DISTRIBUTION AND ORIGIN. 255 



and in all directions by foes, by mountain barriers, by rivers, 

 and by other physical and unknown causes. It will readily be 

 conceded that land species have greater facilities of locomotion 

 than freshwater species, particularly species inhabiting stagnant 

 ponds and ditches ; and it should follow, according to the doctrine 

 of migration, that the further off freshwater species are from the 

 specific centre of a province, the more diminished in number than 

 land species they would be. The very contrary is the fact. Out 

 of five hundred and sixty species of Helix inhabiting the Caucasian 

 province, a very large proportion of which are assembled at its 

 specific centre, we have but twenty-four in Britain, of which only 

 eleven range throughout, The disproportion in number of the 

 species of Clausilia is larger still. This genus is especially popu- 

 lous at its specific centre. Between two and three hundred species 

 inhabit Austria and Hungary, yet we have but four in Britain, of 

 which only one ranges throughout. 



Let us now turn to the sluggish mud-dwelling Lymnwacea of 

 the ponds and ditches of the province. There are not sis species, 

 it may be safely stated, in all Europe more than there are in 

 Britain. They have no particular centre of creation. There is 

 no evidence to show whether the alleged primogenitors of our 

 British species were created in Siberia, Hungary, or Thibet. 

 There is scarcely any variation either in the form or number of the 

 species in those remote localities. Of Planorbis scarcely more 

 than fifteen species inhabit the whole Caucasian province, and we 

 have eleven of them in Britain, all ranging throughout, with the 

 exception that two, P. carinatus and corneus, partial in their distri- 

 bution in Europe, stop short of Scotland. Of Physa and Lymn&a 

 it is extremely doubtful whether there are any species throughout 

 the province more than we have in Britain. Neither of Ancylus, 

 which lives attached, limpet-like, to sticks and stones, and has 

 very limited facilities of migration, are there any species through- 

 out the province more than we have in Britain. The species of 

 these genera described by recent Continental authors as new, are 

 worthless. 



Edward Forbes mentioned, as a strong point in his argument, 

 that the great mass of the flora and fauna of the British Isles, 

 migrated during the post-pliocene epoch, over the elevated bed of 

 the glacial sea, from the Germanic region. He urges in support 

 of this view, that " every plant universally distributed in these 

 islands is Germanic," and " that the great mass of our pulmoni- 



