THE LOCAL EXTINCTION OF MOLLUSCS. 87
On looking over the list of the Land-Mollusca of the British
Isles, we observe how large is the proportion of small, and even
minute, forms. Yet other things being equal, in power of distri-
bution by travel the larger Molluscs have a great advantage. I
once kept a specimen of Helix pomatia, which was a habitual
traveller, and once crept nearly a hundred yards in one night.
Hence, the large species would have a better chance of obtaining
new settlements than small ones. It is probable that the larger
forms, in some cases, suffer, or have suffered, destruction chiefly
on account of their conspicuousness. It is a common expe-
rience of the snail-hunter that at the outset he cannot find the
minute forms, nor, if he had to depend upon his eyes and hands
alone, could he make much progress, except accidentally. Yet
in time he gets to learn that, in point of numbers, many of these
minute species may far outnumber the larger tonus with which
everyone is familiar. The inference here is plain—that, given
any snail-eating bird or other animal, the larger forms are those
that will first succumb, whilst the smaller species will escape
notice. Part of the disparity in numbers between large and
small species may be due directly to their size. There are a
few species that are quoted chiefly from sea-board districts that
may be noticed. That they are not dependent upon a littoral
vegetation or conditions is clear from the circumstance that they
are sometimes found inland. Such are Succinea oblonga, Helix
revelata, H. pisana, H. virgata2 and others. The reason seems to
be that they meet with enemies as the country is penetrated, and
rarely succeed in permanently spreading themselves inland.
It is, however, possible that the enemies of snails in
England now do not very greatly affect their distribution.
Helix nemoralis is everywhere common, and yet it is everywhere
sought for by blackbirds and thrushes. In the immediate past,
as instanced by the shell-marls, it was equally common, and it
has therefore held its own in spite of its enemies, other causes
probably operating to favour its vigorous life and wide distribu-
tion as a species. One effective cause of extinction of Molluscs
is change in the character of the habitat. The variations in my
neighbourhood (Felstead) are due to two causes. One is the
wasting away of a calcareous soil, and the other is the increasing
dryness of the soil due to drainage. We will take the cases due
to calcareous waste first. One is so patent as to have attracted
2 [But Helix virgata is one of the species which Mr. French has observed immigrating
into Felstead. See his remarks below on the introduction of species.—Ed.]