d extension of the ends of the bony arches of
the vertebrae upon the sheath of the notochord; the Carboniferous forms
exhibiting hardly any such expansion, while the Mesozoic genera present
a greater and greater development, until, in the Tertiary forms, the
expanded ends become suturally united so as to form a sort of false
vertebra. Hermann von Meyer, again, to whose luminous researches we
are indebted for our present large knowledge of the organization of the
older Labyrinthodonts, has proved that the Carboniferous 'Archegosaurus'
had very imperfectly developed vertebral centra, while the Triassic
'Mastodonsaurus' had the same parts completely ossified.* ([Footnote]
*As the Address is passing through the press (March 7, 1862),
evidence lies before me of the existence of a new Labyrinthodont
('Pholidogaster'), from the Edinburgh coal-field, with well-ossified
vertebral centra.)

The regularity and evenness of the dentition of the 'Anoplotherium', as
contrasted with that of existing Artiodactyles, and the assumed nearer
approach of the dentition of certain ancient Carnivores to the typical
arrangement, have also been cited as exemplifications of a law of
progressive development, but I know of no other cases based on positive
evidence which are worthy of particular notice.

What then does an impartial survey of the positively ascertained
truths of paleontology testify in relation to the common doctrines of
progressive modification, which suppose that modification to have taken
place by a necessary progress from more to less embryonic forms, or
from more to less generalized types, within the limits of the period
represented by the fossiliferous rocks?

It negatives those doctrines; for it either shows us no evidence of any
such modification, or demonstrates it to have been very slight; and as
to the nature of that modification, it yields no evidence whatsoever
that the earlier members of any long-continued group were more
generalized in structure than the later ones. To a certain extent,
indeed, it may be said that imperfect ossification of the vertebral
column is an embryonic character; but, on the other hand, it would be
extremely incorrect to suppose that the vertebral columns of the older
Vertebrata are in any sense embryonic in their whole structure.

Obviously, if the earliest fossiliferous rocks now known are coeval
with the commencement of life, and if their contents give us any just
conception of the nature