34 
IRVING W. BAILEY AND EDMUND W. SINNOTT 
leaved species in mesophytic cold-temperate regions may be types 
which (i) have avoided the customary effects of prevailing climatic 
and edaphic influences by hidden ecological or physiological means, 
or (2) have been subjected to the modifying influences of a new 
habitat for too limited a period of time for factors of environment to 
neutralize those of heredity (as, for example, natural selection tending, 
more or less rapidly, to eliminate unfavorable forms; variations pro- 
duced by heterogenesis, orthogenesis, or the inheritance of acquired 
characters) . 
Numerous illustrations of the effects of historical factors, such as 
migration, isolation, etc., have been encountered in tracing the distribu- 
tion of the two types of leaf-margins. For example, the low percen- 
tages of entire-leaved arborescent plants in cold-temperate Eurasia, as 
compared with similar regions in North America, are due in all prob- 
ability to the extermination of many of these forms during the glacial 
period ; and to the fact that mountain ranges and other barriers have 
prevented southern types from migrating northward. 
In this connection, it is interesting to note the following comparison 
between the native plants and the naturalized exotics that are recorded 
in Britton and Brown's flora of the northern United States and 
Canada. Here are included species from as far south as southern 
Virginia, Kentucky, and Kansas. The close similarity between the 
percentages in the two floras seems to indicate that the effects of the 
glacial period have been largely neutralized in North America. 
Table V 
Britton and Brown's Flora (2,365 Native Species) 
Britton and Brown' 
3 Flora (501 Naturalized Exotics) 
Percent 
Entire, Percent 
Percent 
Percent 
6 
24 
35 
49 
Trees 
3 
16 
Shrubs 
14 
80 
Shrubs 
7 
44 
42 
Herbs 
Herbs 
90 
As might naturally be expected, historical influences play a more 
important role in intermediate types of environments. For example, 
the high percentages of entire leaved woody plants in Tasmania and 
southern AustraHa, Table I, may be due in part to environment, but 
it should be remembered that these regions have been long isolated, 
and are not in close contact with extensive mesophytic cold-temperate 
regions as are the warm-temperate areas of the northern hemisphere. 
