10 INTRODUCTION. 



that we are prepared to seek for a similar explanation of the 

 facts of human psychology. But wide as is the difference 

 between the mind of a man and the mind of a brute, we must 

 remember that the question is one, not as to degree, but as to 

 kind ; and therefore that our task, as serious enquirers after 

 truth, is calmly and honestly to examine the character of the 

 difference which is presented, in order to determine whether 

 it is really beyond the bounds of rational credibibty that the 

 enormous interval which now separates these two divisions of 

 mind can ever have been bridged over, by numberless inter- 

 mediate gradations, during the untold ages of the past. 



While writing the first chapters of the present volume, I 

 intended that the latter half of it should be devoted to a con- 

 sideration of this question, and therefore in " Animal Intelli- 

 gence" I said that such would be the case. But as the 

 work proceeded it soon became evident that a full treat- 

 ment of this question would require more space than could 

 be allowed in a single volume, without seriously curtailing 

 both the consideration of this question itself and also that of 

 Mental Evolution, as this is exhibited in the animal kingdom. 

 I therefore determined on restricting the present essay to a 

 consideration of Mental Evolution in Animals, and on reserv- 

 ing for subsequent publication all the material which I have 

 collected bearing on Mental Evolution in Man. I cannot 

 yet say how long it will be before I can feel that I am justified 

 in publishing my researches concerning this branch of my 

 subject ; for the more that I have investigated it, the more 

 have I found that it grows, as it were, in three dimensions — 

 in depth, width, and complexity. But at whatever time I 

 shall be able to publish the third and final instalment of 

 my work, it will of course rest upon the basis supplied by 

 the present essay, as this rests upon the basis supplied by 

 the previous one. 



It being understood, then, that the present essay is 

 restricted to a consideration of Mental Evolution in Animals 



