or taking up particles of their own structure, and throwing them off. The actions of composition and decomposition are constant whilst life persists; although subject to particular modifications at different periods of existence, and under different circumstances. Again: the inorganic and organized are alike subject to changes during their existence; but the character of these changes, in the two classes, differs essentially. The mineral retains its form, unless acted upon by some mechanical or chemical force. Within, all the particles are at rest, and no internal force exists, which can subject them to modification. There is no succession of conditions that can be termed ages. How different is the case with organized bodies! Internally, there is no rest; from birth to death all is in a state of activity. plant and the animal are subject to incessant changes. Each runs through a succession of conditions or ages. We see it successively develope its structure and functions, attain maturity, and finally decay. The Characteristic differences likewise exist in the external conformation of the beings of the two divisions, as well as in their mode of increase. Inorganic bodies have no covering to defend them; no exterior envelope to preserve their form; a stone is the same at its centre as at its circumference; whilst organized bodies are protected by an elastic and extensible covering, differing from the parts beneath, and inservient to valuable purposes in the economy. Every change to which an inorganic body is liable must occur at its surface. It is there that the particles are added or abstracted when it experiences increase or diminution. Increase-for growth it can scarcely be termed-takes place by accretion or juxtaposition, that is, by the successive application of fresh particles upon those that form the nucleus; and diminution in bulk is produced by the removal of the external layers or particles. In organized substances, increase or growth is caused by particles deposited internally, and diminution by particles subtracted from within. We see them, likewise, under. two conditions, to which there is nothing similar in the mineral kingdomhealth, and disease. In the former, the functions are executed with freedom and energy; in the latter, with oppression and restraint. 7. Termination.-Every body, inorganic or organized, may cease to exist, but the mode of cessation varies greatly in the two classes. The mineral is broken down by mechanical violence; or it ceases to exist in consequence of modifications in the affinities, which held it concrete. It has no fixed duration; and its existence may be terminated at any moment, when the circumstances, that retained it in aggregation, are destroyed. The vegetable and the animal, on the other hand, carry on their functions for a period only which is fixed and determinate for each species. For a time, new particles are deposited internally. The bulk is augmented, and the external envelope distended, until maturity or full developement is attained; but, after this, decay commences; the functions are exerted with gradually diminishing energy; the fluids decrease in quantity; and the solids become more rigid,-circumstances premonitory of the cessation of vitality. This term of duration is different in different species. Whilst many of the lower classes of ani mals and vegetables have but an ephemeral existence, some of the more elevated individuals of the two kingdoms outlive a century. 8. Motive forces.-Lastly, observation has satisfactorily proved, that there are certain forces, which affect matter in general, inorganic as well as organized; and that, in addition to these, organized bodies possess a peculiar force or forces, which modify them in a remarkable manner. Hence, we have general forces; and special or vital; the first acting upon all matter, the dead and the living, and including the forces of gravitation, cohesion, chemical affinity, &c.; the latter appertaining exclusively to living beings. Such are the chief distinctions to be drawn between the two great divisions of natural bodies; the inorganic and the organized. By the comparison which has been instituted, the objects of physiology have been indicated. To inquire into the mode in which a living being is born, nourished, reproduced, and dies, is the legitimate object of the science. We have, however, entered only into a comparison between the inorganic and the organized. The two divisions constituting the latter class differ also materially from each other. Into these differences we shall now inquire. 2. DIFFERENCE BETWEEN ANIMALS AND VEGETABLES. The distinctions between the divisions of organized bodies are not so rigidly fixed, or so readily appreciated, as those between the inorganic and the organized. There are certain functions possessed by both; hence called vegetative, plastic, or organic,-nutrition and reproduction, for example; but vegetables are endowed with these only. All organized bodies must have the power of assimilating foreign matters to their own substance, and of producing a living being similar to themselves; otherwise, the species, having a limited duration, would perish. In addition to these common functions, animals have sensation and voluntary motion; by the possession of which they are said to be animated. Hence, they are termed animals, and the condition is called animality. This division of the functions into animal and organic has been adopted, with more or less modification, by most physiologists. Between animals and vegetables, situate high in their respective scales, no confusion can exist. The characters are obvious at sight. No one can confound the horse with the oak; the butterfly with the potato. It is on the lower confines of the two kingdoms, that we are liable to be deceived. Many of the zoophytes have alternately been considered vegetable and animal; but we are generally able to classify any doubtful substance with accuracy; and the following are the principal points of difference. 1. Composition.-It was long supposed, that the essential difference between animal and vegetable substances consists in the former containing nitrogen; whilst the latter do not. Modern researches have, however, satisfactorily shown, that the organized portions of animals and vegetables are essentially alike; and consist of the four elements, -carbon, oxygen, hydrogen, and nitrogen; whilst the unorganizedas the fat of the animal, and the starch of the vegetable-are composed of three elements only-carbon, oxygen, and hydrogen. Still, their intimate composition must vary greatly; for, when burning, the animal substance is readily known from the vegetable;-a fact, which, as Dr. Fleming' has remarked, is interesting to the young naturalist, if uncertain to which kingdom to refer any substance met with in his researches. The smell of a burnt sponge, of coral, or other zoophytic animal, is so peculiar, that it can scarcely be mistaken for that of a vegetable body in combustion. According to Mulder,2 there is this real difference between plants and animals in composition, that cellulose (C24H2O21) forms the principal part of the cellular mass in plants; whilst in animals the primary material is gelatin (C13H1oN2O5); and to this rule, he says, no exception has yet been discovered either among animals or plants. 2. Texture. In this respect, important differences are observable. Both animals and vegetables consist of solid and fluid parts. In the former, however, the fluids bear a large proportion: in the latter, the solids. This is the cause, why decomposition occurs so much more rapidly in the animal than in the vegetable; and in the succulent more than in the dry vegetable. If we analyze the structure of the vegetable, we cannot succeed in detecting more than one elementary tissue, which is vesicular or areolar, or arranged in vesicles or areolæ, and appears to form every organ of the body; whilst, in the animal, we discover at least three of these anatomical elements, the areolaranalogous to that of the vegetable;-the muscular, and the nervous. The vegetable again has no great splanchnic cavities containing the chief organs of the body. It has a smaller number of organs, and none that are destined for sensation or volition; in other words, no brain, no nerves, no muscular system; and the organs of which it consists are simple, and readily convertible into each other. But these differences in organization, striking as they may appear, are not sufficient for rigid discrimination, as they are applicable only to the upper classes of each kingdom. In many vegetables, the fluids appear to preponderate over the solids; numerous animals are devoid of muscular and nervous tissues, and apparently of vessels and distinct organs; whilst MM. Dutrochet,3 Brachet, and others, admit the existence of a rudimental nervous system even in vegetables. 3. Sensation and voluntary motion.-There is one manifest distinction between animals and vegetables. Whilst the latter receive their nutrition from the objects around them-irresistibly and without volition, or the participation of mind; and whilst the function of reproduction is effected without the union of the sexes, both volition and sensation are necessary for the nutrition of the former, and for acts that are requisite for the reproduction of the species. Hence, the necessity 'Philosophy of Zoology, i. 41. Edinburgh, 1822. The Chemistry of Animal and Vegetable Physiology; translated by Fromberg, p. 91. Edinburgh and London, 1849. 3 Recherches Anatomiques et Physiologiques sur la Structure Intime des Animaux, et des Végétaux, et sur leur Motilité. Paris, 1824. 4 Recherches Expérimentales sur les Fonctions du Système Nerveux Ganglionnaire, &c. 2d édit. Paris et Lyons, 1837. Sir J. E. Smith, Introduction to Botany, 7th edit., by Sir W. J. Hooker, p. 40. Lond. of two faculties or functions in the animal, that are wanting in the vegetable, sensibility, or the faculty of consciousness and feeling; and motility, or the power of moving at will the whole body or any of its parts. Vegetables are possessed of spontaneous, but not of voluntary motion. Of the former we have numerous examples in the direction of the branches and upper surfaces of the leaves, although repeatedly disturbed, to the light; and in the unfolding and closing of flowers at stated periods of the day. This, however, is distinct from the sensibility and motility that characterize the animal. By sensibility man feels his own existence, becomes acquainted with the universe,-appreciates the bodies that compose it; and experiences all the desires and inward feelings that solicit him to the performance of those external actions, which are requisite for his preservation as an individual, and as a species; and by motility he executes those external actions which his sensibility may suggest to him. By some naturalists it has been maintained, that those plants, which are borne about on the waves, and fructify in that situation, exhibit examples of the locomotility, which is described as characteristic of the animal. One of the most interesting novelties in the monotonous occurrences of a voyage across the Atlantic towards the Gulf of Florida is the almost interminable quantity of Fucus natans, Florida weed or Gulf weed, with which the surface of the ocean is covered. But how different is this from the locomotion of animals! It is a subtlety to conceive them identical. The weed is passively and unconsciously borne whithersoever the winds and the waves may urge it; whilst animal locomotion requires the direct agency of volition, of a nervous system that can excite, and of muscles that can act under such excitement. The spontaneity and perceptivity of plants must also be explained in a different manner from the elevated function of sensibility on which we shall have to dwell. These properties must be referred to the fact of certain vegetables being possessed of the faculty of contracting on the application of a stimulus, independently of sensation or consciousness. If we touch the leaf of the sensitive plant, Mimosa pudica, the various leaflets collapse in rapid succession. In the barberry bush, Berberis vulgaris, we have another example of the possession of this faculty. In the flower, the six stamens, spreading moderately, are sheltered under the concave tips of the petals, till some extraneous body, as the feet or trunk of an insect in search of honey, touches the inner part of each filament, near the bottom. The susceptibility of this part is such, that the filament immediately contracts, and strikes its anther, full of pollen, against the stigma. Any other part of the filament may be touched without this result, provided no concussion be given to the whole. After a while, the filament retires gradually, and may be again. stimulated; and when each petal, with its annexed filament, has fallen to the ground, the latter, on being touched, shows as much sensibility as ever.1 These singular effects are produced by the power of contractility or Sir J. E. Smith's Introduction to Botany, p. 325. irritability, the nature of which will fall under consideration hereafter. It is possessed equally by animals and vegetables, and is essentially organic and vital. This power, we shall see, needs not the intervention of volition: it is constantly exerted in the animal without consciousness, and therefore necessarily without volition. Its existence in vegetables does not, consequently, demonstrate that they are possessed of consciousness. 4. Nutrition. A great difference exists between plants and animals in this respect. The plant, being fixed to the soil, cannot search after food. It must be passive; and obtain its supplies from the materials around, and in contact with it; and the absorbing vessels of nutrition must necessarily open on its exterior. In the animal, on the other hand, the aliment is scarcely ever found in a state fit for absorption: it is crude, and in general-Ehrenberg' thinks always--requires to be received into a central organ or stomach, for the purpose of undergoing changes, by a process termed digestion, which adapts it for the nutrition of the individual. The absorbing vessels of nutrition arise, in this case, from the internal or lining membrane of the alimentary tube. The analogy that exists between these two kinds of absorption is great, and had not escaped the attention of the ancients:-Quemadmodum terra arboribus, ita animalibus ventriculus sicut humus, was an aphoristic expression of universal reception. With similar feelings, Boerhaave asserts, that animals have their roots of nutrition in their intestines; and Dr. Alston has fancifully termed a plant an inverted animal. After all, however, the most essential difference consists in the steps that are preliminary to the reception of food. These, in the animal, are voluntary,-requiring prehension; often locomotion; and always consciousness. 5. Reproduction.-In this function we find a striking analogy between animals and vegetables; but differences exist, which must be referred to the same cause that produced many of the distinctions already pointed out,-the possession, by the animal, of sensibility and locomotility. For example, every part of the generative act, as before remarked, is, in the vegetable, without the perception or volition of the being: the union of the sexes, fecundation, and the birth of the new individual are alike automatic. In the animal, on the other hand, the approximation of the sexes is always voluntary and effected consciously: -the birth of the new individual being not only perceived, but somewhat aided by volition. Fecundation alone is involuntary and irresistible. Again, in the vegetable the sexual organs do not exist at an early period; and are not developed until reproduction is practicable. They are capable of acting for once only, and perish after fecundation; and if the plant be vivacious, they fall off after each reproduction, and are annually renewed. In the animal, on the contrary, they exist from the earliest period of foetal development, survive repeated fecundations, and continue during the life of the individual. Edinb. New Philosophical Journal, for Sept. 1831; and Jan. 1838, p. 232. 'Tirocinium Botanicum Edinburgense, 8vo., Edinb. 1753. |