30 THE DARWINIAN THEORY 



(v.) All races of domestic pigeons are perfectly 

 fertile when crossed, and their mongrel offspring are 

 also fertile. Hybrids between even closely allied 

 species of pigeons are, on the other hand, sterile. 



(vi.) All domestic pigeons have a remarkable ten- 

 dency to revert in minute details of colouring to the 

 blue rock-pigeon. This is of a slate-blue colour, 

 with two bars on the wings, and a black bar near the 

 end of the tail. The outer webs of the outer tail- 

 feathers are edged with white : these markings are 

 not seen together in any other species of the family. 

 This tendency to revert was demonstrated by 

 Darwin as follows : He first crossed a white fantail 

 with a black barb ; then a black barb with a red spot 

 (a white bird with a red tail and a red spot on the 

 forehead). He then succeeded in crossing the 

 mongrel barb-fan tail with the mongrel barb-spot, 

 and the birds produced were blue, with markings 

 on the tail and wings exactly like those of the 

 ancestral rock-pigeon. Thus two black barbs, a red 

 spot, and a white fantail, produced as grandchildren 

 birds having every characteristic of Columba livia, 

 including markings found in no other wild pigeon. 



BLUE 

 ROCK 



FANTAIL 



WHITE 



