THE PRESENT EVOLUTION OF MAN — PHYSICAL 279 



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 patho- 

 genic micro-organisms, which, however, i« 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. 



