172 SOME MINUTE ANIMAL PARASITES 



acquire immunity if they survive childhood, but both 

 they and the native races lose their immunity when 

 away from the centre of infection, and are susceptible 

 to it on their return. 



The main remedy for yellow fever, then, seems to 

 be a campaign for greater sanitation and for mosquito 

 destruction on a systematic scale. Destruction of the 

 carrier is easily accomplished by putting paraffin or 

 petroleum into pools, etc., where the larvae are found. 

 Breeding can be stopped by the removal of water 

 from superfluous vessels of all sorts, by screening 

 the entire household water-supply, and by proper 

 clearance of bush, etc., around compounds. The 

 separation of the dwellings of Europeans and natives 

 is very sound policy. By these means, the enormous 

 wealth of tropical countries can be obtained with 

 fewer of the tragedies that have hitherto marked the 

 progress of white men and civilization in the tropical 

 world. 



