CH. XIV] FORMATIONS AND ASSOCIATIONS 583 



In the study of the constituents of the vegetation of a 

 country, it is customary to make records partly by photog- 

 raphy, and partly on maps, showing by suitable signs the 

 areal and zonal distribution. Particularly effective, how- 

 ever, are diagrammatic TRANSECTS, or optical sections across 

 vegetation and habitats, whereby may be shown vividly 

 both the leading features of the vegetation form, and also 

 the association of the different forms. 



It is a familiar fact that while the principal formation in 

 some countries is forest, in others it is prairie, or desert, etc., 

 though minor formations may be similar in all. It is well 

 known that climate, i.e. temperature and rainfall, determine 

 the nature of these greater formations, which therefore are 

 often called CLIMATIC FORMATIONS. The minor ones, how- 

 ever, marsh, swamp, etc., are determined rather by local 

 soil conditions, and are called EDAPHIC. The distinction has 

 its chief importance in this fact, that while the climate of a 

 country, and therefore the nature of its principal formation, 

 alters very slowly, the soil conditions, and therefore the 

 edaphic formations, are in much more rapid change. The 

 change, however, is in a definite direction, because, for 

 geological reasons, the physical conditions are tending to 

 reduce a country to a base level, i.e. hills are being leveled, 

 lakes filled, etc. Accordingly the formations are likewise 

 approximating always towards one kind, which is the leading 

 formation permitted by the climate. Thus the vegetation of 

 each country is all tending towards a uniform climatic climax 

 formation. In New England this is a mixed forest, further 

 south a deciduous forest, farther north an evergreen forest, 

 while in the west, it is prairie, plains, or desert. 



Since formations are made up of vegetation forms, which 

 depend upon fitness to habitats, it ought to- be true that 

 widely separated countries which have similar conditions of 

 climate and soil should have similar vegetation forms and 

 formations, even though the floras are entirely different. 

 This is actually true, as shown by the fact that the vegetation 



