darwin and modern science-第80章
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ibution must have been laid in a preceding age。 We now know from the discoveries of the last fifty years that the remains of the Neocomian flora occur over an area extending through 30 deg of latitude。 The conclusion is irresistible that within this was its centre of distribution and probably of origin。
Darwin was immensely impressed with the outburst on the world of a fully fledged angiospermous vegetation。 He warmly approved the brilliant theory of Saporta that this happened 〃as soon （as） flower…frequenting insects were developed and favoured intercrossing。〃 （〃More Letters〃； II。 page 21。） Writing to him in 1877 he says： 〃Your idea that dicotyledonous plants were not developed in force until sucking insects had been evolved seems to me a splendid one。 I am surprised that the idea never occurred to me； but this is always the case when one first hears a new and simple explanation of some mysterious phenomenon。〃 （〃Life and Letters〃； III。 page 285。 Substantially the same idea had occurred earlier to F。W。A。 Miquel。 Remarking that 〃sucking insects （Haustellata）。。。perform in nature the important duty of maintaining the existence of the vegetable kingdom； at least as far as the higher orders are concerned；〃 he points our that 〃the appearance in great numbers of haustellate insects occurs at and after the Cretaceous epoch； when the plants with pollen and closed carpels （Angiosperms） are found； and acquire little by little the preponderance in the vegetable kingdom。〃 〃Archives Neerlandaises〃； III。 （1868）。 English translation in 〃Journ。 of Bot。〃 1869； page 101。）
Even with this help the abruptness still remains an almost insoluble problem； though a forecast of floral structure is now recognised in some Jurassic and Lower Cretaceous plants。 But the gap between this and the structural complexity and diversity of angiosperms is enormous。 Darwin thought that the evolution might have been accomplished during a period of prolonged isolation。 Writing to Hooker （1881） he says： 〃Nothing is more extraordinary in the history of the Vegetable Kingdom； as it seems to me； than the APPARENTLY very sudden or abrupt development of the higher plants。 I have sometimes speculated whether there did not exist somewhere during long ages an extremely isolated continent； perhaps near the South Pole。〃 （〃Life and Letters〃； III。 page 248。）
The present trend of evidence is； however； all in favour of a northern origin for flowering plants； and we can only appeal to the imperfection of the geological record as a last resource to extricate us from the difficulty of tracing the process。 But Darwin's instinct that at some time or other the southern hemisphere had played an important part in the evolution of the vegetable kingdom did not mislead him。 Nothing probably would have given him greater satisfaction than the masterly summary in which Seward has brought together the evidence for the origin of the Glossopteris flora in Gondwana land。
〃A vast continental area； of which remnants are preserved in Australia； South Africa and South America。。。A tract of enormous extent occupying an area； part of which has since given place to a southern ocean； while detached masses persist as portions of more modern continents； which have enabled us to read in their fossil plants and ice…scratched boulders the records of a lost continent； in which the Mesozoic vegetation of the northern continent had its birth。〃 （〃Encycl。 Brit。〃 （10th edition 1902）； Vol。 XXXI。 （〃Palaeobotany； Mesozoic〃）； page 422。） Darwin would probably have demurred on physical grounds to the extent of the continent； and preferred to account for the transoceanic distribution of its flora by the same means which must have accomplished it on land。
It must in fairness be added that Guppy's later views give some support to the conjectural existence of the 〃lost continent。〃 〃The distribution of the genus Dammara〃 （Agathis） led him to modify his earlier conclusions。 He tells us：〃In my volume on the geology of Vanua Levu it was shown that the Tertiary period was an age of submergence in the Western Pacific； and a disbelief in any previous continental condition was expressed。 My later view is more in accordance with that of Wichmann； who； on geological grounds； contended that the islands of the Western Pacific were in a continental condition during the Palaeozoic and Mesozoic periods； and that their submergence and subsequent emergence took place in Tertiary times。〃 （Guppy； op。 cit。 II。 page 304。）
The weight of the geological evidence I am unable to scrutinise。 But though I must admit the possibility of some unconscious bias in my own mind on the subject； I am impressed with the fact that the known distribution of the Glossopteris flora in the southern hemisphere is precisely paralleled by that of Proteaceae and Restiaceae in it at the present time。 It is not unreasonable to suppose that both phenomena； so similar； may admit of the same explanation。 I confess it would not surprise me if fresh discoveries in the distribution of the Glossopteris flora were to point to the possibility of its also having migrated southwards from a centre of origin in the northern hemisphere。
Darwin； however； remained sceptical 〃about the travelling of plants from the north EXCEPT DURING THE TERTIARY PERIOD。〃 But he added； 〃such speculations seem to me hardly scientific； seeing how little we know of the old floras。〃 （〃Life and Letters〃； III。 page 247。） That in later geological times the south has been the grave of the weakened offspring of the aggressive north can hardly be doubted。 But if we look to the Glossopteris flora for the ancestry of Angiosperms during the Secondary period； Darwin's prevision might be justified； though he has given us no clue as to how he arrived at it。
It may be true that technically Darwin was not a botanist。 But in two pages of the 〃Origin〃 he has given us a masterly explanation of 〃the relationship； with very little identity； between the productions of North America and Europe。〃 （Pages 333； 334。） He showed that this could be accounted for by their migration southwards 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 la
