54 K. GREIG-SMITH. 



in his latest paper, 1 a reply to Goodey's claim that the 

 protozoa are not responsible for the limiting factor, Russell 

 says that the protozoa are the only organisms fulfilling the 

 conditions that fit the limiting factor, and that there is the 

 closest possible relation between the extinction of the 

 protozoa and of the limiting factor. 



They considered that the limiting factor could not be a 

 toxin, for if it were, it would be sure to affect the more 

 sensitive nitrifying bacteria rather than the sturdy ammoni- 

 fying bacteria. In raw soils, nitrates may accumulate but 

 ammonia never does, and, as there can be nothing toxic to 

 the nitrifiers, it was very unlikely that there could be any- 

 thing toxic to the ammonifiers. Again, as no difference 

 was seen in the growth of barley seedlings grown in extracts 

 of raw or toluened soils, there could be no toxic substance 

 in the raw soil, or the effect would have been shown upon 

 the seedlings. The fact that treatment short of partial 

 sterilisation brought about a certain but slow limitation of 

 the bacteria, appeared to be an objection to any toxin 

 hypothesis, because one could not conceive a partially 

 destroyed toxin revivifying itself. 



It apparently did not occur to them that toxins may be 

 secreted by bacteria and moulds, and that almost complete 

 destruction of a particular toxin-producing species or group, 

 might account for the slow revivification of toxin. 



Among all the experiments bearing upon the protozoal 

 hypothesis, the most important are those in which it has 

 been shown that the limiting factor could not be introduced 

 into partially sterilised soils in any other manner than as 

 raw soil. Pure cultures of certain prominent bacteria were 

 without effect, and the so-called pure cultures of protozoa 

 were likewise inoperative. This is very important, as 



1 Proc. Eoy. Soc, (1915) B. 89, 16. 



