Page:The Habitat of the Eurypterida.djvu/216

208 4. In deposits which, from the study of their lithogenesis can be shown to have come from the same Palaeozoic continents, should be found remains of eurypterids in circumscribed areas as stated in "2" above, and the genera and species, while not necessarily having any near relatives in adjoining deposits, may be identical with forms whose remains are found in a formation perhaps two or three thousand miles distant, but on the same ancient continent. Such relationships are to be accounted for by migration from a common source where the headwaters of two or more river systems interlace (see p. 205 above).

5. The distribution of eurypterids would not have had any necessary connection with those organisms living in marine chronofaunas, and consequently, except when eurypterid-bearing deposits merge into thalassigenous ones, or when fragments or stray eurypterids have been washed out to sea, when intercalation between marine deposits would give the age, eurypterids would not serve as good index fossils.

6. Eurypterids would not suffer rapid changes in evolution, since it is a well known fact, that fluviatile types are often persistent for a long period of time. Thus the cray-fish Cambarus primaevus Packard of the Green River beds (Eocenic) of Wyoming, is a near relative of the modern C. affinis of the same region, a similarity due no doubt to the persistence of the type in essentially the same river basin during the interval.

Zoölogists and palaeontologists who have made detailed studies of the distribution of modern freshwater faunas are thoroughly agreed that accurate results are not to be obtained merely from observations on present distribution. It is an absolute necessity to study the fossil faunas and especially the palæogeography. The reason for this will be evident after a very little thought. If in the Lower Cretacic when there existed the Nearctic continent, comprising most of North America, and continuing across the North Atlantic through Greenland and western Europe, and including the Scandinavian mass, a family of some fluviatile organisms had arisen in the central Canadian area, quickly spreading from one river system to another and finally reaching Europe, we would find in the rocks of that period, that many of the genera on the two modern continents were the same, and that there would be quite a goodly number of identical species. The descendants of these Lower Cretacic organisms would develop on the two continents, (i.e., the two sides of this old nearctic land mass), and the species in the lower reaches of the rivers would diverge in their characters more and more from the parent stock. Those forms