THE LOCAL EXTINCTION OF MOLLUSCS. 89
dew at night. This allowed the Molluscs an excursion daily,
and, in the long run, further peregrinations and a wider field for
the distribution of species. In the arable and drained tracts of
the present day, drought is much more prevalent, and dewy
nights are possibly not so frequent ; as a consequence, the snail
is much limited in its range and opportunities. In short, where
once the whole summer was humid, there are now only the
heavy dews of the spring and autumn. We cannot tell whether
the food is correspondingly diminished. There is a little
evidence in that some species frequent gardens where young
juicy vegetation is more predominant, but it is equally true that
many species do not go into a garden at all.
A drained tract acts directly in breaking up colonies and
isolating individuals. All travel instinctively away from the
drained tract, and consequently, many travel in opposite direc-
tions, never to unite again. This sub-division goes on more and
more as the colonies diminish, till eventually they die out, from
want of opportunities to pair. There are two cases which are
obviously due to the effect of drainage, in which two species of
Mollusca, formerly very abundant in Essex, are now fast hasten-
ing to local extinction. One is Helix arbustorum, and the other is
Zonites purus. Both require to Jive in very moist situations. I
have been fortunate enough to hunt up three small colonies of
H. arbustorum, and in one case I had seen the colony die out to
its last member. Zonites purus is now only found as single speci-
mens in my neighbourhood, and these are few and far between.
The shell-marls and pear-bogs of this district, although not
extensive, are very instructive with regard to the proportions in
which the various species are represented. Some of these marls
enclose Roman and other remains, and we can therefore learn
approximately their age. I here copy a table which I made out
for this journal in 1889,5 in which I have made a slight alteration
in the cases of H. arbustorum and Cyclostoma elegans, required by
the subsequent observations alluded to above. I believe that in
other respects it is correct. It will be seen by this table that
local extinction may be a somewhat rapid process at times :—
5 "On the Mollusca of the Shell-Marl occurring at Felstead and in other parts of Essex."
—Essex Naturalist, vol. iii., p. 14.