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If x anopheles be sufficient to cause 100 per cent. of infection;
then x anopheles, it is obvious, cannot further increase the mischief
but we have no data as to the figure x in different conditions. Our
own experience seems to point to its being in the Duars at any
rate very small, so small that the probable success of human efforts
to reduce anopheles to below this figure does not seem very great in
this district.
This aspect of affairs has not apparently been recognised in
numerous instances where the malaria of a community has been
judged as necessarily directly proportionate to the number of ano-
pheles present, and where it has been supposed that a partial reduc-
tion of anopheles was in itself bound to influence malaria to an appreci.
able extent in any given community.
The general nature of the causes. bringing about residual malaria
we have already indicated in the introduction, and since these causes
seem in the Duars at once most concerned and most controllable
they are those upon which we shall have to lay most stress.
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