In 1894 I was engaged in working at malaria, following out Golgi’s work and that of other Italians. I was particularly struck by the phenomena of exflagellation and more particularly by the fact that it occurred only when the blood had been removed from and was outside the human body. I reasoned that if this exflagellation occurs only outside the body, the purpose of the flagellated body must lie outside the human body, and that therefore the flagellated body must be the first phase of the malarial parasite outside the body, must be the first step that the malarial parasite takes in passing from one human host to another. There seemed to me to be a sort of logic in this. But how was the malarial parasite to pass from one human being to another? It was not provided while inside the human body with any organs of locomotion or penetration; as far as we know the parasite is never extruded in the excreta, neither does it habitually escape in hæmorrhages. Therefore, the idea of a spontaneous escape of the parasite from the human body had to be dismissed. I therefore concluded that some extraneous agency must remove the parasite from the human body, so as to afford the opportunity for this flagellation which I had concluded must constitute the first step in its extra-corporeal life. In casting about for an organism which could effect this removal I, for many reasons similar in some respects to those that influenced the savage African, the Italian peasant, King, Laveran and others, came to the conclusion that the medium of removal and transit must be the mosquito. I was so impressed with the probabilities of this double hypothesis and with its extreme practical value, should it prove to be correct, that I endeavored to leave England for a time and to visit British Guiana or some such suitable malarial country where I might work out the idea. Unfortunately, that could not be accomplished, so I published my theory in the hope that it would appeal to someone who might enjoy the opportunities denied to me. At that time Surgeon-Major Ross was at home from India and we had many conversations on the subject. I described to him my hypothesis, the probabilities of which and the possibilities of which powerfully appealed to his highly logical and practical mind. He undertook, when he returned to India, to do his best either to establish or confute it. Accordingly he set to work in India experimenting with mosquitoes and malaria.