166 Distribution of the North American Flora. [ March, 
appears to me to be at variance with the fact that when imported 
into it they thrive luxuriantly. 
The explanation which I have to offer will be best understood 
by a reference to the section (p. 163), which shows the western half 
of the continent to be enormously elevated as compared with the 
eastern, and to have been singularly adapted for the retention 
of vast bodies of ice for long after the Glacial period. We find 
. there a valley (the desert region), upwards of four hundred miles 
- broad, and upwards of four thousand feet elevation, with many 
ranges of over eight thousand feet in it, bounded by broad and 
lofty mountains, together occupying at least two-thirds of the 
breadth of the western half of the continent. We further know 
that these mountains were clothed with ice during the Glacial 
epoch, and that the valley was then occupied by a vast lake; for 
on the uppermost of the many shelves which the retiring waters 
of this lake cut on the flanks of the Rocky mountains and Sierra 
Nevada, the skull of the musk-ox, the most arctic of land quad- 
rupeds, has been found. 
It is obvious that this whole western region must have retained 
its glacial mantle for an incalculable period after Eastern America 
had been sufficiently warmed to admit of the northward return of 
the plants that had been driven southward in it; and that this 
glaciated condition must have effectually barred a similar return 
of the same plants in those western meridians, these must have 
perished, in short, on reaching Southern California. Long.ages 
after, when the western ice disappeared, and the climate of the 
valleys warmed, the Mexican and more southern plants would, as _ 
a matter of course, take possession of the unoccupied soil, and 
advance northward till they encountered the boreal vegetation of 
North-western America, with which they now commingle. 
I have said that the extinction of East Asiatic types in Western 
America was not total; a few escapes are found in the valleys of 
the Rocky mountains and Sierra Nevada,! and also along the 
coast of the Pacific, the warming influence of which favored their 
preservation during the northern migration. 
The Sequoias.—Two instances of these escapes are of such inter- 
est that I shall, in concluding this lecture, bring them under your 
1 And also on the highlands of Central Mexico, where some Asiatic types remain 
which have not migrated farther north or south in Anterica. Such are the — 
Asiatic genera Bocconia, Meliosma, Photinia, Cotoneaster, Deutzia and Abelia i 
