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.