Page:Popular Science Monthly Volume 68.djvu/351

Rh antelope, bison and ox; these animals roam the open plain and upland, and the outer toes would be of no use except for the occasional support of the foot when these animals enter the water to drink. We find, therefore, that only the small hoofs of the reduced toes persist. In the foot of the camel and giraffe even these vestiges have disappeared, as their habitat has long been the dry, sandy plain.

From these observations it seems plain that among ungulates the functions of the digits have been affected by the habitat of the various animals; and that there is a direct relation between the degree of digital reduction and the character of the country traversed. The use of the foot as an organ of locomotion alone, and the assumption of the digitigrade posture, were the primary factors producing reductions of the toes; the degree of such reductions and the type of foot produced have been dependent upon the habitat of the animals. The artiodactyl foot was formed in adaptation to semi-aquatic habits, and as the animals changed their habitat to 'terra firma' a further reduction of the digits resulted. The perissodactyl, or odd-toed, type of foot, began with the assumption of the digitigrade posture by animals which traversed solid ground, and the digits were further reduced as the digitigrade posture was developed to perfection.

It has been assumed that those digits which were useless would disappear. There is evidence that if they were not reduced they would be not only useless, but of distinct disadvantage to the animals. The writer has observed that the extra hallux rarely occurs on the pes of hunting dogs; when it does occur it is frequently injured and sometimes completely torn away. It is also noteworthy that the extra toe is most often found on the pes of the St. Bernard and Newfoundland, in which breeds it may be of some use for swimming, and walking through deep snows. It has been observed, too, that the small hoofs of the sheep and deer grow rapidly on the second and fifth digits, but are normally worn away by daily use. If these animals are kept in unnatural surroundings, as when sheep are deprived of rocky pasture, or deer kept in zoological gardens, the hoofs of the reduced digits will grow long, curved and twisted to such an extent as seriously to impede locomotion. We can readily see that should the wild deer or sheep change its habitat to the smooth footing of the open plains, the same abnormal growths might occur and hinder rapid locomotion. Variations tending toward the reduction of these digits would favor the survival of their possessors, and give rise to the type of foot found among the antelopes, cattle and giraffes.

In conclusion it may be of interest to speak briefly of the digital reductions which have taken place in the foot of the running birds (Ratitæ). The most primitive of the birds exhibited the digitigrade posture, but walked upon the ball of the foot. This may have caused the reduction of the fifth digit (which early disappeared), and