INTRODUCTION. 15 



In 1834 the parasite of itch (Acarus scabiei, the itch mite, an 

 arachnid, related to the insects) was discovered, and the cause 

 of one contagious malady determined. 



Quite early in the nineteenth century also the relatively large 

 fungi of thrush and some of the parasitic skin diseases were 

 discovered. The bacilli of anthrax, which are also relatively 

 large, were seen in the blood of animals by Pollender in 1855 

 and Davaine in 1863. 



Davaine produced anthrax in animals by injecting into them 

 blood containing anthrax bacilli. But complete proof that 

 these bacilli were the cause of the disease required that they 

 should produce it when injected alone and when freed from the 

 smallest trace of material derived from the first diseased 

 animal. Unless these conditions were complied with, some 

 other material, for example an enzyme or ferment, might be 

 supposed to be carried from the first to the second animal and 

 to be the real cause of the disease. For this purpose it was 

 necessary to cultivate the bacilli outside the animal body in an 

 artificial medium of some kind, such as meat broth, as was 

 done by Pasteur. It then became possible to demonstrate 

 that their properties could remain unaltered after being grown 

 in successive generations on different lots of broth. As bacteria 

 of two or three species were often encountered in mixtures, it 

 became most important to secure a method by which the dif- 

 ferent species could be separated from one another and be 

 propagated as separate "pure cultures." This was done suc- 

 cessfully by diluting such mixtures greatly, so that a drop 

 planted in a new tube of broth should contain only a single 

 organism. The growth ensuing would of course consist of the 

 same kind of organism exclusively. Such procedures were 

 uncertain and very laborious. 



Koch introduced in 1881 his method of separating bacteria 

 by "plating," described below (Part L, Chapter V.), and this 

 is probably the most important contribution to bacteriological 



