Page:The Habitat of the Eurypterida.djvu/193

Rh Devonic fishes of North America here referred to have been shown, from their occurrence and distribution, to be mostly if not entirely fluviatile (Grabau 87, 88).

Macnair and Reid have with great justification brought forward many objections to the "Lake theory" advocated by Geikie, but their logic fails them when they contend that because the Old Red fish and eurypterids could not have been lacustrine forms, therefore they must have been marine.

The river origin seems never to have occurred to these two writers, or else if it did they considered that the same objections were open to it as to the lake origin. One of the arguments which they advance against the lake theory is the difficulty of the origin and distribution of the fish and eurypterids. They argue thus: these forms were present in the Siluric and so it is not strange that they should occur also in the Devonic; "but of the genera Osteolepis, Dipteris, Glyptolepis, and other fishes of the Old Red Sandstone no undoubted plates or scales occur in the preceding formation. The question therefore arises, whence came these highly organized fishes of the Old Red Sandstone? More especially, from what fresh-water region did they migrate? Not only so, but as the same genera of fishes occur in the Devonian of North America and the St. Lawrence basin, we have an equal right to know by what fresh-water pathway of distribution they were enabled to migrate some 3000 miles between one point and another" (160, 218, 219). But surely such facts of distribution should not be distressing; many a case could be cited in the recent fresh-water fish fauna of the same genera occurring more than 3000 miles apart, and with perhaps no related genera in the intervening area. One may mention the case of the genus Umbra, a form so peculiar as to be made the type of a family in which are only two species, these being most closely allied, and yet one occurs in the rivers of the Atlantic states of North America and the other in the Danube system, some thousands of miles distant. Even more remarkable is the genus Scaphirhynchus among the sturgeons, which likewise has two species: one in the Mississippi system, the other in Central Asia. In the same family is the genus Polyodon, with two species only, one in the Mississippi, the other in the Yangtse-kiang. But one need not confine the illustrations to genera which are identical in distant regions; species offer even more surprising examples. Perca fluviatilis, Gastrosteus pungitius, Lota vulgaris, Salmo salar, and many others might be mentioned, inhabiting both the rivers of eastern North America and of Europe. For