142 Survey of Soil Fauna of Agricultural Land 



sufficient to cause a strenuous competition for food and consequent 

 migration of members of species to less populated areas, such as arable 

 land. The invading species will have at the same time to face the more 

 unsuitable environmental conditions found on arable soil. Accidental 

 migration will take place but will hardly provide an adequate explana- 

 tion. 



(3) There is no characteristic fauna of arable land. The predominant 

 species on arable are those commonly found on pasture. It is not possible 

 in an investigation where only a limited number of species are obtained, 

 to discover whether any of them are particularly susceptible to cultural 

 operations and are eUminated either directly or by deprivation of food 

 material. Even where the absence of some commonly occurring species 

 is shown on arable land the environmental conditions as a whole rather 

 than cultivation as such may provide the true explanation. 



It will be understood that owing to the particular nature and uni- 

 formity of the vegetation on arable land different species will pre- 

 dominate from year to year. These, however, cannot be considered to 

 form a special fauna since the effect is due to the overwhelming import- 

 ance of perhaps one factor in the environment, such as the particular 

 crop grown. 



The great disadvantage in taking samples to determine the population 

 of an area is that a large error either way must be allowed for in the 

 computation. The tendency in sampling is that dominant species are 

 stressed, and rarer species, since they may occur only in a certain par- 

 ticular section of the area, may be overlooked. The elimination of species 

 by cultivation can only be determined by the comparison of the results 

 from a large number of separate surveys. 



I am indebted to Professor S. J. Hickson for the kind interest he 

 has always taken in the work and to Mr R. A. Wardle, M.Sc, Lecturer 

 in Zoology (Manchester University), at whose suggestion the survey was 

 undertaken. 



With regard to the classification of soil fauna I have to acknowledge 

 the valuable assistance of Mr H. Britten (Manchester Museum) and the 

 Rev. G. Brade-Burke, M.Sc. (South-Eastern Agricultural College, Wye, 

 Kent). 



