412 CHARLES R. STOCKARD AND E. M. VICARI 



The records of six animals from the backcross by the F x 

 hybrid on the dachshund parent show most widely variable 

 relative amounts of thyroid tissue. The lowest of these pro- 

 portions is less than 200 milligrams per kilogram of body 

 weight, while the two highest specimens are 700 and 940 

 milligrams. Here again one doubtless finds entirely new con- 

 stitutional complexes producing relative amounts of thyroid 

 material entirely out of proportion with those shown for any 

 of the pure dog breeds. 



Text-figure 79, as a whole, clearly indicates that the relative 

 amount of thyroid tissue in the dog is a widely variable 

 proportion, and our examinations of the Boston terrier-dachs- 

 hund cross again show that these differences in proportion 

 are without significance in the determination of body form 

 and type. 



The relative sizes of thyroids in hybrids between the basset- 

 hound and English bulldog are represented in text-figure 80. 

 Thyroid sizes in these two pure breeds overlap, though there 

 is some indication that the 'bulldog has a smaller average 

 amount of thyroid per kilogram of body weight than does 

 the bassethound. Both breeds are widely variable in relative 

 thyroid size. 



The third series of bars in' text-figure 80 represents gross 

 thyroid proportions in nine F t bassethound-bulldog hybrids. 

 The Fj thyroids are fairly uniform in relative sizes and lie 

 within the range of the parent breeds. 



Members of the second hybrid generation from the cross 

 between these two widely contrasted stocks are remarkably 

 uniform in the relative amounts of thyroid tissue present. 

 The records for forty-two individuals of this generation are 

 indicated by the fourth series of bars in this figure. Thyroid 

 sizes in thirty-eight of these forty-two individuals are rep- 

 resented by a gradually lengthening series of bars having 

 values of from about 100 milligrams per kilogram of body 

 weight to about 225 milligrams, all being within the bulldog 

 range. In view of the wide variability in relative thyroid 

 size occurring in both parent stocks, these F 2 records show 



