216 COLIN CLOUT'S CALENDAR. 



Britain it occurs on the main summits of the Scotch 

 highlands, descends more scantily into Wales or Cum- 

 berland, and hardly loiters on upon a few bleak hill-tops 

 in Ireland among the Ulster heights. This moor 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 unin- 

 terruptedly 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 (however 

 a few bad seasons may prejudice us to the contrary), 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 intercommuni- 

 cation 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 mathematicians 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 



