30 
REPORT OF THE MALARIA EXPEDITION. 
true cause, or at least the whole cause — because it sometimes happened in the Indian experiments 
that out of a large batch of Culex (according to Giles, C. fatigam [23]) fed on birds with 
H. relicta, every insect would become infected — or almost every insect. It is scarcely likely that 
the eighteen insects we refer to in paragraph 9 were all immune, while 25^ of Anopheles of the 
same species caught at Wilberforce contained parasites. 
The explanation which suggests itself is that something was omitted in the experiments, 
which is present under natural conditions, and which is essential to the cultivation of the 
parasites. What this is we cannot say for certain ; but it is noteworthy that all the successful 
experiments made by one of us in India, without exception, were made with insects fed in 
mosquito-nets in the presence of males ; while his negative experiments with various species of 
Anopheles fed on human subjects were nearly all made with females isolated in test-tubes, 
and therefore not fertilized. 
Paragraph 16 will suggest the reason why fertilization should affect the question. The 
ingested blood is really required for the nutrition of the eggs. If the ova are not fertilized the 
blood cannot be much needed by the insect, and is possibly evacuated without some digestive 
process which perhaps is necessary to the vitality of the zygotes. We know that very subtle 
diflferences influence the vitality of the zygotes, because otherwise they would not perish in some 
species of gnats and live in others. 
The point requires careful study, because until it is elucidated negative experiments 
cannot be entirely relied on. 
The Indian experiments have sliown that there is considerable variation in the number 
of zygotes found in gnats fed even on the same subject at the same time. This variation 
must depend on differences in the quantity of blood ingested by different individuals ; and 
also possibly on some differences of quality in their digestive juices. But, at the same time, it 
was demonstrated that few individuals of a hospitable species entirely escape infection if fed 
in a natural manner. 
21. Known Facts about Malaria Explained. — Caution must always be used before 
accepting popular theories regarding the diffusion of disease ; but it is remarkable that many 
such ideas respecting malaria are explained and justified by the discovery that the disease is 
carried by Anopheles. We will briefly indicate the principal points. 
(i.) Malaria is endemic. — Anopheles are more numerous at some spots than at others, 
and are entirely absent from many places. 
(2.) Swampy ground favours malaria. — Anopheles breed in surface puddles. 
(3.) Level ground favours malaria. — Pools suitable for Anopheles must be more common 
on level than on sloping ground, e.g., on plains and in valleys, than on mountain sides. 
(4.) Rain favours malaria. — Anopheles pools largely depend on rainfall. 
(5.) Malaria is connected with decaying rock. — Anopheles may breed in hollows in rocks. 
(6.) Malaria often occurs when the soil is disturbed. — Digging must often result in the 
formation of puddles suitable for Anopheles. 
