led up by a close succession
of species, since these must have spread over so many ages, and have
existed in such numbers, that it seems impossible to account for their
total absence from deposits in which great numbers of species belonging
to other groups are preserved and have been discovered. In order to
appreciate the force, or weakness, of these objections, we must inquire
into the character and completeness of that record of the past life of
the earth which geology has unfolded, and ascertain the nature and
amount of the evidence which, under actual conditions, we may expect to
find.

_The Number of known Species of Extinct Animals._

When we state that the known fossil mollusca are considerably more
numerous than those which now live on the earth, it appears at first
sight that our knowledge is very complete, but this is far from being
the case. The species have been continually changing throughout
geological time, and at each period have probably been as numerous as
they are now. If we divide the fossiliferous strata into twelve great
divisions--the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias,
Trias, Permian, Carboniferous, Devonian, Silurian, and Cambrian,--we
find not only that each has a very distinct and characteristic molluscan
fauna, but that the different subdivisions often present a widely
different series of species; so that although a certain number of
species are common to two or more of the great divisions, the totality
of the species that have lived upon the earth must be very much more
than twelve times--perhaps even thirty or forty times--the number now
living. In like manner, although the species of fossil mammals now
recognised by more or less fragmentary fossil remains may not be much
less numerous than the living species, yet the duration of existence of
these was comparatively so short that they were almost completely
changed, perhaps six or seven times, during the Tertiary period; and
this is certainly only a fragment of the geological time during which
mammalia existed on the globe.

There is also reason to believe that the higher animals were much more
abundant in species during past geological epochs than now, owing to the
greater equability of the climate which rendered even the arctic regions
as habitable as the temperate zones are in our time.

The same equable climate would probably cause a more uniform
distribution of moisture, and render what are now desert regions cap