INSTINCT. Every organism is born with a number of tendencies to behave in certain specific ways when confronted with stimuli to which they are adequate. These tendencies not only increase in number and complexity as we pass from the protozoa to the primates, but also vary similarly within a single organism. The amceba inherits three simple reactions; tendencies to move toward certain stimuli, to turn away from others and to engulf still others for food. Man, at the other extreme, has innumerable innate dispositions, and they vary in complexity, as, for example, tendencies to cough and to sneeze, to fear and to become angry, to associate with his kind and to think to a conclusion. All these tendencies are instinc tive, and instinct may be defined as °thegeneral name for these innate tendencies" (Titchener), or as a °congenital mode of behavior depend ent upon inherited dispositions within the lower brain centres" (Morgan), or again as ea combination of congenital responses unfolding serially under appropriate stimulation" (Wat son). Despite differences in emphasis and systematic treatment these three definitions are in agreement as regards essentials, and all three ultimately refer instinct to the nervous system. It is a mistake, therefore, to suppose, as is popularly supposed, that instinct is a faculty provided by Providence to direct the actions of animals and opposed to the faculty of reason furnished for the guidance of man. Such a view purports to explain but, in fact, explains nothing, and it overlooks the fact that man probably has more instincts than the lower animals. If, then, an instinct is a tendency which in the final analysis belongs to the nerv ous system, then it follows that we must go to biology, particularly physiology, for a de tailed description. Nevertheless, instinct has also a psychological aspect, instinctive acts are accompanied by mental processes, and some mental patterns have as their physiological cor relates certain instinctive tendencies. Much needless controversy has arisen through a con fusion of the biological and psychological points of view, and confusion can only be avoided by considering the two aspects separately.
Biology.— The biologists have as yet done little more than to clear the ground for future work; the detailed study of instincts is yet to come. Furthermore, no classification of in stincts has thus far found general acceptance. Instinct may, however, be roughly marked off from the physiological reflex on the one hand and from habit on the other. The former, of
which the knee-jerk, heart-beat and eye-wink will serve as examples, is as a rule confined to a single group of muscles whereas instinct usually involves the entire organism. Habit is a tendency which is acquired during the life time of the organism. We cannot, of course, push these distinctions too far; there is no line of demarkation between reflexes and instincts, and not only are many habits based upon instinctive tendencies but many in stincts are not perfected at their first appear ance. The results of the experimental investi gations may be summarized as follows: (1) Not all instincts make their first appearance at birth; in the vertebrates, at least, new tend encies appear at different stages of growth par ticularly between birth and puberty. In white rats, for example, the instinctive act of °face washing') does not appear until the 12th day, play about the 15th day and sexual activities about the 65th day; the monkey does not as a rule begin to walk before the third week, to make characteristic vocal sounds before the ninth week and the sexual instincts do not ap pear until the end of the first year. (2) We have seen that instincts are not always per fected when they make their first appearance. When this is the case, it sometimes happens that the characteristic act of a species may be modified or even completely inhibited by sub sequent experience. Baltimore orioles reared in captivity and in isolation developed a song quite different from the usual song of their kind; English sparrows reared with canaries gave up the chirp of the sparrow for the peep of the canary; the tendency of a pike to strike at a minnow was completely inhibited after successively bumping against a glass plate in serted in the aquarium and in front of the minnow. (3) The less selective instincts like rest, sleep and play are extraordinarily persist ent although in the human organism they may repeatedly find new forms of expression; others, however, which seem to be conditioned upon the bodily state of the organism as, for example, the various instincts of the mother in• caring for her young, may wane and fade out. (4) Yerkes has found that some instincts like savageness, wildness and timidity in rats, and the direction of whirl in the dancing mouse are hereditary traits.