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〃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 admitted that 〃equatorial refrigeration。。。must have been small。〃 （〃More Letters〃； I。 page 177。） It may prove possible to dispense with it altogether。 One cannot but regret that as he wrote to Bates：〃the sketch in the 'Origin' gives a very meagre account of my fuller MS。 essay on this subject。〃 （Loc。 cit。） Wallace fully accepted 〃the effect of the Glacial epoch in bringing about the present distribution of Alpine and Arctic plants in the NORTHERN HEMISPHERE；〃 but rejected 〃the lowering of the temperature of the tropical regions during the Glacial period〃 in order to account for their presence in the SOUTHERN hemisphere。 （〃More Letters〃； II。 page 25 （footnote 1）。） The divergence however does not lie very deep。 Wallace attaches more importance to ordinary means of transport。 〃If plants can pass in considerable numbers and variety over wide seas and oceans； it must be yet more easy for them to traverse continuous areas of land； wherever mountain…chains o