MENDELISM AND SELECTION 371 



phenomenon, and might, therefore, have played in the past an 

 important role in the origin of species (see Chapter III.). 



Similarly, Hugo de Vries showed in most convincing detail 

 that sudden discontinuous variations or mutations not infre- 

 quently occur among plants and give rise to true-breeding 

 varieties (see Chapter III.). 



Now it is evident that, if Mendel's Law applies in such cases, 

 the mutation, once present, is not likely to be lost or swamped 

 by inbreeding with the normal types. Thus, through Mendel's 

 discovery we are led to a new view of organic evolution, in which 

 we attach less importance to the minute fluctuations on which 

 Darwin relied, and more importance to mutations or saltatory 

 variations. 



Mendelism in Relation to Selection. — The facts of Mendelism 

 are in several ways important in relation to natural selection: — 

 (i) The facts warrant us in believing in the possibility of the 

 particular evolution of unit characters while the rest of the 

 organism remains stable. (2) When a variation is, through 

 inherent stability or through inbreeding, prepotent — i.e. when 

 its possessors breed true inter se — we can understand how it is 

 that even crossing with variants having an antagonistic character 

 need not imply any diminution of the dominance of the character 

 in question. The inbreeding of the hybrids simply results in 

 the sifting out of the pure parental types. (3) Suppose Mendelian 

 phenomena to occur in a series of generations, and suppose 

 that natural selection favours the possessor of the dominant 

 character, they will ex hypothesi prevail as elimination proceeds. 

 But it should also be noted that, apart from selection,, the 

 possessors of the dominant character wiU be in a gradually 

 increasing majority, since extracted dominants and dominant- 

 recessives (practically indistinguishable as far as natural selection 

 goes) are always to recessives in the proportion of 3 : i. 



In the beautiful case of the two nettles given by Correns, the 

 plants with entire leaf-margins are markedly more susceptible 



