Mr Lamb, Insect Oases 349 



there, and in somewhat similar isolation, namely the dolichopids 

 Dolichopus signifer and Acropsiliis niger, but these are so far only- 

 known to be of Central European distribution, though the occur- 

 rence of the former in Ireland many years ago makes it possible 

 that it is another of the former class. 



The most probable reason for the extremely restricted distri- 

 bution of these insects is that we are in the presence of the last 

 stage of the extinction of a species. With creatures of such com- 

 paratively sedentary dispositions, once an island is formed it has 

 little chance of increase. There must be a minimum population 

 density which is such that when the density approaches that 

 minimum the chance of reproduction is practically zero; as there 

 is no evidence that any of the species have sex attracting powers, 

 this minimum density might well be attained at a comparatively 

 small distance from the centre of the colony. 



VOL. XX. PART III. -"* 



