VOYAGE UP WHITE NILE 51 



Negative evidence has a value, and the absence of bird- 

 forms which might reasonably be expected here is worth 

 passing note. Thus our British mallard stops short at 

 the Sahara ; while its South African representatives, the 

 yellow-bill, Anas undulata, and A. sparsa halt about the 

 Equator. 1 For the gadwall, and for pochards also, I had 

 looked in vain till, on my last voyage, I detected a group 

 of four white-eyed pochards, associated with a dozen 

 garganey-drakes. This was near the Iron Gates, March 



IN THE TWILIGHT CRANES. 

 (Instead of Three, picture Three Thousand.) 



1 5th, 1919. Mr Butler tells me he once shot a white-eyed 

 pochard near Khartoum a sufficient proof of its rarity. 

 Possibly these and others may yet be found when 

 a more comprehensive census shall have been taken. 

 Amidst huge aggregations, chance units may easily be 

 overlooked on casual surveys such as mine. But one 

 conspicuous absentee could not be overlooked that 



1 Captain F. Burges tells me Jhat during many years' experience of duck- 

 shooting near Khartoum he only once killed a mallard on White Nile, and 

 saw another shot the same season near Shendi, 50 miles north therefrom. 

 Mr Butler's experience corresponds. 



