30 HEREDITY [ch. 



turbing the equilibrium similar to that produced by 

 change of environment. To some extent this is 

 doubtless true, but recent developments of the 

 theory of heredity have afforded a more exact ex- 

 planation, in the recombination of the different 

 characters of the two races which are crossed. A 

 fuller account of ' variation induced by crossing ' 

 must therefore be postponed until the principles of 

 heredity have been discussed. 



One further question should be mentioned before 

 proceeding to the subject of heredity, namely, the 

 relative importance of 'inherent' and 'acquired' 

 characters in making up the sum of characters of a 

 mature individual. It is often assumed, especially in 

 human cases, that the environment has a prepon- 

 derating influence in shaping the individual. In a 

 certain sense this is true, for many characters can 

 only develop in a suitable environment ; muscles 

 must be exercised to be properly formed and the 

 mind cannot develop its full powers if it is never 

 used. But the study of variation leads inevitably 

 to the conclusion that the inherent characteristics 

 are all-important, and that the effect of environment 

 is not much more than to give them opportunity to 

 develop. This is perhaps most impressively seen in 

 the case of 'identical twins,' as has been shown by 

 Galton [12]. There is reason to believe that such 

 twins are produced by the division of one ovum, and 



