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 
4 International Scientific Series 5 to bring out the second 
division of the work as a separate treatise, under the title 
4 Mental Evolution . 5 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 desirable 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- 
