. A practical course in botany, with especial reference to its bearings on agriculture, economics, and sanitation. Botany. CRYPTOGAMS 361 to conceive of a state, as our planet approaches the period of cosmic debility and decay, when the conditions of existence may become progressively more and more unfavorable. In this case the course of evolution would be reversed, the higher types gradually dying out as the struggle for life became more severe, and the tendency would be constantly toward lower and simpler forms, until finally all life would become extinct on our planet. We have no right, how
. A practical course in botany, with especial reference to its bearings on agriculture, economics, and sanitation. Botany. CRYPTOGAMS 361 to conceive of a state, as our planet approaches the period of cosmic debility and decay, when the conditions of existence may become progressively more and more unfavorable. In this case the course of evolution would be reversed, the higher types gradually dying out as the struggle for life became more severe, and the tendency would be constantly toward lower and simpler forms, until finally all life would become extinct on our planet. We have no right, how- A J ever, to assume that during such a course of retrogressive evolution the same forms would be repeated in reverse order as have already appeared, because there is no reason to believe that the condi- tions brought about by planetary decline and " old age " would be the same as those at- tending planetary birth and adolescence. 419. Explanation of the diagram. — An at- tempt to show the general course of plant evolution up to the present time is made in the accompany- ing diagram. The four great divisions, Thallophytes, Bryo- phytes, Pteridophytes, and Spermatophytes, are represented by spaces between four horizontal lines arranged one above the other in the order of their succession in time and com- plexity of organization. It should be borne in mind that these dividing lines are not sharply defined in nature, but overlap or indent the territory between them with vary-. Thallophytes Fig. 511. — Diagram showing the supposed course of plant Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Andrews, Eliza Frances, b. 1840; Lloyd, Francis Ernest, 1868-1947. ed. New York, Cincinnati [etc. ] American book company
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