D.—ZOOLOGY. 117 
points out, in yellow fever being restricted to rapidly diminishing, 
isolated areas, and this disease seems to be one which by persistent effort 
can be brought completely under control. 
In 1895 Bruce went to Zululand to investigate the tsetse-fly disease 
which had made large tracts of Africa uninhabitable for stock, and 
near the end of the same year he issued his preliminary report in which 
he showed that the disease was not caused by some poison elaborated 
by the fly—as had been formerly believed—but was due to a minute 
flagellate organism, a trypanosome, conveyed from affected to healthy 
animals by a tsetse-fly (Glossina morsitans). In 1901 Forde noticed 
an active organism in the blood of an Englishman in Gambia suffering 
from irregularly intermittent fever, and Dutton (1902) recognised it 
as a trypanosome, which he named T'rypanosoma gambiense. In 1902 
Castellani found trypanosomes in the blood and cerebro-spinal fluid of 
natives with sleeping sickness in Uganda, and suggested that the 
trypanosome was the causal organism of the disease. The Sleeping 
Sickness Commission (Bruce and his colleagues) confirmed this view, 
and showed that a tsetse-fly, Glossina palpalis, was the transmitter. 
Since then much has been learnt regarding the multiplication of the 
trypansosome in the fly and its transference to man. For some years 
this was believed to take place by the direct method, but in 1908 Kleine 
demonstrated ‘ cyclical’ transmission, and this was shown later to be 
the principal means of transference of T. gambiense. In 1916 
Stephens and Fantham described from an Englishman, who had 
become infected in Rhodesia, a trypanosome which, from its morpho- 
logical characters and greater virulence, they regarded as a new species, 
T. rhodesiense, and its ‘cyclical’ transmission by Glossina morsitans 
was proved by Kinghorn and Yorke. Recent reports by Duke 
and Swynnerton (1923) of investigations in Tanganyika Territory suggest 
that direct rather than cyclical transmission by a new species of Glossina 
is there mainly responsible for the spread of a trypanosome of the 
rhodesiense type. The impossibility of distinguishing by their morpho- 
logy what are considered to be different species of trypanosomes, and 
the difficulty of attacking the fly, are handicaps to progress in the 
campaign against sleeping sickness, which presents some of the most 
subtle problems in present day entomology and protozoology. Here 
also we come upon perplexing conditions due apparently to the different 
virulence of separate strains of the same species of trypanosome and 
the varying tolerance of individual hosts—on which subjects much 
further work is required. 
The relation of fleas to plague provides one of the best and most 
recent illustrations of the necessity for careful work on the systematics 
and on the structure and bionomics of insects concerned in carrying 
pathogenic organisms. Plague was introduced into Bombay in autumn 
1896, and during the next two years extended over the greater part 
of Bombay Presidency and was carried to distant provinces. The Indian 
Government requested that a Commission should be sent out to investi- 
gate the conditions. This Commission, which visited India in 1898-99, 
came to the conclusion (1901) that rats spread plague and that infection 
of man took place through the skin, but—and this is amazing to us at 
