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524 GEOGRAPHICAL ZOOLOGY. [part iv.
absence of genera confined to, and characteristic of Africa and
India. One small sub-genus of Helix, (Bachis), and one of Acha-
tina, {Homorus), appear to have this distribution,—a fact of but
little significance when we find another sub-genus of Helix,
(Hapalus), common and confined to Guinea and the Philippine
Islands ;
and when we consider the many other cases of scattered
distribution which cannot be held to indicate any real connection
between the countries implicated. No genus is confined to the
Palsearctic and Nearctic regions as a whole. A large number
of sub-genera, many of them of considerable extent, are peculiar
to one or other of these regions, but only 3 sub-genera of Helix
and 2 of Pupa are common and peculiar to the two combined,
and these are always such as have an Arctic range and whose
distribution therefore offers no difficulty.
We find, then, that each of our six regions and almost all of
our sub-regions are distinctly confirmed by the distribution of the
terrestrial mollusca ; while the different combinations of them
which have at various times been suggested, receive little or no
support whatever. Even those remarkably isolated sub-regions,
New Zealand and Madagascar, have no strictly peculiar genera of
land-shells, although they both possess several peculiar subgenera
; being thus inferior in isolation to some single West
Indian Islands, to the Sandwich Islands, and even to the North
Atlantic Islands (Canaries, Madeira, and Azores), each of which
have peculiar genera. This of course, only indicates that the
means by which land moUusca have been dispersed are somewhat
special and peculiar. To determine in what this speciality
consists we must consider some of the features of the specific
distribution of this group.
The range of genera, and even of sub-genera is, as we have
seen, often wide and erratic, but as a general rule the species
have a very restricted area.
Hardly a small island on the globe but has some land-shells
peculiar to it. Juan Fernandez has 20 species, all peculiar.
Madeira and Porto Santo have 109 peculiar species out of a total
of 134. Every little valley, plain, or hill-top, in the Sandwich
Islands, though only a few square miles in extent, has its