I70 THE DOCTRINE OF DESCENT. 



colour and shape transformed into new creatures, 

 which yet from time to time spontaneously, or by 

 crossing with other conspicuous races, produce birds 

 which, in colouring and characteristic pencilings of 

 black bars on wings and tail, resemble the wild rock- 

 pigeon. 



" I paired," says Darwin,^' " a mongrel female barb- 

 fantail with a mongrel male barb-spot, neither of which 

 mongrels had the least blue about them. Let it 

 be remembered that blue barbs are excessively rare; that 

 spots, as has been already stated, were perfectly char- 

 acterized in the year 1676, and breed perfectly true; this 

 likewise is the case with white fantails, so much so that 

 I have never heard of white fantails showing any other 

 colour. Nevertheless, the offspring from the above two 

 mongrels was of exactly the same blue tint over the 

 whole back and wings as that of the wild rock- 

 pigeon of the Shetland Islands; the double black 

 wing-bars were equally, conspicuous; the tail was exactly 

 alike in all its characters, and the croup was pure white." 



Another reversion frequently to be observed is the 

 striping of the feral domestic cat of Europe, in which 

 it resembles the wild-cat so closely as to be scarcely 

 distingtiishable. Darwin has collected evidence from 

 which we may infer that the wild aHcestral stock of 

 the horse was striped, and this evidence includes the 

 appearance of striped individuals. But yet another 

 strange phenomenon in horses may be interpreted by 

 atavism. Foals are occasionally born with supernumerary 

 toes. This " monstrosity " can be explained only by 

 reversion to the three-toed historical ancestors of t"he 

 present genus. These vouchers are sufficient. 



