The life of animals is dependent on many conditions. Among these rank warmth, atmospheric air, and moisture, along with sufficient nourishment. Light also is essen tial to many, though most of the colorless animals of the lower classes can dispense with it. With regard to outward pressure, the limits are wide, as is seen in the condor soaring to a height of 20,000 ft., and the whale descending to a depth of 1000 ft. below the surface of the sea. But individual animals are confined to much narrower limits; often to one circumscribed range of climate, one species of food, one medium. To go beyond those limits, though it does not always occasion death, yet gives rise to various degrees of degeneracy, from which even man with all his powers of adaptation is not exempt.
Most animals give more or less strong indications of mind : in those high in the scale, this mental life rises to intellect capable of cultivation, while in the lower classes it appears as instinct confined to a few operations. For communicating with the outer world, vertebrated animals are provided with a nervous system in connection with a central brain—a cerebral nervous system; the ganglionk nervous system of the lower animals seems to serve this purpose less and less as we descend in the scale. The impres sions from without are received immediately by the organs of sense, which become more numerous and complex the higher the animal stands in the scale; among the highest, five senses are usually distinguished, which are variously developed in different species—in none so harmoniously as in man.
Nocturnal sleep, being the means of gathering strength for the activity of the waking hours, stands in intimate relation to that activity, and therefore is wanting in beings low in the scale. Winter sleep, or hibernation (q.v.), serves many animals instead of
migration, to enable them to outlive the cold and hunger of winter. Analogous is the summer sleep of serpents and crocodiles, which lie buried in the dry mud during the summer droughts of the tropics.
Of the other vital manifestations of animals may be mentioned the faculty of giving light (glow-worm, meduste). and that of developing electricity, both possessed only by a few; also voice, belonging almost exclusively to vertebrate animals, and of them chiefly to the warm-blooded.
A very remarkable peculiarity occurs in some of the lowest kinds of animals, in what may be termed a composite life; individuals which separately manifest many of the powers of life being united in part of their frame, many of them together into one living mass. Of this, examples are numerous among the zoophytes (q.v.), some of which have already been noticed in the article ALCYONIU3i.
Apart from the transforming and modifying influence of man, the animals and plants of a district—its fauna atid ifora,—give it life and character. To man himself. animals stand in a variety of relations of the highest importance. Some are directly useful to him.for labor, food, the chase, etc.; others hurtful, as destroyers of vegeta tion, as beasts of prey, as vermin, or by their poisons.—The number of known species of animals amounts at present to about 130,000. To describe and classify these on scientific principles, is the object of zoology (q.v.).