Page:Popular Science Monthly Volume 76.djvu/597

Rh such there may be in the present seas about 250,000 kinds, of which about 25,000 have been named. Therefore on this basis we can say that the student of Jurassic faunas knows 1 species in every 54 of shelled animals that lived during this period.

This admittedly great imperfection of the life record needs to be further explained so that the reader will not arrive at the erroneous conclusion that modern stratigraphy rests upon very insecure foundations. The stratigrapher in determining the age of a given deposit, and in the identification of it from place to place and from country to country, and even across the great oceans, deals in his work not with quantity of species, but with comparatively small numbers of constantly recurring hard parts of certain species that are more often of marine than of land origin. Many of these forms have but local value but others have spread thousands of miles, and some of the long enduring species range over the greater part of the earth. Some of the best guide fossils in the Paleozoic are the brachiopods because they are present in nearly all the strata of this era. The writer in 1897 listed 1,859 forms then known from these rocks of North America. Of these about 28 per cent., or 537 species, had great geographic distribution. 117 species are found in the Rocky Mountain area, the Mississippi valley and the Appalachian region, and of these 36 are also known to occur in foreign countries. The number of species common to North America and other continents, however, is 121. It is upon faunal assemblages of this quantity and nature that the stratigrapher relies most in deciphering the former extent of the continental seas.

In the making of paleogeographic maps or in the determination of geologic time, using fossils as the essential basis, we have guidance in those of marine faunas, and the floras and faunas of the land and its fresh waters. Of these widely differing realms or habitats we now know that the fossils of the marine faunas are the more reliable not only because there are so many more of them than of the land dwellers, but more especially because their geologic succession is far more complete. The conditions of preservation, that is, appropriate burial in sediments, are always at hand in marine waters, but on the land entombment occurs only exceptionally, whereas the life of fresh waters is very meager and almost unchanging during geologic time. Then, too, marine life is "less affected by meteorologic factors, and more dependent upon conditions which affect the whole hydrosphere rather than small areas of it. The struggle for life is less intense, the food supply generally more adequate, enemies less vigorous, and dangerous fluctuations of temperature far less frequent, in the sea than on land. The same features make the land fauna more clearly indicative of minor divisions of the scale, and of the progress of organic evolution