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 dif crences in physical traits, in intelligence and disposi- 
tion, and the almost identical surroundings in which thew 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.” 
