Reference has been made already to the difference between periodical malarial fevers (intermittent, etc.) and yellow fever, in that an attack of the latter does, and one of the former does not, protect the individual, usually, from liability to the disease on exposure to its cause. Does this protection extend to offspring of parents who have been "acclimatized" to yellow fever? Facts on this point are not easy to obtain. While, however, there appears to be no proof that a single generation can ever suffice to outgrow (so to speak) liability to this disease, it is well known that creoles in Louisiana and the West Indies are less susceptible to it than recent white residents, and that the negroes are much less so, as a race, than the whites. Furthermore, negroes whose ancestors have long been domesticated in our Southern States appear to re-acquire susceptibility to yellow fever in a degree more nearly like that of white people than is observed in natives of Western Africa imported within one or two generations. As to autumnal malarial fevers (remittent, intermittent), the black race exhibits a sort of race-acclimatization, giving negroes, both in Africa and in America, a much less degree of liability than is common to all races of European descent. How far any similar modification may occur in the course of generations in regard to susceptibility to small-pox and allied diseases remains at present a matter of speculation. Some authors insist that there must be at least a kind of natural selection, according to which a great epidemic of variola, destroying the lives of many of those most predisposed to suffer from it, will leave the remaining population less likely to be attacked by it. The endeavor has even been made to explain away in this manner much of the diminution of mortality from small-pox commonly credited to vaccination. But the statistics of the ravages of variola in different countries before and after the introduction of vaccination show that, while we cannot deny that some alternation (of generations respectively more and less susceptible) may occur, no such law can compare in influence with that of vaccination in the protection of individuals subjected to it. Indeed, the argument may be inverted; thus: if in the days before Jenner small-pox itself weeded out the persons most liable to it, or in some way prepared a partial family- or race-protection, such a protection ought to be gradually conferred upon a whole population through universal and persistent vaccination carried on for several generations.