29c SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



Edouard van Beneden expresses this idea in the following 

 words : — * II semble que la faculte que possedent les cellules, 

 de se multiplier par division soit limitee : il arrive un moment 

 ou elles ne sont plus capables de se diviser ulterieurement, 

 a moins qu'elles ne subissent le phcnomene du rajeunissement 

 par le fait de la fecondation. Chez les animaux et les plantes 

 les seules cellules capables d'etre rajeunies sont les oeufs ; les 

 seules capables de rajeunir sont les spermatocytes. Toutes 

 les autres parties de I'individu sont vouees a la mort. La fe- 

 condation 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 &gg must be fertilized, was for- 

 mulated 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 

 rejuvenescence, although very ingenious, has something un- 

 certain about it, and can hardly be brought into accordance 

 with the usual conception of life as based upon phj-sical and 

 mechanical forces. How can any one imagine that an Infuso- 

 rian, which by continued division had lost its power of repro- 

 duction, 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 



