238 POPULAR SCIENCE MONTHLY. 



harmless — as the experimental evidence now stands. Yellow fever epi- 

 demics are terminated by cold weather because then the mosquitoes die 

 or become torpid. The sanitary condition of our southern seaport 

 cities is no better in winter than in summer and if the infection 

 attached to clothing and bedding it is difficult to understand why the 

 first frosts of autumn should arrest the progress of an epidemic. But 

 all this is explained now that the mode of transmission has been demon- 

 strated. 



Insanitary local conditions may, however, have a certain influence in 

 the propagation of the disease, for it has been ascertained that the 

 species of mosquito which serves as an intermediate host for the yellow 

 fever germ may breed in cesspools and sewers, as well as in stagnant 

 pools of water. If, therefore, the streets of a city^are unpaved and un- 

 graded and there are open spaces where water may accumulate in pools, 

 as well as open cesspools to serve as breeding places for Culex fasciatus, 

 that city will present conditions more favorable for the propagation of 

 yellow fever than it would if well paved and drained and sewered. 



The question whether yellow fever may be transmitted by any other 

 species of mosquito than Culex fasciatus has not been determined. 

 Facts relating to the propagation of the disease indicate that the mos- 

 quito which serves as an intermediate host for the yellow- fever germ has 

 a somewhat restricted geographical range and is to be found especially 

 upon the sea-coast and the margins of rivers in the so-called ^yellow 

 fever zone.' While occasional epidemics have occurred upon the south- 

 west coast of the Iberian peninsula, the disease, as an epidemic, is un- 

 known elsewhere in Europe, and there is no evidence that it has ever 

 invaded the great and populous continent of Asia. In Africa it is 

 limited to the west coast. In North America, although it has occa- 

 sionally prevailed as an epidemic in every one of our seaport cities as 

 far north as Boston, and in the Mississippi Valley as far north as St. 

 Louis, it has never established itself as an endemic disease within the 

 limits of the United States. Vera Cruz, and probably other points on 

 the Gulf coast of Mexico, are, however, at the present time endemic 

 foci of the disease. In South America it has prevailed as an epidemic 

 at all of the seaports on the Gulf and Atlantic Coasts, as far south as 

 Montevideo and Buenos Ayres, and on the Pacific along the coast of 

 Peru. 



The region in which the disease has had the greatest and most fre- 

 quent prevalence is bounded by the shores of the Gulf of Mexico, and 

 includes the West India islands. Within the past few years yellow 

 fever has been carried to the west coast of North American, and has pre- 

 vailed as an epidemic as far north as the Mexican port of Guaymas, on 

 the Gulf of California. 



It must not be supposed that Culex fasciatus is only found where 



