xni VARIATION AND EVOLUTION 127 



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 accord- 

 ing to the Mendelian scheme. Provided that the 

 constitution of the gametes is unchanged, the heredity 

 of such variation 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 

 markedly, 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, superior education may 

 lead to a more intelligent man. But although the 

 changed conditions produce a direct effect upon the 

 individual, we have no indisputable evidence 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 successive generation. We are led, there- 

 fore, to the conclusion that two sorts of variations 

 exist, those which are due to the presence of specific 

 factors in the organism and those which are due to 

 the direct effect of the environment during its life- 

 time. The former are known as mutations, and are 

 inherited according to the Mendelian scheme ; the 



