SELECTION WITHIN PURE LINES OF PESTALOZZIA 



179 



is greatest in the fifth period, falling to a minus value in the sixth period. 

 The mean for shell diameter is the only one which is equal to three times 

 its probable error. Apparently the two lines are not demonstrably dis- 

 tinct. 



In family 314 also, JENNINGS attempted to isolate lines distinct for spine 

 number. Selection was continued for seven periods in the same way as 

 within family 304. The means of the "high"-selected line and of the 

 "low"-selected line were separately computed for each generation. The 

 differences between the two lines varies from generation to generation as 

 in family 304. It is noteworthy that the greatest divergence (1.24 spines) 

 appears in the first selection period; the lowest deviation in the sixth period. 

 The difference between the mean numbers of spines for the two groups is 



TABLE 11 



Differences between "low" and "high" selected groups of family 

 303 of Difflugia. 



.42 .2 5 spines. Since the divergence is less than twice its probable 

 error, we can only conclude that the lines are not distinct and that the 

 apparent difference is due to the independent fluctuation of the two groups. 



In another and more extensive experiment, JENNINGS sought to isolate 

 distinct lines by selection within family 326. Again spine number was the 

 character selected, and selection was continued for twelve periods. For 

 the first six periods selection was ineffective and both plus and minus diver- 

 gences between the "low" and the "high" groups appeared. 



As in one of HEGNER'S experiments, a change in the method of selection, 

 made at this time, prevents us from knowing what would have resulted 

 from further selections in which large numbers of individuals were used. 

 Following the change in method two series distinct for spine number were 



GENETICS 7: Mr 1922 



