CHAP. XXII.] 
MOLLUSOA. 
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 u 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 truly 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 France ; 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 
