CHAPTER I

YALE UNIVERSITY

MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES

PROBLEMS OF GENETICS

SILLIMAN MEMORIAL LECTURES

PUBLISHED BY YALE UNIVERSITY PRESS

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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."