228 THE CONTINUITY OF THE GERM-PLASM AS THE 



if we may further assume that the quantity of germ- plasm in the 

 segmentation nucleus varies in different cases ; then we should be 

 able to understand why one egg can only develope after fertiliza- 

 tion, while another can begin its development without fertilization, 

 but cannot finish it, and why a third is even able to complete its 

 development. We should also understand why one egg only passes 

 through the first stages of segmentation and is then arrested, while 

 another reaches a few more stages in advance, and a third de- 

 velopes so far that the embryo is nearly completely formed. These 

 differences would depend upon the extent to which the germ-plasm, 

 originally present in the egg, was sufficient for the development of 

 the latter ; development will be arrested as soon as the nucleoplasm 

 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 theoiy 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 Daphnidae 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 heredity, 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 which 

 I attempted to support this conclusion 1 . Such a conclusion may 

 be almost certainly accepted for the Daphnidae, because partheno- 

 genesis does not occur in their still living ancestors, the Phyllo- 

 pods, and especially the Estheridae. In Daphnidae the cause and 

 object of the phyletic development 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 



1 Weismann, 'Beitrage zur Naturgeschichte der Daphnoiden,' Leipzig, 1876-79, 

 Abhandlung VII, and ' Zeitschrift fur wissenschaftliche Zoologie,' Bd. XXXIII. 



