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diff --git a/44582-0.txt b/44582-0.txt new file mode 100644 index 0000000..88b1001 --- /dev/null +++ b/44582-0.txt @@ -0,0 +1,10049 @@ +*** 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 *** |