THE DURATION OF LIFE. 7 



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 years, and 

 maturity is only 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 in- 

 creases 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 experienced 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 equivalent 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 years, 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 

 physiological : it is the rate at which the animal lives, the rapidity 

 with which assimilation and the other vital processes take place. 

 Upon this point Lotze remarks in his Microcosmus ' Active and 

 restless mobility destroys the organized 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 a few vigorous efforts.' ' The inertness of the Amphibia is, on 

 the other hand, accompanied by relatively great length of life.' 



There is certainly some truth in these observations, and yet it 

 would be a great mistake to assume that activity necessarily implies 



