vii.] THE PROTOZOA OF THE SOIL 235 



numbers of bacteria in check, why otherwise do they 

 not run up indefinitely when the soil is placed under 

 favourable conditions as to temperature and moisture, 

 since the food supply is adequate ? The heating and 

 other treatment gets rid of this inhibiting factor, as it 

 does of the majority of the bacteria ; but those which 

 remain, being free from the check previously imposed 

 upon their development, multiply with great rapidity 

 to an extent that they never previously could attain. 



The experiments indicated that the factor in question 

 must be a vital one, and Russell and Hutchinson con- 

 cluded that it resided in a group of organisms whose 

 existence in the soil, though known, had hitherto been 

 regarded as without significance. These are certain 

 organisms, large as compared with bacteria, definitely 

 belonging to the animal kingdom, and called, from their 

 simplicity of structure and primitiveness of type, the 

 Protozoa. The protozoa, of which three main groups 

 are found in soil amoebae, ciliates, and flagellates, are 

 single-celled organisms, varying in size from 5 to 50 jj., 

 which exist in the water films surrounding the soil 

 particles and can only be active when there is sufficient 

 moisture in the soil. They are mobile, moving, in the 

 case of amoeba by pushing out arms and flowing into 

 them, in the other cases by their cilia or flagellum, and 

 they obtain their nutriment by consuming bacteria. 

 Though they are found numerously in all decaying 

 organic materials and in putrefactive liquids, they cannot 

 feed upon organic matter alone, but only upon the 

 bacteria present in the material. The experiments 

 show that there is an active population of protozoa in 

 moist soil, increasing with its moisture and richness in 

 organic matter. It is difficult to estimate numbers, 

 but it is something in the order of 50,000 to 100,000 per 

 gram of ordinary soil. They are killed off by relatively 



