darwin and modern science-第102章
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
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 partly by wind and partly by insects。 But he held the view； and that was； indeed； the natural assumption； that self… fertilisation usually occurs in a flower； in other words that the pollen of a flower reaches the stigma of the same flower。 He demonstrated； however； certain cases in which cross…pollination occurs； that is in which the pollen of another flower of the same species is conveyed to the stigma。 He was familiar with the phenomenon； exhibited by numerous flowers； to which Sprengel afterwards applied the term Dichogamy； expressing the fact that the anthers and stigmas of a flower often ripen at different times； a peculiarity which is now recognised as one of the commonest means of ensuring cross…pollination。
With far greater thoroughness and with astonishing power of observation C。K。 Sprengel （1750…1816） investigated the conditions of pollination of flowers。 Darwin was introduced by that eminent botanist Robert Brown to Sprengel's then but little appreciated work；〃Das entdeckte Geheimniss der Natur im Bau und in der Befruchtung der Blumen〃 （Berlin； 1793）； this is by no means the least service to Botany rendered by Robert Brown。
Sprengel proceeded from a naive teleological point of view。 He firmly believed 〃that the wise Author of nature had not created a single hair without a definite purpose。〃 He succeeded in demonstrating a number of beautiful adaptations in flowers for ensuring pollination； but his work exercised but little influence on his contemporaries and indeed for a long time after his death。 It was through Darwin that Sprengel's work first achieved a well deserved though belated fame。 Even such botanists as concerned themselves with researches into the biology of flowers appear to have formerly attached much less value to Sprengel's work than it has received since Darwin's time。 In illustration of this we may quote C。F。 Gartner whose name is rightly held in the highest esteem as that of one of the most eminent hybridologists。 In his work 〃Versuche und Beobachtungen uder die Befruchtungsorgane der vollkommeneren Gewachse und uber die naturliche und kunstliche Befruchtung durch den eigenen Pollen〃 he also deals with flower…pollination。 He recognised the action of the wind； but he believed； in spite of the fact that he both knew and quoted Kolreuter and Sprengel； that while insects assist pollination； they do so only occasionally； and he held that insects are responsible for the conveyance of pollen； thorough investigations would show 〃that a very small proportion of the plants included in this category require this assistance in their native habitat。〃 （Gartner； 〃Versucher und Beobachtungen。。。〃； page 335； Stuttgart； 1844。） In the majority of plants self…pollination occurs。
Seeing that even investigators who had worked for several decades at fertilisation…phenomena had not advanced the biology of flowers beyond the initial stage； we cannot be surprised that other botanists followed to even a less extent the lines laid down by Kolreuter and Sprengel。 This was in part the result of Sprengel's supernatural teleology and in part due to the fact that his book appeared at a time when other lines of inquiry exerted a dominating influence。
At the hands of Linnaeus systematic botany reached a vigorous development； and at the beginning of the nineteenth century the anatomy and physiology of plants grew from small beginnings to a flourishing branch of science。 Those who concerned themselves with flowers endeavoured to investigate their development and structure or the most minute phenomena connected with fertilisation and the formation of the embryo。 No room was left for the extension of the biology of flowers on the lines marked out by Kolreuter and Sprengel。 Darwin was the first to give new life and a deeper significance to this subject； chiefly because he took as his starting…point the above…mentioned problems； the importance of which is at once admitted by all naturalists。
The further development of floral biology by Darwin is in the first place closely connected with the book on the fertilisation of Orchids。 It is noteworthy that the title includes the sentence；〃and on the good effects of intercrossing。〃
The purpose of the book is clearly stated in the introduction：〃The object of the following work is to show that the contrivances by which Orchids are fertilised； are as varied and almost as perfect as any of the most beautiful adaptations in the animal kingdom； and； secondly； to show that these contrivances have for their main object the fertilisation of each flower by the pollen of another flower。〃 （〃Fertilisation of Orchids〃； page 1。） Orchids constituted a particularly suitable family for such researches。 Their flowers exhibit a striking wealth of forms； the question； therefore； whether the great variety in floral structure bears any relation to fertilisation （In the older botanical literature the word fertilisation is usually employed in cases where POLLINATION is really in question： as Darwin used it in this sense it is so used here。） must in this case possess special interest。
Darwin succeeded in showing that in most of the orchids examined self… fertilisation is either an impossibility； or； under natural conditions； occurs only exceptionally。 On the other hand these plants present a series of extraordinarily beautiful and remarkable adaptations which ensure the transference of pollen by insects from one flower to another。 It is impossible to describe adequately in a few words the wealth of facts contained in the Orchid book。 A few examples may； however； be quoted in illustration of the delicacy of the observations and of the perspicuity employed in interpreting the facts。
The majority of orchids differ from other seed plants （with the exception of the Asclepiads） in having no dust…like pollen。 The pollen； or more correctly； the pollen…tetrads； remain fastened together as club…shaped pollinia usually borne on a slender pedicel。 At the base of the pedicel is a small viscid disc by which the pollinium is attached to the head or proboscis of one of the insects which visi
