SELECTION 149 



dogs and cattle may have arisen from more 

 than one species; but probably those of 

 horses and fowls, and clearly those of rabbits, 

 ducks and pigeons, are each descended from 

 a single wild species. At least a score of 

 varieties of pigeon might be chosen which 

 differ so thoroughly, internally as well as 

 externally, that an ornithologist, treating 

 them as wild birds, would be compelled to 

 grant them specific, and even distinct ge- 

 neric rank. Yet, since all these have indis- 

 putably arisen from the wild rock-dove, it 

 is clear that naturalists who admit a unity 

 to such domestic races, which professed 

 breeders have often laughed to scorn, should 

 in turn be cautious before deriding the unity 

 of wild ones. 



SELECTION^ How, then, haye 

 p By external 



conditions or habits alone? Qng^of their 



tell-tale fffltnrps i*g in 



not to thpir own gnnrl W tn mnn^ neo nr 



fancy. We know that all the breeds were 

 not produced in their present state of per- 

 fection, and the key is man'p 



lection. Nature ffiv^s successive 



acTcls ti^nr^ up. rpalflng-fQ^ himself yse= 

 ful breeds. Skilful breeders speak of the 

 organization as plastic and under control, 

 and have effected great changes within our 



