XII.] CONJUGATION AND SEXUAL REPRODUCTION. 209 



life could not arise without amphimixis, i. e. that reproduction 

 was always associated with fertilization. 



As I have already said, I regard the power of living on 

 indefinitely when the vital processes have once begun, as the 

 fundamental peculiarity of living matter. But this principle 

 fails in so many organisms that its very existence was, for a 

 long time, entirely overlooked, and hence the limited duration 

 of life, together with its termination in natural death, were 

 regarded as laws dominating all living beings. Undoubtedly 

 the capability of unending life has been lost in very many 

 organisms of greater or less complexity, and it is, I think, 

 interesting to trace the causes which have led to this loss, and 

 have rendered it necessary and even advantageous. 



I will very briefly recall the manner in which the mortality 

 of Metazoa may be explained, for this has been treated in 

 earlier essays, and my views on the point have undergone no 

 essential change. The immortality of Protozoa was carried 

 over to the germ-cells of Metazoa and Metaphyta whether 

 they are sexual, i. e. adapted for amphimixis, or not. In either 

 case they posses potential immortality, i. e. they can, under 

 the conditions imposed upon them by their constitution, con- 

 tinue without limit to exhibit the phenomena of life. The 

 conditions under which the sexually differentiated germ-cells 

 live include the fusion of two in amphimixis, but it is not 

 generally included among the conditions imposed upon agamic 

 or parthenogenetic germ-cells, and, when imposed, it only 

 requires to be fulfilled again after the lapse of a certain 

 period. 



I will not repeat the reasons which, I believe, explain why 

 the Metazoan soma has been permitted to lose, or has been 

 compelled to lose, the power of unending life, and why natural 

 death has made its appearance. I will only call to mind the fact 

 that, according to the principle of panmixia, every faculty must 

 disappear as soon as it ceases to be necessary. As soon as differ- 

 entiation into soma and germ-cells, — viz. the formation of 

 Metazoa and Metaphyta,— took place, this principle began to 

 act, for the species could be maintained without the immor- 

 tality of single individuals. Whether this immortality is in any 

 way compatible with the high differentiation of the Metazoan 

 body, and if so, whether it would be useful, are questions 



VOL. II. p 



