CONDITIONS INFLUENCING PREVALENCE 219 



their eggs, but a more probable suggestion, for which I am 

 indebted to Dr. St. George Gray, of St. Lucia, is that the 

 larvse require plenty of Hght. 



It is in level country such as the Indian alluvia that 

 conditions such as the above are generally found. On 

 undulating ground surface drainage may usually mainly be 

 left to care for itself, and in such a place as Freetown, 

 Sierra Leone, to judge from the reports of the two Malaria 

 Commissions, no extensive drainage measures can be 

 expected to be of any proportionate value. Here the 

 breeding pools are basins of solid rock which necessarily 

 cannot be drained, and as far as one can judge, the 

 measures that suggest themselves are the regrading of the 

 banks of the rocky watercourses, and the improvement of 

 the roads and other places where the rock lies bare, and 

 the formation of these peculiar puddles thereby becomes 

 possible. Possibly, where not exposed to traffic, many of 

 these pools might be rendered innocuous for a considerable 

 period by filling them with sand ; but this aside, the main 

 reliance must be placed on larvicides, and I hardly under- 

 stand why Drs. Stephens and Christophers should be dis- 

 appointed in the results of their employment (E.S.M.C., 

 p. 43) on the ground that the larvae reappeared as soon as 

 the use of the larvicides was discontinued. The effect of 

 such agents is at most a matter of days, and there is no 

 possible reason for the Mosquitoes not returning to pools so 

 treated the moment their effects have disappeared. The 

 main objection to the use of these agents is not that they 

 are wanting in efficiency, but that success can only be 

 obtained by continuous trouble and expense, but where the 

 breeding pools are in manageable numbers, and they cannot 

 be done away with except at prohibitory expense, it is as 

 reasonable to keep up a staff of puddle oilers as of scavengers, 

 for the result of the labours of the street sweepers is no 

 more permanent than that of the larvse destroyers, and it 

 is as fair to object on this score to the continuous employ- 

 ment of the one municipal servant as the other. I feel 

 perfectly sure that Ross never expected to obtain any 

 advantage from the use of such agents without their 



