Chapter VII: Part II: Darwinism and the Atomistic Interpretation of Inheritance (1)

SUMMARY

The failure to recognize that biology no less than physics is an ethically neutral science is a heritage of the evolutionary controversy. The doctrine of organic evolution evoked intense religious hostility in the middle of the nineteenth century. Biologists were compelled to fight for their right to speculate on their own lines. Forced into the forum as a propagandist the biologist gave less attention to the logical structure of the new theory than to its apparent implications for social philosophy. The ethical concept of progress became entangled in the evolutionary idea. In the writings of Herbert Spencer and the evolutionist philosophers Darwinism has left a lasting impress upon contemporary thought. Experimental biology in this generation has undertaken the task of reducing the problems of organic evolution to an exact science. This must necessitate a re-examination of many traditional biological concepts and many philosophical and sociological inferences which have been extracted from an earlier phase in the development of the evolutionary doctrine.

V. THE METHODOLOGY OF EVOLUTION

“Chemistry is not so far from physics as the generation before ours
thought. Biology, through bio-physics and bio-chemistry, no longer
stands aloof from the methods and procedures of physical science.
And these new alliances cannot be made without modifications in
the logical construction of the separate concepts upon which these
various sciences previously took their stands. This is a task which
laboratory practice alone cannot undertake.”--_Dorothy Wrinch_

§1

From Aristotle to our own time biologists have been too preoccupied with collecting information about the extremely complex phenomena which they study to pay very much attention to the logical structure of the hypotheses they adopt. This not only tends to make controversy between the mechanist and vitalist barren, but also explains why much that has recently been written and said about evolution is both unsatisfactory and perplexing to the intelligent layman. Many of the views which gained well-nigh universal assent among biologists in the latter half of the nineteenth century have been undermined by the discoveries of the Mendelian renaissance. When the onlooker asks the biologist for a straightforward exposition of the present status of the evolutionary hypothesis, he is frequently met with the guarded statement that biologists are no longer so sure that they know _how_ evolution occurred, but are more certain than ever that it _has_ occurred. Such a statement might conceivably have a logically admissible meaning, though if so, it belongs to the category of things which were better said otherwise. On the face of it, the layman has very good reason for wondering whether it means anything at all. It is logically permissible to say we know that common salt is soluble, but we do not know how it happens that common salt should possess this property. But evolution is not a simple property. It is a process. We cannot very well know of the existence of a process unless we can say in what the process consists.

The doctrine of evolution which deals with the way in which living matter has come to exist in the manifold forms which biologists call species is one which can only be placed on the same footing as the great generalizations of physics and chemistry, when it is examined from the experimental standpoint. From that standpoint the particular phase in the growth of the evolutionary hypothesis associated with the names of Darwin and Wallace has less significance than is customarily attached to it. From a purely experimental point of view Darwin and Wallace brought to bear on the discussion of the evolutionary doctrine nothing which their predecessors Buffon, Erasmus Darwin, Lamarck, St. Hilaire, Goethe and Oken lacked. The importance of their work lies in the history of the controversy. Under Cuvier’s influence biology had turned away from premature speculation to industrious study of the nature of species differences from every available standpoint. Darwin and Wallace brought together the fruits of the progress resulting from a generation of intensive research on such lines, and formulated the evolutionary problem in a much more explicit form than _les philosophes_ were in a position to do. The particular answer that they gave to the problem they formulated is the least significant part of the contribution which Darwin and Wallace made to biological science. The biological world did not begin to examine the experimental implications of the selectionist solution until the rediscovery of Mendel’s laws by Correns, de Vries and Tschermak, and their extension to animals by Bateson and Cuenot in the opening years of the present century. The Mendelian renaissance provoked considerable hostility from a generation of biologists untrained in experimental methods. It is only now becoming possible to re-examine the selectionist doctrine with detachment and candour.

It must not be implied that antagonism to the new movement was a mere disinclination to face the effort of learning new methods of attacking the problem. In the nineteenth century biologists had to fight for their right to speculate freely in their own field. The generation in whose memory the struggles of that period were fresh not unnaturally resented the suggestion that biologists were no longer unanimous among themselves. It was heresy to betray the policy of a united front. If such schisms were permitted, and the truth were allowed to leak out to the general public, the church somnolent might again become the church militant. The recently published biography of the late William Bateson shows how keenly this was felt. In the end hostility towards the new movement which followed the rediscovery of Mendel’s work gave place to a comfortable compromise, based on the attractive device of inventing a word for human ignorance. This device is not peculiar to biological science. There were from the start physicists who entertained the most profound suspicion of the ether on that account. It was agreed to state that inheritance in animals and plants is of two kinds, Mendelian and non-Mendelian. Study of the former was to be encouraged because it was useful to stock breeders, horticulturalists, and pigeon fanciers. The latter was the peculiar speciality of the evolutionist. Apart from that, the impenitent selectionist did not attempt to define exactly what non-Mendelian inheritance was. Its sphere was progressively encroached upon by the Mendelian variety, until nothing was left of it but a comfortable corner for those highly variable characters which were somewhat vaguely referred to under the term “quantitative inheritance,” i.e. hereditary differences in size so subject to fluctuating variability in response to external conditions that they are only definable by reference to a statistical average for a particular inbred stock. Naturally experiment first turned to the analysis of clear-cut hereditary differences such as colour, where little trouble is requisite in standardizing external conditions, so that an hereditary difference will be apparent in the individual. Since mathematical analysis has been brought to bear on the study of size inheritance in such work as that of East and Jones, there can no longer be any justification for doubting that the atomistic conception of heredity which Mendel formulated covers the whole domain of biparental inheritance.

While experimental analysis was progressing towards a recognition of the universal validity of the Mendelian conception, the brilliant work of Morgan’s school was leading to an exact theory of the inter-relation of genetical factors based on the observed behaviour of the chromosomes. Experiment now equipped with a definite criterion of genetic purity could assert that new forms do come into existence discontinuously in nature. It could state the conditions which determine whether a new genetic character will persist. When chromosome maps of several allied species of the fruit-fly were constructed by Metz and Sturtevant seven years ago the whole discussion of the problem of species formation entered on an entirely new phase. To-day we must approach the discussion of evolution on the assumption that in Mendel’s atomistic conception of the hereditary process must be sought the correct interpretation of how new characters, having come into being, may be transmitted to future generations.

§2

To appreciate at once the greatness and the limitations of Darwin’s contribution to evolutionary thought it is essential to see the question in its historical perspective. Many of the steps which have led to the construction of the evolutionary hypothesis are now only of historical interest. Only with an understanding of the history of the doctrine is it possible to gain a clear idea of the logical status it occupies in scientific thought. In approaching it, one has to remember that the discussion of organic evolution aroused a good deal of prejudice from religious quarters, and that in consequence many issues, e.g. Recapitulation, which were not strictly relevant to a straightforward presentation of the problem occupied a prominent place in the controversies that raged around it. In forming an estimate of the present status of the evolutionary hypothesis, let us, as far as possible, eliminate these irrelevant questions, and deal only with the steps which have made a definite constructive contribution to the present state of knowledge. These may be treated under four headings: (_a_) The Principle of Biogenesis, (_b_) The Principle of Unity of Type, (_c_) The Principle of Succession, and (_d_) The Principle of Genetic Variation.

_The Principle of Biogenesis_ is simply the recognition that animals and plants only arise in our immediate experience from other animals and plants through the process of reproduction. Linnæus accepted it in his doctrine of the fixity of species as generally true with regard to animals in the ordinary sense. Not until the middle of the nineteenth century did the work of Pasteur demonstrate its validity for micro-organisms. Linnæus and Ray were among the first to recognize the general truth of the commonplace that “like begets like.” The Aristotelian influence which predominated during the Renaissance had lingered on until the beginning of the seventeenth century. The fascinating legend of the goose barnacle contained in the concluding passage of Gerrard’s _Herbal_ (1594) is illustrated by an actual woodcut of the Goose and its Barnacle Progenitor. The passage reads:

“But what our eyes have seene; and hands have touched we shall
declare. There is a small Island in Lancashire called the Pile of
Foulders, wherein are found the broken pieces of old and bruised
ships, some whereof have beene cast thither by shipwracke, and also
the trunks and bodies with the branches of old and rotten trees,
cast up there likewise, whereon is found a certain spume or froth
that in time breedith unto certain shells, in shape like those of
the Muskle, but sharper pointed, and of whitish colour, wherein is
contained a thing in forme like lace of silke finely woven as it
were together, of a whitish colour, one end whereof is fastened
unto the inside of the shell, even as the fish of Oisters and
Muskels are; the other end is made fast unto the belly of a rude
masse or lumpe which in time commeth to the shape of a Bird; when
it is perfectly formed the shell gapeth open, and the first thing
that appeareth is the foresaid lace or string; next come the legs
of the bird hanging out, and as it groweth greater it openeth the
shell by degrees, til at length it is all come forth and hangeth
onely by the bill: in short space after it cometh to full maturitie
and falleth unto the sea, where it gathereth feathers, and groweth
to a fowle bigger than a Mallard and less than a goose having
blacke legs and bill or beak, and feathers blacke and white,
spotted in such manner as is our magpie.... For the truth thereof
if any doubt, may it please them to repaire unto me, and I shall
satisfie them by the testimonie of good witnesses.... The bordes
and rotten planks whereon are found these shells breeding the
Barnacle are taken up on a small Island adjoyning Lancashier, halfe
a mile from the main land, called the Pile of Foulders. They spawn
as it were in March and April; the Geese are formed in May and
June, and come to fulnesse of feathers in the month after. And thus
having through God’s assistance discoursed somewhat at large of
Grasses, herbs, Shrubs, trees and Mosses, and certain Excrescences
of the earth, with other things moe, incident to the historie
thereof, we conclude and end our present Volume, with this Wonder
of England. For the which God’s Name be ever honoured and praised.”

The legend of the goose and the barnacle died a slow death, and many diverting citations might be added. That canny Scot, Sir Robert Moray, wrote concerning the mystery surrounding the reproductive habits of geese and barnacles so late as 1678 in the following words, which occur in a paper actually published in the _Royal Society’s Transactions_. After describing the barnacle shells washed up on the coast of Scotland, he refers to their “little bill like that of a goose, the eyes marked, the head, neck, breast, wings, tail and feet formed, the feathers everywhere perfectly shaped and blackish coloured, and the feet like those of other water fowl to my best remembrance.”

Writing in the middle of the seventeenth century Sir Thomas Browne states (_Vulgar Errors_, bk. 3):

“Concerning the generation of frogs we shall briefly deliver that
account which observation hath taught us. By frogs I understand
not such, as arising from putrefaction are bred without copulation
and because they subsist not long are called temporariæ (Rana
temporaria, the common frog), nor do I mean the little frog of an
excellent parrot green that usually sits on trees and bushes, and
is therefore called Rananculus viridis (the tree frog) but hereby I
understand the aquatile or water frog, whereof, we may behold many
millions every spring in England.”

Referring to the doubt expressed by the author of _Vulgar Errors_ concerning Aristotle’s belief that mice arise from putrefaction, Alexander Ross commented:

“So may one doubt whether in cheese and timber worms are generated;
or if beetles and wasps in cow’s dung; or if butterflies, locusts,
grasshoppers, shell fish, snails, eels and such like be procreated
of putrefied matter which is apt to receive the form of that
creature to which it is by formative powers disposed. To question
this is to question reason, sense, and experience. If he doubt
of this let him go to Egypt, and there he will find the fields
swarming with mice, begot of the mud of Nylus, to the great
calamity of the inhabitants.”

During the sixteenth century under the influence of Vesalius, Fallopius and Servetus experimental investigation liberated medicine from the paralysing tradition of Galenic teleology. The effect of this change of outlook became evident in the revival of natural history in the seventeenth century. Redi (1688) turns to experiment to decide whether maggots can be produced from putrescent meat, if flies are prevented from depositing their eggs on it. “Reason, sense and experience” were at length forced to capitulate to experiment. The comparative study of animal life after centuries of stagnation following the publication of Aristotle’s Natural History entered on a new phase. So long as innumerable _ad hoc_ accounts of the origin of species existed the general problem with which the evolutionary hypothesis deals could not be envisaged. Thus the work of Linnæus is the starting-point of the modern theory of evolution.

More than a century elapsed before the essential features common to sexual reproduction in all animals were understood. Leeuwenhoek, a Hollander, in 1668 had first seen the minute spermatozoa in the seminal fluid. A little over a century later the ingenious Abbot Spallanzani gave experimental proof that it is to the spermatozoan that the seminal fluid owes its fertilizing power. Only in 1879 did Hertwig and Fol independently observe beneath the microscope that only one sperm normally fertilizes one egg. Their observations were made on sea-urchins, but we now know that their conclusions are true for all animals. Thus the recognition that everything implied in the term inheritance has reference to the material substance of the egg and sperm, a concept fundamental to any exact theory of hereditary transmission, did not emerge with clarity till more than fifteen years after the _Origin of Species_ was published.

The formal classification of organisms codified by Linnæus introduced a new era of intensive investigation into the character of species differences and so ushered in the great age of comparative anatomy. Thus we come to the second step in the historical development of the Evolution theory, the _Principle of Unity of Type_. This generalization was the special contribution of the school of French and German comparative anatomists whose foremost exponent was Georges Cuvier. The work of Linnæus gave a great impetus to the study of the structural differences between animals, at a time when anatomy like any young branch of knowledge was still dominated by teleology. Some instructive examples of the happy combination of piety and anatomy are given in the _Speculum Mundi_ published by John Swan in 1635. In an old translation of Pliny the Elder there occurs the following information about the elephant:

“Their skin is covered with haire or bristle, no, not so much as in
their taile, which might serve them in good steade to drive away
the busie and troublesome flies (for as vast and huge a beast as he
is, the flie haunteth and stingeth him), but full their skin is of
crosse wrinckles lattiswise; and besides that, the smell thereof
is able to draw and allure such vermine to it, and therefore when
they are laid stretched along, and perceive the flies by whole
swarmes settled on their skin, sodainly they draw those cranies
and crevices together close, and so crush them all to death. This
serves them instead of taile, maine and long hairs....”

This citation is not an isolated instance of the way in which a pagan philosopher could employ the study of natural history to justify the ways of God to men. During the Middle Ages the influence of ecclesiasticism reinforced the teleological attitude from which Aristotle’s Natural History is comparatively speaking free. At a later date Deism had its scientific complement in a tradition which identified the pursuit of Natural History with Natural Religion. The first classifications were based on comparatively superficial points of resemblance. As the study of animal structure progressed in the two generations that followed the labours of Ray and Linnæus, it became increasingly evident that the teleological standpoint in comparative anatomy is inadequate. If animals had been specially designed to suit their conditions of life, it would be expected that the greatest degree of similarity would be found in animals pursuing a similar mode of existence. This is not what is actually found. On the contrary, as we make the greatest degree of similarity in structure the basis of our attempts to classify animals, our units of classification resolve themselves into collections of forms which show the greatest diversity of habit, locality, diet, means of progression or anything else which might be significant from a purposive standpoint. Animals can be classified in groups based on striking similarity in architecture and development involving complex constellations of physiological units. Within these groups the utmost variety of habitat, climate, locomotion, nutrition, etc., are encountered. The underlying similarity of the bones of the limb and its musculature in a whale, a bird and an elephant, as contrasted with the limb structures of a beetle, a fish or a squid illustrate this conclusion. The whole study of systematic zoology bears witness to it. Van Baer extended the principle of Unity of Type to embryonic forms in 1834.

The importance of the principle of Unity of Type to the Evolutionary hypothesis lies in the attitude which it promoted. By discouraging the teleological approach to the diversity of animal life, it paved the way for a naturalistic investigation of the problem. The net result of the intensive study of comparative anatomy which progressed under the influence of Cuvier in France and Johannes Müller in Germany was also to show that the task of classifying animals in hard and fast categories is at all turns embarrassed by the existence of anomalous intermediate forms like the duck-billed platypus or the worm-like arthropod Peripatus. Thus biological thought was becoming more and more sympathetic towards the existence of a process of species modification. This tendency became more sharply defined as biology took the third great step in the development of the modern theory of evolution.

This step has been called the _Principle of Succession_. When the Law of Unity of Type first obtained recognition, many fossils were known, but geologists had not arrived at a general agreement concerning the order in which the various strata had been deposited nor the magnitude of the time which their formation occupied. By the middle of the nineteenth century the modern doctrine (“Uniformitarianism”) had gained assent. It now became apparent from studying the distribution of animals in space and time, that divergent forms which exist on the earth’s surface to-day were preceded by widely distributed forms of a more generalized type in the past. The further we go back in the history of any group of animals, the less do we find the same pronounced differences as are displayed by existing members of the same assemblage. The differentiation of species is inferred from the record of the rocks to have been a continuous process in space and time. This doctrine in its modern form was explicitly put forward in 1855 by Wallace.

The masterly way in which Darwin marshalled the facts at his disposal in presenting this aspect of the case constitutes his chief claim to have made an enduring contribution to the Doctrine of descent. From ancient times, but more especially from the end of the seventeenth century onwards, the hard remains of animals were discovered and described. Shells of molluscs which only live in water were found far inland remote from lake, river or sea. Such relics were attributed by the current mythology of Christian countries to the deluge that overwhelmed the contemporaries of the Noah family. Sceptics like Voltaire, who ventured to offer more naturalistic hypotheses, were not more felicitous in their speculations. An exception must be made in favour of Xenophanes (B.C. _circa_ 500) and the Arab physician Avicenna, who, it appears, recognized fossils as remains of animals formerly alive, and saw in them evidence of the existence of oceans where there is now only land. A giant fossil salamander which occurs abundantly in the Upper Miocene of Switzerland, closely related to the Japanese salamander _Cryptobranchus japonicus_, was unearthed by Scheuchzer in 1726, and named _Homo diluvii testis_. The motto attached to the figure reads:

Betrübtes Beingerust von einem alten Sünder
Erweiche Herz und Sinn der neuen Bösheitskinder.

This has been translated:

Oh sad remains of bone, frame of poor Man of Sin,
Soften the heart and mind of recent sinful kin.

After the Renaissance it seems that priority in the recognition of fossils as remains of what were once living animals is due to Steno (1699), a Danish anatomist who taught at Padua. More than a century later, Cuvier’s monograph on fossil remains initiated the epoch of systematic palæontology. The effect of the researches which it initiated was not felt till the Uniformitarian doctrine, i.e. the view that successive strata have been deposited by a continuous process, was generally accepted, mainly through the work of Lyell (1830). The impiety of this new geology promoted violent controversy. In the minutes of a meeting of the Geological Society of Great Britain in 1840, we are told that the retiring president, Dr. Buckland, “with a look and tone of triumph pronounced upon his opponents who dared to question the orthodoxy of the scratches and grooves of the glacial mountains the pains of eternal itch without the privilege of scratching” (_Hist. Geol. Soc. Lond._, p. 142). By the middle of the nineteenth century geologists were universally convinced that the various strata of which the earth’s crust is composed have been laid down in orderly succession during periods of time compared with which that occupied by the history of human society is of negligible duration. Once this conclusion was accepted, the study of fossils received a new impetus and progressed rapidly under the leadership of men like Owen, Cope and his contemporaries. Students of fossils now began to compare the characteristics of animals in different geological epochs, and to elucidate evidences of a continuous succession of new forms of life transmitted to posterity in the record of the rocks. Out of their studies the principle of succession took shape.

The geological succession of animal and plant life is demonstrated by two features of the record. Many of the more highly specialized and successful groups of the present day are not found to have existed at earlier periods of the earth’s history, and were preceded by forms which are intermediate between them and representatives of surviving groups that were already existent before them. It is also found that the earliest members of the great groups are usually found to be of a more generalized type of structure than existing types. Adequate material for drawing these conclusions is provided only by forms which have resistant structures, such as the vertebrates, shellfish and vascular plants.

Before we can fully appreciate the continuity of the geological record, we have to take into account the fact that the same animals are not found in all the different parts of the globe. One group of animals may be confined, like the kangaroos, to Australia; one group, like the monotypic order, in which the ant-bear is placed, to South Africa. If, then, we know that there existed in, let us say, the Chalk Age, a small mammal which was of a type so generalized as to form a link between the kangaroo and the ant bear, it is most important to know whether the barriers of ocean that now separate Australia and South Africa were as impassable in those times as they are now; or whether this architypal mammal lived in a situation from which it could have access to both of these promising lands of settlement for its family. We are thus led to ask if the process of geological succession was a continuous one both in time and space.

To answer this question demanded a comprehensive survey of the existing distribution of animal life on the earth, perhaps the most significant contribution that Darwin and Wallace made to the evolutionary doctrine. In their writings the facts of geographical distribution, facts which were very largely based on their own first-hand observations, and not like their erroneous views upon heredity collected from the testimony of other persons, first received critical examination. They were forced to conclude that no amount of ingenuity could successfully interpret the geographical distribution of animals on a purely teleological basis. The habitat of different kinds of animals is not uniquely determined by their special suitability for the locality in which they occur. This statement is attested by many species that were at one time restricted to a very definite area. When introduced into other parts by man they have flourished phenomenally. A familiar instance is the introduction of rabbits into Australia. The facts about the geographical distribution of living and fossil species collected by Darwin and Wallace resulted in an extension of the principle of geological succession. This is especially associated with the name of Wallace. Wallace’s law (1855) is stated briefly at the conclusion of the memoir entitled _On the Law which has Regulated the Introduction of New Species_. “Every species has come into existence coincident both in space and time with a pre-existing closely allied species.”

With the statement of this law and its confirmation by more carefully sifted and comprehensive data the positive contribution of the nineteenth century to the development of the modern theory of evolution ended. The picture of a progressive gradual differentiation of animal life, as it spread over different parts of the earth in successive geological epochs became a commonplace of the naturalistic outlook. It remained for the Mendelian renaissance to clarify the conception of this gradual differentiation as an outcome of the agency of natural generation. It must be remembered that the Principle of Succession is still only a step in the formulation of a theory of Evolution. We have still to ascertain what is the natural process by which this progressive differentiation has been effected. The principle of Biogenesis forces us to look to the reproductive process for the answer; but only experiment can arbitrate in this field. The weaving together of principles derived from anatomy, embryology and geology in the light that experiment throws on the nature of the reproductive process is necessary to the completion of the evolutionary argument.

Let us now examine how much we have proved up to this point. We have seen that animals only come into being in our immediate experience through the agency of natural generation. We have also seen that similarity which animals display in their hereditable properties must be interpreted primarily in terms of the hereditability of the properties themselves, and not in terms of a purposive agency. Finally we have found evidence of a gradual and accumulative divergence in the hereditable properties of animals continuing over vast geological epochs. We have still to interpret this divergence in terms of the only agency through which living matter in our experience is brought into being. We are thus led to the fourth step in our argument, the enunciation of the _Principle of Genetic Variation_.

This states the experimental fact that units of living matter with new hereditable properties do actually come into being in the normal operations of the process of natural generation. In using the word _experimental_ in this connexion we lay bare a sharp divergence of standpoint between Darwin’s generation and our own. Darwin collected a good deal of information about the origin of domesticated plants and animals. This seemed to his immediate successors to constitute sufficient evidence for believing that new hereditable properties arise in nature. The development of Mendelian analysis has shown that this is far from certain. Unless we have studied the parent stock under experimental conditions which safeguard its purity, we cannot be sure that a new domesticated variety is anything more than a new combination of genetical characters already present in pre-existing varieties. In other words it may only have arisen through hybridization. We have now at our disposal a clear concept of genetical purity and well-defined methods for establishing the purity of a stock. The whole question has been placed on a new foundation during the past three years by the _artificial_ production of _mutants_ or sports by X-rays in pure stocks of the fruit-fly Drosophila reared under experimental conditions.

A further discussion of the Principle of Genetical Variation with special reference to the Selection doctrine will be undertaken in a subsequent essay, when the possibility of building up new varieties into the units which biologists call species will be dealt with more fully. To return to the discussion of the logical status of the evolutionary doctrine, we may assume that the Principle of Genetical Variation is established. On this assumption we may state the conclusion of the foregoing survey in the following terms. Animals with new hereditable properties have appeared successively with increasing divergence of type in the past history of the earth. Animals only arise in our experience by reproduction from pre-existing animals. Animals with new hereditable properties can arise in our immediate experience by reproduction from pre-existing animals with different hereditable properties. It is therefore natural to conclude that the existing divergence of specific characteristics is the outcome of a natural process of generation operating over long periods of geological time.

§3

Darwin’s generation was in the main satisfied with the evidence derived from domestication. This was embodied as an _argumentum ad hominem_ in the Selection hypothesis. The immediate effect of Darwin’s influence was thus, as Punnett has remarked, “to divert interest from the study of the origin of species” as an experimental issue. Zoology and physiology became divorced in Great Britain. One resolved itself into a Somerset House for the Animal Kingdom, tracing pedigrees on a purely armorial basis. The other tended to develop in association with narrowly clinical objectives, till the rise of modern experimental zoology in the twentieth century. In the light of modern research the Selection hypothesis presents some interesting methodological aspects discussed elsewhere. Let us here confine ourselves to the evolutionary hypothesis in broad outline.

There are two fundamental results of the present enquiry which must be emphasized in any discussion of the logical structure of the evolutionary doctrine. One is the necessity of distinguishing between the Principle of Succession and the evolutionary hypothesis itself. The other is the recognition that in the last resort the validity of the evolutionary hypothesis rests on the issue of experiment. The first of these may sound like a platitude. It is frequently overlooked. Presumably when a biologist says we are more certain than ever to-day that evolution has occurred, but less certain about _how_ it has occurred, he really means that the enormous extension of our knowledge of fossils has placed the Principle of Succession on a much firmer foundation than it enjoyed in Darwin’s time. The mass of new information about comparative anatomy now available is more than ever inexplicable on a crudely teleological basis and more than ever consistent with an evolutionary interpretation, if such an interpretation is permissible. But evolution is more than succession. It is the interpretation of succession in terms of genetical variation. If experiment does not justify this interpretation, in other words if we do not know _how_ evolution occurred, it is evident that we cannot be more certain that it has occurred.

The critique of evolution is not exhausted by a logical analysis of the experimental postulates of the hypothesis, because the doctrine of succession is more than a question of fact. It also implies the validity of current geological doctrines, whose logical status lies outside our present enquiry.[5] What are ordinarily called scientific hypotheses may be classified in two categories according to the test of validity which is applied to them. They might be called respectively _prospective_ and _interpretative_ for lack of existing terminology which makes the distinction which is relevant to our present object. If the consequences of one or other of a set of hypotheses each capable of accounting for a given series of data are uniquely capable of yielding verifiable conclusions about other realms of our experience, we accept the hypothesis which leads us to the new and previously undiscovered fact. We do this even if, in the absence of the new fact, the hypothesis so verified is a _less economical_ one than others which satisfied the original data but do not account for the new one. By _prospective_ hypotheses I mean hypotheses to which this test is applicable. They are such as permit us to make verifiable predictions in other fields of experience. In everyday language they assist us to prophesy correctly about future events. They constitute a hierarchy of socialized beliefs. By their aid mankind has been permitted to construct modern civilization. They possess to a pre-eminent degree the quality of _publicity_ defined in an earlier essay. Mendel’s hypothesis and the kinetic theory of gases belong to this category. Some writers, among others William James, have tended to imply that all so-called scientific hypotheses are of this type. This is not so. There are hypotheses whose justification resides only in the fact that they conform to the requirements of economy of thought. Such hypotheses are accepted because alternative hypotheses are less economical. They are incapable of yielding any verifiable consequences which follow uniquely from them. It is these to which I refer by the term _interpretative_ hypotheses. We construct them, not because they are practically serviceable to us, but because they are conformable with the intellectual requirements of a civilization which is the practical outcome of the application of science. They share two pre-eminent characteristics of the prospective type, economy of hypothesis and ethical neutrality. The need for them resides in our curiosity. They represent one aspect of the secularization of human life and the obsolescence of animistic ideas. We construct them for their _philosophic_ interest alone. The evolutionary doctrine belongs to this category.

Few biologists would admit so heretical a conclusion. They would argue that every new missing link whose discovery is almost daily announced in the press provides verification of the predictions of the evolutionary hypothesis. But there is a fallacy in this contention. The discovery of missing links is not a unique consequence of the evolutionary doctrine. It might be inferred from the Principle of Succession, even if the evolutionary interpretation of the Principle of Succession turned out to be incorrect. Given the experimental postulates of the evolutionary hypothesis as established facts, the evolutionary hypothesis does not belong to the same hierarchy of scientific generalizations as the kinetic theory of gases or Mendel’s Law, because as yet we are not able to predict with the aid of it any unique consequences which can be made the issue of decisive tests.

There is an interesting consequence of these considerations, and one which has a more comprehensive significance. Biology deals with two kinds of relations: relations between living and non-living matter and relations between different kinds of living matter. The Mechanistic Conception of Life is a secular extension of experimental analysis of the former, just as the evolutionary hypothesis is a secular extension of experimental study of the latter. Both belong to the category of interpretative hypotheses in the sense defined above. Why is it then that so many prefer the luxury of scepticism concerning the first issue, and resent the exercise of a suspicion of scepticism concerning the second? Perhaps the answer is that evolution has already become incorporated in the apparatus of what Robert Briffault calls custom thought. I do not think that the physiologist who adopts the attitude of Gallio towards the mechanistic conception of life, affecting to despise all mere philosophy, is consistent, unless he is prepared to dismiss the doctrine of Organic Descent in the same manner. I have yet to meet one who does. Evolution is a philosophy.

VI. THE PROBLEM OF SPECIES

“The effect of Darwin’s _Origin of Species_ was to divert attention
from the way in which species originate.”--R. C. Punnett,
_Mendelism_

§1

In a letter to H. de Varigny dated November 25, 1891, Thomas Henry Huxley wrote: “I shall be very glad to have your book on Experimental Evolution. I insisted on the necessity of obtaining experimental proof of the possibility of obtaining virtually infertile breeds from a common stock in 1860.... From the first I told Darwin this was the weak point of his case from the point of view of scientific logic. But in this matter we are just where we were thirty years ago.” In this passage Huxley explicitly draws attention to the fact that Darwin never came to grips with the historic problem of the Origin of Species, as it had been propounded by Linnæus. Three years later he is writing to acknowledge the receipt of Bateson’s _Materials for the Study of Variation_, a book which laid the philosophical foundations of the present era of experimental enquiry into evolutionary problems. “I see,” he notes, “you are inclined to advocate the possibility of considerable _saltus_ on the part of Dame Nature in her variations. I always took the same view, much to Darwin’s disgust, and we used often to debate it.” Another thirty years passed by, and Bateson ventured to appeal to his contemporaries for a reconsideration of the traditional species problem in the light of the accumulated results of investigation based on Mendel’s methods. He was rebuffed by a veritable storm of criticism from Huxley’s followers. Evolution had become Darwin, as geometry has become Euclid. Had Huxley been living, it hardly seems likely that he would have taken the same side as his devoted disciples in the controversy which ensued.

During the latter half of the eighteenth and the beginning of the nineteenth century biological science progressed towards a clear definition of the problem of Man’s secular origin. This progress involved the rejection of many of the teleological concepts which had been current since the Middle Ages. In the light of recent advances in the study of inheritance and variation, we know that much of the evidence which seemed adequate for an understanding of the evolutionary process fifty years ago must be re-examined to-day and supplemented from other sources. The final court of appeal in the case for an evolutionary interpretation of the origin of species is experiment. Only experiment can place the Principle of Genetic Variation, i.e. the origin of new genetic types in the normal course of procreation, on a sure foundation. A detailed examination of the evidence for this conclusion is essential to a satisfactory examination of the logical status of evolution in the light of modern knowledge. Four separate issues suggest themselves for discussion in a critical enquiry into the experimental evidence for the Principle of Genetic Variation. We must first ask whether the origin of new hereditable types under experimentally controlled conditions is an established fact. We must then decide what natural agency ensures that new types having so arisen will be preserved. This leads us to ask if the appearance of new types is an occurrence of sufficient frequency to have accounted for all the divergency of specific form that has come about in the interval of time which geology places at our disposal. Finally we are faced with the task of deciding how new genetic types can be segregated into the units which biologists call species.

First let us consider the origin of new hereditable types. Thirty years of controlled experiment on the lines suggested by Mendel’s work has given abundant proof that from time to time there do arise in pure stocks individuals which have entirely new hereditable properties. Such individuals are called _mutants_ or sports, a term used synonymously by some writers with the alternative word mutations. The word mutation was originally employed by De Vries in a somewhat different sense from that in which the term mutant is now used. It is preferable to avoid perpetuating this confusion.[6] A new phase in this aspect of the evolutionary problem has been initiated by the recent work of Müller. A controllable agency, exposure of parents to X-rays, has been shown to produce mutants in the fruit-fly Drosophila.

Darwin and Wallace are usually given the credit of first emphasizing the fact of genetical variation. A careful study of their works shows that they did not clearly apprehend the essential aspect of the problem or realize the imperative necessity of subjecting the issue to direct experimental test. When they spoke of variation they included both genetical variation, i.e. the production of mutants as defined above, and differences between parents and offspring which result from the influence of external agencies in early development. The small differences of which Darwin was thinking were mainly of bodily rather than germinal origin. As such they have nothing to do with the problem of evolution unless, as Darwin himself did, we accept the Lamarckian doctrine. In the Introduction to the _Origin of Species_ Darwin states his position thus: “Any being, if it vary in any manner profitable to itself, under the complex and sometimes varying conditions on life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.” What he meant by the strong principle of inheritance Darwin never states in exact terms. Experimental knowledge was not ripe. Biology was still in the phase of _a priori_ reasoning from “common-sense” principles. That he did not distinguish between bodily and germinal differences is shown by the following passage from Chapter 3 of the _Origin of Species_:

“_Variations, however slight, and from whatever cause_ proceeding,
if they be in any degree profitable to the individuals of a
species, in their infinitely complex relations to the individuals
of a species... will tend to the preservation of such individuals
and will generally be inherited by the offspring. The offspring
also will have a better chance of surviving, for of the many
individuals of a species which are periodically born, but a small
number can survive. _I have called this principle, by which each
slight variation if useful is preserved, by the term Natural
Selection._”--(Italics inserted.)

§2

The second aspect of the problem of genetical variation, formulated above, is the special issue raised by the selection hypothesis of Darwin and Wallace. Mendel might perhaps more justly be given priority for clearly envisaging the essence of the problem.

“Those,” wrote Mendel, “who survey the work done in this department
will arrive at the conviction that among all the numerous
experiments made not one has been carried out to such an extent
and in such a way as to make it possible to determine the number
of different forms under which the offspring of hybrids appear, or
to arrange these forms with certainty according to their separate
generations or definitely to ascertain their statistical relations.
It requires indeed some courage to undertake a labour of such
far-reaching extent. This appears, however, to be _the only right
way by which we can finally_ reach the solution of a question the
importance of which cannot be overestimated in connexion with the
history of _the evolution of organic forms_.”

Mendel’s method shows us that so long as they attain sexual maturity and bear offspring, new forms having once arisen, transmit their hereditable properties unchanged. The new hereditary type will sooner or later appear among subsequent generations in its original purity.

This prompts us to ask what chance a given mutant has of surviving to sexual maturity. The question demands serious consideration. We know that a very small percentage of animals that are born into the world do actually survive till the age at which reproduction is possible. It has been calculated that if all the progeny of a single female aphis (the green plant louse) survived in every generation the total of individuals produced in twelve generations would be 10^{22}. Since a single aphis is about a tenth of an inch long, this number would cover the face of the globe. Twelve generations in a family of aphids would appear in less than three years. Evidently the chance that a given mutant will survive depends on two things. One is whether it possesses any characteristics which favour its survival in preference to the parent form. The other is whether it appears once or many times in the same stock. We now know that the same mutants appear again and again. There seem to be definite _loci of instability_ on the chromosomes. Which of these two considerations is of greater importance is at present problematical. Most biologists incline with good reason to regard the former as more significant. The significance of the second is increasingly realized.

Many contemporary authors use the term Natural Selection to imply that competition for the means of existence permits some mutants to live, and weeds out others. On grounds of priority this can hardly be regarded as justified by the writings of the Selectionist writers of the nineteenth century. It is not supported by the actual words Darwin used to define the term Natural Selection which he himself introduced. Goodrich in his admirable book entitled _Living Organisms_, makes the following statement with regard to Darwin’s position:

“It is often said that of late years Darwinism has lost ground,
and that natural selection cannot be regarded as a satisfying
explanation of, or even as an important factor in, the process of
evolution. Doubtless there is some truth in the saying, at all
events in so far as it appears that the doctrine is not what some
misguided enthusiasts may have represented it to be, that it does
not explain everything, that many problems remain unsolved. Yet
the Darwinian theory still stands unassailable as the one and only
rational scientific explanation of evolution by ‘natural’ forces
whose action can be observed, tested and measured. Nevertheless,
the critics are quite right in demanding convincing evidence for
every step in the argument. The modern developments of the study of
hereditary and variation on Mendelian lines, far from weakening the
case for natural selection, seem to have definitely disposed of the
only rival theory, the doctrine of Lamarck, founded on the supposed
‘inheritance of acquired characters.’ Fortuitous changes in the
inherited organization, in the complex of factors transmitted, are
left as the only elements of primary importance, the only stones of
which the edifice is built.”

These remarks imply that Darwin’s successors went much further than Darwin in asserting the creative, preservative, accumulative and continuous character of the selection process. This is true; but Darwin himself, in the _Origin of Species_, expressly stated what he meant by Natural Selection in two quotations which have already been given; and neither of these agree with what Goodrich or any modern geneticist means when he says that he believes in natural selection. Since Darwin introduced the term he has priority in defining its meaning. If later biologists mean something different, when they speak of natural selection, it would avoid confusion to coin a new term. Elsewhere Goodrich says: “What selection alone can do is to preserve variations;” and he quotes Darwin’s words in support. Darwin meant by _preserving_ variations something different from what a modern geneticist believes. The modern geneticist believes that _individuals_ who possess certain advantageous characters will survive in virtue of these advantages. Darwin and Wallace meant that _hereditary characteristics could only survive_ if the supposed tendency to dilution of characters by crossing were counteracted by the elimination of individuals at the other end of the scale of variability.

Apart from what Darwin himself said on the subject we owe some consideration to the sense in which his contemporaries understood his argument. Since I may be accused of tilting with a lance of straw at a windmill of my own construction, let us refer to the section on swamping in Wallace’s _Darwinism_. “He (Darwin) had always considered that the chief part and, latterly, the whole of the materials with which natural selection works was afforded by individual variations or that amount of ever-fluctuating variability which exists in all organisms and in all their parts...” Wallace then proceeds to quote Romanes as saying that “if a sufficient number of individuals were thus simultaneously and similarly modified, there need no longer be any danger of the variety becoming _swamped by inter-crossing_.” Wallace himself wrote as follows:

“I have already shown that every part of an organism in common
species does vary to a very considerable amount in a large number
of individuals and in the same locality; the only point that
remains to be discussed is whether any or most of these variations
are ‘beneficial.’ But every one of these consists either in
increase or diminution of size or power of the organ or faculty,
that varies.... If less size of body would be beneficial, then as
half the variations in size are above and half below the mean or
existing standard of the species, there would be ample beneficial
variations.”

The implication is that natural selection by cutting off the other half--the ample non-beneficial variations--prevents the swamping of the beneficial ones out of existence. We know to-day that the traditional belief in the swamping effects of intercrossing is false. With its rejection the _argumentum ad hominem_ which made the struggle for existence an essential agency for preserving new hereditary properties becomes unnecessary.

However much importance Darwin himself attributed to this aspect of his theory of Natural Selection, he makes clear his attitude in several passages. It cannot be doubted that the assent which he received from his contemporaries was in large measure due to it. Accepting the prevailing misconceptions about swamping, he showed how an evolutionary process could and, as it then appeared, must operate. Experimental evidence for the hereditability of the kind of variations on which Darwin seems to have relied was not brought forward. We now know that the kind of variations which Darwin regarded as the raw materials for the selective process are not generally hereditable. The wisdom of retaining the term Natural Selection may therefore be questioned. In all probability there is another reason which in part explains the popularity of Darwin’s theory as contrasted with the neglect of Mendel’s pioneer labours. Natural selection was suggested by the analogy of industrial conditions in the nineteenth century. Once formulated as a universal principle of nature it appealed to the dominant political theories of the period. The Origin of Species became the bible of _laissez faire_. It triumphed as classical humanism triumphed during the Middle Ages in part at least for reasons which were primarily political. The idea that the struggle for existence is a constructive process played a prominent part in the social theories of the Selectionist School.