Superimposed on the daily variations in numbers there are seasonal changes, as is clearly shown when fourteen day averages are made of the numbers for each species. Bacteria have long been known to show autumn and spring rises, but recent research has demonstrated that the protozoan population also rises to a maximum at the end of November, with a less marked spring rise at the end of March and beginning of April (Figs. 14 and 15). It has sometimes been claimed that the numbers of soil organisms are closely linked with the soil moisture, but no support for this view was found during the course of the experiment. Similarly, as in the case of the daily variations, no connection could be traced between the seasonal changes and any of the external conditions considered. It is interesting to note, however, that the seasonal variations in the numbers of soil organisms is very similar to those recorded for many aquatic organisms. Miss Delf,[8] for instance, found that in ponds at Hampstead the algæ are most numerous in spring and again in the autumn, and like changes are recorded in British lakes by West and West[25] and in the Illinois river by Kofoid.[14] [Illustration: FIG. 14.--Fortnightly averages of total numbers of Oicomonas, Species γ, and Species α, and of bacteria, moisture, and temperature. (From Phil. Trans. Roy. Soc., vol. ccxi.) X-axis: Fortnight beginning 1920. July. Aug. Sep^t. Oct. Nov. Dec. Jan 1921. Feb^y. Mch. April. May. June. Y-axis (bottom left): Percentage of moisture Y-axis (top left): Logarithms of numbers of active protozoa per gramme of soil Y-axis (bottom right): Temperature F Y-axis (top right): Bacteria in millions per gramme Legend: Oicomonas Species γ Species α Bacteria Temperature Moisture] It is difficult to resist the conclusion that these annual variations are produced by similar causes, from which it follows that the increase in the numbers of protozoa in the soil is not wholly conditioned by an increased food supply--the bacteria--for the algæ are not dependent on such a form of nourishment. This is substantiated by the fact that the numbers of protozoa, except those of _Oicomonas_, rose during March, whereas the corresponding increase in the bacteria was delayed till the early part of April. [Illustration: FIG. 15.--Fortnightly averages of total numbers of Heteromita, Cercomonas, and Dimastigamœba and of bacteria, moisture, and temperature. (From Phil. Trans. Roy. Soc., vol. ccxi.) X-axis: Fortnight beginning July 1920. Aug. Sep^t. Oct. Nov. Dec. Jan. 1921. Feb. Mar. April May June Y-axis (bottom left): Percentage of moisture. Y-axis (top left): Logarithms of numbers of active protozoa per gramme of soil. Y-axis (bottom right): Temperature F Y-axis (top right): Bacteria in millions per gramme Legend: Heteromita Cercomonas Dimastigamoeba Bacteria Temperature Moisture]