The Project Gutenberg eBook ofProblems of GeneticsThis ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.Title: Problems of GeneticsAuthor: William BatesonRelease date: January 4, 2014 [eBook #44582]Most recently updated: October 23, 2024Language: EnglishCredits: Produced by Chris Curnow, Paul Marshall and the OnlineDistributed Proofreading Team at http://www.pgdp.net (Thisfile was produced from images generously made availableby The Internet Archive)*** START OF THE PROJECT GUTENBERG EBOOK PROBLEMS OF GENETICS ***
This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.
Title: Problems of GeneticsAuthor: William BatesonRelease date: January 4, 2014 [eBook #44582]Most recently updated: October 23, 2024Language: EnglishCredits: Produced by Chris Curnow, Paul Marshall and the OnlineDistributed Proofreading Team at http://www.pgdp.net (Thisfile was produced from images generously made availableby The Internet Archive)
Title: Problems of Genetics
Author: William Bateson
Author: William Bateson
Release date: January 4, 2014 [eBook #44582]Most recently updated: October 23, 2024
Language: English
Credits: Produced by Chris Curnow, Paul Marshall and the OnlineDistributed Proofreading Team at http://www.pgdp.net (Thisfile was produced from images generously made availableby The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK PROBLEMS OF GENETICS ***
YALE UNIVERSITY
MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES
SILLIMAN MEMORIAL LECTURESPUBLISHED BY YALE UNIVERSITY PRESSELECTRICITY AND MATTER.ByJoseph 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.ByCharles 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.ByErnest 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 OFTHERMODYNAMICS TO CHEMISTRY.ByDr. 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.ByWilliam 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 MotionsDetermined by Means of the Spectrograph.ByWilliamWallace Campbell, sc.d., ll.d.,Director of the Lick Observatory, University of California.Price $4.00 net; postage 30 cents extra.THEORIES OF SOLUTIONS.BySvante August Arrhenius,ph.d., sc.d., m.d.,Director of the Physico-Chemical Departmentof the Nobel Institute, Stockholm, Sweden.Price $2.25 net; postage 15 cents extra.IRRITABILITY.A Physiological Analysis of the GeneralEffect of Stimuli in Living Substances.ByMax Verworn,Professor at Bonn Physiological Institute.Price $3.50 net; postage 20 cents extra.THE EVOLUTION OF MODERN MEDICINE.BySir William Osler, Bart., m.d., ll.d., sc.d.,Regius Professor of Medicine, Oxford University.Price $3.00 net; postage 40 cents extra.
SILLIMAN MEMORIAL LECTURES
PUBLISHED BY YALE UNIVERSITY PRESS
ELECTRICITY AND MATTER.ByJoseph 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.ByCharles 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.ByErnest 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 OFTHERMODYNAMICS TO CHEMISTRY.ByDr. 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.ByWilliam 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 MotionsDetermined by Means of the Spectrograph.ByWilliamWallace Campbell, sc.d., ll.d.,Director of the Lick Observatory, University of California.Price $4.00 net; postage 30 cents extra.THEORIES OF SOLUTIONS.BySvante August Arrhenius,ph.d., sc.d., m.d.,Director of the Physico-Chemical Departmentof the Nobel Institute, Stockholm, Sweden.Price $2.25 net; postage 15 cents extra.IRRITABILITY.A Physiological Analysis of the GeneralEffect of Stimuli in Living Substances.ByMax Verworn,Professor at Bonn Physiological Institute.Price $3.50 net; postage 20 cents extra.THE EVOLUTION OF MODERN MEDICINE.BySir William Osler, Bart., m.d., ll.d., sc.d.,Regius Professor of Medicine, Oxford University.Price $3.00 net; postage 40 cents extra.
ELECTRICITY AND MATTER.ByJoseph 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.ByCharles 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.ByErnest 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 OFTHERMODYNAMICS TO CHEMISTRY.ByDr. 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.ByWilliam 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 MotionsDetermined by Means of the Spectrograph.ByWilliamWallace Campbell, sc.d., ll.d.,Director of the Lick Observatory, University of California.Price $4.00 net; postage 30 cents extra.
THEORIES OF SOLUTIONS.BySvante August Arrhenius,ph.d., sc.d., m.d.,Director of the Physico-Chemical Departmentof the Nobel Institute, Stockholm, Sweden.Price $2.25 net; postage 15 cents extra.
IRRITABILITY.A Physiological Analysis of the GeneralEffect of Stimuli in Living Substances.ByMax Verworn,Professor at Bonn Physiological Institute.Price $3.50 net; postage 20 cents extra.
THE EVOLUTION OF MODERN MEDICINE.BySir 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
logo
New Haven: Yale University PressLondon: Humphrey MilfordOxford University Press
MCMXIII
Copyright, 1913ByYale University
First printed August, 1913, 1000 copies
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.
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 thenumber 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 ofColaptes, and to authorize the loan of the Plate representing the various forms ofHelminthophila, which is taken from hisNorth American Warblers.
I am under obligation to Messrs. Macmillan & Co., for permission to reproduce several figures fromMaterials 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.
LIST OF ILLUSTRATIONS.
PROBLEMS OF GENETICS
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 toDarwin 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 prejudiceto 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.
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 seventeenthcenturies 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 declaredStirpium mutatioto 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 theHistoria 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 nearlyallied 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 ofMercurialis, 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 ofspecies 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 theSystema Naturaea 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 theSystema Naturaenaively 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 thePhilosophia Botanica, "Species tot numeramus quot diversae formae in principio sunt creatae," with the associated passage "Varietates sunt plantae ejusdem speciei mutatae a caussaquacunque occasionali." Those sayings however do not stand alone. In several places, notably in the famous dissertation on the peloricLinariahe 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 laciniateMercurialis, 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 VegetabileMedullareprincipiis constitutivis diversiCorticalisunde tot difformia individua, quotOrdinesNaturales prognata.
2.Classicashas (1) plantas Omnipotens miscuit inter se, unde totGeneraordinum, quot inde plantae.
3.Genericashas (2) miscuit Natura, unde totSpeciescongeneres quot hodie existunt.
4.Specieshas 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 stillmore obscure if read in connection with the essay on "Generatio Ambigena," where he expresses the conviction that theMedullais contributed by the mother, and theCortexby 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 individualplantae 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 producedSpecies 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 betweenCreator,Natura, andCasus, 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 theSpecies 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 speciesof 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 ofHieraciumwrote, "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 inrerum naturaas 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 productsof 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 theOriginhe 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 theBiologia Centrali-Americana, Ridgway'sBirds of North America, theFauna 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 proficientsystematists 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 tobiological 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 distinctionshas 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 ofGallus) 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. Thedeclaration 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 persistenceof 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 isor 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 theagrestissection of the genusVeronica, namelyTournefortii,agrestis, andpolita.
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 thatpolitais a mere variety ofagrestis.
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 facta 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 thattolerancehas 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:Specificdistinction.
Let us examine a somewhat different set of facts in the case of another pair of nearly allied speciesLychnis diurnaandvespertina. 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. diurnais a plant of hedgerows and sheltered situations.L. vespertinais common in fields and open spaces, wherediurnais hardly ever found; but not rarelyvespertinaoccurs in association withdiurnain the places which that plant frequents. In this case I do not doubt that we have to do with organisms of somewhat different aptitudes. ThatL. vespertinahas powers whichdiurnahas not is shown very clearly by the fact thatdiurnais sometimes entirely absent from areas wherevespertinacan 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 theLychnispopulation of such a locality be examined it will be found to consist of many undoubted and unmodifieddiurna, a number—sometimes few, sometimes many—of similarly unmodifiedvespertina, 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 ofLychnis, nor can I conceive of any process, whether gradual or sudden, by whichdiurnacould have come out ofvespertina, orvespertinaout ofdiurna. Both however may no doubt have been derived from some original third type. It is conceivable thatLychnis macrocarpaof Boissier, a native of Southern Spain and Morocco, may be this original form. This species is said to combine a white flower (like that ofL. vespertina), with capsule-teeth rolled back (like those ofdiurna).[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 originalform 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 ofdiurnaandvespertina.[11]For they may be reproduced exactly in F1or 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 ofvespertina. Not a hundred yards inland is a lane containingdiurna, and among thevespertinaon the sand-bank are always some of the hybrid form, doubtless the result of fertilisation from the neighbouringdiurnapopulation. Seed saved from these hybrids gavevespertinaand hybrids again, having obviously been fertilised by othervespertinaor by other hybrids, and I have no doubt that such hybrid plants if fertilised bydiurnawould have shown somediurnaoffspring. The absence ofdiurnain such localities may fairly be construed as an indication thatdiurnais there at a real disadvantage in the competition for life.
But if, admitting this, we proceed to consider how the special aptitude ofvespertinais constituted, or what it is that putsdiurnaat a disadvantage, we find ourselves quite unable to show the slightest connexion between the success of one or thefailure of the other on the one hand, andthe specific characteristicswhich distinguish the two forms on the other. The orthodox Selectionist would, as usual, appeal to ignorance. We ask what canvespertinagain by its white flowers, its more lanceolate leaves, its grey seeds, its almost erect capsule-teeth, its longer fruits, whichdiurnaloses 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 thingsmayaffect 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 twoLychnisspecies, the series of "intermediates" was taken to be a palpable proof thatvespertina"graded" todiurna. It is this fact, doubtless, upon which Bentham would have relied in suggestingthat 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 instancemontanushas a black patch on the cheek which is absent indomesticus. 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. Indomesticusthe two sexes are strongly differentiated, the cock being more ornate than the hen. On the other hand the two sexes inmontanusare alike, and, if we take a standard fromdomesticus, we may fairly say that inmontanusthe 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 domesticusis a species in which individuals more or less pied occur with especial frequency, but inP. montanussuch variation is extremely rare if it occurs at all. The writer of the section on Birds in theRoyal 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 vulgarisandVespa 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. vulgarisuses rotten wood from which the nest derives a characteristic yellow colour, whileV. germanicascrapes 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 ofgermanicapasses freely through a hole in the external envelope, butvulgarisunites 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 invulgarisbut not emarginate ingermanica.V. vulgarisoften has black spots on the tibiae while ingermanicathetibiae are usually plain yellow. In both species there is a horizontal yellow stripe on the thorax, but whereas invulgaristhis is a plain narrow stripe, it is ingermanicaenlarged 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 ofvariabilityis no less paradoxical, and without investigation of the particular case no one can say what will befound 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.