Page:Early Man in Britain and His Place in the Tertiary Period.djvu/48

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From the coast-line above mentioned the land extended northwards and westwards, far away from our present shores, and is proved by the examination of the forests and of the animals to have been continuous with the North American continent. The chief botanists of the present time—Hooker, Dyer, Saporta, Dawson, and Asa Gray—are agreed that the north polar region was the centre from which the Tertiary floras have been dispersed over the new and old worlds. According to Saporta, the marked difference between summer and winter in the polar regions has first left its impress in the dicotyledons with deciduous leaves, which were unknown on the earth before the Cretaceous age. Their introduction was the greatest revolution in the vegetable kingdom, as yet observed, and when once evolved they increased rapidly in numbers and diversity of forms. As the polar temperature was lowered, the trees of warm habit were pushed farther and farther south, away from their original birthplace, and ultimately only survived in isolated districts, now separated from each other by great tracts of sea, or great areas of desert or of mountain. In consequence of this there is an almost unbroken sequence to be observed in the floras succeeding each other in the northern hemisphere, from the Eocene age down to the present day. "There is no great break," as Mr. Starkie Gardner observes, "in passing from one to the other, when we compare them over many latitudes, and but little change beyond that brought about by altered