THE DUEATION OF LIFE. 



the spring 1 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, duration of life is not ex- 

 clusively determined by the size of the animal, the complexity 

 of its structure, and the rate of its metabolism. 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 dis- 

 tinguished by any great difference in size or complexity of body, 

 or in the rate of metabolism. In all these three particulars 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 deter- 

 mines the duration of its life ; but it is nevertheless useful because 

 f it emphasises the fact that the duration of life is forced upon 

 the organism by causes outside 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 de- 

 pendent 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 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 regulating- 

 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 process oi' 



