Baseleveling and Organic Evolution. — Woodworth. '213 
say himself, in his contribution to our knowledge of the work 
of meteoric agents in England. This awakening of the En- 
glish school in regard to the work of rivers has gone on until 
now the reduction of lands to baselevel by meteoric agents is 
almost admitted.* 
Views of the American School. 
Powell, Gilbert, Davis, McGee, and others, have established 
much in the history of the larger rivers of America. As a 
result of these studies, it is held that rivers and their valleys 
pass through a cycle of existence, ranging from youth in a 
country newly elevated above the sea as a plain or mountain 
belt, through adolescence, maturity, and so on into old age, 
when, if the land remain tolerably stable, the country will be 
flattened down into a plain of subaerial erosion, or a peneplain. 
Renovation of the land by uplift, and the consequent revival 
of stream work, will introduce a new cycle. From the point 
of view of organic life, which it is proposed to exploit in this 
paper, the complete cycle may be assumed to begin and to end 
in uplands of sutticient relief to be characterized as a mountain- 
ous habitat, although the first topography may be construc- 
tional and the last wholly subsequent and polygenetic, the 
peneplain occupying the middle and culminating part in this 
series of progressive geographical changes. 
It has been shown by these river studies that in the reduc- 
tion of a region, either partially or completely, to baselevel, 
mutual adjustments of streams take place, whereby consider- 
able changes in course and direction are brought about. 
Systems are dismembered, the headwaters of one being cap- 
tured and diverted into another through the shifting of 
divides. By changes in level, some streams have been reversed. 
In some cases, an axis of uplift has developed across the path 
of a master river, but the uplift has gone forward so slowly 
and regularly that the stream has been able to maintain its 
course across the completed mountain chain. In other instances, 
changes in the relations of streams have arisen from the pro- 
cess of alluviation in the lower course of a river. It remains 
now to treat of the bearing of these streams on the dispersion 
•Consult Sir Arch. Geikie's Text-Book of Geology, third ed., 1893, pp. 
t<>f>-4?0: The Scenery of Scotland, second ed. Also sec H. 15. Wood- 
ward's Geology of England and Wales, Hrsi ed., 1*7<>. eh. \in. 
