INTRODUCTION. 1 7 



caused by a protozoon. Theobald Smith in the United States 

 discovered that the parasite of Texas fever is conveyed from one 

 animal to another by the cattle-tick. Since then it has been 

 shown (by Manson, Ross and others) that malaria is conveyed 

 from a person having the disease to one not affected by means of 

 mosquitoes. It has also been shown by Reed and Carroll that 

 a similar relation exists between mosquitoes and yellow fever. 

 The part played by flies and other insects in carrying disease 

 germs is still receiving active attention and the future may show 

 that these play a most important part in diseases other than those 

 already mentioned. 



In 1903 Novy and McNeil succeeded in making pure cultures 

 of pathogenic protozoa grow in tubes, in nearly the same way 

 that cultures of bacteria are propagated (see Appendix). 



It is encouraging to reflect that the progress of bacteriology 

 has been made by gradual and logical steps. The great dis- 

 coveries have not been lucky accidents, but have been worked 

 out patiently and with deUberation. 



