100 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
as that in the vicinity of Orono and Burlington, the higher levels of the 
Santa Catalinas being occupied by pine, spruce, maple, alder, elder, cornel, 
brake, columbine, violet, etc. F‘urthermore, plants in Maine and Ver- 
mont were in about the same condition as those at our upper station 
at this time of year.’’ Results of this kind suggest the value and probable 
extensive use of this method of measuring evaporation as a means of 
determining ecological equivalency, whether at near or remote points, 
a matter of fundamental importance, which as yet has received but little 
attention. 
Recent studies of Transeau (1905, 1908) on the relation of plant so- 
cieties to evaporation further emphasize the value of these methods of 
investigation and the importance of rate of evaporation in determining 
the distribution and succession of plants. Earlier papers by the same 
author go to show that a close relation may be traced between the dis- 
tribution of forests in the United States and the combined figures for 
rainfall and evaporation. With no attempt to minimize the edaphic, 
geographic, and historical factors, it is made evident that climatic differ- 
ences which may be expressed in ratios obtained by dividing the mean 
annual rainfall by the depth of evaporation at the same station, correspond 
in a very striking way with the relative position of forest, prairie, and 
arid plains. 
