. Nature sketches in temperate America, a series of sketches and a popular account of insects, birds, and plants, treated from some aspects of their evolution and ecological relations . Map of the Great Lakes Region, showing the former glaciated area. White portion indicates ice sheet; the fine vertical lines, older drift; fine dots, driftless area. Chicago, Dune Park, and Lakeside, Michigan, noted. 320 NATURE SKETCHES IN TEMPERATE AMERICA Levelling and Organic Evolution, has shown that base levellingprocesses influence the evolution of a species by erecting newand destroying old barriers. In
. Nature sketches in temperate America, a series of sketches and a popular account of insects, birds, and plants, treated from some aspects of their evolution and ecological relations . Map of the Great Lakes Region, showing the former glaciated area. White portion indicates ice sheet; the fine vertical lines, older drift; fine dots, driftless area. Chicago, Dune Park, and Lakeside, Michigan, noted. 320 NATURE SKETCHES IN TEMPERATE AMERICA Levelling and Organic Evolution, has shown that base levellingprocesses influence the evolution of a species by erecting newand destroying old barriers. In this way there is causedisolation, or intervention of crossing between a separatedsection of a species or kind on the one hajad, and interminglingof species on the other. Clements ^ remarks that the beginningof all the primary and many secondary successions of plantlife is to be sought in physiographic processes which produce. A view on the Des Plaines River. At the edge of the stream aquatic plants such as Sagittaria and reeds ahoundy awarding a perfect habitat for dragon-flies and diptera. The shores are slcirted with these hydrophytic plants, and farther back are shrubs and tree societies. new habitats or modify old ones. On the other hand, most ofthe reactions which continue successions exert a direct influenceupon the form of land. Along this line, Cowles ^ has recentlyasserted that, according to well-deflned laws governingtopographic geography, namely, the action of water in pro-ducing denudation and deposition and ultimate base levelling,there occurs at the same time a succession of plant societieswhich, after a time, reach a climax stage. As years pass by, Research Methods in Ecology.^ Botanical Gazette, 1901. ECOLOGY — INTERPRETATION OF ENVIRONMENT 321 one plant society must necessarily be supplanted by another,though the one passes into the other by imperceptible grada-tions. This implies that e
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