Page:Popular Science Monthly Volume 48.djvu/745

Rh pro mille from 1837 to 1848, so that Boudin, Bertillon, and Knox doubted if the French could ever colonize there. At the present time the birth-rate even exceeds that in France itself; and the death-rate is but little above the normal. In Tunis also the birth-rate was 35·6 pro mille in 1890-'92, greatly exceeding the ruling death-rate of 25·7 per thousand. In America it is in the uplands of Mexico, Peru, and Bolivia, or along the arid coast of the Pacific, and not in the real tropical climate of Brazil, where the Spaniards have succeeded most fully. They have also done well in Cuba, to be sure, but the cases are entirely dissimilar. And to reason, from the French success in Algeria, that the same would ensue in the Congo basin, in Madagascar, or in Cochin China is totally to misconceive the real limitations of a tropical climate. The relative difficulties to be encountered in these several cases may be roughly indicated by the mortality of soldiers. In Cochin China it is almost exactly double that in Tunis; and this is, roughly speaking, a measure of the difference between a mere torrid climate as distinguished from one which is very humid as well as hot, for humidity means that malaria is superadded to all the other difficulties inherent in climate alone.

The heat in a tropical climate becomes important but indirectly, because it is the cause of humidity and generally accompanies it. In the temperate regions humidity goes with cool weather except in the dog days, while within the tropics heat prevails just when radiation through perspiration is most retarded by moisture in the atmosphere. This, in combination with the enforced lack of exercise and its attendant excretion, forms the double cause of physiologic disturbances. The blood is not properly purified and anæmia ensues, if the more immediate effects do. not manifest themselves in intestinal disorders.

Everything which conduces to give a variety to the climate of the tropics affords relief. The alternating sea and land breezes of islands make them more amenable to European civilization. Especially when these islands are volcanic or mountainous is the strength of these tempering elements increased. This, in fact, is