CHAPTER XIII

SUMMARY AND GENERAL CONCLUSIONS

THE question of the origin of the adaptations with which the last three chapters have so largely dealt is one of the most difficult problems in the whole range of biology, and yet it is one whose immense interest has tempted philosophers in the past, and will no doubt continue to excite the imagination of biologists for many years to come. No pretence has been made in the preceding pages to account for the cause of a single useful variation. We have examined the evidence, and from this we believe the assumption justified that such variations do sometimes appear. The more fundamental question as to the origin of these variations has not been taken up, except in those cases in which the adaptive response appeared directly in connection with a known external cause. But these kinds of responses do not appear to have been the source of the other adaptations of the organic world. Our discussion has been largely confined to the problem of the widespread occurrence of adaptation in living things, and to the most probable kinds of known variations that could have given rise to these adaptations. But, to repeat, we have made no attempt to account for the causes or the origin of the different kinds of variation.

Nägeli, in speaking of the methods of the earlier theorists. in Germany, remarks with much acumen: "We might have expected that after the period of the Nature-philosophizers, which in Germany crippled the best forces that might have been used for the advance of the science, we should have learnt something from experience, and have carefully guarded

the field of real scientific work from philosophical speculation. But the outcome has shown that, in general, the philosophi cal, philological, and æsthetic expression always gets the upper hand, and a fundamental and exact treatment of scientific questions remains limited to a small circle. The public at large always shows a distinct preference for the so-called idealistic, poetic, and speculative modes of expression." The truth of this statement can scarcely be doubted when in our own time we have seen more than once the same method employed with great public applause. Nowhere is this more apparent than in the writings of many of the followers of Darwin in respect to the adaptations of living things. To imagine that a particular organ is useful to its possessor, and to account for its origin because of the imagined benefit conferred, is the general procedure of the followers of this school. Although protests have from time to time been raised against this unwarrantable way of settling the matter, they have been largely ignored and forgotten. The fallacy of the argument has, for example, been admirably pointed out by Bateson in the following statement: "In examining cases of variation I have not thought it necessary to speculate on the usefulness or harmfulness of the variations described. For reasons given in Section II such speculation, whether applied to normal structures or to variation, is barren and profitless. If any one is curious on these questions of Adaptation, he may easily thus exercise his imagination. In any case of Variation there are a hundred ways in which it may be beneficial or detrimental. For instance, if the 'hairy' variety of the moor-hen became established on an island, as many strange varieties have been, I do not doubt that ingenious persons would invite us to see how the hairiness fitted the bird in some special way for life in that island in particular. Their contention would be hard to deny, for on this class of speculation the only limitations are those of the ingenuity of the

1 "Materials for the Study of Variation."

author. While the only test of utility is the success of the organism, even this does not indicate the utility of one part of the economy, but rather the net fitness of the whole." Keeping in mind the admonitions contained in the two preceding quotations, let us pass in review and attempt to analyze more fully the different points that have been considered in the preceding chapters.

It has been pointed out that the evidence in favor of the theory of evolution appears to establish this theory with great probability, although a closer examination shows that we are almost completely in the dark as to how the process has come about. For example, we have not yet been able to determine whether the great groups of animals and plants owe their resemblance to descent from a single original species or from a large number of species. The former view is more plausible, because on it we appear to be furnished with a better explanation of resemblances as due to divergence of character. Yet even here a closer scrutiny of the homologies of comparative anatomy shows that this explanation may be more apparent than real. If discontinuous variation represents the steps by which evolution has taken place, the artificiality of the explanation is apparent, at least to a certain degree.

Admitting that the theory of evolution is the most probable view that we have to account for the facts, we next meet with two further questions, - the origin of species and the meaning of adaptation. These are two separate and distinct questions, and not one and the same as the Darwinian theory claims. The fact that all organisms are more or less adapted to live in some environment appears from our examination to have no direct connection with the origin of the adaptation, for, in the first place, it seems probable that, in general, organisms do not respond adaptively to the environment and produce new species in this way; and, in the second place, there is no evidence to show that variation

from internal causes is so regulated that only adaptive structures arise (although only adaptive ones may survive).

Our general conclusion is then as follows: A species does not arise from another one because it is better adapted. Selection, in other words, does not account for the origin of new species; and adaptation cannot be taken as the measure of a species.

It may sound like a commonplace to state that only those individuals survive and propagate themselves that can find some place in nature where they can exist and leave descendants; and yet this statement may contain all that it is necessary to assume, in order to account for the fact that organisms are, on the whole, adapted. Let us see how this view differs from the Darwinian statement of the origination of new forms through a process of natural selection.

According to Darwin's view of the origin of species, each new species is gradually formed out of an older one, because of the advantage that the new individual may have over the parent form. Each step forward is acquired, because it better adapts the organism to the old, or to a new set of conditions. In contrast to this, I have urged that the formation of the new species is, as a rule, quite independent of its adaptive value in regard to the parent species. But after it has appeared, its survival will depend upon whether it can find a place in nature where it can exist and leave. descendants. If it should be well adapted to an environment, it will be represented in it by a large number of individuals. If it is poorly adapted, it may only barely succeed in existing, and leave correspondingly fewer descendants. If its adaptiveness falls below a certain point, it can never get a permanent foothold, however often it may appear. Thus the test of survival determines which species can remain in existence, and which cannot, but new species are not manufactured in this way. How far subsequent variations may be supposed to be determined by the survival of certain species and the destruction of others will be discussed presently.

The difference between the two points of view that we are contrasting can be best brought out by considering the two other kinds of selection which Darwin supposes to have been at work; namely, artificial and sexual selection.

Darwin thinks that the results of artificial selection are brought about by the breeder picking out fluctuating variations. It appears that he has probably overestimated the extent to which this process can be carried; for while there can be no doubt that a certain standard, or fixity of type, can be obtained by selecting fluctuating variations, yet it now seems quite certain that the extent to which this can be carried is very limited. It appears that other factors have also played an important rôle; amongst these the occasional appearance of discontinuous variation, also the bringing under cultivation of the numerous "smaller species" of De Vries, or the so-called "single variations" of Darwin. Further, the effects of intercrossing in all combinations of the above forms of variations, followed by the selection of certain of the new forms obtained, has been largely employed, and also the direct influence of food and of other external conditions, which may be necessary to keep the race up to a certain standard, have played a part in some cases. outcome is, therefore, by no means so simple as one might infer from Darwin's treatment of the subject in his "Origin of Species." For these reasons, as well as for others that have been given, it will be evident that the process of artificial selection cannot be expected to give a very clear idea of how natural selection could act.

The

It is, however, the process of sexual selection that brings out in the strongest contrast the difference between Darwin's main idea of natural selection and the law of the survival of species. In sexual selection the competition is supposed to be always between the individuals of the same species and of the same sex. There can be no doubt in one's mind, after reading "The Descent of Man," that Darwin