Page:Popular Science Monthly Volume 76.djvu/596

592 cent., or 115,000 species, live in the sea, and 71,000 have their habitat on the land or in the waters of the land. Of the 115,000 kinds of known animals inhabiting the seas nearly 70 per cent, are Cœlenterata, Echinodermata, Molluscoidea and Mollusca, the types of organisms most often found by the stratigrapher and on which he is largely dependent in deciphering the ancient geography.

Let us now examine into the number of available fossil forms made known by the paleontologists. As early as 1868, Bigsby in his "Thesaurus Siluricus" listed 8,897 species from the strata beneath the Devonic, and in his "Thesaurus Devonico-Carboniferous" of 1878, he further enumerated about 5,600 Devonic and 8,700 Carbonic forms. In 1889 Neumayr concluded that there were then known about 10,000 Jurassic species. We may therefore estimate that the paleontologists of to-day have access to at least 100,000 species of fossils. Their numbers in the geologic scale are about as follows: Cambric 2,000, Ordovicic 8,000, Siluric 8,000, Devonic 9,000, Lower Carbonic 7,000, Upper Carbonic 8,000, Permic 4,000, Triassic 6,000, Jurassic 15,000, Cretacic 10,000 and Tertiary 25,000. The end of species-making is not at all in sight, and the day will come when paleontologists will deal with ten times as many species as are now known.

Stiles tells us that zoologists know but from 10 to 20 per cent, of the living forms, and there should therefore be from 3,760,000 to 4,700,000 different kinds of animals alive to-day, ranging from the protozoa to man. Now let us compare the abundance of living animals with those of the geologic ages, and especially with the Jurassic period, of which life we have probably a better knowledge than of any time back of the Tertiary. The European Jurassic has long been divided into 33 zones (Buckman hints at a probable 100), and if we hold that each one of these times had only one quarter as many species as in the lowest estimate of the present world, there must have lived during the entire Jurassic something like 31,000,000 kinds of animals. Yet paleontologists have described not more than 15,000 Jurassic forms. The great imperfection of the extinct life record is thus forcibly brought to our attention, and we learn from these estimates that for each kind of animal preserved in the rocks more than 2,000 other kinds are utterly blotted out of the geologic record.

Much of this more apparent than real imperfection, however, is due to the vast number of insect species now living—animals that must have been comparatively few in the Jurassic, due in the main to the absence of flowering plants. From these figures, however, we must not conclude that the geologic record is equally imperfect throughout; for the paleontologist studying marine fossils well knows that he can not, as a rule, hope to study other than those kinds of animals that have hard and calcareous or siliceous external or internal skeletons. Of