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biological and of this there can be no doubt. Russell and 

 Hutchinson's experiment, showing that the so-called pure 

 cultures of protozoa, when introduced into sterilised soil, 

 did not limit the growth of bacteria, was confirmed by me 

 and has since been proved by Goodey, 1 who in testing soils, 

 which had been stored in bottles for many years, found 

 that only those soils which had not been air-dried contained 

 the limiting factor. A soil which contained amoebse and 

 flagellates, upon being moistened, behaved as if the limiting 

 factor were absent. He showed that the presence of 10,000 

 amoebse per gram of soil was not enough to reduce the 

 bacterial content to the level of a similar soil containing 

 no protozoa, and he therefore concluded that the protozoa 

 cannot functionate as the limiting factor. I have shown 

 that air-drying, combined with antiseptic treatment, does 

 not vitally affect the typical soil protozoa. 2 



The factor may be an obscure member of the protozoa, 

 just as it may be a filterable micro-organism, but at this 

 stage of progress in the matter such a contingency cannot 

 be entertained. We are therefore driven to consider the 

 bacteria and moulds as being the probable agents. The 

 conditions that destroy the limiting factor destroy some of 

 the bacteria and moulds. The bacteria affected include 

 the nitrifying organisms and the sulphur-oxidising bacteria 

 and many other delicate forms. The net result is the 

 temporary establishment of a new flora, and it appears to 

 be only when the old and natural flora is re-established that 

 the limiting factor returns. It would be hopeless to con- 

 sider the matter as pertaining to the whole flora, because 

 this would lead us to nowhere. It is to the single members 

 or the single classes that we must turn our attention. 

 Hiltner and Stormer early showed that the denitrifiers and 

 the streptothrix varieties were almost completely destroyed 



1 Proc. Koy. Soc, B. 606 (88) p. 437. 

 8 Proc. Lion. Soc. New South Wales, 1914, 839. 



