EVOLUTION OF THE HUMAN HAND. 463 



and other parallel fingered species soon after their adoption of the 

 arboreal habit — or at least before the appearance of any important 

 modification of the earlier structural relations of their limbs — took 

 to a form of locomotion in which the body was partly supported from 

 beneath by the hind limbs and partly steadied or suspended from above 

 by the grasp of the fore limbs; so that the peculiar modification which 

 the arboreal form of life contributed to the animal type was incorpo- 

 rated in the hind limbs alone. The anthropoid apes, on the contrary, 

 which show this specialization in fore as well as hind limbs, we shall 

 conceive to have persisted in the quadrupedal habit during a period 

 the continuance of which was sufficiently protracted to allow of the 

 appearance of similar modifications in all four limbs. Only subse- 

 quent to this process of adaptation should we imagine the progenitor 

 of man to have arisen from the quadrupedal position and to have used 

 the fore limbs for the secondary support of the body by grasping the 

 upper branches. 



In this new function the limb specialized by opposition had prob- 

 ably little advantage over the more primitive hand of the monkey, in 

 so far as suspensional support was concerned. In respect to those 

 other uses upon which the subsequent development of man in all kinds 

 of mechanical skill depends, this new structural variation was of the 

 highest significance. The monkey tribe gave up the habit of walking 

 on all-fours too early and is suffering from the consequences to the 

 present day. 



This stage of development, however, represents a condition in which 

 the factors of further evolution are confused and the various parts of 

 the organism imperfectly adapted to the functions they are hereafter 

 to perform. Hands and feet conform to the same architectural type. 

 Both share in the unitary process of locomotion; the hands are capable 

 of supporting the fore part of the body in moving, the feet are still 

 prehensile organs. There is no exclusive functional specialization by 

 which fore and hind limbs may be set off from each other. This sub- 

 division of labor must come about through a development of the lower 

 limbs by which they become capable of the sole support of the body 

 at rest and in progress. In other words, the hands can not be released 

 from their office of steadying and supporting the body until sufficient 

 skeletal changes and muscular growth have taken place in the lower 

 limbs to enable them to carry on the function of locomotion alone. 

 The freeing of the hand for exclusively manipulative purposes thus 

 depends upon the replacement of the semi-erect posture by a fully 

 erect one, in which process the calf develops, the joints are straight- 

 ened and the whole limb rotates upon its point of attachment to the 

 body until the main axes of the two are parallel and each is vertical 

 in position. 



These changes could hardly have taken place during the continuance 



