FOUNDATION OF A THEORY OF HEBEDITY. 241 



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 colonies at once, such as the winter cold, or the drying- 

 up of small ponds in summer. It is unnecessary for me to contro- 

 vert Diising in detail, as I have already taken this course in the 

 case of Herbert Spencer 1 , who had also formed the hypothesis 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 Moina. Females of this genus which 

 possess sexual eggs in their ovaries, and which would have con- 

 tinued 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, deter- 

 mines the formation of parthenogenetic eggs. This apparent con- 

 clusion 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 

 always 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 $o 

 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 Dapknidae which have come under my notice, the 

 females frequently enter again upon the formation of partheno- 

 genetic summer-eggs, after having laid fertilized resting eggs, 

 upon one or more occasions. This is the case, for instance, in all 

 the species of DapJmia with which I am acquainted, and such 

 a fact at once proves that the abnormal increase in nutrition 

 produced by the absorption of winter-eggs cannot be the cause of 

 the succeeding parthenogenesis. It also supports the proof that 



1 Weismann, ' Daphniden,' Abhandlung, VII. p. 329; Herbert Spencer, 'The 

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



B 



