22 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



Equally good examples occur in the various deposits of Devonian time, 

 when invasions from the Arctic, across Hudsons Basin and from the north- 

 west, or from the Gulf of Mexico, successively penetrated to the center of 

 the continental surface. 1 



Here arises at once the question of the source and routes of movements 

 of the migrant forms. The determination of the identity or distinctness of 

 two faunae, as the resident and the migrant, depends upon their composition 

 and immediately introduces the question whether a fauna is to be charac- 

 terized by the similarity of a majority of its species to those of another 

 region (matching of species) or by the presence of a few unique forms. 

 These questions have been fully discussed by competent authorities and is 

 further considered under the subject of correlation below. 2 



The sudden appearance in a bed of new forms similar to the original 

 ones, but recognizable as migrants from another region, and the absence 

 of new types of life implies very similar conditions of water, temperature, 

 food-supply, etc., in the old and new homes of the migrant forms and in all 

 intervening places on the route of migration. The only disturbing factors 

 would be those arising from competition between resident and migrant 

 faunae. Such an invasion could only arise when the migrations were made 

 possible by very gentle movements between regions similar in all general 

 conditions. It is not unlikely that similar temperature conditions prevailed 

 widely over the earth in Paleozoic times, 3 but differences in bottom, food- 

 supply, and so forth, could easily vary as the waters bordered on different 

 terranes. These suggestions will be made plainer by a consideration of the 

 faunae of any part of the continuous seacoast of any continent to-day. 



Shore-lines extending across latitude lines have very different faunae, 

 controlled by temperature, though other factors are also present, and 

 ocean currents parallel to the coast may extend the range of faunae to the 

 north or south beyond the effect that would be produced by latitude alone. 

 The effect of the Greenland Current and the Gulf Stream on the east coast 

 of North America is a good example. The use of this principle in paleo- 

 geography is illustrated by Willis's paleogeographic maps, in which an 

 attempt is made to indicate the course of currents. 4 



It must be recognized that the migration of invertebrates is largely 

 accomplished by currents. The free-swimming forms would be borne by 



1 See Ulrich's Revision of the Paleozoic Systems and Schuchert's Paleogeography of North 



America, already referred to, for many instances. 



2 Ulrich, E. O., Revision of the Paleozoic Systems, Bull. Geol. Soc. Amer., vol. 22, p. 506, 



1911. 



Williams, H. S., Bearing of Some New Paleontologic Facts on Nomenclature and Classi- 

 fication of Sedimentary Formations, Bull. Geol. Soc. Amer., vol. 16, p. 137, 1905. 



Smith, G. P., Principles of Paleontologic Correlation, Jour, of Geol., vol. VIII, p. 673, 1900. 



3 White, David, and F. H. Knowlton, Evidences of Paleobotany as to Geological Climate, 



Science, vol. 31, p. 760, 1910. 



4 Willis, Bailey, and R. H. Salisbury, Outlines of Geologic History. Maps by Willis, 1910. 



