200 SCIENCE AND THE HUMAN MIND 



by parasites that turned out to be malaria germs 

 in an early stage of development. Thus the correct 

 method of attacking the ravages of malarial fever 

 is neither to search for new drugs nor to hunt down 

 existing mosquitoes, but to drain the marshes and to 

 take note of all pools of standing water which provide 

 suitable breeding-places, in order to destroy the larva 

 by a thin coating of oil or other deterrent. 



Similarly Maltese or Mediterranean fever has been 

 traced to the action of a microbe which passes part of 

 its life in goats, and is communicated to man by means 

 of the goats' milk, the goats themselves remaining 

 meanwhile apparently perfectly healthy. The con- 

 nection between bubonic plague, rats, and the fleas 

 or other parasites which help to bring the infection 

 from the rats to human beings gives another instance 

 of the indirect methods by which disease can now be 

 fought with greatest success. By taking precautions 

 founded on such new investigations, vast tracts of 

 the earth's surface formerly uninhabitable, or only 

 habitable at great cost of life and health, have been 

 made fit for occupation. Of late years our general 

 knowledge of microbic diseases, knowledge won by 

 laborious and in some cases dangerous experiments, 

 has led to ceaseless improvements in sanitation and 

 preventive medicine on the one hand, and on the 

 other, by the application of Pasteur's results by Lord 

 Lister to surgery, has, in conjunction with the dis- 

 covery of anaesthetics, made possible the wonderful 

 operations now carried out to relieve human suffering. 

 The practical results of these discoveries in hygiene, 

 medicine and surgery are perhaps most clearly shown 

 in the reduction of the annual death-rate of cities 



