220 CHARLES R, STOCKARD AND A. L. JOHNSON 



might indicate that the development of these characteristics 

 in the bulldog skull is quite susceptible to variations in in- 

 ternal environmental conditions, such as functional modifica- 

 tion in the endocrine glands. Or variations in such characters 

 might with equal probability be due to lack of homozygosity 

 in the multiple factor complex underlying the completed 

 patterns of the bulldog skull. Whether one or both these 

 possibilities are responsible for this variability in pattern 

 we shall attempt to determine by a study of hybrid conditions. 



The differences between the long type of skull and the 

 shortened bulldog type are brought out with even greater 

 clarity in the curves shown in text-figure 55. Six different 

 pure breeds having long typed skulls — the German shepherd, 

 Saluki, bassethound, St. Bernard, great Dane and dachshund 

 — are represented by the six continuous or solid lines in the 

 chart. Round typed skulls from five pure breeds — the English 

 bulldog, French bulldog, Boston terrier, Pekingese and Brus- 

 sels griffon — are indicated by the broken lines. The figures 

 in the vertical column give the values of the indices and 

 the proportional percentages for the twelve features listed 

 at the bottom of the chart. These curves indicate in a re- 

 markably clear fashion the characters in which the two types 

 of skulls differ strongly, those which are not so widely con- 

 trasted, and finally the features which are quite similar for 

 all skull types. 



Examining this figure in detail we find that there is only 

 a slight difference in the cranial indices of these two groups ; 

 skull indices differ more, palatal indices still more and snout 

 indices differ to the greatest degree. The snout indices for 

 the long skull group range from 53 to 75, but in the short 

 skull group they range from a low of 155 up to a high of 

 183. The differences in snout index between the long skulls 

 and the short skulls are seen to be almost three times as 

 great as the range of this index within either group. 



The upper facial index, again calculated on the reverse 

 basis of nasal length to palatal width, is very different in the 

 two types. With this manner of calculation, the short skulls 



