HEBEDITY AND VAEIATION 97 



each variation is individual only, and may or may 

 not die out with the individual. Individual varia- 

 tions are often transmitted from one generation to 

 another; and the oftener they are transmitted, the 

 more constant they become; until, at last, a large 

 number of individuals constantly acquire the same 

 character. Here we have cases of indefinite 

 changing into definite variation, showing that the 

 two are fundamentally the same ; the difference be- 

 tween them lying in the strength and constancy 

 of the directing cause. 



Variations must be due either (1), to the action of 

 the environment, directly on the individuals, or in- 

 directly, by causing a change of habit, and thus 

 leading to the greater or less use of certain organs. 

 Or (2) , to internal causes affecting the action of some 

 law of growth which counteracts the law of heredity. 

 Each of these causes may possibly give rise to inde- 

 finite or to definite variation, according to the 

 strength with which it acts. We have no reason to 

 suppose that external causes must act identically 

 on different individuals, or that because one indivi- 

 dual changes its habits, therefore many must do the 

 same ; but no doubt the environment, if it acts at all, 

 would generally affect a large number of individuals 

 simultaneously, and post-natal variations would 

 generally be definite. On the other hand, indefinite 

 variations would generally arise during the develop- 

 ment of the individual ; and consequently they must 

 generally be congenital in origin, although the effects 

 may not show until long after birth. 



Definite variation is due to the directive force being 



