meee 
1894.] The Meaning of Tree-Life. 573 
north and south polar regions; whereas, on the contrary, we 
find identical types to the far corners of both hemispheres. 
It is a vitally important consideration that a slight increase 
in general atmospheric humidity would have the effect of 
converting the atmosphere into a heat-distributing oven. 
We cannot indulge in the absurdity of asserting separate 
centers of identically similar development, and we know that 
the torrid zone of even the present would be impassable to per- 
haps 99 7; of our far north temperate flora ; so here is proof suffi- 
cient of relatively great homogeneity in the conditions of the far 
past, and increasing heterogeneity thence down to the present. 
Aside from the greater stability and ruggedness of modern 
continents, the change that has wrought an all important 
effect upon vegetation, has been the development of the 
modern widely extended continental land-areas, producing a 
secular diminution in the general humidity of the earth's 
atmosphere, with the consequent full development of the great 
climatic zones, the polar, temperate, and torrid. Probably in 
the later Mesozoic and early Tertiary, this change began to 
make its influence most strongly felt, and through the Tertiary 
down to the present its effect has steadily and rapidly become 
more and more obvious. The fact is of course not to be lost 
sight of, that the highly specialized Mesozoic and Tertiary 
floras would be far more susceptible than the more lowly 
Paleozoie to climatie changes. But the working of these 
changes has been all-powerful in making most of the problems 
of geographie botany that are before us in the present, and so 
we may here fittingly turn the course of our discussion in this 
direction. 
The progressive changes from the comparative homogeneity 
of conditions in remote ages to the world-wide heterogeneity 
of the present, have been recorded in the development of more 
and more complex tension systems between the various factors 
of vegetation. Of these systems, the most primitive was that 
belonging to each individual forest, —a central stronghold of old 
established types, merging into a tensional margin line of 
newer, weaker forms. Wherever vegetation existed, this ten- 
sion system must have existed; but while we see it in the 
