第165章

The conjecture appears to be unnecessary. Love shows that "the force that keeps the Pacific Ocean on one side of the earth is gravity, directed more towards the centre of gravity than the centre of the figure." ("Report of the 77th Meeting of the British Association" (Leicester, 1907), London, 1908, page 431.) I can only summarise the conclusions of a technical but masterly discussion. "The broad general features of the distribution of continent and ocean can be regarded as the consequences of simple causes of a dynamical character," and finally, "As regards the contour of the great ocean basins, we seem to be justified in saying that the earth is approximately an oblate spheroid, but more nearly an ellipsoid with three unequal axes, having its surface furrowed according to the formula for a certain spherical harmonic of the third degree" (Ibid. page 436.), and he shows that this furrowed surface must be produced "if the density is greater in one hemispheroid than in the other, so that the position of the centre of gravity is eccentric." (Ibid. page 431.) Such a modelling of the earth's surface can only be referred to a primitive period of plasticity. If the furrows account for the great ocean basins, the disposition of the continents seems equally to follow. Sir George Darwin has pointed out that they necessarily "arise from a supposed primitive viscosity or plasticity of the earth's mass. For during this course of evolution the earth's mass must have suffered a screwing motion, so that the polar regions have travelled a little from west to east relatively to the equator. This affords a possible explanation of the north and south trend of our great continents." ("Encycl. Brit." (9th edition), Vol. XXIII. "Tides", page 379.)

It would be trespassing on the province of the geologist to pursue the subject at any length. But as Wallace ("Island Life" (2nd edition), 1895, page 103.), who has admirably vindicated Darwin's position, points out, the "question of the permanence of our continents...lies at the root of all our inquiries into the great changes of the earth and its inhabitants." But he proceeds: "The very same evidence which has been adduced to prove the GENERAL stability and permanence of our continental areas also goes to prove that they have been subjected to wonderful and repeated changes in DETAIL." (Loc. cit. page 101.) Darwin of course would have admitted this, for with a happy expression he insisted to Lyell (1856) that "the skeletons, at least, of our continents are ancient." ("More Letters", II. page 135.) It is impossible not to admire the courage and tenacity with which he carried on the conflict single-handed. But he failed to convince Lyell. For we still find him maintaining in the last edition of the "Principles": "Continents therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages."(Lyell's "Principles of Geology" (11th edition), London, 1872, I. page 258.)Evidence, however, steadily accumulates in Darwin's support. His position still remains inexpugnable that it is not permissible to invoke geographical change to explain difficulties in distribution without valid geological and physical support. Writing to Mellard Reade, who in 1878 had said, "While believing that the ocean-depths are of enormous age, it is impossible to reject other evidences that they have once been land," he pointed out "the statement from the 'Challenger' that all sediment is deposited within one or two hundred miles from the shores." ("More Letters", II. page 146.) The following year Sir Archibald Geikie ("Geographical Evolution", "Proc. R. Geogr. Soc." 1879, page 427.) informed the Royal Geographical Society that "No part of the results obtained by the 'Challenger' expedition has a profounder interest for geologists and geographers than the proof which they furnish that the floor of the ocean basins has no real analogy among the sedimentary formations which form most of the framework of the land."Nor has Darwin's earlier argument ever been upset. "The fact which Ipointed out many years ago, that all oceanic islands are volcanic (except St Paul's, and now that is viewed by some as the nucleus of an ancient volcano), seem to me a strong argument that no continent ever occupied the great oceans." ("More Letters", II. page 146.)Dr Guppy, who devoted several years to geological and botanical investigations in the Pacific, found himself forced to similar conclusions.