more generalized type, than the modern Opossum; or a
_Lophiodon_, or a _Palaeotherium_, than a modern _Tapirus_ or _Hyrax_?

These examples might be almost indefinitely multiplied, but surely they
are sufficient to prove that the only safe and unquestionable testimony
we can procure--positive evidence--fails to demonstrate any sort of
progressive modification towards a less embryonic, or less generalized,
type in a great many groups of animals of long-continued geological
existence. In these groups there is abundant evidence of variation--none
of what is ordinarily understood as progression; and, if the known
geological record is to be regarded as even any considerable fragment of
the whole, it is inconceivable that any theory of a necessarily
progressive development can stand, 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, coexisting with them, under the same
conditions, in which more or less distinct indications of such a process
seem 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 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 fr