GENETIC TYPE AND THE ENDOCRINES 37 



standpoint of physiologic differences or of the symptoms 

 indicating modified internal secretions. 



Hybridization for new combinations and recombinations of 

 modified glands and type tendencies. The strongly contrasted 

 types and the sharply localized structural distortions which 

 occur among the dog" breeds are often correlated or associated 

 with definite peculiarities of the endocrine glands. These 

 conditions offer probably the most unique possibilities to be 

 found among the mammals for making new combinations of 

 normal and modified glands and of contrasted growth ten- 

 dencies. Through crossing or hybridizing the various dog 

 breeds, the experimenter is able to break down certain com- 

 plexes of structural and functional modifications and to build 

 up new complexes or recombinations of characters in the 

 second hybrid generation. Thus he is able to devise crucial 

 tests for determining the functions and degrees of influence 

 of the endocrine secretions in regulating the development and 

 growth of tissues and organs in different genetic constitu- 

 tions. The results of such tests are to be recorded in detail 

 in the following sections. 



Since prehistoric time, hybrid breedings of many kinds 

 have occurred at random among the different races of human 

 beings. Such race crossings may have tended to stimulate 

 mutations and genie instability, thus bringing about freak 

 reactions and functional disharmonies just as are found to 

 occur among dogs. The chief difference has been that in 

 dogs a master hand has selected the freak individuals ac- 

 cording to fancy and purified them into the various dog breeds. 

 No such force regulates the mongrel mixing of human beings, 

 and dwarf, giant, achondroplasia and acromegalic tendencies 

 have not been selected out or established in pure form. On 

 the contrary, individuals carrying different degrees of these 

 tendencies are constantly being absorbed into the general 

 human stock, possibly to render the hybridized races less 

 stable and less harmonious in their structural and functional 

 complexes than were the original races from which they were 

 derived. Mongrelization among widely different human stocks 



