Edouard van Beneden expresses this idea in the following words:—‘Il semble que la faculté que possèdent les cellules, de se multiplier par division soit limitée: il arrive un moment où elles ne sont plus capables de se diviser ultérieurement, à moins qu’elles ne subissent le phénomène du rajeunissement par le fait de la fécondation. Chez les animaux et les plantes les seules cellules capables d’être rajeunies sont les œufs; les seules capables de rajeunir sont les spermatocytes. Toutes les autres parties de l’individu sont vouées à la mort. La fécondation est la condition de la continuité de la vie. Par elle le générateur échappe à la mort’ (l. 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 (l. 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.