460 POPULAR SCIENCE MONTHLY. 



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 de- 

 grees 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 ex- 

 tremity 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 con- 

 sist 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. 



