126 EARTHWORMS AND THEIR ALLIES [ch. 



in some similar way the species of Notiodriliis have 

 been wafted from shore to shore of those lands which 

 are washed by the Antarctic Ocean. Dr Benham, 

 however, in criticising this, calls attention to the 

 violent gales and disturbances of the ocean surface 

 which are so prevalent in those stormy regions, and 

 doubts much whether these animals could retain a 

 safe hold upon some travelling tree trunk. Moreover 

 it is only in this antarctic region where the earthworm 

 fauna of the various continents and islands are so 

 very similar. 



Facilities of Migration. 



The above brief account of physical features which 

 affect the range in space of the terrestrial Oligochaeta 

 seem to show that the only really important barrier 

 is the ocean ; and even a narrow tract of sea water 

 would, as it appears, act fatally in preventing the 

 successful immigration of a race inhabiting one shore 

 to the opposite shore. On the other hand we do un- 

 doubtedly find in different countries even when 

 separated by a large expanse of ocean closely related 

 forms. The most striking instance of this is that 

 afforded by a consideration of the antarctic species 

 of Notiodriliis and Chilota. Can this interchange of 

 Oligochaetous faunas be explained by any means which 

 earthworms possess of crossing tracts of sea by the 



