Or to turn to the higher Vertebrata--in what sense are the Liassic
Chelonia inferior to those which now exist? How are the Cretaceous
Ichthyosauria, Plesiosauria, or Pterosauria less embryonic, or more
differentiated, species than those of the Lias?

Or lastly, in what circumstance is the 'Phascolotherium' more embryonic,
or of a more generalized type, than the modern Opossum; or a
'Lophiodon', or a 'Paleotherium', 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 generalised,
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, 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 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