14 HISTORY OF THE HUMAN BODY 



its proliferations in the form of scales, horns and claws. The 

 great increase in the size and strength of the limbs, begun in 

 the previous period, reached here a high degree of perfection, 

 and towards the end of this period vertebrates were for the 

 first time enabled by the help of these to lift their bodies com- 

 pletely from the ground and exchange the crawling move- 

 ments for a definite walk. 



But the most important of all the changes produced by a 

 land environment has been the rapid increase in the size and 

 efficiency of^ the central nervous system, which became de- 

 veloped in part through the need of controlling the larger limb 

 muscles, and in part in response to the far more varied 

 environment afforded by the land surfaces and the consequent 

 necessity of recording a larger number of sensory impressions. 

 By a curious and indirect method this development, especially 

 that of the perceptive centers of the brain, has been still more 

 encouraged in a certain group of rather generalized mammals 

 through the occupation of an arboreal environment. The 

 direct result of this was, that in these animals, which were, in 

 the main, large enough to grasp the boughs in climbing, a 

 prehensile paw with an opposable first digit was developed on 

 both anterior and posterior limbs, and this new tool, especially 

 the anterior set, which became hands, from now on allowed 

 the animals to grasp all sorts of objects, and expose them to 

 a more careful scrutiny, thus causing a continually greater 

 development of the recording centers of the brain. 



As this arboreal environment has been the latest in the 

 line of human history previous to the assumption of a strictly 

 terrestrial life, there are still in man's body more evidences 

 of this than of the earlier stages, but these, because they are 

 the latest, are also the most superficial, and consist of such 

 characters as the flattened nails, the pectoral position of the 

 mammae, and the opposable thumbs. 



The latest change of all, the assumption of an erect position 

 and the emancipation of the anterior limbs from all locomotive 

 functions, has necessitated a few modifications, especially 

 changes in the pelvic girdle and in the relative size and 



