xiii VARIATION AND EVOLUTION 139 



plant, and it is in this latter capacity that it affects its off- 

 spring. Wherever, as in plants and mammals, the organ- 

 ism is parasitic upon the mother during its earlier stages, 

 the state of nutrition of the latter will almost certainly 

 react upon it, and in this way a semblance of transmitted 

 weakness or vigour is brought about. Such a connection 

 between mother and offspring is purely one of environ- 

 ment, and it cannot be too strongly emphasised that it 

 has nothing to do with the ordinary process of heredity. 

 The distinction between these two kinds of varia- 

 tion, so entirely different in their causation, renders 

 it possible to obtain a clearer view of the process of evo- 

 lution than that recently prevalent. As Darwin long 

 ago reaUsed, any theory of evolution must be based upon 

 the facts of heredity and variation. Evolution only 

 comes about through the survival of certain variations 

 and the elimination of others. But to be of any moment 

 in evolutionary change a variation must be inherited. 

 And to be inherited it must be represented in the gametes. 

 This, as we have seen, is the case for those variations 

 which we have termed mutations. For the inheritance 

 of fluctuations, on the other hand, of the variations 

 which result from the direct action of the environment 

 upon the individual, there is no indisputable evidence. 

 Consequently we have no reason for regarding them as 

 playing any part in the production of that succession of 

 temporarily stable forms which we term evolution. In 



