THE WORK OF RUNNING WATER. 101 



growth and survival. At an early stage of its erosion history the 

 number of small valleys in a given area is often great, while at a later 

 stage the number is less and the size of the survivors greater. 



One phase of the struggle for existence is often well illustrated 

 on a freshly exposed slope of clay. The number of miniature gulhes 



which develop on such a slope, even 

 in a single shower, may be very large 

 (Fig. 84); but the history of many 

 of them is ephemeral. If two ad- 



FiG. 85. — Diagram illustrating the ab- . , r i a 4-U +v,« 



sorption of one gully by another by J^cent ones are of Unequal depth the 

 lateral erosion. The successive lines widening of the deeper narrows and 

 represent successive cross-sections. finally eliminates the divide between 

 them, and the two become one (Fig. 85). 



Another phase of the struggle for existence is shown in other situa- 

 tions. Examination of a good map of the north shore of Lake Superior 

 or the west shore of Lake Michigan shows a large number of small streams 

 and gulhes (Fig. 1, PI. IV). The valleys are short and narrow, and 

 between and beyond them are considerable areas untouched by 

 erosion. The drainage near the lake is therefore young, and each of 

 the small valleys is growing. This condition of things is perhaps 

 typical of that which has been, is, or will be along the average coast 

 at a certain stage in its erosion history. No equal stretch of coast- 

 line where erosion is far advanced can boast of a number of large rivers 

 comparable to that of the many small ones along the coasts mentioned. 

 It therefore seems evident that of these many small streams a few 

 only will attain considerable size. 



Some of the methods by which the growth of the many is arrested 

 are easily understood. Some of the young valleys on a given coast 

 will work their heads back into the land faster than others because 

 of inequalities of slope and material. This will be true of the tribu- 

 taries no less than of their mains. If valleys develop in ways other 

 than by head erosion (see p. 73) the chances are also against their 

 equality of growth. If two streams, such as a and c, Fig. 86, develop 

 faster than the intermediate stream h, it is clear that their tributaries 

 may work back into the territory which at the outset drained into h, 

 so as to cut off the supply of water from the latter stream (compare 

 a'h'c^, Fig. 87). As a result, the growth of h will be checked, and ulti- 

 mately stopped. Similarly other valleys, such as /, will get the better 

 of their neighbors, and many of the competitors, as h, d, e, and g will 



