In the following chapters on the Pigeon we shall see that in pure breeds of various colours, when a blue bird is occasionally produced, certain black marks invariably appear on the wings and tail; so again, when variously coloured breeds are crossed, blue birds with the same black marks are frequently produced. We shall further see that these facts are explained by, and afford strong evidence in favour of, the view that all the breeds are descended from the rock-pigeon, or _Columba livia_, which is thus coloured and marked. But the appearance of the stripes on the various breeds of the horse, when of a dun-colour, does not afford nearly such good evidence of their descent from a single primitive stock as in the case of the pigeon; because no certainly wild horse is known as a standard of comparison; because the stripes when they do appear are variable in character; because there is far from sufficient evidence of the appearance of the stripes from the crossing of distinct breeds; and lastly, because all the species of the genus Equus have the spinal stripe, and several have shoulder and leg stripes. Nevertheless the similarity in the most distinct breeds in their general range of colour, in their dappling, and in the occasional appearance, especially in duns, of leg-stripes and of double or triple shoulder-stripes, taken together, indicate the probability of the descent of all the existing races from a single, dun-coloured, more or less striped, primitive stock, to which our horses still occasionally revert. {62} THE ASS.