356 The Theory of Natural Selection 



menter has fixed his attention; (2) that a change in a given 

 organ may be brought about by alterations in any one of two 

 or more genes. It is therefore in accord with the known 

 facts to assume that selecting for a particular trait may carry 

 in its wake far-reaching alterations in other traits. It is also 

 in agreement with the known facts to suppose that systematic 

 selection results in segregating factors which will determine 

 the appearance of distinct traits in successive generations. It 

 remains certain at any rate ( i ) that the most reliable experi- 

 mental selection has never resulted in transcending the nor- 

 mal range of variation of the parental type, and (2) that the 

 apparent establishment of new types through systematic 

 selection can be explained on the theory of factorial deter- 

 mination of characters. 



Darwin's Own Misgivings 



Darwin never claimed quite so much for the doctrine 

 of natural selection as did many of his enthusiastic followers. 

 Certainly he did not claim for natural selection that it was 

 the exclusive means by which species are modified, but only 

 that it was the most important. Certain types of problems 

 troubled him particularly although he remained confident 

 that a more thorough study of the facts would support 

 his theory. 



The first of these difficulties was the consideration that 

 if natural selection were actually taking place, it should lead 

 to the presence of indefinite intergrading forms instead of 

 to definite fixed species. The accumulation of knowledge 

 shows (i) that there are actually more intergrading in- 

 definite forms than Darwin suspected; and (2) that there 

 are many species that are far from fixed. 



Darwin saw a second difficulty in the development of 

 trivial, relatively insignificant organs, such as a tail, as 

 well as highly complex structures, such as the vertebrate 

 eye. It was difficult in the latter case to conceive what 

 competitive advantage would come from the rudiments of 



