794 THE POPULAR SCIENCE MONTHLY. 



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 ex- 

 pected to show. Therefore, it is a matter of observable fact that, 

 whether or not human intelligence differs from animal in kind, it 

 certainly does admit of gradual development from a zero level. 

 This I posit as the second consideration. 



Again, so long as it is passing through the lower phases of its 

 development, the human mind assuredly ascends through a scale 

 of mental faculties which are parallel with those that are perma- 

 nently presented by the psychological species of the animal king- 

 dom. A glance at the accompanying diagram will serve to show 

 in how strikingly quantitative, as well as qualitative, a manner the 

 development of an individual human mind follows the order of 

 mental evolution in the animal kingdom. And when we remember 

 that, at all events up to the level where this parallel ends, the 

 diagram is not an expression of any psychological theory, but of 

 well-observed and undeniable psychological fact, I think every 

 reasonable man must allow that, whatever the explanation of this 

 remarkable coincidence may be, it certainly must admit of some 

 explanation — i. e., can not be ascribed to mere chance. But, if so, 

 the only explanation available is that which is furnished by the 

 theory of descent. These facts, which I present as a third consid- 

 eration, tend still further — and, I think, most strongly— to increase 

 the force of antecedent presumption against any hypothesis which 

 supposes that the process of evolution can have been discontinuous 

 in the region of mind. 



Lastly, it is likewise a matter of observation, as I shall fully 

 show in the next installment of this work, that in the history of our 

 race — as recorded in documents, traditions, antiquarian remains, 

 and flint implements — the intelligence of the race has been sub- 

 ject to a steady process of gradual development. The force of this 

 consideration lies in its proving that, if the process of mental evo- 

 lution was suspended between the anthropoid apes and primitive 

 man, it was again resumed with primitive man, and has since con- 

 tinued as uninterruptedly in the human species as it previously 

 did in the animal species. Now, upon the face of these facts, or 

 from a merely antecedent point of view, such appears to me, to say 

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

 tainly is not the kind of supposition which men of science are 

 disposed to regard with favor 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 several a priori considerations together, 

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



