74 On the Lichen-Flora of Epping Forest,
And the last cause to be noticed of the diminution of the
lichen-flora of the Forest is the increase and extension of
villages in the neighbourhood. It is true that about old
rural villages and small towns in most parts of the country a
very fair number of lichens may always be gathered, especially
of such species as usually occur in cultivated tracts. But
where building operations on an extensive scale are being
conducted, and the physical features of the old villages are
thus entirely altered, and new ones here and there springing
into existence, a large decrease, if not the total extinction of
previously existing species necessarily results. And this not
only from the destruction of old trees, pales, and walls in
fields and gardens, but also from the alterations produced
upon the surroundings of the natal site of such lichens as
remain. For so impatient are they of such alterations which
prevent free access of air, that the old habitats become no
longer suitable for their growth, and they languish and decay.
Hence about Hoe Street, Walthamstow, and Loughton, where
numerous species were collected by Forster, as also about
Epping Town, where several were formerly gathered by
myself, we would at present vainly search for any in a fully-
developed condition. The paucity of lichen-growth in these
localities, it may here also be observed, is attributable to the
atmosphere being in some states of the weather more or less
impregnated with smoke from the increased number of human
habitations on the outskirts of the Forest, the acids contained
in which are most destructive to lichens. Add to this that
the direction of the prevailing winds being from the S.W.,
the smoke and fogs of London and its suburbs extend their
deleterious influence at certain seasons of the year to the
nearer portions of the Forest, and even considerably beyond;
and we cannot be surprised that the result should be a great
decrease in the numbers and change upon the condition of
its lichens. In fact, in so far as I have more recently
observed, these exist only in leprarioid or imperfect states
about the villages, the only exception almost being Lecidea
myriocarpa, which seems better adapted than any other
species (unless perhaps Lecanora galactina) to withstand the
influence of smoke.