1893.] Cretaceous Plant Population. 339 
There is a tendency for plants of a higher phylogenetic type 
to accumulate in the tension-line area than characterizes 
either of the adjacent formations. Thus in the border of the 
wood and prairie one finds Asters, Helianthi, Solidagos, pre- 
dominant over the lower Archichlamydeae or Monocotyledones. 
There is a transition from typically prairie groups of grasses 
to the typical tension-line groups of species, and similarly from 
the typically forest groups of trees, in the hardwood belt. 
The tension line is characterized by a greater comparative 
richness of species and luxuriance of individuals, in the mat- 
ter of number, than the average of the adjacent formations. 
The tension-line is not a fixed, but a moving line, and 
usually one formation or the other is in a condition of 
encroachment while its adjacent formation is in a condition 
of retreat. 
The movement of the tension-line is felt some distance from 
the contact line of the two formations, so that, if forest is 
encroaching upon prairie, the characteristic composites of the 
tension-line are thrust out farther into the prairie. 
From the interior regions of both adjacent formations plants 
“work out” into the tension-line area. Such plants are gen- 
erally the imperfectly established. 
The last point lends itself very easily to direct experiment. 
If a field of some well established perennial plant, as for - 
instance, blue-grass (Poa pratensis) be selected, and several 
sunflower seeds, or thistle seeds, be planted in the center of it, 
it will happen that one or two plants of the composite species 
may reach maturity, and will then distribute seed outward in 
every direction. In three years time the composites will prob- 
ably be ejected to the periphery of the field, and will have ` 
gained there a considerable foothold, while in the center of the 
field they will have gained but a slight foothold, and perhaps 
will not have retained what they had given them at first. A 
strong formation thus ejects to its periphery the plants that 
not in harmony with it, still strive to establish themselves. In 
the work cited above, I have termed this “the law of the ejec- 
tion of the weaker.” It may be seen operating in any wood or 
field, and is, I believe, one of the most important factors in the 
