IV.] FOUNDATION OF A THEORY OF HEREDITY. 233 



depend upon the extent to which the germ-plasm, originally 

 present in the ^gg, was sufficient for the development of the 

 latter; development will be arrested as soon as the nucleo- 

 plasm is no longer capable of producing the succeeding stage, 

 and is thus unable to enter upon the following nuclear division. 



From a general point of view such a theory would explain 

 many difficulties, and it would render possible an explanation 

 of the phyletic origin of parthenogenesis, and an adequate 

 understanding of the strange and often apparently abrupt and 

 arbitrary manner of its occurrence. In my works on Daplniidae 

 I have already laid especial stress upon the proposition that 

 parthenogenesis in insects and Crustacea certainly cannot be 

 an ancestral condition which has been transmitted by hcrcdit}', 

 but that it has been derived from a sexual condition. In what 

 other way can we explain the fact that parthenogenesis is 

 present in certain species or genera, but absent in others 

 closely allied to them ; or the fact that males are entirely 

 wanting in species of which the females possess a complete 

 apparatus for fertilization ? I will not repeat all the arguments 

 with w^hich I attempted to support this conclusion ^ Such a 

 conclusion may be almost certainly accepted for the Dap/inidaey 

 because parthenogenesis does not occur in their still living 

 ancestors, the Phyllopods, and especially the Estheridae. In 

 DapJinidae the cause and object of the phyletic development of 

 parthenogenesis may be traced more clearly than in any other 

 group of animals. In Daphtiidae we can accept the conclusion 

 with greater certainty than in all other groups, except perhaps 

 the Aphidae, that parthenogenesis is extremely advantageous 

 to species in certain conditions of life ; and that it has only 

 been adopted when, and as far as, it has been beneficial ; and 

 further, that at least in this group parthenogenesis became 

 possible, and was adopted, in each species as soon as it became 

 useful. Such a result can be easily understood if it is only the 

 presence of more or less germ-plasm which decides whether an 

 Qgg is, or is not, capable of development without fertilization. 



If we now examine the foundations of this hypothesis we 

 shall find that we may at once accept one of its assumptions, 



^ Weismann, ' Beitriige zur Naturgeschichte der Daphnoidcn,' Leipzig. 

 1876-79, Abhandlung VII, and ' Zeitschrift fiir wisscnschaftliche Zoo- 

 logie/ Bd. XXXIII. 



