574 The American Naturalist. [July, 
present world under an indefinite variety of aspects, probably 
in Paleozoic times a study of the tensions of one forest would 
have been, in the main, a study of all others. The far more 
homogeneous climatic and physiographic conditions then pre- 
vailing, must have meant almost as striking world-wide simi- 
larity between all forest tracts, as there is now bewildering 
diversity, New forms were far more rapidly dispersed from 
the localities where they originated, and whereverthey migrated 
they found conditions practically similar and hence equally 
favorable. Thus within a comparatively brief range of time, 
closely similar floras might have been found in widely separated 
regions. But another factor came into play at an early period to 
greatly complicate the problem—the physiographic irregulari- 
ties in continental surfaces. The increasing stability of physio- 
graphic features from remote toward modern times, has made 
these features vastly more complicated and diverse now than 
in ages past, and consequently their influence on vegetation 
has become more and more profound. The earliest, as well as 
all the subsequent manifestation of this influence, was 
the development of a second great system of tensions—ten- 
sions between the unlike vegetations of adjacent unlike coun- 
try surfaces, between the swamp and the dryer plain, the flat 
country and the hills the mountain sides and the valleys. 
Here the tensional margin lines. of two diverse hosts of vege- 
tation met and formed another tension line between their own, 
and on this, the struggle for the mastery waxed fiercest, and 
the evolution of highly specialized forms was most active. 
Such were the two tension systems of preeminent import- 
ance in the early history of plant-life; later a third came 
upon the stage, brought into existence through the develop- 
ment of the great climatic zones. Probably this first began to 
assume decided importance,as has been pointed out,sometimein 
the later Mesozoie, and increased the range of its influence 
through the Cretaceous and Tertiary, till in modern times, it 
has culminated in producing the broadest and most funda- 
mental division of the world into great botanical realms. 
That there were regions of glacial cold in Australia, India, 
and Cape Colony in Carboniferous times is an undoubted fact; 
