nd, for the numerous orders and families
cited afford no trace of such a process.

But it is a most remarkable fact, that, while the groups which have been
mentioned, and many besides, exhibit no sign of progressive modification,
there are others, co-existing with them, under the same conditions, in
which 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
Siphonostone 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 palaeozoic 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 generalised to a more specialised 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,