in which they live. As an illustration of how such adjustments are supposed to occur I shall take a single example, from Weismann's writings, although it would be an easy matter to give endless examples, similar to this one, from the writings of other Darwinians, who, as a rule, have been only too prone to make use of the same argument in accounting for the origin of the adaptations of animals and plants.
Length of Life as an Adaptation.
Weismann has written a most interesting essay on 'The Duration of Life,' in which he attempts to show that the length of life of the individuals of a species has been regulated by natural selection of individual variations. While it may be granted that in many respects Weismann has out-Darwined Darwin, yet his method in this instance is the same as that which the Darwinians have always employed whenever an occasion occurred.
It has often been pointed out that the life of larger animals is longer than that of smaller ones, and this seems not unreasonable, since in many cases it takes a longer time for a larger animal to reach maturity; yet this relation is by no means universal, as Weismann points out, for, while it is true that an elephant may live two hundred years and a horse not more than forty, yet a man lives longer than a horse, and a cat and a toad may also live forty years. A pig may live no longer than a crayfish. Flourens tried to show that the length of life of an animal is about five times its growing period, but this does not generally hold, since a horse may live ten times as long as it takes to reach maturity.
Complexity of structure can not explain the result, because some very simple forms live to a great age. Weismann also concludes that the length of life is not determined by the 'constitution' of the animal, for, while a queen bee may live for several years, the male lives for only a few weeks. Therefore, Weismann concludes, it is 'proved that physiological considerations alone can not determine the duration of life.'
Thus by an apparent process of exclusion Weismann reaches the conclusion, 'that duration of life is really dependent upon adaptation to external conditions, that its length, whether longer or shorter, is governed by the needs of the species.' In support of this view he points out that 'life does not greatly outlast the period of reproduction except in those species that tend, their young, and as a matter of fact we find that this is the case.' How then has this regulation been brought about? Weismann's answer is that the duration depends first on the time required to reach maturity, and since the longer this time the more the chance for destruction, the number of descendents produced must be greater in proportion as the duration of the reproductive period
