THE PRESENT EVOLXJTION OF MAN — PHYSICAL 291 



races negroes are resistant to it. It is possible, how- 

 ever, that negroes owe their superior powers of resist- 

 ance 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 simul- 

 taneously 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 



