208 Investigations in Plant Pathology 



the time of the Greeks, but we must carefully distinguish between hypo- 

 thesis and proof. What Arthur and Findlay and others suspected detracts 

 in no way from the honor due to the men who first experimentally demon- 

 strated the existence of carriers of diseases. . . . Merton B. Waite, working 

 on pear blight, proved conclusively the transmission of the disease by 

 bees, from whose mouth parts he again cultivated the parasite (1889- 

 1891). . . . Theobald Smith, working on Texas fever of cattle, discovered 

 the parasite to be a protozoan inhabiting the red blood corpuscles and he 

 and Kilbourn demonstrated that the carrier of the disease from infected 

 to healthy cattle was a blood sucking tick (1889-1893). 



The search for intermediary hosts in the transmission of plant 

 diseases was not new. Recollect Farlow's work on the cedar ball 

 fungus which begins its life on cedars, spends its first year there, 

 in the spring emits gelatinous projections which when rain falls 

 becomes a mass, and from which spores, windborne, infect fruit 

 trees miles away with orchard rust, then in the late summer to 

 begin another life cycle. Recall the work of DeBary, Ward, and 

 others, tracing fungous life histories through developmental stages 

 and more than one host. 



Proof of disease transmission by insects, however, was new. 

 Before the end of the century, an Englishman, Sir Ronald Ross, 

 would demonstrate that the mosquito, as an intermediary host, 

 transmits malaria. Others earlier had suggested this. But Ross's 

 work, supplying knowledge of the method of action of the trans- 

 mitting agency as well as the cause of the disease, made possible 

 a period of practical prevention and extermination of the malady 

 in seriously infected regions. *^^ He, for the value of his proof, 

 received one of the first Nobel prizes for medicine. 



During the summer of 1889, to test the prevailing theory that 

 ticks caused southern cattle fever, Dr. F. L. Kilborne of the 

 Bureau of Animal Industry arranged at the Bureau's Washington 

 Experiment Station some preliminary experiments. In one en- 

 closure he placed North Carolina cattle with native cattle, and 

 picked off the ticks of all southern stock. This was done to prevent 

 any ticks from maturing or depositing eggs which might later 

 infect the animals. In another enclosure the ticks were not 

 removed from the southern cattle. Native cattle, mixed with these, 

 contracted Texas fever and died, whereas the cattle of the first 

 enclosure lived, unharmed by the disease. In September further 



'^H. Gushing, The life of Sir William Osier, op. cit., 1: 274; 2: 164. 



