6 CHARLES R. ST0CKARD 



of type must involve both genetic and developmental studies 

 upon one specific type of material. It has been shown 

 (Stockard, '21), for instance, that the eggs from two closely 

 related species may differ in so gross a performance as the 

 production of twin embryos in response to the same modi- 

 fication in environment. 



Among human beings it is well known that the most exag- 

 gerated deviations in type and constitution are associated 

 with various modifications and diseases of the glands of 

 internal secretion. There is strong probability in many cases 

 that to a large extent the diseased gland is the causative 

 agent, bringing about the marked changes in physical type, and 

 in certain of these cases there is indication that hybridization 

 or other genetic modifications are very probably concerned. 

 Certain clinical investigators have tended to emphasize the 

 possibility that human giants, dwarfs, midgets and other 

 peculiarly modified individuals owe their unusual conditions 

 directly to functional deviations in their endocrine systems. 

 However, that the conditions may in general be largely 

 hereditary has not been clearly recognized, and the character 

 and details of such heredities have remained almost entirely 

 unknown. 



In considering such problems it becomes evident that an 

 analysis of the inheritance and endocrinic influences involved 

 in the origin of these types would be difficult, and could not 

 be undertaken with any hope of completion through a study 

 of human beings. However, this fact does not seriously affect 

 the problem. Among the species of several lower mammals 

 we find distortions of the normal anatomical form closely 

 similar to those found in man. I have recognized for many 

 years ('23 a) that the numerous breeds of dogs probably 

 furnish the best example of such distortions. Many of the 

 present dog breeds have been carefully preserved for gen- 

 erations and now constitute the most ideal material for an 

 extensive investigation and analysis of the interrelations of 

 heredity and the influences of internal secretions in bringing 

 about distortion of structural types and behavioristic modi- 



