Page:The Habitat of the Eurypterida.djvu/214

206 Lakes trout, Salmo macdonaldi. Always the form next away from the parent stock is onward in space across the barrier" (Jordan, 134, 547). Migration from the headwaters of one system to those of another only a few miles distant is accomplished: (a) as a result of river capture, (b) by the accidental transportation of the eggs of fishes, by birds, from one stream to another, (c) by the temporary formation of connecting streams or lakes between two river systems in a period of torrential rains, (d) by the temporary or permanent shifting of the watershed between two systems by a slight geological change, (e) by actual migration of fishes over areas where there are not continuous waterways. "Some fishes, provided with gill-openings so narrow that the water moistening the gills cannot readily evaporate; and endowed, besides, with an extraordinary degree of vitality, like many Siluroids (Clarias, Callichthys), eels, etc., are enabled to wander for some distance over land, and may thus reach a watercourse leading them thousands of miles from their original home" (Günther, 97, 212).

3. A shallow body of salt water between two continents may, by a very slight negative eustatic movement, be drawn off and a dry land connection will be afforded which will enable easy migration for freshwater fishes from one continent to the other. A subsequent positive eustatic movement would conceal the route of migration and one would have to deal with some apparently inexplicable occurrences of identical species.

4. "From the great number of freshwater forms which we see at this present day acclimatised in, gradually acclimatising themselves in, or periodically or sporadically migrating into, the sea, we must conclude that, under certain circumstances, salt water may cease to be an impassable barrier at some period of the existence of freshwater species, and that many of them have passed from one river through salt water into another" (Günther, 97, 211).

These facts which have been found out in connection with the distribution of freshwater fish of the present are essentially true for those inhabiting the rivers of all earlier continents. They may, furthermore, be considered as equally true for the eurypterids who were highly organized gill-breathers and many of whom were powerful swimmers. While they lacked one of the modes of transportation from the headwaters of one river system to those of another in not having the possibility of accidental portage by birds they had, on the other hand, a far more important means, for they had walking legs,