I 



GENETIC TYPE AND THE ENDOCRIXES 409 



On the other hand, the distinctly lower average for relative 

 thyroid proportions shown by the giant breeds, as well as 

 the fact that there is no interbreed overlapping of individual 

 thyroid proportions between these and the several smaller 

 sized breeds, might possibly indicate that a low thyroid ratio 

 is conducive to large body size. Many facts concerning the 

 role of thyroid influence on growth and differentiation in 

 lower vertebrates, and in mammals as well, support such a 

 possibility. Nevertheless, we should not lose sight of the fact 

 that the breed with the largest average thyroid has only 

 three times more thyroid than the one with the smallest. This 

 difference in thyroid proportion may be unimportant when 

 it is recalled that one dachshund may have relatively more 

 than three times as much thyroid as another, and that one 

 bulldog may possess six times more thyroid than another 

 member of his breed. 



Although quantity of thyroid tissue, within necessary func- 

 tional limits, would seem to play no role in type determina- 

 tion, the relative size of the thyroid may yet be a definitely 

 hereditary characteristic within itself. The three charts al- 

 ready referred to in connection with relative thyroid sizes 

 for the pure breeds also contain data of interest in connec- 

 tion with the inheritance of thyroid size and its probable lack 

 of correlation with physical type in the hybrids of widely 

 different breeds. 



The relative sizes of thyroids in hybrids between pure breeds 

 of contrasted types. The relative thyroid sizes in the hybrid 

 generations of the cross between the Boston terrier and the 

 dachshund are represented in text-figure 79, to which we have 

 already referred for pure breed records. Relative thyroid 

 sizes for these two pure breeds are not very different, and 

 the individual records overlap considerably. The thyroids in 

 five adult Fj Boston terrier-dachshund hybrids are shown in 

 the chart to average somewhat greater in relative size than 

 do the thyroids of the pure bred parent stocks. In general 

 body type, the F l hybrids, as seen from life in plate 29 

 (p. 127), are nearer the dachshund than the Boston terrier, 



