OF SOME SPECIES AT THE EDGES OF THEIR RANGES 47 
and successfully reproduce when planted under conditions widely 
different from those of their natural habitats if only competing 
plants are removed. Professor J. H. Schaffner informs me that 
Cactus missouriensis in Kansas at the eastern edge of its range 
is confined to the dry tops of the hills where nothing else can grow. 
But when he transplanted it to a rich shaded yard it grew far 
more rapidly than before and attained twice the size of wild 
specimens as long as grass was kept away from it. 
The case of the hemlock isa good illustratiom of one very im- 
portant factor which has not been touched on—the historical 
factor.* The reasons that the hemlock occupies its present range 
are primarily historical. It is here because this particular species 
and not some other analogous conifer like the Norway spruce 
was present in eastern North America during the Glacial Period 
and because it has not yet been completely displaced by the post- 
glacial flora. It occupies the canyon of Queer Creek not because 
it is better suited for that situation than any other plant of the 
region but simply because the invading hardwood forest has not 
had so good an opportunity to gain a foothold there as elsewhere. 
There are thus apparently fension zones between different 
species somewhat similar to the familiar tension zones between 
plant societies. These zones are as critical in the study of plant 
geography as are formational tension zones in ecology. For in 
these zones it can be seen that some species are advancing while 
others are being beaten back. Just as one can determine the 
course of ecological succession from the behavior of plant societies 
in the tension zones so one can determine the trend of geographical 
movements{ by a study of the tension zones at the edge of plant 
ranges. ; 
And yet there can be no doubt that climatic conditions are of 
fundamental importance in determining the ranges of plants. It 
is apparent that plants tend to spread and would finally come to 
For a discussion of the importance of the historical factor, see Adams, C. C. 
The Postglacial dispersal of North American Biota. Biol. Bull. 9: 53-71. 1905 
+ Adams (Southeastern United States as a center of geographical distribution 
of flora and fauna. Biol. Bull. 3: 115-131. 1902) has insisted that eign 
must be put on a dynamic basis. Bessey, C. E. (Plant migration studies. Uni 
Neb. Studies 5: 11-27. 1905) has shown shine the forest trees are invading sapecaitie 
from all sides, largely on account of the elimination of prairie fires. 
