NATURAL SELECTION 



They will often freely intercross in breeding, while, as a 

 usual thing, natural species will not do so. This brings us 



FlG. 6. The rock pigeon (Columba livia) of northern Africa, from which the different vari- 

 eties of domestic pigeons have been derived by artificial selection. From Brehm's Thierleben. 



to a discussion of some of the objections urged against 

 natural selection as a widely effective factor in evolution. 



Objections to natural selection as a factor in evolution. 



To Huxley the inability of artificial selection to produce 

 races which are sterile when crossed, seemed the strongest 

 objection to the certainty of effectiveness in natural selec- 

 tion to produce true species, which in nature are so generally 

 characterized by inability to breed together, or at least by 

 infertility in their hybrid offspring, in cases in which hybrids 

 can be obtained. Doubtless mutually infertile races could 

 be produced by artificial selection if breeders should care- 



