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 A 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 desirable that there should be something resembling a textbook of the facts of Comparative Psychology, 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 endeavor of assigning these levels has been almost exclusively in the hands of popular writers and as these have, for the most part, merely strung together, with discrimination more or less inadequate, innumerable anecdotes of the display of animal intelligence, their books ire valueless as works of reference. So much, indeed, is this the case, that Comparative Psychology has been virtually excluded from the hierarchy of the sciences. If we except the methodical researches of a few distinguished naturalists, it would appear that the phenomena of mind in animals, having constituted so much and so long the theme of unscientific authors, are now considered well nigh unworthy of serious treatment by scientific methods.