ary strata, and the gaps that occur 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
it is manifest that the title, _Palaeozoic_, as applied to the earliest
known fossiliferous strata, involves a _petitio principii_; and that,
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, and that Life in general has been more heterogeneously
manifested as time has advanced. Let us cite, in illustration, the one
case of the _vertebrata_. The earliest known vertebrate remains are
those of Fishes; and Fishes are the most homogeneous of the vertebrata.
Later and more heterogeneous are Reptiles. Later still, and more
heterogeneous still, are Mammals and Birds. If it be said, as it may
fairly be said, that the Palaeozoic deposits, not being estuary deposits,
are not likely to contain the remains of terrestrial vertebrata, which
may nevertheless have existed at that era, we reply that we are merely
pointing to the leading facts, _such as they are_.

But to avoid any such criticism, let us take the mammalian subdivision
only. The earliest known remains of mammals are those of small
marsupials, which are the lowest of the mammalian type; while,
conversely, the highest of the mammalian type--Man--is the most recent.
The evidence