Before leaving the discussion of daily variations in numbers of protozoa, reference must be made to the flagellate species. As already mentioned, their active numbers fluctuate rapidly, and for the most part entirely irregularly. One species, however, _Oicomonas termo_, is characterised by possessing a periodic change; high active numbers on one day being succeeded by low, which are again followed by high on the third day. This rhythm was maintained, with few exceptions, for 365 days (Fig. 16), and has been shown to take place in artificial culture kept under controlled laboratory conditions (Fig. 17). [Illustration: FIG. 17.--Daily variations in the numbers of active individuals of _Oicomonas termo_ (Ehrenb.) in artificial culture media kept at a constant temperature of 20° C. A, in hay infusion; B, in egg albumen. X-axis: Days Y-axis: Thousands] It was thought that an explanation of this phenomenon might be found in alternate excystation and encystation, since the latter is a constituent part of the animals’ life history (see p. 73). This, however, does not hold, for the cyst curve is not the inverse of that of the active; and, moreover, statistical treatment demonstrated that cyst formation is wholly unperiodic in character. An explanation must therefore be sought in the changes in the organisms during the active period of their life, and the deduction can be drawn that, increased active numbers tend to be followed by death, conjugation, or both, while decreases in the active numbers are followed by rises in total numbers, i.e., reproduction, and this rhythmically. This somewhat surprising conclusion appears to hold, in a lesser degree, for other soil protozoa, and is of sufficient importance to warrant further research. The direction in which this is being pursued is by a study of the reproductive rates of pure cultures of certain ciliates and flagellates under varying external conditions. Space does not admit of adequate discussion of this problem, but the results already obtained justify the view that such lines of work will elucidate some of the baffling problems of soil micro-biology. SOIL REACTION. The development of the artificial fertiliser industry has in many ways revolutionised farm practice, with the inevitable result that new problems have arisen, not the least of which are biological in character.