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Show 222 ISLAND LIFE. [PART I. archaic forms, we may mention the mud-fishes and the ganoids, confined to limited fresh-water areas; the frogs and toads, which still maintain themselves vigorously in competition with higher forms; and among mammals the Ornithorhynchus and Echidna of Australia; the whole order of Marsupials-which, out of Australia where they are quite free from competition, only exist abundantly in South America, which was certainly long isolated from the northern continents; the Insectivora, which, though widely scattered, are generally nocturnal or subterranean in their habits; and the Lemurs, which are most abundant in Madagascar, where they have long been isolated, and almost removed from the competition of higher forms. Climatal Revolutions as an agent in pToducing 0Tganic Ohanges.-The geographical and geological changes we have been considering are probably those which have been most effective in bringing about the great features of the distribution of animals, as well as the larger movements in the development of organised beings; but it is to the alternations of warm ann cold, or of uniform and excessive climates-of almost perpetual spring in arctic as well as in temperate lands, with occasional phases of cold culminating at remote intervals in glacial epochs, -that we must impute some of the more remarkable changes both in the specific characters and in the distribution of organisms.1 Although the geological evidence is opposed to the belief in early glacial epochs except at very remote and distant intervals, there is nothing which contradicts tho occurrence of repeated changes of climate, which, though too small in amount to produce any well-marked physical or organic ?hange, would yet be amply sufficient to keep the organic world m a constant state of movement, and which, by subjecting the whole flora and fauna of a country at compar~tively short intervals to decided changes of physical conditions, would supply that stimulus and motive power which as we have seen ' ' 1 Agassiz appears to have been the first to suggest that the principal epochs of life extermination were epochs of cold ; and Dana thinks that two at least such epochs may he recognised, at the close of the Palreozoic and of the Cretaceous periods-to which we may add the laat glacial epoch. CUAL'. X.] 'l'IIE llA'rE OF ORGANIC CHARGE. 223 is all that is necessary to keep the processes of ((natural selection" in constant operation. The frequent recurrence of periods of hicrh and of low excen- • • b tne1ty must certainly have produced chanO'es of climate of cons~derabl~ import.ance to the life of ani~als and plants. Durmg penods of high excentricity with summer in perihelion, t~at season would be certainly very much hotter, while the wmters would be longer and colder than at present; and al~ hough geographical conditions might prevent any permanent mcrease of snow and ice even in the extreme north, yet we cannot doubt that the whole northern hemisphere would then have a .very different climate than when the changing phase of precessiOn brought a very cool summer and a very mild winter -a perpetual. spring, in fact. Now, such a change of climate would certainly be calculated to bring about a considerable change of species, both by modification and migration, without any such decided change of type either in the veO'etation or • 0 the ammals that we could say from their fossil remains that any change of climate had taken place. Let us suppose, for instance, that the climate of England and that of Canada were to be mutually exchanged, and that the change took five or six thousand years to bring about, it cannot be doubted that considerable modifications in the fauna and flora of both countries would be the result, although it is impossible to predict what the precise changes would be. We can safely say, however, that some species would stand the change better than others, while it is highly probable that some would be actually benefited by it, while others would be injured. But the benefited would certainly increase, and the injured decrease, in consequence, and thus a series of changes would be initiated that miO'ht lead to • • 0 most Important r-esults. Agam, we are sure that some species would become modified in adaptation to the change of climate more readily than others, and these modified species would therefore increase at the expense of others not so readily modified, and hence would arise on the one hand extinction of species, and on the other the production of new forms. But this is the very least amount of change of climate that would certainly occur every 10,500 years when there was a high |