XIII VARIATION AND EVOLUTION 137 



be determined in each case by the ordinary methods of 

 Mendelian analysis. For in every instance the variation 

 depends upon the presence or absence of definite factors 

 carried in by the gametes from whose union the individual 

 results. And' as these factors separate out cleanly in the 

 gametes which the individual forms, such variations as 

 depend upon them are transmitted strictly according to 

 the Mendelian scheme. Provided that the constitution 

 of the gametes is unchanged, the heredity of such varia- 

 tion is independent of any change in the conditions of 

 nutrition or environment which may operate upon the 

 individual producing the gametes. 



But, as everybody knows, an individual organism, 

 whether plant or animal, reacts, and often reacts mark- 

 edly, to the environmental conditions under which its 

 life is passed. More especially is this to be seen where 

 such characters as size or weight are concerned. More 

 sunlight or a richer soil may mean stronger growth in a 

 plant, better nutrition may result in a finer animal, su- 

 perior education may lead to a more intelligent man. 

 But although the changed conditions produce a direct 

 efifect upon the individual, we have no indisputable evi- 

 dence that such alterations are connected with alterations 

 in the nature of the gametes which the individual pro- 

 duces. And without this such variations cannot be 

 perpetuated through heredity, but the conditions which 

 produce the effect must always be renewed in each suqces- 



