32 PLANT ASSOCIATIONS 
three great vegetation types of forest, grassland, and 
desert. A rainfall gradient, on the other hand, will 
cause a progressive change in vegetation type, as 
may be seen in crossing North America from east to 
west, where the forests of the New England States 
give way as precipitation diminishes to the prairies 
of the middle States, and these again to the deserts 
which stretch far over the west. It is only in the 
extreme north that temperature, apart from precipita- 
tion, becomes the dominant influence in determining 
the presence or absence of vegetation, or its char- 
acter. 
Within any one climatic region— say within 
the British Islands—the soil in which the plants grow 
is the controlling factor in determining the character 
of the plant population. And while a classification 
by plant form—such as woodland, grassland—is often 
convenient, when we come to analyze the various 
plant associations which colonize the ground, it will 
be found that similarity of form-type does not 
necessarily imply affinity as regards either physio- 
logical conditions or floristic constituents. Thus, a 
Beech wood on the Chalk has really no affinity with 
an Oak wood on the Coal-measures, save that they 
are both woods: they shelter plant groups of quite 
different composition, one a constituent association 
of the Limestone Formation, and the other of the 
Formation of Clays and Loams, according to modern 
English classification. Similarly, the Hazel copse 
which covers the screes of Farleton Fell has no close 
relation to the Hazel copses along the Westmorland 
becks, although the dominant plant—the Hazel—is 
the same in both cases: soil is the controlling factor, 
