JoLY — The Abundance of Life. 75 



action of natural selection, which led to a differentiation into 

 perishable somatic cells and immortal reproductive cells. The 

 time-limit of any particular organism ultimately depends upon 

 the number of somatic cell- generations and the duration of 

 each generation. These quantities are " predestined in the germ 

 itself" which gives rise to each individual. "The existence 

 of immortal metazoan organisms is conceivable," but their 

 capacity for existence is influenced by conditions of the ex- 

 ternal world; this renders necessary the process of adaptation. 

 In fact, in the differentiation of somatic from reproductive cells, 

 material was provided upon which natural selection could operate 

 to shorten or lengthen the life of the individual in accordance with 

 the needs of the species. The soma is in a sense " a secondary 

 appendage of the real bearer of life — the reproductive cells." The 

 somatic cells probably lost their immortal qualities, on this immor- 

 tality becoming useless to the species, by the cessation of the 

 operation of natural selection. Their mortality may have been a 

 mere consequence of their differentiation {loc. cit., p. 140), itself 

 due to natural selection. "Natural death was not," in fact, 

 "introduced from absolute intrinsic necessity inherent in the 

 nature of living matter, but on grounds of utility, that is from 

 necessities which sprang up, not from the general conditions of life, 

 but from those special conditions which dominate the life of multi- 

 cellular organisms." 



Into this question we cannot fully enter here ; it is not so 

 much the precise causes which may have operated to introduce 

 death as the fact of the fundamentally immortal nature of life 

 with which we are concerned. Of course on the assumption of the 

 complete isolation of the germ from somatic influences the view 

 adopted in the foregoing pages as to the cause of inherited death 

 must be erroneous. On this view of heredity, however, authorities 

 differ; indeed, I believe, few go so far as Professor Weismann. 

 On the other hand, the probability of such events arising as we 

 have supposed appears when we consider the effects of the un- 

 checked multiplication of any organism. Even where natural death 

 is continually removing the useless individuals the struggle between 

 those that remain is everywhere intense — as among the ants heaped 

 in living mounds among the pines. 



Further, natural selection will in all cases favour the fertility 



