240 Experimental Zoology 



results. Weismann cites the following experiments in support 

 of his statement. Six young daphnias, bom parthenogenetically 

 from the same mother, were isolated each in a separate glass. 

 Fourteen days later No. i had produced three parthenogenetic 

 young; No. 2, eight of the same; No. 3, six; No. 4, five; 

 No. 5, seven 'sexual females and three males; No. 6, five 

 parthenogenetic females. 



In another experiment a male and a female were placed in the 

 same small glass on March 5. They belonged to the fourth 

 generation from the winter egg. They w^ere not observed to 

 pair. Three weeks later 8 parthenogenetic females were pres- 

 ent. After another 3 weeks 50 young were counted, of which 

 many were males. Ten days later there were 18 sexual females 

 and 5 males. A week later numerous young were found, and 9 

 days later still, when they w^ere counted, there were found 3 

 females wdth winter eggs, 20 parthenogenetic females, i male, 

 and 79 young. In this experiment the sexual forms did not ap- 

 pear for some time ; but in a parallel experiment beginning with 

 25 males and 8 females (March 24) the sexual forms appeared 

 at once. It is difficult to understand why Weismann should con- 

 clude from these experiments that the hfe-cycle is independent 

 of the environment. It is more probable that there was present 

 some unknown condition that caused the difference in the results. 

 One such condition may have been the crowding with the conse- 

 quent decrease in the available food. 



Weismann has laid a good deal of emphasis on the differences 

 in the Hfe history of species of Daphnidae that live under different 

 conditions. Thus monocyclic species in which the sexual genera- 

 tion appears only once in the course of the year are found in 

 large bodies of water that never dry up, — in lakes, ponds, 

 swamps, and in the ocean. Sida crystahina is a species of this 

 kind. Throughout the summer only parthenogenetic indi- 

 viduals appear, and in the autumn males and sexual females. 

 PolycycHc species inhabit puddles and small swamps that are 

 often dried up. The winter eggs can withstand the drying, but 

 not the parthenogenetic adults; therefore in order to exist in 



