PHYSICAL GEOGRAPHY OF THE PLEISTOCENE 597 
floras more difficult than in the eastern part, though perhaps less 
difficult than in Europe. In eastern Asia and in eastern America, 
where migration was relatively easy, the forest flora is much larger 
than in Europe or western North America. Thus Atlantic America 
and Pacific Asia have each 66 genera of forest trees, while Pacific 
America and Europe have but 31 and 33 genera respectively, and the 
number of species is approximately in keeping with the number of 
genera. 
Vulcanism has been regarded as a factor which decreased the flora 
in the western part of North America as compared with the eastern; 
but since the floras were much the same throughout the Tertiary in 
all northern lands, and since the climax of Cenozoic vulcanism came 
as early as the Miocene, the importance of this factor in impoverishing 
the Pleistocene life of the western part of the United States may be 
questioned. Furthermore, it has little or no application to Europe, 
where the flora was equally reduced. 
The to-and-fro movements of the land faunas and floras must have 
introduced an elaborate series of superpositions, giving an elaborate, 
orderly, and unusual succession. ‘The record of this succession has 
not been worked out in its completeness, and unfortunately there is 
little chance that it will be worked out in its fulness unless by the 
most persistent care. In the regions which were glaciated repeatedly, 
the advance of each ice-sheet destroyed, in most places, the record of 
the successive floras and faunas which had lived since the preceding 
retreat, so that, within the area glaciated, the succession of successions 
is hardly likely to be found in its entirety in any one place, and perhaps 
not in all places. Outside the area which was glaciated, especially 
near the borders of the regions occupied by the successive ice-sheets, 
there is better chance that a complete record of the biological changes 
may have been preserved. The peat bogs of such regions might be 
expected to give complete records if they had endured continuously 
since the time of the first glaciation; but peat bogs are themselves 
temporary, and it is perhaps too much to expect that complete record 
of the migrations of life during the successive epochs of the glacial 
period will ever be found at any one place. 
The records, however, of the post-glacial peat bogs are such as to 
give some indication of the results which would probably be found 
