In the foregoing table we see that, in comparison with the wild duck, the reduction in the length of the bones of the wing, relatively to those of the legs, though slight, is universal. The reduction is least in the Call-duck, which has the power and the habit of frequently flying.
In weight there is a greater relative difference between the bones of the leg and wing, as may be seen in the following table:—
| Name of Breed. | Weight of Femur, Tibia, and Metatarsus | Weight of Humerus, Radius, and Metacarpus | Or as |
| Grains. | Grains. | ||
| Wild mallard | 54 | 97 | 100 : 179 |
| Aylesbury | 164 | 204 | 100 : 124 |
| Hooked-bill | 107 | 160 | 100 : 149 |
| Tufted (Dutch) | 111 | 148 | 100 : 133 |
| Penguin | 75 | 90.5 | 100 : 120 |
| Labrador | 141 | 165 | 100 : 117 |
| Call | 57 | 93 | 100 : 163 |
| Weight of all the Bones of the Leg and Foot. | Weight of all the Bones of the Wing. | ||
| Grains. | Grains. | ||
| Wild duck (another specimen) | 66 | 115 | 100 : 173 |
| Common domestic duck | 127 | 158 | 100 : 124 |
In these domesticated birds, the considerably lessened weight of the bones of the wing (i.e. on an average, twenty-five per cent. of their proper proportional weight), as well as their slightly lessened length, relatively to the leg-bones, might follow, not from any actual decrease in the wing-bones, but from the increased weight and length of the bones of the legs. The first of the two tables on the next page shows that the leg-bones relatively to the weight of the entire skeleton have really increased in weight; but the second table shows that according to the same standard the wing-bones have also really decreased in weight; so that the relative disproportion shown in the foregoing tables between the wing and leg bones, in comparison with those of the wild duck, is partly due to the increase in weight and length of the leg-bones, and partly to the decrease in weight and length of the wing-bones.
With respect to the two following tables, I may first state that I tested them by taking another skeleton of a wild duck and of a common domestic duck, and by comparing the weight of all the bones of the leg with all those of the wings, and the result was the same. In the first of these tables we see that the leg-bones in each case have increased in actual weight. It might have been expected that, with the increased or decreased weight of the entire skeleton, the leg-bones would have become proportionally heavier or lighter; but their greater weight in all the breeds relatively to the other bones can be accounted for only by these domestic birds having used their legs in walking and standing much more than the wild, for they never fly, and the more artificial breeds rarely swim. In the second table we see, with the exception of one case, a plain reduction in the weight of the bones of the wing, and this no doubt has resulted from their lessened use. The one exceptional case, namely, in one of the Call-ducks, is in truth no exception, for this bird was constantly in the habit of flying about: and I have seen it day after day rise from my grounds, and fly for a long time in circles of more than a mile in diameter. In this Call-duck there is not only no decrease, but an actual increase in the weight of the wing-bones relatively to those of the wild duck; and this probably is consequent on the remarkable lightness and thinness of all the bones of the skeleton.
| Name of Breed. | Weight of entire Skeleton. (N.B. One Metatarsus and Foot was removed from each skeleton, as it had been accidentally lost in two cases.) | Weight of Femur, Tibia, and Metatarsus. | Or as |
| Grains. | Grains. | ||
| Wild mallard | 839 | 54 | 1000 : 64 |
| Aylesbury | 1925 | 164 | 1000 : 85 |
| Tufted (Dutch) | 1404 | 111 | 1000 : 79 |
| Penguin | 871 | 75 | 1000 : 86 |
| Call (from Mr. Fox) | 717 | 57 | 1000 : 79 |
| Weight of Skeleton as above. | Weight of Humerus, Radius and Ulna, and Metacarpus. | ||
| Grains. | Grains. | ||
| Wild mallard | 839 | 97 | 1000 : 115 |
| Aylesbury | 1925 | 204 | 1000 : 105 |
| Tufted (Dutch) | 1404 | 148 | 1000 : 105 |
| Penguin | 871 | 90 | 1000 : 103 |
| Call (from Mr. Baker) | 914 | 100 | 1000 : 109 |
| Call (from Mr. Fox) | 717 | 92 | 1000 : 129 |
Lastly, I weighed the furcula, coracoids, and scapula of a wild duck and of a common domestic duck, and I found that their weight, relatively to that of the whole skeleton, was as one hundred in the former to eighty-nine in the latter; this shows that these bones in the domestic duck have been reduced eleven per cent. of their due proportional weight. The prominence of the crest of the sternum, relatively to its length, is also much reduced in all the domestic breeds. These changes have evidently been caused by the lessened use of the wings.
It is well known that several birds, belonging to different Orders, and inhabiting oceanic islands, have their wings greatly reduced in size and are incapable of flight. I suggested in my 'Origin of Species' that, as these birds are not persecuted by any enemies, the reduction of their wings has probably been caused by gradual disuse. Hence, during the earlier stages of the
process of reduction, such birds might be expected to resemble in the state of their organs of flight our domesticated ducks. This is the case with the water-hen (Gallinula nesiotis) of Tristan d'Acunha, which "can flutter a little, but obviously uses its legs, and not its wings, as a mode of escape." Now Mr. Sclater[[454]] finds in this bird that the wings, sternum, and coracoids, are all reduced in length, and the crest of the sternum in depth, in comparison with the same bones in the European water-hen (G. chloropus). On the other hand, the thigh-bones and pelvis are increased in length, the former by four lines, relatively to the same bones in the common water-hen. Hence in the skeleton of this natural species nearly the same changes have occurred, only carried a little further, as with our domestic ducks, and in this latter case I presume no one will dispute that they have resulted from the lessened use of the wings and the increased use of the legs.
The Goose.
This bird deserves some notice, as hardly any other anciently domesticated bird or quadruped has varied so little. That geese were anciently domesticated we know from certain verses in Homer; and from these birds having been kept (388 B.C.) in the Capitol at Rome as sacred to Juno, which sacredness implies great antiquity[[455]]. That the goose has varied in some degree, we may infer from naturalists not being unanimous with respect to its wild parent-form; though the difficulty is chiefly due to the existence of three or four closely allied wild European species[[456]]. A large majority of capable judges are convinced that our geese are descended from the wild Grey-lag goose (A. ferus); the young of which can easily be tamed,[[457]] and are domesticated by the Laplanders. This species, when crossed with the domestic goose, produced in the Zoological Gardens, as I was assured in
1849, perfectly fertile offspring.[[458]] Yarrell[[459]] has observed that the lower part of the trachea of the domestic goose is sometimes flattened, and that a ring of white feathers sometimes surrounds the base of the beak. These characters seem at first good indications of a cross at some former period with the white-fronted goose (A. albifrons); but the white ring is variable in this latter species, and we must not overlook the law of analogous variation; that is, of one species assuming some of the characters of allied species.