any particular period.  Considering the perishable nature of many of the lower organic forms, the metamorphosis of many sedimentary 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 that the vertebrate fauna, as a whole, has become more heterogeneous, is considerably stronger.  To the argument that the vertebrate fauna of the Palaeozoic period, consisting, so far as we know, entirely of Fishes, was less heterogeneous than the modern vertebrate fauna, which includes Reptiles, Birds, and Mammals, of multitudinous genera, it may be replied, as before, that estuary deposits of the