ANIMALS AND THEIR ENVIRONMENTS ADAMS. 537 



same time as the land is degraded to a lowland by running water, in 

 the water itself a series of habitats is developing, and thus all the 

 environment is heing transformed, along relatively distinct hut mu- 

 tually interdependent lines, toward the same general direction or 

 condition — a relative 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 activ- 

 ity, and that the deposition of the degraded materials may be a cause 

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

 soluble materials from the land are washed into the sea and only 

 lighter materials remain behind, the increased stress resulting be- 

 tween 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 com- 

 plementary phases of the degradation of the land. The elevations 

 and depressions of the surface of the land with regard to the sea 

 level may thus initiate new cycles of transformation in all environ- 

 ments. 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 over- 

 looked in large parts of the world but in others is very prominent. 

 Only one more comprehensive physical factor will be mentioned; 

 that due to alterations in the atmosphere — climatic changes. Al- 

 though the temperate humid climate has been made the basis for the 

 preceding discussion, it must be remembered 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 ampli- 

 tude 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 many 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 hardwood forests differ 

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



