here introduce the mental factor because the facts seem to justify the inference。 Thus there are some modes of behaviour which are wholly and solely dependent upon inherited racial preparation； there are other modes of behaviour which are also dependent； in part at least； on individual preparation。 In the former case the behaviour is adaptive on the first occurrence of the appropriate presentation； in the latter case accommodation to circumstances is only reached after a greater or less amount of acquired organic modification of structure； often accompanied （as we assume） in the higher animals by acquired experience。 Logically and biologically the two classes of behaviour are clearly distinguishable： but the analysis of complex cases of behaviour where the two factors cooperate； is difficult and requires careful and critical study of life…history。
The foundations of the mental life are laid in the conscious experience that accompanies those modes of behaviour； dependent entirely on racial preparation； which may broadly be described as instinctive。 In the eighth chapter of 〃The Origin of Species〃 Darwin says （〃Origin of Species〃 （6th edition）； page 205。）； 〃I will not attempt any definition of instinct。。。Every one understands what is meant； when it is said that instinct impels the cuckoo to migrate and to lay her eggs in other birds' nests。 An action； which we ourselves require experience to enable us to perform； when performed by an animal； more especially by a very young one； without experience； and when performed by many individuals in the same way； without their knowing for what purpose it is performed； is usually said to be instinctive。〃 And in the summary at the close of the chapter he says （〃Origin of Species〃 （6th edition）； page 233。）； 〃I have endeavoured briefly to show that the mental qualities of our domestic animals vary； and that the variations are inherited。 Still more briefly I have attempted to show that instincts vary slightly in a state of nature。 No one will dispute that instincts are of the highest importance to each animal。 Therefore there is no real difficulty； under changing conditions of life； in natural selection accumulating to any extent slight modifications of instinct which are in any way useful。 In many cases habit or use and disuse have probably come into play。〃
Into the details of Darwin's treatment there is neither space nor need to enter。 There are some ambiguous passages； but it may be said that for him； as for his followers to…day； instinctive behaviour is wholly the result of racial preparation transmitted through organic heredity。 For the performance of the instinctive act no individual preparation under the guidance of personal experience is necessary。 It is true that Darwin quotes with approval Huber's saying that 〃a little dose of judgment or reason often comes into play； even with animals low in the scale of nature。〃 （Ibid。 page 205。） But we may fairly interpret his meaning to be that in behaviour； which is commonly called instinctive； some element of intelligent guidance is often combined。 If this be conceded the strictly instinctive performance （or part of the performance） is the outcome of heredity and due to the direct transmission of parental or ancestral aptitudes。 Hence the instinctive response as such depends entirely on how the nervous mechanism has been built up through heredity； while intelligent behaviour； or the intelligent factor in behaviour； depends also on how the nervous mechanism has been modified and moulded by use during its development and concurrently with the growth of individual experience in the customary situations of daily life。 Of course it is essential to the Darwinian thesis that what Sir E。 Ray Lankester has termed 〃educability；〃 not less than instinct； is hereditary。 But it is also essential to the understanding of this thesis that the differentiae of the hereditary factors should be clearly grasped。
For Darwin there were two modes of racial preparation； （1） natural selection； and （2） the establishment of individually acquired 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： coordinat