Page:The American Cyclopædia (1879) Volume XII.djvu/828

814 in the same latitudes, for the older floras of the temperate zones exhibit in many respects a tropical aspect. In the cretaceous and tertiary periods, the flora embraces many genera of the existing flora of the temperate zones. In the successive faunas also, even of the ocean bed, we are to take into consideration the existing physical conditions. In the very early periods coarse and fine sediments are found, indicating, if not shore lines, at least shallow and disturbed water on one side, and deeper seas with quiet water and finer sediments on the other. The geographical extension of species does not always correspond with the nature of the sediments; for while in the Trenton period we have a large number of brachiopoda extending over wide areas, even as far west as the formation is known, the same is not true of the Hamilton group, although the physical characters of the two formations appear to have been equally uniform. This fact, however, does not furnish an argument in favor of gradual climatic or other permanent changes; for again in the carboniferous period certain forms of brachiopoda have even a wider range than in any preceding period.—The causes affecting the distribution of the faunas and floras of the several geological periods cannot be discussed in a sketch like the present; but that these have successively appeared and disappeared is ascertained in every part of the habitable globe. Of the succession or coming in of new species we have everywhere abundant evidence; and in a great proportion of instances they could not have been derived from sources very far from where we find their imbedded exuviæ.