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bulletin: museum of comparative zoology. 



than others and these, on the Blue Nile, for example, already show 

 through their difference of habits, compared to their congeners of the 

 upper Dinder, an adaptation to the changing conditions. 



The entire country up to the Abyssinian border is monotonously 

 flat, and covered largely with an open forest of thorn trees among 

 which the red-barked gum-arabic tree is conspicuous. A very few 



Fig. 1. — Sketch map of the Blue Nile Valley. 



small and isolated hills or 'gebels' project here and there abruptly 

 from the plain, and alone break its monotony. The Blue Nile has cut 

 a channel through this broad plain, but so steep are its banks for many 

 miles in succession, that access to the water is difficult, and hardly to 

 be obtained except where gullies, cut down during the torrential rains 



