278 THE FORMATION 



different series of zones in the one case, and merely two distinct zones in the 

 other. 



336. Physiographic symmetry. The physical symmetry of a habitat 

 depends upon the distribution of water in it, and this is profoundly affected 

 by the soil and the physiography. The influence of precipitation is slight or 

 lacking, as it is nearly uniform throughout the habitat; the effects of wind 

 and humidity are more localized. Differences of soil rarely obtain within 

 a single habitat, though often occurring in a zoned series. The strikingly 



Fig. 73. Regional zones on a spur of Pike's Peak (3.800 m.) ; the 

 forest consists of Picea engclmannii and Pinus aristata, the forewold is 

 Salix pscudolapponum, and the grassland, alpine meadow (Carex-Cam- 

 t>anula-coryphium). 



zonal structure or arrangement of habitats is nearly always due to differ- 

 ences in water-content produced by physiographic factors, slope, exposure, 

 surface, and altitude. The effect of these upon water-content and humidity 

 is obvious. Wherever appreciable physiographic differences occur, there 

 will be central areas of excess and deficiency in water-content, between 

 which there is a symmetrical modification of this factor. Peaks are typical 

 examples of areas of deficiency, lakes and oceans of areas of excess. When 

 these areas are extreme and close to each other, the resulting zonation will be 

 marked; when they are moderate, particularly if they are widely separated. 



