to conceive this occurring in plants which have no power of circumnutatingand； as already pointed out； physiologists do as a fact neglect circumnutation as a factor in the evolution of movements。 Whatever may be the fate of Darwin's theory of circumnutation there is no doubt that the research he carried out in support of； and by the light of； this hypothesis has had a powerful influence in guiding the modern theories of the behaviour of plants。 Pfeffer （〃The Physiology of Plants〃； Eng。 Tr。 III。 page 11。）； who more than any one man has impressed on the world a rational view of the reactions of plants； has acknowledged in generous words the great value of Darwin's work in the same direction。 The older view was that； for instance； curvature towards the light is the direct mechanical result of the difference of illumination on the lighted and shaded surfaces of the plant。 This has been proved to be an incorrect explanation of the fact； and Darwin by his work on the transmission of stimuli has greatly contributed to the current belief that stimuli act indirectly。 Thus we now believe that in a root and a stem the mechanism for the perception of gravitation is identical； but the resulting movements are different because the motor…irritabilities are dissimilar in the two cases。 We must come back； in fact； to Darwin's comparison of plants to animals。 In both there is perceptive machinery by which they are made delicately alive to their environment； in both the existing survivors are those whose internal constitution has enabled them to respond in a beneficial way to the disturbance originating in their sense…organs。
XX。 THE BIOLOGY OF FLOWERS。
By K。 GOEBEL； Ph。D。 Professor of Botany in the University of Munich。
There is scarcely any subject to which Darwin devoted so much time and work as to his researches into the biology of flowers； or； in other words； to the consideration of the question to what extent the structural and physiological characters of flowers are correlated with their function of producing fruits and seeds。 We know from his own words what fascination these studies possessed for him。 We repeatedly find； for example； in his letters expressions such as this：〃Nothing in my life has ever interested me more than the fertilisation of such plants as Primula and Lythrum； or again Anacamptis or Listera。〃 （〃More Letters of Charles Darwin〃； Vol。 II。 page 419。）
Expressions of this kind coming from a man whose theories exerted an epoch… making influence； would be unintelligible if his researches into the biology of flowers had been concerned only with records of isolated facts； however interesting these might be。 We may at once take it for granted that the investigations were undertaken with the view of following up important problems of general interest； problems which are briefly dealt with in this essay。
Darwin published the results of his researches in several papers and in three larger works； （i） 〃On the various contrivances by which British and Foreign Orchids are fertilised by insects〃 （First edition； London； 1862； second edition； 1877； popular edition； 1904。） （ii） 〃The effects of Cross and Self fertilisation in the vegetable kingdom〃 （First edition； 1876； second edition； 1878）。 （iii） 〃The different forms of Flowers on plants of the same species〃 （First edition； 1877； second edition； 1880）。
Although the influence of his work is considered later； we may here point out that it was almost without a parallel； not only does it include a mass of purely scientific observations； but it awakened interest in very wide circles； as is shown by the fact that we find the results of Darwin's investigations in floral biology universally quoted in school books； they are even willingly accepted by those who； as regards other questions； are opposed to Darwin's views。
The works which we have mentioned are； however； not only of special interest because of the facts they contribute； but because of the MANNER in which the facts are expressed。 A superficial reader seeking merely for catch…words will； for instance； probably find the book on cross and self… fertilisation rather dry because of the numerous details which it contains： it is； indeed； not easy to compress into a few words the general conclusions of this volume。 But on closer examination； we cannot be sufficiently grateful to the author for the exactness and objectivity with which he enables us to participate in the scheme of his researches。 He never tries to persuade us； but only to convince us that his conclusions are based on facts； he always gives prominence to such facts as appear to be in opposition to his opinions；a feature of his work in accordance with a maxim which he laid down：〃It is a golden rule； which I try to follow； to put every fact which is opposed to one's preconceived opinion in the strongest light。〃 （〃More Letters〃； Vol。 II。 page 324。）
The result of this method of presentation is that the works mentioned above represent a collection of most valuable documents even for those who feel impelled to draw from the data other conclusions than those of the author。 Each investigation is the outcome of a definite question； a 〃preconceived opinion；〃 which is either supported by the facts or must be abandoned。 〃How odd it is that anyone should not see that all observation must be for or against some view if it is to be of any service！〃 （Ibid。 Vol。 I。 page 195。）
The points of view which Darwin had before him were principally the following。 In the first place the proof that a large number of the peculiarities in the structure of flowers are not useless； but of the greatest significance in pollination must be of considerable importance for the interpretation of adaptations； 〃The use of each trifling detail of structure is far from a barren search to those who believe in natural selection。〃 （〃Fertilisation of Orchids〃 （1st edition）； page 351； （2nd edition 1904） page 286。） Further； if these structural relations are shown to be useful； they may have been acquired because from the many variations which have occurred along different lines； those have been preserved by natural selection 〃which are beneficial to the organism under the complex and ever…varying conditions of life。〃 （Ibid。 page 351。） But in the case of flowers there is not only the question of adaptation to fertilisation to be considered。 Darwin； indeed； soon formed the opinion which he has expressed in the following sentence；〃From my own observations on plants； guided to a certain extent by the experience of the breeders of animals； I became convinced many years ago that it is a general law of nature that flowers are adapted to be crossed； at least occasionally； by pollen from a distinct plant。〃 （〃Cross and Self fertilisation〃 （1st edition）； page 6。）
The experience of animal breeders pointed to the conclusion that continual in…breeding is injurious。 If this is correct； it raises the question whether the same conclusion holds for plants。 As most flowers are hermaphrodite； plants afford much more favourable material than animals for an experimental solution of the question； what results follow from the union of nearly related sexual cells as compared with those obtained by the introduction of new blood。 The answer to this question must； moreover； possess the greatest significance for the correct understanding of sexual reproduction in general。
We see； therefore； that the problems which Darwin had before him in his researches into the biology of flowers were of the greatest importance； and at the same time that the point of view from which he attacked the problems was essentially a teleological one。
We may next inquire in what condition he found the biology of flowers at the time of his first researches； which were undertaken about the year 1838。 In his autobiography he writes；〃During the summer of 1839； and； I believe； during the previous summer； I was led to attend to the cross… fertilisation of flowers by the aid of insects； from having come to the conclusion in my speculations on the origin of species； that crossing played an important part in keeping specific forms constant。〃 （〃The Life and Letters of Charles Darwin〃； Vol。 I。 page 90； London； 1888。） In 1841 he became acquainted with Sprengel's work： his researches into the biology of flowers were thus continued for about forty years。
It is obvious that there could only be a biology of flowers after it had been demonstrated that the formation of seeds and fruit in the flower is dependent on pollination and subsequent fertilisation。 This proof was supplied at the end of the seventeenth century by R。J。 Camerarius （1665… 1721）。 He showed that normally seeds and fruits are developed only when the pollen reaches the stigma。 The manner in which this happens was first thoroughly investigated by J。G。 Kolreuter （1733…1806 （Kolreuter； 〃Vorlaufige Nachricht von einigen das Geschlecht der Planzen betreffenden Versuchen und Beobachtungen〃； Leipzig； 1761； with three supplements； 1763… 66。 Also； 〃Mem。 de l'acad。 St Petersbourg〃； Vol。 XV。 1809。））； the same observer to whom we owe the earliest experiments in hybridisation of real scientific interest。 Kolreuter mentioned that pollen may be carried from one flower to another p