maintain his theory which, in collaboration with Delgado, he attempted 

 though unsuccessful^, to prove. 



To Reed, Carroll, Agramonte, and Lazear is due the credit for the 

 masterly experiments which converted a discredited hypothesis into 

 an established doctrine. 



The transmission of the disease by the mosquito is not, as Finlay 

 thought, a simple mechanical transfer from one individual to another, 

 such as occurs at times in plague through the instrumentality of fleas 

 or in surra through biting flies. In these diseases neither the flea nor 

 the fly .is necessary, but in yellow fever not only is the mosquito nec- 

 essary, but it is essential that the mosquito be of a particular species 

 or at least of a particular genus. Thus, attempts to transmit the disease 

 by means of mosquitoes of other than the genus Stegomyia " have not 

 been successful. 



It has been found, furthermore, that in yellow fever, unlike either 

 surra or sleeping sickness, a certain period must elapse after the infect- 

 ing feed before the mosquito is capable of communicating the disease.* 

 Experimentally this interval appears to be not less than 12 c days, so 

 that a susceptible individual may expose himself with impunity, to 

 repeated stings within the first 10 or (?) 11 days d after th*e mosquito 

 has fed on a person sick with the disease. This is the period of 

 "extrinsic incubation" of Carter, whose painstaking observations at 

 Orwood and Taylor, Miss., in 1898, resulted in his tentatively fixing 

 this interval as "usually in excess of 10 days" and served, in the 

 light of the then recent discovery of the mosquito transmission of 

 malaria, to direct the attention of the Army Commission to Finlay's 

 mosquito as a possible " intermediary host " for this disease. 



The duration of this period of "extrinsic incubation" is decidedly 

 influenced by the temperature of the air. It is at its minimum at 

 temperatures above 26° C (80° F), but becomes progressively longer 

 as the temperature declines below this point. 



The period of the disease at which the mosquito bites is another 

 essential factor in the latter's power to transmit the disease. Thus 

 all attempts to produce an attack by means of the bites of mosqui- 

 toes that had previously fed on cases after the third day of the 



a No experiments have as yet been recorded with any species of this genus other 

 than 8. calopus. 



6 In surra and sleeping sickness for example, no such interval exists. On the 

 contrary, it is only during the two days immediately following the infecting feed 

 that the tsetse flies concerned can transmit these diseases. After the third day their 

 bite is perfectly harmless. In dengue this interval appears likewise to be absent. 



C I say "appears to be," because the recorded experimental evidence, is not suffi- 

 cient to prove that it may not under favorable conditions be a (very) little shorter 

 than 32 days. 



d Nor do such bites during this period confer, as Finlay believed; an immunity 

 from subsequent attack. 



