24: CHARLES B. STOCKARD 



legs in the far removed creeper fowl. Changes in size and 

 structural quality are the phenomena which occur in the 

 evolution of animal species, judging only from the nature of 

 those specimens living today. Modifications in structural 

 quality of bone, skin, endocrine glands, et cetera are the proc- 

 esses which we are attempting to analyze in our study of 

 the different dog breeds and their hybrids. The further these 

 studies progress the more certain it becomes that along with 

 structural qualities the functions and behaviors of individuals 

 are the products of a definite genetic constitution interacting 

 within a correlated chemical environment regulated to an 

 important degree by the endocrine glands. This position has 

 gradually been reached through the recently accumulated 

 knowledge of genetics and the illuminating experimental 

 studies by many workers on the influences of the endocrine 

 secretions on the growth processes and the functional reactions 

 of the organs and tissues within the body. Slight disturbances, 

 as well as normally rhythmical variations in endocrine 

 secretions, bring about prompt modifications and altera- 

 tions in the functional reactions of the tissues, and particu- 

 larly in the instinctive behaviors and the reactions of the 

 nervous systems. One may logically suppose that any genetic 

 change or mutation which affects the endocrine glands, either 

 directly or indirectly, must very probably be of the highest 

 importance in giving rise to new species as well as to new 

 domestic breeds. This interplay within the single endocrine 

 system may be the reason for the reappearance of the same 

 or comparable patterns among very widely separated families 

 and classes of animals as w r ell as among closely related 

 species. 



Genetic mutations in dogs and the evolution of structurally 

 modified species. At the present time peculiar genetic changes 

 are taking place within some breeds of dogs which may paral- 

 lel in ways and degrees the evolutionary processes which give 

 rise to strangely modified animal species. Several such type 

 modifications and glandular alterations have been referred 

 to above. I recently ('36 a) reported an hereditary loss of 



