Page:Popular Science Monthly Volume 68.djvu/348

344 mammals were plantigrades, resting the whole surface of the foot upon the ground in running. This posture is not favorable for rapid locomotion, as instanced by the lumbering gait of the bear. It has been retained only by animals which, through burrowing, swimming, climbing or means other than speed are enabled to escape their enemies or obtain their food. But both to beasts of prey and to their quarry increased speed and leaping power would be of great advantage in the struggle for existence. To obtain this advantage they had recourse to the same expedient to which on occasion plantigrade man still resorts—they ran upon their toes. If by variation, the digitigrade position became gradually, or suddenly, the fixed posture of the foot in progression, the structure of the digits would soon be affected. Provided that the feet were used only in locomotion, the shorter digits would not reach the ground. Being useless, they might soon disappear. The reduction of the digits has, therefore, been ascribed simply to the adoption of the digitigrade posture. This is, indeed, the chief, but it is not the only factor. It does not explain why the hallux of the dog and cat has atrophied, while the pollex persists; why the pig and waterdeer have four digits, the giraffe and camel only two, though all are digitigrade.

There are evidently three factors upon which the degree of digital reduction depends: (1) the specialization of the extremity for locomotion; (2) the degree of perfection to which the digitigrade posture is carried; (3) the character of the ground which the animals traverse.

The hallux of the dog and cat has been reduced, because the pes is used only for progression and in the digitigrade position. The pollex of these animals has been retained, as the claw is useful to the cats in climbing and in catching their prey—to the dogs and wolves in burrowing and in holding their prey. It is interesting to note that the pollex is no longer a functional organ in the manus of the hyena, an animal which feeds chiefly on carrion.

To the Carnivora, which are beasts of prey, a padded foot and sharp claws are necessary structures. We, therefore, find that the digitigrade posture is not developed to the extreme. These animals run upon the ball of the foot; whatever may be the character of the country traversed, all four toes are used in progression, and no further reductions have taken place.

To the herbivorous ungulates claws are useless structures, and in escaping from their foes noise is no drawback. Speed is their chief requirement and this is increased by leaping from the tips of the toes. This method of progression would blunt the claws, which would then be modified to protect the toes.

If the digitigrade position is not well developed (as is the case with the slow, heavy ungulates like the elephant, tapir and rhinoceros) all the toes, or all but the first, may reach the ground and function in