170 THE OAK 
ceous strata, being coeval with the first undoubted dico- 
tyledons that have been found. Many have been found 
in the Tertiary also, and we have to conclude that the 
' oaks were probably already a well-developed group of 
plants before the higher mammalia existed—i.e. so far 
as we can judge from the fragmentary records of the 
rocks. It seems that even the present species of oaks 
were already in existence in Tertiary times, and possibly 
some of their varieties also. 
From the evidence of their fossil remains, together 
with the facts of their present distribution, it is at least 
exceedingly probable that the European oaks, including 
our English oak, came into existence somewhere in the 
Kast, and that, after spreading from Asia towards the 
West, they are now slowly retreating before competing 
forms—e.g. the beech. Meanwhile the English oak 
(Q. Robur) has been giving rise to several varieties, of 
which three at least (viz. pedunculata, sessilijlora, and 
pubescens) have become sufficiently marked to be re- 
garded as species by those who do not consider the con- 
necting forms. 
It is not improbable that this migration of the 
Huropean oaks from Asia was completed before the 
islands of Sicily, Sardinia, Corsica, and Britain were 
separated from the mainland of the Continent. More- 
over, our English oak is not distantly related to certain 
species of Hastern Asia and of Western North America, 
and it has been surmised that all these related forms 
sprang from a common ancestor not unlike our English 
