573 
Dispei' sal of Herbaceous Angiosperms. 
States, and northern Mexico. In Britton and Brown’s ‘ Flora ’ there are 
recorded, as entirely or almost entirely confined to this general region 
of temperate North America, 183 dicotyledonous genera comprising 1,411 
species which, although not including by any means all the genera endemic 
in this territory, may well be taken as a fair sample of them. Only about 
8 per cent, of this presumably ancient portion of the flora, however, is com- 
posed of woody plants, in contrast to the 33 per cent, of such forms in the total 
indigenous flora. In other words, roughly two- thirds of the woody genera 
of the United States also occur in Europe or some other part of the world and 
almost all the endemic genera of temperate North America are herbaceous 
in habit. Quercus , Betula , Populus , Salix , and many other woody plants, 
however, which are common to North America and Europe, and are there- 
fore endemic in neither, and which we know from fossil evidence to be very 
ancient, are in all probability much older than scores of herbaceous genera 
which are not common to both regions and which therefore constitute the 
bulk of the endemic flora of each. It seems much more likely that these 
plants have developed since an interchange of vegetation between Europe 
and North America was interrupted by arctic refrigeration, and that they 
have consequently been limited in their distribution to one hemisphere 
or the other. In short, new varieties, species, and genera are apparently 
produced much more rapidly among herbs than among woody plants. This 
is only natural, since the life-cycle among herbs is annual or at the most 
biennial, instead of covering a much longer period of years as it does 
in woody plants. Therefore, if the amount of change in each generation is 
approximately the same in the two groups, it will accumulate much more 
rapidly in an herbaceous species than in a woody one. Endemism among 
herbaceous plants is consequently an important criterion of antiquity when 
one is considering such plants alone, but is of very much less value in 
determining the relative ages of herbs and woody plants in a flora composed 
of both. Endemism among woody forms, however, may almost always be 
considered an excellent indication of antiquity, because such plants usually 
change with comparative slowness. With this caution in mind, let us com- 
pare the endemic, or presumably ancient, and the non-endemic, or presum- 
ably recent, elements of the floras of various regions with regard to the 
proportion of herbs and woody plants which they contain. 
Insular floras are of particular value for such a study, since they have 
not been exposed freely to a flood of immigration but have developed 
in comparative isolation and therefore possess a large endemic element. 
Very many oceanic islands are known to possess faunas and floras which are 
very ancient in type and which may well be regarded as remnants of the 
organic life of a much earlier period in the earth’s history. 
