468 The American Naturalist. [June, 
has been steadily waxing in importance from primeval times, 
while the other has begun to wane. In the midst of the 
Silurian jungle, where the energy of plant-life was strained to 
the utmost limit of activity, new forms originated. What was 
their fate? They could not overpower the strongly established 
older forms crowding all about them, so either they must per- 
ish or push outward toward the open margin, where there was 
room to fight. Thus the swamp-margin became the tension- 
line between the uninhabitable higher land and the old strong 
hold of the jungle, and on this tension-line stood the vanguard 
of the world’s future vegetation. On its outer edge the tension- 
flora faced a new and untried habitat, and then, as now, & 
highly specialized habitat meant highly specialized inhabit- 
ants. The untried ground could not be conquered by sheer 
force of vegetative luxuriance, for by their very nature the new 
conditions were physically opposed to the established order of 
things in plant-life. The all-powerful factor in accomplishing 
the conquest was increased capacity for variation and the adap- 
tive evolution of oldstructural types into higher stages of organi- 
zation. Clearly, this tendency predominated and pervaded the 
whole tension-line flora, but its maximum was toward the outer 
edge. So here were ranked the low-growing herbaceous fore- 
runners of coming ages,—forms that were humble in- their 
growth, because of the physical obstacles opposing them ; and 
highly specialized, because their structure did not possess the 
obstinate stability of the patriarchal tree-life behind them. 
For the same reasons the character of the undergrowth in the 
jungle must have always been ages in advance of the arboreal 
monarchs towering overhead. But on the inner side of the 
tension-line, vegetative luxuriance was not only possible and 
potent but also obviously a necessity, for there could be no 
abrupt demarkation between the marginal and central region 
Here, then, where the jungle-flora merged into the tension-flora, 
was the stronghold of the rising generations, the newer higher 
types, of tree-life. Here, in early Silurian times, must epi 
stood the ancestral types of the great tree-ferns and calamites 
and conifers that were. to be supreme in the Carboniferous 
and early Mesozoic. 
