80 



Mr. E. J. Russell. 



out of action ; they are not completely killed, but only suppressed to a 

 greater or less degree, when the detrimental factor is only partly put out of 

 action.* 



Now this parallelism between the properties of the detrimental factor and 

 the protozoa is not proof that the protozoa constitute the limiting factor, but 

 it affords sufficient presumptive evidence to justify further examination. The 

 obvious test of adding cultures of protozoa to partially sterilised soil was made, 

 but no depression in bacterial numbers was obtained, instead there was some- 

 times a rise. But in view of the history of investigations on malaria and 

 other protozoan diseases no great significance was attached to this early 

 failure. 



No attempt had been made in any of the above experiments to identify the 

 protozoa or even to ascertain whether any particular form existed in the soil 

 in the trophic state or as cysts. The variety of forms was considerable, and 

 it soon became evident that a definite protozoological survey of the soil was 

 required. 



This was accordingly put in hand. In order to give the survey as 

 permanent a value as possible the investigations were not confined to the 

 narrow issue whether soil protozoa do or do not interfere with soil bacteria, 

 but they are put on the broader and safer lines of ascertaining whether a 

 trophic protozoan fauna normally occurs in soil, and, if so, how the protozoa 

 live, and what is their relation to other soil inhabitants. 



The first experiments, made by Goodey,f indicated that the protozoa 

 were present only as cysts. Subsequent investigations, however, by Martin 

 and Lewin have established the following conclusions.^ 



1. A protozoan fauna in a trophic state normally occurs in soils. 



2. The trophic fauna found in the soil differs from that developing when 

 soil is inoculated into hay infusions : the forms which appear to predominate 

 in the soil do not predominate in the hay infusions, and vice versa the forms 

 predominating in the hay infusions do not necessarily figure largely in the soil. 



3. The trophic fauna is most readily demonstrated, and is therefore 

 presumably most numerous, in moist soils well supplied with organic manures, 

 e.g., in dunged soils, greenhouse soils, sewage soils and especially glasshouse 

 " sick " soils. 



It is obvious that the protozoa which live largely on bacteria must function 

 as a factor limiting the numbers of bacteria. The problem is therefore reduced 



* Buddin, ' Journ. Agric. Sci.,' vol. 6, pp. 417-451 (1914). 

 t Goodey, ' Boy. Soc. Broc.,' B, vol. 84, p. 165 (1911). 



% Martin and Lewin, 1 Phil. Trans.,' vol. 205, pp. 77-94 (1914), and ' Journ. Agric. 

 Sci.,' vol. 7, pp. 106-119 (1915). 



