other modification from the same root, which may be of higher or
lower organization, more or less numerous in species, and more or less
varied in form and structure than either of those which preceded it.
Again, each of these groups may not have become totally extinct, but may
have left a few species, the modified prototypes of which have existed
in each succeeding period, a faint memorial of their former grandeur and
luxuriance. Thus every case of apparent retrogression may be in reality
a progress, though an interrupted one: when some monarch of the forest
loses a limb, it may be replaced by a feeble and sickly substitute. The
foregoing remarks appear to apply to the case of the Mollusca, which, at
a very early period, had reached a high organization and a great
development of forms and species in the testaceous Cephalopoda. In each
succeeding age modified species and genera replaced the former ones
which had become extinct, and as we approach the present aera, but few
and small representatives of the group remain, while the Gasteropods and
Bivalves have acquired an immense preponderance. In the long series of
changes the earth has undergone, the process of peopling it with organic
beings has been continually going on, and whenever any of the higher
groups have become nearly or quite extinct, the lower forms which have
better resisted the modified physical conditions have served as the
antitypes on which to found the new races. In this manner alone, it is
believed, can the representative groups at successive periods, and the
risings and fallings in the scale of organization, be in every case
explained.

_Objections to Forbes' Theory of Polarity._

The hypothesis of polarity, recently put forward by Professor Edward
Forbes to account for the abundance of generic forms at a very early
period and at present, while in the intermediate epochs there is a
gradual diminution and impoverishment, till the minimum occurred at the
confines of the Palaeozoic and Secondary epochs, appears to us quite
unnecessary, as the facts may be readily accounted for on the principles
already laid down. Between the Palaeozoic and Neozoic periods of
Professor Forbes, there is scarcely a species in common, and the greater
part of the genera and families also disappear to be replaced by new
ones. It is almost universally admitted that such a change in the
organic world must have occupied a vast period of time. Of this interval
we have no record