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ble teaching of what is remarkably skillful and marvelously adapted conduct。＇1＇ ＇1＇ The nature of instinct has been a subject of discussion for centuries； but it is only within the last fifty years or thereabouts that instinctive actions have really been studied。 I refer the reader to the works of Darwin； Romanes； Lloyd Morgan； the Peckhams； Fabre； Hobhouse； and McDougall for details as to the controversies and the facts obtained。
Herbert Spencer considered the instinct as a series of inevitable reflexes。 The carrion fly； when gravid； deposits her eggs in putrid meat in order that the larvae may have appropriate food； although she never sees the larvae or cannot know through experience their needs。 〃The smell of putrid meat attracts the gravid carrion fly。 That is； it sets up motions of the wings which bring the fly to it； and the fly having arrived； the smell； and the contact combined stimulate the functions of oviposition。〃＇1＇ But as all the critics have pointed out； the theory of compound reflex action leaves out of account that there are any number of stimuli pouring in on the carrion fly at the same time that the meat attracts her。 The real mystery lies in that internal condition which makes the smell of the meat act so inevitably。 ＇1＇ Hobhouse。
In fact； it is this internal condition in the living creature that is the most important single link in instinct。 In the non…mating season the sight of the female has no effect on the male。 But periodically his internal organs become tense with procreative cells； these change his coenaesthesia； that starts desire； and desire sets going the mechanisms of search； courtship； the sexual act and the care of the female while she is gravid。 All instinctive acts have back of them either a tension or a deficit of some kind or other； brought about by the awakening of function of some glandular structure； so that the organism becomes ready to respond to some appropriate outside stimulus and inaccessible to others。 During the mating season； with certain animals； the stimulus of food has no effect until there is effected the purposes of the sexual hunger。 Changes in the body due to the activity of sex glands or gastric juices or any other organic product have two effects。 They increase the stimulation that comes from the thing sought and decrease the stimulation that comes from other things。 In physiological language； the threshold for the first is lowered and for the other it is raised。 But this does not explain HOW the changes in glands MAKE the animal seek this or that； except by saying that the animal has hereditary structures all primed to explode in the right way。 We may fall back on Bergson's mystical idea that all life is a unity； and that instinct； which makes one living thing know what to do with anotherto kill it in a scientific way for the good of the posterity of the killeris merely the knowledge； unconscious； that life has of life。 That pleasant explanation projects us back to a darker problem than ever： how life knows life and why one part of life so obviously seeks to circumvent the purpose of another part of life。 For us it is best to say that instinct arises out of the racial and individual needs； that physically there occur changes in the glands and tissues； that these set up desires which arouse into action simple or elaborate mechanisms which finally satisfy the need of the organs and tissues。＇1＇ ＇1＇ Kempf in his book on the vegetative nervous system goes into great detail the way the visceral needs force the animal or human to satisfy them。 Life is a sort of war between the vegetative and the central nervous system。 There is just enough truth in this point of view to make it very entertaining。
Even in the low forms of life instincts are not perfect at the start； or perfect in details； and almost every member of a species will show individuality in dealing with an obstacle to an instinctive action。 In other words； though there is instinct and this furnishes the basis for action in the lowest forms of life； there is also the capacity for learning by experience；and this is Intelligence。 〃The basis of instinct is heredity and we can impute an action to pure instinct only if it is hereditary。 The other class of actions are those devised by the individual animal for himself on the basis of his own experience and these are called generally intelligent。 Of intelligence operating within the sphere of instinct there is ample evidence。 There are modifications of instinctive action directly traceable to experience which cannot be explained by the interaction of purely hereditary tendencies and there are cases in which the whole structure of the instinct is profoundly modified by the experience of the individual。〃 Hobhouse； whom I quote； goes on to give many examples of instinctive action modified by experience and intelligence in the insect and lower animal world。 What I wish especially to point out is that man has many instinctive bases for conduct； but instincts as such are not often seen in pure form in man。 They are constantly modified by other instincts and through them runs the influence of intelligence。 The function of intelligence is to control instincts； to choose ways and means for the fulfillment of instincts that are blocked； etc。 Moreover； the effects of teachings； ethics； social organization and tradition； operating through the social instincts； are to repress； inhibit and whip into conformity every mode of instinctive conduct。 The main instincts are those relating to nutrition and reproduction； the care of the young； to averting danger or destroying it； to play and organized activity； to acquiring； perhaps to teaching and learning and to the social relations generally。 But manners creep in to regulate our methods of eating and the things we shall eat； and we may not eat at all unless we agree to get the things to eat a certain way。 We may not cohabit except under tremendous restriction； and marriage with its aims and purposes is sexual in origin but modified largely and almost beyond recognition by social consideration； taste； esthetic matters； taboos and economic conditions。 We may not treat our enemy as instinct bids us do；for only in war may one kill and here one kills without any personal purpose or anger； almost without instinct。 We may be compelled through social exigencies to treat our enemy politely； eat with him； sleep with him and help him out of difficulties and thus completely thwart one instinctive set of reactions。 Play becomes regulated by rules and customs； becomes motivated by the desire for superiority； or the desire for gain； and may even leave the physical field entirely and become purely mental。 And so on。 It does no special practical good to discuss instincts as if they operated in man as such。 They become purposes。 Therefore we shall defer the consideration of instincts and purposes in detail until later chapters of this book。 Since instincts are too rigid to meet the needs of the social and traditional life of man； they become intellectualized and socialized into purposes and ambitions； sometimes almost beyond recognition。 Nevertheless； the driving force of instinct is behind every purpose； every ambition； even though the individual himself has not the slightest idea of the force that is at work。 This does not mean that instinct acts as a sort of cellar… plotter； roving around in a subconsciousness； or at least no such semi…diabolical personality need be postulated； any more than it need be postulated for the automatic mechanism that regulates heartbeat or digestion。 The organic tensions and depressions that constitute instinct are not conscious or subconscious； they affect our conscious personalities so that we desire something； we fit that desire in with the rest of our desires； we seek the means of gratifying that desire first in accordance with means that Nature has given us and second in accordance with social teaching and our intelligence。 If the desire brings us sharply in contact with obstacles imposed either by circumstances or more precious desire； we inhibit that desire；and thus the instinct。 Because organic tensions and depressions are periodic and are dependent upon the activities of glands and tissues not within our control； the d