REPRODUCTION AND HEKEDITY 265 



It is clear from what I have already said that the partheno- 

 genetic ' summer eggs ' serve chiefly the purpose of rapid 

 increase and distribution, but the fertilized ' winter eggs,' 

 able to resist equally well drought and frost and to develop 

 even after a rest period extending over years, serve to 

 preserve the species in times of necessity. It appears that 

 in the daphnids the number of the successive asexual genera- 

 tions is the lower the more frequently the colony of the 

 species concerned is visited by periods of destruction. Thus the 

 species which are threatened by severe frost only once in a year 

 have the largest number of successive parthenogenetic genera- 

 tions. These are, of course, above all those daphnids which 

 live in the large waters. On the other hand, those species of 

 water-fleas which have their domicile in shallow, easily drying 

 pools or ditches, have only very few, and sometimes only one, 

 generation of parthenogenesis which is immediately followed 

 by the appearance of males and, resulting from that, asexual 

 generation. 



According to the most recent investigation by Issakowitsch, 

 the causes that govern the question of sex lie exclusively in the 

 external conditions, i.e., in the changes of temperature and 

 the conditions of nutrition. In order to decide the question, 

 this investigator chose the method of experiment which naturally 

 offers the greatest prospect of success if a question is rationally 

 put. He started several cultures, one at a temperature of 

 + 24 C., one at room temperature, i.e., + 16 C., and one at 

 -h 8 C. The result of these experiments was that with decreas- 

 ing warmth the tendency towards the formation of sex-animals 

 increased, while with an increasing temperature the ova developed 

 into an asexual generation of females. 



In accordance with these results were the experiments made 

 by Issakowitsch with a view to determining the influences of 

 nutrition. Here, too, it was found that hunger-cultures produced 

 only sex-animals, quite independent of the number of the 

 asexual generations that had preceded them. Issakowitsch sums 

 up the results of his investigations in these words : "When the 

 nutrition of the maternal organism has become so low that it 

 is no longer able to provide the ovum with the food sufficient 

 for its development into a female, a less exacting male develops. 



