14 MICROORGANISMS IN SOILS AND WATER BASINS 



other instances are found in soil and water of an organism preparing 

 the substrate for another, ranging from distinct symbioticism, where 

 one organism depends absolutely for its living processes upon the ac- 

 tivities of another (symbiosis), to association, where one organism 

 merely is favored by the growth of another (metabiosis), to the injury 

 of one organism by another (antagonism), and finally, to the actual 

 destruction of one by another (parasitism). 



INTRODUCTION OF DI SE ASE- PRODUCI N G 

 MICROORGANISMS INTO THE SOIL 



Ever since higher forms of life first made their appearance on this 

 planet they have been subject to attack by microbes. These microscopic 

 organisms must have gained, at an early stage in the development of 

 the higher forms, the capacity of attacking them in one manner or an- 

 other. There is no plant or animal now living that is not subject to in- 

 fection by different bacteria, fungi, and protozoa. The more advanced 

 the animal body is in the stage of evolution, the more numerous are its 

 ills, most of which are caused directly or indirectly by microorganisms. 



The microbial agents causing thousands of diseases of plant and ani- 

 mal life have now been recognized and even isolated and described. In 

 many cases these disease-producing agents are closely related morpho- 

 logically to those which lead a harmless existence in soils or water 

 basins J many of the saprophytes, for instance, are found to be of great 

 benefit to man and to his domesticated plants and animals. This sug- 

 gests the probability that pathogenic microorganisms represent certain 

 strains of soil and water-inhabiting types that have become adjusted to 

 a parasitic existence. During their life in the host, they multiply at a 

 rapid rate and produce substances toxic to the body of the host. The re- 

 sult is that the host is incapacitated for a certain period of time, until it 

 succeeds in building up resistance against the invading organisms. It 

 may thus overcome the injurious effect of the pathogen or it may be 

 killed if such resistance cannot be effected. In the first instance, a tem- 

 porary or permanent immunity against the specific disease-producing 

 microbe or its close relatives may result. The host is often able to sur- 

 vive the attack without being able to destroy the invading microbes j if 



