The remarkable conditions represented in plants with three kinds of flowers, such as Lythrum and Oxalis, agree in essentials with those in Primula. These cannot be considered in detail here; it need only be noted that the investigation of these cases was still more laborious. In order to establish the relative fertility of the different unions in Lythrum salicaria 223 different fertilisations were made, each flower being deprived of its male organs and then dusted with the appropriate pollen.
In the book containing the account of heterostyled plants other species are dealt with which, in addition to flowers opening normally (chasmogamous), also possess flowers which remain closed but are capable of producing fruit. These cleistogamous flowers afford a striking example of habitual self-pollination, and H. von Mohl drew special attention to them as such shortly after the appearance of Darwin's Orchid book. If it were only a question of producing seed in the simplest way, cleistogamous flowers would be the most conveniently constructed. The corolla and frequently other parts of the flower are reduced; the development of the seed may, therefore, be accomplished with a smaller expenditure of building material than in chasmogamous flowers; there is also no loss of pollen, and thus a smaller amount suffices for fertilisation.
Almost all these plants, as Darwin pointed out, have also chasmogamous flowers which render cross-fertilisation possible. His view that cleistogamous flowers are derived from originally chasmogamous flowers has been confirmed by more recent researches. Conditions of nutrition in the broader sense are the factors which determine whether chasmogamous or cleistogamous flowers are produced, assuming, of course, that the plants in question have the power of developing both forms of flower. The former may fail to appear for some time, but are eventually developed under favourable conditions of nourishment. The belief of many authors that there are plants with only cleistogamous flowers cannot therefore be accepted as authoritative without thorough experimental proof, as we are concerned with extra-european plants for which it is often difficult to provide appropriate conditions in cultivation.
Darwin sees in cleistogamous flowers an adaptation to a good supply of seeds with a small expenditure of material, while chasmogamous flowers of the same species are usually cross-fertilised and "their offspring will thus be invigorated, as we may infer from a wide-spread analogy." ("Forms of Flowers" (1st edition), page 341.) Direct proof in support of this has hitherto been supplied in a few cases only; we shall often find that the example set by Darwin in solving such problems as these by laborious experiment has unfortunately been little imitated.
Another chapter of this book treats of the distribution of the sexes in polygamous, dioecious, and gyno-dioecious plants (the last term, now in common use, we owe to Darwin). It contains a number of important facts and discussions and has inspired the experimental researches of Correns and others.
The most important of Darwin's work on floral biology is, however, that on cross and self-fertilisation, chiefly because it states the results of experimental investigations extending over many years. Only such experiments, as we have pointed out, could determine whether cross-fertilisation is in itself beneficial, and self-fertilisation on the other hand injurious; a conclusion which a merely comparative examination of pollination-mechanisms renders in the highest degree probable. Later floral biologists have unfortunately almost entirely confined themselves to observations on floral mechanisms. But there is little more to be gained by this kind of work than an assumption long ago made by C.K. Sprengel that "very many flowers have the sexes separate and probably at least as many hermaphrodite flowers are dichogamous; it would thus appear that Nature was unwilling that any flower should be fertilised by its own pollen."
It was an accidental observation which inspired Darwin's experiments on the effect of cross and self-fertilisation. Plants of Linaria vulgaris were grown in two adjacent beds; in the one were plants produced by cross-fertilisation, that is, from seeds obtained after fertilisation by pollen of another plant of the same species; in the other grew plants produced by self-fertilisation, that is from seed produced as the result of pollination of the same flower. The first were obviously superior to the latter.
Darwin was surprised by this observation, as he had expected a prejudicial influence of self-fertilisation to manifest itself after a series of generations: "I always supposed until lately that no evil effects would be visible until after several generations of self-fertilisation, but now I see that one generation sometimes suffices and the existence of dimorphic plants and all the wonderful contrivances of orchids are quite intelligible to me." ("More Letters", Vol. II. page 373.)
The observations on Linaria and the investigations of the results of legitimate and illegitimate fertilisation in heterostyled plants were apparently the beginning of a long series of experiments. These were concerned with plants of different families and led to results which are of fundamental importance for a true explanation of sexual reproduction.
The experiments were so arranged that plants were shielded from insect-visits by a net. Some flowers were then pollinated with their own pollen, others with pollen from another plant of the same species. The seeds were germinated on moist sand; two seedlings of the same age, one from a cross and the other from a self-fertilised flower, were selected and planted on opposite sides of the same pot. They grew therefore under identical external conditions; it was thus possible to compare their peculiarities such as height, weight, fruiting capacity, etc. In other cases the seedlings were placed near to one another in the open and in this way their capacity of resisting unfavourable external conditions was tested. The experiments were in some cases continued to the tenth generation and the flowers were crossed in different ways. We see, therefore, that this book also represents an enormous amount of most careful and patient original work.
The general result obtained is that plants produced as the result of cross-fertilisation are superior, in the majority of cases, to those produced as the result of self-fertilisation, in height, resistance to external injurious influences, and in seed-production.
Ipomoea purpurea may be quoted as an example. If we express the result of cross-fertilisation by 100, we obtain the following numbers for the fertilised plants.
Taking the average, the ratio as regards growth is 100:77. The considerable superiority of the crossed plants is apparent in the first generation and is not increased in the following generations; but there is some fluctuation about the average ratio. The numbers representing the fertility of crossed and self-fertilised plants are more difficult to compare with accuracy; the superiority of the crossed plants is chiefly explained by the fact that they produce a much larger number of capsules, not because there are on the average more seeds in each capsule. The ratio of the capsules was, e.g. in the third generation, 100:38, that of the seeds in the capsules 100:94. It is also especially noteworthy that in the self-fertilised plants the anthers were smaller and contained a smaller amount of pollen, and in the eighth generation the reduced fertility showed itself in a form which is often found in hybrids, that is the first flowers were sterile. (Complete sterility was not found in any of the plants investigated by Darwin. Others appear to be more sensitive; Cluer found Zea Mais "almost sterile" after three generations of self-fertilisation. (Cf. Fruwirth, "Die Zuchtung der Landwirtschaftlichen Kulturpflanzen", Berlin, 1904, II. page 6.))
The superiority of crossed individuals is not exhibited in the same way in all plants. For example in Eschscholzia californica the crossed seedlings do not exceed the self-fertilised in height and vigour, but the crossing considerably increases the plant's capacity for flower-production, and the seedlings from such a mother-plant are more fertile.
The conception implied by the term crossing requires a closer analysis. As in the majority of plants, a large number of flowers are in bloom at the same time on one and the same plant, it follows that insects visiting the flowers often carry pollen from one flower to another of the same stock. Has this method, which is spoken of as Geitonogamy, the same influence as crossing with pollen from another plant? The results of Darwin's experiments with different plants (Ipomoea purpurea, Digitalis purpurea, Mimulus luteus, Pelargonium, Origanum) were not in complete agreement; but on the whole they pointed to the conclusion that Geitonogamy shows no superiority over self-fertilisation (Autogamy). (Similarly crossing in the case of flowers of Pelargonium zonale, which belong to plants raised from cuttings from the same parent, shows no superiority over self-fertilisation.) Darwin, however, considered it possible that this may sometimes be the case. "The sexual elements in the flowers on the same plant can rarely have been differentiated, though this is possible, as flower-buds are in one sense distinct individuals, sometimes varying and differing from one another in structure or constitution." ("Cross and Self fertilisation" (1st edition), page 444.)
As regards the importance of this question from the point of view of the significance of cross-fertilisation in general, it may be noted that later observers have definitely discovered a difference between the results of autogamy and geitonogamy. Gilley and Fruwirth found that in Brassica Napus, the length and weight of the fruits as also the total weight of the seeds in a single fruit were less in the case of autogamy than in geitonogamy. With Sinapis alba a better crop of seeds was obtained after geitonogamy, and in the Sugar Beet the average weight of a fruit in the case of a self-fertilised plant was 0.009 gr., from geitonogamy 0.012 gr., and on cross-fertilisation 0.013 gr.
On the whole, however, the results of geitonogamy show that the favourable effects of cross-fertilisation do not depend simply on the fact that the pollen of one flower is conveyed to the stigma of another. But the plants which are crossed must in some way be different. If plants of Ipomoea purpurea (and Mimulus luteus) which have been self-fertilised for seven generations and grown under the same conditions of cultivation are crossed together, the plants so crossed would not be superior to the self-fertilised; on the other hand crossing with a fresh stock at once proves very advantageous. The favourable effect of crossing is only apparent, therefore, if the parent plants are grown under different conditions or if they belong to different varieties. "It is really wonderful what an effect pollen from a distinct seedling plant, which has been exposed to different conditions of life, has on the offspring in comparison with pollen from the same flower or from a distinct individual, but which has been long subjected to the same conditions. The subject bears on the very principle of life, which seems almost to require changes in the conditions." ("More Letters", Vol. II. page 406.)
The fertility—measured by the number or weight of the seeds produced by an equal number of plants—noticed under different conditions of fertilisation may be quoted in illustration.
Crossing under very similar conditions shows, therefore, that the difference between the sexual cells is smaller and thus the result of crossing is only slightly superior to that given by self-fertilisation. Is, then, the favourable result of crossing with a foreign stock to be attributed to the fact that this belongs to another systematic entity or to the fact that the plants, though belonging to the same entity were exposed to different conditions? This is a point on which further researches must be taken into account, especially since the analysis of the systematic entities has been much more thorough than formerly. (In the case of garden plants, as Darwin to a large extent claimed, it is not easy to say whether two individuals really belong to the same variety, as they are usually of hybrid origin. In some instances (Petunia, Iberis) the fresh stock employed by Darwin possessed flowers differing in colour from those of the plant crossed with it.) We know that most of Linneaus's species are compound species, frequently consisting of a very large number of smaller or elementary species formerly included under the comprehensive term varieties. Hybridisation has in most cases affected our garden and cultivated plants so that they do not represent pure species but a mixture of species.
But this consideration has no essential bearing on Darwin's point of view, according to which the nature of the sexual cells is influenced by external conditions. Even individuals growing close to one another are only apparently exposed to identical conditions. Their sexual cells may therefore be differently influenced and thus give favourable results on crossing, as "the benefits which so generally follow from a cross between two plants apparently depend on the two differing somewhat in constitution or character." As a matter of fact we are familiar with a large number of cases in which the condition of the reproductive organs is influenced by external conditions. Darwin has himself demonstrated this for self-sterile plants, that is plants in which self-fertilisation produces no result. This self-sterility is affected by climatic conditions: thus in Brazil Eschscholzia californica is absolutely sterile to the pollen of its own flowers; the descendants of Brazilian plants in Darwin's cultures were partially self-fertile in one generation and in a second generation still more so. If one has any doubt in this case whether it is a question of the condition of the style and stigma, which possibly prevents the entrance of the pollen-tube or even its development, rather than that of the actual sexual cells, in other cases there is no doubt that an influence is exerted on the latter.
Janczewski (Janczewski, "Sur les antheres steriles des Groseilliers", "Bull. de l'acad. des sciences de Cracovie", June, 1908.) has recently shown that species of Ribes cultivated under unnatural conditions frequently produce a mixed (i.e. partly useless) or completely sterile pollen, precisely as happens with hybrids. There are, therefore, substantial reasons for the conclusion that conditions of life exert an influence on the sexual cells. "Thus the proposition that the benefit from cross-fertilisation depends on the plants which are crossed having been subjected during previous generations to somewhat different conditions, or to their having varied from some unknown cause as if they had been thus subjected, is securely fortified on all sides." ("Cross and Self fertilisation" (1st edition), page 444.)
We thus obtain an insight into the significance of sexuality. If an occasional and slight alteration in the conditions under which plants and animals live is beneficial (Reasons for this are given by Darwin in "Variation under Domestication" (2nd edition), Vol. II. page 127.), crossing between organisms which have been exposed to different conditions becomes still more advantageous. The entire constitution is in this way influenced from the beginning, at a time when the whole organisation is in a highly plastic state. The total life-energy, so to speak, is increased, a gain which is not produced by asexual reproduction or by the union of sexual cells of plants which have lived under the same or only slightly different conditions. All the wonderful arrangements for cross-fertilisation now appear to be useful adaptations. Darwin was, however, far from giving undue prominence to this point of view, though this has been to some extent done by others. He particularly emphasised the following consideration:—"But we should always keep in mind that two somewhat opposed ends have to be gained; the first and more important one being the production of seeds by any means, and the second, cross-fertilisation." ("Cross and Self fertilisation" (1st edition), page 371.) Just as in some orchids and cleistogamic flowers self-pollination regularly occurs, so it may also occur in other cases. Darwin showed that Pisum sativum and Lathyrus odoratus belong to plants in which self-pollination is regularly effected, and that this accounts for the constancy of certain sorts of these plants, while a variety of form is produced by crossing. Indeed among his culture plants were some which derived no benefit from crossing. Thus in the sixth self-fertilised generation of his Ipomoea cultures the "Hero" made its appearance, a form slightly exceeding its crossed companion in height; this was in the highest degree self-fertile and handed on its characteristics to both children and grandchildren. Similar forms were found in Mimulus luteus and Nicotiana (In Pisum sativum also the crossing of two individuals of the same variety produced no advantage; Darwin attributed this to the fact that the plants had for several generations been self-fertilised and in each generation cultivated under almost the same conditions. Tschermak ("Ueber kunstliche Kreuzung an Pisum sativum") afterwards recorded the same result; but he found on crossing different varieties that usually there was no superiority as regards height over the products of self-fertilisation, while Darwin found a greater height represented by the ratios 100:75 and 100:60.), types which, after self-fertilisation, have an enhanced power of seed-production and of attaining a greater height than the plants of the corresponding generation which are crossed together and self-fertilised and grown under the same conditions. "Some observations made on other plants lead me to suspect that self-fertilisation is in some respects beneficial; although the benefit thus derived is as a rule very small compared with that from a cross with a distinct plant." ("Cross and Self fertilisation", page 350.) We are as ignorant of the reason why plants behave differently when crossed and self-fertilised as we are in regard to the nature of the differentiation of the sexual cells, which determines whether a union of the sexual cells will prove favourable or unfavourable.
It is impossible to discuss the different results of cross-fertilisation; one point must, however, be emphasised, because Darwin attached considerable importance to it. It is inevitable that pollen of different kinds must reach the stigma. It was known that pollen of the same "species" is dominant over the pollen of another species, that, in other words, it is prepotent. Even if the pollen of the same species reaches the stigma rather later than that of another species, the latter does not effect fertilisation.
Darwin showed that the fertilising power of the pollen of another variety or of another individual is greater than that of the plant's own pollen. ("Cross and Self fertilisation", page 391.) This has been demonstrated in the case of Mimulus luteus (for the fixed white-flowering variety) and Iberis umbellata with pollen of another variety, and observations on cultivated plants, such as cabbage, horseradish, etc. gave similar results. It is, however, especially remarkable that pollen of another individual of the same variety may be prepotent over the plant's own pollen. This results from the superiority of plants crossed in this manner over self-fertilised plants. "Scarcely any result from my experiments has surprised me so much as this of the prepotency of pollen from a distinct individual over each plant's own pollen, as proved by the greater constitutional vigour of the crossed seedlings." (Ibid. page 397.) Similarly, in self-fertile plants the flowers of which have not been deprived of the male organs, pollen brought to the stigma by the wind or by insects from another plant effects fertilisation, even if the plant's own pollen has reached the stigma somewhat earlier.
Have the results of his experimental investigations modified the point of view from which Darwin entered on his researches, or not? In the first place the question is, whether or not the opinion expressed in the Orchid book that there is "Something injurious" connected with self-fertilisation, has been confirmed. We can, at all events, affirm that Darwin adhered in essentials to his original position; but self-fertilisation afterwards assumed a greater importance than it formerly possessed. Darwin emphasised the fact that "the difference between the self-fertilised and crossed plants raised by me cannot be attributed to the superiority of the crossed, but to the inferiority of the self-fertilised seedlings, due to the injurious effects of self-fertilisation." (Ibid. page 437.) But he had no doubt that in favourable circumstances self-fertilised plants were able to persist for several generations without crossing. An occasional crossing appears to be useful but not indispensable in all cases; its sporadic occurrence in plants in which self-pollination habitually occurs is not excluded. Self-fertilisation is for the most part relatively and not absolutely injurious and always better than no fertilisation. "Nature abhors perpetual self-fertilisation" (It is incorrect to say, as a writer has lately said, that the aphorism expressed by Darwin in 1859 and 1862, "Nature abhors perpetual self-fertilisation," is not repeated in his later works. The sentence is repeated in "Cross and Self fertilisation" (page 8), with the addition, "If the word perpetual had been omitted, the aphorism would have been false. As it stands, I believe that it is true, though perhaps rather too strongly expressed.") is, however, a pregnant expression of the fact that cross-fertilisation is exceedingly widespread and has been shown in the majority of cases to be beneficial, and that in those plants in which we find self-pollination regularly occurring cross-pollination may occasionally take place.
An attempt has been made to express in brief the main results of Darwin's work on the biology of flowers. We have seen that his object was to elucidate important general questions, particularly the question of the significance of sexual reproduction.
It remains to consider what influence his work has had on botanical science. That this influence has been very considerable, is shown by a glance at the literature on the biology of flowers published since Darwin wrote. Before the book on orchids was published there was nothing but the old and almost forgotten works of Kolreuter and Sprengel with the exception of a few scattered references. Darwin's investigations gave the first stimulus to the development of an extensive literature on floral biology. In Knuth's "Handbuch der Blutenbiologie" ("Handbook of Flower Pollination", Oxford, 1906) as many as 3792 papers on this subject are enumerated as having been published before January 1, 1904. These describe not only the different mechanisms of flowers, but deal also with a series of remarkable adaptations in the pollinating insects. As a fertilising rain quickly calls into existence the most varied assortment of plants on a barren steppe, so activity now reigns in a field which men formerly left deserted. This development of the biology of flowers is of importance not only on theoretical grounds but also from a practical point of view. The rational breeding of plants is possible only if the flower-biology of the plants in question (i.e. the question of the possibility of self-pollination, self-sterility, etc.) is accurately known. And it is also essential for plant-breeders that they should have "the power of fixing each fleeting variety of colour, if they will fertilise the flowers of the desired kind with their own pollen for half-a-dozen generations, and grow the seedlings under the same conditions." ("Cross and Self fertilisation" (1st edition), page 460.)
But the influence of Darwin on floral biology was not confined to the development of this branch of Botany. Darwin's activity in this domain has brought about (as Asa Gray correctly pointed out) the revival of teleology in Botany and Zoology. Attempts were now made to determine, not only in the case of flowers but also in vegetative organs, in what relation the form and function of organs stand to one another and to what extent their morphological characters exhibit adaptation to environment. A branch of Botany, which has since been called Ecology (not a very happy term) has been stimulated to vigorous growth by floral biology.
While the influence of the work on the biology of flowers was extraordinarily great, it could not fail to elicit opinions at variance with Darwin's conclusions. The opposition was based partly on reasons valueless as counterarguments, partly on problems which have still to be solved; to some extent also on that tendency against teleological conceptions which has recently become current. This opposing trend of thought is due to the fact that many biologists are content with teleological explanations, unsupported by proof; it is also closely connected with the fact that many authors estimate the importance of natural selection less highly than Darwin did. We may describe the objections which are based on the widespread occurrence of self-fertilisation and geitonogamy as of little importance. Darwin did not deny the occurrence of self-fertilisation, even for a long series of generations; his law states only that "Nature abhors PERPETUAL self-fertilisation." (It is impossible (as has been attempted) to express Darwin's point of view in a single sentence, such as H. Muller's statement of the "Knight-Darwin law." The conditions of life in organisms are so various and complex that laws, such as are formulated in physics and chemistry, can hardly be conceived.) An exception to this rule would therefore occur only in the case of plants in which the possibility of cross-pollination is excluded. Some of the plants with cleistogamous flowers might afford examples of such cases. We have already seen, however, that such a case has not as yet been shown to occur. Burck believed that he had found an instance in certain tropical plants (Anonaceae, Myrmecodia) of the complete exclusion of cross-fertilisation. The flowers of these plants, in which, however,—in contrast to the cleistogamous flowers—the corolla is well developed, remain closed and fruit is produced.
Loew (E. Loew, "Bemerkungen zu Burck... ", "Biolog. Centralbl." XXVI. (1906).) has shown that cases occur in which cross-fertilisation may be effected even in these "cleistopetalous" flowers: humming birds visit the permanently closed flowers of certain species of Nidularium and transport the pollen. The fact that the formation of hybrids may occur as the result of this shows that pollination may be accomplished.
The existence of plants for which self-pollination is of greater importance than it is for others is by no means contradictory to Darwin's view. Self-fertilisation is, for example, of greater importance for annuals than for perennials as without it seeds might fail to be produced. Even in the case of annual plants with small inconspicuous flowers in which self-fertilisation usually occurs, such as Senecio vulgaris, Capsella bursa-pastoris and Stellaria media, A. Bateson (Anna Bateson, "The effects of cross-fertilisation on inconspicuous flowers", "Annals of Botany", Vol. I. 1888, page 255.) found that cross-fertilisation gave a beneficial result, although only in a slight degree. If the favourable effects of sexual reproduction, according to Darwin's view, are correlated with change of environment, it is quite possible that this is of less importance in plants which die after ripening their seeds ("hapaxanthic") and which in any case constantly change their situation. Objections which are based on the proof of the prevalence of self-fertilisation are not, therefore, pertinent. At first sight another point of view, which has been more recently urged, appears to have more weight.
W. Burck (Burck, "Darwin's Kreuzeungsgesetz... ", "Biol. Centralbl". XXVIII. 1908, page 177.) has expressed the opinion that the beneficial results of cross-fertilisation demonstrated by Darwin concern only hybrid plants. These alone become weaker by self-pollination; while pure species derive no advantage from crossing and no disadvantage from self-fertilisation. It is certain that some of the plants used by Darwin were of hybrid origin. (It is questionable if this was always the case.) This is evident from his statements, which are models of clearness and precision; he says that his Ipomoea plants "were probably the offspring of a cross." ("Cross and Self fertilisation" (1st edition), page 55.) The fixed forms of this plant, such as Hero, which was produced by self-fertilisation, and a form of Mimulus with white flowers spotted with red probably resulted from splitting of the hybrids. It is true that the phenomena observed in self-pollination, e.g. in Ipomoea, agree with those which are often noticed in hybrids; Darwin himself drew attention to this.
Let us next call to mind some of the peculiarities connected with hybridisation. We know that hybrids are often characterized by their large size, rapidity of growth, earlier production of flowers, wealth of flower-production and a longer life; hybrids, if crossed with one of the two parent forms, are usually more fertile than when they are crossed together or with another hybrid. But the characters which hybrids exhibit on self-fertilisation are rather variable. The following instance 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 ground—a union of sexual cells possessing somewhat different characters—as 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.
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" 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 instincts—those of neuter insects—do 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 own—for 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