{70 THE DOCTRINE OF DESCENT, 
are races of pigeons purely bred for several centuries, 
and in 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 Icast blue about them. 
Let it be remembered that blue barbs are excessively 
rare; that spots, as has been already stated, were 
perfectly characterized 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 
distinguishable. Darwin has collected evidence from 
which we may infer that the wild ancestral 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 
