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 hfe. Scien- 

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

 pathological character will make hfe 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 hving 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-hved animal, but the difficulty is that animals equal 

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

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

 should be able to survive very much longer than an intensely 

 hving 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 hfe with size, 

 with intensity of hfe, with the duration of the growing 

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

 could be generahzed. He was led to the conclusion that 

 length of hfe, hke size, is an adaptive character gradually 

 defined in relation to the conditions of hfe of the species. 

 If a species is endangered in the struggle for existence, 

 and shows a dechne of population — ^too high a death-rate 

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

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

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



