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132 The basis of malaria prophylaxis is the fact that particular species of mosquitoes are indispensable for the propagation of the parasites. Practical measures, therefore, have for their object the extermination of these insects, or, failing this, the prevention of their bites. War need not be waged against all mosquitoes; our present knowledge seems to indicate that only the Anophelinæ have to be considered. As the members of this sub-family are easily recognized (p. 146), and as they are somewhat fastidious in their habits, their extermination in limited areas is by no means a hopeless task. Drainage, cultivation, and flooding.—— Experience has shown that much can be done to free a locality of malaria. Drainage and cultivation where the land will repay the expenditure, permanent and complete flooding where it will not and where such flooding is possible, proper paving and draining of unhealthy towns, and the filling-in of stagnant, swampy pools these are the more important things to be striven for in attempting the permanent sanitation of malarious districts. In England, in Holland, in France, in Algeria, in America, and in many other places, enormous tracts of country which formerly were useless and pestilential have been rendere-9d healthy and productive by such means.

In carrying out extensive public works care should be exercised to provide good subsoil drainage in connection with irrigation, to provide efficient drainage to carry off superfluous water before introducing a larger water supply into a town previously inadequately watered, and to avoid interfering with the natural drainage of a district in constructing railways and so forth. To do anything that may raise the level of the subsoil water in potentially malarial districts is most dangerous. Equally so is the neglect to fill up or provide for the drainage of excavations, such as the " borrow-pits " in railway construction, or similar holes in which rain-water may accumulate and create breeding pools for mosquitoes.