BRISTLE INHERITANCE IN DROSOPHILA 421 



some connection between the degree of bristling and the genetic 

 constitution before selection had started and again after a cross 

 with normals. 



It has been shown that only in the early generations are 

 especially high parental averages associated with exceptionally 

 high filial averages. Since the offspring can never be raised 

 under an environment identical with that of the parents, the 

 failure to find any correspondence between the means of the 

 parents and offspring in later generations may be due to the dif- 

 ferent environments in successive generations. However, the 

 highest-grade fhes may have been produced by the highest- 

 grade parents, however well the environment may have con- 

 cealed such a relationship between the means when different 

 generations were compared. The whole frequency distribution 

 of the offspring may be centered about quite a different mean 

 from that of the parents, yet the relative positions of parents 

 and offspring in their own distributions may be the same. The 

 study of the relationship between parents and offspring in indi- 

 vidual families offers a different line of attacking the problem. 

 Such an approach goes directly to the heart of the question 

 that selection seeks to answer empirically: are there genetic dif- 

 ferences between flies with different numbers of extra bristles 

 and, if so, do such differences arise continuously? If the grades 

 of the parents and offspring bear any direct relationship to each 

 other, selection can progress; if such a relationship appears at 

 first and then disappears, one may conclude that selection will 

 not continue to be successful. The demonstration that these 

 are the facts of the relationships between parents and offspring 

 in the various selected generations will strengthen considerably 

 the conclusions drawn from the results of selection based on the 

 means. 



CORRELATION 



The obvious method of investigating the relationship be- 

 tween the parents and offspring, is that of the correlation table, 

 with the coefficients of correlation and regression calculated 

 from it. This method affords a clear description of large masses 



