It has usually been assumed that the changes in the external conditions markedly affect the density of the soil population. To test this the environmental conditions--temperature, moisture content and rainfall were examined; but contrary to all expectation no connection could be traced between any of these and the daily changes in numbers of any of the organisms investigated, and moreover the species of protozoa appeared in the main to be living independently of one another. It is difficult to believe that external conditions are as inoperative as appears from the above; and in view of the known complexity of the soil it is possible that further research will show that certain combinations of external conditions are important agents in effecting the changes. [Illustration: FIG. 11.--Numbers of active amœbæ (Dimastigamœba and Species α) and bacteria to 1 gram of field soil for typical periods in February and April, 1921. (From Phil. Trans. Roy. Soc., vol. ccxi.) X-axis: Feby. Feby. April Y-axis (left): Amoebae Active numbers per gramme of soil Y-axis (right): Bacteria millions Legend: Dimastigamoeba Species α Bacteria] In the case of the bacteria, however, the agent causing the fluctuations is mainly the active amœbæ. This was well shown during the year’s count, for with only 14 per cent. of exceptions, 10 per cent. of which can be explained as due to rapid excystation or encystation, a definite inverse relationship was established between the active numbers of amœbæ and the number of bacteria (Figs. 11 and 12). Thus a rise from one day to the next in the amœbic population was correlated with a fall in the numbers of bacteria and vice versa. It must not be supposed that the flagellates are of no account in this process; some species, known to eat bacteria, undoubtedly induce slight depressions, but, owing to their small size, any effect is masked by the greater one of the amœbæ. [Illustration: FIG. 12.--Numbers of active amœbæ (Dimastigamœba and Species α) and bacteria in 1 gram of field soil for typical periods in September, October, and November, 1920. X-axis: August September October Y-axis (left): Amoebae, thousands Y-axis (right): Millions, Bacteria Legend: Dimastigamoeba Species α Bacteria] These experiments seem to admit of no doubt that in field soil the active protozoa are instrumental in keeping down, below the level they might otherwise have attained, the numbers of bacteria; but a further proof of this contention ought to be obtained by inoculation experiments. It should be possible, by inoculating sterile soil with bacteria alone and with bacteria plus protozoa, to demonstrate fluctuations in bacterial numbers in the latter, while those of the former remained constant. This admittedly crucial test has often been tried, but owing to difficulties in technique, etc., has always failed. Recently, however, by using new methods confirmatory results have been obtained.[5]