206 THE HABITAT OF THE EURYPTERIDA 



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 an- 

 other 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 con- 

 tinuous 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 lead- 

 ing them thousands of miles from their original home" (Gunther, 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" (Gunther, 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, fur- 

 thermore, 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, 



