536 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



We have now outlined the developmental sequence of wet depres- 

 vsions, the lake-pond-swamp series, and the running water, the brook- 

 creek-river series, these two series including the main inland animal 

 environments in a liquid medium in a humid climate. We have yet 

 to consider the animal environments of land animals proper, those 

 which live in the gaseous medium of air. The complexity of condi- 

 tions upon land is much greater than that in water, either fresh or 

 salt. In other words the land habitats are the most complex on earth. 

 For simplicity in handling this involved problem, an ideal series will 

 also be followed, and instead of attempting to discuss all the prin- 

 ciples involved, only such will be mentioned as may be illustrated by 

 a single example. This will serve to show the application of the 

 method. We shall consider the process of degradation of the land, 

 such as might be developed during a topographic cycle of erosion, 

 and as applied to a snowcapped conical mountain in a temperate 

 humid region. 



Let us consider the series of processes which operate upon such a 

 mountain. The snow and rain which fall upon it are in unstable equi- 

 librium, the snow creeps or plunges down the slopes, and the water 

 flows down. In the zone of ice and snow physical and mechanical 

 changes preponderate ; but at lower altitudes, with the melting of the 

 snow and ice, on account of the higher temperature, chemical changes 

 become more prominent and supplement the mechanical work of run- 

 ning water. Here, also, plants and animals become an important fac- 

 tor in modifying the processes of change by hastening or retarding 

 the processes of degradation. We thus see that on different parts of 

 a mountain there are important modifications in the processes of 

 degradation. The same general processes which operate to form 

 lakes, ponds, swamps, brooks, creeks, and rivers, are also at the same 

 time producing changes in the land habitats. The entire surface of 

 such a mountain is undergoing change, but because of the concentra- 

 tion of degradative progress near its base, particularly on account of 

 the concentration of the drainage there, ravines and valleys develop 

 here more rapidly and converge toward the main divide, the moun- 

 tain top. As these ravines and valleys enlarge, the mountain is low- 

 ered ; and ultimately all is reduced to a plain, and to baselevel. The 

 condition of stress which existed upon the slopes of such a mountain 

 as degradation progressed, became relatively adjusted at that pla^e^ 

 but where the degraded materials were deposited a stress was becom- 

 ing cumulative., and it is this ever-changing adjustment of stresses 

 which makes natural processes unending. 



With the degradation of the mountain, progressively higher zones 

 are lowered; the snow cap disappears; the region above the tree 

 limit, and later the lower parts, are spread over a large area, and the 

 mountainous character is largely gone. In this manner and at the 



