen in der Ebene gewisse Nachwirkungen
+   zeigen. Aber es ist bisher kein sicherer. Fall bekannt in den
+   der kunstliche herbeigeführte Charakter _mehrere Generationen
+   hindurch unter der gewöhnlichen "normalen" Bedingungen_
+   übertragen worden ist.
+
+   Auf der andere Seite sind diese negativen Resultaten nicht
+   entscheidend. Denn wie wenig ist in dieser Beziehung überhaupt
+   ernstlich versucht worden! Und zweifellos geht die Sache nicht so
+   einfach.
+
+   Ich versuche es mit anderen Pflanzen weil ich der Meinung bin
+   dass es möglich sein müsse wenigstens solche neuen Varietäten zu
+   erzeugen, wie sie die Gartenvarietäten entsprechen.
+
+   Aber bis jetzt leider sind die Versuche nicht gelungen, weder
+   mir noch irgend einem anderen.
+
+FOOTNOTES:
+
+[1] Semon, R., Der Stand der Frage nach der Vererbung erworbener
+Eigenschaften, published in _Fortschr. der naturw. Forschung._, Bd. 11,
+1910.
+
+[2] Standfuss, M., _Denks. Schweiz. naturf. Ges._, XXXVI, 1898, p. 32.
+
+[3] Fischer, E., _Allg. Ztschr. f. Entomologie_, Bd. VI, 1901.
+
+[4] Out of 12 pupae treated 8 died and of the 4 survivors, one only was
+affected. See M. v. Linden, _Archiv. Rassen. u. Gesells._, 1904, I.
+
+[5] For illustrations see _Oberthur's Études d'Entom._, 1896, where many
+of these curious aberrations are represented; also Barrett, _Lepid.
+Brit. Islands_, II, pp. 71 and 72.
+
+[6] Schübeler, F. C., _Die Culturpflanzen Norwegens_, 1862, especially
+pp. 24 and 28.
+
+[7] I am obliged to him and to Dr. E. Gold for much trouble taken to
+answer my questions. Some idea of the kind of weather indicated by an
+average of 2.76° C. above the mean may be got from a comparison with the
+year 1911, which most people will remember as one of the hottest summers
+they have known. The July of that year was in east and southeast England
+about 4° F. above the mean but 2.67 C. means about 4.8° F. above the
+mean. At Greenwich July, 1859, was about 6.5° F. above the average.
+
+[8] Wille, N., _Biol. Cbltt._, XXV, 1905, p. 521.
+
+[9] Wettstein, R. von. _Der Neo-marckismus u. seine Beziehungen zum
+Darwinismus_, Jena, 1903.
+
+[10] T. Graham Brown, _Proc. Roy. Soc._, 1912, vol. 84, B, p. 555. This
+paper gives full reference to the previous literature of the subject.
+
+[11] Morgan, T. H., _Evolution and Adaptation_, New York, 1903.
+
+[12] Kammerer's chief paper on this subject is in _Arch. f. Entwm._,
+1909, XXVIII, p. 447, and it is to this that the paginal references in
+the present text relate. His previous paper appeared, _ibid._, 1906,
+XXII, p. 48. An account of his further experiments with _Alytes_ is
+given in _Natur_, 1909-10, Heft 6, p. 95.
+
+[13] In reply to my letter Dr. Kammerer who was then away from home
+very kindly replied that he was not quite sure whether he had killed
+specimens of _Alytes_ with "_Brunftschwielen_" or whether he only
+had living males of the fourth generation, but that he would send
+illustrative material.
+
+[14] Kammerer, P., _ Natur_, 12 December, 1909, Heft 6, p. 95, repeated
+in _12 Flugschrift d. Deutsch Ges. f. Züchtungskunde_, Berlin, 1910.
+
+[15] _Festschrift zum Andenken an Gregor Mendel_, being vol. XLIX of the
+_Verh. Naturf. Ver. in Brünn_, 1911, p. 98.
+
+[16] Kammerer's chief papers on this subject are _Archiv fur Entwm._,
+XVII, 1904, and _ibid._, XXV, 1907. An epitome of results is also given
+by him in _12 Flugschrift d. Deutsch. Ges. f. Züchtungskunde_, Berlin,
+1910.
+
+[17] "_Bei Fortdauer der Versuchsbedingungen sind als Vollmolche
+geborene Salamandra maculosa_ gleich bei der ersten Geburt _abermals
+voll molchgebärend_, benutzen zum Geburtsakt das trockene Land,
+und zwar unter Erreichung der (bei _Salamandra atra_ normalen)
+_Embryonen-Zweizahl_," Kammerer, 1907, p. 49.
+
+[18] 1904, p. 56.
+
+[19] Throughout Kammerer's papers this is used almost as a technical
+term. It means, I presume, that the feature was manifested more than
+once.
+
+[20] It should be stated that the papers contain a quantity of detail,
+especially descriptive of the state of the larvae, which I have not
+attempted to represent, but the account here given contains all that
+seemed essential to an understanding of the more important features of
+the account.
+
+[21] The first appeared in _Natur_, 1909-10, Heft 6, p. 94; and the
+second, which contains coloured plates of the animals, in the lecture
+already referred to, _12 Flugschr. d. Deut. Ges. f. Züchtungkunde_,
+Berlin, 1910, p. 26. In the paper in _Mendel Festschrift_, 1911, the
+subject is continued, but no more is added as to this part of the
+experiment.
+
+[22] E. G. Boulenger, _Proc. Zool. Soc._, 1911, p. 323.
+
+[23] _Mendel Festschrift_, 1911, p. 84.
+
+[24] _12 Flugschrift. Deut. Ges. Züchtungskunde_, 1910, Fig. 15, _P.
+Reihe_.
+
+[25] _Mendel Festschrift_, 1911, p. 83.
+
+[26] Field, 1912, 30 March.
+
+[27] _Ibid._, 1904, p. 863.
+
+[28] _Mitth. Naturw. Ver. a. d. Univ. Wien_, 1908, p. 53.
+
+[29] As to the variations of _Lacerta muralis_ in Western Europe and
+North Africa see Boulenger, G. A., _Trans. Zool. Soc._, 1905, vol. XVII,
+p. 351.
+
+[30] As to the experiments of Klebs relating to the transmission of
+acquired characters, see Appendix.
+
+
+
+
+                           CHAPTER X
+
+            EFFECTS OF CHANGED CONDITIONS CONTINUED
+
+
+                THE CAUSES OF GENETIC VARIATION
+
+In the last chapter we examined some of the evidence offered in support
+of the belief that adaptation in highly organised forms is a consequence
+of the inheritance of adaptative changes induced by the influence of
+external conditions. The state of knowledge of this whole subject is, as
+I have said, most unsatisfactory, chiefly for the reason that in none of
+the cases which are alleged to show a positive result have two observers
+been over the same ground, or as yet confirmed each other. In the wider
+consideration respecting the causes of variation at large we find
+ourselves still in the same difficulty. The study has thus far proved
+sadly unfruitful. In spite of the considerable efforts lately made by
+many observers to induce genetic variation in highly organised plants
+or animals, and though successes have occasionally been announced, I
+do not know a single case which has been established and confirmed in
+such a way that we could with confidence expect to witness the alleged
+phenomena if we were to repeat the experiment. Abundant illustrations
+are available in which individuals exposed to novel conditions manifest
+considerable changes in characters or properties, but as yet there is
+no certain means of determining that germ-cells of a new type shall be
+formed.
+
+Of the direct effect of conditions the lower organisms, especially
+bacteria, offer the best examples, the alterations of virulence which
+can be produced in so many distinct ways being the most striking
+and familiar. That attenuation of virulence can be produced by high
+temperatures or by exposure to chemical agents, and that this diminution
+in virulence may remain permanent is, from our point of view, not
+surprising; but the fact that in many cases the full virulence can by
+suitable cultivation be restored is difficult to understand. Similar
+variations have been observed in power of pigment production and other
+properties.
+
+These phenomena naturally raise the question whether any cases of
+apparent loss of factors in higher forms may be comparable.
+
+The subject of variations in the lower organisms and their dependence on
+conditions is a highly special one, and I have no knowledge which can
+justify me in offering any discussion of them, but I understand that
+hitherto little beyond empirical recognition of the phenomena has been
+attempted. A useful summary of observations made by many investigators
+was lately published by Hans Pringsheim,[1] who enumerates the different
+agencies which have been observed to produce modifications, and the
+various ways in which these changes are manifested. One of the most
+comprehensive studies of the subject from the genetic point of view is
+that made by F. Wolf.[2] In his extensive cultivations of _Bacillus
+prodigiosus_, _Staphylococcus pyogenes_ and _Myxococcus_ he succeeded
+in producing many strains with modified properties. In most of these
+the modifications arose in consequence of the application of high or
+low temperatures or of the addition of various chemical substances
+to the culture-media. Some of the variations, which are for the most
+part in the powers of pigment-formation, persisted when the strains
+were returned to normal conditions, and others did not. In reference
+especially to the variations witnessed in the Cocci the reader should
+consult the critical account of variation in that group published by
+the Winslows,[3] where much information on the subject is to be found.
+The authors attempted to determine the systematic relationships of the
+several forms, as far as possible, by the application of statistical
+methods. The result is interesting as showing that the problem of
+species in its main features is presented by these organisms in a form
+identical with that which we know so well in the higher animals and
+plants, whatever properties be selected as the diagnostic characters.
+There are many types perfectly distinct and others which intergrade.
+Some of the types change greatly with conditions while others do not.
+This is exactly what we encounter whenever we study the problem of
+species on an extended scale among the higher forms of life.
+
+There is now practically complete agreement among bacteriologists
+that the observations made first by Massini on the change in color of
+_Bacterium coli mutabile_ grown in Endo's medium, associated with the
+acquisition of the power to ferment lactose, are perfectly reliable
+and free from possibilities of mistake. The work has been extended and
+confirmed by many workers, especially R. Müller, who finds that this
+bacterium can similarly acquire and maintain the power to ferment other
+sugars. A careful account of the whole subject written by Müller for the
+information of biologists will be found in _Zts. für Abstammungsl._,
+VIII, 1912. After discussing the biological significance of the facts,
+he concludes with a caution to the effect that bacteria are so different
+from all other living things that generalizations from their behavior
+must not be indiscriminately applied to animals and plants.
+
+In all work with this class of material there is obviously danger of
+error through foreign infection of the cultures, but there can be no
+doubt that though some of the "mutations" recorded may be due to this
+cause, the majority of the instances observed under stringent conditions
+are genuine.
+
+Another and equally serious difficulty besetting work with bacteria
+and fungi cultivated from spores is that the appearance of variation
+may in reality be due to the selection of a special strain previously
+living masked among other strains. This possibility must be remembered
+especially in those instances which are claimed as exemplifying the
+effects of acclimatisation. Manifestly this consideration can be urged
+with most force when the strain which gave rise to the novelty was
+not raised from a single individual spore. Moreover, when once the
+possibility of spontaneous variation is admitted, it must be difficult
+to be quite confident that any given variation observed is in reality
+due to the novel conditions applied, and as I understand the evidence,
+the appearance of the mutational forms does not with any regularity
+follow upon the application of the changed conditions.
+
+Researches into the variation of these lower forms will, no doubt, be
+continued on a comprehensive scale. So long as the instances recorded
+are each isolated examples it is impossible to know what value they
+possess. If they could be coordinated in such a way as to provide some
+general conception of the types of variation in properties to which
+bacteria, or any considerable group of them, are habitually liable, the
+knowledge might greatly advance the elucidation of genetic problems.
+
+Of mutational changes directly produced with regularity in
+micro-organisms by treatment, the experiments with trypanosomes provide
+some of the clearest examples. A summary of the evidence was lately
+published by Dobell,[4] from which the present account is taken. The
+most definite fact of this kind established is that certain dyes
+introduced into the blood of the host have the effect of destroying
+the small organ known as the "kinetonucleus" in the trypanosomes. The
+trypanosomes thus altered continue to breed, and give rise to races
+destitute of kinetonuclei. This observation was originally made by
+Werbitzki and has been confirmed by several observers. The exact way
+in which this alteration is effected in the trypanosomes is not quite
+definitely made out, but there is good reason for supposing that
+the dyes have a direct and specific action upon the kinetonucleus
+itself, and circumstances make it improbable that in some division a
+daughter-organism without that body is produced, or that any selection
+of a pre-existing defective variety occurs.
+
+Ehrlich has suggested with great probability that the dyes which possess
+this action owe it to the fact that they have the particular chemical
+linkage which he calls "ortho-quinoid." In outward respects, such as
+motility and general appearance, the modified organisms are unchanged,
+but their virulence is diminished. As regards the possibility of the
+defective strain reacquiring the kinetonucleus, Werbitzki states
+that in one case passage through 50 animals and treatment with dyes
+left the strain unaltered; but that in another case at the sixteenth
+passage 7 per cent. of the trypanosomes were found to have re-acquired
+the organ, and in subsequent passages the percentage increased, until
+at the twenty-seventh passage practically all had re-acquired it.
+Kudicke, however, in similar experiments did not succeed in causing
+re-acquisition by transplantation.
+
+By the action of various drugs and anti-bodies races of trypanosomes
+resistant to those substances have been obtained. These breed true, at
+least when kept in the same species of animal in which the resistance
+was acquired. As to whether change of virulence is produced by passage
+through certain animals or not, there is as yet no general agreement.
+
+Other changes, especially in size and some points of structure, are said
+to occur when certain trypanosomes proper to mammals are passed through
+cold-blooded vertebrates (Wendelstadt and Fellmer), and it is stated
+that these changes persist, but the observations have not yet been
+confirmed.
+
+Experiments lately conducted by Woltereck with _Daphnia_ are interesting
+as having given a definite positive result, in so far, at least, as the
+ova were affected by conditions before leaving the bodies of the parent
+individuals. The observations relate to the offspring resulting from
+_parthenogenetic_ eggs. Females bearing ephippia (fertilised eggs) were
+isolated until the ephippia were dropped, and in this way the offspring
+of fertilisation were excluded. Males, of course, appeared from time
+to time in the cultures, but as fertilised eggs were rejected, their
+presence did not disturb the result. The most remarkable observations
+related to _Daphnia longispina_.
+
+This species as found in the lower lake at Lunz had the front end of
+the body blunt and nearly round in profile; but on being cultivated in
+a warm temperature and with abundant nourishment the front end of the
+body became produced into an elongated "helmet," as Woltereck calls it.
+Experiment showed that the change was primarily due to the abundance of
+food, and owing to temperature in a subordinate degree.
+
+This distinction arose as soon as the species was taken into the
+hothouse, but when the modified individuals were put back into the
+original conditions, a lower temperature and scanty food-supply,
+the next generation returned to their original form. After being
+cultivated for two years and about 40 generations in the more favourable
+conditions, when similarly put back into the lower temperature with
+scanty food the _first generation_ born in these conditions was helmeted
+like the modified parents. Woltereck is of opinion that the ova were
+still unformed at the time the parents were put back, and the influence
+of the favourable conditions upon the unformed ova he speaks of as a
+"prae-induction." The effect never extended beyond the one generation,
+after which the strain returned to its original state.
+
+The fact that the influence on the offspring was not manifested at
+first led Woltereck to expect that by more prolonged cultivation in the
+favourable conditions a further extension of this influence would be
+produced, but this expectation was never fulfilled, though the attempt
+was made again and again.
+
+Similar experiments were made with _Hyalodaphnia cucullata_, which is
+far more sensitive to cultural influences, and in nature manifests
+a considerable elongation of the helmet as a seasonal modification,
+but the results were essentially the same as in the preceding case,
+no modification extending beyond the first generation born after the
+restoration to _normal conditions_.[5]
+
+The only criticism of these extremely interesting results which suggests
+itself is that perhaps the original appearance of the modification was
+not in reality due to an _accumulated_ effect of the conditions, but
+to some change in the conditions themselves which was not noticed. It
+is difficult to see how length of time or even the lapse of several
+generations could have so specific an effect on the race. It is no
+doubt often vaguely supposed by many that a long period of time may be
+necessary for the effect of climate or of other environmental conditions
+to be produced in an organism which does not thus respond at first. I
+have never been able to see any reason for this opinion nor how it is
+to be translated into terms of physiological fact, and I imagine that
+in those cases in which the lapse of time is really required for the
+production of an effect, the influence of the prolongation is rather
+on the conditions than on the organisms. The response of the organisms
+thus probably indicates not that the creature is at length feeling the
+effects because of their accumulated action on itself, but that the
+conditions have at length ripened.
+
+As this sheet is passing through the press Agar has published[6] an
+abstract of evidence as to another comparable case in a parthenogenetic
+strain in the daphnid, _Simocephalus vetulus_. When fed on certain
+abnormal foods the shape of the body is changed, the edges of the
+carapace being rolled backwards so as to expose the appendages. The
+offspring of animals thus modified showed similar modification in the
+first, and to a very slight degree, in the second generation, though the
+original mothers were removed to normal conditions before their eggs
+were laid. In the third generation there was "a very pronounced reaction
+in the opposite direction." Agar suggests that the change may be due to
+some toxin-like substances, carried on passively by the egg into the
+next generation, against which the protoplasm eventually produces an
+anti-body.
+
+The experiments which have been in recent years regarded by evolutionary
+writers as the most conclusive proof that direct environmental action
+may produce germinal variation are those of Professor W. L. Tower, of
+Chicago, on _Leptinotarsa_, the potato beetles. This work has attained
+considerable celebrity and has been generally accepted as making a
+definite extension of knowledge. After frequently reading Tower's papers
+and after having been privileged to see some of the experiments in
+progress (in 1907) I am still in doubt as to the weight which should be
+assigned to this contribution.
+
+The work is described in two chief publications, the first of which
+appeared in 1906.[7] This treatise contains a vast amount of information
+about numerous species and varieties of these beetles which the author
+has observed and bred in many parts of their distribution throughout
+the United States, Mexico and Central America. The part of the book
+which has naturally excited the greatest interest is that in which Tower
+states that by subjecting the beetles to change in temperature and
+moisture, he caused them to produce offspring quite unlike themselves,
+which in several cases bred true.
+
+It is much to be regretted that the author did not happen to become
+acquainted with Mendelian analysis at an earlier stage in the
+investigation. The evidence might then have been handled in a much more
+orderly and comprehensive way, and a watch would have been kept for
+several possibilities of error.
+
+The headquarters of the genus is evidently as Tower states, in Mexico
+and the adjoining countries. In this region there is a great profusion
+of forms, some very local, some as for instance the well-known
+_decemlineata_,[8] more widely spread. The distinctions are almost all
+found in peculiarities of colour and pattern, and the limits of species
+are even more indefinable than is usual in multiform animals. Tower
+arranges the various types into seven groups of which the one most
+studied is that which he calls the _lineata_ group. To this group belong
+all the forms to which reference is here made, and, as I understand,
+they differ among themselves entirely in size, colour and pattern.
+There is no suggestion of infertility in the crosses made between the
+several forms of the _lineata_ group; in fact they present, like many
+Chrysomelidae, a good example of what most of us would now call a
+polymorphic species, consisting of many types, some found existing in
+the same locality, others being geographically isolated.
+
+A series of experiments was devoted to the attempt to fix strains
+corresponding to the extremes of continuous variations. For example,
+those with most black pigment and those with least black taken from
+a population continuously varying in this respect, were separately
+bred; but almost always the selection led to no sensible change in the
+position of the mean of the population. The variations in these cases
+were evidently fluctuational. In some instances, however, real genetic
+differences were met with, and strains exhibiting them were, as usual,
+rapidly fixed.
+
+Tower points out that several of the varieties (or species, as he
+prefers to call them) were obviously recessive to _decemlineata_. This
+is most clearly demonstrated in the case of the form called _pallida_,
+which is a pale depauperated-looking creature, with the orange of
+the thorax almost white and the eyes devoid of pigment.[9] This form
+behaved as an ordinary Mendelian recessive, breeding true whenever it
+appeared in the cultures, or when individuals found wild were studied
+in captivity. A black form which Tower names _melanicum_ was similarly
+shown to be a Mendelian recessive. Wild specimens of this variety of
+opposite sexes were not found simultaneously in nature, and there was
+thus no opportunity of breeding them together, but the hereditary
+behaviour was seen in the F_{2} generation from a _melanicum_ found
+coupled with _decemlineata_. Experiments also occurred giving indication
+that a variety with the stripes anastomosing in pairs (_tortuosa_), was
+another recessive, and that a variety--called "_rubri-vittata_"--gave an
+intermediate F_{1} with subsequent segregation. All these are forms of
+_decemlineata_ Stål.
+
+Similar observations were made regarding forms recessive to
+_multitaeniata_ Stål. Of these two were thrown by _multitaeniata_
+itself, namely a form named by Stål _melanothorax_, and regarded by him
+as a species, and one which Tower names _rubicunda_ n. sp. The facts
+proving the recessive behaviour of their several forms will be found in
+the following places in Tower's book:
+
+  _pallida_,                      pp. 273-278.
+  _melanicum_,                     p. 279.
+  _tortuosa_,                      p. 280.
+  _rubrivittata_,                 pp. 280-281.
+  _melanothorax_ and _rubicunda_, pp. 283-285.
+
+Following this evidence of recessive nature of the six forms
+enumerated, Tower describes experiments showing, as he believes, that
+some of them may be caused to appear by applying special treatment
+to the parents during the "growth and fertilisation" (p. 287) of the
+eggs. The most striking example is that in which 4 males and 4 females
+of _decemlineata_ were kept very hot (average 35° C.) and dry, and at
+low atmospheric pressure (19-21 inches). The eggs laid were restored
+to natural conditions. These gave 506 larvae, from which emerged 14
+normal, 82 _pallida_ and 2 "_immaculothorax_," viz., without pigment
+on the pronotum. The account of the rest of the experiment is somewhat
+involved, but I understand that the _pallida_, of which two only
+survived, behaved as normal recessives when bred to the type: also that
+the parents, after having laid the eggs whose history has been given,
+were restored to normal conditions and laid 319 eggs which gave 61
+normals.
+
+In another case normal parents laid 409 eggs in the hot and dry
+conditions, and on restoration to normal conditions, the same parents
+laid 840 eggs. Then 409 eggs gave 64 adults as follows:
+
+                     _Males_          _Females_
+
+  _decemlineata_        12                 8
+  _pallida_             10                13
+  _immaculothorax_       2                 3
+  _albida_               9                 7
+                        ---               ---
+                        33                31
+
+The 840 eggs laid in normal conditions gave 123 normal _decemlineata_.
+
+Similar experiments were made with _multitaeniata_ and gave comparable
+results, the two recessives (_melanothorax_, _rubicunda_) being produced
+in large numbers when the parents were subjected to heat, but in this
+case the atmosphere was kept _saturated_ with moisture, instead of dry,
+as in the previous instance. The same parents transferred to normal
+conditions gave normals only.
+
+Lastly the form _undecimlineata_ was exposed "to an extreme stimulus of
+high temperature, 10° C. above the average," and a dry atmosphere, with
+the result that from 190 eggs there emerged 11 beetles, all of the form
+_angustovittata_ Jacoby, which subsequently bred true to that type (see
+p. 295).
+
+In the results of these experiments, as described, there is one feature
+which I regard as quite unaccountable. Tower makes no comment upon
+it. Indeed, from the general tenour of the paper, I infer, not only
+that he does not perceive that he is recounting anything contrary to
+usual experience, but rather that he regards the result as conforming
+to expectations previously formed. The point in question is the
+genetic behaviour of the dominant normals produced under the abnormal
+conditions. These normals were the result of the breeding of parents
+declared to be at the same time giving off many recessive gametes. Some
+of these normals must be expected therefore to be heterozygous unless
+some selective fertilisation occurs. Nevertheless in every case they and
+their offspring are reported to have continually bred true. I allude
+especially to the tables given on pp. 288, 289, 292, and 293. Tower does
+not mention any misgiving about this result, and I think he regards
+himself as recounting phenomena in general harmony with the ideas of
+mutation expressed by De Vries. This they may be; but to anyone familiar
+with analytical breeding the course of these experiments must seem so
+surprising as to call for most careful, independent confirmation.
+
+In 1910[10] Tower published an account of further experiments with
+_Leptinotarsa_. The work described related to two subjects. Crosses
+were made between three forms, _undecimlineata_ Stål, _signaticollis_
+Stål and "_diversa_" named by Tower as a new species. The distinctions
+between these three depend partly on characters of the adults and
+partly on those of the larvae. The adults of _undecimlineata_ and
+_diversa_ have the elytra striped, but the elytra of _signaticollis_
+are unstriped. The larvae of _signaticollis_ and of _diversa_ are
+yellow, but those of _undecimlineata_ are white.[11] Moreover, in
+_signaticollis_ and _diversa_ the black increases in the third
+stage of the larvae to form transverse bands which are absent in
+_undecimlineata_. The general course of the experiments shows that these
+differences may be approximately represented as due to the action of
+three factors, any of which may be independently present or absent. The
+stripings of the elytra and of the larvae are each due to a separate
+factor. As regards the distinction between the yellow and the white
+larvae the evidence does not prove that there is decided dominance of
+either colour and I infer that the heterozygotes are often intermediate.
+
+The chief contribution which this new paper claims to make relates to
+differences in the results which ensue from crosses effected between
+these three types at different average temperatures.
+
+We are first concerned with four experiments which I number (1), (2),
+(3), (4):
+
+1. _Signaticollis_ [F] × _diversa_ [M] bred at an average temperature
+of 80º F. by day and 75° F. by night, gave two groups in about equal
+numbers. The first (49) was pure _signaticollis_ and bred true. The
+second (53) was of an intermediate type, which on being bred together
+gave the typical Mendelian result--1 _sig._: 2 _intermediate_: 1 _div_.
+
+2. Next, as the account originally stood in the published paper, we
+are told that _sig_ [F] × _div_ [M] bred together at a day-temp.
+average 75° F. and night average 50° F. gave an _intermediate_ only,
+which subsequently produced a normal 1:2:1 ratio. The two crosses were
+repeated eleven times with identical results.
+
+In a further experiment (3) _signaticollis_ [F] × _diversa_ [M] were
+bred under the same conditions as those used in expt. (1). They again
+gave _sig._ and intermediates as before in fairly equal numbers. The
+_sig._ as before bred true, and the intermediate gave 1:2:1, all exactly
+as in expt. (1).
+
+In expt. (4) _the same parents used_ in (3) were again mated under
+conditions of expt. (2) at the lower temperature, and this time gave
+_signaticollis_ exclusively, which bred true for four generations. This
+experiment was repeated seven times with uniform results.
+
+Diagrams are given representing all these histories in graphic fashion.
+
+From these observations, Tower concludes that the determination of
+dominance, and the ensuing type of behaviour, is clearly a function of
+the conditions incident upon the combining germ plasms.
+
+It will be observed that expts. (1) and (3) gave identical results
+but (2) and (4), though much the same conditions were applied, are
+at variance, for (2) gave all intermediates, while (4) gave all
+_signaticollis_. In _Amer. Nat._, XLIV, 1910, p. 747, Professor T.
+D. A. Cockerell commented on this paper of Tower's and pointed out
+that there must be an error somewhere, for when he discusses these
+experiments Tower speaks of (2) and (4) as confirming each other. To
+this Tower replied[12] that there had been a mistake. He states that
+in preparing the paper "certain minor experiments were taken from
+a larger series and combined to illustrate a general point in the
+behaviour of alternative characters in inheritance," and that expt. (2)
+was introduced inadvertently in place of another which he desires to
+substitute. In this, which I number (5), _signaticollis_ [F] × _diversa_
+[M] from exactly the same stocks as those used in (1), were mated at the
+lower temperatures specified for (2), day average 75° F., night average
+50° F. These gave all of the _signaticollis_ type with a narrow range of
+variability, which bred true, in some cases to F_{6}. Tower says he has
+repeated this experiment six times with identical results.
+
+Nevertheless he proceeds to say that the description of expt. (2), which
+was repeated eleven times with identical results, was correct "as far as
+given." That experiment was "from a second series of cultures parallel
+to the one given, but in which there are other factors involved, which
+in H. 410 [my (2)] are productive of a typical Mendelian behaviour." He
+adds he does "not care at this time to make any statement of what these
+factors are, nor of their relations to the behaviours given in the H.
+409, H. 411, H. 409/11 series [my (1), (5) and (3)--(4)] which are the
+simplest and most easily presented series obtained in the crossing of
+_signaticollis_ and _diversa_."
+
+Professor Cockerell's intervention has thus elicited the fact that
+we have as yet only a small selected part of the evidence before us,
+even as concerning the effect of temperature on the cross between
+_signaticollis_ [F] × _diversa_ [M]. We learn that at the lower
+temperatures the result was eleven times the expected one, and six times
+an unexpected one; further, that we owe it to the author's inadvertence
+that we have come to hear of the expected result at all, and that though
+he knows the factors which determine the discrepancy, he declines for
+the present to name them. In these circumstances we can scarcely venture
+as yet to estimate the significance of these records.
+
+The paper goes on to recount somewhat comparable, but more complex
+instances in which the descent of the colour of adults and of larvae
+was affected by temperature in crosses between _undecimlineata_ and
+_signaticollis_. As they stand the results are very striking and
+unexpected, but I think, in view of what has been admitted respecting
+the former part of the paper, full discussion may be postponed till
+confirmation is forthcoming.
+
+One feature, however, calls for remark. This second paper is written
+apparently without any reference to the discoveries related by Tower in
+his previous book, to which no allusion is made. This is most noticeable
+in the case of an experiment in which (p. 296, H. 700A) _undecimlineata_
+[F] (the dominant) was mated to _signaticollis_ [M] with the result
+that all the offspring were _undecimlineata_ and bred true to that
+type (Parthenogenesis was tested for, but never found to occur). This
+experiment was made at a temperature averaging 95° F. ± 3.5° by day
+and 89° F. ± 4.8° by night, and in a humidity given as 84 per cent. by
+day and 100 per cent. by night; but in the previous book (p. 294) we
+are told that pure _undecimlineata_ bred together "under an extreme
+stimulus of high temperature, 10° C. above the average" and a relative
+humidity of 40 per cent. gave 11 beetles only, all _angustovittata_.
+But reference to the Plate 16, Fig. 2, shows that _angustovittata_
+must be exceedingly like _signaticollis_, having, like it, the elytral
+stripes obsolete, and if there is any marked difference at all, it can
+only be in the larvae. It seems strange that if _undecimlineata_ really
+gives off ova of this recessive type at high temperatures, the fact
+should not be alluded to in connection with expt. H. 700A, where, as
+the father was _signaticollis_, having the same recessive character,
+their appearance might have been expected not to pass unobserved. The
+temperature in the older experiment is, of course, not given with the
+great accuracy used in the second, and it may have been higher still.
+The humidity also was widely different. Still, in discussing the
+phenomena we should expect some reference to the very remarkable and
+closely cognate discovery which Tower himself had previously reported in
+regard to the same species.[13]
+
+The hesitation which I had come to feel respecting these two
+publications of Tower's has been, I confess, increased by the appearance
+of a destructive criticism by Gortner[14] who has examined the parts
+of Chapter III of Tower's book, in which he discusses at some length
+the chemistry of the pigments in _Leptinotarsa_ and other animals. As
+Gortner has shown, this discussion, though offered with every show of
+confidence, exhibits such elementary ignorance, both of the special
+subject and of chemistry in general, that it cannot be taken into
+serious consideration.
+
+Some observations made by Dr. W. T. Macdougal[15] have also been
+interpreted as showing the actual causation of genetic variation by
+chemical treatment. Of these perhaps the least open to objection
+were the experiments with _Raimannia odorata_, a Patagonian plant
+closely allied to _Oenothera_. The ovaries were injected with various
+substances and from some of the seeds which subsequently formed in them
+a remarkable new variety was raised. This varying or mutational form
+was strikingly different from the parental type, with which it was not
+connected by any intergradational forms, and it bred true. It made
+no rosette, growing to a much smaller size than the parent, and was
+totally glabrous instead of being very hairy as the parental type is.
+I was shown specimens of these plants by the kindness of Dr. Britton
+in the Bronx Park Botanic Garden in 1907 and can testify to their very
+remarkable peculiarities. They had a somewhat weakly look, and might
+at first sight be thought to be a pathological product, but they had
+bred true for several generations. From the evidence, however, I am by
+no means satisfied that their original appearance was a consequence
+of the treatment applied. This treatment was of a most miscellaneous
+description. Two of the mutants came from an ovary which had been
+treated with a ten per cent. sugar solution. Ten came from one into
+which a 0.1 per cent. solution of calcium nitrate had been injected. One
+was from a capsule which "had been exposed to the action of a radium
+pencil." Macdougal speaks of these results as decisive, but clearly
+before such evidence can be admitted even for consideration it must be
+shown by control experiments that the individual plants which threw
+the mutant were themselves breeding true in ordinary circumstances.
+Nothing is more likely than that the mutant was an ordinary recessive.
+I may add that Mr. R. H. Compton made a number of experiments with
+_Raimannia odorata_, raised from seeds kindly given me by Dr. Britton,
+injecting the ovaries with a variety of substances, including those
+named by Macdougal; but though a numerous progeny was raised from the
+ovaries treated, all were normal. Macdougal relates also that some
+mutational forms came from ovaries of _Oenothera Lamarckiana_ exposed
+to radium pencils, and also from _Oenothera biennis_ injected with
+zinc sulphate a peculiar mutant was raised, but taking into account
+the frequency of these occurrences in those species, he very properly
+regarded this evidence as of doubtful application. In a later paper,[16]
+however, he has returned to the subject and affirms his conviction that
+the appearance of a mutant among seedlings raised from an ovary of
+_Oenothera biennis_ treated with zinc sulphate was really a consequence
+of the injection, saying that the variation previously observed in
+the species was afterwards shown to be due to fungoid disease. The
+circumstances to which he mainly points in support of his view is
+that the mutation bred true, but this is only evidence of its genetic
+distinctness, which may, of course, be admitted by those who remain
+unconvinced as to the original cause of its appearance. He adds that he
+is making similar experiments with some twenty genera; but what is more
+urgently needed is repeated confirmation of the original observation.
+When it has been shown that this mutation can be produced with any
+regularity from a plant which does not otherwise produce it on normal
+self-fertilisation, the enquiry may be profitably extended to other
+plants.
+
+A curious and novel experiment, which however, led ultimately to a
+negative result, was made by F. Payne. Many discussions have been held
+respecting the blindness of cave animals. The phenomenon is one of the
+well-known difficulties, and most of us would admit that the theory of
+evolution by the natural selection of small differences does not offer a
+really satisfying account of it. Those who believe in the causation of
+such modifications by environmental influences and in their hereditary
+transmission make, of course, the simple suggestion that the darkness
+is the cause of the loss of sight, and that disuse has led to the
+reduction of the visual organs. Payne bred _Drosophila ampelophila_,
+the pomace-fly (which is easy to keep in confinement, fed on fermenting
+bananas), for sixty-nine generations in darkness. At the end of that
+period there was no perceptible change in the structure of the eyes, or
+in any other respect. The number of generations may possibly be regarded
+as insufficient to prove anything, but comparing them, as he does, with
+the generations of mankind, we see that they correspond with a period of
+about two thousand years, an interval far longer than those which many
+writers in particular cases have deemed sufficient.
+
+In his first paper Payne states that, though no structural difference
+could be perceived, the flies which had been bred in the dark reacted
+less readily to light than those which had been reared under normal
+conditions, and he inclined to think that the treatment had thus
+produced a definite effect. After more careful tests, however, he
+withdrew this opinion. It proved that both individual flies and
+individual groups of flies, both of those bred in the light and of
+those bred in the dark, differed greatly in their reactions, which were
+measured by counting the time that it took for a fly to travel to the
+light end of a covered tube, various sources of error being eliminated.
+He found further that these differences of behaviour were not inherited
+in any simple way, but he is disposed to attribute them to accidental
+differences in the nature of the food, an account which seems probable
+enough.[17]
+
+In several recent publications Blaringhem[18] has described the origin
+of many abnormal forms of plants, especially of maize, which he
+attributes to various mutilations practised upon the parents. Respecting
+these the same difficulty which has been expressed in other cases
+reappears, that before drawing any conclusion as to the value of such
+evidence we require to know that the plants treated belong to a really
+pure line, which if left to nature in the ordinary circumstances of its
+life in that locality would have had normal offspring. Abnormalities
+abound in the experience of everyone who examines pans of seedlings
+of almost any species of plant, and in maize they are well known to
+be exceptionally common. Some of those which we meet with when we
+attempt to ripen maize in this country are very similar to those which
+Blaringhem describes, consisting in irregularities in the distribution
+of the sexes, in the shapes of the panicles, etc. Many of these are
+doubtless imperfections of development, due to the dullness of our
+climate, but others are presumably genetic and would recur in the
+offspring however treated. If some one working in a climate where maize
+could be raised in perfection would repeat these experiments, and show
+that a strain which was thoroughly reliable and normal in its genetic
+behaviour did, after mutilation, throw the miscellaneous types observed
+by Blaringhem, that would be evidence at least that the development of
+the seed could be so influenced by injury to the parental tissues that
+its properties were changed. Such evidence could be used for what it is
+worth; but pending an inquiry of this kind I am disposed to regard these
+observations of variation following on parental injury as suggestive
+rather than convincing.
+
+Some evidence of a remarkably interesting kind has been collected by
+J. H. Powers[19] respecting the structure and habits of _Amblystoma
+tigrinum_, which led him to the conclusion that striking differences
+in the form, anatomy, and developmental processes could be effected
+directly by change in the conditions of life. It is well known that
+a profusion of forms, distinct in various degrees, is grouped round
+_Amblystoma tigrinum_. Some of these are believed to be geographically
+isolated, others occur together in the same waters, and, as usual,
+authorities have differed greatly as to the number of names to be given.
+These forms were studied in detail by Cope who described them in the
+_Batrachia of North America_. The view which he inclined to take was
+that the individual variations of _Amblystoma tigrinum_ resulted from
+variations in the time and completeness of the metamorphosis, and these
+were regarded as due to external causes, such as differences in season,
+temperature, and geographical conditions. Powers, however, states that
+collecting within a radius of six or eight miles he found almost if
+not quite the whole "gamut of recorded variation in this species."
+Some, however, as he states, occurred rarely except under experimental
+conditions, but considerable differences in temperature were not found
+necessary in producing them. Every year, he says, he has been able to
+add to the number of peculiar types found in the same small area in
+nature, until the amount of natural variation at least equals that seen
+by Cope in the collections of the National Museum and those of the
+Philadelphia Academy.
+
+Powers states that his observations by no means confirm Cope's view
+that these differences are in the main referable to variation in
+the completeness of metamorphosis, and on the contrary, he regards
+metamorphosis as on the whole a levelling process, tending to obliterate
+diversity. The enormous differences in size and proportions which he
+describes can only be appreciated by reference to his figures. They
+affect almost all features of bodily organisation. These striking
+differences he looks upon as brought about by differences in nutrition,
+"diversities in habitual locomotion," and diversity in the age at which
+metamorphosis occurs, and to sexual difference. Apart from sexual
+difference he regards the chief distinctions, in brief, as "acquired
+variations of the larva."
+
+As an example he gives the great elongation of some of the forms as
+"due first to slow growth, second to the free-swimming habit, third
+to the prolongation of larval life, and finally to the assumption of
+sexual maturity as males," either in the branchiate or non-branchiate
+condition. He describes the rapid growth of some and the slow growth
+of others. A larva of intermediate type may grow about a centimeter a
+month, but a rapidly growing specimen may grow more than four times
+as much. The slower rate of growth may, he says, be induced by winter
+feeding, and other treatment.[20]
+
+When, however, he goes on to describe the influences which he regards
+as exerted by the habit of freely swimming, I am led to wonder whether
+after all in most of these illustrations, the primary distinctions
+are not in reality genetic. "Specimens raised in the same aquarium or
+in similar aquaria, side by side with all conditions as uniform as it
+is possible to make them, seldom fail to furnish striking examples of
+broad-headed, short-bodied, and short-tailed types which are habitually
+found at the bottom, while others, slender and elongated, are free
+swimmers, and maintain themselves in almost as continual suspension
+and motion as does a gold fish." Later, again, he writes, "Yet despite
+the uniformity of these favourable conditions, the larvae soon began
+to split up into two noticeably distinct groups, the one of unusually
+compact proportions, the other of uniform intermediate build, such
+is most commonly met with." It is to my mind scarcely possible to
+resist the inference that, though there may be definite responses
+to certain conditions, yet the chief distinctions are genetic, and
+that it is these distinctions which confer the power to respond. The
+parts respectively played by cause and effect are always difficult to
+assign; but when it is stated that "a weak-limbed, long-bodied and
+long-tailed animal becomes well nigh perforce an undulatory swimmer,
+while the strong-limbed, short-tailed, heavy-bodied specimen, when
+these characteristics are rapidly forced upon it, is, under certain
+circumstances, just as forcibly induced to become a crawler," we feel
+how erroneous any estimates of causation are likely to be.
+
+One of the most remarkable and interesting sections of Powers' paper
+is that in which he describes the differences in bodily structure and
+habits which he attributes to cannibalism, and the whole account of the
+phenomena should be read in the original. It appears that there are
+two extremely distinct types of larvae, those with narrow heads and
+slender bodies which live for the most part on small Crustacea such
+as _Daphnias_, and those with huge mouths and very wide heads, which
+disregard such small animals altogether and live on amphibian larvae,
+whether of their own or other species. As the illustrations show, the
+differences between these two types are very great, and the differences
+in instinct and behaviour are no less. The cannibals take no heed of the
+pelagic crustacea, lying sluggishly at the bottom, rousing themselves
+immediately to a violent attack on the larger living things which
+approach them. Nothing but the most incontrovertible evidence based on
+abundant control experiments should convince us that such differences
+are not primarily genetic, and in the present state of knowledge I
+incline to think that the families really consist of individuals which
+are ready to assume the cannibal habit if opportunity offers, and
+others which are congenitally incapable of it. It may readily be that
+if all chance of cannibal diet be excluded, the full development of the
+wide head and mouth, or the other peculiarities, would never become
+pronounced, but I doubt whether such change could be induced in any
+individual taken at random.
+
+FOOTNOTES:
+
+[1] Pringsheim, H., _Die Variabilität niederer Organismen_, Berlin, 1910.
+
+[2] F. Wolf, Modifikationen u. Mutationen von Bakterien, _Zts. F.
+indukt. Abstam. u. Vererbungslehre_, II, 1909, p. 90.
+
+[3] Winslow, C. E. A. and A. R., _Systematic Relationships of the
+Coccaceae_. New York. 1909.
+
+[4] C. C. Dobell, _Jour. Genetics_, 1912, II, p. 201, where full
+references are given.
+
+Still more recently the same author has contributed an excellent summary
+of the evidence relating to bacteria (_ibid._, II. 1913, p. 325).
+
+[5] See Woltereck, _Verh. d. Deut. Zool. Ges._, 1909, p. 110; and 1911,
+p. 142. This is a subject which can only be properly appreciated on
+reference to the original papers. Several complications are involved to
+which I have not here alluded.
+
+[6] _Proc. Roy. Soc._, B, Vol. 86, 1913, p. 113.
+
+[7] _An Investigation of Evolution in Chrysomelid Beetles of the Genus
+Leptinotarsa_, Carnegie Publications, 1906, No. 48.
+
+[8] This is the famous Colorado beetle or potato-bug, which has caused
+such serious destruction in potato crops. There seems to be no doubt
+that this insect, formerly unknown in the eastern States, made its way
+east along the mining trails when the west was opened up.
+
+[9] This is indicated in the coloured plate, but I have not found any
+explicit statement to this effect in the text, and am not sure if the
+absence of pigment was regarded as complete.
+
+[10] _Biol. Bull._, XVIII, 1910, p. 285.
+
+[11] This description does not quite agree with the representation of
+the larvae in Pl. 17 of the book _Evolution in the Genus Leptinotarsa_
+for there the larva of _undecimlineata_ is shown as white in the second
+stage, but yellowish in the third stage; perhaps there is an error in
+printing.
+
+[12] _Biol. Bull._, XX, 1910, p. 67.
+
+[13] As to the interrelations of these three forms, Tower states (1906,
+p. 18) that _angustovittata_, which he reared from _undecimlineata_, is
+intermediate between it and _signaticollis_. Compare Stål, "_Monogr.
+des Chrysomélides_," 1862, p. 163; and Jacoby, _Biol. Centr. Amer.
+Celeopt._, vi, Pt. 1, p. 234, Pl. xiii, fig. 20; Tab. 41, fig. 15;
+_ibid._, Suppl., p. 253. All these forms are evidently very closely
+related, and the delimitation of species is quite arbitrary. Jacoby
+indeed suggests that _undecimlineata_ may be a variety of _decemlineata_.
+
+[14] Gortner, _Amer. Nat._, Dec., 1911, XLV, p. 743.
+
+[15] _Mutations, Variations, and Relationships of the Oenotheras_,
+Carnegie Institution Publication No. 81, 1907, pp. 61-64.
+
+[16] Macdougal, D. T., "Alterations in Heredity induced by Ovarial
+Treatments", _Bot. Gaz._, vol. 51, 1911, p. 241.
+
+[17] Payne, Fernandus, _Biol. Bull._, XVIII, 1910, p. 188, and _ibid._,
+XXI, 1911, p. 297.
+
+[18] See especially, _Mutation et Traumatismes_, Paris, Felix Alcan,
+1908.
+
+[19] J. H. Powers, "Morphological Variation and its Causes in
+_Amblystoma tigrinum_." _Studies from the Zoological Laboratory. _ The
+University of Nebraska, No. 71, 1907.
+
+[20] In connexion with this case I would refer the reader to some
+remarkable observations of Dr. T. A. Chapman on various types of larvae
+which he reared from the moth _Arctia caja_ (_Ent. Rec._, IV, 1893,
+p. 265, and following parts). From a single mother he raised a great
+diversity of forms, some which fed up rapidly and passed through their
+development without assuming certain stages, and others which were, as
+he called them, "laggards," moulting more times than their brethren and
+developing at a much slower rate. It is greatly to be hoped that such a
+case may be critically investigated by analytical breeding.
+
+
+
+
+                          CHAPTER XI.
+
+           STERILITY OF HYBRIDS. CONCLUDING REMARKS.
+
+When we consider the bearing of recent discoveries on those
+comprehensive schemes of evolution with which we were formerly
+satisfied, we find that certain details of the process are more easy
+to imagine. We readily now understand how varieties once formed,
+can persist, but at the same time difficulties hitherto faced with
+complacency become formidable in the light of the new knowledge. So
+generally is this admitted by those familiar with modern genetic
+research that most are rightly inclined to postpone the discussion. The
+premisses, indeed, on which such a discussion must be based are almost
+wholly wanting.
+
+The difficulties to which I chiefly refer are not those created by the
+phenomena of adaptation, though they are serious enough. In treating
+of that subject I have felt obliged to express scepticism as to the
+validity of nearly all the new evidence for the transmission of acquired
+characters. At the present time the utmost we are bound to accept is the
+proof that (1) in some parthenogenetic forms variations, or perhaps we
+may say malformations, produced in response to special conditions, recur
+in one or perhaps two generations asexually produced after removal to
+other conditions. (2) That violent maltreatment may in rare instances
+so affect the germ-cells contained in the parents as to cause the
+individuals resulting from the fertilisation of those cells to exhibit
+an arrest of development similar to that which their parents underwent.
+
+I do not doubt that evidence of this type will be greatly extended. As
+a contribution to genetic physiology these facts are very important
+and interesting, but I cannot think that any one, on reflexion, will
+feel encouraged by such indications to revive old beliefs in the direct
+origin of adaptations.
+
+In these respects we are simply left where we were. The force of
+objections based upon the existence of adaptative mechanisms is
+no greater than it has always been. On the contrary the fact that
+variations can now so generally be recognized as definite is some
+alleviation of the difficulty. We can moreover disabuse ourselves of
+the notion that for all characters which are definite or fixed, some
+utilitarian rationale may be presumed. Upon that point the study of
+variation has provided a perfectly clear answer.
+
+In frankly recognizing that the fixity of characters in general need
+not connote usefulness to their possessors we deliver ourselves of a
+distracting pre-occupation and prepare our minds for an investigation
+of the properties of living organisms in the same spirit as that
+in which the chemist and the physicist examine the properties of
+unorganized materials. The creature persists not merely by virtue of its
+characteristics but in spite of them, and the fact of its persistence
+proves no more than that on the whole the balance of its properties
+leaves something in its favour.
+
+It may be noted by the way that the fact that the structures of living
+things are on the whole adaptative was not always obvious. Though to
+naturalists of this generation it is a truism, we have only to turn to
+Buffon to find that in his philosophy of nature it played no essential
+part. The passage in which Buffon describes what he regards as the
+forlorn and degraded condition of the Woodpecker is well known. We have
+come to think of the Woodpecker as a capital example of adaptation to
+the mode of life; but Buffon after enumerating the hard features of
+the bird's existence, forced to earn its living by piercing the bark
+of trees in an attitude of perpetual constraint, remarks[1] "Tel est
+l'instinct étroit et grossier d'un oiseau borné a une vie triste et
+chétive. Il a reçu de la Nature des organes et des instrumens appropriés
+a cette destinée _ou plutôt il tient cette destinée même des organes
+avec lesquels il est né_" (my italics). His reflexions on the Stilt
+(_Himantopus_) read even more strangely to us, accustomed as we are to
+see in the prodigious length and thinness of the shanks and in the other
+features of its organisation palpable adaptations to a wading life. For
+Buffon, however, this curious bird seemed a poor, neglected production,
+extravagant in its disproportions, one of the misfits of creation,
+left as a shadow in the picture composed of nature's more successful
+efforts.[2] This theme he develops at some length, being evidently well
+pleased with the idea.
+
+Our way of regarding these things is doubtless sounder and more fruitful
+than Buffon's, but it is well to remember that what seems so obvious to
+us looked quite differently to other excellent observers; and stupid
+as it may have been to have overlooked plain examples of adaptation,
+it is a far worse mistake to see adaptation everywhere. I do not seek
+to minimise the real and permanent difficulty which the existence of
+adaptations creates, but by the suggestion that all normal specific
+differences are adaptational that difficulty was quite gratuitously
+increased.
+
+In these respects it may be claimed that progress has been made, even if
+that progress seem outwardly of small account.
+
+But all constructive theories of evolution have been built on the
+understanding that what we know of the relation of varieties to species
+justifies the assumption that the one phenomenon is a phase of the
+other, and that each species arises or has arisen from another species
+either by one or several genetic steps. In the varieties we have
+accustomed ourselves to think that we see those steps. We still know
+little enough of the mode of occurrence of variation, but we do begin to
+know something, and if we ask ourselves whether our knowledge, such as
+it is, conforms at all readily with our former expectations, we cannot
+with any confidence assert that it does. Among the plants and animals
+genetically investigated are many illustrations of very striking and
+distinct varieties. Many of these might readily enough be accepted as
+species by even the most exacting systematists, and not a few have
+been so treated in classification; but when we have examined their
+relationship to each other we feel not merely that they are not species
+in any strict sense but that the distinctions they present cannot be
+regarded as stages in the direction of specific difference. Complete
+fertility of the results of inter-crossing is and I think must rightly
+be regarded as inconsistent with actual specific difference; and of
+variations leading to that consequence no clear indication has yet been
+found. As an example of possible exceptions mention should perhaps be
+made of the case of a giant form of _Primula sinensis_ investigated by
+Keeble.[3] It arose from a "Star" Primula of normal size, and though
+fertile with its own pollen all attempts to fertilise it with the pollen
+of other forms failed. Miss Pellew, who did these fertilisations,
+tells me that very extensive trials were made, and repeated in several
+seasons. Ultimately two plants were raised from it fertilised with a
+plant of the strain from which it sprang, and these proved sterile.
+In the light of modern experience the significance of such isolated
+instances is doubtful.
+
+All the strains known as "Giants" are, as Messrs. Sutton have always
+found, more or less sterile, and their sterility is presumably due to
+some negative defect.
+
+In regard to the fertility of Primula species there are several
+paradoxes. For example the long-styled varieties, apart from giants, are
+fertile with their own pollen, and for many years short-styled plants
+have not been used in most strains. Auriculas and Polyanthuses, on the
+contrary, are generally if not always bred from short-styled plants,
+as the florists have decided that the long-styled are inadmissible.
+Mr. R. P. Gregory tells me that, though most strains of _P. sinensis_
+give seed enough when only long-styled plants are used, he finds
+nevertheless that when a "legitimate" union is made the amount of seed
+usually increases much as Darwin observed. Darwin's statement that
+plants of "illegitimate" origin are less fertile than the "legitimately"
+raised plants is also in general confirmed by his experience. To
+this rule there were some marked exceptions in strains derived from
+_long_-styled plants, which though illegitimate showed a high degree
+of fertility, but illegitimate unions between _short_-styled plants
+always produced comparatively sterile offspring. I have no records of
+the behavior of Auriculas and Polyanthuses. It would be interesting to
+know whether among them pure strains of short-styled plants (dominants)
+have appeared, and, if so, how their fertility is affected. Without
+much more critical data I suppose no one would nowadays be inclined
+to follow Darwin in instituting a comparison between the sterility
+of hybrids and that of illegitimately raised plants of heterostyle
+species.[4] It is even difficult to imagine any essential resemblance
+between these two phenomena, nor has evidence ever been produced to
+show that illegitimately raised plants have bad pollen grains, which is
+the usual symptom of sterility in hybrid plants and the consequence,
+as we believe, of failure of some essential division in the process of
+maturation.
+
+The difficulty that we have no knowledge of the contemporary origin of
+forms, from a common stock, which when crossed together give a sterile
+product, is one of the objections constantly and prominently adduced
+from the time of the first promulgation of evolutionary ideas. In the
+light of recent work the objection has gathered strength. Why, if
+we are able to produce instances of variation colourably simulating
+specific difference in almost all other respects, do we never find
+an original appearance of this most widely spread of all specific
+characteristics? No doubt all breeders know that sterile animals
+and plants occasionally appear in their cultures, but it is more in
+accordance with probability that the sterility in these sporadic
+instances should be regarded as due to defect than that it should be
+thought comparable with that of the sterile hybrids. For their sterility
+must, by all analogy with results elsewhere seen, be attributed not
+to the absence of something, but to the presence and operation of
+complementary factors leading to the production of inhibition of
+division; and consistently with that interpretation, we find that when
+from a partially sterile hybrid comparatively fertile offspring can be
+raised, their comparative fertility continues in the posterity generally
+if not always without diminution. The distinction between these several
+kinds of sterility was of course not understood in Darwin's time. The
+comparison, for example, which he instituted[5] between the sterility
+of "contabescent" anthers and that of hybrids no longer holds, for at
+least in those cases in which the nature of contabescent anthers have
+been genetically investigated (Sweet Pea, _Tropaeolum_) they proved
+to be a simple recessive character. Nor can we now easily suppose that
+the attempt there made by Darwin to suggest resemblance between the
+sterility produced by unnatural conditions and that of hybrids has any
+physiological justification.
+
+In regarding the power to produce a sterile or partially sterile hybrid
+as a distinction in kind, of a nature other than those which we perceive
+among our varieties, I am aware that I am laying stress on an impression
+which may hereafter prove false. The distinction nevertheless is so
+striking and so continually before the eyes of a practical breeder that
+he can scarcely avoid the inference that when he meets a considerable
+degree of sterility in a cross-bred he is dealing with something
+belonging to a distinct category, and not merely a varietal feature of
+an exceptional kind.
+
+Besides the sterility of hybrids appeal has often been made to the
+phenomenon of incompatibility, in its several stages of completeness,
+as distinguishing species. No one doubts that incompatibility may
+arise from a variety of causes of most diverse degrees of importance,
+but though sometimes referred to as an extreme case of interspecific
+sterility, it is really a very different matter. In regard to one phase
+of this incompatibility, that associated with self-sterility, some
+progress has been made, and we are not wholly without experimental
+evidence of its being within the range of contemporary variation.
+
+Given the outline of Mendelian teaching as to gametic differentiation
+and the classification of individuals in a mixed population, it
+seemed highly probable that what we call self-sterility must mean
+that the species really consisted of _classes_, some of which are
+capable of interbreeding with others while others are not. According
+to the received account every individual, though incapable of
+fertilising itself, was supposed to be able both to fertilise and to
+be fertilised by any other individual. This notion has always seemed
+to me a self-evident absurdity, for it would imply that there can be
+as many categories as individuals. Such experiments, however, as I
+made did certainly give results consistent with that belief. I first
+tried Cinerarias, which are usually self-sterile, but I found no
+incompatible pairs of plants. Whether I was deceived by the consequences
+of apogamy, or whether the pollen of certain plants may belong to more
+than one class I do not know. The results were confused in various
+ways. Usually the self-fertilised plants set little or nothing, and
+cross-fertilised they set fully with such uniformity that the few
+failures could plausibly be attributed to mistakes in manipulation
+or to other extraneous causes. Later de Vries announced[6] (without
+giving particulars) that he had proved the existence of such classes in
+_Linaria vulgaris_; but on making experiments with that species I again
+got no positive results, and I came to the conclusion that in spite of
+inherent improbability the conventional belief must be substantially
+true. At last, however, the work of Correns, lately published,[7] does
+definitely show that in one species, _Cardamine pratensis_, classes of
+individuals exist such that individuals of the same class are incapable
+of fertilising themselves or each other, but fertilisation made between
+the classes is usually completely effective. Many complications were
+encountered and some contradictory evidence is recorded, but the general
+bearing of the results was positive and indubitable.
+
+We know far too little of this phenomenon as yet to be able to
+understand its significance, but I suppose we may anticipate with some
+confidence that it will be found to be a manifestation of dissimilarity
+between the male and female gametes of the same individual, comparable
+with that first seen in the Stocks (_Matthiola_) which throw doubles--a
+state of things in all likelihood to be found widely spread among
+hermaphrodite organisms. Whether the incompatibility between species
+is to be associated with that of the self-steriles also cannot be
+positively asserted, though it seems not unreasonable to expect that
+such an association will be discovered.
+
+The case of the apple and the pear is an impressive illustration of
+this possibility. The two species are of course exceedingly alike in
+all outward respects, but nevertheless the pollen of each is entirely
+without effect on the other. Presumably we should interpret this fact
+as meaning not so much that the apple and the pear are in reality
+very wide apart, but rather that either, each is lacking in one of
+two complementary elements, or that each possesses a factor with an
+inhibitory effect. Their incompatibility may well be of the same nature
+as that of the classes in _Cardamine pratensis_.
+
+Returning now to the problem of inter-specific sterility; we note,
+as I have said, the absence of contemporary evidence that variation
+can confer on a variety the power to form a sterile hybrid with the
+parent species. The considerations based on this want of evidence have
+for a long while been familiar to all who have discussed evolutionary
+theories, and it is worth observing the exact reason why the difficulty
+strikes us now with a new and special force. In pre-Mendelian times
+all that was known was that some forms could freely interbreed without
+diminution of fertility in the product, while others could not. But now
+we find that, by virtue of segregation, from one and the same pair of
+parents, or even, in the case of hermaphrodites, from one and the same
+individual, offspring commonly arises showing among themselves exactly
+such differences as distinguish species--and very good species too. This
+we see happening again and again. But to forms capable of arising as
+brethren in one family the title species has never been meant to apply,
+and if we are going to use the term in application to fraternal groups
+we must definitely recognise that by "specific" difference is to be
+understood simply _difference_, without any immediate or even ulterior
+physiological limitation whatever. Naturally, therefore, we begin to
+think of the appearance of sterility in crosses as something apart, and
+as a manifestation which distinguishes certain kinds of unions in a very
+special way.
+
+I am perfectly aware that there are gradations in the sterility of
+hybrids as in every other characteristic upon which it has been proposed
+to base specific definitions; but, as also so often happens in the
+matter of defining intergrading categories, the difficulty in practice
+is not often such as to lead to actual ambiguity. I am speaking of
+course of those examples which are amenable to genetic experiment.
+As to the rest there is complete and permanent uncertainty. But the
+experience of the practical breeder does, I think, on the whole, support
+the contention to which systematists have so steadily clung under all
+the assaults of evolutionary philosophers, that, though we cannot
+strictly define species, they yet have properties which varieties have
+not, and that the distinction is not merely a matter of degree.
+
+The first step is to discover the nature of the factors which by their
+complementary action inhibit the critical divisions and so cause
+the sterility of the hybrid. Thus expressed, we see the problem of
+inter-specific sterility in its right place; and the question why we
+do not now find contemporary instances of varieties lately arisen in
+domestication, which when crossed back with their parents, or with their
+coderivatives, can produce sterile products, is perceived to be only a
+special case of a problem which in its more general form is that of the
+origin of new and additional factors.
+
+For the requisite evidence no comprehensive search has been made, but
+perhaps it will yet be found. All that we can say at the present time
+is that the incidence both of hybrid sterility, and of incompatibility
+also, is most capricious; and provided that two forms have such features
+in common that a cross between them seems not altogether out of the
+question, no one can predict without experiment whether such a cross
+is feasible, and if feasible whether the product will be fertile, or
+sterile more or less completely. For instance, though probably all
+the British and some Foreign Finches (Fringillidae) have been crossed
+together, and some of these crosses, as for instance, the various
+Canary-mules have been made in thousands, I believe no quite clear
+example of a fertile hybrid can be produced. Many species of Anatidae
+cross readily and produce fertile hybrids: others give results uniformly
+sterile. Though most of the Equidae can be crossed and some of the
+hybrids are among the commonest of domesticated animals there is no
+certain record of a fertile mule. Among the Canidae the dogs, wolves and
+jackals all give fertile hybrids, but there is no clearly authenticated
+instance of a cross between any of these forms and the European fox.
+In spite of their close anatomical resemblance it is doubtful if the
+rabbit and the hare have ever interbred. Many of the wild species of
+_Bos_ have been crossed and recrossed both with each other and with
+many domesticated races, but I understand that no cross with the Indian
+buffalo (_Bos bubalus_) has yet been successful even in producing a
+live calf.[8] In the genus _Primula_ many hybrids are known and several
+of them occur in nature, but hitherto no certain hybrid between _P.
+sinensis_ and any other species has been made, in spite of repeated
+attempts.
+
+In _Nicotiana_ many--doubtless all--the various forms of _N. tabacum_
+can be crossed together without diminution of fertility, though some
+are very distinct in appearance, but crosses between _tabacum_ and
+_sylvestris_ are highly sterile (in my experience totally sterile[9]),
+though the distinctions between them are not to outward observation
+nearly so great as those which can be found between the various races of
+_Primula sinensis_.
+
+Recently some remarkable experiments bearing closely on these questions
+have been published by F. Rosen.[10] They concern the forms of _Erophila
+(Draba) verna_, celebrated in the history of evolutionary theory as the
+plants especially chosen by Alexis Jordan for the exposition of his
+views on these subjects.
+
+The "species" contains a profusion of forms dissimilar in many
+structural characters, such as the size and shape of leaves, flowers,
+fruits, etc. Of these forms many grow in association. Jordan found, on
+experiment, that each, to the number of some two hundred, bred true, and
+that therefore, the conventional assumption that polymorphism of this
+kind must mean great contemporary variability had no foundation in fact.
+So far indeed is the evidence from favouring the belief that such forms
+are in any way transitional or indeterminate, that, as is well known,
+Jordan used it with every plausibility to support the doctrine of the
+fixity of species. To certain aspects of Jordan's work we will return
+later in this chapter, but the matter is in the present connection
+of especial interest for the reason that Rosen has lately found by
+experiment that some of these presumably very closely allied forms,
+crossed together, gave hybrids more or less sterile. In the case of the
+offspring of one pair of forms only (_E. cochleata_ and _stricta_) was
+the fertility undiminished, and the various degrees of sterility found
+in the other crosses ranged up to the extreme infertility of the hybrids
+between _E. stricta_ × _elata_. From this cross ten plants were bred.
+Of these the four strongest were chosen to breed from, but two of the
+four proved totally sterile; one had only bad seeds; and from the fourth
+a single seedling was raised which in its turn proved to be sterile.
+From the less sterile hybrids F_{2} families were raised, with the
+usual experience that in this and subsequent generations the sterility
+diminished among extracted forms, new and true-breeding types with
+complete fertility being thus derived from the original cross.[11]
+
+The production of sterility as a consequence of crossing plants so
+nearly approaching each other as these _Erophila_ "species" do is
+not a little interesting, and the fact well exemplifies the futility
+of the various attempts to frame general expressions as to specific
+properties or behaviour. Commenting on his results Rosen argues that
+the polymorphic group commonly called by systematists _Erophila (Draba)
+verna_ may now be regarded as having arisen by crossing, as did his own
+types mentioned above. The question, however, _what_ species were the
+original progenitors of the group cannot be answered. Rosen considers
+that no form which he knows satisfies the requirements, and that
+it or they must be supposed to be lost. This conclusion will recall
+the similar problem raised by the _Oenothera_ mutants (Chap. V); and
+unsatisfactory as it may be to have recourse to such hypotheses we
+must remember the possibility that as a consequence of hybridisation,
+subsequent segregation and recombination of factors, species may
+have thus actually, as we may say, exploded, and left nothing but a
+polymorphic group of miscellaneous types to represent them in posterity.
+If this way of regarding the phenomena be a true one, the sterility
+now seen when some of the group are re-crossed, becomes analogous to
+that "reversion or crossing" which we now so well understand to be
+a consequence of the recombination of characters separated at some
+previous point in the history of descent. In the partial sterility of
+the contemporary hybrid we see this character reappearing, formed now
+as it was on the occasion of the original cross, by the meeting of
+complementary factors.
+
+Another case that may be mentioned in this connection is that of the
+crosses between various culinary peas (_Pisum sativum_) and a peculiar
+form found by Mr. Arthur Sutton growing ostensibly in a wild state in
+Palestine. This Palestine Pea is low growing, rarely reaching 18 inches.
+It is in general appearance like a small and poorly grown field pea.
+The stems are thin and rather hard. The most obvious differences which
+distinguish this from other field peas are the marked serration of the
+stipules, and the development of pith in the pods. Such pith is often
+present in the pods of peas more or less, but in the Palestines it is
+so strongly developed as almost to form a lomentum. Curiously enough,
+though the flowers are purple much as those of ordinary field peas,
+there is no coloured spot in the axils. On the other hand, the stems
+have coloured stripes running up from the axils. Though this plant
+differs so little from domesticated peas, all crosses with them either
+failed, or produced hybrids quite or almost quite sterile. This was Mr.
+Sutton's experience, and on repeating the experiments with material
+kindly given by him I found the same result.[12]
+
+In a large series of crosses some seeds died or gave rise to feeble
+plants. Of the plants which lived, few gave any seed. The seed, however,
+that was obtained from F_{1} plants grew well enough, and the F_{2}
+plants proved, as often in such cases, fertile. In these, indeed, no
+sign of sterility was noticeable. The experiment is being repeated in
+various ways, for, as the genetic behaviour of peas is comparatively
+well known, the subject is an exceptionally favourable one for these
+investigations.
+
+Such an example shows the confusion produced the moment we attempt to
+harmonize conceptions of specific difference with results attained by
+experimental methods. It has been usual to regard the field pea (_P.
+arvense_) as a species distinct from the edible pea (_P. sativum_).
+De Candolle and others regard the field pea as derived from a form
+wild in Italy, but the origin of the edible pea is considered to be
+unknown. From breeding experiments we find no sterility whatever in the
+crosses between the various _arvense_ and _sativum_ types, nor in the
+crosses made between them and several other peculiar types from various
+countries; whereas this Palestine Pea, which only differs from a small
+_arvense_ in what might have been thought trivial characters,[13] either
+fails to cross altogether or gives a sterile product, whatever type be
+chosen as the other parent.
+
+Examples of this kind have at least the merit that they lead to more
+precise delimitations of the problem. We are confronted with two
+distinct alternatives.
+
+1. We may apply the term Species promiscuously to all distinct forms.
+If we do so it must be clearly understood that we cannot even rule out
+the several combinations of "presences and absences" represented by the
+various types whether wild or domesticated. For we may feel perfectly
+assured that at least all the _arvense_ and all the _sativum_ types yet
+subjected to experimental tests are on precisely the same level in this
+respect. There is no distinction, logical or physiological, to be drawn
+between them. Some contain more factors, and others contain fewer. In
+some the re-combinations have been brought about by natural variation or
+crossing, while the same consequences in the others have resulted from
+man's interference.
+
+2. We may follow the conventions of systematists and distinguish the
+outstanding or conspicuous forms such as _arvense_, _quadratum_,
+_sativum_ and perhaps a few more as species, and leave the rest
+unheeded. If this course is followed it must be clearly understood and
+permitted as a piece of pure pragmatism, deliberately adopted for the
+convenience of cataloguers and collectors, without regard to any natural
+fact or system whatsoever.
+
+But while following either the one plan or the other we shall be still
+awaiting the answer, which only genetic experiment can provide, to the
+question whether among the various types there are some which differ
+from the rest in a peculiar way: whether by having groups of characters
+linked together in especially durable combinations, or by possessing
+ingredients which cause greater or less disturbance in the processes of
+cell-division, and especially in the processes of gametic maturation,
+when they are united by fertilisation with complementary ingredients.
+
+Before any but the vaguest ideas regarding the nature and significance
+of inter-specific sterility can be formed, a vast amount of detailed
+work must be done. Sterility as a result of crossing, as well as
+that which is alleged sometimes to arise in consequence of changed
+conditions, is at best a negative characteristic, and there are endless
+opportunities for mistake and misinterpretation in studying features
+of this kind. No one, I suppose, would now feel any great confidence
+in most of the data which from time to time are resuscitated for the
+purpose of such discussions. Even the best collections of evidence, such
+as those given by Darwin in _Forms of Flowers_, cannot be regarded as
+critical when judged by present-day standards. Nothing short of the most
+familiar acquaintance with the habitual behaviour of individuals, and of
+strains kept under constant scrutiny for several years would enable the
+experimenter to form reliable judgments as to the value to be attached
+to observations of this class.
+
+The admission must, however, be faced that nothing in recent work
+materially tends to diminish the surprise which has always been felt
+at the absence of sterility in the crosses between co-derivatives. We
+should expect such groups of forms to behave like the _Erophila_ types,
+and frequently to produce sterile products on crossing. Whatever be
+the explanation, the fact remains that such evidence is wanting almost
+completely. In spite of all that we know of variability nothing readily
+comparable with the power to produce a sterile hybrid on crossing with
+a near ally, has yet been observed spontaneously arising, though that
+characteristic of specificity is one of the most widely distributed
+in nature. It may be that the lacuna in our evidence is due merely to
+want of attention to this special aspect of genetic inquiry, and on the
+whole that is the most acceptable view which can be proposed. But seeing
+that naturalists are more and more driven to believe the domesticated
+animals and plants to be poly-phyletic in origin--the descendants, that
+is to say, of several wild forms--the difficulty is proportionately
+greater than it was formerly, when variation spontaneously occurring was
+regarded as a sufficient account of their diversity.
+
+
+                        CONCLUDING REMARKS.
+
+The many converging lines of evidence point so clearly to the central
+fact of the origin of the forms of life by an evolutionary process
+that we are compelled to accept this deduction, but as to almost all
+the essential features, whether of cause or mode, by which specific
+diversity has become what we perceive it to be, we have to confess an
+ignorance nearly total. The transformation of masses of population by
+imperceptible steps guided by selection, is, as most of us now see, so
+inapplicable to the facts, whether of variation or of specificity, that
+we can only marvel both at the want of penetration displayed by the
+advocates of such a proposition, and at the forensic skill by which it
+was made to appear acceptable even for a time.
+
+In place of this doctrine we have little teaching of a positive kind
+to offer. We have direct perception that new forms of life may arise
+sporadically, and that they differ from their progenitors quite
+sufficiently to pass for species. By the success and maintenance of
+such sporadically arising forms, moreover, there is no reasonable doubt
+that innumerable strains, whether in isolation or in community with
+their co-derivatives, have as a fact arisen, which now pass in the
+lists of systematists as species. For an excellent account of typical
+illustrations I would refer the reader to the book lately published by
+R. E. Lloyd[14] on the rat-population of India. The observations there
+recorded are typical of the state of things disclosed whenever the
+variations of large numbers of individuals are closely investigated,
+whether in domestication or in natural conditions.
+
+Guided by such clues we may get a good way into the problem. We see
+the origin of colourable species in abundance. Then, however, doubt
+arises whether though these new forms are as good species as many
+which are accepted as such by even cautious systematists, there may
+not be a stricter physiological sense in which the term species can be
+consistently used, which would exclude the whole mass of these _petites
+espèces_.
+
+If further we find that we have, with certain somewhat doubtful
+exceptions, never seen the contemporary origin of a dominant factor, or
+of inter-racial sterility between indubitable co-derivatives, it needs
+no elaboration of argument to show that the root of the matter has not
+been reached.
+
+Examination of the inter-relations of unquestionably distinct species
+nearly allied, such as the two common species of _Lychnis_, leads to
+the same disquieting conclusion, and the best suggestion we can make
+as to their origin is that _conceivably_ they may have arisen as two
+re-combinations of factors brought together by the crossing of parent
+species, one or both of which must be supposed to be lost.
+
+All this is, as need hardly be said, an unsatisfying conclusion. To
+those permanently engaged in systematics it may well bring despair.
+The best course for them is once for all to recognise that whether
+or no specific distinction may prove hereafter to have any actual
+physiological meaning, it is impossible for the systematist with the
+means at his disposal to form a judgment of value in any given case.
+Their business is purely that of the cataloguer, and beyond that they
+cannot go. They will serve science best by giving names freely and by
+describing everything to which their successors may possibly want to
+refer, and generally by subdividing their material into as many species
+as they can induce any responsible society or journal to publish.
+Between Jordan with his 200 odd species for _Erophila_, and Grenier and
+Godron with one, there is no hesitation possible. Jordan's view, as he
+again and again declares with vehemence, is at least a view of natural
+facts, whereas the collective species is a mere abstraction, convenient
+indeed for librarians and beginners, but an insidious misrepresentation
+of natural truth, perhaps more than any other the source of the
+plausible fallacies regarding evolution that have so long obstructed
+progress.
+
+Nevertheless though we have been compelled to retreat from the
+speculative position to which scientific opinion had rashly advanced,
+the prospect of permanent progress is greatly better than it was. With
+the development of genetic research clear conceptions have at length
+been formed of the kind of knowledge required and of the methods by
+which it is to be attained. If we no longer see how varieties give
+rise to species, we may feel confident that a minute study of genetic
+physiology of varieties and species is the necessary beginning of any
+critical perception of their inter-relations. It is little more than a
+century since no valid distinction between a mechanical mixture and a
+chemical combination could be perceived, and in regard to the forms of
+life we may well be in a somewhat similar confusion.
+
+As yet the genetic behaviour of animals and plants has only been
+sampled. When the work has been done on a scale so large as to provide
+generalisations, we may be in a position to declare whether specific
+difference is or is not a physiological reality.
+
+FOOTNOTES:
+
+[1] Buffon, _Hist. Nat._, Oiseaux, 1780, VII, p. 3.
+
+[2] Ibid., VIII, p. 115.
+
+[3] Keeble, _Jour. Gen._, 1912, II, p. 173.
+
+[4] _Animals and Plants_, ed. 1, 1868, II, pp. 180-5.
+
+[5] _Animals and Plants_, ed. 1, 1868, II, p. 165.
+
+[6] _Species and Varieties_, 1905, p. 471.
+
+[7] Correns, _Festschr. med.-nat. Ges. zur 84 Versamml. Deutsch. Naturf.
+u. Aertze. Münster i. W._, 1912.
+
+[8] This is a case of a somewhat different order and I mention it partly
+for that reason as an illustration of the complexity which such negative
+instances may present. The difficulty is that though the buffalo and
+the zebu can breed together, the foetus is too large to be born alive.
+(See Ackermann _Ber. d. Ver. f. Naturk._, Kassel, 1898, p. 69. Prof. S.
+Nathusius, of Halle, who has great experience in crossing Bovidae, tells
+me that he has always failed to cross the buffalo with other species.)
+
+[9] In a paper to be published in the Report of the Genetic Conference,
+Paris, 1911, Bellair states that he obtained some partially fertile
+hybrids in the cross _N. sylvestris_ × _tabacum_. As to the various
+degrees of sterility in hybrids between _Nicotiana_ species see Lock, R.
+H., _Ann. Roy. Bot. Gardens_. Peradeniya, IV, 1909, p. 195.
+
+[10] _Beitrage zur Biol. der Pflanzen._, X, 1911, p. 379.
+
+[11] One very peculiar feature was observed, namely, that all the new
+forms in F_{2} which were bred from came true. As I understand, this
+statement applied to five such new types, and they were represented
+by 76 individuals in F_{3}, but further details on this point are
+desirable. Another curious fact was observed, namely that one of the
+F_{1} forms (_cochleata_ × _radiata_) when fertilised by _cochleata_
+gave a highly polymorphic family, but fertilised by _radiata_ the
+resulting offspring were almost uniform.
+
+[12] I also had a few F_{1} seeds given me by Mr. R. H. Lock.
+
+[13] In a paper about to appear in _Jour. Linn. Soc._ Mr. A. W. Sutton
+identifies this Palestine pea as _Pisum humile_ of Boissier and Noé.
+
+[14] Lloyd, R. E., _The Growth of Groups in the Animal Kingdom_, London,
+1912.
+
+
+
+
+                         INDEX OF SUBJECTS
+
+                                                         PAGE
+ Abraxa grossulariata,                               105,193
+ Aceras hircina, local variability,                      123
+ Achatinellidae, local forms of,                         133
+ Acquired characters, inheritance of,    188 et seq.,217,233
+ Acronycta psi, melanic,                                 138
+ Adaptation, problem of,                             187,234
+ Agelaius, local forms,                                  120
+ Agrotis, fixed and variable species,                     25
+ Alkaptonuria,                                            83
+ Alpine Plants, growing larger, if protected,            183
+ Alpine Varieties,                                       165
+ Alytes obstetricans, Kammerer's experiments on,     199,210
+ Amblystoma, races of,                                   230
+ Amphidasys betularia, melanic form,                 136,138
+   dimorphic larvae,                                     141
+ Anodonta, polymorphism of,                              130
+ Antirrhinum, striped,                                    57
+   species-hybrids,                                       99
+   albinos,                                              110
+ Apple, will not cross with pear,                        239
+ Arctia caja, effects of temperature,                    192
+   larval variation in,                                  231
+ Arctic varieties,                                       165
+ Argynnis paphia and valesina in Italy,                  121
+ Armadillo, polyembryony,                                 42
+ Artistic faculty,                                        89
+ Arum, rights and lefts,                                  57
+ Auriculas, short-styled selected,                       236
+ Axis of symmetry in hand and foot,                       48
+ Axolotl, alleged effect of conditions,                  230
+ Azalea, bud-sports,                                      55
+
+ Bacillus anthracis, unsegmented form,                    71
+ Bacillus prodigiosus, variation in,                     213
+ Bacteria, variation in,                                 212
+ Bacterium coli, variation in,                           214
+ Baeolophus, geographical races of,                      159
+ Barley, right and left-handed,                           58
+ Basilarchia, geographical races of,                     161
+ Begonia phyllomaniaca,                                   50
+   hybrids,                                               51
+ Bizarre Carnation, genetics of,                          54
+ Black, as a variation from red,                         148
+ Blackbird, varying,                                     150
+ Black Cock, fixity of,                                   28
+ Boarmia repandata, melanic form,                        136
+   rhomboidaria,                                     137,139
+ Botrytis susceptibility to,                             108
+ Bovidae, hybrid,                                        242
+ Brachydactyly,                                        89,95
+ Bradypus, vertebral variation,                           68
+ Bud-sports geometrically irregular,                   54-57
+ Buffalo, attempts to hybridize,                         242
+ Bullfinch, gynandromorph,                                45
+ Bulimus detritus, local variation of,                   126
+
+ Canary, asymmetrical markings in,                       154
+ Canidae, hybrid,                                        241
+ Capsella,                                               100
+ Cardamine pratensis,                                    239
+ Cat, Polydactylism,                                      53
+ Carnation, Picotees and bizarres compared,            54,58
+ Cataract, hereditary,                                    89
+ Certhiola, melanic,                                     142
+ Chladni figures,                                         60
+ Choloepus, vertebral variation in,                       68
+   local variation in,                                   119
+ Cinerarias, self-sterility in,                          238
+ Cistudo, local variation in,                            119
+ Climatic varieties,                                     164
+ Coccaceae, variation in,                                213
+ Coenonympha arcania, climatic forms of,                 179
+   satyrion,                                             180
+ Coereba, melanic,                                       142
+ Colaptes, geographical races,                           147 et seq.
+   chrysoides,                                           154
+ Colloids, growth in,                                     65
+ Colorado beetles, experiments on,                       218
+ Colour blindness in twins,                               44
+ Continuous variation, possible example of,              173
+ Coracias, geographical races of,                        160
+ Cotton, genetics of,                                 98,100
+ Coupling,                                               110
+ Crab, extra claws,                                       74
+ Crustacean appendages and Serial Homology,               63
+ Crystals, analogy with,                                  78
+ Cyclopian monsters, artificial,                          50
+
+ Daphnia, changed by environment,                        216
+ Dasypus, polyembryony,                                   42
+ Dianthoecia, fixed and variable species,                 25
+ Disease-resistance,                                      87
+ Division, power of,
+   a fundamental attribute of living things,              38
+   Genetics of,                                        46,50
+ Dogger Bank, large varieties on,                        125
+ Dogs, hybrid,                                           241
+ Dominance, nature of,                                    95
+ Dominants, origin of new,                          88,90,95
+ Double monsters,                                         42
+ Draba, experiments with,                                242
+ Drosophila,                                              91
+   Payne's experiments on,                               228
+
+ Earthworm, regeneration,                                 77
+ Elephant, tusk segmented,                                38
+ Entelechy,                                               80
+ Environmental treatment, effects of,                    188 et seq.
+ Enzymes and genetic factors,                             86
+ Epilepsy, inheritance of traumatic,                     197
+ Equidae, sterility of hybrid,                           241
+ Erophila, experiments with,                             242
+   species,                                              249
+ Exacum, right and left,                                  57
+ Euphonia elegantissima, local forms,                    120
+ Eupithecia rectangulata, melanic form,                  137
+
+ Factors, new,                                            88
+   loss of,                                               96
+ Factorial representation of varieties,              158,165
+ Falcons, geographical races,                            147
+ Fasciation,                                              49
+ Ferments, Boyle on,                                      54
+ Finger-prints of twins,                                  44
+ Fixity and Variability in species,                       25
+ Flax, climatic experiments,                             197
+ Fowl, Silky,                                             84
+   Leghorn,                                            85,90
+   Dominant white,                                        94
+   Wyandotte,                                             97
+   Rumpless,                                              46
+ Foxes, incompatibility with dogs,                       241
+ Free-martin,                                             44
+ Fringillidae, sterility of hybrid,                      241
+ Fundulus, cyclopian,                                     50
+
+ Gallus, invariability of wild species,                   13
+   and origin of poultry,                              90,97
+ Genitalia, a basis for classification in insects,        13
+ Gentians, climatic experiments,                         197
+ Geometrical structure and differentiation,            54,56
+ Geometrical distinction between germ-cells
+         and somatic cells,                               58
+ Gladiolus, right and left,                               57
+ Gnophus obscurata, protective colouring,                141
+ Goldfinch, geographical races,                          147
+ Gonioctena variabilis, variation in sexes of,           121
+ Gouldian Finch, polymorphism,                       148,149
+ Gracilaria stigmatella, experiments on,                 193
+ Grantia, large varieties of,                            125
+ Ground-Squirrels, local forms of,                       132
+ Grouse, red, variation,                                  29
+ Guillemot, Ringed,                                      150
+ Guinea-pig, Brown-Séquard's experiments on,             198
+ Gynandromorphs,                                          45
+
+ Heliconius erato, forms of,                         122,164
+ Helix lapicida, local variation of,                     126
+   striata,                                              127
+   Heripensis,                                           127
+   Caespitum,                                            127
+   trochoides,                                           127
+   nemoralis and hortensis,                              128
+ Helminthophila, geographical races of,                  157
+ Hemerophila abruptaria, melanic,                        142
+ Hepialus humuli, in Shetland,                           119
+ Heterostyle plants,                                     236
+ Hieracium,                                                9
+ Himantopus,                                             234
+ Homoeosis,                                               68
+ Hybernia progemmaria,                                   139
+ Hybrids, sterility of,                                  233 et seq.
+
+ Incompatibility between certain allied species,         239
+ Individual, geometrical independence of,                 58
+ Inhibiting Factors,                                      95
+ Intermediates, nature of,                           131,135
+ Isolation, consequences of,                             118
+
+ Lacerta muralis, Kammerer's experiments on,             209
+   fiumana,                                              210
+ Leptinotarsa, Power's experiments on,                   218
+ Limbs, extra, in pairs,                                  72
+ Limnaea, sinistral,                                     134
+ Linaria vulgaris, self-sterility,                       239
+ Loasa fruits, right and left,                            57
+ Lobster, extra claws,                                    76
+ Locality, variation connected with,  14,118,146 et seq.,208
+ Lumbricus, regeneration,                                 77
+ Lychnis dioica and vespertina, inter-relations of,       18
+   macrocarpa, possibly a common parent of,               19
+
+ Machetes pugnax, polymorphism of male,                   28
+ Maize, Blaringhem's experiments on,                     229
+ Maize, cumulative factors in,                           116
+ Malformations, dominants, arising de novo,               89
+ Manx Cat, heredity,                                      46
+ Matthiola,                                       84,104,113
+ Melanic varieties,                                      135 et seq.
+ Memory, analogy with heredity,                          190
+ Meristic variation,                                69,83,86
+ Mirabilis, striped,                                      57
+ Models of segmentation,                               59,60
+ "Modes," Coutagne's conception of,                       126
+ Mödling, peculiar race of _Pieris napi_ at,        178
+ Mole, albino,                                         27,28
+ Mule, Linnaeus on,                                        8
+ Mutation, Matthioli on,                                   4
+   in Mercurialis,                                         5
+   in Kales,                                               5
+   alleged in bulbs,                                       5
+   Theory,                                                97
+   periods of,                                           114
+   in Bacteria,                                          214
+ Mutilation, consequences of,                             71
+   alleged effect of, on offspring,                      229
+ Myxococcus, variation in,                               213
+
+ Narwhal, asymmetry of tusks,                             44
+ Nemesia strumosa,                                        91
+ Neuration, a basis for classification,                   13
+ Nicotiana, sterility of hybrid,                         242
+ Nightjars, varying,                                     150
+ Noctuidae, fixity and variability,                       25
+ Noctua, polymorphic and fixed species,                   25
+ Noctua castanea, local forms of,                        122
+ Nomenclature, future of,                             94,245
+ Notonecta, variations of,                               130
+
+ Odontoptera bidentata, melanic form,                    137
+ Oedipodidae, protectively coloured,                     140
+ Oenothera, new dominant in,                              92
+   rubricalyx and rubrinervis,                         92,95
+   Lamarckiana,                                       92,101
+   origin of,                                        102,244
+   has bad pollen-grains,                                102
+   factorial analysis of,                                103
+   pollen and egg-cells genetically dissimilar,          104
+ Oenothera, "twin hybrids",                              105
+   laeta and velutina,                                   105
+   reciprocal crosses in,                                105 et seq.
+   possible coupling in,                                 111
+   dwarfs,                                           112,114
+  "Triple hybrids",                                      114
+   alleged variation due to treatment,                   227
+ Ophrys, local variability,                              125
+ Orange, polyembryony,                                    45
+ Osmotic growth,                                          65
+ Overlapping forms,                                  146,174
+
+ Papilio, geographical races of,                         162
+ Papilio turnus, variation of,                           144
+ Pararge egeria, geographical forms,                     166 et seq.
+ Parthenogenesis,                                         50
+ Partula, local forms of,                                133
+ Passer domesticus and montanus, distinctions,            22
+ Pea, round and wrinkled,                                 95
+ Pear, will not cross with apple,                        239
+ Pelargonium, variegated,                                 55
+   bud-sports,                                            56
+ Periodic phenomena in structure,                         63
+ Peronea, fixed and variable species,                     26
+ "Petites espèces",                                       248
+ Petunia, double,                                        104
+ Phalanger maculatus, local variation,                   119
+ Pheasant, fixity of,                                     29
+ Phigalia pilosaria, melanic,                        139,140
+ Phratora vitellinae, experiments on,                    193
+ Phyllotaxis,                                             69
+ Pied varieties common in Passer domesticus
+   unknown in Montanus,                                   23
+ Pieris napi and bryoniae,                               174 et seq.
+ Pig, mule-footed,                                        46
+ Pigeon, web-footed,                                   46,49
+   Indian Rock, a recessive form,                         98
+ Pigments, nature of,                                     83
+ Pisum humile, hybrids with culinary peas,               244
+   species,                                              246
+ Planarian, regeneration of,                           71,77
+ Plotheia frontalis, polymorphic,                      26,29
+ Plusia, fixity and variation in,                         26
+ Poephila gouldiae, variation of,                    148,149
+ Polarity of individual,                                  44
+ Polia chi, melanic,                                     138
+ Polyanthus, short-styled selected,                      236
+ Polydactylism in Cat,                                 52,53
+ Polyembryony,                                            45
+ Potato, variation in,                                    91
+ Poultry, evolution of,                                   90
+ Primula obconica,                                        91
+ Primula sinensis, flaked,                                57
+   Leaf-shapes,                                           70
+   new dominant in,                                       92
+   sterility in,                                         236
+  "Giants",                                              236
+ Primula, species-hybrids,                               242
+ Protective coloration,                                  140
+ Pyrrhulagra, local forms,                               120
+ Python, twin-vertebrae,                                  60
+
+ Quiscalus, geographical races of,                       156
+
+ Rabbit, Angora,                                          46
+   colours of,                                            93
+   Incompatibility with hare,                            242
+ Raimannia odorata, Macdougal's experiments on,          226
+ Rats, Variation in,                                     248
+ Recessives, origin of,                                   90
+ Reciprocal crosses, giving distinct results,            105 et seq.
+ Regeneration,                                            70
+ Repulsion,                                              110
+ Reversal on Regeneration,                                77
+ Rhamphocoelus, geographical forms,                  159,184
+ Rhinoptera, variation in jaws of,                        38
+ Rhythm in repetition,                                    69
+ Ribs, variation of,                                      68
+ Rights and Lefts,                                     57-58
+ Ripples, analogous to segments,                    60,66,67
+   regeneration of,                                       79
+ Rollers, geographical races of,                         160
+ Ruff, polymorphism of male,                              28
+
+ Salamandra, maculosa and atra,                  182,199,203
+   spotted and striped,                                  207
+   geographical variation of,                            208
+ Segmentation, nature of,                                 63
+   simulated mechanically,                                64
+   compared with rippling,                                65
+   analogies with,                                        68
+ Segmentation of normally unsegmented structures,         38
+ Selection, Natural, an insufficient
+         cause of definiteness of types,          17,134,142
+ Sempervivum,                                            250
+ Serial Homology, the true nature of,                  62,66
+ Setina, Alpine varieties,                               181
+ Sex of Twins,                                            44
+ Sex-factors, possible coupling of,                      111
+ Sexual characters, variation in,                        119 et seq.
+ Siamese twins,                                           44
+ Silky Fowl,                                           84,85
+ Simocephalus, changed by environment,                   218
+ Sinistral forms,                                      33-34
+ Situs transversus,                                       43
+ Skate's jaws, variation in,                              38
+ Sloths, vertebral variation,                             68
+ Species, conceptions of,                    3,94,99,240,245
+   allied, distribution of,                              185
+   alternative uses of the term,                         245
+ Specific difference, universality of,                    12
+   of organisms compared with those
+       of inorganic materials,                            15
+   failure of theory of Selection
+       to explain,                                18,134,247
+ Sphyropicus varius,                                 149,156
+ Spilosoma lubricipeda, varieties of,                    181
+   Zatima, Heligoland form,                              181
+ Spinal nerves, segmentation of,                          67
+ Sporadic variation,                             131,134,248
+ Squashes, polymorphism of,                              100
+ Staphylococcus pyogenes, variation in,                  213
+ Sterility of hybrids, in general,                       233
+   in Lychnis hybrids,                                    20 et seq.
+   in crossing forms of Draba,                           243
+   Significance of,                                      244
+   Self,                                                 238
+ Stilt,                                                  234
+ Stocks,                                          84,104,113
+ Striped varieties,                                       57
+ Substantive variation,                                   84
+ Subtraction-stages,                                      93
+ Supernumerary limbs,                                  72-76
+ Sweet pea, variation of,                                 91
+   sterile anthers in,                                   237
+ Symmetry compared with heredity,                         41
+ Symmetry of body approximate,                            78
+ Syndactyly,                                              47
+   in foot,                                               48
+ Synthetic formulae, in nomenclature,                     94
+
+ Taeniocampa, fixed and variable species,                 25
+ Tamias, local forms of,                                 132
+ Tanagers, geographical races of,                        159
+ Teeth, variation in,                                  67,39
+ Tephrosia consortaria and consonaria,           137,139,140
+ Tephrosia species, separated by season,                 119
+ Terminal members, variation of,                          68
+ Thais rumina, local variation in,                        27
+ Tolerance, persistence of diversity due to,          17,134
+ Tomato, number of cells in fruit,                        46
+ Transitional populations, rarity of,                    165
+   an example,                                           178
+ Tropaeolum, sterile anthers in,                         237
+ Trypanosomes, variation in,                             215
+ Tusk, of Elephant, segmented,                            38
+   of Narwhal,                                            44
+ Twinning,                                          41,44,71
+   heredity of,                                           45
+   in organs,                                             46
+
+ Uria troile, variety of,                                150
+
+ Vanessa urticae, effects of temperature,                191
+ Variation, a medley of phenomena,                     14,15
+   sporadic,                                         131,134
+   and locality,                                         118
+   Causes of genetic,                          86,87,131,212
+   Substantive and meristic,                              83
+ Veronica, specific difference in,                        16
+   intermediates between species,                         17
+ Vertebrae, division in,                               60,61
+   homologies of,                                         66
+ Vespa, specific difference in,                           23
+ Vortex, living organism compared with,                   40
+
+ Wave-motion compared with repetition of parts,     62,67,79
+ Wheat, cumulative factors in,                           116
+   climatic experiments on,                              195
+ Woodpecker,                                             234
+
+ Zebra, pattern of stripes compared with ripples,         38
+
+
+
+
+                INDEX OF PERSONS
+
+                                         PAGE
+  Ackermann,                              242
+  Agar,                                   218
+  Allen, J. A.,                   132,147,159
+  Annandale,                               47
+  Arrigoni degli Oddi,                    167
+
+  Backhouse,                               50
+  Baker, G. T.,                           166
+  Bangs, Outram,                  120,142,155
+  Barrett,             26,136,167,173,178,193
+  Baur, E.,                             55,99
+  Baur, G.,                               119
+  Beneden, van,                            75
+  Bentham, on species of Veronica,         16
+    Lychnis,                               21
+    Primula,                               22
+  Bernadin,                                42
+  Bishop, L. B.,                      153,157
+  Blaringhem,                             229
+  Bobart,                                   5
+  Boisduval,                              182
+  Boissier,                                19
+  Borradaile,                           74,75
+  Boulenger, E. G.,                       208
+  Boulenger, G. A.,               182,207,209
+  Boyle,                                 5,54
+  Brewster, W.,                       149,150
+  Britton,                                227
+  Brown, T. Graham,                       198
+  Brown-Séquard,                          197 et seq.
+  Bruant, P.,                              51
+  Buffon,                                 234
+  Butler, S.,                         189,190
+  Buysson, R. du,                          24
+
+  Candolle, de,                           245
+  Carpenter, J. H.,                       172
+  Chapman, F. M.,             148,156,157,158
+  Chapman, T. A.,              13,167,182,231
+  Church, A. H.,                           69
+  Cieslar,                                197
+  Clark, Austin,                      142,144
+  Cockayne, E. A.,                         43
+  Cockerell, T. D. A.,                    224
+  Compton, R. H.,                   50,58,227
+  Cope,                                   230
+  Cory,                                   142
+  Correns,                                239
+  Coutagne,                               125 et seq.
+
+  Darwin, on Variation,                   1,2
+    Systematics,                           10
+    Selection,                        134,139
+    Heterostyle plants,               236,237
+  Darwin, F.,                             190
+  Darwin, Sir G.,                          41
+  Davenport,                               46
+  Davis, H. M.,                           102
+  Delcourt,                               130
+  Deschange,                              181
+  Dobell,                                 215
+  Doncaster,                      105,121,136
+  Driesch,                              80,81
+  Duchartre,                               51
+
+  East,                                91,116
+  Edwards, W. H.,                         162
+  Ehrlich,                                215
+
+  Fellmer,                                215
+  Field, W. L. W.,                        161
+  Fischer, E.,                            192
+  Fleck,                              171,174
+  Fletcher, W. H. B.,                  26,181
+  Foster, Sir N.,                          39
+
+  Gallé,                                  123
+  Garrod,                                  83
+  Gates,                            92,95,102
+  Gayner, F.,                             177
+  Godron,                                 249
+  Gold, E.,                               196
+  Goldschmidt,                            116
+  Goodwin, E.,                            137
+  Gortner,                                226
+  Greene, E. L.,                            8
+  Gregory, R. P.,                  92,100,236
+  Grenier,                                249
+  Grover,                                 173
+  Gruber,                                  48
+  Gulick,                             119,133
+
+  Hamling,                                142
+  Hampson, Sir G.,                         26
+  Harris,                                 142
+  Hartlaub,                               182
+  Herbst,                                  42
+  Heribert-Nilsson,                       116
+  Hewett,                                 182
+  Honing,                                 105
+  Hunter, John,                            44
+
+  Jakowatz,                               197
+  Janet,                                   24
+  Jeans,                                   41
+  Jenkinson,                               40
+  Jentink,                                120
+  Johannsen,                              195
+  Jordan,                         185,242,249
+
+  Kammere,                                199 et seq.
+  Keeble,                                 236
+  Klebs,                                  250
+  Krancher,                               182
+  Küchenmeister,                           44
+  Kudicke,                                215
+
+  Lamarck,                                  9
+  Lang, A.,                               128
+  Lawrence, W. N.,                    142,145
+  Leake, H. Martin,                    98,100
+  Leavitt,                                185
+  Lecoq,                                   99
+  Lederer,                                167
+  Leduc,                             64,65,80
+  Leydig,                                 182
+  Linden, M. von,                         192
+  Linnaeus,                             6,7,8
+  Lloyd, R. E.,                           248
+  Locard,                                 130
+  Lock, R. H.,                        242,244
+  Loeb,                        42,45,50,71,77
+  Lotsy,                                   99
+  Lowe, P. R.,                            143
+
+  Macdougal, W. T.,                   102,226
+  Marchant,                                 7
+  Mathew,                                 171
+  Matthioli,                                4
+  Mayer, A. G.,                           133
+  Mendel, Rediscovery of,                   2
+    On Fasciation,                         49
+  Merrifield, 169,                        172
+  Miller, W. D.,                      120,149
+  Morgan,                        71,77,91,198
+  Moggridge,                              125
+
+  Nathusius, S.,                          242
+  Nettleship.,                             44
+  Newman, H. H.,                           42
+  Newsholme,                               48
+  Nilsson-Ehle,                       116,169
+  Norman, A. M.,                      125,156
+
+  Ober,                                   142
+  Oberthür,                       168,170,193
+  Oliver, J.,                              45
+
+  Page, H. E.,                        167,180
+  Patterson, J. T.,                        42
+  Payne, F.,                              278
+  Pellew,                                 236
+  Poll,                                    45
+  Porritt,                                136
+  Poulton,                                141
+  Powers, J. H.,                          230
+  Pringsheim, H.,                         213
+  Przibram,             72,78,178,194,197,199
+  Punnett,                                110
+
+  Ray,                                    4,5
+  Raynor,                                 105
+  Ridgway,                             10,120
+  Roedelius,                              195
+  Rolfe,                                   20
+  Rosen, F.,                              242
+  Rosner,                                  42
+  Rowland-Brown, H.,                  167,180
+
+  Sargent,                                185
+  Saunders, E. R.,                 84,104,112
+  Schima,                                 177
+  Schröder,                           193,194
+  Schübeler,                              195
+  Semon, R.,                              190 et seq.
+  Sharrock,                                 5
+  Shull,                                  100
+  Speyer, A.,                     166,170,181
+  Spillman,                                47
+  Standfuss,                      135,181,191
+  Staples-Browne,                       49,98
+  Staudinger,                         170,179
+  Stockard,                             50,71
+  Sutton,                             236,244
+
+  Tornier,                                 72
+  Tower, W. L.,                       218-226
+  Trechmann,                              133
+  Tugwell,                                181
+  Tutt, J. W. On Definiteness of Species,  13
+    On Plusia interrogationis,             26
+    On Tephrosia,                         119
+    On N. castanea,                       122
+    On Pararge egeria,                    167 et seq.
+
+  Verity, R.,                         171,177
+  Vries, H. de,               101-115,222,239
+
+  Walker, G,                               49
+  Weir, Jenner,                           119
+  Weismann,                           176,188
+  Wendelstadt,                            215
+  Werbitzki,                              215
+  Werner,                                 209
+  Wettstein,                              197
+  Wheeler, G.,                        168,171
+  Wheldale,                                83
+  Wilder,                                  44
+  Wille,                                  197
+  Williams, H.,                       167,172
+  Windle, B. C. A.,                        43
+  Winslow,                                213
+  Wolf, F.,                               213
+  Woodforde,                              123
+  Woltereck,                              215
+
+  Zeijlstra,                              114
+
+
+
+
+
+End of the Project Gutenberg EBook of Problems of Genetics, by William Bateson
+
+*** END OF THE PROJECT GUTENBERG EBOOK 44582 ***
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+&emsp;Cambridge, Cavendish Professor of Experimental Physics, Cambridge</i>.<br />
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+
+<p><b>THE INTEGRATIVE ACTION OF THE NERVOUS SYSTEM.</b><br />
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+&emsp;<span style="margin-left: 0.5em;">d.sc., m.d., hon. ll.d., tor., f.r.s.</span>,</span><br />
+&emsp;<i>Holt Professor of Physiology in the University of Liverpool</i>.<br />
+<i>Price $3.50 net; postage 25 cents extra.</i></p>
+
+<p><b>RADIOACTIVE TRANSFORMATIONS</b>. <i>By</i> <span class="smcap">Ernest Rutherford,<br />
+&emsp;d.sc., ll.d., f.r.s.</span>, <i>Macdonald Professor of Physics, McGill University</i>.<br />
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+&emsp;<i>By</i> <span class="smcap">Dr. Walther Nernst,</span><br />
+&emsp;<i>Professor and Director of the Institute of Physical Chemistry
+&emsp;in the University of Berlin</i>.<br />
+<i>Price $1.25 net; postage 10 cents extra.</i></p>
+
+<p><b>THE PROBLEMS OF GENETICS</b>. <i>By</i> <span class="smcap">William Bateson, m.a.,<br />
+&emsp;f.r.s.</span>, <i>Director of the John Innes Horticultural Institution,<br />
+&emsp;Merton Park, Surrey, England</i>.<br />
+<i>Price $4.00 net; postage 25 cents extra.</i></p>
+
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+&emsp;Wallace Campbell, sc.d., ll.d.</span>,<br />
+&emsp;<i>Director of the Lick Observatory, University of California</i>.<br />
+<i>Price $4.00 net; postage 30 cents extra.</i></p>
+
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+&emsp;ph.d., sc.d., m.d.</span>, <i>Director of the Physico-Chemical Department<br />
+&emsp;of the Nobel Institute, Stockholm, Sweden</i>.<br />
+<i>Price $2.25 net; postage 15 cents extra.</i></p>
+
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+&emsp;Effect of Stimuli in Living Substances</span>.<br />
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+&emsp;<i>Professor at Bonn Physiological Institute</i>.<br />
+<i>Price $3.50 net; postage 20 cents extra.</i></p>
+
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+&emsp;<i>By</i> <span class="smcap">Sir William Osler, Bart., m.d., ll.d., sc.d.</span>,<br />
+&emsp;<i>Regius Professor of Medicine, Oxford University</i>.<br />
+<i>Price $3.00 net; postage 40 cents extra.</i></p>
+</div></div>
+
+<p style="font-size: 150%; text-align: center;"><b>PROBLEMS OF GENETICS</b></p>
+<p style="font-size: 90%; text-align: center;">BY</p>
+<p style="font-size: 150%; text-align: center;"><b><span class="smcap">William Bateson, m.a., f.r.s.</span></b></p>
+
+<p class="space-above" style="font-size: 90%; text-align: center;">
+DIRECTOR OF THE JOHN INNES HORTICULTURAL INSTITUTION,
+HON. FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE,
+AND FORMERLY PROFESSOR OF BIOLOGY IN THE UNIVERSITY</p>
+
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+
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+</div>
+
+<p class="space-above" style="font-size: 110%; text-align: center;">
+<span class="smcap">New Haven: Yale University Press</span><br />
+<span class="smcap">London: Humphrey Milford</span><br />
+<span class="smcap">Oxford University Press</span></p>
+
+<p class="space-above" style="font-size: 130%; text-align: center;"><b>MCMXIII</b></p>
+
+<hr class="r5" />
+<p class="center">Copyright, 1913<br />
+By <span class="smcap">Yale University</span></p>
+
+<p class="center space-above space-below">First printed August, 1913, 1000 copies</p>
+
+<hr class="chap" />
+<p style="font-size: 150%; text-align: center;"><b>THE SILLIMAN FOUNDATION</b></p>
+
+<p class="indent">In the year 1883 a legacy of about eighty-five thousand
+dollars was left to the President and Fellows of Yale College
+in the city of New Haven, to be held in trust, as a gift from her
+children, in memory of their beloved and honored mother, Mrs.
+Hepsa Ely Silliman.</p>
+
+<p class="indent">On this foundation Yale College was requested and directed
+to establish an annual course of lectures designed to illustrate
+the presence and providence, the wisdom and goodness of God,
+as manifested in the natural and moral world. These were to be
+designated as the Mrs. Hepsa Ely Silliman Memorial Lectures.
+It was the belief of the testator that any orderly presentation
+of the facts of nature or history contributed to the end of this
+foundation more effectively than any attempt to emphasize the
+elements of doctrine or of creed; and he therefore provided that
+lectures on dogmatic or polemical theology should be excluded
+from the scope of this foundation, and that the subjects should be
+selected rather from the domains of natural science and history,
+giving special prominence to astronomy, chemistry, geology, and anatomy.</p>
+
+<p class="indent">It was further directed that each annual course should
+be made the basis of a volume to form part of a series constituting a
+memorial to Mrs. Silliman. The memorial fund came into the
+possession of the Corporation of Yale University in the year 1901;
+and the present volume constitutes the fifth of the series of
+memorial lectures.</p>
+
+<p><span class="pagenum"><a name="Page_i" id="Page_i">[Pg i]</a></span></p>
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_ii" id="Page_ii">[Pg ii]</a></span></p>
+
+<h2>PREFACE</h2>
+
+<p class="indent">This book gives the substance of a series of lectures delivered
+in Yale University, where I had the privilege of holding the office
+of Silliman Lecturer in 1907.</p>
+
+<p class="indent">The delay in publication was brought about by a variety of causes.</p>
+
+<p class="indent">Inasmuch as the purpose of the lectures is to discuss
+some of the wider problems of biology in the light of knowledge acquired
+by Mendelian methods of analysis, it was essential that a fairly
+full account of the conclusions established by them should first
+be undertaken and I therefore postponed the present work till
+a book on Mendel's Principles had been completed.</p>
+
+<p class="indent">On attempting a more general discussion of the
+bearing of the phenomena on the theory of Evolution, I found myself
+continually hindered by the consciousness that such treatment
+is premature, and by doubt whether it were not better that the
+debate should for the present stand indefinitely adjourned.
+That species have come into existence by an evolutionary
+process no one seriously doubts; but few who are familiar with
+the facts that genetic research has revealed are now inclined to
+speculate as to the manner by which the process has been
+accomplished. Our knowledge of the nature and properties of
+living things is far too meagre to justify any such attempts.
+Suggestions of course can be made: though, however, these
+ideas may have a stimulating value in the lecture room, they
+look weak and thin when set out in print. The work which may
+one day give them a body has yet to be done.</p>
+
+<p class="indent">The development of negations is always an ungrateful
+task apt to be postponed for the positive business of experiment.
+Such work is happily now going forward in most of the centers
+of scientific life. Of many of the subjects here treated we already
+know more than we did in 1907. The delay in production has
+made it possible to incorporate these new contributions.</p>
+
+<p class="indent">The book makes no pretence at being a treatise and the
+<span class="pagenum"><a name="Page_iii" id="Page_iii">[Pg iii]</a></span>
+number of illustrative cases has been kept within a moderate
+compass. A good many of the examples have been chosen from
+American natural history, as being appropriate to a book intended
+primarily for American readers. The facts are largely
+given on the authority of others, and I wish to express my
+gratitude for the abundant assistance received from American
+colleagues, especially from the staffs of the American Museum
+in New York, and of the Boston Museum of Natural History.
+In connexion with the particular subjects personal acknowledgments
+are made.</p>
+
+<p class="indent">Dr. F. M. Chapman was so good as to supervise the preparation
+of the coloured Plate of <i>Colaptes</i>, and to authorize the loan
+of the Plate representing the various forms of <i>Helminthophila</i>,
+which is taken from his <i>North American Warblers</i>.</p>
+
+<p class="indent">I am under obligation to Messrs. Macmillan &amp; Co.,
+for permission to reproduce several figures from <i>Materials for the Study
+of Variation</i>, illustrating subjects which I wished to treat in
+new associations, and to M. Leduc for leave to use Fig. 9.</p>
+
+<p class="indent">In conclusion I thank my friends in Yale for the high
+honour they did me by their invitation to contribute to the series of
+Silliman Lectures, and for much kindness received during a
+delightful sojourn in that genial home of learning.
+<span class="pagenum"><a name="Page_iv" id="Page_iv">[Pg iv]</a></span></p>
+<hr class="chap" />
+<p class="u" style="font-size: 150%; text-align: center;"><b>TABLE OF CONTENTS</b>.</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="TOC" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr"><b>CHAPTER</b></td>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdr"><b>PAGE</b></td>
+    </tr><tr>
+      <td class="tdr"><b>I.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Introductory. The Problem of Species and Variety</span></b>&nbsp;&nbsp;</td>
+      <td class="tdr"><a href="#Page_1">&nbsp;&nbsp;1</a></td>
+    </tr><tr>
+      <td class="tdr"><b>II.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Meristic Phenomena</span></b></td>
+      <td class="tdr"><a href="#Page_31">&nbsp;31</a></td>
+    </tr><tr>
+      <td class="tdr"><b>III.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Segmentation, Organic and Mechanical</span></b></td>
+      <td class="tdr"><a href="#Page_60">&nbsp;60</a></td>
+    </tr><tr>
+      <td class="tdr"><b>IV.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">The Classification of Variation and the Nature</span></b></td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;<b><span class="smcap">of Substantive Variation</span></b></td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Note to Chapter IV</span></b></td>
+      <td class="tdr"><a href="#Page_94">&nbsp;94</a></td>
+    </tr><tr>
+      <td class="tdr"><b>V.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">The Mutation Theory</span></b></td>
+      <td class="tdr"><a href="#Page_97">&nbsp;97</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Note to Chapter V</span></b></td>
+      <td class="tdr"><a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdr"><b>VI.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Variation and Locality</span></b></td>
+      <td class="tdr"><a href="#Page_118">118</a></td>
+    </tr><tr>
+      <td class="tdr"><b>VII.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Local Differentiation</span>&mdash;<i>continued</i>.</b></td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;<b><span class="smcap">Overlapping Forms</span></b></td>
+      <td class="tdr"><a href="#Page_146">146</a></td>
+    </tr><tr>
+      <td class="tdr"><b>VIII.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Locally Differentiated Forms</span>&mdash;<i>continued</i>.</b></td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b>&emsp;&emsp;<span class="smcap">Climatic Varieties</span></b></td>
+      <td class="tdr"><a href="#Page_164">164</a></td>
+    </tr><tr>
+      <td class="tdr"><b>IX.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">The Effects of Changed Conditions</span></b></td>
+      <td class="tdr"><a href="#Page_187">187</a></td>
+   </tr><tr>
+      <td class="tdr"><b>X.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">The Effects of Changed Conditions</span>&mdash;<i>continued</i>.</b></td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b>&emsp;&emsp;<span class="smcap">The Causes of Genetic Variation</span></b></td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+   </tr><tr>
+      <td class="tdr"><b>XI.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">The Sterility of Hybrids. Concluding Remarks</span></b></td>
+      <td class="tdr"><a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Concluding Remarks</span></b></td>
+      <td class="tdr"><a href="#Page_248">248</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Index of Subjects</span></b></td>
+      <td class="tdr"><a href="#Page_251">251</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Index of Persons</span></b></td>
+      <td class="tdr"><a href="#Page_252">252</a></td>
+    </tr>
+ </tbody>
+</table>
+
+<hr class="chap" />
+<p class="u" style="font-size: 150%; text-align: center;"><b>LIST OF ILLUSTRATIONS</b>.</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="LOC" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdl"><b><span class="smcap">Figure</span></b></td>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdr"><b><span class="smcap">Page</span></b></td>
+    </tr><tr>
+      <td class="tdr"><b>1.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Tusk of Indian Elephant.</b></td>
+      <td class="tdr"><a href="#Page_37">&nbsp;37</a></td>
+    </tr><tr>
+      <td class="tdr"><b>2.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Jaws of Skates.</b></td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdr"><b>3.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Syndactyly of Human Hand and Foot.</b>&nbsp;&nbsp;</td>
+      <td class="tdr"><a href="#Page_47">&nbsp;47</a></td>
+    </tr><tr>
+      <td class="tdr"><b>4.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Syndactyly of the Human Foot.</b></td>
+      <td class="tdr"><a href="#Page_48">&nbsp;48</a></td>
+    </tr><tr>
+      <td class="tdr"><b>5.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Petiole of</b> <i>Begonia phyllomaniaca</i>.</td>
+      <td class="tdr"><a href="#Page_51">&nbsp;51</a></td>
+    </tr><tr>
+      <td class="tdr"><b>6.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Feet of Polydactyle Cats.</b></td>
+      <td class="tdr"><a href="#Page_52">&nbsp;52</a></td>
+    </tr><tr>
+      <td class="tdr"><b>7.</b> &amp; <b>8.</b></td>
+      <td class="tdl">&nbsp;<b>Vertebrae of Python.</b></td>
+      <td class="tdr"><a href="#Page_61">&nbsp;61</a></td>
+    </tr><tr>
+      <td class="tdr"><b>9.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Osmotic Growths.</b></td>
+      <td class="tdr"><a href="#Page_64">&nbsp;64</a></td>
+    </tr><tr>
+      <td class="tdr"><b>10.&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Leaf type in</b> <i>Primula sinensis</i>.</td>
+      <td class="tdr"><a href="#Page_70">&nbsp;70</a></td>
+    </tr><tr>
+      <td class="tdr"><b>11.&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Geometrical relations in</b> <i>Arthropoda</i>.</td>
+      <td class="tdr"><a href="#Page_73">&nbsp;73</a></td>
+    </tr><tr>
+      <td class="tdr"><b>12.&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Right claw of Lobster.</b></td>
+      <td class="tdr"><a href="#Page_76">&nbsp;76</a></td>
+    </tr><tr>
+      <td class="tdr"><b>13.</b>&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl">&nbsp;<b>Forms of</b> <i>Aceras hircina</i>.</td>
+      <td class="tdr"><a href="#Page_124">124</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&nbsp;<b>Variation in Warblers.</b></td>
+      <td class="tdr"><a href="#Page_159">159</a></td>
+    </tr>
+ </tbody>
+</table>
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
+
+<p style="font-size: 150%; text-align: center;"><b>PROBLEMS OF GENETICS</b></p>
+<hr class="r5" />
+<h2>CHAPTER I</h2>
+<h3><span class="smcap">Introductory</span></h3>
+
+<p class="indent">The purpose of these lectures is to discuss some of
+the familiar phenomena of biology in the light of modern discoveries.
+In the last decade of the nineteenth century many of us perceived
+that if any serious advance was to be made with the group
+of problems generally spoken of as the Theory of Evolution,
+methods of investigation must be devised and applied of a kind
+more direct and more penetrating than those which after the
+general acceptance of the Darwinian views had been deemed
+adequate. Such methods obviously were to be found in a
+critical and exhaustive study of the facts of variation and heredity,
+upon which all conceptions of evolution are based. To construct
+a true synthetic theory of Evolution it was necessary that variation
+and heredity instead of being merely postulated as axioms
+should be minutely examined as phenomena. Such a study
+Darwin himself had indeed tentatively begun, but work of a
+more thorough and comprehensive quality was required. In
+the conventional view which the orthodoxy of the day prescribed,
+the terms variation and heredity stood for processes so vague
+and indefinite that no analytical investigation of them could be
+contemplated. So soon, however, as systematic inquiry into
+the natural facts was begun it was at once found that the accepted
+ideas of variation were unfounded. Variation was seen
+very frequently to be a definite and specific phenomenon, affecting
+different forms of life in different ways, but in all its
+diversity showing manifold and often obvious indications of
+regularity. This observation was not in its essence novel.
+Several examples of definite variation had been well known to
+<span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span>
+Darwin and others, but many, especially Darwin himself in his
+later years, had nevertheless been disposed to depreciate the
+significance of such facts. They consequently then lapsed into
+general disparagement. Upon more careful inquiry the abundance
+of such phenomena proved to be far greater than was
+currently supposed, and a discussion of their nature brought
+into prominence a consideration of greater weight, namely
+that the differences by which these definite or discontinuous
+variations are constituted again and again approximate to and
+are comparable with the class of differences by which species
+are distinguished from each other.</p>
+
+<p class="indent">The interest of such observations could no longer be
+denied. The more they were examined the more apparent it became that
+by means of the facts of variation a new light was obtained on
+the physiological composition and capabilities of living things.
+Genetics thus cease to be merely a method of investigating
+theories of evolution or of the origin of species but provide a
+novel and hitherto untried instrument by which the nature of
+the living organism may be explored. Just as in the study of
+non-living matter science began by regarding the external
+properties of weight, opacity, colour, hardness, mode of occurrence,
+etc., noting only such evidences of chemical attributes
+and powers as chance spontaneously revealed; and much later
+proceeded to the discovery that these casual manifestations of
+chemical properties, rightly interpreted, afford a key to the
+intrinsic nature of the diversity of matter, so in biology, having
+examined those features of living things which ordinary observations
+can perceive, we come at last to realize that when studied
+for their own sake the properties of living organisms in respect
+of heredity and variation are indications of their inner nature
+and provide evidences of that nature which can be obtained from
+no other source.</p>
+
+<p class="indent">While such ideas were gradually forming in our minds,
+came the rediscovery of Mendel's work. Investigations which before
+had only been imagined as desirable now became easy to pursue,
+and questions as to the genetic inter-relations and compositions
+of varieties can now be definitely answered. Without prejudice
+<span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span>
+to what the future may disclose whether by way of limitation
+or extension of Mendelian method, it can be declared with
+confidence and certainty that we have now the means of beginning
+an analysis of living organisms, and distinguishing many
+of the units or factors which essentially determine and cause the
+development of their several attributes.</p>
+
+<p class="indent">Briefly put, the essence of Mendelism lies in the
+discovery of the existence of unit characters or factors. For an
+account of the Mendelian method, how it is applied and what it has
+already accomplished, reference must be made to other works.<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a>
+With this part of the subject I shall assume a sufficient acquaintance.
+In these lectures I have rather set myself the task
+of considering how certain problems appear when viewed from
+the standpoint to which the application of these methods has
+led us. It is indeed somewhat premature to discuss such questions.
+The work of Mendelian analysis is progressing with
+great rapidity and anything I can say may very soon be superseded
+as out of date. Nevertheless a discussion of this kind
+may be of at least temporary service in directing inquiry to the
+points of special interest.</p>
+
+<h3><span class="smcap">The Problem of Species and Variety</span></h3>
+
+<p class="indent">Nowhere does our new knowledge of heredity and variation
+apply more directly than to the problem what is a species and
+what is a variety? I cannot assert that we are already in a
+position to answer this important question, but as will presently
+appear, our mode of attack and the answers we expect to receive
+are not those that were contemplated by our predecessors.
+If we glance at the history of the scientific conception of Species
+we find many signs that it was not till comparatively recent
+times that the definiteness of species became a strict canon of
+the scientific faith and that attempts were made to give precise
+limits to that conception. When the diversity of living things
+began to be accurately studied in the sixteenth and seventeenth
+<span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span>
+centuries names were applied in the loosest fashion, and in giving
+a name to an animal or a plant the naturalists of those times had
+no ulterior intention. Names were bestowed on those creatures
+about which the writer proposed to speak. When Gesner or Aldrovandi
+refer to all the kinds of horses, unicorns, dogs, mermaids,
+etc., which they had seen or read of, giving to each a descriptive
+name, they do not mean to "elevate" each named kind to "specific
+rank"; and if anyone had asked them what they meant by
+a species, it is practically certain that they would have had not
+the slightest idea what the question might imply, or any suspicion
+that it raised a fundamental problem of nature.</p>
+
+<p class="indent">Spontaneous generation being a matter of daily observation,
+then unquestioned, and supernatural events of all kinds being
+commonly reported by many witnesses, transmutation of species
+had no inherent improbability. Matthioli,<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a> for instance, did not
+expect to be charged with heresy when he declared <i>Stirpium
+mutatio</i> to be of ordinary occurrence. After giving instances
+of induced modifications he wrote, "Tantum enim in plantis
+naturae germanitas potest, ut non solum saepe praedictos
+praestet effectus, sed etiam ut alteram in alteram stirpem facile
+vertat, ut cassiam in cinnamomum, sisymbrium in mentham,
+triticum in lolium, hordeum in avenam, et ocymum in serpyllum."</p>
+
+<p class="indent">I do not know who first emphasized the need for a
+clear understanding of the sense in which the term species is to be
+applied. In the second half of the seventeenth century Ray
+shows some degree of concern on this matter. In the introduction
+to the <i>Historia Plantarum</i>, 1686, he discusses some
+of the difficulties and lays down the principle that varieties
+which can be produced from the seed of the same plant are to
+be regarded as belonging to one species, being, I believe, the
+first to suggest this definition. That new species can come into
+existence he denies as inconsistent with Genesis 2, in which it is
+declared that God finished the work of Creation in six days.
+Nevertheless he does not wholly discredit the possibility of a
+"transmutation" of species, such that one species may as an
+exceptional occurrence give rise by seed to another and nearly
+<span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span>
+allied species. Of such a phenomenon he gives illustrations the
+authenticity of which he says he is, against his will, compelled to
+admit. He adds that some might doubt whether in the cases
+quoted the two forms concerned are really distinct species, but
+the passage is none the less of value for it shews that the conception
+of species as being distinct unchangeable entities was not
+to Ray the dogma sacrosanct and unquestionable which it
+afterwards became.<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a>
+</p>
+
+<p class="indent">In the beginning of the eighteenth century Marchant,<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a>
+having observed the sudden appearance of a lacinated variety of
+<i>Mercurialis</i>, makes the suggestion that species in general may have
+arisen by similar mutations. Indeed from various passages it is
+manifest that to the authors of the seventeenth and early eighteenth
+centuries species appeared simply as groups more or less
+definite, the boundaries of which it was unnecessary to determine
+with great exactitude. Such views were in accord with the
+general scientific conception of the time. The mutability of
+<span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span>
+species is for example sometimes likened (see for instance Sharrock,
+loc. cit.) to the metamorphoses of insects, and it is to be
+remembered that the search for the Philosopher's Stone by which
+the transmutation of metals was to be effected had only recently
+fallen into discredit as a pursuit.</p>
+
+<p class="indent">The notion indeed of a peculiar, fixed meaning to be
+attached to species as distinct from variety is I think but rarely to be
+found categorically expressed in prae-Linnaean writings.</p>
+
+<p class="indent">But with the appearance of the <i>Systema Naturae</i>
+a great change supervened. Linnaeus was before all a man of order.
+Foreseeing the immense practical gain to science that must come
+from a codification of nomenclature, he invented such a system.</p>
+
+<p class="indent">It is not in question that Linnaeus did great things for
+us and made Natural History a manageable and accessible collection of
+facts instead of a disorderly heap; but orderliness of mind has
+another side, and inventors and interpreters of systems soon attribute
+to them a force and a precision which in fact they have not.</p>
+
+<p class="indent">The systematist is primarily a giver of names, as
+Ray with his broader views perceived. Linnaeus too in the exordium to
+the <i>Systema Naturae</i> naively remarks, that he is setting out to
+continue the work which Adam began in the Golden Age, to give
+names to the living creatures. Naming however involves very
+delicate processes of mind and of logic. Carried out by the light
+of meagre and imperfect knowledge it entails all the mischievous
+consequences of premature definition, and promotes facile
+illusions of finality. So was it with the Linnaean system. An
+interesting piece of biological history might be written respecting
+the growth and gradual hardening of the conception of Species.
+To readers of Linnaeus's own writings it is well known that his
+views cannot be summarized in a few words. Expressed as they
+were at various times during a long life and in various connexions,
+they present those divers inconsistencies which commonly
+reflect a mind retaining the power of development. Nothing
+certainly could be clearer than the often quoted declaration of the
+<i>Philosophia Botanica</i>, "Species tot numeramus quot diversae
+formae in principio sunt creatae," with the associated passage
+"Varietates sunt plantae ejusdem speciei mutatae a caussa
+<span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span>
+quacunque occasionali." Those sayings however do not stand
+alone. In several places, notably in the famous dissertation
+on the peloric <i>Linaria</i> he explicitly contemplates the possibility
+that new species may arise by crossing, declaring nevertheless
+that he thinks such an event to be improbable. In that essay
+he refers to Marchant's observation on a laciniate <i>Mercurialis</i>,
+but though he states clearly that that plant should only be
+regarded as a variety of the normal, he does not express any
+opinion that the contemporary genesis of new species must be an
+impossibility. In the later dissertation on Hybrid Plants he
+returns to the same topic. Again though he states the belief
+that species cannot be generated by cross-breedings, he treats
+the subject not as heretical absurdity but as one deserving
+respectful consideration.</p>
+
+<p class="indent">The significance of the aphorisms that precede the
+lectures on the Natural Orders is not easy to apprehend. These are
+expressed with the utmost formality, and we cannot doubt that
+in them we have Linnaeus's own words, though for the record
+we are dependent on the transcripts of his pupils.</p>
+
+<p class="blockquot">The text of the first five is as follows:</p>
+
+<p class="blockquot">1. Creator T. O. in primordio vestiit Vegetabile <i>Medullare</i>
+principiis constitutivis diversi <i>Corticalis</i> unde tot difformia
+individua, quot <i>Ordines</i> Naturales prognata.</p>
+
+<p class="blockquot">2. <i>Classicas</i> has (1) plantas Omnipotens miscuit inter se,
+unde tot <i>Genera</i> ordinum, quot inde plantae.</p>
+
+<p class="blockquot">3. <i>Genericas</i> has (2) miscuit Natura, unde tot <i>Species</i> congeneres
+quot hodie existunt.</p>
+
+<p class="blockquot">4. <i>Species</i> has miscuit Casus, unde totidem quot passim
+occurrunt, <i>Varietates</i>.</p>
+
+<p class="blockquot">5. Suadent haec (1-4) Creatoris leges a simplicibus ad
+Composita.</p>
+
+<p class="blockquot">Naturae leges generationis in hybridis.</p>
+
+<p class="blockquot">Hominis leges ex observatis a posteriori.</p>
+
+<p class="indent">I am not clear as to the parts assigned in the first sentence
+respectively to the "<i>Medulla</i>" and the "<i>Cortex</i>," beyond that
+Linnaeus conceived that multiformity was first brought about
+by diversity in the "<i>Cortex</i>." The passage is rendered still
+<span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span>
+more obscure if read in connection with the essay on "<i>Generatio
+Ambigena</i>," where he expresses the conviction that the <i>Medulla</i>
+is contributed by the mother, and the <i>Cortex</i> by the father, both
+in plants and animals.<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a></p>
+
+<p class="indent">But however that may be, he regards this original diversity
+as resulting in the constitution of the Natural Orders, each represented
+by one individual.</p>
+
+<p class="indent">In the second aphorism the Omnipotent is represented as
+creating the genera by intermixing the individual <i>plantae classicae</i>,
+or prototypes of the Natural Orders.</p>
+
+<p class="indent">The third statement is the most remarkable, for in
+it he declares that Species were formed by the act of Nature, who by
+inter-mixing the genera produced <i>Species congeneres</i>, namely
+species inside each genus, to the number which now exist.
+Lastly, Chance or Accident, intermixing the species, produced
+as many varieties as there are about us.</p>
+
+<p class="indent">Linnaeus thus evidently regarded the intermixing
+of an originally limited number of types as the sufficient cause of
+all subsequent diversity, and it is clear that he draws an antithesis
+between <i>Creator</i>, <i>Natura</i>, and <i>Casus</i>, assigning to each
+a special part in the operations. The acts resulting in the
+formation of genera are obviously regarded as completed within
+the days of the Creation, but the words do not definitely show
+that the parts played by Nature and Chance were so limited.</p>
+
+<p class="indent">Recently also E. L. Greene<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a>
+has called attention to some curious utterances buried in the <i>Species Plantarum</i>,
+in which Linnaeus refers to intermediate and transitional species, using
+language that even suggests evolutionary proclivities of a
+modern kind, and it is not easy to interpret them otherwise.</p>
+
+<p class="indent">Whatever Linnaeus himself believed to be the truth, the
+effect of his writings was to induce a conviction that the species
+<span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span>
+of animals and plants were immutably fixed. Linnaeus had
+reduced the whole mass of names to order and the old fantastical
+transformations with the growth of knowledge had lapsed into
+discredit; the fixity of species was taken for granted, but not
+till the overt proclamation of evolutionary doctrine by Lamarck
+do we find the strenuous and passionate assertions of immutability
+characteristic of the first half of the nineteenth century.</p>
+
+<p class="indent">It is not to be supposed that the champions of fixity
+were unacquainted with varietal differences and with the problem
+thus created, but in their view these difficulties were apparent
+merely, and by sufficiently careful observation they supposed
+that the critical and permanent distinctions of the true species
+could be discovered, and the impermanent variations detected
+and set aside.</p>
+
+<p class="indent">This at all events was the opinion formed by the great
+body of naturalists at the end of the eighteenth and beginning of the
+nineteenth centuries, and to all intents and purposes in spite
+of the growth of evolutionary ideas, it remains the guiding
+principle of systematists to the present day. There are 'good
+species' and 'bad species' and the systematists of Europe and
+America spend most of their time in making and debating them.</p>
+
+<p class="indent">In some of its aspects the problem of course confronted
+earlier naturalists. Parkinson for instance (1640) in introducing his
+treatment of <i>Hieracium</i> wrote, "To set forth the whole family of
+the Hawkeweedes in due forme and order is such a world of
+worke that I am in much doubt of mine own abilitie, it having
+lyen heavie on his shoudiers that hath already waded through
+them ... for such a multitude of varieties in forme pertaining
+to one herbe is not to be found againe in <i>rerum natura</i> as I
+thinke," and the same idea, that the difficulty lay rather in
+man's imperfect powers of discrimination than in the nature of
+the materials to be discriminated, is reflected in many treatises
+early and late.</p>
+
+<p class="indent">It was however with the great ouburst of scientific
+activity which followed Linnaeus that the difficulty became acute.
+Simultaneously vast masses of new material were being collected
+from all parts of the world into the museums, and the products
+<span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span>
+of the older countries were re-examined with a fresh zeal and on
+a scale of quantity previously unattempted. But the problem
+how to name the forms and where to draw lines, how much
+should be included under one name and where a new name was
+required, all this was felt, rather as a cataloguer's difficulty
+than as a physiological problem. And so we still hear on the
+one hand of the confusion caused by excessive "splitting" and
+subdivisions, and on the other of the uncritical "lumpers" who
+associate together under one name forms which another collector
+or observer would like to see distinguished.</p>
+
+<p class="indent">In spite of Darwin's hopes, the acceptance of his views
+has led to no real improvement&mdash;scarcely indeed to any change at
+all in either the practice or aims of systematists. In a famous
+passage in the <i>Origin</i> he confidently declares that when his
+interpretation is generally adopted "Systematists will be able
+to pursue their labours as at present; but they will not be incessantly
+haunted by the shadowy doubt whether this or that
+form be a true species. This, I feel sure, and I speak after
+experience, will be no slight relief. The endless disputes whether
+or not some fifty species of British brambles are good species
+will cease." Those disputes nevertheless proceed almost exactly
+as before. It is true that biologists in general do not,
+as formerly, participate in these discussions because they have
+abandoned systematics altogether; but those who are engaged
+in the actual work of naming and cataloguing animals and
+plants usually debate the old questions in the old way. There
+is still the same divergence of opinion and of practice, some inclining
+to make much of small differences, others to neglect them.</p>
+
+<p class="indent">Not only does the work of the systematists as a whole proceed
+as if Darwin had never written but their attitude towards these
+problems is but little changed. In support of this statement I may
+refer to several British Museum Catalogues, much of the <i>Biologia
+Centrali-Americana</i>, Ridgway's <i>Birds of North America</i>, the
+<i>Fauna Hawaiensis</i>, indeed to almost any of the most important
+systematic publications of England, America, or any other
+country. These works are compiled by the most proficient
+<span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span>
+systematists of all countries in the several groups, but with
+rare exceptions they show little misgiving as to the fundamental
+reality of specific differences. That the systematists consider
+the species-unit as of primary importance is shown by the
+fact that the whole business of collection and distribution of
+specimens is arranged with regard to it.</p>
+
+<p class="indent">Almost always the collections are arranged in such a
+way that the phenomena of variation are masked. Forms intermediate
+between two species are, if possible, sorted into separate boxes
+under a third specific name. If a species is liable to be constantly
+associated with a mutational form, the mutants are picked out,
+regardless of the circumstances of their origin, from the samples
+among which they were captured, and put apart under a special
+name. Only by a minute study of the original labels of the
+specimens and by redistributing them according to locality and
+dates, can their natural relations be traced. The published
+accounts of these collections often take no notice of variations,
+others make them the subject of casual reference. Very few
+indeed treat them as of much importance. From such indications
+it is surely evident that the systematists attach to the
+conception of species a significance altogether different from that
+which Darwin contemplated.</p>
+
+<p class="indent">I am well aware that some very eminent systematists
+regard the whole problem as solved. They hold as Darwin did that
+specific diversity has no physiological foundation or causation
+apart from fitness, and that species are impermanent groups,
+the delimitations of which are ultimately determined by environmental
+exigency or "fitness." The specific diversity of
+living things is thus regarded as being something quite different
+in nature from the specific diversity of inorganic substances.
+In practice those who share these opinions are, as might be anticipated,
+to be found among the 'lumpers' rather than among
+the 'splitters.' In their work, certainly, the Darwinian theory
+is actually followed as a guiding principle; unanalysed inter-gradations
+of all kinds are accepted as impugning the integrity
+of species; the underlying physiological problem is forgotten,
+and while the product is almost valueless as a contribution to
+<span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span>
+biological research, I can scarcely suppose that it aids greatly
+in the advances of other branches of our science.</p>
+
+<p class="indent">But why is it that, with these exceptions, the consequences
+of the admittedly general acceptance of a theory of evolution
+are so little reflected in the systematic treatment of living things?
+Surely the reason is that though the systematist may be convinced
+of the general truth of the evolution theory at large, he
+is still of opinion that species are really distinct things. For
+him there are still 'good' species and 'bad' species and his experience
+tells him that the distinction between the two is not
+simply a question of degree or a matter of opinion.</p>
+
+<p class="indent">To some it may seem that this is mere perversity, a refusal
+to see obvious truth, a manifestation of the spirit of the collector
+rather than of the naturalist. But while recognising that from
+a magnification of the conception of species the systematists
+are occasionally led into absurdity I do not think the grounds
+for their belief have in recent times been examined with the
+consideration they deserve. The phenomenon of specific
+diversity is manifested to a similar degree by living things belonging
+to all the great groups, from the highest to the lowest,
+Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae,
+and Bacteria, all present diversities of such a kind that among
+them the existence of specific differences can on the whole be
+recognised with a similar degree of success and with very similar
+limitations. In all these groups there are many species quite
+definite and unmistakable, and others practically indefinite.
+The universal presence of specificity, as we may call it, similarly
+limited and characterised, is one of its most remarkable
+features. Not only is this specificity thus universally present
+among the different forms of life, but it manifests itself in respect
+of the most diverse characteristics which living things display.
+Species may thus be distinguished by peculiarities of form, of
+number, of geometrical arrangement, of chemical constitution
+and properties, of sexual differentiation, of development, and of
+many other properties. In any one or in several of these features
+together, species may be found distinguished from other species.
+It is also to be observed that the definiteness of these distinctions
+<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span>
+has no essential dependence on the nature of the characteristic
+which manifests them. It is for example sometimes said that
+colour-distinctions are of small systematic importance, but every
+systematist is familiar with examples (like that of the wild species
+of <i>Gallus</i>) in which colours though complex, show very little
+variation. On the other hand features of structure, sexual
+differentiation, and other attributes which by our standards are
+estimated as essential, may be declared to show much variation
+or little, not according to any principle which can be detected,
+but simply as the attention happens to be applied to one species
+or group of species, or to another. In many groups of animals and
+plants observers have hit upon characters which were for a time
+thought to be finally diagnostic of species. The Lepidoptera and
+Diptera for instance, have been re-classified according to their
+neuration. Through a considerable range of forms determinations
+may be easily made on these characters, but as is now well
+known, neuration is no more immune from variation than any
+other feature of organisation, and in some species great variability
+is the rule. Again it was once believed by some that the
+genitalia of the Lepidoptera provided a basis of final determination&mdash;with
+a similar sequel. In some groups, for example the Lycaenidae,
+or the Hesperidae, there are forms almost or quite indistinguishable
+on external examination, but a glance at the
+genitalia suffices to distinguish numerous species, while on the
+contrary among Pieridae a great range of species show scarcely
+any difference in these respects: and again in occasional species
+the genitalia show very considerable variations.</p>
+
+<p class="indent">The proposition that animals and plants are on the whole
+divisible into definite and recognisable species is an approximation
+to the truth. Such a statement is readily defensible,
+whereas to assert the contrary would be palpably absurd. For
+example, a very competent authority lately wrote: "In the
+whole Lepidopterous fauna of England there is no species of
+really uncertain limits."<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a>
+Others may be disposed to make
+certain reservations, but such exceptions would be so few as
+scarcely to impair the validity of the general statement. The
+<span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span>
+declaration might be extended to other orders and other lands.</p>
+
+<p class="indent">We know, of course, that the phenomenon of specific diversity
+is complicated by local differentiation: that, in general, forms
+which cannot disperse themselves freely exhibit a multitude of
+local races, and that of these some are obviously adaptative,
+and that a few even owe their peculiarity to direct environmental
+effects. Every systematist also is perfectly aware that in dealing
+with collections from little explored countries the occurrence
+of polymorphism or even of sporadic variation may make the
+practical business of distinguishing the species difficult and
+perhaps for the time impossible; still, conceding that a great
+part of the diversity is due to geographical differentiation, and
+that some is sporadic variation, our experience of our own floras
+and faunas encourages the belief that if we were thoroughly
+familiar with these exotic productions it would usually be
+possible to assign their specific limitations with an approach
+to certainty.</p>
+
+<p class="indent">For apart from any question of the justice of these
+wider inferences, if we examine the phenomenon of specificity as it
+appears in those examples which are nearest to hand, surely we
+find signs in plenty that specific distinction is no mere consequence
+of Natural Selection. The strength of this proposition has
+lain mainly in the appeal to ignorance. Steadily with the growth
+of knowledge has its cogency diminished, and such a belief
+could only have been formulated at a time when the facts of
+variation were unknown.</p>
+
+<p class="indent">In Darwin's time no serious attempt had been made
+to examine the manifestations of variability. A vast assemblage of
+miscellaneous facts could formerly be adduced as seemingly
+comparable illustrations of the phenomenon "Variation."
+Time has shown this mass of evidence to be capable of analysis.
+When first promulgated it produced the impression that variability
+was a phenomenon generally distributed amongst living
+things in such a way that the specific divisions must be arbitrary.
+When this variability is sorted out, and is seen to be in part a
+result of hybridisation, in part a consequence of the persistence
+<span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span>
+of hybrids by parthenogenetic reproduction, a polymorphism
+due to the continued presence of individuals representing various
+combinations of Mendelian allelomorphs, partly also the transient
+effect of alteration in external circumstances, we see how
+cautious we must be in drawing inferences as to the indefiniteness
+of specific limits from a bare knowledge that intermediates exist.
+Conversely, from the accident of collocation or from a misleading
+resemblance in features we deem essential, forms genetically
+distinct are often confounded together, and thus the divergence
+of such forms in their other features, which we declare to be
+non-essential, passes as an example of variation. Lastly, and
+this is perhaps the most fertile of all the sources of confusion,
+the impression of the indefiniteness of species is created by the
+existence of numerous local forms, isolated geographically from
+each other, forms whose differences may be referable to any one
+of the categories I have enumerated.</p>
+
+<p class="indent">The advance has been from many sides. Something has
+come from the work of systematists, something from cultural
+experiments, something from the direct study of variation as it
+appears in nature, but progress is especially due to experimental
+investigation of heredity. From all these lines of inquiry we
+get the same answer; that what the naturalists of fifty years
+ago regarded as variation is not one phenomenon but many,
+and that what they would have adduced as evidence against
+the definiteness of species may not in fact be capable of this
+construction at all.</p>
+
+<p class="indent">If we may once more introduce a physical analogy,
+the distinctions with which the systematic naturalist is concerned in
+the study of living things are as multifarious as those by which
+chemists were confronted in the early days of their science.
+Diversities due to mechanical mixtures, to allotropy, to differences
+of temperature and pressure, or to degree of hydration, had all
+to be severally distinguished before the essential diversity due
+to variety of chemical constitution stood out clearly, and I
+surmise that not till a stricter analysis of the diversities of animals
+and plants has been made on a comprehensive scale, shall we
+be in a position to declare with any confidence whether there is
+<span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span>
+or is not a natural and physiological distinction between species
+and variety.</p>
+
+<p class="indent">As I have said above, it is in the cases nearest to
+hand that the problem may be most effectively studied. Comparison
+between forms from dissimilar situations contributes something;
+but it is by a close examination of the behaviour, especially the
+genetic behaviour, of familiar species when living in the presence
+of their nearest allies that the most direct light on the problem
+is to be obtained. I cannot understand the attitude of those who,
+contemplating such facts as this examination elicits, can complacently
+declare that specific difference is a mere question of
+degree. With the spread of evolutionary ideas to speak much
+of the fixity of species has become unfashionable, and yet how
+striking and inscrutable are the manifestations of that fixity!</p>
+
+<p class="indent">Consider the group of species composing the <i>agrestis</i>
+section of the genus <i>Veronica</i>, namely <i>Tournefortii</i>,
+<i>agrestis</i>, and <i>polita</i>.</p>
+
+<p class="indent">These three grow side by side in my garden, as they
+do in suitable situations over a vast area of the temperate regions.
+I have for years noticed them with some care and become familiar
+with their distinctions and resemblances. Never is there any
+real doubt as to the identity of any plant. The species show
+some variability, but I have never seen one which assumed any
+of the distinguishing features of the others. A glance at the
+fruits decides at once to which species a plant belongs. I find
+it impossible to believe that the fixity of these distinctions is
+directly dependent on their value as aids in the struggle for
+existence. The mode of existence of the three forms in so far
+as we can tell is closely similar. By whatever standard we reckon
+systematic affinity I suppose we shall agree that these species
+come very near indeed to each other. Bentham even takes
+the view that <i>polita</i> is a mere variety of <i>agrestis</i>.</p>
+
+<p class="indent">Now in such cases as this it has been argued that the
+specific features of the several types have been separately developed
+in as many distinct localities, and that their present association
+is due to subsequent redistribution. Of these Veronicas indeed
+we know that one, <i>Tournefortii</i> (= <i>Buxbaumii</i>) is as a matter of fact
+<span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span>
+a recent introduction from the east.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a>
+But this course of argument leads to still further difficulties.
+For if it is true that the peculiarities
+of the several species have been perfected and preserved on
+account of their survival-value to their possessors, it follows
+that there must be many ways of attaining the same result.
+But since sufficient adaptation may be ensured in so many ways,
+the disappearance of the common parent of these forms is difficult
+to understand. Obviously it must have been a plant
+very similar in general construction to its modern representatives.
+Like them it must have been an annual weed, with an organisation
+conformable to that mode of life. Why then, after having been
+duly perfected for that existence should it have been entirely
+superseded in favour of a number of other distinct contrivances
+for doing the same thing, and&mdash;if a gradual transition be predicated&mdash;not
+only by them, but by each intermediate stage
+between them and the original progenitor? Surely the obvious
+inference from such facts is that the burden cast upon the theory
+of gradual selection is far greater than it can bear; that adaptation
+is not in practice a very close fit, and that the distinctions
+between these several species of Veronica have not arisen on
+account of their survival-value but rather because none of their
+diversities was so damaging as to lead to the extermination of
+its possessor. When we see these various Veronicas each rigidly
+reproducing its parental type, all comfortably surviving in
+competition with each other, are we not forced to the conclusion
+that <i>tolerance</i> has as much to do with the diversity of species
+as the stringency of Selection? Certainly these species owe their
+continued existence to the fact that they are each good enough
+to live, but how shall we refer the distinctions between them directly
+or indirectly to the determination of Natural Selection?
+<span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span></p>
+
+<p class="indent">The control of Selection is loose while the conformity to
+specific distinction is often very strict and precise, and no less
+so even when several closely related species co-exist in the same
+area and in the same circumstances.</p>
+
+<p class="indent">The theory of Selection fails at exactly the point
+where it was devised to help: <i>Specific</i> distinction.</p>
+
+<p class="indent">Let us examine a somewhat different set of facts in the
+case of another pair of nearly allied species <i>Lychnis diurna</i> and
+<i>vespertina</i>. The two plants have much in common. Both are
+dioecious perennials, with somewhat similar flowers, the one
+crimson, the other white. Each however has its peculiarities
+which are discernible in almost any part of its structure, whether
+flower, leaf, fruit or seed, distinctions which would enable a
+person thoroughly familiar with the plants to determine at once
+from which species even a small piece had been taken. There
+is so much resemblance however as readily to support the surmise
+that the two were mere varieties of one species. Bentham,
+following Linnaeus, in fact actually makes this suggestion,
+with what propriety we will afterwards consider. Now this case
+is typical of many. The two forms have a wide distribution,
+occurring sometimes separately, sometimes in juxtaposition.
+<i>L. diurna</i> is a plant of hedgerows and sheltered situations. <i>L.
+vespertina</i> is common in fields and open spaces, where <i>diurna</i>
+is hardly ever found; but not rarely <i>vespertina</i> occurs in association
+with <i>diurna</i> in the places which that plant frequents. In
+this case I do not doubt that we have to do with organisms of
+somewhat different aptitudes. That <i>L. vespertina</i> has powers
+which <i>diurna</i> has not is shown very clearly by the fact that
+<i>diurna</i> is sometimes entirely absent from areas where <i>vespertina</i>
+can abound.<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a>
+But in order to understand the true genetic
+relations of the two plants to each other it is necessary to observe
+their behaviour when they meet as they not unfrequently do.
+<span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span>
+If the <i>Lychnis</i> population of such a locality be examined it will
+be found to consist of many undoubted and unmodified <i>diurna</i>,
+a number&mdash;sometimes few, sometimes many&mdash;of similarly
+unmodified <i>vespertina</i>, and an uncertain but usually rather small
+proportion of plants obviously hybrids between the two. How
+is it possible to reconcile these facts with the view that specific
+distinction has no natural basis apart from environmental exigency?</p>
+
+<p class="indent">Darwinian orthodoxy suggests that by a gradual process
+of Natural Selection either one of these two types was evolved from
+the other, or both from a third type. I cannot imagine that
+anyone familiar with the facts would propose the first hypothesis
+in the case of <i>Lychnis</i>, nor can I conceive of any process, whether
+gradual or sudden, by which <i>diurna</i> could have come out of
+<i>vespertina</i>, or <i>vespertina</i> out of <i>diurna</i>. Both however may no
+doubt have been derived from some original third type. It is
+conceivable that <i>Lychnis macrocarpa</i> of Boissier, a native of
+Southern Spain and Morocco, may be this original form. This
+species is said to combine a white flower (like that of <i>L. vespertina</i>),
+with capsule-teeth rolled back (like those of <i>diurna</i>).<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a>
+But whatever the common progenitor may have been, if we are
+to believe that these two species have been evolved from it by
+a gradual process of Natural Selection based on adaptation,
+enormous assumptions must be made regarding the special fitness
+of these two forms and the special unfitness of the common
+parent, and these assumptions must be specially invoked and
+repeated for each several feature of structure or habits distinguishing
+the three forms.</p>
+
+<p class="indent">Why, if the common parent was strong enough to live to
+give rise to these two species, is it either altogether lost now, or at
+least absent from the whole of Northern Europe? Its two
+putative descendants, though so distinct from each other, are,
+as we have seen, able often to occupy the same ground. If
+they were gradually derived from a common progenitor&mdash;necessarily
+very like themselves&mdash;can we believe that this original
+<span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span>
+form should always, in all the diversities of soil and situation
+which they inhabit, be unable to exist? Some one may fancy
+that the hybrids which are found in the situations occupied by
+both forms are this original parental species. But nothing can
+be more certain than that these plants are simply heterozygous
+combinations made by the union of gametes bearing the characters
+of <i>diurna</i> and <i>vespertina</i>.<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a>
+For they may be reproduced exactly in F<sub>1</sub> or in
+later generations of that cross when it is artificially
+made; when bred from their families exhibit palpable phenomena
+of segregation more or less complex; and usually, if perhaps not
+always, they are partially sterile.<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a>
+In a locality on the Norfolk coast that I know well,
+there is a strip of rough ground chiefly
+sand-bank, which runs along the shore. This ground is full of
+<i>vespertina</i>. Not a hundred yards inland is a lane containing
+<i>diurna</i>, and among the <i>vespertina</i> on the sand-bank are always
+some of the hybrid form, doubtless the result of fertilisation
+from the neighbouring <i>diurna</i> population. Seed saved from these
+hybrids gave <i>vespertina</i> and hybrids again, having obviously been
+fertilised by other <i>vespertina</i> or by other hybrids, and I have
+no doubt that such hybrid plants if fertilised by <i>diurna</i> would
+have shown some <i>diurna</i> offspring. The absence of <i>diurna</i>
+in such localities may fairly be construed as an indication that
+<i>diurna</i> is there at a real disadvantage in the competition for life.</p>
+
+<p class="indent">But if, admitting this, we proceed to consider how the special
+aptitude of <i>vespertina</i> is constituted, or what it is that puts
+<i>diurna</i> at a disadvantage, we find ourselves quite unable to
+show the slightest connexion between the success of one or the
+<span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span>
+failure of the other on the one hand, and <i>the specific characteristics</i>
+which distinguish the two forms on the other. The orthodox
+Selectionist would, as usual, appeal to ignorance. We
+ask what can <i>vespertina</i> gain by its white flowers, its more lanceolate
+leaves, its grey seeds, its almost erect capsule-teeth,
+its longer fruits, which <i>diurna</i> loses by reason of its red flowers,
+more ovate leaves, dark seeds, capsule-teeth rolled back, and
+shorter fruits? We are told that each of these things <i>may</i>
+affect the viability of their possessors. We cannot assert that
+this is untrue, but we should like to have evidence that it is true.
+The same problem confronts us in thousands upon thousands
+of examples, and as time goes on we begin to feel that speculative
+appeals to ignorance, though dialectically admissible, provide
+an insufficient basis for a proposition which, if granted, is to
+become the foundation of a vast scheme of positive construction.</p>
+
+<p class="indent">One thing must be abundantly clear to all, that to
+treat two forms so profoundly different as one, because intermediates of
+unknown nature can be shown to exist between them, is a mere
+shirking of the difficulties, and this course indeed creates artificial
+obstacles in the way of those who are seeking to discover the
+origin of organic diversity.</p>
+
+<p class="indent">In the enthusiasm with which evolutionary ideas were
+received the specificity of living things was almost forgotten.
+The exactitude with which the members of a species so often
+conform in the diagnostic, specific features passed out of account;
+and the scientific world by dwelling with a constant emphasis
+on the fact of variability, persuaded itself readily that species
+had after all been a mere figment of the human mind.
+Without presuming to declare what future research only can
+reveal, I anticipate that, when variation has been properly
+examined and the several kinds of variability have been successfully
+distinguished according to their respective natures,
+the result will render the natural definiteness of species increasingly
+apparent. Formerly in such a case as that of the two
+<i>Lychnis</i> species, the series of "intermediates" was taken to be a
+palpable proof that <i>vespertina</i> "graded" to <i>diurna</i>. It is this
+fact, doubtless, upon which Bentham would have relied in suggesting
+<span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span>
+that both may be one species.<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a>
+Genetic tests, though as yet imperfectly applied, make it almost certain
+that these inter-grading forms are not in any true sense variations from either
+species in the direction of the other, but combinations of elements
+derived from both.</p>
+
+<p class="indent">The points in which very closely allied species are
+distinguished from each other may be found in the most diverse
+features of their organisation. Sometimes specific difference
+is to be seen in a character which we can believe to be important
+in the struggle, but at least as often it is some little detail that
+we cannot but regard as trivial which suffices to differentiate
+the two species. Even when the diagnostic point is of such a
+nature that we can imagine it to make a serious difference in the
+economy we are absolutely at a loss to suggest why this feature
+should be a necessity to species A and unnecessary to species B
+its nearest ally. The house sparrow (<i>Passer domesticus</i>) is in
+general structure very like the tree sparrow (<i>P. montanus</i>).
+They differ in small points of colour. For instance <i>montanus</i>
+has a black patch on the cheek which is absent in <i>domesticus</i>.
+The presence in the one species and the absence in the other
+are equally definite, and in both cases we are equally unable to
+suggest any consideration of utility in relation to these features.
+The two species are distinguished also by a characteristic that
+may well be supposed to be of great significance. In <i>domesticus</i>
+the two sexes are strongly differentiated, the cock being more
+ornate than the hen. On the other hand the two sexes in <i>montanus</i>
+are alike, and, if we take a standard from <i>domesticus</i>, we
+may fairly say that in <i>montanus</i> the hen has the colouration of
+the male. It is not unreasonable to suppose that such a distinction
+may betoken some great difference in physiological
+economy, but the economical significance of this perhaps important
+distinction is just as unaccountable as that of the seemingly
+trivial but equally diagnostic colour-point.
+<span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span></p>
+
+<p class="indent">I have spoken of the fixed characteristics of the two species.
+If we turn to a very different feature, their respective liability
+to albinistic variation, we find ourselves in precisely similar
+difficulty. <i>Passer domesticus</i> is a species in which individuals
+more or less pied occur with especial frequency, but in <i>P. montanus</i>
+such variation is extremely rare if it occurs at all. The
+writer of the section on Birds in the <i>Royal Natural History</i>
+(III., 1894-5, p. 393) calls attention to this fact and remarks
+that in that species he knows no such instance.</p>
+
+<p class="indent">The two species therefore, apart from any differences that we
+can suppose to be related to their respective habits, are characterised
+by small fixed distinctions in colour-markings, by a
+striking difference in secondary sexual characters, and by a
+difference in variability. In all these respects we can form
+no surmise as to any economic reason why the one species
+should be differentiated in the one way and the other in the other
+way, and I believe it is mere self-deception which suggests the
+hope that with fuller knowledge reasons of this nature would be
+discovered.</p>
+
+<p class="indent">The two common British wasps, <i>Vespa vulgaris</i> and
+<i>Vespa germanica</i>, are another pair of species closely allied although
+sharply distinguished, which suggest similar reflexions. Both
+usually make subterranean nests but of somewhat different
+materials. <i>V. vulgaris</i> uses rotten wood from which the nest
+derives a characteristic yellow colour, while <i>V. germanica</i> scrapes
+off the weathered surfaces of palings and other exposed timber,
+material which is converted into the grey walls of the nest. The
+stalk by which the nest is suspended (usually to a root) in the
+case of <i>germanica</i> passes freely through a hole in the external
+envelope, but <i>vulgaris</i> unites this external wall solidly to the
+stalk. In bodily appearance and structure the two species are
+so much alike that they have often been confounded even by
+naturalists, and to the untrained observer they are quite indistinguishable.
+There are nevertheless small points of difference
+which almost though not quite always suffice to distinguish the
+two forms. For example the yellow part of the sinus of the eyes
+is emarginate in <i>vulgaris</i> but not emarginate in <i>germanica</i>. <i>V.
+vulgaris</i> often has black spots on the tibiae while in <i>germanica</i> the
+<span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span>
+tibiae are usually plain yellow. In both species there is a horizontal
+yellow stripe on the thorax, but whereas in <i>vulgaris</i> this is
+a plain narrow stripe, it is in <i>germanica</i> enlarged downwards in
+the middle. These and other apparently trivial details of colouration,
+though not absolutely constant, are yet so nearly constant
+that irregularities in these respects are quite exceptional. Lastly
+the genitalia of the males, though not very different, present
+small structural points of distinction which are enough to distinguish
+the two species at a glance.<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a>
+</p>
+
+<p class="indent">In considering the meaning of the distinctions between
+these two wasps we meet the old problem illustrated by the Sparrows.
+The two species have somewhat different habits of life and we
+should readily expect to find differences of bodily organisation
+corresponding with the differences of habits. But is that what
+we do find? Surely not. To suppose that there is a correspondence
+between the little points of colour and structure which
+we see and the respective modes of life of the two species is
+perfectly gratuitous. We have no inkling of the nature of such
+a correspondence, how it can be constituted, or in what it may
+consist.</p>
+
+<p class="indent">Is it not time to abandon these fanciful expectations
+which are never realised? Everywhere both among animals and plants
+does the problem of specific difference reiterate itself in the same
+form. In view of such facts as I have related and might indefinitely
+multiply, the fixity of specific characters cannot readily
+be held to be a measure of their economic importance to their
+possessors. The incidence of specific fixity is arbitrary and
+capricious, sometimes lighting on a feature or a property which
+can be supposed to matter much, but as often is it attached to the
+most trifling of superficial peculiarities.</p>
+
+<p class="indent">The incidence of <i>variability</i> is no less paradoxical,
+and without investigation of the particular case no one can say what will be
+<span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span>
+found to show much or little variability. The very characteristic
+which in one species may exhibit extreme variability
+may in an allied species show extreme constancy. Illustrations
+will occur to any naturalist, but nowhere is this truth more
+strikingly presented than in the British Noctuid Moths. Many
+are so variable that, in the common phrase, "scarcely two can
+be found alike," while others show comparatively slight variation.
+It need scarcely be remarked that, in the instances I have in
+mind, the evidence of great variability is in no way due to the
+abundance with which the particular species occurs, for common
+species may show constancy, and less abundant species may show
+great variability. The polymorphism seems to be now at least
+a general property of the variable species, as the fixity is a
+property of the fixed species. In illustration I may refer to the
+following examples.</p>
+
+<p class="indent"><i>Dianthoecia capsincola</i> is a common and widely distributed
+moth which feeds on <i>Lychnis</i>. It shows little variation. <i>Dianthoecia
+carpophaga</i> is another species which feeds chiefly on
+<i>Silene</i>. Its habits are very similar to those of <i>capsincola</i>. Like
+that species it has a wide geographical range and is abundant
+in its localities, but in contrast to the fixity of <i>capsincola</i>, <i>carpophaga</i>
+exhibits a complex series of varieties. <i>Agrotis suffusa</i>
+(= <i>ypsilon</i>) is a moth widely spread through the southern half
+of England. It is very constant in colour and markings. <i>Agrotis
+segetum</i> and <i>tritici</i> are excessively variable both in ground
+colour and markings, being found in an immense profusion of
+dissimilar forms throughout their distribution. Of these and
+several other species of <i>Agrotis</i> there are many named varieties,
+some of which have by various writers been regarded as specifically
+distinct. Of the genus <i>Noctua</i> many species (e. g. <i>festiva</i>)
+show a similar polymorphism, but <i>N. triangulum</i>, though showing
+some variation in certain respects, is usually very constant to
+its type, and the same is true of <i>N. umbrosa</i>.</p>
+
+<p class="indent">In several species of <i>Taeniocampa</i>, especially <i>instabilis</i>,
+the multiplicity of forms is extreme, while <i>cruda</i> (= <i>pulverulenta</i>)
+is a comparatively constant species. The genus <i>Plusia</i> contains
+a number of constant species, but in <i>Plusia interrogationis</i> we
+<span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span>
+meet the fact that the central silvery mark undergoes endless
+variation. "Truly no two are alike," says Mr. Tutt, "and to
+look down a long series of <i>interrogationis</i> is something like looking
+at a series of Chinese characters." In contrast to this we have
+the fact that in <i>Plusia gamma</i> the very similar silvery mark is
+by no means variable.</p>
+
+<p class="indent">I have taken this series of cases from the Noctuid moths,
+but it would be as easy to illustrate the same proposition from
+the Geometridae or the Micro-Lepidoptera.<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a>
+I have a longseries of <i>Peronea cristana</i>, for example, which was given to
+me by Mr. W. H. B. Fletcher, of Bognor. All were beaten out of the same
+hedge, and their polymorphism is such that no one unaccustomed
+to such examples could suppose that they belonged to a single
+species. Another common form, <i>P. schalleriana</i>, which lives in
+similar circumstances, exhibits comparatively slight variability.</p>
+
+<p class="indent">It should be expressly noted that the variation of
+which I am speaking is a genuine polymorphism. Several of the species
+enumerated exhibit also geographical variation, possessing definite
+and often strikingly distinct races peculiar to certain
+localities; but apart from the existence of such local differentiation,
+stands out the fact upon which I would lay stress, that
+some species are excessively variable while others are by comparison
+constant, in circumstances that we may fairly regard as comparable.</p>
+
+<p class="indent">This fact is difficult to reconcile with the conventional
+view that specific type is directly determined by Natural Selection
+<span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span>
+and that the precision with which a species conforms to its
+pattern is an indication of the closeness of that control. Anyone
+familiar with the characteristics of Moths will agree that the
+Noctuids, Geometrids and Tortricids are creatures whose existence
+depends in some degree on the success with which they can escape
+detection by their enemies in the imaginal state. We are therefore
+not surprised to find that some species of these orders
+exhibit definite geographical variation in conformity with the
+character of the ground, which may reasonably be supposed to
+aid in their protection. If this were all, there would be nothing to
+cause surprise. We might even be disposed to allow that variability
+might contribute to the perpetuation of animals so situated,
+on the principle that among a variety of surroundings
+some would probably be in harmony with the objects on which
+they rest. But we cannot admit the plausibility of an argument
+which demands on the one hand that the extreme precision with
+which species A adheres in the minutest details of its colour and
+pattern to a certain type shall be ascribed to the protective fitness
+of those details, and on the other hand that the abundant variability
+of species B shall be ascribed to the same determination.
+If it is absolutely necessary for A to conform to one type how
+comes it that B may range through some twenty distinct forms,
+any two of which differ more from each other than the regular
+species of many other genera? The only reply I can conceive
+is a suggestion that there <i>may</i> be some circumstance which
+differentiates the various classes of cases, that the exigencies
+of the fixed species <i>may</i> be different from those of the variable.
+Those who make such appeals to ignorance do not always perhaps
+realise whither this course of reasoning may lead. If admissible
+here the same argument would lead us to suggest that because
+albino moles have for an indefinite period occurred on a certain
+land near Bath there may be something in the soil or in the
+conditions of life near Bath which requires a proportion of albinos
+in its mole population. Or again, because the butterfly <i>Thais
+rumina</i> in one locality, Digne in the south of France, has a percentage
+of individuals of the variety <i>Honoratii</i> (with certain
+normally yellow spots on the hind wing coloured bright red)
+<span class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></span>
+and nowhere else throughout its distribution, that therefore we
+may suggest that there is some difference in the condition of
+life at Digne which makes the continuance of <i>Honoratii</i> there
+possible and beneficial.</p>
+
+<p class="indent">A polymorphism offering a parallel to that of the
+variable moths is afforded by the breeding plumage of the Ruff, the
+male of <i>Machetes pugnax</i>. The variety of plumage which these
+cocks exhibit is such that the statement that no two can be
+found alike is only a venial exaggeration. Newton remarks<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a>
+"that all this wonderful 'show' is the consequence of the polygamous
+habit of the Ruff can scarcely be doubtful"; but even if
+it be conceded that the great external differentiation of the
+cocks may be a result of sexual selection, the problem of their
+<i>polymorphism</i> remains unsolved, for, as we are well aware,
+polygamy is not usually associated with polymorphism of the
+male. The Black Cock (<i>Tetrao tetrix</i>), for example, is as polygamous
+as the Ruff, but in that and countless other cases, both
+sexes are constant to one type of plumage.</p>
+
+<p class="indent">When we thus compare the polymorphism of one species
+with the fixity of another, and attempt to determine the causes which
+have led to these extraordinary contrasts, two distinct lines of
+argument are open to us. We may ascribe the difference either
+to causes external to the organisms, primarily, that is to say,
+to a difference in the exigencies of Adaptation under Natural
+Selection; or on the other hand we may conceive the difference
+as due to innate distinctions in the chemical and physiological
+constitutions of the fixed and the variable respectively. There
+is truth undoubtedly in both conceptions. If the mole were
+physiologically incapable of producing an albino that variety
+would not have come into being, and if the albino were totally
+incapable of getting its living it would not be able to hold its
+<span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span>
+own. Were <i>Plotheia frontalis</i> constructed on a chemical plan
+which admitted of no variation, the countless varieties would
+not have been produced; and if one of its varieties had an overwhelming
+success out of all proportion to that of the rest, then
+the species would soon become monomorphic again. We
+cannot declare that Natural Selection has no part in the determination
+of fixity or variability; nevertheless looking at the whole
+mass of fact which a study of the incidence of variation provides,
+I incline to the view that the variability of polymorphic forms
+should be regarded rather as a thing tolerated than as an element
+contributing directly to their chances of life; and on the other
+hand that the fixity of the monomorphic forms should be looked
+upon not so much as a proof that Natural Selection controls
+them with a greater stringency, but rather as evidence of a natural
+and intrinsic stability of chemical constitution.</p>
+
+<p class="indent">Compare the condition of a variable form like the male
+Ruff (or in a less degree the Red Grouse in both its sexes) with
+that of the common Pheasant which is comparatively constant.
+In the Pheasant no doubt variations do occur as in other wild
+birds, but apart from the effects of mongrelisation the species
+is unquestionably uniform. Could it seriously be proposed
+that we should regard the constancy of the pheasant's plumage
+in this country as depending on the special fitness of that type
+of colouration? Even if the pheasant be not an alien in Western
+Europe, it has certainly been protected for centuries, and for a
+considerable period has existed in a state of semi-domestication.
+Such conditions should give good opportunity for polymorphism
+to be produced. In some coverts various aberrations do of
+course occur and persist, yet there is nothing indicative of a
+general relaxation of the fixity of the specific type, and the pheasant
+remains substantially a fixed species.<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a>
+The common pheasant (<i>Phasianus colchicus</i>) even shows little of that disposition to
+<span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span>
+form local races which appears in the species of Further India.
+Are we not then on safer ground in regarding the fixity of our
+species as a property inherent in its own nature and constitution?
+Just as in ages of domestication no rose has ever given
+off a blue variety so has the pheasant never broken out into the
+polymorphism of the Ruff.</p>
+
+<p class="indent">As soon as it is realised how largely the phenomena
+of variation and stability must be an index of the internal constitution
+of organisms, and not mere consequences of their relations to
+the outer world, such phenomena acquire a new and more profound significance.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_31" id="Page_31">[Pg 31]</a></span></p>
+
+<h2>CHAPTER II</h2>
+<h3><span class="smcap">Meristic Phenomena</span></h3>
+
+<p class="indent">Twenty years ago in describing the facts of Variation,
+argument was necessary to show that these phenomena had a special
+value in the sciences of Zoology and Botany. This value is
+now universally understood and appreciated. In spite however
+of the general attention devoted to the study of Variation, and
+the accumulation of material bearing on the problem, no satisfactory
+or searching classification of the phenomena is possible.
+The reason for this failure is that a real classification must
+presuppose knowledge of the chemistry and physics of living things
+which at present is quite beyond our reach.</p>
+
+<p class="indent">It is however becoming probable that if more knowledge
+of the chemical and physical structure of organisms is to be attained,
+the clue will be found through Genetics, and thus that
+even in the uncoordinated accumulation of facts of Variation
+we are providing the means of analysis applicable not only to
+them, but to the problems of normality also.</p>
+
+<p class="indent">The only classification that we can yet institute
+with any confidence among the phenomena of Variation is that which
+distinguishes on the one hand variations in the processes of
+division from variations in the nature of the substances divided.</p>
+
+<p class="indent">Variations in the processes of division are most often
+made apparent by a change in the number of the parts, and are therefore
+called <i>Meristic</i> Variations, while the changes in actual composition
+of material are spoken of as <i>Substantive</i> Variations. The Meristic
+Variations form on the whole a natural and fairly well
+defined group, but the Substantive Variations are obviously
+a heterogeneous assemblage.</p>
+
+<p class="indent">Though this distinction does not go very far, it is
+useful, and in all probability fundamental. It is of value inasmuch as
+it brings into prominence the distinct and peculiar part which
+<span class="pagenum"><a name="Page_32" id="Page_32">[Pg 32]</a></span>
+the process of division, or, more generally, repetition of parts,
+plays in the constitution of the forms of living things.</p>
+
+<p class="indent">That there may be a real independence between the
+Meristic and the Substantive phenomena is evident from the fact both
+that Meristic changes may occur without Substantive Variation,
+and that the substances composing an organism may change
+without any perceptible alteration in its meristic structure.
+When the distinction between these two classes of phenomena
+is perceived it will be realised that the study of genetics has on
+the one hand a physical, or perhaps more strictly a mechanical
+aspect, which relates to the manner in which material is divided
+and distributed; and also a chemical aspect, which relates to
+the constitution of the materials themselves. Somewhat as
+the philosophers of the seventeenth and eighteenth centuries
+were awaiting both a chemical and a mechanical discovery which
+should serve as a key to the problems of unorganised matter,
+so have biologists been awaiting two several clues. In Mendelian
+analysis we have now, it is true, something comparable with the
+clue of chemistry, but there is still little prospect of penetrating
+the obscurity which envelops the mechanical aspect of our phenomena.
+To make clear the application of the terms chemical
+and mechanical to the problem of Genetics the nature of that
+problem must be more fully described. In its most concrete
+form this problem is expressed in the question, how does a cell
+divide? If the organism is unicellular, and the single cell is
+the whole body, then the process of heredity is accomplished
+in the single operation of cell-division. Similarly in animals and
+plants whose bodies are made up of many cells, the whole process
+of heredity is accomplished in the cell-divisions by which the
+germ-cells are formed. When therefore we see a cell dividing,
+we are witnessing the process by which the form and the properties
+of the daughter-cells are determined.</p>
+
+<p class="indent">Now this process has the two aspects which I have
+called mechanical and chemical. The term "<i>Entwicklungsmechanik</i>"
+has familiarised us with the application of the word mechanics
+to these processes, but on reflexion it will be seen that this comprehensive
+term includes two sorts of events which are sometimes
+<span class="pagenum"><a name="Page_33" id="Page_33">[Pg 33]</a></span>
+readily distinguishable. There is the event by which the cell
+<i>divides</i>, and the event by which the two halves or their descendants
+are or may be <i>differentiated</i>. It is common knowledge that
+in some cell-divisions two similar halves, indistinguishable in
+appearance, properties, and subsequent fate, may be produced,
+while in other divisions daughter-cells with distinct properties
+and powers are formed. We cannot imagine but that in the
+first case, when the resulting cells are identical, the division
+is a mechanical process by which the mother-cell is simply
+cut in two; while in order that two differentiated halves may be
+produced, some event must have taken place by which a chemical
+distinction between the two halves is effected.<a name="FNanchor_1_18" id="FNanchor_1_18"></a><a href="#Footnote_1_18" class="fnanchor">[1]</a>
+In any ordinary Mendelian case we have a clear proof that such a
+chemical difference may be established between germ-cells. The facts
+of colour-inheritance for instance prove that germ-cells, otherwise
+identical, may be formed <i>possessing</i> the chromogen-factor which
+is necessary to the formation of colour in the flowers, or <i>destitute</i>
+of that factor. Similarly the germ-cells may possess the ferment
+which, by its action on the chromogenic substance, produces
+the colour, or they may be without that ferment. The same
+line of argument applied to a great range of cases. Nevertheless,
+though differences in chemical properties are often thus constituted
+by cell-divisions, and though we are thus able to make a
+quasi-chemical analysis of the individual by determining and
+enumerating these properties, yet it is evident that the distribution
+of these factors is not itself a chemical process. This
+is proved by the fact that similar divisions may be effected between
+halves which are exactly alike, and also by the fact that
+the numbers in which the various types of germ-cells are formed
+negative any suggestion of valency between them. The recognition
+of the unit-factors may lead&mdash;indeed must lead&mdash;to great
+advances in chemical physiology which without that clue would
+have been impossible, but in causation the chemical phenomena
+of heredity must be regarded as secondary to the physical or
+<span class="pagenum"><a name="Page_34" id="Page_34">[Pg 34]</a></span>
+mechanical phenomena by which the cells and their constituents
+are divided and separated. When therefore we speak of the
+<i>essential</i> phenomena of heredity we mean the mechanics of division,
+especially, though not, as we shall see, exclusively, of <i>cell</i>-division;
+and in the relation between the two halves of the dividing
+cell we have the problem presented in what seems to be its simplest form.</p>
+
+<p class="indent">In attempting to form some conception of the processes
+by which bodily characteristics are transmitted, or&mdash;to avoid that
+confusing metaphor of "transmission"&mdash;how it comes about
+that the offspring can grow to resemble its parent, continuity of
+the germ-substance which in some animals is a visible phenomenon,<a name="FNanchor_2_19" id="FNanchor_2_19"></a><a href="#Footnote_2_19" class="fnanchor">[2]</a>
+gives at least apparent help. An egg for example on becoming
+adult develops in certain parts a particular pigment.
+The eggs of that adult when they reach the appropriate age develop
+the same pigment. We have no clear picture of the
+mechanism by which this process is effected, but when we realise
+that the pigment results from the interaction of certain substances,
+and that since all the eggs are in reality pieces of the same
+material, it seems, unless we inquire closely, not unnatural that
+the several pieces of the material should exhibit the same colours
+at the same periods of their development. The continuity of
+the material of the germs suggests that there is a continuity
+of the materials from which the pigment is formed, and that
+thus an actual bit of those substances passes into each egg
+ready at the appropriate moment to generate the pigment.
+The argument thus outlined applies to all <i>substantive</i> characteristics.
+In each case we can imagine, if we will, the appearance
+of that characteristic as due to the contribution of its rudiment
+from the germ tissues.</p>
+
+<p class="indent">When we consider more critically it becomes evident
+that the aid given by this mental picture is of very doubtful reality,
+for even if it were true that any predestined particle actually
+corresponding with the pigment-forming materials is definitely
+<span class="pagenum"><a name="Page_35" id="Page_35">[Pg 35]</a></span>
+passed on from germ to germ, yet the power of increase which
+must be attributed to it remains so incomprehensible that the
+mystery is hardly at all illuminated.</p>
+
+<p class="indent space-below">When however we pass from the substantive to the
+meristic characters, the conception that the character depends on the
+possession by the germ of a particle of a specific material becomes
+even less plausible. Hardly by any effort of imagination can we
+see any way by which the division of the vertebral column into <i>x</i>
+segments or into <i>y</i> segments, or of a Medusa into 4 segments or
+into 6, can be determined by the possession or by the want of a
+material particle. The distinction must surely be of a different
+order. If we are to look for a physical analogy at all we should
+rather be led to suppose that these differences in segmental
+numbers corresponded with changes in the amplitude or number
+of dividing waves than with any change in the substance or
+material divided.</p>
+
+<h3><span class="smcap">Phenomena of Division</span></h3>
+
+<p class="indent space-below">I have said that in the division of a cell we seem to
+see the problem in its simplest form, but it is important to observe that
+the problem of division may be presented by the bodies of animals
+and plants in forms which are independent of the divisions between
+cells. The existence of pattern implies a repetition of
+parts, and repetition of parts when developed in a material
+originally homogeneous can only be created by division. Cell-division
+is probably only a special case of a process similar to
+that by which the pattern of the skeleton is laid down in a unicellular
+body such as that of a Radiolarian or Foraminiferan.
+Attempts have lately been made to apply mathematical treatment
+to problems of biology. It has sometimes seemed to
+me that it is in the geometrical phenomena of life that the most
+hopeful field for the introduction of mathematics will be found.
+If anyone will compare one of our animal patterns, say that of a
+zebra's hide, with patterns known to be of purely mechanical
+production, he will need no argument to convince him that there
+must be an essential similarity between the processes by which
+the two kinds of patterns were made and that parts at least of
+<span class="pagenum"><a name="Page_36" id="Page_36">[Pg 36]</a></span>
+the analysis applicable to the mechanical patterns are applicable
+to the zebra stripes also. Patterns mechanically produced are
+of many and very diverse kinds. One of the most familiar
+examples, and one presenting some especially striking analogies
+to organic patterns, is that provided by the ripples of a mackerel
+sky, or those made in a flat sandy beach by the wind or the ebbing
+tide. With a little search we can find among the ripple-marks, and
+in other patterns produced by simple physical means, the closest
+parallels to all the phenomena of striping as we see them in our
+animals. The forking of the stripes, the differentiation of two
+"faces," the deflections round the limbs and so forth, which in the
+body we know to be phenomena of division, are common both to
+the mechanical and the animal patterns. We cannot tell what in
+the zebra corresponds to the wind or the flow of the current, but
+we can perceive that in the distribution of the pigments, that is
+to say, of the chromogen-substances or of the ferments which
+act upon them, a rhythmical disturbance has been set up which
+has produced the pattern we see; and I think we are entitled to
+the inference that in the formation of patterns in animals and
+plants mechanical forces are operating which ought to be, and
+will prove to be, capable of mathematical analysis. The comparison
+between the striping of a living organism and the sand-ripples
+will serve us yet a little farther, for a pattern may either
+be formed by actual cell-divisions, and the distribution of differentiation
+coincidently determined, or&mdash;as visibly in the pigmentation
+of many animal and plant tissues&mdash;the pattern may
+be laid down and the pigment (for example) distributed through
+a tissue across or independently of the cell-divisions of the tissue.
+Our tissues therefore are like a beach composed of sands of
+different kinds, and different kinds of sands may show distinct
+and interpenetrating ripples. When the essential analogy between
+these various classes of phenomena is perceived, no one
+will be astonished at, or reluctant to admit, the reality of discontinuity
+in Variation, and if we are as far as ever from knowing
+the actual causation of pattern we ought not to feel surprised that
+it may arise suddenly or be suddenly modified in descent. Biologists
+have felt it easier to conceive the evolution of a striped
+animal like a zebra from a self-coloured type like a horse (or
+of the self-coloured from the striped) as a process involving many
+intergradational steps; but so far as the <i>pattern</i> is concerned, the
+change may have been decided by a single event, just as the
+multitudinous and ordered rippling of a beach may be created
+or obliterated at one tide.
+<span class="pagenum"><a name="Page_37" id="Page_37">[Pg 37]</a></span></p>
+
+<div class="figcenter" >
+  <img src="images/i_050.jpg" alt="Elephant Tusk" width="600" height="283" />
+</div>
+<p class="space-below" style="font-size: 120%; text-align: center;">
+   <b><span class="smcap">Fig. 1.</span> Tusk of Indian elephant, showing an abnormal segmentation.</b></p>
+<p><span class="pagenum"><a name="Page_38" id="Page_38">[Pg 38]</a></span></p>
+
+<p class="indent">This point is well illustrated by the tusk of an Indian
+elephant which I lately found in a London sale-room. This tusk is by
+some unknown cause, presumably a chronic inflammation,
+thrown up into thirteen well-marked ridges which closely simulate
+a series of segments (Fig. 1). Whatever the cause the condition
+shows how easily a normally unsegmented structure may be
+converted into a series of repeated parts.</p>
+
+<p class="indent space-below">The spread of segmentation through tissues normally
+unsegmented is very clearly exemplified in the skates' jaws shown in
+in Fig. 2. The right side of the upper figure shows the normal
+arrangement in the species <i>Rhinoptera jussieui</i>, but the structure
+on the left side is very different. The probable relations of the
+several rows of teeth to the normal rows is indicated by the lettering,
+but it is evident that by the appearance of new planes
+of division constituting separate centers of growth, the series has
+been recast. The pattern of the left side is so definite that had
+the variation affected the right side also, no systematist would
+have hesitated to give the specimen a new specific name. The
+other two drawings show similar variations of a less extensive
+kind, the nature of which is explained by the lettering of the
+rows of teeth.</p>
+
+<div class="figcenter" >
+  <img src="images/i_052.jpg" alt="Jaws of Skates" width="600" height="674" />
+</div>
+<p class="space-below" style="font-size: 120%; text-align: center;">
+   <b><span class="smcap">Fig. 2.</span> Jaws of Skates</b> (<i>Rhinoptera</i>)
+<b>showing meristic variation.</b><br />
+(For a detailed discussion see <i>Materials for the Study of Variation</i>, p. 259.)</p>
+
+<p class="indent">This power to divide is a fundamental attribute of
+life, and of that power cell-division is a special example. In regard to
+almost all the chief vital phenomena we can say with truth that
+science has made some progress. If I mention respiration, metabolism,
+digestion, each of these words calls to mind something
+more than a bare statement that such acts are performed by an
+animal or a plant. Each stands for volumes of successful experiment
+and research, But the expression cell-division, the
+fundamental act which typifies the rest, and on which they all
+depend, remains a bare name. We can see with the microscope
+<span class="pagenum"><a name="Page_39" id="Page_39">[Pg 39]</a></span>
+the outward symptoms of division, but we have no surmise as
+to the nature of the process by which the division is begun or
+accomplished. I know nothing which to a man well trained in
+scientific knowledge and method brings so vivid a realisation
+of our ignorance of the nature of life as the mystery of cell-division.
+What is a living thing? The best answer in few words
+that I know is one which my old teacher, Michael Foster, used
+to give in his lectures introductory to biology. "A living thing
+is a vortex of chemical and molecular change." This description
+gives much, if not all, that is of the essence of life. The living
+thing is unlike ordinary matter in the fact that, through it, matter
+is always passing. Matter is essential to it; but, provided
+that the flow in and out is unimpeded, the life-process can go
+on so far as we know indefinitely. Yet the living "vortex"
+differs from all others in the fact that it can divide and throw
+off other "vortices," through which again matter continually swirls.
+<span class="pagenum"><a name="Page_40" id="Page_40">[Pg 40]</a></span></p>
+
+<p class="indent">We may perhaps take the parallel a stage further. A simple
+vortex, like a smoke-ring, if projected in a suitable way will twist
+and form two rings. If each loop as it is formed could grow and
+then twist again to form more loops, we should have a model
+representing several of the essential features of living things.</p>
+
+<p class="indent">It is this power of spontaneous division which most
+sharply distinguishes the living from the non-living. In the excellent
+book dealing with the problems of development, lately published
+by Mr. Jenkinson a special emphasis is very properly laid on the
+distinction between the processes of division, and those of
+differentiation. Too often in discussions of the developmental
+processes the distinction is obscured. He regards differentiation
+as the "central difficulty." "Growth and division of the nucleus
+and the cells," he tells us, are side-issues. This view is quite
+defensible, but I suspect that the division <i>is</i> the central
+difficulty, and that if we could get a rationale of what is happening
+in cell-division we should not be long before we had a clue to the
+nature of differentiation. It may be self-deception, but I do
+not feel it impossible to form some hypothesis as to the mode of
+differentiation, but in no mood of freest speculation are we ever
+able to form a guess as to the nature of the division. We see
+differentiations occurring in the course of chemical action, in
+some phenomena of vibration and so forth: but where do we
+see anything like the spontaneous division of the living cell?
+Excite a gold-leaf electroscope, and the leaves separate, but we
+know that is because they were double before. In electrolysis
+various substances separate out at the positive and negative
+poles respectively. Now if in cell-division the two daughter-cells
+<span class="pagenum"><a name="Page_41" id="Page_41">[Pg 41]</a></span>
+were always dissimilar&mdash;that is to say, if differentiation
+always occurred&mdash;we could conceive some rough comparison
+with such dissociations. But we know the dissimilarity between
+daughter-cells is not essential. In the reproduction of unicellular
+organisms and many other cases, the products formed at the
+two poles are, so far as we can tell, identical. Any assumption
+to the contrary, if we were disposed to make it, would involve
+us in difficulties still more serious. At any rate, therefore, if
+differentiation be really the central difficulty in development,
+it is division which is the essential problem of heredity.</p>
+
+<p class="indent">Sir George Darwin and Professor Jeans tell us that
+"gravitational instability" consequent on the condensation of gases is
+"the primary agent at work in the actual evolution of the universe,"
+which has led to the division of the heavenly bodies. The
+greatest advance I can conceive in biology would be the discovery
+of the nature of the instability which leads to the continual
+division of the cell. When I look at a dividing cell I feel as an
+astronomer might do if he beheld the formation of a double
+star: that an original act of creation is taking place before me.
+Enigmatical as the phenomenon seems, I am not without hope
+that, if it were studied for its own sake, dissociated from the
+complications which obscure it when regarded as a mere incident
+in development, some hint as to the nature of division could be
+found. It is I fear a problem rather for the physicist than for
+the biologist. The sentiment may not be a popular one to utter
+before an assembly of biologists, but looking at the truth impersonally
+I suspect that when at length minds of first rate analytical
+power are attracted to biological problems, some advance will
+be made of the kind which we are awaiting.</p>
+
+<p class="indent">The study of the phenomena of bodily symmetry offers
+perhaps the most hopeful point of attack. The essential fact
+in reproduction is cell-division, and the essential basis of hereditary
+resemblance is the symmetry of cell-division. The phenomena
+of twinning provide a convincing demonstration that this
+is so. By twinning we mean the production of equivalent structures
+by division. The process is one which may affect the whole
+body of an animal or plant, or certain of its parts. The term
+<span class="pagenum"><a name="Page_42" id="Page_42">[Pg 42]</a></span>
+twin as ordinarily used refers to the simultaneous birth of two
+individuals. Those who are naturalists know that such twins
+are of two kinds, (1) twins that are not more alike than any other
+two members of the same family, and (2) twins that are so much
+alike that even intimate friends mistake them. These latter
+twins, except in imaginative literature, are always of the same sex.</p>
+
+<p class="indent">It is scarcely necessary for me to repeat the evidence
+from which it has been concluded that without doubt such twins arise
+by division of the same fertilised ovum. There is a perfect
+series of gradations connecting them with the various forms of
+double monsters united by homologous parts. They have been
+shown several times to be enclosed in the same chorion, and the
+proofs of experimental embryology show that in several animals
+by the separation of the two first hemispheres of a dividing egg
+twins can be produced. Lastly we have recently had the extraordinarily
+interesting demonstration of Loeb, to which I may
+specially refer. Herbst some years ago found that in sea water,
+from which all lime salts had been removed, the segments of the
+living egg fall apart as they are formed. Using this method
+Loeb has shown that a temporary immersion in lime-free sea
+water may result in the production of 90 per cent. of twins.
+We are therefore safe in regarding the homologous or "identical"
+twins as resulting from the divisions of one fertilised egg, while
+the non-identical or "fraternal" twins, as they are called, arise
+by the fertilisation of two separate ova.<a name="FNanchor_3_20" id="FNanchor_3_20"></a><a href="#Footnote_3_20" class="fnanchor">[3]</a>
+<span class="pagenum"><a name="Page_43" id="Page_43">[Pg 43]</a></span></p>
+
+<p class="indent">In the resemblance of identical twins we have an extreme case
+of hereditary likeness<a name="FNanchor_4_21" id="FNanchor_4_21"></a><a href="#Footnote_4_21" class="fnanchor">[4]</a>
+and a proof, if any were needed, that the
+cause of individual variation is to be sought in the differentiation
+of germ-cells. The resemblance of identical twins depends on
+two circumstances, First, since only two germ-cells take part
+in their production, difference between the germ cells of the
+same individual cannot affect them. Secondly the division of
+the fertilised ovum, the process by which they became two instead
+of one, must have been a symmetrical division. The
+structure of twins raises however one extremely significant
+difficulty, which as yet we cannot in any way explain. The
+resemblance between twins is a phenomenon of symmetry,
+like the resemblance between the two sides of a bilaterally symmetrical
+body. Not only is the general resemblance readily
+so interpreted, but we know also that in double monsters, namely
+unseparated twins, various anatomical abnormalities shown by
+the one half-body are frequently shown by the other half-also.<a name="FNanchor_5_22" id="FNanchor_5_22"></a><a href="#Footnote_5_22" class="fnanchor">[5]</a>
+The two belong to one system of symmetry. How then
+does it happen that the body of one of a pair of twins does not
+show a transposition of viscera? We know that the relation of
+right and left implies that the one should be the mirror-image of
+the other. Such a relation of images may be maintained even
+in minute details. For example if the same pattern of finger-print
+is given by the fingers of the two hands, one is the reverse
+of the other. In double monsters, namely unseparated twins,
+there is evidence that an inversion of viscera does occur with
+some frequency. Evidence from such cases is not so clear and
+simple as might be expected, because as a matter of fact, the
+heart and stomach, upon which the asymmetry of the viscera
+chiefly depend, are usually common to the two bodies. Duplicity
+generally affects either the anterior end alone, or the posterior
+end alone. The division is generally <i>from the heart forwards</i>,
+giving two heads and two pairs of anterior limbs on a common
+trunk, or <i>from the heart backwards</i>, giving two pairs of posterior
+limbs with the anterior body common. In either case, though
+<span class="pagenum"><a name="Page_44" id="Page_44">[Pg 44]</a></span>
+the bodies may be grouped in a common system of symmetry,
+neither can be proved to show definite reversal of the parts. To
+see that reversal recourse must be had to more extreme duplications,
+such as the famous Siamese Twins. They, as a matter of
+fact, were an excellent instance of the proposition that twins
+are related as mirror-images, for both of them had eleven pairs
+of ribs instead of the normal twelve, and one of them had a partial reversal
+of viscera.<a name="FNanchor_6_23" id="FNanchor_6_23"></a><a href="#Footnote_6_23" class="fnanchor">[6]</a>
+(Küchenmeister, <i>Verlagerung</i>, etc., p. 204.)</p>
+
+<p class="indent">If anyone could show how it is that neither of a
+pair of twins has transposition of viscera the whole mystery of division
+would, I expect, be greatly illuminated.<a name="FNanchor_7_24" id="FNanchor_7_24"></a><a href="#Footnote_7_24" class="fnanchor">[7]</a>
+At present we have simply to accept the fact that twins, by virtue of
+their detachment from each other, have the power of resuming the polarity
+which is proper to any normal individual. It was nevertheless
+with great interest that I read Wilder's recent observation<a name="FNanchor_8_25" id="FNanchor_8_25"></a><a href="#Footnote_8_25" class="fnanchor">[8]</a>
+that occasionally in identical twins the finger-print of one or both
+the index-fingers may be reversed, showing that there is after
+all some truth in the notion that reversal should occur in them.</p>
+
+<p class="indent">There is another phenomenon by twinning which, if we
+could understand it, might help. I refer to the free-martin, the subject
+of one of John Hunter's masterpieces of anatomical description.
+In horned cattle twin births are rare, and when twins of opposite
+sexes are born, the male is perfect and normal, but the reproductive
+<span class="pagenum"><a name="Page_45" id="Page_45">[Pg 45]</a></span>
+organs of the female are deformed and sterile, being
+known as a free-martin. The same thing occasionally happens
+in sheep, suggesting that in sheep also twins may be formed by
+the division of one ovum; for it is impossible to suppose that
+mere development in juxtaposition can produce a change of this
+character. I mention the free-martin because it raises a question
+of absorbing interest. It is conceivable that we should interpret
+it by reference to the phenomenon of gynandromorphism, seen
+occasionally in insects, and also in birds as a great rarity. In
+the gynandromorph one side of the body is male, the other female.
+A bullfinch for instance has been described with a sharp line of
+division down the breast between the red feathers of the cock
+on one side and the brown feathers of the hen on the other.
+(Poll, H., <i>SB. Ges. Nat. Fr.</i>, Berlin, 1909, p. 338.) In such cases
+neither side is sexually perfect. If the halves of such a gynandromorph
+came apart, perhaps one would be a free-martin.</p>
+
+<p class="indent">The behaviour of homologous twinning in heredity has been
+little studied. It does not exist as a normal feature in any animal
+which is amenable to experiment, and we cannot positively
+assert that a comparable phenomenon exists in plants; for in
+them&mdash;the Orange, for example&mdash;polyembryony may evidently
+be produced by a parthenogenetic development of nucellar tissue.
+It is possible that in Man twinning is due to a peculiarity of the
+mother, not of the father. It may and not rarely does descend
+from mother to daughter, but whether it can be passed on
+through a male generation to a daughter again, there is not
+sufficient evidence to show. The facts as far as they go are
+consistent with the inference which may be drawn from Loeb's
+experiment, that the twinning of a fertilized ovum may be determined
+not by the germ-cells which united to form it, but by
+the environment in which it begins to develop. The opinion that
+twinning may descend through the male directly has been lately
+expressed by Dr. J. Oliver in the <i>Eugenics Review</i> (1912), on the
+evidence of cases in which twins had occurred among the relations
+of fathers of twins, but I do not know of any comprehensive
+collection of evidence bearing on the subject.
+<span class="pagenum"><a name="Page_46" id="Page_46">[Pg 46]</a></span></p>
+
+<p class="indent">Besides twinning of the whole body a comparable duplicity
+of various parts of the same body may occur. Such divisions
+affect especially those organs which have an axis of bilateral
+symmetry, such as the thumb, a cotyledon, a median petal,
+the frond of a fern or the anal fin of a fish. From the little
+yet known it is clear that the genetic analysis of these conditions
+must be very difficult, but evidence of any kind regarding them
+will be valuable. We want especially to know whether these
+divisions are due to the <i>addition</i> of some factor or power which
+enables the part to divide, or whether the division results from
+the <i>absence</i> of something which in the normal body prevents
+the part from dividing. Breeding experiments, so far as they
+go, suggest that the less divided state is usually dominant to
+the more divided.<a name="FNanchor_9_26" id="FNanchor_9_26"></a><a href="#Footnote_9_26" class="fnanchor">[9]</a>
+The two-celled Tomato fruit is dominant to
+the many-celled type. The Manx Cat's tail, with its suppression
+of caudal segmentation is a partial dominant over the normal
+tail. The tail of the Fowl in what is called the "Rumpless"
+condition is at least superficially comparable with that of the
+Manx Cat, and though the evidence is not wholly consistent,
+Davenport obtained facts indicating that this suppressed condition
+of the caudal vertebrae is an imperfect dominant.<a name="FNanchor_10_27" id="FNanchor_10_27"></a><a href="#Footnote_10_27" class="fnanchor">[10]</a>
+</p>
+
+<p class="indent">Some evidence may also be derived from other examples
+of differences which at first sight appear to be substantive though
+they are more probably meristic in ultimate nature. The
+distinction between the normal and the "Angora" hair of the
+Rabbit is a case in point. We can scarcely doubt that one of
+the essential differences between these two types is that in the
+Angora coat the hair-follicles are more finely divided than they
+are in the normal coat, and we know that the normal, or less-divided
+condition, is dominant to the Angora, or more finely divided.</p>
+<p><span class="pagenum"><a name="Page_47" id="Page_47">[Pg 47]</a></span></p>
+
+<div class="figcenter" >
+  <img src="images/i_060.jpg" alt="Elephant Tusk" width="500" height="602" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+   <b><span class="smcap">Fig. 3.</span>&emsp; <i>I</i>, <i>II</i>, <i>III</i>,</b>
+various degrees of syndactyly affecting the medius and annularis in the hand;
+<b><i>IV</i></b>, syndactyly affecting the index and medius in the foot. (After Annandale.)</p>
+
+<p class="indent space-below">In the case of the solid-hoofed or "mule-footed" swine, the
+evidence shows, as Spillman has lately pointed out,<a name="FNanchor_11_28" id="FNanchor_11_28"></a><a href="#Footnote_11_28" class="fnanchor">[11]</a>
+that the condition behaves as a dominant. The essential feature of
+this abnormality is that the digits III and IV are partially
+united. The union is greatest peripherally. Sometimes the
+third phalanges only are joined to form one bone, but the second
+and even the first phalanges may also be compounded together.
+Here the variation is obviously meristic and consists in a failure
+to divide, the normal separation of the median digits of the foot being suppressed.
+<span class="pagenum"><a name="Page_48" id="Page_48">[Pg 48]</a></span></p>
+
+<div class="figcenter" >
+  <img src="images/i_061.jpg" alt="Syndactyly in the Foot" width="600" height="315" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Fig. 4.</span></b>&emsp; Case of complete syndactyly in the foot. <b><i>II</i></b>
+and <b><i>III</i></b>, digit apparently representing the index and medius.
+<b><i>c</i><sup>2</sup> + <i>c</i><sup>3</sup></b>, bone apparently representing the
+middle and external cuneiform; <b><i>cb</i></b>, cuboid; <b><i>c</i><sup>1</sup></b>,
+internal cuneiform. (After Gruber.)</p>
+
+<p class="indent">Webbing between the digits, in at least some of its
+manifestations, is a variation of similar nature. The family recorded
+by Newsholme<a name="FNanchor_12_29" id="FNanchor_12_29"></a><a href="#Footnote_12_29" class="fnanchor">[12]</a>
+very clearly shows the dominance of this condition.
+The case is morphologically of great interest and must
+undoubtedly have a bearing on the problems of the mechanics
+of Division. In discussing the phenomena of syndactylism
+I pointed out some years ago that the digits most frequently
+united in the human hand are III and IV, while in the foot,
+union most frequently takes place between II and III.<a name="FNanchor_13_30" id="FNanchor_13_30"></a><a href="#Footnote_13_30" class="fnanchor">[13]</a>
+In Newsholme's family the union was always between II and III
+of the foot, except in the case of one male who had the digits
+III and IV of the right <i>hand</i> alone webbed together. There
+can be little doubt that the geometrical system on which the
+foot is planned has an axis of symmetry passing between the
+digits II and III, while the corresponding axis in the hand passes
+between III and IV. Union between such digits may therefore
+be regarded as comparable with any non-division or "coalescence"
+of lateral structures in a middle line, and when as in these examples
+<span class="pagenum"><a name="Page_49" id="Page_49">[Pg 49]</a></span>
+such a condition is shown to be a dominant we cannot
+avoid the inference that some concrete factor has the power of
+suppressing or inhibiting this division. Figs. 3 and 4 illustrate
+degrees of union between digits in the human hand and foot.</p>
+
+<p class="indent">It is not in question that various other forms of irregular
+webbing and coalescence of digits exist, and respecting the genetic
+behaviour of these practically nothing is as yet known. Such
+a case is described by Walker,<a name="FNanchor_14_31" id="FNanchor_14_31"></a><a href="#Footnote_14_31" class="fnanchor">[14]</a>
+in which the first and second
+metacarpals of both feet were fused in mother and daughter,
+and several more are found in literature. Contrasted with these
+phenomena we have the curious fact that in the Pigeon, Staples-Browne
+found webbing of the toes a <i>recessive</i> character. The
+question thus arises whether this webbing is of the same nature
+as that shown to be a dominant in Man, and indeed whether the
+phenomenon in pigeons is really meristic at all. There is some
+difference perceptible between the two conditions; for in Man
+there is not so much a development of a special web-like skin
+uniting the digits as a want of proper division between the digits
+themselves, and in extreme cases two digits may be represented
+by a single one. In the Pigeon I am not aware that a real
+union of this kind has ever been observed, and though the web-like
+skin may extend the whole length of the digits and be so narrow
+as to prevent the spread of the toes, it may, I think, be maintained
+that the unity of the digits is unimpaired. For the
+present the nature of this variation in the pigeon's feet must be
+regarded as doubtful, and we should note that if it is actually
+an example of a more perfect division being dominant to a less
+perfect division, the case is a marked exception to the general
+rule that non-division is dominant to division.</p>
+
+<p class="indent">Reference must also be made to the phenomenon of fasciation
+in the stems of plants. As Mendel showed in the case of <i>Pisum</i>
+this condition is often a recessive. The appearances suggest
+that the difference between a normal and a fasciated plant
+consists in the inability of the fasciated plant to separate its
+lateral branches. The nature of the condition is however very
+<span class="pagenum"><a name="Page_50" id="Page_50">[Pg 50]</a></span>
+obscure and it is equally likely that some multiplication of the
+growing point is the essential phenomenon.<a name="FNanchor_15_32" id="FNanchor_15_32"></a><a href="#Footnote_15_32" class="fnanchor">[15]</a>
+</p>
+
+<p>Stockard's interesting experiments<a name="FNanchor_16_33" id="FNanchor_16_33"></a><a href="#Footnote_16_33" class="fnanchor">[16]</a>
+illustrate this question. He showed that by treating the embryos of a fish
+(<i>Fundulus heteroclitus</i>) with a dilute solution of magnesium salts,
+various cyclopian monstrosities were frequently produced. These have
+been called cases of <i>fusion</i> of the optic vesicles. I would prefer
+to regard them as cases of a division suppressed or restricted by
+the control of the environment. Conversely, the splendid discovery
+of Loeb, that an unfertilised egg will divide and develop
+parthenogenetically without fertilisation, as a consequence of
+exposure to various media, may be interpreted as suggesting that
+the action of those media releases the strains already present
+in the ovum, though I admit that an interpretation based on the
+converse hypothesis, that the medium acts as a stimulus, is as
+yet by no means excluded.</p>
+
+<p class="indent">In these cases we come nearest to the direct causation
+or the direct inhibition of a division, but the meaning of the
+evidence is still ambiguous. I incline to compare Loeb's parthenogenesis
+with the development (and of course accompanying
+cell-division) of dormant buds on stems which have been cut back.</p>
+
+<p class="indent">It is interesting to note that sometimes as an abnormality,
+the faculty of division gets out of hand and runs a course apparently
+uncontrolled. A remarkable instance of this condition
+is seen in <i>Begonia</i> "<i>phyllomaniaca</i>", which breaks out into buds
+at any point on the stem, petioles, or leaves, each bud having,
+like other buds, the power of becoming a new plant if removed.
+We would give much to know the genetic properties of <i>B. phyllomaniaca</i>,
+and in conjunction with Mr. W. O. Backhouse I have
+for some time been experimenting with this plant. It proved
+totally sterile. Its own anthers produce no pollen, and all attempts
+to fertilise it with other species failed though the pollen
+of a great number of forms was tried.
+<span class="pagenum"><a name="Page_51" id="Page_51">[Pg 51]</a></span></p>
+
+<p class="indent space-below">Recently however we have succeeded in making plants
+which are in every respect <i>Begonia phyllomaniaca</i>, so far as the characters
+of stems and leaves are concerned. These plants, of
+which we have sixteen, were made by fertilising <i>B. heracleifolia</i>
+with <i>B. polyantha</i>. They are all beginning to break out in
+"phyllomania." As yet they have not flowered, but as they
+agree in all details with <i>phyllomaniaca</i> there can be little doubt
+that the original plant bearing that name was a hybrid similarly
+produced. The production of "phyllomania" on a hybrid
+Begonia has also been previously recorded by Duchartre.<a name="FNanchor_17_34" id="FNanchor_17_34"></a><a href="#Footnote_17_34" class="fnanchor">[17]</a>
+In this case the cross was made between <i>B. incarnata</i> and <i>lucida</i>.
+The synonymy of the last species is unfortunately obscure, and
+I have not succeeded in repeating the experiment.</p>
+
+<div class="figcenter" >
+  <img src="images/i_064.jpg" alt="Petiole" width="600" height="223" />
+</div>
+<p class="space-below" style="font-size: 120%; text-align: center;">
+<b><span class="smcap">Fig. 5.</span>&emsp;Piece of petiole of</b> <i>Begonia phyllomaniaca</i>.
+<b>The proximal end is to the right of the figure.</b></p>
+
+<p class="indent space-below">From these facts it seems practically certain that
+the condition is one which is due to the meeting of complementary factors.
+At first sight we may incline to think that the phyllomania is in
+some way due to the sterility. This however cannot be seriously
+maintained; for not only is sterility in plants not usually associated
+with such manifestations, but we know a Begonia called
+"Wilhelma" which is exactly <i>phyllomaniaca</i> and equally sterile,
+though it has no trace of phyllomania. This plant arose in the
+nurseries of MM. P. Bruant of Poitiers, and has generally been
+described as a seedling of <i>phyllomaniaca</i>, but from the total
+sterility of that form this account of its origin must be set aside.
+<span class="pagenum"><a name="Page_52" id="Page_52">[Pg 52]</a></span></p>
+
+<div class="figcenter" >
+  <img src="images/i_065.jpg" alt="Petiole" width="600" height="560" />
+</div>
+<p style="font-size: 120%;">
+<b><span class="smcap">Fig. 6.</span></b>&emsp;Two right hind feet of polydactyle
+cats. <b><i>II</i></b> shows the lowest development of the condition yet recorded.
+The digit, <b><i>d</i><sup>1</sup></b>, which stands as hallux is
+fully formed and has three phalanges. Both it and the digit marked
+<b><i>d</i><sup>2</sup></b> are formed as <b><i>left</i></b> digits. In the
+normal hind foot of the cat the hallux is represented by a rudiment only.</p>
+
+<p class="space-below" style="font-size: 120%;"><b><i>I</i></b>
+shows a further development of the condition. In this foot
+there are <i>six</i> digits. <b><i>d</i><sup>1</sup></b> has two phalanges, but both
+it and <b><i>d</i><sup>2</sup></b> and <b><i>d</i><sup>3</sup></b> are shaped as left digits.
+Thus <b><i>d</i><sup>3</sup></b>, which in the normal foot would be shaped as a right digit,
+is transformed so as to look like a <b><i>left</i></b> digit.</p>
+
+<p class="indent">The phenomenon in this case can hardly be regarded
+as due to the excitation of dormant buds, for it is apparent on
+examination that the new growths are not placed in any fixed
+geometrical relation to the original plant. They arise on the
+petiole, for example, as small green outgrowths each of which
+gradually becomes a tiny leaf. The attitude of these leaves is
+quite indeterminate, and they may point in any direction,
+<span class="pagenum"><a name="Page_53" id="Page_53">[Pg 53]</a></span>
+some having their apices turned peripherally, some centrally, and
+others in various oblique or transverse positions (Fig. 5). These
+little leaves are thus comparable with seedlings, in that their
+polarity is not related to, or consequent upon that of the parent
+plant. They have in fact that "individuality," which we associate
+with germinal reproduction.</p>
+
+<p class="indent">There are many curious phenomena seen in the behaviour
+of parts normally repeated in bilateral symmetry which may some
+day guide us towards an understanding of the mechanics of
+division. A part like a hand, which needs the other hand to
+complete its symmetry, cannot twin by mere division, yet by
+proliferation and special modifications on the radial side of the
+same limb, even a hand may be twinned. In the well known polydactyle
+cats a change of this kind is very common and indeed
+almost the rule. When extra digits appear at the inner (tibial)
+side of the limb, they are shaped as digits of the other side, and
+even the normal digit II (index) is usually converted into the
+mirror-image of its normal self. The limb then develops a
+new symmetry in itself. Nevertheless it is not easy to interpret
+these facts as meaning that there has been some interruption
+in the control which one side of the body exercises over the
+other. The heredity of polydactylism is complex but there is
+little doubt that the condition familiar in the Cat is a dominant.
+In some human cases also the descent is that of a dominant, but
+irregularities are so frequent that no general rule can yet be
+perceived. The dominance of such a condition is an exception
+to the principle that the less-divided is usually dominant to the
+more-divided, a fact which probably should be interpreted as
+meaning that divisions are of more than one kind.</p>
+
+<p class="indent">Among ordinary somatic divisions, whether of organs,
+cells, or patterns of differentiation, the control of symmetry is usually
+manifested. There is however one class of somatic differentiations
+which are exceptionally interesting from the fact that they
+may show a complete independence of such geometrical control.
+The most familiar examples of these geometrically uncontrolled
+Variations are to be seen in bud-sports. The normal differentiation
+of the organs of a plant is arranged on a definite geometrical
+<span class="pagenum"><a name="Page_54" id="Page_54">[Pg 54]</a></span>
+system, which to those who have never given special
+attention to such things before, will often seem surprisingly
+precise. The arrangement of the leaves on uninjured, free-growing
+shoots can generally be seen to follow a very definite
+order, just as do the flowers or the parts of the flowers. If
+however bud sports occur, then though the parts included in
+the sports show all the geometrical peculiarities proper to the
+sport-variety, yet the sporting-buds themselves are not related
+to each other according to any geometrical plan.</p>
+
+<p class="indent">A very familiar illustration is provided by the
+distribution of colour in those Carnations that are not self-coloured.
+The pigment may, as in Picotees, be distributed peripherally with
+great regularity to the edges of the petals; or, as in Bizarres and
+Flakes, it may be scattered in radial sectors which show no
+geometrical regularity. Now in this case the pigments are the
+same in both types of flower, and the chemical factors concerned
+in their production must surely be the same. The difference
+must lie in the mechanical processes of distribution of the pigment.
+In the Picotee we see the orderly differentiation which we
+associate with normality; in the Bizarre we see the disorderly
+differentiation characteristic of bud-sports. The distribution of
+colour in this case lies outside the scheme of symmetry of the plant.</p>
+
+<p class="indent">Such a distribution is characteristic of bud-sports,
+and of certain other differentiations in both plants and animals, which
+I cannot on this occasion discuss. Now reflexion will show that
+these facts have an intimate bearing on the mechanical problems
+of heredity. For first in the bud-sports we are witnessing the
+distribution of factors which distinguish genetic varieties. We
+do not know the physical nature of those factors, but if we must
+give them a name, I suppose we should call them "ferments"
+exactly as Boyle did in 1666. He is discussing how it comes about
+that a bud, budded on a stock, becomes a branch bearing the
+fruit of its special kind. He notes that though the bud inserted
+be "not so big oftentimes as a Pea," yet "whether by the help
+of some peculiar kind of Strainer or by the Operation of some
+powerful Ferment lodged in it, or by both these, or some other
+<span class="pagenum"><a name="Page_55" id="Page_55">[Pg 55]</a></span>
+cause," the sap is "so far changed as to constitute a Fruit quite
+otherwise qualify'd."<a name="FNanchor_18_35" id="FNanchor_18_35"></a><a href="#Footnote_18_35" class="fnanchor">[18]</a>
+We can add nothing to his speculation,
+and we believe still that by a differential distribution of "ferments"
+the sports are produced. All the factors are together
+present in the normal parts; some are left out in the sport.
+In an analogous case however, that of a variegated <i>Pelargonium</i>
+which has green and also albino shoots, Baur proved that the
+shoots pure in colour are also pure in their posterity. There
+can be no doubt that the sports of Carnations, Azaleas, Chrysanthemums,
+etc., would behave in the same way.</p>
+
+<p class="indent">The well-known Azaleas Perle de Ledeburg, President
+Kerchove, and <i>Vervaeana</i> are familiar illustrations. Perle de
+Ledeburg is predominantly white, but it has red streaks in some
+of its flowers. It not very rarely gives off a self-red sport. This
+is evidently due to the development of a bud in a red-bearing
+area of the stem. The red in this plant is not under "geometrical
+control." Many plants have white flowers with no markings,
+but if the red markings are geometrically ordered differentiations,
+no self-coloured sports are formed. The case of <i>Vervaeana</i> is a
+good illustration of this proposition. It has white flowers with
+red markings arranged in an orderly manner on the lower parts
+of the petals, especially on the dorsal petals. This is one of the
+Azaleas most liable to have red sports, and at first sight it might
+seem that the sport represented the red of the central marks.
+Examination however of a good many flowers shows that irregular
+red streaks like those of Perle de Ledeburg occur, about as commonly
+as in that variety. <i>Vervaeana</i> in fact is Perle de Ledeburg
+with <i>definite</i> red markings added, and its red sports obviously are
+those branches the germs of which came in a patch of the stem
+bearing these red elements. That this is the true account is
+rendered quite obvious by the fact that the red of the sport is a
+colour somewhat different from that of the definite marks, and
+that these marks are still present on the red ground of the sporting
+flowers.</p>
+
+<p class="indent">It will be understood that these remarks apply to those
+cases in which the production of sports is habitual or frequent, and
+<span class="pagenum"><a name="Page_56" id="Page_56">[Pg 56]</a></span>
+I imagine in all such examples it will be found that there are
+indications of irregularity in the distribution of the differentiations
+such as to justify the view that they are not under that
+geometrical control which governs the normal differentiation
+of the parts. The question next arises whether these considerations
+apply also to the production of a bud-sport as a rare
+exception, but by the nature of the case it is not possible to
+say positively whether the appearance of an exceptional sport
+is due to the unsuspected presence of a pre-existing fragment of
+material having a special constitution, or to the origin, <i>de novo</i>,
+of such a material. For instance one of the garden forms of
+<i>Pelargonium</i> known as <i>altum</i> is liable perhaps once in some
+hundreds of flowers to have one or two magenta petals. The
+normal colour is a brilliant red; and as we may be fairly sure that
+this red is recessive to magenta the interpretation would be
+quite different according as the appearance of the magenta is
+regarded as due to the presence of small areas endowed with
+magentaness, or to the spontaneous generation of the factor
+for that pigment. Either interpretation is possible on the facts,
+but the view that the whole plant has in it scarce mosaic particles
+of magenta seems on the whole more consistent with present knowledge.</p>
+
+<p class="indent">In <i>Pelargonium altum</i> the enzyme causing the magenta
+colours must be distributed in very small areas, but a case in which the
+magenta is similarly arranged in a much coarser patchwork
+may be seen in the <i>Pelargonium</i> "Don Juan," which often bears
+whole trusses or branches of red flowers upon plants having the
+normal dominant magenta trusses. In most cases there is little
+doubt that though the magenta flowered parts can "sport" to
+red, the red parts could not produce the magenta flowers.</p>
+
+<p class="indent">The asymmetrical, or to speak more precisely, the disorderly,
+mingling of the colours in the somatic parts is thus an indication
+of a similarly disorderly mixing of the factors for those colours
+in the germ-tissues, so that some of the gametes bear enough of
+the colour-factors to make a self-coloured plant, while others
+bear so little that the plant to which they give rise is a patchwork.
+If this view is correct we may extend it so far as to consider
+<span class="pagenum"><a name="Page_57" id="Page_57">[Pg 57]</a></span>
+whether the fineness or coarseness of the mixture visible
+in the flowers or leaves may not give an indication of the degree
+to which the factors are subdivided among the germ-cells. We
+know very little about the genetic properties of striped varieties.
+In both <i>Antirrhinum</i> and <i>Mirabilis</i> it has been found that the
+striped may occasionally and irregularly throw self-coloured
+plants, and therefore the striping cannot be regarded simply as
+a recessive character. On the other hand in <i>Primula Sinensis</i>
+there are well-known flaked varieties which ordinarily at least
+breed true. Whether these ever throw selfs I do not know,
+but if they do it must be quite exceptionally. The power of
+these flaked plants to breed true is, I suspect, connected with
+the fact that in their flowers the coloured and white parts are
+<i>intimately</i> mixed, this intimate mixture thus being an indication
+of a similarly intimate mixture in the germ-cells. It would be
+important to ascertain whether self-fertilised seed from the occasional
+flowers in which the colour has run together to join a large patch gives
+more self-coloured plants than the intimately flaked flowers do.</p>
+
+<p class="indent">The next fact may eventually prove of great importance.
+We have seen that in bud-sports the differentiation is of the same
+nature as that between pure types, and also that in the sporting
+plant this differentiation is distributed without any reference
+to the plant's axis, or any other consideration of symmetry.
+Now among the germ-cells of a Mendelian hybrid exactly such
+characters are being distributed allelomorphically, and there
+again we have strong evidence for believing that the distribution
+obeys no pattern. For example, we can in the case of seeds still
+<i>in situ</i> perceive how the characters were distributed among the
+germ-cells, and there is certainly no obvious pattern connecting
+them, nor can we suppose that there is an actual pattern obscured.</p>
+
+<p class="indent">Of this one illustration is especially curious. Individual
+plants of the same species are, as regards the decussations of
+their leaves and in other respects, <i>either rights or lefts</i>.
+The fact is not emphasized in modern botany and is in some danger of
+being forgotten. When, as in the flowers of Arum, some <i>Gladioli</i>,
+<i>Exacum</i>, <i>St. Paulia</i>, or the fruits of <i>Loasa</i>, rights and lefts occur
+<span class="pagenum"><a name="Page_58" id="Page_58">[Pg 58]</a></span>
+on the same stem, they come off alternately. But if, as in the
+seedlings of Barley the twist of the first leaf be examined, it
+will be seen to be either a right-or left-handed screw. An
+ear of barley, say a two-row barley, is a definitely symmetrical
+structure. The seeds stand in their envelopes back to back in
+definite positions. Each has its organs placed in perfectly
+definite places. <i>If these seeds were buds</i> their differentiations
+would be grouped into a common plan. One might expect that
+the differentiations of these embryos would still fall into the
+pattern; but they do not, and so far as I have tested them, any
+one may be a right or a left, just as each may carry any of the
+Mendelian allelomorphs possessed by the parent plant, without
+reference to the differentiation of any other seed. The fertilisation
+may be responsible, but our experience of the allelomorphic
+characters suggest that the irregularity is in the egg-cells
+themselves.<a name="FNanchor_19_36" id="FNanchor_19_36"></a><a href="#Footnote_19_36" class="fnanchor">[19]</a>
+</p>
+
+<p class="indent"><i>Germ cells thus differ from somatic cells in the fact
+that their differentiations are outside the geometrical order which governs
+the differentiation of the somatic cells.</i> I can think of possible
+exceptions, but I have confidence that the rule is true and I
+regard it as of great significance.</p>
+
+<p class="indent">The old riddle, what is an individual, finds at least a partial
+solution in the reply that an individual is a group of parts differentiated
+in a geometrically interdependent order. With the
+germ-cell a new geometrical order, with independent polarity is
+almost if not quite always, begun, and with this geometrical independence
+the power of rejuvenescence may possibly be associated.</p>
+
+<p class="indent">The problems thus raised are unsolved, but they do
+not look insoluble. The solution may be nearer than we have thought.
+In a study of the geometry of differentiation, germinal and
+somatic, there is a way of watching and perhaps analyzing what
+may be distinguished as the mechanical phenomena of heredity.
+If any one could in the cases of the Picotee and the Bizarre Carnation,
+respectively, detect the real distinction between the two
+<span class="pagenum"><a name="Page_59" id="Page_59">[Pg 59]</a></span>
+types of distribution, he would make a most notable advance.
+Any one acquainted with mechanical devices can construct a
+model which will reproduce some of these distinctions more or less
+faithfully. The point I would not lose sight of is that the analogy
+with such models must for a long way be a true and valuable
+guide. I trust that some one with the right intellectual equipment
+will endeavor to follow this guide; and I am sanguine enough
+to think that a comprehensive study of the geometrical phenomena
+of differentiation will suggest to a penetrative mind that critical
+experiment which may one day reveal the meaning of spontaneous
+division, the mystery through which lies the road, perhaps the
+most hopeful, to a knowledge of the nature of life.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_60" id="Page_60">[Pg 60]</a></span></p>
+<h2>CHAPTER III</h2>
+<h3><span class="smcap">Segmentation, Organic And Mechanical</span></h3>
+
+<p class="indent">Models may be and often have been devised imitating
+some of the phenomena of division, but none of them have reproduced
+the peculiarity which characterises divisions of living tissues,
+that <i>the position of chemical differentiation</i> is <i>determined by
+those divisions</i>. For example, models of segmentation, whether
+radial or linear, may be made by the vibration of plates as in
+the familiar Chladni figures of the physical laboratory, or by
+the bowing of a tube dusted on the inside with lycopodium
+powder, and in various other ways. The sand or the powder
+will be heaped up in the nodes or regions of least movement, and
+the patterns thus formed reproduce many of the geometrical
+features of segmentation. But in the segmentations of living
+things the nodes and internodes, once determined by the dividing
+forces, would each become the seat of appropriate and distinct
+chemical processes leading to the differentiation of the parts,
+and the deposition of the bones, petals, spines, hairs, and other
+organs in relation to the meristic ground-plan. The "ripples" of
+meristic division not merely divide but differentiate, and when
+a "ripple" forks the result is not merely a division but a reduplication
+of the organ through which the fork runs. An
+example illustrating such a consequence is that of the half-vertebrae
+of the Python. On the left side the vertebra is single
+(Fig. 7) and bears a single rib, but on the right side a division
+has occurred with the result that two half-vertebrae, each
+bearing a rib, are formed, one standing in succession to the
+other. We cannot, indeed, imagine any operation of physiological
+division carried out in such an organ as a vertebra, passing
+through a plane at right angles to the long axis of the body, which
+does not necessarily involve the further process of reduplication.</p>
+
+<p class="indent space-below">As the meristic system of distribution spreads through
+the body, chemical differentiations follow in its track, with
+<span class="pagenum"><a name="Page_61" id="Page_61">[Pg 61]</a></span>
+segmentation and pattern as the visible result. Could we analyse
+these simultaneous phenomena and show how it is that the places
+of chemical differentiation are determined by the system of
+division, progress would then be rapid. It is here that all
+speculation fails.</p>
+
+<div class="figcenter" >
+  <img src="images/i_074.jpg" alt="" width="600" height="654" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Figs.</span> 7 and 8.</b>&emsp;Two examples of
+imperfect division in the vertebræ of a python. <b><i>I</i></b>, the
+vertebræ <b>147-150</b> from the right side, showing imperfect division between
+the <b>148th</b> and <b>149th</b>. The condition on the left side of this
+vertebra was the same. <b><i>II</i></b>, the dorsal surface of vertebræ
+<b>165-167</b>. On the right side the <b>166th</b> is double and bears two ribs,
+but on the left side it is normal and has one rib only.</p>
+
+<p class="indent">Many attempts have been made to interpret the processes
+of division and repetition, in terms of mechanics, or at least to
+refer them to their nearest mechanical analogies, so far with
+<span class="pagenum"><a name="Page_62" id="Page_62">[Pg 62]</a></span>
+little success. The problem is beset with difficulties as yet
+insurmountable and of these one must be especially noticed. In
+the living thing the process by which repetition and patterns
+come into being consists partly in division but partly also in
+growth. We have no means of studying the phenomena of
+pattern-formation except in association with that of growth.
+Growth soon ceases unless division takes place, and if growth is
+impossible division soon ceases also. In consequence of this
+fact that the final pattern is partly a product of growth, it can
+never be used as unimpeachable evidence of the primary geometrical
+relations of the members as laid down in the divisions.</p>
+
+<p class="indent">In the last chapter in referring to the problem of
+repetition I introduced an analogy, comparing the patterns of the organic
+world with those produced in unorganised materials by wave-motion.
+In the preliminary stage of ignorance, having no more
+trustworthy clue, I do not think it wholly unprofitable to consider
+the applicability of this analogy somewhat more fully. It
+possesses, as I hope to show, at least so much validity as to
+encourage the belief that morphology may safely discard one
+source of long-standing error and confusion.</p>
+
+<p class="indent">Those who have studied the structure of parts repeated
+in series will have encountered the old morphological problem of
+"Serial Homology," which has absorbed so much of the attention
+of naturalists and especially of zoologists at various periods.
+This problem includes two separate questions. The first of
+these is the origin in evolution of the resemblance between two
+organs occurring in a repeated series, of which the fore and hind
+limbs of Vertebrates are the prerogative instance. From the
+fact that these resemblances can be traced very far, often into
+minute details of structure, many anatomists have inclined to
+the opinion that the resemblance must originally have been still
+more complete, and that the two limbs, for instance, must have
+acquired their present forms by the differentiation of two identical
+groups of parts.</p>
+
+<p class="indent">Similar questions arise whenever parts are repeated in
+series, whether the series be linear or radial, and, though less obviously,
+even when the repetition is bilateral only. In each such example
+<span class="pagenum"><a name="Page_63" id="Page_63">[Pg 63]</a></span>
+the question arises, is the resemblance between the parts the
+remains of a still closer resemblance, or is differentiation original?
+Sometimes the view that these parts have arisen by the differentiation
+of a series of identical parts is plausible enough,
+as for example when the peculiarities of various appendages of a
+Decapod Crustacean are referred to modifications of the Phyllopod
+series. In application to other cases however we soon meet
+with difficulty, and the suggestion that the segments of a vertebrate
+were originally all alike is seen at once to be absurd, for
+the reason that a creature so constituted could not exist, and that,
+differentiation of at least one anterior and one posterior segment,
+is an essential condition of a viable organism consisting of parts
+repeated in a linear series. Between these two terminal segments
+it is possible to imagine the addition of one segment, or of a
+series of approximately similar segments; but when once it is
+realised that the terminals must have been differentiated from
+the beginning, it will be seen that the problem of the origin of
+the resemblance between segments is not rendered more comprehensible
+by the suggestion that even the intervening members
+were originally alike. Seeing indeed that some differentiation
+must have existed primordially it is as easy to imagine that the
+original body was composed of a series grading from the condition
+of the anterior segment to that of the posterior, as any other
+arrangement. The existence of a linear or successive series in
+fact postulates a polarity of the whole, and in such a system the
+conception of an ideal segment containing all the parts represented
+in the others has manifestly no place. The introduction of that
+conception though sanctioned by the great masters of comparative
+anatomy, has, as I think, really delayed the progress of
+a rational study of the phenomena of division. The same notion
+has been applied to every class of repetition both in animals and
+plants, generally with the same unhappy results. In the cruder
+forms in which this doctrine was taught thirty years ago it is
+now seldom expressed, but modified presentations of it still
+survive and confuse our judgments.
+<span class="pagenum"><a name="Page_64" id="Page_64">[Pg 64]</a></span></p>
+
+<p class="indent space-below">The process of repetition of parts in the bodies
+of organisms is however a periodic phenomenon. This much, provided we
+remain free from prejudice as to the nature and causation of the
+period or rhythm, we may safely declare, and a comparison may
+thus be instituted between the consequences of meristic repetition
+in the bodies of living things and those repetitions which in
+the inorganic world are due to rhythmical processes. Of such
+processes there is a practically unlimited diversity and we have
+nothing to indicate with which of them our repetitions should
+rather be compared.</p>
+
+<div class="figcenter" >
+  <img src="images/i_077.jpg" alt="" width="600" height="665" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Fig. 9.</span></b>&emsp; Osmotic growths simulating segmentation. (After Leduc.)</p>
+
+<p><span class="pagenum"><a name="Page_65" id="Page_65">[Pg 65]</a></span></p>
+
+<p class="indent">In some respects perhaps the best models of living organisms
+yet made are the "osmotic growths" produced by Leduc.<a name="FNanchor_1_37" id="FNanchor_1_37"></a><a href="#Footnote_1_37" class="fnanchor">[1]</a>
+These curious structures were formed by placing a fragment of
+a salt, for instance calcium chloride, in a solution of some colloidal
+substance. As the solid takes up water from the solution
+a permeable pellicle or membrane is formed around it. The vesicle
+thus enclosed grows by further absorption of water, often
+extending in a linear direction, and in many examples this growth
+occurs by a series of rhythmically interrupted extensions. Some
+of the growths thus formed are remarkably like organic structures,
+and might pass for a series of antennary segments or many other
+organs consisting of a linear series of repeated parts. In admitting
+the essential resemblance between these "osmotic growths"
+and living bodies or their organs I lay less stress on the general
+conformation of the growths, which often as Leduc points out, recall
+the forms of fungi or hydroids, but rather on the fact that the
+interruptions in the development of these systems are so closely
+analogous to the segmentations or repetitions of parts characteristic
+of living things (Fig. 9). In the same way I am less impressed
+by Leduc's models of Karyokinesis, wonderful as they
+nevertheless are, for the division is here imitated by putting
+separate drops on the gelatine film. What we most want to know
+is how in the living creature one drop becomes two. The models
+of linear segmentation have the remarkable merit that they do in
+some measure imitate the process of actual division or repetition.
+So in a somewhat modified method Leduc, by causing the diffusion
+of a solution in a gelatine film, produced rhythmical or periodic
+precipitations strikingly reminiscent of various organic tissues,
+for here also the process of periodic repetition is imitated with
+success.</p>
+
+<p class="indent">It is a feature common to these and to all other
+rhythmical repetitions produced by purely mechanical forces that there
+is resemblance between the members of the series, and that this
+similarity of conformation may be maintained in most complex
+detail. When however in the mechanical series some of the
+members differ from the rest we have no difficulty in recognising
+<span class="pagenum"><a name="Page_66" id="Page_66">[Pg 66]</a></span>
+that these differences&mdash;which correspond with the differentiations
+of the organic series&mdash;are due to special heterogeneity in
+the conditions or in the materials, and it never occurs to us to
+suppose that all the members must have been primordially alike.
+For example, in the case of ripple-marks on the sand, which I
+choose as one of the most familiar and obvious illustrations
+of a repeated series due to mechanical agencies, if we notice
+one ripple different in form from those adjacent to it, we do
+not suppose that this variation must have been brought about by
+deformation of a ripple which was at first formed like the others,
+but we ascribe it to a difference in the sand at that point, or to a
+difference in the way in which the wind or the tide dealt with it.
+We may press the analogy further by observing that in as much
+as such a series of waves has a beginning and an end, it possesses
+polarity like that of the various linear series of parts in organisms,
+and even the formation of each member must influence the
+shape of its successor. Since in an organism the beginning and
+end of the series are always included, some differentiation among
+the repetitions must be inevitable. If therefore it be conceded,
+as I think it must, that segmentation and pattern are the consequence
+of a periodic process we realize that it is at least as
+easy to imagine the formation of such a series of parts having
+family likeness combined with differentiation as it would be to
+conceive of their arising primordially as a series of identical repetitions.
+The suggestion that the likenesses which we now perceive
+are the remains of a still more complete resemblance
+is a substitution of a more complex conception for a simpler one.</p>
+
+<p class="indent">The other question raised by the problem of Serial
+Homology is how far there is a correspondence between individual members
+of series when the series differ from each other either in the
+number of parts, or in the mode of distribution of differentiation
+among them. Students, for example, of vertebrate morphology
+debate whether the <i>n</i>th vertebra which carries the pelvic girdle
+in Lizard A is individually homologous with the <i>n</i> + <i>x</i>th vertebra
+which fulfils this function in Lizard B, or whether it is not more
+truly homologous with the vertebra standing in the <i>n</i>th ordinal
+position, though that vertebra in Lizard B is free.
+<span class="pagenum"><a name="Page_67" id="Page_67">[Pg 67]</a></span></p>
+
+<p class="indent">In various and more complex aspects the same question
+is debated in regard to the cranial and spinal nerves, the branches
+of the aorta, the appendages of Arthropoda, and indeed in regard
+to all such series of differentiated parts in linear or successive
+repetition. Persons exercised with these problems
+should before making up their minds consider how similar
+questions would be answered in the case of any series of rhythmical
+repetitions formed by mechanical agencies. In the case
+of our illustration of the ripples in the sand, given the same forces
+acting on the same materials in the same area, the number of
+ripples produced will be the same, and the <i>n</i>th ripple counting
+from the end of the series will stand in the same place whenever
+the series is evoked. If any of the conditions be changed, the
+number and shapes can be changed too, and a fresh "distribution
+of differentiation" created. Stated in this form it is evident
+that the considerations which would guide the judgment in the
+case of the sand ripples are not essentially different from those
+which govern the problem of individual homology in its application
+to vertebrae, nerves, or digits.</p>
+
+<p class="indent">The fact that the unit of repetition is also the unit
+of growth is the source of the obscurity which veils the process. When we
+compare the skeleton of a long-tailed monkey with that of a
+short-tailed or tailless ape we see at once how readily the additional
+series of caudal segments may be described as a consequence
+of the propagation of the "waves" of segmentation
+beyond the point where they die out in the shorter column, and
+we see that with an extension of the series of repetitions there is
+growth and extension of material.</p>
+
+<p class="indent">The considerations which apply to this example will
+be found operating in many cases of the variation of terminal members of
+linear series. Some of these series, like the teeth of the dog,
+end in a terminal member of a size greatly reduced below that of
+the next to it. Even when there is thus a definite specialisation
+of the last member of the series it not infrequently happens that
+the addition, by variation, of a member beyond the normal
+terminal, is accompanied by a very palpable increase in size of
+the member which stands numerically in the place of the normal
+<span class="pagenum"><a name="Page_68" id="Page_68">[Pg 68]</a></span>
+terminal.<a name="FNanchor_2_38" id="FNanchor_2_38"></a><a href="#Footnote_2_38" class="fnanchor">[2]</a>
+So also with variation in the number of ribs, when a
+lumbar vertebra varies homoeotically into the likeness of the
+last dorsal and bears a rib, the rib placed next in front of this,
+which in the normal trunk is the last, shows a definite increase
+in development.</p>
+
+<p class="indent">The consequences of such homoeoses are sometimes
+very extensive, involving readjustments of differentiation affecting
+a long series of members, as may easily be seen by comparing
+the vertebral columns of several individual Sloths<a name="FNanchor_3_39" id="FNanchor_3_39"></a><a href="#Footnote_3_39" class="fnanchor">[3]</a>
+(whether <i>Bradypus</i> or <i>Choloepus</i>) to take a specially striking example.</p>
+
+<p class="indent">It may be urged that no feature as yet enables us
+to perceive wherein lies the primary distinction which determines such
+variation, whether it is due to a difference in the dividing forces
+or in the material to be divided. If for instance we were to
+imitate such a series of segments by pressing hanging drops of
+a viscous fluid out of a paint-tube by successive squeezes, the
+number of times the tube is contracted before it is empty will give
+the number of the segments, but their size may depend either
+on the force of the contractions or on the capacity of the tube,
+or on various other factors. Nevertheless in the case of the
+variation of terminal members, whatever be the nature of the
+rhythmical impulse which produces the series of organs, the elevation
+of the normally terminal member in correspondence with
+the addition of another is what we should expect.</p>
+
+<p class="indent">If the organism acquired its full size first and the
+delimitation of the parts took place afterwards, there might be some hope
+that the resemblance between living patterns and those mechanically
+caused by wave-motion might be shown to be a consequence of
+some real similarity of causation, but in view of the part played
+by growth, appeal to these mechanical phenomena cannot be
+declared to have more than illustrative value. Similarly in as
+much as living patterns appear, and almost certainly do in reality
+come into existence by a rhythmical process, comparisons of
+these patterns with those developed in crystalline structures, and
+in the various fields of force are, as it seems to me, inadmissible,
+or at least inappropriate.
+<span class="pagenum"><a name="Page_69" id="Page_69">[Pg 69]</a></span></p>
+
+<p class="indent">However their intermittence be determined, the
+rhythms of division must be looked upon as the immediate source of
+those geometrically ordered repetitions universally characteristic of
+organic life. In the same category we may thus group the segmentation
+of the Vertebrates and of the Arthropods, the concentric
+growth of the Lamellibranch shells or of Fishes' scales, the
+ripples on the horns of a goat, or the skeletons of the Foraminifera
+or of the Heliozoa. In the case of plant-structures Church<a name="FNanchor_4_40" id="FNanchor_4_40"></a><a href="#Footnote_4_40" class="fnanchor">[4]</a>
+has admirably shown, with an abundance of detail, how on analysis
+the definiteness of phyllotaxis is an expression of such rhythm in
+the division of the apical tissues, and how the spirals and "orthostichies"
+displayed in the grown plant are its ultimate consequences.
+The problem thus narrows itself down to the question
+of the mode whereby these rhythms are determined.</p>
+
+<p class="indent">It is natural that we should incline to refer them to a
+chemical source. If we think of the illustration just given, of the
+segmentation of a viscous fluid into drops by successive contractions
+of a soft-walled tube we can, I think, conceive of such rhythmic
+contractions as due to summations of chemical stimuli, somewhat
+as are the beats of the heart. But when we recognize the vast
+diversity of materials the distribution of which is determined by
+an ostensibly similar rhythmic process it seems hopeless to look
+forward to a directly chemical solution. That the chemical
+degradation of protoplasm or of materials which it contains is
+the source of the energy used in the divisions cannot be in dispute,
+but that these divisions can be themselves the manifestations
+of chemical action seems in the highest degree improbable.</p>
+
+<p class="indent">We may therefore insist with some confidence on the
+distinction between the Meristic and the substantive constitution
+of organisms, between, that is to say, the system according to
+which the materials are divided and the essential composition
+of the materials, conscious of the fact that the energy of division
+is supplied from the materials, and that in the ontogeny the
+manner in which the divisions are effected must depend secondarily
+on the nature of the substances to be divided. The
+<span class="pagenum"><a name="Page_70" id="Page_70">[Pg 70]</a></span>
+mechanical processes of division remain a distinguishable group of
+phenomena, and variations in the substances to be distributed in
+division may be independent of variations in the system by which
+the distribution is effected.</p>
+
+<p class="indent space-below">Modern genetic analysis supplies many remarkable
+examples of this distinction. When formerly we compared the leaves of a
+normal palmatifid Chinese Primula with the pinnatifid leaves<a name="FNanchor_5_41" id="FNanchor_5_41"></a><a href="#Footnote_5_41" class="fnanchor">[5]</a>
+of its fern-leaved variety we were quite unable to say whether the
+difference between the two types of leaf was due to a difference
+in the material cut up in the process of division or to a difference
+in that process itself. Knowledge that the distinction is determined
+by a single segregable factor tends to prove that the
+critical difference is one of substance. So also in the Silky fowl
+we know that the condition of its feathers is due to the absence
+of some one factor present in the normal form. We may conceive
+such differences as due to change of form in the successive
+"waves" of division, but we cannot yet imagine segregation
+otherwise than as acting by the removal or retention of a material
+element. Future observation by some novel method may suggest
+some other possibility, but such cases bring before us very clearly
+the difficulties by which the problem is beset.</p>
+
+<div class="figcenter" >
+  <img src="images/i_084.jpg" alt="Primula Sinensis Leaves" width="600" height="389" />
+</div>
+<p class="space-below" style="font-size: 120%; text-align: center;">
+<b><span class="smcap">Fig. 10.</span></b>&emsp;The palm-and fern type of leaf in
+<i>Primula Sinensis</i>.<br />&emsp;&emsp;&emsp;The palm is dominant and the fern is recessive.</p>
+
+<p class="indent">In another region of observation phenomena occur which
+as it seems to me put it beyond question that the meristic forces are
+essentially independent of the materials upon which they act,
+save, in the remoter sense, in so far as these materials are the
+sources of energy. The physiology of those regenerations and
+repetitions which follow upon mutilation supplies a group of
+facts which both stimulate and limit speculation. No satisfactory
+interpretations of these extraordinary occurrences has
+ever been found, but we already know enough to feel sure that
+in them we are witnessing indications which should lead to the
+discovery of the true mechanics of repetition and pattern.
+The consequences of mutilation in causing new growth or perhaps
+more strictly in enabling new growth to take place, are such that
+they cannot be interpreted as responses to chemical stimuli in
+<span class="pagenum"><a name="Page_71" id="Page_71">[Pg 71]</a></span>
+any sense which the word chemical at present connotes. Powers
+are released by mutilation of which in the normal conditions of
+life no sign can be detected. All who have tried to analyse the
+phenomena of regeneration are compelled to have recourse to the
+metaphor of equilibrium, speaking of the normal body as in a state
+of strain or tension (Morgan) which when disturbed by mutilation
+results in new division and growth. The forces of division
+are inacessible to ordinary means of stimulation. Applications,
+for example, of heat or of electricity excite no responses of a
+positive kind unless the stimuli are so violent as to bring about
+actual destruction.<a name="FNanchor_6_42" id="FNanchor_6_42"></a><a href="#Footnote_6_42" class="fnanchor">[6]</a>
+These agents do not, to use a loose expression,
+come into touch with the meristic forces. Changes in the
+chemical environment of cells may, as in the experiments of
+Loeb and of Stockard produce definite effects, but the facts
+suggest that these effects are due rather to alterations in the
+living material than to influence exerted directly on the forces
+of division themselves.</p>
+
+<p class="indent">By destruction of tissue however the forces both of
+growth and of division also may often be called into action with a
+resulting regeneration. Interruption of the solid connexion
+between the parts may produce the same effects, as for example
+when the new heads or tails grow on the divided edges of Planarians
+(Morgan), or when from each half embryo partially separated
+from its normally corresponding half, a new half is formed
+with a twin monster as the result.</p>
+
+<p class="indent">Often classed with regenerations but in reality
+quite distinct from them are those special and most interesting examples
+where the growth of a <i>paired</i> structure is excited by a simple
+<span class="pagenum"><a name="Page_72" id="Page_72">[Pg 72]</a></span>
+wound. Some of the best known of these instances are presented
+by the paired extra appendages of Insects and Crustacea. Some
+years ago I made an examination of all the examples of such
+monstrosities to which access was to be obtained, and it was with
+no ordinary feeling of excitement that I found that these supernumerary
+structures were commonly disposed on a recognizable
+geometrical plan, having definite spatial relations both to each
+other and to the normal limb from which they grew. The more
+recent researches of Tornier<a name="FNanchor_7_43" id="FNanchor_7_43"></a><a href="#Footnote_7_43" class="fnanchor">[7]</a>
+and especially his experiments on
+the Frog have shown that a cut into the posterior limb-bud
+induces the outgrowth of such a <i>pair</i> of limbs at the wounded
+place. Few observations can compare with this in novelty or
+significance; and though we cannot yet interpret these phenomena
+or place them in their proper relations with normal occurrences,
+we feel convinced that here is an observation which is no mere
+isolated curiosity but a discovery destined to throw a new light
+on biological mechanics. The supernumerary legs of the Frog
+are evidently grouped in a system of symmetry similar to that
+which those of the Arthropods exhibit, and though in Arthropods
+paired repetitions have not been actually produced by injury
+under experimental conditions we need now have no hesitation
+in referring them to these causes as Przibram has done.</p>
+
+<p class="indent">At this point some of the special features of
+the supernumerary appendages become important. First they may arise
+at any point on the normal limb, being found in all situations
+from the base to the apex. Nor are they limited as to the surface
+from which they spring, arising sometimes from the dorsal,
+anterior, ventral, or posterior surfaces, or at points intermediate
+between these principal surfaces.</p>
+
+<p class="indent space-below">With rare and dubious exceptions, the parts which are
+contained in these extra appendages are only those which lie <i>peripheral
+to their point of origin</i>. Thus when the point of origin is
+in the apical joint of the tarsus, the extra growth if completely
+developed consists of a double tarsal apex bearing two pairs
+of claws. If they arise from the tibia, two complete tarsi are
+<span class="pagenum"><a name="Page_73" id="Page_73">[Pg 73]</a></span>
+added. If they spring from the actual base of the appendage
+then two complete appendages may be developed in addition to
+the normal one. We must therefore conclude that in any point
+on a normal appendage the power exists which, if released, may
+produce a bud containing in it a paired set of the parts peripheral
+to this point.</p>
+
+<div class="figcenter" >
+  <img src="images/i_088.jpg" alt="Arthropoda" width="600" height="590" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Fig. 11.</span></b>&emsp;Diagrams of
+the geometrical relations which are generally exhibited by extra pairs of
+appendages in Arthropoda. The sections are supposed to be those of the apex
+of a tibia in a beetle. <b><i>A</i></b>, anterior, <b><i>P</i></b>, posterior, <b><i>D</i></b>,
+dorsal, <b><i>V</i></b>, ventral. <b><i>M</i><sup>1</sup></b>, <b><i>M</i><sup>2</sup></b> are
+the imaginary planes of reflexion. The shaded figure is in each case a limb
+formed like that of the other side of the body, and the outer unshaded figures
+are shaped like the normal for the side on which the appendages are. On the
+several radii are shown the extra pairs in their several possible relations to
+the normal from which they arise. The normal is drawn in thick lines in the center.</p>
+
+<p class="indent">Next the geometrical relations of the halves of the
+supernumerary pair are determined by the position in which they stand
+<span class="pagenum"><a name="Page_74" id="Page_74">[Pg 74]</a></span>
+in regard to the original appendage. These relations are best
+explained by the diagram (Fig. 11), from which it will be seen
+that the two supernumerary appendages stand as images of each
+other; and, of them, that which is adjacent to the normal appendage
+forms an image of it. Thus if the supernumerary pair
+arise from a point on the dorsal surface of the normal appendage,
+the two <i>ventral</i> surfaces of the extra pair will face each other.
+If they arise on the anterior surface of the normal appendage,
+their morphologically posterior surfaces will be adjacent, and so on.</p>
+
+<p class="indent">These facts give us a view of the relations of the two
+halves of a dividing bud very different from that which is to be derived
+from the exclusive study of normal structures. Ordinary morphological
+conceptions no longer apply. The distribution of the
+parts shows that the bud or rudiment which becomes the supernumerary
+pair may break or open out in various ways according
+to its relations to the normal limb. Its planes of division are
+decided by its geometrical relations to the normal body.</p>
+
+<p class="indent">Especially curious are some of the cases in which
+the extra pair are imperfectly formed. The appearance produced is then
+that of two limbs in various stages of coalescence, though in
+reality of course they are stages of imperfect separation. The
+plane of "coalescence" may fall anywhere, and the two appendages
+may thus be compounded with each other much as an
+object partially immersed in mercury "compounds" with its
+optical image reflected from the surface.</p>
+
+<p class="indent">Supernumerary paired structures are not usually, if
+ever, formed when an appendage is simply amputated. Cases occasionally
+are seen which nevertheless seem to be of this nature.
+Borradaile,<a name="FNanchor_8_44" id="FNanchor_8_44"></a><a href="#Footnote_8_44" class="fnanchor">[8]</a>
+for example, described a crab (<i>Cancer pagurus</i>)
+having in place of the right chela three <i>small</i> chelae arising from
+a common base, where the appearances suggested that the three
+reduced limbs replaced a single normal limb. From the details reported
+however it seems still possible that one of the chelae
+(that lettered F. I in Borradaile's figure) may be the normal
+one, and the other two an extra pair. The chela which I suspect
+to be the normal is in several respects deformed as well as being
+<span class="pagenum"><a name="Page_75" id="Page_75">[Pg 75]</a></span>
+reduced in size, and this deformity may perhaps have ensued as
+a consequence of the same wound which excited the growth of the
+extra pair. Its reduced size may be due to the same injury,
+which may quite well have checked its growth to full proportions.</p>
+
+<p class="indent">Admitting doubt in these ambiguous cases it seems to
+be a general rule that for the production of the extra pair the normal
+limb should persist in connexion with the body. Moreover it is
+practically certain that in no case can a <i>single</i>, viz. an unpaired,
+duplicate of the normal appendage grow from it. Many examples
+have been described as of this nature, but all of them may be with
+confidence regarded as instances of a supernumerary pair in
+which only the two morphologically anterior or the two morphologically
+posterior surfaces are developed. We have thus
+the paradox that a limb of one side of the body, say the right,
+has in it the power to form a pair of limbs, right and left, as an
+outgrowth of itself, but cannot form a second left limb alone.</p>
+
+<p class="indent">A very interesting question arises whether it is strictly
+correct to describe the extra pair as a right and a left, or whether
+they are not rather two lefts or two rights of which one is reversed.
+This question did not occur to me when in former years I studied
+these subjects. It was suggested to me by Dr. Przibram.
+The answer might have an important bearing on biological
+mechanics, but I know no evidence from which the point can
+be determined with certainty. In order to decide this question
+it would be necessary to have cases in which the paired repetition
+affected a limb markedly differentiated on the two sides of the
+body, and of course the development of the extra parts in order
+to be decisive must be fairly complete. One example only is
+known to me which at all satisfies these requirements, that of the
+lobster's chela figured (after Van Beneden) in <i>Materials for the
+Study of Variation</i>, p. 531, Fig. 184, III.</p>
+
+<p class="indent space-below">Here the drawing distinctly suggests that one of
+the extra dactylopodites, namely that lettered R, is differentiated
+as a left and not merely a reversed right. For the teeth on this
+dactylopodite are those of a cutting claw, not of a crushing claw,
+whereas the dactylopodites R' and L' bear crushing teeth. The
+figure makes it fairly certain also that the limb affected was a
+<span class="pagenum"><a name="Page_76" id="Page_76">[Pg 76]</a></span>
+crushing claw. Accepting this interpretation, we reach the
+remarkable conclusion that the bud of new growth consisted of
+halves differentiated into cutter and crusher as the normal claws
+are, and that the extra crusher is geometrically a left but physiologically
+a right. Though shaped as a left in respect of the
+direction in which it points, the extra crusher is really an optically
+reversed right, while the dactylopodite R, which is
+placed pointing like a right, is really a reversed left (Fig. 12).</p>
+
+<div class="figcenter" >
+  <img src="images/i_091.jpg" alt="Lobster Claw" width="600" height="550" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Fig. 12.</span></b>&emsp;Right claw of lobster bearing
+a pair of extra dactylopodites (after van Beneden). The fine toothing on R suggests
+that this is part of a cutting claw, though the limb bearing it is a crusher.</p>
+
+<p class="indent">If these indications are reliable<a name="FNanchor_9_45" id="FNanchor_9_45"></a><a href="#Footnote_9_45" class="fnanchor">[9]</a>
+and are established by further
+observation we shall be led to the conclusion that the bud which
+becomes an extra pair of limbs does not merely contain the parts
+proper to the side on which it grows, but is comparable with
+the original zygotic cell, and consists not simply of two halves,
+but of two halves differentiated as a right and a left like the two
+halves of the normal body.</p>
+
+<p class="indent">Phenomena of this kind, evoked by mutilation or
+injury, together with the cognate observations on regeneration throw
+<span class="pagenum"><a name="Page_77" id="Page_77">[Pg 77]</a></span>
+very curious lights on the nature of living things. To an understanding
+of the nature of the mechanics of living matter and its
+relation to matter at large they offer the most hopeful line of
+approach. I allude especially to the examples in which it has
+been established that the part which is produced after mutilation
+is a structure different from that which was removed. The
+term "regeneration" was introduced before such phenomena
+were discovered, and though every one recognizes its inapplicability
+to these remarkable cases, the word still misleads us by
+presenting a wrong picture to the mind. The expression "heteromorphosis"
+(Loeb) has been appropriately applied to various
+phenomena of this kind, and Morgan has given the name "morphallaxis"
+to another group of cases in which the renewal occurs
+by the transformation of a previously existing part.<a name="FNanchor_10_46" id="FNanchor_10_46"></a><a href="#Footnote_10_46" class="fnanchor">[10]</a>
+But we must continually remember that all these occurrences which
+we know only as abnormalities and curiosities must in reality be
+exemplifications of the normal mechanics of division and growth.
+The conditions needed to call them forth are abnormal, but the
+responses which the system makes are evidences of its normal
+constitution. When therefore, for example, the posterior end
+of a worm produces a reversed tail from its cut end we have a
+proof that there must be in the normal body forces ready to
+cause this outgrowth. The new structure is not an ill-shaped
+head-end, for, as Morgan shows, the nephridial ducts have their
+funnels perforating the segments in a reversed direction. The
+"tension" of growth is actually reversed.<a name="FNanchor_11_47" id="FNanchor_11_47"></a><a href="#Footnote_11_47" class="fnanchor">[11]</a>
+So also when in a Planarian amputation of the body immediately behind
+the head leads to the formation of a new reversed head at the back of
+the normal head, while amputation further back leads to the
+regeneration of a new tail, these responses give indications
+of forces normally present in the body of the Planarian. Such
+facts open up a great field of speculation and research. Especially
+important it would be to determine where the critical
+region may be at which the one response is replaced by the
+<span class="pagenum"><a name="Page_78" id="Page_78">[Pg 78]</a></span>
+other. I suppose it is even possible that there is some neutral
+zone in which neither kind of response is made.</p>
+
+<p class="indent">Physical parallels to the phenomena of regeneration are
+not easy to find and we still cannot penetrate beyond the empirical
+facts. Przibram has laid stress on the general resemblance
+between the new growth of an amputated part in an animal and
+the way in which a broken crystal repairs itself when placed in
+the mother-solution. That the two processes have interesting
+points of likeness cannot be denied. It must however never be
+forgotten that there is one feature strongly distinguishing the
+two; for I believe it is universally recognized by physicists that
+all the phenomena of geometrical regularity which crystals
+display are ultimately dependent on the forms of the particles
+of the crystalline body. This cannot in any sense be supposed
+to hold in regard to protoplasm or its constituents. The definiteness
+of crystals is also an unlikely guide for the reason that
+it is absolute and perfect, or in other words because this kind of
+regularity cannot be disturbed at all without a change so great
+that the substance itself is altered; whereas we know that the
+forms of living things are capable of such changes, great and small,
+that we must regard perfection of form, whether manifested in
+symmetry or in number, as an ideal which will only be produced
+in the absence of disturbance. The symmetry of the living
+things is like the symmetry of the concentric waves in a pool
+caused by a splash. Perfect circles are made only in the imaginary
+case of mathematical uniformity, but the system maintains
+an approximate symmetry though liable to manifold deformation.</p>
+
+<p class="indent">Since the geometrical order of the living body cannot
+be a direct function of the materials it must be referred to some more
+proximate control. In renewing a part the body must possess
+the power of seizing particles of many dissimilar kinds, and whirl
+them into their several and proper places. The action in renewal,
+like that of original growth, may be compared&mdash;very
+crudely&mdash;with the action of a separator which simultaneously
+distributes a variety of heterogeneous materials in an orderly
+fashion; but in the living body the thing distributed must rather
+be the <i>appetency</i> for special materials, not the materials themselves.
+<span class="pagenum"><a name="Page_79" id="Page_79">[Pg 79]</a></span></p>
+
+<p class="indent">If the analogy of crystals be set aside and we seek
+for other parallels to regeneration there are none very obvious. I have
+sometimes wondered whether it might not be possible to institute
+a fruitful comparison between the renewal of parts and the reformation
+of waves of certain classes after obliteration. In several
+respects, as I have already said, some curious resemblances with
+the repetitions formed by wave-motion are to be traced in our
+organic phenomena, and though admitting that I cannot develop
+these comparisons, I think nevertheless they may be worth
+bearing in mind. When, after obliteration, an eddy in a stream,
+or a ripple-mark (a more complex case of eddy-formation) in
+blown sand is re-formed, we have an example in which pattern is
+reconstituted and growth takes place not by virtue of the composition
+of the materials&mdash;in this case the water or the sand&mdash;but
+by the way in which they are acted upon by extraneous forces.</p>
+
+<p class="indent">A feature in the actual mode by which ripple-marks
+are reconstituted may not be without interest in connexion with
+our phenomena of regeneration. When, for example, the wind
+is blowing steadily over a surface of fine, dry sand, the familiar
+ripple-marks are formed by a heaping of the sand in lines transverse
+to the direction of the wind. The heaping is due to the
+formation of eddies corresponding with positions of instability.
+When the wind is steady and the sand homogeneous, the distances
+between the ripples, or wave-lengths, are sensibly equal.
+If while the wind continues to blow, the ripples are obliterated
+with a soft brush they will quickly be re-formed over the whole
+area, but I have noticed that at first their wave-length is approximately
+half that of the ripples in the undisturbed parts of
+the system.<a name="FNanchor_12_48" id="FNanchor_12_48"></a><a href="#Footnote_12_48" class="fnanchor">[12]</a>
+The normal wave-length is restored by the gradual
+accentuation of alternate ripples. Of course the sand-ripples are
+in reality slowly travelling forward in the direction towards
+which the wind is blowing, and for this our living segmentations
+afford no obvious parallel, but the appearances in the area of
+<span class="pagenum"><a name="Page_80" id="Page_80">[Pg 80]</a></span>
+reformation, and especially the forking of the old ridges where
+they join the new ones, are curiously reminiscent of the irregularities
+of segmentation seen in regenerated structures. The
+value of the considerations adduced in the chapter is, I admit,
+very small. The utmost that can be claimed for them is that
+mechanical segmentations, like those seen in ripple-mark, or
+in Leduc's osmotic growths, show how by the action of a continuous
+force in one direction, repeated and serially homologous
+divisions can be produced having features of similarity common
+to those repetitions by which organic forms and patterns are
+characterised. The analogy supplies a vicarious picture of the
+phenomena which in default of one more true may in a slight
+degree assist our thoughts. It suggests that the rhythms of
+segmentation may be the consequence of a single force definite
+in direction and continuously acting during the time of growth.
+The polarity of the organism would thus be the expression of
+the fact that this meristic force is definitely directed after it has
+once been excited, and the reversal seen in some products of regeneration
+suggest further that it is capable of being reflected.
+This polarity cannot be a property of the material, as such,
+but is determined by a force acting on that material, just as the
+polarity of a magnet is not determined by the arrangement of its
+particles, but by the direction in which the current flows.</p>
+
+<p class="indent">To some it may appear that even to embark on such discussions
+as this is to enter into a perilous flirtation with vitalistic theories.
+How, they may ask, can any force competent to produce chemical
+and geometrical differentiation in the body be distinguished
+from the "Entelechy" of Driesch? Let me admit that in this
+reflexion there is one element of truth. If those who proclaim
+a vitalistic faith intend thereby to affirm that in the processes
+by which growth and division are effected in the body, a part is
+played by an orderly force which we cannot <i>now</i> translate into
+terms of any known mechanics, what observant man is not a
+vitalist? Driesch's first volume, putting as it does into intelligible
+language that positive deduction from the facts&mdash;especially
+of regeneration&mdash;should carry a vivid realisation of this
+truth to any mind. If after their existence is realised, it is
+<span class="pagenum"><a name="Page_81" id="Page_81">[Pg 81]</a></span>
+desired that these unknown forces of order should have a name,
+and the word entelechy is proposed, the only objection I have to
+make is that the adoption of a term from Aristotelian philosophy
+carries a plain hint that we propose to relegate the future study
+of the problem to metaphysic.</p>
+
+<p class="indent">From this implication the vitalist does not shrink.
+But I cannot find in the facts yet known to us any justification of so
+hopeless a course. It was but yesterday that the study of
+<i>Entwicklungsmechanik</i> was begun, and if in our slight survey
+we have not yet seen how the living machine is to be expressed
+in terms of natural knowledge that is poor cause for despair.
+Driesch sums up his argument thus:<a name="FNanchor_13_49" id="FNanchor_13_49"></a><a href="#Footnote_13_49" class="fnanchor">[13]</a>
+</p>
+
+<p class="blockquot">"It seems to me that there is only one conclusion possible.
+If we are going to explain what happens in our harmonious-equipotential
+systems by the aid of causality based upon the
+constellation of single chemical factors and events, there <i>must</i>
+be some such thing as a machine. Now the assumption of the
+existence of a machine proves to be absolutely absurd in the
+light of the experimental facts. <i>Therefore there can be neither
+any sort of a machine nor any sort of causality based upon constellation
+underlying the differentiation of harmonious-equipotential systems.</i>"</p>
+
+<p class="blockquot">"For a machine, typical with regard to the three chief dimensions
+of space, cannot remain itself if you remove parts of it
+or if you rearrange its parts at will."</p>
+
+<p>To the last clause a note is added as follows:</p>
+
+<p class="blockquot">"The pressure experiments and the dislocation experiments
+come into account here; for the sake of simplicity they have
+not been alluded to in the main line of our argument."</p>
+
+<p class="indent">I doubt whether any man has sufficient knowledge of
+all possible machines to give reality to this statement. In spite also
+of the astonishing results of experiments in dislocation, doubt
+may further be expressed as to whether they have been tried in
+such variety or on such a scale as to justify the suggestion that
+the living organism remains itself if its parts are rearranged at
+<span class="pagenum"><a name="Page_82" id="Page_82">[Pg 82]</a></span>
+will. All we know is that it can "remain itself" when much is
+removed, and when much rearrangement has been affected,
+which is a different thing altogether.</p>
+
+<p class="indent">I scarcely like to venture into a region of which my
+ignorance is so profound, but remembering the powers of eddies to re-form
+after partial obliteration or disturbance, I almost wonder whether
+they are not essentially machines which remain themselves
+when parts of them are removed.</p>
+
+<p class="indent">Real progress in this most obscure province is not likely
+to be made till it attracts the attention of physicists; and though they
+for long may have to forego the application of exact quantitative
+methods, I confidently anticipate that careful comparison
+between the phenomena of repetition formed in living organisms
+and the various kinds of segmentation produced by mechanical
+agencies would be productive of illuminating discoveries.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_83" id="Page_83">[Pg 83]</a></span></p>
+
+<h2>CHAPTER IV</h2>
+<h3><span class="smcap">The Classification Of Variation And<br />The Nature Of Substantive Factors</span></h3>
+
+<p class="indent">We have now seen that among the normal physiological
+processes the phenomena of division form a recognisable, and
+in all likelihood a naturally distinct group. Variations in these
+respects may thus be regarded as constituting a special class
+among variations in general.</p>
+
+<p class="indent">The substantive variations have only one property in
+common&mdash;the negative one that they are not Meristic. The work
+of classifying them and distinguishing them according to their
+several types demands a knowledge of the chemistry of life far
+higher than that to which science has yet attained. In reference
+to some of the simplest variations Garrod has introduced the
+appropriate term "Chemical sports." The condition in man
+known as Alkaptonuria in which the urine is red is due especially
+to the absence of the enzyme which decomposes the excretory
+substance, alkapton. The "chemical sport" here consists in the
+inability to break up the benzene ring. The chemical feature
+which distinguishes and is the proximate cause of several colour-varieties
+can now in a few cases be declared. The work of Miss
+Wheldale has shown that colour-varieties may be produced by
+the absence of the chromogen compound the oxidation of which
+gives rise to sap-colours, by differences in the completeness of
+this process of oxidation, and by a process of reduction supervening
+on or perhaps suppressing the oxidation. Some of these
+processes moreover may be brought about by the combined action
+of two bodies, the one an enzyme, for example an oxygenase, and
+the other a substance regarded as a peroxide, contributing the
+oxygen necessary for the oxidation to take place. Variation in
+colour may thus be brought about by the addition or omission
+of any one of the bodies concerned in the action.
+<span class="pagenum"><a name="Page_84" id="Page_84">[Pg 84]</a></span></p>
+
+<p class="indent">Similar variations, or rather similar series of variations will
+undoubtedly hereafter be identified in reference to all the various
+kinds of chemical processes upon which the structure and functions
+of living things depend. The identification of these processes
+and of the bodies concerned in them will lead to a real
+classification of Substantive Variations.</p>
+
+<p class="indent">To forecast the lines on which such classification
+will proceed is to look too far ahead. We may nevertheless anticipate
+with some confidence that future analysis will recognise among the
+contributing elements, some which are intrinsic and inalienable,
+and others which are extrinsic and superadded.</p>
+
+<p class="indent">We already know that there may be such interdependence
+among the substantive characters that to disentangle them will
+be a work of extreme difficulty. The mere fact that in our
+estimation characters belong to distinct physiological systems is
+no proof of their actual independence. In illustration may be
+mentioned the sap-colour in Stocks and the development of
+hoariness on the leaves and stems, which Miss Saunders's experiments
+have shown to be intimately connected, so that in certain
+varieties no hoariness is produced unless the elements for sap-colour
+are already present in the individual plant.</p>
+
+<p class="indent">The first step in the classification of substantive
+variations is therefore to determine which are due to the addition of
+new elements or factors, and which are produced by the omission of
+old ones. <i>A priori</i> there is no valid criterion by which this can
+be known, and actual experiments in analytical breeding can
+alone provide the knowledge required. Some very curious results
+have by this method been obtained, which throw an altogether
+unexpected light on these problems. For example, in order that
+the remarkable development of mesoblastic black pigment characteristic
+of the Silky Fowl should be developed, it is practically
+certain that two distinct variations from such a type as <i>Gallus
+bankiva</i> must have occurred. I assume, as is reasonable, that
+<i>G. bankiva</i> has genetic properties similar to those of the Brown
+Leghorn breed which has been used in the experiments which Mr.
+Punnett and I have conducted. <i>Gallus bankiva</i> was not available
+but the Brown Leghorn agrees with it very closely in colouration,
+and probably in the general physiology of its pigmentation.
+<span class="pagenum"><a name="Page_85" id="Page_85">[Pg 85]</a></span>
+Setting aside the various structural differences between the two
+breeds, the Silky is immediately distinguished from the Leghorn
+by the fact that the skin of the whole body including that of the
+face and comb appears to be of a deep purplish colour. The
+face and comb of the Leghorn are red and the skin of the body
+is whitish yellow. On examination it is found that the purple
+colour of the Silky is in reality due to the distribution of a deep
+black pigment in the mesoblastic membranes throughout the
+body. The somatopleura, the pleura, <i>pia mater</i>, the dermis, and
+in most organs the connective tissue and the sheaths of the blood-vessels,
+are thus impregnated with black. No such pigmentation
+exists in the Leghorn. As the result of an elaborate series of
+experimental matings we have proved that the distinction between
+the Leghorn and the Silky consists primarily in the fact
+that the Silky possesses a pigment-producing factor, <i>P</i>, which is
+not present in the Leghorn.</p>
+
+<p class="indent">This variation must undoubtedly have been one of <i>addition</i>.
+But besides this there is another difference of an altogether dissimilar
+nature; for the Brown Leghorn possesses a factor which
+has the power of partially or completely restricting the operation
+of the pigment-producing factor, <i>P</i>. Moreover in respect of
+this pigment-restricting factor which we may call <i>D</i>, the sexes
+of the Brown Leghorn differ, for the male is homozygous or <i>DD</i>,
+but the female is heterozygous, <i>Dd</i>. Thus in order that the
+black-skinned breed could be evolved from such a type as a
+Brown Leghorn it must be necessary <i>both</i> that <i>P</i> should be added
+and that <i>D</i> should drop out. We have not the faintest conception
+of the process by which either of these events have come to
+pass, but there is no reasonable doubt that in the evolution of
+the Silky fowl they did actually happen.</p>
+
+<p class="indent">We may anticipate that numerous interdependences of this
+kind will be discovered.</p>
+
+<p class="indent">Before any indisputable progress can be made with
+the problem of evolution it is necessary that we should acquire some
+real knowledge of the genesis of that class of phenomena which
+formed the subject of the last chapter. So long as the process
+of division remains entirely mysterious we can form no conception
+<span class="pagenum"><a name="Page_86" id="Page_86">[Pg 86]</a></span>
+even of the haziest sort as to the nature of living organisms, or
+of the proximate causes which determine their forms, still less
+can we attempt any answer to those remoter questions of origin
+and destiny which form the subject of the philosopher's contemplation.
+It is in no spirit of dogmatism that I have ventured
+to indicate the direction in which I look for a solution, though
+I have none to offer. It may well be that before any solution
+is attained, our knowledge of the nature of unorganised matter
+must first be increased. For a long time yet we may have to
+halt, but we none the less do well to prepare ourselves to utilise
+any means of advance that may be offered, by carefully reconnoitering
+the ground we have to traverse. The real difficulty
+which blocks our progress is ignorance of the nature of division,
+or to use the more general term, of repetition.</p>
+
+<p class="indent">Let us turn to the more familiar problem of the
+causes of variation. Now since variation consists as much in meristic
+change as in alteration in substance or material, there is one
+great range of problems of causation from which we are as yet
+entirely cut off. We know nothing of the causation of division,
+and we have scarcely an observation, experiment or surmise
+touching the causes by which the meristic processes may be altered.</p>
+
+<p class="indent">Of the way in which variations in the substantive
+composition of organisms are caused we have almost as little real evidence,
+but we are beginning to know in what such variations must consist.
+These changes must occur either by the addition or loss of factors.</p>
+
+<p class="indent">We must not lose sight of the fact that though the
+factors operate by the production of enzymes, of bodies on which these
+enzymes can act, and of intermediary substances necessary to
+complete the enzyme-action, yet these bodies themselves can
+scarcely be themselves genetic factors, but consequences of their
+existence. What then are the factors themselves? Whence do
+they come? How do they become integral parts of the organism?
+Whence, for example, came the power which is present in a White
+Leghorn of destroying&mdash;probably reducing&mdash;the pigment in its
+feathers? That power is now a definite possession of the breed,
+<span class="pagenum"><a name="Page_87" id="Page_87">[Pg 87]</a></span>
+present in all its germ-cells, male and female, taking part in their
+symmetrical divisions, and passed on equally to all as much as
+is the protoplasm or any other attribute of the breed. From the
+body of the bird the critical and efficient substance could in all
+likelihood be isolated by suitable means, just as the glycogen
+of the liver can be. But even when this extraction has been
+accomplished and the reducing body isolated, we shall know no
+more than we did before respecting the mode by which the power
+to produce it was conferred on the fowl, any more than we know
+how the walls of its blood-vessels acquired the power to form a
+fibrin-ferment.</p>
+
+<p class="indent">It is when the scope of such considerations as this are
+fully grasped that we realise the fatuousness of the conventional treatment
+which the problem of the causes of variation commonly
+receives. Environmental change, chemical injury, differences in
+food supply, in temperature, in moisture, or the like have been
+proposed as "causes." Admitting as we must do, that changes
+may be produced&mdash;usually inhibitions of development&mdash;by
+subjecting living things to changes in these respects, how can we
+suppose it in the smallest degree likely that very precise, new,
+and adaptative powers can be conferred on the germs by such
+treatment? Reports of positive genetic consequences observed
+comparable with those I have mentioned, become from time to
+time current. We should I think regard them with the gravest
+doubt. Few, so far as I am aware, have ever been confirmed,
+though clear and repeated confirmation should be demanded
+before we suffer ourselves at all to build upon such evidence.
+In a subsequent chapter some of these cases will be considered in
+detail.</p>
+
+<p class="indent">In no class of cases would the transmission of an acquired
+character superficially appear so probable as in those where power
+of resisting the attack of a pathogenic organism is acquired in
+the lifetime of the zygote. The possession of such a power is
+moreover a distinction comparable with those which differentiate
+varieties and species. It is due to the development in the blood
+of specific substances which pervade the whole fluid. This
+development is exactly one of those "appropriate responses to
+<span class="pagenum"><a name="Page_88" id="Page_88">[Pg 88]</a></span>
+stimuli" which naturalists who incline to regard adaptation as
+a direct consequence of an environmental influence might most
+readily invoke as an illustration of their views. And yet all
+evidence is definitely unfavourable to the suggestion of an
+inheritance of the acquired power of resistance. Such change as
+can be perceived in the virulence of the attacks on successive
+generations may be most easily regarded as due to the extermination
+of the more susceptible strains, and perhaps in some
+measure to variation in the invading organisms themselves, an
+"acquired character" of quite different import.</p>
+
+<p class="indent">The specific "anti-body" may have been produced in
+response to the stimulus of disease, but the power to produce it
+without this special stimulus is not included in the germ-cells
+any more than a pigment. All that they bear is the <i>power to
+produce</i> the anti-bodies when the stimulus is applied.</p>
+
+<p class="indent">If we could conceive of an organism like one of
+those to which disease may be due becoming actually incorporated with
+the system of its host, so as to form a constituent of its germ-cells
+and to take part in the symmetry of their divisions, we should
+have something analogous to the case of a species which acquires
+a new factor and emits a dominant variety. When we see the
+phenomenon in this light we realise the obscurity of the problem.
+The appearance of recessive varieties is comparatively easy to
+understand. All that is implied is the omission of a constituent.
+How precisely the omission is effected we cannot suggest, but
+it is not very difficult to suppose that by some mechanical fault
+of cell-division a power may be lost. Such variation by unpacking,
+or analysis of a previously existing complex, though unaccountable,
+is not inconceivable. But whence come the new dominants?
+Whether we imagine that they are created by some
+rearrangement or other change internal to the organism, or
+whether we try to conceive them as due to the assumption of
+something from without we are confronted by equally hopeless
+difficulty.</p>
+
+<p class="indent">The mystery of the origin of a dominant increases when
+it is realised that there is scarcely any recent and authentic account
+of such an event occurring under critical observation, which can
+<span class="pagenum"><a name="Page_89" id="Page_89">[Pg 89]</a></span>
+be taken as a basis for discussion. The literature of horticulture
+for example abounds in cases alleged, but I do not think anyone
+can produce an illustration quite free from doubt. Such evidence
+is usually open to the suspicion that the plant was either introduced
+by some accident, or that it arose from a cross with a pre-existing
+dominant, or that it owed its origin to the meeting of
+complementary factors. In medical literature almost alone however,
+there are numerous records of the spontaneous origin of
+various abnormal conditions in man which habitually behave
+as dominants, and of the authenticity of some of these there
+can be no doubt.</p>
+
+<p class="indent">When we know that such conditions as hereditary
+cataract or various deformities of the fingers behave as dominants,
+we recognize that those conditions must be due to the addition of
+some element to the constitution of the normal man. In the
+collections of pedigrees relating to such pathological dominants
+there are usually to be found alleged instances of the origin of the
+condition <i>de novo</i>. Not only do these records occur with such
+frequency that they cannot be readily set aside as errors, but from
+general considerations it must be obvious that as these malformations
+are not common to normal humanity they must at
+some moment of time have been introduced. The lay reader
+may not be so much impressed with the difficulty as we are. He
+is accustomed to regard the origin of <i>any</i> new character as
+equally mysterious, but when once dominants are distinguished from
+recessives the problem wears a new aspect. Thus the appearance
+of high artistic gifts, whether as an attribute of a race or as a
+sporadic event among the children of parents destitute of such
+faculties, is not very surprising, for we feel fairly sure that the
+faculty is a recessive, due to the loss of a controlling or inhibiting
+factor; but the <i>de novo</i> origin of brachydactylous fingers in a
+child of normal parents is of quite a different nature, and must
+indicate the action of some new specific cause.</p>
+
+<p class="indent">Whether such evidence is applicable to the general
+problem of evolution may with some plausibility be questioned; but
+there is an obvious significance in the fact that it is among these
+pathological occurrences that we meet with phenomena most
+<span class="pagenum"><a name="Page_90" id="Page_90">[Pg 90]</a></span>
+nearly resembling the spontaneous origin of dominant factors,
+and I cannot see such pedigrees as these without recalling Virchow's
+aphorism that every variation owes its origin to some
+pathological accident. In the evolution of domestic poultry,
+if <i>Gallus bankiva</i> be indeed the parent form of all our breeds,
+at least some half dozen new factors must have been added
+during the process. In <i>bankiva</i> there is, for example, no factor
+for rose comb, pea comb, barring on the feathers, or for the
+various dominant types of dark plumage. Whence came all
+these? It is, I think, by no means impossible that some other
+wild species now extinct did take part in the constitution of
+domestic poultry. It seems indeed to me improbable that the
+heavy breeds descend from <i>bankiva</i>. Both in regard to domestic
+races of fowls, pigeons, and some other forms, the belief in origin
+within the period of human civilization from one simple primitive
+wild type seems on a balance of probabilities insecurely
+founded, but allowing something for multiplicity of origin we
+still fall far short of the requisite total of factors. Elements
+exist in our domesticated breeds which we may feel with confidence
+have come in since their captivity began. Such elements
+in fowls are dominant whiteness, extra toe, feathered
+leg, frizzling, etc., so that even hypothetical extension of the
+range of origin is only a slight alleviation of the difficulty.</p>
+
+<p class="indent">Somehow or other, therefore, we must recognize that
+dominant factors do arise. Whether they are created by internal change,
+or whether, as seems to me not wholly beyond possibility, they
+obtain entrance from without, there is no evidence to show.
+If they were proved to enter from without, like pathogenic
+organisms, we should have to account for the extraordinary
+fact that they are distributed with fair constancy to half the
+gametes of the heterozygote.</p>
+
+<p class="indent">In proportion as the nature of dominants grows more
+clear so does it become increasingly difficult to make any plausible
+suggestion as to their possible derivation. On the other hand
+the origin of a recessive variety by the loss of a factor is a process
+so readily imagined that our wonder is rather that the phenomenon
+is not observed far more often. Some slip in the accurate
+<span class="pagenum"><a name="Page_91" id="Page_91">[Pg 91]</a></span>
+working of the mechanical process of division, and a factor
+gets left out, the loss being attested by the appearance of a
+recessive variety in some subsequent generation.</p>
+
+<p class="indent">Consistently with this presentation of the facts we find
+that, as in our domesticated animals and plants, a diversity of recessives
+may appear within a moderately short period, and that when
+variations come they often do not come alone. Witness the
+cultural history of the Sweet Pea, <i>Primula Sinensis</i>, <i>Primula
+obconica</i>, <i>Nemesia strumosa</i> and many such examples in which
+variation when it did come was abundant. The fact cannot
+be too often emphasized that in the vast proportion of these
+examples of substantive variation under domestication, as well
+as of substantive variation in the natural state, the change
+has come about by omission, not by addition. To take, for
+example, the case of the Potato, in which so many spontaneous
+bud-variations have been recorded, East after a careful study
+of the evidence has lately declared his belief that all are of this
+nature, and the opinion might be extended to many other groups
+of cases whether of bud or seminal variation. Morgan draws
+the same conclusion in reference to the many varieties he has
+studied in <i>Drosophila</i>.</p>
+
+<p class="indent">In the Sweet Pea, a form which is beyond suspicion
+of having been crossed with anything else, and has certainly produced
+all the multitude of types which we now possess by variations
+from one wild species, there is only one character of the modern
+types which could, with any plausibility, be referred to a factor
+not originally forming part of the constituents of the wild species.
+This is the waved edge, so characteristic of the "Spencer"
+varieties; for the cross between a smooth-edged and a waved
+type gives an intermediate not unfrequently. Nevertheless
+there is practically no doubt that this is merely an imperfection
+in the dominance of the smooth edge, and we may feel sure
+that any plant homozygous for smooth edge would show no wave
+at all. Hence it is quite possible that even the appearance of
+the original waved type, Countess Spencer, was due to the loss
+of one of the factors for smooth edge at some time in the history
+of the Sweet Pea.
+<span class="pagenum"><a name="Page_92" id="Page_92">[Pg 92]</a></span></p>
+
+<p class="indent">In the case of the Chinese Primrose (<i>Primula Sinensis</i>)
+one dominant factor has been introduced in modern times, probably
+within the last six years at most. This is the factor which
+causes suppression of the yellow eye, giving rise to the curious
+type known as "Queen Alexandra." Mr. R. P. Gregory's
+experiments proved that this was a very definite dominant, and
+the element responsible for this development is undoubtedly an
+addition to the original ingredient-properties, with which the
+species was endowed. Unfortunately, as happens in almost every
+case of the kind, the origin of this important novelty appears
+to be lost. Its behaviour, however, when crossed with various
+other types is that of a simple dominant giving an ordinary 3:1
+ratio. There is therefore no real doubt that it came into existence
+by the definite addition of a new factor, for if it was simply
+a case of the appearance of a new character made by combination
+of two previously existing complementary factors we should
+expect that when Queen Alexandra was self-fertilised a 9:7 ratio
+would be a fairly common result, which is not in practice found.</p>
+
+<p class="indent">In <i>Oenothera</i> Gates<a name="FNanchor_1_50" id="FNanchor_1_50"></a><a href="#Footnote_1_50" class="fnanchor">[1]</a>
+has observed the appearance, in a large sowing of
+about 1,000 <i>Oenothera rubrinervis</i>, of a single individual
+having considerably more red pigment in the calyx than is usual
+in <i>rubrinervis</i>. The whole of the hypanthium in the flowers of
+this plant was red instead of green as in <i>rubrinervis</i>, and the
+whole of the sepals were red in the bud-stage, except for small
+green areas at the base. This type behaved as a dominant over
+<i>rubrinervis</i>, but so far a pure-breeding individual was not found.
+Admittedly the variation of this plant from the type of <i>rubrinervis</i>
+can be represented as one of degree, though there is a very
+sensible gap in the series between the new form which Gates
+names "<i>rubricalyx</i>" and the reddest <i>rubrinervis</i> seen in his
+cultures. It must certainly be recognised as a new dominant.
+Gates, rightly as I consider, regards the distinction between
+<i>rubrinervis</i> and <i>rubricalyx</i> as a quantitative one, and the same
+remark applies to certain other types differing in the amount of
+anthocyanin which they produce. I do not understand the argument
+<span class="pagenum"><a name="Page_93" id="Page_93">[Pg 93]</a></span>
+which Gates introduces to the effect that the difference
+between such quantitative types cannot be represented in terms
+of presence and absence. We are quite accustomed to the fact
+that in the rabbit self-colour segregates from the Dutch-marked
+type. These two types differ in a manner which we may reasonably
+regard as quantitative. It is no doubt possible that the
+self-coloured type contains an ingredient which enables the colour
+to spread over the whole body, but it is, I think, perhaps more
+easy to regard the Dutch type as a form from which a part of
+the colour is absent. It may be spoken of in terms I have used,
+as a <i>subtraction-stage</i> in colour. Following a similar method we
+may regard <i>rubricalyx</i> as an addition-stage in colour-variation.
+The fact that crosses between <i>rubrinervis</i>, or <i>rubricalyx</i> and
+<i>Lamarckiana</i> give a mixture of types in F<sub>1</sub>, does not I
+think show, as Gates declares, that there is any system here at work to
+which a factorial or Mendelian analysis does not apply; but that
+question may be more fitly discussed in connexion with the other
+problems raised by the behaviour of <i>Oenothera</i> species in their crosses.</p>
+
+<p class="indent">I do, however, feel that, interesting as this case must
+be admitted to be, we cannot quite satisfactorily discuss it as an
+illustration of the <i>de novo</i> origin of a dominant factor. The
+difference between the novelty and the type is quantitative, and
+it is not unreasonable to think of such a difference being brought
+about by some "pathological accident" in a cell-division.</p>
+
+<p class="indent">Recognition of the distinction between dominant and
+recessive characters has, it must be conceded, created a very serious
+obstacle in the way of any rational and concrete theory of evolution.
+While variations of all kinds could be regarded as manifestations
+of some mysterious instability of organisms this difficulty
+did not occur to the mind of evolutionists. To most of
+those who have taken part in genetic analysis it has become a
+permanent and continual obsession. With regard to the origin
+of recessive variations, there is, as we have seen, no special
+difficulty. They are negative and are due to absences, but as
+soon as it is understood that dominants are caused by an addition
+we are completely at a loss to account for their origin, for we
+<span class="pagenum"><a name="Page_94" id="Page_94">[Pg 94]</a></span>
+cannot surmise any source from which they may have been
+derived. Just as when typhoid fever breaks out in his district
+the medical officer of health knows for certain that the bacillus of
+typhoid fever has by some means been brought into that district
+so do we know that when first dominant white fowls arose in the
+evolution of the domestic breeds, by some means the factor for
+dominant whiteness got into a bird, or into at least one of its
+germ-cells. Whence it came we cannot surmise.</p>
+
+<p class="indent">Whether we look to the outer world or to some rearrangement
+within the organism itself, the prospect of finding a source of
+such new elements is equally hopeless.</p>
+
+<p class="indent">Leaving this fundamental question aside as one which
+it is as yet quite unprofitable to discuss, we are on safe ground in
+foreseeing that the future classification of substantive variations,
+which genetic research must before long make possible, will be
+based on a reference to the modes of action of the several factors.
+Some will be seen to produce their effects by oxidation, some by
+reduction, some by generating substances of various types,
+sugars, enzymes, activators, and so forth. It may thus be
+anticipated that the relation of varieties to each other and to
+types from which they are derived will be expressible in terms
+of definite synthetical formulae. Clearly it will not for an indefinite
+time be possible to do this in practice for more than a
+few species and for characters especially amenable to experimental
+tests, but as soon as the applicability of such treatment
+is generally understood the influence on systematics must be
+immediate and profound, for the nature of the problem will at
+length be clear and, though the ideal may be unattainable, its
+significance cannot be gainsaid.</p>
+
+<hr class="tab" />
+<p class="blockquot"><b><i>Note.</i></b>&mdash;With hesitation I allow this
+chapter to appear in the form in which it was printed a year ago, but in
+passing it for the press after that interval I feel it necessary to call
+attention to a possible line of argument not hitherto introduced.</p>
+<hr class="tab" />
+
+<p class="indent">In all our discussions we have felt justified in declaring
+that the dominance of any character indicates that some factor is
+<span class="pagenum"><a name="Page_95" id="Page_95">[Pg 95]</a></span>
+present which is responsible for the production of that character.
+Where there is no definite dominance and the heterozygote is
+of an intermediate nature we should be unable to declare on
+which side the factor concerned was present and from which side
+it was absent. The degree of dominance becomes thus the
+deciding criterion by which we distinguish the existence of factors.
+But it should be clearly realized that in any given case the argument
+can with perfect logic be inverted. We already recognize
+cases in which by the presence of an inhibiting factor a character
+may be suppressed and purely as a matter of symbolical expression
+we might apply the same conception of inhibition to any
+example of factorial influence whatever. For instance we say
+that in as much as two normal persons do not have brachydactylous
+children, there must be some factor in these abnormal persons
+which causes the modification. Our conclusion is based on the
+observed fact that the modification is a dominant. But it may
+be that normal persons are homozygous in respect of some factor
+<i>N</i>, which prevents the appearance of brachydactyly, and that in
+any one heterozygous, <i>Nn</i>, for this inhibiting factor, brachydactyly
+can appear. Similarly the round pea we say contains
+<i>R</i>, a factor which confers this property of roundness, without
+which its seeds would be wrinkled. But here we know that the
+wrinkled seed is in reality one having compound starch-grains,
+and that the heterozygote, though outwardly round enough, is
+intermediate in that starch-character. If we chose to say that
+the compoundness of the grains is due to a factor <i>C</i> and that two
+doses of it are needed to make the seed wrinkled, I know no
+evidence by which such a thesis could be actually refuted. That
+such reasoning is seemingly perverse must be conceded; but
+when we consider the extraordinary difficulties which beset
+any attempt to conceive the mode of origin of a new dominant
+factor, we are bound to remember that there is this other line of
+argument which avoids that difficulty altogether. In the case of
+the "Alexandra"-eye in <i>Primula</i>, or the red calyx in Gates's
+<i>Oenothera</i>, inverting the reasoning adopted in the text, we may
+see that only the <i>Primula</i> homozygous for the yellow eye can
+develop it and that two doses of the factor for the <i>rubrinervis</i>
+calyx are required to prevent that part of the plant from being red.
+<span class="pagenum"><a name="Page_96" id="Page_96">[Pg 96]</a></span></p>
+
+<p class="indent">We may proceed further and extend this mode of reasoning
+to all cases of genetic variation, and thus conceive of all alike as
+due to loss of factors present in the original complex. Until we
+can recognize factors by means more direct than are provided by
+a perception of their effects, this doubt cannot be positively
+removed. For all practical purposes of symbolic expression we
+may still continue to use in our analyses the modes of representation
+hitherto adopted, but we must not, merely on the ground of
+its apparent perversity, refuse to admit that the line of argument
+here indicated may some day prove sound.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_97" id="Page_97">[Pg 97]</a></span></p>
+<h2>CHAPTER V</h2>
+<h3><span class="smcap">The Mutation Theory</span></h3>
+
+<p class="indent">When with the thoughts suggested in the last chapter
+we contemplate the problem of Evolution at large the hope at the
+present time of constructing even a mental picture of that process
+grows weak almost to the point of vanishing. We are left
+wondering that so lately men in general, whether scientific or
+lay, were so easily satisfied. Our satisfaction, as we now see,
+was chiefly founded on ignorance.</p>
+
+<p class="indent">Every specific evolutionary change must represent
+a definite event in the construction of the living complex. That event
+may be a disturbance in the meristic system, showing itself in
+a change in the frequency of the repetitions or in the distribution
+of differentiation among them, or again it may be a chemical
+change, adding or removing some factor from the sum total.</p>
+
+<p class="indent">If an attempt be made to apply these conceptions to an
+actual series of allied species the complexity of the problem is such that
+the mind is appalled. Ideas which in the abstract are apprehended
+and accepted with facility fade away before the concrete
+case. It is easy to imagine how Man was evolved from an
+<i>Amoeba</i>, but we cannot form a plausible guess as to how <i>Veronica
+agrestis</i> and <i>Veronica polita</i> were evolved, either one from the
+other, or both from a common form. We have not even an
+inkling of the steps by which a Silver Wyandotte fowl descended
+from <i>Gallus Bankiva</i>, and we can scarcely even believe that it
+did. The Wyandotte has its enormous size, its rose comb, its
+silver lacing, its tame spirit, and its high egg production. The
+tameness and the high egg production are probably enough both
+recessives, and though we cannot guess how the corresponding
+dominant factors have got lost, it is not very difficult to imagine
+that they were lost somehow. But the rose comb and the silver
+colour are <i>dominants</i>. The heavy weight also appears in the
+crosses with Leghorns, but we need not at once conclude that it
+<span class="pagenum"><a name="Page_98" id="Page_98">[Pg 98]</a></span>
+depends on a simple dominant factor, because the big size of
+the crosses may be a consequence of the cross and may depend on
+other elements.</p>
+
+<p class="indent">Now no wild fowl known to us has these qualities.
+May we suppose that some extinct wild species had them? If so, may
+we again make the same supposition in all similar cases? To do
+so is little gain, for we are left with the further problem, whence
+did those lost wild species acquire those dominants? Suppositions
+of this kind help no more than did the once famous
+conjecture as to the origin of living things&mdash;that perhaps they
+came to earth on a meteorite. The unpacking of an original
+complex, the loss of various elements, and the recombination of
+pre-existing materials may all be invoked as sources of specific
+diversity. Undoubtedly the range of possibilities thus opened
+up is large. It will even cover an immense number of actual
+examples which in practice pass as illustrations of specific
+distinction. The Indian Rock pigeon which has a blue rump
+may quite reasonably be regarded as a geographically separated
+recessive form of our own <i>Columba livia</i>, for as Staples-Browne
+has shown the white rump of <i>livia</i> is due to a dominant factor.
+The various degrees to which the leaves of Indian Cottons are
+incised have, as Leake says, been freely used as a means of
+classification. The diversities thus caused are very remarkable,
+and when taken together with diversities in habit, whether
+sympodial or monopodial, the various combinations of points
+of difference are sufficiently distinctive to justify any botanist
+in making a considerable number of species by reference to them
+alone. Nevertheless Leake's work goes far to prove that all of
+these forms represent the re-combinations of a very small number
+of factors. The classical example of <i>Primula Sinensis</i> and its
+multiform races is in fact for a long way a true guide as to the
+actual interrelations of the species which systematists have
+made. That they did make them was due to no mistake in
+judgment or in principle, but simply to the want of that extended
+knowledge of the physiological nature of the specific
+cases which we now know to be a prime necessity.
+<span class="pagenum"><a name="Page_99" id="Page_99">[Pg 99]</a></span></p>
+
+<p class="indent">But will such analysis cover all or even most of the
+ordinary cases of specific diversity between near allies? Postponing the
+problem of the interrelations of the larger divisions as altogether
+beyond present comprehension, can we suppose, that in general,
+closely allied species and varieties represent the various consequences
+of the presence or absence of allelomorphic factors
+in their several combinations? The difficulty in making a
+positive answer lies in the fact that in most of the examples in
+which it has been possible to institute breeding experiments with
+a view to testing the question, a greater or less sterility is
+encountered. Where, however, no such sterility is met with, as
+for instance in the crosses made by E. Baur among the species
+of <i>Antirrhinum</i> there is every reason to think that the whole
+mass of differences can and will eventually be expressed in terms
+of ordinary Mendelian factors. Baur has for example crossed
+species so unlike as <i>Antirrhinum majus</i> and <i>molle</i>, forms differing
+from each other in almost every feature of organisation.<a name="FNanchor_1_51" id="FNanchor_1_51"></a><a href="#Footnote_1_51" class="fnanchor">[1]</a>
+The F<sub>2</sub> generation from this cross presents an amazingly motley
+array of types which might easily if met with in nature be described
+as many distinct species. Yet all are fertile and there
+is not the slightest difficulty in believing that they can all be
+reduced to terms of factorial analysis.</p>
+
+<p class="indent">If allowance be made for the complicating effects of
+sterility, is there anything which prevents us from supposing that such
+good species as those of <i>Veronica</i> or of any other genus comprising
+well-defined forms may not be similarly related? I do not know
+any reason which can be pointed to as finally excluding such a
+possibility. Nevertheless it has been urged with some plausibility
+that good species are distinguished by <i>groups</i> of differentiating
+characters, whereas if they were really related as the
+terms of a Mendelian F<sub>2</sub> family are, we should expect to find
+not groups of characters in association, but rather series of forms
+corresponding to the presence and absence of the integral factors
+composing the groups of characters. I am not well enough
+versed in systematic work to be able to decide with confidence
+how much weight should be attached to this consideration. Some
+<span class="pagenum"><a name="Page_100" id="Page_100">[Pg 100]</a></span>
+weight it certainly has, but I cannot yet regard it as forming a
+fatal objection to the application of factorial conceptions on
+the grand scale. It may be recalled that we are no longer under
+any difficulty in supposing that differences of all classes may be
+caused by the presence or absence of factors. It seemed at first
+for example that such characters as those of leaf shape might
+be too subtle and complex to be reducible to a limited number of
+factors. But first the work of Gregory on <i>Primula Sinensis</i>
+showed that several very distinct types of leaves were related
+to each other in the simplest way. In that particular example,
+intermediates are so rare as to be negligible, but subsequently
+Shull dealing with such a complicated example as <i>Capsella</i>, and
+Leake in regard to Cottons, both forms in which intergrades occur
+in abundance, have shown that a simple factorial scheme is
+applicable. We need not therefore, to take an extreme case,
+doubt that if it were possible to examine the various forms of
+fruit seen in the Squashes by really comprehensive breeding
+tests, even this excessive polymorphism in respect of structural
+features would be similarly reducible to factorial order.</p>
+
+<p class="indent">It must always be remembered also that in a vast number
+of cases, nearly allied forms which are distinct, occupy distinct
+ground. Moreover, by whatever of the many available mechanisms
+that end be attained, it is clear that nature very often does
+succeed in preventing intercrossing between distinct forms so
+far that the occurrence of that phenomenon is a rarity under
+natural conditions. The facts may, I think, fairly be summarized
+in the statement that species are on the whole distinct and not
+intergrading, and that the distinctions between them are usually
+such as might be caused by the presence, absence, or inter-combination
+of groups of Mendelian factors; but that they are so caused the evidence
+is not yet sufficient to prove in more than a very few instances.</p>
+
+<p class="indent">The alternative, be it explicitly stated, is not to
+return to the view formerly so widely held, that the distinctions between
+species have arisen by the accumulation of minute or insensible
+differences. The further we proceed with our analyses the more
+inadequate and untenable does that conception of evolutionary
+<span class="pagenum"><a name="Page_101" id="Page_101">[Pg 101]</a></span>
+change become. If the differences between species have not
+come about by the addition or loss of factors one at a time, then
+we must suppose that the changes have been effected by even
+larger steps, and variations including groups of characters, must
+be invoked.</p>
+
+<p class="indent">That changes of this latter order are really those
+by which species arise, is the view with which de Vries has now made
+us familiar by his writings on the Mutation Theory. In so far as
+mutations may consist in meristic changes of many kinds and
+in the loss of factors it is unnecessary to repeat that we have
+abundant evidence of their frequent occurrence. That they may
+also more rarely occur by the addition of a factor we are, I think,
+compelled to believe, though as yet the evidence is almost entirely
+circumstantial rather than direct. The evidence for the
+occurrence of those mutations of higher order, by which new
+species characterized by several distinct features are created,
+is far less strong, and after the best study of the records which
+I have been able to make, I find myself unconvinced. The facts
+alleged appear capable of other interpretations.</p>
+
+<p class="indent">The most famous and best studied examples are of course the
+forms of <i>Oenothera</i> raised by de Vries from <i>Oenothera Lamarckiana</i>
+in circumstances well known to all readers of genetic literature.
+Whatever be the true significance of these extraordinary "mutations"
+there can be no question about the great interest which
+attaches to them, and the historical importance which they will
+long preserve. Apart also from these considerations it is becoming
+more and more evident that in their peculiarities they
+provide illustrations of physiological phenomena of the highest
+consequence in the study of genetics at large.</p>
+
+<p class="indent">De Vries found, as is well known, that <i>Oenothera Lamarckiana</i>
+gives off plants unlike itself. These mutational forms are of
+several distinct and recognizable types which recur, and several
+of them breed true from their first appearance. The obvious
+difficulty, which in my judgment should make us unwilling at
+present to accept these occurrences as proof of the genesis of new
+species by mutation, is that we have as yet no certainty that the
+appearance of the new forms is not an effect of the recombination
+<span class="pagenum"><a name="Page_102" id="Page_102">[Pg 102]</a></span>
+of factors, such as is to be seen in so many generations of plants
+derived from a cross involving many genetic elements. The
+first question is what is <i>Oenothera Lamarckiana</i>? Is it itself a
+plant of hybrid origin? To this fundamental question no satisfactory
+answer has yet been given. All attempts to find it as a
+wild plant in America have failed. It existed in Europe in the
+latter half of the eighteenth century. Whence it came is still
+uncertain, but the view that it came into existence in Europe and
+perhaps in Paris, seems on the whole the most probable. The
+question has been debated by Macdougal, Gates, and Davis.
+From historical sources there is little expectation of further
+light. Those who favour the notion of a hybrid origin look on
+<i>Oenothera biennis</i> as one of the putative parents. It has been
+conjectured that a species called <i>grandiflora</i> lately re-discovered
+on the Alabama river was the other parent. Experiments have
+been instituted by Davis to discover whether <i>Lamarckiana</i> can
+be made artificially by crossing these two species. The results
+so far have shown that while plants approximating in various
+respects to <i>Lamarckiana</i> have thus been produced, none agree
+exactly with that form. Davis, to whom reference should be
+made for a full account of the present state of the enquiry,
+points out that there are many strains of <i>biennis</i> in existence
+and that it is by no means impossible that by using others of
+these strains a still closer approximation can be made. None
+of Davis's artificial productions as yet breed at all true, as
+<i>Lamarckiana</i> on the whole does. In such a case, however, where
+several characters are involved, this is perhaps hardly to be expected.</p>
+
+<p class="indent">One feature of the <i>Oenotheras</i> is very curious.
+Not only <i>Lamarckiana</i>, but all the allied species so far as I am
+aware, have a considerable proportion of bad and shrivelled pollen
+grains. This is undoubtedly true of species living in the wild
+state as well as of those in cultivation. I have had opportunities
+of verifying this for myself in the United States. No one looking
+at the pollen of an <i>Oenothera</i> would doubt that it was taken from
+some hybrid plant exhibiting partial sterility. On the other
+hand, it is difficult to suppose that numbers, perhaps all, of the
+<span class="pagenum"><a name="Page_103" id="Page_103">[Pg 103]</a></span>
+"species" of the genus are really hybrids, and many of them breed
+substantially true. I regard this constant presence of bad
+pollen grains as an indication that the genetic physiology of
+<i>Oenothera</i> is in some way abnormal, and as we shall presently
+see, there are several other signs which point in the same direction.</p>
+
+<p class="indent">Discussion of the whole series of phenomena is rendered
+exceedingly difficult first, by reason of the actual nature of the
+material. The characteristics of many of the types which de Vries
+has named are evasive. A few of these types, for instance, <i>gigas</i>,
+<i>nanella</i>, <i>albida</i>, <i>brevistylis</i>, and perhaps a few more are
+evidently clear enough, but we have as yet no figures and descriptions
+precise enough to enable a reader to appreciate exactly the peculiarities
+of the vast number of forms which have now to be considered in any
+attempt to gain a comprehensive view of the whole mass of facts.
+It is also not in dispute that the forms are susceptible of great
+variations due simply to soil and cultural influences.</p>
+
+<p class="indent">The fact that no Mendelian analysis has yet been found
+applicable to this group of <i>Oenotheras</i> as a whole is perhaps largely
+due to the fact that until recently such analysis has not been
+seriously attempted. Following the system which he had
+adopted before the rediscovery of Mendelism, or at all events,
+before the development of that method of analysis, de Vries has
+freely applied <i>names</i> to special combinations of characters and
+has scarcely ever instituted a factorial analysis. Before we can
+get much further this must be attempted. It may fail, but we
+must know exactly where and how this failure comes about.
+There are several indications that such a recognition of factorial
+characters, could be carried some way. For example, the height,
+the size of the flowers, the crinkling of the leaves, the brittleness
+of the stems, perhaps even the red stripes on stems and fruits,
+and many more, are all characters which may or may not depend
+on distinct factors, but if such characters are really transmitted
+in unresolved groups, the limitations of those groups should be
+carefully determined. The free use of names for the several
+forms, rather than for the characters, has greatly contributed
+to deepen the obscurity which veils the whole subject.
+<span class="pagenum"><a name="Page_104" id="Page_104">[Pg 104]</a></span></p>
+
+<p class="indent">I do not mean to suggest that these <i>Oenotheras</i>
+follow a simple Mendelian system. All that we know of them goes to show
+that there are curious complications involved. One of these, probably
+the most important of all, has lately been recognized by
+de Vries himself, namely, that in certain types the characters
+borne by the female and the male germ-cells of the same plant
+are demonstrably different. There can be little doubt that
+further research will reveal cognate phenomena in many unsuspected
+places. The first example in which such a state of things
+was proved to exist is that of the Stocks investigated by Miss
+Saunders.<a name="FNanchor_2_52" id="FNanchor_2_52"></a><a href="#Footnote_2_52" class="fnanchor">[2]</a>
+By a long course of analysis she succeeded in establishing
+in 1908 the fact that if a plant of <i>Matthiola</i> is of that
+eversporting kind which gives a large proportion of double-flowered
+plants among its offspring (produced by self-fertilisation),
+then the egg-cells of such a plant are mixed in type, but
+the pollen of the same plant is homogeneous. Some of the egg-cells
+have in them the two factors for singleness, but some of
+them are short of one or both of these factors. The pollen-grains,
+however, are all recessives, containing neither of these
+factors. The egg-cells, in other words, are mixed, "singles" and
+"doubles," while the pollen-grains are all "doubles." The same
+is true of the factor differentiating "white," or colourless plastids
+from cream-coloured plastids in <i>Matthiola</i>, the egg-cells being
+mixed "whites" and "creams," while the pollen-grains are all
+"creams," viz: recessives. Later in the same year (1908)
+de Vries<a name="FNanchor_3_53" id="FNanchor_3_53"></a><a href="#Footnote_3_53" class="fnanchor">[3]</a>
+announced a remarkable case which will be discussed
+in detail subsequently. It relates to certain <i>Oenotheras</i> heterozygous
+for dwarfness, in which (p. 113) the ovules were mixed,
+tails and dwarfs, while the pollen is all dwarf.</p>
+
+<p class="indent">Again in <i>Petunia</i> Miss Saunders's<a name="FNanchor_4_54" id="FNanchor_4_54"></a><a href="#Footnote_4_54" class="fnanchor">[4]</a>
+work has shown that a somewhat similar state of things exists, but with
+this remarkable difference, that though the egg-cells are mixed, singles and
+doubles, the pollen-grains are all <i>singles</i>, viz: dominants. All
+the <i>Petunias</i> yet examined have been in this condition, including
+<span class="pagenum"><a name="Page_105" id="Page_105">[Pg 105]</a></span>
+some which in botanic gardens pass for original species. Whether
+actual wild plants from their native habitats are in the same
+state, is not yet known, but it is by no means improbable. The
+case may be compared with that of the moth <i>Abraxas grossulariata</i>
+studied by Doncaster and Raynor, in which the females
+are all heterozygous, or we may almost say "hybrids" of <i>grossulariata</i>
+and the variety <i>lacticolor</i>. Similarly we may say that at
+least garden Petunias are heterozygous in respect of singleness.
+The proof of this is of course that when fertilised with the pollen
+of doubles they throw a mixture of doubles and singles. The
+statements which de Vries has published regarding the behaviour
+of several of the <i>Oenotheras</i> go far to show that they must have
+a somewhat similar organisation. On the present evidence it is
+still quite impossible to construct a coherent scheme which will
+represent all the phenomena in their interrelations, and among
+the facts are several which, as will appear, seem mutually incompatible.
+The first indication that the <i>Oenotheras</i> may have
+either mixed ovules or mixed pollen appears in the fact that
+<i>Lamarckiana</i> and several of its "mutants" used as males, with
+several other forms as females, give a mixed offspring. For
+example, de Vries (1907) found that</p>
+
+<p class="blockquot"><b>
+<i>biennis</i> &#9792;&emsp;×&emsp;<i>Lamarckiana</i> &#9794;<br />
+<i>biennis cruciata</i> &#9792;&emsp;×&emsp;<i>Lamarckiana</i> &#9794;<br />
+<i>muricata</i> &#9792;&emsp;×&emsp;<i>Lamarckiana</i> &#9794;<br />
+<i>biennis</i> &#9792;&emsp;×&emsp;<i>rubrinervis</i> &#9794;<br />
+<i>biennis cruciata</i> &#9792;&emsp;×&emsp;<i>rubrinervis</i> &#9794;<br />
+</b></p>
+
+<p>all give a mixture of two distinct types which he names <i>laeta</i>
+and <i>velutina</i>, consisting of about equal numbers of each. On
+account of the fact that the two forms are produced in association
+de Vries has called these forms "twin hybrids," a designation
+which is not fortunate, seeing that it is impossible to imagine
+that any kind of twinning is concerned in their production. The
+distinction between these two seems to be considerable, <i>laeta</i>
+having leaves broader, bright green in colour, and flat, with
+pollen scanty, while <i>velutina</i> has leaves narrower, grayish green,
+more hairy, and furrow-shaped, with pollen abundant.
+<span class="pagenum"><a name="Page_106" id="Page_106">[Pg 106]</a></span></p>
+
+<p class="indent">We next meet the remarkable fact that these two forms,
+<i>laeta</i> and <i>velutina</i> breed true to their respective types, and do not
+reproduce the parent-types among their offspring resulting from
+self-fertilisation. This statement must be qualified in two
+respects. When <i>muricata</i> &#9794; is fertilised by <i>brevistylis</i> the forms
+<i>laeta</i> and <i>velutina</i> are produced, but each of them subsequently
+throws the short-styled form as a recessive (de Vries, 1907,
+p. 406). It may be remembered that de Vries's previous publications
+had already shown that the short style of <i>brevistylis</i>,
+one of the <i>Lamarckiana</i> "mutants," behaves as a recessive
+habitually (<i>Mutationstheorie</i>, II, p. 178, etc.).</p>
+
+<p class="indent">Also when <i>nanella</i>, the dwarf "mutant" of <i>Lamarckiana</i> is
+used as male on <i>muricata</i> as female, <i>laeta</i> and <i>velutina</i> are produced,
+but one only of these, namely, <i>velutina</i>, subsequently
+throws dwarfs on self-fertilisation. The dwarfs thus thrown are
+said to form about 50 per cent. of the families in which they
+occur (de Vries, 1908, p. 668). The fact that the two forms,
+<i>laeta</i> and <i>velutina</i>, are produced by many matings in which
+<i>Lamarckiana</i> and its mutant <i>rubrinervis</i> are used as males is
+confirmed abundantly by Honing, who has carried out extensive
+researches on the subject. After carefully reading his paper,
+I have failed to understand the main purport of the argument
+respecting the "double nature" of <i>Lamarckiana</i> which he founds
+on these results, but I gather that in some way <i>laeta</i> is shown to
+partake especially of the nature of <i>Lamarckiana</i>, while <i>velutina</i>
+is a form of <i>rubrinervis</i>. The paper contains many records which
+will be of value in subsequent analysis of these forms.</p>
+
+<p class="indent">Before considering the possible meaning of these
+facts we must have in our minds the next and most novel of the recent
+extensions of knowledge as to the genetic properties of the
+<i>Oenotheras</i>. In the previous statement we have been concerned
+with the results of using either <i>Lamarckiana</i> itself or one of its
+"mutants" <i>rubrinervis</i>, <i>brevistylis</i>, or <i>nanella</i> as male, on one of
+the species <i>biennis</i> or <i>muricata</i>. The new experiments relate
+to crosses between the two species <i>biennis</i> and <i>muricata</i> themselves.
+<span class="pagenum"><a name="Page_107" id="Page_107">[Pg 107]</a></span></p>
+
+<p class="indent">De Vries found:</p>
+
+<p class="blockquot">1. That the reciprocal hybrids from these two species differed,
+
+<i>biennis</i> × <i>muricata</i> producing one type of F<sub>1</sub> and <i>muricata</i> ×
+<i>biennis</i> producing another. Each F<sub>1</sub> resembled the father more
+than the mother.</p>
+
+<p class="blockquot">2. That each of the hybrids so produced breeds true on self-fertilisation.</p>
+
+<p class="blockquot">3. That if we speak of the hybrid from <i>biennis</i> × <i>muricata</i>
+as <i>BM</i> and of the reciprocal as <i>MB</i>, then</p>
+
+<p class="center"><b><i>BM</i> × <i>MB</i></b></p>
+
+<p class="blockquot">gives exclusively offspring of <i>biennis</i> type but that</p>
+
+<p class="center"><b><i>MB</i> × <i>BM</i></b></p>
+
+<p class="blockquot">gives exclusively offspring of <i>muricata</i> type.
+Evidently, apart from all controversy as to the significance of the "mutants"
+of <i>Lamarckiana</i>, we have here a series of observations of the first importance.</p>
+
+<p class="indent">The fact that reciprocal crossings give constantly
+distinct results must be taken to indicate that the male and female sides
+of one, if not of both, of the parents are different in respect of
+characters which they bear. This is de Vries's view, and he
+concludes rightly, I think, that the evidence from all the experiments
+shows that both <i>biennis</i> and <i>muricata</i> are in this condition,
+having one set of characters represented in their pollen-grains
+and another in their ovules. The plants breed true, but their
+somatic structures are compounded of the two sets of elements
+which pass into them from their maternal and paternal sides
+respectively. This possibility that species may exist of which
+the males really belong to one form and the females to another,
+is one which it was evident from the first announcement of the
+discovery of Mendelian segregation might be found realised in nature.<a name="FNanchor_5_55" id="FNanchor_5_55"></a><a href="#Footnote_5_55" class="fnanchor">[5]</a>
+</p>
+
+<p class="indent"><i>Oe. biennis</i> and <i>muricata</i> were crossed
+reciprocally with each other and with a number of other species, and the
+behaviour of each, when used as mother, was consistently different from its
+behaviour when used as father. De Vries is evidently justified
+<span class="pagenum"><a name="Page_108" id="Page_108">[Pg 108]</a></span>
+by the results of this series of experiments in stating that the
+"Bild," as he terms it, or composition of the male and female
+sides of these two species, <i>biennis</i> and <i>muricata</i>, are distinct.
+On the evidence before us it is not, however, possible to form a
+perfectly clear idea of each, and until details are published, a
+reader without personal knowledge of the material cannot do
+more than follow the general course of the argument. For fuller
+comprehension a proper analysis of the characters with a clear
+statement of how they are distributed among the several types
+and crosses is absolutely necessary. According to de Vries the
+female of <i>biennis</i> possesses a group of characters which he defines
+as "<i>conica</i>" in allusion to the shape of the flower-buds. Besides
+the conical buds, this group of features includes imperfect
+development of wood, rendering the plant very liable to attacks
+of <i>Botrytis</i>, and comparatively narrow leaves.</p>
+
+<p class="indent">The female of <i>muricata</i> carries a group of features which he
+calls "<i>frigida</i>," and, though this is not quite explicitly stated in a
+definition of that type, it is to be inferred<a name="FNanchor_6_56" id="FNanchor_6_56"></a><a href="#Footnote_6_56" class="fnanchor">[6]</a>
+that its characteristics are regarded as greater height, strong development
+of wood with comparative resistance to <i>Botrytis</i>, and broad leaves.</p>
+
+<p class="indent">The characters borne by the male parts of the two
+species are in general those by which they are outwardly distinguished.
+For example, the leaves of <i>Oe. biennis</i> are comparatively broad
+and are bright green, while those of <i>muricata</i> are much narrower
+and of a glaucous green, and I understand that de Vries regards
+these properties as contributed by the male side in each case and
+to be carried by the male cells of each species. The suggestion
+as regards <i>biennis</i> and <i>muricata</i> comes near the conception often
+expressed by naturalists in former times (<i>e. g.</i>, Linnaeus) and
+not rarely entertained by breeders at the present day, that the
+internal structure is contributed by the mother and the external
+by the father.</p>
+
+<p class="indent">On the other hand, the offspring of each species when
+used as mother is regarded as possessing in the main the features of
+the maternal "Bild," but the matter is naturally complicated
+by the introduction of features from the father's side, and it is
+<span class="pagenum"><a name="Page_109" id="Page_109">[Pg 109]</a></span>
+here especially that the account provided is at present unsatisfactory
+and inconclusive. There seems, however, to be no serious
+doubt that <i>biennis</i> and <i>muricata</i> each in their outward appearance
+exhibit on the whole the features which their pollens respectively
+carry, and that the features borne by their ovules are in many respects distinct.</p>
+
+<p class="indent">The <i>types</i> are thus "hybrids" which breed true.
+The results of intercrossing them each way are again "hybrids" which breed
+true. It will be remembered that on former occasions de Vries
+has formulated a general rule that <i>species</i>-hybrids breed true,
+but that the cross-breds raised by interbreeding <i>varieties</i> do not.
+One of these very cases was quoted<a name="FNanchor_7_57" id="FNanchor_7_57"></a><a href="#Footnote_7_57" class="fnanchor">[7]</a>
+as an illustration of thisprinciple, viz: <i>muricata</i> × <i>biennis</i>.
+The grounds for this general statement have always appeared to me insufficient,
+and with the further knowledge which the new evidence provides we are
+encouraged to hope that when a proper factorial analysis of the
+types is instituted we shall find that the phenomenon of a constant
+hybrid will be readily brought into line with the systems
+of descent already worked out for such cases as that of the Stocks,
+and others already mentioned.</p>
+
+<p class="indent space-below">In further discussion of these facts de Vries makes a suggestion
+which seems to me improbable. Since the egg-cells of <i>muricata</i>,
+for instance, bear a certain group of features which are missing
+on the male side, and conversely the pollen bears features absent
+from the female side, he is inclined to regard the <i>bad pollen grains</i>
+as the bearers of the missing elements of the male side and to
+infer that there must similarly be defective ovules representing
+the missing elements of the female side. No consideration is
+adduced in support of this view beyond the simple fact that the
+characters borne by male and female are dissimilar, whereas
+it would be more in accord with preconception if the same sets
+of combinations were represented in each&mdash;as in a normal
+Mendelian case. There is as yet no instance in which the absence
+of any particular class of gametes has been shown with any
+plausibility to be due to defective viability, though there are, of
+course, cases in which certain classes of zygotes do not survive
+<span class="pagenum"><a name="Page_110" id="Page_110">[Pg 110]</a></span>
+owing to defective constitution (<i>e. g.</i>, the albinos of <i>Antirrhinum</i>
+studied by Baur, and the homozygous yellow mice). I am
+rather inclined to suppose that in these examples of hybrids
+breeding true we shall find a state of things comparable with
+that to which we formerly applied the terms "coupling" and
+"repulsion." In these cases certain of the possible combinations
+of factors occur in the gametic series with special frequency,
+being in excess, while the gametes representing other combinations
+are comparatively few. In a recent paper on these cases
+Professor Punnett and I have shown that these curious results
+vary according to the manner in which the factors are grouped
+in the parents. If <i>A</i> and <i>B</i> are two factors which exhibit these
+phenomena we find that the gametic series of the double heterozygote
+differs according as the combination is made by crossing
+<i>AB × ab</i>, or by crossing <i>AB × aB</i>. In a normal Mendelian case
+the F<sub>1</sub> form, <i>AaBb</i>, produces gametes <i>AB</i>, <i>Ab</i>,
+<i>aB</i>, <i>ab</i>, in equal numbers;
+but in these peculiar cases those gametes which contain</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc"><b>Gametic</b></td>
+      <td class="tdc"><b>series</b></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;&nbsp;<b># of gametes</b></td>
+      <td class="tdr">&nbsp;&nbsp;<b># of zygotes</b></td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr"><b><i>AB</i></b></td>
+      <td class="tdr"><b><i>Ab</i></b></td>
+      <td class="tdr"><b><i>aB</i></b></td>
+      <td class="tdr"><b><i>ab</i></b></td>
+      <td class="tdr"><b>in series</b>&nbsp;&nbsp;</td>
+      <td class="tdr"><b>in series</b>&nbsp;&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">1</td>
+      <td class="tdr">(<i>n</i>-1)</td>
+      <td class="tdr">(<i>n</i>-1)</td>
+      <td class="tdr">1</td>
+      <td class="tdr"><i>2n</i></td>
+      <td class="tdr"><i>4n<sup>2</sup></i></td>
+    </tr><tr>
+      <td class="tdc"><i>Partial repulsion</i></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">1</td>
+      <td class="tdr">31</td>
+      <td class="tdr">31</td>
+      <td class="tdr">1</td>
+      <td class="tdr">64</td>
+      <td class="tdr">4096</td>
+    </tr><tr>
+      <td class="tdc">from zygote</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">1</td>
+      <td class="tdr">15</td>
+      <td class="tdr">15</td>
+      <td class="tdr">1</td>
+      <td class="tdr">32</td>
+      <td class="tdr">1024</td>
+    </tr><tr>
+      <td class="tdc">of form</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">1</td>
+      <td class="tdr">7</td>
+      <td class="tdr">7</td>
+      <td class="tdr">1</td>
+      <td class="tdr">16</td>
+      <td class="tdr">256</td>
+    </tr><tr>
+      <td class="tdc"><b><i>Ab × aB</i></b></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">1</td>
+      <td class="tdr">3</td>
+      <td class="tdr">3</td>
+      <td class="tdr">1</td>
+      <td class="tdr">8</td>
+      <td class="tdr">64</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">4</td>
+      <td class="tdr">16</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">3</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">3</td>
+      <td class="tdr">8</td>
+      <td class="tdr">64</td>
+    </tr><tr>
+      <td class="tdc"><i>Partial coupling</i></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">7</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">7</td>
+      <td class="tdr">16</td>
+      <td class="tdr">256</td>
+    </tr><tr>
+      <td class="tdc">from zygote</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">15</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">15</td>
+      <td class="tdr">32</td>
+      <td class="tdr">1024</td>
+    </tr><tr>
+      <td class="tdc">of form</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">31</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">31</td>
+      <td class="tdr">64</td>
+      <td class="tdr">4096</td>
+    </tr><tr>
+      <td class="tdc"><b><i>AB × ab</i></b></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">63</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">63</td>
+      <td class="tdr">128</td>
+      <td class="tdr">16384</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">(<i>n</i>-1)</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;(<i>n</i>-1)</td>
+      <td class="tdr"><i>2n</i></td>
+      <td class="tdr"><i>4n<sup>2</sup></i></td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr"><b>Nature</b></td>
+      <td class="tdc"><b>of</b></td>
+      <td class="tdl"><b>zygotic</b></td>
+      <td class="tdc"><b>series</b></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr"><b><i>AB</i></b></td>
+      <td class="tdr"><b><i>Ab</i></b></td>
+      <td class="tdr"><b><i>aB</i></b></td>
+      <td class="tdr"><b><i>ab</i></b></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc"><i>Partial repulsion</i></td>
+      <td class="tdr"> { </td>
+      <td class="tdr"><i>2n<sup>2</sup></i>+1</td>
+      <td class="tdr"><i>n<sup>2</sup></i>-1</td>
+      <td class="tdr"><i>n<sup>2</sup></i>-1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">from zygote</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">2049</td>
+      <td class="tdr">1023</td>
+      <td class="tdr">1023</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">of form</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">513</td>
+      <td class="tdr">255</td>
+      <td class="tdr">255</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc"><b><i>Ab × aB</i></b></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">33</td>
+      <td class="tdr">15</td>
+      <td class="tdr">15</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">9</td>
+      <td class="tdr">3</td>
+      <td class="tdr">3</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">41</td>
+      <td class="tdr">7</td>
+      <td class="tdr">7</td>
+      <td class="tdr">9</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc"><i>Partial coupling</i></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">177</td>
+      <td class="tdr">15</td>
+      <td class="tdr">15</td>
+      <td class="tdr">49</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">from zygote</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">737</td>
+      <td class="tdr">31</td>
+      <td class="tdr">31</td>
+      <td class="tdr">225</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">of form</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">3009</td>
+      <td class="tdr">63</td>
+      <td class="tdr">63</td>
+      <td class="tdr">961</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc"><b><i>AB × ab</i></b></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">12161</td>
+      <td class="tdr">127</td>
+      <td class="tdr">127</td>
+      <td class="tdr">3969</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">(<i>3n<sup>2</sup></i>-(<i>2n</i>-1)</td>
+      <td class="tdr">(<i>2n</i>-1)</td>
+      <td class="tdr">(<i>2n</i>-1)</td>
+      <td class="tdr">&nbsp;&nbsp;<i>n<sup>2</sup></i>-(<i>2n</i>-1)</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr>
+ </tbody>
+</table>
+<p class="space-above"><span class="pagenum"><a name="Page_111" id="Page_111">[Pg 111]</a></span>
+the <i>parental combinations</i> are in excess. This excess almost
+certainly follows the system indicated by the accompanying
+table. In the general expressions <i>n</i> is half the number of gametes
+required to express the whole system. Now if we imagine that
+sex-factors are involved with the others concerned in such a relationship
+as this we have a system of distribution approximating
+to that found in <i>biennis</i> and <i>muricata</i>. The difference in
+reciprocals is represented in a not improbable way. It cannot yet
+be said that the rarer terms in the series are formed at all, and
+perhaps they are not. As we pointed out in our discussion of
+these phenomena, the peculiar distribution of factors in these
+cases must be taken to mean that the planes of division at some
+critical stage in the segregation are determined with reference
+to the parental groups of factors, or in other words, that the
+whole system has a polarity, and that the distribution of factors
+with reference to this polarity differs according to the grouping
+of factors in the gametes which united in fertilization to produce
+the plant. Subsequent proliferation of cells representing certain
+combinations would then lead to excess of the gametes bearing
+them. It is on similar lines that I anticipate we shall hereafter
+find the interpretation of the curious facts discovered by de Vries,
+though it is evident that a long course of experiment and analysis
+must be carried through before any certainty is reached. The
+work must be begun by a careful study of the descent of some
+single factor, for example, that causing the broader leaf of
+<i>biennis</i>, and we may hope that the study of <i>Oenothera</i>
+by proper analytical methods will no longer be deferred.</p>
+
+<p class="indent">We have now to return to the relations of <i>laeta</i>
+and <i>velutina</i>. These two forms, it will be remembered are frequently
+produced when <i>Lamarckiana</i> or one of its derivatives is used as male,
+and the most unexpected feature in their behaviour is that <i>both
+breed true as regards their essential characteristics, on
+self-fertilisation</i>. If one only bred true the case might, in view
+of the approximate numerical equality of the two types, be difficult
+to interpret on ordinary lines, but as both breed true it must be
+clear that some quite special system of segregation is at work.
+What this may be cannot be detected on the evidence, but with
+<span class="pagenum"><a name="Page_112" id="Page_112">[Pg 112]</a></span>
+the results from the <i>biennis-muricata</i> experiments before us,
+it is natural to suspect that we may here again have to recognise
+a process of allocation of different factors to the male and female
+sides in <i>laeta</i> and <i>velutina</i>. That some such system is in
+operation becomes the more probable from the new fact which de Vries
+states in describing the group of characters which he calls <i>conica</i>,
+namely that this type is the same as that of <i>velutina</i>.</p>
+
+<p class="indent">There are many collateral observations recorded
+both by de Vries and others which have a bearing on the problems,
+but they do not yet fall into a coherent scheme. For example, we
+cannot yet represent the formation of <i>laeta</i> and <i>velutina</i> from the
+various species fertilised by <i>Lamarckiana</i> &#9794;. That this is not
+due to any special property associated with the pollen of <i>Lamarckiana</i>
+is shown by the fact that a species called <i>Hookeri</i>
+gives <i>laeta</i> and <i>velutina</i> in both its reciprocal crosses with
+<i>Lamarckiana</i> (de Vries, 1909, p. 3), and also by the similar fact that
+<i>Lamarckiana</i> &#9792; fertilised by the pollen of a peculiar race of
+<i>biennis</i> named <i>biennis Chicago</i> throws the same types. Before
+these very complicated phenomena can be usefully discussed
+particulars must be provided as to the individuality of the various
+plants used. This criticism applies to much of the work which
+de Vries has lately published, for, as we now know familiarly,
+plants to which the same name applies can be quite different in
+genetic composition.</p>
+
+<p class="indent">Attention should also be called to one curiously paradoxical
+series of results. When the dwarf "mutant" of <i>Lamarckiana</i>
+which de Vries names "<i>nanella</i>" is used as father on <i>muricata</i>,
+F<sub>1</sub> consists of <i>laeta</i> and <i>velutina</i> in approximately equal numbers.
+Both forms breed true to their special characteristics, but
+<i>velutina</i> throws dwarfs of its own type, while <i>laeta</i> does not
+throw dwarfs. Subsequent investigation of the properties of
+these types has led to some remarkable conclusions, and it was
+in a study of these plants that de Vries first came upon the phenomena
+of dissimilarity between the factors borne by the male
+and female cells of the same plant, a condition which had been
+recently detected in the Stocks as a result of Miss Saunders's
+investigations. The details are very remarkable. We have
+<span class="pagenum"><a name="Page_113" id="Page_113">[Pg 113]</a></span>
+first the fact that <i>muricata</i> &#9792; × dwarf <i>nanella</i> &#9794; gives about
+50 per cent. <i>laeta</i> and about 50 per cent. of <i>velutina</i>.</p>
+
+<p class="indent">As regards <i>Velutina</i> it was shown that:</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc"><b>Talls,</b></td>
+      <td class="tdc"><b>Dwarfs,</b></td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdc"><b>per cent.</b></td>
+      <td class="tdc"><b>per cent.</b></td>
+    </tr><tr>
+      <td class="tdr">1.&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><i>Velutina</i> selfed gave</td>
+      <td class="tdc">38</td>
+      <td class="tdc">62</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdl"><i>Velutina</i> &#9792; × dwarf <i>nanella</i> &#9794; gave</td>
+      <td class="tdc">39</td>
+      <td class="tdc">61</td>
+    </tr><tr>
+      <td class="tdr">2.&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdl">&nbsp;&emsp;do.&emsp;&emsp;&nbsp;×&emsp;&emsp;do.&emsp;&emsp;&emsp;&emsp;gave</td>
+      <td class="tdc">49</td>
+      <td class="tdc">51</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdl">&nbsp;&emsp;do.&emsp;&emsp;&nbsp;×&emsp;&emsp;do. &#9794; derived from <i>velutina</i> gave</td>
+      <td class="tdc">43</td>
+      <td class="tdc">57</td>
+    </tr><tr>
+      <td class="tdr">3.&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">Dwarfs&emsp;&emsp;×&emsp;<i>velutina</i> &#9794; gave</td>
+      <td class="tdc">&mdash;</td>
+      <td class="tdc">all dwarfs</td>
+    </tr>
+ </tbody>
+</table>
+
+<p class="space-above">The three experiments taken together prove, as de Vries
+says, that the ovules of <i>velutina</i> are mixed, talls and dwarfs, and that
+the pollen is all dwarf. The condition is almost the same as
+that of the Stocks. It may be noted also that in the Stocks the
+egg-cells of the "double" type are in excess, being approximately
+9 to 7 of the "single" type, but de Vries regards the two types
+in <i>velutina</i> as probably equal in number. The figures (169:231)
+rather suggest some excess of the recessives, perhaps 9:7, and
+the point would be worth a further investigation.</p>
+
+<p class="indent">As regards <i>laeta</i>, by self-fertilisation <i>no dwarfs were
+produced</i>, but in all other respects it behaved almost exactly like <i>velutina</i>.
+The ovules are evidently mixed talls and dwarfs, and whether
+fertilised by dwarfs or by the pollen of <i>velutina</i>, which is already
+proved to be all dwarf, the result was a steady 50 per cent. of
+talls and 50 per cent. of dwarfs. The pollen of <i>laeta</i> used on
+dwarfs gives nothing but dwarfs, and in three series of such experiments
+226 dwarfs were produced.</p>
+
+<p class="indent">We are thus faced with this difficulty. Since the egg-cells
+of <i>laeta</i> are evidently mixed, talls and dwarfs, and the pollen used
+on dwarfs gives all dwarfs, why does not self-fertilisation give
+a mixed result, talls and dwarfs, instead of <i>all talls</i>? De Vries
+regards the result of self-fertilisation as showing the real nature
+of the pollen, and declares it to be all talls, while he represents
+the behaviour of the same pollen used on dwarfs by stating that
+in these combinations the dwarf character dominates. This
+does not seem to me a natural interpretation. I should regard
+the pollen of <i>laeta</i> as identical with that of <i>velutina</i>, namely
+dwarf, and I suspect the difficulty is really created by the behaviour of
+<i>laeta</i> on self-fertilisation. Until a proper analysis is made in
+<span class="pagenum"><a name="Page_114" id="Page_114">[Pg 114]</a></span>
+which the identity of the different individuals used is recorded,
+no further discussion is possible.<a name="FNanchor_8_58" id="FNanchor_8_58"></a><a href="#Footnote_8_58" class="fnanchor">[8]</a>
+</p>
+
+<p class="indent">Other results of a complicated kind involving production
+of <i>laeta</i> and <i>velutina</i> together with a third form have been
+published by de Vries in his paper on "Triple Hybrids." To these also
+the same criticism applies. Some of the observations seem capable
+of simple factorial representation and others are conflicting.</p>
+
+<p class="indent">Taking the work on <i>Oenothera</i> as a whole we see
+in it continually glimpses of order which further on are still blocked by
+difficulties and apparent inconsistencies. Through such a stage
+all the successful researches in complicated factorial analysis
+have passed and I see no reason for supposing that with the
+application of more stringent methods this more difficult set of
+problems will be found incapable of similar solutions. To
+return to the original question whether in <i>Oenothera</i> we can claim
+to see a special contemporaneous output of new species in actual
+process of creation, it will be obvious that while the interrelation
+of the several types is still so little understood, such a claim has
+no adequate support. It is true that many of the "mutants"
+of <i>Lamarckiana</i> can well pass for species, but this is equally true
+of many new combinations of pre-existing factors as we have
+seen in <i>Primula Sinensis</i> and other cases. Still less can it be
+admitted that these facts of uncertain import supply a justification
+for the conception which has played a prominent part
+in the scheme of the <i>Mutationstheorie</i>, namely that there are
+special periods of Mutation, when the parent-species has peculiar
+genetic properties. To conclude: The impression which the
+evidence leaves most definitely on the mind is that further discussion
+of the bearing which the <i>Oenotheras</i> may have on the
+problem of evolution should be postponed until we have before
+us the results of a searching analysis applied to a limited part of
+the field. In such an analysis it is to be especially remembered
+that we have now a new clue in the well-ascertained fact that the
+genetic composition of the male and female germ-cells of the
+<span class="pagenum"><a name="Page_115" id="Page_115">[Pg 115]</a></span>
+same individual may be quite different. When with this possibility
+in view the behaviour of the types is re-examined I
+anticipate that many of the difficulties will be removed.</p>
+
+<p class="indent">Outside the evidence from <i>Oenothera</i>, which, as we
+have seen, is still ambiguous, I know no considerable body of facts favourable
+to that special view of Mutation which de Vries has promulgated.
+Of variation, or if we will, Mutation, in respect of
+some one character, or resulting from recombination, there is
+proof in abundance; but of that simultaneous variation in several
+independent respects to which de Vries especially attributes the
+origin of new specific types I know only casual records which
+have yet to undergo the process of criticism.</p>
+
+<hr class="tb" />
+
+<p class="indent">Besides de Vries's "<i>Mutationstheorie</i>" and "Species
+and Varieties" the chief publications relating to the subject of the behaviour
+of <i>Oenothera</i> are the following: (Many other papers
+relating especially to the cytology of the forms have appeared.)</p>
+
+<p>
+<span style="margin-left: 4em;">Davis, B. M.</span><br />
+<span style="margin-left: 6em;">Genetical Studies on <i>Oenothera</i>, I. <i>Amer. Nat.</i>, XLIV, 1910, p. 108.</span><br />
+<span style="margin-left: 6em;">Genetical Studies on <i>Oenothera</i>, II. <i>Ibid.</i>, XLV, 1911, p. 193.</span>
+<br /><br />
+<span style="margin-left: 4em;">Gates, R. R.</span><br />
+<span style="margin-left: 6em;">An Analytical Key to some of the Segregates of Oenothera.</span><br />
+<span style="margin-left: 7em;"><i>Twentieth Annual Report of the Missouri Botanical Garden</i>, 1909.</span><br />
+<span style="margin-left: 6em;">Studies on the Variability and Heritability of Pigmentation in <i>Oenothera</i>.</span><br />
+<span style="margin-left: 7em;"><i>Ztsch. f. Abstammungslehre</i>, 1911, IV, p. 337.</span>
+<br /><br />
+<span style="margin-left: 4em;">Honing, J. A.</span><br />
+<span style="margin-left: 6em;">Die Doppelnatur der <i>Oenothera Lamarckiana</i>.</span><br />
+<span style="margin-left: 7em;"><i>Ztsch. f. Abstammungslehre</i>, 1911, IV, p. 227.</span>
+<br /><br />
+<span style="margin-left: 4em;">Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K. Small).</span><br />
+<span style="margin-left: 6em;">Mutants and Hybrids of the <i>Oenotheras</i>.</span><br />
+<span style="margin-left: 7em;"><i>Carnegie Institution's Publication</i>, No. 24, 1905.</span>
+<br /><br />
+<span style="margin-left: 4em;">Macdougal, D. T., Vail, A. M., Shull, J. H.</span><br />
+<span style="margin-left: 6em;">Mutations, Variations and Relationships of the <i>Oenotheras</i>.</span><br />
+<span style="margin-left: 7em;"><i>Carnegie Institution's Publication</i>, No. 81, 1907.</span>
+<br /><br />
+<span style="margin-left: 4em;">de Vries, H.</span><br />
+<span style="margin-left: 6em;">On Atavistic Variation in <i>Oenothera cruciata</i>.</span><br />
+<span style="margin-left: 7em;"><i>Bull. Torrey Club</i>, 1903, Vol. 30, p. 75.</span><br />
+<span style="margin-left: 6em;">On Twin Hybrids,</span><br />
+<span style="margin-left: 7em;"><i>Bot. Gaz.</i>, Vol. 44, 1907, p. 401.</span><br />
+<span style="margin-left: 4em;">Ueber die Zwillingsbastarde von <i>Oenothera nanella</i>.</span><br />
+<span style="margin-left: 6em;"><i>Ber. Deut. Bot. Ges.</i>, 1908, XXVI, <i>a</i>, p. 667.</span>
+<br /><br />
+<span style="margin-left: 4em;">Bastarde von <i>Oenothera gigas</i>. <i>Ibid.</i>, p. 754.</span>
+<br /><br />
+<span style="margin-left: 4em;">On Triple Hybrids. <i>Bot. Gaz.</i>, 1909, Vol. 47, p. 1.</span><br />
+<span style="margin-left: 4em;">Ueb. doppeltreziproke Bastarde von <i>Oenothera biennis</i> L. und <i>Oenothera muricata</i> L.</span><br />
+<span style="margin-left: 6em;"><i>Biol. Cbltt.</i>, 1911, XXXI, p. 97.</span>
+<br /><br />
+<span style="margin-left: 4em;">Zeijlstra, H. H.</span><br />
+<span style="margin-left: 6em;"><i>Oenothera nanella</i> de Vries, eine krankhafte Pflanzenart.</span><br />
+<span style="margin-left: 7em;"><i>Biol. Cbltt.</i>, 1911, XXXI, p. 129.</span>
+<span class="pagenum"><a name="Page_116" id="Page_116">[Pg 116]</a></span></p>
+
+<h3><span class="smcap">Note.</span></h3>
+
+<p class="indent">Since this chapter was written two contributions of special
+importance have been made to the study of the <i>Oenothera</i> problems.
+The first is that of Heribert-Nilsson.<a name="FNanchor_9_59" id="FNanchor_9_59"></a><a href="#Footnote_9_59" class="fnanchor">[9]</a>
+The author begins by giving a critical account of the evidence for
+de Vries's interpretation of the nature of the mutants. In general this
+criticism pursues lines similar to those sketched in the foregoing chapter,
+concluding, as I have done, that the chief reason why factorial
+analysis has been declared to be inapplicable to the <i>Oenothera</i>
+mutants is because no one has hitherto set about this analysis
+in the right way. He has also himself made a valuable beginning
+of such an analysis and gives good evidential reasons for the belief
+that at least the red veining depends on a definite factor which
+also influences the size of certain parts of the plant. He argues
+further that many of the distinctions between the mutants are
+quantitative in nature. With great plausibility he suggests that
+the system of cumulative factors which Nilsson-Ehle discovered
+in the case of wheat (subsequently traced by East in regard to maize)
+may be operating also in these <i>Oenotheras</i>. According
+to this system several factors having similar powers may coexist
+in the same individual, and together produce a cumulative effect.
+Scope would thus be given for the production of the curious and
+seemingly irregular numbers so often recorded in the "mutating" families.</p>
+
+<p class="indent">Another remarkable observation relating to the crosses of
+<i>muricata</i> and <i>biennis</i> has been published by Goldschmidt.<a name="FNanchor_10_60" id="FNanchor_10_60"></a><a href="#Footnote_10_60" class="fnanchor">[10]</a>
+He finds that in the formation of this cross the female pronucleus
+takes no part in the development of the zygotic cell, but that
+when the male pronucleus enters, the female pronucleus is
+pushed aside and degenerates. As de Vries observed, the reciprocal
+hybrids are in each case very like the father ("<i>stark
+patroklin</i>"), a consequence which finds a natural explanation in
+the phenomenon witnessed by Goldschmidt. The results of
+the subsequent matings can also be readily interpreted on the
+same lines. Indications of maternal characters are nevertheless
+<span class="pagenum"><a name="Page_117" id="Page_117">[Pg 117]</a></span>
+mentioned by de Vries, and if Goldschmidt's account of the
+cytology is confirmed, these must presumably be referred to the
+influence of the maternal cytoplasm. Clearly this new work
+opens up lines of exceptional interest. The interpretation I
+have offered above must probably be reconsidered. The distinction
+between the male and female cells of the types may no doubt be
+ultimately factorial, but it is difficult to regard such a distinction
+as created by a differential distribution of the ordinary factors.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_118" id="Page_118">[Pg 118]</a></span></p>
+
+<h2>CHAPTER VI</h2>
+
+<h3>Variation And Locality</h3>
+
+<p class="indent">In all discussions of the modes of Evolution the
+phenomena of Geographical Distribution have been admitted to be of
+paramount importance. First came the broad question, were the
+facts of distribution consistent with the Doctrine of Descent?
+I suppose all naturalists are now agreed that they are thus
+consistent, and that though some very curious and as yet inexplicable
+cases remain to be accounted for, the distribution of
+animal and plant life on the face of the earth is much what we
+might expect as a result of a process of descent with modification.
+Passing from this general admission to the more particular question
+whether the facts of distribution favour one special conception
+of the mode of progress of evolution rather than another,
+no agreement has yet been reached. One outstanding feature
+is hardly in dispute, namely that prolonged isolation is generally
+followed by greater or less change in the population isolated.
+Groups of individuals which from various causes are debarred
+from free intermixture with other groups almost always exhibit
+peculiarities, but on the other hand, cosmopolitan types which
+range over wide areas are on the whole uniform, or nearly so
+throughout their distribution. Examples of these two categories
+will be familiar to all naturalists. The barriers to intercourse
+may be seas, deserts, prairies, mountain-chains, or circumstances
+of a much less obvious character which isolate quite as effectually.
+The local unit is not necessarily an island, a district, or an area
+of special geological formation, but may, as every collector knows,
+be a valley, a pond, a creek, a "bank" in the sea, a clump of
+trees, a group of rocks in a bay, or a particular patch of ground
+on a mountain side. All the great groups provide examples of
+such specially isolated forms. The botanist knows them well;
+the conchologist, the entomologist, the ornithologist and the
+student of marine life are all equally aware that special varieties
+<span class="pagenum"><a name="Page_119" id="Page_119">[Pg 119]</a></span>
+or special species come from special places and from nowhere
+else. In one remarkable case the season of appearance plainly
+acts as the isolating barrier. <i>Tephrosia bistortata</i> is a small
+Geometrid moth which has two broods, appearing in <i>March</i>
+and <i>July</i> respectively. It is closely allied to <i>T. crepuscularia</i>
+which emerges in <i>May</i> and <i>June</i>. From the fact that occasional
+specimens cannot be quite certainly referred to one or other of
+the two, many have held that the two are one species. Nevertheless,
+in general they present distinctions which are plain
+enough. Some localities have one form only, but in several
+woods they co-exist. Experiment has shown that the two can
+be crossed, and that the cross-breds can breed <i>inter se</i> and with
+at least one of the parent stocks.<a name="FNanchor_1_61" id="FNanchor_1_61"></a><a href="#Footnote_1_61" class="fnanchor">[1]</a>
+Some diminution in fertility
+was observed, but perhaps not more than is commonly encountered
+when wild forms are bred in captivity. In such a case it can
+scarcely be doubted that the distinctness of the two forms in
+the places where they co-exist is maintained by the seasonal isolation.</p>
+
+<p class="indent">Just as the consequences of isolation are to be seen in the
+most different forms of life so may they also affect the most diverse
+features of organisation, such as size, colour, sculpture,
+shape, or number of parts. In the Sloth (<i>Choloepus</i>) the geographical
+races differ in the number of cervical vertebrae&mdash;or
+in other words, in the distribution of vertebral differentiation.
+The geographical races of <i>Cistudo</i> differ in the number of claws
+and phalanges.<a name="FNanchor_2_62" id="FNanchor_2_62"></a><a href="#Footnote_2_62" class="fnanchor">[2]</a>
+</p>
+
+<p class="indent">In Shetland, the males of <i>Hepialus humuli</i> (the Ghost Moth)
+are not sharply differentiated in colour from the females, as they
+are elsewhere, but in varying degrees resemble them.<a name="FNanchor_3_63" id="FNanchor_3_63"></a><a href="#Footnote_3_63" class="fnanchor">[3]</a>
+No such males are found in other localities, and even in the other Scottish
+islands they are normal. In the island of Waigiu the converse
+phenomenon has been observed in <i>Phalanger maculatus</i>.
+<span class="pagenum"><a name="Page_120" id="Page_120">[Pg 120]</a></span>
+Generally the male is spotted with white, and the female is without
+spots, but in Waigiu the females are spotted like the males.<a name="FNanchor_4_64" id="FNanchor_4_64"></a><a href="#Footnote_4_64" class="fnanchor">[4]</a>
+</p>
+
+<p class="indent">The following striking illustration was pointed out to
+me by Dr. W. D. Miller. <i>Euphonia elegantissima</i> as it occurs in Mexico
+and Central America has the two sexes very distinct from each
+other. The male has the lower parts orange and the upper
+parts a dark indigo blue, with a bright turquoise-blue head and
+neck. The female, except for the head, is of a bright olive green.
+A form in which the sexes are similarly differentiated exists in
+Porto Rico and is known as <i>E. Sclateri</i>. But in many of the
+other West Indian islands the representative "species" (<i>E.
+flavifrons</i>) has the two sexes closely resembling the <i>female</i> of
+<i>E. elegantissima</i>. This form is found in Antigua, Barbados,
+St. Vincent, and Guadeloupe, from which localities the British
+Museum has specimens. All three so-called species are very
+much alike otherwise.</p>
+
+<p class="indent">In the genus <i>Pyrrhulagra</i> (<i>Loxigilla</i>) to which Mr. Outram
+Bangs called my attention, several distinct and alternative possibilities
+occur. The genus has many local species occurring
+on the various West Indian islands. These species are characterized
+by differences in size, colour, and the shape of the bill.
+The colours have a narrow range, being black or greyish, with
+or without chestnut marks about the head and throat. In
+most of the islands the males are in general colour a full black,
+and the females are distinctly grey. They are thus found in
+San Domingo, Jamaica, Bahama, and most of the Lesser Antilles.
+In Porto Rico we meet the peculiarity that the hens are almost
+as black as the males (Ridgway describes the black of the hens
+as slightly less intense). This form is called <i>portoricensis</i>.
+A larger type, known as <i>grandis</i>, similarly coloured, inhabits
+St. Kitt's. Then, on the contrary, in Barbados, <i>both sexes</i> are
+a dull blackish grey, like the hens of the Lesser Antilles in general.</p>
+
+<p class="indent">The local species of <i>Agelaius</i> show similarly
+capricious distinctions. <i>A. phoeniceus</i> is a widely spread species, found over
+a great part of North America. The male is black with red-orange
+<span class="pagenum"><a name="Page_121" id="Page_121">[Pg 121]</a></span>
+bars on the wings, but the female is somewhat thrush-like in
+colour. In the island of Porto Rico there is a form called <i>xanthomus</i>,
+in which <i>both sexes</i> are like the males of the mainland.
+A similar species called <i>humeralis</i>, also with both sexes male-like,
+lives in Cuba. The island of Cuba, curiously enough, has also a
+distinct species named <i>assimilis</i>, in which the female is a dull
+black all over, though the male is like the mainland type.</p>
+
+<p class="indent">So also may local races differ in respect of variability.
+<i>Argynnis paphia</i>, the Silver Washed Fritillary, through a great
+part of its distribution has only one female form. In the English
+New Forest a second female form, <i>valesina</i>, co-exists with the
+ordinary <i>paphia</i> female. But in the southern valleys of the Alps
+the <i>valesina</i> female is much the commoner of the two, and indeed
+in some localities where the species is abundant, I have seen no
+<i>paphia</i> females in many days collecting.</p>
+
+<p class="indent">The beetle <i>Gonioctena variabilis</i> furnishes an
+illustration of a comparable phenomenon affecting the male sex. In 1894 and
+1895 I studied the curious colour variations of this species especially
+in the neighbourhood of Granada, and Mr. Doncaster
+ten years later repeated the observations on the same ground,
+and also collected the insect in other places in the south of Spain.
+The distinctions are not easy to give in words and the reader is
+referred to the colour plate accompanying my paper.<a name="FNanchor_5_65" id="FNanchor_5_65"></a><a href="#Footnote_5_65" class="fnanchor">[5]</a>
+The essential fact is that the males commonly have the elytra <i>red
+with black spots</i> and the females for the most part have greenish
+grey elytra with black stripes. In some localities a large minority
+of males closely resemble the female type, being identical in
+colour and then only distinguishable by structural differences.
+In two Granada localities I found the proportion of such males
+quite different. In the Darro valley about 38 per cent. (in 718)
+<span class="pagenum"><a name="Page_122" id="Page_122">[Pg 122]</a></span>
+were of this feminine type, but on the hills some 300 feet
+above only 19 per cent. (in 3,230) were like the females. At
+Castillejo, not far from Toledo I found no such male in 75 specimens.</p>
+
+<p class="indent">Mr. Doncaster collected from several localities,
+especially from two areas near Malaga, about 5 miles apart. In one of
+these the female-like males were, as usual, in a minority, but
+in the other these were actually in great excess, amounting to
+about 81 per cent. in the 173 taken. Doncaster found a doubtful
+indication that the composition of the population varies with
+the season, which is quite possible, but it is most interesting
+to note that in my chief locality after the lapse of ten years he
+found the proportions very much the same as I had done at the
+same season, for where I had 19 per cent. of the female-like males
+his collecting gave 16 per cent. In other respects also, his statistics
+corresponded very closely with mine.<a name="FNanchor_6_66" id="FNanchor_6_66"></a><a href="#Footnote_6_66" class="fnanchor">[6]</a>
+</p>
+
+<p class="indent">The various forms of <i>Heliconius erato</i> are well known
+to entomologists. They are strikingly distinguished by the colours
+of the strong comb-like marking on the hind wing, which may be
+red, yellow, green or blue. In various parts of the distribution
+in South America sometimes two and sometimes three of these
+distinct types co-exist.<a name="FNanchor_7_67" id="FNanchor_7_67"></a><a href="#Footnote_7_67" class="fnanchor">[7]</a>
+</p>
+
+<p class="indent">The distribution of the varieties of <i>Noctua castanea</i>
+typifies a large range of cases. The form which is reckoned the normal
+of the species has red fore-wings. It is practically restricted to
+Great Britain and Germany, according to Tutt. The other
+common form, <i>neglecta</i>, has grey fore-wings, and in this pattern
+it ranges through West Central Europe from North Italy to
+Germany. In the British Isles it extends up to Orkney. In
+Britain this grey form is by far the commoner, occurring
+<span class="pagenum"><a name="Page_123" id="Page_123">[Pg 123]</a></span>
+wherever the species is found. The red form is much scarcer in
+England, and does not occur at all in many localities where the
+grey form is common. Mr. Woodforde, from whom this account
+is taken,<a name="FNanchor_8_68" id="FNanchor_8_68"></a><a href="#Footnote_8_68" class="fnanchor">[8]</a>
+states that in August, 1899, he saw considerably over a
+hundred of the grey in the New Forest at sugar, but only two
+red ones. In Staffordshire however the red is proportionately
+more numerous and he estimates them as 40 per cent. of the
+population. Lastly a form has been taken in Staffordshire as a
+rarity in which the red is replaced by yellow, and this has hitherto
+been seen nowhere else. It is beyond our immediate purposes
+to discuss the genetic relationships of such forms, but the details
+of this case are interesting as making fairly clear the fact that
+the distinctions between <i>castanea</i> and <i>neglecta</i> are due to
+combinations of the presence of and absence of two pairs of factors,
+of which one produces a red pigment in the ground colour of the
+forewing and the other irrorates the same region with black
+scales. Mr. Woodforde states that all intermediates exist,
+and that in Staffordshire the greys always have a pinkish tinge.
+The yellow is doubtless another recessive to the red.</p>
+
+<p class="indent">Species which are uniform in some localities may be
+polymorphic in others. Such a phenomenon is well exemplified by
+the orchid <i>Aceras hircina</i>. Of this species distinct varieties had
+previously been known in Germany, but Gallé<a name="FNanchor_9_69" id="FNanchor_9_69"></a><a href="#Footnote_9_69" class="fnanchor">[9]</a>
+has lately given a detailed account of a number of most diverse forms found
+growing in a district of Eastern France. Without reference to his plates
+it is impossible to give any adequate conception of the profusion
+of types which the flowers of the species there assume. In some
+the lip is elongated to many times its usual length, twisting
+and dividing in a fashion suggesting some of the strangest of the
+Tropical Orchids. In others the labellum and the lateral petals
+are all comparatively short and wide (Fig. 13). Intermediates,
+combining these qualities in various degrees, were abundant, and
+the condition of the species, which was the only representative of
+the genus in the locality, recalls the extreme polymorphism of
+many of the Noctuid Moths.
+<span class="pagenum"><a name="Page_124" id="Page_124">[Pg 124]</a></span></p>
+
+<div class="figcenter" >
+  <img src="images/i_139.jpg" alt="Aceras hircina" width="600" height="847" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Fig. 13.</span></b>&emsp;Various forms of <i>Aceras hircina</i>.
+(After Gallé.) This figure only shows a few of the more striking forms
+illustrated in Gallé's plates.</p>
+
+<p class="indent"><span class="pagenum"><a name="Page_125" id="Page_125">[Pg 125]</a></span>
+Somewhat comparable variability has been seen in another
+Orchid genus <i>Ophrys</i>. In Great Britain the species <i>apifera</i>,
+<i>aranifera</i> and <i>muscifera</i> though variable are fairly distinct,
+but Moggridge has published two series of plates<a name="FNanchor_10_70" id="FNanchor_10_70"></a><a href="#Footnote_10_70" class="fnanchor">[10]</a>
+showing a very different state of things as regards the <i>Ophrys</i> population
+of the Riviera. Here the outward diversity is such that the ordinary
+specific names cannot be applied with any confidence and the
+limits of the species are quite uncertain. It may well be supposed
+that these Riviera plants are interbreeding, and indeed we may
+safely assume that they are. It is, however, to be remembered
+that Darwin showed <i>apifera</i> in this country to be habitually
+self-fertilised, so that the different behaviour on the Riviera may
+itself constitute a local peculiarity. Moreover it is to be gathered
+from Moggridge's account that in the districts which he examined
+the condition was not to be described by the statement that our
+three types were there co-existing and hybridising, but rather
+we should say that the population was polymorphic, containing
+these three types amongst others. Conchologists are aware
+that on the Dogger Bank <i>Modiola</i> attains a size unparalleled
+elsewhere. The same is true of the sponges <i>Grantia compressa</i>
+and <i>Grantia ciliata</i> in the estuary of the Orwell.<a name="FNanchor_11_71" id="FNanchor_11_71"></a><a href="#Footnote_11_71" class="fnanchor">[11]</a>
+Conversely, as we know so well in the case of Man, dwarf races occur in
+several special localities. Such examples may be multiplied indefinitely.</p>
+
+<p class="indent">The relation of local forms to species has often
+been discussed from many points of view, but I know no treatment of
+the subject clearer or more comprehensive than an excellent
+account of some of the various manifestations of local differentiation
+as they appear in Helicidæ published by Coutagne<a name="FNanchor_12_72" id="FNanchor_12_72"></a><a href="#Footnote_12_72" class="fnanchor">[12]</a>
+and a reader interested in the problem which they raise would
+<span class="pagenum"><a name="Page_126" id="Page_126">[Pg 126]</a></span>
+do well to make himself acquainted with the original from
+which the following notes are taken. He speaks for example
+of <i>Helix lapicida</i>. This is on the whole a constant form ranging
+up to the altitude of 1,300 m., common all over France except
+at great heights and in the Olive regions where it is restricted
+to moist places. Though subjected to such diverse conditions
+it shows only trivial variations in colour and other respects
+throughout its distribution, excepting that on both sides of the
+Pyrenees it has a very distinct sporadic variety called <i>Andorrica</i>
+or <i>microporus</i>. This variety occurs here and there, together
+with the type-form sometimes in colonies (pp. 26-30 and 86).</p>
+
+<p class="indent"><i>Bulimus detritus</i> though more restricted in
+geographical range is a much more variable form. It exhibits great
+variations in colour, form, and size, and as Coutagne well insists,
+these are independent of each other. Foreshadowing the methods of
+factorial analysis he suggests that distinctions in each respect,
+the "modes" as he calls them, should be denoted by a letter,
+or if desired, by a name, and the several combinations of differences
+might thus be most logically and usefully expressed. Of
+such combinations he says there are at least 18, all of which can
+be found. The whole possible series does not necessarily occur
+in the same place, and various localities are characterised by
+the presence or absence of certain of the combinations as Coutagne
+calls them, and by the relative frequency with which they
+occur. The ideas thus enunciated are much in advance of the
+ordinary practice of systematists, who give names to forms which
+are nothing but accidental combinations of factors, just as the
+horticulturists for practical reasons give names to similar combinations,
+which as we now know are merely specially noticeable
+terms in a long series of possibilities. In each case it is rather
+the <i>factors</i> which should be named than the forms which are
+constituted by their casual collocation. In this special example
+of <i>Bulimus detritus</i> the 18 forms are made by the combinations
+of three pairs of independent factors. Besides these combinations
+which may occur anywhere or almost anywhere in the distribution
+there are two more distinct local forms, each of which
+is regarded by Coutagne as probably constituting a fresh "mode,"
+perhaps compatible with the others.
+<span class="pagenum"><a name="Page_127" id="Page_127">[Pg 127]</a></span></p>
+
+<p class="indent"><i>Helix striata</i> (Draparnauld)<a name="FNanchor_13_73" id="FNanchor_13_73"></a><a href="#Footnote_13_73" class="fnanchor">[13]</a>
+is truly polymorphic; and its
+various forms have been described under various specific names.
+It abounds in the calcareous hills of Provence and Languedoc,
+disappearing in the alluvial lowlands and equally in the upper
+levels at about 800-1,000 m. From this district it extends
+through regions of similar altitude over a great part of France
+(details given).</p>
+
+<p class="indent">Locard in his monograph of this group, which he calls
+collectively the group of <i>Helix Heripensis</i>, tabulates 27 distinct
+named forms. The characteristics in which these forms differ
+have been reckoned as 17, and as several of these vary in degree
+of development, the number of modes may be increased to 109.
+For practical purposes however Coutagne considers that the
+various developments of 7 characteristics in their several combinations
+are enough to express the various forms, and he gives
+examples of this method of definition. As he observes, though
+names may be required to define the modes, no one need be
+alarmed at that, for the same names of modes will be applicable
+to a great range of distinct species, and the formulae expressing
+their combinations will replace the varietal names.</p>
+
+<p class="indent">This particular example of polymorphism is but little
+limited by locality. Occasional colonies present some special physiognomy
+which may in a given place seem almost invariable, though
+in this very respect the colonies found elsewhere may be highly
+variable, but such limitations are exceptional for <i>H. striata</i>.</p>
+
+<p class="indent">Some distinct and obvious susceptibilities to the
+influence of soil and climate are however noticeable. For example on
+siliceous ground the shells are thinner, while on calcareous soils
+they are thicker; similarly those from the Northern districts
+attain a larger size than those from further South. Moreover
+those subjected to curtailed development, whether from drought,
+heat or cold often show a shortening of the spire. In contrast
+with this case Coutagne describes the varieties of <i>Helix caespitum</i>,
+which he says are for the most part localised, quoting many illustrative cases.</p>
+
+<p class="indent">Another remarkable case in which locality plays a curious
+part is provided by the two species <i>Helix trochoides</i> and
+<span class="pagenum"><a name="Page_128" id="Page_128">[Pg 128]</a></span>
+<i>pyramidata</i>. In France generally they are distinct enough from
+each other, <i>trochoides</i> being smaller and having a characteristic
+keel. Coutagne says that after having collected these species
+from more than a score of localities he came upon a colony of
+<i>trochoides</i> on the island of Pomègues in which the shells were
+relatively enormous, most of them having only a slight keel,
+and a few none at all. On the other hand he received a consignment
+of <i>pyramidata</i> from four localities in Sicily, all small,
+and one of them exactly like the <i>trochoides</i> from Pomègues.
+Judging by the samples received from Sicily, <i>trochoides</i> is there
+not more variable than it is in Provence, while the Sicilian
+<i>pyramidata</i> is protean.</p>
+
+<p class="indent">The relations of the two species <i>Helix nemoralis</i> and
+<i>hortensis</i> provide an illustration of another kind of manifestation of local
+peculiarity. <i>H. hortensis</i> and <i>nemoralis</i> as usually met with,
+are two very distinct forms. <i>H. hortensis</i> is smaller and duller,
+and its peristome is white. <i>H. nemoralis</i> is larger and more shiny,
+and its peristome is brown. In several anatomical points,
+moreover, especially in the shape of the dart, there are great
+differences. For a full account of these peculiarities of the two
+forms and a discussion of their inter-relations the reader is referred
+to the elaborate work of A. Lang<a name="FNanchor_14_74" id="FNanchor_14_74"></a><a href="#Footnote_14_74" class="fnanchor">[14]</a>
+who has studied them extensively and has also succeeded in experimentally
+raising hybrids between them. These hybrids were in a slight degree
+fertile with both the parent species, but up to the time of publication
+no young had been reared from hybrids <i>inter se</i>.</p>
+
+<p class="indent">Coutagne describes the result of collections made in 62
+French localities. Some had exclusively <i>hortensis</i>, some exclusively
+<i>nemoralis</i>, and in some the two were found in association.
+He gives details of five of these collections from which I
+take the following summary of the more essential facts, omitting
+much that is almost equally significant.</p>
+
+<p class="indent"><i>Locality A</i>, near Honfleur. Both forms present, each
+sharply and normally distinguished, without any intermediates. They
+<span class="pagenum"><a name="Page_129" id="Page_129">[Pg 129]</a></span>
+are thus found in many places. Coutagne instances Müller's
+observations in Denmark, his own series from the Jura, etc.</p>
+
+<p class="indent"><i>Locality B.</i> Vonges (Côte d'Or), 242 <i>hortensis</i>
+taken at random, showed 128 with light peristomes (either more or less pinkish
+or quite white) and 114 with dark <i>brown</i> peristomes; together
+with 26 <i>nemoralis</i> all with the usual brown peristomes.</p>
+
+<p class="indent">Of the <i>hortensis</i> 50 were in ground-colour
+<i>opalescens</i> and 1 <i>roseus</i>; and in shape 5 were <i>umbilicatus</i>.</p>
+
+<p class="indent"><i>Locality C</i>, about 3 kilometres from <i>B</i>. There were
+found 35 <i>hortensis</i>, of which 20 had light peristomes and 15 brown; together
+with 7 <i>nemoralis</i>.</p>
+
+<p class="indent">Of the <i>hortensis</i> none were <i>opalescens</i>; 18 were
+<i>roseus</i> and none has the shape of <i>umbilicatus</i>.</p>
+
+<p class="indent"><i>Locality D</i>, about 1,200 metres from <i>B</i>. 147
+<i>hortensis</i>, of which 4 had light peristomes and 143 had brown.
+No <i>nemoralis</i> were found.</p>
+
+<p class="indent">None of the <i>hortensis</i> were <i>opalescens</i> or
+<i>roseus</i>, but 30 were <i>umbilicatus</i>.</p>
+
+<p class="indent">In these localities intermediates of every grade existed between
+the well-characterised <i>opalescens</i>, <i>roseus</i>, or <i>umbilicatus</i>,
+and the other forms, but there were no intergrades between the
+other <i>nemoralis</i> and the smaller <i>hortensis</i>, about which there
+was no hesitation. In the next locality a very different state
+of things was found.</p>
+
+<p class="indent"><i>Locality E.</i> Banks of the Yvette at Orsay (Seine-et-Oise).
+The actual numbers are not given, but we are told that 58 per
+cent. were <i>hortensis</i>, 33 per cent. <i>nemoralis</i>, and 9 per cent.
+intermediate. As at Honfleur, the <i>hortensis</i> had white peristomes,
+and the <i>nemoralis</i> brown. Coutagne's visits to this locality were
+in 1878 and 1880, and he calls attention to the fact that Pascal
+found similar intermediates in the same neighbourhood in 1873.</p>
+
+<p class="indent">The two species, in Coutagne's view, when they
+occur together, can generally be sorted from each other with perfect
+confidence, and it is only in exceptional localities that these
+intermediates occur. Whether they are hybrids, or whether
+sometimes the species in their variations transgress their usual
+limitations is regarded both by Coutagne and by Lang as a
+<span class="pagenum"><a name="Page_130" id="Page_130">[Pg 130]</a></span>
+question not yet answerable with certainty. Coutagne moreover
+lays stress on the fact that although each species may be easily
+known from the other <i>in its own district</i>, yet when shells from
+different districts are brought together it is sometimes impossible
+to sort them. He mentions an example of such casual intermixture
+occurring under natural conditions on an island in the
+Rhone, to which it may well be supposed that floods had brought
+immigrants from miscellaneous localities. This population contained
+a very large number of uncertain specimens, and as he
+says, it was much as if he were to mix the shells from his 62 localities,
+after which it would certainly be impossible to separate
+the two species again.<a name="FNanchor_15_75" id="FNanchor_15_75"></a><a href="#Footnote_15_75" class="fnanchor">[15]</a>
+</p>
+
+<p class="indent">Further evidence is given in the same treatise as to
+other examples of polymorphism, especially in the genus <i>Anodonta</i>,
+of which Locard made 251 species for France alone. Here again
+are cases like those already given, and many forms or "modes"
+are found restricted to special localities, while occasionally
+in the same locality dissimilar forms are found, collectively
+forming a colony, without intermediates.</p>
+
+<p class="indent">Taken as a whole the evidence shows the following conclusions
+to be true. Local races, whether of animals or plants, may be
+distinguished by characters which we are compelled to regard
+as trivial, or again by features of such magnitude that if they
+were known to us only as the characteristics of a uniform species
+they would certainly be assumed without hesitation to be essential
+for its maintenance. Local forms may be sharply differentiated
+from the corresponding populations of other localities or they
+may be connected with them by numbers of intermediates.
+Not rarely also we find a fact which has always seemed to me of
+special significance, that the peculiarity of the local population
+or colony may show itself in a special liability to variation, and
+this variability may show itself in one of many degrees, either
+in the constant possession of a definite aberration, in a dimorphism,
+or in an extreme polymorphism.
+<span class="pagenum"><a name="Page_131" id="Page_131">[Pg 131]</a></span></p>
+
+<p class="indent">At this stage attention should be called to two points.
+First, that when the details of the geographical distribution of
+any variable species are studied in that thorough and minute
+fashion which is necessary for any true knowledge of the interrelations
+of the several forms, the conception of a species invented
+by the popular expositions of Evolution under Selection is found
+to be rarely if ever realised in nature.</p>
+
+<p class="indent">A species in this generalised sense is an aggregate
+of individuals, none exactly alike, but varying round a normal type,
+the characters of which are fixed in so far as they are adapted to
+environmental exigency. In nature, however, the occurrence of
+the varieties, and even the occurrence of the variability is
+sporadic. In one place a population may be perfectly uniform.
+In another it may be again uniform but distinct. In others
+the two forms may occur together, sometimes with and sometimes
+without intergrades. In some localities a sporadic variety
+may be an element of the population, persisting through long
+periods of time. In other localities there may be several such
+aberrations occurring together which are absent elsewhere.</p>
+
+<p class="indent">Secondly, I would remind the reader that in the light
+of genetic analysis we know that intergrades, when they do occur, cannot
+be assumed to represent conditions through which the species
+must pass or has passed on its way to the extreme and definite forms.</p>
+
+<p class="indent">Often, perhaps generally, they are nothing but heterozygous
+forms, and often also they are conditions corresponding with the
+presence of factors in their reduction-stages.</p>
+
+<p class="indent">A broad survey of the facts shows beyond question that
+it is impossible to reconcile the mode of distribution of local forms
+with any belief that they are on the whole adaptational. Their
+peculiarities are occasionally the result of direct environmental
+influence, as we shall hereafter notice in certain cases, but none
+can attribute such sporadic and irregular phenomena to causes
+uniformly acting.</p>
+
+<p class="indent">Writers on systematics, especially those of former generations
+often conjecture or assert that local distinctions are caused by
+"differences of climate, soil, food, etc.," in vague general terms.
+It is usually safe to assume that these remarks do not represent
+<span class="pagenum"><a name="Page_132" id="Page_132">[Pg 132]</a></span>
+conclusions drawn from actual evidence, for only rarely can they
+be translated into more precise language. So thoroughly have
+the biological sciences become permeated with the belief that all
+distinctions are dependent upon adaptation, that the mere
+existence of definite distinctions is felt by many to be sufficient
+ground to warrant an assumption that these distinctions are
+directly or indirectly due to special local conditions. For
+example, Dr. J. A. Allen, who has done so much careful and valuable
+work in delimiting the local forms of the United States
+fauna, writes of the Ground Squirrels (Tamias)<a name="FNanchor_16_76" id="FNanchor_16_76"></a><a href="#Footnote_16_76" class="fnanchor">[16]</a>
+as follows:&mdash;</p>
+
+<p class="blockquot">"From the extreme susceptibility of this plastic
+group to the influences of environment, it is one of the most instructive
+and fascinating groups among North American mammals. No
+one can doubt its comparatively recent differentiation from a
+common stock, and its dispersion from some common centre.
+Whether the type originated at some point in North America,
+or in the Northern part of Eurasia, it is perhaps idle to speculate,
+but that it has increased, multiplied, spread, and become differentiated
+to a wonderful degree in North America is beyond
+question; as it is found from the Arctic regions to the high
+mountain ranges of Central Mexico, and has developed some
+twenty to thirty very palpable local phases."</p>
+
+<p class="blockquot">"Some of them easily take rank as species, others
+as subspecies. Probably a more striking illustration of evolution by
+environment cannot be cited."</p>
+
+<p class="indent">He proceeds to point out that the habits of these
+creatures are such as lead to isolation. This may well be admitted, and
+indeed no exception can possibly be taken to the passage as a
+whole, save in the one respect that there is no real proof that
+the local diversity is due to "evolution by environment" or an
+indication of "susceptibility to the influences of environment."</p>
+
+<p class="indent">Dr. Allen does indeed adduce the fact that California
+"extending through 800 miles of latitude, with numerous sharply
+contrasted physiographic regions, has apparently no less than
+six strongly differentiated forms, while the region east of the
+Rocky Mountains from a little below the northern boundary of
+<span class="pagenum"><a name="Page_133" id="Page_133">[Pg 133]</a></span>
+the United States northward to the limit of trees&mdash;a slightly
+diversified region of at least ten times the area of California&mdash;has
+only one"! But when one comes to ask how the various forms
+are adaptational, and how the influences of environment have
+led to their production, only conjectures of a preliminary and
+tentative character could be expected in reply. Desert forms
+are no doubt pallid as in so many instances, and forest forms are
+more fully coloured, and we may readily enough accept such facts
+as indications of a connection between bodily features and the
+conditions of life, but further than that no one can go; so that
+when we find size, length of ears or of tail, the number of dorsal
+stripes, the pattern of the colours, not to speak of differences in
+the pigments themselves, all exhibiting large modifications, we
+cannot refer these peculiarities to the causation of environmental
+difference, save as a simple expression of faith. I incline far
+more to agree with Gulick who, after years of study of the local
+variations of the Achatinellidae, came to the conclusion that it
+was useless to expect that such local differentiation can be
+referred to adaptation in any sense.<a name="FNanchor_17_77" id="FNanchor_17_77"></a><a href="#Footnote_17_77" class="fnanchor">[17]</a>
+Even the most convinced Selectionist must hesitate before such facts as
+those related by A. G. Mayer regarding the distribution of <i>Partula otaheitana</i>,
+one of these Achatinellidae. The island of Tahiti has been scored
+by erosion so that a series of separated valleys radiate to the coast.
+From four successive valleys Mayer collected the species, and
+found that in the first (Tipaerui) valley all the shells were
+dextral (115, containing 73 young); in the second valley
+(Fautaua) 54 per cent. of adults and 55.5 per cent. of the young
+contained were sinistral; in the third valley (Hamuta) 69 per
+cent. of adults and 73 per cent. of young contained in them were
+sinistral; and lastly, in the fourth valley (Pirae) all the shells
+(131, containing 62 young) were sinistral.<a name="FNanchor_18_78" id="FNanchor_18_78"></a><a href="#Footnote_18_78" class="fnanchor">[18]</a>
+In connection with these observations I may mention the fact that in a certain
+pond in the North of England<a name="FNanchor_19_79" id="FNanchor_19_79"></a><a href="#Footnote_19_79" class="fnanchor">[19]</a>
+the sinistral form of <i>Limnaea peregra</i>
+<span class="pagenum"><a name="Page_134" id="Page_134">[Pg 134]</a></span>
+has been known to occur for about fifty years. Visiting
+it lately I found the left-handed shells to be about 3 per cent. of
+the population. The species is the commonest British freshwater
+shell, but left-handed specimens are exceedingly rare.
+Will anyone ask us to suppose that the persistence of a percentage
+of this rarity in the same place is an indication of some specially
+favouring circumstance in the waters of that pond? It is a
+horse-pond to all appearances exactly like any other horse-pond;
+and I believe that in perfect confidence we may accept the
+suggestion of common sense, which teaches us that there is
+nothing particular in the circumstances which either calls such
+varieties into existence or contributes in any direct way to their
+survival. Had the phenomenon of local variation been studied
+in detail before Darwin wrote, the attempt to make selection
+responsible for fixity wherever found, could never have been
+made. The proposition that not only the definiteness of local
+forms but their variability also is sporadic, can be established
+by countless illustrations taken from any group of either the
+animal or the vegetable kingdoms. Only exceptionally can the
+fixed differences be even suspected of contributing to adaptation,
+and sporadic variability, which is a no less positive fact, must
+manifestly lie outside the range of such suspicions. It is open
+to any one to suggest speculatively that the persistence of
+special varieties or of special variability in special places is an
+indication that in those places the conditions of life are such
+that the forms in question are tolerated though elsewhere
+the same types are exterminated; but that consideration, even
+if it could be proved to be well founded, is not one which lends
+much force to the thesis that definiteness of type is a consequence
+of Natural Selection. On the contrary, recourse to such reasoning
+implies the inevitable but very damaging admission that
+the stringency of Selection is frequently so far relaxed that two
+or more equally definite forms of the same species can persist
+side by side. There is no doubt that this is the simple truth,
+but when once that truth is perceived it is useless to invoke the
+control of Selection as the factor to which definiteness of type
+in general must be referred.
+<span class="pagenum"><a name="Page_135" id="Page_135">[Pg 135]</a></span></p>
+
+<p class="indent">The genetic relations of local forms to each other
+cannot in the absence of actual breeding experiments be often ascertained.
+Standfuss formerly enunciated as a general principle that when
+two forms co-exist in the same locality and are able to interbreed,
+they do not produce intermediates; but that when the forms are
+geographically separated as local races, crosses between them
+result in a series of intermediates.<a name="FNanchor_20_80" id="FNanchor_20_80"></a><a href="#Footnote_20_80" class="fnanchor">[20]</a>
+In this aphorism there is a good deal of truth,
+but if in the light of Mendelian principles we
+examine the two statements we see now that the first is in reality
+only another way of saying that the distinctness of an aberrational
+form co-existing with another is due to segregation, accompanied
+by some degree of dominance of one type. Whether, however,
+one geographically isolated race will give intermediates when
+bred with another must depend entirely on the genetic physiology
+of the special case, and no general rule can be laid down. It
+may well be that, inasmuch as the distinctness of the variety is
+maintained by isolation, the difference in factorial composition
+between it and the representative form in another area is neither
+simple nor sharp; but when two varieties co-exist, though interbreeding,
+it is now clear that their differences must depend on
+the segregation of simple factors. Plainly such aberrations may
+in one place co-exist with another type, and elsewhere be separated
+from it as local races.</p>
+
+<p class="indent">Excellent illustrations of these two stages in
+evolution are provided by the melanic varieties of British Lepidoptera.
+The fact that black or blackish varieties of many species especially
+of Geometridae have come into existence in recent years is well
+known to British collectors, and it is not in dispute that they
+have in several instances replaced the older type more or less
+completely in certain districts. In the year 1900 the Evolution
+Committee of the Royal Society instituted a collective inquiry
+as to the contemporary distribution of these dark varieties. As
+the change had happened within living memory and had greatly
+progressed in recent years it was hoped that a record of the
+existing distribution would serve as a point of departure for
+future comparison. The records thus obtained were tabulated
+<span class="pagenum"><a name="Page_136" id="Page_136">[Pg 136]</a></span>
+by Mr. L. Doncaster.<a name="FNanchor_21_81" id="FNanchor_21_81"></a><a href="#Footnote_21_81" class="fnanchor">[21]</a>
+From that account and from the statements
+in Barrett's British Lepidoptera<a name="FNanchor_22_82" id="FNanchor_22_82"></a><a href="#Footnote_22_82" class="fnanchor">[22]</a>
+this description of some of the more notable cases is taken.</p>
+
+<p class="indent">The most striking and familiar case is that of <i>Amphidasys
+betularia</i>, of which only the ordinary type was known in any
+locality until about 1848-1850, when the totally black var.
+<i>doubledayaria</i> first appeared in the neighbourhood of Manchester.
+This black form was subsequently recorded in Huddersfield
+between 1860 and 1870; Kendal about 1870; Cannock Chase,
+1878; Berkshire, 1885; Norfolk, Essex and Cambridge about
+1892; Suffolk, 1894; London, 1897. For the Southern Counties
+of England, except in the London district, there are still very few
+records. It cannot of course be asserted positively that the
+variety spread from its place of first appearance into the other
+localities, and that it did not arise <i>de novo</i> in them, but there
+can be little doubt that the process was one of colonisation.
+On the European Continent the first records are from Hanover
+in 1884, Belgium 1886 and 1894, Crefeld 188-, Berlin 1903,
+Dresden about the same date.</p>
+
+<p class="indent">As regards the increase of the variety we have the
+fact that in Lancashire, Cheshire and the West Riding of Yorkshire the
+black is now the prevalent form; and in some places, as for example,
+Huddersfield, the black alone is now found, though it was unknown
+there till between 1860 and 1870. About 1870 at Newport,
+Monmouth, the two forms were in about equal numbers,
+but a few years later the type had almost vanished. Similarly
+in Crefeld, where the black form was still very rare in the eighties,
+it now forms about 50 per cent. of the population. In the
+London district the black remains scarce and at the date of the
+report it was still very scarce. From Ireland there is only one
+record and there are hardly any from Scotland.</p>
+
+<p class="indent"><i>Boarmia repandata</i> is another species which is behaving
+in a somewhat similar way. Unlike <i>betularia</i>, however, the species
+is a variable one, and has several colour-forms, amongst them
+the banded var. <i>conversaria</i>, and many others. In addition
+<span class="pagenum"><a name="Page_137" id="Page_137">[Pg 137]</a></span>
+to these there is a black form in the North of England which
+seems to be spreading. In Huddersfield the black was first
+recorded in 1888, and in 1900 20-25 per cent. were black. At
+Rotherham the black or very dark are now prevalent and have
+increased in the last 15 years. From the Midlands, East Anglia
+and Southern Counties the returns show only the light and
+medium forms.</p>
+
+<p class="indent">Of <i>Odontoptera bidentata</i> several intergrading
+dark forms exist, and these are found exclusively in the North and the
+Midlands. Unicolorous blacks have been found recently in the Lancashire
+mosses and at Wakefield. At Huddersfield 50 years ago the
+light forms were prevalent, but now a rather dark brown, not
+infrequently suffused with black, is the commonest. In Southern
+Counties only light forms are known.</p>
+
+<p class="indent"><i>Phigalia pilosaria</i> in South England is always
+light, but in the North the prevalent form is darker. About 35 years ago
+a form with unicolorous sooty fore-wings and dull grey hind
+wings was first seen in Yorkshire and a similar form is now taken
+regularly in South Wales.</p>
+
+<p class="indent">In the following cases the dark varieties were
+found originally only in the South.</p>
+
+<p class="indent"><i>Boarmia rhomboidaria</i> gave rise about 40 years ago
+to a unicolorous smoky variety called <i>perfumaria</i>. This was at first
+peculiar to the London district, but it has since been taken in
+Birmingham and other large cities. More lately coal-black
+specimens have been found at Norwich, and others similar but
+hardly so dark were taken in the South of Scotland and at Cannock Chase.</p>
+
+<p class="indent"><i>Eupithecia rectangulata</i> is a similar case.
+Formerly the light forms were prevalent but within sixty years they have
+almost entirely been replaced in the South of London by a nearly black form.</p>
+
+<p class="indent"><i>Tephrosia</i> (<i>Boarmia</i>) <i>consortaria</i> and
+<i>Tephrosia consonaria</i> are exceptionally interesting, for they have both
+given off dark forms in the same wood near Maidstone, which is far from the
+usual "centres of melanism." They were discovered in this
+locality by Mr. E. Goodwin. That of <i>consortaria</i> is a dark
+<span class="pagenum"><a name="Page_138" id="Page_138">[Pg 138]</a></span>
+grey, but that of <i>consonaria</i> is a full black, and nothing like
+either has been found anywhere else.</p>
+
+<p class="indent">These examples are all taken from the Geometridae but
+others, though of a less conspicuous kind, could be given from
+the Noctuidae or the Micro-Lepidoptera. <i>Acronycta psi</i>, for
+instance, has a suffused form which is believed to be becoming
+more frequent in the London district. <i>Polia chi</i> has two dark
+forms, <i>olivacea</i>, a yellowish grey with dark markings, and <i>suffusa</i>
+which is a darker, blackish-slate colour. Both occur in the North
+of England, sometimes together, sometimes separately, or mixed
+with the type and many intermediates. The distribution is
+peculiarly irregular. At Huddersfield, where the very dark form
+appeared suddenly about 1890, some 30 per cent. are said to be
+now dark and about 6-7 per cent. very dark, but at Saddleworth,
+12 miles away, only the pale forms occur.</p>
+
+<p class="indent">Several questions of interest arise in regard to this
+evidence. This progressive Melanism has arisen in certain families only,
+and may be confined to certain species only, within those families.
+As in almost all other examples in which variation has been much
+observed, its incidence is capricious and specific. A collateral
+line of inquiry relates to the degree of discontinuity which the
+variation manifests. Here again there is no rule. Generally
+speaking, in <i>A. betularia</i>, to take the case most fully studied, the
+variation is discontinuous. Real intermediates between <i>betularia</i>
+and <i>doubledayaria</i> are in most localities absent or rare.
+The black spots of <i>betularia</i> may often be larger or more numerous
+than in the normal, but this variation has nothing to do with
+<i>doubledayaria</i>, and is not an intermediate stage towards it,
+though sometimes wrongly so described. <i>Doubledayaria</i> owes its
+characteristic appearance to a factor which blurs the surface
+of the wings with a layer of black. Sometimes this blurring is
+slighter than in the real <i>doubledayaria</i>, and these forms are real
+intermediates. Occasionally the fore-wings alone are thus blurred.
+These intermediates are clearly due to reduction-stages of the
+<i>doubledayaria</i> factor, and are related to it as a blue mouse is to
+a black, or a dutch rabbit to a self-colour. It cannot positively
+be asserted that the full <i>doubledayaria</i> existed before the
+<span class="pagenum"><a name="Page_139" id="Page_139">[Pg 139]</a></span>
+intermediate, but it almost certainly did. In certain places as for
+instance in Belgium, there is evidence that intermediates have
+at various times been fairly abundant, but they have never become
+common, nor are they known to exist in the absence of
+<i>doubledayaria</i>. When the black variety and the light type breed
+together they do not usually have intermediates among their
+offspring, and the evidence is consistent with the view that the
+black is a complete dominant. The same is probably true of
+<i>Tephrosia consonaria</i>.</p>
+
+<p class="indent">In some of the other species we know that the darkest
+forms did not appear first. For example in <i>Phigalia pilosaria</i> and
+<i>Boarmia rhomboidaria</i> dark forms existed and are believed to
+have increased in number before the darkest made its appearance.
+<i>Hybernia progemmaria</i> is said to have become darker gradually
+both in Cheshire and in the West Riding, and a uniformly smoky
+variety appeared in South Yorkshire less than 45 years ago which
+has spread to neighbouring counties. The dark medium has
+become the commonest form in Huddersfield district, where the
+very dark variety is now about 20 per cent. of the population,
+though the light form is still common.</p>
+
+<p class="indent">Taking the evidence together we find it consistent with
+the view that dark forms have appeared sporadically, in some species
+the very dark appearing first and intermediates later, in others
+the moderately dark came first and the darkest later in time. It
+is practically certain that the change has in general come about
+not by a gradual change supervening on the population at large,
+but by the sporadic appearance of dark specimens as a new element
+in the population, and strains derived from these dark
+individuals have gradually superseded the normal type more or
+less completely.</p>
+
+<p class="indent">If it could be shown that these melanic novelties
+had a definite advantage in the struggle for existence they would provide
+an instance of evolution proceeding much in the way which
+Darwin contemplated. The whole process would differ from
+that conceived by him as the normal method of evolution only
+in so far as the change has come about with great rapidity and
+in some instances largely by the appearance and success of
+<span class="pagenum"><a name="Page_140" id="Page_140">[Pg 140]</a></span>
+discontinuous varieties. The question, however, must be asked
+whether the dark form can reasonably be supposed to have
+an advantage by reason of their darkness. Some naturalists
+believe that the darkness of the colours does thus definitely contribute
+to their protection by making the insects less conspicuous
+and thus more likely to escape the search of birds. In support
+of this view it may be pointed out that it is in the manufacturing
+districts of Lancashire and Yorkshire, and again in the London
+area that the melanics have attained their greatest development.
+Consistently with this argument also, it is in the neighbourhood of
+Crefeld and Essen, the black country of Germany, that they have
+chiefly established themselves on the Continent, and <i>Phigalia
+pilosaria</i> in the black form is now at home in South Wales. Thus
+superficially regarded, the evidence looks rather strong, but it
+is difficult to apply the reasoning in detail. We have first the
+difficulty that the black form of <i>betularia</i> for instance has
+established itself in thoroughly rural districts, notably near King's
+Lynn in Norfolk, and in the neighbourhood of Kendal and
+Windermere. The black form of <i>consonaria</i> and the dark
+<i>consortaria</i> appeared in a wood near Maidstone, far from town
+smoke, and the black <i>rhomboidaria</i> was first found at Norwich,
+which, as towns go, is clean. Then again the spread of the
+melanics is very irregular and unaccountable. The black <i>pilosaria</i>
+is found both in the West Riding and in the Swansea
+district, but not yet elsewhere. It rapidly increased at Huddersfield,
+but made no noticeable progress at Sheffield though recorded
+there for ten years. It is also a remarkable fact that no
+similar melanic development has been observed in America,
+and, so far as I am aware, comparable melanic varieties have not
+appeared on the European continent except in the case of the
+few sorts which possibly may have come from England.</p>
+
+<p class="indent">The whole subject is beset with complications. It must
+not be forgotten that in a few species of moths there is an obvious
+and recognised conformity between the colours of the perfect
+insect and that of the soil on which they live, comparable with
+that which is so striking in the case of some Oedipodidae and
+other grasshoppers. Of this phenomenon the clearest example
+<span class="pagenum"><a name="Page_141" id="Page_141">[Pg 141]</a></span>
+is <i>Gnophos obscurata</i>, which is a most variable species with many
+local forms. Of these a well-known dark variety lives on the
+peaty heaths of the New Forest and other districts, but on the
+chalk hills of Kent, Sussex and Surrey various light varieties
+are found, of which one is a bright silvery white, very near in
+colour to the colour of a chalky bank. This case does not seem
+to be one of direct environmental action,<a name="FNanchor_23_83" id="FNanchor_23_83"></a><a href="#Footnote_23_83" class="fnanchor">[23]</a>
+for Poulton found no change induced by rearing larvae among either
+white or black surrounding objects. No one however can doubt that
+there is some indirect connection between the colour of the
+ground and that of the moths.</p>
+
+<p class="indent">To my mind there is a serious objection to the theory
+of protective resemblance in application to such a case as that of the
+<i>betularia</i> forms, which arises from the fact that the black
+<i>doubledayaria</i> is a fairly conspicuous insect anywhere except
+perhaps on actually black materials, which are not common in any
+locality. Tree trunks and walls are dirty in smoky districts but
+they are not often black, and I doubt whether in the neighbourhood
+of Rotherham, for instance, which is one of the great
+melanic centres, <i>doubledayaria</i> can be harder for a bird to find
+than <i>betularia</i> would be. After all, too, many of the species
+much affected are not urban insects. They live in country
+places between the towns, and the general tone of these places
+even in Lancashire and the West Riding is not very different
+from that of similar places elsewhere. As against the objection
+that the black varieties are much blacker than the case requires
+it may be replied that we know nothing of the senses of birds,
+and that perhaps to their eyes blackness does constitute a disguise
+even though the surroundings are much less dark. This is
+undeniable, but recourse to such an argument is dangerous; for
+if the sight of the insect-eating birds is so dull that it does not
+distinguish dark things from dingy grey, we cannot subsequently
+regard the keen sight of birds as the sufficient control which has
+led to the minute and detailed resemblance of many insects to
+their surroundings. Those who see in such cases examples of
+the omnipotence of Selection must frequently find themselves in this dilemma.
+<span class="pagenum"><a name="Page_142" id="Page_142">[Pg 142]</a></span></p>
+
+<p class="indent">Taking the evidence as a whole, we may say that it
+fairly suggests the existence of some connection between modern urban
+developments and the appearance and rise of the melanic varieties.
+More than that we cannot yet affirm. It is a subject
+in which problems open up on every side, and all of them are
+profitable subjects for investigation. Unhappily such animals
+are difficult to rear successfully in captivity for many generations,
+owing to their extreme liability to disease. Not the least interesting
+feature of the melanics is the fact that the black varieties
+provide about the best and clearest example of a new dominant
+factor attaching itself to a wild species in recent times.
+None of the cases are satisfactorily recorded or analysed as yet,
+but the evidence is clear that <i>doubledayaria</i> is a dominant to its
+type, and in several other dark varieties, though the pigment
+deposited is not black, the records show that the increased
+amount of the pigment almost certainly is due to a positive factor.
+Of this, <i>Hemerophila abruptaria</i> is a good example.<a name="FNanchor_24_84" id="FNanchor_24_84"></a><a href="#Footnote_24_84" class="fnanchor">[24]</a>
+There are some irregularities in the results, but taken together they
+leave little doubt that the dark brown variety is a dominant and the
+light, yellowish brown a recessive.</p>
+
+<p class="indent">A curious parallel to the rise of the melanic moths
+in England is provided by the case of the Honey-creepers or Sugar-birds,
+in certain West Indian islands.<a name="FNanchor_25_85" id="FNanchor_25_85"></a><a href="#Footnote_25_85" class="fnanchor">[25]</a>
+These birds of the genus <i>Coereba</i> (<i>Certhiola</i>) range from Southern
+Mexico to the Northern parts of South America and through the whole chain
+of the West Indian islands and Bahamas except Cuba. There are numerous
+local forms, and many of the islands have types peculiar to themselves,
+as is usual in such cases. Some of the types or species
+range through several islands, but according to Austin Clark<a name="FNanchor_26_86" id="FNanchor_26_86"></a><a href="#Footnote_26_86" class="fnanchor">[26]</a>
+no island has more than one of them. Cory<a name="FNanchor_27_87" id="FNanchor_27_87"></a><a href="#Footnote_27_87" class="fnanchor">[27]</a>
+reckoned twelve such species within the Antillean region. They are small birds
+<span class="pagenum"><a name="Page_143" id="Page_143">[Pg 143]</a></span>
+about the size of a nuthatch with a general colouring of black,
+yellow, and white. From the island of St. Vincent the Smithsonian
+Institution received in the late seventies of last century
+several completely black specimens in addition to two of the usual
+type of colouring. The black were described by W. N. Lawrence
+as <i>atrata</i>, and those marked with the usual yellow and white
+were called <i>saccharina</i>. The collector (Mr. F. A. Ober) reported
+that the black form was common, and that the <i>saccharina</i> form
+was rarer. Lawrence remarks, "Had there been only a single
+example (of the black form) I should have considered it as probably
+a case of abnormal colouring, but it seems to be a representative
+form of the genus in this island."<a name="FNanchor_28_88" id="FNanchor_28_88"></a><a href="#Footnote_28_88" class="fnanchor">[28]</a>
+There is of course no doubt of the correctness of the view taken by Austin Clark
+that "<i>atrata</i>" is a black variety. The black bird is in every respect,
+other than colour, identical with <i>saccharina</i>, and it is even possible
+to detect a greenish colour in the areas which would normally be
+yellow, showing plainly enough the yellow pigment obscured by the black.</p>
+
+<p class="indent">We have next the interesting fact that like our melanic
+moths the dark form is replacing the "type." At the time of Ober's
+visit the type was already in a minority, but now it is nearly
+or perhaps actually extinct, though the black form is one of the
+commonest birds on the island. Austin Clark found no specimen
+when he collected there in 1903-4, though formerly it was not
+uncommon in the vicinity of Kingston and in the immediate
+windward district of St. Vincent.</p>
+
+<p class="indent">The Grenadines are geographically just south of
+St. Vincent, though separated by a deep channel. In these islands no
+black forms have yet been taken, but Grenada, the next island to the
+south, has both normals and blacks. There are trifling differences
+of size between the Grenada birds and those from St.
+Vincent, the Grenada specimens being slightly smaller and for
+this reason they have received distinct names, the form marked
+with yellow and white being called <i>Godmani</i> (Cory) and the black,
+<i>Wellsi</i> (Cory), but this merely introduces a useless complication.
+<span class="pagenum"><a name="Page_144" id="Page_144">[Pg 144]</a></span>
+There is evidence that in Grenada, as in St. Vincent, the black is
+gradually ousting the original type, but the process has not gone
+so far as in St. Vincent. Austin Clark very properly compares
+this case of the Sugar-birds with that of <i>Papilio turnus</i>, which as
+is well-known, has a black female in the southern parts of its distribution,
+in addition to a female of the yellow type, but in the
+Northern States the black female does not occur.</p>
+
+<p class="indent">During the present year P. R. Lowe, who lately studied
+<i>Coerebas</i> on a large scale in the West Indies, has published an
+important paper on the subject.<a name="FNanchor_29_89" id="FNanchor_29_89"></a><a href="#Footnote_29_89" class="fnanchor">[29]</a>
+He calls attention to the fact that Cory recently found
+a black form of <i>Coereba</i> on Los Roques Islands,
+and he himself discovered another on the
+Testigos Islands. Both localities are on the coast of Venezuela,
+far from St. Vincent and Grenada. The whole problem is thus
+further complicated by the fact that the black varieties have,
+as we are almost driven to admit, arisen independently in remote
+places. Improbable as this conclusion may be, it is still more
+difficult to regard all the black forms as derived from one source.
+For first, they present definite small differences from each other;
+and secondly we have to remember a consideration of greater
+importance, that the very fact that each island has its own type
+must be accepted as proving that the localities are effectively
+isolated from each other, and that migration must be a very rare event.</p>
+
+<p class="indent">The rarity of such illustrative cases is, I believe,
+more apparent than real. It is probably due to the extreme reluctance
+of systematists to admit that such things can be, and of course
+to the almost complete absence of knowledge as to the genetic
+behaviour of wild animals and plants. Only in such examples
+as this of the <i>Coereba</i>, where colour constitutes the sole
+difference, or that of the moths which have been minutely studied by
+many collectors, does the significance of the facts appear. The
+arrangement of catalogues and collections is such that much
+practical difficulty of a quite unnecessary kind is introduced. For
+example, in this very case of <i>Coereba</i>, I find the British Museum
+has a fine series from Grenada including 3 normals and 11 black,
+<span class="pagenum"><a name="Page_145" id="Page_145">[Pg 145]</a></span>
+and also 16 blacks from St. Vincent. If the black specimens from
+Grenada were put with the normals which are almost certainly
+nothing but a recessive form of the same bird, the variation would
+strike the eye on even a superficial glance at the drawer. But
+following the notions so naively expressed in the passage quoted
+above from W. N. Lawrence, the blacks from Grenada are put
+apart together with the other blacks from St. Vincent, though
+two of them were shot on the same date as one of the normals.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_146" id="Page_146">[Pg 146]</a></span></p>
+
+<h2>CHAPTER VII</h2>
+<h3><span class="smcap">Local Differentiation.</span> <i>Continued</i></h3>
+
+<p style="font-size: 120%; text-align: center;">
+<b><span class="smcap">Overlapping Forms</span></b></p>
+
+<p class="indent">The facts of the distribution of local forms on the whole
+are consistent with the view that these forms come into existence by the
+sporadic appearance of varieties in a population, rather than by
+transformation of the population as a whole. Of such sporadically
+occurring varieties there are examples in great abundance,
+though by the nature of the case it can be but rarely that we are
+able to produce evidence of a previous type being actually superseded
+by the variety. When the two forms are found co-existing
+in the same area they are usually recorded as one species if intergrades
+are observed, and as two species if the intergrades are
+absent. On the other hand when two forms are found occupying
+separate areas, when, that is, the process of replacement is completed
+in one of the areas, then forthwith each is named separately
+either as species or subspecies. Successive observations carried
+out through considerable periods of time would be necessary to
+establish beyond question that the history proceeds in one way
+rather than another. Such continuity of observation has for
+the most part never been attempted. The kind of information
+wanted has indeed only been lately recognized, and really critical
+collecting is a thing of only the last few decades. The methods of
+the older collectors, who aimed at bringing together a few typical
+specimens of all distinct forms, are of little service in this class
+of inquiry, which is better promoted by the indiscriminate collection
+of large numbers of common forms from many localities.
+When this has been done on a comprehensive scale we shall be
+in a position to form much more confident judgments as to the
+general theory of evolution.</p>
+
+<p class="indent">Some little work of the kind has however been done and
+the results are already of great value. Seeing that the differentiation
+of local forms is only made possible by isolation, it necessarily
+<span class="pagenum"><a name="Page_147" id="Page_147">[Pg 147]</a></span>
+happens that the collector finds one form in one locality
+and another in a distinct locality, and there is no evidence as to
+the behaviour which the two representative species might exhibit
+if they came into touch with each other. In the most familiar
+examples of such distinction each inhabits an island, completely
+occupying it to the exclusion of any other similar form. It can
+only be when the two representative species occupy parts of a
+continental area connected with each other by regions habitable
+for the organism in question, that there is a chance of seeing the
+two forms in contact. Often also, even where this condition is
+satisfied, the habits, social organisation, or some other special
+cause may act as a barrier which prevents the distinguishable
+forms from ever coming into such complete contact as to interbreed
+or to behave as a genetically continuous race. When
+genetic continuity is ensured by a constant diffusion of the population
+over the whole area which they inhabit there will manifestly
+be no formation of local races. The practical uniformity,
+for example, of so many species of birds which inhabit widely
+extended ranges of Western Europe is doubtless maintained by
+such constant diffusion. When, as in the case of the Falcons,
+many localities have peculiar forms, the fact may be taken as
+conclusive evidence that there is little or no diffusion; and when
+we find in such a species as the Goldfinch that in spite of migratory
+fluctuations there are nevertheless geographical races
+fairly well differentiated, it may similarly be inferred that these
+fluctuations habitually move up and down on paths which do
+not intermingle. There are however a few examples of animals,
+not given to much irregular wandering, which occupy a wide and
+continuous range of diversified country and are differentiated as
+local races in two or more districts, though the distinct races
+meet in intervening areas. Of these the most notorious illustration
+which has been investigated with any thoroughness is
+that of the species of <i>Colaptes</i> (Woodpeckers) known in the United
+States as Flickers. The study of the variations of these forms,
+made by J. A. Allen<a name="FNanchor_1_90" id="FNanchor_1_90"></a><a href="#Footnote_1_90" class="fnanchor">[1]</a>
+is an admirable piece of work, with which
+<span class="pagenum"><a name="Page_148" id="Page_148">[Pg 148]</a></span>
+every student of variation and evolutionary problems should
+make himself familiar. The two forms with which we are most
+concerned are known as <i>C. auratus</i> and <i>C. cafer</i>, and are very
+strikingly different in appearance. In size, proportions, general
+pattern of colouration, habits, and notes, the two are alike, but
+they differ in the following seven respects as stated by Allen.</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Woodpeckers" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>Auratus</i></td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>Cafer</i></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">1.&nbsp;&nbsp;</td>
+      <td class="tdl">Quills <i>yellow</i>.</td>
+      <td class="tdr">1.&nbsp;&nbsp;</td>
+      <td class="tdl">Quills <i>red</i>.</td>
+    </tr><tr>
+      <td class="tdr">2.&nbsp;&nbsp;</td>
+      <td class="tdl">Male with a <i>black</i> malar stripe.</td>
+      <td class="tdr">2.&nbsp;&nbsp;</td>
+      <td class="tdl">Male with a <i>red</i> malar stripe.</td>
+    </tr><tr>
+      <td class="tdr">3.&nbsp;&nbsp;</td>
+      <td class="tdl">Adult female with <i>no</i> malar stripe.</td>
+      <td class="tdr">3.&nbsp;&nbsp;</td>
+      <td class="tdl">Adult female with usually a <i>brown</i> malar stripe.</td>
+    </tr><tr>
+      <td class="tdr">4.&nbsp;&nbsp;</td>
+      <td class="tdl">A <i>scarlet</i> nuchal crescent in both sexes.</td>
+      <td class="tdr">4.&nbsp;&nbsp;</td>
+      <td class="tdl"><i>No</i> nuchal crescent in either sex.</td>
+    </tr><tr>
+      <td class="tdr">5.&nbsp;&nbsp;</td>
+      <td class="tdl">Throat and fore neck <i>brown</i>.</td>
+      <td class="tdr">5.&nbsp;&nbsp;</td>
+      <td class="tdl">Throat and fore neck <i>grey</i>.</td>
+    </tr><tr>
+      <td class="tdr">6.&nbsp;&nbsp;</td>
+      <td class="tdl">Whole top of head and hind neck <i>grey</i>.</td>
+      <td class="tdr">6.&nbsp;&nbsp;</td>
+      <td class="tdl">Whole top of neck and hind neck <i>brown</i>.</td>
+    </tr><tr>
+      <td class="tdr">7.&nbsp;&nbsp;</td>
+      <td class="tdl">General plumage with an <i>olivaceous</i> cast.&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdr">7.&nbsp;&nbsp;</td>
+      <td class="tdl">General plumage with a <i>rufescent</i> cast.</td>
+    </tr>
+ </tbody>
+</table>
+
+<p>These differences are illustrated in the accompanying coloured
+plate, which has been most kindly prepared for me under the instructions
+of Dr. F. M. Chapman of the American Museum of
+Natural History. Before going further it is worth considering the
+nature of these differences a little more closely. All but the last
+are large differences which no one would overlook even in a hasty
+glance at the birds. If the only distinction lay in the colour of the
+quills we might feel fairly sure that <i>auratus</i> was a recessive form
+of <i>cafer</i>, and so probably it is in this respect. Similarly the black
+malar stripe of <i>auratus</i> is in all probability recessive to the red
+malar stripe of <i>cafer</i> and I imagine the pigments concerned are
+comparable with those in the Gouldian Finch (<i>Poephila gouldiae</i>)
+of Australia. Both sexes in that species may have the head black,
+red, or, less often, yellow, and though it is not any longer in
+question that birds may breed in either plumage, I believe that
+the young are always black-headed and I imagine that those
+which become red-headed possess a dominant factor absent from
+the permanently black-headed birds.<a name="FNanchor_2_91" id="FNanchor_2_91"></a><a href="#Footnote_2_91" class="fnanchor">[2]</a>
+<span class="pagenum"><a name="Page_149" id="Page_149">[Pg 149]</a></span>
+Yellow as a recessive form of a red is certainly very common, but red
+and black as variants of the same pigment are less usual. In the Gouldian
+Finch we seem to have a case where a pigment can assume all
+three forms. It would be interesting to know whether the red
+of the malar stripes in <i>Colaptes</i> is a pigment of the same nature
+as the red of the quills. Both in <i>Colaptes</i> and in <i>Poephila
+gouldiae</i> I have seen specimens intermediate between the black
+and the red, and the appearance of the part affected was exactly
+alike in the two cases, red feathers coming up among the black
+ones, and many feathers containing both red and black pigments
+mixed together.</p>
+
+<p class="indent">The development of the scarlet nuchal crescent in <i>auratus</i>
+and the absence of this conspicuous mark in <i>cafer</i> constitute from
+the physiological point of view the most remarkable pair of differences.
+When the red crescent is not formed, the feathers
+which would bear it are exactly like the rest, and no special
+pigment is visible in them which one can regard as ready to be
+modified into red. If the crescent is due to a factor it must
+therefore be supposed that this factor has the power of modifying
+the pigment of the neck in one special place alone. Dr. W. D.
+Miller called my attention to the fact that a similar variation
+occurs in another American woodpecker, the Sapsucker,
+<i>Sphyropicus varius</i>.<a name="FNanchor_3_92" id="FNanchor_3_92"></a><a href="#Footnote_3_92" class="fnanchor">[3]</a>
+</p>
+
+<p class="indent">I do not suggest that such variations are without parallel:
+indeed in <i>P. gouldiae</i> the factor which turns the black of the head
+into scarlet affects one special region of the black only, being
+sharply distinct from the unmodified black of the throat. These
+regions of the head are however often the seat of special colours
+in birds.<a name="FNanchor_4_93" id="FNanchor_4_93"></a><a href="#Footnote_4_93" class="fnanchor">[4]</a>
+So also may be instanced the variety of the Common
+<span class="pagenum"><a name="Page_150" id="Page_150">[Pg 150]</a></span>
+Guillemot (<i>Uria troile</i>) which has a white line round the eyes and
+at the sides of the head where the normal has no such mark; but
+this line is formed in a very special place, the groove joining the
+eye to the ear, whereas the feathers of the nuchal crescent are
+not ostensibly distinguished from those adjacent.<a name="FNanchor_5_94" id="FNanchor_5_94"></a><a href="#Footnote_5_94" class="fnanchor">[5]</a>
+</p>
+
+<p class="indent">The transposition of the brown and the grey on the back
+and front of the neck also constitutes a very remarkable difference.
+If either grey or brown depends on a factor then it must be supposed
+that <i>auratus</i> has one of these factors and <i>cafer</i> the other.</p>
+
+<p class="indent">From these several considerations it is quite clear that if
+<i>auratus</i> and <i>cafer</i> are modifications of the same type produced
+by presence or absence of factors, several independent elements
+must be concerned, and to unravel their inter-relations would be
+most difficult even if it were possible to breed the types under observation,
+which is of course quite beyond present possibilities.</p>
+
+<p class="indent">The distribution of the two is as follows. On the east side
+of the Continent <i>C. auratus</i>, relatively pure, occupies the whole of
+Canada and the States from the North to Galveston. Westward
+it extends across the whole continent in the more northern
+region to Alaska, but in its pure form it only reaches down the
+Pacific coast to about the northern border of British Columbia.
+Its southern and western limit is thus roughly a line drawn from
+north of Vancouver, southeast to North Dakota and then south
+to Galveston. <i>C. cafer</i> in the comparatively pure form inhabits
+Mexico, Arizona, California (except Lower California and the
+opposite coast), central and western Nevada, Utah, Oregon, and
+is bounded on the east by a line drawn from the Pacific south of
+Washington, south and eastward through Colorado to the mouth
+<span class="pagenum"><a name="Page_151" id="Page_151">[Pg 151]</a></span>
+of the Rio Grande or the Gulf of Mexico. Between the two
+lines thus roughly defined is a band of country about 1,200-1,300
+miles long and 300-400 miles wide, which contains some normal
+birds of each type, but chiefly birds exhibiting the characters of
+both, mixed together in various and irregular ways. Even in
+the areas occupied by the pure forms occasional birds are recorded
+with more or less indication of characteristics of the other
+form, but within the area in which the two forms are conterminous,
+the mixed birds are in the majority. The condition of these birds
+of mixed character is described by Allen as follows:</p>
+
+<p class="blockquot">"As has been long known&mdash;indeed, as shown by Baird
+in 1858&mdash;the 'intermediates' or 'hybrids' present ever-varying
+combinations of the characters of the two birds, from individuals
+of <i>C. auratus</i> presenting only the slightest traces of the characters
+of <i>C. cafer</i>, or, conversely&mdash;individuals of <i>C. cafer</i>
+presenting only the slightest traces of the characters of
+<i>C. auratus</i>&mdash;to birds in which the characters of the two are about
+equally blended. Thus we may have <i>C. auratus</i> with merely a few red
+feathers in the black malar stripe, or with the quills merely
+slightly flushed with orange, or <i>C. cafer</i> with either merely a few
+black feathers in the red malar stripe, or a few red feathers at
+the sides of the nape, or an incipient, barely traceable scarlet
+nuchal crescent. Where the blending of the characters is more
+strongly marked, the quills may be orange-yellow or orange-red,
+or of any shade between yellow and red, with the other features
+of the two birds about equally blended. But such examples are
+exceptional, an unsymmetrical blending being the rule, the two
+sides of the same bird being often unlike. The quills of the tail,
+for example, may be part red and part yellow, the number of
+yellow or red feathers varying in different individuals, and very
+often in the opposite sides of the tail in the same bird. The
+same irregularity occurs also, but apparently less frequently,
+in the quills of the wings. In such cases the quills may be mostly
+yellow with a few red or orange quills intermixed, or red with a
+similar mixture of yellow. A bird may have the general colouration
+of true <i>cafer</i> combined with a well-developed nuchal crescent,
+or nearly pure <i>auratus</i> with the red malar stripes of a <i>cafer</i>.
+<span class="pagenum"><a name="Page_152" id="Page_152">[Pg 152]</a></span>
+Sometimes the body plumage is that of <i>C. auratus</i> with the head
+nearly as in pure <i>cafer</i>, or exactly the reverse may occur. Or we
+may have the general plumage as in <i>cafer</i> with the throat and
+crown as in <i>auratus</i>, and the malar stripe either red or black,
+or mixed red and black, and so on in almost endless variations,
+it being rare to find, even in birds of the same nest, two individuals
+alike in all their features of colouration. Usually the
+first trace of <i>cafer</i> seen in <i>auratus</i> manifests itself as a mixture of
+red in the black malar stripe, either as a few red feathers, or as a
+tipping of the black feathers with red, or with merely the basal
+portion of the feathers red. Sometimes, however, there is a
+mixture of orange or reddish quills, while the malar stripe remains
+normal. In <i>C. cafer</i> the traces of <i>auratus</i> are usually shown by a
+tendency to an incipient nuchal crescent, represented often by
+merely a few red-tipped feathers on the sides of the nape; at
+other times by a slight mixture of black in the red malar stripe."</p>
+
+<p class="indent">Such a state of things accords very imperfectly with
+expectations under any received theory of Evolution. As in some of
+the instances discussed in the first chapter we have here two
+fairly definite forms, nearly allied, which on any evolutionary
+hypothesis must have been evolved either the one from the other,
+or both from a third form at a time not very remote from the
+present, as time must be measured in evolution. Yet though
+intermediates exist in some quantity, no one can for a moment
+suggest that they are that definite intermediate from which
+<i>auratus</i> and <i>cafer</i> descend in common. One cannot imagine
+that the immediate ancestor of these birds was a mosaic, made up of
+asymmetrical patches of each sort: but that is what many of the
+intermediates are. It is not much easier to suppose the ancestor
+to have been a nondescript, with a compromise between the
+developed characters of each, with quills buff, malar stripes
+neither black nor red, with a trace of nuchal crescent, and so on.
+Such Frankenstein-monsters have played, a considerable part in
+the imaginations of evolutionary philosophers, but if it were
+true that there was once a population of these monsters capable
+of successful existence, surely they should now be found as a
+population occupying the neutral zone between the two modern
+<span class="pagenum"><a name="Page_153" id="Page_153">[Pg 153]</a></span>
+forms. Yet, though much remains to be done in clearing up the
+facts, one thing is certain, namely that the neutral zone has not a
+definite and normally intermediate population, but on the contrary
+it is peopled by fragments of the two definite types and
+miscellaneous mongrels between them.</p>
+
+<p class="indent">On the other hand, one cannot readily suppose that
+either form was the parent of the other. The process must have
+involved both addition and loss of factors, for whatever hypothesis
+be adopted, such changes must be supposed to have occurred.
+A careful statistical tabulation of the way in which the characters
+are distributed in the population of the mixed zone would be
+of great value, and till that has been done there is little that can
+be said with certainty as to the genetics of these characters.
+In the collection of Dr. Bishop of New Haven I was very kindly
+allowed to examine a sample, all taken at random, near together,
+in Saskatchewan. There were females 4 adult, 2 young; males
+4 adult and 5 young. This number, though of course insufficient,
+is enough to give some guide as to the degree of definiteness which
+the characters generally show in their variations. Of the 15
+birds, 8 had simply yellow quills; 2 had red; 1 was almost red
+but had one yellow tail-quill; 3 were intermediate and 1 was
+buff. As regards the malar patch, which can only be determined
+properly in the adult males, 1 was red, 1 was approximately red,
+2 intermediate. As to nuchal crescent 4 females had none, 2
+females very slight; 7 males had it, 1 had only a slight crescent,
+and 1 had none. In point of quills therefore 10 were definite
+out of 15; in point of crescent, 11 were definite out of 15; and in
+point of malar patch 1 only was definite out of 4. The last is a
+feature directly dependent on age and so counts for less, but as
+regards the other two features there is some indication that the
+factors show definiteness in their behaviour. It must be remembered
+that we have no knowledge what the heterozygous
+form may be, and in the case of red and yellow it is probably a
+reddish buff. The patch-works are no doubt to be compared with
+other well-known pied forms, and in these we must suppose the
+active factor broken up, which it probably can be very easily.
+The asymmetry, which Allen notices as so marked a feature, in the
+<span class="pagenum"><a name="Page_154" id="Page_154">[Pg 154]</a></span>
+distribution of the red and yellow quills of the tail especially,
+recalls that of the black markings in the pied Canaries. As is well
+known to students of variations <i>some</i> pigment-factors in <i>some</i>
+animals are apparently uncontrolled by symmetry, while in other
+specific cases symmetry is the rule. On the other hand the
+blackness or redness of the malar patches is, I think, as a rule
+nearly symmetrical. It should be mentioned that two of Dr.
+Bishop's young birds belonged to the same nest, one a female
+with <i>red</i> quills, the other a male with <i>yellow</i>.
+Both are without crescent.</p>
+
+<p class="indent">As to the question whether certain combinations of characters
+occur with special frequency, the evidence is insufficient to give a
+definite answer. Among all the birds I have seen in America
+or in England I have not yet found one having the malar patches
+black without any nuchal crescent. Of Dr. Bishop's 8 adults
+not one, however, showed the combination of the three chief
+features normal for <i>auratus</i> or for <i>cafer</i>.</p>
+
+<p class="indent">Besides the two forms that we have hitherto considered,
+several other local types exist, and these throw some further
+light on the problem. Of these the most important in this
+connexion is <i>chrysoides</i>, which inhabits the whole of southern
+California and the mainland opposite. This remarkable form
+is as Allen says, very different from <i>auratus</i> except that it has
+the quills yellow like <i>auratus</i>, not red like <i>cafer</i>. So that
+we find here in the extreme west of the whole distribution a type
+agreeing in one of its chief features with the eastern type. Between
+this and <i>cafer</i> intergrades have, according to Allen, not
+been found. The relations of this <i>chrysoides</i> are, Allen thinks,
+rather with <i>mexicanoides</i>, a southern, smaller race with colours
+more intense, which inhabits Guatemala, but however that may
+be, it must be regarded as a <i>cafer</i> which has lost its red quills.
+The island of Guadeloupe off Lower California has an island
+form. Beyond the other side of the continent there is also an
+island form of <i>auratus</i>, inhabiting Cuba, so that clearly the yellow
+quills can extend into the tropics.</p>
+
+<p class="indent">The above account is in many respects incomplete, but
+it suffices to give an outline of the chief facts. The whole problem
+<span class="pagenum"><a name="Page_155" id="Page_155">[Pg 155]</a></span>
+is complicated by the undoubted effects of an uncertain amount
+of migration, and in many, perhaps all, districts, the winter
+population differs from the summer population of the same
+localities. The existence of these seasonal ebbs and flows is
+now well known to ornithologists, and most of the bird species
+of temperate regions are subject to them.</p>
+
+<p class="indent">Difficult as it may be to conceive the actual process of
+origin of the two types <i>auratus</i> and <i>cafer</i>, it is I think still
+harder to suggest any possible circumstance which can have determined
+their development as distinct races, or which can maintain
+that distinctness when created. Some will no doubt be disposed
+to appeal once more to our ignorance and suggest that if we only
+knew more we should see that the yellow quills, the black
+"moustache" and the red crescent, specially qualify <i>auratus</i>
+for the north and eastern region, and the red quills, red "moustache"
+and absence of crescent fit <i>cafer</i> to the conditions of
+its homes. Each can judge for himself, but my own view is
+that this is a vain delusion, and that to cherish it merely blunts
+the receptivity of the mind, which if unoccupied with such fancies
+would be more ready to perceive the truth when at last it shall
+appear. Think of the range of conditions prevailing in the
+country occupied by <i>auratus</i>&mdash;a triangle with its apex in Florida
+and its base the whole Arctic region of North America. Is it
+seriously suggested that there is some element common to the
+"conditions" of such an area which demands a nuchal crescent
+in the Flickers, though the birds of the <i>cafer</i> area, almost equally
+varied, can dispense with the same character? Curiously enough,
+the geographical variation of <i>Sphyropicus varius</i>, another though
+a very different Woodpecker<a name="FNanchor_6_95" id="FNanchor_6_95"></a><a href="#Footnote_6_95" class="fnanchor">[6]</a>
+shows that conversely the nuchal crescent can be dispensed with
+in the Eastern form though it is assumed by the Western.<a name="FNanchor_7_96" id="FNanchor_7_96"></a><a href="#Footnote_7_96" class="fnanchor">[7]</a>
+<span class="pagenum"><a name="Page_156" id="Page_156">[Pg 156]</a></span></p>
+
+<p class="indent">Allen points out the interesting additional fact that superposed
+upon each of the two distinct forms, <i>auratus</i> and <i>cafer</i>,
+are many geographical variations which can very naturally be
+regarded as climatic. Each decreases in size from the North
+southward, as so many species do.<a name="FNanchor_8_97" id="FNanchor_8_97"></a><a href="#Footnote_8_97" class="fnanchor">[8]</a>
+They become paler in the arid plains, and show the
+ordinary phases which are seen in other birds having
+the same distribution. Such differences
+we may well suppose to be determined directly or indirectly, by
+environment, and we may anticipate with fuller knowledge it
+will be possible to distinguish variations of this nature as in the
+broad sense environmental, from the larger differences separating
+the two main types of <i>Colaptes</i>, which I surmise are altogether
+independent of such influences.</p>
+
+<p class="indent">It is generally supposed that phenomena like those now so
+well established in the case of <i>Colaptes</i> are very exceptional, and
+as has already been stated a number of circumstances must
+combine in order that they may be produced. I suspect however
+that the examples are more numerous than is commonly thought.
+In all likelihood the three forms <i>Sphyropicus varius</i>, <i>nuchalis</i>
+and <i>ruber</i> are in a very similar condition though the details
+have not, so far as I know, been worked out. A complex example
+which is closely parallel to the case of <i>Colaptes</i> was described by
+F. M. Chapman<a name="FNanchor_9_98" id="FNanchor_9_98"></a><a href="#Footnote_9_98" class="fnanchor">[9]</a>
+at the same date as Allen's work. This is the
+case of <i>Quiscalus</i>, the Grackles, which in the North American
+Continent have three fairly distinct forms which Chapman speaks
+of as <i>Q. aeneus</i>, <i>Q. quiscula</i>, and <i>Q. quiscula aglaeus</i>.
+The birds are all, so far as pigment is concerned, dark blackish brown, but
+the head and mantle have superposed a metallic sheen of interference-colours
+which in the various forms take different tints,
+<span class="pagenum"><a name="Page_157" id="Page_157">[Pg 157]</a></span>
+bluish green, bronze green, or bronze purple. The details are
+complicated and difficult to appreciate without actual specimens,
+but the two common types are sufficiently distinct. The birds
+inhabit the whole area east of the Rockies, <i>quiscula aglaeus</i>
+occupying Florida and the Southern States southwest of a band
+of country about a hundred miles broad extending roughly from
+Connecticut to the mouth of the Mississippi; and <i>aeneus</i> taking
+the area north and west of this band. In discussing this case
+Chapman expresses the same view as Allen does in the <i>Colaptes</i>
+case, that there are two distinct populations, substantially fixed,
+and that the band of country in which they meet each other
+has a mongrel population, with no consistent type, but showing
+miscellaneous combinations of the character of the two chief types.</p>
+
+<p class="indent">The warblers of the genus <i>Helminthophila</i> provide
+another illustration which has points of special interest. The two chief
+species are <i>H. pinus</i>, which has a yellow mantle and lower parts,
+white bars on the wings, a black patch behind the eyes and a
+broad black mark on the throat; and <i>H. chrysoptera</i> with dark grey
+mantle and pale whitish grey lower parts, yellow bars on the
+wings, and grey marks on cheeks and throat where <i>pinus</i> has
+black. These two birds are exceeding distinct, and in addition
+their songs are quite unlike. <i>H. pinus</i> ranges through the eastern
+United States up to Connecticut and Iowa. <i>H. chrysoptera</i> is a
+northern form extending down to Connecticut and New Jersey.
+Both are migrants.</p>
+
+<p class="indent space-below">In these two States, where the two types overlap,
+certain forms have been repeatedly found which have been described as
+two distinct species, <i>Lawrencei</i> and <i>leucobronchialis</i>. Dr. L. B.
+Bishop and Mr. Brewster showed me two long series of <i>Helminthophila</i>
+containing various intergrades between the four
+named kinds, and details regarding these may be found in
+Chapman's <i>North American Warblers</i> and in Dr. Bishop's paper
+in Auk, 1905, XXII. Though the characters evidently break
+up to some extent, the series can be represented as due to
+recombinations of definite factors more easily than the others
+which I have described. The differentiating characters are:
+<span class="pagenum"><a name="Page_158" id="Page_158">[Pg 158]</a></span></p>
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Warblers" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>Pinus</i></td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>Chrysoptera</i></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">1.&nbsp;&nbsp;</td>
+      <td class="tdl">Mantle and lower parts <i>yellow</i> (<b>Y<sup>1</sup></b>).&nbsp;&nbsp;</td>
+      <td class="tdr">1.&nbsp;&nbsp;</td>
+      <td class="tdl">Mantle and lower parts <i>grey</i> (<b>y<sup>1</sup></b>).</td>
+    </tr><tr>
+      <td class="tdr">2.&nbsp;&nbsp;</td>
+      <td class="tdl">Wing-bars <i>white</i> (<b>y<sup>2</sup></b>).</td>
+      <td class="tdr">2.&nbsp;&nbsp;</td>
+      <td class="tdl">Wing-bars <i>yellow</i> (<b>Y<sup>2</sup></b>).</td>
+    </tr><tr>
+      <td class="tdr">3.&nbsp;&nbsp;</td>
+      <td class="tdl">Cheek and throat <i>not black</i> (<b>b</b>).</td>
+      <td class="tdr">3.&nbsp;&nbsp;</td>
+      <td class="tdl">Cheek and throat <i>black</i> (<b>B</b>).</td>
+    </tr>
+ </tbody>
+</table>
+
+<p class="indent space-above">The grey pigment of the mantle is common to both, but is
+masked by the yellow in <i>pinus</i>, the net result being an olive-green.<a name="FNanchor_10_99" id="FNanchor_10_99"></a><a href="#Footnote_10_99" class="fnanchor">[10]</a>
+</p>
+
+<p class="indent">I am much indebted to Dr. F. M. Chapman for the loan
+of the coloured plate in which these distinctions are shown. It first
+appeared in his book, <i>North American Warblers</i>.</p>
+
+<p class="indent">We cannot tell whether <i>yellow</i> or <i>not-yellow</i>
+is due to the presence of a factor, but we may suppose that one or other
+gives the special colour to the parts. The black of character 3
+is no doubt a dominant. Thus <i>pinus</i> becomes Y<sup>1</sup>y<sup>2</sup>b and <i>chrysoptera</i>
+in y<sup>1</sup>Y<sup>2</sup>B. The <i>Lawrencei</i> which has the underparts
+<i>yellow</i>, wing-bars <i>white</i>, and <i>black</i> patches is Y<sup>1</sup>y<sup>2</sup>B and
+<i>leucobronchialis</i> which has mantle and underparts <i>not-yellow</i>, wing-bars
+<i>yellow</i> and <i>no black patches</i> is y<sup>1</sup>Y<sup>2</sup>b. This representation,
+it should be clearly understood, is tentative and approximate
+only. The characters are not really sharp, for there is much
+grading; but allowing for the effects of heterozygosis and for some
+actual breaking-up of factors I believe it gives a fairly correct
+view of the case. In particular we can see how it meets the difficulty
+which Chapman felt in accepting <i>leucobronchialis</i> as in
+any sense derived from <i>pinus</i> which has a yellow breast, and
+<i>chrysoptera</i> which has a black throat, seeing that <i>leucobronchialis</i>
+has neither. We now recognize at once that this form could be
+produced by ordinary re-combination of the absence of Y<sup>1</sup> with
+the absence of B.</p>
+
+<p class="indent space-below">I note also with great interest that the modern
+observers agree that the so-called hybrids may have the song either of
+the one species, or of the other, or a song intermediate between
+the two. It may also be added that these two types have several
+times been seen, in the breeding season, paired with each other
+or with one of the other combinations.</p>
+
+<p><span class="pagenum"><a name="Page_159" id="Page_159">[Pg 159]</a></span></p>
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 1.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl"><i>Helminthophila pinus</i>, male.</td>
+    </tr><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 2.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl"><i>Helminthophila pinus</i>, female.</td>
+    </tr><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 3.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl">"Lawrence's Warbler," male; one of the integrading forms.</td>
+    </tr><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 4.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl">"Brewster's Warbler," male; another of the integrading forms.</td>
+    </tr><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 5.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl"><i>Helminthophila chrysoptera</i>, male.</td>
+    </tr><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 6.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl"><i>Helminthophila chrysoptera</i>, female.</td>
+    </tr>
+ </tbody>
+</table>
+
+<div class="figcenter" >
+  <img src="images/i_175.jpg" alt="Warblers" width="600" height="941" />
+</div>
+
+<p class="indent space-above space-below">Allen<a name="FNanchor_11_100" id="FNanchor_11_100"></a><a href="#Footnote_11_100" class="fnanchor">[11]</a>
+has described another excellent American example,
+the Tits of the group <i>Baeolophus bicolor-atricristatus</i>. The form
+<i>bicolor</i> belongs to the eastern States and ranges from the Atlantic
+coast to the Great Plains, and <i>atricristatus</i>, of east Mexico,
+extends from Vera Cruz to central Texas. In southern and
+central Texas the breeding ranges adjoin, and in this country
+various intermediates occur. The chief types differ in two main points.</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Warblers" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>B. bicolor</i></td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>B. atricristatus</i></td>
+    </tr><tr>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+    </tr><tr>
+      <td class="tdl">Forehead varies from <i>deep black</i></td>
+      <td class="tdl">&emsp;Forehead <i>white</i> to <i>buffish white</i>.</td>
+    </tr><tr>
+      <td class="tdl">&emsp;to <i>dull black</i>, suffused with <i>rusty brown</i>.</td>
+      <td class="tdl">&nbsp;</td>
+    </tr><tr>
+      <td class="tdl">Crown and crest <i>grey</i>,</td>
+      <td class="tdl">&emsp;Crown and crest <i>black</i>, abruptly</td>
+    </tr><tr>
+      <td class="tdl">&emsp;slightly darker than the back.</td>
+      <td class="tdl">&emsp;&emsp;contrasting with the back.</td>
+    </tr>
+ </tbody>
+</table>
+
+<p class="space-above">The intergrades between the two have, as usual,
+received specific names. A detailed description is given by Allen, from which
+it appears that the gradation is very complete. In one case a
+series of 16 adults were all intermediates. It is not stated whether
+the collector took these at random, but from the local lists it is
+clear that the types are found not far away from the place where
+the intergrades were shot.</p>
+
+<p class="indent">Another very striking case is that of the Tanagers, of the
+genus <i>Rhamphocoelus</i>. In this group there are several local forms
+which are related to each other in remarkable ways. The forms
+known as <i>passerinii</i> and <i>icteronotus</i> exhibit the clearest
+phenomena of intergradation. The species <i>passerinii</i> has a
+brilliant scarlet and black male, and it inhabits Honduras and Nicaragua.
+Proceeding southwards along the isthmus we find next <i>costaricensis</i>
+which has a male like that of <i>passerinii</i> (but a female
+with more orange than the olive-grey female of <i>passerinii</i>).
+Next we come to Panama which is occupied by <i>icteronotus</i>,
+sharply distinguished from <i>passerinii</i> by the fact that the <i>scarlet
+is replaced by lemon-yellow</i>. This same <i>icteronotus</i> occurs again
+as a pure type in Ecuador and many other parts of South America;
+but Colombia, <i>between Panama and Ecuador</i>, contains scarlets
+like <i>passerinii</i>, yellows like <i>icteronotus</i>, and various intergrades
+<span class="pagenum"><a name="Page_160" id="Page_160">[Pg 160]</a></span>
+of several shades of orange. The <i>passerinii</i> males from Nicaragua
+are indistinguishable from those of Colombia, and the
+<i>icteronotus</i> of Ecuador are the same as those in Panama. The
+orange intergrades, doubtless heterozygous forms, though collected
+at the same locality (Medellin in Colombia) as several
+pure yellows and pure scarlets, are in the British Museum series
+sorted out as a separate species under the name <i>chrysonotus</i>!
+Complications are introduced by the relations of these forms to
+another named type, <i>flammigerus</i>, but we may for our purpose
+leave that out of consideration, and say that the order of geographical
+sequence from Honduras to Ecuador is (1) scarlet, (2) yellow,
+(3) mixture of types, scarlet, yellow, orange, (4)yellow.</p>
+
+<p class="indent space-below">Similar examples exist in the birds of the old world,
+but I do not know of any that have been studied so fully as those of
+America. The best known is that of the two Rollers, <i>Coracias
+indicus</i> which spreads from Asia Minor through Persia, Baluchistan,
+the Indian Peninsula and Ceylon, and <i>affinis</i> which
+ranges from Nepal, through Assam, Tenasserim and the Indo-Chinese
+countries. The two types are very different and may be
+distinguished as follows:</p>
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Rollers" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>C. indicus</i></td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>C. affinis</i></td>
+    </tr><tr>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+    </tr><tr>
+      <td class="tdl"><i>Mantle</i>&emsp;drab brown-chestnut.</td>
+      <td class="tdl">Dark olive-green.</td>
+    </tr><tr>
+      <td class="tdl"><i>Breast</i>&emsp;chestnut.</td>
+      <td class="tdl">Dull purple brown.</td>
+    </tr><tr>
+      <td class="tdl"><i>Throat</i>&emsp;purplish, streaked with white.&nbsp;&nbsp;</td>
+      <td class="tdl">Purple, streaked with blue.</td>
+    </tr><tr>
+      <td class="tdl"><i>Upper tail-coverts</i>&emsp;indigo.</td>
+      <td class="tdl">Turquoise.</td>
+    </tr>
+ </tbody>
+</table>
+
+<p class="indent space-above">The wings are the same in both. In the provinces
+of Nepal, Sikhim, and Darjiling the two species coexist, with the result
+that intergrades have been frequently recorded. The line of
+intergradation extends to the coast, and birds showing various
+combinations of the two types from the Calcutta district exist
+in collections.<a name="FNanchor_12_101" id="FNanchor_12_101"></a><a href="#Footnote_12_101" class="fnanchor">[12]</a>
+The case is interesting inasmuch as like that of
+<i>Quiscalus</i> it shows a series of combinations of various metallic
+colours. Some of these are probably evoked by the development
+of pigment behind striations or other interferences already existing,
+but in the present state of knowledge it would be quite impossible
+<span class="pagenum"><a name="Page_161" id="Page_161">[Pg 161]</a></span>
+to suggest what the actual factors producing these appearances may be.</p>
+
+<p class="indent">There are, naturally, many other cases among birds
+which are suspected of being in reality comparable, but in most of them
+the evidence is still inadequate. Among Lepidoptera also there
+are a few of these; perhaps the most striking is that of <i>Basilarchia
+"proserpina."</i><a name="FNanchor_13_102" id="FNanchor_13_102"></a><a href="#Footnote_13_102" class="fnanchor">[13]</a>
+The genus is well known to European collectors
+under the name <i>Limenitis</i>, of which we in England have
+one species, <i>L. sibylla</i>, the "White Admiral." A species very
+like <i>sibylla</i> in general appearance is common in the northern
+parts of the United States, ranging through Canada and Northern
+New England, but rarely south of Boston. This species has
+the conspicuous white bands across both wings like our <i>sibylla</i>.</p>
+
+<p class="indent">There is also a more Southern type known as <i>astyanax</i>,
+which is very different in its appearance, being without the white
+bands and having a broad irroration of blue scales on the posterior
+border of the hind wings. The two are so distinct that one would
+not be tempted to suspect any very close relation between them.
+In its distribution <i>astyanax</i> is described by Field as replacing
+arthemis south of latitude 42°. About Boston it is much more
+common than <i>arthemis</i>.</p>
+
+<p class="indent">The two forms encroach but little on each other's territory,
+but where they do coexist, a third form, known as <i>proserpina</i>, is
+found which is almost intermediate, with the white bands much reduced.
+There is now no doubt that this <i>proserpina</i> is a heterozygous
+form, resulting from a combination of the characters of
+<i>arthemis</i> and <i>astyanax</i>. Field succeeded in rearing a brood of 16
+from a <i>proserpina</i> mother caught wild which laid 31 eggs, and of
+these, nine (five males, four females) resembled the mother,
+being <i>proserpina</i>, and seven (four males, three females) were
+<i>arthemis</i>. There can be no question therefore that the mother
+had been fertilised by a male <i>arthemis</i> and that <i>no-white-band</i>
+is a factor partially dominant over the <i>white band</i>. Another
+point of interest which Field observed was that the <i>proserpina</i>
+female refused to lay on birch, poplar or willow, but accepted
+<span class="pagenum"><a name="Page_162" id="Page_162">[Pg 162]</a></span>
+wild cherry (<i>Prunus serotina</i>) a species on which <i>astyanax</i> can
+live, though that tree is not known to be eaten by <i>arthemis</i>.
+Incidentally also the observations show that sterility cannot be
+supposed to be the bar which maintains the distinctness of
+<i>arthemis</i> and <i>astyanax</i>.</p>
+
+<p class="indent">In this connection <i>Papilio oregonia</i> and <i>bairdii</i> should be
+mentioned.<a name="FNanchor_14_103" id="FNanchor_14_103"></a><a href="#Footnote_14_103" class="fnanchor">[14]</a>
+<i>P. oregonia</i> is one of the numerous forms like
+<i>machaon</i>, but rather paler. It is a northern insect, inhabiting
+British Colombia east of the Cascade Range, and reaching to
+Colorado. <i>P. bairdii</i> is a much darker butterfly, representing
+the <i>asterias</i> group of the genus <i>Papilio</i>. Like <i>asterias</i>
+it has the abdomen spotted at the sides, not banded as in the <i>machaon</i>
+group. It belongs to Arizona and Utah extending into Colorado.
+From Colorado the form <i>brucei</i> is described, more or less intermediate,
+like <i>bairdii</i> but with the abdomen banded as in <i>oregonia</i>.
+W. H. Edwards records the results of rearing the offspring of the
+<i>bairdii</i>-like and of the <i>oregonia</i>-like mothers. Each was found
+able to have offspring of both kinds, that is to say, <i>bairdii</i>
+females gave both forms, and <i>oregonia</i> females gave both forms.
+It is not possible to say which is dominant, since the fathers were
+unknown. On general grounds one may expect that the <i>bairdii</i>
+form will be found to dominate, but this is quite doubtful.</p>
+
+<p class="indent">From this particular discussion I omit reference to
+those examples in which the permanently established types are obviously
+associated with special conditions of life. Where considerable
+climatic differences exist between localities, or when we
+pass from South to North, or from the plains into Alpine levels
+we often find that in correspondence with the change of climate
+there is a change in the characteristics of a species common to
+both. When I say "species" in such a connection I am obviously
+using the term in the inclusive sense. Some would prefer to
+say that in the two sets of conditions two <i>representative species</i>
+exist. Whichever expression be preferred it is plain that such
+examples present another phase of the problem we have been
+just considering, and in them also we have an opportunity of
+<span class="pagenum"><a name="Page_163" id="Page_163">[Pg 163]</a></span>
+observing the consequences of the overlap of two closely related
+types, but there are advantages in considering them separately.
+In the examples hitherto given, with the possible exception of
+the Papilios,<a name="FNanchor_15_104" id="FNanchor_15_104"></a><a href="#Footnote_15_104" class="fnanchor">[15]</a>
+the two fixed types severally range over so extensive
+a region that it may fairly be supposed that in the different
+parts they are subject to considerable diversities of climate.
+There is no outstanding difference that we know distinguishing
+the habitats of the two forms; but in comparing Alpine with
+Lowland forms, or essentially northern with essentially southern
+forms we do know an external circumstance, temperature, that
+may reasonably be supposed to have an influence, direct or indirect,
+on the population.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_164" id="Page_164">[Pg 164]</a></span></p>
+<h2>CHAPTER VIII</h2>
+
+<h3>LOCALLY DIFFERENTIATED FORMS. <i>Continued.</i></h3>
+<p style="font-size: 120%; text-align: center;">
+<b><span class="smcap">Climatic Varieties</span></b></p>
+
+<p class="indent">In this chapter we will examine certain cases which
+illustrate phenomena comparable with those just considered, though as
+I have already indicated, they form to some extent a special
+group. The outstanding fact that emerges prominently from
+the study of the local forms is that when two definite types,
+nearly allied, and capable of interbreeding with production of
+fertile offspring, meet together in the region where their
+distributions overlap, though intergrades are habitually found,
+there is no normally or uniformly intermediate population occupying
+the area of intergradation. Such phenomena as these
+must, I think, be admitted to have great weight in any attempt
+to construct a theory of evolution. True we must hesitate in
+asserting their positive significance, but I see no escape from the
+conclusion that they throw grave doubt on conventional views.
+Again and again the same question presents itself. If <i>A</i> and
+<i>B</i> lately emerged from a common form why is that common form
+so utterly lost that it does not even maintain itself in the region
+of overlapping? Almost equally difficult is it, in the cases which
+I have numerated, to apply concrete suggestions based on any
+factorial scheme. We may see that in <i>Heliconius erato</i> the type
+with the red mark on the hind wing probably contains a dominant
+factor, and that where the red mark is absent the metallic colours
+are exposed; and that similarly the green metallic colour may have
+another factor which distinguishes it from the blue. In this way
+we can fairly easily represent the various types of <i>erato</i> on a
+factorial system as the result of the various possible combinations
+of two pairs of factors. But there we stop, and we are quite
+unable to suggest any reason why one area should have the red
+and the green type while another should have the blue also. So
+again with <i>Colaptes</i> or the Warblers. By application of a
+<span class="pagenum"><a name="Page_165" id="Page_165">[Pg 165]</a></span>
+factorial system, admittedly in a somewhat lax fashion, the genetic
+interrelations of the types can be represented; but how it comes
+about that each type maintains a high degree of integrity in
+its own region we can only imagine. Each has in actual fact
+a stability which the intermediate forms have not, but we cannot
+yet analyse the nature of that stability. Mendelian conceptions
+show us how by segregation the integrity of the factors can be in
+some degree maintained, but not why certain combinations of
+factors should be exceptionally stable. All that is left us to
+fall back on is the old unsatisfying suggestions that some combinations
+<i>may</i> have greater viability than others, that there
+<i>may</i> be a tendency for like to mate with like, and so forth.</p>
+
+<p class="indent">These difficulties acquire more than ordinary force
+in those cases in which the two fixed types inhabit regions differing
+in some respect so obvious and definite that we are compelled to
+regard each type as climatic and as specially adapted to the
+conditions. When for example an animal has a distinct type
+never met with except in Arctic or Alpine conditions, and another
+type proper to the plains and temperate regions, what are the
+characteristics of the population of intermediate latitudes or
+at intermediate levels? Some of the examples discussed in the
+last chapter may be instances of this very nature, but even if
+they are not, others are forthcoming which certainly are. The
+evidence of these cases leads to the suspicion that with further
+knowledge they will be found to consist of two classes, some in
+which the observer as he passes from the one climate to the other
+will find the intermediate area actually occupied by a population
+of intermediate character, and others in which, though we may
+presume the maintenance of intermediate conditions in the transitional
+area, there is no definite transitional population. This
+interrupted or discontinuous distribution seems, so far as I
+have means of judging, to be by far the more common of the two.
+I do not doubt that by sufficient search individuals representing
+every or almost every transitional form can be found, but it is
+apparently rare that <i>populations</i> corresponding to these several
+grades can be seen. The question has in few if any cases been
+studied with precision sufficient to provide a positive answer;
+<span class="pagenum"><a name="Page_166" id="Page_166">[Pg 166]</a></span>
+but I suspect that real and complete continuity, in the sense
+thus defined, will only be found where the character of the local
+populations depends <i>directly</i> on the conditions of life, and shows
+an immediate response to changes in them apart from that postponed
+response which we suppose to be achieved by selection.
+Obviously the character must be one, like size for instance,
+capable of sensibly complete gradation.</p>
+
+<p class="indent">The only example I have met with of the phenomenon
+of anything like a complete intergradation between local types
+really distinct in kind is that provided by the butterfly <i>Pararge
+egeria</i>. It is well known to entomologists that this insect exists
+in two very different types, a northern one, the "Speckled Wood"
+of England, in which the spots are a pale whitish yellow, and a
+southern type having the full fulvous colour that we know as
+characteristic of <i>megaera</i>, the "Gatekeeper." It appears that
+Linnaeus gave the name <i>egeria</i> to the southern type,<a name="FNanchor_1_105" id="FNanchor_1_105"></a><a href="#Footnote_1_105" class="fnanchor">[1]</a>
+and our own is now called <i>egerides</i>. Broadly speaking, so far
+as Great Britain, France, and the Spanish Peninsula are concerned, the
+tawny-coloured <i>egeria</i> occupies Spain and western France up
+to the latitude of Poitiers and the pale yellow <i>egerides</i> extends from
+Scotland, where it has a scanty distribution, through southern
+England, where in suitable localities it is common, and the north
+of France to Paris.<a name="FNanchor_2_106" id="FNanchor_2_106"></a><a href="#Footnote_2_106" class="fnanchor">[2]</a>
+The two types when placed side by side
+are strikingly different from each other, and are an excellent
+illustration of what is meant by climatic variation. The insect
+is not a great traveller and probably scarcely ever wanders far
+from its home. It should therefore be possible by collecting
+from north to south to find out how the transition is effected,
+whether suddenly or gradually. This at various times I have
+endeavoured to do, but I am still without exact information as
+to the population in certain critical areas. In addition to the
+information derived from specimens which I have collected or
+seen in the collections of others there is a good account of the
+general distribution in Europe given by the Speyers,<a name="FNanchor_3_107" id="FNanchor_3_107"></a><a href="#Footnote_3_107" class="fnanchor">[3]</a>
+<span class="pagenum"><a name="Page_167" id="Page_167">[Pg 167]</a></span>
+who evidently paid more attention to the subject than most lepidopterists
+have done, and many more recent records. In particular
+Oberthür<a name="FNanchor_4_108" id="FNanchor_4_108"></a><a href="#Footnote_4_108" class="fnanchor">[4]</a>
+has published many details as to the distribution in
+western France and I am especially indebted to Mr. H. Rowland-Brown
+for a long series of notes as to the distribution in France
+generally, and to Mr. H. E. Page and Dr. T. A. Chapman, Mr.
+Oberthür Prof. Arrigoni degli Oddi, Mr. H. Williams and other
+correspondents, for showing me forms from many localities. The
+butterfly is attached for the most part to woods of deciduous trees
+and to country abounding in tall hedges or rough scrub. It is not
+usually to be found in highly cultivated districts or in very dry
+regions. Hence there is necessarily some want of continuity in
+the distribution at the present time and I should think a mile or
+two of arable land without big hedges would constitute a barrier
+hardly ever passed. The larva feeds on several coarse grasses,
+especially <i>Dactylis glomerata</i>. Barrett mentions also <i>Triticum
+repens</i>. In this country the winter is usually passed in the larval
+stage, but I have found that in captivity, at least, there is much
+irregularity. The larvæ feed whenever the weather is not very
+cold and may pupate, but if sharp cold comes on when they are pupating
+or nearly full-grown they often get killed unless protected.</p>
+
+<p class="indent">Some writers speak of a difference between the early
+and later broods, but I have never noticed this, and I do not think
+that the general tone of the yellow is affected by the seasons
+(see Tutt, <i>Ent. Rec.</i>, IX, 1897, p. 37).<a name="FNanchor_5_109" id="FNanchor_5_109"></a><a href="#Footnote_5_109" class="fnanchor">[5]</a>
+</p>
+
+<p class="indent">Beginning at the south of Spain the thoroughly fulvous
+type <i>egeria</i> is common at Gibraltar in the Cork woods, at Granada,
+and doubtless generally. Lederer is said to have found only
+this type in Spain (Speyer), and though I have no precise information
+as to other places in the Peninsula north of Jaen I feel
+tolerably sure that there is no change from south to north.<a name="FNanchor_6_110" id="FNanchor_6_110"></a><a href="#Footnote_6_110" class="fnanchor">[6]</a>
+<span class="pagenum"><a name="Page_168" id="Page_168">[Pg 168]</a></span>
+Immediately north of the Pyrenees we still meet <i>egeria</i> exclusively,
+and up to Poitiers at least there is no noticeable change. But
+somewhere between Poitiers and the bottom of the Loire valley
+at Tours, the genuine southern type comes to an end, and the
+whole population begins at the Loire to be of an intermediate
+type, easy to distinguish both from <i>egeria</i> and from <i>egerides</i>.
+As to the exact condition of the species in the fifty miles separating
+St. Savin on the Vienne from places on the Loire I have no adequate
+information. I have only one small sample from there,
+but it does contain insects both of the southern and intermediate
+types taken on the same day, in a wood near Preuilly. Oberthür
+also states that at Nantes the true southern form exists in company
+with the northern. From this I infer that the southern
+form extends up the coast further than it does inland, but I
+imagine the representative spoken of as northern would be of
+usual Brittany or intermediate type.</p>
+
+<p class="indent">The Vienne river joins the Loire, so the true southern
+type reaches over into the basin of the Loire. From the Loire (Tours,
+Corméry) north to Calvados (Balleroy) only the intermediate
+is found, so far as I know, and the same type extends over
+Brittany.<a name="FNanchor_7_111" id="FNanchor_7_111"></a><a href="#Footnote_7_111" class="fnanchor">[7]</a>
+In general, however, the woods near Paris have the
+thoroughly northern type <i>egerides</i>, but at St. Germain-en-Laye
+and at Etampes (Oberthür) the population approaches the intermediate type.</p>
+
+<p class="indent">On the whole the intermediate type is certainly less
+homogeneous than either of the extremes, and females with the two
+central spots either paler or more fulvous than the rest are not
+uncommon, but I have never taken one on the Loire or in Brittany
+which I should class with either of the extreme types.</p>
+
+<p class="indent">Before speaking of the distribution in other parts
+of France and in Europe generally I will briefly state the results of my
+breeding experiments. The work was done many years ago before
+we had the Mendelian clue, and it is greatly to be hoped that
+some one will find opportunities of repeating it. Crossing the
+English and the thoroughly southern type the families produced
+<span class="pagenum"><a name="Page_169" id="Page_169">[Pg 169]</a></span>
+agree entirely with the intermediates of Brittany and the Loire.
+Reciprocals are alike. Of F<sub>2</sub> I only succeeded in raising very
+few and of those that I had (about 30) nearly all were intermediate
+in character, though perhaps rather less uniform than F<sub>1</sub>. One
+family alone, containing only 4 specimens, had one <i>egerides</i>,
+and three fulvous intermediates. As the case stands alone I
+hesitate whether or not to suppose it due to some mistake. Moreover
+from F<sub>1</sub> crossed back with the respective parental types I
+had fairly long series, especially from F<sub>1</sub> × the southern type, and
+looking at these families I cannot see any clear evidence of segregation.
+On the contrary, I think that though there are slight
+irregularities, they would, taken as a whole, be classed as coming
+between the intermediate type and the extreme form used as the
+second parent. This at least is true when the second parent was
+of the southern type.</p>
+
+<p class="indent">On this evidence I have regarded the case as one in
+which there is no good evidence of segregation and as conforming most
+nearly with the conventional view of gradual transition in response
+to climatic influences. Such influence must however be
+indirect; for I reared five generations of the northern type in
+England, and these, though they included several abnormal-looking
+specimens in the last generation and then died out, did
+not show any noticeable change from the fulvous colour of the
+wild type. Merrifield<a name="FNanchor_8_112" id="FNanchor_8_112"></a><a href="#Footnote_8_112" class="fnanchor">[8]</a>
+also found that heat applied to pupae of the northern type
+produced no approach to the southern type.</p>
+
+<p class="indent">Looking at the facts now in the light of more experience
+it seems to me just possible that the case may be one in which, as
+in Nilson-Ehle's Wheats, the dominant differs from the recessive
+in having two pairs of factors with similar effects. The
+fulvous type for example may have two or more elements in
+separate pairs which together produce the full effect, and the
+intermediate may have one of these. If this were so, some
+segregation should of course eventually be observable, but the
+proportion of the various fulvous and fulvous-intermediate
+individuals would be large, and the reappearance of actual
+<span class="pagenum"><a name="Page_170" id="Page_170">[Pg 170]</a></span>
+representatives of the northern type might be rare. I admit that this
+is a somewhat strained interpretation of the facts, and as yet it
+is not entitled to serious consideration. Nevertheless I am led
+to form some such expectation partly from the great difficulty
+in the way of any other, partly from the evidence of the small
+mixed sample found at Preuilly and partly from the statements
+given by Oberthür. There are moreover other features in the
+general distribution of the species which make it improbable
+that the dependence on climate can after all be so close. Published
+lists are unfortunately of little use in deciding which form
+occurs at a particular place, because, since the name <i>Meone</i> has
+ceased to be used for the southern form, there is no complete
+unanimity among authors as to the application of the names
+<i>egeria</i> and <i>egerides</i>, and unless more particulars are given,
+either name may be used for either form. Besides this, difficulty arises
+from the fact that the intermediate type is not generally distinguished
+at all, and English collectors finding it, may easily
+record it as the southern type. From Staudinger's note on the
+distribution, I gather that he, on the contrary, reckoned the
+intermediate with the northern type, as do the Speyers also.
+The late Mr. J. W. Tutt was careful to distinguish the three
+forms and has left several useful records. Easy therefore as it
+might seem to be to make out the distribution of such a familiar
+insect in its various modifications, there are serious practical
+difficulties, and until long series are brought together with this
+special object in view many obscurities will remain.</p>
+
+<p class="indent">With only the series from England, the west of France,
+and Spain before one it would be easy to regard the successive series
+of tones as a fair measure of climate; the brighter the colour, the
+hotter might one expect the locality to be. Such rough correspondence
+is often to be observed in butterflies and birds. It
+becomes impossible to take these simple views in the light of
+more complete knowledge. Beginning with France the fulvous
+<i>egeria</i> occupies the lower valley of the Rhone, probably from well
+above Lyon, though I have no exact information respecting the
+country above Avignon. According to Speyer it also takes the
+department of Lozère. The same authority says that Puy-de-Dôme
+<span class="pagenum"><a name="Page_171" id="Page_171">[Pg 171]</a></span>
+has "<i>egeria</i>," meaning perhaps the intermediate form,
+with the fulvous form much less commonly. Next comes the
+curious fact that though the Lower Rhone (Avignon, Tarascon,
+Nîmes) has the true fulvous form, Hyères, Cannes, Grasse, Nice,
+Digne, and Alassio have <i>the intermediate</i>. Savoy has the
+intermediate (Chambéry) and even <i>egerides</i> perhaps, though
+in the same latitude on the west of France there is nothing but
+the fulvous type. At Chalseul and Besançon (Doubs) the ordinary
+northern type is found. Switzerland generally, I believe, has the
+northern type, but Staudinger gives <i>egeria</i> for Valais and the
+intermediate occurs in Vaud.<a name="FNanchor_9_113" id="FNanchor_9_113"></a><a href="#Footnote_9_113" class="fnanchor">[9]</a>
+The south side of the Alps has probably colonies of the
+pale <i>egerides</i>, and of intermediates. Orta, with a
+very hot summer, has the English type (Tutt, <i>Ent.
+Rec.</i>, XII, 1900, p. 328). Locarno has the intermediate (<i>ibid.</i>,
+XV, 1903, p. 321). North Italy in general and western Piedmont
+have the intermediate; but further south <i>egeria</i> begins,
+at what region I do not know. Speyer gives on his own authority
+the remarkable statement that at Florence both extremes occur,
+but chiefly intermediates between the two. Mr. R. Verity
+however kindly informs me that in his experience this is not so,
+and that neither the real southern type nor the northern occur
+there. Sardinia, Sicily, Crete all have the southern type.
+Greece probably has various types. Staudinger (<i>Hor. Ross.</i>, VII,
+1870, p. 78) says intermediates resembling Nice types common
+everywhere, but from "Greece" the British Museum has a series
+that would pass for English specimens; and the same type occurs
+near Constantinople. The island of Corfu has a pale intermediate,
+distinct from <i>egerides</i> but approaching it. In Roumania
+all three forms are recorded from various places: <i>egeria</i> in the
+Dobrutscha; not quite typical (presumably an intermediate)
+at Bukharest; intermediate in various mountainous localities
+as well as in Macedonia and Dalmatia; but <i>egerides</i> in Azuga
+at about 3,000 feet.<a name="FNanchor_10_114" id="FNanchor_10_114"></a><a href="#Footnote_10_114" class="fnanchor">[10]</a>
+Hungary has the true <i>egerides</i> also.
+(Cf. Caradja, <i>Deut. Ent. Zt.</i>, IX, p. 58.) Mathew records the same
+<span class="pagenum"><a name="Page_172" id="Page_172">[Pg 172]</a></span>
+from Gallipoli (<i>E. M. M.</i>, 1881, p. 95). Staudinger does not
+distinguish the intermediates from the northern, but he gives
+"<i>egerides</i>" for Armenia and Fergana (Central Asia). As against
+the mere proximity of a great mountain chain being the influence
+which keeps the Riviera population intermediate may be
+mentioned the fact that the northern foothills of the Pyrenees
+have the pure southern type, and the climate of Cambo must
+surely be far cooler than that of Nice. The exact locality of
+the Greek specimens is not given, but there can be no part of
+Greece which is not much hotter in summer than Brittany, or
+Calvados, which have the intermediate, not the English type.</p>
+
+<p class="indent">In face of these facts it can scarcely be maintained
+that average temperature is the efficient cause of the particular tone
+of colour which the butterfly shows in a given region. Nevertheless
+it is clear that climate counts for much in determining
+the distribution. It is noticeable that though the pale <i>egerides</i>
+can be established in a warm climate we never find <i>egeria</i> in cold
+climates, and even the intermediate is not found in places that
+have a hard winter. I suspect that the distribution of the
+broods through the year and the condition of the animal at the
+onset of hard frost are features which really determine whether
+a strain can live in a particular place or not. Though the truth
+of the suggestion cannot be tested by experiments in captivity,
+which at once introduce disturbances, I incline to the idea that
+<i>egeria</i> has not got the right periodicity for northern climates.
+If it could arrange its life so that the population consisted either
+of young larvae, or perhaps of thoroughly formed pupae<a name="FNanchor_11_115" id="FNanchor_11_115"></a><a href="#Footnote_11_115" class="fnanchor">[11]</a>
+at the onset of winter, it might, for any obvious reason to the
+contrary, be able to live in England. It is irregularly "polyvoltine,"
+as the silk-worm breeders say, and as soon as a little
+warmth encourages it, a new generation starts into being, which
+if the frost comes at an untimely moment, is immediately
+<span class="pagenum"><a name="Page_173" id="Page_173">[Pg 173]</a></span>
+destroyed. Many species are continually throwing off individuals
+which feed up fast<a name="FNanchor_12_116" id="FNanchor_12_116"></a><a href="#Footnote_12_116" class="fnanchor">[12]</a>
+and emerge at once if the temperature permits, and
+I imagine a species of Satyrid wholly or largely
+represented by such individuals could scarcely survive in a
+country which had a hard winter. For such a climate some
+definite periodicity in the appearance of the broods may well be
+indispensable. But assuming that <i>egeria</i> is cut off from cold
+climates for such a reason, there is nothing yet to connect these
+habits with the fulvous colour, and until breeding can be carried
+out on a satisfactory scale there is no more to be said.</p>
+
+<p class="indent">From time to time records appear of individual specimens
+more or less fulvous being caught in southern England, especially
+in the New Forest.<a name="FNanchor_13_117" id="FNanchor_13_117"></a><a href="#Footnote_13_117" class="fnanchor">[13]</a>
+It would be interesting to know what offspring such
+individuals might produce. From the evidence
+now given some notion both of the strength and the weakness
+of the case considered as one of continuous climatic variation
+can be formed. I know no other equally satisfactory. Whether
+or not definite mixture of the intermediates with either of the
+extremes will be proved to occur, the case differs materially from
+those considered in the last chapter in the fact that at all events
+there is no general overlapping of forms. In a species so little
+given to wandering, overlapping could indeed scarcely be expected
+to occur. It is this circumstance which makes the species
+preeminently suitable as a subject for the study of climatic
+influences, and I trust that entomologists with the right opportunities
+may be disposed to explore the facts further.</p>
+
+<p class="indent">Just as many species, like <i>egeria</i>, have varieties
+which can be regarded as adapted to northern and southern regions, so
+there are also several which have lowland and Alpine forms quite
+distinct from each other. Every such case presents an example
+of the problem we have been considering. As the collector
+passes from the plains to the Alpine region, how will he find the
+<span class="pagenum"><a name="Page_174" id="Page_174">[Pg 174]</a></span>
+transition from one form to the other effected? Does the lowland
+form give place to the Alpine form suddenly, with a region
+in which the two are mixed, or will he find a zone inhabited by
+an intermediate population? I have spent a good deal of time
+examining the facts in the case of <i>Pieris napi</i> and its Alpine
+female variety <i>bryoniae</i>, and though there are many complications
+which still have to be cleared up, no doubt is possible
+as to the main lines of the answer. If in any valley in the Alps
+inhabited by both <i>napi</i> and <i>bryoniae</i> the collector catches
+every specimen he can, beginning at the bottom and working up to
+7,000 feet, he will at first get nothing but <i>napi</i>. At about 2,500
+feet, he may catch an occasional <i>bryoniae</i> flying with the <i>napi</i>.
+After 3,000 feet <i>napi</i> usually ceases, and only <i>bryoniae</i> are found.
+As an exception a colony of <i>napi</i> may be met with at much
+greater heights. I once found them in numbers at about 6,000
+feet.<a name="FNanchor_14_118" id="FNanchor_14_118"></a><a href="#Footnote_14_118" class="fnanchor">[14]</a>
+Not only were they free from any trace of modification
+in the direction of <i>bryoniae</i>, but they were of the thoroughly
+southern type of <i>napi</i>, being a late brood of that large and very
+pale kind (<i>meridionalis</i>) almost destitute both of dark veining
+above and of green veining below, which are common on the
+shores of Lago Maggiore and in other hot southern localities.
+Not far off at the same level were typical <i>bryoniae</i> in fair
+abundance. Occasionally an intermediate may be met with. I have
+taken a few, for example, at Macugnaga and at Fobello. These,
+however, in my experience are rarities in the Alps. Fleck<a name="FNanchor_15_119" id="FNanchor_15_119"></a><a href="#Footnote_15_119" class="fnanchor">[15]</a>
+gives notes on the distribution in Roumania which shows the
+same state of things. The lowland form is not transformed
+though found at great heights, and at Azuga (nearly 3,000 feet)
+<i>bryoniae</i> occurs with only occasional "<i>flavescens</i>,"
+viz., intermediates of the second brood.</p>
+
+<p class="indent">If this were all the evidence we should be satisfied
+that the lowland and Alpine types keep practically distinct, overlapping
+occasionally, but rarely interbreeding. The problem would
+remain, how is the distinctness of the two types maintained in
+the region of overlapping? Nowadays, I suppose, we should
+<span class="pagenum"><a name="Page_175" id="Page_175">[Pg 175]</a></span>
+incline to answer this question by reference to segregation, and
+perhaps by an appeal to selective mating. The suggestion that
+segregation does take place is certainly true to some extent.
+There are, however, difficulties in the way, and the whole subject
+is one of great complexity. My own experiments were made in
+pre-Mendelian times and were not arranged with the simplicity
+which we now know to be essential. The results are neither
+extensive enough nor clear enough to settle the many collateral
+questions which have to be considered, and the work ought to
+be done again. Nevertheless, some notes of the observations
+may have a suggestive value.</p>
+
+<p class="indent">When I began, I did not sufficiently appreciate that
+the "<i>napi</i>" group, omitting the North American forms, and the
+Asiatic representatives, has at least three chief types in western
+Europe. The differences we have to deal with are manifested
+by the females only, so in this account particulars as to the males
+are omitted for the most part. These are (1) our own British
+<i>napi</i>; (2) the form found in the south, from the Loire downwards,
+and in the Italian Alps, which I think may be spoken of as
+<i>meridionalis</i>; (3) <i>bryoniae</i>, which is a form clearly recognizable
+in the <i>female</i> only, and is found only in the arctic regions and
+in the Alps above 2,500 feet. The first two have several broods,
+two, three, or more, according to opportunity, and the first
+brood is different from the later ones. In <i>napi</i> the markings on
+the upper surface are a dark grey but in <i>meridionalis</i> they are a
+pale silvery grey and much less extensive. In the later broods
+of <i>napi</i> there is much less general irroration of the veins, and the
+spots stand out as more defined and blacker. These differences
+vary greatly in degree of emphasis. In <i>meridionalis</i> the later
+broods are entirely different from the first. Instead of having
+silvery markings they have the ground colour quite white, with
+the spots large and a full black. On the under side of the hind
+wings the usual green veins are almost absent, and I have seen
+individuals which could scarcely be distinguished from <i>rapae</i>.
+To these later broods the term <i>napaeae</i> is sometimes applied,
+but I here use <i>meridionalis</i> for the southern race in general as
+applicable to all broods.
+<span class="pagenum"><a name="Page_176" id="Page_176">[Pg 176]</a></span></p>
+
+<p class="indent">The female <i>bryoniae</i> is totally unlike the others.
+The ground colour is a full yellow, and each nervure is thickly irrorated
+with a brown pigment often spreading so far as to hide the ground
+almost entirely in the fore-wings. The males corresponding with
+these females are not certainly distinguishable from those of our
+own <i>napi</i>. Both sexes have the green veining of the underside
+of the hind wing fully developed, rather more than is usual in the
+lowland races, but this is not really diagnostic of the variety.
+The first serious difficulty arises in regard to the second brood
+of <i>bryoniae</i>. It is stated that there is only one brood,<a name="FNanchor_16_120" id="FNanchor_16_120"></a><a href="#Footnote_16_120" class="fnanchor">[16]</a>
+but I feel fairly sure that a second brood is sometimes produced, and
+that the females with a yellow ground and diminished irroration of the
+veins, not very uncommon in the Italian Alps in July to August,
+are generally representatives of it. Such insects would of course
+be classed with <i>bryoniae</i> in collections.</p>
+
+<p class="indent">My experiments began with eggs of true <i>bryoniae</i>
+females caught at about 2,500 feet early in July. These emerged in
+August-September as intermediates with yellow ground and
+about half as much black on the upper surface as <i>bryoniae</i>.
+They are exactly like the intermediates usually found in nature
+and in the light of later experience I regard them as natural F<sub>1</sub>
+forms, and I think the mothers had been fertilised by <i>napi</i> males,
+though I admit that in view of the rarity of natural intermediates
+there is a difficulty in this suggestion. Three of these females
+were mated with males raised from thorough <i>meridionalis</i>
+females, and three families were produced. Two of them
+showed distinct evidence of segregation, some being yellow and
+some white with various intergrades, some being no blacker than
+<i>meridionalis</i> and some ranging up to a dark intermediate type.
+Part emerged in the same autumn; and part overwintered, emerging
+as the spring <i>meridionalis</i> or as the peculiar type which I
+afterwards learnt to know as the spring F<sub>1</sub> form. The distinctions
+were fairly sharp between the several forms. But the offspring
+of the third female gave a series practically continuous from
+<span class="pagenum"><a name="Page_177" id="Page_177">[Pg 177]</a></span>
+<i>meridionalis</i> to the F<sub>1</sub> type. The work of subsequent years
+gave results similarly irregular which could only be described
+adequately at great length. The outcome may however be
+summed up in the statement that there is evidence that both
+the yellow ground and the dark veining are due to factors, but
+that there are several of these and that imperfect segregation
+is not uncommon, producing various reduction-stages. The
+yellow ground may be due to one factor, and the several shades
+may be the result of irregularities in dominance, but the black
+markings when fully developed cannot I think be the result of
+less than three factors, one for the basal darkening, one for general
+irroration, and one for the margins. Probably also the enlargement
+of the spots is produced by a fourth factor.</p>
+
+<p class="indent">There was not, in my experience any great difficulty
+in getting the various forms to pair in captivity. Some attempts were
+made to see whether individuals of either type selected mates
+of their own type in preference to those of the other, but the
+results were inconclusive. There were some indications of such
+a preference; though, from the impossibility of judging how much
+of this may be due to other circumstances, I could not come to
+a positive conclusion on the rather meagre evidence.</p>
+
+<p class="indent">Recently Schima<a name="FNanchor_17_121" id="FNanchor_17_121"></a><a href="#Footnote_17_121" class="fnanchor">[17]</a>
+has given a careful and detailed account
+of all the forms found in Lower Austria which he enumerates
+under 14 distinct varietal names. He gives full references to
+previous accounts, especially to the beautiful plates lately published
+by Roger Verity.<a name="FNanchor_18_122" id="FNanchor_18_122"></a><a href="#Footnote_18_122" class="fnanchor">[18]</a>
+Examination of these and of my own
+specimens strongly suggests that the several forms are due to
+the recombination of the factors I have named. Among those
+which I have bred are representatives of most if not all the types
+enumerated by Schima in addition to other curious forms. For
+example I have <i>bryoniae</i> markings on a ground practically white;
+the dark veins with spots almost obsolete; <i>meridionalis</i> on a
+yellow ground; the intermediate amount of black on a white
+ground, etc. The last-named may occur wild and I have one
+from Macugnaga as well as one given me by Mr. F. Gayner from
+Lulea (Lapmark).
+<span class="pagenum"><a name="Page_178" id="Page_178">[Pg 178]</a></span></p>
+
+<p class="indent">To obtain really exact knowledge of the number of
+factors and their properties it would be necessary to repeat the work.
+After the beginning, I made a mistake in using British <i>napi</i>
+instead of <i>meridionalis</i> and the results were much confused
+thereby. The contrast between <i>meridionalis</i> and the various
+dark forms is much greater and classification of the types would
+have been therefore easier. The British form is presumably
+<i>meridionalis</i> plus the factor for the basal pigmentation. The
+problem is greatly complicated by the differentiation of the
+seasonal forms. The first point to be determined is whether
+<i>bryoniae</i> is capable of producing a second brood when it is
+thoroughly pure-bred, and whether such a second brood is, as I
+suspect, normally intermediate in character.</p>
+
+<p class="indent">In the Alps generally there is no definitely intermediate
+population; nor I believe, is any such population met with in
+the north where the arctic <i>bryoniae</i> meets <i>napi</i>, but as
+to this I have no precise information. One curious fact, however, must
+be mentioned, namely that there is a population that can probably
+be so described with fairness established at Mödling near
+Vienna. This is not in any sense an Alpine locality, and does
+not, as I am told, differ in any obvious way from the other suburbs
+of Vienna. Dr. H. Przibram was so good as to send me a
+set taken at this place, representing a second brood, and they
+were decidedly heterogeneous, ranging from an intermediate
+form such as <i>bryoniae</i> fertilised by <i>napi</i> usually produces,
+to a light yellowish second-brood type with little dark pigment.
+There are also two actual <i>bryoniae</i>. Whether true <i>napi</i>
+also occur there I do not know, but I have no doubt they do. It
+would be well worth while to investigate the Mödling population
+statistically, and to breed from the intermediates which might
+not impossibly prove to be heterozygotes. There are also records
+of such intermediates being occasionally found in some parts of
+Ireland, in the north of Scotland, and in south Wales,<a name="FNanchor_19_123" id="FNanchor_19_123"></a><a href="#Footnote_19_123" class="fnanchor">[19]</a>
+but I do not know of any regular colony of these forms. We can scarcely
+avoid the inference that one or more of the factors which make
+up <i>bryoniae</i> may be carried by these intermediates. It is not
+<span class="pagenum"><a name="Page_179" id="Page_179">[Pg 179]</a></span>
+clear why their interbreeding does not produce actual <i>bryoniae</i>
+occasionally. If this occurred, the probability is that the fact
+would be known to collectors, at least in the British localities.
+The absence of true <i>bryoniae</i> must, I think, be taken to mean
+that some essential factor is absent from these intermediates.</p>
+
+<p class="indent">To sum up the evidence, the facts that are clear
+may be thus enumerated:</p>
+
+<p class="blockquot">1. <i>Napi</i> and <i>bryoniae</i>, or in the
+Italian Alps, <i>napaeae</i> and <i>bryoniae</i> frequently meet each other.</p>
+
+<p class="blockquot">2. They cross without difficulty, producing fertile offspring.</p>
+
+<p class="blockquot">3. But in the levels at which they overlap there is no
+intermediate population, and only occasional intermediate individuals.</p>
+
+<p class="blockquot">4. In certain parts of the distribution of <i>napi</i>
+similar intermediates sometimes occur, and at one place (Mödling) they are
+so frequent as apparently to constitute a colony.</p>
+
+<p class="blockquot">5. As to the genetic relations of the two forms there
+is no complete certainty. Indications of segregation have been observed
+in some cases, but there are several factors concerned and
+they are liable to some disintegration.</p>
+
+<p class="indent">Another form in which I tried to investigate the same
+problem is <i>Coenonympha arcania</i>, which has one Alpine form known as
+<i>darwiniana</i>, and another, <i>satyrion</i>. In calling <i>satyrion</i>
+a form of <i>arcania</i> I follow Staudinger and other authorities, but I
+have never been quite satisfied that it should be so regarded. The
+differences between <i>arcania</i> and <i>darwiniana</i> are essentially
+differences of degree; <i>C. arcania</i> occurs in places where there is
+cover, and reaches up the valleys usually as high as the mixed
+woods of deciduous trees, which is about 2,500 feet. The variety
+<i>darwiniana</i>, on the contrary, is an insect of treeless hillsides,
+and I regard it as a dwarf and possibly a stunted form. It would not
+greatly surprise me to find that with the application of good
+conditions <i>arcania</i> could be raised from <i>darwiniana</i> eggs, or that
+if <i>arcania</i> larvae were starved they might give rise to <i>darwiniana</i>
+butterflies. I have been unsuccessful in trying to rear the species,
+having lost the larvae by disease. Usually one does not catch
+<i>arcania</i> and <i>darwiniana</i> on the same ground, and as <i>Festuca ovina</i>&mdash;a
+typically hill-side grass&mdash;is a common food-plant of <i>darwiniana</i>
+<span class="pagenum"><a name="Page_180" id="Page_180">[Pg 180]</a></span>
+there can be little doubt that <i>arcania</i> feeds on some other
+grass, probably woodland species. Colonies of <i>arcania</i> of varying
+size and brightness are commonly found, and though a sample of
+<i>arcania</i>, finely grown, from a warm Italian wood, presents a
+striking contrast with <i>darwiniana</i> from an Alpine pasture, one
+certainly may get samples which fill all the gradations. Generally
+the sample from a given locality is fairly homogeneous.</p>
+
+<p class="indent">Of <i>satyrion</i> I have little personal experience. I
+only twice found it, namely at Zinal, and at Hallstatt in Austria, but it
+occurs at Zermatt, Arolla, and in several Swiss localities above
+5,000 feet, and I understand that it is the typical Alpine form in
+the Engadine. With its darkened colour and reduced size it
+might well be expected to be a still further stunted form of
+<i>darwiniana</i>. Yet I have never found the one succeed to the
+other at the higher levels. If <i>darwiniana</i> appears when Alpine
+conditions are reached in a valley it will be met with up to the
+highest level at which such butterflies live. Tutt was of opinion
+that <i>satyrion</i> is a distinct species.<a name="FNanchor_20_124" id="FNanchor_20_124"></a><a href="#Footnote_20_124" class="fnanchor">[20]</a>
+I once, at the top of the
+Vorderrheinthal caught a sample of <i>darwiniana</i> a few of which (males)
+were so dark and had the eye spots so poorly developed that they
+looked like transitions to <i>satyrion</i>. Otherwise I never found
+any such transitional forms and they are certainly exceptional.
+There is further a record<a name="FNanchor_21_125" id="FNanchor_21_125"></a><a href="#Footnote_21_125" class="fnanchor">[21]</a>
+of <i>satyrion</i> having been taken flying
+with <i>arcania</i>. This was near Susa, at about 2,000 feet I infer.
+Mr. H. E. Page has similar specimens from Caud and from St.
+Anton (Arlberg). The females, however, both of mine and of
+Mr. Page's samples are a pale brown, quite unlike the females
+both of <i>arcania</i> and of the dark Zinal <i>satyrion</i>. The difficulty
+thus raised has not I think yet been considered by the authorities,
+and it is possible that the Alpine forms of <i>arcania</i> are in reality
+three, not two.</p>
+
+<p class="indent">The evidence taken together suggests, I think, that <i>darwiniana</i>
+is related to <i>arcania</i> much as so many of the Alpine varieties
+<span class="pagenum"><a name="Page_181" id="Page_181">[Pg 181]</a></span>
+of plants are to the well-developed individuals of the lower
+levels. I do not anticipate that factorial differences will be
+found in these insects, and it is by no means impossible that the
+distinctions between them are the direct consequences of altered
+conditions. The relations of <i>arcania</i> to <i>satyrion</i> are more
+doubtful, and in that case a factorial difference may at least be suspected.</p>
+
+<p class="indent">The species of the genus <i>Setina</i> have Alpine forms
+which agree in possessing a characteristic extension of the black pigment
+to form radiating junctions between the spots on the wings.
+Speyer, who discussed the interrelations of these forms in detail,<a name="FNanchor_22_126" id="FNanchor_22_126"></a><a href="#Footnote_22_126" class="fnanchor">[22]</a>
+lays stress on the absence of genuine transitional forms between
+<i>aurita</i> and the variety <i>ramosa</i>. Both are mountain insects but
+<i>ramosa</i> extends to levels higher than that at which <i>aurita</i>
+ceases, which is about 4,000 feet. The two forms are often found flying
+together. Speyer says that his brother searched diligently for
+transitional forms at the level of overlapping, but found none,
+so that at least they may be regarded as rare. The variety
+<i>ramosa</i> is not infrequent at much lower levels (<i>e. g.</i>,
+Chiavenna, 1,020 feet; Reussthal, 1,500 feet) and extends as high as
+the permanent snows. In the British Museum collection, however,
+I have seen several that I should regard as transitional. Speyer
+perhaps would have classed as <i>ramosa</i> all in which the spots of
+the central field were united, and it is by no means unlikely that
+breeding would prove such individuals to be heterozygous.<a name="FNanchor_23_127" id="FNanchor_23_127"></a><a href="#Footnote_23_127" class="fnanchor">[23]</a>
+<span class="pagenum"><a name="Page_182" id="Page_182">[Pg 182]</a></span></p>
+
+<p class="indent">There can scarcely be a doubt that the distinction between
+<i>aurita</i> and <i>ramosa</i> is factorial, the radiate <i>ramosa</i> probably
+having the factor for striping. In support of this view may be mentioned
+the observation of Boisduval,<a name="FNanchor_24_128" id="FNanchor_24_128"></a><a href="#Footnote_24_128" class="fnanchor">[24]</a>
+respecting a gynandromorphous individual, which was <i>aurita</i> male on one
+side, and <i>ramosa</i> female on the other. Speyer makes another excellent
+comment. He points out that the simple notion that the radiation
+is a mere extension of pigmentation consequent on the
+climate of the higher levels, will not fit the facts very easily,
+because the size of the spots varies greatly in <i>aurita</i> itself at
+any level, and lowland specimens may actually have more black
+confined to the spots alone than some <i>ramosa</i> possess on spots
+and lines combined.<a name="FNanchor_25_129" id="FNanchor_25_129"></a><a href="#Footnote_25_129" class="fnanchor">[25]</a>
+</p>
+
+<p class="indent">The two Salamanders, <i>S. maculosa</i> and its Alpine
+form <i>atra</i>, might not improbably furnish evidence bearing on the same
+problem. The two are of course very distinct, not merely in
+colour (<i>maculosa</i> being spotted with yellow or orange while <i>atra</i>
+is entirely black) but also in the mode of reproduction, a feature
+to which reference will be made in the next chapter. I cannot,
+however, find any evidence as to the overlapping of the two forms.
+<i>S. atra</i> occurs from about 3,000 feet or somewhat less, and reaches
+great elevations in the Eastern Alps, but I do not know if the
+two forms ever occur in the same localities. Leydig,<a name="FNanchor_26_130" id="FNanchor_26_130"></a><a href="#Footnote_26_130" class="fnanchor">[26]</a> Boulenger,<a name="FNanchor_27_131" id="FNanchor_27_131"></a><a href="#Footnote_27_131" class="fnanchor">[27]</a>
+and most modern authorities regard the two types as distinct
+species, but they are in any case closely allied, and it would be of
+interest to have exact knowledge of their geographical delimitations.</p>
+
+<p class="indent">The reader who has considered the cases adduced will
+appreciate the difficulties which must be faced in any attempt to
+<span class="pagenum"><a name="Page_183" id="Page_183">[Pg 183]</a></span>
+account for the facts in a rational way. As always in a problem
+of Evolution, two separate questions have to be answered.
+First how did the form under consideration come into existence,
+and secondly, how did it succeed in maintaining itself so as to
+become a race? The evidence from the local forms, though very
+far from giving complete answers to either of these questions
+definitely refutes the popular notion that a new race comes into
+existence by transformation of an older race. If a gradual mass-transformation
+of this kind took place we should certainly expect
+that when two types, nearly allied and capable of interbreeding,
+overlap each other in their geographical distribution, a normally
+intermediate population would exist. If each type can maintain
+itself, and if each came into existence by gradual transformation,
+then there must have been an intermediate capable of
+existing and maintaining itself as a population; and if this had
+ever been, surely in the region of overlapping, that intermediate
+population should continue. Especially should such a population
+be found when the two extreme types are adaptational forms and
+the region of overlap is a region of intermediate conditions.
+But of the examples we have examined there is only one, that of
+<i>Pararge egeria</i> and <i>egerides</i>, which can at all be so interpreted,
+and even in that case it is not impossible that more minute observation
+would reveal discontinuity between the extremes
+and the admittedly normal intermediate population. Granting
+provisionally however that this example, as it stands, is consistent
+with the conventional theory of evolution, I know not
+where we should look for another case equally good. When the
+distinctions are produced by direct influence of conditions operating
+during the lifetime of the individuals, examples of intermediate
+populations occupying the areas of intermediate conditions
+can no doubt be produced. Many turf-like Alpine
+plants, for instance, if protected from exposure and properly
+nourished can grow as large as those of the same species found in
+the valleys, and in the case of such quantitative effects, intermediate
+conditions can doubtless produce intermediate characters.</p>
+
+<p class="indent">Even these examples however are not very abundant,
+and often the intermediate locality has not a form intermediate
+<span class="pagenum"><a name="Page_184" id="Page_184">[Pg 184]</a></span>
+between those of the two extreme localities, but some third
+form distinct from either. This is the case for instance in the
+fauna of brackish waters. We are taught to believe that the
+fresh water fauna was evolved from the marine fauna, which
+it well may have been; but as students of Crustacea and Mollusca
+know familiarly, the brackish water forms are not as a rule intermediates
+between fresh water species and sea species, but more
+usually they are special forms belonging to the brackish waters,
+with the peculiar property that they can tolerate a great range of
+conditions, and live without ostensible variation in waters of
+most various compositions and densities, which very few marine
+or fresh water species are able to do.</p>
+
+<p class="indent">Sometimes the distinction between local races, as in <i>Rhamphocoelus
+passerinii</i> and <i>icteronotus</i> may be regarded with confidence
+as due to one simple Mendelian factor possessed by one
+race and absent from the other, but I think, more often, as in
+<i>Colaptes</i> or in the varieties of <i>Pieris napi</i>, the existence of
+several distinct factors is to be inferred. As we have seen, the races
+of <i>Colaptes</i> show almost beyond doubt that in different areas at
+least three distinct factorial combinations can be perpetuated as races.</p>
+
+<p class="indent">In the distribution of variability we find, I think,
+some hint as to the steps by which the phenomena under consideration
+have come to their present stage, and I am disposed to regard
+the facts so well attested in the case of our own melanic moths
+as a true indication of the process. Following this indication
+we should regard the change in the character of a population
+as beginning sporadically, by the appearance of varying individuals,
+possibly only one varying individual, in, it may be, one
+place only. As to <i>why</i> a variety should increase in numbers we
+have nothing but mere speculation to offer, and for the present
+we must simply recognise the fact that it may. That such survival
+and replacement may reasonably be taken as an indication
+that the replacing race has some superior power of holding its
+own I am quite disposed to admit. Nevertheless it seems in
+the highest degree unlikely that the outward and perceptible
+character or characters which we recognise as differentiating the
+<span class="pagenum"><a name="Page_185" id="Page_185">[Pg 185]</a></span>
+race should be the actual features which contribute effectively
+to that result.</p>
+
+<p class="indent">In discussions of geographical distribution in relation
+to problems of origin it is generally said that very nearly allied
+species usually occupy distinct areas, while other competent
+observers state the exact contrary. Lately, for example, Dr.
+R. G. Leavitt<a name="FNanchor_28_132" id="FNanchor_28_132"></a><a href="#Footnote_28_132" class="fnanchor">[28]</a>
+has published an important collection of evidence
+upholding the latter proposition, taken chiefly from the botanical
+side, showing how in numerous genera two or more closely allied
+species coexist, frequently without intermediates, in the same
+localities, and may even be thus found in company throughout
+their distribution. The difference of opinion evidently arises
+from a confusion as to the sense in which the term "species"
+is understood and applied. Leavitt, for example, is avowedly
+following Jordan and, among moderns, Sargent, in applying
+a close analysis, and denoting as species all forms which are
+distinct and breed true. Against this use of the term I know
+no valid objection<a name="FNanchor_29_133" id="FNanchor_29_133"></a><a href="#Footnote_29_133" class="fnanchor">[29]</a>
+but it must be obvious that if others follow
+a different practice confusion may result when observations are
+summarised in general statements. We will consider this subject
+again in another place, but here it may be sufficient to say that
+there can scarcely now be a doubt that numbers of these associated
+species, such as Jordan discriminated, represent various
+combinations of the presence and absence of Mendelian factors.
+This does not in any way weaken the argument which Leavitt
+founds upon the facts, namely, that the observed distribution
+of these forms is consistent with the supposition of an evolution
+largely discontinuous.</p>
+
+<p class="indent">On the other hand, those who have come to the opinion
+that nearly allied species generally occupy distinct ground are
+presumably more impressed by the characters differentiating the
+geographically distinct or adaptational races, seeing that genuine
+intermediates between them are less commonly found. Those
+geographical races may no doubt contain various differentiated
+forms; but when all live together, occasional intermediates are
+<span class="pagenum"><a name="Page_186" id="Page_186">[Pg 186]</a></span>
+usually to be found even in the case of characters habitually
+segregating. These segregating forms Jordan would certainly
+have determined as species, and it must be conceded that no
+physiological definition has yet been drawn which consistently
+excludes them.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_187" id="Page_187">[Pg 187]</a></span></p>
+
+<h2>CHAPTER IX</h2>
+<h3>THE EFFECTS OF CHANGED CONDITIONS: ADAPTATION</h3>
+
+<p class="indent">In the attempt to conceive a process by which
+Evolution may have come about, the first phenomenon to be recognized
+and accounted for is specific difference. With that recognition
+the outline of the problem is defined. The second prerogative
+fact is adaptation. Forms of life are <i>on the whole</i> divided into
+species, and these species <i>on the whole</i> are adapted and fit the
+places in which they live. To many students of Evolution,
+adaptation has proved so much more interesting and impressive
+than specific diversity that they have preferred it to the first
+place in their considerations.</p>
+
+<p class="indent">Whether this is, as I believe, an inversion of the
+logical order or not, there is one most serious practical objection to
+such preference, that whereas specific diversity is a subject which
+can be investigated both by the study of variation and by the
+analytical apparatus which modern genetic science has developed,
+we have no very effectual means of directly attacking the problems
+of Adaptation.</p>
+
+<p class="indent">The absence of any definite progress in genetics in
+the last century was in great measure due to the exclusive prominence
+given to the problem of Adaptation. Almost all debates on
+heredity centered in that part of the subject. No one disputes
+that the adaptation of organisms to their surroundings is one
+of the great problems of nature, but it is not the primary problem
+of descent. Moreover, until the normal and undisturbed course
+of descent under uniform conditions is ascertained with some
+exactness, it is useless to attempt a survey of the consequences
+of external interference; nor as a rule can it be even possible to
+decide with much confidence whether such interferences have or
+have not definite consequences. Those, for example, who debated
+with enthusiasm whether acquired characters are or are
+not transmitted were constantly engaged in discussing occurrences
+<span class="pagenum"><a name="Page_188" id="Page_188">[Pg 188]</a></span>
+which we now know to be ordinary features of descent
+under uniform conditions, and the origin of variations which
+were certainly not caused directly by circumstances at all. In
+the absence of any factorial analysis, or of any conception of what
+factorial composition means and implies, no one knew what
+varieties might be expected from given parents. The appearance
+of any recessive variety was claimed as a consequence of some
+treatment which might have been applied to the parents. There
+was no possible standard of evidence or means of controlling it,
+and thus the discussion was singularly unfruitful. Before we can
+tell how the course of descent has departed from the normal, we
+must know what the normal would have been if we had let alone.
+We are still far from having such knowledge in adequate measure,
+but it does now exist in some degree, and we are steadily approaching
+a position from which we shall be able to form fairly sound
+estimates of the true significance of evidence for or against the
+proposition that environmental treatment can produce positive
+disturbances in the physiological course of descent.</p>
+
+<p class="indent">Thus described, the field for consideration is very
+wide. Though the effects of changed conditions were especially studied
+in the hope of solving the problem of adaptation by direct observation,
+that, as all are now agreed, is but a part of a more
+general question. We must ask not only do changed conditions
+produce an <i>adaptative</i> response on the part of the offspring, but
+whether they produce any response on the part of the offspring
+at all. It is not in doubt that by violent means, such as starvation
+or poisoning of the reproductive cells, effects of a kind, stunting
+and deformity for instance, can be made evident, just as similar
+effects may follow similar treatment during embryonic or larval
+life. Apart from interferences of this class, are there any that
+may be reasonably invoked as modifying the course of inheritance?</p>
+
+<p class="indent">No epitome of the older evidence for the inheritance
+of adaptative changes is here required. That has often been collected,
+especially by Weismann, who exposed its weaknesses so
+thoroughly as to carry conviction to most minds, and showed
+that whether the phenomenon occurs or not, no one can yet prove
+<span class="pagenum"><a name="Page_189" id="Page_189">[Pg 189]</a></span>
+that it does. Belief in these transmissions, after being almost
+universally held, was with singular unanimity abandoned. This
+change in opinion, though doing credit to the faith of the scientific
+community in evidential reasoning, is the more remarkable
+inasmuch as the strength of the idea was not derived from the
+minute amounts of supposed facts now demolished. On the
+contrary, it was really an instinctive deduction from a wide
+superficial acquaintance with the properties of animals and
+plants. They <i>can</i> accommodate themselves to circumstances.
+They <i>do</i> make responses sometimes marvellously appropriate
+to demands for which they can scarcely have been prepared.
+What more natural than to suppose that the permanent adaptations
+have been achieved by inherited summation of such responses?
+No one had actually been driven to believe in the
+inheritance of adaptative changes because bitches which had
+been docked had been known to give birth to tailless puppies,
+or because certain wheat in Norway was alleged to have become
+acclimatized in a few generations. Evidence of this kind was
+collected and produced rather as an ornamental appendix to a
+proposition already accepted, and held to be plainly demonstrated
+by the facts of nature. Looked at indeed in that preliminary
+and uncritical way, the case is simply overwhelming.
+Those who desire to see how strong it is should turn to Samuel
+Butler's <i>Life and Habit</i>, and even if in reading they reiterate to
+themselves that no experimental evidence exists in support of
+the propositions advanced, the misgiving that none the less they
+may be true is likely to remain. Making every deduction for
+the fact that the wonders of adaptation have been grossly exaggerated,
+and that marvels of fitness and correspondence between
+means and ends have grown out of mere anthropomorphic
+speculations, there is much more left to be accounted for than
+can at all comfortably be accepted as the product of happy
+accidents. So oppressive are these difficulties that we can scarcely
+blame those who imagine that the study of heredity is primarily
+directed to the problem of the transmission of acquired characters,
+a preconception still almost universal among the laity.
+<span class="pagenum"><a name="Page_190" id="Page_190">[Pg 190]</a></span></p>
+
+<p class="indent">But since the belief in transmission of acquired
+adaptations arose from preconception rather than from evidence, it is
+worth observing that, rightly considered, the probability should surely
+be the other way. For the adaptations relate to every variety
+of exigency. To supply themselves with food, to find it, to seize
+and digest it, to protect themselves from predatory enemies
+whether by offence or defence, to counter-balance the changes
+of temperature, or pressure, to provide for mechanical strains,
+to obtain immunity from poison and from invading organisms,
+to bring the sexual elements into contact, to ensure the distribution
+of the type; all these and many more are accomplished
+by organisms in a thousand most diverse and alternative methods.
+Those are the things that are hard to imagine as produced by
+any concatenation of natural events; but the suggestions that
+organisms had had from the beginning innate in them a power
+of modifying themselves, their organs and their instincts so as
+to meet these multifarious requirements does not materially
+differ from the more overt appeals to supernatural intervention.</p>
+
+<p class="indent">The conception, originally introduced by Hering and
+independently by S. Butler, that adaptation is a consequence or
+product of accumulated <i>memory</i> was of late revived by Semon
+and has been received with some approval, especially by F.
+Darwin. I see nothing fantastic in the notion that memory
+may be unconsciously preserved with the same continuity that
+the protoplasmic basis of life possesses. That idea, though
+purely speculative and, as yet, incapable of proof or disproof
+contains nothing which our experience of matter or of life at all
+refutes. On the contrary, we probably do well to retain the
+suggestion as a clue that may some day be of service. But if
+adaptation is to be the product of these accumulated experiences,
+<i>they must in some way be translated into terms of physiological and
+structural change</i>, a process frankly inconceivable.</p>
+
+<p class="indent">To attempt any representation of heredity as a
+product of memory is, moreover, to substitute the more obscure for the
+less. Both are now inscrutable; but while we may not unreasonably
+aspire to analyse heredity into simpler components by ordinary
+methods of research, the case of memory is altogether
+different. Memory is a mystery as deep as any that even
+<span class="pagenum"><a name="Page_191" id="Page_191">[Pg 191]</a></span>
+psychology can propound. Philosophers might perhaps encourage
+themselves to attack the problem of the nature of memory by
+reflecting that after all the process may in some of its aspects
+be comparable with that of inheritance, but the student of genetics,
+as long as he can keep in close touch with a profitable basis
+of material fact, will scarcely be tempted to look for inspiration
+in psychical analogies.</p>
+
+<p class="indent">For a summary of the recent evidence I may refer the reader
+to Semon's paper<a name="FNanchor_1_134" id="FNanchor_1_134"></a><a href="#Footnote_1_134" class="fnanchor">[1]</a>
+where he will find a collection of these
+observations described from the standpoint of a convinced believer.
+At the outset one cannot help being struck by the fact that of
+the instances alleged, very few, even if authentic, show the transmission
+of acquired modifications which can in any sense be regarded
+as adaptative, and many are examples not so much of a
+transmission of characters produced in the parents as of variation
+induced in the offspring as a consequence of treatment to which
+the parents were submitted, the parents themselves remaining
+apparently unmodified. No one questions the great importance
+of evidence of this latter class as touching the problem of the
+causes of variation, but it is not obvious why it is introduced in
+support of the thesis that acquired characters are inherited.</p>
+
+<p class="indent">It is most difficult to form a clear judgment of the
+value of the evidence as a whole. To doubt the validity of testimony
+put forward by reputable authors is to incur a charge of obstinacy
+or caprice; nevertheless in matters of this kind, where the alleged
+phenomena are, if genuine, of such exceptional significance, belief
+should only be extended to evidence after every possible source
+of doubt has been excluded. We believe such things when we
+must, but not before. At the very least we are entitled to require
+that confirmatory evidence should be forthcoming from independent
+witnesses. So far as I have seen, this requirement is
+satisfied in scarcely any of the examples that have been lately
+published, and until it is, judgment may reasonably be suspended.
+<span class="pagenum"><a name="Page_192" id="Page_192">[Pg 192]</a></span></p>
+
+<p class="indent">In some cases, however, the facts are not doubtful. Standfuss,
+by subjecting pupae of <i>Vanessa urticae</i> to cold, produced
+the now well-known temperature-aberrations in which the dark
+pigment is greatly extended. He put together in a breeding-cage
+32 males and 10 females showing this modification in various
+degrees. Two of these females died without leaving young.
+Seven produced exclusively normal offspring. From the eighth
+female 43 butterflies were bred, and of these there were four (all
+males) which to a greater or less extent exhibited the aberrational
+form.<a name="FNanchor_2_135" id="FNanchor_2_135"></a><a href="#Footnote_2_135" class="fnanchor">[2]</a>
+The mother of this family was the most abnormal of the
+10 females originally put in.</p>
+
+<p class="indent">Fischer's experiment with <i>Aretia caja</i> was on
+similar lines. From pupae which had been frozen almost all the moths
+which emerged showed aberrational markings. A pair of these mated
+and produced 173 young which pupated. Those which emerged
+early were all normal, but of those which emerged late, 17 had
+in various degrees abnormal markings like those of the parents.<a name="FNanchor_3_136" id="FNanchor_3_136"></a><a href="#Footnote_3_136" class="fnanchor">[3]</a>
+In neither of these examples is there any question as to the facts.
+Both observers have great experience and give full details of their work.</p>
+
+<p class="indent">As regards <i>Vanessa urticae</i>, however, it must be
+recalled that Fischer himself showed that in Nymphalids somewhat similar
+aberrations could be produced both by heat and by cold, and
+even by centrifuging the pupae. Frl. von Linden produced a
+transitional form of the same aberration in <i>V. urticae</i> by the
+action of carbonic acid gas.<a name="FNanchor_4_137" id="FNanchor_4_137"></a><a href="#Footnote_4_137" class="fnanchor">[4]</a>
+It is highly probable that the appearance
+is due to a morbid change, perhaps an arrest of development,
+which may be brought about by a great diversity of causes.
+In the experiments the cause probably was a diseased condition
+of the tissues of the mother herself. She had been subjected to
+freezing sufficiently severe to prevent the proper development of
+the pigments and some of the ovarian cells presumably suffered
+also. It will be observed that the only specimens which were
+affected were the offspring of the most abnormal female, and of
+them only four out of forty-three showed any change.
+<span class="pagenum"><a name="Page_193" id="Page_193">[Pg 193]</a></span></p>
+
+<p class="indent">The same interpretation probably applies to the cases
+in <i>Arctia caja</i>. In this species the markings are well known to
+be liable to great variation. As Barrett says, even in nature
+individuals are rarely quite alike, and an immense number of
+strange forms occur in collections.<a name="FNanchor_5_138" id="FNanchor_5_138"></a><a href="#Footnote_5_138" class="fnanchor">[5]</a>
+These are greatly sought after by some collectors, especially in England,
+where they fetch high prices at auctions, and it is notorious that most of
+them come from Lancashire and the West Riding of Yorkshire. It is commonly
+supposed that the breeders of that district subject them to
+abnormal conditions, and especially to unnatural feeding, but
+I know no clear evidence that this is true. From whatever cause
+it is certain that the natural pattern is, in some strains at all
+events, very easily disturbed.</p>
+
+<p class="indent">The elaborate experiments of Schröder with
+<i>Abraxas grossulariata</i> are difficult to follow and are complicated
+by the fact that the series which was submitted to abnormal temperatures
+was derived from an abnormal original pair. From the evidence
+given it is not clear to me whether the temperature had a distinct
+effect. This insect, like <i>Arctia caja</i>, produces an immense number
+of variations (especially in the amount of the black pigment)
+and as most of these are, I believe, reared in domestication for
+sale, it is highly probable that the species is easily influenced
+by cultural conditions.</p>
+
+<p class="indent">Schröder describes two other experiments which have
+been accepted by Semon and other supporters of the view that acquired
+characters are transmitted. In the first, <i>Phratora vitellinae</i>, a
+phytophagous beetle living on the undersides of leaves, was used.
+It naturally feeds on <i>Salix fragilis</i>, a species without a felt, or
+tomentum, on the underside of the leaves. Larvae were transferred
+to another willow (near <i>S. viminalis</i>) which has the undersides
+of the leaves felted. The larvae took readily to the new
+food, pushing the tomentum before them as they gnawed the
+leaves. They came to maturity and when they were about to
+lay their eggs they were given a free choice between <i>S. fragilis</i>
+and the tomentose species. The greater number of ovipositions,
+<span class="pagenum"><a name="Page_194" id="Page_194">[Pg 194]</a></span>
+219, took place on <i>fragilis</i>, and there were 127 on the tomentose
+bush, which we are told was six times as large as the <i>fragilis</i>.
+The larvae from <i>fragilis</i> were next put on the tomentose species
+and reared on it. When they became imagines they were similarly
+given their choice, with the result that there were 104
+ovipositions on the tomentose species and only 83 on <i>fragilis</i>.
+In the next generations there were 48 ovipositions on the tomentose
+and 11 on <i>fragilis</i>. Finally the fourth generation made
+15 ovipositions on the tomentose and none on <i>fragilis</i>.</p>
+
+<p class="indent">The difficulty about such experiments is obviously that
+one has no assurance that the change of instinct, in so far as there
+is any, may not be a mere consequence of the captivity. It
+must, besides, be extremely difficult to arrange the experiment so
+that there is really an equal choice between the two bushes, when
+one stands beside the other. Przibram, in quoting this case,
+considers that as the tomentose bush was about six times as
+large as the <i>fragilis</i>, some indication of the relative attractiveness
+of the two may be obtained by dividing the ovipositions on the
+larger bush by six, but I imagine the matter must be much more complex.</p>
+
+<p class="indent">Schröder's second example is not more convincing,
+in my opinion, though Semon regards it as one of the most important
+pieces of evidence. It concerns a leaf-rolling moth, <i>Gracilaria
+stigmatella</i>, the larva of which is said normally to make its house
+by bending over the <i>tips</i> of the sallow leaves on which it feeds.
+Schröder placed larvae on leaves from which the tips had been
+cut, and these larvae made their houses by rolling over the <i>sides</i>
+of the leaves. Their offspring were again fed on leaves without
+tips, and as before, they rolled in the leaf-margins either on one
+side or both. The offspring of this second generation were then
+fed on entire leaves. There were 19 houses made by these (?19)
+larvae, and of them 15 were normal, made by folding down the
+tips of the leaves, while 4 were abnormal, made by rolling in the
+leaf-margins. Schröder says that in nature he has only twice
+seen abnormal houses; but it is clearly essential not only that the
+frequency of such variability in nature should be thoroughly
+examined, but also that we should know whether when the species
+<span class="pagenum"><a name="Page_195" id="Page_195">[Pg 195]</a></span>
+is bred in captivity these irregularities of behaviour do or do not
+occur when the larvae are fed on uninjured leaves.</p>
+
+<p class="indent">The famous case of Schübeler's wheat is revived by Semon.
+The story will be familiar to most readers of the literature of the
+subject. Briefly it is that annuals, especially wheat and maize,
+raised from seed in Central Europe take more time in coming
+to maturity and ripening than similar plants raised in Norway,
+where the summer days are much longer. The received account
+is that he imported seed especially of maize and of wheat from
+Central Europe to Norway and found that in successive years
+the period of growth and ripening was increasingly reduced.
+After two generations seed of the accelerated wheat was sent
+back to Breslau where it was grown, and was found to ripen rather
+more slowly than in Norway, but much more quickly than the
+original stock had done. The facts recorded by Schübeler<a name="FNanchor_6_139" id="FNanchor_6_139"></a><a href="#Footnote_6_139" class="fnanchor">[6]</a>
+are that he received seed from Eldena, which is on the Baltic near
+Greifswald. The variety is described as "<i>100 tägiger Sommer
+Weizen</i>," but no more exact record of its behaviour in Germany
+is given. This wheat, grown at Christiania in 1857, took 103
+days to harvest. Its seed was again grown in Christiania in 1858,
+and took 93 days, and sown again in 1859 it took only 75 days, 28
+days less than in the first year of cultivation in Norway. Seed of
+the 1858 crop was sent to Breslau, and grown there by Roedelius
+in 1859; it took 80 days. Evidently before such a record can be
+used as proving an inheritance of acquired characters numbers of
+particulars should be forthcoming. The view that Johannsen
+has taken is that the result was probably due to unconscious
+selection of the earlier individuals among a population consisting
+of many types of various compositions. Some effect may no
+doubt be ascribed to that cause, but I cannot think that alone
+it would account for the results. My impression is rather that
+they were produced by differences in the cultivation and especially
+in the seasons. Research of an elaborate character would be
+necessary in order to eliminate the various sources of error, and
+nothing of the kind has been done; nor does Semon allude to these
+difficulties in prominently adducing Schübeler's evidence. A
+<span class="pagenum"><a name="Page_196" id="Page_196">[Pg 196]</a></span>
+difference of even three weeks in time of harvesting may easily
+be due to variation in the season. It would in any case be difficult
+to analyse the meteorological conditions, and to decide how
+much effect in postponing or accelerating the harvest might be
+due to cold days, to cloudy days, to wet weather, to fluctuations
+in average temperature, to hot days, and other such incidents
+occurring at the different periods of growth, even if they were
+specially watched while the experiments were in progress, and
+at this distance of time such analysis is practically impossible.
+Without careful simultaneous control-experiments this evidence
+is almost worthless. The director of the Meteorological
+Office<a name="FNanchor_7_140" id="FNanchor_7_140"></a><a href="#Footnote_7_140" class="fnanchor">[7]</a>
+has, however, kindly sent me some details of the weather
+at Breslau from 1857 to 1860, and I notice that as a matter of
+fact July, 1859, was an exceptionally hot month, <i>having an average
+of 2.67° C. above the mean</i> for the twenty years 1848-1867. June
+in that year was slightly (0.31° C.) below the mean and May
+slightly above it (0.18° C.). August was also abnormally hot,
+2.35° C. above the average. The Breslau wheat was sown on
+<i>May 19</i> and harvested on August 6. There was a cold spell from
+May 11 to 14, which this wheat escaped, as it was sown on May
+19. In the other years the cold spell came much later. These
+elements of the weather may possibly have done something to
+hurry the ripening in 1859. It unfortunate that we are not
+told how long similar wheat from Breslau seed took to ripen in
+that year.</p>
+
+<p class="indent">As regards the Norway cultivations we have the
+average monthly temperatures recorded by Schübeler, though he does
+not discuss them in connection with this special problem. It is
+quite clear that 1857, in which the period was 103 days, was an
+exceptionally cold summer, especially as regards the months of
+June and July, but though there was, so far as the temperature
+<span class="pagenum"><a name="Page_197" id="Page_197">[Pg 197]</a></span>
+records go, no great difference between 1858 and 1859, the year
+1859, in which the period of ripening was the shortest, was somewhat
+colder in Norway than 1858. But we have the further
+difficulty that there were ten days difference in sowing, for in
+1858 the sowing was made on May 14, and in 1859 on May 24.
+With all these possibilities uncontrolled, and indeed unconsidered,
+I am surprised that Semon should claim these experiments as one
+of the chief supports for his views.</p>
+
+<p class="indent">Schübeler's other allegations respecting the influence
+of climate on plants grown in various places and especially at different
+elevations in Norway have been destructively criticised by
+Wille<a name="FNanchor_8_141" id="FNanchor_8_141"></a><a href="#Footnote_8_141" class="fnanchor">[8]</a>
+to whose paper readers interested in the subject should refer.</p>
+
+<p class="indent">Before the appearance of Wille's criticisms Wettstein<a name="FNanchor_9_142" id="FNanchor_9_142"></a><a href="#Footnote_9_142" class="fnanchor">[9]</a>
+made a favourable reference to Schübeler's work, accepting his
+conclusion. He states also that he has himself made analogous
+experiments with flax, finding that the length of the period of
+development and a series of morphological characters show an
+adaptation to local conditions, and that on transference of seed
+to other conditions the previous effects are maintained. No
+details, however, are given, and I do not know if anything more
+on the subject has appeared since. The other examples cited
+by Wettstein, such as the observations of Cieslar on forest-trees
+and those of Jakowatz on gentians seem to me open to all the
+usual objections applicable to evidence of this kind. Such work,
+to be of any value for the purpose to which it is applied, must be
+preceded by a study of the normal heredity and of the variations
+of the species.</p>
+
+<p class="indent">Most of the recent writers (Semon, Przibram, etc.) on the
+inheritance of acquired characters accept the story of Brown-Séquard's
+guinea pigs, which are said to have inherited a liability
+to peculiar epileptiform attacks induced in their parents by various
+nervous lesions.</p>
+
+<p class="indent">The question has been often debated and several observers
+have repeated the experiments with varying results, some failing
+<span class="pagenum"><a name="Page_198" id="Page_198">[Pg 198]</a></span>
+to confirm Brown-Séquard, others finding evidence which in
+various degrees supported his conclusions. Recently a new and
+especially valuable paper has been published by Mr. T. Graham
+Brown<a name="FNanchor_10_143" id="FNanchor_10_143"></a><a href="#Footnote_10_143" class="fnanchor">[10]</a>
+which goes far towards settling this outstanding question.
+He states that "the Brown-Séquard phenomenon is nothing more
+or less than a specific instance of the scratch-reflex," and it is
+due to a raised excitability of the mechanism of this reflex. This
+raised excitability is the character acquired as a consequence,
+for instance, of the removal of part of one great sciatic nerve.
+The nature of this raised excitability and its causation are discussed
+and elucidated, but this part of the work is not essential
+to the present consideration. Mr. Graham Brown in his summary
+of conclusions remarks that it is very difficult to see how this
+condition of raised excitability can be transmitted to the offspring,
+and this comment which might be made in reference to any of
+the alleged cases certainly applies with special cogency to the
+present example.</p>
+
+<p class="indent">He then calls special attention to three observations:</p>
+
+<p class="blockquot">1. That guinea pigs which had a "trophic" change in the
+foot, as a result of division of the great sciatic nerve, have repeatedly
+been seen to nibble the feet of other guinea pigs which
+had this change in the foot from the same causes.</p>
+
+<p class="blockquot">2. That accidental injury to the toes may be followed
+by the Brown-Séquard phenomenon in an otherwise normal animal.</p>
+
+<p class="blockquot">3. That in several instances the young of guinea pigs
+which exhibited the phenomenon have been noticed to have one or more
+toes eaten off by the mother.</p>
+
+<p class="indent">Brown-Séquard noticed that almost all his animals
+in which the great sciatic was divided acquired the "epilepsy" and
+nibbled those parts of their feet in which sensation had been lost.
+Of the offspring of such animals he found that a very small proportion
+exhibited a malformation of the feet, and of these some
+showed the "epilepsy." The proportion which showed the
+"epilepsy" was one to two per cent. of the offspring.</p>
+
+<p class="indent">Morgan<a name="FNanchor_11_144" id="FNanchor_11_144"></a><a href="#Footnote_11_144" class="fnanchor">[11]</a>
+is quoted by Graham Brown as having suggested
+<span class="pagenum"><a name="Page_199" id="Page_199">[Pg 199]</a></span>
+that the loss of toes in the offspring may have been due to mutilation
+by the mother, following his experience in a case in which
+the tails of mice in succeeding litters were thus devoured, and
+there can be little doubt that in this suggestion lies the clue to
+the explanation of the whole mystery. Graham Brown concludes
+that it may be supposed with every degree of probability that
+the "transmission" was due to injuries inflicted upon the young
+by their parents. With this conclusion most people will now be
+disposed to agree, and we may hope that we shall hear the last
+of this curious myth&mdash;to the elucidation of which a vast
+quantity of research has been devoted.</p>
+
+<p class="indent">The series of experiments made by Kammerer with
+various Amphibia have attracted much attention and have been
+acclaimed by Semon and other believers in the transmission of
+acquired characters as giving proof of the truth of their views.
+With respect to these observations the chief comment to be made
+is that they are as yet unconfirmed. Many of the results that
+are described, it is scarcely necessary to say, will strike most
+readers as very improbable; but coming from a man of Dr.
+Kammerer's wide experience, and accepted as they are by Dr.
+Przibram, under whose auspices the work was done in the Biologische
+Vesuchsanstalt at Vienna, the published accounts are
+worthy of the most respectful attention.</p>
+
+<p class="indent">The evidence relates chiefly to three distinct groups of occurrences:</p>
+
+<p class="blockquot">1. Modification in <i>Alytes obstetricans</i>, the Midwife Toad,
+affecting both the structure and the mode of reproduction, induced
+by compulsory change of habits.</p>
+
+<p class="blockquot">2. Modification in the mode of reproduction of <i>Salamandra
+atra</i> and <i>maculosa</i> induced by compulsory change of habits.</p>
+
+<p class="blockquot">3. Modification in the colour of <i>Salamandra maculosa</i> induced
+by change in the colour of the soil on which the animals were kept.</p>
+
+<p class="indent">1. I will take first the case of <i>Alytes</i>,<a name="FNanchor_12_145" id="FNanchor_12_145"></a><a href="#Footnote_12_145" class="fnanchor">[12]</a>
+because it is the most definite example, and because it is the case
+<span class="pagenum"><a name="Page_200" id="Page_200">[Pg 200]</a></span>
+which most readily admits of repetition and verification.</p>
+
+<p class="indent">The habits of <i>Alytes obstetricans</i> are well
+known. The animals copulate on land. As the strings of eggs leave the
+female they are entangled by the hind legs of the male, and being
+adhesive they stick to him and undergo their development attached
+to his back and legs. The number of eggs varies from 18
+to 86, a number much smaller than is usual in toads and frogs
+which lay their eggs in water. The eggs are large and full of yolk.</p>
+
+<p class="indent">There are two breeding seasons, one about April and
+the other about September, and a winter hibernation. Not only animals
+brought in from outside, but their offspring reared in domestication
+maintain these normal habits in confinement, if the temperature
+does not exceed 17° C. (pp. 499 and 534).</p>
+
+<p class="indent">If, however, the temperature be artificially raised
+and kept at 25-30° C., the males do not attach the eggs to themselves
+when spawning occurs on land but let them lie. The adhesion
+of the eggs is said to be hindered by the comparatively rapid
+drying of their surfaces.</p>
+
+<p class="indent">More usually in the high temperatures the animals
+<i>take to the water</i> and copulate there. The eggs are ejected into
+the water, and as their gelatinous coverings immediately swell up,
+they do not stick to the males.</p>
+
+<p class="indent">The offspring thus derived from the parents subjected
+to heat for one breeding-period only, whether they were laid in water
+or on land, did not show departures from the normal type.</p>
+
+<p class="indent">Kammerer states next, however, that in subsequent
+breeding-periods the same parents frequently take to the water to
+breed, though they have become quite accustomed to the heated
+chamber; and furthermore that if such animals, having thus lost
+their instinct to brood their young, be transferred to ordinary
+temperatures they do not readily reassume their normal habits,
+but for several breeding seasons&mdash;at least four&mdash;will take to
+the water. These parents lay from 90 to 115 eggs, which are
+small and contain little yolk, and the larvae, on hatching, breathe
+with their embryonic gills until they are absorbed instead of
+being broken off as normally.
+<span class="pagenum"><a name="Page_201" id="Page_201">[Pg 201]</a></span></p>
+
+<p class="indent">The offspring thus abnormally developed when they
+mature are said never to brood their eggs. If they are derived from
+the earlier spawnings of their parents, before, that is to say, the
+parents had been submitted to the changed conditions long enough
+to transmit their effects, they lay on land; but if they are derived
+from the later spawnings, they lay in the water. These changes
+of habit are manifested without the continued application of
+the abnormal experimental conditions, and, as I understand the
+account, in normal conditions of temperature.</p>
+
+<p class="indent">If the abnormal experimental conditions are continued,
+the toads always lay in water, and their eggs become progressively
+smaller and more numerous. The larvae in the fourth generation
+acquire three pairs of gills instead of one pair, and are in other
+respects also different from the normal form.</p>
+
+<p class="indent">Respecting the <i>Alytes</i> bred in this way Kammerer makes
+the very striking statement that <i>the males in the third generation</i>
+(p. 535) <i>have roughened swellings on their thumbs and that in the
+fourth generation</i> (pp. 516 and 535) <i>these swellings develop black
+pigment</i>. Together with the appearance of this secondary sexual
+character there is hypertrophy of the muscles of the fore-arm.
+To my mind this is the critical observation. If it can be
+substantiated it would go far towards proving Kammerer's case.
+<i>Alytes</i>, among toads and frogs, is peculiar in that the males do
+not develop these lumps in the breeding season, and the fact
+may no doubt be taken to be correlated with the breeding habits,
+copulation occurring on land and not in water as is usual with
+Batrachians. It is to be expressly noticed that these lumps on
+the thumbs or arms of male toads and frogs are not merely pigmented
+swellings, but are pads bearing numerous minute horny
+black spines, which are used in holding the females in the water.
+The figures which Kammerer gives (Taf. XVI, figs. 26 and 26a)
+are quite inadequate, and as they merely indicate a dark patch
+on the thumbs it is not possible to form any opinion as to the
+nature of the structure they represent.</p>
+
+<p class="indent">The systematists who have made a special study of Batrachia
+appear to be agreed that <i>Alytes</i> in nature does not have these
+structures; and when individuals possessing them can be
+<span class="pagenum"><a name="Page_202" id="Page_202">[Pg 202]</a></span>
+produced for inspection it will, I think be time to examine the evidence
+for the inheritance of acquired characters more seriously.
+I wrote to Dr. Kammerer in July, 1910, asking him for the loan
+of such a specimen<a name="FNanchor_13_146" id="FNanchor_13_146"></a><a href="#Footnote_13_146" class="fnanchor">[13]</a>
+and on visiting the Biologische Versuchsanstalt
+in September of the same year I made the same request, but
+hitherto none has been produced. In matters of this kind much
+generally depends on interpretations made at the time of observation;
+here, however, is an example which could readily
+be attested by preserved material. I notice with some surprise
+that in a later publication on the same subject no reference to the
+development of these structures is made (see below).</p>
+
+<p class="indent">The statements here given represent but a small
+part of Kammerer's papers on the subject. He gives much further
+information as to the course of the experiments, especially in
+regard to the fate of the eggs laid on land and the aberrations
+induced in them by treatment. The ramifications of the experiments
+are, however, very difficult to follow, and as I am not
+sure that I have always understood them I must refer the reader
+to the original.</p>
+
+<p class="indent">More recently Kammerer has published<a name="FNanchor_14_147" id="FNanchor_14_147"></a><a href="#Footnote_14_147" class="fnanchor">[14]</a>
+a most curious account of experiments in crossing his modified and abnormal
+<i>Alytes</i>, derived from the water-eggs, with normal individuals.</p>
+
+<p class="indent">In the first case the cross was made between a
+<i>normal female</i> and an <i>abnormal male</i>. The offspring were normal
+in their habits. In the next generation bred from these almost exactly
+a quarter showed the abnormal instinct.</p>
+
+<p class="indent">The reciprocal cross was made between an <i>abnormal female</i>
+and a <i>normal male</i>. In this case the offspring were abnormal in
+their behaviour; but the second generation bred from them
+showed three quarters abnormal and one quarter normal.</p>
+
+<p class="indent">Certain details as to numbers and sexes of the various
+families bred in the course of this amazing experiment are given in a
+<span class="pagenum"><a name="Page_203" id="Page_203">[Pg 203]</a></span>
+subsequent publication.<a name="FNanchor_15_148" id="FNanchor_15_148"></a><a href="#Footnote_15_148" class="fnanchor">[15]</a>
+This later paper goes somewhat fully
+into the question of the difference in behaviour between the
+normal and modified individuals, describing the ways in which
+the males and females possessing the acquired character could be
+recognised from the males and females which were normal, but
+in this account I find no reference to the development of the
+"<i>Brunftschwielen</i>"&mdash;the horny pads on the hands of the males.
+As these structures would be of special value in such a diagnosis
+the omission of any allusion to them calls for explanation.
+Kammerer claims the evidence as proof of Mendelian segregation
+in regard to an acquired character, the first example recorded.
+Pending a repetition of the experiments there is no more to be said.</p>
+
+<p class="indent">2. <i>The Mode of Reproduction of Salamandra atra and maculosa.</i><a name="FNanchor_16_149" id="FNanchor_16_149"></a><a href="#Footnote_16_149" class="fnanchor">[16]</a>
+mdash;<i>Salamandra maculosa</i>, the common lowland form, with yellow
+bands or spots, deposits its young in water, generally as gill-bearing
+tadpoles, with a wide, swimming tail, though occasionally
+they are born still enclosed in the egg-capsule out of which they
+soon hatch. Spawning extends over a considerable period,
+often many weeks, and during the season one female may bear
+more than 50 young.</p>
+
+<p class="indent"><i>S. atra</i>, the black Alpine form, produces its young
+on land. They are born without gills, ready to breathe air, and with the
+rounded tail of the adult. These differences may, as Kammerer
+says, naturally be regarded as adaptations to the Alpine conditions.
+Moreover, the female bears <i>only two</i> young in a season,
+and this reduction in the number must be taken to be a consequence
+or condition of viviparity. There are many eggs in the
+ovary, but all except the two which are destined to develop
+degenerate and form a yolk-material on which these two survivors feed.</p>
+
+<p class="indent">Kammerer gives a long account of the various conditions
+to which he subjected both species. The treatment was complicated
+<span class="pagenum"><a name="Page_204" id="Page_204">[Pg 204]</a></span>
+in many ways, but the essential statements are, as regards
+<i>S. maculosa</i>, that when no water was provided in which the young
+might be born, they were dropped on land, larger and in a later
+stage of development and of a darker colour than is normal; that
+the larvae so born gradually diminished in number until only
+two were deposited in each breeding-period; that dissection
+showed that the other ova degenerated to form a yolk-material.
+The larvae so produced reached maturity. The summary of
+results describes their behaviour, stating that they produced:</p>
+
+<p class="indent">(<i>a</i>) <i>In water</i>, either (1) <i>very</i> advanced,
+large-headed larvae 45 mm. long (instead of 25-30 mm.) with gills already
+reduced, which had awkward, embryo-like movements, and in some few
+days metamorphosed into small perfect salamanders; or (2)
+moderately advanced, properly proportioned larvae, 40-41 mm.
+long, provided with large gills of (at first) intrauterine character,
+which were reduced during aquatic life.</p>
+
+<p class="indent">(<i>b</i>) <i>On land</i>, small (26 mm. long) larvae with
+rudimentary gills, having the body rounded instead of being flattened from
+above downwards, and an elongated narrow head, which were
+unable to live in deep water. These larvae changed to the salamander
+colour in 10-12 days, and after four weeks metamorphosed
+into salamanders 29 mm. long.</p>
+
+<p class="indent">(<i>c</i>) In the foregoing cases the experimental
+conditions were not continued, or in other words, basins of water were
+provided in which they could spawn. But if the experimental conditions
+are continued, these <i>Salamandra maculosa</i> which were born
+newt-like (viz., not in a larval condition), are themselves newt-bearing
+from the first time they give birth, using the dry land,
+and bringing forth only two young, the normal number for the
+births of <i>S. atra</i>. These young are 40-41 mm. long, and are
+dark-coloured, resembling greatly the normal new-born <i>S. atra</i>.</p>
+
+<p class="indent">This epitome of the observations illustrating the
+inheritance of acquired characters has been very widely quoted, and may
+not unnaturally be taken to summarize a wide experience of
+the modified animals. Reference to the details given in the
+same paper shows that, as alleged, each of the four types of behaviour
+enumerated was witnessed <i>once</i> only in the case of each
+<span class="pagenum"><a name="Page_205" id="Page_205">[Pg 205]</a></span>
+of four females, no two agreeing with each other. As to the
+number of the males or their habits nothing is said. The first
+female, <i>a</i> (1), bore five young; the second, <i>a</i> (2), bore two, of
+which one was a partial albino; the third, <i>b</i>, produced four young;
+and the fourth, <i>c</i>, two as already stated.</p>
+
+<p class="indent">In the case of <i>c</i> the details show that the female
+gave birth immediately after being transferred from the open-air terrarium
+to one indoors, which contained no basin of water. This is the
+example of the consequences which follow on a continuance of
+the experimental conditions.<a name="FNanchor_17_150" id="FNanchor_17_150"></a><a href="#Footnote_17_150" class="fnanchor">[17]</a>
+</p>
+
+<p class="indent">As regards <i>S. atra</i> the converse is reported.
+Various means were used to induce them to eject their young prematurely
+in water, such as massaging the sides of the mothers, or raising
+the temperature to 25° or 30° C., with various degrees of success.
+But afterwards it was found that specimens collected wild at an
+elevation of about 1,000 metres responded to much simpler
+treatment, and gave birth prematurely in water when they were
+kept in a large shallow basin of water not so deep but that they
+could everywhere touch the bottom with their feet and keep their
+heads above the surface. With specimens collected at higher
+elevations this treatment was inoperative, and the suggestion is
+made that <i>S. atra</i> at the lower confines of its habitat partakes
+more of the nature of <i>maculosa</i> than do the individuals from
+greater heights; for Kammerer argues that pools suitable for
+breeding must be more uncommon at those elevations than they
+are lower down.</p>
+
+<p class="indent">In the earlier paper<a name="FNanchor_18_151" id="FNanchor_18_151"></a><a href="#Footnote_18_151" class="fnanchor">[18]</a>
+Kammerer states that newly caught
+females of <i>S. atra</i> often give birth in the water, and show an
+undoubted preference for doing so. He describes also how he
+once saw several females, wild in their natural habitat, lay their
+young in a rain-puddle at 1,800 metres elevation, but the larvae
+thus born were fully formed.
+<span class="pagenum"><a name="Page_206" id="Page_206">[Pg 206]</a></span></p>
+
+<p class="indent">When the deposition of the young as larvae has become
+"habitual"<a name="FNanchor_19_152" id="FNanchor_19_152"></a><a href="#Footnote_19_152" class="fnanchor">[19]</a>
+with <i>S. atra</i>, three to nine larvae may be produced
+at one spawning period, from 35 to 45 mm. long, with gills at
+most 8 mm. long, and a tail-fin 2-3 mm. broad. Such larvae
+are generally coffee-brown, or grey (instead of black), and show
+other minor differences.</p>
+
+<p class="indent">The summary states that when grown to maturity they
+become in their turn larva-bearing, and go into the water to bring
+forth. Their young are more than two (3 to 5 being the numbers
+observed) with a length of 33-40 mm. or of 21-23 mm. at birth.
+They are light grey, spotted (mottled with lighter and darker
+colour), have relatively short gills (8 to 9 mm. at most) and a
+broad tail-fin (3 mm. wide). At metamorphosis they are relatively
+long (44 mm.) and one of them had some yellow pigment.</p>
+
+<p class="indent">Here again this summary is, as a matter of fact,
+describing the behaviour of two mothers, of which one produced three,
+and the other five young.</p>
+
+<p class="indent">To my mind these experiments suggest that the reproductive
+habits of both species, if closely observed, will be found to be
+subject to considerable variation, and I think it not impossible
+that each species is, especially in confinement, capable of being a
+good deal deflected from its normal behaviour. Moreover, there
+seems to me no great improbability in the idea that there is an
+interdependence between the number of young and the stage of
+maturity in which they are born. But, at the same time, the case
+as told by Kammerer strikes me as proving too much. If each
+species is so sensitive to conditions that the normal procedure
+is gravely modified in one generation, and if that modification
+can reappear in a pronounced form in the next generation without
+a renewal of the disturbing conditions, it becomes extremely
+difficult to understand how the regularity which each species is
+believed to display in nature can be maintained. Surely both
+species might be expected to be in confusion. From a passage
+in Kammerer's earlier paper (1904, p. 55) on the subject, I infer
+that he also would expect considerable irregularity in the natural
+behaviour, but that he has not investigated the point.<a name="FNanchor_20_153" id="FNanchor_20_153"></a><a href="#Footnote_20_153" class="fnanchor">[20]</a>
+<span class="pagenum"><a name="Page_207" id="Page_207">[Pg 207]</a></span></p>
+
+<p class="indent">3. <i>Modification of the Colour of Salamandra maculosa
+induced by Change in the Colour of the Soil on which the Animals were
+kept.</i>&mdash;Kammerer speaks of this as the most convincing of all
+his experiments on the transmission of acquired characters. So
+far, however, no full account of them has been published.<a name="FNanchor_21_154" id="FNanchor_21_154"></a><a href="#Footnote_21_154" class="fnanchor">[21]</a>
+The statement is that when salamanders are kept in yellow surroundings
+the yellow markings gradually in the course of years increase
+in amount relatively to the black ground colour. Conversely by
+keeping the animals on black garden soil, the yellow may be
+greatly diminished in quantity until it largely disappears. (The
+account in <i>Natur</i> adds that very moist conditions also favour the
+increase of yellow, and that with less moist conditions the yellow
+diminishes.) From each kind, the (induced) yellower and the
+(induced) blacker, a second generation was raised, on soil of
+neutral colour, and each family was later divided into two parts,
+half being put on black and half on yellow ground.</p>
+
+<p class="indent">As regards the offspring of those which had lived on <i>black</i>
+soil no positive result had been reached up to the date of publication,
+but it is stated that these young resembled their parents
+in having the yellow distributed in <i>irregular spots</i>.</p>
+
+<p class="indent">As regards the offspring of those which had lived on
+yellow soil the account follows up the story of that part of the offspring
+which were put on yellow soil again. It is stated that these, though
+derived from parents with irregular spots, <i>developed the yellow
+as longitudinal bands</i>.</p>
+
+<p class="indent">This account is given with slight differences of
+expression in the three places to which I have referred. On returning
+from Vienna in 1910 I consulted Mr. G. A. Boulenger in reference to
+the subject, and he very kindly showed me the fine series from
+many localities in the British Museum, and pointed out that in
+nature the colour-varieties can be grouped into two distinct types,
+<span class="pagenum"><a name="Page_208" id="Page_208">[Pg 208]</a></span>
+one in which the yellow of the body is irregularly distributed in
+spots and one in which this yellow is arranged for the most part
+in two longitudinal bands which may be continuous or interrupted.
+<i>The spotted form is, as he showed me, an eastern variety,
+and the striped form belongs to western Europe.</i> Mr. E. G.
+Boulenger<a name="FNanchor_22_155" id="FNanchor_22_155"></a><a href="#Footnote_22_155" class="fnanchor">[22]</a>
+has since published a careful account of the distribution
+of the two forms. The spotted he regards as the typical
+form, var. <i>typica</i>, and for the striped he uses the name var.
+<i>taeniata</i>. The typical form occupies eastern Europe in general,
+including Austria and Italy, extending as far west as parts of
+eastern France. The var. <i>taeniata</i> is found all over France,
+excepting parts of the eastern border, Belgium and western
+Germany, Spain and Portugal. Of the very large series examined
+there was only one specimen (Lausanne) which could not with
+confidence be referred to one or other of the two varieties.
+Mr. E. G. Boulenger points out that both varieties inhabit very
+large areas, and live on soils of most different colours and
+compositions. Both are liable to variations in the amount and the
+shade of the yellow, but that any suggestion that <i>taeniata</i> belongs
+especially to yellow soils and <i>typica</i> to black soils is altogether
+inadmissible. He expresses surprise that Kammerer should not
+allude to these peculiarities in the geographical distribution of
+the two forms. He suggests further that it is more likely that
+some mistake occurred in Kammerer's observations than that the
+east European <i>typica</i> should, in the course of a generation, have
+been transformed into the west European <i>taeniata</i> by the influence
+of yellow clay soil.</p>
+
+<p class="indent">In his last paper on the subject Kammerer states incidentally<a name="FNanchor_23_156" id="FNanchor_23_156"></a><a href="#Footnote_23_156" class="fnanchor">[23]</a>
+that he has found the <i>striped form recessive to the spotted</i>. No
+evidence for this statement is given, and I have not found any
+other reference to crosses effected between the two natural types.
+If, however, this representation is correct, it is conceivable that
+the production of <i>taeniata</i> from <i>typica</i> was in fact the re-appearance
+of a recessive form. The plate which Kammerer gives
+in illustration of his modified parent figures a single animal at
+four stages, and though it is certainly more like the spotted than
+<span class="pagenum"><a name="Page_209" id="Page_209">[Pg 209]</a></span>
+the striped form, it has a certain suggestion of the striped arrangement,
+such as I can well imagine being produced in the heterozygote.<a name="FNanchor_24_157" id="FNanchor_24_157"></a><a href="#Footnote_24_157" class="fnanchor">[24]</a>
+</p>
+
+<p class="indent">In continuation<a name="FNanchor_25_158" id="FNanchor_25_158"></a><a href="#Footnote_25_158" class="fnanchor">[25]</a>
+of the experiments on the colour of <i>S. maculosa</i>
+Kammerer publishes an account of elaborate experiments
+in grafting ovaries of the various forms, modified and unmodified,
+into each other, and describes the offspring which followed.
+Before pursuing this part of the inquiry I am disposed to wait
+until the earlier steps have been made much more secure than
+they yet are.</p>
+
+<p class="indent">More recently Kammerer has published similar statements
+in regard to the inheritance of characters induced in various lizards
+by keeping them in abnormal temperatures, high and low. The
+changes induced affected in some species the colours, in others
+the reproductive habits. Respecting these examples I feel the
+same scepticism that I have indicated in regard to the others,
+somewhat heightened by the fact that insufficient evidence is
+given both regarding the behaviour of these various species in
+captivity when not subjected to abnormal temperatures, and
+in the wild state.</p>
+
+<p class="indent">Respecting this part of the evidence Mr. G. A. Boulenger has
+lately published a criticism<a name="FNanchor_26_159" id="FNanchor_26_159"></a><a href="#Footnote_26_159" class="fnanchor">[26]</a>
+from which I extract the following
+passages. Referring to a previous note<a name="FNanchor_27_160" id="FNanchor_27_160"></a><a href="#Footnote_27_160" class="fnanchor">[27]</a>
+on the question of the
+melanism of the various insular forms of <i>Lacerta muralis</i> he
+writes: "I also alluded (<i>l. c.</i>) to the theories that have been
+propounded to explain the melanism of various insular forms.
+This is a subject which has been lately taken up by Dr. Kammerer
+at the Biologische Versuchsanstalt in Vienna, and he claims
+to have produced nigrinos artificially by a very strong elevation
+of the temperature, accompanied by extreme dryness. Dr.
+Werner<a name="FNanchor_28_161" id="FNanchor_28_161"></a><a href="#Footnote_28_161" class="fnanchor">[28]</a>
+has already opposed his own experiments to those of
+Kammerer, artificial melanism having been produced by him in
+<i>Lacerta oxycephala</i> by keeping two very light specimens from
+<span class="pagenum"><a name="Page_210" id="Page_210">[Pg 210]</a></span>
+Ragusa for a whole summer in very damp conditions. Neither is
+Kammerer's theory in accordance with the distribution of the
+black lizards, as pointed out by Werner. Kammerer also finds
+that those forms which are known to produce melanic races in a
+state of nature, lend themselves more readily than the others
+to the success of his experiments. But he shows himself misinformed
+when he states that the variety called <i>Lacerta fiumana</i>
+belongs to the category of those of which black forms are not
+known. He overlooks the fact, first pointed out by Scherer in
+1904, and which I can confirm, that the black lizard from Melisello
+near Lissa in the Adriatic is unquestionably derived from
+the lizard from Lissa, which he correctly regards as not separable
+from <i>L. fiumana</i>...."</p>
+
+<p class="indent">"Another colour modification which Dr. Kammerer states
+that he obtained by raising the temperature is the assumption by
+the female of the typical <i>Lacerta muralis</i> of the bright red colour
+of the lower parts which often distinguishes the male from the
+female, and which was not shown by the individuals of the latter
+sex kept by him under normal conditions. He quotes various
+authorities to show that the lower parts are never red in the
+females, but he has omitted to consult others who say the contrary.
+Thus Bedriaga (1878 and 1879) remarks that a so-called
+var. <i>rubriventris</i> of the typical wall lizard has the lower parts red
+in both sexes."<a name="FNanchor_29_162" id="FNanchor_29_162"></a><a href="#Footnote_29_162" class="fnanchor">[29]</a>
+</p>
+
+<p class="indent">In reading such papers as those of Semon or Kammerer
+the thought uppermost in my mind is that to multiply illustrations
+of supposed transmission of acquired characters is of little use
+until some one example has been thoroughly investigated. If
+we had certain assurance that even a single unimpeachable case
+could be repeated at will, the whole matter would assume a more
+serious aspect. If, for instance, Kammerer were able to show us
+<i>Alytes</i> males with horny pads on their hands, it would be
+something tangible; still more, if the experiment were repeated by
+others until no doubt remained that the offspring of <i>Alytes</i> which
+had bred in water for some three generations did acquire these
+<span class="pagenum"><a name="Page_211" id="Page_211">[Pg 211]</a></span>
+pads and that they could transmit these novelties to descendants
+raised in normal conditions. Till evidence of this kind is published
+by at least two independent observers investigating similar
+material, I find it easier to believe that mistakes of observation or
+of interpretation have been made than that any genuine transmission
+of acquired characters has been witnessed.</p>
+
+<p class="indent space-below">Meanwhile there is no denying that the origin of adaptational
+features is a very grave difficulty. With the lapse of time since
+evolutionary conceptions have become a universal subject of study
+that difficulty has, so far as I see, been in nowise diminished.
+But I find nothing in the evidence recently put forward which
+justifies departure from the agnostic position which most of us
+have felt obliged to assume.<a name="FNanchor_30_163" id="FNanchor_30_163"></a><a href="#Footnote_30_163" class="fnanchor">[30]</a>
+</p>
+
+<hr class="chap" />
+<h3><span class="smcap">Appendix to Chapter IX.</span></h3>
+
+<p class="indent">Professor G. Klebs, as is well known to students of
+evolutionary phenomena, has for several years been engaged in investigations
+relating to the inheritance of acquired characters. In
+his many publications on the subject the issue has always been
+represented as more or less uncertain.</p>
+
+<p class="indent">Desiring to know how the matter now stands according
+to Professor Klebs' present judgment I wrote to him asking him to
+favour me with a brief general statement. This he most kindly
+sent in a letter dated 8th July, 1912.</p>
+
+<p class="indent">As such a statement will be read with the greatest
+interest by all who are watching the progress of these studies I obtained
+permission to publish it as follows:</p>
+
+<p class="blockquot">
+<span style="margin-left: 38em;">8. Juli 1912</span><br />
+&emsp;Ihre liebenswurdige Anfrage will ich sehr gern
+beantworten, obwohl ich sie nicht so beantworten kann wie ich
+erwünschte. Ihr Skepticismus in der Frage der Uebertragung erworbener
+Charactere auf die Nachkommen ist nur zu berechtigt. Meine
+Versuche mit Veronica sind <i>nicht</i> beweisend, da es mir bisher
+nicht gelungen ist eine einigermasse konstante Varietät mit
+verlaubten Inflorescenze zu erzeugen. In Bezug auf mein
+Semper vivum bin ich allerdings noch heute der Meinung dass
+<span class="pagenum"><a name="Page_212" id="Page_212">[Pg 212]</a></span>
+die starke künstliche Veränderung der Blüte einen Einfluss auf
+einzelnen Nachkommen gehabt hat. Ich habe seither nichts
+darüber veröffentlicht: die Mehrzahl der anormalen gefüllten
+Blüten war leider steril. Von einem weniger veränderten Exemplar
+erhielt ich einige Sämlinge, aber sie haben noch nicht
+geblüht. Es kann sich in diesem Falle nur um eine <i>Nachwirkung
+in der ersten Generation</i> handeln, vergleichbar jenen Fällen in
+denen Samen von Bäumen aus den hohen Alpen in der Ebene
+gewisse Nachwirkungen zeigen. Aber es ist bisher kein sicherer.
+Fall bekannt in den der kunstliche herbeigeführte Charakter
+<i>mehrere Generationen hindurch unter der gewöhnlichen "normalen"
+Bedingungen</i> übertragen worden ist.</p>
+
+<p class="blockquot">&emsp;Auf der andere Seite sind diese negativen Resultaten
+nicht entscheidend. Denn wie wenig ist in dieser Beziehung überhaupt
+ernstlich versucht worden! Und zweifellos geht die
+Sache nicht so einfach.</p>
+
+<p class="blockquot">&emsp;Ich versuche es mit anderen Pflanzen weil ich der
+Meinung bin dass es möglich sein müsse wenigstens solche neuen Varietäten
+zu erzeugen, wie sie die Gartenvarietäten entsprechen.</p>
+
+<p class="blockquot">&emsp;Aber bis jetzt leider sind die Versuche nicht
+gelungen, weder mir noch irgend einem anderen.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_213" id="Page_213">[Pg 213]</a></span></p>
+
+<h2>CHAPTER X</h2>
+<h3><span class="smcap">Effects Of Changed Conditions</span> <i>Continued</i></h3>
+<p style="font-size: 110%; text-align: center;">
+   <b><span class="smcap">The Causes of Genetic Variation</span></b></p>
+
+<p class="indent">In the last chapter we examined some of the evidence
+offered in support of the belief that adaptation in highly organised
+forms is a consequence of the inheritance of adaptative changes induced
+by the influence of external conditions. The state of knowledge
+of this whole subject is, as I have said, most unsatisfactory,
+chiefly for the reason that in none of the cases which are alleged
+to show a positive result have two observers been over the
+same ground, or as yet confirmed each other. In the wider
+consideration respecting the causes of variation at large we find
+ourselves still in the same difficulty. The study has thus far
+proved sadly unfruitful. In spite of the considerable efforts
+lately made by many observers to induce genetic variation in
+highly organised plants or animals, and though successes have
+occasionally been announced, I do not know a single case which
+has been established and confirmed in such a way that we could
+with confidence expect to witness the alleged phenomena if we
+were to repeat the experiment. Abundant illustrations are
+available in which individuals exposed to novel conditions manifest
+considerable changes in characters or properties, but as yet
+there is no certain means of determining that germ-cells of a new
+type shall be formed.</p>
+
+<p class="indent">Of the direct effect of conditions the lower organisms,
+especially bacteria, offer the best examples, the alterations of virulence
+which can be produced in so many distinct ways being the most
+striking and familiar. That attenuation of virulence can be
+produced by high temperatures or by exposure to chemical
+agents, and that this diminution in virulence may remain permanent
+is, from our point of view, not surprising; but the fact that
+in many cases the full virulence can by suitable cultivation be
+<span class="pagenum"><a name="Page_214" id="Page_214">[Pg 214]</a></span>
+restored is difficult to understand. Similar variations have been
+observed in power of pigment production and other properties.</p>
+
+<p class="indent">These phenomena naturally raise the question whether any
+cases of apparent loss of factors in higher forms may be comparable.</p>
+
+<p class="indent">The subject of variations in the lower organisms and
+their dependence on conditions is a highly special one, and I have no
+knowledge which can justify me in offering any discussion of
+them, but I understand that hitherto little beyond empirical
+recognition of the phenomena has been attempted. A useful
+summary of observations made by many investigators was lately
+published by Hans Pringsheim,<a name="FNanchor_1_164" id="FNanchor_1_164"></a><a href="#Footnote_1_164" class="fnanchor">[1]</a>
+who enumerates the different
+agencies which have been observed to produce modifications,
+and the various ways in which these changes are manifested.
+One of the most comprehensive studies of the subject from the
+genetic point of view is that made by F. Wolf.<a name="FNanchor_2_165" id="FNanchor_2_165"></a><a href="#Footnote_2_165" class="fnanchor">[2]</a>
+In his extensive cultivations of <i>Bacillus prodigiosus</i>,
+<i>Staphylococcus pyogenes</i> and <i>Myxococcus</i> he succeeded in producing
+many strains with modified properties. In most of these the modifications arose
+in consequence of the application of high or low temperatures or of
+the addition of various chemical substances to the culture-media.
+Some of the variations, which are for the most part in the powers
+of pigment-formation, persisted when the strains were returned
+to normal conditions, and others did not. In reference especially
+to the variations witnessed in the Cocci the reader should consult
+the critical account of variation in that group published by the
+Winslows,<a name="FNanchor_3_166" id="FNanchor_3_166"></a><a href="#Footnote_3_166" class="fnanchor">[3]</a>
+where much information on the subject is to be
+found. The authors attempted to determine the systematic
+relationships of the several forms, as far as possible, by the
+application of statistical methods. The result is interesting as
+showing that the problem of species in its main features is presented
+by these organisms in a form identical with that which
+we know so well in the higher animals and plants, whatever
+<span class="pagenum"><a name="Page_215" id="Page_215">[Pg 215]</a></span>
+properties be selected as the diagnostic characters. There are
+many types perfectly distinct and others which intergrade.
+Some of the types change greatly with conditions while others do
+not. This is exactly what we encounter whenever we study the
+problem of species on an extended scale among the higher forms of life.</p>
+
+<p class="indent">There is now practically complete agreement among
+bacteriologists that the observations made first by Massini on the
+change in color of <i>Bacterium coli mutabile</i> grown in Endo's
+medium, associated with the acquisition of the power to ferment
+lactose, are perfectly reliable and free from possibilities of mistake.
+The work has been extended and confirmed by many
+workers, especially R. Müller, who finds that this bacterium can
+similarly acquire and maintain the power to ferment other
+sugars. A careful account of the whole subject written by Müller
+for the information of biologists will be found in <i>Zts. für Abstammungsl.</i>,
+VIII, 1912. After discussing the biological significance
+of the facts, he concludes with a caution to the effect that bacteria
+are so different from all other living things that generalizations
+from their behavior must not be indiscriminately applied to animals and plants.</p>
+
+<p class="indent">In all work with this class of material there is
+obviously danger of error through foreign infection of the cultures,
+but there can be no doubt that though some of the "mutations"
+recorded may be due to this cause, the majority of the instances
+observed under stringent conditions are genuine.</p>
+
+<p class="indent">Another and equally serious difficulty besetting work
+with bacteria and fungi cultivated from spores is that the appearance
+of variation may in reality be due to the selection of a special
+strain previously living masked among other strains. This
+possibility must be remembered especially in those instances
+which are claimed as exemplifying the effects of acclimatisation.
+Manifestly this consideration can be urged with most force when
+the strain which gave rise to the novelty was not raised from a
+single individual spore. Moreover, when once the possibility of
+spontaneous variation is admitted, it must be difficult to be quite
+confident that any given variation observed is in reality due to
+<span class="pagenum"><a name="Page_216" id="Page_216">[Pg 216]</a></span>
+the novel conditions applied, and as I understand the evidence,
+the appearance of the mutational forms does not with any
+regularity follow upon the application of the changed conditions.</p>
+
+<p class="indent">Researches into the variation of these lower forms
+will, no doubt, be continued on a comprehensive scale. So long as the
+instances recorded are each isolated examples it is impossible to
+know what value they possess. If they could be coordinated in
+such a way as to provide some general conception of the types of
+variation in properties to which bacteria, or any considerable
+group of them, are habitually liable, the knowledge might
+greatly advance the elucidation of genetic problems.</p>
+
+<p class="indent">Of mutational changes directly produced with regularity
+in micro-organisms by treatment, the experiments with trypanosomes
+provide some of the clearest examples. A summary of
+the evidence was lately published by Dobell,<a name="FNanchor_4_167" id="FNanchor_4_167"></a><a href="#Footnote_4_167" class="fnanchor">[4]</a>
+from which the present account is taken. The most definite fact of this
+kind established is that certain dyes introduced into the blood of the
+host have the effect of destroying the small organ known as the
+"kinetonucleus" in the trypanosomes. The trypanosomes thus
+altered continue to breed, and give rise to races destitute of
+kinetonuclei. This observation was originally made by Werbitzki
+and has been confirmed by several observers. The exact
+way in which this alteration is effected in the trypanosomes is not
+quite definitely made out, but there is good reason for supposing
+that the dyes have a direct and specific action upon the kinetonucleus
+itself, and circumstances make it improbable that in
+some division a daughter-organism without that body is produced,
+or that any selection of a pre-existing defective variety occurs.</p>
+
+<p class="indent">Ehrlich has suggested with great probability that the
+dyes which possess this action owe it to the fact that they have the
+particular chemical linkage which he calls "ortho-quinoid." In
+outward respects, such as motility and general appearance, the
+modified organisms are unchanged, but their virulence is diminished.
+As regards the possibility of the defective strain
+<span class="pagenum"><a name="Page_217" id="Page_217">[Pg 217]</a></span>
+reacquiring the kinetonucleus, Werbitzki states that in one case
+passage through 50 animals and treatment with dyes left the
+strain unaltered; but that in another case at the sixteenth
+passage 7 per cent. of the trypanosomes were found to have
+re-acquired the organ, and in subsequent passages the percentage
+increased, until at the twenty-seventh passage practically all had
+re-acquired it. Kudicke, however, in similar experiments did not
+succeed in causing re-acquisition by transplantation.</p>
+
+<p class="indent">By the action of various drugs and anti-bodies races
+of trypanosomes resistant to those substances have been obtained.
+These breed true, at least when kept in the same species of animal
+in which the resistance was acquired. As to whether change of
+virulence is produced by passage through certain animals or not,
+there is as yet no general agreement.</p>
+
+<p class="indent">Other changes, especially in size and some points of
+structure, are said to occur when certain trypanosomes proper to mammals
+are passed through cold-blooded vertebrates (Wendelstadt and
+Fellmer), and it is stated that these changes persist, but the
+observations have not yet been confirmed.</p>
+
+<p class="indent">Experiments lately conducted by Woltereck with <i>Daphnia</i>
+are interesting as having given a definite positive result, in so far,
+at least, as the ova were affected by conditions before leaving the
+bodies of the parent individuals. The observations relate to
+the offspring resulting from <i>parthenogenetic</i> eggs. Females
+bearing ephippia (fertilised eggs) were isolated until the ephippia
+were dropped, and in this way the offspring of fertilisation were
+excluded. Males, of course, appeared from time to time in the
+cultures, but as fertilised eggs were rejected, their presence did
+not disturb the result. The most remarkable observations
+related to <i>Daphnia longispina</i>.</p>
+
+<p class="indent">This species as found in the lower lake at Lunz had
+the front end of the body blunt and nearly round in profile; but on being
+cultivated in a warm temperature and with abundant nourishment
+the front end of the body became produced into an elongated
+"helmet," as Woltereck calls it. Experiment showed that
+the change was primarily due to the abundance of food, and owing
+to temperature in a subordinate degree.
+<span class="pagenum"><a name="Page_218" id="Page_218">[Pg 218]</a></span></p>
+
+<p class="indent">This distinction arose as soon as the species was taken
+into the hothouse, but when the modified individuals were put back
+into the original conditions, a lower temperature and scanty
+food-supply, the next generation returned to their original form.
+After being cultivated for two years and about 40 generations in
+the more favourable conditions, when similarly put back into
+the lower temperature with scanty food the <i>first generation</i> born
+in these conditions was helmeted like the modified parents.
+Woltereck is of opinion that the ova were still unformed at the
+time the parents were put back, and the influence of the favourable
+conditions upon the unformed ova he speaks of as a "prae-induction."
+The effect never extended beyond the one generation,
+after which the strain returned to its original state.</p>
+
+<p class="indent">The fact that the influence on the offspring was not
+manifested at first led Woltereck to expect that by more prolonged
+cultivation in the favourable conditions a further extension of
+this influence would be produced, but this expectation was never
+fulfilled, though the attempt was made again and again.</p>
+
+<p class="indent">Similar experiments were made with <i>Hyalodaphnia cucullata</i>,
+which is far more sensitive to cultural influences, and in nature
+manifests a considerable elongation of the helmet as a seasonal
+modification, but the results were essentially the same as in the
+preceding case, no modification extending beyond the first
+generation born after the restoration to <i>normal conditions</i>.<a name="FNanchor_5_168" id="FNanchor_5_168"></a><a href="#Footnote_5_168" class="fnanchor">[5]</a>
+</p>
+
+<p class="indent">The only criticism of these extremely interesting results
+which suggests itself is that perhaps the original appearance of the
+modification was not in reality due to an <i>accumulated</i> effect of
+the conditions, but to some change in the conditions themselves
+which was not noticed. It is difficult to see how length of
+time or even the lapse of several generations could have so specific
+an effect on the race. It is no doubt often vaguely supposed
+by many that a long period of time may be necessary for the
+effect of climate or of other environmental conditions to be
+produced in an organism which does not thus respond at first.
+I have never been able to see any reason for this opinion nor how
+<span class="pagenum"><a name="Page_219" id="Page_219">[Pg 219]</a></span>
+it is to be translated into terms of physiological fact, and I
+imagine that in those cases in which the lapse of time is really
+required for the production of an effect, the influence of the
+prolongation is rather on the conditions than on the organisms.
+The response of the organisms thus probably indicates not that
+the creature is at length feeling the effects because of their
+accumulated action on itself, but that the conditions have at
+length ripened.</p>
+
+<p class="indent">As this sheet is passing through the press Agar has published<a name="FNanchor_6_169" id="FNanchor_6_169"></a><a href="#Footnote_6_169" class="fnanchor">[6]</a>
+an abstract of evidence as to another comparable case in a parthenogenetic
+strain in the daphnid, <i>Simocephalus vetulus</i>. When
+fed on certain abnormal foods the shape of the body is changed,
+the edges of the carapace being rolled backwards so as to expose
+the appendages. The offspring of animals thus modified showed
+similar modification in the first, and to a very slight degree, in
+the second generation, though the original mothers were removed
+to normal conditions before their eggs were laid. In the third
+generation there was "a very pronounced reaction in the opposite
+direction." Agar suggests that the change may be due to some
+toxin-like substances, carried on passively by the egg into the
+next generation, against which the protoplasm eventually produces
+an anti-body.</p>
+
+<p class="indent">The experiments which have been in recent years regarded
+by evolutionary writers as the most conclusive proof that direct
+environmental action may produce germinal variation are those
+of Professor W. L. Tower, of Chicago, on <i>Leptinotarsa</i>, the
+potato beetles. This work has attained considerable celebrity
+and has been generally accepted as making a definite extension
+of knowledge. After frequently reading Tower's papers and
+after having been privileged to see some of the experiments in
+progress (in 1907) I am still in doubt as to the weight which
+should be assigned to this contribution.</p>
+
+<p>The work is described in two chief publications, the first of
+which appeared in 1906.<a name="FNanchor_7_170" id="FNanchor_7_170"></a><a href="#Footnote_7_170" class="fnanchor">[7]</a>
+This treatise contains a vast amount
+of information about numerous species and varieties of these
+<span class="pagenum"><a name="Page_220" id="Page_220">[Pg 220]</a></span>
+beetles which the author has observed and bred in many parts of
+their distribution throughout the United States, Mexico and
+Central America. The part of the book which has naturally
+excited the greatest interest is that in which Tower states that
+by subjecting the beetles to change in temperature and moisture,
+he caused them to produce offspring quite unlike themselves,
+which in several cases bred true.</p>
+
+<p class="indent">It is much to be regretted that the author did not
+happen to become acquainted with Mendelian analysis at an earlier stage
+in the investigation. The evidence might then have been
+handled in a much more orderly and comprehensive way, and a
+watch would have been kept for several possibilities of error.</p>
+
+<p class="indent">The headquarters of the genus is evidently as Tower
+states, in Mexico and the adjoining countries. In this region there is
+a great profusion of forms, some very local, some as for instance
+the well-known <i>decemlineata</i>,<a name="FNanchor_8_171" id="FNanchor_8_171"></a><a href="#Footnote_8_171" class="fnanchor">[8]</a>
+more widely spread. The distinctions are almost all found in peculiarities of
+colour and pattern, and the limits of species are even more indefinable than
+is usual in multiform animals. Tower arranges the various types
+into seven groups of which the one most studied is that which
+he calls the <i>lineata</i> group. To this group belong all the forms
+to which reference is here made, and, as I understand, they differ
+among themselves entirely in size, colour and pattern. There
+is no suggestion of infertility in the crosses made between the
+several forms of the <i>lineata</i> group; in fact they present, like many
+Chrysomelidae, a good example of what most of us would now
+call a polymorphic species, consisting of many types, some found
+existing in the same locality, others being geographically isolated.</p>
+
+<p class="indent">A series of experiments was devoted to the attempt to
+fix strains corresponding to the extremes of continuous variations.
+For example, those with most black pigment and those with
+least black taken from a population continuously varying in this
+respect, were separately bred; but almost always the selection
+led to no sensible change in the position of the mean of the
+<span class="pagenum"><a name="Page_221" id="Page_221">[Pg 221]</a></span>
+population. The variations in these cases were evidently fluctuational.
+In some instances, however, real genetic differences
+were met with, and strains exhibiting them were, as usual,
+rapidly fixed.</p>
+
+<p class="indent">Tower points out that several of the varieties (or species,
+as he prefers to call them) were obviously recessive to <i>decemlineata</i>.
+This is most clearly demonstrated in the case of the form called
+<i>pallida</i>, which is a pale depauperated-looking creature, with the
+orange of the thorax almost white and the eyes devoid of pigment.<a name="FNanchor_9_172" id="FNanchor_9_172"></a><a href="#Footnote_9_172" class="fnanchor">[9]</a>
+This form behaved as an ordinary Mendelian recessive,
+breeding true whenever it appeared in the cultures, or when
+individuals found wild were studied in captivity. A black form
+which Tower names <i>melanicum</i> was similarly shown to be a
+Mendelian recessive. Wild specimens of this variety of opposite
+sexes were not found simultaneously in nature, and there was thus
+no opportunity of breeding them together, but the hereditary
+behaviour was seen in the F<sub>2</sub> generation from a <i>melanicum</i> found
+coupled with <i>decemlineata</i>. Experiments also occurred giving
+indication that a variety with the stripes anastomosing in pairs
+(<i>tortuosa</i>), was another recessive, and that a variety&mdash;called
+"<i>rubri-vittata</i>"&mdash;gave an intermediate F<sub>1</sub> with subsequent
+segregation. All these are forms of <i>decemlineata</i> Stål.</p>
+
+<p class="indent">Similar observations were made regarding forms recessive
+to <i>multitaeniata</i> Stål. Of these two were thrown by <i>multitaeniata</i>
+itself, namely a form named by Stål <i>melanothorax</i>, and regarded
+by him as a species, and one which Tower names <i>rubicunda</i> n. sp.
+The facts proving the recessive behaviour of their several forms
+will be found in the following places in Tower's book:</p>
+
+<p>
+<span style="margin-left: 6em;"><i>pallida</i>,&emsp; pp. 273-278.</span><br />
+<span style="margin-left: 6em;"><i>melanicum</i>,&emsp; p. 279.</span><br />
+<span style="margin-left: 6em;"><i>tortuosa</i>,&emsp; p. 280.</span><br />
+<span style="margin-left: 6em;"><i>rubrivittata</i>,&emsp; pp. 280-281.</span><br />
+<span style="margin-left: 6em;"><i>melanothorax</i> and <i>rubicunda</i>,&emsp; pp. 283-285.</span>
+<span class="pagenum"><a name="Page_222" id="Page_222">[Pg 222]</a></span></p>
+
+<p class="indent">
+Following this evidence of recessive nature of the six forms
+enumerated, Tower describes experiments showing, as he believes,
+that some of them may be caused to appear by applying special
+treatment to the parents during the "growth and fertilisation"
+(p. 287) of the eggs. The most striking example is that in which
+4 males and 4 females of <i>decemlineata</i> were kept very hot (average
+35° C.) and dry, and at low atmospheric pressure (19-21 inches).
+The eggs laid were restored to natural conditions. These gave
+506 larvae, from which emerged 14 normal, 82 <i>pallida</i> and 2
+"<i>immaculothorax</i>," viz., without pigment on the pronotum.
+The account of the rest of the experiment is somewhat involved,
+but I understand that the <i>pallida</i>, of which two only survived,
+behaved as normal recessives when bred to the type: also that
+the parents, after having laid the eggs whose history has been
+given, were restored to normal conditions and laid 319 eggs which
+gave 61 normals.</p>
+
+<p class="indent">In another case normal parents laid 409 eggs in the
+hot and dry conditions, and on restoration to normal conditions, the
+same parents laid 840 eggs. Then 409 eggs gave 64 adults as follows:</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">Males</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">Females</td>
+    </tr><tr>
+      <td class="tdl"><i>decemlineata</i></td>
+      <td class="tdc">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdc">12</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">8</td>
+    </tr><tr>
+      <td class="tdl"><i>pallida</i></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">10</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">13</td>
+    </tr><tr>
+      <td class="tdl"><i>immaculothorax</i></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">2</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">3</td>
+    </tr><tr>
+      <td class="tdl"><i>albida</i></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">9</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">7</td>
+    </tr><tr>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">&mdash;&mdash;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">&mdash;&mdash;</td>
+    </tr><tr>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">33</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">31</td>
+    </tr>
+ </tbody>
+</table>
+
+<p class="indent">The 840 eggs laid in normal conditions
+gave 123 normal <i>decemlineata</i>.</p>
+
+<p class="indent">Similar experiments were made with <i>multitaeniata</i> and gave
+comparable results, the two recessives (<i>melanothorax</i>, <i>rubicunda</i>)
+being produced in large numbers when the parents were subjected
+to heat, but in this case the atmosphere was kept <i>saturated</i> with
+moisture, instead of dry, as in the previous instance. The same
+parents transferred to normal conditions gave normals only.</p>
+
+<p class="indent">Lastly the form <i>undecimlineata</i> was exposed "to an
+extreme stimulus of high temperature, 10° C. above the average," and a
+dry atmosphere, with the result that from 190 eggs there emerged
+11 beetles, all of the form <i>angustovittata</i> Jacoby, which subsequently
+bred true to that type (see p. 295).
+<span class="pagenum"><a name="Page_223" id="Page_223">[Pg 223]</a></span></p>
+
+<p class="indent">In the results of these experiments, as described, there
+is one feature which I regard as quite unaccountable. Tower makes no
+comment upon it. Indeed, from the general tenour of the paper,
+I infer, not only that he does not perceive that he is recounting
+anything contrary to usual experience, but rather that he regards
+the result as conforming to expectations previously formed.
+The point in question is the genetic behaviour of the dominant
+normals produced under the abnormal conditions. These
+normals were the result of the breeding of parents declared to be
+at the same time giving off many recessive gametes. Some of
+these normals must be expected therefore to be heterozygous
+unless some selective fertilisation occurs. Nevertheless in every
+case they and their offspring are reported to have continually
+bred true. I allude especially to the tables given on pp. 288, 289,
+292, and 293. Tower does not mention any misgiving about
+this result, and I think he regards himself as recounting phenomena
+in general harmony with the ideas of mutation expressed
+by De Vries. This they may be; but to anyone familiar with
+analytical breeding the course of these experiments must seem so
+surprising as to call for most careful, independent confirmation.</p>
+
+<p class="indent">In 1910<a name="FNanchor_10_173" id="FNanchor_10_173"></a><a href="#Footnote_10_173" class="fnanchor">[10]</a>
+Tower published an account of further experiments
+with <i>Leptinotarsa</i>. The work described related to two subjects.
+Crosses were made between three forms, <i>undecimlineata</i> Stål,
+<i>signaticollis</i> Stål and "<i>diversa</i>" named by Tower as a new
+species. The distinctions between these three depend partly on
+characters of the adults and partly on those of the larvae. The
+adults of <i>undecimlineata</i> and <i>diversa</i> have the elytra striped, but
+the elytra of <i>signaticollis</i> are unstriped. The larvae of <i>signaticollis</i>
+and of <i>diversa</i> are yellow, but those of <i>undecimlineata</i>
+are white.<a name="FNanchor_11_174" id="FNanchor_11_174"></a><a href="#Footnote_11_174" class="fnanchor">[11]</a>
+Moreover, in <i>signaticollis</i> and <i>diversa</i> the black increases
+in the third stage of the larvae to form transverse bands
+which are absent in <i>undecimlineata</i>. The general course of the
+experiments shows that these differences may be approximately
+<span class="pagenum"><a name="Page_224" id="Page_224">[Pg 224]</a></span>
+represented as due to the action of three factors, any of which
+may be independently present or absent. The stripings of the
+elytra and of the larvae are each due to a separate factor. As
+regards the distinction between the yellow and the white larvae
+the evidence does not prove that there is decided dominance of
+either colour and I infer that the heterozygotes are often intermediate.</p>
+
+<p class="indent">The chief contribution which this new paper claims to
+make relates to differences in the results which ensue from crosses
+effected between these three types at different average temperatures.</p>
+
+<p class="indent">We are first concerned with four experiments
+which I number (1), (2), (3), (4):</p>
+
+<p class="indent">1. <i>Signaticollis</i> &#9792; × <i>diversa</i> &#9794; bred at
+an average temperature of 80º F. by day and 75° F. by night, gave two groups in
+about equal numbers. The first (49) was pure <i>signaticollis</i> and
+bred true. The second (53) was of an intermediate type, which
+on being bred together gave the typical Mendelian result&mdash;1 <i>sig.</i>:
+2 <i>intermediate</i>: 1 <i>div</i>.</p>
+
+<p class="indent">2. Next, as the account originally stood in the published
+paper, we are told that <i>sig</i> &#9792; × <i>div</i> &#9794; bred together at
+a day-temp. average 75° F. and night average 50° F. gave an <i>intermediate</i>
+only, which subsequently produced a normal 1:2:1 ratio. The two crosses were
+repeated eleven times with identical results.</p>
+
+<p class="indent">In a further experiment (3)
+<i>signaticollis</i> &#9792; × <i>diversa</i> &#9794;
+were bred under the same conditions as those used in expt. (1). They
+again gave <i>sig.</i> and intermediates as before in fairly equal numbers.
+The <i>sig.</i> as before bred true, and the intermediate gave 1:2:1,
+all exactly as in expt. (1).</p>
+
+<p class="indent">In expt. (4) <i>the same parents used</i> in (3) were again
+mated under conditions of expt. (2) at the lower temperature, and this
+time gave <i>signaticollis</i> exclusively, which bred true for four
+generations. This experiment was repeated seven times with
+uniform results.</p>
+
+<p class="indent">Diagrams are given representing all these histories in graphic fashion.
+<span class="pagenum"><a name="Page_225" id="Page_225">[Pg 225]</a></span></p>
+
+<p class="indent">From these observations, Tower concludes that the
+determination of dominance, and the ensuing type of behaviour, is clearly
+a function of the conditions incident upon the combining germ plasms.</p>
+
+<p class="indent">It will be observed that expts. (1) and (3) gave
+identical results but (2) and (4), though much the same conditions
+were applied, are at variance, for (2) gave all intermediates, while
+(4) gave all <i>signaticollis</i>. In <i>Amer. Nat.</i>, XLIV, 1910, p. 747,
+Professor T. D. A. Cockerell commented on this paper of Tower's
+and pointed out that there must be an error somewhere, for when
+he discusses these experiments Tower speaks of (2) and (4) as
+confirming each other. To this Tower replied<a name="FNanchor_12_175" id="FNanchor_12_175"></a><a href="#Footnote_12_175" class="fnanchor">[12]</a>
+that there had been a mistake. He states that in preparing the paper
+"certain minor experiments were taken from a larger series and combined
+to illustrate a general point in the behaviour of alternative
+characters in inheritance," and that expt. (2) was introduced
+inadvertently in place of another which he desires to substitute.
+In this, which I number (5), <i>signaticollis</i> &#9792; × <i>diversa</i>
+&#9794; from exactly the same stocks as those used in (1), were mated at
+the lower temperatures specified for (2), day average 75° F., night
+average 50° F. These gave all of the <i>signaticollis</i> type with a
+narrow range of variability, which bred true, in some cases to F<sub>6</sub>.
+Tower says he has repeated this experiment six times with identical results.</p>
+
+<p class="indent">Nevertheless he proceeds to say that the description
+of expt. (2), which was repeated eleven times with identical results, was
+correct "as far as given." That experiment was "from a second
+series of cultures parallel to the one given, but in which there are
+other factors involved, which in H. 410 [my (2)] are productive
+of a typical Mendelian behaviour." He adds he does "not care
+at this time to make any statement of what these factors are,
+nor of their relations to the behaviours given in the H. 409, H. 411,
+H. 409/11 series [my (1), (5) and (3)&mdash;(4)] which are the simplest
+and most easily presented series obtained in the crossing of
+<i>signaticollis</i> and <i>diversa</i>."
+<span class="pagenum"><a name="Page_226" id="Page_226">[Pg 226]</a></span></p>
+
+<p class="indent">Professor Cockerell's intervention has thus elicited the fact
+that we have as yet only a small selected part of the evidence
+before us, even as concerning the effect of temperature on the
+cross between <i>signaticollis</i> &#9792; × <i>diversa</i> &#9794;. We learn that at
+the lower temperatures the result was eleven times the expected
+one, and six times an unexpected one; further, that we owe it to
+the author's inadvertence that we have come to hear of the
+expected result at all, and that though he knows the factors
+which determine the discrepancy, he declines for the present to
+name them. In these circumstances we can scarcely venture as
+yet to estimate the significance of these records.</p>
+
+<p class="indent">The paper goes on to recount somewhat comparable, but
+more complex instances in which the descent of the colour of adults and
+of larvae was affected by temperature in crosses between <i>undecimlineata</i>
+and <i>signaticollis</i>. As they stand the results are
+very striking and unexpected, but I think, in view of what has
+been admitted respecting the former part of the paper, full discussion
+may be postponed till confirmation is forthcoming.</p>
+
+<p class="indent">One feature, however, calls for remark. This second
+paper is written apparently without any reference to the discoveries
+related by Tower in his previous book, to which no allusion is
+made. This is most noticeable in the case of an experiment in
+which (p. 296, H. 700A) <i>undecimlineata</i> &#9792; (the dominant)
+was mated to <i>signaticollis</i> &#9794; with the result that all the offspring
+were <i>undecimlineata</i> and bred true to that type (Parthenogenesis
+was tested for, but never found to occur). This experiment was
+made at a temperature averaging 95° F. ± 3.5° by day and 89° F.
+± 4.8° by night, and in a humidity given as 84 per cent. by day
+and 100 per cent. by night; but in the previous book (p. 294) we
+are told that pure <i>undecimlineata</i> bred together "under an extreme
+stimulus of high temperature, 10° C. above the average"
+and a relative humidity of 40 per cent. gave 11 beetles only,
+all <i>angustovittata</i>. But reference to the Plate 16, Fig. 2, shows
+that <i>angustovittata</i> must be exceedingly like <i>signaticollis</i>,
+having, like it, the elytral stripes obsolete, and if there is any marked
+difference at all, it can only be in the larvae. It seems strange
+that if <i>undecimlineata</i> really gives off ova of this recessive type at
+high temperatures, the fact should not be alluded to in connection
+<span class="pagenum"><a name="Page_227" id="Page_227">[Pg 227]</a></span>
+with expt. H. 700A, where, as the father was <i>signaticollis</i>, having
+the same recessive character, their appearance might have been
+expected not to pass unobserved. The temperature in the
+older experiment is, of course, not given with the great accuracy
+used in the second, and it may have been higher still. The humidity
+also was widely different. Still, in discussing the phenomena
+we should expect some reference to the very remarkable and
+closely cognate discovery which Tower himself had previously
+reported in regard to the same species.<a name="FNanchor_13_176" id="FNanchor_13_176"></a><a href="#Footnote_13_176" class="fnanchor">[13]</a>
+</p>
+
+<p class="indent">The hesitation which I had come to feel respecting
+these two publications of Tower's has been, I confess, increased by the
+appearance of a destructive criticism by Gortner<a name="FNanchor_14_177" id="FNanchor_14_177"></a><a href="#Footnote_14_177" class="fnanchor">[14]</a>
+who has examined the parts of Chapter III of Tower's book, in which he
+discusses at some length the chemistry of the pigments in <i>Leptinotarsa</i>
+and other animals. As Gortner has shown, this discussion,
+though offered with every show of confidence, exhibits
+such elementary ignorance, both of the special subject and of
+chemistry in general, that it cannot be taken into serious consideration.</p>
+
+<p class="indent">Some observations made by Dr. W. T. Macdougal<a name="FNanchor_15_178" id="FNanchor_15_178"></a><a href="#Footnote_15_178" class="fnanchor">[15]</a>
+have also been interpreted as showing the actual causation of genetic
+variation by chemical treatment. Of these perhaps the least open
+to objection were the experiments with <i>Raimannia odorata</i>, a
+Patagonian plant closely allied to <i>Oenothera</i>. The ovaries were
+injected with various substances and from some of the seeds
+which subsequently formed in them a remarkable new variety
+was raised. This varying or mutational form was strikingly
+different from the parental type, with which it was not connected
+by any intergradational forms, and it bred true. It made
+<span class="pagenum"><a name="Page_228" id="Page_228">[Pg 228]</a></span>
+no rosette, growing to a much smaller size than the parent, and
+was totally glabrous instead of being very hairy as the parental
+type is. I was shown specimens of these plants by the kindness
+of Dr. Britton in the Bronx Park Botanic Garden in 1907 and
+can testify to their very remarkable peculiarities. They had a
+somewhat weakly look, and might at first sight be thought to be
+a pathological product, but they had bred true for several
+generations. From the evidence, however, I am by no means
+satisfied that their original appearance was a consequence of
+the treatment applied. This treatment was of a most miscellaneous
+description. Two of the mutants came from an ovary
+which had been treated with a ten per cent. sugar solution. Ten
+came from one into which a 0.1 per cent. solution of calcium
+nitrate had been injected. One was from a capsule which "had
+been exposed to the action of a radium pencil." Macdougal
+speaks of these results as decisive, but clearly before such evidence
+can be admitted even for consideration it must be shown by control
+experiments that the individual plants which threw the
+mutant were themselves breeding true in ordinary circumstances.
+Nothing is more likely than that the mutant was an ordinary
+recessive. I may add that Mr. R. H. Compton made a number
+of experiments with <i>Raimannia odorata</i>, raised from seeds kindly
+given me by Dr. Britton, injecting the ovaries with a variety of
+substances, including those named by Macdougal; but though a
+numerous progeny was raised from the ovaries treated, all were
+normal. Macdougal relates also that some mutational forms
+came from ovaries of <i>Oenothera Lamarckiana</i> exposed to radium
+pencils, and also from <i>Oenothera biennis</i> injected with zinc sulphate
+a peculiar mutant was raised, but taking into account the
+frequency of these occurrences in those species, he very properly
+regarded this evidence as of doubtful application. In a later
+paper,<a name="FNanchor_16_179" id="FNanchor_16_179"></a><a href="#Footnote_16_179" class="fnanchor">[16]</a>
+however, he has returned to the subject and affirms his
+conviction that the appearance of a mutant among seedlings
+raised from an ovary of <i>Oenothera biennis</i> treated with zinc
+sulphate was really a consequence of the injection, saying that
+<span class="pagenum"><a name="Page_229" id="Page_229">[Pg 229]</a></span>
+the variation previously observed in the species was afterwards
+shown to be due to fungoid disease. The circumstances to which
+he mainly points in support of his view is that the mutation bred
+true, but this is only evidence of its genetic distinctness, which
+may, of course, be admitted by those who remain unconvinced
+as to the original cause of its appearance. He adds that he is
+making similar experiments with some twenty genera; but what
+is more urgently needed is repeated confirmation of the original
+observation. When it has been shown that this mutation can
+be produced with any regularity from a plant which does not
+otherwise produce it on normal self-fertilisation, the enquiry
+may be profitably extended to other plants.</p>
+
+<p class="indent">A curious and novel experiment, which however, led
+ultimately to a negative result, was made by F. Payne. Many discussions
+have been held respecting the blindness of cave animals.
+The phenomenon is one of the well-known difficulties, and most
+of us would admit that the theory of evolution by the natural
+selection of small differences does not offer a really satisfying account
+of it. Those who believe in the causation of such modifications
+by environmental influences and in their hereditary transmission
+make, of course, the simple suggestion that the darkness
+is the cause of the loss of sight, and that disuse has led to the
+reduction of the visual organs. Payne bred <i>Drosophila ampelophila</i>,
+the pomace-fly (which is easy to keep in confinement, fed
+on fermenting bananas), for sixty-nine generations in darkness.
+At the end of that period there was no perceptible change in the
+structure of the eyes, or in any other respect. The number of
+generations may possibly be regarded as insufficient to prove
+anything, but comparing them, as he does, with the generations
+of mankind, we see that they correspond with a period of about
+two thousand years, an interval far longer than those which
+many writers in particular cases have deemed sufficient.</p>
+
+<p class="indent">In his first paper Payne states that, though no structural
+difference could be perceived, the flies which had been bred in
+the dark reacted less readily to light than those which had been
+reared under normal conditions, and he inclined to think that
+the treatment had thus produced a definite effect. After more
+<span class="pagenum"><a name="Page_230" id="Page_230">[Pg 230]</a></span>
+careful tests, however, he withdrew this opinion. It proved
+that both individual flies and individual groups of flies, both of
+those bred in the light and of those bred in the dark, differed
+greatly in their reactions, which were measured by counting the
+time that it took for a fly to travel to the light end of a covered
+tube, various sources of error being eliminated. He found further
+that these differences of behaviour were not inherited in any
+simple way, but he is disposed to attribute them to accidental
+differences in the nature of the food, an account which seems
+probable enough.<a name="FNanchor_17_180" id="FNanchor_17_180"></a><a href="#Footnote_17_180" class="fnanchor">[17]</a>
+</p>
+
+<p class="indent">In several recent publications Blaringhem<a name="FNanchor_18_181" id="FNanchor_18_181"></a><a href="#Footnote_18_181" class="fnanchor">[18]</a>
+has described the origin of many abnormal forms of plants, especially
+of maize, which he attributes to various mutilations practised upon the
+parents. Respecting these the same difficulty which has been
+expressed in other cases reappears, that before drawing any
+conclusion as to the value of such evidence we require to know
+that the plants treated belong to a really pure line, which if
+left to nature in the ordinary circumstances of its life in that
+locality would have had normal offspring. Abnormalities abound
+in the experience of everyone who examines pans of seedlings
+of almost any species of plant, and in maize they are well known
+to be exceptionally common. Some of those which we meet
+with when we attempt to ripen maize in this country are very
+similar to those which Blaringhem describes, consisting in irregularities
+in the distribution of the sexes, in the shapes of the
+panicles, etc. Many of these are doubtless imperfections of
+development, due to the dullness of our climate, but others are
+presumably genetic and would recur in the offspring however
+treated. If some one working in a climate where maize could
+be raised in perfection would repeat these experiments, and show
+that a strain which was thoroughly reliable and normal in its
+genetic behaviour did, after mutilation, throw the miscellaneous
+types observed by Blaringhem, that would be evidence at least
+that the development of the seed could be so influenced by
+injury to the parental tissues that its properties were changed.
+<span class="pagenum"><a name="Page_231" id="Page_231">[Pg 231]</a></span>
+Such evidence could be used for what it is worth; but pending an
+inquiry of this kind I am disposed to regard these observations of
+variation following on parental injury as suggestive rather than convincing.</p>
+
+<p class="indent">Some evidence of a remarkably interesting kind has been
+collected by J. H. Powers<a name="FNanchor_19_182" id="FNanchor_19_182"></a><a href="#Footnote_19_182" class="fnanchor">[19]</a>
+respecting the structure and habits
+of <i>Amblystoma tigrinum</i>, which led him to the conclusion that
+striking differences in the form, anatomy, and developmental
+processes could be effected directly by change in the conditions
+of life. It is well known that a profusion of forms, distinct in
+various degrees, is grouped round <i>Amblystoma tigrinum</i>. Some
+of these are believed to be geographically isolated, others occur
+together in the same waters, and, as usual, authorities have differed
+greatly as to the number of names to be given. These forms
+were studied in detail by Cope who described them in the <i>Batrachia
+of North America</i>. The view which he inclined to take
+was that the individual variations of <i>Amblystoma tigrinum</i> resulted
+from variations in the time and completeness of the
+metamorphosis, and these were regarded as due to external
+causes, such as differences in season, temperature, and geographical
+conditions. Powers, however, states that collecting
+within a radius of six or eight miles he found almost if not quite
+the whole "gamut of recorded variation in this species." Some,
+however, as he states, occurred rarely except under experimental
+conditions, but considerable differences in temperature were not
+found necessary in producing them. Every year, he says, he
+has been able to add to the number of peculiar types found in
+the same small area in nature, until the amount of natural
+variation at least equals that seen by Cope in the collections of
+the National Museum and those of the Philadelphia Academy.</p>
+
+<p class="indent">Powers states that his observations by no means confirm
+Cope's view that these differences are in the main referable to
+variation in the completeness of metamorphosis, and on the
+contrary, he regards metamorphosis as on the whole a levelling
+process, tending to obliterate diversity. The enormous
+<span class="pagenum"><a name="Page_232" id="Page_232">[Pg 232]</a></span>
+differences in size and proportions which he describes can only be
+appreciated by reference to his figures. They affect almost all
+features of bodily organisation. These striking differences he
+looks upon as brought about by differences in nutrition, "diversities
+in habitual locomotion," and diversity in the age at which
+metamorphosis occurs, and to sexual difference. Apart from
+sexual difference he regards the chief distinctions, in brief, as
+"acquired variations of the larva."</p>
+
+<p class="indent">As an example he gives the great elongation of some
+of the forms as "due first to slow growth, second to the free-swimming
+habit, third to the prolongation of larval life, and finally to the
+assumption of sexual maturity as males," either in the branchiate
+or non-branchiate condition. He describes the rapid growth of
+some and the slow growth of others. A larva of intermediate
+type may grow about a centimeter a month, but a rapidly growing
+specimen may grow more than four times as much. The slower
+rate of growth may, he says, be induced by winter feeding, and
+other treatment.<a name="FNanchor_20_183" id="FNanchor_20_183"></a><a href="#Footnote_20_183" class="fnanchor">[20]</a>
+</p>
+
+<p class="indent">When, however, he goes on to describe the influences
+which he regards as exerted by the habit of freely swimming, I am led
+to wonder whether after all in most of these illustrations, the
+primary distinctions are not in reality genetic. "Specimens
+raised in the same aquarium or in similar aquaria, side by side
+with all conditions as uniform as it is possible to make them,
+seldom fail to furnish striking examples of broad-headed, short-bodied,
+and short-tailed types which are habitually found at the
+bottom, while others, slender and elongated, are free swimmers,
+and maintain themselves in almost as continual suspension and
+motion as does a gold fish." Later, again, he writes, "Yet despite
+the uniformity of these favourable conditions, the larvae soon
+began to split up into two noticeably distinct groups, the one of
+<span class="pagenum"><a name="Page_233" id="Page_233">[Pg 233]</a></span>
+unusually compact proportions, the other of uniform intermediate
+build, such is most commonly met with." It is to my mind
+scarcely possible to resist the inference that, though there may be
+definite responses to certain conditions, yet the chief distinctions
+are genetic, and that it is these distinctions which confer the
+power to respond. The parts respectively played by cause and
+effect are always difficult to assign; but when it is stated that
+"a weak-limbed, long-bodied and long-tailed animal becomes
+well nigh perforce an undulatory swimmer, while the strong-limbed,
+short-tailed, heavy-bodied specimen, when these characteristics
+are rapidly forced upon it, is, under certain circumstances,
+just as forcibly induced to become a crawler," we feel
+how erroneous any estimates of causation are likely to be.</p>
+
+<p class="indent">One of the most remarkable and interesting sections
+of Powers' paper is that in which he describes the differences in
+bodily structure and habits which he attributes to cannibalism,
+and the whole account of the phenomena should be read in the
+original. It appears that there are two extremely distinct
+types of larvae, those with narrow heads and slender bodies
+which live for the most part on small Crustacea such as <i>Daphnias</i>,
+and those with huge mouths and very wide heads, which disregard
+such small animals altogether and live on amphibian
+larvae, whether of their own or other species. As the illustrations
+show, the differences between these two types are very
+great, and the differences in instinct and behaviour are no less.
+The cannibals take no heed of the pelagic crustacea, lying sluggishly
+at the bottom, rousing themselves immediately to a
+violent attack on the larger living things which approach them.
+Nothing but the most incontrovertible evidence based on abundant
+control experiments should convince us that such differences
+are not primarily genetic, and in the present state of knowledge
+I incline to think that the families really consist of individuals
+which are ready to assume the cannibal habit if opportunity
+offers, and others which are congenitally incapable of it. It may
+readily be that if all chance of cannibal diet be excluded, the
+full development of the wide head and mouth, or the other
+peculiarities, would never become pronounced, but I doubt whether
+such change could be induced in any individual taken at random.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_234" id="Page_234">[Pg 234]</a></span></p>
+
+<h2>CHAPTER XI.</h2>
+<h3><span class="smcap">Sterility of Hybrids. Concluding Remarks.</span></h3>
+
+<p class="indent">When we consider the bearing of recent discoveries on
+those comprehensive schemes of evolution with which we were formerly
+satisfied, we find that certain details of the process are more easy
+to imagine. We readily now understand how varieties once
+formed, can persist, but at the same time difficulties hitherto
+faced with complacency become formidable in the light of the
+new knowledge. So generally is this admitted by those familiar
+with modern genetic research that most are rightly inclined to
+postpone the discussion. The premisses, indeed, on which such
+a discussion must be based are almost wholly wanting.</p>
+
+<p class="indent">The difficulties to which I chiefly refer are not those
+ created by the phenomena of adaptation, though they are serious enough.
+In treating of that subject I have felt obliged to express scepticism
+as to the validity of nearly all the new evidence for the
+transmission of acquired characters. At the present time the
+utmost we are bound to accept is the proof that (1) in some
+parthenogenetic forms variations, or perhaps we may say malformations,
+produced in response to special conditions, recur in
+one or perhaps two generations asexually produced after removal
+to other conditions. (2) That violent maltreatment may in rare
+instances so affect the germ-cells contained in the parents as to
+cause the individuals resulting from the fertilisation of those
+cells to exhibit an arrest of development similar to that which
+their parents underwent.</p>
+
+<p class="indent">I do not doubt that evidence of this type will be
+greatly extended. As a contribution to genetic physiology these facts
+are very important and interesting, but I cannot think that any
+one, on reflexion, will feel encouraged by such indications to
+revive old beliefs in the direct origin of adaptations.</p>
+
+<p class="indent">In these respects we are simply left where we
+were. The force of objections based upon the existence of adaptative
+<span class="pagenum"><a name="Page_235" id="Page_235">[Pg 235]</a></span>
+mechanisms is no greater than it has always been. On the contrary
+the fact that variations can now so generally be recognized
+as definite is some alleviation of the difficulty. We can moreover
+disabuse ourselves of the notion that for all characters which are
+definite or fixed, some utilitarian rationale may be presumed.
+Upon that point the study of variation has provided a perfectly
+clear answer.</p>
+
+<p class="indent">In frankly recognizing that the fixity of characters in
+general need not connote usefulness to their possessors we deliver ourselves
+of a distracting pre-occupation and prepare our minds for
+an investigation of the properties of living organisms in the
+same spirit as that in which the chemist and the physicist
+examine the properties of unorganized materials. The creature
+persists not merely by virtue of its characteristics but in spite
+of them, and the fact of its persistence proves no more than
+that on the whole the balance of its properties leaves something
+in its favour.</p>
+
+<p class="indent">It may be noted by the way that the fact that the
+structures of living things are on the whole adaptative was not always
+obvious. Though to naturalists of this generation it is a truism,
+we have only to turn to Buffon to find that in his philosophy of
+nature it played no essential part. The passage in which Buffon
+describes what he regards as the forlorn and degraded condition
+of the Woodpecker is well known. We have come to think of
+the Woodpecker as a capital example of adaptation to the mode
+of life; but Buffon after enumerating the hard features of the
+bird's existence, forced to earn its living by piercing the bark of
+trees in an attitude of perpetual constraint, remarks<a name="FNanchor_1_184" id="FNanchor_1_184"></a><a href="#Footnote_1_184" class="fnanchor">[1]</a>
+"Tel est l'instinct étroit et grossier d'un oiseau borné a une vie triste
+et chétive. Il a reçu de la Nature des organes et des instrumens
+appropriés a cette destinée <i>ou plutôt il tient cette destinée même
+des organes avec lesquels il est né</i>" (my italics). His reflexions
+on the Stilt (<i>Himantopus</i>) read even more strangely to us,
+accustomed as we are to see in the prodigious length and thinness
+of the shanks and in the other features of its organisation palpable
+adaptations to a wading life. For Buffon, however, this
+<span class="pagenum"><a name="Page_236" id="Page_236">[Pg 236]</a></span>
+curious bird seemed a poor, neglected production, extravagant
+in its disproportions, one of the misfits of creation, left as a
+shadow in the picture composed of nature's more successful
+efforts.<a name="FNanchor_2_185" id="FNanchor_2_185"></a><a href="#Footnote_2_185" class="fnanchor">[2]</a> This theme he develops at some length, being evidently
+well pleased with the idea.</p>
+
+<p class="indent">Our way of regarding these things is doubtless sounder
+and more fruitful than Buffon's, but it is well to remember that what
+seems so obvious to us looked quite differently to other excellent
+observers; and stupid as it may have been to have overlooked
+plain examples of adaptation, it is a far worse mistake to see
+adaptation everywhere. I do not seek to minimise the real
+and permanent difficulty which the existence of adaptations
+creates, but by the suggestion that all normal specific differences
+are adaptational that difficulty was quite gratuitously increased.</p>
+
+<p class="indent">In these respects it may be claimed that progress has
+been made, even if that progress seem outwardly of small account.</p>
+
+<p class="indent">But all constructive theories of evolution have been
+built on the understanding that what we know of the relation of varieties
+to species justifies the assumption that the one phenomenon is a
+phase of the other, and that each species arises or has arisen
+from another species either by one or several genetic steps. In
+the varieties we have accustomed ourselves to think that we see
+those steps. We still know little enough of the mode of occurrence
+of variation, but we do begin to know something, and if we
+ask ourselves whether our knowledge, such as it is, conforms at
+all readily with our former expectations, we cannot with any
+confidence assert that it does. Among the plants and animals
+genetically investigated are many illustrations of very striking
+and distinct varieties. Many of these might readily enough be
+accepted as species by even the most exacting systematists, and
+not a few have been so treated in classification; but when we
+have examined their relationship to each other we feel not merely
+that they are not species in any strict sense but that the distinctions
+they present cannot be regarded as stages in the direction
+of specific difference. Complete fertility of the results of
+inter-crossing is and I think must rightly be regarded as
+<span class="pagenum"><a name="Page_237" id="Page_237">[Pg 237]</a></span>
+inconsistent with actual specific difference; and of variations leading
+to that consequence no clear indication has yet been found.
+As an example of possible exceptions mention should perhaps be
+made of the case of a giant form of <i>Primula sinensis</i> investigated
+by Keeble.<a name="FNanchor_3_186" id="FNanchor_3_186"></a><a href="#Footnote_3_186" class="fnanchor">[3]</a>
+It arose from a "Star" Primula of normal size,
+and though fertile with its own pollen all attempts to fertilise
+it with the pollen of other forms failed. Miss Pellew, who did
+these fertilisations, tells me that very extensive trials were made,
+and repeated in several seasons. Ultimately two plants were
+raised from it fertilised with a plant of the strain from which it
+sprang, and these proved sterile. In the light of modern experience
+the significance of such isolated instances is doubtful.</p>
+
+<p class="indent">All the strains known as "Giants" are, as Messrs. Sutton
+have always found, more or less sterile, and their sterility is
+presumably due to some negative defect.</p>
+
+<p class="indent">In regard to the fertility of Primula species there
+are several paradoxes. For example the long-styled varieties, apart from
+giants, are fertile with their own pollen, and for many years
+short-styled plants have not been used in most strains. Auriculas
+and Polyanthuses, on the contrary, are generally if not always
+bred from short-styled plants, as the florists have decided that
+the long-styled are inadmissible. Mr. R. P. Gregory tells me
+that, though most strains of <i>P. sinensis</i> give seed enough
+when only long-styled plants are used, he finds nevertheless
+that when a "legitimate" union is made the amount of seed
+usually increases much as Darwin observed. Darwin's statement
+that plants of "illegitimate" origin are less fertile than the
+"legitimately" raised plants is also in general confirmed by his
+experience. To this rule there were some marked exceptions in
+strains derived from <i>long</i>-styled plants, which though illegitimate
+showed a high degree of fertility, but illegitimate unions between
+<i>short</i>-styled plants always produced comparatively sterile offspring.
+I have no records of the behavior of Auriculas and
+Polyanthuses. It would be interesting to know whether among
+them pure strains of short-styled plants (dominants) have
+appeared, and, if so, how their fertility is affected. Without
+<span class="pagenum"><a name="Page_238" id="Page_238">[Pg 238]</a></span>
+much more critical data I suppose no one would nowadays be
+inclined to follow Darwin in instituting a comparison between
+the sterility of hybrids and that of illegitimately raised plants of
+heterostyle species.<a name="FNanchor_4_187" id="FNanchor_4_187"></a><a href="#Footnote_4_187" class="fnanchor">[4]</a>
+It is even difficult to imagine any essential
+resemblance between these two phenomena, nor has evidence
+ever been produced to show that illegitimately raised plants
+have bad pollen grains, which is the usual symptom of sterility
+in hybrid plants and the consequence, as we believe, of failure
+of some essential division in the process of maturation.</p>
+
+<p class="indent">The difficulty that we have no knowledge of the contemporary
+origin of forms, from a common stock, which when crossed together
+give a sterile product, is one of the objections constantly and
+prominently adduced from the time of the first promulgation of
+evolutionary ideas. In the light of recent work the objection
+has gathered strength. Why, if we are able to produce instances
+of variation colourably simulating specific difference in almost
+all other respects, do we never find an original appearance of this
+most widely spread of all specific characteristics? No doubt all
+breeders know that sterile animals and plants occasionally appear
+in their cultures, but it is more in accordance with probability
+that the sterility in these sporadic instances should be regarded
+as due to defect than that it should be thought comparable
+with that of the sterile hybrids. For their sterility must, by all
+analogy with results elsewhere seen, be attributed not to the
+absence of something, but to the presence and operation of
+complementary factors leading to the production of inhibition
+of division; and consistently with that interpretation, we find
+that when from a partially sterile hybrid comparatively fertile
+offspring can be raised, their comparative fertility continues in
+the posterity generally if not always without diminution. The
+distinction between these several kinds of sterility was of course
+not understood in Darwin's time. The comparison, for example,
+which he instituted<a name="FNanchor_5_188" id="FNanchor_5_188"></a><a href="#Footnote_5_188" class="fnanchor">[5]</a>
+between the sterility of "contabescent"
+anthers and that of hybrids no longer holds, for at least in those
+cases in which the nature of contabescent anthers have been
+genetically investigated (Sweet Pea, <i>Tropaeolum</i>) they proved
+<span class="pagenum"><a name="Page_239" id="Page_239">[Pg 239]</a></span>
+to be a simple recessive character. Nor can we now easily
+suppose that the attempt there made by Darwin to suggest
+resemblance between the sterility produced by unnatural conditions
+and that of hybrids has any physiological justification.</p>
+
+<p class="indent">In regarding the power to produce a sterile or partially
+sterile hybrid as a distinction in kind, of a nature other than
+those which we perceive among our varieties, I am aware that
+I am laying stress on an impression which may hereafter prove
+false. The distinction nevertheless is so striking and so continually
+before the eyes of a practical breeder that he can
+scarcely avoid the inference that when he meets a considerable
+degree of sterility in a cross-bred he is dealing with something
+belonging to a distinct category, and not merely a varietal feature
+of an exceptional kind.</p>
+
+<p class="indent">Besides the sterility of hybrids appeal has often been
+made to the phenomenon of incompatibility, in its several stages of
+completeness, as distinguishing species. No one doubts that
+incompatibility may arise from a variety of causes of most diverse
+degrees of importance, but though sometimes referred to as an
+extreme case of interspecific sterility, it is really a very different
+matter. In regard to one phase of this incompatibility, that
+associated with self-sterility, some progress has been made, and
+we are not wholly without experimental evidence of its being
+within the range of contemporary variation.</p>
+
+<p class="indent">Given the outline of Mendelian teaching as to
+gametic differentiation and the classification of individuals in a mixed
+population, it seemed highly probable that what we call self-sterility
+must mean that the species really consisted of <i>classes</i>,
+some of which are capable of interbreeding with others while
+others are not. According to the received account every individual,
+though incapable of fertilising itself, was supposed to be
+able both to fertilise and to be fertilised by any other individual.
+This notion has always seemed to me a self-evident absurdity,
+for it would imply that there can be as many categories as
+individuals. Such experiments, however, as I made did certainly
+give results consistent with that belief. I first tried
+Cinerarias, which are usually self-sterile, but I found no
+<span class="pagenum"><a name="Page_240" id="Page_240">[Pg 240]</a></span>
+incompatible pairs of plants. Whether I was deceived by the
+consequences of apogamy, or whether the pollen of certain plants
+may belong to more than one class I do not know. The results
+were confused in various ways. Usually the self-fertilised plants
+set little or nothing, and cross-fertilised they set fully with such
+uniformity that the few failures could plausibly be attributed to
+mistakes in manipulation or to other extraneous causes. Later
+de Vries announced<a name="FNanchor_6_189" id="FNanchor_6_189"></a><a href="#Footnote_6_189" class="fnanchor">[6]</a>
+(without giving particulars) that he had
+proved the existence of such classes in <i>Linaria vulgaris</i>; but on
+making experiments with that species I again got no positive
+results, and I came to the conclusion that in spite of inherent
+improbability the conventional belief must be substantially true.
+At last, however, the work of Correns, lately published,<a name="FNanchor_7_190" id="FNanchor_7_190"></a><a href="#Footnote_7_190" class="fnanchor">[7]</a>
+does definitely show that in one species, <i>Cardamine pratensis</i>,
+classes of individuals exist such that individuals of the same class are
+incapable of fertilising themselves or each other, but fertilisation
+made between the classes is usually completely effective. Many
+complications were encountered and some contradictory evidence
+is recorded, but the general bearing of the results was positive
+and indubitable.</p>
+
+<p class="indent">We know far too little of this phenomenon as yet to
+be able to understand its significance, but I suppose we may anticipate
+with some confidence that it will be found to be a manifestation
+of dissimilarity between the male and female gametes of the
+same individual, comparable with that first seen in the Stocks
+(<i>Matthiola</i>) which throw doubles&mdash;a state of things in all likelihood
+to be found widely spread among hermaphrodite organisms.
+Whether the incompatibility between species is to be associated
+with that of the self-steriles also cannot be positively asserted,
+though it seems not unreasonable to expect that such an association
+will be discovered.</p>
+
+<p class="indent">The case of the apple and the pear is an impressive
+illustration of this possibility. The two species are of course exceedingly
+alike in all outward respects, but nevertheless the pollen of each
+is entirely without effect on the other. Presumably we should
+<span class="pagenum"><a name="Page_241" id="Page_241">[Pg 241]</a></span>
+interpret this fact as meaning not so much that the apple and
+the pear are in reality very wide apart, but rather that either,
+each is lacking in one of two complementary elements, or that
+each possesses a factor with an inhibitory effect. Their incompatibility
+may well be of the same nature as that of the classes
+in <i>Cardamine pratensis</i>.</p>
+
+<p class="indent">Returning now to the problem of inter-specific sterility;
+we note, as I have said, the absence of contemporary evidence that
+variation can confer on a variety the power to form a sterile
+hybrid with the parent species. The considerations based on
+this want of evidence have for a long while been familiar to all
+who have discussed evolutionary theories, and it is worth observing
+the exact reason why the difficulty strikes us now with a new
+and special force. In pre-Mendelian times all that was known
+was that some forms could freely interbreed without diminution
+of fertility in the product, while others could not. But now we
+find that, by virtue of segregation, from one and the same pair
+of parents, or even, in the case of hermaphrodites, from one and
+the same individual, offspring commonly arises showing among
+themselves exactly such differences as distinguish species&mdash;and
+very good species too. This we see happening again and again.
+But to forms capable of arising as brethren in one family the
+title species has never been meant to apply, and if we are going
+to use the term in application to fraternal groups we must
+definitely recognise that by "specific" difference is to be understood
+simply <i>difference</i>, without any immediate or even ulterior
+physiological limitation whatever. Naturally, therefore, we begin
+to think of the appearance of sterility in crosses as something
+apart, and as a manifestation which distinguishes certain kinds
+of unions in a very special way.</p>
+
+<p class="indent">I am perfectly aware that there are gradations in the
+sterility of hybrids as in every other characteristic upon which it has
+been proposed to base specific definitions; but, as also so often happens
+in the matter of defining intergrading categories, the difficulty
+in practice is not often such as to lead to actual ambiguity. I
+am speaking of course of those examples which are amenable to
+genetic experiment. As to the rest there is complete and permanent
+<span class="pagenum"><a name="Page_242" id="Page_242">[Pg 242]</a></span>
+uncertainty. But the experience of the practical breeder
+does, I think, on the whole, support the contention to which
+systematists have so steadily clung under all the assaults of
+evolutionary philosophers, that, though we cannot strictly define
+species, they yet have properties which varieties have not, and
+that the distinction is not merely a matter of degree.</p>
+
+<p class="indent">The first step is to discover the nature of the factors
+which by their complementary action inhibit the critical divisions and so
+cause the sterility of the hybrid. Thus expressed, we see the
+problem of inter-specific sterility in its right place; and the
+question why we do not now find contemporary instances of
+varieties lately arisen in domestication, which when crossed back
+with their parents, or with their coderivatives, can produce
+sterile products, is perceived to be only a special case of a problem
+which in its more general form is that of the origin of new and
+additional factors.</p>
+
+<p class="indent">For the requisite evidence no comprehensive search
+has been made, but perhaps it will yet be found. All that we can say
+at the present time is that the incidence both of hybrid sterility,
+and of incompatibility also, is most capricious; and provided
+that two forms have such features in common that a cross between
+them seems not altogether out of the question, no one can predict
+without experiment whether such a cross is feasible, and if
+feasible whether the product will be fertile, or sterile more or
+less completely. For instance, though probably all the British
+and some Foreign Finches (Fringillidae) have been crossed
+together, and some of these crosses, as for instance, the various
+Canary-mules have been made in thousands, I believe no quite
+clear example of a fertile hybrid can be produced. Many species
+of Anatidae cross readily and produce fertile hybrids: others give
+results uniformly sterile. Though most of the Equidae can be
+crossed and some of the hybrids are among the commonest of
+domesticated animals there is no certain record of a fertile mule.
+Among the Canidae the dogs, wolves and jackals all give fertile
+hybrids, but there is no clearly authenticated instance of a cross
+between any of these forms and the European fox. In spite of
+their close anatomical resemblance it is doubtful if the rabbit
+<span class="pagenum"><a name="Page_243" id="Page_243">[Pg 243]</a></span>
+and the hare have ever interbred. Many of the wild species of
+<i>Bos</i> have been crossed and recrossed both with each other and
+with many domesticated races, but I understand that no cross
+with the Indian buffalo (<i>Bos bubalus</i>) has yet been successful
+even in producing a live calf.<a name="FNanchor_8_191" id="FNanchor_8_191"></a><a href="#Footnote_8_191" class="fnanchor">[8]</a>
+In the genus <i>Primula</i> many
+hybrids are known and several of them occur in nature, but
+hitherto no certain hybrid between <i>P. sinensis</i> and any other
+species has been made, in spite of repeated attempts.</p>
+
+<p class="indent">In <i>Nicotiana</i> many&mdash;doubtless all&mdash;the various
+forms of <i>N. tabacum</i> can be crossed together without diminution of fertility,
+though some are very distinct in appearance, but crosses between
+<i>tabacum</i> and <i>sylvestris</i> are highly sterile (in my experience totally
+sterile<a name="FNanchor_9_192" id="FNanchor_9_192"></a><a href="#Footnote_9_192" class="fnanchor">[9]</a>),
+though the distinctions between them are not to outward
+observation nearly so great as those which can be found between
+the various races of <i>Primula sinensis</i>.</p>
+
+<p class="indent">Recently some remarkable experiments bearing closely on
+these questions have been published by F. Rosen.<a name="FNanchor_10_193" id="FNanchor_10_193"></a><a href="#Footnote_10_193" class="fnanchor">[10]</a>
+They concern the forms of <i>Erophila (Draba) verna</i>, celebrated in
+the history of evolutionary theory as the plants especially chosen by
+Alexis Jordan for the exposition of his views on these subjects.</p>
+
+<p class="indent">The "species" contains a profusion of forms dissimilar
+in many structural characters, such as the size and shape of leaves,
+flowers, fruits, etc. Of these forms many grow in association.
+Jordan found, on experiment, that each, to the number of some
+two hundred, bred true, and that therefore, the conventional
+assumption that polymorphism of this kind must mean great
+contemporary variability had no foundation in fact. So far
+<span class="pagenum"><a name="Page_244" id="Page_244">[Pg 244]</a></span>
+indeed is the evidence from favouring the belief that such forms
+are in any way transitional or indeterminate, that, as is well
+known, Jordan used it with every plausibility to support the
+doctrine of the fixity of species. To certain aspects of Jordan's
+work we will return later in this chapter, but the matter is in
+the present connection of especial interest for the reason that
+Rosen has lately found by experiment that some of these presumably
+very closely allied forms, crossed together, gave hybrids
+more or less sterile. In the case of the offspring of one pair of
+forms only (<i>E. cochleata</i> and <i>stricta</i>) was the fertility
+undiminished, and the various degrees of sterility found in the
+other crosses ranged up to the extreme infertility of the hybrids
+between <i>E. stricta</i> × <i>elata</i>. From this cross ten plants were
+bred. Of these the four strongest were chosen to breed from,
+but two of the four proved totally sterile; one had only bad seeds;
+and from the fourth a single seedling was raised which in its
+turn proved to be sterile. From the less sterile hybrids F<sub>2</sub>
+families were raised, with the usual experience that in this and
+subsequent generations the sterility diminished among extracted
+forms, new and true-breeding types with complete fertility being
+thus derived from the original cross.<a name="FNanchor_11_194" id="FNanchor_11_194"></a><a href="#Footnote_11_194" class="fnanchor">[11]</a>
+</p>
+
+<p class="indent">The production of sterility as a consequence of crossing
+plants so nearly approaching each other as these <i>Erophila</i>
+"species" do is not a little interesting, and the fact well exemplifies
+the futility of the various attempts to frame general
+expressions as to specific properties or behaviour. Commenting
+on his results Rosen argues that the polymorphic group commonly
+called by systematists <i>Erophila (Draba) verna</i> may now be
+regarded as having arisen by crossing, as did his own types mentioned
+above. The question, however, <i>what</i> species were the
+original progenitors of the group cannot be answered. Rosen
+considers that no form which he knows satisfies the requirements,
+<span class="pagenum"><a name="Page_245" id="Page_245">[Pg 245]</a></span>
+and that it or they must be supposed to be lost. This conclusion
+will recall the similar problem raised by the <i>Oenothera</i> mutants
+(Chap. V); and unsatisfactory as it may be to have recourse
+to such hypotheses we must remember the possibility that as a
+consequence of hybridisation, subsequent segregation and recombination
+of factors, species may have thus actually, as we
+may say, exploded, and left nothing but a polymorphic group of
+miscellaneous types to represent them in posterity. If this way
+of regarding the phenomena be a true one, the sterility now seen
+when some of the group are re-crossed, becomes analogous to
+that "reversion or crossing" which we now so well understand
+to be a consequence of the recombination of characters separated
+at some previous point in the history of descent. In the partial
+sterility of the contemporary hybrid we see this character reappearing,
+formed now as it was on the occasion of the original
+cross, by the meeting of complementary factors.</p>
+
+<p class="indent">Another case that may be mentioned in this connection is
+that of the crosses between various culinary peas (<i>Pisum sativum</i>)
+and a peculiar form found by Mr. Arthur Sutton growing ostensibly
+in a wild state in Palestine. This Palestine Pea is low
+growing, rarely reaching 18 inches. It is in general appearance
+like a small and poorly grown field pea. The stems are thin and
+rather hard. The most obvious differences which distinguish
+this from other field peas are the marked serration of the stipules,
+and the development of pith in the pods. Such pith is often
+present in the pods of peas more or less, but in the Palestines it
+is so strongly developed as almost to form a lomentum. Curiously
+enough, though the flowers are purple much as those of
+ordinary field peas, there is no coloured spot in the axils. On
+the other hand, the stems have coloured stripes running up
+from the axils. Though this plant differs so little from domesticated
+peas, all crosses with them either failed, or produced
+hybrids quite or almost quite sterile. This was Mr. Sutton's
+experience, and on repeating the experiments with material
+kindly given by him I found the same result.<a name="FNanchor_12_195" id="FNanchor_12_195"></a><a href="#Footnote_12_195" class="fnanchor">[12]</a>
+<span class="pagenum"><a name="Page_246" id="Page_246">[Pg 246]</a></span></p>
+
+<p class="indent">In a large series of crosses some seeds died or gave
+rise to feeble plants. Of the plants which lived, few gave any seed.
+The seed, however, that was obtained from F<sub>1</sub> plants grew well
+enough, and the F<sub>2</sub> plants proved, as often in such cases, fertile.
+In these, indeed, no sign of sterility was noticeable. The experiment
+is being repeated in various ways, for, as the genetic
+behaviour of peas is comparatively well known, the subject is an
+exceptionally favourable one for these investigations.</p>
+
+<p class="indent">Such an example shows the confusion produced the
+moment we attempt to harmonize conceptions of specific difference
+with results attained by experimental methods. It has been usual
+to regard the field pea (<i>P. arvense</i>) as a species distinct from
+the edible pea (<i>P. sativum</i>). De Candolle and others regard the
+field pea as derived from a form wild in Italy, but the origin of
+the edible pea is considered to be unknown. From breeding
+experiments we find no sterility whatever in the crosses between
+the various <i>arvense</i> and <i>sativum</i> types, nor in the crosses
+made between them and several other peculiar types from various
+countries; whereas this Palestine Pea, which only differs from a
+small <i>arvense</i> in what might have been thought trivial characters,<a name="FNanchor_13_196" id="FNanchor_13_196"></a><a href="#Footnote_13_196" class="fnanchor">[13]</a>
+either fails to cross altogether or gives a sterile product, whatever
+type be chosen as the other parent.</p>
+
+<p class="indent">Examples of this kind have at least the merit that
+they lead to more precise delimitations of the problem. We are confronted
+with two distinct alternatives.</p>
+
+<p class="indent">1. We may apply the term Species promiscuously to
+all distinct forms. If we do so it must be clearly understood that
+we cannot even rule out the several combinations of "presences
+and absences" represented by the various types whether wild or
+domesticated. For we may feel perfectly assured that at least
+all the <i>arvense</i> and all the <i>sativum</i> types yet subjected to
+experimental tests are on precisely the same level in this respect. There
+is no distinction, logical or physiological, to be drawn between
+them. Some contain more factors, and others contain fewer.
+In some the re-combinations have been brought about by natural
+variation or crossing, while the same consequences in the others
+have resulted from man's interference.
+<span class="pagenum"><a name="Page_247" id="Page_247">[Pg 247]</a></span></p>
+
+<p class="indent">2. We may follow the conventions of systematists and
+distinguish the outstanding or conspicuous forms such as <i>arvense</i>,
+<i>quadratum</i>, <i>sativum</i> and perhaps a few more as species, and
+leave the rest unheeded. If this course is followed it must be clearly
+understood and permitted as a piece of pure pragmatism, deliberately
+adopted for the convenience of cataloguers and collectors,
+without regard to any natural fact or system whatsoever.</p>
+
+<p class="indent">But while following either the one plan or the other
+we shall be still awaiting the answer, which only genetic experiment
+can provide, to the question whether among the various types
+there are some which differ from the rest in a peculiar way:
+whether by having groups of characters linked together in
+especially durable combinations, or by possessing ingredients
+which cause greater or less disturbance in the processes of cell-division,
+and especially in the processes of gametic maturation,
+when they are united by fertilisation with complementary ingredients.</p>
+
+<p class="indent">Before any but the vaguest ideas regarding the nature
+and significance of inter-specific sterility can be formed, a vast
+amount of detailed work must be done. Sterility as a result of
+crossing, as well as that which is alleged sometimes to arise in
+consequence of changed conditions, is at best a negative characteristic,
+and there are endless opportunities for mistake and misinterpretation
+in studying features of this kind. No one, I
+suppose, would now feel any great confidence in most of the data
+which from time to time are resuscitated for the purpose of such
+discussions. Even the best collections of evidence, such as those
+given by Darwin in <i>Forms of Flowers</i>, cannot be regarded as
+critical when judged by present-day standards. Nothing short
+of the most familiar acquaintance with the habitual behaviour
+of individuals, and of strains kept under constant scrutiny for
+several years would enable the experimenter to form reliable
+judgments as to the value to be attached to observations of
+this class.</p>
+
+<p class="indent">The admission must, however, be faced that nothing in
+recent work materially tends to diminish the surprise which has always
+been felt at the absence of sterility in the crosses between
+<span class="pagenum"><a name="Page_248" id="Page_248">[Pg 248]</a></span>
+co-derivatives. We should expect such groups of forms to behave
+like the <i>Erophila</i> types, and frequently to produce sterile
+products on crossing. Whatever be the explanation, the fact remains
+that such evidence is wanting almost completely. In spite of
+all that we know of variability nothing readily comparable with
+the power to produce a sterile hybrid on crossing with a near
+ally, has yet been observed spontaneously arising, though that
+characteristic of specificity is one of the most widely distributed
+in nature. It may be that the lacuna in our evidence is due
+merely to want of attention to this special aspect of genetic
+inquiry, and on the whole that is the most acceptable view which
+can be proposed. But seeing that naturalists are more and
+more driven to believe the domesticated animals and plants to be
+poly-phyletic in origin&mdash;the descendants, that is to say, of several
+wild forms&mdash;the difficulty is proportionately greater than it was
+formerly, when variation spontaneously occurring was regarded
+as a sufficient account of their diversity.</p>
+
+<hr class="chap" />
+<h2><span class="smcap">Concluding Remarks.</span></h2>
+
+<p class="indent">The many converging lines of evidence point so clearly to
+the central fact of the origin of the forms of life by an evolutionary
+process that we are compelled to accept this deduction, but as
+to almost all the essential features, whether of cause or mode,
+by which specific diversity has become what we perceive it to
+be, we have to confess an ignorance nearly total. The transformation
+of masses of population by imperceptible steps guided
+by selection, is, as most of us now see, so inapplicable to the
+facts, whether of variation or of specificity, that we can only
+marvel both at the want of penetration displayed by the advocates
+of such a proposition, and at the forensic skill by which it
+was made to appear acceptable even for a time.</p>
+
+<p class="indent">In place of this doctrine we have little teaching of a positive
+kind to offer. We have direct perception that new forms of life
+may arise sporadically, and that they differ from their progenitors
+quite sufficiently to pass for species. By the success and maintenance
+of such sporadically arising forms, moreover, there is
+no reasonable doubt that innumerable strains, whether in isolation
+<span class="pagenum"><a name="Page_249" id="Page_249">[Pg 249]</a></span>
+or in community with their co-derivatives, have as a fact
+arisen, which now pass in the lists of systematists as species. For
+an excellent account of typical illustrations I would refer the
+reader to the book lately published by R. E. Lloyd<a name="FNanchor_14_197" id="FNanchor_14_197"></a><a href="#Footnote_14_197" class="fnanchor">[14]</a>
+on the rat-population of India. The observations there recorded are typical
+of the state of things disclosed whenever the variations of large
+numbers of individuals are closely investigated, whether in
+domestication or in natural conditions.</p>
+
+<p class="indent">Guided by such clues we may get a good way into
+the problem. We see the origin of colourable species in abundance.
+Then, however, doubt arises whether though these new forms
+are as good species as many which are accepted as such by even
+cautious systematists, there may not be a stricter physiological
+sense in which the term species can be consistently used, which
+would exclude the whole mass of these <i>petites espèces</i>.</p>
+
+<p class="indent">If further we find that we have, with certain somewhat
+doubtful exceptions, never seen the contemporary origin of a
+dominant factor, or of inter-racial sterility between indubitable
+co-derivatives, it needs no elaboration of argument to show that
+the root of the matter has not been reached.</p>
+
+<p class="indent">Examination of the inter-relations of unquestionably distinct
+species nearly allied, such as the two common species of <i>Lychnis</i>,
+leads to the same disquieting conclusion, and the best suggestion
+we can make as to their origin is that <i>conceivably</i> they may have
+arisen as two re-combinations of factors brought together by the
+crossing of parent species, one or both of which must be supposed
+to be lost.</p>
+
+<p class="indent">All this is, as need hardly be said, an unsatisfying
+conclusion. To those permanently engaged in systematics it may well bring
+despair. The best course for them is once for all to recognise
+that whether or no specific distinction may prove hereafter to
+have any actual physiological meaning, it is impossible for the
+systematist with the means at his disposal to form a judgment of
+value in any given case. Their business is purely that of the
+cataloguer, and beyond that they cannot go. They will serve
+science best by giving names freely and by describing everything
+<span class="pagenum"><a name="Page_250" id="Page_250">[Pg 250]</a></span>
+to which their successors may possibly want to refer, and generally
+by subdividing their material into as many species as
+they can induce any responsible society or journal to publish.
+Between Jordan with his 200 odd species for <i>Erophila</i>, and
+Grenier and Godron with one, there is no hesitation possible.
+Jordan's view, as he again and again declares with vehemence, is
+at least a view of natural facts, whereas the collective species is a
+mere abstraction, convenient indeed for librarians and beginners,
+but an insidious misrepresentation of natural truth, perhaps
+more than any other the source of the plausible fallacies regarding
+evolution that have so long obstructed progress.</p>
+
+<p class="indent">Nevertheless though we have been compelled to retreat
+from the speculative position to which scientific opinion had rashly
+advanced, the prospect of permanent progress is greatly better
+than it was. With the development of genetic research clear
+conceptions have at length been formed of the kind of knowledge
+required and of the methods by which it is to be attained. If
+we no longer see how varieties give rise to species, we may feel
+confident that a minute study of genetic physiology of varieties
+and species is the necessary beginning of any critical perception
+of their inter-relations. It is little more than a century since
+no valid distinction between a mechanical mixture and a chemical
+combination could be perceived, and in regard to the forms of
+life we may well be in a somewhat similar confusion.</p>
+
+<p class="indent">As yet the genetic behaviour of animals and plants
+has only been sampled. When the work has been done on a scale so
+large as to provide generalisations, we may be in a position to
+declare whether specific difference is or is not a physiological reality.
+<span class="pagenum"><a name="Page_251" id="Page_251">[Pg 251]</a></span></p>
+
+<hr class="chap" />
+<h2>INDEX OF SUBJECTS</h2>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Subject Index" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr"><b>PAGE</b></td>
+    </tr><tr>
+      <td class="tdl">Abraxa grossulariata,</td>
+      <td class="tdr"><a href="#Page_105">105</a>,
+                      <a href="#Page_193">193</a></td>
+    </tr><tr>
+      <td class="tdl">Aceras hircina, local variability,</td>
+      <td class="tdr"><a href="#Page_123">123</a></td>
+    </tr><tr>
+      <td class="tdl">Achatinellidae, local forms of,</td>
+      <td class="tdr"><a href="#Page_133">133</a></td>
+    </tr><tr>
+      <td class="tdl">Acquired characters, inheritance of,</td>
+      <td class="tdr"><a href="#Page_188">188 et seq.</a>,
+                      <a href="#Page_217">217</a>,
+                      <a href="#Page_233">233</a></td>
+    </tr><tr>
+      <td class="tdl">Acronycta psi, melanic,</td>
+      <td class="tdr"><a href="#Page_138">138</a></td>
+    </tr><tr>
+      <td class="tdl">Adaptation, problem of,</td>
+      <td class="tdr"><a href="#Page_187">187</a>,
+                      <a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdl">Agelaius, local forms,</td>
+      <td class="tdr"><a href="#Page_120">120</a></td>
+    </tr><tr>
+      <td class="tdl">Agrotis, fixed and variable species,</td>
+      <td class="tdr"><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Alkaptonuria,</td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdl">Alpine Plants, growing larger, if protected,</td>
+      <td class="tdr"><a href="#Page_183">183</a></td>
+    </tr><tr>
+      <td class="tdl">Alpine Varieties,</td>
+      <td class="tdr"><a href="#Page_165">165</a></td>
+    </tr><tr>
+      <td class="tdl">Alytes obstetricans, Kammerer's experiments on,</td>
+      <td class="tdr"><a href="#Page_199">199</a>,
+                      <a href="#Page_210">210</a></td>
+    </tr><tr>
+      <td class="tdl">Amblystoma, races of,</td>
+      <td class="tdr"><a href="#Page_230">230</a></td>
+    </tr><tr>
+      <td class="tdl">Amphidasys betularia, melanic form,</td>
+      <td class="tdr"><a href="#Page_136">136</a>,
+                      <a href="#Page_138">138</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;dimorphic larvae,</td>
+      <td class="tdr"><a href="#Page_141">141</a></td>
+    </tr><tr>
+      <td class="tdl">Anodonta, polymorphism of,</td>
+      <td class="tdr"><a href="#Page_130">130</a></td>
+    </tr><tr>
+      <td class="tdl">Antirrhinum, striped,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;species-hybrids,</td>
+      <td class="tdr"><a href="#Page_99">&nbsp;99</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;albinos,</td>
+      <td class="tdr"><a href="#Page_110">110</a></td>
+    </tr><tr>
+      <td class="tdl">Apple, will not cross with pear,</td>
+      <td class="tdr"><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Arctia caja, effects of temperature,</td>
+      <td class="tdr"><a href="#Page_192">192</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;larval variation in,</td>
+      <td class="tdr"><a href="#Page_231">231</a></td>
+    </tr><tr>
+      <td class="tdl">Arctic varieties,</td>
+      <td class="tdr"><a href="#Page_165">165</a></td>
+    </tr><tr>
+      <td class="tdl">Argynnis paphia and valesina in Italy,</td>
+      <td class="tdr"><a href="#Page_121">121</a></td>
+    </tr><tr>
+      <td class="tdl">Armadillo, polyembryony,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Artistic faculty,</td>
+      <td class="tdr"><a href="#Page_89">&nbsp;89</a></td>
+    </tr><tr>
+      <td class="tdl">Arum, rights and lefts,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Auriculas, short-styled selected,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Axis of symmetry in hand and foot,</td>
+      <td class="tdr"><a href="#Page_48">&nbsp;48</a></td>
+    </tr><tr>
+      <td class="tdl">Axolotl, alleged effect of conditions,</td>
+      <td class="tdr"><a href="#Page_230">230</a></td>
+    </tr><tr>
+      <td class="tdl">Azalea, bud-sports,</td>
+      <td class="tdr"><a href="#Page_55">&nbsp;55</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Bacillus anthracis, unsegmented form,</td>
+      <td class="tdr"><br /><a href="#Page_71">&nbsp;71</a></td>
+    </tr><tr>
+      <td class="tdl">Bacillus prodigiosus, variation in,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Bacteria, variation in,</td>
+      <td class="tdr"><a href="#Page_212">212</a></td>
+    </tr><tr>
+      <td class="tdl">Bacterium coli, variation in,</td>
+      <td class="tdr"><a href="#Page_214">214</a></td>
+    </tr><tr>
+      <td class="tdl">Baeolophus, geographical races of,</td>
+      <td class="tdr"><a href="#Page_159">159</a></td>
+    </tr><tr>
+      <td class="tdl">Barley, right and left-handed,</td>
+      <td class="tdr"><a href="#Page_58">&nbsp;58</a></td>
+    </tr><tr>
+      <td class="tdl">Basilarchia, geographical races of,</td>
+      <td class="tdr"><a href="#Page_161">161</a></td>
+    </tr><tr>
+      <td class="tdl">Begonia phyllomaniaca,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;hybrids,</td>
+      <td class="tdr"><a href="#Page_51">&nbsp;51</a></td>
+    </tr><tr>
+      <td class="tdl">Bizarre Carnation, genetics of,</td>
+      <td class="tdr"><a href="#Page_54">&nbsp;54</a></td>
+    </tr><tr>
+      <td class="tdl">Black, as a variation from red,</td>
+      <td class="tdr"><a href="#Page_148">148</a></td>
+    </tr><tr>
+      <td class="tdl">Blackbird, varying,</td>
+      <td class="tdr"><a href="#Page_150">150</a></td>
+    </tr><tr>
+      <td class="tdl">Black Cock, fixity of,</td>
+      <td class="tdr"><a href="#Page_28">&nbsp;28</a></td>
+    </tr><tr>
+      <td class="tdl">Boarmia repandata, melanic form,</td>
+      <td class="tdr"><a href="#Page_136">136</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;rhomboidaria,</td>
+      <td class="tdr"><a href="#Page_137">137</a>,
+                      <a href="#Page_139">139</a></td>
+    </tr><tr>
+      <td class="tdl">Botrytis susceptibility to,</td>
+      <td class="tdr"><a href="#Page_108">108</a></td>
+    </tr><tr>
+      <td class="tdl">Bovidae, hybrid,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Brachydactyly,</td>
+      <td class="tdr"><a href="#Page_89">&nbsp;89</a>,
+                      <a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">Bradypus, vertebral variation,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Bud-sports geometrically irregular,</td>
+      <td class="tdr"><a href="#Page_54">54-57</a></td>
+    </tr><tr>
+      <td class="tdl">Buffalo, attempts to hybridize,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Bullfinch, gynandromorph,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl">Bulimus detritus, local variation of,</td>
+      <td class="tdr"><a href="#Page_126">126</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Canary, asymmetrical markings in,</td>
+      <td class="tdr"><br /><a href="#Page_154">154</a></td>
+    </tr><tr>
+      <td class="tdl">Canidae, hybrid,</td>
+      <td class="tdr"><a href="#Page_241">241</a></td>
+    </tr><tr>
+      <td class="tdl">Capsella,</td>
+      <td class="tdr"><a href="#Page_100">100</a></td>
+    </tr><tr>
+      <td class="tdl">Cardamine pratensis,</td>
+      <td class="tdr"><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Cat, Polydactylism,</td>
+      <td class="tdr"><a href="#Page_53">&nbsp;53</a></td>
+    </tr><tr>
+      <td class="tdl">Carnation, Picotees and bizarres compared,</td>
+      <td class="tdr"><a href="#Page_54">&nbsp;54</a>,
+                      <a href="#Page_58">&nbsp;58</a></td>
+    </tr><tr>
+      <td class="tdl">Cataract, hereditary,</td>
+      <td class="tdr"><a href="#Page_89">&nbsp;89</a></td>
+    </tr><tr>
+      <td class="tdl">Certhiola, melanic,</td>
+      <td class="tdr"><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Chladni figures,</td>
+      <td class="tdr"><a href="#Page_60">&nbsp;60</a></td>
+    </tr><tr>
+      <td class="tdl">Choloepus, vertebral variation in,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;local variation in,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Cinerarias, self-sterility in,</td>
+      <td class="tdr"><a href="#Page_238">238</a></td>
+    </tr><tr>
+      <td class="tdl">Cistudo, local variation in,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Climatic varieties,</td>
+      <td class="tdr"><a href="#Page_164">164</a></td>
+    </tr><tr>
+      <td class="tdl">Coccaceae, variation in,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Coenonympha arcania, climatic forms of,</td>
+      <td class="tdr"><a href="#Page_179">179</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;satyrion,</td>
+      <td class="tdr"><a href="#Page_180">180</a></td>
+    </tr><tr>
+      <td class="tdl">Coereba, melanic,</td>
+      <td class="tdr"><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Colaptes, geographical races,</td>
+      <td class="tdr"><a href="#Page_147">147 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;chrysoides,</td>
+      <td class="tdr"><a href="#Page_154">154</a></td>
+    </tr><tr>
+      <td class="tdl">Colloids, growth in,</td>
+      <td class="tdr"><a href="#Page_65">&nbsp;65</a></td>
+    </tr><tr>
+      <td class="tdl">Colorado beetles, experiments on,</td>
+      <td class="tdr"><a href="#Page_218">218</a></td>
+    </tr><tr>
+      <td class="tdl">Colour blindness in twins,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Continuous variation, possible example of,</td>
+      <td class="tdr"><a href="#Page_173">173</a></td>
+    </tr><tr>
+      <td class="tdl">Coracias, geographical races of,</td>
+      <td class="tdr"><a href="#Page_160">160</a></td>
+    </tr><tr>
+      <td class="tdl">Cotton, genetics of,</td>
+      <td class="tdr"><a href="#Page_98">&nbsp;98</a>,
+                      <a href="#Page_100">100</a></td>
+    </tr><tr>
+      <td class="tdl">Coupling,</td>
+      <td class="tdr"><a href="#Page_110">110</a></td>
+    </tr><tr>
+      <td class="tdl">Crab, extra claws,</td>
+      <td class="tdr"><a href="#Page_74">&nbsp;74</a></td>
+    </tr><tr>
+      <td class="tdl">Crustacean appendages and Serial Homology,</td>
+      <td class="tdr"><a href="#Page_63">&nbsp;63</a></td>
+    </tr><tr>
+      <td class="tdl">Crystals, analogy with,</td>
+      <td class="tdr"><a href="#Page_78">&nbsp;78</a></td>
+    </tr><tr>
+      <td class="tdl">Cyclopian monsters, artificial,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Daphnia, changed by environment,</td>
+      <td class="tdr"><br /><a href="#Page_217">217</a></td>
+    </tr><tr>
+      <td class="tdl">Dasypus, polyembryony,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Dianthoecia, fixed and variable species,</td>
+      <td class="tdr"><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Disease-resistance,</td>
+      <td class="tdr"><a href="#Page_87">&nbsp;87</a></td>
+    </tr><tr>
+      <td class="tdl">Division, power of,</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdl">&emsp;a fundamental attribute of living things,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Genetics of,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a>,
+                      <a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl">Dogger Bank, large varieties on,</td>
+      <td class="tdr"><a href="#Page_125">125</a></td>
+    </tr><tr>
+      <td class="tdl">Dogs, hybrid,</td>
+      <td class="tdr"><a href="#Page_241">241</a></td>
+    </tr><tr>
+      <td class="tdl">Dominance, nature of,</td>
+      <td class="tdr"><a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">Dominants, origin of new,</td>
+      <td class="tdr"><a href="#Page_88">&nbsp;88</a>,
+                      <a href="#Page_90">&nbsp;90</a>,
+                      <a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">Double monsters,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Draba, experiments with,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Drosophila,</td>
+      <td class="tdr"><a href="#Page_91">&nbsp;91</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Payne's experiments on,</td>
+      <td class="tdr"><a href="#Page_228">228</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Earthworm, regeneration,</td>
+      <td class="tdr"><br /><a href="#Page_77">&nbsp;77</a></td>
+    </tr><tr>
+      <td class="tdl">Elephant, tusk segmented,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">Entelechy,</td>
+      <td class="tdr"><a href="#Page_80">&nbsp;80</a></td>
+    </tr><tr>
+      <td class="tdl">Environmental treatment, effects of,</td>
+      <td class="tdr"><a href="#Page_188">188 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Enzymes and genetic factors,</td>
+      <td class="tdr"><a href="#Page_86">&nbsp;86</a></td>
+    </tr><tr>
+      <td class="tdl">Epilepsy, inheritance of traumatic,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Equidae, sterility of hybrid,</td>
+      <td class="tdr"><a href="#Page_241">241</a></td>
+    </tr><tr>
+      <td class="tdl">Erophila, experiments with,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;species,</td>
+      <td class="tdr"><a href="#Page_249">249</a></td>
+    </tr><tr>
+      <td class="tdl">Exacum, right and left,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Euphonia elegantissima, local forms,</td>
+      <td class="tdr"><a href="#Page_120">120</a></td>
+    </tr><tr>
+      <td class="tdl">Eupithecia rectangulata, melanic form,</td>
+      <td class="tdr"><a href="#Page_137">137</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Factors, new,</td>
+      <td class="tdr"><br /><a href="#Page_88">&nbsp;88</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;loss of,</td>
+      <td class="tdr"><a href="#Page_96">&nbsp;96</a></td>
+    </tr><tr>
+      <td class="tdl">Factorial representation of varieties,</td>
+      <td class="tdr"><a href="#Page_158">158</a>,
+                      <a href="#Page_165">165</a></td>
+    </tr><tr>
+      <td class="tdl">Falcons, geographical races,</td>
+      <td class="tdr"><a href="#Page_147">147</a></td>
+    </tr><tr>
+      <td class="tdl">Fasciation,</td>
+      <td class="tdr"><a href="#Page_49">&nbsp;49</a></td>
+    </tr><tr>
+      <td class="tdl">Ferments, Boyle on,</td>
+      <td class="tdr"><a href="#Page_54">&nbsp;54</a></td>
+    </tr><tr>
+      <td class="tdl">Finger-prints of twins,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Fixity and Variability in species,</td>
+      <td class="tdr"><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Flax, climatic experiments,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Fowl, Silky,</td>
+      <td class="tdr"><a href="#Page_84">&nbsp;84</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Leghorn,</td>
+      <td class="tdr"><a href="#Page_85">&nbsp;85</a>,
+                      <a href="#Page_90">&nbsp;90</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Dominant white,</td>
+      <td class="tdr"><a href="#Page_94">&nbsp;94</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Wyandotte,</td>
+      <td class="tdr"><a href="#Page_97">&nbsp;97</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Rumpless,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">Foxes, incompatibility with dogs,</td>
+      <td class="tdr"><a href="#Page_241">241</a></td>
+    </tr><tr>
+      <td class="tdl">Free-martin,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Fringillidae, sterility of hybrid,</td>
+      <td class="tdr"><a href="#Page_241">241</a></td>
+    </tr><tr>
+      <td class="tdl">Fundulus, cyclopian,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Gallus, invariability of wild species,</td>
+      <td class="tdr"><br /><a href="#Page_13">&nbsp;13</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;and origin of poultry,</td>
+      <td class="tdr"><a href="#Page_90">&nbsp;90</a>,
+                      <a href="#Page_97">&nbsp;97</a></td>
+    </tr><tr>
+      <td class="tdl">Genitalia, a basis for classification in insects,</td>
+      <td class="tdr"><a href="#Page_13">&nbsp;13</a></td>
+    </tr><tr>
+      <td class="tdl">Gentians, climatic experiments,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Geometrical structure and differentiation,</td>
+      <td class="tdr"><a href="#Page_54">&nbsp;54</a>,
+                      <a href="#Page_56">&nbsp;56</a></td>
+    </tr><tr>
+      <td class="tdl">Geometrical distinction between germ-cells and somatic cells,</td>
+      <td class="tdr"><a href="#Page_58">&nbsp;58</a></td>
+    </tr><tr>
+      <td class="tdl">Gladiolus, right and left,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Gnophus obscurata, protective colouring,</td>
+      <td class="tdr"><a href="#Page_141">141</a></td>
+    </tr><tr>
+      <td class="tdl">Goldfinch, geographical races,</td>
+      <td class="tdr"><a href="#Page_147">147</a></td>
+    </tr><tr>
+      <td class="tdl">Gonioctena variabilis, variation in sexes of,</td>
+      <td class="tdr"><a href="#Page_121">121</a></td>
+    </tr><tr>
+      <td class="tdl">Gouldian Finch, polymorphism,</td>
+      <td class="tdr"><a href="#Page_148">148-149</a></td>
+    </tr><tr>
+      <td class="tdl">Gracilaria stigmatella, experiments on,</td>
+      <td class="tdr"><a href="#Page_193">193</a></td>
+    </tr><tr>
+      <td class="tdl">Grantia, large varieties of,</td>
+      <td class="tdr"><a href="#Page_125">125</a></td>
+    </tr><tr>
+      <td class="tdl">Ground-Squirrels, local forms of,</td>
+      <td class="tdr"><a href="#Page_132">132</a></td>
+    </tr><tr>
+      <td class="tdl">Grouse, red, variation,</td>
+      <td class="tdr"><a href="#Page_29">&nbsp;29</a></td>
+    </tr><tr>
+      <td class="tdl">Guillemot, Ringed,</td>
+      <td class="tdr"><a href="#Page_150">150</a></td>
+    </tr><tr>
+      <td class="tdl">Guinea-pig, Brown-Séquard's experiments on,</td>
+      <td class="tdr"><a href="#Page_198">198</a></td>
+    </tr><tr>
+      <td class="tdl">Gynandromorphs,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Heliconius erato, forms of,</td>
+      <td class="tdr"><br /><a href="#Page_122">122</a>,
+                            <a href="#Page_164">164</a></td>
+    </tr><tr>
+      <td class="tdl">Helix lapicida, local variation of,</td>
+      <td class="tdr"><a href="#Page_126">126</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;striata,</td>
+      <td class="tdr"><a href="#Page_127">127</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Heripensis,</td>
+      <td class="tdr"><a href="#Page_127">127</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Caespitum,</td>
+      <td class="tdr"><a href="#Page_127">127</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;trochoides,</td>
+      <td class="tdr"><a href="#Page_127">127</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;nemoralis and hortensis,</td>
+      <td class="tdr"><a href="#Page_128">128</a></td>
+    </tr><tr>
+      <td class="tdl">Helminthophila, geographical races of,</td>
+      <td class="tdr"><a href="#Page_157">157</a></td>
+    </tr><tr>
+      <td class="tdl">Hemerophila abruptaria, melanic,</td>
+      <td class="tdr"><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Hepialus humuli, in Shetland,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Heterostyle plants,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Hieracium,</td>
+      <td class="tdr"><a href="#Page_9">&nbsp;&nbsp;9</a></td>
+    </tr><tr>
+      <td class="tdl">Himantopus,</td>
+      <td class="tdr"><a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdl">Homoeosis,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Hybernia progemmaria,</td>
+      <td class="tdr"><a href="#Page_139">139</a></td>
+    </tr><tr>
+      <td class="tdl">Hybrids, sterility of,</td>
+      <td class="tdr"><a href="#Page_233">233 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Incompatibility between certain allied species,</td>
+      <td class="tdr"><br /><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Individual, geometrical independence of,</td>
+      <td class="tdr"><a href="#Page_58">&nbsp;58</a></td>
+    </tr><tr>
+      <td class="tdl">Inhibiting Factors,</td>
+      <td class="tdr"><a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">Intermediates, nature of,</td>
+      <td class="tdr"><a href="#Page_131">131</a>,
+                      <a href="#Page_135">135</a></td>
+    </tr><tr>
+      <td class="tdl">Isolation, consequences of,</td>
+      <td class="tdr"><a href="#Page_118">118</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Lacerta muralis, Kammerer's experiments on,</td>
+      <td class="tdr"><br /><a href="#Page_209">209</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;fiumana,</td>
+      <td class="tdr"><a href="#Page_210">210</a></td>
+    </tr><tr>
+      <td class="tdl">Leptinotarsa, Power's experiments on,</td>
+      <td class="tdr"><a href="#Page_218">218</a></td>
+    </tr><tr>
+      <td class="tdl">Limbs, extra, in pairs,</td>
+      <td class="tdr"><a href="#Page_72">&nbsp;72</a></td>
+    </tr><tr>
+      <td class="tdl">Limnaea, sinistral,</td>
+      <td class="tdr"><a href="#Page_134">134</a></td>
+    </tr><tr>
+      <td class="tdl">Linaria vulgaris, self-sterility,</td>
+      <td class="tdr"><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Loasa fruits, right and left,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Lobster, extra claws,</td>
+      <td class="tdr"><a href="#Page_76">&nbsp;76</a></td>
+    </tr><tr>
+      <td class="tdl">Locality, variation connected with,</td>
+      <td class="tdr"><a href="#Page_14">&nbsp;14</a>,
+                      <a href="#Page_118">118</a>,
+                      <a href="#Page_146">146 et seq.</a>,
+                      <a href="#Page_208">208</a></td>
+    </tr><tr>
+      <td class="tdl">Lumbricus, regeneration,</td>
+      <td class="tdr"><a href="#Page_77">&nbsp;77</a></td>
+    </tr><tr>
+      <td class="tdl">Lychnis dioica and vespertina, inter-relations of,</td>
+      <td class="tdr"><a href="#Page_18">&nbsp;18</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;macrocarpa, possibly a common parent of,</td>
+      <td class="tdr"><a href="#Page_19">&nbsp;19</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Machetes pugnax, polymorphism of male,</td>
+      <td class="tdr"><br /><a href="#Page_28">&nbsp;28</a></td>
+    </tr><tr>
+      <td class="tdl">Maize, Blaringhem's experiments on,</td>
+      <td class="tdr"><a href="#Page_229">229</a></td>
+    </tr><tr>
+      <td class="tdl">Maize, cumulative factors in,</td>
+      <td class="tdr"><a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdl">Malformations, dominants, arising de novo,</td>
+      <td class="tdr"><a href="#Page_89">&nbsp;89</a></td>
+    </tr><tr>
+      <td class="tdl">Manx Cat, heredity,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">Matthiola,</td>
+      <td class="tdr"><a href="#Page_84">&nbsp;84</a>,
+                      <a href="#Page_104">104</a>,
+                      <a href="#Page_113">113</a></td>
+    </tr><tr>
+      <td class="tdl">Melanic varieties,</td>
+      <td class="tdr"><a href="#Page_135">135 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Memory, analogy with heredity,</td>
+      <td class="tdr"><a href="#Page_190">190</a></td>
+    </tr><tr>
+      <td class="tdl">Meristic variation,</td>
+      <td class="tdr"><a href="#Page_69">&nbsp;69</a>,
+                      <a href="#Page_83">&nbsp;83</a>,
+                      <a href="#Page_86">&nbsp;86</a></td>
+    </tr><tr>
+      <td class="tdl">Mirabilis, striped,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Models of segmentation,</td>
+      <td class="tdr"><a href="#Page_59">&nbsp;59</a>,
+                      <a href="#Page_60">&nbsp;60</a></td>
+    </tr><tr>
+      <td class="tdl">"Modes," Coutagne's conception of,</td>
+      <td class="tdr"><a href="#Page_126">126</a></td>
+    </tr><tr>
+      <td class="tdl">Mödling, peculiar race of <i>Pieris napi</i> at,</td>
+      <td class="tdr"><a href="#Page_178">178</a></td>
+    </tr><tr>
+      <td class="tdl">Mole, albino,</td>
+      <td class="tdr"><a href="#Page_27">27-28</a></td>
+    </tr><tr>
+      <td class="tdl">Mule, Linnaeus on,</td>
+      <td class="tdr"><a href="#Page_8">&nbsp;&nbsp;8</a></td>
+    </tr><tr>
+      <td class="tdl">Mutation, Matthioli on,</td>
+      <td class="tdr"><a href="#Page_4">&nbsp;&nbsp;4</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in Mercurialis,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in Kales,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;alleged in bulbs,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Theory,</td>
+      <td class="tdr"><a href="#Page_97">&nbsp;97</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;periods of,</td>
+      <td class="tdr"><a href="#Page_114">114</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in Bacteria,</td>
+      <td class="tdr"><a href="#Page_214">214</a></td>
+    </tr><tr>
+      <td class="tdl">Mutilation, consequences of,</td>
+      <td class="tdr"><a href="#Page_71">&nbsp;71</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;alleged effect of, on offspring,</td>
+      <td class="tdr"><a href="#Page_229">229</a></td>
+    </tr><tr>
+      <td class="tdl">Myxococcus, variation in,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Narwhal, asymmetry of tusks,</td>
+      <td class="tdr"><br /><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Nemesia strumosa,</td>
+      <td class="tdr"><a href="#Page_91">&nbsp;91</a></td>
+    </tr><tr>
+      <td class="tdl">Neuration, a basis for classification,</td>
+      <td class="tdr"><a href="#Page_13">&nbsp;13</a></td>
+    </tr><tr>
+      <td class="tdl">Nicotiana, sterility of hybrid,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Nightjars, varying,</td>
+      <td class="tdr"><a href="#Page_150">150</a></td>
+    </tr><tr>
+      <td class="tdl">Noctuidae, fixity and variability,</td>
+      <td class="tdr"><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Noctua, polymorphic and fixed species,</td>
+      <td class="tdr"><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Noctua castanea, local forms of,</td>
+      <td class="tdr"><a href="#Page_122">122</a></td>
+    </tr><tr>
+      <td class="tdl">Nomenclature, future of,</td>
+      <td class="tdr"><a href="#Page_94">&nbsp;94</a>,
+                      <a href="#Page_245">245</a></td>
+    </tr><tr>
+      <td class="tdl">Notonecta, variations of,</td>
+      <td class="tdr"><a href="#Page_130">130</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Odontoptera bidentata, melanic form,</td>
+      <td class="tdr"><br /><a href="#Page_137">137</a></td>
+    </tr><tr>
+      <td class="tdl">Oedipodidae, protectively coloured,</td>
+      <td class="tdr"><a href="#Page_140">140</a></td>
+    </tr><tr>
+      <td class="tdl">Oenothera, new dominant in,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;rubricalyx and rubrinervis,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a>,
+                      <a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Lamarckiana,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a>,
+                      <a href="#Page_101">101</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;origin of,</td>
+      <td class="tdr"><a href="#Page_102">102</a>,
+                      <a href="#Page_244">244</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;has bad pollen-grains,</td>
+      <td class="tdr"><a href="#Page_102">102</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;factorial analysis of,</td>
+      <td class="tdr"><a href="#Page_103">103</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;pollen and egg-cells genetically dissimilar,</td>
+      <td class="tdr"><a href="#Page_104">104</a></td>
+    </tr><tr>
+      <td class="tdl">Oenothera, "twin hybrids",</td>
+      <td class="tdr"><a href="#Page_105">105</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;laeta and velutina,</td>
+      <td class="tdr"><a href="#Page_105">105</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;reciprocal crosses in,</td>
+      <td class="tdr"><a href="#Page_105">105 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;possible coupling in,</td>
+      <td class="tdr"><a href="#Page_111">111</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;dwarfs,</td>
+      <td class="tdr"><a href="#Page_112">112</a>,
+                      <a href="#Page_114">114</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;"Triple hybrids",</td>
+      <td class="tdr"><a href="#Page_114">114</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;alleged variation due to treatment,</td>
+      <td class="tdr"><a href="#Page_227">227</a></td>
+    </tr><tr>
+      <td class="tdl">Ophrys, local variability,</td>
+      <td class="tdr"><a href="#Page_125">125</a></td>
+    </tr><tr>
+      <td class="tdl">Orange, polyembryony,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl">Osmotic growth,</td>
+      <td class="tdr"><a href="#Page_65">&nbsp;65</a></td>
+    </tr><tr>
+      <td class="tdl">Overlapping forms,</td>
+      <td class="tdr"><a href="#Page_146">146</a>,
+                      <a href="#Page_174">174</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Papilio, geographical races of,</td>
+      <td class="tdr"><br /><a href="#Page_162">162</a></td>
+    </tr><tr>
+      <td class="tdl">Papilio turnus, variation of,</td>
+      <td class="tdr"><a href="#Page_144">144</a></td>
+    </tr><tr>
+      <td class="tdl">Pararge egeria, geographical forms,</td>
+      <td class="tdr"><a href="#Page_166">166 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Parthenogenesis,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl">Partula, local forms of,</td>
+      <td class="tdr"><a href="#Page_133">133</a></td>
+    </tr><tr>
+      <td class="tdl">Passer domesticus and montanus, distinctions,</td>
+      <td class="tdr"><a href="#Page_22">&nbsp;22</a></td>
+    </tr><tr>
+      <td class="tdl">Pea, round and wrinkled,</td>
+      <td class="tdr"><a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">Pear, will not cross with apple,</td>
+      <td class="tdr"><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Pelargonium, variegated,</td>
+      <td class="tdr"><a href="#Page_55">&nbsp;55</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;bud-sports,</td>
+      <td class="tdr"><a href="#Page_56">&nbsp;56</a></td>
+    </tr><tr>
+      <td class="tdl">Periodic phenomena in structure,</td>
+      <td class="tdr"><a href="#Page_63">&nbsp;63</a></td>
+    </tr><tr>
+      <td class="tdl">Peronea, fixed and variable species,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a></td>
+    </tr><tr>
+      <td class="tdl">"Petites espèces",</td>
+      <td class="tdr"><a href="#Page_248">248</a></td>
+    </tr><tr>
+      <td class="tdl">Petunia, double,</td>
+      <td class="tdr"><a href="#Page_104">104</a></td>
+    </tr><tr>
+      <td class="tdl">Phalanger maculatus, local variation,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Pheasant, fixity of,</td>
+      <td class="tdr"><a href="#Page_29">&nbsp;29</a></td>
+    </tr><tr>
+      <td class="tdl">Phigalia pilosaria, melanic,</td>
+      <td class="tdr"><a href="#Page_139">139-140</a></td>
+    </tr><tr>
+      <td class="tdl">Phratora vitellinae, experiments on,</td>
+      <td class="tdr"><a href="#Page_193">193</a></td>
+    </tr><tr>
+      <td class="tdl">Phyllotaxis,</td>
+      <td class="tdr"><a href="#Page_69">&nbsp;69</a></td>
+    </tr><tr>
+      <td class="tdl">Pied varieties common in Passer domesticus unknown in Montanus,</td>
+      <td class="tdr"><a href="#Page_23">&nbsp;23</a></td>
+    </tr><tr>
+      <td class="tdl">Pieris napi and bryoniae,</td>
+      <td class="tdr"><a href="#Page_174">174</a></td>
+    </tr><tr>
+      <td class="tdl">Pig, mule-footed,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">Pigeon, web-footed,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a>,
+                      <a href="#Page_49">&nbsp;49</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Indian Rock, a recessive form,</td>
+      <td class="tdr"><a href="#Page_98">&nbsp;98</a></td>
+    </tr><tr>
+      <td class="tdl">Pigments, nature of,</td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdl">Pisum humile, hybrids with culinary peas,</td>
+      <td class="tdr"><a href="#Page_244">244</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;species,</td>
+      <td class="tdr"><a href="#Page_246">246</a></td>
+    </tr><tr>
+      <td class="tdl">Planarian, regeneration of,</td>
+      <td class="tdr"><a href="#Page_71">&nbsp;71</a>,
+                      <a href="#Page_77">&nbsp;77</a></td>
+    </tr><tr>
+      <td class="tdl">Plotheia frontalis, polymorphic,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a>,
+                      <a href="#Page_29">&nbsp;29</a></td>
+    </tr><tr>
+      <td class="tdl">Plusia, fixity and variation in,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a></td>
+    </tr><tr>
+      <td class="tdl">Poephila gouldiae, variation of,</td>
+      <td class="tdr"><a href="#Page_148">148-149</a></td>
+    </tr><tr>
+      <td class="tdl">Polarity of individual,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Polia chi, melanic,</td>
+      <td class="tdr"><a href="#Page_138">138</a></td>
+    </tr><tr>
+      <td class="tdl">Polyanthus, short-styled selected,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Polydactylism in Cat,</td>
+      <td class="tdr"><a href="#Page_52">52-53</a></td>
+    </tr><tr>
+      <td class="tdl">Polyembryony,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl">Potato, variation in,</td>
+      <td class="tdr"><a href="#Page_91">&nbsp;91</a></td>
+    </tr><tr>
+      <td class="tdl">Poultry, evolution of,</td>
+      <td class="tdr"><a href="#Page_90">&nbsp;90</a></td>
+    </tr><tr>
+      <td class="tdl">Primula obconica,</td>
+      <td class="tdr"><a href="#Page_91">&nbsp;91</a></td>
+    </tr><tr>
+      <td class="tdl">Primula sinensis, flaked,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Leaf-shapes,</td>
+      <td class="tdr"><a href="#Page_70">&nbsp;70</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;new dominant in,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;sterility in,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;"Giants",</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Primula, species-hybrids,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Protective coloration,</td>
+      <td class="tdr"><a href="#Page_140">140</a></td>
+    </tr><tr>
+      <td class="tdl">Pyrrhulagra, local forms,</td>
+      <td class="tdr"><a href="#Page_120">120</a></td>
+    </tr><tr>
+      <td class="tdl">Python, twin-vertebrae,</td>
+      <td class="tdr"><a href="#Page_60">&nbsp;60</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Quiscalus, geographical races of,</td>
+      <td class="tdr"><br /><a href="#Page_156">156</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Rabbit, Angora,</td>
+      <td class="tdr"><br /><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;colours of,</td>
+      <td class="tdr"><a href="#Page_93">&nbsp;93</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Incompatibility with hare,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Raimannia odorata, Macdougal's experiments on,</td>
+      <td class="tdr"><a href="#Page_226">226</a></td>
+    </tr><tr>
+      <td class="tdl">Rats, Variation in,</td>
+      <td class="tdr"><a href="#Page_248">248</a></td>
+    </tr><tr>
+      <td class="tdl">Recessives, origin of,</td>
+      <td class="tdr"><a href="#Page_90">&nbsp;90</a></td>
+    </tr><tr>
+      <td class="tdl">Reciprocal crosses, giving distinct results,</td>
+      <td class="tdr"><a href="#Page_105">105 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Regeneration,</td>
+      <td class="tdr"><a href="#Page_70">&nbsp;70</a></td>
+    </tr><tr>
+      <td class="tdl">Repulsion,</td>
+      <td class="tdr"><a href="#Page_110">110</a></td>
+    </tr><tr>
+      <td class="tdl">Reversal on Regeneration,</td>
+      <td class="tdr"><a href="#Page_77">&nbsp;77</a></td>
+    </tr><tr>
+      <td class="tdl">Rhamphocoelus, geographical forms,</td>
+      <td class="tdr"><a href="#Page_159">159</a>,
+                      <a href="#Page_184">184</a></td>
+    </tr><tr>
+      <td class="tdl">Rhinoptera, variation in jaws of,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">Rhythm in repetition,</td>
+      <td class="tdr"><a href="#Page_69">&nbsp;69</a></td>
+    </tr><tr>
+      <td class="tdl">Ribs, variation of,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Rights and Lefts,</td>
+      <td class="tdr"><a href="#Page_57">57-58</a></td>
+    </tr><tr>
+      <td class="tdl">Ripples, analogous to segments,</td>
+      <td class="tdr"><a href="#Page_60">&nbsp;60</a>,
+                      <a href="#Page_66">&nbsp;66-67</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;regeneration of,</td>
+      <td class="tdr"><a href="#Page_79">&nbsp;79</a></td>
+    </tr><tr>
+      <td class="tdl">Rollers, geographical races of,</td>
+      <td class="tdr"><a href="#Page_160">160</a></td>
+    </tr><tr>
+      <td class="tdl">Ruff, polymorphism of male,</td>
+      <td class="tdr"><a href="#Page_28">&nbsp;28</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Salamandra, maculosa and atra,</td>
+      <td class="tdr"><br /><a href="#Page_182">182</a>,
+                      <a href="#Page_199">199</a>,
+                      <a href="#Page_203">203</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;spotted and striped,</td>
+      <td class="tdr"><a href="#Page_207">207</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;geographical variation of,</td>
+      <td class="tdr"><a href="#Page_208">208</a></td>
+    </tr><tr>
+      <td class="tdl">Segmentation, nature of,</td>
+      <td class="tdr"><a href="#Page_63">&nbsp;63</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;simulated mechanically,</td>
+      <td class="tdr"><a href="#Page_64">&nbsp;64</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;compared with rippling,</td>
+      <td class="tdr"><a href="#Page_65">&nbsp;65</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;analogies with,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Segmentation of normally unsegmented structures,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">Selection, Natural, an insufficient cause of definiteness of types,</td>
+      <td class="tdr"><a href="#Page_17">&nbsp;17</a>,
+                      <a href="#Page_134">134</a>,
+                      <a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Sempervivum,</td>
+      <td class="tdr"><a href="#Page_250">250</a></td>
+    </tr><tr>
+      <td class="tdl">Serial Homology, the true nature of,</td>
+      <td class="tdr"><a href="#Page_62">&nbsp;62</a>,
+                      <a href="#Page_66">&nbsp;66</a></td>
+    </tr><tr>
+      <td class="tdl">Setina, Alpine varieties,</td>
+      <td class="tdr"><a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Sex of Twins,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Sex-factors, possible coupling of,</td>
+      <td class="tdr"><a href="#Page_111">111</a></td>
+    </tr><tr>
+      <td class="tdl">Sexual characters, variation in,</td>
+      <td class="tdr"><a href="#Page_119">119 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Siamese twins,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Silky Fowl,</td>
+      <td class="tdr"><a href="#Page_84">84-85</a></td>
+    </tr><tr>
+      <td class="tdl">Simocephalus, changed by environment,</td>
+      <td class="tdr"><a href="#Page_218">218</a></td>
+    </tr><tr>
+      <td class="tdl">Sinistral forms,</td>
+      <td class="tdr"><a href="#Page_33">33-34</a></td>
+    </tr><tr>
+      <td class="tdl">Situs transversus,</td>
+      <td class="tdr"><a href="#Page_43">&nbsp;43</a></td>
+    </tr><tr>
+      <td class="tdl">Skate's jaws, variation in,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">Sloths, vertebral variation,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Species, conceptions of,</td>
+      <td class="tdr"><a href="#Page_3">&nbsp;&nbsp;3</a>,
+                      <a href="#Page_94">&nbsp;94</a>,
+                      <a href="#Page_99">&nbsp;99</a>,
+                      <a href="#Page_240">240</a>,
+                      <a href="#Page_245">245</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;allied, distribution of,</td>
+      <td class="tdr"><a href="#Page_185">185</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;alternative uses of the term,</td>
+      <td class="tdr"><a href="#Page_245">245</a></td>
+    </tr><tr>
+      <td class="tdl">Specific difference, universality of,</td>
+      <td class="tdr"><a href="#Page_12">&nbsp;12</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;of organisms compared with those of inorganic materials,</td>
+      <td class="tdr"><a href="#Page_15">&nbsp;15</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;failure of theory of Selection to explain,</td>
+      <td class="tdr"><a href="#Page_18">&nbsp;18</a>,
+                      <a href="#Page_134">134</a>,
+                      <a href="#Page_247">247</a></td>
+    </tr><tr>
+      <td class="tdl">Sphyropicus varius,</td>
+      <td class="tdr"><a href="#Page_149">149</a>,
+                      <a href="#Page_156">156</a></td>
+    </tr><tr>
+      <td class="tdl">Spilosoma lubricipeda, varieties of,</td>
+      <td class="tdr"><a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Zatima, Heligoland form,</td>
+      <td class="tdr"><a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Spinal nerves, segmentation of,</td>
+      <td class="tdr"><a href="#Page_67">&nbsp;67</a></td>
+    </tr><tr>
+      <td class="tdl">Sporadic variation,</td>
+      <td class="tdr"><a href="#Page_131">131</a>,
+                      <a href="#Page_134">134</a>,
+                      <a href="#Page_248">248</a></td>
+    </tr><tr>
+      <td class="tdl">Squashes, polymorphism of,</td>
+      <td class="tdr"><a href="#Page_100">100</a></td>
+    </tr><tr>
+      <td class="tdl">Staphylococcus pyogenes, variation in,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Sterility of hybrids, in general,</td>
+      <td class="tdr"><a href="#Page_233">233</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in Lychnis hybrids,</td>
+      <td class="tdr"><a href="#Page_20">20 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in crossing forms of Draba,</td>
+      <td class="tdr"><a href="#Page_243">243</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Significance of,</td>
+      <td class="tdr"><a href="#Page_244">244</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Self,</td>
+      <td class="tdr"><a href="#Page_238">238</a></td>
+    </tr><tr>
+      <td class="tdl">Stilt,</td>
+      <td class="tdr"><a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdl">Stocks,</td>
+      <td class="tdr"><a href="#Page_84">&nbsp;84</a>,
+                      <a href="#Page_104">104</a>,
+                      <a href="#Page_113">113</a></td>
+    </tr><tr>
+      <td class="tdl">Striped varieties,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Substantive variation,</td>
+      <td class="tdr"><a href="#Page_84">&nbsp;84</a></td>
+    </tr><tr>
+      <td class="tdl">Subtraction-stages,</td>
+      <td class="tdr"><a href="#Page_93">&nbsp;93</a></td>
+    </tr><tr>
+      <td class="tdl">Supernumerary limbs,</td>
+      <td class="tdr"><a href="#Page_72">72-76</a></td>
+    </tr><tr>
+      <td class="tdl">Sweet pea, variation of,</td>
+      <td class="tdr"><a href="#Page_91">&nbsp;91</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;sterile anthers in,</td>
+      <td class="tdr"><a href="#Page_237">237</a></td>
+    </tr><tr>
+      <td class="tdl">Symmetry compared with heredity,</td>
+      <td class="tdr"><a href="#Page_41">&nbsp;41</a></td>
+    </tr><tr>
+      <td class="tdl">Symmetry of body approximate,</td>
+      <td class="tdr"><a href="#Page_78">&nbsp;78</a></td>
+    </tr><tr>
+      <td class="tdl">Syndactyly,</td>
+      <td class="tdr"><a href="#Page_47">&nbsp;47</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in foot,</td>
+      <td class="tdr"><a href="#Page_48">&nbsp;48</a></td>
+    </tr><tr>
+      <td class="tdl">Synthetic formulae, in nomenclature,</td>
+      <td class="tdr"><a href="#Page_94">&nbsp;94</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Taeniocampa, fixed and variable species,</td>
+      <td class="tdr"><br /><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Tamias, local forms of,</td>
+      <td class="tdr"><a href="#Page_132">132</a></td>
+    </tr><tr>
+      <td class="tdl">Tanagers, geographical races of,</td>
+      <td class="tdr"><a href="#Page_159">159</a></td>
+    </tr><tr>
+      <td class="tdl">Teeth, variation in,</td>
+      <td class="tdr"><a href="#Page_39">&nbsp;39</a>,
+                      <a href="#Page_67">&nbsp;67</a></td>
+    </tr><tr>
+      <td class="tdl">Tephrosia consortaria and consonaria,</td>
+      <td class="tdr"><a href="#Page_137">137</a>,
+                      <a href="#Page_139">139</a>,
+                      <a href="#Page_140">140</a></td>
+    </tr><tr>
+      <td class="tdl">Tephrosia species, separated by season,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Terminal members, variation of,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Thais rumina, local variation in,</td>
+      <td class="tdr"><a href="#Page_27">&nbsp;27</a></td>
+    </tr><tr>
+      <td class="tdl">Tolerance, persistence of diversity due to,</td>
+      <td class="tdr"><a href="#Page_17">&nbsp;17</a>,
+                      <a href="#Page_134">134</a></td>
+    </tr><tr>
+      <td class="tdl">Tomato, number of cells in fruit,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">Transitional populations, rarity of,</td>
+      <td class="tdr"><a href="#Page_165">165</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;an example,</td>
+      <td class="tdr"><a href="#Page_178">178</a></td>
+    </tr><tr>
+      <td class="tdl">Tropaeolum, sterile anthers in,</td>
+      <td class="tdr"><a href="#Page_237">237</a></td>
+    </tr><tr>
+      <td class="tdl">Trypanosomes, variation in,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl">Tusk, of Elephant, segmented,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;of Narwhal,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Twinning,</td>
+      <td class="tdr"><a href="#Page_41">&nbsp;41</a>,
+                      <a href="#Page_44">&nbsp;44</a>,
+                      <a href="#Page_71">&nbsp;71</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;heredity of,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in organs,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Uria troile, variety of,</td>
+      <td class="tdr"><br /><a href="#Page_150">150</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Vanessa urticae, effects of temperature,</td>
+      <td class="tdr"><br /><a href="#Page_191">191</a></td>
+    </tr><tr>
+      <td class="tdl">Variation, a medley of phenomena,</td>
+      <td class="tdr"><a href="#Page_14">14-15</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;sporadic,</td>
+      <td class="tdr"><a href="#Page_131">131</a>,
+                      <a href="#Page_134">134</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;and locality,</td>
+      <td class="tdr"><a href="#Page_118">118</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Causes of genetic,</td>
+      <td class="tdr"><a href="#Page_86">&nbsp;86</a>,
+                      <a href="#Page_87">&nbsp;87</a>,
+                      <a href="#Page_131">131</a>,
+                      <a href="#Page_212">212</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Substantive and meristic,</td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdl">Veronica, specific difference in,</td>
+      <td class="tdr"><a href="#Page_16">&nbsp;16</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;intermediates between species,</td>
+      <td class="tdr"><a href="#Page_17">&nbsp;17</a></td>
+    </tr><tr>
+      <td class="tdl">Vertebrae, division in,</td>
+      <td class="tdr"><a href="#Page_60">60-61</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;homologies of,</td>
+      <td class="tdr"><a href="#Page_66">&nbsp;66</a></td>
+    </tr><tr>
+      <td class="tdl">Vespa, specific difference in,</td>
+      <td class="tdr"><a href="#Page_23">&nbsp;23</a></td>
+    </tr><tr>
+      <td class="tdl">Vortex, living organism compared with,</td>
+      <td class="tdr"><a href="#Page_40">&nbsp;40</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Wave-motion compared with repetition of parts,</td>
+      <td class="tdr"><br /><a href="#Page_62">&nbsp;62</a>,
+                            <a href="#Page_67">&nbsp;67</a>,
+                            <a href="#Page_79">&nbsp;79</a></td>
+    </tr><tr>
+      <td class="tdl">Wheat, cumulative factors in,</td>
+      <td class="tdr"><a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;climatic experiments on,</td>
+      <td class="tdr"><a href="#Page_195">195</a></td>
+    </tr><tr>
+      <td class="tdl">Woodpecker,</td>
+      <td class="tdr"><a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Zebra, pattern of stripes compared with ripples,</td>
+      <td class="tdr"><br /><a href="#Page_38">&nbsp;38</a></td>
+    </tr>
+ </tbody>
+</table>
+<p><span class="pagenum"><a name="Page_252" id="Page_252">[Pg 252]</a></span></p>
+<hr class="chap" />
+<h2>INDEX OF PERSONS</h2>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Person Index" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr"><b>PAGE</b></td>
+    </tr><tr>
+      <td class="tdl">Ackermann,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Agar,</td>
+      <td class="tdr"><a href="#Page_218">218</a></td>
+    </tr><tr>
+      <td class="tdl">Allen, J. A.,</td>
+      <td class="tdr"><a href="#Page_132">132</a>,
+                      <a href="#Page_147">147</a>,
+                      <a href="#Page_159">159</a></td>
+    </tr><tr>
+      <td class="tdl">Annandale,</td>
+      <td class="tdr"><a href="#Page_47">&nbsp;47</a></td>
+    </tr><tr>
+      <td class="tdl">Arrigoni degli Oddi,</td>
+      <td class="tdr"><a href="#Page_167">167</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Backhouse,</td>
+      <td class="tdr"><br /><a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl">Baker, G. T.,</td>
+      <td class="tdr"><a href="#Page_166">166</a></td>
+    </tr><tr>
+      <td class="tdl">Bangs, Outram,</td>
+      <td class="tdr"><a href="#Page_120">120</a>,
+                      <a href="#Page_142">142</a>,
+                      <a href="#Page_155">155</a></td>
+    </tr><tr>
+      <td class="tdl">Barrett,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a>,
+                      <a href="#Page_136">136</a>,
+                      <a href="#Page_167">167</a>,
+                      <a href="#Page_173">173</a>,
+                      <a href="#Page_178">178</a>,
+                      <a href="#Page_193">193</a></td>
+    </tr><tr>
+      <td class="tdl">Baur, E.,</td>
+      <td class="tdr"><a href="#Page_55">&nbsp;55</a>,
+                      <a href="#Page_99">&nbsp;99</a></td>
+    </tr><tr>
+      <td class="tdl">Baur, G.,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Beneden, van,</td>
+      <td class="tdr"><a href="#Page_75">&nbsp;75</a></td>
+    </tr><tr>
+      <td class="tdl">Bentham, on species of Veronica,</td>
+      <td class="tdr"><a href="#Page_16">&nbsp;16</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Lychnis,</td>
+      <td class="tdr"><a href="#Page_21">&nbsp;21</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Primula,</td>
+      <td class="tdr"><a href="#Page_22">&nbsp;22</a></td>
+    </tr><tr>
+      <td class="tdl">Bernadin,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Bishop, L. B.,</td>
+      <td class="tdr"><a href="#Page_153">153</a>,
+                      <a href="#Page_157">157</a></td>
+    </tr><tr>
+      <td class="tdl">Blaringhem,</td>
+      <td class="tdr"><a href="#Page_229">229</a></td>
+    </tr><tr>
+      <td class="tdl">Bobart,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a></td>
+    </tr><tr>
+      <td class="tdl">Boisduval,</td>
+      <td class="tdr"><a href="#Page_182">182</a></td>
+    </tr><tr>
+      <td class="tdl">Boissier,</td>
+      <td class="tdr"><a href="#Page_19">&nbsp;19</a></td>
+    </tr><tr>
+      <td class="tdl">Borradaile,</td>
+      <td class="tdr"><a href="#Page_74">74-75</a></td>
+    </tr><tr>
+      <td class="tdl">Boulenger, E. G.,</td>
+      <td class="tdr"><a href="#Page_208">208</a></td>
+    </tr><tr>
+      <td class="tdl">Boulenger, G. A.,</td>
+      <td class="tdr"><a href="#Page_182">182</a>,
+                      <a href="#Page_207">207</a>,
+                      <a href="#Page_209">209</a></td>
+    </tr><tr>
+      <td class="tdl">Boyle,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a>,
+                      <a href="#Page_54">&nbsp;54</a></td>
+    </tr><tr>
+      <td class="tdl">Brewster, W.,</td>
+      <td class="tdr"><a href="#Page_149">149-150</a></td>
+    </tr><tr>
+      <td class="tdl">Britton,</td>
+      <td class="tdr"><a href="#Page_227">227</a></td>
+    </tr><tr>
+      <td class="tdl">Brown, T. Graham,</td>
+      <td class="tdr"><a href="#Page_198">198</a></td>
+    </tr><tr>
+      <td class="tdl">Brown-Séquard,</td>
+      <td class="tdr"><a href="#Page_197">197 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Bruant, P.,</td>
+      <td class="tdr"><a href="#Page_51">&nbsp;51</a></td>
+    </tr><tr>
+      <td class="tdl">Buffon,</td>
+      <td class="tdr"><a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdl">Butler, S.,</td>
+      <td class="tdr"><a href="#Page_189">189-190</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Candolle, de,</td>
+      <td class="tdr"><br /><a href="#Page_245">245</a></td>
+    </tr><tr>
+      <td class="tdl">Carpenter, J. H.,</td>
+      <td class="tdr"><a href="#Page_172">172</a></td>
+    </tr><tr>
+      <td class="tdl">Chapman, F. M.,</td>
+      <td class="tdr"><a href="#Page_148">148</a>,
+                      <a href="#Page_156">156-158</a></td>
+    </tr><tr>
+      <td class="tdl">Chapman, T. A.,</td>
+      <td class="tdr"><a href="#Page_13">&nbsp;13</a>,
+                      <a href="#Page_167">167</a>,
+                      <a href="#Page_182">182</a>,
+                      <a href="#Page_231">231</a></td>
+    </tr><tr>
+      <td class="tdl">Church, A. H.,</td>
+      <td class="tdr"><a href="#Page_69">&nbsp;69</a></td>
+    </tr><tr>
+      <td class="tdl">Cieslar,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Clark, Austin,</td>
+      <td class="tdr"><a href="#Page_142">142</a>,
+                      <a href="#Page_144">144</a></td>
+    </tr><tr>
+      <td class="tdl">Cockayne, E. A.,</td>
+      <td class="tdr"><a href="#Page_43">&nbsp;43</a></td>
+    </tr><tr>
+      <td class="tdl">Cockerell, T. D. A.,</td>
+      <td class="tdr"><a href="#Page_224">224</a></td>
+    </tr><tr>
+      <td class="tdl">Compton, R. H.,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a>,
+                      <a href="#Page_58">&nbsp;58</a>,
+                      <a href="#Page_227">227</a></td>
+    </tr><tr>
+      <td class="tdl">Cope,</td>
+      <td class="tdr"><a href="#Page_230">230</a></td>
+    </tr><tr>
+      <td class="tdl">Cory,</td>
+      <td class="tdr"><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Correns,</td>
+      <td class="tdr"><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Coutagne,</td>
+      <td class="tdr"><a href="#Page_125">125 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Darwin, on Variation,</td>
+      <td class="tdr"><br /><a href="#Page_1">&nbsp;&nbsp;1</a>,
+                      <a href="#Page_2">&nbsp;&nbsp;2</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Systematics,</td>
+      <td class="tdr"><a href="#Page_10">&nbsp;10</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Selection,</td>
+      <td class="tdr"><a href="#Page_134">134</a>,
+                      <a href="#Page_139">139</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Heterostyle plants,</td>
+      <td class="tdr"><a href="#Page_236">236-237</a></td>
+    </tr><tr>
+      <td class="tdl">Darwin, F.,</td>
+      <td class="tdr"><a href="#Page_190">190</a></td>
+    </tr><tr>
+      <td class="tdl">Darwin, Sir G.,</td>
+      <td class="tdr"><a href="#Page_41">&nbsp;41</a></td>
+    </tr><tr>
+      <td class="tdl">Davenport,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">Davis, H. M.,</td>
+      <td class="tdr"><a href="#Page_102">102</a></td>
+    </tr><tr>
+      <td class="tdl">Delcourt,</td>
+      <td class="tdr"><a href="#Page_130">130</a></td>
+    </tr><tr>
+      <td class="tdl">Deschange,</td>
+      <td class="tdr"><a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Dobell,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl">Doncaster,</td>
+      <td class="tdr"><a href="#Page_105">105</a>,
+                      <a href="#Page_121">121</a>,
+                      <a href="#Page_136">136</a></td>
+    </tr><tr>
+      <td class="tdl">Driesch,</td>
+      <td class="tdr"><a href="#Page_80">80-81</a></td>
+    </tr><tr>
+      <td class="tdl">Duchartre,</td>
+      <td class="tdr"><a href="#Page_51">&nbsp;51</a></td>
+    </tr><tr>
+      <td class="tdl"><br />East,</td>
+      <td class="tdr"><br /><a href="#Page_91">&nbsp;91</a>,
+                            <a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdl">Edwards, W. H.,</td>
+      <td class="tdr"><a href="#Page_162">162</a></td>
+    </tr><tr>
+      <td class="tdl">Ehrlich,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Fellmer,</td>
+      <td class="tdr"><br /><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl">Field, W. L. W.,</td>
+      <td class="tdr"><a href="#Page_161">161</a></td>
+    </tr><tr>
+      <td class="tdl">Fischer, E.,</td>
+      <td class="tdr"><a href="#Page_192">192</a></td>
+    </tr><tr>
+      <td class="tdl">Fleck,</td>
+      <td class="tdr"><a href="#Page_171">171</a>,
+                      <a href="#Page_174">174</a></td>
+    </tr><tr>
+      <td class="tdl">Fletcher, W. H. B.,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a>,
+                      <a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Foster, Sir N.,</td>
+      <td class="tdr"><a href="#Page_39">&nbsp;39</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Gallé,</td>
+      <td class="tdr"><br /><a href="#Page_123">123</a></td>
+    </tr><tr>
+      <td class="tdl">Garrod,</td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdl">Gates,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a>,
+                      <a href="#Page_95">&nbsp;95</a>,
+                      <a href="#Page_102">102</a></td>
+    </tr><tr>
+      <td class="tdl">Gayner, F.,</td>
+      <td class="tdr"><a href="#Page_177">177</a></td>
+    </tr><tr>
+      <td class="tdl">Godron,</td>
+      <td class="tdr"><a href="#Page_249">249</a></td>
+    </tr><tr>
+      <td class="tdl">Gold, E.,</td>
+      <td class="tdr"><a href="#Page_196">196</a></td>
+    </tr><tr>
+      <td class="tdl">Goldschmidt,</td>
+      <td class="tdr"><a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdl">Goodwin, E.,</td>
+      <td class="tdr"><a href="#Page_137">137</a></td>
+    </tr><tr>
+      <td class="tdl">Gortner,</td>
+      <td class="tdr"><a href="#Page_226">226</a></td>
+    </tr><tr>
+      <td class="tdl">Greene, E. L.,</td>
+      <td class="tdr"><a href="#Page_8">&nbsp;&nbsp;8</a></td>
+    </tr><tr>
+      <td class="tdl">Gregory, R. P.,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a>,
+                      <a href="#Page_100">100</a>,
+                      <a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Grenier,</td>
+      <td class="tdr"><a href="#Page_249">249</a></td>
+    </tr><tr>
+      <td class="tdl">Grover,</td>
+      <td class="tdr"><a href="#Page_173">173</a></td>
+    </tr><tr>
+      <td class="tdl">Gruber,</td>
+      <td class="tdr"><a href="#Page_48">&nbsp;48</a></td>
+    </tr><tr>
+      <td class="tdl">Gulick,</td>
+      <td class="tdr"><a href="#Page_119">119</a>,
+                      <a href="#Page_133">133</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Hamling,</td>
+      <td class="tdr"><br /><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Hampson, Sir G.,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a></td>
+    </tr><tr>
+      <td class="tdl">Harris,</td>
+      <td class="tdr"><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Hartlaub,</td>
+      <td class="tdr"><a href="#Page_182">182</a></td>
+    </tr><tr>
+      <td class="tdl">Herbst,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Heribert-Nilsson,</td>
+      <td class="tdr"><a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdl">Hewett,</td>
+      <td class="tdr"><a href="#Page_182">182</a></td>
+    </tr><tr>
+      <td class="tdl">Honing,</td>
+      <td class="tdr"><a href="#Page_105">105</a></td>
+    </tr><tr>
+      <td class="tdl">Hunter, John,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Jakowatz,</td>
+      <td class="tdr"><br /><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Janet,</td>
+      <td class="tdr"><a href="#Page_24">&nbsp;24</a></td>
+    </tr><tr>
+      <td class="tdl">Jeans,</td>
+      <td class="tdr"><a href="#Page_41">&nbsp;41</a></td>
+    </tr><tr>
+      <td class="tdl">Jenkinson,</td>
+      <td class="tdr"><a href="#Page_40">&nbsp;40</a></td>
+    </tr><tr>
+      <td class="tdl">Jentink,</td>
+      <td class="tdr"><a href="#Page_120">120</a></td>
+    </tr><tr>
+      <td class="tdl">Johannsen,</td>
+      <td class="tdr"><a href="#Page_195">195</a></td>
+    </tr><tr>
+      <td class="tdl">Jordan,</td>
+      <td class="tdr"><a href="#Page_185">185</a>,
+                      <a href="#Page_242">242</a>,
+                      <a href="#Page_249">249</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Kammere,</td>
+      <td class="tdr"><br /><a href="#Page_199">199 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Keeble,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Klebs,</td>
+      <td class="tdr"><a href="#Page_250">250</a></td>
+    </tr><tr>
+      <td class="tdl">Krancher,</td>
+      <td class="tdr"><a href="#Page_182">182</a></td>
+    </tr><tr>
+      <td class="tdl">Küchenmeister,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Kudicke,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Lamarck,</td>
+      <td class="tdr"><br /><a href="#Page_9">&nbsp;&nbsp;9</a></td>
+    </tr><tr>
+      <td class="tdl">Lang, A.,</td>
+      <td class="tdr"><a href="#Page_128">128</a></td>
+    </tr><tr>
+      <td class="tdl">Lawrence, W. N.,</td>
+      <td class="tdr"><a href="#Page_142">142</a>,
+                      <a href="#Page_145">145</a></td>
+    </tr><tr>
+      <td class="tdl">Leake, H. Martin,</td>
+      <td class="tdr"><a href="#Page_98">&nbsp;98</a>,
+                      <a href="#Page_100">100</a></td>
+    </tr><tr>
+      <td class="tdl">Leavitt,</td>
+      <td class="tdr"><a href="#Page_185">185</a></td>
+    </tr><tr>
+      <td class="tdl">Lecoq,</td>
+      <td class="tdr"><a href="#Page_99">&nbsp;99</a></td>
+    </tr><tr>
+      <td class="tdl">Lederer,</td>
+      <td class="tdr"><a href="#Page_167">167</a></td>
+    </tr><tr>
+      <td class="tdl">Leduc,</td>
+      <td class="tdr"><a href="#Page_64">&nbsp;64-65</a>,
+                      <a href="#Page_80">&nbsp;80</a></td>
+    </tr><tr>
+      <td class="tdl">Leydig,</td>
+      <td class="tdr"><a href="#Page_182">182</a></td>
+    </tr><tr>
+      <td class="tdl">Linden, M. von,</td>
+      <td class="tdr"><a href="#Page_192">192</a></td>
+    </tr><tr>
+      <td class="tdl">Linnaeus,</td>
+      <td class="tdr"><a href="#Page_6">6-8</a></td>
+    </tr><tr>
+      <td class="tdl">Lloyd, R. E.,</td>
+      <td class="tdr"><a href="#Page_248">248</a></td>
+    </tr><tr>
+      <td class="tdl">Locard,</td>
+      <td class="tdr"><a href="#Page_130">130</a></td>
+    </tr><tr>
+      <td class="tdl">Lock, R. H.,</td>
+      <td class="tdr"><a href="#Page_242">242</a>,
+                      <a href="#Page_244">244</a></td>
+    </tr><tr>
+      <td class="tdl">Loeb,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a>,
+                      <a href="#Page_45">&nbsp;45</a>,
+                      <a href="#Page_50">&nbsp;50</a>,
+                      <a href="#Page_71">&nbsp;71</a>,
+                      <a href="#Page_77">&nbsp;77</a></td>
+    </tr><tr>
+      <td class="tdl">Lotsy,</td>
+      <td class="tdr"><a href="#Page_99">&nbsp;99</a></td>
+    </tr><tr>
+      <td class="tdl">Lowe, P. R.,</td>
+      <td class="tdr"><a href="#Page_143">143</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Macdougal, W. T.,</td>
+      <td class="tdr"><br /><a href="#Page_102">102</a>,
+                            <a href="#Page_226">226</a></td>
+    </tr><tr>
+      <td class="tdl">Marchant,</td>
+      <td class="tdr"><a href="#Page_7">&nbsp;&nbsp;7</a></td>
+    </tr><tr>
+      <td class="tdl">Mathew,</td>
+      <td class="tdr"><a href="#Page_171">171</a></td>
+    </tr><tr>
+      <td class="tdl">Matthioli,</td>
+      <td class="tdr"><a href="#Page_4">&nbsp;&nbsp;4</a></td>
+    </tr><tr>
+      <td class="tdl">Mayer, A. G.,</td>
+      <td class="tdr"><a href="#Page_133">133</a></td>
+    </tr><tr>
+      <td class="tdl">Mendel, Rediscovery of,</td>
+      <td class="tdr"><a href="#Page_2">&nbsp;&nbsp;2</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;On Fasciation,</td>
+      <td class="tdr"><a href="#Page_49">&nbsp;49</a></td>
+    </tr><tr>
+      <td class="tdl">Merrifield,</td>
+      <td class="tdr"><a href="#Page_169">169</a>,
+                      <a href="#Page_172">172</a></td>
+    </tr><tr>
+      <td class="tdl">Miller, W. D.,</td>
+      <td class="tdr"><a href="#Page_120">120</a>,
+                      <a href="#Page_149">149</a></td>
+    </tr><tr>
+      <td class="tdl">Morgan,</td>
+      <td class="tdr"><a href="#Page_71">&nbsp;71</a>,
+                      <a href="#Page_77">&nbsp;77</a>,
+                      <a href="#Page_91">&nbsp;91</a>,
+                      <a href="#Page_198">198</a></td>
+    </tr><tr>
+      <td class="tdl">Moggridge,</td>
+      <td class="tdr"><a href="#Page_125">125</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Nathusius, S.,</td>
+      <td class="tdr"><br /><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Nettleship,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Newman, H. H.,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Newsholme,</td>
+      <td class="tdr"><a href="#Page_48">&nbsp;48</a></td>
+    </tr><tr>
+      <td class="tdl">Nilsson-Ehle,</td>
+      <td class="tdr"><a href="#Page_116">116</a>,
+                      <a href="#Page_169">169</a></td>
+    </tr><tr>
+      <td class="tdl">Norman, A. M.,</td>
+      <td class="tdr"><a href="#Page_125">125</a>,
+                      <a href="#Page_156">156</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Ober,</td>
+      <td class="tdr"><br /><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Oberthür,</td>
+      <td class="tdr"><a href="#Page_168">168</a>,
+                      <a href="#Page_170">170</a>,
+                      <a href="#Page_193">193</a></td>
+    </tr><tr>
+      <td class="tdl">Oliver, J.,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Page, H. E.,</td>
+      <td class="tdr"><br /><a href="#Page_167">167</a>,
+                            <a href="#Page_180">180</a></td>
+    </tr><tr>
+      <td class="tdl">Patterson, J. T.,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Payne, F.,</td>
+      <td class="tdr"><a href="#Page_229">229</a></td>
+    </tr><tr>
+      <td class="tdl">Pellew,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Poll,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl">Porritt,</td>
+      <td class="tdr"><a href="#Page_136">136</a></td>
+    </tr><tr>
+      <td class="tdl">Poulton,</td>
+      <td class="tdr"><a href="#Page_141">141</a></td>
+    </tr><tr>
+      <td class="tdl">Powers, J. H.,</td>
+      <td class="tdr"><a href="#Page_230">230</a></td>
+    </tr><tr>
+      <td class="tdl">Pringsheim, H.,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Przibram,</td>
+      <td class="tdr"><a href="#Page_72">&nbsp;72</a>,
+                      <a href="#Page_78">&nbsp;78</a>,
+                      <a href="#Page_178">178</a>,
+                      <a href="#Page_194">194</a>,
+                      <a href="#Page_197">197</a>,
+                      <a href="#Page_199">199</a></td>
+    </tr><tr>
+      <td class="tdl">Punnett,</td>
+      <td class="tdr"><a href="#Page_110">110</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Ray,</td>
+      <td class="tdr"><br /><a href="#Page_4">4-5</a></td>
+    </tr><tr>
+      <td class="tdl">Raynor,</td>
+      <td class="tdr"><a href="#Page_105">105</a></td>
+    </tr><tr>
+      <td class="tdl">Ridgway,</td>
+      <td class="tdr"><a href="#Page_10">&nbsp;10</a>,
+                      <a href="#Page_120">120</a></td>
+    </tr><tr>
+      <td class="tdl">Roedelius,</td>
+      <td class="tdr"><a href="#Page_195">195</a></td>
+    </tr><tr>
+      <td class="tdl">Rolfe,</td>
+      <td class="tdr"><a href="#Page_20">&nbsp;20</a></td>
+    </tr><tr>
+      <td class="tdl">Rosen, F.,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Rosner,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Rowland-Brown, H.,</td>
+      <td class="tdr"><a href="#Page_167">167</a>,
+                      <a href="#Page_180">180</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Sargent,</td>
+      <td class="tdr"><br /><a href="#Page_185">185</a></td>
+    </tr><tr>
+      <td class="tdl">Saunders, E. R.,</td>
+      <td class="tdr"><a href="#Page_84">&nbsp;84</a>,
+                      <a href="#Page_104">104</a>,
+                      <a href="#Page_112">112</a></td>
+    </tr><tr>
+      <td class="tdl">Schima,</td>
+      <td class="tdr"><a href="#Page_177">177</a></td>
+    </tr><tr>
+      <td class="tdl">Schröder,</td>
+      <td class="tdr"><a href="#Page_193">193-194</a></td>
+    </tr><tr>
+      <td class="tdl">Schübeler,</td>
+      <td class="tdr"><a href="#Page_195">195</a></td>
+    </tr><tr>
+      <td class="tdl">Semon, R.,</td>
+      <td class="tdr"><a href="#Page_190">190 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Sharrock,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a></td>
+    </tr><tr>
+      <td class="tdl">Shull,</td>
+      <td class="tdr"><a href="#Page_100">100</a></td>
+    </tr><tr>
+      <td class="tdl">Speyer, A.,</td>
+      <td class="tdr"><a href="#Page_166">166</a>,
+                      <a href="#Page_170">170</a>,
+                      <a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Spillman,</td>
+      <td class="tdr"><a href="#Page_47">&nbsp;47</a></td>
+    </tr><tr>
+      <td class="tdl">Standfuss,</td>
+      <td class="tdr"><a href="#Page_135">135</a>,
+                      <a href="#Page_181">181</a>,
+                      <a href="#Page_191">191</a></td>
+    </tr><tr>
+      <td class="tdl">Staples-Browne,</td>
+      <td class="tdr"><a href="#Page_49">&nbsp;49</a>,
+                      <a href="#Page_98">&nbsp;98</a></td>
+    </tr><tr>
+      <td class="tdl">Staudinger,</td>
+      <td class="tdr"><a href="#Page_170">170</a>,
+                      <a href="#Page_179">179</a></td>
+    </tr><tr>
+      <td class="tdl">Stockard,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a>,
+                      <a href="#Page_71">&nbsp;71</a></td>
+    </tr><tr>
+      <td class="tdl">Sutton,</td>
+      <td class="tdr"><a href="#Page_236">236</a>,
+                      <a href="#Page_244">244</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Tornier,</td>
+      <td class="tdr"><br /><a href="#Page_72">&nbsp;72</a></td>
+    </tr><tr>
+      <td class="tdl">Tower, W. L.,</td>
+      <td class="tdr"><a href="#Page_218">218-226</a></td>
+    </tr><tr>
+      <td class="tdl">Trechmann,</td>
+      <td class="tdr"><a href="#Page_133">133</a></td>
+    </tr><tr>
+      <td class="tdl">Tugwell,</td>
+      <td class="tdr"><a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Tutt, J. W. On Definiteness of Species,</td>
+      <td class="tdr"><a href="#Page_13">&nbsp;13</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;On Plusia interrogationis,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;On Tephrosia,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;On N. castanea,</td>
+      <td class="tdr"><a href="#Page_122">122</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;On Pararge egeria,</td>
+      <td class="tdr"><a href="#Page_167">167 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Verity, R.,</td>
+      <td class="tdr"><br /><a href="#Page_171">171</a>,
+                            <a href="#Page_177">177</a></td>
+    </tr><tr>
+      <td class="tdl">Vries, H. de,</td>
+      <td class="tdr"><a href="#Page_101">101-115</a>,
+                      <a href="#Page_222">222</a>,
+                      <a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Walker, G,</td>
+      <td class="tdr"><br /><a href="#Page_49">&nbsp;49</a></td>
+    </tr><tr>
+      <td class="tdl">Weir, Jenner,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Weismann,</td>
+      <td class="tdr"><a href="#Page_176">176</a>,
+                      <a href="#Page_188">188</a></td>
+    </tr><tr>
+      <td class="tdl">Wendelstadt,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl">Werbitzki,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl">Werner,</td>
+      <td class="tdr"><a href="#Page_209">209</a></td>
+    </tr><tr>
+      <td class="tdl">Wettstein,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Wheeler, G.,</td>
+      <td class="tdr"><a href="#Page_168">168</a>,
+                      <a href="#Page_171">171</a></td>
+    </tr><tr>
+      <td class="tdl">Wheldale,</td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdl">Wilder,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Wille,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Williams, H.,</td>
+      <td class="tdr"><a href="#Page_167">167</a>,
+                      <a href="#Page_172">172</a></td>
+    </tr><tr>
+      <td class="tdl">Windle, B. C. A.,</td>
+      <td class="tdr"><a href="#Page_43">&nbsp;43</a></td>
+    </tr><tr>
+      <td class="tdl">Winslow,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Wolf, F.,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Woodforde,</td>
+      <td class="tdr"><a href="#Page_123">123</a></td>
+    </tr><tr>
+      <td class="tdl">Woltereck,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Zeijlstra,</td>
+      <td class="tdr"><br /><a href="#Page_114">114</a></td>
+    </tr>
+ </tbody>
+</table>
+<hr class="chap space-below" />
+<div class="transnote">
+<p style="font-size: 150%; text-align: center;"><i>Transcriber's Notes</i>:</p>
+<p class="indent">
+   The illustrations have been moved so that they do not break up
+   paragraphs and so that they are next to the text they illustrate.</p>
+<p class="indent">
+   Errors in punctuation and inconsistent hyphenation were not corrected
+   unless otherwise noted.</p>
+</div>
+<hr class="chap" />
+<h2 class="u">FOOTNOTES:</h2>
+
+<div class="footnotes"><h3>FOOTNOTES:&emsp;CHAPTER I.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a>
+<a href="#FNanchor_1_1"><span class="label">[1]</span></a>
+In <i>Mendel's Principles of Heredity</i> (Cambridge University Press, 1909)
+I have dealt with this subject, giving an account of the principal
+facts discovered up to the beginning of 1909.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a>
+<a href="#FNanchor_2_2"><span class="label">[2]</span></a>
+Matthioli Opera, Ed. 1598, p. 8, originally published 1565.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a>
+<a href="#FNanchor_3_3"><span class="label">[3]</span></a>
+Ray's instances relate to Kales, and in most of these examples we can
+see that there was no question of mutation or transmutation at all, but that the
+occurrence was due either to mistake or to cross-fertilisation. Sharrock, to whom
+Ray refers, was inclined to discredit stories of transmutation, but he has also this
+passage (<i>History of the Propagation and Improvement of Vegetables by the
+Concurrence of Art and Nature</i>, Oxford, 1660, p. 29):</p>
+<p>"It is indeed grown to be a great question, whether the transmutation of a
+species be possible either in the vegetable, Animal, or Minerall Kingdome. For the
+possibility of it in the vegetable; I have heard <i>Mr. Bobart</i> and his <i>Son</i> often report it,
+and proffer to make oath that the Crocus and Gladiolus, as likewise the Leucoium,
+and Hyacinths by a long standing without replanting have in his garden changed
+from one kind to the other: and for satisfaction about the curiosity in the presence
+of <i>Mr. Boyle</i> I tooke up some bulbs of the very numericall roots whereof the relation
+was made, though the alteration was perfected before, where we saw the
+diverse bulbs growing as it were on the same stoole, close together, but no bulb
+half of the one kind, and the other half of the other: But the changetime being past
+it was reason we should believe the report of good artists in matters of their own
+faculty."</p>
+<p>Robert Sharrock was a fellow of New College, Oxford. Both the Bobarts were
+professional botanists, the father was author of a Catalogue of the plants in the
+Hortus Medicus at Oxford, and the son was afterwards Curator of the Oxford Garden.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a>
+<a href="#FNanchor_4_4"><span class="label">[4]</span></a>
+<i>Mém. Ac. roy. des Sci.</i> for 1719 (1721), p. 59.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a>
+<a href="#FNanchor_5_5"><span class="label">[5]</span></a>
+<i>Amoen. Acad.</i>, 1789, vol. 6. I do not know whether attention has been called
+to the curious mistake which Linnaeus makes in the course of this argument. He
+cites the differences between the Mule and the Hinny in illustration of his thesis,
+pointing out that the Mule is externally more like a horse and the Hinny more like
+an ass. This, he says, is because the Mule has the horse for a father, and the
+Hinny the ass, thus inverting the actual facts!</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a>
+<a href="#FNanchor_6_6"><span class="label">[6]</span></a>
+<i>Proc. Washington Ac. Sci.</i>, 1909, XI, pp. 17-26.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a>
+<a href="#FNanchor_7_7"><span class="label">[7]</span></a>
+J. W. Tutt, in <i>Ent. Rec.</i>, 1909, XXI, p. 185.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a>
+<a href="#FNanchor_8_8"><span class="label">[8]</span></a>
+E. Lehmann (<i>Bull. l'Herb. Boissier</i>, Ser. 2, VIII, 1908, p. 229) has published
+an admirable paper on the interrelationships of these species and has instituted
+cultural experiments which will probably much elucidate the nature of their specific
+distinctness. As regards the existence of intermediate forms he comes to the conclusion
+that two only can be so regarded. The first was described by Kuntze from
+specimens found on a flower-pot on board a Caspian steamer, from which Lehmann
+proposes the new specific name <i>Siaretensis</i>. This comes between <i>polita</i>
+and <i>filiformis</i>, a close ally of <i>Tournefortii</i>. The other, which combines
+some of the features of both <i>polita</i> and <i>Tournefortii</i>, was found in the
+province of Asterabad.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a>
+<a href="#FNanchor_9_9"><span class="label">[9]</span></a>
+In Cambridgeshire for example <i>vespertina</i> is common but <i>diurna</i> is absent.
+Whether this absence is connected with the general presence of chalk I cannot say.
+When introduced artificially <i>diurna</i> establishes itself, for a time at least,
+without any apparent difficulty and occasionally escapes from the garden on to the
+neighbouring roadside.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_10" id="Footnote_10_10"></a>
+<a href="#FNanchor_10_10"><span class="label">[10]</span></a>
+Conceivably however it may be a segregated combination. For an account
+of this plant see Boissier, <i>Voy. Bot. Midi de l'Espagne</i>, 1839, II, 722.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_11" id="Footnote_11_11"></a>
+<a href="#FNanchor_11_11"><span class="label">[11]</span></a>
+A discussion of this subject with references to literature is given by Rolfe,
+in an excellent paper on "Hybridisation viewed from the standpoint of Systematic
+Botany" (<i>Jour. R. Hort. Soc.</i>, XXIV, 1900, p. 197). He concludes: "The simple
+fact is that the two plants (<i>L. diurna</i> and <i>vespertina</i>) are thoroughly distinct
+in numerous particulars, and affect such different habitats that in some localities
+one or the other of them is completely wanting. But when their stations are
+adjacent they hybridise together very readily, and it is here that these intermediate
+forms occur which have puzzled botanists so much." The same paper contains
+valuable information concerning several cognate illustrations.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_12" id="Footnote_12_12"></a>
+<a href="#FNanchor_12_12"><span class="label">[12]</span></a>
+In only two cases have I seen such plants (both females) completely sterile.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_13" id="Footnote_13_13"></a>
+<a href="#FNanchor_13_13"><span class="label">[13]</span></a>
+As is well known, in an even more notorious example, he proposed to unite
+<i>Primula vulgaris</i>, <i>P. elatior</i>, and <i>P. acaulis</i>,
+similarly relying on the existence of "intermediates,"
+which we now well know to be mongrels between the species.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_14" id="Footnote_14_14"></a>
+<a href="#FNanchor_14_14"><span class="label">[14]</span></a>
+For an account of the distinctions between <i>Vespa vulgaris</i> and <i>germanica</i>
+see Ch. Janet, <i>Études sur les Fourmis, les Guêpes et les Abeilles</i>, 11<sup>e</sup>,
+Note. Sur <i>Vespa germanica</i> et <i>V. vulgaris</i>. Limoges (Ducourtieux),
+1895; and R. du Buysson, Monographie des Guêpes, <i>Ann. Soc. Ent. France</i>,
+1903, Vol. LXXII, p. 603, Pl. VIII.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_15" id="Footnote_15_15"></a>
+<a href="#FNanchor_15_15"><span class="label">[15]</span></a>
+The statements made above are for the most part taken from Barrett, C. G.,
+<i>Lepidoptera of the British Islands</i>, and from Tutt, J. W., <i>The British Noctuae and
+their Varieties</i>. The reader who is unfamiliar with the amazing polymorphism
+exhibited by some of these moths should if possible take an opportunity of looking
+over a long series in a collection, or, if that be impossible, refer to the admirable
+coloured plates published by Barrett. It may not be superfluous to observe that
+plenty of similar examples are known in other countries. For instance <i>Plotheia
+frontalis</i>, a Noctuid which often abounds in Ceylon, shows an equally bewildering
+wealth of forms. If a dozen specimens of such a species were to be brought home
+from some little known country, each individual would almost certainly be described
+as the type of a distinct species. (See the coloured plate published by Sir G. Hampson,
+Cat. Brit. Mus., Heterocera, Vol. IX.)</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_16" id="Footnote_16_16"></a>
+<a href="#FNanchor_16_16"><span class="label">[16]</span></a>
+<i>Dict. of Birds</i>, p. 800. It would be interesting and profitable to attempt in
+a long series of Ruffs to determine the Mendelian factors which by their combinations
+give rise to this complex assemblage of varietal forms. A few such factors both of
+colour and pattern can be at once distinguished, and it is noticeable that some of
+the resulting types of barring, spangling and penciling show a perceptible correspondence
+with some of the types of colouration found in the breeds of domestic fowls.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_17" id="Footnote_17_17"></a>
+<a href="#FNanchor_17_17"><span class="label">[17]</span></a>
+Howard Saunders (<i>Illust. Manual of British Birds</i>, 1899, p. 499) states that
+there is evidence that the pheasant had become naturalized in the south of England
+before the Norman invasion. He adds, "little, if any, deviation from the typical
+<i>P. colchicus</i> took place up to the end of last century, when the introduction of
+the Chinese Ring-necked <i>P. torquatus</i> commenced, which has left almost indelible
+marks, especially with regard to the characteristic white collar."</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER II.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_18" id="Footnote_1_18"></a>
+<a href="#FNanchor_1_18"><span class="label">[1]</span></a>
+In saying this we make no assumption as to the particular cell-division at
+which differentiation occurs. This may be one of the maturation-divisions, or it
+may perhaps be much earlier.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_19" id="Footnote_2_19"></a>
+<a href="#FNanchor_2_19"><span class="label">[2]</span></a>
+From the recent discoveries of Erwin Baur we are led to surmise that in the
+flowering plants the sub-epidermal layer, or some of its elements, may legitimately
+be regarded as a similar germ-substance, continuous in Weismann's sense.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_20" id="Footnote_3_20"></a>
+<a href="#FNanchor_3_20"><span class="label">[3]</span></a>
+These fraternal twins, which show no special resemblance to each other,
+are like the multiple births of other animals, and there is no disposition for them to
+be of the same sex. In the sheep, for example, statistics show that the frequency
+of pairs of twins, male and female, is approximately double that of the frequency
+of pairs, both male or both female, as it should be if the sex-distribution were fortuitous.
+For instance Bernadin (<i>La Bergerie de Rambouillet</i>, 1890, p. 100) gives
+the following figures for twin-lambs in Merinos: both male, 87; both female, 83;
+sexes mixed, 187. The 9-banded Armadillo (<i>Dasypus novemcinctus</i>), in which
+the young born in one litter are said to be always of one sex, is the only known
+exception in Vertebrates, and is presumably a genuine case of normal polyembryony
+(see especially, Rosner, <i>Bull. Ac. Soc. Cracovie</i>, 1901, p. 443, and Newman and
+Patterson, <i>Biol. Bull.</i>, XVII, 1909, p. 181), and an important paper lately published
+by H. H. Newman and J. T. Patterson, <i>Jour. Morph.</i>, 1911, XXII, p. 855.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_21" id="Footnote_4_21"></a>
+<a href="#FNanchor_4_21"><span class="label">[4]</span></a>
+A good collection of evidence as to disease in homologous twins was lately
+published by E. A. Cockayne, <i>Brit. Jour. Child. Diseases</i>, Nov., 1911.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_22" id="Footnote_5_22"></a>
+<a href="#FNanchor_5_22"><span class="label">[5]</span></a>
+Cp. Windle, B. C. A., <i>Jour. Anal. Phys.</i>, XXVI, p. 295.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_23" id="Footnote_6_23"></a>
+<a href="#FNanchor_6_23"><span class="label">[6]</span></a>
+Mr. E. Nettleship tells me that in the course of collecting pedigrees of families
+containing colour-blind members he has discovered two cases (shortly to be published)
+of pairs of twins, which on account of their very close resemblances must
+be deemed homologous, one of each pair being colour-blind and the other normal.
+Such a distinction between closely similar twins is most curious and unexpected.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_24" id="Footnote_7_24"></a>
+<a href="#FNanchor_7_24"><span class="label">[7]</span></a>
+Another paradoxical phenomenon of the same nature occurs in the Narwhal
+The males normally have the <i>left</i> tusk alone developed, the corresponding right
+tusk remaining as an undeveloped rudiment in its socket. The left tusk is a
+left-handed screw. Occasionally the right tusk is also developed and grows to
+the same length as that of the left side, but in such specimens the right tusk is
+also a left-hand screw like the tusk of the other side, instead of being reversed
+as we should certainly have expected. It need scarcely be remarked that in the
+case of the horns of antelopes, and in other examples of spiral organs arranged in
+pairs, that of one side of the body is the mirror image of that on the other side.
+The Narwhal's tusks in being both twisted in the same direction are thus highly
+anomalous, and are comparable with pairs of twins.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_25" id="Footnote_8_25"></a>
+<a href="#FNanchor_8_25"><span class="label">[8]</span></a>
+Wilder, H. H., <i>Amer. Jour. Anat.</i>, 1904, III, p. 452.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_26" id="Footnote_9_26"></a>
+<a href="#FNanchor_9_26"><span class="label">[9]</span></a>
+Polydactylism which is often a dominant and the web-foot of Pigeons which
+is recessive should be remembered as possible exceptions (see p. 49).</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_27" id="Footnote_10_27"></a>
+<a href="#FNanchor_10_27"><span class="label">[10]</span></a>
+Davenport inclined at first to regard rumplessness as a recessive, but in his
+latest publication on the subject he definitely concludes that it is an imperfect
+dominant. This conclusion accords well with evidence quoted by Darwin (<i>An.
+and Plts.</i>, II, ed. 2, p. 4) that rumpless fowls may throw tailed offspring.
+(<i>Amer. Nat.</i>, 1910, XLIV, p. 134.)</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_28" id="Footnote_11_28"></a>
+<a href="#FNanchor_11_28"><span class="label">[11]</span></a>
+Spillman, W. J., <i>Amer. Breeders Mag.</i>, 1910, I, p. 178.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_29" id="Footnote_12_29"></a>
+<a href="#FNanchor_12_29"><span class="label">[12]</span></a>
+Newsholme, <i>Lancet</i>, December 10, 1910, p. 1690.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_30" id="Footnote_13_30"></a>
+<a href="#FNanchor_13_30"><span class="label">[13]</span></a>
+<i>Materials for the Study of Variation</i>, 1894, p. 358.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_31" id="Footnote_14_31"></a>
+<a href="#FNanchor_14_31"><span class="label">[14]</span></a>
+Walker, G., <i>Johns Hopkins Hospital Bulletin</i>, XII, 1901, p. 129.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_32" id="Footnote_15_32"></a>
+<a href="#FNanchor_15_32"><span class="label">[15]</span></a>
+Cp. R. H. Compton, <i>New Phytologist</i>, 1911, p. 249.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_33" id="Footnote_16_33"></a>
+<a href="#FNanchor_16_33"><span class="label">[16]</span></a>
+<i>Arch. f. Entwickelungsmech.</i>, 1907, XXIII, p. 249.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_34" id="Footnote_17_34"></a>
+<a href="#FNanchor_17_34"><span class="label">[17]</span></a>
+Bull. Soc. Bot. de France, xxxiv, 1887, p. 182.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_35" id="Footnote_18_35"></a>
+<a href="#FNanchor_18_35"><span class="label">[18]</span></a>
+R. Boyle, <i>The Origine of Formes and Qualities</i>, Oxford, 1666.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_36" id="Footnote_19_36"></a>
+<a href="#FNanchor_19_36"><span class="label">[19]</span></a>
+Remarkable experiments on this question have lately been carried out by
+R. H. Compton (<i>Camb. Phil. Soc.</i>, XV, 1910, p. 495), showing that in a certain
+Barley, "Plumage Corn," the average ratio of left to right is about 1.5. A fuller
+paper has since been published by Compton, <i>Jour. Genetics</i>, 1912, II, I, p. 53.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER III.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_37" id="Footnote_1_37"></a>
+<a href="#FNanchor_1_37"><span class="label">[1]</span></a>
+Stéphane Leduc, <i>Théorie Physico-Chymique de la Vie</i>, Paris, 1910.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_38" id="Footnote_2_38"></a>
+<a href="#FNanchor_2_38"><span class="label">[2]</span></a>
+<i>Materials for the Study of Variation</i>, No. 249, p. 217; and p. 272.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_39" id="Footnote_3_39"></a>
+<a href="#FNanchor_3_39"><span class="label">[3]</span></a>
+<i>Materials</i>, p. 118.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_40" id="Footnote_4_40"></a>
+<a href="#FNanchor_4_40"><span class="label">[4]</span></a>
+Church, A. H., <i>On the Relation of Phyllotaxis to Mechanical Laws</i>, London, 1904.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_41" id="Footnote_5_41"></a>
+<a href="#FNanchor_5_41"><span class="label">[5]</span></a>
+It is a question whether the dominance of the palmatifid leaf
+over the pinnatifid is not really an example of the dominance of a lower
+number of segmentations over a higher. From the uncertainty whether two
+given leaves of two separate plants are actually comparable one cannot
+institute quite satisfactory numerical comparisons, but I think the view
+that the "Fern" leaf has more lobes than an otherwise similar "Palm" leaf
+may be fairly maintained. If this be admitted, the "Palm" leaf represents
+the dominant low number and its round shape is a consequence of the greater
+powers of growth which are so often possessed by the members of a shorter series.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_42" id="Footnote_6_42"></a>
+<a href="#FNanchor_6_42"><span class="label">[6]</span></a>
+It is perhaps of importance to remember that in certain species of bacteria
+(e. g. <i>Bacillus Anthracis</i>) division may cease where the organism is
+cultivated under certain artificial conditions though growth continues.
+In this way very long unsegmented threads are produced.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_43" id="Footnote_7_43"></a>
+<a href="#FNanchor_7_43"><span class="label">[7]</span></a>
+<i>Arch. f. Entwm.</i>, XX, 1905, p. 76;
+<i>Sitzungsb. d. Ges. Naturf.</i>, Berlin, 1907, p. 41, etc.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_44" id="Footnote_8_44"></a>
+<a href="#FNanchor_8_44"><span class="label">[8]</span></a>
+Borradaile, L. A., <i>Jour. Marine Zool.</i>, 1897, No. 8.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_45" id="Footnote_9_45"></a>
+<a href="#FNanchor_9_45"><span class="label">[9]</span></a>
+Dr. Przibram, I should mention, concludes that on the whole the facts are
+against this interpretation, but as more evidence is certainly required,
+I call attention to the possibility.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_46" id="Footnote_10_46"></a>
+<a href="#FNanchor_10_46"><span class="label">[10]</span></a>
+ Morgan, T. H., <i>Regeneration</i>, 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_47" id="Footnote_11_47"></a>
+<a href="#FNanchor_11_47"><span class="label">[11]</span></a>
+It would be interesting to know whether growth continues at the original
+posterior end after the new "posterior" end has been formed in front.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_48" id="Footnote_12_48"></a>
+<a href="#FNanchor_12_48"><span class="label">[12]</span></a>
+In the actual case observed, the ripples unsmoothed had a wave-length of
+about 2-1/2 inches; and when the new ones were first formed, there were
+about 30 ridges in the length originally traversed by 15 or 16.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_49" id="Footnote_13_49"></a>
+<a href="#FNanchor_13_49"><span class="label">[13]</span></a>
+<i>The Science and Philosophy of the Organism</i>;
+Gifford Lectures, 1907. London, 1908, p. 141.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER IV.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_50" id="Footnote_1_50"></a>
+<a href="#FNanchor_1_50"><span class="label">[1]</span></a>
+Gates, R. R., <i>Zts. f. Abstammungslehre</i>, 1911, IV, pp. 341 and 361.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER V.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_51" id="Footnote_1_51"></a>
+<a href="#FNanchor_1_51"><span class="label">[1]</span></a>
+See Lotsy and Baur, Rep. Genetics Conf., Paris, 1911, pp. 416-426.
+Compare Lecoq on <i>Mirabilis jalapa</i> × <i>longiflora</i>,
+Fécondation des Végétaux, 1862, p. 311.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_52" id="Footnote_2_52"></a>
+<a href="#FNanchor_2_52"><span class="label">[2]</span></a>
+<i>Rep. Evol. Ctee. R. S.</i>, IV, 1908, p. 38.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_53" id="Footnote_3_53"></a>
+<a href="#FNanchor_3_53"><span class="label">[3]</span></a>
+<i>Ber. Deut. Bot. Ges.</i>, 1908, XXVI, <i>a</i>, p. 672.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_54" id="Footnote_4_54"></a>
+<a href="#FNanchor_4_54"><span class="label">[4]</span></a>
+<i>Jour. Genetics</i>, 1, 1910, p. 57.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_55" id="Footnote_5_55"></a>
+<a href="#FNanchor_5_55"><span class="label">[5]</span></a>
+In Rep. 1 to Evol. Committee, 1902, p. 132, attention was called
+to this possibility, though of course at that date it was in sexual animals alone
+that it was supposed to exist. It had not occurred to me that even a hermaphrodite
+plant might be in this condition.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_56" id="Footnote_6_56"></a>
+<a href="#FNanchor_6_56"><span class="label">[6]</span></a>
+From the description of the offspring of <i>muricata</i> used as mother.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_57" id="Footnote_7_57"></a>
+<a href="#FNanchor_7_57"><span class="label">[7]</span></a>
+de Vries, <i>Species and Varieties</i>, 1905, p. 259.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_58" id="Footnote_8_58"></a>
+<a href="#FNanchor_8_58"><span class="label">[8]</span></a>
+Zeijlstra in a recent paper announces that many <i>nanella</i>
+plants are the subject of a bacterial disease to which he attributes their
+dwarfness. I gather that this does not apply to all <i>nanella</i> plants and
+that some are dwarfs apart from disease. The matter may no doubt be further
+complicated from this cause.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_59" id="Footnote_9_59"></a>
+<a href="#FNanchor_9_59"><span class="label">[9]</span></a>
+<i>Zts. f. Abstamm.</i>, 1912, VIII.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_60" id="Footnote_10_60"></a>
+<a href="#FNanchor_10_60"><span class="label">[10]</span></a>
+<i>Arch. f. Zellforschung</i>, 1912, IX, p. 331.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER VI.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_61" id="Footnote_1_61"></a>
+<a href="#FNanchor_1_61"><span class="label">[1]</span></a>
+For the evidence see Tutt, J. W., <i>Trans. Ent. Soc.</i>, 1898, p. 17.
+Compare the remarkable case given by Gulick
+(<i>Evolution Racial and Habitudinal</i>, p. 123) of the two races of
+<i>Cicada</i>, which are separated by reason of their life-cycles,
+one having a period of 13, the other 17 years.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_62" id="Footnote_2_62"></a>
+<a href="#FNanchor_2_62"><span class="label">[2]</span></a>
+For references see <i>Materials</i>, p. 396, and also G. Baur,
+<i>Amer. Nat.</i>, 1893, July, p. 677.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_63" id="Footnote_3_63"></a>
+<a href="#FNanchor_3_63"><span class="label">[3]</span></a>
+Jenner Weir, <i>Entomologist</i>, 1880, XIII, p. 251.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_64" id="Footnote_4_64"></a>
+<a href="#FNanchor_4_64"><span class="label">[4]</span></a>
+Jentink, <i>Notes Leyden Mus.</i>, 1885, VII, p. 111.
+Specimens illustrating this peculiarity are in the British Museum.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_65" id="Footnote_5_65"></a>
+<a href="#FNanchor_5_65"><span class="label">[5]</span></a>
+<i>Proc. Zool. Soc.</i>, 1895, p. 850. Plate. Many points beyond that mentioned
+above are involved in this remarkable case. For example, not only are there males
+like females, but a small proportion of females resemble the ordinary male type.
+The stripes are not merely the spots produced, for they occupy different anatomical
+positions. The spots almost always go with a black ventral surface, but the striped
+forms nearly always have that region testaceous. <i>Spartium retama</i>, the food-plant,
+will not grow in England, but if it could be naturalised in America the whole problem
+might be investigated there and results of exceptional interest would almost certainly
+be attained.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_66" id="Footnote_6_66"></a>
+<a href="#FNanchor_6_66"><span class="label">[6]</span></a>
+Doncaster, L., <i>Proc. Zool. Soc.</i>, 1905, II, p. 528.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_67" id="Footnote_7_67"></a>
+<a href="#FNanchor_7_67"><span class="label">[7]</span></a>
+I am not aware that the details of this striking case have ever been worked
+out. It should be noted that the green and blue forms are not due to simple modification
+of the red pigment; for these colours, due to interference, fork over the
+area occupied by the red lines. The distinctions between these forms cannot
+therefore be simply chemical, as we may suppose them to be, for instance, in the
+case of many red and yellow forms, and the genetic relationships of the <i>Heliconid</i>
+varieties would raise many novel problems and be well worth studying experimentally.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_68" id="Footnote_8_68"></a>
+<a href="#FNanchor_8_68"><span class="label">[8]</span></a>
+Woodeforde, F. C., <i>Trans. North Staffordshire Field Club</i>, XXXV, 1901, Plate.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_69" id="Footnote_9_69"></a>
+<a href="#FNanchor_9_69"><span class="label">[9]</span></a>
+E. Gallé, <i>Compte Rendus du Congres Internat. de Bot. a l'Expos. Univ.</i>,
+1900, p. 112.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_70" id="Footnote_10_70"></a>
+<a href="#FNanchor_10_70"><span class="label">[10]</span></a>
+Flora of Mentone, 1864-8, <i>Nova Acta Acad. Caes.</i>, XXXV, 1869.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_71" id="Footnote_11_71"></a>
+<a href="#FNanchor_11_71"><span class="label">[11]</span></a>
+I owe these facts to Canon A. M. Norman, who showed me illustrative
+specimens. They were originally described by Bowerbank (<i>Monogr. Brit. Spongiadae</i>,
+vol. II, pp. 18 and XX; vol. III, Pls. I and III). A specimen of <i>G. compressa</i>
+measured 5 inches, with a greatest width of 3-1/4 in. <i>G. ciliata</i> was found measuring
+3 in. long and 3/4 in. wide. These dimensions are many times those of normal
+specimens.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_72" id="Footnote_12_72"></a>
+<a href="#FNanchor_12_72"><span class="label">[12]</span></a>
+Coutagne, G., <i>Recherches sur le Polymorphisme des Mollusques de France</i>,
+<i>Annales Soc. d'Agric. Sci. et Industr. Lyon</i>, 1895.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_73" id="Footnote_13_73"></a>
+<a href="#FNanchor_13_73"><span class="label">[13]</span></a>
+As to the synonymy and references see Coutagne, p. 45.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_74" id="Footnote_14_74"></a>
+<a href="#FNanchor_14_74"><span class="label">[14]</span></a>
+A. Lang, <i>Die Bastarde von H. hortensis Muller H. nemoralis L.</i> Jena, G. Fischer, 1908;
+with a fine coloured plate showing the varieties of the species and their hybrids.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_75" id="Footnote_15_75"></a>
+<a href="#FNanchor_15_75"><span class="label">[15]</span></a>
+With this evidence compare that given by A. Delcourt in
+his valuable papers lately published relating to the variations of
+<i>Notonecta</i>. See especially <i>Bull. Sci. Fr. Belg.</i>, 1909,
+XLIII, p. 443; and <i>C. R. Soc. Biol.</i>, 1909, LXVI, p. 589.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_76" id="Footnote_16_76"></a>
+<a href="#FNanchor_16_76"><span class="label">[16]</span></a>
+Allen, J. A., <i>Bull. Amer. Mus. N. H.</i>, III, 1891, pp. 51-54.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_77" id="Footnote_17_77"></a>
+<a href="#FNanchor_17_77"><span class="label">[17]</span></a>
+J. T. Gulick, <i>Evolution, Racial and Habitudinal</i>,
+Carnegie Institution, Publication No. 25, 1905.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_78" id="Footnote_18_78"></a>
+<a href="#FNanchor_18_78"><span class="label">[18]</span></a>
+A. G. Mayer, <i>Mem. Mus. Comp. Anat. Harvard</i>, Vol. XXVI, 1902, p. 117.
+From the tables given I cannot ascertain the actual numbers from the two
+intermediate valleys, but they were considerable.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_79" id="Footnote_19_79"></a>
+<a href="#FNanchor_19_79"><span class="label">[19]</span></a>
+To which I was very kindly guided by Mr. C. T. Trechmann.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_80" id="Footnote_20_80"></a>
+<a href="#FNanchor_20_80"><span class="label">[20]</span></a>
+Standfuss, <i>Handbuch d. paläarkt Gross-schmet</i>, 1896, p. 321.</p></div>
+
+<div class="footnote"><p><a name="Footnote_21_81" id="Footnote_21_81"></a>
+<a href="#FNanchor_21_81"><span class="label">[21]</span></a>
+<i>Ent. Rec.</i>, XVIII, No. 7, 1906.</p></div>
+
+<div class="footnote"><p><a name="Footnote_22_82" id="Footnote_22_82"></a>
+<a href="#FNanchor_22_82"><span class="label">[22]</span></a>
+This evidence was largely collected by Mr. G. T. Porritt,
+who has given much attention to the subject.</p></div>
+
+<div class="footnote"><p><a name="Footnote_23_83" id="Footnote_23_83"></a>
+<a href="#FNanchor_23_83"><span class="label">[23]</span></a>
+Such direct action has of course been proved to occur in the
+case of several dimorphic larvae (<i>e. g.</i>, <i>A. betularia</i>,
+itself) and pupae.</p></div>
+
+<div class="footnote"><p><a name="Footnote_24_84" id="Footnote_24_84"></a>
+<a href="#FNanchor_24_84"><span class="label">[24]</span></a>
+See Harris, <i>Proc. Ent. Soc. London</i>, 1904, p. lxxii, and 1905, p. lxiii;
+also Hamling, <i>Trans. City of London Ent. Soc.</i>, 1905, p. 5.</p></div>
+
+<div class="footnote"><p><a name="Footnote_25_85" id="Footnote_25_85"></a>
+<a href="#FNanchor_25_85"><span class="label">[25]</span></a>
+I am indebted to Mr. Outram Bangs of the Harvard Museum
+for calling my attention to this remarkable case.</p></div>
+
+<div class="footnote"><p><a name="Footnote_26_86" id="Footnote_26_86"></a>
+<a href="#FNanchor_26_86"><span class="label">[26]</span></a>
+<i>Auk</i>, 1889, VI, p. 219.</p></div>
+
+<div class="footnote"><p><a name="Footnote_27_87" id="Footnote_27_87"></a>
+<a href="#FNanchor_27_87"><span class="label">[27]</span></a>
+<i>Ann. N. Y. Acad. Sci.</i>, 1878, I, p. 149.</p></div>
+
+<div class="footnote"><p><a name="Footnote_28_88" id="Footnote_28_88"></a>
+<a href="#FNanchor_28_88"><span class="label">[28]</span></a>
+<i>Ann. N. Y. Acad. Sci.</i>, 1878, I, p. 149.</p></div>
+
+<div class="footnote"><p><a name="Footnote_29_89" id="Footnote_29_89"></a>
+<a href="#FNanchor_29_89"><span class="label">[29]</span></a>
+<i>Ibid</i>, 1912, pp. 523-8.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER VII.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_90" id="Footnote_1_90"></a>
+<a href="#FNanchor_1_90"><span class="label">[1]</span></a>
+J. A. Allen, <i>The North American Species of the Genus Colaptes,
+Considered with Special Reference to the Relationships of C. auratus and
+C. cafer</i>. Bull. Am. Mus. Nat. Hist., IV, 1892.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_91" id="Footnote_2_91"></a>
+<a href="#FNanchor_2_91"><span class="label">[2]</span></a>
+For a case in which a red-headed female × a black-headed male
+gave a black-headed female and a red-headed male, see <i>Avian Mag.</i>,
+N. S., IV, pp. 49 and 329</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_92" id="Footnote_3_92"></a>
+<a href="#FNanchor_3_92"><span class="label">[3]</span></a>
+The other variations of this bird are also interesting and important. The
+normal male has a red head and a red throat. The female has a red head and a
+white throat, but varieties of the female are known with a black head, thus again
+illustrating the change from black to red. It should be noted that this is not a
+mere retention of a juvenile character, but, as the birds mature, the red feathers
+come up, or as an exception, the black. There is also a western species, <i>ruber</i>,
+in which both sexes have a great extension of red, and are alike. The male of
+<i>nuchalis</i> intergrades with this type, but the female does not.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_93" id="Footnote_4_93"></a>
+<a href="#FNanchor_4_93"><span class="label">[4]</span></a>
+Dr. W. Brewster, for example, has a remarkable specimen
+of the Teal (<i>Nettion carolinense</i>) with a white collar strongly
+developed at the front and sides of the neck, in a place where the
+normal has no such mark.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_94" id="Footnote_5_94"></a>
+<a href="#FNanchor_5_94"><span class="label">[5]</span></a>
+This variety is spoken of as the Ringed Guillemot and is sometimes regarded
+as a distinct species to which the name <i>ringvia</i> was given by Brünnich. In support
+of this view Dr. William Brewster, to whom I am indebted for much assistance
+in regard to the variation of birds, called my attention to observations of his
+own and also of Maynard's, that the ringed birds were sometimes mated together,
+though in a small minority (see Brewster, <i>Proc. Boston Soc. N. H.</i>, XXII, 1883, p.
+410). It would however be possible to produce many instances of varieties mated
+together though surrounded by a typical population (<i>e. g.</i>, two varying Blackbirds,
+<i>Zoologist</i>, p. 2765; two varying Nightjars, <i>ibid.</i>, p. 5278). I am inclined to
+believe that in nature matings between brothers and sisters are frequent in many
+species of animals, and that the production of sporadically varying colonies is
+thus greatly assisted.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_95" id="Footnote_6_95"></a>
+<a href="#FNanchor_6_95"><span class="label">[6]</span></a>
+The Sap-suckers feed on trees and somewhat resemble our Spotted Woodpeckers
+in general appearance. <i>Colaptes</i> feeds on the ground and corresponds
+perhaps rather with the European Green Woodpecker.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_96" id="Footnote_7_96"></a>
+<a href="#FNanchor_7_96"><span class="label">[7]</span></a>
+For an introduction to this example I am indebted to Mr. W. D. Miller of
+the American Museum of Natural History. Some account of the facts is given by
+Baird, Brewer, and Ridgway (<i>A Hist. of N. Amer. Birds</i>. 1874, II, pp. 540, 544,
+etc.). <i>S. varius</i> occupies the whole country in suitable places from the Atlantic
+to the eastern slopes of the Rockies, and all Mexico to Guatemala. <i>S. nuchalis</i> was
+first known from the Southern Rockies only, but many were afterwards taken in
+Utah. <i>S. ruber</i> is restricted to the Pacific coast. In Ridgway's opinion all three
+are geographical forms of one species. In <i>ruber</i> the sexes are alike having both a
+great extension of the red in the throat, and a red crescent. The male of <i>nuchalis</i>
+grades to the <i>ruber</i> form, but the female does not. This female has some red in
+the throat like the male of <i>varius</i>, whereas the female of <i>varius</i> has
+a whitish throat.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_97" id="Footnote_8_97"></a>
+<a href="#FNanchor_8_97"><span class="label">[8]</span></a>
+Not only vertebrates but the marine Crustacea and Mollusca illustrate
+this curious "principle" of variation, as Canon Norman formerly pointed out to me
+with abundant illustrations. There are of course cases to the contrary also.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_98" id="Footnote_9_98"></a>
+<a href="#FNanchor_9_98"><span class="label">[9]</span></a>
+Chapman, F. M., <i>Bull. Amer. Mus.</i>, IV, 1892, p. 1;
+see also Ridgway, <i>Birds of North and Middle America</i>, 1902, Part II, p. 214.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_99" id="Footnote_10_99"></a>
+<a href="#FNanchor_10_99"><span class="label">[10]</span></a>
+It would aid greatly in factorial analysis if the descriptive term "green"
+could be avoided in application to cases where the green effect is due only to a mixture
+of black and yellow pigments. The absence of yellow is the sole difference
+between the mantle and underparts of <i>pinus</i> and <i>chrysoptera</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_100" id="Footnote_11_100"></a>
+<a href="#FNanchor_11_100"><span class="label">[11]</span></a>
+<i>Bull. Amer. Mus. Nat. Hist.</i>, XXIII, 1907, p. 467.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_101" id="Footnote_12_101"></a>
+<a href="#FNanchor_12_101"><span class="label">[12]</span></a>
+References on this subject will be found in
+<i>Brit. Mus. Cat. Birds</i>, XVII, p. 13.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_102" id="Footnote_13_102"></a>
+<a href="#FNanchor_13_102"><span class="label">[13]</span></a>
+For these facts I am indebted to Mr. W. L. W. Field,
+who has lately published an account of his observations and experiments.
+See especially, <i>Psyche</i>, 1910, XVII, No. 3, where full references
+to previous publications are given.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_103" id="Footnote_14_103"></a>
+<a href="#FNanchor_14_103"><span class="label">[14]</span></a>
+For the facts and further references see W. H. Edwards,
+<i>Butterflies of N. America</i>, 2d series, Papilio VII and X;
+3d series, 1897, Papilio IV, <i>Can. Entom.</i>, 1895, XXVII, p. 239.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_104" id="Footnote_15_104"></a>
+<a href="#FNanchor_15_104"><span class="label">[15]</span></a>
+I think this case is fairly included because the <i>machaon</i>
+type is so widespread that it cannot be regarded as a product of a Northern
+climate, nor can <i>asterias</i> be claimed as especially a warm country form,
+seeing that <i>brevicauda</i>, which is scarcely distinguishable from
+<i>asterias</i>, inhabits Newfoundland (having a curious phase there in which
+the yellow is largely replaced by red).</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER VIII.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_105" id="Footnote_1_105"></a>
+<a href="#FNanchor_1_105"><span class="label">[1]</span></a>
+Often referred to by older writers as <i>Meone</i>, Esper's name.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_106" id="Footnote_2_106"></a>
+<a href="#FNanchor_2_106"><span class="label">[2]</span></a>
+There are also two distinct island forms, unlike the European,
+<i>Xiphia</i> of Madeira, and a smaller variety, <i>Xiphioides</i> of Canary.
+See especially, Baker, G. T., <i>Trans. Ent. Soc. London</i>, 1891, p. 292.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_107" id="Footnote_3_107"></a>
+<a href="#FNanchor_3_107"><span class="label">[3]</span></a>
+Speyer, Adolf, and August. <i>Verbreitung der Schmetterlinge</i>, 1858, I, p. 217.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_108" id="Footnote_4_108"></a>
+<a href="#FNanchor_4_108"><span class="label">[4]</span></a>
+<i>Lepid. Comparée</i>, fsc. III, p. 372.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_109" id="Footnote_5_109"></a>
+<a href="#FNanchor_5_109"><span class="label">[5]</span></a>
+Mr. Rowland-Brown has called my attention to a statement by Dr. Vaillantin
+(<i>Petites Nouv. Ent.</i>, II, 235) that in Indre-et-Cher the first brood is of the
+northern type and the second of the southern. My experience is that in captivity these
+distinctions do not occur, and I have true <i>egeria</i> as first brood from Vienne and
+as the late brood from the Landes. I never collected in Indre-et-Cher.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_110" id="Footnote_6_110"></a>
+<a href="#FNanchor_6_110"><span class="label">[6]</span></a>
+I have since seen true <i>egeria</i> from Ferrol in the
+extreme northwest, which was in Mr. Tutt's collection.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_111" id="Footnote_7_111"></a>
+<a href="#FNanchor_7_111"><span class="label">[7]</span></a>
+Mr. G. Wheeler kindly showed me a series identical with
+this type, from Guernsey, and others from near Laon.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_112" id="Footnote_8_112"></a>
+<a href="#FNanchor_8_112"><span class="label">[8]</span></a>
+<i>Ent. Rec.</i>, V, 1894, p. 134.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_113" id="Footnote_9_113"></a>
+<a href="#FNanchor_9_113"><span class="label">[9]</span></a>
+Mr. Wheeler has some pale but rather worn specimens
+from the Rhone Valley at Vernayaz.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_114" id="Footnote_10_114"></a>
+<a href="#FNanchor_10_114"><span class="label">[10]</span></a>
+See Fleck, E., Die Macrolep. Rumäniens,
+<i>Bul. Soc. Sciinte</i>, VIII, 1899, p. 720.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_115" id="Footnote_11_115"></a>
+<a href="#FNanchor_11_115"><span class="label">[11]</span></a>
+My experience agrees with that of Mr. H. Williams (<i>Ent. Rec.</i>, VIII, 1896, p. 181)
+that pupae, well-formed, can stand considerable frost; but I used to find
+that half-grown larvae usually died if unprotected, and I believe that larvae which
+attempted to pupate in warm autumn weather and then got caught by frosts,
+always died. Small larvae which can creep into shelter at the bottom of the plants
+survived, and I expect that in the north the winter is usually passed in that state
+(see also Merrifield, F., <i>Ent. Rec.</i>, VIII, 1896, p. 168, and
+Carpenter, J. H., <i>ibid.</i>).</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_116" id="Footnote_12_116"></a>
+<a href="#FNanchor_12_116"><span class="label">[12]</span></a>
+Some most unlikely species do this. I once had a larva
+of <i>Parnassius delius</i>, found at about 5,500 feet, which emerged
+late in the autumn (in October I believe), a season at which it must
+have perished in its own country.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_117" id="Footnote_13_117"></a>
+<a href="#FNanchor_13_117"><span class="label">[13]</span></a>
+See, for examples, Barrett, G. C., <i>Lepidoptera of the Brit. Islands</i>,
+I, 1893, p. 229; also Grover, W., <i>Ent. Rec.</i>, IX, 1897, p. 314; Williams, H.,
+<i>Proc. Ent. Soc.</i>, 1898, who reared several specimens from the New Forest
+which would pass for Bretons, though the rest of the family were true <i>egerides</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_118" id="Footnote_14_118"></a>
+<a href="#FNanchor_14_118"><span class="label">[14]</span></a>
+Above the Tosa falls.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_119" id="Footnote_15_119"></a>
+<a href="#FNanchor_15_119"><span class="label">[15]</span></a>
+<i>Bul. Soc. Sciinte</i>, VIII, 1899, p. 691.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_120" id="Footnote_16_120"></a>
+<a href="#FNanchor_16_120"><span class="label">[16]</span></a>
+The fact that Weismann by heating pupæ obtained only one
+autumn specimen seems to me to show rather that a second brood can be
+produced than that it cannot, which is the inference usually drawn.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_121" id="Footnote_17_121"></a>
+<a href="#FNanchor_17_121"><span class="label">[17]</span></a>
+Schima, K., <i>Verh. Zool. bot. Ges. Wien</i>, LX, 1910, p. 268.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_122" id="Footnote_18_122"></a>
+<a href="#FNanchor_18_122"><span class="label">[18]</span></a>
+<i>Rhopalocera Palaearctica</i>, Florence, 1905-11, especially Pl. XXXII.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_123" id="Footnote_19_123"></a>
+<a href="#FNanchor_19_123"><span class="label">[19]</span></a>
+See figures in Barrett, G. C., <i>Lepidoptera of Brit. Islands</i>, I, pt. 3, p. 25.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_124" id="Footnote_20_124"></a>
+<a href="#FNanchor_20_124"><span class="label">[20]</span></a>
+Tutt, J. W., <i>Ent. Rec.</i>, XVIII, 1905, p. 5.
+In the same place he states that on the Mendel Pass <i>arcania</i>
+"runs into" <i>darwiniana</i> and that in the Tyrolean localities the
+transition is especially evident. Wheeler (<i>ibid.</i>, XIII, 1901, p. 121)
+expresses the contrary opinion, that <i>satyrion</i> does grade to <i>arcania</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_21_125" id="Footnote_21_125"></a>
+<a href="#FNanchor_21_125"><span class="label">[21]</span></a>
+H. Rowland-Brown, <i>Ent. Rec.</i>, XI, 1899, p. 293.</p></div>
+
+<div class="footnote"><p><a name="Footnote_22_126" id="Footnote_22_126"></a>
+<a href="#FNanchor_22_126"><span class="label">[22]</span></a>
+Speyer, Stettiner, <i>Ent. Ztg.</i>, XXXI, 1870, p. 63.</p></div>
+
+<div class="footnote"><p><a name="Footnote_23_127" id="Footnote_23_127"></a>
+<a href="#FNanchor_23_127"><span class="label">[23]</span></a>
+In regard to the closely analogous case of <i>Spilosoma lubricipeda</i>, Standfuss
+makes a similar statement. He bred the type on a large scale with the radiate form
+which he calls <i>intermedia</i>, and says that in four years of miscellaneous crossing he
+never obtained really transitional forms. Nevertheless after examining large series,
+especially those of Mr. W. H. B. Fletcher, I came to the conclusion that several
+might be so classed, but I am quite prepared to find that such specimens are heterozygous.
+(See Standfuss, <i>Handb. d. Gross-Schmet.</i>, 1896, p. 307.) It is by no means
+unlikely that various dark forms of <i>lubricipeda</i> correspond with a progressive series
+of factorial additions. Many of the stages have been named, and of these the most
+definite are the <i>intermedia</i> of Standfuss (probably = <i>eboraci</i> of Tugwell) and the
+very dark <i>Zatima</i> of Heligoland, in which only the thorax, the nervures and a small
+field in the fore-wings remain yellow. A form was bred by Deschange from <i>Zatima</i>
+in which even the field in the forewing is obliterated. The exact circumstances in
+which <i>Zatima</i> occurs in Heligoland would be worthy of special investigation, for the
+normal <i>lubricipeda</i> is also found on the island. For references as to the British
+occurrences see especially, Hewett, W., <i>Naturalist</i>, 1894, p. 353. As to <i>Zatima</i> see
+especially Krancher, <i>Soc. Ent.</i>, II, 1887-8, p. 26. I am indebted to Dr. Hartlaub
+for information as to the Heligoland types.</p></div>
+
+<div class="footnote"><p><a name="Footnote_24_128" id="Footnote_24_128"></a>
+<a href="#FNanchor_24_128"><span class="label">[24]</span></a>
+Boisduval, <i>Bull. Soc. Ent. Fr.</i>, III, 1834, p. 5.</p></div>
+
+<div class="footnote"><p><a name="Footnote_25_129" id="Footnote_25_129"></a>
+<a href="#FNanchor_25_129"><span class="label">[25]</span></a>
+The systematics of <i>Setina</i> have been much controverted,
+but no one I believe doubts that <i>aurita</i> and <i>ramosa</i> are forms
+of one species. See also Chapman, A. T., <i>Ent. Rec.</i>, XIII, 1901, p. 139.</p></div>
+
+<div class="footnote"><p><a name="Footnote_26_130" id="Footnote_26_130"></a>
+<a href="#FNanchor_26_130"><span class="label">[26]</span></a>
+<i>Arch. Naturg.</i>, 33, 1867, p. 116.</p></div>
+
+<div class="footnote"><p><a name="Footnote_27_131" id="Footnote_27_131"></a>
+<a href="#FNanchor_27_131"><span class="label">[27]</span></a>
+<i>Brit. Mus. Cat., Batrachia Gradientia</i>, 1882.</p></div>
+
+<div class="footnote"><p><a name="Footnote_28_132" id="Footnote_28_132"></a>
+<a href="#FNanchor_28_132"><span class="label">[28]</span></a>
+The Geographical Distribution of nearly related Species.
+<i>Amer. Nat.</i>, XLI. 1907, p. 207.</p></div>
+
+<div class="footnote"><p><a name="Footnote_29_133" id="Footnote_29_133"></a>
+<a href="#FNanchor_29_133"><span class="label">[29]</span></a>
+See later, p. 242.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER IX.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_134" id="Footnote_1_134"></a>
+<a href="#FNanchor_1_134"><span class="label">[1]</span></a>
+Semon, R., Der Stand der Frage nach der Vererbung
+erworbener Eigenschaften, published in <i>Fortschr. der naturw.
+Forschung.</i>, Bd. 11, 1910.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_135" id="Footnote_2_135"></a>
+<a href="#FNanchor_2_135"><span class="label">[2]</span></a>
+Standfuss, M., <i>Denks. Schweiz. naturf. Ges.</i>, XXXVI, 1898, p. 32.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_136" id="Footnote_3_136"></a>
+<a href="#FNanchor_3_136"><span class="label">[3]</span></a>
+Fischer, E., <i>Allg. Ztschr. f. Entomologie</i>, Bd. VI, 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_137" id="Footnote_4_137"></a>
+<a href="#FNanchor_4_137"><span class="label">[4]</span></a>
+Out of 12 pupae treated 8 died and of the 4 survivors, one only was affected.
+See M. v. Linden, <i>Archiv. Rassen. u. Gesells.</i>, 1904, I.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_138" id="Footnote_5_138"></a>
+<a href="#FNanchor_5_138"><span class="label">[5]</span></a>
+For illustrations see <i>Oberthur's Études d'Entom.</i>, 1896,
+where many of these curious aberrations are represented;
+also Barrett, <i>Lepid. Brit. Islands</i>, II, pp. 71 and 72.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_139" id="Footnote_6_139"></a>
+<a href="#FNanchor_6_139"><span class="label">[6]</span></a>
+Schübeler, F. C., <i>Die Culturpflanzen Norwegens</i>,
+1862, especially pp. 24 and 28.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_140" id="Footnote_7_140"></a>
+<a href="#FNanchor_7_140"><span class="label">[7]</span></a>
+I am obliged to him and to Dr. E. Gold for much trouble
+taken to answer my questions. Some idea of the kind of weather indicated
+by an average of 2.76° C. above the mean may be got from a comparison
+with the year 1911, which most people will remember as one of the hottest
+summers they have known. The July of that year was in east and southeast
+England about 4° F. above the mean but 2.67 C. means about 4.8° F. above
+the mean. At Greenwich July, 1859, was about 6.5° F. above the average.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_141" id="Footnote_8_141"></a>
+<a href="#FNanchor_8_141"><span class="label">[8]</span></a>
+Wille, N., <i>Biol. Cbltt.</i>, XXV, 1905, p. 521.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_142" id="Footnote_9_142"></a>
+<a href="#FNanchor_9_142"><span class="label">[9]</span></a>
+Wettstein, R. von. <i>Der Neo-marckismus u. seine
+Beziehungen zum Darwinismus</i>, Jena, 1903.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_143" id="Footnote_10_143"></a>
+<a href="#FNanchor_10_143"><span class="label">[10]</span></a>
+T. Graham Brown, <i>Proc. Roy. Soc.</i>, 1912, vol. 84, B, p. 555.
+This paper gives full reference to the previous literature of the subject.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_144" id="Footnote_11_144"></a>
+<a href="#FNanchor_11_144"><span class="label">[11]</span></a>
+Morgan, T. H., <i>Evolution and Adaptation</i>, New York, 1903.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_145" id="Footnote_12_145"></a>
+<a href="#FNanchor_12_145"><span class="label">[12]</span></a>
+Kammerer's chief paper on this subject is in <i>Arch. f. Entwm.</i>,
+1909, XXVIII, p. 447, and it is to this that the paginal references in the present
+text relate. His previous paper appeared, <i>ibid.</i>, 1906, XXII, p. 48.
+An account of his further experiments with <i>Alytes</i> is given in
+<i>Natur</i>, 1909-10, Heft 6, p. 95.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_146" id="Footnote_13_146"></a>
+<a href="#FNanchor_13_146"><span class="label">[13]</span></a>
+In reply to my letter Dr. Kammerer who was then away from home very
+kindly replied that he was not quite sure whether he had killed specimens of <i>Alytes</i>
+with "<i>Brunftschwielen</i>" or whether he only had living males of the fourth generation,
+but that he would send illustrative material.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_147" id="Footnote_14_147"></a>
+<a href="#FNanchor_14_147"><span class="label">[14]</span></a>
+Kammerer, P., <i> Natur</i>, 12 December, 1909, Heft 6, p. 95, repeated in
+<i>12 Flugschrift d. Deutsch Ges. f. Züchtungskunde</i>, Berlin, 1910.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_148" id="Footnote_15_148"></a>
+<a href="#FNanchor_15_148"><span class="label">[15]</span></a>
+<i>Festschrift zum Andenken an Gregor Mendel</i>, being
+vol. XLIX of the <i>Verh. Naturf. Ver. in Brünn</i>, 1911, p. 98.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_149" id="Footnote_16_149"></a>
+<a href="#FNanchor_16_149"><span class="label">[16]</span></a>
+Kammerer's chief papers on this subject are <i>Archiv fur Entwm.</i>, XVII,
+1904, and <i>ibid.</i>, XXV, 1907. An epitome of results is also given by him in
+<i>12 Flugschrift d. Deutsch. Ges. f. Züchtungskunde</i>, Berlin, 1910.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_150" id="Footnote_17_150"></a>
+<a href="#FNanchor_17_150"><span class="label">[17]</span></a>
+"<i>Bei Fortdauer der Versuchsbedingungen sind als Vollmolche
+geborene Salamandra maculosa</i> gleich bei der ersten Geburt <i>abermals
+voll molchgebärend</i>, benutzen zum Geburtsakt das trockene Land,
+und zwar unter Erreichung der (bei <i>Salamandra atra</i> normalen)
+<i>Embryonen-Zweizahl</i>," Kammerer, 1907, p. 49.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_151" id="Footnote_18_151"></a>
+<a href="#FNanchor_18_151"><span class="label">[18]</span></a> 1904, p. 56.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_152" id="Footnote_19_152"></a>
+<a href="#FNanchor_19_152"><span class="label">[19]</span></a>
+Throughout Kammerer's papers this is used almost as a technical term.
+It means, I presume, that the feature was manifested more than once.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_153" id="Footnote_20_153"></a>
+<a href="#FNanchor_20_153"><span class="label">[20]</span></a>
+It should be stated that the papers contain a quantity of detail, especially
+descriptive of the state of the larvae, which I have not attempted to represent, but
+the account here given contains all that seemed essential to an understanding of
+the more important features of the account.</p></div>
+
+<div class="footnote"><p><a name="Footnote_21_154" id="Footnote_21_154"></a>
+<a href="#FNanchor_21_154"><span class="label">[21]</span></a>
+The first appeared in <i>Natur</i>, 1909-10, Heft 6, p. 94;
+and the second, which contains coloured plates of the animals, in the lecture
+already referred to, <i>12 Flugschr. d. Deut. Ges. f. Züchtungkunde</i>,
+Berlin, 1910, p. 26. In the paper in <i>Mendel Festschrift</i>, 1911,
+the subject is continued, but no more is added as to this part of the experiment.</p></div>
+
+<div class="footnote"><p><a name="Footnote_22_155" id="Footnote_22_155"></a>
+<a href="#FNanchor_22_155"><span class="label">[22]</span></a>
+E. G. Boulenger, <i>Proc. Zool. Soc.</i>, 1911, p. 323.</p></div>
+
+<div class="footnote"><p><a name="Footnote_23_156" id="Footnote_23_156"></a>
+<a href="#FNanchor_23_156"><span class="label">[23]</span></a>
+<i>Mendel Festschrift</i>, 1911, p. 84.</p></div>
+
+<div class="footnote"><p><a name="Footnote_24_157" id="Footnote_24_157"></a>
+<a href="#FNanchor_24_157"><span class="label">[24]</span></a>
+<i>12 Flugschrift. Deut. Ges. Züchtungskunde</i>, 1910, Fig. 15, <i>P. Reihe</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_25_158" id="Footnote_25_158"></a>
+<a href="#FNanchor_25_158"><span class="label">[25]</span></a>
+<i>Mendel Festschrift</i>, 1911, p. 83.</p></div>
+
+<div class="footnote"><p><a name="Footnote_26_159" id="Footnote_26_159"></a>
+<a href="#FNanchor_26_159"><span class="label">[26]</span></a>
+Field, 1912, 30 March.</p></div>
+
+<div class="footnote"><p><a name="Footnote_27_160" id="Footnote_27_160"></a>
+<a href="#FNanchor_27_160"><span class="label">[27]</span></a>
+<i>Ibid.</i>, 1904, p. 863.</p></div>
+
+<div class="footnote"><p><a name="Footnote_28_161" id="Footnote_28_161"></a>
+<a href="#FNanchor_28_161"><span class="label">[28]</span></a>
+<i>Mitth. Naturw. Ver. a. d. Univ. Wien</i>, 1908, p. 53.</p></div>
+
+<div class="footnote"><p><a name="Footnote_29_162" id="Footnote_29_162"></a>
+<a href="#FNanchor_29_162"><span class="label">[29]</span></a>
+As to the variations of <i>Lacerta muralis</i> in Western Europe and
+North Africa see Boulenger, G. A., <i>Trans. Zool. Soc.</i>, 1905, vol. XVII, p. 351.</p></div>
+
+<div class="footnote"><p><a name="Footnote_30_163" id="Footnote_30_163"></a>
+<a href="#FNanchor_30_163"><span class="label">[30]</span></a>
+As to the experiments of Klebs relating to the
+transmission of acquired characters, see Appendix.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER X.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_164" id="Footnote_1_164"></a>
+<a href="#FNanchor_1_164"><span class="label">[1]</span></a>
+Pringsheim, H., <i>Die Variabilität niederer Organismen</i>, Berlin, 1910.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_165" id="Footnote_2_165"></a>
+<a href="#FNanchor_2_165"><span class="label">[2]</span></a>
+ F. Wolf, Modifikationen u. Mutationen von Bakterien,
+<i>Zts. F. indukt. Abstam. u. Vererbungslehre</i>, II, 1909, p. 90.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_166" id="Footnote_3_166"></a>
+<a href="#FNanchor_3_166"><span class="label">[3]</span></a>
+Winslow, C. E. A. and A. R.,<i>Systematic Relationships of the Coccaceae</i>.
+New York. 1909.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_167" id="Footnote_4_167"></a>
+<a href="#FNanchor_4_167"><span class="label">[4]</span></a>
+C. C. Dobell, <i>Jour. Genetics</i>, 1912, II, p. 201,
+where full references are given.<br />
+Still more recently the same author has contributed an excellent summary of
+the evidence relating to bacteria (<i>ibid.</i>, II. 1913, p. 325).</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_168" id="Footnote_5_168"></a>
+<a href="#FNanchor_5_168"><span class="label">[5]</span></a>
+See Woltereck, <i>Verh. d. Deut. Zool. Ges.</i>, 1909, p. 110;
+and 1911, p. 142. This is a subject which can only be
+properly appreciated on reference to the original papers.
+Several complications are involved to which I have not here alluded.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_169" id="Footnote_6_169"></a>
+<a href="#FNanchor_6_169"><span class="label">[6]</span></a>
+<i>Proc. Roy. Soc.</i>, B, Vol. 86, 1913, p. 113.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_170" id="Footnote_7_170"></a>
+<a href="#FNanchor_7_170"><span class="label">[7]</span></a>
+<i>An Investigation of Evolution in Chrysomelid Beetles
+of the Genus Leptinotarsa</i>, Carnegie Publications, 1906, No. 48.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_171" id="Footnote_8_171"></a>
+<a href="#FNanchor_8_171"><span class="label">[8]</span></a>
+This is the famous Colorado beetle or potato-bug, which has
+caused such serious destruction in potato crops. There seems to be no doubt
+that this insect, formerly unknown in the eastern States, made its way east
+along the mining trails when the west was opened up.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_172" id="Footnote_9_172"></a>
+<a href="#FNanchor_9_172"><span class="label">[9]</span></a>
+This is indicated in the coloured plate, but I have not found
+any explicit statement to this effect in the text, and am not sure
+if the absence of pigment was regarded as complete.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_173" id="Footnote_10_173"></a>
+<a href="#FNanchor_10_173"><span class="label">[10]</span></a>
+<i>Biol. Bull.</i>, XVIII, 1910, p. 285.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_174" id="Footnote_11_174"></a>
+<a href="#FNanchor_11_174"><span class="label">[11]</span></a>
+This description does not quite agree with the representation of the larvae
+in Pl. 17 of the book <i>Evolution in the Genus Leptinotarsa</i> for there the larva of
+<i>undecimlineata</i> is shown as white in the second stage, but yellowish in the third
+stage; perhaps there is an error in printing.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_175" id="Footnote_12_175"></a>
+<a href="#FNanchor_12_175"><span class="label">[12]</span></a>
+<i>Biol. Bull.</i>, XX, 1910, p. 67.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_176" id="Footnote_13_176"></a>
+<a href="#FNanchor_13_176"><span class="label">[13]</span></a>
+As to the interrelations of these three forms, Tower states (1906, p. 18) that
+<i>angustovittata</i>, which he reared from <i>undecimlineata</i>, is intermediate between it
+and <i>signaticollis</i>. Compare Stål, "<i>Monogr. des Chrysomélides</i>," 1862, p. 163; and
+Jacoby, <i>Biol. Centr. Amer. Celeopt.</i>, vi, Pt. 1, p. 234, Pl. xiii, fig. 20; Tab. 41, fig.
+15; <i>ibid.</i>, Suppl., p. 253. All these forms are evidently very closely related, and
+the delimitation of species is quite arbitrary. Jacoby indeed suggests that
+<i>undecimlineata</i> may be a variety of <i>decemlineata</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_177" id="Footnote_14_177"></a>
+<a href="#FNanchor_14_177"><span class="label">[14]</span></a>
+Gortner, <i>Amer. Nat.</i>, Dec., 1911, XLV, p. 743.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_178" id="Footnote_15_178"></a>
+<a href="#FNanchor_15_178"><span class="label">[15]</span></a>
+<i>Mutations, Variations, and Relationships of the Oenotheras</i>,
+Carnegie Institution Publication No. 81, 1907, pp. 61-64.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_179" id="Footnote_16_179"></a>
+<a href="#FNanchor_16_179"><span class="label">[16]</span></a>
+Macdougal, D. T., "Alterations in Heredity induced by Ovarial Treatments",
+<i>Bot. Gaz.</i>, vol. 51, 1911, p. 241.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_180" id="Footnote_17_180"></a>
+<a href="#FNanchor_17_180"><span class="label">[17]</span></a>
+Payne, Fernandus, <i>Biol. Bull.</i>, XVIII, 1910,
+p. 188, and <i>ibid.</i>, XXI, 1911, p. 297.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_181" id="Footnote_18_181"></a>
+<a href="#FNanchor_18_181"><span class="label">[18]</span></a>
+See especially, <i>Mutation et Traumatismes</i>, Paris, Felix Alcan, 1908.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_182" id="Footnote_19_182"></a>
+<a href="#FNanchor_19_182"><span class="label">[19]</span></a>
+J. H. Powers, "Morphological Variation and its Causes in
+<i>Amblystoma tigrinum</i>." <i>Studies from the Zoological Laboratory.
+</i> The University of Nebraska, No. 71, 1907.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_183" id="Footnote_20_183"></a>
+<a href="#FNanchor_20_183"><span class="label">[20]</span></a>
+In connexion with this case I would refer the reader to some remarkable
+observations of Dr. T. A. Chapman on various types of larvae which he reared
+from the moth <i>Arctia caja</i> (<i>Ent. Rec.</i>, IV, 1893, p. 265, and following parts).
+From a single mother he raised a great diversity of forms, some which fed up rapidly
+and passed through their development without assuming certain stages, and others
+which were, as he called them, "laggards," moulting more times than their brethren
+and developing at a much slower rate. It is greatly to be hoped that such a case
+may be critically investigated by analytical breeding.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER X.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_184" id="Footnote_1_184"></a>
+<a href="#FNanchor_1_184"><span class="label">[1]</span></a>
+Buffon, <i>Hist. Nat.</i>, Oiseaux, 1780, VII, p. 3.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_185" id="Footnote_2_185"></a>
+<a href="#FNanchor_2_185"><span class="label">[2]</span></a>
+Ibid., VIII, p. 115.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_186" id="Footnote_3_186"></a>
+<a href="#FNanchor_3_186"><span class="label">[3]</span></a>
+Keeble, <i>Jour. Gen.</i>, 1912, II, p. 173.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_187" id="Footnote_4_187"></a>
+<a href="#FNanchor_4_187"><span class="label">[4]</span></a>
+<i>Animals and Plants</i>, ed. 1, 1868, II, pp. 180-5.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_188" id="Footnote_5_188"></a>
+<a href="#FNanchor_5_188"><span class="label">[5]</span></a>
+<i>Animals and Plants</i>, ed. 1, 1868, II, p. 165.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_189" id="Footnote_6_189"></a>
+<a href="#FNanchor_6_189"><span class="label">[6]</span></a>
+<i>Species and Varieties</i>, 1905, p. 471.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_190" id="Footnote_7_190"></a>
+<a href="#FNanchor_7_190"><span class="label">[7]</span></a>
+Correns, <i>Festschr. med.-nat. Ges. zur 84 Versamml. Deutsch.
+Naturf. u. Aertze. Münster i. W.</i>, 1912.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_191" id="Footnote_8_191"></a>
+<a href="#FNanchor_8_191"><span class="label">[8]</span></a>
+This is a case of a somewhat different order and I mention it partly for that
+reason as an illustration of the complexity which such negative instances may
+present. The difficulty is that though the buffalo and the zebu can breed together,
+the foetus is too large to be born alive. (See Ackermann <i>Ber. d. Ver. f. Naturk.</i>,
+Kassel, 1898, p. 69. Prof. S. Nathusius, of Halle, who has great experience in
+crossing Bovidae, tells me that he has always failed to cross the buffalo with
+other species.)</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_192" id="Footnote_9_192"></a>
+<a href="#FNanchor_9_192"><span class="label">[9]</span></a>
+In a paper to be published in the Report of the Genetic Conference, Paris,
+1911, Bellair states that he obtained some partially fertile hybrids in the cross
+<i>N. sylvestris</i> × <i>tabacum</i>. As to the various degrees of sterility in
+hybrids between <i>Nicotiana</i> species
+see Lock, R. H., <i>Ann. Roy. Bot. Gardens</i>. Peradeniya, IV, 1909, p. 195.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_193" id="Footnote_10_193"></a>
+<a href="#FNanchor_10_193"><span class="label">[10]</span></a>
+<i>Beitrage zur Biol. der Pflanzen.</i>, X, 1911, p. 379.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_194" id="Footnote_11_194"></a>
+<a href="#FNanchor_11_194"><span class="label">[11]</span></a>
+One very peculiar feature was observed, namely, that all the new forms in F<sub>2</sub>
+which were bred from came true. As I understand, this statement applied to five
+such new types, and they were represented by 76 individuals in F<sub>3</sub>, but further
+details on this point are desirable. Another curious fact was observed, namely
+that one of the F<sub>1</sub> forms (<i>cochleata</i> × <i>radiata</i>) when fertilised by
+<i>cochleata</i> gave a highly polymorphic family, but fertilised by <i>radiata</i>
+the resulting offspring were almost uniform.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_195" id="Footnote_12_195"></a>
+<a href="#FNanchor_12_195"><span class="label">[12]</span></a>
+I also had a few F<sub>1</sub> seeds given me by Mr. R. H. Lock.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_196" id="Footnote_13_196"></a>
+<a href="#FNanchor_13_196"><span class="label">[13]</span></a>
+In a paper about to appear in <i>Jour. Linn. Soc.</i> Mr. A. W. Sutton identifies
+this Palestine pea as <i>Pisum humile</i> of Boissier and Noé.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_197" id="Footnote_14_197"></a>
+<a href="#FNanchor_14_197"><span class="label">[14]</span></a>
+Lloyd, R. E., <i>The Growth of Groups in the Animal Kingdom</i>, London, 1912.</p></div>
+
+</div>
+
+<div>*** END OF THE PROJECT GUTENBERG EBOOK 44582 ***</div>
+</body>
+</html>
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+status under the laws that apply to them.
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+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #44582 (https://www.gutenberg.org/ebooks/44582)
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+The Project Gutenberg EBook of Problems of Genetics, by William Bateson
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org/license
+
+
+Title: Problems of Genetics
+
+Author: William Bateson
+
+Release Date: January 4, 2014 [EBook #44582]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK PROBLEMS OF GENETICS ***
+
+
+
+
+Produced by Chris Curnow, Paul Marshall and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+                    YALE UNIVERSITY
+
+        MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES
+
+
+                  PROBLEMS OF GENETICS
+
+              SILLIMAN MEMORIAL LECTURES
+          PUBLISHED BY YALE UNIVERSITY PRESS
+
+  ELECTRICITY AND MATTER. _By_ JOSEPH JOHN THOMSON,
+    D.SC., LL.D., PH.D., F.R.S., _Fellow of Trinity College,
+    Cambridge, Cavendish Professor of Experimental Physics, Cambridge_.
+  _Price $1.25 net; postage 10 cents extra._
+
+  THE INTEGRATIVE ACTION OF THE NERVOUS SYSTEM.
+    _By_ CHARLES S. SHERRINGTON,
+     D.SC., M.D., HON. LL.D., TOR., F.R.S.,
+    _Holt Professor of Physiology in the University of Liverpool_.
+  _Price $3.50 net; postage 25 cents extra._
+
+  RADIOACTIVE TRANSFORMATIONS. _By_ ERNEST RUTHERFORD,
+    D.SC., LL.D., F.R.S., _Macdonald Professor of Physics,
+    McGill University_.
+  _Price $3.50 net; postage 22 cents extra._
+
+  EXPERIMENTAL AND THEORETICAL APPLICATIONS OF
+      THERMODYNAMICS TO CHEMISTRY.
+  _By_ DR. WALTHER NERNST, _Professor and Director of the
+    Institute of Physical Chemistry in the University of Berlin_.
+  _Price $1.25 net; postage 10 cents extra._
+
+  THE PROBLEMS OF GENETICS. _By_ WILLIAM BATESON, M.A.,
+    F.R.S., _Director of the John Innes Horticultural Institution,
+    Merton Park, Surrey, England_.
+  _Price $4.00 net; postage 25 cents extra._
+
+  STELLAR MOTIONS.
+    WITH SPECIAL REFERENCE TO MOTIONS DETERMINED BY MEANS OF
+    THE SPECTROGRAPH. _By_ WILLIAM WALLACE CAMPBELL, SC.D., LL.D.,
+   _Director of the Lick Observatory, University of California_.
+  _Price $4.00 net; postage 30 cents extra._
+
+  THEORIES OF SOLUTIONS. _By_ SVANTE AUGUST ARRHENIUS,
+    PH.D., SC.D., M.D., _Director of the Physico-Chemical
+    Department of the Nobel Institute, Stockholm, Sweden_.
+  _Price $2.25 net; postage 15 cents extra._
+
+  IRRITABILITY.
+       A PHYSIOLOGICAL ANALYSIS OF THE GENERAL EFFECT OF
+       STIMULI IN LIVING SUBSTANCES.
+    _By_ MAX VERWORN,
+    _Professor at Bonn Physiological Institute_.
+  _Price $3.50 net; postage 20 cents extra._
+
+  THE EVOLUTION OF MODERN MEDICINE.
+    _By_ SIR WILLIAM OSLER, BART., M.D., LL.D., SC.D.,
+    _Regius Professor of Medicine, Oxford University_.
+  _Price $3.00 net; postage 40 cents extra._
+
+
+
+
+                     PROBLEMS OF GENETICS
+
+                              BY
+                 WILLIAM BATESON, M.A., F.R.S.
+
+        DIRECTOR OF THE JOHN INNES HORTICULTURAL INSTITUTION,
+            HON. FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE,
+        AND FORMERLY PROFESSOR OF BIOLOGY IN THE UNIVERSITY
+
+                     _WITH ILLUSTRATIONS_
+
+                         [Illustration]
+
+                NEW HAVEN: YALE UNIVERSITY PRESS
+                    LONDON: HUMPHREY MILFORD
+                     OXFORD UNIVERSITY PRESS
+
+                             MCMXIII
+
+
+                         Copyright, 1913
+                       By YALE UNIVERSITY
+
+                First printed August, 1913, 1000 copies
+
+
+[** Transcriber's Note:
+     Underscores "_" before and after a word or phrase indicate ITALICS
+       in the original text.
+     Hyphenation was used inconsistently by the author and has been
+        left as in the original text. ]
+
+
+
+
+                    THE SILLIMAN FOUNDATION
+
+In the year 1883 a legacy of about eighty-five thousand dollars was left
+to the President and Fellows of Yale College in the city of New Haven,
+to be held in trust, as a gift from her children, in memory of their
+beloved and honored mother, Mrs. Hepsa Ely Silliman.
+
+On this foundation Yale College was requested and directed to establish
+an annual course of lectures designed to illustrate the presence and
+providence, the wisdom and goodness of God, as manifested in the natural
+and moral world. These were to be designated as the Mrs. Hepsa Ely
+Silliman Memorial Lectures. It was the belief of the testator that any
+orderly presentation of the facts of nature or history contributed
+to the end of this foundation more effectively than any attempt to
+emphasize the elements of doctrine or of creed; and he therefore
+provided that lectures on dogmatic or polemical theology should be
+excluded from the scope of this foundation, and that the subjects should
+be selected rather from the domains of natural science and history,
+giving special prominence to astronomy, chemistry, geology, and anatomy.
+
+It was further directed that each annual course should be made the basis
+of a volume to form part of a series constituting a memorial to Mrs.
+Silliman. The memorial fund came into the possession of the Corporation
+of Yale University in the year 1901; and the present volume constitutes
+the fifth of the series of memorial lectures.
+
+
+
+
+                           PREFACE
+
+This book gives the substance of a series of lectures delivered in Yale
+University, where I had the privilege of holding the office of Silliman
+Lecturer in 1907.
+
+The delay in publication was brought about by a variety of causes.
+
+Inasmuch as the purpose of the lectures is to discuss some of the wider
+problems of biology in the light of knowledge acquired by Mendelian
+methods of analysis, it was essential that a fairly full account of
+the conclusions established by them should first be undertaken and I
+therefore postponed the present work till a book on Mendel's Principles
+had been completed.
+
+On attempting a more general discussion of the bearing of the phenomena
+on the theory of Evolution, I found myself continually hindered by the
+consciousness that such treatment is premature, and by doubt whether
+it were not better that the debate should for the present stand
+indefinitely adjourned. That species have come into existence by an
+evolutionary process no one seriously doubts; but few who are familiar
+with the facts that genetic research has revealed are now inclined to
+speculate as to the manner by which the process has been accomplished.
+Our knowledge of the nature and properties of living things is far too
+meagre to justify any such attempts. Suggestions of course can be made:
+though, however, these ideas may have a stimulating value in the lecture
+room, they look weak and thin when set out in print. The work which may
+one day give them a body has yet to be done.
+
+The development of negations is always an ungrateful task apt to be
+postponed for the positive business of experiment. Such work is happily
+now going forward in most of the centers of scientific life. Of many
+of the subjects here treated we already know more than we did in 1907.
+The delay in production has made it possible to incorporate these new
+contributions.
+
+The book makes no pretence at being a treatise and the number of
+illustrative cases has been kept within a moderate compass. A good many
+of the examples have been chosen from American natural history, as being
+appropriate to a book intended primarily for American readers. The facts
+are largely given on the authority of others, and I wish to express my
+gratitude for the abundant assistance received from American colleagues,
+especially from the staffs of the American Museum in New York, and of
+the Boston Museum of Natural History. In connexion with the particular
+subjects personal acknowledgments are made.
+
+Dr. F. M. Chapman was so good as to supervise the preparation of the
+coloured Plate of _Colaptes_, and to authorize the loan of the Plate
+representing the various forms of _Helminthophila_, which is taken from
+his _North American Warblers_.
+
+I am under obligation to Messrs. Macmillan & Co., for permission to
+reproduce several figures from _Materials for the Study of Variation_,
+illustrating subjects which I wished to treat in new associations, and
+to M. Leduc for leave to use Fig. 9.
+
+In conclusion I thank my friends in Yale for the high honour they did me
+by their invitation to contribute to the series of Silliman Lectures,
+and for much kindness received during a delightful sojourn in that
+genial home of learning.
+
+
+
+
+                    TABLE OF CONTENTS.
+
+  CHAPTER                                                  PAGE
+
+     I. INTRODUCTORY. THE PROBLEM OF SPECIES AND VARIETY     1
+    II. MERISTIC PHENOMENA                                  31
+   III. SEGMENTATION, ORGANIC AND MECHANICAL                60
+    IV. THE CLASSIFICATION OF VARIATION AND THE NATURE
+                OF SUBSTANTIVE VARIATION                    83
+        NOTE TO CHAPTER IV                                  94
+     V. THE MUTATION THEORY                                 97
+        NOTE TO CHAPTER V                                  116
+    VI. VARIATION AND LOCALITY                             118
+   VII. LOCAL DIFFERENTIATION--_continued_.
+              OVERLAPPING FORMS                            146
+  VIII. LOCALLY DIFFERENTIATED FORMS--_continued_.
+              CLIMATIC VARIETIES                           164
+    IX. THE EFFECTS OF CHANGED CONDITIONS                  187
+     X. THE EFFECTS OF CHANGED CONDITIONS--_continued_.
+              THE CAUSES OF GENETIC VARIATION              212
+    XI. THE STERILITY OF HYBRIDS. CONCLUDING REMARKS       233
+        APPENDIX TO CHAPTER X                              250
+        INDEX                                              251
+
+
+
+
+
+
+                        PROBLEMS OF GENETICS
+
+
+
+
+                              CHAPTER I
+
+                             INTRODUCTORY
+
+The purpose of these lectures is to discuss some of the familiar
+phenomena of biology in the light of modern discoveries. In the last
+decade of the nineteenth century many of us perceived that if any
+serious advance was to be made with the group of problems generally
+spoken of as the Theory of Evolution, methods of investigation must be
+devised and applied of a kind more direct and more penetrating than
+those which after the general acceptance of the Darwinian views had been
+deemed adequate. Such methods obviously were to be found in a critical
+and exhaustive study of the facts of variation and heredity, upon which
+all conceptions of evolution are based. To construct a true synthetic
+theory of Evolution it was necessary that variation and heredity instead
+of being merely postulated as axioms should be minutely examined as
+phenomena. Such a study Darwin himself had indeed tentatively begun, but
+work of a more thorough and comprehensive quality was required. In the
+conventional view which the orthodoxy of the day prescribed, the terms
+variation and heredity stood for processes so vague and indefinite that
+no analytical investigation of them could be contemplated. So soon,
+however, as systematic inquiry into the natural facts was begun it was
+at once found that the accepted ideas of variation were unfounded.
+Variation was seen very frequently to be a definite and specific
+phenomenon, affecting different forms of life in different ways, but
+in all its diversity showing manifold and often obvious indications
+of regularity. This observation was not in its essence novel. Several
+examples of definite variation had been well known to Darwin and
+others, but many, especially Darwin himself in his later years, had
+nevertheless been disposed to depreciate the significance of such
+facts. They consequently then lapsed into general disparagement. Upon
+more careful inquiry the abundance of such phenomena proved to be far
+greater than was currently supposed, and a discussion of their nature
+brought into prominence a consideration of greater weight, namely that
+the differences by which these definite or discontinuous variations are
+constituted again and again approximate to and are comparable with the
+class of differences by which species are distinguished from each other.
+
+The interest of such observations could no longer be denied. The
+more they were examined the more apparent it became that by means of
+the facts of variation a new light was obtained on the physiological
+composition and capabilities of living things. Genetics thus cease to be
+merely a method of investigating theories of evolution or of the origin
+of species but provide a novel and hitherto untried instrument by which
+the nature of the living organism may be explored. Just as in the study
+of non-living matter science began by regarding the external properties
+of weight, opacity, colour, hardness, mode of occurrence, etc., noting
+only such evidences of chemical attributes and powers as chance
+spontaneously revealed; and much later proceeded to the discovery that
+these casual manifestations of chemical properties, rightly interpreted,
+afford a key to the intrinsic nature of the diversity of matter, so in
+biology, having examined those features of living things which ordinary
+observations can perceive, we come at last to realize that when studied
+for their own sake the properties of living organisms in respect of
+heredity and variation are indications of their inner nature and provide
+evidences of that nature which can be obtained from no other source.
+
+While such ideas were gradually forming in our minds, came the
+rediscovery of Mendel's work. Investigations which before had only
+been imagined as desirable now became easy to pursue, and questions
+as to the genetic inter-relations and compositions of varieties can
+now be definitely answered. Without prejudice to what the future may
+disclose whether by way of limitation or extension of Mendelian method,
+it can be declared with confidence and certainty that we have now the
+means of beginning an analysis of living organisms, and distinguishing
+many of the units or factors which essentially determine and cause the
+development of their several attributes.
+
+Briefly put, the essence of Mendelism lies in the discovery of the
+existence of unit characters or factors. For an account of the Mendelian
+method, how it is applied and what it has already accomplished,
+reference must be made to other works.[1] With this part of the subject
+I shall assume a sufficient acquaintance. In these lectures I have
+rather set myself the task of considering how certain problems appear
+when viewed from the standpoint to which the application of these
+methods has led us. It is indeed somewhat premature to discuss such
+questions. The work of Mendelian analysis is progressing with great
+rapidity and anything I can say may very soon be superseded as out
+of date. Nevertheless a discussion of this kind may be of at least
+temporary service in directing inquiry to the points of special interest.
+
+
+             THE PROBLEM OF SPECIES AND VARIETY
+
+Nowhere does our new knowledge of heredity and variation apply more
+directly than to the problem what is a species and what is a variety? I
+cannot assert that we are already in a position to answer this important
+question, but as will presently appear, our mode of attack and the
+answers we expect to receive are not those that were contemplated by our
+predecessors. If we glance at the history of the scientific conception
+of Species we find many signs that it was not till comparatively recent
+times that the definiteness of species became a strict canon of the
+scientific faith and that attempts were made to give precise limits
+to that conception. When the diversity of living things began to be
+accurately studied in the sixteenth and seventeenth centuries names
+were applied in the loosest fashion, and in giving a name to an animal
+or a plant the naturalists of those times had no ulterior intention.
+Names were bestowed on those creatures about which the writer proposed
+to speak. When Gesner or Aldrovandi refer to all the kinds of horses,
+unicorns, dogs, mermaids, etc., which they had seen or read of, giving
+to each a descriptive name, they do not mean to "elevate" each named
+kind to "specific rank"; and if anyone had asked them what they meant by
+a species, it is practically certain that they would have had not the
+slightest idea what the question might imply, or any suspicion that it
+raised a fundamental problem of nature.
+
+Spontaneous generation being a matter of daily observation, then
+unquestioned, and supernatural events of all kinds being commonly
+reported by many witnesses, transmutation of species had no inherent
+improbability. Matthioli,[2] for instance, did not expect to be charged
+with heresy when he declared _Stirpium mutatio_ to be of ordinary
+occurrence. After giving instances of induced modifications he wrote,
+"Tantum enim in plantis naturae germanitas potest, ut non solum saepe
+praedictos praestet effectus, sed etiam ut alteram in alteram stirpem
+facile vertat, ut cassiam in cinnamomum, sisymbrium in mentham, triticum
+in lolium, hordeum in avenam, et ocymum in serpyllum."
+
+I do not know who first emphasized the need for a clear understanding
+of the sense in which the term species is to be applied. In the second
+half of the seventeenth century Ray shows some degree of concern on
+this matter. In the introduction to the _Historia Plantarum_, 1686, he
+discusses some of the difficulties and lays down the principle that
+varieties which can be produced from the seed of the same plant are to
+be regarded as belonging to one species, being, I believe, the first
+to suggest this definition. That new species can come into existence
+he denies as inconsistent with Genesis 2, in which it is declared that
+God finished the work of Creation in six days. Nevertheless he does not
+wholly discredit the possibility of a "transmutation" of species, such
+that one species may as an exceptional occurrence give rise by seed
+to another and nearly allied species. Of such a phenomenon he gives
+illustrations the authenticity of which he says he is, against his will,
+compelled to admit. He adds that some might doubt whether in the cases
+quoted the two forms concerned are really distinct species, but the
+passage is none the less of value for it shews that the conception of
+species as being distinct unchangeable entities was not to Ray the dogma
+sacrosanct and unquestionable which it afterwards became.[3]
+
+In the beginning of the eighteenth century Marchant,[4] having observed
+the sudden appearance of a lacinated variety of _Mercurialis_, makes the
+suggestion that species in general may have arisen by similar mutations.
+Indeed from various passages it is manifest that to the authors of the
+seventeenth and early eighteenth centuries species appeared simply as
+groups more or less definite, the boundaries of which it was unnecessary
+to determine with great exactitude. Such views were in accord with the
+general scientific conception of the time. The mutability of species is
+for example sometimes likened (see for instance Sharrock, loc. cit.) to
+the metamorphoses of insects, and it is to be remembered that the search
+for the Philosopher's Stone by which the transmutation of metals was to
+be effected had only recently fallen into discredit as a pursuit.
+
+The notion indeed of a peculiar, fixed meaning to be attached to species
+as distinct from variety is I think but rarely to be found categorically
+expressed in prae-Linnaean writings.
+
+But with the appearance of the _Systema Naturae_ a great change
+supervened. Linnaeus was before all a man of order. Foreseeing the
+immense practical gain to science that must come from a codification of
+nomenclature, he invented such a system.
+
+It is not in question that Linnaeus did great things for us and made
+Natural History a manageable and accessible collection of facts instead
+of a disorderly heap; but orderliness of mind has another side, and
+inventors and interpreters of systems soon attribute to them a force and
+a precision which in fact they have not.
+
+The systematist is primarily a giver of names, as Ray with his broader
+views perceived. Linnaeus too in the exordium to the _Systema Naturae_
+naively remarks, that he is setting out to continue the work which
+Adam began in the Golden Age, to give names to the living creatures.
+Naming however involves very delicate processes of mind and of logic.
+Carried out by the light of meagre and imperfect knowledge it entails
+all the mischievous consequences of premature definition, and promotes
+facile illusions of finality. So was it with the Linnaean system. An
+interesting piece of biological history might be written respecting the
+growth and gradual hardening of the conception of Species. To readers
+of Linnaeus's own writings it is well known that his views cannot be
+summarized in a few words. Expressed as they were at various times
+during a long life and in various connexions, they present those divers
+inconsistencies which commonly reflect a mind retaining the power of
+development. Nothing certainly could be clearer than the often quoted
+declaration of the _Philosophia Botanica_, "Species tot numeramus quot
+diversae formae in principio sunt creatae," with the associated passage
+"Varietates sunt plantae ejusdem speciei mutatae a caussa quacunque
+occasionali." Those sayings however do not stand alone. In several
+places, notably in the famous dissertation on the peloric _Linaria_
+he explicitly contemplates the possibility that new species may arise
+by crossing, declaring nevertheless that he thinks such an event to
+be improbable. In that essay he refers to Marchant's observation on a
+laciniate _Mercurialis_, but though he states clearly that that plant
+should only be regarded as a variety of the normal, he does not express
+any opinion that the contemporary genesis of new species must be an
+impossibility. In the later dissertation on Hybrid Plants he returns to
+the same topic. Again though he states the belief that species cannot
+be generated by cross-breedings, he treats the subject not as heretical
+absurdity but as one deserving respectful consideration.
+
+The significance of the aphorisms that precede the lectures on the
+Natural Orders is not easy to apprehend. These are expressed with the
+utmost formality, and we cannot doubt that in them we have Linnaeus's
+own words, though for the record we are dependent on the transcripts of
+his pupils.
+
+   The text of the first five is as follows:
+
+   1. Creator T. O. in primordio vestiit Vegetabile _Medullare_
+   principiis constitutivis diversi _Corticalis_ unde tot difformia
+   individua, quot _Ordines_ Naturales prognata.
+
+   2. _Classicas_ has (1) plantas Omnipotens miscuit inter se,
+   unde tot _Genera_ ordinum, quot inde plantae.
+
+   3. _Genericas_ has (2) miscuit Natura, unde tot _Species_
+   congeneres quot hodie existunt.
+
+   4. _Species_ has miscuit Casus, unde totidem quot passim
+   occurrunt, _Varietates_.
+
+   5. Suadent haec (1-4) Creatoris leges a simplicibus ad
+   Composita.
+
+   Naturae leges generationis in hybridis.
+
+   Hominis leges ex observatis a posteriori.
+
+I am not clear as to the parts assigned in the first sentence
+respectively to the "_Medulla_" and the "_Cortex_," beyond that Linnaeus
+conceived that multiformity was first brought about by diversity in
+the "_Cortex_." The passage is rendered still more obscure if read in
+connection with the essay on "_Generatio Ambigena_," where he expresses
+the conviction that the _Medulla_ is contributed by the mother, and the
+_Cortex_ by the father, both in plants and animals.[5]
+
+But however that may be, he regards this original diversity as resulting
+in the constitution of the Natural Orders, each represented by one
+individual.
+
+In the second aphorism the Omnipotent is represented as creating the
+genera by intermixing the individual _plantae classicae_, or prototypes
+of the Natural Orders.
+
+The third statement is the most remarkable, for in it he declares that
+Species were formed by the act of Nature, who by inter-mixing the genera
+produced _Species congeneres_, namely species inside each genus, to the
+number which now exist. Lastly, Chance or Accident, intermixing the
+species, produced as many varieties as there are about us.
+
+Linnaeus thus evidently regarded the intermixing of an originally
+limited number of types as the sufficient cause of all subsequent
+diversity, and it is clear that he draws an antithesis between
+_Creator_, _Natura_, and _Casus_, assigning to each a special part
+in the operations. The acts resulting in the formation of genera are
+obviously regarded as completed within the days of the Creation, but the
+words do not definitely show that the parts played by Nature and Chance
+were so limited.
+
+Recently also E. L. Greene[6] has called attention to some curious
+utterances buried in the _Species Plantarum_, in which Linnaeus refers
+to intermediate and transitional species, using language that even
+suggests evolutionary proclivities of a modern kind, and it is not easy
+to interpret them otherwise.
+
+Whatever Linnaeus himself believed to be the truth, the effect of his
+writings was to induce a conviction that the species of animals and
+plants were immutably fixed. Linnaeus had reduced the whole mass of
+names to order and the old fantastical transformations with the growth
+of knowledge had lapsed into discredit; the fixity of species was
+taken for granted, but not till the overt proclamation of evolutionary
+doctrine by Lamarck do we find the strenuous and passionate assertions
+of immutability characteristic of the first half of the nineteenth
+century.
+
+It is not to be supposed that the champions of fixity were unacquainted
+with varietal differences and with the problem thus created, but in
+their view these difficulties were apparent merely, and by sufficiently
+careful observation they supposed that the critical and permanent
+distinctions of the true species could be discovered, and the
+impermanent variations detected and set aside.
+
+This at all events was the opinion formed by the great body of
+naturalists at the end of the eighteenth and beginning of the nineteenth
+centuries, and to all intents and purposes in spite of the growth of
+evolutionary ideas, it remains the guiding principle of systematists
+to the present day. There are 'good species' and 'bad species' and the
+systematists of Europe and America spend most of their time in making
+and debating them.
+
+In some of its aspects the problem of course confronted earlier
+naturalists. Parkinson for instance (1640) in introducing his treatment
+of _Hieracium_ wrote, "To set forth the whole family of the Hawkeweedes
+in due forme and order is such a world of worke that I am in much doubt
+of mine own abilitie, it having lyen heavie on his shoudiers that
+hath already waded through them ... for such a multitude of varieties
+in forme pertaining to one herbe is not to be found againe in _rerum
+natura_ as I thinke," and the same idea, that the difficulty lay rather
+in man's imperfect powers of discrimination than in the nature of the
+materials to be discriminated, is reflected in many treatises early and
+late.
+
+It was however with the great ouburst of scientific activity which
+followed Linnaeus that the difficulty became acute. Simultaneously
+vast masses of new material were being collected from all parts of the
+world into the museums, and the products of the older countries were
+re-examined with a fresh zeal and on a scale of quantity previously
+unattempted. But the problem how to name the forms and where to draw
+lines, how much should be included under one name and where a new name
+was required, all this was felt, rather as a cataloguer's difficulty
+than as a physiological problem. And so we still hear on the one hand
+of the confusion caused by excessive "splitting" and subdivisions, and
+on the other of the uncritical "lumpers" who associate together under
+one name forms which another collector or observer would like to see
+distinguished.
+
+In spite of Darwin's hopes, the acceptance of his views has led to no
+real improvement--scarcely indeed to any change at all in either the
+practice or aims of systematists. In a famous passage in the _Origin_ he
+confidently declares that when his interpretation is generally adopted
+"Systematists will be able to pursue their labours as at present; but
+they will not be incessantly haunted by the shadowy doubt whether this
+or that form be a true species. This, I feel sure, and I speak after
+experience, will be no slight relief. The endless disputes whether or
+not some fifty species of British brambles are good species will cease."
+Those disputes nevertheless proceed almost exactly as before. It is
+true that biologists in general do not, as formerly, participate in
+these discussions because they have abandoned systematics altogether;
+but those who are engaged in the actual work of naming and cataloguing
+animals and plants usually debate the old questions in the old way.
+There is still the same divergence of opinion and of practice, some
+inclining to make much of small differences, others to neglect them.
+
+Not only does the work of the systematists as a whole proceed as if
+Darwin had never written but their attitude towards these problems is
+but little changed. In support of this statement I may refer to several
+British Museum Catalogues, much of the _Biologia Centrali-Americana_,
+Ridgway's _Birds of North America_, the _Fauna Hawaiensis_, indeed to
+almost any of the most important systematic publications of England,
+America, or any other country. These works are compiled by the most
+proficient systematists of all countries in the several groups, but
+with rare exceptions they show little misgiving as to the fundamental
+reality of specific differences. That the systematists consider the
+species-unit as of primary importance is shown by the fact that the
+whole business of collection and distribution of specimens is arranged
+with regard to it.
+
+Almost always the collections are arranged in such a way that the
+phenomena of variation are masked. Forms intermediate between two
+species are, if possible, sorted into separate boxes under a third
+specific name. If a species is liable to be constantly associated
+with a mutational form, the mutants are picked out, regardless of the
+circumstances of their origin, from the samples among which they were
+captured, and put apart under a special name. Only by a minute study
+of the original labels of the specimens and by redistributing them
+according to locality and dates, can their natural relations be traced.
+The published accounts of these collections often take no notice of
+variations, others make them the subject of casual reference. Very few
+indeed treat them as of much importance. From such indications it is
+surely evident that the systematists attach to the conception of species
+a significance altogether different from that which Darwin contemplated.
+
+I am well aware that some very eminent systematists regard the whole
+problem as solved. They hold as Darwin did that specific diversity
+has no physiological foundation or causation apart from fitness, and
+that species are impermanent groups, the delimitations of which are
+ultimately determined by environmental exigency or "fitness." The
+specific diversity of living things is thus regarded as being something
+quite different in nature from the specific diversity of inorganic
+substances. In practice those who share these opinions are, as might
+be anticipated, to be found among the 'lumpers' rather than among the
+'splitters.' In their work, certainly, the Darwinian theory is actually
+followed as a guiding principle; unanalysed inter-gradations of all
+kinds are accepted as impugning the integrity of species; the underlying
+physiological problem is forgotten, and while the product is almost
+valueless as a contribution to biological research, I can scarcely
+suppose that it aids greatly in the advances of other branches of our
+science.
+
+But why is it that, with these exceptions, the consequences of the
+admittedly general acceptance of a theory of evolution are so little
+reflected in the systematic treatment of living things? Surely the
+reason is that though the systematist may be convinced of the general
+truth of the evolution theory at large, he is still of opinion that
+species are really distinct things. For him there are still 'good'
+species and 'bad' species and his experience tells him that the
+distinction between the two is not simply a question of degree or a
+matter of opinion.
+
+To some it may seem that this is mere perversity, a refusal to see
+obvious truth, a manifestation of the spirit of the collector rather
+than of the naturalist. But while recognising that from a magnification
+of the conception of species the systematists are occasionally led
+into absurdity I do not think the grounds for their belief have in
+recent times been examined with the consideration they deserve. The
+phenomenon of specific diversity is manifested to a similar degree by
+living things belonging to all the great groups, from the highest to the
+lowest, Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae,
+and Bacteria, all present diversities of such a kind that among them the
+existence of specific differences can on the whole be recognised with
+a similar degree of success and with very similar limitations. In all
+these groups there are many species quite definite and unmistakable, and
+others practically indefinite. The universal presence of specificity,
+as we may call it, similarly limited and characterised, is one of its
+most remarkable features. Not only is this specificity thus universally
+present among the different forms of life, but it manifests itself
+in respect of the most diverse characteristics which living things
+display. Species may thus be distinguished by peculiarities of form,
+of number, of geometrical arrangement, of chemical constitution and
+properties, of sexual differentiation, of development, and of many
+other properties. In any one or in several of these features together,
+species may be found distinguished from other species. It is also to be
+observed that the definiteness of these distinctions has no essential
+dependence on the nature of the characteristic which manifests them.
+It is for example sometimes said that colour-distinctions are of small
+systematic importance, but every systematist is familiar with examples
+(like that of the wild species of _Gallus_) in which colours though
+complex, show very little variation. On the other hand features of
+structure, sexual differentiation, and other attributes which by our
+standards are estimated as essential, may be declared to show much
+variation or little, not according to any principle which can be
+detected, but simply as the attention happens to be applied to one
+species or group of species, or to another. In many groups of animals
+and plants observers have hit upon characters which were for a time
+thought to be finally diagnostic of species. The Lepidoptera and Diptera
+for instance, have been re-classified according to their neuration.
+Through a considerable range of forms determinations may be easily made
+on these characters, but as is now well known, neuration is no more
+immune from variation than any other feature of organisation, and in
+some species great variability is the rule. Again it was once believed
+by some that the genitalia of the Lepidoptera provided a basis of final
+determination--with a similar sequel. In some groups, for example
+the Lycaenidae, or the Hesperidae, there are forms almost or quite
+indistinguishable on external examination, but a glance at the genitalia
+suffices to distinguish numerous species, while on the contrary among
+Pieridae a great range of species show scarcely any difference in these
+respects: and again in occasional species the genitalia show very
+considerable variations.
+
+The proposition that animals and plants are on the whole divisible into
+definite and recognisable species is an approximation to the truth.
+Such a statement is readily defensible, whereas to assert the contrary
+would be palpably absurd. For example, a very competent authority
+lately wrote: "In the whole Lepidopterous fauna of England there is no
+species of really uncertain limits."[7] Others may be disposed to make
+certain reservations, but such exceptions would be so few as scarcely to
+impair the validity of the general statement. The declaration might be
+extended to other orders and other lands.
+
+We know, of course, that the phenomenon of specific diversity is
+complicated by local differentiation: that, in general, forms which
+cannot disperse themselves freely exhibit a multitude of local races,
+and that of these some are obviously adaptative, and that a few even owe
+their peculiarity to direct environmental effects. Every systematist
+also is perfectly aware that in dealing with collections from little
+explored countries the occurrence of polymorphism or even of sporadic
+variation may make the practical business of distinguishing the species
+difficult and perhaps for the time impossible; still, conceding that a
+great part of the diversity is due to geographical differentiation, and
+that some is sporadic variation, our experience of our own floras and
+faunas encourages the belief that if we were thoroughly familiar with
+these exotic productions it would usually be possible to assign their
+specific limitations with an approach to certainty.
+
+For apart from any question of the justice of these wider inferences,
+if we examine the phenomenon of specificity as it appears in those
+examples which are nearest to hand, surely we find signs in plenty that
+specific distinction is no mere consequence of Natural Selection. The
+strength of this proposition has lain mainly in the appeal to ignorance.
+Steadily with the growth of knowledge has its cogency diminished, and
+such a belief could only have been formulated at a time when the facts
+of variation were unknown.
+
+In Darwin's time no serious attempt had been made to examine the
+manifestations of variability. A vast assemblage of miscellaneous facts
+could formerly be adduced as seemingly comparable illustrations of the
+phenomenon "Variation." Time has shown this mass of evidence to be
+capable of analysis. When first promulgated it produced the impression
+that variability was a phenomenon generally distributed amongst living
+things in such a way that the specific divisions must be arbitrary.
+When this variability is sorted out, and is seen to be in part a result
+of hybridisation, in part a consequence of the persistence of hybrids
+by parthenogenetic reproduction, a polymorphism due to the continued
+presence of individuals representing various combinations of Mendelian
+allelomorphs, partly also the transient effect of alteration in external
+circumstances, we see how cautious we must be in drawing inferences as
+to the indefiniteness of specific limits from a bare knowledge that
+intermediates exist. Conversely, from the accident of collocation or
+from a misleading resemblance in features we deem essential, forms
+genetically distinct are often confounded together, and thus the
+divergence of such forms in their other features, which we declare
+to be non-essential, passes as an example of variation. Lastly, and
+this is perhaps the most fertile of all the sources of confusion, the
+impression of the indefiniteness of species is created by the existence
+of numerous local forms, isolated geographically from each other, forms
+whose differences may be referable to any one of the categories I have
+enumerated.
+
+The advance has been from many sides. Something has come from the work
+of systematists, something from cultural experiments, something from
+the direct study of variation as it appears in nature, but progress
+is especially due to experimental investigation of heredity. From all
+these lines of inquiry we get the same answer; that what the naturalists
+of fifty years ago regarded as variation is not one phenomenon but
+many, and that what they would have adduced as evidence against the
+definiteness of species may not in fact be capable of this construction
+at all.
+
+If we may once more introduce a physical analogy, the distinctions with
+which the systematic naturalist is concerned in the study of living
+things are as multifarious as those by which chemists were confronted in
+the early days of their science. Diversities due to mechanical mixtures,
+to allotropy, to differences of temperature and pressure, or to degree
+of hydration, had all to be severally distinguished before the essential
+diversity due to variety of chemical constitution stood out clearly,
+and I surmise that not till a stricter analysis of the diversities of
+animals and plants has been made on a comprehensive scale, shall we be
+in a position to declare with any confidence whether there is or is not
+a natural and physiological distinction between species and variety.
+
+As I have said above, it is in the cases nearest to hand that the
+problem may be most effectively studied. Comparison between forms from
+dissimilar situations contributes something; but it is by a close
+examination of the behaviour, especially the genetic behaviour, of
+familiar species when living in the presence of their nearest allies
+that the most direct light on the problem is to be obtained. I cannot
+understand the attitude of those who, contemplating such facts as this
+examination elicits, can complacently declare that specific difference
+is a mere question of degree. With the spread of evolutionary ideas to
+speak much of the fixity of species has become unfashionable, and yet
+how striking and inscrutable are the manifestations of that fixity!
+
+Consider the group of species composing the _agrestis_ section of the
+genus _Veronica_, namely _Tournefortii_, _agrestis_, and _polita_.
+
+These three grow side by side in my garden, as they do in suitable
+situations over a vast area of the temperate regions. I have for years
+noticed them with some care and become familiar with their distinctions
+and resemblances. Never is there any real doubt as to the identity of
+any plant. The species show some variability, but I have never seen
+one which assumed any of the distinguishing features of the others. A
+glance at the fruits decides at once to which species a plant belongs. I
+find it impossible to believe that the fixity of these distinctions is
+directly dependent on their value as aids in the struggle for existence.
+The mode of existence of the three forms in so far as we can tell is
+closely similar. By whatever standard we reckon systematic affinity I
+suppose we shall agree that these species come very near indeed to each
+other. Bentham even takes the view that _polita_ is a mere variety of
+_agrestis_.
+
+Now in such cases as this it has been argued that the specific
+features of the several types have been separately developed in as
+many distinct localities, and that their present association is due
+to subsequent redistribution. Of these Veronicas indeed we know that
+one, _Tournefortii_ (= _Buxbaumii_) is as a matter of fact a recent
+introduction from the east.[8] But this course of argument leads to
+still further difficulties. For if it is true that the peculiarities
+of the several species have been perfected and preserved on account
+of their survival-value to their possessors, it follows that there
+must be many ways of attaining the same result. But since sufficient
+adaptation may be ensured in so many ways, the disappearance of the
+common parent of these forms is difficult to understand. Obviously it
+must have been a plant very similar in general construction to its
+modern representatives. Like them it must have been an annual weed, with
+an organisation conformable to that mode of life. Why then, after having
+been duly perfected for that existence should it have been entirely
+superseded in favour of a number of other distinct contrivances for
+doing the same thing, and--if a gradual transition be predicated--not
+only by them, but by each intermediate stage between them and the
+original progenitor? Surely the obvious inference from such facts is
+that the burden cast upon the theory of gradual selection is far greater
+than it can bear; that adaptation is not in practice a very close fit,
+and that the distinctions between these several species of Veronica have
+not arisen on account of their survival-value but rather because none
+of their diversities was so damaging as to lead to the extermination
+of its possessor. When we see these various Veronicas each rigidly
+reproducing its parental type, all comfortably surviving in competition
+with each other, are we not forced to the conclusion that _tolerance_
+has as much to do with the diversity of species as the stringency of
+Selection? Certainly these species owe their continued existence to the
+fact that they are each good enough to live, but how shall we refer the
+distinctions between them directly or indirectly to the determination of
+Natural Selection?
+
+The control of Selection is loose while the conformity to specific
+distinction is often very strict and precise, and no less so even when
+several closely related species co-exist in the same area and in the
+same circumstances.
+
+The theory of Selection fails at exactly the point where it was devised
+to help: _Specific_ distinction.
+
+Let us examine a somewhat different set of facts in the case of another
+pair of nearly allied species _Lychnis diurna_ and _vespertina_. The
+two plants have much in common. Both are dioecious perennials, with
+somewhat similar flowers, the one crimson, the other white. Each however
+has its peculiarities which are discernible in almost any part of its
+structure, whether flower, leaf, fruit or seed, distinctions which
+would enable a person thoroughly familiar with the plants to determine
+at once from which species even a small piece had been taken. There is
+so much resemblance however as readily to support the surmise that the
+two were mere varieties of one species. Bentham, following Linnaeus,
+in fact actually makes this suggestion, with what propriety we will
+afterwards consider. Now this case is typical of many. The two forms
+have a wide distribution, occurring sometimes separately, sometimes
+in juxtaposition. _L. diurna_ is a plant of hedgerows and sheltered
+situations. _L. vespertina_ is common in fields and open spaces, where
+_diurna_ is hardly ever found; but not rarely _vespertina_ occurs in
+association with _diurna_ in the places which that plant frequents. In
+this case I do not doubt that we have to do with organisms of somewhat
+different aptitudes. That _L. vespertina_ has powers which _diurna_
+has not is shown very clearly by the fact that _diurna_ is sometimes
+entirely absent from areas where _vespertina_ can abound.[9] But in
+order to understand the true genetic relations of the two plants to
+each other it is necessary to observe their behaviour when they meet
+as they not unfrequently do. If the _Lychnis_ population of such a
+locality be examined it will be found to consist of many undoubted
+and unmodified _diurna_, a number--sometimes few, sometimes many--of
+similarly unmodified _vespertina_, and an uncertain but usually rather
+small proportion of plants obviously hybrids between the two. How
+is it possible to reconcile these facts with the view that specific
+distinction has no natural basis apart from environmental exigency?
+
+Darwinian orthodoxy suggests that by a gradual process of Natural
+Selection either one of these two types was evolved from the other,
+or both from a third type. I cannot imagine that anyone familiar with
+the facts would propose the first hypothesis in the case of _Lychnis_,
+nor can I conceive of any process, whether gradual or sudden, by
+which _diurna_ could have come out of _vespertina_, or _vespertina_
+out of _diurna_. Both however may no doubt have been derived from
+some original third type. It is conceivable that _Lychnis macrocarpa_
+of Boissier, a native of Southern Spain and Morocco, may be this
+original form. This species is said to combine a white flower (like
+that of _L. vespertina_), with capsule-teeth rolled back (like those
+of _diurna_).[10] But whatever the common progenitor may have been, if
+we are to believe that these two species have been evolved from it by
+a gradual process of Natural Selection based on adaptation, enormous
+assumptions must be made regarding the special fitness of these two
+forms and the special unfitness of the common parent, and these
+assumptions must be specially invoked and repeated for each several
+feature of structure or habits distinguishing the three forms.
+
+Why, if the common parent was strong enough to live to give rise to
+these two species, is it either altogether lost now, or at least absent
+from the whole of Northern Europe? Its two putative descendants, though
+so distinct from each other, are, as we have seen, able often to
+occupy the same ground. If they were gradually derived from a common
+progenitor--necessarily very like themselves--can we believe that
+this original form should always, in all the diversities of soil and
+situation which they inhabit, be unable to exist? Some one may fancy
+that the hybrids which are found in the situations occupied by both
+forms are this original parental species. But nothing can be more
+certain than that these plants are simply heterozygous combinations
+made by the union of gametes bearing the characters of _diurna_ and
+_vespertina_.[11] For they may be reproduced exactly in F_{1} or in
+later generations of that cross when it is artificially made; when bred
+from their families exhibit palpable phenomena of segregation more or
+less complex; and usually, if perhaps not always, they are partially
+sterile.[12] In a locality on the Norfolk coast that I know well,
+there is a strip of rough ground chiefly sand-bank, which runs along
+the shore. This ground is full of _vespertina_. Not a hundred yards
+inland is a lane containing _diurna_, and among the _vespertina_ on
+the sand-bank are always some of the hybrid form, doubtless the result
+of fertilisation from the neighbouring _diurna_ population. Seed saved
+from these hybrids gave _vespertina_ and hybrids again, having obviously
+been fertilised by other _vespertina_ or by other hybrids, and I have no
+doubt that such hybrid plants if fertilised by _diurna_ would have shown
+some _diurna_ offspring. The absence of _diurna_ in such localities may
+fairly be construed as an indication that _diurna_ is there at a real
+disadvantage in the competition for life.
+
+But if, admitting this, we proceed to consider how the special aptitude
+of _vespertina_ is constituted, or what it is that puts _diurna_ at
+a disadvantage, we find ourselves quite unable to show the slightest
+connexion between the success of one or the failure of the other on the
+one hand, and _the specific characteristics_ which distinguish the two
+forms on the other. The orthodox Selectionist would, as usual, appeal to
+ignorance. We ask what can _vespertina_ gain by its white flowers, its
+more lanceolate leaves, its grey seeds, its almost erect capsule-teeth,
+its longer fruits, which _diurna_ loses by reason of its red flowers,
+more ovate leaves, dark seeds, capsule-teeth rolled back, and shorter
+fruits? We are told that each of these things _may_ affect the viability
+of their possessors. We cannot assert that this is untrue, but we should
+like to have evidence that it is true. The same problem confronts us
+in thousands upon thousands of examples, and as time goes on we begin
+to feel that speculative appeals to ignorance, though dialectically
+admissible, provide an insufficient basis for a proposition which,
+if granted, is to become the foundation of a vast scheme of positive
+construction.
+
+One thing must be abundantly clear to all, that to treat two forms so
+profoundly different as one, because intermediates of unknown nature can
+be shown to exist between them, is a mere shirking of the difficulties,
+and this course indeed creates artificial obstacles in the way of those
+who are seeking to discover the origin of organic diversity.
+
+In the enthusiasm with which evolutionary ideas were received the
+specificity of living things was almost forgotten. The exactitude with
+which the members of a species so often conform in the diagnostic,
+specific features passed out of account; and the scientific world by
+dwelling with a constant emphasis on the fact of variability, persuaded
+itself readily that species had after all been a mere figment of
+the human mind. Without presuming to declare what future research
+only can reveal, I anticipate that, when variation has been properly
+examined and the several kinds of variability have been successfully
+distinguished according to their respective natures, the result will
+render the natural definiteness of species increasingly apparent.
+Formerly in such a case as that of the two _Lychnis_ species, the series
+of "intermediates" was taken to be a palpable proof that _vespertina_
+"graded" to _diurna_. It is this fact, doubtless, upon which Bentham
+would have relied in suggesting that both may be one species.[13]
+Genetic tests, though as yet imperfectly applied, make it almost certain
+that these inter-grading forms are not in any true sense variations
+from either species in the direction of the other, but combinations of
+elements derived from both.
+
+The points in which very closely allied species are distinguished
+from each other may be found in the most diverse features of their
+organisation. Sometimes specific difference is to be seen in a
+character which we can believe to be important in the struggle, but at
+least as often it is some little detail that we cannot but regard as
+trivial which suffices to differentiate the two species. Even when the
+diagnostic point is of such a nature that we can imagine it to make a
+serious difference in the economy we are absolutely at a loss to suggest
+why this feature should be a necessity to species A and unnecessary to
+species B its nearest ally. The house sparrow (_Passer domesticus_) is
+in general structure very like the tree sparrow (_P. montanus_). They
+differ in small points of colour. For instance _montanus_ has a black
+patch on the cheek which is absent in _domesticus_. The presence in the
+one species and the absence in the other are equally definite, and in
+both cases we are equally unable to suggest any consideration of utility
+in relation to these features. The two species are distinguished also by
+a characteristic that may well be supposed to be of great significance.
+In _domesticus_ the two sexes are strongly differentiated, the cock
+being more ornate than the hen. On the other hand the two sexes in
+_montanus_ are alike, and, if we take a standard from _domesticus_,
+we may fairly say that in _montanus_ the hen has the colouration of
+the male. It is not unreasonable to suppose that such a distinction
+may betoken some great difference in physiological economy, but the
+economical significance of this perhaps important distinction is just as
+unaccountable as that of the seemingly trivial but equally diagnostic
+colour-point.
+
+I have spoken of the fixed characteristics of the two species. If
+we turn to a very different feature, their respective liability to
+albinistic variation, we find ourselves in precisely similar difficulty.
+_Passer domesticus_ is a species in which individuals more or less pied
+occur with especial frequency, but in _P. montanus_ such variation is
+extremely rare if it occurs at all. The writer of the section on Birds
+in the _Royal Natural History_ (III., 1894-5, p. 393) calls attention to
+this fact and remarks that in that species he knows no such instance.
+
+The two species therefore, apart from any differences that we can
+suppose to be related to their respective habits, are characterised by
+small fixed distinctions in colour-markings, by a striking difference
+in secondary sexual characters, and by a difference in variability. In
+all these respects we can form no surmise as to any economic reason why
+the one species should be differentiated in the one way and the other in
+the other way, and I believe it is mere self-deception which suggests
+the hope that with fuller knowledge reasons of this nature would be
+discovered.
+
+The two common British wasps, _Vespa vulgaris_ and _Vespa germanica_,
+are another pair of species closely allied although sharply
+distinguished, which suggest similar reflexions. Both usually make
+subterranean nests but of somewhat different materials. _V. vulgaris_
+uses rotten wood from which the nest derives a characteristic yellow
+colour, while _V. germanica_ scrapes off the weathered surfaces of
+palings and other exposed timber, material which is converted into
+the grey walls of the nest. The stalk by which the nest is suspended
+(usually to a root) in the case of _germanica_ passes freely through
+a hole in the external envelope, but _vulgaris_ unites this external
+wall solidly to the stalk. In bodily appearance and structure the
+two species are so much alike that they have often been confounded
+even by naturalists, and to the untrained observer they are quite
+indistinguishable. There are nevertheless small points of difference
+which almost though not quite always suffice to distinguish the
+two forms. For example the yellow part of the sinus of the eyes is
+emarginate in _vulgaris_ but not emarginate in _germanica_. _V.
+vulgaris_ often has black spots on the tibiae while in _germanica_ the
+tibiae are usually plain yellow. In both species there is a horizontal
+yellow stripe on the thorax, but whereas in _vulgaris_ this is a plain
+narrow stripe, it is in _germanica_ enlarged downwards in the middle.
+These and other apparently trivial details of colouration, though not
+absolutely constant, are yet so nearly constant that irregularities
+in these respects are quite exceptional. Lastly the genitalia of the
+males, though not very different, present small structural points
+of distinction which are enough to distinguish the two species at a
+glance.[14]
+
+In considering the meaning of the distinctions between these two wasps
+we meet the old problem illustrated by the Sparrows. The two species
+have somewhat different habits of life and we should readily expect
+to find differences of bodily organisation corresponding with the
+differences of habits. But is that what we do find? Surely not. To
+suppose that there is a correspondence between the little points of
+colour and structure which we see and the respective modes of life of
+the two species is perfectly gratuitous. We have no inkling of the
+nature of such a correspondence, how it can be constituted, or in what
+it may consist.
+
+Is it not time to abandon these fanciful expectations which are never
+realised? Everywhere both among animals and plants does the problem of
+specific difference reiterate itself in the same form. In view of such
+facts as I have related and might indefinitely multiply, the fixity of
+specific characters cannot readily be held to be a measure of their
+economic importance to their possessors. The incidence of specific
+fixity is arbitrary and capricious, sometimes lighting on a feature or
+a property which can be supposed to matter much, but as often is it
+attached to the most trifling of superficial peculiarities.
+
+The incidence of _variability_ is no less paradoxical, and without
+investigation of the particular case no one can say what will be found
+to show much or little variability. The very characteristic which in
+one species may exhibit extreme variability may in an allied species
+show extreme constancy. Illustrations will occur to any naturalist, but
+nowhere is this truth more strikingly presented than in the British
+Noctuid Moths. Many are so variable that, in the common phrase,
+"scarcely two can be found alike," while others show comparatively
+slight variation. It need scarcely be remarked that, in the instances
+I have in mind, the evidence of great variability is in no way due to
+the abundance with which the particular species occurs, for common
+species may show constancy, and less abundant species may show great
+variability. The polymorphism seems to be now at least a general
+property of the variable species, as the fixity is a property of the
+fixed species. In illustration I may refer to the following examples.
+
+_Dianthoecia capsincola_ is a common and widely distributed moth which
+feeds on _Lychnis_. It shows little variation. _Dianthoecia carpophaga_
+is another species which feeds chiefly on _Silene_. Its habits are
+very similar to those of _capsincola_. Like that species it has a wide
+geographical range and is abundant in its localities, but in contrast
+to the fixity of _capsincola_, _carpophaga_ exhibits a complex series
+of varieties. _Agrotis suffusa_ (= _ypsilon_) is a moth widely spread
+through the southern half of England. It is very constant in colour and
+markings. _Agrotis segetum_ and _tritici_ are excessively variable both
+in ground colour and markings, being found in an immense profusion of
+dissimilar forms throughout their distribution. Of these and several
+other species of _Agrotis_ there are many named varieties, some of
+which have by various writers been regarded as specifically distinct.
+Of the genus _Noctua_ many species (e. g. _festiva_) show a similar
+polymorphism, but _N. triangulum_, though showing some variation in
+certain respects, is usually very constant to its type, and the same is
+true of _N. umbrosa_.
+
+In several species of _Taeniocampa_, especially _instabilis_, the
+multiplicity of forms is extreme, while _cruda_ (= _pulverulenta_) is
+a comparatively constant species. The genus _Plusia_ contains a number
+of constant species, but in _Plusia interrogationis_ we meet the fact
+that the central silvery mark undergoes endless variation. "Truly no
+two are alike," says Mr. Tutt, "and to look down a long series of
+_interrogationis_ is something like looking at a series of Chinese
+characters." In contrast to this we have the fact that in _Plusia gamma_
+the very similar silvery mark is by no means variable.
+
+I have taken this series of cases from the Noctuid moths, but it would
+be as easy to illustrate the same proposition from the Geometridae or
+the Micro-Lepidoptera.[15] I have a long series of _Peronea cristana_,
+for example, which was given to me by Mr. W. H. B. Fletcher, of Bognor.
+All were beaten out of the same hedge, and their polymorphism is such
+that no one unaccustomed to such examples could suppose that they
+belonged to a single species. Another common form, _P. schalleriana_,
+which lives in similar circumstances, exhibits comparatively slight
+variability.
+
+It should be expressly noted that the variation of which I am speaking
+is a genuine polymorphism. Several of the species enumerated exhibit
+also geographical variation, possessing definite and often strikingly
+distinct races peculiar to certain localities; but apart from the
+existence of such local differentiation, stands out the fact upon which
+I would lay stress, that some species are excessively variable while
+others are by comparison constant, in circumstances that we may fairly
+regard as comparable.
+
+This fact is difficult to reconcile with the conventional view that
+specific type is directly determined by Natural Selection and that
+the precision with which a species conforms to its pattern is an
+indication of the closeness of that control. Anyone familiar with the
+characteristics of Moths will agree that the Noctuids, Geometrids and
+Tortricids are creatures whose existence depends in some degree on the
+success with which they can escape detection by their enemies in the
+imaginal state. We are therefore not surprised to find that some species
+of these orders exhibit definite geographical variation in conformity
+with the character of the ground, which may reasonably be supposed to
+aid in their protection. If this were all, there would be nothing to
+cause surprise. We might even be disposed to allow that variability
+might contribute to the perpetuation of animals so situated, on the
+principle that among a variety of surroundings some would probably be
+in harmony with the objects on which they rest. But we cannot admit
+the plausibility of an argument which demands on the one hand that
+the extreme precision with which species A adheres in the minutest
+details of its colour and pattern to a certain type shall be ascribed
+to the protective fitness of those details, and on the other hand that
+the abundant variability of species B shall be ascribed to the same
+determination. If it is absolutely necessary for A to conform to one
+type how comes it that B may range through some twenty distinct forms,
+any two of which differ more from each other than the regular species
+of many other genera? The only reply I can conceive is a suggestion
+that there _may_ be some circumstance which differentiates the various
+classes of cases, that the exigencies of the fixed species _may_ be
+different from those of the variable. Those who make such appeals to
+ignorance do not always perhaps realise whither this course of reasoning
+may lead. If admissible here the same argument would lead us to suggest
+that because albino moles have for an indefinite period occurred on a
+certain land near Bath there may be something in the soil or in the
+conditions of life near Bath which requires a proportion of albinos in
+its mole population. Or again, because the butterfly _Thais rumina_
+in one locality, Digne in the south of France, has a percentage of
+individuals of the variety _Honoratii_ (with certain normally yellow
+spots on the hind wing coloured bright red) and nowhere else throughout
+its distribution, that therefore we may suggest that there is some
+difference in the condition of life at Digne which makes the continuance
+of _Honoratii_ there possible and beneficial.
+
+A polymorphism offering a parallel to that of the variable moths is
+afforded by the breeding plumage of the Ruff, the male of _Machetes
+pugnax_. The variety of plumage which these cocks exhibit is such
+that the statement that no two can be found alike is only a venial
+exaggeration. Newton remarks[16] "that all this wonderful 'show' is
+the consequence of the polygamous habit of the Ruff can scarcely
+be doubtful"; but even if it be conceded that the great external
+differentiation of the cocks may be a result of sexual selection, the
+problem of their _polymorphism_ remains unsolved, for, as we are well
+aware, polygamy is not usually associated with polymorphism of the male.
+The Black Cock (_Tetrao tetrix_), for example, is as polygamous as the
+Ruff, but in that and countless other cases, both sexes are constant to
+one type of plumage.
+
+When we thus compare the polymorphism of one species with the fixity
+of another, and attempt to determine the causes which have led to
+these extraordinary contrasts, two distinct lines of argument are
+open to us. We may ascribe the difference either to causes external
+to the organisms, primarily, that is to say, to a difference in the
+exigencies of Adaptation under Natural Selection; or on the other hand
+we may conceive the difference as due to innate distinctions in the
+chemical and physiological constitutions of the fixed and the variable
+respectively. There is truth undoubtedly in both conceptions. If the
+mole were physiologically incapable of producing an albino that variety
+would not have come into being, and if the albino were totally incapable
+of getting its living it would not be able to hold its own. Were
+_Plotheia frontalis_ constructed on a chemical plan which admitted of no
+variation, the countless varieties would not have been produced; and if
+one of its varieties had an overwhelming success out of all proportion
+to that of the rest, then the species would soon become monomorphic
+again. We cannot declare that Natural Selection has no part in the
+determination of fixity or variability; nevertheless looking at the
+whole mass of fact which a study of the incidence of variation provides,
+I incline to the view that the variability of polymorphic forms should
+be regarded rather as a thing tolerated than as an element contributing
+directly to their chances of life; and on the other hand that the fixity
+of the monomorphic forms should be looked upon not so much as a proof
+that Natural Selection controls them with a greater stringency, but
+rather as evidence of a natural and intrinsic stability of chemical
+constitution.
+
+Compare the condition of a variable form like the male Ruff (or in
+a less degree the Red Grouse in both its sexes) with that of the
+common Pheasant which is comparatively constant. In the Pheasant no
+doubt variations do occur as in other wild birds, but apart from the
+effects of mongrelisation the species is unquestionably uniform. Could
+it seriously be proposed that we should regard the constancy of the
+pheasant's plumage in this country as depending on the special fitness
+of that type of colouration? Even if the pheasant be not an alien in
+Western Europe, it has certainly been protected for centuries, and for a
+considerable period has existed in a state of semi-domestication. Such
+conditions should give good opportunity for polymorphism to be produced.
+In some coverts various aberrations do of course occur and persist,
+yet there is nothing indicative of a general relaxation of the fixity
+of the specific type, and the pheasant remains substantially a fixed
+species.[17] The common pheasant (_Phasianus colchicus_) even shows
+little of that disposition to form local races which appears in the
+species of Further India. Are we not then on safer ground in regarding
+the fixity of our species as a property inherent in its own nature
+and constitution? Just as in ages of domestication no rose has ever
+given off a blue variety so has the pheasant never broken out into the
+polymorphism of the Ruff.
+
+As soon as it is realised how largely the phenomena of variation and
+stability must be an index of the internal constitution of organisms,
+and not mere consequences of their relations to the outer world, such
+phenomena acquire a new and more profound significance.
+
+FOOTNOTES:
+
+[1] In _Mendel's Principles of Heredity_ (Cambridge University Press,
+1909) I have dealt with this subject, giving an account of the principal
+facts discovered up to the beginning of 1909.
+
+[2] Matthioli Opera, Ed. 1598, p. 8, originally published 1565.
+
+[3] Ray's instances relate to Kales, and in most of these examples
+we can see that there was no question of mutation or transmutation
+at all, but that the occurrence was due either to mistake or to
+cross-fertilisation. Sharrock, to whom Ray refers, was inclined to
+discredit stories of transmutation, but he has also this passage
+(_History of the Propagation and Improvement of Vegetables by the
+Concurrence of Art and Nature_, Oxford, 1660, p. 29):
+
+"It is indeed growen to be a great question, whether the transmutation
+of a species be possible either in the vegetable, Animal, or Minerall
+Kingdome. For the possibility of it in the vegetable; I have heard _Mr.
+Bobart_ and his _Son_ often report it, and proffer to make oath that the
+Crocus and Gladiolus, as likewise the Leucoium, and Hyacinths by a long
+standing without replanting have in his garden changed from one kind to
+the other: and for satisfaction about the curiosity in the presence of
+_Mr. Boyle_ I tooke up some bulbs of the very numericall roots whereof
+the relation was made, though the alteration was perfected before, where
+we saw the diverse bulbs growing as it were on the same stoole, close
+together, but no bulb half of the one kind, and the other half of the
+other: But the changetime being past it was reason we should believe the
+report of good artists in matters of their own faculty."
+
+Robert Sharrock was a fellow of New College, Oxford. Both the Bobarts
+were professional botanists, the father was author of a Catalogue of
+the plants in the Hortus Medicus at Oxford, and the son was afterwards
+Curator of the Oxford Garden.
+
+[4] _Mém. Ac. roy. des Sci._ for 1719 (1721), p. 59.
+
+[5] _Amoen. Acad._, 1789, vol. 6. I do not know whether attention has
+been called to the curious mistake which Linnaeus makes in the course of
+this argument. He cites the differences between the Mule and the Hinny
+in illustration of his thesis, pointing out that the Mule is externally
+more like a horse and the Hinny more like an ass. This, he says, is
+because the Mule has the horse for a father, and the Hinny the ass, thus
+inverting the actual facts!
+
+[6] _Proc. Washington Ac. Sci._, 1909, XI, pp. 17-26.
+
+[7] J. W. Tutt, in _Ent. Rec._, 1909, XXI, p. 185.
+
+[8] E. Lehmann (_Bull. l'Herb. Boissier_, Ser. 2, VIII, 1908, p. 229)
+has published an admirable paper on the interrelationships of these
+species and has instituted cultural experiments which will probably
+much elucidate the nature of their specific distinctness. As regards
+the existence of intermediate forms he comes to the conclusion that
+two only can be so regarded. The first was described by Kuntze from
+specimens found on a flower-pot on board a Caspian steamer, from which
+Lehmann proposes the new specific name _Siaretensis_. This comes between
+_polita_ and _filiformis_, a close ally of _Tournefortii_. The other,
+which combines some of the features of both _polita_ and _Tournefortii_,
+was found in the province of Asterabad.
+
+[9] In Cambridgeshire for example _vespertina_ is common but _diurna_ is
+absent. Whether this absence is connected with the general presence of
+chalk I cannot say. When introduced artificially _diurna_ establishes
+itself, for a time at least, without any apparent difficulty and
+occasionally escapes from the garden on to the neighbouring roadside.
+
+[10] Conceivably however it may be a segregated combination. For an
+account of this plant see Boissier, _Voy. Bot. Midi de l'Espagne_, 1839,
+II, 722.
+
+[11] A discussion of this subject with references to literature is
+given by Rolfe, in an excellent paper on "Hybridisation viewed from the
+standpoint of Systematic Botany" (_Jour. R. Hort. Soc._, XXIV, 1900, p.
+197). He concludes: "The simple fact is that the two plants (_L. diurna_
+and _vespertina_) are thoroughly distinct in numerous particulars, and
+affect such different habitats that in some localities one or the other
+of them is completely wanting. But when their stations are adjacent they
+hybridise together very readily, and it is here that these intermediate
+forms occur which have puzzled botanists so much." The same paper
+contains valuable information concerning several cognate illustrations.
+
+[12] In only two cases have I seen such plants (both females) completely
+sterile.
+
+[13] As is well known, in an even more notorious example, he proposed
+to unite _Primula vulgaris_, _P. elatior_, and _P. acaulis_, similarly
+relying on the existence of "intermediates," which we now well know to
+be mongrels between the species.
+
+[14] For an account of the distinctions between _Vespa vulgaris_ and
+_germanica_ see Ch. Janet, _Études sur les Fourmis, les Guêpes et les
+Abeilles_, 11^e, Note. Sur _Vespa germanica_ et _V. vulgaris_. Limoges
+(Ducourtieux), 1895; and R. du Buysson, Monographie des Guêpes, _Ann.
+Soc. Ent. France_, 1903, Vol. LXXII, p. 603, Pl. VIII.
+
+[15] The statements made above are for the most part taken from Barrett,
+C. G., _Lepidoptera of the British Islands_, and from Tutt, J. W., _The
+British Noctuae and their Varieties_. The reader who is unfamiliar
+with the amazing polymorphism exhibited by some of these moths should
+if possible take an opportunity of looking over a long series in a
+collection, or, if that be impossible, refer to the admirable coloured
+plates published by Barrett. It may not be superfluous to observe that
+plenty of similar examples are known in other countries. For instance
+_Plotheia frontalis_, a Noctuid which often abounds in Ceylon, shows
+an equally bewildering wealth of forms. If a dozen specimens of such a
+species were to be brought home from some little known country, each
+individual would almost certainly be described as the type of a distinct
+species. (See the coloured plate published by Sir G. Hampson, Cat. Brit.
+Mus., Heterocera, Vol. IX.)
+
+[16] _Dict. of Birds_, p. 800. It would be interesting and profitable
+to attempt in a long series of Ruffs to determine the Mendelian factors
+which by their combinations give rise to this complex assemblage
+of varietal forms. A few such factors both of colour and pattern
+can be at once distinguished, and it is noticeable that some of the
+resulting types of barring, spangling and penciling show a perceptible
+correspondence with some of the types of colouration found in the breeds
+of domestic fowls.
+
+[17] Howard Saunders (_Illust. Manual of British Birds_, 1899, p. 499)
+states that there is evidence that the pheasant had become naturalized
+in the south of England before the Norman invasion. He adds, "little, if
+any, deviation from the typical _P. colchicus_ took place up to the end
+of last century, when the introduction of the Chinese Ring-necked _P.
+torquatus_ commenced, which has left almost indelible marks, especially
+with regard to the characteristic white collar."
+
+
+
+
+                         CHAPTER II
+
+                     MERISTIC PHENOMENA
+
+Twenty years ago in describing the facts of Variation, argument was
+necessary to show that these phenomena had a special value in the
+sciences of Zoology and Botany. This value is now universally understood
+and appreciated. In spite however of the general attention devoted to
+the study of Variation, and the accumulation of material bearing on the
+problem, no satisfactory or searching classification of the phenomena is
+possible. The reason for this failure is that a real classification must
+presuppose knowledge of the chemistry and physics of living things which
+at present is quite beyond our reach.
+
+It is however becoming probable that if more knowledge of the chemical
+and physical structure of organisms is to be attained, the clue will
+be found through Genetics, and thus that even in the uncoordinated
+accumulation of facts of Variation we are providing the means of
+analysis applicable not only to them, but to the problems of normality
+also.
+
+The only classification that we can yet institute with any confidence
+among the phenomena of Variation is that which distinguishes on the one
+hand variations in the processes of division from variations in the
+nature of the substances divided.
+
+Variations in the processes of division are most often made apparent by
+a change in the number of the parts, and are therefore called _Meristic_
+Variations, while the changes in actual composition of material are
+spoken of as _Substantive_ Variations. The Meristic Variations form on
+the whole a natural and fairly well defined group, but the Substantive
+Variations are obviously a heterogeneous assemblage.
+
+Though this distinction does not go very far, it is useful, and in
+all probability fundamental. It is of value inasmuch as it brings
+into prominence the distinct and peculiar part which the process
+of division, or, more generally, repetition of parts, plays in the
+constitution of the forms of living things.
+
+That there may be a real independence between the Meristic and the
+Substantive phenomena is evident from the fact both that Meristic
+changes may occur without Substantive Variation, and that the substances
+composing an organism may change without any perceptible alteration
+in its meristic structure. When the distinction between these two
+classes of phenomena is perceived it will be realised that the study
+of genetics has on the one hand a physical, or perhaps more strictly
+a mechanical aspect, which relates to the manner in which material is
+divided and distributed; and also a chemical aspect, which relates
+to the constitution of the materials themselves. Somewhat as the
+philosophers of the seventeenth and eighteenth centuries were awaiting
+both a chemical and a mechanical discovery which should serve as a
+key to the problems of unorganised matter, so have biologists been
+awaiting two several clues. In Mendelian analysis we have now, it is
+true, something comparable with the clue of chemistry, but there is
+still little prospect of penetrating the obscurity which envelops the
+mechanical aspect of our phenomena. To make clear the application of
+the terms chemical and mechanical to the problem of Genetics the nature
+of that problem must be more fully described. In its most concrete form
+this problem is expressed in the question, how does a cell divide? If
+the organism is unicellular, and the single cell is the whole body,
+then the process of heredity is accomplished in the single operation of
+cell-division. Similarly in animals and plants whose bodies are made
+up of many cells, the whole process of heredity is accomplished in the
+cell-divisions by which the germ-cells are formed. When therefore we see
+a cell dividing, we are witnessing the process by which the form and the
+properties of the daughter-cells are determined.
+
+Now this process has the two aspects which I have called mechanical
+and chemical. The term "_Entwicklungsmechanik_" has familiarised us
+with the application of the word mechanics to these processes, but on
+reflexion it will be seen that this comprehensive term includes two
+sorts of events which are sometimes readily distinguishable. There
+is the event by which the cell _divides_, and the event by which the
+two halves or their descendants are or may be _differentiated_. It
+is common knowledge that in some cell-divisions two similar halves,
+indistinguishable in appearance, properties, and subsequent fate, may
+be produced, while in other divisions daughter-cells with distinct
+properties and powers are formed. We cannot imagine but that in the
+first case, when the resulting cells are identical, the division is a
+mechanical process by which the mother-cell is simply cut in two; while
+in order that two differentiated halves may be produced, some event must
+have taken place by which a chemical distinction between the two halves
+is effected.[1] In any ordinary Mendelian case we have a clear proof
+that such a chemical difference may be established between germ-cells.
+The facts of colour-inheritance for instance prove that germ-cells,
+otherwise identical, may be formed _possessing_ the chromogen-factor
+which is necessary to the formation of colour in the flowers, or
+_destitute_ of that factor. Similarly the germ-cells may possess the
+ferment which, by its action on the chromogenic substance, produces the
+colour, or they may be without that ferment. The same line of argument
+applied to a great range of cases. Nevertheless, though differences
+in chemical properties are often thus constituted by cell-divisions,
+and though we are thus able to make a quasi-chemical analysis of the
+individual by determining and enumerating these properties, yet it is
+evident that the distribution of these factors is not itself a chemical
+process. This is proved by the fact that similar divisions may be
+effected between halves which are exactly alike, and also by the fact
+that the numbers in which the various types of germ-cells are formed
+negative any suggestion of valency between them. The recognition of the
+unit-factors may lead--indeed must lead--to great advances in chemical
+physiology which without that clue would have been impossible, but
+in causation the chemical phenomena of heredity must be regarded as
+secondary to the physical or mechanical phenomena by which the cells
+and their constituents are divided and separated. When therefore we
+speak of the _essential_ phenomena of heredity we mean the mechanics
+of division, especially, though not, as we shall see, exclusively, of
+_cell_-division; and in the relation between the two halves of the
+dividing cell we have the problem presented in what seems to be its
+simplest form.
+
+In attempting to form some conception of the processes by which bodily
+characteristics are transmitted, or--to avoid that confusing metaphor
+of "transmission"--how it comes about that the offspring can grow to
+resemble its parent, continuity of the germ-substance which in some
+animals is a visible phenomenon,[2] gives at least apparent help. An egg
+for example on becoming adult develops in certain parts a particular
+pigment. The eggs of that adult when they reach the appropriate age
+develop the same pigment. We have no clear picture of the mechanism by
+which this process is effected, but when we realise that the pigment
+results from the interaction of certain substances, and that since
+all the eggs are in reality pieces of the same material, it seems,
+unless we inquire closely, not unnatural that the several pieces of the
+material should exhibit the same colours at the same periods of their
+development. The continuity of the material of the germs suggests that
+there is a continuity of the materials from which the pigment is formed,
+and that thus an actual bit of those substances passes into each egg
+ready at the appropriate moment to generate the pigment. The argument
+thus outlined applies to all _substantive_ characteristics. In each case
+we can imagine, if we will, the appearance of that characteristic as due
+to the contribution of its rudiment from the germ tissues.
+
+When we consider more critically it becomes evident that the aid given
+by this mental picture is of very doubtful reality, for even if it were
+true that any predestined particle actually corresponding with the
+pigment-forming materials is definitely passed on from germ to germ,
+yet the power of increase which must be attributed to it remains so
+incomprehensible that the mystery is hardly at all illuminated.
+
+When however we pass from the substantive to the meristic characters,
+the conception that the character depends on the possession by the germ
+of a particle of a specific material becomes even less plausible. Hardly
+by any effort of imagination can we see any way by which the division
+of the vertebral column into _x_ segments or into _y_ segments, or of a
+Medusa into 4 segments or into 6, can be determined by the possession
+or by the want of a material particle. The distinction must surely be
+of a different order. If we are to look for a physical analogy at all
+we should rather be led to suppose that these differences in segmental
+numbers corresponded with changes in the amplitude or number of dividing
+waves than with any change in the substance or material divided.
+
+
+                     PHENOMENA OF DIVISION
+
+I have said that in the division of a cell we seem to see the problem in
+its simplest form, but it is important to observe that the problem of
+division may be presented by the bodies of animals and plants in forms
+which are independent of the divisions between cells. The existence of
+pattern implies a repetition of parts, and repetition of parts when
+developed in a material originally homogeneous can only be created by
+division. Cell-division is probably only a special case of a process
+similar to that by which the pattern of the skeleton is laid down in
+a unicellular body such as that of a Radiolarian or Foraminiferan.
+Attempts have lately been made to apply mathematical treatment to
+problems of biology. It has sometimes seemed to me that it is in the
+geometrical phenomena of life that the most hopeful field for the
+introduction of mathematics will be found. If anyone will compare one
+of our animal patterns, say that of a zebra's hide, with patterns known
+to be of purely mechanical production, he will need no argument to
+convince him that there must be an essential similarity between the
+processes by which the two kinds of patterns were made and that parts
+at least of the analysis applicable to the mechanical patterns are
+applicable to the zebra stripes also. Patterns mechanically produced are
+of many and very diverse kinds. One of the most familiar examples, and
+one presenting some especially striking analogies to organic patterns,
+is that provided by the ripples of a mackerel sky, or those made in a
+flat sandy beach by the wind or the ebbing tide. With a little search
+we can find among the ripple-marks, and in other patterns produced by
+simple physical means, the closest parallels to all the phenomena of
+striping as we see them in our animals. The forking of the stripes, the
+differentiation of two "faces," the deflections round the limbs and
+so forth, which in the body we know to be phenomena of division, are
+common both to the mechanical and the animal patterns. We cannot tell
+what in the zebra corresponds to the wind or the flow of the current,
+but we can perceive that in the distribution of the pigments, that
+is to say, of the chromogen-substances or of the ferments which act
+upon them, a rhythmical disturbance has been set up which has produced
+the pattern we see; and I think we are entitled to the inference that
+in the formation of patterns in animals and plants mechanical forces
+are operating which ought to be, and will prove to be, capable of
+mathematical analysis. The comparison between the striping of a living
+organism and the sand-ripples will serve us yet a little farther,
+for a pattern may either be formed by actual cell-divisions, and
+the distribution of differentiation coincidently determined, or--as
+visibly in the pigmentation of many animal and plant tissues--the
+pattern may be laid down and the pigment (for example) distributed
+through a tissue across or independently of the cell-divisions of the
+tissue. Our tissues therefore are like a beach composed of sands of
+different kinds, and different kinds of sands may show distinct and
+interpenetrating ripples. When the essential analogy between these
+various classes of phenomena is perceived, no one will be astonished at,
+or reluctant to admit, the reality of discontinuity in Variation, and if
+we are as far as ever from knowing the actual causation of pattern we
+ought not to feel surprised that it may arise suddenly or be suddenly
+modified in descent. Biologists have felt it easier to conceive the
+evolution of a striped animal like a zebra from a self-coloured type
+like a horse (or of the self-coloured from the striped) as a process
+involving many intergradational steps; but so far as the _pattern_ is
+concerned, the change may have been decided by a single event, just as
+the multitudinous and ordered rippling of a beach may be created or
+obliterated at one tide.
+
+[Illustration: FIG. 1. Tusk of Indian elephant, showing an abnormal
+segmentation.]
+
+This point is well illustrated by the tusk of an Indian elephant which
+I lately found in a London sale-room. This tusk is by some unknown
+cause, presumably a chronic inflammation, thrown up into thirteen
+well-marked ridges which closely simulate a series of segments (Fig. 1).
+Whatever the cause the condition shows how easily a normally unsegmented
+structure may be converted into a series of repeated parts.
+
+The spread of segmentation through tissues normally unsegmented is very
+clearly exemplified in the skates' jaws shown in Fig. 2. The right
+side of the upper figure shows the normal arrangement in the species
+_Rhinoptera jussieui_, but the structure on the left side is very
+different. The probable relations of the several rows of teeth to the
+normal rows is indicated by the lettering, but it is evident that by
+the appearance of new planes of division constituting separate centers
+of growth, the series has been recast. The pattern of the left side is
+so definite that had the variation affected the right side also, no
+systematist would have hesitated to give the specimen a new specific
+name. The other two drawings show similar variations of a less extensive
+kind, the nature of which is explained by the lettering of the rows of
+teeth.
+
+[Illustration: FIG. 2. Jaws of Skates (_Rhinoptera_) showing meristic
+variation. (For a detailed discussion see _Materials for the Study of
+Variation_, p. 259.)]
+
+This power to divide is a fundamental attribute of life, and of that
+power cell-division is a special example. In regard to almost all the
+chief vital phenomena we can say with truth that science has made some
+progress. If I mention respiration, metabolism, digestion, each of these
+words calls to mind something more than a bare statement that such
+acts are performed by an animal or a plant. Each stands for volumes of
+successful experiment and research, But the expression cell-division,
+the fundamental act which typifies the rest, and on which they all
+depend, remains a bare name. We can see with the microscope the outward
+symptoms of division, but we have no surmise as to the nature of the
+process by which the division is begun or accomplished. I know nothing
+which to a man well trained in scientific knowledge and method brings
+so vivid a realisation of our ignorance of the nature of life as the
+mystery of cell-division. What is a living thing? The best answer in
+few words that I know is one which my old teacher, Michael Foster, used
+to give in his lectures introductory to biology. "A living thing is a
+vortex of chemical and molecular change." This description gives much,
+if not all, that is of the essence of life. The living thing is unlike
+ordinary matter in the fact that, through it, matter is always passing.
+Matter is essential to it; but, provided that the flow in and out is
+unimpeded, the life-process can go on so far as we know indefinitely.
+Yet the living "vortex" differs from all others in the fact that it
+can divide and throw off other "vortices," through which again matter
+continually swirls.
+
+We may perhaps take the parallel a stage further. A simple vortex, like
+a smoke-ring, if projected in a suitable way will twist and form two
+rings. If each loop as it is formed could grow and then twist again to
+form more loops, we should have a model representing several of the
+essential features of living things.
+
+It is this power of spontaneous division which most sharply
+distinguishes the living from the non-living. In the excellent book
+dealing with the problems of development, lately published by Mr.
+Jenkinson a special emphasis is very properly laid on the distinction
+between the processes of division, and those of differentiation. Too
+often in discussions of the developmental processes the distinction
+is obscured. He regards differentiation as the "central difficulty."
+"Growth and division of the nucleus and the cells," he tells us,
+are side-issues. This view is quite defensible, but I suspect that
+the division _is_ the central difficulty, and that if we could get
+a rationale of what is happening in cell-division we should not be
+long before we had a clue to the nature of differentiation. It may be
+self-deception, but I do not feel it impossible to form some hypothesis
+as to the mode of differentiation, but in no mood of freest speculation
+are we ever able to form a guess as to the nature of the division. We
+see differentiations occurring in the course of chemical action, in
+some phenomena of vibration and so forth: but where do we see anything
+like the spontaneous division of the living cell? Excite a gold-leaf
+electroscope, and the leaves separate, but we know that is because they
+were double before. In electrolysis various substances separate out at
+the positive and negative poles respectively. Now if in cell-division
+the two daughter-cells were always dissimilar--that is to say, if
+differentiation always occurred--we could conceive some rough comparison
+with such dissociations. But we know the dissimilarity between
+daughter-cells is not essential. In the reproduction of unicellular
+organisms and many other cases, the products formed at the two poles
+are, so far as we can tell, identical. Any assumption to the contrary,
+if we were disposed to make it, would involve us in difficulties still
+more serious. At any rate, therefore, if differentiation be really the
+central difficulty in development, it is division which is the essential
+problem of heredity.
+
+Sir George Darwin and Professor Jeans tell us that "gravitational
+instability" consequent on the condensation of gases is "the primary
+agent at work in the actual evolution of the universe," which has led to
+the division of the heavenly bodies. The greatest advance I can conceive
+in biology would be the discovery of the nature of the instability which
+leads to the continual division of the cell. When I look at a dividing
+cell I feel as an astronomer might do if he beheld the formation of a
+double star: that an original act of creation is taking place before me.
+Enigmatical as the phenomenon seems, I am not without hope that, if it
+were studied for its own sake, dissociated from the complications which
+obscure it when regarded as a mere incident in development, some hint as
+to the nature of division could be found. It is I fear a problem rather
+for the physicist than for the biologist. The sentiment may not be a
+popular one to utter before an assembly of biologists, but looking at
+the truth impersonally I suspect that when at length minds of first rate
+analytical power are attracted to biological problems, some advance will
+be made of the kind which we are awaiting.
+
+The study of the phenomena of bodily symmetry offers perhaps the
+most hopeful point of attack. The essential fact in reproduction is
+cell-division, and the essential basis of hereditary resemblance is
+the symmetry of cell-division. The phenomena of twinning provide a
+convincing demonstration that this is so. By twinning we mean the
+production of equivalent structures by division. The process is one
+which may affect the whole body of an animal or plant, or certain of
+its parts. The term twin as ordinarily used refers to the simultaneous
+birth of two individuals. Those who are naturalists know that such twins
+are of two kinds, (1) twins that are not more alike than any other
+two members of the same family, and (2) twins that are so much alike
+that even intimate friends mistake them. These latter twins, except in
+imaginative literature, are always of the same sex.
+
+It is scarcely necessary for me to repeat the evidence from which it has
+been concluded that without doubt such twins arise by division of the
+same fertilised ovum. There is a perfect series of gradations connecting
+them with the various forms of double monsters united by homologous
+parts. They have been shown several times to be enclosed in the same
+chorion, and the proofs of experimental embryology show that in several
+animals by the separation of the two first hemispheres of a dividing egg
+twins can be produced. Lastly we have recently had the extraordinarily
+interesting demonstration of Loeb, to which I may specially refer.
+Herbst some years ago found that in sea water, from which all lime salts
+had been removed, the segments of the living egg fall apart as they are
+formed. Using this method Loeb has shown that a temporary immersion in
+lime-free sea water may result in the production of 90 per cent. of
+twins. We are therefore safe in regarding the homologous or "identical"
+twins as resulting from the divisions of one fertilised egg, while the
+non-identical or "fraternal" twins, as they are called, arise by the
+fertilisation of two separate ova.[3]
+
+In the resemblance of identical twins we have an extreme case of
+hereditary likeness[4] and a proof, if any were needed, that the
+cause of individual variation is to be sought in the differentiation
+of germ-cells. The resemblance of identical twins depends on two
+circumstances, First, since only two germ-cells take part in their
+production, difference between the germ cells of the same individual
+cannot affect them. Secondly the division of the fertilised ovum,
+the process by which they became two instead of one, must have been
+a symmetrical division. The structure of twins raises however one
+extremely significant difficulty, which as yet we cannot in any way
+explain. The resemblance between twins is a phenomenon of symmetry,
+like the resemblance between the two sides of a bilaterally symmetrical
+body. Not only is the general resemblance readily so interpreted, but
+we know also that in double monsters, namely unseparated twins, various
+anatomical abnormalities shown by the one half-body are frequently shown
+by the other half-also.[5] The two belong to one system of symmetry.
+How then does it happen that the body of one of a pair of twins does
+not show a transposition of viscera? We know that the relation of right
+and left implies that the one should be the mirror-image of the other.
+Such a relation of images may be maintained even in minute details.
+For example if the same pattern of finger-print is given by the fingers
+of the two hands, one is the reverse of the other. In double monsters,
+namely unseparated twins, there is evidence that an inversion of viscera
+does occur with some frequency. Evidence from such cases is not so clear
+and simple as might be expected, because as a matter of fact, the heart
+and stomach, upon which the asymmetry of the viscera chiefly depend, are
+usually common to the two bodies. Duplicity generally affects either
+the anterior end alone, or the posterior end alone. The division is
+generally _from the heart forwards_, giving two heads and two pairs of
+anterior limbs on a common trunk, or _from the heart backwards_, giving
+two pairs of posterior limbs with the anterior body common. In either
+case, though the bodies may be grouped in a common system of symmetry,
+neither can be proved to show definite reversal of the parts. To see
+that reversal recourse must be had to more extreme duplications, such as
+the famous Siamese Twins. They, as a matter of fact, were an excellent
+instance of the proposition that twins are related as mirror-images,
+for both of them had eleven pairs of ribs instead of the normal twelve,
+and one of them had a partial reversal of viscera.[6] (Küchenmeister,
+_Verlagerung_, etc., p. 204.)
+
+If anyone could show how it is that neither of a pair of twins has
+transposition of viscera the whole mystery of division would, I expect,
+be greatly illuminated.[7] At present we have simply to accept the fact
+that twins, by virtue of their detachment from each other, have the
+power of resuming the polarity which is proper to any normal individual.
+It was nevertheless with great interest that I read Wilder's recent
+observation[8] that occasionally in identical twins the finger-print of
+one or both the index-fingers may be reversed, showing that there is
+after all some truth in the notion that reversal should occur in them.
+
+There is another phenomenon by twinning which, if we could understand
+it, might help. I refer to the free-martin, the subject of one of John
+Hunter's masterpieces of anatomical description. In horned cattle twin
+births are rare, and when twins of opposite sexes are born, the male
+is perfect and normal, but the reproductive organs of the female are
+deformed and sterile, being known as a free-martin. The same thing
+occasionally happens in sheep, suggesting that in sheep also twins may
+be formed by the division of one ovum; for it is impossible to suppose
+that mere development in juxtaposition can produce a change of this
+character. I mention the free-martin because it raises a question of
+absorbing interest. It is conceivable that we should interpret it by
+reference to the phenomenon of gynandromorphism, seen occasionally in
+insects, and also in birds as a great rarity. In the gynandromorph one
+side of the body is male, the other female. A bullfinch for instance has
+been described with a sharp line of division down the breast between the
+red feathers of the cock on one side and the brown feathers of the hen
+on the other. (Poll, H., _SB. Ges. Nat. Fr._, Berlin, 1909, p. 338.) In
+such cases neither side is sexually perfect. If the halves of such a
+gynandromorph came apart, perhaps one would be a free-martin.
+
+The behaviour of homologous twinning in heredity has been little
+studied. It does not exist as a normal feature in any animal which
+is amenable to experiment, and we cannot positively assert that a
+comparable phenomenon exists in plants; for in them--the Orange, for
+example--polyembryony may evidently be produced by a parthenogenetic
+development of nucellar tissue. It is possible that in Man twinning
+is due to a peculiarity of the mother, not of the father. It may and
+not rarely does descend from mother to daughter, but whether it can be
+passed on through a male generation to a daughter again, there is not
+sufficient evidence to show. The facts as far as they go are consistent
+with the inference which may be drawn from Loeb's experiment, that the
+twinning of a fertilized ovum may be determined not by the germ-cells
+which united to form it, but by the environment in which it begins to
+develop. The opinion that twinning may descend through the male directly
+has been lately expressed by Dr. J. Oliver in the _Eugenics Review_
+(1912), on the evidence of cases in which twins had occurred among the
+relations of fathers of twins, but I do not know of any comprehensive
+collection of evidence bearing on the subject.
+
+Besides twinning of the whole body a comparable duplicity of various
+parts of the same body may occur. Such divisions affect especially those
+organs which have an axis of bilateral symmetry, such as the thumb, a
+cotyledon, a median petal, the frond of a fern or the anal fin of a
+fish. From the little yet known it is clear that the genetic analysis
+of these conditions must be very difficult, but evidence of any kind
+regarding them will be valuable. We want especially to know whether
+these divisions are due to the _addition_ of some factor or power which
+enables the part to divide, or whether the division results from the
+_absence_ of something which in the normal body prevents the part from
+dividing. Breeding experiments, so far as they go, suggest that the less
+divided state is usually dominant to the more divided.[9] The two-celled
+Tomato fruit is dominant to the many-celled type. The Manx Cat's tail,
+with its suppression of caudal segmentation is a partial dominant over
+the normal tail. The tail of the Fowl in what is called the "Rumpless"
+condition is at least superficially comparable with that of the Manx
+Cat, and though the evidence is not wholly consistent, Davenport
+obtained facts indicating that this suppressed condition of the caudal
+vertebrae is an imperfect dominant.[10]
+
+Some evidence may also be derived from other examples of differences
+which at first sight appear to be substantive though they are more
+probably meristic in ultimate nature. The distinction between the
+normal and the "Angora" hair of the Rabbit is a case in point. We can
+scarcely doubt that one of the essential differences between these two
+types is that in the Angora coat the hair-follicles are more finely
+divided than they are in the normal coat, and we know that the normal,
+or less-divided condition, is dominant to the Angora, or more finely
+divided.
+
+[Illustration: FIG. 3. _I_, _II_, _III_, various degrees of syndactyly
+affecting the medius and annularis in the hand; _IV_, syndactyly
+affecting the index and medius in the foot. (After Annandale.)]
+
+In the case of the solid-hoofed or "mule-footed" swine, the evidence
+shows, as Spillman has lately pointed out,[11] that the condition
+behaves as a dominant. The essential feature of this abnormality is
+that the digits III and IV are partially united. The union is greatest
+peripherally. Sometimes the third phalanges only are joined to form one
+bone, but the second and even the first phalanges may also be compounded
+together. Here the variation is obviously meristic and consists in a
+failure to divide, the normal separation of the median digits of the
+foot being suppressed.
+
+[Illustration: FIG. 4. Case of complete syndactyly in the foot. _II_ and
+_III_, digit apparently representing the index and medius. _c_^{2} +
+_c_^{3}, bone apparently representing the middle and external cuneiform;
+_cb_, cuboid; _c_^{1}, internal cuneiform. (After Gruber.)]
+
+Webbing between the digits, in at least some of its manifestations, is
+a variation of similar nature. The family recorded by Newsholme[12]
+very clearly shows the dominance of this condition. The case is
+morphologically of great interest and must undoubtedly have a bearing on
+the problems of the mechanics of Division. In discussing the phenomena
+of syndactylism I pointed out some years ago that the digits most
+frequently united in the human hand are III and IV, while in the foot,
+union most frequently takes place between II and III.[13] In Newsholme's
+family the union was always between II and III of the foot, except in
+the case of one male who had the digits III and IV of the right _hand_
+alone webbed together. There can be little doubt that the geometrical
+system on which the foot is planned has an axis of symmetry passing
+between the digits II and III, while the corresponding axis in the hand
+passes between III and IV. Union between such digits may therefore be
+regarded as comparable with any non-division or "coalescence" of lateral
+structures in a middle line, and when as in these examples such a
+condition is shown to be a dominant we cannot avoid the inference that
+some concrete factor has the power of suppressing or inhibiting this
+division. Figs. 3 and 4 illustrate degrees of union between digits in
+the human hand and foot.
+
+It is not in question that various other forms of irregular webbing
+and coalescence of digits exist, and respecting the genetic behaviour
+of these practically nothing is as yet known. Such a case is described
+by Walker,[14] in which the first and second metacarpals of both
+feet were fused in mother and daughter, and several more are found
+in literature. Contrasted with these phenomena we have the curious
+fact that in the Pigeon, Staples-Browne found webbing of the toes a
+_recessive_ character. The question thus arises whether this webbing is
+of the same nature as that shown to be a dominant in Man, and indeed
+whether the phenomenon in pigeons is really meristic at all. There is
+some difference perceptible between the two conditions; for in Man
+there is not so much a development of a special web-like skin uniting
+the digits as a want of proper division between the digits themselves,
+and in extreme cases two digits may be represented by a single one. In
+the Pigeon I am not aware that a real union of this kind has ever been
+observed, and though the web-like skin may extend the whole length of
+the digits and be so narrow as to prevent the spread of the toes, it
+may, I think, be maintained that the unity of the digits is unimpaired.
+For the present the nature of this variation in the pigeon's feet must
+be regarded as doubtful, and we should note that if it is actually an
+example of a more perfect division being dominant to a less perfect
+division, the case is a marked exception to the general rule that
+non-division is dominant to division.
+
+Reference must also be made to the phenomenon of fasciation in the stems
+of plants. As Mendel showed in the case of _Pisum_ this condition is
+often a recessive. The appearances suggest that the difference between a
+normal and a fasciated plant consists in the inability of the fasciated
+plant to separate its lateral branches. The nature of the condition is
+however very obscure and it is equally likely that some multiplication
+of the growing point is the essential phenomenon.[15]
+
+Stockard's interesting experiments[16] illustrate this question. He
+showed that by treating the embryos of a fish (_Fundulus heteroclitus_)
+with a dilute solution of magnesium salts, various cyclopian
+monstrosities were frequently produced. These have been called cases
+of _fusion_ of the optic vesicles. I would prefer to regard them as
+cases of a division suppressed or restricted by the control of the
+environment. Conversely, the splendid discovery of Loeb, that an
+unfertilised egg will divide and develop parthenogenetically without
+fertilisation, as a consequence of exposure to various media, may be
+interpreted as suggesting that the action of those media releases
+the strains already present in the ovum, though I admit that an
+interpretation based on the converse hypothesis, that the medium acts as
+a stimulus, is as yet by no means excluded.
+
+In these cases we come nearest to the direct causation or the direct
+inhibition of a division, but the meaning of the evidence is still
+ambiguous. I incline to compare Loeb's parthenogenesis with the
+development (and of course accompanying cell-division) of dormant buds
+on stems which have been cut back.
+
+It is interesting to note that sometimes as an abnormality, the
+faculty of division gets out of hand and runs a course apparently
+uncontrolled. A remarkable instance of this condition is seen in
+_Begonia_ "_phyllomaniaca_", which breaks out into buds at any point on
+the stem, petioles, or leaves, each bud having, like other buds, the
+power of becoming a new plant if removed. We would give much to know the
+genetic properties of _B. phyllomaniaca_, and in conjunction with Mr. W.
+O. Backhouse I have for some time been experimenting with this plant.
+It proved totally sterile. Its own anthers produce no pollen, and all
+attempts to fertilise it with other species failed though the pollen of
+a great number of forms was tried.
+
+Recently however we have succeeded in making plants which are in every
+respect _Begonia phyllomaniaca_, so far as the characters of stems and
+leaves are concerned. These plants, of which we have sixteen, were made
+by fertilising _B. heracleifolia_ with _B. polyantha_. They are all
+beginning to break out in "phyllomania." As yet they have not flowered,
+but as they agree in all details with _phyllomaniaca_ there can be
+little doubt that the original plant bearing that name was a hybrid
+similarly produced. The production of "phyllomania" on a hybrid Begonia
+has also been previously recorded by Duchartre.[17] In this case the
+cross was made between _B. incarnata_ and _lucida_. The synonymy of
+the last species is unfortunately obscure, and I have not succeeded in
+repeating the experiment.
+
+[Illustration: FIG. 5. Piece of petiole of _Begonia phyllomaniaca_. The
+proximal end is to the right of the figure.]
+
+From these facts it seems practically certain that the condition is one
+which is due to the meeting of complementary factors. At first sight
+we may incline to think that the phyllomania is in some way due to the
+sterility. This however cannot be seriously maintained; for not only is
+sterility in plants not usually associated with such manifestations, but
+we know a Begonia called "Wilhelma" which is exactly _phyllomaniaca_ and
+equally sterile, though it has no trace of phyllomania. This plant arose
+in the nurseries of MM. P. Bruant of Poitiers, and has generally been
+described as a seedling of _phyllomaniaca_, but from the total sterility
+of that form this account of its origin must be set aside.
+
+[Illustration: FIG. 6. Two right hind feet of polydactyle cats. _II_
+shows the lowest development of the condition yet recorded. The digit,
+_d_^{1}, which stands as hallux is fully formed and has three phalanges.
+Both it and the digit marked _d_^{2} are formed as _left_ digits. In the
+normal hind foot of the cat the hallux is represented by a rudiment only.
+
+_I_ shows a further development of the condition. In this foot there are
+_six_ digits. _d_^{1} has two phalanges, but both it and _d_^{2} and
+_d_^{3} are shaped as left digits. Thus _d_^{3}, which in the normal
+foot would be shaped as a right digit, is transformed so as to look like
+a _left_ digit.]
+
+The phenomenon in this case can hardly be regarded as due to the
+excitation of dormant buds, for it is apparent on examination that
+the new growths are not placed in any fixed geometrical relation to
+the original plant. They arise on the petiole, for example, as small
+green outgrowths each of which gradually becomes a tiny leaf. The
+attitude of these leaves is quite indeterminate, and they may point
+in any direction, some having their apices turned peripherally, some
+centrally, and others in various oblique or transverse positions (Fig.
+5). These little leaves are thus comparable with seedlings, in that
+their polarity is not related to, or consequent upon that of the parent
+plant. They have in fact that "individuality," which we associate with
+germinal reproduction.
+
+There are many curious phenomena seen in the behaviour of parts normally
+repeated in bilateral symmetry which may some day guide us towards
+an understanding of the mechanics of division. A part like a hand,
+which needs the other hand to complete its symmetry, cannot twin by
+mere division, yet by proliferation and special modifications on the
+radial side of the same limb, even a hand may be twinned. In the well
+known polydactyle cats a change of this kind is very common and indeed
+almost the rule. When extra digits appear at the inner (tibial) side
+of the limb, they are shaped as digits of the other side, and even the
+normal digit II (index) is usually converted into the mirror-image
+of its normal self. The limb then develops a new symmetry in itself.
+Nevertheless it is not easy to interpret these facts as meaning that
+there has been some interruption in the control which one side of the
+body exercises over the other. The heredity of polydactylism is complex
+but there is little doubt that the condition familiar in the Cat is a
+dominant. In some human cases also the descent is that of a dominant,
+but irregularities are so frequent that no general rule can yet be
+perceived. The dominance of such a condition is an exception to the
+principle that the less-divided is usually dominant to the more-divided,
+a fact which probably should be interpreted as meaning that divisions
+are of more than one kind.
+
+Among ordinary somatic divisions, whether of organs, cells, or patterns
+of differentiation, the control of symmetry is usually manifested. There
+is however one class of somatic differentiations which are exceptionally
+interesting from the fact that they may show a complete independence
+of such geometrical control. The most familiar examples of these
+geometrically uncontrolled Variations are to be seen in bud-sports.
+The normal differentiation of the organs of a plant is arranged on
+a definite geometrical system, which to those who have never given
+special attention to such things before, will often seem surprisingly
+precise. The arrangement of the leaves on uninjured, free-growing shoots
+can generally be seen to follow a very definite order, just as do the
+flowers or the parts of the flowers. If however bud sports occur,
+then though the parts included in the sports show all the geometrical
+peculiarities proper to the sport-variety, yet the sporting-buds
+themselves are not related to each other according to any geometrical
+plan.
+
+A very familiar illustration is provided by the distribution of colour
+in those Carnations that are not self-coloured. The pigment may, as in
+Picotees, be distributed peripherally with great regularity to the edges
+of the petals; or, as in Bizarres and Flakes, it may be scattered in
+radial sectors which show no geometrical regularity. Now in this case
+the pigments are the same in both types of flower, and the chemical
+factors concerned in their production must surely be the same. The
+difference must lie in the mechanical processes of distribution of
+the pigment. In the Picotee we see the orderly differentiation which
+we associate with normality; in the Bizarre we see the disorderly
+differentiation characteristic of bud-sports. The distribution of colour
+in this case lies outside the scheme of symmetry of the plant.
+
+Such a distribution is characteristic of bud-sports, and of certain
+other differentiations in both plants and animals, which I cannot on
+this occasion discuss. Now reflexion will show that these facts have
+an intimate bearing on the mechanical problems of heredity. For first
+in the bud-sports we are witnessing the distribution of factors which
+distinguish genetic varieties. We do not know the physical nature of
+those factors, but if we must give them a name, I suppose we should call
+them "ferments" exactly as Boyle did in 1666. He is discussing how it
+comes about that a bud, budded on a stock, becomes a branch bearing the
+fruit of its special kind. He notes that though the bud inserted be "not
+so big oftentimes as a Pea," yet "whether by the help of some peculiar
+kind of Strainer or by the Operation of some powerful Ferment lodged in
+it, or by both these, or some other cause," the sap is "so far changed
+as to constitute a Fruit quite otherwise qualify'd."[18] We can add
+nothing to his speculation, and we believe still that by a differential
+distribution of "ferments" the sports are produced. All the factors are
+together present in the normal parts; some are left out in the sport. In
+an analogous case however, that of a variegated _Pelargonium_ which has
+green and also albino shoots, Baur proved that the shoots pure in colour
+are also pure in their posterity. There can be no doubt that the sports
+of Carnations, Azaleas, Chrysanthemums, etc., would behave in the same
+way.
+
+The well-known Azaleas Perle de Ledeburg, President Kerchove,
+and _Vervaeana_ are familiar illustrations. Perle de Ledeburg is
+predominantly white, but it has red streaks in some of its flowers. It
+not very rarely gives off a self-red sport. This is evidently due to
+the development of a bud in a red-bearing area of the stem. The red in
+this plant is not under "geometrical control." Many plants have white
+flowers with no markings, but if the red markings are geometrically
+ordered differentiations, no self-coloured sports are formed. The case
+of _Vervaeana_ is a good illustration of this proposition. It has white
+flowers with red markings arranged in an orderly manner on the lower
+parts of the petals, especially on the dorsal petals. This is one of the
+Azaleas most liable to have red sports, and at first sight it might seem
+that the sport represented the red of the central marks. Examination
+however of a good many flowers shows that irregular red streaks like
+those of Perle de Ledeburg occur, about as commonly as in that variety.
+_Vervaeana_ in fact is Perle de Ledeburg with _definite_ red markings
+added, and its red sports obviously are those branches the germs of
+which came in a patch of the stem bearing these red elements. That this
+is the true account is rendered quite obvious by the fact that the red
+of the sport is a colour somewhat different from that of the definite
+marks, and that these marks are still present on the red ground of the
+sporting flowers.
+
+It will be understood that these remarks apply to those cases in which
+the production of sports is habitual or frequent, and I imagine in
+all such examples it will be found that there are indications of
+irregularity in the distribution of the differentiations such as to
+justify the view that they are not under that geometrical control which
+governs the normal differentiation of the parts. The question next
+arises whether these considerations apply also to the production of a
+bud-sport as a rare exception, but by the nature of the case it is not
+possible to say positively whether the appearance of an exceptional
+sport is due to the unsuspected presence of a pre-existing fragment of
+material having a special constitution, or to the origin, _de novo_, of
+such a material. For instance one of the garden forms of _Pelargonium_
+known as _altum_ is liable perhaps once in some hundreds of flowers to
+have one or two magenta petals. The normal colour is a brilliant red;
+and as we may be fairly sure that this red is recessive to magenta the
+interpretation would be quite different according as the appearance of
+the magenta is regarded as due to the presence of small areas endowed
+with magentaness, or to the spontaneous generation of the factor for
+that pigment. Either interpretation is possible on the facts, but the
+view that the whole plant has in it scarce mosaic particles of magenta
+seems on the whole more consistent with present knowledge.
+
+In _Pelargonium altum_ the enzyme causing the magenta colours must
+be distributed in very small areas, but a case in which the magenta
+is similarly arranged in a much coarser patchwork may be seen in the
+_Pelargonium_ "Don Juan," which often bears whole trusses or branches of
+red flowers upon plants having the normal dominant magenta trusses. In
+most cases there is little doubt that though the magenta flowered parts
+can "sport" to red, the red parts could not produce the magenta flowers.
+
+The asymmetrical, or to speak more precisely, the disorderly, mingling
+of the colours in the somatic parts is thus an indication of a similarly
+disorderly mixing of the factors for those colours in the germ-tissues,
+so that some of the gametes bear enough of the colour-factors to make a
+self-coloured plant, while others bear so little that the plant to which
+they give rise is a patchwork. If this view is correct we may extend it
+so far as to consider whether the fineness or coarseness of the mixture
+visible in the flowers or leaves may not give an indication of the
+degree to which the factors are subdivided among the germ-cells. We know
+very little about the genetic properties of striped varieties. In both
+_Antirrhinum_ and _Mirabilis_ it has been found that the striped may
+occasionally and irregularly throw self-coloured plants, and therefore
+the striping cannot be regarded simply as a recessive character. On the
+other hand in _Primula Sinensis_ there are well-known flaked varieties
+which ordinarily at least breed true. Whether these ever throw selfs I
+do not know, but if they do it must be quite exceptionally. The power
+of these flaked plants to breed true is, I suspect, connected with the
+fact that in their flowers the coloured and white parts are _intimately_
+mixed, this intimate mixture thus being an indication of a similarly
+intimate mixture in the germ-cells. It would be important to ascertain
+whether self-fertilised seed from the occasional flowers in which the
+colour has run together to join a large patch gives more self-coloured
+plants than the intimately flaked flowers do.
+
+The next fact may eventually prove of great importance. We have seen
+that in bud-sports the differentiation is of the same nature as
+that between pure types, and also that in the sporting plant this
+differentiation is distributed without any reference to the plant's
+axis, or any other consideration of symmetry. Now among the germ-cells
+of a Mendelian hybrid exactly such characters are being distributed
+allelomorphically, and there again we have strong evidence for believing
+that the distribution obeys no pattern. For example, we can in the case
+of seeds still _in situ_ perceive how the characters were distributed
+among the germ-cells, and there is certainly no obvious pattern
+connecting them, nor can we suppose that there is an actual pattern
+obscured.
+
+Of this one illustration is especially curious. Individual plants of
+the same species are, as regards the decussations of their leaves and
+in other respects, _either rights or lefts_. The fact is not emphasized
+in modern botany and is in some danger of being forgotten. When, as
+in the flowers of Arum, some _Gladioli_, _Exacum_, _St. Paulia_, or
+the fruits of _Loasa_, rights and lefts occur on the same stem,
+they come off alternately. But if, as in the seedlings of Barley the
+twist of the first leaf be examined, it will be seen to be either a
+right-or left-handed screw. An ear of barley, say a two-row barley,
+is a definitely symmetrical structure. The seeds stand in their
+envelopes back to back in definite positions. Each has its organs
+placed in perfectly definite places. _If these seeds were buds_ their
+differentiations would be grouped into a common plan. One might expect
+that the differentiations of these embryos would still fall into the
+pattern; but they do not, and so far as I have tested them, any one
+may be a right or a left, just as each may carry any of the Mendelian
+allelomorphs possessed by the parent plant, without reference to the
+differentiation of any other seed. The fertilisation may be responsible,
+but our experience of the allelomorphic characters suggest that the
+irregularity is in the egg-cells themselves.[19]
+
+_Germ cells thus differ from somatic cells in the fact that their
+differentiations are outside the geometrical order which governs
+the differentiation of the somatic cells._ I can think of possible
+exceptions, but I have confidence that the rule is true and I regard it
+as of great significance.
+
+The old riddle, what is an individual, finds at least a partial solution
+in the reply that an individual is a group of parts differentiated in a
+geometrically interdependent order. With the germ-cell a new geometrical
+order, with independent polarity is almost if not quite always, begun,
+and with this geometrical independence the power of rejuvenescence may
+possibly be associated.
+
+The problems thus raised are unsolved, but they do not look insoluble.
+The solution may be nearer than we have thought. In a study of the
+geometry of differentiation, germinal and somatic, there is a way
+of watching and perhaps analyzing what may be distinguished as the
+mechanical phenomena of heredity. If any one could in the cases of
+the Picotee and the Bizarre Carnation, respectively, detect the real
+distinction between the two types of distribution, he would make a
+most notable advance. Any one acquainted with mechanical devices can
+construct a model which will reproduce some of these distinctions more
+or less faithfully. The point I would not lose sight of is that the
+analogy with such models must for a long way be a true and valuable
+guide. I trust that some one with the right intellectual equipment will
+endeavor to follow this guide; and I am sanguine enough to think that a
+comprehensive study of the geometrical phenomena of differentiation will
+suggest to a penetrative mind that critical experiment which may one day
+reveal the meaning of spontaneous division, the mystery through which
+lies the road, perhaps the most hopeful, to a knowledge of the nature of
+life.
+
+FOOTNOTES:
+
+[1] In saying this we make no assumption as to the particular
+cell-division at which differentiation occurs. This may be one of the
+maturation-divisions, or it may perhaps be much earlier.
+
+[2] From the recent discoveries of Erwin Baur we are led to surmise
+that in the flowering plants the sub-epidermal layer, or some of its
+elements, may legitimately be regarded as a similar germ-substance,
+continuous in Weismann's sense.
+
+[3] These fraternal twins, which show no special resemblance to each
+other, are like the multiple births of other animals, and there is no
+disposition for them to be of the same sex. In the sheep, for example,
+statistics show that the frequency of pairs of twins, male and female,
+is approximately double that of the frequency of pairs, both male or
+both female, as it should be if the sex-distribution were fortuitous.
+For instance Bernadin (_La Bergerie de Rambouillet_, 1890, p. 100)
+gives the following figures for twin-lambs in Merinos: both male, 87;
+both female, 83; sexes mixed, 187. The 9-banded Armadillo (_Dasypus
+novemcinctus_), in which the young born in one litter are said to be
+always of one sex, is the only known exception in Vertebrates, and
+is presumably a genuine case of normal polyembryony (see especially,
+Rosner, _Bull. Ac. Soc. Cracovie_, 1901, p. 443, and Newman and
+Patterson, _Biol. Bull._, XVII, 1909, p. 181), and an important paper
+lately published by H. H. Newman and J. T. Patterson, _Jour. Morph._,
+1911, XXII, p. 855.
+
+[4] A good collection of evidence as to disease in homologous twins was
+lately published by E. A. Cockayne, _Brit. Jour. Child. Diseases_, Nov.,
+1911.
+
+[5] Cp. Windle, B. C. A., _Jour. Anal. Phys._, XXVI, p. 295.
+
+[6] Mr. E. Nettleship tells me that in the course of collecting
+pedigrees of families containing colour-blind members he has discovered
+two cases (shortly to be published) of pairs of twins, which on account
+of their very close resemblances must be deemed homologous, one of each
+pair being colour-blind and the other normal. Such a distinction between
+closely similar twins is most curious and unexpected.
+
+[7] Another paradoxical phenomenon of the same nature occurs in the
+Narwhal The males normally have the _left_ tusk alone developed, the
+corresponding right tusk remaining as an undeveloped rudiment in its
+socket. The left tusk is a left-handed screw. Occasionally the right
+tusk is also developed and grows to the same length as that of the
+left side, but in such specimens the right tusk is also a left-hand
+screw like the tusk of the other side, instead of being reversed as we
+should certainly have expected. It need scarcely be remarked that in the
+case of the horns of antelopes, and in other examples of spiral organs
+arranged in pairs, that of one side of the body is the mirror image of
+that on the other side. The Narwhal's tusks in being both twisted in the
+same direction are thus highly anomalous, and are comparable with pairs
+of twins.
+
+[8] Wilder, H. H., _Amer. Jour. Anat._, 1904, III, p. 452.
+
+[9] Polydactylism which is often a dominant and the web-foot of Pigeons
+which is recessive should be remembered as possible exceptions (see p.
+49).
+
+[10] Davenport inclined at first to regard rumplessness as a recessive,
+but in his latest publication on the subject he definitely concludes
+that it is an imperfect dominant. This conclusion accords well with
+evidence quoted by Darwin (_An. and Plts._, II, ed. 2, p. 4) that
+rumpless fowls may throw tailed offspring. (_Amer. Nat._, 1910, XLIV, p.
+134.)
+
+[11] Spillman, W. J., _Amer. Breeders Mag._, 1910, I, p. 178.
+
+[12] Newsholme, _Lancet_, December 10, 1910, p. 1690.
+
+[13] _Materials for the Study of Variation_, 1894, p. 358.
+
+[14] Walker, G., _Johns Hopkins Hospital Bulletin_, XII, 1901, p. 129.
+
+[15] Cp. R. H. Compton, _New Phytologist_, 1911, p. 249.
+
+[16] _Arch. f. Entwickelungsmech._, 1907, XXIII, p. 249.
+
+[17] Bull. Soc. Bot. de France, xxxiv, 1887, p. 182.
+
+[18] R. Boyle, _The Origine of Formes and Qualities_, Oxford, 1666.
+
+[19] Remarkable experiments on this question have lately been carried
+out by R. H. Compton (_Camb. Phil. Soc._, XV, 1910, p. 495), showing
+that in a certain Barley, "Plumage Corn," the average ratio of left to
+right is about 1.5. A fuller paper has since been published by Compton,
+_Jour. Genetics_, 1912, II, I, p. 53.
+
+
+
+
+                                CHAPTER III
+
+                    SEGMENTATION, ORGANIC AND MECHANICAL
+
+Models may be and often have been devised imitating some of the
+phenomena of division, but none of them have reproduced the peculiarity
+which characterises divisions of living tissues, that _the position
+of chemical differentiation_ is _determined by those divisions_. For
+example, models of segmentation, whether radial or linear, may be made
+by the vibration of plates as in the familiar Chladni figures of the
+physical laboratory, or by the bowing of a tube dusted on the inside
+with lycopodium powder, and in various other ways. The sand or the
+powder will be heaped up in the nodes or regions of least movement, and
+the patterns thus formed reproduce many of the geometrical features
+of segmentation. But in the segmentations of living things the nodes
+and internodes, once determined by the dividing forces, would each
+become the seat of appropriate and distinct chemical processes leading
+to the differentiation of the parts, and the deposition of the bones,
+petals, spines, hairs, and other organs in relation to the meristic
+ground-plan. The "ripples" of meristic division not merely divide but
+differentiate, and when a "ripple" forks the result is not merely a
+division but a reduplication of the organ through which the fork runs.
+An example illustrating such a consequence is that of the half-vertebrae
+of the Python. On the left side the vertebra is single (Fig. 7) and
+bears a single rib, but on the right side a division has occurred with
+the result that two half-vertebrae, each bearing a rib, are formed, one
+standing in succession to the other. We cannot, indeed, imagine any
+operation of physiological division carried out in such an organ as a
+vertebra, passing through a plane at right angles to the long axis of
+the body, which does not necessarily involve the further process of
+reduplication.
+
+As the meristic system of distribution spreads through the body,
+chemical differentiations follow in its track, with segmentation and
+pattern as the visible result. Could we analyse these simultaneous
+phenomena and show how it is that the places of chemical differentiation
+are determined by the system of division, progress would then be rapid.
+It is here that all speculation fails.
+
+[Illustration: FIGS. 7 and 8. Two examples of imperfect division in the
+vertebræ of a python. _I_, the vertebræ 147-150 from the right side,
+showing imperfect division between the 148th and 149th. The condition on
+the left side of this vertebra was the same. _II_, the dorsal surface of
+vertebræ 165-167. On the right side the 166th is double and bears two
+ribs, but on the left side it is normal and has one rib only.]
+
+Many attempts have been made to interpret the processes of division
+and repetition, in terms of mechanics, or at least to refer them to
+their nearest mechanical analogies, so far with little success. The
+problem is beset with difficulties as yet insurmountable and of these
+one must be especially noticed. In the living thing the process by which
+repetition and patterns come into being consists partly in division but
+partly also in growth. We have no means of studying the phenomena of
+pattern-formation except in association with that of growth. Growth soon
+ceases unless division takes place, and if growth is impossible division
+soon ceases also. In consequence of this fact that the final pattern
+is partly a product of growth, it can never be used as unimpeachable
+evidence of the primary geometrical relations of the members as laid
+down in the divisions.
+
+In the last chapter in referring to the problem of repetition I
+introduced an analogy, comparing the patterns of the organic world
+with those produced in unorganised materials by wave-motion. In the
+preliminary stage of ignorance, having no more trustworthy clue, I do
+not think it wholly unprofitable to consider the applicability of this
+analogy somewhat more fully. It possesses, as I hope to show, at least
+so much validity as to encourage the belief that morphology may safely
+discard one source of long-standing error and confusion.
+
+Those who have studied the structure of parts repeated in series will
+have encountered the old morphological problem of "Serial Homology,"
+which has absorbed so much of the attention of naturalists and
+especially of zoologists at various periods. This problem includes
+two separate questions. The first of these is the origin in evolution
+of the resemblance between two organs occurring in a repeated series,
+of which the fore and hind limbs of Vertebrates are the prerogative
+instance. From the fact that these resemblances can be traced very far,
+often into minute details of structure, many anatomists have inclined to
+the opinion that the resemblance must originally have been still more
+complete, and that the two limbs, for instance, must have acquired their
+present forms by the differentiation of two identical groups of parts.
+
+Similar questions arise whenever parts are repeated in series, whether
+the series be linear or radial, and, though less obviously, even when
+the repetition is bilateral only. In each such example the question
+arises, is the resemblance between the parts the remains of a still
+closer resemblance, or is differentiation original? Sometimes the
+view that these parts have arisen by the differentiation of a series
+of identical parts is plausible enough, as for example when the
+peculiarities of various appendages of a Decapod Crustacean are referred
+to modifications of the Phyllopod series. In application to other cases
+however we soon meet with difficulty, and the suggestion that the
+segments of a vertebrate were originally all alike is seen at once to be
+absurd, for the reason that a creature so constituted could not exist,
+and that, differentiation of at least one anterior and one posterior
+segment, is an essential condition of a viable organism consisting of
+parts repeated in a linear series. Between these two terminal segments
+it is possible to imagine the addition of one segment, or of a series
+of approximately similar segments; but when once it is realised that
+the terminals must have been differentiated from the beginning, it
+will be seen that the problem of the origin of the resemblance between
+segments is not rendered more comprehensible by the suggestion that
+even the intervening members were originally alike. Seeing indeed that
+some differentiation must have existed primordially it is as easy to
+imagine that the original body was composed of a series grading from the
+condition of the anterior segment to that of the posterior, as any other
+arrangement. The existence of a linear or successive series in fact
+postulates a polarity of the whole, and in such a system the conception
+of an ideal segment containing all the parts represented in the others
+has manifestly no place. The introduction of that conception though
+sanctioned by the great masters of comparative anatomy, has, as I think,
+really delayed the progress of a rational study of the phenomena of
+division. The same notion has been applied to every class of repetition
+both in animals and plants, generally with the same unhappy results. In
+the cruder forms in which this doctrine was taught thirty years ago it
+is now seldom expressed, but modified presentations of it still survive
+and confuse our judgments.
+
+The process of repetition of parts in the bodies of organisms is
+however a periodic phenomenon. This much, provided we remain free from
+prejudice as to the nature and causation of the period or rhythm, we
+may safely declare, and a comparison may thus be instituted between the
+consequences of meristic repetition in the bodies of living things and
+those repetitions which in the inorganic world are due to rhythmical
+processes. Of such processes there is a practically unlimited diversity
+and we have nothing to indicate with which of them our repetitions
+should rather be compared.
+
+[Illustration: FIG. 9. Osmotic growths simulating segmentation. (After
+Leduc.)]
+
+In some respects perhaps the best models of living organisms yet made
+are the "osmotic growths" produced by Leduc.[1] These curious structures
+were formed by placing a fragment of a salt, for instance calcium
+chloride, in a solution of some colloidal substance. As the solid takes
+up water from the solution a permeable pellicle or membrane is formed
+around it. The vesicle thus enclosed grows by further absorption of
+water, often extending in a linear direction, and in many examples this
+growth occurs by a series of rhythmically interrupted extensions. Some
+of the growths thus formed are remarkably like organic structures,
+and might pass for a series of antennary segments or many other
+organs consisting of a linear series of repeated parts. In admitting
+the essential resemblance between these "osmotic growths" and living
+bodies or their organs I lay less stress on the general conformation
+of the growths, which often as Leduc points out, recall the forms
+of fungi or hydroids, but rather on the fact that the interruptions
+in the development of these systems are so closely analogous to the
+segmentations or repetitions of parts characteristic of living things
+(Fig. 9). In the same way I am less impressed by Leduc's models of
+Karyokinesis, wonderful as they nevertheless are, for the division is
+here imitated by putting separate drops on the gelatine film. What we
+most want to know is how in the living creature one drop becomes two.
+The models of linear segmentation have the remarkable merit that they do
+in some measure imitate the process of actual division or repetition.
+So in a somewhat modified method Leduc, by causing the diffusion
+of a solution in a gelatine film, produced rhythmical or periodic
+precipitations strikingly reminiscent of various organic tissues, for
+here also the process of periodic repetition is imitated with success.
+
+It is a feature common to these and to all other rhythmical repetitions
+produced by purely mechanical forces that there is resemblance between
+the members of the series, and that this similarity of conformation may
+be maintained in most complex detail. When however in the mechanical
+series some of the members differ from the rest we have no difficulty
+in recognising that these differences--which correspond with the
+differentiations of the organic series--are due to special heterogeneity
+in the conditions or in the materials, and it never occurs to us to
+suppose that all the members must have been primordially alike. For
+example, in the case of ripple-marks on the sand, which I choose as one
+of the most familiar and obvious illustrations of a repeated series
+due to mechanical agencies, if we notice one ripple different in form
+from those adjacent to it, we do not suppose that this variation must
+have been brought about by deformation of a ripple which was at first
+formed like the others, but we ascribe it to a difference in the sand at
+that point, or to a difference in the way in which the wind or the tide
+dealt with it. We may press the analogy further by observing that in as
+much as such a series of waves has a beginning and an end, it possesses
+polarity like that of the various linear series of parts in organisms,
+and even the formation of each member must influence the shape of its
+successor. Since in an organism the beginning and end of the series
+are always included, some differentiation among the repetitions must
+be inevitable. If therefore it be conceded, as I think it must, that
+segmentation and pattern are the consequence of a periodic process we
+realize that it is at least as easy to imagine the formation of such a
+series of parts having family likeness combined with differentiation
+as it would be to conceive of their arising primordially as a series
+of identical repetitions. The suggestion that the likenesses which we
+now perceive are the remains of a still more complete resemblance is a
+substitution of a more complex conception for a simpler one.
+
+The other question raised by the problem of Serial Homology is how far
+there is a correspondence between individual members of series when
+the series differ from each other either in the number of parts, or in
+the mode of distribution of differentiation among them. Students, for
+example, of vertebrate morphology debate whether the _n_th vertebra
+which carries the pelvic girdle in Lizard A is individually homologous
+with the _n_ + _x_th vertebra which fulfils this function in Lizard B,
+or whether it is not more truly homologous with the vertebra standing in
+the _n_th ordinal position, though that vertebra in Lizard B is free.
+
+In various and more complex aspects the same question is debated in
+regard to the cranial and spinal nerves, the branches of the aorta,
+the appendages of Arthropoda, and indeed in regard to all such series
+of differentiated parts in linear or successive repetition. Persons
+exercised with these problems should before making up their minds
+consider how similar questions would be answered in the case of any
+series of rhythmical repetitions formed by mechanical agencies. In the
+case of our illustration of the ripples in the sand, given the same
+forces acting on the same materials in the same area, the number of
+ripples produced will be the same, and the _n_th ripple counting from
+the end of the series will stand in the same place whenever the series
+is evoked. If any of the conditions be changed, the number and shapes
+can be changed too, and a fresh "distribution of differentiation"
+created. Stated in this form it is evident that the considerations
+which would guide the judgment in the case of the sand ripples are not
+essentially different from those which govern the problem of individual
+homology in its application to vertebrae, nerves, or digits.
+
+The fact that the unit of repetition is also the unit of growth is the
+source of the obscurity which veils the process. When we compare the
+skeleton of a long-tailed monkey with that of a short-tailed or tailless
+ape we see at once how readily the additional series of caudal segments
+may be described as a consequence of the propagation of the "waves" of
+segmentation beyond the point where they die out in the shorter column,
+and we see that with an extension of the series of repetitions there is
+growth and extension of material.
+
+The considerations which apply to this example will be found operating
+in many cases of the variation of terminal members of linear series.
+Some of these series, like the teeth of the dog, end in a terminal
+member of a size greatly reduced below that of the next to it. Even when
+there is thus a definite specialisation of the last member of the series
+it not infrequently happens that the addition, by variation, of a member
+beyond the normal terminal, is accompanied by a very palpable increase
+in size of the member which stands numerically in the place of the
+normal terminal.[2] So also with variation in the number of ribs, when
+a lumbar vertebra varies homoeotically into the likeness of the last
+dorsal and bears a rib, the rib placed next in front of this, which in
+the normal trunk is the last, shows a definite increase in development.
+
+The consequences of such homoeoses are sometimes very extensive,
+involving readjustments of differentiation affecting a long series of
+members, as may easily be seen by comparing the vertebral columns of
+several individual Sloths[3] (whether _Bradypus_ or _Choloepus_) to take
+a specially striking example.
+
+It may be urged that no feature as yet enables us to perceive wherein
+lies the primary distinction which determines such variation, whether it
+is due to a difference in the dividing forces or in the material to be
+divided. If for instance we were to imitate such a series of segments
+by pressing hanging drops of a viscous fluid out of a paint-tube by
+successive squeezes, the number of times the tube is contracted before
+it is empty will give the number of the segments, but their size may
+depend either on the force of the contractions or on the capacity of
+the tube, or on various other factors. Nevertheless in the case of the
+variation of terminal members, whatever be the nature of the rhythmical
+impulse which produces the series of organs, the elevation of the
+normally terminal member in correspondence with the addition of another
+is what we should expect.
+
+If the organism acquired its full size first and the delimitation of
+the parts took place afterwards, there might be some hope that the
+resemblance between living patterns and those mechanically caused by
+wave-motion might be shown to be a consequence of some real similarity
+of causation, but in view of the part played by growth, appeal to these
+mechanical phenomena cannot be declared to have more than illustrative
+value. Similarly in as much as living patterns appear, and almost
+certainly do in reality come into existence by a rhythmical process,
+comparisons of these patterns with those developed in crystalline
+structures, and in the various fields of force are, as it seems to me,
+inadmissible, or at least inappropriate.
+
+However their intermittence be determined, the rhythms of division must
+be looked upon as the immediate source of those geometrically ordered
+repetitions universally characteristic of organic life. In the same
+category we may thus group the segmentation of the Vertebrates and of
+the Arthropods, the concentric growth of the Lamellibranch shells or of
+Fishes' scales, the ripples on the horns of a goat, or the skeletons of
+the Foraminifera or of the Heliozoa. In the case of plant-structures
+Church[4] has admirably shown, with an abundance of detail, how on
+analysis the definiteness of phyllotaxis is an expression of such
+rhythm in the division of the apical tissues, and how the spirals
+and "orthostichies" displayed in the grown plant are its ultimate
+consequences. The problem thus narrows itself down to the question of
+the mode whereby these rhythms are determined.
+
+It is natural that we should incline to refer them to a chemical source.
+If we think of the illustration just given, of the segmentation of a
+viscous fluid into drops by successive contractions of a soft-walled
+tube we can, I think, conceive of such rhythmic contractions as due to
+summations of chemical stimuli, somewhat as are the beats of the heart.
+But when we recognize the vast diversity of materials the distribution
+of which is determined by an ostensibly similar rhythmic process it
+seems hopeless to look forward to a directly chemical solution. That the
+chemical degradation of protoplasm or of materials which it contains is
+the source of the energy used in the divisions cannot be in dispute, but
+that these divisions can be themselves the manifestations of chemical
+action seems in the highest degree improbable.
+
+We may therefore insist with some confidence on the distinction between
+the Meristic and the substantive constitution of organisms, between,
+that is to say, the system according to which the materials are divided
+and the essential composition of the materials, conscious of the fact
+that the energy of division is supplied from the materials, and that
+in the ontogeny the manner in which the divisions are effected must
+depend secondarily on the nature of the substances to be divided.
+The mechanical processes of division remain a distinguishable group
+of phenomena, and variations in the substances to be distributed in
+division may be independent of variations in the system by which the
+distribution is effected.
+
+Modern genetic analysis supplies many remarkable examples of this
+distinction. When formerly we compared the leaves of a normal palmatifid
+Chinese Primula with the pinnatifid leaves[5] of its fern-leaved variety
+we were quite unable to say whether the difference between the two types
+of leaf was due to a difference in the material cut up in the process
+of division or to a difference in that process itself. Knowledge that
+the distinction is determined by a single segregable factor tends to
+prove that the critical difference is one of substance. So also in the
+Silky fowl we know that the condition of its feathers is due to the
+absence of some one factor present in the normal form. We may conceive
+such differences as due to change of form in the successive "waves" of
+division, but we cannot yet imagine segregation otherwise than as acting
+by the removal or retention of a material element. Future observation
+by some novel method may suggest some other possibility, but such cases
+bring before us very clearly the difficulties by which the problem is
+beset.
+
+[Illustration: FIG. 10. The palm-and fern type of leaf in _Primula
+Sinensis_. The palm is dominant and the fern is recessive.]
+
+In another region of observation phenomena occur which as it seems to
+me put it beyond question that the meristic forces are essentially
+independent of the materials upon which they act, save, in the remoter
+sense, in so far as these materials are the sources of energy. The
+physiology of those regenerations and repetitions which follow upon
+mutilation supplies a group of facts which both stimulate and limit
+speculation. No satisfactory interpretations of these extraordinary
+occurrences has ever been found, but we already know enough to feel
+sure that in them we are witnessing indications which should lead
+to the discovery of the true mechanics of repetition and pattern.
+The consequences of mutilation in causing new growth or perhaps more
+strictly in enabling new growth to take place, are such that they cannot
+be interpreted as responses to chemical stimuli in any sense which
+the word chemical at present connotes. Powers are released by mutilation
+of which in the normal conditions of life no sign can be detected. All
+who have tried to analyse the phenomena of regeneration are compelled
+to have recourse to the metaphor of equilibrium, speaking of the normal
+body as in a state of strain or tension (Morgan) which when disturbed by
+mutilation results in new division and growth. The forces of division
+are inacessible to ordinary means of stimulation. Applications, for
+example, of heat or of electricity excite no responses of a positive
+kind unless the stimuli are so violent as to bring about actual
+destruction.[6] These agents do not, to use a loose expression, come
+into touch with the meristic forces. Changes in the chemical environment
+of cells may, as in the experiments of Loeb and of Stockard produce
+definite effects, but the facts suggest that these effects are due
+rather to alterations in the living material than to influence exerted
+directly on the forces of division themselves.
+
+By destruction of tissue however the forces both of growth and of
+division also may often be called into action with a resulting
+regeneration. Interruption of the solid connexion between the parts may
+produce the same effects, as for example when the new heads or tails
+grow on the divided edges of Planarians (Morgan), or when from each half
+embryo partially separated from its normally corresponding half, a new
+half is formed with a twin monster as the result.
+
+Often classed with regenerations but in reality quite distinct from
+them are those special and most interesting examples where the growth
+of a _paired_ structure is excited by a simple wound. Some of the best
+known of these instances are presented by the paired extra appendages
+of Insects and Crustacea. Some years ago I made an examination of all
+the examples of such monstrosities to which access was to be obtained,
+and it was with no ordinary feeling of excitement that I found that
+these supernumerary structures were commonly disposed on a recognizable
+geometrical plan, having definite spatial relations both to each other
+and to the normal limb from which they grew. The more recent researches
+of Tornier[7] and especially his experiments on the Frog have shown
+that a cut into the posterior limb-bud induces the outgrowth of such a
+_pair_ of limbs at the wounded place. Few observations can compare with
+this in novelty or significance; and though we cannot yet interpret
+these phenomena or place them in their proper relations with normal
+occurrences, we feel convinced that here is an observation which is no
+mere isolated curiosity but a discovery destined to throw a new light on
+biological mechanics. The supernumerary legs of the Frog are evidently
+grouped in a system of symmetry similar to that which those of the
+Arthropods exhibit, and though in Arthropods paired repetitions have not
+been actually produced by injury under experimental conditions we need
+now have no hesitation in referring them to these causes as Przibram has
+done.
+
+At this point some of the special features of the supernumerary
+appendages become important. First they may arise at any point on the
+normal limb, being found in all situations from the base to the apex.
+Nor are they limited as to the surface from which they spring, arising
+sometimes from the dorsal, anterior, ventral, or posterior surfaces, or
+at points intermediate between these principal surfaces.
+
+With rare and dubious exceptions, the parts which are contained in these
+extra appendages are only those which lie _peripheral to their point of
+origin_. Thus when the point of origin is in the apical joint of the
+tarsus, the extra growth if completely developed consists of a double
+tarsal apex bearing two pairs of claws. If they arise from the tibia,
+two complete tarsi are added. If they spring from the actual base of
+the appendage then two complete appendages may be developed in addition
+to the normal one. We must therefore conclude that in any point on a
+normal appendage the power exists which, if released, may produce a bud
+containing in it a paired set of the parts peripheral to this point.
+
+[Illustration: FIG. 11. Diagrams of the geometrical relations which are
+generally exhibited by extra pairs of appendages in Arthropoda. The
+sections are supposed to be those of the apex of a tibia in a beetle.
+_A_, anterior, _P_, posterior, _D_, dorsal, _V_, ventral. _M_^{1},
+_M_^{2} are the imaginary planes of reflexion. The shaded figure is in
+each case a limb formed like that of the other side of the body, and the
+outer unshaded figures are shaped like the normal for the side on which
+the appendages are. On the several radii are shown the extra pairs in
+their several possible relations to the normal from which they arise.
+The normal is drawn in thick lines in the center.]
+
+Next the geometrical relations of the halves of the supernumerary pair
+are determined by the position in which they stand in regard to the
+original appendage. These relations are best explained by the diagram
+(Fig. 11), from which it will be seen that the two supernumerary
+appendages stand as images of each other; and, of them, that which is
+adjacent to the normal appendage forms an image of it. Thus if the
+supernumerary pair arise from a point on the dorsal surface of the
+normal appendage, the two _ventral_ surfaces of the extra pair will
+face each other. If they arise on the anterior surface of the normal
+appendage, their morphologically posterior surfaces will be adjacent,
+and so on.
+
+These facts give us a view of the relations of the two halves of a
+dividing bud very different from that which is to be derived from the
+exclusive study of normal structures. Ordinary morphological conceptions
+no longer apply. The distribution of the parts shows that the bud or
+rudiment which becomes the supernumerary pair may break or open out in
+various ways according to its relations to the normal limb. Its planes
+of division are decided by its geometrical relations to the normal body.
+
+Especially curious are some of the cases in which the extra pair are
+imperfectly formed. The appearance produced is then that of two limbs
+in various stages of coalescence, though in reality of course they are
+stages of imperfect separation. The plane of "coalescence" may fall
+anywhere, and the two appendages may thus be compounded with each other
+much as an object partially immersed in mercury "compounds" with its
+optical image reflected from the surface.
+
+Supernumerary paired structures are not usually, if ever, formed when
+an appendage is simply amputated. Cases occasionally are seen which
+nevertheless seem to be of this nature. Borradaile,[8] for example,
+described a crab (_Cancer pagurus_) having in place of the right chela
+three _small_ chelae arising from a common base, where the appearances
+suggested that the three reduced limbs replaced a single normal limb.
+From the details reported however it seems still possible that one of
+the chelae (that lettered F. I in Borradaile's figure) may be the normal
+one, and the other two an extra pair. The chela which I suspect to be
+the normal is in several respects deformed as well as being reduced in
+size, and this deformity may perhaps have ensued as a consequence of the
+same wound which excited the growth of the extra pair. Its reduced size
+may be due to the same injury, which may quite well have checked its
+growth to full proportions.
+
+Admitting doubt in these ambiguous cases it seems to be a general rule
+that for the production of the extra pair the normal limb should persist
+in connexion with the body. Moreover it is practically certain that
+in no case can a _single_, viz. an unpaired, duplicate of the normal
+appendage grow from it. Many examples have been described as of this
+nature, but all of them may be with confidence regarded as instances of
+a supernumerary pair in which only the two morphologically anterior or
+the two morphologically posterior surfaces are developed. We have thus
+the paradox that a limb of one side of the body, say the right, has in
+it the power to form a pair of limbs, right and left, as an outgrowth of
+itself, but cannot form a second left limb alone.
+
+A very interesting question arises whether it is strictly correct to
+describe the extra pair as a right and a left, or whether they are not
+rather two lefts or two rights of which one is reversed. This question
+did not occur to me when in former years I studied these subjects. It
+was suggested to me by Dr. Przibram. The answer might have an important
+bearing on biological mechanics, but I know no evidence from which the
+point can be determined with certainty. In order to decide this question
+it would be necessary to have cases in which the paired repetition
+affected a limb markedly differentiated on the two sides of the body,
+and of course the development of the extra parts in order to be decisive
+must be fairly complete. One example only is known to me which at all
+satisfies these requirements, that of the lobster's chela figured (after
+Van Beneden) in _Materials for the Study of Variation_, p. 531, Fig.
+184, III.
+
+Here the drawing distinctly suggests that one of the extra
+dactylopodites, namely that lettered R, is differentiated as a left
+and not merely a reversed right. For the teeth on this dactylopodite
+are those of a cutting claw, not of a crushing claw, whereas the
+dactylopodites R' and L' bear crushing teeth. The figure makes it fairly
+certain also that the limb affected was a crushing claw. Accepting this
+interpretation, we reach the remarkable conclusion that the bud of new
+growth consisted of halves differentiated into cutter and crusher as the
+normal claws are, and that the extra crusher is geometrically a left
+but physiologically a right. Though shaped as a left in respect of the
+direction in which it points, the extra crusher is really an optically
+reversed right, while the dactylopodite R, which is placed pointing like
+a right, is really a reversed left (Fig. 12).
+
+[Illustration: FIG. 12. Right claw of lobster bearing a pair of extra
+dactylopodites (after van Beneden). The fine toothing on R suggests that
+this is part of a cutting claw, though the limb bearing it is a crusher.]
+
+If these indications are reliable[9] and are established by further
+observation we shall be led to the conclusion that the bud which
+becomes an extra pair of limbs does not merely contain the parts proper
+to the side on which it grows, but is comparable with the original
+zygotic cell, and consists not simply of two halves, but of two halves
+differentiated as a right and a left like the two halves of the normal
+body.
+
+Phenomena of this kind, evoked by mutilation or injury, together with
+the cognate observations on regeneration throw very curious lights
+on the nature of living things. To an understanding of the nature of
+the mechanics of living matter and its relation to matter at large
+they offer the most hopeful line of approach. I allude especially to
+the examples in which it has been established that the part which is
+produced after mutilation is a structure different from that which was
+removed. The term "regeneration" was introduced before such phenomena
+were discovered, and though every one recognizes its inapplicability
+to these remarkable cases, the word still misleads us by presenting
+a wrong picture to the mind. The expression "heteromorphosis" (Loeb)
+has been appropriately applied to various phenomena of this kind, and
+Morgan has given the name "morphallaxis" to another group of cases in
+which the renewal occurs by the transformation of a previously existing
+part.[10] But we must continually remember that all these occurrences
+which we know only as abnormalities and curiosities must in reality be
+exemplifications of the normal mechanics of division and growth. The
+conditions needed to call them forth are abnormal, but the responses
+which the system makes are evidences of its normal constitution. When
+therefore, for example, the posterior end of a worm produces a reversed
+tail from its cut end we have a proof that there must be in the normal
+body forces ready to cause this outgrowth. The new structure is not
+an ill-shaped head-end, for, as Morgan shows, the nephridial ducts
+have their funnels perforating the segments in a reversed direction.
+The "tension" of growth is actually reversed.[11] So also when in a
+Planarian amputation of the body immediately behind the head leads to
+the formation of a new reversed head at the back of the normal head,
+while amputation further back leads to the regeneration of a new tail,
+these responses give indications of forces normally present in the
+body of the Planarian. Such facts open up a great field of speculation
+and research. Especially important it would be to determine where the
+critical region may be at which the one response is replaced by the
+other. I suppose it is even possible that there is some neutral zone in
+which neither kind of response is made.
+
+Physical parallels to the phenomena of regeneration are not easy
+to find and we still cannot penetrate beyond the empirical facts.
+Przibram has laid stress on the general resemblance between the new
+growth of an amputated part in an animal and the way in which a broken
+crystal repairs itself when placed in the mother-solution. That the
+two processes have interesting points of likeness cannot be denied.
+It must however never be forgotten that there is one feature strongly
+distinguishing the two; for I believe it is universally recognized
+by physicists that all the phenomena of geometrical regularity which
+crystals display are ultimately dependent on the forms of the particles
+of the crystalline body. This cannot in any sense be supposed to hold
+in regard to protoplasm or its constituents. The definiteness of
+crystals is also an unlikely guide for the reason that it is absolute
+and perfect, or in other words because this kind of regularity cannot
+be disturbed at all without a change so great that the substance itself
+is altered; whereas we know that the forms of living things are capable
+of such changes, great and small, that we must regard perfection of
+form, whether manifested in symmetry or in number, as an ideal which
+will only be produced in the absence of disturbance. The symmetry of the
+living things is like the symmetry of the concentric waves in a pool
+caused by a splash. Perfect circles are made only in the imaginary case
+of mathematical uniformity, but the system maintains an approximate
+symmetry though liable to manifold deformation.
+
+Since the geometrical order of the living body cannot be a direct
+function of the materials it must be referred to some more proximate
+control. In renewing a part the body must possess the power of seizing
+particles of many dissimilar kinds, and whirl them into their several
+and proper places. The action in renewal, like that of original growth,
+may be compared--very crudely--with the action of a separator which
+simultaneously distributes a variety of heterogeneous materials in an
+orderly fashion; but in the living body the thing distributed must
+rather be the _appetency_ for special materials, not the materials
+themselves.
+
+If the analogy of crystals be set aside and we seek for other parallels
+to regeneration there are none very obvious. I have sometimes wondered
+whether it might not be possible to institute a fruitful comparison
+between the renewal of parts and the reformation of waves of certain
+classes after obliteration. In several respects, as I have already said,
+some curious resemblances with the repetitions formed by wave-motion
+are to be traced in our organic phenomena, and though admitting that
+I cannot develop these comparisons, I think nevertheless they may be
+worth bearing in mind. When, after obliteration, an eddy in a stream,
+or a ripple-mark (a more complex case of eddy-formation) in blown sand
+is re-formed, we have an example in which pattern is reconstituted and
+growth takes place not by virtue of the composition of the materials--in
+this case the water or the sand--but by the way in which they are acted
+upon by extraneous forces.
+
+A feature in the actual mode by which ripple-marks are reconstituted may
+not be without interest in connexion with our phenomena of regeneration.
+When, for example, the wind is blowing steadily over a surface of fine,
+dry sand, the familiar ripple-marks are formed by a heaping of the sand
+in lines transverse to the direction of the wind. The heaping is due to
+the formation of eddies corresponding with positions of instability.
+When the wind is steady and the sand homogeneous, the distances between
+the ripples, or wave-lengths, are sensibly equal. If while the wind
+continues to blow, the ripples are obliterated with a soft brush they
+will quickly be re-formed over the whole area, but I have noticed that
+at first their wave-length is approximately half that of the ripples
+in the undisturbed parts of the system.[12] The normal wave-length is
+restored by the gradual accentuation of alternate ripples. Of course
+the sand-ripples are in reality slowly travelling forward in the
+direction towards which the wind is blowing, and for this our living
+segmentations afford no obvious parallel, but the appearances in the
+area of reformation, and especially the forking of the old ridges where
+they join the new ones, are curiously reminiscent of the irregularities
+of segmentation seen in regenerated structures. The value of the
+considerations adduced in the chapter is, I admit, very small. The
+utmost that can be claimed for them is that mechanical segmentations,
+like those seen in ripple-mark, or in Leduc's osmotic growths, show
+how by the action of a continuous force in one direction, repeated
+and serially homologous divisions can be produced having features of
+similarity common to those repetitions by which organic forms and
+patterns are characterised. The analogy supplies a vicarious picture of
+the phenomena which in default of one more true may in a slight degree
+assist our thoughts. It suggests that the rhythms of segmentation may be
+the consequence of a single force definite in direction and continuously
+acting during the time of growth. The polarity of the organism would
+thus be the expression of the fact that this meristic force is
+definitely directed after it has once been excited, and the reversal
+seen in some products of regeneration suggest further that it is capable
+of being reflected. This polarity cannot be a property of the material,
+as such, but is determined by a force acting on that material, just as
+the polarity of a magnet is not determined by the arrangement of its
+particles, but by the direction in which the current flows.
+
+To some it may appear that even to embark on such discussions as this is
+to enter into a perilous flirtation with vitalistic theories. How, they
+may ask, can any force competent to produce chemical and geometrical
+differentiation in the body be distinguished from the "Entelechy" of
+Driesch? Let me admit that in this reflexion there is one element of
+truth. If those who proclaim a vitalistic faith intend thereby to
+affirm that in the processes by which growth and division are effected
+in the body, a part is played by an orderly force which we cannot
+_now_ translate into terms of any known mechanics, what observant man
+is not a vitalist? Driesch's first volume, putting as it does into
+intelligible language that positive deduction from the facts--especially
+of regeneration--should carry a vivid realisation of this truth to any
+mind. If after their existence is realised, it is desired that these
+unknown forces of order should have a name, and the word entelechy is
+proposed, the only objection I have to make is that the adoption of a
+term from Aristotelian philosophy carries a plain hint that we propose
+to relegate the future study of the problem to metaphysic.
+
+From this implication the vitalist does not shrink. But I cannot find in
+the facts yet known to us any justification of so hopeless a course. It
+was but yesterday that the study of _Entwicklungsmechanik_ was begun,
+and if in our slight survey we have not yet seen how the living machine
+is to be expressed in terms of natural knowledge that is poor cause for
+despair. Driesch sums up his argument thus:[13]
+
+"It seems to me that there is only one conclusion possible. If we are
+going to explain what happens in our harmonious-equipotential systems
+by the aid of causality based upon the constellation of single chemical
+factors and events, there _must_ be some such thing as a machine. Now
+the assumption of the existence of a machine proves to be absolutely
+absurd in the light of the experimental facts. _Therefore there can
+be neither any sort of a machine nor any sort of causality based upon
+constellation underlying the differentiation of harmonious-equipotential
+systems._"
+
+"For a machine, typical with regard to the three chief dimensions
+of space, cannot remain itself if you remove parts of it or if you
+rearrange its parts at will."
+
+To the last clause a note is added as follows:
+
+"The pressure experiments and the dislocation experiments come into
+account here; for the sake of simplicity they have not been alluded to
+in the main line of our argument."
+
+I doubt whether any man has sufficient knowledge of all possible
+machines to give reality to this statement. In spite also of the
+astonishing results of experiments in dislocation, doubt may further be
+expressed as to whether they have been tried in such variety or on such
+a scale as to justify the suggestion that the living organism remains
+itself if its parts are rearranged at will. All we know is that it can
+"remain itself" when much is removed, and when much rearrangement has
+been affected, which is a different thing altogether.
+
+I scarcely like to venture into a region of which my ignorance is so
+profound, but remembering the powers of eddies to re-form after partial
+obliteration or disturbance, I almost wonder whether they are not
+essentially machines which remain themselves when parts of them are
+removed.
+
+Real progress in this most obscure province is not likely to be made
+till it attracts the attention of physicists; and though they for long
+may have to forego the application of exact quantitative methods, I
+confidently anticipate that careful comparison between the phenomena
+of repetition formed in living organisms and the various kinds of
+segmentation produced by mechanical agencies would be productive of
+illuminating discoveries.
+
+FOOTNOTES:
+
+[1] Stéphane Leduc, _Théorie Physico-Chymique de la Vie_, Paris, 1910.
+
+[2] _Materials for the Study of Variation_, No. 249, p. 217; and p. 272.
+
+[3] _Materials_, p. 118.
+
+[4] Church, A. H., _On the Relation of Phyllotaxis to Mechanical Laws_,
+London, 1904.
+
+[5] It is a question whether the dominance of the palmatifid leaf over
+the pinnatifid is not really an example of the dominance of a lower
+number of segmentations over a higher. From the uncertainty whether two
+given leaves of two separate plants are actually comparable one cannot
+institute quite satisfactory numerical comparisons, but I think the view
+that the "Fern" leaf has more lobes than an otherwise similar "Palm"
+leaf may be fairly maintained. If this be admitted, the "Palm" leaf
+represents the dominant low number and its round shape is a consequence
+of the greater powers of growth which are so often possessed by the
+members of a shorter series.
+
+[6] It is perhaps of importance to remember that in certain species
+of bacteria (e. g. _Bacillus Anthracis_) division may cease where the
+organism is cultivated under certain artificial conditions though growth
+continues. In this way very long unsegmented threads are produced.
+
+[7] _Arch. f. Entwm._, XX, 1905, p. 76; _Sitzungsb. d. Ges. Naturf._,
+Berlin, 1907, p. 41, etc.
+
+[8] Borradaile, L. A., _Jour. Marine Zool._, 1897, No. 8.
+
+[9] Dr. Przibram, I should mention, concludes that on the whole the
+facts are against this interpretation, but as more evidence is certainly
+required, I call attention to the possibility.
+
+[10] Morgan, T. H., _Regeneration_, 1901.
+
+[11] It would be interesting to know whether growth continues at the
+original posterior end after the new "posterior" end has been formed in
+front.
+
+[12] In the actual case observed, the ripples unsmoothed had a
+wave-length of about 2-1/2 inches; and when the new ones were first
+formed, there were about 30 ridges in the length originally traversed by
+15 or 16.
+
+[13] _The Science and Philosophy of the Organism_; Gifford Lectures,
+1907. London, 1908, p. 141.
+
+
+
+
+                       CHAPTER IV
+
+       THE CLASSIFICATION OF VARIATION AND THE NATURE OF
+                   SUBSTANTIVE FACTORS
+
+We have now seen that among the normal physiological processes the
+phenomena of division form a recognisable, and in all likelihood a
+naturally distinct group. Variations in these respects may thus be
+regarded as constituting a special class among variations in general.
+
+The substantive variations have only one property in common--the
+negative one that they are not Meristic. The work of classifying them
+and distinguishing them according to their several types demands a
+knowledge of the chemistry of life far higher than that to which science
+has yet attained. In reference to some of the simplest variations Garrod
+has introduced the appropriate term "Chemical sports." The condition in
+man known as Alkaptonuria in which the urine is red is due especially
+to the absence of the enzyme which decomposes the excretory substance,
+alkapton. The "chemical sport" here consists in the inability to break
+up the benzene ring. The chemical feature which distinguishes and is the
+proximate cause of several colour-varieties can now in a few cases be
+declared. The work of Miss Wheldale has shown that colour-varieties may
+be produced by the absence of the chromogen compound the oxidation of
+which gives rise to sap-colours, by differences in the completeness of
+this process of oxidation, and by a process of reduction supervening on
+or perhaps suppressing the oxidation. Some of these processes moreover
+may be brought about by the combined action of two bodies, the one an
+enzyme, for example an oxygenase, and the other a substance regarded as
+a peroxide, contributing the oxygen necessary for the oxidation to take
+place. Variation in colour may thus be brought about by the addition or
+omission of any one of the bodies concerned in the action.
+
+Similar variations, or rather similar series of variations will
+undoubtedly hereafter be identified in reference to all the various
+kinds of chemical processes upon which the structure and functions of
+living things depend. The identification of these processes and of
+the bodies concerned in them will lead to a real classification of
+Substantive Variations.
+
+To forecast the lines on which such classification will proceed is to
+look too far ahead. We may nevertheless anticipate with some confidence
+that future analysis will recognise among the contributing elements,
+some which are intrinsic and inalienable, and others which are extrinsic
+and superadded.
+
+We already know that there may be such interdependence among the
+substantive characters that to disentangle them will be a work of
+extreme difficulty. The mere fact that in our estimation characters
+belong to distinct physiological systems is no proof of their actual
+independence. In illustration may be mentioned the sap-colour in Stocks
+and the development of hoariness on the leaves and stems, which Miss
+Saunders's experiments have shown to be intimately connected, so that
+in certain varieties no hoariness is produced unless the elements for
+sap-colour are already present in the individual plant.
+
+The first step in the classification of substantive variations is
+therefore to determine which are due to the addition of new elements
+or factors, and which are produced by the omission of old ones. _A
+priori_ there is no valid criterion by which this can be known, and
+actual experiments in analytical breeding can alone provide the
+knowledge required. Some very curious results have by this method been
+obtained, which throw an altogether unexpected light on these problems.
+For example, in order that the remarkable development of mesoblastic
+black pigment characteristic of the Silky Fowl should be developed, it
+is practically certain that two distinct variations from such a type
+as _Gallus bankiva_ must have occurred. I assume, as is reasonable,
+that _G. bankiva_ has genetic properties similar to those of the Brown
+Leghorn breed which has been used in the experiments which Mr. Punnett
+and I have conducted. _Gallus bankiva_ was not available but the Brown
+Leghorn agrees with it very closely in colouration, and probably in
+the general physiology of its pigmentation. Setting aside the various
+structural differences between the two breeds, the Silky is immediately
+distinguished from the Leghorn by the fact that the skin of the whole
+body including that of the face and comb appears to be of a deep
+purplish colour. The face and comb of the Leghorn are red and the skin
+of the body is whitish yellow. On examination it is found that the
+purple colour of the Silky is in reality due to the distribution of a
+deep black pigment in the mesoblastic membranes throughout the body. The
+somatopleura, the pleura, _pia mater_, the dermis, and in most organs
+the connective tissue and the sheaths of the blood-vessels, are thus
+impregnated with black. No such pigmentation exists in the Leghorn.
+As the result of an elaborate series of experimental matings we have
+proved that the distinction between the Leghorn and the Silky consists
+primarily in the fact that the Silky possesses a pigment-producing
+factor, _P_, which is not present in the Leghorn.
+
+This variation must undoubtedly have been one of _addition_. But besides
+this there is another difference of an altogether dissimilar nature; for
+the Brown Leghorn possesses a factor which has the power of partially or
+completely restricting the operation of the pigment-producing factor,
+_P_. Moreover in respect of this pigment-restricting factor which we
+may call _D_, the sexes of the Brown Leghorn differ, for the male is
+homozygous or _DD_, but the female is heterozygous, _Dd_. Thus in order
+that the black-skinned breed could be evolved from such a type as a
+Brown Leghorn it must be necessary _both_ that _P_ should be added and
+that _D_ should drop out. We have not the faintest conception of the
+process by which either of these events have come to pass, but there is
+no reasonable doubt that in the evolution of the Silky fowl they did
+actually happen.
+
+We may anticipate that numerous interdependences of this kind will be
+discovered.
+
+Before any indisputable progress can be made with the problem of
+evolution it is necessary that we should acquire some real knowledge
+of the genesis of that class of phenomena which formed the subject of
+the last chapter. So long as the process of division remains entirely
+mysterious we can form no conception even of the haziest sort as
+to the nature of living organisms, or of the proximate causes which
+determine their forms, still less can we attempt any answer to those
+remoter questions of origin and destiny which form the subject of the
+philosopher's contemplation. It is in no spirit of dogmatism that I
+have ventured to indicate the direction in which I look for a solution,
+though I have none to offer. It may well be that before any solution is
+attained, our knowledge of the nature of unorganised matter must first
+be increased. For a long time yet we may have to halt, but we none
+the less do well to prepare ourselves to utilise any means of advance
+that may be offered, by carefully reconnoitering the ground we have to
+traverse. The real difficulty which blocks our progress is ignorance of
+the nature of division, or to use the more general term, of repetition.
+
+Let us turn to the more familiar problem of the causes of variation. Now
+since variation consists as much in meristic change as in alteration in
+substance or material, there is one great range of problems of causation
+from which we are as yet entirely cut off. We know nothing of the
+causation of division, and we have scarcely an observation, experiment
+or surmise touching the causes by which the meristic processes may be
+altered.
+
+Of the way in which variations in the substantive composition of
+organisms are caused we have almost as little real evidence, but we are
+beginning to know in what such variations must consist. These changes
+must occur either by the addition or loss of factors.
+
+We must not lose sight of the fact that though the factors operate by
+the production of enzymes, of bodies on which these enzymes can act, and
+of intermediary substances necessary to complete the enzyme-action, yet
+these bodies themselves can scarcely be themselves genetic factors, but
+consequences of their existence. What then are the factors themselves?
+Whence do they come? How do they become integral parts of the organism?
+Whence, for example, came the power which is present in a White Leghorn
+of destroying--probably reducing--the pigment in its feathers? That
+power is now a definite possession of the breed, present in all its
+germ-cells, male and female, taking part in their symmetrical divisions,
+and passed on equally to all as much as is the protoplasm or any other
+attribute of the breed. From the body of the bird the critical and
+efficient substance could in all likelihood be isolated by suitable
+means, just as the glycogen of the liver can be. But even when this
+extraction has been accomplished and the reducing body isolated, we
+shall know no more than we did before respecting the mode by which
+the power to produce it was conferred on the fowl, any more than we
+know how the walls of its blood-vessels acquired the power to form a
+fibrin-ferment.
+
+It is when the scope of such considerations as this are fully grasped
+that we realise the fatuousness of the conventional treatment which the
+problem of the causes of variation commonly receives. Environmental
+change, chemical injury, differences in food supply, in temperature,
+in moisture, or the like have been proposed as "causes." Admitting
+as we must do, that changes may be produced--usually inhibitions of
+development--by subjecting living things to changes in these respects,
+how can we suppose it in the smallest degree likely that very precise,
+new, and adaptative powers can be conferred on the germs by such
+treatment? Reports of positive genetic consequences observed comparable
+with those I have mentioned, become from time to time current. We
+should I think regard them with the gravest doubt. Few, so far as I am
+aware, have ever been confirmed, though clear and repeated confirmation
+should be demanded before we suffer ourselves at all to build upon such
+evidence. In a subsequent chapter some of these cases will be considered
+in detail.
+
+In no class of cases would the transmission of an acquired character
+superficially appear so probable as in those where power of resisting
+the attack of a pathogenic organism is acquired in the lifetime of
+the zygote. The possession of such a power is moreover a distinction
+comparable with those which differentiate varieties and species. It
+is due to the development in the blood of specific substances which
+pervade the whole fluid. This development is exactly one of those
+"appropriate responses to stimuli" which naturalists who incline to
+regard adaptation as a direct consequence of an environmental influence
+might most readily invoke as an illustration of their views. And yet all
+evidence is definitely unfavourable to the suggestion of an inheritance
+of the acquired power of resistance. Such change as can be perceived
+in the virulence of the attacks on successive generations may be most
+easily regarded as due to the extermination of the more susceptible
+strains, and perhaps in some measure to variation in the invading
+organisms themselves, an "acquired character" of quite different import.
+
+The specific "anti-body" may have been produced in response to the
+stimulus of disease, but the power to produce it without this special
+stimulus is not included in the germ-cells any more than a pigment.
+All that they bear is the _power to produce_ the anti-bodies when the
+stimulus is applied.
+
+If we could conceive of an organism like one of those to which disease
+may be due becoming actually incorporated with the system of its host,
+so as to form a constituent of its germ-cells and to take part in the
+symmetry of their divisions, we should have something analogous to the
+case of a species which acquires a new factor and emits a dominant
+variety. When we see the phenomenon in this light we realise the
+obscurity of the problem. The appearance of recessive varieties is
+comparatively easy to understand. All that is implied is the omission of
+a constituent. How precisely the omission is effected we cannot suggest,
+but it is not very difficult to suppose that by some mechanical fault
+of cell-division a power may be lost. Such variation by unpacking,
+or analysis of a previously existing complex, though unaccountable,
+is not inconceivable. But whence come the new dominants? Whether we
+imagine that they are created by some rearrangement or other change
+internal to the organism, or whether we try to conceive them as due to
+the assumption of something from without we are confronted by equally
+hopeless difficulty.
+
+The mystery of the origin of a dominant increases when it is realised
+that there is scarcely any recent and authentic account of such an event
+occurring under critical observation, which can be taken as a basis for
+discussion. The literature of horticulture for example abounds in cases
+alleged, but I do not think anyone can produce an illustration quite
+free from doubt. Such evidence is usually open to the suspicion that the
+plant was either introduced by some accident, or that it arose from a
+cross with a pre-existing dominant, or that it owed its origin to the
+meeting of complementary factors. In medical literature almost alone
+however, there are numerous records of the spontaneous origin of various
+abnormal conditions in man which habitually behave as dominants, and of
+the authenticity of some of these there can be no doubt.
+
+When we know that such conditions as hereditary cataract or various
+deformities of the fingers behave as dominants, we recognize that
+those conditions must be due to the addition of some element to the
+constitution of the normal man. In the collections of pedigrees relating
+to such pathological dominants there are usually to be found alleged
+instances of the origin of the condition _de novo_. Not only do these
+records occur with such frequency that they cannot be readily set aside
+as errors, but from general considerations it must be obvious that as
+these malformations are not common to normal humanity they must at
+some moment of time have been introduced. The lay reader may not be
+so much impressed with the difficulty as we are. He is accustomed to
+regard the origin of _any_ new character as equally mysterious, but when
+once dominants are distinguished from recessives the problem wears a
+new aspect. Thus the appearance of high artistic gifts, whether as an
+attribute of a race or as a sporadic event among the children of parents
+destitute of such faculties, is not very surprising, for we feel fairly
+sure that the faculty is a recessive, due to the loss of a controlling
+or inhibiting factor; but the _de novo_ origin of brachydactylous
+fingers in a child of normal parents is of quite a different nature, and
+must indicate the action of some new specific cause.
+
+Whether such evidence is applicable to the general problem of evolution
+may with some plausibility be questioned; but there is an obvious
+significance in the fact that it is among these pathological occurrences
+that we meet with phenomena most nearly resembling the spontaneous
+origin of dominant factors, and I cannot see such pedigrees as these
+without recalling Virchow's aphorism that every variation owes its
+origin to some pathological accident. In the evolution of domestic
+poultry, if _Gallus bankiva_ be indeed the parent form of all our
+breeds, at least some half dozen new factors must have been added during
+the process. In _bankiva_ there is, for example, no factor for rose
+comb, pea comb, barring on the feathers, or for the various dominant
+types of dark plumage. Whence came all these? It is, I think, by no
+means impossible that some other wild species now extinct did take
+part in the constitution of domestic poultry. It seems indeed to me
+improbable that the heavy breeds descend from _bankiva_. Both in regard
+to domestic races of fowls, pigeons, and some other forms, the belief in
+origin within the period of human civilization from one simple primitive
+wild type seems on a balance of probabilities insecurely founded, but
+allowing something for multiplicity of origin we still fall far short
+of the requisite total of factors. Elements exist in our domesticated
+breeds which we may feel with confidence have come in since their
+captivity began. Such elements in fowls are dominant whiteness, extra
+toe, feathered leg, frizzling, etc., so that even hypothetical extension
+of the range of origin is only a slight alleviation of the difficulty.
+
+Somehow or other, therefore, we must recognize that dominant factors
+do arise. Whether they are created by internal change, or whether, as
+seems to me not wholly beyond possibility, they obtain entrance from
+without, there is no evidence to show. If they were proved to enter from
+without, like pathogenic organisms, we should have to account for the
+extraordinary fact that they are distributed with fair constancy to half
+the gametes of the heterozygote.
+
+In proportion as the nature of dominants grows more clear so does it
+become increasingly difficult to make any plausible suggestion as to
+their possible derivation. On the other hand the origin of a recessive
+variety by the loss of a factor is a process so readily imagined that
+our wonder is rather that the phenomenon is not observed far more
+often. Some slip in the accurate working of the mechanical process of
+division, and a factor gets left out, the loss being attested by the
+appearance of a recessive variety in some subsequent generation.
+
+Consistently with this presentation of the facts we find that, as in our
+domesticated animals and plants, a diversity of recessives may appear
+within a moderately short period, and that when variations come they
+often do not come alone. Witness the cultural history of the Sweet Pea,
+_Primula Sinensis_, _Primula obconica_, _Nemesia strumosa_ and many such
+examples in which variation when it did come was abundant. The fact
+cannot be too often emphasized that in the vast proportion of these
+examples of substantive variation under domestication, as well as of
+substantive variation in the natural state, the change has come about by
+omission, not by addition. To take, for example, the case of the Potato,
+in which so many spontaneous bud-variations have been recorded, East
+after a careful study of the evidence has lately declared his belief
+that all are of this nature, and the opinion might be extended to many
+other groups of cases whether of bud or seminal variation. Morgan draws
+the same conclusion in reference to the many varieties he has studied in
+_Drosophila_.
+
+In the Sweet Pea, a form which is beyond suspicion of having been
+crossed with anything else, and has certainly produced all the multitude
+of types which we now possess by variations from one wild species,
+there is only one character of the modern types which could, with any
+plausibility, be referred to a factor not originally forming part
+of the constituents of the wild species. This is the waved edge, so
+characteristic of the "Spencer" varieties; for the cross between a
+smooth-edged and a waved type gives an intermediate not unfrequently.
+Nevertheless there is practically no doubt that this is merely an
+imperfection in the dominance of the smooth edge, and we may feel sure
+that any plant homozygous for smooth edge would show no wave at all.
+Hence it is quite possible that even the appearance of the original
+waved type, Countess Spencer, was due to the loss of one of the factors
+for smooth edge at some time in the history of the Sweet Pea.
+
+In the case of the Chinese Primrose (_Primula Sinensis_) one dominant
+factor has been introduced in modern times, probably within the last six
+years at most. This is the factor which causes suppression of the yellow
+eye, giving rise to the curious type known as "Queen Alexandra." Mr. R.
+P. Gregory's experiments proved that this was a very definite dominant,
+and the element responsible for this development is undoubtedly an
+addition to the original ingredient-properties, with which the species
+was endowed. Unfortunately, as happens in almost every case of the kind,
+the origin of this important novelty appears to be lost. Its behaviour,
+however, when crossed with various other types is that of a simple
+dominant giving an ordinary 3:1 ratio. There is therefore no real doubt
+that it came into existence by the definite addition of a new factor,
+for if it was simply a case of the appearance of a new character made by
+combination of two previously existing complementary factors we should
+expect that when Queen Alexandra was self-fertilised a 9:7 ratio would
+be a fairly common result, which is not in practice found.
+
+In _Oenothera_ Gates[1] has observed the appearance, in a large
+sowing of about 1,000 _Oenothera rubrinervis_, of a single individual
+having considerably more red pigment in the calyx than is usual in
+_rubrinervis_. The whole of the hypanthium in the flowers of this
+plant was red instead of green as in _rubrinervis_, and the whole of
+the sepals were red in the bud-stage, except for small green areas at
+the base. This type behaved as a dominant over _rubrinervis_, but so
+far a pure-breeding individual was not found. Admittedly the variation
+of this plant from the type of _rubrinervis_ can be represented as
+one of degree, though there is a very sensible gap in the series
+between the new form which Gates names "_rubricalyx_" and the reddest
+_rubrinervis_ seen in his cultures. It must certainly be recognised as
+a new dominant. Gates, rightly as I consider, regards the distinction
+between _rubrinervis_ and _rubricalyx_ as a quantitative one, and the
+same remark applies to certain other types differing in the amount
+of anthocyanin which they produce. I do not understand the argument
+which Gates introduces to the effect that the difference between
+such quantitative types cannot be represented in terms of presence
+and absence. We are quite accustomed to the fact that in the rabbit
+self-colour segregates from the Dutch-marked type. These two types
+differ in a manner which we may reasonably regard as quantitative. It
+is no doubt possible that the self-coloured type contains an ingredient
+which enables the colour to spread over the whole body, but it is, I
+think, perhaps more easy to regard the Dutch type as a form from which a
+part of the colour is absent. It may be spoken of in terms I have used,
+as a _subtraction-stage_ in colour. Following a similar method we may
+regard _rubricalyx_ as an addition-stage in colour-variation. The fact
+that crosses between _rubrinervis_, or _rubricalyx_ and _Lamarckiana_
+give a mixture of types in F_{1}, does not I think show, as Gates
+declares, that there is any system here at work to which a factorial or
+Mendelian analysis does not apply; but that question may be more fitly
+discussed in connexion with the other problems raised by the behaviour
+of _Oenothera_ species in their crosses.
+
+I do, however, feel that, interesting as this case must be admitted to
+be, we cannot quite satisfactorily discuss it as an illustration of
+the _de novo_ origin of a dominant factor. The difference between the
+novelty and the type is quantitative, and it is not unreasonable to
+think of such a difference being brought about by some "pathological
+accident" in a cell-division.
+
+Recognition of the distinction between dominant and recessive characters
+has, it must be conceded, created a very serious obstacle in the way
+of any rational and concrete theory of evolution. While variations
+of all kinds could be regarded as manifestations of some mysterious
+instability of organisms this difficulty did not occur to the mind of
+evolutionists. To most of those who have taken part in genetic analysis
+it has become a permanent and continual obsession. With regard to the
+origin of recessive variations, there is, as we have seen, no special
+difficulty. They are negative and are due to absences, but as soon as it
+is understood that dominants are caused by an addition we are completely
+at a loss to account for their origin, for we cannot surmise any source
+from which they may have been derived. Just as when typhoid fever breaks
+out in his district the medical officer of health knows for certain that
+the bacillus of typhoid fever has by some means been brought into that
+district so do we know that when first dominant white fowls arose in the
+evolution of the domestic breeds, by some means the factor for dominant
+whiteness got into a bird, or into at least one of its germ-cells.
+Whence it came we cannot surmise.
+
+Whether we look to the outer world or to some rearrangement within the
+organism itself, the prospect of finding a source of such new elements
+is equally hopeless.
+
+Leaving this fundamental question aside as one which it is as yet quite
+unprofitable to discuss, we are on safe ground in foreseeing that the
+future classification of substantive variations, which genetic research
+must before long make possible, will be based on a reference to the
+modes of action of the several factors. Some will be seen to produce
+their effects by oxidation, some by reduction, some by generating
+substances of various types, sugars, enzymes, activators, and so forth.
+It may thus be anticipated that the relation of varieties to each other
+and to types from which they are derived will be expressible in terms
+of definite synthetical formulae. Clearly it will not for an indefinite
+time be possible to do this in practice for more than a few species
+and for characters especially amenable to experimental tests, but as
+soon as the applicability of such treatment is generally understood the
+influence on systematics must be immediate and profound, for the nature
+of the problem will at length be clear and, though the ideal may be
+unattainable, its significance cannot be gainsaid.
+
+                 *       *       *       *       *
+
+_Note._--With hesitation I allow this chapter to appear in the form in
+which it was printed a year ago, but in passing it for the press after
+that interval I feel it necessary to call attention to a possible line
+of argument not hitherto introduced.
+
+In all our discussions we have felt justified in declaring that the
+dominance of any character indicates that some factor is present which
+is responsible for the production of that character. Where there is no
+definite dominance and the heterozygote is of an intermediate nature
+we should be unable to declare on which side the factor concerned was
+present and from which side it was absent. The degree of dominance
+becomes thus the deciding criterion by which we distinguish the
+existence of factors. But it should be clearly realized that in any
+given case the argument can with perfect logic be inverted. We already
+recognize cases in which by the presence of an inhibiting factor a
+character may be suppressed and purely as a matter of symbolical
+expression we might apply the same conception of inhibition to any
+example of factorial influence whatever. For instance we say that in
+as much as two normal persons do not have brachydactylous children,
+there must be some factor in these abnormal persons which causes the
+modification. Our conclusion is based on the observed fact that the
+modification is a dominant. But it may be that normal persons are
+homozygous in respect of some factor _N_, which prevents the appearance
+of brachydactyly, and that in any one heterozygous, _Nn_, for this
+inhibiting factor, brachydactyly can appear. Similarly the round pea we
+say contains _R_, a factor which confers this property of roundness,
+without which its seeds would be wrinkled. But here we know that the
+wrinkled seed is in reality one having compound starch-grains, and
+that the heterozygote, though outwardly round enough, is intermediate
+in that starch-character. If we chose to say that the compoundness of
+the grains is due to a factor _C_ and that two doses of it are needed
+to make the seed wrinkled, I know no evidence by which such a thesis
+could be actually refuted. That such reasoning is seemingly perverse
+must be conceded; but when we consider the extraordinary difficulties
+which beset any attempt to conceive the mode of origin of a new dominant
+factor, we are bound to remember that there is this other line of
+argument which avoids that difficulty altogether. In the case of the
+"Alexandra"-eye in _Primula_, or the red calyx in Gates's _Oenothera_,
+inverting the reasoning adopted in the text, we may see that only the
+_Primula_ homozygous for the yellow eye can develop it and that two
+doses of the factor for the _rubrinervis_ calyx are required to prevent
+that part of the plant from being red.
+
+We may proceed further and extend this mode of reasoning to all cases
+of genetic variation, and thus conceive of all alike as due to loss of
+factors present in the original complex. Until we can recognize factors
+by means more direct than are provided by a perception of their effects,
+this doubt cannot be positively removed. For all practical purposes of
+symbolic expression we may still continue to use in our analyses the
+modes of representation hitherto adopted, but we must not, merely on
+the ground of its apparent perversity, refuse to admit that the line of
+argument here indicated may some day prove sound.
+
+FOOTNOTES:
+
+[1] Gates, R. R., _Zts. f. Abstammungslehre_, 1911, IV, pp. 341 and 361.
+
+
+
+
+                        CHAPTER V
+
+                    THE MUTATION THEORY
+
+When with the thoughts suggested in the last chapter we contemplate
+the problem of Evolution at large the hope at the present time of
+constructing even a mental picture of that process grows weak almost
+to the point of vanishing. We are left wondering that so lately men
+in general, whether scientific or lay, were so easily satisfied. Our
+satisfaction, as we now see, was chiefly founded on ignorance.
+
+Every specific evolutionary change must represent a definite event in
+the construction of the living complex. That event may be a disturbance
+in the meristic system, showing itself in a change in the frequency of
+the repetitions or in the distribution of differentiation among them, or
+again it may be a chemical change, adding or removing some factor from
+the sum total.
+
+If an attempt be made to apply these conceptions to an actual series
+of allied species the complexity of the problem is such that the mind
+is appalled. Ideas which in the abstract are apprehended and accepted
+with facility fade away before the concrete case. It is easy to imagine
+how Man was evolved from an _Amoeba_, but we cannot form a plausible
+guess as to how _Veronica agrestis_ and _Veronica polita_ were evolved,
+either one from the other, or both from a common form. We have not even
+an inkling of the steps by which a Silver Wyandotte fowl descended from
+_Gallus Bankiva_, and we can scarcely even believe that it did. The
+Wyandotte has its enormous size, its rose comb, its silver lacing, its
+tame spirit, and its high egg production. The tameness and the high egg
+production are probably enough both recessives, and though we cannot
+guess how the corresponding dominant factors have got lost, it is not
+very difficult to imagine that they were lost somehow. But the rose comb
+and the silver colour are _dominants_. The heavy weight also appears in
+the crosses with Leghorns, but we need not at once conclude that it
+depends on a simple dominant factor, because the big size of the crosses
+may be a consequence of the cross and may depend on other elements.
+
+Now no wild fowl known to us has these qualities. May we suppose that
+some extinct wild species had them? If so, may we again make the same
+supposition in all similar cases? To do so is little gain, for we are
+left with the further problem, whence did those lost wild species
+acquire those dominants? Suppositions of this kind help no more than
+did the once famous conjecture as to the origin of living things--that
+perhaps they came to earth on a meteorite. The unpacking of an
+original complex, the loss of various elements, and the recombination
+of pre-existing materials may all be invoked as sources of specific
+diversity. Undoubtedly the range of possibilities thus opened up is
+large. It will even cover an immense number of actual examples which
+in practice pass as illustrations of specific distinction. The Indian
+Rock pigeon which has a blue rump may quite reasonably be regarded as
+a geographically separated recessive form of our own _Columba livia_,
+for as Staples-Browne has shown the white rump of _livia_ is due to
+a dominant factor. The various degrees to which the leaves of Indian
+Cottons are incised have, as Leake says, been freely used as a means
+of classification. The diversities thus caused are very remarkable,
+and when taken together with diversities in habit, whether sympodial
+or monopodial, the various combinations of points of difference
+are sufficiently distinctive to justify any botanist in making a
+considerable number of species by reference to them alone. Nevertheless
+Leake's work goes far to prove that all of these forms represent the
+re-combinations of a very small number of factors. The classical example
+of _Primula Sinensis_ and its multiform races is in fact for a long
+way a true guide as to the actual interrelations of the species which
+systematists have made. That they did make them was due to no mistake
+in judgment or in principle, but simply to the want of that extended
+knowledge of the physiological nature of the specific cases which we now
+know to be a prime necessity.
+
+But will such analysis cover all or even most of the ordinary cases
+of specific diversity between near allies? Postponing the problem of
+the interrelations of the larger divisions as altogether beyond present
+comprehension, can we suppose, that in general, closely allied species
+and varieties represent the various consequences of the presence or
+absence of allelomorphic factors in their several combinations? The
+difficulty in making a positive answer lies in the fact that in most
+of the examples in which it has been possible to institute breeding
+experiments with a view to testing the question, a greater or less
+sterility is encountered. Where, however, no such sterility is met
+with, as for instance in the crosses made by E. Baur among the species
+of _Antirrhinum_ there is every reason to think that the whole mass of
+differences can and will eventually be expressed in terms of ordinary
+Mendelian factors. Baur has for example crossed species so unlike as
+_Antirrhinum majus_ and _molle_, forms differing from each other in
+almost every feature of organisation.[1] The F_{2} generation from this
+cross presents an amazingly motley array of types which might easily if
+met with in nature be described as many distinct species. Yet all are
+fertile and there is not the slightest difficulty in believing that they
+can all be reduced to terms of factorial analysis.
+
+If allowance be made for the complicating effects of sterility, is there
+anything which prevents us from supposing that such good species as
+those of _Veronica_ or of any other genus comprising well-defined forms
+may not be similarly related? I do not know any reason which can be
+pointed to as finally excluding such a possibility. Nevertheless it has
+been urged with some plausibility that good species are distinguished
+by _groups_ of differentiating characters, whereas if they were really
+related as the terms of a Mendelian F_{2} family are, we should expect
+to find not groups of characters in association, but rather series of
+forms corresponding to the presence and absence of the integral factors
+composing the groups of characters. I am not well enough versed in
+systematic work to be able to decide with confidence how much weight
+should be attached to this consideration. Some weight it certainly
+has, but I cannot yet regard it as forming a fatal objection to the
+application of factorial conceptions on the grand scale. It may be
+recalled that we are no longer under any difficulty in supposing that
+differences of all classes may be caused by the presence or absence of
+factors. It seemed at first for example that such characters as those of
+leaf shape might be too subtle and complex to be reducible to a limited
+number of factors. But first the work of Gregory on _Primula Sinensis_
+showed that several very distinct types of leaves were related to each
+other in the simplest way. In that particular example, intermediates are
+so rare as to be negligible, but subsequently Shull dealing with such
+a complicated example as _Capsella_, and Leake in regard to Cottons,
+both forms in which intergrades occur in abundance, have shown that a
+simple factorial scheme is applicable. We need not therefore, to take
+an extreme case, doubt that if it were possible to examine the various
+forms of fruit seen in the Squashes by really comprehensive breeding
+tests, even this excessive polymorphism in respect of structural
+features would be similarly reducible to factorial order.
+
+It must always be remembered also that in a vast number of cases, nearly
+allied forms which are distinct, occupy distinct ground. Moreover, by
+whatever of the many available mechanisms that end be attained, it is
+clear that nature very often does succeed in preventing intercrossing
+between distinct forms so far that the occurrence of that phenomenon is
+a rarity under natural conditions. The facts may, I think, fairly be
+summarized in the statement that species are on the whole distinct and
+not intergrading, and that the distinctions between them are usually
+such as might be caused by the presence, absence, or inter-combination
+of groups of Mendelian factors; but that they are so caused the evidence
+is not yet sufficient to prove in more than a very few instances.
+
+The alternative, be it explicitly stated, is not to return to the view
+formerly so widely held, that the distinctions between species have
+arisen by the accumulation of minute or insensible differences. The
+further we proceed with our analyses the more inadequate and untenable
+does that conception of evolutionary change become. If the differences
+between species have not come about by the addition or loss of factors
+one at a time, then we must suppose that the changes have been effected
+by even larger steps, and variations including groups of characters,
+must be invoked.
+
+That changes of this latter order are really those by which species
+arise, is the view with which de Vries has now made us familiar by his
+writings on the Mutation Theory. In so far as mutations may consist
+in meristic changes of many kinds and in the loss of factors it is
+unnecessary to repeat that we have abundant evidence of their frequent
+occurrence. That they may also more rarely occur by the addition of a
+factor we are, I think, compelled to believe, though as yet the evidence
+is almost entirely circumstantial rather than direct. The evidence for
+the occurrence of those mutations of higher order, by which new species
+characterized by several distinct features are created, is far less
+strong, and after the best study of the records which I have been able
+to make, I find myself unconvinced. The facts alleged appear capable of
+other interpretations.
+
+The most famous and best studied examples are of course the forms
+of _Oenothera_ raised by de Vries from _Oenothera Lamarckiana_ in
+circumstances well known to all readers of genetic literature. Whatever
+be the true significance of these extraordinary "mutations" there can
+be no question about the great interest which attaches to them, and the
+historical importance which they will long preserve. Apart also from
+these considerations it is becoming more and more evident that in their
+peculiarities they provide illustrations of physiological phenomena of
+the highest consequence in the study of genetics at large.
+
+De Vries found, as is well known, that _Oenothera Lamarckiana_ gives off
+plants unlike itself. These mutational forms are of several distinct
+and recognizable types which recur, and several of them breed true from
+their first appearance. The obvious difficulty, which in my judgment
+should make us unwilling at present to accept these occurrences as proof
+of the genesis of new species by mutation, is that we have as yet no
+certainty that the appearance of the new forms is not an effect of the
+recombination of factors, such as is to be seen in so many generations
+of plants derived from a cross involving many genetic elements. The
+first question is what is _Oenothera Lamarckiana_? Is it itself a plant
+of hybrid origin? To this fundamental question no satisfactory answer
+has yet been given. All attempts to find it as a wild plant in America
+have failed. It existed in Europe in the latter half of the eighteenth
+century. Whence it came is still uncertain, but the view that it came
+into existence in Europe and perhaps in Paris, seems on the whole the
+most probable. The question has been debated by Macdougal, Gates, and
+Davis. From historical sources there is little expectation of further
+light. Those who favour the notion of a hybrid origin look on _Oenothera
+biennis_ as one of the putative parents. It has been conjectured that
+a species called _grandiflora_ lately re-discovered on the Alabama
+river was the other parent. Experiments have been instituted by Davis
+to discover whether _Lamarckiana_ can be made artificially by crossing
+these two species. The results so far have shown that while plants
+approximating in various respects to _Lamarckiana_ have thus been
+produced, none agree exactly with that form. Davis, to whom reference
+should be made for a full account of the present state of the enquiry,
+points out that there are many strains of _biennis_ in existence and
+that it is by no means impossible that by using others of these strains
+a still closer approximation can be made. None of Davis's artificial
+productions as yet breed at all true, as _Lamarckiana_ on the whole
+does. In such a case, however, where several characters are involved,
+this is perhaps hardly to be expected.
+
+One feature of the _Oenotheras_ is very curious. Not only _Lamarckiana_,
+but all the allied species so far as I am aware, have a considerable
+proportion of bad and shrivelled pollen grains. This is undoubtedly true
+of species living in the wild state as well as of those in cultivation.
+I have had opportunities of verifying this for myself in the United
+States. No one looking at the pollen of an _Oenothera_ would doubt that
+it was taken from some hybrid plant exhibiting partial sterility. On the
+other hand, it is difficult to suppose that numbers, perhaps all, of
+the "species" of the genus are really hybrids, and many of them breed
+substantially true. I regard this constant presence of bad pollen grains
+as an indication that the genetic physiology of _Oenothera_ is in some
+way abnormal, and as we shall presently see, there are several other
+signs which point in the same direction.
+
+Discussion of the whole series of phenomena is rendered exceedingly
+difficult first, by reason of the actual nature of the material. The
+characteristics of many of the types which de Vries has named are
+evasive. A few of these types, for instance, _gigas_, _nanella_,
+_albida_, _brevistylis_, and perhaps a few more are evidently clear
+enough, but we have as yet no figures and descriptions precise enough
+to enable a reader to appreciate exactly the peculiarities of the vast
+number of forms which have now to be considered in any attempt to gain a
+comprehensive view of the whole mass of facts. It is also not in dispute
+that the forms are susceptible of great variations due simply to soil
+and cultural influences.
+
+The fact that no Mendelian analysis has yet been found applicable to
+this group of _Oenotheras_ as a whole is perhaps largely due to the fact
+that until recently such analysis has not been seriously attempted.
+Following the system which he had adopted before the rediscovery of
+Mendelism, or at all events, before the development of that method of
+analysis, de Vries has freely applied _names_ to special combinations of
+characters and has scarcely ever instituted a factorial analysis. Before
+we can get much further this must be attempted. It may fail, but we must
+know exactly where and how this failure comes about. There are several
+indications that such a recognition of factorial characters, could be
+carried some way. For example, the height, the size of the flowers, the
+crinkling of the leaves, the brittleness of the stems, perhaps even the
+red stripes on stems and fruits, and many more, are all characters which
+may or may not depend on distinct factors, but if such characters are
+really transmitted in unresolved groups, the limitations of those groups
+should be carefully determined. The free use of names for the several
+forms, rather than for the characters, has greatly contributed to deepen
+the obscurity which veils the whole subject.
+
+I do not mean to suggest that these _Oenotheras_ follow a simple
+Mendelian system. All that we know of them goes to show that there
+are curious complications involved. One of these, probably the most
+important of all, has lately been recognized by de Vries himself,
+namely, that in certain types the characters borne by the female and
+the male germ-cells of the same plant are demonstrably different. There
+can be little doubt that further research will reveal cognate phenomena
+in many unsuspected places. The first example in which such a state of
+things was proved to exist is that of the Stocks investigated by Miss
+Saunders.[2] By a long course of analysis she succeeded in establishing
+in 1908 the fact that if a plant of _Matthiola_ is of that eversporting
+kind which gives a large proportion of double-flowered plants among
+its offspring (produced by self-fertilisation), then the egg-cells of
+such a plant are mixed in type, but the pollen of the same plant is
+homogeneous. Some of the egg-cells have in them the two factors for
+singleness, but some of them are short of one or both of these factors.
+The pollen-grains, however, are all recessives, containing neither of
+these factors. The egg-cells, in other words, are mixed, "singles"
+and "doubles," while the pollen-grains are all "doubles." The same is
+true of the factor differentiating "white," or colourless plastids
+from cream-coloured plastids in _Matthiola_, the egg-cells being mixed
+"whites" and "creams," while the pollen-grains are all "creams," viz:
+recessives. Later in the same year (1908) de Vries[3] announced a
+remarkable case which will be discussed in detail subsequently. It
+relates to certain _Oenotheras_ heterozygous for dwarfness, in which (p.
+113) the ovules were mixed, tails and dwarfs, while the pollen is all
+dwarf.
+
+Again in _Petunia_ Miss Saunders's[4] work has shown that a somewhat
+similar state of things exists, but with this remarkable difference,
+that though the egg-cells are mixed, singles and doubles, the
+pollen-grains are all _singles_, viz: dominants. All the _Petunias_ yet
+examined have been in this condition, including some which in botanic
+gardens pass for original species. Whether actual wild plants from
+their native habitats are in the same state, is not yet known, but it
+is by no means improbable. The case may be compared with that of the
+moth _Abraxas grossulariata_ studied by Doncaster and Raynor, in which
+the females are all heterozygous, or we may almost say "hybrids" of
+_grossulariata_ and the variety _lacticolor_. Similarly we may say that
+at least garden Petunias are heterozygous in respect of singleness.
+The proof of this is of course that when fertilised with the pollen of
+doubles they throw a mixture of doubles and singles. The statements
+which de Vries has published regarding the behaviour of several of the
+_Oenotheras_ go far to show that they must have a somewhat similar
+organisation. On the present evidence it is still quite impossible to
+construct a coherent scheme which will represent all the phenomena in
+their interrelations, and among the facts are several which, as will
+appear, seem mutually incompatible. The first indication that the
+_Oenotheras_ may have either mixed ovules or mixed pollen appears in the
+fact that _Lamarckiana_ and several of its "mutants" used as males, with
+several other forms as females, give a mixed offspring. For example, de
+Vries (1907) found that
+
+          _biennis_ [F] × _Lamarckiana_ [M]
+          _biennis cruciata_ [F] × _Lamarckiana_ [M]
+          _muricata_ [F] × _Lamarckiana_ [M]
+          _biennis_ [F] × _rubrinervis_ [M]
+          _biennis cruciata_ [F] × _rubrinervis_ [M]
+
+all give a mixture of two distinct types which he names _laeta_ and
+_velutina_, consisting of about equal numbers of each. On account of the
+fact that the two forms are produced in association de Vries has called
+these forms "twin hybrids," a designation which is not fortunate, seeing
+that it is impossible to imagine that any kind of twinning is concerned
+in their production. The distinction between these two seems to be
+considerable, _laeta_ having leaves broader, bright green in colour, and
+flat, with pollen scanty, while _velutina_ has leaves narrower, grayish
+green, more hairy, and furrow-shaped, with pollen abundant.
+
+We next meet the remarkable fact that these two forms, _laeta_
+and _velutina_ breed true to their respective types, and do not
+reproduce the parent-types among their offspring resulting from
+self-fertilisation. This statement must be qualified in two respects.
+When _muricata_ [M] is fertilised by _brevistylis_ the forms _laeta_
+and _velutina_ are produced, but each of them subsequently throws the
+short-styled form as a recessive (de Vries, 1907, p. 406). It may be
+remembered that de Vries's previous publications had already shown that
+the short style of _brevistylis_, one of the _Lamarckiana_ "mutants,"
+behaves as a recessive habitually (_Mutationstheorie_, II, p. 178, etc.).
+
+Also when _nanella_, the dwarf "mutant" of _Lamarckiana_ is used as
+male on _muricata_ as female, _laeta_ and _velutina_ are produced, but
+one only of these, namely, _velutina_, subsequently throws dwarfs on
+self-fertilisation. The dwarfs thus thrown are said to form about 50 per
+cent. of the families in which they occur (de Vries, 1908, p. 668). The
+fact that the two forms, _laeta_ and _velutina_, are produced by many
+matings in which _Lamarckiana_ and its mutant _rubrinervis_ are used as
+males is confirmed abundantly by Honing, who has carried out extensive
+researches on the subject. After carefully reading his paper, I have
+failed to understand the main purport of the argument respecting the
+"double nature" of _Lamarckiana_ which he founds on these results, but
+I gather that in some way _laeta_ is shown to partake especially of the
+nature of _Lamarckiana_, while _velutina_ is a form of _rubrinervis_.
+The paper contains many records which will be of value in subsequent
+analysis of these forms.
+
+Before considering the possible meaning of these facts we must have
+in our minds the next and most novel of the recent extensions of
+knowledge as to the genetic properties of the _Oenotheras_. In the
+previous statement we have been concerned with the results of using
+either _Lamarckiana_ itself or one of its "mutants" _rubrinervis_,
+_brevistylis_, or _nanella_ as male, on one of the species _biennis_
+or _muricata_. The new experiments relate to crosses between the two
+species _biennis_ and _muricata_ themselves.
+
+De Vries found:
+
+1. That the reciprocal hybrids from these two species differed,
+_biennis_ × _muricata_ producing one type of F_{1} and _muricata_ ×
+_biennis_ producing another. Each F_{1} resembled the father more than
+the mother.
+
+2. That each of the hybrids so produced breeds true on
+self-fertilisation.
+
+3. That if we speak of the hybrid from _biennis_ × _muricata_ as _BM_
+and of the reciprocal as _MB_, then
+
+          _BM_ × _MB_
+
+gives exclusively offspring of _biennis_ type but that
+
+          _MB_ × _BM_
+
+gives exclusively offspring of _muricata_ type. Evidently, apart
+from all controversy as to the significance of the "mutants" of
+_Lamarckiana_, we have here a series of observations of the first
+importance.
+
+The fact that reciprocal crossings give constantly distinct results
+must be taken to indicate that the male and female sides of one, if not
+of both, of the parents are different in respect of characters which
+they bear. This is de Vries's view, and he concludes rightly, I think,
+that the evidence from all the experiments shows that both _biennis_
+and _muricata_ are in this condition, having one set of characters
+represented in their pollen-grains and another in their ovules. The
+plants breed true, but their somatic structures are compounded of the
+two sets of elements which pass into them from their maternal and
+paternal sides respectively. This possibility that species may exist of
+which the males really belong to one form and the females to another, is
+one which it was evident from the first announcement of the discovery of
+Mendelian segregation might be found realised in nature.[5]
+
+_Oe. biennis_ and _muricata_ were crossed reciprocally with each other
+and with a number of other species, and the behaviour of each, when
+used as mother, was consistently different from its behaviour when
+used as father. De Vries is evidently justified by the results of
+this series of experiments in stating that the "Bild," as he terms
+it, or composition of the male and female sides of these two species,
+_biennis_ and _muricata_, are distinct. On the evidence before us it
+is not, however, possible to form a perfectly clear idea of each, and
+until details are published, a reader without personal knowledge of the
+material cannot do more than follow the general course of the argument.
+For fuller comprehension a proper analysis of the characters with a
+clear statement of how they are distributed among the several types and
+crosses is absolutely necessary. According to de Vries the female of
+_biennis_ possesses a group of characters which he defines as "_conica_"
+in allusion to the shape of the flower-buds. Besides the conical buds,
+this group of features includes imperfect development of wood, rendering
+the plant very liable to attacks of _Botrytis_, and comparatively narrow
+leaves.
+
+The female of _muricata_ carries a group of features which he
+calls "_frigida_," and, though this is not quite explicitly stated
+in a definition of that type, it is to be inferred[6] that its
+characteristics are regarded as greater height, strong development of
+wood with comparative resistance to _Botrytis_, and broad leaves.
+
+The characters borne by the male parts of the two species are in general
+those by which they are outwardly distinguished. For example, the
+leaves of _Oe. biennis_ are comparatively broad and are bright green,
+while those of _muricata_ are much narrower and of a glaucous green,
+and I understand that de Vries regards these properties as contributed
+by the male side in each case and to be carried by the male cells of
+each species. The suggestion as regards _biennis_ and _muricata_ comes
+near the conception often expressed by naturalists in former times (_e.
+g._, Linnaeus) and not rarely entertained by breeders at the present
+day, that the internal structure is contributed by the mother and the
+external by the father.
+
+On the other hand, the offspring of each species when used as mother is
+regarded as possessing in the main the features of the maternal "Bild,"
+but the matter is naturally complicated by the introduction of features
+from the father's side, and it is here especially that the account
+provided is at present unsatisfactory and inconclusive. There seems,
+however, to be no serious doubt that _biennis_ and _muricata_ each in
+their outward appearance exhibit on the whole the features which their
+pollens respectively carry, and that the features borne by their ovules
+are in many respects distinct.
+
+The _types_ are thus "hybrids" which breed true. The results of
+intercrossing them each way are again "hybrids" which breed true. It
+will be remembered that on former occasions de Vries has formulated a
+general rule that _species_-hybrids breed true, but that the cross-breds
+raised by interbreeding _varieties_ do not. One of these very cases
+was quoted[7] as an illustration of this principle, viz: _muricata_ ×
+_biennis_. The grounds for this general statement have always appeared
+to me insufficient, and with the further knowledge which the new
+evidence provides we are encouraged to hope that when a proper factorial
+analysis of the types is instituted we shall find that the phenomenon
+of a constant hybrid will be readily brought into line with the systems
+of descent already worked out for such cases as that of the Stocks, and
+others already mentioned.
+
+In further discussion of these facts de Vries makes a suggestion which
+seems to me improbable. Since the egg-cells of _muricata_, for instance,
+bear a certain group of features which are missing on the male side,
+and conversely the pollen bears features absent from the female side,
+he is inclined to regard the _bad pollen grains_ as the bearers of the
+missing elements of the male side and to infer that there must similarly
+be defective ovules representing the missing elements of the female
+side. No consideration is adduced in support of this view beyond the
+simple fact that the characters borne by male and female are dissimilar,
+whereas it would be more in accord with preconception if the same sets
+of combinations were represented in each--as in a normal Mendelian case.
+There is as yet no instance in which the absence of any particular
+class of gametes has been shown with any plausibility to be due to
+defective viability, though there are, of course, cases in which certain
+classes of zygotes do not survive owing to defective constitution (_e.
+g._, the albinos of _Antirrhinum_ studied by Baur, and the homozygous
+yellow mice). I am rather inclined to suppose that in these examples of
+hybrids breeding true we shall find a state of things comparable with
+that to which we formerly applied the terms "coupling" and "repulsion."
+In these cases certain of the possible combinations of factors occur
+in the gametic series with special frequency, being in excess, while
+the gametes representing other combinations are comparatively few.
+In a recent paper on these cases Professor Punnett and I have shown
+that these curious results vary according to the manner in which the
+factors are grouped in the parents. If _A_ and _B_ are two factors which
+exhibit these phenomena we find that the gametic series of the double
+heterozygote differs according as the combination is made by crossing
+_AB × ab_, or by crossing _AB × aB_. In a normal Mendelian case the
+F_{1} form, _AaBb_, produces gametes _AB_, _Ab_, _aB_, _ab_, in equal
+numbers; but in these peculiar cases those gametes which contain
+
+                          Gametic series      Number of   Number of
+                      ---------------------    gametes     zygotes
+                       AB    Ab    aB    ab   in series    formed
+
+  Partial repulsion {   1  (n-1) (n-1)    1      2n         4n^{2}
+     from zygote    {   1    31    31     1      64         4096
+       of form      {   1    15    15     1      32         1024
+        Ab×aB       {   1     7     7     1      16          256
+                    {   1     3     3     1       8           64
+                        1     1     1     1       4           16
+  Partial coupling  {   3     1     1     3       8           64
+    from zygote     {   7     1     1     7      16          256
+      of form       {  15     1     1    15      32         1024
+       AB×ab        {  31     1     1    31      64         4096
+                    {  63     1     1    63     128        16384
+                    { (n-1)   1     1   (n-1)    2n         4n^{2}
+
+
+                            Nature of zygotic series
+                      ---------------------------------------
+                             AB        Ab     aB        ab
+  Partial repulsion {     2n^{2}+1  n^{2}-1  n^{2}-1     1
+     from zygote    {       2049      1023    1023       1
+       of form      {        513       255     255       1
+        Ab×aB       {        129        63      63       1
+                    {         33        15      15       1
+                               9         3       3       1
+                    {         41         7       7       9
+  Partial coupling  {        177        15      15      49
+     from zygote    {        737        31      31     225
+       of form      {       3009        63      63     961
+        AB×ab       {      12161       127     127    3969
+                    { 3n^{2}-(2n - 1) 2n-1  2n-1  n^{2}-(2n-1)
+
+
+the _parental combinations_ are in excess. This excess almost certainly
+follows the system indicated by the accompanying table. In the general
+expressions _n_ is half the number of gametes required to express the
+whole system. Now if we imagine that sex-factors are involved with the
+others concerned in such a relationship as this we have a system of
+distribution approximating to that found in _biennis_ and _muricata_.
+The difference in reciprocals is represented in a not improbable way.
+It cannot yet be said that the rarer terms in the series are formed at
+all, and perhaps they are not. As we pointed out in our discussion of
+these phenomena, the peculiar distribution of factors in these cases
+must be taken to mean that the planes of division at some critical stage
+in the segregation are determined with reference to the parental groups
+of factors, or in other words, that the whole system has a polarity,
+and that the distribution of factors with reference to this polarity
+differs according to the grouping of factors in the gametes which united
+in fertilization to produce the plant. Subsequent proliferation of
+cells representing certain combinations would then lead to excess of
+the gametes bearing them. It is on similar lines that I anticipate we
+shall hereafter find the interpretation of the curious facts discovered
+by de Vries, though it is evident that a long course of experiment and
+analysis must be carried through before any certainty is reached. The
+work must be begun by a careful study of the descent of some single
+factor, for example, that causing the broader leaf of _biennis_, and we
+may hope that the study of _Oenothera_ by proper analytical methods will
+no longer be deferred.
+
+We have now to return to the relations of _laeta_ and _velutina_.
+These two forms, it will be remembered are frequently produced when
+_Lamarckiana_ or one of its derivatives is used as male, and the most
+unexpected feature in their behaviour is that _both breed true as
+regards their essential characteristics, on self-fertilisation_. If one
+only bred true the case might, in view of the approximate numerical
+equality of the two types, be difficult to interpret on ordinary lines,
+but as both breed true it must be clear that some quite special system
+of segregation is at work. What this may be cannot be detected on the
+evidence, but with the results from the _biennis-muricata_ experiments
+before us, it is natural to suspect that we may here again have to
+recognise a process of allocation of different factors to the male and
+female sides in _laeta_ and _velutina_. That some such system is in
+operation becomes the more probable from the new fact which de Vries
+states in describing the group of characters which he calls _conica_,
+namely that this type is the same as that of _velutina_.
+
+There are many collateral observations recorded both by de Vries and
+others which have a bearing on the problems, but they do not yet fall
+into a coherent scheme. For example, we cannot yet represent the
+formation of _laeta_ and _velutina_ from the various species fertilised
+by _Lamarckiana_ [M]. That this is not due to any special property
+associated with the pollen of _Lamarckiana_ is shown by the fact that
+a species called _Hookeri_ gives _laeta_ and _velutina_ in both its
+reciprocal crosses with _Lamarckiana_ (de Vries, 1909, p. 3), and also
+by the similar fact that _Lamarckiana_ [F] fertilised by the pollen of
+a peculiar race of _biennis_ named _biennis Chicago_ throws the same
+types. Before these very complicated phenomena can be usefully discussed
+particulars must be provided as to the individuality of the various
+plants used. This criticism applies to much of the work which de Vries
+has lately published, for, as we now know familiarly, plants to which
+the same name applies can be quite different in genetic composition.
+
+Attention should also be called to one curiously paradoxical series of
+results. When the dwarf "mutant" of _Lamarckiana_ which de Vries names
+"_nanella_" is used as father on _muricata_, F_{1} consists of _laeta_
+and _velutina_ in approximately equal numbers. Both forms breed true to
+their special characteristics, but _velutina_ throws dwarfs of its own
+type, while _laeta_ does not throw dwarfs. Subsequent investigation of
+the properties of these types has led to some remarkable conclusions,
+and it was in a study of these plants that de Vries first came upon the
+phenomena of dissimilarity between the factors borne by the male and
+female cells of the same plant, a condition which had been recently
+detected in the Stocks as a result of Miss Saunders's investigations.
+The details are very remarkable. We have first the fact that _muricata_
+[F] × dwarf _nanella_ [M] gives about 50 per cent. _laeta_ and about 50
+per cent. of _velutina_.
+
+As regards _Velutina_ it was shown that:
+
+                                                 Talls,     Dwarfs,
+                                                per cent.  per cent.
+
+  1. Velutina selfed gave                          38         62
+
+    {Velutina [F] × dwarf nanella [M] gave         39         61
+  2.{    do.      × do.               gave         49         51
+    {    do.      × dwarf [M] derived from
+                                 velutina gave     43         57
+  3. Dwarfs × velutina [M] gave                    --     all dwarfs
+
+The three experiments taken together prove, as de Vries says, that the
+ovules of _velutina_ are mixed, talls and dwarfs, and that the pollen is
+all dwarf. The condition is almost the same as that of the Stocks. It
+may be noted also that in the Stocks the egg-cells of the "double" type
+are in excess, being approximately 9 to 7 of the "single" type, but de
+Vries regards the two types in _velutina_ as probably equal in number.
+The figures (169:231) rather suggest some excess of the recessives,
+perhaps 9:7, and the point would be worth a further investigation.
+
+As regards _laeta_, by self-fertilisation _no dwarfs were produced_, but
+in all other respects it behaved almost exactly like _velutina_. The
+ovules are evidently mixed talls and dwarfs, and whether fertilised by
+dwarfs or by the pollen of _velutina_, which is already proved to be all
+dwarf, the result was a steady 50 per cent. of talls and 50 per cent. of
+dwarfs. The pollen of _laeta_ used on dwarfs gives nothing but dwarfs,
+and in three series of such experiments 226 dwarfs were produced.
+
+We are thus faced with this difficulty. Since the egg-cells of _laeta_
+are evidently mixed, talls and dwarfs, and the pollen used on dwarfs
+gives all dwarfs, why does not self-fertilisation give a mixed result,
+talls and dwarfs, instead of _all talls_? De Vries regards the result
+of self-fertilisation as showing the real nature of the pollen, and
+declares it to be all talls, while he represents the behaviour of
+the same pollen used on dwarfs by stating that in these combinations
+the dwarf character dominates. This does not seem to me a natural
+interpretation. I should regard the pollen of _laeta_ as identical
+with that of _velutina_, namely dwarf, and I suspect the difficulty
+is really created by the behaviour of _laeta_ on self-fertilisation.
+Until a proper analysis is made in which the identity of the different
+individuals used is recorded, no further discussion is possible.[8]
+
+Other results of a complicated kind involving production of _laeta_ and
+_velutina_ together with a third form have been published by de Vries in
+his paper on "Triple Hybrids." To these also the same criticism applies.
+Some of the observations seem capable of simple factorial representation
+and others are conflicting.
+
+Taking the work on _Oenothera_ as a whole we see in it continually
+glimpses of order which further on are still blocked by difficulties
+and apparent inconsistencies. Through such a stage all the successful
+researches in complicated factorial analysis have passed and I see
+no reason for supposing that with the application of more stringent
+methods this more difficult set of problems will be found incapable
+of similar solutions. To return to the original question whether in
+_Oenothera_ we can claim to see a special contemporaneous output of new
+species in actual process of creation, it will be obvious that while
+the interrelation of the several types is still so little understood,
+such a claim has no adequate support. It is true that many of the
+"mutants" of _Lamarckiana_ can well pass for species, but this is
+equally true of many new combinations of pre-existing factors as we
+have seen in _Primula Sinensis_ and other cases. Still less can it be
+admitted that these facts of uncertain import supply a justification for
+the conception which has played a prominent part in the scheme of the
+_Mutationstheorie_, namely that there are special periods of Mutation,
+when the parent-species has peculiar genetic properties. To conclude:
+The impression which the evidence leaves most definitely on the mind is
+that further discussion of the bearing which the _Oenotheras_ may have
+on the problem of evolution should be postponed until we have before
+us the results of a searching analysis applied to a limited part of
+the field. In such an analysis it is to be especially remembered that
+we have now a new clue in the well-ascertained fact that the genetic
+composition of the male and female germ-cells of the same individual
+may be quite different. When with this possibility in view the behaviour
+of the types is re-examined I anticipate that many of the difficulties
+will be removed.
+
+Outside the evidence from _Oenothera_, which, as we have seen, is
+still ambiguous, I know no considerable body of facts favourable to
+that special view of Mutation which de Vries has promulgated. Of
+variation, or if we will, Mutation, in respect of some one character,
+or resulting from recombination, there is proof in abundance; but of
+that simultaneous variation in several independent respects to which de
+Vries especially attributes the origin of new specific types I know only
+casual records which have yet to undergo the process of criticism.
+
+                *       *       *       *       *
+
+Besides de Vries's "_Mutationstheorie_" and "Species and Varieties"
+the chief publications relating to the subject of the behaviour of
+_Oenothera_ are the following: (Many other papers relating especially to
+the cytology of the forms have appeared.)
+
+   Davis, B. M. Genetical Studies on _Oenothera_,
+     I. _Amer. Nat._, XLIV, 1910, p. 108. Genetical Studies on
+     _Oenothera_, II. _Ibid._, XLV, 1911, p. 193.
+
+   Gates, R. R. An Analytical Key to some of the Segregates of
+     Oenothera. _Twentieth Annual Report of the Missouri Botanical
+     Garden_, 1909.
+
+   Studies on the Variability and Heritability of Pigmentation in
+     _Oenothera_. _Ztsch. f. Abstammungslehre_, 1911, IV, p. 337.
+
+   Honing, J. A. Die Doppelnatur der _Oenothera Lamarckiana_.
+     _Ztsch. f. Abstammungslehre_, 1911, IV, p. 227.
+
+   Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K.
+     Small). Mutants and Hybrids of the _Oenotheras_. _Carnegie
+     Institution's Publication_, No. 24, 1905.
+
+   Macdougal, D. T., Vail, A. M., Shull, J. H. Mutations,
+     Variations and Relationships of the _Oenotheras_. _Carnegie
+     Institution's Publication_, No. 81, 1907.
+
+   de Vries, H. On Atavistic Variation in _Oenothera cruciata_.
+     _Bull. Torrey Club_, 1903, Vol. 30, p. 75.
+
+   On Twin Hybrids, _Bot. Gaz._, Vol. 44, 1907, p. 401.
+
+   Ueber die Zwillingsbastarde von _Oenothera nanella_. _Ber.
+     Deut. Bot. Ges._, 1908, XXVI, _a_, p. 667.
+
+   Bastarde von _Oenothera gigas_. _Ibid._, p. 754.
+
+   On Triple Hybrids. _Bot. Gaz._, 1909, Vol. 47, p. 1.
+
+   Ueb. doppeltreziproke Bastarde von _Oenothera biennis_ L. und
+     _Oenothera muricata_ L. _Biol. Cbltt._, 1911, XXXI, p. 97.
+
+   Zeijlstra, H. H. _Oenothera nanella_ de Vries, eine krankhafte
+     Pflanzenart. _Biol. Cbltt._, 1911, XXXI, p. 129.
+
+
+                              NOTE.
+
+Since this chapter was written two contributions of special importance
+have been made to the study of the _Oenothera_ problems. The first is
+that of Heribert-Nilsson.[9] The author begins by giving a critical
+account of the evidence for de Vries's interpretation of the nature of
+the mutants. In general this criticism pursues lines similar to those
+sketched in the foregoing chapter, concluding, as I have done, that the
+chief reason why factorial analysis has been declared to be inapplicable
+to the _Oenothera_ mutants is because no one has hitherto set about this
+analysis in the right way. He has also himself made a valuable beginning
+of such an analysis and gives good evidential reasons for the belief
+that at least the red veining depends on a definite factor which also
+influences the size of certain parts of the plant. He argues further
+that many of the distinctions between the mutants are quantitative
+in nature. With great plausibility he suggests that the system of
+cumulative factors which Nilsson-Ehle discovered in the case of wheat
+(subsequently traced by East in regard to maize) may be operating also
+in these _Oenotheras_. According to this system several factors having
+similar powers may coexist in the same individual, and together produce
+a cumulative effect. Scope would thus be given for the production of
+the curious and seemingly irregular numbers so often recorded in the
+"mutating" families.
+
+Another remarkable observation relating to the crosses of _muricata_
+and _biennis_ has been published by Goldschmidt.[10] He finds that in
+the formation of this cross the female pronucleus takes no part in the
+development of the zygotic cell, but that when the male pronucleus
+enters, the female pronucleus is pushed aside and degenerates. As de
+Vries observed, the reciprocal hybrids are in each case very like the
+father ("_stark patroklin_"), a consequence which finds a natural
+explanation in the phenomenon witnessed by Goldschmidt. The results
+of the subsequent matings can also be readily interpreted on the same
+lines. Indications of maternal characters are nevertheless mentioned
+by de Vries, and if Goldschmidt's account of the cytology is confirmed,
+these must presumably be referred to the influence of the maternal
+cytoplasm. Clearly this new work opens up lines of exceptional interest.
+The interpretation I have offered above must probably be reconsidered.
+The distinction between the male and female cells of the types may no
+doubt be ultimately factorial, but it is difficult to regard such a
+distinction as created by a differential distribution of the ordinary
+factors.
+
+FOOTNOTES:
+
+[1] See Lotsy and Baur, Rep. Genetics Conf., Paris, 1911, pp. 416-426.
+Compare Lecoq on _Mirabilis jalapa_ × _longiflora_, Fécondation des
+Végétaux, 1862, p. 311.
+
+[2] _Rep. Evol. Ctee. R. S._, IV, 1908, p. 38.
+
+[3] _Ber. Deut. Bot. Ges._, 1908, XXVI, _a_, p. 672.
+
+[4] _Jour. Genetics_, 1, 1910, p. 57.
+
+[5] In Rep. 1 to Evol. Committee, 1902, p. 132, attention was called to
+this possibility, though of course at that date it was in sexual animals
+alone that it was supposed to exist. It had not occurred to me that even
+a hermaphrodite plant might be in this condition.
+
+[6] From the description of the offspring of _muricata_ used as mother.
+
+[7] de Vries, _Species and Varieties_, 1905, p. 259.
+
+[8] Zeijlstra in a recent paper announces that many _nanella_ plants
+are the subject of a bacterial disease to which he attributes their
+dwarfness. I gather that this does not apply to all _nanella_ plants
+and that some are dwarfs apart from disease. The matter may no doubt be
+further complicated from this cause.
+
+[9] _Zts. f. Abstamm._, 1912, VIII.
+
+[10] _Arch. f. Zellforschung_, 1912, IX, p. 331.
+
+
+
+
+                       CHAPTER VI
+
+                  VARIATION AND LOCALITY
+
+In all discussions of the modes of Evolution the phenomena of
+Geographical Distribution have been admitted to be of paramount
+importance. First came the broad question, were the facts of
+distribution consistent with the Doctrine of Descent? I suppose all
+naturalists are now agreed that they are thus consistent, and that
+though some very curious and as yet inexplicable cases remain to be
+accounted for, the distribution of animal and plant life on the face
+of the earth is much what we might expect as a result of a process of
+descent with modification. Passing from this general admission to the
+more particular question whether the facts of distribution favour one
+special conception of the mode of progress of evolution rather than
+another, no agreement has yet been reached. One outstanding feature
+is hardly in dispute, namely that prolonged isolation is generally
+followed by greater or less change in the population isolated. Groups
+of individuals which from various causes are debarred from free
+intermixture with other groups almost always exhibit peculiarities,
+but on the other hand, cosmopolitan types which range over wide areas
+are on the whole uniform, or nearly so throughout their distribution.
+Examples of these two categories will be familiar to all naturalists.
+The barriers to intercourse may be seas, deserts, prairies,
+mountain-chains, or circumstances of a much less obvious character
+which isolate quite as effectually. The local unit is not necessarily
+an island, a district, or an area of special geological formation,
+but may, as every collector knows, be a valley, a pond, a creek, a
+"bank" in the sea, a clump of trees, a group of rocks in a bay, or a
+particular patch of ground on a mountain side. All the great groups
+provide examples of such specially isolated forms. The botanist knows
+them well; the conchologist, the entomologist, the ornithologist and the
+student of marine life are all equally aware that special varieties or
+special species come from special places and from nowhere else. In one
+remarkable case the season of appearance plainly acts as the isolating
+barrier. _Tephrosia bistortata_ is a small Geometrid moth which has two
+broods, appearing in _March_ and _July_ respectively. It is closely
+allied to _T. crepuscularia_ which emerges in _May_ and _June_. From the
+fact that occasional specimens cannot be quite certainly referred to
+one or other of the two, many have held that the two are one species.
+Nevertheless, in general they present distinctions which are plain
+enough. Some localities have one form only, but in several woods they
+co-exist. Experiment has shown that the two can be crossed, and that the
+cross-breds can breed _inter se_ and with at least one of the parent
+stocks.[1] Some diminution in fertility was observed, but perhaps not
+more than is commonly encountered when wild forms are bred in captivity.
+In such a case it can scarcely be doubted that the distinctness of
+the two forms in the places where they co-exist is maintained by the
+seasonal isolation.
+
+Just as the consequences of isolation are to be seen in the most
+different forms of life so may they also affect the most diverse
+features of organisation, such as size, colour, sculpture, shape, or
+number of parts. In the Sloth (_Choloepus_) the geographical races
+differ in the number of cervical vertebrae--or in other words, in the
+distribution of vertebral differentiation. The geographical races of
+_Cistudo_ differ in the number of claws and phalanges.[2]
+
+In Shetland, the males of _Hepialus humuli_ (the Ghost Moth) are
+not sharply differentiated in colour from the females, as they are
+elsewhere, but in varying degrees resemble them.[3] No such males are
+found in other localities, and even in the other Scottish islands they
+are normal. In the island of Waigiu the converse phenomenon has been
+observed in _Phalanger maculatus_. Generally the male is spotted with
+white, and the female is without spots, but in Waigiu the females are
+spotted like the males.[4]
+
+The following striking illustration was pointed out to me by Dr. W. D.
+Miller. _Euphonia elegantissima_ as it occurs in Mexico and Central
+America has the two sexes very distinct from each other. The male has
+the lower parts orange and the upper parts a dark indigo blue, with a
+bright turquoise-blue head and neck. The female, except for the head,
+is of a bright olive green. A form in which the sexes are similarly
+differentiated exists in Porto Rico and is known as _E. Sclateri_. But
+in many of the other West Indian islands the representative "species"
+(_E. flavifrons_) has the two sexes closely resembling the _female_
+of _E. elegantissima_. This form is found in Antigua, Barbados, St.
+Vincent, and Guadeloupe, from which localities the British Museum has
+specimens. All three so-called species are very much alike otherwise.
+
+In the genus _Pyrrhulagra_ (_Loxigilla_) to which Mr. Outram Bangs
+called my attention, several distinct and alternative possibilities
+occur. The genus has many local species occurring on the various West
+Indian islands. These species are characterized by differences in size,
+colour, and the shape of the bill. The colours have a narrow range,
+being black or greyish, with or without chestnut marks about the head
+and throat. In most of the islands the males are in general colour a
+full black, and the females are distinctly grey. They are thus found
+in San Domingo, Jamaica, Bahama, and most of the Lesser Antilles. In
+Porto Rico we meet the peculiarity that the hens are almost as black
+as the males (Ridgway describes the black of the hens as slightly less
+intense). This form is called _portoricensis_. A larger type, known
+as _grandis_, similarly coloured, inhabits St. Kitt's. Then, on the
+contrary, in Barbados, _both sexes_ are a dull blackish grey, like the
+hens of the Lesser Antilles in general.
+
+The local species of _Agelaius_ show similarly capricious distinctions.
+_A. phoeniceus_ is a widely spread species, found over a great part of
+North America. The male is black with red-orange bars on the wings, but
+the female is somewhat thrush-like in colour. In the island of Porto
+Rico there is a form called _xanthomus_, in which _both sexes_ are like
+the males of the mainland. A similar species called _humeralis_, also
+with both sexes male-like, lives in Cuba. The island of Cuba, curiously
+enough, has also a distinct species named _assimilis_, in which the
+female is a dull black all over, though the male is like the mainland
+type.
+
+So also may local races differ in respect of variability. _Argynnis
+paphia_, the Silver Washed Fritillary, through a great part of its
+distribution has only one female form. In the English New Forest a
+second female form, _valesina_, co-exists with the ordinary _paphia_
+female. But in the southern valleys of the Alps the _valesina_ female
+is much the commoner of the two, and indeed in some localities where
+the species is abundant, I have seen no _paphia_ females in many days
+collecting.
+
+The beetle _Gonioctena variabilis_ furnishes an illustration of a
+comparable phenomenon affecting the male sex. In 1894 and 1895 I
+studied the curious colour variations of this species especially in the
+neighbourhood of Granada, and Mr. Doncaster ten years later repeated the
+observations on the same ground, and also collected the insect in other
+places in the south of Spain. The distinctions are not easy to give in
+words and the reader is referred to the colour plate accompanying my
+paper.[5] The essential fact is that the males commonly have the elytra
+_red with black spots_ and the females for the most part have greenish
+grey elytra with black stripes. In some localities a large minority of
+males closely resemble the female type, being identical in colour and
+then only distinguishable by structural differences. In two Granada
+localities I found the proportion of such males quite different. In the
+Darro valley about 38 per cent. (in 718) were of this feminine type,
+but on the hills some 300 feet above only 19 per cent. (in 3,230) were
+like the females. At Castillejo, not far from Toledo I found no such
+male in 75 specimens.
+
+Mr. Doncaster collected from several localities, especially from two
+areas near Malaga, about 5 miles apart. In one of these the female-like
+males were, as usual, in a minority, but in the other these were
+actually in great excess, amounting to about 81 per cent. in the 173
+taken. Doncaster found a doubtful indication that the composition of the
+population varies with the season, which is quite possible, but it is
+most interesting to note that in my chief locality after the lapse of
+ten years he found the proportions very much the same as I had done at
+the same season, for where I had 19 per cent. of the female-like males
+his collecting gave 16 per cent. In other respects also, his statistics
+corresponded very closely with mine.[6]
+
+The various forms of _Heliconius erato_ are well known to entomologists.
+They are strikingly distinguished by the colours of the strong comb-like
+marking on the hind wing, which may be red, yellow, green or blue. In
+various parts of the distribution in South America sometimes two and
+sometimes three of these distinct types co-exist.[7]
+
+The distribution of the varieties of _Noctua castanea_ typifies a large
+range of cases. The form which is reckoned the normal of the species
+has red fore-wings. It is practically restricted to Great Britain and
+Germany, according to Tutt. The other common form, _neglecta_, has grey
+fore-wings, and in this pattern it ranges through West Central Europe
+from North Italy to Germany. In the British Isles it extends up to
+Orkney. In Britain this grey form is by far the commoner, occurring
+wherever the species is found. The red form is much scarcer in England,
+and does not occur at all in many localities where the grey form is
+common. Mr. Woodforde, from whom this account is taken,[8] states that
+in August, 1899, he saw considerably over a hundred of the grey in the
+New Forest at sugar, but only two red ones. In Staffordshire however the
+red is proportionately more numerous and he estimates them as 40 per
+cent. of the population. Lastly a form has been taken in Staffordshire
+as a rarity in which the red is replaced by yellow, and this has
+hitherto been seen nowhere else. It is beyond our immediate purposes
+to discuss the genetic relationships of such forms, but the details
+of this case are interesting as making fairly clear the fact that the
+distinctions between _castanea_ and _neglecta_ are due to combinations
+of the presence of and absence of two pairs of factors, of which one
+produces a red pigment in the ground colour of the forewing and the
+other irrorates the same region with black scales. Mr. Woodforde states
+that all intermediates exist, and that in Staffordshire the greys always
+have a pinkish tinge. The yellow is doubtless another recessive to the
+red.
+
+Species which are uniform in some localities may be polymorphic in
+others. Such a phenomenon is well exemplified by the orchid _Aceras
+hircina_. Of this species distinct varieties had previously been known
+in Germany, but Gallé[9] has lately given a detailed account of a
+number of most diverse forms found growing in a district of Eastern
+France. Without reference to his plates it is impossible to give any
+adequate conception of the profusion of types which the flowers of
+the species there assume. In some the lip is elongated to many times
+its usual length, twisting and dividing in a fashion suggesting some
+of the strangest of the Tropical Orchids. In others the labellum
+and the lateral petals are all comparatively short and wide (Fig.
+13). Intermediates, combining these qualities in various degrees,
+were abundant, and the condition of the species, which was the only
+representative of the genus in the locality, recalls the extreme
+polymorphism of many of the Noctuid Moths.
+
+[Illustration: FIG. 13. Various forms of _Aceras hircina_. (After
+Gallé.) This figure only shows a few of the more striking forms
+illustrated in Gallé's plates.]
+
+Somewhat comparable variability has been seen in another Orchid genus
+_Ophrys_. In Great Britain the species _apifera_, _aranifera_ and
+_muscifera_ though variable are fairly distinct, but Moggridge has
+published two series of plates[10] showing a very different state of
+things as regards the _Ophrys_ population of the Riviera. Here the
+outward diversity is such that the ordinary specific names cannot be
+applied with any confidence and the limits of the species are quite
+uncertain. It may well be supposed that these Riviera plants are
+interbreeding, and indeed we may safely assume that they are. It is,
+however, to be remembered that Darwin showed _apifera_ in this country
+to be habitually self-fertilised, so that the different behaviour on
+the Riviera may itself constitute a local peculiarity. Moreover it is
+to be gathered from Moggridge's account that in the districts which he
+examined the condition was not to be described by the statement that
+our three types were there co-existing and hybridising, but rather we
+should say that the population was polymorphic, containing these three
+types amongst others. Conchologists are aware that on the Dogger Bank
+_Modiola_ attains a size unparalleled elsewhere. The same is true of
+the sponges _Grantia compressa_ and _Grantia ciliata_ in the estuary
+of the Orwell.[11] Conversely, as we know so well in the case of Man,
+dwarf races occur in several special localities. Such examples may be
+multiplied indefinitely.
+
+The relation of local forms to species has often been discussed from
+many points of view, but I know no treatment of the subject clearer or
+more comprehensive than an excellent account of some of the various
+manifestations of local differentiation as they appear in Helicidæ
+published by Coutagne[12] and a reader interested in the problem which
+they raise would do well to make himself acquainted with the original
+from which the following notes are taken. He speaks for example of
+_Helix lapicida_. This is on the whole a constant form ranging up to the
+altitude of 1,300 m., common all over France except at great heights
+and in the Olive regions where it is restricted to moist places. Though
+subjected to such diverse conditions it shows only trivial variations in
+colour and other respects throughout its distribution, excepting that
+on both sides of the Pyrenees it has a very distinct sporadic variety
+called _Andorrica_ or _microporus_. This variety occurs here and there,
+together with the type-form sometimes in colonies (pp. 26-30 and 86).
+
+_Bulimus detritus_ though more restricted in geographical range is a
+much more variable form. It exhibits great variations in colour, form,
+and size, and as Coutagne well insists, these are independent of each
+other. Foreshadowing the methods of factorial analysis he suggests that
+distinctions in each respect, the "modes" as he calls them, should
+be denoted by a letter, or if desired, by a name, and the several
+combinations of differences might thus be most logically and usefully
+expressed. Of such combinations he says there are at least 18, all of
+which can be found. The whole possible series does not necessarily
+occur in the same place, and various localities are characterised by
+the presence or absence of certain of the combinations as Coutagne
+calls them, and by the relative frequency with which they occur. The
+ideas thus enunciated are much in advance of the ordinary practice of
+systematists, who give names to forms which are nothing but accidental
+combinations of factors, just as the horticulturists for practical
+reasons give names to similar combinations, which as we now know are
+merely specially noticeable terms in a long series of possibilities.
+In each case it is rather the _factors_ which should be named than the
+forms which are constituted by their casual collocation. In this special
+example of _Bulimus detritus_ the 18 forms are made by the combinations
+of three pairs of independent factors. Besides these combinations which
+may occur anywhere or almost anywhere in the distribution there are
+two more distinct local forms, each of which is regarded by Coutagne
+as probably constituting a fresh "mode," perhaps compatible with the
+others.
+
+_Helix striata_ (Draparnauld)[13] is truly polymorphic; and its various
+forms have been described under various specific names. It abounds in
+the calcareous hills of Provence and Languedoc, disappearing in the
+alluvial lowlands and equally in the upper levels at about 800-1,000 m.
+From this district it extends through regions of similar altitude over a
+great part of France (details given).
+
+Locard in his monograph of this group, which he calls collectively the
+group of _Helix Heripensis_, tabulates 27 distinct named forms. The
+characteristics in which these forms differ have been reckoned as 17,
+and as several of these vary in degree of development, the number of
+modes may be increased to 109. For practical purposes however Coutagne
+considers that the various developments of 7 characteristics in their
+several combinations are enough to express the various forms, and he
+gives examples of this method of definition. As he observes, though
+names may be required to define the modes, no one need be alarmed at
+that, for the same names of modes will be applicable to a great range of
+distinct species, and the formulae expressing their combinations will
+replace the varietal names.
+
+This particular example of polymorphism is but little limited by
+locality. Occasional colonies present some special physiognomy which may
+in a given place seem almost invariable, though in this very respect the
+colonies found elsewhere may be highly variable, but such limitations
+are exceptional for _H. striata_.
+
+Some distinct and obvious susceptibilities to the influence of soil
+and climate are however noticeable. For example on siliceous ground
+the shells are thinner, while on calcareous soils they are thicker;
+similarly those from the Northern districts attain a larger size
+than those from further South. Moreover those subjected to curtailed
+development, whether from drought, heat or cold often show a shortening
+of the spire. In contrast with this case Coutagne describes the
+varieties of _Helix caespitum_, which he says are for the most part
+localised, quoting many illustrative cases.
+
+Another remarkable case in which locality plays a curious part is
+provided by the two species _Helix trochoides_ and _pyramidata_. In
+France generally they are distinct enough from each other, _trochoides_
+being smaller and having a characteristic keel. Coutagne says that after
+having collected these species from more than a score of localities he
+came upon a colony of _trochoides_ on the island of Pomègues in which
+the shells were relatively enormous, most of them having only a slight
+keel, and a few none at all. On the other hand he received a consignment
+of _pyramidata_ from four localities in Sicily, all small, and one of
+them exactly like the _trochoides_ from Pomègues. Judging by the samples
+received from Sicily, _trochoides_ is there not more variable than it is
+in Provence, while the Sicilian _pyramidata_ is protean.
+
+The relations of the two species _Helix nemoralis_ and _hortensis_
+provide an illustration of another kind of manifestation of local
+peculiarity. _H. hortensis_ and _nemoralis_ as usually met with, are
+two very distinct forms. _H. hortensis_ is smaller and duller, and its
+peristome is white. _H. nemoralis_ is larger and more shiny, and its
+peristome is brown. In several anatomical points, moreover, especially
+in the shape of the dart, there are great differences. For a full
+account of these peculiarities of the two forms and a discussion of
+their inter-relations the reader is referred to the elaborate work of
+A. Lang[14] who has studied them extensively and has also succeeded in
+experimentally raising hybrids between them. These hybrids were in a
+slight degree fertile with both the parent species, but up to the time
+of publication no young had been reared from hybrids _inter se_.
+
+Coutagne describes the result of collections made in 62 French
+localities. Some had exclusively _hortensis_, some exclusively
+_nemoralis_, and in some the two were found in association. He gives
+details of five of these collections from which I take the following
+summary of the more essential facts, omitting much that is almost
+equally significant.
+
+_Locality A_, near Honfleur. Both forms present, each sharply and
+normally distinguished, without any intermediates. They are thus found
+in many places. Coutagne instances Müller's observations in Denmark, his
+own series from the Jura, etc.
+
+_Locality B._ Vonges (Côte d'Or), 242 _hortensis_ taken at random,
+showed 128 with light peristomes (either more or less pinkish or
+quite white) and 114 with dark _brown_ peristomes; together with 26
+_nemoralis_ all with the usual brown peristomes.
+
+Of the _hortensis_ 50 were in ground-colour _opalescens_ and 1 _roseus_;
+and in shape 5 were _umbilicatus_.
+
+_Locality C_, about 3 kilometres from _B_. There were found 35
+_hortensis_, of which 20 had light peristomes and 15 brown; together
+with 7 _nemoralis_.
+
+Of the _hortensis_ none were _opalescens_; 18 were _roseus_ and none has
+the shape of _umbilicatus_.
+
+_Locality D_, about 1,200 metres from _B_. 147 _hortensis_, of which 4
+had light peristomes and 143 had brown. No _nemoralis_ were found.
+
+None of the _hortensis_ were _opalescens_ or _roseus_, but 30 were
+_umbilicatus_.
+
+In these localities intermediates of every grade existed between the
+well-characterised _opalescens_, _roseus_, or _umbilicatus_, and the
+other forms, but there were no intergrades between the other _nemoralis_
+and the smaller _hortensis_, about which there was no hesitation. In the
+next locality a very different state of things was found.
+
+_Locality E._ Banks of the Yvette at Orsay (Seine-et-Oise). The
+actual numbers are not given, but we are told that 58 per cent. were
+_hortensis_, 33 per cent. _nemoralis_, and 9 per cent. intermediate. As
+at Honfleur, the _hortensis_ had white peristomes, and the _nemoralis_
+brown. Coutagne's visits to this locality were in 1878 and 1880, and he
+calls attention to the fact that Pascal found similar intermediates in
+the same neighbourhood in 1873.
+
+The two species, in Coutagne's view, when they occur together, can
+generally be sorted from each other with perfect confidence, and it is
+only in exceptional localities that these intermediates occur. Whether
+they are hybrids, or whether sometimes the species in their variations
+transgress their usual limitations is regarded both by Coutagne and
+by Lang as a question not yet answerable with certainty. Coutagne
+moreover lays stress on the fact that although each species may be
+easily known from the other _in its own district_, yet when shells from
+different districts are brought together it is sometimes impossible to
+sort them. He mentions an example of such casual intermixture occurring
+under natural conditions on an island in the Rhone, to which it may
+well be supposed that floods had brought immigrants from miscellaneous
+localities. This population contained a very large number of uncertain
+specimens, and as he says, it was much as if he were to mix the shells
+from his 62 localities, after which it would certainly be impossible to
+separate the two species again.[15]
+
+Further evidence is given in the same treatise as to other examples
+of polymorphism, especially in the genus _Anodonta_, of which Locard
+made 251 species for France alone. Here again are cases like those
+already given, and many forms or "modes" are found restricted to special
+localities, while occasionally in the same locality dissimilar forms are
+found, collectively forming a colony, without intermediates.
+
+Taken as a whole the evidence shows the following conclusions to be
+true. Local races, whether of animals or plants, may be distinguished
+by characters which we are compelled to regard as trivial, or again by
+features of such magnitude that if they were known to us only as the
+characteristics of a uniform species they would certainly be assumed
+without hesitation to be essential for its maintenance. Local forms
+may be sharply differentiated from the corresponding populations of
+other localities or they may be connected with them by numbers of
+intermediates. Not rarely also we find a fact which has always seemed to
+me of special significance, that the peculiarity of the local population
+or colony may show itself in a special liability to variation, and
+this variability may show itself in one of many degrees, either in the
+constant possession of a definite aberration, in a dimorphism, or in an
+extreme polymorphism.
+
+At this stage attention should be called to two points. First, that
+when the details of the geographical distribution of any variable
+species are studied in that thorough and minute fashion which is
+necessary for any true knowledge of the interrelations of the several
+forms, the conception of a species invented by the popular expositions
+of Evolution under Selection is found to be rarely if ever realised in
+nature.
+
+A species in this generalised sense is an aggregate of individuals,
+none exactly alike, but varying round a normal type, the characters of
+which are fixed in so far as they are adapted to environmental exigency.
+In nature, however, the occurrence of the varieties, and even the
+occurrence of the variability is sporadic. In one place a population may
+be perfectly uniform. In another it may be again uniform but distinct.
+In others the two forms may occur together, sometimes with and sometimes
+without intergrades. In some localities a sporadic variety may be an
+element of the population, persisting through long periods of time.
+In other localities there may be several such aberrations occurring
+together which are absent elsewhere.
+
+Secondly, I would remind the reader that in the light of genetic
+analysis we know that intergrades, when they do occur, cannot be assumed
+to represent conditions through which the species must pass or has
+passed on its way to the extreme and definite forms.
+
+Often, perhaps generally, they are nothing but heterozygous forms,
+and often also they are conditions corresponding with the presence of
+factors in their reduction-stages.
+
+A broad survey of the facts shows beyond question that it is impossible
+to reconcile the mode of distribution of local forms with any belief
+that they are on the whole adaptational. Their peculiarities are
+occasionally the result of direct environmental influence, as we shall
+hereafter notice in certain cases, but none can attribute such sporadic
+and irregular phenomena to causes uniformly acting.
+
+Writers on systematics, especially those of former generations often
+conjecture or assert that local distinctions are caused by "differences
+of climate, soil, food, etc.," in vague general terms. It is usually
+safe to assume that these remarks do not represent conclusions drawn
+from actual evidence, for only rarely can they be translated into more
+precise language. So thoroughly have the biological sciences become
+permeated with the belief that all distinctions are dependent upon
+adaptation, that the mere existence of definite distinctions is felt
+by many to be sufficient ground to warrant an assumption that these
+distinctions are directly or indirectly due to special local conditions.
+For example, Dr. J. A. Allen, who has done so much careful and valuable
+work in delimiting the local forms of the United States fauna, writes of
+the Ground Squirrels (Tamias)[16] as follows:--
+
+   "From the extreme susceptibility of this plastic
+   group to the influences of environment, it is one of the most
+   instructive and fascinating groups among North American mammals.
+   No one can doubt its comparatively recent differentiation from
+   a common stock, and its dispersion from some common centre.
+   Whether the type originated at some point in North America, or in
+   the Northern part of Eurasia, it is perhaps idle to speculate,
+   but that it has increased, multiplied, spread, and become
+   differentiated to a wonderful degree in North America is beyond
+   question; as it is found from the Arctic regions to the high
+   mountain ranges of Central Mexico, and has developed some twenty
+   to thirty very palpable local phases."
+
+   "Some of them easily take rank as species, others as
+   subspecies. Probably a more striking illustration of evolution by
+   environment cannot be cited."
+
+He proceeds to point out that the habits of these creatures are such as
+lead to isolation. This may well be admitted, and indeed no exception
+can possibly be taken to the passage as a whole, save in the one
+respect that there is no real proof that the local diversity is due to
+"evolution by environment" or an indication of "susceptibility to the
+influences of environment."
+
+Dr. Allen does indeed adduce the fact that California "extending through
+800 miles of latitude, with numerous sharply contrasted physiographic
+regions, has apparently no less than six strongly differentiated forms,
+while the region east of the Rocky Mountains from a little below the
+northern boundary of the United States northward to the limit of
+trees--a slightly diversified region of at least ten times the area of
+California--has only one"! But when one comes to ask how the various
+forms are adaptational, and how the influences of environment have led
+to their production, only conjectures of a preliminary and tentative
+character could be expected in reply. Desert forms are no doubt pallid
+as in so many instances, and forest forms are more fully coloured, and
+we may readily enough accept such facts as indications of a connection
+between bodily features and the conditions of life, but further than
+that no one can go; so that when we find size, length of ears or of
+tail, the number of dorsal stripes, the pattern of the colours, not to
+speak of differences in the pigments themselves, all exhibiting large
+modifications, we cannot refer these peculiarities to the causation
+of environmental difference, save as a simple expression of faith. I
+incline far more to agree with Gulick who, after years of study of the
+local variations of the Achatinellidae, came to the conclusion that it
+was useless to expect that such local differentiation can be referred to
+adaptation in any sense.[17] Even the most convinced Selectionist must
+hesitate before such facts as those related by A. G. Mayer regarding
+the distribution of _Partula otaheitana_, one of these Achatinellidae.
+The island of Tahiti has been scored by erosion so that a series of
+separated valleys radiate to the coast. From four successive valleys
+Mayer collected the species, and found that in the first (Tipaerui)
+valley all the shells were dextral (115, containing 73 young); in the
+second valley (Fautaua) 54 per cent. of adults and 55.5 per cent. of
+the young contained were sinistral; in the third valley (Hamuta) 69
+per cent. of adults and 73 per cent. of young contained in them were
+sinistral; and lastly, in the fourth valley (Pirae) all the shells
+(131, containing 62 young) were sinistral.[18] In connection with
+these observations I may mention the fact that in a certain pond in
+the North of England[19] the sinistral form of _Limnaea peregra_ has
+been known to occur for about fifty years. Visiting it lately I found
+the left-handed shells to be about 3 per cent. of the population. The
+species is the commonest British freshwater shell, but left-handed
+specimens are exceedingly rare. Will anyone ask us to suppose that the
+persistence of a percentage of this rarity in the same place is an
+indication of some specially favouring circumstance in the waters of
+that pond? It is a horse-pond to all appearances exactly like any other
+horse-pond; and I believe that in perfect confidence we may accept the
+suggestion of common sense, which teaches us that there is nothing
+particular in the circumstances which either calls such varieties into
+existence or contributes in any direct way to their survival. Had the
+phenomenon of local variation been studied in detail before Darwin
+wrote, the attempt to make selection responsible for fixity wherever
+found, could never have been made. The proposition that not only the
+definiteness of local forms but their variability also is sporadic,
+can be established by countless illustrations taken from any group of
+either the animal or the vegetable kingdoms. Only exceptionally can the
+fixed differences be even suspected of contributing to adaptation, and
+sporadic variability, which is a no less positive fact, must manifestly
+lie outside the range of such suspicions. It is open to any one to
+suggest speculatively that the persistence of special varieties or of
+special variability in special places is an indication that in those
+places the conditions of life are such that the forms in question are
+tolerated though elsewhere the same types are exterminated; but that
+consideration, even if it could be proved to be well founded, is not
+one which lends much force to the thesis that definiteness of type is
+a consequence of Natural Selection. On the contrary, recourse to such
+reasoning implies the inevitable but very damaging admission that the
+stringency of Selection is frequently so far relaxed that two or more
+equally definite forms of the same species can persist side by side.
+There is no doubt that this is the simple truth, but when once that
+truth is perceived it is useless to invoke the control of Selection as
+the factor to which definiteness of type in general must be referred.
+
+The genetic relations of local forms to each other cannot in the
+absence of actual breeding experiments be often ascertained. Standfuss
+formerly enunciated as a general principle that when two forms
+co-exist in the same locality and are able to interbreed, they do not
+produce intermediates; but that when the forms are geographically
+separated as local races, crosses between them result in a series of
+intermediates.[20] In this aphorism there is a good deal of truth, but
+if in the light of Mendelian principles we examine the two statements
+we see now that the first is in reality only another way of saying
+that the distinctness of an aberrational form co-existing with another
+is due to segregation, accompanied by some degree of dominance of one
+type. Whether, however, one geographically isolated race will give
+intermediates when bred with another must depend entirely on the genetic
+physiology of the special case, and no general rule can be laid down.
+It may well be that, inasmuch as the distinctness of the variety is
+maintained by isolation, the difference in factorial composition between
+it and the representative form in another area is neither simple nor
+sharp; but when two varieties co-exist, though interbreeding, it is now
+clear that their differences must depend on the segregation of simple
+factors. Plainly such aberrations may in one place co-exist with another
+type, and elsewhere be separated from it as local races.
+
+Excellent illustrations of these two stages in evolution are provided
+by the melanic varieties of British Lepidoptera. The fact that black or
+blackish varieties of many species especially of Geometridae have come
+into existence in recent years is well known to British collectors,
+and it is not in dispute that they have in several instances replaced
+the older type more or less completely in certain districts. In the
+year 1900 the Evolution Committee of the Royal Society instituted a
+collective inquiry as to the contemporary distribution of these dark
+varieties. As the change had happened within living memory and had
+greatly progressed in recent years it was hoped that a record of the
+existing distribution would serve as a point of departure for future
+comparison. The records thus obtained were tabulated by Mr. L.
+Doncaster.[21] From that account and from the statements in Barrett's
+British Lepidoptera[22] this description of some of the more notable
+cases is taken.
+
+The most striking and familiar case is that of _Amphidasys betularia_,
+of which only the ordinary type was known in any locality until about
+1848-1850, when the totally black var. _doubledayaria_ first appeared
+in the neighbourhood of Manchester. This black form was subsequently
+recorded in Huddersfield between 1860 and 1870; Kendal about 1870;
+Cannock Chase, 1878; Berkshire, 1885; Norfolk, Essex and Cambridge about
+1892; Suffolk, 1894; London, 1897. For the Southern Counties of England,
+except in the London district, there are still very few records. It
+cannot of course be asserted positively that the variety spread from its
+place of first appearance into the other localities, and that it did not
+arise _de novo_ in them, but there can be little doubt that the process
+was one of colonisation. On the European Continent the first records are
+from Hanover in 1884, Belgium 1886 and 1894, Crefeld 188-, Berlin 1903,
+Dresden about the same date.
+
+As regards the increase of the variety we have the fact that in
+Lancashire, Cheshire and the West Riding of Yorkshire the black is now
+the prevalent form; and in some places, as for example, Huddersfield,
+the black alone is now found, though it was unknown there till between
+1860 and 1870. About 1870 at Newport, Monmouth, the two forms were in
+about equal numbers, but a few years later the type had almost vanished.
+Similarly in Crefeld, where the black form was still very rare in the
+eighties, it now forms about 50 per cent. of the population. In the
+London district the black remains scarce and at the date of the report
+it was still very scarce. From Ireland there is only one record and
+there are hardly any from Scotland.
+
+_Boarmia repandata_ is another species which is behaving in a somewhat
+similar way. Unlike _betularia_, however, the species is a variable
+one, and has several colour-forms, amongst them the banded var.
+_conversaria_, and many others. In addition to these there is a
+black form in the North of England which seems to be spreading. In
+Huddersfield the black was first recorded in 1888, and in 1900 20-25 per
+cent. were black. At Rotherham the black or very dark are now prevalent
+and have increased in the last 15 years. From the Midlands, East Anglia
+and Southern Counties the returns show only the light and medium forms.
+
+Of _Odontoptera bidentata_ several intergrading dark forms exist, and
+these are found exclusively in the North and the Midlands. Unicolorous
+blacks have been found recently in the Lancashire mosses and at
+Wakefield. At Huddersfield 50 years ago the light forms were prevalent,
+but now a rather dark brown, not infrequently suffused with black, is
+the commonest. In Southern Counties only light forms are known.
+
+_Phigalia pilosaria_ in South England is always light, but in the North
+the prevalent form is darker. About 35 years ago a form with unicolorous
+sooty fore-wings and dull grey hind wings was first seen in Yorkshire
+and a similar form is now taken regularly in South Wales.
+
+In the following cases the dark varieties were found originally only in
+the South.
+
+_Boarmia rhomboidaria_ gave rise about 40 years ago to a unicolorous
+smoky variety called _perfumaria_. This was at first peculiar to the
+London district, but it has since been taken in Birmingham and other
+large cities. More lately coal-black specimens have been found at
+Norwich, and others similar but hardly so dark were taken in the South
+of Scotland and at Cannock Chase.
+
+_Eupithecia rectangulata_ is a similar case. Formerly the light forms
+were prevalent but within sixty years they have almost entirely been
+replaced in the South of London by a nearly black form.
+
+_Tephrosia_ (_Boarmia_) _consortaria_ and _Tephrosia consonaria_ are
+exceptionally interesting, for they have both given off dark forms in
+the same wood near Maidstone, which is far from the usual "centres of
+melanism." They were discovered in this locality by Mr. E. Goodwin. That
+of _consortaria_ is a dark grey, but that of _consonaria_ is a full
+black, and nothing like either has been found anywhere else.
+
+These examples are all taken from the Geometridae but others, though
+of a less conspicuous kind, could be given from the Noctuidae or the
+Micro-Lepidoptera. _Acronycta psi_, for instance, has a suffused form
+which is believed to be becoming more frequent in the London district.
+_Polia chi_ has two dark forms, _olivacea_, a yellowish grey with dark
+markings, and _suffusa_ which is a darker, blackish-slate colour. Both
+occur in the North of England, sometimes together, sometimes separately,
+or mixed with the type and many intermediates. The distribution is
+peculiarly irregular. At Huddersfield, where the very dark form appeared
+suddenly about 1890, some 30 per cent. are said to be now dark and about
+6-7 per cent. very dark, but at Saddleworth, 12 miles away, only the
+pale forms occur.
+
+Several questions of interest arise in regard to this evidence. This
+progressive Melanism has arisen in certain families only, and may be
+confined to certain species only, within those families. As in almost
+all other examples in which variation has been much observed, its
+incidence is capricious and specific. A collateral line of inquiry
+relates to the degree of discontinuity which the variation manifests.
+Here again there is no rule. Generally speaking, in _A. betularia_,
+to take the case most fully studied, the variation is discontinuous.
+Real intermediates between _betularia_ and _doubledayaria_ are in most
+localities absent or rare. The black spots of _betularia_ may often
+be larger or more numerous than in the normal, but this variation has
+nothing to do with _doubledayaria_, and is not an intermediate stage
+towards it, though sometimes wrongly so described. _Doubledayaria_ owes
+its characteristic appearance to a factor which blurs the surface of the
+wings with a layer of black. Sometimes this blurring is slighter than
+in the real _doubledayaria_, and these forms are real intermediates.
+Occasionally the fore-wings alone are thus blurred. These intermediates
+are clearly due to reduction-stages of the _doubledayaria_ factor,
+and are related to it as a blue mouse is to a black, or a dutch
+rabbit to a self-colour. It cannot positively be asserted that the
+full _doubledayaria_ existed before the intermediate, but it almost
+certainly did. In certain places as for instance in Belgium, there is
+evidence that intermediates have at various times been fairly abundant,
+but they have never become common, nor are they known to exist in the
+absence of _doubledayaria_. When the black variety and the light type
+breed together they do not usually have intermediates among their
+offspring, and the evidence is consistent with the view that the
+black is a complete dominant. The same is probably true of _Tephrosia
+consonaria_.
+
+In some of the other species we know that the darkest forms did
+not appear first. For example in _Phigalia pilosaria_ and _Boarmia
+rhomboidaria_ dark forms existed and are believed to have increased in
+number before the darkest made its appearance. _Hybernia progemmaria_ is
+said to have become darker gradually both in Cheshire and in the West
+Riding, and a uniformly smoky variety appeared in South Yorkshire less
+than 45 years ago which has spread to neighbouring counties. The dark
+medium has become the commonest form in Huddersfield district, where the
+very dark variety is now about 20 per cent. of the population, though
+the light form is still common.
+
+Taking the evidence together we find it consistent with the view that
+dark forms have appeared sporadically, in some species the very dark
+appearing first and intermediates later, in others the moderately dark
+came first and the darkest later in time. It is practically certain that
+the change has in general come about not by a gradual change supervening
+on the population at large, but by the sporadic appearance of dark
+specimens as a new element in the population, and strains derived from
+these dark individuals have gradually superseded the normal type more or
+less completely.
+
+If it could be shown that these melanic novelties had a definite
+advantage in the struggle for existence they would provide an instance
+of evolution proceeding much in the way which Darwin contemplated. The
+whole process would differ from that conceived by him as the normal
+method of evolution only in so far as the change has come about with
+great rapidity and in some instances largely by the appearance and
+success of discontinuous varieties. The question, however, must be
+asked whether the dark form can reasonably be supposed to have an
+advantage by reason of their darkness. Some naturalists believe that
+the darkness of the colours does thus definitely contribute to their
+protection by making the insects less conspicuous and thus more likely
+to escape the search of birds. In support of this view it may be
+pointed out that it is in the manufacturing districts of Lancashire and
+Yorkshire, and again in the London area that the melanics have attained
+their greatest development. Consistently with this argument also, it is
+in the neighbourhood of Crefeld and Essen, the black country of Germany,
+that they have chiefly established themselves on the Continent, and
+_Phigalia pilosaria_ in the black form is now at home in South Wales.
+Thus superficially regarded, the evidence looks rather strong, but it is
+difficult to apply the reasoning in detail. We have first the difficulty
+that the black form of _betularia_ for instance has established itself
+in thoroughly rural districts, notably near King's Lynn in Norfolk,
+and in the neighbourhood of Kendal and Windermere. The black form
+of _consonaria_ and the dark _consortaria_ appeared in a wood near
+Maidstone, far from town smoke, and the black _rhomboidaria_ was first
+found at Norwich, which, as towns go, is clean. Then again the spread of
+the melanics is very irregular and unaccountable. The black _pilosaria_
+is found both in the West Riding and in the Swansea district, but
+not yet elsewhere. It rapidly increased at Huddersfield, but made no
+noticeable progress at Sheffield though recorded there for ten years.
+It is also a remarkable fact that no similar melanic development has
+been observed in America, and, so far as I am aware, comparable melanic
+varieties have not appeared on the European continent except in the case
+of the few sorts which possibly may have come from England.
+
+The whole subject is beset with complications. It must not be forgotten
+that in a few species of moths there is an obvious and recognised
+conformity between the colours of the perfect insect and that of the
+soil on which they live, comparable with that which is so striking in
+the case of some Oedipodidae and other grasshoppers. Of this phenomenon
+the clearest example is _Gnophos obscurata_, which is a most variable
+species with many local forms. Of these a well-known dark variety lives
+on the peaty heaths of the New Forest and other districts, but on the
+chalk hills of Kent, Sussex and Surrey various light varieties are
+found, of which one is a bright silvery white, very near in colour to
+the colour of a chalky bank. This case does not seem to be one of direct
+environmental action,[23] for Poulton found no change induced by rearing
+larvae among either white or black surrounding objects. No one however
+can doubt that there is some indirect connection between the colour of
+the ground and that of the moths.
+
+To my mind there is a serious objection to the theory of protective
+resemblance in application to such a case as that of the _betularia_
+forms, which arises from the fact that the black _doubledayaria_ is a
+fairly conspicuous insect anywhere except perhaps on actually black
+materials, which are not common in any locality. Tree trunks and walls
+are dirty in smoky districts but they are not often black, and I doubt
+whether in the neighbourhood of Rotherham, for instance, which is one
+of the great melanic centres, _doubledayaria_ can be harder for a bird
+to find than _betularia_ would be. After all, too, many of the species
+much affected are not urban insects. They live in country places between
+the towns, and the general tone of these places even in Lancashire
+and the West Riding is not very different from that of similar places
+elsewhere. As against the objection that the black varieties are much
+blacker than the case requires it may be replied that we know nothing
+of the senses of birds, and that perhaps to their eyes blackness does
+constitute a disguise even though the surroundings are much less dark.
+This is undeniable, but recourse to such an argument is dangerous; for
+if the sight of the insect-eating birds is so dull that it does not
+distinguish dark things from dingy grey, we cannot subsequently regard
+the keen sight of birds as the sufficient control which has led to the
+minute and detailed resemblance of many insects to their surroundings.
+Those who see in such cases examples of the omnipotence of Selection
+must frequently find themselves in this dilemma.
+
+Taking the evidence as a whole, we may say that it fairly suggests the
+existence of some connection between modern urban developments and the
+appearance and rise of the melanic varieties. More than that we cannot
+yet affirm. It is a subject in which problems open up on every side,
+and all of them are profitable subjects for investigation. Unhappily
+such animals are difficult to rear successfully in captivity for many
+generations, owing to their extreme liability to disease. Not the least
+interesting feature of the melanics is the fact that the black varieties
+provide about the best and clearest example of a new dominant factor
+attaching itself to a wild species in recent times. None of the cases
+are satisfactorily recorded or analysed as yet, but the evidence is
+clear that _doubledayaria_ is a dominant to its type, and in several
+other dark varieties, though the pigment deposited is not black, the
+records show that the increased amount of the pigment almost certainly
+is due to a positive factor. Of this, _Hemerophila abruptaria_ is a
+good example.[24] There are some irregularities in the results, but
+taken together they leave little doubt that the dark brown variety is a
+dominant and the light, yellowish brown a recessive.
+
+A curious parallel to the rise of the melanic moths in England
+is provided by the case of the Honey-creepers or Sugar-birds, in
+certain West Indian islands.[25] These birds of the genus _Coereba_
+(_Certhiola_) range from Southern Mexico to the Northern parts of
+South America and through the whole chain of the West Indian islands
+and Bahamas except Cuba. There are numerous local forms, and many of
+the islands have types peculiar to themselves, as is usual in such
+cases. Some of the types or species range through several islands, but
+according to Austin Clark[26] no island has more than one of them.
+Cory[27] reckoned twelve such species within the Antillean region. They
+are small birds about the size of a nuthatch with a general colouring of
+black, yellow, and white. From the island of St. Vincent the Smithsonian
+Institution received in the late seventies of last century several
+completely black specimens in addition to two of the usual type of
+colouring. The black were described by W. N. Lawrence as _atrata_, and
+those marked with the usual yellow and white were called _saccharina_.
+The collector (Mr. F. A. Ober) reported that the black form was common,
+and that the _saccharina_ form was rarer. Lawrence remarks, "Had there
+been only a single example (of the black form) I should have considered
+it as probably a case of abnormal colouring, but it seems to be a
+representative form of the genus in this island."[28] There is of course
+no doubt of the correctness of the view taken by Austin Clark that
+"_atrata_" is a black variety. The black bird is in every respect, other
+than colour, identical with _saccharina_, and it is even possible to
+detect a greenish colour in the areas which would normally be yellow,
+showing plainly enough the yellow pigment obscured by the black.
+
+We have next the interesting fact that like our melanic moths the dark
+form is replacing the "type." At the time of Ober's visit the type was
+already in a minority, but now it is nearly or perhaps actually extinct,
+though the black form is one of the commonest birds on the island.
+Austin Clark found no specimen when he collected there in 1903-4, though
+formerly it was not uncommon in the vicinity of Kingston and in the
+immediate windward district of St. Vincent.
+
+The Grenadines are geographically just south of St. Vincent, though
+separated by a deep channel. In these islands no black forms have
+yet been taken, but Grenada, the next island to the south, has both
+normals and blacks. There are trifling differences of size between the
+Grenada birds and those from St. Vincent, the Grenada specimens being
+slightly smaller and for this reason they have received distinct names,
+the form marked with yellow and white being called _Godmani_ (Cory)
+and the black, _Wellsi_ (Cory), but this merely introduces a useless
+complication. There is evidence that in Grenada, as in St. Vincent,
+the black is gradually ousting the original type, but the process has
+not gone so far as in St. Vincent. Austin Clark very properly compares
+this case of the Sugar-birds with that of _Papilio turnus_, which
+as is well-known, has a black female in the southern parts of its
+distribution, in addition to a female of the yellow type, but in the
+Northern States the black female does not occur.
+
+During the present year P. R. Lowe, who lately studied _Coerebas_ on
+a large scale in the West Indies, has published an important paper on
+the subject.[29] He calls attention to the fact that Cory recently
+found a black form of _Coereba_ on Los Roques Islands, and he himself
+discovered another on the Testigos Islands. Both localities are on the
+coast of Venezuela, far from St. Vincent and Grenada. The whole problem
+is thus further complicated by the fact that the black varieties have,
+as we are almost driven to admit, arisen independently in remote places.
+Improbable as this conclusion may be, it is still more difficult to
+regard all the black forms as derived from one source. For first, they
+present definite small differences from each other; and secondly we have
+to remember a consideration of greater importance, that the very fact
+that each island has its own type must be accepted as proving that the
+localities are effectively isolated from each other, and that migration
+must be a very rare event.
+
+The rarity of such illustrative cases is, I believe, more apparent than
+real. It is probably due to the extreme reluctance of systematists to
+admit that such things can be, and of course to the almost complete
+absence of knowledge as to the genetic behaviour of wild animals and
+plants. Only in such examples as this of the _Coereba_, where colour
+constitutes the sole difference, or that of the moths which have been
+minutely studied by many collectors, does the significance of the facts
+appear. The arrangement of catalogues and collections is such that much
+practical difficulty of a quite unnecessary kind is introduced. For
+example, in this very case of _Coereba_, I find the British Museum has
+a fine series from Grenada including 3 normals and 11 black, and also
+16 blacks from St. Vincent. If the black specimens from Grenada were
+put with the normals which are almost certainly nothing but a recessive
+form of the same bird, the variation would strike the eye on even a
+superficial glance at the drawer. But following the notions so naively
+expressed in the passage quoted above from W. N. Lawrence, the blacks
+from Grenada are put apart together with the other blacks from St.
+Vincent, though two of them were shot on the same date as one of the
+normals.
+
+FOOTNOTES:
+
+[1] For the evidence see Tutt, J. W., _Trans. Ent. Soc._, 1898, p. 17.
+Compare the remarkable case given by Gulick (_Evolution Racial and
+Habitudinal_, p. 123) of the two races of _Cicada_, which are separated
+by reason of their life-cycles, one having a period of 13, the other 17
+years.
+
+[2] For references see _Materials_, p. 396, and also G. Baur, _Amer.
+Nat._, 1893, July, p. 677.
+
+[3] Jenner Weir, _Entomologist_, 1880, XIII, p. 251.
+
+[4] Jentink, _Notes Leyden Mus._, 1885, VII, p. 111. Specimens
+illustrating this peculiarity are in the British Museum.
+
+[5] _Proc. Zool. Soc._, 1895, p. 850. Plate. Many points beyond that
+mentioned above are involved in this remarkable case. For example, not
+only are there males like females, but a small proportion of females
+resemble the ordinary male type. The stripes are not merely the spots
+produced, for they occupy different anatomical positions. The spots
+almost always go with a black ventral surface, but the striped forms
+nearly always have that region testaceous. _Spartium retama_, the
+food-plant, will not grow in England, but if it could be naturalised in
+America the whole problem might be investigated there and results of
+exceptional interest would almost certainly be attained.
+
+[6] Doncaster, L., _Proc. Zool. Soc._, 1905, II, p. 528.
+
+[7] I am not aware that the details of this striking case have ever
+been worked out. It should be noted that the green and blue forms are
+not due to simple modification of the red pigment; for these colours,
+due to interference, fork over the area occupied by the red lines. The
+distinctions between these forms cannot therefore be simply chemical,
+as we may suppose them to be, for instance, in the case of many red
+and yellow forms, and the genetic relationships of the _Heliconid_
+varieties would raise many novel problems and be well worth studying
+experimentally.
+
+[8] Woodeforde, F. C., _Trans. North Staffordshire Field Club_, XXXV,
+1901, Plate.
+
+[9] E. Gallé, _Compte Rendus du Congres Internat. de Bot. a l'Expos.
+Univ._, 1900, p. 112.
+
+[10] Flora of Mentone, 1864-8, _Nova Acta Acad. Caes._, XXXV, 1869.
+
+[11] I owe these facts to Canon A. M. Norman, who showed me illustrative
+specimens. They were originally described by Bowerbank (_Monogr. Brit.
+Spongiadae_, vol. II, pp. 18 and XX; vol. III, Pls. I and III). A
+specimen of _G. compressa_ measured 5 inches, with a greatest width of
+3-1/4 in. _G. ciliata_ was found measuring 3 in. long and 3/4 in. wide.
+These dimensions are many times those of normal specimens.
+
+[12] Coutagne, G., _Recherches sur le Polymorphisme des Mollusques de
+France_, _Annales Soc. d'Agric. Sci. et Industr. Lyon_, 1895.
+
+[13] As to the synonymy and references see Coutagne, p. 45.
+
+[14] A. Lang, _Die Bastarde von H. hortensis Muller H. nemoralis L._
+Jena, G. Fischer, 1908; with a fine coloured plate showing the varieties
+of the species and their hybrids.
+
+[15] With this evidence compare that given by A. Delcourt in his
+valuable papers lately published relating to the variations of
+_Notonecta_. See especially _Bull. Sci. Fr. Belg._, 1909, XLIII, p. 443;
+and _C. R. Soc. Biol._, 1909, LXVI, p. 589.
+
+[16] Allen, J. A., _Bull. Amer. Mus. N. H._, III, 1891, pp. 51-54.
+
+[17] J. T. Gulick, _Evolution, Racial and Habitudinal_, Carnegie
+Institution, Publication No. 25, 1905.
+
+[18] A. G. Mayer, _Mem. Mus. Comp. Anat. Harvard_, Vol. XXVI, 1902, p.
+117. From the tables given I cannot ascertain the actual numbers from
+the two intermediate valleys, but they were considerable.
+
+[19] To which I was very kindly guided by Mr. C. T. Trechmann.
+
+[20] Standfuss, _Handbuch d. paläarkt Gross-schmet_, 1896, p. 321.
+
+[21] _Ent. Rec._, XVIII, No. 7, 1906.
+
+[22] This evidence was largely collected by Mr. G. T. Porritt, who has
+given much attention to the subject.
+
+[23] Such direct action has of course been proved to occur in the case
+of several dimorphic larvae (_e. g._, _A. betularia_, itself) and pupae.
+
+[24] See Harris, _Proc. Ent. Soc. London_, 1904, p. lxxii, and 1905, p.
+lxiii; also Hamling, _Trans. City of London Ent. Soc._, 1905, p. 5.
+
+[25] I am indebted to Mr. Outram Bangs of the Harvard Museum for calling
+my attention to this remarkable case.
+
+[26] _Auk_, 1889, VI, p. 219.
+
+[27] _Ann. N. Y. Acad. Sci._, 1878, I, p. 149.
+
+[28] _Ann. N. Y. Acad. Sci._, 1878, I, p. 149.
+
+[29] _Ibid_, 1912, pp. 523-8.
+
+
+
+
+                        CHAPTER VII
+
+                 LOCAL DIFFERENTIATION. _Continued_
+
+
+                       OVERLAPPING FORMS
+
+The facts of the distribution of local forms on the whole are consistent
+with the view that these forms come into existence by the sporadic
+appearance of varieties in a population, rather than by transformation
+of the population as a whole. Of such sporadically occurring varieties
+there are examples in great abundance, though by the nature of the
+case it can be but rarely that we are able to produce evidence of a
+previous type being actually superseded by the variety. When the two
+forms are found co-existing in the same area they are usually recorded
+as one species if intergrades are observed, and as two species if the
+intergrades are absent. On the other hand when two forms are found
+occupying separate areas, when, that is, the process of replacement is
+completed in one of the areas, then forthwith each is named separately
+either as species or subspecies. Successive observations carried out
+through considerable periods of time would be necessary to establish
+beyond question that the history proceeds in one way rather than
+another. Such continuity of observation has for the most part never
+been attempted. The kind of information wanted has indeed only been
+lately recognized, and really critical collecting is a thing of only
+the last few decades. The methods of the older collectors, who aimed at
+bringing together a few typical specimens of all distinct forms, are of
+little service in this class of inquiry, which is better promoted by the
+indiscriminate collection of large numbers of common forms from many
+localities. When this has been done on a comprehensive scale we shall be
+in a position to form much more confident judgments as to the general
+theory of evolution.
+
+Some little work of the kind has however been done and the results are
+already of great value. Seeing that the differentiation of local forms
+is only made possible by isolation, it necessarily happens that the
+collector finds one form in one locality and another in a distinct
+locality, and there is no evidence as to the behaviour which the two
+representative species might exhibit if they came into touch with each
+other. In the most familiar examples of such distinction each inhabits
+an island, completely occupying it to the exclusion of any other similar
+form. It can only be when the two representative species occupy parts
+of a continental area connected with each other by regions habitable
+for the organism in question, that there is a chance of seeing the two
+forms in contact. Often also, even where this condition is satisfied,
+the habits, social organisation, or some other special cause may
+act as a barrier which prevents the distinguishable forms from ever
+coming into such complete contact as to interbreed or to behave as a
+genetically continuous race. When genetic continuity is ensured by a
+constant diffusion of the population over the whole area which they
+inhabit there will manifestly be no formation of local races. The
+practical uniformity, for example, of so many species of birds which
+inhabit widely extended ranges of Western Europe is doubtless maintained
+by such constant diffusion. When, as in the case of the Falcons, many
+localities have peculiar forms, the fact may be taken as conclusive
+evidence that there is little or no diffusion; and when we find in such
+a species as the Goldfinch that in spite of migratory fluctuations there
+are nevertheless geographical races fairly well differentiated, it may
+similarly be inferred that these fluctuations habitually move up and
+down on paths which do not intermingle. There are however a few examples
+of animals, not given to much irregular wandering, which occupy a wide
+and continuous range of diversified country and are differentiated as
+local races in two or more districts, though the distinct races meet
+in intervening areas. Of these the most notorious illustration which
+has been investigated with any thoroughness is that of the species of
+_Colaptes_ (Woodpeckers) known in the United States as Flickers. The
+study of the variations of these forms, made by J. A. Allen[1] is an
+admirable piece of work, with which every student of variation and
+evolutionary problems should make himself familiar. The two forms with
+which we are most concerned are known as _C. auratus_ and _C. cafer_,
+and are very strikingly different in appearance. In size, proportions,
+general pattern of colouration, habits, and notes, the two are alike,
+but they differ in the following seven respects as stated by Allen.
+
+          _Auratus_                          _Cafer_
+
+  1. Quills _yellow_.                   1. Quills _red_.
+  2. Male with a _black_ malar          2. Male with a _red_
+       stripe.                               malar stripe.
+  3. Adult female with _no_             3. Adult female with usually a
+       malar stripe.                         brown malar stripe.
+  4. _A scarlet nuchal crescent         4. No nuchal crescent in
+       in both sexes._                       either sex.
+  5. Throat and fore neck               5. Throat and fore neck
+       _brown_.                               _grey_.
+  6. Whole top of head and hind neck    6. Whole top of neck and hind
+       _grey_.                                neck _brown_.
+  7. General plumage with an            7. General plumage with a
+       _olivaceous_ cast.                     _rufescent_ cast.
+
+These differences are illustrated in the accompanying coloured plate,
+which has been most kindly prepared for me under the instructions of
+Dr. F. M. Chapman of the American Museum of Natural History. Before
+going further it is worth considering the nature of these differences
+a little more closely. All but the last are large differences which no
+one would overlook even in a hasty glance at the birds. If the only
+distinction lay in the colour of the quills we might feel fairly sure
+that _auratus_ was a recessive form of _cafer_, and so probably it is in
+this respect. Similarly the black malar stripe of _auratus_ is in all
+probability recessive to the red malar stripe of _cafer_ and I imagine
+the pigments concerned are comparable with those in the Gouldian Finch
+(_Poephila gouldiae_) of Australia. Both sexes in that species may have
+the head black, red, or, less often, yellow, and though it is not any
+longer in question that birds may breed in either plumage, I believe
+that the young are always black-headed and I imagine that those which
+become red-headed possess a dominant factor absent from the permanently
+black-headed birds.[2] Yellow as a recessive form of a red is certainly
+very common, but red and black as variants of the same pigment are less
+usual. In the Gouldian Finch we seem to have a case where a pigment can
+assume all three forms. It would be interesting to know whether the red
+of the malar stripes in _Colaptes_ is a pigment of the same nature as
+the red of the quills. Both in _Colaptes_ and in _Poephila gouldiae_ I
+have seen specimens intermediate between the black and the red, and the
+appearance of the part affected was exactly alike in the two cases, red
+feathers coming up among the black ones, and many feathers containing
+both red and black pigments mixed together. The development of the
+scarlet nuchal crescent in _auratus_ and the absence of this conspicuous
+mark in _cafer_ constitute from the physiological point of view the most
+remarkable pair of differences. When the red crescent is not formed, the
+feathers which would bear it are exactly like the rest, and no special
+pigment is visible in them which one can regard as ready to be modified
+into red. If the crescent is due to a factor it must therefore be
+supposed that this factor has the power of modifying the pigment of the
+neck in one special place alone. Dr. W. D. Miller called my attention to
+the fact that a similar variation occurs in another American woodpecker,
+the Sapsucker, _Sphyropicus varius_.[3]
+
+I do not suggest that such variations are without parallel: indeed in
+_P. gouldiae_ the factor which turns the black of the head into scarlet
+affects one special region of the black only, being sharply distinct
+from the unmodified black of the throat. These regions of the head are
+however often the seat of special colours in birds.[4] So also may be
+instanced the variety of the Common Guillemot (_Uria troile_) which
+has a white line round the eyes and at the sides of the head where the
+normal has no such mark; but this line is formed in a very special
+place, the groove joining the eye to the ear, whereas the feathers
+of the nuchal crescent are not ostensibly distinguished from those
+adjacent.[5]
+
+The transposition of the brown and the grey on the back and front of the
+neck also constitutes a very remarkable difference. If either grey or
+brown depends on a factor then it must be supposed that _auratus_ has
+one of these factors and _cafer_ the other.
+
+From these several considerations it is quite clear that if _auratus_
+and _cafer_ are modifications of the same type produced by presence or
+absence of factors, several independent elements must be concerned, and
+to unravel their inter-relations would be most difficult even if it were
+possible to breed the types under observation, which is of course quite
+beyond present possibilities.
+
+The distribution of the two is as follows. On the east side of the
+Continent _C. auratus_, relatively pure, occupies the whole of Canada
+and the States from the North to Galveston. Westward it extends across
+the whole continent in the more northern region to Alaska, but in its
+pure form it only reaches down the Pacific coast to about the northern
+border of British Columbia. Its southern and western limit is thus
+roughly a line drawn from north of Vancouver, southeast to North Dakota
+and then south to Galveston. _C. cafer_ in the comparatively pure
+form inhabits Mexico, Arizona, California (except Lower California
+and the opposite coast), central and western Nevada, Utah, Oregon,
+and is bounded on the east by a line drawn from the Pacific south of
+Washington, south and eastward through Colorado to the mouth of the
+Rio Grande or the Gulf of Mexico. Between the two lines thus roughly
+defined is a band of country about 1,200-1,300 miles long and 300-400
+miles wide, which contains some normal birds of each type, but chiefly
+birds exhibiting the characters of both, mixed together in various and
+irregular ways. Even in the areas occupied by the pure forms occasional
+birds are recorded with more or less indication of characteristics
+of the other form, but within the area in which the two forms are
+conterminous, the mixed birds are in the majority. The condition of
+these birds of mixed character is described by Allen as follows:
+
+   "As has been long known--indeed, as shown by Baird
+   in 1858--the 'intermediates' or 'hybrids' present ever-varying
+   combinations of the characters of the two birds, from individuals
+   of _C. auratus_ presenting only the slightest traces of the
+   characters of _C. cafer_, or, conversely--individuals of _C.
+   cafer_ presenting only the slightest traces of the characters
+   of _C. auratus_--to birds in which the characters of the two
+   are about equally blended. Thus we may have _C. auratus_ with
+   merely a few red feathers in the black malar stripe, or with the
+   quills merely slightly flushed with orange, or _C. cafer_ with
+   either merely a few black feathers in the red malar stripe, or
+   a few red feathers at the sides of the nape, or an incipient,
+   barely traceable scarlet nuchal crescent. Where the blending
+   of the characters is more strongly marked, the quills may be
+   orange-yellow or orange-red, or of any shade between yellow and
+   red, with the other features of the two birds about equally
+   blended. But such examples are exceptional, an unsymmetrical
+   blending being the rule, the two sides of the same bird being
+   often unlike. The quills of the tail, for example, may be part
+   red and part yellow, the number of yellow or red feathers varying
+   in different individuals, and very often in the opposite sides of
+   the tail in the same bird. The same irregularity occurs also, but
+   apparently less frequently, in the quills of the wings. In such
+   cases the quills may be mostly yellow with a few red or orange
+   quills intermixed, or red with a similar mixture of yellow. A
+   bird may have the general colouration of true _cafer_ combined
+   with a well-developed nuchal crescent, or nearly pure _auratus_
+   with the red malar stripes of a _cafer_. Sometimes the body
+   plumage is that of _C. auratus_ with the head nearly as in pure
+   _cafer_, or exactly the reverse may occur. Or we may have the
+   general plumage as in _cafer_ with the throat and crown as in
+   _auratus_, and the malar stripe either red or black, or mixed
+   red and black, and so on in almost endless variations, it being
+   rare to find, even in birds of the same nest, two individuals
+   alike in all their features of colouration. Usually the first
+   trace of _cafer_ seen in _auratus_ manifests itself as a mixture
+   of red in the black malar stripe, either as a few red feathers,
+   or as a tipping of the black feathers with red, or with merely
+   the basal portion of the feathers red. Sometimes, however, there
+   is a mixture of orange or reddish quills, while the malar stripe
+   remains normal. In _C. cafer_ the traces of _auratus_ are usually
+   shown by a tendency to an incipient nuchal crescent, represented
+   often by merely a few red-tipped feathers on the sides of the
+   nape; at other times by a slight mixture of black in the red
+   malar stripe."
+
+Such a state of things accords very imperfectly with expectations under
+any received theory of Evolution. As in some of the instances discussed
+in the first chapter we have here two fairly definite forms, nearly
+allied, which on any evolutionary hypothesis must have been evolved
+either the one from the other, or both from a third form at a time not
+very remote from the present, as time must be measured in evolution. Yet
+though intermediates exist in some quantity, no one can for a moment
+suggest that they are that definite intermediate from which _auratus_
+and _cafer_ descend in common. One cannot imagine that the immediate
+ancestor of these birds was a mosaic, made up of asymmetrical patches
+of each sort: but that is what many of the intermediates are. It is not
+much easier to suppose the ancestor to have been a nondescript, with a
+compromise between the developed characters of each, with quills buff,
+malar stripes neither black nor red, with a trace of nuchal crescent,
+and so on. Such Frankenstein-monsters have played, a considerable part
+in the imaginations of evolutionary philosophers, but if it were true
+that there was once a population of these monsters capable of successful
+existence, surely they should now be found as a population occupying the
+neutral zone between the two modern forms. Yet, though much remains to
+be done in clearing up the facts, one thing is certain, namely that the
+neutral zone has not a definite and normally intermediate population,
+but on the contrary it is peopled by fragments of the two definite types
+and miscellaneous mongrels between them.
+
+On the other hand, one cannot readily suppose that either form was
+the parent of the other. The process must have involved both addition
+and loss of factors, for whatever hypothesis be adopted, such changes
+must be supposed to have occurred. A careful statistical tabulation of
+the way in which the characters are distributed in the population of
+the mixed zone would be of great value, and till that has been done
+there is little that can be said with certainty as to the genetics of
+these characters. In the collection of Dr. Bishop of New Haven I was
+very kindly allowed to examine a sample, all taken at random, near
+together, in Saskatchewan. There were females 4 adult, 2 young; males
+4 adult and 5 young. This number, though of course insufficient, is
+enough to give some guide as to the degree of definiteness which the
+characters generally show in their variations. Of the 15 birds, 8 had
+simply yellow quills; 2 had red; 1 was almost red but had one yellow
+tail-quill; 3 were intermediate and 1 was buff. As regards the malar
+patch, which can only be determined properly in the adult males, 1 was
+red, 1 was approximately red, 2 intermediate. As to nuchal crescent 4
+females had none, 2 females very slight; 7 males had it, 1 had only a
+slight crescent, and 1 had none. In point of quills therefore 10 were
+definite out of 15; in point of crescent, 11 were definite out of 15;
+and in point of malar patch 1 only was definite out of 4. The last is a
+feature directly dependent on age and so counts for less, but as regards
+the other two features there is some indication that the factors show
+definiteness in their behaviour. It must be remembered that we have no
+knowledge what the heterozygous form may be, and in the case of red
+and yellow it is probably a reddish buff. The patch-works are no doubt
+to be compared with other well-known pied forms, and in these we must
+suppose the active factor broken up, which it probably can be very
+easily. The asymmetry, which Allen notices as so marked a feature, in
+the distribution of the red and yellow quills of the tail especially,
+recalls that of the black markings in the pied Canaries. As is well
+known to students of variations _some_ pigment-factors in _some_ animals
+are apparently uncontrolled by symmetry, while in other specific cases
+symmetry is the rule. On the other hand the blackness or redness of
+the malar patches is, I think, as a rule nearly symmetrical. It should
+be mentioned that two of Dr. Bishop's young birds belonged to the same
+nest, one a female with _red_ quills, the other a male with _yellow_.
+Both are without crescent.
+
+As to the question whether certain combinations of characters occur
+with special frequency, the evidence is insufficient to give a definite
+answer. Among all the birds I have seen in America or in England I
+have not yet found one having the malar patches black without any
+nuchal crescent. Of Dr. Bishop's 8 adults not one, however, showed the
+combination of the three chief features normal for _auratus_ or for
+_cafer_.
+
+Besides the two forms that we have hitherto considered, several other
+local types exist, and these throw some further light on the problem.
+Of these the most important in this connexion is _chrysoides_, which
+inhabits the whole of southern California and the mainland opposite.
+This remarkable form is as Allen says, very different from _auratus_
+except that it has the quills yellow like _auratus_, not red like
+_cafer_. So that we find here in the extreme west of the whole
+distribution a type agreeing in one of its chief features with the
+eastern type. Between this and _cafer_ intergrades have, according to
+Allen, not been found. The relations of this _chrysoides_ are, Allen
+thinks, rather with _mexicanoides_, a southern, smaller race with
+colours more intense, which inhabits Guatemala, but however that may
+be, it must be regarded as a _cafer_ which has lost its red quills. The
+island of Guadeloupe off Lower California has an island form. Beyond the
+other side of the continent there is also an island form of _auratus_,
+inhabiting Cuba, so that clearly the yellow quills can extend into the
+tropics.
+
+The above account is in many respects incomplete, but it suffices to
+give an outline of the chief facts. The whole problem is complicated by
+the undoubted effects of an uncertain amount of migration, and in many,
+perhaps all, districts, the winter population differs from the summer
+population of the same localities. The existence of these seasonal ebbs
+and flows is now well known to ornithologists, and most of the bird
+species of temperate regions are subject to them.
+
+Difficult as it may be to conceive the actual process of origin of the
+two types _auratus_ and _cafer_, it is I think still harder to suggest
+any possible circumstance which can have determined their development as
+distinct races, or which can maintain that distinctness when created.
+Some will no doubt be disposed to appeal once more to our ignorance
+and suggest that if we only knew more we should see that the yellow
+quills, the black "moustache" and the red crescent, specially qualify
+_auratus_ for the north and eastern region, and the red quills, red
+"moustache" and absence of crescent fit _cafer_ to the conditions of its
+homes. Each can judge for himself, but my own view is that this is a
+vain delusion, and that to cherish it merely blunts the receptivity of
+the mind, which if unoccupied with such fancies would be more ready to
+perceive the truth when at last it shall appear. Think of the range of
+conditions prevailing in the country occupied by _auratus_--a triangle
+with its apex in Florida and its base the whole Arctic region of North
+America. Is it seriously suggested that there is some element common to
+the "conditions" of such an area which demands a nuchal crescent in the
+Flickers, though the birds of the _cafer_ area, almost equally varied,
+can dispense with the same character? Curiously enough, the geographical
+variation of _Sphyropicus varius_, another though a very different
+Woodpecker[6] shows that conversely the nuchal crescent can be dispensed
+with in the Eastern form though it is assumed by the Western.[7]
+
+Allen points out the interesting additional fact that superposed
+upon each of the two distinct forms, _auratus_ and _cafer_, are
+many geographical variations which can very naturally be regarded
+as climatic. Each decreases in size from the North southward, as so
+many species do.[8] They become paler in the arid plains, and show
+the ordinary phases which are seen in other birds having the same
+distribution. Such differences we may well suppose to be determined
+directly or indirectly, by environment, and we may anticipate with
+fuller knowledge it will be possible to distinguish variations of this
+nature as in the broad sense environmental, from the larger differences
+separating the two main types of _Colaptes_, which I surmise are
+altogether independent of such influences.
+
+It is generally supposed that phenomena like those now so well
+established in the case of _Colaptes_ are very exceptional, and as has
+already been stated a number of circumstances must combine in order
+that they may be produced. I suspect however that the examples are
+more numerous than is commonly thought. In all likelihood the three
+forms _Sphyropicus varius_, _nuchalis_ and _ruber_ are in a very
+similar condition though the details have not, so far as I know, been
+worked out. A complex example which is closely parallel to the case of
+_Colaptes_ was described by F. M. Chapman[9] at the same date as Allen's
+work. This is the case of _Quiscalus_, the Grackles, which in the North
+American Continent have three fairly distinct forms which Chapman speaks
+of as _Q. aeneus_, _Q. quiscula_, and _Q. quiscula aglaeus_. The birds
+are all, so far as pigment is concerned, dark blackish brown, but the
+head and mantle have superposed a metallic sheen of interference-colours
+which in the various forms take different tints, bluish green, bronze
+green, or bronze purple. The details are complicated and difficult
+to appreciate without actual specimens, but the two common types are
+sufficiently distinct. The birds inhabit the whole area east of the
+Rockies, _quiscula aglaeus_ occupying Florida and the Southern States
+southwest of a band of country about a hundred miles broad extending
+roughly from Connecticut to the mouth of the Mississippi; and _aeneus_
+taking the area north and west of this band. In discussing this case
+Chapman expresses the same view as Allen does in the _Colaptes_ case,
+that there are two distinct populations, substantially fixed, and
+that the band of country in which they meet each other has a mongrel
+population, with no consistent type, but showing miscellaneous
+combinations of the character of the two chief types.
+
+The warblers of the genus _Helminthophila_ provide another illustration
+which has points of special interest. The two chief species are _H.
+pinus_, which has a yellow mantle and lower parts, white bars on the
+wings, a black patch behind the eyes and a broad black mark on the
+throat; and _H. chrysoptera_ with dark grey mantle and pale whitish grey
+lower parts, yellow bars on the wings, and grey marks on cheeks and
+throat where _pinus_ has black. These two birds are exceeding distinct,
+and in addition their songs are quite unlike. _H. pinus_ ranges through
+the eastern United States up to Connecticut and Iowa. _H. chrysoptera_
+is a northern form extending down to Connecticut and New Jersey. Both
+are migrants.
+
+In these two States, where the two types overlap, certain forms have
+been repeatedly found which have been described as two distinct species,
+_Lawrencei_ and _leucobronchialis_. Dr. L. B. Bishop and Mr. Brewster
+showed me two long series of _Helminthophila_ containing various
+intergrades between the four named kinds, and details regarding these
+may be found in Chapman's _North American Warblers_ and in Dr. Bishop's
+paper in Auk, 1905, XXII. Though the characters evidently break up to
+some extent, the series can be represented as due to recombinations of
+definite factors more easily than the others which I have described. The
+differentiating characters are:
+
+               _Pinus_
+
+  1.  Mantle and lower parts _yellow_ (Y^1).
+  2.  Wing-bars _white_ (y^2).
+  3.  Cheek and throat _not black_ (b).
+
+
+             _Chrysoptera_
+
+  1. Mantle and lower parts _grey_ (y^1).
+  2. Wing-bars _yellow_ (Y^2).
+  3. Cheek and throat _black_ (B).
+
+The grey pigment of the mantle is common to both, but is masked by the
+yellow in _pinus_, the net result being an olive-green.[10]
+
+I am much indebted to Dr. F. M. Chapman for the loan of the coloured
+plate in which these distinctions are shown. It first appeared in his
+book, _North American Warblers_.
+
+We cannot tell whether _yellow_ or _not-yellow_ is due to the presence
+of a factor, but we may suppose that one or other gives the special
+colour to the parts. The black of character 3 is no doubt a dominant.
+Thus _pinus_ becomes Y^{1}y^{2}b and _chrysoptera_ in y^{1}Y^{2}B. The
+_Lawrencei_ which has the underparts _yellow_, wing-bars _white_, and
+_black_ patches is Y^{1}y^{2}B and _leucobronchialis_ which has mantle
+and underparts _not-yellow_, wing-bars _yellow_ and _no black patches_
+is y^{1}Y^{2}b. This representation, it should be clearly understood,
+is tentative and approximate only. The characters are not really sharp,
+for there is much grading; but allowing for the effects of heterozygosis
+and for some actual breaking-up of factors I believe it gives a fairly
+correct view of the case. In particular we can see how it meets the
+difficulty which Chapman felt in accepting _leucobronchialis_ as in any
+sense derived from _pinus_ which has a yellow breast, and _chrysoptera_
+which has a black throat, seeing that _leucobronchialis_ has neither.
+We now recognize at once that this form could be produced by ordinary
+re-combination of the absence of Y^{1} with the absence of B.
+
+I note also with great interest that the modern observers agree that the
+so-called hybrids may have the song either of the one species, or of the
+other, or a song intermediate between the two. It may also be added that
+these two types have several times been seen, in the breeding season,
+paired with each other or with one of the other combinations.
+
+[Illustration: FIG. 1. _Helminthophila pinus_, male.
+
+FIG. 2. _Helminthophila pinus_, female.
+
+FIG. 3. "Lawrence's Warbler," male; one of the integrading forms.
+
+FIG. 4. "Brewster's Warbler," male; another of the integrading forms.
+
+FIG. 5. _Helminthophila chrysoptera_, male.
+
+FIG. 6. _Helminthophila chrysoptera_, female.]
+
+Allen[11] has described another excellent American example, the Tits of
+the group _Baeolophus bicolor-atricristatus_. The form _bicolor_ belongs
+to the eastern States and ranges from the Atlantic coast to the Great
+Plains, and _atricristatus_, of east Mexico, extends from Vera Cruz to
+central Texas. In southern and central Texas the breeding ranges adjoin,
+and in this country various intermediates occur. The chief types differ
+in two main points.
+
+         _B. bicolor_
+
+  Forehead varies from deep _black_
+    to dull black, suffused with
+    rusty brown.
+
+  Crown and crest _grey_,
+    slightly darker than the back.
+
+
+        _B. atricristatus_
+
+  Forehead _white_ to buffish white.
+
+  Crown and crest _black_, abruptly
+    contrasting with the back.
+
+The intergrades between the two have, as usual, received specific names.
+A detailed description is given by Allen, from which it appears that
+the gradation is very complete. In one case a series of 16 adults were
+all intermediates. It is not stated whether the collector took these at
+random, but from the local lists it is clear that the types are found
+not far away from the place where the intergrades were shot.
+
+Another very striking case is that of the Tanagers, of the genus
+_Rhamphocoelus_. In this group there are several local forms which
+are related to each other in remarkable ways. The forms known as
+_passerinii_ and _icteronotus_ exhibit the clearest phenomena of
+intergradation. The species _passerinii_ has a brilliant scarlet
+and black male, and it inhabits Honduras and Nicaragua. Proceeding
+southwards along the isthmus we find next _costaricensis_ which has
+a male like that of _passerinii_ (but a female with more orange than
+the olive-grey female of _passerinii_). Next we come to Panama which
+is occupied by _icteronotus_, sharply distinguished from _passerinii_
+by the fact that the _scarlet is replaced by lemon-yellow_. This same
+_icteronotus_ occurs again as a pure type in Ecuador and many other
+parts of South America; but Colombia, _between Panama and Ecuador_,
+contains scarlets like _passerinii_, yellows like _icteronotus_, and
+various intergrades of several shades of orange. The _passerinii_ males
+from Nicaragua are indistinguishable from those of Colombia, and the
+_icteronotus_ of Ecuador are the same as those in Panama. The orange
+intergrades, doubtless heterozygous forms, though collected at the
+same locality (Medellin in Colombia) as several pure yellows and pure
+scarlets, are in the British Museum series sorted out as a separate
+species under the name _chrysonotus_! Complications are introduced by
+the relations of these forms to another named type, _flammigerus_, but
+we may for our purpose leave that out of consideration, and say that the
+order of geographical sequence from Honduras to Ecuador is (1) scarlet,
+(2) yellow, (3) mixture of types, scarlet, yellow, orange, (4)yellow.
+
+Similar examples exist in the birds of the old world, but I do not know
+of any that have been studied so fully as those of America. The best
+known is that of the two Rollers, _Coracias indicus_ which spreads from
+Asia Minor through Persia, Baluchistan, the Indian Peninsula and Ceylon,
+and _affinis_ which ranges from Nepal, through Assam, Tenasserim and
+the Indo-Chinese countries. The two types are very different and may be
+distinguished as follows:
+
+        _C. indicus_
+
+  _Mantle_ drab brown-chestnut.
+  _Breast_ chestnut.
+  _Throat_ purplish, streaked with white.
+  _Upper tail-coverts_ indigo.
+
+
+        _C. affinis_
+
+  Dark olive-green.
+  Dull purple brown.
+  Purple, streaked with blue.
+  Turquoise.
+
+The wings are the same in both. In the provinces of Nepal, Sikhim, and
+Darjiling the two species coexist, with the result that intergrades
+have been frequently recorded. The line of intergradation extends to
+the coast, and birds showing various combinations of the two types from
+the Calcutta district exist in collections.[12] The case is interesting
+inasmuch as like that of _Quiscalus_ it shows a series of combinations
+of various metallic colours. Some of these are probably evoked by
+the development of pigment behind striations or other interferences
+already existing, but in the present state of knowledge it would be
+quite impossible to suggest what the actual factors producing these
+appearances may be.
+
+There are, naturally, many other cases among birds which are suspected
+of being in reality comparable, but in most of them the evidence is
+still inadequate. Among Lepidoptera also there are a few of these;
+perhaps the most striking is that of _Basilarchia "proserpina."_[13] The
+genus is well known to European collectors under the name _Limenitis_,
+of which we in England have one species, _L. sibylla_, the "White
+Admiral." A species very like _sibylla_ in general appearance is common
+in the northern parts of the United States, ranging through Canada and
+Northern New England, but rarely south of Boston. This species has the
+conspicuous white bands across both wings like our _sibylla_.
+
+There is also a more Southern type known as _astyanax_, which is very
+different in its appearance, being without the white bands and having
+a broad irroration of blue scales on the posterior border of the hind
+wings. The two are so distinct that one would not be tempted to suspect
+any very close relation between them. In its distribution _astyanax_ is
+described by Field as replacing arthemis south of latitude 42°. About
+Boston it is much more common than _arthemis_.
+
+The two forms encroach but little on each other's territory, but where
+they do coexist, a third form, known as _proserpina_, is found which is
+almost intermediate, with the white bands much reduced. There is now
+no doubt that this _proserpina_ is a heterozygous form, resulting from
+a combination of the characters of _arthemis_ and _astyanax_. Field
+succeeded in rearing a brood of 16 from a _proserpina_ mother caught
+wild which laid 31 eggs, and of these, nine (five males, four females)
+resembled the mother, being _proserpina_, and seven (four males, three
+females) were _arthemis_. There can be no question therefore that the
+mother had been fertilised by a male _arthemis_ and that _no-white-band_
+is a factor partially dominant over the _white band_. Another point of
+interest which Field observed was that the _proserpina_ female refused
+to lay on birch, poplar or willow, but accepted wild cherry (_Prunus
+serotina_) a species on which _astyanax_ can live, though that tree is
+not known to be eaten by _arthemis_. Incidentally also the observations
+show that sterility cannot be supposed to be the bar which maintains the
+distinctness of _arthemis_ and _astyanax_.
+
+In this connection _Papilio oregonia_ and _bairdii_ should be
+mentioned.[14] _P. oregonia_ is one of the numerous forms like
+_machaon_, but rather paler. It is a northern insect, inhabiting British
+Colombia east of the Cascade Range, and reaching to Colorado. _P.
+bairdii_ is a much darker butterfly, representing the _asterias_ group
+of the genus _Papilio_. Like _asterias_ it has the abdomen spotted at
+the sides, not banded as in the _machaon_ group. It belongs to Arizona
+and Utah extending into Colorado. From Colorado the form _brucei_ is
+described, more or less intermediate, like _bairdii_ but with the
+abdomen banded as in _oregonia_. W. H. Edwards records the results of
+rearing the offspring of the _bairdii_-like and of the _oregonia_-like
+mothers. Each was found able to have offspring of both kinds, that is to
+say, _bairdii_ females gave both forms, and _oregonia_ females gave both
+forms. It is not possible to say which is dominant, since the fathers
+were unknown. On general grounds one may expect that the _bairdii_ form
+will be found to dominate, but this is quite doubtful.
+
+From this particular discussion I omit reference to those examples in
+which the permanently established types are obviously associated with
+special conditions of life. Where considerable climatic differences
+exist between localities, or when we pass from South to North, or from
+the plains into Alpine levels we often find that in correspondence
+with the change of climate there is a change in the characteristics of
+a species common to both. When I say "species" in such a connection I
+am obviously using the term in the inclusive sense. Some would prefer
+to say that in the two sets of conditions two _representative species_
+exist. Whichever expression be preferred it is plain that such examples
+present another phase of the problem we have been just considering, and
+in them also we have an opportunity of observing the consequences of
+the overlap of two closely related types, but there are advantages in
+considering them separately. In the examples hitherto given, with the
+possible exception of the Papilios,[15] the two fixed types severally
+range over so extensive a region that it may fairly be supposed that
+in the different parts they are subject to considerable diversities of
+climate. There is no outstanding difference that we know distinguishing
+the habitats of the two forms; but in comparing Alpine with Lowland
+forms, or essentially northern with essentially southern forms we do
+know an external circumstance, temperature, that may reasonably be
+supposed to have an influence, direct or indirect, on the population.
+
+FOOTNOTES:
+
+[1] J. A. Allen, _The North American Species of the Genus Colaptes,
+Considered with Special Reference to the Relationships of C. auratus and
+C. cafer_. Bull. Am. Mus. Nat. Hist., IV, 1892.
+
+[2] For a case in which a red-headed female × a black-headed male gave a
+black-headed female and a red-headed male, see _Avian Mag._, N. S., IV,
+pp. 49 and 329
+
+[3] The other variations of this bird are also interesting and
+important. The normal male has a red head and a red throat. The female
+has a red head and a white throat, but varieties of the female are known
+with a black head, thus again illustrating the change from black to
+red. It should be noted that this is not a mere retention of a juvenile
+character, but, as the birds mature, the red feathers come up, or as an
+exception, the black. There is also a western species, _ruber_, in which
+both sexes have a great extension of red, and are alike. The male of
+_nuchalis_ intergrades with this type, but the female does not.
+
+[4] Dr. W. Brewster, for example, has a remarkable specimen of the Teal
+(_Nettion carolinense_) with a white collar strongly developed at the
+front and sides of the neck, in a place where the normal has no such
+mark.
+
+[5] This variety is spoken of as the Ringed Guillemot and is sometimes
+regarded as a distinct species to which the name _ringvia_ was given by
+Brünnich. In support of this view Dr. William Brewster, to whom I am
+indebted for much assistance in regard to the variation of birds, called
+my attention to observations of his own and also of Maynard's, that the
+ringed birds were sometimes mated together, though in a small minority
+(see Brewster, _Proc. Boston Soc. N. H._, XXII, 1883, p. 410). It
+would however be possible to produce many instances of varieties mated
+together though surrounded by a typical population (_e. g._, two varying
+Blackbirds, _Zoologist_, p. 2765; two varying Nightjars, _ibid._, p.
+5278). I am inclined to believe that in nature matings between brothers
+and sisters are frequent in many species of animals, and that the
+production of sporadically varying colonies is thus greatly assisted.
+
+[6] The Sap-suckers feed on trees and somewhat resemble our Spotted
+Woodpeckers in general appearance. _Colaptes_ feeds on the ground and
+corresponds perhaps rather with the European Green Woodpecker.
+
+[7] For an introduction to this example I am indebted to Mr. W. D.
+Miller of the American Museum of Natural History. Some account of the
+facts is given by Baird, Brewer, and Ridgway (_A Hist. of N. Amer.
+Birds_. 1874, II, pp. 540, 544, etc.). _S. varius_ occupies the whole
+country in suitable places from the Atlantic to the eastern slopes of
+the Rockies, and all Mexico to Guatemala. _S. nuchalis_ was first known
+from the Southern Rockies only, but many were afterwards taken in Utah.
+_S. ruber_ is restricted to the Pacific coast. In Ridgway's opinion all
+three are geographical forms of one species. In _ruber_ the sexes are
+alike having both a great extension of the red in the throat, and a red
+crescent. The male of _nuchalis_ grades to the _ruber_ form, but the
+female does not. This female has some red in the throat like the male of
+_varius_, whereas the female of _varius_ has a whitish throat.
+
+[8] Not only vertebrates but the marine Crustacea and Mollusca
+illustrate this curious "principle" of variation, as Canon Norman
+formerly pointed out to me with abundant illustrations. There are of
+course cases to the contrary also.
+
+[9] Chapman, F. M., _Bull. Amer. Mus._, IV, 1892, p. 1; see also
+Ridgway, _Birds of North and Middle America_, 1902, Part II, p. 214.
+
+[10] It would aid greatly in factorial analysis if the descriptive term
+"green" could be avoided in application to cases where the green effect
+is due only to a mixture of black and yellow pigments. The absence of
+yellow is the sole difference between the mantle and underparts of
+_pinus_ and _chrysoptera_.
+
+[11] _Bull. Amer. Mus. Nat. Hist._, XXIII, 1907, p. 467.
+
+[12] References on this subject will be found in _Brit. Mus. Cat.
+Birds_, XVII, p. 13.
+
+[13] For these facts I am indebted to Mr. W. L. W. Field, who has
+lately published an account of his observations and experiments. See
+especially, _Psyche_, 1910, XVII, No. 3, where full references to
+previous publications are given.
+
+[14] For the facts and further references see W. H. Edwards,
+_Butterflies of N. America_, 2d series, Papilio VII and X; 3d series,
+1897, Papilio IV, _Can. Entom._, 1895, XXVII, p. 239.
+
+[15] I think this case is fairly included because the _machaon_ type
+is so widespread that it cannot be regarded as a product of a Northern
+climate, nor can _asterias_ be claimed as especially a warm country
+form, seeing that _brevicauda_, which is scarcely distinguishable from
+_asterias_, inhabits Newfoundland (having a curious phase there in which
+the yellow is largely replaced by red).
+
+
+
+
+                        CHAPTER VIII
+
+              LOCALLY DIFFERENTIATED FORMS. _Continued._
+
+
+                    CLIMATIC VARIETIES
+
+In this chapter we will examine certain cases which illustrate phenomena
+comparable with those just considered, though as I have already
+indicated, they form to some extent a special group. The outstanding
+fact that emerges prominently from the study of the local forms is that
+when two definite types, nearly allied, and capable of interbreeding
+with production of fertile offspring, meet together in the region
+where their distributions overlap, though intergrades are habitually
+found, there is no normally or uniformly intermediate population
+occupying the area of intergradation. Such phenomena as these must, I
+think, be admitted to have great weight in any attempt to construct a
+theory of evolution. True we must hesitate in asserting their positive
+significance, but I see no escape from the conclusion that they throw
+grave doubt on conventional views. Again and again the same question
+presents itself. If _A_ and _B_ lately emerged from a common form why is
+that common form so utterly lost that it does not even maintain itself
+in the region of overlapping? Almost equally difficult is it, in the
+cases which I have numerated, to apply concrete suggestions based on
+any factorial scheme. We may see that in _Heliconius erato_ the type
+with the red mark on the hind wing probably contains a dominant factor,
+and that where the red mark is absent the metallic colours are exposed;
+and that similarly the green metallic colour may have another factor
+which distinguishes it from the blue. In this way we can fairly easily
+represent the various types of _erato_ on a factorial system as the
+result of the various possible combinations of two pairs of factors. But
+there we stop, and we are quite unable to suggest any reason why one
+area should have the red and the green type while another should have
+the blue also. So again with _Colaptes_ or the Warblers. By application
+of a factorial system, admittedly in a somewhat lax fashion, the
+genetic interrelations of the types can be represented; but how it comes
+about that each type maintains a high degree of integrity in its own
+region we can only imagine. Each has in actual fact a stability which
+the intermediate forms have not, but we cannot yet analyse the nature
+of that stability. Mendelian conceptions show us how by segregation the
+integrity of the factors can be in some degree maintained, but not why
+certain combinations of factors should be exceptionally stable. All that
+is left us to fall back on is the old unsatisfying suggestions that some
+combinations _may_ have greater viability than others, that there _may_
+be a tendency for like to mate with like, and so forth.
+
+These difficulties acquire more than ordinary force in those cases in
+which the two fixed types inhabit regions differing in some respect
+so obvious and definite that we are compelled to regard each type as
+climatic and as specially adapted to the conditions. When for example
+an animal has a distinct type never met with except in Arctic or Alpine
+conditions, and another type proper to the plains and temperate regions,
+what are the characteristics of the population of intermediate latitudes
+or at intermediate levels? Some of the examples discussed in the last
+chapter may be instances of this very nature, but even if they are not,
+others are forthcoming which certainly are. The evidence of these cases
+leads to the suspicion that with further knowledge they will be found
+to consist of two classes, some in which the observer as he passes from
+the one climate to the other will find the intermediate area actually
+occupied by a population of intermediate character, and others in which,
+though we may presume the maintenance of intermediate conditions in
+the transitional area, there is no definite transitional population.
+This interrupted or discontinuous distribution seems, so far as I have
+means of judging, to be by far the more common of the two. I do not
+doubt that by sufficient search individuals representing every or almost
+every transitional form can be found, but it is apparently rare that
+_populations_ corresponding to these several grades can be seen. The
+question has in few if any cases been studied with precision sufficient
+to provide a positive answer; but I suspect that real and complete
+continuity, in the sense thus defined, will only be found where the
+character of the local populations depends _directly_ on the conditions
+of life, and shows an immediate response to changes in them apart from
+that postponed response which we suppose to be achieved by selection.
+Obviously the character must be one, like size for instance, capable of
+sensibly complete gradation.
+
+The only example I have met with of the phenomenon of anything like a
+complete intergradation between local types really distinct in kind is
+that provided by the butterfly _Pararge egeria_. It is well known to
+entomologists that this insect exists in two very different types, a
+northern one, the "Speckled Wood" of England, in which the spots are
+a pale whitish yellow, and a southern type having the full fulvous
+colour that we know as characteristic of _megaera_, the "Gatekeeper."
+It appears that Linnaeus gave the name _egeria_ to the southern
+type,[1] and our own is now called _egerides_. Broadly speaking, so
+far as Great Britain, France, and the Spanish Peninsula are concerned,
+the tawny-coloured _egeria_ occupies Spain and western France up to
+the latitude of Poitiers and the pale yellow _egerides_ extends from
+Scotland, where it has a scanty distribution, through southern England,
+where in suitable localities it is common, and the north of France
+to Paris.[2] The two types when placed side by side are strikingly
+different from each other, and are an excellent illustration of what is
+meant by climatic variation. The insect is not a great traveller and
+probably scarcely ever wanders far from its home. It should therefore
+be possible by collecting from north to south to find out how the
+transition is effected, whether suddenly or gradually. This at various
+times I have endeavoured to do, but I am still without exact information
+as to the population in certain critical areas. In addition to the
+information derived from specimens which I have collected or seen
+in the collections of others there is a good account of the general
+distribution in Europe given by the Speyers,[3] who evidently paid
+more attention to the subject than most lepidopterists have done, and
+many more recent records. In particular Oberthür[4] has published many
+details as to the distribution in western France and I am especially
+indebted to Mr. H. Rowland-Brown for a long series of notes as to the
+distribution in France generally, and to Mr. H. E. Page and Dr. T. A.
+Chapman, Mr. Oberthür Prof. Arrigoni degli Oddi, Mr. H. Williams and
+other correspondents, for showing me forms from many localities. The
+butterfly is attached for the most part to woods of deciduous trees and
+to country abounding in tall hedges or rough scrub. It is not usually to
+be found in highly cultivated districts or in very dry regions. Hence
+there is necessarily some want of continuity in the distribution at the
+present time and I should think a mile or two of arable land without big
+hedges would constitute a barrier hardly ever passed. The larva feeds
+on several coarse grasses, especially _Dactylis glomerata_. Barrett
+mentions also _Triticum repens_. In this country the winter is usually
+passed in the larval stage, but I have found that in captivity, at
+least, there is much irregularity. The larvæ feed whenever the weather
+is not very cold and may pupate, but if sharp cold comes on when they
+are pupating or nearly full-grown they often get killed unless protected.
+
+Some writers speak of a difference between the early and later broods,
+but I have never noticed this, and I do not think that the general tone
+of the yellow is affected by the seasons (see Tutt, _Ent. Rec._, IX,
+1897, p. 37).[5]
+
+Beginning at the south of Spain the thoroughly fulvous type _egeria_
+is common at Gibraltar in the Cork woods, at Granada, and doubtless
+generally. Lederer is said to have found only this type in Spain
+(Speyer), and though I have no precise information as to other places
+in the Peninsula north of Jaen I feel tolerably sure that there is no
+change from south to north.[6] Immediately north of the Pyrenees we
+still meet _egeria_ exclusively, and up to Poitiers at least there is no
+noticeable change. But somewhere between Poitiers and the bottom of the
+Loire valley at Tours, the genuine southern type comes to an end, and
+the whole population begins at the Loire to be of an intermediate type,
+easy to distinguish both from _egeria_ and from _egerides_. As to the
+exact condition of the species in the fifty miles separating St. Savin
+on the Vienne from places on the Loire I have no adequate information. I
+have only one small sample from there, but it does contain insects both
+of the southern and intermediate types taken on the same day, in a wood
+near Preuilly. Oberthür also states that at Nantes the true southern
+form exists in company with the northern. From this I infer that the
+southern form extends up the coast further than it does inland, but
+I imagine the representative spoken of as northern would be of usual
+Brittany or intermediate type.
+
+The Vienne river joins the Loire, so the true southern type reaches over
+into the basin of the Loire. From the Loire (Tours, Corméry) north to
+Calvados (Balleroy) only the intermediate is found, so far as I know,
+and the same type extends over Brittany.[7] In general, however, the
+woods near Paris have the thoroughly northern type _egerides_, but at
+St. Germain-en-Laye and at Etampes (Oberthür) the population approaches
+the intermediate type.
+
+On the whole the intermediate type is certainly less homogeneous than
+either of the extremes, and females with the two central spots either
+paler or more fulvous than the rest are not uncommon, but I have never
+taken one on the Loire or in Brittany which I should class with either
+of the extreme types.
+
+Before speaking of the distribution in other parts of France and in
+Europe generally I will briefly state the results of my breeding
+experiments. The work was done many years ago before we had the
+Mendelian clue, and it is greatly to be hoped that some one will
+find opportunities of repeating it. Crossing the English and the
+thoroughly southern type the families produced agree entirely with
+the intermediates of Brittany and the Loire. Reciprocals are alike.
+Of F_{2} I only succeeded in raising very few and of those that I had
+(about 30) nearly all were intermediate in character, though perhaps
+rather less uniform than F_{1}. One family alone, containing only 4
+specimens, had one _egerides_, and three fulvous intermediates. As the
+case stands alone I hesitate whether or not to suppose it due to some
+mistake. Moreover from F_{1} crossed back with the respective parental
+types I had fairly long series, especially from F_{1} × the southern
+type, and looking at these families I cannot see any clear evidence
+of segregation. On the contrary, I think that though there are slight
+irregularities, they would, taken as a whole, be classed as coming
+between the intermediate type and the extreme form used as the second
+parent. This at least is true when the second parent was of the southern
+type.
+
+On this evidence I have regarded the case as one in which there is
+no good evidence of segregation and as conforming most nearly with
+the conventional view of gradual transition in response to climatic
+influences. Such influence must however be indirect; for I reared five
+generations of the northern type in England, and these, though they
+included several abnormal-looking specimens in the last generation and
+then died out, did not show any noticeable change from the fulvous
+colour of the wild type. Merrifield[8] also found that heat applied to
+pupae of the northern type produced no approach to the southern type.
+
+Looking at the facts now in the light of more experience it seems to me
+just possible that the case may be one in which, as in Nilson-Ehle's
+Wheats, the dominant differs from the recessive in having two pairs of
+factors with similar effects. The fulvous type for example may have
+two or more elements in separate pairs which together produce the full
+effect, and the intermediate may have one of these. If this were so,
+some segregation should of course eventually be observable, but the
+proportion of the various fulvous and fulvous-intermediate individuals
+would be large, and the reappearance of actual representatives of the
+northern type might be rare. I admit that this is a somewhat strained
+interpretation of the facts, and as yet it is not entitled to serious
+consideration. Nevertheless I am led to form some such expectation
+partly from the great difficulty in the way of any other, partly from
+the evidence of the small mixed sample found at Preuilly and partly from
+the statements given by Oberthür. There are moreover other features in
+the general distribution of the species which make it improbable that
+the dependence on climate can after all be so close. Published lists
+are unfortunately of little use in deciding which form occurs at a
+particular place, because, since the name _Meone_ has ceased to be used
+for the southern form, there is no complete unanimity among authors as
+to the application of the names _egeria_ and _egerides_, and unless
+more particulars are given, either name may be used for either form.
+Besides this, difficulty arises from the fact that the intermediate
+type is not generally distinguished at all, and English collectors
+finding it, may easily record it as the southern type. From Staudinger's
+note on the distribution, I gather that he, on the contrary, reckoned
+the intermediate with the northern type, as do the Speyers also. The
+late Mr. J. W. Tutt was careful to distinguish the three forms and has
+left several useful records. Easy therefore as it might seem to be to
+make out the distribution of such a familiar insect in its various
+modifications, there are serious practical difficulties, and until
+long series are brought together with this special object in view many
+obscurities will remain.
+
+With only the series from England, the west of France, and Spain before
+one it would be easy to regard the successive series of tones as a fair
+measure of climate; the brighter the colour, the hotter might one expect
+the locality to be. Such rough correspondence is often to be observed
+in butterflies and birds. It becomes impossible to take these simple
+views in the light of more complete knowledge. Beginning with France
+the fulvous _egeria_ occupies the lower valley of the Rhone, probably
+from well above Lyon, though I have no exact information respecting the
+country above Avignon. According to Speyer it also takes the department
+of Lozère. The same authority says that Puy-de-Dôme has "_egeria_,"
+meaning perhaps the intermediate form, with the fulvous form much less
+commonly. Next comes the curious fact that though the Lower Rhone
+(Avignon, Tarascon, Nîmes) has the true fulvous form, Hyères, Cannes,
+Grasse, Nice, Digne, and Alassio have _the intermediate_. Savoy has the
+intermediate (Chambéry) and even _egerides_ perhaps, though in the same
+latitude on the west of France there is nothing but the fulvous type.
+At Chalseul and Besançon (Doubs) the ordinary northern type is found.
+Switzerland generally, I believe, has the northern type, but Staudinger
+gives _egeria_ for Valais and the intermediate occurs in Vaud.[9] The
+south side of the Alps has probably colonies of the pale _egerides_, and
+of intermediates. Orta, with a very hot summer, has the English type
+(Tutt, _Ent. Rec._, XII, 1900, p. 328). Locarno has the intermediate
+(_ibid._, XV, 1903, p. 321). North Italy in general and western
+Piedmont have the intermediate; but further south _egeria_ begins,
+at what region I do not know. Speyer gives on his own authority the
+remarkable statement that at Florence both extremes occur, but chiefly
+intermediates between the two. Mr. R. Verity however kindly informs
+me that in his experience this is not so, and that neither the real
+southern type nor the northern occur there. Sardinia, Sicily, Crete all
+have the southern type. Greece probably has various types. Staudinger
+(_Hor. Ross._, VII, 1870, p. 78) says intermediates resembling Nice
+types common everywhere, but from "Greece" the British Museum has a
+series that would pass for English specimens; and the same type occurs
+near Constantinople. The island of Corfu has a pale intermediate,
+distinct from _egerides_ but approaching it. In Roumania all three
+forms are recorded from various places: _egeria_ in the Dobrutscha; not
+quite typical (presumably an intermediate) at Bukharest; intermediate
+in various mountainous localities as well as in Macedonia and Dalmatia;
+but _egerides_ in Azuga at about 3,000 feet.[10] Hungary has the true
+_egerides_ also. (Cf. Caradja, _Deut. Ent. Zt._, IX, p. 58.) Mathew
+records the same from Gallipoli (_E. M. M._, 1881, p. 95). Staudinger
+does not distinguish the intermediates from the northern, but he gives
+"_egerides_" for Armenia and Fergana (Central Asia). As against the mere
+proximity of a great mountain chain being the influence which keeps
+the Riviera population intermediate may be mentioned the fact that the
+northern foothills of the Pyrenees have the pure southern type, and the
+climate of Cambo must surely be far cooler than that of Nice. The exact
+locality of the Greek specimens is not given, but there can be no part
+of Greece which is not much hotter in summer than Brittany, or Calvados,
+which have the intermediate, not the English type.
+
+In face of these facts it can scarcely be maintained that average
+temperature is the efficient cause of the particular tone of colour
+which the butterfly shows in a given region. Nevertheless it is clear
+that climate counts for much in determining the distribution. It is
+noticeable that though the pale _egerides_ can be established in a
+warm climate we never find _egeria_ in cold climates, and even the
+intermediate is not found in places that have a hard winter. I suspect
+that the distribution of the broods through the year and the condition
+of the animal at the onset of hard frost are features which really
+determine whether a strain can live in a particular place or not.
+Though the truth of the suggestion cannot be tested by experiments in
+captivity, which at once introduce disturbances, I incline to the idea
+that _egeria_ has not got the right periodicity for northern climates.
+If it could arrange its life so that the population consisted either
+of young larvae, or perhaps of thoroughly formed pupae[11] at the
+onset of winter, it might, for any obvious reason to the contrary,
+be able to live in England. It is irregularly "polyvoltine," as the
+silk-worm breeders say, and as soon as a little warmth encourages it,
+a new generation starts into being, which if the frost comes at an
+untimely moment, is immediately destroyed. Many species are continually
+throwing off individuals which feed up fast[12] and emerge at once if
+the temperature permits, and I imagine a species of Satyrid wholly
+or largely represented by such individuals could scarcely survive in
+a country which had a hard winter. For such a climate some definite
+periodicity in the appearance of the broods may well be indispensable.
+But assuming that _egeria_ is cut off from cold climates for such a
+reason, there is nothing yet to connect these habits with the fulvous
+colour, and until breeding can be carried out on a satisfactory scale
+there is no more to be said.
+
+From time to time records appear of individual specimens more or
+less fulvous being caught in southern England, especially in the
+New Forest.[13] It would be interesting to know what offspring such
+individuals might produce. From the evidence now given some notion
+both of the strength and the weakness of the case considered as one of
+continuous climatic variation can be formed. I know no other equally
+satisfactory. Whether or not definite mixture of the intermediates
+with either of the extremes will be proved to occur, the case differs
+materially from those considered in the last chapter in the fact that
+at all events there is no general overlapping of forms. In a species so
+little given to wandering, overlapping could indeed scarcely be expected
+to occur. It is this circumstance which makes the species preeminently
+suitable as a subject for the study of climatic influences, and I trust
+that entomologists with the right opportunities may be disposed to
+explore the facts further.
+
+Just as many species, like _egeria_, have varieties which can be
+regarded as adapted to northern and southern regions, so there are
+also several which have lowland and Alpine forms quite distinct from
+each other. Every such case presents an example of the problem we
+have been considering. As the collector passes from the plains to the
+Alpine region, how will he find the transition from one form to the
+other effected? Does the lowland form give place to the Alpine form
+suddenly, with a region in which the two are mixed, or will he find a
+zone inhabited by an intermediate population? I have spent a good deal
+of time examining the facts in the case of _Pieris napi_ and its Alpine
+female variety _bryoniae_, and though there are many complications which
+still have to be cleared up, no doubt is possible as to the main lines
+of the answer. If in any valley in the Alps inhabited by both _napi_ and
+_bryoniae_ the collector catches every specimen he can, beginning at
+the bottom and working up to 7,000 feet, he will at first get nothing
+but _napi_. At about 2,500 feet, he may catch an occasional _bryoniae_
+flying with the _napi_. After 3,000 feet _napi_ usually ceases, and
+only _bryoniae_ are found. As an exception a colony of _napi_ may be
+met with at much greater heights. I once found them in numbers at about
+6,000 feet.[14] Not only were they free from any trace of modification
+in the direction of _bryoniae_, but they were of the thoroughly southern
+type of _napi_, being a late brood of that large and very pale kind
+(_meridionalis_) almost destitute both of dark veining above and of
+green veining below, which are common on the shores of Lago Maggiore
+and in other hot southern localities. Not far off at the same level
+were typical _bryoniae_ in fair abundance. Occasionally an intermediate
+may be met with. I have taken a few, for example, at Macugnaga and at
+Fobello. These, however, in my experience are rarities in the Alps.
+Fleck[15] gives notes on the distribution in Roumania which shows the
+same state of things. The lowland form is not transformed though found
+at great heights, and at Azuga (nearly 3,000 feet) _bryoniae_ occurs
+with only occasional "_flavescens_," viz., intermediates of the second
+brood.
+
+If this were all the evidence we should be satisfied that the lowland
+and Alpine types keep practically distinct, overlapping occasionally,
+but rarely interbreeding. The problem would remain, how is the
+distinctness of the two types maintained in the region of overlapping?
+Nowadays, I suppose, we should incline to answer this question by
+reference to segregation, and perhaps by an appeal to selective mating.
+The suggestion that segregation does take place is certainly true to
+some extent. There are, however, difficulties in the way, and the whole
+subject is one of great complexity. My own experiments were made in
+pre-Mendelian times and were not arranged with the simplicity which we
+now know to be essential. The results are neither extensive enough nor
+clear enough to settle the many collateral questions which have to be
+considered, and the work ought to be done again. Nevertheless, some
+notes of the observations may have a suggestive value.
+
+When I began, I did not sufficiently appreciate that the "_napi_" group,
+omitting the North American forms, and the Asiatic representatives, has
+at least three chief types in western Europe. The differences we have
+to deal with are manifested by the females only, so in this account
+particulars as to the males are omitted for the most part. These are (1)
+our own British _napi_; (2) the form found in the south, from the Loire
+downwards, and in the Italian Alps, which I think may be spoken of as
+_meridionalis_; (3) _bryoniae_, which is a form clearly recognizable in
+the _female_ only, and is found only in the arctic regions and in the
+Alps above 2,500 feet. The first two have several broods, two, three,
+or more, according to opportunity, and the first brood is different
+from the later ones. In _napi_ the markings on the upper surface are
+a dark grey but in _meridionalis_ they are a pale silvery grey and
+much less extensive. In the later broods of _napi_ there is much less
+general irroration of the veins, and the spots stand out as more defined
+and blacker. These differences vary greatly in degree of emphasis. In
+_meridionalis_ the later broods are entirely different from the first.
+Instead of having silvery markings they have the ground colour quite
+white, with the spots large and a full black. On the under side of the
+hind wings the usual green veins are almost absent, and I have seen
+individuals which could scarcely be distinguished from _rapae_. To these
+later broods the term _napaeae_ is sometimes applied, but I here use
+_meridionalis_ for the southern race in general as applicable to all
+broods.
+
+The female _bryoniae_ is totally unlike the others. The ground colour
+is a full yellow, and each nervure is thickly irrorated with a brown
+pigment often spreading so far as to hide the ground almost entirely
+in the fore-wings. The males corresponding with these females are not
+certainly distinguishable from those of our own _napi_. Both sexes have
+the green veining of the underside of the hind wing fully developed,
+rather more than is usual in the lowland races, but this is not really
+diagnostic of the variety. The first serious difficulty arises in regard
+to the second brood of _bryoniae_. It is stated that there is only one
+brood,[16] but I feel fairly sure that a second brood is sometimes
+produced, and that the females with a yellow ground and diminished
+irroration of the veins, not very uncommon in the Italian Alps in July
+to August, are generally representatives of it. Such insects would of
+course be classed with _bryoniae_ in collections.
+
+My experiments began with eggs of true _bryoniae_ females caught at
+about 2,500 feet early in July. These emerged in August-September as
+intermediates with yellow ground and about half as much black on the
+upper surface as _bryoniae_. They are exactly like the intermediates
+usually found in nature and in the light of later experience I regard
+them as natural F_{1} forms, and I think the mothers had been fertilised
+by _napi_ males, though I admit that in view of the rarity of natural
+intermediates there is a difficulty in this suggestion. Three of these
+females were mated with males raised from thorough _meridionalis_
+females, and three families were produced. Two of them showed distinct
+evidence of segregation, some being yellow and some white with various
+intergrades, some being no blacker than _meridionalis_ and some ranging
+up to a dark intermediate type. Part emerged in the same autumn; and
+part overwintered, emerging as the spring _meridionalis_ or as the
+peculiar type which I afterwards learnt to know as the spring F_{1}
+form. The distinctions were fairly sharp between the several forms. But
+the offspring of the third female gave a series practically continuous
+from _meridionalis_ to the F_{1} type. The work of subsequent years
+gave results similarly irregular which could only be described
+adequately at great length. The outcome may however be summed up in
+the statement that there is evidence that both the yellow ground and
+the dark veining are due to factors, but that there are several of
+these and that imperfect segregation is not uncommon, producing various
+reduction-stages. The yellow ground may be due to one factor, and the
+several shades may be the result of irregularities in dominance, but
+the black markings when fully developed cannot I think be the result of
+less than three factors, one for the basal darkening, one for general
+irroration, and one for the margins. Probably also the enlargement of
+the spots is produced by a fourth factor.
+
+There was not, in my experience any great difficulty in getting the
+various forms to pair in captivity. Some attempts were made to see
+whether individuals of either type selected mates of their own type in
+preference to those of the other, but the results were inconclusive.
+There were some indications of such a preference; though, from
+the impossibility of judging how much of this may be due to other
+circumstances, I could not come to a positive conclusion on the rather
+meagre evidence.
+
+Recently Schima[17] has given a careful and detailed account of all
+the forms found in Lower Austria which he enumerates under 14 distinct
+varietal names. He gives full references to previous accounts,
+especially to the beautiful plates lately published by Roger Verity.[18]
+Examination of these and of my own specimens strongly suggests that
+the several forms are due to the recombination of the factors I have
+named. Among those which I have bred are representatives of most if not
+all the types enumerated by Schima in addition to other curious forms.
+For example I have _bryoniae_ markings on a ground practically white;
+the dark veins with spots almost obsolete; _meridionalis_ on a yellow
+ground; the intermediate amount of black on a white ground, etc. The
+last-named may occur wild and I have one from Macugnaga as well as one
+given me by Mr. F. Gayner from Lulea (Lapmark).
+
+To obtain really exact knowledge of the number of factors and
+their properties it would be necessary to repeat the work. After
+the beginning, I made a mistake in using British _napi_ instead of
+_meridionalis_ and the results were much confused thereby. The contrast
+between _meridionalis_ and the various dark forms is much greater and
+classification of the types would have been therefore easier. The
+British form is presumably _meridionalis_ plus the factor for the basal
+pigmentation. The problem is greatly complicated by the differentiation
+of the seasonal forms. The first point to be determined is whether
+_bryoniae_ is capable of producing a second brood when it is thoroughly
+pure-bred, and whether such a second brood is, as I suspect, normally
+intermediate in character.
+
+In the Alps generally there is no definitely intermediate population;
+nor I believe, is any such population met with in the north where
+the arctic _bryoniae_ meets _napi_, but as to this I have no precise
+information. One curious fact, however, must be mentioned, namely that
+there is a population that can probably be so described with fairness
+established at Mödling near Vienna. This is not in any sense an Alpine
+locality, and does not, as I am told, differ in any obvious way from
+the other suburbs of Vienna. Dr. H. Przibram was so good as to send me
+a set taken at this place, representing a second brood, and they were
+decidedly heterogeneous, ranging from an intermediate form such as
+_bryoniae_ fertilised by _napi_ usually produces, to a light yellowish
+second-brood type with little dark pigment. There are also two actual
+_bryoniae_. Whether true _napi_ also occur there I do not know, but I
+have no doubt they do. It would be well worth while to investigate the
+Mödling population statistically, and to breed from the intermediates
+which might not impossibly prove to be heterozygotes. There are also
+records of such intermediates being occasionally found in some parts
+of Ireland, in the north of Scotland, and in south Wales,[19] but I do
+not know of any regular colony of these forms. We can scarcely avoid
+the inference that one or more of the factors which make up _bryoniae_
+may be carried by these intermediates. It is not clear why their
+interbreeding does not produce actual _bryoniae_ occasionally. If this
+occurred, the probability is that the fact would be known to collectors,
+at least in the British localities. The absence of true _bryoniae_ must,
+I think, be taken to mean that some essential factor is absent from
+these intermediates.
+
+To sum up the evidence, the facts that are clear may be thus enumerated:
+
+   1. _Napi_ and _bryoniae_, or in the Italian Alps,
+      _napaeae_ and _bryoniae_ frequently meet each other.
+
+   2. They cross without difficulty, producing fertile offspring.
+
+   3. But in the levels at which they overlap there is no
+      intermediate population, and only occasional intermediate
+      individuals.
+
+   4. In certain parts of the distribution of _napi_ similar
+      intermediates sometimes occur, and at one place (Mödling) they
+      are so frequent as apparently to constitute a colony.
+
+   5. As to the genetic relations of the two forms there is no
+      complete certainty. Indications of segregation have been observed
+      in some cases, but there are several factors concerned and they
+      are liable to some disintegration.
+
+Another form in which I tried to investigate the same problem is
+_Coenonympha arcania_, which has one Alpine form known as _darwiniana_,
+and another, _satyrion_. In calling _satyrion_ a form of _arcania_ I
+follow Staudinger and other authorities, but I have never been quite
+satisfied that it should be so regarded. The differences between
+_arcania_ and _darwiniana_ are essentially differences of degree; _C.
+arcania_ occurs in places where there is cover, and reaches up the
+valleys usually as high as the mixed woods of deciduous trees, which
+is about 2,500 feet. The variety _darwiniana_, on the contrary, is an
+insect of treeless hillsides, and I regard it as a dwarf and possibly
+a stunted form. It would not greatly surprise me to find that with
+the application of good conditions _arcania_ could be raised from
+_darwiniana_ eggs, or that if _arcania_ larvae were starved they might
+give rise to _darwiniana_ butterflies. I have been unsuccessful in
+trying to rear the species, having lost the larvae by disease. Usually
+one does not catch _arcania_ and _darwiniana_ on the same ground, and
+as _Festuca ovina_--a typically hill-side grass--is a common food-plant
+of _darwiniana_ there can be little doubt that _arcania_ feeds on
+some other grass, probably woodland species. Colonies of _arcania_ of
+varying size and brightness are commonly found, and though a sample of
+_arcania_, finely grown, from a warm Italian wood, presents a striking
+contrast with _darwiniana_ from an Alpine pasture, one certainly may get
+samples which fill all the gradations. Generally the sample from a given
+locality is fairly homogeneous.
+
+Of _satyrion_ I have little personal experience. I only twice found
+it, namely at Zinal, and at Hallstatt in Austria, but it occurs at
+Zermatt, Arolla, and in several Swiss localities above 5,000 feet, and
+I understand that it is the typical Alpine form in the Engadine. With
+its darkened colour and reduced size it might well be expected to be a
+still further stunted form of _darwiniana_. Yet I have never found the
+one succeed to the other at the higher levels. If _darwiniana_ appears
+when Alpine conditions are reached in a valley it will be met with
+up to the highest level at which such butterflies live. Tutt was of
+opinion that _satyrion_ is a distinct species.[20] I once, at the top
+of the Vorderrheinthal caught a sample of _darwiniana_ a few of which
+(males) were so dark and had the eye spots so poorly developed that they
+looked like transitions to _satyrion_. Otherwise I never found any such
+transitional forms and they are certainly exceptional. There is further
+a record[21] of _satyrion_ having been taken flying with _arcania_.
+This was near Susa, at about 2,000 feet I infer. Mr. H. E. Page has
+similar specimens from Caud and from St. Anton (Arlberg). The females,
+however, both of mine and of Mr. Page's samples are a pale brown, quite
+unlike the females both of _arcania_ and of the dark Zinal _satyrion_.
+The difficulty thus raised has not I think yet been considered by the
+authorities, and it is possible that the Alpine forms of _arcania_ are
+in reality three, not two.
+
+The evidence taken together suggests, I think, that _darwiniana_ is
+related to _arcania_ much as so many of the Alpine varieties of plants
+are to the well-developed individuals of the lower levels. I do not
+anticipate that factorial differences will be found in these insects,
+and it is by no means impossible that the distinctions between them
+are the direct consequences of altered conditions. The relations of
+_arcania_ to _satyrion_ are more doubtful, and in that case a factorial
+difference may at least be suspected.
+
+The species of the genus _Setina_ have Alpine forms which agree in
+possessing a characteristic extension of the black pigment to form
+radiating junctions between the spots on the wings. Speyer, who
+discussed the interrelations of these forms in detail,[22] lays stress
+on the absence of genuine transitional forms between _aurita_ and the
+variety _ramosa_. Both are mountain insects but _ramosa_ extends to
+levels higher than that at which _aurita_ ceases, which is about 4,000
+feet. The two forms are often found flying together. Speyer says that
+his brother searched diligently for transitional forms at the level of
+overlapping, but found none, so that at least they may be regarded as
+rare. The variety _ramosa_ is not infrequent at much lower levels (_e.
+g._, Chiavenna, 1,020 feet; Reussthal, 1,500 feet) and extends as high
+as the permanent snows. In the British Museum collection, however, I
+have seen several that I should regard as transitional. Speyer perhaps
+would have classed as _ramosa_ all in which the spots of the central
+field were united, and it is by no means unlikely that breeding would
+prove such individuals to be heterozygous.[23]
+
+There can scarcely be a doubt that the distinction between _aurita_ and
+_ramosa_ is factorial, the radiate _ramosa_ probably having the factor
+for striping. In support of this view may be mentioned the observation
+of Boisduval,[24] respecting a gynandromorphous individual, which was
+_aurita_ male on one side, and _ramosa_ female on the other. Speyer
+makes another excellent comment. He points out that the simple notion
+that the radiation is a mere extension of pigmentation consequent on
+the climate of the higher levels, will not fit the facts very easily,
+because the size of the spots varies greatly in _aurita_ itself at any
+level, and lowland specimens may actually have more black confined
+to the spots alone than some _ramosa_ possess on spots and lines
+combined.[25]
+
+The two Salamanders, _S. maculosa_ and its Alpine form _atra_, might
+not improbably furnish evidence bearing on the same problem. The two
+are of course very distinct, not merely in colour (_maculosa_ being
+spotted with yellow or orange while _atra_ is entirely black) but also
+in the mode of reproduction, a feature to which reference will be made
+in the next chapter. I cannot, however, find any evidence as to the
+overlapping of the two forms. _S. atra_ occurs from about 3,000 feet
+or somewhat less, and reaches great elevations in the Eastern Alps,
+but I do not know if the two forms ever occur in the same localities.
+Leydig,[26] Boulenger,[27] and most modern authorities regard the two
+types as distinct species, but they are in any case closely allied, and
+it would be of interest to have exact knowledge of their geographical
+delimitations.
+
+The reader who has considered the cases adduced will appreciate the
+difficulties which must be faced in any attempt to account for the
+facts in a rational way. As always in a problem of Evolution, two
+separate questions have to be answered. First how did the form under
+consideration come into existence, and secondly, how did it succeed
+in maintaining itself so as to become a race? The evidence from the
+local forms, though very far from giving complete answers to either of
+these questions definitely refutes the popular notion that a new race
+comes into existence by transformation of an older race. If a gradual
+mass-transformation of this kind took place we should certainly expect
+that when two types, nearly allied and capable of interbreeding, overlap
+each other in their geographical distribution, a normally intermediate
+population would exist. If each type can maintain itself, and if
+each came into existence by gradual transformation, then there must
+have been an intermediate capable of existing and maintaining itself
+as a population; and if this had ever been, surely in the region of
+overlapping, that intermediate population should continue. Especially
+should such a population be found when the two extreme types are
+adaptational forms and the region of overlap is a region of intermediate
+conditions. But of the examples we have examined there is only one, that
+of _Pararge egeria_ and _egerides_, which can at all be so interpreted,
+and even in that case it is not impossible that more minute observation
+would reveal discontinuity between the extremes and the admittedly
+normal intermediate population. Granting provisionally however that
+this example, as it stands, is consistent with the conventional theory
+of evolution, I know not where we should look for another case equally
+good. When the distinctions are produced by direct influence of
+conditions operating during the lifetime of the individuals, examples of
+intermediate populations occupying the areas of intermediate conditions
+can no doubt be produced. Many turf-like Alpine plants, for instance,
+if protected from exposure and properly nourished can grow as large as
+those of the same species found in the valleys, and in the case of such
+quantitative effects, intermediate conditions can doubtless produce
+intermediate characters.
+
+Even these examples however are not very abundant, and often the
+intermediate locality has not a form intermediate between those of
+the two extreme localities, but some third form distinct from either.
+This is the case for instance in the fauna of brackish waters. We are
+taught to believe that the fresh water fauna was evolved from the
+marine fauna, which it well may have been; but as students of Crustacea
+and Mollusca know familiarly, the brackish water forms are not as a
+rule intermediates between fresh water species and sea species, but
+more usually they are special forms belonging to the brackish waters,
+with the peculiar property that they can tolerate a great range of
+conditions, and live without ostensible variation in waters of most
+various compositions and densities, which very few marine or fresh water
+species are able to do.
+
+Sometimes the distinction between local races, as in _Rhamphocoelus
+passerinii_ and _icteronotus_ may be regarded with confidence as due to
+one simple Mendelian factor possessed by one race and absent from the
+other, but I think, more often, as in _Colaptes_ or in the varieties
+of _Pieris napi_, the existence of several distinct factors is to
+be inferred. As we have seen, the races of _Colaptes_ show almost
+beyond doubt that in different areas at least three distinct factorial
+combinations can be perpetuated as races.
+
+In the distribution of variability we find, I think, some hint as to
+the steps by which the phenomena under consideration have come to their
+present stage, and I am disposed to regard the facts so well attested
+in the case of our own melanic moths as a true indication of the
+process. Following this indication we should regard the change in the
+character of a population as beginning sporadically, by the appearance
+of varying individuals, possibly only one varying individual, in, it may
+be, one place only. As to _why_ a variety should increase in numbers
+we have nothing but mere speculation to offer, and for the present we
+must simply recognise the fact that it may. That such survival and
+replacement may reasonably be taken as an indication that the replacing
+race has some superior power of holding its own I am quite disposed to
+admit. Nevertheless it seems in the highest degree unlikely that the
+outward and perceptible character or characters which we recognise as
+differentiating the race should be the actual features which contribute
+effectively to that result.
+
+In discussions of geographical distribution in relation to problems of
+origin it is generally said that very nearly allied species usually
+occupy distinct areas, while other competent observers state the exact
+contrary. Lately, for example, Dr. R. G. Leavitt[28] has published an
+important collection of evidence upholding the latter proposition, taken
+chiefly from the botanical side, showing how in numerous genera two or
+more closely allied species coexist, frequently without intermediates,
+in the same localities, and may even be thus found in company throughout
+their distribution. The difference of opinion evidently arises from a
+confusion as to the sense in which the term "species" is understood and
+applied. Leavitt, for example, is avowedly following Jordan and, among
+moderns, Sargent, in applying a close analysis, and denoting as species
+all forms which are distinct and breed true. Against this use of the
+term I know no valid objection[29] but it must be obvious that if others
+follow a different practice confusion may result when observations are
+summarised in general statements. We will consider this subject again
+in another place, but here it may be sufficient to say that there can
+scarcely now be a doubt that numbers of these associated species, such
+as Jordan discriminated, represent various combinations of the presence
+and absence of Mendelian factors. This does not in any way weaken the
+argument which Leavitt founds upon the facts, namely, that the observed
+distribution of these forms is consistent with the supposition of an
+evolution largely discontinuous.
+
+On the other hand, those who have come to the opinion that nearly
+allied species generally occupy distinct ground are presumably more
+impressed by the characters differentiating the geographically distinct
+or adaptational races, seeing that genuine intermediates between
+them are less commonly found. Those geographical races may no doubt
+contain various differentiated forms; but when all live together,
+occasional intermediates are usually to be found even in the case of
+characters habitually segregating. These segregating forms Jordan would
+certainly have determined as species, and it must be conceded that no
+physiological definition has yet been drawn which consistently excludes
+them.
+
+FOOTNOTES:
+
+[1] Often referred to by older writers as _Meone_, Esper's name.
+
+[2] There are also two distinct island forms, unlike the European,
+_Xiphia_ of Madeira, and a smaller variety, _Xiphioides_ of Canary. See
+especially, Baker, G. T., _Trans. Ent. Soc. London_, 1891, p. 292.
+
+[3] Speyer, Adolf, and August. _Verbreitung der Schmetterlinge_, 1858,
+I, p. 217.
+
+[4] _Lepid. Comparée_, fsc. III, p. 372.
+
+[5] Mr. Rowland-Brown has called my attention to a statement by Dr.
+Vaillantin (_Petites Nouv. Ent._, II, 235) that in Indre-et-Cher the
+first brood is of the northern type and the second of the southern. My
+experience is that in captivity these distinctions do not occur, and I
+have true _egeria_ as first brood from Vienne and as the late brood from
+the Landes. I never collected in Indre-et-Cher.
+
+[6] I have since seen true _egeria_ from Ferrol in the extreme
+northwest, which was in Mr. Tutt's collection.
+
+[7] Mr. G. Wheeler kindly showed me a series identical with this type,
+from Guernsey, and others from near Laon.
+
+[8] _Ent. Rec._, V, 1894, p. 134.
+
+[9] Mr. Wheeler has some pale but rather worn specimens from the Rhone
+Valley at Vernayaz.
+
+[10] See Fleck, E., Die Macrolep. Rumäniens, _Bul. Soc. Sciinte_, VIII,
+1899, p. 720.
+
+[11] My experience agrees with that of Mr. H. Williams (_Ent. Rec._,
+VIII, 1896, p. 181) that pupae, well-formed, can stand considerable
+frost; but I used to find that half-grown larvae usually died if
+unprotected, and I believe that larvae which attempted to pupate in warm
+autumn weather and then got caught by frosts, always died. Small larvae
+which can creep into shelter at the bottom of the plants survived, and I
+expect that in the north the winter is usually passed in that state (see
+also Merrifield, F., _Ent. Rec._, VIII, 1896, p. 168, and Carpenter, J.
+H., _ibid._).
+
+[12] Some most unlikely species do this. I once had a larva of
+_Parnassius delius_, found at about 5,500 feet, which emerged late
+in the autumn (in October I believe), a season at which it must have
+perished in its own country.
+
+[13] See, for examples, Barrett, G. C., _Lepidoptera of the Brit.
+Islands_, I, 1893, p. 229; also Grover, W., _Ent. Rec._, IX, 1897, p.
+314; Williams, H., _Proc. Ent. Soc._, 1898, who reared several specimens
+from the New Forest which would pass for Bretons, though the rest of the
+family were true _egerides_.
+
+[14] Above the Tosa falls.
+
+[15] _Bul. Soc. Sciinte_, VIII, 1899, p. 691.
+
+[16] The fact that Weismann by heating pupæ obtained only one autumn
+specimen seems to me to show rather that a second brood can be produced
+than that it cannot, which is the inference usually drawn.
+
+[17] Schima, K., _Verh. Zool. bot. Ges. Wien_, LX, 1910, p. 268.
+
+[18] _Rhopalocera Palaearctica_, Florence, 1905-11, especially Pl. XXXII.
+
+[19] See figures in Barrett, G. C., _Lepidoptera of Brit. Islands_, I,
+pt. 3, p. 25.
+
+[20] Tutt, J. W., _Ent. Rec._, XVIII, 1905, p. 5. In the same place he
+states that on the Mendel Pass _arcania_ "runs into" _darwiniana_ and
+that in the Tyrolean localities the transition is especially evident.
+Wheeler (_ibid._, XIII, 1901, p. 121) expresses the contrary opinion,
+that _satyrion_ does grade to _arcania_.
+
+[21] H. Rowland-Brown, _Ent. Rec._, XI, 1899, p. 293.
+
+[22] Speyer, Stettiner, _Ent. Ztg._, XXXI, 1870, p. 63.
+
+[23] In regard to the closely analogous case of _Spilosoma lubricipeda_,
+Standfuss makes a similar statement. He bred the type on a large scale
+with the radiate form which he calls _intermedia_, and says that in four
+years of miscellaneous crossing he never obtained really transitional
+forms. Nevertheless after examining large series, especially those of
+Mr. W. H. B. Fletcher, I came to the conclusion that several might be
+so classed, but I am quite prepared to find that such specimens are
+heterozygous. (See Standfuss, _Handb. d. Gross-Schmet._, 1896, p. 307.)
+It is by no means unlikely that various dark forms of _lubricipeda_
+correspond with a progressive series of factorial additions. Many of
+the stages have been named, and of these the most definite are the
+_intermedia_ of Standfuss (probably = _eboraci_ of Tugwell) and the very
+dark _Zatima_ of Heligoland, in which only the thorax, the nervures
+and a small field in the fore-wings remain yellow. A form was bred
+by Deschange from _Zatima_ in which even the field in the forewing
+is obliterated. The exact circumstances in which _Zatima_ occurs in
+Heligoland would be worthy of special investigation, for the normal
+_lubricipeda_ is also found on the island. For references as to the
+British occurrences see especially, Hewett, W., _Naturalist_, 1894, p.
+353. As to _Zatima_ see especially Krancher, _Soc. Ent._, II, 1887-8, p.
+26. I am indebted to Dr. Hartlaub for information as to the Heligoland
+types.
+
+[24] Boisduval, _Bull. Soc. Ent. Fr._, III, 1834, p. 5.
+
+[25] The systematics of _Setina_ have been much controverted, but no one
+I believe doubts that _aurita_ and _ramosa_ are forms of one species.
+See also Chapman, A. T., _Ent. Rec._, XIII, 1901, p. 139.
+
+[26] _Arch. Naturg._, 33, 1867, p. 116.
+
+[27] _Brit. Mus. Cat., Batrachia Gradientia_, 1882.
+
+[28] The Geographical Distribution of nearly related Species. _Amer.
+Nat._, XLI. 1907, p. 207.
+
+[29] See later, p. 242.
+
+
+
+
+                        CHAPTER IX
+
+         THE EFFECTS OF CHANGED CONDITIONS: ADAPTATION
+
+In the attempt to conceive a process by which Evolution may have come
+about, the first phenomenon to be recognized and accounted for is
+specific difference. With that recognition the outline of the problem is
+defined. The second prerogative fact is adaptation. Forms of life are
+_on the whole_ divided into species, and these species _on the whole_
+are adapted and fit the places in which they live. To many students of
+Evolution, adaptation has proved so much more interesting and impressive
+than specific diversity that they have preferred it to the first place
+in their considerations.
+
+Whether this is, as I believe, an inversion of the logical order or
+not, there is one most serious practical objection to such preference,
+that whereas specific diversity is a subject which can be investigated
+both by the study of variation and by the analytical apparatus which
+modern genetic science has developed, we have no very effectual means of
+directly attacking the problems of Adaptation.
+
+The absence of any definite progress in genetics in the last century was
+in great measure due to the exclusive prominence given to the problem
+of Adaptation. Almost all debates on heredity centered in that part of
+the subject. No one disputes that the adaptation of organisms to their
+surroundings is one of the great problems of nature, but it is not the
+primary problem of descent. Moreover, until the normal and undisturbed
+course of descent under uniform conditions is ascertained with some
+exactness, it is useless to attempt a survey of the consequences of
+external interference; nor as a rule can it be even possible to decide
+with much confidence whether such interferences have or have not
+definite consequences. Those, for example, who debated with enthusiasm
+whether acquired characters are or are not transmitted were constantly
+engaged in discussing occurrences which we now know to be ordinary
+features of descent under uniform conditions, and the origin of
+variations which were certainly not caused directly by circumstances at
+all. In the absence of any factorial analysis, or of any conception of
+what factorial composition means and implies, no one knew what varieties
+might be expected from given parents. The appearance of any recessive
+variety was claimed as a consequence of some treatment which might have
+been applied to the parents. There was no possible standard of evidence
+or means of controlling it, and thus the discussion was singularly
+unfruitful. Before we can tell how the course of descent has departed
+from the normal, we must know what the normal would have been if we
+had let alone. We are still far from having such knowledge in adequate
+measure, but it does now exist in some degree, and we are steadily
+approaching a position from which we shall be able to form fairly
+sound estimates of the true significance of evidence for or against
+the proposition that environmental treatment can produce positive
+disturbances in the physiological course of descent.
+
+Thus described, the field for consideration is very wide. Though the
+effects of changed conditions were especially studied in the hope of
+solving the problem of adaptation by direct observation, that, as all
+are now agreed, is but a part of a more general question. We must ask
+not only do changed conditions produce an _adaptative_ response on the
+part of the offspring, but whether they produce any response on the
+part of the offspring at all. It is not in doubt that by violent means,
+such as starvation or poisoning of the reproductive cells, effects of a
+kind, stunting and deformity for instance, can be made evident, just as
+similar effects may follow similar treatment during embryonic or larval
+life. Apart from interferences of this class, are there any that may be
+reasonably invoked as modifying the course of inheritance?
+
+No epitome of the older evidence for the inheritance of adaptative
+changes is here required. That has often been collected, especially
+by Weismann, who exposed its weaknesses so thoroughly as to carry
+conviction to most minds, and showed that whether the phenomenon
+occurs or not, no one can yet prove that it does. Belief in these
+transmissions, after being almost universally held, was with singular
+unanimity abandoned. This change in opinion, though doing credit to the
+faith of the scientific community in evidential reasoning, is the more
+remarkable inasmuch as the strength of the idea was not derived from the
+minute amounts of supposed facts now demolished. On the contrary, it was
+really an instinctive deduction from a wide superficial acquaintance
+with the properties of animals and plants. They _can_ accommodate
+themselves to circumstances. They _do_ make responses sometimes
+marvellously appropriate to demands for which they can scarcely have
+been prepared. What more natural than to suppose that the permanent
+adaptations have been achieved by inherited summation of such responses?
+No one had actually been driven to believe in the inheritance of
+adaptative changes because bitches which had been docked had been known
+to give birth to tailless puppies, or because certain wheat in Norway
+was alleged to have become acclimatized in a few generations. Evidence
+of this kind was collected and produced rather as an ornamental appendix
+to a proposition already accepted, and held to be plainly demonstrated
+by the facts of nature. Looked at indeed in that preliminary and
+uncritical way, the case is simply overwhelming. Those who desire to see
+how strong it is should turn to Samuel Butler's _Life and Habit_, and
+even if in reading they reiterate to themselves that no experimental
+evidence exists in support of the propositions advanced, the misgiving
+that none the less they may be true is likely to remain. Making every
+deduction for the fact that the wonders of adaptation have been grossly
+exaggerated, and that marvels of fitness and correspondence between
+means and ends have grown out of mere anthropomorphic speculations,
+there is much more left to be accounted for than can at all comfortably
+be accepted as the product of happy accidents. So oppressive are these
+difficulties that we can scarcely blame those who imagine that the study
+of heredity is primarily directed to the problem of the transmission of
+acquired characters, a preconception still almost universal among the
+laity.
+
+But since the belief in transmission of acquired adaptations arose
+from preconception rather than from evidence, it is worth observing
+that, rightly considered, the probability should surely be the other
+way. For the adaptations relate to every variety of exigency. To supply
+themselves with food, to find it, to seize and digest it, to protect
+themselves from predatory enemies whether by offence or defence, to
+counter-balance the changes of temperature, or pressure, to provide for
+mechanical strains, to obtain immunity from poison and from invading
+organisms, to bring the sexual elements into contact, to ensure the
+distribution of the type; all these and many more are accomplished by
+organisms in a thousand most diverse and alternative methods. Those are
+the things that are hard to imagine as produced by any concatenation
+of natural events; but the suggestions that organisms had had from the
+beginning innate in them a power of modifying themselves, their organs
+and their instincts so as to meet these multifarious requirements does
+not materially differ from the more overt appeals to supernatural
+intervention.
+
+The conception, originally introduced by Hering and independently by
+S. Butler, that adaptation is a consequence or product of accumulated
+_memory_ was of late revived by Semon and has been received with some
+approval, especially by F. Darwin. I see nothing fantastic in the notion
+that memory may be unconsciously preserved with the same continuity
+that the protoplasmic basis of life possesses. That idea, though purely
+speculative and, as yet, incapable of proof or disproof contains
+nothing which our experience of matter or of life at all refutes. On
+the contrary, we probably do well to retain the suggestion as a clue
+that may some day be of service. But if adaptation is to be the product
+of these accumulated experiences, _they must in some way be translated
+into terms of physiological and structural change_, a process frankly
+inconceivable.
+
+To attempt any representation of heredity as a product of memory is,
+moreover, to substitute the more obscure for the less. Both are now
+inscrutable; but while we may not unreasonably aspire to analyse
+heredity into simpler components by ordinary methods of research, the
+case of memory is altogether different. Memory is a mystery as deep
+as any that even psychology can propound. Philosophers might perhaps
+encourage themselves to attack the problem of the nature of memory by
+reflecting that after all the process may in some of its aspects be
+comparable with that of inheritance, but the student of genetics, as
+long as he can keep in close touch with a profitable basis of material
+fact, will scarcely be tempted to look for inspiration in psychical
+analogies.
+
+For a summary of the recent evidence I may refer the reader to Semon's
+paper[1] where he will find a collection of these observations described
+from the standpoint of a convinced believer. At the outset one cannot
+help being struck by the fact that of the instances alleged, very few,
+even if authentic, show the transmission of acquired modifications
+which can in any sense be regarded as adaptative, and many are examples
+not so much of a transmission of characters produced in the parents as
+of variation induced in the offspring as a consequence of treatment
+to which the parents were submitted, the parents themselves remaining
+apparently unmodified. No one questions the great importance of evidence
+of this latter class as touching the problem of the causes of variation,
+but it is not obvious why it is introduced in support of the thesis that
+acquired characters are inherited.
+
+It is most difficult to form a clear judgment of the value of the
+evidence as a whole. To doubt the validity of testimony put forward
+by reputable authors is to incur a charge of obstinacy or caprice;
+nevertheless in matters of this kind, where the alleged phenomena are,
+if genuine, of such exceptional significance, belief should only be
+extended to evidence after every possible source of doubt has been
+excluded. We believe such things when we must, but not before. At the
+very least we are entitled to require that confirmatory evidence should
+be forthcoming from independent witnesses. So far as I have seen, this
+requirement is satisfied in scarcely any of the examples that have been
+lately published, and until it is, judgment may reasonably be suspended.
+
+In some cases, however, the facts are not doubtful. Standfuss, by
+subjecting pupae of _Vanessa urticae_ to cold, produced the now
+well-known temperature-aberrations in which the dark pigment is greatly
+extended. He put together in a breeding-cage 32 males and 10 females
+showing this modification in various degrees. Two of these females died
+without leaving young. Seven produced exclusively normal offspring. From
+the eighth female 43 butterflies were bred, and of these there were four
+(all males) which to a greater or less extent exhibited the aberrational
+form.[2] The mother of this family was the most abnormal of the 10
+females originally put in.
+
+Fischer's experiment with _Aretia caja_ was on similar lines. From
+pupae which had been frozen almost all the moths which emerged showed
+aberrational markings. A pair of these mated and produced 173 young
+which pupated. Those which emerged early were all normal, but of those
+which emerged late, 17 had in various degrees abnormal markings like
+those of the parents.[3] In neither of these examples is there any
+question as to the facts. Both observers have great experience and give
+full details of their work.
+
+As regards _Vanessa urticae_, however, it must be recalled that Fischer
+himself showed that in Nymphalids somewhat similar aberrations could be
+produced both by heat and by cold, and even by centrifuging the pupae.
+Frl. von Linden produced a transitional form of the same aberration
+in _V. urticae_ by the action of carbonic acid gas.[4] It is highly
+probable that the appearance is due to a morbid change, perhaps an
+arrest of development, which may be brought about by a great diversity
+of causes. In the experiments the cause probably was a diseased
+condition of the tissues of the mother herself. She had been subjected
+to freezing sufficiently severe to prevent the proper development of
+the pigments and some of the ovarian cells presumably suffered also.
+It will be observed that the only specimens which were affected were
+the offspring of the most abnormal female, and of them only four out of
+forty-three showed any change.
+
+The same interpretation probably applies to the cases in _Arctia caja_.
+In this species the markings are well known to be liable to great
+variation. As Barrett says, even in nature individuals are rarely quite
+alike, and an immense number of strange forms occur in collections.[5]
+These are greatly sought after by some collectors, especially in
+England, where they fetch high prices at auctions, and it is notorious
+that most of them come from Lancashire and the West Riding of Yorkshire.
+It is commonly supposed that the breeders of that district subject them
+to abnormal conditions, and especially to unnatural feeding, but I know
+no clear evidence that this is true. From whatever cause it is certain
+that the natural pattern is, in some strains at all events, very easily
+disturbed.
+
+The elaborate experiments of Schröder with _Abraxas grossulariata_ are
+difficult to follow and are complicated by the fact that the series
+which was submitted to abnormal temperatures was derived from an
+abnormal original pair. From the evidence given it is not clear to me
+whether the temperature had a distinct effect. This insect, like _Arctia
+caja_, produces an immense number of variations (especially in the
+amount of the black pigment) and as most of these are, I believe, reared
+in domestication for sale, it is highly probable that the species is
+easily influenced by cultural conditions.
+
+Schröder describes two other experiments which have been accepted
+by Semon and other supporters of the view that acquired characters
+are transmitted. In the first, _Phratora vitellinae_, a phytophagous
+beetle living on the undersides of leaves, was used. It naturally
+feeds on _Salix fragilis_, a species without a felt, or tomentum, on
+the underside of the leaves. Larvae were transferred to another willow
+(near _S. viminalis_) which has the undersides of the leaves felted. The
+larvae took readily to the new food, pushing the tomentum before them as
+they gnawed the leaves. They came to maturity and when they were about
+to lay their eggs they were given a free choice between _S. fragilis_
+and the tomentose species. The greater number of ovipositions, 219,
+took place on _fragilis_, and there were 127 on the tomentose bush,
+which we are told was six times as large as the _fragilis_. The larvae
+from _fragilis_ were next put on the tomentose species and reared on
+it. When they became imagines they were similarly given their choice,
+with the result that there were 104 ovipositions on the tomentose
+species and only 83 on _fragilis_. In the next generations there were 48
+ovipositions on the tomentose and 11 on _fragilis_. Finally the fourth
+generation made 15 ovipositions on the tomentose and none on _fragilis_.
+
+The difficulty about such experiments is obviously that one has no
+assurance that the change of instinct, in so far as there is any,
+may not be a mere consequence of the captivity. It must, besides, be
+extremely difficult to arrange the experiment so that there is really
+an equal choice between the two bushes, when one stands beside the
+other. Przibram, in quoting this case, considers that as the tomentose
+bush was about six times as large as the _fragilis_, some indication of
+the relative attractiveness of the two may be obtained by dividing the
+ovipositions on the larger bush by six, but I imagine the matter must be
+much more complex.
+
+Schröder's second example is not more convincing, in my opinion, though
+Semon regards it as one of the most important pieces of evidence. It
+concerns a leaf-rolling moth, _Gracilaria stigmatella_, the larva of
+which is said normally to make its house by bending over the _tips_ of
+the sallow leaves on which it feeds. Schröder placed larvae on leaves
+from which the tips had been cut, and these larvae made their houses by
+rolling over the _sides_ of the leaves. Their offspring were again fed
+on leaves without tips, and as before, they rolled in the leaf-margins
+either on one side or both. The offspring of this second generation were
+then fed on entire leaves. There were 19 houses made by these (?19)
+larvae, and of them 15 were normal, made by folding down the tips of
+the leaves, while 4 were abnormal, made by rolling in the leaf-margins.
+Schröder says that in nature he has only twice seen abnormal houses; but
+it is clearly essential not only that the frequency of such variability
+in nature should be thoroughly examined, but also that we should know
+whether when the species is bred in captivity these irregularities of
+behaviour do or do not occur when the larvae are fed on uninjured leaves.
+
+The famous case of Schübeler's wheat is revived by Semon. The story will
+be familiar to most readers of the literature of the subject. Briefly
+it is that annuals, especially wheat and maize, raised from seed in
+Central Europe take more time in coming to maturity and ripening than
+similar plants raised in Norway, where the summer days are much longer.
+The received account is that he imported seed especially of maize and
+of wheat from Central Europe to Norway and found that in successive
+years the period of growth and ripening was increasingly reduced. After
+two generations seed of the accelerated wheat was sent back to Breslau
+where it was grown, and was found to ripen rather more slowly than in
+Norway, but much more quickly than the original stock had done. The
+facts recorded by Schübeler[6] are that he received seed from Eldena,
+which is on the Baltic near Greifswald. The variety is described as
+"_100 tägiger Sommer Weizen_," but no more exact record of its behaviour
+in Germany is given. This wheat, grown at Christiania in 1857, took
+103 days to harvest. Its seed was again grown in Christiania in 1858,
+and took 93 days, and sown again in 1859 it took only 75 days, 28 days
+less than in the first year of cultivation in Norway. Seed of the 1858
+crop was sent to Breslau, and grown there by Roedelius in 1859; it
+took 80 days. Evidently before such a record can be used as proving an
+inheritance of acquired characters numbers of particulars should be
+forthcoming. The view that Johannsen has taken is that the result was
+probably due to unconscious selection of the earlier individuals among a
+population consisting of many types of various compositions. Some effect
+may no doubt be ascribed to that cause, but I cannot think that alone
+it would account for the results. My impression is rather that they
+were produced by differences in the cultivation and especially in the
+seasons. Research of an elaborate character would be necessary in order
+to eliminate the various sources of error, and nothing of the kind has
+been done; nor does Semon allude to these difficulties in prominently
+adducing Schübeler's evidence. A difference of even three weeks in time
+of harvesting may easily be due to variation in the season. It would in
+any case be difficult to analyse the meteorological conditions, and to
+decide how much effect in postponing or accelerating the harvest might
+be due to cold days, to cloudy days, to wet weather, to fluctuations in
+average temperature, to hot days, and other such incidents occurring at
+the different periods of growth, even if they were specially watched
+while the experiments were in progress, and at this distance of time
+such analysis is practically impossible. Without careful simultaneous
+control-experiments this evidence is almost worthless. The director of
+the Meteorological Office[7] has, however, kindly sent me some details
+of the weather at Breslau from 1857 to 1860, and I notice that as a
+matter of fact July, 1859, was an exceptionally hot month, _having an
+average of 2.67° C. above the mean_ for the twenty years 1848-1867. June
+in that year was slightly (0.31° C.) below the mean and May slightly
+above it (0.18° C.). August was also abnormally hot, 2.35° C. above the
+average. The Breslau wheat was sown on _May 19_ and harvested on August
+6. There was a cold spell from May 11 to 14, which this wheat escaped,
+as it was sown on May 19. In the other years the cold spell came much
+later. These elements of the weather may possibly have done something to
+hurry the ripening in 1859. It unfortunate that we are not told how long
+similar wheat from Breslau seed took to ripen in that year.
+
+As regards the Norway cultivations we have the average monthly
+temperatures recorded by Schübeler, though he does not discuss them
+in connection with this special problem. It is quite clear that 1857,
+in which the period was 103 days, was an exceptionally cold summer,
+especially as regards the months of June and July, but though there
+was, so far as the temperature records go, no great difference between
+1858 and 1859, the year 1859, in which the period of ripening was the
+shortest, was somewhat colder in Norway than 1858. But we have the
+further difficulty that there were ten days difference in sowing, for
+in 1858 the sowing was made on May 14, and in 1859 on May 24. With
+all these possibilities uncontrolled, and indeed unconsidered, I am
+surprised that Semon should claim these experiments as one of the chief
+supports for his views.
+
+Schübeler's other allegations respecting the influence of climate on
+plants grown in various places and especially at different elevations
+in Norway have been destructively criticised by Wille[8] to whose paper
+readers interested in the subject should refer.
+
+Before the appearance of Wille's criticisms Wettstein[9] made a
+favourable reference to Schübeler's work, accepting his conclusion. He
+states also that he has himself made analogous experiments with flax,
+finding that the length of the period of development and a series of
+morphological characters show an adaptation to local conditions, and
+that on transference of seed to other conditions the previous effects
+are maintained. No details, however, are given, and I do not know if
+anything more on the subject has appeared since. The other examples
+cited by Wettstein, such as the observations of Cieslar on forest-trees
+and those of Jakowatz on gentians seem to me open to all the usual
+objections applicable to evidence of this kind. Such work, to be of any
+value for the purpose to which it is applied, must be preceded by a
+study of the normal heredity and of the variations of the species.
+
+Most of the recent writers (Semon, Przibram, etc.) on the inheritance
+of acquired characters accept the story of Brown-Séquard's guinea pigs,
+which are said to have inherited a liability to peculiar epileptiform
+attacks induced in their parents by various nervous lesions.
+
+The question has been often debated and several observers have repeated
+the experiments with varying results, some failing to confirm
+Brown-Séquard, others finding evidence which in various degrees
+supported his conclusions. Recently a new and especially valuable paper
+has been published by Mr. T. Graham Brown[10] which goes far towards
+settling this outstanding question. He states that "the Brown-Séquard
+phenomenon is nothing more or less than a specific instance of the
+scratch-reflex," and it is due to a raised excitability of the mechanism
+of this reflex. This raised excitability is the character acquired as a
+consequence, for instance, of the removal of part of one great sciatic
+nerve. The nature of this raised excitability and its causation are
+discussed and elucidated, but this part of the work is not essential
+to the present consideration. Mr. Graham Brown in his summary of
+conclusions remarks that it is very difficult to see how this condition
+of raised excitability can be transmitted to the offspring, and this
+comment which might be made in reference to any of the alleged cases
+certainly applies with special cogency to the present example.
+
+He then calls special attention to three observations:
+
+1. That guinea pigs which had a "trophic" change in the foot, as a
+result of division of the great sciatic nerve, have repeatedly been seen
+to nibble the feet of other guinea pigs which had this change in the
+foot from the same causes.
+
+2. That accidental injury to the toes may be followed by the
+Brown-Séquard phenomenon in an otherwise normal animal.
+
+3. That in several instances the young of guinea pigs which exhibited
+the phenomenon have been noticed to have one or more toes eaten off by
+the mother.
+
+Brown-Séquard noticed that almost all his animals in which the great
+sciatic was divided acquired the "epilepsy" and nibbled those parts of
+their feet in which sensation had been lost. Of the offspring of such
+animals he found that a very small proportion exhibited a malformation
+of the feet, and of these some showed the "epilepsy." The proportion
+which showed the "epilepsy" was one to two per cent. of the offspring.
+
+Morgan[11] is quoted by Graham Brown as having suggested that the
+loss of toes in the offspring may have been due to mutilation by the
+mother, following his experience in a case in which the tails of mice
+in succeeding litters were thus devoured, and there can be little doubt
+that in this suggestion lies the clue to the explanation of the whole
+mystery. Graham Brown concludes that it may be supposed with every
+degree of probability that the "transmission" was due to injuries
+inflicted upon the young by their parents. With this conclusion most
+people will now be disposed to agree, and we may hope that we shall
+hear the last of this curious myth--to the elucidation of which a vast
+quantity of research has been devoted.
+
+The series of experiments made by Kammerer with various Amphibia have
+attracted much attention and have been acclaimed by Semon and other
+believers in the transmission of acquired characters as giving proof of
+the truth of their views. With respect to these observations the chief
+comment to be made is that they are as yet unconfirmed. Many of the
+results that are described, it is scarcely necessary to say, will strike
+most readers as very improbable; but coming from a man of Dr. Kammerer's
+wide experience, and accepted as they are by Dr. Przibram, under whose
+auspices the work was done in the Biologische Vesuchsanstalt at Vienna,
+the published accounts are worthy of the most respectful attention.
+
+The evidence relates chiefly to three distinct groups of occurrences:
+
+  1. Modification in _Alytes obstetricans_, the Midwife Toad, affecting
+     both the structure and the mode of reproduction, induced by compulsory
+     change of habits.
+
+  2. Modification in the mode of reproduction of _Salamandra atra_ and
+     _maculosa_ induced by compulsory change of habits.
+
+  3. Modification in the colour of _Salamandra maculosa_ induced by change
+     in the colour of the soil on which the animals were kept.
+
+1. I will take first the case of _Alytes_,[12] because it is the most
+definite example, and because it is the case which most readily admits
+of repetition and verification.
+
+The habits of _Alytes obstetricans_ are well known. The animals copulate
+on land. As the strings of eggs leave the female they are entangled by
+the hind legs of the male, and being adhesive they stick to him and
+undergo their development attached to his back and legs. The number of
+eggs varies from 18 to 86, a number much smaller than is usual in toads
+and frogs which lay their eggs in water. The eggs are large and full of
+yolk.
+
+There are two breeding seasons, one about April and the other about
+September, and a winter hibernation. Not only animals brought in from
+outside, but their offspring reared in domestication maintain these
+normal habits in confinement, if the temperature does not exceed 17° C.
+(pp. 499 and 534).
+
+If, however, the temperature be artificially raised and kept at 25-30°
+C., the males do not attach the eggs to themselves when spawning occurs
+on land but let them lie. The adhesion of the eggs is said to be
+hindered by the comparatively rapid drying of their surfaces.
+
+More usually in the high temperatures the animals _take to the water_
+and copulate there. The eggs are ejected into the water, and as their
+gelatinous coverings immediately swell up, they do not stick to the
+males.
+
+The offspring thus derived from the parents subjected to heat for one
+breeding-period only, whether they were laid in water or on land, did
+not show departures from the normal type.
+
+Kammerer states next, however, that in subsequent breeding-periods the
+same parents frequently take to the water to breed, though they have
+become quite accustomed to the heated chamber; and furthermore that if
+such animals, having thus lost their instinct to brood their young, be
+transferred to ordinary temperatures they do not readily reassume their
+normal habits, but for several breeding seasons--at least four--will
+take to the water. These parents lay from 90 to 115 eggs, which are
+small and contain little yolk, and the larvae, on hatching, breathe with
+their embryonic gills until they are absorbed instead of being broken
+off as normally.
+
+The offspring thus abnormally developed when they mature are said never
+to brood their eggs. If they are derived from the earlier spawnings of
+their parents, before, that is to say, the parents had been submitted
+to the changed conditions long enough to transmit their effects, they
+lay on land; but if they are derived from the later spawnings, they
+lay in the water. These changes of habit are manifested without the
+continued application of the abnormal experimental conditions, and, as I
+understand the account, in normal conditions of temperature.
+
+If the abnormal experimental conditions are continued, the toads always
+lay in water, and their eggs become progressively smaller and more
+numerous. The larvae in the fourth generation acquire three pairs of
+gills instead of one pair, and are in other respects also different from
+the normal form.
+
+Respecting the _Alytes_ bred in this way Kammerer makes the very
+striking statement that _the males in the third generation_ (p. 535)
+_have roughened swellings on their thumbs and that in the fourth
+generation_ (pp. 516 and 535) _these swellings develop black pigment_.
+Together with the appearance of this secondary sexual character there
+is hypertrophy of the muscles of the fore-arm. To my mind this is
+the critical observation. If it can be substantiated it would go far
+towards proving Kammerer's case. _Alytes_, among toads and frogs, is
+peculiar in that the males do not develop these lumps in the breeding
+season, and the fact may no doubt be taken to be correlated with the
+breeding habits, copulation occurring on land and not in water as is
+usual with Batrachians. It is to be expressly noticed that these lumps
+on the thumbs or arms of male toads and frogs are not merely pigmented
+swellings, but are pads bearing numerous minute horny black spines,
+which are used in holding the females in the water. The figures which
+Kammerer gives (Taf. XVI, figs. 26 and 26a) are quite inadequate, and as
+they merely indicate a dark patch on the thumbs it is not possible to
+form any opinion as to the nature of the structure they represent.
+
+The systematists who have made a special study of Batrachia appear to
+be agreed that _Alytes_ in nature does not have these structures; and
+when individuals possessing them can be produced for inspection it
+will, I think be time to examine the evidence for the inheritance of
+acquired characters more seriously. I wrote to Dr. Kammerer in July,
+1910, asking him for the loan of such a specimen[13] and on visiting
+the Biologische Versuchsanstalt in September of the same year I made
+the same request, but hitherto none has been produced. In matters of
+this kind much generally depends on interpretations made at the time
+of observation; here, however, is an example which could readily be
+attested by preserved material. I notice with some surprise that in a
+later publication on the same subject no reference to the development of
+these structures is made (see below).
+
+The statements here given represent but a small part of Kammerer's
+papers on the subject. He gives much further information as to the
+course of the experiments, especially in regard to the fate of the eggs
+laid on land and the aberrations induced in them by treatment. The
+ramifications of the experiments are, however, very difficult to follow,
+and as I am not sure that I have always understood them I must refer the
+reader to the original.
+
+More recently Kammerer has published[14] a most curious account of
+experiments in crossing his modified and abnormal _Alytes_, derived from
+the water-eggs, with normal individuals.
+
+In the first case the cross was made between a _normal female_ and an
+_abnormal male_. The offspring were normal in their habits. In the next
+generation bred from these almost exactly a quarter showed the abnormal
+instinct.
+
+The reciprocal cross was made between an _abnormal female_ and a _normal
+male_. In this case the offspring were abnormal in their behaviour; but
+the second generation bred from them showed three quarters abnormal and
+one quarter normal.
+
+Certain details as to numbers and sexes of the various families bred
+in the course of this amazing experiment are given in a subsequent
+publication.[15] This later paper goes somewhat fully into the question
+of the difference in behaviour between the normal and modified
+individuals, describing the ways in which the males and females
+possessing the acquired character could be recognised from the males and
+females which were normal, but in this account I find no reference to
+the development of the "_Brunftschwielen_"--the horny pads on the hands
+of the males. As these structures would be of special value in such a
+diagnosis the omission of any allusion to them calls for explanation.
+Kammerer claims the evidence as proof of Mendelian segregation in
+regard to an acquired character, the first example recorded. Pending a
+repetition of the experiments there is no more to be said.
+
+2. _The Mode of Reproduction of Salamandra atra and
+maculosa._[16]--_Salamandra maculosa_, the common lowland form, with
+yellow bands or spots, deposits its young in water, generally as
+gill-bearing tadpoles, with a wide, swimming tail, though occasionally
+they are born still enclosed in the egg-capsule out of which they soon
+hatch. Spawning extends over a considerable period, often many weeks,
+and during the season one female may bear more than 50 young.
+
+_S. atra_, the black Alpine form, produces its young on land. They are
+born without gills, ready to breathe air, and with the rounded tail
+of the adult. These differences may, as Kammerer says, naturally be
+regarded as adaptations to the Alpine conditions. Moreover, the female
+bears _only two_ young in a season, and this reduction in the number
+must be taken to be a consequence or condition of viviparity. There are
+many eggs in the ovary, but all except the two which are destined to
+develop degenerate and form a yolk-material on which these two survivors
+feed.
+
+Kammerer gives a long account of the various conditions to which he
+subjected both species. The treatment was complicated in many ways, but
+the essential statements are, as regards _S. maculosa_, that when no
+water was provided in which the young might be born, they were dropped
+on land, larger and in a later stage of development and of a darker
+colour than is normal; that the larvae so born gradually diminished
+in number until only two were deposited in each breeding-period;
+that dissection showed that the other ova degenerated to form a
+yolk-material. The larvae so produced reached maturity. The summary of
+results describes their behaviour, stating that they produced:
+
+(_a_) _In water_, either (1) _very_ advanced, large-headed larvae 45
+mm. long (instead of 25-30 mm.) with gills already reduced, which had
+awkward, embryo-like movements, and in some few days metamorphosed
+into small perfect salamanders; or (2) moderately advanced, properly
+proportioned larvae, 40-41 mm. long, provided with large gills of (at
+first) intrauterine character, which were reduced during aquatic life.
+
+(_b_) _On land_, small (26 mm. long) larvae with rudimentary gills,
+having the body rounded instead of being flattened from above downwards,
+and an elongated narrow head, which were unable to live in deep water.
+These larvae changed to the salamander colour in 10-12 days, and after
+four weeks metamorphosed into salamanders 29 mm. long.
+
+(_c_) In the foregoing cases the experimental conditions were not
+continued, or in other words, basins of water were provided in which
+they could spawn. But if the experimental conditions are continued,
+these _Salamandra maculosa_ which were born newt-like (viz., not in a
+larval condition), are themselves newt-bearing from the first time they
+give birth, using the dry land, and bringing forth only two young, the
+normal number for the births of _S. atra_. These young are 40-41 mm.
+long, and are dark-coloured, resembling greatly the normal new-born _S.
+atra_.
+
+This epitome of the observations illustrating the inheritance of
+acquired characters has been very widely quoted, and may not unnaturally
+be taken to summarize a wide experience of the modified animals.
+Reference to the details given in the same paper shows that, as alleged,
+each of the four types of behaviour enumerated was witnessed _once_ only
+in the case of each of four females, no two agreeing with each other.
+As to the number of the males or their habits nothing is said. The first
+female, _a_ (1), bore five young; the second, _a_ (2), bore two, of
+which one was a partial albino; the third, _b_, produced four young; and
+the fourth, _c_, two as already stated.
+
+In the case of _c_ the details show that the female gave birth
+immediately after being transferred from the open-air terrarium to
+one indoors, which contained no basin of water. This is the example
+of the consequences which follow on a continuance of the experimental
+conditions.[17]
+
+As regards _S. atra_ the converse is reported. Various means were
+used to induce them to eject their young prematurely in water, such
+as massaging the sides of the mothers, or raising the temperature to
+25° or 30° C., with various degrees of success. But afterwards it was
+found that specimens collected wild at an elevation of about 1,000
+metres responded to much simpler treatment, and gave birth prematurely
+in water when they were kept in a large shallow basin of water not so
+deep but that they could everywhere touch the bottom with their feet and
+keep their heads above the surface. With specimens collected at higher
+elevations this treatment was inoperative, and the suggestion is made
+that _S. atra_ at the lower confines of its habitat partakes more of the
+nature of _maculosa_ than do the individuals from greater heights; for
+Kammerer argues that pools suitable for breeding must be more uncommon
+at those elevations than they are lower down.
+
+In the earlier paper[18] Kammerer states that newly caught females
+of _S. atra_ often give birth in the water, and show an undoubted
+preference for doing so. He describes also how he once saw several
+females, wild in their natural habitat, lay their young in a rain-puddle
+at 1,800 metres elevation, but the larvae thus born were fully formed.
+
+When the deposition of the young as larvae has become "habitual"[19]
+with _S. atra_, three to nine larvae may be produced at one spawning
+period, from 35 to 45 mm. long, with gills at most 8 mm. long, and a
+tail-fin 2-3 mm. broad. Such larvae are generally coffee-brown, or grey
+(instead of black), and show other minor differences.
+
+The summary states that when grown to maturity they become in their turn
+larva-bearing, and go into the water to bring forth. Their young are
+more than two (3 to 5 being the numbers observed) with a length of 33-40
+mm. or of 21-23 mm. at birth. They are light grey, spotted (mottled with
+lighter and darker colour), have relatively short gills (8 to 9 mm.
+at most) and a broad tail-fin (3 mm. wide). At metamorphosis they are
+relatively long (44 mm.) and one of them had some yellow pigment.
+
+Here again this summary is, as a matter of fact, describing the
+behaviour of two mothers, of which one produced three, and the other
+five young.
+
+To my mind these experiments suggest that the reproductive habits
+of both species, if closely observed, will be found to be subject
+to considerable variation, and I think it not impossible that each
+species is, especially in confinement, capable of being a good deal
+deflected from its normal behaviour. Moreover, there seems to me no
+great improbability in the idea that there is an interdependence
+between the number of young and the stage of maturity in which they
+are born. But, at the same time, the case as told by Kammerer strikes
+me as proving too much. If each species is so sensitive to conditions
+that the normal procedure is gravely modified in one generation, and
+if that modification can reappear in a pronounced form in the next
+generation without a renewal of the disturbing conditions, it becomes
+extremely difficult to understand how the regularity which each species
+is believed to display in nature can be maintained. Surely both species
+might be expected to be in confusion. From a passage in Kammerer's
+earlier paper (1904, p. 55) on the subject, I infer that he also would
+expect considerable irregularity in the natural behaviour, but that he
+has not investigated the point.[20]
+
+3. _Modification of the Colour of Salamandra maculosa induced by Change
+in the Colour of the Soil on which the Animals were kept._--Kammerer
+speaks of this as the most convincing of all his experiments on the
+transmission of acquired characters. So far, however, no full account of
+them has been published.[21] The statement is that when salamanders are
+kept in yellow surroundings the yellow markings gradually in the course
+of years increase in amount relatively to the black ground colour.
+Conversely by keeping the animals on black garden soil, the yellow may
+be greatly diminished in quantity until it largely disappears. (The
+account in _Natur_ adds that very moist conditions also favour the
+increase of yellow, and that with less moist conditions the yellow
+diminishes.) From each kind, the (induced) yellower and the (induced)
+blacker, a second generation was raised, on soil of neutral colour, and
+each family was later divided into two parts, half being put on black
+and half on yellow ground.
+
+As regards the offspring of those which had lived on _black_ soil no
+positive result had been reached up to the date of publication, but it
+is stated that these young resembled their parents in having the yellow
+distributed in _irregular spots_.
+
+As regards the offspring of those which had lived on yellow soil the
+account follows up the story of that part of the offspring which were
+put on yellow soil again. It is stated that these, though derived from
+parents with irregular spots, _developed the yellow as longitudinal
+bands_.
+
+This account is given with slight differences of expression in the
+three places to which I have referred. On returning from Vienna in
+1910 I consulted Mr. G. A. Boulenger in reference to the subject, and
+he very kindly showed me the fine series from many localities in the
+British Museum, and pointed out that in nature the colour-varieties
+can be grouped into two distinct types, one in which the yellow of
+the body is irregularly distributed in spots and one in which this
+yellow is arranged for the most part in two longitudinal bands which
+may be continuous or interrupted. _The spotted form is, as he showed
+me, an eastern variety, and the striped form belongs to western
+Europe._ Mr. E. G. Boulenger[22] has since published a careful account
+of the distribution of the two forms. The spotted he regards as the
+typical form, var. _typica_, and for the striped he uses the name
+var. _taeniata_. The typical form occupies eastern Europe in general,
+including Austria and Italy, extending as far west as parts of eastern
+France. The var. _taeniata_ is found all over France, excepting parts of
+the eastern border, Belgium and western Germany, Spain and Portugal. Of
+the very large series examined there was only one specimen (Lausanne)
+which could not with confidence be referred to one or other of the
+two varieties. Mr. E. G. Boulenger points out that both varieties
+inhabit very large areas, and live on soils of most different colours
+and compositions. Both are liable to variations in the amount and the
+shade of the yellow, but that any suggestion that _taeniata_ belongs
+especially to yellow soils and _typica_ to black soils is altogether
+inadmissible. He expresses surprise that Kammerer should not allude to
+these peculiarities in the geographical distribution of the two forms.
+He suggests further that it is more likely that some mistake occurred in
+Kammerer's observations than that the east European _typica_ should, in
+the course of a generation, have been transformed into the west European
+_taeniata_ by the influence of yellow clay soil.
+
+In his last paper on the subject Kammerer states incidentally[23] that
+he has found the _striped form recessive to the spotted_. No evidence
+for this statement is given, and I have not found any other reference
+to crosses effected between the two natural types. If, however, this
+representation is correct, it is conceivable that the production of
+_taeniata_ from _typica_ was in fact the re-appearance of a recessive
+form. The plate which Kammerer gives in illustration of his modified
+parent figures a single animal at four stages, and though it is
+certainly more like the spotted than the striped form, it has a certain
+suggestion of the striped arrangement, such as I can well imagine being
+produced in the heterozygote.[24]
+
+In continuation[25] of the experiments on the colour of _S. maculosa_
+Kammerer publishes an account of elaborate experiments in grafting
+ovaries of the various forms, modified and unmodified, into each other,
+and describes the offspring which followed. Before pursuing this part of
+the inquiry I am disposed to wait until the earlier steps have been made
+much more secure than they yet are.
+
+More recently Kammerer has published similar statements in regard to the
+inheritance of characters induced in various lizards by keeping them in
+abnormal temperatures, high and low. The changes induced affected in
+some species the colours, in others the reproductive habits. Respecting
+these examples I feel the same scepticism that I have indicated in
+regard to the others, somewhat heightened by the fact that insufficient
+evidence is given both regarding the behaviour of these various species
+in captivity when not subjected to abnormal temperatures, and in the
+wild state.
+
+Respecting this part of the evidence Mr. G. A. Boulenger has lately
+published a criticism[26] from which I extract the following passages.
+Referring to a previous note[27] on the question of the melanism of the
+various insular forms of _Lacerta muralis_ he writes: "I also alluded
+(_l. c._) to the theories that have been propounded to explain the
+melanism of various insular forms. This is a subject which has been
+lately taken up by Dr. Kammerer at the Biologische Versuchsanstalt in
+Vienna, and he claims to have produced nigrinos artificially by a very
+strong elevation of the temperature, accompanied by extreme dryness. Dr.
+Werner[28] has already opposed his own experiments to those of Kammerer,
+artificial melanism having been produced by him in _Lacerta oxycephala_
+by keeping two very light specimens from Ragusa for a whole summer
+in very damp conditions. Neither is Kammerer's theory in accordance
+with the distribution of the black lizards, as pointed out by Werner.
+Kammerer also finds that those forms which are known to produce melanic
+races in a state of nature, lend themselves more readily than the others
+to the success of his experiments. But he shows himself misinformed
+when he states that the variety called _Lacerta fiumana_ belongs to the
+category of those of which black forms are not known. He overlooks the
+fact, first pointed out by Scherer in 1904, and which I can confirm,
+that the black lizard from Melisello near Lissa in the Adriatic is
+unquestionably derived from the lizard from Lissa, which he correctly
+regards as not separable from _L. fiumana_...."
+
+"Another colour modification which Dr. Kammerer states that he obtained
+by raising the temperature is the assumption by the female of the
+typical _Lacerta muralis_ of the bright red colour of the lower parts
+which often distinguishes the male from the female, and which was not
+shown by the individuals of the latter sex kept by him under normal
+conditions. He quotes various authorities to show that the lower parts
+are never red in the females, but he has omitted to consult others who
+say the contrary. Thus Bedriaga (1878 and 1879) remarks that a so-called
+var. _rubriventris_ of the typical wall lizard has the lower parts red
+in both sexes."[29]
+
+In reading such papers as those of Semon or Kammerer the thought
+uppermost in my mind is that to multiply illustrations of supposed
+transmission of acquired characters is of little use until some one
+example has been thoroughly investigated. If we had certain assurance
+that even a single unimpeachable case could be repeated at will, the
+whole matter would assume a more serious aspect. If, for instance,
+Kammerer were able to show us _Alytes_ males with horny pads on their
+hands, it would be something tangible; still more, if the experiment
+were repeated by others until no doubt remained that the offspring
+of _Alytes_ which had bred in water for some three generations did
+acquire these pads and that they could transmit these novelties to
+descendants raised in normal conditions. Till evidence of this kind is
+published by at least two independent observers investigating similar
+material, I find it easier to believe that mistakes of observation or
+of interpretation have been made than that any genuine transmission of
+acquired characters has been witnessed.
+
+Meanwhile there is no denying that the origin of adaptational features
+is a very grave difficulty. With the lapse of time since evolutionary
+conceptions have become a universal subject of study that difficulty
+has, so far as I see, been in nowise diminished. But I find nothing in
+the evidence recently put forward which justifies departure from the
+agnostic position which most of us have felt obliged to assume.[30]
+
+
+                 APPENDIX TO CHAPTER IX.
+
+Professor G. Klebs, as is well known to students of evolutionary
+phenomena, has for several years been engaged in investigations relating
+to the inheritance of acquired characters. In his many publications
+on the subject the issue has always been represented as more or less
+uncertain.
+
+Desiring to know how the matter now stands according to Professor Klebs'
+present judgment I wrote to him asking him to favour me with a brief
+general statement. This he most kindly sent in a letter dated 8th July,
+1912.
+
+As such a statement will be read with the greatest interest by all who
+are watching the progress of these studies I obtained permission to
+publish it as follows:
+
+                                                    8. Juli 1912
+
+   Ihre liebenswurdige Anfrage will ich sehr gern
+   beantworten, obwohl ich sie nicht so beantworten kann wie ich
+   erwünschte. Ihr Skepticismus in der Frage der Uebertragung
+   erworbener Charactere auf die Nachkommen ist nur zu berechtigt.
+   Meine Versuche mit Veronica sind _nicht_ beweisend, da es mir
+   bisher nicht gelungen ist eine einigermasse konstante Varietät
+   mit verlaubten Inflorescenze zu erzeugen. In Bezug auf mein
+   Semper vivum bin ich allerdings noch heute der Meinung dass
+   die starke künstliche Veränderung der Blüte einen Einfluss auf
+   einzelnen Nachkommen gehabt hat. Ich habe seither nichts darüber
+   veröffentlicht: die Mehrzahl der anormalen gefüllten Blüten war
+   leider steril. Von einem weniger veränderten Exemplar erhielt
+   ich einige Sämlinge, aber sie haben noch nicht geblüht. Es kann
+   sich in diesem Falle nur um eine _Nachwirkung in der ersten
+   Generation_ handeln, vergleichbar jenen Fällen in denen Samen von
+   Bäumen aus den hohen Alpen in der Ebene gewisse Nachwirkungen
+   zeigen. Aber es ist bisher kein sicherer. Fall bekannt in den
+   der kunstliche herbeigeführte Charakter _mehrere Generationen
+   hindurch unter der gewöhnlichen "normalen" Bedingungen_
+   übertragen worden ist.
+
+   Auf der andere Seite sind diese negativen Resultaten nicht
+   entscheidend. Denn wie wenig ist in dieser Beziehung überhaupt
+   ernstlich versucht worden! Und zweifellos geht die Sache nicht so
+   einfach.
+
+   Ich versuche es mit anderen Pflanzen weil ich der Meinung bin
+   dass es möglich sein müsse wenigstens solche neuen Varietäten zu
+   erzeugen, wie sie die Gartenvarietäten entsprechen.
+
+   Aber bis jetzt leider sind die Versuche nicht gelungen, weder
+   mir noch irgend einem anderen.
+
+FOOTNOTES:
+
+[1] Semon, R., Der Stand der Frage nach der Vererbung erworbener
+Eigenschaften, published in _Fortschr. der naturw. Forschung._, Bd. 11,
+1910.
+
+[2] Standfuss, M., _Denks. Schweiz. naturf. Ges._, XXXVI, 1898, p. 32.
+
+[3] Fischer, E., _Allg. Ztschr. f. Entomologie_, Bd. VI, 1901.
+
+[4] Out of 12 pupae treated 8 died and of the 4 survivors, one only was
+affected. See M. v. Linden, _Archiv. Rassen. u. Gesells._, 1904, I.
+
+[5] For illustrations see _Oberthur's Études d'Entom._, 1896, where many
+of these curious aberrations are represented; also Barrett, _Lepid.
+Brit. Islands_, II, pp. 71 and 72.
+
+[6] Schübeler, F. C., _Die Culturpflanzen Norwegens_, 1862, especially
+pp. 24 and 28.
+
+[7] I am obliged to him and to Dr. E. Gold for much trouble taken to
+answer my questions. Some idea of the kind of weather indicated by an
+average of 2.76° C. above the mean may be got from a comparison with the
+year 1911, which most people will remember as one of the hottest summers
+they have known. The July of that year was in east and southeast England
+about 4° F. above the mean but 2.67 C. means about 4.8° F. above the
+mean. At Greenwich July, 1859, was about 6.5° F. above the average.
+
+[8] Wille, N., _Biol. Cbltt._, XXV, 1905, p. 521.
+
+[9] Wettstein, R. von. _Der Neo-marckismus u. seine Beziehungen zum
+Darwinismus_, Jena, 1903.
+
+[10] T. Graham Brown, _Proc. Roy. Soc._, 1912, vol. 84, B, p. 555. This
+paper gives full reference to the previous literature of the subject.
+
+[11] Morgan, T. H., _Evolution and Adaptation_, New York, 1903.
+
+[12] Kammerer's chief paper on this subject is in _Arch. f. Entwm._,
+1909, XXVIII, p. 447, and it is to this that the paginal references in
+the present text relate. His previous paper appeared, _ibid._, 1906,
+XXII, p. 48. An account of his further experiments with _Alytes_ is
+given in _Natur_, 1909-10, Heft 6, p. 95.
+
+[13] In reply to my letter Dr. Kammerer who was then away from home
+very kindly replied that he was not quite sure whether he had killed
+specimens of _Alytes_ with "_Brunftschwielen_" or whether he only
+had living males of the fourth generation, but that he would send
+illustrative material.
+
+[14] Kammerer, P., _ Natur_, 12 December, 1909, Heft 6, p. 95, repeated
+in _12 Flugschrift d. Deutsch Ges. f. Züchtungskunde_, Berlin, 1910.
+
+[15] _Festschrift zum Andenken an Gregor Mendel_, being vol. XLIX of the
+_Verh. Naturf. Ver. in Brünn_, 1911, p. 98.
+
+[16] Kammerer's chief papers on this subject are _Archiv fur Entwm._,
+XVII, 1904, and _ibid._, XXV, 1907. An epitome of results is also given
+by him in _12 Flugschrift d. Deutsch. Ges. f. Züchtungskunde_, Berlin,
+1910.
+
+[17] "_Bei Fortdauer der Versuchsbedingungen sind als Vollmolche
+geborene Salamandra maculosa_ gleich bei der ersten Geburt _abermals
+voll molchgebärend_, benutzen zum Geburtsakt das trockene Land,
+und zwar unter Erreichung der (bei _Salamandra atra_ normalen)
+_Embryonen-Zweizahl_," Kammerer, 1907, p. 49.
+
+[18] 1904, p. 56.
+
+[19] Throughout Kammerer's papers this is used almost as a technical
+term. It means, I presume, that the feature was manifested more than
+once.
+
+[20] It should be stated that the papers contain a quantity of detail,
+especially descriptive of the state of the larvae, which I have not
+attempted to represent, but the account here given contains all that
+seemed essential to an understanding of the more important features of
+the account.
+
+[21] The first appeared in _Natur_, 1909-10, Heft 6, p. 94; and the
+second, which contains coloured plates of the animals, in the lecture
+already referred to, _12 Flugschr. d. Deut. Ges. f. Züchtungkunde_,
+Berlin, 1910, p. 26. In the paper in _Mendel Festschrift_, 1911, the
+subject is continued, but no more is added as to this part of the
+experiment.
+
+[22] E. G. Boulenger, _Proc. Zool. Soc._, 1911, p. 323.
+
+[23] _Mendel Festschrift_, 1911, p. 84.
+
+[24] _12 Flugschrift. Deut. Ges. Züchtungskunde_, 1910, Fig. 15, _P.
+Reihe_.
+
+[25] _Mendel Festschrift_, 1911, p. 83.
+
+[26] Field, 1912, 30 March.
+
+[27] _Ibid._, 1904, p. 863.
+
+[28] _Mitth. Naturw. Ver. a. d. Univ. Wien_, 1908, p. 53.
+
+[29] As to the variations of _Lacerta muralis_ in Western Europe and
+North Africa see Boulenger, G. A., _Trans. Zool. Soc._, 1905, vol. XVII,
+p. 351.
+
+[30] As to the experiments of Klebs relating to the transmission of
+acquired characters, see Appendix.
+
+
+
+
+                           CHAPTER X
+
+            EFFECTS OF CHANGED CONDITIONS CONTINUED
+
+
+                THE CAUSES OF GENETIC VARIATION
+
+In the last chapter we examined some of the evidence offered in support
+of the belief that adaptation in highly organised forms is a consequence
+of the inheritance of adaptative changes induced by the influence of
+external conditions. The state of knowledge of this whole subject is, as
+I have said, most unsatisfactory, chiefly for the reason that in none of
+the cases which are alleged to show a positive result have two observers
+been over the same ground, or as yet confirmed each other. In the wider
+consideration respecting the causes of variation at large we find
+ourselves still in the same difficulty. The study has thus far proved
+sadly unfruitful. In spite of the considerable efforts lately made by
+many observers to induce genetic variation in highly organised plants
+or animals, and though successes have occasionally been announced, I
+do not know a single case which has been established and confirmed in
+such a way that we could with confidence expect to witness the alleged
+phenomena if we were to repeat the experiment. Abundant illustrations
+are available in which individuals exposed to novel conditions manifest
+considerable changes in characters or properties, but as yet there is
+no certain means of determining that germ-cells of a new type shall be
+formed.
+
+Of the direct effect of conditions the lower organisms, especially
+bacteria, offer the best examples, the alterations of virulence which
+can be produced in so many distinct ways being the most striking
+and familiar. That attenuation of virulence can be produced by high
+temperatures or by exposure to chemical agents, and that this diminution
+in virulence may remain permanent is, from our point of view, not
+surprising; but the fact that in many cases the full virulence can by
+suitable cultivation be restored is difficult to understand. Similar
+variations have been observed in power of pigment production and other
+properties.
+
+These phenomena naturally raise the question whether any cases of
+apparent loss of factors in higher forms may be comparable.
+
+The subject of variations in the lower organisms and their dependence on
+conditions is a highly special one, and I have no knowledge which can
+justify me in offering any discussion of them, but I understand that
+hitherto little beyond empirical recognition of the phenomena has been
+attempted. A useful summary of observations made by many investigators
+was lately published by Hans Pringsheim,[1] who enumerates the different
+agencies which have been observed to produce modifications, and the
+various ways in which these changes are manifested. One of the most
+comprehensive studies of the subject from the genetic point of view is
+that made by F. Wolf.[2] In his extensive cultivations of _Bacillus
+prodigiosus_, _Staphylococcus pyogenes_ and _Myxococcus_ he succeeded
+in producing many strains with modified properties. In most of these
+the modifications arose in consequence of the application of high or
+low temperatures or of the addition of various chemical substances
+to the culture-media. Some of the variations, which are for the most
+part in the powers of pigment-formation, persisted when the strains
+were returned to normal conditions, and others did not. In reference
+especially to the variations witnessed in the Cocci the reader should
+consult the critical account of variation in that group published by
+the Winslows,[3] where much information on the subject is to be found.
+The authors attempted to determine the systematic relationships of the
+several forms, as far as possible, by the application of statistical
+methods. The result is interesting as showing that the problem of
+species in its main features is presented by these organisms in a form
+identical with that which we know so well in the higher animals and
+plants, whatever properties be selected as the diagnostic characters.
+There are many types perfectly distinct and others which intergrade.
+Some of the types change greatly with conditions while others do not.
+This is exactly what we encounter whenever we study the problem of
+species on an extended scale among the higher forms of life.
+
+There is now practically complete agreement among bacteriologists
+that the observations made first by Massini on the change in color of
+_Bacterium coli mutabile_ grown in Endo's medium, associated with the
+acquisition of the power to ferment lactose, are perfectly reliable
+and free from possibilities of mistake. The work has been extended and
+confirmed by many workers, especially R. Müller, who finds that this
+bacterium can similarly acquire and maintain the power to ferment other
+sugars. A careful account of the whole subject written by Müller for the
+information of biologists will be found in _Zts. für Abstammungsl._,
+VIII, 1912. After discussing the biological significance of the facts,
+he concludes with a caution to the effect that bacteria are so different
+from all other living things that generalizations from their behavior
+must not be indiscriminately applied to animals and plants.
+
+In all work with this class of material there is obviously danger of
+error through foreign infection of the cultures, but there can be no
+doubt that though some of the "mutations" recorded may be due to this
+cause, the majority of the instances observed under stringent conditions
+are genuine.
+
+Another and equally serious difficulty besetting work with bacteria
+and fungi cultivated from spores is that the appearance of variation
+may in reality be due to the selection of a special strain previously
+living masked among other strains. This possibility must be remembered
+especially in those instances which are claimed as exemplifying the
+effects of acclimatisation. Manifestly this consideration can be urged
+with most force when the strain which gave rise to the novelty was
+not raised from a single individual spore. Moreover, when once the
+possibility of spontaneous variation is admitted, it must be difficult
+to be quite confident that any given variation observed is in reality
+due to the novel conditions applied, and as I understand the evidence,
+the appearance of the mutational forms does not with any regularity
+follow upon the application of the changed conditions.
+
+Researches into the variation of these lower forms will, no doubt, be
+continued on a comprehensive scale. So long as the instances recorded
+are each isolated examples it is impossible to know what value they
+possess. If they could be coordinated in such a way as to provide some
+general conception of the types of variation in properties to which
+bacteria, or any considerable group of them, are habitually liable, the
+knowledge might greatly advance the elucidation of genetic problems.
+
+Of mutational changes directly produced with regularity in
+micro-organisms by treatment, the experiments with trypanosomes provide
+some of the clearest examples. A summary of the evidence was lately
+published by Dobell,[4] from which the present account is taken. The
+most definite fact of this kind established is that certain dyes
+introduced into the blood of the host have the effect of destroying
+the small organ known as the "kinetonucleus" in the trypanosomes. The
+trypanosomes thus altered continue to breed, and give rise to races
+destitute of kinetonuclei. This observation was originally made by
+Werbitzki and has been confirmed by several observers. The exact way
+in which this alteration is effected in the trypanosomes is not quite
+definitely made out, but there is good reason for supposing that
+the dyes have a direct and specific action upon the kinetonucleus
+itself, and circumstances make it improbable that in some division a
+daughter-organism without that body is produced, or that any selection
+of a pre-existing defective variety occurs.
+
+Ehrlich has suggested with great probability that the dyes which possess
+this action owe it to the fact that they have the particular chemical
+linkage which he calls "ortho-quinoid." In outward respects, such as
+motility and general appearance, the modified organisms are unchanged,
+but their virulence is diminished. As regards the possibility of the
+defective strain reacquiring the kinetonucleus, Werbitzki states
+that in one case passage through 50 animals and treatment with dyes
+left the strain unaltered; but that in another case at the sixteenth
+passage 7 per cent. of the trypanosomes were found to have re-acquired
+the organ, and in subsequent passages the percentage increased, until
+at the twenty-seventh passage practically all had re-acquired it.
+Kudicke, however, in similar experiments did not succeed in causing
+re-acquisition by transplantation.
+
+By the action of various drugs and anti-bodies races of trypanosomes
+resistant to those substances have been obtained. These breed true, at
+least when kept in the same species of animal in which the resistance
+was acquired. As to whether change of virulence is produced by passage
+through certain animals or not, there is as yet no general agreement.
+
+Other changes, especially in size and some points of structure, are said
+to occur when certain trypanosomes proper to mammals are passed through
+cold-blooded vertebrates (Wendelstadt and Fellmer), and it is stated
+that these changes persist, but the observations have not yet been
+confirmed.
+
+Experiments lately conducted by Woltereck with _Daphnia_ are interesting
+as having given a definite positive result, in so far, at least, as the
+ova were affected by conditions before leaving the bodies of the parent
+individuals. The observations relate to the offspring resulting from
+_parthenogenetic_ eggs. Females bearing ephippia (fertilised eggs) were
+isolated until the ephippia were dropped, and in this way the offspring
+of fertilisation were excluded. Males, of course, appeared from time
+to time in the cultures, but as fertilised eggs were rejected, their
+presence did not disturb the result. The most remarkable observations
+related to _Daphnia longispina_.
+
+This species as found in the lower lake at Lunz had the front end of
+the body blunt and nearly round in profile; but on being cultivated in
+a warm temperature and with abundant nourishment the front end of the
+body became produced into an elongated "helmet," as Woltereck calls it.
+Experiment showed that the change was primarily due to the abundance of
+food, and owing to temperature in a subordinate degree.
+
+This distinction arose as soon as the species was taken into the
+hothouse, but when the modified individuals were put back into the
+original conditions, a lower temperature and scanty food-supply,
+the next generation returned to their original form. After being
+cultivated for two years and about 40 generations in the more favourable
+conditions, when similarly put back into the lower temperature with
+scanty food the _first generation_ born in these conditions was helmeted
+like the modified parents. Woltereck is of opinion that the ova were
+still unformed at the time the parents were put back, and the influence
+of the favourable conditions upon the unformed ova he speaks of as a
+"prae-induction." The effect never extended beyond the one generation,
+after which the strain returned to its original state.
+
+The fact that the influence on the offspring was not manifested at
+first led Woltereck to expect that by more prolonged cultivation in the
+favourable conditions a further extension of this influence would be
+produced, but this expectation was never fulfilled, though the attempt
+was made again and again.
+
+Similar experiments were made with _Hyalodaphnia cucullata_, which is
+far more sensitive to cultural influences, and in nature manifests
+a considerable elongation of the helmet as a seasonal modification,
+but the results were essentially the same as in the preceding case,
+no modification extending beyond the first generation born after the
+restoration to _normal conditions_.[5]
+
+The only criticism of these extremely interesting results which suggests
+itself is that perhaps the original appearance of the modification was
+not in reality due to an _accumulated_ effect of the conditions, but
+to some change in the conditions themselves which was not noticed. It
+is difficult to see how length of time or even the lapse of several
+generations could have so specific an effect on the race. It is no
+doubt often vaguely supposed by many that a long period of time may be
+necessary for the effect of climate or of other environmental conditions
+to be produced in an organism which does not thus respond at first. I
+have never been able to see any reason for this opinion nor how it is
+to be translated into terms of physiological fact, and I imagine that
+in those cases in which the lapse of time is really required for the
+production of an effect, the influence of the prolongation is rather
+on the conditions than on the organisms. The response of the organisms
+thus probably indicates not that the creature is at length feeling the
+effects because of their accumulated action on itself, but that the
+conditions have at length ripened.
+
+As this sheet is passing through the press Agar has published[6] an
+abstract of evidence as to another comparable case in a parthenogenetic
+strain in the daphnid, _Simocephalus vetulus_. When fed on certain
+abnormal foods the shape of the body is changed, the edges of the
+carapace being rolled backwards so as to expose the appendages. The
+offspring of animals thus modified showed similar modification in the
+first, and to a very slight degree, in the second generation, though the
+original mothers were removed to normal conditions before their eggs
+were laid. In the third generation there was "a very pronounced reaction
+in the opposite direction." Agar suggests that the change may be due to
+some toxin-like substances, carried on passively by the egg into the
+next generation, against which the protoplasm eventually produces an
+anti-body.
+
+The experiments which have been in recent years regarded by evolutionary
+writers as the most conclusive proof that direct environmental action
+may produce germinal variation are those of Professor W. L. Tower, of
+Chicago, on _Leptinotarsa_, the potato beetles. This work has attained
+considerable celebrity and has been generally accepted as making a
+definite extension of knowledge. After frequently reading Tower's papers
+and after having been privileged to see some of the experiments in
+progress (in 1907) I am still in doubt as to the weight which should be
+assigned to this contribution.
+
+The work is described in two chief publications, the first of which
+appeared in 1906.[7] This treatise contains a vast amount of information
+about numerous species and varieties of these beetles which the author
+has observed and bred in many parts of their distribution throughout
+the United States, Mexico and Central America. The part of the book
+which has naturally excited the greatest interest is that in which Tower
+states that by subjecting the beetles to change in temperature and
+moisture, he caused them to produce offspring quite unlike themselves,
+which in several cases bred true.
+
+It is much to be regretted that the author did not happen to become
+acquainted with Mendelian analysis at an earlier stage in the
+investigation. The evidence might then have been handled in a much more
+orderly and comprehensive way, and a watch would have been kept for
+several possibilities of error.
+
+The headquarters of the genus is evidently as Tower states, in Mexico
+and the adjoining countries. In this region there is a great profusion
+of forms, some very local, some as for instance the well-known
+_decemlineata_,[8] more widely spread. The distinctions are almost all
+found in peculiarities of colour and pattern, and the limits of species
+are even more indefinable than is usual in multiform animals. Tower
+arranges the various types into seven groups of which the one most
+studied is that which he calls the _lineata_ group. To this group belong
+all the forms to which reference is here made, and, as I understand,
+they differ among themselves entirely in size, colour and pattern.
+There is no suggestion of infertility in the crosses made between the
+several forms of the _lineata_ group; in fact they present, like many
+Chrysomelidae, a good example of what most of us would now call a
+polymorphic species, consisting of many types, some found existing in
+the same locality, others being geographically isolated.
+
+A series of experiments was devoted to the attempt to fix strains
+corresponding to the extremes of continuous variations. For example,
+those with most black pigment and those with least black taken from
+a population continuously varying in this respect, were separately
+bred; but almost always the selection led to no sensible change in the
+position of the mean of the population. The variations in these cases
+were evidently fluctuational. In some instances, however, real genetic
+differences were met with, and strains exhibiting them were, as usual,
+rapidly fixed.
+
+Tower points out that several of the varieties (or species, as he
+prefers to call them) were obviously recessive to _decemlineata_. This
+is most clearly demonstrated in the case of the form called _pallida_,
+which is a pale depauperated-looking creature, with the orange of
+the thorax almost white and the eyes devoid of pigment.[9] This form
+behaved as an ordinary Mendelian recessive, breeding true whenever it
+appeared in the cultures, or when individuals found wild were studied
+in captivity. A black form which Tower names _melanicum_ was similarly
+shown to be a Mendelian recessive. Wild specimens of this variety of
+opposite sexes were not found simultaneously in nature, and there was
+thus no opportunity of breeding them together, but the hereditary
+behaviour was seen in the F_{2} generation from a _melanicum_ found
+coupled with _decemlineata_. Experiments also occurred giving indication
+that a variety with the stripes anastomosing in pairs (_tortuosa_), was
+another recessive, and that a variety--called "_rubri-vittata_"--gave an
+intermediate F_{1} with subsequent segregation. All these are forms of
+_decemlineata_ Stål.
+
+Similar observations were made regarding forms recessive to
+_multitaeniata_ Stål. Of these two were thrown by _multitaeniata_
+itself, namely a form named by Stål _melanothorax_, and regarded by him
+as a species, and one which Tower names _rubicunda_ n. sp. The facts
+proving the recessive behaviour of their several forms will be found in
+the following places in Tower's book:
+
+  _pallida_,                      pp. 273-278.
+  _melanicum_,                     p. 279.
+  _tortuosa_,                      p. 280.
+  _rubrivittata_,                 pp. 280-281.
+  _melanothorax_ and _rubicunda_, pp. 283-285.
+
+Following this evidence of recessive nature of the six forms
+enumerated, Tower describes experiments showing, as he believes, that
+some of them may be caused to appear by applying special treatment
+to the parents during the "growth and fertilisation" (p. 287) of the
+eggs. The most striking example is that in which 4 males and 4 females
+of _decemlineata_ were kept very hot (average 35° C.) and dry, and at
+low atmospheric pressure (19-21 inches). The eggs laid were restored
+to natural conditions. These gave 506 larvae, from which emerged 14
+normal, 82 _pallida_ and 2 "_immaculothorax_," viz., without pigment
+on the pronotum. The account of the rest of the experiment is somewhat
+involved, but I understand that the _pallida_, of which two only
+survived, behaved as normal recessives when bred to the type: also that
+the parents, after having laid the eggs whose history has been given,
+were restored to normal conditions and laid 319 eggs which gave 61
+normals.
+
+In another case normal parents laid 409 eggs in the hot and dry
+conditions, and on restoration to normal conditions, the same parents
+laid 840 eggs. Then 409 eggs gave 64 adults as follows:
+
+                     _Males_          _Females_
+
+  _decemlineata_        12                 8
+  _pallida_             10                13
+  _immaculothorax_       2                 3
+  _albida_               9                 7
+                        ---               ---
+                        33                31
+
+The 840 eggs laid in normal conditions gave 123 normal _decemlineata_.
+
+Similar experiments were made with _multitaeniata_ and gave comparable
+results, the two recessives (_melanothorax_, _rubicunda_) being produced
+in large numbers when the parents were subjected to heat, but in this
+case the atmosphere was kept _saturated_ with moisture, instead of dry,
+as in the previous instance. The same parents transferred to normal
+conditions gave normals only.
+
+Lastly the form _undecimlineata_ was exposed "to an extreme stimulus of
+high temperature, 10° C. above the average," and a dry atmosphere, with
+the result that from 190 eggs there emerged 11 beetles, all of the form
+_angustovittata_ Jacoby, which subsequently bred true to that type (see
+p. 295).
+
+In the results of these experiments, as described, there is one feature
+which I regard as quite unaccountable. Tower makes no comment upon
+it. Indeed, from the general tenour of the paper, I infer, not only
+that he does not perceive that he is recounting anything contrary to
+usual experience, but rather that he regards the result as conforming
+to expectations previously formed. The point in question is the
+genetic behaviour of the dominant normals produced under the abnormal
+conditions. These normals were the result of the breeding of parents
+declared to be at the same time giving off many recessive gametes. Some
+of these normals must be expected therefore to be heterozygous unless
+some selective fertilisation occurs. Nevertheless in every case they and
+their offspring are reported to have continually bred true. I allude
+especially to the tables given on pp. 288, 289, 292, and 293. Tower does
+not mention any misgiving about this result, and I think he regards
+himself as recounting phenomena in general harmony with the ideas of
+mutation expressed by De Vries. This they may be; but to anyone familiar
+with analytical breeding the course of these experiments must seem so
+surprising as to call for most careful, independent confirmation.
+
+In 1910[10] Tower published an account of further experiments with
+_Leptinotarsa_. The work described related to two subjects. Crosses
+were made between three forms, _undecimlineata_ Stål, _signaticollis_
+Stål and "_diversa_" named by Tower as a new species. The distinctions
+between these three depend partly on characters of the adults and
+partly on those of the larvae. The adults of _undecimlineata_ and
+_diversa_ have the elytra striped, but the elytra of _signaticollis_
+are unstriped. The larvae of _signaticollis_ and of _diversa_ are
+yellow, but those of _undecimlineata_ are white.[11] Moreover, in
+_signaticollis_ and _diversa_ the black increases in the third
+stage of the larvae to form transverse bands which are absent in
+_undecimlineata_. The general course of the experiments shows that these
+differences may be approximately represented as due to the action of
+three factors, any of which may be independently present or absent. The
+stripings of the elytra and of the larvae are each due to a separate
+factor. As regards the distinction between the yellow and the white
+larvae the evidence does not prove that there is decided dominance of
+either colour and I infer that the heterozygotes are often intermediate.
+
+The chief contribution which this new paper claims to make relates to
+differences in the results which ensue from crosses effected between
+these three types at different average temperatures.
+
+We are first concerned with four experiments which I number (1), (2),
+(3), (4):
+
+1. _Signaticollis_ [F] × _diversa_ [M] bred at an average temperature
+of 80º F. by day and 75° F. by night, gave two groups in about equal
+numbers. The first (49) was pure _signaticollis_ and bred true. The
+second (53) was of an intermediate type, which on being bred together
+gave the typical Mendelian result--1 _sig._: 2 _intermediate_: 1 _div_.
+
+2. Next, as the account originally stood in the published paper, we
+are told that _sig_ [F] × _div_ [M] bred together at a day-temp.
+average 75° F. and night average 50° F. gave an _intermediate_ only,
+which subsequently produced a normal 1:2:1 ratio. The two crosses were
+repeated eleven times with identical results.
+
+In a further experiment (3) _signaticollis_ [F] × _diversa_ [M] were
+bred under the same conditions as those used in expt. (1). They again
+gave _sig._ and intermediates as before in fairly equal numbers. The
+_sig._ as before bred true, and the intermediate gave 1:2:1, all exactly
+as in expt. (1).
+
+In expt. (4) _the same parents used_ in (3) were again mated under
+conditions of expt. (2) at the lower temperature, and this time gave
+_signaticollis_ exclusively, which bred true for four generations. This
+experiment was repeated seven times with uniform results.
+
+Diagrams are given representing all these histories in graphic fashion.
+
+From these observations, Tower concludes that the determination of
+dominance, and the ensuing type of behaviour, is clearly a function of
+the conditions incident upon the combining germ plasms.
+
+It will be observed that expts. (1) and (3) gave identical results
+but (2) and (4), though much the same conditions were applied, are
+at variance, for (2) gave all intermediates, while (4) gave all
+_signaticollis_. In _Amer. Nat._, XLIV, 1910, p. 747, Professor T.
+D. A. Cockerell commented on this paper of Tower's and pointed out
+that there must be an error somewhere, for when he discusses these
+experiments Tower speaks of (2) and (4) as confirming each other. To
+this Tower replied[12] that there had been a mistake. He states that
+in preparing the paper "certain minor experiments were taken from
+a larger series and combined to illustrate a general point in the
+behaviour of alternative characters in inheritance," and that expt. (2)
+was introduced inadvertently in place of another which he desires to
+substitute. In this, which I number (5), _signaticollis_ [F] × _diversa_
+[M] from exactly the same stocks as those used in (1), were mated at the
+lower temperatures specified for (2), day average 75° F., night average
+50° F. These gave all of the _signaticollis_ type with a narrow range of
+variability, which bred true, in some cases to F_{6}. Tower says he has
+repeated this experiment six times with identical results.
+
+Nevertheless he proceeds to say that the description of expt. (2), which
+was repeated eleven times with identical results, was correct "as far as
+given." That experiment was "from a second series of cultures parallel
+to the one given, but in which there are other factors involved, which
+in H. 410 [my (2)] are productive of a typical Mendelian behaviour." He
+adds he does "not care at this time to make any statement of what these
+factors are, nor of their relations to the behaviours given in the H.
+409, H. 411, H. 409/11 series [my (1), (5) and (3)--(4)] which are the
+simplest and most easily presented series obtained in the crossing of
+_signaticollis_ and _diversa_."
+
+Professor Cockerell's intervention has thus elicited the fact that
+we have as yet only a small selected part of the evidence before us,
+even as concerning the effect of temperature on the cross between
+_signaticollis_ [F] × _diversa_ [M]. We learn that at the lower
+temperatures the result was eleven times the expected one, and six times
+an unexpected one; further, that we owe it to the author's inadvertence
+that we have come to hear of the expected result at all, and that though
+he knows the factors which determine the discrepancy, he declines for
+the present to name them. In these circumstances we can scarcely venture
+as yet to estimate the significance of these records.
+
+The paper goes on to recount somewhat comparable, but more complex
+instances in which the descent of the colour of adults and of larvae
+was affected by temperature in crosses between _undecimlineata_ and
+_signaticollis_. As they stand the results are very striking and
+unexpected, but I think, in view of what has been admitted respecting
+the former part of the paper, full discussion may be postponed till
+confirmation is forthcoming.
+
+One feature, however, calls for remark. This second paper is written
+apparently without any reference to the discoveries related by Tower in
+his previous book, to which no allusion is made. This is most noticeable
+in the case of an experiment in which (p. 296, H. 700A) _undecimlineata_
+[F] (the dominant) was mated to _signaticollis_ [M] with the result
+that all the offspring were _undecimlineata_ and bred true to that
+type (Parthenogenesis was tested for, but never found to occur). This
+experiment was made at a temperature averaging 95° F. ± 3.5° by day
+and 89° F. ± 4.8° by night, and in a humidity given as 84 per cent. by
+day and 100 per cent. by night; but in the previous book (p. 294) we
+are told that pure _undecimlineata_ bred together "under an extreme
+stimulus of high temperature, 10° C. above the average" and a relative
+humidity of 40 per cent. gave 11 beetles only, all _angustovittata_.
+But reference to the Plate 16, Fig. 2, shows that _angustovittata_
+must be exceedingly like _signaticollis_, having, like it, the elytral
+stripes obsolete, and if there is any marked difference at all, it can
+only be in the larvae. It seems strange that if _undecimlineata_ really
+gives off ova of this recessive type at high temperatures, the fact
+should not be alluded to in connection with expt. H. 700A, where, as
+the father was _signaticollis_, having the same recessive character,
+their appearance might have been expected not to pass unobserved. The
+temperature in the older experiment is, of course, not given with the
+great accuracy used in the second, and it may have been higher still.
+The humidity also was widely different. Still, in discussing the
+phenomena we should expect some reference to the very remarkable and
+closely cognate discovery which Tower himself had previously reported in
+regard to the same species.[13]
+
+The hesitation which I had come to feel respecting these two
+publications of Tower's has been, I confess, increased by the appearance
+of a destructive criticism by Gortner[14] who has examined the parts
+of Chapter III of Tower's book, in which he discusses at some length
+the chemistry of the pigments in _Leptinotarsa_ and other animals. As
+Gortner has shown, this discussion, though offered with every show of
+confidence, exhibits such elementary ignorance, both of the special
+subject and of chemistry in general, that it cannot be taken into
+serious consideration.
+
+Some observations made by Dr. W. T. Macdougal[15] have also been
+interpreted as showing the actual causation of genetic variation by
+chemical treatment. Of these perhaps the least open to objection
+were the experiments with _Raimannia odorata_, a Patagonian plant
+closely allied to _Oenothera_. The ovaries were injected with various
+substances and from some of the seeds which subsequently formed in them
+a remarkable new variety was raised. This varying or mutational form
+was strikingly different from the parental type, with which it was not
+connected by any intergradational forms, and it bred true. It made
+no rosette, growing to a much smaller size than the parent, and was
+totally glabrous instead of being very hairy as the parental type is.
+I was shown specimens of these plants by the kindness of Dr. Britton
+in the Bronx Park Botanic Garden in 1907 and can testify to their very
+remarkable peculiarities. They had a somewhat weakly look, and might
+at first sight be thought to be a pathological product, but they had
+bred true for several generations. From the evidence, however, I am by
+no means satisfied that their original appearance was a consequence
+of the treatment applied. This treatment was of a most miscellaneous
+description. Two of the mutants came from an ovary which had been
+treated with a ten per cent. sugar solution. Ten came from one into
+which a 0.1 per cent. solution of calcium nitrate had been injected. One
+was from a capsule which "had been exposed to the action of a radium
+pencil." Macdougal speaks of these results as decisive, but clearly
+before such evidence can be admitted even for consideration it must be
+shown by control experiments that the individual plants which threw
+the mutant were themselves breeding true in ordinary circumstances.
+Nothing is more likely than that the mutant was an ordinary recessive.
+I may add that Mr. R. H. Compton made a number of experiments with
+_Raimannia odorata_, raised from seeds kindly given me by Dr. Britton,
+injecting the ovaries with a variety of substances, including those
+named by Macdougal; but though a numerous progeny was raised from the
+ovaries treated, all were normal. Macdougal relates also that some
+mutational forms came from ovaries of _Oenothera Lamarckiana_ exposed
+to radium pencils, and also from _Oenothera biennis_ injected with
+zinc sulphate a peculiar mutant was raised, but taking into account
+the frequency of these occurrences in those species, he very properly
+regarded this evidence as of doubtful application. In a later paper,[16]
+however, he has returned to the subject and affirms his conviction that
+the appearance of a mutant among seedlings raised from an ovary of
+_Oenothera biennis_ treated with zinc sulphate was really a consequence
+of the injection, saying that the variation previously observed in
+the species was afterwards shown to be due to fungoid disease. The
+circumstances to which he mainly points in support of his view is
+that the mutation bred true, but this is only evidence of its genetic
+distinctness, which may, of course, be admitted by those who remain
+unconvinced as to the original cause of its appearance. He adds that he
+is making similar experiments with some twenty genera; but what is more
+urgently needed is repeated confirmation of the original observation.
+When it has been shown that this mutation can be produced with any
+regularity from a plant which does not otherwise produce it on normal
+self-fertilisation, the enquiry may be profitably extended to other
+plants.
+
+A curious and novel experiment, which however, led ultimately to a
+negative result, was made by F. Payne. Many discussions have been held
+respecting the blindness of cave animals. The phenomenon is one of the
+well-known difficulties, and most of us would admit that the theory of
+evolution by the natural selection of small differences does not offer a
+really satisfying account of it. Those who believe in the causation of
+such modifications by environmental influences and in their hereditary
+transmission make, of course, the simple suggestion that the darkness
+is the cause of the loss of sight, and that disuse has led to the
+reduction of the visual organs. Payne bred _Drosophila ampelophila_,
+the pomace-fly (which is easy to keep in confinement, fed on fermenting
+bananas), for sixty-nine generations in darkness. At the end of that
+period there was no perceptible change in the structure of the eyes, or
+in any other respect. The number of generations may possibly be regarded
+as insufficient to prove anything, but comparing them, as he does, with
+the generations of mankind, we see that they correspond with a period of
+about two thousand years, an interval far longer than those which many
+writers in particular cases have deemed sufficient.
+
+In his first paper Payne states that, though no structural difference
+could be perceived, the flies which had been bred in the dark reacted
+less readily to light than those which had been reared under normal
+conditions, and he inclined to think that the treatment had thus
+produced a definite effect. After more careful tests, however, he
+withdrew this opinion. It proved that both individual flies and
+individual groups of flies, both of those bred in the light and of
+those bred in the dark, differed greatly in their reactions, which were
+measured by counting the time that it took for a fly to travel to the
+light end of a covered tube, various sources of error being eliminated.
+He found further that these differences of behaviour were not inherited
+in any simple way, but he is disposed to attribute them to accidental
+differences in the nature of the food, an account which seems probable
+enough.[17]
+
+In several recent publications Blaringhem[18] has described the origin
+of many abnormal forms of plants, especially of maize, which he
+attributes to various mutilations practised upon the parents. Respecting
+these the same difficulty which has been expressed in other cases
+reappears, that before drawing any conclusion as to the value of such
+evidence we require to know that the plants treated belong to a really
+pure line, which if left to nature in the ordinary circumstances of its
+life in that locality would have had normal offspring. Abnormalities
+abound in the experience of everyone who examines pans of seedlings
+of almost any species of plant, and in maize they are well known to
+be exceptionally common. Some of those which we meet with when we
+attempt to ripen maize in this country are very similar to those which
+Blaringhem describes, consisting in irregularities in the distribution
+of the sexes, in the shapes of the panicles, etc. Many of these are
+doubtless imperfections of development, due to the dullness of our
+climate, but others are presumably genetic and would recur in the
+offspring however treated. If some one working in a climate where maize
+could be raised in perfection would repeat these experiments, and show
+that a strain which was thoroughly reliable and normal in its genetic
+behaviour did, after mutilation, throw the miscellaneous types observed
+by Blaringhem, that would be evidence at least that the development of
+the seed could be so influenced by injury to the parental tissues that
+its properties were changed. Such evidence could be used for what it is
+worth; but pending an inquiry of this kind I am disposed to regard these
+observations of variation following on parental injury as suggestive
+rather than convincing.
+
+Some evidence of a remarkably interesting kind has been collected by
+J. H. Powers[19] respecting the structure and habits of _Amblystoma
+tigrinum_, which led him to the conclusion that striking differences
+in the form, anatomy, and developmental processes could be effected
+directly by change in the conditions of life. It is well known that
+a profusion of forms, distinct in various degrees, is grouped round
+_Amblystoma tigrinum_. Some of these are believed to be geographically
+isolated, others occur together in the same waters, and, as usual,
+authorities have differed greatly as to the number of names to be given.
+These forms were studied in detail by Cope who described them in the
+_Batrachia of North America_. The view which he inclined to take was
+that the individual variations of _Amblystoma tigrinum_ resulted from
+variations in the time and completeness of the metamorphosis, and these
+were regarded as due to external causes, such as differences in season,
+temperature, and geographical conditions. Powers, however, states that
+collecting within a radius of six or eight miles he found almost if
+not quite the whole "gamut of recorded variation in this species."
+Some, however, as he states, occurred rarely except under experimental
+conditions, but considerable differences in temperature were not found
+necessary in producing them. Every year, he says, he has been able to
+add to the number of peculiar types found in the same small area in
+nature, until the amount of natural variation at least equals that seen
+by Cope in the collections of the National Museum and those of the
+Philadelphia Academy.
+
+Powers states that his observations by no means confirm Cope's view
+that these differences are in the main referable to variation in
+the completeness of metamorphosis, and on the contrary, he regards
+metamorphosis as on the whole a levelling process, tending to obliterate
+diversity. The enormous differences in size and proportions which he
+describes can only be appreciated by reference to his figures. They
+affect almost all features of bodily organisation. These striking
+differences he looks upon as brought about by differences in nutrition,
+"diversities in habitual locomotion," and diversity in the age at which
+metamorphosis occurs, and to sexual difference. Apart from sexual
+difference he regards the chief distinctions, in brief, as "acquired
+variations of the larva."
+
+As an example he gives the great elongation of some of the forms as
+"due first to slow growth, second to the free-swimming habit, third
+to the prolongation of larval life, and finally to the assumption of
+sexual maturity as males," either in the branchiate or non-branchiate
+condition. He describes the rapid growth of some and the slow growth
+of others. A larva of intermediate type may grow about a centimeter a
+month, but a rapidly growing specimen may grow more than four times
+as much. The slower rate of growth may, he says, be induced by winter
+feeding, and other treatment.[20]
+
+When, however, he goes on to describe the influences which he regards
+as exerted by the habit of freely swimming, I am led to wonder whether
+after all in most of these illustrations, the primary distinctions
+are not in reality genetic. "Specimens raised in the same aquarium or
+in similar aquaria, side by side with all conditions as uniform as it
+is possible to make them, seldom fail to furnish striking examples of
+broad-headed, short-bodied, and short-tailed types which are habitually
+found at the bottom, while others, slender and elongated, are free
+swimmers, and maintain themselves in almost as continual suspension
+and motion as does a gold fish." Later, again, he writes, "Yet despite
+the uniformity of these favourable conditions, the larvae soon began
+to split up into two noticeably distinct groups, the one of unusually
+compact proportions, the other of uniform intermediate build, such
+is most commonly met with." It is to my mind scarcely possible to
+resist the inference that, though there may be definite responses
+to certain conditions, yet the chief distinctions are genetic, and
+that it is these distinctions which confer the power to respond. The
+parts respectively played by cause and effect are always difficult to
+assign; but when it is stated that "a weak-limbed, long-bodied and
+long-tailed animal becomes well nigh perforce an undulatory swimmer,
+while the strong-limbed, short-tailed, heavy-bodied specimen, when
+these characteristics are rapidly forced upon it, is, under certain
+circumstances, just as forcibly induced to become a crawler," we feel
+how erroneous any estimates of causation are likely to be.
+
+One of the most remarkable and interesting sections of Powers' paper
+is that in which he describes the differences in bodily structure and
+habits which he attributes to cannibalism, and the whole account of the
+phenomena should be read in the original. It appears that there are
+two extremely distinct types of larvae, those with narrow heads and
+slender bodies which live for the most part on small Crustacea such
+as _Daphnias_, and those with huge mouths and very wide heads, which
+disregard such small animals altogether and live on amphibian larvae,
+whether of their own or other species. As the illustrations show, the
+differences between these two types are very great, and the differences
+in instinct and behaviour are no less. The cannibals take no heed of the
+pelagic crustacea, lying sluggishly at the bottom, rousing themselves
+immediately to a violent attack on the larger living things which
+approach them. Nothing but the most incontrovertible evidence based on
+abundant control experiments should convince us that such differences
+are not primarily genetic, and in the present state of knowledge I
+incline to think that the families really consist of individuals which
+are ready to assume the cannibal habit if opportunity offers, and
+others which are congenitally incapable of it. It may readily be that
+if all chance of cannibal diet be excluded, the full development of the
+wide head and mouth, or the other peculiarities, would never become
+pronounced, but I doubt whether such change could be induced in any
+individual taken at random.
+
+FOOTNOTES:
+
+[1] Pringsheim, H., _Die Variabilität niederer Organismen_, Berlin, 1910.
+
+[2] F. Wolf, Modifikationen u. Mutationen von Bakterien, _Zts. F.
+indukt. Abstam. u. Vererbungslehre_, II, 1909, p. 90.
+
+[3] Winslow, C. E. A. and A. R., _Systematic Relationships of the
+Coccaceae_. New York. 1909.
+
+[4] C. C. Dobell, _Jour. Genetics_, 1912, II, p. 201, where full
+references are given.
+
+Still more recently the same author has contributed an excellent summary
+of the evidence relating to bacteria (_ibid._, II. 1913, p. 325).
+
+[5] See Woltereck, _Verh. d. Deut. Zool. Ges._, 1909, p. 110; and 1911,
+p. 142. This is a subject which can only be properly appreciated on
+reference to the original papers. Several complications are involved to
+which I have not here alluded.
+
+[6] _Proc. Roy. Soc._, B, Vol. 86, 1913, p. 113.
+
+[7] _An Investigation of Evolution in Chrysomelid Beetles of the Genus
+Leptinotarsa_, Carnegie Publications, 1906, No. 48.
+
+[8] This is the famous Colorado beetle or potato-bug, which has caused
+such serious destruction in potato crops. There seems to be no doubt
+that this insect, formerly unknown in the eastern States, made its way
+east along the mining trails when the west was opened up.
+
+[9] This is indicated in the coloured plate, but I have not found any
+explicit statement to this effect in the text, and am not sure if the
+absence of pigment was regarded as complete.
+
+[10] _Biol. Bull._, XVIII, 1910, p. 285.
+
+[11] This description does not quite agree with the representation of
+the larvae in Pl. 17 of the book _Evolution in the Genus Leptinotarsa_
+for there the larva of _undecimlineata_ is shown as white in the second
+stage, but yellowish in the third stage; perhaps there is an error in
+printing.
+
+[12] _Biol. Bull._, XX, 1910, p. 67.
+
+[13] As to the interrelations of these three forms, Tower states (1906,
+p. 18) that _angustovittata_, which he reared from _undecimlineata_, is
+intermediate between it and _signaticollis_. Compare Stål, "_Monogr.
+des Chrysomélides_," 1862, p. 163; and Jacoby, _Biol. Centr. Amer.
+Celeopt._, vi, Pt. 1, p. 234, Pl. xiii, fig. 20; Tab. 41, fig. 15;
+_ibid._, Suppl., p. 253. All these forms are evidently very closely
+related, and the delimitation of species is quite arbitrary. Jacoby
+indeed suggests that _undecimlineata_ may be a variety of _decemlineata_.
+
+[14] Gortner, _Amer. Nat._, Dec., 1911, XLV, p. 743.
+
+[15] _Mutations, Variations, and Relationships of the Oenotheras_,
+Carnegie Institution Publication No. 81, 1907, pp. 61-64.
+
+[16] Macdougal, D. T., "Alterations in Heredity induced by Ovarial
+Treatments", _Bot. Gaz._, vol. 51, 1911, p. 241.
+
+[17] Payne, Fernandus, _Biol. Bull._, XVIII, 1910, p. 188, and _ibid._,
+XXI, 1911, p. 297.
+
+[18] See especially, _Mutation et Traumatismes_, Paris, Felix Alcan,
+1908.
+
+[19] J. H. Powers, "Morphological Variation and its Causes in
+_Amblystoma tigrinum_." _Studies from the Zoological Laboratory. _ The
+University of Nebraska, No. 71, 1907.
+
+[20] In connexion with this case I would refer the reader to some
+remarkable observations of Dr. T. A. Chapman on various types of larvae
+which he reared from the moth _Arctia caja_ (_Ent. Rec._, IV, 1893,
+p. 265, and following parts). From a single mother he raised a great
+diversity of forms, some which fed up rapidly and passed through their
+development without assuming certain stages, and others which were, as
+he called them, "laggards," moulting more times than their brethren and
+developing at a much slower rate. It is greatly to be hoped that such a
+case may be critically investigated by analytical breeding.
+
+
+
+
+                          CHAPTER XI.
+
+           STERILITY OF HYBRIDS. CONCLUDING REMARKS.
+
+When we consider the bearing of recent discoveries on those
+comprehensive schemes of evolution with which we were formerly
+satisfied, we find that certain details of the process are more easy
+to imagine. We readily now understand how varieties once formed,
+can persist, but at the same time difficulties hitherto faced with
+complacency become formidable in the light of the new knowledge. So
+generally is this admitted by those familiar with modern genetic
+research that most are rightly inclined to postpone the discussion. The
+premisses, indeed, on which such a discussion must be based are almost
+wholly wanting.
+
+The difficulties to which I chiefly refer are not those created by the
+phenomena of adaptation, though they are serious enough. In treating
+of that subject I have felt obliged to express scepticism as to the
+validity of nearly all the new evidence for the transmission of acquired
+characters. At the present time the utmost we are bound to accept is the
+proof that (1) in some parthenogenetic forms variations, or perhaps we
+may say malformations, produced in response to special conditions, recur
+in one or perhaps two generations asexually produced after removal to
+other conditions. (2) That violent maltreatment may in rare instances
+so affect the germ-cells contained in the parents as to cause the
+individuals resulting from the fertilisation of those cells to exhibit
+an arrest of development similar to that which their parents underwent.
+
+I do not doubt that evidence of this type will be greatly extended. As
+a contribution to genetic physiology these facts are very important
+and interesting, but I cannot think that any one, on reflexion, will
+feel encouraged by such indications to revive old beliefs in the direct
+origin of adaptations.
+
+In these respects we are simply left where we were. The force of
+objections based upon the existence of adaptative mechanisms is
+no greater than it has always been. On the contrary the fact that
+variations can now so generally be recognized as definite is some
+alleviation of the difficulty. We can moreover disabuse ourselves of
+the notion that for all characters which are definite or fixed, some
+utilitarian rationale may be presumed. Upon that point the study of
+variation has provided a perfectly clear answer.
+
+In frankly recognizing that the fixity of characters in general need
+not connote usefulness to their possessors we deliver ourselves of a
+distracting pre-occupation and prepare our minds for an investigation
+of the properties of living organisms in the same spirit as that
+in which the chemist and the physicist examine the properties of
+unorganized materials. The creature persists not merely by virtue of its
+characteristics but in spite of them, and the fact of its persistence
+proves no more than that on the whole the balance of its properties
+leaves something in its favour.
+
+It may be noted by the way that the fact that the structures of living
+things are on the whole adaptative was not always obvious. Though to
+naturalists of this generation it is a truism, we have only to turn to
+Buffon to find that in his philosophy of nature it played no essential
+part. The passage in which Buffon describes what he regards as the
+forlorn and degraded condition of the Woodpecker is well known. We have
+come to think of the Woodpecker as a capital example of adaptation to
+the mode of life; but Buffon after enumerating the hard features of
+the bird's existence, forced to earn its living by piercing the bark
+of trees in an attitude of perpetual constraint, remarks[1] "Tel est
+l'instinct étroit et grossier d'un oiseau borné a une vie triste et
+chétive. Il a reçu de la Nature des organes et des instrumens appropriés
+a cette destinée _ou plutôt il tient cette destinée même des organes
+avec lesquels il est né_" (my italics). His reflexions on the Stilt
+(_Himantopus_) read even more strangely to us, accustomed as we are to
+see in the prodigious length and thinness of the shanks and in the other
+features of its organisation palpable adaptations to a wading life. For
+Buffon, however, this curious bird seemed a poor, neglected production,
+extravagant in its disproportions, one of the misfits of creation,
+left as a shadow in the picture composed of nature's more successful
+efforts.[2] This theme he develops at some length, being evidently well
+pleased with the idea.
+
+Our way of regarding these things is doubtless sounder and more fruitful
+than Buffon's, but it is well to remember that what seems so obvious to
+us looked quite differently to other excellent observers; and stupid
+as it may have been to have overlooked plain examples of adaptation,
+it is a far worse mistake to see adaptation everywhere. I do not seek
+to minimise the real and permanent difficulty which the existence of
+adaptations creates, but by the suggestion that all normal specific
+differences are adaptational that difficulty was quite gratuitously
+increased.
+
+In these respects it may be claimed that progress has been made, even if
+that progress seem outwardly of small account.
+
+But all constructive theories of evolution have been built on the
+understanding that what we know of the relation of varieties to species
+justifies the assumption that the one phenomenon is a phase of the
+other, and that each species arises or has arisen from another species
+either by one or several genetic steps. In the varieties we have
+accustomed ourselves to think that we see those steps. We still know
+little enough of the mode of occurrence of variation, but we do begin to
+know something, and if we ask ourselves whether our knowledge, such as
+it is, conforms at all readily with our former expectations, we cannot
+with any confidence assert that it does. Among the plants and animals
+genetically investigated are many illustrations of very striking and
+distinct varieties. Many of these might readily enough be accepted as
+species by even the most exacting systematists, and not a few have
+been so treated in classification; but when we have examined their
+relationship to each other we feel not merely that they are not species
+in any strict sense but that the distinctions they present cannot be
+regarded as stages in the direction of specific difference. Complete
+fertility of the results of inter-crossing is and I think must rightly
+be regarded as inconsistent with actual specific difference; and of
+variations leading to that consequence no clear indication has yet been
+found. As an example of possible exceptions mention should perhaps be
+made of the case of a giant form of _Primula sinensis_ investigated by
+Keeble.[3] It arose from a "Star" Primula of normal size, and though
+fertile with its own pollen all attempts to fertilise it with the pollen
+of other forms failed. Miss Pellew, who did these fertilisations,
+tells me that very extensive trials were made, and repeated in several
+seasons. Ultimately two plants were raised from it fertilised with a
+plant of the strain from which it sprang, and these proved sterile.
+In the light of modern experience the significance of such isolated
+instances is doubtful.
+
+All the strains known as "Giants" are, as Messrs. Sutton have always
+found, more or less sterile, and their sterility is presumably due to
+some negative defect.
+
+In regard to the fertility of Primula species there are several
+paradoxes. For example the long-styled varieties, apart from giants, are
+fertile with their own pollen, and for many years short-styled plants
+have not been used in most strains. Auriculas and Polyanthuses, on the
+contrary, are generally if not always bred from short-styled plants,
+as the florists have decided that the long-styled are inadmissible.
+Mr. R. P. Gregory tells me that, though most strains of _P. sinensis_
+give seed enough when only long-styled plants are used, he finds
+nevertheless that when a "legitimate" union is made the amount of seed
+usually increases much as Darwin observed. Darwin's statement that
+plants of "illegitimate" origin are less fertile than the "legitimately"
+raised plants is also in general confirmed by his experience. To
+this rule there were some marked exceptions in strains derived from
+_long_-styled plants, which though illegitimate showed a high degree
+of fertility, but illegitimate unions between _short_-styled plants
+always produced comparatively sterile offspring. I have no records of
+the behavior of Auriculas and Polyanthuses. It would be interesting to
+know whether among them pure strains of short-styled plants (dominants)
+have appeared, and, if so, how their fertility is affected. Without
+much more critical data I suppose no one would nowadays be inclined
+to follow Darwin in instituting a comparison between the sterility
+of hybrids and that of illegitimately raised plants of heterostyle
+species.[4] It is even difficult to imagine any essential resemblance
+between these two phenomena, nor has evidence ever been produced to
+show that illegitimately raised plants have bad pollen grains, which is
+the usual symptom of sterility in hybrid plants and the consequence,
+as we believe, of failure of some essential division in the process of
+maturation.
+
+The difficulty that we have no knowledge of the contemporary origin of
+forms, from a common stock, which when crossed together give a sterile
+product, is one of the objections constantly and prominently adduced
+from the time of the first promulgation of evolutionary ideas. In the
+light of recent work the objection has gathered strength. Why, if
+we are able to produce instances of variation colourably simulating
+specific difference in almost all other respects, do we never find
+an original appearance of this most widely spread of all specific
+characteristics? No doubt all breeders know that sterile animals
+and plants occasionally appear in their cultures, but it is more in
+accordance with probability that the sterility in these sporadic
+instances should be regarded as due to defect than that it should be
+thought comparable with that of the sterile hybrids. For their sterility
+must, by all analogy with results elsewhere seen, be attributed not
+to the absence of something, but to the presence and operation of
+complementary factors leading to the production of inhibition of
+division; and consistently with that interpretation, we find that when
+from a partially sterile hybrid comparatively fertile offspring can be
+raised, their comparative fertility continues in the posterity generally
+if not always without diminution. The distinction between these several
+kinds of sterility was of course not understood in Darwin's time. The
+comparison, for example, which he instituted[5] between the sterility
+of "contabescent" anthers and that of hybrids no longer holds, for at
+least in those cases in which the nature of contabescent anthers have
+been genetically investigated (Sweet Pea, _Tropaeolum_) they proved
+to be a simple recessive character. Nor can we now easily suppose that
+the attempt there made by Darwin to suggest resemblance between the
+sterility produced by unnatural conditions and that of hybrids has any
+physiological justification.
+
+In regarding the power to produce a sterile or partially sterile hybrid
+as a distinction in kind, of a nature other than those which we perceive
+among our varieties, I am aware that I am laying stress on an impression
+which may hereafter prove false. The distinction nevertheless is so
+striking and so continually before the eyes of a practical breeder that
+he can scarcely avoid the inference that when he meets a considerable
+degree of sterility in a cross-bred he is dealing with something
+belonging to a distinct category, and not merely a varietal feature of
+an exceptional kind.
+
+Besides the sterility of hybrids appeal has often been made to the
+phenomenon of incompatibility, in its several stages of completeness,
+as distinguishing species. No one doubts that incompatibility may
+arise from a variety of causes of most diverse degrees of importance,
+but though sometimes referred to as an extreme case of interspecific
+sterility, it is really a very different matter. In regard to one phase
+of this incompatibility, that associated with self-sterility, some
+progress has been made, and we are not wholly without experimental
+evidence of its being within the range of contemporary variation.
+
+Given the outline of Mendelian teaching as to gametic differentiation
+and the classification of individuals in a mixed population, it
+seemed highly probable that what we call self-sterility must mean
+that the species really consisted of _classes_, some of which are
+capable of interbreeding with others while others are not. According
+to the received account every individual, though incapable of
+fertilising itself, was supposed to be able both to fertilise and to
+be fertilised by any other individual. This notion has always seemed
+to me a self-evident absurdity, for it would imply that there can be
+as many categories as individuals. Such experiments, however, as I
+made did certainly give results consistent with that belief. I first
+tried Cinerarias, which are usually self-sterile, but I found no
+incompatible pairs of plants. Whether I was deceived by the consequences
+of apogamy, or whether the pollen of certain plants may belong to more
+than one class I do not know. The results were confused in various
+ways. Usually the self-fertilised plants set little or nothing, and
+cross-fertilised they set fully with such uniformity that the few
+failures could plausibly be attributed to mistakes in manipulation
+or to other extraneous causes. Later de Vries announced[6] (without
+giving particulars) that he had proved the existence of such classes in
+_Linaria vulgaris_; but on making experiments with that species I again
+got no positive results, and I came to the conclusion that in spite of
+inherent improbability the conventional belief must be substantially
+true. At last, however, the work of Correns, lately published,[7] does
+definitely show that in one species, _Cardamine pratensis_, classes of
+individuals exist such that individuals of the same class are incapable
+of fertilising themselves or each other, but fertilisation made between
+the classes is usually completely effective. Many complications were
+encountered and some contradictory evidence is recorded, but the general
+bearing of the results was positive and indubitable.
+
+We know far too little of this phenomenon as yet to be able to
+understand its significance, but I suppose we may anticipate with some
+confidence that it will be found to be a manifestation of dissimilarity
+between the male and female gametes of the same individual, comparable
+with that first seen in the Stocks (_Matthiola_) which throw doubles--a
+state of things in all likelihood to be found widely spread among
+hermaphrodite organisms. Whether the incompatibility between species
+is to be associated with that of the self-steriles also cannot be
+positively asserted, though it seems not unreasonable to expect that
+such an association will be discovered.
+
+The case of the apple and the pear is an impressive illustration of
+this possibility. The two species are of course exceedingly alike in
+all outward respects, but nevertheless the pollen of each is entirely
+without effect on the other. Presumably we should interpret this fact
+as meaning not so much that the apple and the pear are in reality
+very wide apart, but rather that either, each is lacking in one of
+two complementary elements, or that each possesses a factor with an
+inhibitory effect. Their incompatibility may well be of the same nature
+as that of the classes in _Cardamine pratensis_.
+
+Returning now to the problem of inter-specific sterility; we note,
+as I have said, the absence of contemporary evidence that variation
+can confer on a variety the power to form a sterile hybrid with the
+parent species. The considerations based on this want of evidence have
+for a long while been familiar to all who have discussed evolutionary
+theories, and it is worth observing the exact reason why the difficulty
+strikes us now with a new and special force. In pre-Mendelian times
+all that was known was that some forms could freely interbreed without
+diminution of fertility in the product, while others could not. But now
+we find that, by virtue of segregation, from one and the same pair of
+parents, or even, in the case of hermaphrodites, from one and the same
+individual, offspring commonly arises showing among themselves exactly
+such differences as distinguish species--and very good species too. This
+we see happening again and again. But to forms capable of arising as
+brethren in one family the title species has never been meant to apply,
+and if we are going to use the term in application to fraternal groups
+we must definitely recognise that by "specific" difference is to be
+understood simply _difference_, without any immediate or even ulterior
+physiological limitation whatever. Naturally, therefore, we begin to
+think of the appearance of sterility in crosses as something apart, and
+as a manifestation which distinguishes certain kinds of unions in a very
+special way.
+
+I am perfectly aware that there are gradations in the sterility of
+hybrids as in every other characteristic upon which it has been proposed
+to base specific definitions; but, as also so often happens in the
+matter of defining intergrading categories, the difficulty in practice
+is not often such as to lead to actual ambiguity. I am speaking of
+course of those examples which are amenable to genetic experiment.
+As to the rest there is complete and permanent uncertainty. But the
+experience of the practical breeder does, I think, on the whole, support
+the contention to which systematists have so steadily clung under all
+the assaults of evolutionary philosophers, that, though we cannot
+strictly define species, they yet have properties which varieties have
+not, and that the distinction is not merely a matter of degree.
+
+The first step is to discover the nature of the factors which by their
+complementary action inhibit the critical divisions and so cause
+the sterility of the hybrid. Thus expressed, we see the problem of
+inter-specific sterility in its right place; and the question why we
+do not now find contemporary instances of varieties lately arisen in
+domestication, which when crossed back with their parents, or with their
+coderivatives, can produce sterile products, is perceived to be only a
+special case of a problem which in its more general form is that of the
+origin of new and additional factors.
+
+For the requisite evidence no comprehensive search has been made, but
+perhaps it will yet be found. All that we can say at the present time
+is that the incidence both of hybrid sterility, and of incompatibility
+also, is most capricious; and provided that two forms have such features
+in common that a cross between them seems not altogether out of the
+question, no one can predict without experiment whether such a cross
+is feasible, and if feasible whether the product will be fertile, or
+sterile more or less completely. For instance, though probably all
+the British and some Foreign Finches (Fringillidae) have been crossed
+together, and some of these crosses, as for instance, the various
+Canary-mules have been made in thousands, I believe no quite clear
+example of a fertile hybrid can be produced. Many species of Anatidae
+cross readily and produce fertile hybrids: others give results uniformly
+sterile. Though most of the Equidae can be crossed and some of the
+hybrids are among the commonest of domesticated animals there is no
+certain record of a fertile mule. Among the Canidae the dogs, wolves and
+jackals all give fertile hybrids, but there is no clearly authenticated
+instance of a cross between any of these forms and the European fox.
+In spite of their close anatomical resemblance it is doubtful if the
+rabbit and the hare have ever interbred. Many of the wild species of
+_Bos_ have been crossed and recrossed both with each other and with
+many domesticated races, but I understand that no cross with the Indian
+buffalo (_Bos bubalus_) has yet been successful even in producing a
+live calf.[8] In the genus _Primula_ many hybrids are known and several
+of them occur in nature, but hitherto no certain hybrid between _P.
+sinensis_ and any other species has been made, in spite of repeated
+attempts.
+
+In _Nicotiana_ many--doubtless all--the various forms of _N. tabacum_
+can be crossed together without diminution of fertility, though some
+are very distinct in appearance, but crosses between _tabacum_ and
+_sylvestris_ are highly sterile (in my experience totally sterile[9]),
+though the distinctions between them are not to outward observation
+nearly so great as those which can be found between the various races of
+_Primula sinensis_.
+
+Recently some remarkable experiments bearing closely on these questions
+have been published by F. Rosen.[10] They concern the forms of _Erophila
+(Draba) verna_, celebrated in the history of evolutionary theory as the
+plants especially chosen by Alexis Jordan for the exposition of his
+views on these subjects.
+
+The "species" contains a profusion of forms dissimilar in many
+structural characters, such as the size and shape of leaves, flowers,
+fruits, etc. Of these forms many grow in association. Jordan found, on
+experiment, that each, to the number of some two hundred, bred true, and
+that therefore, the conventional assumption that polymorphism of this
+kind must mean great contemporary variability had no foundation in fact.
+So far indeed is the evidence from favouring the belief that such forms
+are in any way transitional or indeterminate, that, as is well known,
+Jordan used it with every plausibility to support the doctrine of the
+fixity of species. To certain aspects of Jordan's work we will return
+later in this chapter, but the matter is in the present connection
+of especial interest for the reason that Rosen has lately found by
+experiment that some of these presumably very closely allied forms,
+crossed together, gave hybrids more or less sterile. In the case of the
+offspring of one pair of forms only (_E. cochleata_ and _stricta_) was
+the fertility undiminished, and the various degrees of sterility found
+in the other crosses ranged up to the extreme infertility of the hybrids
+between _E. stricta_ × _elata_. From this cross ten plants were bred.
+Of these the four strongest were chosen to breed from, but two of the
+four proved totally sterile; one had only bad seeds; and from the fourth
+a single seedling was raised which in its turn proved to be sterile.
+From the less sterile hybrids F_{2} families were raised, with the
+usual experience that in this and subsequent generations the sterility
+diminished among extracted forms, new and true-breeding types with
+complete fertility being thus derived from the original cross.[11]
+
+The production of sterility as a consequence of crossing plants so
+nearly approaching each other as these _Erophila_ "species" do is
+not a little interesting, and the fact well exemplifies the futility
+of the various attempts to frame general expressions as to specific
+properties or behaviour. Commenting on his results Rosen argues that
+the polymorphic group commonly called by systematists _Erophila (Draba)
+verna_ may now be regarded as having arisen by crossing, as did his own
+types mentioned above. The question, however, _what_ species were the
+original progenitors of the group cannot be answered. Rosen considers
+that no form which he knows satisfies the requirements, and that
+it or they must be supposed to be lost. This conclusion will recall
+the similar problem raised by the _Oenothera_ mutants (Chap. V); and
+unsatisfactory as it may be to have recourse to such hypotheses we
+must remember the possibility that as a consequence of hybridisation,
+subsequent segregation and recombination of factors, species may
+have thus actually, as we may say, exploded, and left nothing but a
+polymorphic group of miscellaneous types to represent them in posterity.
+If this way of regarding the phenomena be a true one, the sterility
+now seen when some of the group are re-crossed, becomes analogous to
+that "reversion or crossing" which we now so well understand to be
+a consequence of the recombination of characters separated at some
+previous point in the history of descent. In the partial sterility of
+the contemporary hybrid we see this character reappearing, formed now
+as it was on the occasion of the original cross, by the meeting of
+complementary factors.
+
+Another case that may be mentioned in this connection is that of the
+crosses between various culinary peas (_Pisum sativum_) and a peculiar
+form found by Mr. Arthur Sutton growing ostensibly in a wild state in
+Palestine. This Palestine Pea is low growing, rarely reaching 18 inches.
+It is in general appearance like a small and poorly grown field pea.
+The stems are thin and rather hard. The most obvious differences which
+distinguish this from other field peas are the marked serration of the
+stipules, and the development of pith in the pods. Such pith is often
+present in the pods of peas more or less, but in the Palestines it is
+so strongly developed as almost to form a lomentum. Curiously enough,
+though the flowers are purple much as those of ordinary field peas,
+there is no coloured spot in the axils. On the other hand, the stems
+have coloured stripes running up from the axils. Though this plant
+differs so little from domesticated peas, all crosses with them either
+failed, or produced hybrids quite or almost quite sterile. This was Mr.
+Sutton's experience, and on repeating the experiments with material
+kindly given by him I found the same result.[12]
+
+In a large series of crosses some seeds died or gave rise to feeble
+plants. Of the plants which lived, few gave any seed. The seed, however,
+that was obtained from F_{1} plants grew well enough, and the F_{2}
+plants proved, as often in such cases, fertile. In these, indeed, no
+sign of sterility was noticeable. The experiment is being repeated in
+various ways, for, as the genetic behaviour of peas is comparatively
+well known, the subject is an exceptionally favourable one for these
+investigations.
+
+Such an example shows the confusion produced the moment we attempt to
+harmonize conceptions of specific difference with results attained by
+experimental methods. It has been usual to regard the field pea (_P.
+arvense_) as a species distinct from the edible pea (_P. sativum_).
+De Candolle and others regard the field pea as derived from a form
+wild in Italy, but the origin of the edible pea is considered to be
+unknown. From breeding experiments we find no sterility whatever in the
+crosses between the various _arvense_ and _sativum_ types, nor in the
+crosses made between them and several other peculiar types from various
+countries; whereas this Palestine Pea, which only differs from a small
+_arvense_ in what might have been thought trivial characters,[13] either
+fails to cross altogether or gives a sterile product, whatever type be
+chosen as the other parent.
+
+Examples of this kind have at least the merit that they lead to more
+precise delimitations of the problem. We are confronted with two
+distinct alternatives.
+
+1. We may apply the term Species promiscuously to all distinct forms.
+If we do so it must be clearly understood that we cannot even rule out
+the several combinations of "presences and absences" represented by the
+various types whether wild or domesticated. For we may feel perfectly
+assured that at least all the _arvense_ and all the _sativum_ types yet
+subjected to experimental tests are on precisely the same level in this
+respect. There is no distinction, logical or physiological, to be drawn
+between them. Some contain more factors, and others contain fewer. In
+some the re-combinations have been brought about by natural variation or
+crossing, while the same consequences in the others have resulted from
+man's interference.
+
+2. We may follow the conventions of systematists and distinguish the
+outstanding or conspicuous forms such as _arvense_, _quadratum_,
+_sativum_ and perhaps a few more as species, and leave the rest
+unheeded. If this course is followed it must be clearly understood and
+permitted as a piece of pure pragmatism, deliberately adopted for the
+convenience of cataloguers and collectors, without regard to any natural
+fact or system whatsoever.
+
+But while following either the one plan or the other we shall be still
+awaiting the answer, which only genetic experiment can provide, to the
+question whether among the various types there are some which differ
+from the rest in a peculiar way: whether by having groups of characters
+linked together in especially durable combinations, or by possessing
+ingredients which cause greater or less disturbance in the processes of
+cell-division, and especially in the processes of gametic maturation,
+when they are united by fertilisation with complementary ingredients.
+
+Before any but the vaguest ideas regarding the nature and significance
+of inter-specific sterility can be formed, a vast amount of detailed
+work must be done. Sterility as a result of crossing, as well as
+that which is alleged sometimes to arise in consequence of changed
+conditions, is at best a negative characteristic, and there are endless
+opportunities for mistake and misinterpretation in studying features
+of this kind. No one, I suppose, would now feel any great confidence
+in most of the data which from time to time are resuscitated for the
+purpose of such discussions. Even the best collections of evidence, such
+as those given by Darwin in _Forms of Flowers_, cannot be regarded as
+critical when judged by present-day standards. Nothing short of the most
+familiar acquaintance with the habitual behaviour of individuals, and of
+strains kept under constant scrutiny for several years would enable the
+experimenter to form reliable judgments as to the value to be attached
+to observations of this class.
+
+The admission must, however, be faced that nothing in recent work
+materially tends to diminish the surprise which has always been felt
+at the absence of sterility in the crosses between co-derivatives. We
+should expect such groups of forms to behave like the _Erophila_ types,
+and frequently to produce sterile products on crossing. Whatever be
+the explanation, the fact remains that such evidence is wanting almost
+completely. In spite of all that we know of variability nothing readily
+comparable with the power to produce a sterile hybrid on crossing with
+a near ally, has yet been observed spontaneously arising, though that
+characteristic of specificity is one of the most widely distributed
+in nature. It may be that the lacuna in our evidence is due merely to
+want of attention to this special aspect of genetic inquiry, and on the
+whole that is the most acceptable view which can be proposed. But seeing
+that naturalists are more and more driven to believe the domesticated
+animals and plants to be poly-phyletic in origin--the descendants, that
+is to say, of several wild forms--the difficulty is proportionately
+greater than it was formerly, when variation spontaneously occurring was
+regarded as a sufficient account of their diversity.
+
+
+                        CONCLUDING REMARKS.
+
+The many converging lines of evidence point so clearly to the central
+fact of the origin of the forms of life by an evolutionary process
+that we are compelled to accept this deduction, but as to almost all
+the essential features, whether of cause or mode, by which specific
+diversity has become what we perceive it to be, we have to confess an
+ignorance nearly total. The transformation of masses of population by
+imperceptible steps guided by selection, is, as most of us now see, so
+inapplicable to the facts, whether of variation or of specificity, that
+we can only marvel both at the want of penetration displayed by the
+advocates of such a proposition, and at the forensic skill by which it
+was made to appear acceptable even for a time.
+
+In place of this doctrine we have little teaching of a positive kind
+to offer. We have direct perception that new forms of life may arise
+sporadically, and that they differ from their progenitors quite
+sufficiently to pass for species. By the success and maintenance of
+such sporadically arising forms, moreover, there is no reasonable doubt
+that innumerable strains, whether in isolation or in community with
+their co-derivatives, have as a fact arisen, which now pass in the
+lists of systematists as species. For an excellent account of typical
+illustrations I would refer the reader to the book lately published by
+R. E. Lloyd[14] on the rat-population of India. The observations there
+recorded are typical of the state of things disclosed whenever the
+variations of large numbers of individuals are closely investigated,
+whether in domestication or in natural conditions.
+
+Guided by such clues we may get a good way into the problem. We see
+the origin of colourable species in abundance. Then, however, doubt
+arises whether though these new forms are as good species as many
+which are accepted as such by even cautious systematists, there may
+not be a stricter physiological sense in which the term species can be
+consistently used, which would exclude the whole mass of these _petites
+espèces_.
+
+If further we find that we have, with certain somewhat doubtful
+exceptions, never seen the contemporary origin of a dominant factor, or
+of inter-racial sterility between indubitable co-derivatives, it needs
+no elaboration of argument to show that the root of the matter has not
+been reached.
+
+Examination of the inter-relations of unquestionably distinct species
+nearly allied, such as the two common species of _Lychnis_, leads to
+the same disquieting conclusion, and the best suggestion we can make
+as to their origin is that _conceivably_ they may have arisen as two
+re-combinations of factors brought together by the crossing of parent
+species, one or both of which must be supposed to be lost.
+
+All this is, as need hardly be said, an unsatisfying conclusion. To
+those permanently engaged in systematics it may well bring despair.
+The best course for them is once for all to recognise that whether
+or no specific distinction may prove hereafter to have any actual
+physiological meaning, it is impossible for the systematist with the
+means at his disposal to form a judgment of value in any given case.
+Their business is purely that of the cataloguer, and beyond that they
+cannot go. They will serve science best by giving names freely and by
+describing everything to which their successors may possibly want to
+refer, and generally by subdividing their material into as many species
+as they can induce any responsible society or journal to publish.
+Between Jordan with his 200 odd species for _Erophila_, and Grenier and
+Godron with one, there is no hesitation possible. Jordan's view, as he
+again and again declares with vehemence, is at least a view of natural
+facts, whereas the collective species is a mere abstraction, convenient
+indeed for librarians and beginners, but an insidious misrepresentation
+of natural truth, perhaps more than any other the source of the
+plausible fallacies regarding evolution that have so long obstructed
+progress.
+
+Nevertheless though we have been compelled to retreat from the
+speculative position to which scientific opinion had rashly advanced,
+the prospect of permanent progress is greatly better than it was. With
+the development of genetic research clear conceptions have at length
+been formed of the kind of knowledge required and of the methods by
+which it is to be attained. If we no longer see how varieties give
+rise to species, we may feel confident that a minute study of genetic
+physiology of varieties and species is the necessary beginning of any
+critical perception of their inter-relations. It is little more than a
+century since no valid distinction between a mechanical mixture and a
+chemical combination could be perceived, and in regard to the forms of
+life we may well be in a somewhat similar confusion.
+
+As yet the genetic behaviour of animals and plants has only been
+sampled. When the work has been done on a scale so large as to provide
+generalisations, we may be in a position to declare whether specific
+difference is or is not a physiological reality.
+
+FOOTNOTES:
+
+[1] Buffon, _Hist. Nat._, Oiseaux, 1780, VII, p. 3.
+
+[2] Ibid., VIII, p. 115.
+
+[3] Keeble, _Jour. Gen._, 1912, II, p. 173.
+
+[4] _Animals and Plants_, ed. 1, 1868, II, pp. 180-5.
+
+[5] _Animals and Plants_, ed. 1, 1868, II, p. 165.
+
+[6] _Species and Varieties_, 1905, p. 471.
+
+[7] Correns, _Festschr. med.-nat. Ges. zur 84 Versamml. Deutsch. Naturf.
+u. Aertze. Münster i. W._, 1912.
+
+[8] This is a case of a somewhat different order and I mention it partly
+for that reason as an illustration of the complexity which such negative
+instances may present. The difficulty is that though the buffalo and
+the zebu can breed together, the foetus is too large to be born alive.
+(See Ackermann _Ber. d. Ver. f. Naturk._, Kassel, 1898, p. 69. Prof. S.
+Nathusius, of Halle, who has great experience in crossing Bovidae, tells
+me that he has always failed to cross the buffalo with other species.)
+
+[9] In a paper to be published in the Report of the Genetic Conference,
+Paris, 1911, Bellair states that he obtained some partially fertile
+hybrids in the cross _N. sylvestris_ × _tabacum_. As to the various
+degrees of sterility in hybrids between _Nicotiana_ species see Lock, R.
+H., _Ann. Roy. Bot. Gardens_. Peradeniya, IV, 1909, p. 195.
+
+[10] _Beitrage zur Biol. der Pflanzen._, X, 1911, p. 379.
+
+[11] One very peculiar feature was observed, namely, that all the new
+forms in F_{2} which were bred from came true. As I understand, this
+statement applied to five such new types, and they were represented
+by 76 individuals in F_{3}, but further details on this point are
+desirable. Another curious fact was observed, namely that one of the
+F_{1} forms (_cochleata_ × _radiata_) when fertilised by _cochleata_
+gave a highly polymorphic family, but fertilised by _radiata_ the
+resulting offspring were almost uniform.
+
+[12] I also had a few F_{1} seeds given me by Mr. R. H. Lock.
+
+[13] In a paper about to appear in _Jour. Linn. Soc._ Mr. A. W. Sutton
+identifies this Palestine pea as _Pisum humile_ of Boissier and Noé.
+
+[14] Lloyd, R. E., _The Growth of Groups in the Animal Kingdom_, London,
+1912.
+
+
+
+
+                         INDEX OF SUBJECTS
+
+                                                         PAGE
+ Abraxa grossulariata,                               105,193
+ Aceras hircina, local variability,                      123
+ Achatinellidae, local forms of,                         133
+ Acquired characters, inheritance of,    188 et seq.,217,233
+ Acronycta psi, melanic,                                 138
+ Adaptation, problem of,                             187,234
+ Agelaius, local forms,                                  120
+ Agrotis, fixed and variable species,                     25
+ Alkaptonuria,                                            83
+ Alpine Plants, growing larger, if protected,            183
+ Alpine Varieties,                                       165
+ Alytes obstetricans, Kammerer's experiments on,     199,210
+ Amblystoma, races of,                                   230
+ Amphidasys betularia, melanic form,                 136,138
+   dimorphic larvae,                                     141
+ Anodonta, polymorphism of,                              130
+ Antirrhinum, striped,                                    57
+   species-hybrids,                                       99
+   albinos,                                              110
+ Apple, will not cross with pear,                        239
+ Arctia caja, effects of temperature,                    192
+   larval variation in,                                  231
+ Arctic varieties,                                       165
+ Argynnis paphia and valesina in Italy,                  121
+ Armadillo, polyembryony,                                 42
+ Artistic faculty,                                        89
+ Arum, rights and lefts,                                  57
+ Auriculas, short-styled selected,                       236
+ Axis of symmetry in hand and foot,                       48
+ Axolotl, alleged effect of conditions,                  230
+ Azalea, bud-sports,                                      55
+
+ Bacillus anthracis, unsegmented form,                    71
+ Bacillus prodigiosus, variation in,                     213
+ Bacteria, variation in,                                 212
+ Bacterium coli, variation in,                           214
+ Baeolophus, geographical races of,                      159
+ Barley, right and left-handed,                           58
+ Basilarchia, geographical races of,                     161
+ Begonia phyllomaniaca,                                   50
+   hybrids,                                               51
+ Bizarre Carnation, genetics of,                          54
+ Black, as a variation from red,                         148
+ Blackbird, varying,                                     150
+ Black Cock, fixity of,                                   28
+ Boarmia repandata, melanic form,                        136
+   rhomboidaria,                                     137,139
+ Botrytis susceptibility to,                             108
+ Bovidae, hybrid,                                        242
+ Brachydactyly,                                        89,95
+ Bradypus, vertebral variation,                           68
+ Bud-sports geometrically irregular,                   54-57
+ Buffalo, attempts to hybridize,                         242
+ Bullfinch, gynandromorph,                                45
+ Bulimus detritus, local variation of,                   126
+
+ Canary, asymmetrical markings in,                       154
+ Canidae, hybrid,                                        241
+ Capsella,                                               100
+ Cardamine pratensis,                                    239
+ Cat, Polydactylism,                                      53
+ Carnation, Picotees and bizarres compared,            54,58
+ Cataract, hereditary,                                    89
+ Certhiola, melanic,                                     142
+ Chladni figures,                                         60
+ Choloepus, vertebral variation in,                       68
+   local variation in,                                   119
+ Cinerarias, self-sterility in,                          238
+ Cistudo, local variation in,                            119
+ Climatic varieties,                                     164
+ Coccaceae, variation in,                                213
+ Coenonympha arcania, climatic forms of,                 179
+   satyrion,                                             180
+ Coereba, melanic,                                       142
+ Colaptes, geographical races,                           147 et seq.
+   chrysoides,                                           154
+ Colloids, growth in,                                     65
+ Colorado beetles, experiments on,                       218
+ Colour blindness in twins,                               44
+ Continuous variation, possible example of,              173
+ Coracias, geographical races of,                        160
+ Cotton, genetics of,                                 98,100
+ Coupling,                                               110
+ Crab, extra claws,                                       74
+ Crustacean appendages and Serial Homology,               63
+ Crystals, analogy with,                                  78
+ Cyclopian monsters, artificial,                          50
+
+ Daphnia, changed by environment,                        216
+ Dasypus, polyembryony,                                   42
+ Dianthoecia, fixed and variable species,                 25
+ Disease-resistance,                                      87
+ Division, power of,
+   a fundamental attribute of living things,              38
+   Genetics of,                                        46,50
+ Dogger Bank, large varieties on,                        125
+ Dogs, hybrid,                                           241
+ Dominance, nature of,                                    95
+ Dominants, origin of new,                          88,90,95
+ Double monsters,                                         42
+ Draba, experiments with,                                242
+ Drosophila,                                              91
+   Payne's experiments on,                               228
+
+ Earthworm, regeneration,                                 77
+ Elephant, tusk segmented,                                38
+ Entelechy,                                               80
+ Environmental treatment, effects of,                    188 et seq.
+ Enzymes and genetic factors,                             86
+ Epilepsy, inheritance of traumatic,                     197
+ Equidae, sterility of hybrid,                           241
+ Erophila, experiments with,                             242
+   species,                                              249
+ Exacum, right and left,                                  57
+ Euphonia elegantissima, local forms,                    120
+ Eupithecia rectangulata, melanic form,                  137
+
+ Factors, new,                                            88
+   loss of,                                               96
+ Factorial representation of varieties,              158,165
+ Falcons, geographical races,                            147
+ Fasciation,                                              49
+ Ferments, Boyle on,                                      54
+ Finger-prints of twins,                                  44
+ Fixity and Variability in species,                       25
+ Flax, climatic experiments,                             197
+ Fowl, Silky,                                             84
+   Leghorn,                                            85,90
+   Dominant white,                                        94
+   Wyandotte,                                             97
+   Rumpless,                                              46
+ Foxes, incompatibility with dogs,                       241
+ Free-martin,                                             44
+ Fringillidae, sterility of hybrid,                      241
+ Fundulus, cyclopian,                                     50
+
+ Gallus, invariability of wild species,                   13
+   and origin of poultry,                              90,97
+ Genitalia, a basis for classification in insects,        13
+ Gentians, climatic experiments,                         197
+ Geometrical structure and differentiation,            54,56
+ Geometrical distinction between germ-cells
+         and somatic cells,                               58
+ Gladiolus, right and left,                               57
+ Gnophus obscurata, protective colouring,                141
+ Goldfinch, geographical races,                          147
+ Gonioctena variabilis, variation in sexes of,           121
+ Gouldian Finch, polymorphism,                       148,149
+ Gracilaria stigmatella, experiments on,                 193
+ Grantia, large varieties of,                            125
+ Ground-Squirrels, local forms of,                       132
+ Grouse, red, variation,                                  29
+ Guillemot, Ringed,                                      150
+ Guinea-pig, Brown-Séquard's experiments on,             198
+ Gynandromorphs,                                          45
+
+ Heliconius erato, forms of,                         122,164
+ Helix lapicida, local variation of,                     126
+   striata,                                              127
+   Heripensis,                                           127
+   Caespitum,                                            127
+   trochoides,                                           127
+   nemoralis and hortensis,                              128
+ Helminthophila, geographical races of,                  157
+ Hemerophila abruptaria, melanic,                        142
+ Hepialus humuli, in Shetland,                           119
+ Heterostyle plants,                                     236
+ Hieracium,                                                9
+ Himantopus,                                             234
+ Homoeosis,                                               68
+ Hybernia progemmaria,                                   139
+ Hybrids, sterility of,                                  233 et seq.
+
+ Incompatibility between certain allied species,         239
+ Individual, geometrical independence of,                 58
+ Inhibiting Factors,                                      95
+ Intermediates, nature of,                           131,135
+ Isolation, consequences of,                             118
+
+ Lacerta muralis, Kammerer's experiments on,             209
+   fiumana,                                              210
+ Leptinotarsa, Power's experiments on,                   218
+ Limbs, extra, in pairs,                                  72
+ Limnaea, sinistral,                                     134
+ Linaria vulgaris, self-sterility,                       239
+ Loasa fruits, right and left,                            57
+ Lobster, extra claws,                                    76
+ Locality, variation connected with,  14,118,146 et seq.,208
+ Lumbricus, regeneration,                                 77
+ Lychnis dioica and vespertina, inter-relations of,       18
+   macrocarpa, possibly a common parent of,               19
+
+ Machetes pugnax, polymorphism of male,                   28
+ Maize, Blaringhem's experiments on,                     229
+ Maize, cumulative factors in,                           116
+ Malformations, dominants, arising de novo,               89
+ Manx Cat, heredity,                                      46
+ Matthiola,                                       84,104,113
+ Melanic varieties,                                      135 et seq.
+ Memory, analogy with heredity,                          190
+ Meristic variation,                                69,83,86
+ Mirabilis, striped,                                      57
+ Models of segmentation,                               59,60
+ "Modes," Coutagne's conception of,                       126
+ Mödling, peculiar race of _Pieris napi_ at,        178
+ Mole, albino,                                         27,28
+ Mule, Linnaeus on,                                        8
+ Mutation, Matthioli on,                                   4
+   in Mercurialis,                                         5
+   in Kales,                                               5
+   alleged in bulbs,                                       5
+   Theory,                                                97
+   periods of,                                           114
+   in Bacteria,                                          214
+ Mutilation, consequences of,                             71
+   alleged effect of, on offspring,                      229
+ Myxococcus, variation in,                               213
+
+ Narwhal, asymmetry of tusks,                             44
+ Nemesia strumosa,                                        91
+ Neuration, a basis for classification,                   13
+ Nicotiana, sterility of hybrid,                         242
+ Nightjars, varying,                                     150
+ Noctuidae, fixity and variability,                       25
+ Noctua, polymorphic and fixed species,                   25
+ Noctua castanea, local forms of,                        122
+ Nomenclature, future of,                             94,245
+ Notonecta, variations of,                               130
+
+ Odontoptera bidentata, melanic form,                    137
+ Oedipodidae, protectively coloured,                     140
+ Oenothera, new dominant in,                              92
+   rubricalyx and rubrinervis,                         92,95
+   Lamarckiana,                                       92,101
+   origin of,                                        102,244
+   has bad pollen-grains,                                102
+   factorial analysis of,                                103
+   pollen and egg-cells genetically dissimilar,          104
+ Oenothera, "twin hybrids",                              105
+   laeta and velutina,                                   105
+   reciprocal crosses in,                                105 et seq.
+   possible coupling in,                                 111
+   dwarfs,                                           112,114
+  "Triple hybrids",                                      114
+   alleged variation due to treatment,                   227
+ Ophrys, local variability,                              125
+ Orange, polyembryony,                                    45
+ Osmotic growth,                                          65
+ Overlapping forms,                                  146,174
+
+ Papilio, geographical races of,                         162
+ Papilio turnus, variation of,                           144
+ Pararge egeria, geographical forms,                     166 et seq.
+ Parthenogenesis,                                         50
+ Partula, local forms of,                                133
+ Passer domesticus and montanus, distinctions,            22
+ Pea, round and wrinkled,                                 95
+ Pear, will not cross with apple,                        239
+ Pelargonium, variegated,                                 55
+   bud-sports,                                            56
+ Periodic phenomena in structure,                         63
+ Peronea, fixed and variable species,                     26
+ "Petites espèces",                                       248
+ Petunia, double,                                        104
+ Phalanger maculatus, local variation,                   119
+ Pheasant, fixity of,                                     29
+ Phigalia pilosaria, melanic,                        139,140
+ Phratora vitellinae, experiments on,                    193
+ Phyllotaxis,                                             69
+ Pied varieties common in Passer domesticus
+   unknown in Montanus,                                   23
+ Pieris napi and bryoniae,                               174 et seq.
+ Pig, mule-footed,                                        46
+ Pigeon, web-footed,                                   46,49
+   Indian Rock, a recessive form,                         98
+ Pigments, nature of,                                     83
+ Pisum humile, hybrids with culinary peas,               244
+   species,                                              246
+ Planarian, regeneration of,                           71,77
+ Plotheia frontalis, polymorphic,                      26,29
+ Plusia, fixity and variation in,                         26
+ Poephila gouldiae, variation of,                    148,149
+ Polarity of individual,                                  44
+ Polia chi, melanic,                                     138
+ Polyanthus, short-styled selected,                      236
+ Polydactylism in Cat,                                 52,53
+ Polyembryony,                                            45
+ Potato, variation in,                                    91
+ Poultry, evolution of,                                   90
+ Primula obconica,                                        91
+ Primula sinensis, flaked,                                57
+   Leaf-shapes,                                           70
+   new dominant in,                                       92
+   sterility in,                                         236
+  "Giants",                                              236
+ Primula, species-hybrids,                               242
+ Protective coloration,                                  140
+ Pyrrhulagra, local forms,                               120
+ Python, twin-vertebrae,                                  60
+
+ Quiscalus, geographical races of,                       156
+
+ Rabbit, Angora,                                          46
+   colours of,                                            93
+   Incompatibility with hare,                            242
+ Raimannia odorata, Macdougal's experiments on,          226
+ Rats, Variation in,                                     248
+ Recessives, origin of,                                   90
+ Reciprocal crosses, giving distinct results,            105 et seq.
+ Regeneration,                                            70
+ Repulsion,                                              110
+ Reversal on Regeneration,                                77
+ Rhamphocoelus, geographical forms,                  159,184
+ Rhinoptera, variation in jaws of,                        38
+ Rhythm in repetition,                                    69
+ Ribs, variation of,                                      68
+ Rights and Lefts,                                     57-58
+ Ripples, analogous to segments,                    60,66,67
+   regeneration of,                                       79
+ Rollers, geographical races of,                         160
+ Ruff, polymorphism of male,                              28
+
+ Salamandra, maculosa and atra,                  182,199,203
+   spotted and striped,                                  207
+   geographical variation of,                            208
+ Segmentation, nature of,                                 63
+   simulated mechanically,                                64
+   compared with rippling,                                65
+   analogies with,                                        68
+ Segmentation of normally unsegmented structures,         38
+ Selection, Natural, an insufficient
+         cause of definiteness of types,          17,134,142
+ Sempervivum,                                            250
+ Serial Homology, the true nature of,                  62,66
+ Setina, Alpine varieties,                               181
+ Sex of Twins,                                            44
+ Sex-factors, possible coupling of,                      111
+ Sexual characters, variation in,                        119 et seq.
+ Siamese twins,                                           44
+ Silky Fowl,                                           84,85
+ Simocephalus, changed by environment,                   218
+ Sinistral forms,                                      33-34
+ Situs transversus,                                       43
+ Skate's jaws, variation in,                              38
+ Sloths, vertebral variation,                             68
+ Species, conceptions of,                    3,94,99,240,245
+   allied, distribution of,                              185
+   alternative uses of the term,                         245
+ Specific difference, universality of,                    12
+   of organisms compared with those
+       of inorganic materials,                            15
+   failure of theory of Selection
+       to explain,                                18,134,247
+ Sphyropicus varius,                                 149,156
+ Spilosoma lubricipeda, varieties of,                    181
+   Zatima, Heligoland form,                              181
+ Spinal nerves, segmentation of,                          67
+ Sporadic variation,                             131,134,248
+ Squashes, polymorphism of,                              100
+ Staphylococcus pyogenes, variation in,                  213
+ Sterility of hybrids, in general,                       233
+   in Lychnis hybrids,                                    20 et seq.
+   in crossing forms of Draba,                           243
+   Significance of,                                      244
+   Self,                                                 238
+ Stilt,                                                  234
+ Stocks,                                          84,104,113
+ Striped varieties,                                       57
+ Substantive variation,                                   84
+ Subtraction-stages,                                      93
+ Supernumerary limbs,                                  72-76
+ Sweet pea, variation of,                                 91
+   sterile anthers in,                                   237
+ Symmetry compared with heredity,                         41
+ Symmetry of body approximate,                            78
+ Syndactyly,                                              47
+   in foot,                                               48
+ Synthetic formulae, in nomenclature,                     94
+
+ Taeniocampa, fixed and variable species,                 25
+ Tamias, local forms of,                                 132
+ Tanagers, geographical races of,                        159
+ Teeth, variation in,                                  67,39
+ Tephrosia consortaria and consonaria,           137,139,140
+ Tephrosia species, separated by season,                 119
+ Terminal members, variation of,                          68
+ Thais rumina, local variation in,                        27
+ Tolerance, persistence of diversity due to,          17,134
+ Tomato, number of cells in fruit,                        46
+ Transitional populations, rarity of,                    165
+   an example,                                           178
+ Tropaeolum, sterile anthers in,                         237
+ Trypanosomes, variation in,                             215
+ Tusk, of Elephant, segmented,                            38
+   of Narwhal,                                            44
+ Twinning,                                          41,44,71
+   heredity of,                                           45
+   in organs,                                             46
+
+ Uria troile, variety of,                                150
+
+ Vanessa urticae, effects of temperature,                191
+ Variation, a medley of phenomena,                     14,15
+   sporadic,                                         131,134
+   and locality,                                         118
+   Causes of genetic,                          86,87,131,212
+   Substantive and meristic,                              83
+ Veronica, specific difference in,                        16
+   intermediates between species,                         17
+ Vertebrae, division in,                               60,61
+   homologies of,                                         66
+ Vespa, specific difference in,                           23
+ Vortex, living organism compared with,                   40
+
+ Wave-motion compared with repetition of parts,     62,67,79
+ Wheat, cumulative factors in,                           116
+   climatic experiments on,                              195
+ Woodpecker,                                             234
+
+ Zebra, pattern of stripes compared with ripples,         38
+
+
+
+
+                INDEX OF PERSONS
+
+                                         PAGE
+  Ackermann,                              242
+  Agar,                                   218
+  Allen, J. A.,                   132,147,159
+  Annandale,                               47
+  Arrigoni degli Oddi,                    167
+
+  Backhouse,                               50
+  Baker, G. T.,                           166
+  Bangs, Outram,                  120,142,155
+  Barrett,             26,136,167,173,178,193
+  Baur, E.,                             55,99
+  Baur, G.,                               119
+  Beneden, van,                            75
+  Bentham, on species of Veronica,         16
+    Lychnis,                               21
+    Primula,                               22
+  Bernadin,                                42
+  Bishop, L. B.,                      153,157
+  Blaringhem,                             229
+  Bobart,                                   5
+  Boisduval,                              182
+  Boissier,                                19
+  Borradaile,                           74,75
+  Boulenger, E. G.,                       208
+  Boulenger, G. A.,               182,207,209
+  Boyle,                                 5,54
+  Brewster, W.,                       149,150
+  Britton,                                227
+  Brown, T. Graham,                       198
+  Brown-Séquard,                          197 et seq.
+  Bruant, P.,                              51
+  Buffon,                                 234
+  Butler, S.,                         189,190
+  Buysson, R. du,                          24
+
+  Candolle, de,                           245
+  Carpenter, J. H.,                       172
+  Chapman, F. M.,             148,156,157,158
+  Chapman, T. A.,              13,167,182,231
+  Church, A. H.,                           69
+  Cieslar,                                197
+  Clark, Austin,                      142,144
+  Cockayne, E. A.,                         43
+  Cockerell, T. D. A.,                    224
+  Compton, R. H.,                   50,58,227
+  Cope,                                   230
+  Cory,                                   142
+  Correns,                                239
+  Coutagne,                               125 et seq.
+
+  Darwin, on Variation,                   1,2
+    Systematics,                           10
+    Selection,                        134,139
+    Heterostyle plants,               236,237
+  Darwin, F.,                             190
+  Darwin, Sir G.,                          41
+  Davenport,                               46
+  Davis, H. M.,                           102
+  Delcourt,                               130
+  Deschange,                              181
+  Dobell,                                 215
+  Doncaster,                      105,121,136
+  Driesch,                              80,81
+  Duchartre,                               51
+
+  East,                                91,116
+  Edwards, W. H.,                         162
+  Ehrlich,                                215
+
+  Fellmer,                                215
+  Field, W. L. W.,                        161
+  Fischer, E.,                            192
+  Fleck,                              171,174
+  Fletcher, W. H. B.,                  26,181
+  Foster, Sir N.,                          39
+
+  Gallé,                                  123
+  Garrod,                                  83
+  Gates,                            92,95,102
+  Gayner, F.,                             177
+  Godron,                                 249
+  Gold, E.,                               196
+  Goldschmidt,                            116
+  Goodwin, E.,                            137
+  Gortner,                                226
+  Greene, E. L.,                            8
+  Gregory, R. P.,                  92,100,236
+  Grenier,                                249
+  Grover,                                 173
+  Gruber,                                  48
+  Gulick,                             119,133
+
+  Hamling,                                142
+  Hampson, Sir G.,                         26
+  Harris,                                 142
+  Hartlaub,                               182
+  Herbst,                                  42
+  Heribert-Nilsson,                       116
+  Hewett,                                 182
+  Honing,                                 105
+  Hunter, John,                            44
+
+  Jakowatz,                               197
+  Janet,                                   24
+  Jeans,                                   41
+  Jenkinson,                               40
+  Jentink,                                120
+  Johannsen,                              195
+  Jordan,                         185,242,249
+
+  Kammere,                                199 et seq.
+  Keeble,                                 236
+  Klebs,                                  250
+  Krancher,                               182
+  Küchenmeister,                           44
+  Kudicke,                                215
+
+  Lamarck,                                  9
+  Lang, A.,                               128
+  Lawrence, W. N.,                    142,145
+  Leake, H. Martin,                    98,100
+  Leavitt,                                185
+  Lecoq,                                   99
+  Lederer,                                167
+  Leduc,                             64,65,80
+  Leydig,                                 182
+  Linden, M. von,                         192
+  Linnaeus,                             6,7,8
+  Lloyd, R. E.,                           248
+  Locard,                                 130
+  Lock, R. H.,                        242,244
+  Loeb,                        42,45,50,71,77
+  Lotsy,                                   99
+  Lowe, P. R.,                            143
+
+  Macdougal, W. T.,                   102,226
+  Marchant,                                 7
+  Mathew,                                 171
+  Matthioli,                                4
+  Mayer, A. G.,                           133
+  Mendel, Rediscovery of,                   2
+    On Fasciation,                         49
+  Merrifield, 169,                        172
+  Miller, W. D.,                      120,149
+  Morgan,                        71,77,91,198
+  Moggridge,                              125
+
+  Nathusius, S.,                          242
+  Nettleship.,                             44
+  Newman, H. H.,                           42
+  Newsholme,                               48
+  Nilsson-Ehle,                       116,169
+  Norman, A. M.,                      125,156
+
+  Ober,                                   142
+  Oberthür,                       168,170,193
+  Oliver, J.,                              45
+
+  Page, H. E.,                        167,180
+  Patterson, J. T.,                        42
+  Payne, F.,                              278
+  Pellew,                                 236
+  Poll,                                    45
+  Porritt,                                136
+  Poulton,                                141
+  Powers, J. H.,                          230
+  Pringsheim, H.,                         213
+  Przibram,             72,78,178,194,197,199
+  Punnett,                                110
+
+  Ray,                                    4,5
+  Raynor,                                 105
+  Ridgway,                             10,120
+  Roedelius,                              195
+  Rolfe,                                   20
+  Rosen, F.,                              242
+  Rosner,                                  42
+  Rowland-Brown, H.,                  167,180
+
+  Sargent,                                185
+  Saunders, E. R.,                 84,104,112
+  Schima,                                 177
+  Schröder,                           193,194
+  Schübeler,                              195
+  Semon, R.,                              190 et seq.
+  Sharrock,                                 5
+  Shull,                                  100
+  Speyer, A.,                     166,170,181
+  Spillman,                                47
+  Standfuss,                      135,181,191
+  Staples-Browne,                       49,98
+  Staudinger,                         170,179
+  Stockard,                             50,71
+  Sutton,                             236,244
+
+  Tornier,                                 72
+  Tower, W. L.,                       218-226
+  Trechmann,                              133
+  Tugwell,                                181
+  Tutt, J. W. On Definiteness of Species,  13
+    On Plusia interrogationis,             26
+    On Tephrosia,                         119
+    On N. castanea,                       122
+    On Pararge egeria,                    167 et seq.
+
+  Verity, R.,                         171,177
+  Vries, H. de,               101-115,222,239
+
+  Walker, G,                               49
+  Weir, Jenner,                           119
+  Weismann,                           176,188
+  Wendelstadt,                            215
+  Werbitzki,                              215
+  Werner,                                 209
+  Wettstein,                              197
+  Wheeler, G.,                        168,171
+  Wheldale,                                83
+  Wilder,                                  44
+  Wille,                                  197
+  Williams, H.,                       167,172
+  Windle, B. C. A.,                        43
+  Winslow,                                213
+  Wolf, F.,                               213
+  Woodforde,                              123
+  Woltereck,                              215
+
+  Zeijlstra,                              114
+
+
+
+
+
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+
+The Project Gutenberg EBook of Problems of Genetics, by William Bateson
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org/license
+
+
+Title: Problems of Genetics
+
+Author: William Bateson
+
+Release Date: January 4, 2014 [EBook #44582]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK PROBLEMS OF GENETICS ***
+
+
+
+
+Produced by Chris Curnow, Paul Marshall and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+</pre>
+
+<div class="figcenter" >
+  <img src="images/titlepage.jpg" alt="logo" width="500" height="778" />
+</div>
+<hr class="chap" />
+<p  class="space-above" style="font-size: 110%; text-align: center;"><b>YALE UNIVERSITY</b></p>
+<p  class= "space-below" style="font-size: 110%; text-align: center;"><b>
+                MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES</b></p>
+<hr class="r5" />
+<h1>PROBLEMS OF GENETICS</h1>
+
+<div class="bt bl br bb">
+<p style="font-size: 150%; text-align: center;"><b>SILLIMAN MEMORIAL LECTURES</b></p>
+<p style="font-size: 120%; text-align: center;"><b>PUBLISHED BY YALE UNIVERSITY PRESS</b></p>
+<hr class="full" />
+<div class="advert">
+<p><b>ELECTRICITY AND MATTER.</b> <i>By</i> <span class="smcap">Joseph John Thomson,<br />
+&emsp;d.sc., ll.d., ph.d., f.r.s.</span>, <i>Fellow of Trinity College,<br />
+&emsp;Cambridge, Cavendish Professor of Experimental Physics, Cambridge</i>.<br />
+<i>Price $1.25 net; postage 10 cents extra.</i></p>
+
+<p><b>THE INTEGRATIVE ACTION OF THE NERVOUS SYSTEM.</b><br />
+&emsp;<i>By</i> <span class="smcap">Charles S. Sherrington,<br />
+&emsp;<span style="margin-left: 0.5em;">d.sc., m.d., hon. ll.d., tor., f.r.s.</span>,</span><br />
+&emsp;<i>Holt Professor of Physiology in the University of Liverpool</i>.<br />
+<i>Price $3.50 net; postage 25 cents extra.</i></p>
+
+<p><b>RADIOACTIVE TRANSFORMATIONS</b>. <i>By</i> <span class="smcap">Ernest Rutherford,<br />
+&emsp;d.sc., ll.d., f.r.s.</span>, <i>Macdonald Professor of Physics, McGill University</i>.<br />
+<i>Price $3.50 net; postage 22 cents extra.</i></p>
+
+<p><b>EXPERIMENTAL AND THEORETICAL APPLICATIONS OF<br />
+ &emsp;&emsp;&emsp;THERMODYNAMICS TO CHEMISTRY</b>.<br />
+&emsp;<i>By</i> <span class="smcap">Dr. Walther Nernst,</span><br />
+&emsp;<i>Professor and Director of the Institute of Physical Chemistry
+&emsp;in the University of Berlin</i>.<br />
+<i>Price $1.25 net; postage 10 cents extra.</i></p>
+
+<p><b>THE PROBLEMS OF GENETICS</b>. <i>By</i> <span class="smcap">William Bateson, m.a.,<br />
+&emsp;f.r.s.</span>, <i>Director of the John Innes Horticultural Institution,<br />
+&emsp;Merton Park, Surrey, England</i>.<br />
+<i>Price $4.00 net; postage 25 cents extra.</i></p>
+
+<p><b>STELLAR MOTIONS</b>. <span class="smcap">With Special Reference to Motions<br />
+&emsp;Determined by Means of the Spectrograph.</span> <i>By</i> <span class="smcap">William<br />
+&emsp;Wallace Campbell, sc.d., ll.d.</span>,<br />
+&emsp;<i>Director of the Lick Observatory, University of California</i>.<br />
+<i>Price $4.00 net; postage 30 cents extra.</i></p>
+
+<p><b>THEORIES OF SOLUTIONS</b>. <i>By</i> <span class="smcap">Svante August Arrhenius,<br />
+&emsp;ph.d., sc.d., m.d.</span>, <i>Director of the Physico-Chemical Department<br />
+&emsp;of the Nobel Institute, Stockholm, Sweden</i>.<br />
+<i>Price $2.25 net; postage 15 cents extra.</i></p>
+
+<p><b>IRRITABILITY</b>. <span class="smcap">A Physiological Analysis of the General<br />
+&emsp;Effect of Stimuli in Living Substances</span>.<br />
+&emsp;<i>By</i> <span class="smcap">Max Verworn</span>,<br />
+&emsp;<i>Professor at Bonn Physiological Institute</i>.<br />
+<i>Price $3.50 net; postage 20 cents extra.</i></p>
+
+<p><b>THE EVOLUTION OF MODERN MEDICINE</b>.<br />
+&emsp;<i>By</i> <span class="smcap">Sir William Osler, Bart., m.d., ll.d., sc.d.</span>,<br />
+&emsp;<i>Regius Professor of Medicine, Oxford University</i>.<br />
+<i>Price $3.00 net; postage 40 cents extra.</i></p>
+</div></div>
+
+<p style="font-size: 150%; text-align: center;"><b>PROBLEMS OF GENETICS</b></p>
+<p style="font-size: 90%; text-align: center;">BY</p>
+<p style="font-size: 150%; text-align: center;"><b><span class="smcap">William Bateson, m.a., f.r.s.</span></b></p>
+
+<p class="space-above" style="font-size: 90%; text-align: center;">
+DIRECTOR OF THE JOHN INNES HORTICULTURAL INSTITUTION,
+HON. FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE,
+AND FORMERLY PROFESSOR OF BIOLOGY IN THE UNIVERSITY</p>
+
+<p style="font-size: 120%; text-align: center;"><i>WITH ILLUSTRATIONS</i></p>
+
+<div class="figcenter" >
+  <img src="images/i_004.jpg" alt="logo" width="200" height="206" />
+</div>
+
+<p class="space-above" style="font-size: 110%; text-align: center;">
+<span class="smcap">New Haven: Yale University Press</span><br />
+<span class="smcap">London: Humphrey Milford</span><br />
+<span class="smcap">Oxford University Press</span></p>
+
+<p class="space-above" style="font-size: 130%; text-align: center;"><b>MCMXIII</b></p>
+
+<hr class="r5" />
+<p class="center">Copyright, 1913<br />
+By <span class="smcap">Yale University</span></p>
+
+<p class="center space-above space-below">First printed August, 1913, 1000 copies</p>
+
+<hr class="chap" />
+<p style="font-size: 150%; text-align: center;"><b>THE SILLIMAN FOUNDATION</b></p>
+
+<p class="indent">In the year 1883 a legacy of about eighty-five thousand
+dollars was left to the President and Fellows of Yale College
+in the city of New Haven, to be held in trust, as a gift from her
+children, in memory of their beloved and honored mother, Mrs.
+Hepsa Ely Silliman.</p>
+
+<p class="indent">On this foundation Yale College was requested and directed
+to establish an annual course of lectures designed to illustrate
+the presence and providence, the wisdom and goodness of God,
+as manifested in the natural and moral world. These were to be
+designated as the Mrs. Hepsa Ely Silliman Memorial Lectures.
+It was the belief of the testator that any orderly presentation
+of the facts of nature or history contributed to the end of this
+foundation more effectively than any attempt to emphasize the
+elements of doctrine or of creed; and he therefore provided that
+lectures on dogmatic or polemical theology should be excluded
+from the scope of this foundation, and that the subjects should be
+selected rather from the domains of natural science and history,
+giving special prominence to astronomy, chemistry, geology, and anatomy.</p>
+
+<p class="indent">It was further directed that each annual course should
+be made the basis of a volume to form part of a series constituting a
+memorial to Mrs. Silliman. The memorial fund came into the
+possession of the Corporation of Yale University in the year 1901;
+and the present volume constitutes the fifth of the series of
+memorial lectures.</p>
+
+<p><span class="pagenum"><a name="Page_i" id="Page_i">[Pg i]</a></span></p>
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_ii" id="Page_ii">[Pg ii]</a></span></p>
+
+<h2>PREFACE</h2>
+
+<p class="indent">This book gives the substance of a series of lectures delivered
+in Yale University, where I had the privilege of holding the office
+of Silliman Lecturer in 1907.</p>
+
+<p class="indent">The delay in publication was brought about by a variety of causes.</p>
+
+<p class="indent">Inasmuch as the purpose of the lectures is to discuss
+some of the wider problems of biology in the light of knowledge acquired
+by Mendelian methods of analysis, it was essential that a fairly
+full account of the conclusions established by them should first
+be undertaken and I therefore postponed the present work till
+a book on Mendel's Principles had been completed.</p>
+
+<p class="indent">On attempting a more general discussion of the
+bearing of the phenomena on the theory of Evolution, I found myself
+continually hindered by the consciousness that such treatment
+is premature, and by doubt whether it were not better that the
+debate should for the present stand indefinitely adjourned.
+That species have come into existence by an evolutionary
+process no one seriously doubts; but few who are familiar with
+the facts that genetic research has revealed are now inclined to
+speculate as to the manner by which the process has been
+accomplished. Our knowledge of the nature and properties of
+living things is far too meagre to justify any such attempts.
+Suggestions of course can be made: though, however, these
+ideas may have a stimulating value in the lecture room, they
+look weak and thin when set out in print. The work which may
+one day give them a body has yet to be done.</p>
+
+<p class="indent">The development of negations is always an ungrateful
+task apt to be postponed for the positive business of experiment.
+Such work is happily now going forward in most of the centers
+of scientific life. Of many of the subjects here treated we already
+know more than we did in 1907. The delay in production has
+made it possible to incorporate these new contributions.</p>
+
+<p class="indent">The book makes no pretence at being a treatise and the
+<span class="pagenum"><a name="Page_iii" id="Page_iii">[Pg iii]</a></span>
+number of illustrative cases has been kept within a moderate
+compass. A good many of the examples have been chosen from
+American natural history, as being appropriate to a book intended
+primarily for American readers. The facts are largely
+given on the authority of others, and I wish to express my
+gratitude for the abundant assistance received from American
+colleagues, especially from the staffs of the American Museum
+in New York, and of the Boston Museum of Natural History.
+In connexion with the particular subjects personal acknowledgments
+are made.</p>
+
+<p class="indent">Dr. F. M. Chapman was so good as to supervise the preparation
+of the coloured Plate of <i>Colaptes</i>, and to authorize the loan
+of the Plate representing the various forms of <i>Helminthophila</i>,
+which is taken from his <i>North American Warblers</i>.</p>
+
+<p class="indent">I am under obligation to Messrs. Macmillan &amp; Co.,
+for permission to reproduce several figures from <i>Materials for the Study
+of Variation</i>, illustrating subjects which I wished to treat in
+new associations, and to M. Leduc for leave to use Fig. 9.</p>
+
+<p class="indent">In conclusion I thank my friends in Yale for the high
+honour they did me by their invitation to contribute to the series of
+Silliman Lectures, and for much kindness received during a
+delightful sojourn in that genial home of learning.
+<span class="pagenum"><a name="Page_iv" id="Page_iv">[Pg iv]</a></span></p>
+<hr class="chap" />
+<p class="u" style="font-size: 150%; text-align: center;"><b>TABLE OF CONTENTS</b>.</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="TOC" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr"><b>CHAPTER</b></td>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdr"><b>PAGE</b></td>
+    </tr><tr>
+      <td class="tdr"><b>I.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Introductory. The Problem of Species and Variety</span></b>&nbsp;&nbsp;</td>
+      <td class="tdr"><a href="#Page_1">&nbsp;&nbsp;1</a></td>
+    </tr><tr>
+      <td class="tdr"><b>II.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Meristic Phenomena</span></b></td>
+      <td class="tdr"><a href="#Page_31">&nbsp;31</a></td>
+    </tr><tr>
+      <td class="tdr"><b>III.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Segmentation, Organic and Mechanical</span></b></td>
+      <td class="tdr"><a href="#Page_60">&nbsp;60</a></td>
+    </tr><tr>
+      <td class="tdr"><b>IV.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">The Classification of Variation and the Nature</span></b></td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;<b><span class="smcap">of Substantive Variation</span></b></td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Note to Chapter IV</span></b></td>
+      <td class="tdr"><a href="#Page_94">&nbsp;94</a></td>
+    </tr><tr>
+      <td class="tdr"><b>V.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">The Mutation Theory</span></b></td>
+      <td class="tdr"><a href="#Page_97">&nbsp;97</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Note to Chapter V</span></b></td>
+      <td class="tdr"><a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdr"><b>VI.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Variation and Locality</span></b></td>
+      <td class="tdr"><a href="#Page_118">118</a></td>
+    </tr><tr>
+      <td class="tdr"><b>VII.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Local Differentiation</span>&mdash;<i>continued</i>.</b></td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;<b><span class="smcap">Overlapping Forms</span></b></td>
+      <td class="tdr"><a href="#Page_146">146</a></td>
+    </tr><tr>
+      <td class="tdr"><b>VIII.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Locally Differentiated Forms</span>&mdash;<i>continued</i>.</b></td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b>&emsp;&emsp;<span class="smcap">Climatic Varieties</span></b></td>
+      <td class="tdr"><a href="#Page_164">164</a></td>
+    </tr><tr>
+      <td class="tdr"><b>IX.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">The Effects of Changed Conditions</span></b></td>
+      <td class="tdr"><a href="#Page_187">187</a></td>
+   </tr><tr>
+      <td class="tdr"><b>X.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">The Effects of Changed Conditions</span>&mdash;<i>continued</i>.</b></td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b>&emsp;&emsp;<span class="smcap">The Causes of Genetic Variation</span></b></td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+   </tr><tr>
+      <td class="tdr"><b>XI.</b>&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">The Sterility of Hybrids. Concluding Remarks</span></b></td>
+      <td class="tdr"><a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Concluding Remarks</span></b></td>
+      <td class="tdr"><a href="#Page_248">248</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Index of Subjects</span></b></td>
+      <td class="tdr"><a href="#Page_251">251</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><b><span class="smcap">Index of Persons</span></b></td>
+      <td class="tdr"><a href="#Page_252">252</a></td>
+    </tr>
+ </tbody>
+</table>
+
+<hr class="chap" />
+<p class="u" style="font-size: 150%; text-align: center;"><b>LIST OF ILLUSTRATIONS</b>.</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="LOC" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdl"><b><span class="smcap">Figure</span></b></td>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdr"><b><span class="smcap">Page</span></b></td>
+    </tr><tr>
+      <td class="tdr"><b>1.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Tusk of Indian Elephant.</b></td>
+      <td class="tdr"><a href="#Page_37">&nbsp;37</a></td>
+    </tr><tr>
+      <td class="tdr"><b>2.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Jaws of Skates.</b></td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdr"><b>3.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Syndactyly of Human Hand and Foot.</b>&nbsp;&nbsp;</td>
+      <td class="tdr"><a href="#Page_47">&nbsp;47</a></td>
+    </tr><tr>
+      <td class="tdr"><b>4.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Syndactyly of the Human Foot.</b></td>
+      <td class="tdr"><a href="#Page_48">&nbsp;48</a></td>
+    </tr><tr>
+      <td class="tdr"><b>5.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Petiole of</b> <i>Begonia phyllomaniaca</i>.</td>
+      <td class="tdr"><a href="#Page_51">&nbsp;51</a></td>
+    </tr><tr>
+      <td class="tdr"><b>6.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Feet of Polydactyle Cats.</b></td>
+      <td class="tdr"><a href="#Page_52">&nbsp;52</a></td>
+    </tr><tr>
+      <td class="tdr"><b>7.</b> &amp; <b>8.</b></td>
+      <td class="tdl">&nbsp;<b>Vertebrae of Python.</b></td>
+      <td class="tdr"><a href="#Page_61">&nbsp;61</a></td>
+    </tr><tr>
+      <td class="tdr"><b>9.&nbsp;&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Osmotic Growths.</b></td>
+      <td class="tdr"><a href="#Page_64">&nbsp;64</a></td>
+    </tr><tr>
+      <td class="tdr"><b>10.&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Leaf type in</b> <i>Primula sinensis</i>.</td>
+      <td class="tdr"><a href="#Page_70">&nbsp;70</a></td>
+    </tr><tr>
+      <td class="tdr"><b>11.&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Geometrical relations in</b> <i>Arthropoda</i>.</td>
+      <td class="tdr"><a href="#Page_73">&nbsp;73</a></td>
+    </tr><tr>
+      <td class="tdr"><b>12.&nbsp;&nbsp;&nbsp;</b></td>
+      <td class="tdl">&nbsp;<b>Right claw of Lobster.</b></td>
+      <td class="tdr"><a href="#Page_76">&nbsp;76</a></td>
+    </tr><tr>
+      <td class="tdr"><b>13.</b>&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdl">&nbsp;<b>Forms of</b> <i>Aceras hircina</i>.</td>
+      <td class="tdr"><a href="#Page_124">124</a></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&nbsp;<b>Variation in Warblers.</b></td>
+      <td class="tdr"><a href="#Page_159">159</a></td>
+    </tr>
+ </tbody>
+</table>
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
+
+<p style="font-size: 150%; text-align: center;"><b>PROBLEMS OF GENETICS</b></p>
+<hr class="r5" />
+<h2>CHAPTER I</h2>
+<h3><span class="smcap">Introductory</span></h3>
+
+<p class="indent">The purpose of these lectures is to discuss some of
+the familiar phenomena of biology in the light of modern discoveries.
+In the last decade of the nineteenth century many of us perceived
+that if any serious advance was to be made with the group
+of problems generally spoken of as the Theory of Evolution,
+methods of investigation must be devised and applied of a kind
+more direct and more penetrating than those which after the
+general acceptance of the Darwinian views had been deemed
+adequate. Such methods obviously were to be found in a
+critical and exhaustive study of the facts of variation and heredity,
+upon which all conceptions of evolution are based. To construct
+a true synthetic theory of Evolution it was necessary that variation
+and heredity instead of being merely postulated as axioms
+should be minutely examined as phenomena. Such a study
+Darwin himself had indeed tentatively begun, but work of a
+more thorough and comprehensive quality was required. In
+the conventional view which the orthodoxy of the day prescribed,
+the terms variation and heredity stood for processes so vague
+and indefinite that no analytical investigation of them could be
+contemplated. So soon, however, as systematic inquiry into
+the natural facts was begun it was at once found that the accepted
+ideas of variation were unfounded. Variation was seen
+very frequently to be a definite and specific phenomenon, affecting
+different forms of life in different ways, but in all its
+diversity showing manifold and often obvious indications of
+regularity. This observation was not in its essence novel.
+Several examples of definite variation had been well known to
+<span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span>
+Darwin and others, but many, especially Darwin himself in his
+later years, had nevertheless been disposed to depreciate the
+significance of such facts. They consequently then lapsed into
+general disparagement. Upon more careful inquiry the abundance
+of such phenomena proved to be far greater than was
+currently supposed, and a discussion of their nature brought
+into prominence a consideration of greater weight, namely
+that the differences by which these definite or discontinuous
+variations are constituted again and again approximate to and
+are comparable with the class of differences by which species
+are distinguished from each other.</p>
+
+<p class="indent">The interest of such observations could no longer be
+denied. The more they were examined the more apparent it became that
+by means of the facts of variation a new light was obtained on
+the physiological composition and capabilities of living things.
+Genetics thus cease to be merely a method of investigating
+theories of evolution or of the origin of species but provide a
+novel and hitherto untried instrument by which the nature of
+the living organism may be explored. Just as in the study of
+non-living matter science began by regarding the external
+properties of weight, opacity, colour, hardness, mode of occurrence,
+etc., noting only such evidences of chemical attributes
+and powers as chance spontaneously revealed; and much later
+proceeded to the discovery that these casual manifestations of
+chemical properties, rightly interpreted, afford a key to the
+intrinsic nature of the diversity of matter, so in biology, having
+examined those features of living things which ordinary observations
+can perceive, we come at last to realize that when studied
+for their own sake the properties of living organisms in respect
+of heredity and variation are indications of their inner nature
+and provide evidences of that nature which can be obtained from
+no other source.</p>
+
+<p class="indent">While such ideas were gradually forming in our minds,
+came the rediscovery of Mendel's work. Investigations which before
+had only been imagined as desirable now became easy to pursue,
+and questions as to the genetic inter-relations and compositions
+of varieties can now be definitely answered. Without prejudice
+<span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span>
+to what the future may disclose whether by way of limitation
+or extension of Mendelian method, it can be declared with
+confidence and certainty that we have now the means of beginning
+an analysis of living organisms, and distinguishing many
+of the units or factors which essentially determine and cause the
+development of their several attributes.</p>
+
+<p class="indent">Briefly put, the essence of Mendelism lies in the
+discovery of the existence of unit characters or factors. For an
+account of the Mendelian method, how it is applied and what it has
+already accomplished, reference must be made to other works.<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a>
+With this part of the subject I shall assume a sufficient acquaintance.
+In these lectures I have rather set myself the task
+of considering how certain problems appear when viewed from
+the standpoint to which the application of these methods has
+led us. It is indeed somewhat premature to discuss such questions.
+The work of Mendelian analysis is progressing with
+great rapidity and anything I can say may very soon be superseded
+as out of date. Nevertheless a discussion of this kind
+may be of at least temporary service in directing inquiry to the
+points of special interest.</p>
+
+<h3><span class="smcap">The Problem of Species and Variety</span></h3>
+
+<p class="indent">Nowhere does our new knowledge of heredity and variation
+apply more directly than to the problem what is a species and
+what is a variety? I cannot assert that we are already in a
+position to answer this important question, but as will presently
+appear, our mode of attack and the answers we expect to receive
+are not those that were contemplated by our predecessors.
+If we glance at the history of the scientific conception of Species
+we find many signs that it was not till comparatively recent
+times that the definiteness of species became a strict canon of
+the scientific faith and that attempts were made to give precise
+limits to that conception. When the diversity of living things
+began to be accurately studied in the sixteenth and seventeenth
+<span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span>
+centuries names were applied in the loosest fashion, and in giving
+a name to an animal or a plant the naturalists of those times had
+no ulterior intention. Names were bestowed on those creatures
+about which the writer proposed to speak. When Gesner or Aldrovandi
+refer to all the kinds of horses, unicorns, dogs, mermaids,
+etc., which they had seen or read of, giving to each a descriptive
+name, they do not mean to "elevate" each named kind to "specific
+rank"; and if anyone had asked them what they meant by
+a species, it is practically certain that they would have had not
+the slightest idea what the question might imply, or any suspicion
+that it raised a fundamental problem of nature.</p>
+
+<p class="indent">Spontaneous generation being a matter of daily observation,
+then unquestioned, and supernatural events of all kinds being
+commonly reported by many witnesses, transmutation of species
+had no inherent improbability. Matthioli,<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a> for instance, did not
+expect to be charged with heresy when he declared <i>Stirpium
+mutatio</i> to be of ordinary occurrence. After giving instances
+of induced modifications he wrote, "Tantum enim in plantis
+naturae germanitas potest, ut non solum saepe praedictos
+praestet effectus, sed etiam ut alteram in alteram stirpem facile
+vertat, ut cassiam in cinnamomum, sisymbrium in mentham,
+triticum in lolium, hordeum in avenam, et ocymum in serpyllum."</p>
+
+<p class="indent">I do not know who first emphasized the need for a
+clear understanding of the sense in which the term species is to be
+applied. In the second half of the seventeenth century Ray
+shows some degree of concern on this matter. In the introduction
+to the <i>Historia Plantarum</i>, 1686, he discusses some
+of the difficulties and lays down the principle that varieties
+which can be produced from the seed of the same plant are to
+be regarded as belonging to one species, being, I believe, the
+first to suggest this definition. That new species can come into
+existence he denies as inconsistent with Genesis 2, in which it is
+declared that God finished the work of Creation in six days.
+Nevertheless he does not wholly discredit the possibility of a
+"transmutation" of species, such that one species may as an
+exceptional occurrence give rise by seed to another and nearly
+<span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span>
+allied species. Of such a phenomenon he gives illustrations the
+authenticity of which he says he is, against his will, compelled to
+admit. He adds that some might doubt whether in the cases
+quoted the two forms concerned are really distinct species, but
+the passage is none the less of value for it shews that the conception
+of species as being distinct unchangeable entities was not
+to Ray the dogma sacrosanct and unquestionable which it
+afterwards became.<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a>
+</p>
+
+<p class="indent">In the beginning of the eighteenth century Marchant,<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a>
+having observed the sudden appearance of a lacinated variety of
+<i>Mercurialis</i>, makes the suggestion that species in general may have
+arisen by similar mutations. Indeed from various passages it is
+manifest that to the authors of the seventeenth and early eighteenth
+centuries species appeared simply as groups more or less
+definite, the boundaries of which it was unnecessary to determine
+with great exactitude. Such views were in accord with the
+general scientific conception of the time. The mutability of
+<span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span>
+species is for example sometimes likened (see for instance Sharrock,
+loc. cit.) to the metamorphoses of insects, and it is to be
+remembered that the search for the Philosopher's Stone by which
+the transmutation of metals was to be effected had only recently
+fallen into discredit as a pursuit.</p>
+
+<p class="indent">The notion indeed of a peculiar, fixed meaning to be
+attached to species as distinct from variety is I think but rarely to be
+found categorically expressed in prae-Linnaean writings.</p>
+
+<p class="indent">But with the appearance of the <i>Systema Naturae</i>
+a great change supervened. Linnaeus was before all a man of order.
+Foreseeing the immense practical gain to science that must come
+from a codification of nomenclature, he invented such a system.</p>
+
+<p class="indent">It is not in question that Linnaeus did great things for
+us and made Natural History a manageable and accessible collection of
+facts instead of a disorderly heap; but orderliness of mind has
+another side, and inventors and interpreters of systems soon attribute
+to them a force and a precision which in fact they have not.</p>
+
+<p class="indent">The systematist is primarily a giver of names, as
+Ray with his broader views perceived. Linnaeus too in the exordium to
+the <i>Systema Naturae</i> naively remarks, that he is setting out to
+continue the work which Adam began in the Golden Age, to give
+names to the living creatures. Naming however involves very
+delicate processes of mind and of logic. Carried out by the light
+of meagre and imperfect knowledge it entails all the mischievous
+consequences of premature definition, and promotes facile
+illusions of finality. So was it with the Linnaean system. An
+interesting piece of biological history might be written respecting
+the growth and gradual hardening of the conception of Species.
+To readers of Linnaeus's own writings it is well known that his
+views cannot be summarized in a few words. Expressed as they
+were at various times during a long life and in various connexions,
+they present those divers inconsistencies which commonly
+reflect a mind retaining the power of development. Nothing
+certainly could be clearer than the often quoted declaration of the
+<i>Philosophia Botanica</i>, "Species tot numeramus quot diversae
+formae in principio sunt creatae," with the associated passage
+"Varietates sunt plantae ejusdem speciei mutatae a caussa
+<span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span>
+quacunque occasionali." Those sayings however do not stand
+alone. In several places, notably in the famous dissertation
+on the peloric <i>Linaria</i> he explicitly contemplates the possibility
+that new species may arise by crossing, declaring nevertheless
+that he thinks such an event to be improbable. In that essay
+he refers to Marchant's observation on a laciniate <i>Mercurialis</i>,
+but though he states clearly that that plant should only be
+regarded as a variety of the normal, he does not express any
+opinion that the contemporary genesis of new species must be an
+impossibility. In the later dissertation on Hybrid Plants he
+returns to the same topic. Again though he states the belief
+that species cannot be generated by cross-breedings, he treats
+the subject not as heretical absurdity but as one deserving
+respectful consideration.</p>
+
+<p class="indent">The significance of the aphorisms that precede the
+lectures on the Natural Orders is not easy to apprehend. These are
+expressed with the utmost formality, and we cannot doubt that
+in them we have Linnaeus's own words, though for the record
+we are dependent on the transcripts of his pupils.</p>
+
+<p class="blockquot">The text of the first five is as follows:</p>
+
+<p class="blockquot">1. Creator T. O. in primordio vestiit Vegetabile <i>Medullare</i>
+principiis constitutivis diversi <i>Corticalis</i> unde tot difformia
+individua, quot <i>Ordines</i> Naturales prognata.</p>
+
+<p class="blockquot">2. <i>Classicas</i> has (1) plantas Omnipotens miscuit inter se,
+unde tot <i>Genera</i> ordinum, quot inde plantae.</p>
+
+<p class="blockquot">3. <i>Genericas</i> has (2) miscuit Natura, unde tot <i>Species</i> congeneres
+quot hodie existunt.</p>
+
+<p class="blockquot">4. <i>Species</i> has miscuit Casus, unde totidem quot passim
+occurrunt, <i>Varietates</i>.</p>
+
+<p class="blockquot">5. Suadent haec (1-4) Creatoris leges a simplicibus ad
+Composita.</p>
+
+<p class="blockquot">Naturae leges generationis in hybridis.</p>
+
+<p class="blockquot">Hominis leges ex observatis a posteriori.</p>
+
+<p class="indent">I am not clear as to the parts assigned in the first sentence
+respectively to the "<i>Medulla</i>" and the "<i>Cortex</i>," beyond that
+Linnaeus conceived that multiformity was first brought about
+by diversity in the "<i>Cortex</i>." The passage is rendered still
+<span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span>
+more obscure if read in connection with the essay on "<i>Generatio
+Ambigena</i>," where he expresses the conviction that the <i>Medulla</i>
+is contributed by the mother, and the <i>Cortex</i> by the father, both
+in plants and animals.<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a></p>
+
+<p class="indent">But however that may be, he regards this original diversity
+as resulting in the constitution of the Natural Orders, each represented
+by one individual.</p>
+
+<p class="indent">In the second aphorism the Omnipotent is represented as
+creating the genera by intermixing the individual <i>plantae classicae</i>,
+or prototypes of the Natural Orders.</p>
+
+<p class="indent">The third statement is the most remarkable, for in
+it he declares that Species were formed by the act of Nature, who by
+inter-mixing the genera produced <i>Species congeneres</i>, namely
+species inside each genus, to the number which now exist.
+Lastly, Chance or Accident, intermixing the species, produced
+as many varieties as there are about us.</p>
+
+<p class="indent">Linnaeus thus evidently regarded the intermixing
+of an originally limited number of types as the sufficient cause of
+all subsequent diversity, and it is clear that he draws an antithesis
+between <i>Creator</i>, <i>Natura</i>, and <i>Casus</i>, assigning to each
+a special part in the operations. The acts resulting in the
+formation of genera are obviously regarded as completed within
+the days of the Creation, but the words do not definitely show
+that the parts played by Nature and Chance were so limited.</p>
+
+<p class="indent">Recently also E. L. Greene<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a>
+has called attention to some curious utterances buried in the <i>Species Plantarum</i>,
+in which Linnaeus refers to intermediate and transitional species, using
+language that even suggests evolutionary proclivities of a
+modern kind, and it is not easy to interpret them otherwise.</p>
+
+<p class="indent">Whatever Linnaeus himself believed to be the truth, the
+effect of his writings was to induce a conviction that the species
+<span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span>
+of animals and plants were immutably fixed. Linnaeus had
+reduced the whole mass of names to order and the old fantastical
+transformations with the growth of knowledge had lapsed into
+discredit; the fixity of species was taken for granted, but not
+till the overt proclamation of evolutionary doctrine by Lamarck
+do we find the strenuous and passionate assertions of immutability
+characteristic of the first half of the nineteenth century.</p>
+
+<p class="indent">It is not to be supposed that the champions of fixity
+were unacquainted with varietal differences and with the problem
+thus created, but in their view these difficulties were apparent
+merely, and by sufficiently careful observation they supposed
+that the critical and permanent distinctions of the true species
+could be discovered, and the impermanent variations detected
+and set aside.</p>
+
+<p class="indent">This at all events was the opinion formed by the great
+body of naturalists at the end of the eighteenth and beginning of the
+nineteenth centuries, and to all intents and purposes in spite
+of the growth of evolutionary ideas, it remains the guiding
+principle of systematists to the present day. There are 'good
+species' and 'bad species' and the systematists of Europe and
+America spend most of their time in making and debating them.</p>
+
+<p class="indent">In some of its aspects the problem of course confronted
+earlier naturalists. Parkinson for instance (1640) in introducing his
+treatment of <i>Hieracium</i> wrote, "To set forth the whole family of
+the Hawkeweedes in due forme and order is such a world of
+worke that I am in much doubt of mine own abilitie, it having
+lyen heavie on his shoudiers that hath already waded through
+them ... for such a multitude of varieties in forme pertaining
+to one herbe is not to be found againe in <i>rerum natura</i> as I
+thinke," and the same idea, that the difficulty lay rather in
+man's imperfect powers of discrimination than in the nature of
+the materials to be discriminated, is reflected in many treatises
+early and late.</p>
+
+<p class="indent">It was however with the great ouburst of scientific
+activity which followed Linnaeus that the difficulty became acute.
+Simultaneously vast masses of new material were being collected
+from all parts of the world into the museums, and the products
+<span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span>
+of the older countries were re-examined with a fresh zeal and on
+a scale of quantity previously unattempted. But the problem
+how to name the forms and where to draw lines, how much
+should be included under one name and where a new name was
+required, all this was felt, rather as a cataloguer's difficulty
+than as a physiological problem. And so we still hear on the
+one hand of the confusion caused by excessive "splitting" and
+subdivisions, and on the other of the uncritical "lumpers" who
+associate together under one name forms which another collector
+or observer would like to see distinguished.</p>
+
+<p class="indent">In spite of Darwin's hopes, the acceptance of his views
+has led to no real improvement&mdash;scarcely indeed to any change at
+all in either the practice or aims of systematists. In a famous
+passage in the <i>Origin</i> he confidently declares that when his
+interpretation is generally adopted "Systematists will be able
+to pursue their labours as at present; but they will not be incessantly
+haunted by the shadowy doubt whether this or that
+form be a true species. This, I feel sure, and I speak after
+experience, will be no slight relief. The endless disputes whether
+or not some fifty species of British brambles are good species
+will cease." Those disputes nevertheless proceed almost exactly
+as before. It is true that biologists in general do not,
+as formerly, participate in these discussions because they have
+abandoned systematics altogether; but those who are engaged
+in the actual work of naming and cataloguing animals and
+plants usually debate the old questions in the old way. There
+is still the same divergence of opinion and of practice, some inclining
+to make much of small differences, others to neglect them.</p>
+
+<p class="indent">Not only does the work of the systematists as a whole proceed
+as if Darwin had never written but their attitude towards these
+problems is but little changed. In support of this statement I may
+refer to several British Museum Catalogues, much of the <i>Biologia
+Centrali-Americana</i>, Ridgway's <i>Birds of North America</i>, the
+<i>Fauna Hawaiensis</i>, indeed to almost any of the most important
+systematic publications of England, America, or any other
+country. These works are compiled by the most proficient
+<span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span>
+systematists of all countries in the several groups, but with
+rare exceptions they show little misgiving as to the fundamental
+reality of specific differences. That the systematists consider
+the species-unit as of primary importance is shown by the
+fact that the whole business of collection and distribution of
+specimens is arranged with regard to it.</p>
+
+<p class="indent">Almost always the collections are arranged in such a
+way that the phenomena of variation are masked. Forms intermediate
+between two species are, if possible, sorted into separate boxes
+under a third specific name. If a species is liable to be constantly
+associated with a mutational form, the mutants are picked out,
+regardless of the circumstances of their origin, from the samples
+among which they were captured, and put apart under a special
+name. Only by a minute study of the original labels of the
+specimens and by redistributing them according to locality and
+dates, can their natural relations be traced. The published
+accounts of these collections often take no notice of variations,
+others make them the subject of casual reference. Very few
+indeed treat them as of much importance. From such indications
+it is surely evident that the systematists attach to the
+conception of species a significance altogether different from that
+which Darwin contemplated.</p>
+
+<p class="indent">I am well aware that some very eminent systematists
+regard the whole problem as solved. They hold as Darwin did that
+specific diversity has no physiological foundation or causation
+apart from fitness, and that species are impermanent groups,
+the delimitations of which are ultimately determined by environmental
+exigency or "fitness." The specific diversity of
+living things is thus regarded as being something quite different
+in nature from the specific diversity of inorganic substances.
+In practice those who share these opinions are, as might be anticipated,
+to be found among the 'lumpers' rather than among
+the 'splitters.' In their work, certainly, the Darwinian theory
+is actually followed as a guiding principle; unanalysed inter-gradations
+of all kinds are accepted as impugning the integrity
+of species; the underlying physiological problem is forgotten,
+and while the product is almost valueless as a contribution to
+<span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span>
+biological research, I can scarcely suppose that it aids greatly
+in the advances of other branches of our science.</p>
+
+<p class="indent">But why is it that, with these exceptions, the consequences
+of the admittedly general acceptance of a theory of evolution
+are so little reflected in the systematic treatment of living things?
+Surely the reason is that though the systematist may be convinced
+of the general truth of the evolution theory at large, he
+is still of opinion that species are really distinct things. For
+him there are still 'good' species and 'bad' species and his experience
+tells him that the distinction between the two is not
+simply a question of degree or a matter of opinion.</p>
+
+<p class="indent">To some it may seem that this is mere perversity, a refusal
+to see obvious truth, a manifestation of the spirit of the collector
+rather than of the naturalist. But while recognising that from
+a magnification of the conception of species the systematists
+are occasionally led into absurdity I do not think the grounds
+for their belief have in recent times been examined with the
+consideration they deserve. The phenomenon of specific
+diversity is manifested to a similar degree by living things belonging
+to all the great groups, from the highest to the lowest,
+Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae,
+and Bacteria, all present diversities of such a kind that among
+them the existence of specific differences can on the whole be
+recognised with a similar degree of success and with very similar
+limitations. In all these groups there are many species quite
+definite and unmistakable, and others practically indefinite.
+The universal presence of specificity, as we may call it, similarly
+limited and characterised, is one of its most remarkable
+features. Not only is this specificity thus universally present
+among the different forms of life, but it manifests itself in respect
+of the most diverse characteristics which living things display.
+Species may thus be distinguished by peculiarities of form, of
+number, of geometrical arrangement, of chemical constitution
+and properties, of sexual differentiation, of development, and of
+many other properties. In any one or in several of these features
+together, species may be found distinguished from other species.
+It is also to be observed that the definiteness of these distinctions
+<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span>
+has no essential dependence on the nature of the characteristic
+which manifests them. It is for example sometimes said that
+colour-distinctions are of small systematic importance, but every
+systematist is familiar with examples (like that of the wild species
+of <i>Gallus</i>) in which colours though complex, show very little
+variation. On the other hand features of structure, sexual
+differentiation, and other attributes which by our standards are
+estimated as essential, may be declared to show much variation
+or little, not according to any principle which can be detected,
+but simply as the attention happens to be applied to one species
+or group of species, or to another. In many groups of animals and
+plants observers have hit upon characters which were for a time
+thought to be finally diagnostic of species. The Lepidoptera and
+Diptera for instance, have been re-classified according to their
+neuration. Through a considerable range of forms determinations
+may be easily made on these characters, but as is now well
+known, neuration is no more immune from variation than any
+other feature of organisation, and in some species great variability
+is the rule. Again it was once believed by some that the
+genitalia of the Lepidoptera provided a basis of final determination&mdash;with
+a similar sequel. In some groups, for example the Lycaenidae,
+or the Hesperidae, there are forms almost or quite indistinguishable
+on external examination, but a glance at the
+genitalia suffices to distinguish numerous species, while on the
+contrary among Pieridae a great range of species show scarcely
+any difference in these respects: and again in occasional species
+the genitalia show very considerable variations.</p>
+
+<p class="indent">The proposition that animals and plants are on the whole
+divisible into definite and recognisable species is an approximation
+to the truth. Such a statement is readily defensible,
+whereas to assert the contrary would be palpably absurd. For
+example, a very competent authority lately wrote: "In the
+whole Lepidopterous fauna of England there is no species of
+really uncertain limits."<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a>
+Others may be disposed to make
+certain reservations, but such exceptions would be so few as
+scarcely to impair the validity of the general statement. The
+<span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span>
+declaration might be extended to other orders and other lands.</p>
+
+<p class="indent">We know, of course, that the phenomenon of specific diversity
+is complicated by local differentiation: that, in general, forms
+which cannot disperse themselves freely exhibit a multitude of
+local races, and that of these some are obviously adaptative,
+and that a few even owe their peculiarity to direct environmental
+effects. Every systematist also is perfectly aware that in dealing
+with collections from little explored countries the occurrence
+of polymorphism or even of sporadic variation may make the
+practical business of distinguishing the species difficult and
+perhaps for the time impossible; still, conceding that a great
+part of the diversity is due to geographical differentiation, and
+that some is sporadic variation, our experience of our own floras
+and faunas encourages the belief that if we were thoroughly
+familiar with these exotic productions it would usually be
+possible to assign their specific limitations with an approach
+to certainty.</p>
+
+<p class="indent">For apart from any question of the justice of these
+wider inferences, if we examine the phenomenon of specificity as it
+appears in those examples which are nearest to hand, surely we
+find signs in plenty that specific distinction is no mere consequence
+of Natural Selection. The strength of this proposition has
+lain mainly in the appeal to ignorance. Steadily with the growth
+of knowledge has its cogency diminished, and such a belief
+could only have been formulated at a time when the facts of
+variation were unknown.</p>
+
+<p class="indent">In Darwin's time no serious attempt had been made
+to examine the manifestations of variability. A vast assemblage of
+miscellaneous facts could formerly be adduced as seemingly
+comparable illustrations of the phenomenon "Variation."
+Time has shown this mass of evidence to be capable of analysis.
+When first promulgated it produced the impression that variability
+was a phenomenon generally distributed amongst living
+things in such a way that the specific divisions must be arbitrary.
+When this variability is sorted out, and is seen to be in part a
+result of hybridisation, in part a consequence of the persistence
+<span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span>
+of hybrids by parthenogenetic reproduction, a polymorphism
+due to the continued presence of individuals representing various
+combinations of Mendelian allelomorphs, partly also the transient
+effect of alteration in external circumstances, we see how
+cautious we must be in drawing inferences as to the indefiniteness
+of specific limits from a bare knowledge that intermediates exist.
+Conversely, from the accident of collocation or from a misleading
+resemblance in features we deem essential, forms genetically
+distinct are often confounded together, and thus the divergence
+of such forms in their other features, which we declare to be
+non-essential, passes as an example of variation. Lastly, and
+this is perhaps the most fertile of all the sources of confusion,
+the impression of the indefiniteness of species is created by the
+existence of numerous local forms, isolated geographically from
+each other, forms whose differences may be referable to any one
+of the categories I have enumerated.</p>
+
+<p class="indent">The advance has been from many sides. Something has
+come from the work of systematists, something from cultural
+experiments, something from the direct study of variation as it
+appears in nature, but progress is especially due to experimental
+investigation of heredity. From all these lines of inquiry we
+get the same answer; that what the naturalists of fifty years
+ago regarded as variation is not one phenomenon but many,
+and that what they would have adduced as evidence against
+the definiteness of species may not in fact be capable of this
+construction at all.</p>
+
+<p class="indent">If we may once more introduce a physical analogy,
+the distinctions with which the systematic naturalist is concerned in
+the study of living things are as multifarious as those by which
+chemists were confronted in the early days of their science.
+Diversities due to mechanical mixtures, to allotropy, to differences
+of temperature and pressure, or to degree of hydration, had all
+to be severally distinguished before the essential diversity due
+to variety of chemical constitution stood out clearly, and I
+surmise that not till a stricter analysis of the diversities of animals
+and plants has been made on a comprehensive scale, shall we
+be in a position to declare with any confidence whether there is
+<span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span>
+or is not a natural and physiological distinction between species
+and variety.</p>
+
+<p class="indent">As I have said above, it is in the cases nearest to
+hand that the problem may be most effectively studied. Comparison
+between forms from dissimilar situations contributes something;
+but it is by a close examination of the behaviour, especially the
+genetic behaviour, of familiar species when living in the presence
+of their nearest allies that the most direct light on the problem
+is to be obtained. I cannot understand the attitude of those who,
+contemplating such facts as this examination elicits, can complacently
+declare that specific difference is a mere question of
+degree. With the spread of evolutionary ideas to speak much
+of the fixity of species has become unfashionable, and yet how
+striking and inscrutable are the manifestations of that fixity!</p>
+
+<p class="indent">Consider the group of species composing the <i>agrestis</i>
+section of the genus <i>Veronica</i>, namely <i>Tournefortii</i>,
+<i>agrestis</i>, and <i>polita</i>.</p>
+
+<p class="indent">These three grow side by side in my garden, as they
+do in suitable situations over a vast area of the temperate regions.
+I have for years noticed them with some care and become familiar
+with their distinctions and resemblances. Never is there any
+real doubt as to the identity of any plant. The species show
+some variability, but I have never seen one which assumed any
+of the distinguishing features of the others. A glance at the
+fruits decides at once to which species a plant belongs. I find
+it impossible to believe that the fixity of these distinctions is
+directly dependent on their value as aids in the struggle for
+existence. The mode of existence of the three forms in so far
+as we can tell is closely similar. By whatever standard we reckon
+systematic affinity I suppose we shall agree that these species
+come very near indeed to each other. Bentham even takes
+the view that <i>polita</i> is a mere variety of <i>agrestis</i>.</p>
+
+<p class="indent">Now in such cases as this it has been argued that the
+specific features of the several types have been separately developed
+in as many distinct localities, and that their present association
+is due to subsequent redistribution. Of these Veronicas indeed
+we know that one, <i>Tournefortii</i> (= <i>Buxbaumii</i>) is as a matter of fact
+<span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span>
+a recent introduction from the east.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a>
+But this course of argument leads to still further difficulties.
+For if it is true that the peculiarities
+of the several species have been perfected and preserved on
+account of their survival-value to their possessors, it follows
+that there must be many ways of attaining the same result.
+But since sufficient adaptation may be ensured in so many ways,
+the disappearance of the common parent of these forms is difficult
+to understand. Obviously it must have been a plant
+very similar in general construction to its modern representatives.
+Like them it must have been an annual weed, with an organisation
+conformable to that mode of life. Why then, after having been
+duly perfected for that existence should it have been entirely
+superseded in favour of a number of other distinct contrivances
+for doing the same thing, and&mdash;if a gradual transition be predicated&mdash;not
+only by them, but by each intermediate stage
+between them and the original progenitor? Surely the obvious
+inference from such facts is that the burden cast upon the theory
+of gradual selection is far greater than it can bear; that adaptation
+is not in practice a very close fit, and that the distinctions
+between these several species of Veronica have not arisen on
+account of their survival-value but rather because none of their
+diversities was so damaging as to lead to the extermination of
+its possessor. When we see these various Veronicas each rigidly
+reproducing its parental type, all comfortably surviving in
+competition with each other, are we not forced to the conclusion
+that <i>tolerance</i> has as much to do with the diversity of species
+as the stringency of Selection? Certainly these species owe their
+continued existence to the fact that they are each good enough
+to live, but how shall we refer the distinctions between them directly
+or indirectly to the determination of Natural Selection?
+<span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span></p>
+
+<p class="indent">The control of Selection is loose while the conformity to
+specific distinction is often very strict and precise, and no less
+so even when several closely related species co-exist in the same
+area and in the same circumstances.</p>
+
+<p class="indent">The theory of Selection fails at exactly the point
+where it was devised to help: <i>Specific</i> distinction.</p>
+
+<p class="indent">Let us examine a somewhat different set of facts in the
+case of another pair of nearly allied species <i>Lychnis diurna</i> and
+<i>vespertina</i>. The two plants have much in common. Both are
+dioecious perennials, with somewhat similar flowers, the one
+crimson, the other white. Each however has its peculiarities
+which are discernible in almost any part of its structure, whether
+flower, leaf, fruit or seed, distinctions which would enable a
+person thoroughly familiar with the plants to determine at once
+from which species even a small piece had been taken. There
+is so much resemblance however as readily to support the surmise
+that the two were mere varieties of one species. Bentham,
+following Linnaeus, in fact actually makes this suggestion,
+with what propriety we will afterwards consider. Now this case
+is typical of many. The two forms have a wide distribution,
+occurring sometimes separately, sometimes in juxtaposition.
+<i>L. diurna</i> is a plant of hedgerows and sheltered situations. <i>L.
+vespertina</i> is common in fields and open spaces, where <i>diurna</i>
+is hardly ever found; but not rarely <i>vespertina</i> occurs in association
+with <i>diurna</i> in the places which that plant frequents. In
+this case I do not doubt that we have to do with organisms of
+somewhat different aptitudes. That <i>L. vespertina</i> has powers
+which <i>diurna</i> has not is shown very clearly by the fact that
+<i>diurna</i> is sometimes entirely absent from areas where <i>vespertina</i>
+can abound.<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a>
+But in order to understand the true genetic
+relations of the two plants to each other it is necessary to observe
+their behaviour when they meet as they not unfrequently do.
+<span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span>
+If the <i>Lychnis</i> population of such a locality be examined it will
+be found to consist of many undoubted and unmodified <i>diurna</i>,
+a number&mdash;sometimes few, sometimes many&mdash;of similarly
+unmodified <i>vespertina</i>, and an uncertain but usually rather small
+proportion of plants obviously hybrids between the two. How
+is it possible to reconcile these facts with the view that specific
+distinction has no natural basis apart from environmental exigency?</p>
+
+<p class="indent">Darwinian orthodoxy suggests that by a gradual process
+of Natural Selection either one of these two types was evolved from
+the other, or both from a third type. I cannot imagine that
+anyone familiar with the facts would propose the first hypothesis
+in the case of <i>Lychnis</i>, nor can I conceive of any process, whether
+gradual or sudden, by which <i>diurna</i> could have come out of
+<i>vespertina</i>, or <i>vespertina</i> out of <i>diurna</i>. Both however may no
+doubt have been derived from some original third type. It is
+conceivable that <i>Lychnis macrocarpa</i> of Boissier, a native of
+Southern Spain and Morocco, may be this original form. This
+species is said to combine a white flower (like that of <i>L. vespertina</i>),
+with capsule-teeth rolled back (like those of <i>diurna</i>).<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a>
+But whatever the common progenitor may have been, if we are
+to believe that these two species have been evolved from it by
+a gradual process of Natural Selection based on adaptation,
+enormous assumptions must be made regarding the special fitness
+of these two forms and the special unfitness of the common
+parent, and these assumptions must be specially invoked and
+repeated for each several feature of structure or habits distinguishing
+the three forms.</p>
+
+<p class="indent">Why, if the common parent was strong enough to live to
+give rise to these two species, is it either altogether lost now, or at
+least absent from the whole of Northern Europe? Its two
+putative descendants, though so distinct from each other, are,
+as we have seen, able often to occupy the same ground. If
+they were gradually derived from a common progenitor&mdash;necessarily
+very like themselves&mdash;can we believe that this original
+<span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span>
+form should always, in all the diversities of soil and situation
+which they inhabit, be unable to exist? Some one may fancy
+that the hybrids which are found in the situations occupied by
+both forms are this original parental species. But nothing can
+be more certain than that these plants are simply heterozygous
+combinations made by the union of gametes bearing the characters
+of <i>diurna</i> and <i>vespertina</i>.<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a>
+For they may be reproduced exactly in F<sub>1</sub> or in
+later generations of that cross when it is artificially
+made; when bred from their families exhibit palpable phenomena
+of segregation more or less complex; and usually, if perhaps not
+always, they are partially sterile.<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a>
+In a locality on the Norfolk coast that I know well,
+there is a strip of rough ground chiefly
+sand-bank, which runs along the shore. This ground is full of
+<i>vespertina</i>. Not a hundred yards inland is a lane containing
+<i>diurna</i>, and among the <i>vespertina</i> on the sand-bank are always
+some of the hybrid form, doubtless the result of fertilisation
+from the neighbouring <i>diurna</i> population. Seed saved from these
+hybrids gave <i>vespertina</i> and hybrids again, having obviously been
+fertilised by other <i>vespertina</i> or by other hybrids, and I have
+no doubt that such hybrid plants if fertilised by <i>diurna</i> would
+have shown some <i>diurna</i> offspring. The absence of <i>diurna</i>
+in such localities may fairly be construed as an indication that
+<i>diurna</i> is there at a real disadvantage in the competition for life.</p>
+
+<p class="indent">But if, admitting this, we proceed to consider how the special
+aptitude of <i>vespertina</i> is constituted, or what it is that puts
+<i>diurna</i> at a disadvantage, we find ourselves quite unable to
+show the slightest connexion between the success of one or the
+<span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span>
+failure of the other on the one hand, and <i>the specific characteristics</i>
+which distinguish the two forms on the other. The orthodox
+Selectionist would, as usual, appeal to ignorance. We
+ask what can <i>vespertina</i> gain by its white flowers, its more lanceolate
+leaves, its grey seeds, its almost erect capsule-teeth,
+its longer fruits, which <i>diurna</i> loses by reason of its red flowers,
+more ovate leaves, dark seeds, capsule-teeth rolled back, and
+shorter fruits? We are told that each of these things <i>may</i>
+affect the viability of their possessors. We cannot assert that
+this is untrue, but we should like to have evidence that it is true.
+The same problem confronts us in thousands upon thousands
+of examples, and as time goes on we begin to feel that speculative
+appeals to ignorance, though dialectically admissible, provide
+an insufficient basis for a proposition which, if granted, is to
+become the foundation of a vast scheme of positive construction.</p>
+
+<p class="indent">One thing must be abundantly clear to all, that to
+treat two forms so profoundly different as one, because intermediates of
+unknown nature can be shown to exist between them, is a mere
+shirking of the difficulties, and this course indeed creates artificial
+obstacles in the way of those who are seeking to discover the
+origin of organic diversity.</p>
+
+<p class="indent">In the enthusiasm with which evolutionary ideas were
+received the specificity of living things was almost forgotten.
+The exactitude with which the members of a species so often
+conform in the diagnostic, specific features passed out of account;
+and the scientific world by dwelling with a constant emphasis
+on the fact of variability, persuaded itself readily that species
+had after all been a mere figment of the human mind.
+Without presuming to declare what future research only can
+reveal, I anticipate that, when variation has been properly
+examined and the several kinds of variability have been successfully
+distinguished according to their respective natures,
+the result will render the natural definiteness of species increasingly
+apparent. Formerly in such a case as that of the two
+<i>Lychnis</i> species, the series of "intermediates" was taken to be a
+palpable proof that <i>vespertina</i> "graded" to <i>diurna</i>. It is this
+fact, doubtless, upon which Bentham would have relied in suggesting
+<span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span>
+that both may be one species.<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a>
+Genetic tests, though as yet imperfectly applied, make it almost certain
+that these inter-grading forms are not in any true sense variations from either
+species in the direction of the other, but combinations of elements
+derived from both.</p>
+
+<p class="indent">The points in which very closely allied species are
+distinguished from each other may be found in the most diverse
+features of their organisation. Sometimes specific difference
+is to be seen in a character which we can believe to be important
+in the struggle, but at least as often it is some little detail that
+we cannot but regard as trivial which suffices to differentiate
+the two species. Even when the diagnostic point is of such a
+nature that we can imagine it to make a serious difference in the
+economy we are absolutely at a loss to suggest why this feature
+should be a necessity to species A and unnecessary to species B
+its nearest ally. The house sparrow (<i>Passer domesticus</i>) is in
+general structure very like the tree sparrow (<i>P. montanus</i>).
+They differ in small points of colour. For instance <i>montanus</i>
+has a black patch on the cheek which is absent in <i>domesticus</i>.
+The presence in the one species and the absence in the other
+are equally definite, and in both cases we are equally unable to
+suggest any consideration of utility in relation to these features.
+The two species are distinguished also by a characteristic that
+may well be supposed to be of great significance. In <i>domesticus</i>
+the two sexes are strongly differentiated, the cock being more
+ornate than the hen. On the other hand the two sexes in <i>montanus</i>
+are alike, and, if we take a standard from <i>domesticus</i>, we
+may fairly say that in <i>montanus</i> the hen has the colouration of
+the male. It is not unreasonable to suppose that such a distinction
+may betoken some great difference in physiological
+economy, but the economical significance of this perhaps important
+distinction is just as unaccountable as that of the seemingly
+trivial but equally diagnostic colour-point.
+<span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span></p>
+
+<p class="indent">I have spoken of the fixed characteristics of the two species.
+If we turn to a very different feature, their respective liability
+to albinistic variation, we find ourselves in precisely similar
+difficulty. <i>Passer domesticus</i> is a species in which individuals
+more or less pied occur with especial frequency, but in <i>P. montanus</i>
+such variation is extremely rare if it occurs at all. The
+writer of the section on Birds in the <i>Royal Natural History</i>
+(III., 1894-5, p. 393) calls attention to this fact and remarks
+that in that species he knows no such instance.</p>
+
+<p class="indent">The two species therefore, apart from any differences that we
+can suppose to be related to their respective habits, are characterised
+by small fixed distinctions in colour-markings, by a
+striking difference in secondary sexual characters, and by a
+difference in variability. In all these respects we can form
+no surmise as to any economic reason why the one species
+should be differentiated in the one way and the other in the other
+way, and I believe it is mere self-deception which suggests the
+hope that with fuller knowledge reasons of this nature would be
+discovered.</p>
+
+<p class="indent">The two common British wasps, <i>Vespa vulgaris</i> and
+<i>Vespa germanica</i>, are another pair of species closely allied although
+sharply distinguished, which suggest similar reflexions. Both
+usually make subterranean nests but of somewhat different
+materials. <i>V. vulgaris</i> uses rotten wood from which the nest
+derives a characteristic yellow colour, while <i>V. germanica</i> scrapes
+off the weathered surfaces of palings and other exposed timber,
+material which is converted into the grey walls of the nest. The
+stalk by which the nest is suspended (usually to a root) in the
+case of <i>germanica</i> passes freely through a hole in the external
+envelope, but <i>vulgaris</i> unites this external wall solidly to the
+stalk. In bodily appearance and structure the two species are
+so much alike that they have often been confounded even by
+naturalists, and to the untrained observer they are quite indistinguishable.
+There are nevertheless small points of difference
+which almost though not quite always suffice to distinguish the
+two forms. For example the yellow part of the sinus of the eyes
+is emarginate in <i>vulgaris</i> but not emarginate in <i>germanica</i>. <i>V.
+vulgaris</i> often has black spots on the tibiae while in <i>germanica</i> the
+<span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span>
+tibiae are usually plain yellow. In both species there is a horizontal
+yellow stripe on the thorax, but whereas in <i>vulgaris</i> this is
+a plain narrow stripe, it is in <i>germanica</i> enlarged downwards in
+the middle. These and other apparently trivial details of colouration,
+though not absolutely constant, are yet so nearly constant
+that irregularities in these respects are quite exceptional. Lastly
+the genitalia of the males, though not very different, present
+small structural points of distinction which are enough to distinguish
+the two species at a glance.<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a>
+</p>
+
+<p class="indent">In considering the meaning of the distinctions between
+these two wasps we meet the old problem illustrated by the Sparrows.
+The two species have somewhat different habits of life and we
+should readily expect to find differences of bodily organisation
+corresponding with the differences of habits. But is that what
+we do find? Surely not. To suppose that there is a correspondence
+between the little points of colour and structure which
+we see and the respective modes of life of the two species is
+perfectly gratuitous. We have no inkling of the nature of such
+a correspondence, how it can be constituted, or in what it may
+consist.</p>
+
+<p class="indent">Is it not time to abandon these fanciful expectations
+which are never realised? Everywhere both among animals and plants
+does the problem of specific difference reiterate itself in the same
+form. In view of such facts as I have related and might indefinitely
+multiply, the fixity of specific characters cannot readily
+be held to be a measure of their economic importance to their
+possessors. The incidence of specific fixity is arbitrary and
+capricious, sometimes lighting on a feature or a property which
+can be supposed to matter much, but as often is it attached to the
+most trifling of superficial peculiarities.</p>
+
+<p class="indent">The incidence of <i>variability</i> is no less paradoxical,
+and without investigation of the particular case no one can say what will be
+<span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span>
+found to show much or little variability. The very characteristic
+which in one species may exhibit extreme variability
+may in an allied species show extreme constancy. Illustrations
+will occur to any naturalist, but nowhere is this truth more
+strikingly presented than in the British Noctuid Moths. Many
+are so variable that, in the common phrase, "scarcely two can
+be found alike," while others show comparatively slight variation.
+It need scarcely be remarked that, in the instances I have in
+mind, the evidence of great variability is in no way due to the
+abundance with which the particular species occurs, for common
+species may show constancy, and less abundant species may show
+great variability. The polymorphism seems to be now at least
+a general property of the variable species, as the fixity is a
+property of the fixed species. In illustration I may refer to the
+following examples.</p>
+
+<p class="indent"><i>Dianthoecia capsincola</i> is a common and widely distributed
+moth which feeds on <i>Lychnis</i>. It shows little variation. <i>Dianthoecia
+carpophaga</i> is another species which feeds chiefly on
+<i>Silene</i>. Its habits are very similar to those of <i>capsincola</i>. Like
+that species it has a wide geographical range and is abundant
+in its localities, but in contrast to the fixity of <i>capsincola</i>, <i>carpophaga</i>
+exhibits a complex series of varieties. <i>Agrotis suffusa</i>
+(= <i>ypsilon</i>) is a moth widely spread through the southern half
+of England. It is very constant in colour and markings. <i>Agrotis
+segetum</i> and <i>tritici</i> are excessively variable both in ground
+colour and markings, being found in an immense profusion of
+dissimilar forms throughout their distribution. Of these and
+several other species of <i>Agrotis</i> there are many named varieties,
+some of which have by various writers been regarded as specifically
+distinct. Of the genus <i>Noctua</i> many species (e. g. <i>festiva</i>)
+show a similar polymorphism, but <i>N. triangulum</i>, though showing
+some variation in certain respects, is usually very constant to
+its type, and the same is true of <i>N. umbrosa</i>.</p>
+
+<p class="indent">In several species of <i>Taeniocampa</i>, especially <i>instabilis</i>,
+the multiplicity of forms is extreme, while <i>cruda</i> (= <i>pulverulenta</i>)
+is a comparatively constant species. The genus <i>Plusia</i> contains
+a number of constant species, but in <i>Plusia interrogationis</i> we
+<span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span>
+meet the fact that the central silvery mark undergoes endless
+variation. "Truly no two are alike," says Mr. Tutt, "and to
+look down a long series of <i>interrogationis</i> is something like looking
+at a series of Chinese characters." In contrast to this we have
+the fact that in <i>Plusia gamma</i> the very similar silvery mark is
+by no means variable.</p>
+
+<p class="indent">I have taken this series of cases from the Noctuid moths,
+but it would be as easy to illustrate the same proposition from
+the Geometridae or the Micro-Lepidoptera.<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a>
+I have a longseries of <i>Peronea cristana</i>, for example, which was given to
+me by Mr. W. H. B. Fletcher, of Bognor. All were beaten out of the same
+hedge, and their polymorphism is such that no one unaccustomed
+to such examples could suppose that they belonged to a single
+species. Another common form, <i>P. schalleriana</i>, which lives in
+similar circumstances, exhibits comparatively slight variability.</p>
+
+<p class="indent">It should be expressly noted that the variation of
+which I am speaking is a genuine polymorphism. Several of the species
+enumerated exhibit also geographical variation, possessing definite
+and often strikingly distinct races peculiar to certain
+localities; but apart from the existence of such local differentiation,
+stands out the fact upon which I would lay stress, that
+some species are excessively variable while others are by comparison
+constant, in circumstances that we may fairly regard as comparable.</p>
+
+<p class="indent">This fact is difficult to reconcile with the conventional
+view that specific type is directly determined by Natural Selection
+<span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span>
+and that the precision with which a species conforms to its
+pattern is an indication of the closeness of that control. Anyone
+familiar with the characteristics of Moths will agree that the
+Noctuids, Geometrids and Tortricids are creatures whose existence
+depends in some degree on the success with which they can escape
+detection by their enemies in the imaginal state. We are therefore
+not surprised to find that some species of these orders
+exhibit definite geographical variation in conformity with the
+character of the ground, which may reasonably be supposed to
+aid in their protection. If this were all, there would be nothing to
+cause surprise. We might even be disposed to allow that variability
+might contribute to the perpetuation of animals so situated,
+on the principle that among a variety of surroundings
+some would probably be in harmony with the objects on which
+they rest. But we cannot admit the plausibility of an argument
+which demands on the one hand that the extreme precision with
+which species A adheres in the minutest details of its colour and
+pattern to a certain type shall be ascribed to the protective fitness
+of those details, and on the other hand that the abundant variability
+of species B shall be ascribed to the same determination.
+If it is absolutely necessary for A to conform to one type how
+comes it that B may range through some twenty distinct forms,
+any two of which differ more from each other than the regular
+species of many other genera? The only reply I can conceive
+is a suggestion that there <i>may</i> be some circumstance which
+differentiates the various classes of cases, that the exigencies
+of the fixed species <i>may</i> be different from those of the variable.
+Those who make such appeals to ignorance do not always perhaps
+realise whither this course of reasoning may lead. If admissible
+here the same argument would lead us to suggest that because
+albino moles have for an indefinite period occurred on a certain
+land near Bath there may be something in the soil or in the
+conditions of life near Bath which requires a proportion of albinos
+in its mole population. Or again, because the butterfly <i>Thais
+rumina</i> in one locality, Digne in the south of France, has a percentage
+of individuals of the variety <i>Honoratii</i> (with certain
+normally yellow spots on the hind wing coloured bright red)
+<span class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></span>
+and nowhere else throughout its distribution, that therefore we
+may suggest that there is some difference in the condition of
+life at Digne which makes the continuance of <i>Honoratii</i> there
+possible and beneficial.</p>
+
+<p class="indent">A polymorphism offering a parallel to that of the
+variable moths is afforded by the breeding plumage of the Ruff, the
+male of <i>Machetes pugnax</i>. The variety of plumage which these
+cocks exhibit is such that the statement that no two can be
+found alike is only a venial exaggeration. Newton remarks<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a>
+"that all this wonderful 'show' is the consequence of the polygamous
+habit of the Ruff can scarcely be doubtful"; but even if
+it be conceded that the great external differentiation of the
+cocks may be a result of sexual selection, the problem of their
+<i>polymorphism</i> remains unsolved, for, as we are well aware,
+polygamy is not usually associated with polymorphism of the
+male. The Black Cock (<i>Tetrao tetrix</i>), for example, is as polygamous
+as the Ruff, but in that and countless other cases, both
+sexes are constant to one type of plumage.</p>
+
+<p class="indent">When we thus compare the polymorphism of one species
+with the fixity of another, and attempt to determine the causes which
+have led to these extraordinary contrasts, two distinct lines of
+argument are open to us. We may ascribe the difference either
+to causes external to the organisms, primarily, that is to say,
+to a difference in the exigencies of Adaptation under Natural
+Selection; or on the other hand we may conceive the difference
+as due to innate distinctions in the chemical and physiological
+constitutions of the fixed and the variable respectively. There
+is truth undoubtedly in both conceptions. If the mole were
+physiologically incapable of producing an albino that variety
+would not have come into being, and if the albino were totally
+incapable of getting its living it would not be able to hold its
+<span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span>
+own. Were <i>Plotheia frontalis</i> constructed on a chemical plan
+which admitted of no variation, the countless varieties would
+not have been produced; and if one of its varieties had an overwhelming
+success out of all proportion to that of the rest, then
+the species would soon become monomorphic again. We
+cannot declare that Natural Selection has no part in the determination
+of fixity or variability; nevertheless looking at the whole
+mass of fact which a study of the incidence of variation provides,
+I incline to the view that the variability of polymorphic forms
+should be regarded rather as a thing tolerated than as an element
+contributing directly to their chances of life; and on the other
+hand that the fixity of the monomorphic forms should be looked
+upon not so much as a proof that Natural Selection controls
+them with a greater stringency, but rather as evidence of a natural
+and intrinsic stability of chemical constitution.</p>
+
+<p class="indent">Compare the condition of a variable form like the male
+Ruff (or in a less degree the Red Grouse in both its sexes) with
+that of the common Pheasant which is comparatively constant.
+In the Pheasant no doubt variations do occur as in other wild
+birds, but apart from the effects of mongrelisation the species
+is unquestionably uniform. Could it seriously be proposed
+that we should regard the constancy of the pheasant's plumage
+in this country as depending on the special fitness of that type
+of colouration? Even if the pheasant be not an alien in Western
+Europe, it has certainly been protected for centuries, and for a
+considerable period has existed in a state of semi-domestication.
+Such conditions should give good opportunity for polymorphism
+to be produced. In some coverts various aberrations do of
+course occur and persist, yet there is nothing indicative of a
+general relaxation of the fixity of the specific type, and the pheasant
+remains substantially a fixed species.<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a>
+The common pheasant (<i>Phasianus colchicus</i>) even shows little of that disposition to
+<span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span>
+form local races which appears in the species of Further India.
+Are we not then on safer ground in regarding the fixity of our
+species as a property inherent in its own nature and constitution?
+Just as in ages of domestication no rose has ever given
+off a blue variety so has the pheasant never broken out into the
+polymorphism of the Ruff.</p>
+
+<p class="indent">As soon as it is realised how largely the phenomena
+of variation and stability must be an index of the internal constitution
+of organisms, and not mere consequences of their relations to
+the outer world, such phenomena acquire a new and more profound significance.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_31" id="Page_31">[Pg 31]</a></span></p>
+
+<h2>CHAPTER II</h2>
+<h3><span class="smcap">Meristic Phenomena</span></h3>
+
+<p class="indent">Twenty years ago in describing the facts of Variation,
+argument was necessary to show that these phenomena had a special
+value in the sciences of Zoology and Botany. This value is
+now universally understood and appreciated. In spite however
+of the general attention devoted to the study of Variation, and
+the accumulation of material bearing on the problem, no satisfactory
+or searching classification of the phenomena is possible.
+The reason for this failure is that a real classification must
+presuppose knowledge of the chemistry and physics of living things
+which at present is quite beyond our reach.</p>
+
+<p class="indent">It is however becoming probable that if more knowledge
+of the chemical and physical structure of organisms is to be attained,
+the clue will be found through Genetics, and thus that
+even in the uncoordinated accumulation of facts of Variation
+we are providing the means of analysis applicable not only to
+them, but to the problems of normality also.</p>
+
+<p class="indent">The only classification that we can yet institute
+with any confidence among the phenomena of Variation is that which
+distinguishes on the one hand variations in the processes of
+division from variations in the nature of the substances divided.</p>
+
+<p class="indent">Variations in the processes of division are most often
+made apparent by a change in the number of the parts, and are therefore
+called <i>Meristic</i> Variations, while the changes in actual composition
+of material are spoken of as <i>Substantive</i> Variations. The Meristic
+Variations form on the whole a natural and fairly well
+defined group, but the Substantive Variations are obviously
+a heterogeneous assemblage.</p>
+
+<p class="indent">Though this distinction does not go very far, it is
+useful, and in all probability fundamental. It is of value inasmuch as
+it brings into prominence the distinct and peculiar part which
+<span class="pagenum"><a name="Page_32" id="Page_32">[Pg 32]</a></span>
+the process of division, or, more generally, repetition of parts,
+plays in the constitution of the forms of living things.</p>
+
+<p class="indent">That there may be a real independence between the
+Meristic and the Substantive phenomena is evident from the fact both
+that Meristic changes may occur without Substantive Variation,
+and that the substances composing an organism may change
+without any perceptible alteration in its meristic structure.
+When the distinction between these two classes of phenomena
+is perceived it will be realised that the study of genetics has on
+the one hand a physical, or perhaps more strictly a mechanical
+aspect, which relates to the manner in which material is divided
+and distributed; and also a chemical aspect, which relates to
+the constitution of the materials themselves. Somewhat as
+the philosophers of the seventeenth and eighteenth centuries
+were awaiting both a chemical and a mechanical discovery which
+should serve as a key to the problems of unorganised matter,
+so have biologists been awaiting two several clues. In Mendelian
+analysis we have now, it is true, something comparable with the
+clue of chemistry, but there is still little prospect of penetrating
+the obscurity which envelops the mechanical aspect of our phenomena.
+To make clear the application of the terms chemical
+and mechanical to the problem of Genetics the nature of that
+problem must be more fully described. In its most concrete
+form this problem is expressed in the question, how does a cell
+divide? If the organism is unicellular, and the single cell is
+the whole body, then the process of heredity is accomplished
+in the single operation of cell-division. Similarly in animals and
+plants whose bodies are made up of many cells, the whole process
+of heredity is accomplished in the cell-divisions by which the
+germ-cells are formed. When therefore we see a cell dividing,
+we are witnessing the process by which the form and the properties
+of the daughter-cells are determined.</p>
+
+<p class="indent">Now this process has the two aspects which I have
+called mechanical and chemical. The term "<i>Entwicklungsmechanik</i>"
+has familiarised us with the application of the word mechanics
+to these processes, but on reflexion it will be seen that this comprehensive
+term includes two sorts of events which are sometimes
+<span class="pagenum"><a name="Page_33" id="Page_33">[Pg 33]</a></span>
+readily distinguishable. There is the event by which the cell
+<i>divides</i>, and the event by which the two halves or their descendants
+are or may be <i>differentiated</i>. It is common knowledge that
+in some cell-divisions two similar halves, indistinguishable in
+appearance, properties, and subsequent fate, may be produced,
+while in other divisions daughter-cells with distinct properties
+and powers are formed. We cannot imagine but that in the
+first case, when the resulting cells are identical, the division
+is a mechanical process by which the mother-cell is simply
+cut in two; while in order that two differentiated halves may be
+produced, some event must have taken place by which a chemical
+distinction between the two halves is effected.<a name="FNanchor_1_18" id="FNanchor_1_18"></a><a href="#Footnote_1_18" class="fnanchor">[1]</a>
+In any ordinary Mendelian case we have a clear proof that such a
+chemical difference may be established between germ-cells. The facts
+of colour-inheritance for instance prove that germ-cells, otherwise
+identical, may be formed <i>possessing</i> the chromogen-factor which
+is necessary to the formation of colour in the flowers, or <i>destitute</i>
+of that factor. Similarly the germ-cells may possess the ferment
+which, by its action on the chromogenic substance, produces
+the colour, or they may be without that ferment. The same
+line of argument applied to a great range of cases. Nevertheless,
+though differences in chemical properties are often thus constituted
+by cell-divisions, and though we are thus able to make a
+quasi-chemical analysis of the individual by determining and
+enumerating these properties, yet it is evident that the distribution
+of these factors is not itself a chemical process. This
+is proved by the fact that similar divisions may be effected between
+halves which are exactly alike, and also by the fact that
+the numbers in which the various types of germ-cells are formed
+negative any suggestion of valency between them. The recognition
+of the unit-factors may lead&mdash;indeed must lead&mdash;to great
+advances in chemical physiology which without that clue would
+have been impossible, but in causation the chemical phenomena
+of heredity must be regarded as secondary to the physical or
+<span class="pagenum"><a name="Page_34" id="Page_34">[Pg 34]</a></span>
+mechanical phenomena by which the cells and their constituents
+are divided and separated. When therefore we speak of the
+<i>essential</i> phenomena of heredity we mean the mechanics of division,
+especially, though not, as we shall see, exclusively, of <i>cell</i>-division;
+and in the relation between the two halves of the dividing
+cell we have the problem presented in what seems to be its simplest form.</p>
+
+<p class="indent">In attempting to form some conception of the processes
+by which bodily characteristics are transmitted, or&mdash;to avoid that
+confusing metaphor of "transmission"&mdash;how it comes about
+that the offspring can grow to resemble its parent, continuity of
+the germ-substance which in some animals is a visible phenomenon,<a name="FNanchor_2_19" id="FNanchor_2_19"></a><a href="#Footnote_2_19" class="fnanchor">[2]</a>
+gives at least apparent help. An egg for example on becoming
+adult develops in certain parts a particular pigment.
+The eggs of that adult when they reach the appropriate age develop
+the same pigment. We have no clear picture of the
+mechanism by which this process is effected, but when we realise
+that the pigment results from the interaction of certain substances,
+and that since all the eggs are in reality pieces of the same
+material, it seems, unless we inquire closely, not unnatural that
+the several pieces of the material should exhibit the same colours
+at the same periods of their development. The continuity of
+the material of the germs suggests that there is a continuity
+of the materials from which the pigment is formed, and that
+thus an actual bit of those substances passes into each egg
+ready at the appropriate moment to generate the pigment.
+The argument thus outlined applies to all <i>substantive</i> characteristics.
+In each case we can imagine, if we will, the appearance
+of that characteristic as due to the contribution of its rudiment
+from the germ tissues.</p>
+
+<p class="indent">When we consider more critically it becomes evident
+that the aid given by this mental picture is of very doubtful reality,
+for even if it were true that any predestined particle actually
+corresponding with the pigment-forming materials is definitely
+<span class="pagenum"><a name="Page_35" id="Page_35">[Pg 35]</a></span>
+passed on from germ to germ, yet the power of increase which
+must be attributed to it remains so incomprehensible that the
+mystery is hardly at all illuminated.</p>
+
+<p class="indent space-below">When however we pass from the substantive to the
+meristic characters, the conception that the character depends on the
+possession by the germ of a particle of a specific material becomes
+even less plausible. Hardly by any effort of imagination can we
+see any way by which the division of the vertebral column into <i>x</i>
+segments or into <i>y</i> segments, or of a Medusa into 4 segments or
+into 6, can be determined by the possession or by the want of a
+material particle. The distinction must surely be of a different
+order. If we are to look for a physical analogy at all we should
+rather be led to suppose that these differences in segmental
+numbers corresponded with changes in the amplitude or number
+of dividing waves than with any change in the substance or
+material divided.</p>
+
+<h3><span class="smcap">Phenomena of Division</span></h3>
+
+<p class="indent space-below">I have said that in the division of a cell we seem to
+see the problem in its simplest form, but it is important to observe that
+the problem of division may be presented by the bodies of animals
+and plants in forms which are independent of the divisions between
+cells. The existence of pattern implies a repetition of
+parts, and repetition of parts when developed in a material
+originally homogeneous can only be created by division. Cell-division
+is probably only a special case of a process similar to
+that by which the pattern of the skeleton is laid down in a unicellular
+body such as that of a Radiolarian or Foraminiferan.
+Attempts have lately been made to apply mathematical treatment
+to problems of biology. It has sometimes seemed to
+me that it is in the geometrical phenomena of life that the most
+hopeful field for the introduction of mathematics will be found.
+If anyone will compare one of our animal patterns, say that of a
+zebra's hide, with patterns known to be of purely mechanical
+production, he will need no argument to convince him that there
+must be an essential similarity between the processes by which
+the two kinds of patterns were made and that parts at least of
+<span class="pagenum"><a name="Page_36" id="Page_36">[Pg 36]</a></span>
+the analysis applicable to the mechanical patterns are applicable
+to the zebra stripes also. Patterns mechanically produced are
+of many and very diverse kinds. One of the most familiar
+examples, and one presenting some especially striking analogies
+to organic patterns, is that provided by the ripples of a mackerel
+sky, or those made in a flat sandy beach by the wind or the ebbing
+tide. With a little search we can find among the ripple-marks, and
+in other patterns produced by simple physical means, the closest
+parallels to all the phenomena of striping as we see them in our
+animals. The forking of the stripes, the differentiation of two
+"faces," the deflections round the limbs and so forth, which in the
+body we know to be phenomena of division, are common both to
+the mechanical and the animal patterns. We cannot tell what in
+the zebra corresponds to the wind or the flow of the current, but
+we can perceive that in the distribution of the pigments, that is
+to say, of the chromogen-substances or of the ferments which
+act upon them, a rhythmical disturbance has been set up which
+has produced the pattern we see; and I think we are entitled to
+the inference that in the formation of patterns in animals and
+plants mechanical forces are operating which ought to be, and
+will prove to be, capable of mathematical analysis. The comparison
+between the striping of a living organism and the sand-ripples
+will serve us yet a little farther, for a pattern may either
+be formed by actual cell-divisions, and the distribution of differentiation
+coincidently determined, or&mdash;as visibly in the pigmentation
+of many animal and plant tissues&mdash;the pattern may
+be laid down and the pigment (for example) distributed through
+a tissue across or independently of the cell-divisions of the tissue.
+Our tissues therefore are like a beach composed of sands of
+different kinds, and different kinds of sands may show distinct
+and interpenetrating ripples. When the essential analogy between
+these various classes of phenomena is perceived, no one
+will be astonished at, or reluctant to admit, the reality of discontinuity
+in Variation, and if we are as far as ever from knowing
+the actual causation of pattern we ought not to feel surprised that
+it may arise suddenly or be suddenly modified in descent. Biologists
+have felt it easier to conceive the evolution of a striped
+animal like a zebra from a self-coloured type like a horse (or
+of the self-coloured from the striped) as a process involving many
+intergradational steps; but so far as the <i>pattern</i> is concerned, the
+change may have been decided by a single event, just as the
+multitudinous and ordered rippling of a beach may be created
+or obliterated at one tide.
+<span class="pagenum"><a name="Page_37" id="Page_37">[Pg 37]</a></span></p>
+
+<div class="figcenter" >
+  <img src="images/i_050.jpg" alt="Elephant Tusk" width="600" height="283" />
+</div>
+<p class="space-below" style="font-size: 120%; text-align: center;">
+   <b><span class="smcap">Fig. 1.</span> Tusk of Indian elephant, showing an abnormal segmentation.</b></p>
+<p><span class="pagenum"><a name="Page_38" id="Page_38">[Pg 38]</a></span></p>
+
+<p class="indent">This point is well illustrated by the tusk of an Indian
+elephant which I lately found in a London sale-room. This tusk is by
+some unknown cause, presumably a chronic inflammation,
+thrown up into thirteen well-marked ridges which closely simulate
+a series of segments (Fig. 1). Whatever the cause the condition
+shows how easily a normally unsegmented structure may be
+converted into a series of repeated parts.</p>
+
+<p class="indent space-below">The spread of segmentation through tissues normally
+unsegmented is very clearly exemplified in the skates' jaws shown in
+in Fig. 2. The right side of the upper figure shows the normal
+arrangement in the species <i>Rhinoptera jussieui</i>, but the structure
+on the left side is very different. The probable relations of the
+several rows of teeth to the normal rows is indicated by the lettering,
+but it is evident that by the appearance of new planes
+of division constituting separate centers of growth, the series has
+been recast. The pattern of the left side is so definite that had
+the variation affected the right side also, no systematist would
+have hesitated to give the specimen a new specific name. The
+other two drawings show similar variations of a less extensive
+kind, the nature of which is explained by the lettering of the
+rows of teeth.</p>
+
+<div class="figcenter" >
+  <img src="images/i_052.jpg" alt="Jaws of Skates" width="600" height="674" />
+</div>
+<p class="space-below" style="font-size: 120%; text-align: center;">
+   <b><span class="smcap">Fig. 2.</span> Jaws of Skates</b> (<i>Rhinoptera</i>)
+<b>showing meristic variation.</b><br />
+(For a detailed discussion see <i>Materials for the Study of Variation</i>, p. 259.)</p>
+
+<p class="indent">This power to divide is a fundamental attribute of
+life, and of that power cell-division is a special example. In regard to
+almost all the chief vital phenomena we can say with truth that
+science has made some progress. If I mention respiration, metabolism,
+digestion, each of these words calls to mind something
+more than a bare statement that such acts are performed by an
+animal or a plant. Each stands for volumes of successful experiment
+and research, But the expression cell-division, the
+fundamental act which typifies the rest, and on which they all
+depend, remains a bare name. We can see with the microscope
+<span class="pagenum"><a name="Page_39" id="Page_39">[Pg 39]</a></span>
+the outward symptoms of division, but we have no surmise as
+to the nature of the process by which the division is begun or
+accomplished. I know nothing which to a man well trained in
+scientific knowledge and method brings so vivid a realisation
+of our ignorance of the nature of life as the mystery of cell-division.
+What is a living thing? The best answer in few words
+that I know is one which my old teacher, Michael Foster, used
+to give in his lectures introductory to biology. "A living thing
+is a vortex of chemical and molecular change." This description
+gives much, if not all, that is of the essence of life. The living
+thing is unlike ordinary matter in the fact that, through it, matter
+is always passing. Matter is essential to it; but, provided
+that the flow in and out is unimpeded, the life-process can go
+on so far as we know indefinitely. Yet the living "vortex"
+differs from all others in the fact that it can divide and throw
+off other "vortices," through which again matter continually swirls.
+<span class="pagenum"><a name="Page_40" id="Page_40">[Pg 40]</a></span></p>
+
+<p class="indent">We may perhaps take the parallel a stage further. A simple
+vortex, like a smoke-ring, if projected in a suitable way will twist
+and form two rings. If each loop as it is formed could grow and
+then twist again to form more loops, we should have a model
+representing several of the essential features of living things.</p>
+
+<p class="indent">It is this power of spontaneous division which most
+sharply distinguishes the living from the non-living. In the excellent
+book dealing with the problems of development, lately published
+by Mr. Jenkinson a special emphasis is very properly laid on the
+distinction between the processes of division, and those of
+differentiation. Too often in discussions of the developmental
+processes the distinction is obscured. He regards differentiation
+as the "central difficulty." "Growth and division of the nucleus
+and the cells," he tells us, are side-issues. This view is quite
+defensible, but I suspect that the division <i>is</i> the central
+difficulty, and that if we could get a rationale of what is happening
+in cell-division we should not be long before we had a clue to the
+nature of differentiation. It may be self-deception, but I do
+not feel it impossible to form some hypothesis as to the mode of
+differentiation, but in no mood of freest speculation are we ever
+able to form a guess as to the nature of the division. We see
+differentiations occurring in the course of chemical action, in
+some phenomena of vibration and so forth: but where do we
+see anything like the spontaneous division of the living cell?
+Excite a gold-leaf electroscope, and the leaves separate, but we
+know that is because they were double before. In electrolysis
+various substances separate out at the positive and negative
+poles respectively. Now if in cell-division the two daughter-cells
+<span class="pagenum"><a name="Page_41" id="Page_41">[Pg 41]</a></span>
+were always dissimilar&mdash;that is to say, if differentiation
+always occurred&mdash;we could conceive some rough comparison
+with such dissociations. But we know the dissimilarity between
+daughter-cells is not essential. In the reproduction of unicellular
+organisms and many other cases, the products formed at the
+two poles are, so far as we can tell, identical. Any assumption
+to the contrary, if we were disposed to make it, would involve
+us in difficulties still more serious. At any rate, therefore, if
+differentiation be really the central difficulty in development,
+it is division which is the essential problem of heredity.</p>
+
+<p class="indent">Sir George Darwin and Professor Jeans tell us that
+"gravitational instability" consequent on the condensation of gases is
+"the primary agent at work in the actual evolution of the universe,"
+which has led to the division of the heavenly bodies. The
+greatest advance I can conceive in biology would be the discovery
+of the nature of the instability which leads to the continual
+division of the cell. When I look at a dividing cell I feel as an
+astronomer might do if he beheld the formation of a double
+star: that an original act of creation is taking place before me.
+Enigmatical as the phenomenon seems, I am not without hope
+that, if it were studied for its own sake, dissociated from the
+complications which obscure it when regarded as a mere incident
+in development, some hint as to the nature of division could be
+found. It is I fear a problem rather for the physicist than for
+the biologist. The sentiment may not be a popular one to utter
+before an assembly of biologists, but looking at the truth impersonally
+I suspect that when at length minds of first rate analytical
+power are attracted to biological problems, some advance will
+be made of the kind which we are awaiting.</p>
+
+<p class="indent">The study of the phenomena of bodily symmetry offers
+perhaps the most hopeful point of attack. The essential fact
+in reproduction is cell-division, and the essential basis of hereditary
+resemblance is the symmetry of cell-division. The phenomena
+of twinning provide a convincing demonstration that this
+is so. By twinning we mean the production of equivalent structures
+by division. The process is one which may affect the whole
+body of an animal or plant, or certain of its parts. The term
+<span class="pagenum"><a name="Page_42" id="Page_42">[Pg 42]</a></span>
+twin as ordinarily used refers to the simultaneous birth of two
+individuals. Those who are naturalists know that such twins
+are of two kinds, (1) twins that are not more alike than any other
+two members of the same family, and (2) twins that are so much
+alike that even intimate friends mistake them. These latter
+twins, except in imaginative literature, are always of the same sex.</p>
+
+<p class="indent">It is scarcely necessary for me to repeat the evidence
+from which it has been concluded that without doubt such twins arise
+by division of the same fertilised ovum. There is a perfect
+series of gradations connecting them with the various forms of
+double monsters united by homologous parts. They have been
+shown several times to be enclosed in the same chorion, and the
+proofs of experimental embryology show that in several animals
+by the separation of the two first hemispheres of a dividing egg
+twins can be produced. Lastly we have recently had the extraordinarily
+interesting demonstration of Loeb, to which I may
+specially refer. Herbst some years ago found that in sea water,
+from which all lime salts had been removed, the segments of the
+living egg fall apart as they are formed. Using this method
+Loeb has shown that a temporary immersion in lime-free sea
+water may result in the production of 90 per cent. of twins.
+We are therefore safe in regarding the homologous or "identical"
+twins as resulting from the divisions of one fertilised egg, while
+the non-identical or "fraternal" twins, as they are called, arise
+by the fertilisation of two separate ova.<a name="FNanchor_3_20" id="FNanchor_3_20"></a><a href="#Footnote_3_20" class="fnanchor">[3]</a>
+<span class="pagenum"><a name="Page_43" id="Page_43">[Pg 43]</a></span></p>
+
+<p class="indent">In the resemblance of identical twins we have an extreme case
+of hereditary likeness<a name="FNanchor_4_21" id="FNanchor_4_21"></a><a href="#Footnote_4_21" class="fnanchor">[4]</a>
+and a proof, if any were needed, that the
+cause of individual variation is to be sought in the differentiation
+of germ-cells. The resemblance of identical twins depends on
+two circumstances, First, since only two germ-cells take part
+in their production, difference between the germ cells of the
+same individual cannot affect them. Secondly the division of
+the fertilised ovum, the process by which they became two instead
+of one, must have been a symmetrical division. The
+structure of twins raises however one extremely significant
+difficulty, which as yet we cannot in any way explain. The
+resemblance between twins is a phenomenon of symmetry,
+like the resemblance between the two sides of a bilaterally symmetrical
+body. Not only is the general resemblance readily
+so interpreted, but we know also that in double monsters, namely
+unseparated twins, various anatomical abnormalities shown by
+the one half-body are frequently shown by the other half-also.<a name="FNanchor_5_22" id="FNanchor_5_22"></a><a href="#Footnote_5_22" class="fnanchor">[5]</a>
+The two belong to one system of symmetry. How then
+does it happen that the body of one of a pair of twins does not
+show a transposition of viscera? We know that the relation of
+right and left implies that the one should be the mirror-image of
+the other. Such a relation of images may be maintained even
+in minute details. For example if the same pattern of finger-print
+is given by the fingers of the two hands, one is the reverse
+of the other. In double monsters, namely unseparated twins,
+there is evidence that an inversion of viscera does occur with
+some frequency. Evidence from such cases is not so clear and
+simple as might be expected, because as a matter of fact, the
+heart and stomach, upon which the asymmetry of the viscera
+chiefly depend, are usually common to the two bodies. Duplicity
+generally affects either the anterior end alone, or the posterior
+end alone. The division is generally <i>from the heart forwards</i>,
+giving two heads and two pairs of anterior limbs on a common
+trunk, or <i>from the heart backwards</i>, giving two pairs of posterior
+limbs with the anterior body common. In either case, though
+<span class="pagenum"><a name="Page_44" id="Page_44">[Pg 44]</a></span>
+the bodies may be grouped in a common system of symmetry,
+neither can be proved to show definite reversal of the parts. To
+see that reversal recourse must be had to more extreme duplications,
+such as the famous Siamese Twins. They, as a matter of
+fact, were an excellent instance of the proposition that twins
+are related as mirror-images, for both of them had eleven pairs
+of ribs instead of the normal twelve, and one of them had a partial reversal
+of viscera.<a name="FNanchor_6_23" id="FNanchor_6_23"></a><a href="#Footnote_6_23" class="fnanchor">[6]</a>
+(Küchenmeister, <i>Verlagerung</i>, etc., p. 204.)</p>
+
+<p class="indent">If anyone could show how it is that neither of a
+pair of twins has transposition of viscera the whole mystery of division
+would, I expect, be greatly illuminated.<a name="FNanchor_7_24" id="FNanchor_7_24"></a><a href="#Footnote_7_24" class="fnanchor">[7]</a>
+At present we have simply to accept the fact that twins, by virtue of
+their detachment from each other, have the power of resuming the polarity
+which is proper to any normal individual. It was nevertheless
+with great interest that I read Wilder's recent observation<a name="FNanchor_8_25" id="FNanchor_8_25"></a><a href="#Footnote_8_25" class="fnanchor">[8]</a>
+that occasionally in identical twins the finger-print of one or both
+the index-fingers may be reversed, showing that there is after
+all some truth in the notion that reversal should occur in them.</p>
+
+<p class="indent">There is another phenomenon by twinning which, if we
+could understand it, might help. I refer to the free-martin, the subject
+of one of John Hunter's masterpieces of anatomical description.
+In horned cattle twin births are rare, and when twins of opposite
+sexes are born, the male is perfect and normal, but the reproductive
+<span class="pagenum"><a name="Page_45" id="Page_45">[Pg 45]</a></span>
+organs of the female are deformed and sterile, being
+known as a free-martin. The same thing occasionally happens
+in sheep, suggesting that in sheep also twins may be formed by
+the division of one ovum; for it is impossible to suppose that
+mere development in juxtaposition can produce a change of this
+character. I mention the free-martin because it raises a question
+of absorbing interest. It is conceivable that we should interpret
+it by reference to the phenomenon of gynandromorphism, seen
+occasionally in insects, and also in birds as a great rarity. In
+the gynandromorph one side of the body is male, the other female.
+A bullfinch for instance has been described with a sharp line of
+division down the breast between the red feathers of the cock
+on one side and the brown feathers of the hen on the other.
+(Poll, H., <i>SB. Ges. Nat. Fr.</i>, Berlin, 1909, p. 338.) In such cases
+neither side is sexually perfect. If the halves of such a gynandromorph
+came apart, perhaps one would be a free-martin.</p>
+
+<p class="indent">The behaviour of homologous twinning in heredity has been
+little studied. It does not exist as a normal feature in any animal
+which is amenable to experiment, and we cannot positively
+assert that a comparable phenomenon exists in plants; for in
+them&mdash;the Orange, for example&mdash;polyembryony may evidently
+be produced by a parthenogenetic development of nucellar tissue.
+It is possible that in Man twinning is due to a peculiarity of the
+mother, not of the father. It may and not rarely does descend
+from mother to daughter, but whether it can be passed on
+through a male generation to a daughter again, there is not
+sufficient evidence to show. The facts as far as they go are
+consistent with the inference which may be drawn from Loeb's
+experiment, that the twinning of a fertilized ovum may be determined
+not by the germ-cells which united to form it, but by
+the environment in which it begins to develop. The opinion that
+twinning may descend through the male directly has been lately
+expressed by Dr. J. Oliver in the <i>Eugenics Review</i> (1912), on the
+evidence of cases in which twins had occurred among the relations
+of fathers of twins, but I do not know of any comprehensive
+collection of evidence bearing on the subject.
+<span class="pagenum"><a name="Page_46" id="Page_46">[Pg 46]</a></span></p>
+
+<p class="indent">Besides twinning of the whole body a comparable duplicity
+of various parts of the same body may occur. Such divisions
+affect especially those organs which have an axis of bilateral
+symmetry, such as the thumb, a cotyledon, a median petal,
+the frond of a fern or the anal fin of a fish. From the little
+yet known it is clear that the genetic analysis of these conditions
+must be very difficult, but evidence of any kind regarding them
+will be valuable. We want especially to know whether these
+divisions are due to the <i>addition</i> of some factor or power which
+enables the part to divide, or whether the division results from
+the <i>absence</i> of something which in the normal body prevents
+the part from dividing. Breeding experiments, so far as they
+go, suggest that the less divided state is usually dominant to
+the more divided.<a name="FNanchor_9_26" id="FNanchor_9_26"></a><a href="#Footnote_9_26" class="fnanchor">[9]</a>
+The two-celled Tomato fruit is dominant to
+the many-celled type. The Manx Cat's tail, with its suppression
+of caudal segmentation is a partial dominant over the normal
+tail. The tail of the Fowl in what is called the "Rumpless"
+condition is at least superficially comparable with that of the
+Manx Cat, and though the evidence is not wholly consistent,
+Davenport obtained facts indicating that this suppressed condition
+of the caudal vertebrae is an imperfect dominant.<a name="FNanchor_10_27" id="FNanchor_10_27"></a><a href="#Footnote_10_27" class="fnanchor">[10]</a>
+</p>
+
+<p class="indent">Some evidence may also be derived from other examples
+of differences which at first sight appear to be substantive though
+they are more probably meristic in ultimate nature. The
+distinction between the normal and the "Angora" hair of the
+Rabbit is a case in point. We can scarcely doubt that one of
+the essential differences between these two types is that in the
+Angora coat the hair-follicles are more finely divided than they
+are in the normal coat, and we know that the normal, or less-divided
+condition, is dominant to the Angora, or more finely divided.</p>
+<p><span class="pagenum"><a name="Page_47" id="Page_47">[Pg 47]</a></span></p>
+
+<div class="figcenter" >
+  <img src="images/i_060.jpg" alt="Elephant Tusk" width="500" height="602" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+   <b><span class="smcap">Fig. 3.</span>&emsp; <i>I</i>, <i>II</i>, <i>III</i>,</b>
+various degrees of syndactyly affecting the medius and annularis in the hand;
+<b><i>IV</i></b>, syndactyly affecting the index and medius in the foot. (After Annandale.)</p>
+
+<p class="indent space-below">In the case of the solid-hoofed or "mule-footed" swine, the
+evidence shows, as Spillman has lately pointed out,<a name="FNanchor_11_28" id="FNanchor_11_28"></a><a href="#Footnote_11_28" class="fnanchor">[11]</a>
+that the condition behaves as a dominant. The essential feature of
+this abnormality is that the digits III and IV are partially
+united. The union is greatest peripherally. Sometimes the
+third phalanges only are joined to form one bone, but the second
+and even the first phalanges may also be compounded together.
+Here the variation is obviously meristic and consists in a failure
+to divide, the normal separation of the median digits of the foot being suppressed.
+<span class="pagenum"><a name="Page_48" id="Page_48">[Pg 48]</a></span></p>
+
+<div class="figcenter" >
+  <img src="images/i_061.jpg" alt="Syndactyly in the Foot" width="600" height="315" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Fig. 4.</span></b>&emsp; Case of complete syndactyly in the foot. <b><i>II</i></b>
+and <b><i>III</i></b>, digit apparently representing the index and medius.
+<b><i>c</i><sup>2</sup> + <i>c</i><sup>3</sup></b>, bone apparently representing the
+middle and external cuneiform; <b><i>cb</i></b>, cuboid; <b><i>c</i><sup>1</sup></b>,
+internal cuneiform. (After Gruber.)</p>
+
+<p class="indent">Webbing between the digits, in at least some of its
+manifestations, is a variation of similar nature. The family recorded
+by Newsholme<a name="FNanchor_12_29" id="FNanchor_12_29"></a><a href="#Footnote_12_29" class="fnanchor">[12]</a>
+very clearly shows the dominance of this condition.
+The case is morphologically of great interest and must
+undoubtedly have a bearing on the problems of the mechanics
+of Division. In discussing the phenomena of syndactylism
+I pointed out some years ago that the digits most frequently
+united in the human hand are III and IV, while in the foot,
+union most frequently takes place between II and III.<a name="FNanchor_13_30" id="FNanchor_13_30"></a><a href="#Footnote_13_30" class="fnanchor">[13]</a>
+In Newsholme's family the union was always between II and III
+of the foot, except in the case of one male who had the digits
+III and IV of the right <i>hand</i> alone webbed together. There
+can be little doubt that the geometrical system on which the
+foot is planned has an axis of symmetry passing between the
+digits II and III, while the corresponding axis in the hand passes
+between III and IV. Union between such digits may therefore
+be regarded as comparable with any non-division or "coalescence"
+of lateral structures in a middle line, and when as in these examples
+<span class="pagenum"><a name="Page_49" id="Page_49">[Pg 49]</a></span>
+such a condition is shown to be a dominant we cannot
+avoid the inference that some concrete factor has the power of
+suppressing or inhibiting this division. Figs. 3 and 4 illustrate
+degrees of union between digits in the human hand and foot.</p>
+
+<p class="indent">It is not in question that various other forms of irregular
+webbing and coalescence of digits exist, and respecting the genetic
+behaviour of these practically nothing is as yet known. Such
+a case is described by Walker,<a name="FNanchor_14_31" id="FNanchor_14_31"></a><a href="#Footnote_14_31" class="fnanchor">[14]</a>
+in which the first and second
+metacarpals of both feet were fused in mother and daughter,
+and several more are found in literature. Contrasted with these
+phenomena we have the curious fact that in the Pigeon, Staples-Browne
+found webbing of the toes a <i>recessive</i> character. The
+question thus arises whether this webbing is of the same nature
+as that shown to be a dominant in Man, and indeed whether the
+phenomenon in pigeons is really meristic at all. There is some
+difference perceptible between the two conditions; for in Man
+there is not so much a development of a special web-like skin
+uniting the digits as a want of proper division between the digits
+themselves, and in extreme cases two digits may be represented
+by a single one. In the Pigeon I am not aware that a real
+union of this kind has ever been observed, and though the web-like
+skin may extend the whole length of the digits and be so narrow
+as to prevent the spread of the toes, it may, I think, be maintained
+that the unity of the digits is unimpaired. For the
+present the nature of this variation in the pigeon's feet must be
+regarded as doubtful, and we should note that if it is actually
+an example of a more perfect division being dominant to a less
+perfect division, the case is a marked exception to the general
+rule that non-division is dominant to division.</p>
+
+<p class="indent">Reference must also be made to the phenomenon of fasciation
+in the stems of plants. As Mendel showed in the case of <i>Pisum</i>
+this condition is often a recessive. The appearances suggest
+that the difference between a normal and a fasciated plant
+consists in the inability of the fasciated plant to separate its
+lateral branches. The nature of the condition is however very
+<span class="pagenum"><a name="Page_50" id="Page_50">[Pg 50]</a></span>
+obscure and it is equally likely that some multiplication of the
+growing point is the essential phenomenon.<a name="FNanchor_15_32" id="FNanchor_15_32"></a><a href="#Footnote_15_32" class="fnanchor">[15]</a>
+</p>
+
+<p>Stockard's interesting experiments<a name="FNanchor_16_33" id="FNanchor_16_33"></a><a href="#Footnote_16_33" class="fnanchor">[16]</a>
+illustrate this question. He showed that by treating the embryos of a fish
+(<i>Fundulus heteroclitus</i>) with a dilute solution of magnesium salts,
+various cyclopian monstrosities were frequently produced. These have
+been called cases of <i>fusion</i> of the optic vesicles. I would prefer
+to regard them as cases of a division suppressed or restricted by
+the control of the environment. Conversely, the splendid discovery
+of Loeb, that an unfertilised egg will divide and develop
+parthenogenetically without fertilisation, as a consequence of
+exposure to various media, may be interpreted as suggesting that
+the action of those media releases the strains already present
+in the ovum, though I admit that an interpretation based on the
+converse hypothesis, that the medium acts as a stimulus, is as
+yet by no means excluded.</p>
+
+<p class="indent">In these cases we come nearest to the direct causation
+or the direct inhibition of a division, but the meaning of the
+evidence is still ambiguous. I incline to compare Loeb's parthenogenesis
+with the development (and of course accompanying
+cell-division) of dormant buds on stems which have been cut back.</p>
+
+<p class="indent">It is interesting to note that sometimes as an abnormality,
+the faculty of division gets out of hand and runs a course apparently
+uncontrolled. A remarkable instance of this condition
+is seen in <i>Begonia</i> "<i>phyllomaniaca</i>", which breaks out into buds
+at any point on the stem, petioles, or leaves, each bud having,
+like other buds, the power of becoming a new plant if removed.
+We would give much to know the genetic properties of <i>B. phyllomaniaca</i>,
+and in conjunction with Mr. W. O. Backhouse I have
+for some time been experimenting with this plant. It proved
+totally sterile. Its own anthers produce no pollen, and all attempts
+to fertilise it with other species failed though the pollen
+of a great number of forms was tried.
+<span class="pagenum"><a name="Page_51" id="Page_51">[Pg 51]</a></span></p>
+
+<p class="indent space-below">Recently however we have succeeded in making plants
+which are in every respect <i>Begonia phyllomaniaca</i>, so far as the characters
+of stems and leaves are concerned. These plants, of
+which we have sixteen, were made by fertilising <i>B. heracleifolia</i>
+with <i>B. polyantha</i>. They are all beginning to break out in
+"phyllomania." As yet they have not flowered, but as they
+agree in all details with <i>phyllomaniaca</i> there can be little doubt
+that the original plant bearing that name was a hybrid similarly
+produced. The production of "phyllomania" on a hybrid
+Begonia has also been previously recorded by Duchartre.<a name="FNanchor_17_34" id="FNanchor_17_34"></a><a href="#Footnote_17_34" class="fnanchor">[17]</a>
+In this case the cross was made between <i>B. incarnata</i> and <i>lucida</i>.
+The synonymy of the last species is unfortunately obscure, and
+I have not succeeded in repeating the experiment.</p>
+
+<div class="figcenter" >
+  <img src="images/i_064.jpg" alt="Petiole" width="600" height="223" />
+</div>
+<p class="space-below" style="font-size: 120%; text-align: center;">
+<b><span class="smcap">Fig. 5.</span>&emsp;Piece of petiole of</b> <i>Begonia phyllomaniaca</i>.
+<b>The proximal end is to the right of the figure.</b></p>
+
+<p class="indent space-below">From these facts it seems practically certain that
+the condition is one which is due to the meeting of complementary factors.
+At first sight we may incline to think that the phyllomania is in
+some way due to the sterility. This however cannot be seriously
+maintained; for not only is sterility in plants not usually associated
+with such manifestations, but we know a Begonia called
+"Wilhelma" which is exactly <i>phyllomaniaca</i> and equally sterile,
+though it has no trace of phyllomania. This plant arose in the
+nurseries of MM. P. Bruant of Poitiers, and has generally been
+described as a seedling of <i>phyllomaniaca</i>, but from the total
+sterility of that form this account of its origin must be set aside.
+<span class="pagenum"><a name="Page_52" id="Page_52">[Pg 52]</a></span></p>
+
+<div class="figcenter" >
+  <img src="images/i_065.jpg" alt="Petiole" width="600" height="560" />
+</div>
+<p style="font-size: 120%;">
+<b><span class="smcap">Fig. 6.</span></b>&emsp;Two right hind feet of polydactyle
+cats. <b><i>II</i></b> shows the lowest development of the condition yet recorded.
+The digit, <b><i>d</i><sup>1</sup></b>, which stands as hallux is
+fully formed and has three phalanges. Both it and the digit marked
+<b><i>d</i><sup>2</sup></b> are formed as <b><i>left</i></b> digits. In the
+normal hind foot of the cat the hallux is represented by a rudiment only.</p>
+
+<p class="space-below" style="font-size: 120%;"><b><i>I</i></b>
+shows a further development of the condition. In this foot
+there are <i>six</i> digits. <b><i>d</i><sup>1</sup></b> has two phalanges, but both
+it and <b><i>d</i><sup>2</sup></b> and <b><i>d</i><sup>3</sup></b> are shaped as left digits.
+Thus <b><i>d</i><sup>3</sup></b>, which in the normal foot would be shaped as a right digit,
+is transformed so as to look like a <b><i>left</i></b> digit.</p>
+
+<p class="indent">The phenomenon in this case can hardly be regarded
+as due to the excitation of dormant buds, for it is apparent on
+examination that the new growths are not placed in any fixed
+geometrical relation to the original plant. They arise on the
+petiole, for example, as small green outgrowths each of which
+gradually becomes a tiny leaf. The attitude of these leaves is
+quite indeterminate, and they may point in any direction,
+<span class="pagenum"><a name="Page_53" id="Page_53">[Pg 53]</a></span>
+some having their apices turned peripherally, some centrally, and
+others in various oblique or transverse positions (Fig. 5). These
+little leaves are thus comparable with seedlings, in that their
+polarity is not related to, or consequent upon that of the parent
+plant. They have in fact that "individuality," which we associate
+with germinal reproduction.</p>
+
+<p class="indent">There are many curious phenomena seen in the behaviour
+of parts normally repeated in bilateral symmetry which may some
+day guide us towards an understanding of the mechanics of
+division. A part like a hand, which needs the other hand to
+complete its symmetry, cannot twin by mere division, yet by
+proliferation and special modifications on the radial side of the
+same limb, even a hand may be twinned. In the well known polydactyle
+cats a change of this kind is very common and indeed
+almost the rule. When extra digits appear at the inner (tibial)
+side of the limb, they are shaped as digits of the other side, and
+even the normal digit II (index) is usually converted into the
+mirror-image of its normal self. The limb then develops a
+new symmetry in itself. Nevertheless it is not easy to interpret
+these facts as meaning that there has been some interruption
+in the control which one side of the body exercises over the
+other. The heredity of polydactylism is complex but there is
+little doubt that the condition familiar in the Cat is a dominant.
+In some human cases also the descent is that of a dominant, but
+irregularities are so frequent that no general rule can yet be
+perceived. The dominance of such a condition is an exception
+to the principle that the less-divided is usually dominant to the
+more-divided, a fact which probably should be interpreted as
+meaning that divisions are of more than one kind.</p>
+
+<p class="indent">Among ordinary somatic divisions, whether of organs,
+cells, or patterns of differentiation, the control of symmetry is usually
+manifested. There is however one class of somatic differentiations
+which are exceptionally interesting from the fact that they
+may show a complete independence of such geometrical control.
+The most familiar examples of these geometrically uncontrolled
+Variations are to be seen in bud-sports. The normal differentiation
+of the organs of a plant is arranged on a definite geometrical
+<span class="pagenum"><a name="Page_54" id="Page_54">[Pg 54]</a></span>
+system, which to those who have never given special
+attention to such things before, will often seem surprisingly
+precise. The arrangement of the leaves on uninjured, free-growing
+shoots can generally be seen to follow a very definite
+order, just as do the flowers or the parts of the flowers. If
+however bud sports occur, then though the parts included in
+the sports show all the geometrical peculiarities proper to the
+sport-variety, yet the sporting-buds themselves are not related
+to each other according to any geometrical plan.</p>
+
+<p class="indent">A very familiar illustration is provided by the
+distribution of colour in those Carnations that are not self-coloured.
+The pigment may, as in Picotees, be distributed peripherally with
+great regularity to the edges of the petals; or, as in Bizarres and
+Flakes, it may be scattered in radial sectors which show no
+geometrical regularity. Now in this case the pigments are the
+same in both types of flower, and the chemical factors concerned
+in their production must surely be the same. The difference
+must lie in the mechanical processes of distribution of the pigment.
+In the Picotee we see the orderly differentiation which we
+associate with normality; in the Bizarre we see the disorderly
+differentiation characteristic of bud-sports. The distribution of
+colour in this case lies outside the scheme of symmetry of the plant.</p>
+
+<p class="indent">Such a distribution is characteristic of bud-sports,
+and of certain other differentiations in both plants and animals, which
+I cannot on this occasion discuss. Now reflexion will show that
+these facts have an intimate bearing on the mechanical problems
+of heredity. For first in the bud-sports we are witnessing the
+distribution of factors which distinguish genetic varieties. We
+do not know the physical nature of those factors, but if we must
+give them a name, I suppose we should call them "ferments"
+exactly as Boyle did in 1666. He is discussing how it comes about
+that a bud, budded on a stock, becomes a branch bearing the
+fruit of its special kind. He notes that though the bud inserted
+be "not so big oftentimes as a Pea," yet "whether by the help
+of some peculiar kind of Strainer or by the Operation of some
+powerful Ferment lodged in it, or by both these, or some other
+<span class="pagenum"><a name="Page_55" id="Page_55">[Pg 55]</a></span>
+cause," the sap is "so far changed as to constitute a Fruit quite
+otherwise qualify'd."<a name="FNanchor_18_35" id="FNanchor_18_35"></a><a href="#Footnote_18_35" class="fnanchor">[18]</a>
+We can add nothing to his speculation,
+and we believe still that by a differential distribution of "ferments"
+the sports are produced. All the factors are together
+present in the normal parts; some are left out in the sport.
+In an analogous case however, that of a variegated <i>Pelargonium</i>
+which has green and also albino shoots, Baur proved that the
+shoots pure in colour are also pure in their posterity. There
+can be no doubt that the sports of Carnations, Azaleas, Chrysanthemums,
+etc., would behave in the same way.</p>
+
+<p class="indent">The well-known Azaleas Perle de Ledeburg, President
+Kerchove, and <i>Vervaeana</i> are familiar illustrations. Perle de
+Ledeburg is predominantly white, but it has red streaks in some
+of its flowers. It not very rarely gives off a self-red sport. This
+is evidently due to the development of a bud in a red-bearing
+area of the stem. The red in this plant is not under "geometrical
+control." Many plants have white flowers with no markings,
+but if the red markings are geometrically ordered differentiations,
+no self-coloured sports are formed. The case of <i>Vervaeana</i> is a
+good illustration of this proposition. It has white flowers with
+red markings arranged in an orderly manner on the lower parts
+of the petals, especially on the dorsal petals. This is one of the
+Azaleas most liable to have red sports, and at first sight it might
+seem that the sport represented the red of the central marks.
+Examination however of a good many flowers shows that irregular
+red streaks like those of Perle de Ledeburg occur, about as commonly
+as in that variety. <i>Vervaeana</i> in fact is Perle de Ledeburg
+with <i>definite</i> red markings added, and its red sports obviously are
+those branches the germs of which came in a patch of the stem
+bearing these red elements. That this is the true account is
+rendered quite obvious by the fact that the red of the sport is a
+colour somewhat different from that of the definite marks, and
+that these marks are still present on the red ground of the sporting
+flowers.</p>
+
+<p class="indent">It will be understood that these remarks apply to those
+cases in which the production of sports is habitual or frequent, and
+<span class="pagenum"><a name="Page_56" id="Page_56">[Pg 56]</a></span>
+I imagine in all such examples it will be found that there are
+indications of irregularity in the distribution of the differentiations
+such as to justify the view that they are not under that
+geometrical control which governs the normal differentiation
+of the parts. The question next arises whether these considerations
+apply also to the production of a bud-sport as a rare
+exception, but by the nature of the case it is not possible to
+say positively whether the appearance of an exceptional sport
+is due to the unsuspected presence of a pre-existing fragment of
+material having a special constitution, or to the origin, <i>de novo</i>,
+of such a material. For instance one of the garden forms of
+<i>Pelargonium</i> known as <i>altum</i> is liable perhaps once in some
+hundreds of flowers to have one or two magenta petals. The
+normal colour is a brilliant red; and as we may be fairly sure that
+this red is recessive to magenta the interpretation would be
+quite different according as the appearance of the magenta is
+regarded as due to the presence of small areas endowed with
+magentaness, or to the spontaneous generation of the factor
+for that pigment. Either interpretation is possible on the facts,
+but the view that the whole plant has in it scarce mosaic particles
+of magenta seems on the whole more consistent with present knowledge.</p>
+
+<p class="indent">In <i>Pelargonium altum</i> the enzyme causing the magenta
+colours must be distributed in very small areas, but a case in which the
+magenta is similarly arranged in a much coarser patchwork
+may be seen in the <i>Pelargonium</i> "Don Juan," which often bears
+whole trusses or branches of red flowers upon plants having the
+normal dominant magenta trusses. In most cases there is little
+doubt that though the magenta flowered parts can "sport" to
+red, the red parts could not produce the magenta flowers.</p>
+
+<p class="indent">The asymmetrical, or to speak more precisely, the disorderly,
+mingling of the colours in the somatic parts is thus an indication
+of a similarly disorderly mixing of the factors for those colours
+in the germ-tissues, so that some of the gametes bear enough of
+the colour-factors to make a self-coloured plant, while others
+bear so little that the plant to which they give rise is a patchwork.
+If this view is correct we may extend it so far as to consider
+<span class="pagenum"><a name="Page_57" id="Page_57">[Pg 57]</a></span>
+whether the fineness or coarseness of the mixture visible
+in the flowers or leaves may not give an indication of the degree
+to which the factors are subdivided among the germ-cells. We
+know very little about the genetic properties of striped varieties.
+In both <i>Antirrhinum</i> and <i>Mirabilis</i> it has been found that the
+striped may occasionally and irregularly throw self-coloured
+plants, and therefore the striping cannot be regarded simply as
+a recessive character. On the other hand in <i>Primula Sinensis</i>
+there are well-known flaked varieties which ordinarily at least
+breed true. Whether these ever throw selfs I do not know,
+but if they do it must be quite exceptionally. The power of
+these flaked plants to breed true is, I suspect, connected with
+the fact that in their flowers the coloured and white parts are
+<i>intimately</i> mixed, this intimate mixture thus being an indication
+of a similarly intimate mixture in the germ-cells. It would be
+important to ascertain whether self-fertilised seed from the occasional
+flowers in which the colour has run together to join a large patch gives
+more self-coloured plants than the intimately flaked flowers do.</p>
+
+<p class="indent">The next fact may eventually prove of great importance.
+We have seen that in bud-sports the differentiation is of the same
+nature as that between pure types, and also that in the sporting
+plant this differentiation is distributed without any reference
+to the plant's axis, or any other consideration of symmetry.
+Now among the germ-cells of a Mendelian hybrid exactly such
+characters are being distributed allelomorphically, and there
+again we have strong evidence for believing that the distribution
+obeys no pattern. For example, we can in the case of seeds still
+<i>in situ</i> perceive how the characters were distributed among the
+germ-cells, and there is certainly no obvious pattern connecting
+them, nor can we suppose that there is an actual pattern obscured.</p>
+
+<p class="indent">Of this one illustration is especially curious. Individual
+plants of the same species are, as regards the decussations of
+their leaves and in other respects, <i>either rights or lefts</i>.
+The fact is not emphasized in modern botany and is in some danger of
+being forgotten. When, as in the flowers of Arum, some <i>Gladioli</i>,
+<i>Exacum</i>, <i>St. Paulia</i>, or the fruits of <i>Loasa</i>, rights and lefts occur
+<span class="pagenum"><a name="Page_58" id="Page_58">[Pg 58]</a></span>
+on the same stem, they come off alternately. But if, as in the
+seedlings of Barley the twist of the first leaf be examined, it
+will be seen to be either a right-or left-handed screw. An
+ear of barley, say a two-row barley, is a definitely symmetrical
+structure. The seeds stand in their envelopes back to back in
+definite positions. Each has its organs placed in perfectly
+definite places. <i>If these seeds were buds</i> their differentiations
+would be grouped into a common plan. One might expect that
+the differentiations of these embryos would still fall into the
+pattern; but they do not, and so far as I have tested them, any
+one may be a right or a left, just as each may carry any of the
+Mendelian allelomorphs possessed by the parent plant, without
+reference to the differentiation of any other seed. The fertilisation
+may be responsible, but our experience of the allelomorphic
+characters suggest that the irregularity is in the egg-cells
+themselves.<a name="FNanchor_19_36" id="FNanchor_19_36"></a><a href="#Footnote_19_36" class="fnanchor">[19]</a>
+</p>
+
+<p class="indent"><i>Germ cells thus differ from somatic cells in the fact
+that their differentiations are outside the geometrical order which governs
+the differentiation of the somatic cells.</i> I can think of possible
+exceptions, but I have confidence that the rule is true and I
+regard it as of great significance.</p>
+
+<p class="indent">The old riddle, what is an individual, finds at least a partial
+solution in the reply that an individual is a group of parts differentiated
+in a geometrically interdependent order. With the
+germ-cell a new geometrical order, with independent polarity is
+almost if not quite always, begun, and with this geometrical independence
+the power of rejuvenescence may possibly be associated.</p>
+
+<p class="indent">The problems thus raised are unsolved, but they do
+not look insoluble. The solution may be nearer than we have thought.
+In a study of the geometry of differentiation, germinal and
+somatic, there is a way of watching and perhaps analyzing what
+may be distinguished as the mechanical phenomena of heredity.
+If any one could in the cases of the Picotee and the Bizarre Carnation,
+respectively, detect the real distinction between the two
+<span class="pagenum"><a name="Page_59" id="Page_59">[Pg 59]</a></span>
+types of distribution, he would make a most notable advance.
+Any one acquainted with mechanical devices can construct a
+model which will reproduce some of these distinctions more or less
+faithfully. The point I would not lose sight of is that the analogy
+with such models must for a long way be a true and valuable
+guide. I trust that some one with the right intellectual equipment
+will endeavor to follow this guide; and I am sanguine enough
+to think that a comprehensive study of the geometrical phenomena
+of differentiation will suggest to a penetrative mind that critical
+experiment which may one day reveal the meaning of spontaneous
+division, the mystery through which lies the road, perhaps the
+most hopeful, to a knowledge of the nature of life.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_60" id="Page_60">[Pg 60]</a></span></p>
+<h2>CHAPTER III</h2>
+<h3><span class="smcap">Segmentation, Organic And Mechanical</span></h3>
+
+<p class="indent">Models may be and often have been devised imitating
+some of the phenomena of division, but none of them have reproduced
+the peculiarity which characterises divisions of living tissues,
+that <i>the position of chemical differentiation</i> is <i>determined by
+those divisions</i>. For example, models of segmentation, whether
+radial or linear, may be made by the vibration of plates as in
+the familiar Chladni figures of the physical laboratory, or by
+the bowing of a tube dusted on the inside with lycopodium
+powder, and in various other ways. The sand or the powder
+will be heaped up in the nodes or regions of least movement, and
+the patterns thus formed reproduce many of the geometrical
+features of segmentation. But in the segmentations of living
+things the nodes and internodes, once determined by the dividing
+forces, would each become the seat of appropriate and distinct
+chemical processes leading to the differentiation of the parts,
+and the deposition of the bones, petals, spines, hairs, and other
+organs in relation to the meristic ground-plan. The "ripples" of
+meristic division not merely divide but differentiate, and when
+a "ripple" forks the result is not merely a division but a reduplication
+of the organ through which the fork runs. An
+example illustrating such a consequence is that of the half-vertebrae
+of the Python. On the left side the vertebra is single
+(Fig. 7) and bears a single rib, but on the right side a division
+has occurred with the result that two half-vertebrae, each
+bearing a rib, are formed, one standing in succession to the
+other. We cannot, indeed, imagine any operation of physiological
+division carried out in such an organ as a vertebra, passing
+through a plane at right angles to the long axis of the body, which
+does not necessarily involve the further process of reduplication.</p>
+
+<p class="indent space-below">As the meristic system of distribution spreads through
+the body, chemical differentiations follow in its track, with
+<span class="pagenum"><a name="Page_61" id="Page_61">[Pg 61]</a></span>
+segmentation and pattern as the visible result. Could we analyse
+these simultaneous phenomena and show how it is that the places
+of chemical differentiation are determined by the system of
+division, progress would then be rapid. It is here that all
+speculation fails.</p>
+
+<div class="figcenter" >
+  <img src="images/i_074.jpg" alt="" width="600" height="654" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Figs.</span> 7 and 8.</b>&emsp;Two examples of
+imperfect division in the vertebræ of a python. <b><i>I</i></b>, the
+vertebræ <b>147-150</b> from the right side, showing imperfect division between
+the <b>148th</b> and <b>149th</b>. The condition on the left side of this
+vertebra was the same. <b><i>II</i></b>, the dorsal surface of vertebræ
+<b>165-167</b>. On the right side the <b>166th</b> is double and bears two ribs,
+but on the left side it is normal and has one rib only.</p>
+
+<p class="indent">Many attempts have been made to interpret the processes
+of division and repetition, in terms of mechanics, or at least to
+refer them to their nearest mechanical analogies, so far with
+<span class="pagenum"><a name="Page_62" id="Page_62">[Pg 62]</a></span>
+little success. The problem is beset with difficulties as yet
+insurmountable and of these one must be especially noticed. In
+the living thing the process by which repetition and patterns
+come into being consists partly in division but partly also in
+growth. We have no means of studying the phenomena of
+pattern-formation except in association with that of growth.
+Growth soon ceases unless division takes place, and if growth is
+impossible division soon ceases also. In consequence of this
+fact that the final pattern is partly a product of growth, it can
+never be used as unimpeachable evidence of the primary geometrical
+relations of the members as laid down in the divisions.</p>
+
+<p class="indent">In the last chapter in referring to the problem of
+repetition I introduced an analogy, comparing the patterns of the organic
+world with those produced in unorganised materials by wave-motion.
+In the preliminary stage of ignorance, having no more
+trustworthy clue, I do not think it wholly unprofitable to consider
+the applicability of this analogy somewhat more fully. It
+possesses, as I hope to show, at least so much validity as to
+encourage the belief that morphology may safely discard one
+source of long-standing error and confusion.</p>
+
+<p class="indent">Those who have studied the structure of parts repeated
+in series will have encountered the old morphological problem of
+"Serial Homology," which has absorbed so much of the attention
+of naturalists and especially of zoologists at various periods.
+This problem includes two separate questions. The first of
+these is the origin in evolution of the resemblance between two
+organs occurring in a repeated series, of which the fore and hind
+limbs of Vertebrates are the prerogative instance. From the
+fact that these resemblances can be traced very far, often into
+minute details of structure, many anatomists have inclined to
+the opinion that the resemblance must originally have been still
+more complete, and that the two limbs, for instance, must have
+acquired their present forms by the differentiation of two identical
+groups of parts.</p>
+
+<p class="indent">Similar questions arise whenever parts are repeated in
+series, whether the series be linear or radial, and, though less obviously,
+even when the repetition is bilateral only. In each such example
+<span class="pagenum"><a name="Page_63" id="Page_63">[Pg 63]</a></span>
+the question arises, is the resemblance between the parts the
+remains of a still closer resemblance, or is differentiation original?
+Sometimes the view that these parts have arisen by the differentiation
+of a series of identical parts is plausible enough,
+as for example when the peculiarities of various appendages of a
+Decapod Crustacean are referred to modifications of the Phyllopod
+series. In application to other cases however we soon meet
+with difficulty, and the suggestion that the segments of a vertebrate
+were originally all alike is seen at once to be absurd, for
+the reason that a creature so constituted could not exist, and that,
+differentiation of at least one anterior and one posterior segment,
+is an essential condition of a viable organism consisting of parts
+repeated in a linear series. Between these two terminal segments
+it is possible to imagine the addition of one segment, or of a
+series of approximately similar segments; but when once it is
+realised that the terminals must have been differentiated from
+the beginning, it will be seen that the problem of the origin of
+the resemblance between segments is not rendered more comprehensible
+by the suggestion that even the intervening members
+were originally alike. Seeing indeed that some differentiation
+must have existed primordially it is as easy to imagine that the
+original body was composed of a series grading from the condition
+of the anterior segment to that of the posterior, as any other
+arrangement. The existence of a linear or successive series in
+fact postulates a polarity of the whole, and in such a system the
+conception of an ideal segment containing all the parts represented
+in the others has manifestly no place. The introduction of that
+conception though sanctioned by the great masters of comparative
+anatomy, has, as I think, really delayed the progress of
+a rational study of the phenomena of division. The same notion
+has been applied to every class of repetition both in animals and
+plants, generally with the same unhappy results. In the cruder
+forms in which this doctrine was taught thirty years ago it is
+now seldom expressed, but modified presentations of it still
+survive and confuse our judgments.
+<span class="pagenum"><a name="Page_64" id="Page_64">[Pg 64]</a></span></p>
+
+<p class="indent space-below">The process of repetition of parts in the bodies
+of organisms is however a periodic phenomenon. This much, provided we
+remain free from prejudice as to the nature and causation of the
+period or rhythm, we may safely declare, and a comparison may
+thus be instituted between the consequences of meristic repetition
+in the bodies of living things and those repetitions which in
+the inorganic world are due to rhythmical processes. Of such
+processes there is a practically unlimited diversity and we have
+nothing to indicate with which of them our repetitions should
+rather be compared.</p>
+
+<div class="figcenter" >
+  <img src="images/i_077.jpg" alt="" width="600" height="665" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Fig. 9.</span></b>&emsp; Osmotic growths simulating segmentation. (After Leduc.)</p>
+
+<p><span class="pagenum"><a name="Page_65" id="Page_65">[Pg 65]</a></span></p>
+
+<p class="indent">In some respects perhaps the best models of living organisms
+yet made are the "osmotic growths" produced by Leduc.<a name="FNanchor_1_37" id="FNanchor_1_37"></a><a href="#Footnote_1_37" class="fnanchor">[1]</a>
+These curious structures were formed by placing a fragment of
+a salt, for instance calcium chloride, in a solution of some colloidal
+substance. As the solid takes up water from the solution
+a permeable pellicle or membrane is formed around it. The vesicle
+thus enclosed grows by further absorption of water, often
+extending in a linear direction, and in many examples this growth
+occurs by a series of rhythmically interrupted extensions. Some
+of the growths thus formed are remarkably like organic structures,
+and might pass for a series of antennary segments or many other
+organs consisting of a linear series of repeated parts. In admitting
+the essential resemblance between these "osmotic growths"
+and living bodies or their organs I lay less stress on the general
+conformation of the growths, which often as Leduc points out, recall
+the forms of fungi or hydroids, but rather on the fact that the
+interruptions in the development of these systems are so closely
+analogous to the segmentations or repetitions of parts characteristic
+of living things (Fig. 9). In the same way I am less impressed
+by Leduc's models of Karyokinesis, wonderful as they
+nevertheless are, for the division is here imitated by putting
+separate drops on the gelatine film. What we most want to know
+is how in the living creature one drop becomes two. The models
+of linear segmentation have the remarkable merit that they do in
+some measure imitate the process of actual division or repetition.
+So in a somewhat modified method Leduc, by causing the diffusion
+of a solution in a gelatine film, produced rhythmical or periodic
+precipitations strikingly reminiscent of various organic tissues,
+for here also the process of periodic repetition is imitated with
+success.</p>
+
+<p class="indent">It is a feature common to these and to all other
+rhythmical repetitions produced by purely mechanical forces that there
+is resemblance between the members of the series, and that this
+similarity of conformation may be maintained in most complex
+detail. When however in the mechanical series some of the
+members differ from the rest we have no difficulty in recognising
+<span class="pagenum"><a name="Page_66" id="Page_66">[Pg 66]</a></span>
+that these differences&mdash;which correspond with the differentiations
+of the organic series&mdash;are due to special heterogeneity in
+the conditions or in the materials, and it never occurs to us to
+suppose that all the members must have been primordially alike.
+For example, in the case of ripple-marks on the sand, which I
+choose as one of the most familiar and obvious illustrations
+of a repeated series due to mechanical agencies, if we notice
+one ripple different in form from those adjacent to it, we do
+not suppose that this variation must have been brought about by
+deformation of a ripple which was at first formed like the others,
+but we ascribe it to a difference in the sand at that point, or to a
+difference in the way in which the wind or the tide dealt with it.
+We may press the analogy further by observing that in as much
+as such a series of waves has a beginning and an end, it possesses
+polarity like that of the various linear series of parts in organisms,
+and even the formation of each member must influence the
+shape of its successor. Since in an organism the beginning and
+end of the series are always included, some differentiation among
+the repetitions must be inevitable. If therefore it be conceded,
+as I think it must, that segmentation and pattern are the consequence
+of a periodic process we realize that it is at least as
+easy to imagine the formation of such a series of parts having
+family likeness combined with differentiation as it would be to
+conceive of their arising primordially as a series of identical repetitions.
+The suggestion that the likenesses which we now perceive
+are the remains of a still more complete resemblance
+is a substitution of a more complex conception for a simpler one.</p>
+
+<p class="indent">The other question raised by the problem of Serial
+Homology is how far there is a correspondence between individual members
+of series when the series differ from each other either in the
+number of parts, or in the mode of distribution of differentiation
+among them. Students, for example, of vertebrate morphology
+debate whether the <i>n</i>th vertebra which carries the pelvic girdle
+in Lizard A is individually homologous with the <i>n</i> + <i>x</i>th vertebra
+which fulfils this function in Lizard B, or whether it is not more
+truly homologous with the vertebra standing in the <i>n</i>th ordinal
+position, though that vertebra in Lizard B is free.
+<span class="pagenum"><a name="Page_67" id="Page_67">[Pg 67]</a></span></p>
+
+<p class="indent">In various and more complex aspects the same question
+is debated in regard to the cranial and spinal nerves, the branches
+of the aorta, the appendages of Arthropoda, and indeed in regard
+to all such series of differentiated parts in linear or successive
+repetition. Persons exercised with these problems
+should before making up their minds consider how similar
+questions would be answered in the case of any series of rhythmical
+repetitions formed by mechanical agencies. In the case
+of our illustration of the ripples in the sand, given the same forces
+acting on the same materials in the same area, the number of
+ripples produced will be the same, and the <i>n</i>th ripple counting
+from the end of the series will stand in the same place whenever
+the series is evoked. If any of the conditions be changed, the
+number and shapes can be changed too, and a fresh "distribution
+of differentiation" created. Stated in this form it is evident
+that the considerations which would guide the judgment in the
+case of the sand ripples are not essentially different from those
+which govern the problem of individual homology in its application
+to vertebrae, nerves, or digits.</p>
+
+<p class="indent">The fact that the unit of repetition is also the unit
+of growth is the source of the obscurity which veils the process. When we
+compare the skeleton of a long-tailed monkey with that of a
+short-tailed or tailless ape we see at once how readily the additional
+series of caudal segments may be described as a consequence
+of the propagation of the "waves" of segmentation
+beyond the point where they die out in the shorter column, and
+we see that with an extension of the series of repetitions there is
+growth and extension of material.</p>
+
+<p class="indent">The considerations which apply to this example will
+be found operating in many cases of the variation of terminal members of
+linear series. Some of these series, like the teeth of the dog,
+end in a terminal member of a size greatly reduced below that of
+the next to it. Even when there is thus a definite specialisation
+of the last member of the series it not infrequently happens that
+the addition, by variation, of a member beyond the normal
+terminal, is accompanied by a very palpable increase in size of
+the member which stands numerically in the place of the normal
+<span class="pagenum"><a name="Page_68" id="Page_68">[Pg 68]</a></span>
+terminal.<a name="FNanchor_2_38" id="FNanchor_2_38"></a><a href="#Footnote_2_38" class="fnanchor">[2]</a>
+So also with variation in the number of ribs, when a
+lumbar vertebra varies homoeotically into the likeness of the
+last dorsal and bears a rib, the rib placed next in front of this,
+which in the normal trunk is the last, shows a definite increase
+in development.</p>
+
+<p class="indent">The consequences of such homoeoses are sometimes
+very extensive, involving readjustments of differentiation affecting
+a long series of members, as may easily be seen by comparing
+the vertebral columns of several individual Sloths<a name="FNanchor_3_39" id="FNanchor_3_39"></a><a href="#Footnote_3_39" class="fnanchor">[3]</a>
+(whether <i>Bradypus</i> or <i>Choloepus</i>) to take a specially striking example.</p>
+
+<p class="indent">It may be urged that no feature as yet enables us
+to perceive wherein lies the primary distinction which determines such
+variation, whether it is due to a difference in the dividing forces
+or in the material to be divided. If for instance we were to
+imitate such a series of segments by pressing hanging drops of
+a viscous fluid out of a paint-tube by successive squeezes, the
+number of times the tube is contracted before it is empty will give
+the number of the segments, but their size may depend either
+on the force of the contractions or on the capacity of the tube,
+or on various other factors. Nevertheless in the case of the
+variation of terminal members, whatever be the nature of the
+rhythmical impulse which produces the series of organs, the elevation
+of the normally terminal member in correspondence with
+the addition of another is what we should expect.</p>
+
+<p class="indent">If the organism acquired its full size first and the
+delimitation of the parts took place afterwards, there might be some hope
+that the resemblance between living patterns and those mechanically
+caused by wave-motion might be shown to be a consequence of
+some real similarity of causation, but in view of the part played
+by growth, appeal to these mechanical phenomena cannot be
+declared to have more than illustrative value. Similarly in as
+much as living patterns appear, and almost certainly do in reality
+come into existence by a rhythmical process, comparisons of
+these patterns with those developed in crystalline structures, and
+in the various fields of force are, as it seems to me, inadmissible,
+or at least inappropriate.
+<span class="pagenum"><a name="Page_69" id="Page_69">[Pg 69]</a></span></p>
+
+<p class="indent">However their intermittence be determined, the
+rhythms of division must be looked upon as the immediate source of
+those geometrically ordered repetitions universally characteristic of
+organic life. In the same category we may thus group the segmentation
+of the Vertebrates and of the Arthropods, the concentric
+growth of the Lamellibranch shells or of Fishes' scales, the
+ripples on the horns of a goat, or the skeletons of the Foraminifera
+or of the Heliozoa. In the case of plant-structures Church<a name="FNanchor_4_40" id="FNanchor_4_40"></a><a href="#Footnote_4_40" class="fnanchor">[4]</a>
+has admirably shown, with an abundance of detail, how on analysis
+the definiteness of phyllotaxis is an expression of such rhythm in
+the division of the apical tissues, and how the spirals and "orthostichies"
+displayed in the grown plant are its ultimate consequences.
+The problem thus narrows itself down to the question
+of the mode whereby these rhythms are determined.</p>
+
+<p class="indent">It is natural that we should incline to refer them to a
+chemical source. If we think of the illustration just given, of the
+segmentation of a viscous fluid into drops by successive contractions
+of a soft-walled tube we can, I think, conceive of such rhythmic
+contractions as due to summations of chemical stimuli, somewhat
+as are the beats of the heart. But when we recognize the vast
+diversity of materials the distribution of which is determined by
+an ostensibly similar rhythmic process it seems hopeless to look
+forward to a directly chemical solution. That the chemical
+degradation of protoplasm or of materials which it contains is
+the source of the energy used in the divisions cannot be in dispute,
+but that these divisions can be themselves the manifestations
+of chemical action seems in the highest degree improbable.</p>
+
+<p class="indent">We may therefore insist with some confidence on the
+distinction between the Meristic and the substantive constitution
+of organisms, between, that is to say, the system according to
+which the materials are divided and the essential composition
+of the materials, conscious of the fact that the energy of division
+is supplied from the materials, and that in the ontogeny the
+manner in which the divisions are effected must depend secondarily
+on the nature of the substances to be divided. The
+<span class="pagenum"><a name="Page_70" id="Page_70">[Pg 70]</a></span>
+mechanical processes of division remain a distinguishable group of
+phenomena, and variations in the substances to be distributed in
+division may be independent of variations in the system by which
+the distribution is effected.</p>
+
+<p class="indent space-below">Modern genetic analysis supplies many remarkable
+examples of this distinction. When formerly we compared the leaves of a
+normal palmatifid Chinese Primula with the pinnatifid leaves<a name="FNanchor_5_41" id="FNanchor_5_41"></a><a href="#Footnote_5_41" class="fnanchor">[5]</a>
+of its fern-leaved variety we were quite unable to say whether the
+difference between the two types of leaf was due to a difference
+in the material cut up in the process of division or to a difference
+in that process itself. Knowledge that the distinction is determined
+by a single segregable factor tends to prove that the
+critical difference is one of substance. So also in the Silky fowl
+we know that the condition of its feathers is due to the absence
+of some one factor present in the normal form. We may conceive
+such differences as due to change of form in the successive
+"waves" of division, but we cannot yet imagine segregation
+otherwise than as acting by the removal or retention of a material
+element. Future observation by some novel method may suggest
+some other possibility, but such cases bring before us very clearly
+the difficulties by which the problem is beset.</p>
+
+<div class="figcenter" >
+  <img src="images/i_084.jpg" alt="Primula Sinensis Leaves" width="600" height="389" />
+</div>
+<p class="space-below" style="font-size: 120%; text-align: center;">
+<b><span class="smcap">Fig. 10.</span></b>&emsp;The palm-and fern type of leaf in
+<i>Primula Sinensis</i>.<br />&emsp;&emsp;&emsp;The palm is dominant and the fern is recessive.</p>
+
+<p class="indent">In another region of observation phenomena occur which
+as it seems to me put it beyond question that the meristic forces are
+essentially independent of the materials upon which they act,
+save, in the remoter sense, in so far as these materials are the
+sources of energy. The physiology of those regenerations and
+repetitions which follow upon mutilation supplies a group of
+facts which both stimulate and limit speculation. No satisfactory
+interpretations of these extraordinary occurrences has
+ever been found, but we already know enough to feel sure that
+in them we are witnessing indications which should lead to the
+discovery of the true mechanics of repetition and pattern.
+The consequences of mutilation in causing new growth or perhaps
+more strictly in enabling new growth to take place, are such that
+they cannot be interpreted as responses to chemical stimuli in
+<span class="pagenum"><a name="Page_71" id="Page_71">[Pg 71]</a></span>
+any sense which the word chemical at present connotes. Powers
+are released by mutilation of which in the normal conditions of
+life no sign can be detected. All who have tried to analyse the
+phenomena of regeneration are compelled to have recourse to the
+metaphor of equilibrium, speaking of the normal body as in a state
+of strain or tension (Morgan) which when disturbed by mutilation
+results in new division and growth. The forces of division
+are inacessible to ordinary means of stimulation. Applications,
+for example, of heat or of electricity excite no responses of a
+positive kind unless the stimuli are so violent as to bring about
+actual destruction.<a name="FNanchor_6_42" id="FNanchor_6_42"></a><a href="#Footnote_6_42" class="fnanchor">[6]</a>
+These agents do not, to use a loose expression,
+come into touch with the meristic forces. Changes in the
+chemical environment of cells may, as in the experiments of
+Loeb and of Stockard produce definite effects, but the facts
+suggest that these effects are due rather to alterations in the
+living material than to influence exerted directly on the forces
+of division themselves.</p>
+
+<p class="indent">By destruction of tissue however the forces both of
+growth and of division also may often be called into action with a
+resulting regeneration. Interruption of the solid connexion
+between the parts may produce the same effects, as for example
+when the new heads or tails grow on the divided edges of Planarians
+(Morgan), or when from each half embryo partially separated
+from its normally corresponding half, a new half is formed
+with a twin monster as the result.</p>
+
+<p class="indent">Often classed with regenerations but in reality
+quite distinct from them are those special and most interesting examples
+where the growth of a <i>paired</i> structure is excited by a simple
+<span class="pagenum"><a name="Page_72" id="Page_72">[Pg 72]</a></span>
+wound. Some of the best known of these instances are presented
+by the paired extra appendages of Insects and Crustacea. Some
+years ago I made an examination of all the examples of such
+monstrosities to which access was to be obtained, and it was with
+no ordinary feeling of excitement that I found that these supernumerary
+structures were commonly disposed on a recognizable
+geometrical plan, having definite spatial relations both to each
+other and to the normal limb from which they grew. The more
+recent researches of Tornier<a name="FNanchor_7_43" id="FNanchor_7_43"></a><a href="#Footnote_7_43" class="fnanchor">[7]</a>
+and especially his experiments on
+the Frog have shown that a cut into the posterior limb-bud
+induces the outgrowth of such a <i>pair</i> of limbs at the wounded
+place. Few observations can compare with this in novelty or
+significance; and though we cannot yet interpret these phenomena
+or place them in their proper relations with normal occurrences,
+we feel convinced that here is an observation which is no mere
+isolated curiosity but a discovery destined to throw a new light
+on biological mechanics. The supernumerary legs of the Frog
+are evidently grouped in a system of symmetry similar to that
+which those of the Arthropods exhibit, and though in Arthropods
+paired repetitions have not been actually produced by injury
+under experimental conditions we need now have no hesitation
+in referring them to these causes as Przibram has done.</p>
+
+<p class="indent">At this point some of the special features of
+the supernumerary appendages become important. First they may arise
+at any point on the normal limb, being found in all situations
+from the base to the apex. Nor are they limited as to the surface
+from which they spring, arising sometimes from the dorsal,
+anterior, ventral, or posterior surfaces, or at points intermediate
+between these principal surfaces.</p>
+
+<p class="indent space-below">With rare and dubious exceptions, the parts which are
+contained in these extra appendages are only those which lie <i>peripheral
+to their point of origin</i>. Thus when the point of origin is
+in the apical joint of the tarsus, the extra growth if completely
+developed consists of a double tarsal apex bearing two pairs
+of claws. If they arise from the tibia, two complete tarsi are
+<span class="pagenum"><a name="Page_73" id="Page_73">[Pg 73]</a></span>
+added. If they spring from the actual base of the appendage
+then two complete appendages may be developed in addition to
+the normal one. We must therefore conclude that in any point
+on a normal appendage the power exists which, if released, may
+produce a bud containing in it a paired set of the parts peripheral
+to this point.</p>
+
+<div class="figcenter" >
+  <img src="images/i_088.jpg" alt="Arthropoda" width="600" height="590" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Fig. 11.</span></b>&emsp;Diagrams of
+the geometrical relations which are generally exhibited by extra pairs of
+appendages in Arthropoda. The sections are supposed to be those of the apex
+of a tibia in a beetle. <b><i>A</i></b>, anterior, <b><i>P</i></b>, posterior, <b><i>D</i></b>,
+dorsal, <b><i>V</i></b>, ventral. <b><i>M</i><sup>1</sup></b>, <b><i>M</i><sup>2</sup></b> are
+the imaginary planes of reflexion. The shaded figure is in each case a limb
+formed like that of the other side of the body, and the outer unshaded figures
+are shaped like the normal for the side on which the appendages are. On the
+several radii are shown the extra pairs in their several possible relations to
+the normal from which they arise. The normal is drawn in thick lines in the center.</p>
+
+<p class="indent">Next the geometrical relations of the halves of the
+supernumerary pair are determined by the position in which they stand
+<span class="pagenum"><a name="Page_74" id="Page_74">[Pg 74]</a></span>
+in regard to the original appendage. These relations are best
+explained by the diagram (Fig. 11), from which it will be seen
+that the two supernumerary appendages stand as images of each
+other; and, of them, that which is adjacent to the normal appendage
+forms an image of it. Thus if the supernumerary pair
+arise from a point on the dorsal surface of the normal appendage,
+the two <i>ventral</i> surfaces of the extra pair will face each other.
+If they arise on the anterior surface of the normal appendage,
+their morphologically posterior surfaces will be adjacent, and so on.</p>
+
+<p class="indent">These facts give us a view of the relations of the two
+halves of a dividing bud very different from that which is to be derived
+from the exclusive study of normal structures. Ordinary morphological
+conceptions no longer apply. The distribution of the
+parts shows that the bud or rudiment which becomes the supernumerary
+pair may break or open out in various ways according
+to its relations to the normal limb. Its planes of division are
+decided by its geometrical relations to the normal body.</p>
+
+<p class="indent">Especially curious are some of the cases in which
+the extra pair are imperfectly formed. The appearance produced is then
+that of two limbs in various stages of coalescence, though in
+reality of course they are stages of imperfect separation. The
+plane of "coalescence" may fall anywhere, and the two appendages
+may thus be compounded with each other much as an
+object partially immersed in mercury "compounds" with its
+optical image reflected from the surface.</p>
+
+<p class="indent">Supernumerary paired structures are not usually, if
+ever, formed when an appendage is simply amputated. Cases occasionally
+are seen which nevertheless seem to be of this nature.
+Borradaile,<a name="FNanchor_8_44" id="FNanchor_8_44"></a><a href="#Footnote_8_44" class="fnanchor">[8]</a>
+for example, described a crab (<i>Cancer pagurus</i>)
+having in place of the right chela three <i>small</i> chelae arising from
+a common base, where the appearances suggested that the three
+reduced limbs replaced a single normal limb. From the details reported
+however it seems still possible that one of the chelae
+(that lettered F. I in Borradaile's figure) may be the normal
+one, and the other two an extra pair. The chela which I suspect
+to be the normal is in several respects deformed as well as being
+<span class="pagenum"><a name="Page_75" id="Page_75">[Pg 75]</a></span>
+reduced in size, and this deformity may perhaps have ensued as
+a consequence of the same wound which excited the growth of the
+extra pair. Its reduced size may be due to the same injury,
+which may quite well have checked its growth to full proportions.</p>
+
+<p class="indent">Admitting doubt in these ambiguous cases it seems to
+be a general rule that for the production of the extra pair the normal
+limb should persist in connexion with the body. Moreover it is
+practically certain that in no case can a <i>single</i>, viz. an unpaired,
+duplicate of the normal appendage grow from it. Many examples
+have been described as of this nature, but all of them may be with
+confidence regarded as instances of a supernumerary pair in
+which only the two morphologically anterior or the two morphologically
+posterior surfaces are developed. We have thus
+the paradox that a limb of one side of the body, say the right,
+has in it the power to form a pair of limbs, right and left, as an
+outgrowth of itself, but cannot form a second left limb alone.</p>
+
+<p class="indent">A very interesting question arises whether it is strictly
+correct to describe the extra pair as a right and a left, or whether
+they are not rather two lefts or two rights of which one is reversed.
+This question did not occur to me when in former years I studied
+these subjects. It was suggested to me by Dr. Przibram.
+The answer might have an important bearing on biological
+mechanics, but I know no evidence from which the point can
+be determined with certainty. In order to decide this question
+it would be necessary to have cases in which the paired repetition
+affected a limb markedly differentiated on the two sides of the
+body, and of course the development of the extra parts in order
+to be decisive must be fairly complete. One example only is
+known to me which at all satisfies these requirements, that of the
+lobster's chela figured (after Van Beneden) in <i>Materials for the
+Study of Variation</i>, p. 531, Fig. 184, III.</p>
+
+<p class="indent space-below">Here the drawing distinctly suggests that one of
+the extra dactylopodites, namely that lettered R, is differentiated
+as a left and not merely a reversed right. For the teeth on this
+dactylopodite are those of a cutting claw, not of a crushing claw,
+whereas the dactylopodites R' and L' bear crushing teeth. The
+figure makes it fairly certain also that the limb affected was a
+<span class="pagenum"><a name="Page_76" id="Page_76">[Pg 76]</a></span>
+crushing claw. Accepting this interpretation, we reach the
+remarkable conclusion that the bud of new growth consisted of
+halves differentiated into cutter and crusher as the normal claws
+are, and that the extra crusher is geometrically a left but physiologically
+a right. Though shaped as a left in respect of the
+direction in which it points, the extra crusher is really an optically
+reversed right, while the dactylopodite R, which is
+placed pointing like a right, is really a reversed left (Fig. 12).</p>
+
+<div class="figcenter" >
+  <img src="images/i_091.jpg" alt="Lobster Claw" width="600" height="550" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Fig. 12.</span></b>&emsp;Right claw of lobster bearing
+a pair of extra dactylopodites (after van Beneden). The fine toothing on R suggests
+that this is part of a cutting claw, though the limb bearing it is a crusher.</p>
+
+<p class="indent">If these indications are reliable<a name="FNanchor_9_45" id="FNanchor_9_45"></a><a href="#Footnote_9_45" class="fnanchor">[9]</a>
+and are established by further
+observation we shall be led to the conclusion that the bud which
+becomes an extra pair of limbs does not merely contain the parts
+proper to the side on which it grows, but is comparable with
+the original zygotic cell, and consists not simply of two halves,
+but of two halves differentiated as a right and a left like the two
+halves of the normal body.</p>
+
+<p class="indent">Phenomena of this kind, evoked by mutilation or
+injury, together with the cognate observations on regeneration throw
+<span class="pagenum"><a name="Page_77" id="Page_77">[Pg 77]</a></span>
+very curious lights on the nature of living things. To an understanding
+of the nature of the mechanics of living matter and its
+relation to matter at large they offer the most hopeful line of
+approach. I allude especially to the examples in which it has
+been established that the part which is produced after mutilation
+is a structure different from that which was removed. The
+term "regeneration" was introduced before such phenomena
+were discovered, and though every one recognizes its inapplicability
+to these remarkable cases, the word still misleads us by
+presenting a wrong picture to the mind. The expression "heteromorphosis"
+(Loeb) has been appropriately applied to various
+phenomena of this kind, and Morgan has given the name "morphallaxis"
+to another group of cases in which the renewal occurs
+by the transformation of a previously existing part.<a name="FNanchor_10_46" id="FNanchor_10_46"></a><a href="#Footnote_10_46" class="fnanchor">[10]</a>
+But we must continually remember that all these occurrences which
+we know only as abnormalities and curiosities must in reality be
+exemplifications of the normal mechanics of division and growth.
+The conditions needed to call them forth are abnormal, but the
+responses which the system makes are evidences of its normal
+constitution. When therefore, for example, the posterior end
+of a worm produces a reversed tail from its cut end we have a
+proof that there must be in the normal body forces ready to
+cause this outgrowth. The new structure is not an ill-shaped
+head-end, for, as Morgan shows, the nephridial ducts have their
+funnels perforating the segments in a reversed direction. The
+"tension" of growth is actually reversed.<a name="FNanchor_11_47" id="FNanchor_11_47"></a><a href="#Footnote_11_47" class="fnanchor">[11]</a>
+So also when in a Planarian amputation of the body immediately behind
+the head leads to the formation of a new reversed head at the back of
+the normal head, while amputation further back leads to the
+regeneration of a new tail, these responses give indications
+of forces normally present in the body of the Planarian. Such
+facts open up a great field of speculation and research. Especially
+important it would be to determine where the critical
+region may be at which the one response is replaced by the
+<span class="pagenum"><a name="Page_78" id="Page_78">[Pg 78]</a></span>
+other. I suppose it is even possible that there is some neutral
+zone in which neither kind of response is made.</p>
+
+<p class="indent">Physical parallels to the phenomena of regeneration are
+not easy to find and we still cannot penetrate beyond the empirical
+facts. Przibram has laid stress on the general resemblance
+between the new growth of an amputated part in an animal and
+the way in which a broken crystal repairs itself when placed in
+the mother-solution. That the two processes have interesting
+points of likeness cannot be denied. It must however never be
+forgotten that there is one feature strongly distinguishing the
+two; for I believe it is universally recognized by physicists that
+all the phenomena of geometrical regularity which crystals
+display are ultimately dependent on the forms of the particles
+of the crystalline body. This cannot in any sense be supposed
+to hold in regard to protoplasm or its constituents. The definiteness
+of crystals is also an unlikely guide for the reason that
+it is absolute and perfect, or in other words because this kind of
+regularity cannot be disturbed at all without a change so great
+that the substance itself is altered; whereas we know that the
+forms of living things are capable of such changes, great and small,
+that we must regard perfection of form, whether manifested in
+symmetry or in number, as an ideal which will only be produced
+in the absence of disturbance. The symmetry of the living
+things is like the symmetry of the concentric waves in a pool
+caused by a splash. Perfect circles are made only in the imaginary
+case of mathematical uniformity, but the system maintains
+an approximate symmetry though liable to manifold deformation.</p>
+
+<p class="indent">Since the geometrical order of the living body cannot
+be a direct function of the materials it must be referred to some more
+proximate control. In renewing a part the body must possess
+the power of seizing particles of many dissimilar kinds, and whirl
+them into their several and proper places. The action in renewal,
+like that of original growth, may be compared&mdash;very
+crudely&mdash;with the action of a separator which simultaneously
+distributes a variety of heterogeneous materials in an orderly
+fashion; but in the living body the thing distributed must rather
+be the <i>appetency</i> for special materials, not the materials themselves.
+<span class="pagenum"><a name="Page_79" id="Page_79">[Pg 79]</a></span></p>
+
+<p class="indent">If the analogy of crystals be set aside and we seek
+for other parallels to regeneration there are none very obvious. I have
+sometimes wondered whether it might not be possible to institute
+a fruitful comparison between the renewal of parts and the reformation
+of waves of certain classes after obliteration. In several
+respects, as I have already said, some curious resemblances with
+the repetitions formed by wave-motion are to be traced in our
+organic phenomena, and though admitting that I cannot develop
+these comparisons, I think nevertheless they may be worth
+bearing in mind. When, after obliteration, an eddy in a stream,
+or a ripple-mark (a more complex case of eddy-formation) in
+blown sand is re-formed, we have an example in which pattern is
+reconstituted and growth takes place not by virtue of the composition
+of the materials&mdash;in this case the water or the sand&mdash;but
+by the way in which they are acted upon by extraneous forces.</p>
+
+<p class="indent">A feature in the actual mode by which ripple-marks
+are reconstituted may not be without interest in connexion with
+our phenomena of regeneration. When, for example, the wind
+is blowing steadily over a surface of fine, dry sand, the familiar
+ripple-marks are formed by a heaping of the sand in lines transverse
+to the direction of the wind. The heaping is due to the
+formation of eddies corresponding with positions of instability.
+When the wind is steady and the sand homogeneous, the distances
+between the ripples, or wave-lengths, are sensibly equal.
+If while the wind continues to blow, the ripples are obliterated
+with a soft brush they will quickly be re-formed over the whole
+area, but I have noticed that at first their wave-length is approximately
+half that of the ripples in the undisturbed parts of
+the system.<a name="FNanchor_12_48" id="FNanchor_12_48"></a><a href="#Footnote_12_48" class="fnanchor">[12]</a>
+The normal wave-length is restored by the gradual
+accentuation of alternate ripples. Of course the sand-ripples are
+in reality slowly travelling forward in the direction towards
+which the wind is blowing, and for this our living segmentations
+afford no obvious parallel, but the appearances in the area of
+<span class="pagenum"><a name="Page_80" id="Page_80">[Pg 80]</a></span>
+reformation, and especially the forking of the old ridges where
+they join the new ones, are curiously reminiscent of the irregularities
+of segmentation seen in regenerated structures. The
+value of the considerations adduced in the chapter is, I admit,
+very small. The utmost that can be claimed for them is that
+mechanical segmentations, like those seen in ripple-mark, or
+in Leduc's osmotic growths, show how by the action of a continuous
+force in one direction, repeated and serially homologous
+divisions can be produced having features of similarity common
+to those repetitions by which organic forms and patterns are
+characterised. The analogy supplies a vicarious picture of the
+phenomena which in default of one more true may in a slight
+degree assist our thoughts. It suggests that the rhythms of
+segmentation may be the consequence of a single force definite
+in direction and continuously acting during the time of growth.
+The polarity of the organism would thus be the expression of
+the fact that this meristic force is definitely directed after it has
+once been excited, and the reversal seen in some products of regeneration
+suggest further that it is capable of being reflected.
+This polarity cannot be a property of the material, as such,
+but is determined by a force acting on that material, just as the
+polarity of a magnet is not determined by the arrangement of its
+particles, but by the direction in which the current flows.</p>
+
+<p class="indent">To some it may appear that even to embark on such discussions
+as this is to enter into a perilous flirtation with vitalistic theories.
+How, they may ask, can any force competent to produce chemical
+and geometrical differentiation in the body be distinguished
+from the "Entelechy" of Driesch? Let me admit that in this
+reflexion there is one element of truth. If those who proclaim
+a vitalistic faith intend thereby to affirm that in the processes
+by which growth and division are effected in the body, a part is
+played by an orderly force which we cannot <i>now</i> translate into
+terms of any known mechanics, what observant man is not a
+vitalist? Driesch's first volume, putting as it does into intelligible
+language that positive deduction from the facts&mdash;especially
+of regeneration&mdash;should carry a vivid realisation of this
+truth to any mind. If after their existence is realised, it is
+<span class="pagenum"><a name="Page_81" id="Page_81">[Pg 81]</a></span>
+desired that these unknown forces of order should have a name,
+and the word entelechy is proposed, the only objection I have to
+make is that the adoption of a term from Aristotelian philosophy
+carries a plain hint that we propose to relegate the future study
+of the problem to metaphysic.</p>
+
+<p class="indent">From this implication the vitalist does not shrink.
+But I cannot find in the facts yet known to us any justification of so
+hopeless a course. It was but yesterday that the study of
+<i>Entwicklungsmechanik</i> was begun, and if in our slight survey
+we have not yet seen how the living machine is to be expressed
+in terms of natural knowledge that is poor cause for despair.
+Driesch sums up his argument thus:<a name="FNanchor_13_49" id="FNanchor_13_49"></a><a href="#Footnote_13_49" class="fnanchor">[13]</a>
+</p>
+
+<p class="blockquot">"It seems to me that there is only one conclusion possible.
+If we are going to explain what happens in our harmonious-equipotential
+systems by the aid of causality based upon the
+constellation of single chemical factors and events, there <i>must</i>
+be some such thing as a machine. Now the assumption of the
+existence of a machine proves to be absolutely absurd in the
+light of the experimental facts. <i>Therefore there can be neither
+any sort of a machine nor any sort of causality based upon constellation
+underlying the differentiation of harmonious-equipotential systems.</i>"</p>
+
+<p class="blockquot">"For a machine, typical with regard to the three chief dimensions
+of space, cannot remain itself if you remove parts of it
+or if you rearrange its parts at will."</p>
+
+<p>To the last clause a note is added as follows:</p>
+
+<p class="blockquot">"The pressure experiments and the dislocation experiments
+come into account here; for the sake of simplicity they have
+not been alluded to in the main line of our argument."</p>
+
+<p class="indent">I doubt whether any man has sufficient knowledge of
+all possible machines to give reality to this statement. In spite also
+of the astonishing results of experiments in dislocation, doubt
+may further be expressed as to whether they have been tried in
+such variety or on such a scale as to justify the suggestion that
+the living organism remains itself if its parts are rearranged at
+<span class="pagenum"><a name="Page_82" id="Page_82">[Pg 82]</a></span>
+will. All we know is that it can "remain itself" when much is
+removed, and when much rearrangement has been affected,
+which is a different thing altogether.</p>
+
+<p class="indent">I scarcely like to venture into a region of which my
+ignorance is so profound, but remembering the powers of eddies to re-form
+after partial obliteration or disturbance, I almost wonder whether
+they are not essentially machines which remain themselves
+when parts of them are removed.</p>
+
+<p class="indent">Real progress in this most obscure province is not likely
+to be made till it attracts the attention of physicists; and though they
+for long may have to forego the application of exact quantitative
+methods, I confidently anticipate that careful comparison
+between the phenomena of repetition formed in living organisms
+and the various kinds of segmentation produced by mechanical
+agencies would be productive of illuminating discoveries.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_83" id="Page_83">[Pg 83]</a></span></p>
+
+<h2>CHAPTER IV</h2>
+<h3><span class="smcap">The Classification Of Variation And<br />The Nature Of Substantive Factors</span></h3>
+
+<p class="indent">We have now seen that among the normal physiological
+processes the phenomena of division form a recognisable, and
+in all likelihood a naturally distinct group. Variations in these
+respects may thus be regarded as constituting a special class
+among variations in general.</p>
+
+<p class="indent">The substantive variations have only one property in
+common&mdash;the negative one that they are not Meristic. The work
+of classifying them and distinguishing them according to their
+several types demands a knowledge of the chemistry of life far
+higher than that to which science has yet attained. In reference
+to some of the simplest variations Garrod has introduced the
+appropriate term "Chemical sports." The condition in man
+known as Alkaptonuria in which the urine is red is due especially
+to the absence of the enzyme which decomposes the excretory
+substance, alkapton. The "chemical sport" here consists in the
+inability to break up the benzene ring. The chemical feature
+which distinguishes and is the proximate cause of several colour-varieties
+can now in a few cases be declared. The work of Miss
+Wheldale has shown that colour-varieties may be produced by
+the absence of the chromogen compound the oxidation of which
+gives rise to sap-colours, by differences in the completeness of
+this process of oxidation, and by a process of reduction supervening
+on or perhaps suppressing the oxidation. Some of these
+processes moreover may be brought about by the combined action
+of two bodies, the one an enzyme, for example an oxygenase, and
+the other a substance regarded as a peroxide, contributing the
+oxygen necessary for the oxidation to take place. Variation in
+colour may thus be brought about by the addition or omission
+of any one of the bodies concerned in the action.
+<span class="pagenum"><a name="Page_84" id="Page_84">[Pg 84]</a></span></p>
+
+<p class="indent">Similar variations, or rather similar series of variations will
+undoubtedly hereafter be identified in reference to all the various
+kinds of chemical processes upon which the structure and functions
+of living things depend. The identification of these processes
+and of the bodies concerned in them will lead to a real
+classification of Substantive Variations.</p>
+
+<p class="indent">To forecast the lines on which such classification
+will proceed is to look too far ahead. We may nevertheless anticipate
+with some confidence that future analysis will recognise among the
+contributing elements, some which are intrinsic and inalienable,
+and others which are extrinsic and superadded.</p>
+
+<p class="indent">We already know that there may be such interdependence
+among the substantive characters that to disentangle them will
+be a work of extreme difficulty. The mere fact that in our
+estimation characters belong to distinct physiological systems is
+no proof of their actual independence. In illustration may be
+mentioned the sap-colour in Stocks and the development of
+hoariness on the leaves and stems, which Miss Saunders's experiments
+have shown to be intimately connected, so that in certain
+varieties no hoariness is produced unless the elements for sap-colour
+are already present in the individual plant.</p>
+
+<p class="indent">The first step in the classification of substantive
+variations is therefore to determine which are due to the addition of
+new elements or factors, and which are produced by the omission of
+old ones. <i>A priori</i> there is no valid criterion by which this can
+be known, and actual experiments in analytical breeding can
+alone provide the knowledge required. Some very curious results
+have by this method been obtained, which throw an altogether
+unexpected light on these problems. For example, in order that
+the remarkable development of mesoblastic black pigment characteristic
+of the Silky Fowl should be developed, it is practically
+certain that two distinct variations from such a type as <i>Gallus
+bankiva</i> must have occurred. I assume, as is reasonable, that
+<i>G. bankiva</i> has genetic properties similar to those of the Brown
+Leghorn breed which has been used in the experiments which Mr.
+Punnett and I have conducted. <i>Gallus bankiva</i> was not available
+but the Brown Leghorn agrees with it very closely in colouration,
+and probably in the general physiology of its pigmentation.
+<span class="pagenum"><a name="Page_85" id="Page_85">[Pg 85]</a></span>
+Setting aside the various structural differences between the two
+breeds, the Silky is immediately distinguished from the Leghorn
+by the fact that the skin of the whole body including that of the
+face and comb appears to be of a deep purplish colour. The
+face and comb of the Leghorn are red and the skin of the body
+is whitish yellow. On examination it is found that the purple
+colour of the Silky is in reality due to the distribution of a deep
+black pigment in the mesoblastic membranes throughout the
+body. The somatopleura, the pleura, <i>pia mater</i>, the dermis, and
+in most organs the connective tissue and the sheaths of the blood-vessels,
+are thus impregnated with black. No such pigmentation
+exists in the Leghorn. As the result of an elaborate series of
+experimental matings we have proved that the distinction between
+the Leghorn and the Silky consists primarily in the fact
+that the Silky possesses a pigment-producing factor, <i>P</i>, which is
+not present in the Leghorn.</p>
+
+<p class="indent">This variation must undoubtedly have been one of <i>addition</i>.
+But besides this there is another difference of an altogether dissimilar
+nature; for the Brown Leghorn possesses a factor which
+has the power of partially or completely restricting the operation
+of the pigment-producing factor, <i>P</i>. Moreover in respect of
+this pigment-restricting factor which we may call <i>D</i>, the sexes
+of the Brown Leghorn differ, for the male is homozygous or <i>DD</i>,
+but the female is heterozygous, <i>Dd</i>. Thus in order that the
+black-skinned breed could be evolved from such a type as a
+Brown Leghorn it must be necessary <i>both</i> that <i>P</i> should be added
+and that <i>D</i> should drop out. We have not the faintest conception
+of the process by which either of these events have come to
+pass, but there is no reasonable doubt that in the evolution of
+the Silky fowl they did actually happen.</p>
+
+<p class="indent">We may anticipate that numerous interdependences of this
+kind will be discovered.</p>
+
+<p class="indent">Before any indisputable progress can be made with
+the problem of evolution it is necessary that we should acquire some
+real knowledge of the genesis of that class of phenomena which
+formed the subject of the last chapter. So long as the process
+of division remains entirely mysterious we can form no conception
+<span class="pagenum"><a name="Page_86" id="Page_86">[Pg 86]</a></span>
+even of the haziest sort as to the nature of living organisms, or
+of the proximate causes which determine their forms, still less
+can we attempt any answer to those remoter questions of origin
+and destiny which form the subject of the philosopher's contemplation.
+It is in no spirit of dogmatism that I have ventured
+to indicate the direction in which I look for a solution, though
+I have none to offer. It may well be that before any solution
+is attained, our knowledge of the nature of unorganised matter
+must first be increased. For a long time yet we may have to
+halt, but we none the less do well to prepare ourselves to utilise
+any means of advance that may be offered, by carefully reconnoitering
+the ground we have to traverse. The real difficulty
+which blocks our progress is ignorance of the nature of division,
+or to use the more general term, of repetition.</p>
+
+<p class="indent">Let us turn to the more familiar problem of the
+causes of variation. Now since variation consists as much in meristic
+change as in alteration in substance or material, there is one
+great range of problems of causation from which we are as yet
+entirely cut off. We know nothing of the causation of division,
+and we have scarcely an observation, experiment or surmise
+touching the causes by which the meristic processes may be altered.</p>
+
+<p class="indent">Of the way in which variations in the substantive
+composition of organisms are caused we have almost as little real evidence,
+but we are beginning to know in what such variations must consist.
+These changes must occur either by the addition or loss of factors.</p>
+
+<p class="indent">We must not lose sight of the fact that though the
+factors operate by the production of enzymes, of bodies on which these
+enzymes can act, and of intermediary substances necessary to
+complete the enzyme-action, yet these bodies themselves can
+scarcely be themselves genetic factors, but consequences of their
+existence. What then are the factors themselves? Whence do
+they come? How do they become integral parts of the organism?
+Whence, for example, came the power which is present in a White
+Leghorn of destroying&mdash;probably reducing&mdash;the pigment in its
+feathers? That power is now a definite possession of the breed,
+<span class="pagenum"><a name="Page_87" id="Page_87">[Pg 87]</a></span>
+present in all its germ-cells, male and female, taking part in their
+symmetrical divisions, and passed on equally to all as much as
+is the protoplasm or any other attribute of the breed. From the
+body of the bird the critical and efficient substance could in all
+likelihood be isolated by suitable means, just as the glycogen
+of the liver can be. But even when this extraction has been
+accomplished and the reducing body isolated, we shall know no
+more than we did before respecting the mode by which the power
+to produce it was conferred on the fowl, any more than we know
+how the walls of its blood-vessels acquired the power to form a
+fibrin-ferment.</p>
+
+<p class="indent">It is when the scope of such considerations as this are
+fully grasped that we realise the fatuousness of the conventional treatment
+which the problem of the causes of variation commonly
+receives. Environmental change, chemical injury, differences in
+food supply, in temperature, in moisture, or the like have been
+proposed as "causes." Admitting as we must do, that changes
+may be produced&mdash;usually inhibitions of development&mdash;by
+subjecting living things to changes in these respects, how can we
+suppose it in the smallest degree likely that very precise, new,
+and adaptative powers can be conferred on the germs by such
+treatment? Reports of positive genetic consequences observed
+comparable with those I have mentioned, become from time to
+time current. We should I think regard them with the gravest
+doubt. Few, so far as I am aware, have ever been confirmed,
+though clear and repeated confirmation should be demanded
+before we suffer ourselves at all to build upon such evidence.
+In a subsequent chapter some of these cases will be considered in
+detail.</p>
+
+<p class="indent">In no class of cases would the transmission of an acquired
+character superficially appear so probable as in those where power
+of resisting the attack of a pathogenic organism is acquired in
+the lifetime of the zygote. The possession of such a power is
+moreover a distinction comparable with those which differentiate
+varieties and species. It is due to the development in the blood
+of specific substances which pervade the whole fluid. This
+development is exactly one of those "appropriate responses to
+<span class="pagenum"><a name="Page_88" id="Page_88">[Pg 88]</a></span>
+stimuli" which naturalists who incline to regard adaptation as
+a direct consequence of an environmental influence might most
+readily invoke as an illustration of their views. And yet all
+evidence is definitely unfavourable to the suggestion of an
+inheritance of the acquired power of resistance. Such change as
+can be perceived in the virulence of the attacks on successive
+generations may be most easily regarded as due to the extermination
+of the more susceptible strains, and perhaps in some
+measure to variation in the invading organisms themselves, an
+"acquired character" of quite different import.</p>
+
+<p class="indent">The specific "anti-body" may have been produced in
+response to the stimulus of disease, but the power to produce it
+without this special stimulus is not included in the germ-cells
+any more than a pigment. All that they bear is the <i>power to
+produce</i> the anti-bodies when the stimulus is applied.</p>
+
+<p class="indent">If we could conceive of an organism like one of
+those to which disease may be due becoming actually incorporated with
+the system of its host, so as to form a constituent of its germ-cells
+and to take part in the symmetry of their divisions, we should
+have something analogous to the case of a species which acquires
+a new factor and emits a dominant variety. When we see the
+phenomenon in this light we realise the obscurity of the problem.
+The appearance of recessive varieties is comparatively easy to
+understand. All that is implied is the omission of a constituent.
+How precisely the omission is effected we cannot suggest, but
+it is not very difficult to suppose that by some mechanical fault
+of cell-division a power may be lost. Such variation by unpacking,
+or analysis of a previously existing complex, though unaccountable,
+is not inconceivable. But whence come the new dominants?
+Whether we imagine that they are created by some
+rearrangement or other change internal to the organism, or
+whether we try to conceive them as due to the assumption of
+something from without we are confronted by equally hopeless
+difficulty.</p>
+
+<p class="indent">The mystery of the origin of a dominant increases when
+it is realised that there is scarcely any recent and authentic account
+of such an event occurring under critical observation, which can
+<span class="pagenum"><a name="Page_89" id="Page_89">[Pg 89]</a></span>
+be taken as a basis for discussion. The literature of horticulture
+for example abounds in cases alleged, but I do not think anyone
+can produce an illustration quite free from doubt. Such evidence
+is usually open to the suspicion that the plant was either introduced
+by some accident, or that it arose from a cross with a pre-existing
+dominant, or that it owed its origin to the meeting of
+complementary factors. In medical literature almost alone however,
+there are numerous records of the spontaneous origin of
+various abnormal conditions in man which habitually behave
+as dominants, and of the authenticity of some of these there
+can be no doubt.</p>
+
+<p class="indent">When we know that such conditions as hereditary
+cataract or various deformities of the fingers behave as dominants,
+we recognize that those conditions must be due to the addition of
+some element to the constitution of the normal man. In the
+collections of pedigrees relating to such pathological dominants
+there are usually to be found alleged instances of the origin of the
+condition <i>de novo</i>. Not only do these records occur with such
+frequency that they cannot be readily set aside as errors, but from
+general considerations it must be obvious that as these malformations
+are not common to normal humanity they must at
+some moment of time have been introduced. The lay reader
+may not be so much impressed with the difficulty as we are. He
+is accustomed to regard the origin of <i>any</i> new character as
+equally mysterious, but when once dominants are distinguished from
+recessives the problem wears a new aspect. Thus the appearance
+of high artistic gifts, whether as an attribute of a race or as a
+sporadic event among the children of parents destitute of such
+faculties, is not very surprising, for we feel fairly sure that the
+faculty is a recessive, due to the loss of a controlling or inhibiting
+factor; but the <i>de novo</i> origin of brachydactylous fingers in a
+child of normal parents is of quite a different nature, and must
+indicate the action of some new specific cause.</p>
+
+<p class="indent">Whether such evidence is applicable to the general
+problem of evolution may with some plausibility be questioned; but
+there is an obvious significance in the fact that it is among these
+pathological occurrences that we meet with phenomena most
+<span class="pagenum"><a name="Page_90" id="Page_90">[Pg 90]</a></span>
+nearly resembling the spontaneous origin of dominant factors,
+and I cannot see such pedigrees as these without recalling Virchow's
+aphorism that every variation owes its origin to some
+pathological accident. In the evolution of domestic poultry,
+if <i>Gallus bankiva</i> be indeed the parent form of all our breeds,
+at least some half dozen new factors must have been added
+during the process. In <i>bankiva</i> there is, for example, no factor
+for rose comb, pea comb, barring on the feathers, or for the
+various dominant types of dark plumage. Whence came all
+these? It is, I think, by no means impossible that some other
+wild species now extinct did take part in the constitution of
+domestic poultry. It seems indeed to me improbable that the
+heavy breeds descend from <i>bankiva</i>. Both in regard to domestic
+races of fowls, pigeons, and some other forms, the belief in origin
+within the period of human civilization from one simple primitive
+wild type seems on a balance of probabilities insecurely
+founded, but allowing something for multiplicity of origin we
+still fall far short of the requisite total of factors. Elements
+exist in our domesticated breeds which we may feel with confidence
+have come in since their captivity began. Such elements
+in fowls are dominant whiteness, extra toe, feathered
+leg, frizzling, etc., so that even hypothetical extension of the
+range of origin is only a slight alleviation of the difficulty.</p>
+
+<p class="indent">Somehow or other, therefore, we must recognize that
+dominant factors do arise. Whether they are created by internal change,
+or whether, as seems to me not wholly beyond possibility, they
+obtain entrance from without, there is no evidence to show.
+If they were proved to enter from without, like pathogenic
+organisms, we should have to account for the extraordinary
+fact that they are distributed with fair constancy to half the
+gametes of the heterozygote.</p>
+
+<p class="indent">In proportion as the nature of dominants grows more
+clear so does it become increasingly difficult to make any plausible
+suggestion as to their possible derivation. On the other hand
+the origin of a recessive variety by the loss of a factor is a process
+so readily imagined that our wonder is rather that the phenomenon
+is not observed far more often. Some slip in the accurate
+<span class="pagenum"><a name="Page_91" id="Page_91">[Pg 91]</a></span>
+working of the mechanical process of division, and a factor
+gets left out, the loss being attested by the appearance of a
+recessive variety in some subsequent generation.</p>
+
+<p class="indent">Consistently with this presentation of the facts we find
+that, as in our domesticated animals and plants, a diversity of recessives
+may appear within a moderately short period, and that when
+variations come they often do not come alone. Witness the
+cultural history of the Sweet Pea, <i>Primula Sinensis</i>, <i>Primula
+obconica</i>, <i>Nemesia strumosa</i> and many such examples in which
+variation when it did come was abundant. The fact cannot
+be too often emphasized that in the vast proportion of these
+examples of substantive variation under domestication, as well
+as of substantive variation in the natural state, the change
+has come about by omission, not by addition. To take, for
+example, the case of the Potato, in which so many spontaneous
+bud-variations have been recorded, East after a careful study
+of the evidence has lately declared his belief that all are of this
+nature, and the opinion might be extended to many other groups
+of cases whether of bud or seminal variation. Morgan draws
+the same conclusion in reference to the many varieties he has
+studied in <i>Drosophila</i>.</p>
+
+<p class="indent">In the Sweet Pea, a form which is beyond suspicion
+of having been crossed with anything else, and has certainly produced
+all the multitude of types which we now possess by variations
+from one wild species, there is only one character of the modern
+types which could, with any plausibility, be referred to a factor
+not originally forming part of the constituents of the wild species.
+This is the waved edge, so characteristic of the "Spencer"
+varieties; for the cross between a smooth-edged and a waved
+type gives an intermediate not unfrequently. Nevertheless
+there is practically no doubt that this is merely an imperfection
+in the dominance of the smooth edge, and we may feel sure
+that any plant homozygous for smooth edge would show no wave
+at all. Hence it is quite possible that even the appearance of
+the original waved type, Countess Spencer, was due to the loss
+of one of the factors for smooth edge at some time in the history
+of the Sweet Pea.
+<span class="pagenum"><a name="Page_92" id="Page_92">[Pg 92]</a></span></p>
+
+<p class="indent">In the case of the Chinese Primrose (<i>Primula Sinensis</i>)
+one dominant factor has been introduced in modern times, probably
+within the last six years at most. This is the factor which
+causes suppression of the yellow eye, giving rise to the curious
+type known as "Queen Alexandra." Mr. R. P. Gregory's
+experiments proved that this was a very definite dominant, and
+the element responsible for this development is undoubtedly an
+addition to the original ingredient-properties, with which the
+species was endowed. Unfortunately, as happens in almost every
+case of the kind, the origin of this important novelty appears
+to be lost. Its behaviour, however, when crossed with various
+other types is that of a simple dominant giving an ordinary 3:1
+ratio. There is therefore no real doubt that it came into existence
+by the definite addition of a new factor, for if it was simply
+a case of the appearance of a new character made by combination
+of two previously existing complementary factors we should
+expect that when Queen Alexandra was self-fertilised a 9:7 ratio
+would be a fairly common result, which is not in practice found.</p>
+
+<p class="indent">In <i>Oenothera</i> Gates<a name="FNanchor_1_50" id="FNanchor_1_50"></a><a href="#Footnote_1_50" class="fnanchor">[1]</a>
+has observed the appearance, in a large sowing of
+about 1,000 <i>Oenothera rubrinervis</i>, of a single individual
+having considerably more red pigment in the calyx than is usual
+in <i>rubrinervis</i>. The whole of the hypanthium in the flowers of
+this plant was red instead of green as in <i>rubrinervis</i>, and the
+whole of the sepals were red in the bud-stage, except for small
+green areas at the base. This type behaved as a dominant over
+<i>rubrinervis</i>, but so far a pure-breeding individual was not found.
+Admittedly the variation of this plant from the type of <i>rubrinervis</i>
+can be represented as one of degree, though there is a very
+sensible gap in the series between the new form which Gates
+names "<i>rubricalyx</i>" and the reddest <i>rubrinervis</i> seen in his
+cultures. It must certainly be recognised as a new dominant.
+Gates, rightly as I consider, regards the distinction between
+<i>rubrinervis</i> and <i>rubricalyx</i> as a quantitative one, and the same
+remark applies to certain other types differing in the amount of
+anthocyanin which they produce. I do not understand the argument
+<span class="pagenum"><a name="Page_93" id="Page_93">[Pg 93]</a></span>
+which Gates introduces to the effect that the difference
+between such quantitative types cannot be represented in terms
+of presence and absence. We are quite accustomed to the fact
+that in the rabbit self-colour segregates from the Dutch-marked
+type. These two types differ in a manner which we may reasonably
+regard as quantitative. It is no doubt possible that the
+self-coloured type contains an ingredient which enables the colour
+to spread over the whole body, but it is, I think, perhaps more
+easy to regard the Dutch type as a form from which a part of
+the colour is absent. It may be spoken of in terms I have used,
+as a <i>subtraction-stage</i> in colour. Following a similar method we
+may regard <i>rubricalyx</i> as an addition-stage in colour-variation.
+The fact that crosses between <i>rubrinervis</i>, or <i>rubricalyx</i> and
+<i>Lamarckiana</i> give a mixture of types in F<sub>1</sub>, does not I
+think show, as Gates declares, that there is any system here at work to
+which a factorial or Mendelian analysis does not apply; but that
+question may be more fitly discussed in connexion with the other
+problems raised by the behaviour of <i>Oenothera</i> species in their crosses.</p>
+
+<p class="indent">I do, however, feel that, interesting as this case must
+be admitted to be, we cannot quite satisfactorily discuss it as an
+illustration of the <i>de novo</i> origin of a dominant factor. The
+difference between the novelty and the type is quantitative, and
+it is not unreasonable to think of such a difference being brought
+about by some "pathological accident" in a cell-division.</p>
+
+<p class="indent">Recognition of the distinction between dominant and
+recessive characters has, it must be conceded, created a very serious
+obstacle in the way of any rational and concrete theory of evolution.
+While variations of all kinds could be regarded as manifestations
+of some mysterious instability of organisms this difficulty
+did not occur to the mind of evolutionists. To most of
+those who have taken part in genetic analysis it has become a
+permanent and continual obsession. With regard to the origin
+of recessive variations, there is, as we have seen, no special
+difficulty. They are negative and are due to absences, but as
+soon as it is understood that dominants are caused by an addition
+we are completely at a loss to account for their origin, for we
+<span class="pagenum"><a name="Page_94" id="Page_94">[Pg 94]</a></span>
+cannot surmise any source from which they may have been
+derived. Just as when typhoid fever breaks out in his district
+the medical officer of health knows for certain that the bacillus of
+typhoid fever has by some means been brought into that district
+so do we know that when first dominant white fowls arose in the
+evolution of the domestic breeds, by some means the factor for
+dominant whiteness got into a bird, or into at least one of its
+germ-cells. Whence it came we cannot surmise.</p>
+
+<p class="indent">Whether we look to the outer world or to some rearrangement
+within the organism itself, the prospect of finding a source of
+such new elements is equally hopeless.</p>
+
+<p class="indent">Leaving this fundamental question aside as one which
+it is as yet quite unprofitable to discuss, we are on safe ground in
+foreseeing that the future classification of substantive variations,
+which genetic research must before long make possible, will be
+based on a reference to the modes of action of the several factors.
+Some will be seen to produce their effects by oxidation, some by
+reduction, some by generating substances of various types,
+sugars, enzymes, activators, and so forth. It may thus be
+anticipated that the relation of varieties to each other and to
+types from which they are derived will be expressible in terms
+of definite synthetical formulae. Clearly it will not for an indefinite
+time be possible to do this in practice for more than a
+few species and for characters especially amenable to experimental
+tests, but as soon as the applicability of such treatment
+is generally understood the influence on systematics must be
+immediate and profound, for the nature of the problem will at
+length be clear and, though the ideal may be unattainable, its
+significance cannot be gainsaid.</p>
+
+<hr class="tab" />
+<p class="blockquot"><b><i>Note.</i></b>&mdash;With hesitation I allow this
+chapter to appear in the form in which it was printed a year ago, but in
+passing it for the press after that interval I feel it necessary to call
+attention to a possible line of argument not hitherto introduced.</p>
+<hr class="tab" />
+
+<p class="indent">In all our discussions we have felt justified in declaring
+that the dominance of any character indicates that some factor is
+<span class="pagenum"><a name="Page_95" id="Page_95">[Pg 95]</a></span>
+present which is responsible for the production of that character.
+Where there is no definite dominance and the heterozygote is
+of an intermediate nature we should be unable to declare on
+which side the factor concerned was present and from which side
+it was absent. The degree of dominance becomes thus the
+deciding criterion by which we distinguish the existence of factors.
+But it should be clearly realized that in any given case the argument
+can with perfect logic be inverted. We already recognize
+cases in which by the presence of an inhibiting factor a character
+may be suppressed and purely as a matter of symbolical expression
+we might apply the same conception of inhibition to any
+example of factorial influence whatever. For instance we say
+that in as much as two normal persons do not have brachydactylous
+children, there must be some factor in these abnormal persons
+which causes the modification. Our conclusion is based on the
+observed fact that the modification is a dominant. But it may
+be that normal persons are homozygous in respect of some factor
+<i>N</i>, which prevents the appearance of brachydactyly, and that in
+any one heterozygous, <i>Nn</i>, for this inhibiting factor, brachydactyly
+can appear. Similarly the round pea we say contains
+<i>R</i>, a factor which confers this property of roundness, without
+which its seeds would be wrinkled. But here we know that the
+wrinkled seed is in reality one having compound starch-grains,
+and that the heterozygote, though outwardly round enough, is
+intermediate in that starch-character. If we chose to say that
+the compoundness of the grains is due to a factor <i>C</i> and that two
+doses of it are needed to make the seed wrinkled, I know no
+evidence by which such a thesis could be actually refuted. That
+such reasoning is seemingly perverse must be conceded; but
+when we consider the extraordinary difficulties which beset
+any attempt to conceive the mode of origin of a new dominant
+factor, we are bound to remember that there is this other line of
+argument which avoids that difficulty altogether. In the case of
+the "Alexandra"-eye in <i>Primula</i>, or the red calyx in Gates's
+<i>Oenothera</i>, inverting the reasoning adopted in the text, we may
+see that only the <i>Primula</i> homozygous for the yellow eye can
+develop it and that two doses of the factor for the <i>rubrinervis</i>
+calyx are required to prevent that part of the plant from being red.
+<span class="pagenum"><a name="Page_96" id="Page_96">[Pg 96]</a></span></p>
+
+<p class="indent">We may proceed further and extend this mode of reasoning
+to all cases of genetic variation, and thus conceive of all alike as
+due to loss of factors present in the original complex. Until we
+can recognize factors by means more direct than are provided by
+a perception of their effects, this doubt cannot be positively
+removed. For all practical purposes of symbolic expression we
+may still continue to use in our analyses the modes of representation
+hitherto adopted, but we must not, merely on the ground of
+its apparent perversity, refuse to admit that the line of argument
+here indicated may some day prove sound.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_97" id="Page_97">[Pg 97]</a></span></p>
+<h2>CHAPTER V</h2>
+<h3><span class="smcap">The Mutation Theory</span></h3>
+
+<p class="indent">When with the thoughts suggested in the last chapter
+we contemplate the problem of Evolution at large the hope at the
+present time of constructing even a mental picture of that process
+grows weak almost to the point of vanishing. We are left
+wondering that so lately men in general, whether scientific or
+lay, were so easily satisfied. Our satisfaction, as we now see,
+was chiefly founded on ignorance.</p>
+
+<p class="indent">Every specific evolutionary change must represent
+a definite event in the construction of the living complex. That event
+may be a disturbance in the meristic system, showing itself in
+a change in the frequency of the repetitions or in the distribution
+of differentiation among them, or again it may be a chemical
+change, adding or removing some factor from the sum total.</p>
+
+<p class="indent">If an attempt be made to apply these conceptions to an
+actual series of allied species the complexity of the problem is such that
+the mind is appalled. Ideas which in the abstract are apprehended
+and accepted with facility fade away before the concrete
+case. It is easy to imagine how Man was evolved from an
+<i>Amoeba</i>, but we cannot form a plausible guess as to how <i>Veronica
+agrestis</i> and <i>Veronica polita</i> were evolved, either one from the
+other, or both from a common form. We have not even an
+inkling of the steps by which a Silver Wyandotte fowl descended
+from <i>Gallus Bankiva</i>, and we can scarcely even believe that it
+did. The Wyandotte has its enormous size, its rose comb, its
+silver lacing, its tame spirit, and its high egg production. The
+tameness and the high egg production are probably enough both
+recessives, and though we cannot guess how the corresponding
+dominant factors have got lost, it is not very difficult to imagine
+that they were lost somehow. But the rose comb and the silver
+colour are <i>dominants</i>. The heavy weight also appears in the
+crosses with Leghorns, but we need not at once conclude that it
+<span class="pagenum"><a name="Page_98" id="Page_98">[Pg 98]</a></span>
+depends on a simple dominant factor, because the big size of
+the crosses may be a consequence of the cross and may depend on
+other elements.</p>
+
+<p class="indent">Now no wild fowl known to us has these qualities.
+May we suppose that some extinct wild species had them? If so, may
+we again make the same supposition in all similar cases? To do
+so is little gain, for we are left with the further problem, whence
+did those lost wild species acquire those dominants? Suppositions
+of this kind help no more than did the once famous
+conjecture as to the origin of living things&mdash;that perhaps they
+came to earth on a meteorite. The unpacking of an original
+complex, the loss of various elements, and the recombination of
+pre-existing materials may all be invoked as sources of specific
+diversity. Undoubtedly the range of possibilities thus opened
+up is large. It will even cover an immense number of actual
+examples which in practice pass as illustrations of specific
+distinction. The Indian Rock pigeon which has a blue rump
+may quite reasonably be regarded as a geographically separated
+recessive form of our own <i>Columba livia</i>, for as Staples-Browne
+has shown the white rump of <i>livia</i> is due to a dominant factor.
+The various degrees to which the leaves of Indian Cottons are
+incised have, as Leake says, been freely used as a means of
+classification. The diversities thus caused are very remarkable,
+and when taken together with diversities in habit, whether
+sympodial or monopodial, the various combinations of points
+of difference are sufficiently distinctive to justify any botanist
+in making a considerable number of species by reference to them
+alone. Nevertheless Leake's work goes far to prove that all of
+these forms represent the re-combinations of a very small number
+of factors. The classical example of <i>Primula Sinensis</i> and its
+multiform races is in fact for a long way a true guide as to the
+actual interrelations of the species which systematists have
+made. That they did make them was due to no mistake in
+judgment or in principle, but simply to the want of that extended
+knowledge of the physiological nature of the specific
+cases which we now know to be a prime necessity.
+<span class="pagenum"><a name="Page_99" id="Page_99">[Pg 99]</a></span></p>
+
+<p class="indent">But will such analysis cover all or even most of the
+ordinary cases of specific diversity between near allies? Postponing the
+problem of the interrelations of the larger divisions as altogether
+beyond present comprehension, can we suppose, that in general,
+closely allied species and varieties represent the various consequences
+of the presence or absence of allelomorphic factors
+in their several combinations? The difficulty in making a
+positive answer lies in the fact that in most of the examples in
+which it has been possible to institute breeding experiments with
+a view to testing the question, a greater or less sterility is
+encountered. Where, however, no such sterility is met with, as
+for instance in the crosses made by E. Baur among the species
+of <i>Antirrhinum</i> there is every reason to think that the whole
+mass of differences can and will eventually be expressed in terms
+of ordinary Mendelian factors. Baur has for example crossed
+species so unlike as <i>Antirrhinum majus</i> and <i>molle</i>, forms differing
+from each other in almost every feature of organisation.<a name="FNanchor_1_51" id="FNanchor_1_51"></a><a href="#Footnote_1_51" class="fnanchor">[1]</a>
+The F<sub>2</sub> generation from this cross presents an amazingly motley
+array of types which might easily if met with in nature be described
+as many distinct species. Yet all are fertile and there
+is not the slightest difficulty in believing that they can all be
+reduced to terms of factorial analysis.</p>
+
+<p class="indent">If allowance be made for the complicating effects of
+sterility, is there anything which prevents us from supposing that such
+good species as those of <i>Veronica</i> or of any other genus comprising
+well-defined forms may not be similarly related? I do not know
+any reason which can be pointed to as finally excluding such a
+possibility. Nevertheless it has been urged with some plausibility
+that good species are distinguished by <i>groups</i> of differentiating
+characters, whereas if they were really related as the
+terms of a Mendelian F<sub>2</sub> family are, we should expect to find
+not groups of characters in association, but rather series of forms
+corresponding to the presence and absence of the integral factors
+composing the groups of characters. I am not well enough
+versed in systematic work to be able to decide with confidence
+how much weight should be attached to this consideration. Some
+<span class="pagenum"><a name="Page_100" id="Page_100">[Pg 100]</a></span>
+weight it certainly has, but I cannot yet regard it as forming a
+fatal objection to the application of factorial conceptions on
+the grand scale. It may be recalled that we are no longer under
+any difficulty in supposing that differences of all classes may be
+caused by the presence or absence of factors. It seemed at first
+for example that such characters as those of leaf shape might
+be too subtle and complex to be reducible to a limited number of
+factors. But first the work of Gregory on <i>Primula Sinensis</i>
+showed that several very distinct types of leaves were related
+to each other in the simplest way. In that particular example,
+intermediates are so rare as to be negligible, but subsequently
+Shull dealing with such a complicated example as <i>Capsella</i>, and
+Leake in regard to Cottons, both forms in which intergrades occur
+in abundance, have shown that a simple factorial scheme is
+applicable. We need not therefore, to take an extreme case,
+doubt that if it were possible to examine the various forms of
+fruit seen in the Squashes by really comprehensive breeding
+tests, even this excessive polymorphism in respect of structural
+features would be similarly reducible to factorial order.</p>
+
+<p class="indent">It must always be remembered also that in a vast number
+of cases, nearly allied forms which are distinct, occupy distinct
+ground. Moreover, by whatever of the many available mechanisms
+that end be attained, it is clear that nature very often does
+succeed in preventing intercrossing between distinct forms so
+far that the occurrence of that phenomenon is a rarity under
+natural conditions. The facts may, I think, fairly be summarized
+in the statement that species are on the whole distinct and not
+intergrading, and that the distinctions between them are usually
+such as might be caused by the presence, absence, or inter-combination
+of groups of Mendelian factors; but that they are so caused the evidence
+is not yet sufficient to prove in more than a very few instances.</p>
+
+<p class="indent">The alternative, be it explicitly stated, is not to
+return to the view formerly so widely held, that the distinctions between
+species have arisen by the accumulation of minute or insensible
+differences. The further we proceed with our analyses the more
+inadequate and untenable does that conception of evolutionary
+<span class="pagenum"><a name="Page_101" id="Page_101">[Pg 101]</a></span>
+change become. If the differences between species have not
+come about by the addition or loss of factors one at a time, then
+we must suppose that the changes have been effected by even
+larger steps, and variations including groups of characters, must
+be invoked.</p>
+
+<p class="indent">That changes of this latter order are really those
+by which species arise, is the view with which de Vries has now made
+us familiar by his writings on the Mutation Theory. In so far as
+mutations may consist in meristic changes of many kinds and
+in the loss of factors it is unnecessary to repeat that we have
+abundant evidence of their frequent occurrence. That they may
+also more rarely occur by the addition of a factor we are, I think,
+compelled to believe, though as yet the evidence is almost entirely
+circumstantial rather than direct. The evidence for the
+occurrence of those mutations of higher order, by which new
+species characterized by several distinct features are created,
+is far less strong, and after the best study of the records which
+I have been able to make, I find myself unconvinced. The facts
+alleged appear capable of other interpretations.</p>
+
+<p class="indent">The most famous and best studied examples are of course the
+forms of <i>Oenothera</i> raised by de Vries from <i>Oenothera Lamarckiana</i>
+in circumstances well known to all readers of genetic literature.
+Whatever be the true significance of these extraordinary "mutations"
+there can be no question about the great interest which
+attaches to them, and the historical importance which they will
+long preserve. Apart also from these considerations it is becoming
+more and more evident that in their peculiarities they
+provide illustrations of physiological phenomena of the highest
+consequence in the study of genetics at large.</p>
+
+<p class="indent">De Vries found, as is well known, that <i>Oenothera Lamarckiana</i>
+gives off plants unlike itself. These mutational forms are of
+several distinct and recognizable types which recur, and several
+of them breed true from their first appearance. The obvious
+difficulty, which in my judgment should make us unwilling at
+present to accept these occurrences as proof of the genesis of new
+species by mutation, is that we have as yet no certainty that the
+appearance of the new forms is not an effect of the recombination
+<span class="pagenum"><a name="Page_102" id="Page_102">[Pg 102]</a></span>
+of factors, such as is to be seen in so many generations of plants
+derived from a cross involving many genetic elements. The
+first question is what is <i>Oenothera Lamarckiana</i>? Is it itself a
+plant of hybrid origin? To this fundamental question no satisfactory
+answer has yet been given. All attempts to find it as a
+wild plant in America have failed. It existed in Europe in the
+latter half of the eighteenth century. Whence it came is still
+uncertain, but the view that it came into existence in Europe and
+perhaps in Paris, seems on the whole the most probable. The
+question has been debated by Macdougal, Gates, and Davis.
+From historical sources there is little expectation of further
+light. Those who favour the notion of a hybrid origin look on
+<i>Oenothera biennis</i> as one of the putative parents. It has been
+conjectured that a species called <i>grandiflora</i> lately re-discovered
+on the Alabama river was the other parent. Experiments have
+been instituted by Davis to discover whether <i>Lamarckiana</i> can
+be made artificially by crossing these two species. The results
+so far have shown that while plants approximating in various
+respects to <i>Lamarckiana</i> have thus been produced, none agree
+exactly with that form. Davis, to whom reference should be
+made for a full account of the present state of the enquiry,
+points out that there are many strains of <i>biennis</i> in existence
+and that it is by no means impossible that by using others of
+these strains a still closer approximation can be made. None
+of Davis's artificial productions as yet breed at all true, as
+<i>Lamarckiana</i> on the whole does. In such a case, however, where
+several characters are involved, this is perhaps hardly to be expected.</p>
+
+<p class="indent">One feature of the <i>Oenotheras</i> is very curious.
+Not only <i>Lamarckiana</i>, but all the allied species so far as I am
+aware, have a considerable proportion of bad and shrivelled pollen
+grains. This is undoubtedly true of species living in the wild
+state as well as of those in cultivation. I have had opportunities
+of verifying this for myself in the United States. No one looking
+at the pollen of an <i>Oenothera</i> would doubt that it was taken from
+some hybrid plant exhibiting partial sterility. On the other
+hand, it is difficult to suppose that numbers, perhaps all, of the
+<span class="pagenum"><a name="Page_103" id="Page_103">[Pg 103]</a></span>
+"species" of the genus are really hybrids, and many of them breed
+substantially true. I regard this constant presence of bad
+pollen grains as an indication that the genetic physiology of
+<i>Oenothera</i> is in some way abnormal, and as we shall presently
+see, there are several other signs which point in the same direction.</p>
+
+<p class="indent">Discussion of the whole series of phenomena is rendered
+exceedingly difficult first, by reason of the actual nature of the
+material. The characteristics of many of the types which de Vries
+has named are evasive. A few of these types, for instance, <i>gigas</i>,
+<i>nanella</i>, <i>albida</i>, <i>brevistylis</i>, and perhaps a few more are
+evidently clear enough, but we have as yet no figures and descriptions
+precise enough to enable a reader to appreciate exactly the peculiarities
+of the vast number of forms which have now to be considered in any
+attempt to gain a comprehensive view of the whole mass of facts.
+It is also not in dispute that the forms are susceptible of great
+variations due simply to soil and cultural influences.</p>
+
+<p class="indent">The fact that no Mendelian analysis has yet been found
+applicable to this group of <i>Oenotheras</i> as a whole is perhaps largely
+due to the fact that until recently such analysis has not been
+seriously attempted. Following the system which he had
+adopted before the rediscovery of Mendelism, or at all events,
+before the development of that method of analysis, de Vries has
+freely applied <i>names</i> to special combinations of characters and
+has scarcely ever instituted a factorial analysis. Before we can
+get much further this must be attempted. It may fail, but we
+must know exactly where and how this failure comes about.
+There are several indications that such a recognition of factorial
+characters, could be carried some way. For example, the height,
+the size of the flowers, the crinkling of the leaves, the brittleness
+of the stems, perhaps even the red stripes on stems and fruits,
+and many more, are all characters which may or may not depend
+on distinct factors, but if such characters are really transmitted
+in unresolved groups, the limitations of those groups should be
+carefully determined. The free use of names for the several
+forms, rather than for the characters, has greatly contributed
+to deepen the obscurity which veils the whole subject.
+<span class="pagenum"><a name="Page_104" id="Page_104">[Pg 104]</a></span></p>
+
+<p class="indent">I do not mean to suggest that these <i>Oenotheras</i>
+follow a simple Mendelian system. All that we know of them goes to show
+that there are curious complications involved. One of these, probably
+the most important of all, has lately been recognized by
+de Vries himself, namely, that in certain types the characters
+borne by the female and the male germ-cells of the same plant
+are demonstrably different. There can be little doubt that
+further research will reveal cognate phenomena in many unsuspected
+places. The first example in which such a state of things
+was proved to exist is that of the Stocks investigated by Miss
+Saunders.<a name="FNanchor_2_52" id="FNanchor_2_52"></a><a href="#Footnote_2_52" class="fnanchor">[2]</a>
+By a long course of analysis she succeeded in establishing
+in 1908 the fact that if a plant of <i>Matthiola</i> is of that
+eversporting kind which gives a large proportion of double-flowered
+plants among its offspring (produced by self-fertilisation),
+then the egg-cells of such a plant are mixed in type, but
+the pollen of the same plant is homogeneous. Some of the egg-cells
+have in them the two factors for singleness, but some of
+them are short of one or both of these factors. The pollen-grains,
+however, are all recessives, containing neither of these
+factors. The egg-cells, in other words, are mixed, "singles" and
+"doubles," while the pollen-grains are all "doubles." The same
+is true of the factor differentiating "white," or colourless plastids
+from cream-coloured plastids in <i>Matthiola</i>, the egg-cells being
+mixed "whites" and "creams," while the pollen-grains are all
+"creams," viz: recessives. Later in the same year (1908)
+de Vries<a name="FNanchor_3_53" id="FNanchor_3_53"></a><a href="#Footnote_3_53" class="fnanchor">[3]</a>
+announced a remarkable case which will be discussed
+in detail subsequently. It relates to certain <i>Oenotheras</i> heterozygous
+for dwarfness, in which (p. 113) the ovules were mixed,
+tails and dwarfs, while the pollen is all dwarf.</p>
+
+<p class="indent">Again in <i>Petunia</i> Miss Saunders's<a name="FNanchor_4_54" id="FNanchor_4_54"></a><a href="#Footnote_4_54" class="fnanchor">[4]</a>
+work has shown that a somewhat similar state of things exists, but with
+this remarkable difference, that though the egg-cells are mixed, singles and
+doubles, the pollen-grains are all <i>singles</i>, viz: dominants. All
+the <i>Petunias</i> yet examined have been in this condition, including
+<span class="pagenum"><a name="Page_105" id="Page_105">[Pg 105]</a></span>
+some which in botanic gardens pass for original species. Whether
+actual wild plants from their native habitats are in the same
+state, is not yet known, but it is by no means improbable. The
+case may be compared with that of the moth <i>Abraxas grossulariata</i>
+studied by Doncaster and Raynor, in which the females
+are all heterozygous, or we may almost say "hybrids" of <i>grossulariata</i>
+and the variety <i>lacticolor</i>. Similarly we may say that at
+least garden Petunias are heterozygous in respect of singleness.
+The proof of this is of course that when fertilised with the pollen
+of doubles they throw a mixture of doubles and singles. The
+statements which de Vries has published regarding the behaviour
+of several of the <i>Oenotheras</i> go far to show that they must have
+a somewhat similar organisation. On the present evidence it is
+still quite impossible to construct a coherent scheme which will
+represent all the phenomena in their interrelations, and among
+the facts are several which, as will appear, seem mutually incompatible.
+The first indication that the <i>Oenotheras</i> may have
+either mixed ovules or mixed pollen appears in the fact that
+<i>Lamarckiana</i> and several of its "mutants" used as males, with
+several other forms as females, give a mixed offspring. For
+example, de Vries (1907) found that</p>
+
+<p class="blockquot"><b>
+<i>biennis</i> &#9792;&emsp;×&emsp;<i>Lamarckiana</i> &#9794;<br />
+<i>biennis cruciata</i> &#9792;&emsp;×&emsp;<i>Lamarckiana</i> &#9794;<br />
+<i>muricata</i> &#9792;&emsp;×&emsp;<i>Lamarckiana</i> &#9794;<br />
+<i>biennis</i> &#9792;&emsp;×&emsp;<i>rubrinervis</i> &#9794;<br />
+<i>biennis cruciata</i> &#9792;&emsp;×&emsp;<i>rubrinervis</i> &#9794;<br />
+</b></p>
+
+<p>all give a mixture of two distinct types which he names <i>laeta</i>
+and <i>velutina</i>, consisting of about equal numbers of each. On
+account of the fact that the two forms are produced in association
+de Vries has called these forms "twin hybrids," a designation
+which is not fortunate, seeing that it is impossible to imagine
+that any kind of twinning is concerned in their production. The
+distinction between these two seems to be considerable, <i>laeta</i>
+having leaves broader, bright green in colour, and flat, with
+pollen scanty, while <i>velutina</i> has leaves narrower, grayish green,
+more hairy, and furrow-shaped, with pollen abundant.
+<span class="pagenum"><a name="Page_106" id="Page_106">[Pg 106]</a></span></p>
+
+<p class="indent">We next meet the remarkable fact that these two forms,
+<i>laeta</i> and <i>velutina</i> breed true to their respective types, and do not
+reproduce the parent-types among their offspring resulting from
+self-fertilisation. This statement must be qualified in two
+respects. When <i>muricata</i> &#9794; is fertilised by <i>brevistylis</i> the forms
+<i>laeta</i> and <i>velutina</i> are produced, but each of them subsequently
+throws the short-styled form as a recessive (de Vries, 1907,
+p. 406). It may be remembered that de Vries's previous publications
+had already shown that the short style of <i>brevistylis</i>,
+one of the <i>Lamarckiana</i> "mutants," behaves as a recessive
+habitually (<i>Mutationstheorie</i>, II, p. 178, etc.).</p>
+
+<p class="indent">Also when <i>nanella</i>, the dwarf "mutant" of <i>Lamarckiana</i> is
+used as male on <i>muricata</i> as female, <i>laeta</i> and <i>velutina</i> are produced,
+but one only of these, namely, <i>velutina</i>, subsequently
+throws dwarfs on self-fertilisation. The dwarfs thus thrown are
+said to form about 50 per cent. of the families in which they
+occur (de Vries, 1908, p. 668). The fact that the two forms,
+<i>laeta</i> and <i>velutina</i>, are produced by many matings in which
+<i>Lamarckiana</i> and its mutant <i>rubrinervis</i> are used as males is
+confirmed abundantly by Honing, who has carried out extensive
+researches on the subject. After carefully reading his paper,
+I have failed to understand the main purport of the argument
+respecting the "double nature" of <i>Lamarckiana</i> which he founds
+on these results, but I gather that in some way <i>laeta</i> is shown to
+partake especially of the nature of <i>Lamarckiana</i>, while <i>velutina</i>
+is a form of <i>rubrinervis</i>. The paper contains many records which
+will be of value in subsequent analysis of these forms.</p>
+
+<p class="indent">Before considering the possible meaning of these
+facts we must have in our minds the next and most novel of the recent
+extensions of knowledge as to the genetic properties of the
+<i>Oenotheras</i>. In the previous statement we have been concerned
+with the results of using either <i>Lamarckiana</i> itself or one of its
+"mutants" <i>rubrinervis</i>, <i>brevistylis</i>, or <i>nanella</i> as male, on one of
+the species <i>biennis</i> or <i>muricata</i>. The new experiments relate
+to crosses between the two species <i>biennis</i> and <i>muricata</i> themselves.
+<span class="pagenum"><a name="Page_107" id="Page_107">[Pg 107]</a></span></p>
+
+<p class="indent">De Vries found:</p>
+
+<p class="blockquot">1. That the reciprocal hybrids from these two species differed,
+
+<i>biennis</i> × <i>muricata</i> producing one type of F<sub>1</sub> and <i>muricata</i> ×
+<i>biennis</i> producing another. Each F<sub>1</sub> resembled the father more
+than the mother.</p>
+
+<p class="blockquot">2. That each of the hybrids so produced breeds true on self-fertilisation.</p>
+
+<p class="blockquot">3. That if we speak of the hybrid from <i>biennis</i> × <i>muricata</i>
+as <i>BM</i> and of the reciprocal as <i>MB</i>, then</p>
+
+<p class="center"><b><i>BM</i> × <i>MB</i></b></p>
+
+<p class="blockquot">gives exclusively offspring of <i>biennis</i> type but that</p>
+
+<p class="center"><b><i>MB</i> × <i>BM</i></b></p>
+
+<p class="blockquot">gives exclusively offspring of <i>muricata</i> type.
+Evidently, apart from all controversy as to the significance of the "mutants"
+of <i>Lamarckiana</i>, we have here a series of observations of the first importance.</p>
+
+<p class="indent">The fact that reciprocal crossings give constantly
+distinct results must be taken to indicate that the male and female sides
+of one, if not of both, of the parents are different in respect of
+characters which they bear. This is de Vries's view, and he
+concludes rightly, I think, that the evidence from all the experiments
+shows that both <i>biennis</i> and <i>muricata</i> are in this condition,
+having one set of characters represented in their pollen-grains
+and another in their ovules. The plants breed true, but their
+somatic structures are compounded of the two sets of elements
+which pass into them from their maternal and paternal sides
+respectively. This possibility that species may exist of which
+the males really belong to one form and the females to another,
+is one which it was evident from the first announcement of the
+discovery of Mendelian segregation might be found realised in nature.<a name="FNanchor_5_55" id="FNanchor_5_55"></a><a href="#Footnote_5_55" class="fnanchor">[5]</a>
+</p>
+
+<p class="indent"><i>Oe. biennis</i> and <i>muricata</i> were crossed
+reciprocally with each other and with a number of other species, and the
+behaviour of each, when used as mother, was consistently different from its
+behaviour when used as father. De Vries is evidently justified
+<span class="pagenum"><a name="Page_108" id="Page_108">[Pg 108]</a></span>
+by the results of this series of experiments in stating that the
+"Bild," as he terms it, or composition of the male and female
+sides of these two species, <i>biennis</i> and <i>muricata</i>, are distinct.
+On the evidence before us it is not, however, possible to form a
+perfectly clear idea of each, and until details are published, a
+reader without personal knowledge of the material cannot do
+more than follow the general course of the argument. For fuller
+comprehension a proper analysis of the characters with a clear
+statement of how they are distributed among the several types
+and crosses is absolutely necessary. According to de Vries the
+female of <i>biennis</i> possesses a group of characters which he defines
+as "<i>conica</i>" in allusion to the shape of the flower-buds. Besides
+the conical buds, this group of features includes imperfect
+development of wood, rendering the plant very liable to attacks
+of <i>Botrytis</i>, and comparatively narrow leaves.</p>
+
+<p class="indent">The female of <i>muricata</i> carries a group of features which he
+calls "<i>frigida</i>," and, though this is not quite explicitly stated in a
+definition of that type, it is to be inferred<a name="FNanchor_6_56" id="FNanchor_6_56"></a><a href="#Footnote_6_56" class="fnanchor">[6]</a>
+that its characteristics are regarded as greater height, strong development
+of wood with comparative resistance to <i>Botrytis</i>, and broad leaves.</p>
+
+<p class="indent">The characters borne by the male parts of the two
+species are in general those by which they are outwardly distinguished.
+For example, the leaves of <i>Oe. biennis</i> are comparatively broad
+and are bright green, while those of <i>muricata</i> are much narrower
+and of a glaucous green, and I understand that de Vries regards
+these properties as contributed by the male side in each case and
+to be carried by the male cells of each species. The suggestion
+as regards <i>biennis</i> and <i>muricata</i> comes near the conception often
+expressed by naturalists in former times (<i>e. g.</i>, Linnaeus) and
+not rarely entertained by breeders at the present day, that the
+internal structure is contributed by the mother and the external
+by the father.</p>
+
+<p class="indent">On the other hand, the offspring of each species when
+used as mother is regarded as possessing in the main the features of
+the maternal "Bild," but the matter is naturally complicated
+by the introduction of features from the father's side, and it is
+<span class="pagenum"><a name="Page_109" id="Page_109">[Pg 109]</a></span>
+here especially that the account provided is at present unsatisfactory
+and inconclusive. There seems, however, to be no serious
+doubt that <i>biennis</i> and <i>muricata</i> each in their outward appearance
+exhibit on the whole the features which their pollens respectively
+carry, and that the features borne by their ovules are in many respects distinct.</p>
+
+<p class="indent">The <i>types</i> are thus "hybrids" which breed true.
+The results of intercrossing them each way are again "hybrids" which breed
+true. It will be remembered that on former occasions de Vries
+has formulated a general rule that <i>species</i>-hybrids breed true,
+but that the cross-breds raised by interbreeding <i>varieties</i> do not.
+One of these very cases was quoted<a name="FNanchor_7_57" id="FNanchor_7_57"></a><a href="#Footnote_7_57" class="fnanchor">[7]</a>
+as an illustration of thisprinciple, viz: <i>muricata</i> × <i>biennis</i>.
+The grounds for this general statement have always appeared to me insufficient,
+and with the further knowledge which the new evidence provides we are
+encouraged to hope that when a proper factorial analysis of the
+types is instituted we shall find that the phenomenon of a constant
+hybrid will be readily brought into line with the systems
+of descent already worked out for such cases as that of the Stocks,
+and others already mentioned.</p>
+
+<p class="indent space-below">In further discussion of these facts de Vries makes a suggestion
+which seems to me improbable. Since the egg-cells of <i>muricata</i>,
+for instance, bear a certain group of features which are missing
+on the male side, and conversely the pollen bears features absent
+from the female side, he is inclined to regard the <i>bad pollen grains</i>
+as the bearers of the missing elements of the male side and to
+infer that there must similarly be defective ovules representing
+the missing elements of the female side. No consideration is
+adduced in support of this view beyond the simple fact that the
+characters borne by male and female are dissimilar, whereas
+it would be more in accord with preconception if the same sets
+of combinations were represented in each&mdash;as in a normal
+Mendelian case. There is as yet no instance in which the absence
+of any particular class of gametes has been shown with any
+plausibility to be due to defective viability, though there are, of
+course, cases in which certain classes of zygotes do not survive
+<span class="pagenum"><a name="Page_110" id="Page_110">[Pg 110]</a></span>
+owing to defective constitution (<i>e. g.</i>, the albinos of <i>Antirrhinum</i>
+studied by Baur, and the homozygous yellow mice). I am
+rather inclined to suppose that in these examples of hybrids
+breeding true we shall find a state of things comparable with
+that to which we formerly applied the terms "coupling" and
+"repulsion." In these cases certain of the possible combinations
+of factors occur in the gametic series with special frequency,
+being in excess, while the gametes representing other combinations
+are comparatively few. In a recent paper on these cases
+Professor Punnett and I have shown that these curious results
+vary according to the manner in which the factors are grouped
+in the parents. If <i>A</i> and <i>B</i> are two factors which exhibit these
+phenomena we find that the gametic series of the double heterozygote
+differs according as the combination is made by crossing
+<i>AB × ab</i>, or by crossing <i>AB × aB</i>. In a normal Mendelian case
+the F<sub>1</sub> form, <i>AaBb</i>, produces gametes <i>AB</i>, <i>Ab</i>,
+<i>aB</i>, <i>ab</i>, in equal numbers;
+but in these peculiar cases those gametes which contain</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc"><b>Gametic</b></td>
+      <td class="tdc"><b>series</b></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;&nbsp;<b># of gametes</b></td>
+      <td class="tdr">&nbsp;&nbsp;<b># of zygotes</b></td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr"><b><i>AB</i></b></td>
+      <td class="tdr"><b><i>Ab</i></b></td>
+      <td class="tdr"><b><i>aB</i></b></td>
+      <td class="tdr"><b><i>ab</i></b></td>
+      <td class="tdr"><b>in series</b>&nbsp;&nbsp;</td>
+      <td class="tdr"><b>in series</b>&nbsp;&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">1</td>
+      <td class="tdr">(<i>n</i>-1)</td>
+      <td class="tdr">(<i>n</i>-1)</td>
+      <td class="tdr">1</td>
+      <td class="tdr"><i>2n</i></td>
+      <td class="tdr"><i>4n<sup>2</sup></i></td>
+    </tr><tr>
+      <td class="tdc"><i>Partial repulsion</i></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">1</td>
+      <td class="tdr">31</td>
+      <td class="tdr">31</td>
+      <td class="tdr">1</td>
+      <td class="tdr">64</td>
+      <td class="tdr">4096</td>
+    </tr><tr>
+      <td class="tdc">from zygote</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">1</td>
+      <td class="tdr">15</td>
+      <td class="tdr">15</td>
+      <td class="tdr">1</td>
+      <td class="tdr">32</td>
+      <td class="tdr">1024</td>
+    </tr><tr>
+      <td class="tdc">of form</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">1</td>
+      <td class="tdr">7</td>
+      <td class="tdr">7</td>
+      <td class="tdr">1</td>
+      <td class="tdr">16</td>
+      <td class="tdr">256</td>
+    </tr><tr>
+      <td class="tdc"><b><i>Ab × aB</i></b></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">1</td>
+      <td class="tdr">3</td>
+      <td class="tdr">3</td>
+      <td class="tdr">1</td>
+      <td class="tdr">8</td>
+      <td class="tdr">64</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">4</td>
+      <td class="tdr">16</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">3</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">3</td>
+      <td class="tdr">8</td>
+      <td class="tdr">64</td>
+    </tr><tr>
+      <td class="tdc"><i>Partial coupling</i></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">7</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">7</td>
+      <td class="tdr">16</td>
+      <td class="tdr">256</td>
+    </tr><tr>
+      <td class="tdc">from zygote</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">15</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">15</td>
+      <td class="tdr">32</td>
+      <td class="tdr">1024</td>
+    </tr><tr>
+      <td class="tdc">of form</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">31</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">31</td>
+      <td class="tdr">64</td>
+      <td class="tdr">4096</td>
+    </tr><tr>
+      <td class="tdc"><b><i>AB × ab</i></b></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">63</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">63</td>
+      <td class="tdr">128</td>
+      <td class="tdr">16384</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">(<i>n</i>-1)</td>
+      <td class="tdr">1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;(<i>n</i>-1)</td>
+      <td class="tdr"><i>2n</i></td>
+      <td class="tdr"><i>4n<sup>2</sup></i></td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr"><b>Nature</b></td>
+      <td class="tdc"><b>of</b></td>
+      <td class="tdl"><b>zygotic</b></td>
+      <td class="tdc"><b>series</b></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr"><b><i>AB</i></b></td>
+      <td class="tdr"><b><i>Ab</i></b></td>
+      <td class="tdr"><b><i>aB</i></b></td>
+      <td class="tdr"><b><i>ab</i></b></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc"><i>Partial repulsion</i></td>
+      <td class="tdr"> { </td>
+      <td class="tdr"><i>2n<sup>2</sup></i>+1</td>
+      <td class="tdr"><i>n<sup>2</sup></i>-1</td>
+      <td class="tdr"><i>n<sup>2</sup></i>-1</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">from zygote</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">2049</td>
+      <td class="tdr">1023</td>
+      <td class="tdr">1023</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">of form</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">513</td>
+      <td class="tdr">255</td>
+      <td class="tdr">255</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc"><b><i>Ab × aB</i></b></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">33</td>
+      <td class="tdr">15</td>
+      <td class="tdr">15</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">9</td>
+      <td class="tdr">3</td>
+      <td class="tdr">3</td>
+      <td class="tdr">1</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">41</td>
+      <td class="tdr">7</td>
+      <td class="tdr">7</td>
+      <td class="tdr">9</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc"><i>Partial coupling</i></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">177</td>
+      <td class="tdr">15</td>
+      <td class="tdr">15</td>
+      <td class="tdr">49</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">from zygote</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">737</td>
+      <td class="tdr">31</td>
+      <td class="tdr">31</td>
+      <td class="tdr">225</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">of form</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">3009</td>
+      <td class="tdr">63</td>
+      <td class="tdr">63</td>
+      <td class="tdr">961</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc"><b><i>AB × ab</i></b></td>
+      <td class="tdr"> { </td>
+      <td class="tdr">12161</td>
+      <td class="tdr">127</td>
+      <td class="tdr">127</td>
+      <td class="tdr">3969</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdr">(<i>3n<sup>2</sup></i>-(<i>2n</i>-1)</td>
+      <td class="tdr">(<i>2n</i>-1)</td>
+      <td class="tdr">(<i>2n</i>-1)</td>
+      <td class="tdr">&nbsp;&nbsp;<i>n<sup>2</sup></i>-(<i>2n</i>-1)</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+    </tr>
+ </tbody>
+</table>
+<p class="space-above"><span class="pagenum"><a name="Page_111" id="Page_111">[Pg 111]</a></span>
+the <i>parental combinations</i> are in excess. This excess almost
+certainly follows the system indicated by the accompanying
+table. In the general expressions <i>n</i> is half the number of gametes
+required to express the whole system. Now if we imagine that
+sex-factors are involved with the others concerned in such a relationship
+as this we have a system of distribution approximating
+to that found in <i>biennis</i> and <i>muricata</i>. The difference in
+reciprocals is represented in a not improbable way. It cannot yet
+be said that the rarer terms in the series are formed at all, and
+perhaps they are not. As we pointed out in our discussion of
+these phenomena, the peculiar distribution of factors in these
+cases must be taken to mean that the planes of division at some
+critical stage in the segregation are determined with reference
+to the parental groups of factors, or in other words, that the
+whole system has a polarity, and that the distribution of factors
+with reference to this polarity differs according to the grouping
+of factors in the gametes which united in fertilization to produce
+the plant. Subsequent proliferation of cells representing certain
+combinations would then lead to excess of the gametes bearing
+them. It is on similar lines that I anticipate we shall hereafter
+find the interpretation of the curious facts discovered by de Vries,
+though it is evident that a long course of experiment and analysis
+must be carried through before any certainty is reached. The
+work must be begun by a careful study of the descent of some
+single factor, for example, that causing the broader leaf of
+<i>biennis</i>, and we may hope that the study of <i>Oenothera</i>
+by proper analytical methods will no longer be deferred.</p>
+
+<p class="indent">We have now to return to the relations of <i>laeta</i>
+and <i>velutina</i>. These two forms, it will be remembered are frequently
+produced when <i>Lamarckiana</i> or one of its derivatives is used as male,
+and the most unexpected feature in their behaviour is that <i>both
+breed true as regards their essential characteristics, on
+self-fertilisation</i>. If one only bred true the case might, in view
+of the approximate numerical equality of the two types, be difficult
+to interpret on ordinary lines, but as both breed true it must be
+clear that some quite special system of segregation is at work.
+What this may be cannot be detected on the evidence, but with
+<span class="pagenum"><a name="Page_112" id="Page_112">[Pg 112]</a></span>
+the results from the <i>biennis-muricata</i> experiments before us,
+it is natural to suspect that we may here again have to recognise
+a process of allocation of different factors to the male and female
+sides in <i>laeta</i> and <i>velutina</i>. That some such system is in
+operation becomes the more probable from the new fact which de Vries
+states in describing the group of characters which he calls <i>conica</i>,
+namely that this type is the same as that of <i>velutina</i>.</p>
+
+<p class="indent">There are many collateral observations recorded
+both by de Vries and others which have a bearing on the problems,
+but they do not yet fall into a coherent scheme. For example, we
+cannot yet represent the formation of <i>laeta</i> and <i>velutina</i> from the
+various species fertilised by <i>Lamarckiana</i> &#9794;. That this is not
+due to any special property associated with the pollen of <i>Lamarckiana</i>
+is shown by the fact that a species called <i>Hookeri</i>
+gives <i>laeta</i> and <i>velutina</i> in both its reciprocal crosses with
+<i>Lamarckiana</i> (de Vries, 1909, p. 3), and also by the similar fact that
+<i>Lamarckiana</i> &#9792; fertilised by the pollen of a peculiar race of
+<i>biennis</i> named <i>biennis Chicago</i> throws the same types. Before
+these very complicated phenomena can be usefully discussed
+particulars must be provided as to the individuality of the various
+plants used. This criticism applies to much of the work which
+de Vries has lately published, for, as we now know familiarly,
+plants to which the same name applies can be quite different in
+genetic composition.</p>
+
+<p class="indent">Attention should also be called to one curiously paradoxical
+series of results. When the dwarf "mutant" of <i>Lamarckiana</i>
+which de Vries names "<i>nanella</i>" is used as father on <i>muricata</i>,
+F<sub>1</sub> consists of <i>laeta</i> and <i>velutina</i> in approximately equal numbers.
+Both forms breed true to their special characteristics, but
+<i>velutina</i> throws dwarfs of its own type, while <i>laeta</i> does not
+throw dwarfs. Subsequent investigation of the properties of
+these types has led to some remarkable conclusions, and it was
+in a study of these plants that de Vries first came upon the phenomena
+of dissimilarity between the factors borne by the male
+and female cells of the same plant, a condition which had been
+recently detected in the Stocks as a result of Miss Saunders's
+investigations. The details are very remarkable. We have
+<span class="pagenum"><a name="Page_113" id="Page_113">[Pg 113]</a></span>
+first the fact that <i>muricata</i> &#9792; × dwarf <i>nanella</i> &#9794; gives about
+50 per cent. <i>laeta</i> and about 50 per cent. of <i>velutina</i>.</p>
+
+<p class="indent">As regards <i>Velutina</i> it was shown that:</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc"><b>Talls,</b></td>
+      <td class="tdc"><b>Dwarfs,</b></td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdc"><b>per cent.</b></td>
+      <td class="tdc"><b>per cent.</b></td>
+    </tr><tr>
+      <td class="tdr">1.&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl"><i>Velutina</i> selfed gave</td>
+      <td class="tdc">38</td>
+      <td class="tdc">62</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdl"><i>Velutina</i> &#9792; × dwarf <i>nanella</i> &#9794; gave</td>
+      <td class="tdc">39</td>
+      <td class="tdc">61</td>
+    </tr><tr>
+      <td class="tdr">2.&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdl">&nbsp;&emsp;do.&emsp;&emsp;&nbsp;×&emsp;&emsp;do.&emsp;&emsp;&emsp;&emsp;gave</td>
+      <td class="tdc">49</td>
+      <td class="tdc">51</td>
+    </tr><tr>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdr"> { </td>
+      <td class="tdl">&nbsp;&emsp;do.&emsp;&emsp;&nbsp;×&emsp;&emsp;do. &#9794; derived from <i>velutina</i> gave</td>
+      <td class="tdc">43</td>
+      <td class="tdc">57</td>
+    </tr><tr>
+      <td class="tdr">3.&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">Dwarfs&emsp;&emsp;×&emsp;<i>velutina</i> &#9794; gave</td>
+      <td class="tdc">&mdash;</td>
+      <td class="tdc">all dwarfs</td>
+    </tr>
+ </tbody>
+</table>
+
+<p class="space-above">The three experiments taken together prove, as de Vries
+says, that the ovules of <i>velutina</i> are mixed, talls and dwarfs, and that
+the pollen is all dwarf. The condition is almost the same as
+that of the Stocks. It may be noted also that in the Stocks the
+egg-cells of the "double" type are in excess, being approximately
+9 to 7 of the "single" type, but de Vries regards the two types
+in <i>velutina</i> as probably equal in number. The figures (169:231)
+rather suggest some excess of the recessives, perhaps 9:7, and
+the point would be worth a further investigation.</p>
+
+<p class="indent">As regards <i>laeta</i>, by self-fertilisation <i>no dwarfs were
+produced</i>, but in all other respects it behaved almost exactly like <i>velutina</i>.
+The ovules are evidently mixed talls and dwarfs, and whether
+fertilised by dwarfs or by the pollen of <i>velutina</i>, which is already
+proved to be all dwarf, the result was a steady 50 per cent. of
+talls and 50 per cent. of dwarfs. The pollen of <i>laeta</i> used on
+dwarfs gives nothing but dwarfs, and in three series of such experiments
+226 dwarfs were produced.</p>
+
+<p class="indent">We are thus faced with this difficulty. Since the egg-cells
+of <i>laeta</i> are evidently mixed, talls and dwarfs, and the pollen used
+on dwarfs gives all dwarfs, why does not self-fertilisation give
+a mixed result, talls and dwarfs, instead of <i>all talls</i>? De Vries
+regards the result of self-fertilisation as showing the real nature
+of the pollen, and declares it to be all talls, while he represents
+the behaviour of the same pollen used on dwarfs by stating that
+in these combinations the dwarf character dominates. This
+does not seem to me a natural interpretation. I should regard
+the pollen of <i>laeta</i> as identical with that of <i>velutina</i>, namely
+dwarf, and I suspect the difficulty is really created by the behaviour of
+<i>laeta</i> on self-fertilisation. Until a proper analysis is made in
+<span class="pagenum"><a name="Page_114" id="Page_114">[Pg 114]</a></span>
+which the identity of the different individuals used is recorded,
+no further discussion is possible.<a name="FNanchor_8_58" id="FNanchor_8_58"></a><a href="#Footnote_8_58" class="fnanchor">[8]</a>
+</p>
+
+<p class="indent">Other results of a complicated kind involving production
+of <i>laeta</i> and <i>velutina</i> together with a third form have been
+published by de Vries in his paper on "Triple Hybrids." To these also
+the same criticism applies. Some of the observations seem capable
+of simple factorial representation and others are conflicting.</p>
+
+<p class="indent">Taking the work on <i>Oenothera</i> as a whole we see
+in it continually glimpses of order which further on are still blocked by
+difficulties and apparent inconsistencies. Through such a stage
+all the successful researches in complicated factorial analysis
+have passed and I see no reason for supposing that with the
+application of more stringent methods this more difficult set of
+problems will be found incapable of similar solutions. To
+return to the original question whether in <i>Oenothera</i> we can claim
+to see a special contemporaneous output of new species in actual
+process of creation, it will be obvious that while the interrelation
+of the several types is still so little understood, such a claim has
+no adequate support. It is true that many of the "mutants"
+of <i>Lamarckiana</i> can well pass for species, but this is equally true
+of many new combinations of pre-existing factors as we have
+seen in <i>Primula Sinensis</i> and other cases. Still less can it be
+admitted that these facts of uncertain import supply a justification
+for the conception which has played a prominent part
+in the scheme of the <i>Mutationstheorie</i>, namely that there are
+special periods of Mutation, when the parent-species has peculiar
+genetic properties. To conclude: The impression which the
+evidence leaves most definitely on the mind is that further discussion
+of the bearing which the <i>Oenotheras</i> may have on the
+problem of evolution should be postponed until we have before
+us the results of a searching analysis applied to a limited part of
+the field. In such an analysis it is to be especially remembered
+that we have now a new clue in the well-ascertained fact that the
+genetic composition of the male and female germ-cells of the
+<span class="pagenum"><a name="Page_115" id="Page_115">[Pg 115]</a></span>
+same individual may be quite different. When with this possibility
+in view the behaviour of the types is re-examined I
+anticipate that many of the difficulties will be removed.</p>
+
+<p class="indent">Outside the evidence from <i>Oenothera</i>, which, as we
+have seen, is still ambiguous, I know no considerable body of facts favourable
+to that special view of Mutation which de Vries has promulgated.
+Of variation, or if we will, Mutation, in respect of
+some one character, or resulting from recombination, there is
+proof in abundance; but of that simultaneous variation in several
+independent respects to which de Vries especially attributes the
+origin of new specific types I know only casual records which
+have yet to undergo the process of criticism.</p>
+
+<hr class="tb" />
+
+<p class="indent">Besides de Vries's "<i>Mutationstheorie</i>" and "Species
+and Varieties" the chief publications relating to the subject of the behaviour
+of <i>Oenothera</i> are the following: (Many other papers
+relating especially to the cytology of the forms have appeared.)</p>
+
+<p>
+<span style="margin-left: 4em;">Davis, B. M.</span><br />
+<span style="margin-left: 6em;">Genetical Studies on <i>Oenothera</i>, I. <i>Amer. Nat.</i>, XLIV, 1910, p. 108.</span><br />
+<span style="margin-left: 6em;">Genetical Studies on <i>Oenothera</i>, II. <i>Ibid.</i>, XLV, 1911, p. 193.</span>
+<br /><br />
+<span style="margin-left: 4em;">Gates, R. R.</span><br />
+<span style="margin-left: 6em;">An Analytical Key to some of the Segregates of Oenothera.</span><br />
+<span style="margin-left: 7em;"><i>Twentieth Annual Report of the Missouri Botanical Garden</i>, 1909.</span><br />
+<span style="margin-left: 6em;">Studies on the Variability and Heritability of Pigmentation in <i>Oenothera</i>.</span><br />
+<span style="margin-left: 7em;"><i>Ztsch. f. Abstammungslehre</i>, 1911, IV, p. 337.</span>
+<br /><br />
+<span style="margin-left: 4em;">Honing, J. A.</span><br />
+<span style="margin-left: 6em;">Die Doppelnatur der <i>Oenothera Lamarckiana</i>.</span><br />
+<span style="margin-left: 7em;"><i>Ztsch. f. Abstammungslehre</i>, 1911, IV, p. 227.</span>
+<br /><br />
+<span style="margin-left: 4em;">Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K. Small).</span><br />
+<span style="margin-left: 6em;">Mutants and Hybrids of the <i>Oenotheras</i>.</span><br />
+<span style="margin-left: 7em;"><i>Carnegie Institution's Publication</i>, No. 24, 1905.</span>
+<br /><br />
+<span style="margin-left: 4em;">Macdougal, D. T., Vail, A. M., Shull, J. H.</span><br />
+<span style="margin-left: 6em;">Mutations, Variations and Relationships of the <i>Oenotheras</i>.</span><br />
+<span style="margin-left: 7em;"><i>Carnegie Institution's Publication</i>, No. 81, 1907.</span>
+<br /><br />
+<span style="margin-left: 4em;">de Vries, H.</span><br />
+<span style="margin-left: 6em;">On Atavistic Variation in <i>Oenothera cruciata</i>.</span><br />
+<span style="margin-left: 7em;"><i>Bull. Torrey Club</i>, 1903, Vol. 30, p. 75.</span><br />
+<span style="margin-left: 6em;">On Twin Hybrids,</span><br />
+<span style="margin-left: 7em;"><i>Bot. Gaz.</i>, Vol. 44, 1907, p. 401.</span><br />
+<span style="margin-left: 4em;">Ueber die Zwillingsbastarde von <i>Oenothera nanella</i>.</span><br />
+<span style="margin-left: 6em;"><i>Ber. Deut. Bot. Ges.</i>, 1908, XXVI, <i>a</i>, p. 667.</span>
+<br /><br />
+<span style="margin-left: 4em;">Bastarde von <i>Oenothera gigas</i>. <i>Ibid.</i>, p. 754.</span>
+<br /><br />
+<span style="margin-left: 4em;">On Triple Hybrids. <i>Bot. Gaz.</i>, 1909, Vol. 47, p. 1.</span><br />
+<span style="margin-left: 4em;">Ueb. doppeltreziproke Bastarde von <i>Oenothera biennis</i> L. und <i>Oenothera muricata</i> L.</span><br />
+<span style="margin-left: 6em;"><i>Biol. Cbltt.</i>, 1911, XXXI, p. 97.</span>
+<br /><br />
+<span style="margin-left: 4em;">Zeijlstra, H. H.</span><br />
+<span style="margin-left: 6em;"><i>Oenothera nanella</i> de Vries, eine krankhafte Pflanzenart.</span><br />
+<span style="margin-left: 7em;"><i>Biol. Cbltt.</i>, 1911, XXXI, p. 129.</span>
+<span class="pagenum"><a name="Page_116" id="Page_116">[Pg 116]</a></span></p>
+
+<h3><span class="smcap">Note.</span></h3>
+
+<p class="indent">Since this chapter was written two contributions of special
+importance have been made to the study of the <i>Oenothera</i> problems.
+The first is that of Heribert-Nilsson.<a name="FNanchor_9_59" id="FNanchor_9_59"></a><a href="#Footnote_9_59" class="fnanchor">[9]</a>
+The author begins by giving a critical account of the evidence for
+de Vries's interpretation of the nature of the mutants. In general this
+criticism pursues lines similar to those sketched in the foregoing chapter,
+concluding, as I have done, that the chief reason why factorial
+analysis has been declared to be inapplicable to the <i>Oenothera</i>
+mutants is because no one has hitherto set about this analysis
+in the right way. He has also himself made a valuable beginning
+of such an analysis and gives good evidential reasons for the belief
+that at least the red veining depends on a definite factor which
+also influences the size of certain parts of the plant. He argues
+further that many of the distinctions between the mutants are
+quantitative in nature. With great plausibility he suggests that
+the system of cumulative factors which Nilsson-Ehle discovered
+in the case of wheat (subsequently traced by East in regard to maize)
+may be operating also in these <i>Oenotheras</i>. According
+to this system several factors having similar powers may coexist
+in the same individual, and together produce a cumulative effect.
+Scope would thus be given for the production of the curious and
+seemingly irregular numbers so often recorded in the "mutating" families.</p>
+
+<p class="indent">Another remarkable observation relating to the crosses of
+<i>muricata</i> and <i>biennis</i> has been published by Goldschmidt.<a name="FNanchor_10_60" id="FNanchor_10_60"></a><a href="#Footnote_10_60" class="fnanchor">[10]</a>
+He finds that in the formation of this cross the female pronucleus
+takes no part in the development of the zygotic cell, but that
+when the male pronucleus enters, the female pronucleus is
+pushed aside and degenerates. As de Vries observed, the reciprocal
+hybrids are in each case very like the father ("<i>stark
+patroklin</i>"), a consequence which finds a natural explanation in
+the phenomenon witnessed by Goldschmidt. The results of
+the subsequent matings can also be readily interpreted on the
+same lines. Indications of maternal characters are nevertheless
+<span class="pagenum"><a name="Page_117" id="Page_117">[Pg 117]</a></span>
+mentioned by de Vries, and if Goldschmidt's account of the
+cytology is confirmed, these must presumably be referred to the
+influence of the maternal cytoplasm. Clearly this new work
+opens up lines of exceptional interest. The interpretation I
+have offered above must probably be reconsidered. The distinction
+between the male and female cells of the types may no doubt be
+ultimately factorial, but it is difficult to regard such a distinction
+as created by a differential distribution of the ordinary factors.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_118" id="Page_118">[Pg 118]</a></span></p>
+
+<h2>CHAPTER VI</h2>
+
+<h3>Variation And Locality</h3>
+
+<p class="indent">In all discussions of the modes of Evolution the
+phenomena of Geographical Distribution have been admitted to be of
+paramount importance. First came the broad question, were the
+facts of distribution consistent with the Doctrine of Descent?
+I suppose all naturalists are now agreed that they are thus
+consistent, and that though some very curious and as yet inexplicable
+cases remain to be accounted for, the distribution of
+animal and plant life on the face of the earth is much what we
+might expect as a result of a process of descent with modification.
+Passing from this general admission to the more particular question
+whether the facts of distribution favour one special conception
+of the mode of progress of evolution rather than another,
+no agreement has yet been reached. One outstanding feature
+is hardly in dispute, namely that prolonged isolation is generally
+followed by greater or less change in the population isolated.
+Groups of individuals which from various causes are debarred
+from free intermixture with other groups almost always exhibit
+peculiarities, but on the other hand, cosmopolitan types which
+range over wide areas are on the whole uniform, or nearly so
+throughout their distribution. Examples of these two categories
+will be familiar to all naturalists. The barriers to intercourse
+may be seas, deserts, prairies, mountain-chains, or circumstances
+of a much less obvious character which isolate quite as effectually.
+The local unit is not necessarily an island, a district, or an area
+of special geological formation, but may, as every collector knows,
+be a valley, a pond, a creek, a "bank" in the sea, a clump of
+trees, a group of rocks in a bay, or a particular patch of ground
+on a mountain side. All the great groups provide examples of
+such specially isolated forms. The botanist knows them well;
+the conchologist, the entomologist, the ornithologist and the
+student of marine life are all equally aware that special varieties
+<span class="pagenum"><a name="Page_119" id="Page_119">[Pg 119]</a></span>
+or special species come from special places and from nowhere
+else. In one remarkable case the season of appearance plainly
+acts as the isolating barrier. <i>Tephrosia bistortata</i> is a small
+Geometrid moth which has two broods, appearing in <i>March</i>
+and <i>July</i> respectively. It is closely allied to <i>T. crepuscularia</i>
+which emerges in <i>May</i> and <i>June</i>. From the fact that occasional
+specimens cannot be quite certainly referred to one or other of
+the two, many have held that the two are one species. Nevertheless,
+in general they present distinctions which are plain
+enough. Some localities have one form only, but in several
+woods they co-exist. Experiment has shown that the two can
+be crossed, and that the cross-breds can breed <i>inter se</i> and with
+at least one of the parent stocks.<a name="FNanchor_1_61" id="FNanchor_1_61"></a><a href="#Footnote_1_61" class="fnanchor">[1]</a>
+Some diminution in fertility
+was observed, but perhaps not more than is commonly encountered
+when wild forms are bred in captivity. In such a case it can
+scarcely be doubted that the distinctness of the two forms in
+the places where they co-exist is maintained by the seasonal isolation.</p>
+
+<p class="indent">Just as the consequences of isolation are to be seen in the
+most different forms of life so may they also affect the most diverse
+features of organisation, such as size, colour, sculpture,
+shape, or number of parts. In the Sloth (<i>Choloepus</i>) the geographical
+races differ in the number of cervical vertebrae&mdash;or
+in other words, in the distribution of vertebral differentiation.
+The geographical races of <i>Cistudo</i> differ in the number of claws
+and phalanges.<a name="FNanchor_2_62" id="FNanchor_2_62"></a><a href="#Footnote_2_62" class="fnanchor">[2]</a>
+</p>
+
+<p class="indent">In Shetland, the males of <i>Hepialus humuli</i> (the Ghost Moth)
+are not sharply differentiated in colour from the females, as they
+are elsewhere, but in varying degrees resemble them.<a name="FNanchor_3_63" id="FNanchor_3_63"></a><a href="#Footnote_3_63" class="fnanchor">[3]</a>
+No such males are found in other localities, and even in the other Scottish
+islands they are normal. In the island of Waigiu the converse
+phenomenon has been observed in <i>Phalanger maculatus</i>.
+<span class="pagenum"><a name="Page_120" id="Page_120">[Pg 120]</a></span>
+Generally the male is spotted with white, and the female is without
+spots, but in Waigiu the females are spotted like the males.<a name="FNanchor_4_64" id="FNanchor_4_64"></a><a href="#Footnote_4_64" class="fnanchor">[4]</a>
+</p>
+
+<p class="indent">The following striking illustration was pointed out to
+me by Dr. W. D. Miller. <i>Euphonia elegantissima</i> as it occurs in Mexico
+and Central America has the two sexes very distinct from each
+other. The male has the lower parts orange and the upper
+parts a dark indigo blue, with a bright turquoise-blue head and
+neck. The female, except for the head, is of a bright olive green.
+A form in which the sexes are similarly differentiated exists in
+Porto Rico and is known as <i>E. Sclateri</i>. But in many of the
+other West Indian islands the representative "species" (<i>E.
+flavifrons</i>) has the two sexes closely resembling the <i>female</i> of
+<i>E. elegantissima</i>. This form is found in Antigua, Barbados,
+St. Vincent, and Guadeloupe, from which localities the British
+Museum has specimens. All three so-called species are very
+much alike otherwise.</p>
+
+<p class="indent">In the genus <i>Pyrrhulagra</i> (<i>Loxigilla</i>) to which Mr. Outram
+Bangs called my attention, several distinct and alternative possibilities
+occur. The genus has many local species occurring
+on the various West Indian islands. These species are characterized
+by differences in size, colour, and the shape of the bill.
+The colours have a narrow range, being black or greyish, with
+or without chestnut marks about the head and throat. In
+most of the islands the males are in general colour a full black,
+and the females are distinctly grey. They are thus found in
+San Domingo, Jamaica, Bahama, and most of the Lesser Antilles.
+In Porto Rico we meet the peculiarity that the hens are almost
+as black as the males (Ridgway describes the black of the hens
+as slightly less intense). This form is called <i>portoricensis</i>.
+A larger type, known as <i>grandis</i>, similarly coloured, inhabits
+St. Kitt's. Then, on the contrary, in Barbados, <i>both sexes</i> are
+a dull blackish grey, like the hens of the Lesser Antilles in general.</p>
+
+<p class="indent">The local species of <i>Agelaius</i> show similarly
+capricious distinctions. <i>A. phoeniceus</i> is a widely spread species, found over
+a great part of North America. The male is black with red-orange
+<span class="pagenum"><a name="Page_121" id="Page_121">[Pg 121]</a></span>
+bars on the wings, but the female is somewhat thrush-like in
+colour. In the island of Porto Rico there is a form called <i>xanthomus</i>,
+in which <i>both sexes</i> are like the males of the mainland.
+A similar species called <i>humeralis</i>, also with both sexes male-like,
+lives in Cuba. The island of Cuba, curiously enough, has also a
+distinct species named <i>assimilis</i>, in which the female is a dull
+black all over, though the male is like the mainland type.</p>
+
+<p class="indent">So also may local races differ in respect of variability.
+<i>Argynnis paphia</i>, the Silver Washed Fritillary, through a great
+part of its distribution has only one female form. In the English
+New Forest a second female form, <i>valesina</i>, co-exists with the
+ordinary <i>paphia</i> female. But in the southern valleys of the Alps
+the <i>valesina</i> female is much the commoner of the two, and indeed
+in some localities where the species is abundant, I have seen no
+<i>paphia</i> females in many days collecting.</p>
+
+<p class="indent">The beetle <i>Gonioctena variabilis</i> furnishes an
+illustration of a comparable phenomenon affecting the male sex. In 1894 and
+1895 I studied the curious colour variations of this species especially
+in the neighbourhood of Granada, and Mr. Doncaster
+ten years later repeated the observations on the same ground,
+and also collected the insect in other places in the south of Spain.
+The distinctions are not easy to give in words and the reader is
+referred to the colour plate accompanying my paper.<a name="FNanchor_5_65" id="FNanchor_5_65"></a><a href="#Footnote_5_65" class="fnanchor">[5]</a>
+The essential fact is that the males commonly have the elytra <i>red
+with black spots</i> and the females for the most part have greenish
+grey elytra with black stripes. In some localities a large minority
+of males closely resemble the female type, being identical in
+colour and then only distinguishable by structural differences.
+In two Granada localities I found the proportion of such males
+quite different. In the Darro valley about 38 per cent. (in 718)
+<span class="pagenum"><a name="Page_122" id="Page_122">[Pg 122]</a></span>
+were of this feminine type, but on the hills some 300 feet
+above only 19 per cent. (in 3,230) were like the females. At
+Castillejo, not far from Toledo I found no such male in 75 specimens.</p>
+
+<p class="indent">Mr. Doncaster collected from several localities,
+especially from two areas near Malaga, about 5 miles apart. In one of
+these the female-like males were, as usual, in a minority, but
+in the other these were actually in great excess, amounting to
+about 81 per cent. in the 173 taken. Doncaster found a doubtful
+indication that the composition of the population varies with
+the season, which is quite possible, but it is most interesting
+to note that in my chief locality after the lapse of ten years he
+found the proportions very much the same as I had done at the
+same season, for where I had 19 per cent. of the female-like males
+his collecting gave 16 per cent. In other respects also, his statistics
+corresponded very closely with mine.<a name="FNanchor_6_66" id="FNanchor_6_66"></a><a href="#Footnote_6_66" class="fnanchor">[6]</a>
+</p>
+
+<p class="indent">The various forms of <i>Heliconius erato</i> are well known
+to entomologists. They are strikingly distinguished by the colours
+of the strong comb-like marking on the hind wing, which may be
+red, yellow, green or blue. In various parts of the distribution
+in South America sometimes two and sometimes three of these
+distinct types co-exist.<a name="FNanchor_7_67" id="FNanchor_7_67"></a><a href="#Footnote_7_67" class="fnanchor">[7]</a>
+</p>
+
+<p class="indent">The distribution of the varieties of <i>Noctua castanea</i>
+typifies a large range of cases. The form which is reckoned the normal
+of the species has red fore-wings. It is practically restricted to
+Great Britain and Germany, according to Tutt. The other
+common form, <i>neglecta</i>, has grey fore-wings, and in this pattern
+it ranges through West Central Europe from North Italy to
+Germany. In the British Isles it extends up to Orkney. In
+Britain this grey form is by far the commoner, occurring
+<span class="pagenum"><a name="Page_123" id="Page_123">[Pg 123]</a></span>
+wherever the species is found. The red form is much scarcer in
+England, and does not occur at all in many localities where the
+grey form is common. Mr. Woodforde, from whom this account
+is taken,<a name="FNanchor_8_68" id="FNanchor_8_68"></a><a href="#Footnote_8_68" class="fnanchor">[8]</a>
+states that in August, 1899, he saw considerably over a
+hundred of the grey in the New Forest at sugar, but only two
+red ones. In Staffordshire however the red is proportionately
+more numerous and he estimates them as 40 per cent. of the
+population. Lastly a form has been taken in Staffordshire as a
+rarity in which the red is replaced by yellow, and this has hitherto
+been seen nowhere else. It is beyond our immediate purposes
+to discuss the genetic relationships of such forms, but the details
+of this case are interesting as making fairly clear the fact that
+the distinctions between <i>castanea</i> and <i>neglecta</i> are due to
+combinations of the presence of and absence of two pairs of factors,
+of which one produces a red pigment in the ground colour of the
+forewing and the other irrorates the same region with black
+scales. Mr. Woodforde states that all intermediates exist,
+and that in Staffordshire the greys always have a pinkish tinge.
+The yellow is doubtless another recessive to the red.</p>
+
+<p class="indent">Species which are uniform in some localities may be
+polymorphic in others. Such a phenomenon is well exemplified by
+the orchid <i>Aceras hircina</i>. Of this species distinct varieties had
+previously been known in Germany, but Gallé<a name="FNanchor_9_69" id="FNanchor_9_69"></a><a href="#Footnote_9_69" class="fnanchor">[9]</a>
+has lately given a detailed account of a number of most diverse forms found
+growing in a district of Eastern France. Without reference to his plates
+it is impossible to give any adequate conception of the profusion
+of types which the flowers of the species there assume. In some
+the lip is elongated to many times its usual length, twisting
+and dividing in a fashion suggesting some of the strangest of the
+Tropical Orchids. In others the labellum and the lateral petals
+are all comparatively short and wide (Fig. 13). Intermediates,
+combining these qualities in various degrees, were abundant, and
+the condition of the species, which was the only representative of
+the genus in the locality, recalls the extreme polymorphism of
+many of the Noctuid Moths.
+<span class="pagenum"><a name="Page_124" id="Page_124">[Pg 124]</a></span></p>
+
+<div class="figcenter" >
+  <img src="images/i_139.jpg" alt="Aceras hircina" width="600" height="847" />
+</div>
+<p class="space-below" style="font-size: 120%;">
+<b><span class="smcap">Fig. 13.</span></b>&emsp;Various forms of <i>Aceras hircina</i>.
+(After Gallé.) This figure only shows a few of the more striking forms
+illustrated in Gallé's plates.</p>
+
+<p class="indent"><span class="pagenum"><a name="Page_125" id="Page_125">[Pg 125]</a></span>
+Somewhat comparable variability has been seen in another
+Orchid genus <i>Ophrys</i>. In Great Britain the species <i>apifera</i>,
+<i>aranifera</i> and <i>muscifera</i> though variable are fairly distinct,
+but Moggridge has published two series of plates<a name="FNanchor_10_70" id="FNanchor_10_70"></a><a href="#Footnote_10_70" class="fnanchor">[10]</a>
+showing a very different state of things as regards the <i>Ophrys</i> population
+of the Riviera. Here the outward diversity is such that the ordinary
+specific names cannot be applied with any confidence and the
+limits of the species are quite uncertain. It may well be supposed
+that these Riviera plants are interbreeding, and indeed we may
+safely assume that they are. It is, however, to be remembered
+that Darwin showed <i>apifera</i> in this country to be habitually
+self-fertilised, so that the different behaviour on the Riviera may
+itself constitute a local peculiarity. Moreover it is to be gathered
+from Moggridge's account that in the districts which he examined
+the condition was not to be described by the statement that our
+three types were there co-existing and hybridising, but rather
+we should say that the population was polymorphic, containing
+these three types amongst others. Conchologists are aware
+that on the Dogger Bank <i>Modiola</i> attains a size unparalleled
+elsewhere. The same is true of the sponges <i>Grantia compressa</i>
+and <i>Grantia ciliata</i> in the estuary of the Orwell.<a name="FNanchor_11_71" id="FNanchor_11_71"></a><a href="#Footnote_11_71" class="fnanchor">[11]</a>
+Conversely, as we know so well in the case of Man, dwarf races occur in
+several special localities. Such examples may be multiplied indefinitely.</p>
+
+<p class="indent">The relation of local forms to species has often
+been discussed from many points of view, but I know no treatment of
+the subject clearer or more comprehensive than an excellent
+account of some of the various manifestations of local differentiation
+as they appear in Helicidæ published by Coutagne<a name="FNanchor_12_72" id="FNanchor_12_72"></a><a href="#Footnote_12_72" class="fnanchor">[12]</a>
+and a reader interested in the problem which they raise would
+<span class="pagenum"><a name="Page_126" id="Page_126">[Pg 126]</a></span>
+do well to make himself acquainted with the original from
+which the following notes are taken. He speaks for example
+of <i>Helix lapicida</i>. This is on the whole a constant form ranging
+up to the altitude of 1,300 m., common all over France except
+at great heights and in the Olive regions where it is restricted
+to moist places. Though subjected to such diverse conditions
+it shows only trivial variations in colour and other respects
+throughout its distribution, excepting that on both sides of the
+Pyrenees it has a very distinct sporadic variety called <i>Andorrica</i>
+or <i>microporus</i>. This variety occurs here and there, together
+with the type-form sometimes in colonies (pp. 26-30 and 86).</p>
+
+<p class="indent"><i>Bulimus detritus</i> though more restricted in
+geographical range is a much more variable form. It exhibits great
+variations in colour, form, and size, and as Coutagne well insists,
+these are independent of each other. Foreshadowing the methods of
+factorial analysis he suggests that distinctions in each respect,
+the "modes" as he calls them, should be denoted by a letter,
+or if desired, by a name, and the several combinations of differences
+might thus be most logically and usefully expressed. Of
+such combinations he says there are at least 18, all of which can
+be found. The whole possible series does not necessarily occur
+in the same place, and various localities are characterised by
+the presence or absence of certain of the combinations as Coutagne
+calls them, and by the relative frequency with which they
+occur. The ideas thus enunciated are much in advance of the
+ordinary practice of systematists, who give names to forms which
+are nothing but accidental combinations of factors, just as the
+horticulturists for practical reasons give names to similar combinations,
+which as we now know are merely specially noticeable
+terms in a long series of possibilities. In each case it is rather
+the <i>factors</i> which should be named than the forms which are
+constituted by their casual collocation. In this special example
+of <i>Bulimus detritus</i> the 18 forms are made by the combinations
+of three pairs of independent factors. Besides these combinations
+which may occur anywhere or almost anywhere in the distribution
+there are two more distinct local forms, each of which
+is regarded by Coutagne as probably constituting a fresh "mode,"
+perhaps compatible with the others.
+<span class="pagenum"><a name="Page_127" id="Page_127">[Pg 127]</a></span></p>
+
+<p class="indent"><i>Helix striata</i> (Draparnauld)<a name="FNanchor_13_73" id="FNanchor_13_73"></a><a href="#Footnote_13_73" class="fnanchor">[13]</a>
+is truly polymorphic; and its
+various forms have been described under various specific names.
+It abounds in the calcareous hills of Provence and Languedoc,
+disappearing in the alluvial lowlands and equally in the upper
+levels at about 800-1,000 m. From this district it extends
+through regions of similar altitude over a great part of France
+(details given).</p>
+
+<p class="indent">Locard in his monograph of this group, which he calls
+collectively the group of <i>Helix Heripensis</i>, tabulates 27 distinct
+named forms. The characteristics in which these forms differ
+have been reckoned as 17, and as several of these vary in degree
+of development, the number of modes may be increased to 109.
+For practical purposes however Coutagne considers that the
+various developments of 7 characteristics in their several combinations
+are enough to express the various forms, and he gives
+examples of this method of definition. As he observes, though
+names may be required to define the modes, no one need be
+alarmed at that, for the same names of modes will be applicable
+to a great range of distinct species, and the formulae expressing
+their combinations will replace the varietal names.</p>
+
+<p class="indent">This particular example of polymorphism is but little
+limited by locality. Occasional colonies present some special physiognomy
+which may in a given place seem almost invariable, though
+in this very respect the colonies found elsewhere may be highly
+variable, but such limitations are exceptional for <i>H. striata</i>.</p>
+
+<p class="indent">Some distinct and obvious susceptibilities to the
+influence of soil and climate are however noticeable. For example on
+siliceous ground the shells are thinner, while on calcareous soils
+they are thicker; similarly those from the Northern districts
+attain a larger size than those from further South. Moreover
+those subjected to curtailed development, whether from drought,
+heat or cold often show a shortening of the spire. In contrast
+with this case Coutagne describes the varieties of <i>Helix caespitum</i>,
+which he says are for the most part localised, quoting many illustrative cases.</p>
+
+<p class="indent">Another remarkable case in which locality plays a curious
+part is provided by the two species <i>Helix trochoides</i> and
+<span class="pagenum"><a name="Page_128" id="Page_128">[Pg 128]</a></span>
+<i>pyramidata</i>. In France generally they are distinct enough from
+each other, <i>trochoides</i> being smaller and having a characteristic
+keel. Coutagne says that after having collected these species
+from more than a score of localities he came upon a colony of
+<i>trochoides</i> on the island of Pomègues in which the shells were
+relatively enormous, most of them having only a slight keel,
+and a few none at all. On the other hand he received a consignment
+of <i>pyramidata</i> from four localities in Sicily, all small,
+and one of them exactly like the <i>trochoides</i> from Pomègues.
+Judging by the samples received from Sicily, <i>trochoides</i> is there
+not more variable than it is in Provence, while the Sicilian
+<i>pyramidata</i> is protean.</p>
+
+<p class="indent">The relations of the two species <i>Helix nemoralis</i> and
+<i>hortensis</i> provide an illustration of another kind of manifestation of local
+peculiarity. <i>H. hortensis</i> and <i>nemoralis</i> as usually met with,
+are two very distinct forms. <i>H. hortensis</i> is smaller and duller,
+and its peristome is white. <i>H. nemoralis</i> is larger and more shiny,
+and its peristome is brown. In several anatomical points,
+moreover, especially in the shape of the dart, there are great
+differences. For a full account of these peculiarities of the two
+forms and a discussion of their inter-relations the reader is referred
+to the elaborate work of A. Lang<a name="FNanchor_14_74" id="FNanchor_14_74"></a><a href="#Footnote_14_74" class="fnanchor">[14]</a>
+who has studied them extensively and has also succeeded in experimentally
+raising hybrids between them. These hybrids were in a slight degree
+fertile with both the parent species, but up to the time of publication
+no young had been reared from hybrids <i>inter se</i>.</p>
+
+<p class="indent">Coutagne describes the result of collections made in 62
+French localities. Some had exclusively <i>hortensis</i>, some exclusively
+<i>nemoralis</i>, and in some the two were found in association.
+He gives details of five of these collections from which I
+take the following summary of the more essential facts, omitting
+much that is almost equally significant.</p>
+
+<p class="indent"><i>Locality A</i>, near Honfleur. Both forms present, each
+sharply and normally distinguished, without any intermediates. They
+<span class="pagenum"><a name="Page_129" id="Page_129">[Pg 129]</a></span>
+are thus found in many places. Coutagne instances Müller's
+observations in Denmark, his own series from the Jura, etc.</p>
+
+<p class="indent"><i>Locality B.</i> Vonges (Côte d'Or), 242 <i>hortensis</i>
+taken at random, showed 128 with light peristomes (either more or less pinkish
+or quite white) and 114 with dark <i>brown</i> peristomes; together
+with 26 <i>nemoralis</i> all with the usual brown peristomes.</p>
+
+<p class="indent">Of the <i>hortensis</i> 50 were in ground-colour
+<i>opalescens</i> and 1 <i>roseus</i>; and in shape 5 were <i>umbilicatus</i>.</p>
+
+<p class="indent"><i>Locality C</i>, about 3 kilometres from <i>B</i>. There were
+found 35 <i>hortensis</i>, of which 20 had light peristomes and 15 brown; together
+with 7 <i>nemoralis</i>.</p>
+
+<p class="indent">Of the <i>hortensis</i> none were <i>opalescens</i>; 18 were
+<i>roseus</i> and none has the shape of <i>umbilicatus</i>.</p>
+
+<p class="indent"><i>Locality D</i>, about 1,200 metres from <i>B</i>. 147
+<i>hortensis</i>, of which 4 had light peristomes and 143 had brown.
+No <i>nemoralis</i> were found.</p>
+
+<p class="indent">None of the <i>hortensis</i> were <i>opalescens</i> or
+<i>roseus</i>, but 30 were <i>umbilicatus</i>.</p>
+
+<p class="indent">In these localities intermediates of every grade existed between
+the well-characterised <i>opalescens</i>, <i>roseus</i>, or <i>umbilicatus</i>,
+and the other forms, but there were no intergrades between the
+other <i>nemoralis</i> and the smaller <i>hortensis</i>, about which there
+was no hesitation. In the next locality a very different state
+of things was found.</p>
+
+<p class="indent"><i>Locality E.</i> Banks of the Yvette at Orsay (Seine-et-Oise).
+The actual numbers are not given, but we are told that 58 per
+cent. were <i>hortensis</i>, 33 per cent. <i>nemoralis</i>, and 9 per cent.
+intermediate. As at Honfleur, the <i>hortensis</i> had white peristomes,
+and the <i>nemoralis</i> brown. Coutagne's visits to this locality were
+in 1878 and 1880, and he calls attention to the fact that Pascal
+found similar intermediates in the same neighbourhood in 1873.</p>
+
+<p class="indent">The two species, in Coutagne's view, when they
+occur together, can generally be sorted from each other with perfect
+confidence, and it is only in exceptional localities that these
+intermediates occur. Whether they are hybrids, or whether
+sometimes the species in their variations transgress their usual
+limitations is regarded both by Coutagne and by Lang as a
+<span class="pagenum"><a name="Page_130" id="Page_130">[Pg 130]</a></span>
+question not yet answerable with certainty. Coutagne moreover
+lays stress on the fact that although each species may be easily
+known from the other <i>in its own district</i>, yet when shells from
+different districts are brought together it is sometimes impossible
+to sort them. He mentions an example of such casual intermixture
+occurring under natural conditions on an island in the
+Rhone, to which it may well be supposed that floods had brought
+immigrants from miscellaneous localities. This population contained
+a very large number of uncertain specimens, and as he
+says, it was much as if he were to mix the shells from his 62 localities,
+after which it would certainly be impossible to separate
+the two species again.<a name="FNanchor_15_75" id="FNanchor_15_75"></a><a href="#Footnote_15_75" class="fnanchor">[15]</a>
+</p>
+
+<p class="indent">Further evidence is given in the same treatise as to
+other examples of polymorphism, especially in the genus <i>Anodonta</i>,
+of which Locard made 251 species for France alone. Here again
+are cases like those already given, and many forms or "modes"
+are found restricted to special localities, while occasionally
+in the same locality dissimilar forms are found, collectively
+forming a colony, without intermediates.</p>
+
+<p class="indent">Taken as a whole the evidence shows the following conclusions
+to be true. Local races, whether of animals or plants, may be
+distinguished by characters which we are compelled to regard
+as trivial, or again by features of such magnitude that if they
+were known to us only as the characteristics of a uniform species
+they would certainly be assumed without hesitation to be essential
+for its maintenance. Local forms may be sharply differentiated
+from the corresponding populations of other localities or they
+may be connected with them by numbers of intermediates.
+Not rarely also we find a fact which has always seemed to me of
+special significance, that the peculiarity of the local population
+or colony may show itself in a special liability to variation, and
+this variability may show itself in one of many degrees, either
+in the constant possession of a definite aberration, in a dimorphism,
+or in an extreme polymorphism.
+<span class="pagenum"><a name="Page_131" id="Page_131">[Pg 131]</a></span></p>
+
+<p class="indent">At this stage attention should be called to two points.
+First, that when the details of the geographical distribution of
+any variable species are studied in that thorough and minute
+fashion which is necessary for any true knowledge of the interrelations
+of the several forms, the conception of a species invented
+by the popular expositions of Evolution under Selection is found
+to be rarely if ever realised in nature.</p>
+
+<p class="indent">A species in this generalised sense is an aggregate
+of individuals, none exactly alike, but varying round a normal type,
+the characters of which are fixed in so far as they are adapted to
+environmental exigency. In nature, however, the occurrence of
+the varieties, and even the occurrence of the variability is
+sporadic. In one place a population may be perfectly uniform.
+In another it may be again uniform but distinct. In others
+the two forms may occur together, sometimes with and sometimes
+without intergrades. In some localities a sporadic variety
+may be an element of the population, persisting through long
+periods of time. In other localities there may be several such
+aberrations occurring together which are absent elsewhere.</p>
+
+<p class="indent">Secondly, I would remind the reader that in the light
+of genetic analysis we know that intergrades, when they do occur, cannot
+be assumed to represent conditions through which the species
+must pass or has passed on its way to the extreme and definite forms.</p>
+
+<p class="indent">Often, perhaps generally, they are nothing but heterozygous
+forms, and often also they are conditions corresponding with the
+presence of factors in their reduction-stages.</p>
+
+<p class="indent">A broad survey of the facts shows beyond question that
+it is impossible to reconcile the mode of distribution of local forms
+with any belief that they are on the whole adaptational. Their
+peculiarities are occasionally the result of direct environmental
+influence, as we shall hereafter notice in certain cases, but none
+can attribute such sporadic and irregular phenomena to causes
+uniformly acting.</p>
+
+<p class="indent">Writers on systematics, especially those of former generations
+often conjecture or assert that local distinctions are caused by
+"differences of climate, soil, food, etc.," in vague general terms.
+It is usually safe to assume that these remarks do not represent
+<span class="pagenum"><a name="Page_132" id="Page_132">[Pg 132]</a></span>
+conclusions drawn from actual evidence, for only rarely can they
+be translated into more precise language. So thoroughly have
+the biological sciences become permeated with the belief that all
+distinctions are dependent upon adaptation, that the mere
+existence of definite distinctions is felt by many to be sufficient
+ground to warrant an assumption that these distinctions are
+directly or indirectly due to special local conditions. For
+example, Dr. J. A. Allen, who has done so much careful and valuable
+work in delimiting the local forms of the United States
+fauna, writes of the Ground Squirrels (Tamias)<a name="FNanchor_16_76" id="FNanchor_16_76"></a><a href="#Footnote_16_76" class="fnanchor">[16]</a>
+as follows:&mdash;</p>
+
+<p class="blockquot">"From the extreme susceptibility of this plastic
+group to the influences of environment, it is one of the most instructive
+and fascinating groups among North American mammals. No
+one can doubt its comparatively recent differentiation from a
+common stock, and its dispersion from some common centre.
+Whether the type originated at some point in North America,
+or in the Northern part of Eurasia, it is perhaps idle to speculate,
+but that it has increased, multiplied, spread, and become differentiated
+to a wonderful degree in North America is beyond
+question; as it is found from the Arctic regions to the high
+mountain ranges of Central Mexico, and has developed some
+twenty to thirty very palpable local phases."</p>
+
+<p class="blockquot">"Some of them easily take rank as species, others
+as subspecies. Probably a more striking illustration of evolution by
+environment cannot be cited."</p>
+
+<p class="indent">He proceeds to point out that the habits of these
+creatures are such as lead to isolation. This may well be admitted, and
+indeed no exception can possibly be taken to the passage as a
+whole, save in the one respect that there is no real proof that
+the local diversity is due to "evolution by environment" or an
+indication of "susceptibility to the influences of environment."</p>
+
+<p class="indent">Dr. Allen does indeed adduce the fact that California
+"extending through 800 miles of latitude, with numerous sharply
+contrasted physiographic regions, has apparently no less than
+six strongly differentiated forms, while the region east of the
+Rocky Mountains from a little below the northern boundary of
+<span class="pagenum"><a name="Page_133" id="Page_133">[Pg 133]</a></span>
+the United States northward to the limit of trees&mdash;a slightly
+diversified region of at least ten times the area of California&mdash;has
+only one"! But when one comes to ask how the various forms
+are adaptational, and how the influences of environment have
+led to their production, only conjectures of a preliminary and
+tentative character could be expected in reply. Desert forms
+are no doubt pallid as in so many instances, and forest forms are
+more fully coloured, and we may readily enough accept such facts
+as indications of a connection between bodily features and the
+conditions of life, but further than that no one can go; so that
+when we find size, length of ears or of tail, the number of dorsal
+stripes, the pattern of the colours, not to speak of differences in
+the pigments themselves, all exhibiting large modifications, we
+cannot refer these peculiarities to the causation of environmental
+difference, save as a simple expression of faith. I incline far
+more to agree with Gulick who, after years of study of the local
+variations of the Achatinellidae, came to the conclusion that it
+was useless to expect that such local differentiation can be
+referred to adaptation in any sense.<a name="FNanchor_17_77" id="FNanchor_17_77"></a><a href="#Footnote_17_77" class="fnanchor">[17]</a>
+Even the most convinced Selectionist must hesitate before such facts as
+those related by A. G. Mayer regarding the distribution of <i>Partula otaheitana</i>,
+one of these Achatinellidae. The island of Tahiti has been scored
+by erosion so that a series of separated valleys radiate to the coast.
+From four successive valleys Mayer collected the species, and
+found that in the first (Tipaerui) valley all the shells were
+dextral (115, containing 73 young); in the second valley
+(Fautaua) 54 per cent. of adults and 55.5 per cent. of the young
+contained were sinistral; in the third valley (Hamuta) 69 per
+cent. of adults and 73 per cent. of young contained in them were
+sinistral; and lastly, in the fourth valley (Pirae) all the shells
+(131, containing 62 young) were sinistral.<a name="FNanchor_18_78" id="FNanchor_18_78"></a><a href="#Footnote_18_78" class="fnanchor">[18]</a>
+In connection with these observations I may mention the fact that in a certain
+pond in the North of England<a name="FNanchor_19_79" id="FNanchor_19_79"></a><a href="#Footnote_19_79" class="fnanchor">[19]</a>
+the sinistral form of <i>Limnaea peregra</i>
+<span class="pagenum"><a name="Page_134" id="Page_134">[Pg 134]</a></span>
+has been known to occur for about fifty years. Visiting
+it lately I found the left-handed shells to be about 3 per cent. of
+the population. The species is the commonest British freshwater
+shell, but left-handed specimens are exceedingly rare.
+Will anyone ask us to suppose that the persistence of a percentage
+of this rarity in the same place is an indication of some specially
+favouring circumstance in the waters of that pond? It is a
+horse-pond to all appearances exactly like any other horse-pond;
+and I believe that in perfect confidence we may accept the
+suggestion of common sense, which teaches us that there is
+nothing particular in the circumstances which either calls such
+varieties into existence or contributes in any direct way to their
+survival. Had the phenomenon of local variation been studied
+in detail before Darwin wrote, the attempt to make selection
+responsible for fixity wherever found, could never have been
+made. The proposition that not only the definiteness of local
+forms but their variability also is sporadic, can be established
+by countless illustrations taken from any group of either the
+animal or the vegetable kingdoms. Only exceptionally can the
+fixed differences be even suspected of contributing to adaptation,
+and sporadic variability, which is a no less positive fact, must
+manifestly lie outside the range of such suspicions. It is open
+to any one to suggest speculatively that the persistence of
+special varieties or of special variability in special places is an
+indication that in those places the conditions of life are such
+that the forms in question are tolerated though elsewhere
+the same types are exterminated; but that consideration, even
+if it could be proved to be well founded, is not one which lends
+much force to the thesis that definiteness of type is a consequence
+of Natural Selection. On the contrary, recourse to such reasoning
+implies the inevitable but very damaging admission that
+the stringency of Selection is frequently so far relaxed that two
+or more equally definite forms of the same species can persist
+side by side. There is no doubt that this is the simple truth,
+but when once that truth is perceived it is useless to invoke the
+control of Selection as the factor to which definiteness of type
+in general must be referred.
+<span class="pagenum"><a name="Page_135" id="Page_135">[Pg 135]</a></span></p>
+
+<p class="indent">The genetic relations of local forms to each other
+cannot in the absence of actual breeding experiments be often ascertained.
+Standfuss formerly enunciated as a general principle that when
+two forms co-exist in the same locality and are able to interbreed,
+they do not produce intermediates; but that when the forms are
+geographically separated as local races, crosses between them
+result in a series of intermediates.<a name="FNanchor_20_80" id="FNanchor_20_80"></a><a href="#Footnote_20_80" class="fnanchor">[20]</a>
+In this aphorism there is a good deal of truth,
+but if in the light of Mendelian principles we
+examine the two statements we see now that the first is in reality
+only another way of saying that the distinctness of an aberrational
+form co-existing with another is due to segregation, accompanied
+by some degree of dominance of one type. Whether, however,
+one geographically isolated race will give intermediates when
+bred with another must depend entirely on the genetic physiology
+of the special case, and no general rule can be laid down. It
+may well be that, inasmuch as the distinctness of the variety is
+maintained by isolation, the difference in factorial composition
+between it and the representative form in another area is neither
+simple nor sharp; but when two varieties co-exist, though interbreeding,
+it is now clear that their differences must depend on
+the segregation of simple factors. Plainly such aberrations may
+in one place co-exist with another type, and elsewhere be separated
+from it as local races.</p>
+
+<p class="indent">Excellent illustrations of these two stages in
+evolution are provided by the melanic varieties of British Lepidoptera.
+The fact that black or blackish varieties of many species especially
+of Geometridae have come into existence in recent years is well
+known to British collectors, and it is not in dispute that they
+have in several instances replaced the older type more or less
+completely in certain districts. In the year 1900 the Evolution
+Committee of the Royal Society instituted a collective inquiry
+as to the contemporary distribution of these dark varieties. As
+the change had happened within living memory and had greatly
+progressed in recent years it was hoped that a record of the
+existing distribution would serve as a point of departure for
+future comparison. The records thus obtained were tabulated
+<span class="pagenum"><a name="Page_136" id="Page_136">[Pg 136]</a></span>
+by Mr. L. Doncaster.<a name="FNanchor_21_81" id="FNanchor_21_81"></a><a href="#Footnote_21_81" class="fnanchor">[21]</a>
+From that account and from the statements
+in Barrett's British Lepidoptera<a name="FNanchor_22_82" id="FNanchor_22_82"></a><a href="#Footnote_22_82" class="fnanchor">[22]</a>
+this description of some of the more notable cases is taken.</p>
+
+<p class="indent">The most striking and familiar case is that of <i>Amphidasys
+betularia</i>, of which only the ordinary type was known in any
+locality until about 1848-1850, when the totally black var.
+<i>doubledayaria</i> first appeared in the neighbourhood of Manchester.
+This black form was subsequently recorded in Huddersfield
+between 1860 and 1870; Kendal about 1870; Cannock Chase,
+1878; Berkshire, 1885; Norfolk, Essex and Cambridge about
+1892; Suffolk, 1894; London, 1897. For the Southern Counties
+of England, except in the London district, there are still very few
+records. It cannot of course be asserted positively that the
+variety spread from its place of first appearance into the other
+localities, and that it did not arise <i>de novo</i> in them, but there
+can be little doubt that the process was one of colonisation.
+On the European Continent the first records are from Hanover
+in 1884, Belgium 1886 and 1894, Crefeld 188-, Berlin 1903,
+Dresden about the same date.</p>
+
+<p class="indent">As regards the increase of the variety we have the
+fact that in Lancashire, Cheshire and the West Riding of Yorkshire the
+black is now the prevalent form; and in some places, as for example,
+Huddersfield, the black alone is now found, though it was unknown
+there till between 1860 and 1870. About 1870 at Newport,
+Monmouth, the two forms were in about equal numbers,
+but a few years later the type had almost vanished. Similarly
+in Crefeld, where the black form was still very rare in the eighties,
+it now forms about 50 per cent. of the population. In the
+London district the black remains scarce and at the date of the
+report it was still very scarce. From Ireland there is only one
+record and there are hardly any from Scotland.</p>
+
+<p class="indent"><i>Boarmia repandata</i> is another species which is behaving
+in a somewhat similar way. Unlike <i>betularia</i>, however, the species
+is a variable one, and has several colour-forms, amongst them
+the banded var. <i>conversaria</i>, and many others. In addition
+<span class="pagenum"><a name="Page_137" id="Page_137">[Pg 137]</a></span>
+to these there is a black form in the North of England which
+seems to be spreading. In Huddersfield the black was first
+recorded in 1888, and in 1900 20-25 per cent. were black. At
+Rotherham the black or very dark are now prevalent and have
+increased in the last 15 years. From the Midlands, East Anglia
+and Southern Counties the returns show only the light and
+medium forms.</p>
+
+<p class="indent">Of <i>Odontoptera bidentata</i> several intergrading
+dark forms exist, and these are found exclusively in the North and the
+Midlands. Unicolorous blacks have been found recently in the Lancashire
+mosses and at Wakefield. At Huddersfield 50 years ago the
+light forms were prevalent, but now a rather dark brown, not
+infrequently suffused with black, is the commonest. In Southern
+Counties only light forms are known.</p>
+
+<p class="indent"><i>Phigalia pilosaria</i> in South England is always
+light, but in the North the prevalent form is darker. About 35 years ago
+a form with unicolorous sooty fore-wings and dull grey hind
+wings was first seen in Yorkshire and a similar form is now taken
+regularly in South Wales.</p>
+
+<p class="indent">In the following cases the dark varieties were
+found originally only in the South.</p>
+
+<p class="indent"><i>Boarmia rhomboidaria</i> gave rise about 40 years ago
+to a unicolorous smoky variety called <i>perfumaria</i>. This was at first
+peculiar to the London district, but it has since been taken in
+Birmingham and other large cities. More lately coal-black
+specimens have been found at Norwich, and others similar but
+hardly so dark were taken in the South of Scotland and at Cannock Chase.</p>
+
+<p class="indent"><i>Eupithecia rectangulata</i> is a similar case.
+Formerly the light forms were prevalent but within sixty years they have
+almost entirely been replaced in the South of London by a nearly black form.</p>
+
+<p class="indent"><i>Tephrosia</i> (<i>Boarmia</i>) <i>consortaria</i> and
+<i>Tephrosia consonaria</i> are exceptionally interesting, for they have both
+given off dark forms in the same wood near Maidstone, which is far from the
+usual "centres of melanism." They were discovered in this
+locality by Mr. E. Goodwin. That of <i>consortaria</i> is a dark
+<span class="pagenum"><a name="Page_138" id="Page_138">[Pg 138]</a></span>
+grey, but that of <i>consonaria</i> is a full black, and nothing like
+either has been found anywhere else.</p>
+
+<p class="indent">These examples are all taken from the Geometridae but
+others, though of a less conspicuous kind, could be given from
+the Noctuidae or the Micro-Lepidoptera. <i>Acronycta psi</i>, for
+instance, has a suffused form which is believed to be becoming
+more frequent in the London district. <i>Polia chi</i> has two dark
+forms, <i>olivacea</i>, a yellowish grey with dark markings, and <i>suffusa</i>
+which is a darker, blackish-slate colour. Both occur in the North
+of England, sometimes together, sometimes separately, or mixed
+with the type and many intermediates. The distribution is
+peculiarly irregular. At Huddersfield, where the very dark form
+appeared suddenly about 1890, some 30 per cent. are said to be
+now dark and about 6-7 per cent. very dark, but at Saddleworth,
+12 miles away, only the pale forms occur.</p>
+
+<p class="indent">Several questions of interest arise in regard to this
+evidence. This progressive Melanism has arisen in certain families only,
+and may be confined to certain species only, within those families.
+As in almost all other examples in which variation has been much
+observed, its incidence is capricious and specific. A collateral
+line of inquiry relates to the degree of discontinuity which the
+variation manifests. Here again there is no rule. Generally
+speaking, in <i>A. betularia</i>, to take the case most fully studied, the
+variation is discontinuous. Real intermediates between <i>betularia</i>
+and <i>doubledayaria</i> are in most localities absent or rare.
+The black spots of <i>betularia</i> may often be larger or more numerous
+than in the normal, but this variation has nothing to do with
+<i>doubledayaria</i>, and is not an intermediate stage towards it,
+though sometimes wrongly so described. <i>Doubledayaria</i> owes its
+characteristic appearance to a factor which blurs the surface
+of the wings with a layer of black. Sometimes this blurring is
+slighter than in the real <i>doubledayaria</i>, and these forms are real
+intermediates. Occasionally the fore-wings alone are thus blurred.
+These intermediates are clearly due to reduction-stages of the
+<i>doubledayaria</i> factor, and are related to it as a blue mouse is to
+a black, or a dutch rabbit to a self-colour. It cannot positively
+be asserted that the full <i>doubledayaria</i> existed before the
+<span class="pagenum"><a name="Page_139" id="Page_139">[Pg 139]</a></span>
+intermediate, but it almost certainly did. In certain places as for
+instance in Belgium, there is evidence that intermediates have
+at various times been fairly abundant, but they have never become
+common, nor are they known to exist in the absence of
+<i>doubledayaria</i>. When the black variety and the light type breed
+together they do not usually have intermediates among their
+offspring, and the evidence is consistent with the view that the
+black is a complete dominant. The same is probably true of
+<i>Tephrosia consonaria</i>.</p>
+
+<p class="indent">In some of the other species we know that the darkest
+forms did not appear first. For example in <i>Phigalia pilosaria</i> and
+<i>Boarmia rhomboidaria</i> dark forms existed and are believed to
+have increased in number before the darkest made its appearance.
+<i>Hybernia progemmaria</i> is said to have become darker gradually
+both in Cheshire and in the West Riding, and a uniformly smoky
+variety appeared in South Yorkshire less than 45 years ago which
+has spread to neighbouring counties. The dark medium has
+become the commonest form in Huddersfield district, where the
+very dark variety is now about 20 per cent. of the population,
+though the light form is still common.</p>
+
+<p class="indent">Taking the evidence together we find it consistent with
+the view that dark forms have appeared sporadically, in some species
+the very dark appearing first and intermediates later, in others
+the moderately dark came first and the darkest later in time. It
+is practically certain that the change has in general come about
+not by a gradual change supervening on the population at large,
+but by the sporadic appearance of dark specimens as a new element
+in the population, and strains derived from these dark
+individuals have gradually superseded the normal type more or
+less completely.</p>
+
+<p class="indent">If it could be shown that these melanic novelties
+had a definite advantage in the struggle for existence they would provide
+an instance of evolution proceeding much in the way which
+Darwin contemplated. The whole process would differ from
+that conceived by him as the normal method of evolution only
+in so far as the change has come about with great rapidity and
+in some instances largely by the appearance and success of
+<span class="pagenum"><a name="Page_140" id="Page_140">[Pg 140]</a></span>
+discontinuous varieties. The question, however, must be asked
+whether the dark form can reasonably be supposed to have
+an advantage by reason of their darkness. Some naturalists
+believe that the darkness of the colours does thus definitely contribute
+to their protection by making the insects less conspicuous
+and thus more likely to escape the search of birds. In support
+of this view it may be pointed out that it is in the manufacturing
+districts of Lancashire and Yorkshire, and again in the London
+area that the melanics have attained their greatest development.
+Consistently with this argument also, it is in the neighbourhood of
+Crefeld and Essen, the black country of Germany, that they have
+chiefly established themselves on the Continent, and <i>Phigalia
+pilosaria</i> in the black form is now at home in South Wales. Thus
+superficially regarded, the evidence looks rather strong, but it
+is difficult to apply the reasoning in detail. We have first the
+difficulty that the black form of <i>betularia</i> for instance has
+established itself in thoroughly rural districts, notably near King's
+Lynn in Norfolk, and in the neighbourhood of Kendal and
+Windermere. The black form of <i>consonaria</i> and the dark
+<i>consortaria</i> appeared in a wood near Maidstone, far from town
+smoke, and the black <i>rhomboidaria</i> was first found at Norwich,
+which, as towns go, is clean. Then again the spread of the
+melanics is very irregular and unaccountable. The black <i>pilosaria</i>
+is found both in the West Riding and in the Swansea
+district, but not yet elsewhere. It rapidly increased at Huddersfield,
+but made no noticeable progress at Sheffield though recorded
+there for ten years. It is also a remarkable fact that no
+similar melanic development has been observed in America,
+and, so far as I am aware, comparable melanic varieties have not
+appeared on the European continent except in the case of the
+few sorts which possibly may have come from England.</p>
+
+<p class="indent">The whole subject is beset with complications. It must
+not be forgotten that in a few species of moths there is an obvious
+and recognised conformity between the colours of the perfect
+insect and that of the soil on which they live, comparable with
+that which is so striking in the case of some Oedipodidae and
+other grasshoppers. Of this phenomenon the clearest example
+<span class="pagenum"><a name="Page_141" id="Page_141">[Pg 141]</a></span>
+is <i>Gnophos obscurata</i>, which is a most variable species with many
+local forms. Of these a well-known dark variety lives on the
+peaty heaths of the New Forest and other districts, but on the
+chalk hills of Kent, Sussex and Surrey various light varieties
+are found, of which one is a bright silvery white, very near in
+colour to the colour of a chalky bank. This case does not seem
+to be one of direct environmental action,<a name="FNanchor_23_83" id="FNanchor_23_83"></a><a href="#Footnote_23_83" class="fnanchor">[23]</a>
+for Poulton found no change induced by rearing larvae among either
+white or black surrounding objects. No one however can doubt that
+there is some indirect connection between the colour of the
+ground and that of the moths.</p>
+
+<p class="indent">To my mind there is a serious objection to the theory
+of protective resemblance in application to such a case as that of the
+<i>betularia</i> forms, which arises from the fact that the black
+<i>doubledayaria</i> is a fairly conspicuous insect anywhere except
+perhaps on actually black materials, which are not common in any
+locality. Tree trunks and walls are dirty in smoky districts but
+they are not often black, and I doubt whether in the neighbourhood
+of Rotherham, for instance, which is one of the great
+melanic centres, <i>doubledayaria</i> can be harder for a bird to find
+than <i>betularia</i> would be. After all, too, many of the species
+much affected are not urban insects. They live in country
+places between the towns, and the general tone of these places
+even in Lancashire and the West Riding is not very different
+from that of similar places elsewhere. As against the objection
+that the black varieties are much blacker than the case requires
+it may be replied that we know nothing of the senses of birds,
+and that perhaps to their eyes blackness does constitute a disguise
+even though the surroundings are much less dark. This is
+undeniable, but recourse to such an argument is dangerous; for
+if the sight of the insect-eating birds is so dull that it does not
+distinguish dark things from dingy grey, we cannot subsequently
+regard the keen sight of birds as the sufficient control which has
+led to the minute and detailed resemblance of many insects to
+their surroundings. Those who see in such cases examples of
+the omnipotence of Selection must frequently find themselves in this dilemma.
+<span class="pagenum"><a name="Page_142" id="Page_142">[Pg 142]</a></span></p>
+
+<p class="indent">Taking the evidence as a whole, we may say that it
+fairly suggests the existence of some connection between modern urban
+developments and the appearance and rise of the melanic varieties.
+More than that we cannot yet affirm. It is a subject
+in which problems open up on every side, and all of them are
+profitable subjects for investigation. Unhappily such animals
+are difficult to rear successfully in captivity for many generations,
+owing to their extreme liability to disease. Not the least interesting
+feature of the melanics is the fact that the black varieties
+provide about the best and clearest example of a new dominant
+factor attaching itself to a wild species in recent times.
+None of the cases are satisfactorily recorded or analysed as yet,
+but the evidence is clear that <i>doubledayaria</i> is a dominant to its
+type, and in several other dark varieties, though the pigment
+deposited is not black, the records show that the increased
+amount of the pigment almost certainly is due to a positive factor.
+Of this, <i>Hemerophila abruptaria</i> is a good example.<a name="FNanchor_24_84" id="FNanchor_24_84"></a><a href="#Footnote_24_84" class="fnanchor">[24]</a>
+There are some irregularities in the results, but taken together they
+leave little doubt that the dark brown variety is a dominant and the
+light, yellowish brown a recessive.</p>
+
+<p class="indent">A curious parallel to the rise of the melanic moths
+in England is provided by the case of the Honey-creepers or Sugar-birds,
+in certain West Indian islands.<a name="FNanchor_25_85" id="FNanchor_25_85"></a><a href="#Footnote_25_85" class="fnanchor">[25]</a>
+These birds of the genus <i>Coereba</i> (<i>Certhiola</i>) range from Southern
+Mexico to the Northern parts of South America and through the whole chain
+of the West Indian islands and Bahamas except Cuba. There are numerous
+local forms, and many of the islands have types peculiar to themselves,
+as is usual in such cases. Some of the types or species
+range through several islands, but according to Austin Clark<a name="FNanchor_26_86" id="FNanchor_26_86"></a><a href="#Footnote_26_86" class="fnanchor">[26]</a>
+no island has more than one of them. Cory<a name="FNanchor_27_87" id="FNanchor_27_87"></a><a href="#Footnote_27_87" class="fnanchor">[27]</a>
+reckoned twelve such species within the Antillean region. They are small birds
+<span class="pagenum"><a name="Page_143" id="Page_143">[Pg 143]</a></span>
+about the size of a nuthatch with a general colouring of black,
+yellow, and white. From the island of St. Vincent the Smithsonian
+Institution received in the late seventies of last century
+several completely black specimens in addition to two of the usual
+type of colouring. The black were described by W. N. Lawrence
+as <i>atrata</i>, and those marked with the usual yellow and white
+were called <i>saccharina</i>. The collector (Mr. F. A. Ober) reported
+that the black form was common, and that the <i>saccharina</i> form
+was rarer. Lawrence remarks, "Had there been only a single
+example (of the black form) I should have considered it as probably
+a case of abnormal colouring, but it seems to be a representative
+form of the genus in this island."<a name="FNanchor_28_88" id="FNanchor_28_88"></a><a href="#Footnote_28_88" class="fnanchor">[28]</a>
+There is of course no doubt of the correctness of the view taken by Austin Clark
+that "<i>atrata</i>" is a black variety. The black bird is in every respect,
+other than colour, identical with <i>saccharina</i>, and it is even possible
+to detect a greenish colour in the areas which would normally be
+yellow, showing plainly enough the yellow pigment obscured by the black.</p>
+
+<p class="indent">We have next the interesting fact that like our melanic
+moths the dark form is replacing the "type." At the time of Ober's
+visit the type was already in a minority, but now it is nearly
+or perhaps actually extinct, though the black form is one of the
+commonest birds on the island. Austin Clark found no specimen
+when he collected there in 1903-4, though formerly it was not
+uncommon in the vicinity of Kingston and in the immediate
+windward district of St. Vincent.</p>
+
+<p class="indent">The Grenadines are geographically just south of
+St. Vincent, though separated by a deep channel. In these islands no
+black forms have yet been taken, but Grenada, the next island to the
+south, has both normals and blacks. There are trifling differences
+of size between the Grenada birds and those from St.
+Vincent, the Grenada specimens being slightly smaller and for
+this reason they have received distinct names, the form marked
+with yellow and white being called <i>Godmani</i> (Cory) and the black,
+<i>Wellsi</i> (Cory), but this merely introduces a useless complication.
+<span class="pagenum"><a name="Page_144" id="Page_144">[Pg 144]</a></span>
+There is evidence that in Grenada, as in St. Vincent, the black is
+gradually ousting the original type, but the process has not gone
+so far as in St. Vincent. Austin Clark very properly compares
+this case of the Sugar-birds with that of <i>Papilio turnus</i>, which as
+is well-known, has a black female in the southern parts of its distribution,
+in addition to a female of the yellow type, but in the
+Northern States the black female does not occur.</p>
+
+<p class="indent">During the present year P. R. Lowe, who lately studied
+<i>Coerebas</i> on a large scale in the West Indies, has published an
+important paper on the subject.<a name="FNanchor_29_89" id="FNanchor_29_89"></a><a href="#Footnote_29_89" class="fnanchor">[29]</a>
+He calls attention to the fact that Cory recently found
+a black form of <i>Coereba</i> on Los Roques Islands,
+and he himself discovered another on the
+Testigos Islands. Both localities are on the coast of Venezuela,
+far from St. Vincent and Grenada. The whole problem is thus
+further complicated by the fact that the black varieties have,
+as we are almost driven to admit, arisen independently in remote
+places. Improbable as this conclusion may be, it is still more
+difficult to regard all the black forms as derived from one source.
+For first, they present definite small differences from each other;
+and secondly we have to remember a consideration of greater
+importance, that the very fact that each island has its own type
+must be accepted as proving that the localities are effectively
+isolated from each other, and that migration must be a very rare event.</p>
+
+<p class="indent">The rarity of such illustrative cases is, I believe,
+more apparent than real. It is probably due to the extreme reluctance
+of systematists to admit that such things can be, and of course
+to the almost complete absence of knowledge as to the genetic
+behaviour of wild animals and plants. Only in such examples
+as this of the <i>Coereba</i>, where colour constitutes the sole
+difference, or that of the moths which have been minutely studied by
+many collectors, does the significance of the facts appear. The
+arrangement of catalogues and collections is such that much
+practical difficulty of a quite unnecessary kind is introduced. For
+example, in this very case of <i>Coereba</i>, I find the British Museum
+has a fine series from Grenada including 3 normals and 11 black,
+<span class="pagenum"><a name="Page_145" id="Page_145">[Pg 145]</a></span>
+and also 16 blacks from St. Vincent. If the black specimens from
+Grenada were put with the normals which are almost certainly
+nothing but a recessive form of the same bird, the variation would
+strike the eye on even a superficial glance at the drawer. But
+following the notions so naively expressed in the passage quoted
+above from W. N. Lawrence, the blacks from Grenada are put
+apart together with the other blacks from St. Vincent, though
+two of them were shot on the same date as one of the normals.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_146" id="Page_146">[Pg 146]</a></span></p>
+
+<h2>CHAPTER VII</h2>
+<h3><span class="smcap">Local Differentiation.</span> <i>Continued</i></h3>
+
+<p style="font-size: 120%; text-align: center;">
+<b><span class="smcap">Overlapping Forms</span></b></p>
+
+<p class="indent">The facts of the distribution of local forms on the whole
+are consistent with the view that these forms come into existence by the
+sporadic appearance of varieties in a population, rather than by
+transformation of the population as a whole. Of such sporadically
+occurring varieties there are examples in great abundance,
+though by the nature of the case it can be but rarely that we are
+able to produce evidence of a previous type being actually superseded
+by the variety. When the two forms are found co-existing
+in the same area they are usually recorded as one species if intergrades
+are observed, and as two species if the intergrades are
+absent. On the other hand when two forms are found occupying
+separate areas, when, that is, the process of replacement is completed
+in one of the areas, then forthwith each is named separately
+either as species or subspecies. Successive observations carried
+out through considerable periods of time would be necessary to
+establish beyond question that the history proceeds in one way
+rather than another. Such continuity of observation has for
+the most part never been attempted. The kind of information
+wanted has indeed only been lately recognized, and really critical
+collecting is a thing of only the last few decades. The methods of
+the older collectors, who aimed at bringing together a few typical
+specimens of all distinct forms, are of little service in this class
+of inquiry, which is better promoted by the indiscriminate collection
+of large numbers of common forms from many localities.
+When this has been done on a comprehensive scale we shall be
+in a position to form much more confident judgments as to the
+general theory of evolution.</p>
+
+<p class="indent">Some little work of the kind has however been done and
+the results are already of great value. Seeing that the differentiation
+of local forms is only made possible by isolation, it necessarily
+<span class="pagenum"><a name="Page_147" id="Page_147">[Pg 147]</a></span>
+happens that the collector finds one form in one locality
+and another in a distinct locality, and there is no evidence as to
+the behaviour which the two representative species might exhibit
+if they came into touch with each other. In the most familiar
+examples of such distinction each inhabits an island, completely
+occupying it to the exclusion of any other similar form. It can
+only be when the two representative species occupy parts of a
+continental area connected with each other by regions habitable
+for the organism in question, that there is a chance of seeing the
+two forms in contact. Often also, even where this condition is
+satisfied, the habits, social organisation, or some other special
+cause may act as a barrier which prevents the distinguishable
+forms from ever coming into such complete contact as to interbreed
+or to behave as a genetically continuous race. When
+genetic continuity is ensured by a constant diffusion of the population
+over the whole area which they inhabit there will manifestly
+be no formation of local races. The practical uniformity,
+for example, of so many species of birds which inhabit widely
+extended ranges of Western Europe is doubtless maintained by
+such constant diffusion. When, as in the case of the Falcons,
+many localities have peculiar forms, the fact may be taken as
+conclusive evidence that there is little or no diffusion; and when
+we find in such a species as the Goldfinch that in spite of migratory
+fluctuations there are nevertheless geographical races
+fairly well differentiated, it may similarly be inferred that these
+fluctuations habitually move up and down on paths which do
+not intermingle. There are however a few examples of animals,
+not given to much irregular wandering, which occupy a wide and
+continuous range of diversified country and are differentiated as
+local races in two or more districts, though the distinct races
+meet in intervening areas. Of these the most notorious illustration
+which has been investigated with any thoroughness is
+that of the species of <i>Colaptes</i> (Woodpeckers) known in the United
+States as Flickers. The study of the variations of these forms,
+made by J. A. Allen<a name="FNanchor_1_90" id="FNanchor_1_90"></a><a href="#Footnote_1_90" class="fnanchor">[1]</a>
+is an admirable piece of work, with which
+<span class="pagenum"><a name="Page_148" id="Page_148">[Pg 148]</a></span>
+every student of variation and evolutionary problems should
+make himself familiar. The two forms with which we are most
+concerned are known as <i>C. auratus</i> and <i>C. cafer</i>, and are very
+strikingly different in appearance. In size, proportions, general
+pattern of colouration, habits, and notes, the two are alike, but
+they differ in the following seven respects as stated by Allen.</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Woodpeckers" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>Auratus</i></td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>Cafer</i></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">1.&nbsp;&nbsp;</td>
+      <td class="tdl">Quills <i>yellow</i>.</td>
+      <td class="tdr">1.&nbsp;&nbsp;</td>
+      <td class="tdl">Quills <i>red</i>.</td>
+    </tr><tr>
+      <td class="tdr">2.&nbsp;&nbsp;</td>
+      <td class="tdl">Male with a <i>black</i> malar stripe.</td>
+      <td class="tdr">2.&nbsp;&nbsp;</td>
+      <td class="tdl">Male with a <i>red</i> malar stripe.</td>
+    </tr><tr>
+      <td class="tdr">3.&nbsp;&nbsp;</td>
+      <td class="tdl">Adult female with <i>no</i> malar stripe.</td>
+      <td class="tdr">3.&nbsp;&nbsp;</td>
+      <td class="tdl">Adult female with usually a <i>brown</i> malar stripe.</td>
+    </tr><tr>
+      <td class="tdr">4.&nbsp;&nbsp;</td>
+      <td class="tdl">A <i>scarlet</i> nuchal crescent in both sexes.</td>
+      <td class="tdr">4.&nbsp;&nbsp;</td>
+      <td class="tdl"><i>No</i> nuchal crescent in either sex.</td>
+    </tr><tr>
+      <td class="tdr">5.&nbsp;&nbsp;</td>
+      <td class="tdl">Throat and fore neck <i>brown</i>.</td>
+      <td class="tdr">5.&nbsp;&nbsp;</td>
+      <td class="tdl">Throat and fore neck <i>grey</i>.</td>
+    </tr><tr>
+      <td class="tdr">6.&nbsp;&nbsp;</td>
+      <td class="tdl">Whole top of head and hind neck <i>grey</i>.</td>
+      <td class="tdr">6.&nbsp;&nbsp;</td>
+      <td class="tdl">Whole top of neck and hind neck <i>brown</i>.</td>
+    </tr><tr>
+      <td class="tdr">7.&nbsp;&nbsp;</td>
+      <td class="tdl">General plumage with an <i>olivaceous</i> cast.&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdr">7.&nbsp;&nbsp;</td>
+      <td class="tdl">General plumage with a <i>rufescent</i> cast.</td>
+    </tr>
+ </tbody>
+</table>
+
+<p>These differences are illustrated in the accompanying coloured
+plate, which has been most kindly prepared for me under the instructions
+of Dr. F. M. Chapman of the American Museum of
+Natural History. Before going further it is worth considering the
+nature of these differences a little more closely. All but the last
+are large differences which no one would overlook even in a hasty
+glance at the birds. If the only distinction lay in the colour of the
+quills we might feel fairly sure that <i>auratus</i> was a recessive form
+of <i>cafer</i>, and so probably it is in this respect. Similarly the black
+malar stripe of <i>auratus</i> is in all probability recessive to the red
+malar stripe of <i>cafer</i> and I imagine the pigments concerned are
+comparable with those in the Gouldian Finch (<i>Poephila gouldiae</i>)
+of Australia. Both sexes in that species may have the head black,
+red, or, less often, yellow, and though it is not any longer in
+question that birds may breed in either plumage, I believe that
+the young are always black-headed and I imagine that those
+which become red-headed possess a dominant factor absent from
+the permanently black-headed birds.<a name="FNanchor_2_91" id="FNanchor_2_91"></a><a href="#Footnote_2_91" class="fnanchor">[2]</a>
+<span class="pagenum"><a name="Page_149" id="Page_149">[Pg 149]</a></span>
+Yellow as a recessive form of a red is certainly very common, but red
+and black as variants of the same pigment are less usual. In the Gouldian
+Finch we seem to have a case where a pigment can assume all
+three forms. It would be interesting to know whether the red
+of the malar stripes in <i>Colaptes</i> is a pigment of the same nature
+as the red of the quills. Both in <i>Colaptes</i> and in <i>Poephila
+gouldiae</i> I have seen specimens intermediate between the black
+and the red, and the appearance of the part affected was exactly
+alike in the two cases, red feathers coming up among the black
+ones, and many feathers containing both red and black pigments
+mixed together.</p>
+
+<p class="indent">The development of the scarlet nuchal crescent in <i>auratus</i>
+and the absence of this conspicuous mark in <i>cafer</i> constitute from
+the physiological point of view the most remarkable pair of differences.
+When the red crescent is not formed, the feathers
+which would bear it are exactly like the rest, and no special
+pigment is visible in them which one can regard as ready to be
+modified into red. If the crescent is due to a factor it must
+therefore be supposed that this factor has the power of modifying
+the pigment of the neck in one special place alone. Dr. W. D.
+Miller called my attention to the fact that a similar variation
+occurs in another American woodpecker, the Sapsucker,
+<i>Sphyropicus varius</i>.<a name="FNanchor_3_92" id="FNanchor_3_92"></a><a href="#Footnote_3_92" class="fnanchor">[3]</a>
+</p>
+
+<p class="indent">I do not suggest that such variations are without parallel:
+indeed in <i>P. gouldiae</i> the factor which turns the black of the head
+into scarlet affects one special region of the black only, being
+sharply distinct from the unmodified black of the throat. These
+regions of the head are however often the seat of special colours
+in birds.<a name="FNanchor_4_93" id="FNanchor_4_93"></a><a href="#Footnote_4_93" class="fnanchor">[4]</a>
+So also may be instanced the variety of the Common
+<span class="pagenum"><a name="Page_150" id="Page_150">[Pg 150]</a></span>
+Guillemot (<i>Uria troile</i>) which has a white line round the eyes and
+at the sides of the head where the normal has no such mark; but
+this line is formed in a very special place, the groove joining the
+eye to the ear, whereas the feathers of the nuchal crescent are
+not ostensibly distinguished from those adjacent.<a name="FNanchor_5_94" id="FNanchor_5_94"></a><a href="#Footnote_5_94" class="fnanchor">[5]</a>
+</p>
+
+<p class="indent">The transposition of the brown and the grey on the back
+and front of the neck also constitutes a very remarkable difference.
+If either grey or brown depends on a factor then it must be supposed
+that <i>auratus</i> has one of these factors and <i>cafer</i> the other.</p>
+
+<p class="indent">From these several considerations it is quite clear that if
+<i>auratus</i> and <i>cafer</i> are modifications of the same type produced
+by presence or absence of factors, several independent elements
+must be concerned, and to unravel their inter-relations would be
+most difficult even if it were possible to breed the types under observation,
+which is of course quite beyond present possibilities.</p>
+
+<p class="indent">The distribution of the two is as follows. On the east side
+of the Continent <i>C. auratus</i>, relatively pure, occupies the whole of
+Canada and the States from the North to Galveston. Westward
+it extends across the whole continent in the more northern
+region to Alaska, but in its pure form it only reaches down the
+Pacific coast to about the northern border of British Columbia.
+Its southern and western limit is thus roughly a line drawn from
+north of Vancouver, southeast to North Dakota and then south
+to Galveston. <i>C. cafer</i> in the comparatively pure form inhabits
+Mexico, Arizona, California (except Lower California and the
+opposite coast), central and western Nevada, Utah, Oregon, and
+is bounded on the east by a line drawn from the Pacific south of
+Washington, south and eastward through Colorado to the mouth
+<span class="pagenum"><a name="Page_151" id="Page_151">[Pg 151]</a></span>
+of the Rio Grande or the Gulf of Mexico. Between the two
+lines thus roughly defined is a band of country about 1,200-1,300
+miles long and 300-400 miles wide, which contains some normal
+birds of each type, but chiefly birds exhibiting the characters of
+both, mixed together in various and irregular ways. Even in
+the areas occupied by the pure forms occasional birds are recorded
+with more or less indication of characteristics of the other
+form, but within the area in which the two forms are conterminous,
+the mixed birds are in the majority. The condition of these birds
+of mixed character is described by Allen as follows:</p>
+
+<p class="blockquot">"As has been long known&mdash;indeed, as shown by Baird
+in 1858&mdash;the 'intermediates' or 'hybrids' present ever-varying
+combinations of the characters of the two birds, from individuals
+of <i>C. auratus</i> presenting only the slightest traces of the characters
+of <i>C. cafer</i>, or, conversely&mdash;individuals of <i>C. cafer</i>
+presenting only the slightest traces of the characters of
+<i>C. auratus</i>&mdash;to birds in which the characters of the two are about
+equally blended. Thus we may have <i>C. auratus</i> with merely a few red
+feathers in the black malar stripe, or with the quills merely
+slightly flushed with orange, or <i>C. cafer</i> with either merely a few
+black feathers in the red malar stripe, or a few red feathers at
+the sides of the nape, or an incipient, barely traceable scarlet
+nuchal crescent. Where the blending of the characters is more
+strongly marked, the quills may be orange-yellow or orange-red,
+or of any shade between yellow and red, with the other features
+of the two birds about equally blended. But such examples are
+exceptional, an unsymmetrical blending being the rule, the two
+sides of the same bird being often unlike. The quills of the tail,
+for example, may be part red and part yellow, the number of
+yellow or red feathers varying in different individuals, and very
+often in the opposite sides of the tail in the same bird. The
+same irregularity occurs also, but apparently less frequently,
+in the quills of the wings. In such cases the quills may be mostly
+yellow with a few red or orange quills intermixed, or red with a
+similar mixture of yellow. A bird may have the general colouration
+of true <i>cafer</i> combined with a well-developed nuchal crescent,
+or nearly pure <i>auratus</i> with the red malar stripes of a <i>cafer</i>.
+<span class="pagenum"><a name="Page_152" id="Page_152">[Pg 152]</a></span>
+Sometimes the body plumage is that of <i>C. auratus</i> with the head
+nearly as in pure <i>cafer</i>, or exactly the reverse may occur. Or we
+may have the general plumage as in <i>cafer</i> with the throat and
+crown as in <i>auratus</i>, and the malar stripe either red or black,
+or mixed red and black, and so on in almost endless variations,
+it being rare to find, even in birds of the same nest, two individuals
+alike in all their features of colouration. Usually the
+first trace of <i>cafer</i> seen in <i>auratus</i> manifests itself as a mixture of
+red in the black malar stripe, either as a few red feathers, or as a
+tipping of the black feathers with red, or with merely the basal
+portion of the feathers red. Sometimes, however, there is a
+mixture of orange or reddish quills, while the malar stripe remains
+normal. In <i>C. cafer</i> the traces of <i>auratus</i> are usually shown by a
+tendency to an incipient nuchal crescent, represented often by
+merely a few red-tipped feathers on the sides of the nape; at
+other times by a slight mixture of black in the red malar stripe."</p>
+
+<p class="indent">Such a state of things accords very imperfectly with
+expectations under any received theory of Evolution. As in some of
+the instances discussed in the first chapter we have here two
+fairly definite forms, nearly allied, which on any evolutionary
+hypothesis must have been evolved either the one from the other,
+or both from a third form at a time not very remote from the
+present, as time must be measured in evolution. Yet though
+intermediates exist in some quantity, no one can for a moment
+suggest that they are that definite intermediate from which
+<i>auratus</i> and <i>cafer</i> descend in common. One cannot imagine
+that the immediate ancestor of these birds was a mosaic, made up of
+asymmetrical patches of each sort: but that is what many of the
+intermediates are. It is not much easier to suppose the ancestor
+to have been a nondescript, with a compromise between the
+developed characters of each, with quills buff, malar stripes
+neither black nor red, with a trace of nuchal crescent, and so on.
+Such Frankenstein-monsters have played, a considerable part in
+the imaginations of evolutionary philosophers, but if it were
+true that there was once a population of these monsters capable
+of successful existence, surely they should now be found as a
+population occupying the neutral zone between the two modern
+<span class="pagenum"><a name="Page_153" id="Page_153">[Pg 153]</a></span>
+forms. Yet, though much remains to be done in clearing up the
+facts, one thing is certain, namely that the neutral zone has not a
+definite and normally intermediate population, but on the contrary
+it is peopled by fragments of the two definite types and
+miscellaneous mongrels between them.</p>
+
+<p class="indent">On the other hand, one cannot readily suppose that
+either form was the parent of the other. The process must have
+involved both addition and loss of factors, for whatever hypothesis
+be adopted, such changes must be supposed to have occurred.
+A careful statistical tabulation of the way in which the characters
+are distributed in the population of the mixed zone would be
+of great value, and till that has been done there is little that can
+be said with certainty as to the genetics of these characters.
+In the collection of Dr. Bishop of New Haven I was very kindly
+allowed to examine a sample, all taken at random, near together,
+in Saskatchewan. There were females 4 adult, 2 young; males
+4 adult and 5 young. This number, though of course insufficient,
+is enough to give some guide as to the degree of definiteness which
+the characters generally show in their variations. Of the 15
+birds, 8 had simply yellow quills; 2 had red; 1 was almost red
+but had one yellow tail-quill; 3 were intermediate and 1 was
+buff. As regards the malar patch, which can only be determined
+properly in the adult males, 1 was red, 1 was approximately red,
+2 intermediate. As to nuchal crescent 4 females had none, 2
+females very slight; 7 males had it, 1 had only a slight crescent,
+and 1 had none. In point of quills therefore 10 were definite
+out of 15; in point of crescent, 11 were definite out of 15; and in
+point of malar patch 1 only was definite out of 4. The last is a
+feature directly dependent on age and so counts for less, but as
+regards the other two features there is some indication that the
+factors show definiteness in their behaviour. It must be remembered
+that we have no knowledge what the heterozygous
+form may be, and in the case of red and yellow it is probably a
+reddish buff. The patch-works are no doubt to be compared with
+other well-known pied forms, and in these we must suppose the
+active factor broken up, which it probably can be very easily.
+The asymmetry, which Allen notices as so marked a feature, in the
+<span class="pagenum"><a name="Page_154" id="Page_154">[Pg 154]</a></span>
+distribution of the red and yellow quills of the tail especially,
+recalls that of the black markings in the pied Canaries. As is well
+known to students of variations <i>some</i> pigment-factors in <i>some</i>
+animals are apparently uncontrolled by symmetry, while in other
+specific cases symmetry is the rule. On the other hand the
+blackness or redness of the malar patches is, I think, as a rule
+nearly symmetrical. It should be mentioned that two of Dr.
+Bishop's young birds belonged to the same nest, one a female
+with <i>red</i> quills, the other a male with <i>yellow</i>.
+Both are without crescent.</p>
+
+<p class="indent">As to the question whether certain combinations of characters
+occur with special frequency, the evidence is insufficient to give a
+definite answer. Among all the birds I have seen in America
+or in England I have not yet found one having the malar patches
+black without any nuchal crescent. Of Dr. Bishop's 8 adults
+not one, however, showed the combination of the three chief
+features normal for <i>auratus</i> or for <i>cafer</i>.</p>
+
+<p class="indent">Besides the two forms that we have hitherto considered,
+several other local types exist, and these throw some further
+light on the problem. Of these the most important in this
+connexion is <i>chrysoides</i>, which inhabits the whole of southern
+California and the mainland opposite. This remarkable form
+is as Allen says, very different from <i>auratus</i> except that it has
+the quills yellow like <i>auratus</i>, not red like <i>cafer</i>. So that
+we find here in the extreme west of the whole distribution a type
+agreeing in one of its chief features with the eastern type. Between
+this and <i>cafer</i> intergrades have, according to Allen, not
+been found. The relations of this <i>chrysoides</i> are, Allen thinks,
+rather with <i>mexicanoides</i>, a southern, smaller race with colours
+more intense, which inhabits Guatemala, but however that may
+be, it must be regarded as a <i>cafer</i> which has lost its red quills.
+The island of Guadeloupe off Lower California has an island
+form. Beyond the other side of the continent there is also an
+island form of <i>auratus</i>, inhabiting Cuba, so that clearly the yellow
+quills can extend into the tropics.</p>
+
+<p class="indent">The above account is in many respects incomplete, but
+it suffices to give an outline of the chief facts. The whole problem
+<span class="pagenum"><a name="Page_155" id="Page_155">[Pg 155]</a></span>
+is complicated by the undoubted effects of an uncertain amount
+of migration, and in many, perhaps all, districts, the winter
+population differs from the summer population of the same
+localities. The existence of these seasonal ebbs and flows is
+now well known to ornithologists, and most of the bird species
+of temperate regions are subject to them.</p>
+
+<p class="indent">Difficult as it may be to conceive the actual process of
+origin of the two types <i>auratus</i> and <i>cafer</i>, it is I think still
+harder to suggest any possible circumstance which can have determined
+their development as distinct races, or which can maintain
+that distinctness when created. Some will no doubt be disposed
+to appeal once more to our ignorance and suggest that if we only
+knew more we should see that the yellow quills, the black
+"moustache" and the red crescent, specially qualify <i>auratus</i>
+for the north and eastern region, and the red quills, red "moustache"
+and absence of crescent fit <i>cafer</i> to the conditions of
+its homes. Each can judge for himself, but my own view is
+that this is a vain delusion, and that to cherish it merely blunts
+the receptivity of the mind, which if unoccupied with such fancies
+would be more ready to perceive the truth when at last it shall
+appear. Think of the range of conditions prevailing in the
+country occupied by <i>auratus</i>&mdash;a triangle with its apex in Florida
+and its base the whole Arctic region of North America. Is it
+seriously suggested that there is some element common to the
+"conditions" of such an area which demands a nuchal crescent
+in the Flickers, though the birds of the <i>cafer</i> area, almost equally
+varied, can dispense with the same character? Curiously enough,
+the geographical variation of <i>Sphyropicus varius</i>, another though
+a very different Woodpecker<a name="FNanchor_6_95" id="FNanchor_6_95"></a><a href="#Footnote_6_95" class="fnanchor">[6]</a>
+shows that conversely the nuchal crescent can be dispensed with
+in the Eastern form though it is assumed by the Western.<a name="FNanchor_7_96" id="FNanchor_7_96"></a><a href="#Footnote_7_96" class="fnanchor">[7]</a>
+<span class="pagenum"><a name="Page_156" id="Page_156">[Pg 156]</a></span></p>
+
+<p class="indent">Allen points out the interesting additional fact that superposed
+upon each of the two distinct forms, <i>auratus</i> and <i>cafer</i>,
+are many geographical variations which can very naturally be
+regarded as climatic. Each decreases in size from the North
+southward, as so many species do.<a name="FNanchor_8_97" id="FNanchor_8_97"></a><a href="#Footnote_8_97" class="fnanchor">[8]</a>
+They become paler in the arid plains, and show the
+ordinary phases which are seen in other birds having
+the same distribution. Such differences
+we may well suppose to be determined directly or indirectly, by
+environment, and we may anticipate with fuller knowledge it
+will be possible to distinguish variations of this nature as in the
+broad sense environmental, from the larger differences separating
+the two main types of <i>Colaptes</i>, which I surmise are altogether
+independent of such influences.</p>
+
+<p class="indent">It is generally supposed that phenomena like those now so
+well established in the case of <i>Colaptes</i> are very exceptional, and
+as has already been stated a number of circumstances must
+combine in order that they may be produced. I suspect however
+that the examples are more numerous than is commonly thought.
+In all likelihood the three forms <i>Sphyropicus varius</i>, <i>nuchalis</i>
+and <i>ruber</i> are in a very similar condition though the details
+have not, so far as I know, been worked out. A complex example
+which is closely parallel to the case of <i>Colaptes</i> was described by
+F. M. Chapman<a name="FNanchor_9_98" id="FNanchor_9_98"></a><a href="#Footnote_9_98" class="fnanchor">[9]</a>
+at the same date as Allen's work. This is the
+case of <i>Quiscalus</i>, the Grackles, which in the North American
+Continent have three fairly distinct forms which Chapman speaks
+of as <i>Q. aeneus</i>, <i>Q. quiscula</i>, and <i>Q. quiscula aglaeus</i>.
+The birds are all, so far as pigment is concerned, dark blackish brown, but
+the head and mantle have superposed a metallic sheen of interference-colours
+which in the various forms take different tints,
+<span class="pagenum"><a name="Page_157" id="Page_157">[Pg 157]</a></span>
+bluish green, bronze green, or bronze purple. The details are
+complicated and difficult to appreciate without actual specimens,
+but the two common types are sufficiently distinct. The birds
+inhabit the whole area east of the Rockies, <i>quiscula aglaeus</i>
+occupying Florida and the Southern States southwest of a band
+of country about a hundred miles broad extending roughly from
+Connecticut to the mouth of the Mississippi; and <i>aeneus</i> taking
+the area north and west of this band. In discussing this case
+Chapman expresses the same view as Allen does in the <i>Colaptes</i>
+case, that there are two distinct populations, substantially fixed,
+and that the band of country in which they meet each other
+has a mongrel population, with no consistent type, but showing
+miscellaneous combinations of the character of the two chief types.</p>
+
+<p class="indent">The warblers of the genus <i>Helminthophila</i> provide
+another illustration which has points of special interest. The two chief
+species are <i>H. pinus</i>, which has a yellow mantle and lower parts,
+white bars on the wings, a black patch behind the eyes and a
+broad black mark on the throat; and <i>H. chrysoptera</i> with dark grey
+mantle and pale whitish grey lower parts, yellow bars on the
+wings, and grey marks on cheeks and throat where <i>pinus</i> has
+black. These two birds are exceeding distinct, and in addition
+their songs are quite unlike. <i>H. pinus</i> ranges through the eastern
+United States up to Connecticut and Iowa. <i>H. chrysoptera</i> is a
+northern form extending down to Connecticut and New Jersey.
+Both are migrants.</p>
+
+<p class="indent space-below">In these two States, where the two types overlap,
+certain forms have been repeatedly found which have been described as
+two distinct species, <i>Lawrencei</i> and <i>leucobronchialis</i>. Dr. L. B.
+Bishop and Mr. Brewster showed me two long series of <i>Helminthophila</i>
+containing various intergrades between the four
+named kinds, and details regarding these may be found in
+Chapman's <i>North American Warblers</i> and in Dr. Bishop's paper
+in Auk, 1905, XXII. Though the characters evidently break
+up to some extent, the series can be represented as due to
+recombinations of definite factors more easily than the others
+which I have described. The differentiating characters are:
+<span class="pagenum"><a name="Page_158" id="Page_158">[Pg 158]</a></span></p>
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Warblers" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>Pinus</i></td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>Chrysoptera</i></td>
+    </tr><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+    </tr><tr>
+      <td class="tdr">1.&nbsp;&nbsp;</td>
+      <td class="tdl">Mantle and lower parts <i>yellow</i> (<b>Y<sup>1</sup></b>).&nbsp;&nbsp;</td>
+      <td class="tdr">1.&nbsp;&nbsp;</td>
+      <td class="tdl">Mantle and lower parts <i>grey</i> (<b>y<sup>1</sup></b>).</td>
+    </tr><tr>
+      <td class="tdr">2.&nbsp;&nbsp;</td>
+      <td class="tdl">Wing-bars <i>white</i> (<b>y<sup>2</sup></b>).</td>
+      <td class="tdr">2.&nbsp;&nbsp;</td>
+      <td class="tdl">Wing-bars <i>yellow</i> (<b>Y<sup>2</sup></b>).</td>
+    </tr><tr>
+      <td class="tdr">3.&nbsp;&nbsp;</td>
+      <td class="tdl">Cheek and throat <i>not black</i> (<b>b</b>).</td>
+      <td class="tdr">3.&nbsp;&nbsp;</td>
+      <td class="tdl">Cheek and throat <i>black</i> (<b>B</b>).</td>
+    </tr>
+ </tbody>
+</table>
+
+<p class="indent space-above">The grey pigment of the mantle is common to both, but is
+masked by the yellow in <i>pinus</i>, the net result being an olive-green.<a name="FNanchor_10_99" id="FNanchor_10_99"></a><a href="#Footnote_10_99" class="fnanchor">[10]</a>
+</p>
+
+<p class="indent">I am much indebted to Dr. F. M. Chapman for the loan
+of the coloured plate in which these distinctions are shown. It first
+appeared in his book, <i>North American Warblers</i>.</p>
+
+<p class="indent">We cannot tell whether <i>yellow</i> or <i>not-yellow</i>
+is due to the presence of a factor, but we may suppose that one or other
+gives the special colour to the parts. The black of character 3
+is no doubt a dominant. Thus <i>pinus</i> becomes Y<sup>1</sup>y<sup>2</sup>b and <i>chrysoptera</i>
+in y<sup>1</sup>Y<sup>2</sup>B. The <i>Lawrencei</i> which has the underparts
+<i>yellow</i>, wing-bars <i>white</i>, and <i>black</i> patches is Y<sup>1</sup>y<sup>2</sup>B and
+<i>leucobronchialis</i> which has mantle and underparts <i>not-yellow</i>, wing-bars
+<i>yellow</i> and <i>no black patches</i> is y<sup>1</sup>Y<sup>2</sup>b. This representation,
+it should be clearly understood, is tentative and approximate
+only. The characters are not really sharp, for there is much
+grading; but allowing for the effects of heterozygosis and for some
+actual breaking-up of factors I believe it gives a fairly correct
+view of the case. In particular we can see how it meets the difficulty
+which Chapman felt in accepting <i>leucobronchialis</i> as in
+any sense derived from <i>pinus</i> which has a yellow breast, and
+<i>chrysoptera</i> which has a black throat, seeing that <i>leucobronchialis</i>
+has neither. We now recognize at once that this form could be
+produced by ordinary re-combination of the absence of Y<sup>1</sup> with
+the absence of B.</p>
+
+<p class="indent space-below">I note also with great interest that the modern
+observers agree that the so-called hybrids may have the song either of
+the one species, or of the other, or a song intermediate between
+the two. It may also be added that these two types have several
+times been seen, in the breeding season, paired with each other
+or with one of the other combinations.</p>
+
+<p><span class="pagenum"><a name="Page_159" id="Page_159">[Pg 159]</a></span></p>
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 1.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl"><i>Helminthophila pinus</i>, male.</td>
+    </tr><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 2.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl"><i>Helminthophila pinus</i>, female.</td>
+    </tr><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 3.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl">"Lawrence's Warbler," male; one of the integrading forms.</td>
+    </tr><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 4.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl">"Brewster's Warbler," male; another of the integrading forms.</td>
+    </tr><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 5.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl"><i>Helminthophila chrysoptera</i>, male.</td>
+    </tr><tr>
+      <td class="tdr"><span class="smcap"><b>Fig. 6.</b>&nbsp;&nbsp;</span></td>
+      <td class="tdl"><i>Helminthophila chrysoptera</i>, female.</td>
+    </tr>
+ </tbody>
+</table>
+
+<div class="figcenter" >
+  <img src="images/i_175.jpg" alt="Warblers" width="600" height="941" />
+</div>
+
+<p class="indent space-above space-below">Allen<a name="FNanchor_11_100" id="FNanchor_11_100"></a><a href="#Footnote_11_100" class="fnanchor">[11]</a>
+has described another excellent American example,
+the Tits of the group <i>Baeolophus bicolor-atricristatus</i>. The form
+<i>bicolor</i> belongs to the eastern States and ranges from the Atlantic
+coast to the Great Plains, and <i>atricristatus</i>, of east Mexico,
+extends from Vera Cruz to central Texas. In southern and
+central Texas the breeding ranges adjoin, and in this country
+various intermediates occur. The chief types differ in two main points.</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Warblers" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>B. bicolor</i></td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>B. atricristatus</i></td>
+    </tr><tr>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+    </tr><tr>
+      <td class="tdl">Forehead varies from <i>deep black</i></td>
+      <td class="tdl">&emsp;Forehead <i>white</i> to <i>buffish white</i>.</td>
+    </tr><tr>
+      <td class="tdl">&emsp;to <i>dull black</i>, suffused with <i>rusty brown</i>.</td>
+      <td class="tdl">&nbsp;</td>
+    </tr><tr>
+      <td class="tdl">Crown and crest <i>grey</i>,</td>
+      <td class="tdl">&emsp;Crown and crest <i>black</i>, abruptly</td>
+    </tr><tr>
+      <td class="tdl">&emsp;slightly darker than the back.</td>
+      <td class="tdl">&emsp;&emsp;contrasting with the back.</td>
+    </tr>
+ </tbody>
+</table>
+
+<p class="space-above">The intergrades between the two have, as usual,
+received specific names. A detailed description is given by Allen, from which
+it appears that the gradation is very complete. In one case a
+series of 16 adults were all intermediates. It is not stated whether
+the collector took these at random, but from the local lists it is
+clear that the types are found not far away from the place where
+the intergrades were shot.</p>
+
+<p class="indent">Another very striking case is that of the Tanagers, of the
+genus <i>Rhamphocoelus</i>. In this group there are several local forms
+which are related to each other in remarkable ways. The forms
+known as <i>passerinii</i> and <i>icteronotus</i> exhibit the clearest
+phenomena of intergradation. The species <i>passerinii</i> has a
+brilliant scarlet and black male, and it inhabits Honduras and Nicaragua.
+Proceeding southwards along the isthmus we find next <i>costaricensis</i>
+which has a male like that of <i>passerinii</i> (but a female
+with more orange than the olive-grey female of <i>passerinii</i>).
+Next we come to Panama which is occupied by <i>icteronotus</i>,
+sharply distinguished from <i>passerinii</i> by the fact that the <i>scarlet
+is replaced by lemon-yellow</i>. This same <i>icteronotus</i> occurs again
+as a pure type in Ecuador and many other parts of South America;
+but Colombia, <i>between Panama and Ecuador</i>, contains scarlets
+like <i>passerinii</i>, yellows like <i>icteronotus</i>, and various intergrades
+<span class="pagenum"><a name="Page_160" id="Page_160">[Pg 160]</a></span>
+of several shades of orange. The <i>passerinii</i> males from Nicaragua
+are indistinguishable from those of Colombia, and the
+<i>icteronotus</i> of Ecuador are the same as those in Panama. The
+orange intergrades, doubtless heterozygous forms, though collected
+at the same locality (Medellin in Colombia) as several
+pure yellows and pure scarlets, are in the British Museum series
+sorted out as a separate species under the name <i>chrysonotus</i>!
+Complications are introduced by the relations of these forms to
+another named type, <i>flammigerus</i>, but we may for our purpose
+leave that out of consideration, and say that the order of geographical
+sequence from Honduras to Ecuador is (1) scarlet, (2) yellow,
+(3) mixture of types, scarlet, yellow, orange, (4)yellow.</p>
+
+<p class="indent space-below">Similar examples exist in the birds of the old world,
+but I do not know of any that have been studied so fully as those of
+America. The best known is that of the two Rollers, <i>Coracias
+indicus</i> which spreads from Asia Minor through Persia, Baluchistan,
+the Indian Peninsula and Ceylon, and <i>affinis</i> which
+ranges from Nepal, through Assam, Tenasserim and the Indo-Chinese
+countries. The two types are very different and may be
+distinguished as follows:</p>
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Rollers" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>C. indicus</i></td>
+      <td class="tdl">&emsp;&emsp;&emsp;<i>C. affinis</i></td>
+    </tr><tr>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdl">&nbsp;</td>
+    </tr><tr>
+      <td class="tdl"><i>Mantle</i>&emsp;drab brown-chestnut.</td>
+      <td class="tdl">Dark olive-green.</td>
+    </tr><tr>
+      <td class="tdl"><i>Breast</i>&emsp;chestnut.</td>
+      <td class="tdl">Dull purple brown.</td>
+    </tr><tr>
+      <td class="tdl"><i>Throat</i>&emsp;purplish, streaked with white.&nbsp;&nbsp;</td>
+      <td class="tdl">Purple, streaked with blue.</td>
+    </tr><tr>
+      <td class="tdl"><i>Upper tail-coverts</i>&emsp;indigo.</td>
+      <td class="tdl">Turquoise.</td>
+    </tr>
+ </tbody>
+</table>
+
+<p class="indent space-above">The wings are the same in both. In the provinces
+of Nepal, Sikhim, and Darjiling the two species coexist, with the result
+that intergrades have been frequently recorded. The line of
+intergradation extends to the coast, and birds showing various
+combinations of the two types from the Calcutta district exist
+in collections.<a name="FNanchor_12_101" id="FNanchor_12_101"></a><a href="#Footnote_12_101" class="fnanchor">[12]</a>
+The case is interesting inasmuch as like that of
+<i>Quiscalus</i> it shows a series of combinations of various metallic
+colours. Some of these are probably evoked by the development
+of pigment behind striations or other interferences already existing,
+but in the present state of knowledge it would be quite impossible
+<span class="pagenum"><a name="Page_161" id="Page_161">[Pg 161]</a></span>
+to suggest what the actual factors producing these appearances may be.</p>
+
+<p class="indent">There are, naturally, many other cases among birds
+which are suspected of being in reality comparable, but in most of them
+the evidence is still inadequate. Among Lepidoptera also there
+are a few of these; perhaps the most striking is that of <i>Basilarchia
+"proserpina."</i><a name="FNanchor_13_102" id="FNanchor_13_102"></a><a href="#Footnote_13_102" class="fnanchor">[13]</a>
+The genus is well known to European collectors
+under the name <i>Limenitis</i>, of which we in England have
+one species, <i>L. sibylla</i>, the "White Admiral." A species very
+like <i>sibylla</i> in general appearance is common in the northern
+parts of the United States, ranging through Canada and Northern
+New England, but rarely south of Boston. This species has
+the conspicuous white bands across both wings like our <i>sibylla</i>.</p>
+
+<p class="indent">There is also a more Southern type known as <i>astyanax</i>,
+which is very different in its appearance, being without the white
+bands and having a broad irroration of blue scales on the posterior
+border of the hind wings. The two are so distinct that one would
+not be tempted to suspect any very close relation between them.
+In its distribution <i>astyanax</i> is described by Field as replacing
+arthemis south of latitude 42°. About Boston it is much more
+common than <i>arthemis</i>.</p>
+
+<p class="indent">The two forms encroach but little on each other's territory,
+but where they do coexist, a third form, known as <i>proserpina</i>, is
+found which is almost intermediate, with the white bands much reduced.
+There is now no doubt that this <i>proserpina</i> is a heterozygous
+form, resulting from a combination of the characters of
+<i>arthemis</i> and <i>astyanax</i>. Field succeeded in rearing a brood of 16
+from a <i>proserpina</i> mother caught wild which laid 31 eggs, and of
+these, nine (five males, four females) resembled the mother,
+being <i>proserpina</i>, and seven (four males, three females) were
+<i>arthemis</i>. There can be no question therefore that the mother
+had been fertilised by a male <i>arthemis</i> and that <i>no-white-band</i>
+is a factor partially dominant over the <i>white band</i>. Another
+point of interest which Field observed was that the <i>proserpina</i>
+female refused to lay on birch, poplar or willow, but accepted
+<span class="pagenum"><a name="Page_162" id="Page_162">[Pg 162]</a></span>
+wild cherry (<i>Prunus serotina</i>) a species on which <i>astyanax</i> can
+live, though that tree is not known to be eaten by <i>arthemis</i>.
+Incidentally also the observations show that sterility cannot be
+supposed to be the bar which maintains the distinctness of
+<i>arthemis</i> and <i>astyanax</i>.</p>
+
+<p class="indent">In this connection <i>Papilio oregonia</i> and <i>bairdii</i> should be
+mentioned.<a name="FNanchor_14_103" id="FNanchor_14_103"></a><a href="#Footnote_14_103" class="fnanchor">[14]</a>
+<i>P. oregonia</i> is one of the numerous forms like
+<i>machaon</i>, but rather paler. It is a northern insect, inhabiting
+British Colombia east of the Cascade Range, and reaching to
+Colorado. <i>P. bairdii</i> is a much darker butterfly, representing
+the <i>asterias</i> group of the genus <i>Papilio</i>. Like <i>asterias</i>
+it has the abdomen spotted at the sides, not banded as in the <i>machaon</i>
+group. It belongs to Arizona and Utah extending into Colorado.
+From Colorado the form <i>brucei</i> is described, more or less intermediate,
+like <i>bairdii</i> but with the abdomen banded as in <i>oregonia</i>.
+W. H. Edwards records the results of rearing the offspring of the
+<i>bairdii</i>-like and of the <i>oregonia</i>-like mothers. Each was found
+able to have offspring of both kinds, that is to say, <i>bairdii</i>
+females gave both forms, and <i>oregonia</i> females gave both forms.
+It is not possible to say which is dominant, since the fathers were
+unknown. On general grounds one may expect that the <i>bairdii</i>
+form will be found to dominate, but this is quite doubtful.</p>
+
+<p class="indent">From this particular discussion I omit reference to
+those examples in which the permanently established types are obviously
+associated with special conditions of life. Where considerable
+climatic differences exist between localities, or when we
+pass from South to North, or from the plains into Alpine levels
+we often find that in correspondence with the change of climate
+there is a change in the characteristics of a species common to
+both. When I say "species" in such a connection I am obviously
+using the term in the inclusive sense. Some would prefer to
+say that in the two sets of conditions two <i>representative species</i>
+exist. Whichever expression be preferred it is plain that such
+examples present another phase of the problem we have been
+just considering, and in them also we have an opportunity of
+<span class="pagenum"><a name="Page_163" id="Page_163">[Pg 163]</a></span>
+observing the consequences of the overlap of two closely related
+types, but there are advantages in considering them separately.
+In the examples hitherto given, with the possible exception of
+the Papilios,<a name="FNanchor_15_104" id="FNanchor_15_104"></a><a href="#Footnote_15_104" class="fnanchor">[15]</a>
+the two fixed types severally range over so extensive
+a region that it may fairly be supposed that in the different
+parts they are subject to considerable diversities of climate.
+There is no outstanding difference that we know distinguishing
+the habitats of the two forms; but in comparing Alpine with
+Lowland forms, or essentially northern with essentially southern
+forms we do know an external circumstance, temperature, that
+may reasonably be supposed to have an influence, direct or indirect,
+on the population.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_164" id="Page_164">[Pg 164]</a></span></p>
+<h2>CHAPTER VIII</h2>
+
+<h3>LOCALLY DIFFERENTIATED FORMS. <i>Continued.</i></h3>
+<p style="font-size: 120%; text-align: center;">
+<b><span class="smcap">Climatic Varieties</span></b></p>
+
+<p class="indent">In this chapter we will examine certain cases which
+illustrate phenomena comparable with those just considered, though as
+I have already indicated, they form to some extent a special
+group. The outstanding fact that emerges prominently from
+the study of the local forms is that when two definite types,
+nearly allied, and capable of interbreeding with production of
+fertile offspring, meet together in the region where their
+distributions overlap, though intergrades are habitually found,
+there is no normally or uniformly intermediate population occupying
+the area of intergradation. Such phenomena as these
+must, I think, be admitted to have great weight in any attempt
+to construct a theory of evolution. True we must hesitate in
+asserting their positive significance, but I see no escape from the
+conclusion that they throw grave doubt on conventional views.
+Again and again the same question presents itself. If <i>A</i> and
+<i>B</i> lately emerged from a common form why is that common form
+so utterly lost that it does not even maintain itself in the region
+of overlapping? Almost equally difficult is it, in the cases which
+I have numerated, to apply concrete suggestions based on any
+factorial scheme. We may see that in <i>Heliconius erato</i> the type
+with the red mark on the hind wing probably contains a dominant
+factor, and that where the red mark is absent the metallic colours
+are exposed; and that similarly the green metallic colour may have
+another factor which distinguishes it from the blue. In this way
+we can fairly easily represent the various types of <i>erato</i> on a
+factorial system as the result of the various possible combinations
+of two pairs of factors. But there we stop, and we are quite
+unable to suggest any reason why one area should have the red
+and the green type while another should have the blue also. So
+again with <i>Colaptes</i> or the Warblers. By application of a
+<span class="pagenum"><a name="Page_165" id="Page_165">[Pg 165]</a></span>
+factorial system, admittedly in a somewhat lax fashion, the genetic
+interrelations of the types can be represented; but how it comes
+about that each type maintains a high degree of integrity in
+its own region we can only imagine. Each has in actual fact
+a stability which the intermediate forms have not, but we cannot
+yet analyse the nature of that stability. Mendelian conceptions
+show us how by segregation the integrity of the factors can be in
+some degree maintained, but not why certain combinations of
+factors should be exceptionally stable. All that is left us to
+fall back on is the old unsatisfying suggestions that some combinations
+<i>may</i> have greater viability than others, that there
+<i>may</i> be a tendency for like to mate with like, and so forth.</p>
+
+<p class="indent">These difficulties acquire more than ordinary force
+in those cases in which the two fixed types inhabit regions differing
+in some respect so obvious and definite that we are compelled to
+regard each type as climatic and as specially adapted to the
+conditions. When for example an animal has a distinct type
+never met with except in Arctic or Alpine conditions, and another
+type proper to the plains and temperate regions, what are the
+characteristics of the population of intermediate latitudes or
+at intermediate levels? Some of the examples discussed in the
+last chapter may be instances of this very nature, but even if
+they are not, others are forthcoming which certainly are. The
+evidence of these cases leads to the suspicion that with further
+knowledge they will be found to consist of two classes, some in
+which the observer as he passes from the one climate to the other
+will find the intermediate area actually occupied by a population
+of intermediate character, and others in which, though we may
+presume the maintenance of intermediate conditions in the transitional
+area, there is no definite transitional population. This
+interrupted or discontinuous distribution seems, so far as I
+have means of judging, to be by far the more common of the two.
+I do not doubt that by sufficient search individuals representing
+every or almost every transitional form can be found, but it is
+apparently rare that <i>populations</i> corresponding to these several
+grades can be seen. The question has in few if any cases been
+studied with precision sufficient to provide a positive answer;
+<span class="pagenum"><a name="Page_166" id="Page_166">[Pg 166]</a></span>
+but I suspect that real and complete continuity, in the sense
+thus defined, will only be found where the character of the local
+populations depends <i>directly</i> on the conditions of life, and shows
+an immediate response to changes in them apart from that postponed
+response which we suppose to be achieved by selection.
+Obviously the character must be one, like size for instance,
+capable of sensibly complete gradation.</p>
+
+<p class="indent">The only example I have met with of the phenomenon
+of anything like a complete intergradation between local types
+really distinct in kind is that provided by the butterfly <i>Pararge
+egeria</i>. It is well known to entomologists that this insect exists
+in two very different types, a northern one, the "Speckled Wood"
+of England, in which the spots are a pale whitish yellow, and a
+southern type having the full fulvous colour that we know as
+characteristic of <i>megaera</i>, the "Gatekeeper." It appears that
+Linnaeus gave the name <i>egeria</i> to the southern type,<a name="FNanchor_1_105" id="FNanchor_1_105"></a><a href="#Footnote_1_105" class="fnanchor">[1]</a>
+and our own is now called <i>egerides</i>. Broadly speaking, so far
+as Great Britain, France, and the Spanish Peninsula are concerned, the
+tawny-coloured <i>egeria</i> occupies Spain and western France up
+to the latitude of Poitiers and the pale yellow <i>egerides</i> extends from
+Scotland, where it has a scanty distribution, through southern
+England, where in suitable localities it is common, and the north
+of France to Paris.<a name="FNanchor_2_106" id="FNanchor_2_106"></a><a href="#Footnote_2_106" class="fnanchor">[2]</a>
+The two types when placed side by side
+are strikingly different from each other, and are an excellent
+illustration of what is meant by climatic variation. The insect
+is not a great traveller and probably scarcely ever wanders far
+from its home. It should therefore be possible by collecting
+from north to south to find out how the transition is effected,
+whether suddenly or gradually. This at various times I have
+endeavoured to do, but I am still without exact information as
+to the population in certain critical areas. In addition to the
+information derived from specimens which I have collected or
+seen in the collections of others there is a good account of the
+general distribution in Europe given by the Speyers,<a name="FNanchor_3_107" id="FNanchor_3_107"></a><a href="#Footnote_3_107" class="fnanchor">[3]</a>
+<span class="pagenum"><a name="Page_167" id="Page_167">[Pg 167]</a></span>
+who evidently paid more attention to the subject than most lepidopterists
+have done, and many more recent records. In particular
+Oberthür<a name="FNanchor_4_108" id="FNanchor_4_108"></a><a href="#Footnote_4_108" class="fnanchor">[4]</a>
+has published many details as to the distribution in
+western France and I am especially indebted to Mr. H. Rowland-Brown
+for a long series of notes as to the distribution in France
+generally, and to Mr. H. E. Page and Dr. T. A. Chapman, Mr.
+Oberthür Prof. Arrigoni degli Oddi, Mr. H. Williams and other
+correspondents, for showing me forms from many localities. The
+butterfly is attached for the most part to woods of deciduous trees
+and to country abounding in tall hedges or rough scrub. It is not
+usually to be found in highly cultivated districts or in very dry
+regions. Hence there is necessarily some want of continuity in
+the distribution at the present time and I should think a mile or
+two of arable land without big hedges would constitute a barrier
+hardly ever passed. The larva feeds on several coarse grasses,
+especially <i>Dactylis glomerata</i>. Barrett mentions also <i>Triticum
+repens</i>. In this country the winter is usually passed in the larval
+stage, but I have found that in captivity, at least, there is much
+irregularity. The larvæ feed whenever the weather is not very
+cold and may pupate, but if sharp cold comes on when they are pupating
+or nearly full-grown they often get killed unless protected.</p>
+
+<p class="indent">Some writers speak of a difference between the early
+and later broods, but I have never noticed this, and I do not think
+that the general tone of the yellow is affected by the seasons
+(see Tutt, <i>Ent. Rec.</i>, IX, 1897, p. 37).<a name="FNanchor_5_109" id="FNanchor_5_109"></a><a href="#Footnote_5_109" class="fnanchor">[5]</a>
+</p>
+
+<p class="indent">Beginning at the south of Spain the thoroughly fulvous
+type <i>egeria</i> is common at Gibraltar in the Cork woods, at Granada,
+and doubtless generally. Lederer is said to have found only
+this type in Spain (Speyer), and though I have no precise information
+as to other places in the Peninsula north of Jaen I feel
+tolerably sure that there is no change from south to north.<a name="FNanchor_6_110" id="FNanchor_6_110"></a><a href="#Footnote_6_110" class="fnanchor">[6]</a>
+<span class="pagenum"><a name="Page_168" id="Page_168">[Pg 168]</a></span>
+Immediately north of the Pyrenees we still meet <i>egeria</i> exclusively,
+and up to Poitiers at least there is no noticeable change. But
+somewhere between Poitiers and the bottom of the Loire valley
+at Tours, the genuine southern type comes to an end, and the
+whole population begins at the Loire to be of an intermediate
+type, easy to distinguish both from <i>egeria</i> and from <i>egerides</i>.
+As to the exact condition of the species in the fifty miles separating
+St. Savin on the Vienne from places on the Loire I have no adequate
+information. I have only one small sample from there,
+but it does contain insects both of the southern and intermediate
+types taken on the same day, in a wood near Preuilly. Oberthür
+also states that at Nantes the true southern form exists in company
+with the northern. From this I infer that the southern
+form extends up the coast further than it does inland, but I
+imagine the representative spoken of as northern would be of
+usual Brittany or intermediate type.</p>
+
+<p class="indent">The Vienne river joins the Loire, so the true southern
+type reaches over into the basin of the Loire. From the Loire (Tours,
+Corméry) north to Calvados (Balleroy) only the intermediate
+is found, so far as I know, and the same type extends over
+Brittany.<a name="FNanchor_7_111" id="FNanchor_7_111"></a><a href="#Footnote_7_111" class="fnanchor">[7]</a>
+In general, however, the woods near Paris have the
+thoroughly northern type <i>egerides</i>, but at St. Germain-en-Laye
+and at Etampes (Oberthür) the population approaches the intermediate type.</p>
+
+<p class="indent">On the whole the intermediate type is certainly less
+homogeneous than either of the extremes, and females with the two
+central spots either paler or more fulvous than the rest are not
+uncommon, but I have never taken one on the Loire or in Brittany
+which I should class with either of the extreme types.</p>
+
+<p class="indent">Before speaking of the distribution in other parts
+of France and in Europe generally I will briefly state the results of my
+breeding experiments. The work was done many years ago before
+we had the Mendelian clue, and it is greatly to be hoped that
+some one will find opportunities of repeating it. Crossing the
+English and the thoroughly southern type the families produced
+<span class="pagenum"><a name="Page_169" id="Page_169">[Pg 169]</a></span>
+agree entirely with the intermediates of Brittany and the Loire.
+Reciprocals are alike. Of F<sub>2</sub> I only succeeded in raising very
+few and of those that I had (about 30) nearly all were intermediate
+in character, though perhaps rather less uniform than F<sub>1</sub>. One
+family alone, containing only 4 specimens, had one <i>egerides</i>,
+and three fulvous intermediates. As the case stands alone I
+hesitate whether or not to suppose it due to some mistake. Moreover
+from F<sub>1</sub> crossed back with the respective parental types I
+had fairly long series, especially from F<sub>1</sub> × the southern type, and
+looking at these families I cannot see any clear evidence of segregation.
+On the contrary, I think that though there are slight
+irregularities, they would, taken as a whole, be classed as coming
+between the intermediate type and the extreme form used as the
+second parent. This at least is true when the second parent was
+of the southern type.</p>
+
+<p class="indent">On this evidence I have regarded the case as one in
+which there is no good evidence of segregation and as conforming most
+nearly with the conventional view of gradual transition in response
+to climatic influences. Such influence must however be
+indirect; for I reared five generations of the northern type in
+England, and these, though they included several abnormal-looking
+specimens in the last generation and then died out, did
+not show any noticeable change from the fulvous colour of the
+wild type. Merrifield<a name="FNanchor_8_112" id="FNanchor_8_112"></a><a href="#Footnote_8_112" class="fnanchor">[8]</a>
+also found that heat applied to pupae of the northern type
+produced no approach to the southern type.</p>
+
+<p class="indent">Looking at the facts now in the light of more experience
+it seems to me just possible that the case may be one in which, as
+in Nilson-Ehle's Wheats, the dominant differs from the recessive
+in having two pairs of factors with similar effects. The
+fulvous type for example may have two or more elements in
+separate pairs which together produce the full effect, and the
+intermediate may have one of these. If this were so, some
+segregation should of course eventually be observable, but the
+proportion of the various fulvous and fulvous-intermediate
+individuals would be large, and the reappearance of actual
+<span class="pagenum"><a name="Page_170" id="Page_170">[Pg 170]</a></span>
+representatives of the northern type might be rare. I admit that this
+is a somewhat strained interpretation of the facts, and as yet it
+is not entitled to serious consideration. Nevertheless I am led
+to form some such expectation partly from the great difficulty
+in the way of any other, partly from the evidence of the small
+mixed sample found at Preuilly and partly from the statements
+given by Oberthür. There are moreover other features in the
+general distribution of the species which make it improbable
+that the dependence on climate can after all be so close. Published
+lists are unfortunately of little use in deciding which form
+occurs at a particular place, because, since the name <i>Meone</i> has
+ceased to be used for the southern form, there is no complete
+unanimity among authors as to the application of the names
+<i>egeria</i> and <i>egerides</i>, and unless more particulars are given,
+either name may be used for either form. Besides this, difficulty arises
+from the fact that the intermediate type is not generally distinguished
+at all, and English collectors finding it, may easily
+record it as the southern type. From Staudinger's note on the
+distribution, I gather that he, on the contrary, reckoned the
+intermediate with the northern type, as do the Speyers also.
+The late Mr. J. W. Tutt was careful to distinguish the three
+forms and has left several useful records. Easy therefore as it
+might seem to be to make out the distribution of such a familiar
+insect in its various modifications, there are serious practical
+difficulties, and until long series are brought together with this
+special object in view many obscurities will remain.</p>
+
+<p class="indent">With only the series from England, the west of France,
+and Spain before one it would be easy to regard the successive series
+of tones as a fair measure of climate; the brighter the colour, the
+hotter might one expect the locality to be. Such rough correspondence
+is often to be observed in butterflies and birds. It
+becomes impossible to take these simple views in the light of
+more complete knowledge. Beginning with France the fulvous
+<i>egeria</i> occupies the lower valley of the Rhone, probably from well
+above Lyon, though I have no exact information respecting the
+country above Avignon. According to Speyer it also takes the
+department of Lozère. The same authority says that Puy-de-Dôme
+<span class="pagenum"><a name="Page_171" id="Page_171">[Pg 171]</a></span>
+has "<i>egeria</i>," meaning perhaps the intermediate form,
+with the fulvous form much less commonly. Next comes the
+curious fact that though the Lower Rhone (Avignon, Tarascon,
+Nîmes) has the true fulvous form, Hyères, Cannes, Grasse, Nice,
+Digne, and Alassio have <i>the intermediate</i>. Savoy has the
+intermediate (Chambéry) and even <i>egerides</i> perhaps, though
+in the same latitude on the west of France there is nothing but
+the fulvous type. At Chalseul and Besançon (Doubs) the ordinary
+northern type is found. Switzerland generally, I believe, has the
+northern type, but Staudinger gives <i>egeria</i> for Valais and the
+intermediate occurs in Vaud.<a name="FNanchor_9_113" id="FNanchor_9_113"></a><a href="#Footnote_9_113" class="fnanchor">[9]</a>
+The south side of the Alps has probably colonies of the
+pale <i>egerides</i>, and of intermediates. Orta, with a
+very hot summer, has the English type (Tutt, <i>Ent.
+Rec.</i>, XII, 1900, p. 328). Locarno has the intermediate (<i>ibid.</i>,
+XV, 1903, p. 321). North Italy in general and western Piedmont
+have the intermediate; but further south <i>egeria</i> begins,
+at what region I do not know. Speyer gives on his own authority
+the remarkable statement that at Florence both extremes occur,
+but chiefly intermediates between the two. Mr. R. Verity
+however kindly informs me that in his experience this is not so,
+and that neither the real southern type nor the northern occur
+there. Sardinia, Sicily, Crete all have the southern type.
+Greece probably has various types. Staudinger (<i>Hor. Ross.</i>, VII,
+1870, p. 78) says intermediates resembling Nice types common
+everywhere, but from "Greece" the British Museum has a series
+that would pass for English specimens; and the same type occurs
+near Constantinople. The island of Corfu has a pale intermediate,
+distinct from <i>egerides</i> but approaching it. In Roumania
+all three forms are recorded from various places: <i>egeria</i> in the
+Dobrutscha; not quite typical (presumably an intermediate)
+at Bukharest; intermediate in various mountainous localities
+as well as in Macedonia and Dalmatia; but <i>egerides</i> in Azuga
+at about 3,000 feet.<a name="FNanchor_10_114" id="FNanchor_10_114"></a><a href="#Footnote_10_114" class="fnanchor">[10]</a>
+Hungary has the true <i>egerides</i> also.
+(Cf. Caradja, <i>Deut. Ent. Zt.</i>, IX, p. 58.) Mathew records the same
+<span class="pagenum"><a name="Page_172" id="Page_172">[Pg 172]</a></span>
+from Gallipoli (<i>E. M. M.</i>, 1881, p. 95). Staudinger does not
+distinguish the intermediates from the northern, but he gives
+"<i>egerides</i>" for Armenia and Fergana (Central Asia). As against
+the mere proximity of a great mountain chain being the influence
+which keeps the Riviera population intermediate may be
+mentioned the fact that the northern foothills of the Pyrenees
+have the pure southern type, and the climate of Cambo must
+surely be far cooler than that of Nice. The exact locality of
+the Greek specimens is not given, but there can be no part of
+Greece which is not much hotter in summer than Brittany, or
+Calvados, which have the intermediate, not the English type.</p>
+
+<p class="indent">In face of these facts it can scarcely be maintained
+that average temperature is the efficient cause of the particular tone
+of colour which the butterfly shows in a given region. Nevertheless
+it is clear that climate counts for much in determining
+the distribution. It is noticeable that though the pale <i>egerides</i>
+can be established in a warm climate we never find <i>egeria</i> in cold
+climates, and even the intermediate is not found in places that
+have a hard winter. I suspect that the distribution of the
+broods through the year and the condition of the animal at the
+onset of hard frost are features which really determine whether
+a strain can live in a particular place or not. Though the truth
+of the suggestion cannot be tested by experiments in captivity,
+which at once introduce disturbances, I incline to the idea that
+<i>egeria</i> has not got the right periodicity for northern climates.
+If it could arrange its life so that the population consisted either
+of young larvae, or perhaps of thoroughly formed pupae<a name="FNanchor_11_115" id="FNanchor_11_115"></a><a href="#Footnote_11_115" class="fnanchor">[11]</a>
+at the onset of winter, it might, for any obvious reason to the
+contrary, be able to live in England. It is irregularly "polyvoltine,"
+as the silk-worm breeders say, and as soon as a little
+warmth encourages it, a new generation starts into being, which
+if the frost comes at an untimely moment, is immediately
+<span class="pagenum"><a name="Page_173" id="Page_173">[Pg 173]</a></span>
+destroyed. Many species are continually throwing off individuals
+which feed up fast<a name="FNanchor_12_116" id="FNanchor_12_116"></a><a href="#Footnote_12_116" class="fnanchor">[12]</a>
+and emerge at once if the temperature permits, and
+I imagine a species of Satyrid wholly or largely
+represented by such individuals could scarcely survive in a
+country which had a hard winter. For such a climate some
+definite periodicity in the appearance of the broods may well be
+indispensable. But assuming that <i>egeria</i> is cut off from cold
+climates for such a reason, there is nothing yet to connect these
+habits with the fulvous colour, and until breeding can be carried
+out on a satisfactory scale there is no more to be said.</p>
+
+<p class="indent">From time to time records appear of individual specimens
+more or less fulvous being caught in southern England, especially
+in the New Forest.<a name="FNanchor_13_117" id="FNanchor_13_117"></a><a href="#Footnote_13_117" class="fnanchor">[13]</a>
+It would be interesting to know what offspring such
+individuals might produce. From the evidence
+now given some notion both of the strength and the weakness
+of the case considered as one of continuous climatic variation
+can be formed. I know no other equally satisfactory. Whether
+or not definite mixture of the intermediates with either of the
+extremes will be proved to occur, the case differs materially from
+those considered in the last chapter in the fact that at all events
+there is no general overlapping of forms. In a species so little
+given to wandering, overlapping could indeed scarcely be expected
+to occur. It is this circumstance which makes the species
+preeminently suitable as a subject for the study of climatic
+influences, and I trust that entomologists with the right opportunities
+may be disposed to explore the facts further.</p>
+
+<p class="indent">Just as many species, like <i>egeria</i>, have varieties
+which can be regarded as adapted to northern and southern regions, so
+there are also several which have lowland and Alpine forms quite
+distinct from each other. Every such case presents an example
+of the problem we have been considering. As the collector
+passes from the plains to the Alpine region, how will he find the
+<span class="pagenum"><a name="Page_174" id="Page_174">[Pg 174]</a></span>
+transition from one form to the other effected? Does the lowland
+form give place to the Alpine form suddenly, with a region
+in which the two are mixed, or will he find a zone inhabited by
+an intermediate population? I have spent a good deal of time
+examining the facts in the case of <i>Pieris napi</i> and its Alpine
+female variety <i>bryoniae</i>, and though there are many complications
+which still have to be cleared up, no doubt is possible
+as to the main lines of the answer. If in any valley in the Alps
+inhabited by both <i>napi</i> and <i>bryoniae</i> the collector catches
+every specimen he can, beginning at the bottom and working up to
+7,000 feet, he will at first get nothing but <i>napi</i>. At about 2,500
+feet, he may catch an occasional <i>bryoniae</i> flying with the <i>napi</i>.
+After 3,000 feet <i>napi</i> usually ceases, and only <i>bryoniae</i> are found.
+As an exception a colony of <i>napi</i> may be met with at much
+greater heights. I once found them in numbers at about 6,000
+feet.<a name="FNanchor_14_118" id="FNanchor_14_118"></a><a href="#Footnote_14_118" class="fnanchor">[14]</a>
+Not only were they free from any trace of modification
+in the direction of <i>bryoniae</i>, but they were of the thoroughly
+southern type of <i>napi</i>, being a late brood of that large and very
+pale kind (<i>meridionalis</i>) almost destitute both of dark veining
+above and of green veining below, which are common on the
+shores of Lago Maggiore and in other hot southern localities.
+Not far off at the same level were typical <i>bryoniae</i> in fair
+abundance. Occasionally an intermediate may be met with. I have
+taken a few, for example, at Macugnaga and at Fobello. These,
+however, in my experience are rarities in the Alps. Fleck<a name="FNanchor_15_119" id="FNanchor_15_119"></a><a href="#Footnote_15_119" class="fnanchor">[15]</a>
+gives notes on the distribution in Roumania which shows the
+same state of things. The lowland form is not transformed
+though found at great heights, and at Azuga (nearly 3,000 feet)
+<i>bryoniae</i> occurs with only occasional "<i>flavescens</i>,"
+viz., intermediates of the second brood.</p>
+
+<p class="indent">If this were all the evidence we should be satisfied
+that the lowland and Alpine types keep practically distinct, overlapping
+occasionally, but rarely interbreeding. The problem would
+remain, how is the distinctness of the two types maintained in
+the region of overlapping? Nowadays, I suppose, we should
+<span class="pagenum"><a name="Page_175" id="Page_175">[Pg 175]</a></span>
+incline to answer this question by reference to segregation, and
+perhaps by an appeal to selective mating. The suggestion that
+segregation does take place is certainly true to some extent.
+There are, however, difficulties in the way, and the whole subject
+is one of great complexity. My own experiments were made in
+pre-Mendelian times and were not arranged with the simplicity
+which we now know to be essential. The results are neither
+extensive enough nor clear enough to settle the many collateral
+questions which have to be considered, and the work ought to
+be done again. Nevertheless, some notes of the observations
+may have a suggestive value.</p>
+
+<p class="indent">When I began, I did not sufficiently appreciate that
+the "<i>napi</i>" group, omitting the North American forms, and the
+Asiatic representatives, has at least three chief types in western
+Europe. The differences we have to deal with are manifested
+by the females only, so in this account particulars as to the males
+are omitted for the most part. These are (1) our own British
+<i>napi</i>; (2) the form found in the south, from the Loire downwards,
+and in the Italian Alps, which I think may be spoken of as
+<i>meridionalis</i>; (3) <i>bryoniae</i>, which is a form clearly recognizable
+in the <i>female</i> only, and is found only in the arctic regions and
+in the Alps above 2,500 feet. The first two have several broods,
+two, three, or more, according to opportunity, and the first
+brood is different from the later ones. In <i>napi</i> the markings on
+the upper surface are a dark grey but in <i>meridionalis</i> they are a
+pale silvery grey and much less extensive. In the later broods
+of <i>napi</i> there is much less general irroration of the veins, and the
+spots stand out as more defined and blacker. These differences
+vary greatly in degree of emphasis. In <i>meridionalis</i> the later
+broods are entirely different from the first. Instead of having
+silvery markings they have the ground colour quite white, with
+the spots large and a full black. On the under side of the hind
+wings the usual green veins are almost absent, and I have seen
+individuals which could scarcely be distinguished from <i>rapae</i>.
+To these later broods the term <i>napaeae</i> is sometimes applied,
+but I here use <i>meridionalis</i> for the southern race in general as
+applicable to all broods.
+<span class="pagenum"><a name="Page_176" id="Page_176">[Pg 176]</a></span></p>
+
+<p class="indent">The female <i>bryoniae</i> is totally unlike the others.
+The ground colour is a full yellow, and each nervure is thickly irrorated
+with a brown pigment often spreading so far as to hide the ground
+almost entirely in the fore-wings. The males corresponding with
+these females are not certainly distinguishable from those of our
+own <i>napi</i>. Both sexes have the green veining of the underside
+of the hind wing fully developed, rather more than is usual in the
+lowland races, but this is not really diagnostic of the variety.
+The first serious difficulty arises in regard to the second brood
+of <i>bryoniae</i>. It is stated that there is only one brood,<a name="FNanchor_16_120" id="FNanchor_16_120"></a><a href="#Footnote_16_120" class="fnanchor">[16]</a>
+but I feel fairly sure that a second brood is sometimes produced, and
+that the females with a yellow ground and diminished irroration of the
+veins, not very uncommon in the Italian Alps in July to August,
+are generally representatives of it. Such insects would of course
+be classed with <i>bryoniae</i> in collections.</p>
+
+<p class="indent">My experiments began with eggs of true <i>bryoniae</i>
+females caught at about 2,500 feet early in July. These emerged in
+August-September as intermediates with yellow ground and
+about half as much black on the upper surface as <i>bryoniae</i>.
+They are exactly like the intermediates usually found in nature
+and in the light of later experience I regard them as natural F<sub>1</sub>
+forms, and I think the mothers had been fertilised by <i>napi</i> males,
+though I admit that in view of the rarity of natural intermediates
+there is a difficulty in this suggestion. Three of these females
+were mated with males raised from thorough <i>meridionalis</i>
+females, and three families were produced. Two of them
+showed distinct evidence of segregation, some being yellow and
+some white with various intergrades, some being no blacker than
+<i>meridionalis</i> and some ranging up to a dark intermediate type.
+Part emerged in the same autumn; and part overwintered, emerging
+as the spring <i>meridionalis</i> or as the peculiar type which I
+afterwards learnt to know as the spring F<sub>1</sub> form. The distinctions
+were fairly sharp between the several forms. But the offspring
+of the third female gave a series practically continuous from
+<span class="pagenum"><a name="Page_177" id="Page_177">[Pg 177]</a></span>
+<i>meridionalis</i> to the F<sub>1</sub> type. The work of subsequent years
+gave results similarly irregular which could only be described
+adequately at great length. The outcome may however be
+summed up in the statement that there is evidence that both
+the yellow ground and the dark veining are due to factors, but
+that there are several of these and that imperfect segregation
+is not uncommon, producing various reduction-stages. The
+yellow ground may be due to one factor, and the several shades
+may be the result of irregularities in dominance, but the black
+markings when fully developed cannot I think be the result of
+less than three factors, one for the basal darkening, one for general
+irroration, and one for the margins. Probably also the enlargement
+of the spots is produced by a fourth factor.</p>
+
+<p class="indent">There was not, in my experience any great difficulty
+in getting the various forms to pair in captivity. Some attempts were
+made to see whether individuals of either type selected mates
+of their own type in preference to those of the other, but the
+results were inconclusive. There were some indications of such
+a preference; though, from the impossibility of judging how much
+of this may be due to other circumstances, I could not come to
+a positive conclusion on the rather meagre evidence.</p>
+
+<p class="indent">Recently Schima<a name="FNanchor_17_121" id="FNanchor_17_121"></a><a href="#Footnote_17_121" class="fnanchor">[17]</a>
+has given a careful and detailed account
+of all the forms found in Lower Austria which he enumerates
+under 14 distinct varietal names. He gives full references to
+previous accounts, especially to the beautiful plates lately published
+by Roger Verity.<a name="FNanchor_18_122" id="FNanchor_18_122"></a><a href="#Footnote_18_122" class="fnanchor">[18]</a>
+Examination of these and of my own
+specimens strongly suggests that the several forms are due to
+the recombination of the factors I have named. Among those
+which I have bred are representatives of most if not all the types
+enumerated by Schima in addition to other curious forms. For
+example I have <i>bryoniae</i> markings on a ground practically white;
+the dark veins with spots almost obsolete; <i>meridionalis</i> on a
+yellow ground; the intermediate amount of black on a white
+ground, etc. The last-named may occur wild and I have one
+from Macugnaga as well as one given me by Mr. F. Gayner from
+Lulea (Lapmark).
+<span class="pagenum"><a name="Page_178" id="Page_178">[Pg 178]</a></span></p>
+
+<p class="indent">To obtain really exact knowledge of the number of
+factors and their properties it would be necessary to repeat the work.
+After the beginning, I made a mistake in using British <i>napi</i>
+instead of <i>meridionalis</i> and the results were much confused
+thereby. The contrast between <i>meridionalis</i> and the various
+dark forms is much greater and classification of the types would
+have been therefore easier. The British form is presumably
+<i>meridionalis</i> plus the factor for the basal pigmentation. The
+problem is greatly complicated by the differentiation of the
+seasonal forms. The first point to be determined is whether
+<i>bryoniae</i> is capable of producing a second brood when it is
+thoroughly pure-bred, and whether such a second brood is, as I
+suspect, normally intermediate in character.</p>
+
+<p class="indent">In the Alps generally there is no definitely intermediate
+population; nor I believe, is any such population met with in
+the north where the arctic <i>bryoniae</i> meets <i>napi</i>, but as
+to this I have no precise information. One curious fact, however, must
+be mentioned, namely that there is a population that can probably
+be so described with fairness established at Mödling near
+Vienna. This is not in any sense an Alpine locality, and does
+not, as I am told, differ in any obvious way from the other suburbs
+of Vienna. Dr. H. Przibram was so good as to send me a
+set taken at this place, representing a second brood, and they
+were decidedly heterogeneous, ranging from an intermediate
+form such as <i>bryoniae</i> fertilised by <i>napi</i> usually produces,
+to a light yellowish second-brood type with little dark pigment.
+There are also two actual <i>bryoniae</i>. Whether true <i>napi</i>
+also occur there I do not know, but I have no doubt they do. It
+would be well worth while to investigate the Mödling population
+statistically, and to breed from the intermediates which might
+not impossibly prove to be heterozygotes. There are also records
+of such intermediates being occasionally found in some parts of
+Ireland, in the north of Scotland, and in south Wales,<a name="FNanchor_19_123" id="FNanchor_19_123"></a><a href="#Footnote_19_123" class="fnanchor">[19]</a>
+but I do not know of any regular colony of these forms. We can scarcely
+avoid the inference that one or more of the factors which make
+up <i>bryoniae</i> may be carried by these intermediates. It is not
+<span class="pagenum"><a name="Page_179" id="Page_179">[Pg 179]</a></span>
+clear why their interbreeding does not produce actual <i>bryoniae</i>
+occasionally. If this occurred, the probability is that the fact
+would be known to collectors, at least in the British localities.
+The absence of true <i>bryoniae</i> must, I think, be taken to mean
+that some essential factor is absent from these intermediates.</p>
+
+<p class="indent">To sum up the evidence, the facts that are clear
+may be thus enumerated:</p>
+
+<p class="blockquot">1. <i>Napi</i> and <i>bryoniae</i>, or in the
+Italian Alps, <i>napaeae</i> and <i>bryoniae</i> frequently meet each other.</p>
+
+<p class="blockquot">2. They cross without difficulty, producing fertile offspring.</p>
+
+<p class="blockquot">3. But in the levels at which they overlap there is no
+intermediate population, and only occasional intermediate individuals.</p>
+
+<p class="blockquot">4. In certain parts of the distribution of <i>napi</i>
+similar intermediates sometimes occur, and at one place (Mödling) they are
+so frequent as apparently to constitute a colony.</p>
+
+<p class="blockquot">5. As to the genetic relations of the two forms there
+is no complete certainty. Indications of segregation have been observed
+in some cases, but there are several factors concerned and
+they are liable to some disintegration.</p>
+
+<p class="indent">Another form in which I tried to investigate the same
+problem is <i>Coenonympha arcania</i>, which has one Alpine form known as
+<i>darwiniana</i>, and another, <i>satyrion</i>. In calling <i>satyrion</i>
+a form of <i>arcania</i> I follow Staudinger and other authorities, but I
+have never been quite satisfied that it should be so regarded. The
+differences between <i>arcania</i> and <i>darwiniana</i> are essentially
+differences of degree; <i>C. arcania</i> occurs in places where there is
+cover, and reaches up the valleys usually as high as the mixed
+woods of deciduous trees, which is about 2,500 feet. The variety
+<i>darwiniana</i>, on the contrary, is an insect of treeless hillsides,
+and I regard it as a dwarf and possibly a stunted form. It would not
+greatly surprise me to find that with the application of good
+conditions <i>arcania</i> could be raised from <i>darwiniana</i> eggs, or that
+if <i>arcania</i> larvae were starved they might give rise to <i>darwiniana</i>
+butterflies. I have been unsuccessful in trying to rear the species,
+having lost the larvae by disease. Usually one does not catch
+<i>arcania</i> and <i>darwiniana</i> on the same ground, and as <i>Festuca ovina</i>&mdash;a
+typically hill-side grass&mdash;is a common food-plant of <i>darwiniana</i>
+<span class="pagenum"><a name="Page_180" id="Page_180">[Pg 180]</a></span>
+there can be little doubt that <i>arcania</i> feeds on some other
+grass, probably woodland species. Colonies of <i>arcania</i> of varying
+size and brightness are commonly found, and though a sample of
+<i>arcania</i>, finely grown, from a warm Italian wood, presents a
+striking contrast with <i>darwiniana</i> from an Alpine pasture, one
+certainly may get samples which fill all the gradations. Generally
+the sample from a given locality is fairly homogeneous.</p>
+
+<p class="indent">Of <i>satyrion</i> I have little personal experience. I
+only twice found it, namely at Zinal, and at Hallstatt in Austria, but it
+occurs at Zermatt, Arolla, and in several Swiss localities above
+5,000 feet, and I understand that it is the typical Alpine form in
+the Engadine. With its darkened colour and reduced size it
+might well be expected to be a still further stunted form of
+<i>darwiniana</i>. Yet I have never found the one succeed to the
+other at the higher levels. If <i>darwiniana</i> appears when Alpine
+conditions are reached in a valley it will be met with up to the
+highest level at which such butterflies live. Tutt was of opinion
+that <i>satyrion</i> is a distinct species.<a name="FNanchor_20_124" id="FNanchor_20_124"></a><a href="#Footnote_20_124" class="fnanchor">[20]</a>
+I once, at the top of the
+Vorderrheinthal caught a sample of <i>darwiniana</i> a few of which (males)
+were so dark and had the eye spots so poorly developed that they
+looked like transitions to <i>satyrion</i>. Otherwise I never found
+any such transitional forms and they are certainly exceptional.
+There is further a record<a name="FNanchor_21_125" id="FNanchor_21_125"></a><a href="#Footnote_21_125" class="fnanchor">[21]</a>
+of <i>satyrion</i> having been taken flying
+with <i>arcania</i>. This was near Susa, at about 2,000 feet I infer.
+Mr. H. E. Page has similar specimens from Caud and from St.
+Anton (Arlberg). The females, however, both of mine and of
+Mr. Page's samples are a pale brown, quite unlike the females
+both of <i>arcania</i> and of the dark Zinal <i>satyrion</i>. The difficulty
+thus raised has not I think yet been considered by the authorities,
+and it is possible that the Alpine forms of <i>arcania</i> are in reality
+three, not two.</p>
+
+<p class="indent">The evidence taken together suggests, I think, that <i>darwiniana</i>
+is related to <i>arcania</i> much as so many of the Alpine varieties
+<span class="pagenum"><a name="Page_181" id="Page_181">[Pg 181]</a></span>
+of plants are to the well-developed individuals of the lower
+levels. I do not anticipate that factorial differences will be
+found in these insects, and it is by no means impossible that the
+distinctions between them are the direct consequences of altered
+conditions. The relations of <i>arcania</i> to <i>satyrion</i> are more
+doubtful, and in that case a factorial difference may at least be suspected.</p>
+
+<p class="indent">The species of the genus <i>Setina</i> have Alpine forms
+which agree in possessing a characteristic extension of the black pigment
+to form radiating junctions between the spots on the wings.
+Speyer, who discussed the interrelations of these forms in detail,<a name="FNanchor_22_126" id="FNanchor_22_126"></a><a href="#Footnote_22_126" class="fnanchor">[22]</a>
+lays stress on the absence of genuine transitional forms between
+<i>aurita</i> and the variety <i>ramosa</i>. Both are mountain insects but
+<i>ramosa</i> extends to levels higher than that at which <i>aurita</i>
+ceases, which is about 4,000 feet. The two forms are often found flying
+together. Speyer says that his brother searched diligently for
+transitional forms at the level of overlapping, but found none,
+so that at least they may be regarded as rare. The variety
+<i>ramosa</i> is not infrequent at much lower levels (<i>e. g.</i>,
+Chiavenna, 1,020 feet; Reussthal, 1,500 feet) and extends as high as
+the permanent snows. In the British Museum collection, however,
+I have seen several that I should regard as transitional. Speyer
+perhaps would have classed as <i>ramosa</i> all in which the spots of
+the central field were united, and it is by no means unlikely that
+breeding would prove such individuals to be heterozygous.<a name="FNanchor_23_127" id="FNanchor_23_127"></a><a href="#Footnote_23_127" class="fnanchor">[23]</a>
+<span class="pagenum"><a name="Page_182" id="Page_182">[Pg 182]</a></span></p>
+
+<p class="indent">There can scarcely be a doubt that the distinction between
+<i>aurita</i> and <i>ramosa</i> is factorial, the radiate <i>ramosa</i> probably
+having the factor for striping. In support of this view may be mentioned
+the observation of Boisduval,<a name="FNanchor_24_128" id="FNanchor_24_128"></a><a href="#Footnote_24_128" class="fnanchor">[24]</a>
+respecting a gynandromorphous individual, which was <i>aurita</i> male on one
+side, and <i>ramosa</i> female on the other. Speyer makes another excellent
+comment. He points out that the simple notion that the radiation
+is a mere extension of pigmentation consequent on the
+climate of the higher levels, will not fit the facts very easily,
+because the size of the spots varies greatly in <i>aurita</i> itself at
+any level, and lowland specimens may actually have more black
+confined to the spots alone than some <i>ramosa</i> possess on spots
+and lines combined.<a name="FNanchor_25_129" id="FNanchor_25_129"></a><a href="#Footnote_25_129" class="fnanchor">[25]</a>
+</p>
+
+<p class="indent">The two Salamanders, <i>S. maculosa</i> and its Alpine
+form <i>atra</i>, might not improbably furnish evidence bearing on the same
+problem. The two are of course very distinct, not merely in
+colour (<i>maculosa</i> being spotted with yellow or orange while <i>atra</i>
+is entirely black) but also in the mode of reproduction, a feature
+to which reference will be made in the next chapter. I cannot,
+however, find any evidence as to the overlapping of the two forms.
+<i>S. atra</i> occurs from about 3,000 feet or somewhat less, and reaches
+great elevations in the Eastern Alps, but I do not know if the
+two forms ever occur in the same localities. Leydig,<a name="FNanchor_26_130" id="FNanchor_26_130"></a><a href="#Footnote_26_130" class="fnanchor">[26]</a> Boulenger,<a name="FNanchor_27_131" id="FNanchor_27_131"></a><a href="#Footnote_27_131" class="fnanchor">[27]</a>
+and most modern authorities regard the two types as distinct
+species, but they are in any case closely allied, and it would be of
+interest to have exact knowledge of their geographical delimitations.</p>
+
+<p class="indent">The reader who has considered the cases adduced will
+appreciate the difficulties which must be faced in any attempt to
+<span class="pagenum"><a name="Page_183" id="Page_183">[Pg 183]</a></span>
+account for the facts in a rational way. As always in a problem
+of Evolution, two separate questions have to be answered.
+First how did the form under consideration come into existence,
+and secondly, how did it succeed in maintaining itself so as to
+become a race? The evidence from the local forms, though very
+far from giving complete answers to either of these questions
+definitely refutes the popular notion that a new race comes into
+existence by transformation of an older race. If a gradual mass-transformation
+of this kind took place we should certainly expect
+that when two types, nearly allied and capable of interbreeding,
+overlap each other in their geographical distribution, a normally
+intermediate population would exist. If each type can maintain
+itself, and if each came into existence by gradual transformation,
+then there must have been an intermediate capable of
+existing and maintaining itself as a population; and if this had
+ever been, surely in the region of overlapping, that intermediate
+population should continue. Especially should such a population
+be found when the two extreme types are adaptational forms and
+the region of overlap is a region of intermediate conditions.
+But of the examples we have examined there is only one, that of
+<i>Pararge egeria</i> and <i>egerides</i>, which can at all be so interpreted,
+and even in that case it is not impossible that more minute observation
+would reveal discontinuity between the extremes
+and the admittedly normal intermediate population. Granting
+provisionally however that this example, as it stands, is consistent
+with the conventional theory of evolution, I know not
+where we should look for another case equally good. When the
+distinctions are produced by direct influence of conditions operating
+during the lifetime of the individuals, examples of intermediate
+populations occupying the areas of intermediate conditions
+can no doubt be produced. Many turf-like Alpine
+plants, for instance, if protected from exposure and properly
+nourished can grow as large as those of the same species found in
+the valleys, and in the case of such quantitative effects, intermediate
+conditions can doubtless produce intermediate characters.</p>
+
+<p class="indent">Even these examples however are not very abundant,
+and often the intermediate locality has not a form intermediate
+<span class="pagenum"><a name="Page_184" id="Page_184">[Pg 184]</a></span>
+between those of the two extreme localities, but some third
+form distinct from either. This is the case for instance in the
+fauna of brackish waters. We are taught to believe that the
+fresh water fauna was evolved from the marine fauna, which
+it well may have been; but as students of Crustacea and Mollusca
+know familiarly, the brackish water forms are not as a rule intermediates
+between fresh water species and sea species, but more
+usually they are special forms belonging to the brackish waters,
+with the peculiar property that they can tolerate a great range of
+conditions, and live without ostensible variation in waters of
+most various compositions and densities, which very few marine
+or fresh water species are able to do.</p>
+
+<p class="indent">Sometimes the distinction between local races, as in <i>Rhamphocoelus
+passerinii</i> and <i>icteronotus</i> may be regarded with confidence
+as due to one simple Mendelian factor possessed by one
+race and absent from the other, but I think, more often, as in
+<i>Colaptes</i> or in the varieties of <i>Pieris napi</i>, the existence of
+several distinct factors is to be inferred. As we have seen, the races
+of <i>Colaptes</i> show almost beyond doubt that in different areas at
+least three distinct factorial combinations can be perpetuated as races.</p>
+
+<p class="indent">In the distribution of variability we find, I think,
+some hint as to the steps by which the phenomena under consideration
+have come to their present stage, and I am disposed to regard
+the facts so well attested in the case of our own melanic moths
+as a true indication of the process. Following this indication
+we should regard the change in the character of a population
+as beginning sporadically, by the appearance of varying individuals,
+possibly only one varying individual, in, it may be, one
+place only. As to <i>why</i> a variety should increase in numbers we
+have nothing but mere speculation to offer, and for the present
+we must simply recognise the fact that it may. That such survival
+and replacement may reasonably be taken as an indication
+that the replacing race has some superior power of holding its
+own I am quite disposed to admit. Nevertheless it seems in
+the highest degree unlikely that the outward and perceptible
+character or characters which we recognise as differentiating the
+<span class="pagenum"><a name="Page_185" id="Page_185">[Pg 185]</a></span>
+race should be the actual features which contribute effectively
+to that result.</p>
+
+<p class="indent">In discussions of geographical distribution in relation
+to problems of origin it is generally said that very nearly allied
+species usually occupy distinct areas, while other competent
+observers state the exact contrary. Lately, for example, Dr.
+R. G. Leavitt<a name="FNanchor_28_132" id="FNanchor_28_132"></a><a href="#Footnote_28_132" class="fnanchor">[28]</a>
+has published an important collection of evidence
+upholding the latter proposition, taken chiefly from the botanical
+side, showing how in numerous genera two or more closely allied
+species coexist, frequently without intermediates, in the same
+localities, and may even be thus found in company throughout
+their distribution. The difference of opinion evidently arises
+from a confusion as to the sense in which the term "species"
+is understood and applied. Leavitt, for example, is avowedly
+following Jordan and, among moderns, Sargent, in applying
+a close analysis, and denoting as species all forms which are
+distinct and breed true. Against this use of the term I know
+no valid objection<a name="FNanchor_29_133" id="FNanchor_29_133"></a><a href="#Footnote_29_133" class="fnanchor">[29]</a>
+but it must be obvious that if others follow
+a different practice confusion may result when observations are
+summarised in general statements. We will consider this subject
+again in another place, but here it may be sufficient to say that
+there can scarcely now be a doubt that numbers of these associated
+species, such as Jordan discriminated, represent various
+combinations of the presence and absence of Mendelian factors.
+This does not in any way weaken the argument which Leavitt
+founds upon the facts, namely, that the observed distribution
+of these forms is consistent with the supposition of an evolution
+largely discontinuous.</p>
+
+<p class="indent">On the other hand, those who have come to the opinion
+that nearly allied species generally occupy distinct ground are
+presumably more impressed by the characters differentiating the
+geographically distinct or adaptational races, seeing that genuine
+intermediates between them are less commonly found. Those
+geographical races may no doubt contain various differentiated
+forms; but when all live together, occasional intermediates are
+<span class="pagenum"><a name="Page_186" id="Page_186">[Pg 186]</a></span>
+usually to be found even in the case of characters habitually
+segregating. These segregating forms Jordan would certainly
+have determined as species, and it must be conceded that no
+physiological definition has yet been drawn which consistently
+excludes them.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_187" id="Page_187">[Pg 187]</a></span></p>
+
+<h2>CHAPTER IX</h2>
+<h3>THE EFFECTS OF CHANGED CONDITIONS: ADAPTATION</h3>
+
+<p class="indent">In the attempt to conceive a process by which
+Evolution may have come about, the first phenomenon to be recognized
+and accounted for is specific difference. With that recognition
+the outline of the problem is defined. The second prerogative
+fact is adaptation. Forms of life are <i>on the whole</i> divided into
+species, and these species <i>on the whole</i> are adapted and fit the
+places in which they live. To many students of Evolution,
+adaptation has proved so much more interesting and impressive
+than specific diversity that they have preferred it to the first
+place in their considerations.</p>
+
+<p class="indent">Whether this is, as I believe, an inversion of the
+logical order or not, there is one most serious practical objection to
+such preference, that whereas specific diversity is a subject which
+can be investigated both by the study of variation and by the
+analytical apparatus which modern genetic science has developed,
+we have no very effectual means of directly attacking the problems
+of Adaptation.</p>
+
+<p class="indent">The absence of any definite progress in genetics in
+the last century was in great measure due to the exclusive prominence
+given to the problem of Adaptation. Almost all debates on
+heredity centered in that part of the subject. No one disputes
+that the adaptation of organisms to their surroundings is one
+of the great problems of nature, but it is not the primary problem
+of descent. Moreover, until the normal and undisturbed course
+of descent under uniform conditions is ascertained with some
+exactness, it is useless to attempt a survey of the consequences
+of external interference; nor as a rule can it be even possible to
+decide with much confidence whether such interferences have or
+have not definite consequences. Those, for example, who debated
+with enthusiasm whether acquired characters are or are
+not transmitted were constantly engaged in discussing occurrences
+<span class="pagenum"><a name="Page_188" id="Page_188">[Pg 188]</a></span>
+which we now know to be ordinary features of descent
+under uniform conditions, and the origin of variations which
+were certainly not caused directly by circumstances at all. In
+the absence of any factorial analysis, or of any conception of what
+factorial composition means and implies, no one knew what
+varieties might be expected from given parents. The appearance
+of any recessive variety was claimed as a consequence of some
+treatment which might have been applied to the parents. There
+was no possible standard of evidence or means of controlling it,
+and thus the discussion was singularly unfruitful. Before we can
+tell how the course of descent has departed from the normal, we
+must know what the normal would have been if we had let alone.
+We are still far from having such knowledge in adequate measure,
+but it does now exist in some degree, and we are steadily approaching
+a position from which we shall be able to form fairly sound
+estimates of the true significance of evidence for or against the
+proposition that environmental treatment can produce positive
+disturbances in the physiological course of descent.</p>
+
+<p class="indent">Thus described, the field for consideration is very
+wide. Though the effects of changed conditions were especially studied
+in the hope of solving the problem of adaptation by direct observation,
+that, as all are now agreed, is but a part of a more
+general question. We must ask not only do changed conditions
+produce an <i>adaptative</i> response on the part of the offspring, but
+whether they produce any response on the part of the offspring
+at all. It is not in doubt that by violent means, such as starvation
+or poisoning of the reproductive cells, effects of a kind, stunting
+and deformity for instance, can be made evident, just as similar
+effects may follow similar treatment during embryonic or larval
+life. Apart from interferences of this class, are there any that
+may be reasonably invoked as modifying the course of inheritance?</p>
+
+<p class="indent">No epitome of the older evidence for the inheritance
+of adaptative changes is here required. That has often been collected,
+especially by Weismann, who exposed its weaknesses so
+thoroughly as to carry conviction to most minds, and showed
+that whether the phenomenon occurs or not, no one can yet prove
+<span class="pagenum"><a name="Page_189" id="Page_189">[Pg 189]</a></span>
+that it does. Belief in these transmissions, after being almost
+universally held, was with singular unanimity abandoned. This
+change in opinion, though doing credit to the faith of the scientific
+community in evidential reasoning, is the more remarkable
+inasmuch as the strength of the idea was not derived from the
+minute amounts of supposed facts now demolished. On the
+contrary, it was really an instinctive deduction from a wide
+superficial acquaintance with the properties of animals and
+plants. They <i>can</i> accommodate themselves to circumstances.
+They <i>do</i> make responses sometimes marvellously appropriate
+to demands for which they can scarcely have been prepared.
+What more natural than to suppose that the permanent adaptations
+have been achieved by inherited summation of such responses?
+No one had actually been driven to believe in the
+inheritance of adaptative changes because bitches which had
+been docked had been known to give birth to tailless puppies,
+or because certain wheat in Norway was alleged to have become
+acclimatized in a few generations. Evidence of this kind was
+collected and produced rather as an ornamental appendix to a
+proposition already accepted, and held to be plainly demonstrated
+by the facts of nature. Looked at indeed in that preliminary
+and uncritical way, the case is simply overwhelming.
+Those who desire to see how strong it is should turn to Samuel
+Butler's <i>Life and Habit</i>, and even if in reading they reiterate to
+themselves that no experimental evidence exists in support of
+the propositions advanced, the misgiving that none the less they
+may be true is likely to remain. Making every deduction for
+the fact that the wonders of adaptation have been grossly exaggerated,
+and that marvels of fitness and correspondence between
+means and ends have grown out of mere anthropomorphic
+speculations, there is much more left to be accounted for than
+can at all comfortably be accepted as the product of happy
+accidents. So oppressive are these difficulties that we can scarcely
+blame those who imagine that the study of heredity is primarily
+directed to the problem of the transmission of acquired characters,
+a preconception still almost universal among the laity.
+<span class="pagenum"><a name="Page_190" id="Page_190">[Pg 190]</a></span></p>
+
+<p class="indent">But since the belief in transmission of acquired
+adaptations arose from preconception rather than from evidence, it is
+worth observing that, rightly considered, the probability should surely
+be the other way. For the adaptations relate to every variety
+of exigency. To supply themselves with food, to find it, to seize
+and digest it, to protect themselves from predatory enemies
+whether by offence or defence, to counter-balance the changes
+of temperature, or pressure, to provide for mechanical strains,
+to obtain immunity from poison and from invading organisms,
+to bring the sexual elements into contact, to ensure the distribution
+of the type; all these and many more are accomplished
+by organisms in a thousand most diverse and alternative methods.
+Those are the things that are hard to imagine as produced by
+any concatenation of natural events; but the suggestions that
+organisms had had from the beginning innate in them a power
+of modifying themselves, their organs and their instincts so as
+to meet these multifarious requirements does not materially
+differ from the more overt appeals to supernatural intervention.</p>
+
+<p class="indent">The conception, originally introduced by Hering and
+independently by S. Butler, that adaptation is a consequence or
+product of accumulated <i>memory</i> was of late revived by Semon
+and has been received with some approval, especially by F.
+Darwin. I see nothing fantastic in the notion that memory
+may be unconsciously preserved with the same continuity that
+the protoplasmic basis of life possesses. That idea, though
+purely speculative and, as yet, incapable of proof or disproof
+contains nothing which our experience of matter or of life at all
+refutes. On the contrary, we probably do well to retain the
+suggestion as a clue that may some day be of service. But if
+adaptation is to be the product of these accumulated experiences,
+<i>they must in some way be translated into terms of physiological and
+structural change</i>, a process frankly inconceivable.</p>
+
+<p class="indent">To attempt any representation of heredity as a
+product of memory is, moreover, to substitute the more obscure for the
+less. Both are now inscrutable; but while we may not unreasonably
+aspire to analyse heredity into simpler components by ordinary
+methods of research, the case of memory is altogether
+different. Memory is a mystery as deep as any that even
+<span class="pagenum"><a name="Page_191" id="Page_191">[Pg 191]</a></span>
+psychology can propound. Philosophers might perhaps encourage
+themselves to attack the problem of the nature of memory by
+reflecting that after all the process may in some of its aspects
+be comparable with that of inheritance, but the student of genetics,
+as long as he can keep in close touch with a profitable basis
+of material fact, will scarcely be tempted to look for inspiration
+in psychical analogies.</p>
+
+<p class="indent">For a summary of the recent evidence I may refer the reader
+to Semon's paper<a name="FNanchor_1_134" id="FNanchor_1_134"></a><a href="#Footnote_1_134" class="fnanchor">[1]</a>
+where he will find a collection of these
+observations described from the standpoint of a convinced believer.
+At the outset one cannot help being struck by the fact that of
+the instances alleged, very few, even if authentic, show the transmission
+of acquired modifications which can in any sense be regarded
+as adaptative, and many are examples not so much of a
+transmission of characters produced in the parents as of variation
+induced in the offspring as a consequence of treatment to which
+the parents were submitted, the parents themselves remaining
+apparently unmodified. No one questions the great importance
+of evidence of this latter class as touching the problem of the
+causes of variation, but it is not obvious why it is introduced in
+support of the thesis that acquired characters are inherited.</p>
+
+<p class="indent">It is most difficult to form a clear judgment of the
+value of the evidence as a whole. To doubt the validity of testimony
+put forward by reputable authors is to incur a charge of obstinacy
+or caprice; nevertheless in matters of this kind, where the alleged
+phenomena are, if genuine, of such exceptional significance, belief
+should only be extended to evidence after every possible source
+of doubt has been excluded. We believe such things when we
+must, but not before. At the very least we are entitled to require
+that confirmatory evidence should be forthcoming from independent
+witnesses. So far as I have seen, this requirement is
+satisfied in scarcely any of the examples that have been lately
+published, and until it is, judgment may reasonably be suspended.
+<span class="pagenum"><a name="Page_192" id="Page_192">[Pg 192]</a></span></p>
+
+<p class="indent">In some cases, however, the facts are not doubtful. Standfuss,
+by subjecting pupae of <i>Vanessa urticae</i> to cold, produced
+the now well-known temperature-aberrations in which the dark
+pigment is greatly extended. He put together in a breeding-cage
+32 males and 10 females showing this modification in various
+degrees. Two of these females died without leaving young.
+Seven produced exclusively normal offspring. From the eighth
+female 43 butterflies were bred, and of these there were four (all
+males) which to a greater or less extent exhibited the aberrational
+form.<a name="FNanchor_2_135" id="FNanchor_2_135"></a><a href="#Footnote_2_135" class="fnanchor">[2]</a>
+The mother of this family was the most abnormal of the
+10 females originally put in.</p>
+
+<p class="indent">Fischer's experiment with <i>Aretia caja</i> was on
+similar lines. From pupae which had been frozen almost all the moths
+which emerged showed aberrational markings. A pair of these mated
+and produced 173 young which pupated. Those which emerged
+early were all normal, but of those which emerged late, 17 had
+in various degrees abnormal markings like those of the parents.<a name="FNanchor_3_136" id="FNanchor_3_136"></a><a href="#Footnote_3_136" class="fnanchor">[3]</a>
+In neither of these examples is there any question as to the facts.
+Both observers have great experience and give full details of their work.</p>
+
+<p class="indent">As regards <i>Vanessa urticae</i>, however, it must be
+recalled that Fischer himself showed that in Nymphalids somewhat similar
+aberrations could be produced both by heat and by cold, and
+even by centrifuging the pupae. Frl. von Linden produced a
+transitional form of the same aberration in <i>V. urticae</i> by the
+action of carbonic acid gas.<a name="FNanchor_4_137" id="FNanchor_4_137"></a><a href="#Footnote_4_137" class="fnanchor">[4]</a>
+It is highly probable that the appearance
+is due to a morbid change, perhaps an arrest of development,
+which may be brought about by a great diversity of causes.
+In the experiments the cause probably was a diseased condition
+of the tissues of the mother herself. She had been subjected to
+freezing sufficiently severe to prevent the proper development of
+the pigments and some of the ovarian cells presumably suffered
+also. It will be observed that the only specimens which were
+affected were the offspring of the most abnormal female, and of
+them only four out of forty-three showed any change.
+<span class="pagenum"><a name="Page_193" id="Page_193">[Pg 193]</a></span></p>
+
+<p class="indent">The same interpretation probably applies to the cases
+in <i>Arctia caja</i>. In this species the markings are well known to
+be liable to great variation. As Barrett says, even in nature
+individuals are rarely quite alike, and an immense number of
+strange forms occur in collections.<a name="FNanchor_5_138" id="FNanchor_5_138"></a><a href="#Footnote_5_138" class="fnanchor">[5]</a>
+These are greatly sought after by some collectors, especially in England,
+where they fetch high prices at auctions, and it is notorious that most of
+them come from Lancashire and the West Riding of Yorkshire. It is commonly
+supposed that the breeders of that district subject them to
+abnormal conditions, and especially to unnatural feeding, but
+I know no clear evidence that this is true. From whatever cause
+it is certain that the natural pattern is, in some strains at all
+events, very easily disturbed.</p>
+
+<p class="indent">The elaborate experiments of Schröder with
+<i>Abraxas grossulariata</i> are difficult to follow and are complicated
+by the fact that the series which was submitted to abnormal temperatures
+was derived from an abnormal original pair. From the evidence
+given it is not clear to me whether the temperature had a distinct
+effect. This insect, like <i>Arctia caja</i>, produces an immense number
+of variations (especially in the amount of the black pigment)
+and as most of these are, I believe, reared in domestication for
+sale, it is highly probable that the species is easily influenced
+by cultural conditions.</p>
+
+<p class="indent">Schröder describes two other experiments which have
+been accepted by Semon and other supporters of the view that acquired
+characters are transmitted. In the first, <i>Phratora vitellinae</i>, a
+phytophagous beetle living on the undersides of leaves, was used.
+It naturally feeds on <i>Salix fragilis</i>, a species without a felt, or
+tomentum, on the underside of the leaves. Larvae were transferred
+to another willow (near <i>S. viminalis</i>) which has the undersides
+of the leaves felted. The larvae took readily to the new
+food, pushing the tomentum before them as they gnawed the
+leaves. They came to maturity and when they were about to
+lay their eggs they were given a free choice between <i>S. fragilis</i>
+and the tomentose species. The greater number of ovipositions,
+<span class="pagenum"><a name="Page_194" id="Page_194">[Pg 194]</a></span>
+219, took place on <i>fragilis</i>, and there were 127 on the tomentose
+bush, which we are told was six times as large as the <i>fragilis</i>.
+The larvae from <i>fragilis</i> were next put on the tomentose species
+and reared on it. When they became imagines they were similarly
+given their choice, with the result that there were 104
+ovipositions on the tomentose species and only 83 on <i>fragilis</i>.
+In the next generations there were 48 ovipositions on the tomentose
+and 11 on <i>fragilis</i>. Finally the fourth generation made
+15 ovipositions on the tomentose and none on <i>fragilis</i>.</p>
+
+<p class="indent">The difficulty about such experiments is obviously that
+one has no assurance that the change of instinct, in so far as there
+is any, may not be a mere consequence of the captivity. It
+must, besides, be extremely difficult to arrange the experiment so
+that there is really an equal choice between the two bushes, when
+one stands beside the other. Przibram, in quoting this case,
+considers that as the tomentose bush was about six times as
+large as the <i>fragilis</i>, some indication of the relative attractiveness
+of the two may be obtained by dividing the ovipositions on the
+larger bush by six, but I imagine the matter must be much more complex.</p>
+
+<p class="indent">Schröder's second example is not more convincing,
+in my opinion, though Semon regards it as one of the most important
+pieces of evidence. It concerns a leaf-rolling moth, <i>Gracilaria
+stigmatella</i>, the larva of which is said normally to make its house
+by bending over the <i>tips</i> of the sallow leaves on which it feeds.
+Schröder placed larvae on leaves from which the tips had been
+cut, and these larvae made their houses by rolling over the <i>sides</i>
+of the leaves. Their offspring were again fed on leaves without
+tips, and as before, they rolled in the leaf-margins either on one
+side or both. The offspring of this second generation were then
+fed on entire leaves. There were 19 houses made by these (?19)
+larvae, and of them 15 were normal, made by folding down the
+tips of the leaves, while 4 were abnormal, made by rolling in the
+leaf-margins. Schröder says that in nature he has only twice
+seen abnormal houses; but it is clearly essential not only that the
+frequency of such variability in nature should be thoroughly
+examined, but also that we should know whether when the species
+<span class="pagenum"><a name="Page_195" id="Page_195">[Pg 195]</a></span>
+is bred in captivity these irregularities of behaviour do or do not
+occur when the larvae are fed on uninjured leaves.</p>
+
+<p class="indent">The famous case of Schübeler's wheat is revived by Semon.
+The story will be familiar to most readers of the literature of the
+subject. Briefly it is that annuals, especially wheat and maize,
+raised from seed in Central Europe take more time in coming
+to maturity and ripening than similar plants raised in Norway,
+where the summer days are much longer. The received account
+is that he imported seed especially of maize and of wheat from
+Central Europe to Norway and found that in successive years
+the period of growth and ripening was increasingly reduced.
+After two generations seed of the accelerated wheat was sent
+back to Breslau where it was grown, and was found to ripen rather
+more slowly than in Norway, but much more quickly than the
+original stock had done. The facts recorded by Schübeler<a name="FNanchor_6_139" id="FNanchor_6_139"></a><a href="#Footnote_6_139" class="fnanchor">[6]</a>
+are that he received seed from Eldena, which is on the Baltic near
+Greifswald. The variety is described as "<i>100 tägiger Sommer
+Weizen</i>," but no more exact record of its behaviour in Germany
+is given. This wheat, grown at Christiania in 1857, took 103
+days to harvest. Its seed was again grown in Christiania in 1858,
+and took 93 days, and sown again in 1859 it took only 75 days, 28
+days less than in the first year of cultivation in Norway. Seed of
+the 1858 crop was sent to Breslau, and grown there by Roedelius
+in 1859; it took 80 days. Evidently before such a record can be
+used as proving an inheritance of acquired characters numbers of
+particulars should be forthcoming. The view that Johannsen
+has taken is that the result was probably due to unconscious
+selection of the earlier individuals among a population consisting
+of many types of various compositions. Some effect may no
+doubt be ascribed to that cause, but I cannot think that alone
+it would account for the results. My impression is rather that
+they were produced by differences in the cultivation and especially
+in the seasons. Research of an elaborate character would be
+necessary in order to eliminate the various sources of error, and
+nothing of the kind has been done; nor does Semon allude to these
+difficulties in prominently adducing Schübeler's evidence. A
+<span class="pagenum"><a name="Page_196" id="Page_196">[Pg 196]</a></span>
+difference of even three weeks in time of harvesting may easily
+be due to variation in the season. It would in any case be difficult
+to analyse the meteorological conditions, and to decide how
+much effect in postponing or accelerating the harvest might be
+due to cold days, to cloudy days, to wet weather, to fluctuations
+in average temperature, to hot days, and other such incidents
+occurring at the different periods of growth, even if they were
+specially watched while the experiments were in progress, and
+at this distance of time such analysis is practically impossible.
+Without careful simultaneous control-experiments this evidence
+is almost worthless. The director of the Meteorological
+Office<a name="FNanchor_7_140" id="FNanchor_7_140"></a><a href="#Footnote_7_140" class="fnanchor">[7]</a>
+has, however, kindly sent me some details of the weather
+at Breslau from 1857 to 1860, and I notice that as a matter of
+fact July, 1859, was an exceptionally hot month, <i>having an average
+of 2.67° C. above the mean</i> for the twenty years 1848-1867. June
+in that year was slightly (0.31° C.) below the mean and May
+slightly above it (0.18° C.). August was also abnormally hot,
+2.35° C. above the average. The Breslau wheat was sown on
+<i>May 19</i> and harvested on August 6. There was a cold spell from
+May 11 to 14, which this wheat escaped, as it was sown on May
+19. In the other years the cold spell came much later. These
+elements of the weather may possibly have done something to
+hurry the ripening in 1859. It unfortunate that we are not
+told how long similar wheat from Breslau seed took to ripen in
+that year.</p>
+
+<p class="indent">As regards the Norway cultivations we have the
+average monthly temperatures recorded by Schübeler, though he does
+not discuss them in connection with this special problem. It is
+quite clear that 1857, in which the period was 103 days, was an
+exceptionally cold summer, especially as regards the months of
+June and July, but though there was, so far as the temperature
+<span class="pagenum"><a name="Page_197" id="Page_197">[Pg 197]</a></span>
+records go, no great difference between 1858 and 1859, the year
+1859, in which the period of ripening was the shortest, was somewhat
+colder in Norway than 1858. But we have the further
+difficulty that there were ten days difference in sowing, for in
+1858 the sowing was made on May 14, and in 1859 on May 24.
+With all these possibilities uncontrolled, and indeed unconsidered,
+I am surprised that Semon should claim these experiments as one
+of the chief supports for his views.</p>
+
+<p class="indent">Schübeler's other allegations respecting the influence
+of climate on plants grown in various places and especially at different
+elevations in Norway have been destructively criticised by
+Wille<a name="FNanchor_8_141" id="FNanchor_8_141"></a><a href="#Footnote_8_141" class="fnanchor">[8]</a>
+to whose paper readers interested in the subject should refer.</p>
+
+<p class="indent">Before the appearance of Wille's criticisms Wettstein<a name="FNanchor_9_142" id="FNanchor_9_142"></a><a href="#Footnote_9_142" class="fnanchor">[9]</a>
+made a favourable reference to Schübeler's work, accepting his
+conclusion. He states also that he has himself made analogous
+experiments with flax, finding that the length of the period of
+development and a series of morphological characters show an
+adaptation to local conditions, and that on transference of seed
+to other conditions the previous effects are maintained. No
+details, however, are given, and I do not know if anything more
+on the subject has appeared since. The other examples cited
+by Wettstein, such as the observations of Cieslar on forest-trees
+and those of Jakowatz on gentians seem to me open to all the
+usual objections applicable to evidence of this kind. Such work,
+to be of any value for the purpose to which it is applied, must be
+preceded by a study of the normal heredity and of the variations
+of the species.</p>
+
+<p class="indent">Most of the recent writers (Semon, Przibram, etc.) on the
+inheritance of acquired characters accept the story of Brown-Séquard's
+guinea pigs, which are said to have inherited a liability
+to peculiar epileptiform attacks induced in their parents by various
+nervous lesions.</p>
+
+<p class="indent">The question has been often debated and several observers
+have repeated the experiments with varying results, some failing
+<span class="pagenum"><a name="Page_198" id="Page_198">[Pg 198]</a></span>
+to confirm Brown-Séquard, others finding evidence which in
+various degrees supported his conclusions. Recently a new and
+especially valuable paper has been published by Mr. T. Graham
+Brown<a name="FNanchor_10_143" id="FNanchor_10_143"></a><a href="#Footnote_10_143" class="fnanchor">[10]</a>
+which goes far towards settling this outstanding question.
+He states that "the Brown-Séquard phenomenon is nothing more
+or less than a specific instance of the scratch-reflex," and it is
+due to a raised excitability of the mechanism of this reflex. This
+raised excitability is the character acquired as a consequence,
+for instance, of the removal of part of one great sciatic nerve.
+The nature of this raised excitability and its causation are discussed
+and elucidated, but this part of the work is not essential
+to the present consideration. Mr. Graham Brown in his summary
+of conclusions remarks that it is very difficult to see how this
+condition of raised excitability can be transmitted to the offspring,
+and this comment which might be made in reference to any of
+the alleged cases certainly applies with special cogency to the
+present example.</p>
+
+<p class="indent">He then calls special attention to three observations:</p>
+
+<p class="blockquot">1. That guinea pigs which had a "trophic" change in the
+foot, as a result of division of the great sciatic nerve, have repeatedly
+been seen to nibble the feet of other guinea pigs which
+had this change in the foot from the same causes.</p>
+
+<p class="blockquot">2. That accidental injury to the toes may be followed
+by the Brown-Séquard phenomenon in an otherwise normal animal.</p>
+
+<p class="blockquot">3. That in several instances the young of guinea pigs
+which exhibited the phenomenon have been noticed to have one or more
+toes eaten off by the mother.</p>
+
+<p class="indent">Brown-Séquard noticed that almost all his animals
+in which the great sciatic was divided acquired the "epilepsy" and
+nibbled those parts of their feet in which sensation had been lost.
+Of the offspring of such animals he found that a very small proportion
+exhibited a malformation of the feet, and of these some
+showed the "epilepsy." The proportion which showed the
+"epilepsy" was one to two per cent. of the offspring.</p>
+
+<p class="indent">Morgan<a name="FNanchor_11_144" id="FNanchor_11_144"></a><a href="#Footnote_11_144" class="fnanchor">[11]</a>
+is quoted by Graham Brown as having suggested
+<span class="pagenum"><a name="Page_199" id="Page_199">[Pg 199]</a></span>
+that the loss of toes in the offspring may have been due to mutilation
+by the mother, following his experience in a case in which
+the tails of mice in succeeding litters were thus devoured, and
+there can be little doubt that in this suggestion lies the clue to
+the explanation of the whole mystery. Graham Brown concludes
+that it may be supposed with every degree of probability that
+the "transmission" was due to injuries inflicted upon the young
+by their parents. With this conclusion most people will now be
+disposed to agree, and we may hope that we shall hear the last
+of this curious myth&mdash;to the elucidation of which a vast
+quantity of research has been devoted.</p>
+
+<p class="indent">The series of experiments made by Kammerer with
+various Amphibia have attracted much attention and have been
+acclaimed by Semon and other believers in the transmission of
+acquired characters as giving proof of the truth of their views.
+With respect to these observations the chief comment to be made
+is that they are as yet unconfirmed. Many of the results that
+are described, it is scarcely necessary to say, will strike most
+readers as very improbable; but coming from a man of Dr.
+Kammerer's wide experience, and accepted as they are by Dr.
+Przibram, under whose auspices the work was done in the Biologische
+Vesuchsanstalt at Vienna, the published accounts are
+worthy of the most respectful attention.</p>
+
+<p class="indent">The evidence relates chiefly to three distinct groups of occurrences:</p>
+
+<p class="blockquot">1. Modification in <i>Alytes obstetricans</i>, the Midwife Toad,
+affecting both the structure and the mode of reproduction, induced
+by compulsory change of habits.</p>
+
+<p class="blockquot">2. Modification in the mode of reproduction of <i>Salamandra
+atra</i> and <i>maculosa</i> induced by compulsory change of habits.</p>
+
+<p class="blockquot">3. Modification in the colour of <i>Salamandra maculosa</i> induced
+by change in the colour of the soil on which the animals were kept.</p>
+
+<p class="indent">1. I will take first the case of <i>Alytes</i>,<a name="FNanchor_12_145" id="FNanchor_12_145"></a><a href="#Footnote_12_145" class="fnanchor">[12]</a>
+because it is the most definite example, and because it is the case
+<span class="pagenum"><a name="Page_200" id="Page_200">[Pg 200]</a></span>
+which most readily admits of repetition and verification.</p>
+
+<p class="indent">The habits of <i>Alytes obstetricans</i> are well
+known. The animals copulate on land. As the strings of eggs leave the
+female they are entangled by the hind legs of the male, and being
+adhesive they stick to him and undergo their development attached
+to his back and legs. The number of eggs varies from 18
+to 86, a number much smaller than is usual in toads and frogs
+which lay their eggs in water. The eggs are large and full of yolk.</p>
+
+<p class="indent">There are two breeding seasons, one about April and
+the other about September, and a winter hibernation. Not only animals
+brought in from outside, but their offspring reared in domestication
+maintain these normal habits in confinement, if the temperature
+does not exceed 17° C. (pp. 499 and 534).</p>
+
+<p class="indent">If, however, the temperature be artificially raised
+and kept at 25-30° C., the males do not attach the eggs to themselves
+when spawning occurs on land but let them lie. The adhesion
+of the eggs is said to be hindered by the comparatively rapid
+drying of their surfaces.</p>
+
+<p class="indent">More usually in the high temperatures the animals
+<i>take to the water</i> and copulate there. The eggs are ejected into
+the water, and as their gelatinous coverings immediately swell up,
+they do not stick to the males.</p>
+
+<p class="indent">The offspring thus derived from the parents subjected
+to heat for one breeding-period only, whether they were laid in water
+or on land, did not show departures from the normal type.</p>
+
+<p class="indent">Kammerer states next, however, that in subsequent
+breeding-periods the same parents frequently take to the water to
+breed, though they have become quite accustomed to the heated
+chamber; and furthermore that if such animals, having thus lost
+their instinct to brood their young, be transferred to ordinary
+temperatures they do not readily reassume their normal habits,
+but for several breeding seasons&mdash;at least four&mdash;will take to
+the water. These parents lay from 90 to 115 eggs, which are
+small and contain little yolk, and the larvae, on hatching, breathe
+with their embryonic gills until they are absorbed instead of
+being broken off as normally.
+<span class="pagenum"><a name="Page_201" id="Page_201">[Pg 201]</a></span></p>
+
+<p class="indent">The offspring thus abnormally developed when they
+mature are said never to brood their eggs. If they are derived from
+the earlier spawnings of their parents, before, that is to say, the
+parents had been submitted to the changed conditions long enough
+to transmit their effects, they lay on land; but if they are derived
+from the later spawnings, they lay in the water. These changes
+of habit are manifested without the continued application of
+the abnormal experimental conditions, and, as I understand the
+account, in normal conditions of temperature.</p>
+
+<p class="indent">If the abnormal experimental conditions are continued,
+the toads always lay in water, and their eggs become progressively
+smaller and more numerous. The larvae in the fourth generation
+acquire three pairs of gills instead of one pair, and are in other
+respects also different from the normal form.</p>
+
+<p class="indent">Respecting the <i>Alytes</i> bred in this way Kammerer makes
+the very striking statement that <i>the males in the third generation</i>
+(p. 535) <i>have roughened swellings on their thumbs and that in the
+fourth generation</i> (pp. 516 and 535) <i>these swellings develop black
+pigment</i>. Together with the appearance of this secondary sexual
+character there is hypertrophy of the muscles of the fore-arm.
+To my mind this is the critical observation. If it can be
+substantiated it would go far towards proving Kammerer's case.
+<i>Alytes</i>, among toads and frogs, is peculiar in that the males do
+not develop these lumps in the breeding season, and the fact
+may no doubt be taken to be correlated with the breeding habits,
+copulation occurring on land and not in water as is usual with
+Batrachians. It is to be expressly noticed that these lumps on
+the thumbs or arms of male toads and frogs are not merely pigmented
+swellings, but are pads bearing numerous minute horny
+black spines, which are used in holding the females in the water.
+The figures which Kammerer gives (Taf. XVI, figs. 26 and 26a)
+are quite inadequate, and as they merely indicate a dark patch
+on the thumbs it is not possible to form any opinion as to the
+nature of the structure they represent.</p>
+
+<p class="indent">The systematists who have made a special study of Batrachia
+appear to be agreed that <i>Alytes</i> in nature does not have these
+structures; and when individuals possessing them can be
+<span class="pagenum"><a name="Page_202" id="Page_202">[Pg 202]</a></span>
+produced for inspection it will, I think be time to examine the evidence
+for the inheritance of acquired characters more seriously.
+I wrote to Dr. Kammerer in July, 1910, asking him for the loan
+of such a specimen<a name="FNanchor_13_146" id="FNanchor_13_146"></a><a href="#Footnote_13_146" class="fnanchor">[13]</a>
+and on visiting the Biologische Versuchsanstalt
+in September of the same year I made the same request, but
+hitherto none has been produced. In matters of this kind much
+generally depends on interpretations made at the time of observation;
+here, however, is an example which could readily
+be attested by preserved material. I notice with some surprise
+that in a later publication on the same subject no reference to the
+development of these structures is made (see below).</p>
+
+<p class="indent">The statements here given represent but a small
+part of Kammerer's papers on the subject. He gives much further
+information as to the course of the experiments, especially in
+regard to the fate of the eggs laid on land and the aberrations
+induced in them by treatment. The ramifications of the experiments
+are, however, very difficult to follow, and as I am not
+sure that I have always understood them I must refer the reader
+to the original.</p>
+
+<p class="indent">More recently Kammerer has published<a name="FNanchor_14_147" id="FNanchor_14_147"></a><a href="#Footnote_14_147" class="fnanchor">[14]</a>
+a most curious account of experiments in crossing his modified and abnormal
+<i>Alytes</i>, derived from the water-eggs, with normal individuals.</p>
+
+<p class="indent">In the first case the cross was made between a
+<i>normal female</i> and an <i>abnormal male</i>. The offspring were normal
+in their habits. In the next generation bred from these almost exactly
+a quarter showed the abnormal instinct.</p>
+
+<p class="indent">The reciprocal cross was made between an <i>abnormal female</i>
+and a <i>normal male</i>. In this case the offspring were abnormal in
+their behaviour; but the second generation bred from them
+showed three quarters abnormal and one quarter normal.</p>
+
+<p class="indent">Certain details as to numbers and sexes of the various
+families bred in the course of this amazing experiment are given in a
+<span class="pagenum"><a name="Page_203" id="Page_203">[Pg 203]</a></span>
+subsequent publication.<a name="FNanchor_15_148" id="FNanchor_15_148"></a><a href="#Footnote_15_148" class="fnanchor">[15]</a>
+This later paper goes somewhat fully
+into the question of the difference in behaviour between the
+normal and modified individuals, describing the ways in which
+the males and females possessing the acquired character could be
+recognised from the males and females which were normal, but
+in this account I find no reference to the development of the
+"<i>Brunftschwielen</i>"&mdash;the horny pads on the hands of the males.
+As these structures would be of special value in such a diagnosis
+the omission of any allusion to them calls for explanation.
+Kammerer claims the evidence as proof of Mendelian segregation
+in regard to an acquired character, the first example recorded.
+Pending a repetition of the experiments there is no more to be said.</p>
+
+<p class="indent">2. <i>The Mode of Reproduction of Salamandra atra and maculosa.</i><a name="FNanchor_16_149" id="FNanchor_16_149"></a><a href="#Footnote_16_149" class="fnanchor">[16]</a>
+mdash;<i>Salamandra maculosa</i>, the common lowland form, with yellow
+bands or spots, deposits its young in water, generally as gill-bearing
+tadpoles, with a wide, swimming tail, though occasionally
+they are born still enclosed in the egg-capsule out of which they
+soon hatch. Spawning extends over a considerable period,
+often many weeks, and during the season one female may bear
+more than 50 young.</p>
+
+<p class="indent"><i>S. atra</i>, the black Alpine form, produces its young
+on land. They are born without gills, ready to breathe air, and with the
+rounded tail of the adult. These differences may, as Kammerer
+says, naturally be regarded as adaptations to the Alpine conditions.
+Moreover, the female bears <i>only two</i> young in a season,
+and this reduction in the number must be taken to be a consequence
+or condition of viviparity. There are many eggs in the
+ovary, but all except the two which are destined to develop
+degenerate and form a yolk-material on which these two survivors feed.</p>
+
+<p class="indent">Kammerer gives a long account of the various conditions
+to which he subjected both species. The treatment was complicated
+<span class="pagenum"><a name="Page_204" id="Page_204">[Pg 204]</a></span>
+in many ways, but the essential statements are, as regards
+<i>S. maculosa</i>, that when no water was provided in which the young
+might be born, they were dropped on land, larger and in a later
+stage of development and of a darker colour than is normal; that
+the larvae so born gradually diminished in number until only
+two were deposited in each breeding-period; that dissection
+showed that the other ova degenerated to form a yolk-material.
+The larvae so produced reached maturity. The summary of
+results describes their behaviour, stating that they produced:</p>
+
+<p class="indent">(<i>a</i>) <i>In water</i>, either (1) <i>very</i> advanced,
+large-headed larvae 45 mm. long (instead of 25-30 mm.) with gills already
+reduced, which had awkward, embryo-like movements, and in some few
+days metamorphosed into small perfect salamanders; or (2)
+moderately advanced, properly proportioned larvae, 40-41 mm.
+long, provided with large gills of (at first) intrauterine character,
+which were reduced during aquatic life.</p>
+
+<p class="indent">(<i>b</i>) <i>On land</i>, small (26 mm. long) larvae with
+rudimentary gills, having the body rounded instead of being flattened from
+above downwards, and an elongated narrow head, which were
+unable to live in deep water. These larvae changed to the salamander
+colour in 10-12 days, and after four weeks metamorphosed
+into salamanders 29 mm. long.</p>
+
+<p class="indent">(<i>c</i>) In the foregoing cases the experimental
+conditions were not continued, or in other words, basins of water were
+provided in which they could spawn. But if the experimental conditions
+are continued, these <i>Salamandra maculosa</i> which were born
+newt-like (viz., not in a larval condition), are themselves newt-bearing
+from the first time they give birth, using the dry land,
+and bringing forth only two young, the normal number for the
+births of <i>S. atra</i>. These young are 40-41 mm. long, and are
+dark-coloured, resembling greatly the normal new-born <i>S. atra</i>.</p>
+
+<p class="indent">This epitome of the observations illustrating the
+inheritance of acquired characters has been very widely quoted, and may
+not unnaturally be taken to summarize a wide experience of
+the modified animals. Reference to the details given in the
+same paper shows that, as alleged, each of the four types of behaviour
+enumerated was witnessed <i>once</i> only in the case of each
+<span class="pagenum"><a name="Page_205" id="Page_205">[Pg 205]</a></span>
+of four females, no two agreeing with each other. As to the
+number of the males or their habits nothing is said. The first
+female, <i>a</i> (1), bore five young; the second, <i>a</i> (2), bore two, of
+which one was a partial albino; the third, <i>b</i>, produced four young;
+and the fourth, <i>c</i>, two as already stated.</p>
+
+<p class="indent">In the case of <i>c</i> the details show that the female
+gave birth immediately after being transferred from the open-air terrarium
+to one indoors, which contained no basin of water. This is the
+example of the consequences which follow on a continuance of
+the experimental conditions.<a name="FNanchor_17_150" id="FNanchor_17_150"></a><a href="#Footnote_17_150" class="fnanchor">[17]</a>
+</p>
+
+<p class="indent">As regards <i>S. atra</i> the converse is reported.
+Various means were used to induce them to eject their young prematurely
+in water, such as massaging the sides of the mothers, or raising
+the temperature to 25° or 30° C., with various degrees of success.
+But afterwards it was found that specimens collected wild at an
+elevation of about 1,000 metres responded to much simpler
+treatment, and gave birth prematurely in water when they were
+kept in a large shallow basin of water not so deep but that they
+could everywhere touch the bottom with their feet and keep their
+heads above the surface. With specimens collected at higher
+elevations this treatment was inoperative, and the suggestion is
+made that <i>S. atra</i> at the lower confines of its habitat partakes
+more of the nature of <i>maculosa</i> than do the individuals from
+greater heights; for Kammerer argues that pools suitable for
+breeding must be more uncommon at those elevations than they
+are lower down.</p>
+
+<p class="indent">In the earlier paper<a name="FNanchor_18_151" id="FNanchor_18_151"></a><a href="#Footnote_18_151" class="fnanchor">[18]</a>
+Kammerer states that newly caught
+females of <i>S. atra</i> often give birth in the water, and show an
+undoubted preference for doing so. He describes also how he
+once saw several females, wild in their natural habitat, lay their
+young in a rain-puddle at 1,800 metres elevation, but the larvae
+thus born were fully formed.
+<span class="pagenum"><a name="Page_206" id="Page_206">[Pg 206]</a></span></p>
+
+<p class="indent">When the deposition of the young as larvae has become
+"habitual"<a name="FNanchor_19_152" id="FNanchor_19_152"></a><a href="#Footnote_19_152" class="fnanchor">[19]</a>
+with <i>S. atra</i>, three to nine larvae may be produced
+at one spawning period, from 35 to 45 mm. long, with gills at
+most 8 mm. long, and a tail-fin 2-3 mm. broad. Such larvae
+are generally coffee-brown, or grey (instead of black), and show
+other minor differences.</p>
+
+<p class="indent">The summary states that when grown to maturity they
+become in their turn larva-bearing, and go into the water to bring
+forth. Their young are more than two (3 to 5 being the numbers
+observed) with a length of 33-40 mm. or of 21-23 mm. at birth.
+They are light grey, spotted (mottled with lighter and darker
+colour), have relatively short gills (8 to 9 mm. at most) and a
+broad tail-fin (3 mm. wide). At metamorphosis they are relatively
+long (44 mm.) and one of them had some yellow pigment.</p>
+
+<p class="indent">Here again this summary is, as a matter of fact,
+describing the behaviour of two mothers, of which one produced three,
+and the other five young.</p>
+
+<p class="indent">To my mind these experiments suggest that the reproductive
+habits of both species, if closely observed, will be found to be
+subject to considerable variation, and I think it not impossible
+that each species is, especially in confinement, capable of being a
+good deal deflected from its normal behaviour. Moreover, there
+seems to me no great improbability in the idea that there is an
+interdependence between the number of young and the stage of
+maturity in which they are born. But, at the same time, the case
+as told by Kammerer strikes me as proving too much. If each
+species is so sensitive to conditions that the normal procedure
+is gravely modified in one generation, and if that modification
+can reappear in a pronounced form in the next generation without
+a renewal of the disturbing conditions, it becomes extremely
+difficult to understand how the regularity which each species is
+believed to display in nature can be maintained. Surely both
+species might be expected to be in confusion. From a passage
+in Kammerer's earlier paper (1904, p. 55) on the subject, I infer
+that he also would expect considerable irregularity in the natural
+behaviour, but that he has not investigated the point.<a name="FNanchor_20_153" id="FNanchor_20_153"></a><a href="#Footnote_20_153" class="fnanchor">[20]</a>
+<span class="pagenum"><a name="Page_207" id="Page_207">[Pg 207]</a></span></p>
+
+<p class="indent">3. <i>Modification of the Colour of Salamandra maculosa
+induced by Change in the Colour of the Soil on which the Animals were
+kept.</i>&mdash;Kammerer speaks of this as the most convincing of all
+his experiments on the transmission of acquired characters. So
+far, however, no full account of them has been published.<a name="FNanchor_21_154" id="FNanchor_21_154"></a><a href="#Footnote_21_154" class="fnanchor">[21]</a>
+The statement is that when salamanders are kept in yellow surroundings
+the yellow markings gradually in the course of years increase
+in amount relatively to the black ground colour. Conversely by
+keeping the animals on black garden soil, the yellow may be
+greatly diminished in quantity until it largely disappears. (The
+account in <i>Natur</i> adds that very moist conditions also favour the
+increase of yellow, and that with less moist conditions the yellow
+diminishes.) From each kind, the (induced) yellower and the
+(induced) blacker, a second generation was raised, on soil of
+neutral colour, and each family was later divided into two parts,
+half being put on black and half on yellow ground.</p>
+
+<p class="indent">As regards the offspring of those which had lived on <i>black</i>
+soil no positive result had been reached up to the date of publication,
+but it is stated that these young resembled their parents
+in having the yellow distributed in <i>irregular spots</i>.</p>
+
+<p class="indent">As regards the offspring of those which had lived on
+yellow soil the account follows up the story of that part of the offspring
+which were put on yellow soil again. It is stated that these, though
+derived from parents with irregular spots, <i>developed the yellow
+as longitudinal bands</i>.</p>
+
+<p class="indent">This account is given with slight differences of
+expression in the three places to which I have referred. On returning
+from Vienna in 1910 I consulted Mr. G. A. Boulenger in reference to
+the subject, and he very kindly showed me the fine series from
+many localities in the British Museum, and pointed out that in
+nature the colour-varieties can be grouped into two distinct types,
+<span class="pagenum"><a name="Page_208" id="Page_208">[Pg 208]</a></span>
+one in which the yellow of the body is irregularly distributed in
+spots and one in which this yellow is arranged for the most part
+in two longitudinal bands which may be continuous or interrupted.
+<i>The spotted form is, as he showed me, an eastern variety,
+and the striped form belongs to western Europe.</i> Mr. E. G.
+Boulenger<a name="FNanchor_22_155" id="FNanchor_22_155"></a><a href="#Footnote_22_155" class="fnanchor">[22]</a>
+has since published a careful account of the distribution
+of the two forms. The spotted he regards as the typical
+form, var. <i>typica</i>, and for the striped he uses the name var.
+<i>taeniata</i>. The typical form occupies eastern Europe in general,
+including Austria and Italy, extending as far west as parts of
+eastern France. The var. <i>taeniata</i> is found all over France,
+excepting parts of the eastern border, Belgium and western
+Germany, Spain and Portugal. Of the very large series examined
+there was only one specimen (Lausanne) which could not with
+confidence be referred to one or other of the two varieties.
+Mr. E. G. Boulenger points out that both varieties inhabit very
+large areas, and live on soils of most different colours and
+compositions. Both are liable to variations in the amount and the
+shade of the yellow, but that any suggestion that <i>taeniata</i> belongs
+especially to yellow soils and <i>typica</i> to black soils is altogether
+inadmissible. He expresses surprise that Kammerer should not
+allude to these peculiarities in the geographical distribution of
+the two forms. He suggests further that it is more likely that
+some mistake occurred in Kammerer's observations than that the
+east European <i>typica</i> should, in the course of a generation, have
+been transformed into the west European <i>taeniata</i> by the influence
+of yellow clay soil.</p>
+
+<p class="indent">In his last paper on the subject Kammerer states incidentally<a name="FNanchor_23_156" id="FNanchor_23_156"></a><a href="#Footnote_23_156" class="fnanchor">[23]</a>
+that he has found the <i>striped form recessive to the spotted</i>. No
+evidence for this statement is given, and I have not found any
+other reference to crosses effected between the two natural types.
+If, however, this representation is correct, it is conceivable that
+the production of <i>taeniata</i> from <i>typica</i> was in fact the re-appearance
+of a recessive form. The plate which Kammerer gives
+in illustration of his modified parent figures a single animal at
+four stages, and though it is certainly more like the spotted than
+<span class="pagenum"><a name="Page_209" id="Page_209">[Pg 209]</a></span>
+the striped form, it has a certain suggestion of the striped arrangement,
+such as I can well imagine being produced in the heterozygote.<a name="FNanchor_24_157" id="FNanchor_24_157"></a><a href="#Footnote_24_157" class="fnanchor">[24]</a>
+</p>
+
+<p class="indent">In continuation<a name="FNanchor_25_158" id="FNanchor_25_158"></a><a href="#Footnote_25_158" class="fnanchor">[25]</a>
+of the experiments on the colour of <i>S. maculosa</i>
+Kammerer publishes an account of elaborate experiments
+in grafting ovaries of the various forms, modified and unmodified,
+into each other, and describes the offspring which followed.
+Before pursuing this part of the inquiry I am disposed to wait
+until the earlier steps have been made much more secure than
+they yet are.</p>
+
+<p class="indent">More recently Kammerer has published similar statements
+in regard to the inheritance of characters induced in various lizards
+by keeping them in abnormal temperatures, high and low. The
+changes induced affected in some species the colours, in others
+the reproductive habits. Respecting these examples I feel the
+same scepticism that I have indicated in regard to the others,
+somewhat heightened by the fact that insufficient evidence is
+given both regarding the behaviour of these various species in
+captivity when not subjected to abnormal temperatures, and
+in the wild state.</p>
+
+<p class="indent">Respecting this part of the evidence Mr. G. A. Boulenger has
+lately published a criticism<a name="FNanchor_26_159" id="FNanchor_26_159"></a><a href="#Footnote_26_159" class="fnanchor">[26]</a>
+from which I extract the following
+passages. Referring to a previous note<a name="FNanchor_27_160" id="FNanchor_27_160"></a><a href="#Footnote_27_160" class="fnanchor">[27]</a>
+on the question of the
+melanism of the various insular forms of <i>Lacerta muralis</i> he
+writes: "I also alluded (<i>l. c.</i>) to the theories that have been
+propounded to explain the melanism of various insular forms.
+This is a subject which has been lately taken up by Dr. Kammerer
+at the Biologische Versuchsanstalt in Vienna, and he claims
+to have produced nigrinos artificially by a very strong elevation
+of the temperature, accompanied by extreme dryness. Dr.
+Werner<a name="FNanchor_28_161" id="FNanchor_28_161"></a><a href="#Footnote_28_161" class="fnanchor">[28]</a>
+has already opposed his own experiments to those of
+Kammerer, artificial melanism having been produced by him in
+<i>Lacerta oxycephala</i> by keeping two very light specimens from
+<span class="pagenum"><a name="Page_210" id="Page_210">[Pg 210]</a></span>
+Ragusa for a whole summer in very damp conditions. Neither is
+Kammerer's theory in accordance with the distribution of the
+black lizards, as pointed out by Werner. Kammerer also finds
+that those forms which are known to produce melanic races in a
+state of nature, lend themselves more readily than the others
+to the success of his experiments. But he shows himself misinformed
+when he states that the variety called <i>Lacerta fiumana</i>
+belongs to the category of those of which black forms are not
+known. He overlooks the fact, first pointed out by Scherer in
+1904, and which I can confirm, that the black lizard from Melisello
+near Lissa in the Adriatic is unquestionably derived from
+the lizard from Lissa, which he correctly regards as not separable
+from <i>L. fiumana</i>...."</p>
+
+<p class="indent">"Another colour modification which Dr. Kammerer states
+that he obtained by raising the temperature is the assumption by
+the female of the typical <i>Lacerta muralis</i> of the bright red colour
+of the lower parts which often distinguishes the male from the
+female, and which was not shown by the individuals of the latter
+sex kept by him under normal conditions. He quotes various
+authorities to show that the lower parts are never red in the
+females, but he has omitted to consult others who say the contrary.
+Thus Bedriaga (1878 and 1879) remarks that a so-called
+var. <i>rubriventris</i> of the typical wall lizard has the lower parts red
+in both sexes."<a name="FNanchor_29_162" id="FNanchor_29_162"></a><a href="#Footnote_29_162" class="fnanchor">[29]</a>
+</p>
+
+<p class="indent">In reading such papers as those of Semon or Kammerer
+the thought uppermost in my mind is that to multiply illustrations
+of supposed transmission of acquired characters is of little use
+until some one example has been thoroughly investigated. If
+we had certain assurance that even a single unimpeachable case
+could be repeated at will, the whole matter would assume a more
+serious aspect. If, for instance, Kammerer were able to show us
+<i>Alytes</i> males with horny pads on their hands, it would be
+something tangible; still more, if the experiment were repeated by
+others until no doubt remained that the offspring of <i>Alytes</i> which
+had bred in water for some three generations did acquire these
+<span class="pagenum"><a name="Page_211" id="Page_211">[Pg 211]</a></span>
+pads and that they could transmit these novelties to descendants
+raised in normal conditions. Till evidence of this kind is published
+by at least two independent observers investigating similar
+material, I find it easier to believe that mistakes of observation or
+of interpretation have been made than that any genuine transmission
+of acquired characters has been witnessed.</p>
+
+<p class="indent space-below">Meanwhile there is no denying that the origin of adaptational
+features is a very grave difficulty. With the lapse of time since
+evolutionary conceptions have become a universal subject of study
+that difficulty has, so far as I see, been in nowise diminished.
+But I find nothing in the evidence recently put forward which
+justifies departure from the agnostic position which most of us
+have felt obliged to assume.<a name="FNanchor_30_163" id="FNanchor_30_163"></a><a href="#Footnote_30_163" class="fnanchor">[30]</a>
+</p>
+
+<hr class="chap" />
+<h3><span class="smcap">Appendix to Chapter IX.</span></h3>
+
+<p class="indent">Professor G. Klebs, as is well known to students of
+evolutionary phenomena, has for several years been engaged in investigations
+relating to the inheritance of acquired characters. In
+his many publications on the subject the issue has always been
+represented as more or less uncertain.</p>
+
+<p class="indent">Desiring to know how the matter now stands according
+to Professor Klebs' present judgment I wrote to him asking him to
+favour me with a brief general statement. This he most kindly
+sent in a letter dated 8th July, 1912.</p>
+
+<p class="indent">As such a statement will be read with the greatest
+interest by all who are watching the progress of these studies I obtained
+permission to publish it as follows:</p>
+
+<p class="blockquot">
+<span style="margin-left: 38em;">8. Juli 1912</span><br />
+&emsp;Ihre liebenswurdige Anfrage will ich sehr gern
+beantworten, obwohl ich sie nicht so beantworten kann wie ich
+erwünschte. Ihr Skepticismus in der Frage der Uebertragung erworbener
+Charactere auf die Nachkommen ist nur zu berechtigt. Meine
+Versuche mit Veronica sind <i>nicht</i> beweisend, da es mir bisher
+nicht gelungen ist eine einigermasse konstante Varietät mit
+verlaubten Inflorescenze zu erzeugen. In Bezug auf mein
+Semper vivum bin ich allerdings noch heute der Meinung dass
+<span class="pagenum"><a name="Page_212" id="Page_212">[Pg 212]</a></span>
+die starke künstliche Veränderung der Blüte einen Einfluss auf
+einzelnen Nachkommen gehabt hat. Ich habe seither nichts
+darüber veröffentlicht: die Mehrzahl der anormalen gefüllten
+Blüten war leider steril. Von einem weniger veränderten Exemplar
+erhielt ich einige Sämlinge, aber sie haben noch nicht
+geblüht. Es kann sich in diesem Falle nur um eine <i>Nachwirkung
+in der ersten Generation</i> handeln, vergleichbar jenen Fällen in
+denen Samen von Bäumen aus den hohen Alpen in der Ebene
+gewisse Nachwirkungen zeigen. Aber es ist bisher kein sicherer.
+Fall bekannt in den der kunstliche herbeigeführte Charakter
+<i>mehrere Generationen hindurch unter der gewöhnlichen "normalen"
+Bedingungen</i> übertragen worden ist.</p>
+
+<p class="blockquot">&emsp;Auf der andere Seite sind diese negativen Resultaten
+nicht entscheidend. Denn wie wenig ist in dieser Beziehung überhaupt
+ernstlich versucht worden! Und zweifellos geht die
+Sache nicht so einfach.</p>
+
+<p class="blockquot">&emsp;Ich versuche es mit anderen Pflanzen weil ich der
+Meinung bin dass es möglich sein müsse wenigstens solche neuen Varietäten
+zu erzeugen, wie sie die Gartenvarietäten entsprechen.</p>
+
+<p class="blockquot">&emsp;Aber bis jetzt leider sind die Versuche nicht
+gelungen, weder mir noch irgend einem anderen.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_213" id="Page_213">[Pg 213]</a></span></p>
+
+<h2>CHAPTER X</h2>
+<h3><span class="smcap">Effects Of Changed Conditions</span> <i>Continued</i></h3>
+<p style="font-size: 110%; text-align: center;">
+   <b><span class="smcap">The Causes of Genetic Variation</span></b></p>
+
+<p class="indent">In the last chapter we examined some of the evidence
+offered in support of the belief that adaptation in highly organised
+forms is a consequence of the inheritance of adaptative changes induced
+by the influence of external conditions. The state of knowledge
+of this whole subject is, as I have said, most unsatisfactory,
+chiefly for the reason that in none of the cases which are alleged
+to show a positive result have two observers been over the
+same ground, or as yet confirmed each other. In the wider
+consideration respecting the causes of variation at large we find
+ourselves still in the same difficulty. The study has thus far
+proved sadly unfruitful. In spite of the considerable efforts
+lately made by many observers to induce genetic variation in
+highly organised plants or animals, and though successes have
+occasionally been announced, I do not know a single case which
+has been established and confirmed in such a way that we could
+with confidence expect to witness the alleged phenomena if we
+were to repeat the experiment. Abundant illustrations are
+available in which individuals exposed to novel conditions manifest
+considerable changes in characters or properties, but as yet
+there is no certain means of determining that germ-cells of a new
+type shall be formed.</p>
+
+<p class="indent">Of the direct effect of conditions the lower organisms,
+especially bacteria, offer the best examples, the alterations of virulence
+which can be produced in so many distinct ways being the most
+striking and familiar. That attenuation of virulence can be
+produced by high temperatures or by exposure to chemical
+agents, and that this diminution in virulence may remain permanent
+is, from our point of view, not surprising; but the fact that
+in many cases the full virulence can by suitable cultivation be
+<span class="pagenum"><a name="Page_214" id="Page_214">[Pg 214]</a></span>
+restored is difficult to understand. Similar variations have been
+observed in power of pigment production and other properties.</p>
+
+<p class="indent">These phenomena naturally raise the question whether any
+cases of apparent loss of factors in higher forms may be comparable.</p>
+
+<p class="indent">The subject of variations in the lower organisms and
+their dependence on conditions is a highly special one, and I have no
+knowledge which can justify me in offering any discussion of
+them, but I understand that hitherto little beyond empirical
+recognition of the phenomena has been attempted. A useful
+summary of observations made by many investigators was lately
+published by Hans Pringsheim,<a name="FNanchor_1_164" id="FNanchor_1_164"></a><a href="#Footnote_1_164" class="fnanchor">[1]</a>
+who enumerates the different
+agencies which have been observed to produce modifications,
+and the various ways in which these changes are manifested.
+One of the most comprehensive studies of the subject from the
+genetic point of view is that made by F. Wolf.<a name="FNanchor_2_165" id="FNanchor_2_165"></a><a href="#Footnote_2_165" class="fnanchor">[2]</a>
+In his extensive cultivations of <i>Bacillus prodigiosus</i>,
+<i>Staphylococcus pyogenes</i> and <i>Myxococcus</i> he succeeded in producing
+many strains with modified properties. In most of these the modifications arose
+in consequence of the application of high or low temperatures or of
+the addition of various chemical substances to the culture-media.
+Some of the variations, which are for the most part in the powers
+of pigment-formation, persisted when the strains were returned
+to normal conditions, and others did not. In reference especially
+to the variations witnessed in the Cocci the reader should consult
+the critical account of variation in that group published by the
+Winslows,<a name="FNanchor_3_166" id="FNanchor_3_166"></a><a href="#Footnote_3_166" class="fnanchor">[3]</a>
+where much information on the subject is to be
+found. The authors attempted to determine the systematic
+relationships of the several forms, as far as possible, by the
+application of statistical methods. The result is interesting as
+showing that the problem of species in its main features is presented
+by these organisms in a form identical with that which
+we know so well in the higher animals and plants, whatever
+<span class="pagenum"><a name="Page_215" id="Page_215">[Pg 215]</a></span>
+properties be selected as the diagnostic characters. There are
+many types perfectly distinct and others which intergrade.
+Some of the types change greatly with conditions while others do
+not. This is exactly what we encounter whenever we study the
+problem of species on an extended scale among the higher forms of life.</p>
+
+<p class="indent">There is now practically complete agreement among
+bacteriologists that the observations made first by Massini on the
+change in color of <i>Bacterium coli mutabile</i> grown in Endo's
+medium, associated with the acquisition of the power to ferment
+lactose, are perfectly reliable and free from possibilities of mistake.
+The work has been extended and confirmed by many
+workers, especially R. Müller, who finds that this bacterium can
+similarly acquire and maintain the power to ferment other
+sugars. A careful account of the whole subject written by Müller
+for the information of biologists will be found in <i>Zts. für Abstammungsl.</i>,
+VIII, 1912. After discussing the biological significance
+of the facts, he concludes with a caution to the effect that bacteria
+are so different from all other living things that generalizations
+from their behavior must not be indiscriminately applied to animals and plants.</p>
+
+<p class="indent">In all work with this class of material there is
+obviously danger of error through foreign infection of the cultures,
+but there can be no doubt that though some of the "mutations"
+recorded may be due to this cause, the majority of the instances
+observed under stringent conditions are genuine.</p>
+
+<p class="indent">Another and equally serious difficulty besetting work
+with bacteria and fungi cultivated from spores is that the appearance
+of variation may in reality be due to the selection of a special
+strain previously living masked among other strains. This
+possibility must be remembered especially in those instances
+which are claimed as exemplifying the effects of acclimatisation.
+Manifestly this consideration can be urged with most force when
+the strain which gave rise to the novelty was not raised from a
+single individual spore. Moreover, when once the possibility of
+spontaneous variation is admitted, it must be difficult to be quite
+confident that any given variation observed is in reality due to
+<span class="pagenum"><a name="Page_216" id="Page_216">[Pg 216]</a></span>
+the novel conditions applied, and as I understand the evidence,
+the appearance of the mutational forms does not with any
+regularity follow upon the application of the changed conditions.</p>
+
+<p class="indent">Researches into the variation of these lower forms
+will, no doubt, be continued on a comprehensive scale. So long as the
+instances recorded are each isolated examples it is impossible to
+know what value they possess. If they could be coordinated in
+such a way as to provide some general conception of the types of
+variation in properties to which bacteria, or any considerable
+group of them, are habitually liable, the knowledge might
+greatly advance the elucidation of genetic problems.</p>
+
+<p class="indent">Of mutational changes directly produced with regularity
+in micro-organisms by treatment, the experiments with trypanosomes
+provide some of the clearest examples. A summary of
+the evidence was lately published by Dobell,<a name="FNanchor_4_167" id="FNanchor_4_167"></a><a href="#Footnote_4_167" class="fnanchor">[4]</a>
+from which the present account is taken. The most definite fact of this
+kind established is that certain dyes introduced into the blood of the
+host have the effect of destroying the small organ known as the
+"kinetonucleus" in the trypanosomes. The trypanosomes thus
+altered continue to breed, and give rise to races destitute of
+kinetonuclei. This observation was originally made by Werbitzki
+and has been confirmed by several observers. The exact
+way in which this alteration is effected in the trypanosomes is not
+quite definitely made out, but there is good reason for supposing
+that the dyes have a direct and specific action upon the kinetonucleus
+itself, and circumstances make it improbable that in
+some division a daughter-organism without that body is produced,
+or that any selection of a pre-existing defective variety occurs.</p>
+
+<p class="indent">Ehrlich has suggested with great probability that the
+dyes which possess this action owe it to the fact that they have the
+particular chemical linkage which he calls "ortho-quinoid." In
+outward respects, such as motility and general appearance, the
+modified organisms are unchanged, but their virulence is diminished.
+As regards the possibility of the defective strain
+<span class="pagenum"><a name="Page_217" id="Page_217">[Pg 217]</a></span>
+reacquiring the kinetonucleus, Werbitzki states that in one case
+passage through 50 animals and treatment with dyes left the
+strain unaltered; but that in another case at the sixteenth
+passage 7 per cent. of the trypanosomes were found to have
+re-acquired the organ, and in subsequent passages the percentage
+increased, until at the twenty-seventh passage practically all had
+re-acquired it. Kudicke, however, in similar experiments did not
+succeed in causing re-acquisition by transplantation.</p>
+
+<p class="indent">By the action of various drugs and anti-bodies races
+of trypanosomes resistant to those substances have been obtained.
+These breed true, at least when kept in the same species of animal
+in which the resistance was acquired. As to whether change of
+virulence is produced by passage through certain animals or not,
+there is as yet no general agreement.</p>
+
+<p class="indent">Other changes, especially in size and some points of
+structure, are said to occur when certain trypanosomes proper to mammals
+are passed through cold-blooded vertebrates (Wendelstadt and
+Fellmer), and it is stated that these changes persist, but the
+observations have not yet been confirmed.</p>
+
+<p class="indent">Experiments lately conducted by Woltereck with <i>Daphnia</i>
+are interesting as having given a definite positive result, in so far,
+at least, as the ova were affected by conditions before leaving the
+bodies of the parent individuals. The observations relate to
+the offspring resulting from <i>parthenogenetic</i> eggs. Females
+bearing ephippia (fertilised eggs) were isolated until the ephippia
+were dropped, and in this way the offspring of fertilisation were
+excluded. Males, of course, appeared from time to time in the
+cultures, but as fertilised eggs were rejected, their presence did
+not disturb the result. The most remarkable observations
+related to <i>Daphnia longispina</i>.</p>
+
+<p class="indent">This species as found in the lower lake at Lunz had
+the front end of the body blunt and nearly round in profile; but on being
+cultivated in a warm temperature and with abundant nourishment
+the front end of the body became produced into an elongated
+"helmet," as Woltereck calls it. Experiment showed that
+the change was primarily due to the abundance of food, and owing
+to temperature in a subordinate degree.
+<span class="pagenum"><a name="Page_218" id="Page_218">[Pg 218]</a></span></p>
+
+<p class="indent">This distinction arose as soon as the species was taken
+into the hothouse, but when the modified individuals were put back
+into the original conditions, a lower temperature and scanty
+food-supply, the next generation returned to their original form.
+After being cultivated for two years and about 40 generations in
+the more favourable conditions, when similarly put back into
+the lower temperature with scanty food the <i>first generation</i> born
+in these conditions was helmeted like the modified parents.
+Woltereck is of opinion that the ova were still unformed at the
+time the parents were put back, and the influence of the favourable
+conditions upon the unformed ova he speaks of as a "prae-induction."
+The effect never extended beyond the one generation,
+after which the strain returned to its original state.</p>
+
+<p class="indent">The fact that the influence on the offspring was not
+manifested at first led Woltereck to expect that by more prolonged
+cultivation in the favourable conditions a further extension of
+this influence would be produced, but this expectation was never
+fulfilled, though the attempt was made again and again.</p>
+
+<p class="indent">Similar experiments were made with <i>Hyalodaphnia cucullata</i>,
+which is far more sensitive to cultural influences, and in nature
+manifests a considerable elongation of the helmet as a seasonal
+modification, but the results were essentially the same as in the
+preceding case, no modification extending beyond the first
+generation born after the restoration to <i>normal conditions</i>.<a name="FNanchor_5_168" id="FNanchor_5_168"></a><a href="#Footnote_5_168" class="fnanchor">[5]</a>
+</p>
+
+<p class="indent">The only criticism of these extremely interesting results
+which suggests itself is that perhaps the original appearance of the
+modification was not in reality due to an <i>accumulated</i> effect of
+the conditions, but to some change in the conditions themselves
+which was not noticed. It is difficult to see how length of
+time or even the lapse of several generations could have so specific
+an effect on the race. It is no doubt often vaguely supposed
+by many that a long period of time may be necessary for the
+effect of climate or of other environmental conditions to be
+produced in an organism which does not thus respond at first.
+I have never been able to see any reason for this opinion nor how
+<span class="pagenum"><a name="Page_219" id="Page_219">[Pg 219]</a></span>
+it is to be translated into terms of physiological fact, and I
+imagine that in those cases in which the lapse of time is really
+required for the production of an effect, the influence of the
+prolongation is rather on the conditions than on the organisms.
+The response of the organisms thus probably indicates not that
+the creature is at length feeling the effects because of their
+accumulated action on itself, but that the conditions have at
+length ripened.</p>
+
+<p class="indent">As this sheet is passing through the press Agar has published<a name="FNanchor_6_169" id="FNanchor_6_169"></a><a href="#Footnote_6_169" class="fnanchor">[6]</a>
+an abstract of evidence as to another comparable case in a parthenogenetic
+strain in the daphnid, <i>Simocephalus vetulus</i>. When
+fed on certain abnormal foods the shape of the body is changed,
+the edges of the carapace being rolled backwards so as to expose
+the appendages. The offspring of animals thus modified showed
+similar modification in the first, and to a very slight degree, in
+the second generation, though the original mothers were removed
+to normal conditions before their eggs were laid. In the third
+generation there was "a very pronounced reaction in the opposite
+direction." Agar suggests that the change may be due to some
+toxin-like substances, carried on passively by the egg into the
+next generation, against which the protoplasm eventually produces
+an anti-body.</p>
+
+<p class="indent">The experiments which have been in recent years regarded
+by evolutionary writers as the most conclusive proof that direct
+environmental action may produce germinal variation are those
+of Professor W. L. Tower, of Chicago, on <i>Leptinotarsa</i>, the
+potato beetles. This work has attained considerable celebrity
+and has been generally accepted as making a definite extension
+of knowledge. After frequently reading Tower's papers and
+after having been privileged to see some of the experiments in
+progress (in 1907) I am still in doubt as to the weight which
+should be assigned to this contribution.</p>
+
+<p>The work is described in two chief publications, the first of
+which appeared in 1906.<a name="FNanchor_7_170" id="FNanchor_7_170"></a><a href="#Footnote_7_170" class="fnanchor">[7]</a>
+This treatise contains a vast amount
+of information about numerous species and varieties of these
+<span class="pagenum"><a name="Page_220" id="Page_220">[Pg 220]</a></span>
+beetles which the author has observed and bred in many parts of
+their distribution throughout the United States, Mexico and
+Central America. The part of the book which has naturally
+excited the greatest interest is that in which Tower states that
+by subjecting the beetles to change in temperature and moisture,
+he caused them to produce offspring quite unlike themselves,
+which in several cases bred true.</p>
+
+<p class="indent">It is much to be regretted that the author did not
+happen to become acquainted with Mendelian analysis at an earlier stage
+in the investigation. The evidence might then have been
+handled in a much more orderly and comprehensive way, and a
+watch would have been kept for several possibilities of error.</p>
+
+<p class="indent">The headquarters of the genus is evidently as Tower
+states, in Mexico and the adjoining countries. In this region there is
+a great profusion of forms, some very local, some as for instance
+the well-known <i>decemlineata</i>,<a name="FNanchor_8_171" id="FNanchor_8_171"></a><a href="#Footnote_8_171" class="fnanchor">[8]</a>
+more widely spread. The distinctions are almost all found in peculiarities of
+colour and pattern, and the limits of species are even more indefinable than
+is usual in multiform animals. Tower arranges the various types
+into seven groups of which the one most studied is that which
+he calls the <i>lineata</i> group. To this group belong all the forms
+to which reference is here made, and, as I understand, they differ
+among themselves entirely in size, colour and pattern. There
+is no suggestion of infertility in the crosses made between the
+several forms of the <i>lineata</i> group; in fact they present, like many
+Chrysomelidae, a good example of what most of us would now
+call a polymorphic species, consisting of many types, some found
+existing in the same locality, others being geographically isolated.</p>
+
+<p class="indent">A series of experiments was devoted to the attempt to
+fix strains corresponding to the extremes of continuous variations.
+For example, those with most black pigment and those with
+least black taken from a population continuously varying in this
+respect, were separately bred; but almost always the selection
+led to no sensible change in the position of the mean of the
+<span class="pagenum"><a name="Page_221" id="Page_221">[Pg 221]</a></span>
+population. The variations in these cases were evidently fluctuational.
+In some instances, however, real genetic differences
+were met with, and strains exhibiting them were, as usual,
+rapidly fixed.</p>
+
+<p class="indent">Tower points out that several of the varieties (or species,
+as he prefers to call them) were obviously recessive to <i>decemlineata</i>.
+This is most clearly demonstrated in the case of the form called
+<i>pallida</i>, which is a pale depauperated-looking creature, with the
+orange of the thorax almost white and the eyes devoid of pigment.<a name="FNanchor_9_172" id="FNanchor_9_172"></a><a href="#Footnote_9_172" class="fnanchor">[9]</a>
+This form behaved as an ordinary Mendelian recessive,
+breeding true whenever it appeared in the cultures, or when
+individuals found wild were studied in captivity. A black form
+which Tower names <i>melanicum</i> was similarly shown to be a
+Mendelian recessive. Wild specimens of this variety of opposite
+sexes were not found simultaneously in nature, and there was thus
+no opportunity of breeding them together, but the hereditary
+behaviour was seen in the F<sub>2</sub> generation from a <i>melanicum</i> found
+coupled with <i>decemlineata</i>. Experiments also occurred giving
+indication that a variety with the stripes anastomosing in pairs
+(<i>tortuosa</i>), was another recessive, and that a variety&mdash;called
+"<i>rubri-vittata</i>"&mdash;gave an intermediate F<sub>1</sub> with subsequent
+segregation. All these are forms of <i>decemlineata</i> Stål.</p>
+
+<p class="indent">Similar observations were made regarding forms recessive
+to <i>multitaeniata</i> Stål. Of these two were thrown by <i>multitaeniata</i>
+itself, namely a form named by Stål <i>melanothorax</i>, and regarded
+by him as a species, and one which Tower names <i>rubicunda</i> n. sp.
+The facts proving the recessive behaviour of their several forms
+will be found in the following places in Tower's book:</p>
+
+<p>
+<span style="margin-left: 6em;"><i>pallida</i>,&emsp; pp. 273-278.</span><br />
+<span style="margin-left: 6em;"><i>melanicum</i>,&emsp; p. 279.</span><br />
+<span style="margin-left: 6em;"><i>tortuosa</i>,&emsp; p. 280.</span><br />
+<span style="margin-left: 6em;"><i>rubrivittata</i>,&emsp; pp. 280-281.</span><br />
+<span style="margin-left: 6em;"><i>melanothorax</i> and <i>rubicunda</i>,&emsp; pp. 283-285.</span>
+<span class="pagenum"><a name="Page_222" id="Page_222">[Pg 222]</a></span></p>
+
+<p class="indent">
+Following this evidence of recessive nature of the six forms
+enumerated, Tower describes experiments showing, as he believes,
+that some of them may be caused to appear by applying special
+treatment to the parents during the "growth and fertilisation"
+(p. 287) of the eggs. The most striking example is that in which
+4 males and 4 females of <i>decemlineata</i> were kept very hot (average
+35° C.) and dry, and at low atmospheric pressure (19-21 inches).
+The eggs laid were restored to natural conditions. These gave
+506 larvae, from which emerged 14 normal, 82 <i>pallida</i> and 2
+"<i>immaculothorax</i>," viz., without pigment on the pronotum.
+The account of the rest of the experiment is somewhat involved,
+but I understand that the <i>pallida</i>, of which two only survived,
+behaved as normal recessives when bred to the type: also that
+the parents, after having laid the eggs whose history has been
+given, were restored to normal conditions and laid 319 eggs which
+gave 61 normals.</p>
+
+<p class="indent">In another case normal parents laid 409 eggs in the
+hot and dry conditions, and on restoration to normal conditions, the
+same parents laid 840 eggs. Then 409 eggs gave 64 adults as follows:</p>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="_" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">Males</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">Females</td>
+    </tr><tr>
+      <td class="tdl"><i>decemlineata</i></td>
+      <td class="tdc">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td>
+      <td class="tdc">12</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">8</td>
+    </tr><tr>
+      <td class="tdl"><i>pallida</i></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">10</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">13</td>
+    </tr><tr>
+      <td class="tdl"><i>immaculothorax</i></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">2</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">3</td>
+    </tr><tr>
+      <td class="tdl"><i>albida</i></td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">9</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">7</td>
+    </tr><tr>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">&mdash;&mdash;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">&mdash;&mdash;</td>
+    </tr><tr>
+      <td class="tdl">&nbsp;</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">33</td>
+      <td class="tdc">&nbsp;</td>
+      <td class="tdc">31</td>
+    </tr>
+ </tbody>
+</table>
+
+<p class="indent">The 840 eggs laid in normal conditions
+gave 123 normal <i>decemlineata</i>.</p>
+
+<p class="indent">Similar experiments were made with <i>multitaeniata</i> and gave
+comparable results, the two recessives (<i>melanothorax</i>, <i>rubicunda</i>)
+being produced in large numbers when the parents were subjected
+to heat, but in this case the atmosphere was kept <i>saturated</i> with
+moisture, instead of dry, as in the previous instance. The same
+parents transferred to normal conditions gave normals only.</p>
+
+<p class="indent">Lastly the form <i>undecimlineata</i> was exposed "to an
+extreme stimulus of high temperature, 10° C. above the average," and a
+dry atmosphere, with the result that from 190 eggs there emerged
+11 beetles, all of the form <i>angustovittata</i> Jacoby, which subsequently
+bred true to that type (see p. 295).
+<span class="pagenum"><a name="Page_223" id="Page_223">[Pg 223]</a></span></p>
+
+<p class="indent">In the results of these experiments, as described, there
+is one feature which I regard as quite unaccountable. Tower makes no
+comment upon it. Indeed, from the general tenour of the paper,
+I infer, not only that he does not perceive that he is recounting
+anything contrary to usual experience, but rather that he regards
+the result as conforming to expectations previously formed.
+The point in question is the genetic behaviour of the dominant
+normals produced under the abnormal conditions. These
+normals were the result of the breeding of parents declared to be
+at the same time giving off many recessive gametes. Some of
+these normals must be expected therefore to be heterozygous
+unless some selective fertilisation occurs. Nevertheless in every
+case they and their offspring are reported to have continually
+bred true. I allude especially to the tables given on pp. 288, 289,
+292, and 293. Tower does not mention any misgiving about
+this result, and I think he regards himself as recounting phenomena
+in general harmony with the ideas of mutation expressed
+by De Vries. This they may be; but to anyone familiar with
+analytical breeding the course of these experiments must seem so
+surprising as to call for most careful, independent confirmation.</p>
+
+<p class="indent">In 1910<a name="FNanchor_10_173" id="FNanchor_10_173"></a><a href="#Footnote_10_173" class="fnanchor">[10]</a>
+Tower published an account of further experiments
+with <i>Leptinotarsa</i>. The work described related to two subjects.
+Crosses were made between three forms, <i>undecimlineata</i> Stål,
+<i>signaticollis</i> Stål and "<i>diversa</i>" named by Tower as a new
+species. The distinctions between these three depend partly on
+characters of the adults and partly on those of the larvae. The
+adults of <i>undecimlineata</i> and <i>diversa</i> have the elytra striped, but
+the elytra of <i>signaticollis</i> are unstriped. The larvae of <i>signaticollis</i>
+and of <i>diversa</i> are yellow, but those of <i>undecimlineata</i>
+are white.<a name="FNanchor_11_174" id="FNanchor_11_174"></a><a href="#Footnote_11_174" class="fnanchor">[11]</a>
+Moreover, in <i>signaticollis</i> and <i>diversa</i> the black increases
+in the third stage of the larvae to form transverse bands
+which are absent in <i>undecimlineata</i>. The general course of the
+experiments shows that these differences may be approximately
+<span class="pagenum"><a name="Page_224" id="Page_224">[Pg 224]</a></span>
+represented as due to the action of three factors, any of which
+may be independently present or absent. The stripings of the
+elytra and of the larvae are each due to a separate factor. As
+regards the distinction between the yellow and the white larvae
+the evidence does not prove that there is decided dominance of
+either colour and I infer that the heterozygotes are often intermediate.</p>
+
+<p class="indent">The chief contribution which this new paper claims to
+make relates to differences in the results which ensue from crosses
+effected between these three types at different average temperatures.</p>
+
+<p class="indent">We are first concerned with four experiments
+which I number (1), (2), (3), (4):</p>
+
+<p class="indent">1. <i>Signaticollis</i> &#9792; × <i>diversa</i> &#9794; bred at
+an average temperature of 80º F. by day and 75° F. by night, gave two groups in
+about equal numbers. The first (49) was pure <i>signaticollis</i> and
+bred true. The second (53) was of an intermediate type, which
+on being bred together gave the typical Mendelian result&mdash;1 <i>sig.</i>:
+2 <i>intermediate</i>: 1 <i>div</i>.</p>
+
+<p class="indent">2. Next, as the account originally stood in the published
+paper, we are told that <i>sig</i> &#9792; × <i>div</i> &#9794; bred together at
+a day-temp. average 75° F. and night average 50° F. gave an <i>intermediate</i>
+only, which subsequently produced a normal 1:2:1 ratio. The two crosses were
+repeated eleven times with identical results.</p>
+
+<p class="indent">In a further experiment (3)
+<i>signaticollis</i> &#9792; × <i>diversa</i> &#9794;
+were bred under the same conditions as those used in expt. (1). They
+again gave <i>sig.</i> and intermediates as before in fairly equal numbers.
+The <i>sig.</i> as before bred true, and the intermediate gave 1:2:1,
+all exactly as in expt. (1).</p>
+
+<p class="indent">In expt. (4) <i>the same parents used</i> in (3) were again
+mated under conditions of expt. (2) at the lower temperature, and this
+time gave <i>signaticollis</i> exclusively, which bred true for four
+generations. This experiment was repeated seven times with
+uniform results.</p>
+
+<p class="indent">Diagrams are given representing all these histories in graphic fashion.
+<span class="pagenum"><a name="Page_225" id="Page_225">[Pg 225]</a></span></p>
+
+<p class="indent">From these observations, Tower concludes that the
+determination of dominance, and the ensuing type of behaviour, is clearly
+a function of the conditions incident upon the combining germ plasms.</p>
+
+<p class="indent">It will be observed that expts. (1) and (3) gave
+identical results but (2) and (4), though much the same conditions
+were applied, are at variance, for (2) gave all intermediates, while
+(4) gave all <i>signaticollis</i>. In <i>Amer. Nat.</i>, XLIV, 1910, p. 747,
+Professor T. D. A. Cockerell commented on this paper of Tower's
+and pointed out that there must be an error somewhere, for when
+he discusses these experiments Tower speaks of (2) and (4) as
+confirming each other. To this Tower replied<a name="FNanchor_12_175" id="FNanchor_12_175"></a><a href="#Footnote_12_175" class="fnanchor">[12]</a>
+that there had been a mistake. He states that in preparing the paper
+"certain minor experiments were taken from a larger series and combined
+to illustrate a general point in the behaviour of alternative
+characters in inheritance," and that expt. (2) was introduced
+inadvertently in place of another which he desires to substitute.
+In this, which I number (5), <i>signaticollis</i> &#9792; × <i>diversa</i>
+&#9794; from exactly the same stocks as those used in (1), were mated at
+the lower temperatures specified for (2), day average 75° F., night
+average 50° F. These gave all of the <i>signaticollis</i> type with a
+narrow range of variability, which bred true, in some cases to F<sub>6</sub>.
+Tower says he has repeated this experiment six times with identical results.</p>
+
+<p class="indent">Nevertheless he proceeds to say that the description
+of expt. (2), which was repeated eleven times with identical results, was
+correct "as far as given." That experiment was "from a second
+series of cultures parallel to the one given, but in which there are
+other factors involved, which in H. 410 [my (2)] are productive
+of a typical Mendelian behaviour." He adds he does "not care
+at this time to make any statement of what these factors are,
+nor of their relations to the behaviours given in the H. 409, H. 411,
+H. 409/11 series [my (1), (5) and (3)&mdash;(4)] which are the simplest
+and most easily presented series obtained in the crossing of
+<i>signaticollis</i> and <i>diversa</i>."
+<span class="pagenum"><a name="Page_226" id="Page_226">[Pg 226]</a></span></p>
+
+<p class="indent">Professor Cockerell's intervention has thus elicited the fact
+that we have as yet only a small selected part of the evidence
+before us, even as concerning the effect of temperature on the
+cross between <i>signaticollis</i> &#9792; × <i>diversa</i> &#9794;. We learn that at
+the lower temperatures the result was eleven times the expected
+one, and six times an unexpected one; further, that we owe it to
+the author's inadvertence that we have come to hear of the
+expected result at all, and that though he knows the factors
+which determine the discrepancy, he declines for the present to
+name them. In these circumstances we can scarcely venture as
+yet to estimate the significance of these records.</p>
+
+<p class="indent">The paper goes on to recount somewhat comparable, but
+more complex instances in which the descent of the colour of adults and
+of larvae was affected by temperature in crosses between <i>undecimlineata</i>
+and <i>signaticollis</i>. As they stand the results are
+very striking and unexpected, but I think, in view of what has
+been admitted respecting the former part of the paper, full discussion
+may be postponed till confirmation is forthcoming.</p>
+
+<p class="indent">One feature, however, calls for remark. This second
+paper is written apparently without any reference to the discoveries
+related by Tower in his previous book, to which no allusion is
+made. This is most noticeable in the case of an experiment in
+which (p. 296, H. 700A) <i>undecimlineata</i> &#9792; (the dominant)
+was mated to <i>signaticollis</i> &#9794; with the result that all the offspring
+were <i>undecimlineata</i> and bred true to that type (Parthenogenesis
+was tested for, but never found to occur). This experiment was
+made at a temperature averaging 95° F. ± 3.5° by day and 89° F.
+± 4.8° by night, and in a humidity given as 84 per cent. by day
+and 100 per cent. by night; but in the previous book (p. 294) we
+are told that pure <i>undecimlineata</i> bred together "under an extreme
+stimulus of high temperature, 10° C. above the average"
+and a relative humidity of 40 per cent. gave 11 beetles only,
+all <i>angustovittata</i>. But reference to the Plate 16, Fig. 2, shows
+that <i>angustovittata</i> must be exceedingly like <i>signaticollis</i>,
+having, like it, the elytral stripes obsolete, and if there is any marked
+difference at all, it can only be in the larvae. It seems strange
+that if <i>undecimlineata</i> really gives off ova of this recessive type at
+high temperatures, the fact should not be alluded to in connection
+<span class="pagenum"><a name="Page_227" id="Page_227">[Pg 227]</a></span>
+with expt. H. 700A, where, as the father was <i>signaticollis</i>, having
+the same recessive character, their appearance might have been
+expected not to pass unobserved. The temperature in the
+older experiment is, of course, not given with the great accuracy
+used in the second, and it may have been higher still. The humidity
+also was widely different. Still, in discussing the phenomena
+we should expect some reference to the very remarkable and
+closely cognate discovery which Tower himself had previously
+reported in regard to the same species.<a name="FNanchor_13_176" id="FNanchor_13_176"></a><a href="#Footnote_13_176" class="fnanchor">[13]</a>
+</p>
+
+<p class="indent">The hesitation which I had come to feel respecting
+these two publications of Tower's has been, I confess, increased by the
+appearance of a destructive criticism by Gortner<a name="FNanchor_14_177" id="FNanchor_14_177"></a><a href="#Footnote_14_177" class="fnanchor">[14]</a>
+who has examined the parts of Chapter III of Tower's book, in which he
+discusses at some length the chemistry of the pigments in <i>Leptinotarsa</i>
+and other animals. As Gortner has shown, this discussion,
+though offered with every show of confidence, exhibits
+such elementary ignorance, both of the special subject and of
+chemistry in general, that it cannot be taken into serious consideration.</p>
+
+<p class="indent">Some observations made by Dr. W. T. Macdougal<a name="FNanchor_15_178" id="FNanchor_15_178"></a><a href="#Footnote_15_178" class="fnanchor">[15]</a>
+have also been interpreted as showing the actual causation of genetic
+variation by chemical treatment. Of these perhaps the least open
+to objection were the experiments with <i>Raimannia odorata</i>, a
+Patagonian plant closely allied to <i>Oenothera</i>. The ovaries were
+injected with various substances and from some of the seeds
+which subsequently formed in them a remarkable new variety
+was raised. This varying or mutational form was strikingly
+different from the parental type, with which it was not connected
+by any intergradational forms, and it bred true. It made
+<span class="pagenum"><a name="Page_228" id="Page_228">[Pg 228]</a></span>
+no rosette, growing to a much smaller size than the parent, and
+was totally glabrous instead of being very hairy as the parental
+type is. I was shown specimens of these plants by the kindness
+of Dr. Britton in the Bronx Park Botanic Garden in 1907 and
+can testify to their very remarkable peculiarities. They had a
+somewhat weakly look, and might at first sight be thought to be
+a pathological product, but they had bred true for several
+generations. From the evidence, however, I am by no means
+satisfied that their original appearance was a consequence of
+the treatment applied. This treatment was of a most miscellaneous
+description. Two of the mutants came from an ovary
+which had been treated with a ten per cent. sugar solution. Ten
+came from one into which a 0.1 per cent. solution of calcium
+nitrate had been injected. One was from a capsule which "had
+been exposed to the action of a radium pencil." Macdougal
+speaks of these results as decisive, but clearly before such evidence
+can be admitted even for consideration it must be shown by control
+experiments that the individual plants which threw the
+mutant were themselves breeding true in ordinary circumstances.
+Nothing is more likely than that the mutant was an ordinary
+recessive. I may add that Mr. R. H. Compton made a number
+of experiments with <i>Raimannia odorata</i>, raised from seeds kindly
+given me by Dr. Britton, injecting the ovaries with a variety of
+substances, including those named by Macdougal; but though a
+numerous progeny was raised from the ovaries treated, all were
+normal. Macdougal relates also that some mutational forms
+came from ovaries of <i>Oenothera Lamarckiana</i> exposed to radium
+pencils, and also from <i>Oenothera biennis</i> injected with zinc sulphate
+a peculiar mutant was raised, but taking into account the
+frequency of these occurrences in those species, he very properly
+regarded this evidence as of doubtful application. In a later
+paper,<a name="FNanchor_16_179" id="FNanchor_16_179"></a><a href="#Footnote_16_179" class="fnanchor">[16]</a>
+however, he has returned to the subject and affirms his
+conviction that the appearance of a mutant among seedlings
+raised from an ovary of <i>Oenothera biennis</i> treated with zinc
+sulphate was really a consequence of the injection, saying that
+<span class="pagenum"><a name="Page_229" id="Page_229">[Pg 229]</a></span>
+the variation previously observed in the species was afterwards
+shown to be due to fungoid disease. The circumstances to which
+he mainly points in support of his view is that the mutation bred
+true, but this is only evidence of its genetic distinctness, which
+may, of course, be admitted by those who remain unconvinced
+as to the original cause of its appearance. He adds that he is
+making similar experiments with some twenty genera; but what
+is more urgently needed is repeated confirmation of the original
+observation. When it has been shown that this mutation can
+be produced with any regularity from a plant which does not
+otherwise produce it on normal self-fertilisation, the enquiry
+may be profitably extended to other plants.</p>
+
+<p class="indent">A curious and novel experiment, which however, led
+ultimately to a negative result, was made by F. Payne. Many discussions
+have been held respecting the blindness of cave animals.
+The phenomenon is one of the well-known difficulties, and most
+of us would admit that the theory of evolution by the natural
+selection of small differences does not offer a really satisfying account
+of it. Those who believe in the causation of such modifications
+by environmental influences and in their hereditary transmission
+make, of course, the simple suggestion that the darkness
+is the cause of the loss of sight, and that disuse has led to the
+reduction of the visual organs. Payne bred <i>Drosophila ampelophila</i>,
+the pomace-fly (which is easy to keep in confinement, fed
+on fermenting bananas), for sixty-nine generations in darkness.
+At the end of that period there was no perceptible change in the
+structure of the eyes, or in any other respect. The number of
+generations may possibly be regarded as insufficient to prove
+anything, but comparing them, as he does, with the generations
+of mankind, we see that they correspond with a period of about
+two thousand years, an interval far longer than those which
+many writers in particular cases have deemed sufficient.</p>
+
+<p class="indent">In his first paper Payne states that, though no structural
+difference could be perceived, the flies which had been bred in
+the dark reacted less readily to light than those which had been
+reared under normal conditions, and he inclined to think that
+the treatment had thus produced a definite effect. After more
+<span class="pagenum"><a name="Page_230" id="Page_230">[Pg 230]</a></span>
+careful tests, however, he withdrew this opinion. It proved
+that both individual flies and individual groups of flies, both of
+those bred in the light and of those bred in the dark, differed
+greatly in their reactions, which were measured by counting the
+time that it took for a fly to travel to the light end of a covered
+tube, various sources of error being eliminated. He found further
+that these differences of behaviour were not inherited in any
+simple way, but he is disposed to attribute them to accidental
+differences in the nature of the food, an account which seems
+probable enough.<a name="FNanchor_17_180" id="FNanchor_17_180"></a><a href="#Footnote_17_180" class="fnanchor">[17]</a>
+</p>
+
+<p class="indent">In several recent publications Blaringhem<a name="FNanchor_18_181" id="FNanchor_18_181"></a><a href="#Footnote_18_181" class="fnanchor">[18]</a>
+has described the origin of many abnormal forms of plants, especially
+of maize, which he attributes to various mutilations practised upon the
+parents. Respecting these the same difficulty which has been
+expressed in other cases reappears, that before drawing any
+conclusion as to the value of such evidence we require to know
+that the plants treated belong to a really pure line, which if
+left to nature in the ordinary circumstances of its life in that
+locality would have had normal offspring. Abnormalities abound
+in the experience of everyone who examines pans of seedlings
+of almost any species of plant, and in maize they are well known
+to be exceptionally common. Some of those which we meet
+with when we attempt to ripen maize in this country are very
+similar to those which Blaringhem describes, consisting in irregularities
+in the distribution of the sexes, in the shapes of the
+panicles, etc. Many of these are doubtless imperfections of
+development, due to the dullness of our climate, but others are
+presumably genetic and would recur in the offspring however
+treated. If some one working in a climate where maize could
+be raised in perfection would repeat these experiments, and show
+that a strain which was thoroughly reliable and normal in its
+genetic behaviour did, after mutilation, throw the miscellaneous
+types observed by Blaringhem, that would be evidence at least
+that the development of the seed could be so influenced by
+injury to the parental tissues that its properties were changed.
+<span class="pagenum"><a name="Page_231" id="Page_231">[Pg 231]</a></span>
+Such evidence could be used for what it is worth; but pending an
+inquiry of this kind I am disposed to regard these observations of
+variation following on parental injury as suggestive rather than convincing.</p>
+
+<p class="indent">Some evidence of a remarkably interesting kind has been
+collected by J. H. Powers<a name="FNanchor_19_182" id="FNanchor_19_182"></a><a href="#Footnote_19_182" class="fnanchor">[19]</a>
+respecting the structure and habits
+of <i>Amblystoma tigrinum</i>, which led him to the conclusion that
+striking differences in the form, anatomy, and developmental
+processes could be effected directly by change in the conditions
+of life. It is well known that a profusion of forms, distinct in
+various degrees, is grouped round <i>Amblystoma tigrinum</i>. Some
+of these are believed to be geographically isolated, others occur
+together in the same waters, and, as usual, authorities have differed
+greatly as to the number of names to be given. These forms
+were studied in detail by Cope who described them in the <i>Batrachia
+of North America</i>. The view which he inclined to take
+was that the individual variations of <i>Amblystoma tigrinum</i> resulted
+from variations in the time and completeness of the
+metamorphosis, and these were regarded as due to external
+causes, such as differences in season, temperature, and geographical
+conditions. Powers, however, states that collecting
+within a radius of six or eight miles he found almost if not quite
+the whole "gamut of recorded variation in this species." Some,
+however, as he states, occurred rarely except under experimental
+conditions, but considerable differences in temperature were not
+found necessary in producing them. Every year, he says, he
+has been able to add to the number of peculiar types found in
+the same small area in nature, until the amount of natural
+variation at least equals that seen by Cope in the collections of
+the National Museum and those of the Philadelphia Academy.</p>
+
+<p class="indent">Powers states that his observations by no means confirm
+Cope's view that these differences are in the main referable to
+variation in the completeness of metamorphosis, and on the
+contrary, he regards metamorphosis as on the whole a levelling
+process, tending to obliterate diversity. The enormous
+<span class="pagenum"><a name="Page_232" id="Page_232">[Pg 232]</a></span>
+differences in size and proportions which he describes can only be
+appreciated by reference to his figures. They affect almost all
+features of bodily organisation. These striking differences he
+looks upon as brought about by differences in nutrition, "diversities
+in habitual locomotion," and diversity in the age at which
+metamorphosis occurs, and to sexual difference. Apart from
+sexual difference he regards the chief distinctions, in brief, as
+"acquired variations of the larva."</p>
+
+<p class="indent">As an example he gives the great elongation of some
+of the forms as "due first to slow growth, second to the free-swimming
+habit, third to the prolongation of larval life, and finally to the
+assumption of sexual maturity as males," either in the branchiate
+or non-branchiate condition. He describes the rapid growth of
+some and the slow growth of others. A larva of intermediate
+type may grow about a centimeter a month, but a rapidly growing
+specimen may grow more than four times as much. The slower
+rate of growth may, he says, be induced by winter feeding, and
+other treatment.<a name="FNanchor_20_183" id="FNanchor_20_183"></a><a href="#Footnote_20_183" class="fnanchor">[20]</a>
+</p>
+
+<p class="indent">When, however, he goes on to describe the influences
+which he regards as exerted by the habit of freely swimming, I am led
+to wonder whether after all in most of these illustrations, the
+primary distinctions are not in reality genetic. "Specimens
+raised in the same aquarium or in similar aquaria, side by side
+with all conditions as uniform as it is possible to make them,
+seldom fail to furnish striking examples of broad-headed, short-bodied,
+and short-tailed types which are habitually found at the
+bottom, while others, slender and elongated, are free swimmers,
+and maintain themselves in almost as continual suspension and
+motion as does a gold fish." Later, again, he writes, "Yet despite
+the uniformity of these favourable conditions, the larvae soon
+began to split up into two noticeably distinct groups, the one of
+<span class="pagenum"><a name="Page_233" id="Page_233">[Pg 233]</a></span>
+unusually compact proportions, the other of uniform intermediate
+build, such is most commonly met with." It is to my mind
+scarcely possible to resist the inference that, though there may be
+definite responses to certain conditions, yet the chief distinctions
+are genetic, and that it is these distinctions which confer the
+power to respond. The parts respectively played by cause and
+effect are always difficult to assign; but when it is stated that
+"a weak-limbed, long-bodied and long-tailed animal becomes
+well nigh perforce an undulatory swimmer, while the strong-limbed,
+short-tailed, heavy-bodied specimen, when these characteristics
+are rapidly forced upon it, is, under certain circumstances,
+just as forcibly induced to become a crawler," we feel
+how erroneous any estimates of causation are likely to be.</p>
+
+<p class="indent">One of the most remarkable and interesting sections
+of Powers' paper is that in which he describes the differences in
+bodily structure and habits which he attributes to cannibalism,
+and the whole account of the phenomena should be read in the
+original. It appears that there are two extremely distinct
+types of larvae, those with narrow heads and slender bodies
+which live for the most part on small Crustacea such as <i>Daphnias</i>,
+and those with huge mouths and very wide heads, which disregard
+such small animals altogether and live on amphibian
+larvae, whether of their own or other species. As the illustrations
+show, the differences between these two types are very
+great, and the differences in instinct and behaviour are no less.
+The cannibals take no heed of the pelagic crustacea, lying sluggishly
+at the bottom, rousing themselves immediately to a
+violent attack on the larger living things which approach them.
+Nothing but the most incontrovertible evidence based on abundant
+control experiments should convince us that such differences
+are not primarily genetic, and in the present state of knowledge
+I incline to think that the families really consist of individuals
+which are ready to assume the cannibal habit if opportunity
+offers, and others which are congenitally incapable of it. It may
+readily be that if all chance of cannibal diet be excluded, the
+full development of the wide head and mouth, or the other
+peculiarities, would never become pronounced, but I doubt whether
+such change could be induced in any individual taken at random.</p>
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_234" id="Page_234">[Pg 234]</a></span></p>
+
+<h2>CHAPTER XI.</h2>
+<h3><span class="smcap">Sterility of Hybrids. Concluding Remarks.</span></h3>
+
+<p class="indent">When we consider the bearing of recent discoveries on
+those comprehensive schemes of evolution with which we were formerly
+satisfied, we find that certain details of the process are more easy
+to imagine. We readily now understand how varieties once
+formed, can persist, but at the same time difficulties hitherto
+faced with complacency become formidable in the light of the
+new knowledge. So generally is this admitted by those familiar
+with modern genetic research that most are rightly inclined to
+postpone the discussion. The premisses, indeed, on which such
+a discussion must be based are almost wholly wanting.</p>
+
+<p class="indent">The difficulties to which I chiefly refer are not those
+ created by the phenomena of adaptation, though they are serious enough.
+In treating of that subject I have felt obliged to express scepticism
+as to the validity of nearly all the new evidence for the
+transmission of acquired characters. At the present time the
+utmost we are bound to accept is the proof that (1) in some
+parthenogenetic forms variations, or perhaps we may say malformations,
+produced in response to special conditions, recur in
+one or perhaps two generations asexually produced after removal
+to other conditions. (2) That violent maltreatment may in rare
+instances so affect the germ-cells contained in the parents as to
+cause the individuals resulting from the fertilisation of those
+cells to exhibit an arrest of development similar to that which
+their parents underwent.</p>
+
+<p class="indent">I do not doubt that evidence of this type will be
+greatly extended. As a contribution to genetic physiology these facts
+are very important and interesting, but I cannot think that any
+one, on reflexion, will feel encouraged by such indications to
+revive old beliefs in the direct origin of adaptations.</p>
+
+<p class="indent">In these respects we are simply left where we
+were. The force of objections based upon the existence of adaptative
+<span class="pagenum"><a name="Page_235" id="Page_235">[Pg 235]</a></span>
+mechanisms is no greater than it has always been. On the contrary
+the fact that variations can now so generally be recognized
+as definite is some alleviation of the difficulty. We can moreover
+disabuse ourselves of the notion that for all characters which are
+definite or fixed, some utilitarian rationale may be presumed.
+Upon that point the study of variation has provided a perfectly
+clear answer.</p>
+
+<p class="indent">In frankly recognizing that the fixity of characters in
+general need not connote usefulness to their possessors we deliver ourselves
+of a distracting pre-occupation and prepare our minds for
+an investigation of the properties of living organisms in the
+same spirit as that in which the chemist and the physicist
+examine the properties of unorganized materials. The creature
+persists not merely by virtue of its characteristics but in spite
+of them, and the fact of its persistence proves no more than
+that on the whole the balance of its properties leaves something
+in its favour.</p>
+
+<p class="indent">It may be noted by the way that the fact that the
+structures of living things are on the whole adaptative was not always
+obvious. Though to naturalists of this generation it is a truism,
+we have only to turn to Buffon to find that in his philosophy of
+nature it played no essential part. The passage in which Buffon
+describes what he regards as the forlorn and degraded condition
+of the Woodpecker is well known. We have come to think of
+the Woodpecker as a capital example of adaptation to the mode
+of life; but Buffon after enumerating the hard features of the
+bird's existence, forced to earn its living by piercing the bark of
+trees in an attitude of perpetual constraint, remarks<a name="FNanchor_1_184" id="FNanchor_1_184"></a><a href="#Footnote_1_184" class="fnanchor">[1]</a>
+"Tel est l'instinct étroit et grossier d'un oiseau borné a une vie triste
+et chétive. Il a reçu de la Nature des organes et des instrumens
+appropriés a cette destinée <i>ou plutôt il tient cette destinée même
+des organes avec lesquels il est né</i>" (my italics). His reflexions
+on the Stilt (<i>Himantopus</i>) read even more strangely to us,
+accustomed as we are to see in the prodigious length and thinness
+of the shanks and in the other features of its organisation palpable
+adaptations to a wading life. For Buffon, however, this
+<span class="pagenum"><a name="Page_236" id="Page_236">[Pg 236]</a></span>
+curious bird seemed a poor, neglected production, extravagant
+in its disproportions, one of the misfits of creation, left as a
+shadow in the picture composed of nature's more successful
+efforts.<a name="FNanchor_2_185" id="FNanchor_2_185"></a><a href="#Footnote_2_185" class="fnanchor">[2]</a> This theme he develops at some length, being evidently
+well pleased with the idea.</p>
+
+<p class="indent">Our way of regarding these things is doubtless sounder
+and more fruitful than Buffon's, but it is well to remember that what
+seems so obvious to us looked quite differently to other excellent
+observers; and stupid as it may have been to have overlooked
+plain examples of adaptation, it is a far worse mistake to see
+adaptation everywhere. I do not seek to minimise the real
+and permanent difficulty which the existence of adaptations
+creates, but by the suggestion that all normal specific differences
+are adaptational that difficulty was quite gratuitously increased.</p>
+
+<p class="indent">In these respects it may be claimed that progress has
+been made, even if that progress seem outwardly of small account.</p>
+
+<p class="indent">But all constructive theories of evolution have been
+built on the understanding that what we know of the relation of varieties
+to species justifies the assumption that the one phenomenon is a
+phase of the other, and that each species arises or has arisen
+from another species either by one or several genetic steps. In
+the varieties we have accustomed ourselves to think that we see
+those steps. We still know little enough of the mode of occurrence
+of variation, but we do begin to know something, and if we
+ask ourselves whether our knowledge, such as it is, conforms at
+all readily with our former expectations, we cannot with any
+confidence assert that it does. Among the plants and animals
+genetically investigated are many illustrations of very striking
+and distinct varieties. Many of these might readily enough be
+accepted as species by even the most exacting systematists, and
+not a few have been so treated in classification; but when we
+have examined their relationship to each other we feel not merely
+that they are not species in any strict sense but that the distinctions
+they present cannot be regarded as stages in the direction
+of specific difference. Complete fertility of the results of
+inter-crossing is and I think must rightly be regarded as
+<span class="pagenum"><a name="Page_237" id="Page_237">[Pg 237]</a></span>
+inconsistent with actual specific difference; and of variations leading
+to that consequence no clear indication has yet been found.
+As an example of possible exceptions mention should perhaps be
+made of the case of a giant form of <i>Primula sinensis</i> investigated
+by Keeble.<a name="FNanchor_3_186" id="FNanchor_3_186"></a><a href="#Footnote_3_186" class="fnanchor">[3]</a>
+It arose from a "Star" Primula of normal size,
+and though fertile with its own pollen all attempts to fertilise
+it with the pollen of other forms failed. Miss Pellew, who did
+these fertilisations, tells me that very extensive trials were made,
+and repeated in several seasons. Ultimately two plants were
+raised from it fertilised with a plant of the strain from which it
+sprang, and these proved sterile. In the light of modern experience
+the significance of such isolated instances is doubtful.</p>
+
+<p class="indent">All the strains known as "Giants" are, as Messrs. Sutton
+have always found, more or less sterile, and their sterility is
+presumably due to some negative defect.</p>
+
+<p class="indent">In regard to the fertility of Primula species there
+are several paradoxes. For example the long-styled varieties, apart from
+giants, are fertile with their own pollen, and for many years
+short-styled plants have not been used in most strains. Auriculas
+and Polyanthuses, on the contrary, are generally if not always
+bred from short-styled plants, as the florists have decided that
+the long-styled are inadmissible. Mr. R. P. Gregory tells me
+that, though most strains of <i>P. sinensis</i> give seed enough
+when only long-styled plants are used, he finds nevertheless
+that when a "legitimate" union is made the amount of seed
+usually increases much as Darwin observed. Darwin's statement
+that plants of "illegitimate" origin are less fertile than the
+"legitimately" raised plants is also in general confirmed by his
+experience. To this rule there were some marked exceptions in
+strains derived from <i>long</i>-styled plants, which though illegitimate
+showed a high degree of fertility, but illegitimate unions between
+<i>short</i>-styled plants always produced comparatively sterile offspring.
+I have no records of the behavior of Auriculas and
+Polyanthuses. It would be interesting to know whether among
+them pure strains of short-styled plants (dominants) have
+appeared, and, if so, how their fertility is affected. Without
+<span class="pagenum"><a name="Page_238" id="Page_238">[Pg 238]</a></span>
+much more critical data I suppose no one would nowadays be
+inclined to follow Darwin in instituting a comparison between
+the sterility of hybrids and that of illegitimately raised plants of
+heterostyle species.<a name="FNanchor_4_187" id="FNanchor_4_187"></a><a href="#Footnote_4_187" class="fnanchor">[4]</a>
+It is even difficult to imagine any essential
+resemblance between these two phenomena, nor has evidence
+ever been produced to show that illegitimately raised plants
+have bad pollen grains, which is the usual symptom of sterility
+in hybrid plants and the consequence, as we believe, of failure
+of some essential division in the process of maturation.</p>
+
+<p class="indent">The difficulty that we have no knowledge of the contemporary
+origin of forms, from a common stock, which when crossed together
+give a sterile product, is one of the objections constantly and
+prominently adduced from the time of the first promulgation of
+evolutionary ideas. In the light of recent work the objection
+has gathered strength. Why, if we are able to produce instances
+of variation colourably simulating specific difference in almost
+all other respects, do we never find an original appearance of this
+most widely spread of all specific characteristics? No doubt all
+breeders know that sterile animals and plants occasionally appear
+in their cultures, but it is more in accordance with probability
+that the sterility in these sporadic instances should be regarded
+as due to defect than that it should be thought comparable
+with that of the sterile hybrids. For their sterility must, by all
+analogy with results elsewhere seen, be attributed not to the
+absence of something, but to the presence and operation of
+complementary factors leading to the production of inhibition
+of division; and consistently with that interpretation, we find
+that when from a partially sterile hybrid comparatively fertile
+offspring can be raised, their comparative fertility continues in
+the posterity generally if not always without diminution. The
+distinction between these several kinds of sterility was of course
+not understood in Darwin's time. The comparison, for example,
+which he instituted<a name="FNanchor_5_188" id="FNanchor_5_188"></a><a href="#Footnote_5_188" class="fnanchor">[5]</a>
+between the sterility of "contabescent"
+anthers and that of hybrids no longer holds, for at least in those
+cases in which the nature of contabescent anthers have been
+genetically investigated (Sweet Pea, <i>Tropaeolum</i>) they proved
+<span class="pagenum"><a name="Page_239" id="Page_239">[Pg 239]</a></span>
+to be a simple recessive character. Nor can we now easily
+suppose that the attempt there made by Darwin to suggest
+resemblance between the sterility produced by unnatural conditions
+and that of hybrids has any physiological justification.</p>
+
+<p class="indent">In regarding the power to produce a sterile or partially
+sterile hybrid as a distinction in kind, of a nature other than
+those which we perceive among our varieties, I am aware that
+I am laying stress on an impression which may hereafter prove
+false. The distinction nevertheless is so striking and so continually
+before the eyes of a practical breeder that he can
+scarcely avoid the inference that when he meets a considerable
+degree of sterility in a cross-bred he is dealing with something
+belonging to a distinct category, and not merely a varietal feature
+of an exceptional kind.</p>
+
+<p class="indent">Besides the sterility of hybrids appeal has often been
+made to the phenomenon of incompatibility, in its several stages of
+completeness, as distinguishing species. No one doubts that
+incompatibility may arise from a variety of causes of most diverse
+degrees of importance, but though sometimes referred to as an
+extreme case of interspecific sterility, it is really a very different
+matter. In regard to one phase of this incompatibility, that
+associated with self-sterility, some progress has been made, and
+we are not wholly without experimental evidence of its being
+within the range of contemporary variation.</p>
+
+<p class="indent">Given the outline of Mendelian teaching as to
+gametic differentiation and the classification of individuals in a mixed
+population, it seemed highly probable that what we call self-sterility
+must mean that the species really consisted of <i>classes</i>,
+some of which are capable of interbreeding with others while
+others are not. According to the received account every individual,
+though incapable of fertilising itself, was supposed to be
+able both to fertilise and to be fertilised by any other individual.
+This notion has always seemed to me a self-evident absurdity,
+for it would imply that there can be as many categories as
+individuals. Such experiments, however, as I made did certainly
+give results consistent with that belief. I first tried
+Cinerarias, which are usually self-sterile, but I found no
+<span class="pagenum"><a name="Page_240" id="Page_240">[Pg 240]</a></span>
+incompatible pairs of plants. Whether I was deceived by the
+consequences of apogamy, or whether the pollen of certain plants
+may belong to more than one class I do not know. The results
+were confused in various ways. Usually the self-fertilised plants
+set little or nothing, and cross-fertilised they set fully with such
+uniformity that the few failures could plausibly be attributed to
+mistakes in manipulation or to other extraneous causes. Later
+de Vries announced<a name="FNanchor_6_189" id="FNanchor_6_189"></a><a href="#Footnote_6_189" class="fnanchor">[6]</a>
+(without giving particulars) that he had
+proved the existence of such classes in <i>Linaria vulgaris</i>; but on
+making experiments with that species I again got no positive
+results, and I came to the conclusion that in spite of inherent
+improbability the conventional belief must be substantially true.
+At last, however, the work of Correns, lately published,<a name="FNanchor_7_190" id="FNanchor_7_190"></a><a href="#Footnote_7_190" class="fnanchor">[7]</a>
+does definitely show that in one species, <i>Cardamine pratensis</i>,
+classes of individuals exist such that individuals of the same class are
+incapable of fertilising themselves or each other, but fertilisation
+made between the classes is usually completely effective. Many
+complications were encountered and some contradictory evidence
+is recorded, but the general bearing of the results was positive
+and indubitable.</p>
+
+<p class="indent">We know far too little of this phenomenon as yet to
+be able to understand its significance, but I suppose we may anticipate
+with some confidence that it will be found to be a manifestation
+of dissimilarity between the male and female gametes of the
+same individual, comparable with that first seen in the Stocks
+(<i>Matthiola</i>) which throw doubles&mdash;a state of things in all likelihood
+to be found widely spread among hermaphrodite organisms.
+Whether the incompatibility between species is to be associated
+with that of the self-steriles also cannot be positively asserted,
+though it seems not unreasonable to expect that such an association
+will be discovered.</p>
+
+<p class="indent">The case of the apple and the pear is an impressive
+illustration of this possibility. The two species are of course exceedingly
+alike in all outward respects, but nevertheless the pollen of each
+is entirely without effect on the other. Presumably we should
+<span class="pagenum"><a name="Page_241" id="Page_241">[Pg 241]</a></span>
+interpret this fact as meaning not so much that the apple and
+the pear are in reality very wide apart, but rather that either,
+each is lacking in one of two complementary elements, or that
+each possesses a factor with an inhibitory effect. Their incompatibility
+may well be of the same nature as that of the classes
+in <i>Cardamine pratensis</i>.</p>
+
+<p class="indent">Returning now to the problem of inter-specific sterility;
+we note, as I have said, the absence of contemporary evidence that
+variation can confer on a variety the power to form a sterile
+hybrid with the parent species. The considerations based on
+this want of evidence have for a long while been familiar to all
+who have discussed evolutionary theories, and it is worth observing
+the exact reason why the difficulty strikes us now with a new
+and special force. In pre-Mendelian times all that was known
+was that some forms could freely interbreed without diminution
+of fertility in the product, while others could not. But now we
+find that, by virtue of segregation, from one and the same pair
+of parents, or even, in the case of hermaphrodites, from one and
+the same individual, offspring commonly arises showing among
+themselves exactly such differences as distinguish species&mdash;and
+very good species too. This we see happening again and again.
+But to forms capable of arising as brethren in one family the
+title species has never been meant to apply, and if we are going
+to use the term in application to fraternal groups we must
+definitely recognise that by "specific" difference is to be understood
+simply <i>difference</i>, without any immediate or even ulterior
+physiological limitation whatever. Naturally, therefore, we begin
+to think of the appearance of sterility in crosses as something
+apart, and as a manifestation which distinguishes certain kinds
+of unions in a very special way.</p>
+
+<p class="indent">I am perfectly aware that there are gradations in the
+sterility of hybrids as in every other characteristic upon which it has
+been proposed to base specific definitions; but, as also so often happens
+in the matter of defining intergrading categories, the difficulty
+in practice is not often such as to lead to actual ambiguity. I
+am speaking of course of those examples which are amenable to
+genetic experiment. As to the rest there is complete and permanent
+<span class="pagenum"><a name="Page_242" id="Page_242">[Pg 242]</a></span>
+uncertainty. But the experience of the practical breeder
+does, I think, on the whole, support the contention to which
+systematists have so steadily clung under all the assaults of
+evolutionary philosophers, that, though we cannot strictly define
+species, they yet have properties which varieties have not, and
+that the distinction is not merely a matter of degree.</p>
+
+<p class="indent">The first step is to discover the nature of the factors
+which by their complementary action inhibit the critical divisions and so
+cause the sterility of the hybrid. Thus expressed, we see the
+problem of inter-specific sterility in its right place; and the
+question why we do not now find contemporary instances of
+varieties lately arisen in domestication, which when crossed back
+with their parents, or with their coderivatives, can produce
+sterile products, is perceived to be only a special case of a problem
+which in its more general form is that of the origin of new and
+additional factors.</p>
+
+<p class="indent">For the requisite evidence no comprehensive search
+has been made, but perhaps it will yet be found. All that we can say
+at the present time is that the incidence both of hybrid sterility,
+and of incompatibility also, is most capricious; and provided
+that two forms have such features in common that a cross between
+them seems not altogether out of the question, no one can predict
+without experiment whether such a cross is feasible, and if
+feasible whether the product will be fertile, or sterile more or
+less completely. For instance, though probably all the British
+and some Foreign Finches (Fringillidae) have been crossed
+together, and some of these crosses, as for instance, the various
+Canary-mules have been made in thousands, I believe no quite
+clear example of a fertile hybrid can be produced. Many species
+of Anatidae cross readily and produce fertile hybrids: others give
+results uniformly sterile. Though most of the Equidae can be
+crossed and some of the hybrids are among the commonest of
+domesticated animals there is no certain record of a fertile mule.
+Among the Canidae the dogs, wolves and jackals all give fertile
+hybrids, but there is no clearly authenticated instance of a cross
+between any of these forms and the European fox. In spite of
+their close anatomical resemblance it is doubtful if the rabbit
+<span class="pagenum"><a name="Page_243" id="Page_243">[Pg 243]</a></span>
+and the hare have ever interbred. Many of the wild species of
+<i>Bos</i> have been crossed and recrossed both with each other and
+with many domesticated races, but I understand that no cross
+with the Indian buffalo (<i>Bos bubalus</i>) has yet been successful
+even in producing a live calf.<a name="FNanchor_8_191" id="FNanchor_8_191"></a><a href="#Footnote_8_191" class="fnanchor">[8]</a>
+In the genus <i>Primula</i> many
+hybrids are known and several of them occur in nature, but
+hitherto no certain hybrid between <i>P. sinensis</i> and any other
+species has been made, in spite of repeated attempts.</p>
+
+<p class="indent">In <i>Nicotiana</i> many&mdash;doubtless all&mdash;the various
+forms of <i>N. tabacum</i> can be crossed together without diminution of fertility,
+though some are very distinct in appearance, but crosses between
+<i>tabacum</i> and <i>sylvestris</i> are highly sterile (in my experience totally
+sterile<a name="FNanchor_9_192" id="FNanchor_9_192"></a><a href="#Footnote_9_192" class="fnanchor">[9]</a>),
+though the distinctions between them are not to outward
+observation nearly so great as those which can be found between
+the various races of <i>Primula sinensis</i>.</p>
+
+<p class="indent">Recently some remarkable experiments bearing closely on
+these questions have been published by F. Rosen.<a name="FNanchor_10_193" id="FNanchor_10_193"></a><a href="#Footnote_10_193" class="fnanchor">[10]</a>
+They concern the forms of <i>Erophila (Draba) verna</i>, celebrated in
+the history of evolutionary theory as the plants especially chosen by
+Alexis Jordan for the exposition of his views on these subjects.</p>
+
+<p class="indent">The "species" contains a profusion of forms dissimilar
+in many structural characters, such as the size and shape of leaves,
+flowers, fruits, etc. Of these forms many grow in association.
+Jordan found, on experiment, that each, to the number of some
+two hundred, bred true, and that therefore, the conventional
+assumption that polymorphism of this kind must mean great
+contemporary variability had no foundation in fact. So far
+<span class="pagenum"><a name="Page_244" id="Page_244">[Pg 244]</a></span>
+indeed is the evidence from favouring the belief that such forms
+are in any way transitional or indeterminate, that, as is well
+known, Jordan used it with every plausibility to support the
+doctrine of the fixity of species. To certain aspects of Jordan's
+work we will return later in this chapter, but the matter is in
+the present connection of especial interest for the reason that
+Rosen has lately found by experiment that some of these presumably
+very closely allied forms, crossed together, gave hybrids
+more or less sterile. In the case of the offspring of one pair of
+forms only (<i>E. cochleata</i> and <i>stricta</i>) was the fertility
+undiminished, and the various degrees of sterility found in the
+other crosses ranged up to the extreme infertility of the hybrids
+between <i>E. stricta</i> × <i>elata</i>. From this cross ten plants were
+bred. Of these the four strongest were chosen to breed from,
+but two of the four proved totally sterile; one had only bad seeds;
+and from the fourth a single seedling was raised which in its
+turn proved to be sterile. From the less sterile hybrids F<sub>2</sub>
+families were raised, with the usual experience that in this and
+subsequent generations the sterility diminished among extracted
+forms, new and true-breeding types with complete fertility being
+thus derived from the original cross.<a name="FNanchor_11_194" id="FNanchor_11_194"></a><a href="#Footnote_11_194" class="fnanchor">[11]</a>
+</p>
+
+<p class="indent">The production of sterility as a consequence of crossing
+plants so nearly approaching each other as these <i>Erophila</i>
+"species" do is not a little interesting, and the fact well exemplifies
+the futility of the various attempts to frame general
+expressions as to specific properties or behaviour. Commenting
+on his results Rosen argues that the polymorphic group commonly
+called by systematists <i>Erophila (Draba) verna</i> may now be
+regarded as having arisen by crossing, as did his own types mentioned
+above. The question, however, <i>what</i> species were the
+original progenitors of the group cannot be answered. Rosen
+considers that no form which he knows satisfies the requirements,
+<span class="pagenum"><a name="Page_245" id="Page_245">[Pg 245]</a></span>
+and that it or they must be supposed to be lost. This conclusion
+will recall the similar problem raised by the <i>Oenothera</i> mutants
+(Chap. V); and unsatisfactory as it may be to have recourse
+to such hypotheses we must remember the possibility that as a
+consequence of hybridisation, subsequent segregation and recombination
+of factors, species may have thus actually, as we
+may say, exploded, and left nothing but a polymorphic group of
+miscellaneous types to represent them in posterity. If this way
+of regarding the phenomena be a true one, the sterility now seen
+when some of the group are re-crossed, becomes analogous to
+that "reversion or crossing" which we now so well understand
+to be a consequence of the recombination of characters separated
+at some previous point in the history of descent. In the partial
+sterility of the contemporary hybrid we see this character reappearing,
+formed now as it was on the occasion of the original
+cross, by the meeting of complementary factors.</p>
+
+<p class="indent">Another case that may be mentioned in this connection is
+that of the crosses between various culinary peas (<i>Pisum sativum</i>)
+and a peculiar form found by Mr. Arthur Sutton growing ostensibly
+in a wild state in Palestine. This Palestine Pea is low
+growing, rarely reaching 18 inches. It is in general appearance
+like a small and poorly grown field pea. The stems are thin and
+rather hard. The most obvious differences which distinguish
+this from other field peas are the marked serration of the stipules,
+and the development of pith in the pods. Such pith is often
+present in the pods of peas more or less, but in the Palestines it
+is so strongly developed as almost to form a lomentum. Curiously
+enough, though the flowers are purple much as those of
+ordinary field peas, there is no coloured spot in the axils. On
+the other hand, the stems have coloured stripes running up
+from the axils. Though this plant differs so little from domesticated
+peas, all crosses with them either failed, or produced
+hybrids quite or almost quite sterile. This was Mr. Sutton's
+experience, and on repeating the experiments with material
+kindly given by him I found the same result.<a name="FNanchor_12_195" id="FNanchor_12_195"></a><a href="#Footnote_12_195" class="fnanchor">[12]</a>
+<span class="pagenum"><a name="Page_246" id="Page_246">[Pg 246]</a></span></p>
+
+<p class="indent">In a large series of crosses some seeds died or gave
+rise to feeble plants. Of the plants which lived, few gave any seed.
+The seed, however, that was obtained from F<sub>1</sub> plants grew well
+enough, and the F<sub>2</sub> plants proved, as often in such cases, fertile.
+In these, indeed, no sign of sterility was noticeable. The experiment
+is being repeated in various ways, for, as the genetic
+behaviour of peas is comparatively well known, the subject is an
+exceptionally favourable one for these investigations.</p>
+
+<p class="indent">Such an example shows the confusion produced the
+moment we attempt to harmonize conceptions of specific difference
+with results attained by experimental methods. It has been usual
+to regard the field pea (<i>P. arvense</i>) as a species distinct from
+the edible pea (<i>P. sativum</i>). De Candolle and others regard the
+field pea as derived from a form wild in Italy, but the origin of
+the edible pea is considered to be unknown. From breeding
+experiments we find no sterility whatever in the crosses between
+the various <i>arvense</i> and <i>sativum</i> types, nor in the crosses
+made between them and several other peculiar types from various
+countries; whereas this Palestine Pea, which only differs from a
+small <i>arvense</i> in what might have been thought trivial characters,<a name="FNanchor_13_196" id="FNanchor_13_196"></a><a href="#Footnote_13_196" class="fnanchor">[13]</a>
+either fails to cross altogether or gives a sterile product, whatever
+type be chosen as the other parent.</p>
+
+<p class="indent">Examples of this kind have at least the merit that
+they lead to more precise delimitations of the problem. We are confronted
+with two distinct alternatives.</p>
+
+<p class="indent">1. We may apply the term Species promiscuously to
+all distinct forms. If we do so it must be clearly understood that
+we cannot even rule out the several combinations of "presences
+and absences" represented by the various types whether wild or
+domesticated. For we may feel perfectly assured that at least
+all the <i>arvense</i> and all the <i>sativum</i> types yet subjected to
+experimental tests are on precisely the same level in this respect. There
+is no distinction, logical or physiological, to be drawn between
+them. Some contain more factors, and others contain fewer.
+In some the re-combinations have been brought about by natural
+variation or crossing, while the same consequences in the others
+have resulted from man's interference.
+<span class="pagenum"><a name="Page_247" id="Page_247">[Pg 247]</a></span></p>
+
+<p class="indent">2. We may follow the conventions of systematists and
+distinguish the outstanding or conspicuous forms such as <i>arvense</i>,
+<i>quadratum</i>, <i>sativum</i> and perhaps a few more as species, and
+leave the rest unheeded. If this course is followed it must be clearly
+understood and permitted as a piece of pure pragmatism, deliberately
+adopted for the convenience of cataloguers and collectors,
+without regard to any natural fact or system whatsoever.</p>
+
+<p class="indent">But while following either the one plan or the other
+we shall be still awaiting the answer, which only genetic experiment
+can provide, to the question whether among the various types
+there are some which differ from the rest in a peculiar way:
+whether by having groups of characters linked together in
+especially durable combinations, or by possessing ingredients
+which cause greater or less disturbance in the processes of cell-division,
+and especially in the processes of gametic maturation,
+when they are united by fertilisation with complementary ingredients.</p>
+
+<p class="indent">Before any but the vaguest ideas regarding the nature
+and significance of inter-specific sterility can be formed, a vast
+amount of detailed work must be done. Sterility as a result of
+crossing, as well as that which is alleged sometimes to arise in
+consequence of changed conditions, is at best a negative characteristic,
+and there are endless opportunities for mistake and misinterpretation
+in studying features of this kind. No one, I
+suppose, would now feel any great confidence in most of the data
+which from time to time are resuscitated for the purpose of such
+discussions. Even the best collections of evidence, such as those
+given by Darwin in <i>Forms of Flowers</i>, cannot be regarded as
+critical when judged by present-day standards. Nothing short
+of the most familiar acquaintance with the habitual behaviour
+of individuals, and of strains kept under constant scrutiny for
+several years would enable the experimenter to form reliable
+judgments as to the value to be attached to observations of
+this class.</p>
+
+<p class="indent">The admission must, however, be faced that nothing in
+recent work materially tends to diminish the surprise which has always
+been felt at the absence of sterility in the crosses between
+<span class="pagenum"><a name="Page_248" id="Page_248">[Pg 248]</a></span>
+co-derivatives. We should expect such groups of forms to behave
+like the <i>Erophila</i> types, and frequently to produce sterile
+products on crossing. Whatever be the explanation, the fact remains
+that such evidence is wanting almost completely. In spite of
+all that we know of variability nothing readily comparable with
+the power to produce a sterile hybrid on crossing with a near
+ally, has yet been observed spontaneously arising, though that
+characteristic of specificity is one of the most widely distributed
+in nature. It may be that the lacuna in our evidence is due
+merely to want of attention to this special aspect of genetic
+inquiry, and on the whole that is the most acceptable view which
+can be proposed. But seeing that naturalists are more and
+more driven to believe the domesticated animals and plants to be
+poly-phyletic in origin&mdash;the descendants, that is to say, of several
+wild forms&mdash;the difficulty is proportionately greater than it was
+formerly, when variation spontaneously occurring was regarded
+as a sufficient account of their diversity.</p>
+
+<hr class="chap" />
+<h2><span class="smcap">Concluding Remarks.</span></h2>
+
+<p class="indent">The many converging lines of evidence point so clearly to
+the central fact of the origin of the forms of life by an evolutionary
+process that we are compelled to accept this deduction, but as
+to almost all the essential features, whether of cause or mode,
+by which specific diversity has become what we perceive it to
+be, we have to confess an ignorance nearly total. The transformation
+of masses of population by imperceptible steps guided
+by selection, is, as most of us now see, so inapplicable to the
+facts, whether of variation or of specificity, that we can only
+marvel both at the want of penetration displayed by the advocates
+of such a proposition, and at the forensic skill by which it
+was made to appear acceptable even for a time.</p>
+
+<p class="indent">In place of this doctrine we have little teaching of a positive
+kind to offer. We have direct perception that new forms of life
+may arise sporadically, and that they differ from their progenitors
+quite sufficiently to pass for species. By the success and maintenance
+of such sporadically arising forms, moreover, there is
+no reasonable doubt that innumerable strains, whether in isolation
+<span class="pagenum"><a name="Page_249" id="Page_249">[Pg 249]</a></span>
+or in community with their co-derivatives, have as a fact
+arisen, which now pass in the lists of systematists as species. For
+an excellent account of typical illustrations I would refer the
+reader to the book lately published by R. E. Lloyd<a name="FNanchor_14_197" id="FNanchor_14_197"></a><a href="#Footnote_14_197" class="fnanchor">[14]</a>
+on the rat-population of India. The observations there recorded are typical
+of the state of things disclosed whenever the variations of large
+numbers of individuals are closely investigated, whether in
+domestication or in natural conditions.</p>
+
+<p class="indent">Guided by such clues we may get a good way into
+the problem. We see the origin of colourable species in abundance.
+Then, however, doubt arises whether though these new forms
+are as good species as many which are accepted as such by even
+cautious systematists, there may not be a stricter physiological
+sense in which the term species can be consistently used, which
+would exclude the whole mass of these <i>petites espèces</i>.</p>
+
+<p class="indent">If further we find that we have, with certain somewhat
+doubtful exceptions, never seen the contemporary origin of a
+dominant factor, or of inter-racial sterility between indubitable
+co-derivatives, it needs no elaboration of argument to show that
+the root of the matter has not been reached.</p>
+
+<p class="indent">Examination of the inter-relations of unquestionably distinct
+species nearly allied, such as the two common species of <i>Lychnis</i>,
+leads to the same disquieting conclusion, and the best suggestion
+we can make as to their origin is that <i>conceivably</i> they may have
+arisen as two re-combinations of factors brought together by the
+crossing of parent species, one or both of which must be supposed
+to be lost.</p>
+
+<p class="indent">All this is, as need hardly be said, an unsatisfying
+conclusion. To those permanently engaged in systematics it may well bring
+despair. The best course for them is once for all to recognise
+that whether or no specific distinction may prove hereafter to
+have any actual physiological meaning, it is impossible for the
+systematist with the means at his disposal to form a judgment of
+value in any given case. Their business is purely that of the
+cataloguer, and beyond that they cannot go. They will serve
+science best by giving names freely and by describing everything
+<span class="pagenum"><a name="Page_250" id="Page_250">[Pg 250]</a></span>
+to which their successors may possibly want to refer, and generally
+by subdividing their material into as many species as
+they can induce any responsible society or journal to publish.
+Between Jordan with his 200 odd species for <i>Erophila</i>, and
+Grenier and Godron with one, there is no hesitation possible.
+Jordan's view, as he again and again declares with vehemence, is
+at least a view of natural facts, whereas the collective species is a
+mere abstraction, convenient indeed for librarians and beginners,
+but an insidious misrepresentation of natural truth, perhaps
+more than any other the source of the plausible fallacies regarding
+evolution that have so long obstructed progress.</p>
+
+<p class="indent">Nevertheless though we have been compelled to retreat
+from the speculative position to which scientific opinion had rashly
+advanced, the prospect of permanent progress is greatly better
+than it was. With the development of genetic research clear
+conceptions have at length been formed of the kind of knowledge
+required and of the methods by which it is to be attained. If
+we no longer see how varieties give rise to species, we may feel
+confident that a minute study of genetic physiology of varieties
+and species is the necessary beginning of any critical perception
+of their inter-relations. It is little more than a century since
+no valid distinction between a mechanical mixture and a chemical
+combination could be perceived, and in regard to the forms of
+life we may well be in a somewhat similar confusion.</p>
+
+<p class="indent">As yet the genetic behaviour of animals and plants
+has only been sampled. When the work has been done on a scale so
+large as to provide generalisations, we may be in a position to
+declare whether specific difference is or is not a physiological reality.
+<span class="pagenum"><a name="Page_251" id="Page_251">[Pg 251]</a></span></p>
+
+<hr class="chap" />
+<h2>INDEX OF SUBJECTS</h2>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Subject Index" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr"><b>PAGE</b></td>
+    </tr><tr>
+      <td class="tdl">Abraxa grossulariata,</td>
+      <td class="tdr"><a href="#Page_105">105</a>,
+                      <a href="#Page_193">193</a></td>
+    </tr><tr>
+      <td class="tdl">Aceras hircina, local variability,</td>
+      <td class="tdr"><a href="#Page_123">123</a></td>
+    </tr><tr>
+      <td class="tdl">Achatinellidae, local forms of,</td>
+      <td class="tdr"><a href="#Page_133">133</a></td>
+    </tr><tr>
+      <td class="tdl">Acquired characters, inheritance of,</td>
+      <td class="tdr"><a href="#Page_188">188 et seq.</a>,
+                      <a href="#Page_217">217</a>,
+                      <a href="#Page_233">233</a></td>
+    </tr><tr>
+      <td class="tdl">Acronycta psi, melanic,</td>
+      <td class="tdr"><a href="#Page_138">138</a></td>
+    </tr><tr>
+      <td class="tdl">Adaptation, problem of,</td>
+      <td class="tdr"><a href="#Page_187">187</a>,
+                      <a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdl">Agelaius, local forms,</td>
+      <td class="tdr"><a href="#Page_120">120</a></td>
+    </tr><tr>
+      <td class="tdl">Agrotis, fixed and variable species,</td>
+      <td class="tdr"><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Alkaptonuria,</td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdl">Alpine Plants, growing larger, if protected,</td>
+      <td class="tdr"><a href="#Page_183">183</a></td>
+    </tr><tr>
+      <td class="tdl">Alpine Varieties,</td>
+      <td class="tdr"><a href="#Page_165">165</a></td>
+    </tr><tr>
+      <td class="tdl">Alytes obstetricans, Kammerer's experiments on,</td>
+      <td class="tdr"><a href="#Page_199">199</a>,
+                      <a href="#Page_210">210</a></td>
+    </tr><tr>
+      <td class="tdl">Amblystoma, races of,</td>
+      <td class="tdr"><a href="#Page_230">230</a></td>
+    </tr><tr>
+      <td class="tdl">Amphidasys betularia, melanic form,</td>
+      <td class="tdr"><a href="#Page_136">136</a>,
+                      <a href="#Page_138">138</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;dimorphic larvae,</td>
+      <td class="tdr"><a href="#Page_141">141</a></td>
+    </tr><tr>
+      <td class="tdl">Anodonta, polymorphism of,</td>
+      <td class="tdr"><a href="#Page_130">130</a></td>
+    </tr><tr>
+      <td class="tdl">Antirrhinum, striped,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;species-hybrids,</td>
+      <td class="tdr"><a href="#Page_99">&nbsp;99</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;albinos,</td>
+      <td class="tdr"><a href="#Page_110">110</a></td>
+    </tr><tr>
+      <td class="tdl">Apple, will not cross with pear,</td>
+      <td class="tdr"><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Arctia caja, effects of temperature,</td>
+      <td class="tdr"><a href="#Page_192">192</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;larval variation in,</td>
+      <td class="tdr"><a href="#Page_231">231</a></td>
+    </tr><tr>
+      <td class="tdl">Arctic varieties,</td>
+      <td class="tdr"><a href="#Page_165">165</a></td>
+    </tr><tr>
+      <td class="tdl">Argynnis paphia and valesina in Italy,</td>
+      <td class="tdr"><a href="#Page_121">121</a></td>
+    </tr><tr>
+      <td class="tdl">Armadillo, polyembryony,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Artistic faculty,</td>
+      <td class="tdr"><a href="#Page_89">&nbsp;89</a></td>
+    </tr><tr>
+      <td class="tdl">Arum, rights and lefts,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Auriculas, short-styled selected,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Axis of symmetry in hand and foot,</td>
+      <td class="tdr"><a href="#Page_48">&nbsp;48</a></td>
+    </tr><tr>
+      <td class="tdl">Axolotl, alleged effect of conditions,</td>
+      <td class="tdr"><a href="#Page_230">230</a></td>
+    </tr><tr>
+      <td class="tdl">Azalea, bud-sports,</td>
+      <td class="tdr"><a href="#Page_55">&nbsp;55</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Bacillus anthracis, unsegmented form,</td>
+      <td class="tdr"><br /><a href="#Page_71">&nbsp;71</a></td>
+    </tr><tr>
+      <td class="tdl">Bacillus prodigiosus, variation in,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Bacteria, variation in,</td>
+      <td class="tdr"><a href="#Page_212">212</a></td>
+    </tr><tr>
+      <td class="tdl">Bacterium coli, variation in,</td>
+      <td class="tdr"><a href="#Page_214">214</a></td>
+    </tr><tr>
+      <td class="tdl">Baeolophus, geographical races of,</td>
+      <td class="tdr"><a href="#Page_159">159</a></td>
+    </tr><tr>
+      <td class="tdl">Barley, right and left-handed,</td>
+      <td class="tdr"><a href="#Page_58">&nbsp;58</a></td>
+    </tr><tr>
+      <td class="tdl">Basilarchia, geographical races of,</td>
+      <td class="tdr"><a href="#Page_161">161</a></td>
+    </tr><tr>
+      <td class="tdl">Begonia phyllomaniaca,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;hybrids,</td>
+      <td class="tdr"><a href="#Page_51">&nbsp;51</a></td>
+    </tr><tr>
+      <td class="tdl">Bizarre Carnation, genetics of,</td>
+      <td class="tdr"><a href="#Page_54">&nbsp;54</a></td>
+    </tr><tr>
+      <td class="tdl">Black, as a variation from red,</td>
+      <td class="tdr"><a href="#Page_148">148</a></td>
+    </tr><tr>
+      <td class="tdl">Blackbird, varying,</td>
+      <td class="tdr"><a href="#Page_150">150</a></td>
+    </tr><tr>
+      <td class="tdl">Black Cock, fixity of,</td>
+      <td class="tdr"><a href="#Page_28">&nbsp;28</a></td>
+    </tr><tr>
+      <td class="tdl">Boarmia repandata, melanic form,</td>
+      <td class="tdr"><a href="#Page_136">136</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;rhomboidaria,</td>
+      <td class="tdr"><a href="#Page_137">137</a>,
+                      <a href="#Page_139">139</a></td>
+    </tr><tr>
+      <td class="tdl">Botrytis susceptibility to,</td>
+      <td class="tdr"><a href="#Page_108">108</a></td>
+    </tr><tr>
+      <td class="tdl">Bovidae, hybrid,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Brachydactyly,</td>
+      <td class="tdr"><a href="#Page_89">&nbsp;89</a>,
+                      <a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">Bradypus, vertebral variation,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Bud-sports geometrically irregular,</td>
+      <td class="tdr"><a href="#Page_54">54-57</a></td>
+    </tr><tr>
+      <td class="tdl">Buffalo, attempts to hybridize,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Bullfinch, gynandromorph,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl">Bulimus detritus, local variation of,</td>
+      <td class="tdr"><a href="#Page_126">126</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Canary, asymmetrical markings in,</td>
+      <td class="tdr"><br /><a href="#Page_154">154</a></td>
+    </tr><tr>
+      <td class="tdl">Canidae, hybrid,</td>
+      <td class="tdr"><a href="#Page_241">241</a></td>
+    </tr><tr>
+      <td class="tdl">Capsella,</td>
+      <td class="tdr"><a href="#Page_100">100</a></td>
+    </tr><tr>
+      <td class="tdl">Cardamine pratensis,</td>
+      <td class="tdr"><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Cat, Polydactylism,</td>
+      <td class="tdr"><a href="#Page_53">&nbsp;53</a></td>
+    </tr><tr>
+      <td class="tdl">Carnation, Picotees and bizarres compared,</td>
+      <td class="tdr"><a href="#Page_54">&nbsp;54</a>,
+                      <a href="#Page_58">&nbsp;58</a></td>
+    </tr><tr>
+      <td class="tdl">Cataract, hereditary,</td>
+      <td class="tdr"><a href="#Page_89">&nbsp;89</a></td>
+    </tr><tr>
+      <td class="tdl">Certhiola, melanic,</td>
+      <td class="tdr"><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Chladni figures,</td>
+      <td class="tdr"><a href="#Page_60">&nbsp;60</a></td>
+    </tr><tr>
+      <td class="tdl">Choloepus, vertebral variation in,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;local variation in,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Cinerarias, self-sterility in,</td>
+      <td class="tdr"><a href="#Page_238">238</a></td>
+    </tr><tr>
+      <td class="tdl">Cistudo, local variation in,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Climatic varieties,</td>
+      <td class="tdr"><a href="#Page_164">164</a></td>
+    </tr><tr>
+      <td class="tdl">Coccaceae, variation in,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Coenonympha arcania, climatic forms of,</td>
+      <td class="tdr"><a href="#Page_179">179</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;satyrion,</td>
+      <td class="tdr"><a href="#Page_180">180</a></td>
+    </tr><tr>
+      <td class="tdl">Coereba, melanic,</td>
+      <td class="tdr"><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Colaptes, geographical races,</td>
+      <td class="tdr"><a href="#Page_147">147 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;chrysoides,</td>
+      <td class="tdr"><a href="#Page_154">154</a></td>
+    </tr><tr>
+      <td class="tdl">Colloids, growth in,</td>
+      <td class="tdr"><a href="#Page_65">&nbsp;65</a></td>
+    </tr><tr>
+      <td class="tdl">Colorado beetles, experiments on,</td>
+      <td class="tdr"><a href="#Page_218">218</a></td>
+    </tr><tr>
+      <td class="tdl">Colour blindness in twins,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Continuous variation, possible example of,</td>
+      <td class="tdr"><a href="#Page_173">173</a></td>
+    </tr><tr>
+      <td class="tdl">Coracias, geographical races of,</td>
+      <td class="tdr"><a href="#Page_160">160</a></td>
+    </tr><tr>
+      <td class="tdl">Cotton, genetics of,</td>
+      <td class="tdr"><a href="#Page_98">&nbsp;98</a>,
+                      <a href="#Page_100">100</a></td>
+    </tr><tr>
+      <td class="tdl">Coupling,</td>
+      <td class="tdr"><a href="#Page_110">110</a></td>
+    </tr><tr>
+      <td class="tdl">Crab, extra claws,</td>
+      <td class="tdr"><a href="#Page_74">&nbsp;74</a></td>
+    </tr><tr>
+      <td class="tdl">Crustacean appendages and Serial Homology,</td>
+      <td class="tdr"><a href="#Page_63">&nbsp;63</a></td>
+    </tr><tr>
+      <td class="tdl">Crystals, analogy with,</td>
+      <td class="tdr"><a href="#Page_78">&nbsp;78</a></td>
+    </tr><tr>
+      <td class="tdl">Cyclopian monsters, artificial,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Daphnia, changed by environment,</td>
+      <td class="tdr"><br /><a href="#Page_217">217</a></td>
+    </tr><tr>
+      <td class="tdl">Dasypus, polyembryony,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Dianthoecia, fixed and variable species,</td>
+      <td class="tdr"><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Disease-resistance,</td>
+      <td class="tdr"><a href="#Page_87">&nbsp;87</a></td>
+    </tr><tr>
+      <td class="tdl">Division, power of,</td>
+      <td class="tdr">&nbsp;</td>
+    </tr><tr>
+      <td class="tdl">&emsp;a fundamental attribute of living things,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Genetics of,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a>,
+                      <a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl">Dogger Bank, large varieties on,</td>
+      <td class="tdr"><a href="#Page_125">125</a></td>
+    </tr><tr>
+      <td class="tdl">Dogs, hybrid,</td>
+      <td class="tdr"><a href="#Page_241">241</a></td>
+    </tr><tr>
+      <td class="tdl">Dominance, nature of,</td>
+      <td class="tdr"><a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">Dominants, origin of new,</td>
+      <td class="tdr"><a href="#Page_88">&nbsp;88</a>,
+                      <a href="#Page_90">&nbsp;90</a>,
+                      <a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">Double monsters,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Draba, experiments with,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Drosophila,</td>
+      <td class="tdr"><a href="#Page_91">&nbsp;91</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Payne's experiments on,</td>
+      <td class="tdr"><a href="#Page_228">228</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Earthworm, regeneration,</td>
+      <td class="tdr"><br /><a href="#Page_77">&nbsp;77</a></td>
+    </tr><tr>
+      <td class="tdl">Elephant, tusk segmented,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">Entelechy,</td>
+      <td class="tdr"><a href="#Page_80">&nbsp;80</a></td>
+    </tr><tr>
+      <td class="tdl">Environmental treatment, effects of,</td>
+      <td class="tdr"><a href="#Page_188">188 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Enzymes and genetic factors,</td>
+      <td class="tdr"><a href="#Page_86">&nbsp;86</a></td>
+    </tr><tr>
+      <td class="tdl">Epilepsy, inheritance of traumatic,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Equidae, sterility of hybrid,</td>
+      <td class="tdr"><a href="#Page_241">241</a></td>
+    </tr><tr>
+      <td class="tdl">Erophila, experiments with,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;species,</td>
+      <td class="tdr"><a href="#Page_249">249</a></td>
+    </tr><tr>
+      <td class="tdl">Exacum, right and left,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Euphonia elegantissima, local forms,</td>
+      <td class="tdr"><a href="#Page_120">120</a></td>
+    </tr><tr>
+      <td class="tdl">Eupithecia rectangulata, melanic form,</td>
+      <td class="tdr"><a href="#Page_137">137</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Factors, new,</td>
+      <td class="tdr"><br /><a href="#Page_88">&nbsp;88</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;loss of,</td>
+      <td class="tdr"><a href="#Page_96">&nbsp;96</a></td>
+    </tr><tr>
+      <td class="tdl">Factorial representation of varieties,</td>
+      <td class="tdr"><a href="#Page_158">158</a>,
+                      <a href="#Page_165">165</a></td>
+    </tr><tr>
+      <td class="tdl">Falcons, geographical races,</td>
+      <td class="tdr"><a href="#Page_147">147</a></td>
+    </tr><tr>
+      <td class="tdl">Fasciation,</td>
+      <td class="tdr"><a href="#Page_49">&nbsp;49</a></td>
+    </tr><tr>
+      <td class="tdl">Ferments, Boyle on,</td>
+      <td class="tdr"><a href="#Page_54">&nbsp;54</a></td>
+    </tr><tr>
+      <td class="tdl">Finger-prints of twins,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Fixity and Variability in species,</td>
+      <td class="tdr"><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Flax, climatic experiments,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Fowl, Silky,</td>
+      <td class="tdr"><a href="#Page_84">&nbsp;84</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Leghorn,</td>
+      <td class="tdr"><a href="#Page_85">&nbsp;85</a>,
+                      <a href="#Page_90">&nbsp;90</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Dominant white,</td>
+      <td class="tdr"><a href="#Page_94">&nbsp;94</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Wyandotte,</td>
+      <td class="tdr"><a href="#Page_97">&nbsp;97</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Rumpless,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">Foxes, incompatibility with dogs,</td>
+      <td class="tdr"><a href="#Page_241">241</a></td>
+    </tr><tr>
+      <td class="tdl">Free-martin,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Fringillidae, sterility of hybrid,</td>
+      <td class="tdr"><a href="#Page_241">241</a></td>
+    </tr><tr>
+      <td class="tdl">Fundulus, cyclopian,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Gallus, invariability of wild species,</td>
+      <td class="tdr"><br /><a href="#Page_13">&nbsp;13</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;and origin of poultry,</td>
+      <td class="tdr"><a href="#Page_90">&nbsp;90</a>,
+                      <a href="#Page_97">&nbsp;97</a></td>
+    </tr><tr>
+      <td class="tdl">Genitalia, a basis for classification in insects,</td>
+      <td class="tdr"><a href="#Page_13">&nbsp;13</a></td>
+    </tr><tr>
+      <td class="tdl">Gentians, climatic experiments,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Geometrical structure and differentiation,</td>
+      <td class="tdr"><a href="#Page_54">&nbsp;54</a>,
+                      <a href="#Page_56">&nbsp;56</a></td>
+    </tr><tr>
+      <td class="tdl">Geometrical distinction between germ-cells and somatic cells,</td>
+      <td class="tdr"><a href="#Page_58">&nbsp;58</a></td>
+    </tr><tr>
+      <td class="tdl">Gladiolus, right and left,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Gnophus obscurata, protective colouring,</td>
+      <td class="tdr"><a href="#Page_141">141</a></td>
+    </tr><tr>
+      <td class="tdl">Goldfinch, geographical races,</td>
+      <td class="tdr"><a href="#Page_147">147</a></td>
+    </tr><tr>
+      <td class="tdl">Gonioctena variabilis, variation in sexes of,</td>
+      <td class="tdr"><a href="#Page_121">121</a></td>
+    </tr><tr>
+      <td class="tdl">Gouldian Finch, polymorphism,</td>
+      <td class="tdr"><a href="#Page_148">148-149</a></td>
+    </tr><tr>
+      <td class="tdl">Gracilaria stigmatella, experiments on,</td>
+      <td class="tdr"><a href="#Page_193">193</a></td>
+    </tr><tr>
+      <td class="tdl">Grantia, large varieties of,</td>
+      <td class="tdr"><a href="#Page_125">125</a></td>
+    </tr><tr>
+      <td class="tdl">Ground-Squirrels, local forms of,</td>
+      <td class="tdr"><a href="#Page_132">132</a></td>
+    </tr><tr>
+      <td class="tdl">Grouse, red, variation,</td>
+      <td class="tdr"><a href="#Page_29">&nbsp;29</a></td>
+    </tr><tr>
+      <td class="tdl">Guillemot, Ringed,</td>
+      <td class="tdr"><a href="#Page_150">150</a></td>
+    </tr><tr>
+      <td class="tdl">Guinea-pig, Brown-Séquard's experiments on,</td>
+      <td class="tdr"><a href="#Page_198">198</a></td>
+    </tr><tr>
+      <td class="tdl">Gynandromorphs,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Heliconius erato, forms of,</td>
+      <td class="tdr"><br /><a href="#Page_122">122</a>,
+                            <a href="#Page_164">164</a></td>
+    </tr><tr>
+      <td class="tdl">Helix lapicida, local variation of,</td>
+      <td class="tdr"><a href="#Page_126">126</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;striata,</td>
+      <td class="tdr"><a href="#Page_127">127</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Heripensis,</td>
+      <td class="tdr"><a href="#Page_127">127</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Caespitum,</td>
+      <td class="tdr"><a href="#Page_127">127</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;trochoides,</td>
+      <td class="tdr"><a href="#Page_127">127</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;nemoralis and hortensis,</td>
+      <td class="tdr"><a href="#Page_128">128</a></td>
+    </tr><tr>
+      <td class="tdl">Helminthophila, geographical races of,</td>
+      <td class="tdr"><a href="#Page_157">157</a></td>
+    </tr><tr>
+      <td class="tdl">Hemerophila abruptaria, melanic,</td>
+      <td class="tdr"><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Hepialus humuli, in Shetland,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Heterostyle plants,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Hieracium,</td>
+      <td class="tdr"><a href="#Page_9">&nbsp;&nbsp;9</a></td>
+    </tr><tr>
+      <td class="tdl">Himantopus,</td>
+      <td class="tdr"><a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdl">Homoeosis,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Hybernia progemmaria,</td>
+      <td class="tdr"><a href="#Page_139">139</a></td>
+    </tr><tr>
+      <td class="tdl">Hybrids, sterility of,</td>
+      <td class="tdr"><a href="#Page_233">233 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Incompatibility between certain allied species,</td>
+      <td class="tdr"><br /><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Individual, geometrical independence of,</td>
+      <td class="tdr"><a href="#Page_58">&nbsp;58</a></td>
+    </tr><tr>
+      <td class="tdl">Inhibiting Factors,</td>
+      <td class="tdr"><a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">Intermediates, nature of,</td>
+      <td class="tdr"><a href="#Page_131">131</a>,
+                      <a href="#Page_135">135</a></td>
+    </tr><tr>
+      <td class="tdl">Isolation, consequences of,</td>
+      <td class="tdr"><a href="#Page_118">118</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Lacerta muralis, Kammerer's experiments on,</td>
+      <td class="tdr"><br /><a href="#Page_209">209</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;fiumana,</td>
+      <td class="tdr"><a href="#Page_210">210</a></td>
+    </tr><tr>
+      <td class="tdl">Leptinotarsa, Power's experiments on,</td>
+      <td class="tdr"><a href="#Page_218">218</a></td>
+    </tr><tr>
+      <td class="tdl">Limbs, extra, in pairs,</td>
+      <td class="tdr"><a href="#Page_72">&nbsp;72</a></td>
+    </tr><tr>
+      <td class="tdl">Limnaea, sinistral,</td>
+      <td class="tdr"><a href="#Page_134">134</a></td>
+    </tr><tr>
+      <td class="tdl">Linaria vulgaris, self-sterility,</td>
+      <td class="tdr"><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Loasa fruits, right and left,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Lobster, extra claws,</td>
+      <td class="tdr"><a href="#Page_76">&nbsp;76</a></td>
+    </tr><tr>
+      <td class="tdl">Locality, variation connected with,</td>
+      <td class="tdr"><a href="#Page_14">&nbsp;14</a>,
+                      <a href="#Page_118">118</a>,
+                      <a href="#Page_146">146 et seq.</a>,
+                      <a href="#Page_208">208</a></td>
+    </tr><tr>
+      <td class="tdl">Lumbricus, regeneration,</td>
+      <td class="tdr"><a href="#Page_77">&nbsp;77</a></td>
+    </tr><tr>
+      <td class="tdl">Lychnis dioica and vespertina, inter-relations of,</td>
+      <td class="tdr"><a href="#Page_18">&nbsp;18</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;macrocarpa, possibly a common parent of,</td>
+      <td class="tdr"><a href="#Page_19">&nbsp;19</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Machetes pugnax, polymorphism of male,</td>
+      <td class="tdr"><br /><a href="#Page_28">&nbsp;28</a></td>
+    </tr><tr>
+      <td class="tdl">Maize, Blaringhem's experiments on,</td>
+      <td class="tdr"><a href="#Page_229">229</a></td>
+    </tr><tr>
+      <td class="tdl">Maize, cumulative factors in,</td>
+      <td class="tdr"><a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdl">Malformations, dominants, arising de novo,</td>
+      <td class="tdr"><a href="#Page_89">&nbsp;89</a></td>
+    </tr><tr>
+      <td class="tdl">Manx Cat, heredity,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">Matthiola,</td>
+      <td class="tdr"><a href="#Page_84">&nbsp;84</a>,
+                      <a href="#Page_104">104</a>,
+                      <a href="#Page_113">113</a></td>
+    </tr><tr>
+      <td class="tdl">Melanic varieties,</td>
+      <td class="tdr"><a href="#Page_135">135 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Memory, analogy with heredity,</td>
+      <td class="tdr"><a href="#Page_190">190</a></td>
+    </tr><tr>
+      <td class="tdl">Meristic variation,</td>
+      <td class="tdr"><a href="#Page_69">&nbsp;69</a>,
+                      <a href="#Page_83">&nbsp;83</a>,
+                      <a href="#Page_86">&nbsp;86</a></td>
+    </tr><tr>
+      <td class="tdl">Mirabilis, striped,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Models of segmentation,</td>
+      <td class="tdr"><a href="#Page_59">&nbsp;59</a>,
+                      <a href="#Page_60">&nbsp;60</a></td>
+    </tr><tr>
+      <td class="tdl">"Modes," Coutagne's conception of,</td>
+      <td class="tdr"><a href="#Page_126">126</a></td>
+    </tr><tr>
+      <td class="tdl">Mödling, peculiar race of <i>Pieris napi</i> at,</td>
+      <td class="tdr"><a href="#Page_178">178</a></td>
+    </tr><tr>
+      <td class="tdl">Mole, albino,</td>
+      <td class="tdr"><a href="#Page_27">27-28</a></td>
+    </tr><tr>
+      <td class="tdl">Mule, Linnaeus on,</td>
+      <td class="tdr"><a href="#Page_8">&nbsp;&nbsp;8</a></td>
+    </tr><tr>
+      <td class="tdl">Mutation, Matthioli on,</td>
+      <td class="tdr"><a href="#Page_4">&nbsp;&nbsp;4</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in Mercurialis,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in Kales,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;alleged in bulbs,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Theory,</td>
+      <td class="tdr"><a href="#Page_97">&nbsp;97</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;periods of,</td>
+      <td class="tdr"><a href="#Page_114">114</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in Bacteria,</td>
+      <td class="tdr"><a href="#Page_214">214</a></td>
+    </tr><tr>
+      <td class="tdl">Mutilation, consequences of,</td>
+      <td class="tdr"><a href="#Page_71">&nbsp;71</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;alleged effect of, on offspring,</td>
+      <td class="tdr"><a href="#Page_229">229</a></td>
+    </tr><tr>
+      <td class="tdl">Myxococcus, variation in,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Narwhal, asymmetry of tusks,</td>
+      <td class="tdr"><br /><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Nemesia strumosa,</td>
+      <td class="tdr"><a href="#Page_91">&nbsp;91</a></td>
+    </tr><tr>
+      <td class="tdl">Neuration, a basis for classification,</td>
+      <td class="tdr"><a href="#Page_13">&nbsp;13</a></td>
+    </tr><tr>
+      <td class="tdl">Nicotiana, sterility of hybrid,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Nightjars, varying,</td>
+      <td class="tdr"><a href="#Page_150">150</a></td>
+    </tr><tr>
+      <td class="tdl">Noctuidae, fixity and variability,</td>
+      <td class="tdr"><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Noctua, polymorphic and fixed species,</td>
+      <td class="tdr"><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Noctua castanea, local forms of,</td>
+      <td class="tdr"><a href="#Page_122">122</a></td>
+    </tr><tr>
+      <td class="tdl">Nomenclature, future of,</td>
+      <td class="tdr"><a href="#Page_94">&nbsp;94</a>,
+                      <a href="#Page_245">245</a></td>
+    </tr><tr>
+      <td class="tdl">Notonecta, variations of,</td>
+      <td class="tdr"><a href="#Page_130">130</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Odontoptera bidentata, melanic form,</td>
+      <td class="tdr"><br /><a href="#Page_137">137</a></td>
+    </tr><tr>
+      <td class="tdl">Oedipodidae, protectively coloured,</td>
+      <td class="tdr"><a href="#Page_140">140</a></td>
+    </tr><tr>
+      <td class="tdl">Oenothera, new dominant in,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;rubricalyx and rubrinervis,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a>,
+                      <a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Lamarckiana,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a>,
+                      <a href="#Page_101">101</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;origin of,</td>
+      <td class="tdr"><a href="#Page_102">102</a>,
+                      <a href="#Page_244">244</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;has bad pollen-grains,</td>
+      <td class="tdr"><a href="#Page_102">102</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;factorial analysis of,</td>
+      <td class="tdr"><a href="#Page_103">103</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;pollen and egg-cells genetically dissimilar,</td>
+      <td class="tdr"><a href="#Page_104">104</a></td>
+    </tr><tr>
+      <td class="tdl">Oenothera, "twin hybrids",</td>
+      <td class="tdr"><a href="#Page_105">105</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;laeta and velutina,</td>
+      <td class="tdr"><a href="#Page_105">105</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;reciprocal crosses in,</td>
+      <td class="tdr"><a href="#Page_105">105 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;possible coupling in,</td>
+      <td class="tdr"><a href="#Page_111">111</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;dwarfs,</td>
+      <td class="tdr"><a href="#Page_112">112</a>,
+                      <a href="#Page_114">114</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;"Triple hybrids",</td>
+      <td class="tdr"><a href="#Page_114">114</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;alleged variation due to treatment,</td>
+      <td class="tdr"><a href="#Page_227">227</a></td>
+    </tr><tr>
+      <td class="tdl">Ophrys, local variability,</td>
+      <td class="tdr"><a href="#Page_125">125</a></td>
+    </tr><tr>
+      <td class="tdl">Orange, polyembryony,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl">Osmotic growth,</td>
+      <td class="tdr"><a href="#Page_65">&nbsp;65</a></td>
+    </tr><tr>
+      <td class="tdl">Overlapping forms,</td>
+      <td class="tdr"><a href="#Page_146">146</a>,
+                      <a href="#Page_174">174</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Papilio, geographical races of,</td>
+      <td class="tdr"><br /><a href="#Page_162">162</a></td>
+    </tr><tr>
+      <td class="tdl">Papilio turnus, variation of,</td>
+      <td class="tdr"><a href="#Page_144">144</a></td>
+    </tr><tr>
+      <td class="tdl">Pararge egeria, geographical forms,</td>
+      <td class="tdr"><a href="#Page_166">166 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Parthenogenesis,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl">Partula, local forms of,</td>
+      <td class="tdr"><a href="#Page_133">133</a></td>
+    </tr><tr>
+      <td class="tdl">Passer domesticus and montanus, distinctions,</td>
+      <td class="tdr"><a href="#Page_22">&nbsp;22</a></td>
+    </tr><tr>
+      <td class="tdl">Pea, round and wrinkled,</td>
+      <td class="tdr"><a href="#Page_95">&nbsp;95</a></td>
+    </tr><tr>
+      <td class="tdl">Pear, will not cross with apple,</td>
+      <td class="tdr"><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Pelargonium, variegated,</td>
+      <td class="tdr"><a href="#Page_55">&nbsp;55</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;bud-sports,</td>
+      <td class="tdr"><a href="#Page_56">&nbsp;56</a></td>
+    </tr><tr>
+      <td class="tdl">Periodic phenomena in structure,</td>
+      <td class="tdr"><a href="#Page_63">&nbsp;63</a></td>
+    </tr><tr>
+      <td class="tdl">Peronea, fixed and variable species,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a></td>
+    </tr><tr>
+      <td class="tdl">"Petites espèces",</td>
+      <td class="tdr"><a href="#Page_248">248</a></td>
+    </tr><tr>
+      <td class="tdl">Petunia, double,</td>
+      <td class="tdr"><a href="#Page_104">104</a></td>
+    </tr><tr>
+      <td class="tdl">Phalanger maculatus, local variation,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Pheasant, fixity of,</td>
+      <td class="tdr"><a href="#Page_29">&nbsp;29</a></td>
+    </tr><tr>
+      <td class="tdl">Phigalia pilosaria, melanic,</td>
+      <td class="tdr"><a href="#Page_139">139-140</a></td>
+    </tr><tr>
+      <td class="tdl">Phratora vitellinae, experiments on,</td>
+      <td class="tdr"><a href="#Page_193">193</a></td>
+    </tr><tr>
+      <td class="tdl">Phyllotaxis,</td>
+      <td class="tdr"><a href="#Page_69">&nbsp;69</a></td>
+    </tr><tr>
+      <td class="tdl">Pied varieties common in Passer domesticus unknown in Montanus,</td>
+      <td class="tdr"><a href="#Page_23">&nbsp;23</a></td>
+    </tr><tr>
+      <td class="tdl">Pieris napi and bryoniae,</td>
+      <td class="tdr"><a href="#Page_174">174</a></td>
+    </tr><tr>
+      <td class="tdl">Pig, mule-footed,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">Pigeon, web-footed,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a>,
+                      <a href="#Page_49">&nbsp;49</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Indian Rock, a recessive form,</td>
+      <td class="tdr"><a href="#Page_98">&nbsp;98</a></td>
+    </tr><tr>
+      <td class="tdl">Pigments, nature of,</td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdl">Pisum humile, hybrids with culinary peas,</td>
+      <td class="tdr"><a href="#Page_244">244</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;species,</td>
+      <td class="tdr"><a href="#Page_246">246</a></td>
+    </tr><tr>
+      <td class="tdl">Planarian, regeneration of,</td>
+      <td class="tdr"><a href="#Page_71">&nbsp;71</a>,
+                      <a href="#Page_77">&nbsp;77</a></td>
+    </tr><tr>
+      <td class="tdl">Plotheia frontalis, polymorphic,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a>,
+                      <a href="#Page_29">&nbsp;29</a></td>
+    </tr><tr>
+      <td class="tdl">Plusia, fixity and variation in,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a></td>
+    </tr><tr>
+      <td class="tdl">Poephila gouldiae, variation of,</td>
+      <td class="tdr"><a href="#Page_148">148-149</a></td>
+    </tr><tr>
+      <td class="tdl">Polarity of individual,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Polia chi, melanic,</td>
+      <td class="tdr"><a href="#Page_138">138</a></td>
+    </tr><tr>
+      <td class="tdl">Polyanthus, short-styled selected,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Polydactylism in Cat,</td>
+      <td class="tdr"><a href="#Page_52">52-53</a></td>
+    </tr><tr>
+      <td class="tdl">Polyembryony,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl">Potato, variation in,</td>
+      <td class="tdr"><a href="#Page_91">&nbsp;91</a></td>
+    </tr><tr>
+      <td class="tdl">Poultry, evolution of,</td>
+      <td class="tdr"><a href="#Page_90">&nbsp;90</a></td>
+    </tr><tr>
+      <td class="tdl">Primula obconica,</td>
+      <td class="tdr"><a href="#Page_91">&nbsp;91</a></td>
+    </tr><tr>
+      <td class="tdl">Primula sinensis, flaked,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Leaf-shapes,</td>
+      <td class="tdr"><a href="#Page_70">&nbsp;70</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;new dominant in,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;sterility in,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;"Giants",</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Primula, species-hybrids,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Protective coloration,</td>
+      <td class="tdr"><a href="#Page_140">140</a></td>
+    </tr><tr>
+      <td class="tdl">Pyrrhulagra, local forms,</td>
+      <td class="tdr"><a href="#Page_120">120</a></td>
+    </tr><tr>
+      <td class="tdl">Python, twin-vertebrae,</td>
+      <td class="tdr"><a href="#Page_60">&nbsp;60</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Quiscalus, geographical races of,</td>
+      <td class="tdr"><br /><a href="#Page_156">156</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Rabbit, Angora,</td>
+      <td class="tdr"><br /><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;colours of,</td>
+      <td class="tdr"><a href="#Page_93">&nbsp;93</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Incompatibility with hare,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Raimannia odorata, Macdougal's experiments on,</td>
+      <td class="tdr"><a href="#Page_226">226</a></td>
+    </tr><tr>
+      <td class="tdl">Rats, Variation in,</td>
+      <td class="tdr"><a href="#Page_248">248</a></td>
+    </tr><tr>
+      <td class="tdl">Recessives, origin of,</td>
+      <td class="tdr"><a href="#Page_90">&nbsp;90</a></td>
+    </tr><tr>
+      <td class="tdl">Reciprocal crosses, giving distinct results,</td>
+      <td class="tdr"><a href="#Page_105">105 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Regeneration,</td>
+      <td class="tdr"><a href="#Page_70">&nbsp;70</a></td>
+    </tr><tr>
+      <td class="tdl">Repulsion,</td>
+      <td class="tdr"><a href="#Page_110">110</a></td>
+    </tr><tr>
+      <td class="tdl">Reversal on Regeneration,</td>
+      <td class="tdr"><a href="#Page_77">&nbsp;77</a></td>
+    </tr><tr>
+      <td class="tdl">Rhamphocoelus, geographical forms,</td>
+      <td class="tdr"><a href="#Page_159">159</a>,
+                      <a href="#Page_184">184</a></td>
+    </tr><tr>
+      <td class="tdl">Rhinoptera, variation in jaws of,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">Rhythm in repetition,</td>
+      <td class="tdr"><a href="#Page_69">&nbsp;69</a></td>
+    </tr><tr>
+      <td class="tdl">Ribs, variation of,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Rights and Lefts,</td>
+      <td class="tdr"><a href="#Page_57">57-58</a></td>
+    </tr><tr>
+      <td class="tdl">Ripples, analogous to segments,</td>
+      <td class="tdr"><a href="#Page_60">&nbsp;60</a>,
+                      <a href="#Page_66">&nbsp;66-67</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;regeneration of,</td>
+      <td class="tdr"><a href="#Page_79">&nbsp;79</a></td>
+    </tr><tr>
+      <td class="tdl">Rollers, geographical races of,</td>
+      <td class="tdr"><a href="#Page_160">160</a></td>
+    </tr><tr>
+      <td class="tdl">Ruff, polymorphism of male,</td>
+      <td class="tdr"><a href="#Page_28">&nbsp;28</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Salamandra, maculosa and atra,</td>
+      <td class="tdr"><br /><a href="#Page_182">182</a>,
+                      <a href="#Page_199">199</a>,
+                      <a href="#Page_203">203</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;spotted and striped,</td>
+      <td class="tdr"><a href="#Page_207">207</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;geographical variation of,</td>
+      <td class="tdr"><a href="#Page_208">208</a></td>
+    </tr><tr>
+      <td class="tdl">Segmentation, nature of,</td>
+      <td class="tdr"><a href="#Page_63">&nbsp;63</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;simulated mechanically,</td>
+      <td class="tdr"><a href="#Page_64">&nbsp;64</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;compared with rippling,</td>
+      <td class="tdr"><a href="#Page_65">&nbsp;65</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;analogies with,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Segmentation of normally unsegmented structures,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">Selection, Natural, an insufficient cause of definiteness of types,</td>
+      <td class="tdr"><a href="#Page_17">&nbsp;17</a>,
+                      <a href="#Page_134">134</a>,
+                      <a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Sempervivum,</td>
+      <td class="tdr"><a href="#Page_250">250</a></td>
+    </tr><tr>
+      <td class="tdl">Serial Homology, the true nature of,</td>
+      <td class="tdr"><a href="#Page_62">&nbsp;62</a>,
+                      <a href="#Page_66">&nbsp;66</a></td>
+    </tr><tr>
+      <td class="tdl">Setina, Alpine varieties,</td>
+      <td class="tdr"><a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Sex of Twins,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Sex-factors, possible coupling of,</td>
+      <td class="tdr"><a href="#Page_111">111</a></td>
+    </tr><tr>
+      <td class="tdl">Sexual characters, variation in,</td>
+      <td class="tdr"><a href="#Page_119">119 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Siamese twins,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Silky Fowl,</td>
+      <td class="tdr"><a href="#Page_84">84-85</a></td>
+    </tr><tr>
+      <td class="tdl">Simocephalus, changed by environment,</td>
+      <td class="tdr"><a href="#Page_218">218</a></td>
+    </tr><tr>
+      <td class="tdl">Sinistral forms,</td>
+      <td class="tdr"><a href="#Page_33">33-34</a></td>
+    </tr><tr>
+      <td class="tdl">Situs transversus,</td>
+      <td class="tdr"><a href="#Page_43">&nbsp;43</a></td>
+    </tr><tr>
+      <td class="tdl">Skate's jaws, variation in,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">Sloths, vertebral variation,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Species, conceptions of,</td>
+      <td class="tdr"><a href="#Page_3">&nbsp;&nbsp;3</a>,
+                      <a href="#Page_94">&nbsp;94</a>,
+                      <a href="#Page_99">&nbsp;99</a>,
+                      <a href="#Page_240">240</a>,
+                      <a href="#Page_245">245</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;allied, distribution of,</td>
+      <td class="tdr"><a href="#Page_185">185</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;alternative uses of the term,</td>
+      <td class="tdr"><a href="#Page_245">245</a></td>
+    </tr><tr>
+      <td class="tdl">Specific difference, universality of,</td>
+      <td class="tdr"><a href="#Page_12">&nbsp;12</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;of organisms compared with those of inorganic materials,</td>
+      <td class="tdr"><a href="#Page_15">&nbsp;15</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;failure of theory of Selection to explain,</td>
+      <td class="tdr"><a href="#Page_18">&nbsp;18</a>,
+                      <a href="#Page_134">134</a>,
+                      <a href="#Page_247">247</a></td>
+    </tr><tr>
+      <td class="tdl">Sphyropicus varius,</td>
+      <td class="tdr"><a href="#Page_149">149</a>,
+                      <a href="#Page_156">156</a></td>
+    </tr><tr>
+      <td class="tdl">Spilosoma lubricipeda, varieties of,</td>
+      <td class="tdr"><a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Zatima, Heligoland form,</td>
+      <td class="tdr"><a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Spinal nerves, segmentation of,</td>
+      <td class="tdr"><a href="#Page_67">&nbsp;67</a></td>
+    </tr><tr>
+      <td class="tdl">Sporadic variation,</td>
+      <td class="tdr"><a href="#Page_131">131</a>,
+                      <a href="#Page_134">134</a>,
+                      <a href="#Page_248">248</a></td>
+    </tr><tr>
+      <td class="tdl">Squashes, polymorphism of,</td>
+      <td class="tdr"><a href="#Page_100">100</a></td>
+    </tr><tr>
+      <td class="tdl">Staphylococcus pyogenes, variation in,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Sterility of hybrids, in general,</td>
+      <td class="tdr"><a href="#Page_233">233</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in Lychnis hybrids,</td>
+      <td class="tdr"><a href="#Page_20">20 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in crossing forms of Draba,</td>
+      <td class="tdr"><a href="#Page_243">243</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Significance of,</td>
+      <td class="tdr"><a href="#Page_244">244</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Self,</td>
+      <td class="tdr"><a href="#Page_238">238</a></td>
+    </tr><tr>
+      <td class="tdl">Stilt,</td>
+      <td class="tdr"><a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdl">Stocks,</td>
+      <td class="tdr"><a href="#Page_84">&nbsp;84</a>,
+                      <a href="#Page_104">104</a>,
+                      <a href="#Page_113">113</a></td>
+    </tr><tr>
+      <td class="tdl">Striped varieties,</td>
+      <td class="tdr"><a href="#Page_57">&nbsp;57</a></td>
+    </tr><tr>
+      <td class="tdl">Substantive variation,</td>
+      <td class="tdr"><a href="#Page_84">&nbsp;84</a></td>
+    </tr><tr>
+      <td class="tdl">Subtraction-stages,</td>
+      <td class="tdr"><a href="#Page_93">&nbsp;93</a></td>
+    </tr><tr>
+      <td class="tdl">Supernumerary limbs,</td>
+      <td class="tdr"><a href="#Page_72">72-76</a></td>
+    </tr><tr>
+      <td class="tdl">Sweet pea, variation of,</td>
+      <td class="tdr"><a href="#Page_91">&nbsp;91</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;sterile anthers in,</td>
+      <td class="tdr"><a href="#Page_237">237</a></td>
+    </tr><tr>
+      <td class="tdl">Symmetry compared with heredity,</td>
+      <td class="tdr"><a href="#Page_41">&nbsp;41</a></td>
+    </tr><tr>
+      <td class="tdl">Symmetry of body approximate,</td>
+      <td class="tdr"><a href="#Page_78">&nbsp;78</a></td>
+    </tr><tr>
+      <td class="tdl">Syndactyly,</td>
+      <td class="tdr"><a href="#Page_47">&nbsp;47</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in foot,</td>
+      <td class="tdr"><a href="#Page_48">&nbsp;48</a></td>
+    </tr><tr>
+      <td class="tdl">Synthetic formulae, in nomenclature,</td>
+      <td class="tdr"><a href="#Page_94">&nbsp;94</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Taeniocampa, fixed and variable species,</td>
+      <td class="tdr"><br /><a href="#Page_25">&nbsp;25</a></td>
+    </tr><tr>
+      <td class="tdl">Tamias, local forms of,</td>
+      <td class="tdr"><a href="#Page_132">132</a></td>
+    </tr><tr>
+      <td class="tdl">Tanagers, geographical races of,</td>
+      <td class="tdr"><a href="#Page_159">159</a></td>
+    </tr><tr>
+      <td class="tdl">Teeth, variation in,</td>
+      <td class="tdr"><a href="#Page_39">&nbsp;39</a>,
+                      <a href="#Page_67">&nbsp;67</a></td>
+    </tr><tr>
+      <td class="tdl">Tephrosia consortaria and consonaria,</td>
+      <td class="tdr"><a href="#Page_137">137</a>,
+                      <a href="#Page_139">139</a>,
+                      <a href="#Page_140">140</a></td>
+    </tr><tr>
+      <td class="tdl">Tephrosia species, separated by season,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Terminal members, variation of,</td>
+      <td class="tdr"><a href="#Page_68">&nbsp;68</a></td>
+    </tr><tr>
+      <td class="tdl">Thais rumina, local variation in,</td>
+      <td class="tdr"><a href="#Page_27">&nbsp;27</a></td>
+    </tr><tr>
+      <td class="tdl">Tolerance, persistence of diversity due to,</td>
+      <td class="tdr"><a href="#Page_17">&nbsp;17</a>,
+                      <a href="#Page_134">134</a></td>
+    </tr><tr>
+      <td class="tdl">Tomato, number of cells in fruit,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">Transitional populations, rarity of,</td>
+      <td class="tdr"><a href="#Page_165">165</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;an example,</td>
+      <td class="tdr"><a href="#Page_178">178</a></td>
+    </tr><tr>
+      <td class="tdl">Tropaeolum, sterile anthers in,</td>
+      <td class="tdr"><a href="#Page_237">237</a></td>
+    </tr><tr>
+      <td class="tdl">Trypanosomes, variation in,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl">Tusk, of Elephant, segmented,</td>
+      <td class="tdr"><a href="#Page_38">&nbsp;38</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;of Narwhal,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Twinning,</td>
+      <td class="tdr"><a href="#Page_41">&nbsp;41</a>,
+                      <a href="#Page_44">&nbsp;44</a>,
+                      <a href="#Page_71">&nbsp;71</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;heredity of,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;in organs,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Uria troile, variety of,</td>
+      <td class="tdr"><br /><a href="#Page_150">150</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Vanessa urticae, effects of temperature,</td>
+      <td class="tdr"><br /><a href="#Page_191">191</a></td>
+    </tr><tr>
+      <td class="tdl">Variation, a medley of phenomena,</td>
+      <td class="tdr"><a href="#Page_14">14-15</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;sporadic,</td>
+      <td class="tdr"><a href="#Page_131">131</a>,
+                      <a href="#Page_134">134</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;and locality,</td>
+      <td class="tdr"><a href="#Page_118">118</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Causes of genetic,</td>
+      <td class="tdr"><a href="#Page_86">&nbsp;86</a>,
+                      <a href="#Page_87">&nbsp;87</a>,
+                      <a href="#Page_131">131</a>,
+                      <a href="#Page_212">212</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Substantive and meristic,</td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdl">Veronica, specific difference in,</td>
+      <td class="tdr"><a href="#Page_16">&nbsp;16</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;intermediates between species,</td>
+      <td class="tdr"><a href="#Page_17">&nbsp;17</a></td>
+    </tr><tr>
+      <td class="tdl">Vertebrae, division in,</td>
+      <td class="tdr"><a href="#Page_60">60-61</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;homologies of,</td>
+      <td class="tdr"><a href="#Page_66">&nbsp;66</a></td>
+    </tr><tr>
+      <td class="tdl">Vespa, specific difference in,</td>
+      <td class="tdr"><a href="#Page_23">&nbsp;23</a></td>
+    </tr><tr>
+      <td class="tdl">Vortex, living organism compared with,</td>
+      <td class="tdr"><a href="#Page_40">&nbsp;40</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Wave-motion compared with repetition of parts,</td>
+      <td class="tdr"><br /><a href="#Page_62">&nbsp;62</a>,
+                            <a href="#Page_67">&nbsp;67</a>,
+                            <a href="#Page_79">&nbsp;79</a></td>
+    </tr><tr>
+      <td class="tdl">Wheat, cumulative factors in,</td>
+      <td class="tdr"><a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;climatic experiments on,</td>
+      <td class="tdr"><a href="#Page_195">195</a></td>
+    </tr><tr>
+      <td class="tdl">Woodpecker,</td>
+      <td class="tdr"><a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Zebra, pattern of stripes compared with ripples,</td>
+      <td class="tdr"><br /><a href="#Page_38">&nbsp;38</a></td>
+    </tr>
+ </tbody>
+</table>
+<p><span class="pagenum"><a name="Page_252" id="Page_252">[Pg 252]</a></span></p>
+<hr class="chap" />
+<h2>INDEX OF PERSONS</h2>
+
+<table border="0" style="max-width: 45em;" cellspacing="2" summary="Person Index" cellpadding="0" >
+  <tbody><tr>
+      <td class="tdr">&nbsp;</td>
+      <td class="tdr"><b>PAGE</b></td>
+    </tr><tr>
+      <td class="tdl">Ackermann,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Agar,</td>
+      <td class="tdr"><a href="#Page_218">218</a></td>
+    </tr><tr>
+      <td class="tdl">Allen, J. A.,</td>
+      <td class="tdr"><a href="#Page_132">132</a>,
+                      <a href="#Page_147">147</a>,
+                      <a href="#Page_159">159</a></td>
+    </tr><tr>
+      <td class="tdl">Annandale,</td>
+      <td class="tdr"><a href="#Page_47">&nbsp;47</a></td>
+    </tr><tr>
+      <td class="tdl">Arrigoni degli Oddi,</td>
+      <td class="tdr"><a href="#Page_167">167</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Backhouse,</td>
+      <td class="tdr"><br /><a href="#Page_50">&nbsp;50</a></td>
+    </tr><tr>
+      <td class="tdl">Baker, G. T.,</td>
+      <td class="tdr"><a href="#Page_166">166</a></td>
+    </tr><tr>
+      <td class="tdl">Bangs, Outram,</td>
+      <td class="tdr"><a href="#Page_120">120</a>,
+                      <a href="#Page_142">142</a>,
+                      <a href="#Page_155">155</a></td>
+    </tr><tr>
+      <td class="tdl">Barrett,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a>,
+                      <a href="#Page_136">136</a>,
+                      <a href="#Page_167">167</a>,
+                      <a href="#Page_173">173</a>,
+                      <a href="#Page_178">178</a>,
+                      <a href="#Page_193">193</a></td>
+    </tr><tr>
+      <td class="tdl">Baur, E.,</td>
+      <td class="tdr"><a href="#Page_55">&nbsp;55</a>,
+                      <a href="#Page_99">&nbsp;99</a></td>
+    </tr><tr>
+      <td class="tdl">Baur, G.,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Beneden, van,</td>
+      <td class="tdr"><a href="#Page_75">&nbsp;75</a></td>
+    </tr><tr>
+      <td class="tdl">Bentham, on species of Veronica,</td>
+      <td class="tdr"><a href="#Page_16">&nbsp;16</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Lychnis,</td>
+      <td class="tdr"><a href="#Page_21">&nbsp;21</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Primula,</td>
+      <td class="tdr"><a href="#Page_22">&nbsp;22</a></td>
+    </tr><tr>
+      <td class="tdl">Bernadin,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Bishop, L. B.,</td>
+      <td class="tdr"><a href="#Page_153">153</a>,
+                      <a href="#Page_157">157</a></td>
+    </tr><tr>
+      <td class="tdl">Blaringhem,</td>
+      <td class="tdr"><a href="#Page_229">229</a></td>
+    </tr><tr>
+      <td class="tdl">Bobart,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a></td>
+    </tr><tr>
+      <td class="tdl">Boisduval,</td>
+      <td class="tdr"><a href="#Page_182">182</a></td>
+    </tr><tr>
+      <td class="tdl">Boissier,</td>
+      <td class="tdr"><a href="#Page_19">&nbsp;19</a></td>
+    </tr><tr>
+      <td class="tdl">Borradaile,</td>
+      <td class="tdr"><a href="#Page_74">74-75</a></td>
+    </tr><tr>
+      <td class="tdl">Boulenger, E. G.,</td>
+      <td class="tdr"><a href="#Page_208">208</a></td>
+    </tr><tr>
+      <td class="tdl">Boulenger, G. A.,</td>
+      <td class="tdr"><a href="#Page_182">182</a>,
+                      <a href="#Page_207">207</a>,
+                      <a href="#Page_209">209</a></td>
+    </tr><tr>
+      <td class="tdl">Boyle,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a>,
+                      <a href="#Page_54">&nbsp;54</a></td>
+    </tr><tr>
+      <td class="tdl">Brewster, W.,</td>
+      <td class="tdr"><a href="#Page_149">149-150</a></td>
+    </tr><tr>
+      <td class="tdl">Britton,</td>
+      <td class="tdr"><a href="#Page_227">227</a></td>
+    </tr><tr>
+      <td class="tdl">Brown, T. Graham,</td>
+      <td class="tdr"><a href="#Page_198">198</a></td>
+    </tr><tr>
+      <td class="tdl">Brown-Séquard,</td>
+      <td class="tdr"><a href="#Page_197">197 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Bruant, P.,</td>
+      <td class="tdr"><a href="#Page_51">&nbsp;51</a></td>
+    </tr><tr>
+      <td class="tdl">Buffon,</td>
+      <td class="tdr"><a href="#Page_234">234</a></td>
+    </tr><tr>
+      <td class="tdl">Butler, S.,</td>
+      <td class="tdr"><a href="#Page_189">189-190</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Candolle, de,</td>
+      <td class="tdr"><br /><a href="#Page_245">245</a></td>
+    </tr><tr>
+      <td class="tdl">Carpenter, J. H.,</td>
+      <td class="tdr"><a href="#Page_172">172</a></td>
+    </tr><tr>
+      <td class="tdl">Chapman, F. M.,</td>
+      <td class="tdr"><a href="#Page_148">148</a>,
+                      <a href="#Page_156">156-158</a></td>
+    </tr><tr>
+      <td class="tdl">Chapman, T. A.,</td>
+      <td class="tdr"><a href="#Page_13">&nbsp;13</a>,
+                      <a href="#Page_167">167</a>,
+                      <a href="#Page_182">182</a>,
+                      <a href="#Page_231">231</a></td>
+    </tr><tr>
+      <td class="tdl">Church, A. H.,</td>
+      <td class="tdr"><a href="#Page_69">&nbsp;69</a></td>
+    </tr><tr>
+      <td class="tdl">Cieslar,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Clark, Austin,</td>
+      <td class="tdr"><a href="#Page_142">142</a>,
+                      <a href="#Page_144">144</a></td>
+    </tr><tr>
+      <td class="tdl">Cockayne, E. A.,</td>
+      <td class="tdr"><a href="#Page_43">&nbsp;43</a></td>
+    </tr><tr>
+      <td class="tdl">Cockerell, T. D. A.,</td>
+      <td class="tdr"><a href="#Page_224">224</a></td>
+    </tr><tr>
+      <td class="tdl">Compton, R. H.,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a>,
+                      <a href="#Page_58">&nbsp;58</a>,
+                      <a href="#Page_227">227</a></td>
+    </tr><tr>
+      <td class="tdl">Cope,</td>
+      <td class="tdr"><a href="#Page_230">230</a></td>
+    </tr><tr>
+      <td class="tdl">Cory,</td>
+      <td class="tdr"><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Correns,</td>
+      <td class="tdr"><a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl">Coutagne,</td>
+      <td class="tdr"><a href="#Page_125">125 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Darwin, on Variation,</td>
+      <td class="tdr"><br /><a href="#Page_1">&nbsp;&nbsp;1</a>,
+                      <a href="#Page_2">&nbsp;&nbsp;2</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Systematics,</td>
+      <td class="tdr"><a href="#Page_10">&nbsp;10</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Selection,</td>
+      <td class="tdr"><a href="#Page_134">134</a>,
+                      <a href="#Page_139">139</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;Heterostyle plants,</td>
+      <td class="tdr"><a href="#Page_236">236-237</a></td>
+    </tr><tr>
+      <td class="tdl">Darwin, F.,</td>
+      <td class="tdr"><a href="#Page_190">190</a></td>
+    </tr><tr>
+      <td class="tdl">Darwin, Sir G.,</td>
+      <td class="tdr"><a href="#Page_41">&nbsp;41</a></td>
+    </tr><tr>
+      <td class="tdl">Davenport,</td>
+      <td class="tdr"><a href="#Page_46">&nbsp;46</a></td>
+    </tr><tr>
+      <td class="tdl">Davis, H. M.,</td>
+      <td class="tdr"><a href="#Page_102">102</a></td>
+    </tr><tr>
+      <td class="tdl">Delcourt,</td>
+      <td class="tdr"><a href="#Page_130">130</a></td>
+    </tr><tr>
+      <td class="tdl">Deschange,</td>
+      <td class="tdr"><a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Dobell,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl">Doncaster,</td>
+      <td class="tdr"><a href="#Page_105">105</a>,
+                      <a href="#Page_121">121</a>,
+                      <a href="#Page_136">136</a></td>
+    </tr><tr>
+      <td class="tdl">Driesch,</td>
+      <td class="tdr"><a href="#Page_80">80-81</a></td>
+    </tr><tr>
+      <td class="tdl">Duchartre,</td>
+      <td class="tdr"><a href="#Page_51">&nbsp;51</a></td>
+    </tr><tr>
+      <td class="tdl"><br />East,</td>
+      <td class="tdr"><br /><a href="#Page_91">&nbsp;91</a>,
+                            <a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdl">Edwards, W. H.,</td>
+      <td class="tdr"><a href="#Page_162">162</a></td>
+    </tr><tr>
+      <td class="tdl">Ehrlich,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Fellmer,</td>
+      <td class="tdr"><br /><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl">Field, W. L. W.,</td>
+      <td class="tdr"><a href="#Page_161">161</a></td>
+    </tr><tr>
+      <td class="tdl">Fischer, E.,</td>
+      <td class="tdr"><a href="#Page_192">192</a></td>
+    </tr><tr>
+      <td class="tdl">Fleck,</td>
+      <td class="tdr"><a href="#Page_171">171</a>,
+                      <a href="#Page_174">174</a></td>
+    </tr><tr>
+      <td class="tdl">Fletcher, W. H. B.,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a>,
+                      <a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Foster, Sir N.,</td>
+      <td class="tdr"><a href="#Page_39">&nbsp;39</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Gallé,</td>
+      <td class="tdr"><br /><a href="#Page_123">123</a></td>
+    </tr><tr>
+      <td class="tdl">Garrod,</td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdl">Gates,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a>,
+                      <a href="#Page_95">&nbsp;95</a>,
+                      <a href="#Page_102">102</a></td>
+    </tr><tr>
+      <td class="tdl">Gayner, F.,</td>
+      <td class="tdr"><a href="#Page_177">177</a></td>
+    </tr><tr>
+      <td class="tdl">Godron,</td>
+      <td class="tdr"><a href="#Page_249">249</a></td>
+    </tr><tr>
+      <td class="tdl">Gold, E.,</td>
+      <td class="tdr"><a href="#Page_196">196</a></td>
+    </tr><tr>
+      <td class="tdl">Goldschmidt,</td>
+      <td class="tdr"><a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdl">Goodwin, E.,</td>
+      <td class="tdr"><a href="#Page_137">137</a></td>
+    </tr><tr>
+      <td class="tdl">Gortner,</td>
+      <td class="tdr"><a href="#Page_226">226</a></td>
+    </tr><tr>
+      <td class="tdl">Greene, E. L.,</td>
+      <td class="tdr"><a href="#Page_8">&nbsp;&nbsp;8</a></td>
+    </tr><tr>
+      <td class="tdl">Gregory, R. P.,</td>
+      <td class="tdr"><a href="#Page_92">&nbsp;92</a>,
+                      <a href="#Page_100">100</a>,
+                      <a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Grenier,</td>
+      <td class="tdr"><a href="#Page_249">249</a></td>
+    </tr><tr>
+      <td class="tdl">Grover,</td>
+      <td class="tdr"><a href="#Page_173">173</a></td>
+    </tr><tr>
+      <td class="tdl">Gruber,</td>
+      <td class="tdr"><a href="#Page_48">&nbsp;48</a></td>
+    </tr><tr>
+      <td class="tdl">Gulick,</td>
+      <td class="tdr"><a href="#Page_119">119</a>,
+                      <a href="#Page_133">133</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Hamling,</td>
+      <td class="tdr"><br /><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Hampson, Sir G.,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a></td>
+    </tr><tr>
+      <td class="tdl">Harris,</td>
+      <td class="tdr"><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Hartlaub,</td>
+      <td class="tdr"><a href="#Page_182">182</a></td>
+    </tr><tr>
+      <td class="tdl">Herbst,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Heribert-Nilsson,</td>
+      <td class="tdr"><a href="#Page_116">116</a></td>
+    </tr><tr>
+      <td class="tdl">Hewett,</td>
+      <td class="tdr"><a href="#Page_182">182</a></td>
+    </tr><tr>
+      <td class="tdl">Honing,</td>
+      <td class="tdr"><a href="#Page_105">105</a></td>
+    </tr><tr>
+      <td class="tdl">Hunter, John,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Jakowatz,</td>
+      <td class="tdr"><br /><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Janet,</td>
+      <td class="tdr"><a href="#Page_24">&nbsp;24</a></td>
+    </tr><tr>
+      <td class="tdl">Jeans,</td>
+      <td class="tdr"><a href="#Page_41">&nbsp;41</a></td>
+    </tr><tr>
+      <td class="tdl">Jenkinson,</td>
+      <td class="tdr"><a href="#Page_40">&nbsp;40</a></td>
+    </tr><tr>
+      <td class="tdl">Jentink,</td>
+      <td class="tdr"><a href="#Page_120">120</a></td>
+    </tr><tr>
+      <td class="tdl">Johannsen,</td>
+      <td class="tdr"><a href="#Page_195">195</a></td>
+    </tr><tr>
+      <td class="tdl">Jordan,</td>
+      <td class="tdr"><a href="#Page_185">185</a>,
+                      <a href="#Page_242">242</a>,
+                      <a href="#Page_249">249</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Kammere,</td>
+      <td class="tdr"><br /><a href="#Page_199">199 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Keeble,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Klebs,</td>
+      <td class="tdr"><a href="#Page_250">250</a></td>
+    </tr><tr>
+      <td class="tdl">Krancher,</td>
+      <td class="tdr"><a href="#Page_182">182</a></td>
+    </tr><tr>
+      <td class="tdl">Küchenmeister,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Kudicke,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Lamarck,</td>
+      <td class="tdr"><br /><a href="#Page_9">&nbsp;&nbsp;9</a></td>
+    </tr><tr>
+      <td class="tdl">Lang, A.,</td>
+      <td class="tdr"><a href="#Page_128">128</a></td>
+    </tr><tr>
+      <td class="tdl">Lawrence, W. N.,</td>
+      <td class="tdr"><a href="#Page_142">142</a>,
+                      <a href="#Page_145">145</a></td>
+    </tr><tr>
+      <td class="tdl">Leake, H. Martin,</td>
+      <td class="tdr"><a href="#Page_98">&nbsp;98</a>,
+                      <a href="#Page_100">100</a></td>
+    </tr><tr>
+      <td class="tdl">Leavitt,</td>
+      <td class="tdr"><a href="#Page_185">185</a></td>
+    </tr><tr>
+      <td class="tdl">Lecoq,</td>
+      <td class="tdr"><a href="#Page_99">&nbsp;99</a></td>
+    </tr><tr>
+      <td class="tdl">Lederer,</td>
+      <td class="tdr"><a href="#Page_167">167</a></td>
+    </tr><tr>
+      <td class="tdl">Leduc,</td>
+      <td class="tdr"><a href="#Page_64">&nbsp;64-65</a>,
+                      <a href="#Page_80">&nbsp;80</a></td>
+    </tr><tr>
+      <td class="tdl">Leydig,</td>
+      <td class="tdr"><a href="#Page_182">182</a></td>
+    </tr><tr>
+      <td class="tdl">Linden, M. von,</td>
+      <td class="tdr"><a href="#Page_192">192</a></td>
+    </tr><tr>
+      <td class="tdl">Linnaeus,</td>
+      <td class="tdr"><a href="#Page_6">6-8</a></td>
+    </tr><tr>
+      <td class="tdl">Lloyd, R. E.,</td>
+      <td class="tdr"><a href="#Page_248">248</a></td>
+    </tr><tr>
+      <td class="tdl">Locard,</td>
+      <td class="tdr"><a href="#Page_130">130</a></td>
+    </tr><tr>
+      <td class="tdl">Lock, R. H.,</td>
+      <td class="tdr"><a href="#Page_242">242</a>,
+                      <a href="#Page_244">244</a></td>
+    </tr><tr>
+      <td class="tdl">Loeb,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a>,
+                      <a href="#Page_45">&nbsp;45</a>,
+                      <a href="#Page_50">&nbsp;50</a>,
+                      <a href="#Page_71">&nbsp;71</a>,
+                      <a href="#Page_77">&nbsp;77</a></td>
+    </tr><tr>
+      <td class="tdl">Lotsy,</td>
+      <td class="tdr"><a href="#Page_99">&nbsp;99</a></td>
+    </tr><tr>
+      <td class="tdl">Lowe, P. R.,</td>
+      <td class="tdr"><a href="#Page_143">143</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Macdougal, W. T.,</td>
+      <td class="tdr"><br /><a href="#Page_102">102</a>,
+                            <a href="#Page_226">226</a></td>
+    </tr><tr>
+      <td class="tdl">Marchant,</td>
+      <td class="tdr"><a href="#Page_7">&nbsp;&nbsp;7</a></td>
+    </tr><tr>
+      <td class="tdl">Mathew,</td>
+      <td class="tdr"><a href="#Page_171">171</a></td>
+    </tr><tr>
+      <td class="tdl">Matthioli,</td>
+      <td class="tdr"><a href="#Page_4">&nbsp;&nbsp;4</a></td>
+    </tr><tr>
+      <td class="tdl">Mayer, A. G.,</td>
+      <td class="tdr"><a href="#Page_133">133</a></td>
+    </tr><tr>
+      <td class="tdl">Mendel, Rediscovery of,</td>
+      <td class="tdr"><a href="#Page_2">&nbsp;&nbsp;2</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;On Fasciation,</td>
+      <td class="tdr"><a href="#Page_49">&nbsp;49</a></td>
+    </tr><tr>
+      <td class="tdl">Merrifield,</td>
+      <td class="tdr"><a href="#Page_169">169</a>,
+                      <a href="#Page_172">172</a></td>
+    </tr><tr>
+      <td class="tdl">Miller, W. D.,</td>
+      <td class="tdr"><a href="#Page_120">120</a>,
+                      <a href="#Page_149">149</a></td>
+    </tr><tr>
+      <td class="tdl">Morgan,</td>
+      <td class="tdr"><a href="#Page_71">&nbsp;71</a>,
+                      <a href="#Page_77">&nbsp;77</a>,
+                      <a href="#Page_91">&nbsp;91</a>,
+                      <a href="#Page_198">198</a></td>
+    </tr><tr>
+      <td class="tdl">Moggridge,</td>
+      <td class="tdr"><a href="#Page_125">125</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Nathusius, S.,</td>
+      <td class="tdr"><br /><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Nettleship,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Newman, H. H.,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Newsholme,</td>
+      <td class="tdr"><a href="#Page_48">&nbsp;48</a></td>
+    </tr><tr>
+      <td class="tdl">Nilsson-Ehle,</td>
+      <td class="tdr"><a href="#Page_116">116</a>,
+                      <a href="#Page_169">169</a></td>
+    </tr><tr>
+      <td class="tdl">Norman, A. M.,</td>
+      <td class="tdr"><a href="#Page_125">125</a>,
+                      <a href="#Page_156">156</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Ober,</td>
+      <td class="tdr"><br /><a href="#Page_142">142</a></td>
+    </tr><tr>
+      <td class="tdl">Oberthür,</td>
+      <td class="tdr"><a href="#Page_168">168</a>,
+                      <a href="#Page_170">170</a>,
+                      <a href="#Page_193">193</a></td>
+    </tr><tr>
+      <td class="tdl">Oliver, J.,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Page, H. E.,</td>
+      <td class="tdr"><br /><a href="#Page_167">167</a>,
+                            <a href="#Page_180">180</a></td>
+    </tr><tr>
+      <td class="tdl">Patterson, J. T.,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Payne, F.,</td>
+      <td class="tdr"><a href="#Page_229">229</a></td>
+    </tr><tr>
+      <td class="tdl">Pellew,</td>
+      <td class="tdr"><a href="#Page_236">236</a></td>
+    </tr><tr>
+      <td class="tdl">Poll,</td>
+      <td class="tdr"><a href="#Page_45">&nbsp;45</a></td>
+    </tr><tr>
+      <td class="tdl">Porritt,</td>
+      <td class="tdr"><a href="#Page_136">136</a></td>
+    </tr><tr>
+      <td class="tdl">Poulton,</td>
+      <td class="tdr"><a href="#Page_141">141</a></td>
+    </tr><tr>
+      <td class="tdl">Powers, J. H.,</td>
+      <td class="tdr"><a href="#Page_230">230</a></td>
+    </tr><tr>
+      <td class="tdl">Pringsheim, H.,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Przibram,</td>
+      <td class="tdr"><a href="#Page_72">&nbsp;72</a>,
+                      <a href="#Page_78">&nbsp;78</a>,
+                      <a href="#Page_178">178</a>,
+                      <a href="#Page_194">194</a>,
+                      <a href="#Page_197">197</a>,
+                      <a href="#Page_199">199</a></td>
+    </tr><tr>
+      <td class="tdl">Punnett,</td>
+      <td class="tdr"><a href="#Page_110">110</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Ray,</td>
+      <td class="tdr"><br /><a href="#Page_4">4-5</a></td>
+    </tr><tr>
+      <td class="tdl">Raynor,</td>
+      <td class="tdr"><a href="#Page_105">105</a></td>
+    </tr><tr>
+      <td class="tdl">Ridgway,</td>
+      <td class="tdr"><a href="#Page_10">&nbsp;10</a>,
+                      <a href="#Page_120">120</a></td>
+    </tr><tr>
+      <td class="tdl">Roedelius,</td>
+      <td class="tdr"><a href="#Page_195">195</a></td>
+    </tr><tr>
+      <td class="tdl">Rolfe,</td>
+      <td class="tdr"><a href="#Page_20">&nbsp;20</a></td>
+    </tr><tr>
+      <td class="tdl">Rosen, F.,</td>
+      <td class="tdr"><a href="#Page_242">242</a></td>
+    </tr><tr>
+      <td class="tdl">Rosner,</td>
+      <td class="tdr"><a href="#Page_42">&nbsp;42</a></td>
+    </tr><tr>
+      <td class="tdl">Rowland-Brown, H.,</td>
+      <td class="tdr"><a href="#Page_167">167</a>,
+                      <a href="#Page_180">180</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Sargent,</td>
+      <td class="tdr"><br /><a href="#Page_185">185</a></td>
+    </tr><tr>
+      <td class="tdl">Saunders, E. R.,</td>
+      <td class="tdr"><a href="#Page_84">&nbsp;84</a>,
+                      <a href="#Page_104">104</a>,
+                      <a href="#Page_112">112</a></td>
+    </tr><tr>
+      <td class="tdl">Schima,</td>
+      <td class="tdr"><a href="#Page_177">177</a></td>
+    </tr><tr>
+      <td class="tdl">Schröder,</td>
+      <td class="tdr"><a href="#Page_193">193-194</a></td>
+    </tr><tr>
+      <td class="tdl">Schübeler,</td>
+      <td class="tdr"><a href="#Page_195">195</a></td>
+    </tr><tr>
+      <td class="tdl">Semon, R.,</td>
+      <td class="tdr"><a href="#Page_190">190 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl">Sharrock,</td>
+      <td class="tdr"><a href="#Page_5">&nbsp;&nbsp;5</a></td>
+    </tr><tr>
+      <td class="tdl">Shull,</td>
+      <td class="tdr"><a href="#Page_100">100</a></td>
+    </tr><tr>
+      <td class="tdl">Speyer, A.,</td>
+      <td class="tdr"><a href="#Page_166">166</a>,
+                      <a href="#Page_170">170</a>,
+                      <a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Spillman,</td>
+      <td class="tdr"><a href="#Page_47">&nbsp;47</a></td>
+    </tr><tr>
+      <td class="tdl">Standfuss,</td>
+      <td class="tdr"><a href="#Page_135">135</a>,
+                      <a href="#Page_181">181</a>,
+                      <a href="#Page_191">191</a></td>
+    </tr><tr>
+      <td class="tdl">Staples-Browne,</td>
+      <td class="tdr"><a href="#Page_49">&nbsp;49</a>,
+                      <a href="#Page_98">&nbsp;98</a></td>
+    </tr><tr>
+      <td class="tdl">Staudinger,</td>
+      <td class="tdr"><a href="#Page_170">170</a>,
+                      <a href="#Page_179">179</a></td>
+    </tr><tr>
+      <td class="tdl">Stockard,</td>
+      <td class="tdr"><a href="#Page_50">&nbsp;50</a>,
+                      <a href="#Page_71">&nbsp;71</a></td>
+    </tr><tr>
+      <td class="tdl">Sutton,</td>
+      <td class="tdr"><a href="#Page_236">236</a>,
+                      <a href="#Page_244">244</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Tornier,</td>
+      <td class="tdr"><br /><a href="#Page_72">&nbsp;72</a></td>
+    </tr><tr>
+      <td class="tdl">Tower, W. L.,</td>
+      <td class="tdr"><a href="#Page_218">218-226</a></td>
+    </tr><tr>
+      <td class="tdl">Trechmann,</td>
+      <td class="tdr"><a href="#Page_133">133</a></td>
+    </tr><tr>
+      <td class="tdl">Tugwell,</td>
+      <td class="tdr"><a href="#Page_181">181</a></td>
+    </tr><tr>
+      <td class="tdl">Tutt, J. W. On Definiteness of Species,</td>
+      <td class="tdr"><a href="#Page_13">&nbsp;13</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;On Plusia interrogationis,</td>
+      <td class="tdr"><a href="#Page_26">&nbsp;26</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;On Tephrosia,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;On N. castanea,</td>
+      <td class="tdr"><a href="#Page_122">122</a></td>
+    </tr><tr>
+      <td class="tdl">&emsp;On Pararge egeria,</td>
+      <td class="tdr"><a href="#Page_167">167 et seq.</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Verity, R.,</td>
+      <td class="tdr"><br /><a href="#Page_171">171</a>,
+                            <a href="#Page_177">177</a></td>
+    </tr><tr>
+      <td class="tdl">Vries, H. de,</td>
+      <td class="tdr"><a href="#Page_101">101-115</a>,
+                      <a href="#Page_222">222</a>,
+                      <a href="#Page_239">239</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Walker, G,</td>
+      <td class="tdr"><br /><a href="#Page_49">&nbsp;49</a></td>
+    </tr><tr>
+      <td class="tdl">Weir, Jenner,</td>
+      <td class="tdr"><a href="#Page_119">119</a></td>
+    </tr><tr>
+      <td class="tdl">Weismann,</td>
+      <td class="tdr"><a href="#Page_176">176</a>,
+                      <a href="#Page_188">188</a></td>
+    </tr><tr>
+      <td class="tdl">Wendelstadt,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl">Werbitzki,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl">Werner,</td>
+      <td class="tdr"><a href="#Page_209">209</a></td>
+    </tr><tr>
+      <td class="tdl">Wettstein,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Wheeler, G.,</td>
+      <td class="tdr"><a href="#Page_168">168</a>,
+                      <a href="#Page_171">171</a></td>
+    </tr><tr>
+      <td class="tdl">Wheldale,</td>
+      <td class="tdr"><a href="#Page_83">&nbsp;83</a></td>
+    </tr><tr>
+      <td class="tdl">Wilder,</td>
+      <td class="tdr"><a href="#Page_44">&nbsp;44</a></td>
+    </tr><tr>
+      <td class="tdl">Wille,</td>
+      <td class="tdr"><a href="#Page_197">197</a></td>
+    </tr><tr>
+      <td class="tdl">Williams, H.,</td>
+      <td class="tdr"><a href="#Page_167">167</a>,
+                      <a href="#Page_172">172</a></td>
+    </tr><tr>
+      <td class="tdl">Windle, B. C. A.,</td>
+      <td class="tdr"><a href="#Page_43">&nbsp;43</a></td>
+    </tr><tr>
+      <td class="tdl">Winslow,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Wolf, F.,</td>
+      <td class="tdr"><a href="#Page_213">213</a></td>
+    </tr><tr>
+      <td class="tdl">Woodforde,</td>
+      <td class="tdr"><a href="#Page_123">123</a></td>
+    </tr><tr>
+      <td class="tdl">Woltereck,</td>
+      <td class="tdr"><a href="#Page_215">215</a></td>
+    </tr><tr>
+      <td class="tdl"><br />Zeijlstra,</td>
+      <td class="tdr"><br /><a href="#Page_114">114</a></td>
+    </tr>
+ </tbody>
+</table>
+<hr class="chap space-below" />
+<div class="transnote">
+<p style="font-size: 150%; text-align: center;"><i>Transcriber's Notes</i>:</p>
+<p class="indent">
+   The illustrations have been moved so that they do not break up
+   paragraphs and so that they are next to the text they illustrate.</p>
+<p class="indent">
+   Errors in punctuation and inconsistent hyphenation were not corrected
+   unless otherwise noted.</p>
+</div>
+<hr class="chap" />
+<h2 class="u">FOOTNOTES:</h2>
+
+<div class="footnotes"><h3>FOOTNOTES:&emsp;CHAPTER I.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a>
+<a href="#FNanchor_1_1"><span class="label">[1]</span></a>
+In <i>Mendel's Principles of Heredity</i> (Cambridge University Press, 1909)
+I have dealt with this subject, giving an account of the principal
+facts discovered up to the beginning of 1909.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a>
+<a href="#FNanchor_2_2"><span class="label">[2]</span></a>
+Matthioli Opera, Ed. 1598, p. 8, originally published 1565.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a>
+<a href="#FNanchor_3_3"><span class="label">[3]</span></a>
+Ray's instances relate to Kales, and in most of these examples we can
+see that there was no question of mutation or transmutation at all, but that the
+occurrence was due either to mistake or to cross-fertilisation. Sharrock, to whom
+Ray refers, was inclined to discredit stories of transmutation, but he has also this
+passage (<i>History of the Propagation and Improvement of Vegetables by the
+Concurrence of Art and Nature</i>, Oxford, 1660, p. 29):</p>
+<p>"It is indeed grown to be a great question, whether the transmutation of a
+species be possible either in the vegetable, Animal, or Minerall Kingdome. For the
+possibility of it in the vegetable; I have heard <i>Mr. Bobart</i> and his <i>Son</i> often report it,
+and proffer to make oath that the Crocus and Gladiolus, as likewise the Leucoium,
+and Hyacinths by a long standing without replanting have in his garden changed
+from one kind to the other: and for satisfaction about the curiosity in the presence
+of <i>Mr. Boyle</i> I tooke up some bulbs of the very numericall roots whereof the relation
+was made, though the alteration was perfected before, where we saw the
+diverse bulbs growing as it were on the same stoole, close together, but no bulb
+half of the one kind, and the other half of the other: But the changetime being past
+it was reason we should believe the report of good artists in matters of their own
+faculty."</p>
+<p>Robert Sharrock was a fellow of New College, Oxford. Both the Bobarts were
+professional botanists, the father was author of a Catalogue of the plants in the
+Hortus Medicus at Oxford, and the son was afterwards Curator of the Oxford Garden.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a>
+<a href="#FNanchor_4_4"><span class="label">[4]</span></a>
+<i>Mém. Ac. roy. des Sci.</i> for 1719 (1721), p. 59.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a>
+<a href="#FNanchor_5_5"><span class="label">[5]</span></a>
+<i>Amoen. Acad.</i>, 1789, vol. 6. I do not know whether attention has been called
+to the curious mistake which Linnaeus makes in the course of this argument. He
+cites the differences between the Mule and the Hinny in illustration of his thesis,
+pointing out that the Mule is externally more like a horse and the Hinny more like
+an ass. This, he says, is because the Mule has the horse for a father, and the
+Hinny the ass, thus inverting the actual facts!</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a>
+<a href="#FNanchor_6_6"><span class="label">[6]</span></a>
+<i>Proc. Washington Ac. Sci.</i>, 1909, XI, pp. 17-26.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a>
+<a href="#FNanchor_7_7"><span class="label">[7]</span></a>
+J. W. Tutt, in <i>Ent. Rec.</i>, 1909, XXI, p. 185.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a>
+<a href="#FNanchor_8_8"><span class="label">[8]</span></a>
+E. Lehmann (<i>Bull. l'Herb. Boissier</i>, Ser. 2, VIII, 1908, p. 229) has published
+an admirable paper on the interrelationships of these species and has instituted
+cultural experiments which will probably much elucidate the nature of their specific
+distinctness. As regards the existence of intermediate forms he comes to the conclusion
+that two only can be so regarded. The first was described by Kuntze from
+specimens found on a flower-pot on board a Caspian steamer, from which Lehmann
+proposes the new specific name <i>Siaretensis</i>. This comes between <i>polita</i>
+and <i>filiformis</i>, a close ally of <i>Tournefortii</i>. The other, which combines
+some of the features of both <i>polita</i> and <i>Tournefortii</i>, was found in the
+province of Asterabad.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a>
+<a href="#FNanchor_9_9"><span class="label">[9]</span></a>
+In Cambridgeshire for example <i>vespertina</i> is common but <i>diurna</i> is absent.
+Whether this absence is connected with the general presence of chalk I cannot say.
+When introduced artificially <i>diurna</i> establishes itself, for a time at least,
+without any apparent difficulty and occasionally escapes from the garden on to the
+neighbouring roadside.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_10" id="Footnote_10_10"></a>
+<a href="#FNanchor_10_10"><span class="label">[10]</span></a>
+Conceivably however it may be a segregated combination. For an account
+of this plant see Boissier, <i>Voy. Bot. Midi de l'Espagne</i>, 1839, II, 722.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_11" id="Footnote_11_11"></a>
+<a href="#FNanchor_11_11"><span class="label">[11]</span></a>
+A discussion of this subject with references to literature is given by Rolfe,
+in an excellent paper on "Hybridisation viewed from the standpoint of Systematic
+Botany" (<i>Jour. R. Hort. Soc.</i>, XXIV, 1900, p. 197). He concludes: "The simple
+fact is that the two plants (<i>L. diurna</i> and <i>vespertina</i>) are thoroughly distinct
+in numerous particulars, and affect such different habitats that in some localities
+one or the other of them is completely wanting. But when their stations are
+adjacent they hybridise together very readily, and it is here that these intermediate
+forms occur which have puzzled botanists so much." The same paper contains
+valuable information concerning several cognate illustrations.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_12" id="Footnote_12_12"></a>
+<a href="#FNanchor_12_12"><span class="label">[12]</span></a>
+In only two cases have I seen such plants (both females) completely sterile.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_13" id="Footnote_13_13"></a>
+<a href="#FNanchor_13_13"><span class="label">[13]</span></a>
+As is well known, in an even more notorious example, he proposed to unite
+<i>Primula vulgaris</i>, <i>P. elatior</i>, and <i>P. acaulis</i>,
+similarly relying on the existence of "intermediates,"
+which we now well know to be mongrels between the species.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_14" id="Footnote_14_14"></a>
+<a href="#FNanchor_14_14"><span class="label">[14]</span></a>
+For an account of the distinctions between <i>Vespa vulgaris</i> and <i>germanica</i>
+see Ch. Janet, <i>Études sur les Fourmis, les Guêpes et les Abeilles</i>, 11<sup>e</sup>,
+Note. Sur <i>Vespa germanica</i> et <i>V. vulgaris</i>. Limoges (Ducourtieux),
+1895; and R. du Buysson, Monographie des Guêpes, <i>Ann. Soc. Ent. France</i>,
+1903, Vol. LXXII, p. 603, Pl. VIII.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_15" id="Footnote_15_15"></a>
+<a href="#FNanchor_15_15"><span class="label">[15]</span></a>
+The statements made above are for the most part taken from Barrett, C. G.,
+<i>Lepidoptera of the British Islands</i>, and from Tutt, J. W., <i>The British Noctuae and
+their Varieties</i>. The reader who is unfamiliar with the amazing polymorphism
+exhibited by some of these moths should if possible take an opportunity of looking
+over a long series in a collection, or, if that be impossible, refer to the admirable
+coloured plates published by Barrett. It may not be superfluous to observe that
+plenty of similar examples are known in other countries. For instance <i>Plotheia
+frontalis</i>, a Noctuid which often abounds in Ceylon, shows an equally bewildering
+wealth of forms. If a dozen specimens of such a species were to be brought home
+from some little known country, each individual would almost certainly be described
+as the type of a distinct species. (See the coloured plate published by Sir G. Hampson,
+Cat. Brit. Mus., Heterocera, Vol. IX.)</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_16" id="Footnote_16_16"></a>
+<a href="#FNanchor_16_16"><span class="label">[16]</span></a>
+<i>Dict. of Birds</i>, p. 800. It would be interesting and profitable to attempt in
+a long series of Ruffs to determine the Mendelian factors which by their combinations
+give rise to this complex assemblage of varietal forms. A few such factors both of
+colour and pattern can be at once distinguished, and it is noticeable that some of
+the resulting types of barring, spangling and penciling show a perceptible correspondence
+with some of the types of colouration found in the breeds of domestic fowls.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_17" id="Footnote_17_17"></a>
+<a href="#FNanchor_17_17"><span class="label">[17]</span></a>
+Howard Saunders (<i>Illust. Manual of British Birds</i>, 1899, p. 499) states that
+there is evidence that the pheasant had become naturalized in the south of England
+before the Norman invasion. He adds, "little, if any, deviation from the typical
+<i>P. colchicus</i> took place up to the end of last century, when the introduction of
+the Chinese Ring-necked <i>P. torquatus</i> commenced, which has left almost indelible
+marks, especially with regard to the characteristic white collar."</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER II.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_18" id="Footnote_1_18"></a>
+<a href="#FNanchor_1_18"><span class="label">[1]</span></a>
+In saying this we make no assumption as to the particular cell-division at
+which differentiation occurs. This may be one of the maturation-divisions, or it
+may perhaps be much earlier.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_19" id="Footnote_2_19"></a>
+<a href="#FNanchor_2_19"><span class="label">[2]</span></a>
+From the recent discoveries of Erwin Baur we are led to surmise that in the
+flowering plants the sub-epidermal layer, or some of its elements, may legitimately
+be regarded as a similar germ-substance, continuous in Weismann's sense.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_20" id="Footnote_3_20"></a>
+<a href="#FNanchor_3_20"><span class="label">[3]</span></a>
+These fraternal twins, which show no special resemblance to each other,
+are like the multiple births of other animals, and there is no disposition for them to
+be of the same sex. In the sheep, for example, statistics show that the frequency
+of pairs of twins, male and female, is approximately double that of the frequency
+of pairs, both male or both female, as it should be if the sex-distribution were fortuitous.
+For instance Bernadin (<i>La Bergerie de Rambouillet</i>, 1890, p. 100) gives
+the following figures for twin-lambs in Merinos: both male, 87; both female, 83;
+sexes mixed, 187. The 9-banded Armadillo (<i>Dasypus novemcinctus</i>), in which
+the young born in one litter are said to be always of one sex, is the only known
+exception in Vertebrates, and is presumably a genuine case of normal polyembryony
+(see especially, Rosner, <i>Bull. Ac. Soc. Cracovie</i>, 1901, p. 443, and Newman and
+Patterson, <i>Biol. Bull.</i>, XVII, 1909, p. 181), and an important paper lately published
+by H. H. Newman and J. T. Patterson, <i>Jour. Morph.</i>, 1911, XXII, p. 855.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_21" id="Footnote_4_21"></a>
+<a href="#FNanchor_4_21"><span class="label">[4]</span></a>
+A good collection of evidence as to disease in homologous twins was lately
+published by E. A. Cockayne, <i>Brit. Jour. Child. Diseases</i>, Nov., 1911.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_22" id="Footnote_5_22"></a>
+<a href="#FNanchor_5_22"><span class="label">[5]</span></a>
+Cp. Windle, B. C. A., <i>Jour. Anal. Phys.</i>, XXVI, p. 295.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_23" id="Footnote_6_23"></a>
+<a href="#FNanchor_6_23"><span class="label">[6]</span></a>
+Mr. E. Nettleship tells me that in the course of collecting pedigrees of families
+containing colour-blind members he has discovered two cases (shortly to be published)
+of pairs of twins, which on account of their very close resemblances must
+be deemed homologous, one of each pair being colour-blind and the other normal.
+Such a distinction between closely similar twins is most curious and unexpected.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_24" id="Footnote_7_24"></a>
+<a href="#FNanchor_7_24"><span class="label">[7]</span></a>
+Another paradoxical phenomenon of the same nature occurs in the Narwhal
+The males normally have the <i>left</i> tusk alone developed, the corresponding right
+tusk remaining as an undeveloped rudiment in its socket. The left tusk is a
+left-handed screw. Occasionally the right tusk is also developed and grows to
+the same length as that of the left side, but in such specimens the right tusk is
+also a left-hand screw like the tusk of the other side, instead of being reversed
+as we should certainly have expected. It need scarcely be remarked that in the
+case of the horns of antelopes, and in other examples of spiral organs arranged in
+pairs, that of one side of the body is the mirror image of that on the other side.
+The Narwhal's tusks in being both twisted in the same direction are thus highly
+anomalous, and are comparable with pairs of twins.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_25" id="Footnote_8_25"></a>
+<a href="#FNanchor_8_25"><span class="label">[8]</span></a>
+Wilder, H. H., <i>Amer. Jour. Anat.</i>, 1904, III, p. 452.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_26" id="Footnote_9_26"></a>
+<a href="#FNanchor_9_26"><span class="label">[9]</span></a>
+Polydactylism which is often a dominant and the web-foot of Pigeons which
+is recessive should be remembered as possible exceptions (see p. 49).</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_27" id="Footnote_10_27"></a>
+<a href="#FNanchor_10_27"><span class="label">[10]</span></a>
+Davenport inclined at first to regard rumplessness as a recessive, but in his
+latest publication on the subject he definitely concludes that it is an imperfect
+dominant. This conclusion accords well with evidence quoted by Darwin (<i>An.
+and Plts.</i>, II, ed. 2, p. 4) that rumpless fowls may throw tailed offspring.
+(<i>Amer. Nat.</i>, 1910, XLIV, p. 134.)</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_28" id="Footnote_11_28"></a>
+<a href="#FNanchor_11_28"><span class="label">[11]</span></a>
+Spillman, W. J., <i>Amer. Breeders Mag.</i>, 1910, I, p. 178.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_29" id="Footnote_12_29"></a>
+<a href="#FNanchor_12_29"><span class="label">[12]</span></a>
+Newsholme, <i>Lancet</i>, December 10, 1910, p. 1690.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_30" id="Footnote_13_30"></a>
+<a href="#FNanchor_13_30"><span class="label">[13]</span></a>
+<i>Materials for the Study of Variation</i>, 1894, p. 358.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_31" id="Footnote_14_31"></a>
+<a href="#FNanchor_14_31"><span class="label">[14]</span></a>
+Walker, G., <i>Johns Hopkins Hospital Bulletin</i>, XII, 1901, p. 129.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_32" id="Footnote_15_32"></a>
+<a href="#FNanchor_15_32"><span class="label">[15]</span></a>
+Cp. R. H. Compton, <i>New Phytologist</i>, 1911, p. 249.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_33" id="Footnote_16_33"></a>
+<a href="#FNanchor_16_33"><span class="label">[16]</span></a>
+<i>Arch. f. Entwickelungsmech.</i>, 1907, XXIII, p. 249.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_34" id="Footnote_17_34"></a>
+<a href="#FNanchor_17_34"><span class="label">[17]</span></a>
+Bull. Soc. Bot. de France, xxxiv, 1887, p. 182.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_35" id="Footnote_18_35"></a>
+<a href="#FNanchor_18_35"><span class="label">[18]</span></a>
+R. Boyle, <i>The Origine of Formes and Qualities</i>, Oxford, 1666.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_36" id="Footnote_19_36"></a>
+<a href="#FNanchor_19_36"><span class="label">[19]</span></a>
+Remarkable experiments on this question have lately been carried out by
+R. H. Compton (<i>Camb. Phil. Soc.</i>, XV, 1910, p. 495), showing that in a certain
+Barley, "Plumage Corn," the average ratio of left to right is about 1.5. A fuller
+paper has since been published by Compton, <i>Jour. Genetics</i>, 1912, II, I, p. 53.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER III.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_37" id="Footnote_1_37"></a>
+<a href="#FNanchor_1_37"><span class="label">[1]</span></a>
+Stéphane Leduc, <i>Théorie Physico-Chymique de la Vie</i>, Paris, 1910.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_38" id="Footnote_2_38"></a>
+<a href="#FNanchor_2_38"><span class="label">[2]</span></a>
+<i>Materials for the Study of Variation</i>, No. 249, p. 217; and p. 272.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_39" id="Footnote_3_39"></a>
+<a href="#FNanchor_3_39"><span class="label">[3]</span></a>
+<i>Materials</i>, p. 118.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_40" id="Footnote_4_40"></a>
+<a href="#FNanchor_4_40"><span class="label">[4]</span></a>
+Church, A. H., <i>On the Relation of Phyllotaxis to Mechanical Laws</i>, London, 1904.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_41" id="Footnote_5_41"></a>
+<a href="#FNanchor_5_41"><span class="label">[5]</span></a>
+It is a question whether the dominance of the palmatifid leaf
+over the pinnatifid is not really an example of the dominance of a lower
+number of segmentations over a higher. From the uncertainty whether two
+given leaves of two separate plants are actually comparable one cannot
+institute quite satisfactory numerical comparisons, but I think the view
+that the "Fern" leaf has more lobes than an otherwise similar "Palm" leaf
+may be fairly maintained. If this be admitted, the "Palm" leaf represents
+the dominant low number and its round shape is a consequence of the greater
+powers of growth which are so often possessed by the members of a shorter series.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_42" id="Footnote_6_42"></a>
+<a href="#FNanchor_6_42"><span class="label">[6]</span></a>
+It is perhaps of importance to remember that in certain species of bacteria
+(e. g. <i>Bacillus Anthracis</i>) division may cease where the organism is
+cultivated under certain artificial conditions though growth continues.
+In this way very long unsegmented threads are produced.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_43" id="Footnote_7_43"></a>
+<a href="#FNanchor_7_43"><span class="label">[7]</span></a>
+<i>Arch. f. Entwm.</i>, XX, 1905, p. 76;
+<i>Sitzungsb. d. Ges. Naturf.</i>, Berlin, 1907, p. 41, etc.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_44" id="Footnote_8_44"></a>
+<a href="#FNanchor_8_44"><span class="label">[8]</span></a>
+Borradaile, L. A., <i>Jour. Marine Zool.</i>, 1897, No. 8.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_45" id="Footnote_9_45"></a>
+<a href="#FNanchor_9_45"><span class="label">[9]</span></a>
+Dr. Przibram, I should mention, concludes that on the whole the facts are
+against this interpretation, but as more evidence is certainly required,
+I call attention to the possibility.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_46" id="Footnote_10_46"></a>
+<a href="#FNanchor_10_46"><span class="label">[10]</span></a>
+ Morgan, T. H., <i>Regeneration</i>, 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_47" id="Footnote_11_47"></a>
+<a href="#FNanchor_11_47"><span class="label">[11]</span></a>
+It would be interesting to know whether growth continues at the original
+posterior end after the new "posterior" end has been formed in front.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_48" id="Footnote_12_48"></a>
+<a href="#FNanchor_12_48"><span class="label">[12]</span></a>
+In the actual case observed, the ripples unsmoothed had a wave-length of
+about 2-1/2 inches; and when the new ones were first formed, there were
+about 30 ridges in the length originally traversed by 15 or 16.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_49" id="Footnote_13_49"></a>
+<a href="#FNanchor_13_49"><span class="label">[13]</span></a>
+<i>The Science and Philosophy of the Organism</i>;
+Gifford Lectures, 1907. London, 1908, p. 141.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER IV.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_50" id="Footnote_1_50"></a>
+<a href="#FNanchor_1_50"><span class="label">[1]</span></a>
+Gates, R. R., <i>Zts. f. Abstammungslehre</i>, 1911, IV, pp. 341 and 361.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER V.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_51" id="Footnote_1_51"></a>
+<a href="#FNanchor_1_51"><span class="label">[1]</span></a>
+See Lotsy and Baur, Rep. Genetics Conf., Paris, 1911, pp. 416-426.
+Compare Lecoq on <i>Mirabilis jalapa</i> × <i>longiflora</i>,
+Fécondation des Végétaux, 1862, p. 311.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_52" id="Footnote_2_52"></a>
+<a href="#FNanchor_2_52"><span class="label">[2]</span></a>
+<i>Rep. Evol. Ctee. R. S.</i>, IV, 1908, p. 38.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_53" id="Footnote_3_53"></a>
+<a href="#FNanchor_3_53"><span class="label">[3]</span></a>
+<i>Ber. Deut. Bot. Ges.</i>, 1908, XXVI, <i>a</i>, p. 672.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_54" id="Footnote_4_54"></a>
+<a href="#FNanchor_4_54"><span class="label">[4]</span></a>
+<i>Jour. Genetics</i>, 1, 1910, p. 57.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_55" id="Footnote_5_55"></a>
+<a href="#FNanchor_5_55"><span class="label">[5]</span></a>
+In Rep. 1 to Evol. Committee, 1902, p. 132, attention was called
+to this possibility, though of course at that date it was in sexual animals alone
+that it was supposed to exist. It had not occurred to me that even a hermaphrodite
+plant might be in this condition.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_56" id="Footnote_6_56"></a>
+<a href="#FNanchor_6_56"><span class="label">[6]</span></a>
+From the description of the offspring of <i>muricata</i> used as mother.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_57" id="Footnote_7_57"></a>
+<a href="#FNanchor_7_57"><span class="label">[7]</span></a>
+de Vries, <i>Species and Varieties</i>, 1905, p. 259.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_58" id="Footnote_8_58"></a>
+<a href="#FNanchor_8_58"><span class="label">[8]</span></a>
+Zeijlstra in a recent paper announces that many <i>nanella</i>
+plants are the subject of a bacterial disease to which he attributes their
+dwarfness. I gather that this does not apply to all <i>nanella</i> plants and
+that some are dwarfs apart from disease. The matter may no doubt be further
+complicated from this cause.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_59" id="Footnote_9_59"></a>
+<a href="#FNanchor_9_59"><span class="label">[9]</span></a>
+<i>Zts. f. Abstamm.</i>, 1912, VIII.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_60" id="Footnote_10_60"></a>
+<a href="#FNanchor_10_60"><span class="label">[10]</span></a>
+<i>Arch. f. Zellforschung</i>, 1912, IX, p. 331.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER VI.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_61" id="Footnote_1_61"></a>
+<a href="#FNanchor_1_61"><span class="label">[1]</span></a>
+For the evidence see Tutt, J. W., <i>Trans. Ent. Soc.</i>, 1898, p. 17.
+Compare the remarkable case given by Gulick
+(<i>Evolution Racial and Habitudinal</i>, p. 123) of the two races of
+<i>Cicada</i>, which are separated by reason of their life-cycles,
+one having a period of 13, the other 17 years.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_62" id="Footnote_2_62"></a>
+<a href="#FNanchor_2_62"><span class="label">[2]</span></a>
+For references see <i>Materials</i>, p. 396, and also G. Baur,
+<i>Amer. Nat.</i>, 1893, July, p. 677.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_63" id="Footnote_3_63"></a>
+<a href="#FNanchor_3_63"><span class="label">[3]</span></a>
+Jenner Weir, <i>Entomologist</i>, 1880, XIII, p. 251.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_64" id="Footnote_4_64"></a>
+<a href="#FNanchor_4_64"><span class="label">[4]</span></a>
+Jentink, <i>Notes Leyden Mus.</i>, 1885, VII, p. 111.
+Specimens illustrating this peculiarity are in the British Museum.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_65" id="Footnote_5_65"></a>
+<a href="#FNanchor_5_65"><span class="label">[5]</span></a>
+<i>Proc. Zool. Soc.</i>, 1895, p. 850. Plate. Many points beyond that mentioned
+above are involved in this remarkable case. For example, not only are there males
+like females, but a small proportion of females resemble the ordinary male type.
+The stripes are not merely the spots produced, for they occupy different anatomical
+positions. The spots almost always go with a black ventral surface, but the striped
+forms nearly always have that region testaceous. <i>Spartium retama</i>, the food-plant,
+will not grow in England, but if it could be naturalised in America the whole problem
+might be investigated there and results of exceptional interest would almost certainly
+be attained.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_66" id="Footnote_6_66"></a>
+<a href="#FNanchor_6_66"><span class="label">[6]</span></a>
+Doncaster, L., <i>Proc. Zool. Soc.</i>, 1905, II, p. 528.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_67" id="Footnote_7_67"></a>
+<a href="#FNanchor_7_67"><span class="label">[7]</span></a>
+I am not aware that the details of this striking case have ever been worked
+out. It should be noted that the green and blue forms are not due to simple modification
+of the red pigment; for these colours, due to interference, fork over the
+area occupied by the red lines. The distinctions between these forms cannot
+therefore be simply chemical, as we may suppose them to be, for instance, in the
+case of many red and yellow forms, and the genetic relationships of the <i>Heliconid</i>
+varieties would raise many novel problems and be well worth studying experimentally.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_68" id="Footnote_8_68"></a>
+<a href="#FNanchor_8_68"><span class="label">[8]</span></a>
+Woodeforde, F. C., <i>Trans. North Staffordshire Field Club</i>, XXXV, 1901, Plate.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_69" id="Footnote_9_69"></a>
+<a href="#FNanchor_9_69"><span class="label">[9]</span></a>
+E. Gallé, <i>Compte Rendus du Congres Internat. de Bot. a l'Expos. Univ.</i>,
+1900, p. 112.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_70" id="Footnote_10_70"></a>
+<a href="#FNanchor_10_70"><span class="label">[10]</span></a>
+Flora of Mentone, 1864-8, <i>Nova Acta Acad. Caes.</i>, XXXV, 1869.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_71" id="Footnote_11_71"></a>
+<a href="#FNanchor_11_71"><span class="label">[11]</span></a>
+I owe these facts to Canon A. M. Norman, who showed me illustrative
+specimens. They were originally described by Bowerbank (<i>Monogr. Brit. Spongiadae</i>,
+vol. II, pp. 18 and XX; vol. III, Pls. I and III). A specimen of <i>G. compressa</i>
+measured 5 inches, with a greatest width of 3-1/4 in. <i>G. ciliata</i> was found measuring
+3 in. long and 3/4 in. wide. These dimensions are many times those of normal
+specimens.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_72" id="Footnote_12_72"></a>
+<a href="#FNanchor_12_72"><span class="label">[12]</span></a>
+Coutagne, G., <i>Recherches sur le Polymorphisme des Mollusques de France</i>,
+<i>Annales Soc. d'Agric. Sci. et Industr. Lyon</i>, 1895.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_73" id="Footnote_13_73"></a>
+<a href="#FNanchor_13_73"><span class="label">[13]</span></a>
+As to the synonymy and references see Coutagne, p. 45.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_74" id="Footnote_14_74"></a>
+<a href="#FNanchor_14_74"><span class="label">[14]</span></a>
+A. Lang, <i>Die Bastarde von H. hortensis Muller H. nemoralis L.</i> Jena, G. Fischer, 1908;
+with a fine coloured plate showing the varieties of the species and their hybrids.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_75" id="Footnote_15_75"></a>
+<a href="#FNanchor_15_75"><span class="label">[15]</span></a>
+With this evidence compare that given by A. Delcourt in
+his valuable papers lately published relating to the variations of
+<i>Notonecta</i>. See especially <i>Bull. Sci. Fr. Belg.</i>, 1909,
+XLIII, p. 443; and <i>C. R. Soc. Biol.</i>, 1909, LXVI, p. 589.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_76" id="Footnote_16_76"></a>
+<a href="#FNanchor_16_76"><span class="label">[16]</span></a>
+Allen, J. A., <i>Bull. Amer. Mus. N. H.</i>, III, 1891, pp. 51-54.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_77" id="Footnote_17_77"></a>
+<a href="#FNanchor_17_77"><span class="label">[17]</span></a>
+J. T. Gulick, <i>Evolution, Racial and Habitudinal</i>,
+Carnegie Institution, Publication No. 25, 1905.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_78" id="Footnote_18_78"></a>
+<a href="#FNanchor_18_78"><span class="label">[18]</span></a>
+A. G. Mayer, <i>Mem. Mus. Comp. Anat. Harvard</i>, Vol. XXVI, 1902, p. 117.
+From the tables given I cannot ascertain the actual numbers from the two
+intermediate valleys, but they were considerable.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_79" id="Footnote_19_79"></a>
+<a href="#FNanchor_19_79"><span class="label">[19]</span></a>
+To which I was very kindly guided by Mr. C. T. Trechmann.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_80" id="Footnote_20_80"></a>
+<a href="#FNanchor_20_80"><span class="label">[20]</span></a>
+Standfuss, <i>Handbuch d. paläarkt Gross-schmet</i>, 1896, p. 321.</p></div>
+
+<div class="footnote"><p><a name="Footnote_21_81" id="Footnote_21_81"></a>
+<a href="#FNanchor_21_81"><span class="label">[21]</span></a>
+<i>Ent. Rec.</i>, XVIII, No. 7, 1906.</p></div>
+
+<div class="footnote"><p><a name="Footnote_22_82" id="Footnote_22_82"></a>
+<a href="#FNanchor_22_82"><span class="label">[22]</span></a>
+This evidence was largely collected by Mr. G. T. Porritt,
+who has given much attention to the subject.</p></div>
+
+<div class="footnote"><p><a name="Footnote_23_83" id="Footnote_23_83"></a>
+<a href="#FNanchor_23_83"><span class="label">[23]</span></a>
+Such direct action has of course been proved to occur in the
+case of several dimorphic larvae (<i>e. g.</i>, <i>A. betularia</i>,
+itself) and pupae.</p></div>
+
+<div class="footnote"><p><a name="Footnote_24_84" id="Footnote_24_84"></a>
+<a href="#FNanchor_24_84"><span class="label">[24]</span></a>
+See Harris, <i>Proc. Ent. Soc. London</i>, 1904, p. lxxii, and 1905, p. lxiii;
+also Hamling, <i>Trans. City of London Ent. Soc.</i>, 1905, p. 5.</p></div>
+
+<div class="footnote"><p><a name="Footnote_25_85" id="Footnote_25_85"></a>
+<a href="#FNanchor_25_85"><span class="label">[25]</span></a>
+I am indebted to Mr. Outram Bangs of the Harvard Museum
+for calling my attention to this remarkable case.</p></div>
+
+<div class="footnote"><p><a name="Footnote_26_86" id="Footnote_26_86"></a>
+<a href="#FNanchor_26_86"><span class="label">[26]</span></a>
+<i>Auk</i>, 1889, VI, p. 219.</p></div>
+
+<div class="footnote"><p><a name="Footnote_27_87" id="Footnote_27_87"></a>
+<a href="#FNanchor_27_87"><span class="label">[27]</span></a>
+<i>Ann. N. Y. Acad. Sci.</i>, 1878, I, p. 149.</p></div>
+
+<div class="footnote"><p><a name="Footnote_28_88" id="Footnote_28_88"></a>
+<a href="#FNanchor_28_88"><span class="label">[28]</span></a>
+<i>Ann. N. Y. Acad. Sci.</i>, 1878, I, p. 149.</p></div>
+
+<div class="footnote"><p><a name="Footnote_29_89" id="Footnote_29_89"></a>
+<a href="#FNanchor_29_89"><span class="label">[29]</span></a>
+<i>Ibid</i>, 1912, pp. 523-8.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER VII.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_90" id="Footnote_1_90"></a>
+<a href="#FNanchor_1_90"><span class="label">[1]</span></a>
+J. A. Allen, <i>The North American Species of the Genus Colaptes,
+Considered with Special Reference to the Relationships of C. auratus and
+C. cafer</i>. Bull. Am. Mus. Nat. Hist., IV, 1892.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_91" id="Footnote_2_91"></a>
+<a href="#FNanchor_2_91"><span class="label">[2]</span></a>
+For a case in which a red-headed female × a black-headed male
+gave a black-headed female and a red-headed male, see <i>Avian Mag.</i>,
+N. S., IV, pp. 49 and 329</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_92" id="Footnote_3_92"></a>
+<a href="#FNanchor_3_92"><span class="label">[3]</span></a>
+The other variations of this bird are also interesting and important. The
+normal male has a red head and a red throat. The female has a red head and a
+white throat, but varieties of the female are known with a black head, thus again
+illustrating the change from black to red. It should be noted that this is not a
+mere retention of a juvenile character, but, as the birds mature, the red feathers
+come up, or as an exception, the black. There is also a western species, <i>ruber</i>,
+in which both sexes have a great extension of red, and are alike. The male of
+<i>nuchalis</i> intergrades with this type, but the female does not.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_93" id="Footnote_4_93"></a>
+<a href="#FNanchor_4_93"><span class="label">[4]</span></a>
+Dr. W. Brewster, for example, has a remarkable specimen
+of the Teal (<i>Nettion carolinense</i>) with a white collar strongly
+developed at the front and sides of the neck, in a place where the
+normal has no such mark.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_94" id="Footnote_5_94"></a>
+<a href="#FNanchor_5_94"><span class="label">[5]</span></a>
+This variety is spoken of as the Ringed Guillemot and is sometimes regarded
+as a distinct species to which the name <i>ringvia</i> was given by Brünnich. In support
+of this view Dr. William Brewster, to whom I am indebted for much assistance
+in regard to the variation of birds, called my attention to observations of his
+own and also of Maynard's, that the ringed birds were sometimes mated together,
+though in a small minority (see Brewster, <i>Proc. Boston Soc. N. H.</i>, XXII, 1883, p.
+410). It would however be possible to produce many instances of varieties mated
+together though surrounded by a typical population (<i>e. g.</i>, two varying Blackbirds,
+<i>Zoologist</i>, p. 2765; two varying Nightjars, <i>ibid.</i>, p. 5278). I am inclined to
+believe that in nature matings between brothers and sisters are frequent in many
+species of animals, and that the production of sporadically varying colonies is
+thus greatly assisted.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_95" id="Footnote_6_95"></a>
+<a href="#FNanchor_6_95"><span class="label">[6]</span></a>
+The Sap-suckers feed on trees and somewhat resemble our Spotted Woodpeckers
+in general appearance. <i>Colaptes</i> feeds on the ground and corresponds
+perhaps rather with the European Green Woodpecker.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_96" id="Footnote_7_96"></a>
+<a href="#FNanchor_7_96"><span class="label">[7]</span></a>
+For an introduction to this example I am indebted to Mr. W. D. Miller of
+the American Museum of Natural History. Some account of the facts is given by
+Baird, Brewer, and Ridgway (<i>A Hist. of N. Amer. Birds</i>. 1874, II, pp. 540, 544,
+etc.). <i>S. varius</i> occupies the whole country in suitable places from the Atlantic
+to the eastern slopes of the Rockies, and all Mexico to Guatemala. <i>S. nuchalis</i> was
+first known from the Southern Rockies only, but many were afterwards taken in
+Utah. <i>S. ruber</i> is restricted to the Pacific coast. In Ridgway's opinion all three
+are geographical forms of one species. In <i>ruber</i> the sexes are alike having both a
+great extension of the red in the throat, and a red crescent. The male of <i>nuchalis</i>
+grades to the <i>ruber</i> form, but the female does not. This female has some red in
+the throat like the male of <i>varius</i>, whereas the female of <i>varius</i> has
+a whitish throat.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_97" id="Footnote_8_97"></a>
+<a href="#FNanchor_8_97"><span class="label">[8]</span></a>
+Not only vertebrates but the marine Crustacea and Mollusca illustrate
+this curious "principle" of variation, as Canon Norman formerly pointed out to me
+with abundant illustrations. There are of course cases to the contrary also.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_98" id="Footnote_9_98"></a>
+<a href="#FNanchor_9_98"><span class="label">[9]</span></a>
+Chapman, F. M., <i>Bull. Amer. Mus.</i>, IV, 1892, p. 1;
+see also Ridgway, <i>Birds of North and Middle America</i>, 1902, Part II, p. 214.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_99" id="Footnote_10_99"></a>
+<a href="#FNanchor_10_99"><span class="label">[10]</span></a>
+It would aid greatly in factorial analysis if the descriptive term "green"
+could be avoided in application to cases where the green effect is due only to a mixture
+of black and yellow pigments. The absence of yellow is the sole difference
+between the mantle and underparts of <i>pinus</i> and <i>chrysoptera</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_100" id="Footnote_11_100"></a>
+<a href="#FNanchor_11_100"><span class="label">[11]</span></a>
+<i>Bull. Amer. Mus. Nat. Hist.</i>, XXIII, 1907, p. 467.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_101" id="Footnote_12_101"></a>
+<a href="#FNanchor_12_101"><span class="label">[12]</span></a>
+References on this subject will be found in
+<i>Brit. Mus. Cat. Birds</i>, XVII, p. 13.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_102" id="Footnote_13_102"></a>
+<a href="#FNanchor_13_102"><span class="label">[13]</span></a>
+For these facts I am indebted to Mr. W. L. W. Field,
+who has lately published an account of his observations and experiments.
+See especially, <i>Psyche</i>, 1910, XVII, No. 3, where full references
+to previous publications are given.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_103" id="Footnote_14_103"></a>
+<a href="#FNanchor_14_103"><span class="label">[14]</span></a>
+For the facts and further references see W. H. Edwards,
+<i>Butterflies of N. America</i>, 2d series, Papilio VII and X;
+3d series, 1897, Papilio IV, <i>Can. Entom.</i>, 1895, XXVII, p. 239.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_104" id="Footnote_15_104"></a>
+<a href="#FNanchor_15_104"><span class="label">[15]</span></a>
+I think this case is fairly included because the <i>machaon</i>
+type is so widespread that it cannot be regarded as a product of a Northern
+climate, nor can <i>asterias</i> be claimed as especially a warm country form,
+seeing that <i>brevicauda</i>, which is scarcely distinguishable from
+<i>asterias</i>, inhabits Newfoundland (having a curious phase there in which
+the yellow is largely replaced by red).</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER VIII.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_105" id="Footnote_1_105"></a>
+<a href="#FNanchor_1_105"><span class="label">[1]</span></a>
+Often referred to by older writers as <i>Meone</i>, Esper's name.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_106" id="Footnote_2_106"></a>
+<a href="#FNanchor_2_106"><span class="label">[2]</span></a>
+There are also two distinct island forms, unlike the European,
+<i>Xiphia</i> of Madeira, and a smaller variety, <i>Xiphioides</i> of Canary.
+See especially, Baker, G. T., <i>Trans. Ent. Soc. London</i>, 1891, p. 292.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_107" id="Footnote_3_107"></a>
+<a href="#FNanchor_3_107"><span class="label">[3]</span></a>
+Speyer, Adolf, and August. <i>Verbreitung der Schmetterlinge</i>, 1858, I, p. 217.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_108" id="Footnote_4_108"></a>
+<a href="#FNanchor_4_108"><span class="label">[4]</span></a>
+<i>Lepid. Comparée</i>, fsc. III, p. 372.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_109" id="Footnote_5_109"></a>
+<a href="#FNanchor_5_109"><span class="label">[5]</span></a>
+Mr. Rowland-Brown has called my attention to a statement by Dr. Vaillantin
+(<i>Petites Nouv. Ent.</i>, II, 235) that in Indre-et-Cher the first brood is of the
+northern type and the second of the southern. My experience is that in captivity these
+distinctions do not occur, and I have true <i>egeria</i> as first brood from Vienne and
+as the late brood from the Landes. I never collected in Indre-et-Cher.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_110" id="Footnote_6_110"></a>
+<a href="#FNanchor_6_110"><span class="label">[6]</span></a>
+I have since seen true <i>egeria</i> from Ferrol in the
+extreme northwest, which was in Mr. Tutt's collection.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_111" id="Footnote_7_111"></a>
+<a href="#FNanchor_7_111"><span class="label">[7]</span></a>
+Mr. G. Wheeler kindly showed me a series identical with
+this type, from Guernsey, and others from near Laon.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_112" id="Footnote_8_112"></a>
+<a href="#FNanchor_8_112"><span class="label">[8]</span></a>
+<i>Ent. Rec.</i>, V, 1894, p. 134.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_113" id="Footnote_9_113"></a>
+<a href="#FNanchor_9_113"><span class="label">[9]</span></a>
+Mr. Wheeler has some pale but rather worn specimens
+from the Rhone Valley at Vernayaz.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_114" id="Footnote_10_114"></a>
+<a href="#FNanchor_10_114"><span class="label">[10]</span></a>
+See Fleck, E., Die Macrolep. Rumäniens,
+<i>Bul. Soc. Sciinte</i>, VIII, 1899, p. 720.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_115" id="Footnote_11_115"></a>
+<a href="#FNanchor_11_115"><span class="label">[11]</span></a>
+My experience agrees with that of Mr. H. Williams (<i>Ent. Rec.</i>, VIII, 1896, p. 181)
+that pupae, well-formed, can stand considerable frost; but I used to find
+that half-grown larvae usually died if unprotected, and I believe that larvae which
+attempted to pupate in warm autumn weather and then got caught by frosts,
+always died. Small larvae which can creep into shelter at the bottom of the plants
+survived, and I expect that in the north the winter is usually passed in that state
+(see also Merrifield, F., <i>Ent. Rec.</i>, VIII, 1896, p. 168, and
+Carpenter, J. H., <i>ibid.</i>).</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_116" id="Footnote_12_116"></a>
+<a href="#FNanchor_12_116"><span class="label">[12]</span></a>
+Some most unlikely species do this. I once had a larva
+of <i>Parnassius delius</i>, found at about 5,500 feet, which emerged
+late in the autumn (in October I believe), a season at which it must
+have perished in its own country.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_117" id="Footnote_13_117"></a>
+<a href="#FNanchor_13_117"><span class="label">[13]</span></a>
+See, for examples, Barrett, G. C., <i>Lepidoptera of the Brit. Islands</i>,
+I, 1893, p. 229; also Grover, W., <i>Ent. Rec.</i>, IX, 1897, p. 314; Williams, H.,
+<i>Proc. Ent. Soc.</i>, 1898, who reared several specimens from the New Forest
+which would pass for Bretons, though the rest of the family were true <i>egerides</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_118" id="Footnote_14_118"></a>
+<a href="#FNanchor_14_118"><span class="label">[14]</span></a>
+Above the Tosa falls.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_119" id="Footnote_15_119"></a>
+<a href="#FNanchor_15_119"><span class="label">[15]</span></a>
+<i>Bul. Soc. Sciinte</i>, VIII, 1899, p. 691.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_120" id="Footnote_16_120"></a>
+<a href="#FNanchor_16_120"><span class="label">[16]</span></a>
+The fact that Weismann by heating pupæ obtained only one
+autumn specimen seems to me to show rather that a second brood can be
+produced than that it cannot, which is the inference usually drawn.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_121" id="Footnote_17_121"></a>
+<a href="#FNanchor_17_121"><span class="label">[17]</span></a>
+Schima, K., <i>Verh. Zool. bot. Ges. Wien</i>, LX, 1910, p. 268.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_122" id="Footnote_18_122"></a>
+<a href="#FNanchor_18_122"><span class="label">[18]</span></a>
+<i>Rhopalocera Palaearctica</i>, Florence, 1905-11, especially Pl. XXXII.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_123" id="Footnote_19_123"></a>
+<a href="#FNanchor_19_123"><span class="label">[19]</span></a>
+See figures in Barrett, G. C., <i>Lepidoptera of Brit. Islands</i>, I, pt. 3, p. 25.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_124" id="Footnote_20_124"></a>
+<a href="#FNanchor_20_124"><span class="label">[20]</span></a>
+Tutt, J. W., <i>Ent. Rec.</i>, XVIII, 1905, p. 5.
+In the same place he states that on the Mendel Pass <i>arcania</i>
+"runs into" <i>darwiniana</i> and that in the Tyrolean localities the
+transition is especially evident. Wheeler (<i>ibid.</i>, XIII, 1901, p. 121)
+expresses the contrary opinion, that <i>satyrion</i> does grade to <i>arcania</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_21_125" id="Footnote_21_125"></a>
+<a href="#FNanchor_21_125"><span class="label">[21]</span></a>
+H. Rowland-Brown, <i>Ent. Rec.</i>, XI, 1899, p. 293.</p></div>
+
+<div class="footnote"><p><a name="Footnote_22_126" id="Footnote_22_126"></a>
+<a href="#FNanchor_22_126"><span class="label">[22]</span></a>
+Speyer, Stettiner, <i>Ent. Ztg.</i>, XXXI, 1870, p. 63.</p></div>
+
+<div class="footnote"><p><a name="Footnote_23_127" id="Footnote_23_127"></a>
+<a href="#FNanchor_23_127"><span class="label">[23]</span></a>
+In regard to the closely analogous case of <i>Spilosoma lubricipeda</i>, Standfuss
+makes a similar statement. He bred the type on a large scale with the radiate form
+which he calls <i>intermedia</i>, and says that in four years of miscellaneous crossing he
+never obtained really transitional forms. Nevertheless after examining large series,
+especially those of Mr. W. H. B. Fletcher, I came to the conclusion that several
+might be so classed, but I am quite prepared to find that such specimens are heterozygous.
+(See Standfuss, <i>Handb. d. Gross-Schmet.</i>, 1896, p. 307.) It is by no means
+unlikely that various dark forms of <i>lubricipeda</i> correspond with a progressive series
+of factorial additions. Many of the stages have been named, and of these the most
+definite are the <i>intermedia</i> of Standfuss (probably = <i>eboraci</i> of Tugwell) and the
+very dark <i>Zatima</i> of Heligoland, in which only the thorax, the nervures and a small
+field in the fore-wings remain yellow. A form was bred by Deschange from <i>Zatima</i>
+in which even the field in the forewing is obliterated. The exact circumstances in
+which <i>Zatima</i> occurs in Heligoland would be worthy of special investigation, for the
+normal <i>lubricipeda</i> is also found on the island. For references as to the British
+occurrences see especially, Hewett, W., <i>Naturalist</i>, 1894, p. 353. As to <i>Zatima</i> see
+especially Krancher, <i>Soc. Ent.</i>, II, 1887-8, p. 26. I am indebted to Dr. Hartlaub
+for information as to the Heligoland types.</p></div>
+
+<div class="footnote"><p><a name="Footnote_24_128" id="Footnote_24_128"></a>
+<a href="#FNanchor_24_128"><span class="label">[24]</span></a>
+Boisduval, <i>Bull. Soc. Ent. Fr.</i>, III, 1834, p. 5.</p></div>
+
+<div class="footnote"><p><a name="Footnote_25_129" id="Footnote_25_129"></a>
+<a href="#FNanchor_25_129"><span class="label">[25]</span></a>
+The systematics of <i>Setina</i> have been much controverted,
+but no one I believe doubts that <i>aurita</i> and <i>ramosa</i> are forms
+of one species. See also Chapman, A. T., <i>Ent. Rec.</i>, XIII, 1901, p. 139.</p></div>
+
+<div class="footnote"><p><a name="Footnote_26_130" id="Footnote_26_130"></a>
+<a href="#FNanchor_26_130"><span class="label">[26]</span></a>
+<i>Arch. Naturg.</i>, 33, 1867, p. 116.</p></div>
+
+<div class="footnote"><p><a name="Footnote_27_131" id="Footnote_27_131"></a>
+<a href="#FNanchor_27_131"><span class="label">[27]</span></a>
+<i>Brit. Mus. Cat., Batrachia Gradientia</i>, 1882.</p></div>
+
+<div class="footnote"><p><a name="Footnote_28_132" id="Footnote_28_132"></a>
+<a href="#FNanchor_28_132"><span class="label">[28]</span></a>
+The Geographical Distribution of nearly related Species.
+<i>Amer. Nat.</i>, XLI. 1907, p. 207.</p></div>
+
+<div class="footnote"><p><a name="Footnote_29_133" id="Footnote_29_133"></a>
+<a href="#FNanchor_29_133"><span class="label">[29]</span></a>
+See later, p. 242.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER IX.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_134" id="Footnote_1_134"></a>
+<a href="#FNanchor_1_134"><span class="label">[1]</span></a>
+Semon, R., Der Stand der Frage nach der Vererbung
+erworbener Eigenschaften, published in <i>Fortschr. der naturw.
+Forschung.</i>, Bd. 11, 1910.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_135" id="Footnote_2_135"></a>
+<a href="#FNanchor_2_135"><span class="label">[2]</span></a>
+Standfuss, M., <i>Denks. Schweiz. naturf. Ges.</i>, XXXVI, 1898, p. 32.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_136" id="Footnote_3_136"></a>
+<a href="#FNanchor_3_136"><span class="label">[3]</span></a>
+Fischer, E., <i>Allg. Ztschr. f. Entomologie</i>, Bd. VI, 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_137" id="Footnote_4_137"></a>
+<a href="#FNanchor_4_137"><span class="label">[4]</span></a>
+Out of 12 pupae treated 8 died and of the 4 survivors, one only was affected.
+See M. v. Linden, <i>Archiv. Rassen. u. Gesells.</i>, 1904, I.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_138" id="Footnote_5_138"></a>
+<a href="#FNanchor_5_138"><span class="label">[5]</span></a>
+For illustrations see <i>Oberthur's Études d'Entom.</i>, 1896,
+where many of these curious aberrations are represented;
+also Barrett, <i>Lepid. Brit. Islands</i>, II, pp. 71 and 72.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_139" id="Footnote_6_139"></a>
+<a href="#FNanchor_6_139"><span class="label">[6]</span></a>
+Schübeler, F. C., <i>Die Culturpflanzen Norwegens</i>,
+1862, especially pp. 24 and 28.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_140" id="Footnote_7_140"></a>
+<a href="#FNanchor_7_140"><span class="label">[7]</span></a>
+I am obliged to him and to Dr. E. Gold for much trouble
+taken to answer my questions. Some idea of the kind of weather indicated
+by an average of 2.76° C. above the mean may be got from a comparison
+with the year 1911, which most people will remember as one of the hottest
+summers they have known. The July of that year was in east and southeast
+England about 4° F. above the mean but 2.67 C. means about 4.8° F. above
+the mean. At Greenwich July, 1859, was about 6.5° F. above the average.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_141" id="Footnote_8_141"></a>
+<a href="#FNanchor_8_141"><span class="label">[8]</span></a>
+Wille, N., <i>Biol. Cbltt.</i>, XXV, 1905, p. 521.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_142" id="Footnote_9_142"></a>
+<a href="#FNanchor_9_142"><span class="label">[9]</span></a>
+Wettstein, R. von. <i>Der Neo-marckismus u. seine
+Beziehungen zum Darwinismus</i>, Jena, 1903.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_143" id="Footnote_10_143"></a>
+<a href="#FNanchor_10_143"><span class="label">[10]</span></a>
+T. Graham Brown, <i>Proc. Roy. Soc.</i>, 1912, vol. 84, B, p. 555.
+This paper gives full reference to the previous literature of the subject.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_144" id="Footnote_11_144"></a>
+<a href="#FNanchor_11_144"><span class="label">[11]</span></a>
+Morgan, T. H., <i>Evolution and Adaptation</i>, New York, 1903.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_145" id="Footnote_12_145"></a>
+<a href="#FNanchor_12_145"><span class="label">[12]</span></a>
+Kammerer's chief paper on this subject is in <i>Arch. f. Entwm.</i>,
+1909, XXVIII, p. 447, and it is to this that the paginal references in the present
+text relate. His previous paper appeared, <i>ibid.</i>, 1906, XXII, p. 48.
+An account of his further experiments with <i>Alytes</i> is given in
+<i>Natur</i>, 1909-10, Heft 6, p. 95.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_146" id="Footnote_13_146"></a>
+<a href="#FNanchor_13_146"><span class="label">[13]</span></a>
+In reply to my letter Dr. Kammerer who was then away from home very
+kindly replied that he was not quite sure whether he had killed specimens of <i>Alytes</i>
+with "<i>Brunftschwielen</i>" or whether he only had living males of the fourth generation,
+but that he would send illustrative material.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_147" id="Footnote_14_147"></a>
+<a href="#FNanchor_14_147"><span class="label">[14]</span></a>
+Kammerer, P., <i> Natur</i>, 12 December, 1909, Heft 6, p. 95, repeated in
+<i>12 Flugschrift d. Deutsch Ges. f. Züchtungskunde</i>, Berlin, 1910.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_148" id="Footnote_15_148"></a>
+<a href="#FNanchor_15_148"><span class="label">[15]</span></a>
+<i>Festschrift zum Andenken an Gregor Mendel</i>, being
+vol. XLIX of the <i>Verh. Naturf. Ver. in Brünn</i>, 1911, p. 98.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_149" id="Footnote_16_149"></a>
+<a href="#FNanchor_16_149"><span class="label">[16]</span></a>
+Kammerer's chief papers on this subject are <i>Archiv fur Entwm.</i>, XVII,
+1904, and <i>ibid.</i>, XXV, 1907. An epitome of results is also given by him in
+<i>12 Flugschrift d. Deutsch. Ges. f. Züchtungskunde</i>, Berlin, 1910.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_150" id="Footnote_17_150"></a>
+<a href="#FNanchor_17_150"><span class="label">[17]</span></a>
+"<i>Bei Fortdauer der Versuchsbedingungen sind als Vollmolche
+geborene Salamandra maculosa</i> gleich bei der ersten Geburt <i>abermals
+voll molchgebärend</i>, benutzen zum Geburtsakt das trockene Land,
+und zwar unter Erreichung der (bei <i>Salamandra atra</i> normalen)
+<i>Embryonen-Zweizahl</i>," Kammerer, 1907, p. 49.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_151" id="Footnote_18_151"></a>
+<a href="#FNanchor_18_151"><span class="label">[18]</span></a> 1904, p. 56.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_152" id="Footnote_19_152"></a>
+<a href="#FNanchor_19_152"><span class="label">[19]</span></a>
+Throughout Kammerer's papers this is used almost as a technical term.
+It means, I presume, that the feature was manifested more than once.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_153" id="Footnote_20_153"></a>
+<a href="#FNanchor_20_153"><span class="label">[20]</span></a>
+It should be stated that the papers contain a quantity of detail, especially
+descriptive of the state of the larvae, which I have not attempted to represent, but
+the account here given contains all that seemed essential to an understanding of
+the more important features of the account.</p></div>
+
+<div class="footnote"><p><a name="Footnote_21_154" id="Footnote_21_154"></a>
+<a href="#FNanchor_21_154"><span class="label">[21]</span></a>
+The first appeared in <i>Natur</i>, 1909-10, Heft 6, p. 94;
+and the second, which contains coloured plates of the animals, in the lecture
+already referred to, <i>12 Flugschr. d. Deut. Ges. f. Züchtungkunde</i>,
+Berlin, 1910, p. 26. In the paper in <i>Mendel Festschrift</i>, 1911,
+the subject is continued, but no more is added as to this part of the experiment.</p></div>
+
+<div class="footnote"><p><a name="Footnote_22_155" id="Footnote_22_155"></a>
+<a href="#FNanchor_22_155"><span class="label">[22]</span></a>
+E. G. Boulenger, <i>Proc. Zool. Soc.</i>, 1911, p. 323.</p></div>
+
+<div class="footnote"><p><a name="Footnote_23_156" id="Footnote_23_156"></a>
+<a href="#FNanchor_23_156"><span class="label">[23]</span></a>
+<i>Mendel Festschrift</i>, 1911, p. 84.</p></div>
+
+<div class="footnote"><p><a name="Footnote_24_157" id="Footnote_24_157"></a>
+<a href="#FNanchor_24_157"><span class="label">[24]</span></a>
+<i>12 Flugschrift. Deut. Ges. Züchtungskunde</i>, 1910, Fig. 15, <i>P. Reihe</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_25_158" id="Footnote_25_158"></a>
+<a href="#FNanchor_25_158"><span class="label">[25]</span></a>
+<i>Mendel Festschrift</i>, 1911, p. 83.</p></div>
+
+<div class="footnote"><p><a name="Footnote_26_159" id="Footnote_26_159"></a>
+<a href="#FNanchor_26_159"><span class="label">[26]</span></a>
+Field, 1912, 30 March.</p></div>
+
+<div class="footnote"><p><a name="Footnote_27_160" id="Footnote_27_160"></a>
+<a href="#FNanchor_27_160"><span class="label">[27]</span></a>
+<i>Ibid.</i>, 1904, p. 863.</p></div>
+
+<div class="footnote"><p><a name="Footnote_28_161" id="Footnote_28_161"></a>
+<a href="#FNanchor_28_161"><span class="label">[28]</span></a>
+<i>Mitth. Naturw. Ver. a. d. Univ. Wien</i>, 1908, p. 53.</p></div>
+
+<div class="footnote"><p><a name="Footnote_29_162" id="Footnote_29_162"></a>
+<a href="#FNanchor_29_162"><span class="label">[29]</span></a>
+As to the variations of <i>Lacerta muralis</i> in Western Europe and
+North Africa see Boulenger, G. A., <i>Trans. Zool. Soc.</i>, 1905, vol. XVII, p. 351.</p></div>
+
+<div class="footnote"><p><a name="Footnote_30_163" id="Footnote_30_163"></a>
+<a href="#FNanchor_30_163"><span class="label">[30]</span></a>
+As to the experiments of Klebs relating to the
+transmission of acquired characters, see Appendix.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER X.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_164" id="Footnote_1_164"></a>
+<a href="#FNanchor_1_164"><span class="label">[1]</span></a>
+Pringsheim, H., <i>Die Variabilität niederer Organismen</i>, Berlin, 1910.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_165" id="Footnote_2_165"></a>
+<a href="#FNanchor_2_165"><span class="label">[2]</span></a>
+ F. Wolf, Modifikationen u. Mutationen von Bakterien,
+<i>Zts. F. indukt. Abstam. u. Vererbungslehre</i>, II, 1909, p. 90.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_166" id="Footnote_3_166"></a>
+<a href="#FNanchor_3_166"><span class="label">[3]</span></a>
+Winslow, C. E. A. and A. R.,<i>Systematic Relationships of the Coccaceae</i>.
+New York. 1909.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_167" id="Footnote_4_167"></a>
+<a href="#FNanchor_4_167"><span class="label">[4]</span></a>
+C. C. Dobell, <i>Jour. Genetics</i>, 1912, II, p. 201,
+where full references are given.<br />
+Still more recently the same author has contributed an excellent summary of
+the evidence relating to bacteria (<i>ibid.</i>, II. 1913, p. 325).</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_168" id="Footnote_5_168"></a>
+<a href="#FNanchor_5_168"><span class="label">[5]</span></a>
+See Woltereck, <i>Verh. d. Deut. Zool. Ges.</i>, 1909, p. 110;
+and 1911, p. 142. This is a subject which can only be
+properly appreciated on reference to the original papers.
+Several complications are involved to which I have not here alluded.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_169" id="Footnote_6_169"></a>
+<a href="#FNanchor_6_169"><span class="label">[6]</span></a>
+<i>Proc. Roy. Soc.</i>, B, Vol. 86, 1913, p. 113.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_170" id="Footnote_7_170"></a>
+<a href="#FNanchor_7_170"><span class="label">[7]</span></a>
+<i>An Investigation of Evolution in Chrysomelid Beetles
+of the Genus Leptinotarsa</i>, Carnegie Publications, 1906, No. 48.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_171" id="Footnote_8_171"></a>
+<a href="#FNanchor_8_171"><span class="label">[8]</span></a>
+This is the famous Colorado beetle or potato-bug, which has
+caused such serious destruction in potato crops. There seems to be no doubt
+that this insect, formerly unknown in the eastern States, made its way east
+along the mining trails when the west was opened up.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_172" id="Footnote_9_172"></a>
+<a href="#FNanchor_9_172"><span class="label">[9]</span></a>
+This is indicated in the coloured plate, but I have not found
+any explicit statement to this effect in the text, and am not sure
+if the absence of pigment was regarded as complete.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_173" id="Footnote_10_173"></a>
+<a href="#FNanchor_10_173"><span class="label">[10]</span></a>
+<i>Biol. Bull.</i>, XVIII, 1910, p. 285.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_174" id="Footnote_11_174"></a>
+<a href="#FNanchor_11_174"><span class="label">[11]</span></a>
+This description does not quite agree with the representation of the larvae
+in Pl. 17 of the book <i>Evolution in the Genus Leptinotarsa</i> for there the larva of
+<i>undecimlineata</i> is shown as white in the second stage, but yellowish in the third
+stage; perhaps there is an error in printing.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_175" id="Footnote_12_175"></a>
+<a href="#FNanchor_12_175"><span class="label">[12]</span></a>
+<i>Biol. Bull.</i>, XX, 1910, p. 67.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_176" id="Footnote_13_176"></a>
+<a href="#FNanchor_13_176"><span class="label">[13]</span></a>
+As to the interrelations of these three forms, Tower states (1906, p. 18) that
+<i>angustovittata</i>, which he reared from <i>undecimlineata</i>, is intermediate between it
+and <i>signaticollis</i>. Compare Stål, "<i>Monogr. des Chrysomélides</i>," 1862, p. 163; and
+Jacoby, <i>Biol. Centr. Amer. Celeopt.</i>, vi, Pt. 1, p. 234, Pl. xiii, fig. 20; Tab. 41, fig.
+15; <i>ibid.</i>, Suppl., p. 253. All these forms are evidently very closely related, and
+the delimitation of species is quite arbitrary. Jacoby indeed suggests that
+<i>undecimlineata</i> may be a variety of <i>decemlineata</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_177" id="Footnote_14_177"></a>
+<a href="#FNanchor_14_177"><span class="label">[14]</span></a>
+Gortner, <i>Amer. Nat.</i>, Dec., 1911, XLV, p. 743.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_178" id="Footnote_15_178"></a>
+<a href="#FNanchor_15_178"><span class="label">[15]</span></a>
+<i>Mutations, Variations, and Relationships of the Oenotheras</i>,
+Carnegie Institution Publication No. 81, 1907, pp. 61-64.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_179" id="Footnote_16_179"></a>
+<a href="#FNanchor_16_179"><span class="label">[16]</span></a>
+Macdougal, D. T., "Alterations in Heredity induced by Ovarial Treatments",
+<i>Bot. Gaz.</i>, vol. 51, 1911, p. 241.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_180" id="Footnote_17_180"></a>
+<a href="#FNanchor_17_180"><span class="label">[17]</span></a>
+Payne, Fernandus, <i>Biol. Bull.</i>, XVIII, 1910,
+p. 188, and <i>ibid.</i>, XXI, 1911, p. 297.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_181" id="Footnote_18_181"></a>
+<a href="#FNanchor_18_181"><span class="label">[18]</span></a>
+See especially, <i>Mutation et Traumatismes</i>, Paris, Felix Alcan, 1908.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_182" id="Footnote_19_182"></a>
+<a href="#FNanchor_19_182"><span class="label">[19]</span></a>
+J. H. Powers, "Morphological Variation and its Causes in
+<i>Amblystoma tigrinum</i>." <i>Studies from the Zoological Laboratory.
+</i> The University of Nebraska, No. 71, 1907.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_183" id="Footnote_20_183"></a>
+<a href="#FNanchor_20_183"><span class="label">[20]</span></a>
+In connexion with this case I would refer the reader to some remarkable
+observations of Dr. T. A. Chapman on various types of larvae which he reared
+from the moth <i>Arctia caja</i> (<i>Ent. Rec.</i>, IV, 1893, p. 265, and following parts).
+From a single mother he raised a great diversity of forms, some which fed up rapidly
+and passed through their development without assuming certain stages, and others
+which were, as he called them, "laggards," moulting more times than their brethren
+and developing at a much slower rate. It is greatly to be hoped that such a case
+may be critically investigated by analytical breeding.</p></div>
+
+<hr class="chap" />
+<h3>FOOTNOTES:&emsp;CHAPTER X.</h3>
+
+<div class="footnote"><p><a name="Footnote_1_184" id="Footnote_1_184"></a>
+<a href="#FNanchor_1_184"><span class="label">[1]</span></a>
+Buffon, <i>Hist. Nat.</i>, Oiseaux, 1780, VII, p. 3.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_185" id="Footnote_2_185"></a>
+<a href="#FNanchor_2_185"><span class="label">[2]</span></a>
+Ibid., VIII, p. 115.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_186" id="Footnote_3_186"></a>
+<a href="#FNanchor_3_186"><span class="label">[3]</span></a>
+Keeble, <i>Jour. Gen.</i>, 1912, II, p. 173.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_187" id="Footnote_4_187"></a>
+<a href="#FNanchor_4_187"><span class="label">[4]</span></a>
+<i>Animals and Plants</i>, ed. 1, 1868, II, pp. 180-5.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_188" id="Footnote_5_188"></a>
+<a href="#FNanchor_5_188"><span class="label">[5]</span></a>
+<i>Animals and Plants</i>, ed. 1, 1868, II, p. 165.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_189" id="Footnote_6_189"></a>
+<a href="#FNanchor_6_189"><span class="label">[6]</span></a>
+<i>Species and Varieties</i>, 1905, p. 471.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_190" id="Footnote_7_190"></a>
+<a href="#FNanchor_7_190"><span class="label">[7]</span></a>
+Correns, <i>Festschr. med.-nat. Ges. zur 84 Versamml. Deutsch.
+Naturf. u. Aertze. Münster i. W.</i>, 1912.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_191" id="Footnote_8_191"></a>
+<a href="#FNanchor_8_191"><span class="label">[8]</span></a>
+This is a case of a somewhat different order and I mention it partly for that
+reason as an illustration of the complexity which such negative instances may
+present. The difficulty is that though the buffalo and the zebu can breed together,
+the foetus is too large to be born alive. (See Ackermann <i>Ber. d. Ver. f. Naturk.</i>,
+Kassel, 1898, p. 69. Prof. S. Nathusius, of Halle, who has great experience in
+crossing Bovidae, tells me that he has always failed to cross the buffalo with
+other species.)</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_192" id="Footnote_9_192"></a>
+<a href="#FNanchor_9_192"><span class="label">[9]</span></a>
+In a paper to be published in the Report of the Genetic Conference, Paris,
+1911, Bellair states that he obtained some partially fertile hybrids in the cross
+<i>N. sylvestris</i> × <i>tabacum</i>. As to the various degrees of sterility in
+hybrids between <i>Nicotiana</i> species
+see Lock, R. H., <i>Ann. Roy. Bot. Gardens</i>. Peradeniya, IV, 1909, p. 195.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_193" id="Footnote_10_193"></a>
+<a href="#FNanchor_10_193"><span class="label">[10]</span></a>
+<i>Beitrage zur Biol. der Pflanzen.</i>, X, 1911, p. 379.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_194" id="Footnote_11_194"></a>
+<a href="#FNanchor_11_194"><span class="label">[11]</span></a>
+One very peculiar feature was observed, namely, that all the new forms in F<sub>2</sub>
+which were bred from came true. As I understand, this statement applied to five
+such new types, and they were represented by 76 individuals in F<sub>3</sub>, but further
+details on this point are desirable. Another curious fact was observed, namely
+that one of the F<sub>1</sub> forms (<i>cochleata</i> × <i>radiata</i>) when fertilised by
+<i>cochleata</i> gave a highly polymorphic family, but fertilised by <i>radiata</i>
+the resulting offspring were almost uniform.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_195" id="Footnote_12_195"></a>
+<a href="#FNanchor_12_195"><span class="label">[12]</span></a>
+I also had a few F<sub>1</sub> seeds given me by Mr. R. H. Lock.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_196" id="Footnote_13_196"></a>
+<a href="#FNanchor_13_196"><span class="label">[13]</span></a>
+In a paper about to appear in <i>Jour. Linn. Soc.</i> Mr. A. W. Sutton identifies
+this Palestine pea as <i>Pisum humile</i> of Boissier and Noé.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_197" id="Footnote_14_197"></a>
+<a href="#FNanchor_14_197"><span class="label">[14]</span></a>
+Lloyd, R. E., <i>The Growth of Groups in the Animal Kingdom</i>, London, 1912.</p></div>
+
+</div>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
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+</pre>
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+The Project Gutenberg EBook of Problems of Genetics, by William Bateson
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org/license
+
+
+Title: Problems of Genetics
+
+Author: William Bateson
+
+Release Date: January 4, 2014 [EBook #44582]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK PROBLEMS OF GENETICS ***
+
+
+
+
+Produced by Chris Curnow, Paul Marshall and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+                    YALE UNIVERSITY
+
+        MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES
+
+
+                  PROBLEMS OF GENETICS
+
+              SILLIMAN MEMORIAL LECTURES
+          PUBLISHED BY YALE UNIVERSITY PRESS
+
+  ELECTRICITY AND MATTER. _By_ JOSEPH JOHN THOMSON,
+    D.SC., LL.D., PH.D., F.R.S., _Fellow of Trinity College,
+    Cambridge, Cavendish Professor of Experimental Physics, Cambridge_.
+  _Price $1.25 net; postage 10 cents extra._
+
+  THE INTEGRATIVE ACTION OF THE NERVOUS SYSTEM.
+    _By_ CHARLES S. SHERRINGTON,
+     D.SC., M.D., HON. LL.D., TOR., F.R.S.,
+    _Holt Professor of Physiology in the University of Liverpool_.
+  _Price $3.50 net; postage 25 cents extra._
+
+  RADIOACTIVE TRANSFORMATIONS. _By_ ERNEST RUTHERFORD,
+    D.SC., LL.D., F.R.S., _Macdonald Professor of Physics,
+    McGill University_.
+  _Price $3.50 net; postage 22 cents extra._
+
+  EXPERIMENTAL AND THEORETICAL APPLICATIONS OF
+      THERMODYNAMICS TO CHEMISTRY.
+  _By_ DR. WALTHER NERNST, _Professor and Director of the
+    Institute of Physical Chemistry in the University of Berlin_.
+  _Price $1.25 net; postage 10 cents extra._
+
+  THE PROBLEMS OF GENETICS. _By_ WILLIAM BATESON, M.A.,
+    F.R.S., _Director of the John Innes Horticultural Institution,
+    Merton Park, Surrey, England_.
+  _Price $4.00 net; postage 25 cents extra._
+
+  STELLAR MOTIONS.
+    WITH SPECIAL REFERENCE TO MOTIONS DETERMINED BY MEANS OF
+    THE SPECTROGRAPH. _By_ WILLIAM WALLACE CAMPBELL, SC.D., LL.D.,
+   _Director of the Lick Observatory, University of California_.
+  _Price $4.00 net; postage 30 cents extra._
+
+  THEORIES OF SOLUTIONS. _By_ SVANTE AUGUST ARRHENIUS,
+    PH.D., SC.D., M.D., _Director of the Physico-Chemical
+    Department of the Nobel Institute, Stockholm, Sweden_.
+  _Price $2.25 net; postage 15 cents extra._
+
+  IRRITABILITY.
+       A PHYSIOLOGICAL ANALYSIS OF THE GENERAL EFFECT OF
+       STIMULI IN LIVING SUBSTANCES.
+    _By_ MAX VERWORN,
+    _Professor at Bonn Physiological Institute_.
+  _Price $3.50 net; postage 20 cents extra._
+
+  THE EVOLUTION OF MODERN MEDICINE.
+    _By_ SIR WILLIAM OSLER, BART., M.D., LL.D., SC.D.,
+    _Regius Professor of Medicine, Oxford University_.
+  _Price $3.00 net; postage 40 cents extra._
+
+
+
+
+                     PROBLEMS OF GENETICS
+
+                              BY
+                 WILLIAM BATESON, M.A., F.R.S.
+
+        DIRECTOR OF THE JOHN INNES HORTICULTURAL INSTITUTION,
+            HON. FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE,
+        AND FORMERLY PROFESSOR OF BIOLOGY IN THE UNIVERSITY
+
+                     _WITH ILLUSTRATIONS_
+
+                         [Illustration]
+
+                NEW HAVEN: YALE UNIVERSITY PRESS
+                    LONDON: HUMPHREY MILFORD
+                     OXFORD UNIVERSITY PRESS
+
+                             MCMXIII
+
+
+                         Copyright, 1913
+                       By YALE UNIVERSITY
+
+                First printed August, 1913, 1000 copies
+
+
+[** Transcriber's Note:
+     Underscores "_" before and after a word or phrase indicate ITALICS
+       in the original text.
+     Hyphenation was used inconsistently by the author and has been
+        left as in the original text. ]
+
+
+
+
+                    THE SILLIMAN FOUNDATION
+
+In the year 1883 a legacy of about eighty-five thousand dollars was left
+to the President and Fellows of Yale College in the city of New Haven,
+to be held in trust, as a gift from her children, in memory of their
+beloved and honored mother, Mrs. Hepsa Ely Silliman.
+
+On this foundation Yale College was requested and directed to establish
+an annual course of lectures designed to illustrate the presence and
+providence, the wisdom and goodness of God, as manifested in the natural
+and moral world. These were to be designated as the Mrs. Hepsa Ely
+Silliman Memorial Lectures. It was the belief of the testator that any
+orderly presentation of the facts of nature or history contributed
+to the end of this foundation more effectively than any attempt to
+emphasize the elements of doctrine or of creed; and he therefore
+provided that lectures on dogmatic or polemical theology should be
+excluded from the scope of this foundation, and that the subjects should
+be selected rather from the domains of natural science and history,
+giving special prominence to astronomy, chemistry, geology, and anatomy.
+
+It was further directed that each annual course should be made the basis
+of a volume to form part of a series constituting a memorial to Mrs.
+Silliman. The memorial fund came into the possession of the Corporation
+of Yale University in the year 1901; and the present volume constitutes
+the fifth of the series of memorial lectures.
+
+
+
+
+                           PREFACE
+
+This book gives the substance of a series of lectures delivered in Yale
+University, where I had the privilege of holding the office of Silliman
+Lecturer in 1907.
+
+The delay in publication was brought about by a variety of causes.
+
+Inasmuch as the purpose of the lectures is to discuss some of the wider
+problems of biology in the light of knowledge acquired by Mendelian
+methods of analysis, it was essential that a fairly full account of
+the conclusions established by them should first be undertaken and I
+therefore postponed the present work till a book on Mendel's Principles
+had been completed.
+
+On attempting a more general discussion of the bearing of the phenomena
+on the theory of Evolution, I found myself continually hindered by the
+consciousness that such treatment is premature, and by doubt whether
+it were not better that the debate should for the present stand
+indefinitely adjourned. That species have come into existence by an
+evolutionary process no one seriously doubts; but few who are familiar
+with the facts that genetic research has revealed are now inclined to
+speculate as to the manner by which the process has been accomplished.
+Our knowledge of the nature and properties of living things is far too
+meagre to justify any such attempts. Suggestions of course can be made:
+though, however, these ideas may have a stimulating value in the lecture
+room, they look weak and thin when set out in print. The work which may
+one day give them a body has yet to be done.
+
+The development of negations is always an ungrateful task apt to be
+postponed for the positive business of experiment. Such work is happily
+now going forward in most of the centers of scientific life. Of many
+of the subjects here treated we already know more than we did in 1907.
+The delay in production has made it possible to incorporate these new
+contributions.
+
+The book makes no pretence at being a treatise and the number of
+illustrative cases has been kept within a moderate compass. A good many
+of the examples have been chosen from American natural history, as being
+appropriate to a book intended primarily for American readers. The facts
+are largely given on the authority of others, and I wish to express my
+gratitude for the abundant assistance received from American colleagues,
+especially from the staffs of the American Museum in New York, and of
+the Boston Museum of Natural History. In connexion with the particular
+subjects personal acknowledgments are made.
+
+Dr. F. M. Chapman was so good as to supervise the preparation of the
+coloured Plate of _Colaptes_, and to authorize the loan of the Plate
+representing the various forms of _Helminthophila_, which is taken from
+his _North American Warblers_.
+
+I am under obligation to Messrs. Macmillan & Co., for permission to
+reproduce several figures from _Materials for the Study of Variation_,
+illustrating subjects which I wished to treat in new associations, and
+to M. Leduc for leave to use Fig. 9.
+
+In conclusion I thank my friends in Yale for the high honour they did me
+by their invitation to contribute to the series of Silliman Lectures,
+and for much kindness received during a delightful sojourn in that
+genial home of learning.
+
+
+
+
+                    TABLE OF CONTENTS.
+
+  CHAPTER                                                  PAGE
+
+     I. INTRODUCTORY. THE PROBLEM OF SPECIES AND VARIETY     1
+    II. MERISTIC PHENOMENA                                  31
+   III. SEGMENTATION, ORGANIC AND MECHANICAL                60
+    IV. THE CLASSIFICATION OF VARIATION AND THE NATURE
+                OF SUBSTANTIVE VARIATION                    83
+        NOTE TO CHAPTER IV                                  94
+     V. THE MUTATION THEORY                                 97
+        NOTE TO CHAPTER V                                  116
+    VI. VARIATION AND LOCALITY                             118
+   VII. LOCAL DIFFERENTIATION--_continued_.
+              OVERLAPPING FORMS                            146
+  VIII. LOCALLY DIFFERENTIATED FORMS--_continued_.
+              CLIMATIC VARIETIES                           164
+    IX. THE EFFECTS OF CHANGED CONDITIONS                  187
+     X. THE EFFECTS OF CHANGED CONDITIONS--_continued_.
+              THE CAUSES OF GENETIC VARIATION              212
+    XI. THE STERILITY OF HYBRIDS. CONCLUDING REMARKS       233
+        APPENDIX TO CHAPTER X                              250
+        INDEX                                              251
+
+
+
+
+
+
+                        PROBLEMS OF GENETICS
+
+
+
+
+                              CHAPTER I
+
+                             INTRODUCTORY
+
+The purpose of these lectures is to discuss some of the familiar
+phenomena of biology in the light of modern discoveries. In the last
+decade of the nineteenth century many of us perceived that if any
+serious advance was to be made with the group of problems generally
+spoken of as the Theory of Evolution, methods of investigation must be
+devised and applied of a kind more direct and more penetrating than
+those which after the general acceptance of the Darwinian views had been
+deemed adequate. Such methods obviously were to be found in a critical
+and exhaustive study of the facts of variation and heredity, upon which
+all conceptions of evolution are based. To construct a true synthetic
+theory of Evolution it was necessary that variation and heredity instead
+of being merely postulated as axioms should be minutely examined as
+phenomena. Such a study Darwin himself had indeed tentatively begun, but
+work of a more thorough and comprehensive quality was required. In the
+conventional view which the orthodoxy of the day prescribed, the terms
+variation and heredity stood for processes so vague and indefinite that
+no analytical investigation of them could be contemplated. So soon,
+however, as systematic inquiry into the natural facts was begun it was
+at once found that the accepted ideas of variation were unfounded.
+Variation was seen very frequently to be a definite and specific
+phenomenon, affecting different forms of life in different ways, but
+in all its diversity showing manifold and often obvious indications
+of regularity. This observation was not in its essence novel. Several
+examples of definite variation had been well known to Darwin and
+others, but many, especially Darwin himself in his later years, had
+nevertheless been disposed to depreciate the significance of such
+facts. They consequently then lapsed into general disparagement. Upon
+more careful inquiry the abundance of such phenomena proved to be far
+greater than was currently supposed, and a discussion of their nature
+brought into prominence a consideration of greater weight, namely that
+the differences by which these definite or discontinuous variations are
+constituted again and again approximate to and are comparable with the
+class of differences by which species are distinguished from each other.
+
+The interest of such observations could no longer be denied. The
+more they were examined the more apparent it became that by means of
+the facts of variation a new light was obtained on the physiological
+composition and capabilities of living things. Genetics thus cease to be
+merely a method of investigating theories of evolution or of the origin
+of species but provide a novel and hitherto untried instrument by which
+the nature of the living organism may be explored. Just as in the study
+of non-living matter science began by regarding the external properties
+of weight, opacity, colour, hardness, mode of occurrence, etc., noting
+only such evidences of chemical attributes and powers as chance
+spontaneously revealed; and much later proceeded to the discovery that
+these casual manifestations of chemical properties, rightly interpreted,
+afford a key to the intrinsic nature of the diversity of matter, so in
+biology, having examined those features of living things which ordinary
+observations can perceive, we come at last to realize that when studied
+for their own sake the properties of living organisms in respect of
+heredity and variation are indications of their inner nature and provide
+evidences of that nature which can be obtained from no other source.
+
+While such ideas were gradually forming in our minds, came the
+rediscovery of Mendel's work. Investigations which before had only
+been imagined as desirable now became easy to pursue, and questions
+as to the genetic inter-relations and compositions of varieties can
+now be definitely answered. Without prejudice to what the future may
+disclose whether by way of limitation or extension of Mendelian method,
+it can be declared with confidence and certainty that we have now the
+means of beginning an analysis of living organisms, and distinguishing
+many of the units or factors which essentially determine and cause the
+development of their several attributes.
+
+Briefly put, the essence of Mendelism lies in the discovery of the
+existence of unit characters or factors. For an account of the Mendelian
+method, how it is applied and what it has already accomplished,
+reference must be made to other works.[1] With this part of the subject
+I shall assume a sufficient acquaintance. In these lectures I have
+rather set myself the task of considering how certain problems appear
+when viewed from the standpoint to which the application of these
+methods has led us. It is indeed somewhat premature to discuss such
+questions. The work of Mendelian analysis is progressing with great
+rapidity and anything I can say may very soon be superseded as out
+of date. Nevertheless a discussion of this kind may be of at least
+temporary service in directing inquiry to the points of special interest.
+
+
+             THE PROBLEM OF SPECIES AND VARIETY
+
+Nowhere does our new knowledge of heredity and variation apply more
+directly than to the problem what is a species and what is a variety? I
+cannot assert that we are already in a position to answer this important
+question, but as will presently appear, our mode of attack and the
+answers we expect to receive are not those that were contemplated by our
+predecessors. If we glance at the history of the scientific conception
+of Species we find many signs that it was not till comparatively recent
+times that the definiteness of species became a strict canon of the
+scientific faith and that attempts were made to give precise limits
+to that conception. When the diversity of living things began to be
+accurately studied in the sixteenth and seventeenth centuries names
+were applied in the loosest fashion, and in giving a name to an animal
+or a plant the naturalists of those times had no ulterior intention.
+Names were bestowed on those creatures about which the writer proposed
+to speak. When Gesner or Aldrovandi refer to all the kinds of horses,
+unicorns, dogs, mermaids, etc., which they had seen or read of, giving
+to each a descriptive name, they do not mean to "elevate" each named
+kind to "specific rank"; and if anyone had asked them what they meant by
+a species, it is practically certain that they would have had not the
+slightest idea what the question might imply, or any suspicion that it
+raised a fundamental problem of nature.
+
+Spontaneous generation being a matter of daily observation, then
+unquestioned, and supernatural events of all kinds being commonly
+reported by many witnesses, transmutation of species had no inherent
+improbability. Matthioli,[2] for instance, did not expect to be charged
+with heresy when he declared _Stirpium mutatio_ to be of ordinary
+occurrence. After giving instances of induced modifications he wrote,
+"Tantum enim in plantis naturae germanitas potest, ut non solum saepe
+praedictos praestet effectus, sed etiam ut alteram in alteram stirpem
+facile vertat, ut cassiam in cinnamomum, sisymbrium in mentham, triticum
+in lolium, hordeum in avenam, et ocymum in serpyllum."
+
+I do not know who first emphasized the need for a clear understanding
+of the sense in which the term species is to be applied. In the second
+half of the seventeenth century Ray shows some degree of concern on
+this matter. In the introduction to the _Historia Plantarum_, 1686, he
+discusses some of the difficulties and lays down the principle that
+varieties which can be produced from the seed of the same plant are to
+be regarded as belonging to one species, being, I believe, the first
+to suggest this definition. That new species can come into existence
+he denies as inconsistent with Genesis 2, in which it is declared that
+God finished the work of Creation in six days. Nevertheless he does not
+wholly discredit the possibility of a "transmutation" of species, such
+that one species may as an exceptional occurrence give rise by seed
+to another and nearly allied species. Of such a phenomenon he gives
+illustrations the authenticity of which he says he is, against his will,
+compelled to admit. He adds that some might doubt whether in the cases
+quoted the two forms concerned are really distinct species, but the
+passage is none the less of value for it shews that the conception of
+species as being distinct unchangeable entities was not to Ray the dogma
+sacrosanct and unquestionable which it afterwards became.[3]
+
+In the beginning of the eighteenth century Marchant,[4] having observed
+the sudden appearance of a lacinated variety of _Mercurialis_, makes the
+suggestion that species in general may have arisen by similar mutations.
+Indeed from various passages it is manifest that to the authors of the
+seventeenth and early eighteenth centuries species appeared simply as
+groups more or less definite, the boundaries of which it was unnecessary
+to determine with great exactitude. Such views were in accord with the
+general scientific conception of the time. The mutability of species is
+for example sometimes likened (see for instance Sharrock, loc. cit.) to
+the metamorphoses of insects, and it is to be remembered that the search
+for the Philosopher's Stone by which the transmutation of metals was to
+be effected had only recently fallen into discredit as a pursuit.
+
+The notion indeed of a peculiar, fixed meaning to be attached to species
+as distinct from variety is I think but rarely to be found categorically
+expressed in prae-Linnaean writings.
+
+But with the appearance of the _Systema Naturae_ a great change
+supervened. Linnaeus was before all a man of order. Foreseeing the
+immense practical gain to science that must come from a codification of
+nomenclature, he invented such a system.
+
+It is not in question that Linnaeus did great things for us and made
+Natural History a manageable and accessible collection of facts instead
+of a disorderly heap; but orderliness of mind has another side, and
+inventors and interpreters of systems soon attribute to them a force and
+a precision which in fact they have not.
+
+The systematist is primarily a giver of names, as Ray with his broader
+views perceived. Linnaeus too in the exordium to the _Systema Naturae_
+naively remarks, that he is setting out to continue the work which
+Adam began in the Golden Age, to give names to the living creatures.
+Naming however involves very delicate processes of mind and of logic.
+Carried out by the light of meagre and imperfect knowledge it entails
+all the mischievous consequences of premature definition, and promotes
+facile illusions of finality. So was it with the Linnaean system. An
+interesting piece of biological history might be written respecting the
+growth and gradual hardening of the conception of Species. To readers
+of Linnaeus's own writings it is well known that his views cannot be
+summarized in a few words. Expressed as they were at various times
+during a long life and in various connexions, they present those divers
+inconsistencies which commonly reflect a mind retaining the power of
+development. Nothing certainly could be clearer than the often quoted
+declaration of the _Philosophia Botanica_, "Species tot numeramus quot
+diversae formae in principio sunt creatae," with the associated passage
+"Varietates sunt plantae ejusdem speciei mutatae a caussa quacunque
+occasionali." Those sayings however do not stand alone. In several
+places, notably in the famous dissertation on the peloric _Linaria_
+he explicitly contemplates the possibility that new species may arise
+by crossing, declaring nevertheless that he thinks such an event to
+be improbable. In that essay he refers to Marchant's observation on a
+laciniate _Mercurialis_, but though he states clearly that that plant
+should only be regarded as a variety of the normal, he does not express
+any opinion that the contemporary genesis of new species must be an
+impossibility. In the later dissertation on Hybrid Plants he returns to
+the same topic. Again though he states the belief that species cannot
+be generated by cross-breedings, he treats the subject not as heretical
+absurdity but as one deserving respectful consideration.
+
+The significance of the aphorisms that precede the lectures on the
+Natural Orders is not easy to apprehend. These are expressed with the
+utmost formality, and we cannot doubt that in them we have Linnaeus's
+own words, though for the record we are dependent on the transcripts of
+his pupils.
+
+   The text of the first five is as follows:
+
+   1. Creator T. O. in primordio vestiit Vegetabile _Medullare_
+   principiis constitutivis diversi _Corticalis_ unde tot difformia
+   individua, quot _Ordines_ Naturales prognata.
+
+   2. _Classicas_ has (1) plantas Omnipotens miscuit inter se,
+   unde tot _Genera_ ordinum, quot inde plantae.
+
+   3. _Genericas_ has (2) miscuit Natura, unde tot _Species_
+   congeneres quot hodie existunt.
+
+   4. _Species_ has miscuit Casus, unde totidem quot passim
+   occurrunt, _Varietates_.
+
+   5. Suadent haec (1-4) Creatoris leges a simplicibus ad
+   Composita.
+
+   Naturae leges generationis in hybridis.
+
+   Hominis leges ex observatis a posteriori.
+
+I am not clear as to the parts assigned in the first sentence
+respectively to the "_Medulla_" and the "_Cortex_," beyond that Linnaeus
+conceived that multiformity was first brought about by diversity in
+the "_Cortex_." The passage is rendered still more obscure if read in
+connection with the essay on "_Generatio Ambigena_," where he expresses
+the conviction that the _Medulla_ is contributed by the mother, and the
+_Cortex_ by the father, both in plants and animals.[5]
+
+But however that may be, he regards this original diversity as resulting
+in the constitution of the Natural Orders, each represented by one
+individual.
+
+In the second aphorism the Omnipotent is represented as creating the
+genera by intermixing the individual _plantae classicae_, or prototypes
+of the Natural Orders.
+
+The third statement is the most remarkable, for in it he declares that
+Species were formed by the act of Nature, who by inter-mixing the genera
+produced _Species congeneres_, namely species inside each genus, to the
+number which now exist. Lastly, Chance or Accident, intermixing the
+species, produced as many varieties as there are about us.
+
+Linnaeus thus evidently regarded the intermixing of an originally
+limited number of types as the sufficient cause of all subsequent
+diversity, and it is clear that he draws an antithesis between
+_Creator_, _Natura_, and _Casus_, assigning to each a special part
+in the operations. The acts resulting in the formation of genera are
+obviously regarded as completed within the days of the Creation, but the
+words do not definitely show that the parts played by Nature and Chance
+were so limited.
+
+Recently also E. L. Greene[6] has called attention to some curious
+utterances buried in the _Species Plantarum_, in which Linnaeus refers
+to intermediate and transitional species, using language that even
+suggests evolutionary proclivities of a modern kind, and it is not easy
+to interpret them otherwise.
+
+Whatever Linnaeus himself believed to be the truth, the effect of his
+writings was to induce a conviction that the species of animals and
+plants were immutably fixed. Linnaeus had reduced the whole mass of
+names to order and the old fantastical transformations with the growth
+of knowledge had lapsed into discredit; the fixity of species was
+taken for granted, but not till the overt proclamation of evolutionary
+doctrine by Lamarck do we find the strenuous and passionate assertions
+of immutability characteristic of the first half of the nineteenth
+century.
+
+It is not to be supposed that the champions of fixity were unacquainted
+with varietal differences and with the problem thus created, but in
+their view these difficulties were apparent merely, and by sufficiently
+careful observation they supposed that the critical and permanent
+distinctions of the true species could be discovered, and the
+impermanent variations detected and set aside.
+
+This at all events was the opinion formed by the great body of
+naturalists at the end of the eighteenth and beginning of the nineteenth
+centuries, and to all intents and purposes in spite of the growth of
+evolutionary ideas, it remains the guiding principle of systematists
+to the present day. There are 'good species' and 'bad species' and the
+systematists of Europe and America spend most of their time in making
+and debating them.
+
+In some of its aspects the problem of course confronted earlier
+naturalists. Parkinson for instance (1640) in introducing his treatment
+of _Hieracium_ wrote, "To set forth the whole family of the Hawkeweedes
+in due forme and order is such a world of worke that I am in much doubt
+of mine own abilitie, it having lyen heavie on his shoudiers that
+hath already waded through them ... for such a multitude of varieties
+in forme pertaining to one herbe is not to be found againe in _rerum
+natura_ as I thinke," and the same idea, that the difficulty lay rather
+in man's imperfect powers of discrimination than in the nature of the
+materials to be discriminated, is reflected in many treatises early and
+late.
+
+It was however with the great ouburst of scientific activity which
+followed Linnaeus that the difficulty became acute. Simultaneously
+vast masses of new material were being collected from all parts of the
+world into the museums, and the products of the older countries were
+re-examined with a fresh zeal and on a scale of quantity previously
+unattempted. But the problem how to name the forms and where to draw
+lines, how much should be included under one name and where a new name
+was required, all this was felt, rather as a cataloguer's difficulty
+than as a physiological problem. And so we still hear on the one hand
+of the confusion caused by excessive "splitting" and subdivisions, and
+on the other of the uncritical "lumpers" who associate together under
+one name forms which another collector or observer would like to see
+distinguished.
+
+In spite of Darwin's hopes, the acceptance of his views has led to no
+real improvement--scarcely indeed to any change at all in either the
+practice or aims of systematists. In a famous passage in the _Origin_ he
+confidently declares that when his interpretation is generally adopted
+"Systematists will be able to pursue their labours as at present; but
+they will not be incessantly haunted by the shadowy doubt whether this
+or that form be a true species. This, I feel sure, and I speak after
+experience, will be no slight relief. The endless disputes whether or
+not some fifty species of British brambles are good species will cease."
+Those disputes nevertheless proceed almost exactly as before. It is
+true that biologists in general do not, as formerly, participate in
+these discussions because they have abandoned systematics altogether;
+but those who are engaged in the actual work of naming and cataloguing
+animals and plants usually debate the old questions in the old way.
+There is still the same divergence of opinion and of practice, some
+inclining to make much of small differences, others to neglect them.
+
+Not only does the work of the systematists as a whole proceed as if
+Darwin had never written but their attitude towards these problems is
+but little changed. In support of this statement I may refer to several
+British Museum Catalogues, much of the _Biologia Centrali-Americana_,
+Ridgway's _Birds of North America_, the _Fauna Hawaiensis_, indeed to
+almost any of the most important systematic publications of England,
+America, or any other country. These works are compiled by the most
+proficient systematists of all countries in the several groups, but
+with rare exceptions they show little misgiving as to the fundamental
+reality of specific differences. That the systematists consider the
+species-unit as of primary importance is shown by the fact that the
+whole business of collection and distribution of specimens is arranged
+with regard to it.
+
+Almost always the collections are arranged in such a way that the
+phenomena of variation are masked. Forms intermediate between two
+species are, if possible, sorted into separate boxes under a third
+specific name. If a species is liable to be constantly associated
+with a mutational form, the mutants are picked out, regardless of the
+circumstances of their origin, from the samples among which they were
+captured, and put apart under a special name. Only by a minute study
+of the original labels of the specimens and by redistributing them
+according to locality and dates, can their natural relations be traced.
+The published accounts of these collections often take no notice of
+variations, others make them the subject of casual reference. Very few
+indeed treat them as of much importance. From such indications it is
+surely evident that the systematists attach to the conception of species
+a significance altogether different from that which Darwin contemplated.
+
+I am well aware that some very eminent systematists regard the whole
+problem as solved. They hold as Darwin did that specific diversity
+has no physiological foundation or causation apart from fitness, and
+that species are impermanent groups, the delimitations of which are
+ultimately determined by environmental exigency or "fitness." The
+specific diversity of living things is thus regarded as being something
+quite different in nature from the specific diversity of inorganic
+substances. In practice those who share these opinions are, as might
+be anticipated, to be found among the 'lumpers' rather than among the
+'splitters.' In their work, certainly, the Darwinian theory is actually
+followed as a guiding principle; unanalysed inter-gradations of all
+kinds are accepted as impugning the integrity of species; the underlying
+physiological problem is forgotten, and while the product is almost
+valueless as a contribution to biological research, I can scarcely
+suppose that it aids greatly in the advances of other branches of our
+science.
+
+But why is it that, with these exceptions, the consequences of the
+admittedly general acceptance of a theory of evolution are so little
+reflected in the systematic treatment of living things? Surely the
+reason is that though the systematist may be convinced of the general
+truth of the evolution theory at large, he is still of opinion that
+species are really distinct things. For him there are still 'good'
+species and 'bad' species and his experience tells him that the
+distinction between the two is not simply a question of degree or a
+matter of opinion.
+
+To some it may seem that this is mere perversity, a refusal to see
+obvious truth, a manifestation of the spirit of the collector rather
+than of the naturalist. But while recognising that from a magnification
+of the conception of species the systematists are occasionally led
+into absurdity I do not think the grounds for their belief have in
+recent times been examined with the consideration they deserve. The
+phenomenon of specific diversity is manifested to a similar degree by
+living things belonging to all the great groups, from the highest to the
+lowest, Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae,
+and Bacteria, all present diversities of such a kind that among them the
+existence of specific differences can on the whole be recognised with
+a similar degree of success and with very similar limitations. In all
+these groups there are many species quite definite and unmistakable, and
+others practically indefinite. The universal presence of specificity,
+as we may call it, similarly limited and characterised, is one of its
+most remarkable features. Not only is this specificity thus universally
+present among the different forms of life, but it manifests itself
+in respect of the most diverse characteristics which living things
+display. Species may thus be distinguished by peculiarities of form,
+of number, of geometrical arrangement, of chemical constitution and
+properties, of sexual differentiation, of development, and of many
+other properties. In any one or in several of these features together,
+species may be found distinguished from other species. It is also to be
+observed that the definiteness of these distinctions has no essential
+dependence on the nature of the characteristic which manifests them.
+It is for example sometimes said that colour-distinctions are of small
+systematic importance, but every systematist is familiar with examples
+(like that of the wild species of _Gallus_) in which colours though
+complex, show very little variation. On the other hand features of
+structure, sexual differentiation, and other attributes which by our
+standards are estimated as essential, may be declared to show much
+variation or little, not according to any principle which can be
+detected, but simply as the attention happens to be applied to one
+species or group of species, or to another. In many groups of animals
+and plants observers have hit upon characters which were for a time
+thought to be finally diagnostic of species. The Lepidoptera and Diptera
+for instance, have been re-classified according to their neuration.
+Through a considerable range of forms determinations may be easily made
+on these characters, but as is now well known, neuration is no more
+immune from variation than any other feature of organisation, and in
+some species great variability is the rule. Again it was once believed
+by some that the genitalia of the Lepidoptera provided a basis of final
+determination--with a similar sequel. In some groups, for example
+the Lycaenidae, or the Hesperidae, there are forms almost or quite
+indistinguishable on external examination, but a glance at the genitalia
+suffices to distinguish numerous species, while on the contrary among
+Pieridae a great range of species show scarcely any difference in these
+respects: and again in occasional species the genitalia show very
+considerable variations.
+
+The proposition that animals and plants are on the whole divisible into
+definite and recognisable species is an approximation to the truth.
+Such a statement is readily defensible, whereas to assert the contrary
+would be palpably absurd. For example, a very competent authority
+lately wrote: "In the whole Lepidopterous fauna of England there is no
+species of really uncertain limits."[7] Others may be disposed to make
+certain reservations, but such exceptions would be so few as scarcely to
+impair the validity of the general statement. The declaration might be
+extended to other orders and other lands.
+
+We know, of course, that the phenomenon of specific diversity is
+complicated by local differentiation: that, in general, forms which
+cannot disperse themselves freely exhibit a multitude of local races,
+and that of these some are obviously adaptative, and that a few even owe
+their peculiarity to direct environmental effects. Every systematist
+also is perfectly aware that in dealing with collections from little
+explored countries the occurrence of polymorphism or even of sporadic
+variation may make the practical business of distinguishing the species
+difficult and perhaps for the time impossible; still, conceding that a
+great part of the diversity is due to geographical differentiation, and
+that some is sporadic variation, our experience of our own floras and
+faunas encourages the belief that if we were thoroughly familiar with
+these exotic productions it would usually be possible to assign their
+specific limitations with an approach to certainty.
+
+For apart from any question of the justice of these wider inferences,
+if we examine the phenomenon of specificity as it appears in those
+examples which are nearest to hand, surely we find signs in plenty that
+specific distinction is no mere consequence of Natural Selection. The
+strength of this proposition has lain mainly in the appeal to ignorance.
+Steadily with the growth of knowledge has its cogency diminished, and
+such a belief could only have been formulated at a time when the facts
+of variation were unknown.
+
+In Darwin's time no serious attempt had been made to examine the
+manifestations of variability. A vast assemblage of miscellaneous facts
+could formerly be adduced as seemingly comparable illustrations of the
+phenomenon "Variation." Time has shown this mass of evidence to be
+capable of analysis. When first promulgated it produced the impression
+that variability was a phenomenon generally distributed amongst living
+things in such a way that the specific divisions must be arbitrary.
+When this variability is sorted out, and is seen to be in part a result
+of hybridisation, in part a consequence of the persistence of hybrids
+by parthenogenetic reproduction, a polymorphism due to the continued
+presence of individuals representing various combinations of Mendelian
+allelomorphs, partly also the transient effect of alteration in external
+circumstances, we see how cautious we must be in drawing inferences as
+to the indefiniteness of specific limits from a bare knowledge that
+intermediates exist. Conversely, from the accident of collocation or
+from a misleading resemblance in features we deem essential, forms
+genetically distinct are often confounded together, and thus the
+divergence of such forms in their other features, which we declare
+to be non-essential, passes as an example of variation. Lastly, and
+this is perhaps the most fertile of all the sources of confusion, the
+impression of the indefiniteness of species is created by the existence
+of numerous local forms, isolated geographically from each other, forms
+whose differences may be referable to any one of the categories I have
+enumerated.
+
+The advance has been from many sides. Something has come from the work
+of systematists, something from cultural experiments, something from
+the direct study of variation as it appears in nature, but progress
+is especially due to experimental investigation of heredity. From all
+these lines of inquiry we get the same answer; that what the naturalists
+of fifty years ago regarded as variation is not one phenomenon but
+many, and that what they would have adduced as evidence against the
+definiteness of species may not in fact be capable of this construction
+at all.
+
+If we may once more introduce a physical analogy, the distinctions with
+which the systematic naturalist is concerned in the study of living
+things are as multifarious as those by which chemists were confronted in
+the early days of their science. Diversities due to mechanical mixtures,
+to allotropy, to differences of temperature and pressure, or to degree
+of hydration, had all to be severally distinguished before the essential
+diversity due to variety of chemical constitution stood out clearly,
+and I surmise that not till a stricter analysis of the diversities of
+animals and plants has been made on a comprehensive scale, shall we be
+in a position to declare with any confidence whether there is or is not
+a natural and physiological distinction between species and variety.
+
+As I have said above, it is in the cases nearest to hand that the
+problem may be most effectively studied. Comparison between forms from
+dissimilar situations contributes something; but it is by a close
+examination of the behaviour, especially the genetic behaviour, of
+familiar species when living in the presence of their nearest allies
+that the most direct light on the problem is to be obtained. I cannot
+understand the attitude of those who, contemplating such facts as this
+examination elicits, can complacently declare that specific difference
+is a mere question of degree. With the spread of evolutionary ideas to
+speak much of the fixity of species has become unfashionable, and yet
+how striking and inscrutable are the manifestations of that fixity!
+
+Consider the group of species composing the _agrestis_ section of the
+genus _Veronica_, namely _Tournefortii_, _agrestis_, and _polita_.
+
+These three grow side by side in my garden, as they do in suitable
+situations over a vast area of the temperate regions. I have for years
+noticed them with some care and become familiar with their distinctions
+and resemblances. Never is there any real doubt as to the identity of
+any plant. The species show some variability, but I have never seen
+one which assumed any of the distinguishing features of the others. A
+glance at the fruits decides at once to which species a plant belongs. I
+find it impossible to believe that the fixity of these distinctions is
+directly dependent on their value as aids in the struggle for existence.
+The mode of existence of the three forms in so far as we can tell is
+closely similar. By whatever standard we reckon systematic affinity I
+suppose we shall agree that these species come very near indeed to each
+other. Bentham even takes the view that _polita_ is a mere variety of
+_agrestis_.
+
+Now in such cases as this it has been argued that the specific
+features of the several types have been separately developed in as
+many distinct localities, and that their present association is due
+to subsequent redistribution. Of these Veronicas indeed we know that
+one, _Tournefortii_ (= _Buxbaumii_) is as a matter of fact a recent
+introduction from the east.[8] But this course of argument leads to
+still further difficulties. For if it is true that the peculiarities
+of the several species have been perfected and preserved on account
+of their survival-value to their possessors, it follows that there
+must be many ways of attaining the same result. But since sufficient
+adaptation may be ensured in so many ways, the disappearance of the
+common parent of these forms is difficult to understand. Obviously it
+must have been a plant very similar in general construction to its
+modern representatives. Like them it must have been an annual weed, with
+an organisation conformable to that mode of life. Why then, after having
+been duly perfected for that existence should it have been entirely
+superseded in favour of a number of other distinct contrivances for
+doing the same thing, and--if a gradual transition be predicated--not
+only by them, but by each intermediate stage between them and the
+original progenitor? Surely the obvious inference from such facts is
+that the burden cast upon the theory of gradual selection is far greater
+than it can bear; that adaptation is not in practice a very close fit,
+and that the distinctions between these several species of Veronica have
+not arisen on account of their survival-value but rather because none
+of their diversities was so damaging as to lead to the extermination
+of its possessor. When we see these various Veronicas each rigidly
+reproducing its parental type, all comfortably surviving in competition
+with each other, are we not forced to the conclusion that _tolerance_
+has as much to do with the diversity of species as the stringency of
+Selection? Certainly these species owe their continued existence to the
+fact that they are each good enough to live, but how shall we refer the
+distinctions between them directly or indirectly to the determination of
+Natural Selection?
+
+The control of Selection is loose while the conformity to specific
+distinction is often very strict and precise, and no less so even when
+several closely related species co-exist in the same area and in the
+same circumstances.
+
+The theory of Selection fails at exactly the point where it was devised
+to help: _Specific_ distinction.
+
+Let us examine a somewhat different set of facts in the case of another
+pair of nearly allied species _Lychnis diurna_ and _vespertina_. The
+two plants have much in common. Both are dioecious perennials, with
+somewhat similar flowers, the one crimson, the other white. Each however
+has its peculiarities which are discernible in almost any part of its
+structure, whether flower, leaf, fruit or seed, distinctions which
+would enable a person thoroughly familiar with the plants to determine
+at once from which species even a small piece had been taken. There is
+so much resemblance however as readily to support the surmise that the
+two were mere varieties of one species. Bentham, following Linnaeus,
+in fact actually makes this suggestion, with what propriety we will
+afterwards consider. Now this case is typical of many. The two forms
+have a wide distribution, occurring sometimes separately, sometimes
+in juxtaposition. _L. diurna_ is a plant of hedgerows and sheltered
+situations. _L. vespertina_ is common in fields and open spaces, where
+_diurna_ is hardly ever found; but not rarely _vespertina_ occurs in
+association with _diurna_ in the places which that plant frequents. In
+this case I do not doubt that we have to do with organisms of somewhat
+different aptitudes. That _L. vespertina_ has powers which _diurna_
+has not is shown very clearly by the fact that _diurna_ is sometimes
+entirely absent from areas where _vespertina_ can abound.[9] But in
+order to understand the true genetic relations of the two plants to
+each other it is necessary to observe their behaviour when they meet
+as they not unfrequently do. If the _Lychnis_ population of such a
+locality be examined it will be found to consist of many undoubted
+and unmodified _diurna_, a number--sometimes few, sometimes many--of
+similarly unmodified _vespertina_, and an uncertain but usually rather
+small proportion of plants obviously hybrids between the two. How
+is it possible to reconcile these facts with the view that specific
+distinction has no natural basis apart from environmental exigency?
+
+Darwinian orthodoxy suggests that by a gradual process of Natural
+Selection either one of these two types was evolved from the other,
+or both from a third type. I cannot imagine that anyone familiar with
+the facts would propose the first hypothesis in the case of _Lychnis_,
+nor can I conceive of any process, whether gradual or sudden, by
+which _diurna_ could have come out of _vespertina_, or _vespertina_
+out of _diurna_. Both however may no doubt have been derived from
+some original third type. It is conceivable that _Lychnis macrocarpa_
+of Boissier, a native of Southern Spain and Morocco, may be this
+original form. This species is said to combine a white flower (like
+that of _L. vespertina_), with capsule-teeth rolled back (like those
+of _diurna_).[10] But whatever the common progenitor may have been, if
+we are to believe that these two species have been evolved from it by
+a gradual process of Natural Selection based on adaptation, enormous
+assumptions must be made regarding the special fitness of these two
+forms and the special unfitness of the common parent, and these
+assumptions must be specially invoked and repeated for each several
+feature of structure or habits distinguishing the three forms.
+
+Why, if the common parent was strong enough to live to give rise to
+these two species, is it either altogether lost now, or at least absent
+from the whole of Northern Europe? Its two putative descendants, though
+so distinct from each other, are, as we have seen, able often to
+occupy the same ground. If they were gradually derived from a common
+progenitor--necessarily very like themselves--can we believe that
+this original form should always, in all the diversities of soil and
+situation which they inhabit, be unable to exist? Some one may fancy
+that the hybrids which are found in the situations occupied by both
+forms are this original parental species. But nothing can be more
+certain than that these plants are simply heterozygous combinations
+made by the union of gametes bearing the characters of _diurna_ and
+_vespertina_.[11] For they may be reproduced exactly in F_{1} or in
+later generations of that cross when it is artificially made; when bred
+from their families exhibit palpable phenomena of segregation more or
+less complex; and usually, if perhaps not always, they are partially
+sterile.[12] In a locality on the Norfolk coast that I know well,
+there is a strip of rough ground chiefly sand-bank, which runs along
+the shore. This ground is full of _vespertina_. Not a hundred yards
+inland is a lane containing _diurna_, and among the _vespertina_ on
+the sand-bank are always some of the hybrid form, doubtless the result
+of fertilisation from the neighbouring _diurna_ population. Seed saved
+from these hybrids gave _vespertina_ and hybrids again, having obviously
+been fertilised by other _vespertina_ or by other hybrids, and I have no
+doubt that such hybrid plants if fertilised by _diurna_ would have shown
+some _diurna_ offspring. The absence of _diurna_ in such localities may
+fairly be construed as an indication that _diurna_ is there at a real
+disadvantage in the competition for life.
+
+But if, admitting this, we proceed to consider how the special aptitude
+of _vespertina_ is constituted, or what it is that puts _diurna_ at
+a disadvantage, we find ourselves quite unable to show the slightest
+connexion between the success of one or the failure of the other on the
+one hand, and _the specific characteristics_ which distinguish the two
+forms on the other. The orthodox Selectionist would, as usual, appeal to
+ignorance. We ask what can _vespertina_ gain by its white flowers, its
+more lanceolate leaves, its grey seeds, its almost erect capsule-teeth,
+its longer fruits, which _diurna_ loses by reason of its red flowers,
+more ovate leaves, dark seeds, capsule-teeth rolled back, and shorter
+fruits? We are told that each of these things _may_ affect the viability
+of their possessors. We cannot assert that this is untrue, but we should
+like to have evidence that it is true. The same problem confronts us
+in thousands upon thousands of examples, and as time goes on we begin
+to feel that speculative appeals to ignorance, though dialectically
+admissible, provide an insufficient basis for a proposition which,
+if granted, is to become the foundation of a vast scheme of positive
+construction.
+
+One thing must be abundantly clear to all, that to treat two forms so
+profoundly different as one, because intermediates of unknown nature can
+be shown to exist between them, is a mere shirking of the difficulties,
+and this course indeed creates artificial obstacles in the way of those
+who are seeking to discover the origin of organic diversity.
+
+In the enthusiasm with which evolutionary ideas were received the
+specificity of living things was almost forgotten. The exactitude with
+which the members of a species so often conform in the diagnostic,
+specific features passed out of account; and the scientific world by
+dwelling with a constant emphasis on the fact of variability, persuaded
+itself readily that species had after all been a mere figment of
+the human mind. Without presuming to declare what future research
+only can reveal, I anticipate that, when variation has been properly
+examined and the several kinds of variability have been successfully
+distinguished according to their respective natures, the result will
+render the natural definiteness of species increasingly apparent.
+Formerly in such a case as that of the two _Lychnis_ species, the series
+of "intermediates" was taken to be a palpable proof that _vespertina_
+"graded" to _diurna_. It is this fact, doubtless, upon which Bentham
+would have relied in suggesting that both may be one species.[13]
+Genetic tests, though as yet imperfectly applied, make it almost certain
+that these inter-grading forms are not in any true sense variations
+from either species in the direction of the other, but combinations of
+elements derived from both.
+
+The points in which very closely allied species are distinguished
+from each other may be found in the most diverse features of their
+organisation. Sometimes specific difference is to be seen in a
+character which we can believe to be important in the struggle, but at
+least as often it is some little detail that we cannot but regard as
+trivial which suffices to differentiate the two species. Even when the
+diagnostic point is of such a nature that we can imagine it to make a
+serious difference in the economy we are absolutely at a loss to suggest
+why this feature should be a necessity to species A and unnecessary to
+species B its nearest ally. The house sparrow (_Passer domesticus_) is
+in general structure very like the tree sparrow (_P. montanus_). They
+differ in small points of colour. For instance _montanus_ has a black
+patch on the cheek which is absent in _domesticus_. The presence in the
+one species and the absence in the other are equally definite, and in
+both cases we are equally unable to suggest any consideration of utility
+in relation to these features. The two species are distinguished also by
+a characteristic that may well be supposed to be of great significance.
+In _domesticus_ the two sexes are strongly differentiated, the cock
+being more ornate than the hen. On the other hand the two sexes in
+_montanus_ are alike, and, if we take a standard from _domesticus_,
+we may fairly say that in _montanus_ the hen has the colouration of
+the male. It is not unreasonable to suppose that such a distinction
+may betoken some great difference in physiological economy, but the
+economical significance of this perhaps important distinction is just as
+unaccountable as that of the seemingly trivial but equally diagnostic
+colour-point.
+
+I have spoken of the fixed characteristics of the two species. If
+we turn to a very different feature, their respective liability to
+albinistic variation, we find ourselves in precisely similar difficulty.
+_Passer domesticus_ is a species in which individuals more or less pied
+occur with especial frequency, but in _P. montanus_ such variation is
+extremely rare if it occurs at all. The writer of the section on Birds
+in the _Royal Natural History_ (III., 1894-5, p. 393) calls attention to
+this fact and remarks that in that species he knows no such instance.
+
+The two species therefore, apart from any differences that we can
+suppose to be related to their respective habits, are characterised by
+small fixed distinctions in colour-markings, by a striking difference
+in secondary sexual characters, and by a difference in variability. In
+all these respects we can form no surmise as to any economic reason why
+the one species should be differentiated in the one way and the other in
+the other way, and I believe it is mere self-deception which suggests
+the hope that with fuller knowledge reasons of this nature would be
+discovered.
+
+The two common British wasps, _Vespa vulgaris_ and _Vespa germanica_,
+are another pair of species closely allied although sharply
+distinguished, which suggest similar reflexions. Both usually make
+subterranean nests but of somewhat different materials. _V. vulgaris_
+uses rotten wood from which the nest derives a characteristic yellow
+colour, while _V. germanica_ scrapes off the weathered surfaces of
+palings and other exposed timber, material which is converted into
+the grey walls of the nest. The stalk by which the nest is suspended
+(usually to a root) in the case of _germanica_ passes freely through
+a hole in the external envelope, but _vulgaris_ unites this external
+wall solidly to the stalk. In bodily appearance and structure the
+two species are so much alike that they have often been confounded
+even by naturalists, and to the untrained observer they are quite
+indistinguishable. There are nevertheless small points of difference
+which almost though not quite always suffice to distinguish the
+two forms. For example the yellow part of the sinus of the eyes is
+emarginate in _vulgaris_ but not emarginate in _germanica_. _V.
+vulgaris_ often has black spots on the tibiae while in _germanica_ the
+tibiae are usually plain yellow. In both species there is a horizontal
+yellow stripe on the thorax, but whereas in _vulgaris_ this is a plain
+narrow stripe, it is in _germanica_ enlarged downwards in the middle.
+These and other apparently trivial details of colouration, though not
+absolutely constant, are yet so nearly constant that irregularities
+in these respects are quite exceptional. Lastly the genitalia of the
+males, though not very different, present small structural points
+of distinction which are enough to distinguish the two species at a
+glance.[14]
+
+In considering the meaning of the distinctions between these two wasps
+we meet the old problem illustrated by the Sparrows. The two species
+have somewhat different habits of life and we should readily expect
+to find differences of bodily organisation corresponding with the
+differences of habits. But is that what we do find? Surely not. To
+suppose that there is a correspondence between the little points of
+colour and structure which we see and the respective modes of life of
+the two species is perfectly gratuitous. We have no inkling of the
+nature of such a correspondence, how it can be constituted, or in what
+it may consist.
+
+Is it not time to abandon these fanciful expectations which are never
+realised? Everywhere both among animals and plants does the problem of
+specific difference reiterate itself in the same form. In view of such
+facts as I have related and might indefinitely multiply, the fixity of
+specific characters cannot readily be held to be a measure of their
+economic importance to their possessors. The incidence of specific
+fixity is arbitrary and capricious, sometimes lighting on a feature or
+a property which can be supposed to matter much, but as often is it
+attached to the most trifling of superficial peculiarities.
+
+The incidence of _variability_ is no less paradoxical, and without
+investigation of the particular case no one can say what will be found
+to show much or little variability. The very characteristic which in
+one species may exhibit extreme variability may in an allied species
+show extreme constancy. Illustrations will occur to any naturalist, but
+nowhere is this truth more strikingly presented than in the British
+Noctuid Moths. Many are so variable that, in the common phrase,
+"scarcely two can be found alike," while others show comparatively
+slight variation. It need scarcely be remarked that, in the instances
+I have in mind, the evidence of great variability is in no way due to
+the abundance with which the particular species occurs, for common
+species may show constancy, and less abundant species may show great
+variability. The polymorphism seems to be now at least a general
+property of the variable species, as the fixity is a property of the
+fixed species. In illustration I may refer to the following examples.
+
+_Dianthoecia capsincola_ is a common and widely distributed moth which
+feeds on _Lychnis_. It shows little variation. _Dianthoecia carpophaga_
+is another species which feeds chiefly on _Silene_. Its habits are
+very similar to those of _capsincola_. Like that species it has a wide
+geographical range and is abundant in its localities, but in contrast
+to the fixity of _capsincola_, _carpophaga_ exhibits a complex series
+of varieties. _Agrotis suffusa_ (= _ypsilon_) is a moth widely spread
+through the southern half of England. It is very constant in colour and
+markings. _Agrotis segetum_ and _tritici_ are excessively variable both
+in ground colour and markings, being found in an immense profusion of
+dissimilar forms throughout their distribution. Of these and several
+other species of _Agrotis_ there are many named varieties, some of
+which have by various writers been regarded as specifically distinct.
+Of the genus _Noctua_ many species (e. g. _festiva_) show a similar
+polymorphism, but _N. triangulum_, though showing some variation in
+certain respects, is usually very constant to its type, and the same is
+true of _N. umbrosa_.
+
+In several species of _Taeniocampa_, especially _instabilis_, the
+multiplicity of forms is extreme, while _cruda_ (= _pulverulenta_) is
+a comparatively constant species. The genus _Plusia_ contains a number
+of constant species, but in _Plusia interrogationis_ we meet the fact
+that the central silvery mark undergoes endless variation. "Truly no
+two are alike," says Mr. Tutt, "and to look down a long series of
+_interrogationis_ is something like looking at a series of Chinese
+characters." In contrast to this we have the fact that in _Plusia gamma_
+the very similar silvery mark is by no means variable.
+
+I have taken this series of cases from the Noctuid moths, but it would
+be as easy to illustrate the same proposition from the Geometridae or
+the Micro-Lepidoptera.[15] I have a long series of _Peronea cristana_,
+for example, which was given to me by Mr. W. H. B. Fletcher, of Bognor.
+All were beaten out of the same hedge, and their polymorphism is such
+that no one unaccustomed to such examples could suppose that they
+belonged to a single species. Another common form, _P. schalleriana_,
+which lives in similar circumstances, exhibits comparatively slight
+variability.
+
+It should be expressly noted that the variation of which I am speaking
+is a genuine polymorphism. Several of the species enumerated exhibit
+also geographical variation, possessing definite and often strikingly
+distinct races peculiar to certain localities; but apart from the
+existence of such local differentiation, stands out the fact upon which
+I would lay stress, that some species are excessively variable while
+others are by comparison constant, in circumstances that we may fairly
+regard as comparable.
+
+This fact is difficult to reconcile with the conventional view that
+specific type is directly determined by Natural Selection and that
+the precision with which a species conforms to its pattern is an
+indication of the closeness of that control. Anyone familiar with the
+characteristics of Moths will agree that the Noctuids, Geometrids and
+Tortricids are creatures whose existence depends in some degree on the
+success with which they can escape detection by their enemies in the
+imaginal state. We are therefore not surprised to find that some species
+of these orders exhibit definite geographical variation in conformity
+with the character of the ground, which may reasonably be supposed to
+aid in their protection. If this were all, there would be nothing to
+cause surprise. We might even be disposed to allow that variability
+might contribute to the perpetuation of animals so situated, on the
+principle that among a variety of surroundings some would probably be
+in harmony with the objects on which they rest. But we cannot admit
+the plausibility of an argument which demands on the one hand that
+the extreme precision with which species A adheres in the minutest
+details of its colour and pattern to a certain type shall be ascribed
+to the protective fitness of those details, and on the other hand that
+the abundant variability of species B shall be ascribed to the same
+determination. If it is absolutely necessary for A to conform to one
+type how comes it that B may range through some twenty distinct forms,
+any two of which differ more from each other than the regular species
+of many other genera? The only reply I can conceive is a suggestion
+that there _may_ be some circumstance which differentiates the various
+classes of cases, that the exigencies of the fixed species _may_ be
+different from those of the variable. Those who make such appeals to
+ignorance do not always perhaps realise whither this course of reasoning
+may lead. If admissible here the same argument would lead us to suggest
+that because albino moles have for an indefinite period occurred on a
+certain land near Bath there may be something in the soil or in the
+conditions of life near Bath which requires a proportion of albinos in
+its mole population. Or again, because the butterfly _Thais rumina_
+in one locality, Digne in the south of France, has a percentage of
+individuals of the variety _Honoratii_ (with certain normally yellow
+spots on the hind wing coloured bright red) and nowhere else throughout
+its distribution, that therefore we may suggest that there is some
+difference in the condition of life at Digne which makes the continuance
+of _Honoratii_ there possible and beneficial.
+
+A polymorphism offering a parallel to that of the variable moths is
+afforded by the breeding plumage of the Ruff, the male of _Machetes
+pugnax_. The variety of plumage which these cocks exhibit is such
+that the statement that no two can be found alike is only a venial
+exaggeration. Newton remarks[16] "that all this wonderful 'show' is
+the consequence of the polygamous habit of the Ruff can scarcely
+be doubtful"; but even if it be conceded that the great external
+differentiation of the cocks may be a result of sexual selection, the
+problem of their _polymorphism_ remains unsolved, for, as we are well
+aware, polygamy is not usually associated with polymorphism of the male.
+The Black Cock (_Tetrao tetrix_), for example, is as polygamous as the
+Ruff, but in that and countless other cases, both sexes are constant to
+one type of plumage.
+
+When we thus compare the polymorphism of one species with the fixity
+of another, and attempt to determine the causes which have led to
+these extraordinary contrasts, two distinct lines of argument are
+open to us. We may ascribe the difference either to causes external
+to the organisms, primarily, that is to say, to a difference in the
+exigencies of Adaptation under Natural Selection; or on the other hand
+we may conceive the difference as due to innate distinctions in the
+chemical and physiological constitutions of the fixed and the variable
+respectively. There is truth undoubtedly in both conceptions. If the
+mole were physiologically incapable of producing an albino that variety
+would not have come into being, and if the albino were totally incapable
+of getting its living it would not be able to hold its own. Were
+_Plotheia frontalis_ constructed on a chemical plan which admitted of no
+variation, the countless varieties would not have been produced; and if
+one of its varieties had an overwhelming success out of all proportion
+to that of the rest, then the species would soon become monomorphic
+again. We cannot declare that Natural Selection has no part in the
+determination of fixity or variability; nevertheless looking at the
+whole mass of fact which a study of the incidence of variation provides,
+I incline to the view that the variability of polymorphic forms should
+be regarded rather as a thing tolerated than as an element contributing
+directly to their chances of life; and on the other hand that the fixity
+of the monomorphic forms should be looked upon not so much as a proof
+that Natural Selection controls them with a greater stringency, but
+rather as evidence of a natural and intrinsic stability of chemical
+constitution.
+
+Compare the condition of a variable form like the male Ruff (or in
+a less degree the Red Grouse in both its sexes) with that of the
+common Pheasant which is comparatively constant. In the Pheasant no
+doubt variations do occur as in other wild birds, but apart from the
+effects of mongrelisation the species is unquestionably uniform. Could
+it seriously be proposed that we should regard the constancy of the
+pheasant's plumage in this country as depending on the special fitness
+of that type of colouration? Even if the pheasant be not an alien in
+Western Europe, it has certainly been protected for centuries, and for a
+considerable period has existed in a state of semi-domestication. Such
+conditions should give good opportunity for polymorphism to be produced.
+In some coverts various aberrations do of course occur and persist,
+yet there is nothing indicative of a general relaxation of the fixity
+of the specific type, and the pheasant remains substantially a fixed
+species.[17] The common pheasant (_Phasianus colchicus_) even shows
+little of that disposition to form local races which appears in the
+species of Further India. Are we not then on safer ground in regarding
+the fixity of our species as a property inherent in its own nature
+and constitution? Just as in ages of domestication no rose has ever
+given off a blue variety so has the pheasant never broken out into the
+polymorphism of the Ruff.
+
+As soon as it is realised how largely the phenomena of variation and
+stability must be an index of the internal constitution of organisms,
+and not mere consequences of their relations to the outer world, such
+phenomena acquire a new and more profound significance.
+
+FOOTNOTES:
+
+[1] In _Mendel's Principles of Heredity_ (Cambridge University Press,
+1909) I have dealt with this subject, giving an account of the principal
+facts discovered up to the beginning of 1909.
+
+[2] Matthioli Opera, Ed. 1598, p. 8, originally published 1565.
+
+[3] Ray's instances relate to Kales, and in most of these examples
+we can see that there was no question of mutation or transmutation
+at all, but that the occurrence was due either to mistake or to
+cross-fertilisation. Sharrock, to whom Ray refers, was inclined to
+discredit stories of transmutation, but he has also this passage
+(_History of the Propagation and Improvement of Vegetables by the
+Concurrence of Art and Nature_, Oxford, 1660, p. 29):
+
+"It is indeed growen to be a great question, whether the transmutation
+of a species be possible either in the vegetable, Animal, or Minerall
+Kingdome. For the possibility of it in the vegetable; I have heard _Mr.
+Bobart_ and his _Son_ often report it, and proffer to make oath that the
+Crocus and Gladiolus, as likewise the Leucoium, and Hyacinths by a long
+standing without replanting have in his garden changed from one kind to
+the other: and for satisfaction about the curiosity in the presence of
+_Mr. Boyle_ I tooke up some bulbs of the very numericall roots whereof
+the relation was made, though the alteration was perfected before, where
+we saw the diverse bulbs growing as it were on the same stoole, close
+together, but no bulb half of the one kind, and the other half of the
+other: But the changetime being past it was reason we should believe the
+report of good artists in matters of their own faculty."
+
+Robert Sharrock was a fellow of New College, Oxford. Both the Bobarts
+were professional botanists, the father was author of a Catalogue of
+the plants in the Hortus Medicus at Oxford, and the son was afterwards
+Curator of the Oxford Garden.
+
+[4] _Mem. Ac. roy. des Sci._ for 1719 (1721), p. 59.
+
+[5] _Amoen. Acad._, 1789, vol. 6. I do not know whether attention has
+been called to the curious mistake which Linnaeus makes in the course of
+this argument. He cites the differences between the Mule and the Hinny
+in illustration of his thesis, pointing out that the Mule is externally
+more like a horse and the Hinny more like an ass. This, he says, is
+because the Mule has the horse for a father, and the Hinny the ass, thus
+inverting the actual facts!
+
+[6] _Proc. Washington Ac. Sci._, 1909, XI, pp. 17-26.
+
+[7] J. W. Tutt, in _Ent. Rec._, 1909, XXI, p. 185.
+
+[8] E. Lehmann (_Bull. l'Herb. Boissier_, Ser. 2, VIII, 1908, p. 229)
+has published an admirable paper on the interrelationships of these
+species and has instituted cultural experiments which will probably
+much elucidate the nature of their specific distinctness. As regards
+the existence of intermediate forms he comes to the conclusion that
+two only can be so regarded. The first was described by Kuntze from
+specimens found on a flower-pot on board a Caspian steamer, from which
+Lehmann proposes the new specific name _Siaretensis_. This comes between
+_polita_ and _filiformis_, a close ally of _Tournefortii_. The other,
+which combines some of the features of both _polita_ and _Tournefortii_,
+was found in the province of Asterabad.
+
+[9] In Cambridgeshire for example _vespertina_ is common but _diurna_ is
+absent. Whether this absence is connected with the general presence of
+chalk I cannot say. When introduced artificially _diurna_ establishes
+itself, for a time at least, without any apparent difficulty and
+occasionally escapes from the garden on to the neighbouring roadside.
+
+[10] Conceivably however it may be a segregated combination. For an
+account of this plant see Boissier, _Voy. Bot. Midi de l'Espagne_, 1839,
+II, 722.
+
+[11] A discussion of this subject with references to literature is
+given by Rolfe, in an excellent paper on "Hybridisation viewed from the
+standpoint of Systematic Botany" (_Jour. R. Hort. Soc._, XXIV, 1900, p.
+197). He concludes: "The simple fact is that the two plants (_L. diurna_
+and _vespertina_) are thoroughly distinct in numerous particulars, and
+affect such different habitats that in some localities one or the other
+of them is completely wanting. But when their stations are adjacent they
+hybridise together very readily, and it is here that these intermediate
+forms occur which have puzzled botanists so much." The same paper
+contains valuable information concerning several cognate illustrations.
+
+[12] In only two cases have I seen such plants (both females) completely
+sterile.
+
+[13] As is well known, in an even more notorious example, he proposed
+to unite _Primula vulgaris_, _P. elatior_, and _P. acaulis_, similarly
+relying on the existence of "intermediates," which we now well know to
+be mongrels between the species.
+
+[14] For an account of the distinctions between _Vespa vulgaris_ and
+_germanica_ see Ch. Janet, _Etudes sur les Fourmis, les Guepes et les
+Abeilles_, 11^e, Note. Sur _Vespa germanica_ et _V. vulgaris_. Limoges
+(Ducourtieux), 1895; and R. du Buysson, Monographie des Guepes, _Ann.
+Soc. Ent. France_, 1903, Vol. LXXII, p. 603, Pl. VIII.
+
+[15] The statements made above are for the most part taken from Barrett,
+C. G., _Lepidoptera of the British Islands_, and from Tutt, J. W., _The
+British Noctuae and their Varieties_. The reader who is unfamiliar
+with the amazing polymorphism exhibited by some of these moths should
+if possible take an opportunity of looking over a long series in a
+collection, or, if that be impossible, refer to the admirable coloured
+plates published by Barrett. It may not be superfluous to observe that
+plenty of similar examples are known in other countries. For instance
+_Plotheia frontalis_, a Noctuid which often abounds in Ceylon, shows
+an equally bewildering wealth of forms. If a dozen specimens of such a
+species were to be brought home from some little known country, each
+individual would almost certainly be described as the type of a distinct
+species. (See the coloured plate published by Sir G. Hampson, Cat. Brit.
+Mus., Heterocera, Vol. IX.)
+
+[16] _Dict. of Birds_, p. 800. It would be interesting and profitable
+to attempt in a long series of Ruffs to determine the Mendelian factors
+which by their combinations give rise to this complex assemblage
+of varietal forms. A few such factors both of colour and pattern
+can be at once distinguished, and it is noticeable that some of the
+resulting types of barring, spangling and penciling show a perceptible
+correspondence with some of the types of colouration found in the breeds
+of domestic fowls.
+
+[17] Howard Saunders (_Illust. Manual of British Birds_, 1899, p. 499)
+states that there is evidence that the pheasant had become naturalized
+in the south of England before the Norman invasion. He adds, "little, if
+any, deviation from the typical _P. colchicus_ took place up to the end
+of last century, when the introduction of the Chinese Ring-necked _P.
+torquatus_ commenced, which has left almost indelible marks, especially
+with regard to the characteristic white collar."
+
+
+
+
+                         CHAPTER II
+
+                     MERISTIC PHENOMENA
+
+Twenty years ago in describing the facts of Variation, argument was
+necessary to show that these phenomena had a special value in the
+sciences of Zoology and Botany. This value is now universally understood
+and appreciated. In spite however of the general attention devoted to
+the study of Variation, and the accumulation of material bearing on the
+problem, no satisfactory or searching classification of the phenomena is
+possible. The reason for this failure is that a real classification must
+presuppose knowledge of the chemistry and physics of living things which
+at present is quite beyond our reach.
+
+It is however becoming probable that if more knowledge of the chemical
+and physical structure of organisms is to be attained, the clue will
+be found through Genetics, and thus that even in the uncoordinated
+accumulation of facts of Variation we are providing the means of
+analysis applicable not only to them, but to the problems of normality
+also.
+
+The only classification that we can yet institute with any confidence
+among the phenomena of Variation is that which distinguishes on the one
+hand variations in the processes of division from variations in the
+nature of the substances divided.
+
+Variations in the processes of division are most often made apparent by
+a change in the number of the parts, and are therefore called _Meristic_
+Variations, while the changes in actual composition of material are
+spoken of as _Substantive_ Variations. The Meristic Variations form on
+the whole a natural and fairly well defined group, but the Substantive
+Variations are obviously a heterogeneous assemblage.
+
+Though this distinction does not go very far, it is useful, and in
+all probability fundamental. It is of value inasmuch as it brings
+into prominence the distinct and peculiar part which the process
+of division, or, more generally, repetition of parts, plays in the
+constitution of the forms of living things.
+
+That there may be a real independence between the Meristic and the
+Substantive phenomena is evident from the fact both that Meristic
+changes may occur without Substantive Variation, and that the substances
+composing an organism may change without any perceptible alteration
+in its meristic structure. When the distinction between these two
+classes of phenomena is perceived it will be realised that the study
+of genetics has on the one hand a physical, or perhaps more strictly
+a mechanical aspect, which relates to the manner in which material is
+divided and distributed; and also a chemical aspect, which relates
+to the constitution of the materials themselves. Somewhat as the
+philosophers of the seventeenth and eighteenth centuries were awaiting
+both a chemical and a mechanical discovery which should serve as a
+key to the problems of unorganised matter, so have biologists been
+awaiting two several clues. In Mendelian analysis we have now, it is
+true, something comparable with the clue of chemistry, but there is
+still little prospect of penetrating the obscurity which envelops the
+mechanical aspect of our phenomena. To make clear the application of
+the terms chemical and mechanical to the problem of Genetics the nature
+of that problem must be more fully described. In its most concrete form
+this problem is expressed in the question, how does a cell divide? If
+the organism is unicellular, and the single cell is the whole body,
+then the process of heredity is accomplished in the single operation of
+cell-division. Similarly in animals and plants whose bodies are made
+up of many cells, the whole process of heredity is accomplished in the
+cell-divisions by which the germ-cells are formed. When therefore we see
+a cell dividing, we are witnessing the process by which the form and the
+properties of the daughter-cells are determined.
+
+Now this process has the two aspects which I have called mechanical
+and chemical. The term "_Entwicklungsmechanik_" has familiarised us
+with the application of the word mechanics to these processes, but on
+reflexion it will be seen that this comprehensive term includes two
+sorts of events which are sometimes readily distinguishable. There
+is the event by which the cell _divides_, and the event by which the
+two halves or their descendants are or may be _differentiated_. It
+is common knowledge that in some cell-divisions two similar halves,
+indistinguishable in appearance, properties, and subsequent fate, may
+be produced, while in other divisions daughter-cells with distinct
+properties and powers are formed. We cannot imagine but that in the
+first case, when the resulting cells are identical, the division is a
+mechanical process by which the mother-cell is simply cut in two; while
+in order that two differentiated halves may be produced, some event must
+have taken place by which a chemical distinction between the two halves
+is effected.[1] In any ordinary Mendelian case we have a clear proof
+that such a chemical difference may be established between germ-cells.
+The facts of colour-inheritance for instance prove that germ-cells,
+otherwise identical, may be formed _possessing_ the chromogen-factor
+which is necessary to the formation of colour in the flowers, or
+_destitute_ of that factor. Similarly the germ-cells may possess the
+ferment which, by its action on the chromogenic substance, produces the
+colour, or they may be without that ferment. The same line of argument
+applied to a great range of cases. Nevertheless, though differences
+in chemical properties are often thus constituted by cell-divisions,
+and though we are thus able to make a quasi-chemical analysis of the
+individual by determining and enumerating these properties, yet it is
+evident that the distribution of these factors is not itself a chemical
+process. This is proved by the fact that similar divisions may be
+effected between halves which are exactly alike, and also by the fact
+that the numbers in which the various types of germ-cells are formed
+negative any suggestion of valency between them. The recognition of the
+unit-factors may lead--indeed must lead--to great advances in chemical
+physiology which without that clue would have been impossible, but
+in causation the chemical phenomena of heredity must be regarded as
+secondary to the physical or mechanical phenomena by which the cells
+and their constituents are divided and separated. When therefore we
+speak of the _essential_ phenomena of heredity we mean the mechanics
+of division, especially, though not, as we shall see, exclusively, of
+_cell_-division; and in the relation between the two halves of the
+dividing cell we have the problem presented in what seems to be its
+simplest form.
+
+In attempting to form some conception of the processes by which bodily
+characteristics are transmitted, or--to avoid that confusing metaphor
+of "transmission"--how it comes about that the offspring can grow to
+resemble its parent, continuity of the germ-substance which in some
+animals is a visible phenomenon,[2] gives at least apparent help. An egg
+for example on becoming adult develops in certain parts a particular
+pigment. The eggs of that adult when they reach the appropriate age
+develop the same pigment. We have no clear picture of the mechanism by
+which this process is effected, but when we realise that the pigment
+results from the interaction of certain substances, and that since
+all the eggs are in reality pieces of the same material, it seems,
+unless we inquire closely, not unnatural that the several pieces of the
+material should exhibit the same colours at the same periods of their
+development. The continuity of the material of the germs suggests that
+there is a continuity of the materials from which the pigment is formed,
+and that thus an actual bit of those substances passes into each egg
+ready at the appropriate moment to generate the pigment. The argument
+thus outlined applies to all _substantive_ characteristics. In each case
+we can imagine, if we will, the appearance of that characteristic as due
+to the contribution of its rudiment from the germ tissues.
+
+When we consider more critically it becomes evident that the aid given
+by this mental picture is of very doubtful reality, for even if it were
+true that any predestined particle actually corresponding with the
+pigment-forming materials is definitely passed on from germ to germ,
+yet the power of increase which must be attributed to it remains so
+incomprehensible that the mystery is hardly at all illuminated.
+
+When however we pass from the substantive to the meristic characters,
+the conception that the character depends on the possession by the germ
+of a particle of a specific material becomes even less plausible. Hardly
+by any effort of imagination can we see any way by which the division
+of the vertebral column into _x_ segments or into _y_ segments, or of a
+Medusa into 4 segments or into 6, can be determined by the possession
+or by the want of a material particle. The distinction must surely be
+of a different order. If we are to look for a physical analogy at all
+we should rather be led to suppose that these differences in segmental
+numbers corresponded with changes in the amplitude or number of dividing
+waves than with any change in the substance or material divided.
+
+
+                     PHENOMENA OF DIVISION
+
+I have said that in the division of a cell we seem to see the problem in
+its simplest form, but it is important to observe that the problem of
+division may be presented by the bodies of animals and plants in forms
+which are independent of the divisions between cells. The existence of
+pattern implies a repetition of parts, and repetition of parts when
+developed in a material originally homogeneous can only be created by
+division. Cell-division is probably only a special case of a process
+similar to that by which the pattern of the skeleton is laid down in
+a unicellular body such as that of a Radiolarian or Foraminiferan.
+Attempts have lately been made to apply mathematical treatment to
+problems of biology. It has sometimes seemed to me that it is in the
+geometrical phenomena of life that the most hopeful field for the
+introduction of mathematics will be found. If anyone will compare one
+of our animal patterns, say that of a zebra's hide, with patterns known
+to be of purely mechanical production, he will need no argument to
+convince him that there must be an essential similarity between the
+processes by which the two kinds of patterns were made and that parts
+at least of the analysis applicable to the mechanical patterns are
+applicable to the zebra stripes also. Patterns mechanically produced are
+of many and very diverse kinds. One of the most familiar examples, and
+one presenting some especially striking analogies to organic patterns,
+is that provided by the ripples of a mackerel sky, or those made in a
+flat sandy beach by the wind or the ebbing tide. With a little search
+we can find among the ripple-marks, and in other patterns produced by
+simple physical means, the closest parallels to all the phenomena of
+striping as we see them in our animals. The forking of the stripes, the
+differentiation of two "faces," the deflections round the limbs and
+so forth, which in the body we know to be phenomena of division, are
+common both to the mechanical and the animal patterns. We cannot tell
+what in the zebra corresponds to the wind or the flow of the current,
+but we can perceive that in the distribution of the pigments, that
+is to say, of the chromogen-substances or of the ferments which act
+upon them, a rhythmical disturbance has been set up which has produced
+the pattern we see; and I think we are entitled to the inference that
+in the formation of patterns in animals and plants mechanical forces
+are operating which ought to be, and will prove to be, capable of
+mathematical analysis. The comparison between the striping of a living
+organism and the sand-ripples will serve us yet a little farther,
+for a pattern may either be formed by actual cell-divisions, and
+the distribution of differentiation coincidently determined, or--as
+visibly in the pigmentation of many animal and plant tissues--the
+pattern may be laid down and the pigment (for example) distributed
+through a tissue across or independently of the cell-divisions of the
+tissue. Our tissues therefore are like a beach composed of sands of
+different kinds, and different kinds of sands may show distinct and
+interpenetrating ripples. When the essential analogy between these
+various classes of phenomena is perceived, no one will be astonished at,
+or reluctant to admit, the reality of discontinuity in Variation, and if
+we are as far as ever from knowing the actual causation of pattern we
+ought not to feel surprised that it may arise suddenly or be suddenly
+modified in descent. Biologists have felt it easier to conceive the
+evolution of a striped animal like a zebra from a self-coloured type
+like a horse (or of the self-coloured from the striped) as a process
+involving many intergradational steps; but so far as the _pattern_ is
+concerned, the change may have been decided by a single event, just as
+the multitudinous and ordered rippling of a beach may be created or
+obliterated at one tide.
+
+[Illustration: FIG. 1. Tusk of Indian elephant, showing an abnormal
+segmentation.]
+
+This point is well illustrated by the tusk of an Indian elephant which
+I lately found in a London sale-room. This tusk is by some unknown
+cause, presumably a chronic inflammation, thrown up into thirteen
+well-marked ridges which closely simulate a series of segments (Fig. 1).
+Whatever the cause the condition shows how easily a normally unsegmented
+structure may be converted into a series of repeated parts.
+
+The spread of segmentation through tissues normally unsegmented is very
+clearly exemplified in the skates' jaws shown in Fig. 2. The right
+side of the upper figure shows the normal arrangement in the species
+_Rhinoptera jussieui_, but the structure on the left side is very
+different. The probable relations of the several rows of teeth to the
+normal rows is indicated by the lettering, but it is evident that by
+the appearance of new planes of division constituting separate centers
+of growth, the series has been recast. The pattern of the left side is
+so definite that had the variation affected the right side also, no
+systematist would have hesitated to give the specimen a new specific
+name. The other two drawings show similar variations of a less extensive
+kind, the nature of which is explained by the lettering of the rows of
+teeth.
+
+[Illustration: FIG. 2. Jaws of Skates (_Rhinoptera_) showing meristic
+variation. (For a detailed discussion see _Materials for the Study of
+Variation_, p. 259.)]
+
+This power to divide is a fundamental attribute of life, and of that
+power cell-division is a special example. In regard to almost all the
+chief vital phenomena we can say with truth that science has made some
+progress. If I mention respiration, metabolism, digestion, each of these
+words calls to mind something more than a bare statement that such
+acts are performed by an animal or a plant. Each stands for volumes of
+successful experiment and research, But the expression cell-division,
+the fundamental act which typifies the rest, and on which they all
+depend, remains a bare name. We can see with the microscope the outward
+symptoms of division, but we have no surmise as to the nature of the
+process by which the division is begun or accomplished. I know nothing
+which to a man well trained in scientific knowledge and method brings
+so vivid a realisation of our ignorance of the nature of life as the
+mystery of cell-division. What is a living thing? The best answer in
+few words that I know is one which my old teacher, Michael Foster, used
+to give in his lectures introductory to biology. "A living thing is a
+vortex of chemical and molecular change." This description gives much,
+if not all, that is of the essence of life. The living thing is unlike
+ordinary matter in the fact that, through it, matter is always passing.
+Matter is essential to it; but, provided that the flow in and out is
+unimpeded, the life-process can go on so far as we know indefinitely.
+Yet the living "vortex" differs from all others in the fact that it
+can divide and throw off other "vortices," through which again matter
+continually swirls.
+
+We may perhaps take the parallel a stage further. A simple vortex, like
+a smoke-ring, if projected in a suitable way will twist and form two
+rings. If each loop as it is formed could grow and then twist again to
+form more loops, we should have a model representing several of the
+essential features of living things.
+
+It is this power of spontaneous division which most sharply
+distinguishes the living from the non-living. In the excellent book
+dealing with the problems of development, lately published by Mr.
+Jenkinson a special emphasis is very properly laid on the distinction
+between the processes of division, and those of differentiation. Too
+often in discussions of the developmental processes the distinction
+is obscured. He regards differentiation as the "central difficulty."
+"Growth and division of the nucleus and the cells," he tells us,
+are side-issues. This view is quite defensible, but I suspect that
+the division _is_ the central difficulty, and that if we could get
+a rationale of what is happening in cell-division we should not be
+long before we had a clue to the nature of differentiation. It may be
+self-deception, but I do not feel it impossible to form some hypothesis
+as to the mode of differentiation, but in no mood of freest speculation
+are we ever able to form a guess as to the nature of the division. We
+see differentiations occurring in the course of chemical action, in
+some phenomena of vibration and so forth: but where do we see anything
+like the spontaneous division of the living cell? Excite a gold-leaf
+electroscope, and the leaves separate, but we know that is because they
+were double before. In electrolysis various substances separate out at
+the positive and negative poles respectively. Now if in cell-division
+the two daughter-cells were always dissimilar--that is to say, if
+differentiation always occurred--we could conceive some rough comparison
+with such dissociations. But we know the dissimilarity between
+daughter-cells is not essential. In the reproduction of unicellular
+organisms and many other cases, the products formed at the two poles
+are, so far as we can tell, identical. Any assumption to the contrary,
+if we were disposed to make it, would involve us in difficulties still
+more serious. At any rate, therefore, if differentiation be really the
+central difficulty in development, it is division which is the essential
+problem of heredity.
+
+Sir George Darwin and Professor Jeans tell us that "gravitational
+instability" consequent on the condensation of gases is "the primary
+agent at work in the actual evolution of the universe," which has led to
+the division of the heavenly bodies. The greatest advance I can conceive
+in biology would be the discovery of the nature of the instability which
+leads to the continual division of the cell. When I look at a dividing
+cell I feel as an astronomer might do if he beheld the formation of a
+double star: that an original act of creation is taking place before me.
+Enigmatical as the phenomenon seems, I am not without hope that, if it
+were studied for its own sake, dissociated from the complications which
+obscure it when regarded as a mere incident in development, some hint as
+to the nature of division could be found. It is I fear a problem rather
+for the physicist than for the biologist. The sentiment may not be a
+popular one to utter before an assembly of biologists, but looking at
+the truth impersonally I suspect that when at length minds of first rate
+analytical power are attracted to biological problems, some advance will
+be made of the kind which we are awaiting.
+
+The study of the phenomena of bodily symmetry offers perhaps the
+most hopeful point of attack. The essential fact in reproduction is
+cell-division, and the essential basis of hereditary resemblance is
+the symmetry of cell-division. The phenomena of twinning provide a
+convincing demonstration that this is so. By twinning we mean the
+production of equivalent structures by division. The process is one
+which may affect the whole body of an animal or plant, or certain of
+its parts. The term twin as ordinarily used refers to the simultaneous
+birth of two individuals. Those who are naturalists know that such twins
+are of two kinds, (1) twins that are not more alike than any other
+two members of the same family, and (2) twins that are so much alike
+that even intimate friends mistake them. These latter twins, except in
+imaginative literature, are always of the same sex.
+
+It is scarcely necessary for me to repeat the evidence from which it has
+been concluded that without doubt such twins arise by division of the
+same fertilised ovum. There is a perfect series of gradations connecting
+them with the various forms of double monsters united by homologous
+parts. They have been shown several times to be enclosed in the same
+chorion, and the proofs of experimental embryology show that in several
+animals by the separation of the two first hemispheres of a dividing egg
+twins can be produced. Lastly we have recently had the extraordinarily
+interesting demonstration of Loeb, to which I may specially refer.
+Herbst some years ago found that in sea water, from which all lime salts
+had been removed, the segments of the living egg fall apart as they are
+formed. Using this method Loeb has shown that a temporary immersion in
+lime-free sea water may result in the production of 90 per cent. of
+twins. We are therefore safe in regarding the homologous or "identical"
+twins as resulting from the divisions of one fertilised egg, while the
+non-identical or "fraternal" twins, as they are called, arise by the
+fertilisation of two separate ova.[3]
+
+In the resemblance of identical twins we have an extreme case of
+hereditary likeness[4] and a proof, if any were needed, that the
+cause of individual variation is to be sought in the differentiation
+of germ-cells. The resemblance of identical twins depends on two
+circumstances, First, since only two germ-cells take part in their
+production, difference between the germ cells of the same individual
+cannot affect them. Secondly the division of the fertilised ovum,
+the process by which they became two instead of one, must have been
+a symmetrical division. The structure of twins raises however one
+extremely significant difficulty, which as yet we cannot in any way
+explain. The resemblance between twins is a phenomenon of symmetry,
+like the resemblance between the two sides of a bilaterally symmetrical
+body. Not only is the general resemblance readily so interpreted, but
+we know also that in double monsters, namely unseparated twins, various
+anatomical abnormalities shown by the one half-body are frequently shown
+by the other half-also.[5] The two belong to one system of symmetry.
+How then does it happen that the body of one of a pair of twins does
+not show a transposition of viscera? We know that the relation of right
+and left implies that the one should be the mirror-image of the other.
+Such a relation of images may be maintained even in minute details.
+For example if the same pattern of finger-print is given by the fingers
+of the two hands, one is the reverse of the other. In double monsters,
+namely unseparated twins, there is evidence that an inversion of viscera
+does occur with some frequency. Evidence from such cases is not so clear
+and simple as might be expected, because as a matter of fact, the heart
+and stomach, upon which the asymmetry of the viscera chiefly depend, are
+usually common to the two bodies. Duplicity generally affects either
+the anterior end alone, or the posterior end alone. The division is
+generally _from the heart forwards_, giving two heads and two pairs of
+anterior limbs on a common trunk, or _from the heart backwards_, giving
+two pairs of posterior limbs with the anterior body common. In either
+case, though the bodies may be grouped in a common system of symmetry,
+neither can be proved to show definite reversal of the parts. To see
+that reversal recourse must be had to more extreme duplications, such as
+the famous Siamese Twins. They, as a matter of fact, were an excellent
+instance of the proposition that twins are related as mirror-images,
+for both of them had eleven pairs of ribs instead of the normal twelve,
+and one of them had a partial reversal of viscera.[6] (Kuechenmeister,
+_Verlagerung_, etc., p. 204.)
+
+If anyone could show how it is that neither of a pair of twins has
+transposition of viscera the whole mystery of division would, I expect,
+be greatly illuminated.[7] At present we have simply to accept the fact
+that twins, by virtue of their detachment from each other, have the
+power of resuming the polarity which is proper to any normal individual.
+It was nevertheless with great interest that I read Wilder's recent
+observation[8] that occasionally in identical twins the finger-print of
+one or both the index-fingers may be reversed, showing that there is
+after all some truth in the notion that reversal should occur in them.
+
+There is another phenomenon by twinning which, if we could understand
+it, might help. I refer to the free-martin, the subject of one of John
+Hunter's masterpieces of anatomical description. In horned cattle twin
+births are rare, and when twins of opposite sexes are born, the male
+is perfect and normal, but the reproductive organs of the female are
+deformed and sterile, being known as a free-martin. The same thing
+occasionally happens in sheep, suggesting that in sheep also twins may
+be formed by the division of one ovum; for it is impossible to suppose
+that mere development in juxtaposition can produce a change of this
+character. I mention the free-martin because it raises a question of
+absorbing interest. It is conceivable that we should interpret it by
+reference to the phenomenon of gynandromorphism, seen occasionally in
+insects, and also in birds as a great rarity. In the gynandromorph one
+side of the body is male, the other female. A bullfinch for instance has
+been described with a sharp line of division down the breast between the
+red feathers of the cock on one side and the brown feathers of the hen
+on the other. (Poll, H., _SB. Ges. Nat. Fr._, Berlin, 1909, p. 338.) In
+such cases neither side is sexually perfect. If the halves of such a
+gynandromorph came apart, perhaps one would be a free-martin.
+
+The behaviour of homologous twinning in heredity has been little
+studied. It does not exist as a normal feature in any animal which
+is amenable to experiment, and we cannot positively assert that a
+comparable phenomenon exists in plants; for in them--the Orange, for
+example--polyembryony may evidently be produced by a parthenogenetic
+development of nucellar tissue. It is possible that in Man twinning
+is due to a peculiarity of the mother, not of the father. It may and
+not rarely does descend from mother to daughter, but whether it can be
+passed on through a male generation to a daughter again, there is not
+sufficient evidence to show. The facts as far as they go are consistent
+with the inference which may be drawn from Loeb's experiment, that the
+twinning of a fertilized ovum may be determined not by the germ-cells
+which united to form it, but by the environment in which it begins to
+develop. The opinion that twinning may descend through the male directly
+has been lately expressed by Dr. J. Oliver in the _Eugenics Review_
+(1912), on the evidence of cases in which twins had occurred among the
+relations of fathers of twins, but I do not know of any comprehensive
+collection of evidence bearing on the subject.
+
+Besides twinning of the whole body a comparable duplicity of various
+parts of the same body may occur. Such divisions affect especially those
+organs which have an axis of bilateral symmetry, such as the thumb, a
+cotyledon, a median petal, the frond of a fern or the anal fin of a
+fish. From the little yet known it is clear that the genetic analysis
+of these conditions must be very difficult, but evidence of any kind
+regarding them will be valuable. We want especially to know whether
+these divisions are due to the _addition_ of some factor or power which
+enables the part to divide, or whether the division results from the
+_absence_ of something which in the normal body prevents the part from
+dividing. Breeding experiments, so far as they go, suggest that the less
+divided state is usually dominant to the more divided.[9] The two-celled
+Tomato fruit is dominant to the many-celled type. The Manx Cat's tail,
+with its suppression of caudal segmentation is a partial dominant over
+the normal tail. The tail of the Fowl in what is called the "Rumpless"
+condition is at least superficially comparable with that of the Manx
+Cat, and though the evidence is not wholly consistent, Davenport
+obtained facts indicating that this suppressed condition of the caudal
+vertebrae is an imperfect dominant.[10]
+
+Some evidence may also be derived from other examples of differences
+which at first sight appear to be substantive though they are more
+probably meristic in ultimate nature. The distinction between the
+normal and the "Angora" hair of the Rabbit is a case in point. We can
+scarcely doubt that one of the essential differences between these two
+types is that in the Angora coat the hair-follicles are more finely
+divided than they are in the normal coat, and we know that the normal,
+or less-divided condition, is dominant to the Angora, or more finely
+divided.
+
+[Illustration: FIG. 3. _I_, _II_, _III_, various degrees of syndactyly
+affecting the medius and annularis in the hand; _IV_, syndactyly
+affecting the index and medius in the foot. (After Annandale.)]
+
+In the case of the solid-hoofed or "mule-footed" swine, the evidence
+shows, as Spillman has lately pointed out,[11] that the condition
+behaves as a dominant. The essential feature of this abnormality is
+that the digits III and IV are partially united. The union is greatest
+peripherally. Sometimes the third phalanges only are joined to form one
+bone, but the second and even the first phalanges may also be compounded
+together. Here the variation is obviously meristic and consists in a
+failure to divide, the normal separation of the median digits of the
+foot being suppressed.
+
+[Illustration: FIG. 4. Case of complete syndactyly in the foot. _II_ and
+_III_, digit apparently representing the index and medius. _c_^{2} +
+_c_^{3}, bone apparently representing the middle and external cuneiform;
+_cb_, cuboid; _c_^{1}, internal cuneiform. (After Gruber.)]
+
+Webbing between the digits, in at least some of its manifestations, is
+a variation of similar nature. The family recorded by Newsholme[12]
+very clearly shows the dominance of this condition. The case is
+morphologically of great interest and must undoubtedly have a bearing on
+the problems of the mechanics of Division. In discussing the phenomena
+of syndactylism I pointed out some years ago that the digits most
+frequently united in the human hand are III and IV, while in the foot,
+union most frequently takes place between II and III.[13] In Newsholme's
+family the union was always between II and III of the foot, except in
+the case of one male who had the digits III and IV of the right _hand_
+alone webbed together. There can be little doubt that the geometrical
+system on which the foot is planned has an axis of symmetry passing
+between the digits II and III, while the corresponding axis in the hand
+passes between III and IV. Union between such digits may therefore be
+regarded as comparable with any non-division or "coalescence" of lateral
+structures in a middle line, and when as in these examples such a
+condition is shown to be a dominant we cannot avoid the inference that
+some concrete factor has the power of suppressing or inhibiting this
+division. Figs. 3 and 4 illustrate degrees of union between digits in
+the human hand and foot.
+
+It is not in question that various other forms of irregular webbing
+and coalescence of digits exist, and respecting the genetic behaviour
+of these practically nothing is as yet known. Such a case is described
+by Walker,[14] in which the first and second metacarpals of both
+feet were fused in mother and daughter, and several more are found
+in literature. Contrasted with these phenomena we have the curious
+fact that in the Pigeon, Staples-Browne found webbing of the toes a
+_recessive_ character. The question thus arises whether this webbing is
+of the same nature as that shown to be a dominant in Man, and indeed
+whether the phenomenon in pigeons is really meristic at all. There is
+some difference perceptible between the two conditions; for in Man
+there is not so much a development of a special web-like skin uniting
+the digits as a want of proper division between the digits themselves,
+and in extreme cases two digits may be represented by a single one. In
+the Pigeon I am not aware that a real union of this kind has ever been
+observed, and though the web-like skin may extend the whole length of
+the digits and be so narrow as to prevent the spread of the toes, it
+may, I think, be maintained that the unity of the digits is unimpaired.
+For the present the nature of this variation in the pigeon's feet must
+be regarded as doubtful, and we should note that if it is actually an
+example of a more perfect division being dominant to a less perfect
+division, the case is a marked exception to the general rule that
+non-division is dominant to division.
+
+Reference must also be made to the phenomenon of fasciation in the stems
+of plants. As Mendel showed in the case of _Pisum_ this condition is
+often a recessive. The appearances suggest that the difference between a
+normal and a fasciated plant consists in the inability of the fasciated
+plant to separate its lateral branches. The nature of the condition is
+however very obscure and it is equally likely that some multiplication
+of the growing point is the essential phenomenon.[15]
+
+Stockard's interesting experiments[16] illustrate this question. He
+showed that by treating the embryos of a fish (_Fundulus heteroclitus_)
+with a dilute solution of magnesium salts, various cyclopian
+monstrosities were frequently produced. These have been called cases
+of _fusion_ of the optic vesicles. I would prefer to regard them as
+cases of a division suppressed or restricted by the control of the
+environment. Conversely, the splendid discovery of Loeb, that an
+unfertilised egg will divide and develop parthenogenetically without
+fertilisation, as a consequence of exposure to various media, may be
+interpreted as suggesting that the action of those media releases
+the strains already present in the ovum, though I admit that an
+interpretation based on the converse hypothesis, that the medium acts as
+a stimulus, is as yet by no means excluded.
+
+In these cases we come nearest to the direct causation or the direct
+inhibition of a division, but the meaning of the evidence is still
+ambiguous. I incline to compare Loeb's parthenogenesis with the
+development (and of course accompanying cell-division) of dormant buds
+on stems which have been cut back.
+
+It is interesting to note that sometimes as an abnormality, the
+faculty of division gets out of hand and runs a course apparently
+uncontrolled. A remarkable instance of this condition is seen in
+_Begonia_ "_phyllomaniaca_", which breaks out into buds at any point on
+the stem, petioles, or leaves, each bud having, like other buds, the
+power of becoming a new plant if removed. We would give much to know the
+genetic properties of _B. phyllomaniaca_, and in conjunction with Mr. W.
+O. Backhouse I have for some time been experimenting with this plant.
+It proved totally sterile. Its own anthers produce no pollen, and all
+attempts to fertilise it with other species failed though the pollen of
+a great number of forms was tried.
+
+Recently however we have succeeded in making plants which are in every
+respect _Begonia phyllomaniaca_, so far as the characters of stems and
+leaves are concerned. These plants, of which we have sixteen, were made
+by fertilising _B. heracleifolia_ with _B. polyantha_. They are all
+beginning to break out in "phyllomania." As yet they have not flowered,
+but as they agree in all details with _phyllomaniaca_ there can be
+little doubt that the original plant bearing that name was a hybrid
+similarly produced. The production of "phyllomania" on a hybrid Begonia
+has also been previously recorded by Duchartre.[17] In this case the
+cross was made between _B. incarnata_ and _lucida_. The synonymy of
+the last species is unfortunately obscure, and I have not succeeded in
+repeating the experiment.
+
+[Illustration: FIG. 5. Piece of petiole of _Begonia phyllomaniaca_. The
+proximal end is to the right of the figure.]
+
+From these facts it seems practically certain that the condition is one
+which is due to the meeting of complementary factors. At first sight
+we may incline to think that the phyllomania is in some way due to the
+sterility. This however cannot be seriously maintained; for not only is
+sterility in plants not usually associated with such manifestations, but
+we know a Begonia called "Wilhelma" which is exactly _phyllomaniaca_ and
+equally sterile, though it has no trace of phyllomania. This plant arose
+in the nurseries of MM. P. Bruant of Poitiers, and has generally been
+described as a seedling of _phyllomaniaca_, but from the total sterility
+of that form this account of its origin must be set aside.
+
+[Illustration: FIG. 6. Two right hind feet of polydactyle cats. _II_
+shows the lowest development of the condition yet recorded. The digit,
+_d_^{1}, which stands as hallux is fully formed and has three phalanges.
+Both it and the digit marked _d_^{2} are formed as _left_ digits. In the
+normal hind foot of the cat the hallux is represented by a rudiment only.
+
+_I_ shows a further development of the condition. In this foot there are
+_six_ digits. _d_^{1} has two phalanges, but both it and _d_^{2} and
+_d_^{3} are shaped as left digits. Thus _d_^{3}, which in the normal
+foot would be shaped as a right digit, is transformed so as to look like
+a _left_ digit.]
+
+The phenomenon in this case can hardly be regarded as due to the
+excitation of dormant buds, for it is apparent on examination that
+the new growths are not placed in any fixed geometrical relation to
+the original plant. They arise on the petiole, for example, as small
+green outgrowths each of which gradually becomes a tiny leaf. The
+attitude of these leaves is quite indeterminate, and they may point
+in any direction, some having their apices turned peripherally, some
+centrally, and others in various oblique or transverse positions (Fig.
+5). These little leaves are thus comparable with seedlings, in that
+their polarity is not related to, or consequent upon that of the parent
+plant. They have in fact that "individuality," which we associate with
+germinal reproduction.
+
+There are many curious phenomena seen in the behaviour of parts normally
+repeated in bilateral symmetry which may some day guide us towards
+an understanding of the mechanics of division. A part like a hand,
+which needs the other hand to complete its symmetry, cannot twin by
+mere division, yet by proliferation and special modifications on the
+radial side of the same limb, even a hand may be twinned. In the well
+known polydactyle cats a change of this kind is very common and indeed
+almost the rule. When extra digits appear at the inner (tibial) side
+of the limb, they are shaped as digits of the other side, and even the
+normal digit II (index) is usually converted into the mirror-image
+of its normal self. The limb then develops a new symmetry in itself.
+Nevertheless it is not easy to interpret these facts as meaning that
+there has been some interruption in the control which one side of the
+body exercises over the other. The heredity of polydactylism is complex
+but there is little doubt that the condition familiar in the Cat is a
+dominant. In some human cases also the descent is that of a dominant,
+but irregularities are so frequent that no general rule can yet be
+perceived. The dominance of such a condition is an exception to the
+principle that the less-divided is usually dominant to the more-divided,
+a fact which probably should be interpreted as meaning that divisions
+are of more than one kind.
+
+Among ordinary somatic divisions, whether of organs, cells, or patterns
+of differentiation, the control of symmetry is usually manifested. There
+is however one class of somatic differentiations which are exceptionally
+interesting from the fact that they may show a complete independence
+of such geometrical control. The most familiar examples of these
+geometrically uncontrolled Variations are to be seen in bud-sports.
+The normal differentiation of the organs of a plant is arranged on
+a definite geometrical system, which to those who have never given
+special attention to such things before, will often seem surprisingly
+precise. The arrangement of the leaves on uninjured, free-growing shoots
+can generally be seen to follow a very definite order, just as do the
+flowers or the parts of the flowers. If however bud sports occur,
+then though the parts included in the sports show all the geometrical
+peculiarities proper to the sport-variety, yet the sporting-buds
+themselves are not related to each other according to any geometrical
+plan.
+
+A very familiar illustration is provided by the distribution of colour
+in those Carnations that are not self-coloured. The pigment may, as in
+Picotees, be distributed peripherally with great regularity to the edges
+of the petals; or, as in Bizarres and Flakes, it may be scattered in
+radial sectors which show no geometrical regularity. Now in this case
+the pigments are the same in both types of flower, and the chemical
+factors concerned in their production must surely be the same. The
+difference must lie in the mechanical processes of distribution of
+the pigment. In the Picotee we see the orderly differentiation which
+we associate with normality; in the Bizarre we see the disorderly
+differentiation characteristic of bud-sports. The distribution of colour
+in this case lies outside the scheme of symmetry of the plant.
+
+Such a distribution is characteristic of bud-sports, and of certain
+other differentiations in both plants and animals, which I cannot on
+this occasion discuss. Now reflexion will show that these facts have
+an intimate bearing on the mechanical problems of heredity. For first
+in the bud-sports we are witnessing the distribution of factors which
+distinguish genetic varieties. We do not know the physical nature of
+those factors, but if we must give them a name, I suppose we should call
+them "ferments" exactly as Boyle did in 1666. He is discussing how it
+comes about that a bud, budded on a stock, becomes a branch bearing the
+fruit of its special kind. He notes that though the bud inserted be "not
+so big oftentimes as a Pea," yet "whether by the help of some peculiar
+kind of Strainer or by the Operation of some powerful Ferment lodged in
+it, or by both these, or some other cause," the sap is "so far changed
+as to constitute a Fruit quite otherwise qualify'd."[18] We can add
+nothing to his speculation, and we believe still that by a differential
+distribution of "ferments" the sports are produced. All the factors are
+together present in the normal parts; some are left out in the sport. In
+an analogous case however, that of a variegated _Pelargonium_ which has
+green and also albino shoots, Baur proved that the shoots pure in colour
+are also pure in their posterity. There can be no doubt that the sports
+of Carnations, Azaleas, Chrysanthemums, etc., would behave in the same
+way.
+
+The well-known Azaleas Perle de Ledeburg, President Kerchove,
+and _Vervaeana_ are familiar illustrations. Perle de Ledeburg is
+predominantly white, but it has red streaks in some of its flowers. It
+not very rarely gives off a self-red sport. This is evidently due to
+the development of a bud in a red-bearing area of the stem. The red in
+this plant is not under "geometrical control." Many plants have white
+flowers with no markings, but if the red markings are geometrically
+ordered differentiations, no self-coloured sports are formed. The case
+of _Vervaeana_ is a good illustration of this proposition. It has white
+flowers with red markings arranged in an orderly manner on the lower
+parts of the petals, especially on the dorsal petals. This is one of the
+Azaleas most liable to have red sports, and at first sight it might seem
+that the sport represented the red of the central marks. Examination
+however of a good many flowers shows that irregular red streaks like
+those of Perle de Ledeburg occur, about as commonly as in that variety.
+_Vervaeana_ in fact is Perle de Ledeburg with _definite_ red markings
+added, and its red sports obviously are those branches the germs of
+which came in a patch of the stem bearing these red elements. That this
+is the true account is rendered quite obvious by the fact that the red
+of the sport is a colour somewhat different from that of the definite
+marks, and that these marks are still present on the red ground of the
+sporting flowers.
+
+It will be understood that these remarks apply to those cases in which
+the production of sports is habitual or frequent, and I imagine in
+all such examples it will be found that there are indications of
+irregularity in the distribution of the differentiations such as to
+justify the view that they are not under that geometrical control which
+governs the normal differentiation of the parts. The question next
+arises whether these considerations apply also to the production of a
+bud-sport as a rare exception, but by the nature of the case it is not
+possible to say positively whether the appearance of an exceptional
+sport is due to the unsuspected presence of a pre-existing fragment of
+material having a special constitution, or to the origin, _de novo_, of
+such a material. For instance one of the garden forms of _Pelargonium_
+known as _altum_ is liable perhaps once in some hundreds of flowers to
+have one or two magenta petals. The normal colour is a brilliant red;
+and as we may be fairly sure that this red is recessive to magenta the
+interpretation would be quite different according as the appearance of
+the magenta is regarded as due to the presence of small areas endowed
+with magentaness, or to the spontaneous generation of the factor for
+that pigment. Either interpretation is possible on the facts, but the
+view that the whole plant has in it scarce mosaic particles of magenta
+seems on the whole more consistent with present knowledge.
+
+In _Pelargonium altum_ the enzyme causing the magenta colours must
+be distributed in very small areas, but a case in which the magenta
+is similarly arranged in a much coarser patchwork may be seen in the
+_Pelargonium_ "Don Juan," which often bears whole trusses or branches of
+red flowers upon plants having the normal dominant magenta trusses. In
+most cases there is little doubt that though the magenta flowered parts
+can "sport" to red, the red parts could not produce the magenta flowers.
+
+The asymmetrical, or to speak more precisely, the disorderly, mingling
+of the colours in the somatic parts is thus an indication of a similarly
+disorderly mixing of the factors for those colours in the germ-tissues,
+so that some of the gametes bear enough of the colour-factors to make a
+self-coloured plant, while others bear so little that the plant to which
+they give rise is a patchwork. If this view is correct we may extend it
+so far as to consider whether the fineness or coarseness of the mixture
+visible in the flowers or leaves may not give an indication of the
+degree to which the factors are subdivided among the germ-cells. We know
+very little about the genetic properties of striped varieties. In both
+_Antirrhinum_ and _Mirabilis_ it has been found that the striped may
+occasionally and irregularly throw self-coloured plants, and therefore
+the striping cannot be regarded simply as a recessive character. On the
+other hand in _Primula Sinensis_ there are well-known flaked varieties
+which ordinarily at least breed true. Whether these ever throw selfs I
+do not know, but if they do it must be quite exceptionally. The power
+of these flaked plants to breed true is, I suspect, connected with the
+fact that in their flowers the coloured and white parts are _intimately_
+mixed, this intimate mixture thus being an indication of a similarly
+intimate mixture in the germ-cells. It would be important to ascertain
+whether self-fertilised seed from the occasional flowers in which the
+colour has run together to join a large patch gives more self-coloured
+plants than the intimately flaked flowers do.
+
+The next fact may eventually prove of great importance. We have seen
+that in bud-sports the differentiation is of the same nature as
+that between pure types, and also that in the sporting plant this
+differentiation is distributed without any reference to the plant's
+axis, or any other consideration of symmetry. Now among the germ-cells
+of a Mendelian hybrid exactly such characters are being distributed
+allelomorphically, and there again we have strong evidence for believing
+that the distribution obeys no pattern. For example, we can in the case
+of seeds still _in situ_ perceive how the characters were distributed
+among the germ-cells, and there is certainly no obvious pattern
+connecting them, nor can we suppose that there is an actual pattern
+obscured.
+
+Of this one illustration is especially curious. Individual plants of
+the same species are, as regards the decussations of their leaves and
+in other respects, _either rights or lefts_. The fact is not emphasized
+in modern botany and is in some danger of being forgotten. When, as
+in the flowers of Arum, some _Gladioli_, _Exacum_, _St. Paulia_, or
+the fruits of _Loasa_, rights and lefts occur on the same stem,
+they come off alternately. But if, as in the seedlings of Barley the
+twist of the first leaf be examined, it will be seen to be either a
+right-or left-handed screw. An ear of barley, say a two-row barley,
+is a definitely symmetrical structure. The seeds stand in their
+envelopes back to back in definite positions. Each has its organs
+placed in perfectly definite places. _If these seeds were buds_ their
+differentiations would be grouped into a common plan. One might expect
+that the differentiations of these embryos would still fall into the
+pattern; but they do not, and so far as I have tested them, any one
+may be a right or a left, just as each may carry any of the Mendelian
+allelomorphs possessed by the parent plant, without reference to the
+differentiation of any other seed. The fertilisation may be responsible,
+but our experience of the allelomorphic characters suggest that the
+irregularity is in the egg-cells themselves.[19]
+
+_Germ cells thus differ from somatic cells in the fact that their
+differentiations are outside the geometrical order which governs
+the differentiation of the somatic cells._ I can think of possible
+exceptions, but I have confidence that the rule is true and I regard it
+as of great significance.
+
+The old riddle, what is an individual, finds at least a partial solution
+in the reply that an individual is a group of parts differentiated in a
+geometrically interdependent order. With the germ-cell a new geometrical
+order, with independent polarity is almost if not quite always, begun,
+and with this geometrical independence the power of rejuvenescence may
+possibly be associated.
+
+The problems thus raised are unsolved, but they do not look insoluble.
+The solution may be nearer than we have thought. In a study of the
+geometry of differentiation, germinal and somatic, there is a way
+of watching and perhaps analyzing what may be distinguished as the
+mechanical phenomena of heredity. If any one could in the cases of
+the Picotee and the Bizarre Carnation, respectively, detect the real
+distinction between the two types of distribution, he would make a
+most notable advance. Any one acquainted with mechanical devices can
+construct a model which will reproduce some of these distinctions more
+or less faithfully. The point I would not lose sight of is that the
+analogy with such models must for a long way be a true and valuable
+guide. I trust that some one with the right intellectual equipment will
+endeavor to follow this guide; and I am sanguine enough to think that a
+comprehensive study of the geometrical phenomena of differentiation will
+suggest to a penetrative mind that critical experiment which may one day
+reveal the meaning of spontaneous division, the mystery through which
+lies the road, perhaps the most hopeful, to a knowledge of the nature of
+life.
+
+FOOTNOTES:
+
+[1] In saying this we make no assumption as to the particular
+cell-division at which differentiation occurs. This may be one of the
+maturation-divisions, or it may perhaps be much earlier.
+
+[2] From the recent discoveries of Erwin Baur we are led to surmise
+that in the flowering plants the sub-epidermal layer, or some of its
+elements, may legitimately be regarded as a similar germ-substance,
+continuous in Weismann's sense.
+
+[3] These fraternal twins, which show no special resemblance to each
+other, are like the multiple births of other animals, and there is no
+disposition for them to be of the same sex. In the sheep, for example,
+statistics show that the frequency of pairs of twins, male and female,
+is approximately double that of the frequency of pairs, both male or
+both female, as it should be if the sex-distribution were fortuitous.
+For instance Bernadin (_La Bergerie de Rambouillet_, 1890, p. 100)
+gives the following figures for twin-lambs in Merinos: both male, 87;
+both female, 83; sexes mixed, 187. The 9-banded Armadillo (_Dasypus
+novemcinctus_), in which the young born in one litter are said to be
+always of one sex, is the only known exception in Vertebrates, and
+is presumably a genuine case of normal polyembryony (see especially,
+Rosner, _Bull. Ac. Soc. Cracovie_, 1901, p. 443, and Newman and
+Patterson, _Biol. Bull._, XVII, 1909, p. 181), and an important paper
+lately published by H. H. Newman and J. T. Patterson, _Jour. Morph._,
+1911, XXII, p. 855.
+
+[4] A good collection of evidence as to disease in homologous twins was
+lately published by E. A. Cockayne, _Brit. Jour. Child. Diseases_, Nov.,
+1911.
+
+[5] Cp. Windle, B. C. A., _Jour. Anal. Phys._, XXVI, p. 295.
+
+[6] Mr. E. Nettleship tells me that in the course of collecting
+pedigrees of families containing colour-blind members he has discovered
+two cases (shortly to be published) of pairs of twins, which on account
+of their very close resemblances must be deemed homologous, one of each
+pair being colour-blind and the other normal. Such a distinction between
+closely similar twins is most curious and unexpected.
+
+[7] Another paradoxical phenomenon of the same nature occurs in the
+Narwhal The males normally have the _left_ tusk alone developed, the
+corresponding right tusk remaining as an undeveloped rudiment in its
+socket. The left tusk is a left-handed screw. Occasionally the right
+tusk is also developed and grows to the same length as that of the
+left side, but in such specimens the right tusk is also a left-hand
+screw like the tusk of the other side, instead of being reversed as we
+should certainly have expected. It need scarcely be remarked that in the
+case of the horns of antelopes, and in other examples of spiral organs
+arranged in pairs, that of one side of the body is the mirror image of
+that on the other side. The Narwhal's tusks in being both twisted in the
+same direction are thus highly anomalous, and are comparable with pairs
+of twins.
+
+[8] Wilder, H. H., _Amer. Jour. Anat._, 1904, III, p. 452.
+
+[9] Polydactylism which is often a dominant and the web-foot of Pigeons
+which is recessive should be remembered as possible exceptions (see p.
+49).
+
+[10] Davenport inclined at first to regard rumplessness as a recessive,
+but in his latest publication on the subject he definitely concludes
+that it is an imperfect dominant. This conclusion accords well with
+evidence quoted by Darwin (_An. and Plts._, II, ed. 2, p. 4) that
+rumpless fowls may throw tailed offspring. (_Amer. Nat._, 1910, XLIV, p.
+134.)
+
+[11] Spillman, W. J., _Amer. Breeders Mag._, 1910, I, p. 178.
+
+[12] Newsholme, _Lancet_, December 10, 1910, p. 1690.
+
+[13] _Materials for the Study of Variation_, 1894, p. 358.
+
+[14] Walker, G., _Johns Hopkins Hospital Bulletin_, XII, 1901, p. 129.
+
+[15] Cp. R. H. Compton, _New Phytologist_, 1911, p. 249.
+
+[16] _Arch. f. Entwickelungsmech._, 1907, XXIII, p. 249.
+
+[17] Bull. Soc. Bot. de France, xxxiv, 1887, p. 182.
+
+[18] R. Boyle, _The Origine of Formes and Qualities_, Oxford, 1666.
+
+[19] Remarkable experiments on this question have lately been carried
+out by R. H. Compton (_Camb. Phil. Soc._, XV, 1910, p. 495), showing
+that in a certain Barley, "Plumage Corn," the average ratio of left to
+right is about 1.5. A fuller paper has since been published by Compton,
+_Jour. Genetics_, 1912, II, I, p. 53.
+
+
+
+
+                                CHAPTER III
+
+                    SEGMENTATION, ORGANIC AND MECHANICAL
+
+Models may be and often have been devised imitating some of the
+phenomena of division, but none of them have reproduced the peculiarity
+which characterises divisions of living tissues, that _the position
+of chemical differentiation_ is _determined by those divisions_. For
+example, models of segmentation, whether radial or linear, may be made
+by the vibration of plates as in the familiar Chladni figures of the
+physical laboratory, or by the bowing of a tube dusted on the inside
+with lycopodium powder, and in various other ways. The sand or the
+powder will be heaped up in the nodes or regions of least movement, and
+the patterns thus formed reproduce many of the geometrical features
+of segmentation. But in the segmentations of living things the nodes
+and internodes, once determined by the dividing forces, would each
+become the seat of appropriate and distinct chemical processes leading
+to the differentiation of the parts, and the deposition of the bones,
+petals, spines, hairs, and other organs in relation to the meristic
+ground-plan. The "ripples" of meristic division not merely divide but
+differentiate, and when a "ripple" forks the result is not merely a
+division but a reduplication of the organ through which the fork runs.
+An example illustrating such a consequence is that of the half-vertebrae
+of the Python. On the left side the vertebra is single (Fig. 7) and
+bears a single rib, but on the right side a division has occurred with
+the result that two half-vertebrae, each bearing a rib, are formed, one
+standing in succession to the other. We cannot, indeed, imagine any
+operation of physiological division carried out in such an organ as a
+vertebra, passing through a plane at right angles to the long axis of
+the body, which does not necessarily involve the further process of
+reduplication.
+
+As the meristic system of distribution spreads through the body,
+chemical differentiations follow in its track, with segmentation and
+pattern as the visible result. Could we analyse these simultaneous
+phenomena and show how it is that the places of chemical differentiation
+are determined by the system of division, progress would then be rapid.
+It is here that all speculation fails.
+
+[Illustration: FIGS. 7 and 8. Two examples of imperfect division in the
+vertebrae of a python. _I_, the vertebrae 147-150 from the right side,
+showing imperfect division between the 148th and 149th. The condition on
+the left side of this vertebra was the same. _II_, the dorsal surface of
+vertebrae 165-167. On the right side the 166th is double and bears two
+ribs, but on the left side it is normal and has one rib only.]
+
+Many attempts have been made to interpret the processes of division
+and repetition, in terms of mechanics, or at least to refer them to
+their nearest mechanical analogies, so far with little success. The
+problem is beset with difficulties as yet insurmountable and of these
+one must be especially noticed. In the living thing the process by which
+repetition and patterns come into being consists partly in division but
+partly also in growth. We have no means of studying the phenomena of
+pattern-formation except in association with that of growth. Growth soon
+ceases unless division takes place, and if growth is impossible division
+soon ceases also. In consequence of this fact that the final pattern
+is partly a product of growth, it can never be used as unimpeachable
+evidence of the primary geometrical relations of the members as laid
+down in the divisions.
+
+In the last chapter in referring to the problem of repetition I
+introduced an analogy, comparing the patterns of the organic world
+with those produced in unorganised materials by wave-motion. In the
+preliminary stage of ignorance, having no more trustworthy clue, I do
+not think it wholly unprofitable to consider the applicability of this
+analogy somewhat more fully. It possesses, as I hope to show, at least
+so much validity as to encourage the belief that morphology may safely
+discard one source of long-standing error and confusion.
+
+Those who have studied the structure of parts repeated in series will
+have encountered the old morphological problem of "Serial Homology,"
+which has absorbed so much of the attention of naturalists and
+especially of zoologists at various periods. This problem includes
+two separate questions. The first of these is the origin in evolution
+of the resemblance between two organs occurring in a repeated series,
+of which the fore and hind limbs of Vertebrates are the prerogative
+instance. From the fact that these resemblances can be traced very far,
+often into minute details of structure, many anatomists have inclined to
+the opinion that the resemblance must originally have been still more
+complete, and that the two limbs, for instance, must have acquired their
+present forms by the differentiation of two identical groups of parts.
+
+Similar questions arise whenever parts are repeated in series, whether
+the series be linear or radial, and, though less obviously, even when
+the repetition is bilateral only. In each such example the question
+arises, is the resemblance between the parts the remains of a still
+closer resemblance, or is differentiation original? Sometimes the
+view that these parts have arisen by the differentiation of a series
+of identical parts is plausible enough, as for example when the
+peculiarities of various appendages of a Decapod Crustacean are referred
+to modifications of the Phyllopod series. In application to other cases
+however we soon meet with difficulty, and the suggestion that the
+segments of a vertebrate were originally all alike is seen at once to be
+absurd, for the reason that a creature so constituted could not exist,
+and that, differentiation of at least one anterior and one posterior
+segment, is an essential condition of a viable organism consisting of
+parts repeated in a linear series. Between these two terminal segments
+it is possible to imagine the addition of one segment, or of a series
+of approximately similar segments; but when once it is realised that
+the terminals must have been differentiated from the beginning, it
+will be seen that the problem of the origin of the resemblance between
+segments is not rendered more comprehensible by the suggestion that
+even the intervening members were originally alike. Seeing indeed that
+some differentiation must have existed primordially it is as easy to
+imagine that the original body was composed of a series grading from the
+condition of the anterior segment to that of the posterior, as any other
+arrangement. The existence of a linear or successive series in fact
+postulates a polarity of the whole, and in such a system the conception
+of an ideal segment containing all the parts represented in the others
+has manifestly no place. The introduction of that conception though
+sanctioned by the great masters of comparative anatomy, has, as I think,
+really delayed the progress of a rational study of the phenomena of
+division. The same notion has been applied to every class of repetition
+both in animals and plants, generally with the same unhappy results. In
+the cruder forms in which this doctrine was taught thirty years ago it
+is now seldom expressed, but modified presentations of it still survive
+and confuse our judgments.
+
+The process of repetition of parts in the bodies of organisms is
+however a periodic phenomenon. This much, provided we remain free from
+prejudice as to the nature and causation of the period or rhythm, we
+may safely declare, and a comparison may thus be instituted between the
+consequences of meristic repetition in the bodies of living things and
+those repetitions which in the inorganic world are due to rhythmical
+processes. Of such processes there is a practically unlimited diversity
+and we have nothing to indicate with which of them our repetitions
+should rather be compared.
+
+[Illustration: FIG. 9. Osmotic growths simulating segmentation. (After
+Leduc.)]
+
+In some respects perhaps the best models of living organisms yet made
+are the "osmotic growths" produced by Leduc.[1] These curious structures
+were formed by placing a fragment of a salt, for instance calcium
+chloride, in a solution of some colloidal substance. As the solid takes
+up water from the solution a permeable pellicle or membrane is formed
+around it. The vesicle thus enclosed grows by further absorption of
+water, often extending in a linear direction, and in many examples this
+growth occurs by a series of rhythmically interrupted extensions. Some
+of the growths thus formed are remarkably like organic structures,
+and might pass for a series of antennary segments or many other
+organs consisting of a linear series of repeated parts. In admitting
+the essential resemblance between these "osmotic growths" and living
+bodies or their organs I lay less stress on the general conformation
+of the growths, which often as Leduc points out, recall the forms
+of fungi or hydroids, but rather on the fact that the interruptions
+in the development of these systems are so closely analogous to the
+segmentations or repetitions of parts characteristic of living things
+(Fig. 9). In the same way I am less impressed by Leduc's models of
+Karyokinesis, wonderful as they nevertheless are, for the division is
+here imitated by putting separate drops on the gelatine film. What we
+most want to know is how in the living creature one drop becomes two.
+The models of linear segmentation have the remarkable merit that they do
+in some measure imitate the process of actual division or repetition.
+So in a somewhat modified method Leduc, by causing the diffusion
+of a solution in a gelatine film, produced rhythmical or periodic
+precipitations strikingly reminiscent of various organic tissues, for
+here also the process of periodic repetition is imitated with success.
+
+It is a feature common to these and to all other rhythmical repetitions
+produced by purely mechanical forces that there is resemblance between
+the members of the series, and that this similarity of conformation may
+be maintained in most complex detail. When however in the mechanical
+series some of the members differ from the rest we have no difficulty
+in recognising that these differences--which correspond with the
+differentiations of the organic series--are due to special heterogeneity
+in the conditions or in the materials, and it never occurs to us to
+suppose that all the members must have been primordially alike. For
+example, in the case of ripple-marks on the sand, which I choose as one
+of the most familiar and obvious illustrations of a repeated series
+due to mechanical agencies, if we notice one ripple different in form
+from those adjacent to it, we do not suppose that this variation must
+have been brought about by deformation of a ripple which was at first
+formed like the others, but we ascribe it to a difference in the sand at
+that point, or to a difference in the way in which the wind or the tide
+dealt with it. We may press the analogy further by observing that in as
+much as such a series of waves has a beginning and an end, it possesses
+polarity like that of the various linear series of parts in organisms,
+and even the formation of each member must influence the shape of its
+successor. Since in an organism the beginning and end of the series
+are always included, some differentiation among the repetitions must
+be inevitable. If therefore it be conceded, as I think it must, that
+segmentation and pattern are the consequence of a periodic process we
+realize that it is at least as easy to imagine the formation of such a
+series of parts having family likeness combined with differentiation
+as it would be to conceive of their arising primordially as a series
+of identical repetitions. The suggestion that the likenesses which we
+now perceive are the remains of a still more complete resemblance is a
+substitution of a more complex conception for a simpler one.
+
+The other question raised by the problem of Serial Homology is how far
+there is a correspondence between individual members of series when
+the series differ from each other either in the number of parts, or in
+the mode of distribution of differentiation among them. Students, for
+example, of vertebrate morphology debate whether the _n_th vertebra
+which carries the pelvic girdle in Lizard A is individually homologous
+with the _n_ + _x_th vertebra which fulfils this function in Lizard B,
+or whether it is not more truly homologous with the vertebra standing in
+the _n_th ordinal position, though that vertebra in Lizard B is free.
+
+In various and more complex aspects the same question is debated in
+regard to the cranial and spinal nerves, the branches of the aorta,
+the appendages of Arthropoda, and indeed in regard to all such series
+of differentiated parts in linear or successive repetition. Persons
+exercised with these problems should before making up their minds
+consider how similar questions would be answered in the case of any
+series of rhythmical repetitions formed by mechanical agencies. In the
+case of our illustration of the ripples in the sand, given the same
+forces acting on the same materials in the same area, the number of
+ripples produced will be the same, and the _n_th ripple counting from
+the end of the series will stand in the same place whenever the series
+is evoked. If any of the conditions be changed, the number and shapes
+can be changed too, and a fresh "distribution of differentiation"
+created. Stated in this form it is evident that the considerations
+which would guide the judgment in the case of the sand ripples are not
+essentially different from those which govern the problem of individual
+homology in its application to vertebrae, nerves, or digits.
+
+The fact that the unit of repetition is also the unit of growth is the
+source of the obscurity which veils the process. When we compare the
+skeleton of a long-tailed monkey with that of a short-tailed or tailless
+ape we see at once how readily the additional series of caudal segments
+may be described as a consequence of the propagation of the "waves" of
+segmentation beyond the point where they die out in the shorter column,
+and we see that with an extension of the series of repetitions there is
+growth and extension of material.
+
+The considerations which apply to this example will be found operating
+in many cases of the variation of terminal members of linear series.
+Some of these series, like the teeth of the dog, end in a terminal
+member of a size greatly reduced below that of the next to it. Even when
+there is thus a definite specialisation of the last member of the series
+it not infrequently happens that the addition, by variation, of a member
+beyond the normal terminal, is accompanied by a very palpable increase
+in size of the member which stands numerically in the place of the
+normal terminal.[2] So also with variation in the number of ribs, when
+a lumbar vertebra varies homoeotically into the likeness of the last
+dorsal and bears a rib, the rib placed next in front of this, which in
+the normal trunk is the last, shows a definite increase in development.
+
+The consequences of such homoeoses are sometimes very extensive,
+involving readjustments of differentiation affecting a long series of
+members, as may easily be seen by comparing the vertebral columns of
+several individual Sloths[3] (whether _Bradypus_ or _Choloepus_) to take
+a specially striking example.
+
+It may be urged that no feature as yet enables us to perceive wherein
+lies the primary distinction which determines such variation, whether it
+is due to a difference in the dividing forces or in the material to be
+divided. If for instance we were to imitate such a series of segments
+by pressing hanging drops of a viscous fluid out of a paint-tube by
+successive squeezes, the number of times the tube is contracted before
+it is empty will give the number of the segments, but their size may
+depend either on the force of the contractions or on the capacity of
+the tube, or on various other factors. Nevertheless in the case of the
+variation of terminal members, whatever be the nature of the rhythmical
+impulse which produces the series of organs, the elevation of the
+normally terminal member in correspondence with the addition of another
+is what we should expect.
+
+If the organism acquired its full size first and the delimitation of
+the parts took place afterwards, there might be some hope that the
+resemblance between living patterns and those mechanically caused by
+wave-motion might be shown to be a consequence of some real similarity
+of causation, but in view of the part played by growth, appeal to these
+mechanical phenomena cannot be declared to have more than illustrative
+value. Similarly in as much as living patterns appear, and almost
+certainly do in reality come into existence by a rhythmical process,
+comparisons of these patterns with those developed in crystalline
+structures, and in the various fields of force are, as it seems to me,
+inadmissible, or at least inappropriate.
+
+However their intermittence be determined, the rhythms of division must
+be looked upon as the immediate source of those geometrically ordered
+repetitions universally characteristic of organic life. In the same
+category we may thus group the segmentation of the Vertebrates and of
+the Arthropods, the concentric growth of the Lamellibranch shells or of
+Fishes' scales, the ripples on the horns of a goat, or the skeletons of
+the Foraminifera or of the Heliozoa. In the case of plant-structures
+Church[4] has admirably shown, with an abundance of detail, how on
+analysis the definiteness of phyllotaxis is an expression of such
+rhythm in the division of the apical tissues, and how the spirals
+and "orthostichies" displayed in the grown plant are its ultimate
+consequences. The problem thus narrows itself down to the question of
+the mode whereby these rhythms are determined.
+
+It is natural that we should incline to refer them to a chemical source.
+If we think of the illustration just given, of the segmentation of a
+viscous fluid into drops by successive contractions of a soft-walled
+tube we can, I think, conceive of such rhythmic contractions as due to
+summations of chemical stimuli, somewhat as are the beats of the heart.
+But when we recognize the vast diversity of materials the distribution
+of which is determined by an ostensibly similar rhythmic process it
+seems hopeless to look forward to a directly chemical solution. That the
+chemical degradation of protoplasm or of materials which it contains is
+the source of the energy used in the divisions cannot be in dispute, but
+that these divisions can be themselves the manifestations of chemical
+action seems in the highest degree improbable.
+
+We may therefore insist with some confidence on the distinction between
+the Meristic and the substantive constitution of organisms, between,
+that is to say, the system according to which the materials are divided
+and the essential composition of the materials, conscious of the fact
+that the energy of division is supplied from the materials, and that
+in the ontogeny the manner in which the divisions are effected must
+depend secondarily on the nature of the substances to be divided.
+The mechanical processes of division remain a distinguishable group
+of phenomena, and variations in the substances to be distributed in
+division may be independent of variations in the system by which the
+distribution is effected.
+
+Modern genetic analysis supplies many remarkable examples of this
+distinction. When formerly we compared the leaves of a normal palmatifid
+Chinese Primula with the pinnatifid leaves[5] of its fern-leaved variety
+we were quite unable to say whether the difference between the two types
+of leaf was due to a difference in the material cut up in the process
+of division or to a difference in that process itself. Knowledge that
+the distinction is determined by a single segregable factor tends to
+prove that the critical difference is one of substance. So also in the
+Silky fowl we know that the condition of its feathers is due to the
+absence of some one factor present in the normal form. We may conceive
+such differences as due to change of form in the successive "waves" of
+division, but we cannot yet imagine segregation otherwise than as acting
+by the removal or retention of a material element. Future observation
+by some novel method may suggest some other possibility, but such cases
+bring before us very clearly the difficulties by which the problem is
+beset.
+
+[Illustration: FIG. 10. The palm-and fern type of leaf in _Primula
+Sinensis_. The palm is dominant and the fern is recessive.]
+
+In another region of observation phenomena occur which as it seems to
+me put it beyond question that the meristic forces are essentially
+independent of the materials upon which they act, save, in the remoter
+sense, in so far as these materials are the sources of energy. The
+physiology of those regenerations and repetitions which follow upon
+mutilation supplies a group of facts which both stimulate and limit
+speculation. No satisfactory interpretations of these extraordinary
+occurrences has ever been found, but we already know enough to feel
+sure that in them we are witnessing indications which should lead
+to the discovery of the true mechanics of repetition and pattern.
+The consequences of mutilation in causing new growth or perhaps more
+strictly in enabling new growth to take place, are such that they cannot
+be interpreted as responses to chemical stimuli in any sense which
+the word chemical at present connotes. Powers are released by mutilation
+of which in the normal conditions of life no sign can be detected. All
+who have tried to analyse the phenomena of regeneration are compelled
+to have recourse to the metaphor of equilibrium, speaking of the normal
+body as in a state of strain or tension (Morgan) which when disturbed by
+mutilation results in new division and growth. The forces of division
+are inacessible to ordinary means of stimulation. Applications, for
+example, of heat or of electricity excite no responses of a positive
+kind unless the stimuli are so violent as to bring about actual
+destruction.[6] These agents do not, to use a loose expression, come
+into touch with the meristic forces. Changes in the chemical environment
+of cells may, as in the experiments of Loeb and of Stockard produce
+definite effects, but the facts suggest that these effects are due
+rather to alterations in the living material than to influence exerted
+directly on the forces of division themselves.
+
+By destruction of tissue however the forces both of growth and of
+division also may often be called into action with a resulting
+regeneration. Interruption of the solid connexion between the parts may
+produce the same effects, as for example when the new heads or tails
+grow on the divided edges of Planarians (Morgan), or when from each half
+embryo partially separated from its normally corresponding half, a new
+half is formed with a twin monster as the result.
+
+Often classed with regenerations but in reality quite distinct from
+them are those special and most interesting examples where the growth
+of a _paired_ structure is excited by a simple wound. Some of the best
+known of these instances are presented by the paired extra appendages
+of Insects and Crustacea. Some years ago I made an examination of all
+the examples of such monstrosities to which access was to be obtained,
+and it was with no ordinary feeling of excitement that I found that
+these supernumerary structures were commonly disposed on a recognizable
+geometrical plan, having definite spatial relations both to each other
+and to the normal limb from which they grew. The more recent researches
+of Tornier[7] and especially his experiments on the Frog have shown
+that a cut into the posterior limb-bud induces the outgrowth of such a
+_pair_ of limbs at the wounded place. Few observations can compare with
+this in novelty or significance; and though we cannot yet interpret
+these phenomena or place them in their proper relations with normal
+occurrences, we feel convinced that here is an observation which is no
+mere isolated curiosity but a discovery destined to throw a new light on
+biological mechanics. The supernumerary legs of the Frog are evidently
+grouped in a system of symmetry similar to that which those of the
+Arthropods exhibit, and though in Arthropods paired repetitions have not
+been actually produced by injury under experimental conditions we need
+now have no hesitation in referring them to these causes as Przibram has
+done.
+
+At this point some of the special features of the supernumerary
+appendages become important. First they may arise at any point on the
+normal limb, being found in all situations from the base to the apex.
+Nor are they limited as to the surface from which they spring, arising
+sometimes from the dorsal, anterior, ventral, or posterior surfaces, or
+at points intermediate between these principal surfaces.
+
+With rare and dubious exceptions, the parts which are contained in these
+extra appendages are only those which lie _peripheral to their point of
+origin_. Thus when the point of origin is in the apical joint of the
+tarsus, the extra growth if completely developed consists of a double
+tarsal apex bearing two pairs of claws. If they arise from the tibia,
+two complete tarsi are added. If they spring from the actual base of
+the appendage then two complete appendages may be developed in addition
+to the normal one. We must therefore conclude that in any point on a
+normal appendage the power exists which, if released, may produce a bud
+containing in it a paired set of the parts peripheral to this point.
+
+[Illustration: FIG. 11. Diagrams of the geometrical relations which are
+generally exhibited by extra pairs of appendages in Arthropoda. The
+sections are supposed to be those of the apex of a tibia in a beetle.
+_A_, anterior, _P_, posterior, _D_, dorsal, _V_, ventral. _M_^{1},
+_M_^{2} are the imaginary planes of reflexion. The shaded figure is in
+each case a limb formed like that of the other side of the body, and the
+outer unshaded figures are shaped like the normal for the side on which
+the appendages are. On the several radii are shown the extra pairs in
+their several possible relations to the normal from which they arise.
+The normal is drawn in thick lines in the center.]
+
+Next the geometrical relations of the halves of the supernumerary pair
+are determined by the position in which they stand in regard to the
+original appendage. These relations are best explained by the diagram
+(Fig. 11), from which it will be seen that the two supernumerary
+appendages stand as images of each other; and, of them, that which is
+adjacent to the normal appendage forms an image of it. Thus if the
+supernumerary pair arise from a point on the dorsal surface of the
+normal appendage, the two _ventral_ surfaces of the extra pair will
+face each other. If they arise on the anterior surface of the normal
+appendage, their morphologically posterior surfaces will be adjacent,
+and so on.
+
+These facts give us a view of the relations of the two halves of a
+dividing bud very different from that which is to be derived from the
+exclusive study of normal structures. Ordinary morphological conceptions
+no longer apply. The distribution of the parts shows that the bud or
+rudiment which becomes the supernumerary pair may break or open out in
+various ways according to its relations to the normal limb. Its planes
+of division are decided by its geometrical relations to the normal body.
+
+Especially curious are some of the cases in which the extra pair are
+imperfectly formed. The appearance produced is then that of two limbs
+in various stages of coalescence, though in reality of course they are
+stages of imperfect separation. The plane of "coalescence" may fall
+anywhere, and the two appendages may thus be compounded with each other
+much as an object partially immersed in mercury "compounds" with its
+optical image reflected from the surface.
+
+Supernumerary paired structures are not usually, if ever, formed when
+an appendage is simply amputated. Cases occasionally are seen which
+nevertheless seem to be of this nature. Borradaile,[8] for example,
+described a crab (_Cancer pagurus_) having in place of the right chela
+three _small_ chelae arising from a common base, where the appearances
+suggested that the three reduced limbs replaced a single normal limb.
+From the details reported however it seems still possible that one of
+the chelae (that lettered F. I in Borradaile's figure) may be the normal
+one, and the other two an extra pair. The chela which I suspect to be
+the normal is in several respects deformed as well as being reduced in
+size, and this deformity may perhaps have ensued as a consequence of the
+same wound which excited the growth of the extra pair. Its reduced size
+may be due to the same injury, which may quite well have checked its
+growth to full proportions.
+
+Admitting doubt in these ambiguous cases it seems to be a general rule
+that for the production of the extra pair the normal limb should persist
+in connexion with the body. Moreover it is practically certain that
+in no case can a _single_, viz. an unpaired, duplicate of the normal
+appendage grow from it. Many examples have been described as of this
+nature, but all of them may be with confidence regarded as instances of
+a supernumerary pair in which only the two morphologically anterior or
+the two morphologically posterior surfaces are developed. We have thus
+the paradox that a limb of one side of the body, say the right, has in
+it the power to form a pair of limbs, right and left, as an outgrowth of
+itself, but cannot form a second left limb alone.
+
+A very interesting question arises whether it is strictly correct to
+describe the extra pair as a right and a left, or whether they are not
+rather two lefts or two rights of which one is reversed. This question
+did not occur to me when in former years I studied these subjects. It
+was suggested to me by Dr. Przibram. The answer might have an important
+bearing on biological mechanics, but I know no evidence from which the
+point can be determined with certainty. In order to decide this question
+it would be necessary to have cases in which the paired repetition
+affected a limb markedly differentiated on the two sides of the body,
+and of course the development of the extra parts in order to be decisive
+must be fairly complete. One example only is known to me which at all
+satisfies these requirements, that of the lobster's chela figured (after
+Van Beneden) in _Materials for the Study of Variation_, p. 531, Fig.
+184, III.
+
+Here the drawing distinctly suggests that one of the extra
+dactylopodites, namely that lettered R, is differentiated as a left
+and not merely a reversed right. For the teeth on this dactylopodite
+are those of a cutting claw, not of a crushing claw, whereas the
+dactylopodites R' and L' bear crushing teeth. The figure makes it fairly
+certain also that the limb affected was a crushing claw. Accepting this
+interpretation, we reach the remarkable conclusion that the bud of new
+growth consisted of halves differentiated into cutter and crusher as the
+normal claws are, and that the extra crusher is geometrically a left
+but physiologically a right. Though shaped as a left in respect of the
+direction in which it points, the extra crusher is really an optically
+reversed right, while the dactylopodite R, which is placed pointing like
+a right, is really a reversed left (Fig. 12).
+
+[Illustration: FIG. 12. Right claw of lobster bearing a pair of extra
+dactylopodites (after van Beneden). The fine toothing on R suggests that
+this is part of a cutting claw, though the limb bearing it is a crusher.]
+
+If these indications are reliable[9] and are established by further
+observation we shall be led to the conclusion that the bud which
+becomes an extra pair of limbs does not merely contain the parts proper
+to the side on which it grows, but is comparable with the original
+zygotic cell, and consists not simply of two halves, but of two halves
+differentiated as a right and a left like the two halves of the normal
+body.
+
+Phenomena of this kind, evoked by mutilation or injury, together with
+the cognate observations on regeneration throw very curious lights
+on the nature of living things. To an understanding of the nature of
+the mechanics of living matter and its relation to matter at large
+they offer the most hopeful line of approach. I allude especially to
+the examples in which it has been established that the part which is
+produced after mutilation is a structure different from that which was
+removed. The term "regeneration" was introduced before such phenomena
+were discovered, and though every one recognizes its inapplicability
+to these remarkable cases, the word still misleads us by presenting
+a wrong picture to the mind. The expression "heteromorphosis" (Loeb)
+has been appropriately applied to various phenomena of this kind, and
+Morgan has given the name "morphallaxis" to another group of cases in
+which the renewal occurs by the transformation of a previously existing
+part.[10] But we must continually remember that all these occurrences
+which we know only as abnormalities and curiosities must in reality be
+exemplifications of the normal mechanics of division and growth. The
+conditions needed to call them forth are abnormal, but the responses
+which the system makes are evidences of its normal constitution. When
+therefore, for example, the posterior end of a worm produces a reversed
+tail from its cut end we have a proof that there must be in the normal
+body forces ready to cause this outgrowth. The new structure is not
+an ill-shaped head-end, for, as Morgan shows, the nephridial ducts
+have their funnels perforating the segments in a reversed direction.
+The "tension" of growth is actually reversed.[11] So also when in a
+Planarian amputation of the body immediately behind the head leads to
+the formation of a new reversed head at the back of the normal head,
+while amputation further back leads to the regeneration of a new tail,
+these responses give indications of forces normally present in the
+body of the Planarian. Such facts open up a great field of speculation
+and research. Especially important it would be to determine where the
+critical region may be at which the one response is replaced by the
+other. I suppose it is even possible that there is some neutral zone in
+which neither kind of response is made.
+
+Physical parallels to the phenomena of regeneration are not easy
+to find and we still cannot penetrate beyond the empirical facts.
+Przibram has laid stress on the general resemblance between the new
+growth of an amputated part in an animal and the way in which a broken
+crystal repairs itself when placed in the mother-solution. That the
+two processes have interesting points of likeness cannot be denied.
+It must however never be forgotten that there is one feature strongly
+distinguishing the two; for I believe it is universally recognized
+by physicists that all the phenomena of geometrical regularity which
+crystals display are ultimately dependent on the forms of the particles
+of the crystalline body. This cannot in any sense be supposed to hold
+in regard to protoplasm or its constituents. The definiteness of
+crystals is also an unlikely guide for the reason that it is absolute
+and perfect, or in other words because this kind of regularity cannot
+be disturbed at all without a change so great that the substance itself
+is altered; whereas we know that the forms of living things are capable
+of such changes, great and small, that we must regard perfection of
+form, whether manifested in symmetry or in number, as an ideal which
+will only be produced in the absence of disturbance. The symmetry of the
+living things is like the symmetry of the concentric waves in a pool
+caused by a splash. Perfect circles are made only in the imaginary case
+of mathematical uniformity, but the system maintains an approximate
+symmetry though liable to manifold deformation.
+
+Since the geometrical order of the living body cannot be a direct
+function of the materials it must be referred to some more proximate
+control. In renewing a part the body must possess the power of seizing
+particles of many dissimilar kinds, and whirl them into their several
+and proper places. The action in renewal, like that of original growth,
+may be compared--very crudely--with the action of a separator which
+simultaneously distributes a variety of heterogeneous materials in an
+orderly fashion; but in the living body the thing distributed must
+rather be the _appetency_ for special materials, not the materials
+themselves.
+
+If the analogy of crystals be set aside and we seek for other parallels
+to regeneration there are none very obvious. I have sometimes wondered
+whether it might not be possible to institute a fruitful comparison
+between the renewal of parts and the reformation of waves of certain
+classes after obliteration. In several respects, as I have already said,
+some curious resemblances with the repetitions formed by wave-motion
+are to be traced in our organic phenomena, and though admitting that
+I cannot develop these comparisons, I think nevertheless they may be
+worth bearing in mind. When, after obliteration, an eddy in a stream,
+or a ripple-mark (a more complex case of eddy-formation) in blown sand
+is re-formed, we have an example in which pattern is reconstituted and
+growth takes place not by virtue of the composition of the materials--in
+this case the water or the sand--but by the way in which they are acted
+upon by extraneous forces.
+
+A feature in the actual mode by which ripple-marks are reconstituted may
+not be without interest in connexion with our phenomena of regeneration.
+When, for example, the wind is blowing steadily over a surface of fine,
+dry sand, the familiar ripple-marks are formed by a heaping of the sand
+in lines transverse to the direction of the wind. The heaping is due to
+the formation of eddies corresponding with positions of instability.
+When the wind is steady and the sand homogeneous, the distances between
+the ripples, or wave-lengths, are sensibly equal. If while the wind
+continues to blow, the ripples are obliterated with a soft brush they
+will quickly be re-formed over the whole area, but I have noticed that
+at first their wave-length is approximately half that of the ripples
+in the undisturbed parts of the system.[12] The normal wave-length is
+restored by the gradual accentuation of alternate ripples. Of course
+the sand-ripples are in reality slowly travelling forward in the
+direction towards which the wind is blowing, and for this our living
+segmentations afford no obvious parallel, but the appearances in the
+area of reformation, and especially the forking of the old ridges where
+they join the new ones, are curiously reminiscent of the irregularities
+of segmentation seen in regenerated structures. The value of the
+considerations adduced in the chapter is, I admit, very small. The
+utmost that can be claimed for them is that mechanical segmentations,
+like those seen in ripple-mark, or in Leduc's osmotic growths, show
+how by the action of a continuous force in one direction, repeated
+and serially homologous divisions can be produced having features of
+similarity common to those repetitions by which organic forms and
+patterns are characterised. The analogy supplies a vicarious picture of
+the phenomena which in default of one more true may in a slight degree
+assist our thoughts. It suggests that the rhythms of segmentation may be
+the consequence of a single force definite in direction and continuously
+acting during the time of growth. The polarity of the organism would
+thus be the expression of the fact that this meristic force is
+definitely directed after it has once been excited, and the reversal
+seen in some products of regeneration suggest further that it is capable
+of being reflected. This polarity cannot be a property of the material,
+as such, but is determined by a force acting on that material, just as
+the polarity of a magnet is not determined by the arrangement of its
+particles, but by the direction in which the current flows.
+
+To some it may appear that even to embark on such discussions as this is
+to enter into a perilous flirtation with vitalistic theories. How, they
+may ask, can any force competent to produce chemical and geometrical
+differentiation in the body be distinguished from the "Entelechy" of
+Driesch? Let me admit that in this reflexion there is one element of
+truth. If those who proclaim a vitalistic faith intend thereby to
+affirm that in the processes by which growth and division are effected
+in the body, a part is played by an orderly force which we cannot
+_now_ translate into terms of any known mechanics, what observant man
+is not a vitalist? Driesch's first volume, putting as it does into
+intelligible language that positive deduction from the facts--especially
+of regeneration--should carry a vivid realisation of this truth to any
+mind. If after their existence is realised, it is desired that these
+unknown forces of order should have a name, and the word entelechy is
+proposed, the only objection I have to make is that the adoption of a
+term from Aristotelian philosophy carries a plain hint that we propose
+to relegate the future study of the problem to metaphysic.
+
+From this implication the vitalist does not shrink. But I cannot find in
+the facts yet known to us any justification of so hopeless a course. It
+was but yesterday that the study of _Entwicklungsmechanik_ was begun,
+and if in our slight survey we have not yet seen how the living machine
+is to be expressed in terms of natural knowledge that is poor cause for
+despair. Driesch sums up his argument thus:[13]
+
+"It seems to me that there is only one conclusion possible. If we are
+going to explain what happens in our harmonious-equipotential systems
+by the aid of causality based upon the constellation of single chemical
+factors and events, there _must_ be some such thing as a machine. Now
+the assumption of the existence of a machine proves to be absolutely
+absurd in the light of the experimental facts. _Therefore there can
+be neither any sort of a machine nor any sort of causality based upon
+constellation underlying the differentiation of harmonious-equipotential
+systems._"
+
+"For a machine, typical with regard to the three chief dimensions
+of space, cannot remain itself if you remove parts of it or if you
+rearrange its parts at will."
+
+To the last clause a note is added as follows:
+
+"The pressure experiments and the dislocation experiments come into
+account here; for the sake of simplicity they have not been alluded to
+in the main line of our argument."
+
+I doubt whether any man has sufficient knowledge of all possible
+machines to give reality to this statement. In spite also of the
+astonishing results of experiments in dislocation, doubt may further be
+expressed as to whether they have been tried in such variety or on such
+a scale as to justify the suggestion that the living organism remains
+itself if its parts are rearranged at will. All we know is that it can
+"remain itself" when much is removed, and when much rearrangement has
+been affected, which is a different thing altogether.
+
+I scarcely like to venture into a region of which my ignorance is so
+profound, but remembering the powers of eddies to re-form after partial
+obliteration or disturbance, I almost wonder whether they are not
+essentially machines which remain themselves when parts of them are
+removed.
+
+Real progress in this most obscure province is not likely to be made
+till it attracts the attention of physicists; and though they for long
+may have to forego the application of exact quantitative methods, I
+confidently anticipate that careful comparison between the phenomena
+of repetition formed in living organisms and the various kinds of
+segmentation produced by mechanical agencies would be productive of
+illuminating discoveries.
+
+FOOTNOTES:
+
+[1] Stephane Leduc, _Theorie Physico-Chymique de la Vie_, Paris, 1910.
+
+[2] _Materials for the Study of Variation_, No. 249, p. 217; and p. 272.
+
+[3] _Materials_, p. 118.
+
+[4] Church, A. H., _On the Relation of Phyllotaxis to Mechanical Laws_,
+London, 1904.
+
+[5] It is a question whether the dominance of the palmatifid leaf over
+the pinnatifid is not really an example of the dominance of a lower
+number of segmentations over a higher. From the uncertainty whether two
+given leaves of two separate plants are actually comparable one cannot
+institute quite satisfactory numerical comparisons, but I think the view
+that the "Fern" leaf has more lobes than an otherwise similar "Palm"
+leaf may be fairly maintained. If this be admitted, the "Palm" leaf
+represents the dominant low number and its round shape is a consequence
+of the greater powers of growth which are so often possessed by the
+members of a shorter series.
+
+[6] It is perhaps of importance to remember that in certain species
+of bacteria (e. g. _Bacillus Anthracis_) division may cease where the
+organism is cultivated under certain artificial conditions though growth
+continues. In this way very long unsegmented threads are produced.
+
+[7] _Arch. f. Entwm._, XX, 1905, p. 76; _Sitzungsb. d. Ges. Naturf._,
+Berlin, 1907, p. 41, etc.
+
+[8] Borradaile, L. A., _Jour. Marine Zool._, 1897, No. 8.
+
+[9] Dr. Przibram, I should mention, concludes that on the whole the
+facts are against this interpretation, but as more evidence is certainly
+required, I call attention to the possibility.
+
+[10] Morgan, T. H., _Regeneration_, 1901.
+
+[11] It would be interesting to know whether growth continues at the
+original posterior end after the new "posterior" end has been formed in
+front.
+
+[12] In the actual case observed, the ripples unsmoothed had a
+wave-length of about 2-1/2 inches; and when the new ones were first
+formed, there were about 30 ridges in the length originally traversed by
+15 or 16.
+
+[13] _The Science and Philosophy of the Organism_; Gifford Lectures,
+1907. London, 1908, p. 141.
+
+
+
+
+                       CHAPTER IV
+
+       THE CLASSIFICATION OF VARIATION AND THE NATURE OF
+                   SUBSTANTIVE FACTORS
+
+We have now seen that among the normal physiological processes the
+phenomena of division form a recognisable, and in all likelihood a
+naturally distinct group. Variations in these respects may thus be
+regarded as constituting a special class among variations in general.
+
+The substantive variations have only one property in common--the
+negative one that they are not Meristic. The work of classifying them
+and distinguishing them according to their several types demands a
+knowledge of the chemistry of life far higher than that to which science
+has yet attained. In reference to some of the simplest variations Garrod
+has introduced the appropriate term "Chemical sports." The condition in
+man known as Alkaptonuria in which the urine is red is due especially
+to the absence of the enzyme which decomposes the excretory substance,
+alkapton. The "chemical sport" here consists in the inability to break
+up the benzene ring. The chemical feature which distinguishes and is the
+proximate cause of several colour-varieties can now in a few cases be
+declared. The work of Miss Wheldale has shown that colour-varieties may
+be produced by the absence of the chromogen compound the oxidation of
+which gives rise to sap-colours, by differences in the completeness of
+this process of oxidation, and by a process of reduction supervening on
+or perhaps suppressing the oxidation. Some of these processes moreover
+may be brought about by the combined action of two bodies, the one an
+enzyme, for example an oxygenase, and the other a substance regarded as
+a peroxide, contributing the oxygen necessary for the oxidation to take
+place. Variation in colour may thus be brought about by the addition or
+omission of any one of the bodies concerned in the action.
+
+Similar variations, or rather similar series of variations will
+undoubtedly hereafter be identified in reference to all the various
+kinds of chemical processes upon which the structure and functions of
+living things depend. The identification of these processes and of
+the bodies concerned in them will lead to a real classification of
+Substantive Variations.
+
+To forecast the lines on which such classification will proceed is to
+look too far ahead. We may nevertheless anticipate with some confidence
+that future analysis will recognise among the contributing elements,
+some which are intrinsic and inalienable, and others which are extrinsic
+and superadded.
+
+We already know that there may be such interdependence among the
+substantive characters that to disentangle them will be a work of
+extreme difficulty. The mere fact that in our estimation characters
+belong to distinct physiological systems is no proof of their actual
+independence. In illustration may be mentioned the sap-colour in Stocks
+and the development of hoariness on the leaves and stems, which Miss
+Saunders's experiments have shown to be intimately connected, so that
+in certain varieties no hoariness is produced unless the elements for
+sap-colour are already present in the individual plant.
+
+The first step in the classification of substantive variations is
+therefore to determine which are due to the addition of new elements
+or factors, and which are produced by the omission of old ones. _A
+priori_ there is no valid criterion by which this can be known, and
+actual experiments in analytical breeding can alone provide the
+knowledge required. Some very curious results have by this method been
+obtained, which throw an altogether unexpected light on these problems.
+For example, in order that the remarkable development of mesoblastic
+black pigment characteristic of the Silky Fowl should be developed, it
+is practically certain that two distinct variations from such a type
+as _Gallus bankiva_ must have occurred. I assume, as is reasonable,
+that _G. bankiva_ has genetic properties similar to those of the Brown
+Leghorn breed which has been used in the experiments which Mr. Punnett
+and I have conducted. _Gallus bankiva_ was not available but the Brown
+Leghorn agrees with it very closely in colouration, and probably in
+the general physiology of its pigmentation. Setting aside the various
+structural differences between the two breeds, the Silky is immediately
+distinguished from the Leghorn by the fact that the skin of the whole
+body including that of the face and comb appears to be of a deep
+purplish colour. The face and comb of the Leghorn are red and the skin
+of the body is whitish yellow. On examination it is found that the
+purple colour of the Silky is in reality due to the distribution of a
+deep black pigment in the mesoblastic membranes throughout the body. The
+somatopleura, the pleura, _pia mater_, the dermis, and in most organs
+the connective tissue and the sheaths of the blood-vessels, are thus
+impregnated with black. No such pigmentation exists in the Leghorn.
+As the result of an elaborate series of experimental matings we have
+proved that the distinction between the Leghorn and the Silky consists
+primarily in the fact that the Silky possesses a pigment-producing
+factor, _P_, which is not present in the Leghorn.
+
+This variation must undoubtedly have been one of _addition_. But besides
+this there is another difference of an altogether dissimilar nature; for
+the Brown Leghorn possesses a factor which has the power of partially or
+completely restricting the operation of the pigment-producing factor,
+_P_. Moreover in respect of this pigment-restricting factor which we
+may call _D_, the sexes of the Brown Leghorn differ, for the male is
+homozygous or _DD_, but the female is heterozygous, _Dd_. Thus in order
+that the black-skinned breed could be evolved from such a type as a
+Brown Leghorn it must be necessary _both_ that _P_ should be added and
+that _D_ should drop out. We have not the faintest conception of the
+process by which either of these events have come to pass, but there is
+no reasonable doubt that in the evolution of the Silky fowl they did
+actually happen.
+
+We may anticipate that numerous interdependences of this kind will be
+discovered.
+
+Before any indisputable progress can be made with the problem of
+evolution it is necessary that we should acquire some real knowledge
+of the genesis of that class of phenomena which formed the subject of
+the last chapter. So long as the process of division remains entirely
+mysterious we can form no conception even of the haziest sort as
+to the nature of living organisms, or of the proximate causes which
+determine their forms, still less can we attempt any answer to those
+remoter questions of origin and destiny which form the subject of the
+philosopher's contemplation. It is in no spirit of dogmatism that I
+have ventured to indicate the direction in which I look for a solution,
+though I have none to offer. It may well be that before any solution is
+attained, our knowledge of the nature of unorganised matter must first
+be increased. For a long time yet we may have to halt, but we none
+the less do well to prepare ourselves to utilise any means of advance
+that may be offered, by carefully reconnoitering the ground we have to
+traverse. The real difficulty which blocks our progress is ignorance of
+the nature of division, or to use the more general term, of repetition.
+
+Let us turn to the more familiar problem of the causes of variation. Now
+since variation consists as much in meristic change as in alteration in
+substance or material, there is one great range of problems of causation
+from which we are as yet entirely cut off. We know nothing of the
+causation of division, and we have scarcely an observation, experiment
+or surmise touching the causes by which the meristic processes may be
+altered.
+
+Of the way in which variations in the substantive composition of
+organisms are caused we have almost as little real evidence, but we are
+beginning to know in what such variations must consist. These changes
+must occur either by the addition or loss of factors.
+
+We must not lose sight of the fact that though the factors operate by
+the production of enzymes, of bodies on which these enzymes can act, and
+of intermediary substances necessary to complete the enzyme-action, yet
+these bodies themselves can scarcely be themselves genetic factors, but
+consequences of their existence. What then are the factors themselves?
+Whence do they come? How do they become integral parts of the organism?
+Whence, for example, came the power which is present in a White Leghorn
+of destroying--probably reducing--the pigment in its feathers? That
+power is now a definite possession of the breed, present in all its
+germ-cells, male and female, taking part in their symmetrical divisions,
+and passed on equally to all as much as is the protoplasm or any other
+attribute of the breed. From the body of the bird the critical and
+efficient substance could in all likelihood be isolated by suitable
+means, just as the glycogen of the liver can be. But even when this
+extraction has been accomplished and the reducing body isolated, we
+shall know no more than we did before respecting the mode by which
+the power to produce it was conferred on the fowl, any more than we
+know how the walls of its blood-vessels acquired the power to form a
+fibrin-ferment.
+
+It is when the scope of such considerations as this are fully grasped
+that we realise the fatuousness of the conventional treatment which the
+problem of the causes of variation commonly receives. Environmental
+change, chemical injury, differences in food supply, in temperature,
+in moisture, or the like have been proposed as "causes." Admitting
+as we must do, that changes may be produced--usually inhibitions of
+development--by subjecting living things to changes in these respects,
+how can we suppose it in the smallest degree likely that very precise,
+new, and adaptative powers can be conferred on the germs by such
+treatment? Reports of positive genetic consequences observed comparable
+with those I have mentioned, become from time to time current. We
+should I think regard them with the gravest doubt. Few, so far as I am
+aware, have ever been confirmed, though clear and repeated confirmation
+should be demanded before we suffer ourselves at all to build upon such
+evidence. In a subsequent chapter some of these cases will be considered
+in detail.
+
+In no class of cases would the transmission of an acquired character
+superficially appear so probable as in those where power of resisting
+the attack of a pathogenic organism is acquired in the lifetime of
+the zygote. The possession of such a power is moreover a distinction
+comparable with those which differentiate varieties and species. It
+is due to the development in the blood of specific substances which
+pervade the whole fluid. This development is exactly one of those
+"appropriate responses to stimuli" which naturalists who incline to
+regard adaptation as a direct consequence of an environmental influence
+might most readily invoke as an illustration of their views. And yet all
+evidence is definitely unfavourable to the suggestion of an inheritance
+of the acquired power of resistance. Such change as can be perceived
+in the virulence of the attacks on successive generations may be most
+easily regarded as due to the extermination of the more susceptible
+strains, and perhaps in some measure to variation in the invading
+organisms themselves, an "acquired character" of quite different import.
+
+The specific "anti-body" may have been produced in response to the
+stimulus of disease, but the power to produce it without this special
+stimulus is not included in the germ-cells any more than a pigment.
+All that they bear is the _power to produce_ the anti-bodies when the
+stimulus is applied.
+
+If we could conceive of an organism like one of those to which disease
+may be due becoming actually incorporated with the system of its host,
+so as to form a constituent of its germ-cells and to take part in the
+symmetry of their divisions, we should have something analogous to the
+case of a species which acquires a new factor and emits a dominant
+variety. When we see the phenomenon in this light we realise the
+obscurity of the problem. The appearance of recessive varieties is
+comparatively easy to understand. All that is implied is the omission of
+a constituent. How precisely the omission is effected we cannot suggest,
+but it is not very difficult to suppose that by some mechanical fault
+of cell-division a power may be lost. Such variation by unpacking,
+or analysis of a previously existing complex, though unaccountable,
+is not inconceivable. But whence come the new dominants? Whether we
+imagine that they are created by some rearrangement or other change
+internal to the organism, or whether we try to conceive them as due to
+the assumption of something from without we are confronted by equally
+hopeless difficulty.
+
+The mystery of the origin of a dominant increases when it is realised
+that there is scarcely any recent and authentic account of such an event
+occurring under critical observation, which can be taken as a basis for
+discussion. The literature of horticulture for example abounds in cases
+alleged, but I do not think anyone can produce an illustration quite
+free from doubt. Such evidence is usually open to the suspicion that the
+plant was either introduced by some accident, or that it arose from a
+cross with a pre-existing dominant, or that it owed its origin to the
+meeting of complementary factors. In medical literature almost alone
+however, there are numerous records of the spontaneous origin of various
+abnormal conditions in man which habitually behave as dominants, and of
+the authenticity of some of these there can be no doubt.
+
+When we know that such conditions as hereditary cataract or various
+deformities of the fingers behave as dominants, we recognize that
+those conditions must be due to the addition of some element to the
+constitution of the normal man. In the collections of pedigrees relating
+to such pathological dominants there are usually to be found alleged
+instances of the origin of the condition _de novo_. Not only do these
+records occur with such frequency that they cannot be readily set aside
+as errors, but from general considerations it must be obvious that as
+these malformations are not common to normal humanity they must at
+some moment of time have been introduced. The lay reader may not be
+so much impressed with the difficulty as we are. He is accustomed to
+regard the origin of _any_ new character as equally mysterious, but when
+once dominants are distinguished from recessives the problem wears a
+new aspect. Thus the appearance of high artistic gifts, whether as an
+attribute of a race or as a sporadic event among the children of parents
+destitute of such faculties, is not very surprising, for we feel fairly
+sure that the faculty is a recessive, due to the loss of a controlling
+or inhibiting factor; but the _de novo_ origin of brachydactylous
+fingers in a child of normal parents is of quite a different nature, and
+must indicate the action of some new specific cause.
+
+Whether such evidence is applicable to the general problem of evolution
+may with some plausibility be questioned; but there is an obvious
+significance in the fact that it is among these pathological occurrences
+that we meet with phenomena most nearly resembling the spontaneous
+origin of dominant factors, and I cannot see such pedigrees as these
+without recalling Virchow's aphorism that every variation owes its
+origin to some pathological accident. In the evolution of domestic
+poultry, if _Gallus bankiva_ be indeed the parent form of all our
+breeds, at least some half dozen new factors must have been added during
+the process. In _bankiva_ there is, for example, no factor for rose
+comb, pea comb, barring on the feathers, or for the various dominant
+types of dark plumage. Whence came all these? It is, I think, by no
+means impossible that some other wild species now extinct did take
+part in the constitution of domestic poultry. It seems indeed to me
+improbable that the heavy breeds descend from _bankiva_. Both in regard
+to domestic races of fowls, pigeons, and some other forms, the belief in
+origin within the period of human civilization from one simple primitive
+wild type seems on a balance of probabilities insecurely founded, but
+allowing something for multiplicity of origin we still fall far short
+of the requisite total of factors. Elements exist in our domesticated
+breeds which we may feel with confidence have come in since their
+captivity began. Such elements in fowls are dominant whiteness, extra
+toe, feathered leg, frizzling, etc., so that even hypothetical extension
+of the range of origin is only a slight alleviation of the difficulty.
+
+Somehow or other, therefore, we must recognize that dominant factors
+do arise. Whether they are created by internal change, or whether, as
+seems to me not wholly beyond possibility, they obtain entrance from
+without, there is no evidence to show. If they were proved to enter from
+without, like pathogenic organisms, we should have to account for the
+extraordinary fact that they are distributed with fair constancy to half
+the gametes of the heterozygote.
+
+In proportion as the nature of dominants grows more clear so does it
+become increasingly difficult to make any plausible suggestion as to
+their possible derivation. On the other hand the origin of a recessive
+variety by the loss of a factor is a process so readily imagined that
+our wonder is rather that the phenomenon is not observed far more
+often. Some slip in the accurate working of the mechanical process of
+division, and a factor gets left out, the loss being attested by the
+appearance of a recessive variety in some subsequent generation.
+
+Consistently with this presentation of the facts we find that, as in our
+domesticated animals and plants, a diversity of recessives may appear
+within a moderately short period, and that when variations come they
+often do not come alone. Witness the cultural history of the Sweet Pea,
+_Primula Sinensis_, _Primula obconica_, _Nemesia strumosa_ and many such
+examples in which variation when it did come was abundant. The fact
+cannot be too often emphasized that in the vast proportion of these
+examples of substantive variation under domestication, as well as of
+substantive variation in the natural state, the change has come about by
+omission, not by addition. To take, for example, the case of the Potato,
+in which so many spontaneous bud-variations have been recorded, East
+after a careful study of the evidence has lately declared his belief
+that all are of this nature, and the opinion might be extended to many
+other groups of cases whether of bud or seminal variation. Morgan draws
+the same conclusion in reference to the many varieties he has studied in
+_Drosophila_.
+
+In the Sweet Pea, a form which is beyond suspicion of having been
+crossed with anything else, and has certainly produced all the multitude
+of types which we now possess by variations from one wild species,
+there is only one character of the modern types which could, with any
+plausibility, be referred to a factor not originally forming part
+of the constituents of the wild species. This is the waved edge, so
+characteristic of the "Spencer" varieties; for the cross between a
+smooth-edged and a waved type gives an intermediate not unfrequently.
+Nevertheless there is practically no doubt that this is merely an
+imperfection in the dominance of the smooth edge, and we may feel sure
+that any plant homozygous for smooth edge would show no wave at all.
+Hence it is quite possible that even the appearance of the original
+waved type, Countess Spencer, was due to the loss of one of the factors
+for smooth edge at some time in the history of the Sweet Pea.
+
+In the case of the Chinese Primrose (_Primula Sinensis_) one dominant
+factor has been introduced in modern times, probably within the last six
+years at most. This is the factor which causes suppression of the yellow
+eye, giving rise to the curious type known as "Queen Alexandra." Mr. R.
+P. Gregory's experiments proved that this was a very definite dominant,
+and the element responsible for this development is undoubtedly an
+addition to the original ingredient-properties, with which the species
+was endowed. Unfortunately, as happens in almost every case of the kind,
+the origin of this important novelty appears to be lost. Its behaviour,
+however, when crossed with various other types is that of a simple
+dominant giving an ordinary 3:1 ratio. There is therefore no real doubt
+that it came into existence by the definite addition of a new factor,
+for if it was simply a case of the appearance of a new character made by
+combination of two previously existing complementary factors we should
+expect that when Queen Alexandra was self-fertilised a 9:7 ratio would
+be a fairly common result, which is not in practice found.
+
+In _Oenothera_ Gates[1] has observed the appearance, in a large
+sowing of about 1,000 _Oenothera rubrinervis_, of a single individual
+having considerably more red pigment in the calyx than is usual in
+_rubrinervis_. The whole of the hypanthium in the flowers of this
+plant was red instead of green as in _rubrinervis_, and the whole of
+the sepals were red in the bud-stage, except for small green areas at
+the base. This type behaved as a dominant over _rubrinervis_, but so
+far a pure-breeding individual was not found. Admittedly the variation
+of this plant from the type of _rubrinervis_ can be represented as
+one of degree, though there is a very sensible gap in the series
+between the new form which Gates names "_rubricalyx_" and the reddest
+_rubrinervis_ seen in his cultures. It must certainly be recognised as
+a new dominant. Gates, rightly as I consider, regards the distinction
+between _rubrinervis_ and _rubricalyx_ as a quantitative one, and the
+same remark applies to certain other types differing in the amount
+of anthocyanin which they produce. I do not understand the argument
+which Gates introduces to the effect that the difference between
+such quantitative types cannot be represented in terms of presence
+and absence. We are quite accustomed to the fact that in the rabbit
+self-colour segregates from the Dutch-marked type. These two types
+differ in a manner which we may reasonably regard as quantitative. It
+is no doubt possible that the self-coloured type contains an ingredient
+which enables the colour to spread over the whole body, but it is, I
+think, perhaps more easy to regard the Dutch type as a form from which a
+part of the colour is absent. It may be spoken of in terms I have used,
+as a _subtraction-stage_ in colour. Following a similar method we may
+regard _rubricalyx_ as an addition-stage in colour-variation. The fact
+that crosses between _rubrinervis_, or _rubricalyx_ and _Lamarckiana_
+give a mixture of types in F_{1}, does not I think show, as Gates
+declares, that there is any system here at work to which a factorial or
+Mendelian analysis does not apply; but that question may be more fitly
+discussed in connexion with the other problems raised by the behaviour
+of _Oenothera_ species in their crosses.
+
+I do, however, feel that, interesting as this case must be admitted to
+be, we cannot quite satisfactorily discuss it as an illustration of
+the _de novo_ origin of a dominant factor. The difference between the
+novelty and the type is quantitative, and it is not unreasonable to
+think of such a difference being brought about by some "pathological
+accident" in a cell-division.
+
+Recognition of the distinction between dominant and recessive characters
+has, it must be conceded, created a very serious obstacle in the way
+of any rational and concrete theory of evolution. While variations
+of all kinds could be regarded as manifestations of some mysterious
+instability of organisms this difficulty did not occur to the mind of
+evolutionists. To most of those who have taken part in genetic analysis
+it has become a permanent and continual obsession. With regard to the
+origin of recessive variations, there is, as we have seen, no special
+difficulty. They are negative and are due to absences, but as soon as it
+is understood that dominants are caused by an addition we are completely
+at a loss to account for their origin, for we cannot surmise any source
+from which they may have been derived. Just as when typhoid fever breaks
+out in his district the medical officer of health knows for certain that
+the bacillus of typhoid fever has by some means been brought into that
+district so do we know that when first dominant white fowls arose in the
+evolution of the domestic breeds, by some means the factor for dominant
+whiteness got into a bird, or into at least one of its germ-cells.
+Whence it came we cannot surmise.
+
+Whether we look to the outer world or to some rearrangement within the
+organism itself, the prospect of finding a source of such new elements
+is equally hopeless.
+
+Leaving this fundamental question aside as one which it is as yet quite
+unprofitable to discuss, we are on safe ground in foreseeing that the
+future classification of substantive variations, which genetic research
+must before long make possible, will be based on a reference to the
+modes of action of the several factors. Some will be seen to produce
+their effects by oxidation, some by reduction, some by generating
+substances of various types, sugars, enzymes, activators, and so forth.
+It may thus be anticipated that the relation of varieties to each other
+and to types from which they are derived will be expressible in terms
+of definite synthetical formulae. Clearly it will not for an indefinite
+time be possible to do this in practice for more than a few species
+and for characters especially amenable to experimental tests, but as
+soon as the applicability of such treatment is generally understood the
+influence on systematics must be immediate and profound, for the nature
+of the problem will at length be clear and, though the ideal may be
+unattainable, its significance cannot be gainsaid.
+
+                 *       *       *       *       *
+
+_Note._--With hesitation I allow this chapter to appear in the form in
+which it was printed a year ago, but in passing it for the press after
+that interval I feel it necessary to call attention to a possible line
+of argument not hitherto introduced.
+
+In all our discussions we have felt justified in declaring that the
+dominance of any character indicates that some factor is present which
+is responsible for the production of that character. Where there is no
+definite dominance and the heterozygote is of an intermediate nature
+we should be unable to declare on which side the factor concerned was
+present and from which side it was absent. The degree of dominance
+becomes thus the deciding criterion by which we distinguish the
+existence of factors. But it should be clearly realized that in any
+given case the argument can with perfect logic be inverted. We already
+recognize cases in which by the presence of an inhibiting factor a
+character may be suppressed and purely as a matter of symbolical
+expression we might apply the same conception of inhibition to any
+example of factorial influence whatever. For instance we say that in
+as much as two normal persons do not have brachydactylous children,
+there must be some factor in these abnormal persons which causes the
+modification. Our conclusion is based on the observed fact that the
+modification is a dominant. But it may be that normal persons are
+homozygous in respect of some factor _N_, which prevents the appearance
+of brachydactyly, and that in any one heterozygous, _Nn_, for this
+inhibiting factor, brachydactyly can appear. Similarly the round pea we
+say contains _R_, a factor which confers this property of roundness,
+without which its seeds would be wrinkled. But here we know that the
+wrinkled seed is in reality one having compound starch-grains, and
+that the heterozygote, though outwardly round enough, is intermediate
+in that starch-character. If we chose to say that the compoundness of
+the grains is due to a factor _C_ and that two doses of it are needed
+to make the seed wrinkled, I know no evidence by which such a thesis
+could be actually refuted. That such reasoning is seemingly perverse
+must be conceded; but when we consider the extraordinary difficulties
+which beset any attempt to conceive the mode of origin of a new dominant
+factor, we are bound to remember that there is this other line of
+argument which avoids that difficulty altogether. In the case of the
+"Alexandra"-eye in _Primula_, or the red calyx in Gates's _Oenothera_,
+inverting the reasoning adopted in the text, we may see that only the
+_Primula_ homozygous for the yellow eye can develop it and that two
+doses of the factor for the _rubrinervis_ calyx are required to prevent
+that part of the plant from being red.
+
+We may proceed further and extend this mode of reasoning to all cases
+of genetic variation, and thus conceive of all alike as due to loss of
+factors present in the original complex. Until we can recognize factors
+by means more direct than are provided by a perception of their effects,
+this doubt cannot be positively removed. For all practical purposes of
+symbolic expression we may still continue to use in our analyses the
+modes of representation hitherto adopted, but we must not, merely on
+the ground of its apparent perversity, refuse to admit that the line of
+argument here indicated may some day prove sound.
+
+FOOTNOTES:
+
+[1] Gates, R. R., _Zts. f. Abstammungslehre_, 1911, IV, pp. 341 and 361.
+
+
+
+
+                        CHAPTER V
+
+                    THE MUTATION THEORY
+
+When with the thoughts suggested in the last chapter we contemplate
+the problem of Evolution at large the hope at the present time of
+constructing even a mental picture of that process grows weak almost
+to the point of vanishing. We are left wondering that so lately men
+in general, whether scientific or lay, were so easily satisfied. Our
+satisfaction, as we now see, was chiefly founded on ignorance.
+
+Every specific evolutionary change must represent a definite event in
+the construction of the living complex. That event may be a disturbance
+in the meristic system, showing itself in a change in the frequency of
+the repetitions or in the distribution of differentiation among them, or
+again it may be a chemical change, adding or removing some factor from
+the sum total.
+
+If an attempt be made to apply these conceptions to an actual series
+of allied species the complexity of the problem is such that the mind
+is appalled. Ideas which in the abstract are apprehended and accepted
+with facility fade away before the concrete case. It is easy to imagine
+how Man was evolved from an _Amoeba_, but we cannot form a plausible
+guess as to how _Veronica agrestis_ and _Veronica polita_ were evolved,
+either one from the other, or both from a common form. We have not even
+an inkling of the steps by which a Silver Wyandotte fowl descended from
+_Gallus Bankiva_, and we can scarcely even believe that it did. The
+Wyandotte has its enormous size, its rose comb, its silver lacing, its
+tame spirit, and its high egg production. The tameness and the high egg
+production are probably enough both recessives, and though we cannot
+guess how the corresponding dominant factors have got lost, it is not
+very difficult to imagine that they were lost somehow. But the rose comb
+and the silver colour are _dominants_. The heavy weight also appears in
+the crosses with Leghorns, but we need not at once conclude that it
+depends on a simple dominant factor, because the big size of the crosses
+may be a consequence of the cross and may depend on other elements.
+
+Now no wild fowl known to us has these qualities. May we suppose that
+some extinct wild species had them? If so, may we again make the same
+supposition in all similar cases? To do so is little gain, for we are
+left with the further problem, whence did those lost wild species
+acquire those dominants? Suppositions of this kind help no more than
+did the once famous conjecture as to the origin of living things--that
+perhaps they came to earth on a meteorite. The unpacking of an
+original complex, the loss of various elements, and the recombination
+of pre-existing materials may all be invoked as sources of specific
+diversity. Undoubtedly the range of possibilities thus opened up is
+large. It will even cover an immense number of actual examples which
+in practice pass as illustrations of specific distinction. The Indian
+Rock pigeon which has a blue rump may quite reasonably be regarded as
+a geographically separated recessive form of our own _Columba livia_,
+for as Staples-Browne has shown the white rump of _livia_ is due to
+a dominant factor. The various degrees to which the leaves of Indian
+Cottons are incised have, as Leake says, been freely used as a means
+of classification. The diversities thus caused are very remarkable,
+and when taken together with diversities in habit, whether sympodial
+or monopodial, the various combinations of points of difference
+are sufficiently distinctive to justify any botanist in making a
+considerable number of species by reference to them alone. Nevertheless
+Leake's work goes far to prove that all of these forms represent the
+re-combinations of a very small number of factors. The classical example
+of _Primula Sinensis_ and its multiform races is in fact for a long
+way a true guide as to the actual interrelations of the species which
+systematists have made. That they did make them was due to no mistake
+in judgment or in principle, but simply to the want of that extended
+knowledge of the physiological nature of the specific cases which we now
+know to be a prime necessity.
+
+But will such analysis cover all or even most of the ordinary cases
+of specific diversity between near allies? Postponing the problem of
+the interrelations of the larger divisions as altogether beyond present
+comprehension, can we suppose, that in general, closely allied species
+and varieties represent the various consequences of the presence or
+absence of allelomorphic factors in their several combinations? The
+difficulty in making a positive answer lies in the fact that in most
+of the examples in which it has been possible to institute breeding
+experiments with a view to testing the question, a greater or less
+sterility is encountered. Where, however, no such sterility is met
+with, as for instance in the crosses made by E. Baur among the species
+of _Antirrhinum_ there is every reason to think that the whole mass of
+differences can and will eventually be expressed in terms of ordinary
+Mendelian factors. Baur has for example crossed species so unlike as
+_Antirrhinum majus_ and _molle_, forms differing from each other in
+almost every feature of organisation.[1] The F_{2} generation from this
+cross presents an amazingly motley array of types which might easily if
+met with in nature be described as many distinct species. Yet all are
+fertile and there is not the slightest difficulty in believing that they
+can all be reduced to terms of factorial analysis.
+
+If allowance be made for the complicating effects of sterility, is there
+anything which prevents us from supposing that such good species as
+those of _Veronica_ or of any other genus comprising well-defined forms
+may not be similarly related? I do not know any reason which can be
+pointed to as finally excluding such a possibility. Nevertheless it has
+been urged with some plausibility that good species are distinguished
+by _groups_ of differentiating characters, whereas if they were really
+related as the terms of a Mendelian F_{2} family are, we should expect
+to find not groups of characters in association, but rather series of
+forms corresponding to the presence and absence of the integral factors
+composing the groups of characters. I am not well enough versed in
+systematic work to be able to decide with confidence how much weight
+should be attached to this consideration. Some weight it certainly
+has, but I cannot yet regard it as forming a fatal objection to the
+application of factorial conceptions on the grand scale. It may be
+recalled that we are no longer under any difficulty in supposing that
+differences of all classes may be caused by the presence or absence of
+factors. It seemed at first for example that such characters as those of
+leaf shape might be too subtle and complex to be reducible to a limited
+number of factors. But first the work of Gregory on _Primula Sinensis_
+showed that several very distinct types of leaves were related to each
+other in the simplest way. In that particular example, intermediates are
+so rare as to be negligible, but subsequently Shull dealing with such
+a complicated example as _Capsella_, and Leake in regard to Cottons,
+both forms in which intergrades occur in abundance, have shown that a
+simple factorial scheme is applicable. We need not therefore, to take
+an extreme case, doubt that if it were possible to examine the various
+forms of fruit seen in the Squashes by really comprehensive breeding
+tests, even this excessive polymorphism in respect of structural
+features would be similarly reducible to factorial order.
+
+It must always be remembered also that in a vast number of cases, nearly
+allied forms which are distinct, occupy distinct ground. Moreover, by
+whatever of the many available mechanisms that end be attained, it is
+clear that nature very often does succeed in preventing intercrossing
+between distinct forms so far that the occurrence of that phenomenon is
+a rarity under natural conditions. The facts may, I think, fairly be
+summarized in the statement that species are on the whole distinct and
+not intergrading, and that the distinctions between them are usually
+such as might be caused by the presence, absence, or inter-combination
+of groups of Mendelian factors; but that they are so caused the evidence
+is not yet sufficient to prove in more than a very few instances.
+
+The alternative, be it explicitly stated, is not to return to the view
+formerly so widely held, that the distinctions between species have
+arisen by the accumulation of minute or insensible differences. The
+further we proceed with our analyses the more inadequate and untenable
+does that conception of evolutionary change become. If the differences
+between species have not come about by the addition or loss of factors
+one at a time, then we must suppose that the changes have been effected
+by even larger steps, and variations including groups of characters,
+must be invoked.
+
+That changes of this latter order are really those by which species
+arise, is the view with which de Vries has now made us familiar by his
+writings on the Mutation Theory. In so far as mutations may consist
+in meristic changes of many kinds and in the loss of factors it is
+unnecessary to repeat that we have abundant evidence of their frequent
+occurrence. That they may also more rarely occur by the addition of a
+factor we are, I think, compelled to believe, though as yet the evidence
+is almost entirely circumstantial rather than direct. The evidence for
+the occurrence of those mutations of higher order, by which new species
+characterized by several distinct features are created, is far less
+strong, and after the best study of the records which I have been able
+to make, I find myself unconvinced. The facts alleged appear capable of
+other interpretations.
+
+The most famous and best studied examples are of course the forms
+of _Oenothera_ raised by de Vries from _Oenothera Lamarckiana_ in
+circumstances well known to all readers of genetic literature. Whatever
+be the true significance of these extraordinary "mutations" there can
+be no question about the great interest which attaches to them, and the
+historical importance which they will long preserve. Apart also from
+these considerations it is becoming more and more evident that in their
+peculiarities they provide illustrations of physiological phenomena of
+the highest consequence in the study of genetics at large.
+
+De Vries found, as is well known, that _Oenothera Lamarckiana_ gives off
+plants unlike itself. These mutational forms are of several distinct
+and recognizable types which recur, and several of them breed true from
+their first appearance. The obvious difficulty, which in my judgment
+should make us unwilling at present to accept these occurrences as proof
+of the genesis of new species by mutation, is that we have as yet no
+certainty that the appearance of the new forms is not an effect of the
+recombination of factors, such as is to be seen in so many generations
+of plants derived from a cross involving many genetic elements. The
+first question is what is _Oenothera Lamarckiana_? Is it itself a plant
+of hybrid origin? To this fundamental question no satisfactory answer
+has yet been given. All attempts to find it as a wild plant in America
+have failed. It existed in Europe in the latter half of the eighteenth
+century. Whence it came is still uncertain, but the view that it came
+into existence in Europe and perhaps in Paris, seems on the whole the
+most probable. The question has been debated by Macdougal, Gates, and
+Davis. From historical sources there is little expectation of further
+light. Those who favour the notion of a hybrid origin look on _Oenothera
+biennis_ as one of the putative parents. It has been conjectured that
+a species called _grandiflora_ lately re-discovered on the Alabama
+river was the other parent. Experiments have been instituted by Davis
+to discover whether _Lamarckiana_ can be made artificially by crossing
+these two species. The results so far have shown that while plants
+approximating in various respects to _Lamarckiana_ have thus been
+produced, none agree exactly with that form. Davis, to whom reference
+should be made for a full account of the present state of the enquiry,
+points out that there are many strains of _biennis_ in existence and
+that it is by no means impossible that by using others of these strains
+a still closer approximation can be made. None of Davis's artificial
+productions as yet breed at all true, as _Lamarckiana_ on the whole
+does. In such a case, however, where several characters are involved,
+this is perhaps hardly to be expected.
+
+One feature of the _Oenotheras_ is very curious. Not only _Lamarckiana_,
+but all the allied species so far as I am aware, have a considerable
+proportion of bad and shrivelled pollen grains. This is undoubtedly true
+of species living in the wild state as well as of those in cultivation.
+I have had opportunities of verifying this for myself in the United
+States. No one looking at the pollen of an _Oenothera_ would doubt that
+it was taken from some hybrid plant exhibiting partial sterility. On the
+other hand, it is difficult to suppose that numbers, perhaps all, of
+the "species" of the genus are really hybrids, and many of them breed
+substantially true. I regard this constant presence of bad pollen grains
+as an indication that the genetic physiology of _Oenothera_ is in some
+way abnormal, and as we shall presently see, there are several other
+signs which point in the same direction.
+
+Discussion of the whole series of phenomena is rendered exceedingly
+difficult first, by reason of the actual nature of the material. The
+characteristics of many of the types which de Vries has named are
+evasive. A few of these types, for instance, _gigas_, _nanella_,
+_albida_, _brevistylis_, and perhaps a few more are evidently clear
+enough, but we have as yet no figures and descriptions precise enough
+to enable a reader to appreciate exactly the peculiarities of the vast
+number of forms which have now to be considered in any attempt to gain a
+comprehensive view of the whole mass of facts. It is also not in dispute
+that the forms are susceptible of great variations due simply to soil
+and cultural influences.
+
+The fact that no Mendelian analysis has yet been found applicable to
+this group of _Oenotheras_ as a whole is perhaps largely due to the fact
+that until recently such analysis has not been seriously attempted.
+Following the system which he had adopted before the rediscovery of
+Mendelism, or at all events, before the development of that method of
+analysis, de Vries has freely applied _names_ to special combinations of
+characters and has scarcely ever instituted a factorial analysis. Before
+we can get much further this must be attempted. It may fail, but we must
+know exactly where and how this failure comes about. There are several
+indications that such a recognition of factorial characters, could be
+carried some way. For example, the height, the size of the flowers, the
+crinkling of the leaves, the brittleness of the stems, perhaps even the
+red stripes on stems and fruits, and many more, are all characters which
+may or may not depend on distinct factors, but if such characters are
+really transmitted in unresolved groups, the limitations of those groups
+should be carefully determined. The free use of names for the several
+forms, rather than for the characters, has greatly contributed to deepen
+the obscurity which veils the whole subject.
+
+I do not mean to suggest that these _Oenotheras_ follow a simple
+Mendelian system. All that we know of them goes to show that there
+are curious complications involved. One of these, probably the most
+important of all, has lately been recognized by de Vries himself,
+namely, that in certain types the characters borne by the female and
+the male germ-cells of the same plant are demonstrably different. There
+can be little doubt that further research will reveal cognate phenomena
+in many unsuspected places. The first example in which such a state of
+things was proved to exist is that of the Stocks investigated by Miss
+Saunders.[2] By a long course of analysis she succeeded in establishing
+in 1908 the fact that if a plant of _Matthiola_ is of that eversporting
+kind which gives a large proportion of double-flowered plants among
+its offspring (produced by self-fertilisation), then the egg-cells of
+such a plant are mixed in type, but the pollen of the same plant is
+homogeneous. Some of the egg-cells have in them the two factors for
+singleness, but some of them are short of one or both of these factors.
+The pollen-grains, however, are all recessives, containing neither of
+these factors. The egg-cells, in other words, are mixed, "singles"
+and "doubles," while the pollen-grains are all "doubles." The same is
+true of the factor differentiating "white," or colourless plastids
+from cream-coloured plastids in _Matthiola_, the egg-cells being mixed
+"whites" and "creams," while the pollen-grains are all "creams," viz:
+recessives. Later in the same year (1908) de Vries[3] announced a
+remarkable case which will be discussed in detail subsequently. It
+relates to certain _Oenotheras_ heterozygous for dwarfness, in which (p.
+113) the ovules were mixed, tails and dwarfs, while the pollen is all
+dwarf.
+
+Again in _Petunia_ Miss Saunders's[4] work has shown that a somewhat
+similar state of things exists, but with this remarkable difference,
+that though the egg-cells are mixed, singles and doubles, the
+pollen-grains are all _singles_, viz: dominants. All the _Petunias_ yet
+examined have been in this condition, including some which in botanic
+gardens pass for original species. Whether actual wild plants from
+their native habitats are in the same state, is not yet known, but it
+is by no means improbable. The case may be compared with that of the
+moth _Abraxas grossulariata_ studied by Doncaster and Raynor, in which
+the females are all heterozygous, or we may almost say "hybrids" of
+_grossulariata_ and the variety _lacticolor_. Similarly we may say that
+at least garden Petunias are heterozygous in respect of singleness.
+The proof of this is of course that when fertilised with the pollen of
+doubles they throw a mixture of doubles and singles. The statements
+which de Vries has published regarding the behaviour of several of the
+_Oenotheras_ go far to show that they must have a somewhat similar
+organisation. On the present evidence it is still quite impossible to
+construct a coherent scheme which will represent all the phenomena in
+their interrelations, and among the facts are several which, as will
+appear, seem mutually incompatible. The first indication that the
+_Oenotheras_ may have either mixed ovules or mixed pollen appears in the
+fact that _Lamarckiana_ and several of its "mutants" used as males, with
+several other forms as females, give a mixed offspring. For example, de
+Vries (1907) found that
+
+          _biennis_ [F] x _Lamarckiana_ [M]
+          _biennis cruciata_ [F] x _Lamarckiana_ [M]
+          _muricata_ [F] x _Lamarckiana_ [M]
+          _biennis_ [F] x _rubrinervis_ [M]
+          _biennis cruciata_ [F] x _rubrinervis_ [M]
+
+all give a mixture of two distinct types which he names _laeta_ and
+_velutina_, consisting of about equal numbers of each. On account of the
+fact that the two forms are produced in association de Vries has called
+these forms "twin hybrids," a designation which is not fortunate, seeing
+that it is impossible to imagine that any kind of twinning is concerned
+in their production. The distinction between these two seems to be
+considerable, _laeta_ having leaves broader, bright green in colour, and
+flat, with pollen scanty, while _velutina_ has leaves narrower, grayish
+green, more hairy, and furrow-shaped, with pollen abundant.
+
+We next meet the remarkable fact that these two forms, _laeta_
+and _velutina_ breed true to their respective types, and do not
+reproduce the parent-types among their offspring resulting from
+self-fertilisation. This statement must be qualified in two respects.
+When _muricata_ [M] is fertilised by _brevistylis_ the forms _laeta_
+and _velutina_ are produced, but each of them subsequently throws the
+short-styled form as a recessive (de Vries, 1907, p. 406). It may be
+remembered that de Vries's previous publications had already shown that
+the short style of _brevistylis_, one of the _Lamarckiana_ "mutants,"
+behaves as a recessive habitually (_Mutationstheorie_, II, p. 178, etc.).
+
+Also when _nanella_, the dwarf "mutant" of _Lamarckiana_ is used as
+male on _muricata_ as female, _laeta_ and _velutina_ are produced, but
+one only of these, namely, _velutina_, subsequently throws dwarfs on
+self-fertilisation. The dwarfs thus thrown are said to form about 50 per
+cent. of the families in which they occur (de Vries, 1908, p. 668). The
+fact that the two forms, _laeta_ and _velutina_, are produced by many
+matings in which _Lamarckiana_ and its mutant _rubrinervis_ are used as
+males is confirmed abundantly by Honing, who has carried out extensive
+researches on the subject. After carefully reading his paper, I have
+failed to understand the main purport of the argument respecting the
+"double nature" of _Lamarckiana_ which he founds on these results, but
+I gather that in some way _laeta_ is shown to partake especially of the
+nature of _Lamarckiana_, while _velutina_ is a form of _rubrinervis_.
+The paper contains many records which will be of value in subsequent
+analysis of these forms.
+
+Before considering the possible meaning of these facts we must have
+in our minds the next and most novel of the recent extensions of
+knowledge as to the genetic properties of the _Oenotheras_. In the
+previous statement we have been concerned with the results of using
+either _Lamarckiana_ itself or one of its "mutants" _rubrinervis_,
+_brevistylis_, or _nanella_ as male, on one of the species _biennis_
+or _muricata_. The new experiments relate to crosses between the two
+species _biennis_ and _muricata_ themselves.
+
+De Vries found:
+
+1. That the reciprocal hybrids from these two species differed,
+_biennis_ x _muricata_ producing one type of F_{1} and _muricata_ x
+_biennis_ producing another. Each F_{1} resembled the father more than
+the mother.
+
+2. That each of the hybrids so produced breeds true on
+self-fertilisation.
+
+3. That if we speak of the hybrid from _biennis_ x _muricata_ as _BM_
+and of the reciprocal as _MB_, then
+
+          _BM_ x _MB_
+
+gives exclusively offspring of _biennis_ type but that
+
+          _MB_ x _BM_
+
+gives exclusively offspring of _muricata_ type. Evidently, apart
+from all controversy as to the significance of the "mutants" of
+_Lamarckiana_, we have here a series of observations of the first
+importance.
+
+The fact that reciprocal crossings give constantly distinct results
+must be taken to indicate that the male and female sides of one, if not
+of both, of the parents are different in respect of characters which
+they bear. This is de Vries's view, and he concludes rightly, I think,
+that the evidence from all the experiments shows that both _biennis_
+and _muricata_ are in this condition, having one set of characters
+represented in their pollen-grains and another in their ovules. The
+plants breed true, but their somatic structures are compounded of the
+two sets of elements which pass into them from their maternal and
+paternal sides respectively. This possibility that species may exist of
+which the males really belong to one form and the females to another, is
+one which it was evident from the first announcement of the discovery of
+Mendelian segregation might be found realised in nature.[5]
+
+_Oe. biennis_ and _muricata_ were crossed reciprocally with each other
+and with a number of other species, and the behaviour of each, when
+used as mother, was consistently different from its behaviour when
+used as father. De Vries is evidently justified by the results of
+this series of experiments in stating that the "Bild," as he terms
+it, or composition of the male and female sides of these two species,
+_biennis_ and _muricata_, are distinct. On the evidence before us it
+is not, however, possible to form a perfectly clear idea of each, and
+until details are published, a reader without personal knowledge of the
+material cannot do more than follow the general course of the argument.
+For fuller comprehension a proper analysis of the characters with a
+clear statement of how they are distributed among the several types and
+crosses is absolutely necessary. According to de Vries the female of
+_biennis_ possesses a group of characters which he defines as "_conica_"
+in allusion to the shape of the flower-buds. Besides the conical buds,
+this group of features includes imperfect development of wood, rendering
+the plant very liable to attacks of _Botrytis_, and comparatively narrow
+leaves.
+
+The female of _muricata_ carries a group of features which he
+calls "_frigida_," and, though this is not quite explicitly stated
+in a definition of that type, it is to be inferred[6] that its
+characteristics are regarded as greater height, strong development of
+wood with comparative resistance to _Botrytis_, and broad leaves.
+
+The characters borne by the male parts of the two species are in general
+those by which they are outwardly distinguished. For example, the
+leaves of _Oe. biennis_ are comparatively broad and are bright green,
+while those of _muricata_ are much narrower and of a glaucous green,
+and I understand that de Vries regards these properties as contributed
+by the male side in each case and to be carried by the male cells of
+each species. The suggestion as regards _biennis_ and _muricata_ comes
+near the conception often expressed by naturalists in former times (_e.
+g._, Linnaeus) and not rarely entertained by breeders at the present
+day, that the internal structure is contributed by the mother and the
+external by the father.
+
+On the other hand, the offspring of each species when used as mother is
+regarded as possessing in the main the features of the maternal "Bild,"
+but the matter is naturally complicated by the introduction of features
+from the father's side, and it is here especially that the account
+provided is at present unsatisfactory and inconclusive. There seems,
+however, to be no serious doubt that _biennis_ and _muricata_ each in
+their outward appearance exhibit on the whole the features which their
+pollens respectively carry, and that the features borne by their ovules
+are in many respects distinct.
+
+The _types_ are thus "hybrids" which breed true. The results of
+intercrossing them each way are again "hybrids" which breed true. It
+will be remembered that on former occasions de Vries has formulated a
+general rule that _species_-hybrids breed true, but that the cross-breds
+raised by interbreeding _varieties_ do not. One of these very cases
+was quoted[7] as an illustration of this principle, viz: _muricata_ x
+_biennis_. The grounds for this general statement have always appeared
+to me insufficient, and with the further knowledge which the new
+evidence provides we are encouraged to hope that when a proper factorial
+analysis of the types is instituted we shall find that the phenomenon
+of a constant hybrid will be readily brought into line with the systems
+of descent already worked out for such cases as that of the Stocks, and
+others already mentioned.
+
+In further discussion of these facts de Vries makes a suggestion which
+seems to me improbable. Since the egg-cells of _muricata_, for instance,
+bear a certain group of features which are missing on the male side,
+and conversely the pollen bears features absent from the female side,
+he is inclined to regard the _bad pollen grains_ as the bearers of the
+missing elements of the male side and to infer that there must similarly
+be defective ovules representing the missing elements of the female
+side. No consideration is adduced in support of this view beyond the
+simple fact that the characters borne by male and female are dissimilar,
+whereas it would be more in accord with preconception if the same sets
+of combinations were represented in each--as in a normal Mendelian case.
+There is as yet no instance in which the absence of any particular
+class of gametes has been shown with any plausibility to be due to
+defective viability, though there are, of course, cases in which certain
+classes of zygotes do not survive owing to defective constitution (_e.
+g._, the albinos of _Antirrhinum_ studied by Baur, and the homozygous
+yellow mice). I am rather inclined to suppose that in these examples of
+hybrids breeding true we shall find a state of things comparable with
+that to which we formerly applied the terms "coupling" and "repulsion."
+In these cases certain of the possible combinations of factors occur
+in the gametic series with special frequency, being in excess, while
+the gametes representing other combinations are comparatively few.
+In a recent paper on these cases Professor Punnett and I have shown
+that these curious results vary according to the manner in which the
+factors are grouped in the parents. If _A_ and _B_ are two factors which
+exhibit these phenomena we find that the gametic series of the double
+heterozygote differs according as the combination is made by crossing
+_AB x ab_, or by crossing _AB x aB_. In a normal Mendelian case the
+F_{1} form, _AaBb_, produces gametes _AB_, _Ab_, _aB_, _ab_, in equal
+numbers; but in these peculiar cases those gametes which contain
+
+                          Gametic series      Number of   Number of
+                      ---------------------    gametes     zygotes
+                       AB    Ab    aB    ab   in series    formed
+
+  Partial repulsion {   1  (n-1) (n-1)    1      2n         4n^{2}
+     from zygote    {   1    31    31     1      64         4096
+       of form      {   1    15    15     1      32         1024
+        AbxaB       {   1     7     7     1      16          256
+                    {   1     3     3     1       8           64
+                        1     1     1     1       4           16
+  Partial coupling  {   3     1     1     3       8           64
+    from zygote     {   7     1     1     7      16          256
+      of form       {  15     1     1    15      32         1024
+       ABxab        {  31     1     1    31      64         4096
+                    {  63     1     1    63     128        16384
+                    { (n-1)   1     1   (n-1)    2n         4n^{2}
+
+
+                            Nature of zygotic series
+                      ---------------------------------------
+                             AB        Ab     aB        ab
+  Partial repulsion {     2n^{2}+1  n^{2}-1  n^{2}-1     1
+     from zygote    {       2049      1023    1023       1
+       of form      {        513       255     255       1
+        AbxaB       {        129        63      63       1
+                    {         33        15      15       1
+                               9         3       3       1
+                    {         41         7       7       9
+  Partial coupling  {        177        15      15      49
+     from zygote    {        737        31      31     225
+       of form      {       3009        63      63     961
+        ABxab       {      12161       127     127    3969
+                    { 3n^{2}-(2n - 1) 2n-1  2n-1  n^{2}-(2n-1)
+
+
+the _parental combinations_ are in excess. This excess almost certainly
+follows the system indicated by the accompanying table. In the general
+expressions _n_ is half the number of gametes required to express the
+whole system. Now if we imagine that sex-factors are involved with the
+others concerned in such a relationship as this we have a system of
+distribution approximating to that found in _biennis_ and _muricata_.
+The difference in reciprocals is represented in a not improbable way.
+It cannot yet be said that the rarer terms in the series are formed at
+all, and perhaps they are not. As we pointed out in our discussion of
+these phenomena, the peculiar distribution of factors in these cases
+must be taken to mean that the planes of division at some critical stage
+in the segregation are determined with reference to the parental groups
+of factors, or in other words, that the whole system has a polarity,
+and that the distribution of factors with reference to this polarity
+differs according to the grouping of factors in the gametes which united
+in fertilization to produce the plant. Subsequent proliferation of
+cells representing certain combinations would then lead to excess of
+the gametes bearing them. It is on similar lines that I anticipate we
+shall hereafter find the interpretation of the curious facts discovered
+by de Vries, though it is evident that a long course of experiment and
+analysis must be carried through before any certainty is reached. The
+work must be begun by a careful study of the descent of some single
+factor, for example, that causing the broader leaf of _biennis_, and we
+may hope that the study of _Oenothera_ by proper analytical methods will
+no longer be deferred.
+
+We have now to return to the relations of _laeta_ and _velutina_.
+These two forms, it will be remembered are frequently produced when
+_Lamarckiana_ or one of its derivatives is used as male, and the most
+unexpected feature in their behaviour is that _both breed true as
+regards their essential characteristics, on self-fertilisation_. If one
+only bred true the case might, in view of the approximate numerical
+equality of the two types, be difficult to interpret on ordinary lines,
+but as both breed true it must be clear that some quite special system
+of segregation is at work. What this may be cannot be detected on the
+evidence, but with the results from the _biennis-muricata_ experiments
+before us, it is natural to suspect that we may here again have to
+recognise a process of allocation of different factors to the male and
+female sides in _laeta_ and _velutina_. That some such system is in
+operation becomes the more probable from the new fact which de Vries
+states in describing the group of characters which he calls _conica_,
+namely that this type is the same as that of _velutina_.
+
+There are many collateral observations recorded both by de Vries and
+others which have a bearing on the problems, but they do not yet fall
+into a coherent scheme. For example, we cannot yet represent the
+formation of _laeta_ and _velutina_ from the various species fertilised
+by _Lamarckiana_ [M]. That this is not due to any special property
+associated with the pollen of _Lamarckiana_ is shown by the fact that
+a species called _Hookeri_ gives _laeta_ and _velutina_ in both its
+reciprocal crosses with _Lamarckiana_ (de Vries, 1909, p. 3), and also
+by the similar fact that _Lamarckiana_ [F] fertilised by the pollen of
+a peculiar race of _biennis_ named _biennis Chicago_ throws the same
+types. Before these very complicated phenomena can be usefully discussed
+particulars must be provided as to the individuality of the various
+plants used. This criticism applies to much of the work which de Vries
+has lately published, for, as we now know familiarly, plants to which
+the same name applies can be quite different in genetic composition.
+
+Attention should also be called to one curiously paradoxical series of
+results. When the dwarf "mutant" of _Lamarckiana_ which de Vries names
+"_nanella_" is used as father on _muricata_, F_{1} consists of _laeta_
+and _velutina_ in approximately equal numbers. Both forms breed true to
+their special characteristics, but _velutina_ throws dwarfs of its own
+type, while _laeta_ does not throw dwarfs. Subsequent investigation of
+the properties of these types has led to some remarkable conclusions,
+and it was in a study of these plants that de Vries first came upon the
+phenomena of dissimilarity between the factors borne by the male and
+female cells of the same plant, a condition which had been recently
+detected in the Stocks as a result of Miss Saunders's investigations.
+The details are very remarkable. We have first the fact that _muricata_
+[F] x dwarf _nanella_ [M] gives about 50 per cent. _laeta_ and about 50
+per cent. of _velutina_.
+
+As regards _Velutina_ it was shown that:
+
+                                                 Talls,     Dwarfs,
+                                                per cent.  per cent.
+
+  1. Velutina selfed gave                          38         62
+
+    {Velutina [F] x dwarf nanella [M] gave         39         61
+  2.{    do.      x do.               gave         49         51
+    {    do.      x dwarf [M] derived from
+                                 velutina gave     43         57
+  3. Dwarfs x velutina [M] gave                    --     all dwarfs
+
+The three experiments taken together prove, as de Vries says, that the
+ovules of _velutina_ are mixed, talls and dwarfs, and that the pollen is
+all dwarf. The condition is almost the same as that of the Stocks. It
+may be noted also that in the Stocks the egg-cells of the "double" type
+are in excess, being approximately 9 to 7 of the "single" type, but de
+Vries regards the two types in _velutina_ as probably equal in number.
+The figures (169:231) rather suggest some excess of the recessives,
+perhaps 9:7, and the point would be worth a further investigation.
+
+As regards _laeta_, by self-fertilisation _no dwarfs were produced_, but
+in all other respects it behaved almost exactly like _velutina_. The
+ovules are evidently mixed talls and dwarfs, and whether fertilised by
+dwarfs or by the pollen of _velutina_, which is already proved to be all
+dwarf, the result was a steady 50 per cent. of talls and 50 per cent. of
+dwarfs. The pollen of _laeta_ used on dwarfs gives nothing but dwarfs,
+and in three series of such experiments 226 dwarfs were produced.
+
+We are thus faced with this difficulty. Since the egg-cells of _laeta_
+are evidently mixed, talls and dwarfs, and the pollen used on dwarfs
+gives all dwarfs, why does not self-fertilisation give a mixed result,
+talls and dwarfs, instead of _all talls_? De Vries regards the result
+of self-fertilisation as showing the real nature of the pollen, and
+declares it to be all talls, while he represents the behaviour of
+the same pollen used on dwarfs by stating that in these combinations
+the dwarf character dominates. This does not seem to me a natural
+interpretation. I should regard the pollen of _laeta_ as identical
+with that of _velutina_, namely dwarf, and I suspect the difficulty
+is really created by the behaviour of _laeta_ on self-fertilisation.
+Until a proper analysis is made in which the identity of the different
+individuals used is recorded, no further discussion is possible.[8]
+
+Other results of a complicated kind involving production of _laeta_ and
+_velutina_ together with a third form have been published by de Vries in
+his paper on "Triple Hybrids." To these also the same criticism applies.
+Some of the observations seem capable of simple factorial representation
+and others are conflicting.
+
+Taking the work on _Oenothera_ as a whole we see in it continually
+glimpses of order which further on are still blocked by difficulties
+and apparent inconsistencies. Through such a stage all the successful
+researches in complicated factorial analysis have passed and I see
+no reason for supposing that with the application of more stringent
+methods this more difficult set of problems will be found incapable
+of similar solutions. To return to the original question whether in
+_Oenothera_ we can claim to see a special contemporaneous output of new
+species in actual process of creation, it will be obvious that while
+the interrelation of the several types is still so little understood,
+such a claim has no adequate support. It is true that many of the
+"mutants" of _Lamarckiana_ can well pass for species, but this is
+equally true of many new combinations of pre-existing factors as we
+have seen in _Primula Sinensis_ and other cases. Still less can it be
+admitted that these facts of uncertain import supply a justification for
+the conception which has played a prominent part in the scheme of the
+_Mutationstheorie_, namely that there are special periods of Mutation,
+when the parent-species has peculiar genetic properties. To conclude:
+The impression which the evidence leaves most definitely on the mind is
+that further discussion of the bearing which the _Oenotheras_ may have
+on the problem of evolution should be postponed until we have before
+us the results of a searching analysis applied to a limited part of
+the field. In such an analysis it is to be especially remembered that
+we have now a new clue in the well-ascertained fact that the genetic
+composition of the male and female germ-cells of the same individual
+may be quite different. When with this possibility in view the behaviour
+of the types is re-examined I anticipate that many of the difficulties
+will be removed.
+
+Outside the evidence from _Oenothera_, which, as we have seen, is
+still ambiguous, I know no considerable body of facts favourable to
+that special view of Mutation which de Vries has promulgated. Of
+variation, or if we will, Mutation, in respect of some one character,
+or resulting from recombination, there is proof in abundance; but of
+that simultaneous variation in several independent respects to which de
+Vries especially attributes the origin of new specific types I know only
+casual records which have yet to undergo the process of criticism.
+
+                *       *       *       *       *
+
+Besides de Vries's "_Mutationstheorie_" and "Species and Varieties"
+the chief publications relating to the subject of the behaviour of
+_Oenothera_ are the following: (Many other papers relating especially to
+the cytology of the forms have appeared.)
+
+   Davis, B. M. Genetical Studies on _Oenothera_,
+     I. _Amer. Nat._, XLIV, 1910, p. 108. Genetical Studies on
+     _Oenothera_, II. _Ibid._, XLV, 1911, p. 193.
+
+   Gates, R. R. An Analytical Key to some of the Segregates of
+     Oenothera. _Twentieth Annual Report of the Missouri Botanical
+     Garden_, 1909.
+
+   Studies on the Variability and Heritability of Pigmentation in
+     _Oenothera_. _Ztsch. f. Abstammungslehre_, 1911, IV, p. 337.
+
+   Honing, J. A. Die Doppelnatur der _Oenothera Lamarckiana_.
+     _Ztsch. f. Abstammungslehre_, 1911, IV, p. 227.
+
+   Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K.
+     Small). Mutants and Hybrids of the _Oenotheras_. _Carnegie
+     Institution's Publication_, No. 24, 1905.
+
+   Macdougal, D. T., Vail, A. M., Shull, J. H. Mutations,
+     Variations and Relationships of the _Oenotheras_. _Carnegie
+     Institution's Publication_, No. 81, 1907.
+
+   de Vries, H. On Atavistic Variation in _Oenothera cruciata_.
+     _Bull. Torrey Club_, 1903, Vol. 30, p. 75.
+
+   On Twin Hybrids, _Bot. Gaz._, Vol. 44, 1907, p. 401.
+
+   Ueber die Zwillingsbastarde von _Oenothera nanella_. _Ber.
+     Deut. Bot. Ges._, 1908, XXVI, _a_, p. 667.
+
+   Bastarde von _Oenothera gigas_. _Ibid._, p. 754.
+
+   On Triple Hybrids. _Bot. Gaz._, 1909, Vol. 47, p. 1.
+
+   Ueb. doppeltreziproke Bastarde von _Oenothera biennis_ L. und
+     _Oenothera muricata_ L. _Biol. Cbltt._, 1911, XXXI, p. 97.
+
+   Zeijlstra, H. H. _Oenothera nanella_ de Vries, eine krankhafte
+     Pflanzenart. _Biol. Cbltt._, 1911, XXXI, p. 129.
+
+
+                              NOTE.
+
+Since this chapter was written two contributions of special importance
+have been made to the study of the _Oenothera_ problems. The first is
+that of Heribert-Nilsson.[9] The author begins by giving a critical
+account of the evidence for de Vries's interpretation of the nature of
+the mutants. In general this criticism pursues lines similar to those
+sketched in the foregoing chapter, concluding, as I have done, that the
+chief reason why factorial analysis has been declared to be inapplicable
+to the _Oenothera_ mutants is because no one has hitherto set about this
+analysis in the right way. He has also himself made a valuable beginning
+of such an analysis and gives good evidential reasons for the belief
+that at least the red veining depends on a definite factor which also
+influences the size of certain parts of the plant. He argues further
+that many of the distinctions between the mutants are quantitative
+in nature. With great plausibility he suggests that the system of
+cumulative factors which Nilsson-Ehle discovered in the case of wheat
+(subsequently traced by East in regard to maize) may be operating also
+in these _Oenotheras_. According to this system several factors having
+similar powers may coexist in the same individual, and together produce
+a cumulative effect. Scope would thus be given for the production of
+the curious and seemingly irregular numbers so often recorded in the
+"mutating" families.
+
+Another remarkable observation relating to the crosses of _muricata_
+and _biennis_ has been published by Goldschmidt.[10] He finds that in
+the formation of this cross the female pronucleus takes no part in the
+development of the zygotic cell, but that when the male pronucleus
+enters, the female pronucleus is pushed aside and degenerates. As de
+Vries observed, the reciprocal hybrids are in each case very like the
+father ("_stark patroklin_"), a consequence which finds a natural
+explanation in the phenomenon witnessed by Goldschmidt. The results
+of the subsequent matings can also be readily interpreted on the same
+lines. Indications of maternal characters are nevertheless mentioned
+by de Vries, and if Goldschmidt's account of the cytology is confirmed,
+these must presumably be referred to the influence of the maternal
+cytoplasm. Clearly this new work opens up lines of exceptional interest.
+The interpretation I have offered above must probably be reconsidered.
+The distinction between the male and female cells of the types may no
+doubt be ultimately factorial, but it is difficult to regard such a
+distinction as created by a differential distribution of the ordinary
+factors.
+
+FOOTNOTES:
+
+[1] See Lotsy and Baur, Rep. Genetics Conf., Paris, 1911, pp. 416-426.
+Compare Lecoq on _Mirabilis jalapa_ x _longiflora_, Fecondation des
+Vegetaux, 1862, p. 311.
+
+[2] _Rep. Evol. Ctee. R. S._, IV, 1908, p. 38.
+
+[3] _Ber. Deut. Bot. Ges._, 1908, XXVI, _a_, p. 672.
+
+[4] _Jour. Genetics_, 1, 1910, p. 57.
+
+[5] In Rep. 1 to Evol. Committee, 1902, p. 132, attention was called to
+this possibility, though of course at that date it was in sexual animals
+alone that it was supposed to exist. It had not occurred to me that even
+a hermaphrodite plant might be in this condition.
+
+[6] From the description of the offspring of _muricata_ used as mother.
+
+[7] de Vries, _Species and Varieties_, 1905, p. 259.
+
+[8] Zeijlstra in a recent paper announces that many _nanella_ plants
+are the subject of a bacterial disease to which he attributes their
+dwarfness. I gather that this does not apply to all _nanella_ plants
+and that some are dwarfs apart from disease. The matter may no doubt be
+further complicated from this cause.
+
+[9] _Zts. f. Abstamm._, 1912, VIII.
+
+[10] _Arch. f. Zellforschung_, 1912, IX, p. 331.
+
+
+
+
+                       CHAPTER VI
+
+                  VARIATION AND LOCALITY
+
+In all discussions of the modes of Evolution the phenomena of
+Geographical Distribution have been admitted to be of paramount
+importance. First came the broad question, were the facts of
+distribution consistent with the Doctrine of Descent? I suppose all
+naturalists are now agreed that they are thus consistent, and that
+though some very curious and as yet inexplicable cases remain to be
+accounted for, the distribution of animal and plant life on the face
+of the earth is much what we might expect as a result of a process of
+descent with modification. Passing from this general admission to the
+more particular question whether the facts of distribution favour one
+special conception of the mode of progress of evolution rather than
+another, no agreement has yet been reached. One outstanding feature
+is hardly in dispute, namely that prolonged isolation is generally
+followed by greater or less change in the population isolated. Groups
+of individuals which from various causes are debarred from free
+intermixture with other groups almost always exhibit peculiarities,
+but on the other hand, cosmopolitan types which range over wide areas
+are on the whole uniform, or nearly so throughout their distribution.
+Examples of these two categories will be familiar to all naturalists.
+The barriers to intercourse may be seas, deserts, prairies,
+mountain-chains, or circumstances of a much less obvious character
+which isolate quite as effectually. The local unit is not necessarily
+an island, a district, or an area of special geological formation,
+but may, as every collector knows, be a valley, a pond, a creek, a
+"bank" in the sea, a clump of trees, a group of rocks in a bay, or a
+particular patch of ground on a mountain side. All the great groups
+provide examples of such specially isolated forms. The botanist knows
+them well; the conchologist, the entomologist, the ornithologist and the
+student of marine life are all equally aware that special varieties or
+special species come from special places and from nowhere else. In one
+remarkable case the season of appearance plainly acts as the isolating
+barrier. _Tephrosia bistortata_ is a small Geometrid moth which has two
+broods, appearing in _March_ and _July_ respectively. It is closely
+allied to _T. crepuscularia_ which emerges in _May_ and _June_. From the
+fact that occasional specimens cannot be quite certainly referred to
+one or other of the two, many have held that the two are one species.
+Nevertheless, in general they present distinctions which are plain
+enough. Some localities have one form only, but in several woods they
+co-exist. Experiment has shown that the two can be crossed, and that the
+cross-breds can breed _inter se_ and with at least one of the parent
+stocks.[1] Some diminution in fertility was observed, but perhaps not
+more than is commonly encountered when wild forms are bred in captivity.
+In such a case it can scarcely be doubted that the distinctness of
+the two forms in the places where they co-exist is maintained by the
+seasonal isolation.
+
+Just as the consequences of isolation are to be seen in the most
+different forms of life so may they also affect the most diverse
+features of organisation, such as size, colour, sculpture, shape, or
+number of parts. In the Sloth (_Choloepus_) the geographical races
+differ in the number of cervical vertebrae--or in other words, in the
+distribution of vertebral differentiation. The geographical races of
+_Cistudo_ differ in the number of claws and phalanges.[2]
+
+In Shetland, the males of _Hepialus humuli_ (the Ghost Moth) are
+not sharply differentiated in colour from the females, as they are
+elsewhere, but in varying degrees resemble them.[3] No such males are
+found in other localities, and even in the other Scottish islands they
+are normal. In the island of Waigiu the converse phenomenon has been
+observed in _Phalanger maculatus_. Generally the male is spotted with
+white, and the female is without spots, but in Waigiu the females are
+spotted like the males.[4]
+
+The following striking illustration was pointed out to me by Dr. W. D.
+Miller. _Euphonia elegantissima_ as it occurs in Mexico and Central
+America has the two sexes very distinct from each other. The male has
+the lower parts orange and the upper parts a dark indigo blue, with a
+bright turquoise-blue head and neck. The female, except for the head,
+is of a bright olive green. A form in which the sexes are similarly
+differentiated exists in Porto Rico and is known as _E. Sclateri_. But
+in many of the other West Indian islands the representative "species"
+(_E. flavifrons_) has the two sexes closely resembling the _female_
+of _E. elegantissima_. This form is found in Antigua, Barbados, St.
+Vincent, and Guadeloupe, from which localities the British Museum has
+specimens. All three so-called species are very much alike otherwise.
+
+In the genus _Pyrrhulagra_ (_Loxigilla_) to which Mr. Outram Bangs
+called my attention, several distinct and alternative possibilities
+occur. The genus has many local species occurring on the various West
+Indian islands. These species are characterized by differences in size,
+colour, and the shape of the bill. The colours have a narrow range,
+being black or greyish, with or without chestnut marks about the head
+and throat. In most of the islands the males are in general colour a
+full black, and the females are distinctly grey. They are thus found
+in San Domingo, Jamaica, Bahama, and most of the Lesser Antilles. In
+Porto Rico we meet the peculiarity that the hens are almost as black
+as the males (Ridgway describes the black of the hens as slightly less
+intense). This form is called _portoricensis_. A larger type, known
+as _grandis_, similarly coloured, inhabits St. Kitt's. Then, on the
+contrary, in Barbados, _both sexes_ are a dull blackish grey, like the
+hens of the Lesser Antilles in general.
+
+The local species of _Agelaius_ show similarly capricious distinctions.
+_A. phoeniceus_ is a widely spread species, found over a great part of
+North America. The male is black with red-orange bars on the wings, but
+the female is somewhat thrush-like in colour. In the island of Porto
+Rico there is a form called _xanthomus_, in which _both sexes_ are like
+the males of the mainland. A similar species called _humeralis_, also
+with both sexes male-like, lives in Cuba. The island of Cuba, curiously
+enough, has also a distinct species named _assimilis_, in which the
+female is a dull black all over, though the male is like the mainland
+type.
+
+So also may local races differ in respect of variability. _Argynnis
+paphia_, the Silver Washed Fritillary, through a great part of its
+distribution has only one female form. In the English New Forest a
+second female form, _valesina_, co-exists with the ordinary _paphia_
+female. But in the southern valleys of the Alps the _valesina_ female
+is much the commoner of the two, and indeed in some localities where
+the species is abundant, I have seen no _paphia_ females in many days
+collecting.
+
+The beetle _Gonioctena variabilis_ furnishes an illustration of a
+comparable phenomenon affecting the male sex. In 1894 and 1895 I
+studied the curious colour variations of this species especially in the
+neighbourhood of Granada, and Mr. Doncaster ten years later repeated the
+observations on the same ground, and also collected the insect in other
+places in the south of Spain. The distinctions are not easy to give in
+words and the reader is referred to the colour plate accompanying my
+paper.[5] The essential fact is that the males commonly have the elytra
+_red with black spots_ and the females for the most part have greenish
+grey elytra with black stripes. In some localities a large minority of
+males closely resemble the female type, being identical in colour and
+then only distinguishable by structural differences. In two Granada
+localities I found the proportion of such males quite different. In the
+Darro valley about 38 per cent. (in 718) were of this feminine type,
+but on the hills some 300 feet above only 19 per cent. (in 3,230) were
+like the females. At Castillejo, not far from Toledo I found no such
+male in 75 specimens.
+
+Mr. Doncaster collected from several localities, especially from two
+areas near Malaga, about 5 miles apart. In one of these the female-like
+males were, as usual, in a minority, but in the other these were
+actually in great excess, amounting to about 81 per cent. in the 173
+taken. Doncaster found a doubtful indication that the composition of the
+population varies with the season, which is quite possible, but it is
+most interesting to note that in my chief locality after the lapse of
+ten years he found the proportions very much the same as I had done at
+the same season, for where I had 19 per cent. of the female-like males
+his collecting gave 16 per cent. In other respects also, his statistics
+corresponded very closely with mine.[6]
+
+The various forms of _Heliconius erato_ are well known to entomologists.
+They are strikingly distinguished by the colours of the strong comb-like
+marking on the hind wing, which may be red, yellow, green or blue. In
+various parts of the distribution in South America sometimes two and
+sometimes three of these distinct types co-exist.[7]
+
+The distribution of the varieties of _Noctua castanea_ typifies a large
+range of cases. The form which is reckoned the normal of the species
+has red fore-wings. It is practically restricted to Great Britain and
+Germany, according to Tutt. The other common form, _neglecta_, has grey
+fore-wings, and in this pattern it ranges through West Central Europe
+from North Italy to Germany. In the British Isles it extends up to
+Orkney. In Britain this grey form is by far the commoner, occurring
+wherever the species is found. The red form is much scarcer in England,
+and does not occur at all in many localities where the grey form is
+common. Mr. Woodforde, from whom this account is taken,[8] states that
+in August, 1899, he saw considerably over a hundred of the grey in the
+New Forest at sugar, but only two red ones. In Staffordshire however the
+red is proportionately more numerous and he estimates them as 40 per
+cent. of the population. Lastly a form has been taken in Staffordshire
+as a rarity in which the red is replaced by yellow, and this has
+hitherto been seen nowhere else. It is beyond our immediate purposes
+to discuss the genetic relationships of such forms, but the details
+of this case are interesting as making fairly clear the fact that the
+distinctions between _castanea_ and _neglecta_ are due to combinations
+of the presence of and absence of two pairs of factors, of which one
+produces a red pigment in the ground colour of the forewing and the
+other irrorates the same region with black scales. Mr. Woodforde states
+that all intermediates exist, and that in Staffordshire the greys always
+have a pinkish tinge. The yellow is doubtless another recessive to the
+red.
+
+Species which are uniform in some localities may be polymorphic in
+others. Such a phenomenon is well exemplified by the orchid _Aceras
+hircina_. Of this species distinct varieties had previously been known
+in Germany, but Galle[9] has lately given a detailed account of a
+number of most diverse forms found growing in a district of Eastern
+France. Without reference to his plates it is impossible to give any
+adequate conception of the profusion of types which the flowers of
+the species there assume. In some the lip is elongated to many times
+its usual length, twisting and dividing in a fashion suggesting some
+of the strangest of the Tropical Orchids. In others the labellum
+and the lateral petals are all comparatively short and wide (Fig.
+13). Intermediates, combining these qualities in various degrees,
+were abundant, and the condition of the species, which was the only
+representative of the genus in the locality, recalls the extreme
+polymorphism of many of the Noctuid Moths.
+
+[Illustration: FIG. 13. Various forms of _Aceras hircina_. (After
+Galle.) This figure only shows a few of the more striking forms
+illustrated in Galle's plates.]
+
+Somewhat comparable variability has been seen in another Orchid genus
+_Ophrys_. In Great Britain the species _apifera_, _aranifera_ and
+_muscifera_ though variable are fairly distinct, but Moggridge has
+published two series of plates[10] showing a very different state of
+things as regards the _Ophrys_ population of the Riviera. Here the
+outward diversity is such that the ordinary specific names cannot be
+applied with any confidence and the limits of the species are quite
+uncertain. It may well be supposed that these Riviera plants are
+interbreeding, and indeed we may safely assume that they are. It is,
+however, to be remembered that Darwin showed _apifera_ in this country
+to be habitually self-fertilised, so that the different behaviour on
+the Riviera may itself constitute a local peculiarity. Moreover it is
+to be gathered from Moggridge's account that in the districts which he
+examined the condition was not to be described by the statement that
+our three types were there co-existing and hybridising, but rather we
+should say that the population was polymorphic, containing these three
+types amongst others. Conchologists are aware that on the Dogger Bank
+_Modiola_ attains a size unparalleled elsewhere. The same is true of
+the sponges _Grantia compressa_ and _Grantia ciliata_ in the estuary
+of the Orwell.[11] Conversely, as we know so well in the case of Man,
+dwarf races occur in several special localities. Such examples may be
+multiplied indefinitely.
+
+The relation of local forms to species has often been discussed from
+many points of view, but I know no treatment of the subject clearer or
+more comprehensive than an excellent account of some of the various
+manifestations of local differentiation as they appear in Helicidae
+published by Coutagne[12] and a reader interested in the problem which
+they raise would do well to make himself acquainted with the original
+from which the following notes are taken. He speaks for example of
+_Helix lapicida_. This is on the whole a constant form ranging up to the
+altitude of 1,300 m., common all over France except at great heights
+and in the Olive regions where it is restricted to moist places. Though
+subjected to such diverse conditions it shows only trivial variations in
+colour and other respects throughout its distribution, excepting that
+on both sides of the Pyrenees it has a very distinct sporadic variety
+called _Andorrica_ or _microporus_. This variety occurs here and there,
+together with the type-form sometimes in colonies (pp. 26-30 and 86).
+
+_Bulimus detritus_ though more restricted in geographical range is a
+much more variable form. It exhibits great variations in colour, form,
+and size, and as Coutagne well insists, these are independent of each
+other. Foreshadowing the methods of factorial analysis he suggests that
+distinctions in each respect, the "modes" as he calls them, should
+be denoted by a letter, or if desired, by a name, and the several
+combinations of differences might thus be most logically and usefully
+expressed. Of such combinations he says there are at least 18, all of
+which can be found. The whole possible series does not necessarily
+occur in the same place, and various localities are characterised by
+the presence or absence of certain of the combinations as Coutagne
+calls them, and by the relative frequency with which they occur. The
+ideas thus enunciated are much in advance of the ordinary practice of
+systematists, who give names to forms which are nothing but accidental
+combinations of factors, just as the horticulturists for practical
+reasons give names to similar combinations, which as we now know are
+merely specially noticeable terms in a long series of possibilities.
+In each case it is rather the _factors_ which should be named than the
+forms which are constituted by their casual collocation. In this special
+example of _Bulimus detritus_ the 18 forms are made by the combinations
+of three pairs of independent factors. Besides these combinations which
+may occur anywhere or almost anywhere in the distribution there are
+two more distinct local forms, each of which is regarded by Coutagne
+as probably constituting a fresh "mode," perhaps compatible with the
+others.
+
+_Helix striata_ (Draparnauld)[13] is truly polymorphic; and its various
+forms have been described under various specific names. It abounds in
+the calcareous hills of Provence and Languedoc, disappearing in the
+alluvial lowlands and equally in the upper levels at about 800-1,000 m.
+From this district it extends through regions of similar altitude over a
+great part of France (details given).
+
+Locard in his monograph of this group, which he calls collectively the
+group of _Helix Heripensis_, tabulates 27 distinct named forms. The
+characteristics in which these forms differ have been reckoned as 17,
+and as several of these vary in degree of development, the number of
+modes may be increased to 109. For practical purposes however Coutagne
+considers that the various developments of 7 characteristics in their
+several combinations are enough to express the various forms, and he
+gives examples of this method of definition. As he observes, though
+names may be required to define the modes, no one need be alarmed at
+that, for the same names of modes will be applicable to a great range of
+distinct species, and the formulae expressing their combinations will
+replace the varietal names.
+
+This particular example of polymorphism is but little limited by
+locality. Occasional colonies present some special physiognomy which may
+in a given place seem almost invariable, though in this very respect the
+colonies found elsewhere may be highly variable, but such limitations
+are exceptional for _H. striata_.
+
+Some distinct and obvious susceptibilities to the influence of soil
+and climate are however noticeable. For example on siliceous ground
+the shells are thinner, while on calcareous soils they are thicker;
+similarly those from the Northern districts attain a larger size
+than those from further South. Moreover those subjected to curtailed
+development, whether from drought, heat or cold often show a shortening
+of the spire. In contrast with this case Coutagne describes the
+varieties of _Helix caespitum_, which he says are for the most part
+localised, quoting many illustrative cases.
+
+Another remarkable case in which locality plays a curious part is
+provided by the two species _Helix trochoides_ and _pyramidata_. In
+France generally they are distinct enough from each other, _trochoides_
+being smaller and having a characteristic keel. Coutagne says that after
+having collected these species from more than a score of localities he
+came upon a colony of _trochoides_ on the island of Pomegues in which
+the shells were relatively enormous, most of them having only a slight
+keel, and a few none at all. On the other hand he received a consignment
+of _pyramidata_ from four localities in Sicily, all small, and one of
+them exactly like the _trochoides_ from Pomegues. Judging by the samples
+received from Sicily, _trochoides_ is there not more variable than it is
+in Provence, while the Sicilian _pyramidata_ is protean.
+
+The relations of the two species _Helix nemoralis_ and _hortensis_
+provide an illustration of another kind of manifestation of local
+peculiarity. _H. hortensis_ and _nemoralis_ as usually met with, are
+two very distinct forms. _H. hortensis_ is smaller and duller, and its
+peristome is white. _H. nemoralis_ is larger and more shiny, and its
+peristome is brown. In several anatomical points, moreover, especially
+in the shape of the dart, there are great differences. For a full
+account of these peculiarities of the two forms and a discussion of
+their inter-relations the reader is referred to the elaborate work of
+A. Lang[14] who has studied them extensively and has also succeeded in
+experimentally raising hybrids between them. These hybrids were in a
+slight degree fertile with both the parent species, but up to the time
+of publication no young had been reared from hybrids _inter se_.
+
+Coutagne describes the result of collections made in 62 French
+localities. Some had exclusively _hortensis_, some exclusively
+_nemoralis_, and in some the two were found in association. He gives
+details of five of these collections from which I take the following
+summary of the more essential facts, omitting much that is almost
+equally significant.
+
+_Locality A_, near Honfleur. Both forms present, each sharply and
+normally distinguished, without any intermediates. They are thus found
+in many places. Coutagne instances Mueller's observations in Denmark, his
+own series from the Jura, etc.
+
+_Locality B._ Vonges (Cote d'Or), 242 _hortensis_ taken at random,
+showed 128 with light peristomes (either more or less pinkish or
+quite white) and 114 with dark _brown_ peristomes; together with 26
+_nemoralis_ all with the usual brown peristomes.
+
+Of the _hortensis_ 50 were in ground-colour _opalescens_ and 1 _roseus_;
+and in shape 5 were _umbilicatus_.
+
+_Locality C_, about 3 kilometres from _B_. There were found 35
+_hortensis_, of which 20 had light peristomes and 15 brown; together
+with 7 _nemoralis_.
+
+Of the _hortensis_ none were _opalescens_; 18 were _roseus_ and none has
+the shape of _umbilicatus_.
+
+_Locality D_, about 1,200 metres from _B_. 147 _hortensis_, of which 4
+had light peristomes and 143 had brown. No _nemoralis_ were found.
+
+None of the _hortensis_ were _opalescens_ or _roseus_, but 30 were
+_umbilicatus_.
+
+In these localities intermediates of every grade existed between the
+well-characterised _opalescens_, _roseus_, or _umbilicatus_, and the
+other forms, but there were no intergrades between the other _nemoralis_
+and the smaller _hortensis_, about which there was no hesitation. In the
+next locality a very different state of things was found.
+
+_Locality E._ Banks of the Yvette at Orsay (Seine-et-Oise). The
+actual numbers are not given, but we are told that 58 per cent. were
+_hortensis_, 33 per cent. _nemoralis_, and 9 per cent. intermediate. As
+at Honfleur, the _hortensis_ had white peristomes, and the _nemoralis_
+brown. Coutagne's visits to this locality were in 1878 and 1880, and he
+calls attention to the fact that Pascal found similar intermediates in
+the same neighbourhood in 1873.
+
+The two species, in Coutagne's view, when they occur together, can
+generally be sorted from each other with perfect confidence, and it is
+only in exceptional localities that these intermediates occur. Whether
+they are hybrids, or whether sometimes the species in their variations
+transgress their usual limitations is regarded both by Coutagne and
+by Lang as a question not yet answerable with certainty. Coutagne
+moreover lays stress on the fact that although each species may be
+easily known from the other _in its own district_, yet when shells from
+different districts are brought together it is sometimes impossible to
+sort them. He mentions an example of such casual intermixture occurring
+under natural conditions on an island in the Rhone, to which it may
+well be supposed that floods had brought immigrants from miscellaneous
+localities. This population contained a very large number of uncertain
+specimens, and as he says, it was much as if he were to mix the shells
+from his 62 localities, after which it would certainly be impossible to
+separate the two species again.[15]
+
+Further evidence is given in the same treatise as to other examples
+of polymorphism, especially in the genus _Anodonta_, of which Locard
+made 251 species for France alone. Here again are cases like those
+already given, and many forms or "modes" are found restricted to special
+localities, while occasionally in the same locality dissimilar forms are
+found, collectively forming a colony, without intermediates.
+
+Taken as a whole the evidence shows the following conclusions to be
+true. Local races, whether of animals or plants, may be distinguished
+by characters which we are compelled to regard as trivial, or again by
+features of such magnitude that if they were known to us only as the
+characteristics of a uniform species they would certainly be assumed
+without hesitation to be essential for its maintenance. Local forms
+may be sharply differentiated from the corresponding populations of
+other localities or they may be connected with them by numbers of
+intermediates. Not rarely also we find a fact which has always seemed to
+me of special significance, that the peculiarity of the local population
+or colony may show itself in a special liability to variation, and
+this variability may show itself in one of many degrees, either in the
+constant possession of a definite aberration, in a dimorphism, or in an
+extreme polymorphism.
+
+At this stage attention should be called to two points. First, that
+when the details of the geographical distribution of any variable
+species are studied in that thorough and minute fashion which is
+necessary for any true knowledge of the interrelations of the several
+forms, the conception of a species invented by the popular expositions
+of Evolution under Selection is found to be rarely if ever realised in
+nature.
+
+A species in this generalised sense is an aggregate of individuals,
+none exactly alike, but varying round a normal type, the characters of
+which are fixed in so far as they are adapted to environmental exigency.
+In nature, however, the occurrence of the varieties, and even the
+occurrence of the variability is sporadic. In one place a population may
+be perfectly uniform. In another it may be again uniform but distinct.
+In others the two forms may occur together, sometimes with and sometimes
+without intergrades. In some localities a sporadic variety may be an
+element of the population, persisting through long periods of time.
+In other localities there may be several such aberrations occurring
+together which are absent elsewhere.
+
+Secondly, I would remind the reader that in the light of genetic
+analysis we know that intergrades, when they do occur, cannot be assumed
+to represent conditions through which the species must pass or has
+passed on its way to the extreme and definite forms.
+
+Often, perhaps generally, they are nothing but heterozygous forms,
+and often also they are conditions corresponding with the presence of
+factors in their reduction-stages.
+
+A broad survey of the facts shows beyond question that it is impossible
+to reconcile the mode of distribution of local forms with any belief
+that they are on the whole adaptational. Their peculiarities are
+occasionally the result of direct environmental influence, as we shall
+hereafter notice in certain cases, but none can attribute such sporadic
+and irregular phenomena to causes uniformly acting.
+
+Writers on systematics, especially those of former generations often
+conjecture or assert that local distinctions are caused by "differences
+of climate, soil, food, etc.," in vague general terms. It is usually
+safe to assume that these remarks do not represent conclusions drawn
+from actual evidence, for only rarely can they be translated into more
+precise language. So thoroughly have the biological sciences become
+permeated with the belief that all distinctions are dependent upon
+adaptation, that the mere existence of definite distinctions is felt
+by many to be sufficient ground to warrant an assumption that these
+distinctions are directly or indirectly due to special local conditions.
+For example, Dr. J. A. Allen, who has done so much careful and valuable
+work in delimiting the local forms of the United States fauna, writes of
+the Ground Squirrels (Tamias)[16] as follows:--
+
+   "From the extreme susceptibility of this plastic
+   group to the influences of environment, it is one of the most
+   instructive and fascinating groups among North American mammals.
+   No one can doubt its comparatively recent differentiation from
+   a common stock, and its dispersion from some common centre.
+   Whether the type originated at some point in North America, or in
+   the Northern part of Eurasia, it is perhaps idle to speculate,
+   but that it has increased, multiplied, spread, and become
+   differentiated to a wonderful degree in North America is beyond
+   question; as it is found from the Arctic regions to the high
+   mountain ranges of Central Mexico, and has developed some twenty
+   to thirty very palpable local phases."
+
+   "Some of them easily take rank as species, others as
+   subspecies. Probably a more striking illustration of evolution by
+   environment cannot be cited."
+
+He proceeds to point out that the habits of these creatures are such as
+lead to isolation. This may well be admitted, and indeed no exception
+can possibly be taken to the passage as a whole, save in the one
+respect that there is no real proof that the local diversity is due to
+"evolution by environment" or an indication of "susceptibility to the
+influences of environment."
+
+Dr. Allen does indeed adduce the fact that California "extending through
+800 miles of latitude, with numerous sharply contrasted physiographic
+regions, has apparently no less than six strongly differentiated forms,
+while the region east of the Rocky Mountains from a little below the
+northern boundary of the United States northward to the limit of
+trees--a slightly diversified region of at least ten times the area of
+California--has only one"! But when one comes to ask how the various
+forms are adaptational, and how the influences of environment have led
+to their production, only conjectures of a preliminary and tentative
+character could be expected in reply. Desert forms are no doubt pallid
+as in so many instances, and forest forms are more fully coloured, and
+we may readily enough accept such facts as indications of a connection
+between bodily features and the conditions of life, but further than
+that no one can go; so that when we find size, length of ears or of
+tail, the number of dorsal stripes, the pattern of the colours, not to
+speak of differences in the pigments themselves, all exhibiting large
+modifications, we cannot refer these peculiarities to the causation
+of environmental difference, save as a simple expression of faith. I
+incline far more to agree with Gulick who, after years of study of the
+local variations of the Achatinellidae, came to the conclusion that it
+was useless to expect that such local differentiation can be referred to
+adaptation in any sense.[17] Even the most convinced Selectionist must
+hesitate before such facts as those related by A. G. Mayer regarding
+the distribution of _Partula otaheitana_, one of these Achatinellidae.
+The island of Tahiti has been scored by erosion so that a series of
+separated valleys radiate to the coast. From four successive valleys
+Mayer collected the species, and found that in the first (Tipaerui)
+valley all the shells were dextral (115, containing 73 young); in the
+second valley (Fautaua) 54 per cent. of adults and 55.5 per cent. of
+the young contained were sinistral; in the third valley (Hamuta) 69
+per cent. of adults and 73 per cent. of young contained in them were
+sinistral; and lastly, in the fourth valley (Pirae) all the shells
+(131, containing 62 young) were sinistral.[18] In connection with
+these observations I may mention the fact that in a certain pond in
+the North of England[19] the sinistral form of _Limnaea peregra_ has
+been known to occur for about fifty years. Visiting it lately I found
+the left-handed shells to be about 3 per cent. of the population. The
+species is the commonest British freshwater shell, but left-handed
+specimens are exceedingly rare. Will anyone ask us to suppose that the
+persistence of a percentage of this rarity in the same place is an
+indication of some specially favouring circumstance in the waters of
+that pond? It is a horse-pond to all appearances exactly like any other
+horse-pond; and I believe that in perfect confidence we may accept the
+suggestion of common sense, which teaches us that there is nothing
+particular in the circumstances which either calls such varieties into
+existence or contributes in any direct way to their survival. Had the
+phenomenon of local variation been studied in detail before Darwin
+wrote, the attempt to make selection responsible for fixity wherever
+found, could never have been made. The proposition that not only the
+definiteness of local forms but their variability also is sporadic,
+can be established by countless illustrations taken from any group of
+either the animal or the vegetable kingdoms. Only exceptionally can the
+fixed differences be even suspected of contributing to adaptation, and
+sporadic variability, which is a no less positive fact, must manifestly
+lie outside the range of such suspicions. It is open to any one to
+suggest speculatively that the persistence of special varieties or of
+special variability in special places is an indication that in those
+places the conditions of life are such that the forms in question are
+tolerated though elsewhere the same types are exterminated; but that
+consideration, even if it could be proved to be well founded, is not
+one which lends much force to the thesis that definiteness of type is
+a consequence of Natural Selection. On the contrary, recourse to such
+reasoning implies the inevitable but very damaging admission that the
+stringency of Selection is frequently so far relaxed that two or more
+equally definite forms of the same species can persist side by side.
+There is no doubt that this is the simple truth, but when once that
+truth is perceived it is useless to invoke the control of Selection as
+the factor to which definiteness of type in general must be referred.
+
+The genetic relations of local forms to each other cannot in the
+absence of actual breeding experiments be often ascertained. Standfuss
+formerly enunciated as a general principle that when two forms
+co-exist in the same locality and are able to interbreed, they do not
+produce intermediates; but that when the forms are geographically
+separated as local races, crosses between them result in a series of
+intermediates.[20] In this aphorism there is a good deal of truth, but
+if in the light of Mendelian principles we examine the two statements
+we see now that the first is in reality only another way of saying
+that the distinctness of an aberrational form co-existing with another
+is due to segregation, accompanied by some degree of dominance of one
+type. Whether, however, one geographically isolated race will give
+intermediates when bred with another must depend entirely on the genetic
+physiology of the special case, and no general rule can be laid down.
+It may well be that, inasmuch as the distinctness of the variety is
+maintained by isolation, the difference in factorial composition between
+it and the representative form in another area is neither simple nor
+sharp; but when two varieties co-exist, though interbreeding, it is now
+clear that their differences must depend on the segregation of simple
+factors. Plainly such aberrations may in one place co-exist with another
+type, and elsewhere be separated from it as local races.
+
+Excellent illustrations of these two stages in evolution are provided
+by the melanic varieties of British Lepidoptera. The fact that black or
+blackish varieties of many species especially of Geometridae have come
+into existence in recent years is well known to British collectors,
+and it is not in dispute that they have in several instances replaced
+the older type more or less completely in certain districts. In the
+year 1900 the Evolution Committee of the Royal Society instituted a
+collective inquiry as to the contemporary distribution of these dark
+varieties. As the change had happened within living memory and had
+greatly progressed in recent years it was hoped that a record of the
+existing distribution would serve as a point of departure for future
+comparison. The records thus obtained were tabulated by Mr. L.
+Doncaster.[21] From that account and from the statements in Barrett's
+British Lepidoptera[22] this description of some of the more notable
+cases is taken.
+
+The most striking and familiar case is that of _Amphidasys betularia_,
+of which only the ordinary type was known in any locality until about
+1848-1850, when the totally black var. _doubledayaria_ first appeared
+in the neighbourhood of Manchester. This black form was subsequently
+recorded in Huddersfield between 1860 and 1870; Kendal about 1870;
+Cannock Chase, 1878; Berkshire, 1885; Norfolk, Essex and Cambridge about
+1892; Suffolk, 1894; London, 1897. For the Southern Counties of England,
+except in the London district, there are still very few records. It
+cannot of course be asserted positively that the variety spread from its
+place of first appearance into the other localities, and that it did not
+arise _de novo_ in them, but there can be little doubt that the process
+was one of colonisation. On the European Continent the first records are
+from Hanover in 1884, Belgium 1886 and 1894, Crefeld 188-, Berlin 1903,
+Dresden about the same date.
+
+As regards the increase of the variety we have the fact that in
+Lancashire, Cheshire and the West Riding of Yorkshire the black is now
+the prevalent form; and in some places, as for example, Huddersfield,
+the black alone is now found, though it was unknown there till between
+1860 and 1870. About 1870 at Newport, Monmouth, the two forms were in
+about equal numbers, but a few years later the type had almost vanished.
+Similarly in Crefeld, where the black form was still very rare in the
+eighties, it now forms about 50 per cent. of the population. In the
+London district the black remains scarce and at the date of the report
+it was still very scarce. From Ireland there is only one record and
+there are hardly any from Scotland.
+
+_Boarmia repandata_ is another species which is behaving in a somewhat
+similar way. Unlike _betularia_, however, the species is a variable
+one, and has several colour-forms, amongst them the banded var.
+_conversaria_, and many others. In addition to these there is a
+black form in the North of England which seems to be spreading. In
+Huddersfield the black was first recorded in 1888, and in 1900 20-25 per
+cent. were black. At Rotherham the black or very dark are now prevalent
+and have increased in the last 15 years. From the Midlands, East Anglia
+and Southern Counties the returns show only the light and medium forms.
+
+Of _Odontoptera bidentata_ several intergrading dark forms exist, and
+these are found exclusively in the North and the Midlands. Unicolorous
+blacks have been found recently in the Lancashire mosses and at
+Wakefield. At Huddersfield 50 years ago the light forms were prevalent,
+but now a rather dark brown, not infrequently suffused with black, is
+the commonest. In Southern Counties only light forms are known.
+
+_Phigalia pilosaria_ in South England is always light, but in the North
+the prevalent form is darker. About 35 years ago a form with unicolorous
+sooty fore-wings and dull grey hind wings was first seen in Yorkshire
+and a similar form is now taken regularly in South Wales.
+
+In the following cases the dark varieties were found originally only in
+the South.
+
+_Boarmia rhomboidaria_ gave rise about 40 years ago to a unicolorous
+smoky variety called _perfumaria_. This was at first peculiar to the
+London district, but it has since been taken in Birmingham and other
+large cities. More lately coal-black specimens have been found at
+Norwich, and others similar but hardly so dark were taken in the South
+of Scotland and at Cannock Chase.
+
+_Eupithecia rectangulata_ is a similar case. Formerly the light forms
+were prevalent but within sixty years they have almost entirely been
+replaced in the South of London by a nearly black form.
+
+_Tephrosia_ (_Boarmia_) _consortaria_ and _Tephrosia consonaria_ are
+exceptionally interesting, for they have both given off dark forms in
+the same wood near Maidstone, which is far from the usual "centres of
+melanism." They were discovered in this locality by Mr. E. Goodwin. That
+of _consortaria_ is a dark grey, but that of _consonaria_ is a full
+black, and nothing like either has been found anywhere else.
+
+These examples are all taken from the Geometridae but others, though
+of a less conspicuous kind, could be given from the Noctuidae or the
+Micro-Lepidoptera. _Acronycta psi_, for instance, has a suffused form
+which is believed to be becoming more frequent in the London district.
+_Polia chi_ has two dark forms, _olivacea_, a yellowish grey with dark
+markings, and _suffusa_ which is a darker, blackish-slate colour. Both
+occur in the North of England, sometimes together, sometimes separately,
+or mixed with the type and many intermediates. The distribution is
+peculiarly irregular. At Huddersfield, where the very dark form appeared
+suddenly about 1890, some 30 per cent. are said to be now dark and about
+6-7 per cent. very dark, but at Saddleworth, 12 miles away, only the
+pale forms occur.
+
+Several questions of interest arise in regard to this evidence. This
+progressive Melanism has arisen in certain families only, and may be
+confined to certain species only, within those families. As in almost
+all other examples in which variation has been much observed, its
+incidence is capricious and specific. A collateral line of inquiry
+relates to the degree of discontinuity which the variation manifests.
+Here again there is no rule. Generally speaking, in _A. betularia_,
+to take the case most fully studied, the variation is discontinuous.
+Real intermediates between _betularia_ and _doubledayaria_ are in most
+localities absent or rare. The black spots of _betularia_ may often
+be larger or more numerous than in the normal, but this variation has
+nothing to do with _doubledayaria_, and is not an intermediate stage
+towards it, though sometimes wrongly so described. _Doubledayaria_ owes
+its characteristic appearance to a factor which blurs the surface of the
+wings with a layer of black. Sometimes this blurring is slighter than
+in the real _doubledayaria_, and these forms are real intermediates.
+Occasionally the fore-wings alone are thus blurred. These intermediates
+are clearly due to reduction-stages of the _doubledayaria_ factor,
+and are related to it as a blue mouse is to a black, or a dutch
+rabbit to a self-colour. It cannot positively be asserted that the
+full _doubledayaria_ existed before the intermediate, but it almost
+certainly did. In certain places as for instance in Belgium, there is
+evidence that intermediates have at various times been fairly abundant,
+but they have never become common, nor are they known to exist in the
+absence of _doubledayaria_. When the black variety and the light type
+breed together they do not usually have intermediates among their
+offspring, and the evidence is consistent with the view that the
+black is a complete dominant. The same is probably true of _Tephrosia
+consonaria_.
+
+In some of the other species we know that the darkest forms did
+not appear first. For example in _Phigalia pilosaria_ and _Boarmia
+rhomboidaria_ dark forms existed and are believed to have increased in
+number before the darkest made its appearance. _Hybernia progemmaria_ is
+said to have become darker gradually both in Cheshire and in the West
+Riding, and a uniformly smoky variety appeared in South Yorkshire less
+than 45 years ago which has spread to neighbouring counties. The dark
+medium has become the commonest form in Huddersfield district, where the
+very dark variety is now about 20 per cent. of the population, though
+the light form is still common.
+
+Taking the evidence together we find it consistent with the view that
+dark forms have appeared sporadically, in some species the very dark
+appearing first and intermediates later, in others the moderately dark
+came first and the darkest later in time. It is practically certain that
+the change has in general come about not by a gradual change supervening
+on the population at large, but by the sporadic appearance of dark
+specimens as a new element in the population, and strains derived from
+these dark individuals have gradually superseded the normal type more or
+less completely.
+
+If it could be shown that these melanic novelties had a definite
+advantage in the struggle for existence they would provide an instance
+of evolution proceeding much in the way which Darwin contemplated. The
+whole process would differ from that conceived by him as the normal
+method of evolution only in so far as the change has come about with
+great rapidity and in some instances largely by the appearance and
+success of discontinuous varieties. The question, however, must be
+asked whether the dark form can reasonably be supposed to have an
+advantage by reason of their darkness. Some naturalists believe that
+the darkness of the colours does thus definitely contribute to their
+protection by making the insects less conspicuous and thus more likely
+to escape the search of birds. In support of this view it may be
+pointed out that it is in the manufacturing districts of Lancashire and
+Yorkshire, and again in the London area that the melanics have attained
+their greatest development. Consistently with this argument also, it is
+in the neighbourhood of Crefeld and Essen, the black country of Germany,
+that they have chiefly established themselves on the Continent, and
+_Phigalia pilosaria_ in the black form is now at home in South Wales.
+Thus superficially regarded, the evidence looks rather strong, but it is
+difficult to apply the reasoning in detail. We have first the difficulty
+that the black form of _betularia_ for instance has established itself
+in thoroughly rural districts, notably near King's Lynn in Norfolk,
+and in the neighbourhood of Kendal and Windermere. The black form
+of _consonaria_ and the dark _consortaria_ appeared in a wood near
+Maidstone, far from town smoke, and the black _rhomboidaria_ was first
+found at Norwich, which, as towns go, is clean. Then again the spread of
+the melanics is very irregular and unaccountable. The black _pilosaria_
+is found both in the West Riding and in the Swansea district, but
+not yet elsewhere. It rapidly increased at Huddersfield, but made no
+noticeable progress at Sheffield though recorded there for ten years.
+It is also a remarkable fact that no similar melanic development has
+been observed in America, and, so far as I am aware, comparable melanic
+varieties have not appeared on the European continent except in the case
+of the few sorts which possibly may have come from England.
+
+The whole subject is beset with complications. It must not be forgotten
+that in a few species of moths there is an obvious and recognised
+conformity between the colours of the perfect insect and that of the
+soil on which they live, comparable with that which is so striking in
+the case of some Oedipodidae and other grasshoppers. Of this phenomenon
+the clearest example is _Gnophos obscurata_, which is a most variable
+species with many local forms. Of these a well-known dark variety lives
+on the peaty heaths of the New Forest and other districts, but on the
+chalk hills of Kent, Sussex and Surrey various light varieties are
+found, of which one is a bright silvery white, very near in colour to
+the colour of a chalky bank. This case does not seem to be one of direct
+environmental action,[23] for Poulton found no change induced by rearing
+larvae among either white or black surrounding objects. No one however
+can doubt that there is some indirect connection between the colour of
+the ground and that of the moths.
+
+To my mind there is a serious objection to the theory of protective
+resemblance in application to such a case as that of the _betularia_
+forms, which arises from the fact that the black _doubledayaria_ is a
+fairly conspicuous insect anywhere except perhaps on actually black
+materials, which are not common in any locality. Tree trunks and walls
+are dirty in smoky districts but they are not often black, and I doubt
+whether in the neighbourhood of Rotherham, for instance, which is one
+of the great melanic centres, _doubledayaria_ can be harder for a bird
+to find than _betularia_ would be. After all, too, many of the species
+much affected are not urban insects. They live in country places between
+the towns, and the general tone of these places even in Lancashire
+and the West Riding is not very different from that of similar places
+elsewhere. As against the objection that the black varieties are much
+blacker than the case requires it may be replied that we know nothing
+of the senses of birds, and that perhaps to their eyes blackness does
+constitute a disguise even though the surroundings are much less dark.
+This is undeniable, but recourse to such an argument is dangerous; for
+if the sight of the insect-eating birds is so dull that it does not
+distinguish dark things from dingy grey, we cannot subsequently regard
+the keen sight of birds as the sufficient control which has led to the
+minute and detailed resemblance of many insects to their surroundings.
+Those who see in such cases examples of the omnipotence of Selection
+must frequently find themselves in this dilemma.
+
+Taking the evidence as a whole, we may say that it fairly suggests the
+existence of some connection between modern urban developments and the
+appearance and rise of the melanic varieties. More than that we cannot
+yet affirm. It is a subject in which problems open up on every side,
+and all of them are profitable subjects for investigation. Unhappily
+such animals are difficult to rear successfully in captivity for many
+generations, owing to their extreme liability to disease. Not the least
+interesting feature of the melanics is the fact that the black varieties
+provide about the best and clearest example of a new dominant factor
+attaching itself to a wild species in recent times. None of the cases
+are satisfactorily recorded or analysed as yet, but the evidence is
+clear that _doubledayaria_ is a dominant to its type, and in several
+other dark varieties, though the pigment deposited is not black, the
+records show that the increased amount of the pigment almost certainly
+is due to a positive factor. Of this, _Hemerophila abruptaria_ is a
+good example.[24] There are some irregularities in the results, but
+taken together they leave little doubt that the dark brown variety is a
+dominant and the light, yellowish brown a recessive.
+
+A curious parallel to the rise of the melanic moths in England
+is provided by the case of the Honey-creepers or Sugar-birds, in
+certain West Indian islands.[25] These birds of the genus _Coereba_
+(_Certhiola_) range from Southern Mexico to the Northern parts of
+South America and through the whole chain of the West Indian islands
+and Bahamas except Cuba. There are numerous local forms, and many of
+the islands have types peculiar to themselves, as is usual in such
+cases. Some of the types or species range through several islands, but
+according to Austin Clark[26] no island has more than one of them.
+Cory[27] reckoned twelve such species within the Antillean region. They
+are small birds about the size of a nuthatch with a general colouring of
+black, yellow, and white. From the island of St. Vincent the Smithsonian
+Institution received in the late seventies of last century several
+completely black specimens in addition to two of the usual type of
+colouring. The black were described by W. N. Lawrence as _atrata_, and
+those marked with the usual yellow and white were called _saccharina_.
+The collector (Mr. F. A. Ober) reported that the black form was common,
+and that the _saccharina_ form was rarer. Lawrence remarks, "Had there
+been only a single example (of the black form) I should have considered
+it as probably a case of abnormal colouring, but it seems to be a
+representative form of the genus in this island."[28] There is of course
+no doubt of the correctness of the view taken by Austin Clark that
+"_atrata_" is a black variety. The black bird is in every respect, other
+than colour, identical with _saccharina_, and it is even possible to
+detect a greenish colour in the areas which would normally be yellow,
+showing plainly enough the yellow pigment obscured by the black.
+
+We have next the interesting fact that like our melanic moths the dark
+form is replacing the "type." At the time of Ober's visit the type was
+already in a minority, but now it is nearly or perhaps actually extinct,
+though the black form is one of the commonest birds on the island.
+Austin Clark found no specimen when he collected there in 1903-4, though
+formerly it was not uncommon in the vicinity of Kingston and in the
+immediate windward district of St. Vincent.
+
+The Grenadines are geographically just south of St. Vincent, though
+separated by a deep channel. In these islands no black forms have
+yet been taken, but Grenada, the next island to the south, has both
+normals and blacks. There are trifling differences of size between the
+Grenada birds and those from St. Vincent, the Grenada specimens being
+slightly smaller and for this reason they have received distinct names,
+the form marked with yellow and white being called _Godmani_ (Cory)
+and the black, _Wellsi_ (Cory), but this merely introduces a useless
+complication. There is evidence that in Grenada, as in St. Vincent,
+the black is gradually ousting the original type, but the process has
+not gone so far as in St. Vincent. Austin Clark very properly compares
+this case of the Sugar-birds with that of _Papilio turnus_, which
+as is well-known, has a black female in the southern parts of its
+distribution, in addition to a female of the yellow type, but in the
+Northern States the black female does not occur.
+
+During the present year P. R. Lowe, who lately studied _Coerebas_ on
+a large scale in the West Indies, has published an important paper on
+the subject.[29] He calls attention to the fact that Cory recently
+found a black form of _Coereba_ on Los Roques Islands, and he himself
+discovered another on the Testigos Islands. Both localities are on the
+coast of Venezuela, far from St. Vincent and Grenada. The whole problem
+is thus further complicated by the fact that the black varieties have,
+as we are almost driven to admit, arisen independently in remote places.
+Improbable as this conclusion may be, it is still more difficult to
+regard all the black forms as derived from one source. For first, they
+present definite small differences from each other; and secondly we have
+to remember a consideration of greater importance, that the very fact
+that each island has its own type must be accepted as proving that the
+localities are effectively isolated from each other, and that migration
+must be a very rare event.
+
+The rarity of such illustrative cases is, I believe, more apparent than
+real. It is probably due to the extreme reluctance of systematists to
+admit that such things can be, and of course to the almost complete
+absence of knowledge as to the genetic behaviour of wild animals and
+plants. Only in such examples as this of the _Coereba_, where colour
+constitutes the sole difference, or that of the moths which have been
+minutely studied by many collectors, does the significance of the facts
+appear. The arrangement of catalogues and collections is such that much
+practical difficulty of a quite unnecessary kind is introduced. For
+example, in this very case of _Coereba_, I find the British Museum has
+a fine series from Grenada including 3 normals and 11 black, and also
+16 blacks from St. Vincent. If the black specimens from Grenada were
+put with the normals which are almost certainly nothing but a recessive
+form of the same bird, the variation would strike the eye on even a
+superficial glance at the drawer. But following the notions so naively
+expressed in the passage quoted above from W. N. Lawrence, the blacks
+from Grenada are put apart together with the other blacks from St.
+Vincent, though two of them were shot on the same date as one of the
+normals.
+
+FOOTNOTES:
+
+[1] For the evidence see Tutt, J. W., _Trans. Ent. Soc._, 1898, p. 17.
+Compare the remarkable case given by Gulick (_Evolution Racial and
+Habitudinal_, p. 123) of the two races of _Cicada_, which are separated
+by reason of their life-cycles, one having a period of 13, the other 17
+years.
+
+[2] For references see _Materials_, p. 396, and also G. Baur, _Amer.
+Nat._, 1893, July, p. 677.
+
+[3] Jenner Weir, _Entomologist_, 1880, XIII, p. 251.
+
+[4] Jentink, _Notes Leyden Mus._, 1885, VII, p. 111. Specimens
+illustrating this peculiarity are in the British Museum.
+
+[5] _Proc. Zool. Soc._, 1895, p. 850. Plate. Many points beyond that
+mentioned above are involved in this remarkable case. For example, not
+only are there males like females, but a small proportion of females
+resemble the ordinary male type. The stripes are not merely the spots
+produced, for they occupy different anatomical positions. The spots
+almost always go with a black ventral surface, but the striped forms
+nearly always have that region testaceous. _Spartium retama_, the
+food-plant, will not grow in England, but if it could be naturalised in
+America the whole problem might be investigated there and results of
+exceptional interest would almost certainly be attained.
+
+[6] Doncaster, L., _Proc. Zool. Soc._, 1905, II, p. 528.
+
+[7] I am not aware that the details of this striking case have ever
+been worked out. It should be noted that the green and blue forms are
+not due to simple modification of the red pigment; for these colours,
+due to interference, fork over the area occupied by the red lines. The
+distinctions between these forms cannot therefore be simply chemical,
+as we may suppose them to be, for instance, in the case of many red
+and yellow forms, and the genetic relationships of the _Heliconid_
+varieties would raise many novel problems and be well worth studying
+experimentally.
+
+[8] Woodeforde, F. C., _Trans. North Staffordshire Field Club_, XXXV,
+1901, Plate.
+
+[9] E. Galle, _Compte Rendus du Congres Internat. de Bot. a l'Expos.
+Univ._, 1900, p. 112.
+
+[10] Flora of Mentone, 1864-8, _Nova Acta Acad. Caes._, XXXV, 1869.
+
+[11] I owe these facts to Canon A. M. Norman, who showed me illustrative
+specimens. They were originally described by Bowerbank (_Monogr. Brit.
+Spongiadae_, vol. II, pp. 18 and XX; vol. III, Pls. I and III). A
+specimen of _G. compressa_ measured 5 inches, with a greatest width of
+3-1/4 in. _G. ciliata_ was found measuring 3 in. long and 3/4 in. wide.
+These dimensions are many times those of normal specimens.
+
+[12] Coutagne, G., _Recherches sur le Polymorphisme des Mollusques de
+France_, _Annales Soc. d'Agric. Sci. et Industr. Lyon_, 1895.
+
+[13] As to the synonymy and references see Coutagne, p. 45.
+
+[14] A. Lang, _Die Bastarde von H. hortensis Muller H. nemoralis L._
+Jena, G. Fischer, 1908; with a fine coloured plate showing the varieties
+of the species and their hybrids.
+
+[15] With this evidence compare that given by A. Delcourt in his
+valuable papers lately published relating to the variations of
+_Notonecta_. See especially _Bull. Sci. Fr. Belg._, 1909, XLIII, p. 443;
+and _C. R. Soc. Biol._, 1909, LXVI, p. 589.
+
+[16] Allen, J. A., _Bull. Amer. Mus. N. H._, III, 1891, pp. 51-54.
+
+[17] J. T. Gulick, _Evolution, Racial and Habitudinal_, Carnegie
+Institution, Publication No. 25, 1905.
+
+[18] A. G. Mayer, _Mem. Mus. Comp. Anat. Harvard_, Vol. XXVI, 1902, p.
+117. From the tables given I cannot ascertain the actual numbers from
+the two intermediate valleys, but they were considerable.
+
+[19] To which I was very kindly guided by Mr. C. T. Trechmann.
+
+[20] Standfuss, _Handbuch d. palaearkt Gross-schmet_, 1896, p. 321.
+
+[21] _Ent. Rec._, XVIII, No. 7, 1906.
+
+[22] This evidence was largely collected by Mr. G. T. Porritt, who has
+given much attention to the subject.
+
+[23] Such direct action has of course been proved to occur in the case
+of several dimorphic larvae (_e. g._, _A. betularia_, itself) and pupae.
+
+[24] See Harris, _Proc. Ent. Soc. London_, 1904, p. lxxii, and 1905, p.
+lxiii; also Hamling, _Trans. City of London Ent. Soc._, 1905, p. 5.
+
+[25] I am indebted to Mr. Outram Bangs of the Harvard Museum for calling
+my attention to this remarkable case.
+
+[26] _Auk_, 1889, VI, p. 219.
+
+[27] _Ann. N. Y. Acad. Sci._, 1878, I, p. 149.
+
+[28] _Ann. N. Y. Acad. Sci._, 1878, I, p. 149.
+
+[29] _Ibid_, 1912, pp. 523-8.
+
+
+
+
+                        CHAPTER VII
+
+                 LOCAL DIFFERENTIATION. _Continued_
+
+
+                       OVERLAPPING FORMS
+
+The facts of the distribution of local forms on the whole are consistent
+with the view that these forms come into existence by the sporadic
+appearance of varieties in a population, rather than by transformation
+of the population as a whole. Of such sporadically occurring varieties
+there are examples in great abundance, though by the nature of the
+case it can be but rarely that we are able to produce evidence of a
+previous type being actually superseded by the variety. When the two
+forms are found co-existing in the same area they are usually recorded
+as one species if intergrades are observed, and as two species if the
+intergrades are absent. On the other hand when two forms are found
+occupying separate areas, when, that is, the process of replacement is
+completed in one of the areas, then forthwith each is named separately
+either as species or subspecies. Successive observations carried out
+through considerable periods of time would be necessary to establish
+beyond question that the history proceeds in one way rather than
+another. Such continuity of observation has for the most part never
+been attempted. The kind of information wanted has indeed only been
+lately recognized, and really critical collecting is a thing of only
+the last few decades. The methods of the older collectors, who aimed at
+bringing together a few typical specimens of all distinct forms, are of
+little service in this class of inquiry, which is better promoted by the
+indiscriminate collection of large numbers of common forms from many
+localities. When this has been done on a comprehensive scale we shall be
+in a position to form much more confident judgments as to the general
+theory of evolution.
+
+Some little work of the kind has however been done and the results are
+already of great value. Seeing that the differentiation of local forms
+is only made possible by isolation, it necessarily happens that the
+collector finds one form in one locality and another in a distinct
+locality, and there is no evidence as to the behaviour which the two
+representative species might exhibit if they came into touch with each
+other. In the most familiar examples of such distinction each inhabits
+an island, completely occupying it to the exclusion of any other similar
+form. It can only be when the two representative species occupy parts
+of a continental area connected with each other by regions habitable
+for the organism in question, that there is a chance of seeing the two
+forms in contact. Often also, even where this condition is satisfied,
+the habits, social organisation, or some other special cause may
+act as a barrier which prevents the distinguishable forms from ever
+coming into such complete contact as to interbreed or to behave as a
+genetically continuous race. When genetic continuity is ensured by a
+constant diffusion of the population over the whole area which they
+inhabit there will manifestly be no formation of local races. The
+practical uniformity, for example, of so many species of birds which
+inhabit widely extended ranges of Western Europe is doubtless maintained
+by such constant diffusion. When, as in the case of the Falcons, many
+localities have peculiar forms, the fact may be taken as conclusive
+evidence that there is little or no diffusion; and when we find in such
+a species as the Goldfinch that in spite of migratory fluctuations there
+are nevertheless geographical races fairly well differentiated, it may
+similarly be inferred that these fluctuations habitually move up and
+down on paths which do not intermingle. There are however a few examples
+of animals, not given to much irregular wandering, which occupy a wide
+and continuous range of diversified country and are differentiated as
+local races in two or more districts, though the distinct races meet
+in intervening areas. Of these the most notorious illustration which
+has been investigated with any thoroughness is that of the species of
+_Colaptes_ (Woodpeckers) known in the United States as Flickers. The
+study of the variations of these forms, made by J. A. Allen[1] is an
+admirable piece of work, with which every student of variation and
+evolutionary problems should make himself familiar. The two forms with
+which we are most concerned are known as _C. auratus_ and _C. cafer_,
+and are very strikingly different in appearance. In size, proportions,
+general pattern of colouration, habits, and notes, the two are alike,
+but they differ in the following seven respects as stated by Allen.
+
+          _Auratus_                          _Cafer_
+
+  1. Quills _yellow_.                   1. Quills _red_.
+  2. Male with a _black_ malar          2. Male with a _red_
+       stripe.                               malar stripe.
+  3. Adult female with _no_             3. Adult female with usually a
+       malar stripe.                         brown malar stripe.
+  4. _A scarlet nuchal crescent         4. No nuchal crescent in
+       in both sexes._                       either sex.
+  5. Throat and fore neck               5. Throat and fore neck
+       _brown_.                               _grey_.
+  6. Whole top of head and hind neck    6. Whole top of neck and hind
+       _grey_.                                neck _brown_.
+  7. General plumage with an            7. General plumage with a
+       _olivaceous_ cast.                     _rufescent_ cast.
+
+These differences are illustrated in the accompanying coloured plate,
+which has been most kindly prepared for me under the instructions of
+Dr. F. M. Chapman of the American Museum of Natural History. Before
+going further it is worth considering the nature of these differences
+a little more closely. All but the last are large differences which no
+one would overlook even in a hasty glance at the birds. If the only
+distinction lay in the colour of the quills we might feel fairly sure
+that _auratus_ was a recessive form of _cafer_, and so probably it is in
+this respect. Similarly the black malar stripe of _auratus_ is in all
+probability recessive to the red malar stripe of _cafer_ and I imagine
+the pigments concerned are comparable with those in the Gouldian Finch
+(_Poephila gouldiae_) of Australia. Both sexes in that species may have
+the head black, red, or, less often, yellow, and though it is not any
+longer in question that birds may breed in either plumage, I believe
+that the young are always black-headed and I imagine that those which
+become red-headed possess a dominant factor absent from the permanently
+black-headed birds.[2] Yellow as a recessive form of a red is certainly
+very common, but red and black as variants of the same pigment are less
+usual. In the Gouldian Finch we seem to have a case where a pigment can
+assume all three forms. It would be interesting to know whether the red
+of the malar stripes in _Colaptes_ is a pigment of the same nature as
+the red of the quills. Both in _Colaptes_ and in _Poephila gouldiae_ I
+have seen specimens intermediate between the black and the red, and the
+appearance of the part affected was exactly alike in the two cases, red
+feathers coming up among the black ones, and many feathers containing
+both red and black pigments mixed together. The development of the
+scarlet nuchal crescent in _auratus_ and the absence of this conspicuous
+mark in _cafer_ constitute from the physiological point of view the most
+remarkable pair of differences. When the red crescent is not formed, the
+feathers which would bear it are exactly like the rest, and no special
+pigment is visible in them which one can regard as ready to be modified
+into red. If the crescent is due to a factor it must therefore be
+supposed that this factor has the power of modifying the pigment of the
+neck in one special place alone. Dr. W. D. Miller called my attention to
+the fact that a similar variation occurs in another American woodpecker,
+the Sapsucker, _Sphyropicus varius_.[3]
+
+I do not suggest that such variations are without parallel: indeed in
+_P. gouldiae_ the factor which turns the black of the head into scarlet
+affects one special region of the black only, being sharply distinct
+from the unmodified black of the throat. These regions of the head are
+however often the seat of special colours in birds.[4] So also may be
+instanced the variety of the Common Guillemot (_Uria troile_) which
+has a white line round the eyes and at the sides of the head where the
+normal has no such mark; but this line is formed in a very special
+place, the groove joining the eye to the ear, whereas the feathers
+of the nuchal crescent are not ostensibly distinguished from those
+adjacent.[5]
+
+The transposition of the brown and the grey on the back and front of the
+neck also constitutes a very remarkable difference. If either grey or
+brown depends on a factor then it must be supposed that _auratus_ has
+one of these factors and _cafer_ the other.
+
+From these several considerations it is quite clear that if _auratus_
+and _cafer_ are modifications of the same type produced by presence or
+absence of factors, several independent elements must be concerned, and
+to unravel their inter-relations would be most difficult even if it were
+possible to breed the types under observation, which is of course quite
+beyond present possibilities.
+
+The distribution of the two is as follows. On the east side of the
+Continent _C. auratus_, relatively pure, occupies the whole of Canada
+and the States from the North to Galveston. Westward it extends across
+the whole continent in the more northern region to Alaska, but in its
+pure form it only reaches down the Pacific coast to about the northern
+border of British Columbia. Its southern and western limit is thus
+roughly a line drawn from north of Vancouver, southeast to North Dakota
+and then south to Galveston. _C. cafer_ in the comparatively pure
+form inhabits Mexico, Arizona, California (except Lower California
+and the opposite coast), central and western Nevada, Utah, Oregon,
+and is bounded on the east by a line drawn from the Pacific south of
+Washington, south and eastward through Colorado to the mouth of the
+Rio Grande or the Gulf of Mexico. Between the two lines thus roughly
+defined is a band of country about 1,200-1,300 miles long and 300-400
+miles wide, which contains some normal birds of each type, but chiefly
+birds exhibiting the characters of both, mixed together in various and
+irregular ways. Even in the areas occupied by the pure forms occasional
+birds are recorded with more or less indication of characteristics
+of the other form, but within the area in which the two forms are
+conterminous, the mixed birds are in the majority. The condition of
+these birds of mixed character is described by Allen as follows:
+
+   "As has been long known--indeed, as shown by Baird
+   in 1858--the 'intermediates' or 'hybrids' present ever-varying
+   combinations of the characters of the two birds, from individuals
+   of _C. auratus_ presenting only the slightest traces of the
+   characters of _C. cafer_, or, conversely--individuals of _C.
+   cafer_ presenting only the slightest traces of the characters
+   of _C. auratus_--to birds in which the characters of the two
+   are about equally blended. Thus we may have _C. auratus_ with
+   merely a few red feathers in the black malar stripe, or with the
+   quills merely slightly flushed with orange, or _C. cafer_ with
+   either merely a few black feathers in the red malar stripe, or
+   a few red feathers at the sides of the nape, or an incipient,
+   barely traceable scarlet nuchal crescent. Where the blending
+   of the characters is more strongly marked, the quills may be
+   orange-yellow or orange-red, or of any shade between yellow and
+   red, with the other features of the two birds about equally
+   blended. But such examples are exceptional, an unsymmetrical
+   blending being the rule, the two sides of the same bird being
+   often unlike. The quills of the tail, for example, may be part
+   red and part yellow, the number of yellow or red feathers varying
+   in different individuals, and very often in the opposite sides of
+   the tail in the same bird. The same irregularity occurs also, but
+   apparently less frequently, in the quills of the wings. In such
+   cases the quills may be mostly yellow with a few red or orange
+   quills intermixed, or red with a similar mixture of yellow. A
+   bird may have the general colouration of true _cafer_ combined
+   with a well-developed nuchal crescent, or nearly pure _auratus_
+   with the red malar stripes of a _cafer_. Sometimes the body
+   plumage is that of _C. auratus_ with the head nearly as in pure
+   _cafer_, or exactly the reverse may occur. Or we may have the
+   general plumage as in _cafer_ with the throat and crown as in
+   _auratus_, and the malar stripe either red or black, or mixed
+   red and black, and so on in almost endless variations, it being
+   rare to find, even in birds of the same nest, two individuals
+   alike in all their features of colouration. Usually the first
+   trace of _cafer_ seen in _auratus_ manifests itself as a mixture
+   of red in the black malar stripe, either as a few red feathers,
+   or as a tipping of the black feathers with red, or with merely
+   the basal portion of the feathers red. Sometimes, however, there
+   is a mixture of orange or reddish quills, while the malar stripe
+   remains normal. In _C. cafer_ the traces of _auratus_ are usually
+   shown by a tendency to an incipient nuchal crescent, represented
+   often by merely a few red-tipped feathers on the sides of the
+   nape; at other times by a slight mixture of black in the red
+   malar stripe."
+
+Such a state of things accords very imperfectly with expectations under
+any received theory of Evolution. As in some of the instances discussed
+in the first chapter we have here two fairly definite forms, nearly
+allied, which on any evolutionary hypothesis must have been evolved
+either the one from the other, or both from a third form at a time not
+very remote from the present, as time must be measured in evolution. Yet
+though intermediates exist in some quantity, no one can for a moment
+suggest that they are that definite intermediate from which _auratus_
+and _cafer_ descend in common. One cannot imagine that the immediate
+ancestor of these birds was a mosaic, made up of asymmetrical patches
+of each sort: but that is what many of the intermediates are. It is not
+much easier to suppose the ancestor to have been a nondescript, with a
+compromise between the developed characters of each, with quills buff,
+malar stripes neither black nor red, with a trace of nuchal crescent,
+and so on. Such Frankenstein-monsters have played, a considerable part
+in the imaginations of evolutionary philosophers, but if it were true
+that there was once a population of these monsters capable of successful
+existence, surely they should now be found as a population occupying the
+neutral zone between the two modern forms. Yet, though much remains to
+be done in clearing up the facts, one thing is certain, namely that the
+neutral zone has not a definite and normally intermediate population,
+but on the contrary it is peopled by fragments of the two definite types
+and miscellaneous mongrels between them.
+
+On the other hand, one cannot readily suppose that either form was
+the parent of the other. The process must have involved both addition
+and loss of factors, for whatever hypothesis be adopted, such changes
+must be supposed to have occurred. A careful statistical tabulation of
+the way in which the characters are distributed in the population of
+the mixed zone would be of great value, and till that has been done
+there is little that can be said with certainty as to the genetics of
+these characters. In the collection of Dr. Bishop of New Haven I was
+very kindly allowed to examine a sample, all taken at random, near
+together, in Saskatchewan. There were females 4 adult, 2 young; males
+4 adult and 5 young. This number, though of course insufficient, is
+enough to give some guide as to the degree of definiteness which the
+characters generally show in their variations. Of the 15 birds, 8 had
+simply yellow quills; 2 had red; 1 was almost red but had one yellow
+tail-quill; 3 were intermediate and 1 was buff. As regards the malar
+patch, which can only be determined properly in the adult males, 1 was
+red, 1 was approximately red, 2 intermediate. As to nuchal crescent 4
+females had none, 2 females very slight; 7 males had it, 1 had only a
+slight crescent, and 1 had none. In point of quills therefore 10 were
+definite out of 15; in point of crescent, 11 were definite out of 15;
+and in point of malar patch 1 only was definite out of 4. The last is a
+feature directly dependent on age and so counts for less, but as regards
+the other two features there is some indication that the factors show
+definiteness in their behaviour. It must be remembered that we have no
+knowledge what the heterozygous form may be, and in the case of red
+and yellow it is probably a reddish buff. The patch-works are no doubt
+to be compared with other well-known pied forms, and in these we must
+suppose the active factor broken up, which it probably can be very
+easily. The asymmetry, which Allen notices as so marked a feature, in
+the distribution of the red and yellow quills of the tail especially,
+recalls that of the black markings in the pied Canaries. As is well
+known to students of variations _some_ pigment-factors in _some_ animals
+are apparently uncontrolled by symmetry, while in other specific cases
+symmetry is the rule. On the other hand the blackness or redness of
+the malar patches is, I think, as a rule nearly symmetrical. It should
+be mentioned that two of Dr. Bishop's young birds belonged to the same
+nest, one a female with _red_ quills, the other a male with _yellow_.
+Both are without crescent.
+
+As to the question whether certain combinations of characters occur
+with special frequency, the evidence is insufficient to give a definite
+answer. Among all the birds I have seen in America or in England I
+have not yet found one having the malar patches black without any
+nuchal crescent. Of Dr. Bishop's 8 adults not one, however, showed the
+combination of the three chief features normal for _auratus_ or for
+_cafer_.
+
+Besides the two forms that we have hitherto considered, several other
+local types exist, and these throw some further light on the problem.
+Of these the most important in this connexion is _chrysoides_, which
+inhabits the whole of southern California and the mainland opposite.
+This remarkable form is as Allen says, very different from _auratus_
+except that it has the quills yellow like _auratus_, not red like
+_cafer_. So that we find here in the extreme west of the whole
+distribution a type agreeing in one of its chief features with the
+eastern type. Between this and _cafer_ intergrades have, according to
+Allen, not been found. The relations of this _chrysoides_ are, Allen
+thinks, rather with _mexicanoides_, a southern, smaller race with
+colours more intense, which inhabits Guatemala, but however that may
+be, it must be regarded as a _cafer_ which has lost its red quills. The
+island of Guadeloupe off Lower California has an island form. Beyond the
+other side of the continent there is also an island form of _auratus_,
+inhabiting Cuba, so that clearly the yellow quills can extend into the
+tropics.
+
+The above account is in many respects incomplete, but it suffices to
+give an outline of the chief facts. The whole problem is complicated by
+the undoubted effects of an uncertain amount of migration, and in many,
+perhaps all, districts, the winter population differs from the summer
+population of the same localities. The existence of these seasonal ebbs
+and flows is now well known to ornithologists, and most of the bird
+species of temperate regions are subject to them.
+
+Difficult as it may be to conceive the actual process of origin of the
+two types _auratus_ and _cafer_, it is I think still harder to suggest
+any possible circumstance which can have determined their development as
+distinct races, or which can maintain that distinctness when created.
+Some will no doubt be disposed to appeal once more to our ignorance
+and suggest that if we only knew more we should see that the yellow
+quills, the black "moustache" and the red crescent, specially qualify
+_auratus_ for the north and eastern region, and the red quills, red
+"moustache" and absence of crescent fit _cafer_ to the conditions of its
+homes. Each can judge for himself, but my own view is that this is a
+vain delusion, and that to cherish it merely blunts the receptivity of
+the mind, which if unoccupied with such fancies would be more ready to
+perceive the truth when at last it shall appear. Think of the range of
+conditions prevailing in the country occupied by _auratus_--a triangle
+with its apex in Florida and its base the whole Arctic region of North
+America. Is it seriously suggested that there is some element common to
+the "conditions" of such an area which demands a nuchal crescent in the
+Flickers, though the birds of the _cafer_ area, almost equally varied,
+can dispense with the same character? Curiously enough, the geographical
+variation of _Sphyropicus varius_, another though a very different
+Woodpecker[6] shows that conversely the nuchal crescent can be dispensed
+with in the Eastern form though it is assumed by the Western.[7]
+
+Allen points out the interesting additional fact that superposed
+upon each of the two distinct forms, _auratus_ and _cafer_, are
+many geographical variations which can very naturally be regarded
+as climatic. Each decreases in size from the North southward, as so
+many species do.[8] They become paler in the arid plains, and show
+the ordinary phases which are seen in other birds having the same
+distribution. Such differences we may well suppose to be determined
+directly or indirectly, by environment, and we may anticipate with
+fuller knowledge it will be possible to distinguish variations of this
+nature as in the broad sense environmental, from the larger differences
+separating the two main types of _Colaptes_, which I surmise are
+altogether independent of such influences.
+
+It is generally supposed that phenomena like those now so well
+established in the case of _Colaptes_ are very exceptional, and as has
+already been stated a number of circumstances must combine in order
+that they may be produced. I suspect however that the examples are
+more numerous than is commonly thought. In all likelihood the three
+forms _Sphyropicus varius_, _nuchalis_ and _ruber_ are in a very
+similar condition though the details have not, so far as I know, been
+worked out. A complex example which is closely parallel to the case of
+_Colaptes_ was described by F. M. Chapman[9] at the same date as Allen's
+work. This is the case of _Quiscalus_, the Grackles, which in the North
+American Continent have three fairly distinct forms which Chapman speaks
+of as _Q. aeneus_, _Q. quiscula_, and _Q. quiscula aglaeus_. The birds
+are all, so far as pigment is concerned, dark blackish brown, but the
+head and mantle have superposed a metallic sheen of interference-colours
+which in the various forms take different tints, bluish green, bronze
+green, or bronze purple. The details are complicated and difficult
+to appreciate without actual specimens, but the two common types are
+sufficiently distinct. The birds inhabit the whole area east of the
+Rockies, _quiscula aglaeus_ occupying Florida and the Southern States
+southwest of a band of country about a hundred miles broad extending
+roughly from Connecticut to the mouth of the Mississippi; and _aeneus_
+taking the area north and west of this band. In discussing this case
+Chapman expresses the same view as Allen does in the _Colaptes_ case,
+that there are two distinct populations, substantially fixed, and
+that the band of country in which they meet each other has a mongrel
+population, with no consistent type, but showing miscellaneous
+combinations of the character of the two chief types.
+
+The warblers of the genus _Helminthophila_ provide another illustration
+which has points of special interest. The two chief species are _H.
+pinus_, which has a yellow mantle and lower parts, white bars on the
+wings, a black patch behind the eyes and a broad black mark on the
+throat; and _H. chrysoptera_ with dark grey mantle and pale whitish grey
+lower parts, yellow bars on the wings, and grey marks on cheeks and
+throat where _pinus_ has black. These two birds are exceeding distinct,
+and in addition their songs are quite unlike. _H. pinus_ ranges through
+the eastern United States up to Connecticut and Iowa. _H. chrysoptera_
+is a northern form extending down to Connecticut and New Jersey. Both
+are migrants.
+
+In these two States, where the two types overlap, certain forms have
+been repeatedly found which have been described as two distinct species,
+_Lawrencei_ and _leucobronchialis_. Dr. L. B. Bishop and Mr. Brewster
+showed me two long series of _Helminthophila_ containing various
+intergrades between the four named kinds, and details regarding these
+may be found in Chapman's _North American Warblers_ and in Dr. Bishop's
+paper in Auk, 1905, XXII. Though the characters evidently break up to
+some extent, the series can be represented as due to recombinations of
+definite factors more easily than the others which I have described. The
+differentiating characters are:
+
+               _Pinus_
+
+  1.  Mantle and lower parts _yellow_ (Y^1).
+  2.  Wing-bars _white_ (y^2).
+  3.  Cheek and throat _not black_ (b).
+
+
+             _Chrysoptera_
+
+  1. Mantle and lower parts _grey_ (y^1).
+  2. Wing-bars _yellow_ (Y^2).
+  3. Cheek and throat _black_ (B).
+
+The grey pigment of the mantle is common to both, but is masked by the
+yellow in _pinus_, the net result being an olive-green.[10]
+
+I am much indebted to Dr. F. M. Chapman for the loan of the coloured
+plate in which these distinctions are shown. It first appeared in his
+book, _North American Warblers_.
+
+We cannot tell whether _yellow_ or _not-yellow_ is due to the presence
+of a factor, but we may suppose that one or other gives the special
+colour to the parts. The black of character 3 is no doubt a dominant.
+Thus _pinus_ becomes Y^{1}y^{2}b and _chrysoptera_ in y^{1}Y^{2}B. The
+_Lawrencei_ which has the underparts _yellow_, wing-bars _white_, and
+_black_ patches is Y^{1}y^{2}B and _leucobronchialis_ which has mantle
+and underparts _not-yellow_, wing-bars _yellow_ and _no black patches_
+is y^{1}Y^{2}b. This representation, it should be clearly understood,
+is tentative and approximate only. The characters are not really sharp,
+for there is much grading; but allowing for the effects of heterozygosis
+and for some actual breaking-up of factors I believe it gives a fairly
+correct view of the case. In particular we can see how it meets the
+difficulty which Chapman felt in accepting _leucobronchialis_ as in any
+sense derived from _pinus_ which has a yellow breast, and _chrysoptera_
+which has a black throat, seeing that _leucobronchialis_ has neither.
+We now recognize at once that this form could be produced by ordinary
+re-combination of the absence of Y^{1} with the absence of B.
+
+I note also with great interest that the modern observers agree that the
+so-called hybrids may have the song either of the one species, or of the
+other, or a song intermediate between the two. It may also be added that
+these two types have several times been seen, in the breeding season,
+paired with each other or with one of the other combinations.
+
+[Illustration: FIG. 1. _Helminthophila pinus_, male.
+
+FIG. 2. _Helminthophila pinus_, female.
+
+FIG. 3. "Lawrence's Warbler," male; one of the integrading forms.
+
+FIG. 4. "Brewster's Warbler," male; another of the integrading forms.
+
+FIG. 5. _Helminthophila chrysoptera_, male.
+
+FIG. 6. _Helminthophila chrysoptera_, female.]
+
+Allen[11] has described another excellent American example, the Tits of
+the group _Baeolophus bicolor-atricristatus_. The form _bicolor_ belongs
+to the eastern States and ranges from the Atlantic coast to the Great
+Plains, and _atricristatus_, of east Mexico, extends from Vera Cruz to
+central Texas. In southern and central Texas the breeding ranges adjoin,
+and in this country various intermediates occur. The chief types differ
+in two main points.
+
+         _B. bicolor_
+
+  Forehead varies from deep _black_
+    to dull black, suffused with
+    rusty brown.
+
+  Crown and crest _grey_,
+    slightly darker than the back.
+
+
+        _B. atricristatus_
+
+  Forehead _white_ to buffish white.
+
+  Crown and crest _black_, abruptly
+    contrasting with the back.
+
+The intergrades between the two have, as usual, received specific names.
+A detailed description is given by Allen, from which it appears that
+the gradation is very complete. In one case a series of 16 adults were
+all intermediates. It is not stated whether the collector took these at
+random, but from the local lists it is clear that the types are found
+not far away from the place where the intergrades were shot.
+
+Another very striking case is that of the Tanagers, of the genus
+_Rhamphocoelus_. In this group there are several local forms which
+are related to each other in remarkable ways. The forms known as
+_passerinii_ and _icteronotus_ exhibit the clearest phenomena of
+intergradation. The species _passerinii_ has a brilliant scarlet
+and black male, and it inhabits Honduras and Nicaragua. Proceeding
+southwards along the isthmus we find next _costaricensis_ which has
+a male like that of _passerinii_ (but a female with more orange than
+the olive-grey female of _passerinii_). Next we come to Panama which
+is occupied by _icteronotus_, sharply distinguished from _passerinii_
+by the fact that the _scarlet is replaced by lemon-yellow_. This same
+_icteronotus_ occurs again as a pure type in Ecuador and many other
+parts of South America; but Colombia, _between Panama and Ecuador_,
+contains scarlets like _passerinii_, yellows like _icteronotus_, and
+various intergrades of several shades of orange. The _passerinii_ males
+from Nicaragua are indistinguishable from those of Colombia, and the
+_icteronotus_ of Ecuador are the same as those in Panama. The orange
+intergrades, doubtless heterozygous forms, though collected at the
+same locality (Medellin in Colombia) as several pure yellows and pure
+scarlets, are in the British Museum series sorted out as a separate
+species under the name _chrysonotus_! Complications are introduced by
+the relations of these forms to another named type, _flammigerus_, but
+we may for our purpose leave that out of consideration, and say that the
+order of geographical sequence from Honduras to Ecuador is (1) scarlet,
+(2) yellow, (3) mixture of types, scarlet, yellow, orange, (4)yellow.
+
+Similar examples exist in the birds of the old world, but I do not know
+of any that have been studied so fully as those of America. The best
+known is that of the two Rollers, _Coracias indicus_ which spreads from
+Asia Minor through Persia, Baluchistan, the Indian Peninsula and Ceylon,
+and _affinis_ which ranges from Nepal, through Assam, Tenasserim and
+the Indo-Chinese countries. The two types are very different and may be
+distinguished as follows:
+
+        _C. indicus_
+
+  _Mantle_ drab brown-chestnut.
+  _Breast_ chestnut.
+  _Throat_ purplish, streaked with white.
+  _Upper tail-coverts_ indigo.
+
+
+        _C. affinis_
+
+  Dark olive-green.
+  Dull purple brown.
+  Purple, streaked with blue.
+  Turquoise.
+
+The wings are the same in both. In the provinces of Nepal, Sikhim, and
+Darjiling the two species coexist, with the result that intergrades
+have been frequently recorded. The line of intergradation extends to
+the coast, and birds showing various combinations of the two types from
+the Calcutta district exist in collections.[12] The case is interesting
+inasmuch as like that of _Quiscalus_ it shows a series of combinations
+of various metallic colours. Some of these are probably evoked by
+the development of pigment behind striations or other interferences
+already existing, but in the present state of knowledge it would be
+quite impossible to suggest what the actual factors producing these
+appearances may be.
+
+There are, naturally, many other cases among birds which are suspected
+of being in reality comparable, but in most of them the evidence is
+still inadequate. Among Lepidoptera also there are a few of these;
+perhaps the most striking is that of _Basilarchia "proserpina."_[13] The
+genus is well known to European collectors under the name _Limenitis_,
+of which we in England have one species, _L. sibylla_, the "White
+Admiral." A species very like _sibylla_ in general appearance is common
+in the northern parts of the United States, ranging through Canada and
+Northern New England, but rarely south of Boston. This species has the
+conspicuous white bands across both wings like our _sibylla_.
+
+There is also a more Southern type known as _astyanax_, which is very
+different in its appearance, being without the white bands and having
+a broad irroration of blue scales on the posterior border of the hind
+wings. The two are so distinct that one would not be tempted to suspect
+any very close relation between them. In its distribution _astyanax_ is
+described by Field as replacing arthemis south of latitude 42 deg.. About
+Boston it is much more common than _arthemis_.
+
+The two forms encroach but little on each other's territory, but where
+they do coexist, a third form, known as _proserpina_, is found which is
+almost intermediate, with the white bands much reduced. There is now
+no doubt that this _proserpina_ is a heterozygous form, resulting from
+a combination of the characters of _arthemis_ and _astyanax_. Field
+succeeded in rearing a brood of 16 from a _proserpina_ mother caught
+wild which laid 31 eggs, and of these, nine (five males, four females)
+resembled the mother, being _proserpina_, and seven (four males, three
+females) were _arthemis_. There can be no question therefore that the
+mother had been fertilised by a male _arthemis_ and that _no-white-band_
+is a factor partially dominant over the _white band_. Another point of
+interest which Field observed was that the _proserpina_ female refused
+to lay on birch, poplar or willow, but accepted wild cherry (_Prunus
+serotina_) a species on which _astyanax_ can live, though that tree is
+not known to be eaten by _arthemis_. Incidentally also the observations
+show that sterility cannot be supposed to be the bar which maintains the
+distinctness of _arthemis_ and _astyanax_.
+
+In this connection _Papilio oregonia_ and _bairdii_ should be
+mentioned.[14] _P. oregonia_ is one of the numerous forms like
+_machaon_, but rather paler. It is a northern insect, inhabiting British
+Colombia east of the Cascade Range, and reaching to Colorado. _P.
+bairdii_ is a much darker butterfly, representing the _asterias_ group
+of the genus _Papilio_. Like _asterias_ it has the abdomen spotted at
+the sides, not banded as in the _machaon_ group. It belongs to Arizona
+and Utah extending into Colorado. From Colorado the form _brucei_ is
+described, more or less intermediate, like _bairdii_ but with the
+abdomen banded as in _oregonia_. W. H. Edwards records the results of
+rearing the offspring of the _bairdii_-like and of the _oregonia_-like
+mothers. Each was found able to have offspring of both kinds, that is to
+say, _bairdii_ females gave both forms, and _oregonia_ females gave both
+forms. It is not possible to say which is dominant, since the fathers
+were unknown. On general grounds one may expect that the _bairdii_ form
+will be found to dominate, but this is quite doubtful.
+
+From this particular discussion I omit reference to those examples in
+which the permanently established types are obviously associated with
+special conditions of life. Where considerable climatic differences
+exist between localities, or when we pass from South to North, or from
+the plains into Alpine levels we often find that in correspondence
+with the change of climate there is a change in the characteristics of
+a species common to both. When I say "species" in such a connection I
+am obviously using the term in the inclusive sense. Some would prefer
+to say that in the two sets of conditions two _representative species_
+exist. Whichever expression be preferred it is plain that such examples
+present another phase of the problem we have been just considering, and
+in them also we have an opportunity of observing the consequences of
+the overlap of two closely related types, but there are advantages in
+considering them separately. In the examples hitherto given, with the
+possible exception of the Papilios,[15] the two fixed types severally
+range over so extensive a region that it may fairly be supposed that
+in the different parts they are subject to considerable diversities of
+climate. There is no outstanding difference that we know distinguishing
+the habitats of the two forms; but in comparing Alpine with Lowland
+forms, or essentially northern with essentially southern forms we do
+know an external circumstance, temperature, that may reasonably be
+supposed to have an influence, direct or indirect, on the population.
+
+FOOTNOTES:
+
+[1] J. A. Allen, _The North American Species of the Genus Colaptes,
+Considered with Special Reference to the Relationships of C. auratus and
+C. cafer_. Bull. Am. Mus. Nat. Hist., IV, 1892.
+
+[2] For a case in which a red-headed female x a black-headed male gave a
+black-headed female and a red-headed male, see _Avian Mag._, N. S., IV,
+pp. 49 and 329
+
+[3] The other variations of this bird are also interesting and
+important. The normal male has a red head and a red throat. The female
+has a red head and a white throat, but varieties of the female are known
+with a black head, thus again illustrating the change from black to
+red. It should be noted that this is not a mere retention of a juvenile
+character, but, as the birds mature, the red feathers come up, or as an
+exception, the black. There is also a western species, _ruber_, in which
+both sexes have a great extension of red, and are alike. The male of
+_nuchalis_ intergrades with this type, but the female does not.
+
+[4] Dr. W. Brewster, for example, has a remarkable specimen of the Teal
+(_Nettion carolinense_) with a white collar strongly developed at the
+front and sides of the neck, in a place where the normal has no such
+mark.
+
+[5] This variety is spoken of as the Ringed Guillemot and is sometimes
+regarded as a distinct species to which the name _ringvia_ was given by
+Bruennich. In support of this view Dr. William Brewster, to whom I am
+indebted for much assistance in regard to the variation of birds, called
+my attention to observations of his own and also of Maynard's, that the
+ringed birds were sometimes mated together, though in a small minority
+(see Brewster, _Proc. Boston Soc. N. H._, XXII, 1883, p. 410). It
+would however be possible to produce many instances of varieties mated
+together though surrounded by a typical population (_e. g._, two varying
+Blackbirds, _Zoologist_, p. 2765; two varying Nightjars, _ibid._, p.
+5278). I am inclined to believe that in nature matings between brothers
+and sisters are frequent in many species of animals, and that the
+production of sporadically varying colonies is thus greatly assisted.
+
+[6] The Sap-suckers feed on trees and somewhat resemble our Spotted
+Woodpeckers in general appearance. _Colaptes_ feeds on the ground and
+corresponds perhaps rather with the European Green Woodpecker.
+
+[7] For an introduction to this example I am indebted to Mr. W. D.
+Miller of the American Museum of Natural History. Some account of the
+facts is given by Baird, Brewer, and Ridgway (_A Hist. of N. Amer.
+Birds_. 1874, II, pp. 540, 544, etc.). _S. varius_ occupies the whole
+country in suitable places from the Atlantic to the eastern slopes of
+the Rockies, and all Mexico to Guatemala. _S. nuchalis_ was first known
+from the Southern Rockies only, but many were afterwards taken in Utah.
+_S. ruber_ is restricted to the Pacific coast. In Ridgway's opinion all
+three are geographical forms of one species. In _ruber_ the sexes are
+alike having both a great extension of the red in the throat, and a red
+crescent. The male of _nuchalis_ grades to the _ruber_ form, but the
+female does not. This female has some red in the throat like the male of
+_varius_, whereas the female of _varius_ has a whitish throat.
+
+[8] Not only vertebrates but the marine Crustacea and Mollusca
+illustrate this curious "principle" of variation, as Canon Norman
+formerly pointed out to me with abundant illustrations. There are of
+course cases to the contrary also.
+
+[9] Chapman, F. M., _Bull. Amer. Mus._, IV, 1892, p. 1; see also
+Ridgway, _Birds of North and Middle America_, 1902, Part II, p. 214.
+
+[10] It would aid greatly in factorial analysis if the descriptive term
+"green" could be avoided in application to cases where the green effect
+is due only to a mixture of black and yellow pigments. The absence of
+yellow is the sole difference between the mantle and underparts of
+_pinus_ and _chrysoptera_.
+
+[11] _Bull. Amer. Mus. Nat. Hist._, XXIII, 1907, p. 467.
+
+[12] References on this subject will be found in _Brit. Mus. Cat.
+Birds_, XVII, p. 13.
+
+[13] For these facts I am indebted to Mr. W. L. W. Field, who has
+lately published an account of his observations and experiments. See
+especially, _Psyche_, 1910, XVII, No. 3, where full references to
+previous publications are given.
+
+[14] For the facts and further references see W. H. Edwards,
+_Butterflies of N. America_, 2d series, Papilio VII and X; 3d series,
+1897, Papilio IV, _Can. Entom._, 1895, XXVII, p. 239.
+
+[15] I think this case is fairly included because the _machaon_ type
+is so widespread that it cannot be regarded as a product of a Northern
+climate, nor can _asterias_ be claimed as especially a warm country
+form, seeing that _brevicauda_, which is scarcely distinguishable from
+_asterias_, inhabits Newfoundland (having a curious phase there in which
+the yellow is largely replaced by red).
+
+
+
+
+                        CHAPTER VIII
+
+              LOCALLY DIFFERENTIATED FORMS. _Continued._
+
+
+                    CLIMATIC VARIETIES
+
+In this chapter we will examine certain cases which illustrate phenomena
+comparable with those just considered, though as I have already
+indicated, they form to some extent a special group. The outstanding
+fact that emerges prominently from the study of the local forms is that
+when two definite types, nearly allied, and capable of interbreeding
+with production of fertile offspring, meet together in the region
+where their distributions overlap, though intergrades are habitually
+found, there is no normally or uniformly intermediate population
+occupying the area of intergradation. Such phenomena as these must, I
+think, be admitted to have great weight in any attempt to construct a
+theory of evolution. True we must hesitate in asserting their positive
+significance, but I see no escape from the conclusion that they throw
+grave doubt on conventional views. Again and again the same question
+presents itself. If _A_ and _B_ lately emerged from a common form why is
+that common form so utterly lost that it does not even maintain itself
+in the region of overlapping? Almost equally difficult is it, in the
+cases which I have numerated, to apply concrete suggestions based on
+any factorial scheme. We may see that in _Heliconius erato_ the type
+with the red mark on the hind wing probably contains a dominant factor,
+and that where the red mark is absent the metallic colours are exposed;
+and that similarly the green metallic colour may have another factor
+which distinguishes it from the blue. In this way we can fairly easily
+represent the various types of _erato_ on a factorial system as the
+result of the various possible combinations of two pairs of factors. But
+there we stop, and we are quite unable to suggest any reason why one
+area should have the red and the green type while another should have
+the blue also. So again with _Colaptes_ or the Warblers. By application
+of a factorial system, admittedly in a somewhat lax fashion, the
+genetic interrelations of the types can be represented; but how it comes
+about that each type maintains a high degree of integrity in its own
+region we can only imagine. Each has in actual fact a stability which
+the intermediate forms have not, but we cannot yet analyse the nature
+of that stability. Mendelian conceptions show us how by segregation the
+integrity of the factors can be in some degree maintained, but not why
+certain combinations of factors should be exceptionally stable. All that
+is left us to fall back on is the old unsatisfying suggestions that some
+combinations _may_ have greater viability than others, that there _may_
+be a tendency for like to mate with like, and so forth.
+
+These difficulties acquire more than ordinary force in those cases in
+which the two fixed types inhabit regions differing in some respect
+so obvious and definite that we are compelled to regard each type as
+climatic and as specially adapted to the conditions. When for example
+an animal has a distinct type never met with except in Arctic or Alpine
+conditions, and another type proper to the plains and temperate regions,
+what are the characteristics of the population of intermediate latitudes
+or at intermediate levels? Some of the examples discussed in the last
+chapter may be instances of this very nature, but even if they are not,
+others are forthcoming which certainly are. The evidence of these cases
+leads to the suspicion that with further knowledge they will be found
+to consist of two classes, some in which the observer as he passes from
+the one climate to the other will find the intermediate area actually
+occupied by a population of intermediate character, and others in which,
+though we may presume the maintenance of intermediate conditions in
+the transitional area, there is no definite transitional population.
+This interrupted or discontinuous distribution seems, so far as I have
+means of judging, to be by far the more common of the two. I do not
+doubt that by sufficient search individuals representing every or almost
+every transitional form can be found, but it is apparently rare that
+_populations_ corresponding to these several grades can be seen. The
+question has in few if any cases been studied with precision sufficient
+to provide a positive answer; but I suspect that real and complete
+continuity, in the sense thus defined, will only be found where the
+character of the local populations depends _directly_ on the conditions
+of life, and shows an immediate response to changes in them apart from
+that postponed response which we suppose to be achieved by selection.
+Obviously the character must be one, like size for instance, capable of
+sensibly complete gradation.
+
+The only example I have met with of the phenomenon of anything like a
+complete intergradation between local types really distinct in kind is
+that provided by the butterfly _Pararge egeria_. It is well known to
+entomologists that this insect exists in two very different types, a
+northern one, the "Speckled Wood" of England, in which the spots are
+a pale whitish yellow, and a southern type having the full fulvous
+colour that we know as characteristic of _megaera_, the "Gatekeeper."
+It appears that Linnaeus gave the name _egeria_ to the southern
+type,[1] and our own is now called _egerides_. Broadly speaking, so
+far as Great Britain, France, and the Spanish Peninsula are concerned,
+the tawny-coloured _egeria_ occupies Spain and western France up to
+the latitude of Poitiers and the pale yellow _egerides_ extends from
+Scotland, where it has a scanty distribution, through southern England,
+where in suitable localities it is common, and the north of France
+to Paris.[2] The two types when placed side by side are strikingly
+different from each other, and are an excellent illustration of what is
+meant by climatic variation. The insect is not a great traveller and
+probably scarcely ever wanders far from its home. It should therefore
+be possible by collecting from north to south to find out how the
+transition is effected, whether suddenly or gradually. This at various
+times I have endeavoured to do, but I am still without exact information
+as to the population in certain critical areas. In addition to the
+information derived from specimens which I have collected or seen
+in the collections of others there is a good account of the general
+distribution in Europe given by the Speyers,[3] who evidently paid
+more attention to the subject than most lepidopterists have done, and
+many more recent records. In particular Oberthuer[4] has published many
+details as to the distribution in western France and I am especially
+indebted to Mr. H. Rowland-Brown for a long series of notes as to the
+distribution in France generally, and to Mr. H. E. Page and Dr. T. A.
+Chapman, Mr. Oberthuer Prof. Arrigoni degli Oddi, Mr. H. Williams and
+other correspondents, for showing me forms from many localities. The
+butterfly is attached for the most part to woods of deciduous trees and
+to country abounding in tall hedges or rough scrub. It is not usually to
+be found in highly cultivated districts or in very dry regions. Hence
+there is necessarily some want of continuity in the distribution at the
+present time and I should think a mile or two of arable land without big
+hedges would constitute a barrier hardly ever passed. The larva feeds
+on several coarse grasses, especially _Dactylis glomerata_. Barrett
+mentions also _Triticum repens_. In this country the winter is usually
+passed in the larval stage, but I have found that in captivity, at
+least, there is much irregularity. The larvae feed whenever the weather
+is not very cold and may pupate, but if sharp cold comes on when they
+are pupating or nearly full-grown they often get killed unless protected.
+
+Some writers speak of a difference between the early and later broods,
+but I have never noticed this, and I do not think that the general tone
+of the yellow is affected by the seasons (see Tutt, _Ent. Rec._, IX,
+1897, p. 37).[5]
+
+Beginning at the south of Spain the thoroughly fulvous type _egeria_
+is common at Gibraltar in the Cork woods, at Granada, and doubtless
+generally. Lederer is said to have found only this type in Spain
+(Speyer), and though I have no precise information as to other places
+in the Peninsula north of Jaen I feel tolerably sure that there is no
+change from south to north.[6] Immediately north of the Pyrenees we
+still meet _egeria_ exclusively, and up to Poitiers at least there is no
+noticeable change. But somewhere between Poitiers and the bottom of the
+Loire valley at Tours, the genuine southern type comes to an end, and
+the whole population begins at the Loire to be of an intermediate type,
+easy to distinguish both from _egeria_ and from _egerides_. As to the
+exact condition of the species in the fifty miles separating St. Savin
+on the Vienne from places on the Loire I have no adequate information. I
+have only one small sample from there, but it does contain insects both
+of the southern and intermediate types taken on the same day, in a wood
+near Preuilly. Oberthuer also states that at Nantes the true southern
+form exists in company with the northern. From this I infer that the
+southern form extends up the coast further than it does inland, but
+I imagine the representative spoken of as northern would be of usual
+Brittany or intermediate type.
+
+The Vienne river joins the Loire, so the true southern type reaches over
+into the basin of the Loire. From the Loire (Tours, Cormery) north to
+Calvados (Balleroy) only the intermediate is found, so far as I know,
+and the same type extends over Brittany.[7] In general, however, the
+woods near Paris have the thoroughly northern type _egerides_, but at
+St. Germain-en-Laye and at Etampes (Oberthuer) the population approaches
+the intermediate type.
+
+On the whole the intermediate type is certainly less homogeneous than
+either of the extremes, and females with the two central spots either
+paler or more fulvous than the rest are not uncommon, but I have never
+taken one on the Loire or in Brittany which I should class with either
+of the extreme types.
+
+Before speaking of the distribution in other parts of France and in
+Europe generally I will briefly state the results of my breeding
+experiments. The work was done many years ago before we had the
+Mendelian clue, and it is greatly to be hoped that some one will
+find opportunities of repeating it. Crossing the English and the
+thoroughly southern type the families produced agree entirely with
+the intermediates of Brittany and the Loire. Reciprocals are alike.
+Of F_{2} I only succeeded in raising very few and of those that I had
+(about 30) nearly all were intermediate in character, though perhaps
+rather less uniform than F_{1}. One family alone, containing only 4
+specimens, had one _egerides_, and three fulvous intermediates. As the
+case stands alone I hesitate whether or not to suppose it due to some
+mistake. Moreover from F_{1} crossed back with the respective parental
+types I had fairly long series, especially from F_{1} x the southern
+type, and looking at these families I cannot see any clear evidence
+of segregation. On the contrary, I think that though there are slight
+irregularities, they would, taken as a whole, be classed as coming
+between the intermediate type and the extreme form used as the second
+parent. This at least is true when the second parent was of the southern
+type.
+
+On this evidence I have regarded the case as one in which there is
+no good evidence of segregation and as conforming most nearly with
+the conventional view of gradual transition in response to climatic
+influences. Such influence must however be indirect; for I reared five
+generations of the northern type in England, and these, though they
+included several abnormal-looking specimens in the last generation and
+then died out, did not show any noticeable change from the fulvous
+colour of the wild type. Merrifield[8] also found that heat applied to
+pupae of the northern type produced no approach to the southern type.
+
+Looking at the facts now in the light of more experience it seems to me
+just possible that the case may be one in which, as in Nilson-Ehle's
+Wheats, the dominant differs from the recessive in having two pairs of
+factors with similar effects. The fulvous type for example may have
+two or more elements in separate pairs which together produce the full
+effect, and the intermediate may have one of these. If this were so,
+some segregation should of course eventually be observable, but the
+proportion of the various fulvous and fulvous-intermediate individuals
+would be large, and the reappearance of actual representatives of the
+northern type might be rare. I admit that this is a somewhat strained
+interpretation of the facts, and as yet it is not entitled to serious
+consideration. Nevertheless I am led to form some such expectation
+partly from the great difficulty in the way of any other, partly from
+the evidence of the small mixed sample found at Preuilly and partly from
+the statements given by Oberthuer. There are moreover other features in
+the general distribution of the species which make it improbable that
+the dependence on climate can after all be so close. Published lists
+are unfortunately of little use in deciding which form occurs at a
+particular place, because, since the name _Meone_ has ceased to be used
+for the southern form, there is no complete unanimity among authors as
+to the application of the names _egeria_ and _egerides_, and unless
+more particulars are given, either name may be used for either form.
+Besides this, difficulty arises from the fact that the intermediate
+type is not generally distinguished at all, and English collectors
+finding it, may easily record it as the southern type. From Staudinger's
+note on the distribution, I gather that he, on the contrary, reckoned
+the intermediate with the northern type, as do the Speyers also. The
+late Mr. J. W. Tutt was careful to distinguish the three forms and has
+left several useful records. Easy therefore as it might seem to be to
+make out the distribution of such a familiar insect in its various
+modifications, there are serious practical difficulties, and until
+long series are brought together with this special object in view many
+obscurities will remain.
+
+With only the series from England, the west of France, and Spain before
+one it would be easy to regard the successive series of tones as a fair
+measure of climate; the brighter the colour, the hotter might one expect
+the locality to be. Such rough correspondence is often to be observed
+in butterflies and birds. It becomes impossible to take these simple
+views in the light of more complete knowledge. Beginning with France
+the fulvous _egeria_ occupies the lower valley of the Rhone, probably
+from well above Lyon, though I have no exact information respecting the
+country above Avignon. According to Speyer it also takes the department
+of Lozere. The same authority says that Puy-de-Dome has "_egeria_,"
+meaning perhaps the intermediate form, with the fulvous form much less
+commonly. Next comes the curious fact that though the Lower Rhone
+(Avignon, Tarascon, Nimes) has the true fulvous form, Hyeres, Cannes,
+Grasse, Nice, Digne, and Alassio have _the intermediate_. Savoy has the
+intermediate (Chambery) and even _egerides_ perhaps, though in the same
+latitude on the west of France there is nothing but the fulvous type.
+At Chalseul and Besancon (Doubs) the ordinary northern type is found.
+Switzerland generally, I believe, has the northern type, but Staudinger
+gives _egeria_ for Valais and the intermediate occurs in Vaud.[9] The
+south side of the Alps has probably colonies of the pale _egerides_, and
+of intermediates. Orta, with a very hot summer, has the English type
+(Tutt, _Ent. Rec._, XII, 1900, p. 328). Locarno has the intermediate
+(_ibid._, XV, 1903, p. 321). North Italy in general and western
+Piedmont have the intermediate; but further south _egeria_ begins,
+at what region I do not know. Speyer gives on his own authority the
+remarkable statement that at Florence both extremes occur, but chiefly
+intermediates between the two. Mr. R. Verity however kindly informs
+me that in his experience this is not so, and that neither the real
+southern type nor the northern occur there. Sardinia, Sicily, Crete all
+have the southern type. Greece probably has various types. Staudinger
+(_Hor. Ross._, VII, 1870, p. 78) says intermediates resembling Nice
+types common everywhere, but from "Greece" the British Museum has a
+series that would pass for English specimens; and the same type occurs
+near Constantinople. The island of Corfu has a pale intermediate,
+distinct from _egerides_ but approaching it. In Roumania all three
+forms are recorded from various places: _egeria_ in the Dobrutscha; not
+quite typical (presumably an intermediate) at Bukharest; intermediate
+in various mountainous localities as well as in Macedonia and Dalmatia;
+but _egerides_ in Azuga at about 3,000 feet.[10] Hungary has the true
+_egerides_ also. (Cf. Caradja, _Deut. Ent. Zt._, IX, p. 58.) Mathew
+records the same from Gallipoli (_E. M. M._, 1881, p. 95). Staudinger
+does not distinguish the intermediates from the northern, but he gives
+"_egerides_" for Armenia and Fergana (Central Asia). As against the mere
+proximity of a great mountain chain being the influence which keeps
+the Riviera population intermediate may be mentioned the fact that the
+northern foothills of the Pyrenees have the pure southern type, and the
+climate of Cambo must surely be far cooler than that of Nice. The exact
+locality of the Greek specimens is not given, but there can be no part
+of Greece which is not much hotter in summer than Brittany, or Calvados,
+which have the intermediate, not the English type.
+
+In face of these facts it can scarcely be maintained that average
+temperature is the efficient cause of the particular tone of colour
+which the butterfly shows in a given region. Nevertheless it is clear
+that climate counts for much in determining the distribution. It is
+noticeable that though the pale _egerides_ can be established in a
+warm climate we never find _egeria_ in cold climates, and even the
+intermediate is not found in places that have a hard winter. I suspect
+that the distribution of the broods through the year and the condition
+of the animal at the onset of hard frost are features which really
+determine whether a strain can live in a particular place or not.
+Though the truth of the suggestion cannot be tested by experiments in
+captivity, which at once introduce disturbances, I incline to the idea
+that _egeria_ has not got the right periodicity for northern climates.
+If it could arrange its life so that the population consisted either
+of young larvae, or perhaps of thoroughly formed pupae[11] at the
+onset of winter, it might, for any obvious reason to the contrary,
+be able to live in England. It is irregularly "polyvoltine," as the
+silk-worm breeders say, and as soon as a little warmth encourages it,
+a new generation starts into being, which if the frost comes at an
+untimely moment, is immediately destroyed. Many species are continually
+throwing off individuals which feed up fast[12] and emerge at once if
+the temperature permits, and I imagine a species of Satyrid wholly
+or largely represented by such individuals could scarcely survive in
+a country which had a hard winter. For such a climate some definite
+periodicity in the appearance of the broods may well be indispensable.
+But assuming that _egeria_ is cut off from cold climates for such a
+reason, there is nothing yet to connect these habits with the fulvous
+colour, and until breeding can be carried out on a satisfactory scale
+there is no more to be said.
+
+From time to time records appear of individual specimens more or
+less fulvous being caught in southern England, especially in the
+New Forest.[13] It would be interesting to know what offspring such
+individuals might produce. From the evidence now given some notion
+both of the strength and the weakness of the case considered as one of
+continuous climatic variation can be formed. I know no other equally
+satisfactory. Whether or not definite mixture of the intermediates
+with either of the extremes will be proved to occur, the case differs
+materially from those considered in the last chapter in the fact that
+at all events there is no general overlapping of forms. In a species so
+little given to wandering, overlapping could indeed scarcely be expected
+to occur. It is this circumstance which makes the species preeminently
+suitable as a subject for the study of climatic influences, and I trust
+that entomologists with the right opportunities may be disposed to
+explore the facts further.
+
+Just as many species, like _egeria_, have varieties which can be
+regarded as adapted to northern and southern regions, so there are
+also several which have lowland and Alpine forms quite distinct from
+each other. Every such case presents an example of the problem we
+have been considering. As the collector passes from the plains to the
+Alpine region, how will he find the transition from one form to the
+other effected? Does the lowland form give place to the Alpine form
+suddenly, with a region in which the two are mixed, or will he find a
+zone inhabited by an intermediate population? I have spent a good deal
+of time examining the facts in the case of _Pieris napi_ and its Alpine
+female variety _bryoniae_, and though there are many complications which
+still have to be cleared up, no doubt is possible as to the main lines
+of the answer. If in any valley in the Alps inhabited by both _napi_ and
+_bryoniae_ the collector catches every specimen he can, beginning at
+the bottom and working up to 7,000 feet, he will at first get nothing
+but _napi_. At about 2,500 feet, he may catch an occasional _bryoniae_
+flying with the _napi_. After 3,000 feet _napi_ usually ceases, and
+only _bryoniae_ are found. As an exception a colony of _napi_ may be
+met with at much greater heights. I once found them in numbers at about
+6,000 feet.[14] Not only were they free from any trace of modification
+in the direction of _bryoniae_, but they were of the thoroughly southern
+type of _napi_, being a late brood of that large and very pale kind
+(_meridionalis_) almost destitute both of dark veining above and of
+green veining below, which are common on the shores of Lago Maggiore
+and in other hot southern localities. Not far off at the same level
+were typical _bryoniae_ in fair abundance. Occasionally an intermediate
+may be met with. I have taken a few, for example, at Macugnaga and at
+Fobello. These, however, in my experience are rarities in the Alps.
+Fleck[15] gives notes on the distribution in Roumania which shows the
+same state of things. The lowland form is not transformed though found
+at great heights, and at Azuga (nearly 3,000 feet) _bryoniae_ occurs
+with only occasional "_flavescens_," viz., intermediates of the second
+brood.
+
+If this were all the evidence we should be satisfied that the lowland
+and Alpine types keep practically distinct, overlapping occasionally,
+but rarely interbreeding. The problem would remain, how is the
+distinctness of the two types maintained in the region of overlapping?
+Nowadays, I suppose, we should incline to answer this question by
+reference to segregation, and perhaps by an appeal to selective mating.
+The suggestion that segregation does take place is certainly true to
+some extent. There are, however, difficulties in the way, and the whole
+subject is one of great complexity. My own experiments were made in
+pre-Mendelian times and were not arranged with the simplicity which we
+now know to be essential. The results are neither extensive enough nor
+clear enough to settle the many collateral questions which have to be
+considered, and the work ought to be done again. Nevertheless, some
+notes of the observations may have a suggestive value.
+
+When I began, I did not sufficiently appreciate that the "_napi_" group,
+omitting the North American forms, and the Asiatic representatives, has
+at least three chief types in western Europe. The differences we have
+to deal with are manifested by the females only, so in this account
+particulars as to the males are omitted for the most part. These are (1)
+our own British _napi_; (2) the form found in the south, from the Loire
+downwards, and in the Italian Alps, which I think may be spoken of as
+_meridionalis_; (3) _bryoniae_, which is a form clearly recognizable in
+the _female_ only, and is found only in the arctic regions and in the
+Alps above 2,500 feet. The first two have several broods, two, three,
+or more, according to opportunity, and the first brood is different
+from the later ones. In _napi_ the markings on the upper surface are
+a dark grey but in _meridionalis_ they are a pale silvery grey and
+much less extensive. In the later broods of _napi_ there is much less
+general irroration of the veins, and the spots stand out as more defined
+and blacker. These differences vary greatly in degree of emphasis. In
+_meridionalis_ the later broods are entirely different from the first.
+Instead of having silvery markings they have the ground colour quite
+white, with the spots large and a full black. On the under side of the
+hind wings the usual green veins are almost absent, and I have seen
+individuals which could scarcely be distinguished from _rapae_. To these
+later broods the term _napaeae_ is sometimes applied, but I here use
+_meridionalis_ for the southern race in general as applicable to all
+broods.
+
+The female _bryoniae_ is totally unlike the others. The ground colour
+is a full yellow, and each nervure is thickly irrorated with a brown
+pigment often spreading so far as to hide the ground almost entirely
+in the fore-wings. The males corresponding with these females are not
+certainly distinguishable from those of our own _napi_. Both sexes have
+the green veining of the underside of the hind wing fully developed,
+rather more than is usual in the lowland races, but this is not really
+diagnostic of the variety. The first serious difficulty arises in regard
+to the second brood of _bryoniae_. It is stated that there is only one
+brood,[16] but I feel fairly sure that a second brood is sometimes
+produced, and that the females with a yellow ground and diminished
+irroration of the veins, not very uncommon in the Italian Alps in July
+to August, are generally representatives of it. Such insects would of
+course be classed with _bryoniae_ in collections.
+
+My experiments began with eggs of true _bryoniae_ females caught at
+about 2,500 feet early in July. These emerged in August-September as
+intermediates with yellow ground and about half as much black on the
+upper surface as _bryoniae_. They are exactly like the intermediates
+usually found in nature and in the light of later experience I regard
+them as natural F_{1} forms, and I think the mothers had been fertilised
+by _napi_ males, though I admit that in view of the rarity of natural
+intermediates there is a difficulty in this suggestion. Three of these
+females were mated with males raised from thorough _meridionalis_
+females, and three families were produced. Two of them showed distinct
+evidence of segregation, some being yellow and some white with various
+intergrades, some being no blacker than _meridionalis_ and some ranging
+up to a dark intermediate type. Part emerged in the same autumn; and
+part overwintered, emerging as the spring _meridionalis_ or as the
+peculiar type which I afterwards learnt to know as the spring F_{1}
+form. The distinctions were fairly sharp between the several forms. But
+the offspring of the third female gave a series practically continuous
+from _meridionalis_ to the F_{1} type. The work of subsequent years
+gave results similarly irregular which could only be described
+adequately at great length. The outcome may however be summed up in
+the statement that there is evidence that both the yellow ground and
+the dark veining are due to factors, but that there are several of
+these and that imperfect segregation is not uncommon, producing various
+reduction-stages. The yellow ground may be due to one factor, and the
+several shades may be the result of irregularities in dominance, but
+the black markings when fully developed cannot I think be the result of
+less than three factors, one for the basal darkening, one for general
+irroration, and one for the margins. Probably also the enlargement of
+the spots is produced by a fourth factor.
+
+There was not, in my experience any great difficulty in getting the
+various forms to pair in captivity. Some attempts were made to see
+whether individuals of either type selected mates of their own type in
+preference to those of the other, but the results were inconclusive.
+There were some indications of such a preference; though, from
+the impossibility of judging how much of this may be due to other
+circumstances, I could not come to a positive conclusion on the rather
+meagre evidence.
+
+Recently Schima[17] has given a careful and detailed account of all
+the forms found in Lower Austria which he enumerates under 14 distinct
+varietal names. He gives full references to previous accounts,
+especially to the beautiful plates lately published by Roger Verity.[18]
+Examination of these and of my own specimens strongly suggests that
+the several forms are due to the recombination of the factors I have
+named. Among those which I have bred are representatives of most if not
+all the types enumerated by Schima in addition to other curious forms.
+For example I have _bryoniae_ markings on a ground practically white;
+the dark veins with spots almost obsolete; _meridionalis_ on a yellow
+ground; the intermediate amount of black on a white ground, etc. The
+last-named may occur wild and I have one from Macugnaga as well as one
+given me by Mr. F. Gayner from Lulea (Lapmark).
+
+To obtain really exact knowledge of the number of factors and
+their properties it would be necessary to repeat the work. After
+the beginning, I made a mistake in using British _napi_ instead of
+_meridionalis_ and the results were much confused thereby. The contrast
+between _meridionalis_ and the various dark forms is much greater and
+classification of the types would have been therefore easier. The
+British form is presumably _meridionalis_ plus the factor for the basal
+pigmentation. The problem is greatly complicated by the differentiation
+of the seasonal forms. The first point to be determined is whether
+_bryoniae_ is capable of producing a second brood when it is thoroughly
+pure-bred, and whether such a second brood is, as I suspect, normally
+intermediate in character.
+
+In the Alps generally there is no definitely intermediate population;
+nor I believe, is any such population met with in the north where
+the arctic _bryoniae_ meets _napi_, but as to this I have no precise
+information. One curious fact, however, must be mentioned, namely that
+there is a population that can probably be so described with fairness
+established at Moedling near Vienna. This is not in any sense an Alpine
+locality, and does not, as I am told, differ in any obvious way from
+the other suburbs of Vienna. Dr. H. Przibram was so good as to send me
+a set taken at this place, representing a second brood, and they were
+decidedly heterogeneous, ranging from an intermediate form such as
+_bryoniae_ fertilised by _napi_ usually produces, to a light yellowish
+second-brood type with little dark pigment. There are also two actual
+_bryoniae_. Whether true _napi_ also occur there I do not know, but I
+have no doubt they do. It would be well worth while to investigate the
+Moedling population statistically, and to breed from the intermediates
+which might not impossibly prove to be heterozygotes. There are also
+records of such intermediates being occasionally found in some parts
+of Ireland, in the north of Scotland, and in south Wales,[19] but I do
+not know of any regular colony of these forms. We can scarcely avoid
+the inference that one or more of the factors which make up _bryoniae_
+may be carried by these intermediates. It is not clear why their
+interbreeding does not produce actual _bryoniae_ occasionally. If this
+occurred, the probability is that the fact would be known to collectors,
+at least in the British localities. The absence of true _bryoniae_ must,
+I think, be taken to mean that some essential factor is absent from
+these intermediates.
+
+To sum up the evidence, the facts that are clear may be thus enumerated:
+
+   1. _Napi_ and _bryoniae_, or in the Italian Alps,
+      _napaeae_ and _bryoniae_ frequently meet each other.
+
+   2. They cross without difficulty, producing fertile offspring.
+
+   3. But in the levels at which they overlap there is no
+      intermediate population, and only occasional intermediate
+      individuals.
+
+   4. In certain parts of the distribution of _napi_ similar
+      intermediates sometimes occur, and at one place (Moedling) they
+      are so frequent as apparently to constitute a colony.
+
+   5. As to the genetic relations of the two forms there is no
+      complete certainty. Indications of segregation have been observed
+      in some cases, but there are several factors concerned and they
+      are liable to some disintegration.
+
+Another form in which I tried to investigate the same problem is
+_Coenonympha arcania_, which has one Alpine form known as _darwiniana_,
+and another, _satyrion_. In calling _satyrion_ a form of _arcania_ I
+follow Staudinger and other authorities, but I have never been quite
+satisfied that it should be so regarded. The differences between
+_arcania_ and _darwiniana_ are essentially differences of degree; _C.
+arcania_ occurs in places where there is cover, and reaches up the
+valleys usually as high as the mixed woods of deciduous trees, which
+is about 2,500 feet. The variety _darwiniana_, on the contrary, is an
+insect of treeless hillsides, and I regard it as a dwarf and possibly
+a stunted form. It would not greatly surprise me to find that with
+the application of good conditions _arcania_ could be raised from
+_darwiniana_ eggs, or that if _arcania_ larvae were starved they might
+give rise to _darwiniana_ butterflies. I have been unsuccessful in
+trying to rear the species, having lost the larvae by disease. Usually
+one does not catch _arcania_ and _darwiniana_ on the same ground, and
+as _Festuca ovina_--a typically hill-side grass--is a common food-plant
+of _darwiniana_ there can be little doubt that _arcania_ feeds on
+some other grass, probably woodland species. Colonies of _arcania_ of
+varying size and brightness are commonly found, and though a sample of
+_arcania_, finely grown, from a warm Italian wood, presents a striking
+contrast with _darwiniana_ from an Alpine pasture, one certainly may get
+samples which fill all the gradations. Generally the sample from a given
+locality is fairly homogeneous.
+
+Of _satyrion_ I have little personal experience. I only twice found
+it, namely at Zinal, and at Hallstatt in Austria, but it occurs at
+Zermatt, Arolla, and in several Swiss localities above 5,000 feet, and
+I understand that it is the typical Alpine form in the Engadine. With
+its darkened colour and reduced size it might well be expected to be a
+still further stunted form of _darwiniana_. Yet I have never found the
+one succeed to the other at the higher levels. If _darwiniana_ appears
+when Alpine conditions are reached in a valley it will be met with
+up to the highest level at which such butterflies live. Tutt was of
+opinion that _satyrion_ is a distinct species.[20] I once, at the top
+of the Vorderrheinthal caught a sample of _darwiniana_ a few of which
+(males) were so dark and had the eye spots so poorly developed that they
+looked like transitions to _satyrion_. Otherwise I never found any such
+transitional forms and they are certainly exceptional. There is further
+a record[21] of _satyrion_ having been taken flying with _arcania_.
+This was near Susa, at about 2,000 feet I infer. Mr. H. E. Page has
+similar specimens from Caud and from St. Anton (Arlberg). The females,
+however, both of mine and of Mr. Page's samples are a pale brown, quite
+unlike the females both of _arcania_ and of the dark Zinal _satyrion_.
+The difficulty thus raised has not I think yet been considered by the
+authorities, and it is possible that the Alpine forms of _arcania_ are
+in reality three, not two.
+
+The evidence taken together suggests, I think, that _darwiniana_ is
+related to _arcania_ much as so many of the Alpine varieties of plants
+are to the well-developed individuals of the lower levels. I do not
+anticipate that factorial differences will be found in these insects,
+and it is by no means impossible that the distinctions between them
+are the direct consequences of altered conditions. The relations of
+_arcania_ to _satyrion_ are more doubtful, and in that case a factorial
+difference may at least be suspected.
+
+The species of the genus _Setina_ have Alpine forms which agree in
+possessing a characteristic extension of the black pigment to form
+radiating junctions between the spots on the wings. Speyer, who
+discussed the interrelations of these forms in detail,[22] lays stress
+on the absence of genuine transitional forms between _aurita_ and the
+variety _ramosa_. Both are mountain insects but _ramosa_ extends to
+levels higher than that at which _aurita_ ceases, which is about 4,000
+feet. The two forms are often found flying together. Speyer says that
+his brother searched diligently for transitional forms at the level of
+overlapping, but found none, so that at least they may be regarded as
+rare. The variety _ramosa_ is not infrequent at much lower levels (_e.
+g._, Chiavenna, 1,020 feet; Reussthal, 1,500 feet) and extends as high
+as the permanent snows. In the British Museum collection, however, I
+have seen several that I should regard as transitional. Speyer perhaps
+would have classed as _ramosa_ all in which the spots of the central
+field were united, and it is by no means unlikely that breeding would
+prove such individuals to be heterozygous.[23]
+
+There can scarcely be a doubt that the distinction between _aurita_ and
+_ramosa_ is factorial, the radiate _ramosa_ probably having the factor
+for striping. In support of this view may be mentioned the observation
+of Boisduval,[24] respecting a gynandromorphous individual, which was
+_aurita_ male on one side, and _ramosa_ female on the other. Speyer
+makes another excellent comment. He points out that the simple notion
+that the radiation is a mere extension of pigmentation consequent on
+the climate of the higher levels, will not fit the facts very easily,
+because the size of the spots varies greatly in _aurita_ itself at any
+level, and lowland specimens may actually have more black confined
+to the spots alone than some _ramosa_ possess on spots and lines
+combined.[25]
+
+The two Salamanders, _S. maculosa_ and its Alpine form _atra_, might
+not improbably furnish evidence bearing on the same problem. The two
+are of course very distinct, not merely in colour (_maculosa_ being
+spotted with yellow or orange while _atra_ is entirely black) but also
+in the mode of reproduction, a feature to which reference will be made
+in the next chapter. I cannot, however, find any evidence as to the
+overlapping of the two forms. _S. atra_ occurs from about 3,000 feet
+or somewhat less, and reaches great elevations in the Eastern Alps,
+but I do not know if the two forms ever occur in the same localities.
+Leydig,[26] Boulenger,[27] and most modern authorities regard the two
+types as distinct species, but they are in any case closely allied, and
+it would be of interest to have exact knowledge of their geographical
+delimitations.
+
+The reader who has considered the cases adduced will appreciate the
+difficulties which must be faced in any attempt to account for the
+facts in a rational way. As always in a problem of Evolution, two
+separate questions have to be answered. First how did the form under
+consideration come into existence, and secondly, how did it succeed
+in maintaining itself so as to become a race? The evidence from the
+local forms, though very far from giving complete answers to either of
+these questions definitely refutes the popular notion that a new race
+comes into existence by transformation of an older race. If a gradual
+mass-transformation of this kind took place we should certainly expect
+that when two types, nearly allied and capable of interbreeding, overlap
+each other in their geographical distribution, a normally intermediate
+population would exist. If each type can maintain itself, and if
+each came into existence by gradual transformation, then there must
+have been an intermediate capable of existing and maintaining itself
+as a population; and if this had ever been, surely in the region of
+overlapping, that intermediate population should continue. Especially
+should such a population be found when the two extreme types are
+adaptational forms and the region of overlap is a region of intermediate
+conditions. But of the examples we have examined there is only one, that
+of _Pararge egeria_ and _egerides_, which can at all be so interpreted,
+and even in that case it is not impossible that more minute observation
+would reveal discontinuity between the extremes and the admittedly
+normal intermediate population. Granting provisionally however that
+this example, as it stands, is consistent with the conventional theory
+of evolution, I know not where we should look for another case equally
+good. When the distinctions are produced by direct influence of
+conditions operating during the lifetime of the individuals, examples of
+intermediate populations occupying the areas of intermediate conditions
+can no doubt be produced. Many turf-like Alpine plants, for instance,
+if protected from exposure and properly nourished can grow as large as
+those of the same species found in the valleys, and in the case of such
+quantitative effects, intermediate conditions can doubtless produce
+intermediate characters.
+
+Even these examples however are not very abundant, and often the
+intermediate locality has not a form intermediate between those of
+the two extreme localities, but some third form distinct from either.
+This is the case for instance in the fauna of brackish waters. We are
+taught to believe that the fresh water fauna was evolved from the
+marine fauna, which it well may have been; but as students of Crustacea
+and Mollusca know familiarly, the brackish water forms are not as a
+rule intermediates between fresh water species and sea species, but
+more usually they are special forms belonging to the brackish waters,
+with the peculiar property that they can tolerate a great range of
+conditions, and live without ostensible variation in waters of most
+various compositions and densities, which very few marine or fresh water
+species are able to do.
+
+Sometimes the distinction between local races, as in _Rhamphocoelus
+passerinii_ and _icteronotus_ may be regarded with confidence as due to
+one simple Mendelian factor possessed by one race and absent from the
+other, but I think, more often, as in _Colaptes_ or in the varieties
+of _Pieris napi_, the existence of several distinct factors is to
+be inferred. As we have seen, the races of _Colaptes_ show almost
+beyond doubt that in different areas at least three distinct factorial
+combinations can be perpetuated as races.
+
+In the distribution of variability we find, I think, some hint as to
+the steps by which the phenomena under consideration have come to their
+present stage, and I am disposed to regard the facts so well attested
+in the case of our own melanic moths as a true indication of the
+process. Following this indication we should regard the change in the
+character of a population as beginning sporadically, by the appearance
+of varying individuals, possibly only one varying individual, in, it may
+be, one place only. As to _why_ a variety should increase in numbers
+we have nothing but mere speculation to offer, and for the present we
+must simply recognise the fact that it may. That such survival and
+replacement may reasonably be taken as an indication that the replacing
+race has some superior power of holding its own I am quite disposed to
+admit. Nevertheless it seems in the highest degree unlikely that the
+outward and perceptible character or characters which we recognise as
+differentiating the race should be the actual features which contribute
+effectively to that result.
+
+In discussions of geographical distribution in relation to problems of
+origin it is generally said that very nearly allied species usually
+occupy distinct areas, while other competent observers state the exact
+contrary. Lately, for example, Dr. R. G. Leavitt[28] has published an
+important collection of evidence upholding the latter proposition, taken
+chiefly from the botanical side, showing how in numerous genera two or
+more closely allied species coexist, frequently without intermediates,
+in the same localities, and may even be thus found in company throughout
+their distribution. The difference of opinion evidently arises from a
+confusion as to the sense in which the term "species" is understood and
+applied. Leavitt, for example, is avowedly following Jordan and, among
+moderns, Sargent, in applying a close analysis, and denoting as species
+all forms which are distinct and breed true. Against this use of the
+term I know no valid objection[29] but it must be obvious that if others
+follow a different practice confusion may result when observations are
+summarised in general statements. We will consider this subject again
+in another place, but here it may be sufficient to say that there can
+scarcely now be a doubt that numbers of these associated species, such
+as Jordan discriminated, represent various combinations of the presence
+and absence of Mendelian factors. This does not in any way weaken the
+argument which Leavitt founds upon the facts, namely, that the observed
+distribution of these forms is consistent with the supposition of an
+evolution largely discontinuous.
+
+On the other hand, those who have come to the opinion that nearly
+allied species generally occupy distinct ground are presumably more
+impressed by the characters differentiating the geographically distinct
+or adaptational races, seeing that genuine intermediates between
+them are less commonly found. Those geographical races may no doubt
+contain various differentiated forms; but when all live together,
+occasional intermediates are usually to be found even in the case of
+characters habitually segregating. These segregating forms Jordan would
+certainly have determined as species, and it must be conceded that no
+physiological definition has yet been drawn which consistently excludes
+them.
+
+FOOTNOTES:
+
+[1] Often referred to by older writers as _Meone_, Esper's name.
+
+[2] There are also two distinct island forms, unlike the European,
+_Xiphia_ of Madeira, and a smaller variety, _Xiphioides_ of Canary. See
+especially, Baker, G. T., _Trans. Ent. Soc. London_, 1891, p. 292.
+
+[3] Speyer, Adolf, and August. _Verbreitung der Schmetterlinge_, 1858,
+I, p. 217.
+
+[4] _Lepid. Comparee_, fsc. III, p. 372.
+
+[5] Mr. Rowland-Brown has called my attention to a statement by Dr.
+Vaillantin (_Petites Nouv. Ent._, II, 235) that in Indre-et-Cher the
+first brood is of the northern type and the second of the southern. My
+experience is that in captivity these distinctions do not occur, and I
+have true _egeria_ as first brood from Vienne and as the late brood from
+the Landes. I never collected in Indre-et-Cher.
+
+[6] I have since seen true _egeria_ from Ferrol in the extreme
+northwest, which was in Mr. Tutt's collection.
+
+[7] Mr. G. Wheeler kindly showed me a series identical with this type,
+from Guernsey, and others from near Laon.
+
+[8] _Ent. Rec._, V, 1894, p. 134.
+
+[9] Mr. Wheeler has some pale but rather worn specimens from the Rhone
+Valley at Vernayaz.
+
+[10] See Fleck, E., Die Macrolep. Rumaeniens, _Bul. Soc. Sciinte_, VIII,
+1899, p. 720.
+
+[11] My experience agrees with that of Mr. H. Williams (_Ent. Rec._,
+VIII, 1896, p. 181) that pupae, well-formed, can stand considerable
+frost; but I used to find that half-grown larvae usually died if
+unprotected, and I believe that larvae which attempted to pupate in warm
+autumn weather and then got caught by frosts, always died. Small larvae
+which can creep into shelter at the bottom of the plants survived, and I
+expect that in the north the winter is usually passed in that state (see
+also Merrifield, F., _Ent. Rec._, VIII, 1896, p. 168, and Carpenter, J.
+H., _ibid._).
+
+[12] Some most unlikely species do this. I once had a larva of
+_Parnassius delius_, found at about 5,500 feet, which emerged late
+in the autumn (in October I believe), a season at which it must have
+perished in its own country.
+
+[13] See, for examples, Barrett, G. C., _Lepidoptera of the Brit.
+Islands_, I, 1893, p. 229; also Grover, W., _Ent. Rec._, IX, 1897, p.
+314; Williams, H., _Proc. Ent. Soc._, 1898, who reared several specimens
+from the New Forest which would pass for Bretons, though the rest of the
+family were true _egerides_.
+
+[14] Above the Tosa falls.
+
+[15] _Bul. Soc. Sciinte_, VIII, 1899, p. 691.
+
+[16] The fact that Weismann by heating pupae obtained only one autumn
+specimen seems to me to show rather that a second brood can be produced
+than that it cannot, which is the inference usually drawn.
+
+[17] Schima, K., _Verh. Zool. bot. Ges. Wien_, LX, 1910, p. 268.
+
+[18] _Rhopalocera Palaearctica_, Florence, 1905-11, especially Pl. XXXII.
+
+[19] See figures in Barrett, G. C., _Lepidoptera of Brit. Islands_, I,
+pt. 3, p. 25.
+
+[20] Tutt, J. W., _Ent. Rec._, XVIII, 1905, p. 5. In the same place he
+states that on the Mendel Pass _arcania_ "runs into" _darwiniana_ and
+that in the Tyrolean localities the transition is especially evident.
+Wheeler (_ibid._, XIII, 1901, p. 121) expresses the contrary opinion,
+that _satyrion_ does grade to _arcania_.
+
+[21] H. Rowland-Brown, _Ent. Rec._, XI, 1899, p. 293.
+
+[22] Speyer, Stettiner, _Ent. Ztg._, XXXI, 1870, p. 63.
+
+[23] In regard to the closely analogous case of _Spilosoma lubricipeda_,
+Standfuss makes a similar statement. He bred the type on a large scale
+with the radiate form which he calls _intermedia_, and says that in four
+years of miscellaneous crossing he never obtained really transitional
+forms. Nevertheless after examining large series, especially those of
+Mr. W. H. B. Fletcher, I came to the conclusion that several might be
+so classed, but I am quite prepared to find that such specimens are
+heterozygous. (See Standfuss, _Handb. d. Gross-Schmet._, 1896, p. 307.)
+It is by no means unlikely that various dark forms of _lubricipeda_
+correspond with a progressive series of factorial additions. Many of
+the stages have been named, and of these the most definite are the
+_intermedia_ of Standfuss (probably = _eboraci_ of Tugwell) and the very
+dark _Zatima_ of Heligoland, in which only the thorax, the nervures
+and a small field in the fore-wings remain yellow. A form was bred
+by Deschange from _Zatima_ in which even the field in the forewing
+is obliterated. The exact circumstances in which _Zatima_ occurs in
+Heligoland would be worthy of special investigation, for the normal
+_lubricipeda_ is also found on the island. For references as to the
+British occurrences see especially, Hewett, W., _Naturalist_, 1894, p.
+353. As to _Zatima_ see especially Krancher, _Soc. Ent._, II, 1887-8, p.
+26. I am indebted to Dr. Hartlaub for information as to the Heligoland
+types.
+
+[24] Boisduval, _Bull. Soc. Ent. Fr._, III, 1834, p. 5.
+
+[25] The systematics of _Setina_ have been much controverted, but no one
+I believe doubts that _aurita_ and _ramosa_ are forms of one species.
+See also Chapman, A. T., _Ent. Rec._, XIII, 1901, p. 139.
+
+[26] _Arch. Naturg._, 33, 1867, p. 116.
+
+[27] _Brit. Mus. Cat., Batrachia Gradientia_, 1882.
+
+[28] The Geographical Distribution of nearly related Species. _Amer.
+Nat._, XLI. 1907, p. 207.
+
+[29] See later, p. 242.
+
+
+
+
+                        CHAPTER IX
+
+         THE EFFECTS OF CHANGED CONDITIONS: ADAPTATION
+
+In the attempt to conceive a process by which Evolution may have come
+about, the first phenomenon to be recognized and accounted for is
+specific difference. With that recognition the outline of the problem is
+defined. The second prerogative fact is adaptation. Forms of life are
+_on the whole_ divided into species, and these species _on the whole_
+are adapted and fit the places in which they live. To many students of
+Evolution, adaptation has proved so much more interesting and impressive
+than specific diversity that they have preferred it to the first place
+in their considerations.
+
+Whether this is, as I believe, an inversion of the logical order or
+not, there is one most serious practical objection to such preference,
+that whereas specific diversity is a subject which can be investigated
+both by the study of variation and by the analytical apparatus which
+modern genetic science has developed, we have no very effectual means of
+directly attacking the problems of Adaptation.
+
+The absence of any definite progress in genetics in the last century was
+in great measure due to the exclusive prominence given to the problem
+of Adaptation. Almost all debates on heredity centered in that part of
+the subject. No one disputes that the adaptation of organisms to their
+surroundings is one of the great problems of nature, but it is not the
+primary problem of descent. Moreover, until the normal and undisturbed
+course of descent under uniform conditions is ascertained with some
+exactness, it is useless to attempt a survey of the consequences of
+external interference; nor as a rule can it be even possible to decide
+with much confidence whether such interferences have or have not
+definite consequences. Those, for example, who debated with enthusiasm
+whether acquired characters are or are not transmitted were constantly
+engaged in discussing occurrences which we now know to be ordinary
+features of descent under uniform conditions, and the origin of
+variations which were certainly not caused directly by circumstances at
+all. In the absence of any factorial analysis, or of any conception of
+what factorial composition means and implies, no one knew what varieties
+might be expected from given parents. The appearance of any recessive
+variety was claimed as a consequence of some treatment which might have
+been applied to the parents. There was no possible standard of evidence
+or means of controlling it, and thus the discussion was singularly
+unfruitful. Before we can tell how the course of descent has departed
+from the normal, we must know what the normal would have been if we
+had let alone. We are still far from having such knowledge in adequate
+measure, but it does now exist in some degree, and we are steadily
+approaching a position from which we shall be able to form fairly
+sound estimates of the true significance of evidence for or against
+the proposition that environmental treatment can produce positive
+disturbances in the physiological course of descent.
+
+Thus described, the field for consideration is very wide. Though the
+effects of changed conditions were especially studied in the hope of
+solving the problem of adaptation by direct observation, that, as all
+are now agreed, is but a part of a more general question. We must ask
+not only do changed conditions produce an _adaptative_ response on the
+part of the offspring, but whether they produce any response on the
+part of the offspring at all. It is not in doubt that by violent means,
+such as starvation or poisoning of the reproductive cells, effects of a
+kind, stunting and deformity for instance, can be made evident, just as
+similar effects may follow similar treatment during embryonic or larval
+life. Apart from interferences of this class, are there any that may be
+reasonably invoked as modifying the course of inheritance?
+
+No epitome of the older evidence for the inheritance of adaptative
+changes is here required. That has often been collected, especially
+by Weismann, who exposed its weaknesses so thoroughly as to carry
+conviction to most minds, and showed that whether the phenomenon
+occurs or not, no one can yet prove that it does. Belief in these
+transmissions, after being almost universally held, was with singular
+unanimity abandoned. This change in opinion, though doing credit to the
+faith of the scientific community in evidential reasoning, is the more
+remarkable inasmuch as the strength of the idea was not derived from the
+minute amounts of supposed facts now demolished. On the contrary, it was
+really an instinctive deduction from a wide superficial acquaintance
+with the properties of animals and plants. They _can_ accommodate
+themselves to circumstances. They _do_ make responses sometimes
+marvellously appropriate to demands for which they can scarcely have
+been prepared. What more natural than to suppose that the permanent
+adaptations have been achieved by inherited summation of such responses?
+No one had actually been driven to believe in the inheritance of
+adaptative changes because bitches which had been docked had been known
+to give birth to tailless puppies, or because certain wheat in Norway
+was alleged to have become acclimatized in a few generations. Evidence
+of this kind was collected and produced rather as an ornamental appendix
+to a proposition already accepted, and held to be plainly demonstrated
+by the facts of nature. Looked at indeed in that preliminary and
+uncritical way, the case is simply overwhelming. Those who desire to see
+how strong it is should turn to Samuel Butler's _Life and Habit_, and
+even if in reading they reiterate to themselves that no experimental
+evidence exists in support of the propositions advanced, the misgiving
+that none the less they may be true is likely to remain. Making every
+deduction for the fact that the wonders of adaptation have been grossly
+exaggerated, and that marvels of fitness and correspondence between
+means and ends have grown out of mere anthropomorphic speculations,
+there is much more left to be accounted for than can at all comfortably
+be accepted as the product of happy accidents. So oppressive are these
+difficulties that we can scarcely blame those who imagine that the study
+of heredity is primarily directed to the problem of the transmission of
+acquired characters, a preconception still almost universal among the
+laity.
+
+But since the belief in transmission of acquired adaptations arose
+from preconception rather than from evidence, it is worth observing
+that, rightly considered, the probability should surely be the other
+way. For the adaptations relate to every variety of exigency. To supply
+themselves with food, to find it, to seize and digest it, to protect
+themselves from predatory enemies whether by offence or defence, to
+counter-balance the changes of temperature, or pressure, to provide for
+mechanical strains, to obtain immunity from poison and from invading
+organisms, to bring the sexual elements into contact, to ensure the
+distribution of the type; all these and many more are accomplished by
+organisms in a thousand most diverse and alternative methods. Those are
+the things that are hard to imagine as produced by any concatenation
+of natural events; but the suggestions that organisms had had from the
+beginning innate in them a power of modifying themselves, their organs
+and their instincts so as to meet these multifarious requirements does
+not materially differ from the more overt appeals to supernatural
+intervention.
+
+The conception, originally introduced by Hering and independently by
+S. Butler, that adaptation is a consequence or product of accumulated
+_memory_ was of late revived by Semon and has been received with some
+approval, especially by F. Darwin. I see nothing fantastic in the notion
+that memory may be unconsciously preserved with the same continuity
+that the protoplasmic basis of life possesses. That idea, though purely
+speculative and, as yet, incapable of proof or disproof contains
+nothing which our experience of matter or of life at all refutes. On
+the contrary, we probably do well to retain the suggestion as a clue
+that may some day be of service. But if adaptation is to be the product
+of these accumulated experiences, _they must in some way be translated
+into terms of physiological and structural change_, a process frankly
+inconceivable.
+
+To attempt any representation of heredity as a product of memory is,
+moreover, to substitute the more obscure for the less. Both are now
+inscrutable; but while we may not unreasonably aspire to analyse
+heredity into simpler components by ordinary methods of research, the
+case of memory is altogether different. Memory is a mystery as deep
+as any that even psychology can propound. Philosophers might perhaps
+encourage themselves to attack the problem of the nature of memory by
+reflecting that after all the process may in some of its aspects be
+comparable with that of inheritance, but the student of genetics, as
+long as he can keep in close touch with a profitable basis of material
+fact, will scarcely be tempted to look for inspiration in psychical
+analogies.
+
+For a summary of the recent evidence I may refer the reader to Semon's
+paper[1] where he will find a collection of these observations described
+from the standpoint of a convinced believer. At the outset one cannot
+help being struck by the fact that of the instances alleged, very few,
+even if authentic, show the transmission of acquired modifications
+which can in any sense be regarded as adaptative, and many are examples
+not so much of a transmission of characters produced in the parents as
+of variation induced in the offspring as a consequence of treatment
+to which the parents were submitted, the parents themselves remaining
+apparently unmodified. No one questions the great importance of evidence
+of this latter class as touching the problem of the causes of variation,
+but it is not obvious why it is introduced in support of the thesis that
+acquired characters are inherited.
+
+It is most difficult to form a clear judgment of the value of the
+evidence as a whole. To doubt the validity of testimony put forward
+by reputable authors is to incur a charge of obstinacy or caprice;
+nevertheless in matters of this kind, where the alleged phenomena are,
+if genuine, of such exceptional significance, belief should only be
+extended to evidence after every possible source of doubt has been
+excluded. We believe such things when we must, but not before. At the
+very least we are entitled to require that confirmatory evidence should
+be forthcoming from independent witnesses. So far as I have seen, this
+requirement is satisfied in scarcely any of the examples that have been
+lately published, and until it is, judgment may reasonably be suspended.
+
+In some cases, however, the facts are not doubtful. Standfuss, by
+subjecting pupae of _Vanessa urticae_ to cold, produced the now
+well-known temperature-aberrations in which the dark pigment is greatly
+extended. He put together in a breeding-cage 32 males and 10 females
+showing this modification in various degrees. Two of these females died
+without leaving young. Seven produced exclusively normal offspring. From
+the eighth female 43 butterflies were bred, and of these there were four
+(all males) which to a greater or less extent exhibited the aberrational
+form.[2] The mother of this family was the most abnormal of the 10
+females originally put in.
+
+Fischer's experiment with _Aretia caja_ was on similar lines. From
+pupae which had been frozen almost all the moths which emerged showed
+aberrational markings. A pair of these mated and produced 173 young
+which pupated. Those which emerged early were all normal, but of those
+which emerged late, 17 had in various degrees abnormal markings like
+those of the parents.[3] In neither of these examples is there any
+question as to the facts. Both observers have great experience and give
+full details of their work.
+
+As regards _Vanessa urticae_, however, it must be recalled that Fischer
+himself showed that in Nymphalids somewhat similar aberrations could be
+produced both by heat and by cold, and even by centrifuging the pupae.
+Frl. von Linden produced a transitional form of the same aberration
+in _V. urticae_ by the action of carbonic acid gas.[4] It is highly
+probable that the appearance is due to a morbid change, perhaps an
+arrest of development, which may be brought about by a great diversity
+of causes. In the experiments the cause probably was a diseased
+condition of the tissues of the mother herself. She had been subjected
+to freezing sufficiently severe to prevent the proper development of
+the pigments and some of the ovarian cells presumably suffered also.
+It will be observed that the only specimens which were affected were
+the offspring of the most abnormal female, and of them only four out of
+forty-three showed any change.
+
+The same interpretation probably applies to the cases in _Arctia caja_.
+In this species the markings are well known to be liable to great
+variation. As Barrett says, even in nature individuals are rarely quite
+alike, and an immense number of strange forms occur in collections.[5]
+These are greatly sought after by some collectors, especially in
+England, where they fetch high prices at auctions, and it is notorious
+that most of them come from Lancashire and the West Riding of Yorkshire.
+It is commonly supposed that the breeders of that district subject them
+to abnormal conditions, and especially to unnatural feeding, but I know
+no clear evidence that this is true. From whatever cause it is certain
+that the natural pattern is, in some strains at all events, very easily
+disturbed.
+
+The elaborate experiments of Schroeder with _Abraxas grossulariata_ are
+difficult to follow and are complicated by the fact that the series
+which was submitted to abnormal temperatures was derived from an
+abnormal original pair. From the evidence given it is not clear to me
+whether the temperature had a distinct effect. This insect, like _Arctia
+caja_, produces an immense number of variations (especially in the
+amount of the black pigment) and as most of these are, I believe, reared
+in domestication for sale, it is highly probable that the species is
+easily influenced by cultural conditions.
+
+Schroeder describes two other experiments which have been accepted
+by Semon and other supporters of the view that acquired characters
+are transmitted. In the first, _Phratora vitellinae_, a phytophagous
+beetle living on the undersides of leaves, was used. It naturally
+feeds on _Salix fragilis_, a species without a felt, or tomentum, on
+the underside of the leaves. Larvae were transferred to another willow
+(near _S. viminalis_) which has the undersides of the leaves felted. The
+larvae took readily to the new food, pushing the tomentum before them as
+they gnawed the leaves. They came to maturity and when they were about
+to lay their eggs they were given a free choice between _S. fragilis_
+and the tomentose species. The greater number of ovipositions, 219,
+took place on _fragilis_, and there were 127 on the tomentose bush,
+which we are told was six times as large as the _fragilis_. The larvae
+from _fragilis_ were next put on the tomentose species and reared on
+it. When they became imagines they were similarly given their choice,
+with the result that there were 104 ovipositions on the tomentose
+species and only 83 on _fragilis_. In the next generations there were 48
+ovipositions on the tomentose and 11 on _fragilis_. Finally the fourth
+generation made 15 ovipositions on the tomentose and none on _fragilis_.
+
+The difficulty about such experiments is obviously that one has no
+assurance that the change of instinct, in so far as there is any,
+may not be a mere consequence of the captivity. It must, besides, be
+extremely difficult to arrange the experiment so that there is really
+an equal choice between the two bushes, when one stands beside the
+other. Przibram, in quoting this case, considers that as the tomentose
+bush was about six times as large as the _fragilis_, some indication of
+the relative attractiveness of the two may be obtained by dividing the
+ovipositions on the larger bush by six, but I imagine the matter must be
+much more complex.
+
+Schroeder's second example is not more convincing, in my opinion, though
+Semon regards it as one of the most important pieces of evidence. It
+concerns a leaf-rolling moth, _Gracilaria stigmatella_, the larva of
+which is said normally to make its house by bending over the _tips_ of
+the sallow leaves on which it feeds. Schroeder placed larvae on leaves
+from which the tips had been cut, and these larvae made their houses by
+rolling over the _sides_ of the leaves. Their offspring were again fed
+on leaves without tips, and as before, they rolled in the leaf-margins
+either on one side or both. The offspring of this second generation were
+then fed on entire leaves. There were 19 houses made by these (?19)
+larvae, and of them 15 were normal, made by folding down the tips of
+the leaves, while 4 were abnormal, made by rolling in the leaf-margins.
+Schroeder says that in nature he has only twice seen abnormal houses; but
+it is clearly essential not only that the frequency of such variability
+in nature should be thoroughly examined, but also that we should know
+whether when the species is bred in captivity these irregularities of
+behaviour do or do not occur when the larvae are fed on uninjured leaves.
+
+The famous case of Schuebeler's wheat is revived by Semon. The story will
+be familiar to most readers of the literature of the subject. Briefly
+it is that annuals, especially wheat and maize, raised from seed in
+Central Europe take more time in coming to maturity and ripening than
+similar plants raised in Norway, where the summer days are much longer.
+The received account is that he imported seed especially of maize and
+of wheat from Central Europe to Norway and found that in successive
+years the period of growth and ripening was increasingly reduced. After
+two generations seed of the accelerated wheat was sent back to Breslau
+where it was grown, and was found to ripen rather more slowly than in
+Norway, but much more quickly than the original stock had done. The
+facts recorded by Schuebeler[6] are that he received seed from Eldena,
+which is on the Baltic near Greifswald. The variety is described as
+"_100 taegiger Sommer Weizen_," but no more exact record of its behaviour
+in Germany is given. This wheat, grown at Christiania in 1857, took
+103 days to harvest. Its seed was again grown in Christiania in 1858,
+and took 93 days, and sown again in 1859 it took only 75 days, 28 days
+less than in the first year of cultivation in Norway. Seed of the 1858
+crop was sent to Breslau, and grown there by Roedelius in 1859; it
+took 80 days. Evidently before such a record can be used as proving an
+inheritance of acquired characters numbers of particulars should be
+forthcoming. The view that Johannsen has taken is that the result was
+probably due to unconscious selection of the earlier individuals among a
+population consisting of many types of various compositions. Some effect
+may no doubt be ascribed to that cause, but I cannot think that alone
+it would account for the results. My impression is rather that they
+were produced by differences in the cultivation and especially in the
+seasons. Research of an elaborate character would be necessary in order
+to eliminate the various sources of error, and nothing of the kind has
+been done; nor does Semon allude to these difficulties in prominently
+adducing Schuebeler's evidence. A difference of even three weeks in time
+of harvesting may easily be due to variation in the season. It would in
+any case be difficult to analyse the meteorological conditions, and to
+decide how much effect in postponing or accelerating the harvest might
+be due to cold days, to cloudy days, to wet weather, to fluctuations in
+average temperature, to hot days, and other such incidents occurring at
+the different periods of growth, even if they were specially watched
+while the experiments were in progress, and at this distance of time
+such analysis is practically impossible. Without careful simultaneous
+control-experiments this evidence is almost worthless. The director of
+the Meteorological Office[7] has, however, kindly sent me some details
+of the weather at Breslau from 1857 to 1860, and I notice that as a
+matter of fact July, 1859, was an exceptionally hot month, _having an
+average of 2.67 deg. C. above the mean_ for the twenty years 1848-1867. June
+in that year was slightly (0.31 deg. C.) below the mean and May slightly
+above it (0.18 deg. C.). August was also abnormally hot, 2.35 deg. C. above the
+average. The Breslau wheat was sown on _May 19_ and harvested on August
+6. There was a cold spell from May 11 to 14, which this wheat escaped,
+as it was sown on May 19. In the other years the cold spell came much
+later. These elements of the weather may possibly have done something to
+hurry the ripening in 1859. It unfortunate that we are not told how long
+similar wheat from Breslau seed took to ripen in that year.
+
+As regards the Norway cultivations we have the average monthly
+temperatures recorded by Schuebeler, though he does not discuss them
+in connection with this special problem. It is quite clear that 1857,
+in which the period was 103 days, was an exceptionally cold summer,
+especially as regards the months of June and July, but though there
+was, so far as the temperature records go, no great difference between
+1858 and 1859, the year 1859, in which the period of ripening was the
+shortest, was somewhat colder in Norway than 1858. But we have the
+further difficulty that there were ten days difference in sowing, for
+in 1858 the sowing was made on May 14, and in 1859 on May 24. With
+all these possibilities uncontrolled, and indeed unconsidered, I am
+surprised that Semon should claim these experiments as one of the chief
+supports for his views.
+
+Schuebeler's other allegations respecting the influence of climate on
+plants grown in various places and especially at different elevations
+in Norway have been destructively criticised by Wille[8] to whose paper
+readers interested in the subject should refer.
+
+Before the appearance of Wille's criticisms Wettstein[9] made a
+favourable reference to Schuebeler's work, accepting his conclusion. He
+states also that he has himself made analogous experiments with flax,
+finding that the length of the period of development and a series of
+morphological characters show an adaptation to local conditions, and
+that on transference of seed to other conditions the previous effects
+are maintained. No details, however, are given, and I do not know if
+anything more on the subject has appeared since. The other examples
+cited by Wettstein, such as the observations of Cieslar on forest-trees
+and those of Jakowatz on gentians seem to me open to all the usual
+objections applicable to evidence of this kind. Such work, to be of any
+value for the purpose to which it is applied, must be preceded by a
+study of the normal heredity and of the variations of the species.
+
+Most of the recent writers (Semon, Przibram, etc.) on the inheritance
+of acquired characters accept the story of Brown-Sequard's guinea pigs,
+which are said to have inherited a liability to peculiar epileptiform
+attacks induced in their parents by various nervous lesions.
+
+The question has been often debated and several observers have repeated
+the experiments with varying results, some failing to confirm
+Brown-Sequard, others finding evidence which in various degrees
+supported his conclusions. Recently a new and especially valuable paper
+has been published by Mr. T. Graham Brown[10] which goes far towards
+settling this outstanding question. He states that "the Brown-Sequard
+phenomenon is nothing more or less than a specific instance of the
+scratch-reflex," and it is due to a raised excitability of the mechanism
+of this reflex. This raised excitability is the character acquired as a
+consequence, for instance, of the removal of part of one great sciatic
+nerve. The nature of this raised excitability and its causation are
+discussed and elucidated, but this part of the work is not essential
+to the present consideration. Mr. Graham Brown in his summary of
+conclusions remarks that it is very difficult to see how this condition
+of raised excitability can be transmitted to the offspring, and this
+comment which might be made in reference to any of the alleged cases
+certainly applies with special cogency to the present example.
+
+He then calls special attention to three observations:
+
+1. That guinea pigs which had a "trophic" change in the foot, as a
+result of division of the great sciatic nerve, have repeatedly been seen
+to nibble the feet of other guinea pigs which had this change in the
+foot from the same causes.
+
+2. That accidental injury to the toes may be followed by the
+Brown-Sequard phenomenon in an otherwise normal animal.
+
+3. That in several instances the young of guinea pigs which exhibited
+the phenomenon have been noticed to have one or more toes eaten off by
+the mother.
+
+Brown-Sequard noticed that almost all his animals in which the great
+sciatic was divided acquired the "epilepsy" and nibbled those parts of
+their feet in which sensation had been lost. Of the offspring of such
+animals he found that a very small proportion exhibited a malformation
+of the feet, and of these some showed the "epilepsy." The proportion
+which showed the "epilepsy" was one to two per cent. of the offspring.
+
+Morgan[11] is quoted by Graham Brown as having suggested that the
+loss of toes in the offspring may have been due to mutilation by the
+mother, following his experience in a case in which the tails of mice
+in succeeding litters were thus devoured, and there can be little doubt
+that in this suggestion lies the clue to the explanation of the whole
+mystery. Graham Brown concludes that it may be supposed with every
+degree of probability that the "transmission" was due to injuries
+inflicted upon the young by their parents. With this conclusion most
+people will now be disposed to agree, and we may hope that we shall
+hear the last of this curious myth--to the elucidation of which a vast
+quantity of research has been devoted.
+
+The series of experiments made by Kammerer with various Amphibia have
+attracted much attention and have been acclaimed by Semon and other
+believers in the transmission of acquired characters as giving proof of
+the truth of their views. With respect to these observations the chief
+comment to be made is that they are as yet unconfirmed. Many of the
+results that are described, it is scarcely necessary to say, will strike
+most readers as very improbable; but coming from a man of Dr. Kammerer's
+wide experience, and accepted as they are by Dr. Przibram, under whose
+auspices the work was done in the Biologische Vesuchsanstalt at Vienna,
+the published accounts are worthy of the most respectful attention.
+
+The evidence relates chiefly to three distinct groups of occurrences:
+
+  1. Modification in _Alytes obstetricans_, the Midwife Toad, affecting
+     both the structure and the mode of reproduction, induced by compulsory
+     change of habits.
+
+  2. Modification in the mode of reproduction of _Salamandra atra_ and
+     _maculosa_ induced by compulsory change of habits.
+
+  3. Modification in the colour of _Salamandra maculosa_ induced by change
+     in the colour of the soil on which the animals were kept.
+
+1. I will take first the case of _Alytes_,[12] because it is the most
+definite example, and because it is the case which most readily admits
+of repetition and verification.
+
+The habits of _Alytes obstetricans_ are well known. The animals copulate
+on land. As the strings of eggs leave the female they are entangled by
+the hind legs of the male, and being adhesive they stick to him and
+undergo their development attached to his back and legs. The number of
+eggs varies from 18 to 86, a number much smaller than is usual in toads
+and frogs which lay their eggs in water. The eggs are large and full of
+yolk.
+
+There are two breeding seasons, one about April and the other about
+September, and a winter hibernation. Not only animals brought in from
+outside, but their offspring reared in domestication maintain these
+normal habits in confinement, if the temperature does not exceed 17 deg. C.
+(pp. 499 and 534).
+
+If, however, the temperature be artificially raised and kept at 25-30 deg.
+C., the males do not attach the eggs to themselves when spawning occurs
+on land but let them lie. The adhesion of the eggs is said to be
+hindered by the comparatively rapid drying of their surfaces.
+
+More usually in the high temperatures the animals _take to the water_
+and copulate there. The eggs are ejected into the water, and as their
+gelatinous coverings immediately swell up, they do not stick to the
+males.
+
+The offspring thus derived from the parents subjected to heat for one
+breeding-period only, whether they were laid in water or on land, did
+not show departures from the normal type.
+
+Kammerer states next, however, that in subsequent breeding-periods the
+same parents frequently take to the water to breed, though they have
+become quite accustomed to the heated chamber; and furthermore that if
+such animals, having thus lost their instinct to brood their young, be
+transferred to ordinary temperatures they do not readily reassume their
+normal habits, but for several breeding seasons--at least four--will
+take to the water. These parents lay from 90 to 115 eggs, which are
+small and contain little yolk, and the larvae, on hatching, breathe with
+their embryonic gills until they are absorbed instead of being broken
+off as normally.
+
+The offspring thus abnormally developed when they mature are said never
+to brood their eggs. If they are derived from the earlier spawnings of
+their parents, before, that is to say, the parents had been submitted
+to the changed conditions long enough to transmit their effects, they
+lay on land; but if they are derived from the later spawnings, they
+lay in the water. These changes of habit are manifested without the
+continued application of the abnormal experimental conditions, and, as I
+understand the account, in normal conditions of temperature.
+
+If the abnormal experimental conditions are continued, the toads always
+lay in water, and their eggs become progressively smaller and more
+numerous. The larvae in the fourth generation acquire three pairs of
+gills instead of one pair, and are in other respects also different from
+the normal form.
+
+Respecting the _Alytes_ bred in this way Kammerer makes the very
+striking statement that _the males in the third generation_ (p. 535)
+_have roughened swellings on their thumbs and that in the fourth
+generation_ (pp. 516 and 535) _these swellings develop black pigment_.
+Together with the appearance of this secondary sexual character there
+is hypertrophy of the muscles of the fore-arm. To my mind this is
+the critical observation. If it can be substantiated it would go far
+towards proving Kammerer's case. _Alytes_, among toads and frogs, is
+peculiar in that the males do not develop these lumps in the breeding
+season, and the fact may no doubt be taken to be correlated with the
+breeding habits, copulation occurring on land and not in water as is
+usual with Batrachians. It is to be expressly noticed that these lumps
+on the thumbs or arms of male toads and frogs are not merely pigmented
+swellings, but are pads bearing numerous minute horny black spines,
+which are used in holding the females in the water. The figures which
+Kammerer gives (Taf. XVI, figs. 26 and 26a) are quite inadequate, and as
+they merely indicate a dark patch on the thumbs it is not possible to
+form any opinion as to the nature of the structure they represent.
+
+The systematists who have made a special study of Batrachia appear to
+be agreed that _Alytes_ in nature does not have these structures; and
+when individuals possessing them can be produced for inspection it
+will, I think be time to examine the evidence for the inheritance of
+acquired characters more seriously. I wrote to Dr. Kammerer in July,
+1910, asking him for the loan of such a specimen[13] and on visiting
+the Biologische Versuchsanstalt in September of the same year I made
+the same request, but hitherto none has been produced. In matters of
+this kind much generally depends on interpretations made at the time
+of observation; here, however, is an example which could readily be
+attested by preserved material. I notice with some surprise that in a
+later publication on the same subject no reference to the development of
+these structures is made (see below).
+
+The statements here given represent but a small part of Kammerer's
+papers on the subject. He gives much further information as to the
+course of the experiments, especially in regard to the fate of the eggs
+laid on land and the aberrations induced in them by treatment. The
+ramifications of the experiments are, however, very difficult to follow,
+and as I am not sure that I have always understood them I must refer the
+reader to the original.
+
+More recently Kammerer has published[14] a most curious account of
+experiments in crossing his modified and abnormal _Alytes_, derived from
+the water-eggs, with normal individuals.
+
+In the first case the cross was made between a _normal female_ and an
+_abnormal male_. The offspring were normal in their habits. In the next
+generation bred from these almost exactly a quarter showed the abnormal
+instinct.
+
+The reciprocal cross was made between an _abnormal female_ and a _normal
+male_. In this case the offspring were abnormal in their behaviour; but
+the second generation bred from them showed three quarters abnormal and
+one quarter normal.
+
+Certain details as to numbers and sexes of the various families bred
+in the course of this amazing experiment are given in a subsequent
+publication.[15] This later paper goes somewhat fully into the question
+of the difference in behaviour between the normal and modified
+individuals, describing the ways in which the males and females
+possessing the acquired character could be recognised from the males and
+females which were normal, but in this account I find no reference to
+the development of the "_Brunftschwielen_"--the horny pads on the hands
+of the males. As these structures would be of special value in such a
+diagnosis the omission of any allusion to them calls for explanation.
+Kammerer claims the evidence as proof of Mendelian segregation in
+regard to an acquired character, the first example recorded. Pending a
+repetition of the experiments there is no more to be said.
+
+2. _The Mode of Reproduction of Salamandra atra and
+maculosa._[16]--_Salamandra maculosa_, the common lowland form, with
+yellow bands or spots, deposits its young in water, generally as
+gill-bearing tadpoles, with a wide, swimming tail, though occasionally
+they are born still enclosed in the egg-capsule out of which they soon
+hatch. Spawning extends over a considerable period, often many weeks,
+and during the season one female may bear more than 50 young.
+
+_S. atra_, the black Alpine form, produces its young on land. They are
+born without gills, ready to breathe air, and with the rounded tail
+of the adult. These differences may, as Kammerer says, naturally be
+regarded as adaptations to the Alpine conditions. Moreover, the female
+bears _only two_ young in a season, and this reduction in the number
+must be taken to be a consequence or condition of viviparity. There are
+many eggs in the ovary, but all except the two which are destined to
+develop degenerate and form a yolk-material on which these two survivors
+feed.
+
+Kammerer gives a long account of the various conditions to which he
+subjected both species. The treatment was complicated in many ways, but
+the essential statements are, as regards _S. maculosa_, that when no
+water was provided in which the young might be born, they were dropped
+on land, larger and in a later stage of development and of a darker
+colour than is normal; that the larvae so born gradually diminished
+in number until only two were deposited in each breeding-period;
+that dissection showed that the other ova degenerated to form a
+yolk-material. The larvae so produced reached maturity. The summary of
+results describes their behaviour, stating that they produced:
+
+(_a_) _In water_, either (1) _very_ advanced, large-headed larvae 45
+mm. long (instead of 25-30 mm.) with gills already reduced, which had
+awkward, embryo-like movements, and in some few days metamorphosed
+into small perfect salamanders; or (2) moderately advanced, properly
+proportioned larvae, 40-41 mm. long, provided with large gills of (at
+first) intrauterine character, which were reduced during aquatic life.
+
+(_b_) _On land_, small (26 mm. long) larvae with rudimentary gills,
+having the body rounded instead of being flattened from above downwards,
+and an elongated narrow head, which were unable to live in deep water.
+These larvae changed to the salamander colour in 10-12 days, and after
+four weeks metamorphosed into salamanders 29 mm. long.
+
+(_c_) In the foregoing cases the experimental conditions were not
+continued, or in other words, basins of water were provided in which
+they could spawn. But if the experimental conditions are continued,
+these _Salamandra maculosa_ which were born newt-like (viz., not in a
+larval condition), are themselves newt-bearing from the first time they
+give birth, using the dry land, and bringing forth only two young, the
+normal number for the births of _S. atra_. These young are 40-41 mm.
+long, and are dark-coloured, resembling greatly the normal new-born _S.
+atra_.
+
+This epitome of the observations illustrating the inheritance of
+acquired characters has been very widely quoted, and may not unnaturally
+be taken to summarize a wide experience of the modified animals.
+Reference to the details given in the same paper shows that, as alleged,
+each of the four types of behaviour enumerated was witnessed _once_ only
+in the case of each of four females, no two agreeing with each other.
+As to the number of the males or their habits nothing is said. The first
+female, _a_ (1), bore five young; the second, _a_ (2), bore two, of
+which one was a partial albino; the third, _b_, produced four young; and
+the fourth, _c_, two as already stated.
+
+In the case of _c_ the details show that the female gave birth
+immediately after being transferred from the open-air terrarium to
+one indoors, which contained no basin of water. This is the example
+of the consequences which follow on a continuance of the experimental
+conditions.[17]
+
+As regards _S. atra_ the converse is reported. Various means were
+used to induce them to eject their young prematurely in water, such
+as massaging the sides of the mothers, or raising the temperature to
+25 deg. or 30 deg. C., with various degrees of success. But afterwards it was
+found that specimens collected wild at an elevation of about 1,000
+metres responded to much simpler treatment, and gave birth prematurely
+in water when they were kept in a large shallow basin of water not so
+deep but that they could everywhere touch the bottom with their feet and
+keep their heads above the surface. With specimens collected at higher
+elevations this treatment was inoperative, and the suggestion is made
+that _S. atra_ at the lower confines of its habitat partakes more of the
+nature of _maculosa_ than do the individuals from greater heights; for
+Kammerer argues that pools suitable for breeding must be more uncommon
+at those elevations than they are lower down.
+
+In the earlier paper[18] Kammerer states that newly caught females
+of _S. atra_ often give birth in the water, and show an undoubted
+preference for doing so. He describes also how he once saw several
+females, wild in their natural habitat, lay their young in a rain-puddle
+at 1,800 metres elevation, but the larvae thus born were fully formed.
+
+When the deposition of the young as larvae has become "habitual"[19]
+with _S. atra_, three to nine larvae may be produced at one spawning
+period, from 35 to 45 mm. long, with gills at most 8 mm. long, and a
+tail-fin 2-3 mm. broad. Such larvae are generally coffee-brown, or grey
+(instead of black), and show other minor differences.
+
+The summary states that when grown to maturity they become in their turn
+larva-bearing, and go into the water to bring forth. Their young are
+more than two (3 to 5 being the numbers observed) with a length of 33-40
+mm. or of 21-23 mm. at birth. They are light grey, spotted (mottled with
+lighter and darker colour), have relatively short gills (8 to 9 mm.
+at most) and a broad tail-fin (3 mm. wide). At metamorphosis they are
+relatively long (44 mm.) and one of them had some yellow pigment.
+
+Here again this summary is, as a matter of fact, describing the
+behaviour of two mothers, of which one produced three, and the other
+five young.
+
+To my mind these experiments suggest that the reproductive habits
+of both species, if closely observed, will be found to be subject
+to considerable variation, and I think it not impossible that each
+species is, especially in confinement, capable of being a good deal
+deflected from its normal behaviour. Moreover, there seems to me no
+great improbability in the idea that there is an interdependence
+between the number of young and the stage of maturity in which they
+are born. But, at the same time, the case as told by Kammerer strikes
+me as proving too much. If each species is so sensitive to conditions
+that the normal procedure is gravely modified in one generation, and
+if that modification can reappear in a pronounced form in the next
+generation without a renewal of the disturbing conditions, it becomes
+extremely difficult to understand how the regularity which each species
+is believed to display in nature can be maintained. Surely both species
+might be expected to be in confusion. From a passage in Kammerer's
+earlier paper (1904, p. 55) on the subject, I infer that he also would
+expect considerable irregularity in the natural behaviour, but that he
+has not investigated the point.[20]
+
+3. _Modification of the Colour of Salamandra maculosa induced by Change
+in the Colour of the Soil on which the Animals were kept._--Kammerer
+speaks of this as the most convincing of all his experiments on the
+transmission of acquired characters. So far, however, no full account of
+them has been published.[21] The statement is that when salamanders are
+kept in yellow surroundings the yellow markings gradually in the course
+of years increase in amount relatively to the black ground colour.
+Conversely by keeping the animals on black garden soil, the yellow may
+be greatly diminished in quantity until it largely disappears. (The
+account in _Natur_ adds that very moist conditions also favour the
+increase of yellow, and that with less moist conditions the yellow
+diminishes.) From each kind, the (induced) yellower and the (induced)
+blacker, a second generation was raised, on soil of neutral colour, and
+each family was later divided into two parts, half being put on black
+and half on yellow ground.
+
+As regards the offspring of those which had lived on _black_ soil no
+positive result had been reached up to the date of publication, but it
+is stated that these young resembled their parents in having the yellow
+distributed in _irregular spots_.
+
+As regards the offspring of those which had lived on yellow soil the
+account follows up the story of that part of the offspring which were
+put on yellow soil again. It is stated that these, though derived from
+parents with irregular spots, _developed the yellow as longitudinal
+bands_.
+
+This account is given with slight differences of expression in the
+three places to which I have referred. On returning from Vienna in
+1910 I consulted Mr. G. A. Boulenger in reference to the subject, and
+he very kindly showed me the fine series from many localities in the
+British Museum, and pointed out that in nature the colour-varieties
+can be grouped into two distinct types, one in which the yellow of
+the body is irregularly distributed in spots and one in which this
+yellow is arranged for the most part in two longitudinal bands which
+may be continuous or interrupted. _The spotted form is, as he showed
+me, an eastern variety, and the striped form belongs to western
+Europe._ Mr. E. G. Boulenger[22] has since published a careful account
+of the distribution of the two forms. The spotted he regards as the
+typical form, var. _typica_, and for the striped he uses the name
+var. _taeniata_. The typical form occupies eastern Europe in general,
+including Austria and Italy, extending as far west as parts of eastern
+France. The var. _taeniata_ is found all over France, excepting parts of
+the eastern border, Belgium and western Germany, Spain and Portugal. Of
+the very large series examined there was only one specimen (Lausanne)
+which could not with confidence be referred to one or other of the
+two varieties. Mr. E. G. Boulenger points out that both varieties
+inhabit very large areas, and live on soils of most different colours
+and compositions. Both are liable to variations in the amount and the
+shade of the yellow, but that any suggestion that _taeniata_ belongs
+especially to yellow soils and _typica_ to black soils is altogether
+inadmissible. He expresses surprise that Kammerer should not allude to
+these peculiarities in the geographical distribution of the two forms.
+He suggests further that it is more likely that some mistake occurred in
+Kammerer's observations than that the east European _typica_ should, in
+the course of a generation, have been transformed into the west European
+_taeniata_ by the influence of yellow clay soil.
+
+In his last paper on the subject Kammerer states incidentally[23] that
+he has found the _striped form recessive to the spotted_. No evidence
+for this statement is given, and I have not found any other reference
+to crosses effected between the two natural types. If, however, this
+representation is correct, it is conceivable that the production of
+_taeniata_ from _typica_ was in fact the re-appearance of a recessive
+form. The plate which Kammerer gives in illustration of his modified
+parent figures a single animal at four stages, and though it is
+certainly more like the spotted than the striped form, it has a certain
+suggestion of the striped arrangement, such as I can well imagine being
+produced in the heterozygote.[24]
+
+In continuation[25] of the experiments on the colour of _S. maculosa_
+Kammerer publishes an account of elaborate experiments in grafting
+ovaries of the various forms, modified and unmodified, into each other,
+and describes the offspring which followed. Before pursuing this part of
+the inquiry I am disposed to wait until the earlier steps have been made
+much more secure than they yet are.
+
+More recently Kammerer has published similar statements in regard to the
+inheritance of characters induced in various lizards by keeping them in
+abnormal temperatures, high and low. The changes induced affected in
+some species the colours, in others the reproductive habits. Respecting
+these examples I feel the same scepticism that I have indicated in
+regard to the others, somewhat heightened by the fact that insufficient
+evidence is given both regarding the behaviour of these various species
+in captivity when not subjected to abnormal temperatures, and in the
+wild state.
+
+Respecting this part of the evidence Mr. G. A. Boulenger has lately
+published a criticism[26] from which I extract the following passages.
+Referring to a previous note[27] on the question of the melanism of the
+various insular forms of _Lacerta muralis_ he writes: "I also alluded
+(_l. c._) to the theories that have been propounded to explain the
+melanism of various insular forms. This is a subject which has been
+lately taken up by Dr. Kammerer at the Biologische Versuchsanstalt in
+Vienna, and he claims to have produced nigrinos artificially by a very
+strong elevation of the temperature, accompanied by extreme dryness. Dr.
+Werner[28] has already opposed his own experiments to those of Kammerer,
+artificial melanism having been produced by him in _Lacerta oxycephala_
+by keeping two very light specimens from Ragusa for a whole summer
+in very damp conditions. Neither is Kammerer's theory in accordance
+with the distribution of the black lizards, as pointed out by Werner.
+Kammerer also finds that those forms which are known to produce melanic
+races in a state of nature, lend themselves more readily than the others
+to the success of his experiments. But he shows himself misinformed
+when he states that the variety called _Lacerta fiumana_ belongs to the
+category of those of which black forms are not known. He overlooks the
+fact, first pointed out by Scherer in 1904, and which I can confirm,
+that the black lizard from Melisello near Lissa in the Adriatic is
+unquestionably derived from the lizard from Lissa, which he correctly
+regards as not separable from _L. fiumana_...."
+
+"Another colour modification which Dr. Kammerer states that he obtained
+by raising the temperature is the assumption by the female of the
+typical _Lacerta muralis_ of the bright red colour of the lower parts
+which often distinguishes the male from the female, and which was not
+shown by the individuals of the latter sex kept by him under normal
+conditions. He quotes various authorities to show that the lower parts
+are never red in the females, but he has omitted to consult others who
+say the contrary. Thus Bedriaga (1878 and 1879) remarks that a so-called
+var. _rubriventris_ of the typical wall lizard has the lower parts red
+in both sexes."[29]
+
+In reading such papers as those of Semon or Kammerer the thought
+uppermost in my mind is that to multiply illustrations of supposed
+transmission of acquired characters is of little use until some one
+example has been thoroughly investigated. If we had certain assurance
+that even a single unimpeachable case could be repeated at will, the
+whole matter would assume a more serious aspect. If, for instance,
+Kammerer were able to show us _Alytes_ males with horny pads on their
+hands, it would be something tangible; still more, if the experiment
+were repeated by others until no doubt remained that the offspring
+of _Alytes_ which had bred in water for some three generations did
+acquire these pads and that they could transmit these novelties to
+descendants raised in normal conditions. Till evidence of this kind is
+published by at least two independent observers investigating similar
+material, I find it easier to believe that mistakes of observation or
+of interpretation have been made than that any genuine transmission of
+acquired characters has been witnessed.
+
+Meanwhile there is no denying that the origin of adaptational features
+is a very grave difficulty. With the lapse of time since evolutionary
+conceptions have become a universal subject of study that difficulty
+has, so far as I see, been in nowise diminished. But I find nothing in
+the evidence recently put forward which justifies departure from the
+agnostic position which most of us have felt obliged to assume.[30]
+
+
+                 APPENDIX TO CHAPTER IX.
+
+Professor G. Klebs, as is well known to students of evolutionary
+phenomena, has for several years been engaged in investigations relating
+to the inheritance of acquired characters. In his many publications
+on the subject the issue has always been represented as more or less
+uncertain.
+
+Desiring to know how the matter now stands according to Professor Klebs'
+present judgment I wrote to him asking him to favour me with a brief
+general statement. This he most kindly sent in a letter dated 8th July,
+1912.
+
+As such a statement will be read with the greatest interest by all who
+are watching the progress of these studies I obtained permission to
+publish it as follows:
+
+                                                    8. Juli 1912
+
+   Ihre liebenswurdige Anfrage will ich sehr gern
+   beantworten, obwohl ich sie nicht so beantworten kann wie ich
+   erwuenschte. Ihr Skepticismus in der Frage der Uebertragung
+   erworbener Charactere auf die Nachkommen ist nur zu berechtigt.
+   Meine Versuche mit Veronica sind _nicht_ beweisend, da es mir
+   bisher nicht gelungen ist eine einigermasse konstante Varietaet
+   mit verlaubten Inflorescenze zu erzeugen. In Bezug auf mein
+   Semper vivum bin ich allerdings noch heute der Meinung dass
+   die starke kuenstliche Veraenderung der Bluete einen Einfluss auf
+   einzelnen Nachkommen gehabt hat. Ich habe seither nichts darueber
+   veroeffentlicht: die Mehrzahl der anormalen gefuellten Blueten war
+   leider steril. Von einem weniger veraenderten Exemplar erhielt
+   ich einige Saemlinge, aber sie haben noch nicht geblueht. Es kann
+   sich in diesem Falle nur um eine _Nachwirkung in der ersten
+   Generation_ handeln, vergleichbar jenen Faellen in denen Samen von
+   Baeumen aus den hohen Alpen in der Ebene gewisse Nachwirkungen
+   zeigen. Aber es ist bisher kein sicherer. Fall bekannt in den
+   der kunstliche herbeigefuehrte Charakter _mehrere Generationen
+   hindurch unter der gewoehnlichen "normalen" Bedingungen_
+   uebertragen worden ist.
+
+   Auf der andere Seite sind diese negativen Resultaten nicht
+   entscheidend. Denn wie wenig ist in dieser Beziehung ueberhaupt
+   ernstlich versucht worden! Und zweifellos geht die Sache nicht so
+   einfach.
+
+   Ich versuche es mit anderen Pflanzen weil ich der Meinung bin
+   dass es moeglich sein muesse wenigstens solche neuen Varietaeten zu
+   erzeugen, wie sie die Gartenvarietaeten entsprechen.
+
+   Aber bis jetzt leider sind die Versuche nicht gelungen, weder
+   mir noch irgend einem anderen.
+
+FOOTNOTES:
+
+[1] Semon, R., Der Stand der Frage nach der Vererbung erworbener
+Eigenschaften, published in _Fortschr. der naturw. Forschung._, Bd. 11,
+1910.
+
+[2] Standfuss, M., _Denks. Schweiz. naturf. Ges._, XXXVI, 1898, p. 32.
+
+[3] Fischer, E., _Allg. Ztschr. f. Entomologie_, Bd. VI, 1901.
+
+[4] Out of 12 pupae treated 8 died and of the 4 survivors, one only was
+affected. See M. v. Linden, _Archiv. Rassen. u. Gesells._, 1904, I.
+
+[5] For illustrations see _Oberthur's Etudes d'Entom._, 1896, where many
+of these curious aberrations are represented; also Barrett, _Lepid.
+Brit. Islands_, II, pp. 71 and 72.
+
+[6] Schuebeler, F. C., _Die Culturpflanzen Norwegens_, 1862, especially
+pp. 24 and 28.
+
+[7] I am obliged to him and to Dr. E. Gold for much trouble taken to
+answer my questions. Some idea of the kind of weather indicated by an
+average of 2.76 deg. C. above the mean may be got from a comparison with the
+year 1911, which most people will remember as one of the hottest summers
+they have known. The July of that year was in east and southeast England
+about 4 deg. F. above the mean but 2.67 C. means about 4.8 deg. F. above the
+mean. At Greenwich July, 1859, was about 6.5 deg. F. above the average.
+
+[8] Wille, N., _Biol. Cbltt._, XXV, 1905, p. 521.
+
+[9] Wettstein, R. von. _Der Neo-marckismus u. seine Beziehungen zum
+Darwinismus_, Jena, 1903.
+
+[10] T. Graham Brown, _Proc. Roy. Soc._, 1912, vol. 84, B, p. 555. This
+paper gives full reference to the previous literature of the subject.
+
+[11] Morgan, T. H., _Evolution and Adaptation_, New York, 1903.
+
+[12] Kammerer's chief paper on this subject is in _Arch. f. Entwm._,
+1909, XXVIII, p. 447, and it is to this that the paginal references in
+the present text relate. His previous paper appeared, _ibid._, 1906,
+XXII, p. 48. An account of his further experiments with _Alytes_ is
+given in _Natur_, 1909-10, Heft 6, p. 95.
+
+[13] In reply to my letter Dr. Kammerer who was then away from home
+very kindly replied that he was not quite sure whether he had killed
+specimens of _Alytes_ with "_Brunftschwielen_" or whether he only
+had living males of the fourth generation, but that he would send
+illustrative material.
+
+[14] Kammerer, P., _ Natur_, 12 December, 1909, Heft 6, p. 95, repeated
+in _12 Flugschrift d. Deutsch Ges. f. Zuechtungskunde_, Berlin, 1910.
+
+[15] _Festschrift zum Andenken an Gregor Mendel_, being vol. XLIX of the
+_Verh. Naturf. Ver. in Bruenn_, 1911, p. 98.
+
+[16] Kammerer's chief papers on this subject are _Archiv fur Entwm._,
+XVII, 1904, and _ibid._, XXV, 1907. An epitome of results is also given
+by him in _12 Flugschrift d. Deutsch. Ges. f. Zuechtungskunde_, Berlin,
+1910.
+
+[17] "_Bei Fortdauer der Versuchsbedingungen sind als Vollmolche
+geborene Salamandra maculosa_ gleich bei der ersten Geburt _abermals
+voll molchgebaerend_, benutzen zum Geburtsakt das trockene Land,
+und zwar unter Erreichung der (bei _Salamandra atra_ normalen)
+_Embryonen-Zweizahl_," Kammerer, 1907, p. 49.
+
+[18] 1904, p. 56.
+
+[19] Throughout Kammerer's papers this is used almost as a technical
+term. It means, I presume, that the feature was manifested more than
+once.
+
+[20] It should be stated that the papers contain a quantity of detail,
+especially descriptive of the state of the larvae, which I have not
+attempted to represent, but the account here given contains all that
+seemed essential to an understanding of the more important features of
+the account.
+
+[21] The first appeared in _Natur_, 1909-10, Heft 6, p. 94; and the
+second, which contains coloured plates of the animals, in the lecture
+already referred to, _12 Flugschr. d. Deut. Ges. f. Zuechtungkunde_,
+Berlin, 1910, p. 26. In the paper in _Mendel Festschrift_, 1911, the
+subject is continued, but no more is added as to this part of the
+experiment.
+
+[22] E. G. Boulenger, _Proc. Zool. Soc._, 1911, p. 323.
+
+[23] _Mendel Festschrift_, 1911, p. 84.
+
+[24] _12 Flugschrift. Deut. Ges. Zuechtungskunde_, 1910, Fig. 15, _P.
+Reihe_.
+
+[25] _Mendel Festschrift_, 1911, p. 83.
+
+[26] Field, 1912, 30 March.
+
+[27] _Ibid._, 1904, p. 863.
+
+[28] _Mitth. Naturw. Ver. a. d. Univ. Wien_, 1908, p. 53.
+
+[29] As to the variations of _Lacerta muralis_ in Western Europe and
+North Africa see Boulenger, G. A., _Trans. Zool. Soc._, 1905, vol. XVII,
+p. 351.
+
+[30] As to the experiments of Klebs relating to the transmission of
+acquired characters, see Appendix.
+
+
+
+
+                           CHAPTER X
+
+            EFFECTS OF CHANGED CONDITIONS CONTINUED
+
+
+                THE CAUSES OF GENETIC VARIATION
+
+In the last chapter we examined some of the evidence offered in support
+of the belief that adaptation in highly organised forms is a consequence
+of the inheritance of adaptative changes induced by the influence of
+external conditions. The state of knowledge of this whole subject is, as
+I have said, most unsatisfactory, chiefly for the reason that in none of
+the cases which are alleged to show a positive result have two observers
+been over the same ground, or as yet confirmed each other. In the wider
+consideration respecting the causes of variation at large we find
+ourselves still in the same difficulty. The study has thus far proved
+sadly unfruitful. In spite of the considerable efforts lately made by
+many observers to induce genetic variation in highly organised plants
+or animals, and though successes have occasionally been announced, I
+do not know a single case which has been established and confirmed in
+such a way that we could with confidence expect to witness the alleged
+phenomena if we were to repeat the experiment. Abundant illustrations
+are available in which individuals exposed to novel conditions manifest
+considerable changes in characters or properties, but as yet there is
+no certain means of determining that germ-cells of a new type shall be
+formed.
+
+Of the direct effect of conditions the lower organisms, especially
+bacteria, offer the best examples, the alterations of virulence which
+can be produced in so many distinct ways being the most striking
+and familiar. That attenuation of virulence can be produced by high
+temperatures or by exposure to chemical agents, and that this diminution
+in virulence may remain permanent is, from our point of view, not
+surprising; but the fact that in many cases the full virulence can by
+suitable cultivation be restored is difficult to understand. Similar
+variations have been observed in power of pigment production and other
+properties.
+
+These phenomena naturally raise the question whether any cases of
+apparent loss of factors in higher forms may be comparable.
+
+The subject of variations in the lower organisms and their dependence on
+conditions is a highly special one, and I have no knowledge which can
+justify me in offering any discussion of them, but I understand that
+hitherto little beyond empirical recognition of the phenomena has been
+attempted. A useful summary of observations made by many investigators
+was lately published by Hans Pringsheim,[1] who enumerates the different
+agencies which have been observed to produce modifications, and the
+various ways in which these changes are manifested. One of the most
+comprehensive studies of the subject from the genetic point of view is
+that made by F. Wolf.[2] In his extensive cultivations of _Bacillus
+prodigiosus_, _Staphylococcus pyogenes_ and _Myxococcus_ he succeeded
+in producing many strains with modified properties. In most of these
+the modifications arose in consequence of the application of high or
+low temperatures or of the addition of various chemical substances
+to the culture-media. Some of the variations, which are for the most
+part in the powers of pigment-formation, persisted when the strains
+were returned to normal conditions, and others did not. In reference
+especially to the variations witnessed in the Cocci the reader should
+consult the critical account of variation in that group published by
+the Winslows,[3] where much information on the subject is to be found.
+The authors attempted to determine the systematic relationships of the
+several forms, as far as possible, by the application of statistical
+methods. The result is interesting as showing that the problem of
+species in its main features is presented by these organisms in a form
+identical with that which we know so well in the higher animals and
+plants, whatever properties be selected as the diagnostic characters.
+There are many types perfectly distinct and others which intergrade.
+Some of the types change greatly with conditions while others do not.
+This is exactly what we encounter whenever we study the problem of
+species on an extended scale among the higher forms of life.
+
+There is now practically complete agreement among bacteriologists
+that the observations made first by Massini on the change in color of
+_Bacterium coli mutabile_ grown in Endo's medium, associated with the
+acquisition of the power to ferment lactose, are perfectly reliable
+and free from possibilities of mistake. The work has been extended and
+confirmed by many workers, especially R. Mueller, who finds that this
+bacterium can similarly acquire and maintain the power to ferment other
+sugars. A careful account of the whole subject written by Mueller for the
+information of biologists will be found in _Zts. fuer Abstammungsl._,
+VIII, 1912. After discussing the biological significance of the facts,
+he concludes with a caution to the effect that bacteria are so different
+from all other living things that generalizations from their behavior
+must not be indiscriminately applied to animals and plants.
+
+In all work with this class of material there is obviously danger of
+error through foreign infection of the cultures, but there can be no
+doubt that though some of the "mutations" recorded may be due to this
+cause, the majority of the instances observed under stringent conditions
+are genuine.
+
+Another and equally serious difficulty besetting work with bacteria
+and fungi cultivated from spores is that the appearance of variation
+may in reality be due to the selection of a special strain previously
+living masked among other strains. This possibility must be remembered
+especially in those instances which are claimed as exemplifying the
+effects of acclimatisation. Manifestly this consideration can be urged
+with most force when the strain which gave rise to the novelty was
+not raised from a single individual spore. Moreover, when once the
+possibility of spontaneous variation is admitted, it must be difficult
+to be quite confident that any given variation observed is in reality
+due to the novel conditions applied, and as I understand the evidence,
+the appearance of the mutational forms does not with any regularity
+follow upon the application of the changed conditions.
+
+Researches into the variation of these lower forms will, no doubt, be
+continued on a comprehensive scale. So long as the instances recorded
+are each isolated examples it is impossible to know what value they
+possess. If they could be coordinated in such a way as to provide some
+general conception of the types of variation in properties to which
+bacteria, or any considerable group of them, are habitually liable, the
+knowledge might greatly advance the elucidation of genetic problems.
+
+Of mutational changes directly produced with regularity in
+micro-organisms by treatment, the experiments with trypanosomes provide
+some of the clearest examples. A summary of the evidence was lately
+published by Dobell,[4] from which the present account is taken. The
+most definite fact of this kind established is that certain dyes
+introduced into the blood of the host have the effect of destroying
+the small organ known as the "kinetonucleus" in the trypanosomes. The
+trypanosomes thus altered continue to breed, and give rise to races
+destitute of kinetonuclei. This observation was originally made by
+Werbitzki and has been confirmed by several observers. The exact way
+in which this alteration is effected in the trypanosomes is not quite
+definitely made out, but there is good reason for supposing that
+the dyes have a direct and specific action upon the kinetonucleus
+itself, and circumstances make it improbable that in some division a
+daughter-organism without that body is produced, or that any selection
+of a pre-existing defective variety occurs.
+
+Ehrlich has suggested with great probability that the dyes which possess
+this action owe it to the fact that they have the particular chemical
+linkage which he calls "ortho-quinoid." In outward respects, such as
+motility and general appearance, the modified organisms are unchanged,
+but their virulence is diminished. As regards the possibility of the
+defective strain reacquiring the kinetonucleus, Werbitzki states
+that in one case passage through 50 animals and treatment with dyes
+left the strain unaltered; but that in another case at the sixteenth
+passage 7 per cent. of the trypanosomes were found to have re-acquired
+the organ, and in subsequent passages the percentage increased, until
+at the twenty-seventh passage practically all had re-acquired it.
+Kudicke, however, in similar experiments did not succeed in causing
+re-acquisition by transplantation.
+
+By the action of various drugs and anti-bodies races of trypanosomes
+resistant to those substances have been obtained. These breed true, at
+least when kept in the same species of animal in which the resistance
+was acquired. As to whether change of virulence is produced by passage
+through certain animals or not, there is as yet no general agreement.
+
+Other changes, especially in size and some points of structure, are said
+to occur when certain trypanosomes proper to mammals are passed through
+cold-blooded vertebrates (Wendelstadt and Fellmer), and it is stated
+that these changes persist, but the observations have not yet been
+confirmed.
+
+Experiments lately conducted by Woltereck with _Daphnia_ are interesting
+as having given a definite positive result, in so far, at least, as the
+ova were affected by conditions before leaving the bodies of the parent
+individuals. The observations relate to the offspring resulting from
+_parthenogenetic_ eggs. Females bearing ephippia (fertilised eggs) were
+isolated until the ephippia were dropped, and in this way the offspring
+of fertilisation were excluded. Males, of course, appeared from time
+to time in the cultures, but as fertilised eggs were rejected, their
+presence did not disturb the result. The most remarkable observations
+related to _Daphnia longispina_.
+
+This species as found in the lower lake at Lunz had the front end of
+the body blunt and nearly round in profile; but on being cultivated in
+a warm temperature and with abundant nourishment the front end of the
+body became produced into an elongated "helmet," as Woltereck calls it.
+Experiment showed that the change was primarily due to the abundance of
+food, and owing to temperature in a subordinate degree.
+
+This distinction arose as soon as the species was taken into the
+hothouse, but when the modified individuals were put back into the
+original conditions, a lower temperature and scanty food-supply,
+the next generation returned to their original form. After being
+cultivated for two years and about 40 generations in the more favourable
+conditions, when similarly put back into the lower temperature with
+scanty food the _first generation_ born in these conditions was helmeted
+like the modified parents. Woltereck is of opinion that the ova were
+still unformed at the time the parents were put back, and the influence
+of the favourable conditions upon the unformed ova he speaks of as a
+"prae-induction." The effect never extended beyond the one generation,
+after which the strain returned to its original state.
+
+The fact that the influence on the offspring was not manifested at
+first led Woltereck to expect that by more prolonged cultivation in the
+favourable conditions a further extension of this influence would be
+produced, but this expectation was never fulfilled, though the attempt
+was made again and again.
+
+Similar experiments were made with _Hyalodaphnia cucullata_, which is
+far more sensitive to cultural influences, and in nature manifests
+a considerable elongation of the helmet as a seasonal modification,
+but the results were essentially the same as in the preceding case,
+no modification extending beyond the first generation born after the
+restoration to _normal conditions_.[5]
+
+The only criticism of these extremely interesting results which suggests
+itself is that perhaps the original appearance of the modification was
+not in reality due to an _accumulated_ effect of the conditions, but
+to some change in the conditions themselves which was not noticed. It
+is difficult to see how length of time or even the lapse of several
+generations could have so specific an effect on the race. It is no
+doubt often vaguely supposed by many that a long period of time may be
+necessary for the effect of climate or of other environmental conditions
+to be produced in an organism which does not thus respond at first. I
+have never been able to see any reason for this opinion nor how it is
+to be translated into terms of physiological fact, and I imagine that
+in those cases in which the lapse of time is really required for the
+production of an effect, the influence of the prolongation is rather
+on the conditions than on the organisms. The response of the organisms
+thus probably indicates not that the creature is at length feeling the
+effects because of their accumulated action on itself, but that the
+conditions have at length ripened.
+
+As this sheet is passing through the press Agar has published[6] an
+abstract of evidence as to another comparable case in a parthenogenetic
+strain in the daphnid, _Simocephalus vetulus_. When fed on certain
+abnormal foods the shape of the body is changed, the edges of the
+carapace being rolled backwards so as to expose the appendages. The
+offspring of animals thus modified showed similar modification in the
+first, and to a very slight degree, in the second generation, though the
+original mothers were removed to normal conditions before their eggs
+were laid. In the third generation there was "a very pronounced reaction
+in the opposite direction." Agar suggests that the change may be due to
+some toxin-like substances, carried on passively by the egg into the
+next generation, against which the protoplasm eventually produces an
+anti-body.
+
+The experiments which have been in recent years regarded by evolutionary
+writers as the most conclusive proof that direct environmental action
+may produce germinal variation are those of Professor W. L. Tower, of
+Chicago, on _Leptinotarsa_, the potato beetles. This work has attained
+considerable celebrity and has been generally accepted as making a
+definite extension of knowledge. After frequently reading Tower's papers
+and after having been privileged to see some of the experiments in
+progress (in 1907) I am still in doubt as to the weight which should be
+assigned to this contribution.
+
+The work is described in two chief publications, the first of which
+appeared in 1906.[7] This treatise contains a vast amount of information
+about numerous species and varieties of these beetles which the author
+has observed and bred in many parts of their distribution throughout
+the United States, Mexico and Central America. The part of the book
+which has naturally excited the greatest interest is that in which Tower
+states that by subjecting the beetles to change in temperature and
+moisture, he caused them to produce offspring quite unlike themselves,
+which in several cases bred true.
+
+It is much to be regretted that the author did not happen to become
+acquainted with Mendelian analysis at an earlier stage in the
+investigation. The evidence might then have been handled in a much more
+orderly and comprehensive way, and a watch would have been kept for
+several possibilities of error.
+
+The headquarters of the genus is evidently as Tower states, in Mexico
+and the adjoining countries. In this region there is a great profusion
+of forms, some very local, some as for instance the well-known
+_decemlineata_,[8] more widely spread. The distinctions are almost all
+found in peculiarities of colour and pattern, and the limits of species
+are even more indefinable than is usual in multiform animals. Tower
+arranges the various types into seven groups of which the one most
+studied is that which he calls the _lineata_ group. To this group belong
+all the forms to which reference is here made, and, as I understand,
+they differ among themselves entirely in size, colour and pattern.
+There is no suggestion of infertility in the crosses made between the
+several forms of the _lineata_ group; in fact they present, like many
+Chrysomelidae, a good example of what most of us would now call a
+polymorphic species, consisting of many types, some found existing in
+the same locality, others being geographically isolated.
+
+A series of experiments was devoted to the attempt to fix strains
+corresponding to the extremes of continuous variations. For example,
+those with most black pigment and those with least black taken from
+a population continuously varying in this respect, were separately
+bred; but almost always the selection led to no sensible change in the
+position of the mean of the population. The variations in these cases
+were evidently fluctuational. In some instances, however, real genetic
+differences were met with, and strains exhibiting them were, as usual,
+rapidly fixed.
+
+Tower points out that several of the varieties (or species, as he
+prefers to call them) were obviously recessive to _decemlineata_. This
+is most clearly demonstrated in the case of the form called _pallida_,
+which is a pale depauperated-looking creature, with the orange of
+the thorax almost white and the eyes devoid of pigment.[9] This form
+behaved as an ordinary Mendelian recessive, breeding true whenever it
+appeared in the cultures, or when individuals found wild were studied
+in captivity. A black form which Tower names _melanicum_ was similarly
+shown to be a Mendelian recessive. Wild specimens of this variety of
+opposite sexes were not found simultaneously in nature, and there was
+thus no opportunity of breeding them together, but the hereditary
+behaviour was seen in the F_{2} generation from a _melanicum_ found
+coupled with _decemlineata_. Experiments also occurred giving indication
+that a variety with the stripes anastomosing in pairs (_tortuosa_), was
+another recessive, and that a variety--called "_rubri-vittata_"--gave an
+intermediate F_{1} with subsequent segregation. All these are forms of
+_decemlineata_ Stal.
+
+Similar observations were made regarding forms recessive to
+_multitaeniata_ Stal. Of these two were thrown by _multitaeniata_
+itself, namely a form named by Stal _melanothorax_, and regarded by him
+as a species, and one which Tower names _rubicunda_ n. sp. The facts
+proving the recessive behaviour of their several forms will be found in
+the following places in Tower's book:
+
+  _pallida_,                      pp. 273-278.
+  _melanicum_,                     p. 279.
+  _tortuosa_,                      p. 280.
+  _rubrivittata_,                 pp. 280-281.
+  _melanothorax_ and _rubicunda_, pp. 283-285.
+
+Following this evidence of recessive nature of the six forms
+enumerated, Tower describes experiments showing, as he believes, that
+some of them may be caused to appear by applying special treatment
+to the parents during the "growth and fertilisation" (p. 287) of the
+eggs. The most striking example is that in which 4 males and 4 females
+of _decemlineata_ were kept very hot (average 35 deg. C.) and dry, and at
+low atmospheric pressure (19-21 inches). The eggs laid were restored
+to natural conditions. These gave 506 larvae, from which emerged 14
+normal, 82 _pallida_ and 2 "_immaculothorax_," viz., without pigment
+on the pronotum. The account of the rest of the experiment is somewhat
+involved, but I understand that the _pallida_, of which two only
+survived, behaved as normal recessives when bred to the type: also that
+the parents, after having laid the eggs whose history has been given,
+were restored to normal conditions and laid 319 eggs which gave 61
+normals.
+
+In another case normal parents laid 409 eggs in the hot and dry
+conditions, and on restoration to normal conditions, the same parents
+laid 840 eggs. Then 409 eggs gave 64 adults as follows:
+
+                     _Males_          _Females_
+
+  _decemlineata_        12                 8
+  _pallida_             10                13
+  _immaculothorax_       2                 3
+  _albida_               9                 7
+                        ---               ---
+                        33                31
+
+The 840 eggs laid in normal conditions gave 123 normal _decemlineata_.
+
+Similar experiments were made with _multitaeniata_ and gave comparable
+results, the two recessives (_melanothorax_, _rubicunda_) being produced
+in large numbers when the parents were subjected to heat, but in this
+case the atmosphere was kept _saturated_ with moisture, instead of dry,
+as in the previous instance. The same parents transferred to normal
+conditions gave normals only.
+
+Lastly the form _undecimlineata_ was exposed "to an extreme stimulus of
+high temperature, 10 deg. C. above the average," and a dry atmosphere, with
+the result that from 190 eggs there emerged 11 beetles, all of the form
+_angustovittata_ Jacoby, which subsequently bred true to that type (see
+p. 295).
+
+In the results of these experiments, as described, there is one feature
+which I regard as quite unaccountable. Tower makes no comment upon
+it. Indeed, from the general tenour of the paper, I infer, not only
+that he does not perceive that he is recounting anything contrary to
+usual experience, but rather that he regards the result as conforming
+to expectations previously formed. The point in question is the
+genetic behaviour of the dominant normals produced under the abnormal
+conditions. These normals were the result of the breeding of parents
+declared to be at the same time giving off many recessive gametes. Some
+of these normals must be expected therefore to be heterozygous unless
+some selective fertilisation occurs. Nevertheless in every case they and
+their offspring are reported to have continually bred true. I allude
+especially to the tables given on pp. 288, 289, 292, and 293. Tower does
+not mention any misgiving about this result, and I think he regards
+himself as recounting phenomena in general harmony with the ideas of
+mutation expressed by De Vries. This they may be; but to anyone familiar
+with analytical breeding the course of these experiments must seem so
+surprising as to call for most careful, independent confirmation.
+
+In 1910[10] Tower published an account of further experiments with
+_Leptinotarsa_. The work described related to two subjects. Crosses
+were made between three forms, _undecimlineata_ Stal, _signaticollis_
+Stal and "_diversa_" named by Tower as a new species. The distinctions
+between these three depend partly on characters of the adults and
+partly on those of the larvae. The adults of _undecimlineata_ and
+_diversa_ have the elytra striped, but the elytra of _signaticollis_
+are unstriped. The larvae of _signaticollis_ and of _diversa_ are
+yellow, but those of _undecimlineata_ are white.[11] Moreover, in
+_signaticollis_ and _diversa_ the black increases in the third
+stage of the larvae to form transverse bands which are absent in
+_undecimlineata_. The general course of the experiments shows that these
+differences may be approximately represented as due to the action of
+three factors, any of which may be independently present or absent. The
+stripings of the elytra and of the larvae are each due to a separate
+factor. As regards the distinction between the yellow and the white
+larvae the evidence does not prove that there is decided dominance of
+either colour and I infer that the heterozygotes are often intermediate.
+
+The chief contribution which this new paper claims to make relates to
+differences in the results which ensue from crosses effected between
+these three types at different average temperatures.
+
+We are first concerned with four experiments which I number (1), (2),
+(3), (4):
+
+1. _Signaticollis_ [F] x _diversa_ [M] bred at an average temperature
+of 80º F. by day and 75 deg. F. by night, gave two groups in about equal
+numbers. The first (49) was pure _signaticollis_ and bred true. The
+second (53) was of an intermediate type, which on being bred together
+gave the typical Mendelian result--1 _sig._: 2 _intermediate_: 1 _div_.
+
+2. Next, as the account originally stood in the published paper, we
+are told that _sig_ [F] x _div_ [M] bred together at a day-temp.
+average 75 deg. F. and night average 50 deg. F. gave an _intermediate_ only,
+which subsequently produced a normal 1:2:1 ratio. The two crosses were
+repeated eleven times with identical results.
+
+In a further experiment (3) _signaticollis_ [F] x _diversa_ [M] were
+bred under the same conditions as those used in expt. (1). They again
+gave _sig._ and intermediates as before in fairly equal numbers. The
+_sig._ as before bred true, and the intermediate gave 1:2:1, all exactly
+as in expt. (1).
+
+In expt. (4) _the same parents used_ in (3) were again mated under
+conditions of expt. (2) at the lower temperature, and this time gave
+_signaticollis_ exclusively, which bred true for four generations. This
+experiment was repeated seven times with uniform results.
+
+Diagrams are given representing all these histories in graphic fashion.
+
+From these observations, Tower concludes that the determination of
+dominance, and the ensuing type of behaviour, is clearly a function of
+the conditions incident upon the combining germ plasms.
+
+It will be observed that expts. (1) and (3) gave identical results
+but (2) and (4), though much the same conditions were applied, are
+at variance, for (2) gave all intermediates, while (4) gave all
+_signaticollis_. In _Amer. Nat._, XLIV, 1910, p. 747, Professor T.
+D. A. Cockerell commented on this paper of Tower's and pointed out
+that there must be an error somewhere, for when he discusses these
+experiments Tower speaks of (2) and (4) as confirming each other. To
+this Tower replied[12] that there had been a mistake. He states that
+in preparing the paper "certain minor experiments were taken from
+a larger series and combined to illustrate a general point in the
+behaviour of alternative characters in inheritance," and that expt. (2)
+was introduced inadvertently in place of another which he desires to
+substitute. In this, which I number (5), _signaticollis_ [F] x _diversa_
+[M] from exactly the same stocks as those used in (1), were mated at the
+lower temperatures specified for (2), day average 75 deg. F., night average
+50 deg. F. These gave all of the _signaticollis_ type with a narrow range of
+variability, which bred true, in some cases to F_{6}. Tower says he has
+repeated this experiment six times with identical results.
+
+Nevertheless he proceeds to say that the description of expt. (2), which
+was repeated eleven times with identical results, was correct "as far as
+given." That experiment was "from a second series of cultures parallel
+to the one given, but in which there are other factors involved, which
+in H. 410 [my (2)] are productive of a typical Mendelian behaviour." He
+adds he does "not care at this time to make any statement of what these
+factors are, nor of their relations to the behaviours given in the H.
+409, H. 411, H. 409/11 series [my (1), (5) and (3)--(4)] which are the
+simplest and most easily presented series obtained in the crossing of
+_signaticollis_ and _diversa_."
+
+Professor Cockerell's intervention has thus elicited the fact that
+we have as yet only a small selected part of the evidence before us,
+even as concerning the effect of temperature on the cross between
+_signaticollis_ [F] x _diversa_ [M]. We learn that at the lower
+temperatures the result was eleven times the expected one, and six times
+an unexpected one; further, that we owe it to the author's inadvertence
+that we have come to hear of the expected result at all, and that though
+he knows the factors which determine the discrepancy, he declines for
+the present to name them. In these circumstances we can scarcely venture
+as yet to estimate the significance of these records.
+
+The paper goes on to recount somewhat comparable, but more complex
+instances in which the descent of the colour of adults and of larvae
+was affected by temperature in crosses between _undecimlineata_ and
+_signaticollis_. As they stand the results are very striking and
+unexpected, but I think, in view of what has been admitted respecting
+the former part of the paper, full discussion may be postponed till
+confirmation is forthcoming.
+
+One feature, however, calls for remark. This second paper is written
+apparently without any reference to the discoveries related by Tower in
+his previous book, to which no allusion is made. This is most noticeable
+in the case of an experiment in which (p. 296, H. 700A) _undecimlineata_
+[F] (the dominant) was mated to _signaticollis_ [M] with the result
+that all the offspring were _undecimlineata_ and bred true to that
+type (Parthenogenesis was tested for, but never found to occur). This
+experiment was made at a temperature averaging 95 deg. F. +- 3.5 deg. by day
+and 89 deg. F. +- 4.8 deg. by night, and in a humidity given as 84 per cent. by
+day and 100 per cent. by night; but in the previous book (p. 294) we
+are told that pure _undecimlineata_ bred together "under an extreme
+stimulus of high temperature, 10 deg. C. above the average" and a relative
+humidity of 40 per cent. gave 11 beetles only, all _angustovittata_.
+But reference to the Plate 16, Fig. 2, shows that _angustovittata_
+must be exceedingly like _signaticollis_, having, like it, the elytral
+stripes obsolete, and if there is any marked difference at all, it can
+only be in the larvae. It seems strange that if _undecimlineata_ really
+gives off ova of this recessive type at high temperatures, the fact
+should not be alluded to in connection with expt. H. 700A, where, as
+the father was _signaticollis_, having the same recessive character,
+their appearance might have been expected not to pass unobserved. The
+temperature in the older experiment is, of course, not given with the
+great accuracy used in the second, and it may have been higher still.
+The humidity also was widely different. Still, in discussing the
+phenomena we should expect some reference to the very remarkable and
+closely cognate discovery which Tower himself had previously reported in
+regard to the same species.[13]
+
+The hesitation which I had come to feel respecting these two
+publications of Tower's has been, I confess, increased by the appearance
+of a destructive criticism by Gortner[14] who has examined the parts
+of Chapter III of Tower's book, in which he discusses at some length
+the chemistry of the pigments in _Leptinotarsa_ and other animals. As
+Gortner has shown, this discussion, though offered with every show of
+confidence, exhibits such elementary ignorance, both of the special
+subject and of chemistry in general, that it cannot be taken into
+serious consideration.
+
+Some observations made by Dr. W. T. Macdougal[15] have also been
+interpreted as showing the actual causation of genetic variation by
+chemical treatment. Of these perhaps the least open to objection
+were the experiments with _Raimannia odorata_, a Patagonian plant
+closely allied to _Oenothera_. The ovaries were injected with various
+substances and from some of the seeds which subsequently formed in them
+a remarkable new variety was raised. This varying or mutational form
+was strikingly different from the parental type, with which it was not
+connected by any intergradational forms, and it bred true. It made
+no rosette, growing to a much smaller size than the parent, and was
+totally glabrous instead of being very hairy as the parental type is.
+I was shown specimens of these plants by the kindness of Dr. Britton
+in the Bronx Park Botanic Garden in 1907 and can testify to their very
+remarkable peculiarities. They had a somewhat weakly look, and might
+at first sight be thought to be a pathological product, but they had
+bred true for several generations. From the evidence, however, I am by
+no means satisfied that their original appearance was a consequence
+of the treatment applied. This treatment was of a most miscellaneous
+description. Two of the mutants came from an ovary which had been
+treated with a ten per cent. sugar solution. Ten came from one into
+which a 0.1 per cent. solution of calcium nitrate had been injected. One
+was from a capsule which "had been exposed to the action of a radium
+pencil." Macdougal speaks of these results as decisive, but clearly
+before such evidence can be admitted even for consideration it must be
+shown by control experiments that the individual plants which threw
+the mutant were themselves breeding true in ordinary circumstances.
+Nothing is more likely than that the mutant was an ordinary recessive.
+I may add that Mr. R. H. Compton made a number of experiments with
+_Raimannia odorata_, raised from seeds kindly given me by Dr. Britton,
+injecting the ovaries with a variety of substances, including those
+named by Macdougal; but though a numerous progeny was raised from the
+ovaries treated, all were normal. Macdougal relates also that some
+mutational forms came from ovaries of _Oenothera Lamarckiana_ exposed
+to radium pencils, and also from _Oenothera biennis_ injected with
+zinc sulphate a peculiar mutant was raised, but taking into account
+the frequency of these occurrences in those species, he very properly
+regarded this evidence as of doubtful application. In a later paper,[16]
+however, he has returned to the subject and affirms his conviction that
+the appearance of a mutant among seedlings raised from an ovary of
+_Oenothera biennis_ treated with zinc sulphate was really a consequence
+of the injection, saying that the variation previously observed in
+the species was afterwards shown to be due to fungoid disease. The
+circumstances to which he mainly points in support of his view is
+that the mutation bred true, but this is only evidence of its genetic
+distinctness, which may, of course, be admitted by those who remain
+unconvinced as to the original cause of its appearance. He adds that he
+is making similar experiments with some twenty genera; but what is more
+urgently needed is repeated confirmation of the original observation.
+When it has been shown that this mutation can be produced with any
+regularity from a plant which does not otherwise produce it on normal
+self-fertilisation, the enquiry may be profitably extended to other
+plants.
+
+A curious and novel experiment, which however, led ultimately to a
+negative result, was made by F. Payne. Many discussions have been held
+respecting the blindness of cave animals. The phenomenon is one of the
+well-known difficulties, and most of us would admit that the theory of
+evolution by the natural selection of small differences does not offer a
+really satisfying account of it. Those who believe in the causation of
+such modifications by environmental influences and in their hereditary
+transmission make, of course, the simple suggestion that the darkness
+is the cause of the loss of sight, and that disuse has led to the
+reduction of the visual organs. Payne bred _Drosophila ampelophila_,
+the pomace-fly (which is easy to keep in confinement, fed on fermenting
+bananas), for sixty-nine generations in darkness. At the end of that
+period there was no perceptible change in the structure of the eyes, or
+in any other respect. The number of generations may possibly be regarded
+as insufficient to prove anything, but comparing them, as he does, with
+the generations of mankind, we see that they correspond with a period of
+about two thousand years, an interval far longer than those which many
+writers in particular cases have deemed sufficient.
+
+In his first paper Payne states that, though no structural difference
+could be perceived, the flies which had been bred in the dark reacted
+less readily to light than those which had been reared under normal
+conditions, and he inclined to think that the treatment had thus
+produced a definite effect. After more careful tests, however, he
+withdrew this opinion. It proved that both individual flies and
+individual groups of flies, both of those bred in the light and of
+those bred in the dark, differed greatly in their reactions, which were
+measured by counting the time that it took for a fly to travel to the
+light end of a covered tube, various sources of error being eliminated.
+He found further that these differences of behaviour were not inherited
+in any simple way, but he is disposed to attribute them to accidental
+differences in the nature of the food, an account which seems probable
+enough.[17]
+
+In several recent publications Blaringhem[18] has described the origin
+of many abnormal forms of plants, especially of maize, which he
+attributes to various mutilations practised upon the parents. Respecting
+these the same difficulty which has been expressed in other cases
+reappears, that before drawing any conclusion as to the value of such
+evidence we require to know that the plants treated belong to a really
+pure line, which if left to nature in the ordinary circumstances of its
+life in that locality would have had normal offspring. Abnormalities
+abound in the experience of everyone who examines pans of seedlings
+of almost any species of plant, and in maize they are well known to
+be exceptionally common. Some of those which we meet with when we
+attempt to ripen maize in this country are very similar to those which
+Blaringhem describes, consisting in irregularities in the distribution
+of the sexes, in the shapes of the panicles, etc. Many of these are
+doubtless imperfections of development, due to the dullness of our
+climate, but others are presumably genetic and would recur in the
+offspring however treated. If some one working in a climate where maize
+could be raised in perfection would repeat these experiments, and show
+that a strain which was thoroughly reliable and normal in its genetic
+behaviour did, after mutilation, throw the miscellaneous types observed
+by Blaringhem, that would be evidence at least that the development of
+the seed could be so influenced by injury to the parental tissues that
+its properties were changed. Such evidence could be used for what it is
+worth; but pending an inquiry of this kind I am disposed to regard these
+observations of variation following on parental injury as suggestive
+rather than convincing.
+
+Some evidence of a remarkably interesting kind has been collected by
+J. H. Powers[19] respecting the structure and habits of _Amblystoma
+tigrinum_, which led him to the conclusion that striking differences
+in the form, anatomy, and developmental processes could be effected
+directly by change in the conditions of life. It is well known that
+a profusion of forms, distinct in various degrees, is grouped round
+_Amblystoma tigrinum_. Some of these are believed to be geographically
+isolated, others occur together in the same waters, and, as usual,
+authorities have differed greatly as to the number of names to be given.
+These forms were studied in detail by Cope who described them in the
+_Batrachia of North America_. The view which he inclined to take was
+that the individual variations of _Amblystoma tigrinum_ resulted from
+variations in the time and completeness of the metamorphosis, and these
+were regarded as due to external causes, such as differences in season,
+temperature, and geographical conditions. Powers, however, states that
+collecting within a radius of six or eight miles he found almost if
+not quite the whole "gamut of recorded variation in this species."
+Some, however, as he states, occurred rarely except under experimental
+conditions, but considerable differences in temperature were not found
+necessary in producing them. Every year, he says, he has been able to
+add to the number of peculiar types found in the same small area in
+nature, until the amount of natural variation at least equals that seen
+by Cope in the collections of the National Museum and those of the
+Philadelphia Academy.
+
+Powers states that his observations by no means confirm Cope's view
+that these differences are in the main referable to variation in
+the completeness of metamorphosis, and on the contrary, he regards
+metamorphosis as on the whole a levelling process, tending to obliterate
+diversity. The enormous differences in size and proportions which he
+describes can only be appreciated by reference to his figures. They
+affect almost all features of bodily organisation. These striking
+differences he looks upon as brought about by differences in nutrition,
+"diversities in habitual locomotion," and diversity in the age at which
+metamorphosis occurs, and to sexual difference. Apart from sexual
+difference he regards the chief distinctions, in brief, as "acquired
+variations of the larva."
+
+As an example he gives the great elongation of some of the forms as
+"due first to slow growth, second to the free-swimming habit, third
+to the prolongation of larval life, and finally to the assumption of
+sexual maturity as males," either in the branchiate or non-branchiate
+condition. He describes the rapid growth of some and the slow growth
+of others. A larva of intermediate type may grow about a centimeter a
+month, but a rapidly growing specimen may grow more than four times
+as much. The slower rate of growth may, he says, be induced by winter
+feeding, and other treatment.[20]
+
+When, however, he goes on to describe the influences which he regards
+as exerted by the habit of freely swimming, I am led to wonder whether
+after all in most of these illustrations, the primary distinctions
+are not in reality genetic. "Specimens raised in the same aquarium or
+in similar aquaria, side by side with all conditions as uniform as it
+is possible to make them, seldom fail to furnish striking examples of
+broad-headed, short-bodied, and short-tailed types which are habitually
+found at the bottom, while others, slender and elongated, are free
+swimmers, and maintain themselves in almost as continual suspension
+and motion as does a gold fish." Later, again, he writes, "Yet despite
+the uniformity of these favourable conditions, the larvae soon began
+to split up into two noticeably distinct groups, the one of unusually
+compact proportions, the other of uniform intermediate build, such
+is most commonly met with." It is to my mind scarcely possible to
+resist the inference that, though there may be definite responses
+to certain conditions, yet the chief distinctions are genetic, and
+that it is these distinctions which confer the power to respond. The
+parts respectively played by cause and effect are always difficult to
+assign; but when it is stated that "a weak-limbed, long-bodied and
+long-tailed animal becomes well nigh perforce an undulatory swimmer,
+while the strong-limbed, short-tailed, heavy-bodied specimen, when
+these characteristics are rapidly forced upon it, is, under certain
+circumstances, just as forcibly induced to become a crawler," we feel
+how erroneous any estimates of causation are likely to be.
+
+One of the most remarkable and interesting sections of Powers' paper
+is that in which he describes the differences in bodily structure and
+habits which he attributes to cannibalism, and the whole account of the
+phenomena should be read in the original. It appears that there are
+two extremely distinct types of larvae, those with narrow heads and
+slender bodies which live for the most part on small Crustacea such
+as _Daphnias_, and those with huge mouths and very wide heads, which
+disregard such small animals altogether and live on amphibian larvae,
+whether of their own or other species. As the illustrations show, the
+differences between these two types are very great, and the differences
+in instinct and behaviour are no less. The cannibals take no heed of the
+pelagic crustacea, lying sluggishly at the bottom, rousing themselves
+immediately to a violent attack on the larger living things which
+approach them. Nothing but the most incontrovertible evidence based on
+abundant control experiments should convince us that such differences
+are not primarily genetic, and in the present state of knowledge I
+incline to think that the families really consist of individuals which
+are ready to assume the cannibal habit if opportunity offers, and
+others which are congenitally incapable of it. It may readily be that
+if all chance of cannibal diet be excluded, the full development of the
+wide head and mouth, or the other peculiarities, would never become
+pronounced, but I doubt whether such change could be induced in any
+individual taken at random.
+
+FOOTNOTES:
+
+[1] Pringsheim, H., _Die Variabilitaet niederer Organismen_, Berlin, 1910.
+
+[2] F. Wolf, Modifikationen u. Mutationen von Bakterien, _Zts. F.
+indukt. Abstam. u. Vererbungslehre_, II, 1909, p. 90.
+
+[3] Winslow, C. E. A. and A. R., _Systematic Relationships of the
+Coccaceae_. New York. 1909.
+
+[4] C. C. Dobell, _Jour. Genetics_, 1912, II, p. 201, where full
+references are given.
+
+Still more recently the same author has contributed an excellent summary
+of the evidence relating to bacteria (_ibid._, II. 1913, p. 325).
+
+[5] See Woltereck, _Verh. d. Deut. Zool. Ges._, 1909, p. 110; and 1911,
+p. 142. This is a subject which can only be properly appreciated on
+reference to the original papers. Several complications are involved to
+which I have not here alluded.
+
+[6] _Proc. Roy. Soc._, B, Vol. 86, 1913, p. 113.
+
+[7] _An Investigation of Evolution in Chrysomelid Beetles of the Genus
+Leptinotarsa_, Carnegie Publications, 1906, No. 48.
+
+[8] This is the famous Colorado beetle or potato-bug, which has caused
+such serious destruction in potato crops. There seems to be no doubt
+that this insect, formerly unknown in the eastern States, made its way
+east along the mining trails when the west was opened up.
+
+[9] This is indicated in the coloured plate, but I have not found any
+explicit statement to this effect in the text, and am not sure if the
+absence of pigment was regarded as complete.
+
+[10] _Biol. Bull._, XVIII, 1910, p. 285.
+
+[11] This description does not quite agree with the representation of
+the larvae in Pl. 17 of the book _Evolution in the Genus Leptinotarsa_
+for there the larva of _undecimlineata_ is shown as white in the second
+stage, but yellowish in the third stage; perhaps there is an error in
+printing.
+
+[12] _Biol. Bull._, XX, 1910, p. 67.
+
+[13] As to the interrelations of these three forms, Tower states (1906,
+p. 18) that _angustovittata_, which he reared from _undecimlineata_, is
+intermediate between it and _signaticollis_. Compare Stal, "_Monogr.
+des Chrysomelides_," 1862, p. 163; and Jacoby, _Biol. Centr. Amer.
+Celeopt._, vi, Pt. 1, p. 234, Pl. xiii, fig. 20; Tab. 41, fig. 15;
+_ibid._, Suppl., p. 253. All these forms are evidently very closely
+related, and the delimitation of species is quite arbitrary. Jacoby
+indeed suggests that _undecimlineata_ may be a variety of _decemlineata_.
+
+[14] Gortner, _Amer. Nat._, Dec., 1911, XLV, p. 743.
+
+[15] _Mutations, Variations, and Relationships of the Oenotheras_,
+Carnegie Institution Publication No. 81, 1907, pp. 61-64.
+
+[16] Macdougal, D. T., "Alterations in Heredity induced by Ovarial
+Treatments", _Bot. Gaz._, vol. 51, 1911, p. 241.
+
+[17] Payne, Fernandus, _Biol. Bull._, XVIII, 1910, p. 188, and _ibid._,
+XXI, 1911, p. 297.
+
+[18] See especially, _Mutation et Traumatismes_, Paris, Felix Alcan,
+1908.
+
+[19] J. H. Powers, "Morphological Variation and its Causes in
+_Amblystoma tigrinum_." _Studies from the Zoological Laboratory. _ The
+University of Nebraska, No. 71, 1907.
+
+[20] In connexion with this case I would refer the reader to some
+remarkable observations of Dr. T. A. Chapman on various types of larvae
+which he reared from the moth _Arctia caja_ (_Ent. Rec._, IV, 1893,
+p. 265, and following parts). From a single mother he raised a great
+diversity of forms, some which fed up rapidly and passed through their
+development without assuming certain stages, and others which were, as
+he called them, "laggards," moulting more times than their brethren and
+developing at a much slower rate. It is greatly to be hoped that such a
+case may be critically investigated by analytical breeding.
+
+
+
+
+                          CHAPTER XI.
+
+           STERILITY OF HYBRIDS. CONCLUDING REMARKS.
+
+When we consider the bearing of recent discoveries on those
+comprehensive schemes of evolution with which we were formerly
+satisfied, we find that certain details of the process are more easy
+to imagine. We readily now understand how varieties once formed,
+can persist, but at the same time difficulties hitherto faced with
+complacency become formidable in the light of the new knowledge. So
+generally is this admitted by those familiar with modern genetic
+research that most are rightly inclined to postpone the discussion. The
+premisses, indeed, on which such a discussion must be based are almost
+wholly wanting.
+
+The difficulties to which I chiefly refer are not those created by the
+phenomena of adaptation, though they are serious enough. In treating
+of that subject I have felt obliged to express scepticism as to the
+validity of nearly all the new evidence for the transmission of acquired
+characters. At the present time the utmost we are bound to accept is the
+proof that (1) in some parthenogenetic forms variations, or perhaps we
+may say malformations, produced in response to special conditions, recur
+in one or perhaps two generations asexually produced after removal to
+other conditions. (2) That violent maltreatment may in rare instances
+so affect the germ-cells contained in the parents as to cause the
+individuals resulting from the fertilisation of those cells to exhibit
+an arrest of development similar to that which their parents underwent.
+
+I do not doubt that evidence of this type will be greatly extended. As
+a contribution to genetic physiology these facts are very important
+and interesting, but I cannot think that any one, on reflexion, will
+feel encouraged by such indications to revive old beliefs in the direct
+origin of adaptations.
+
+In these respects we are simply left where we were. The force of
+objections based upon the existence of adaptative mechanisms is
+no greater than it has always been. On the contrary the fact that
+variations can now so generally be recognized as definite is some
+alleviation of the difficulty. We can moreover disabuse ourselves of
+the notion that for all characters which are definite or fixed, some
+utilitarian rationale may be presumed. Upon that point the study of
+variation has provided a perfectly clear answer.
+
+In frankly recognizing that the fixity of characters in general need
+not connote usefulness to their possessors we deliver ourselves of a
+distracting pre-occupation and prepare our minds for an investigation
+of the properties of living organisms in the same spirit as that
+in which the chemist and the physicist examine the properties of
+unorganized materials. The creature persists not merely by virtue of its
+characteristics but in spite of them, and the fact of its persistence
+proves no more than that on the whole the balance of its properties
+leaves something in its favour.
+
+It may be noted by the way that the fact that the structures of living
+things are on the whole adaptative was not always obvious. Though to
+naturalists of this generation it is a truism, we have only to turn to
+Buffon to find that in his philosophy of nature it played no essential
+part. The passage in which Buffon describes what he regards as the
+forlorn and degraded condition of the Woodpecker is well known. We have
+come to think of the Woodpecker as a capital example of adaptation to
+the mode of life; but Buffon after enumerating the hard features of
+the bird's existence, forced to earn its living by piercing the bark
+of trees in an attitude of perpetual constraint, remarks[1] "Tel est
+l'instinct etroit et grossier d'un oiseau borne a une vie triste et
+chetive. Il a recu de la Nature des organes et des instrumens appropries
+a cette destinee _ou plutot il tient cette destinee meme des organes
+avec lesquels il est ne_" (my italics). His reflexions on the Stilt
+(_Himantopus_) read even more strangely to us, accustomed as we are to
+see in the prodigious length and thinness of the shanks and in the other
+features of its organisation palpable adaptations to a wading life. For
+Buffon, however, this curious bird seemed a poor, neglected production,
+extravagant in its disproportions, one of the misfits of creation,
+left as a shadow in the picture composed of nature's more successful
+efforts.[2] This theme he develops at some length, being evidently well
+pleased with the idea.
+
+Our way of regarding these things is doubtless sounder and more fruitful
+than Buffon's, but it is well to remember that what seems so obvious to
+us looked quite differently to other excellent observers; and stupid
+as it may have been to have overlooked plain examples of adaptation,
+it is a far worse mistake to see adaptation everywhere. I do not seek
+to minimise the real and permanent difficulty which the existence of
+adaptations creates, but by the suggestion that all normal specific
+differences are adaptational that difficulty was quite gratuitously
+increased.
+
+In these respects it may be claimed that progress has been made, even if
+that progress seem outwardly of small account.
+
+But all constructive theories of evolution have been built on the
+understanding that what we know of the relation of varieties to species
+justifies the assumption that the one phenomenon is a phase of the
+other, and that each species arises or has arisen from another species
+either by one or several genetic steps. In the varieties we have
+accustomed ourselves to think that we see those steps. We still know
+little enough of the mode of occurrence of variation, but we do begin to
+know something, and if we ask ourselves whether our knowledge, such as
+it is, conforms at all readily with our former expectations, we cannot
+with any confidence assert that it does. Among the plants and animals
+genetically investigated are many illustrations of very striking and
+distinct varieties. Many of these might readily enough be accepted as
+species by even the most exacting systematists, and not a few have
+been so treated in classification; but when we have examined their
+relationship to each other we feel not merely that they are not species
+in any strict sense but that the distinctions they present cannot be
+regarded as stages in the direction of specific difference. Complete
+fertility of the results of inter-crossing is and I think must rightly
+be regarded as inconsistent with actual specific difference; and of
+variations leading to that consequence no clear indication has yet been
+found. As an example of possible exceptions mention should perhaps be
+made of the case of a giant form of _Primula sinensis_ investigated by
+Keeble.[3] It arose from a "Star" Primula of normal size, and though
+fertile with its own pollen all attempts to fertilise it with the pollen
+of other forms failed. Miss Pellew, who did these fertilisations,
+tells me that very extensive trials were made, and repeated in several
+seasons. Ultimately two plants were raised from it fertilised with a
+plant of the strain from which it sprang, and these proved sterile.
+In the light of modern experience the significance of such isolated
+instances is doubtful.
+
+All the strains known as "Giants" are, as Messrs. Sutton have always
+found, more or less sterile, and their sterility is presumably due to
+some negative defect.
+
+In regard to the fertility of Primula species there are several
+paradoxes. For example the long-styled varieties, apart from giants, are
+fertile with their own pollen, and for many years short-styled plants
+have not been used in most strains. Auriculas and Polyanthuses, on the
+contrary, are generally if not always bred from short-styled plants,
+as the florists have decided that the long-styled are inadmissible.
+Mr. R. P. Gregory tells me that, though most strains of _P. sinensis_
+give seed enough when only long-styled plants are used, he finds
+nevertheless that when a "legitimate" union is made the amount of seed
+usually increases much as Darwin observed. Darwin's statement that
+plants of "illegitimate" origin are less fertile than the "legitimately"
+raised plants is also in general confirmed by his experience. To
+this rule there were some marked exceptions in strains derived from
+_long_-styled plants, which though illegitimate showed a high degree
+of fertility, but illegitimate unions between _short_-styled plants
+always produced comparatively sterile offspring. I have no records of
+the behavior of Auriculas and Polyanthuses. It would be interesting to
+know whether among them pure strains of short-styled plants (dominants)
+have appeared, and, if so, how their fertility is affected. Without
+much more critical data I suppose no one would nowadays be inclined
+to follow Darwin in instituting a comparison between the sterility
+of hybrids and that of illegitimately raised plants of heterostyle
+species.[4] It is even difficult to imagine any essential resemblance
+between these two phenomena, nor has evidence ever been produced to
+show that illegitimately raised plants have bad pollen grains, which is
+the usual symptom of sterility in hybrid plants and the consequence,
+as we believe, of failure of some essential division in the process of
+maturation.
+
+The difficulty that we have no knowledge of the contemporary origin of
+forms, from a common stock, which when crossed together give a sterile
+product, is one of the objections constantly and prominently adduced
+from the time of the first promulgation of evolutionary ideas. In the
+light of recent work the objection has gathered strength. Why, if
+we are able to produce instances of variation colourably simulating
+specific difference in almost all other respects, do we never find
+an original appearance of this most widely spread of all specific
+characteristics? No doubt all breeders know that sterile animals
+and plants occasionally appear in their cultures, but it is more in
+accordance with probability that the sterility in these sporadic
+instances should be regarded as due to defect than that it should be
+thought comparable with that of the sterile hybrids. For their sterility
+must, by all analogy with results elsewhere seen, be attributed not
+to the absence of something, but to the presence and operation of
+complementary factors leading to the production of inhibition of
+division; and consistently with that interpretation, we find that when
+from a partially sterile hybrid comparatively fertile offspring can be
+raised, their comparative fertility continues in the posterity generally
+if not always without diminution. The distinction between these several
+kinds of sterility was of course not understood in Darwin's time. The
+comparison, for example, which he instituted[5] between the sterility
+of "contabescent" anthers and that of hybrids no longer holds, for at
+least in those cases in which the nature of contabescent anthers have
+been genetically investigated (Sweet Pea, _Tropaeolum_) they proved
+to be a simple recessive character. Nor can we now easily suppose that
+the attempt there made by Darwin to suggest resemblance between the
+sterility produced by unnatural conditions and that of hybrids has any
+physiological justification.
+
+In regarding the power to produce a sterile or partially sterile hybrid
+as a distinction in kind, of a nature other than those which we perceive
+among our varieties, I am aware that I am laying stress on an impression
+which may hereafter prove false. The distinction nevertheless is so
+striking and so continually before the eyes of a practical breeder that
+he can scarcely avoid the inference that when he meets a considerable
+degree of sterility in a cross-bred he is dealing with something
+belonging to a distinct category, and not merely a varietal feature of
+an exceptional kind.
+
+Besides the sterility of hybrids appeal has often been made to the
+phenomenon of incompatibility, in its several stages of completeness,
+as distinguishing species. No one doubts that incompatibility may
+arise from a variety of causes of most diverse degrees of importance,
+but though sometimes referred to as an extreme case of interspecific
+sterility, it is really a very different matter. In regard to one phase
+of this incompatibility, that associated with self-sterility, some
+progress has been made, and we are not wholly without experimental
+evidence of its being within the range of contemporary variation.
+
+Given the outline of Mendelian teaching as to gametic differentiation
+and the classification of individuals in a mixed population, it
+seemed highly probable that what we call self-sterility must mean
+that the species really consisted of _classes_, some of which are
+capable of interbreeding with others while others are not. According
+to the received account every individual, though incapable of
+fertilising itself, was supposed to be able both to fertilise and to
+be fertilised by any other individual. This notion has always seemed
+to me a self-evident absurdity, for it would imply that there can be
+as many categories as individuals. Such experiments, however, as I
+made did certainly give results consistent with that belief. I first
+tried Cinerarias, which are usually self-sterile, but I found no
+incompatible pairs of plants. Whether I was deceived by the consequences
+of apogamy, or whether the pollen of certain plants may belong to more
+than one class I do not know. The results were confused in various
+ways. Usually the self-fertilised plants set little or nothing, and
+cross-fertilised they set fully with such uniformity that the few
+failures could plausibly be attributed to mistakes in manipulation
+or to other extraneous causes. Later de Vries announced[6] (without
+giving particulars) that he had proved the existence of such classes in
+_Linaria vulgaris_; but on making experiments with that species I again
+got no positive results, and I came to the conclusion that in spite of
+inherent improbability the conventional belief must be substantially
+true. At last, however, the work of Correns, lately published,[7] does
+definitely show that in one species, _Cardamine pratensis_, classes of
+individuals exist such that individuals of the same class are incapable
+of fertilising themselves or each other, but fertilisation made between
+the classes is usually completely effective. Many complications were
+encountered and some contradictory evidence is recorded, but the general
+bearing of the results was positive and indubitable.
+
+We know far too little of this phenomenon as yet to be able to
+understand its significance, but I suppose we may anticipate with some
+confidence that it will be found to be a manifestation of dissimilarity
+between the male and female gametes of the same individual, comparable
+with that first seen in the Stocks (_Matthiola_) which throw doubles--a
+state of things in all likelihood to be found widely spread among
+hermaphrodite organisms. Whether the incompatibility between species
+is to be associated with that of the self-steriles also cannot be
+positively asserted, though it seems not unreasonable to expect that
+such an association will be discovered.
+
+The case of the apple and the pear is an impressive illustration of
+this possibility. The two species are of course exceedingly alike in
+all outward respects, but nevertheless the pollen of each is entirely
+without effect on the other. Presumably we should interpret this fact
+as meaning not so much that the apple and the pear are in reality
+very wide apart, but rather that either, each is lacking in one of
+two complementary elements, or that each possesses a factor with an
+inhibitory effect. Their incompatibility may well be of the same nature
+as that of the classes in _Cardamine pratensis_.
+
+Returning now to the problem of inter-specific sterility; we note,
+as I have said, the absence of contemporary evidence that variation
+can confer on a variety the power to form a sterile hybrid with the
+parent species. The considerations based on this want of evidence have
+for a long while been familiar to all who have discussed evolutionary
+theories, and it is worth observing the exact reason why the difficulty
+strikes us now with a new and special force. In pre-Mendelian times
+all that was known was that some forms could freely interbreed without
+diminution of fertility in the product, while others could not. But now
+we find that, by virtue of segregation, from one and the same pair of
+parents, or even, in the case of hermaphrodites, from one and the same
+individual, offspring commonly arises showing among themselves exactly
+such differences as distinguish species--and very good species too. This
+we see happening again and again. But to forms capable of arising as
+brethren in one family the title species has never been meant to apply,
+and if we are going to use the term in application to fraternal groups
+we must definitely recognise that by "specific" difference is to be
+understood simply _difference_, without any immediate or even ulterior
+physiological limitation whatever. Naturally, therefore, we begin to
+think of the appearance of sterility in crosses as something apart, and
+as a manifestation which distinguishes certain kinds of unions in a very
+special way.
+
+I am perfectly aware that there are gradations in the sterility of
+hybrids as in every other characteristic upon which it has been proposed
+to base specific definitions; but, as also so often happens in the
+matter of defining intergrading categories, the difficulty in practice
+is not often such as to lead to actual ambiguity. I am speaking of
+course of those examples which are amenable to genetic experiment.
+As to the rest there is complete and permanent uncertainty. But the
+experience of the practical breeder does, I think, on the whole, support
+the contention to which systematists have so steadily clung under all
+the assaults of evolutionary philosophers, that, though we cannot
+strictly define species, they yet have properties which varieties have
+not, and that the distinction is not merely a matter of degree.
+
+The first step is to discover the nature of the factors which by their
+complementary action inhibit the critical divisions and so cause
+the sterility of the hybrid. Thus expressed, we see the problem of
+inter-specific sterility in its right place; and the question why we
+do not now find contemporary instances of varieties lately arisen in
+domestication, which when crossed back with their parents, or with their
+coderivatives, can produce sterile products, is perceived to be only a
+special case of a problem which in its more general form is that of the
+origin of new and additional factors.
+
+For the requisite evidence no comprehensive search has been made, but
+perhaps it will yet be found. All that we can say at the present time
+is that the incidence both of hybrid sterility, and of incompatibility
+also, is most capricious; and provided that two forms have such features
+in common that a cross between them seems not altogether out of the
+question, no one can predict without experiment whether such a cross
+is feasible, and if feasible whether the product will be fertile, or
+sterile more or less completely. For instance, though probably all
+the British and some Foreign Finches (Fringillidae) have been crossed
+together, and some of these crosses, as for instance, the various
+Canary-mules have been made in thousands, I believe no quite clear
+example of a fertile hybrid can be produced. Many species of Anatidae
+cross readily and produce fertile hybrids: others give results uniformly
+sterile. Though most of the Equidae can be crossed and some of the
+hybrids are among the commonest of domesticated animals there is no
+certain record of a fertile mule. Among the Canidae the dogs, wolves and
+jackals all give fertile hybrids, but there is no clearly authenticated
+instance of a cross between any of these forms and the European fox.
+In spite of their close anatomical resemblance it is doubtful if the
+rabbit and the hare have ever interbred. Many of the wild species of
+_Bos_ have been crossed and recrossed both with each other and with
+many domesticated races, but I understand that no cross with the Indian
+buffalo (_Bos bubalus_) has yet been successful even in producing a
+live calf.[8] In the genus _Primula_ many hybrids are known and several
+of them occur in nature, but hitherto no certain hybrid between _P.
+sinensis_ and any other species has been made, in spite of repeated
+attempts.
+
+In _Nicotiana_ many--doubtless all--the various forms of _N. tabacum_
+can be crossed together without diminution of fertility, though some
+are very distinct in appearance, but crosses between _tabacum_ and
+_sylvestris_ are highly sterile (in my experience totally sterile[9]),
+though the distinctions between them are not to outward observation
+nearly so great as those which can be found between the various races of
+_Primula sinensis_.
+
+Recently some remarkable experiments bearing closely on these questions
+have been published by F. Rosen.[10] They concern the forms of _Erophila
+(Draba) verna_, celebrated in the history of evolutionary theory as the
+plants especially chosen by Alexis Jordan for the exposition of his
+views on these subjects.
+
+The "species" contains a profusion of forms dissimilar in many
+structural characters, such as the size and shape of leaves, flowers,
+fruits, etc. Of these forms many grow in association. Jordan found, on
+experiment, that each, to the number of some two hundred, bred true, and
+that therefore, the conventional assumption that polymorphism of this
+kind must mean great contemporary variability had no foundation in fact.
+So far indeed is the evidence from favouring the belief that such forms
+are in any way transitional or indeterminate, that, as is well known,
+Jordan used it with every plausibility to support the doctrine of the
+fixity of species. To certain aspects of Jordan's work we will return
+later in this chapter, but the matter is in the present connection
+of especial interest for the reason that Rosen has lately found by
+experiment that some of these presumably very closely allied forms,
+crossed together, gave hybrids more or less sterile. In the case of the
+offspring of one pair of forms only (_E. cochleata_ and _stricta_) was
+the fertility undiminished, and the various degrees of sterility found
+in the other crosses ranged up to the extreme infertility of the hybrids
+between _E. stricta_ x _elata_. From this cross ten plants were bred.
+Of these the four strongest were chosen to breed from, but two of the
+four proved totally sterile; one had only bad seeds; and from the fourth
+a single seedling was raised which in its turn proved to be sterile.
+From the less sterile hybrids F_{2} families were raised, with the
+usual experience that in this and subsequent generations the sterility
+diminished among extracted forms, new and true-breeding types with
+complete fertility being thus derived from the original cross.[11]
+
+The production of sterility as a consequence of crossing plants so
+nearly approaching each other as these _Erophila_ "species" do is
+not a little interesting, and the fact well exemplifies the futility
+of the various attempts to frame general expressions as to specific
+properties or behaviour. Commenting on his results Rosen argues that
+the polymorphic group commonly called by systematists _Erophila (Draba)
+verna_ may now be regarded as having arisen by crossing, as did his own
+types mentioned above. The question, however, _what_ species were the
+original progenitors of the group cannot be answered. Rosen considers
+that no form which he knows satisfies the requirements, and that
+it or they must be supposed to be lost. This conclusion will recall
+the similar problem raised by the _Oenothera_ mutants (Chap. V); and
+unsatisfactory as it may be to have recourse to such hypotheses we
+must remember the possibility that as a consequence of hybridisation,
+subsequent segregation and recombination of factors, species may
+have thus actually, as we may say, exploded, and left nothing but a
+polymorphic group of miscellaneous types to represent them in posterity.
+If this way of regarding the phenomena be a true one, the sterility
+now seen when some of the group are re-crossed, becomes analogous to
+that "reversion or crossing" which we now so well understand to be
+a consequence of the recombination of characters separated at some
+previous point in the history of descent. In the partial sterility of
+the contemporary hybrid we see this character reappearing, formed now
+as it was on the occasion of the original cross, by the meeting of
+complementary factors.
+
+Another case that may be mentioned in this connection is that of the
+crosses between various culinary peas (_Pisum sativum_) and a peculiar
+form found by Mr. Arthur Sutton growing ostensibly in a wild state in
+Palestine. This Palestine Pea is low growing, rarely reaching 18 inches.
+It is in general appearance like a small and poorly grown field pea.
+The stems are thin and rather hard. The most obvious differences which
+distinguish this from other field peas are the marked serration of the
+stipules, and the development of pith in the pods. Such pith is often
+present in the pods of peas more or less, but in the Palestines it is
+so strongly developed as almost to form a lomentum. Curiously enough,
+though the flowers are purple much as those of ordinary field peas,
+there is no coloured spot in the axils. On the other hand, the stems
+have coloured stripes running up from the axils. Though this plant
+differs so little from domesticated peas, all crosses with them either
+failed, or produced hybrids quite or almost quite sterile. This was Mr.
+Sutton's experience, and on repeating the experiments with material
+kindly given by him I found the same result.[12]
+
+In a large series of crosses some seeds died or gave rise to feeble
+plants. Of the plants which lived, few gave any seed. The seed, however,
+that was obtained from F_{1} plants grew well enough, and the F_{2}
+plants proved, as often in such cases, fertile. In these, indeed, no
+sign of sterility was noticeable. The experiment is being repeated in
+various ways, for, as the genetic behaviour of peas is comparatively
+well known, the subject is an exceptionally favourable one for these
+investigations.
+
+Such an example shows the confusion produced the moment we attempt to
+harmonize conceptions of specific difference with results attained by
+experimental methods. It has been usual to regard the field pea (_P.
+arvense_) as a species distinct from the edible pea (_P. sativum_).
+De Candolle and others regard the field pea as derived from a form
+wild in Italy, but the origin of the edible pea is considered to be
+unknown. From breeding experiments we find no sterility whatever in the
+crosses between the various _arvense_ and _sativum_ types, nor in the
+crosses made between them and several other peculiar types from various
+countries; whereas this Palestine Pea, which only differs from a small
+_arvense_ in what might have been thought trivial characters,[13] either
+fails to cross altogether or gives a sterile product, whatever type be
+chosen as the other parent.
+
+Examples of this kind have at least the merit that they lead to more
+precise delimitations of the problem. We are confronted with two
+distinct alternatives.
+
+1. We may apply the term Species promiscuously to all distinct forms.
+If we do so it must be clearly understood that we cannot even rule out
+the several combinations of "presences and absences" represented by the
+various types whether wild or domesticated. For we may feel perfectly
+assured that at least all the _arvense_ and all the _sativum_ types yet
+subjected to experimental tests are on precisely the same level in this
+respect. There is no distinction, logical or physiological, to be drawn
+between them. Some contain more factors, and others contain fewer. In
+some the re-combinations have been brought about by natural variation or
+crossing, while the same consequences in the others have resulted from
+man's interference.
+
+2. We may follow the conventions of systematists and distinguish the
+outstanding or conspicuous forms such as _arvense_, _quadratum_,
+_sativum_ and perhaps a few more as species, and leave the rest
+unheeded. If this course is followed it must be clearly understood and
+permitted as a piece of pure pragmatism, deliberately adopted for the
+convenience of cataloguers and collectors, without regard to any natural
+fact or system whatsoever.
+
+But while following either the one plan or the other we shall be still
+awaiting the answer, which only genetic experiment can provide, to the
+question whether among the various types there are some which differ
+from the rest in a peculiar way: whether by having groups of characters
+linked together in especially durable combinations, or by possessing
+ingredients which cause greater or less disturbance in the processes of
+cell-division, and especially in the processes of gametic maturation,
+when they are united by fertilisation with complementary ingredients.
+
+Before any but the vaguest ideas regarding the nature and significance
+of inter-specific sterility can be formed, a vast amount of detailed
+work must be done. Sterility as a result of crossing, as well as
+that which is alleged sometimes to arise in consequence of changed
+conditions, is at best a negative characteristic, and there are endless
+opportunities for mistake and misinterpretation in studying features
+of this kind. No one, I suppose, would now feel any great confidence
+in most of the data which from time to time are resuscitated for the
+purpose of such discussions. Even the best collections of evidence, such
+as those given by Darwin in _Forms of Flowers_, cannot be regarded as
+critical when judged by present-day standards. Nothing short of the most
+familiar acquaintance with the habitual behaviour of individuals, and of
+strains kept under constant scrutiny for several years would enable the
+experimenter to form reliable judgments as to the value to be attached
+to observations of this class.
+
+The admission must, however, be faced that nothing in recent work
+materially tends to diminish the surprise which has always been felt
+at the absence of sterility in the crosses between co-derivatives. We
+should expect such groups of forms to behave like the _Erophila_ types,
+and frequently to produce sterile products on crossing. Whatever be
+the explanation, the fact remains that such evidence is wanting almost
+completely. In spite of all that we know of variability nothing readily
+comparable with the power to produce a sterile hybrid on crossing with
+a near ally, has yet been observed spontaneously arising, though that
+characteristic of specificity is one of the most widely distributed
+in nature. It may be that the lacuna in our evidence is due merely to
+want of attention to this special aspect of genetic inquiry, and on the
+whole that is the most acceptable view which can be proposed. But seeing
+that naturalists are more and more driven to believe the domesticated
+animals and plants to be poly-phyletic in origin--the descendants, that
+is to say, of several wild forms--the difficulty is proportionately
+greater than it was formerly, when variation spontaneously occurring was
+regarded as a sufficient account of their diversity.
+
+
+                        CONCLUDING REMARKS.
+
+The many converging lines of evidence point so clearly to the central
+fact of the origin of the forms of life by an evolutionary process
+that we are compelled to accept this deduction, but as to almost all
+the essential features, whether of cause or mode, by which specific
+diversity has become what we perceive it to be, we have to confess an
+ignorance nearly total. The transformation of masses of population by
+imperceptible steps guided by selection, is, as most of us now see, so
+inapplicable to the facts, whether of variation or of specificity, that
+we can only marvel both at the want of penetration displayed by the
+advocates of such a proposition, and at the forensic skill by which it
+was made to appear acceptable even for a time.
+
+In place of this doctrine we have little teaching of a positive kind
+to offer. We have direct perception that new forms of life may arise
+sporadically, and that they differ from their progenitors quite
+sufficiently to pass for species. By the success and maintenance of
+such sporadically arising forms, moreover, there is no reasonable doubt
+that innumerable strains, whether in isolation or in community with
+their co-derivatives, have as a fact arisen, which now pass in the
+lists of systematists as species. For an excellent account of typical
+illustrations I would refer the reader to the book lately published by
+R. E. Lloyd[14] on the rat-population of India. The observations there
+recorded are typical of the state of things disclosed whenever the
+variations of large numbers of individuals are closely investigated,
+whether in domestication or in natural conditions.
+
+Guided by such clues we may get a good way into the problem. We see
+the origin of colourable species in abundance. Then, however, doubt
+arises whether though these new forms are as good species as many
+which are accepted as such by even cautious systematists, there may
+not be a stricter physiological sense in which the term species can be
+consistently used, which would exclude the whole mass of these _petites
+especes_.
+
+If further we find that we have, with certain somewhat doubtful
+exceptions, never seen the contemporary origin of a dominant factor, or
+of inter-racial sterility between indubitable co-derivatives, it needs
+no elaboration of argument to show that the root of the matter has not
+been reached.
+
+Examination of the inter-relations of unquestionably distinct species
+nearly allied, such as the two common species of _Lychnis_, leads to
+the same disquieting conclusion, and the best suggestion we can make
+as to their origin is that _conceivably_ they may have arisen as two
+re-combinations of factors brought together by the crossing of parent
+species, one or both of which must be supposed to be lost.
+
+All this is, as need hardly be said, an unsatisfying conclusion. To
+those permanently engaged in systematics it may well bring despair.
+The best course for them is once for all to recognise that whether
+or no specific distinction may prove hereafter to have any actual
+physiological meaning, it is impossible for the systematist with the
+means at his disposal to form a judgment of value in any given case.
+Their business is purely that of the cataloguer, and beyond that they
+cannot go. They will serve science best by giving names freely and by
+describing everything to which their successors may possibly want to
+refer, and generally by subdividing their material into as many species
+as they can induce any responsible society or journal to publish.
+Between Jordan with his 200 odd species for _Erophila_, and Grenier and
+Godron with one, there is no hesitation possible. Jordan's view, as he
+again and again declares with vehemence, is at least a view of natural
+facts, whereas the collective species is a mere abstraction, convenient
+indeed for librarians and beginners, but an insidious misrepresentation
+of natural truth, perhaps more than any other the source of the
+plausible fallacies regarding evolution that have so long obstructed
+progress.
+
+Nevertheless though we have been compelled to retreat from the
+speculative position to which scientific opinion had rashly advanced,
+the prospect of permanent progress is greatly better than it was. With
+the development of genetic research clear conceptions have at length
+been formed of the kind of knowledge required and of the methods by
+which it is to be attained. If we no longer see how varieties give
+rise to species, we may feel confident that a minute study of genetic
+physiology of varieties and species is the necessary beginning of any
+critical perception of their inter-relations. It is little more than a
+century since no valid distinction between a mechanical mixture and a
+chemical combination could be perceived, and in regard to the forms of
+life we may well be in a somewhat similar confusion.
+
+As yet the genetic behaviour of animals and plants has only been
+sampled. When the work has been done on a scale so large as to provide
+generalisations, we may be in a position to declare whether specific
+difference is or is not a physiological reality.
+
+FOOTNOTES:
+
+[1] Buffon, _Hist. Nat._, Oiseaux, 1780, VII, p. 3.
+
+[2] Ibid., VIII, p. 115.
+
+[3] Keeble, _Jour. Gen._, 1912, II, p. 173.
+
+[4] _Animals and Plants_, ed. 1, 1868, II, pp. 180-5.
+
+[5] _Animals and Plants_, ed. 1, 1868, II, p. 165.
+
+[6] _Species and Varieties_, 1905, p. 471.
+
+[7] Correns, _Festschr. med.-nat. Ges. zur 84 Versamml. Deutsch. Naturf.
+u. Aertze. Muenster i. W._, 1912.
+
+[8] This is a case of a somewhat different order and I mention it partly
+for that reason as an illustration of the complexity which such negative
+instances may present. The difficulty is that though the buffalo and
+the zebu can breed together, the foetus is too large to be born alive.
+(See Ackermann _Ber. d. Ver. f. Naturk._, Kassel, 1898, p. 69. Prof. S.
+Nathusius, of Halle, who has great experience in crossing Bovidae, tells
+me that he has always failed to cross the buffalo with other species.)
+
+[9] In a paper to be published in the Report of the Genetic Conference,
+Paris, 1911, Bellair states that he obtained some partially fertile
+hybrids in the cross _N. sylvestris_ x _tabacum_. As to the various
+degrees of sterility in hybrids between _Nicotiana_ species see Lock, R.
+H., _Ann. Roy. Bot. Gardens_. Peradeniya, IV, 1909, p. 195.
+
+[10] _Beitrage zur Biol. der Pflanzen._, X, 1911, p. 379.
+
+[11] One very peculiar feature was observed, namely, that all the new
+forms in F_{2} which were bred from came true. As I understand, this
+statement applied to five such new types, and they were represented
+by 76 individuals in F_{3}, but further details on this point are
+desirable. Another curious fact was observed, namely that one of the
+F_{1} forms (_cochleata_ x _radiata_) when fertilised by _cochleata_
+gave a highly polymorphic family, but fertilised by _radiata_ the
+resulting offspring were almost uniform.
+
+[12] I also had a few F_{1} seeds given me by Mr. R. H. Lock.
+
+[13] In a paper about to appear in _Jour. Linn. Soc._ Mr. A. W. Sutton
+identifies this Palestine pea as _Pisum humile_ of Boissier and Noe.
+
+[14] Lloyd, R. E., _The Growth of Groups in the Animal Kingdom_, London,
+1912.
+
+
+
+
+                         INDEX OF SUBJECTS
+
+                                                         PAGE
+ Abraxa grossulariata,                               105,193
+ Aceras hircina, local variability,                      123
+ Achatinellidae, local forms of,                         133
+ Acquired characters, inheritance of,    188 et seq.,217,233
+ Acronycta psi, melanic,                                 138
+ Adaptation, problem of,                             187,234
+ Agelaius, local forms,                                  120
+ Agrotis, fixed and variable species,                     25
+ Alkaptonuria,                                            83
+ Alpine Plants, growing larger, if protected,            183
+ Alpine Varieties,                                       165
+ Alytes obstetricans, Kammerer's experiments on,     199,210
+ Amblystoma, races of,                                   230
+ Amphidasys betularia, melanic form,                 136,138
+   dimorphic larvae,                                     141
+ Anodonta, polymorphism of,                              130
+ Antirrhinum, striped,                                    57
+   species-hybrids,                                       99
+   albinos,                                              110
+ Apple, will not cross with pear,                        239
+ Arctia caja, effects of temperature,                    192
+   larval variation in,                                  231
+ Arctic varieties,                                       165
+ Argynnis paphia and valesina in Italy,                  121
+ Armadillo, polyembryony,                                 42
+ Artistic faculty,                                        89
+ Arum, rights and lefts,                                  57
+ Auriculas, short-styled selected,                       236
+ Axis of symmetry in hand and foot,                       48
+ Axolotl, alleged effect of conditions,                  230
+ Azalea, bud-sports,                                      55
+
+ Bacillus anthracis, unsegmented form,                    71
+ Bacillus prodigiosus, variation in,                     213
+ Bacteria, variation in,                                 212
+ Bacterium coli, variation in,                           214
+ Baeolophus, geographical races of,                      159
+ Barley, right and left-handed,                           58
+ Basilarchia, geographical races of,                     161
+ Begonia phyllomaniaca,                                   50
+   hybrids,                                               51
+ Bizarre Carnation, genetics of,                          54
+ Black, as a variation from red,                         148
+ Blackbird, varying,                                     150
+ Black Cock, fixity of,                                   28
+ Boarmia repandata, melanic form,                        136
+   rhomboidaria,                                     137,139
+ Botrytis susceptibility to,                             108
+ Bovidae, hybrid,                                        242
+ Brachydactyly,                                        89,95
+ Bradypus, vertebral variation,                           68
+ Bud-sports geometrically irregular,                   54-57
+ Buffalo, attempts to hybridize,                         242
+ Bullfinch, gynandromorph,                                45
+ Bulimus detritus, local variation of,                   126
+
+ Canary, asymmetrical markings in,                       154
+ Canidae, hybrid,                                        241
+ Capsella,                                               100
+ Cardamine pratensis,                                    239
+ Cat, Polydactylism,                                      53
+ Carnation, Picotees and bizarres compared,            54,58
+ Cataract, hereditary,                                    89
+ Certhiola, melanic,                                     142
+ Chladni figures,                                         60
+ Choloepus, vertebral variation in,                       68
+   local variation in,                                   119
+ Cinerarias, self-sterility in,                          238
+ Cistudo, local variation in,                            119
+ Climatic varieties,                                     164
+ Coccaceae, variation in,                                213
+ Coenonympha arcania, climatic forms of,                 179
+   satyrion,                                             180
+ Coereba, melanic,                                       142
+ Colaptes, geographical races,                           147 et seq.
+   chrysoides,                                           154
+ Colloids, growth in,                                     65
+ Colorado beetles, experiments on,                       218
+ Colour blindness in twins,                               44
+ Continuous variation, possible example of,              173
+ Coracias, geographical races of,                        160
+ Cotton, genetics of,                                 98,100
+ Coupling,                                               110
+ Crab, extra claws,                                       74
+ Crustacean appendages and Serial Homology,               63
+ Crystals, analogy with,                                  78
+ Cyclopian monsters, artificial,                          50
+
+ Daphnia, changed by environment,                        216
+ Dasypus, polyembryony,                                   42
+ Dianthoecia, fixed and variable species,                 25
+ Disease-resistance,                                      87
+ Division, power of,
+   a fundamental attribute of living things,              38
+   Genetics of,                                        46,50
+ Dogger Bank, large varieties on,                        125
+ Dogs, hybrid,                                           241
+ Dominance, nature of,                                    95
+ Dominants, origin of new,                          88,90,95
+ Double monsters,                                         42
+ Draba, experiments with,                                242
+ Drosophila,                                              91
+   Payne's experiments on,                               228
+
+ Earthworm, regeneration,                                 77
+ Elephant, tusk segmented,                                38
+ Entelechy,                                               80
+ Environmental treatment, effects of,                    188 et seq.
+ Enzymes and genetic factors,                             86
+ Epilepsy, inheritance of traumatic,                     197
+ Equidae, sterility of hybrid,                           241
+ Erophila, experiments with,                             242
+   species,                                              249
+ Exacum, right and left,                                  57
+ Euphonia elegantissima, local forms,                    120
+ Eupithecia rectangulata, melanic form,                  137
+
+ Factors, new,                                            88
+   loss of,                                               96
+ Factorial representation of varieties,              158,165
+ Falcons, geographical races,                            147
+ Fasciation,                                              49
+ Ferments, Boyle on,                                      54
+ Finger-prints of twins,                                  44
+ Fixity and Variability in species,                       25
+ Flax, climatic experiments,                             197
+ Fowl, Silky,                                             84
+   Leghorn,                                            85,90
+   Dominant white,                                        94
+   Wyandotte,                                             97
+   Rumpless,                                              46
+ Foxes, incompatibility with dogs,                       241
+ Free-martin,                                             44
+ Fringillidae, sterility of hybrid,                      241
+ Fundulus, cyclopian,                                     50
+
+ Gallus, invariability of wild species,                   13
+   and origin of poultry,                              90,97
+ Genitalia, a basis for classification in insects,        13
+ Gentians, climatic experiments,                         197
+ Geometrical structure and differentiation,            54,56
+ Geometrical distinction between germ-cells
+         and somatic cells,                               58
+ Gladiolus, right and left,                               57
+ Gnophus obscurata, protective colouring,                141
+ Goldfinch, geographical races,                          147
+ Gonioctena variabilis, variation in sexes of,           121
+ Gouldian Finch, polymorphism,                       148,149
+ Gracilaria stigmatella, experiments on,                 193
+ Grantia, large varieties of,                            125
+ Ground-Squirrels, local forms of,                       132
+ Grouse, red, variation,                                  29
+ Guillemot, Ringed,                                      150
+ Guinea-pig, Brown-Sequard's experiments on,             198
+ Gynandromorphs,                                          45
+
+ Heliconius erato, forms of,                         122,164
+ Helix lapicida, local variation of,                     126
+   striata,                                              127
+   Heripensis,                                           127
+   Caespitum,                                            127
+   trochoides,                                           127
+   nemoralis and hortensis,                              128
+ Helminthophila, geographical races of,                  157
+ Hemerophila abruptaria, melanic,                        142
+ Hepialus humuli, in Shetland,                           119
+ Heterostyle plants,                                     236
+ Hieracium,                                                9
+ Himantopus,                                             234
+ Homoeosis,                                               68
+ Hybernia progemmaria,                                   139
+ Hybrids, sterility of,                                  233 et seq.
+
+ Incompatibility between certain allied species,         239
+ Individual, geometrical independence of,                 58
+ Inhibiting Factors,                                      95
+ Intermediates, nature of,                           131,135
+ Isolation, consequences of,                             118
+
+ Lacerta muralis, Kammerer's experiments on,             209
+   fiumana,                                              210
+ Leptinotarsa, Power's experiments on,                   218
+ Limbs, extra, in pairs,                                  72
+ Limnaea, sinistral,                                     134
+ Linaria vulgaris, self-sterility,                       239
+ Loasa fruits, right and left,                            57
+ Lobster, extra claws,                                    76
+ Locality, variation connected with,  14,118,146 et seq.,208
+ Lumbricus, regeneration,                                 77
+ Lychnis dioica and vespertina, inter-relations of,       18
+   macrocarpa, possibly a common parent of,               19
+
+ Machetes pugnax, polymorphism of male,                   28
+ Maize, Blaringhem's experiments on,                     229
+ Maize, cumulative factors in,                           116
+ Malformations, dominants, arising de novo,               89
+ Manx Cat, heredity,                                      46
+ Matthiola,                                       84,104,113
+ Melanic varieties,                                      135 et seq.
+ Memory, analogy with heredity,                          190
+ Meristic variation,                                69,83,86
+ Mirabilis, striped,                                      57
+ Models of segmentation,                               59,60
+ "Modes," Coutagne's conception of,                       126
+ Moedling, peculiar race of _Pieris napi_ at,        178
+ Mole, albino,                                         27,28
+ Mule, Linnaeus on,                                        8
+ Mutation, Matthioli on,                                   4
+   in Mercurialis,                                         5
+   in Kales,                                               5
+   alleged in bulbs,                                       5
+   Theory,                                                97
+   periods of,                                           114
+   in Bacteria,                                          214
+ Mutilation, consequences of,                             71
+   alleged effect of, on offspring,                      229
+ Myxococcus, variation in,                               213
+
+ Narwhal, asymmetry of tusks,                             44
+ Nemesia strumosa,                                        91
+ Neuration, a basis for classification,                   13
+ Nicotiana, sterility of hybrid,                         242
+ Nightjars, varying,                                     150
+ Noctuidae, fixity and variability,                       25
+ Noctua, polymorphic and fixed species,                   25
+ Noctua castanea, local forms of,                        122
+ Nomenclature, future of,                             94,245
+ Notonecta, variations of,                               130
+
+ Odontoptera bidentata, melanic form,                    137
+ Oedipodidae, protectively coloured,                     140
+ Oenothera, new dominant in,                              92
+   rubricalyx and rubrinervis,                         92,95
+   Lamarckiana,                                       92,101
+   origin of,                                        102,244
+   has bad pollen-grains,                                102
+   factorial analysis of,                                103
+   pollen and egg-cells genetically dissimilar,          104
+ Oenothera, "twin hybrids",                              105
+   laeta and velutina,                                   105
+   reciprocal crosses in,                                105 et seq.
+   possible coupling in,                                 111
+   dwarfs,                                           112,114
+  "Triple hybrids",                                      114
+   alleged variation due to treatment,                   227
+ Ophrys, local variability,                              125
+ Orange, polyembryony,                                    45
+ Osmotic growth,                                          65
+ Overlapping forms,                                  146,174
+
+ Papilio, geographical races of,                         162
+ Papilio turnus, variation of,                           144
+ Pararge egeria, geographical forms,                     166 et seq.
+ Parthenogenesis,                                         50
+ Partula, local forms of,                                133
+ Passer domesticus and montanus, distinctions,            22
+ Pea, round and wrinkled,                                 95
+ Pear, will not cross with apple,                        239
+ Pelargonium, variegated,                                 55
+   bud-sports,                                            56
+ Periodic phenomena in structure,                         63
+ Peronea, fixed and variable species,                     26
+ "Petites especes",                                       248
+ Petunia, double,                                        104
+ Phalanger maculatus, local variation,                   119
+ Pheasant, fixity of,                                     29
+ Phigalia pilosaria, melanic,                        139,140
+ Phratora vitellinae, experiments on,                    193
+ Phyllotaxis,                                             69
+ Pied varieties common in Passer domesticus
+   unknown in Montanus,                                   23
+ Pieris napi and bryoniae,                               174 et seq.
+ Pig, mule-footed,                                        46
+ Pigeon, web-footed,                                   46,49
+   Indian Rock, a recessive form,                         98
+ Pigments, nature of,                                     83
+ Pisum humile, hybrids with culinary peas,               244
+   species,                                              246
+ Planarian, regeneration of,                           71,77
+ Plotheia frontalis, polymorphic,                      26,29
+ Plusia, fixity and variation in,                         26
+ Poephila gouldiae, variation of,                    148,149
+ Polarity of individual,                                  44
+ Polia chi, melanic,                                     138
+ Polyanthus, short-styled selected,                      236
+ Polydactylism in Cat,                                 52,53
+ Polyembryony,                                            45
+ Potato, variation in,                                    91
+ Poultry, evolution of,                                   90
+ Primula obconica,                                        91
+ Primula sinensis, flaked,                                57
+   Leaf-shapes,                                           70
+   new dominant in,                                       92
+   sterility in,                                         236
+  "Giants",                                              236
+ Primula, species-hybrids,                               242
+ Protective coloration,                                  140
+ Pyrrhulagra, local forms,                               120
+ Python, twin-vertebrae,                                  60
+
+ Quiscalus, geographical races of,                       156
+
+ Rabbit, Angora,                                          46
+   colours of,                                            93
+   Incompatibility with hare,                            242
+ Raimannia odorata, Macdougal's experiments on,          226
+ Rats, Variation in,                                     248
+ Recessives, origin of,                                   90
+ Reciprocal crosses, giving distinct results,            105 et seq.
+ Regeneration,                                            70
+ Repulsion,                                              110
+ Reversal on Regeneration,                                77
+ Rhamphocoelus, geographical forms,                  159,184
+ Rhinoptera, variation in jaws of,                        38
+ Rhythm in repetition,                                    69
+ Ribs, variation of,                                      68
+ Rights and Lefts,                                     57-58
+ Ripples, analogous to segments,                    60,66,67
+   regeneration of,                                       79
+ Rollers, geographical races of,                         160
+ Ruff, polymorphism of male,                              28
+
+ Salamandra, maculosa and atra,                  182,199,203
+   spotted and striped,                                  207
+   geographical variation of,                            208
+ Segmentation, nature of,                                 63
+   simulated mechanically,                                64
+   compared with rippling,                                65
+   analogies with,                                        68
+ Segmentation of normally unsegmented structures,         38
+ Selection, Natural, an insufficient
+         cause of definiteness of types,          17,134,142
+ Sempervivum,                                            250
+ Serial Homology, the true nature of,                  62,66
+ Setina, Alpine varieties,                               181
+ Sex of Twins,                                            44
+ Sex-factors, possible coupling of,                      111
+ Sexual characters, variation in,                        119 et seq.
+ Siamese twins,                                           44
+ Silky Fowl,                                           84,85
+ Simocephalus, changed by environment,                   218
+ Sinistral forms,                                      33-34
+ Situs transversus,                                       43
+ Skate's jaws, variation in,                              38
+ Sloths, vertebral variation,                             68
+ Species, conceptions of,                    3,94,99,240,245
+   allied, distribution of,                              185
+   alternative uses of the term,                         245
+ Specific difference, universality of,                    12
+   of organisms compared with those
+       of inorganic materials,                            15
+   failure of theory of Selection
+       to explain,                                18,134,247
+ Sphyropicus varius,                                 149,156
+ Spilosoma lubricipeda, varieties of,                    181
+   Zatima, Heligoland form,                              181
+ Spinal nerves, segmentation of,                          67
+ Sporadic variation,                             131,134,248
+ Squashes, polymorphism of,                              100
+ Staphylococcus pyogenes, variation in,                  213
+ Sterility of hybrids, in general,                       233
+   in Lychnis hybrids,                                    20 et seq.
+   in crossing forms of Draba,                           243
+   Significance of,                                      244
+   Self,                                                 238
+ Stilt,                                                  234
+ Stocks,                                          84,104,113
+ Striped varieties,                                       57
+ Substantive variation,                                   84
+ Subtraction-stages,                                      93
+ Supernumerary limbs,                                  72-76
+ Sweet pea, variation of,                                 91
+   sterile anthers in,                                   237
+ Symmetry compared with heredity,                         41
+ Symmetry of body approximate,                            78
+ Syndactyly,                                              47
+   in foot,                                               48
+ Synthetic formulae, in nomenclature,                     94
+
+ Taeniocampa, fixed and variable species,                 25
+ Tamias, local forms of,                                 132
+ Tanagers, geographical races of,                        159
+ Teeth, variation in,                                  67,39
+ Tephrosia consortaria and consonaria,           137,139,140
+ Tephrosia species, separated by season,                 119
+ Terminal members, variation of,                          68
+ Thais rumina, local variation in,                        27
+ Tolerance, persistence of diversity due to,          17,134
+ Tomato, number of cells in fruit,                        46
+ Transitional populations, rarity of,                    165
+   an example,                                           178
+ Tropaeolum, sterile anthers in,                         237
+ Trypanosomes, variation in,                             215
+ Tusk, of Elephant, segmented,                            38
+   of Narwhal,                                            44
+ Twinning,                                          41,44,71
+   heredity of,                                           45
+   in organs,                                             46
+
+ Uria troile, variety of,                                150
+
+ Vanessa urticae, effects of temperature,                191
+ Variation, a medley of phenomena,                     14,15
+   sporadic,                                         131,134
+   and locality,                                         118
+   Causes of genetic,                          86,87,131,212
+   Substantive and meristic,                              83
+ Veronica, specific difference in,                        16
+   intermediates between species,                         17
+ Vertebrae, division in,                               60,61
+   homologies of,                                         66
+ Vespa, specific difference in,                           23
+ Vortex, living organism compared with,                   40
+
+ Wave-motion compared with repetition of parts,     62,67,79
+ Wheat, cumulative factors in,                           116
+   climatic experiments on,                              195
+ Woodpecker,                                             234
+
+ Zebra, pattern of stripes compared with ripples,         38
+
+
+
+
+                INDEX OF PERSONS
+
+                                         PAGE
+  Ackermann,                              242
+  Agar,                                   218
+  Allen, J. A.,                   132,147,159
+  Annandale,                               47
+  Arrigoni degli Oddi,                    167
+
+  Backhouse,                               50
+  Baker, G. T.,                           166
+  Bangs, Outram,                  120,142,155
+  Barrett,             26,136,167,173,178,193
+  Baur, E.,                             55,99
+  Baur, G.,                               119
+  Beneden, van,                            75
+  Bentham, on species of Veronica,         16
+    Lychnis,                               21
+    Primula,                               22
+  Bernadin,                                42
+  Bishop, L. B.,                      153,157
+  Blaringhem,                             229
+  Bobart,                                   5
+  Boisduval,                              182
+  Boissier,                                19
+  Borradaile,                           74,75
+  Boulenger, E. G.,                       208
+  Boulenger, G. A.,               182,207,209
+  Boyle,                                 5,54
+  Brewster, W.,                       149,150
+  Britton,                                227
+  Brown, T. Graham,                       198
+  Brown-Sequard,                          197 et seq.
+  Bruant, P.,                              51
+  Buffon,                                 234
+  Butler, S.,                         189,190
+  Buysson, R. du,                          24
+
+  Candolle, de,                           245
+  Carpenter, J. H.,                       172
+  Chapman, F. M.,             148,156,157,158
+  Chapman, T. A.,              13,167,182,231
+  Church, A. H.,                           69
+  Cieslar,                                197
+  Clark, Austin,                      142,144
+  Cockayne, E. A.,                         43
+  Cockerell, T. D. A.,                    224
+  Compton, R. H.,                   50,58,227
+  Cope,                                   230
+  Cory,                                   142
+  Correns,                                239
+  Coutagne,                               125 et seq.
+
+  Darwin, on Variation,                   1,2
+    Systematics,                           10
+    Selection,                        134,139
+    Heterostyle plants,               236,237
+  Darwin, F.,                             190
+  Darwin, Sir G.,                          41
+  Davenport,                               46
+  Davis, H. M.,                           102
+  Delcourt,                               130
+  Deschange,                              181
+  Dobell,                                 215
+  Doncaster,                      105,121,136
+  Driesch,                              80,81
+  Duchartre,                               51
+
+  East,                                91,116
+  Edwards, W. H.,                         162
+  Ehrlich,                                215
+
+  Fellmer,                                215
+  Field, W. L. W.,                        161
+  Fischer, E.,                            192
+  Fleck,                              171,174
+  Fletcher, W. H. B.,                  26,181
+  Foster, Sir N.,                          39
+
+  Galle,                                  123
+  Garrod,                                  83
+  Gates,                            92,95,102
+  Gayner, F.,                             177
+  Godron,                                 249
+  Gold, E.,                               196
+  Goldschmidt,                            116
+  Goodwin, E.,                            137
+  Gortner,                                226
+  Greene, E. L.,                            8
+  Gregory, R. P.,                  92,100,236
+  Grenier,                                249
+  Grover,                                 173
+  Gruber,                                  48
+  Gulick,                             119,133
+
+  Hamling,                                142
+  Hampson, Sir G.,                         26
+  Harris,                                 142
+  Hartlaub,                               182
+  Herbst,                                  42
+  Heribert-Nilsson,                       116
+  Hewett,                                 182
+  Honing,                                 105
+  Hunter, John,                            44
+
+  Jakowatz,                               197
+  Janet,                                   24
+  Jeans,                                   41
+  Jenkinson,                               40
+  Jentink,                                120
+  Johannsen,                              195
+  Jordan,                         185,242,249
+
+  Kammere,                                199 et seq.
+  Keeble,                                 236
+  Klebs,                                  250
+  Krancher,                               182
+  Kuechenmeister,                           44
+  Kudicke,                                215
+
+  Lamarck,                                  9
+  Lang, A.,                               128
+  Lawrence, W. N.,                    142,145
+  Leake, H. Martin,                    98,100
+  Leavitt,                                185
+  Lecoq,                                   99
+  Lederer,                                167
+  Leduc,                             64,65,80
+  Leydig,                                 182
+  Linden, M. von,                         192
+  Linnaeus,                             6,7,8
+  Lloyd, R. E.,                           248
+  Locard,                                 130
+  Lock, R. H.,                        242,244
+  Loeb,                        42,45,50,71,77
+  Lotsy,                                   99
+  Lowe, P. R.,                            143
+
+  Macdougal, W. T.,                   102,226
+  Marchant,                                 7
+  Mathew,                                 171
+  Matthioli,                                4
+  Mayer, A. G.,                           133
+  Mendel, Rediscovery of,                   2
+    On Fasciation,                         49
+  Merrifield, 169,                        172
+  Miller, W. D.,                      120,149
+  Morgan,                        71,77,91,198
+  Moggridge,                              125
+
+  Nathusius, S.,                          242
+  Nettleship.,                             44
+  Newman, H. H.,                           42
+  Newsholme,                               48
+  Nilsson-Ehle,                       116,169
+  Norman, A. M.,                      125,156
+
+  Ober,                                   142
+  Oberthuer,                       168,170,193
+  Oliver, J.,                              45
+
+  Page, H. E.,                        167,180
+  Patterson, J. T.,                        42
+  Payne, F.,                              278
+  Pellew,                                 236
+  Poll,                                    45
+  Porritt,                                136
+  Poulton,                                141
+  Powers, J. H.,                          230
+  Pringsheim, H.,                         213
+  Przibram,             72,78,178,194,197,199
+  Punnett,                                110
+
+  Ray,                                    4,5
+  Raynor,                                 105
+  Ridgway,                             10,120
+  Roedelius,                              195
+  Rolfe,                                   20
+  Rosen, F.,                              242
+  Rosner,                                  42
+  Rowland-Brown, H.,                  167,180
+
+  Sargent,                                185
+  Saunders, E. R.,                 84,104,112
+  Schima,                                 177
+  Schroeder,                           193,194
+  Schuebeler,                              195
+  Semon, R.,                              190 et seq.
+  Sharrock,                                 5
+  Shull,                                  100
+  Speyer, A.,                     166,170,181
+  Spillman,                                47
+  Standfuss,                      135,181,191
+  Staples-Browne,                       49,98
+  Staudinger,                         170,179
+  Stockard,                             50,71
+  Sutton,                             236,244
+
+  Tornier,                                 72
+  Tower, W. L.,                       218-226
+  Trechmann,                              133
+  Tugwell,                                181
+  Tutt, J. W. On Definiteness of Species,  13
+    On Plusia interrogationis,             26
+    On Tephrosia,                         119
+    On N. castanea,                       122
+    On Pararge egeria,                    167 et seq.
+
+  Verity, R.,                         171,177
+  Vries, H. de,               101-115,222,239
+
+  Walker, G,                               49
+  Weir, Jenner,                           119
+  Weismann,                           176,188
+  Wendelstadt,                            215
+  Werbitzki,                              215
+  Werner,                                 209
+  Wettstein,                              197
+  Wheeler, G.,                        168,171
+  Wheldale,                                83
+  Wilder,                                  44
+  Wille,                                  197
+  Williams, H.,                       167,172
+  Windle, B. C. A.,                        43
+  Winslow,                                213
+  Wolf, F.,                               213
+  Woodforde,                              123
+  Woltereck,                              215
+
+  Zeijlstra,                              114
+
+
+
+
+
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