Page:Popular Science Monthly Volume 76.djvu/594

590 rock layer in which it occurs and which it characterizes, long and successfully employed by the invertebrate paleontologist, must be recognized and used by us also.

I need hardly refer to the fact that the determination of the geological age and the successful correlation of many North American formations, ranging from Mesozoic to Pleistocene, depend in large measure, if not entirely, on vertebrate fossils. I need only contrast the American series of Pleistocene glacial and intergiacial stages, determinable at present only by the strictly stratigraphic method of superposition, with the carefully worked-out series in Europe, where each epoch of ice advance and retreat is characterized by its particular fauna and flora. That even the beginnings of stratigraphic paleontology, as contrasted with the morphological, will lead to immediate and valuable results, is strikingly shown by Professor Calvin's recent paper in the Bulletin of our parent society, the Geological Society, in which he describes the Aftonian mammal fauna from the earliest of American intergiacial stages.

While readily admitting that the slow-moving invertebrate, living, it may be, in the very mud which is destined to become the matrix of its fossil remains, enjoys advantages as a prospective horizon-determiner which the agile vertebrate can more readily, and does most willingly, escape, still the short life of vertebrate species, and their comparatively rapid evolution, fit them for use as index fossils quite admirably. The localization of mammalian faunas, their inability to cross barriers such as ocean basins and great mountain ranges, their dependence on temperature, etc., are comparable to similar conditions circumscribing the free migration of invertebrates. We should not expect to find in the distribution of vertebrate faunas the analogues of the cosmopolitan graptolite zones of the Ordovician or the ammonite zones of the Trias, but we can work out our major zones as recognized by the great migrational movements among vertebrates, expressed in changes in the faunas and the rock succession, which will give us a world scale, and then, by interpolation, fill in the minor and local subdivisions which we probably shall not be able to correlate at once, but which there is every reason to believe we may be able to do later.

The attempt will be accompanied by difficulties which are not appreciated by the invertebrate paleontologist, and I speak feelingly and from experience, for there is a difference between collecting, on the one hand, from a layer a few inches thick, crowded with shells, and, on the other, tramping miles up hill and down over beds hundreds of feet thick, to be rewarded by a few teeth, a lot of useless bone fragments or nothing. Horizons based on vertebrates must include larger stratigraphic units