Page:Popular Science Monthly Volume 77.djvu/73

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PALEONTOLOGIC EVIDENCES OF CLIMATE

O every one climate is an interesting theme. The climates of the past, especially when they can be shown to differ in character or distribution from those of the present, attract the attention of the general public, and they are of importance to the special student of geologic history whether his researches deal with the purely physical aspects of the subject or include some branch of paleontologic study.

The evidence as to former climates comes from many sources. The records of deposition and denudation in themselves sometimes give more or less definite indications concerning variations in temperature or moisture or both; the land floras when compared with those now living by their general characters and by the details of their structure, show more or less clearly the climatic conditions under which they lived; the land animals, especially the higher vertebrates, afford a good basis for inferring their habits and hence indirectly their environment, including climate; marine invertebrates give trustworthy evidence of differences in temperature of oceanic littoral waters at least in the later periods. It is obvious, however that the data furnished by any one of these lines of evidence will make only unconnected fragments of the history of past climates and that the evidence on the climate of any particular epoch, if derived from a single source, is seldom so complete or so convincing that corroborative testimony from other sources is not desirable. The subject is one in which general cooperation is essential.

It should be stated at the outset that the most abundant and most definite evidence comes from paleobotany, and will be outlined in Mr. White's paper. The discussion of the data derived from fossil vertebrates must also be left for some one who is qualified to present it, and the whole Paleozoic era may be passed over with the statement that so far as indications from the animal life are concerned the climate of the whole earth was mild and equable. The proof of local exceptions to this statement comes from other sources.

All inferences from paleontologic evidence as to former climatic conditions rest in the final analysis on a comparison with the present distribution of animals and plants with reference to climate. Such comparisons may be general or specific, direct or indirect, and the