4 MENTAL EVOLUTION IN MAN. 



is concerned, both with respect to bodily and to mental 

 organization ; but in doing this I expressly excluded the 

 mental organization of man, as being a department of com- 

 parative psychology with reference to which I did not feel 

 entitled to assume the principles of evolution. The reason 

 why I made this special exception, I sufficiently explained ; 

 and I shall therefore now proceed, without further introduction, 

 to a full consideration of the problem that is before us. 



First, let us consider the question on purely a priori 

 grounds. In accordance with our original hypothesis — upon 

 which all naturalists of any standing are nowadays agreed — 

 the process of organic and of mental evolution has been 

 continuous throughout the whole region of life and of mind, 

 with the one exception of the mind of man. On grounds of 

 analogy, therefore, we should deem it antecedently improbable 

 that the process of evolution, elsewhere so uniform and 

 ubiquitous, should have been interrupted at its terminal phase. 

 And looking to the very large extent of this analogy, the 

 antecedent presumption which it raises is so considerable, that 

 in my opinion it could only be counterbalanced by some very 

 cogent and unmistakable facts, showing a difference between 

 animal and human psychology so distinctive as to render it in 

 the nature of the case virtually impossible that the one could 

 ever have graduated into the other. This I posit as the first 

 consideration. 



Next, still restricting ourselves to an a priori view, it is 

 unquestionable that human psychology, in the case of every 

 individual human being, presents to actual observation a 

 process of gradual development, or evolution, extending from 

 infancy to manhood ; and that in this process, which begins 

 at a zero level of mental life and may culminate in genius, 

 there is nowhere and never observable a sudden leap of 

 progress, such as the passage from one order of psychical being 

 to another might reasonably be expected to show. Therefore, 

 it is a matter of observable fact that, whether or not human 

 intelligence differs from animal in kind, it certainly does 



