PRESIDENTIAL ADDRESS. 53 



numbers of the bacteria in the raw soil as a standard, and 

 the numbers obtained following treatment with heat or 

 volatile antiseptics as an increase, they reversed the point 

 of view and considered the numbers in the raw soil as being 

 anomalous. The irregularity was caused by something 

 which limited the natural increase, and in casting about 

 for a clue to this limiting factor, they noted from various 

 experiments which they made, that it was put out of action 

 by volatile antiseptics sufficiently potent to kill the nitrify- 

 ing organisms, and by heat above 55°. If the soils were 

 kept free from reinfection, the factor did not reappear, even 

 although the conditions were made very favourable for the 

 growth of bacteria. A lower degree of heat put it only 

 temporarily out of action, and if the soil was kept under 

 favourable conditions it soon reappeared. It could be 

 reintroduced into a treated soil by adding a small quantity 

 of a raw soil. It developed more slowly than bacteria, for 

 the effect was not seen for some time, and yet it appeared 

 to be biological, for the conditions that favoured its activity 

 were precisely those which favoured living organisms. It 

 did not appear to be bacterial nor of the nature of a toxin, 

 nor due to any adverse physical or chemical state of any of 

 the soil constituents; indeed, there was no escape from the 

 conclusion that it was a living organism, and the only 

 organisms that appeared to agree with the observed facts, 

 were the protozoa. 



The phagocytic tendency of the protozoa, such as the 

 larger ciliates, especially Colpoda cucullus and the amoebae, 

 was considered to be the reason for the numbers of bacteria 

 being kept down to a comparatively low level. Although 

 in their first paper, Russell and Hutchinson say that the 

 protozoa are only one reason for the limitation of the 

 bacteria, yet later papers give the impression that they 

 consider the protozoa to be the one and only reason. Indeed 



