按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
nificant importance？ More northerners have established themselves in southern lands than vice versa； because there is such a great mass of land in the north and greater continents imply greater intensity of selection。 〃The productions of real islands have everywhere largely yielded to continental forms。〃 （Ibid。 page 380。）。。。〃The Alpine forms have almost everywhere largely yielded to the more dominant forms generated in the larger areas and more efficient workshops of the North。〃
Let us now pass in rapid survey the influence of the publication of 〃The Origin of Species〃 upon the study of Geographical Distribution in its wider sense。
Hitherto the following thought ran through the minds of most writers： Wherever we examine two or more widely separated countries their respective faunas are very different； but where two faunas can come into contact with each other； they intermingle。 Consequently these faunas represent centres of creation； whence the component creatures have spread peripherally so far as existing boundaries allowed them to do so。 This is of course the fundamental idea of 〃regions。〃 There is not one of the numerous writers who considered the possibility that these intermediate belts might represent not a mixture of species but transitional forms； the result of changes undergone by the most peripheral migrants in adaptation to their new surroundings。 The usual standpoint was also that of Pucheran （〃Note sur l'equateur zoologique〃； 〃Rev。 et Mag。 de Zoologie〃； 1855； also several other papers； ibid。 1865； 1866； and 1867。） in 1855。 But what a change within the next ten years！ Pucheran explains the agreement in coloration between the desert and its fauna as 〃une harmonie post…etablie〃； the Sahara； formerly a marine basin； was peopled by immigrants from the neighbouring countries； and these new animals adapted themselves to the new environment。 He also discusses； among other similar questions； the Isthmus of Panama with regard to its having once been a strait。 From the same author may be quoted the following passage as a strong proof of the new influence： 〃By the radiation of the contemporaneous faunas； each from one centre； whence as the various parts of the world successively were formed and became habitable； they spread and became modified according to the local physical conditions。〃
The 〃multiple〃 origin of each species as advocated by Sclater and Murray； although giving the species a broader basis； suffered from the same difficulties。 There was only one alternative to the old orthodox view of independent creation； namely the bold acceptance of land…connections to an extent for which geological and palaeontological science was not yet ripe。 Those who shrank from either view； gave up the problem as mysterious and beyond the human intellect。 This was the expressed opinion of men like Swainson； Lyell and Humboldt。 Only Darwin had the courage to say that the problem was not insoluble。 If we admit 〃that in the long course of time the individuals of the same species； and likewise of allied species； have proceeded from some one source； then I think all the grand leading facts of geographical distribution are explicable on the theory of migration。。。together with subsequent modification and the multiplication of new forms。〃 We can thus understand how it is that in some countries the inhabitants 〃are linked to the extinct beings which formerly inhabited the same continent。〃 We can see why two areas； having nearly the same physical conditions； should often be inhabited by very different forms of life；。。。and 〃we can see why in two areas； however distant from each other； there should be a correlation； in the presence of identical species。。。and of distinct but representative species。〃 （〃The Origin of Species〃 （1st edition）； pages 408； 409。）
Darwin's reluctance to assume great geological changes； such as a land… connection of Europe with North America； is easily explained by the fact that he restricted himself to the distribution of the present and comparatively recent species。 〃I do not believe that it will ever be proved that within the recent period continents which are now quite separate； have been continuously； or almost continuously； united with each other； and with the many existing oceanic islands。〃 （Ibid。 page 357。） Again； 〃believing。。。that our continents have long remained in nearly the same relative position； though subjected to large； but partial oscillations of level；〃 that means to say within the period of existing species； or 〃within the recent period。〃 （Ibid。 page。 370。） The difficulty was to a great extent one of his own making。 Whilst almost everybody else believed in the immutability of the species； which implies an enormous age； logically since the dawn of creation； to him the actually existing species as the latest results of evolution； were necessarily something very new； so young that only the very latest of the geological epochs could have affected them。 It has since come to our knowledge that a great number of terrestrial 〃recent〃 species； even those of the higher classes of Vertebrates； date much farther back than had been thought possible。 Many of them reach well into the Miocene； a time since which the world seems to have assumed the main outlines of the present continents。
In the year 1866 appeared A。 Murray's work on the 〃Geographical Distribution of Mammals〃； a book which has perhaps received less recognition than it deserves。 His treatment of the general introductory questions marks a considerable advance of our problem； although； and partly because； he did not entirely agree with Darwin's views as laid down in the first edition of 〃The Origin of Species〃； which after all was the great impulse given to Murray's work。 Like Forbes he did not shrink from assuming enormous changes in the configuration of the continents and oceans because the theory of descent； with its necessary postulate of great migrations； required them。 He stated； for instance； 〃that a Miocene Atlantis sufficiently explains the common distribution of animals and plants in Europe and America up to the glacial epoch。〃 And next he considers how； and by what changes； the rehabilitation and distribution of these lands themselves were effected subsequent to that period。 Further； he deserves credit for having cleared up a misunderstanding of the idea of specific centres of creation。 Whilst for instance Schmarda assumed without hesitation that the same species； if occurring at places separated by great distances； or apparently insurmountable barriers； had been there created independently （multiple centres）； Lyell and Darwin held that each species had only one single centre； and with this view most of us agree； but their starting point was to them represented by one individual； or rather one single pair。 According to Murray； on the other hand； this centre of a species is formed by all the individuals of a species； all of which equally undergo those changes which new conditions may impose upon them。 In this respect a new species has a multiple origin； but this in a sense very different from that which was upheld by L。 Agassiz。 As Murray himself puts it： 〃To my multiple origin； communication and dir