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may be quoted from Gartner： 〃There are many hybrids which retain the self…fertility of the first generation during the second and later generations； but very often in a less degree； a considerable number； however； become sterile。〃 But the hybrid varieties may be more fertile in the second generation than in the first； and in some hybrids the fertility with their own pollen increases in the second； third； and following generations。 （K。F。 Gartner； 〃Versuche uber die Bastarderzeugung〃； Stuttgart； 1849； page 149。） As yet it is impossible to lay down rules of general application for the self… fertility of hybrids。 That the beneficial influence of crossing with a fresh stock rests on the same grounda union of sexual cells possessing somewhat different charactersas the fact that many hybrids are distinguished by greater luxuriance； wealth of flowers； etc。 corresponds entirely with Darwin's conclusions。 It seems to me to follow clearly from his investigations that there is no essential difference between cross… 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。 In