Page:Popular Science Monthly Volume 68.djvu/350

346 possesses, as we should expect, four digits, arranged in pairs of equal size. The wild swine, although attached to water and to boggy ground, are nevertheless swift runners, and spend most of their time on more solid footing. The outer digits are retained because they are useful in keeping the foot from sinking in the mud, though they are not functional a good share of the time. The greater strains brought to bear upon the middle digits have resulted in their increased size (by variation) and their monopoly of the carpal and tarsal joints, firm articulation with which is no longer needed by the little-used outer toes. That these changes were advantageous to the swine is shown by the fact that related forms, less adaptive in this respect, have become extinct.

Of the deer family, the water chevrotain and its relatives are the only forms possessing four complete digits. Again we have to do with animals attached to swampy places, and the outer digits, though slender, are retained intact because of the extra support they offer in traversing boggy ground. The fusion of the metacarpal and metatarsal bones of the middle digits, which characterizes the foot of other ruminants, has evidently been prevented in the water deer by the spreading of the toes.

The red deer is one of the swiftest of runners and its usual habitat is the wooded plain and upland. It, however, readily takes to the water, as a large part of its food consists of aquatic plants. In roaming the more solid floor of the forest only the middle digits support the body. These have become relatively larger than those of swine, and are further strengthened hy the union of the metacarpal and metatarsal bones. The outer digits are perfectly useless in ordinary locomotion, but still perform two important functions: they serve to support the foot in yielding ground and give the deer a firm footing when running rapidly, especially down-hill. Any one may observe that in walking on fairly firm ground the foot of the deer leaves but two hoofprints, but that the foot of a running deer leaves four distinct marks. The performance of these functions has caused the retention of the lower portion of the outer toes. But as these digits no longer support any part of the weight of the animal, no proximal articulation is necessary and we find that the upper part of the metacarpal and metatarsal bones has atrophied.

Wild goats and sheep are mountain animals, feeding on rugged and precipitous slopes where the footing is precarious. Both sheep and goats are expert climbers and leapers, but in their ordinary habitat only the middle digits are used for supporting the weight of the body. The outer digits have, therefore, been reduced, but the hoofs and rudiments of two phalanges have been retained, because these small toes are used in climbing, and render the animals more sure-footed. No such function is performed by the second and fifth digits of the