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known animals; all of which agree in possessing the ruminating apparatus, as well as in having the hoof cleft (or rather double); and nearly all in the following dental formula:— incisors, canines, molars,, the last being marked on the surface with crescents, which are formed by ridges of enamel.

X. PACHYDERMATA, or thick-skinned animals; a group that includes a great variety of dissimilar forms, which agree rather in the absence of other characters, than in the possession of any that are common to all. Thus the Elephant, Horse, and Pig, would seem to possess very different types of structure,—the Elephant having the bones of its toes distinct, but included together in a sort of horny skin,-the Pig having four distinct toes, with separate hoofs,-and the Horse having all the bones. of the toes consolidated into a single row, which is terminated by a single hoof. The characters afforded by the teeth are equally various; some having incisors, others none; some having large canines, others small ones, and others being destitute of them; and the number of molars also being subject to great variation. Yet these very dissimilar forms are connected by intermediate links, recent and fossil (§ 28); and it appears as if we were to unite with this order those whale-like animals, whose food is of a vegetable character, their whole conformation being very different from that of the true Cetacea, and more closely resembling that of certain aquatic Pachydermata.

125. The foregoing arrangement of the orders of the viviparous Mammalia is more adapted to distinguish them from each other, than to represent their relative positions in a natural series. Thus, the Edentata and Rodentia are unquestionably lower, in regard to their intelligence and their general conformation, than the Ruminantia and Pachydermata; and the Rodentia are those, among the truly viviparous Mammals, which approach the nearest to the non-placental group, in the low development of their brain; as well as in several points of their general structure. It is impossible to represent their mutual relations by any arrangement of them in a single line; since each group has connections, not only with those before and behind it, but with others at a distance. Nevertheless it is

necessary to follow some order in the more particular description of them; and the following is probably the most correct, in regard especially to the grade which the different groups possess in the scale. I. BIMANA. II. QUADRUMANA. III. CHEIROPTERA. IV. INSECTIVORA. V. CARNIVORA. VI. CETACEA. VII. PACHY DERMATA. VIII. RUMINANTIA. IX. EDENTAta. X. RODENTIA. Thus from Man we pass to the Quadrumana by the Apes which most nearly resemble him in bodily structure. From these we descend to the Lemurs, in which the teeth show an insectivorous character, and certain species exhibit a tendency to the formation of organs of flight; by which characters we are conducted to the Cheiroptera, or Bats. From the insectivorous forms of these, the transition is not difficult to the proper Insectivora; which, again, lead us to the Carnivora; and these, by their aquatic species, to the Cetacea. From the Cetacea we naturally pass to the whale-like Pachydermata; and then, through the terrestrial tribes of these, to certain species which conduct us to the Ruminants. The Edentata and Rodentia are not closely connected, by any existing species, with either of the two last orders; but there are certain fossil forms, which seem to link them to both.

126. The division of non-placental Mammalia contains two orders only, which are chiefly distinguished by certain peculiarities affecting the reproductive apparatus.

XI. MARSUPIALIA, or pouched animals; being those in which the young are carried during the early part of their lives, in a marsupium, formed by folds of the skin of the abdomen, which are supported by peculiar bones. The species included in this group differ greatly among each other, both in the general form of their bodies, and in the conformation of their teeth and digestive apparatus. Indeed they bear, in these respects, a striking resemblance to animals of different orders among the truly viviparous Mammals. Thus, the Opossums have an opposable thumb on all four extremities; so that they are truly quadrumanous, or four-handed. Others, again, bear a close resemblance to the Carnivora; and others to the Rodentia; so that the order Marsupialia was really distributed amongst these groups, when the

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principles of classification were not so well established as they are at present. But it is now almost universally considered, that the absence of the placenta, connected as it is with the low development of the brain, and with other evidences of an inferior grade, is a character of the most essential nature, completely distinguishing the animals by which it is manifested, from any to which they may bear a general resemblance in adaptive characters (§ 25). And this can be hardly otherwise than true; since we find that the Marsupialia, taken as a group, have more points of real agreement with each other, than any of them have with animals of other orders.

XII. MONOTREMATA, or animals with a single outlet ; a character which has been already explained (§ 117). This order is very limited, containing only two genera; which were formerly placed among the Edentata, on account of the absence of teeth in their jaws. They are altogether most remarkable animals; and present several points of the greatest interest both to the Physiologist and Zoologist,—as do generally, indeed, those animals which stand on the borders of two great divisions.

ORDER I.-BIMANA.

127. The name Bimana is the most appropriate that could be found for an order constituted by the human species only; for Man alone is two-handed. That," says Cuvier, "which constitutes the hand, properly so called, is the faculty of opposing the thumb to the other fingers, so as to seize the most minute objects,—a faculty which is carried to its highest degree of perfection in Man, in whom the whole anterior extremity is free, and can be employed in prehension." Some Naturalists refuse the term hand to the extremities of the Monkey tribe, preferring to call them graspers; for it is certainly true that, although most of them possess an opposable thumb, yet they are destitute of the power of performing many of those actions, which we regard as most characteristic of the hand. These actions are dependent upon the size and power of the thumb, which is much more de

veloped in Man, than it is in even the highest Apes. The thumb of the Human hand may be made to meet, at its point, the extremities of each finger singly, or of all in combination; whilst in those Quadrumana, which most nearly approach Man, the thumb is so short and weak, and the fingers so long and slender, that their tips can scarcely be brought in opposition, and can never be opposed in near contact with each other with any degree of force. Hence, although admirably adapted for clinging round bodies of a certain size, such as the small branches of trees, &c., the extremities of the Quadrumana can neither seize very minute objects with that precision, nor support large ones with that firmness, which are essential to the dexterous performance of a variety of operations, for which the hand of Man is admirably adapted. Hence the possession of four hands is not, as might be supposed, a character which raises the animals that possess it above two-handed Man; since none of these four hands are adapted to the same variety of actions of prehension, of which his are capable; and all of them are in some degree required for support. In this respect, their character approaches much nearer to that of the extremities of the lower Mammalia.

128. We ought, then, as Sir Charles Bell remarked, "to define the hand as belonging exclusively to Man." There is in him, what we observe in none of the Mammalia that approach him in other respects, a complete distinction between the functions of his two pairs of extremities; the upper being adapted for prehension alone, the lower for support only. Thus each function is performed with a much higher degree of perfection than it can be, where two such opposite purposes have to be united. The arm of the Ape has as wide a range of motion as that of Man, so far as this can be given by the joints; but it is only when the animal is in the erect attitude, that its arm can have free play. Thus the structure of the whole frame must conform to that of the hand, and must act with reference to it. But it cannot be said with truth, (as some have maintained,) that Man owes his superiority to his hand alone; for without the directing mind, the hand would be comparatively valueless. His elevated position is due to his mind and its instruments con

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jointly; for, if destitute of either, the Human race would either be speedily extinguished altogether, or would be reduced to a kind of life not above the level of that of the brutes.

129. Man is further distinguished from all other animals by his erect attitude; which involves a considerable number of modifications in his general structure. Thus, his head is set upon the top of the spinal column in such a manner, that its weight bears directly downwards upon it; and it is so nearly balanced, owing to the position of the articulation on which it is supported, that very little muscular exertion is required to keep it in the erect position. Now if, with the same form of the head and neck, Man had been destined to walk upon all-fours, the head would have been without proper support; since the neck of Man is destitute of the powerful muscles and ligaments by which the large heads of the herbivorous quadrupeds are sustained in this position, (ANIM. PHYSIOL. § 29): and the face would be directed towards the ground, instead of commanding the horizon around. Among those Quadrumana which most nearly approach Man, we find that the point on which the head is supported is placed much further back than in him; and that it bears obliquely on the spinal column. This corresponds with the semi-erect position which seems natural to them,-the spinal being inclined forwards, so that the body is partly supported upon the anterior extremities; and in this state, the face looks directly forwards. But these animals occasionally assume the completely-erect, or the completely horizontal posture; and they can easily adapt the position of the head to either of these, its usual angle being intermediate between them.

130. The position of the face immediately beneath the brain, so that its front is nearly in the same plane with the forehead,— making the facial angle (ANIM. PHYSIOL. § 719) large,-is peculiarly characteristic of Man; for the brains of the Chimpanzee and Orang, which approach most nearly to that of Man, are almost entirely behind, and not above, the face. In the young Ape, however, there is a much greater resemblance to Man, in this respect, than there is in the adult. For at the time that the second set of teeth appears, the muzzle of the Ape undergoes a