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ards from a common area； and he told Wallace that he 〃doubted much whether the now called Palaearctic and Neartic regions ought to be separated。〃 （〃Life and Letters〃； III。 page 230。） Catkin…bearing deciduous trees had long been seen to justify Darwin's doubt： oaks； chestnuts； beeches； hazels； hornbeams； birches； alders； willows and poplars are common both to the Old and New World。 Newton found that the separate regions could not be sustained for birds； and he is now usually followed in uniting them as the Holartic。 One feels inclined to say in reading the two pages； as Lord Kelvin did to a correspondent who asked for some further development of one of his papers； It is all there。 We have only to apply the principle to previous geological ages to understand why the flora of the Southern United States preserves a Cretaceous facies。 Applying it still further we can understand why； when the northern hemisphere gradually cooled through the Tertiary period； the plants of the Eocene 〃suggest a comparison of the climate and forests with those of the Malay Archipelago and Tropical America。〃 （Clement Reid； 〃Encycl。 Brit。〃 （10th edition）； Vol。 XXXI。 （〃Palaeobotany； Tertiary〃）； page 435。） Writing to Asa Gray in 1856 with respect to the United States flora； Darwin said that 〃nothing has surprised me more than the greater generic and specific affinity with East Asia than with West America。〃 （〃More Letters〃； I。 page 434。） The recent discoveries of a Tulip tree and a Sassafras in China afford fresh illustrations。 A few years later Asa Gray found the explanation in both areas being centres of preservation of the Cretaceous flora from a common origin。 It is interesting to note that the paper in which this was enunciated at once established his reputation。
In Europe the latitudinal range of the great mountain chains gave the Miocene flora no chance of escape during the Glacial period； and the Mediterranean appears to have equally intercepted the flow of alpine plants to the Atlas。 （John Ball in Appendix G； page 438； in 〃Journal of a Tour in Morocco and the Great Atlas〃； J。D。 Hooker and J。 Ball； London； 1878。） In Southern Europe the myrtle； the laurel； the fig and the dwarf…palm are the sole representatives of as many great tropical families。 Another great tropical family； the Gesneraceae has left single representatives from the Pyrenees to the Balkans； and in the former a diminutive yam still lingers。 These are only illustrations of the evidence which constantly accumulates and which finds no rational explanation except that which Darwin has given to it。
The theory of southward migration is the key to the interpretation of the geographical distribution of plants。 It derived enormous support from the researches of Heer and has now become an accepted commonplace。 Saporta in 1888 described the vegetable kingdom as 〃emigrant pour suivre une direction determinee et marcher du nord au sud； a la recherche de regions et de stations plus favorables； mieux appropriees aux adaptations acquises； a meme que la temperature terrestre perd ses conditions premieres。〃 （〃Origine Paleontologique des arbres〃； Paris； 1888； page 28。） If； as is so often the case； the theory now seems to be a priori inevitable； the historian of science will not omit to record that the first germ sprang from the brain of Darwin。
In attempting this sketch of Darwin's influence on Geographical Distribution； I have found it impossible to treat it from an external point of view。 His interest in it was unflagging； all I could say became necessarily a record of that interest and could not be detached from it。 He was in more or less intimate touch with everyone who was working at it。 In reading the letters we move amongst great names。 With an extraordinary charm of persuasive correspondence he was constantly suggesting； criticising and stimulating。 It is hardly an exaggeration to say that from the quiet of his study at Down he was founding and directing a wide…world school。
POSTSCRIPTUM。
Since this essay was put in type Dr Ernst's striking account of the 〃New Flora of the Volcanic Island of Krakatau〃 （Cambridge； 1909。） has reached me。 All botanists must feel a debt of gratitude to Prof。 Seward for his admirable translation of a memoir which in its original form is practically unprocurable and to the liberality of the Cambridge University Press for its publication。 In the preceding pages I have traced the laborious research by which the methods of Plant Dispersal were established by Darwin。 In the island of Krakatau nature has supplied a crucial experiment which； if it had occurred earlier； would have at once secured conviction of their efficiency。 A quarter of a century ago every trace of organic life in the island was 〃destroyed and buried under a thick covering of glowing stones。〃 Now； it is 〃again covered with a mantle of green； the growth being in places so luxuriant that it is necessary to cut one's way laboriously through the vegetation。〃 （Op。 cit。 page 4。） Ernst traces minutely how this has been brought about by the combined action of wind； birds and sea currents； as means of transport。 The process will continue； and he concludes：〃At last after a long interval the vegetation on the desolated island will again acquire that wealth of variety and luxuriance which we see in the fullest development which Nature has reached in the primaeval forest in the tropics。〃 （Op。 cit。 page 72。） The possibility of such a result revealed itself to the insight of Darwin with little encouragement or support from contemporary opinion。
One of the most remarkable facts established by Ernst is that this has not been accomplished by the transport of seeds alone。 〃Tree stems and branches played an important part in the colonisation of Krakatau by plants and animals。 Large piles of floating trees； stems； branches and bamboos are met with everywhere on the beach above high…water mark and often carried a considerable distance inland。 Some of the animals on the island； such as the fat Iguana （Varanus salvator） which suns itself in the beds of streams； may have travelled on floating wood； possibly also the ancestors of the numerous ants； but certainly plants。〃 （Op。 cit。 page 56。） Darwin actually had a prevision of this。 Writing to Hooker he says：〃Would it not be a prodigy if an unstocked island did not in the course of ages receive colonists from coasts whence the currents flow； trees are drifted and birds are driven by gales？〃 （〃More Letters〃； I。 page 483。） And ten years earlier：〃I must believe in the。。。whole plant or branch being washed into the sea； with floods and slips and earthquakes； this must continually be happening。〃 （〃Life and Letters〃； II。 pages 56； 57。） If we give to 〃continually〃 a cosmic measure； can the fact be doubted？ All this； in the light of our present knowledge； is too obvious to us to admit of discussion。 But it seems to me nothing less than pathetic to see how in the teeth of the obsession as to continental extension； Darwin fought single…handed for what we now know to be the truth。
Guppy's heart failed him when he had to deal with the isolated case of Agathis which alone seemed inexplicable by known means of transport。 But when we