6 MENTAL EVOLUTION IN MAN 



the least, a highly improbable supposition. At all events, it 

 certainly is not the kind of supposition which men of science 

 are disposed to regard with favour elsewhere ; for a long and 

 arduous experience has taught us that the most paying kind of 

 supposition which we can bring with us into our study of nature, 

 is that which recognizes in nature the principle of continuity. 



Taking, then, these sevetaX a priori considerations together, 

 they must, in my opinion, be fairly held to make out a very 

 strong primd facie case in favour of the view that there has 

 been no interruption of the developmental process in the 

 course of psychological history ; but that the mind of man, 

 like the mind of animals — and, indeed, like everything else in 

 the domain of living nature — has been evolved. For these 

 considerations show, not only that on analogical grounds any 

 such interruption must be held as in itself improbable ; but 

 also that there is nothing in the constitution of the human 

 mind incompatible with the supposition of its having been 

 slowly evolved, seeing that not only in the case of every 

 individual life, but also during the whole history of our species, 

 the human mind actually does undergo, and has undergone, 

 the process in question. 



In order to overturn so immense a presumption as is thus 

 erected on a priori grounds, the psychologist must fairly be 

 called upon to supply some very powerful considerations of 

 an a posteriori kind, tending to show that there is something 

 in the constitution of the human mind which renders it 

 virtually impossible — or at all events exceedingly difficult to 

 imagine — that it can have proceeded by way of genetic 

 descent from mind of lower orders. I shall therefore proceed 

 to consider, as carefully and as impartially as I can, the 

 arguments which have been adduced in support of this thesis. 



In the introductory chapter of my previous work I 

 observed, that the question whether or not human intelligence 

 has been evolved from animal intelligence can only be dealt 

 with scientifically by comparing the one with the other, in 

 order to ascertain the points wherein they agree and the points 



