RESPONSIVE LIFE OF ORGANISMS 



487 



about. Thus things may be known as objects, and not as 

 mere obtruding features of the general environment. It is 

 hardly conceivable that the specialized hand of a bird or of a 

 fish could be of much use in the educating of its possessor; 

 the variety of sense impressions it could furnish would be 

 very limited. Doubtless the possession of so adaptable a 

 grasping organ has been a large factor in human develop- 

 ment. It has made man a tool-using animal. It is the 

 recognition of this service that has made 

 hand- training (manual training) an integral 

 part of our educational system. 



The human brain is distinguished by a 

 very great development of the cerebral hemi- 

 spheres. These, as we have seen, are rela- 

 tively small in the salamander. They in- 

 crease in size with improvement in mental 

 power in all the higher vertebrates. They 

 overspread first the olfactory and then the 

 optic lobes and the upper side of the cere- 

 bellum, and in the higher mammals their 

 cortex becomes thrown into folds increasing 

 thus its superficial area. They reach their FJG 2?2 Boneg of 

 maximum development in the human the foot of a spotted 



salamander, a, te- 



species, exceeding many times in weight g^ r la . 6> / ibi tl rs ai 

 all other parts of the nervous system put bones; e phalanges, 

 together. 



The hemispheres constitute, as we have seen, the chief con- 

 trol center of the vertebrate nervous system. This added 

 mass of nervous matter, which was not, in the beginning of 

 development, necessary to the organism, and which is still 

 unconcerned with the ordinary performance of the most 

 vital processes of the body (although connected with all), 

 may be conceived of as containing innumerable possible 

 nerve paths, formed by the association together of its ex- 



