“It is difficult to see what agent other than a living organism can fulfil these conditions. Search was therefore made for a larger organism capable of destroying bacteria, and considerable numbers of protozoa were found. The ciliates and amœbæ are killed by partial sterilisation. Whenever they are killed the detrimental factor is found to be put out of action; the bacterial numbers rise and maintain a high level. Whenever the detrimental factor is not put out of action, the protozoa are not killed. To these rules we have found no exception.” From such premises as the above Russell and Hutchinson founded the “protozoa theory of partial sterilisation,” and at Rothamsted there was commenced the serious study of these soil organisms. Goodey was one of the early workers on this new subject, and added considerably to our knowledge of the species living in normal soils, and of the chemical constitution of the cyst wall of ciliates. He also made investigations on the effects of various chemicals on the micro-population of soils, but was unable to draw very definite conclusions.[11] One of the first criticisms raised against the protozoa theory of partial sterilisation was that the protozoa were not normal inhabitants of the soil, and were present only in small numbers, all of them in the cystic, quiescent condition. It was further held that these cysts were carried by the wind from dried-up ponds and streams. It is difficult to trace the origin of this view, since early observers, viz., Ehrenberg and Dujardin, in 1841, were of the opinion that the protozoa were living in the trophic active condition in the soil, and it was not until 1878 that Stein showed that free living protozoa can encyst. To Martin and Lewin, however, must be ascribed the distinction of first proving that the soil possesses an active protozoan population, for by a series of ingenious experiments these observers isolated several flagellates and amœbæ in a trophic condition from certain of the Rothamsted soils.[18] The more recent work in this country has been in the direction of devising new quantitative methods of research, since by this means alone is it possible to elucidate many fundamental questions. In America and elsewhere experiments have been devised for testing the conclusions of Russell and Hutchinson. Cunningham and Löhnis,[2] in America, Truffaut and Bezssonoff,[24] in France, supply evidence in favour of the theory, but most of the American work is in opposition to it. Sherman[23] is perhaps the most prominent in opposing the phagocytic action of protozoa on soil bacteria in spite of the fact that certain of his experimental results apparently show enormous decreases in bacterial numbers in the presence of protozoa. In many of his soil inoculation experiments, however, it was not demonstrated that his active cultures remained alive after entering the soil.