the considerations which have been laid before you have certainly not
tended to increase your estimation of such evidence. It will be
preferable to turn to the positive facts of paleontology, and to inquire
what they tell us.

[footnote] *Anniversary Address for 1851, 'Quart. Journ.
Geol. Soc.' vol. vii.

We are all accustomed to speak of the number and the extent of the
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.*

[footnote] *See Hooker's 'Introductory Essay to the Flora of
Tasmania', p. xxiii.