346 THE FIGHT AGAINST YELLOW FEVER. 



from the capital, at an altitude of 2,800 feet. It is the resi- 

 dence" of the well-to-do population — the diplomatic corps, the 

 principal merchants, financiers, and government officials. Before 

 nightfall all these take the tram for the elevated region, where 

 they find bracing air and healthful security. Petropolis, in fact, is 

 free from yellow fever at the very time when Rio is scourged with it, 

 and yet between these two places there is a lively traffic and constant 

 relations. Even more, some yellow-fever patients are taken there 

 for treatment throughout their illness, yet the disease is not connnuni- 

 cated to anyone. The reason is that the Stegomyia can not live in 

 that climate, and such as come in the railway trains each day perish 

 immediately, for the evenings are cool and during the night the tem- 

 ]Derature often falls below G0°. There is sometimes another explana- 

 tion given for the immunity from fever enjoyed in this Brazilian 

 country resort. It is attributed to the altitude of the region above 

 sea level. This is a mistake. Yellow fever and its mosquito are 

 found Qven at greater heights than Petropolis. It is enough to men- 

 tion the epidemics at Morne Rouge, in Martinique (920 feet eleva- 

 tion) ; :it Camp Jacob, in Guadeloupe (1,800 feet) ; at Newcastle, in 

 Jamaica (4,000 feet). The fact is that thermometrio conditions are 

 paramount. 



It is found that the extreme heat and humidity best adapted to 

 the welfare of the yellow-fever mosquito are most common along the 

 coasts of tropical countries. The lands best suited to the swarming of 

 this insect and to the spread of the plague form a belt around the 

 earth north and south of the equator. If lines be drawn in the north- 

 ern and southern hemispheres corresponding to the forty-third par- 

 allel of latitude, the circles would mark the upper and lower limits 

 of the home of the Stegomyia and, accordingly, of the yellow fever. 

 The region comprised in this vast zone form what MM. Chantamesse 

 and Borel call the " infectible territories," and the more temperate 

 countries on either side of this zone, denied to the mosquito, are the 

 " uninfectible territories." The Stegomyia fasciata considered as a 

 species can not become acclimated beyond this equatorial zone, for 

 it does not find that almost invariable temperature of 82° which is 

 indispensable for the proper exercise of its vital functions, and 

 especially of reproduction. 



The infectible zone represents the actual or virtual habitat of the 

 infectious mosquito. It is the region of epidemics and is also the 

 region of possible endemic or permanent centers of infection. At 

 present the Stegomyia, which is a genus of mosquito very cosmopoli- 

 tan in its habit, lives in widespread districts throughout the infecti- 

 ble zone. Theobald found it in India, the Malay archipelago, in 

 Japan, in Africa, in America, and in all warm countries where he 

 hunted for it. The contagion is capable, if not well guarded, of 



