V. H. Maktin AM) K. H. Lkwin !(»!» 



active, and of the cyst fauna, has been used by L'uiiiiingh;ini (l) as 

 the basis of a method of determining the active fauna. He estimates 

 tlie total fauna of a soil, and, in a second sample, the cyst fauna; the 

 difference between the results is taken as a measure of the active fauna. 



Unfortunately, the I'esults obtained by a dilution method will 

 almost certainly be vitiated by the incomijleteness of the cultural 

 fauna. As has already been pointed out, present cultural methods 

 fail to indicate, or indicate very late indeed, an important class of 

 soil ])rotozf)a, the thecamoebae. Again, the manipulative errors of the 

 successive dilutions, together with the serious risk that shaking will 

 not result in an even distribution of the protozoa through the suspen- 

 sion, introduce a cumulative series of inaccuracies into a troublesome 

 and complicated method. Finally, in common with any other numerical 

 method, it encounters the weighty difficulty of the heterogeneous nature 

 of the soil. 



On the whole, therefore, it seems to us that this type of method 

 will be liable to introduce a specious appearance of accuracy into a 

 subject which bristles with difficulties'. 



A very rough, but still valuable, idea of the relative abundance of 

 active protozoa in soils is given, however, by the richness of the fresh 

 fixed films obtained as described on p. 112. In comparing different 

 types of soil only the most striking differences can be regarded as 

 significant, but in considering the variations in the active fauna of 

 one particular soil under changing conditions of temperature, moisture, 

 etc., it is probable that the index of richness of the films obtained will 

 prove a sound basis for general conclusions, although no hope can be 

 entertained of reaching numerical results by this method. 



II. Methods. 



It is exceedingly difficult, in an examination of any ordinary soil, 

 to get an adequate idea as to the abundance and nature of the active 

 fauna, and for this reason we have thought it well to describe some of 

 the methods we have found helpfid in this work. 



By far the simplest method of fixing and staining soil protozoa, 

 whether in cultures or on fresh films from the soil, is by means of cover- 

 shp films. We have usually stored the films in small corked tubes of 

 height 1^" and diameter 1|", and these tubes have been found very 

 convenient for purposes of fixation and staining. 



' This critioi.sm doe.s not apply to Cunningham's paper, where it is recognised that 

 precise numbers camiot be given. 



