For a generation this truth has been accepted by biologists.* §120. When we pass from individual forms of life to life at large,and ask whether the same law is seen in the ensemble of its manifestations-- whether modern plants and animals have more heterogeneous structures thanancient ones, and whether the Earth's present Flora and Fauna are more heterogeneousthan the Flora and Fauna of the past, -- we find the evidence so fragmentarythat nearly every conclusion is open to dispute. Three-fifths of the Earth'ssurface being covered by water; a great part of the exposed land being inaccessibleto, or untravelled by, the geologist; the most of the remainder having beenscarcely more than glanced at; and even familiar portions, as England, havingbeen so imperfectly explored that a new series of strata has been added withinthese few years; it is clearly impossible to say with any certainty whatcreatures have, and what have not, existed at any particular period. Consideringthe perishable nature of many of the lower organic forms, the metamorphosisof many beds of sediment, and the gaps that occur among the rest, we shallsee further reason for distrusting our deductions. On the one hand, the repeateddiscovery of vertebrate remains in strata previously supposed to containnone -- of reptiles where only fish were thought to exist, and of mammalswhere it was believed there were no creatures higher than reptiles; rendersit daily more manifest how small is the value of negative evidence. On theother hand, the worthlessness of the assumption that we have found the earliest,or anything like the earliest, organic remains, is becoming equally clear.
That the oldest known aqueous formations have been greatly changed by igneousaction, and that still older ones have been totally transformed by it, isbecoming undeniable. And the fact that sedimentary strata earlier than anywe know have been melted up, being admitted, it must also be admitted thatwe cannot say how far back in time this destruction of sedimentary stratahas been going on. For aught we know to the contrary, only the last chaptersof the Earth's biological history may have come down to us.
