Evolution and Adaptation

In the present chapter we may first consider, from the point of view of discontinuous variations as contrasted with the theory of the selection of individual variations, the structural adaptations of animals and plants, i.e. those cases in which the organism has a definite form that adapts it to live in a particular environment. In the second place, we may consider those adaptations that are the result of the adjustment of each individual to its surroundings. In subsequent chapters the adaptations connected with the responses of the nervous system and with the process of sexual reproduction will be considered.

It should be stated here, at the outset, that the term mutation will be used in the following chapters in a very general way, and it is not intended that the word shall convey only the idea which De Vries attaches to it; it is used rather as synonymous with discontinuous and also definite variation of all kinds. The term will be used to include “the single variations” of Darwin, “sports,” and even orthogenic variation, if this has been definite or discontinuous.

Form and Symmetry

Almost without exception, animals and plants have definite and characteristic forms. In other words, they are not amorphous masses of substance. The members of each species conform, more or less, to a sort of ideal type. Our first problem is to examine in what sense the form itself may be looked upon as an adaptation to the surroundings.

It is a well-recognized fact that the forms of many animals appear to stand in a definite relation to the environment. For instance, animals that move in definite directions in relation to their structure have the anterior and the posterior ends quite different, and it is evident that these ends stand in quite different relations to surrounding objects; while, on the other hand, the two sides of the body which are, as a rule, subjected to the same influences are nearly exactly alike. The dorsal and the ventral surfaces of the body are generally exposed to very different external conditions, and are quite different in structure.

The relation is so obvious in most cases that it might lead one quite readily to conclude that the form of the animal had been moulded by its surroundings. Yet this first impression probably gives an entirely wrong conception of how such a relation has been acquired. Before we attempt to discuss this question, let us examine some typical examples.

A radial type of structure is often found in fixed forms, and in some floating forms, like the jellyfish. In a fixed form, a sea-anemone, for instance, the conditions around the free end and the fixed end of the body are entirely different, and we find that these two ends are also different. The free end contains the special sense-organs, the mouth, tentacles, etc.; while the fixed end contains the organ for attachment. It is evident that the free end is exposed to the same conditions in all directions, and it may seem probable that this will account for the radial symmetry of the anemone. There are also a few free forms, the sea-urchin for instance, that have a radial symmetry. Whether their ancestors were fixed forms, for which there is some evidence, we do not know definitely; but, even if this is true, it does not affect the main point, namely, that, although at present free to move, the sea-urchin is radially symmetrical. But when we examine its method of locomotion, we find that it moves indifferently in any direction over a solid surface; that is, it keeps its oral face against a solid object, and moves over the surface in any direction. Under these circumstances the same external conditions will act equally upon all sides of the body. In contrast to these common sea-urchins, there are two other related groups, in which, although traces of a well-marked radial symmetry are found, the external form has been so changed that a secondary bilateral form has been superimposed on it. These are the groups of the clypeasters and the spatangoids, and it is generally supposed that their forefathers were radially symmetrical forms like the ordinary forms of sea-urchins. These bilateral forms move in the direction of their plane of symmetry, but we have no means of knowing whether they first became bilateral and, in consequence, now move in the direction of the median plane, or whether they acquired the habit of moving in one direction, and in consequence acquired a bilateral symmetry. It seems more probable that the form changed first, for otherwise it is difficult to see why a change of movement in one direction should ever have taken place.

The radially symmetrical form is characteristic of many flowers that stand on the ends of their stalks. They also will be subjected to similar external influences in all directions. Many flowers, on the other hand, are bilaterally symmetrical. Some of these forms are of such a sort that they are generally interpreted as having been acquired in connection with the visits of insects. Be this as it may, it is still not clear why, if the flowers are terminal, insects should not approach them equally from every direction. If the flowers are not terminal, as, in fact, many of them are not, their relation to the surroundings is bilateral with respect to internal as well as to external conditions. The former, rather than the latter, may have produced the bilateral form of the flower. Here also we meet with the problem as to whether the flowers, being lateral in position, have assumed a bilateral form because their internal relations were bilateral; or whether an external relation, for example, the visits of insects, has been the principle cause of their becoming bilateral.

In some bilateral forms the right and left sides may be unsymmetrical in certain organs. Right and left handedness in man is the most familiar example, although the structural difference on which this rests is not very obvious. More striking is the difference in the two big claws of the lobster (Fig. 4 A). One of the two claws is flat and has a fine saw-toothed edge. The other is thicker and has rounded knobs instead of teeth. It is said that these two claws are used by the lobster for different purposes,—the heavy one for crushing and for holding on, and the narrower for cutting up the food. If this is true, then we find a symmetrical organism becoming unsymmetrical, and in consequence it takes advantage of its asymmetry by using its right and left claws for different purposes.

More striking still is the difference in the size of the right and left claws in a related form, Alpheus—a crayfish-like form that lives in the sea. With the larger claw (Fig. 4 C) it makes a clicking sound that can be heard for a long distance. In some of the crabs the difference in the size of the two claws is enormous, as in the male fiddler-crab, for example (Fig. 4 B). One of the claws is so big and unwieldy that it must put the animal at a distinct disadvantage. Its use is unknown, although it has been suggested that it is a secondary sexual character.

The asymmetry of the body of the snail is very conspicuous, at least so far as certain organs are concerned. The foot on which the animal crawls and the head have preserved their bilaterality; but the visceral mass of the animal, contained in the spirally wound shell, lying on the middle of the upper surface of the foot, is twisted into a spiral form. Many of the organs of one side of the body are atrophied. The gill, the kidney, the reproductive organ, and one of the auricles of the heart have completely, or almost completely, disappeared. The cause of this loss seems to be connected with the spiral twist of the visceral mass. One of the consequences of the twisting has been to bring the organs of the left side of the body around the posterior end until they come to lie on the right side, the organs of the original right side being carried forward and there atrophying.

There is another remarkable fact connected with the asymmetry of the snail. In some species, Helix pomatia, for example, the twist has been toward the right, i.e. in the direction which the hands of a watch follow when the face is turned upward toward the observer. Individuals twisted in this direction are called dextral. Occasionally there is found an individual with the spiral in the opposite direction (sinistral), and in this the conditions of the internal organs are exactly reversed. It is the left set of organs that is now atrophied, and the right set that is functional. Such changes appear suddenly. Organs of one side of the body that have not been functional for many generations may become fully developed. Moreover, Lang has shown that when a sinistral form breeds with a normal dextral form, or even when sinistral forms are bred with each other, the young are practically all of the ordinary type.

An attempt has been made to connect these facts with the mode of development of the mollusks. It is known that the eggs of a number of gasteropod mollusks segment in a perfectly definite manner. A sort of spiral cleavage is followed by the formation of a large mesodermal cell from the left posterior yolk-cell. From this mesodermal cell nearly all the mesodermal organs of the body are formed. Thus it may appear that the spiral form of the snail is connected with the spiral form of the cleavage. In a few species of marine and fresh-water snails the cleavage spiral is reversed, and the mesoderm arises from the right posterior yolk-cell. It has been shown in several cases that the snail coming from such an egg is twisted in the reverse direction from that of ordinary snails.

It has been suggested, therefore, that the occasional sinistral individual of Helix arises from an egg cleaving in the reverse direction, and there is nothing improbable in an assumption of this kind. No attempt has been made as yet to explain why, in some cases, the cleavage spiral is turned in one direction, and in other cases in the reverse direction; but even leaving this unaccounted for, the assumption of the unusual form of Helix being the result of a reversal of the cleavage throws some light as to how it is possible for the complete reversal of the organs of the adult to arise. If it is assumed that in the early embryo the cells on each side of the median line are alike, and at this time capable of forming adult structures, a simple change of the spiral from right to left might determine on which side of the middle line the mesodermal cell would lie, and its presence on one side rather than on the other might determine which side of the embryo would develop, and which would not. This possibility removes much of the mystery which may appear to surround a sudden change of this sort.

It seems to me that we shall not go far wrong if we assume that it is largely a matter of indifference whether an individual snail is a right-handed or a left-handed form, as far as its relation to the environment is concerned. One form would have as good a chance for existing as the other. If this is granted, we may conclude that, while in most species a perfectly definite type is found, a right or a left spiral, yet neither the one nor the other has been acquired on account of its relation to the environment. This conclusion does not, of course, commit us in any way as to whether the spiral form of the visceral mass has been acquired in relation to the environment, but only to the view that, if a spiral form is to be produced, it is indifferent which way it turns. From the evolutionary point of view this conclusion is of some importance, since it indicates that one of the alternatives has been adopted and has become practically constant in most cases without selection having had anything to do with it.

Somewhat similar conditions are found in the flounders and soles. As is well known, these fishes lie upon one side of the body on the bottom of the ocean. Some species, with the rarest exceptions to be mentioned in a moment, lie always on the right side, others on the left side. A few species are indifferently right or left. At rare intervals a left-sided form is found in a right-sided species, and conversely, a right-sided form in a left-sided species. In such cases the reversed type is as perfectly developed in all respects as the normal form, but with a complete reversal of its right and left sides.

When the young flounders leave the egg, they swim in an upright position, as do ordinary fishes, with both sides equally developed. There cannot be any doubt that the ancestors of these fish were bilaterally symmetrical. Therefore, within the group, both right-handed and left-handed forms have appeared. It seems to me highly improbable that if a right-handed form had been slowly evolved through the selection of favorable variations in this direction, the end result could be suddenly reversed, and a perfect left-sided form appear. Moreover, as has been pointed out, the intermediate stages would have been at a great disadvantage as compared with the parent, and this would lead to their extermination on the selection theory. If, however, we suppose that a variation of this sort appeared at once, and was fixed,—a mutation in other words,—and that whether or not it had an advantage over the parent form, it could still continue to exist, and propagate its kind, then we avoid the chief difficulty of the selection theory. Moreover, we can imagine, at least, that if this variation appeared in the germ and was, in its essential nature, something like the relation seen in the snail, the occasional reversal of the relations of the parts presents no great difficulty.

In this same connection may be mentioned a curious fact first discovered by Przibram and later confirmed by others. If the leg carrying the large claw of a crustacean be removed, then, at the next moult, the leg of the other side that had been the smaller first leg becomes the new big one; and the new leg that has regenerated from the place where the big one was cut off becomes the smaller one.

Wilson has suggested that both claws in the young crustacean have the power to become either sort. We do not know what decides the matter in the adult, after the removal of one of the claws. Some slight difference may turn the balance one way or the other, so that the smaller claw grows into the larger one. At any rate, there is seen a latent power like that in the egg of the snail. Zeleny has found a similar relation to exist for the big and the little opercula of the marine worm, Hydroides.

Let us consider now the more general questions involved in these symmetrical and asymmetrical relations between the organism and its environment. In what sense, it may be asked, is the symmetry of a form an adaptation to its environment? That the kind of symmetry gives to the animal in many cases a certain advantage in relation to its environment is so evident that I think it will not be questioned. The main question is how this relation is supposed to have been attained. Three points of view suggest themselves: First, that the form has resulted directly from the action of the environment upon the organism. This is the Lamarckian point of view, which we rejected as improbable. Second, that the form has been slowly acquired by selecting those individual variations that best suited it to a given set of surrounding conditions. This is the Darwinian view, which we also reject. The third, that the origin of the form has had nothing to do with the environment, but appeared independently of it. Having, however, appeared, it has been able to perpetuate itself under certain conditions.

It should be pointed out that the Darwinian view does not suppose that the environment actually produces any of the new variations which it selects after they have appeared, but in so far as the environment selects individual differences it is supposed to determine the direction in which evolution takes place. On the theory that evolution has taken place independently of selection, this latter is not supposed to be the case; the finished products, so to speak, are offered to the environment; and if they pass muster, even ever so badly, they may continue to propagate themselves.

The asymmetrical form of certain animals living in a symmetrical environment might be used as an argument to show that the relation of symmetry between an animal and its environment can easily be overstepped without danger. The enormous claw of the fiddler-crab must throw the animal out of all symmetrical relation with its environment, and yet the species flourishes. The snail carries around a spiral hump that is entirely out of symmetrical relation with the surroundings of a snail.

These facts, few though they are, yet suffice to show, I believe, that the relation of symmetry between the organism and its environment may be, and is no doubt in many cases, more perfect than the requirements of the situation demand. The fact that animals made unsymmetrical through injuries (as when a crab loses several legs on one side, or a worm its head) can still remain in existence in their natural environment, is in favor of the view that I have just stated. By this I do not mean to maintain that a symmetrical form does not have, on the whole, an advantage over the same form rendered asymmetrical, but that this relation need not have in all forms a selective value, and if not, then it cannot be the outcome of a process of natural selection.

To sum up: it appears probable that the laws determining the symmetry of a form are the outcome of internal factors, and are not the result either of the direct action of the environment, or of a selective process. The finished products and not the different imperfect stages in such a process, are what the inner organization offers to the environment. While the symmetry or asymmetry may be one of the numerous conditions which determine whether a form can persist or not, yet we find that the symmetrical relations may be in some cases more perfect than the environment actually demands; and in other cases, although the form may place the organism at a certain disadvantage, it may still be able to exist in certain localities.

In the white ants, true ants, and bees, we find certain individuals of the community specialized in such a way that their modifications stand in certain useful relations to other members of the community. Amongst the bees, the workers collect the food, make the comb, and look after the young. The queen does little more than lay eggs, and the drone’s only function is to fertilize the queen. In the true ants there are, besides the workers and the queen and the males, the soldier caste. These have large thick heads and large strong jaws. On the Darwinian theory it is assumed that this caste must have an important rôle to play, for otherwise their presence as a distinct group of forms cannot be accounted for; but I do not believe it is necessary to find an excuse for their existence in their supposed utility. From the point of view of the mutation theory, their real value may be very small, but so long as their actual presence is not entirely fatal to the community they may be endured.

In regard to these forms, Sharp writes:^[28] “The soldiers are not alike in any two species of Termitidæ, so far as we know, and it seems impossible to ascribe the differences that exist between the soldiers of different species of Termitidæ to special adaptations for the work they have to perform.” “On the whole, it would be more correct to say that the soldiers are very dissimilar in spite of their having to perform similar work, than to state that they are dissimilar in conformity with the different tasks they carry on.” The soldiers have the same instincts as the workers, and do the same kinds of things to a certain extent. “The soldiers are not such effective combatants as the workers are.” Statements such as these indicate very strongly that the origin of this caste can have very little to do with its importance as a specialized part of the community.

The differences between the castes have gone so far in some of these groups that the majority of the members of the community have even lost the power to reproduce their kind, and this function has devolved upon the queen, whose sole duty is to reproduce the different castes of which the community is composed. This specialization carries with it the idea of the individuals being adapted to each other, so that, taken all together, they form a whole, capable of maintaining and reproducing itself. It does not seem that we must necessarily look upon this union as the result of competition leading to a death struggle between different colonies, so that only those have survived in each generation that carried the work of specialization one step farther. All that is required is to suppose that such specialization has appeared in a group of forms living together, and the group has been able to perpetuate itself. We do not find that all other members of the two great groups to which the white ants and true ants belong have been crowded out because these colonial forms have been evolved. Neither need we suppose that during the evolution of these colonial species there has been a death struggle accompanying each stage in the evolution. If the members of a colonial group began to give rise to different forms through mutations, and if it happened that some of the combinations formed in this way were capable of living together, and perpetuating the group, this is all that is required for such a condition to persist.

The relation of the parents to the offspring presents in some groups a somewhat parallel case to that of these colonial forms. Not only are some of the fundamental instincts of the parents changed, but structures may be present in the parents whose only use is in connection with the young. The marsupial pouch of the kangaroo, in which the immature young are carried and suckled, is a case in point, and the mammary glands of the Mammalia furnish another illustration.

Adaptations of these kinds are clearly connected with the perpetuation of the race. In the case of the mammals the young are born so immature that they are dependent on the parental organs, just spoken of, for their existence. Could we follow this relation through its evolutionary stages, it would no doubt furnish us with important data, but unfortunately we can do no more than guess how this relation became established. The changes in the young and in the parent may have been intimately connected at each stage, or more or less independent. If we suppose the mammary glands to have appeared first, they might have been utilized by the young in order to procure food. Their presence would then make it possible for the young to be born in an immature condition, as is the case with the young of many of the mammals. But this is pure guessing, and until we know more of the actual process of evolution in this case, it is unprofitable to speculate.

Degeneration

In almost every group of the animal kingdom there are forms that are recognized as degenerate. This degeneration is usually associated with the habitat of the animal. In many cases it can be shown with much probability that these degenerate forms have descended from members of the group that are not degenerate. We find there is a loss of those organs that are not useful to the organism in its new environment. The degeneration may involve nearly the whole organization (except as a rule the reproductive system), as seen in the tapeworm, or only certain organs of the body, as the eyes in cave animals. A few examples will bring the main facts before us.

A parasitic existence is nearly always associated with degeneration. Under these conditions, food can generally be obtained without difficulty, at the expense of the host, and apparently associated with this there is a degeneration, and even a complete loss of so important an organ as the digestive tract. Thus the tapeworm has lost all traces of its digestive tract, absorbing the already digested matter of its host through its body wall. Some of the roundworms, that live in the alimentary tracts of other animals, may have their digestive organs reduced. In Trichina, this degeneration has gone so far that the digestive tract is represented, in part, by a single line of endoderm cells, pierced by a cavity. The digestive organs are also absent in certain male rotifers, which are parasitic on the females, and these organs are also very degenerate in the male of Bonellia, a gephyrean worm. A parasitic snail, Entoscolax ludwigii, has its digestive apparatus reduced to a sucking tube ending in a blind sac. The rest of the tract has completely degenerated. The remarkable parasitic crustacean, Sacculina carcini, looks like a tumor attached to the under surface of the abdomen of a crab. It has neither mouth nor digestive tract, and absorbs nourishment from the crab through rootlike outgrowths that penetrate the body. From its development alone we know that it is a degenerate barnacle.

There seems to be in all these cases an apparent connection between the absence of the digestive tract and the presence of an abundant supply of food, that has already been partly digested by the host. Put in a different way, we may say that the presence of this food has furnished the environment in which an animal may live that has a rudimentary digestive tract.

An interesting case of degeneration is found in the rudimentary mouth parts of the insects known as May-flies, or ephemerids. Some of these species live in the adult condition for only a few hours, only long enough to unite and deposit their eggs. In the adult stage the insects do not take any food. In this case the degeneration is obviously not connected with the presence of food, but apparently with the shortness of the adult life.

One of the most familiar cases of degeneration is blindness, associated with life in the dark. The most striking cases are those of cave animals, but this is only an extreme example of what is found everywhere amongst animals that live concealed during the day under stones, etc. The blind fish and the blind crayfish of the Mammoth Cave, the blind proteus of the caves of Carniola, the blind mole that burrows underground, the blind larvæ of many insects that live in the dark, are examples most often cited. Some nocturnal animals, like the earthworm, have no eyes, although they are still able to distinguish light; and some of the deep-sea animals, that live below the depth to which light penetrates, have degenerate eyes. The workers of some ants, that remain in the nests, are blind, but the males and the queens of these forms have well-developed eyes, although the eyes may be of use to them at only one short period of their life, namely, at the time of the marriage flight. This fact is significant and is underestimated by those who believe that disuse accounts for the degeneration of organs.

The wings of the ostrich and of the kiwi are rudimentary structures no longer used for flight, and many insects, belonging to several different orders, have lost their wings, as seen in fleas, some kinds of bugs, and moths, and even in some grasshoppers.

A curious case of degeneration is found in the abdomen of the hermit crab, which is protected by the appropriated shell of a snail. The appendages of one side of the abdomen have nearly disappeared in the male, although in the female the abdominal appendages are used to carry the eggs as in other decapod crustaceans. The abdomen, instead of being covered by a hard cuticle, as in other members of this group, is soft and unprotected except by the shell of the snail.

Cases of these kinds could be added to almost indefinitely, and the explanation of these degenerate structures has been a source of contention amongst zoologists for a long time. The most obvious interpretation is that the degeneration has been the result of disuse. But as I have already discussed this question, and given my reasons for regarding it as improbable that degeneration has arisen in this way, we need not further consider this point here.

The selectionists have offered several suggestions to account for degeneration. In fact, this has been one of the difficulties that has given them most concern. They have suggested, for example, that when an organ is no longer of use to its possessor it would become a source of danger, and hence would be removed through natural selection. They have also suggested that since such organs draw on the general food supply they would place their possessor at a disadvantage, and hence would be removed. Weismann has attempted to meet the difficulty by his theory of “Panmixia,” or universal crossing, by which means the useless structures are imagined to be eliminated.

These attempts will suffice to point out the straits to which the Darwinians have found themselves reduced, and we have by no means exhausted the list of suggestions that have been made. Let us see, if, on any other view, we can avoid some of the difficulties that the selection theory has encountered.

In the first place we shall be justified, I think, in eliminating competition as a factor in the process, since the admission that an organ has become useless carries with it the idea that it has no longer a selective value. If, in its useless condition, it is no longer greatly injurious, as is probably, though not necessarily always, the case, then selection cannot enter into the problem. If in parasitism we assume that an animal finds a lodgement in another animal, where it is able to exist, we may have the first stage of the process introduced at once. If under these conditions a mutation appeared, involving some of the organs that are no longer essential to the life of the individual in its new environment, the new mutation may persist. We need not suppose that the original form becomes crowded out, but only that a more degenerate form has come into existence. As a matter of fact we find in most groups, in which degenerate forms exist, a number of different stages in the degeneration in different species. Mutation after mutation might follow until many of the original organs have disappeared. The connection that appears to exist between the degeneration of a special part and the environment in which the animal lives finds its explanation simply in the fact that the environment makes possible the existence of that sort of mutation in it. We do not know, as yet, whether through mutative changes an organ can completely disappear, although this seems probable from the fact that in a few cases mutations are known to have arisen in which a given part is entirely functionless. If we could assume that, a mutation in the direction of degeneration being once established, further mutations in the same direction would probably occur, the problem would be much simplified; but we lack data, at present, to establish this view.

In the case of blind animals it seems probable that the transition has taken place in such forms as had already established themselves in places more or less removed from the light. Such forms as had the habit of hiding away under stones, or in the ground, living partly in and partly out of the light, might, if a mutation appeared of such a sort that amongst other changes the eyes were less developed, still be capable of leading an existence in the dark, while it might be impossible for them to exist any longer with weakened vision in the light. If such a process took place, the habitat of the new form would be limited, or in other words it would be confined to the locality to which it finds itself adapted; not that it has become adapted to the environment through competition with the original species, or, in fact, with any other.

Thus, from the point of view that is here taken, an animal does not become degenerate because it becomes parasitic, but the environment being given, some forms have found their way there; in fact, we may almost say, have been forced there, for these degenerate forms can only exist under such conditions.

In conclusion, this much at least can be claimed for the mutation theory; that it meets with no serious difficulty in connection with the phenomena of degeneration. It meets with no difficulty, because it makes no pretence to explain the origin of adaptations, but can account for the occurrence of degenerate forms, if it is admitted that these appear as mutations, or as definite variations. Let us, however, not close our eyes to the fact that there is still much to be explained in respect to the degeneration of animals and plants. It is far from my purpose to apply the mutation theory to all adaptations; in fact, it will not be difficult to show that there are many adaptations whose existence can have nothing directly to do with the mutation theory.

Protective Coloration

That many species of animals are protected by their resemblance to their environment no one will probably deny. That we are ignorant in all cases as to how far this protection is necessary for the maintenance of the species must be admitted. That some of the resemblances that have been pointed out have been given fictitious value, I believe very probable.

Resemblance in color between the organism and its environment has given to the modern selectionist some of his most valuable arguments, but we should be on our guard against supposing that, because an animal may be protected by its color, the color has been acquired on this account. On the supposition that the animal has become adapted by degrees, and through selection, we meet with all the objections that have been urged, in general, against the theory of natural selection. But if we assume here also that mutations have occurred without relation to the environment, and, having once appeared, determined in some cases the distribution of the species, we have at least a simple hypothesis that appears to explain the facts. If it be claimed that the resemblance is, in some cases, too close for us to suppose that it has arisen independently of the environment, it may be pointed out that it has not been shown that such a close resemblance is at all necessary for the continued existence of the species, and hence the argument is likely to prove too much. For instance, the most remarkable case of resemblance is that of Kallima, but in the light of a recent statement by Dean it may be seriously asked whether there is absolute need of such a close resemblance to a leaf. Even if it be admitted that to a certain extent the butterfly is at times protected by its resemblance to a leaf, it is not improbable that it could exist almost equally well without such a close resemblance. If this is true, natural selection could never have brought about such a close imitation of a leaf. Cases like these of over-adaptation are not unaccountable on the theory of mutation, for on this view the adaptation may be far ahead of what the actual requirements for protection demand. We meet occasionally, I think, throughout the living world with resemblances that can have no such interpretation, and a number of the kinds of adaptations to be described in this chapter show the same relation.

Some of the cases of mimicry appear also to fall under this head; although I do not doubt that many so-called cases of mimicry are purely imaginary, in the sense that the resemblance has not been acquired on account of its relation to the animal imitated. There is no need to question that in some cases animals may be protected by their resemblance to other animals, but it does not follow, despite the vigorous assertions of some modern Darwinians, that this imitation has been the result of selection. Until it can be shown that the imitating species is dependent on its close imitation for its existence, the evidence is unconvincing; and even if, in some cases, this should prove to be the case, it does not follow that natural selection has brought about the result, or even that it is the most plausible explanation that we have to account for the results. The mutation theory gives, in such cases, an equally good explanation, and at the same time avoids some of the difficulties that appear fatal to the selection theory.

What has been said against the theory of mimicry might be repeated in much stronger terms against the hypothesis of warning colors.

It seems to me, in this connection, that the imagination of the selectionist has sometimes been allowed to “run wild”; and while it may be true that in some cases the colors may serve as a signal to the possible enemies of the animal, it seems strange that it has been thought necessary to explain the origin of such colors as the result of natural selection. Indeed, some of these warning colors appear unnecessarily conspicuous for the purpose they have to perform. In other words, it does not seem plausible that an animal already protected should need to be so conspicuous. If we stop for a moment to consider what an enormous amount of destruction must have occurred, according to Darwin’s theory, in order to bring this warning coloration to its supposed state of perfection, we may well hesitate before committing ourselves to such an extreme view.

That gaudy colors have appeared or been present in animals that are protected in other ways is not improbable, when we consider the rôle that color plays everywhere in nature. That the presence of such colors may, to a certain limited extent, protect its possessor may be admitted without in any degree supposing that natural selection has directed the evolution of such color, or that it has been acquired through a life and death struggle of the individuals of the species.

Sexual Dimorphism^[29] and Trimorphism

It has been found in a few species of animals and plants that two or more forms of one sex may exist, and here we find a condition that appears to be far more readily explained on the mutation theory than on any other. The most important cases, perhaps, are those in plants, but there are also similar cases known amongst animals, and these will be given first.

There is a North American butterfly, Papilio turnus, that appears under at least two forms. In the eastern United States the male has yellow wings with black stripes. There are two kinds of females, one of which resembles the male except that she has also an orange “eye-spot”; the other female is much blacker, and this variety is found particularly in the south and west. The species is dimorphic, therefore, mainly in the latter regions.

The cases of seasonal dimorphism offer somewhat similar illustrations. The European butterfly, Vanessa levana-prorsa, has a spring generation (levana) with a yellow and black pattern on the upper surface of the wings. The summer generation (prorsa) has black wings “with a broad white transverse band, and delicate yellow lines running parallel to the margins.” These two types are sharply separated, and their differences in color do not appear to be associated with any special protection that it confers on the bearer. These facts in regard to Vanessa seem to indicate that differences may arise that are perfectly well marked and sharply defined, which yet appear to be without any useful significance.

We meet with cases in which the same animal has at different times of year different colors, as seen in the summer and winter plumage of the ptarmigan. There is no direct evidence to show how this seasonable change has been brought about; but from the facts in regard to Vanessa we can see that it might have been at least possible for the white winter plumage, for instance, to have appeared without respect to any advantage it conferred on the animal, but after it had appeared it may have been to a certain degree useful to its possessor.

Amongst plants there are some very interesting cases of dimorphism and trimorphism in the structure of the flowers. Darwin has studied some of these cases with great care, and has made out some important points in regard to their powers of cross-fertilization.^[30] The common European cowslip, Primula veris, var. officinalis, is found under two forms, Figure 5 A and B, which are about equally abundant. In one the style is long so that the stigma borne on its end comes to the top of the tube of the corolla. The stamens in this form stand about halfway up the tube. This is called the long-styled form. The other kind, known as the short-styled form, has a style only half as long as the tube of the corolla, and the stamens are attached around the upper end of the tube near its opening. In other words, the position of the end of the style (the stigma) and that of the stamens is exactly reversed in the two forms. The corolla is also somewhat differently shaped in the two forms, and the expanded part of the tube above the stamens is larger in the long-styled than in the short-styled form. Another difference is found in the stigma, which is globular in the long-styled, and depressed on its top in the short-styled, form. The papillæ on the former are twice as long as those on the short-styled form. The most important difference is found in the size of the pollen grains. These are larger in the long-styled form, being in the two cases in the proportion of 100 to 67. The shape of the grains is also different. Furthermore, the long-styled form tends to flower before the other kind, but the short-styled form produces more seeds. The ovules in the long-styled form, even when unfertilized, are considerably larger than those of the short-styled, and this, Darwin suggests, may be connected with the fact that fewer seeds are produced, since there is less room for them. The important point for our present consideration is that intermediate forms do not exist, although there are fluctuating variations about the two types. Moreover, the two kinds of flowers never appear on the same plant.

Darwin tried the effect of fertilizing the long-styled flowers with the pollen from the same flower or from other long-styled flowers. Unions of this sort he calls illegitimate, for reasons that will appear later. He also fertilized the long-styled flowers with pollen from short-styled forms. A union of this sort is called legitimate. Conversely, the short-styled forms were fertilized with their own pollen or with that from another short-styled form. This is also an illegitimate union. Short-styled forms fertilized with pollen from long-styled forms give again legitimate unions.

The outcome of these different crossings are most curious. In the table, page 364, the results of the four combinations are given. It will be seen at once that the legitimate unions give more capsules, and the seeds weigh more, than in the illegitimate unions.

The behavior of the offspring from seeds of legitimate and illegitimate origin is even more astonishing. Darwin found in Primula veris (the form just described) that the seeds from the short-styled form fertilized with pollen from the same form germinated so badly that he obtained only 14 plants, of which 9 were short-styled and 5 long-styled. The long-styled form fertilized with its own-styled pollen produced “in the first generation 3 long-styled plants. From their seed 53 long-styled grandchildren were produced; from their seed 4 long-styled great-grandchildren; from their seed 20 long-styled great-great-grandchildren; and lastly, from their seed 8 long-styled and 2 short-styled great-great-great-grandchildren.”