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cquired habit。 He showed that instincts are subject to hereditary variation； he saw that instincts are also subject to modification through acquisition in the course of individual life。 He believed that not only the variations but also； to some extent； the modifications are inherited。 He therefore held that some instincts （the greater number） are due to natural selection but that others （less numerous） are due； or partly due； to the inheritance of acquired habits。 The latter involve Lamarckian inheritance； which of late years has been the centre of so much controversy。 It is noteworthy however that Darwin laid especial emphasis on the fact that many of the most typical and also the most complex instinctsthose of neuter insectsdo not admit of such an interpretation。 〃I am surprised；〃 he says （〃Origin of Species〃 （6th edition）； page 233。）； 〃that no one has hitherto advanced this demonstrative case of neuter insects； against the well…known doctrine of inherited habit； as advanced by Lamarck。〃 None the less Darwin admitted this doctrine as supplementary to that which was more distinctively his ownfor example in the case of the instincts of domesticated animals。 Still； even in such cases； 〃it may be doubted；〃 he says （Ibid。 pages 210； 211。）； 〃whether any one would have thought of training a dog to point； had not some one dog naturally shown a tendency in this line。。。so that habit and some degree of selection have probably concurred in civilising by inheritance our dogs。〃 But in the interpretation of the instincts of domesticated animals； a more recently suggested hypothesis； that of organic selection （Independently suggested； on somewhat different lines； by Profs。 J。 Mark Baldwin； Henry F。 Osborn and the writer。）； may be helpful。 According to this hypothesis any intelligent modification of behaviour which is subject to selection is probably coincident in direction with an inherited tendency to behave in this fashion。 Hence in such behaviour there are two factors： （1） an incipient variation in the line of such behaviour； and （2） an acquired modification by which the behaviour is carried further along the same line。 Under natural selection those organisms in which the two factors cooperate are likely to survive。 Under artificial selection they are deliberately chosen out from among the rest。
Organic selection has been termed a compromise between the more strictly Darwinian and the Lamarckian principles of interpretation。 But it is not in any sense a compromise。 The principle of interpretation of that which is instinctive and hereditary is wholly Darwinian。 It is true that some of the facts of observation relied upon by Lamarckians are introduced。 For Lamarckians however the modifications which are admittedly factors in survival； are regarded as the parents of inherited variations； for believers in organic selection they are only the foster parents or nurses。 It is because organic selection is the direct outcome of and a natural extension of Darwin's cardinal thesis that some reference to it here is justifiable。 The matter may be put with the utmost brevity as follows。 （1） Variations （V） occur； some of which are in the direction of increased adaptation （＋）； others in the direction of decreased adaptation （…）。 （2） Acquired modifications （M） also occur。 Some of these are in the direction of increased accommodation to circumstances （＋）； while others are in the direction of diminished accommodation （…）。 Four major combinations are
（a） ＋ V with ＋ M； （b） ＋ V with … M； （c） … V with ＋ M； （d） … V with … M。
Of these （d） must inevitably be eliminated while （a） are selected。 The predominant survival of （a） entails the survival of the adaptive variations which are inherited。 The contributory acquisitions （＋M） are not inherited； but they are none the less factors in determining the survival of the coincident variations。 It is surely abundantly clear that this is Darwinism and has no tincture of Lamarck's essential principle； the inheritance of acquired characters。
Whether Darwin himself would have accepted this interpretation of some at least of the evidence put forward by Lamarckians is unfortunately a matter of conjecture。 The fact remains that in his interpretation of instinct and in allied questions he accepted the inheritance of individually acquired modifications of behaviour and structure。
Darwin was chiefly concerned with instinct from the biological rather than from the psychological point of view。 Indeed it must be confessed that； from the latter standpoint； his conception of instinct as a 〃mental faculty〃 which 〃impels〃 an animal to the performance of certain actions； scarcely affords a satisfactory basis for genetic treatment。 To carry out the spirit of Darwin's teaching it is necessary to link more closely biological and psychological evolution。 The first step towards this is to interpret the phenomena of instinctive behaviour in terms of stimulation and response。 It may be well to take a particular case。 Swimming on the part of a duckling is； from the biological point of view； a typical example of instinctive behaviour。 Gently lower a recently hatched bird into water： coordinated movements of the limbs follow in rhythmical sequence。 The behaviour is new to the individual though it is no doubt closely related to that of walking； which is no less instinctive。 There is a group of stimuli afforded by the 〃presentation〃 which results from partial immersion： upon this there follows as a complex response an application of the functional activities in swimming； the sequence of adaptive application on the appropriate presentation is determined by racial preparation。 We know； it is true； but little of the physiological details of what takes place in the central nervous system； but in broad outline the nature of the organic mechanism and the manner of its functioning may at least be provisionally conjectured in the present state of physiological knowledge。 Similarly in the case of the pecking of newly…hatched chicks； there is a visual presentation； there is probably a cooperating group of stimuli from the alimentary tract in need of food； there is an adaptive application of the activities in a definite mode of behaviour。 Like data are afforded in a great number of cases of instinctive procedure； sometimes occurring very early in life； not infrequently deferred until the organism is more fully developed； but all of them dependent upon racial preparation。 No doubt there is some range of variation in the behaviour； just such variation as the theory of natural selection demands。 But there can be no question that the higher animals inherit a bodily organisation and a nervous system； the functional working of which gives rise to those inherited modes of behaviour which are termed instinctive。
It is to be noted that the term 〃instinctive〃 is here employed in the adjectival form as a descriptive heading under which may be grouped many and various modes of behaviour due to racial preparation。 We speak of these as inherited； but in strictness what is transmitted through heredity is the complex of anatomical and physiological conditions under which； in appropriate circumstances； the organism so