Having, in the preceding chapter, taken a general view of the constitution of those great classes into which the creatures of this Globe may be divided, we come now to trace the relation in which these classes stand to one another; or to examine what has been termed the Polity of Nature.

When we attentively consider the present condition of the inorganic kingdom, it will appear obvious, that the continuance of its existence and arrangement does not depend on the presence of organized beings. Independent of its vegetable covering, of the animals by which it is peopled, and of man himself, this globe could still revolve in its orbit, and act its part in the solar system, though naked, silent and lifeless. Changes would, indeed, take place on its surface, in consequence of the laws of chemistry and mechanics, and independent of the aid of living bodies. The prominent parts would be worn down; the hollows would be filled up ; and its outline would assume an inclination every where at right angles with the direction of gravity. These changes have, in part, been accomplished; and have impressed on the different strata the peculiar characters of their structure and superposition.

The organized kingdom, on the other hand, could not exist alone. The beings of which it is composed may be considered as the parasites of this plahet, and depend on its present movements for the exercise of their functions.

By means of the digestive organs, the food is reduced to a pultaceous mass, and mixed with a variety of secreted fluids. In this state it is denominated Chyme; and, whether it owes its origin to vegetable or animal substances, exhibits, in its last condition, a chemical constitution nearly approaching that of blood, into which it is destined to be converted. In this stage of the process, however, it is necessary to effect a separation between that portion of the chyme which is fitted for the use of the system, and the other part, which is either superfluous or useless.

In giving an account of the circulating system, it is necessary that we attend to the state of the blood, and the fluids which enter into its formation, previous to its aeration ; that we investigate the changes which it undergoes during this important process; and, lastly, consider the uses to which it is applied in what is considered its most perfect state.

The vessels in which the blood is collected previous to aeration, are with great propriety denominated Pulmoinc, by Dr Barclay. They have their origin in every part of the body, and terminate in the lungs, or other organs, in which respiration is performed.

The vessels which are destined to absorb the nutritious portion of the chyme, in the higher orders of animals, are termed Lacteals, from the milky appearance of the fluid which they contain, and which is denominated Chyle.

In treating of the appetites, as occupying a conspicuous place among the active powers, we had occasion to delineate the characters of the instinct for food. In this place, we have to investigate those organs over which this instinct presides, and the important purposes which they serve in the animal economy.

Every one knows, that the sensation of hunger is felt when the stomach is empty, but the nature of the connection between these circumstances has not been satisfactorily explained. Some have ascribed the feeling to the fatigue of the muscular fibre from the stomach being contracted, and others to the friction of the rugae of its internal covering, while there have not been wanting many who refer it to the stimulus of the accumulated gastric juice, or to the acrimony of that fluid, arising from its unusual detention. But the total absence of proof in support of any of these opinions, renders it unnecessary to enter upon their examination.

Many circumstances, however, are known, which promote hunger, especially exercise, cold air applied to the skin, and cold, acid, or astringent fluids introduced into the stomach. Inactivity, warm covering, the attention diverted, and warm fluids, have a tendency to allay the sensation. But whether these effects indicate the local state of the stomach as the cause of the sensation, or merely its connection with other circumstances, which give rise to the feeling, can scarcely be determined.

The organs which we have hitherto been considering, refer exclusively to the individual, and are necessary to the support of the system throughout the whole of life; those which now claim our attention, do not refer to the wants of the individual, but are subservient to the continuation of the species. The instincts by which the organs of reproduction are governed, have been already enumerated, when treating of the active powers of the mind. At present, it only remains for us to enquire into the different modes by which animals are propagated, and the various organs which are called into exercise in each.

The simplest mode of generation does not require sexual organs for the accomplishment of its purpose. Part of an individual drops off, and speedily exercises the functions of an independent being. This is termed Generation by Spontaneous Division. In other cases, a bud is produced from the surface, which gradually evolves, drops off from the parent when ripe, and begins to exhibit a separate life. This is termed Gemmiparous Generation.

In those animals which possess peculiar organs for the preparation of the germ or ovum, some are Androgynous, and either have the sexual organs incorporated, and capable of generating without assistance, or the sexual organs are distinct, and the union of two individuals is necessary for mutual impregnation: others have the sexual organs separate, and on different individuals.

If we employ the twofold method of classification recommended in the preceding part of this work, we shall be at no loss to obtain the first divisions of the animal kingdom, vertebral and invertebral, however great difficulty may be experienced in the construction of the subordinate groups. These two divisions depend, the one on a positive, the other on the negative character, and possess the advantage of being easily recognized. In this respect, and indeed in every other, they have the decided superiority. Any other basis of division hitherto employed is faulty, in not including a number of common properties, in effecting unnatural separation among kindred tribes, or in being founded on characters which are merely modifications of some positive quality. Without wasting the time of the reader, in dwelling on the defects of these different systems, we shall proceed at once to an exposition of the characters of the method employed.

VERTEBRATA

Vertebral Animals

CHARACTER.—ANIMALS FURNISHED WITH A SKULL AND VERTEBRAL COLUMN FOR THE PROTECTION OF BRAIN AND SPINAL MARROW.

Description.—The properties which the vertebral aniitials possess in common, are numerous, and clearly indicate the unity of the plan according to which they have been constructed. In reference to the nervous system, indeed, a conformity of character here prevails, which is not observable in that, or any other system of organs among the invertebral tribes.

The infinite variety of species which constitute the organized kingdom, possess many common properties, independent of the remarkable differences which they exhibit in their structure and appearance. By attending to these common properties, we shall be able to discover some of the qualities of that principle to which they owe their character, and by which their arrangements are regulated.

All organized bodies consist of Solids and Fluids. The former exhibit the appearance of fibres or laminae, of which cells and tubes are constructed, destined to contain the fluids. These solids and fluids are very differently arranged in the different classes of organized being, although similar in individuals of the same species.

The Characters of the Vital Principle.–When we examine a plant or an animal as near to the origin of its existence as possible, we witness its embryo or germ, small indeed, but possessing a power capable of developing in succession the destined phenomena of existence. By means of this power, the germ is able to attract towards it particles of inanimate matter, and bestow on them an arrangement widely different from that which the laws of chemistry and mechanics would have assigned them. The same power not only attracts these particles and preserves them in their new situation, but is continually engaged in removing those which, by their presence, might prevent or otherwise derange its operations.

In the numerous references which we have hitherto made to the faculty of Sensation, as a display of the operation of the nervous system, we have considered it as indicating merely the presence of bodies, and as giving no information respecting their character. If we attend more minutely to this faculty, we shall find, that all the sensitive parts of the body, are not equally capable of warning us of the presence of the same kind of objects. The rays of light make no im pression upon the tongue or the fingers, indicating their presence, while they act with energy on the eye. The vibrations of the air make no impression on the eye, the mouth, or the nose, while they instantly act upon the ear. Sensation, therefore, is a generic term, intimating the capability of the nervous system to receive impressions of external objects; and it includes as many species as there are impressions calculated to act on one organ, and not upon another, distinguished by this common property, that they intimate the presence of objects.

The number of impressions which may be regarded as distinct species, is more extensive than is generally imagined, and would justify us in considering the term Sensation as the index of an order or class, rather than of a subordinate division. Philosophers, however, have agreed to reduce our sensations to five kinds, namely, those of Touch, Sight, Hearing, Taste, and Smell, to which I have ventured to add Heat.

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.