2 j8 
Matthews .— The Distribution of certain 
Historical Phytogeography. 
The problem of the origin of the British flora was attacked by Clement 
Reid (1899), employing the historical method, but the imperfection of 
the fossil record made a final conclusion exceedingly difficult. Since that date 
our knowledge of Pliocene and Pleistocene floras has materially increased. 
The researches of C. and E. M. Reid (1908) and those of E. M. Reid 
(1920 tf) on pre-glacial floras of Western Europe provide reliable evidence of 
the nature of the vegetation of this country before the onset of the Pleistocene, 
when the character of the flora underwent a decided change. A comparative 
review of West European Pliocene floras is given by E. M. Reid (1920 b ), who 
shows that the flora during this period changed from one having a high 
percentage of exotic species, many of which are Chinese-North American, 
to a flora (represented by the Cromerian at the top of the Pliocene) about 
95 per cent, of which is composed of species which inhabit East Anglia at 
the present day. The author refers to a suggestion made in an earlier paper 
(1915) in which it is stated that the high land of Central Asia may have 
acted as a second centre for the origin and dispersal of temperate species in 
Miocene and Pliocene times, and is led to make the further suggestion that 
‘ much of the living flora of the lowlands of Western Europe has been derived 
from this source, by dispersal through the Near East, the Caucasus, and the 
mountains of Southern and Central Europe, or by way of the Mediterranean \ 
The identity of most of the Cromerian flora with species now occurring in 
England has been mentioned, and, were it not for the onset of a period marked 
by increasing cold, there would have been probably no special difficulty in 
tracing the progressive emergence of our flora to its present state. But the 
change from heat to cold which culminated in the Ice Age produced 
a migration southwards, and the central problem of distribution in Britain 
turns entirely on the course of events throughout this period of climatic 
change. Information as to what exactly happened during the time of 
maximum glaciation would provide a definite starting-point in any attempt 
to solve the problem, but unfortunately the geological record is incon¬ 
clusive. 
There is, however, a steadily increasing body of evidence to show that 
a redistribution of vegetation must have occurred during Pleistocene times, 
since in place of the temperate flora which inhabited the south of Britain at 
the close of the Pliocene we find post-Pliocene fossil floras containing a very 
pronounced arctic element. The early records of glacial plant-bearing de¬ 
posits are enumerated by C. Reid (1899), who gives full details, and we shall 
here refer only to two recently described arctic floras from low latitudes in 
Pmgland. Reid (1916) gives a list of the plants of the late glacial deposits 
of the Lea Valley, 22 per cent, of which are arctic species. The arctic flora 
revealed by Marr and Gardner (1916) at Barnwell in the Cam Valley and 
