PHYSIOGRAPHIC PROBLEMS OF TODAY 545 



animals; and the other, the reaction of life on its physical environ- 

 ment, and the modification in the relief of the lithosphere and the 

 geography of its surface thus produced. Although man is embraced 

 in each of these categories, there are sufficient reasons for consider- 

 ing his relations to his environment separately from those of the 

 lower forms of life. 



The dependence of life on its physical environment has received 

 much attention from botanists and zoologists, and is perhaps the 

 leading thesis now claiming their attention. So important is this 

 branch of study that a name "ecology," has been coined by which 

 to designate it. The phase of nature-study thus made prominent 

 pertains to the marvelously dehcate adjustment that has been found 

 to exist between the distribution of life and the nature of the region 

 it inhabits. Among the interesting themes involved are topographic 

 relief, degree of comminution and disintegration of the surface 

 blanket of rock waste, depth and freedom of penetration of water 

 and air into the hfe- sustaining film of the earth's surface, and the 

 concurrent changes in life with variations in these and other 

 physical conditions. In this most fascinating branch of study the 

 ecologist borrows freely of the physiographer, and makes payment 

 in peat bogs, living vegetable dams in streams, organic acids service- 

 able for rock disintegration and decay, deposits of calcium carbonate 

 and silica in lakes and about springs, vast incipient coal beds in the 

 tundras of the far north, and numerous other ways. 



From the physiographic point of view, however, the many and 

 intricate ways in which life leads to modifications in the features of 

 the lithosphere are of more direct interest than studies in ecology. 

 Much has been accomplished in this direction, but it is evident that 

 as yet but partially explored paths leading through the borderland 

 between biology and physiography remain to be critically examined. 



In connection with the changes in progress on the earth's surface, 

 due to the influence of organic agencies, and the appHcation of that 

 knowledge in interpreting past changes, the study of the influences 

 exerted by the lowest forms of hfe in both the botanical and the 

 zoological scale seems most promising to the physiographer. 



The secretion of calcium carbonate and sihca by one-cefled 

 organisms, as is well known, has led to the accumulation of vast 



