886 THE AMERICAN NATURALIST. (Vor. XXXIX. 
also be stated that since climates are constantly changing and 
effects may lag far behind their causes, no map of present cli- 
matic conditions can hope to do more than approximate the 
present distribution of plants. Geographic and historical rela- 
tions must be constantly borne in mind. 
SUMMARY. 
It may be stated, by way of summary, that eastern North 
America is occupied by four great forest centers: (1) the North- 
eastern Conifer forest, centering in the St. Lawrence basin, (2) 
the Deciduous forest, centering in the lower Ohio basin and 
Piedmont plateau, (3) the Southeastern Conifer forest, centering 
in the south Atlantic and Gulf coastal plain, and (4) the Insular 
Tropical forest of the southern part of the Florida peninsula, 
centering in the West Indies. The term center as here used, 
implies the idea of distribution about a region where the plants 
attain their best development: Such vegetation divisions are 
not fixed, but move and increase or decrease in extent depending 
upon continental evolution and climatic change. 
Each formation is made up of many societies, bearing a defi- 
nite successional relationship to one another, which being 
dependent upon soil factors may be best correlated with phy- 
siographic changes. In regions intermediate between centers, 
the local order of succession is made up of societies from each 
of the adjoining formations. j 
It has been found that if the ratios produced by dividing the 
amount of rainfall by the depth of evaporation for the same 
station, be plotted on a map they exhibit climatic centers which 
correspond in general with the centers of plant distribution. 
Further, the distribution of grassland, prairie, open forest, and 
dense forest regions is clearly indicated. 
This is explained by the fact that such ratios involve four 
climatic factors which are of the greatest importance to plant 
life, vzz., temperature, relative humidity, wind velocity, and 
rainfall. 
ALMA COLLEGE. ALMA. MICH. 
