GENETIC TYPE AND THE ENDOCRINES 11 



example, there are badger-hounds, rabbit-hounds, foxhounds, 

 and deerhounds, and bird-dogs for land birds and water fowl. 

 On the other hand, some dogs are incapable of hunting any- 

 thing and are prized only for their daintiness as lap dogs, 

 or their extreme grotesqueness, or because they are difficult 

 animals to breed or cultivate. In spite of all these differences 

 among- the many characters of the various breeds of dogs, 

 there is one characteristic which all have in common — a pe- 

 culiar devotion to man. Their association with human beings 

 may differ in degree, but every breed of dog lends itself to 

 human companionship. 



The internal organs and functional reactions among differ- 

 ent breeds show wide deviations from the known wild dog 

 pattern. Certain breeds reproduce with high fecundity and 

 show well-expressed reproductive instincts, while others con- 

 tain many sterile individuals or produce litters of only one 

 to four puppies, as contrasted with eight to fifteen in the 

 breeds with highest fecundity. The maternal instincts are 

 vitiated to various extents among the individuals of some 

 breeds. The glands of internal secretion which are known to 

 influence these behaviors often show remarkable peculiarities, 

 both in gross proportions and microscopic structure. 



In view of these facts, the question arises as to whether 

 the characteristic structural and functional peculiarities found 

 among the various breeds might not be due to genetic muta- 

 tions which brought about primary changes in the glands of 

 internal secretion. The affected glands would then secondari- 

 ly induce the characteristic modifications. Such conjecture 

 would interpose the glands of internal secretion as the modus 

 operandi through which the mutant genes may finally give 

 rise to the many modified types of both anatomical and 

 physiological characters. For example, does the shortened 

 head of the bulldog develop as a result of modified pituitary- 

 thyroid reactions which have arisen from a definite genie 

 mutation? The alternative query would be, does g-enic muta- 

 tion impress each growing tissue in such a way as to cause 

 it to develop modified structural arrangements apart from 



