GENETIC TYPE AND THE EXDOCRIXES 413 



remarkable uniformity. The four bars at the right end of the 

 series break the uniformity, yet even these fall within the 

 upper limits of the range for the bassethound grandparent. 

 This record is very different from the Boston terrier-dachs- 

 hund F., series which extended below and above the ranges 

 of the two parent stocks. Since the Boston terrier-dachshund 

 pure stocks are less variable for thyroid size than are the 

 bassethound-bulldog breeds, it is difficult to account for the 

 direction of differences between these F 2 records except on 

 the grounds that the number of animals is small and the 

 variability in relative thyroid sizes very great. 



The backcross records in this cross are not important. 



Relative thyroid sizes for the Dane-St. Bernard cross are 

 represented in text-figure 81, and it is seen that both these 

 giant breeds have small thyroid glands. The twelve F } hybrids 

 recorded by the third series of bars would seem to give 

 somewhat larger thyroid proportions than either parent stock, 

 although in view of the small numbers the difference is not 

 convincing. 



The series of bars at the right in this figure represents the 

 relative thyroid sizes for forty-two F 2 great Dane-St. Bernard 

 hybrids. These sizes range from a low of about 50 milligrams 

 of thyroid per kilogram of body weight to only 175 milli- 

 grams, showing the consistent tendency for low amounts of 

 thyroid material in these large dogs. A comparison of this 

 Fo chart with the F 2 series in text-figures 79 and 80 shows 

 that hybrids from the Boston terrier-dachshund dwarf breeds 

 have an extremely wide range in relative amounts of thyroid ; 

 while animals of normal body size, the bassethound-bulldog 

 hybrids, are less variable in relative thyroid amounts ; and the 

 giant dogs are least variable, having uniformly low amounts 

 of thyroid tissue. In further support of previous conclusions, 

 no correlation between structural type and relative thyroid 

 size can be found in any of these crosses. If the thyroid gland 

 exerts any specific control over the development of bodily 

 proportions and structural form, it is not through the gross 

 relative quantity of thyroid tissue, and we must examine into 



