H. L. Duke 429 
negative, but relatively little positive evidence, and which for the present is 
chiefly valuable as it suggests the need for further specific enquiry. 
Pending the results of such enquiry this hypothesis affords the best 
answer we can give to what we regard as the main question. It recognises the 
possibility that T. gambiense may vary greatly in pathogenicity to man: it 
takes into account the manner in which peculiarly virulent strains of normally 
less virulent species of insect-borne trypanosomes are developed: it recognises 
also the real probability that such strains may develop in nature as well as in 
the laboratory: it recognises a clear possibility that a peculiarly virulent strain 
of T. gambiense may have been developed in this manner; and, finally, if it 
could be proved well founded, it would indicate that very broad contact 
between fly and population is a prime essential to the occurrence of sleeping 
sickness in the form of a widespread epidemic. 
REFERENCES. 
(1) Colonial Reports — Miscellaneous, No. 65, § 21. 
(2) Ibid. § 22. 
(3) Colonial Reports, No. 65. 
(4) Dispatch 218, Sir H. H. Bell to C.O., 23 Nov., 1906, § 21. 
(5) Colonial Report, No. 65, § 26. 
(6) Reports of S.S. Commission of Royal Society, No. ix. 86. 
(7) Colonial Report, No. 65, § 34. 
(8) Ibid. § 32. 
(9) Reports of S.S. Commission of the Royal Society, No. xm. 15. 
(10) Ibid. 109. 
(11) Gonder (1911). Centralblatt fur Bald. lxi. 102-113. 
(12) Blacklock and Yorke (1913). Ann. Trop. Med. and Parasit. vii. 563-568. 
(13) C. O. Report, No. 65, § 19. 
(14) Ibid. § 19. 
