THE DEVELOPMENT 9 



anthrax and usually succumb. Furthermore, he discovered 

 that in the bodies of animals artificially inoculated there 

 were present the same organisms which he introduced, and 

 that he could get pure cultures again from the tissues of the 

 animal. His proof of the causal relationship of organisms 

 to disease was quite adequate and complete. 



Many investigators began studying other diseases and 

 soon bacteria were discovered which caused many of them. 

 These facts were of course in contradiction to the sup- 

 posed facts which had governed the treatment of disease 

 formerly, and there was much opposition to the acceptance 

 of the statement that bacteria can cause disease. The proof 

 finally became overwhelming, and the whole development of 

 medicine was modified in consequence. The importance 

 of bacteria as causes of disease led to a large amount of 

 study, and gave the needed incentive to the rapid develop- 

 ment of the science of bacteriology, and to the modern con- 

 ception that diseases are of two classes; the so-called in- 

 fectious diseases, which are caused by microorganisms; and 

 the non-infectious diseases, which are not so caused. The 

 development of the germ theory of disease led also to a 

 study of the means by which the animal* body resists dis- 

 ease, that is, to the development of immunology. It re- 

 sulted also in a study of the means by which disease-pro- 

 ducing microorganisms travel from one person to another, 

 that is, to the development of sanitary bacteriology. 



During the growth and development of the science of 

 bacteriology, and in the laboratory studies necessitated by 

 the various controversies noted, many facts of interest and 

 importance to agriculture were discovered. The knowledge 

 that microorganisms cause fermentation and disease led to 

 a study of foods, particularly of milk and milk products, 

 and to the development of dairy bacteriology. A study of 

 bacteria in their relation to the nitrogen of the soil, and the 

 discovery of bacteria living in the roots of leguminous 



