Instinct, the mental aspect of those actions which take rank between unconscious reflex activities and intelligent conduct. When we observe the lowest forms of life gliding slowly towards their food, or the roots of plants overcoming obstacles in their search for soil and moisture, we recognise activities certainly advantageous, yet so comparatively simple that they almost admit of direct physical and chemical explanation. More complex activity is at once apparent when we watch the fly-trap or sundew catching insects, or notice the protective responses which most animals make to provoking or startling stimuli. These imply an inherited and well-established relation of parts (usually nerve and muscle), such that a frequently recurrent form of stimulus provokes an immediate, definite, and more or less appropriate response. Such actions usually depend on what is figuratively called a neuro-muscular 'mechanism'—i.e. on the power that subordinate nerve-centres have of responding to stimuli without bringing the chief centres (or brain) into exercise. They may therefore occur even in cut-off parts of animals, or after the organism is virtually dead. Higher than these, however, are the marvellous activities, most familiar perhaps in insects and birds, for which more or less of a brain is essential, which are so engrained in the organism that they require no practice, which often adapt means to ends, but show little power of adjustment to novel conditions, which are finally the birthright, not of elect individuals, but of all the members of a species. But as we review the animal series in ascending order we become more and more impressed with yet higher actions, for which a head-centre or brain seems essential, which often require to be learned and are perfected by practice, which adapt means to ends in novel circumstances, and vary greatly among individuals.
So far we have kept apart such words as mind, intelligence, instinct, consciousness; but that is no longer possible or desirable, for the last three grades of activity described above are not only observable facts, but are also parts of our personal experience, and must be considered in that light. Like animals, we of course exhibit immediate neuro-muscular responses to external stimuli: witness the sudden withdrawal of our finger from a burning object unwittingly touched. Such responses, for which brains are not necessary, occur 'without our knowing,' and are called reflex. Next on the scale come numerous actions, from the sucking of infancy onwards, which require no practice, deliberation, or effort, but yet have a distinct mental aspect, being usually associated with consciousness, and stimulated rather by perceptions than by sensations. Such actions, learned so long ago that the power of doing them is now entailed by heredity, are more predominant in animals than in ourselves, and are called instinctive. Higher than these, and pre-eminent in man, are the actions which deliberately adapt means to ends, with conscious intention and controlling intelligence. These lead on to the most characteristically human actions, in which we often seem to hold ourselves as unities apart from what is outside us, and in which we are influenced by general ideas and definite ideals, being in fact self-conscious men. So far there is practical unanimity, but difficulties inevitably arise when we begin to project upon animals our own experience of reflex, instinctive, and intelligent actions. We are forced to argue by analogy, and therefore with uncertainty. It is allowed, however, by almost all that the old-fashioned attempt to call all the higher activities of animals instinctive, in sharp contrast to the intelligent conduct of man, merely expresses an ignorant prejudice. No competent observer denies that ant and bee, dog and elephant, beaver and monkey, frequently exhibit actions higher than instinctive, in some cases quite parallel to that human conduct which we call intelligent. This, however, does not of course assert that any animals have attained to the human level of self-conscious intelligence, with its ideas and ideals. In thinking about the grades of action, which are usually regarded as stages in evolution, it is well to distinguish the objective or observable characteristics from the subjective or analogical interpretation; and it is also important to recognise that the grades distinguished are not hard and fast, but simply mark areas on an inclined plane which slopes from the amoeba up to man.
Definitions.—We are not concerned here with the general questions suggested by such descriptions of instinct as refer it to 'immediate impressions from the First Mover or from the divine energy acting in the creature,' nor is it necessary to discuss those which make the term include all the adaptive actions of animals in sharp contrast to the intelligence of man. Some others, however, are more to the point. Thus, Hartmann defines instinct as 'action taken in pursuance of an end, without conscious perception of what the end is.' Spencer calls instinct 'a kind of organised memory;' Samuel Butler says 'instinct is inherited memory;' J. J. Murphy describes it as 'the sum of inherited habits.' According to Eimer, 'instinct is inherited capability, and especially inherited habit; or more exactly, instinct is the inherited power of acting habitually and without deliberation in a purposeful, intelligent fashion, under the influence of internal stimuli, plus or minus others from without.' According to Romanes, 'instinct is reflex action into which there is imported the element of consciousness. The term is therefore a generic one, comprising all those faculties of mind which are concerned in conscious and adaptive action, antecedent to individual experience, without necessary knowledge of the relation between means employed and ends attained, but similarly performed under similar and frequently recurring circumstances by all the individuals of the same species.'
Examples.—Instinctive actions are usually perfect from the first and independent of individual experience. Thus, the butterfly makes the remarkable transition from caterpillar to adult habits without hesitation or failure; the bee rifles flowers on its first flight; and the chick in the first few hours of its open-air life makes successful darts at flies. In other cases, however, practice appears to help, as in the nest-building activities of birds. Nor are instincts always sufficiently perfect, for ants store beads instead of grains, and mistake corn-wheat seeds for their own cocoons; flower-visiting insects also patronise bright-coloured wall-paper; and the lemmings in their instinct for going right ahead will swim straight out to sea. Marvellous are the instincts exhibited by social animals such as ants and beavers, by insects which provide elaborately for young which they never survive to behold, and in the nesting and migration of our common birds. Less pleasant, in fact almost devilish in ingenuity, is the instinct of the Sphex wasps, which provide fresh meat for their future larvae by storing spiders, insects, and caterpillars which they have stung in their chief nerve-centres, with the result that the victims are not killed outright, but only paralysed.
Origin of Instincts.—An approximation to the truth will probably be attained by combining the chief theories. (1) Instincts may be the inherited results of compound reflex actions, and are there- fore in origin unintelligent (Spencer). (2) Natural selection may fix on purposeless habits which chance to be profitable, and convert them into instincts without intelligence being ever concerned in the process (Darwin, Romanes). Weismann points out that not a few instincts are exhibited only once in a lifetime, so that they at least can hardly be the inherited results of practice. He holds that 'all instinct is entirely due to the operation of natural selection, and has its foundation not upon inherited experiences, but upon variations of the germ.' (3) Instincts may arise from habits, which were originally intelligent, becoming by repetition automatic (Darwin, Lewes, Romanes, &c.). Eimer derives instincts from inherited intelligent habits, which are shortened and simplified in evolution, though frequently retaining a trace of deliberation. (4)
The diagram shows a vertical progression of four levels. At the top is 'INTELLIGENT.' with a downward arrow pointing to 'Automatic Habit.' Below 'Automatic Habit.' is 'Instinctive.' with an upward arrow pointing to 'Reflex Action.' at the bottom. A horizontal line connects 'Automatic Habit.' and 'Instinctive.' with a downward arrow pointing from 'Automatic Habit.' to 'Instinctive.' and an upward arrow pointing from 'Instinctive.' to 'Automatic Habit.'.
Diagram illustrating origin of Instinct from reflex action on the one hand, from lapsed intelligence on the other (cf. Romanes).
While instincts may arise by natural selection alone, or by lapsing intelligence alone, 'these principles when working in co-operation have greater influence in evolving instincts than either of them can have when working singly' (Romanes). (5) Imitation, as Wallace insists, and the power of rapid learning, which Eimer emphasises, have probably been of importance in the evolution of some instincts. It seems certain that instincts may arise either from unintelligent or from intelligent habits, that their evolution may be abetted by natural selection, and that the power of instinctive action is conserved by the organic memory of inheritance.
See ANT, BEAVER, BEE, BIRD, CUCKOO, ELEPHANT, &c. for illustrations; also BRAIN, EVOLUTION, HEREDITY. For full illustrations, see especially G. J. Romanes, Animal Intelligence (Inter. Sc. Series, 1882); Couch's Illustrations of Instinct; Lauder Lindsay's Mind in Animals; Büchner's Aus dem Geistesleben der Thiere (trans.), &c. For theory of instinct, see especially Romanes, Mental Evolution in Animals, with a posthumous essay on Instinct by Darwin (1883); compare Darwin's Origin of Species; Wallace's Natural Selection; Spencer's Principles of Psychology and Principles of Biology; G. H. Lewes, Problems of Life and Mind; S. Butler, Life and Habit; J. J. Murphy, Habit and Intelligence; Carpenter, Maudsley, Bastian, Wundt, and others on Mental Physiology; E. Von Hartmann, Das Unbewusste vom Standpunkte der Physiologie (2d ed. 1877); Schneider, Der thierische Wille (1880); Preyer, Die Seele des Kindes (1882); Eimer, Die Entstehung der Arten (1888); Weismann, Papers on Heredity (1889).