6 BIG SPRING PRAIRIE. 
The writer wishes to extend his thanks to Dr. H.C. 
Cowles, of the University of Chicago, under whose direc- 
tion the investigation of this area was conducted: and 
also to Prof. W. A. Kellerman, Ph. D., of the Ohio State 
University, for some of the photographs, and for his per- 
sonal visits to the area and his kind assistance in various 
ways. ‘he writer is also under obligation to Mr. W. 
C. Johnson, Mr. Peter Brayton, and Mr. H. B. Phelps 
for aid rendered or information furnished. 
The principal factors which must be considered in 
prosecuting the lines of investigation indicated naturally 
divide themselves into Climatic, Physiographic, His- 
torical, and Ecological. 
