returned from a residence abroad than do their compatriots, or than they themselves did before they left their homes; but even this acquired character, like all similar characters, depends on an inborn trait developed by natural selection, for negroes exhibit greater powers of acquiring it than do Europeans.
Tuberculosis.—Man's evolution against tuberculosis is not less marked than his evolution against malaria, but owing to the insidious nature of the former disease, the gradual character of the attack, the slowness with which symptoms supervene, it is not so striking to the casual observer. In malaria toxins are present in abundance, and are of great virulence, and therefore the person attacked passes in a few hours from apparent health to extreme illness. Within twenty-four hours of entering an infested country he may manifest the symptoms of a virulent seizure. Ships navigated by men of a race that has undergone no evolution against the disease, may have their whole crews stricken down on entering a malarious port, while the natives around retain their health. Invading armies from beyond the borders of malaria have been decimated, and rendered useless as fighting forces, while the inhabitants of the land were able to pursue their ordinary avocations. Moreover, in malarious countries, the pathogenic micro-organisms are everywhere present in enormous numbers, and therefore no susceptible person escapes infection.
But in tuberculosis the toxins are conspicuously feeble. Infection is not marked by sudden and manifest illness. A slow, long-continued "personal" struggle ensues between the phagocytes and the pathogenic micro-organisms, which, however, is shorter in the less resistant than in the more resistant, in, generally speaking, the men of a race to which the disease is strange than in those of a race to which it is familiar.
