USE AND DISUSE 35 



been acquired by every individual of the race during hundreds 

 of thousands of generations. Plainly then that which is trans- 

 mitted to the infant is not the modification, lut only the power 

 of acquiring the modification under similar circumstances a 

 power which has undergone such an evolution in high animal 

 organisms that, in man, for instance, nearly all the develop- 

 mental changes which occur between infancy and manhood 

 are attributable to it. It follows, therefore, that the exquisite 

 co-ordination of all the parts of a high animal is not due to 

 the inherited effects of use and disuse, but to this great 

 power of acquiring use-modifications along certain definite 

 lines ; so that if an animal varies in such a way as to have 

 one of its structures (e. g. horn, which is wholly inborn) larger 

 than it was in the parent, then all the other structures 

 associated with it, owing to the increased strain (i. e. the 

 increased stimulation) put on them, undergo a corresponding 

 modification, and thus preserve the harmony of all the parts 

 of the whole. So also if the horn be smaller than in the 

 parent, the lesser strain placed by it on the associated 

 structures causes them to develop less than in the parent, 

 whereby again the harmony of the whole is preserved. 



58. The power of acquiring fit modifications in response to 

 appropriate stimulation is that which specially differentiates 

 very high animal organisms from lower types. It enables 

 the animal possessing it to closely adapt itself to all sorts 

 of varying circumstances and environments to acquire, for 

 example, additional muscular powers under the stress of 

 need, or get rid by atrophy of the burdensome addition when 

 the need is past. It provides, in fact, a short-cut by means 

 of which qualities too numerous to be evolved and maintained 

 as inborn traits by Natural Selection (owing to the immense 

 mortality which would be involved) are evoked at need. 

 Without this power and the plasticity which results from it, 

 the multitudinous parts of high animals could not well be 

 co-ordinated, and, therefore, without it the evolution of the 

 higher animals could scarcely have been possible. Indeed it 

 is not too much to say, so vitally important is this power to 

 the higher animals, that, as regards them, the chief aim (if 

 we may use the expression) of Natural Selection has been to 

 evolve it. We shall see this very clearly when we come to 

 the study of Mind. But since this power of developing in 

 response to the stimulation of use operates mainly along 

 certain definite lines which are not quite the same in every 

 species since Nature has endowed animals with the power of 

 making, not all possible acquirements, but only certain fixed 



