NATURAL SELECTION 129 



in the light of the more recent and extensive studies 

 of variation that have been made since Darwin's 

 time. 



Selection and Variation 



If, then, all that selection can do is to produce 

 more individuals of a given type, it may appear that 

 this part of Darwin's evidence fails to support his 

 assumption that the observed variability of animals 

 and plants suffices to furnish selection with its neces- 

 sarv material. It is here that the mutation theoiv 

 has a contribution to make. Since 1900 much evi- 

 dence has been obtained showing that new variations 

 may appear that transcend the extremes of varia- 

 tion of the original ty2)e. These, if they are inherited, 

 are called mutants. In some cases the new type so 

 far transcends the original type that the extreme 

 fluctuations of the two do not overlap ; but in other 

 cases the new type may be nearer to the original one 

 and the fluctuations of each may overlap. 



Darwin knew^ of cases of sudden mutation and 

 called them sports or monstrosities. He thought that 

 they could seldom supply materials for evolution 

 because they changed a part so greatly as to throw 

 the organism as a whole out of harmony with its 

 environment. This argument for rejecting extreme 

 or monstrous forms seems to us todav as valid as it 

 did to Darwin ; but we now recognize that sj^orts are 

 only extreme types of mutation, and that even the 



