RELATIVE POTENCY OF HEREDITY AND ENVIRONMENT 539 



ovulates from each ovary when twins are produced. Each twin 

 develops separately for some time in its own horn of the uterus, and 

 their membranes grow down toward the united part of the uterus. As 

 a rule the chorionic membranes of the two fuse and the blood vessels 

 unite so that their blood supply is in common. It is now known that 

 the male at a very early period develops glandular tissues in its 

 gonads that are a necessary agent in differentiating the characteristic 

 male features, while the female does not develop ovarian gland tissues 

 till a late fetal period. The result is that the substances given off from 

 the male gonads, which we call hormones, pass through the common 

 blood supply to the individual that is by heredity a female, and so 

 profoundly transform this prospective female that in many cases she 

 has more male qualities than female. Apparently it is not possible 

 completely to reverse the sexes, but in some cases reversal is approxi- 

 mated. The male hormones that come to the prospective female 

 from outside her body are obviously environmental factors, and that 

 heredity is profoundly modified by environment cannot be denied in 

 such cases as these. 



What light is thrown upon the human situation by the study of 

 these various types of twins? The influence of environment as an 

 agent capable of modifying heredity is not denied as a general proposi- 

 tion, but it appears that the organism is in general so plastic, almost 

 elastic one might say, that environmental effects within the range of 

 ordinary human experience are able to affect heredity only very slightly 

 or temporarily. By the time an individual is born its characters are 

 already so far advanced toward the definitive condition that only such 

 radical disturbances as serious illness or gross poisoning of one kind or 

 another are capable of altering the course of differentiation that has 

 been shaped by heredity. If we could change the environment during 

 an early embryonic period, we might hope effectively to modify hered- 

 ity, but no such control is yet within our power. Prenatal influences 

 through maternal impressions are believed in by many, but the genet- 

 icist looks upon this notion as among the most patent fallacies with 

 which he comes in contact. 



I suppose that the motive actuating the invitation given me to 

 speak before this representative body of sociologists was the hope that 

 I would emphasize the factor of heredity and thus tend to neutralize to 

 some extent the prevailing overemphasis upon the factor of environ- 

 ment. Perhaps I have in turn overemphasized heredity. Somewhere 

 between these extremes of overemphasis lies the truth. 



