AUGUST WEI S MANN 351 



in their still living ancestors, the Phyllopods, and especially the Es- 

 theridae. In Daphnidae the cause and object of the phyletic de- 

 velopment of parthenogenesis may be traced more clearly than in any 

 other group of animals. In Daphnidae 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 egg 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, viz., that fluctua- 

 tions occur in the quantity of germ-plasm in the segmentation nucleus ; 

 for there can never be absolute equality in any single part of different 

 individuals. As soon therefore as these fluctuations become so great 

 that parthenogenesis is produced, it may become, by the operation of 

 natural selection, the chief mode of reproduction of the species or of 

 certain generations of the species. In order to place this theory upon 

 a firm basis, we have simply to decide whether the quantity of germ- 

 plasm contained in the segmentation nucleus is the factor which de- 

 termines development ; although for the present it will be sufficient if 

 we can render this view to some extent probable, and show that it 

 is not a contradiction of established facts. 



At first sight this hypothesis seems to encounter serious difificulties. 

 It will be objected that neither the beginning nor the end of embryonic 

 development can possibly depend upon the quantity of nucleoplasm in 

 the segmentation nucleus, since the amount may be continually in- 

 creased by growth; for it is well known that during embryonic de- 

 velopment the nuclear substance increases with astonishing rapidity. 

 By an approximate calculation I found that in the egg of a Cynips the 

 quantity of nuclear substance present at the time when the blastoderm 

 was about to be formed, and when there were twenty-six nuclei, w^as 

 even then seven times as great as the quantity which had been con- 

 tained in the segmentation nucleus. How then can we imagine that 

 embryonic development would ever be arrested from want of nuclear 



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