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 gad wall, 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.) 
15th, 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 that 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. 
