there is no impossibility in the possession of a scaly skin by an air-breathing, warm-blooded animal; and we could not feel quite certain that a Whale-like animal might not be hereafter discovered possessing a scaly skin-however improbable such a thing may be.

57. It should be the aim of the Zoologist, therefore, in the choice of the characters which he uses, for the most easy distinction of the animals composing the several natural groups,into which, (it cannot be too often repeated), they must be associated by their general conformity of structure,-to select those which are the most natural, as indicating the nature of their internal structure, in preference to those which are artificial, giving no information beyond that derived from themselves. Hence, in classifying the Mammalia, the conformation of the teeth and extremities afford (in most orders at least) characters of the highest value; since these are intimately connected with the structure of the digestive apparatus, the nature of the food on which the animal lives, the mode in which it is obtained, and, consequently, the entire habits of the species. And, in the subdivision of the class of Birds, the conformation of the bill, wings, and claws, afford characters of similar value. In the arrangement of Insects, on the other hand, it is often necessary to adopt artificial characters for the separation of the several groups; because they are the only ones which are recognised with sufficient facility, and because our comparative ignorance of their internal anatomy, as well as of their economy, prevents, us from understanding, as clearly as in the two classes just mentioned, how far differences apparently trifling in external conformation are essentially connected with those peculiarities, which really characterise the respective groups. Thus, the enormous order Coleoptera (Beetles) is subdivided into sections or sub-orders, according to the number of joints in the tarsus, or foot; a character which, at first sight, appears very trivial, but which really does bring together the families that have the greatest number of points of general agreement, and separates them from others which differ more widely from them. Hence it is probable, that an increased acquaintance with the habits and structure of these animals will show us, why

HARMONY BETWEEN DISTINCT CHARACTERS.

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the presence of five joints in the tarsus should always accompany one set of internal characters; and why four joints should be found in the tarsus of those Beetles, which agree among themselves, and differ from the last, in certain other particulars. When that is the case, the character will deserve to be regarded as in itself a natural one; as it already must be considered in some degree, since the classification founded upon it alone has nothing of the artificial nature, usually seen in arrangements formed upon single characters, and displaying itself so prominently in the Linnæan classification of Plants. (BOTANY, Chap. XIII.)

58. It is from this correspondence between single characters, and general plans of structure, that the Comparative Anatomist. derives the power, already adverted to, of determining the nature of a whole animal from a single fragment of its skeleton, or from one of its teeth. In no animal is the body made up of a number of disconnected parts, united, as it were at hazard; for all its organs have a more or less intimate connection with each other; so that there is a kind of harmony amongst them all, and between every part and the entire structure. Thus, the simple inspection of the tooth represented in the accompanying figure, suffices to disclose to the scientific Naturalist, the following facts regarding the animal to which it belonged. In the first place, there must have been a bony framework, in which this tooth was implanted, and which gave support to the rest of the body; and, OF A LION. as this internal framework does not exist in any other animals than those of the Vertebrated series, we know, by its possession of this tooth, that the animal in question had the brain and spinal cord, the complete set of organs of the senses, the red blood, &c. &c. which belong to that sub-kingdom only. Further, there are certain characters about the roots of this tooth, which enable the Anatomist to feel certain, that it must have been implanted in a deep socket, which is only the case in Reptiles and Mammals; and he may further determine from them, that the animal belonged to this last class, and that it must have therefore possessed the organisation which is peculiar to it. Again,

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by the form of the crown of the tooth, it is easily shown that this tooth was destined to divide animal flesh; and that it consequently belonged to a Carnivorous quadruped. To digest this flesh, the animal must have had a stomach and intestinal canal formed upon a certain plan; and in order to obtain its prey, it must have had appropriate organs of locomotion and of prehension. Its extremities must have terminated in separate fingers, and these must have been armed with claws. The limbs must have been furnished with very powerful muscles, to enable the animal to give chase to its prey, or to spring upon it unawares; and afterwards to drag it to its den. The head, also, must have been connected with the spinal column by ligaments and muscles of great power, attached to elevated projections (spinous processes) of the vertebræ; in order that it may have the power of lifting the heavy bodies, which the animal desires to remove. The lower jaw must have been connected with the upper, by a hinge admitting but a scissars-like action, by which the edges of the cutting-teeth were constantly kept sharp; and the muzzle must not have been very protuberant, otherwise the strength of the muscles that raise the jaw, would be applied at too great a disadvantage (ANIM. PHYSIOL. § 189). The cranial cavity must have been comparatively large, in order that the size of the brain might correspond with the degree of intellect which the habits of the animal required.

59. By inferences of this kind, and under the guidance of our knowledge of the forms at present existing, all the leading peculiarities of an animal may be deduced from any characteristic portion of it; for if any part, essential to the action of the remainder, had been deficient, the animal could not have maintained its existence. An animal with the carnivorous propensity of the Tiger, for example, and the teeth or hoofs of a Horse, could not remain alive, for want of power to obtain and prepare its nutriment; nor would a Horse be better for the long canine teeth of the Tiger, which would prevent the grinding motion of the jaws, required for the trituration of his food. The great principle is, therefore, that every animal is a combination of mutuallyadapted parts, forming an harmonious whole; and that no one of these can be altered, without affecting all the rest, more or less considerably.

CHAPTER II.

GENERAL CHARACTERS OF VERTEBRATED ANIMALS.

60. THE Vertebrated sub-kingdom, including the classes of Mammals, Birds, Reptiles, and Fishes, is characterised by the presence, in all the animals which belong to it, of an internal skeleton, composed of bone or cartilage, and forming an envelope to the nervous centres. In the Articulated classes, there is no vestige of any such structure; and the only Mollusca (some of the Cuttle-fish tribe), in which there is the least approach to it, are sufficiently distinguished by other characters. It is true that, among many of the Radiata,-such as a few of the Jelly-fish tribe, and a large proportion of the Polypes, there is an internal skeleton, sometimes composed of a horny or cartilaginous tissue, and sometimes possessing even a stony hardness; but this gives equal support to the whole fabric, and is not arranged in such a manner as to give the least degree of peculiar protection to the nervous centres; so that, although it may be regarded as a kind of sketch, or shadowing-forth, in this lowest group, of the plan of structure which is characteristic of the highest, it cannot be said to have any real correspondence with it.

61. The animals of the Vertebrated series are, of all sentient beings, those whose faculties are the most varied and the most perfect. The principle of the division of labour is carried out in them to its highest degree; every function to be performed having its own separate organ, whose operations are limited to it alone; consequently, the Vertebrata are, of all animals, those in which the distinct organs are the most numerous and the most complicated. We may encounter many, among the lower tribes, in which the number of parts is as great, or even greater; but,

where this is the case, most of these parts are but repetitions of one another. It is by the variety existing in the form and structure of their several organs, and in the perfection with which each is adapted to perform its allotted function, that the Vertebrata are chiefly characterised. It is manifest that the structure of such animals must be regarded as more elaborate, than that of beings, in which the number of dissimilar parts is small, and every one of them capable of discharging a variety of offices; and that their functions must be performed with more energy and completeness, when carried into effect by instruments peculiarly adapted to each, than when several are the result of the actions of one organ. Hence we are justified in ranking the Vertebrata as the highest group in the Animal scale, independently of its being the one which contains Man. But we are not justified in speaking of the animals composing it, as more perfectly constructed than any others; since, in the eye of the Creator, they must be all equally perfect. In every one, the adaptation between the actions of its several parts must be complete; or it could not maintain its existence. And it should not be less wonderful to us, to meet in the Zoophyte with a simple structure, capable of performing all the functions of absorption, assimilation, respiration, and secretion; than to contemplate the numerous and elaboratelyconstructed organs, by which these several operations are respectively performed in the Vertebrated animal.

62. The development of all the organs in the Vertebrata seems to be subservient to that of the Nervous System ;—their object being to give to it the means of the exercise of its powers. In almost every animal of the group, we find that the brain and spinal cord bear a much larger proportion to the entire bulk of the body, than do the nervous centres of any Invertebrated animal; and in Man, this proportion is greatly increased. It is not only in the actual bulk of the organs, that we see the importance here given to the nervous system; for this is still more shown by the quantity of blood which is transmitted to them. Thus it is calculated that the brain of Man receives about a fifth or sixth part of the whole blood in circulation; although its weight is not more than a fortieth or fiftieth part of that of the entire body and

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