88 HEREDITY [CH. 



the subject, but when it is remembered how wide- 

 spread and in what varied characters segregation has 

 been found, it would not seem improbable that it 

 should occur in such cases also. 



Another subject which Mendelian investigation 

 has brought into prominent notice, and which has led 

 to much controversy, is the kind of variation which 

 has been effective in the process of evolution. Darwin 

 assumed that evolution takes place by the preservation 

 of very small ' continuous ' variations which occur in 

 a direction favourable to the species, but even among 

 his immediate followers, for example Huxley, doubt 

 was expressed whether larger step-like variations or 

 'mutations' may not have been operative. Darwin 

 rejected this idea chiefly on the ground of the rarity 

 of such mutations, which makes it inevitable that the 

 mutating individual should generally mate with one 

 of the normal type, and so it was supposed that the 

 mutation would be diluted and rapidly lost. But 

 Mendelian work shows that this dilution does not 

 occur in simple cases; the offspring of the cross 

 between the mutation and the type produces half its 

 germ-cells bearing the mutation to its full extent, 

 and these will transmit the mutation until the race 

 may become widely infected with it, and not infre- 

 quently individuals both of which possess it will mate 

 together. If the mutation be dominant, as in the 

 case of the well-known black variety of the ' Peppered 



