ch more or less distinct indications of such a process
seems to be traceable. Among such indications I may remind you of the
predominance of Holostome Gasteropoda in the older rocks as compared
with that of Siphonostome Gasteropoda in the later. A case less open
to the objection of negative evidence, however, is that afforded by the
Tetrabranchiate Cephalopoda, the forms of the shells and of the septal
sutures exhibiting a certain increase of complexity in the newer genera.
Here, however, one is met at once with the occurrence of 'Orthoceras'
and 'Baculites' at the two ends of the series, and of the fact that one
of the simplest Genera, 'Nautilus', is that which now exists.

The Crinoidea, in the abundance of stalked forms in the ancient
formations as compared with their present rarity, seem to present us
with a fair case of modification from a more embryonic towards a less
embryonic condition. But then, on careful consideration of the facts,
the objection arises that the stalk, calyx, and arms of the paleozoic
Crinoid are exceedingly different from the corresponding organs of a
larval 'Comatula'; and it might with perfect justice be argued that
'Actinocrinus' and 'Eucalyptocrinus', for example, depart to the full
as widely, in one direction, from the stalked embryo of 'Comatula', as
'Comatula' itself does in the other.

The Echinidea, again, are frequently quoted as exhibiting a gradual
passage from a more generalized to a more specialized type, seeing
that the elongated, or oval, Spatangoids appear after the spheroidal
Echinoids. But here it might be argued, on the other hand, that the
spheroidal Echinoids, in reality, depart further from the general plan
and from the embryonic form than the elongated Spatangoids do; and that
the peculiar dental apparatus and the pedicellariae of the former are
marks of at least as great differentiation as the petaloid ambulacra and
semitae of the latter.

Once more, the prevalence of Macrurous before Brachyurous Podophthalmia
is, apparently, a fair piece of evidence in favour of progressive
modification in the same order of Crustacea; and yet the case will not
stand much sifting, seeing that the Macrurous Podophthalmia depart as
far in one direction from the common type of Podophthalmia, or from any
embryonic condition of the Brachyura, as the Brachyura do in the
other; and that the middle terms between Macrura and Brachyura--the
Anomura--are little better represented in the older