396 THE EVOLUTION OF MEMORY 



able happenings of our daily lives. So, also, if we saw a thousand 

 sheep jump in succession over a gate, we could recall, not the 

 action of every sheep, but in the main only a general notion. 

 The ultimate result is, that our minds are not burdened with a 

 mass of unnecessary details, but store only what is likely to be 

 useful. Our general impressions are concentrated, condensed 

 recollections. Concentrated memories also, as we shall see 

 presently, are all our acquired dexterities in thinking and acting. 

 Moreover, apart from the concentration which occurs in the 

 formation of general impressions, much that is seemingly lost and 

 forgotten is none the less stored, and waits ready to be dragged 

 into the conscious mind by some associated fact or idea. Thus 

 when walking down the street we may see a face which is recorded 

 but not recalled unless a second meeting awakens the memory. 



655. Every animal species is fitted by its structures and their 

 associated faculties to its particular place in nature. In some 

 cases it holds its own largely through the great evolution of some 

 one structure or group of structures in co-adaptation with which 

 the whole mind and body is modified. Thus the bat is especially 

 distinguished by the great development of its fingers and of the 

 web between them, and the elephant by its trunk. The principal 

 distinguishing physical peculiarity of man is the enormous relative 

 size of that upper part of the vertebrate brain which is termed the 

 cerebrum, and which we have every reason to believe constitutes 

 his organ of memory and thought, and of all the mental processes 

 and physical co-ordinations that depend on them. The cerebrum, 

 especially the human cerebrum, is one of those organs which, after 

 birth, develop under the influence of use, its growth being cor- 

 related to the growth of the mind under the stimulus of experience. 

 In evidence of this is the fact, that if one hemisphere of the 

 cerebrum be injured, as by disease, the other tends to hypertrophy 

 under the stimulus of increased use. 



656. Associated in a special way with man's great brain are 

 his organs of speech and manipulation. These three structures, 

 the brain, the vocal apparatus, and the hand, undoubtedly 

 underwent concurrent evolution by the constant survival, during 

 what must have been a period of intense competition, of those 

 individuals who were * naturally ' (i.e. innately) the best capable 

 of receiving and storing experience, of using it for the intelligent 

 manipulation of objects, and of communicating it to their fellows 

 and descendants through the medium of speech. Even the 

 highest of the lower animals are able to learn from one another 



