358 HEREDITY AND ENVIRONMJKNT 



impossible to determine the relative impor- 

 tance of these three factors. So far as intellect 

 and morals are concerned we are all inclined 

 to place greater weight upon the extrinsic than 

 upon the intrinsic factors, but this opinion is 

 not based upon scientific 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 heredity. 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. 76). In 

 ^/ bodily and mental characters such twins are re- 

 markably alike ; the differences which exist are 

 slight and may usually be traced to different 

 environmental and educational influences, 

 and 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 



