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Portions of the British Flora. /. 
referred to a late, if not the latest, stage of the Pleistocene deposits of the 
district has been fully studied by Chandler (1921), who finds that 42 per 
cent, of the species are arctic-alpines. Of these the following are worth 
noting: 
Ranunculus aconitifolius , L., Papavcr alpinum, L., Draba incana , L., 
Arenana sedoides , L., Arcnana biflora, L., Potentilla alpeslris , Hall., P. 
fruticosa , L., Dry as octopetala, L., Saxifraga oppositifolia , L., Vaccinium tdigi- 
nosum , L., Primula scotica , Hook., Armeria arctica, Wallr., Salix Arbusctda, 
Fries., N. Lapponum, L., N. herbacea , L., S. polaris, Wahl., .S', retiadata, L., 
Betida nana, L., Carex lagopina , Wahl., 0. uslalata, Wahl., and C. capil- 
laris , L. To these may be added Oxyria digyna , Hill, Scheuchzeria 
palustris, L., and Carex incurva , Lightf., reported from the Lea Valley, but 
not on record from the Cam Valley. 
The occurrence in England during the Pleistocene of arctic species now 
extinct in Britain and known only from arctic Europe and Greenland is 
particularly interesting, as is also the fact that other species of the Cam 
Valley are unknown from Arctic Europe, occurring only in alpine situations 
in Central and South Europe. 
Whether this arctic-alpine flora existed at the climax of a cold period 
the evidence from the plants alone, as Miss Chandler points out, is insufficient 
to say, but the association points to climatic conditions different from those 
which now prevail in the south of Britain, and clearly indicates that an 
invasion of northern forms had taken place since the time of the Cromerian 
flora. 
Numerous arctic plant-beds occurring at low levels have been described 
from Scotland also, the most extensive investigations being those of Lewis 
(1905-11). Considerable fluctuations in the distribution of vegetation over 
North Britain during post-glacial times are indicated, for the changes 
described by Lewis occurred later than the last ice-sheet and give no 
information about the chain of events during the period of maximum 
glaciation. In Europe, Scandinavian workers have succeeded in tracing 
with some degree of precision successive floras which existed during the 
Pleistocene. A review of the historical sequence is given by Wille (1915) 
and short accounts of numerous continental investigations are included by 
Clements (1916) in his discussion of the ceneosere. The general conclusion 
may be reached (E. M. Reid, 1922) that plant migrations have been brought 
about in the past under stress of climatic change, and, while Britain 
undoubtedly shared in the southward movement developed during the 
glacial period, there is, as already mentioned, no incontrovertible evidence 
of the exact extent of this movement within our own country^. Historical 
phytogeography does not provide a conclusive answer. 
Nevertheless, certain students of the subject have adopted a definite 
standpoint. Engler (1879) assumes the complete or almost complete 
