C. H. Martin and K. R. Lkwin 107 



Under the varying conditions which obtain in a soil there must be 

 continually changing relations between these two faunas, but at any 

 moment only the members of the active fauna of that period can exert 

 any effect on the soil. To guard against possible misunderstanding, 

 it may be well to state that it is very improbable that the line between 

 the active and the cyst fauna of a soil is one between species and 

 species. There is little doubt that under most conditions a species 

 represented in the active fauna will also be represented in the con- 

 temporaneous cyst fauna. 



Since the cultural method fails to distinguish between the above 

 two categories, and even leaves unsettled the question of whether an 

 active fauna is present at all, recourse has been made to other methods 

 of examination, which are fully described in the next section. By 

 their aid it has been completely estabhshed that an active fauna does 

 exist in a variety of soils ranging from the unmanured plot on Broad- 

 balk field at Rothamsted to sewage-farm soil, leaf mould, and soil from 

 a cucumber border. Some of the results obtained by the examination 

 of these soils will be found in section III. 



As regards the forms found, it is improbable that many are generi- 

 cally new; most of them seem to have been described by the older 

 workers on protozoa. Of recent years a very large amount of the 

 literature on protozoa, including the more recent textbooks on proto- 

 zoology, have been devoted almost exclusively to parasitic forms, so 

 that a worker on soil forms must refer back to the excellent papers of 

 the older authors. References to some of these works will be found 

 in the Uterature list. 



Before the effect of protozoa on the soil can be adequately discussed, 

 it is necessary to gather information about the life led by the protozoan 

 famia. In particular the effect exercised on the active fauna by the 

 water content, the density of the bacterial flora, the temperature, etc., 

 must be investigated. 



Now whilst soil temperatures can readily be determined with suffi- 

 cient accuracy, the evaluation of the other two factors presents 

 considerable, and in part unsolved, difficulties, which arise largely 

 from the heterogeneity of the soil. 



Thus the present method for determining water content deals 

 usually with samples taken to a depth of nine inches. It is clear, 

 however, that if in dry weather a crust has been formed on the surface 

 of the soil, the protozoa may be active at a lower level which might 

 still have a relatively high water content, so that the figure obtained 



