Evolution and Adaptation

by Thomas Hunt Morgan

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.