THE WEB OF LIFE 271 



found that when soils were heated or when they were dosed 

 with certain volatile antiseptics, and afterwards brought 

 into conditions favourable for plant growth, they showed a 

 great increase in fertility. Further inquiry showed that the 

 soil Bacteria are first reduced in numbers by the heating 

 or sterilizing, and that after a while they increase enor- 

 mously. To this increase is due a greater production of 

 ammonia in the soil, and to this, of course, the greater 

 fertility. But the puzzle is why the decrease after heating 

 or sterilizing should be followed by a great increase. 



The suggested solution of the puzzle is very interesting, 

 and it is instructive even though it may require subsequent 

 modification. There are many Protozoa in the soil, some 

 of which feed on Bacteria and thus limit their increase. 

 The Protozoa are more sensitive than the Bacteria to the 

 heating or sterilizing. There is a killing off in both camps, 

 but the Protozoa suffer most. In the period of recovery 

 the surviving Bacteria multiply enormously in the relative 

 absence of their enemies. This solution requires verifica- 

 tion, and our knowledge of the soil Protozoa is still too 

 scanty and vague. A great reward certainly awaits the 

 investigator of the Protozoa of the soil. Mr. T. Goodey 

 has listed about thirty already, but eighteen of these are 

 ciliated Infusorians which exist in the soil in an encysted, 

 not in an active state, and cannot therefore function as 

 Bactericides. 



MUTUAL DEPENDENCE FOB THE CONTINUANCE OF LIFE 



Two organisms inhabiting the same area may become 

 linked together in such a way that the continuance of the 

 life of one of the two is dependent on the presence of the 



