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Show 502 ISLAND LIFE. (PART II. the immediate vicinity of land. The general stn.bility of continents has, however, been accompanied by constant changes ?f form and insuln.r conditions have prevailed over every part m succ~ssion; and the effect of such changes on the distribution of organisms is pointed out. We then approach the consideration of another set of chancres-those of climn.te-which have probably been agents b of the first importance in modifying the specific forms as well as the distribution of animals. Here again we find ourselves in the midst of fierce controversies. The occurrence of a recent glacial epoch of great severity in the northern hemisphere is now universally admitted, but the causes which brought it on are mn.tter of dispute. But unless we can arrive at these causes, as well as at those which produced the equally well demonstrated mild climate in the Arctic regions, we shall be , quite unable to determine the nature and amount of the changes of climate which have occurred throughout past ages, n.nd shall thus be left without a most important clue to the explanation of many of the anomalies in the distribution of animals and plants. I have therefore devoted three chapters to a full investigation of this question. I have first given such a sketch of the most salient facts as to render the phenomena of the glacial epoch clear and intelligible. I then review the various suggested explanations, and, taking up the two which alone seem tenable, I endeavour to determine the true principles of each. While adopting generally Mr. Croll's views as to the causes of the "glacial epoch," I have introduced certain limitations and modifications. I have pointed out with more precision than has, I believe, hitherto been done, the very different effects on climate of water in the liquid and in the solid state ; and I have shown, by a variety of evidence, that without high land there can be no permanent snow and ice. From theEe facts and principles the very important conclusion is reached, that the alternate phases of precession-causing the winter of each hemisphere to be in aphelion and peTihelion each 10,500 years-would produce a complete change of climate only where a country was partially snow-cJaJ; while, whenever a large area became almost wholly buried in snow and ICe-as was certainly the CHAP. XXIV.] SUMMARY AND CONCLUSION. 503 case. with N.o:thern Europe during the glacial epoch-then the g.lae1al conditlOns would be continued and perhaps even intensified when the sun approached nearest to the earth in winter instead of there being at that time, as Mr. Croll maintains: an almost perpetual spring. This important result is supported by reference to the existing differences between the climates of the northern and southern hemispheres, and by what is known to have occurred durincr the last glacial epoch· and it • b ' IS shown to be in complete harmony with the geolocrical evidence as to interglacial mild periods. 0 . Disc~s~ing next the evidence for glacial epochs in earlier times, It IS shown that Mr. Croll's views are opposed by a vast body of facts, and that the geological evidence leads irresistibly to the conclusion that during a large portion of the Secondary and Tertiary periods, uninterrupted warm climates prevailed in the north temperate zone, and so far ameliorated the climate of the Arctic regions as to admit of the growth of a luxuriant. vegetation in the highest latitudes yet explored. The geographical condition of the northern hemisphere at these periods is then investigated, and it is shown to have been such as to admit the warm tropical waters freely to penetrate the land, and to reach the Arctic seas by several channels ; and, adopting Mr. Croll's views as to the enormous quantity of heat that would thus be conveyed northwards, it is maintained that the mild Arctic climates are amply accounted for. With ·such favourable geographical conditions, it is shown, that changes of excentricity and of the phases of precession would have no other effect than to cause greater differences of temperature between summer and winter ; but, wherever there was a considerable extent of very lofty mountains the snow-line would be lowered, and the snow-collecting area being thus largely increaRed a considerable amount of local glaciation might result. Thus may be explained the presence of enormous ice-borne rocks in Eocene and Miocene times in Central Europe, while at the very same period all the surrounding country enjoyed- a tropical or subtropical climate. The general conclusion is thus reached, that geographical conditions are the primary causes of great changes _of climate, |