which came under the same environmental conditions might, and experience shows that they would develop along parallel lines, appearing in later geological times as similar or even what might be called identical species. In the course of centuries emigrants from an earlier home centre of distribution would pass from the headwaters of one stream to those of another, and soon these forms which had been passing through their individual modifications under one set of environmental factors would migrate down the rivers and mingle with those, forms which had in an earlier period sought the lower reaches of the rivers where a different complex of environmental factors obtained, and there the old immigrants and the new, would come to live in the same waters. A single family, in this way, would give rise to a certain number of primitive genera, some of which would migrate far from the original centre of distribution. The descendants of these early immigrants might, after a long time and after having suffered profound morphological changes, return to mingle with the descendants of the provincial forms which had never left the ancestral region. Now let us think of such inter-changes going on across the Nearctic continent all through the Tertiary until at the close Europe was separated from North America by an advance of the sea. At once we have two separate continents and two river faunas. Were one to try to account for the distribution of the fiuviatile forms now living in the rivers by a study of the present geography, one would be in despair to account for the similarity or seeming identity of many species on opposite sides of the dividing waters. Evidently the only mode of attack is by the study of successively earlier and earlier fossil faunas and by the slow reconstruction of the palaeogeography for each of those periods. One need not search far to find the application of these hypothetical statements to the eurypterids. If they were river-living organisms then it is clearly impossible to explain their distribution in any particular period without considering their distribution in each immediately preceding period. No one has ever done this because each writer tried to account for eurypterid occurrences on a hypothesis of marine distribution.
The results of migration are very different for marine organisms, because of the fundamental difference between the continuity of the seas and the discontinuity of the lands. Marine faunas, especially, the vagrant benthos of the littoral zone and the pelagic ones, tend to be widespread, for they have greater freedom in the size of life districts available, and in the lesser competition, as compared with the
