eral, and inquire whether the same law is seen in the _ensemble_
of its manifestations,--whether modern plants and animals are of more
heterogeneous structure than ancient ones, and whether the Earth's
present Flora and Fauna are more heterogeneous than the Flora and Fauna
of the past,--we find the evidence so fragmentary, that every conclusion
is open to dispute. Three-fifths of the Earth's surface being covered by
water; a great part of the exposed land being inaccessible to, or
untravelled by, the geologist; the greater part of the remainder having
been scarcely more than glanced at; and even the most familiar portions,
as England, having been so imperfectly explored that a new series of
strata has been added within these four years,--it is impossible for us
to say with certainty what creatures have, and what have not, existed at
any particular period. Considering the perishable nature of many of the
lower organic forms, the metamorphosis of numerous sedimentary strata,
and the great gaps occurring among the rest, we shall see further reason
for distrusting our deductions. On the one hand, the repeated discovery
of vertebrate remains in strata previously supposed to contain none,--of
reptiles where only fish were thought to exist,--of mammals where it was
believed there were no creatures higher than reptiles,--renders it daily
more manifest how small is the value of negative evidence. On the other
hand, the worthlessness of the assumption that we have discovered the
earliest, or anything like the earliest, organic remains, is becoming
equally clear. That the oldest known sedimentary rocks have been greatly
changed by igneous action, and that still older ones have been totally
transformed by it, is becoming undeniable. And the fact that sedimentary
strata earlier than any we know, have been melted up, being admitted, it
must also be admitted that we cannot say how far back in time this
destruction of sedimentary strata has been going on. Thus the title
_Palaeozoic_, as applied to the earliest known fossiliferous strata,
involves a _petitio principii_; and, for aught we know to the contrary,
only the last few chapters of the Earth's biological history may have
come down to us. On neither side, therefore, is the evidence conclusive.
Nevertheless we cannot but think that, scanty as they are, the facts,
taken altogether, tend to show both that the more heterogeneous
organisms have been evolved in the later geologic periods,