Evolution and Adaptation

From other long-styled plants, fertilized with their own-form pollen, 72 plants were raised, which were made up of 68 long-styled and 4 short-styled. In all, 162 illegitimate unions of this sort produced 156 long-styled and 6 short-styled plants. It is evident from these results that the long-form pistils, fertilized with pollen from flowers of the same pistil-form (from other individuals as a rule), tend to produce the same form as their parents, although occasionally the other form. The fertility of these plants from an illegitimate union is found to be very low. Darwin observed that sometimes the male and female organs of these plants were in a very deteriorated condition. It is interesting to notice, in this connection, that in another species, Primula sinensis, illegitimate plants from long-styled parents were vigorous, but the flowers were small and more like the wild form. They were, however, perfectly fertile.

Illegitimate plants from short-styled parents were dwarfed in stature, and often had a weakly constitution. They were not very fertile inter se, and remarkably infertile when legitimately fertilized. This kind of result, where a difference in the power of mutual intercrossing exists between two forms, recalls in many ways the difference in the results of crossing of different species of animals and plants, especially those cases in which a cross can be made in one way more successfully than in the other.

The heterostyled trimorphic plants, of which Lythrum salicaria, Figure 5 C, D, E, may be taken as an example, are even more remarkable. There are three different kinds of flowers: in one the pistil is long and there is a medium and a short set of stamens; in another the pistil is of intermediate length and there is a long set and a short set of stamens; in the third kind the pistil is short, and there is a medium and a long set of stamens. There are possible only six sorts of legitimate unions between these three sets of flowers. No less than twelve kinds of illegitimate unions may occur. In regard to the difference in the sizes of the pollen grains, those from the long-styled form are the largest, from the mid-styled form next, and from the short-styled form the smallest. The extreme difference is as 100 to 60. “Nothing shows more clearly the extraordinary complexity of the reproductive system of this plant than the necessity of making eighteen distinct unions in order to ascertain the relative fertilizing power of the three forms.” Darwin tried the effect of each of these combinations, making 223 unions in all. The results are surprising. Comparing the outcome of the six legitimate unions with the twelve illegitimate ones, the following results were obtained:—

This table shows that the fertility of the legitimate to that of the illegitimate is as 100 to 33, as judged by the flowers that produced capsules; and as 100 to 46 as judged by the average number of seeds per capsule. It is evident, therefore, that “it is only the pollen from the longest stamens that can fully fertilize the longest pistil; only that from the mid-length stamens, the mid-length pistil; and only that from the shortest stamens, the shortest pistil.”

Darwin tries to connect this fact with the visits of insects to the flowers. He says: “And now we can comprehend the meaning of the almost exact correspondence in length between the pistil in each form and of a set of six stamens in two of the other forms; for the stigma of each form is thus rubbed against that part of the insect’s body which becomes charged with the proper pollen.” A further conclusion that Darwin draws is “that the greater the inequality in length between the pistil and the set of stamens, the pollen of which is employed for its fertilization, by so much is the sterility the more increased.” Darwin also makes the following significant comment on the problem here involved: “The correspondence in length between the pistil in each form, and a set of stamens in the other two forms, is probably the direct result of adaptation, as it is of the highest service to the species by leading to full and legitimate fertilization.” He points out, on the other hand, that the increased sterility of the illegitimate unions, in proportion to the inequality in length between the pistil and the stamens employed, can be of no service at all. Neither can this relation have any connection with the facility for self-fertilization. “We are led, therefore, to conclude that the rule of increased sterility in accordance with increased inequality in length between the pistils and stamens is a purposeless result, incidental on those changes through which the species has passed in acquiring certain characters fitted to insure the legitimate fertilization of the three flowers.”

In regard to the plants that were raised from the seeds from legitimate and illegitimate unions, Darwin found in Lythrum that of twelve illegitimate unions two were completely barren, and nearly all showed lessened fertility; only one approached complete fertility. Darwin lays much emphasis on the close resemblance in the sterility of the illegitimate unions, and the sterility of different species when crossed. In both cases every degree of sterility is met with, “from very slightly lessened fertility to absolute barrenness.” The importance of this comparison cannot, I think, be overestimated, for, if admitted, it indicates clearly that the infertility between species cannot be used as a criterion of their distinctness, because here, in individuals belonging to the same species, we find sterility between pistils and stamens of different lengths. If, as I shall urge below, we must consider these different forms of Primula the results of a mutation, and not the outcome of selection as Darwin supposed, then this relation in regard to infertility becomes a point of great interest.

This brings us to the central point of our examination of these cases of dimorphism and trimorphism. How have these forms arisen? Darwin tries to account for them as follows: Since heterostyled plants occur in fourteen different families of plants, it is probable that this condition has been acquired independently in each family, and “that it can be acquired without any great difficulty.” The first step in the process he imagines to have been due to great variability in the length of the pistil and stamens, or of the pistil alone. Flowers in which there is a great deal of variation of this sort are known. “As most plants are occasionally cross-fertilized by the aid of insects, we may assume that this was the case with our supposed varying plant; but that it would have been beneficial to it to have been more regularly cross-fertilized.” “This would have been better accomplished if the stigma and the stamens stood at the same level; but as the stamens and pistil are supposed to have varied much in length, and to be still varying, it might well happen that they could be reduced much more easily through natural selection into two sets of different lengths in different individuals than all to the same length and level in all individuals.” By means of these assumptions, improbable as they may appear, Darwin tries to explain these cases of dimorphism. But when we attempt to apply the same argument to the trimorphic forms, it is manifestly absurd to pretend that three such sharply defined types could ever have been formed as the result of natural selection. But we have not even yet touched the chief difficulty, as Darwin himself points out. “The essential character of a heterostyled plant is that an individual of one form cannot fully fertilize, or be fertilized by, an individual of the same form, but only by one belonging to another form.” This result Darwin admits cannot be explained by the selection theory, for, as he says, “How can it be any advantage to a plant to be sterile with half of its brethren, that is, with the individuals belonging to the same form?” He concludes that this sterility between the individuals of the same form is an incidental and purposeless result. “Inner constitutional differences” between the individuals is the only suggestion that is offered to account for the phenomenon. In other words, it is clearly apparent that the attempt to apply the theory of selection has here broken down, and it is a fortunate circumstance that the Lamarckian theory cannot here be brought to the rescue, as it so often is in Darwin’s writings, when the theory of natural selection fails to give a sufficient explanation.

On the other hand, this is one of the cases that seem to fit in excellently with the mutation theory, for if these two forms of the primrose should appear, as mutations, and if, as is the case, they do not blend when crossed, but are equally inherited, they would both continue to exist as we find them to-day. Whether the similar forms were infertile with each other would be determined at the outset by the nature of the individual variation, and if, despite this obvious disadvantage, the forms could still continue to propagate themselves, the new dimorphic form would remain in existence. Darwin cannot explain the origin of dimorphic forms and trimorphic forms unless he can show that there is some advantage in having two forms, and as we have seen, he fails completely to show that there is an advantage. On the other hand, the result might have been reached on the mutation theory, even if the dimorphic and trimorphic forms were placed at a greater disadvantage than were the parent forms. In such a case fewer individuals might appear, or find a foothold; but as long as the race could be kept up the new forms would remain in existence. Thus, while no attempt is made to explain what has always been, and may possibly long remain, inexplicable to us, namely, the origin of the new form itself, yet granting that such new forms may sometimes appear spontaneously, they may be able to establish themselves, regardless of whether they are a little more or a little less well adapted to the environment than were their parent forms. If it should appear that the question is begged by the assumption that mutations such as these may appear (at one step or by a series of steps is immaterial), it should not be forgotten that the whole Darwinian theory itself also rests on the spontaneous appearance of fluctuating variations, whose origin it does not pretend to explain. In this respect both theories are on the same footing, but where the Darwinian theory meets with difficulties at every turn by assuming that new forms are built up through the action of selection, the mutation theory escapes most of these difficulties, because it applies no such rigid test as that of selection to account for the presence of new forms.

Length of Life as an Adaptation

It has been pointed out in the first chapter that the length of life of the individual has been supposed by some of the most enthusiastic followers of Darwin to be determined by the relation of the individual to the species as a whole. In other words, the doctrine of utility has been applied here also, on the ground that it would be detrimental to the species to have part of the individuals live on to a time when they can no longer propagate the race or protect the young. It is assumed that those varieties or groups of individuals (unfortunately not sharply defined) would have the best chance to survive in which the parent forms died as soon as they had lost the power to produce new individuals. Sometimes interwoven with this idea there is another, namely, that death itself has been acquired because it was more profitable to supplant the old and the injured individuals by new ones, than to have the old forms survive, and thus deprive the reproducing individuals of some of the common food supply.

This insidious form that the selection theory has taken in the hands of its would-be advocates only serves to show to what extremes its disciples are willing to push it. On the whole it would be folly to pursue such a will-o’-the-wisp, when the theory can be examined in much more tangible examples. If in these cases it can be shown to be improbable, the remaining superstructure of quasi-mystical hypothesis will fall without more ado.

That the problem of the length of life may be a real one for physiological investigation will be granted, no doubt, without discussion, and that in some cases the length of life and the coming to maturity of the germ-cell may be, in some way, physiologically connected seems not improbable; but that this relation has been regulated by the competition of species with each other can scarcely be seriously maintained. I will not pretend to say whether the mutation theory can or cannot be made to appear to give the semblance of an explanation of the length of life in each species, but it seems to me fairly certain that this is one of the questions which we are not yet in a position to attempt to consider on any theory of evolution.

Organs of Extreme Perfection

It has often been pointed out that certain organs may be more perfectly developed than the requirements of the surroundings strictly demand. At least we have no good reasons to suppose in some cases that constant selection is keeping certain organs at the highest possible point of development, yet, on the Darwinian theory, as soon as selection ceases to be operative the level of perfection must sink to that which the exigencies of the situation demand. The problem may be expressed in a different way. Does the animal or plant ever possess organs that are more perfectly adapted than the absolute requirements demand? If such organs are the result of fluctuating variations, they will be unable to maintain themselves in subsequent generations without a constant process of selection going on. If, on the other hand, the organs have arisen as mutations, they may become permanently established without respect to the degree of perfection of their adaptation. We can see, therefore, that cases of extreme perfection meet with no difficulty on the mutation theory, while they have proven one of the stumbling-blocks to the selection theory.

There are, in fact, many structures in the animal and plant kingdoms that appear to be more perfect than the requirements seem to demand. The exact symmetry of many forms appears in some cases to be unnecessarily perfect. The perfection of the hand of man, the development of his vocal organs, and certain qualities of his brain, as his musical and mathematical powers, seem to go beyond the required limits. It is not, of course, that these things may not be of some use, but that their development appears to have gone beyond what selection requires of these parts.

Closely related to this group of phenomena are those cases in which certain organs are well developed, but which can scarcely be of use to the animal in proportion to their elaboration. The electric organs of several fishes and skates are excellent examples of this sort of structures. The phosphorescent organs do not appear, in some forms at least, to be useful in proportion to their development. The selection theory fails completely to explain the building up of organs of this kind, but on the mutation theory there is no difficulty at all in accounting for the presence of even highly developed organs that are of little or of no use to the individual. If the organs appeared in the first place as mutations, and their presence was not injurious to the extent of interfering seriously with the existence and propagation of the new form, this new form may remain in existence, and if the mutations continued in the same direction, the organs might become more perfect, and highly developed. The whole class of secondary sexual organs may belong to this category, but a discussion of these organs will be deferred to the following section.

Secondary Sexual Organs as Adaptations

In the sixth chapter we have examined at some length Darwin’s interpretation of the secondary sexual characters. His explanation has been found insufficient in many cases to account for the conditions. That these organs do play in some cases a role in the relation of the sexes to each other may be freely admitted. In other words, in some animals the organs in one sex appear in the light of adaptations to certain instincts in the other sex. It would, perhaps, appear to simplify the problem to deny outright that any such relation exists; but I think, in the light of the evidence that we have, this procedure would be like that of the proverbial ostrich, which is supposed to stick its head in the sand in order to escape an anticipated danger. If we assumed this agnostic position, we might attempt to account for the appearance of secondary sexual organs as mutations that had appeared in one sex, and had no immediate connection with the other sex; and, so long as these organs were not directly and seriously injurious, we might assume that the animals in which such structures had appeared might be able to exist. But, on the other hand, I think that an examination of the evidence will show that this way out of the difficulty is not very satisfactory, for the organs in question appear, in some cases at least, to be closely connected with certain definite responses in the other sex. Moreover, as Darwin has so insistently pointed out, the action of the males is of such a sort that it is evidently associated with the presence of the secondary sexual organs which they often display before the other sex. Furthermore, the greater and often exclusive development of these organs during the sexual period distinctly points to them as in some way connected with the relation of the sexes to each other. And finally, there is a small, although not entirely convincing, body of evidence, indicating that the female is influenced by the action of the male; but I do not think that this evidence shows that she selects one individual at the expense of all other rivals. We meet here with a problem that is as profoundly interesting as it is obscure. In fact, if we admit that this relation exists we have a double set of conditions to deal with: first, the development in the males of certain secondary sexual organs; and secondly, the instinct to display these organs. The supposed influence of the display on the female may also have to be taken into account, although, for all we know to the contrary, the same results might follow were there no secondary sexual character at all, as is, in fact, the case in most animals.

I have a strong suspicion that much that has been written on this subject is imaginative, and in large part fictitious; so that it may, after all, be the wisest course not to attempt to explain how this relation has arisen until we have a more definite conception of what we are really called upon to explain. For example, when we see a gorgeously bedecked male displaying himself before a female, we feel that his finery must have been acquired for this very purpose. On the other hand, when we see an unornamented male also making definite movements before the female, we do not feel called upon to explain the origin of his colors. Now, it is not improbable that the ornaments of the first individual have not been acquired in order to display them before the female, and this view seems to me the more probable. From this standpoint our problem is at least much simplified. What we need to account for is only that the male is excited to undergo certain movements in the presence of the female, and possibly that the female may be influenced by the result. That this view is the more profitable is indicated by the occurrence of secondary sexual characters in the lower forms, as in the insects and crustaceans, in which it appears almost inconceivable that the ornamentation could have been acquired in connection with the æsthetic taste of the other sex. It does not seem to me that the conditions in the higher animals call for any other explanation than that which applies to these lower forms.

My position may be summed up in the statement, that, while in some cases there appears to be a connection between the presence in one sex of secondary sexual organs and their effect on the other sex, yet their origin cannot be explained on account of this connection.

As pointed out in the first chapter, there is a group of adaptations, obviously including several quite different kinds of phenomena, that can at least be conveniently brought together under the general rubric of individual adjustments or regulations. A few examples of these will serve to show in what sense they may be looked upon as adaptations, and how they may be regarded from the evolutionary point of view.

Color Changes as Individual Adaptations

The change in color of certain fish in response to the color of the background, the change in color of some chrysalides also in response to their surroundings, appears to be of some use to the animals in protecting them from their enemies. The change in color from green to brown and from brown back to green in several lizards and in some tree frogs is popularly supposed to be in response to the color of the surroundings, but a more searching examination has shown that, in some cases at least, the response has nothing to do with the color of the background.

In the first cases mentioned above, in which the response appears to be of some advantage to the animal, the question may be asked, how have such responses arisen? The selection theory assumes that those animals that responded at first to a slight degree in a favorable direction have escaped, and this process being repeated, the power to change has been gradually built up. The mutation theory will also account for the result by assuming the response to have appeared as a new quality, but it has been preserved, not because it has been of vital importance to its possessor, but simply because the species possessing it has been able to survive, perhaps in some cases even more easily, although this is not essential. Even if the change were of no direct benefit, or even injurious to a slight degree, it might have been retained, as appears in fact to be the case in the change of color of the green lizards.

Increase of Organs through Use and Decrease through Disuse

We meet here with one of the most characteristic and unique features of living things as contrasted with non-living things. We shall have to dismiss at once the idea that we can explain this attribute of organisms by either the selection or the mutation theory; for we find animals possessing this power that could never be supposed to have acquired it by any experience to which they have been subjected; and since it appears to be so universally present, we cannot account for it as a chance mutation that may have appeared in each species. No doubt Wolff had responses of this kind in view when he made the rather sweeping statement that purposeful adaptation is the most characteristic feature of living things. The statement appears to contain a large amount of truth, if confined to the present group of phenomena.

This power of self-regulation may confer a great benefit on its possessor. The increase in the size and strength of the muscles through use may give the animal just those qualities that make its existence easier. The increase in the power of vision, or at least of visual discrimination through use, of the power of smell and of taste, of hearing and of touch, are familiar examples of this phenomenon.

However much we may be tempted to speculate as to how this property of the animal may have been acquired, we lack the evidence which would justify us in formulating even a working hypothesis. It may be that when we come to know more of what the process of contraction of the muscle involves, the possibility of its development as a consequence of its use may be found to be a very simple phenomenon that requires no special explanation at all to account for its existence in the individual, further than that the muscles are of such a kind that this is a necessary physical result of their action. But until we know more of the physiology involved in the process, it is idle to speculate about the origin of the phenomenon.

Reactions of the Organism to Poisons, etc.

In this case also we meet with a number of responses for whose origin we can give not the shadow of an explanation. On the other hand, the cases are significant in so far as a number of them show quite clearly that the response cannot have been acquired through the experience of the organism, or the selection of those individuals that have best resisted the particular poison. This is true, because in a number of cases the poison is a substance that the animal cannot possibly have met with during the ordinary course of its life, or of that of its ancestors. It may be argued, it is true, that in the case of the poisons produced by certain bacteria the power of resistance has been acquired through the survival of the less susceptible, or more resistant, individuals. Improbable as this may be in some cases, it does not, even if it were true, alter the real issue, for it can be shown, as has just been said, that the same power of responding adaptively is sometimes shown in cases of poisons that are new to the animal.

There is no question that different individuals respond in very different degrees to these poisonous substances, and it is easy to imagine in the case of contagious diseases that a sort of selective process might go on that would bring the race up to the highest point to which fluctuating variations could be carried, even to complete immunity; but even if this were the case, it seems to be true that the moment the selection stopped the race would sink back to the former condition.

All this touches only indirectly the main point that we have under consideration, namely, the existence of this power of resistance in cases where it cannot have been the result of any educative process. Since the responses to new poisons do not appear to be in principle different from the responses to those to which the organism may have possibly been subjected at times in the past, we shall probably not go far wrong if we treat all cases on the same general footing. Whether the power of adaptation to certain substances, such as nicotine, morphine, cocaine, arsenic, alcohol, etc., is brought about by the formation of a counter-substance is as yet unproven. And while it seems not improbable that in some of these instances it may turn out that this is the case, especially for poisons of plant origin, it is better to suspend judgment on this point until each case has been established.

In recent years it has been shown that the animal body has the power of making counter-substances when a very large number of different kinds of things are introduced into the blood. We seem to be here on the threshold of a field for discovery which may, if opened up, give us an insight into some of the most remarkable phenomena of adaptation shown by living things.

It has already been pointed out that it appears to be almost a reductio ad absurdum to speak of animals adapting themselves to poisonous substances. It is curious, too, that in man at least the use of these substances may arouse a craving for the poison, or at any rate the individual may become so dependent on the poison that the depression following its disuse may lead to a desire for a repetition of the dose. The two questions that are raised here must be kept apart, for the adaptation of the individual to the poison and the so-called craving for it may depend on quite different factors. Nevertheless, it seems to be true in the case of morphine and of arsenic, and probably for some other substances as well, that if their use is suddenly stopped the individual may die in consequence. In this respect the organism behaves exactly as it does to an environment to which it has become adapted.

Regeneration

Many animals are able to replace lost parts, and all of them can heal wounds and mend injuries. This power is obviously of great advantage to them, and it has been supposed by Darwin, and more especially by his followers, that the power has been acquired through natural selection. It is not difficult to show that regeneration could not, in many cases, and presumably in none, have been acquired in this way. Since I have treated this subject at some length recently in my book on “Regeneration,” I shall attempt to do no more here than indicate the outline of the argument.

The Darwinians believe that, if some individuals of a species have the power to replace a part that is lost better than have other individuals, it would follow that those would survive that regenerate best, and in this way after a time the power to regenerate perfectly would be acquired.

But the matter is by no means so simple as may appear from this statement. In the first place, it is a matter of common observation that all the individuals of a species are never injured in the same part of the body at the same time. In those cases in which it is known that a special part is often injured, an examination has shown that there are not more than ten per cent of individuals that are injured at any one time, and in the case of the vast majority of animals this estimate is much too great. Thus there will be very little chance for competition of the injured individuals in each generation with each other, and the effects that are imagined to be gained as a result would be entirely lost by crossing with the uninjured individuals. But it is not necessary to consider this possibility, since there is another fact that shows at once that the power to regenerate could not have been gained through selection. The number of uninjured individuals in each generation will be much greater than the injured ones, and these will have so great an advantage over the injured individuals that, if competition approached the degree assumed by the selectionists, the injured individuals should be exterminated. A slight advantage gained through better powers of regeneration would be of little avail in competition, as compared with the competition with the uninjured individuals. Since selection is powerless to accomplish its end without competition, and since with competition all the injured individuals would be eliminated, it is clear that an appeal cannot be made to selection to explain the power of regeneration.

In many cases the power of regeneration could not have been slowly acquired through selection, since the intermediate steps would be of no use. Unless, for example, a limb regenerated from the beginning almost completely, the result would be of no use to the animal. If the limb did regenerate completely the first time it was injured, then the selection hypothesis becomes superfluous.

There are also a few cases known in which a process of regeneration takes place that is of no use to the animal. If, for instance, the earthworm (Allolobophora fœtida) be cut in two in the middle, the posterior piece regenerates at its anterior cut end, not a head, but a tail. Not by the widest stretch of the imagination can such a result be accounted for on the selection theory. Again, we find the reverse case, as it were, in certain planarians. If the head of Planaria lugubris is cut off just behind the eyes, there develops at the cut surface of this head-piece another head turned in the opposite direction. Here again we have the regeneration of a perfect structure, but one that is entirely useless to the individual. The development of an antenna in place of an eye in the shrimp, when the eye stalk is cut off near its base, is another instance of the occurrence of a perfectly constant process, but one that is of no use to the organism.

When we recall that in some organisms regeneration takes place in almost every part of the body, it does not seem possible that this power could have been acquired by selection. And when we find that many internal organs regenerate, that can rarely or never be injured without the animal perishing, it seems impossible that this can be ascribed to the principle of natural selection.

It has also been found that if the first two cells of the egg of a number of animals, jellyfish, sea-urchins, salamanders, etc., be separated, each will produce an entire animal. In some of these cases it is inconceivable that the process could ever have been acquired through selection, because the cells themselves can be separated only by very special and artificial means.

These, and other reasons, indicate with certainty that regeneration cannot be explained by the theory of natural selection.