84 Oscar Drude. 
neighbouring continent by immigration due to dispersal of continental 
species by wind, birds, etc. Without wishing to deny the existence 
of this factor, I consider the mixture of floristic elements already 
present in England at the close of the Baltic ice period, which forms, 
for Germany, the starting point of the last great transformation of 
vegetation, as very important. In the southern part of England, 
which remained free of ice at the time of the greatest extension of 
glaciers, and which in the last (Baltic) ice period would have 
possessed mild climatic conditions on account of its maritime 
situation, J consider the ground stock of species to have been settled. 
The species of this stock afterwards extended further north, and 
constantly won fresh ground from the northern floristic elements. 
According to this view the flora of Great Britain would still to-day 
give an indication as to the distribution of species at that time, a 
distribution which was much more freely and profoundly modified 
on the continent owing to the stronger influence of the Steppe 
period. Thus many species, such as Digitalis purpurea, Meum 
athamanticum, Thlaspi alpestre, Helleborus fetidus, range from the 
Pyrenees and Western Alps through France and Belgium, on the 
one side to Scotland, on the other eastwards to the Harz, Thuringia 
or Saxony, but stop at the Sudetes, which stood completely in the 
line of the Ural-East Baltic invasion characterised by Ledum, 
Vaccinium uliginosum, Carex pauciflora, Betula nana and Pedicularis 
sudetica. In addition there are many other species of common 
mid-European distribution, and the occurrence of these in the 
British Isles with an increasing distribution towards the north and 
west gives very important indications which have hitherto not been 
sufficiently considered. It is also noteworthy that most of the 
plants given in the list above are absent from Cornwall and appear 
first in Devon, Somerset and Dorset (for instance so common a 
plant, distinguished by its preference for cool damp soils, as Geum 
rivale). Others are such common German hill plants of drier 
ground as Potentilla verna, which begins in the English “continental” 
region round Cambridge and preferring the east of England reaches 
as far north as Edinburgh. All this makes very much more the 
impression of an ordered distribution than of chance invasion ; but 
the latter factor has also no doubt been always at work. The 
views expressed by Clement Reid are based mostly on single very 
remarkable species (such as Arbutus, Dabeocia, Erica vagans and 
ciliaris, which are very isolated) and neglect the distributional 
relations of the great bulk of species, which we must consider in 
