j650 GEOLOGY. 



those of natural processes, and these indirect effects are probabl}^ much 

 more important than the direct ones. The removal of the native vege- 

 tation and the cultivation of the soil expose the surface to wash to a 

 degree far beyond that prevalent when the surface was prairie sod, 

 or leaf-carpeted forest, and denudation and transportation have 

 been greatly multiplied in consequence. Not only has this cultivation 

 increased the exposure to erosion, but, by increasing the rate of run-off, it 

 has added to the erosive power of the streams. The ditching of swamps 

 and other tracts of retarded drainage has contributed to this accelera- 

 tion. The naked, soil-less uplands of some of the once populous king- 

 doms of the Orient, notably portions of Syria and Greece, are sad wit- 

 nesses of the accelerated erosion that attends cultivation. The erosion 

 of certain southern fields of the United States in the last forty years is 

 another striking illustration. It is doubtful whether some parts of this 

 region suffered as much erosion in the preceding five centuries as they 

 have during the last one. On the other hand, some compensation is 

 found in the reservoirs established for water-power, and in artificial 

 devices for retarding and steadying stream flow. 



In the light of considerations such as these, man may well be regarded 

 not only as a potent geological agent, but as dangerously so to himself. 

 The hope is that the intelligence that has wrought a change of surface 

 conditions serviceable for the present, but dangerous to the future, 

 will be so enlarged as to inspire a still more intelligent control of surface 

 conditions which shall compass the future welfare as well as transient 

 benefit. 



Human modification of the animal and vegetal kingdoms. — Man's 

 agency is also coming to be felt powerfully in the modification of the 

 plant and animal life of the land and even to some extent of the sea. 

 The larger animals that are not propagated by man are fast approaching 

 extinction. At the present rate of extension of man's dominion, a cen- 

 tury or so will see the disappearance of nearly every large mammal and 

 reptile that he does not choose to protect or propagate. By way of 

 compensation, certain selected animals are increasing and will doubt- 

 less continue to increase. The result is, therefore, likely to be a pecul- 

 iar assemblage of animal life dependent strictly on the choice of a 

 dominant type, a state of things that has apparently never occurred in 

 an equal degree in the past history of the earth. How far the minor 

 forms of life, especially the insect life, and the denizens of the sea, may 



