82 INFECTIOUS AND PARASITIC DISEASES. 



life never cease to excite the admiration and wonder of 

 the traveler. While from the temperate zone, in a 

 direction away from the equator, vegetable and animal 

 life grows less and less abundant and flourishes for a 

 briefer period of each year, as we approach the frigid 

 zones. These diversities in climate have given origin 

 to forms of life peculiar to themselves, and attention is 

 directed to this most obvious phenomenon because 

 similar influences play their part in the production of 

 disease the world over. The living agents of disease 

 are either animal or vegetable, and are therefore sub- 

 ject to the same physical laws as govern forms higher 

 than they. Just as there are plants and animals indig- 

 enous to the several regions, so do we find pathogenic 

 agents in one region that are not encountered in another. 

 Likewise, as many plants native to temperate zones 

 outgrow in size and color all semblance to the original 

 when cultivated under tropical atmospheric conditions, 

 so do certain diseases common in temperate latitudes, 

 where they are relatively mild, assume in the tropics a 

 virulence that makes of them a terrible scourge. But 

 here the simile of flora and fauna, and disease, ends. 

 Tropical plants, unless carefully nurtured, do not 

 prosper when transplanted in temperate climates; 

 whereas tropical diseases do, if circumstances are the 

 least propitious, when they are introduced. The 

 microscopic agents of disease are less sensitive in some 

 respects to physical agents than plants, and moreover 



