(from ἰν, in, and στική, to prick), the name given to that principle in the animal economy which enables all animated beings to perform certain actions for the preservation and reproduction of their species, involuntarily and independently of all experience. It may be conveniently divided into physical and mental instinct; the former, independent of will or mind altogether, although never found except where animal life and consequently mind exists, is exemplified in the action of the heart and the peristaltic motion of the bowels; the latter, independent of reason or the higher faculties of mind, may be exemplified in the formation of beaver-dams by the efforts of that animal itself. The existence of such a principle is best seen in the two instances which have been rendered classic by Lord Brougham,—the formation of hexagonal cells by the bee, and the deposition of grubs over her egg by the solitary wasp. In the former example, antecedently to all experience, the bee constructs a habitation for her brood on mathematical principles of economy and strength, not solved by the reason of man till after a lapse of eighteen centuries of his history. In the latter instance, the solitary wasp is found unerringly to provide for an unknown future, bringing a given number of small grubs, and depositing them over her egg in a hole which she has made, providing exactly grubs enough to maintain the worm that egg will produce when hatched, although all the steps in the process are unknown to her experience and transcend her reason, she having never seen an egg produce a worm, having never seen a worm produced, being herself doomed to death before the worm can be in existence, having never tasted grubs nor used the hole which she has made except for the prospective benefit of the unknown worm she is never to see. These two instances bring into striking prominence the characteristic feature of instinctive action. It is action without knowledge, and in the dark; action in which nothing is designed by the agent, and yet all his labour tends to a certain defined and important purpose; action which works to perfection in its kind, anterior to and independent of the teaching of experience. In all these particulars the instinct of animals differs from the appropriate action of man. He only works well after a course of teaching and experiment; he has a consciousness of the nature of the work which he undertakes, and intends and designs to do what he accomplishes. Perfection and uniformity are seen in the products of instinctive toil, imperfection and variety are the consequences of human labour having the process and the result always within the sphere of its cognition.

From these and similar facts various theories have been formed in regard to the instinctive principle. By some philosophers all instinctive action has been attributed to merely an exquisite kind of mechanism, such as may be brought to explain the actions resulting from mere irritation; while, on the other hand, they have been by others referred not only to thought, but to the most sublime degree of intelligence of which we can form any conception. From the marked infallibility of instinctive labour—which though some have challenged, yet most concede—there has arisen the notion that it is produced immediately by a direct divine inspiration; and in comparing the deceitfulness of reason with the faultless operations of instinct, the maxim has become common with many writers, Deus est anima brutorum. Both these theories, however, are incompetent; the one degrading instinct below its proper rank to the level of machinery, and the other lifting it above its proper place to the highest mood of mind. It seems as unreasonable to claim all that is unerring for the immediate agency of God, as to claim it for the purely mechanical agency of matter. The fact of the instinctive impulse being more precise and unerring than human reason, does not demand that it should be regarded as the effect of an influence superior to it, since in this particular, although excelling human reason, it falls short of the precision secured by mere irritation, as irritation itself is transcended in perfection of result by mere chemical and mechanical attraction and repulsion.

Another theory regards instinct as the result of the animal's faculties actuated by the impressions of his senses. Here it is necessary to distinguish instinctive or natural desires from instinctive operations. The gratification of a sensual desire is something different from the execution of a work. In the one it is not necessary to suppose any moving power other than the strength of the appetency, which finds its gratification in the sensual act; but in the other, when the process is long and complex, a purely sensual cause seems utterly inadequate. Of course, such a theory cannot be disproved, and there is much to afford it countenance in those cases where the instinct has been blunted, the animal being guarded against those sensations which would have called it into play. Thus swallows have been known to lose their impulse to migrate at certain seasons, when a due supply of food was granted, and they were carefully shielded from the ungrateful sensations of cold. This, however, is of as little avail to solve the more intricate problems of the question, as is Leibnitz's doctrine of pre-established harmony to cut the knot of liberty and necessity in man, a theory which though it cannot be redargued, scarcely demands a formal rejection. That a bee should simply gratify herself by the laborious structure of a cell—which as a product occupies a place vastly superior to the motive that called it forth—is a much more inconceivable hypothesis even than an invisible power or influence acting constantly and immediately on the animal. To imagine a sensitive organism so contrived that it shall find gratification only in making a straight line of a certain size at right angles to a plane, and in executing rhomboids inclined at a certain angle, is to imagine a machine to which the analogy of our own nature forbids us to be reconciled. But the theory which would reduce all to sensual titillation demands more than this. It postulates an organism so contrived that each finds pleasure not only in her own work, narrowed as it is into a given mathematical track, but in adjusting that work to the place and direction wanted, in order that when added to the previous and subsequent labours of the rest, a given effect unknown to each may be produced. It errs also in placing on a level those acts in which instinct per- forms a known part only of the process, with these higher acts in which instinct not only procures a gratification which it knows, but accomplishes an end which it does not know and does not intend. The phenomena in fact are more complex than the theory allows or proposes to account for; to use Lord Brougham's comparison, it is as inapplicable to account for the working of the bee as the supposition of merely sensuous gratification would be to account for the skill of a man who in rubbing his hands not only warms himself but moulds a watch, or of a child who, in trying a pencil on a slate for its own amusement, unconsciously writes a classic.

Another theory discerns in instinct the faint glimmerings of intelligence working by the same rules which guide the operations of more developed reason; in fact, regards it as of the same kind as the rational intelligence of man. This, however, is completely at variance with the dicta of our consciousness, which clearly and decisively discriminates the simple agency of instinct from the more elaborate process of reasoning. It is at variance with the fundamental idea of unconscious tendency to a given end, which, from an induction of the phenomena, we cannot help regarding as a necessary element in every instinctive act. But although distinct from reason, with which it coexists generally in the inverse ratio, being regular and perfect in its operation in proportion as reason is dormant or inactive, the instinctive act admits of being modified by the reasoning faculty. That animals do reason, and are capable of abstraction and comparison, both of concepts and judgments, in a faint degree resembling the higher intellectual process, is clear from numerous recorded feats, and is the foundation of their training for the assistance of man. The bee, when interrupted by Huber in her operations, was seen to shorten the length of her cells, to diminish the diameter, gradually making them pass from one state to another, and adapting her building to the unusual circumstances imposed on it. The thirsty crow which threw into the pitcher pebble after pebble, till he raised the surface of the liquid to the level of his beak, showed a power of utilizing the resources of nature that is put forth in its highest degree only by him who is pre-eminently the "tool-making animal." And, to a certain extent the habits, thus originated by the judgment of individuals, become hereditary in the race. Skill acquired by dogs in hunting is found to pass to their pups; and the dogs in South America hunt in line the first time they are taken to the chase, while animals from other countries, heedless of the precaution, rush on singly and are killed. Still this modifying power is found to go only a little way, and from its transitory effects, as seen in numerous experiments, it serves only to show how certainly the operations of instinct can not be referred to reason.

It thus only remains for us to regard instinct as a mental faculty, sui generis, the gift of God to the lower animals, that man in his own person and by them might be relieved from the meanest drudgery of nature.