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CHAP. XXII.] MOLLUSCA. 527
archipelagoes. In the Philippines, for example, the proportion
of the Operculata is a little more than one-seventh ; in the
Mauritius, between one-third and one-fourth ; in Madeira, one-
fourteenth ;
in the whole American continent about one-eighth
but when we come to the Antilles we find them to amount to
nearly five-sixths, about half the Operculata of the globe being
found there
!
Mr. Bland endeavours to ascertain the source of some of the
chief genera found in the "West Indian Islands, on the principle
that " each genus has had its origin where the greatest number
of species is found ;" and then proceeds to determine that some
have had an African, some an Asiatic, and some an American
origin, while others are tridy indigenous. But we fear there is
no such simple way of arriving at So important a result ; and in
the case of groups of extreme antiquity like the genera of mol-
lusca, it would seem quite as possible that the origin of a genus
is generally not where the greatest number of species are now
found. For during the repeated changes of physical conditions
that have everywhere occurred since the Eocene period (to go
no further back) every genus must have made extensive migra-
tions, and have often become largely developed in some other
district than that in which it first appeared. As a proof of this,
we not unfrequently find fossil shells where the species and even
the genus now no longer exists ; as Auricula, found fossil in
Europe, but only living in the Malay and Pacific Islands ;
;
Anas-
toma and Megaspira, now peculiar to Brazil, but fossil in the
Eocene of Prance ; and Proserpina of the West Indies, found in
the Eocene formation of the Isle of Wight. The only means by
which the origin of a genus can satisfactorily be arrived at, is by
tracing back its fossil remains step by step to an earlier form
and this we have at present no means of doing in the case of
the land-shells. Taking existing species as our guide we should
certainly have imagined that the genus Equus originated in
Africa or Central Asia ; but recent discoveries of numerous
extinct species and of less specialized forms of the same type,
seem to indicate that it originated in North America, and that
the whole tribe of " horses " may be, for anything we yet know