DEVELOPMENT OF VALLEYS 



the growth of b will be checked, and ultimately stopped. Sim- 

 ilarly other valleys, such as/ (Fig. 49), will get the better of their 

 neighbors, and many of the competitors, as b', d f , e f , and g' (Fig. 

 50), will soon drop out of the race. Between the stronger streams 



Fig. So 



Fig- Si 



Figs. 49, 50, and 51. Diagrams illustrating successive stages in the struggle for 

 existence among streams. 



ipetition still goes on. If a' and /' (Fig. 50) develop faster than 

 ', its prospective drainage territory will be pre-empted by them 

 (compare Figs. 50 and 51). Thus as the result of the unequal 

 ite at which valleys are lengthened, the larger number of those 

 ich come into existence are arrested in their development. 

 Piracy. Not all streams hold permanently the courses which they 



