OF SOME SPECIES AT THE EDGES OF THEIR RANGES 43 
that originally covered the Sugar Grove region could grow only 
on the cliffs, where the successful forest trees could neither follow 
it nor deprive it of its light. 
We seem, therefore, to have made little progress in the effort 
to determine the causes of the termination of ranges. The feeling 
that it was not caused by such factors as I could observe has also 
been borne in upon me as I have repeatedly studied the plants in 
the field. It has been quite impossible to imagine why a given 
species should be abundant in one valley and absent from the 
next one only a few miles away when all conditions seemed similar. 
It is generally supposed that climatic influences are the most 
important factors in determining plant ranges. By appropriate 
manipulation of climatic data it is possible to draw striking 
parallels with vegetational zones. Thus Merriam* has constructed 
maps from temperature data which conform remarkably closely 
to the life zones of North America. Transeauj likewise by plotting 
the ratio of evaporation to rainfall has made a map conforming to 
the forest areas of eastern North America in scarcely less striking 
fashion. Although dealing with factors to a large extent unre- 
lated to each other, the correspondence of each of these maps 
with biogeography is so striking as to lead almost irresistibly to the 
conclusion that each of them represents the causative factor. 
They are, however, admittedly made to fit the facts of distribu- 
tion and have not the weight that they would possess if con- 
structed a priori by someone unfamiliar with the biogeography of 
orth America. Moreover, it is not possible to state with any 
confidence the climatic requirements of any given species. 
If climate were the principal factor restricting the spread of 
plants the edges of their ranges should exhibit one of two condi- 
tions: (1) Since climate changes vary gradually from place to 
place the controlling conditions should appear very gradually 
and the plants in response finding the conditions of life more and 
more difficult should become scarcer and scarcer and finally fail. 
This, however, happens comparatively seldom in this region. (2) 
The range should be fringed with outliers occupying habitats 
* Merriam, C. Hart. Laws of temperature control of the geographic distribution 
of terrestrial plants and animals. Nat. Geogr. Mag. 6: 229-238. 1894. 
+ Transeau, E. N. Forest centers of eastern America. Am. Nat. 39: 875-889." 
1905. 
