Page:The Habitat of the Eurypterida.djvu/219

Rh understand that a fauna may be made up of individuals which show a fairly close relationship with faunas in neighboring areas, but may contain one species which is identical or nearly so with a species in a fauna three thousand miles distant. If these were marine fossils we could not understand such a thing, because marine faunas show whole groups of species in one region related to groups in another, and contemporaneous marine deposits in the path of migration show similar related groups. But the routes of migration for river forms would almost never be shown to us in the rocks, because rivers in their upper and middle portions degrade and would continually be carrying away the traces of their history which would be recorded only in deltas or flood plains. Thus, contemporaneous and related fluviatile faunas would appear geographically at the outer ends of the spokes of a great wheel which has its hub at the centre of dispersal. The remains of synchronous faunas would of necessity appear scattered over the face of the earth, without any apparent connection; a fact which would be inexplicable if the faunas were interpreted as marine. The only way to solve the problem of the distribution of those forms would be through a study of the palæogeography of the period in which they occurred and of all preceding periods in so far as was possible.

When stratigraphers come fully to appreciate the value of continental deposits and faunas, they will have taken a big step toward the unravelling of the palæogeography of our earth. No one would attempt to restore the conditions of land and sea in the Tertiary without making use of the migrations of mammals and other terrestrial organisms, for it is evident that while a study of marine faunas will show the position of the oceans and epicontinental seas in any period, the exact configurations of the continents, the exact location of land barriers and connections can only be determined by the migrations of the animals and plants living on the land or in the rivers. This applies with as great truth to the Palaeozoic as to the Tertiary, and while the aid of plants cannot there be invoked until the end of the period, I hope to show before concluding this paper that the eurypterids will be of vast service in helping to locate Palaeozoic rivers and routes of migration from one continent to another.