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 
