DEATH 279 



similar instances could be quoted from the vegetable 

 kingdom. 



Weismann's contention that the reproductive 

 cells of Metazoa are immortal in the same sense as 

 an Amoeba, must also be admitted to be established. 

 He however leaves altogether undecided the ques- 

 tion of the way in which death of the somatic cells 

 arose in the first instance ; and we shall find later 

 on that there are strong grounds for holding that 

 natural death appeared first, not as Weismann sup- 

 posed among Metazoa, but in the Protozoa them- 

 selves. 



Before considering these more recent aspects of 

 the problem it will be well to refer briefly to Weis- 

 mann's views, which have already been mentioned, 

 in regard to the causes determining the duration of 

 life in different animals. According to Weismann 

 the duration of life in a given case is that which is 

 most advantageous for the species. In the simpler 

 cases death occurs at the close of the reproductive 

 period. In the silkworm moth, or the May fly, the 

 adult existence is only of a few hours' duration ; 

 the insect laying all its eggs simultaneously, and 

 then dying. In other insects, as in many of the 

 hawkmoths, and in most butterflies, the eggs are 

 laid at intervals and in different places ; in such 

 cases the life of the adult is prolonged until a 

 sufficient number of eggs have been laid to ensure 

 the perpetuation of the species, and then the insect 

 dies. In birds, owing to the small number of the 

 eggs that are produced at any one time, and the 

 great destruction to which the eggs are liable from 



