well-established fact that the negro in Africa, although he does get fever, does not get it so frequently nor so severely as the European; even although the latter, from his hygienic ways of living, is of the two much the less exposed to infection. The Chinese, the Malays, and some other dark-skinned races also appear to enjoy a comparative immunity—— an immunity considerably less pronounced, however, than that enjoyed by the African and West Indian negro. There is some evidence to show that even the individual European, if he survive the process, may after many years, through frequent infection, attain immunity; this, however, does not appear to be transmitted. The inhabitants of the malarious districts of Italy, Corsica, Greece, Turkey, and other South European countries have inherited no marked immunity from malaria in virtue of the thousands of years during which their ancestors lived in malarious districts. But they have inherited experience, and many of them know how to keep clear of the infection they cannot overcome; this probably is, in great measure, the extent of their acclimatization and apparent acquired immunity.
We are indebted to Koch for an important observation, with manifest practical bearings, on this subject; an observation which throws light on the apparent immunity of negroes, Melanesians, and other dark-skinned races living in highly malarious countries. He has shown, and his statements have been abundantly confirmed, that the natives of such districts acquire their immunity from repeated and persistent infection in childhood. In such places the blood of practically every child up to three or four years of age contains malaria parasites. The proportion of infected children gradually becomes smaller with each additional year, until adolescence is approached, when the blood becomes practically parasite-free, and immunity is established. Daniels had already shown, by a comparative study of the prevalence of malarial pigmentation in the cadavers of natives of British Guiana, that such was probably the case; Koch's more direct observations on the
