xii.] TEE ORIGIN OF SPECiES. 271 



vacters 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 com- 

 monly called external circumstances, upon the wild rock- 

 pigeon. On the contrary, from time immemorial, pigeon 

 lanciers 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 

 pio-eons to refute the doctrine which one sees put forth 

 on high authority, that "no other characters than those 

 founded on the development of bone for the attachment 

 of muscles" are capable of variation. In precise con- 

 tradiction 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 im- 

 portant 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 



