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"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.