GENETIC TYPE AND THE ENDOCRINES 249 



these characters that we hope in later chapters to evaluate 

 the relations between the genetic constitution and the influ- 

 ences of various endocrinal modifications in the differentiation 

 of structural and functional types. 



The expression of individual features in the skulls of Boston 

 terrier-dachshund hybrids. The indices and various propor- 

 tional relationships which have been found significant for the 

 differentiation of types among the skulls of the different 

 pure dog breeds have been calculated for each of the several 

 hybrid generations resulting from the Boston terrier-dachs- 

 hund cross. The figures for these indices and proportions 

 and the probable errors and standard deviations for the two 

 parent stocks and their hybrids are recorded in table 2 (p. 227). 

 The pure stocks are strongly contrasted in almost every skull 

 feature, and in a large majority of these features the F, 

 hybrids more nearly approach the dachshund than they do 

 the Boston terrier. 



The figures derived from the F 2 animals are interesting 

 in that the averages for the various dimensions approach 

 in value those recorded for the Fj generation. Such average 

 figures, however, completely conceal or level out the wide 

 differences which exist among the F 2 skulls. The standard 

 deviations also fail to convey a proper appreciation of the 

 variation in skull measurements. In the cases of the most 

 significant indices and proportions, however, the deviations 

 in the F 2 column are larger than those in the F, column, in 

 some instances even being several times as large. 



The backcross of the F x on the Boston terrier also gives 

 wide variations in skull form, and again high standard devia- 

 tions are seen. In contrast to these, the backcross on the 

 dachshund produces skulls with features nearly the same as 

 those of the dachshund and without great variation in pattern. 

 Although the number of skulls is not sufficient to fully demon- 

 strate the latter point, numerous observations from other 

 specimens confirm this statement, 



