FOUNDATION OP A THEOKY OF HEBEDITY. 239 



suppose that parthenogenetic eggs must contain a larger amount of 

 nutritive material in order to facilitate the growth of the nucleus. 

 The parthenogenetic eggs of certain Daplmidae (Bylliotrephes, Poly- 

 phemus] are very much smaller than the winter-eggs, which require 

 fertilization, in the same species. It is also an error for Strasburger 

 to conclude that ' it has been established with certainty that favour- 

 able conditions of nutrition cause parthenogenetic development in 

 Daphnidae, while unfavourable conditions cause the formation of 

 eggs requiring fertilization.' It is true that Carl Diising 1 , in his 

 notable work upon the origin of sex, has attempted, in a most 

 ingenious manner, to prove, from my observations and experiments 

 on the reproduction of Daphnidae, ' that winter or summer-eggs are 

 formed according to the nutritive condition of the ovary.' I do 

 not, however, believe that he has succeeded in this attempt, and 

 at all events it is quite clear that the validity of such conclusions 

 is not fully established. I have observed that the maturing eggs 

 break up in the ovaries and are absorbed in those Daplmidae 

 (Sida] which are starved because sufficient food cannot be pro- 

 vided in captivity. Hence such animals live, as it were, at the 

 expense of their descendants ; but it would be quite erroneous 

 to conclude with Diising, from the similarity which such disap- 

 pearing egg-follicles bear to the groups of germ-cells which 

 normally break up in the formation of winter-eggs, that with 

 a less degree of starvation winter-eggs would have been formed. 

 Diising further quotes my incidental remark that the formation of 

 resting-eggs in Daphnia has been especially frequent in aquaria 

 ' which had been for some time neglected, and in which it was 

 found that a great increase in the number of individuals had 

 taken place.' He is entirely wrong in concluding that there 

 was any want of food in these neglected aquaria ; and if I had 

 foreseen that such conclusions would have been drawn, I might 

 have easily guarded against them by adding that in these very 

 aquaria an undisturbed growth of different algae was flourishing, 

 so that there could have been no deficiency, but, on the contrary, 

 a great abundance of nutritive material. I may add that since 

 that time I have conducted some experiments directly bearing upon 

 this question, by bringing virgin females as near to the verge of 



1 Carl Diising, ' Die Regulirung des Geschlechtsverhal trusses.' Jena, 1884. 



