CREATION BY EVOLUTION
have been almost neglected, is that found in the Don Valley, near Toronto, Canada. Here are found impressions of leaves and other parts of the sycamore, maple, osage orange, and other types that do not to-day quite reach that latitude. Other traces of Pleistocene floras are found in cave deposits, associated with fossil remains of animals, some of which are now extinct. Swamp deposits that were overwhelmed by sand during changes along the coasts yield many species of plants, most of which still exist, such as the bald cypress, loblolly pine, sycamore, poplar, hickory, river birch, and several species of oaks. All these fossils show that the inter-glacial floras scarcely differed from those of to-day except in the details of distribution of the species. During the periods of glaciation these temperate forests retired southward and gave way along the ice front in this country to arctic willows and dwarf birches, which reached southward to about latitude 40°.
The post-glacial amelioration of the climate, the opening to occupation by plants of areas that had been covered with glaciers, the mixing of soils through the action of the ice, all combined to stimulate the evolutionary activity of plants, particularly the herbaceous forms. It seems probable that the herbaceous families that are characteristic of the Temperate Zone originated at this time.
Possibly more potent than natural causes in modifying the character and distribution of the existing vegetation has been the work of man, which includes the action of fire, the ax, and domesticated grazing animals. The forests are now waning. Human intercourse results in surprising feats of plant distribution, such as are shown in our familiar cosmopolitan weeds. Insect and fungal pests are similarly spread, both rapidly and widely, and tend increasingly to restrict or even to exterminate the native vegetation.
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