GENETIC TYPE AND THE ENDOCRINES 223 



the curves for the two types, but there is also a general 

 tendency to be in accord with the maxillary premolar condition. 



The mandibular index is distinctly lower in the long typed 

 skulls than in the short bulldog typed, but the difference for 

 this feature is not very great. The divergence of the rami 

 of the mandible is greater in the bulldog typed skull than 

 in the shepherd dog type. 



We may conclude from the several charts based on skull 

 measurements (text-figs. 51-54) and the curves in text-figure 

 55 that the skulls of the long muzzled dog breeds differ in 

 various respects within their group, yet all exhibit character- 

 istics clearly contrasted with those of the short faced breeds. 

 We may term these two groups, for simplicity, the long-skull 

 and short-skull types. 



The skull characteristics which are most important for 

 distinguishing the long and short types are indicated by the 

 indices rather than the linear measurements, and the curves 

 in text-figure 55 show that for this purpose certain indices 

 are far more significant than others. In some breeds of both 

 groups the cranial indices almost overlap, while the mandi- 

 bular index shows no overlapping between the two types, 

 and the skull index, palatal index and upper facial index are 

 increasingly far apart for the long and the short skulls. The 

 values for snout index show the maximum difference between 

 the two types. 



The only linear comparisons which bring out the differences 

 between the types are those concerning the relations of total 

 anteroposterior width of the premolar teeth to the premolar 

 regions of the upper and lower jaws, and the proportion of 

 palate length to length of skull base. 



Whether the several differences between these skull types 

 are inherited primarily and independently as separate char- 

 acters or whether the divergent patterns of the dog skulls 

 result from definitely modified endocrine functions may be 

 analyzed by a study of the hybrids resulting from crosses 

 between the extreme types. We have attempted to analyze 

 the modifications found in the bulldog skull by selecting pure 



