THE ORIGIN OF GftOUSE. . 189 



and the Irish Sea, the plants and animals of temperato 

 Europe spread by slow degrees across the unoccupied 

 plains and valleys of the British Isles. Their onward 

 course* over the land denuded by the ice-sheet was un- 

 doubtedly very tardy, for many species never succeeded 

 in reaching England at all ; while others, which got as 

 far as our own island, did not travel as much to the west 

 as Ireland before the submergence of St. George's 

 Channel made that part of Britain into a separate island. 

 It is, perhaps, to this accident of position, rather than to 

 the exterminating efforts of St. Patrick, that Ireland 

 owes its famous freedom from the presence of many 

 terrestrial reptiles and amphibians. A little later, before 

 the advanced guard of the European mammalia had fully 

 occupied our eastern coasts, the North Sea and the 

 Straits of Dover were invaded by arms of the Atlantic, 

 and Great Britain finally assumed its insular shape. 

 Thus our existing fauna and flora really represent a 

 mere fraction of the Central European species the few 

 pioneer kinds that had travelled so far on their way into 

 the bare waste before the sea cut us off from the remain- 

 der of the European world. We are comparatively rich 

 in insects, birds, bats, and plants, whose wings, eggs, or 

 seeds give them special opportunities of transport across 

 the sea ; but we are very poor indeed in terrestrial mam- 

 mals and land-amphibians, which cannot readily be 

 transported across wide stretches of intervening water. 



Mr. Wallace has noticed that in all such insulated 

 lands there is a great tendency for species to vary, partly 

 through the special sets of circumstances to which they 

 are thus exposed, and partly through the rarity of crosses 

 with the original stock, which doubtless continues to de- 

 velop and alter on its own part in another direction, 

 under pressure of other influences to which it is exposed 



