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Show 342 DARWINISM I.JHAP. and that during the course of known geologica] time the continents and great oceans had again and again changed places with each other. Sir Charles Lyell, in the last edition of his Principles of Geology (1872), said: "Continents, therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages;" and this m<ty be said to have been the orthodox opinion down to the very recent period when, by means of deep-sea soundings, the natmc of the ocean bottom was made known. The first person to throw doubt on this view appears to have been the veteran American geologist, Professor Dana. In 1849, in the Hcport of vVilke's Exploring Expedition, he adduced the argument against a former continent in the Pacific during the Tertiary period, from the absence of all native quadrupeds. In 1 5G, in articles in the American Journal, he discussed the development of the American continent, and argued for its general permanence; and in his Ma:nztal of Geology in 1863 and later editions, the same views were more fully enforced and were latterly applied to all continents. Darwin, in his Joumal uj Researches, published in 1845, called attention to the fact th<tt all the small islands far from land in the Pacific, Indian, aud Atlantic Oceans are either of coralline or volcanic formation. He excepted, however, the Seychelles and St. Paul's rocks ; but the former have since been shown to be no exception, af:i they consist entirely of coral rock ; and although Darwin himself spent a few hours on St. Paul's rocks on his outwarcl voyage in the Beagle, and believed he had found some portions of them to be of a "cherty," and others of :L " felspathic " nature, this also has been shown to be crronconl:l, and the careful examination of the rocks by the Abbe Renard clearly proves them to be wholly of volcanic origin.l \ V<' have, therefore, at the present time, absolutely no exception whatever to the remarkable fact that all the oceanic island of the globe are either of volcanic or coral formation ; and there is, further, good reason to believe that those of the latter clnss in every case rest upon a volcanic foundation. In his Origin of Species, Darwin further showed that no true oceanic island had any native mammals or batraclJi:~ 1 See .A.. Agassiz, Three Cruises of the Blake (Cam bridge, Mas.' ., 1 S8), vol. i. 1'· 1 '2i, footnote. xu GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 343 when first discovered, this fact constituting the test of the class to which an island belongs; whence he argued that none of them had ever been connected with continents, but all had originated in mid-ocean. These considerations alone render it almost certain that the a.rea. now occupied by the great oceans have never, during known geoloaical time, been occupied by continents, since it is in the highest degree improbable that every fn.Lgment of those continent. should have completely disappeared, and have been replaced by volcanic islands ri ing out of profound ocea.nic aby · ·es; bnt recent research into the depth of tho oceans and the nature of the deposits now forming on their floors, adds greatly to the evidence in this direction, and renders it a.]most a certainty that they represent very ancient if not prim::eval fc<.tturcs of the earth's urface. A very brief outline of the nature of this evidence will be now given. The researches of the Challenger expedition into the nature of the sea-bottom show, tha.t the whole of the land debris brought down by rivers to the ocean (with the exception of pumice and other fto:tting matter), is dcpo. ited comparatively ncar to the shores, a.nd that the fineness of the material is a,n indication of the eli. tance to which it ha · been carried. Everything in the nature of gravel and sand is laid down within a very few miles of land, only the finer muddy sediments being carried out for 20 or 50 mile , and the very finest of all, under the most favourable conditions, rarely extending beyond 150, or at the utmost, 300 miles from land in to the deep oce .1 Beyond these di. tances, and covering the entire ocean floor, are various oozes formed wholly from the dchri. of marine or<ranism. ; while intermingled with these arc found various vobtnic products which have bcmt either carried throw:.'h the air or floated on the surface, and a small but perfectly recognisable quantity of meteoric matter. Icc-borne rocks are al. o found abundantly scattered over the ocean bottom within a definite distance of the arctic and antarctic circles, clearly marking out the limit of floating icebergs in recent geological times. 1 Even the extremely fine Mississippi mud is nowhere foun<l l.Jcyond a hnnrlred miles from the months of the river in the Gulf of Mexico (A. Agassiz, TMee Crui ·es of the Blake, vol. i. p. 128). |
