LIFE AND DEATH. 153 



development ; for the coalescence of entire multicellular organisms 

 in such a manner that fusion would only take place between equi- 

 valent cells, would seem to be impracticable. In the necessity 

 for sexual reproduction, there is therefore also implied the ne- 

 cessity for reverting 1 to the 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 Poly- 

 plastides, 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 l 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 ex- 

 amine 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 in- 

 dividuals lived, the more defective they became, and as a result gave 

 rise to weaker offspring 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. 

 1 See the first essay on 'The Duration of Life,' p. 23 et seq. 



