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this simple fact that life has lasted long enough for 

 the parents to produce offspring. The consideration 

 of this fact has led certain naturalists to the supposi- 

 tion that reproduction is the cause of the termination 

 of life ; but it is not, it seems to me, at all to be so 

 interpreted. We know, in a general way, that large 

 animals live longer than small ones. The elephant is 

 longer lived than the horse, the horse than the mouse, 

 the whale than the fish, 1 the fish than the insect, and 

 so on through innumerable other instances. At first 

 this seems a promising clue, but if we think a moment 

 longer we recognise quickly the fact that a parrot, 

 which is much smaller than a dog, may live one 

 hundred years, whereas a dog is very old at twenty. 

 There are insects which live for many years, like the 

 seventeen-year locusts, and others which live but a 

 single year or a fraction even of one year, and yet the 

 long-lived and the short-lived may be of the same 

 size. It is evident, therefore, that size is not in itself 

 properly a measure of the length of life. 2 Another 

 supposition, which at first sounds very attractive, is 

 that which explains the duration of life by the rate of 

 wear, of the using up, of the wearing out, of the 

 body. This theory has been particularly put forward 

 by Professor Weissmann, who in his writings calls it 

 the Abnutzungstheorie the theory of the wearing out 

 of the body. But the body does not really wear out 



1 But there are some species of fish which outlive whales ; thus the European 

 carp is said to live more than a century. 



2 See Appendix No. IV, F. A. Lucas's letter. 



