20 PLACE OF MICROORGANISMS IN NATURE 



opment it has been possible to progress more rapidly 

 and definitely with bacteria than with protozoa. 

 Bacterial life and activity can be controlled very 

 largely now, but as yet little or nothing is known of 

 the important vital activities of the minute animals. 



As in the case of bacteria, so the earliest records of 

 protozoa are those of Van Leeuwenhoek's animalculse. 

 Their natural history has been gradually developed 

 by Jablot, Dujardin, Prowaczek, and Biitschli, and 

 the present leaders in the field, Calkins and Doflein. 

 However, it is only within the last score of years that 

 we have been familiar enough with these lowest animal 

 forms to be sure of their species identity, and we are 

 yet imperfectly informed as to their vital phenomena. 



PLACE OF MICROORGANISMS IN NATURE. 



The studies of the life history of bacteria and pro- 

 tozoa have been the work of botanists, chemists, and 

 physicians. Through this combined effort it has 

 become known that these minute forms are present 

 in or upon or have something to do with the life of all 

 the higher animals and plants. The number of species 

 in all is legion. The number of species pathogenic 

 for animals is but small. A microorganism is patho- 

 genic when it is capable of producing some form of 

 disease in the animal in which it is a parasite. In 

 Chapter III some of the known relations of non- 

 pathogenic bacteria will be discussed. It is sufficient 

 here to emphasize the difference between the so-called 

 parasites and saprophytes. Parasites are organisms 



