(1). From the earliest periods to the present time, the great desideratum in Zoology has been the establishment of some fundamental system of arrangement which, being universal in its application, should distribute the countless beings surrounding us into natural groups or divisions, such as might be subdivided into classes, orders, and genera, by obvious differences of structure in the tribes composing them, and thus enable the Zoologist at once to indicate the position which any unknown animal ought to occupy in the scale of existence, and its relations with other creatures.

(2). Aristotle, the father of our science, was the first who attempted a scientific division of the animal world *. The outlines of his system were rude in proportion to the necessarily limited knowledge at his disposal, although his efforts were gigantic, and still excite our warmest admiration. This acute observer admitted but two great sections, in one or other of which all known beings were included, - the highest comprehending creatures possessed of blood (i. e. red blood), corresponding to the Vertebeata of modern authors; the lowest embracing animals which in his view were exsangueous, or provided with a colourless fluid instead of blood, and corresponding to the Invertebrata of more recent zoologists 1.

(3). Linnaeus, like Aristotle, selected the circulatory system as the foundation of his arrangement*, dividing the animal creation into three great sections, characterized as follows: -

* Historia Animalium.

On Animal Kingdom Classification 2

I. Animals possessed of warm red blood, and provided with a heart containing four compartments, viz. two auricles and two ventricles. Such are the Mammalia and Birds.

II. Animals with cold red blood, their heart consisting of but one auricle and one ventricle, as he believed to be the case in Reptiles and Fishes.

III. Animals possessed of cold white sanies instead of blood, having a heart consisting of a single cavity, which he designates an auricle: under this head he includes insects and all other invertebrate animals, to which latter he gives the general name of Vermes, Worms.

We shall not in this place comment upon the want of anatomical knowledge conspicuous in the above definitions, or the insufficient data afforded by them for the purposes of Zoology. The apparatus of circulation, being a system of secondary importance in the animal economy, was soon found to be too variable in its arrangement to warrant its being made the basis of zoological classification, and a more permanent criterion was eagerly sought after to supply its place.

(4). Among the most earnest in this search was our distinguished countryman John Hunter, who, not satisfied with the results obtained from the adoption of any one system, seems to have tried all the more vital organs, tabulating the different groups of animals in accordance with the structure of their apparatus of digestion, of their hearts, of their organs of respiration, of their generative organs, and of their nervous system, balancing the relative importance of each, and sketching out with a master hand the outlines of that arrangement since adopted as the most natural and satisfactory 1.

The result of the labours of this illustrious man cannot but be of deep interest to the zoological student, and accordingly an epitome of his ideas upon the present subject is here concisely given.

The apparatus of digestion appears to be among the least efficient for the purpose of a natural division; as the separation of animals into such as have a simple digestive cavity, receiving and expelling its contents by the same orifice, and such as have an aperture for the expulsion of the contents of the alimentary canal distinct from that by which food is taken into the stomach, is by no means of practical utility, although this circumstance, as we shall afterwards see, has been much insisted upon.

Hunter's arrangement of the animal kingdom in conformity with the structure of the heart was a great improvement upon that of Linnaeus, founded upon the same basis. He arranges in this manner all animals in five groups.

* Systema Naturae. Vindobonae, 1767. 13th edition.

1 Descriptive and Illustrated Catalogue of the Physiological Series of Comparative Anatomy contained in the Museum of the Royal College of Surgeons in London, vol. iii. part i. 1835.

I. Creatures whose hearts are divided into four cavities - Mammalia and Birds.

II. Those having a heart consisting of three cavities - Reptiles and Amphibia*.

III. Animals possessing a heart with two cavities - Fishes and most Mollusca.

IV. Animals whose heart consists of a single cavity - Articulated Animals.

V. Creatures in which the functions both of stomach and heart are performed by the same organ, as in Medusae.

We shall pass over Hunter's sketches of arrangements founded on the respiratory and reproductive organs, as offering little that is satisfactory; but the researches of this profound physiologist upon the employment of the nervous system for the purpose of zoological distribution did much to inaugurate a more natural method of classification, afterwards carried out with important results.

(5). The appearance of the "Animal Kingdom distributed in accordance with its organization," of Cuvier, formed a new and important era in Zoology. In this we find all creatures arranged in four great divisions, Vertebrata, Mollusca, Articulata, and Radiata. These divisions, with the exception of the first, are named from the external appearance of the creatures composing them; nevertheless the three first are defined by characters exclusively drawn from their internal organization, the arrangement of the nervous system being essentially the primary character of distinction, and have been found to be strictly natural; whilst the last division, characterized by the appellation of Radiata, in the formation of which the structure of the nervous system has been allowed to give place in importance to other characters of secondary weight, obviously embraces creatures of very dissimilar and incongruous formation.

(6). The Vertebrata are distinguished by the possession of an internal nervous centre or axis, composed of the brain and spinal cord, which is enclosed in an osseous or cartilaginous case, and placed in the median plane of the body, giving off symmetrical nerves, which are distributed to all parts of the system.. This general definition indicates a large division of the animal world, which, by secondary characters drawn from the structure of their organs of respiration and circulation, is separable into mammals, birds, reptiles, amphibia, and fishes.

(7). The Mollusca have a nervous system constructed upon a very different type, and do not possess any vertebral column or articulated skeleton. The nervous centres consist of several detached masses placed in different parts of the body, without regularity of distribution or symmetrical arrangement; and the entire group is obviously natural, although Cuvier has ranged in it some creatures which, in the structure of their nervous system, differ essentially from those comprised in his own definition.

* For the important discovery that the heart of the Amphibia is divided into three cavities, instead of being composed of a single auricle and ventricle, we are indebted to Professor Owen (vide Zool. Trans, vol. i).

(8). The class of Articulated Animals is likewise well characterized by the nervous system, which, in all the members of it, is composed of a double series of ganglia or masses of neurine, arranged in two parallel lines along the abdominal surface of the body, united by communicating cords, and from which nerves are given off to the different segments of which the body consists.

(9). The fourth division of Cuvier, namely that of Zoophytes or Radiated Animals, is confessedly made up of the most heterogeneous materials, comprising creatures differing in too many important points to admit of their being associated in the same group; and the efforts of subsequent zoologists have been mainly directed to the establishment of something like order in this chaotic assemblage.

(10). The evident relation which the perfection of the nervous system bears to that of animal structure, and the success of Cuvier in selecting this as the great point of distinction in the establishment of the higher divisions of the animal kingdom, necessarily led succeeding naturalists still to have recourse to this important part of the economy in making a further subdivision of the Radiata of Cuvier. In some of the radiated forms, indeed, nervous filaments are distinctly visible, and such are among the more perfectly organized of the group; these, therefore, have been classed by themselves, and designated by Professor Owen the Nematoneueose * division of the animal world; while those which are apparently without the least trace of distinct nervous matter have been formed by Mr. Macleay into a group by themselves, to which he has given the denomination of ACRITA1.

(11). There can be no doubt that the nervous matter must be regarded as the very essence or being of all creatures, with which their sensations, volition, and capability of action are inseparably connected; and such being the case, it is a legitimate inference that the capacities and powers of the several tribes are in immediate relation with the development and perfection of this supreme part of their organization, and their entire structure must be in accordance with that of the nervous apparatus which they possess. The nature of the limbs and external members, the existence or non-existence of certain senses, the capability of locomotion, and the means of procuring food must be in strict correspondence with the powers centered in the nervous masses of the body, or in that arrangement of nervous particles which represents or replaces them.

A thread.

A thread.

A nerve.

A nerve.

1 A priv.

To discern.

To discern.

(12). Granting the accuracy of the above view, it is obvious that, if exactly acquainted with the structure and elaboration of the nervous apparatus in any animal, we might to a great extent predicate the most important points in its economy, and form a tolerably correct estimate of its powers and general conformation. But, unfortunately, such knowledge is not always at our disposal: in the lower forms of the animal world especially, we are far from being able to avail ourselves of such a guide; and it will probably be long ere our improved means of research permit us to apply to practice the views which Physiology would lead us to adopt. It is, however, by no means our intention in the present work to enter the arena of discussion relative to the juxtaposition or precedence in the scale of animal existence which ought to be assigned to any particular group as denned by modern zoologists. The classification employed in the following pages is simply adopted as being the most convenient for our present object; we shall therefore arrange our studies in accordance with the following sequence.