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LTHOUGH the mammalian extremities are nicely adapted by their structure to the functions they perform, the number of digits frequently varies from the normal. Moreover, different degrees of digital reduction may be observed in the extremities of animals whose habits are apparently identical. It is generally recognized that the digits of many mammals have been reduced to adapt the foot to rapid locomotion, but the evidence is chiefly circumstantial. In the present paper the writer will attempt to reconcile the various theories accounting for supernumerary digits, to call attention to certain evidences of reversion which may be daily observed, and to point out some little recognized factors concerned in the evolution of the mammalian foot.

We may assume that the primitive and typical mammalian foot was pentadactyl, in spite of Bardeleben's contention that the progenitors of the mammalia possessed not five but seven digits. Bardeleben's assumption was based upon the observation that certain mammals, the whale, for example, have more than five digits; that among five-toed forms six and seven digits occasionally occur; and that in many species small cartilages are present on each side of the hand and foot. These cartilages Bardeleben regards as digital rudiments, and the occurrence of extra digits is explained by him as reversion, a 'turning back' through heredity, to ancestral conditions. Unfortunately, the facts do not support this beautiful theory. Paleontology tells us that the forerunners of the mammalia possessed only five toes. Embryology has shown that the sixth digit of the whale, and the cartilages which Bardeleben supposes to be digital rudiments, develop secondarily some time after the typical five digits have appeared. Finally, observations have proved that the extra digits which occur in polydactylism do not develop from Bardeleben's 'digital rudiments,' but originate in an entirely different manner. We may, therefore, assume that the primitive mammalian foot was pentadactyl, and this being so, the occurrence of six or seven digits on a foot normally five-toed can not be attributed to reversion, unless we assume with Albrecht that it is reversion to the many-rayed fins of the Elasmobranch fishes, an absurd supposition. Such cases of polydactylism are, nevertheless, of frequent occurrence on the appendages of man and the cat. They have been explained as due to bifurcations or duplications of one of the typical five digits. Dissections show that this is really the case, for, though the skeletal