DISTINCTIVE CHARACTERS OF ANIMALS. 25 great as that which exists among the lowest animals; so that no positive line can be drawn between the two kingdoms on the basis of this distinction alone. There is another very important physiological difference, however, between the two kingdoms, which seems to afford an adequate means of settling the true place of those tribes whose position would otherwise be doubtful. This lies in the nature of their food, and the source from which it is obtained. For although it is now known that the primary tissues of plants are originally formed of the same albuminous material as are those of animals (the cellulose layers which constitute the great bulk of the vegetable fabric being a subsequent deposit), yet this material is generated in the Plant by the combination of the elements which it obtains from the carbonic acid, water, and ammonia of the soil or of the atmosphere; whilst the Animal is destitute of all power of thus forming it for itself, and is hence entirely dependent upon the plant for its supplies of nutriment. Thus, whilst the very humblest forms of Vegetation, in common with the highest, are found to have the power of decomposing carbonic acid under the influence of sunlight, setting free its oxygen and retaining its carbon, the humblest forms of Animal life, in common with the highest, derive their nutriment either directly from plants, or from the bodies of other animals which have subsisted on vegetable food, whilst they produce a converse change in the atmosphere by their respiration, absorbing from it oxygen, and giving forth to it carbonic acid. This criterion will serve, it is believed, to distinguish the very lowest forms of Animal life from those humble forms of Vegetation which they most closely resemble in the simplicity of their organization (§ 128); and its application will generally be found to be very easy. There is now no longer any doubt that a large proportion of the beings formerly ranked as Animalcules, are really to be regarded as Plants, notwithstanding that they possess a power of active and apparently spontaneous movement, far greater than that of many unquestionable animals. And generally it may be said that the presence of a bright-green or bright-red colour in any of these simple organisms, where it is not derived from coloured substances taken in as food, affords a strong probability of their vegetable character; these colours being produced in the 26 DISTINCTIVE CHARACTERS OF ANIMALS. course of that series of chemical changes, by which, under the influence of light, the living plant can unite, inorganic elements into organic compounds. 8. Not only do Animals differ from Plants in the nature and sources of their aliment, but also in the mode in which it is taken into their bodies; and this difference is related alike to the character of the food of animals, and to the general conditions of animal existence. For the Plant extends its roots through the soil in search of liquid, and spreads out its leaves to the air for the purpose of imbibing some of its gaseous ingredients. But the Animal could not so exist, and be at the same time endowed with the power of moving from place to place; nor could it appropriate solid nutriment, if it were not provided with some peculiar means of receiving and preparing this. For these purposes, animals (with few exceptions) are provided with an internal cavity or stomach into which the food is received from time to time, in which it can be carried about in the general movements of the body, and within which it can be prepared for being received by absorption into the current of nutrient liquid which circulates through the body. This stomach is nothing else than a bag formed by the prolongation of the external covering of the body into its interior (§ 36); its cavity receives the food introduced into it by the mouth; its walls pour out or secrete a fluid which acts upon the food in such a manner as to dissolve it; and through its walls are absorbed those portions of the food which are fit to be employed as nutriment, while the remainder is cast forth from the cavity, either by the aperture which first admitted it, or by a distinct orifice. The exceptional cases, in which no stomach exists, chiefly occur in one particular tribe of animals, the Entozoa (§ 105), which live either in the intestinal canal or in the substance of the tissues of other animals, and which are supported by the nutrient juices of these; such an organ obviously not being required by creatures which have no power of locomotion, and which can imbibe liquids already prepared for their use, through the whole of the soft surface of their bodies. But there is a large tribe of very simple animals, the Rhizopoda (§ 129), in which, notwithstanding the absence of any regular stomach, the food is received into the very substance of the jelly-like particle of which the body consists; a mouth and DISTINCTIVE CHARACTERS OF ANIMALS. 27 stomach being extemporized, as it were, on each occasion that aliment is ingested; and an anal orifice being extemporized in like manner, when the indigestible residue has to be cast forth. All true Animalcules (§ 133) have a proper mouth, into which food is drawn by the current created by the cilia (§ 45) wherewith it is fringed; and this mouth leads to the general cavity of the body, within which the food is subjected to the digestive process. In Zoophytes (§ 121) which possess a proper stomach, this organ forms so large a part of the animal, that its entire body may be almost said to consist of the stomach and of the prehensile appendages by which it draws in its food. But in all the higher tribes, the stomach, with the alimentary canal proceeding from it, are suspended freely within the general cavity of the body; and we shall find that the space that surrounds these viscera is extremely important in the economy of all but vertebrated animals, as being a sort of reservoir into which the nutrient materials prepared by the digestive process first transude, and from which it is carried into the remoter parts of the system. In vertebrated animals, this cavity-called in them the peritoneal cavity, from its being lined with a serous membrane (§ 28), termed the peritoneum is not subservient to the same purposes; the nutrient materials being taken up from the walls of the digestive cavity, both by the blood-vessels and by special absorbents, and being by them carried into the current of the circulation. It is obvious that until they have found their way, through one or other of these channels, into the general system, the nutrient materials introduced as food into the stomach of an animal are not within its body, properly so called, any more than a fluid is within a plant when it bathes the exterior of its roots, or within an entozoon when in contact with the soft surface of its integument. In each case, the absorption of the fluid is first requisite; and it is with this that its application to the requirements of the living body really commences. 9. But further, when we compare together, not the lowest, but the highest members of the Vegetable and Animal kingdoms respectively-those in which their respective attributes are most characteristically displayed,-we find that they present such differences as to render it quite impossible to confound the one with the other. Although it is easy even 28 DISTINCTIVE CHARACTERS OF ANIMALS. for the scientific naturalist to mistake a Protophyte (or one of the simplest forms of vegetation) for an Animalcule, and although Zoophytes are continually ranked in the popular mind with the Plants they so much resemble in form, no one is in any danger of confounding the Oak and the Elephant, the Palm and the Whale. For among the higher Animals, not only the principal organs, but the greater part of their elementary parts or tissues, are formed upon a plan entirely different from that which prevails in Plants. All the arrangements of their organism or corporeal edifice are made for the purpose of enabling them to perform, in the most advantageous manner possible, those peculiar functions with which they have been endowed,-to receive sensations,-to feel, think, and will, and to move in accordance with the directions of the instinct or the judgment. For these purposes we find a peculiar apparatus, termed the Nervous system, adapted. This apparatus consists of a vast number of fibres, spread out over the surface of the body, and especially collected in certain parts, called Organs of Sense (such as the eye, nose, ear, tongue, lips, and points of the fingers). These have the peculiar property of receiving impressions which are made upon their extremities, and of conveying them to the central masses of nervous matter (known in the higher animals as the Brain and Spinal Cord), by the instrumentality of which they are communicated to the mind. 10. From the Nervous centres, other cords proceed to the various Muscles, by which the body is moved. These muscles, commonly known as "flesh," are composed of a tissue which has the power of contracting suddenly and forcibly, when peculiar stimuli are applied to it. In this respect, it bears a resemblance to the contractile tissues by which the movements of plants are produced (VEGET. PHYS. § 390); but it differs from them in being thrown into action, not only by stimuli that are applied directly to itself, but by an influence conveyed through the nervous system. Thus, in an animal recently dead, we may excite any muscles to contraction, by sending a current of electricity into the nerves supplying them; and in a living animal we may do the same by simply touching those nerves. But the stimulus which these nerves ordinarily convey, originates in an act of the mind, which is connected in some mysterious and inscrutable manner with DISTINCTIVE CHARACTERS OF ANIMALS. 29 the central masses of the nervous system. Thus, we desire to perform a certain movement or set of movements; this desire leads to an act of volition or will; and the will causes a certain force or motor impulse to issue from the brain and travel along the nerves, so as to produce the desired motion, by exciting contractions in the muscles that perform it. Or, again, a certain sensation calls forth an emotion, which prompts a certain muscular movement, and may even cause it to take place against the will,—as when a strong sense of the ludicrous produces laughter, in spite of our desire (owing to the unfitness of the time and place) to restrain it; for the emotion, like the act of volition, produces a change in the nervous centres, which causes a motor impulse to travel along the nerves, and thus calls the muscles into contraction. And it seems to be in the same manner that those instinctive actions are produced, which, although few in adult Man when compared with those resulting from his will, predominate in his infant state, and through the whole life of the lower animals (Chap. xiv.). We shall also find that the nervous and muscular systems of animals are concerned in a class of actions with which the mind has no necessary connexion; these automatic actions, such as those of swallowing (§ 195) and breathing (§ 340), having for their object to assist in the performance of the organic functions, and to protect the body from danger. 11. In the higher Animals, then, the presence of this Nervo-Muscular apparatus is an essential and obvious distinction between their structure and that of Plants; and we find that it constitutes a large part of the bulk of the body. Thus the whole interior of the skull of Man is occupied by his brain; his limbs are composed of the muscles, and of the bones which support them and which are put in motion by them; and it is only in the interior of his trunk, that we find organs corresponding with those which form the entire fabric of the Plant. These organs of Nutrition have for their main purpose, to supply the wants of the organs of animal life; every exercise of which is accompanied by a certain decay or wear of their structure, and which consequently require to be continually nourished and repaired, by the materials provided by what may be termed the vegetative organs. But in the lower of tribes of Animals, we do not find the animal functions to possess this predominance. In fact, among the |