darwin and modern science-第104章
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Although so interested in the subject and notwithstanding his conviction that the hypothesis was sound； Darwin was quite aware that it was probably the most vulnerable part of the 〃Origin〃。 Thus he wrote to H。W。 Bates； April 4； 1861： 〃If I had to cut up myself in a review I would have （worried？） and quizzed sexual selection； therefore； though I am fully convinced that it is largely true； you may imagine how pleased I am at what you say on your belief。〃 （〃More Letters〃； I。 page 183。）
The existence of sound…producing organs in the males of insects was； Darwin considered； the strongest evidence in favour of the operation of sexual selection in this group。 （〃Life and Letters〃； III。 pages 94； 138。） Such a conclusion has received strong support in recent years by the numerous careful observations of Dr F。A。 Dixey （〃Proc。 Ent。 Soc。 Lond。〃 1904； page lvi； 1905； pages xxxvii； liv； 1906； page ii。） and Dr G。B。 Longstaff （〃Proc。 Ent。 Soc。 Lond。〃 1905； page xxxv； 〃Trans。 Ent。 Soc。 Lond。〃 1905； page 136； 1908； page 607。） on the scents of male butterflies。 The experience of these naturalists abundantly confirms and extends the account given by Fritz Muller （〃Jen。 Zeit。〃 Vol。 XI。 1877； page 99； 〃Trans。 Ent。 Soc。 Lond。〃 1878； page 211。） of the scents of certain Brazilian butterflies。 It is a remarkable fact that the apparently epigamic scents of male butterflies should be pleasing to man while the apparently aposematic scents in both sexes of species with warning colours should be displeasing to him。 But the former is far more surprising than the latter。 It is not perhaps astonishing that a scent which is ex hypothesi unpleasant to an insect… eating Vertebrate should be displeasing to the human sense； but it is certainly wonderful that an odour which is ex hypothesi agreeable to a female butterfly should also be agreeable to man。
Entirely new light upon the seasonal appearance of epigamic characters is shed by the recent researches of C。W。 Beebe （〃The American Naturalist〃； Vol。 XLII。 No。 493； Jan。 1908； page 34。）； who caused the scarlet tanager （Piranga erythromelas） and the bobolink （Dolichonyx oryzivorus） to retain their breeding plumage through the whole year by means of fattening food； dim illumination； and reduced activity。 Gradual restoration to the light and the addition of meal…worms to the diet invariably brought back the spring song； even in the middle of winter。 A sudden alteration of temperature； either higher or lower； caused the birds nearly to stop feeding； and one tanager lost weight rapidly and in two weeks moulted into the olive…green winter plumage。 After a year； and at the beginning of the normal breeding season； 〃individual tanagers and bobolinks were gradually brought under normal conditions and activities；〃 and in every case moulted from nuptial plumage to nuptial plumage。 〃The dull colours of the winter season had been skipped。〃 The author justly claims to have established 〃that the sequence of plumage in these birds is not in any way predestined through inheritance。。。； but that it may be interrupted by certain factors in the environmental complex。〃
XVI。 GEOGRAPHICAL DISTRIBUTION OF PLANTS。
By SIR WILLIAM THISELTON…DYER； K。C。M。G。； C。I。E。 Sc。D。； F。R。S。
The publication of 〃The Origin of Species〃 placed the study of Botanical Geography on an entirely new basis。 It is only necessary to study the monumental 〃Geographie Botanique raisonnee〃 of Alphonse De Candolle； published four years earlier （1855）； to realise how profound and far… reaching was the change。 After a masterly and exhaustive discussion of all available data De Candolle in his final conclusions could only arrive at a deadlock。 It is sufficient to quote a few sentences：
〃L'opinion de Lamarck est aujourd'hui abandonee par tous les naturalistes qui ont etudie sagement les modifications possibles des etres organises。。。
〃Et si l'on s'ecarte des exagerations de Lamarck； si l'on suppose un premier type de chaque genre； de chaque famille tout au moins； on se trouve encore a l'egard de l'origine de ces types en presence de la grande question de la creation。
〃Le seul parti a prendre est donc d'envisager les etres organises comme existant depuis certaines epoques； avec leurs qualites particulieres。〃 （Vol。 II。 page 1107。）
Reviewing the position fourteen years afterwards； Bentham remarked：〃These views； generally received by the great majority of naturalists at the time De Candolle wrote； and still maintained by a few； must； if adhered to； check all further enquiry into any connection of facts with causes；〃 and he added； 〃there is little doubt but that if De Candolle were to revise his work； he would follow the example of so many other eminent naturalists； and。。。insist that the present geographical distribution of plants was in most instances a derivative one； altered from a very different former distribution。〃 （〃Pres。 Addr。〃 （1869） 〃Proc。 Linn。 Soc。〃 1868…69； page lxviii。）
Writing to Asa Gray in 1856； Darwin gave a brief preliminary account of his ideas as to the origin of species； and said that geographical distribution must be one of the tests of their validity。 （〃Life and Letters〃； II。 page 78。） What is of supreme interest is that it was also their starting…point。 He tells us：〃When I visited； during the voyage of H。M。S。 〃Beagle〃； the Galapagos Archipelago；。。。I fancied myself brought near to the very act of creation。 I often asked myself how these many peculiar animals and plants had been produced： the simplest answer seemed to be that the inhabitants of the several islands had descended from each other； undergoing modification in the course of their descent。〃 （〃The Variation of Animals and Plants〃 （2nd edition）； 1890； I。 pages 9； 10。） We need not be surprised then； that in writing in 1845 to Sir Joseph Hooker； he speaks of 〃that grand subject； that almost keystone of the laws of creation； Geographical Distribution。〃 （〃Life and Letters〃； I。 page 336。）
Yet De Candolle was； as Bentham saw； unconsciously feeling his way； like Lyell； towards evolution； without being able to grasp it。 They both strove to explain phenomena by means of agencies which they saw actually at work。 If De Candolle gave up the ultimate problem as insoluble：〃La creation ou premiere formation des etres organises echappe； par sa nature et par son anciennete； a nos moyens d'observation〃 （Loc。 cit。 page 1106。）； he steadily endeavoured to minimise its scope。 At least half of his great work is devoted to the researches by which he extricated himself from a belief in species having had a multiple origin； the view which had been held by successive naturalists from Gmelin to Agassiz。 To account for the obvious fact that species constantly occupy dissevered areas； De Candolle made a minute study of their means of transport。 This was found to dispose of the vast majority of cases； and the remainder he accounted for by geographical change。 （Loc。 cit。 page 1116。）
But Darwin strenuously objected to invoking geographical change as a solution of every difficulty。 He had apparently long satisfied himself as to the 〃permanence of continents and great oceans。〃 Dana； he tells us 〃was； I believe； the first man who maintained〃 this （〃Life and Letters〃； III。 page 247。 Dana says：〃The continents and oceans had their general outline or form defined in earliest time；〃 〃Manual of Geology〃； revised edition。 Philadelphia； 1869； page 732。 I have no access to an earlier edition。）； but he had himself probably arrived at it independently。 Modern physical research tends to confirm it。 The earth's centre of gravity； as pointed out by Pratt from the existence of the Pacific Ocean； does not coincide with its centre of figure； and it has been conjectured that the Pacific Ocean dates its origin from the separation of the moon from the earth。
The conjecture appears to be unnecessary。 Love shows that 〃the force that keeps the Pacific Ocean on one side of the earth is gravity； directed more towards the centre of gravity than the centre of the figure。〃 （〃Report of the 77th Meeting of the British Association〃 （Leicester； 1907）； London； 1908； page 431。） I can only summarise the conclusions of a technical but masterly discussion。 〃The broad general features of the distrib
