Evolution and Adaptation

by Thomas Hunt Morgan

Adaptations for the Good of the Species

Aside from the class of adaptations that are for the good of the individual, there is another class connected solely with the preservation of the race. The organs for reproduction are the most important examples of this kind. These organs are of no use to the individual for maintaining its own existence, and, in fact, their presence may even be deleterious to the animal. The instincts connected with the use of these organs may lead inevitably to the death of the individual, as in the case of the California salmon, which, on entering fresh water in order to deposit its eggs, dies after performing this act.

The presence of the organs of reproduction in the individual is obviously connected with the propagation of other individuals. Indeed in many organisms the life of the individual appears to have for its purpose the continuation of the race. In a large number of animals the individual dies after it has deposited its eggs. The most striking case is that of the May-flies, whose life, as mature individuals, may last for only a few hours. The eggs are set free by the bursting of the abdomen, and the insect dies. The male bee also dies after union with the queen. In some annelids, the body is also said to burst when the eggs are set free; and in other forms those parts of the body containing the eggs break off, and, after setting free the eggs, die. These are extreme cases of what is seen in many animals, namely the replacement of the old individuals by a new generation; and while in general there is only a loose connection between the death of the individual and the consummation of its reproductive power, yet the two run a course so nearly parallel that several writers have attempted to explain this connection as one of racial adaptation.

It has also been pointed out that in those higher animals that take care of their young after birth, the life of the individual does not end with the period of birth of the young, but extends at least throughout the time necessary to care for the young. It has even been suggested that this lengthening of the life period has been acquired on account of its use to the species. When, however, as in the case of the vertebrates, the young are born at intervals either in great numbers at a birth, as in fishes and amphibia, or in lots of twos, threes, or fours, as in many birds and mammals, or even only one at a time, as in a few birds and in man, it will be evident that the relation cannot be so simple as has been supposed. It cannot be assumed in these forms that the end of the life of the individual is in any way connected with the ripening of the last eggs, for, on the contrary, hundreds, or even many thousands, of potential eggs may be present in the ovaries when the animal is overtaken by old age, and its power of reproduction lost.

In regard to several of the lower animals, we find, in a number of cases where there are accurate data, that the individual goes on year after year producing young. Whether they ever grow old, in the sense of losing their power of reproduction, has not been definitely determined, but there is, so far as I know, no evidence to show that such a process takes place, and these animals appear to have the power of reproducing themselves indefinitely.

The phenomenon of old age (apart from its possible connection with the cessation of the power of reproduction), which leads to the death of the individual, has been looked upon by a few writers as an adaptation of the individual for the good of the species. It has been pointed out by these writers that the longer an individual lives, the more likely it is to become damaged, and if along with this its powers of reproduction diminish, as compared with younger individuals, then it stands in the way and takes food that might be used by other, younger individuals, that are better able to carry on the propagation of the race. It is assumed, therefore, that the life of the individual has been shortened for the benefit of the race. Whether such a thing is probable is a question that will also be discussed later. We are chiefly concerned here only in recording the different groups of phenomena that have been regarded by biologists as adaptations.

The so-called secondary sexual characters such as the brighter colors of the males, ornaments of different kinds, crests, color-pattern, tail feathers, etc., organs of offence and of defence used in fighting members of the same species, present a rather unique group of adaptations. These characters are supposed to be of use to the individual in conquering its rivals, or in attracting the females. They may be considered as useful to the individual in allowing it to propagate at the expense of its rivals, but whether the race is thereby benefited is a question that will be carefully considered later.

The colors of flowers, that is supposed to attract insects, have been already mentioned. The sweet fluid, or nectar, secreted by many flowers is sought by insects, which on entering the flowers bring about cross-fertilization. Thus while the nectar seems to be of no immediate service to the plant itself, it is useful to the species in bringing about the fertilization of the flowers. The odors of flowers also serve to attract insects, and their presence is one of the means by which insects find the flowers. This also is of advantage to the race.