The most remarkable and instructive example of variation produced by human selection is afforded by the various races and breeds of domestic pigeons, not only because the variations produced are often most extraordinary in amount and diverse in character, but because in this case there is no doubt whatever that all have been derived from one wild species, the common rock-pigeon (Columba livia). As this is a very important point it is well to state the evidence on which the belief is founded. 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. He crossed some white fantails with some black barbs, and the mongrels were black, brown, or mottled. He also crossed a barb with a spot, which is a white bird with a red tail and red spot on the forehead, and the mongrel offspring were dusky and mottled. On now crossing these two sets of mongrels with each other, he obtained a bird of a beautiful blue colour, with the barred and white edged tail, and double-banded wings, so as almost exactly to resemble a wild rock-pigeon. This bird was descended in the second generation from a pure white and pure black bird, both of which when unmixed breed their kind remarkably true. These facts, well known to experienced pigeon-fanciers, together with the habits of the birds, which all like to nest in holes, or dovecots, not in trees like the great majority of wild pigeons, have led to the general belief in the single origin of all the different kinds.
In order to afford some idea of the great differences which exist among domesticated pigeons, it will be well to give a brief abstract of Mr. Darwin's account of them. He divides them into eleven distinct races, most of which have several sub-races.
RACE I. Pouters.—These are especially distinguished by the enormously enlarged crop, which can be so inflated in some birds as almost to conceal the beak. They are very long in the body and legs and stand almost upright, so as to present a very distinct appearance. Their skeleton has become modified, the ribs being broader and the vertebrae more numerous than in other pigeons.
RACE II. Carriers.—These are large, long-necked birds, with a long pointed beak, and the eyes surrounded with a naked carunculated skin or wattle, which is also largely developed at the base of the beak. The opening of the mouth is unusually wide. There are several sub-races, one being called Dragons.
RACE III. Runts.—These are very large-bodied, long-beaked pigeons, with naked skin round the eyes. The wings are usually very long, the legs long, and the feet large, and the skin of the neck is often red. There are several sub-races, and these differ very much, forming a series of links between the wild rock-pigeon and the carrier.
RACE IV. Barbs.—These are remarkable for their very short and thick beak, so unlike that of most pigeons that fanciers compare it with that of a bullfinch. They have also a naked carunculated skin round the eyes, and the skin over the nostrils swollen.
RACE V. Fantails.—Short-bodied and rather small-beaked pigeons, with an enormously developed tail, consisting usually of from fourteen to forty feathers instead of twelve, the regular number in all other pigeons, wild and tame. The tail spreads out like a fan and is usually carried erect, and the bird bends back its slender neck, so that in highly-bred varieties the head touches the tail. The feet are small, and they walk stiffly.
RACE VI. Turbits and Owls.—These are characterised by the feathers of the middle of neck and breast in front spreading out irregularly so as to form a frill. The Turbits also have a crest on the head, and both have the beak exceedingly short.
RACE VII. Tumblers.—These have a small body and short beak, but they are specially distinguished by the singular habit of tumbling over backwards during flight. One of the sub-races, the Indian Lotan or Ground tumbler, if slightly shaken and placed on the ground, will immediately begin tumbling head over heels until taken up and soothed. If not taken up, some of them will go on tumbling till they die. Some English tumblers are almost equally persistent. A writer, quoted by Mr. Darwin, says that these birds generally begin to tumble almost as soon as they can fly; "at three months old they tumble well, but still fly strong; at five or six months they tumble excessively; and in the second year they mostly give up flying, on account of their tumbling so much and so close to the ground. Some fly round with the flock, throwing a clean summersault every few yards till they are obliged to settle from giddiness and exhaustion. These are called Air-tumblers, and they commonly throw from twenty to thirty summersaults in a minute, each clear and clean. I have one red cock that I have on two or three occasions timed by my watch, and counted forty summersaults in the minute. At first they throw a single summersault, then it is double, till it becomes a continuous roll, which puts an end to flying, for if they fly a few yards over they go, and roll till they reach the ground. Thus I had one kill herself, and another broke his leg. Many of them turn over only a few inches from the ground, and will tumble two or three times in flying across their loft. These are called House-tumblers from tumbling in the house. The act of tumbling seems to be one over which they have no control, an involuntary movement which they seem to try to prevent. I have seen a bird sometimes in his struggles fly a yard or two straight upwards, the impulse forcing him backwards while he struggles to go forwards."[34]
The Short-faced tumblers are an improved sub-race which have almost lost the power of tumbling, but are valued for possessing some other characteristics in an extreme degree. They are very small, have almost globular heads, and a very minute beak, so that fanciers say the head of a perfect bird should resemble a cherry with a barleycorn stuck in it. Some of these weigh less than seven ounces, whereas the wild rock-pigeon weighs about fourteen ounces. The feet, too, are very short and small, and the middle toe has twelve or thirteen instead of fourteen or fifteen scutellae. They have often only nine primary wing-feathers instead of ten as in all other pigeons.