242 THE CONTINUITY OF THE GERM-PLASM AS THE 



a high or low nutritive condition of the whole animal can have 

 nothing- to do with the kind of eggs which are produced, for in 

 the above-quoted instance, the nutrition has remained the same 

 throughout, or at all events has not been increased. It is erroneous 

 to always look for the explanation of the mode of egg-formation in 

 the direct action of external causes. Of course there must be 

 direct causes which determine that one germ shall become a winter- 

 egg, and another a summer-egg ; but such causes do not lie outside 

 the animal, and have nothing to do with the nutritive condition of 

 the ovary : they are to be found in those conditions which we are 

 not at present able to analyze further, and which we must, in the 

 meantime, call the specific constitution of the species. In the young 

 males of Daphnidae the testes have precisely the same appearance 

 as the ovaries of the young females x , but the former will, never- 

 theless, produce sperm-cells and not ova. In such cases the sex of 

 the young individual can always be identified by the form of the 

 first antenna and of the first thoracic appendage, both of which 

 are always clawed in the male. But who can point to the direct 

 causes which determine that the sexual cells shall become spcrm- 

 cells in this case, and not egg-cells ? Does the determining cause 

 depend on the conditions of nutrition ? Or, again, in the females, 

 can the state of nutrition determine that the third out of a group 

 of four germ-cells shall become an egg-cell, and that the others 

 shall break up to serve as its food ? 



It is, I think, clear that these are obvious instances of the general 

 conclusion that the direct causes determining the direction of 

 development in each case are not to be looked for in external con- 

 ditions, but in the constitution of the organs concerned. 



We arrive at a like conclusion when we consider the quality of 

 the eggs which are produced. The constitution of one species of 

 Moina contains the cause which determines that each individual 

 shall produce winter-eggs only, or summer-eggs only; while in 

 another species the transition from the formation of sexual eggs to 

 the formation of summer-eggs can take place, but only when the 

 winter-egg remains unfertilized. The latter case appears to me to 

 be notably a special adaptation, in this and other species, to the 

 deficiency of males, which is apt to occur. At all events, it is 



1 The same fact has since been ascertained in specie* belonging to several 

 of aniin.-iK 



