Page:Popular Science Monthly Volume 77.djvu/74

68 that may be drawn from them vary greatly in positiveness. To take a familiar example, the reef-building corals are now restricted to shallow waters in which the mean temperature during the coldest month in the year is not less than 68° F., and such conditions are not found in the northern hemisphere north of latitude 32°. Since late Tertiary corals differ but little from those of the present time it is justifiable to assume that coral reefs in late Tertiary rocks indicate waters of about the temperature stated. But when Jurassic coral reefs are found as far north as latitude 53° it is by no means so certain that they indicate a minimum monthly mean temperature of 68° F., and concerning Devonian and Silurian coral reefs in high latitudes the doubt must be still greater. At the present time large reptiles are mainly confined to hot moist climates, but that fact alone can not be considered proof that the Mesozoic dinosaurs required the same kind of a climate.

The impress of climate on the present fauna is shown in various ways. A tropical fauna contains the greatest number of species and exhibits its luxuriance in other ways. Thus, taking shell-bearing marine mollusks to illustrate the general law, Dall has shown in Bulletin 84, U. S. Geological Survey, that the average tropical fauna in shallow waters consists of over 600 species, while the temperate fauna has less than 500 species, and the boreal fauna only 250. Again, there are certain genera that are characteristic of particular zones, and assemblages of forms that are recognized as belonging only to frigid, or temperate, or tropical waters, and in genera that have a wide range many of the species are restricted to certain limits of temperature.

In the late Tertiary faunas which contain a large proportion of living genera and many living species justifiable inferences as to climate may be made from direct comparison with living faunas. By one or another of the tests just indicated, or by a combination of them, Dall has produced convincing evidence that the Oligocene fauna of the Atlantic states was subtropical and that the Oligocene maintains its' subtropical character even as far north as Arctic Siberia. He has also shown that the Miocene fauna of Maryland indicates a temperate climate and that a similar cool-water fauna extended at that time as far south as Florida. The fossils of the raised Pliocene beaches at Nome, Alaska, according to the same investigator, furnish evidence of warmer climate during Pliocene time even at that high latitude. By similar methods, in a paper published in the Journal of Geology, Vol. XVII., Arnold has recently argued for a series of climatic changes in the late Tertiary and Pleistocene of California.

When the investigation is carried back to the Mesozoic and earlier