； 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 when in explanation of the Scandinavian elements in the Spitzbergen flora he had to choose between a former land connection and the agency of birds； he preferred the bird。〃 （Guppy； op。 cit。 II。 pages 511； 512。）
Darwin objected to 〃continental extensions〃 on geological grounds； but he also objected to Lyell that they do not 〃account for all the phenomena of distribution on islands〃 （〃Life and Letters〃； II。 page 77。）； such for example as the absence of Acacias and Banksias in New Zealand。 He agreed with De Candolle that 〃it is poor work putting together the merely POSSIBLE means of distribution。〃 But he also agreed with him that they were the only practicable door of escape from multiple origins。 If they would not work then 〃every one who believes in single centres will have to admit continental extensions〃 （Ibid。 II。 page 82。）； and that he regarded as a mere counsel of despair：〃to make continents； as easily as a cook does pancakes。〃 （Ibid。 II。 page 74。）
The question of multiple origins however presented itself in another shape where the solution was much more difficult。 The problem； as stated by Darwin； is this：〃The identity of many plants and animals； on mountain… summits； separated from each other by hundreds of miles of lowlands。。。without the apparent possibility of their having migrated from one point to the other。〃 He continues； 〃even as long ago as 1747； such facts led Gmelin to conclude that the same species must have been independently created at several distinct points； and we might have remained in this same belief； had not Agassiz and others called vivid attention to the Glacial period； which affords。。。a simple explanation of the facts。〃 （〃Origin of Species〃 （6th edition） page 330。）
The 〃simple explanation〃 was substantially given by E。 Forbes in 1846。 It is scarcely too much to say that it belongs to the same class of fertile and far…reaching ideas as 〃natural selection〃 itself。 It is an extraordinary instance； if one were wanted at all； of Darwin's magnanimity and intense modesty that though he had arrived at the theory himself； he acquiesced in Forbes receiving the well…merited credit。 〃I have never；〃 he says； 〃of course alluded in print to my having independently worked out this view。〃 But he would have been more than human if he had not added： 〃I was forestalled in。。。one important point； which my vanity has always made me regret。〃 （〃Life and Letters〃； I。 page 88。）
Darwin； however； by applying the theory to trans…tropical migration； went far beyond Forbes。 The first enunciation to this is apparently contained in a letter to Asa Gray in 1858。 The whole is too long to quote； but the pith is contained in one paragraph。 〃There is a considerable body of geological evidence that during the Glacial epoch the whole world was colder； I inferred that；。。。from erratic boulder phenomena carefully observed by me on both the east and west coast of South America。 Now I am so bold as to believe that at the height of the Glacial epoch； AND WHEN ALL TROPICAL PRODUCTIONS MUST HAVE BEEN CONSIDERABLY DISTRESSED； several temperate forms slowly travelled into the heart of the Tropics； and even reached the southern hemisphere； and some few southern forms penetrated in a reverse direction northward。〃 （〃Life and Letters〃； II。 page 136。） Here again it is clear that though he credits Agassiz with having called vivid attention to the Glacial period； he had himself much earlier grasped the idea of periods of refrigeration。
Putting aside the fact； which has only been made known to us since Darwin's death； that he had anticipated Forbes； it is clear that he gave the theory a generality of which the latter had no conception。 This is pointed out by Hooker in his classical paper 〃On the Distribution of Arctic Plants〃 （1860）。 〃The theory of a southern migration of northern types being due to the cold epochs preceding and during the glacial； originated； I believe； with the late Edward Forbes； the extended one； of the trans…tropical migration； is Mr Darwin's。〃 （〃Linn。 Trans。〃 XXIII。 page 253。 The attempt appears to have been made to claim for Heer priority in what I may term for short the arctic…alpine theory （Scharff； 〃European Animals〃； page 128）。 I find no suggestion of his having hit upon it in his correspondence with Darwin or Hooker。 Nor am I aware of any reference to his having done so in his later publications。 I am indebted to his biographer； Professor Schroter； of Zurich； for an examination of his earlier papers with an equally negative result。） Assuming that local races have derived from a common ancestor； Hooker's great paper placed the fact of the migration on an impregnable basis。 And； as he pointed out； Darwin has shown that 〃such an explanation meets the difficulty of accounting for the restriction of so many American and Asiatic arctic types to their own peculiar longitudinal zones； and for what is a far greater difficulty； the representation of the same arctic genera by most closely allied species in different longitudes。〃
The facts of botanical geography were vital to Darwin's argument。 He had to show that they admitted of explanation without assuming multiple origins for species； which would be fatal to the theory of Descent。 He had therefore to strengthen and extend De Candolle's work as to means of transport。 He refused to supplement them by hypothetical geographical changes for which there was no independent evidence： this was simply to attempt to explain ignotum per ignotius。 He found a real and； as it has turned out； a far…reaching solution in climatic change due to cosmical causes which compelled the migration of species as a condition of their existence。 The logical force of the argument consists in dispensing with any violent assumption； and in showing that the principle of descent is adequate to explain the ascertained facts。
It does not； I think； detract from the merit of Darwin's conclusions that the tendency of modern research has been to show that the effects of the Glacial period were less simple； more localised and less general than he perhaps supposed。 He admitte