CONTROL OF HEREDITY: EUGENICS 387 



pure line always fluctuating about the mean 

 of that line (Fig. 44). 



Similarly Tower found in his studies on the 

 potato beetle that he was unable to shift the 

 mean or the extremes of any character by se- 

 lection of extreme forms of an inbred line. 



Pearl also made an extensive study of the 

 records of breeding experiments extending 

 over many years in which the attempt was 

 made to increase the egg-laying capacity of 

 hens by selecting in each generation for breed- 

 ing only those which had a high record for 

 egg production. It was found that certain 

 "blood lines" produced a larger number of 

 eggs than other lines, but by no amount of 

 selection was it possible to increase the egg 

 production within any line. 



On the other hand Castle strenuously de- 

 fends the importance of the selection of fluctu- 

 ations in "building up" a character. From 

 among the descendants of piebald rats and 

 rabbits he has selected through many genera- 

 tions those individuals showing the largest and 

 also those showing the smallest extent of color 

 in the coat and he has thus produced one line 



