THE ORIGINS OF ZYMOTIC DISEASES 175 



Columbus. The subject is involved in obscurity, but, while it 

 is evident that the European adventurers introduced many 

 diseases, there is no clear indication that they found and 

 brought back one. Apparently all the diseases which have 

 been prevalent in Europe and America during the last four 

 hundred years were prevalent in the former continent before 

 the fifteenth century. Syphilis and yellow fever have some- 

 times been regarded as exceptions. But syphilis was well 

 known to the Roman physicians and was common during the 

 Middle Ages. 1 Moreover, the inhabitants of the New World 

 take the disease in a very acute form, and it is not found in 

 remote communities to which Europeans have had no access. 

 Yellow fever was first noted with certainty in the West 

 Indies in the middle of the seventeenth century. The 

 records of the time " tell of the importation of the disease 

 from place to place, and from island to island." 2 Not till 

 more than a century later was it observed on the West Coast 

 of Africa. There can be no doubt that the earlier observers 

 confused yellow fever with bilious malaria, and that it was 

 present both in the West Indies and Africa long before a 

 differential diagnosis was made. 3 The fact that of all races 

 Negroes are most resistant to the disease would seem to 

 indicate West Africa as the place of origin. In any case it 

 is certain that with the exception of malaria, zymotic disease, 

 if not entirely absent, was extremely rare in the New World. 

 That fact is of high significance. It demonstrates the ex- 

 treme difficulty with which saprophytic organisms evolve 

 parasitic habits, and the extreme improbability of the 

 hypothesis that any disease has had multiple origins in 

 more than one centre. 



294. We have no means of judging whether the enormously 

 prolonged separation between the inhabitants of the Old and 

 the New Worlds occurred before or after the evolution of 

 zymotic disease among the former. Even if it had occurred 

 before the separation, zymotic disease could hardly have 

 persisted among the isolated bands of roving hunters, who 

 penetrated into lands where there were no other human 

 beings. Even contagious disease could hardly have persisted 

 under conditions so adverse. The fact that malaria was 

 found on both sides of the Atlantic and the Pacific is very 

 remarkable. It indicates, perhaps, that some at least of the 

 human invasions of America occurred otherwise than by way 

 of the Arctic Zone. Since men have drifted in boats to 



1 Hirsch, vol. ii., p. 60. 2 Op. cit., vol. i., pi. 317-18. 



8 Op. cit., p. 317. 



