246 CON'TTNUITV OF THE GERM-PLASM AS THE [IV. 



very Daphnidac reproduce themselves parthenogenetically. I 

 am far from believing that my experiments on Daphnidae are 

 exhaustive and final, and I have stated this in my published 

 writings on the subject ; but it seems to me that I have estab- 

 lished the fact that direct influences, whether of food or of 

 temperature, acting upon single individuals, do not determine 

 the kind of eggs which are to be produced ; but that such a 

 decisive influence is to be found in the indirect conditions of 

 life, and especially in the average frequency of the recurrence 

 of adverse circumstances which kill whole colonics at once, 

 such as the winter cold, or the drying-up of small ponds in 

 summer. It is unnecessary for me to controvert Diising in 

 detail, as I have already taken this course in the case of Herbert 

 Spencer \ who had also formed the hj^pothesis that diminished 

 nutrition causes sexual reproduction. 



One of my observations seems, indeed, to support such a 

 view, but only when it is considered as an isolated example. 

 I refer to the behaviour of the genus Moiua. Females of this 

 genus which possess sexual eggs in their ovaries, and which 

 would have continued to produce such eggs if males had been 

 present, enter in the absence of the latter upon the formation 

 of parthenogenetic summer-eggs, that is, if the sexual eggs 

 have not all been extruded, but have been re-absorbed in the 

 ovary. At first sight, indeed, such a result appears to indicate 

 that the increase in nutrition, produced by the breaking-up of 

 the large winter-egg in the ovary, determines the formation of 

 parthenogenetic eggs. This apparent conclusion seems to be 

 further confirmed by the following fact. The transition from 

 sexual to parthenogenetic reproduction only occurs in one 

 species of Moina {M. rectirostris), but in this species it occurs 

 always and without exception, while in the other species which 

 I have investigated [M. paradoxa), winter-eggs, when once 

 formed, are alwa^^s laid, and such females can never produce 

 summer-eggs. But in spite of this fact, Diising is mistaken 

 when he explains the continuous formation of sexual eggs in 

 the latter species as due to the absence of any great increase 

 in the amount of nutrition, such as would have followed if the 

 egg had broken up in the ovary. In many other Daphnidae 



' Weismann, ' Daphnidcn,' Abhandlung, VII. p. 329; Herbert 

 Spencer, 'The Principles of Biology,' 1864, vol. i. pp. 229, 230. 



