THE GEOLOGIC FUNCTIONS OF LIFE. ^49 



man is included, the geologic effects require consideration/ but here 

 the third distinctive factor, the mental element, comes into effective 

 play, and we pass to its consideration. 



C. The Mental Element. ; . 



Current opinion does not recognize a mental element as residing in 

 the plant world, and it is divided as to the degree of its development in 

 the lower animal kingdom, but its influential presence in the higher 

 animal orders and in man is beyond legitimate question. Two phases 

 are to be recognized: (1) the material work done under the stimulus 

 and direction of mental impulses, as, for example, excavations, trans- 

 portations, changes of drainage, removal of forests, cultivation of soil, 

 etc., and (2) the intellectual work of the faculties themselves irrespec- 

 tive of material changes. In one view, geology is a purely material 

 science concerned solely with the formation of the earth and with the 

 ph^^sical development and relations of its inhabitants. In another, 

 geology is a comprehensive historical science concerned with every phase 

 of the world's history, and certainly not least with the higher forms of 

 life development, with their psychological, sociological, and other phases 

 of mental attainments, since these are the highest output of the earth's 

 evolution. The latter seems to us the more comprehensive view. 



(1) The material effects of the mental element. — Lyell long since 

 urged that the direct work of man in changing the face of the earth was 

 slight compared with that of the contemporaneous inorganic agencies. 

 He called attention to the relative insignificance of the quarries, pits, 

 cellars, and other excavations of man, compared with the work of streams, 

 waves, and other inorganic agencies. There is justness in this view, 

 but it needs qualification. It is to be observed that the mental era has 

 but just begun, and that its effects are increasing with a rapidity quite 

 phenomenal when measured by the slow pace of most geologic events. 

 The excavations and transportations of material to-day show an enor- 

 mous advance on those of Lyeh's day, which was, geologically speaking, 

 but a moment ago. The mile-tons of industrial freightage in the Missis- 

 sippi basin are to-day not wholly incomparable 'with the drainage trans- 

 portation of the same area a century ago. A century ago is named, 

 because the surface was then covered with natural vegetation, and the 

 normal effect of surface erosion, independent of man, was then experi- 

 enced. At present the indirect effects of man's action are mingled with 



