THE ORIGIN OF SPECIES. 55 



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 when- 

 ever 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 



