The above heading sufficiently indicates a difficulty which has doubtless often occurred to those who have studied the subject of the agency of Anopheles in the causation of malaria. In some cases this difficulty has seemed so formidable that disbelief in any such agency has been boldly professed. The following observations, most of which I owe to the kindness of Dr Gould, who succeeded me at Acre in Palestine, will help to explain the occurrence of malaria at places where there would seem to be little or no breeding-ground for Anopheles, and prolonged observation by residents has not revealed the presence of the perfect insect.