394 



ANIMAL BIOLOGY 



Most inspiring is the long story of heroism and hard work which 

 has made it possible to cope with yellow fever; made possible the 

 building of the Panama Canal, since the earlier attempt by France 

 was unsuccessful largely on account of its ravages. We even have 

 to be reminded that half a million cases occurred in the United 

 States during the past century: the epidemic of 1793 took a total 

 of one-tenth of the population of Philadelphia, and that of 1878 

 killed more than thirteen thousand in the Mississippi Valley alone. 



Fig. 248. — Results of a quarter century of yellow fever control. 

 (From the Rockefeller Foundation.) 



However, while this and more is true, the recent discovery of 

 new sources of infection in the interior of South America and 

 Africa has made the problem much more complex and the complete 

 control of the fever less certain than it appeared to be a decade 

 ago. (Fig. 248.) 



Syphilis. The brilliant investigations, chiefly of the proto- 

 zoologist Schaudinn in 1905, revealed the unicellular parasite, 

 Treponema pallidum, that is the cause of syphilis — one of the 

 greatest scourges of mankind since it became widespread during 

 the sixteenth century. The ravages of the parasite produce many 

 symptoms — frequently a type of paralysis, or paresis, with a 



