races negroes are resistant to it. It is possible, however, that negroes owe their superior powers of resistance in this instance to their long experience of malaria, for it seems to be true that persons immune to malaria are, for some reason, immune to yellow fever also, even when they are emigrants from regions where the latter has never prevailed; in other words, it seems to be true that evolution against yellow fever proceeds on identical, or nearly identical, lines as that against malaria, and therefore that extended racial experience of the one disease confers protective powers against the other. However this may be, it is at least certain that zymotic diseases of the non-malarial type, if not quite unknown, were almost unknown, over the whole of the New World, and were quite unknown over by far the greatest part of it. At the very least it is certain that all the non-malarial zymotic diseases of the Old World were quite unknown throughout the whole of the New World, and this without the exception even of yellow fever, which must have been originally 'either a West Indian or an African disease. It can scarcely have been both, for it is not likely that an identical species of parasitic micro-organism can have been evolved simultaneously and independently in two far-distant parts of the world.
Among the races of the New World, therefore, survival of the fittest can have caused no evolution of resisting power against the non-malarial zymotic diseases of the Old World, since there was no elimination of the unfittest from that cause, and therefore, when these diseases arrived in the train of conquerors, colonists, missionaries, and traders from their ancient seats, there then began the greatest tragedy the world has ever known; a tragedy so tremendous that, beside it, all the combined tragedies caused by all wars in all countries during all time shrink into insignificance. Wherever the races
