D. Ward Cutler and L. M. Crump 21 



Table III gives the number of bacteria per gr. of soil for 14 daily 

 samples. 



Table III. 



The case is different, however, when the trophic amoebae are con- 

 sidered, the numbers of which are given in Table IV. Fig. 5 shows the 

 bacterial and amoebic numbers in 14 daily samples. It is at once 

 evident that there is a very close correlation between the two: without 

 exception when the bacterial numbers are high those of the amoebae 

 are low, and vice versa. Russell and Hutchinson postulated such a re- 

 lationship in their partial sterilisation hypothesis, but we believe this 

 is the first time it has been so clearly demonstrated 1 . By inoculating 

 soil with Amoeba Umax Goodey(7) was able to bring about a de- 

 pression in bacterial numbers; but as far as we know data for a curve 

 from ordinary field soil such as Fig. 5 has never before been obtained. 

 This is primarily due to lack of methods. The earlier workers counted 

 the total numbers of amoebae (trophic and cystic); this as was to be 

 expected, gave variable results, and the method ought to be abandoned 

 in future investigations. The second source of error was the length of 

 time between the taking of the samples : they should be taken at frequent 

 intervals, daily if possible. For example, examination of Fig. 5 shows 

 that if after taking sample 17 an interval of three days had elapsed 

 before another was taken both the bacterial and the amoebic numbers 

 would have gone down, and a false conception of the relationship of the 

 two would have resulted. A similar error would have resulted had no 

 counts been taken between samples 19 and 25. 



Table IV gives the total, active and cystic numbers per gr. of soil for 

 the amoebae. 



Fig. 1 appears to negative the contention of the necessity of daily 

 sampling, for, with one exception the bacterial and amoebic curves fit 



1 Previous work by one of us showed that the experimental error by our mode of 

 counting bacteria was 17 per cent. This, in the present experiments, would not vitiate the 

 conclusion that there is interaction between amoebae and bacteria. 



