Yellow fever, unlike malaria, does not occur in all tropical countries. Its home is the West Indies, Central, and parts of South, America, and, before its extinction in Havana, it has been a serious scourge in the Southern United States. In the New World cases have occurred as far north as Quebec, in Europe cases have occurred in Wales and France, and there have been serious epidemics in Spain. It has never been known east of Genoa, whether in Europe or elsewhere. Thus in Africa it is known on the west but not on the east coast. The fact that it is unknown in India is very remarkable, seeing that _stegomyia_ is a very prevalent variety of mosquito there. It follows from this that if yellow fever once got hold in India it would probably spread and might work great havoc. The same is true of China in an even greater degree, for such preventive measures as have been taken in Panama would be far more difficult to carry out in the great cities of India, and altogether impracticable in those of China. Thus, as Colonel Gorgas has pointed out, if the Canal had been constructed in spite of yellow fever, and if that disease had been allowed then to persist at Panama, the disease might not improbably have been carried to Asia, for the three months of life of _stegomyia_ is ample for the voyage. In this event the Panama Canal might have proved a curse rather than a boon to mankind. [Illustration: CUT SOUTH OF CULEBRA, LANDSLIP ON LEFT.] [Illustration: LOOKING NORTH, THE SCARPED FACE OF GOLDEN HILL ON THE RIGHT.] _Malaria._ The campaign against malaria has been conducted on somewhat different lines. The _anopheles_, which transmits that disease, deposits its larvæ in clean water where grass and algæ grow, and is therefore almost entirely a mosquito of country districts. But Colon and Panama, both small cities, are exposed to the disease, as are about seventeen little towns and forty villages for labourers along the line of the Canal. As the flight of _anopheles_ is not more than one hundred, or possibly two hundred yards, the working population can be in great measure protected from their attack by destroying the breeding places for such a distance on either side of the dwelling and working places. This in itself is a large task, which could not be carried out in a short time, and while in progress the Sanitary Department relied mainly upon the erection of buildings completely screened (including the verandahs) with fine copper gauze, which effectually shields the employees against mosquito attack within doors, and therefore during the particularly dangerous hours of night. In addition, the employees are supplied with quinine, and recommended to take three grains daily while in health. This "cinchonises" the blood and renders it unwholesome to the malarial parasite. The effect of screening is shown by the following example from the report of the Commission, December, 1906:--