Dispersal of Herbaceous Angio sperms. 593 
finally forced to leave Antarctica and now constitute part of the endemic 
herbaceous genera of the south temperate zone. That they have not been 
derived directly from northern herbs but are related rather to the ancient 
vegetation, fragments of which are now preserved in oceanic islands, is made 
clear from their floral structure. Colobanthus , for example, has its closest ally 
in A Isinodendron, one of the shrubby Caryophyllaceae of Hawaii ; and Myoso - 
tidium is nearly related to Selkirkici , a woody member of the Borraginaceae 
from Juan Fernandez. 
It is evident, therefore, that the lowland flora of Antarctica, and 
probably the entire flora up to comparatively recent times, agreed with 
those of other portions of the earth which have long been isolated from the 
continental area of the north temperate zone in being composed almost 
entirely of woody plants ; and that the herbaceous element in the vegetation 
of this Polar continent was derived partly from recent immigration out 
of the north and partly from a similarly recent acquisition of the herbaceous 
habit by certain members of the endemic flora. 
The general results of our study of the present distribution of herbs 
throughout the world indicate that up to the early Tertiary they must 
have formed an insignificant portion of the vegetation ; that the bulk of 
herbs originated in the continental area of the north temperate zone in 
response to the progressive refrigeration of the climate during the 
Tertiary; that this aggressive herbaceous flora spread at every opportunity 
into the temperate regions of the southern hemisphere, and that in these 
Antarctic lands a comparatively small body of herbs had an independent 
origin. 
V. Factors in the Development of Herbs. 
We have spoken of the progressive differentiation and refrigeration of 
climate in the north and south temperate zones as the chief factor in 
the development of the herbaceous habit. The first effect of such refrigera- 
tion was doubtless to stunt and dwarf trees into shrubby plants and thus 
shorten the time necessary for them to reach maturity. This process we 
may still see in arctic and alpine regions. This would first affect the more 
susceptible and warmth-loving plants and would reduce them — if they 
lived — with considerable rapidity, until finally very dwarf forms were 
developed which would produce seed in a single year and which would stand 
being killed back every winter. They would be the primitive perennial 
herbs, and it is significant that the present arctic and alpine vegetation is 
composed almost entirely of perennials. The annual herb seems to have 
been developed from such plants under more favourable environments, 
where a plant growing from seed could become large enough in a single 
season to reproduce itself, and did not need a subterranean food-reservoir to 
give it a rapid start. 
