The wild rock-pigeon is of a slaty-blue colour, the tail has a dark band across the end, the wings have two black bands, and the outer tail-feathers are edged with white at the base.

No other wild pigeon in the world has this combination of characters.

Now in every one of the domestic varieties, even the most extreme, all the above marks, even to the white edging of the outer tail-feathers, are sometimes found perfectly developed.

When birds belonging to two distinct breeds are crossed one or more times, neither of the parents being blue, or having any of the above-named marks, the mongrel offspring are very apt to acquire some of these characters.

Mr.Darwin gives instances which he observed himself.