conditions which obtained in the past. If there were oceans during Palaeozoic time in which large accumulations of clastic material were forming, we are drawn to the reasonable conclusion that there were land masses from which this clastic material was derived. We must also conclude, if we view the matter rationally, that there must have been rivers on those ancient continents and that then, as now, they constituted the principal agents of transportation of material into the sea. And finally, we must believe that if there was any life in those rivers, it must have been subject to the same laws of dispersal as is the life in the rivers today. My statement does not say that because we have life in the rivers now there must have been life in the Palaeozoic rivers; that is obviously untrue. But if there was life in those rivers, then it was subject to the same laws which are operative now. It is advisable, therefore, to consider these laws and to formulate them that we may have certain definite principles for future reference.
MIGRATION AND DISPERSAL OF RECENT FLUVIATILE ORGANISMS
Of existing taxonomic groups, the fish have received more study than any other group of fluviatile organisms, and interesting as well as exceedingly pertinent data are at hand in regard to migration and dispersal of this group. Günther in his Study of Fishes, makes the general statement that: "The Freshwater fishes . . . . have been spread in circumpolar zones, and in but a limited degree from north to south. No family, much less a genus, ranges from the north to the south, whilst a number of families and genera make the entire circuit round the globe within the zone to which they belong. Not even the Cyprinoids and Siluroids, which are most characteristic of the freshwater fauna of our period, are an exception to this. Temperature and climate, indeed, are the principal factors by which the character of the freshwater fauna is determined; they form the barriers which interfere with the unlimited dispersal of the ichthyic type, much more than mountain ranges, deserts, or oceans" (97, 215).
A few illustrations of this widespread dispersal of fishes in circumpolar zones will show that the above statement is not merely theoretical. These illustrations are selected, but taken verbatim from Günther's work (97, 209–211).
A. Species Identical in Distant Continents. 1. A number of species inhabiting Europe and the temperate parts of eastern North America, as Perca fluviatilis, Gastrosteus pungitius, Lota vulgaris,
