43 8 THE POPULAR SCIENCE MONTHLY. 



divided surfaces. This reproduction is but a special manifestation of 

 the reconstructive change which is always taking place in the nervous 

 system ; it being not less obvious to the eye of reason that the ' waste ' 

 occasioned by its functional activity must be constantly repaired by 

 the production of new tissue, than it is to the eye of sense that such 

 reparation supplies an actual loss of substance by disease or injury. 



"Now, in this constant and active reconstruction of the nervous 

 system, we recognize a most marked conformity to the general plan 

 manifested in the nutrition of the organism as a whole. For, in the 

 first place, it is obvious that there is a tendency to the production of a 

 determinate type of structure ; which type is often not merely that of 

 the species, but some special modification of it which characterized 

 one or both of the progenitors. But this type is peculiarly liable to 

 modification during the early period of life ; in which the functional 

 activity of the nervous system (and particularly of the brain) is extraor- 

 dinarily great, and the reconstructive process proportionally active. 

 And this modifiability expresses itself in the formation of the mechan- 

 ism by which those secondarily automatic modes of movement come 

 to be established, which, in man, take the place of those that are con- 

 genital in most of the animals beneath him ; and those modes of sense- 

 perception come to be acquired, which are elsewhere clearly instinctive. 

 For there can be no reasonable doubt that, in both cases, a nervous 

 mechanism is developed in the course of this self -education, correspond- 

 ing with that which the lower animals inherit from their parents. The 

 plan of that rebuilding process, which is necessary to maintain the 

 integrity of the organism generally, and which goes on with peculiar 

 activity in this portion of it, is thus being incessantly modified ; and 

 in this manner all that portion of it which ministers to the external 

 life of sense and motion that is shared by man with the animal king- 

 dom at large, becomes at adult age the expression of the habits which 

 the individual has acquired during the period of growth and develop- 

 ment. Of these habits, some are common to the race generally, while 

 others are peculiar to the individual ; those of the former kind (such 

 as walking erect) being universally acquired, save where physical in- 

 ability prevents ; while for the latter a special training is needed, 

 which is usually the more effective the earlier it is begun — as is re- 

 markably seen in the case of such feats of dexterity as require a con- 

 joint education of the perceptive and of the motor powers. And when 

 thus developed during the period of growth, so as to have become a 

 part of the constitution of the adult, the acquired mechanism is thence- 

 forth maintained in the ordinary course of the nutritive operations, so 

 as to be ready for use when called upon, even after long inaction. 



" What is so clearly true of the nervous apparatus of animal life, can 

 scarcely be -otherwise than true of that which ministers to the automat- 

 ic activity of the mind. For, as already shown, the study of psychol- 

 ogy has evolved no more certain result than that there are uniformi- 



