conjecture what is the nature of the bond of connexion. But between homologous parts—between the fore and hind limbs—between the hair, hoofs, horns, and teeth—we can see that parts which are closely similar during their early development, and which are exposed to similar conditions, would be liable to be modified in the same manner. Homologous parts, from having the same nature, are apt to blend together and, when many exist, to vary in number.
Although every variation is either directly or indirectly caused by some change in the surrounding conditions, we must never forget that the nature of the organisation which is acted on essentially governs the result. Distinct organisms, when placed under similar conditions, vary in different manners, whilst closely-allied organisms under dissimilar conditions often vary in nearly the same manner. We see this in the same modification frequently reappearing at long intervals of time in the same variety, and likewise in the several striking cases given of analogous or parallel varieties. Although some of these latter cases are simply due to reversion, others cannot thus be accounted for.
From the indirect action of changed conditions on the organisation, through the impaired state of the reproductive organs—from the direct action of such conditions (and this will cause the individuals of the same species either to vary in the same manner, or differently in accordance with slight differences in their constitution)—from the effects of the increased or decreased use of parts,—and from correlation,—the variability of our domesticated productions is complicated in an extreme degree. The whole organisation becomes slightly plastic. Although each modification must have its proper exciting cause, and though each is subjected to law, yet we can so rarely trace the precise relation between cause and effect, that we are tempted to speak of variations as if they spontaneously arose. We may even call them accidental, but this must be only in the sense in which we say that a fragment of rock dropped from a height owes its shape to accident.
It may be worth while briefly to consider the results of the exposure to unnatural conditions of a large number of animals of the same species, allowed to cross freely, with no selection of any
kind; and afterwards to consider the results when selection is brought into play. Let us suppose that 500 wild rock-pigeons were confined in their native land in an aviary, and fed in the same manner as pigeons usually are; and that they were not allowed to increase in number. As pigeons propagate so rapidly, I suppose that a thousand or fifteen hundred birds would have to be annually killed by mere chance. After several generations had been thus reared, we may feel sure that some of the young birds would vary, and the variations would tend to be inherited; for at the present day slight deviations of structure often occur, but, as most breeds are already well established, these modifications are rejected as blemishes. It would be tedious even to enumerate the multitude of points which still go on varying or have recently varied. Many variations would occur in correlation, as the length of the wing and tail feathers—the number of the primary wing-feathers, as well as the number and breadth of the ribs, in correlation with the size and form of the body—the number of the scutellæ, with the size of the feet—the length of the tongue, with the length of the beak—the size of the nostrils and eyelids and the form of lower jaw in correlation with the development of wattle—the nakedness of the young with the future colour of the plumage—the size of the feet and beak, and other such points. Lastly, as our birds are supposed to be confined in an aviary, they would use their wings and legs but little, and certain parts of the skeleton, such as the sternum and scapulæ and the feet, would in consequence become slightly reduced in size.
As in our assumed case many birds have to be indiscriminately killed every year, the chances are against any new variety surviving long enough to breed. And as the variations which arise are of an extremely diversified nature, the chances are very great against two birds pairing which have varied in the same manner; nevertheless, a varying bird even when not thus paired would occasionally transmit its character to its young; and these would not only be exposed to the same conditions which first caused the variation in question to appear, but would in addition inherit from their one modified parent a tendency again to vary in the same manner. So that, if the conditions decidedly tended to induce some particular variation, all the birds might
in the course of time become similarly modified. But a far commoner result would be, that one bird would vary in one way and another bird in another way; one would be born with a little longer beak, and another with a shorter beak; one would gain some black feathers, another some white or red feathers. And as these birds would be continually intercrossing, the final result would be a body of individuals differing from each other slightly in many ways, yet far more than did the original rock-pigeons. But there would not be the least tendency to the formation of distinct breeds.
If two separate lots of pigeons were to be treated in the manner just described, one in England and the other in a tropical country, the two lots being supplied with different food, would they, after many generations had passed, differ? When we reflect on the cases given in the twenty-third chapter, and on such facts as the difference in former times between the breeds of cattle, sheep, &c., in almost every district of Europe, we are strongly inclined to admit that the two lots would be differently modified through the influence of climate and food. But the evidence on the definite action of changed conditions is in most cases insufficient; and, with respect to pigeons, I have had the opportunity of examining a large collection of domesticated birds, sent to me by Sir W. Elliot from India, and they varied in a remarkably similar manner with our European birds.
If two distinct breeds were to be confined together in equal numbers, there is reason to suspect that they would to a certain extent prefer pairing with their own kind; but they would likewise intercross. From the greater vigour and fertility of the crossed offspring, the whole body would by this means become interblended sooner than would otherwise have occurred. From certain breeds being prepotent over others, it does not follow that the interblended progeny would be strictly intermediate in character. I have, also, proved that the act of crossing in itself gives a strong tendency to reversion, so that the crossed offspring would tend to revert to the state of the aboriginal rock-pigeon. In the course of time they would probably be not much more heterogeneous in character than in our first case, when birds of the same breed were confined together.