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.