162 THEORY OF EVOLUTION 



i.e., artificial selection, man has undoubtedly 

 brought about changes as remarkable as any 

 shown by wild animals and plants. We know, 

 moreover, a good deal about how these changes 

 have been wrought. 



( 1 ) By crossing different wild species or by 

 crossing wild with races already domesticated 

 new combinations have been made. Parts of 

 one individual have been combined with parts 

 of others, creating new combinations. It is 

 possible even that characters that are entirely 

 new may be jjroduced by the interaction of fac- 

 tors brought into recombination. 



(2) New characters appear from time to 

 time in domesticated and in wild species. 

 These, like the mutants in Drosophila, are fully 

 equipped at the start. Since they breed true 

 and follow JMendel's laws it is possible to com- 

 l)ine them with characters of the wild type or 

 with those of other mutant races. 



Amongst the new mutant factors there 

 may be some whose chief effect is on the char- 

 acter that the breeder is already selecting. 

 Such a modification will be likely to attract at- 

 tention. Superficially it may appear that the 



