GEXETIC TYPE AXD THE ENDOCRINES 411 



The initial letters B and D above the bars in text-figure 

 79 indicate, respectively, Boston terrier and dachshund char- 

 acters in the head; and T and S indicate tall and short for 

 legs. The reader may readily see the irregular distribution 

 of these four characters throughout the slope of relative 

 thyroid sizes as represented for the thirty-three F 2 Boston 

 terrier-dachshund hybrids. Although no relation exists be- 

 tween proportional amounts of thyroid tissue and body form, 

 there is an indication from these F 2 records that members 

 of the same litter tend to have more or less the same relative 

 amounts of thyroid tissue. This tendency might be interpreted 

 to mean that richness in thyroid material is an hereditary 

 or familial character. 



The chart for F 2 thyroid sizes may be further interpreted 

 as indicating that new genetic constitutions have arisen among 

 these hybrid individuals which bring about relative thyroid 

 proportions considerably smaller than either parental stock 

 in some and very much larger in others. The proportionally 

 huge thyroids are more than twice the relative size of the 

 highest recorded for the parent stocks. These distorted pro- 

 portions, deviating in both directions from the standard of 

 the original breeds, are clear indications of the strange com- 

 binations in genetic constitutions which give rise to some 

 of the F, hybrid individuals. The normal adjustment in pro- 

 portional sizes of the various bodily organs is no doubt 

 delicately controlled through both genetic and developmental 

 influences, and in certain of these F 2 animals, organs other 

 than endocrine glands are frequently distorted in proportion, 

 and many of the puppies are non-viable. Several F 2 speci- 

 mens have possessed such disproportionately large heads with 

 internal hydrocephalus that they were unable to survive even 

 though nursed with great care. 



The backcross offspring of the F t hybrid and the Boston 

 terrier parent show uniformly small relative proportions of 

 thyroid material, as is indicated in the chart by the series 

 of bars representing fourteen such backcross animals. 



