94 HISTORY OF THE EUROPEAN FAUNA. 



Europe across Greenland and the old Arctic land- 

 connections. Bathyphantes nigrinus^ Linyphia insignis, 

 and Drapetisca socialis, for instance, are three British 

 species whose range indicates a northern origin, and 

 which also occur, according to Mr. Carpenter, in 

 North America. Mr. Carpenter also tells me that the 

 Collembolan, Isotoma littoralis, is a typical northern 

 migrant He has recently discovered it in the west 

 of Ireland, its only station in the British Islands. 



Among the Crustacea, the genus Apus forms an 

 exceedingly interesting illustration of the northern 

 migration, Apus glacialis having been discovered in 

 a Scottish pleistocene freshwater deposit, whilst it is 

 now almost confined to the Arctic regions. 



To the same group of animals also belong the three 

 remarkable species of freshwater sponges, Ephydatia 

 crateriformis, Heteromeyenia Ryderi, and Tubella pen- 

 sylvanica y which Dr. Hanitsch has described from some 

 lakes in Western Ireland. None of these are known 

 from Great Britain or from the continent of Europe. 

 A few North American plants grow wild in the same 

 district. That any of these should owe their existence 

 in Ireland to accidental introduction appears to me 

 exceedingly improbable. In a former contribu- 

 tion to this subject (a, p. 475) I assumed that 

 these American plants and animals had migrated 

 to Europe at the same time as the other northern 

 forms referred to. My friend Mr. Carpenter, how- 

 ever, takes exception to this (p. 383), and I quite 

 recognise the force of his argument. "Their very 



