… fertilisation and hybridisation。 The heterostyled plants are normally dependent on a process corresponding to hybridisation。 The view that specifically distinct species could at best produce sterile hybrids was always opposed by Darwin。 But if the good results of crossing were EXCLUSIVELY dependent on the fact that we are concerned with hybrids； there must then be a demonstration of two distinct things。 First； that crossing with a fresh stock belonging to the same systematic entity or to the same hybrid； but cultivated for a considerable time under different conditions； shows no superiority over self…fertilisation； and that in pure species crossing gives no better results than self…pollination。 If this were the case； we should be better able to understand why in one plant crossing is advantageous while in others； such as Darwin's Hero and the forms of Mimulus and Nicotiana no advantage is gained； these would then be pure species。 But such a proof has not been supplied； the inference drawn from cleistogamous and cleistopetalous plants is not supported by evidence； and the experiments on geitonogamy and on the advantage of cross…fertilisation in species which are usually self…fertilised are opposed to this view。 There are still but few researches on this point； Darwin found that in Ononis minutissima； which produces cleistogamous as well as self…fertile chasmogamous flowers； the crossed and self…fertilised capsules produced seed in the proportion of 100：65 and that the average bore the proportion 100：86。 Facts previously mentioned are also applicable to this case。 Further； it is certain that the self…sterility exhibited by many plants has nothing to do with hybridisation。 Between self…sterility and reduced fertility as the result of self…fertilisation there is probably no fundamental difference。
It is certain that so difficult a problem as that of the significance of sexual reproduction requires much more investigation。 Darwin was anything but dogmatic and always ready to alter an opinion when it was not based on definite proof： he wrote； 〃But the veil of secrecy is as yet far from lifted； nor will it be； until we can say why it is beneficial that the sexual elements should be differentiated to a certain extent； and why； if the differentiation be carried still further； injury follows。〃 He has also shown us the way along which to follow up this problem； it is that of carefully planned and exact experimental research。 It may be that eventually many things will be viewed in a different light； but Darwin's investigations will always form the foundation of Floral Biology on which the future may continue to build。
XXI。 MENTAL FACTORS IN EVOLUTION。
By C。 LLOYD MORGAN； LL。D。； F。R。S。
In developing his conception of organic evolution Charles Darwin was of necessity brought into contact with some of the problems of mental evolution。 In 〃The Origin of Species〃 he devoted a chapter to 〃the diversities of instinct and of the other mental faculties in animals of the same class。〃 （〃Origin of Species〃 （6th edition）； page 205。） When he passed to the detailed consideration of 〃The Descent of Man〃； it was part of his object to show 〃that there is no fundamental difference between man and the higher mammals in their mental faculties。〃 （〃Descent of Man〃 （2nd edition 1888）； Vol。 I。 page 99； Popular edition page 99。） 〃If no organic being excepting man；〃 he said； 〃had possessed any mental power； or if his powers had been of a wholly different nature from those of the lower animals； then we should never have been able to convince ourselves that our high faculties had been gradually developed。〃 （Ibid。 page 99。） In his discussion of 〃The Expression of the Emotions〃 it was important for his purpose 〃fully to recognise that actions readily become associated with other actions and with various states of the mind。〃 （〃The Expression of the Emotions〃 （2nd edition）； page 32。） His hypothesis of sexual selection is largely dependent upon the exercise of choice on the part of the female and her preference for 〃not only the more attractive but at the same time the more vigorous and victorious males。〃 （〃Descent of Man〃； Vol。 II。 page 435。） Mental processes and physiological processes were for Darwin closely correlated； and he accepted the conclusion 〃that the nervous system not only regulates most of the existing functions of the body； but has indirectly influenced the progressive development of various bodily structures and of certain mental qualities。〃 （Ibid。 pages 437； 438。）
Throughout his treatment； mental evolution was for Darwin incidental to and contributory to organic evolution。 For specialised research in comparative and genetic psychology； as an independent field of investigation； he had neither the time nor the requisite training。 None the less his writings and the spirit of his work have exercised a profound influence on this department of evolutionary thought。 And； for those who follow Darwin's lead； mental evolution is still in a measure subservient to organic evolution。 Mental processes are the accompaniments or concomitants of the functional activity of specially differentiated parts of the organism。 They are in some way dependent on physiological and physical conditions。 But though they are not physical in their nature； and though it is difficult or impossible to conceive that they are physical in their origin； they are； for Darwin and his followers； factors in the evolutionary process in its physical or organic aspect。 By the physiologist within his special and well…defined universe of discourse they may be properly regarded as epiphenomena； but by the naturalist in his more catholic survey of nature they cannot be so regarded； and were not so regarded by Darwin。 Intelligence has contributed to evolution of which it is in a sense a product。
The facts of observation or of inference which Darwin accepted are these： Conscious experience accompanies some of the modes of animal behaviour； it is concomitant with certain physiological processes； these processes are the outcome of development in the individual and evolution in the race； the accompanying mental processes undergo a like development。 Into the subtle philosophical questions which arise out of the naive acceptance of such a creed it was not Darwin's province to enter； 〃I have nothing to do；〃 he said （〃Origin of Species〃 （6th edition）； page 205。）； 〃with the origin of the mental powers； any more than I have with that of life itself。〃 He dealt with the natural history of organisms； including not only their structure but their modes of behaviour； with the natural history of the states of consciousness which accompany some of their actions； and with the relation of behaviour to experience。 We will endeavour to follow Darwin in his modesty and candour in making no pretence to give ultimate explanations。 But we must note one of the implications of this self… denying ordinance of science。 Development and evolution imply continuity。 For Darwin and his followers the continuity is organic through physical heredity。 Apart from speculative hypothesis； legitimate enough in its proper place but here out of court； we know nothing of continuity of mental evolution as such： consciousness appears afresh in each succeeding generation。 Hence it is that for those who follow Darwin's lead； mental evolution is and must ever be； within his universe of discourse； subservient to organic evolution。 Only in so far as conscious experience； or its neural correlate； effects some changes in organic structure can it influence the course of heredity； and conversely only in so far as changes in organic structure are transmitted through heredity； is mental evolution rendered possible。 Such is the logical outcome of Darwin's teaching。
Those who abide by the cardinal results of this teaching are bound to regard all behaviour as the expression of the functional activities of the living tissues of the organism； and all conscious experience as correlated with such activities。 For the purposes of scientific treatment； mental processes are one mode of expression of the same changes of which the physiological processes accompanying behaviour are another mode of expression。 This is simply accepted as a fact which others may seek to explain。 The behaviour itself is the adaptive application of the energies of the organism； it is called forth by some form of presentation or stimulation brought to bear on the organism by the environment。 This presentation is always an individual or personal matter。 But in order that the organism may be fitted to respond to the presentation of the environment it must have undergone in some way a suitable preparation。 According to the theory of evolution this preparation is primarily racial and is transmitted through heredity。 Darwin's main thesis was that the method of preparation is predominantly by natural selection。 Subordinate to racial preparation； and always dependent thereon； is individual or personal preparation through some kind of acquisition； of which the guidance of behaviour through individually won experience is a typical example。 We 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〃 Da