GENETIC TYPE AND THE ENDOCRINES 357 



which differentiate in parallel directions and become func- 

 tionally coordinated would lead one to expect a close cor- 

 respondence in their reactions to stimuli tending to modify 

 their growth and development. Yet the evidence from the 

 dachshund-Pekingese cross, as well as from other crosses 

 already discussed, shows quite clearly that the genetic basis 

 for the pattern of the maxilla is entirely distinct and inde- 

 pendent of the genetic background for the mandibular size 

 and form. And further, the genetic constitution of the in- 

 dividual influences the patterns of the two jaws separately in 

 spite of the quality of the internal environment as influenced 

 by the endocrine glands. The genetic constitution of the tissues 

 themselves determines their mode of growth and develop- 

 ment as long as the environment is sufficiently favorable to 

 permit the expression of the structure concerned. 



Mutations have arisen in the ancestry of several different 

 dog breeds which tend to suppress the growth in length of 

 the face. The mutant factors which shorten the upper face 

 and maxilla are not closely linked with other factors influ- 

 encing the length and pattern of the mandible. Thus, in the 

 English bulldog and the French bulldog, mutations have 

 brought about a severe reduction of the usual canine muzzle, 

 but the mutations for a corresponding reduction in length of 

 the lower jaw have not reached completion. In the Boston 

 terrier, the mandibular mutation is more nearly accomplished 

 and there is only a slight or no undershot condition; and 

 in the Brussels griffon and Pekingese the mandible is .fully 

 shortened in association with the completely flattened face. 

 These three latter breeds with the shortened, wide lower jaw 

 all occasionally show the overshot as well as the undershot 

 condition. The reduction in length of each jaw is influenced 

 by a complex of factors, some of which are dominant and 

 some recessive to the normal jaw length, and not all members 

 of the several short faced breeds are pure or completely 

 homozygous for the entire mutant complex governing the 

 reduction in length of each jaw. The F 2 hybrids arising from 



