THE LAWS OF VARIABILITY. 53 



In attempting to estimate the amount of 

 ' structural difference between allied domestic 

 races, we are soon involved in doubt, from not knowing 

 whether they are descended from one or several parent 

 species. This point, if it could be cleared up, would be 

 interesting ; if, for instance, it could be shown that the 

 greyhound, bloodhound, terrier, spaniel, and bull-dog, 

 which we all know propagate their kind truly, were the 

 offspring of any single species. Then such facts would 

 have great weight in making us doubt about the immu- 

 tability of the many closely allied natural species — for 

 instance, of the many foxes — inhabiting different quar- 

 ters of the world. 



DESCENT OF THE DOMESTIC PIGEON". 



Ori<nn of Great as are the differences between the 



Species, breeds of the pigeon, I am fully convinced that 

 p&se ' the common opinion of naturalists is correct, 

 namely, that all are descended from the rock-pigeon 

 (Columba livid), including under this term several geo- 

 graphical races or subspecies, which differ from each 

 other in the most trifling respects. As several of the rea- 

 sons which have led me to this belief are in some degree 

 applicable in other cases, I will here briefly give them. 

 If the several breeds are not varieties, and have not pro- 

 ceeded from the rock-pigeon, they must have descended 

 from at least seven or eight aboriginal stocks ; for it is 

 impossible to make the present domestic breeds by the 

 crossing of any lesser number : how, for instance, could 

 a pouter be produced by crossing two breeds unless one 

 of the parent-stocks possessed the characteristic enormous 

 crop ? The supposed aboriginal stocks must all have been 

 rock-pigeons — that is, they did not breed or willingly 

 perch on trees. But besides C. livia, with its geographical 



