344 The American Naturalist. [April, 
explains the matter, so far as general explanations are of 
value. As the Metasperme began to fight their way back into 
the solid coniferous fern and cycadean forest, doubtless the same 
phenomena might have been noticed that are apparent to-day. 
The examination of a region where hardwood forest is encroach- 
ing upon coniferous is especially suggestive. The forerunners 
of the hardwood trees, various herbs and shrubs, insinuate 
themselves up the rivers and tributaries, closely followed by 
the trees themselves. In this way the coniferous plants and 
their numerous allies, the various heaths, cornels, club-mosses, 
etc., are isolated into phalanxes of smaller and smaller size. 
At last they become entirely surrounded by the invading 
types, and at the first opportunity are exterminated altogether. 
Without question similar scenes might have been noticed in 
the ancient Cretaceous forest when the tension-line group of 
plants waxed strong and began the backward march carried 
on through Tertiary time and resulting in the domination to- 
day of archichlamydeous trees in the extra-tropical forest 
regions, while metachlamydeous plants are in their turn 
characteristic of the tension-lines. 
To summarize, in conclusion, what is at best a disconnected 
and imperfect attempt to contribute somewhat toward the 
analysis of a very difficult group of problems. A study of 
modern plant formations reveals the presence of tension-line 
areas in which higher types of plants may become aggregated, 
and where the general tendencies are toward a high degree of 
individual and specific variability. Such tension-lines are 
peripheral in position, and may be of different degrees of 
organization as they lie around formations of greater or less 
degree of integration. Newer types of plants are particularly 
likely to be forced into the tension-line habitat, under the law 
of the ejection of the weaker. Both observation and experi- 
ment show this. Such tension-lines doubtless characterized 
the Cretaceous formations as well as those of to-day. In them 
the weaker, because newer, types were aggregated, and in this 
way the littoral regions of the Cretaceous oceans came in all 
cases to be densely metaspermic in the character of their 
plant-population. The generally low elevation of the coasts 
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