Having endeavoured to ascertain those characters which are common to all organized bodies, it now remains that we mark the limits which separate these into the two great kingdoms of Animals and Vegetables, into which they have been divided by the universal consent of mankind. The undertaking may be regarded as peculiarly difficult, if we judge from the variety of definitions which have been given, and the still unsettled state of opinion among naturalists ou the subject. Part of this difficulty, however, may be referred to the employment of ambiguous phrases, and to inattention to the number and character of those properties which are common equally to plants and animals. But the greatest share may be traced to the practice of examining the doubtful objects, without attending to all their relations,–by comparing the less perfect animals with some of the qualities of the most perfect plants, and by allowing our opinions to be influenced by circumstances connected with mere size and form.

When we compare together those animals and plants, which are considered as occupying the highest stations in each kingdom, we perceive that the characters by which they may be distinguished, are obvious and well defined. But when we descend to the animals and plants which occupy the lowest stations, and perceive that they are less complicated in their structure; exercise few functions besides those which are essential to living bodies; and, in consequence, present only obscure points of difference, we may be led into the supposition that, at a certain link of the chain, the two kingdoms coalesce.

When treating of the characters by which organized bodies might be distinguished from inorganic matter, it was stated, that the former always possess a skin or common integument. This organ, as it exists in animals, is now to be the subject of our consideration.

STRUCTURE OF THE SKIN

Animals present remarkable differences with regard to the size, the shape, and the number of their parts; but they all agree in possessing an exterior covering, or skin, to modify their surface, regulate their form, and protect them from the action of the surrounding elements. In the more perfect animals, this organ consists of the following parts: the cuticle,–the corpus mucosum,–the corium,–the panniculus carnosus,–and the cellular web.

1. The Cuticle.– This is likewise termed Epidermis, or Scarf-skin, and appears to be common to all animals. It constitutes the exterior layer of the skin, and covers not only those parts which are exposed to the influence of the atmosphere, or the waters, but the different openings of the extremities of the vessels, and lines the central surface of these with a coating, varying inits nature according to the functions of the organ which it is destined to protect. It is destitute of bloodvessels, nerves, and fibres, and usually consists of thin transparent membrane, possessing little tenacity. Various pores may be observed on its surface, which are the mouths of the vessels of the inner layers of the skin; and it is variously marked by grooves or ridges, depending on the inequalities of the surface which it covers.

As the nervous system contains the organs of sensation and volition, and distinguishes animal from vegetable structures, it has long occupied the attention of anatomists, and is now employed by naturalists as the basis of systematical arrangement. The discoveries with which these investigations have been rewarded, are indeed numerous; but much yet remains to be done, in order to ascertain the structure and actions of various parts of this system, and to reconcile the contradictory statements of different authors. In taking our view of the subject, it will be necessary to avoid all minuteness of detail, and to attend chiefly to those circumstances which characterise the different races of animals, instead of investigating the peculiarities of particular species.

1. Structure of the Nervous System.–The nervous system, as it appears in its most perfect form in the vertebral animals, consists of the Brain, the Spinal Marrow, and the Nerves.

The Brain occurs in the anterior part of the body, surrounded by the bony covering of the skull. Between the brain and the skull, there are three membranes which are considered as the integuments of this part of the nervous system. The exterior of these is termed the Dura mater, and may be considered as the inner periosteum of the skull. It forms various processes for dividing and supporting different portions of the brain, and contains, within its dupKcatures, tortuous cavities for the reception of blood, which are called sinuses.

In order to render complete the history of any species of animal, it is necessary to examine the peculiar character of all those systems of organs which have been noticed in the first volume of this Work, and to ascertain those laws which regulate its physical and geographical distribution. As this method of investigation is both laborious and difficult, it has been successfully practised by few. The greater number of naturalists have rested satisfied with an examination of the external characters of animals, and have overlooked those which are furnished by their internal structure. In order to form a correct opinion of the merits of these different methods of investigation, we ought to bearin mind, that the history of a species is incomplete, when its external characters only have been determined; that many of these characters are liable to change, and are, consequently, apt to mislead. The characters, on the other hand, furnished by structure are more permanent, yield more certain results, and are more engaging to a philosophical mind. On this important subject, however, it will be necessary to go more into detail, and to consider what those different characters are, and how they are ascertained.

The objects which present themselves to the notice of the Natural Historian, on the surface of this globe, exhibit innumerable varieties of form, structure, action and position. But, however diversified in appearance, they readily admit of distribution into various groups, each including numerous species, capable of farther arrangement into subordinate tribes. The most extensive pf these groups, are two in number,–the one called the Organized,–the other the Inorganic Kingdom. The limits which separate these two divisions, are so well defined, that the distinction has been universally received.

Philosophers and poets, in all ages, have been anxious to point out a certain gradation of perfection in earthly objects,–a CHAIN OF BEING, the links of which consist of all created beings, passing by insensible degrees from the simplest to the most complicated, and constituting one harmonious whole, unbroken and dependent. Crystallization, they say, is the highest link of the inanimate part of the chain, and connects the Mineral with the Vegetable Kingdom. The lichen which encrusts the stone, is but one step higher in the scale of being than the stone itself. The mushrooms and corals, form a bond of union between the Vegetable and Animal Kingdoms; and the vast interval by which Man is separated from his Maker, is occupied by different orders of superior intelligences.

The ultimate object which the zoologist has in view, in the employment of the preceding methods of investigation, is to complete the History of Species. For the full accomplishment, however, of this end, it is not only necessary to acquire a knowledge of their structure and functions, but likewise of all their mutual relations. This last task can only be executed, by calling to our aid the principles of arrangement, and by distributing animals into divisions or classes, according to the characters which they exhibit. Attempts of this kind have been made by numerous observers; and the various systems which have been proposed, differing from one another in the characters employed, and the divisions recognised, intimate very plainly the difficulties inseparable from the subject.

The methodical investigation and distribution of Animals, would be comparatively easy, if the forms and modifications of the different systems of organs exhibited constant mutual relations. Thus, if we consider the organs of any system to be in their most perfect state, when they admit into their structure the greatest variety of combinations, and execute the greatest number of motions or functions, does it happen, that, when we have discovered in any species, one system of organs in its most perfect state, all the other systems may be expected to be in the same condition.

In treating of the organs of perception, we endeavoured to ascertain the functions of the six senses, and the kind of information concerning the objects around us, which they are qualified to communicate to the mind. It now remains that we attend to the sensations themselves, in reference to the processes performed upon them by the mind, with the view of becoming acquainted with the faculties of that mysterious part of our nature.

In conducting this inquiry, it is difficult to avoid the use of ambiguous phrases; for almost every term which can be applied to mind, has been loaded with a variety of significations. It will be our aim to render obvious the meaning of the terms here employed, by the descriptions which accompany them.

When we attend to the phenomena displayed by the mind, we perceive that it exhibits certain relations to the sensations of the body, changes resulting from their production, and consequent efforts of volition.

These different states or conditions of the mind have been observed to be regulated by peculiar laws, and to be subservient to particular purposes in the animal economy. They have had bestowed on them specific appellations, to express their characters ; and they have generally been denominated the Attributes, Faculties, or (in consequence of volition producing change) Powers of the Mind. Trivial objections have been urged against the use of these terms, as implying that the mind is composed of different parts; and, consequently, not entitled to its prerogative of unity.

Although the attention of many eminent chemists has been directed to the examination of the composition of animal bodies, a great deal remains undetermined, in this difficult department of experimental research. The elementary principles which occur in the Animal Kingdom, have been ascertained with considerable precision; but the binary, ternary, or other compounds which these form, have not been investigated with so much success. As these various ingredients are brought into union in the animal system, by the agency of the vital principle, their state of combination may be expected to differ widely from the ordinary results of elective attraction. When such compounds of organization are submitted to analysis, the influence of the vital principle having ceased, the product obtained, may be regarded in many cases as modifications of the elements of the substance, occasioned by the processes employed, rather than the display of the number or nature of the ingredients as they existed previous to the analytical operations. Errors, we know, are to be guarded against in the analysis of mineral waters, arising from combinations taking place during the process, which did not exist in the compound previous to analysis. How much more necessary is it, to exercise caution in drawing our conclusions regarding the composition of animal bodies?

In this section I shall enumerate the elementary bodies, which are considered as entering into the composition of the parts of animals, and then consider the combinations which these form, or the substances in which they have been detected.

The organs which we examined in the preceding chapter, are generally considered as destined for protection, and are, therefore, placed on the exterior of the body. The bones occupy a different position. They are seated in the interior ; and while they likewise assist in protecting many important organs, they, at the same time, give stability to the frame, support to the muscles, and afford levers for the execution of locomotion.

COMPOSITION OF BONES

In considering the nature ofbone, it will be of advantage to the reader, to be made acquainted with its composition, in order to understand its mode of growth. The different parts of which bones consist, may be reduced to four; the periosteum, cartilaginous basis, earthy matter, and fat.

1. Periosteum.–This bears the same relation to the bone as the skin to the body, serving as a covering for its surface, and a sheath for the different cavities which enter it. It varies in thickness, according to the nature of the bone. Its texture is obviously fibrous ; and it possesses bloodvessels. Its sensibility indicates the existence of nerves. Where this organ covers cartilaginous processes, it is called Perichondrium.

2. Cartilaginous basis.–This part of bone apparently consists of gelatine and coagulated albumen. The gelatine may be obtained by boiling the bone, previously broken into small fragments, for a considerable time in water. If the liquor be sufficiently concentrated, it will gelatinize when cold.

In preparing this work for the public, the writer was chiefly influenced by a desire to collect the truths of Zoology within a small compass, and to render them more intelligible, by a systematical arrangement. He is not aware that there exists any work in the English language, in which the subject, in its different bearings, has been illustrated in & philosophical manner, or to which a student of Zoology could be referred, as a suitable introduction to the science. There are not wanting, it is true, many disquisitions of great value, on particular departments of the physiology and classification of Animals ; for who can enumerate the names of Tyson, Lister, Willoughby, Ray, Ellis, Hunter, Pennant, Monro, and Montagu, among the dead, and Home, Kirby, and Leach, among the living zoologists of Britain, without regarding them as extensive benefactors of the science. But the writings of these naturalists, and others which have been noticed in the body of the work, are not only rare, but expensive; so that the task of investigating the facts which have been established, or the theories which have been proposed, can scarcely, in ordinary circumstances, be entered upon. The want, indeed, of such an introduction to the study of the Animal Kingdom, as should serve as an index to the doctrines on which the classification is founded, has frequently been the subject of regret, and may probably be considered as the origin of that indifference to the science which is but too apparent in this country.

Each species of Animal is destined, in the absence of disease and accidents, to enjoy existence during a particular period. In no species, however, is this term absolutely limited, as we find some individuals outliving others, by a considerable fraction of their whole life. In order to find the ordinary duration of life of any species, therefore, we must take the average of the lives of a number of individuals, and rest satisfied with the approximation to truth which can thus be obtained.

There is but little resemblance, in respect of longevity, between the different classes, or even species of animals. There is no peculiar structure, by which long-lived species may be distinguished from those which are short-lived. Many species, whose structure is complicated; live but for a few years, as the rabbit, while some of the testaceous mollusca, with more simple organization, have a more extended existence. If longevity is not influenced by structure, neither is it modified by the size of the species. While the horse, greatly larger than the dog, lives to twice its age, man enjoys an existence three times longer than the former.

The circumstances which regulate the term of existence in different species, exhibit so many peculiarities, corresponding to each, that it is difficult to offer any general observations on the subject. Health is precarious, and the origin of diseases generally involved in obscurity.

The observations which we have ventured to offer in the former volume, relate to what may be termed the Motive, the Sentient, the Nutritive, and Reproductive Functions of Animals. The various Organs of the animal frame have been described, their actions investigated, and the important purposes of life, to which they are subservient, have at the same time been pointed out. An equally extensive field of Zoological Science remains to be investigated.

Animals are related to one another, and to the objects which surround them, in such a manner, as to be dependent on a variety of circumstances for the preservation of their existence, their dispersion over the globe, and their power of accommodation to the changes of the seasons. They are likewise to be viewed as admitting of division into classes and subordinate groups, according to the external or internal characters which they exhibit. In the investigation of these characters, a variety of methods are employed, and many rules have been prescribed, to regulate the principles of zoological nomenclature.

In order to enter more fully into these important subjects, we shall distribute the present volume into Four Parts. In the first, we shall consider the Condition of Animals in reference to their Duration, Distribution, and Economical Use. In the second, we shall treat of the Methods of Investigation employed to ascertain their structure and actions.