aio ZONES AND REGIONS 



subject to a more or less rapid change, attributable, as regards the flon 

 parti)' to historical causes and partly to the inequality in the climati 

 humidity, but as regards vegetation, to the climatic humidity exclusivel} 

 According to the nature of the atmospheric precipitations, either th 

 woodland, grassland, or desert type predominates, and, excluding th 

 local effects of soil, the type of vegetation changes its character onl 

 on passing to a climate with a different degree of humidit)'. Tracts c 

 land bounded by such climates may be termed districts'^. The varioi 

 districts of vegetation, in contrast to the floristic zones, exhibit ver 

 irregular distribution and very unequal areas. 



High mountains differ, as regards temperature and humidity, climaticall 

 from the lowlands. Their regions defined by temperature are at the san 

 time those defined by atmospheric precipitations. 



Owing to the great differences between the conditions of vegetatic 

 in mountains and lowlands, and to the manifold oecological relations 

 the mountain vegetation everywhere, it appears advisable to separate tl 

 treatment of high regions from that of zones and districts and to devo 

 a distinct section of the book to the vegetation on mountains. Tl 

 sections on zones therefore, except in cases where the exclusion of lo 

 mountains or of the lower regions of mountains would be unnatural, a 

 devoted to the lowlands. 



' See p. 1 60 and ff. 



