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Show 370 DARWINISM: CHAP. lands j and, in the next place, there is not a particle _of ~irect evidence that any such low!')ring of temperature m . mtertropical lowlands ever took place. The only alleged ev~dence of the ki11d is that adduced by the late Professor Agassiz and Mr. Hartt ; but I am informed by my friend, Mr. J. C. Branner (now State Geologi t of Arkansas, U.S.), ~vho succeeded ~1r. Hartt, and spent several years completi_ng the gcolo_gwtl survey of Brazil, that the supposed morames and glac1ated granite rocks ncar Rio Jn.neiro and else,_vherc, as we~l as the so -called boulder-clay of the same regwn, are entirely explicable as the results of snb-aeriu1 denudation a~d :vea,t~crin g, and that there is no proof wh}ttever of glac1atwn lll any part of Brazil. Lower Tempeml'ure 1znt needed to Explain the Facts. But any . uch vast physical change as th~t s:1ggested by Darwin, involving a,s it does such tremendous 1. sues as regards its effects on the tropical fauna and flora of the whole world, is really quite uncalled for, bcca,usc the facts to be explained are of the same essential n~turc as those presented by remote oceanic i. lands, between _wh1~h and tho nearest COJI tiuents no temperate land connect10n 1s postulated. In proportion to their limited area and extreme isol~tion, the Azon 'K, St. Helena, the Gn.lapa,go , and the Sandw1ch I lands, each possess a fairly rich-the last a very rich-indige_nous fl om ; and the means which sufficed to stock them w1th a, grc;lf variety of plants would probably suffice to transmit other;) from mountain-top to mountain-top in various parts of the globe. In the case of the Azores, we have large number~ of species identical with those of Europe, and others closely allied, forming an exactly parallel case to the species found on the various mountain summits which have been referred to. The distances from Madagascar to the South African moun tains and to Kilimandjaro, and from the latter to Abyssinia, arc no greater than from Spain to the Azores, while there arc othc·r equatorial mountains forming stepping-stones at about :m equal distance to the Cameroons. Between Java and the Himalayas we have the lofty mountains of Sumatra and of North-western Burma, forming steps at about the same di. tance apart ; while between Kini Balu and the Australian Alps we XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 371 have the unexplored snow mountains of New Guinea, the Bcllcnden Ker mountains in Queensland, and the New England and Blue Mountains of Now South Wales. Between Brazil and Bolivia the distances are no greater ; while the unbroken ra.1we of mountains from Arctic America to Tierra-del-Fuego offers the greatest facilities for transmission, the partial gap between the lofty peak of Chiriqui and the high Andes of New Grenada being far less than from Spain to the Azores. Thus, whatever means have sufficed for stocking oceanic islands must have been to some extent effective in transmitting northern form from mountain to mountain, across the equator, to the sonthern hemisphere ; while for this latter form of dispersal there are special facilities, in the abundance of fresh and unoccupied surfaces always occurring in mountain regions, owing to avaJanches, torrents, mountain-slides, and rock-falls, thus affording stations on which air-borne seeds may germinate and find a temporary home till driven out by the inroads of the indigenous vegetation. These temporary stations may be at much lower altitudes than the original habitat of the species, if other conditions are favourable. Alpine plants often descend into the valleys on glacial moraines, while some arctic species grow equally well on mountain summits and on the seashore. The distances above referred to between the loftier mountains may thus be greatly reduced by the occurrence of suitable conditions at lower altitudes, and the facilities for transmission by means of aerial currents proportionally increased.1 Facts Explained by the liVind-Ca1-riage of Seeds. But if we altogether reject aerial transmission of seeds for great distances, except by the agency of birds, it will be difficult, if not impossible, to account for the presence of so many identical species of plants on remote mountain summits, or for that "continuous current of vegetation" described by Sir Joseph Hooker as having apparently long existed from the northern to the southern hemisphere. It may be admitted that we can, possibly, account for the greater portion of the floras of remote oceanic islands by the agency of birds alone ; because, when blown out to sea land-birds must reach some island 1 For a fuller discussion of this subject, see my Island Life, chap. xxiii. |