28 



healthy hefore the disease can be propagated. What I propose to consider 

 is the means by which the morbific cause of yellow fever is enabled to part 

 from the body of the patient and to be implanted into that of a healthy 

 person. The need of an external intervention, apart from the disease itself, 

 in order that the latter may be transmitted is made apparent by numerous 

 considerations; some of them already pointed out by Humboldt and Ben- 

 jamin Rush since the beginning of this century, and now corroborated 

 by recent observations. Yellow fever, at times, will travel across the Ocean 

 to be propagated in distant ports presenting climatic and topographic 

 conditions very different from those of the focus from which the in- 

 fection has proceeded, while, at other times, the disease seems unable to 

 transmit itself outside of a very limited zone, although the meteorology 

 and topography beyond that zone do not appear to differ very materially. 

 Once the need of an agent of transmission is admitted as the only means 

 of accounting for such anomalies, it is evident that all the conditions which 

 have hitherto been recognized essential for the propagation of the 

 disease must be understood to act through their influence upon the 

 said agent. It seemed unlikely, therefore, that this ageut should be found 

 among Micro or Zoophytes, for those lowest orders of animal life are but 

 little affected by such meteorologic variations as are known to influence 

 the development of yellow fever. To satisfy that recpiisite it was necessary 

 to search for it amongs insects. On the other hand, the fact of yellow 

 fever being characterized both clinically and (according to recent find- 

 ings) histologically, by lesions of the blood-vessels and by alterations of the 

 physical and chemical conditions of the blood, suggested that the insect 

 which should convey the infectious particles from the patient to the healthy 

 should be looked for among those which drive their sting into blood-vessels 

 in order to suck human blood. Finally, by reason of other considerations 

 which need not be stated here, I came to think that the mosquito might 

 be the transmitter of yellow fever. 



Such was the hypothesis which led me to undertake the experimental 

 investigation which I shall here relate. 



The application of the auxiliary sciences to Medicine often demands 

 such a minute acquaintance with the different branches of human know- 

 ledge, that one cannot wonder at the length of time which sometimes 

 elapses before certain facts recorded in a special branch can become avail- 

 able I'm- purely medical investigations. This is particularly the case with 

 regard to Natural History; its acquisitions being the outcome of the direct 

 observation of Nature must, as a rule, undergo a complete revision from 

 our own point of view before they can be turned to account in a nosological 

 investigation. It has thus happened that more than a century after 

 Reaumur had written his admirable Memoir on the habits of mosquitoes, 

 justly considered as a model of accurate and keen observation, and which, 



