278 DEATH 



forces, and for this reason alone it is necessary that 

 new and perfect individuals should continually arise 

 and take their place, and this necessity would re- 

 main even if the individuals possessed the power 

 of living eternally." " From this follows, on the 

 one hand the necessity of reproduction, and on the 

 other the utility of death. Worn-out individuals 

 are not only valueless to the species, but they are 

 even harmful, for they take the place of those which 

 are sound. Hence by the operation of natural 

 selection the life of our hypothetically immortal 

 individual would be shortened by the amount which 

 was useless to the species. It would be reduced to 

 a length which would afford the most favourable 

 conditions for the existence of as large a number as 

 possible of vigorous individuals at the same time." 



The passage just quoted is extremely ingenious, 

 but is hardly convincing, for it does not attempt to 

 explain the real nature of death, nor how it came 

 about in the first instance. The distinction between 

 somatic and reproductive cells is a real one in 

 Metazoa. The actual steps by which it was estab- 

 lished have yet to be traced : but it would seem 

 probable that in the earliest Metazoa all the cells 

 originally retained their reproductive power, and 

 that the process by which this power became re- 

 stricted to particular groups of cells was a gradual 

 one. The gemmules of sponges, or the statoblasts 

 of Polyzoa, are perhaps to be interpreted as 

 examples of the retention of reproductive power 

 by groups of cells which in allied animals have 

 become exclusively somatic in character ; and 



