92 THE LOCAL EXTINCTION OF MOLLUSCS.
slight advance in my direction of the two species of Paludina
(vivipara). They are now divided from my neighbourhood by an
insignificant watershed.
A history of the "old,'' but geologically recent, marls does
not give much evidence in favour of new arrivals since that
period ; almost all our present species are found in these
deposits. I cannot, however, trace the species I have named as
new arrivals in any of the Essex lists of the marl fossils, nor
have I found them myself ; and as they are conspicuous forms,
we may safely conclude that they did not live in the neighbour-
hood at the time these marls were deposited. I am credibly
informed that Helix aspersa does not occur in the older marls,
and among the smaller species there may be some other
instances.
We have a few species now that occur in extraordinary
abundance—several of the Helices and Zonites, some species of
Pupa, and among the Freshwater forms some species of Planorbis.
Bythinia, &c.
In considering the variations and wanderings of Molluscs,
many factors enter into the problem, the values of which are not
themselves well understood. Thus we say, and on good evi-
dence, that a species will remain unvaried and without appreci-
able change of distribution for an enormous length of time,
Some, possibly, go further in saying that the gregarious habits
of Molluscs tend to this fixity in that it gives facility for per-
petual intercrossing, and so keeps them bound to an average
type. But, on the other hand, we are acquainted with numerous
variations which may also, to a certain extent, be due to these
gregarious habits. We cannot tell how these variations are set
up—whether it is due to the preservation of some variation in a
diminishing colony, or whether it is due to the change of food
and environment in a full colony. Again, as to their wander-
ings. Like plants, they are sometimes found having a time of
introduction, culmination, and decay. Some spread themselves
over a whole tract of country ; others confine themselves strictly
to a circumscribed locality. We conjecture that the supply of
food is directly concerned in these wanderings, and the more one
notices the habits of Molluscs, the more one becomes impressed
with the idea that snails are very fastidious creatures ; it is
certain that they will in some cases prefer starvation, or rather
hibernation, amounting to death, rather than make any material
change in their diet or be greatly disturbed in their habitat. If