28o DEATH 



the attacks of enemies, several years may elapse 

 before enough eggs are produced to ensure survival 

 of the species ; and the life of the adult is conse- 

 quently prolonged for many years. A further 

 lengthening of life takes place in mammals and other 

 animals which tend or rear their young, either by 

 retaining the eggs or embryos within their bodies 

 for a longer or shorter portion of their development, 

 or by protecting and feeding the young after birth. 



The whole subject thus opened up is of extreme 

 interest, but it would be impossible to treat it 

 adequately on the present occasion. Weismann 

 himself has accumulated a large number of statistics 

 with regard to different groups of animals, which 

 lead him to the conclusion that " the end of the 

 reproductive period is usually more or less coinci- 

 dent with death ; " while in cases in which the 

 duration of life is prolonged, owing to the parent 

 tending or nursing the young after birth, he con- 

 cludes that " as a general rule the increase in 

 length of life is exactly proportional to the time 

 which is demanded by the care of the young." 



With regard to the actual cause of death in 

 Metazoa, Weismann suggested in his earlier essay 

 published in 1881, that death may be due to the 

 somatic cells losing the power of reproduction by 

 cell-division after a certain number of generations. 

 In 1883 in a further essay on life and death, he 

 repeated this suggestion and developed it more 

 fully. After referring to his former view that the 

 varying duration of life in the animal kingdom " is 

 determined in different species by the varying 



