46 LOUIS PASTEUR 



may subsequently emerge and begin the usual 

 course of their lives. A few forms have been kept 

 for several years in the encysted state and subse- 

 quently revived. Some of the soil Amoebae when 

 in these cysts are able to withstand several minutes 

 of boiling without being killed. 



The ability of many Protozoa to withstand 

 drought while in the encysted state greatly favors 

 their wide dissemination. These cysts may be 

 blown in the dust of the air like the spores of mold. 

 If we make an infusion by boiling some animal or 

 vegetable material and then set it aside for a few 

 days exposed to the air, we shall probably find it 

 to be teeming not only with bacteria but with sev- 

 eral species of Protozoa. Or if I soak a bit of 

 dried hay in water for a few days and then examine 

 it with the microscope, I would probably observe 

 a veritable menagerie of small animal forms of the 

 most diverse kind. I well remember when as a boy 

 after becoming the proud possessor of a compound 

 microscope I used to make up all sorts of decoc- 

 tions, malodorous and otherwise, and examine them 

 for the living creatures that somehow mysteriously 

 came to develop in them. This world of life with 

 its rolling ciliated infusorians, swiftly darting 

 flagellates, and sluggish slowly-crawling Amoebae, 



