38 MENTAL EVOLUTION IN ANIMALS. 



have previously occurred, is merely a psychological expres- 

 sion of the physiological fact that lines of discharge become 

 more and more permeable by use. 



We thus see that the most fundamental of physiological 

 principles — the association of ideas — is merely an obverse 

 expression of the most fundamental of neurological principles 

 — reflex action ; and that such, in general terms, is the fact, 

 seems to be proved beyond question by such instances as 

 those above given of the sleeping waiter and Dr. Abercrombie's 

 unconscious patient, &c. ; for such cases prove that actions 

 originally due to a conscious association of ideas may, by a 

 sufficiently long course of ganglionic instruction, cease to be 

 conscious actions, and thei^efore become in no way distin- 

 guishable from reflex actions.* 



But the proof of .the fundamental correlation between 

 ganglionic action and mental action does not end even 

 here. There is another line of evidence which, although 

 perhaps not quite so definite, nevertheless seems to me most 

 cogent, and even more interesting than the considerations 

 already adduced. If we take ideation to be in the same 

 sense an index of -the higher or more complex nervous pro- 

 cesses, as muscular movement is of the lower or less complex, 

 we shall find evidence to show that the development of 

 ideation, or mental evolution, implies a further and continuous 

 development of the corresponding nervous processes, wdiich 

 is precisely the same in kind as that which on the lower 

 plane (that of muscular movement) has led to the advancing 

 developmeut of muscular co-ordination. In other words, if 

 we consent to change the index from muscles to ideas, w^e 

 shall find evidence that the method of nervous evolution has 

 throughout been uniform ; we shall find that the progressive 

 elaboration of nervous structures — which in the one case has 

 found expression in the growing complexity of the muscular 

 syi^tem, and in the other case has been reflected in the 

 advancing phases of mental evolution — we shall find that this 

 progressive elaboration has throughout been pervaded by the 

 same principles of development. 



* A good instance of this may be found in the fact that men always bring 

 ftheir knees together in order to catch a small falling object, such as a coin, 

 while women always spread their knees apart. The reason of course is that 

 the difference of dress has led to a differencse of organized habit — the habit 

 in each case haying been originally due to intelligent adjustment, but now 

 scarcely distinguishable from a reflex. 



