IN THE THEORY OF NATURAL SELECTION. 283 



Chez les animaux et les plantes les seules cellules capables d'etre 

 rajeunies sont les ceufs ; les seules capables de rajeunir sont les 

 spermatocytes. Toutes les autres parties de 1'individu sont vouees 

 a la mort. La fecondation est la condition de la continuite de la 

 vie. Par elle le generateur echappe a la mort ' (1. c., p. 405). 

 Victor Hensen thinks it possible that the germ and its products 

 are prevented from dying by means of normal fertilization : he 

 says that the law which states that every egg must be fertilized, 

 was formulated before the discovery of parthenogenesis and cannot 

 now be maintained, but that we are nevertheless compelled to 

 assume that even the most completely parthenogenetic species 

 requires fertilization after many generations (1. c., p. 236). 



If the theory of rejuvenescence be thoroughly examined, it will 

 be found to be nothing more than the expression of the fact that 

 sexual reproduction persists without any ascertainable limit. From 

 the fact of its general occurrence, the conclusion is, however, drawn 

 that asexual reproduction could not persist indefinitely as the 

 only mode of reproduction in any species of animal. But proofs 

 in support of this opinion are wanting, and it is very probable 

 that it would never have been advanced if it had been possible 

 to explain the general occurrence of sexual reproduction in any 

 other way, if we had been able to ascribe any other significance 

 to this pre-eminently important process. 



But quite apart from the fact that it is impossible to bring 

 forward any proofs, the theory of rejuvenescence seems to me to 

 be unsatisfactory in other ways. The whole conception of re- 

 juvenescence, although very ingenious, has something uncertain 

 about it, and can hardly be brought into accordance with the 

 usual conception of life as based upon physical and mechanical 

 forces. How can any one imagine that an Infusorian, which by 

 continued division had lost its power of reproduction, could regain 

 this power by forming a new individual, after fusion with another 

 Infusorian, which had similarly become incapable of division? 

 Twice nothing cannot make one. If indeed we could assume that 

 each animal contained half the power necessary for reproduc- 

 tion, then both together would certainly form an efficient whole ; 

 but it is hardly possible to apply the term rejuvenescence to a 

 process which is simply an addition, such as would be attained 

 under other circumstances by mere growth ; neglecting, for the 



