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ilisation〃 （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 flowersthe 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 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 corr