progressive modification of the vertebral column, in correspondence
with geological age, are those afforded by the Pycnodonts among fish,
and the Labyrinthodonts among Amphibia.

The late able ichthyologist Heckel pointed out the fact, that, while the
Pycnodonts never possess true vertebral centra, they differ in the
degree of expansion and 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