44 PRELIMINARY INQUIRIES. 



fornia and Oregon, and only sucli a portion of it as might be able to cross the ridge or sea woidd 

 succeed in making good its footing on the eastern side of North America. 



I conceive that the Missouri sea must have been the more important obstacle of the two ; 

 because the place where that sea lay is to this day the limiting boimdary between various 

 species inhabiting the country to the east and west of it. That sea, however, was interrupted near 

 the south, and by that interruption Eastern America doubtless received many of its species. The sea 

 appears to have run up straight to the Arctic Ocean in the line of the Mackenzie River ; but I have a 

 strong expectation that it will be found, when the regions to the north of Vancouver's Island are 

 thoroughly examined, that this sea had another communication with the Pacific perhaps not very 

 far north of or on the site of the tertiary beds near that island, which acted as a barrier isolating the 

 strip of land which lies along the coast of the Pacific between the mountains and the sea. This 

 seems probable from the following facts in the distribution of Asiatic plants in America, or of 

 American types in Asia. 



Professor Gray has shown that the relations of the flora of Eastern Asia (as more particularly 

 expressed by that of Japan) with that of the United States, east of the Mississippi are j)eculiarly 

 intimate. This is evinced by the great number of congeneric, closely representative, and identical 

 species in the two floras. Also that although there is a considerable number of species com- 

 mon to the western side of the American continent and to Japan, yet that the likeness is 

 less strong between their floras than between those of Eastern North America and Japan. 



On the other hand, large American genera (such as Eupatorium, Aster, Solidago, Solanum) 

 are represented in Asia by a small number of species, which diminish or disappear as we approach 

 the Atlantic limits of Europe, whilst the tj'pes peculiar to the extreme west of Europe are wholly 

 deficient in America. " The deficiency," says Dr. Gray, " in the temperate American flora of forms 

 at all peculiar to Western Europe is almost complete, and is most strikingly in contrast with the 

 large number of Eastern American forms repeated or represented in Eastern Asia." 



Professor Gray thus accounts for these facts. First, he adopts the theory that a more extended 

 homogeneous and uniform distribution of plants than we now have existed previous to the glacial 

 epoch ; that during the continuance of thgt epoch, the northern types migrated, or were 

 driven southwards, (although he does not seem to accept the idea of their being driven so com- 

 pletely out of the coimtry as I have supposed), and that, on the retreat of the ice before the 

 returning warmth, the temperate flora which had sui'vived the cold returned to the north, fol- 

 lowing the steps of the ice pari passu; and, what he considers an important point, that they 

 must have advanced further north, and especially north-westward, than they now do, so far, 

 indeed, that the temperate flora of North America and Eastern Asia must have become con- 

 terminous. He- then supjjoses that an epoch, called by Dana the Fluvial epoch, followed the 

 Glacial epoch, which, from whatever cause, was of a milder character than our present climate, 

 as he thinks is proved by the remains of species of Megatherium, Mylodon, Megaxonyx, Mas- 

 todon, and the Mammoth, having been foimd in the deposits of that period. He argues that all 

 the facts known to us, even to tlie limiting of the drift, show that the configuration of the two 

 continents was nearly the same then as now, and the isothermal Knes curved as now (which, 

 so far as regards the isothermal lines, the reader will see to be correct by comparing the limits of 

 the glacial action in Europe and America shown in Blap 4) ; that such a more genial climate would 

 commingle the temperate floras of the two continents hy Bhering's Straits, or perhaps by the still 

 shorter route of a tract of land between Kamtschatka and the Aleutian Islands. 



