On 25th August 1914, the medical officer in charge of a rubber estate in Malaya wrote to the manager a report about a new method of polluting Anopheline breeding-places: “It will interest your Directors to learn that by oiling with the mixture in use on Sungei Way, one of the ravines in the West Divisions has been kept free from Anopheles for several months. Formerly they bred freely in it. I am encouraged to think that this is not merely due to the physical difficulty the larvae would have of getting air when oil is on the water, but that some change has actually taken place in the water as a consequence of the oil, and a change which would make it uncongenial to the stream-breeding mosquito. My reason for thinking this is that in the bottom of the ravine a green slime has developed, in the presence of which I have never found dangerous anophelines” (Watson, M. 1921). Before 1914 oil had been used on standing water, and poisons had been used in running water (Howard, Dyar & Knab, 1912) to reduce the breeding of mosquitoes. But this discovery, that it could be effectively and economically used even on fast-flowing streams to control Anopheles, made the use of oil par excellence the great standby in emergencies of malaria; while it may be the method of choice if control measures are only required for two or three months in each year.