568 report — 1884. 



in the intermediate region, rendering the general absence the more note- 

 worthy. And certain peculiar types are represented in single identical 

 species on the coasts of Oregon and Japan, &c. (such as lysichiton, fatsia, 

 glehnia) ; yet there is less community between these floras than might be 

 expected from their geographical proximity at the north. Of course the 

 high northern flora is not here in view. 



Now, if, as I have maintained, the eastern side of North America and 

 the eastern side of Northern Asia are the favoured heirs of the old boreal 

 flora, and if I have plausibly explained how Europe lost so much of its 

 portion of a common inheritance, it only remains to consider how the 

 western side of North America lost so much more. For that the missing 

 types once existed there, as well as in Europe, has already been indicated 

 in the few fossil explorations that have been made. They have brought 

 to light magnolias, elms, beeches, chestnut, a liquidambar, &c. And 

 living witnesses remain in the two sequoias of California, whose ancestors, 

 along with taxodium, which is similarly preserved on the Atlantic side, 

 appear to have formed no small part of the miocene flora of the Arctic 

 regions. 



Several causes may have conspired in the destruction ; — climatic 

 differences between the two sides of the continent, such as must early 

 have been established (and we know that a difference no greater than the 

 jDresent would be effective) ; geographical configuration, probably con- 

 fining the migration to and fro to a long and narrow tract, little wider, 

 perhaps, than that to which it is now restricted ; the tremendous out- 

 pouring of lava and volcanic ashes just anterior to the glacial period, by 

 which a large part of the region was thickly covered ; and, at length, 

 competition from the Mexican plateau vegetation, — a vegetation beyond 

 the reach of general glacial movement from the north, and climatically 

 well adapted to the south-western portion of the United States. 



It is now becoming obvious that the Mexican plateau vegetation is the 

 proximate source of most of the peculiar elements of the Californian flora, 

 as also of the southern Rocky Mountain region and of the Great Basin 

 between ; and that these plants from the south have competed with those 

 from the north on the eastward plains and prairies. It is from this source 

 that are derived not only our cactea? but our mimoseas, our daleas and 

 petalostemons, our numerous and varied onagraceae, our loasaceaa, a large 

 part of our composita;, especially the eupatoriaceas, belianthoidea?, hele- 

 nioidea?, and mutisiaceas, which are so characteristic of the country, the 

 asclepiadeas, the very numerous polemoniacea?, hydrophyllaceaa, eriogonere, 

 and the like. 



I had formerly recognised this element in our North American flora ; 

 but I have only recently come to apprehend its full significance. With 

 increasing knowledge we may in a good measure discriminate between 

 the descendants of the ancient northern flora, and those which come 

 from the highlands of the south-west. 



