CHAPTER X
THE ORIGIN OF THE DIFFERENT KINDS OF ADAPTATIONS
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.
Mutual Adaptation of Colonial Forms
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.
Footnote 28:
“The Cambridge Natural History,” Vol. V, 1895.
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
Footnote 29:
This term is used here in the sense employed by Darwin. The same term is sometimes used for those cases in which the male departs very greatly from the female in form.
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.
Footnote 30:
Many of the facts as to the occurrence of these cases were known before Darwin worked on them; but very little had been ascertained in regard to the sexual relation between the dimorphic and trimorphic forms, and it was here that Darwin obtained his most interesting results.
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.”
══════════════╤═════════╤═════════╤═════════╤═════════╤═════════ │Number of│ │ Maximum │ Minimum │ Average Nature of │ Flowers │Number of│of Seeds │of Seeds │ No. of Union │Fertilized│ Seed │ in any │ in any │Seeds per │ │Capsules │ one │ one │ Capsule │ │ │ Capsule │ Capsule │ ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── Long-styled │ 10 │ 6 │ 62 │ 34 │ 46.5 form by │ │ │ │ │ pollen of │ │ │ │ │ short-styled│ │ │ │ │ form: │ │ │ │ │ Legitimate │ │ │ │ │ union. │ │ │ │ │ ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── Long-styled │ 20 │ 4 │ 49 │ 2 │ 27.7 form by │ │ │ │ │ own-form │ │ │ │ │ pollen: │ │ │ │ │ Illegitimate│ │ │ │ │ union. │ │ │ │ │ ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── Short-styled │ 10 │ 8 │ 61 │ 37 │ 47.7 form by │ │ │ │ │ pollen of │ │ │ │ │ long-styled │ │ │ │ │ form: │ │ │ │ │ Legitimate │ │ │ │ │ union. │ │ │ │ │ ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── Short-styled │ 17 │ 3 │ 19 │ 6 │ 12.1 form by │ │ │ │ │ own-form │ │ │ │ │ pollen: │ │ │ │ │ Illegitimate│ │ │ │ │ union. │ │ │ │ │ ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── The two │ 20 │ 14 │ 62 │ 37 │ 47.1 legitimate │ │ │ │ │ unions │ │ │ │ │ together. │ │ │ │ │ ──────────────┼─────────┼─────────┼─────────┼─────────┼───────── The two │ 30 │ 7 │ 49 │ 2 │ 35.5 illegitimate│ │ │ │ │ unions │ │ │ │ │ together. │ │ │ │ │ ══════════════╧═════════╧═════════╧═════════╧═════════╧═════════
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:—
═══════════════╤══════════╤══════════╤══════════╤══════════ │Number of │ │ │ Average │ Flowers │Number of │ Average │ No. of Nature of Union│Fertilized│ Capsules │ No. of │Seeds per │ │ Produced │Seeds per │ Flower │ │ │ Capsule │Fertilized ───────────────┼──────────┼──────────┼──────────┼────────── The 6 │ 75 │ 56 │ 96.29 │ 71.89 legitimate │ │ │ │ unions │ │ │ │ ───────────────┼──────────┼──────────┼──────────┼────────── The 12 │ 146 │ 36 │ 44.72 │ 11.03 illegitimate │ │ │ │ unions │ │ │ │ ═══════════════╧══════════╧══════════╧══════════╧══════════
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.
Individual Adjustments as Adaptations
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.