Sex as a Phenomenon of Adaptation

Of what advantage is it to have the individuals of many species separated into males and females? It is obviously a disadvantage from the point of view of propagation to have half of the individuals incapable of producing young, and the other half also incapable of doing so, as a rule, unless the eggs are fertilized by the other sex. Is there any compensation gained because each new individual arises from two parents instead of from one? Many answers have been attempted to these questions.

At the outset it should be recognized that we are by no means forced to assume, as is so often done, that because there is this separation of the sexes it must have arisen on account of its advantage to the species. Whether the result may be of some benefit regardless of how it arose, may be an entirely different question. It would be extremely difficult to weigh the relative advantages (if there are any) and disadvantages (that are obvious as pointed out above), nor is it probable that in this way we can hope to get a final answer to our problem. We may begin by examining some of the modern hypotheses that have been advanced in this connection.

Darwin has brought together a large number of facts which appear to show the beneficial effects of the union of germ-cells from two different individuals. Conversely, it is very generally believed, both by breeders and by some experimenters, that self-fertilization in the case of hermaphroditic forms leads, in many cases, though apparently not in all, to the production of less vigorous offspring. Darwin’s general position is that it is an advantage to the offspring to have been derived from two parents rather than to have come from the union of the germ-cells of the same individual, and he sees, in the manifold contrivances in hermaphroditic animals and plants to insure cross-fertilization, an adaptation for this purpose.

This question of whether self-fertilization is less advantageous than cross-fertilization is, however, a different question from that of whether non-sexual methods of reproduction are less advantageous than sexual ones. Since some plants, like the banana, have been propagated for a very long time solely by non-sexual methods without any obvious detriment to them, it is at first sight not easy to see what other advantage could be gained by the sexual method. The case of the banana shows that some forms do not require a sexual method of propagation. Other forms, however, are so constituted, as we find them, that they cannot reproduce at the present time except by the sexual method. In other words, the latter are now adapted, as it were, to the sexual method, and there is no longer any choice between the two methods. The question of whether a non-sexual form might do better if it had another method of propagation is not, perhaps, a profitable question to discuss.

What we really need to know is whether or not the sexual method was once acquired, because it was an advantage to a particular organism, or to the species to reproduce in this way. It is assumed by many writers that this was the case, but whether they have sufficient ground for forming such an opinion is our chief concern here. On the other hand, it is conceivable, at least, that if the sexual method once became established, it might continue without respect to any superiority it gave over other methods, and might finally become a necessary condition for the propagation of particular species. Thus the method would become essential to propagation without respect to whether the species lost more than it gained. Whichever way the balance should turn, it might make little difference, so long as the species was still able to propagate itself.

Brooks made the interesting and ingenious suggestion that the separation of the sexes has been brought about as a sort of specialization of the individuals in two directions. The male cells are supposed to accumulate the newly acquired characters, and represent, therefore, the progressive element in evolution. The female cells are the conservative element, holding on to what has been gained in the past. It does not seem probable, in the light of more recent work, that this is the function of the two sexes, and it is unlikely that we could account for the origin of the two sexes through the supposed advantage that such a specialization might bring about. A number of writers, Galton, Van Beneden, Bütschli, Maupas, and others, have looked at the process of sexual reproduction as a sort of renewal of youth, or rejuvenescence of the individuals. There is certainly a good deal in the process to suggest that something of this sort takes place, although we must be on our guard against assuming that the rejuvenescence is anything more than the fulfilment of a necessary stage in the life history. Weismann has ridiculed this suggestion on the ground that it is inconceivable that two organisms, decrepit with old age, could renew their youth by uniting. Two spent rockets, he says, cannot be imagined to form a new one by combining. There is apparent soundness in this argument, if the implication is taken in a narrow physical sense. If, on the other hand, the egg is so constituted that at a certain stage in its development an outside change is required to introduce a new phase, then the conception of rejuvenescence does not appear in quite so absurd a light.

This hypothesis of rejuvenescence is based mainly on certain processes that take place in the life history of some of the unicellular animals. Let us now see what this evidence is. The results of certain experiments carried out by Maupas on some of the ciliate protozoans have been fruitful in arousing discussion as to the ultimate meaning of the sexual process. Maupas’ experiments consisted in isolating single individuals, and in following the history of the descendants that were produced non-sexually by division. He found that the descendants of an individual kept on dividing, but showed no tendency to unite with each other. After a large number of generations had been passed through (in Stylonychia pustulata, between 128 and 175; in Leucophys patula, 300 to 450; and in Onychodromus grandis, 140 to 230 generations), the division began to slow down, and finally came to a standstill. Maupas found that if he took one of these run-down individuals, and placed it with another in the same condition from another culture, that had had a different parentage, the two would unite and the so-called process of conjugation take place. This process consists for the species used, in the temporary union and partial fusion of the protoplasm of the two individuals, of an interchange of micronuclei, and of a fusion, in each individual, of the micronucleus received from the other individual with one of its own. The individuals then separate, and a new nucleus (or nuclei) is formed out of the fused pair.

The individuals in question, in which this interchange of micronuclei has taken place, undergo a change, and behave differently from what they did before. They feed, become larger and less vacuolated, and are more active. They soon begin once more to divide. Maupas found that an individual that has conjugated will run through a new cycle of divisions, which will, however, after a time also slow down, unless conjugation with another individual having a different history takes place. If conjugation is prevented, the individual will die after a time. These results seemed to show that the division phase of the life history cannot go on indefinitely, and that through conjugation the individual is again brought back to the starting-point.

Quite recently Calkins has carried out a somewhat similar series of experiments, which have an important bearing on the interpretation of Maupas’ results. The experiment of isolating an individual and tracing the career of its descendants was repeated with the following results: two series were started, the original forms coming from different localities. Of their eight descendants four of each were isolated. The remaining four of each set were kept together as stock material. The rate of division was taken as the measure of vitality. The animals divided more or less regularly from February to July. After each division (or sometimes after two divisions) the individuals were separated. About the 30th of July the paramœcia began to die “at an alarming rate, indicating that a period of depression had apparently set in, or degeneration in Maupas’ sense.” Up to this time the animals had been living in hay infusion, renewed every few days, from which they obtained the bacteria on which they feed. Calkins tried the effect of putting the weakened paramœcia into a new environment. Infusion of vegetables gave no good results, but meat infusions proved successful. “The first experiment with the latter was with teased liver, which was added to the usual hay infusion. The result was very gratifying, for the organisms began immediately to grow and to divide, the rate of division rising from five to nine divisions in successive ten-day periods.” This beneficial effect was not lasting, however, and after ten days the paramœcia began to die off faster than before, and the renewed application of the liver extract failed to revive them. A number of other extracts were then tried without effect. Finally they were transferred to the clear extract of lean beef in tap water. The effect of this medium was interesting, for, although it restored the weakened vitality, there was no rapid increase in the rate of division, as when first treated with the teased liver. The infusoria were, however, now large and vigorous, and did not die unless transferred from the beef medium to the usual hay infusion. “When this was attempted, they would become abnormally active and would finally die. The division rate gradually increased during the month of August until, in the last ten days, they averaged six generations. Finally, in September, the attempts to get them back on the old diet of hay infusion were successful, and then the division rate went up at once to twelve times in ten days, and a month later they were dividing at the rate of fifty times a month.”

“These cultures went on well until December, when the paramœcia began to die again. They were saved once more with the beef extract, and when returned later to the hay infusion continued through another cycle of almost three months. Some of these were treated, once a week for twenty-four hours, with the beef extract, and while the two sets ran a parallel course at first, those kept continuously in the hay infusion died after a time, but those that had been put once a week into the beef extract (which had been stopped, however, in March) continued their high rate of division throughout the period of decline of their sister cells, and did not show signs of diminished vitality until the first period in June.” At this time their rate of division increased rapidly. They were put back into the beef extract, but it failed now to have a beneficial effect, and the animals continued to die at a rapid rate. To judge from the appearance of the organisms the new decline was due to a different cause; for, while in the former periods the food vacuoles contained undigested food, at this period the interior was free from food masses. The protoplasm became granular and different from that of a healthy individual. None of the former remedies were now of any avail. “When the last of the B-series stock had died in the five hundred and seventieth generation (June 16th), it looked as though the cultures were about to come to an end.” Extract of the brain and of the pancreas were then tried. To this a favorable response took place at once. The organisms became normal in appearance and began to divide. After forty-eight hours’ treatment they were returned to the usual hay infusion. Here they continued to multiply and reached on June 28th the six hundred and sixty-fifth generation.

There can be no doubt that the periods of depression that appear in these infusoria kept in cultures can be successfully passed if the animals are introduced into a new environment. Without a change of this sort they will die. Calkins thinks that the effect is produced, not by the new kind of food that is supplied, but by the presence of certain chemical compounds. The beef extract “does not have a direct stimulating effect upon the digestive process and upon division, for, while the organisms are immersed in it, there is a very slow division rate; when transferred again to the hay infusion, however, they divide more rapidly than before.”

This brings us back to the idea of the “renewal of youth” through conjugation. Maupas claimed that union of individuals having the same immediate descent is profitless. Calkins suggests that this is due to the similarity in the chemical composition of the protoplasm of the two individuals. When in nature two individuals that have lived under somewhat different conditions conjugate, the result should be beneficial, since there takes place the commingling of different protoplasms.

Calkins’s work has shown that by means of certain substances much the same effect can be produced as that which is supposed to follow from the conjugation of two unrelated individuals. The presumption, therefore, is in favor of the view that the two results may be brought about in the same way, although we should be careful against a too ready acceptation of this plausible argument; for we have ample evidence to show that many closely similar (if not identical) responses of organisms may be brought about by very different agencies. The experiments seem to indicate that paramœcium might go on indefinitely reproducing by division, provided its environment is changed from time to time. If this is true, it is conceivable that the same thing is accomplished through conjugation. In the light of this possible interpretation much of the mystery connected with the term rejuvenescence is removed, for we see that there is nothing in the process itself except that it brings the organism into a new relation with other substances. Difficult as it assuredly is to understand how this benefits the animal, the experimental fact shows, nevertheless, that such a change is for its good. That there is really nothing in the process of conjugation itself apart from the difference in the constitution of the conjugating individuals is shown by the result that the union of individuals having the same history and kept under the same conditions is of no benefit.

Can we apply this same conception to the process of fertilization in the higher animals and plants? Is the substance of which their bodies are made of such a sort that it cannot go on living indefinitely under the same conditions, but must at times be supplied with a new environment? If this could be established, we could see the advantage of sexual reproduction over the non-sexual method. It would be extremely rash at present to make a generalization of this kind, for there are many forms known in which the only method of propagation that exists is the non-sexual one. In other words, there are no grounds for the assumption that this is a necessary condition for all kinds of protoplasm, but only for certain kinds.

In the insects, crustaceans, rotifers, and in some plants there are a few species whose egg develops without fertilization. This makes it appear probable that the particular kind of protoplasm of these animals does not absolutely require union from time to time with the protoplasm of another individual having a somewhat different constitution.

There is also an interesting parallel between the effects of solutions on the protozoans in Calkins’s experiments and certain results that have been obtained in artificial parthenogenesis. It has been stated, that by brushing the unfertilized eggs of the silkworm moth a larger percentage will develop parthenogenetically; and more recently it has been shown by Matthews that by agitation of the water in which the unfertilized eggs of the starfish have been placed many of them will begin their development. It was first shown by Richard Hertwig that by putting the unfertilized eggs of the sea-urchin in strychnine solutions, they will begin to segment, and I obtained the same results much better by placing the eggs in solutions of magnesium chloride. Loeb then succeeded in carrying the development to a later stage by using a different strength of the same solution, as well as by other solutions. Under the most favorable circumstances some of the eggs may produce larvæ that seem normal in all respects, but whether they can develop into adult sea-urchins has not yet been shown.

These results indicate that one at least of the factors of fertilization is the stimulus given to the egg. On the other hand, the lack of vigor shown by many eggs that have been artificially fertilized indicates that some other result is also accomplished by the normal method of fertilization that is here absent. This may mean no more than that as yet we have not found all the conditions necessary to supply the place of the spermatozoon.

In our study of the phenomena of adaptation we have found that sometimes the adaptation is for the benefit of the individual and at other times for the benefit of the species. May it not be true also that the process of sexual reproduction has more to do with a benefit conferred on the race rather than on the individual? In fact, Weismann has elaborated a view based on the conception that the process of sexual reproduction is beneficial to the race rather than to the individual. His idea, however, is not so much that the result is of direct benefit to a particular species, as it is advantageous to the formation of new species from the original one. In a sense this amounts, perhaps, to nearly the same thing, but in another sense the idea involves a somewhat different point of view.

According to his view “the deeper significance of conjugation” and of sexual reproduction is concerned “with the mingling of the hereditary tendencies of two individuals.” In this way, through the different combinations that are formed, variations which he supposes are indispensable for the action of natural selection originate. The purpose of the sexual process is solely, according to Weismann, to supply the variations for natural selection. If it be asked how this process has been acquired for the purpose of supplying natural selection with the material on which it can work, we find the following reply given by Weismann. “But if amphimixis [by which he means the union of sex-cells from different individuals] is not absolutely necessary, the rarity of purely parthenogenetic reproduction shows that it must have a widespread and deep significance. Its benefits are not to be sought in the single individual; for organisms can arise by agamic methods, without thereby suffering any loss of vital energy; amphimixis must rather be advantageous for the maintenance and modification of species. As soon as we admit that amphimixis confers some such benefits, it is clear that the latter must be augmented, as the method appears more frequently in the course of generations; hence we are led to inquire how nature can best have undertaken to give this amphimixis the widest possible range in the organic world.” Nature, Weismann says, could find no more effectual means of bringing about the union of the sexual cells than by rendering them incapable of developing alone. “The male germ-cells, being specially adapted for seeking and entering the ovum, are, as a rule, so ill provided with nutriment that their unaided development into an individual would be impossible; but with the ovum it is otherwise, and accordingly the ‘reduction division’ removes half the germ-plasm and the power of developing is withdrawn.” It can scarcely be claimed, in the light of more recent discoveries, that the reduction division takes place in order to prevent the development of the ovum, for how then could we explain the corresponding division of the male germ-cells?

Whatever means has been employed to bring about the process of sexual reproduction, the guiding principle is supposed by Weismann to be natural selection as stated in the following paragraph: “If we regard amphimixis as an adaptation of the highest importance, the phenomenon can be explained in a simple way. I only assume that amphimixis is of advantage in the phyletic development of life, and furthermore that it is beneficial in maintaining the level of adaptation, which has been once attained, in every single organism; for this is as dependent upon the continuous activity of natural selection as the coming of new species. According to the frequency with which amphimixis recurs in the life of a species, is the efficiency with which the species is maintained; since so much the more easily will it adapt itself to new conditions of life, and thus become modified.”

Thus we reach the somewhat startling conclusion that through natural selection the germ-cells and their protozoan prototypes have been rendered incapable, through natural selection, of reproducing by non-sexual methods, in order that variations may be supplied for the farther action of this same process of natural selection. The speculation has the appearance of arguing in a circle, although if it were worth the attempt an ingenious mind might perhaps succeed in showing that such a thing is not logically inconceivable.

It seems strange that a claim of this sort should have been made, when it is so apparent that the most immediate effect of intercrossing is to swamp all variations that depart from the average. Even if it were true that new combinations of characters would arise through the union of the germ-cells of two different animals, it is certainly true that in the case of fluctuating variations this new combination would be lost by later crossing with average individuals. Moreover, it is well known that variations occur amongst forms that are produced asexually. On the whole, it does not seem to be a satisfactory solution of the problem to assume that sexual reproduction has been acquired in order to supply natural selection with material on which it may work.

Our examination of the suggestions that have been made and of the speculation indulged in, as to what benefit the process of sexual reproduction confers on the animals and plants that make use of this method of propagation, has failed to show convincingly that any advantage to the individual or to the species is the outcome. This may mean, either that there is no advantage, or that we have as yet failed to understand the meaning of the phenomenon. The only light that has been thrown on the question is that a certain amount of renewed vigor is a consequence of this process, but we cannot explain how this takes place. There is also the suggestion that the union of different cells produces the same beneficial effect as a change in the conditions of life produces on the organism. The bad effects of close interbreeding that seem sometimes to follow is explicable on this view. This, it seems to me, is the most plausible solution of the question that has been advanced; but, even if this should prove to be the correct view, we need not assume that the process has been acquired on account of this advantage, for there is nothing to show that it has been acquired in this way.

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 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.

[36]. “Materials for the Study of Variation.”

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.