1900] MICROSCOPICAL JOURNAL. 20i 



was : (1) That Northern cattle pastured in a field with 

 cattle from the infected district which were infested with 

 ticks contracted Texas fever ; (2) that Northern cattle 

 pastured in a field with cattle from the infected district 

 that were carefully freed from all ticks by hand picking 

 did not contract Texas fever ; (3) that Northern cattle 

 pastured in a field where no cattle from the infected dis- 

 trict had been, but over which had been scattered a large 

 number of ticks, contracted Texas fever. 



The result of these experiments was a distinct and posi- 

 tive advance in our knowledge of the disease. It was now 

 known (1) that the disease was inoculable ; (2) that the 

 blood of diseased animals contained a microscopic proto- 

 zoan parasite ; (3) that ticks picked from Southern cattle 

 and spread upon pastures were a means of communicat- 

 ing the infection. 



It was next important to learn in what manner the ticks 

 conveyed the contagion. From a medical point of view 

 the most plausible theory was that the biting parts of the 

 ticks became soiled with the blood of the Southern cattle, 

 and that these contaminated ticks, migrating to suscepti- 

 ble cattle, carried the virus and inserted it when they be- 

 gan sucking blood from the latter. A study of the life 

 history of the tick showed, however, that this theory was 

 not consistent with the facts. The ticks do not leave one 

 animal and go to another. When they are once upon an 

 animal they remain there until they become mature, and 

 then they drop off, lay their eggs on the surface of the 

 ground, and die. There is no opportunity for this para- 

 site to carry blood directly from the Southern to the 

 Northern animal and inoculate it. 



Another hypothesis was that with the blood sucked from 

 Southern cattle the tick took into its body the virus of the 

 disease, and that, when the mother tick died and became 

 disintegrated upon the pastures, the contagion was liber- 

 ated and the grounds infected. This supposition was en 



