VIII] MOVEMENT AND MIGRATION 117 



have to do with those rather mysterious species 

 which can apparently unduly extend their range and 

 which are known as peregrine forms; for they also 

 occur in other parts of the world besides Africa. We 

 have therefore in Dichogaster the case of a genus 

 which ranges all over the tropical parts of Africa, 

 but whose species are not common to the Atlantic 

 and Indian shores of that continent. 



We will now contrast these conditions, which 

 exemplify certain facts shown by the characteristic 

 Oligochaeta of tropical Africa, with those which 

 obtain in Europe. In this region of the world the 

 prevalent and practically the only genera which 

 need be taken into consideration in surveying the 

 Oligochaetous fauna from the present point of view, 

 are Lumbrlcus and the genus Allolobophora of 

 Eisen which has been variously rearranged into genera 

 and sub-genera known by the names of HelodrUm, 

 Bimastos, Octolaslum, etc. The structural diiferences 

 which divide these genera and sub-genera arc not 

 great; in any case they do not exhibit such a wide 

 range of variation from each other as do two such 

 Eudrilid genera as IStuhl mcumia and llifpi'iiodrilus. 

 We find the genera mentioned not only in Europe 

 but extending themselves over more or less of Asia, 

 even occurring in Japan; while the North xVmerican 

 continent contains also representatives of the same. 

 Not only do we find this comnuuiity of genera over 



