Dispersal of Herbaceous A ngio sperms. 569 
Turning to living plants, we find that a comparison of the percentage 
of herbs in the eastern United States and in Europe north of the Alps (see 
previous table) shows that in the former region about 24 per cent, of the 
dicotyledonous flora is woody, but in the latter only about 13 per cent. 
Professor Gray many years ago called attention to the paucity of trees and 
shrubs in the European flora compared with the American, and suggested 
that the difference was due to the fact that the Alps and the Mediterranean 
prevented the southward escape of the European flora at the advent of the 
glacial period ; but that in America the preglacial flora was able to migrate 
freely to the south and suffered much less extinction. This conclusion, that 
the European flora was decimated relatively more in the glacial invasion 
than was the American, has been suggested by several other writers and 
seems to be a sound one, since we know from the foregoing fossil evidence 
that many families of plants existed in Europe in the middle and latter 
parts of the Tertiary which are now' found there no longer. Many of these, 
however, such as the Ebenaceae, Menispermaceae, and Lauraceae, still exist 
in the United States. Dr. Gray believed that herbaceous plants in Europe 
had lost as much by extinction as had woody ones from the ice invasion, but 
a comparison of the percentage of herbs in the two regions apparently indi- 
cates that such has not been the case, but that woody plants have suffered 
proportionally much the more. This is also suggested by the fact that, 
although almost all the herbaceous families of Dicotyledons in temperate 
North America are represented in Europe north of the Alps (the only ex- 
ceptions being herbaceous members of the Acanthaceae, Capparidaceae, 
Loasaceae, Melastomaceae, Nyctaginaceae, Passifloraceae, Phytolaccaceae, 
and Sarraceniaceae), there are sixteen families of trees, shrubs, and woody 
climbers which are not indigenous to the latter region. These are the 
Anonaceae, Bignoniaceae, Calycanthaceae, Cyrillaceae, Ebenaceae, Ilama- 
melidaceae, Juglandaceae, Lauraceae, Leitneriaceae, Magnoliaceae, Meni- 
spermaceae, Platanaceae, Sapindaceae, Sapotaceae, Styracaceae, Ternstroe- 
miaceae. The only families in Europe which do not occur in America are 
the Resedaceae and Dipsacaceae, both herbaceous. 
It seems quite evident, therefore, that herbs have proven themselves 
much better able to resist unfavourable conditions, such as enforced migra- 
tion and an increasingly cold climate, than woody plants. This is, of 
course, what might naturally be expected from the fact that herbs, which 
are able to reproduce in a single year from seed, can thus be disseminated 
much more rapidly, and, above all, can adapt themselves to a cold climate 
by living over the periods of low temperature underground or in the form 
of resistant seeds. 
This ability of herbaceous plants to thrive in cold climates is also 
emphasized by a study of alpine and arctic floras. We have already stated 
the percentage of herbs in the dicotyledonous floras of Switzerland, the 
