lO THE DURATION OF LIFE. [I. 



mechanical process of regulation as that by which the structure 

 and functions of an organism are adapted to its environment. 



Assuming for the moment that these conclusions are valid, 

 let us ask how the duration of life of any given species can have 

 been determined by their means. In the first place, in regu- 

 lating duration of life, the advantage to the species, and not to 

 the individual, is alone of any importance. This must be 

 obvious to any one who has once thoroughly thought out the pro- 

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

 whether the individual lives longer or shorter, but it is of im- 

 portance that the individual should be enabled to do its work 

 towards the maintenance of the species. This work is repro- 

 duction, 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 ma}' be of 

 advantage to the species for a longer period if it not only pro- 

 duces oft'spring, but tends them for a longer or shorter time, 

 either by protecting, feeding, or instructing them. This last 

 dut}"" is not only undertaken by man, but also by animals, 

 although to a smaller extent; for Instance, birds teach their 

 3'oung 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 3'outh, 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 descendants to ensure the perpetuation of the species. It is 



