38 THE CONTROL OF LIFE 



and malaria is checked. The success of this method 

 has been dramatically illustrated in connection with the 

 making of the Panama Canal and on a smaller scale 

 around places Jike Khartoum. Now, the point is, that 

 this method of controlling life was discovered by turn- 

 ing scientific investigators on to the problem, and by 

 basing practical action on the facts discovered. 



The same kind of story might be told in regard to 

 some other diseases. Thus at the beginning of the 

 twentieth century Walter Eeed showed that a mosquito 

 called Stegomyia fasciata carries the virus of yellow 

 fever. Bitten people die, whilst those who used their 

 bedding but were not bitten did not take the disease. 

 Similarly Bruce showed that the tsetse fly is the carrier 

 of the microscopic animal {Trypanosoma evansi) which 

 causes sleeping sickness, from which 200,000 may die 

 in one year. 



Among the most troublesome of human parasites 

 are the hookworms, insidious Nematodes which are 

 able to make their way through the skin, and are very 

 common in many warm countries. One kind especially 

 affects miners, for the moisture and high temperatures 

 of underground workings are favourable to the develop- 

 ment of the hookworm eggs which are excreted from man. 

 A knowledge of the life-history of the parasite has led 

 to the suggestion of measures to prevent the pollution 

 of water and soil, and, thanks in great part to the Ameri- 

 can Rockefeller Foundation, the disease has been brought 

 under control. The first step is to kill the worms in 

 the individual patients ; the second is to prevent 



