PART II. Darwinism and the Atomistic Interpretation of Inheritance

DARWINISM AND THE ATOMISTIC INTERPRETATION OF INHERITANCE

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

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.

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.

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. 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

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. 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.)

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.

A great deal of confusion can be dispelled if we recognize that Darwin never clearly distinguished between two distinct issues. His herculean labours in the field of geographical distribution urged him to seek a reason for the circumstance that different species of animals exist in different parts of the world. The struggle for existence does explain why some species have died out in one place while others have died out in other places. From that point Darwin went on to generalize about the Origin of Species, i.e. how species come into being. It is unfortunate that, though most of his earlier and enduring contributions to science are concerned with how certain species have ceased to exist, the title of his work laid emphasis on the process by which species are brought into being. It was naturally this part of his theory which made the greatest appeal to his contemporaries. Possibly it was not the one which was most significant to Darwin himself. Darwin used the term Natural Selection in connexion with both problems. With regard to the former his theory is as acceptable as ever. With regard to the latter it has now been superseded by exact experimental enquiry into the mechanism involved in the production and preservation of new hereditable types. The work of Gregor Mendel is the proper starting-point of such enquiry.

A third aspect of the Principle of Genetic Variation concerns the adequacy of geological time. It will only be touched on briefly. When the evolutionary theory was introduced to the biological world, it had to encounter a difficulty that no longer presents itself as a formidable objection. Kelvin had calculated the possible period of time during which life can have existed from considerations derived from the rate of cooling of the earth. The allowance which Kelvin conceded was subject to the qualification that no factors at that time undiscovered enter into the question significantly. Since that time the discovery of radio activity has removed the necessity to place any such restriction on the period of geological time, as Kelvin was led to deduce. To-day we have no reason for believing that geological time is too short to permit us to ascribe the faunistic changes of successive generations to the operations of the natural process of genetical variation. At the same time the Evolution Theory will not stand side by side with the retrospective hypotheses of astronomy in the hierarchy of scientific generalizations, until the frequency of genetical variation and the conditions which determine it have been correlated with more exact knowledge of the duration and climatic features of the intervals corresponding to geological strata.

There remains another aspect of the Principle of Genetic Variation. This is of paramount importance in connexion with the evolutionary hypothesis. It is the Origin of Species sensu stricto. A good deal of confusion has arisen in the discussion of the species problem on account of the equivocal usage of the word species. It is therefore best to begin with a clear definition of the species problem. It is a universal experience that any dog resembles its father and mother in more respects than it resembles any cat or any fish; any cat resembles its father and mother in more respects than it resembles any dog or any fish; any fish resembles its father and mother more closely than it resembles any cat or any dog. We may express this by saying that cats, dogs and fish have certain specific hereditable properties. If we examine these hereditable properties we find that a cat has more hereditable properties in common with any dog than those which it shares with any fish. Thus organisms can be arranged or classified in groups expressing the extent of resemblance in their hereditable properties. The work of Ray and Linnæus in the early half of the eighteenth century led to the general belief that “like begets like,” and the publication of the Systema Naturæ (1757) by the latter author marks the beginning of a century and a half of detailed anatomical studies directed to classification of this kind. According to the degree of resemblance of organisms with respect to their hereditary properties they are customarily grouped in phyla, classes, orders, families, genera and species. To illustrate the meaning of these terms let us consider the reader of this essay. He or she is said to belong to the species sapiens of the genus Homo, which includes all living races of man. The genus Homo includes in addition H. Neanderthalensis, the early stone-age heavy-browed first men, and is grouped with the genera Pithecanthropus and Eoanthropus (the fossil ape man of Java and Pilt Down man) in a family Hominidæ, within the order Primates, that comprises apes, monkeys and marmosets. The order Primates is one of many orders of forms within the class Mammalia that includes hairy animals that suckle their young. The Mammalia, along with birds, reptiles, amphibia (frogs, toads salamanders) and fishes, is placed in the phylum Vertebrata, which includes all forms with a backbone.

The degree of similarity implied by placing two species in the same genus, order, class, etc., is an arbitrary one defined by convenience and general consent. The degree of similarity implied in placing two individuals in the same species in the sense in which the term was defined by Linnæus implies something more than convenience. Linnæus placed within the same species all individuals which breed readily with one another. The structural difference between two Linnæan species of animals and plants may be negligible compared with the immense structural differences that distinguish varieties within a single Linnæan species, as for instance the difference between White Leghorns, Yokohamas, Silkies, Partridge Cochins, etc., which are all members of the species Gallus domesticus.

Though this definition of the species as a unit is the one sanctioned by priority, it is insufficiently emphasized by those who discuss evolution that the creation of new species in the daily routine of a large museum has very little to do with the Linnæan test. Preserved animals are sent by collectors to the taxonomist, who proceeds to classify them in new species, varieties or genera in the vast majority of cases without any experimental knowledge as to their breeding habits. Hence the terms species and variety are in practice used to a large extent interchangeably, though not deliberately. The historic problem of the origin of species is not that of the origin of museum species but of Linnæan species. If we can show that discrete hereditable properties arise, as we know that they do, discontinuously in the normal course of generation, we have all the materials we need to interpret the origin of varieties, genera, orders, families, classes, phyla. Whereas all these are arbitrary groups defined in terms of similarity and difference of the hereditable anatomical properties of animals and plants, the species, as defined by Linnæus, is a group limited not merely by the anatomical resemblance of its individual members but also by their inability to breed successfully with other forms.

What has been said so far about the origin of new hereditable properties bears directly upon the way in which new varieties arise. New varieties will only retain their characteristics if some external agency is employed to prevent them from hybridizing and thereby giving rise to an indefinite number of new combinations of characters. The Yokohama can be made to retain those characteristic differences which distinguish it from a White Leghorn by the mechanical device of separating the two strains with a partition of wire netting. No wire netting is required to prevent a White Leghorn and a turkey from losing their genetic individualities, when they are placed in propinquity to one another as are closely allied species in Nature. There is therefore in addition to the problem of the origin of new varieties a problem of the origin of species incompatibility. This cannot be dismissed as of no importance, so long as our experimental knowledge of the origin of varieties fails to suggest in what way this incompatibility may arise. If we can solve this new problem the evolutionary hypothesis presents no ulterior difficulties in the way of explaining the origin of differences which separate the larger systematic groups. The differences employed in distinguishing genera from varieties, and orders from genera or classes from orders are differences of degree. Between species, superficially at least, there seems to be a difference in kind. On this account the origin of species has always been taken to signify the core of the evolutionary problem.

In a somewhat panegyric vein Mr. H. G. Wells replying to Hilaire Belloc makes the following remark: “Darwin’s book upon the subject was called The Origin of Species. It was a very modest and sufficient title. He did not even go to the length of calling it the origin of genera or orders or classes.” Surely Darwin might much more appropriately have employed the latter. How types which are structurally different arise may or may not be accounted for by the selection hypothesis. How types which will not breed with one another arise within the same stock is not relevant to it. It is true that Darwin and Wallace vaguely referred in their writings to a natural tendency to infertility as forms become more sharply differentiated. This does not meet the difficulties of the case, even if it is a sound experimental doctrine. Bateson has used the following illustration to emphasize the irrelevance of Natural Selection to the species problem in the strict sense of the term:

“Sometimes specific difference (anatomical differences between species) 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 explain why this feature should be necessary 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 point of colour... The two species therefore, apart from any difference that we can suppose to be related to respective habits, are characterized 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 one way and the other in another way, and I believe it is mere self-deception which suggests the hope that with fuller knowledge reasons of this nature would be discovered.”

It is permissible to argue that the final justification of the evolutionary argument will be achieved when intersterile mutants have been shown to appear under experimental conditions. We shall then be able to state that new types which display not only anatomical but specific discontinuity have arisen in the ordinary course of generation. At present it is only possible to say that we have very good reason to believe they can do so. Bateson overemphasized the difficulty of the species problem when he said “the production of an indubitably sterile hybrid from completely fertile parents, which have arisen under critical observation from a common origin... is the event for which we wait.” Although the origin of the species barrier does introduce a novel issue into the discussion of the evolutionary problem, its novelty is not so fundamental as it appears to be at first sight. Morgan remarks with justice:

“The necessity of putting the mutation theory to the test that Bateson calls for seems to me very doubtful, for while this is one of the possible ways in which a mutant might split off at once from the parent type, it is by no means the only way or even, I think, the most probable way in which species have become separated.... There is no one problem of infertility of species and no one problem of the sterility of hybrids, but many problems, each due to differences that have arisen in the germinal material. One or more of these differences may affect the mechanism of fertilization or the process of development, producing some incompatibility.”

Bateson performed a most important task in emphasizing that the problem of species discontinuity exists. He made its solution assume more formidable proportions than the facts merit. There is no mysterious wholeness about the concept of the species barrier. Like other scientific concepts it defines a class of properties. When we examine the characteristics of species barriers, we at once see that they constitute a very heterogeneous assemblage of hereditable properties, many of which are recognizably similar to hereditable properties which we know to arise as mutants in genetic experiments. An individual may be placed in a different species from another individual because of some merely anatomical difference in the structures associated with the copulative act. Owing to the respective absence of neck hackles and tail feathers in two strains known as the Barbadoes and Rumpies, the male of the latter cannot successfully tread the female of the former, though each is interfertile with other breeds of domestic fowls. The origin of such differences does not constitute a problem of a different class from the origin of other varieties. High and low fertility are hereditable properties that can be studied as varieties within the species group. They have arisen as mutant characters in experiment. If there arose within a stock mutants with complementary genes for infertility either type would be infertile with respect to the other. They would constitute separate species in the Linnæan sense, when the parent stock died out. In the case of the donkey and the horse, we can go further and identify the complementary sterility factors in the structure of the chromosomes. Difference of size and shape in the chromosomes of the donkey and horse prevent them from pairing in the reduction division, so that no ripe sperm is formed in the testis of the mule. Mutants differing with respect to chromosome numbers and sizes arising by fragmentation or fusion are known both in plants and animals to have arisen under experimental conditions. In many plants they have been perpetuated by self-fertilization. Plough has raised a mutant strain of Drosophila which is more fertile inter se than with the wild stock. The genetic basis of interspecific sterility, while worthy of much more extensive research, is now reaching a precision which places the experimental data of evolutionary theory beyond the plane of Malthusian speculation.

The foregoing illustrations do not exhaust the variety of biological characteristics which separate one individual from another as a member of a different Linnæan species. Nor do they exhaust the types which can be brought within the realm of experimental treatment. Other cases are discussed at length in Crew’s Animal Genetics. The species barrier is not one thing but many things. In the light of modern research there is no reason to regard the origin of species barriers as an essentially different problem from the origin of varieties. Nevertheless the two issues are superficially distinct. No discussion of the present status of the evolutionary hypothesis is complete unless the distinction is submitted to critical examination in the light of experiment.

In the opening years of the twentieth century it had become the fashion among biologists to treat evolution as a dogma. The growth of experimental study of inheritance and variation tends rather to make us value it as a hypothesis suggestive of further enquiry. The difference between the two attitudes is akin to a difference of method which mankind has adopted throughout the ages in the pursuit of knowledge. One method rationalized in its most rigid form in the philosophy of Hegel is to seek for some proposition to which every one is agreed and proceed by deduction to whatever conclusions may be reached from the starting-point. This method has proved invaluable to politicians and members of the legal profession in the discharge of their vocational activities. It is essentially like that of the schoolmen who would exhaust themselves in untiring search into the writings of the ancients for some authoritative statement regarding the number of teeth which the horse possesses, a statement that no one would dare to question. The scientific method is irreconcilably opposed to the Hegelian method. With no aspirations to good breeding it prefers to look the gift horse in the mouth. It is just those propositions which every one accepts that the scientist is most anxious to examine in the hard light of experience. In attempting to envisage a natural mechanism by which the graded differentiation of animal structure could have been brought about, Lamarck was content to employ the generally accepted belief in the inheritance of acquired characters without bringing it to experimental test. Darwin, fortified with newer knowledge of the historical succession of animals and plants as recorded in the rocks, sought to show that evolution was a necessary consequence of competition and the “strong principle of inheritance.” Darwin did not undertake the task of enquiring into the nature of the “strong principle of inheritance.” It was to him like one of Euclid’s axioms. Mendel alone at this time saw the necessity for an experimental study of inheritance, and pointed the way to a non-dialectical treatment of the problem.

VII. NATURAL SELECTION AND EXPERIMENTAL RESEARCH

“Heredity as something quite incomprehensible cannot be used as an explanation, but only as a designation for the identification of a problem. And the same holds good of adaptability.”--Nietzsche, The Will to Power

To large numbers of people evolution is Darwinism, just as to our fathers geometry was Euclid. In one of his writings Morgan has remarked that “it is not so important to find out whether Darwin’s ideas were as clear as our own, as to make sure that our own ideas are clear.” This is true; but an interest in the history of scientific thought is a blameless pursuit for its own sake; and there are ulterior reasons which justify an historical discussion of the criticisms which experimental discovery has brought to bear on the Selection doctrine in its original form. During the latter half of the nineteenth century the evolutionary hypothesis became entangled with the idea of a moral progress of mankind. On this account some philosophers, who are not biologists themselves, fail to recognize the ethical neutrality of biological enquiry. It is doubtful whether the promulgation of any scientific hypothesis has ever had so profound and, at the same time, so immediate an effect on the attitude of educated people towards personal responsibility and social obligations. The fate of Darwinism is as much the concern of the layman as of academic biologists.

Nor is it easy for those who are not biologists to gain definite enlightenment concerning the extent of the change that has taken place. With the rise of experimental method the discussion of evolution has become more technical owing to the accumulation of new data and on account of the introduction of a more intricate form of logic. It is a quantitative branch of science. There was a time when the biologist thought it worth his while to read and to reply to Samuel Butler. To-day there are biologists who read--and like the present writer enjoy--the works of Mr. Bernard Shaw. They do not feel it necessary to defend their philosophy against the arguments advanced in the preface to Back to Methuselah. Popular expositions of evolution are still written. More often than not one suspects that they are rather too popular to answer the questions which an intelligent reader who is not a biologist is most anxious to hear discussed.

As an exact science biology is still very young. Evolution is in its infancy. Only in our generation has it become the nucleus of a growing body of experimental research. It may be that when the history of the evolutionary hypothesis is written two centuries hence, Bateson’s Materials for the Study of Variation will assume a more prominent place than The Origin of Species. It may be that the name of Thomas Hunt Morgan will be mentioned in its pages more often than that of Charles Darwin. We are too near the footlights to view the matter in its correct historical perspective. It is at least permissible to entertain such a possibility. Ancestor worship has no place in the ritual of science. If any display of sentiment is appropriate in scientific discussion, it might be said that the only fit way in which to honour the memory of a Darwin and a Newton is to press forward in exploring the fields which their labours have fertilized.

Without entering into technicalities I shall make the attempt in this essay to contrast the use of the term Natural Selection in Morgan’s writings with the Darwinian doctrine in its original form. My aim will be neither to justify in the one case nor exculpate in the other, but to discover whether a difference exists, wherein the difference lies, and how the difference has arisen. In contrasting the views held by two men of science it is of the utmost importance to lay emphasis on the type of data which they have respectively studied most. Morgan is an experimental geneticist. Darwin was pre-eminently a geographical naturalist. Morgan’s most brilliant contributions to the advance of science have been focused on the study of those conditions which are significant to the origin and transmission of new hereditable properties in animals. Before the publication of The Origin of Species Darwin’s scientific labours had concentrated more especially on amassing a wealth of information about the way in which species are distributed in different parts of the world. In his long itineraries, it is not difficult to surmise what aspect of the species problem was constantly uppermost in Darwin’s thought. I think it is necessary to appreciate this bias in any attempt to understand the way in which the Selection hypothesis developed. Though Darwin spoke of the Origin of Species, he was interested primarily in why some species happen to be found in one place and other species in different places. Darwin had two distinct problems in view when he set out to write The Origin of Species. In the course of writing it he sometimes lost sight of the distinction between them. One was how different types of animals have come to persist in different parts of the world. The other was how an evolutionary process could take place at all. That the struggle for existence is the key to the former is highly plausible. No facts are known which contradict such a view. It is not really an issue with which the modern experimentalist concerns himself. Up to this point there is no divergence between the Darwinian and the Mendelian standpoint. But Darwin in very unequivocal language committed himself to the view that in building up new specific forms the struggle for existence makes use of all differences between parent and offspring of “whatsoever” origin. He thus implicitly encouraged the view that natural selection is a creative agency. Herein lies a fundamental difference between the standpoint adopted by Morgan and the Darwinian doctrine. Darwin really believed in the Origin of Species by natural selection. Morgan believes in the Origin of Gaps by natural selection.

It is perfectly true that Darwin did not formulate this deduction so explicitly or so prominently as did some of his followers. But it was logically implicit in his earlier writings and very definitely set forth in his later. It was in virtue of this aspect of the Natural Selection hypothesis that evolution captured the support of Darwin’s contemporaries. Till Darwin’s book appeared, biologists did not for the most part believe that evolution could take place. Darwin’s hypothesis demonstrated that evolution must take place in a world in which organisms had to struggle for their existence. The experimental data which Morgan employs as the basis for his conception of the evolutionary process imply that the reasons which led the pre-Darwinian biologist to think that evolution could not take place are unfounded. They also imply that the reasons which Darwin advanced to show that evolution must take place are wrong.

It is easier to make this distinction clear at a later stage with the aid of a concrete example than by stating general propositions. This is because one result of experimental progress has been a change in our use of the concept of “variation.” Darwin used the term variation for any difference between parent and offspring. In affirming that the struggle for existence makes use of all variations for building up species differences, he logically implied one of two things. Either all differences between parents and offspring are genetic in origin, that is to say, referable to differences in the egg or sperm; or alternatively bodily modifications which occur during the lifetime of an individual influence the genetic structure of the offspring so as to produce an analogous result. This principle, usually associated with the name of Lamarck, was accepted by every one in Darwin’s time. Darwin himself, while ridiculing Lamarck’s idea of the modus operandi of evolution, accepted the inheritance of acquired characters. There was therefore no need for him to make a distinction between the two alternatives. Neither the one nor the other is in harmony with the standpoint of a modern geneticist of Morgan’s school; but the difference between the Darwinian standpoint and that of Morgan concerns not only the question of fact but the deductions drawn from it.

The difference between either of these alternatives on the one hand and the Mendelian standpoint on the other can be illustrated by reference to one of Mendel’s original experiments on the hybridization of peas. In crossing pure-bred peas of the variety characterized by a dwarf shoot with the normal tall variety, Mendel obtained only tall types on the first generation, and in the second generation derived from crossing the latter inter se one-quarter were dwarf and the remaining three-quarters tall. Now the individuals of either the tall or the dwarf class are not all alike. Any dwarf shoot grown under ordinary conditions is distinctly smaller than a tall shoot, so that the two classes are discontinuous and quite easily distinguishable; but when the conditions are standardized as much as possible small differences of light, moisture, soil-content, temperature or proximity exert their influence, so that no two dwarf plants are of exactly the same size. What is transmitted through the gametes is something which determines the extent to which an individual is capable of growing under appropriate conditions. This distinction greatly clarifies our thought about the so-called inheritance of acquired characters.

A criticism of the Lamarckian doctrine is irrelevant at this juncture. It is referred to in this connexion because it was only in the eighties, after the Lamarckian view was challenged by Weismann, that the full force of the logical implications of Darwin’s teaching was felt. It is true that his followers were far more definite than the author of The Origin of Species in emphasizing the creative rôle of selection. It is true that the discredit into which the Lamarckian principle fell after the discovery of the nature of fertilization led the Selectionist writers to exaggerate this aspect of Darwin’s hypothesis. Nevertheless Darwin did express himself in unmistakable language with regard to this issue. His followers, forced to be more specific concerning the nature of differences between parents and offspring, made the bold, and, it transpired, unwarranted assumption that all those small differences between parent and offspring now referred to as fluctuating variability are in the main genetic in origin. The Selectionist doctrine thus assumed that hard outline which produced its first vigorous reaction in Bateson’s Materials for the Study of Variation (1894), a work which laid down the main lines of investigation which have been elucidated by the Mendelian renaissance.

To avoid vagueness concerning what Darwin actually did say I shall quote once more from The Origin of Species:

“Any being, if it vary in any manner profitable to itself, under the complex and sometimes varying conditions of 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.” (Introduction.)

“Each of the endless variations which we see in the plumage of fowls must have had some efficient cause; and if the same cause were to act uniformly during a long series of generations on many individuals, all probably would be modified in the same manner.” (Chap. 1.)

“A high degree of variability is obviously favourable as giving the materials for selection to work upon, not that mere individual differences are not amply sufficient, with extreme care, to allow of the accumulation of a large amount of modification in almost any desired direction.” (Chap. 1.)

“Over all these causes of change, the accumulative action of selection, whether applied methodically and quickly, or unconsciously and slowly but more efficiently, seems to have been the predominant power.” (Chap. 1.)

“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.” (Chap. 3.) (Italics inserted.)

If, as Darwin believed, it were true, that variation occurs in every generation, the evolutionary process would be a continuous one. To Morgan the production of mutants is a discontinuous break in a normal routine of stability. To Darwin variation and heredity were co-extensive terms. The offspring are always on the whole like their parents. That resemblance constitutes inheritance. On the other hand they are never quite the same. The difference was what Darwin called variation. To Morgan heredity and variation are not co-extensive terms. The structure of the chromosomes is fundamentally stable. From time to time there occur disturbances of this normally stable equilibrium. New hereditable properties emerge into being in a quite discontinuous fashion. There is no self-evident reason why a particular stock should not remain indefinitely in a phase of stability. To the experimental geneticist there thus exists no difficulty in interpreting the fact that some animals have remained unchanged since the earliest rocks.

To the generation in which Darwin lived there seemed to be only one logical outcome of the view that variation is a continuous process involving all the individuals of every generation. This deduction was never stated very explicitly by Darwin himself, though it was definitely asserted by Wallace. There can be no doubt that this deduction gave the Selection hypothesis such a strong appeal to Darwin’s contemporaries, and contributed largely to the success of the hypothesis of Natural Selection. Before Mendel, investigators in hybridization had treated the individual as the unit for study. From this arose the belief that hybrids are intermediate between the parents. This belief in its turn gave rise to the notion that on crossing a new type back to the parent stock there would be a dilution of the new character, culminating after a number of generations in swamping it out of existence altogether. Evolutionists of the Darwinian period therefore introduced a variety of devices, such as geographical isolation and, above all, the survival of the fittest, to counteract the effect of this swamping and account for the persistence of new types. To Darwin’s generation it seemed that without selection there could be no evolution. The new type would always be swamped out in the long run. In the struggle for existence the less viable variations would tend to be eliminated, and since there would always be less of them on that account, the swamping process would favour the gradual moulding of the species in the direction of more favourable variation. On this view the struggle for existence is the agency which makes species change. Evolution becomes a necessity.

From Morgan’s standpoint evolution is only a necessity in so far as it happens that mutants do from time to time appear. The struggle for existence though eliminating the less viable types has no creative rôle in the Darwinian sense. Mendelian analysis shows that though the first generation of a cross between pure-bred parents may be intermediate between the parental types, both parental types appear in their original purity in the next generation, and will continue to breed true to type, whenever they mate with other individuals similarly constituted. The modern geneticist feels no necessity for an argumentum ad hominem to explain how evolution can occur in spite of a supposed swamping process. To him the swamping process is an illusion based on imperfect knowledge of the facts of hereditary transmission. The importance of this difference in standpoint lies in the fact that the idea of natural selection would never have assumed so powerful an influence over biological thought, unless it had provided the evolutionist with train of reasoning which seemed to prove that evolution must be going on all the time.

This interpretation of the Darwinian standpoint is not a caricature drawn by the pen of an adverse critic. An enthusiastic contemporary exponent of Natural Selection, Mr. H. G. Wells, thus defines the selection theory in his Outline of History:

“the young which a living thing produces... are like the parent living thing. But they are never exactly like it or like each other.... Suppose, for example, there is some little furry whitey-brown animal living in a bitterly cold land which is usually under snow. Such individuals as have the thickest, whitest fur will be least hurt by the cold, less seen by their enemies and less conspicuous as they seek their prey. The fur of this species will thicken and its whiteness increase with every generation, until there is no advantage in carrying any more fur.” (Italics inserted.)

Having cited the above, it is somewhat surprising to note that in replying to Mr. Belloc’s strictures, Mr. Wells makes the following statement with reference to the Natural Selection theory:

“Among questions bearing upon it but not directly attacking it has been the discussion of the individual difference.... What rôle is played by what one might call normal relatively slight differences and what by the sports. Can differences establish themselves while outer necessity remains natural? Can variations amounting to specific differences... be tolerated rather than selected by Nature?... What happens to differences in cases of hybridization?... None of these subsidiary questions affect the stability of this main generalization of biology.”

In explaining the Natural Selection theory, as quoted above, Mr. Wells himself states or implies every one of these “subsidiary” questions, and answers them in his own way.

Let us now see how a modern geneticist would interpret the evolutionary process by taking an analogous concrete example. He would argue somewhat as follows. Supposing a single white mutant hare arises in a grey parent stock, the behaviour of the chromosomes leads us to infer that eventually other white hares, pure for the white gene or genes, will reappear. These mated inter se will breed true to type. On the assumption (not conclusively proved) that it is advantageous for a hare in temperate climates to be grey and in arctic regions to be white, there will be more white hares in the long run in northern countries and more grey ones in temperate countries. If there were no competitive struggle at all, there would in the long run be grey and white hares in northern and grey and white hares in temperate countries. There would have been the same amount of evolution. The only difference that the struggle for existence introduces is that the final picture presents a more discontinuous aspect. This was not at all what Darwin meant by Natural Selection. He would have said that a single mutant would be swamped out of existence by intercrossing. He would have formulated the problem in the following terms. Of all hares born to grey parents some are lighter and others darker. In a region where it is advantageous, the half that are lighter than the mean will have more chance of surviving to maturity. In any given generation there will therefore be more lighter than darker parents. The result of this will be that in every generation the swamping process will always be on the side of the lighter individuals. Darwin postulated that, if this process went on long enough, a white hare would eventually be produced. Such a race would only be produced in the region where natural selection favoured its survival. On this view natural selection is the creative agency, or at least a paramount creative agency in the evolutionary process. Without the struggle for existence hares everywhere would remain grey. In every generation the half that are lighter than their parents would always be swamped by the half that are darker.

To Darwin and more especially to Darwin’s followers selection was the agency which preserved not merely new individuals but new characters, since characters would otherwise be diluted out of existence. For Morgan the preservation of new characters ultimately resides in Mendel’s law of segregation. It has its material basis in the behaviour of the chromosomes. The contrast between the alternatives is at once made clear when we consider what would happen in a universe so large and so abundantly supplied with the necessities of life that no struggle for existence intervenes. Given unlimited time in a Mendelian universe in which natural selection did not operate, all the species we know to-day would be present, and many more besides. Evolution would have occurred; but the pageant of life would present to the taxonomist a more continuous appearance, and the striking gaps which we now see would be filled not by fossil relics but by living forms. Except in so far as he was prepared to invoke the Lamarckian principle to circumvent difficulties inherent in his own hypothesis, natural selection was to Darwin the necessary condition not merely for gaps but for any evolution to take place at all. In a Darwinian universe without natural selection there would be no progressive differentiation of new characters.

When, out of deference to Darwin’s contribution to biological thought, the experimentalist of Morgan’s school asserts his belief in Natural Selection, he is in fact referring to something very different from Darwin’s Natural Selection, indeed to a view of the process which Darwin would have rejected emphatically. Of course it is admitted that all scientific hypotheses become modified as new data accumulate; and phrases imperceptibly change their meaning in the course of time. But the natural selection of Morgan’s school is not a continuous development from the original concept. Within two decades of the publication of The Origin of Species the selection hypothesis had assumed a clarity of outline which had an influence on subsequent developments in biological thought, persisting till the present day, and not likely to disappear for some time. In 1881 Weismann challenged the prevailing belief in the inheritance of acquired characters. Thenceforth in the hands of the Selectionists environment became merely an agency by which the hereditary materials are preserved or rejected. As an aspect of the problem of development it faded into the background of the picture. To question the almightiness of heredity became equivalent to defending the Lamarckian principle, though the two issues are logically independent.

Educated people frequently use the words environment and heredity in a very different sense from that in which they are employed by the biologist. Unless we are accustomed to the study of embryonic and larval life, we are apt to think of an organism as a finished product. The rôle of environment and of heredity as seen through the eyes of a contemporary biologist can be made explicit by reference to recent work on the metamorphosis of tadpoles. We know to-day that the thyroid gland of all vertebrates contains a high percentage of iodine. Barger and Harrington have now prepared in pure crystalline form an iodine compound which has the same therapeutic properties as extracts of the thyroid gland. A few years ago the discovery that frog tadpoles will change very rapidly into adults if fed with thyroid gland, was followed up by the development of a successful technique for removing the rudiment of the thyroid gland in frog embryos. Thyroidless tadpoles never undergo metamorphosis. They continue to grow as tadpoles when the normal tadpole would change into a frog. The change into the adult in the normal tadpole is initiated by the liberation of the thyroid secretion into the circulation. It has also been shown that tadpoles reared on an iodine-free diet in water containing no trace of iodine remain permanently in the larval state. This clarifies what is meant by an environmental factor in development. In contradistinction to the influence of environment the influence of inheritance in development may be illustrated by reference to an American salamander, Amblystoma tigrinum, which has a characteristic larval form. In the lakes around Mexico city there is a local race of this species which never undergoes metamorphosis in nature, reproducing in the larval form. It can be made to develop into the land-dwelling adult in a few weeks, if fed with thyroid gland in the laboratory. Addition of iodine salts to the water in which it lives or to its food will not induce metamorphosis. Its permanent fixation in the larval stage is due to the fact that it inherits from one generation to another a deficient thyroid gland, which cannot make use of the iodine in its surroundings. Absence of iodine in minute quantities from the water, a purely environmental agency, or on the other hand a hereditary difference between two races with respect to the efficiency of thyroid secretion, may either of them be independently instrumental in deciding whether a particular individual shall attain sexual maturity in the form of an air-breathing land-dwelling salamander, or an aquatic half-way house between a salamander and a fish. A geological epoch, if you like to put it in that way, is thus summed up in a mutant gene or in a trace of iodine.

In the attempt to understand the tenacity with which belief in the Lamarckian view persisted in biological thought, it must be borne in mind that embryology is the most recently developed branch of anatomical science. Until the classical researches of von Baer and Meckel were published in the first half of the nineteenth century, the prevailing idea about development was the teleological doctrine that an animal is from the very first complete in all its parts and only needs growth to make its minute structure manifest to the eye. Caspar Wolff in 1759 made observations on the hen’s egg, and was led to state the “epigenetic” as opposed to the prevailing “evolutionary” view. He sought to show that the hen’s egg is at the beginning without any gross anatomical organization and that structural organization within the egg is a gradual development. His work failed to attract attention. Von Baer’s researches on the same subject were published synchronously with the formulation of the Cell doctrine (1832). One might say that until the middle of the nineteenth century, the current conception of inheritance in biology was closely analogous to the legal notion. The parent was supposed to hand on its anatomy to its offspring in the same sense as the well-to-do hand on their belongings. With so erroneous a conception of the nature of development prevailing, it is little wonder that the idea of the inheritance of acquired characters seemed a perfectly reasonable one. It is not surprising that the doctrine of Lamarck should have been first challenged during the decade in which the nature of fertilization and the process of maturation of the germ cells were elucidated.

As stated by its author the Lamarckian principle implied that any reaction of the organism to its environment is carried over to subsequent generations. It was especially adaptive reactions such as the effect of use and disuse which Lamarck emphasized in his evolutionary speculations. When the Lamarckian principle was first challenged, prominent scientists like Cope were willing to assert such fables as the story that a cock deprived of one eye transmitted eye defects to all his offspring. When it was conclusively proved that mutilations effected through several generations left no impress on the hereditable characters of the stock, the Lamarckians fell back on the gratuitous postulate that only “adaptive” changes could be transmitted. The precise meaning of this adjective was never defined, nor was any reason forthcoming to suggest the existence of a mechanism that could discriminate between mutilations and bodily changes that are “adaptive.” This is yet another example of the perils of introducing teleological preoccupations into the construction of biological hypotheses. If recent experimental research conserves any element of truth in the Lamarckian idea, it has robbed it of any special significance to the way in which adaptive structures originate.

Structural changes may arise in the course of development from two conceivable sources. The chromosomes which represent the hereditary materials may find themselves reacting to a different type of “internal environment.” The majority of modifications in the normal course of development undoubtedly come within this category. Modifications of this type, including in all probability relative sizes of organs, all mutilations and habits are clearly not hereditable. Belief in their hereditability was only possible so long as biology was dominated by teleology and the essential features of the reproductive cycle were undiscovered. There is another possibility which was entirely disregarded by Weismann in his Theory of the Germ Plasm. It is a possibility that has no bearing on the problem of adaptation. If environmental agencies can produce mutations by a structural change in the chromosome itself, there is no reason why such structural changes should be confined to the chromosomes of the germ cells. We must therefore preserve an open mind with regard to the possibility of encountering phenomena having a superficial similarity to what is implied in Lamarck’s doctrine. The exposure of young larvæ of the fruit-fly to X-rays has led to the production of individuals which show bodily resemblances to forms which have arisen in the ordinary course of events as mutants. The effect of X-rays may be to change the environment in which the chromosomes operate. But the recent investigations of Patterson indicate the likelihood that the modification is due to the action of the X-rays on the chromosome itself. We know that X-rays will produce mutant changes in the chromosomes of the germ cells. If Patterson’s interpretation is correct, it may well be found that X-rays can simultaneously effect mutant changes in all the chromosomes of the body. If applied sufficiently early in the course of development, radiation with X-rays would then produce bodily changes of a transmissible nature. This possibility resides in the fact that the agent is capable of acting on all the cells of the body in the same way at the same time. There is no inherent unlikelihood that temperature and the chemical constituents of an animal’s food may simultaneously produce bodily and germinal mutations. Strictly speaking this is not the same as the traditional belief in the “inheritance of acquired characters.” The Lamarckian principle completely disregards the distinction between modifications which arise from a change in the internal environment of the chromosomes and a physical change in the chromosomes themselves. It takes no account of the possibility that the environmental agent can act in the same way simultaneously on all the cells of the body.

There are still students of fossil forms who claim that the traditional Lamarckian view is necessary to explain the historic succession of animals by continuous generation. There seems to be no satisfactory reason to justify the statement that evolution can only be satisfactorily explained by assuming the inheritance of acquired characters. If there were, it would not be an argument in favour of the Lamarckian principle. It would be as an argument against the evolution theory. It would imply that the truth of evolution depends on assuming a mechanism whose existence is most unlikely. What is often called the neo-Lamarckian standpoint, the view that acquired characters only gradually become impressed on the hereditary constitution after countless generations, transfers the issue from the plane of verifiable experience to one of pure surmise, rendering further discussion profitless. In such a matter as this when experiment is silent, the student of fossils must also be silent.

The objection rests in fact on a misapprehension. The earlier phase of experimental enquiry along the lines laid down by Mendel was confined to the analysis of simple clear-cut hereditary differences which present themselves in almost any environment in which the animal can live. They were also largely concerned with differences that could be resolved into the simplest arithmetical ratios, or as Morgan would say with mutants that have arisen through a change at a single point on one pair of chromosomes. It is only as technique has progressed that it has been possible to analyse the more complex cases in which single characteristics depend on numerous Mendelian factors, or where the character differences are so variable that they can only be defined in statistical terms. The palæontologist being occupied very largely with size differences is sometimes disappointed, because such phenomena lie outside the scope of the simpler problems, which were once thought to define the scope of the Mendelian hypothesis. Recent progress which has led to the recognition that Mendel’s principle of segregation underlies the inheritance of size is therefore of no little significance to evolutionary theory. As we come to recognize the dependence of hereditary transmission on discrete particles which maintain their entities uncontaminated through all the cell divisions of the body, segregating in their entirety in the formation of the gametes, the unlikelihood of the Lamarckian principle in its traditional form becomes more and more evident.

If the Lamarckian principle in its traditional form was undoubtedly based on a confusion of ideas and an ignorance of fact, the Theory of the Germ Plasm put forward by Weismann shows how facts may be distorted to fit in with preconceived ideas which are in themselves logically flawless. The discredit into which the Lamarckian principle fell, almost as soon as the elementary facts about the nature of fertilization became known, led Darwin’s successors to assume that all those differences between parent and offspring which Darwin had referred to under the term variations are genetic in origin. The assumption was gratuitous, as later experimental analysis has shown. Without that assumption the Selection doctrine would have been robbed of the immense importance it had already begun to assume. From a complete misapprehension of the true rôle of the environment in relation to inheritance, the biological pendulum swung in the opposite direction to a complete disregard of the influence of the environment in relation to development. It is from Weismann’s writings that we can best appreciate the fundamental dissimilarity of Darwin’s Natural Selection and Morgan’s views. For Weismann’s “germinal selection” is the logical outcome of Darwin’s selectionism, once it had been purged of the Lamarckian principle. It is a triumph of Hegelian reasoning applied to biology. There is nothing wrong with it but its premises. Weismann’s theory embodied an atomistic conception of heredity. Unlike Mendel’s it had no connexion with experimental data. Weismann identified his hereditary determinants with the substance of the chromosomes. Unlike Morgan’s hypothesis, Weismann’s speculations were based on incorrect observations about the way in which the chromosomes behave. In the long run the influence of Weismann’s teaching has probably been more sterilizing than the Lamarckian doctrine which he challenged.

Weismann imagined that his atoms of heredity or “determinants” multiply in the cell and in some rather abstract way compete with one another for survival. Hence the hereditary constitution of the individual is never quite the same in two successive generations. Heredity and variation are thus co-extensive, as Darwin’s Natural Selection postulates. Weismann also thought wrongly, it transpired, that the reduction division of the germ cells takes place in such a way that each cell receives half a maternal and half a paternal chromosome of each pair and not, as we now know, a whole paternal or a whole maternal element. Hence he argued that the formation of the germ cells involves not, as Mendel proved by experiment, a segregation but a closer intermingling of the germinal materials. From this the swamping of new characters on crossing became an absolute necessity. To Weismann selection alone could prevent this swamping. Selection must act in every generation, because the mingling of the hereditary materials becomes more intimate with every generation. Only under the influence of continuous selection could any change be brought about. Without it universal stagnation would exist. In short Selection was the creator and the preserver of the benefits of variation. In all this Weismann, with the support of Wallace, went much further than Darwin himself. But the Selectionist doctrine in its main features was implicit in the Origin of Species. The sociological exploits of biologists belong especially to the period in which the Selection doctrine assumed this doctrinaire aspect. Doctrinaire Selectionism has persisted in our own generation in the writings of many eugenists.

We set out in the first place to contrast the views of the modern geneticist with the Selection hypothesis in its original form. The main differences arise in connexion with two issues. One concerns Darwin’s own view that evolution is a continuous process. Darwin believed that selection operates on all the individuals of every generation. This implies either that acquired characters are inherited or alternatively that all differences between parent and offspring are hereditary differences in the modern sense. The views to which modern geneticists have been led by their experiments are diametrically opposed to both conclusions. The other question concerns the creative rôle of selection. This belief arose from ideas about hybridization and artificial selection current among those biologists to whom Darwin addressed his argument. Darwin himself did not stress the point; but it was this corollary of his theory which accounts for the successful appeal which Natural Selection made to Darwin’s contemporaries. They were satisfied that, if a struggle for existence occurs, evolution must be taking place. This was because all biologists before Mendel confused the characters which do blend with the genes that do not. To the modern geneticist this corollary has no significance, because experiment has forced him to reject views about hybridization prevalent before the publication of Mendel’s researches. To Morgan, as to Darwin, selection through the survival of the fitter is essentially like artificial selection. Morgan differs radically from Darwin in his understanding of the way in which artificial selection itself operates. According to Morgan selection has no creative significance. “Selection has not produced anything new, but only more of certain kinds of individuals; Evolution however means producing new things, not more of what already exists.”

Thus from the standpoint of Morgan the status of evolution is more satisfactory in the light of modern research. For there is no need to advance any special device to explain why new types are not swamped out of existence through the blending of characters on crossing. From the point of view of the Darwinians, if they were still with us, the outlook would be disconcerting. The modern geneticist no longer regards evolution as an imperative consequence of the struggle for existence. On the other hand the modern view presents no greater difficulty than the former one in explaining the tendency towards greater adaptation. It is free from the objection that it proves too much. New hereditary types would persist even if there were no struggle for existence. Since there is one, the chance that a given mutant will reach the age at which it can produce offspring will be greater if the mutant character has “survival value.” At present there are insufficient experimental data to make profitable the discussion of the amount of advantage necessary to ensure survival. At the same time it is of interest to record that the application of Mendelian method furnishes materials for a precise statement of what selection can achieve and the rate at which it works, when the extent of differential fertility or mortality in a population is known. The mathematical theory of selection has been made the subject of some illuminating researches by J. B. S. Haldane and by Fisher. Haldane’s calculations have led him to conclusions very different from the dialectical deductions which some eugenists have drawn from the recent decline of the European birth rate.

VIII. THE SURVIVAL OF THE EUGENIST

“I am that ancient hunter of the plains, That raked the shaggy flitches of the bison: Pass, world: I am the dreamer that remains, The Man, clear-cut against the last horizon.” Roy Campbell, Flaming Terrapin

Concerning Vesalius one of his biographers has said: “in dissecting monkeys he became convinced that the many discrepancies between the Galenic teaching and his own observations on the human body were due to the circumstance that Galen had derived most of his knowledge from dissecting monkeys, and had not thought it necessary to mention the fact.” Perhaps the biographer of a future Vesalius who succeeds in laying the foundations of social anatomy will record that “in studying the writings of the Eugenists he became strengthened in the conclusion that they were discussing the habits of fruit flies rather than human beings, but had not thought it necessary to mention the fact.”

I have called this essay The Survival of the Eugenist; but I wish to make it clear that I entertain no lack of sympathy for Eugenics as defined in general terms by Galton, the Galen of social biology. I have chosen this title to lay emphasis on the part which eugenists have played in perpetuating a certain attitude towards human society. This attitude starts from an examination of those characteristics which man shares with all other animals, but neglects the equally important task of defining those characteristics which distinguish man from all other animals. The weakness of all mechanistic systems hitherto proposed lies in their refusal to recognize the existence of anything which does not yet come within the province of scientific method. A mechanistic philosopher can legitimately entertain the hope that the study of human society will become an ethically neutral science, and that the methods of biology will fertilize sociological enquiry, as the methods of physics and chemistry have fertilized biological investigation. He is not entitled to pretend that biology can at present provide a key to the interpretation of human history. I am well aware that there are eugenists who would repudiate any such pretensions. At the same time the general tendency of eugenic propaganda has been to exaggerate, and grossly exaggerate, the applicability of genetic principles to the analysis of human society. This tendency is a legacy of the period in which Eugenic ideas had their origin.

Whatever disadvantages the Christian cosmogony imposed upon the study of human society, it possessed the merit of emphasizing that the proper study of mankind is man. The immediate influence of the evolutionary controversy was a reversion to the Galenic practice in social anatomy. There is nothing surprising in this reaction. To Huxley and Spencer the important fact was that Man is a brute. It was necessary for them to emphasize man’s genetic similarity to other animals in opposition to the traditional view which placed man in a special category apart from other natural objects. How strongly the need to emphasize Man’s new status was felt can be inferred by a well-known dictum in Man’s Place in Nature. “Whatever systems of organs be studied,” wrote Huxley, “the comparison of their modification in the ape series leads to one and the same result--that the structural differences which separate man from the gorilla and the chimpanzee are not so great as those which separate the gorilla from the lower apes.” In his dispute with Owen, Huxley went much further than any modern anatomist would be prepared to follow him. If like Cuvier he had based his objections on the structure of the human foot instead of the hippocampus major, Owen might have made a stronger case. His opponents were too busy disposing of man’s Cartesian spirit to devote much attention to his Cuvierian sole.

The evolution of Thomas Henry Huxley, of Herbert Spencer and of Francis Galton was a precocious baby. Its parents and relatives entertained high hopes of its future career. In that tradition it has been nursed by their loyal disciples who have encouraged it to discourse upon sociology before it has learned to read and write. Huxley, Spencer and Galton were fundamentally right in recognizing that any theory of the development of human society implies certain biological assumptions. Their anticipations of immediate progress in the biological treatment of human society was inevitably coloured by the issues which made the first claim on their attention. Those issues are no longer topical. The experimental biologist of to-day cannot approach the structure of human society from quite the same angle. The pioneers of evolution were goaded by theological opposition to adopt an attitude which is easy to condone but unnecessary to emulate. To justify their right to speculate, they found it necessary to convince the non-scientific public that their speculations were correct. To do so they were driven to minimize the gap between man and the apes and make the best of any evidence pointing to the missing link which popular imagination demanded.

The missing link provided the occasion for one of the first sociological exploits of anatomical science. There is an account of the incident given in Dr. Haddon’s History of Anthropology. Three years after The Origin of Species was published Dr. James Hunt, President of the Anthropological Society, read his paper on “The Negro’s Place in Nature.” In it he maintained that “the analogies are far more numerous between the ape and the negro than between the ape and the European.” In 1866 he recorded a further contribution to the detection of the missing link by asserting that “there is as good reason for classifying the negro as a distinct species from the European as there is for making the ass a distinct species from the zebra.” In this discussion Huxley gave the exponents of the missing link a half-hearted support tempered somewhat by his humane and sceptical disposition. An obituary notice of Dr. Hunt in a New York paper announced in 1870 the “Death of the Best Man in England.” Sixty years after the publication of Hunt’s first communication, a leading American anthropologist, Professor Kroeber, summed up the present state of knowledge in the following terms:

“The only way in which a decision could be arrived at along this line of consideration would be to count all features to see whether the Negro or the Caucasian was the most unape-like in the plurality of cases. It is possible that in such a reckoning the Caucasian would emerge with a lead. But it is even more clear that which ever way the majority fell, it would be a well-divided count.”

Speculation upon the ancestry of man has continued with unabated vigour to the present time. Huxley’s generation had one good excuse for confusing the process of social and organic evolution. It cannot be pleaded by our own. Modern men were known to be associated with the later palæolithic cultures. The Mousterian artefacts had been associated with the Neanderthal type. There was much to encourage the hope that further research would reveal a close parallelism between the physical differentiation of specific or racial types and successive stages of cultural development. It now appears that Mousterian artefacts were also fashioned by types who, as Sir Arthur Keith puts it, “would excite no comment, if dressed in modern garb in any assemblage of modern Europeans.” Our own species has served a long apprenticeship in a much earlier phase of cultural development than that which was at one time attributed specifically to the Neanderthalers. The data presented in Sir Arthur Keith’s book The Antiquity of Man show that it is not easy to press blood relationships out of stone implements. There are already signs of a reaction against the extravagant claims which have been put forward by some physical anthropologists. The most recent hypothesis of the origin of civilization completely breaks with the earlier tradition to harp on the racial aspect of the problem. Professor Elliott Smith is distinguished both as a physical and cultural anthropologist, and it is therefore noteworthy that his theory emphasizes the characteristics of man’s physical environment as the significant factors in the appearance of the first civilized communities of the Nilotic region.

Under Weismann’s influence environment as an aspect of the problem of development assumed a nebulous outline. For a generation biologists were hypnotized by the discredit of the Lamarckian teaching. Eventually the progress of experimental embryology and cell anatomy relegated Weismann’s theory of germinal selection to the same limbo as the Lamarckian hypothesis. In Weismann’s hands the Selection doctrine had assumed a particularly rigid form. Evolution was necessarily a continuous process. All differences between parents and offspring were genetic. Heredity and variation were coextensive processes. From this it followed that a continuous evolutionary process had accompanied the development of social institutions. It was a natural step to confuse the two. The conviction that eugenic legislation is a matter of overwhelming urgency arose as a direct outcome of that step. That the same confusion still dominates eugenic propaganda is illustrated by a statement made by Mr. Lidbetter, a prominent eugenist, in his paper at the World Population Congress of 1927, “It is a platitude,” Mr. Lidbetter stated, “in these days to speak of natural selection as the essential agent in human progress.” It may be a platitude. It is not a truism. It is simply a misuse of terms. Social development is the communication of social tradition and social accomplishment from one generation to another, with the addition of new ingredients in each. Organic evolution is brought about by the transmission through the gametes of new hereditable properties. The mechanism of one is education. The mechanism of the other is sexual reproduction. It is possible that they react upon one another, but the extent to which they do so cannot be ascertained by a priori reasoning. The experimental study of genetic variation has made it abundantly clear that evolution is not a continuous process. At present we do not know the precise conditions relevant to the production of mutant types; consequently it is unjustifiable to make any general assumptions about genetic variation in human societies without recourse to direct experimental inquiry.

That is the task which now lies before the social biologist. Its successful accomplishment will not be facilitated by under-estimating the difficulties inherent in the problem. The study of human inheritance is beset by innumerable obstacles. Man is a slow-breeding animal of low fertility. His chromosomes are numerous. The geneticist cannot control his matings. In spite of these drawbacks some insight into the nature of hereditary transmission within the human species can be gained by formulating the results of random mating on certain hypothetical assumptions. Familial studies of colour blindness, brachydactyly and the blood groups provide clear illustrations of Mendelian phenomena. So long as family pedigrees are employed to demonstrate the inheritance of physical characteristics, it is not difficult to recognize the nature of the environmental influences with which the hereditary materials react, and to make allowance for them. The geneticist is on familiar ground. The constituents of man’s physical environment have been classified by the physicist, the chemist and the bacteriologist. Their effects upon the physical characteristics of an organism form the subject matter of physiology. It is possible to speak of the action of sunlight and humidity, oxygen pressure and diet, infectious and contagious germs, iodine and calcium salts with some measure of confidence. All these things are features of man’s physical environment or of the physical environment of any other animal. The methods for investigating their influence are well tried. The concept of a uniform physical environment is tangible. It can be explained to a pragmatist or a presbyterian, a behaviourist or a bimetallist.

It is another thing to speak about a uniform social environment. The factors which determine man’s social behaviour are obscure and elusive. Even to-day any dogmatism on the relative importance of heredity and environment assumes an almost frivolous aspect when the attitude of the experimental biologist is brought to bear on the evidence. Analogies from the animal kingdom have been pressed into the service of those who emphasize the rôle of either the one or the other. Kropotkin’s Mutual Aid was the reductio ad absurdum of that attitude to social problems. Kropotkin was neither more nor less scientific than the exponents of nature red in tooth and claw. Both were irrelevant. The same irrelevance has been evident whenever biologists have attempted to rationalize their political sentiments. The anti-feminist appeals to the fighting and protective male. The feminist can retort by invoking the worm Bonellia of which the male lives as a parasite in the generative passages of the female. The eugenist pictures the human poultry farm nicely mapped out in pens, each surrounded by its own partition of wire-netting with a few holes here and there. Maybe the Rhode Island Reds have scratched their way into the proper preserve of the Partridge Cochins. Sooner or later the cosmic poultryman, aided by wise statesmen, will put them back where they belong. His opponents can reply that class differences exist in insect communities. The difference between a white ant queen and a termite worker is more striking than the difference between royalty and factory girls; and it is a difference determined by diet. Encouraging illustrations in support of any social doctrine can be brought forward by those who prefer analogy to analysis.

It might be hoped that the study of human history would assist, but the record of history is ambiguous. A striking instance of this ambiguity is to be found in Professor Carr Saunders’ book on the Population Problem. In the course of a temperate and on the whole well-balanced discussion of the racial factor in history, Carr Saunders remarks that the

“Nordic peoples are mostly Protestant and the Mediterranean peoples mostly Catholic and Greek. The fact,” he continues, “that during the Reformation a choice was set before most European nations as to what religion should be adopted--the issue hanging in the balance for some time in many places--seems to indicate that the conditions were more or less equalized and the adoption of the Protestant religion by the Nordic type was influenced by certain innate characters attaching to that type.”

Even if we make a very generous allowance for the genetic homogeneity of the Nordic and Mediterranean populations in mediæval times, an entirely different interpretation of the same facts is equally plausible. At the time when Christianity received official recognition the countries to which Carr Saunders refers as predominantly Nordic lay on the fringe of Roman Imperial domination or completely outside it. The process of christianizing the Nordic geographical region was still in its infancy when the Holy Roman Empire embarked on its ephemeral and inglorious career. It was hardly complete, when controversy within the Western Church began to assume sinister proportions. With the exception of the Saxons the conversion of the Germanic peoples, including the Frisians, took place in the early part of the eighth century. The official conversion of Saxony occurred about A.D. 800. Christianity was accepted by the ruling powers of Denmark towards the end of the tenth century and by those of Norway and Sweden at the beginning of the eleventh century. The conversion of East Prussia, Latvia and Pomerania occurred during the twelfth century, and the conversion of Lithuania did not occur until the middle of the fourteenth century. In those countries which Christianity penetrated last of all the conflict between the ruling houses and the temporal claims of the Papacy was generally most acute. Where reformers could seek protection in the clemency of monarchs at loggerheads with the Pope, they spread their doctrines successfully. Where there only existed a religious movement, it was speedily extinguished. The Reformed doctrines spread in those countries where Christianity had been more recently introduced, and where the political sovereignty of the Pope and the economic power of the Church as a landowner were least firmly entrenched and least agreeable to the secular authorities. Catholicism had taken root in the ancient civilization of the Mediterranean region, when the Nordic peoples were outside the pale. If it is true that the Nordic peoples gravitated to Protestantism, it is equally true that they happened to inhabit the geographical region most remote from Rome. There is no reason to suppose that their choice of locality was determined by any characters peculiar to their type or relevant to the progress of theological discovery.

In seeking to make allowance for the significant factors of man’s social environment there is no body of accredited information to which the geneticist can turn. There are as many schools of psychology as there are schools of philosophy. The introspective psychologist approaches social behaviour from a purely teleological standpoint, interpreting the means in relation to the end it fulfils. The behaviourist adopts a mechanistic attitude, seeking to interpret the end as predestined by the means. One speaks of a directing intelligence and instinctive action. The other speaks of intelligent behaviour and unconditioned response. Between the two schools there is a great gulf fixed. It is that which separates the philosophy of Plato from the teaching of Democritus. It is not merely a difference of perspective or of minor issues. Such differences exist in an exact science. The psychologists disagree about the very nature of inquiry into the basis of social behaviour; and there is no immediate prospect that they will come to terms. Meanwhile the eugenist finds himself impaled on the horns of a dilemma. The methods of animal genetics are mechanistic; but the behaviourist is suspicious of the genetical standpoint; while the introspective psychologist fails to define the characteristics of social behaviour in a form suitable for genetic analysis.

When Binet and Terman published their psychological tests, it seemed that there was a brighter prospect for the objective study of mental inheritance. Of late the psychologists themselves have begun to adopt a less confident attitude. Recently the Stanford school of workers have conceded a conservative allowance of 20 per cent. for the influence of home environment on the intelligence quotient. We have no grounds for believing that the ingenious system of home ratings adopted by Miss Burks (1927) in this investigation include all the significant factors. Consequently this figure represents a minimum. The Chicago school have investigated the intelligence quotients of foster children, and adopt an even more sceptical attitude to the value of the I.Q. as a measure of genetic endowment. Tallman has investigated the intelligence quotients of sixty pairs of identical twins. It was found that the mean difference between pairs of brothers and sisters of different ages on the one hand and pairs of non-identical twins on the other was larger than the difference between pairs of non-identical twins and pairs of identical twins. Accepting the most conservative allowance, it may be stated with some confidence that the contribution of environment to the intelligence quotient is at least as large as the recorded differences between racial and occupational groups subject to different environmental influences.

For two generations eugenists have been writing about mental inheritance. As far as I am aware Professor MacDougall alone has pointed out that the attempt to formulate a concept of mental inheritance raises a very formidable issue which challenges the foundations of current biological philosophy. He himself faces the difficulty by returning to the Lamarckian fold. Lamarck’s position was at least consistent. He conceived heredity in mental terms. His theory was teleological throughout. Galton was not consistent, and his disciples have been less so. Since Weismann’s time the study of heredity has become more and more explicitly materialistic. To the modern geneticist heredity is one aspect of the physical process involved in the production of a new unit of living matter. His hypotheses are conceived in physical units. The gene has space-time dimensions. Mental inheritance is a meaningless collocation of words, unless it is possible to bring the concept of mentality within the mechanistic framework. That is what the behaviourist school in psychology has undertaken to do. The future of social biology depends on the success which attends their efforts.

Fifty years have passed since Francis Galton published Hereditary Genius and An Enquiry into Human Faculty. Since then there have been notable changes in the attitude which scientists have adopted both towards heredity and human faculty. The work of Mendel, Bateson and Morgan has enormously enriched our knowledge of hereditary transmission in animals. The work of Loeb, Sherrington and Pavlov has opened up new horizons in the study of animal behaviour. The biological analysis of social behaviour presupposes that both methods can be brought to bear upon it. It may be premature to adopt a confident attitude to the prospects, but it is legitimate to state that there is no likelihood of solving the problem which Galton propounded so long as eugenists continue to regard it as the exclusive prerogative of the evolutionist. The enthusiasms engendered first by the reception of Darwin’s hypothesis and subsequently by the spectacular advances which have resulted from Mendel’s discovery, encouraged the eugenist to adopt an extremist attitude. New and no less noteworthy developments in the physiology of the nervous system have encouraged the behaviourist to go as far as possible in the opposite direction.

It is not difficult to understand how this has happened. In Galton’s time the analysis of animal conduct had not progressed beyond the recognition of those simple units of behaviour which Pavlov calls “unconditioned” reflexes. The scratch reflex evoked on stimulating the lumbosacral region in the spinal dog is an example of this type. Given the same external situation, it can be elicited in any member of the canine species. There are therefore two principal factors which determine the scratch reflex. One is the immediate stimulus. The other is the inherited structure of the nervous system. Simple reflexes of this kind play very little part in man’s social behaviour; but modern physiology recognizes a more complex type, which Pavlov calls the “conditioned” reflex. The study of these promises to meet some of the requirements of a biological analysis of man’s social behaviour. The conditioned reflex is not characteristic of all the members of a species subjected to the same immediate situation. It depends upon the time relations of other stimuli which have previously acted upon the organism. Within certain limits it is possible both to predict the outcome, when the time relations of previous stimuli are defined, and to account for a totally different pattern of behaviour in two individuals who inherit the same neuromuscular organization. It was natural that Galton’s generation should harp on the hereditary basis of social conduct. They were beginning to understand a type of behaviour in which the genetic factor is the significant variable, and to apply their knowledge to the interpretation of “instinct” in animals. It is not surprising that the behaviourists should adopt the opposite point of view. They are beginning to understand a type of behaviour in which the genetic factor is less important, and to apply the new methods to the study of Man himself.

Even if the behaviourist reaction goes too far in neglecting the genetic aspect of social behaviour, it will have performed one considerable service to social biology. Biology and sociology coincide in the attempt to distinguish what characteristics of human society are related to those characteristics which man shares with all other animals, and what characteristics of human society are related to characteristics which man shares with no other animals. The geneticist is only concerned with the former, since the material basis of inheritance in man and other animals is substantially the same. It is the physiologist who is brought into contact with the characteristics which distinguish man from other animals. Man inherits an immensely developed forebrain; and this circumstance frees him from many of the restrictions which heredity imposes upon the brute creation. The forebrain is the structural basis of conditioned behaviour; and what distinguishes man pre-eminently from all other animals is the extent to which his behaviour is conditioned by previous experience. A truly biological analysis of human society must build on the recognition that man is the most teachable of animals. This is a profound truth which the eugenist has neglected. The behaviourist has reopened the door which the eugenist closed. The selectionists succeeded in presenting evolution in a form acceptable to their contemporaries. Man was dragged down from his celestial eminence. His place among the brutes became an accepted commonplace of the naturalistic outlook. Sentence had been passed upon him. Henceforth he must live within the prison of his own genetic limitations. Before the portals of his primeval dungeon Heredity stood with a flaming sword. In his new surroundings Man could still demand a retrial, because selectionism was the product of his own forebrain. That trial is still in process. Science has not yet promulgated its final verdict. Galton conducts the prosecution. Watson cross-examines for the defence. Man is released on bail, pending the result of his appeal.

In English law there is a wholesome provision which forbids the public discussion of evidence until the case is closed. In science there is no penalty for contempt of court. It is a pity that there is not. The discussion of the genetical foundations of racial and occupational classes in human society calls for discipline, for restraint and for detachment. Nothing could make the exercise of these virtues more difficult than to force the issue into the political arena in the present state of knowledge. This is precisely what the eugenist has done. The result is that social biology is encumbered with a vocabulary of terms which have no place in an ethically neutral science; and a growing literature of inquiries repeats all the shortcomings which animal genetics has outgrown. Of these shortcomings anecdotalism is the least. All biologists recognize the disastrous consequences of constructing evolutionary hypotheses on the testimony of the stock breeder and the pigeon fancier. Only an undue haste to establish conclusions which can be made the basis of legislation has arrested the development of social biology in its anecdotage.

Quotations from well-known contributions by eugenic writers will exempt me from the charge of overstating the danger to which I allude, when I speak of the anecdotal method. Few would deny the desirability of shedding further light on the contribution of heredity to feeblemindedness. It is the concern of the social biologist to do so. Goddard’s familial studies on this problem have been extensively quoted by eugenic writers. In his investigation several hundred individuals in the Vineland training-school for mental defectives were classified by the Binet test as morons. Goddard conducted inquiries into the family histories of these individuals, and records them in his book. He concludes that a certain type of feeblemindedness is determined by a single Mendelian factor. This conclusion is logically untenable apart from the evidence, because his criterion of feeblemindedness was a segment arbitrarily cut off from a normal distribution curve; but the method which he employs rather than the conclusions he infers is the issue to which I would direct attention. Mendel initiated a new epoch in genetics by clearly defining the nature of the character which he studied. That practice is the keystone of the science which has developed from his pioneer labours. The Binet test may be legitimately employed as a means of providing an objective definition of feeblemindedness; but since the Binet test is a recent innovation, it is obvious that Goddard could not employ it to identify feeblemindedness in the parents and grandparents of his cases at the time of writing. The method he adopted is stated in the following passage (Feeblemindedness, p. 20):

“The ease with which it is sometimes possible to get satisfactory evidence on the fifth generation is illustrated in the Kallikak family. The field worker accosts an old farmer--‘Do you remember an old man Martin Kallikak (Jr.) who lived on the mountain edge yonder?’ ‘Do I? Well I guess. Nobody’d forget him. Simple, not quite right here (tapping his head), but inoffensive and kind. All the family was that. Old Moll, simple as she was, would do anything for a neighbour. She finally died, burned to death in a chimney corner. She had come in drunk and sat down there. Whether she fell over in a fit or her clothes caught fire nobody knows. She was burned to a crisp when they found her. That was the worst of them, they would drink. Poverty was their best friend in this respect, or they would have been drunk all the time. Old Martin could never stop as long as he had a drop. Many’s the time he’d rolled off of Billy Parson’s porch. Billy always had a barrel of cider handy. He’d just chuckle to see Martin drink and drink until finally he’d lose his balance and over he’d go.’”

At the conclusion of this recital Goddard asks, “Is there any doubt that Martin was feebleminded?”

It may at least be said for Goddard’s work that it contains some presumptive indications that genetic factors play a significant part in determining certain kinds of feeblemindedness. It is doubtful whether any plausible conclusions can be drawn from the dreary history of the Jukes. In his monograph on the Jukes in 1915, Estabrook only ventures to proffer one definite statement concerning hereditary transmission in the Jukes family. It is that “there is an hereditary factor in licentiousness.” I have searched through his memoir for a single indication of the way in which he defines licentiousness and its allelomorphic opposite chastity. Out of a large number of monotonously similar family case histories I shall quote the only one which contains any suggestion of the meaning he attaches to the latter. This (Case G) is as follows:

“A cousin mating of chaste individuals was followed in the first generation by no licentiousness. In the second generation from the cousin mating no licentiousness appears, although the father of one of the children of this generation had cohabited previous to marriage. Their one daughter was chaste, but she has one daughter brought up in a good home free from bad influences, who was very erotic but is at present chaste. The third child of this cousin mating of chaste people, Addie, married a man who had acquired syphilus and had one son an inefficient syphilitic who died of tuberculosis. Addie died of syphilis at 20. The fourth child Alta V 78 who was always chaste, married but had no children. Horace the only other child of Alfred who reached maturity was reputed chaste but was intemperate: he married a chaste woman and had nine children, all of whom are chaste.”

Before we take the risk of wrecking the machinery of social biology by exceeding the speed limit of rational inquiry, it is desirable to ascertain the reasons for such haste. Dr. Estabrook has recorded his own reasons in quantitative terms. “Dugdale estimated a loss to society of $1,250,000 by the Jukes family from 1800 to 1875. The loss to society caused by mental deficiency, crime, prostitution, syphilis and pauperism of these 2,800 people is now estimated at $2,093,685. If the drink bill is added, this total becomes $2,516,685.” The reason for this addition will be more apparent to a prohibitionist than to a brewer. Mr. Chesterton might retort by asking whether there are no idle young clubmen in New York whose annual upkeep is equivalent to the loss entailed by the Jukes during the last century and a half. Deplorable as the history of the Jukes may be, its consequences to civilization may be less disastrous than half an hour’s conversation between a manufacturer of armaments and a newspaper proprietor. In such matters private values influence our opinions more than those issues which can be discussed in the public forum of science. Estabrook’s arithmetic does not convince me that we should exchange the experimental and sceptical temper of scientific inquiry for the facile slogans of the parliamentary candidate.

The eugenic movement was founded to encourage “the study of agencies under social control that may improve or impair the racial qualities of future generations either physically or mentally.” That aim might be taken as a statement of the scope of social biology, when due allowance is made for the full requirements of a scientific inquiry into the nature of “mental inheritance.” There are a few prominent eugenists who have adhered to this praiseworthy and modest programme. Professor Carr Saunders who has been prominently associated with the eugenic movement in England has consistently expressed himself with discrimination and restraint on the complex issues which the genetic structure of human society involves. If I am disinclined to follow him in the alarmist attitude which he adopts towards the differential fertility which has accompanied the recent decline of the European birth-rate, I entirely agree with him in recognizing that the differential fertility of occupational groups is a matter for careful and comprehensive investigation. To make any satisfactory predictions about the outcome of the present decline it is necessary to ascertain what factors have contributed to the reduction of the birth-rate, what genetic differences distinguish different occupational groups, and how such differences are transmitted. The impressive array of evidence which Beveridge, Stevenson and Carr Saunders have presented strongly suggests that the spread of contraceptive practice has been the main factor in the decline of the birth-rate. The German and Swedish data of Grotjahn and Edin point to the conclusion that contraceptive practice is spreading to all sections of society. If this is so the problem of differential fertility is solving itself. Of genetic differences which distinguish occupational groups we have no definite information. Even if we had, it would be necessary to know how such differences are transmitted before prophesying disaster. Haldane’s mathematical analysis of the effect of selection shows that a selective process must be continued for a very long period in order to produce an appreciable effect on the distribution of a character which depends on the co-operation of several recessive genes. An attitude of calvinistic gloom towards the future of human society is not a necessary consequence of the biological study of human society.

In discussing the influence of eugenic propaganda in this essay I have been primarily concerned with the dangers of speculating upon questions whose philosophical importance is less apparent than their practical interest. I trust that I have made it abundantly clear that I am in no sense hostile to eugenics as defined above. Were I to indulge in the luxury of stating a purely personal opinion about the genetics of human society, it would be somewhat as follows. It is probable that extremes of intellectual accomplishment or defect are significantly determined by genetic variation. It is highly unlikely that extreme types of defective are reproducing disproportionately. It is also doubtful whether genius has ever been biologically fertile. Between the two extremes there is probably a neutral zone in which somatic variability plays a larger part than genetic differences in determining social behaviour. At present it is impossible to assess with precision the mean genotypic endowment of different social groups, whether occupational or racial. Even if it were, the precise significance of the mean would be problematical. I think it highly unlikely that such mean differences as may exist provide any basis for establishing new social barriers or reinforcing old ones, still less for curtailing opportunities of education and the exercise of political responsibility. On the other hand it is not unlikely that there does exist a section of genetic types on the borderline of extreme defect not segregated from the rest of the community and more fertile than others of the same social grade. With Mr. Chesterton I am inclined to doubt whether they represent a larger proportion in one social class than in any other. Unlike Mr. Chesterton I see no reason why society should not deal with this issue as a genetic problem, when it is clearly proved to be a genetic problem. Indeed I think it arguable that it would be wiser not to take any risk of encouraging the feebleminded to breed. At present I see no way of stopping them.

There can be no disagreement concerning the desirability of exploring every avenue in human genetics. This cannot be done without enlarging the scope of the official census with the support of a sympathetic government. Hitherto Eugenic propaganda has been dominated by an explicit social bias which, in England, can only serve to render the Eugenic standpoint unpalatable to a section of the community which for good or ill seems to be assuming the rôle of a governing class. The greatest obstacle to the spread of a sane eugenic point of view is the eugenists themselves. By recklessly antagonizing the leaders of thought among the working classes the protagonists of eugenics have done their best to make eugenics a matter of party politics, with results which can only delay the acceptance of a national minimum of parenthood. These last remarks I repeat are a statement of purely personal opinion. Biologists share the human frailty which prompts all of us to entertain beliefs fortified by insufficient evidence; but there is no reason why the biologist should fail to make it clear, when he is speaking as a professional biologist and when he is speaking as a private citizen. From a purely scientific standpoint the problem of human inheritance can only be regarded as a virgin field in which the prospects of an early and abundant harvest are by no means bright. I believe that the eugenists have performed a useful task in emphasizing the need for a biological analysis of human society. The furtherance of that task will not be promoted by propaganda which overstates the achievements of the present, while underestimating the difficulties which lie ahead. Evolutionary inquiry was brought to an end in ancient Greece, when philosophy became the handmaiden of politics. Further progress was checked when philosophy became the bondservant of theology. Eugenics like Greek philosophy derived its first impulse from natural science. It soon entered into alliance with the politician. It is fast finding its most stalwart supporters among the clergy. It can only realize the aims of its founder by bringing the science of genetics into closer relationship with other methods of studying human biology and annulling the marriage of biological inquiry with political propaganda. As a private citizen the biologist is entitled to his own opinions concerning the merits of sterilizing the unfit, just as he is entitled to his own opinions on the Single tax or the advantages of capital punishment. Such opinions usually belong to his private world. In his public capacity, as a biologist, he is primarily concerned with sterilizing the instruments of research before undertaking surgical operations upon the body politic.