438 THE WONDER OF LIFE 



pression of alcoholism, the second great factor in the pro- 

 duction of senile degeneration of the arteries, will produce 

 a still more marked extension of the term of life. Scien- 

 tific study of old age and of the means of modifying its 

 pathological character will make life longer and happier '. 

 He also quotes the theoretically simple conclusion of Pfliiger 's 

 essay on The Art of Prolonging Human Life ' Avoid the 

 things that are harmful and be moderate in all things '. 

 Attempts have often been made to correlate the duration 

 of an animal's life with its structural or functional character- 

 istics, and up to a certain point this way of looking at it is 

 useful. For the living creature is a consistent unity, and 

 its length of life must be correlated with its whole being. 

 It is evident that a very large animal will not be a very 

 short-lived animal, but the difficulty is that animals equal 

 in size are often very far from equal in length of life. It is 

 natural that a relatively easy-going animal like a sea-anemone 

 should be able to survive very much longer than an intensely 

 living insect, but the difficulty is that equally active insects 

 may differ greatly in their length of life. In his famous 

 essay On the Duration of Life (1881) Weismann considered 

 the various attempts to correlate length of life with size, 

 with intensity of life, with the duration of the growing 

 period, and so on, but found that none of the correlations 

 could be generalized. He was led to the conclusion that 

 length of life, like size, is an adaptive character gradually 

 denned in relation to the conditions of life of the species. 

 If a species is endangered in the struggle for existence, 

 and shows a decline of population too high a death-rate 

 in proportion to its birth-rate then, seeing that length 

 of life is a very variable quality, the species may be saved 

 by the Natural Selection of the longer-lived variants, who 



