14 INTRODUCTION 



portant diseases, such as yellow-fever, dengue fever, poliomy- 

 elitis, measles, typhus fever, small-pox, rabies and hog cholera. 

 Knowledge of this group of organisms is still relatively meagre 

 and many features are still obscure or in controversy. Micro- 

 scopic methods of defining their form and structure are still poorly 

 developed but they cannot with justice be regarded as wholly 

 in the realm of the unknown. 



Agriculture. — The importance of microbes in soil fertility 

 and agriculture has a relatively short history. Duclaux, 1885, 

 showed that plants could not well utilize complex organic matter 

 as food in the absence of microbic life. In addition to ordinary 

 decomposition of organic matter, bacteria also bear an important 

 relation to the nitrogen metabolism of plants. Hellriegel and 

 Wilfarth (1886-88) showed the infectious nature of the nitro- 

 gen-fixing root tubercles of legumes, and the organism B. radicicola 

 was isolated by Beyerinck in 1888. The importance for agricul- 

 ture of other living elements in the soil, such as amebae and 

 nematodes, has been more recently recognized. 



Although it is well to recognize the many important applica- 

 tions of bacteriology, a word of caution may not be amiss, lest 

 we follow too eagerly the alluring applications and neglect the 

 secure foundation of scientific knowledge of the biology and bio- 

 logical relationships of micro-organisms, the proper training in 

 logical thinking concerning these beings and in the technic of 

 dealing with them. 



