THE FIELD NATURALIST'S KNOWLEDGE.
61
which, consequently, we conclude was much warmer than at present.
Even the change of climate from the north to the south side of
the Alps greatly increases reptilian life. In Florence, which is not
far south in the Italian Peninsula, I have seen from the public drive
to the Coscini Gardens along the banks of the Arno, swarms of lizards
on the bounding wall even in the early spring, while in the middle of
summer this abundant reptilian life is not seen in Switzerland.
Insect and molluscan life is in many of its forms still more sen-
sitive to meteorological conditions, and a little difference in average
temperature will give abundance at one place and paucity at another.
The Rev. Dr. Walker, who visited Iceland last summer to study
the entomology of that country, states that in the north-west of the
island ten miles makes the greatest difference in the relative abund-
ance of insect life. As the vegetation is similar, this must be the
result of a difference of climate. The Tsetse-fly of Africa, too,
infests areas having well-defined boundaries. An exception to this
seems to be the mosquito, which, though a hot-climate insect, is in
the short, hot summers of Lapland most abundant in that Arctic
region, though the northern species of Culex is doubtless different
from that of the south.
A remarkable instance of the limitation of a mollusc to certain
latitudes is afforded by that beautiful gasteropod, Haliotis tuberculatus,
which, flourishing in tropical seas, comes diminished in size as far
north as the Channel Islands, but it has not been found living on the
English side of the British Channel, though the shells are sometimes
cast up on the Devonshire coast. A little mollusc, Cyrena flumi-
nalis, that, judging from the Brick-Earths of Essex, was once
abundant here, is now restricted to the warm rivers of Africa.
GEOLOGY AND BOTANY.
The connection between the geological structure of a county or
district and its flora is very intimate and is often very conspicuously
displayed. This is so in the South of England where the Chalk
downs afford very large areas of short herbage without trees, and
where they are covered by thin surface deposits they support chiefly
beech trees. The famous Burnham Beeches, the Knockholt Beeches
and the Cassiobury Beeches are all on Chalk areas.
A very good illustration of the connection which exists between
geology and trees is seen from a tower on the summit of Pinner
Hill, on the borders of Middlesex and Herts, from which is obtained