GROWTH AND SENESCENCE. 873 



elusive results have not been reached. Assuming that the poten- 

 tial immortality exhibited by the reproductive cells was originally a 

 general property of protoplasm, Weissman conceives that the phe- 

 nomenon of senescence and death exhibited by other cells, somato- 

 plasm, is a secondary property, which was acquired as a result of 

 variation and was preserved by natural selection because it is an 

 advantage in the propagation of the species. An actual immor- 

 tality of the entire organism, that is, the property of indefinite 

 existence except as death might be caused by accidental occur- 

 rences of various kinds would be a disadvantage in many ways. 

 The vast increase in the number of individuals might exceed the 

 capacity of nature to provide for, the retention of the maimed and 

 imperfect would make many useless mouths to feed, and perhaps 

 the evolution of higher and more perfect forms by the slow action 

 of variation and natural selection would be retarded. From this 

 point of view senility and natural death constitute a beneficial 

 adaptation, acquired because of its utility to the race, on the one 

 hand, and, on the other, because, after the beginning of a differen- 

 tiation in function among the cells, the possession of immortality 

 by all the cells was no longer of any value to the race, and therefore 

 was not brought under the preserving influence of natural selection. 



