ical
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
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 lea