242 CHARLES K. STOCKARD AND A. L. JOHNSON 



over outlines of the dachshund and the Boston terrier skulls. 

 In the one case (fig. 4), the fit with the dachshund is very 

 close while in the other (fig. 5), the F 2 skull closely approaches 

 the outline of the Boston terrier. The sagittal outlines of 

 these two F 2 skulls are shown for contrast in figure 6. 



The appearance of such divergent patterns in the skulls 

 of Fo litter mates makes it quite certain that external en- 

 vironment and dietary elements have little if any respon- 

 sibility in the differential expression of these types. The 

 various structural responses are due to the differences in 

 the basic constitutional complexes possessed by the several 

 individuals. The chance that two animals in the second hybrid 

 generation will ever receive exactly the same total genetic 

 composition from their FX parents is extremely small, and 

 as a consequence members of this generation differ greatly 

 in size, form and proportions, as the photographs in plate 30 

 very well illustrate. The instincts and behaviors of these 

 F2 animals are also widely variable. 



A minor peculiarity of the mandibular condyle in the dachs- 

 hund skull was found to be inherited in this cross as a single 

 factor recessive character. Figure 1 in plates 41 and 42 

 shows that the articular surface of the condyle in the 

 dachshund mandible is concave rather than convex as in 

 other dog skulls, illustrated for the Boston terrier in figure 

 2 of these plates. Figure 3 illustrates an F l mandibular 

 condyle approaching the Boston terrier in type although the 

 expression is not complete. The four F2 mandibles (figs. 

 4-7) were selected to illustrate the distribution and expres- 

 sion of the character in this generation. About one fourth 

 of the condyles are typically convex as in the Boston terrier, 

 half are slightly flattened as in the F a , and one fourth show 



PLATE 42 



EXPLANATION OF FIGURES 



Enlarged photographs of the condyles of the same mandibles as shown in 

 plate 41 showing in more detail the differences in the articular surfaces in the 

 pure breeds and the resulting expression in the hybrids. 



1 Dachshund 255 $. 2 Boston terrier 536 $?. 3 F, 130 $. 



4 F 2 722 <?. 5 F 2 1522 <?. 6 F, 781 $. 7 F, 810 <?. 



