greater
importance, and now requires to rest on a basis the firmness of which is
thoroughly well assured. Among its fundamental conceptions, there must
be no confusion between what is certain and what is more or less
probable.[33] But, pending the construction of a surer foundation than
palaeontology now possesses, it may be instructive, assuming for the
nonce the general correctness of the ordinary hypothesis of geological
contemporaneity, to consider whether the deductions which are ordinarily
drawn from the whole body of palaeontological facts are justifiable.

The evidence on which such conclusions are based is of two kinds,
negative and positive. The value of negative evidence, in connexion with
this inquiry, has been so fully and clearly discussed in an address from
the chair of this Society,[34] which none of us have forgotten, that
nothing need at present be said about it; the more, as 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 palaeontology, and to inquire what they
tell us.

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.[35]

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, con