changes in the living population of the globe during geological time as
something enormous: and indeed they are so, if we regard only the
negative differences which separate the older rocks from the more modern,
and if we look upon specific and generic changes as great changes, which
from one point of view, they truly are. But leaving the negative
differences out of consideration, and looking only at the positive data
furnished by the fossil world from a broader point of view--from that of
the comparative anatomist who has made the study of the greater
modifications of animal form his chief business--a surprise of another
kind dawns upon the mind; and under _this_ aspect the smallness of the
total change becomes as astonishing as was its greatness under the other.

There are two hundred known orders of plants; of these not one is
certainly known to exist exclusively in the fossil state. The whole lapse
of geological time has as yet yielded not a single new ordinal type of
vegetable structure.[3]

[Footnote 3: See Hooker's _Introductory Essay to the Flora of Tasmania_,
p. xxiii.]

The positive change in passing from the recent to the ancient animal
world is greater, but still singularly small. No fossil animal is so
distinct from those now living as to require to be arranged even in a
separate class from those which contain existing forms. It is only when
we come to the orders, which may be roughly estimated at about a hundred
and thirty, that we meet with fossil animals so distinct from those now
living as to require orders for themselves; and these do not amount, on
the most liberal estimate, to more than about 10 per cent. of the whole.

There is no certainly known extinct order of Protozoa; there is but one
among the Coelenterata--that of the rugose corals; there is none among
the Mollusca; there are three, the Cystidea, Blastoidea, and
Edrioasterida, among the Echinoderms; and two, the Trilobita and
Eurypterida, among the Crustacea; making altogether five for the great
sub-kingdom of Annulosa. Among Vertebrates there is no ordinally distinct
fossil fish: there is only one extinct order of Amphibia--the
Labyrinthodonts; but there are at least four distinct orders of Reptilia,
viz. the Ichthyosauria, Plesiosauria, Pterosauria, Dinosauria, and
perhaps another or two. There is no known extinct order of Birds, and no
certainly known extinct order of Mammals, the ordinal distinctness of the
"Toxodontia" being doubtful