Nos. 622-623] MIGRATION A FACTOR IN EVOLUTION 489 



fore now necessary to review briefly some of the main en- 

 vironmental agencies and processes operative. The geol- 

 ogists and physiographers have made much progress in 

 the dynamic interpretation of their problems, so that it 

 is a relatively simple matter to adapt their results to our 

 purpose. They have shown that the rocks below the 

 ocean are heavier than those of the land and that the 

 present shores of the oceans change as a dynamic equi- 

 librium is established between the heavier sea bottom and 

 the lighter land area (Willis, '11). We have in this a 

 cause for innumerable changes in the physical features of 

 the earth's surface, and in the environments of animals. 

 This beautifully illustrates the fundamental unity and 

 method of interaction between the liquid sea and the solid 

 land systems. This is one of the huge physical cycles 

 which illustrate every dynamic phase, from a condition 

 of stress through the slow process of adjustment to strain 

 —retarded by the rigidity of the earth's crust — on to a 

 dynamic equilibrium. All the land area which remains 

 above sea level is exposed to waves, the disintegrating in- 

 fluences of the atmosphere, and the erosion by wind and 

 running water, all of which tend to cut down all land to 

 sea level, and to deposit the heavy debris on the sea bot- 

 tom, thus cumulatively destroying its relative equilib- 

 rium, and, supplemented by radio-activity, there are in- 

 stituted cycles of stress, in an unending series. 



In the equilibria existing between the land and the sea, 

 the isotatic cycle, and the cycle of erosion or base-leveling, 

 are found two phases of the most important gross influ- 

 ences in the physical causes of animal migration (ef. 

 Woodworth, '94; Adams, '01). To this must be specifi- 

 cally included climatic cycles (Huntington, '14, and oth- 

 ers) whose influence upon animal migration is also pro- 

 found. Intimately related to the preceding physical fac- 

 tors are the cyclic changes in the vegetational covering 

 of the earth, particularly those recognized in recent years 

 by the plant ecologists (Cowles, '11; Clements, '16, and 

 others). The physical changes influence all organisms, 



