The Making of Species 



thus leads us to the conclusion that, although 

 many hybrids are fertile, the crossing of distinct 

 species has exercised little or no effect on the 

 origin of species. Even where allied species, like 

 the pintail and the mallard ducks, whose hybrid 

 offspring is known to be fertile, inhabit the 

 same breeding area and occasionally interbreed 

 in nature, such crossing does not, for some reason 

 or other, appear to affect the purity of the 

 species. 



Very different, of course, is the effect of cross- 

 ing a mutation within a species with the parent 

 form ; the offspring are, as we shall see, likely 

 to resemble one or other of the parents ; so that, 

 if the mutation occur frequently enough and be 

 favourable to the species, the new form may in 

 course of time replace the old one. 



132 



