Ill] CAUSES OF VARIATION 29 



the equilibrium, but the eifects may differ in different 

 individuals. It is the nature of the organism or 

 germ-cell affected which determines whether and 

 to what extent the change shall take place ; the 

 environment merely supplies the stimulus. 



It will be seen that our knowledge of the causes 

 of variation, in so far as these are connected with 

 environment, is very incomplete and unsatisfactory, 

 for although it is fairly clear that conditions may 

 sometimes disturb the equilibrium of the germ-ceUs 

 and provide a stimulus to variation, yet we have no 

 knowledge of the way in which the stimulus acts 

 and can make no prediction as to the direction the 

 variation will take. Before leaving the subject, one 

 other cause of variability must be mentioned — the 

 effect of crossing different races in producing varia- 

 tion, it frequently happens that the result of 

 crossing distinct races is that the crossed individuals 

 differ from either parent ; sometimes in the direction 

 of increased vigour, as was pointed out by Darwin, 

 and other more recent observers ; sometimes by the 

 development of characters apparently not possessed 

 by either parent, as in the case of 'reversion on 

 crossing.' The cause of this latter phenomenon will 

 be discussed in a later chapter. In the subsequent 

 generations from the cross great diversity may often 

 appear, and Darwin supposed that the mingling of 

 two distinct germinal stocks had an effect in dis- 



