III.] LIFE AND DEATH. 155 



original condition of the Polyplastides— that of a single cell— 

 and upon this alone depends the fundamental law of biogenesis. 

 This law is therefore confined to the Polyplastides, and does 

 not apply to the Monoplastides ; and Gotte's suggestion that 

 the latter fall back into the primitive condition of the organism 

 during their encystment (rejuvenescence), finds no support in 

 this aspect of the question. 



I have on a previous occasion^ referred the utility of death to 

 the ultimate fact that the unending life of the Metazoan body 

 would be a useless luxury, and to the fact that the individuals 

 would necessarily become injured in the course of time, and 

 would be therefore ' not only valueless to the species, but . . . 

 even harmful, for they take the place of those which are sound ' 

 (1. c, p. 24). I might also have said that such damaged individuals 

 would sooner or later fall victims to some accidental death, so 

 that there would be no possibility of real immortality, I now 

 propose to examine this statement a little more closely, and to 

 return to a question which has already been alluded to before. 



It is obvious that the advantages above set forth did not form 

 the motive which impelled natural selection to convert the im- 

 mortal life of the Monoplastides into the life of limited duration 

 possessed by the Heteroplastides, or more correctly, which led 

 to the restriction of potential immortality to the reproductive 

 cells of the latter. It is at any rate theoretically conceivable 

 that a struggle might arise between the mortal and immortal 

 individuals of a certain Metazoan species, and that natural 

 selection might secure the success of the former, because the 

 longer the immortal individuals lived, the more defective they 

 became, and as a result gave rise to weaker oftspring in 

 diminished numbers. Probably no one would be bold enough 

 to suggest such a crude example of natural selection. And 

 yet I venture to think that the principle of natural selection is 

 here also to be taken into account, and even plays, although in 

 a negative rather than a positive way, a very essential part 

 in determining the duration of life in the Metazoa. 



When the somatic cells of the first Heteroplastides ceased to 

 be immortal, such a loss would not in any way have precluded 

 them from regaining this condition. Just as, with the differen- 

 tiation of the first somatic cells of the lowest Heteroplastides, 



^ See the first essay on ' The Duration of Life,' p. 23 d seq. 



