240 BIOLOGY? OF DEATH 



responsible for the spread of the infection, and to locate the breeding places 

 of these mosquitoes. Breeding places were exhibited on a community 

 map, and organized effort was centered on their destruction or control. 

 The program of simple measures excluded all major drainage. Barrow 

 pits and shallow ponds were filled or drained; streams were cleared of 

 undergrowth when necessary to let the sunlight in; their margins and beds 

 were cleared of vegetation and obstruction; and they were trained to a 

 narrow channel, thus providing an unobstructed off-flow. Artificial ^con- 

 tainers were removed from premises ; water barrels on bridges were treated 

 with nitre cake. All remaining breeding places were regularly treated by 

 removing vegetation, opening up shallow margins to give free access to 

 small fish, and spraying once a week with road oil by means of automatic 

 drips or a knapsack sprayer. All operations were under the supervision 

 of a trained lay inspector. Care was exercised to eliminate all unnecessary 

 effort and to secure, not the elimination of the last mosquito, but a rea- 

 sonably high degree of control at a minimum cost." 



The results are shown in Figure 57, as measured by 

 a number of physicians' calls for the treatment of ma- 

 laria in the community. 



The second example shows the effectiveness of con- 

 trol of yellow fever, another disease for which definite 

 scientific knowledge exists as to etiology and mode 

 of transmission. 



Nothing could more convincingly demonstrate than 

 does Figure 58 the effectiveness with which this disease 

 can be controlled. The diagram shows the results of 

 the International Health Board's yellow fever work in 

 Guayaquil in 1918-1920. 



! 



THE POPULATION PKOBLEM 



Turning to another phase of the problem, it is appar- 

 ent that if, as a result of sanitary and hygienic activi- 

 ties and natural evolution, the average duration of 

 human life is greater now than it used to be and is getting 

 greater all the time, then clearly there must be more 

 people on the earth at any time, out of a given number 



