PEE FACE 



When I first began to collect materials for this work it 

 was my intention to divide the book into two parts. Of 

 these I intended the first to be concerned only with the 

 facts of animal intelligence, while the second was to have 

 treated of these facts in their relation to the theory of 

 Descent. Finding, however, as I proceeded, 'that the 

 material was too considerable in amount to admit of 

 being comprised within the limits of a single volume, I 

 have made arrangements with the publishers of the 

 ' International Scientific Series ' to bring out the second 

 division of the work as a separate treatise, under the title 

 'Mental Evolution.' This treatise I hope to get ready 

 for press within a year or two. 



My object in the work as a whole is twofold. First, I 

 have thought it desii'able that there should be something 

 resembling a text-book of the facts of Comparative Psy- 

 chology, to which men of science, and also metaphysicians, 

 may turn whenever they may have occasion to acquaint 

 themselves with the particular level of intelligence to 

 which this or that species of animal attains. Hitherto the 

 endeavour of assigning these levels has been almost exclu- 

 sively in the hands of popular writers ; and as these have, 

 for the most part, merely strung together, with dis- 

 crimination more or less inadequate, innumerable anec- 



