154 ZONATION 



obvious that an ecofcone separates two different series of 

 zones in the one case, and merely two distinct zones in the 

 other. 



The physical symmetry of a habitat depends upon the dis- 

 tribution of water in it, and this is profoundly affected by 

 the soil and the physiography. The influence of precipi- 

 tation is slight or lacking, as it is nearly uniform through- 

 out the habitat: the effects of wind and humidity are more 

 localised. Differences of soil rarely obtain within a single 

 habitat, though often occurring in a zoned series. The 

 strikingly zonal structure or arrangement of habitats is 

 nearly always due to differences in water content produced 

 by physiographic factors, slope, exposure, surface and 

 altitude. The effect of these upon water content and hu- 

 midity 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 ot 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, the 

 zones produced are obscure. Asymmetry of a habitat or a 

 region practically does not exist. Central areas of excess 

 and deficiency may be very large and in consequence fail to 

 seem symmetrical, or the space between them so great that 

 the symmetry is not conspicuous, but they are everywhere 

 present, acting as foci for the intervening areas. 



The response of vegetation to habitat is so intimate that 

 physiographic symmetry everywhere produces vegetational 

 symmetry, which finds its ready expression in plant zones. 

 The reaction of vegetation upon habitat causes biological 

 symmetry, typical of growth zones and light zones. From 

 these facts, it is clear that zonation will be regularly char- 

 acteristic of the vegetative covering. The zonal arrange- 

 ment of formations is usually very evident: the zones of a 

 formation are often obscured, or, where the latter occupies 

 a uniform central area of excess or deficiency, they are 



