THE HEREDITARY BASIS 23 



showed very little effect of the different environments to which 

 the children were exposed. Presumably, therefore, differences in 

 vision met with among children are the results of differences of 

 inheritance much more than differences of environment. Whether 

 differences among human beings are due in greater measure to 

 heredity depends very largely on the characters studied. Differ- 

 ences in eye color are due almost entirely to heredity, as the 

 character shows scarcely any effect of ordinary environmental 

 changes. In stature and weight environmental influence is more 

 obvious although heredity is an important factor. In manners 

 and customs environmental influence is more obvious still, and 

 whether a person talks English or Chinese may depend entirely 

 upon the locality in which he is raised. If he had the heredity of 

 a horse or a cow he would be unable to talk either, but if his 

 heredity were such that he could talk any human language, en- 

 vironment would determine what language he would speak or 

 whether or not he would speak any. 



A good illustration of the relative influence of heredity and 

 environment is afforded by the resemblance of so-called identical 

 twins compared with that of twins of the usual kind. The recog- 

 nition of these two classes of twins is due to Francis Galton, who 

 gave several illustrations of striking similarities between twins 

 which he termed identical. Ordinary twins are about as different 

 as other members of the same family. They frequently exhibit 

 marked di^.rences in physical traits, in intelligence and disposi- 

 tion, ar.a the almost identical surroundings in which the" are 

 frequently brought up, fail to overcome their inherited differences 

 which are often conspicuous even in early life. One of Galton's 

 correspondents describes his twin offspring by saying "They have 

 had exactly the same nurture from their birth up to the present 

 time; they are both perfectly healthy and strong, yet they are 

 otherwise as dissimilar as two boys could be, physically, mentally 

 and in their emotional nature." Another correspondent says of a 

 pair of twins, "They were never alike either in body or mind, and 

 their dissimilarity increases daily. The external influences have 

 been identical; they have never been separated." 



