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Such is the native physiography and anthropogenesis of the land of the Oregon.

The evolution of the species-groups of Smilisca was effected through isolation by physical barriers in the Cenozoic; the differentiation of the species was initiated by further isolation of populations by changes in physiography and climate.

It may be concluded that the present physiography of the land is proximately due solely to the action of the denuding agents—rain, frost, rivers, and the sea.

In still more western fields of research much additional light has been thrown since 1875 on the physiography of the great deserts and oases of Arabia.

The physical geography of Canada is so closely bound up with its geology that at least an outline of the geological factors involved in its history is necessary to understand the present physiography.