Evolution and Adaptation

by Thomas Hunt Morgan

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.

[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.