I.] THE DURATION OF LIFE. 7 



able to become even comparatively large unless endowed with 

 a comparatively long duration of life. 



Apart from all other reasons, no one could imagine that the 

 gigantic body of an elephant could be built up like that of a 

 mouse in three weeks, or in a single day like that of the larva 

 of certain flies. The gestation of an elephant lasts for nearly 

 two j^'ears, and maturity is onl}' reached after a lapse of about 

 twenty-four years. 



Furthermore, to ensure the preservation of the species, a 

 longer time is required by a large animal than by a small one, 

 when both have reached maturity. Thus Leuckart and later 

 Herbert Spencer have pointed out that the absorbing surface 

 of an animal only increases as the square of its length, while 

 its size increases as the cube ; and it therefore follows that the 

 larger an animal becomes, the greater will be the difficulty ex- 

 perienced in assimilating any nourishment over and above that 

 which it requires for its own needs, and therefore the more 

 slowly will it reproduce itself. 



But although it may be stated generally that the duration of 

 the period of growth and length of life are longest in the largest 

 animals, it is nevertheless impossible to maintain that there is 

 any fixed relation between the two; and Flourens was mistaken 

 when he considered that the length of life was always equiva- 

 lent to five times the duration of the period of growth. Such a 

 conclusion might be accepted in the case of man if we set his 

 period of growth at twenty years and his length of life at a 

 hundred ; but it cannot be accepted for the majority of other 

 Mammalia. Thus the horse lives from forty to fifty years, and 

 the latter age is at least as frequently reached among horses as 

 a hundred years among men ; but the horse becomes mature 

 in four j^ears, and the length of its life is thus ten or twelve 

 times as long as its period of growth. -- 



The second factor which influences the duration of life is 

 purely ph3'siological : it is the rate at which the animal lives, 

 the rapidity with which assimilation and the other vital pro- 

 cesses take place. Upon this point Lotze remarks in his 

 Microcosmus — ' Active and restless mobility destroys the or- 

 ganized body : the swift-footed animals hunted by man, as also 

 dogs, and even apes, are inferior in length of life to man 

 and the larger beasts of prey, which satisfy their needs by 



