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ourse 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 principle that islands have always been islands has not been shaken；〃 and he entirely rejects 〃the hypothesis of a Pacific continent。〃 He comes back； in full view of the problems on the spot； to the position from which； as has been seen； Darwin started： 〃If the distribution of a particular group of plants or animals does not seem to accord with the present arrangement of the land； it is by far the safest plan； even after exhausting all likely modes of explanation； not to invoke the intervention of geographical changes； and I scarcely think that our knowledge of any one group of organisms is ever sufficiently precise to justify a recourse to hypothetical alterations in the present relations of land and sea。〃 （〃Observations of a Naturalist in the Pacific between 1896 and 1899〃； London； 1903； I。 page 380。） Wallace clinches the matter when he finds 〃almost the whole of the vast areas of the Atlantic； Pacific； Indian； and Southern Oceans； without a solitary relic of the great islands or continents supposed to have sunk beneath their waves。〃 （〃Island Life〃； page 105。）
Writing to Wallace （1876）； Darwin warmly approves the former's 〃protest against sinking imaginary continents in a quite reckless manner； as was stated by Forbes； followed； alas； by Hooker； and caricatured by Wollaston and （Andrew） Murray。〃 （〃Life and Letters〃； III。 page 230。） The transport question thus became of enormously enhanced importance。 We need not be surprised then at his writing to Lyell in 1856：〃I cannot avoid thinking that Forbes's 'Atlantis' was an ill…service to science； as checking a close study of means of dissemination〃 （Ibid。 II。 page 78。）； and Darwin spared no pains to extend our knowledge of them。 He implores Hooker； ten years later； to 〃admit how little is known on the subject；〃 and summarises with some satisfaction what he had himself achieved：〃Remember how recently you and others thought that salt water would soon kill seeds。。。Remember that no one knew that seeds would remain for many hours in the crops of birds and retain their vitality； that fish eat seeds； and that when the fish are devoured by birds the seeds can germinate； etc。 Remember that every year many birds are blown to Madeira and to the Bermudas。 Remember that dust is blown 1000 miles across the Atlantic。〃 （〃More Letters〃； I。 page 483。）
It has always been the fashion to minimise Darwin's conclusions； and these have not escaped objection。 The advocatus diaboli has a useful function in science。 But in attacking Darwin his brief is generally found to be founded on a slender basis of facts。 Thus Winge and Knud Andersen have examined many thousands of migratory birds and found 〃that their crops and stomachs were always empty。 They never observed any seeds adhering to the feathers； beaks or feet of the birds。〃 （R。F。 Scharff； 〃European Animals〃； page 64； London； 1907。） The most considerable investigation of the problem of Plant Dispersal since Darwin is that of Guppy。 He gives a striking illustration of how easily an observer may be led into error by relying on negative evidence。
〃When Ekstam published； in 1895； the results of his observations on the plants of Nova Zembla； he observed that he possessed no data to show whether swimming and wading birds fed on berries； and he attached all importance to dispersal by winds。 On subsequently visiting Spitzbergen he must have been at first inclined； therefore； to the opinion of Nathorst； who； having found only a solitary species of bird （a snow…sparrow） in that region； naturally concluded that birds had been of no importance as agents in the plant…stocking。 However； Ekstam's opportunities were greater； and he tells us that in the craws of six specimens of Lagopus hyperboreus shot in Spitzbergen in August he found represented almost 25 per cent。 of the usual phanerogamic flora of that region in the form of fruits； seeds； bulbils； flower…buds； leaf…buds； etc。。。〃
〃The result of Ekstam's observations in Spitzbergen was to lead him to attach a very considerable importance in plant dispersal to the agency of birds； and