198 DOCTRINE OF EVOLUTION 



and he approaches his further studies with a conviction 

 that evolution, having proved to be universal so far, 

 in all probability will be found equally true in the case 

 of psychological phenomena. This expectation is in- 

 deed realized, and the scope of the doctrine is extended 

 over a new field, when the facts of human psychology 

 are treated as materials for impersonal comparative 

 study ; and this result is not only useful and valuable 

 in and by itself, but it also provides in the principles 

 of mental evolution the transition to the field of social 

 relations and ethical ideas and ideals which are appar- 

 ently the unique possessions of men as individuals and 

 as associated groups. 



The field of comparative psychology might seem at 

 first sight to be a foreign territory to the average well- 

 informed layman in science, but the contrary is really 

 the case. Every one has thought at one time or an- 

 other about his own mental make-up, and about the 

 minds of others. No one can watch a child at play 

 with his toys or at work with his schoolbooks without 

 being struck by many evidences of marked differences 

 between the immature and the experienced types of 

 mind. Every one knows also that the mental " scheme 

 of things " is by no means the same for all nations or 

 races of mankind existing to-day, while furthermore 

 the fact is entirely familiar that the intellectual heritage 

 of a present race has changed in the course of previous 

 ages. Therefore in this field as before we need only to 

 amplify our knowledge of such representative psycholog- 

 ical facts as these by drawing upon the full stores of 

 the special investigator, in order to learn that human 

 thought, like the human frame, has undergone a natural 



