This conclusion aroused considerable controversy. It was maintained that protozoa were not normal inhabitants of the soil, but only occasional visitants, and, in any case, they were only there as cysts; the soil conditions, it was urged, were not suitable to large organisms like protozoa. The objection was not to be treated lightly, but, on the other hand, the experiments seemed quite sound. As neither Dr. Hutchinson nor the writer were protozoologists, Dr. T. Goodey and (after he left) Mr. Kenneth R. Lewin were invited to try and find out, quite independently of the partial sterilisation investigation, whether protozoa are normal inhabitants of the soil, and if so, whether they are in a trophic condition, and what is their mode of life and their relation to soil bacteria. Had it turned out that protozoa had nothing to do with the matter, search would have been made for some other organism. Goodey showed that the ciliates were not particularly important; Lewin soon demonstrated the existence of trophic amœbæ and flagellates. Unfortunately he was killed in the war before he had got far with the work. After the Armistice, Mr. Cutler accepted charge of the work: he will himself relate in Chapters IV. and V. what he has done. At first sight it might be thought comparatively easy to settle a question of this kind by examining soil under a microscope or by sterilising it and introducing successively bacteria and known types of protozoa. Unfortunately neither method is simple in practice. It is impossible to look into the soil with a microscope, and methods of teasing-out small pieces of soil on a slide under the high, or even the low power, give no information, because the particles of soil have the remarkable power of attracting and firmly retaining protozoa, and no doubt bacteria as well; indeed, for protozoa (which have been the more fully investigated) there seems to be something not unlike a saturation capacity (see Fig. 9, p. 78). Further, complete sterilisation of soil cannot be effected without at the same time altering its chemical and physical properties, and changing it as a habitat for micro-organisms. Cutler has, however, overcome the difficulties and shown that the introduction of protozoa into soils sterilised and then reinfected with bacteria considerably reduces the numbers of these organisms. The method adopted, therefore, is to take a census of population and of production. Counting methods are elaborated, and estimates as accurate as possible are made of the numbers of the various organisms in a natural field soil at stated intervals. Simultaneously, wherever possible some measure is taken of the work done. The details of the census are finally arranged in consultation with the Statistical Department, to ensure that the data shall possess adequate statistical value. From the results it is possible to adduce information of great value as to the life of the population, the influence of external conditions, etc.