140 MENDELISM CHAP. 



the light of our present knowledge we must regard the 

 mutation as the basis of evolution as the material upon 

 which natural selection works. For it is the only form 

 of variation of whose heredity we have any certain 

 knowledge. 



It is evident that this view of the process of evolution 

 is in some respects at variance with that generally held 

 during the past half century. There we were given the 

 conception of an abstract type representing the species, 

 and from it most of the individuals diverged in various 

 directions, though, generally speaking, only to a very 

 small extent. It was assumed that any variation, how- 

 ever small, might have a selection value, that is to say, 

 could be transmitted to the offspring. Some of these 

 would possess it in a less and some in a greater degree 

 than the parent. If the variation were a -useful one, 

 those possessing to a rather greater extent would be 

 favoured through the action of natural selection at the 

 expense of their less fortunate brethren, and would leave 

 a greater number of offspring, of whom some possessed it 

 in an even more marked degree than themselves. And 

 so it would go on. The process was a cumulative one. 

 The slightest variation in a favourable direction gave 

 natural selection a starting-point to work on. Through 

 the continued action of natural selection on each succes- 

 sive generation the useful variation was gradually worked 

 up, until at last it reached the magnitude of a specific dis- 



