INTRODUCTION. 7 



been to present a book, if not of inconvenient bnlk, at least 

 quite out of keeping with the size of all the other books in 

 the same series. Moreover, the subject-matter of each work, 

 although intimately related to that of the other, is never- 

 theless quite distinct. The first is a compendium of facts 

 relating to Animal Intelligence, which, while necessary as a 

 basis for the present essay, is in itself a separate and distinct 

 treatise, intended to meet the interest already alluded to as 

 attaching to this subject for its own sake ; while the second 

 treatise, although based upon the former, has to deal with a 

 wider range of subject-matter. 



It is evident that, in entering upon this wider field, I shall 

 frequently have to quit the narrower limits of du^ect obser- 

 vation within which my former work was confined ; and it is 

 chiefiy because I think it desirable clearly to distinguish 

 between the objects of Comparative Psychology as a science, 

 and any inferences or doctrines which may be connected with 

 its study, that I have made so complete a partition of the 

 facts of animal intelligence from the theories which I believe 

 these facts to justify. 



So much, then, for the reasons which have ted to the form 

 of these essays, and the relations which I intend the one to 

 bear to the other. I may now say a few words to indicate 

 the structure and scope of the present essay. 



Every discussion must rest on some basis of assumption ; 

 every thesis must have some hypothesis. The hypothesis 

 which I shall take is that of the truth of the general theory 

 of Evolution : I shall assume the truth of this theory so far 

 as I feel that all competent persons of the present day will 

 be prepared to allow me. I must therefore first define what 

 degree of latitude I suppose to be thus conceded. 



I take it for granted, then, that all my readers accept the 

 doctrine of Organic Evolution, or the belief that all species of 

 plants and animals have had a derivative mode of origin by 

 way of natural descent ; and, moreover, that one great law or 



