287 
Portions of the British Flora. I. 
only in Sweden and Switzerland, may be considered a glacial relic, a mem¬ 
ber of a residual flora, the bulk of which is found farther north, although in 
Pleistocene times it reached the south of England. Perhaps the other 
montane or arctic-alpine species are also old, although the fossil evidence 
is not yet forthcoming. On the other hand, a plant like Ammophila baltica 
of the Durham and Norfolk coasts may have arrived recently, if it has not 
actually originated in these localities, since it is often regarded as a hybrid. 
Again, Potamogeton rutilis in Anglesea may be a recent immigrant. 1 Such 
outliers and others that will be mentioned later show the necessity for 
taking into consideration all the facts which bear on plant-distribution 
problems. They seem to point in some cases to survival, in other cases to 
sporadic introduction, rather than to any particular, directed movement. 
They will interfere, therefore, with the ‘ Age and Area ’ scheme in detail. 
But it is impossible to ignore the fact that most of the very rare flora in 
England is concentrated in the south-west, south, and south-east. West 
Cornwall stands highest with 28 species. East Kent has 19, yet this vice¬ 
county possesses the largest number when the total of 266 species is 
analysed. Thus the important point emerges that all the more widely- 
distributed English species, once having reached Kent, have remained 
established there. There is no suggestion of a wholesale destruction once 
colonization has been effected. The dotted areas in Diagram 2 showing 
density need not be considered the last strongholds of assemblages of plants 
once more widespread. It seems more reasonable to suppose that they are 
points at which plants have arrived and have secured a foothold. 
If this view is correct, it may be instructive to inquire whether any 
floral or geographical alliance exists amongst those areas which are 
especially rich in species. Guided by the results of Diagram 2, we may 
select a few districts for the purpose of illustrating this point. Four areas 
are shown enclosed by dotted lines. Each includes several vice-counties, 
so that the districts compared may not be too small nor the analysis too 
elaborate. The four regions do not correspond exactly in size with the 
provinces employed by Watson, being smaller, but it is convenient for 
reference to use his names, viz. Ouse, Thames, Channel, and Peninsula. 
Of the 129 rare species under consideration, 37 occur in the Ouse 
province, 29 in the Thames, 39 in the Channel, and 41 in the Peninsula. 
We may now trace the distribution of these assemblages, beginning with 
the Ouse group. Diagram 3 gives the vice-county details, but, taking the 
four larger areas proposed, we find that of the 37 species centred in the 
Ouse province, 11 occur in the Thames, 9 in the Channel, while 7 reach 
the Peninsula. In the same way Diagram 4 illustrates the distribution of 
the Thames group of 29 species, of which 11 pass northwards to the Ouse, 
13 occur in the Channel, and 7 in the Peninsula. The range of the 39 species 
1 Lately recorded for Shetland also. 
U 
