types very much higher than any that had yet
existed. [Sidenote: Cold-blooded animals succeeded by hot.] In the old
heavy atmosphere, full of a noxious gas, none but slowly-respiring
cold-blooded animals could maintain themselves; but after the great
change in the constitution of the air had been accomplished, the
quickly-respiring and hot-blooded forms might exist. Hitherto the
highest advancement that animal life could reach was in batrachian and
lizard-like organisms; yet even these were destined to participate in
the change, increasing in magnitude and vital capacity. The pterodactyl
of the chalk, a flying lizard, measures nearly seventeen feet from tip
to tip of its wings. The air had now become suitable for mammals, both
placental and implacental, and for birds. One after another, in their
due order, appeared the highest vertebrates: marine, as the cetacean;
aerial, as the bat; and in the terrestrial, reaching, in the Eocene,
quadrumanous animals, but not, until after the Pliocene, man.

[Sidenote: The date of organisms may change, but the order not.]
Although the advance of geology may hereafter lead to a correction of
some of the conclusions thus attained to respecting the first dates of
different organic forms, and carry them back to more ancient times, it
is scarcely likely that any material modification of their order of
occurrence will ever be made. Birds, mammals, reptiles, fishes, and
invertebrates may each be detected in earlier strata; even in some of
those formations now regarded as non-fossiliferous, organisms may be
found; but it is not at all probable that the preponderance of reptiles
will ever cease to be the essential characteristic of the Secondary
rocks, or that of mammals of the Tertiary, or that a preceding period of
vast duration, in which the type of life had been the invertebrate, will
ever be doubted. Nothing, probably, will ever be discovered to
invalidate the physical conclusion that, while there was an excess of
carbonic acid in the air, the Flora would tend to be Cryptogamic and
Gymnospermic, and that there would be a scarcity of monocotyledons and
dicotyledonous angiosperms in the coal; nothing to disprove the fact
that the animals were slow-breathing and cold-blooded; and that it was
not until after the oxygen of the air had increased and the mean
temperature had declined that birds made their appearance. Though both
placental and marsupial animals may hereafter be found earlier than