The presence of the Nummulitic and other marine Eocene and Miocene formations over such extensive areas, and at such great elevations, is a conclusive proof that a great part of our existing continents were then at the bottom of deep oceans. The Alps were certainly 10,000 feet lower than their present level, and the Himalayas more so; and when this was the case a great part of Europe and Asia must have been sea, in which only a few of the highest peaks and elevated plateaux stood up as islands. The Pacific and Indian Oceans as well as the Atlantic might then have poured their Gulf Streams into the Polar basin, and prevalent southerly and westerly winds, blowing over wide expanses of water, have deposited their vapour in genial showers instead of in solid snow. The effect of such geographical conditions in producing both heat and cold is admirably worked out by Wallace in his _Island Life_, and few who read it can doubt that Lyell was right in saying that they have been the principal causes of the vicissitudes of climate. And here I may say a word to express my admiration of the innate sagacity with which Lyell, many years ago, and with comparatively few facts to work upon, sketched out the leading lines of geology, which have been confirmed by subsequent research. Details may have been corrected or added, but his main theories have stood the fullest test of the survival of the fittest. His law of the uniformity of natural causes, continued for long intervals of time, holds the field unchallenged. These causes may have operated a little more quickly or slowly in former ages than at present, but they have been of the same order. The waste of continents, instead, of averaging one foot in 4000 years, may have averaged ten or twenty feet during certain periods, and certain portions of the earth's crust may have been elevated or depressed at a quicker rate than is now going on in Scandinavia; but no one any longer believes in paroxysms throwing up mountain chains or sinking continents below the ocean at a single blow. In like manner later geologists have corrected details in the distribution of land and sea suggested by Lyell to account for the glacial period, but his main law has only received confirmation--viz. that this distribution, and especially high land in high latitudes, has been the principal cause of such periods. At the same time there is a pretty general consensus of the best and latest geologists, that, as Lyell himself suggested, elevation and depression and other geographical changes, though the principal, are not the sole causes of the glacial period. The main argument is, that the phases of this period, though not exactly simultaneous over the whole world, are too nearly so to be due to mere local movements, and require the intervention of some general cosmic cause. We have already seen that of such causes there is none which appears feasible except Croll's theory of the effects of precession combined with high eccentricity.