268 Heredity and Environment 



was made to increase the egg-laying capacity of hens by selecting 

 for breeding in each generation 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 pro- 

 duction within any line. 



On the other hand Castle strenuously defended the im- 

 portance of the selection of fluctuations in "building up" a char- 

 acter. From among the descendants of piebald rats and rab- 

 bits he selected through many generations individuals showing 

 the largest and others showing the smallest extent of color in the 

 coat and he thus produced one line which was nearly all-black 

 and another nearly all-white. He maintained that not only in- 

 herited characters, but also their factors are variable and that by 

 means of selection of plus or minus variations the mode, or mean, 

 of a character may be shifted in one direction or the other. This 

 is the old view which flourished before a distinction was made 

 between fluctuations and mutations. Breeders have long been 

 acquainted with similar results of selection from a mixed popu- 

 lation containing different hereditary lines ; however Castle was 

 careful to employ as pure a race of rats and of rabbits as he 

 could obtain, but it is not possible to get as pure a race of these 

 animals or of any organisms in which cross fertilization occurs 

 as in the case of self-fertilizing plants such as beans. Johannsen 

 defines a "pure line" as "all individuals which are derived from a 

 single, absolutely self-fertilizing, homozygous individual" and 

 within such a pure line he maintains that selection is unable to 

 change any character. 



Adherents of the "pure line" hypothesis explain Castle's re- 

 sults in one of two or three ways : either his material may not have 

 been genetically pure, or mutations may have occurred during the 

 course of his experiments and his selection served merely to iso- 

 late distinct hereditary lines already present; or, more probably, 

 extent of color in his animals may depend upon multiple factors, 

 or modifying factors, which are more numerous in some individu- 



