Russel Wallace and Dr. Brehm have agitated the question, but failed to settle it,--even to their own satisfaction. The reason, I believe, is that the exponents of the different theories have failed to agree on a definite standard of comparison. The mathematical principle implied in the construction of a honey-comb, we are told, can challenge comparison with the ripest results of human science. The acumen of a well-trained elk-hound, a philosophical sportsman assures us, comes nearer to human reason than any other manifestation of animal sagacity. Elephant-trainers, too, adduce instances that almost pass the line of distinction between intuitive prudence and the results of reflection. Yet if those distinctions suffice to define the difference between reason and the primitive instincts, they should reduce the scope of the question in so far as to make it clear that, instead of measuring the degree of the development of special faculties of the animal mind, we should _ascertain the direction_ of those faculties. Instinct tends to promote the interests of the species, and is limited to the more or less skilful, but monotonous, performance of a special task. Within that limited sphere its competence is perfect. Reason may be often at fault, but its capacity enlarges with practice, and the scope of its application is unlimited. It may be exerted in the interest of the species, of the tribe, of the family; it may devote itself to the service of an abstract principle or subserve the purposes of individual caprice. It differs from instinct as a piano differs from a barrel-organ. The pianist has to master his art by years of toil, but can apply it to all possible variations or extravaganzas of music. The organ-grinder can delight his audience as much by his first as by his last performance, but his _répertoire_ is limited. Reason is indefinite, free, and versatile. Instinct is exact, but circumscribed. Tested by that standard, the difference between the intelligence of the higher _quadrumana_--the anthropoid apes, the baboons, and several species of the macaques--and that of their dumb fellow-creatures is so pronounced that it amounts to a difference of kind as well as of degree. _Borné_, literally limited, but used in French as a synonyme of short-witted, is the term that best characterizes the actions of all other animals, as compared with the graceless but amazingly versatile and well-planned pranks of our nearest relatives. The standard of _usefulness_ would, indeed, degrade the perpetrators of these pranks below the rank of the dullest donkey; but as a criterion of intelligence the application of that test should rather be reversed.