GENETIC TYPE AND THE ENDOCRINES 349 



cases. The almost complete absence of prognathism of the 

 mandible in this Fj hybrid is in contrast to the strong preva- 

 lence of this condition in the F l bulldog-bassethounds, and 

 emphasizes the fact that in the bulldog the largely dominant 

 mutation for shortening the mandible has only begun to 

 arise, although it seems to have been almost entirely accom- 

 plished in the Pekingese. In this connection we should also 

 recall that the F x hybrids between the flat muzzled Brussels 

 griffon and the dachshund are in most cases decidedly under- 

 shot. Although it has probably not been derived from the 

 same stock as the typical bulldog, and even though the lower 

 jaw shows greater reduction than this breed, yet the Brussels 

 griffon has not attained the same degree of genetic purity 

 for the mutant factors determining the short mandible as 

 has the far more ancient Pekingese breed. This interpretation 

 for these variations is far more plausible than would be the 

 simple assumption that the lower jaw of the Pekingese is 

 more dominant in character than that of the bulldog. It is 

 not more dominant; it is more fully modified by mutations 

 for shortness. We shall return to this matter in connection 

 with the disharmony between the jaws in members of the 

 second hybrid generation, in which both the genetic and 

 developmental independence of maxilla and mandible are 

 clearly demonstrated. 



The variety of head characters in the second generation 

 dachshund-Pekingese hybrids. Four F! bitches were mated 

 with four Fj males, and eight litters of F 2 dachshund-Peking- 

 ese hybrids were whelped. These litters contained eight, 

 eight, five, eight, four, one, eight and five individuals, a total 

 of forty-seven, more than half of which lived to adult age. 

 Photographs of seven of these F 2 hybrids are shown in plate 69. 



The F 2 hybrids vary considerably in size, ranging from 

 small, even midget-like, individuals to dogs equal in size to 

 the dachshund. Not one member of this generation possesses 

 a complete expression of the typical Pekingese coat; the hair 

 is either short, intermediate, or moderately long and varies 



