THE ORIGIN OF SPECIES 155 



domestic pigeons, of which there are certainly more than a 

 hundred well-marked races. The most noteworthy of 

 these races are, the four great stocks known to the " fancy " 

 as tumblers, pouters, carriers, and fantails ; birds which 

 not only differ most singularly in size, colour, and habits, 

 but in the form of the beak and of the skull : in the pro- 

 portions of the beak to the skull ; in the number of tail- 

 feathers ; in the absolute and relative size of the feet ; in 

 the presence or absence of the uropygial gland ; in the 

 number of vertebrae in the back ; in short, in precisely 

 those characters in which the genera and species of birds 

 differ from one another. 



And it is most remarkable and instructive to observe, that 

 none of these races can be shown to have been originated 

 by the action of changes in what are commonly called 

 external circumstances, upon the wild rock-pigeon. On 

 the contrary, from time immemorial, pigeon fanciers have 

 had essentially similar methods of treating their pets, 

 which have been housed, fed, protected and cared for in 

 much the same way in all pigeonries. In fact, there is no 

 case better adapted than that of the pigeons to refute the 

 doctrine which one sees put forth on high authority, that 

 " no other characters than those founded on the develop- 

 ment of bone for the attachment of muscles " are capable of 

 variation. In precise contradiction of this hasty assertion, 

 Mr. Darwin's researches prove that the skeleton of the 

 wings in domestic pigeons has hardly varied at all from 

 that of the wild type ; while, on the other hand, it is in 

 exactly those respects, such as the relative length of the 

 beak and skull, the number of the vertebrae, and the 

 number of the tail-feathers, in which muscular exertion can 

 have no important influence, that the utmost amount of 

 variation has taken place. 



We have said that the following out of the properties 

 exhibited by physiological species would lead us into 

 difficulties, and at this point they begin to be obvious ; 

 for if, as the result of spontaneous variation and of selective 

 breeding, the progeny of a common stock may become 

 separated into groups distinguished from one another by 

 constant, not sexual, morphological characters, it is clear 

 that the physiological definition of species is likely to 

 clash with the morphological definition. No one would 

 hesitate to describe the pouter and the tumbler as distinct 



