256 CHARLES R. STOCKARD AND A. L. JOHNSON 



proach to the indices for the pure Boston terrier. The single 

 skull from the F t backcrossed on the dachshund parent is 

 close to the pure dachshund level. 



The five charts for the several skull indices in text-figures 

 57 and 58 clearly indicate that these characters, all widely 

 contrasted in the pure dachshund and Boston terrier breeds, 

 are expressed in the hybrid generations in such ways as to 

 suggest that they are strictly genetic in nature and not the 

 results of dietary and environmental responses. Such records, 

 however, are not sufficient for determining whether endocrine 

 modifications or typical internal environmental changes are 

 the primarily inherited qualities underlying the different 

 growth and structural reactions indicated through these in- 

 dices. Only the gross and microscopic examination of the 

 endocrine glands and the observations on their functional 

 reactions, which are to be presented in following chapters, 

 can supply the additional necessary evidence from which to 

 draw satisfactory deductions regarding these matters. 



The curves in text-figures 59 and 60 give a clear picture 

 of the records for the five indices just considered, and illus- 

 trate as well seven other skull features from the different 

 generations of this cross breeding. In text-figure 59 the solid 

 line represents the dachshund parent and the dash-dot line 

 the Boston terrier parent; the average of skull features for 

 the F! hybrid generation is shown by the line of dashes, and 

 the dotted line illustrates the average for the second, F 2 , 

 generation. The figures at the left vertical border of the 

 graph indicate the values for the several points along the 

 curves. The different indices and proportions considered are 

 listed immediately below the points on the curves which 

 represent their values. The cranial indices of the two parent 

 stocks are not widely different and the average indices for 

 both hybrid generations fall about half way between them. 

 Other readings on the graph are readily understood. The 

 main points of interest are that the average indices for every 



