10 THE DUKATION OF LIFE. 



natural selection. It is of no importance to the species whether 

 the individual lives longer or shorter, but it is of importance 

 that the individual should be enabled to do its work towards the 

 maintenance of the species. This work is reproduction, or the 

 formation of a sufficient number of new individuals to compensate 

 the species for those which die. As soon as the individual has 

 performed its share in this work of compensation, it ceases to be 

 of any value to the species, it has fulfilled its duty and may die. 

 But the individual may be of advantage to the species for a longer 

 period if it not only produces offspring, but tends them for a 

 longer or shorter time, either by protecting, feeding, or instructing 

 them. This last duty is not only undertaken by man, but also 

 by animals, although to a smaller extent ; for instance, birds teach 

 their young to fly, and so on. 



We should therefore expect to find that, as a rule, life does not 

 greatly outlast the period of reproduction except in those species 

 which tend their young ; and as a matter of fact we find that this 

 is the case. 



All mammals and birds outlive the period of reproduction, but 

 this never occurs among insects except in those species which 

 tend their young. Furthermore, the life of all the lower animals 

 ceases also with the end of the reproductive period, as far as we 

 can judge. 



Duration of life is not however determined in this way, but 

 only the point at which its termination occurs relatively to the 

 cessation of reproduction. The duration itself depends first upon 

 the length of time which is required for the animal to reach 

 maturity that is, the duration of its youth, and, secondly, upon 

 the length of the period of fertility that is the time which is 

 necessary for the individual to produce a sufficient number of de- 

 scendants to ensure the perpetuation of the species. It is precisely 

 this latter point which is determined by external conditions. 



There is no species of animal which is not exposed to de- 

 struction through various accidental agencies by hunger or 

 cold, by drought or flood, by epidemics, or by enemies, whether 

 beasts of prey or parasites. We also know that these causes of 

 death are only apparently accidental, or at least that they can 

 only be called accidental as far as a single individual is concerned. 

 As a matter of fact a far greater number of individuals perish 



