two-, three-and four-toed forms, arose finally the five-toed generalized type of mammalian limb. The subsequent modifications of this organ, if we omit the divergent series of adaptations which gave rise to the pterosaurians and finally to the birds, present forms of specialization connected with the following modes of progression, namely, swimming, running, leaping and climbing. The first, exhibited in different degrees by the whale and the dolphin, we may pass by, both because it follows a process of adaptation unlike that of the group of animals to which man belongs, and because the change may be regarded as degenerative, inasmuch as the animal returns to a medium which makes less demand upon the structural resistance of the organism than did that which was relinquished. Adaptation to running finds its extreme form in the hoofed animals, in which the body is poised upon the extremity of the limbs, thereby conserving their full length for the purpose of rapid movement by employing the utmost length of stride; and in which the number of functioning toes is progressively reduced until, as in the horse, only a single massive and horn-shod central digit forms the body of the so-called foot. In adaptation to leaping, which is presented both by animals which have passed through an arboreal stage, as the kangaroo, and by others which have always been terrestrial, like the hare and jerboa, the structural modifications consist primarily in an increase in the size of and strength of the posterior limbs, with a concomitant degeneration of the fore limbs as they are less and less called upon to share in the function of supporting the body. Along with this primary modification goes a greater or less degree of specialization in the extremities of the limb, by which, as in the case of the running animals, one or more of these take upon themselves the chief support of the body and the rest suffer functional atrophy. In the jerboa, for example, one toe only is thus degenerate, while in the kangaroo three are rudimentary.
It is with the modification of the five-toed limb for the purpose of climbing that we are here especially concerned, since it is in the arboreal group of animals only that the specialization of the fore limb in the form of a hand appears, and since it is to the adaptations fostered by this form of existence that man owes the early development of his own dextrous and accomplished manipulative organ. This modification consists, first, in the modeling of the extremities of the limbs to a form which made the act of grasping possible; secondly, in the separation of the whole system of limb terminations into two opposable groups, by which primarily a more perfect grasp was secured, and later the refined manipulation of objects was made possible; and finally, in the differentiation of hind and fore limbs, by which the former were made to provide secure and rapid locomotion and the latter were left free for specialization controlled by the sole condition of prehension and manipulation.