22 K. S. LASHLEY 



distinct lines of descent were represented. The complete his- 

 tory of selection is given in table 1. 



When approaching winter again threatened a reduction of the 

 food supply, selection was stopped and the initial numbers of 

 tentacles of all buds produced by the last selected generation 

 were recorded, until at least ten buds had been obtained from 

 each of the 48 lines. These were bred under conditions as nearly 

 uniform for the two groups as it is possible to make them for 

 animals with such complex food habits as Hydra. The data ob- 

 tained from these buds furnished the basis for a trial of the ef- 

 fects of the preceding selection. 



The entire experiment extended over somewhat more than a 

 year. After the test previously reported, selection was con- 

 tinued for an average of 17.13 generations in the group selected 

 for 6 or less tentacles and for an average of 9.38 generations in 

 the group selected for 7 or more, an average of 13.25 generations 

 for the two groups. The greater number of generations in the 

 group selected for few tentacles is due to the fact that buds with 

 few tentacles are produced early in each fraternity while buds 

 with many tentacles appear only after the parents have fully 

 matured. Before the first test an average of 6 selected genera- 

 tions was obtained and this, with the 13.25 later selections, gives 

 an average of 19.25 generations rigidly selected during the ex- 

 periment. The average number of tentacles of all selected par- 

 ents of the minus-selected group is 5.567, that of the plus-se- 

 lected group is 6.888, giving an average difference of 1.321 ten- 

 tacles per generation as the extent of selection. A total of 366 

 buds was obtained from the last selected generation of the group 

 selected for 6 or less tentacles, and of 358 in the group selected 

 for 7 or more. The distribution of variations in the initial num- 

 ber of tentacles of these unselected buds is given in table 2. 

 The constants determined from the table are: 



For the group selected for Mean <t 



6 or less 6.584 ± 0.022 0.6408 tentacles 



For the group selected for 



7 or more 6.544*0.023 0.6569 tentacles 



Difference 0.040*0.031 



