26 



tive equilibrium resulting from the balancing of all stresses near sea 

 level. 



In the preceding discussion no emphasis has been placed on the fact 

 that degradation of the land is only a part of a large cycle of activity, 

 and that the deposition of the degraded materials may he a cause of so 

 much stress as to initiate an elevation of the land. If the heavy solu- 

 ble materials from the land are washed into the sea and only lighter 

 materials remain behind, the increased stress resulting between the sea 

 and the land will tend to elevate the lighter areas until an equilibrium 

 is established between the heavy sea and the lighter land ; therefore, 

 some crustal movements, at least, may be complementary phases of the 

 degradation of the land. The elevations and depressions of the sur- 

 face of the land with regard to the sea level may thus initiate new 

 cycles of transformation in all environments. These processes do not 

 need amplification here, although they should be noted ; but this lack 

 of amplification does not imply a minor influence of this factor. Still 

 another cycle may be initiated by the processes of vulcanism, a factor 

 the influence of which is easily overlooked in large parts of the world 

 but in others is very prominent. Only one more comprehensive physi- 

 cal factor will be mentioned ; that due to alterations in the atmos- 

 phere — climatic changes. Although the temperate humid climate has 

 been made the basis for the preceding discussion, it must be remem- 

 bered not only that there are other kinds of climates, but that these 

 undergo transformation or changes from such extremes as the cold 

 arctic deserts on the one hand, to the dry hot deserts on the other. 

 Within this great amplitude of climatic possibility is found one of the 

 greatest causes both of complexity in land environment and of many 

 local differences in the transformation of habitats. 



To simplify this sketch of the operation of the physical features of 

 the environment the organic factors have been neglected, and these 

 should now be considered. On account of the ultimate dependence of 

 animals for food upon vegetation, many intimate relations exist be- 

 tween plants and animals ; furthermore, in addition to the food rela- 

 tions there are manv other important ones, such as the physical and 

 chemical influence of the vegetation upon the soil, its influence upon 

 the temperature and humidity of the air and on light ; and, finally, 

 there is qualification of these influences by the different kinds of vege- 

 tation. A vegetational cover of grass has a very different effect from 

 one of shrubs or a forest cover; conifers and hard-wood forests differ 

 in effect also; and the succession of plant societies varies, not only with 

 different kinds of vegetation but also in different climates, and with 

 different physiographic conditions. As Cowles ('ii) has shown, there 

 are several cycles or series of successions of vegetation. Many of 



