Animal. The popular classification of all bodies into three 'kingdoms'—the animal, the vegetable, and the mineral—only assumed authority in comparatively recent times, and has done much mischief in exaggerating the apparent differences between plants and animals on the one hand, and in obscuring the fundamental distinction between these and minerals on the other. There are in reality only two kingdoms of nature, the living and the non-living—the organic and the inorganic. The famous aphorism of Linnaeus, 'Stones grow; plants grow and live; animals grow, live, and feel,' is no longer satisfactory, for growth is of two distinct kinds. While growth in minerals takes place merely by accretion—addition of new particles to the external surface, that of living matter is by intussusception—the interposition of new molecules between those formerly present. Again, living matter, or protoplasm, is clearly distinguished by its chemical composition, it being composed of very highly complex compounds, or mixture of compounds of carbon, nitrogen, hydrogen, oxygen, and sulphur, together with water and salts. During life, it is incessantly disintegrating and combining with the oxygen of the atmosphere, many products of change, chiefly carbonic acid, water, and nitrogenous waste being evolved; and reintegration must therefore take place by intussusception, for which purpose new matter containing the necessary elements must be taken up, either from other organisms or from the inorganic world. Certain cyclical changes are also exhibited by all forms of living matter—that is to say, each arises as a detached portion of some previous organism; develops into a form similar to that from which it arose; tends to reproduce itself; and, finally, ceases to live, when its protoplasm breaks up, and its elements ultimately return in a highly oxidised state to the inorganic world. Moreover, certain conditions of temperature, pressure, presence of oxygen, &c., variable only within comparatively slender limits, are essential to the maintenance of life.
While living bodies are thus clearly distinguishable from inorganic, every attempt to erect a similarly sharp distinction between plants and animals completely breaks down. Vast numbers of animals are destitute of the power of locomotion, so that, for instance, corals were unhesitatingly referred to the vegetable kingdom until about a century ago; while diatoms, and many embryonic algae and fungi, which possess marked powers of locomotion, would thus require to be ranked as animals. Nor is sensibility a purely animal characteristic; the well-known sensitive plant, the sun-dew, and Venus's fly-trap, exhibiting it in the most marked degree. Cellulose, again, which forms the coating of the vegetable cell, was regarded as completely characteristic of this; but many algae and fungi are naked at some period of their lives, while the thick external tunic of those degraded vertebrates known as Ascidians has essentially the chemical composition of plant cellulose. Chlorophyll, the green colouring matter of plants, is absent from fungi and from many flowering parasites, and is yet present in infusorians, in Hydra and some other invertebrates, which are thus enabled to vegetate in sunshine, forming starch and evolving oxygen.
Animals thus do not necessarily feed; while the well-known insectivorous plants (see DIONEA, SUN-DEW) capture animals, and frequently digest them.
The attempt to establish a difference in structure is equally unsuccessful; for although the students of higher forms have no difficulty in grouping their flowers and ferns, their birds and beasts, into distinct series, the microscopist finds that these two great stems arise from a common root. It has therefore repeatedly been proposed to divide living forms into three groups—Animals, Plants, and Protista—a solution which, while decidedly gaining adoption on account of its great service in treating together the lowest forms hitherto separated as Protozoa and Protophytes, of course raises minor difficulties—that of distinguishing on the one hand between Protists and Animals, and on the other, between Protists and Plants. And thus every attempt to limit and define its forms has really resulted in proving the fundamental unity of life. The general study of the phenomena of life constitutes the science of Biology, of which the sub-sciences are usually reckoned as four: (1) Morphology, dealing with the structure of organisms, and including Anatomy and Embryology; (2) Distribution, dealing with the time and place of their occurrence on the earth; (3) Physiology, dealing with the study of their functions; (4) Etiology, dealing with the explanation of the preceding facts by the rival hypotheses of Creation and Evolution. These subjects are divided between botanist and zoologist, and their labours, while starting, as has been shown, from a common point, thence diverge widely. The results of Animal Morphology are outlined in the articles ZOOLOGY, VERTEBRATA, &c.; and those of the study of Animal Physiology under the heads of the separate functions, DIGESTION, NUTRITION, REPRODUCTION, &c. See also GEOGRAPHICAL DISTRIBUTION, EVOLUTION, &c. For Animal Intelligence, see INSTINCT.