Each of these grand divisions has been subdivided into several minor groups or genera; but zoologists have hitherto been by no means unanimous with respect to the principles on which this subdivision ought to be effected. The arrangement which appears to be most generally adopted at the present day is that of M. Cuvier and M. Geoffroy-Saint-Hilaire, which is essentially founded on the application of an imaginary rule, first employed by Camper for ascertaining the degree of intelligence, and consequently of ideal beauty, expressed by the human face in its various gradations of elevation or debasement, and called by him the facial angle. Unfortunately, however, the operations of nature in the animal creation can never be subjected to geometrical laws; nor can her innumerable phases be expressed with the precision of a mathematical theorem. This assumed point of comparison varies almost indefinitely, not merely in different species, but even in the same individual; and the Oran-Otang himself, who is supposed to approach most nearly to the human form, offers the most striking illustration of the truth of this observation; inasmuch as in his young and intellectual state his facial angle is equal to 65°, while in his aged and debased condition, in which he has actually been repeatedly described as a different animal under the name of Pongo, it sinks below 30°; degrading him even beneath the level of the most savage and stupid of the Baboons. In the foregoing observations we may perhaps be considered as giving too much space to the generalities of the subject; an objection to which we can only answer that nearly the whole of our knowledge of the Monkey tribes consists in generalities. Of the great number of species, upwards of one hundred, which are now known and characterized, very few are distinguished from their immediate fellows by striking and strongly-marked characters, either physical or moral. The groups too are connected by such gradual and easy transitions, that although the typical forms of each, isolated from the mass and placed in contrast with each other, unquestionably exhibit many broadly distinguishing peculiarities, yet the entire series offers a chain so nearly complete and unbroken as scarcely to admit of being treated of in any other way than as one homogeneous whole. A no less striking than apposite instance of the close affinity between the species, and of the difficulty of distinguishing them from each other, especially in their young state, is furnished by the animals whose figures stand at the head of the present article. They are all three very evidently young individuals, and have not yet reached the period when it would be safe to pronounce with positiveness upon the species, or, were we to adopt the Cuvierian system in its full extent, upon the genera even, to which they respectively belong.