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Show 368 DARWINISM CHAP. already shown that limitations of are~.~re almo~t always ~ue to the competition of allied forms, facilities for dispersal bemg only one of many factors in d~term~ning the wide ra~ge of species. It is, however, a. specially I~I?ortant £.actor m th~ case of the inhabitants of remote ocea,mc -Islands, smce, whethei they are peculiar species or not, they or their r.emote ancestor:s must at some time or other have reached their present position by natural means. I have already shown elsewhere, that t~e fl.or~ of the Azores strikingly supports the view of the sp~ciCs havmg been introduced by aerial transmission only, that 1s, by the agc.ncy of birds and the wind, because all plants that co~ld not possibly have been carried by these means are a~sent. In t?e sa~e way we may a.ccount for the extrem~ rar~ty of Legummosre m all oceanic islands. Mr. Hemsley, m his Report on Insular Floras, says that they "are. wanting in. a large nu;,nber of oceanic islands where there IS no true httoral flora, as St. Helena Juan Fernandez, and all the islands of the South Atlantic and South Indian Oceans. Even in the tropical islands such as Mauritius and Bourbon, there are no endemic species: and very few in the Galapagos and the remoter Pacific I lands. All these facts are quite in accordance with the absence of facilities for transmission through the air, either by birds or the wind, owing to the comparatively large size and weight of the seeds ; and an additional proof is thus afforded of the extreme rarity of the successful floating of seeds for great distances across the ocean.2 Explanation of North Temperate Plants in the Southern Hemisphen If we now admit that many seeds which are either minute in size of thin texture or wavy form, or so fringed or margin~d as to afford a good hold to th.e air, are cap~ble of being carried for many hundreds of miles by exceptiOnally 1 SP-e lslctnd Life, p. 251. . . . ~ Mr. Hemsley suggests that it is not so much the difficulty of transmissiOn by floating, as the bad conditions the seeds are usually exposed to when they reach laml. Many, even if they germinate, are destroyed by the waves, as Burchell noticed at St. Helena ; while even a flat and sreltered shore would be an unsuitable position for many inlanu plants. Air-borne see<ls, on t1Ie other hand, may be carried far inland, and so scattered that some of them are likely to reach suitable stations. XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 369 violent and long-continued gales of wind, we shall not only be hettcr a,blc to account for the floras of some of the remotest oceanic islands, but shall also find in the fact a sufficient explanation of the wide diffusion of many genera, a.nd even species, of arctic and north temperate plants in the southern hemisphere or on the summits of tropical mountains. ..Nearly fifty of the flowering plants of Tierra-del-Fuego are found also in Korth America or Europe, but in no intermediate country; while fifty eight species are common to New Zealand and Northern Europe; thirty-eight to Australia, Northern Europe, and Asia; and no less than seventy-seven common to New Zealand, Australia, and , 'outh America. 1 On lofty mountains far removed from each other, identical or closely allied plants often occur. Thus the fine Primula imperialis of a single mountain peak in Java has been found (or a closely allied species) in the Himalayas ; and many other plants of the high mountains of Java, Ceylon, and North India are either identical or closely allied forms. So, in Africa, some species, found on the sumrriits of the Cameroons and Fernando Po in \Vest Africa, are closely allied to species in the Abyssinian highbnds and in Temperate Europe; while other Abyssinian and Cameroons species have recently been found on the mountains of Madagascar. Some peculiar Australian forms have been found represented on the summit of Kini Balu in Borneo. Again, on the summit of the Organ mountains in Brazil there arc species allied to those of the Andes, but not found in the intervening lowlands. No P?'oof of Recent Lowe1· Ten1,pemtuTe in the T?'opics. Now all these facts, and numerous others of like character, were supposed by Mr. Darwin to be due to a lowering of temperature during glacial epochs, which allowed these temperate forms to migrate across the intervening tropical lowlands. But any such change within the epoch of existing species is almost inconceivable. In the first place, it would necessitate the extinction of much of the tropical flora (and with it of the insect life), because without such extinction alpine herbaceous plants could certainly never spread over tropical forest low- 1 For fuller particulars, see Sir J. Hooker's l ntToduction to Floras of New Zealand and Austmlia, and a sumnutry in my lslaucl Life, chaps. xxii. xxiii. 2B |
