is in itself sufficient to show that Australia has been geographically isolated for a long time, so that mammals of the higher, more advanced groups were unable to reach it.
The fact that zoölogical regions, as pictured on the map, when marked by differences of colour, seem to make a patchwork or a meaningless pattern on the map is due to the long, varied, and complicated changes in the geography of the regions and in their climate as recorded in their geological structure. Land areas that are now united were for ages separated by arms of the sea, and lands that are now separated were formerly united. Not very long ago, as geological time is reckoned, Great Britain and Ireland were joined to each other and to the continent of Europe, and the North Sea was a wide terrestrial plain, which stood not far above sea level. At about the same time the great islands of the East Indies were parts of the mainland of Asia, from which they became separated at different times. Borneo, Sumatra, Java, Celebes, the Philippines, the Japanese group, and the Kurile Islands were once joined to Asia. In the course of the ages Alaska and Siberia have been often connected and as often separated by uplift and depression of the land, and North and South America were brought together by the Isthmus at a relatively late period. Before that period the two continents had long been separated by a broad sea, which covered the site of the Isthmus and most of Central America.
We must also note the remarkable changes of climate to which the rocks and fossils bear unequivocal testimony. During the greater part of the Tertiary period the climate of the earth was mild and genial, a fact indicated by the fossil plants of the Arctic regions. This climate gradually gave way to one that was colder and that culminated in the glacial period, when so many lands were buried under sheets of moving ice, as Greenland is to-day.
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