"Assimilating the disease to small-pox and to vaccination, it occurred to me that in order to inoculate yellow fever it would be necessary to pick out the inoculable material from within the blood vessels of a yellow fever patient and to carry it likewise into the interior of a blood vessel of a person who was to be inoculated. All of which conditions the mosquito satisfies most admirably through its bite." In the course of his study of the problem, Finlay made detailed studies of the life history and habits of the common mosquitoes at Havana, and arrived at the conclusion that the carrier of the yellow fever was the _Culex mosquito_ or _Aëdes calopus_, as it is now known. With this species he undertook direct experimental tests, and believed that he succeeded in transmitting the disease by the bite of infected mosquitoes in three cases. Unfortunately, possibility of other exposure was not absolutely excluded, and the experiments attracted little attention. Throughout the next twenty years Finlay continued his work on yellow fever, modifying his original theory somewhat as time went on. Among his later suggestions was that in the light of Smith's work on Texas fever, his theory must be "somewhat modified so as to include the important circumstance that the faculty of transmitting the yellow fever germ need not be limited to the parent insect, directly contaminated by stinging a yellow fever patient (or perhaps by contact with or feeding from his discharges), but may be likewise inherited by the next generation of mosquitoes issued from the contaminated parent." He believed that the bite of a single mosquito produced a light attack of the disease and was thus effective in immunizing the patient. Throughout the period, many apparently successful attempts to transmit the disease by mosquitoes were made. In the light of present day knowledge we must regard these as defective not only because possibility of other infection was not absolutely excluded but because no account was taken of the incubation period within the body of the mosquito. In 1900, while the American army was stationed in Cuba there occurred an epidemic of yellow fever and an army medical board was appointed for "the purpose of pursuing scientific investigations with reference to the acute infectious diseases prevalent on the island." This was headed by Walter Reed and associated with him were James Carroll, Jesse W. Lazear and Aristides Agramonte, the latter a Cuban immune. For a detailed summary of this work the lay reader cannot do better than read Dr. Kelly's fascinating biography "Walter Reed and Yellow Fever."