darwin and modern science-第76章
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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 distribution of continent and ocean can be regarded as the consequences of simple causes of a dynamical character；〃 and finally； 〃As regards the contour of the great ocean basins； we seem to be justified in saying that the earth is approximately an oblate spheroid； but more nearly an ellipsoid with three unequal axes； having its surface furrowed according to the formula for a certain spherical harmonic of the third degree〃 （Ibid。 page 436。）； and he shows that this furrowed surface must be produced 〃if the density is greater in one hemispheroid than in the other； so that the position of the centre of gravity is eccentric。〃 （Ibid。 page 431。） Such a modelling of the earth's surface can only be referred to a primitive period of plasticity。 If the furrows account for the great ocean basins； the disposition of the continents seems equally to follow。 Sir George Darwin has pointed out that they necessarily 〃arise from a supposed primitive viscosity or plasticity of the earth's mass。 For during this course of evolution the earth's mass must have suffered a screwing motion； so that the polar regions have travelled a little from west to east relatively to the equator。 This affords a possible explanation of the north and south trend of our great continents。〃 （〃Encycl。 Brit。〃 （9th edition）； Vol。 XXIII。 〃Tides〃； page 379。）
It would be trespassing on the province of the geologist to pursue the subject at any length。 But as Wallace （〃Island Life〃 （2nd edition）； 1895； page 103。）； who has admirably vindicated Darwin's position； points out； the 〃question of the permanence of our continents。。。lies at the root of all our inquiries into the great changes of the earth and its inhabitants。〃 But he proceeds： 〃The very same evidence which has been adduced to prove the GENERAL stability and permanence of our continental areas also goes to prove that they have been subjected to wonderful and repeated changes in DETAIL。〃 （Loc。 cit。 page 101。） Darwin of course would have admitted this； for with a happy expression he insisted to Lyell （1856） that 〃the skeletons； at least； of our continents are ancient。〃 （〃More Letters〃； II。 page 135。） It is impossible not to admire the courage and tenacity with which he carried on the conflict single…handed。 But he failed to convince Lyell。 For we still find him maintaining in the last edition of the 〃Principles〃： 〃Continents therefore； although permanent for whole geological epochs； shift their positions entirely in the course of ages。〃 （Lyell's 〃Principles of Geology〃 （11th edition）； London； 1872； I。 page 258。）
Evidence； however； steadily accumulates in Darwin's support。 His position still remains inexpugnable that it is not permissible to invoke geographical change to explain difficulties in distribution without valid geological and physical support。 Writing to Mellard Reade； who in 1878 had said； 〃While believing that the ocean…depths are of enormous age； it is impossible to reject other evidences that they have once been land；〃 he pointed out 〃the statement from the 'Challenger' that all sediment is deposited within one or two hundred miles from the shores。〃 （〃More Letters〃； II。 page 146。） The following year Sir Archibald Geikie （〃Geographical Evolution〃； 〃Proc。 R。 Geogr。 Soc。〃 1879； page 427。） informed the Royal Geographical Society that 〃No part of the results obtained by the 'Challenger' expedition has a profounder interest for geologists and geographers than the proof which they furnish that the floor of the ocean basins has no real analogy among the sedimentary formations which form most of the framework of the land。〃
Nor has Darwin's earlier argument ever been upset。 〃The fact which I pointed out many years ago； that all oceanic islands are volcanic （except St Paul's； and now that is viewed by some as the nucleus of an ancient volcano）； seem to me a strong argument that no continent ever occupied the great oceans。〃 （〃More Letters〃； II。 page 146。）
Dr Guppy； who devoted several years to geological and botanical investigations in the Pacific； found himself forced to similar conclusions。 〃It may be at once observed；〃 he says； 〃that my belief in the general princ
