92 Voyage of the Novara. 



But, so long as dense forest, creeping plants, and swamps 

 encumber the soil, there can be no country within the tropics 

 favourable to the health of man, and all immigrants or other 

 persons who make a sufficiently long stay in such localities, 

 prepare themselves for being visited by maladies of the most 

 formidable nature, among which fever and dysentery play 

 the most conspicuous part. 



Similar conditions are occasionally met with in certain 

 parts of Europe where swamp and uncultivated land are 

 exposed to the influences of a high tem^^erature, of which ex- 

 amples enough are furnished in the malaria of Italy, and the 

 marsh fever of the lagoons of Venice and along the coasts 

 of Istria. And if such visitations make less im]3ression upon 

 us in Europe, it is not that there is little danger, but simply 

 because, as habit is second nature, the regularity of their 

 return has ceased to attract attention. 



This is precisely what the English have experienced in 

 the East Indies, it is what the German emigrant is now 

 going through on the banks of the Mississippi and Ohio, in 

 Brazil and in Peru, until the forests are cleared and rendered 

 productive, until, in short, advancing cultivation has dis- 

 pelled those miasmata, which are inevitably developed amid 

 the undisturbed voluptuousness of nature. 



Wlien at certain seasons of the year the vital principles of 

 millions upon millions of organisms begin to be active, they 

 throw off oxygen into the atmosphere, rej)lacing it by ab- 

 sorbing carbonic acid; while, on the other hand, different 



