576 TRANSACTIONS OF SECTION K. 



Ujb North Sea with Holland and Denmark. But 20 or 25 metres seems to have 

 been the approximate extent of the rise in the south of England. I have 

 searched in vain for evidence of a greater movement. A shallowing of the sea 

 by 25 metres is not nearly sufficient to connect Ireland with England or Scot- 

 land, 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 view there are, however, other archipelagos 

 besides those surrounded by water. No doubt if we can postulate sufficient oro- 

 graphic 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 carboniferous limestone, must have been 

 isolated from far-distant geological periods, from times before the present flora 

 of Britain had any existence. We have a still more difficult problem than this. 

 Britain is divided into numerous river-basins, for most of which any connection 

 with other basins in post-Glacial times is unthinkable. Yet each basin yields 

 numerous aquatic plants and animals of the same species as those found in other 

 basins cut off by high hills. Isolated lakes have their aquatic flora; and even 

 artificial ponds, such as the dew-ponds 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 con- 

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

 aquatic plants, why need we postulate a land-connection for the land-plants, or 

 a bridge of limestone to aid the migration of the limestone plants from crag to 

 distant crag? Aquatic plants and limestone plants must obviously in most 

 cases have taken leaps of many miles to arrive at their present stations. Our 

 plants have far greater power of crossing deserts and seas than most botanists 

 are willing to allow. 



Let us examine the present distribution of one of the most interesting groups 

 of British plants. The Atlantic or Lusitanian plants form an assemblage be- 

 longing mainly to the Pyrenees, and found also in the S.W. of England, and 

 acain in S.W. Ireland. But they do not occur in the intermediate districts. 

 If we look more closely into the composition of this Atlantic flora, as it is 

 represented in Britain, we find that only plants with small seeds have been able 

 to cross to Cornwall and Ireland, those with large seeds being left behind on 

 the Continent. There is only one tree among them, and that is the Arbutus, 

 one of the few trees with minute seeds now living in Europe. A further exam- 

 ination confronts us with the puzzle that, whilst various Pyrenean species are 

 found also in Cornwall and Kerry, the species occurring in Cornwall and Ire- 

 land are not the same. The Arbutus is a case in point; it is wild in Ireland, 

 but in no part of England. Erica ciliaris and E. vagans are English, and not 

 Irish; E. mcditerranca is Irish, and not English. 



The local distribution of these plants is equally strange. A few, like 

 Pinguecula lusitanica have spread throughout the West Country, wherever the 

 conditions are suitable. Most occur, however, in quite different fashion; they 

 are abundant over certain limited areas, to which they are strictly confined, 

 but they are absent from other adjoining areas, though equally suited. I have 

 mapped and examined a good many of these areas, and the plants seem in most 

 places to be spreading vigorously from certain definite centres, to which chance 

 has transported a seed. Thus, Erica ciliaris is confined to three areas, in 

 Cornwall, Devon, and Dorset. E. vagans occurs abundantly in the Lizard and 



