d is
unknown except in such places. My correspondent adds:--"This autumn I went
back about thirty-five miles through a dense forest, along a track marked
by some prospectors the year before, and in one spot where they had camped,
and the fire had burnt the fallen logs, &c., there was a fine crop of
'fire-weed.' All around for many miles was a forest of the largest trees
and dense scrub." Here we have a case in which burnt soil and ashes favour
the germination of a particular plant, whose seeds are easily carried by
the wind, and it is not difficult to see how this peculiarity might favour
the dispersal of the species for enormous distances, by enabling it
temporarily to grow and produce seeds on burnt spots.

3. In answer to an inquiry on this subject, Mr. H. C. Watson has been kind
enough to send me a detailed account of the progress of vegetation on the
railway banks and cuttings about Thames Ditton. This account is written
from memory, but as Mr. Watson states that he took a great interest in
watching the process year by year, there can be no reason to doubt the
accuracy of his memory. I give a few extracts which bear especially on the
subject we are discussing.

"One rather remarkable biennial plant appeared early (the second year, as I
recollect) and renewed itself either two or three years, namely, _Isatis
tinctoria_--a species usually supposed, to be one of our introduced, but
pretty well naturalised, plants. The nearest stations then or since known
to me for this _Isatis_ are on chalk about Guildford, twenty miles distant.
There were two or three plants of it at first, never more than half a
dozen. Once since I saw a plant of _Isatis_ on the railway bank near
Vauxhall.

"Close by Ditton Station three species appeared which may be called
interlopers. The biennial _Barbarea precox_, one of these, is the least
remarkable, because it might have come as seed in the earth from some
garden, or possibly in the Thames gravel (used as ballast). At first it
increased to several plants, then became less numerous, and will soon, in
all probability, become extinct, crowded out by other plants. The biennial
_Petroselinum segetum_ was at first one very luxuriant plant on the slope
of the embankment. It increased by seed into a dozen or a score, and is now
nearly if not quite extinct. The third species is _Linaria purpurea_, not
strictly a British plant, but one established in some places on old walls.
A single root of it appe