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 engelmannii and Pinus aristata, the forewold is 
Salix pseudolapponum, and the grassland, alpine meadow (Carex-Cam- 
panula-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 thev are moderate, particularly if they are widely separated, 
