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i] In certain bloods the flagellated body is easily procured; in others the opposite is the case. As regards the crescents, doubtless success depends in a measure on the degree of maturity of the parasite, young or effete crescents failing to evolve. There are other conditions affecting the process, however, which are as yet unknown.

Popular and early views.—The idea that some relationship exists between the mosquito and malaria has long been entertained, not only by medical men but also by the inhabitants of malarial countries. In parts of Italy the peasants for centuries have believed that fever is produced by the bite of the mosquito a fact noted by Lancisi; and, as Koch has pointed out, in German East Africa the natives of the highlands declare that when they visit the unhealthy lowlands they are bitten by an insect they call Mbu (mosquito), with the result that they get fever, which they also call Mbu. King and others long ago pointed out how a mosquito-malaria hypothesis could best explain such things as the connection of malaria with swamps and with high atmospheric temperatures ; the well-known danger of night exposure in malarial countries; the influence of trees, of an expanse of water, and of altitude on the diffusion of the germ; and many other points. As to how the mosquito intervened they had no reasonable hypothesis to offer. Laveran, Koch, and Pfeiffer suggested that the mosquito might stand in the same relation to the malaria parasite as it stands to the larva of Filaria bancrofti; but as to the particular phase of the parasite it subserved, and as to the exact way in which the insect operated, they were silent.

A mosquito - malaria theory definitely formulated.—In 1894, and again in 1896, I formulated a definite hypothesis on the subject. Being a parasite, the germ of malaria, to keep in existence as a species, must pass from host to host; in other words, must at some time have an extracorporeal life. From the fact that the flagellated body does not come into existence until the blood has left the blood-vessels—