DISTRIBUTION OF ANIMALS AND PLANTS IN FELSTEAD, ESSEX. 195
adapting themselves to some untoward circumstances which would
mean death to other organisms, the very general restriction of certain
forms to ponds, and others to swift streams, leaves no other solution
than that the stillness or temperature of the water is the important
factor. Given, therefore, the river basin and the low-lying lands,
another period of lakes and morasses, and perhaps some increase of
woodland to assist in the preservation of moisture, there would seem
to be nothing to hinder the universal distribution of all the forms
now peculiar to their several localities.
Taking now the distribution of the land mollusca, we are assisted
by a historical data which appertain sparingly to the water molluscs,
and not at all to the fish. These are the fossil remains of some shell-
marls. In vol. iii., page 14, of The Essex Naturalist, I gave
a table of thirteen species from these deposits. Seven of these
species are now locally extinct, four are very rare and probably
locally extinct, and two, frequent or common in the past, are now
rare. The substantial truth of that list I have since verified more
particularly, and although the number of species in it may be
increased,1 it will not need material alteration. It is very difficult to
account for this change among the land molluscs, the more so that
two species, now very rare, seem to have been common, say in Roman
times or somewhat earlier. These are Helix arbustorum and
Cyclostoma elegans, both having been found in shell-marl connected
with an artificial lake (see E. N., vol. vi., page 35). Of the former
Mr. Christy wrote me some time ago that in his neighbourhood
(Chignal St, James) he was astonished at the former abundance of
these shells, and their present rarity, which is also my experience.
Yet the conditions, except that the land is better drained, so far as
we know, have not altered. Of the latter species (C. elegans) I have
given some account (E. N., vol. iv., page 92). It is true that in
some places the Boulder-clay has wasted, and if it is true that the
animal requires a limestone habitat, this fact may, in some measure,
account for its disappearance. It would be interesting to know
whether Cyclostoma occurs as a fossil in Essex south of the line of
Boulder-clay. A peculiarity of its distribution is that, although
common on the Chalk hills of Charlton, on the opposite bank of the
Thames, it does not now live in South Essex.
For the disappearance of the thirteen forms mentioned in the
1 Mr. Christy calls attention to Helix aspersa as not having been named, but it appears to be
unrepresented in the older marls.