120 SUPPLEMENT. 



conversion to last very long : there is already apparent a backsliding to the 

 earlier faith of steady flow, to which undisturbed rivers adhere closely through- 

 out their lives. 



Water-surface is, for the needs of man, so unlike land-surface, that it is 

 natural enough to include all water-basins under the single geographic term, 

 ' lakes.' Wherever they occur, — in narrow mountain-valleys or on broad, level 

 plains ; on divides or on deltas ; in solid rock or in alluvium, — they are all 

 given one name. But if we in imagination lengthen our life so that we wit- 

 ness the growth of a river-system as we now watch the growth of plants, we 

 must then as readily perceive and as little confuse the several physiographic 

 kinds of lakes as we now distinguish the cotyledons, the leaves, the galls, and 

 the flowers, of a quickly growing annual that produces all these forms in 

 appropriate order and position in the brief course of a single summer. 



W. M. DAVIS. 

 Cambridge, Mass., September 7, 1887. 



