Conditions 39 



surrounding country most of the sub-surface 

 water went toward it. The, dumps where the 

 excavated material was "wasted" often blocked 

 natural drainage channels and ditches. Water 

 collected constantly and proposed excavations and 

 fills interfered with or prevented permanent drain- 

 age schemes. As the Canal neared completion, 

 the bottom became too fiat to drain economically 

 and numerous small pools were left. Hundreds 

 of acres of this territory had to be inspected 

 and oiled each week. The hilltop of to-day be- 

 comes a depression a month later, and permanent 

 drainage is often out of the question. The history 

 of tropical malaria is generally alike east or west.. 

 It develops most rapidly when the soil is dis- 

 turbed by large and extensive excavations and 

 fills accompanied by the introduction of non- 

 immune labor housed near the site of their 

 work. Until now, no precautions had ever 

 been taken in similar construction work in 

 the tropics to prevent an increase in the num- 

 ber of places suitable for the development of 

 malaria-conveying mosquitoes. Pools and stag- 

 nant water were brought into existence by fills 

 that had settled. By the time the surface of 

 a dump was in condition to hold water it was 

 covered with rank vegetation, often more than 



