2o6 The Irish Naturalist. Deceuiber, 



evidence of a greater movement. A shallowing of the sea 

 b}- 25 metres is not nearly sufficient to connect Ireland 

 with England or Scotland, or the Isles of Scilly with England. 

 Still less would it suffice to connect the West of Ireland or 

 Cornwall with the Pyrenees, where the peculiar plants find 

 their home. A rise of land to this amount would not even 

 bring Scilly and the Land's End appreciably nearer together. 



This limitation of the extent to which we can bridge over 

 the gaps between our islands is, however, a point on which 

 there is much difference of opinion, and I will not insist on 

 the conclusiveness of the evidence as to the extent of the 

 oscillations. 



From the botanist's point of \iew there are, however, 

 other archipelagos besides those surrounded by water. No 

 doubt if we can postulate sufficient orographic changes plants 

 would spread slowly from land to land during the few 

 thousand years that have elapsed since the cold died away. 

 But — and this ' but ' is all-important — they would only do 

 so if the soils were suitable. An isolated tract of limestone 

 surrounded by clay or by sand is as much an island, as far 

 as many of our most peculiar plants are concerned, as if it 

 were surrounded by water. We have many such islands — 

 or oases is perhaps a more suitable term for them — and no 

 possible ups and downs of the land will connect them. Many 

 of them, like the central limestone district of Ireland, or the 

 Peak District in Derbyshire, or the West Yorkshire car- 

 boniferous limestone, must have been isolated from far- 

 distant geological periods, from times before the present llora 

 of Britain had an\' existence. We ha\-e a still more difficult 

 problem than this. Britain is divided into mmierous rix'cr- 

 basins, for most of which any connection with other basins 

 in post-Glacial times is unthinkable. \'et each basin yields 

 numerous aquatic 1 lants and animals of the same species as 

 those found in other basins cut off by high hills. Isolated 

 lakes have their aquatic flora ; and ev^en artihcial ponds, 

 such as the dew-])onds of our high chalk downs, have a fauna 

 and flora closely proportionate in the number of species 

 with the time that has elapsed since the pond was made, or 

 since it last dried up. If no actual connection between 

 river-basins or isolated ponds is needed for the spread of 



