Page:The Habitat of the Eurypterida.djvu/186

178 fissures and other structural features to be mentioned below was taken as indicative of the mud flats along shore, which were from time to time inundated by the waters of the lake. Plant, insect, and crustaceous remains, as well as the abundant fish fauna, were correctly pointed out as showing the near presence of land. The distinctness of the fish and merostome faunas in the Caledonian and Orcadian rocks was cited as proof of the distinctness of the lakes in which these organisms had lived. It is surprising, and therefore, worthy of note, that Geikie came so very near to the recognition of the Old Red fish and eurypterids as river dwellers that one marvels at his not having reached that conclusion. The arguments which he cites to account for the differences of the ichthyic fauna of his Lake Orcadie and Lake Caledonia, which were supposed to have been separated by the Grampians, are illustrations taken from modern river faunas; and, if application were made directly to the Old Red faunas, one would have to say that the fish in the two Devonic lakes were different because they came from rivers whose headwaters were separated by a divide. I shall give Geikie's statement in order to show how near he came to the discovery that the Old Red Fauna came from the rivers, and how he failed to realize this because he was so intent on the theory of lakes.

"In the second place," he says, "there does not seem to be any valid reason why the ichthyic fauna of two adjacent but completely disconnected water-basins should not have differed considerably in Old Red Sandstone times, as they do at the present day. Even in the same river-system it is well known that the fishes of the higher portions of the basin are sometimes far from corresponding with those in the maritime parts of the area. Neighboring drainage-basins, divided by a comparatively unimportant watershed, sometimes show a remarkable contrast in their fishes. This has been well pointed out by Professor E. D. Cope, in a suggestive paper "On the Distribution of Fresh-water Fishes in the Alleghany Region of South-western Virginia." The James and Roanoke rivers descend the eastern slope of the continent and discharge into the Atlantic. In their upper waters they have only four species of fish in common. In the upper waters of the rivers Holston and Kanawha, which flow south-westwards into the Mississippi basin, there are only two species alike. Between those eastern and western pairs of rivers runs the more marked water-parting of the Alleghany chain. Out of fifty-six species