Dispersal of Herbaceous A ngiosperms. 571 
the flora of northern Europe is much smaller than it was before the advent 
of the glacial invasion, and that the proportion of woody plants to herbs in 
the United States is a much closer approximation to the preglacial ratio. 
There is evidence, however, that even in the latter region woody plants 
suffered heavily during the Ice Age, and that they now form a much smaller 
part of the whole flora than they did before the glacial period. This 
evidence is derived from a comparison of the flora of eastern North America 
and that of eastern Asia. Dr. Gray (6) in 1889 called attention to the fact 
that the floras of these two regions were strikingly similar in many ways, 
and that they contained in common a large number of genera and species 
which were entirely absent in Europe and but scatteringly represented in 
western North America. Dr. Gray’s theory, which has since received 
further support, regards these two very similar floras as remnants of the 
preglacial vegetation of the great north temperate land-mass, a vegetation 
which was pushed south by the advance of the glacier, and which in Europe 
suffered much by extinction. That part of their flora which eastern 
America and eastern Asia to-day possess in common may therefore be 
taken to represent at least a part of that which in ancient times covered the 
arctic lands. A study of its composition as to percentages of herbs and 
woody plants is consequently of much importance in throwing light on the 
composition of the preglacial boreal flora. 
Dr. Gray has published a list of the genera and species of eastern 
North America which are absent in Europe, but which are represented by 
identical or closely related forms in eastern Asia. This list may therefore 
serve as a rough approximation to a portion of the ancient preglacial 
vegetation. It comprises 142 genera of Dicotyledons, of which 70 are 
woody or predominantly so, and 340 species, of which 138 are woody. The 
flora is therefore just about equally divided between herbs and woody 
plants. 
An analysis of the present flora of Japan, one which has probably 
suffered very little, if at all, from glacial invasions and may thus be 
expected to display a ratio between herbs and woody plants much like 
that of the preglacial boreal flora, shows that of the Dicotyledons 
comprised within it only 54 per cent, are herbaceous. 
These two pieces of evidence both point to the conclusion that the 
flora of the great northern land-mass just before the glacial period contained 
a much greater proportion of woody plants than it does to-day, but that even 
in these ancient times about 50 per cent, of the dicotyledonous vegetation 
was herbaceous. We have already brought forward evidence that at the 
close of the Mesozoic, at any rate, herbaceous vegetation was apparently 
very inconspicuous. What was the factor, then, which led to the develop- 
ment of such a large number of herbs in so many families of plants during 
the course of Tertiary time ? 
