40 



fever may be propagated: 1. The existence of a yellow fever patient into 

 whose capillaries the mosquito is able to drive its sting and to impregnate 

 it with the virulent particles, at an appropiate stage of the disease. 2. That 

 the life of the mosquito be spared after its bite upon the patient until it 

 has a chance of biting the person in whom the disease is to be reproduced. 

 3. The coincidence that some of the persons whom the same mosquito 

 happens to bite thereafter shall be susceptible of contrating the disease. 



The first of these conditions, since Dr. Ambrosio G. del Valle has 

 been publishing his valuable mortuary tables, we may be sure, has never 

 failed to be satisfied in Havana. With regard to the 2d and 3d, it is evident 

 that the probabilities of their being satisfied will depend on the abundance 

 of mosquitoes and on the number of susceptible persons present in the 

 locality. I firmly believe that the three above mentioned conditions have, 

 indeed, always coincided in years when yellow fever has made its greatest 

 ravages. 



Such is, Gentlemen, my theory; and I consider that it has been 

 singularly strengthened by the numerous historical, geographical, 

 ethnological and meteorological coincidences which occur between the data 

 which I have collected regarding the mosquito and those which are recorded 

 about the yellow fever ; while, at the same time, we are enabled by it to 

 account for circumstances which have until now been considered inexplicable 

 under the prevailing theories. Yellow fever was unknown to the white race 

 before the discovery of America, and, according to Humboldt, it is a 

 traditional opinion in Veracruz that the disease has been prevailing there 

 ever since the first Spanish explorers landed on its shores. There also, as 

 we have seen, the Spaniards since their first landing have recorded the 

 presence of mosquitoes ; and with greater insistance than in any other 

 place in America, in the identical sand-mounds of San Juan de Ulloa 

 (the present site of Veracruz). The races which are most susceptible to 

 Yellow fever are also the ones who suffer most from the bites of mosquitoes. 

 The meteorological conditions which are most favorable to the development 

 of yellow fever are those which contribute to increase the number of 

 mosquitoes ; in proof of which I can cite several local epidemics regarding 

 which competent authorities assert that the number of mosquitoes during 

 the prevalence of the yellow fever was much greater than on other 

 occasions; indeed, it is stated in one instance that the mosquitoes were of 

 a different kind from those which were usually observed in the locality, 

 having gray rings around their bodies. Regarding the topography of the 

 yellow fever, Humboldt points out the altitudes beyond which mosquitoes 

 cease to appear, and in another passage gives the limits above the sea-level 

 within which the yellow fever may be propagated. Finally, in the notorious 

 case of the U. S. Steamship Plymouth, in which two cases of yellow fever 

 occurred at sea, after the vessel had been disinfected and frozen during the 



