94 



This attenuation of intensities, above and below the surface, pro- 

 duces vertical layers of relatively equal strength or pressure. Thus 

 the attenuation of temperature in gases (air) and in liquids (water) 

 causes different densities in air and in water which modify to an im- 

 portant degree the physical and chemical conditions in these media. 

 This results in their stratification: when the heavier layers are below, 

 stability is the tendency ; and when the reverse order obtains, a 

 change takes place toward the stable condition. With stratification, 

 fiowage tends to occur within the strata, and to be horizontal rather 

 than vertical; additional pressure is therefore necessary to cause the 

 vertical currents or circulation under such conditions. This is why 

 carbonic acid accumulates in the soil and in small deep lakes abound- 

 ing in organic debris, this accumulation being largely due, in both 

 cases, to the slow rate of exchange caused by the stratification pro- 

 duced by differences in density. This same relative stagnation is a 

 primary ^factor in the vertical differences in the relative evaporating 

 power of the air within a vegetable layer of the prairie or the forest. 

 Though on the prairie the vegetational layer is generally but a few 

 inches or a few feet thick, in the forest it is about eighty feet, or 

 more, thick ; and the forest thus influences atmospheric conditions 

 solely as a thick layer of vegetation. 



Differences, then, in the character, structure, or composition of the 

 surface of the substratum are of fundamental importance in under- 

 standing its relative influence upon animals. Primarily these differ- 

 ences are due to temperature, secondarily to temperature in combina- 

 tion with moisture ; and they result in the relative humidity and the 

 relative evaporating power of the air. The most important difference 

 in the surface layer in the Charleston region is that of prairie and for- 

 est, and therefore the main features of these habitats will now be sum- 

 marized. It should not be overlooked that conditions on the prairie are 

 likely to be quite representative of open places in general, though they 

 will probably be somewhat unrepresentative in the case of open places 

 having wet or extremely dry substrata. It is also true that the condi- 

 tions produced by the forest are comparable, in some degree, with 

 those due to the influence of an elevation. 



