44 MENTAL EVOLUTION IN ANIMALS. 



lutely indissoluble. And similarly, of course, associations of 

 ideas acquired only during the life-history of the individual 

 need to be more or less constantly maintained by repetition, 

 just as muscular co-ordinations similarly acquired can only 

 be maintained by practice. 



Upon the whole, therefore, it is impossible that there could 

 be a more precise parallelism between these two manifesta- 

 tions of nervous machinery, and it is one which for recog- 

 nition in a general way does not require scientific analysis ; 

 it has been perceived by the common sense of mankind — 

 witness, for instance, the term " gymnastics " having become 

 applicable to mental no less than to muscular co-ordinations. 

 But, for the sake of systematic completeness, I shall conclude 

 this exposition by briefly pointing out that all those patho- 

 logical derangements which occur in the nervous centres that 

 preside over muscular activities, have their parallels in 

 similar derangements which occur in the nervous centres 

 that are concerned in mental activities. Thus "nervous- 

 ness," or a disturbance of the normal balance of nerve- 

 centres, has a strikingly analogous effect in confusing the 

 ideas and in perturbing muscular co-ordinations. Idiotcy has 

 its parallel in inability to perform complex muscular move- 

 ments, with which inability, indeed, idiotcy is itself almost 

 invariably associated. Lunacy has it counterpart in an un- 

 balanced, or badly correlated power of muscular co-ordina- 

 tion, which in its graver manifestations is known as ataxy ; 

 while mania is mental convulsion, and unconsciousness 

 mental paralysis. 



I must not, however, take leave of this branch of our 

 subject without briefly alluding to a difliculty which may 

 occur to some minds, and which has been well stated by 

 Professor Calderwood in his recently published work.* The 

 difliculty to which I allude arises from there being an 

 absence of such a constant relationship between the size or 

 mass of the brain, and the degree of intelligence displayed 

 by it, as the foregoing teaching would reasonably lead us to 

 expect. 



Now, I do not deny that the relation of intelligence to 

 size, mass, or weight of brain is a perplexing matter when 

 we look to the animal kingdom as a whole; for although 

 there is unquestionably a general relation of a quantitative 



* Pp. 211—216. 



