40 MENTAL EVOLUTION IN ANIMALS. 



the nervous processes. All that physiology can see in these 

 processes is a greatly improved power of discriminating 

 between stimuli, and of issuing impulses to a correspondingly 

 greater number and variety of adaptive movements ; the 

 mental changes which accompany these nervous processes are 

 as wholly without the ken of physiology as these nervous 

 processes are without the ken of subjectivity. Therefore it 

 is that when we speak of an idea as the analogue of a muscle, 

 we feel the incongruity of confusing two things which are 

 separated from one another by the whole interval that 

 divides subject from object. But although in speaking of an 

 idea as the analogue of a muscle, we do and ought to feel the 

 incongruity, let it not be supposed that by thus speaking we 

 are allowing ourselves to be betrayed into any confusion of 

 thought. I speak of a mental change as the analogue of a 

 muscular contraction only with reference to its being the ter- 

 minal event invariably associated (whether by way of 

 causality or not) with the activity of a nervous structure. 

 And if we do not seek to press the analogy further than this, 

 there is no fear of our confusing ideas which ought always to 

 be kept fundamentally distinct. 



So much, then, by way of introduction to the point which 

 I have to make plain. Now it admits of being abundantly 

 proved that throughout the animal kingdom, so long as we 

 regard the muscular system as our index of the structural 

 advances taking place in the nervous system, we find this 

 index to consist in the growing complexity of the muscular 

 system, and the consequent increase in the number and 

 variety of co-ordinated movements which this system is 

 enabled to execute. Therefore the point which I have to 

 prove will be proved if I can make it clear that the process 

 of mental evolution bears some such resemblance to that of 

 muscular evolution as we should expect that it ought to bear, 

 if they are both dependent on a similar process of nervous 

 evolution. In other words, I have to show that the process 

 of mental evolution consists essentially in a progressive 

 co-ordination of progressively developing mental faculties, 

 analogous to that which takes place in muscular movements. 



Beginning with the faculties of simple sensation, we 

 know, for instance, that when a note of music is struck, it 

 appears to produce a single vibration, and yet physical ana- 

 lysis shows that the sound is not a single vibration, but a 



