I.] THE DURATION OF LIFE. 9 



before reproduction begins. The period before reproduction is 

 however only part of the whole life of an animal, which of course 

 extends over the total period during which the animal exists. 



Hitherto it has always been assumed that the duration of 

 this total period is solely determined by the constitution of 

 the animal's body. But the assumption is erroneous. The 

 strength of the spring which drives the wheel of life does not 

 solely depend upon the size of the wheel itself or upon the 

 material of which it is made ; and, leaving the metaphor, dura- 

 tion of life is not exclusively determined by the size of the 

 animal, the complexity of its structure, and the rate of its meta- 

 bolism. The facts are plainly and clearly opposed to such a 

 supposition. 



How, for instance, can we explain from this point of view the 

 fact that the queen-ant and the workers live for many years, 

 while the males live for a few weeks at most ? The sexes are 

 not distinguished by any great difference in size or complexity 

 of body, or in the rate of metabolism. In all these three par- 

 ticulars they must be looked upon as precisely the same, and 

 yet there is this immense difference between the lengths of their 

 lives. 



I shall return later on to this and other similar cases, and for 

 the present I assume it to be proved that physiological con- 

 siderations alone cannot determine the duration of life. It is 

 not these which alone determine the strength of the spring 

 which moves the machinery of life ; we know that springs of 

 different strengths may be fixed in machines of the same kind 

 and quality. This metaphor is however imperfect, because we 

 cannot imagine the existence of any special force in an 

 organism which determines the duration of its life ; but it is 

 nevertheless useful because it emphasises the fact that the 

 duration of life is forced upon the organism by causes out- 

 side itself, just as the spring is fixed in its place by forces 

 outside the machine, and not only fixed in its place, but chosen 

 of a certain strength so that it will run down after a certain 

 time. 



To put it briefly, I consider 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, and that it is determined by precisely the same 



