The condensed moisture of the dew on the fluff of the meshes in some way attracting the germs or dissolving some assumedly gaseous emanation, was a favourite so-called explanation, and was, I believe, that adopted by the late Prof. Maclean, of Netley, who in his lectures was always careful to impress upon us the protection afforded by mosquito nets, as a well-established, though ill-understood, fact. This was before the date at which the French military surgeon, Laveran, discovered the fact that malaria was due to the presence in the blood of certain animal parasites (_Protozoa_), and although this discovery was made in 1880, it was many years before its truth was accepted by the general body of the medical profession, who have somehow always exhibited a curious reluctance to admit the harmfulness of animal parasites. Some years later Sir Patrick Manson, F.R.S., then a hard-working doctor in practice in China, made the remarkable discovery that the blood-worm disease, which is very common in those parts, was conveyed from man to man by the agency of mosquitoes; and as the parasitic origin of malarial fever became more and more firmly established, the idea suggested itself to him that this disease, too, might very well be transmitted by the intervention of the same insects. At this time Sir Patrick had left China, to work harder than ever in London, so that he was unable to personally test the truth of his surmise, which he, however, communicated to Major Ronald Ross, I.M.S., who after prolonged work, was able to establish the truth of Manson’s suggestion, which was a remarkable instance of the value of imagination in science. Almost immediately after Rossi’s work was confirmed and amplified by Prof. Grassi, of Rome, and by several other naturalists, and the fact that malarial diseases are communicated by the agency of mosquitoes, and can be carried from man to man in no other manner, is now absolutely established. Medical science has commonly to accept, as a working theory, whatever hypothesis of the causation of disease may appear most tenable, but in the present case there is no room for doubt, and, like the protective power of vaccination, the carriage of malaria by mosquitoes only, may be taken as one of the few absolutely proven facts of medicine.