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Show 1. 84 APPEARANCE OF NEW SPECIES. [Ch. XI. f these q uestl.o ns we can even now return a satis~ To some 0 . . and w.i t1 ·d to the rest, we have some 1 regai factory answer' . . d t enable us to speculate with t uide conJecture, an ° . . . data 0 g . ld b mature to anticipate such d1s-d t . but 1t wou e pre a va.n age. t'l e have 1a l' d b e.l1o! re the reader an ample body cusslons .un I w d b the industry of modern geologists. of matenals amasse Y CHAPTER XII. Efftlcts produced by the powers of vitality on the state of the earth's smfaceModifications in physical geography caused by organic beings on dry land in· ferior to those caused in the subaqneous regions-Why the vegetable Hoil does not augment in thickness-Organic matter drifted annually to the sea, and buried in subaqueous strata-Loss of nourishment from this source, how suppliedThe theory, that vegetation is an antagonist power counterbalancing the degra· dation caused by running water, untenable-That the igneous causes are the true antagonist powers, and not the action of animal and vegetable life-Con. servative influence of vegetation-Its bearing on the theory of the fo.rmation of valleys, and on the age of the cones of certain extinct volcanos-Rain diminished by the felling of forests-Distribution of the American forests dependent on the direction of the predominant winds-Influence of man in modifying the physical geography of the globe, THE second branch of our inquiry, respecting the changes of the organic world, relates to the effects produced by the powers of vitality on the state of the earth's surface, and on the material constituents of its crust . . Dy the effects produced on the surface, we mean those modi: fications in physical geography of which the existence of organic beings is the direct cause,-as when the growth of certain plants covers the slope of a mountain with peat~ or converts a swamp into dry land; or when vegetation prevents the soil, in certain localities, from being washed away by running water. By the agency of the powers of vitality on the material constituents of the earth's crust, we mean those permanent modi: fications in the composition and structure of new strata, which result from the imbedding therein of animal and vegetable remains. In this case, organic beings may not give rise immediately to any new features in the physical geography of certain tracts, which would not equally have resulted from the mere operation of inorganic causes ; as, for example, if a lake be filling up with sediment, held in suspension by the waters |