390 CARTER : THE EVOLUTION OF THE DON RIVER-SYSTEM, 
has not previously been treated of in a paper in the Proceedings 
of the Yorkshire Geological Society, it may not be out of place 
to give a brief resume of the theory of its action. 
The principle of river struggle with the survival of the fittest 
was first applied by Professor Jukes to explain certain anomalies 
of the Blackwater and other rivers of Southern Ireland.* The 
American geologists have expanded his suggestions into a scien- 
tific scheme for dealing with their extensive river problems, 
and have by this principle solved many of the comphcated 
questions of American physical geography. Professor W. M. 
Davis, t Mr. Jukes-Browne, J Professor P. F. Kendall, 1 1 and Mr. 
Cowper Reed§ have also applied the same method to the ex- 
planation of the erratic courses of certain English rivers. In 
considering the conditions of the struggle of rivers for the 
possession of a watershed it is evident that some will be equipped 
more advantageously for the conflict than their neighbours. 
The three chief conditions of success are : — 
1. Slope ; the greater the fall of a river the faster Avill it 
flow and the greater wiU be its cutting power. Hence the steeper 
the slope the more rapidly will the stream intrench itself, and 
thus obtain a commanding place in the watershed. 
2. Volume ; the larger the volume of a river the greater 
will be its excavating power. 
3. Softness of bed ; the material of the rocks over which 
a river flows will be a considerable factor in deciding the 
rate of denudation, and therefore the river that flows over the 
softer strata will have a considerable advantage in the struggle 
over neighbours flowing over harder rocks. 
These principles make it evident that a river that was 
advantaged in two or three of these respects would be able to 
cut down its channel more rapidly than a rival less advan- 
tageously situated, and in turn to cut back further into the 
watershed, and thus obtain a larger area of the rainfall-catchment. 
This advantage would be shared by each of the predominant 
*Q.J.G.S., xviii., 1862. ~ 
j Geographical Joiu-nal, v., 1895, p. 127. 
+ Q.J.G.S., 1883, p. 596. 
II Chairman's Address, Yorks. Geol. Soc. Whitby Meeting, Aug. 1st, 1896. 
§ The Geological History of the Rivers of East Yorkshire. 1901. 
