IV.] FOUNDATION OF A THEORY OF HEREDITY. 247 



which have come under my notice, the females frequently enter 

 again upon the formation of parthenogenetic summer-eggs, 

 after having laid fertilized resting eggs, upon one or more 

 occasions. This is the case, for instance, in all the species of 

 Daphnia 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 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 \ but the former will, nevertheless, 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 deter- 

 mine that the sexual cells shall become sperm-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, anid 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 conditions, but in the constitution of the organs 

 concerned. 



^ The same fact has since been ascertained in species belonging to 

 several groups of animals. 



