Geographical Distribution of British Plants. 27 
not essential to the existence of the species. These vegeta- 
bles came from the north-west of France, and the formation 
of the strait will indicate the period of their isolation. If, 
as is probable, the rupture of the strata took place before the 
destruction of the great Germanic plain which favoured the 
migration of the fifth flora, we may, says Mr Forbes (page 
346) regard the flora of Kent as very ancient, perhaps even 
anterior to the second, that of Cornwall, Devonshire, the 
south-east of Ireland, the Channel Islands, and the west 
of France, which has a more southern character than the 
third. 
We have already seen that geological and zoological data 
unite in placing the separation of England from the Conti- 
nent at the epoch of the destruction of the fauna of the large 
mammifera, that is to say, at the end of the phenomenon 
which accumulated the drift ; which ill agrees with the anti- 
quity which Mr Forbes ascribes to this rupture, relatively to 
an emerged plain, whose existence nothing geological, hydro- 
graphical, or orographical tends, in any way, to confirm. 
The geological characters of the districts occupied by the 
second flora are connected with the remains of a great de- 
stroyed barrier, which likewise marked the southern limit of 
the Iey Sea. But what is this great barrier evoked by the 
author? Is it the North-Down chain of hills of which we have 
alreadyspoken? Besides, the northern limit of the second flora, 
represented by a rose-coloured tint on the chart (Plate VI. of 
Professor Forbes’s Memoir), certainly does not coincide with 
any character, either physical or geological, in the soil of 
France and England. The shore of the Jey Sea is not in uni- 
son with this supposed limit; for, with the exception we have 
pointed out, the erratic phenomenon of the north is not seen to 
the south of a line drawn from the mouth of the Thames to 
Dusseldorf.* The comparative examination of the relief, of 
the disposition and relative thickness of the tertiary and 
* The limit is, in fact, likewise indicated in Plate VII. of the Memoir, Not- 
withstanding this assertion, true in general, we have seen that M. Mantell men- 
tions fragments of primary rocks in the second diluvial bed of the steep shores 
at Brighton, 
