206 COLU* CLOUT'S CALENDAR. 



on which I have discovered it to-day probably represents 

 its furthest southern colony in the British Isles. There 

 was a time, doubtless, when its ancestors spread uninter- 

 ruptedly over the whole of central Europe, from the 

 Caucasus and the Urals to the Asturias and the Kerry 

 hills ; but with the gradual and still continuous improve- 

 ment in the climate of the northern hemisphere (how- 

 ever a few bad seasons may prejudice us to the con- 

 trary), it has been driven to the arctic regions or to the 

 very tops of the higher mountains ; and it now survives 

 as a whole series of distinct colonies, between which in- 

 tercommunication can only be effected at rare intervals 

 (if at all) by seeds carried across the intervening warm 

 tracts through the agency of Alpine birds. So very 

 small a community as this upon whose territory I have 

 just lighted may be regarded as almost certainly self- 

 contained ; for the chances of an occasional cross are 

 here so remote as to represent really what mathemati- 

 cians would describe as a vanishing quantity. 



Of course, it might plausibly be argued that this little 

 group of Alpine rock-cresses on this small patch of hill- 

 top may itself be due to such a solitary accident, and 

 that it may very likely have originated from a single 

 seed dropped on this congenial spot. That is quite a 

 possible explanation in any such individual case, and it 

 may, perhaps, even be the right one in this particular 

 instance. But no number of accidents of the kind could 

 ever account for the persistence with which almost every 

 higher summit in Great Britain or Ireland still presents 

 examples of little isolated groups belonging to the arctic 

 or glacial flora. We know from the analogy of oceanic 

 isles that a fauna or flora entirely dependent upon such 

 waifs and strays is always fragmentary and heteroge- 

 neous in the extreme ; it contains only those casual 



