44 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



forward by the paleobiologists, in the presence of common fossils or the 

 absence of groups in definite localities, but in some cases, at least, the 

 physical geologists have given equally important evidence. The continent 

 which occupied the North Atlantic Ocean basin is vouched for by both and 

 its presence up to at least mid-Miocene fully accepted. The Mediterranean 

 Tethys is equally well established. Gondwana Land and the Antarctic 

 connection between Africa, Australia, and South America depend more 

 definitely upon biological evidence and await full confirmation. 



For a discussion of the general principles of the subject we may follow 

 the majority of geologists in accepting the permanence of the great conti- 

 nental blocks and ocean basins and so dismiss for the time all questions of 

 the movement of life forms upon the blocks or within the basins. Only 

 the cases where "bridges" have been suggested as existing across permanent 

 ocean basins need be mentioned. 



The condition of the surface of the earth before Mesozoic time is, to say 

 the least, uncertain. The tetrahedral theory suggests a practical reversal 

 of the land and water conditions; 1 in the Paleozoic the bulk of the land lay 

 in the southern hemisphere; after the Paleozoic it lay in the northern, 

 but the great Mediterranean Tethys lay always in an approximately equa- 

 torial position. If it shall ever be fully demonstrated that this reversal 

 took place, we shall have a rational explanation for the proposition that 

 land life of the Paleozoic developed largely in the southern hemisphere and 

 migrated northward, while in the Mesozoic and Cenozoic the land life 

 reached its maximum in the north and pressed southward. Somewhere 

 must be found the bridge across the Tethys. Perhaps the edges of the tetra- 

 hedroid, which would be in the same position before and after the reversal, 

 will reveal the clue. The deepening of the ocean basin in the Mesozoic, 

 suggested by Walther, and the contraction and elevation of the continental 

 blocks, suggested by Wegener, 2 account for the present wide separation of 

 the land by uncrossable barriers of deep sea; but these are but develop- 

 ments of previous conditions and it is not proven that the deepening of the 

 basins took place in the Mesozoic. 



The uncertainty as to the mode of origin of geosynclines and their later 

 elevation into mountains of sedimentary rock introduces another element of 

 uncertainty. Haug believes that such geosynclines lie between great conti- 

 nental masses and would use their presence as an evidence for the former 

 existence of a continent in the Pacific Ocean basin ; others contend only for 

 their presence upon the edges of continental blocks. However this may be, 

 such geosynclines to-day, so far as we can now determine, lie upon the edges 

 of continental blocks and are the origin of bordering mountain ranges, 



1 Gregory, J. W., The Plan of the Earth and Its Causes, Geog. Jour., vol. xm, p. 225, 1899. 



Reprint in Annual Report Secretary Smithsonian Institution for 1898, p. 363. 



2 Dacque, E., Grundlagen und Methoden der Palaogeographie, chaps, iv, v, and vi. 



