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 philosophical, 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:^[36] “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 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. The 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.

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 held firmly to the belief that the individual differences, or fluctuating variations, furnish the material for selection. In this way it could never happen that two competing species could exterminate each other, because in the one the males were better adorned, or killed each other off on a larger scale, owing to the presence of special weapons of warfare. It is clear that on the law of the survival of species, secondary sexual characters cannot be supposed to have evolved because of their value. Their origin is totally inexplicable on this view. In fact, the presence of the ornaments must be in some cases injurious to the existence of the species. The interpretation of this means, I think, that individual competition cannot be as severe as Darwin believed, and cannot lead to the results that he imagined it does. For this reason it seemed important to make as careful an examination of the claims of the theory of sexual selection as possible, and I hope that the outcome of the examination has shown quite definitely that the theory is incompetent to account for the facts that it claims to explain. It is certain in this case that we are dealing with a phenomenon that must be studied quite apart from any selective value that the secondary sexual organs may have. If this is granted, it will be seen that there is here a wide field for experimental investigation that is practically untouched.

It is evident that the first step that will clear the way to a fuller understanding of the problem of evolution must be a more thorough examination of the question of variation. Darwin himself fully appreciated this fact, yet until within the last fifteen years the study of variation has been largely neglected. With a fuller knowledge of the nature of fluctuating variation as the outcome of the studies of Galton, Pearson, De Vries, and others, and with a fuller knowledge of the possibilities of discontinuous variation as emphasized by Bateson and by De Vries, and, further, with a better knowledge of some of the laws of inheritance in these cases, we have begun to get a different conception of how evolution has come about. It may be well, therefore, to go once more over the main points in regard to the different kinds of variation.

While it has been found that no two individuals of a species are exactly alike, yet, taken as a group, the variations appear as though they followed the law of chance. The descendants of the group show the same differences. Thus the group as a whole appears constant, while the individuals fluctuate continually in all directions. This is what we understand by fluctuating variation. If the external conditions are changed, a new “mode” may appear, but the change is generally only a temporary one, and lasts only as long as the new conditions remain. Thus, while the direct influence of the environment may show for a time, the result is transient. Even if it were permanent, there is no evidence that the adaptation of organisms could be accounted for in this way unless the response were useful. It appears that this sometimes really occurs, especially in responses to temperature, to moisture, to the amount of salts in solution, to poisonous substances, etc. In this way, one kind of adaptation is brought about, but there is no evidence that the great number of structural adaptations have thus arisen.

The Lamarckian principle of the inheritance of acquired characters has also been supposed by many writers to be an important source of adaptive variation. An examination of this theory is not found to inspire confidence. We have, therefore, eliminated this hypothesis on the ground that it lacks evidence in its favor, and also because it appears improbable that in this way many of the adaptations in organisms could have been acquired.

Finally, there is the group of discontinuous variations. Of these there may be several kinds, and there is some evidence showing that there are such. For the present we may include all the different sorts under the term mutation, meaning that the new character or group of characters suddenly appears, and is inherited in its new form. From the results of De Vries it appears that mutations are sometimes scattering, at least in the case of the evening primrose. From such scattering mutations, the smaller species or varieties (in so far as these do not depend on local conditions) arise. There is here an important point of agreement with Darwin’s idea in regard to evolution, inasmuch as he supposed that varieties are incipient species. Our point of view is different, however, in that we do not suppose these varieties (mutations) to have been gradually formed out of fluctuating variations by a process of selection, but to have arisen at once by a single mutation. It also appears that in some cases a single new mutation may develop in a species. We may suppose that the new form might in such a case supplant the parent species by absorbing it, or both may go on living side by side, as will be more likely the case if they are adapted to somewhat different conditions.

A number of writers have supposed that evolution marches steadily forward toward its final goal, which may even lead in some cases to the final but inevitable destruction of the species. By certain writers this view has been called orthogenesis, although at other times the idea is not so much that there is advance in a straight line, as advance in all directions. This appears to be NÄgeli’s view. It gives a splendid picture of the organic world, as irresistibly marching toward its goal,—a relentless process in some cases, leading to final annihilation, a beneficent process in other cases, leading to the fullest perfection of form of which the type is capable. Compared with the vacillating progress which is supposed to be the outcome of individual selection, this view of progression has a grandeur that appeals directly to the imagination. We must be guided, however, by evidence, rather than by sentiment. The case will, moreover, bear closer scrutiny. If evolution has indeed taken place by the survival of a series of mutations, whose origin has no connection with their value, does not this in the end amount to nearly the same thing as maintaining that evolution of organisms has been a steady progress forward,—a progress undirected by external forces, but the outcome of internal development? Admitting that innumerable creations have been lopped off, because they could find no foothold, yet, as NÄgeli points out, the result is that, instead of a dense tangle of forms, there has been left relatively few that have been found capable of existing,—those that have found some place in which they can live and leave progeny. From this point of view it may appear, at first thought, that the idea of evolution through mutations involves a fundamentally different view from that of the Darwinian school of selection; but in so far as selection also depends on the spontaneous appearance of fluctuating variations, the same point of view is to some extent involved,—only the steps are supposed to be smaller. This point is usually ignored and passed over in silence by the Darwinians, but, as Wigand has pointed out, it makes very little difference whether the stages in the process of evolution are imagined to be very small or somewhat larger, so long as they are spontaneous. Selection does not do more than determine the survival of what is offered to it, and does not create anything new.

It is true that if the fluctuating variations that are selected be connected by very slight differences with an almost continuous series of other forms, and if little by little such a series be advanced in a given direction by selection, we get the idea of a continuity, whose advance is determined by selection. It is this conception that appears to give the theory of natural selection a creative power, which in reality it does not possess, and certainly not in the modified form in which the theory was finally left by Darwin. For Darwin found himself forced to admit that, unless a very considerable number of individuals varied at the same time and in the same direction, the formation of new species could not take place, and this idea of many individuals varying at the same time, and in the same direction, at once involves the conception that evolution moves forward by some force residing in the organism, driving it forwards or backwards. Instability comes, perhaps, nearer to expressing this idea than any other term, and yet to evolve from a protozoan to a man implies the idea of something more than simple unstableness.

The idea that Weismann has touched upon in this connection, namely, that the survival of a given form determines the future course of evolution for that form, is very plausible, and also fits in well with the results of our experience in the field of the inheritance of variations. We see new variations or mutations departing in some or in many characters from the original type, apparently by new combinations or perturbations of those already present. We never expect to see a bird emerge from the egg of an alligator. Thus it appears that by the survival of certain forms the future course of evolution is determined in so far as the new types of mutation are thereby limited. Weismann means, however, that in this way new plus or minus steps will be indefinitely determined amongst the new fluctuating variations, but this statement is contradicted by our experience of the results of artificial selection. The upper limit does not keep on pushing out indefinitely in the direction determined by the first selection, but is soon brought to a standstill. So that, as far as Weismann’s hypothesis is concerned, the idea appears to have no special value. On the other hand, this idea may be fruitful if applied to mutations, but here unfortunately we have not sufficient experience to guide us, and we do not know definitely whether a new character that appears as a mutation will be more likely, in subsequent mutations, to go on increasing in some of the descendants. Thus, while the mutation theory must assume that some new characters will go on heaping up, we lack the experimental evidence to show that this really occurs. It would be also equally important to determine, whether, if after several mutations have successively appeared in the same direction, there would be an established tendency to go on in the same direction in some of the future mutations. But here again we must wait until we have more data before we attempt to build up a theory on such a basis.

The attacks on the Darwinian school by the followers of the modern school of experimentalists are with few exceptions based on the assumption that the natural selectionists pretend that their principle is a sort of creative force,—a factor in evolution in the sense of being an active agent. This assumption of the selectionists has led many of them to ignore a fundamental weakness of their theory, namely, the origin of the variations themselves, although Darwin did not overlook or ignore this side of the problem, or fail to realize its importance, as have some of his more ardent, but less critical, followers. They have contented themselves, as a rule, with pointing out that certain structures are useful, and this has seemed to them sufficient proof that the structures must have been acquired because of their value. In contrast to this complacency of the selectionists, we find here and there naturalists who have, from time to time, insisted that the scientific problem of evolution is not to be found in the principle of selection, but in the origin of the variations themselves. It will be clear, from what has been said, that this is our position also, and for us adaptation itself does not appear to be any more a problem that can be examined by scientific methods, than the lack of adaptation. The causes of the change of whatever kind should be our immediate quest. The destruction of the unfit, because they can find no place where they can exist, does not explain the origin of the fit.

Over and beyond the primary question of the origin of the adaptive, or non-adaptive, structure is the fact that we find that the great majority of animals and plants show distinct evidence of being suited or adapted to live in a special environment, i.e. their structure and their responses are such that they can live and leave descendants behind them. I can see but two ways in which to account for this condition, either (1) teleologically, by assuming that only adaptive variations arise, or (2) by the survival of only those mutations that are sufficiently adapted to get a foothold. Against the former view is to be urged that the evidence shows quite clearly that variations (mutations) arise that are not adaptive. On the latter view the dual nature of the problem that we have to deal with becomes evident, for we assume that, while the origin of the adaptive structures must be due to purely physical principles in the widest sense, yet whether an organism that arises in this way shall persist depends on whether it can find a suitable environment. This latter is in one sense selection, although the word has come to have a different significance, and, therefore, I prefer to use the term survival of species.

The origin of a new form and its survival after it has appeared have been often confused by the Darwinian school and have given the critics of this school a fair chance for ridiculing the selection theory. The Darwinian school has supposed that it could explain the origin of adaptations on the basis of their usefulness. In this it seems to me they are wrong. Their opponents, on the other hand, have, I believe, gone too far when they state that the present condition of animals and plants can be explained without applying the test of survival, or in a broad sense the principle of selection amongst species.

It will be clear, therefore, in spite of the criticism that I have not hesitated to apply to many of the phases of the selection theory, especially in relation to the selection of the individuals of a species, that I am not unappreciative of the great value of that part of Darwin’s idea which claims that the condition of the organic world, as we find it, cannot be accounted for entirely without applying the principle of selection in one form or another. This idea will remain, I think, a most important contribution to the theory of evolution. We may sum up our position categorically in the following statements:

Animals and plants are not changed in this or in that part in order to become better adjusted to a given environment, as the Darwinian theory postulates. Species exist that are in some respects very poorly adapted to the environment in which they must live. If competition were as severe as the selection theory assumes, this imperfection would not exist.

In other cases a structure may be more perfect than the requirements of selection demand. We must admit, therefore, that we cannot measure the organic world by the measure of utility alone. If it be granted that selection is not a moulding force in the organic world, we can more easily understand how both less perfection and greater perfection may be present than the demands of survival require.

If we suppose that new mutations and “definitely” inherited variations suddenly appear, some of which will find an environment to which they are more or less well fitted, we can see how evolution may have gone on without assuming new species have been formed through a process of competition. Nature’s supreme test is survival. She makes new forms to bring them to this test through mutation, and does not remodel old forms through a process of individual selection.