), that propagate by eggs or spawn,--chiefly by the former.
Then appear warm-blooded vertebrates (birds), that propagate by eggs
exclusively. Then warm-blooded vertebrates come upon the stage, that
produce _eggs_ without shells, which have to be subjected for months to
a species of extra-placental incubation. And last of all the true
placental mammals appear. And thus, tried by the test of perfect
reproduction, the great vertebral division receives its full development
in creation.

[Illustration: Fig. 70.

ANOPLTHERIUM COMMUNE.

(_Eocene._)]

The placental mammals make their appearance, as I have said, in the
earliest ages of the great Tertiary division, and exhibit in the group
an aspect very unlike that which they at present bear. The Eocene ages
were peculiarly the ages of the Palaeotheres,--strange animals of that
pachydermatous or thick-skinned order to which the elephants, the
tapirs, the hogs, and the horses belong. It had been remarked by
naturalists, that there are fewer families of this order in living
nature than of almost any other, and that, of the existing genera, not a
few are widely separated in their analogies from the others. But in the
Palaeotheres of the Eocene, which ranged in size from a large horse to a
hare, not a few of the missing links have been found,--links connecting
the tapirs to the hogs, and the hogs to the Palaeotheres proper; and
there is at least one species suggestive of an union of some of the more
peculiar traits of the tapirs and the horses. It was among these extinct
Pachydermata of the Paris basin that Cuvier effected his wonderful
restorations, and produced those figures in outline which are now as
familiar to the geologist as any of the forms of the existing animals.
The London Clay and the Eocene of the Isle of Wight have also yielded
numerous specimens of those pachyderms, whose identity with the
Continental ones has been established by Owen; but they are more
fragmentary, and their state of keeping less perfect, than those
furnished by the gypsum quarries of Velay and Montmartre. In these the
smaller animals occur often in a state of preservation so peculiar and
partial as to excite the curiosity of even the untaught workmen. Only
half the skeleton is present. The limbs and ribs of the under side are
found lying in nearly their proper places; while of the limbs and ribs
of the upper side usually not a trace can be detected,--even the upper
side of the skull is often a