v] RELATIVE FREQUENCY 71 



Two preliminary remarks are necessary. The 

 opinions of naturalists vary as to the limits of 

 genera ; and a species may be a species to one and 

 a mere variety to another. Thus it Avill be impossible 

 to give a summary of the facts to be enumerated 

 presently, which will be either absolutely accurate or 

 which will satisfy everyone in every detail. But it is 

 asserted that the following survey is substantially 

 correct. 



In the second place it is often possible to elim- 

 inate from the fauna of a given region those species 

 and even genera which have been accidentally 

 imported, a matter which Avill receive careful con- 

 sideration on a later page. Such forms are therefore, 

 in those cases at any rate where the evidence seems 

 to be overwhelming, withdrawn from the list. In 

 other cases, particularly in the Eastern region of the 

 world, it has been found less easy to rectify the 

 catalogues by removing what Dr Michaelsen has 

 termed 'peregrine' forms. 



We shall commence with a census of South 

 America ; the entire Continent will be divided for 

 the present purpose into three divisions, viz. South 

 America, Central and North America, and in the 

 third place the West Indian Islands. 



In South America we find that the bulk of the 

 indigenous earthworms belong to the family Geoscole- 

 cidae and to a definite sub-family, viz. Geoscolecinae. 



