Influence of Environment 253 



greater weight upon the extrinsic than upon the intrinsic factors, 

 but this opinion is not based upon demonstrable evidence. So far 

 as organisms below man are concerned there is general agreement 

 that heredity is the most important factor, and this opinion is held 

 also for man by those who have made a thorough study of hered- 

 ity. Galton has made the best scientific study of this subject in 

 the case of identical twins, in which as we know heredity is the 

 same in the two, both individuals having come from the same 

 oosperm (Fig. 81). In bodily and mental characters such twins 

 are remarkable alike; the differences which exist are slight and 

 may usually be traced to different environmental and educational 

 influences, arid particularly to different illnesses. Galton sums 

 up his study with these words: "There is no escape from the 

 conclusion that nature prevails enormously over nurture when the 

 differences of nurture do not exceed what is commonly to be 

 found among persons of the same rank of society and in the same 

 country." 



A 



X 



' 



v / 



^L/ 



Heredity 



FIG. 88. DIAGRAM TO SHOW THE INFLUENCE OF HEREDITY, ENVIRONMENT 

 AND TRAINING IN THE DEVELOPMENT OF AN INDIVIDUAL. Various types 

 of individuals (represented by the triangles) may be produced from the 

 same germ cells (heredity) if the environment and training are variable. 



