Page:Popular Science Monthly Volume 68.djvu/352

348 had to do with the reduction of the hallux. The hallux has, however, been retained by most flying birds because it is used in perching, in prehension and in swimming. As the legs of birds are set well forward on the body, they are more widely separated above than below. This position has thrown the greatest strains upon the outer or fourth digit, which is always longer than the second toe. Now in the foot of running birds like the emu, the digits are used only in locomotion and the hallux has disappeared, as it was useless in progression. But the digitigrade posture of the emu is the same as that of the flying birds; the ball of the foot touches the ground and the remaining three digits, being all functional, are well developed. The ostrich, however, has increased its swiftness by running (and walking) upon the tips of its toes. This posture would throw the weight of the body and the work of locomotion upon the longer third and fourth digits. As the foot of the ostrich is used only in locomotion, and as the birds traverse the smooth floor of the desert, the shorter second digit would fail to reach the ground and eventually disappear. As a result we find that the ostrich has only two functional digits.

The digits of birds, therefore, show structural changes which are exactly paralleled by those exhibited by various ungulates, and the digital reductions which have taken place may be attributed to the same factors in each case.