We may infer that during each period of the past, as at the present, only a very insignificant fraction of the innumerable organisms of sea and land escaped destruction and left in continental and oceanic deposits permanent records of their existence.  Scanty as these original life records must have been, they have been largely destroyed by metamorphism of the rocks in which they were imbedded, by solution in underground waters, and by the vast denudation under which the sediments of earlier periods have been eroded to furnish materials for the sedimentary records of later times.  Moreover, very much of what has escaped destruction still remains undiscovered.  The immense bulk of the stratified rocks is buried and inaccessible, and the records of the past which it contains can never be known.  Comparatively few outcrops have been thoroughly searched for fossils.  Although new species are constantly being discovered, each discovery may be considered as the outcome of a series of happy accidents,—­that the remains of individuals of this particular species happened to be imbedded and fossilized, that they happened to escape destruction during long ages, and that they happened to be exposed and found.

Some inferences from the records of the history of life upon the planet.  Meager as are these records, they set forth plainly some important truths which we will now briefly mention.

1.  Each series of the stratified rocks, except the very deepest, contains vestiges of life.  Hence the earth was tenanted by living creatures for an uncalculated length of time before human history began.

2.  Life on the earth has been EVERCHANGING.  The youngest strata hold the remains of existing species of animals and plants and those of species and varieties closely allied to them.  Strata somewhat older contain fewer existing species, and in strata of a still earlier, but by no means an ancient epoch, no existing species are to be found; the species of that epoch and of previous epochs have vanished from the living world.  During all geological time since life began on earth old species have constantly become extinct and with them the genera and families to which they belong, and other species, genera, and families have replaced them.  The fossils of each formation differ on the whole from those of every other.  The assemblage of animals and plants (the fauna-flora) of each epoch differs from that of every other epoch.

In many cases the extinction of a type has been gradual; in other instances apparently abrupt.  There is no evidence that any organism once become extinct has ever reappeared.  The duration of a species in time, or its “vertical range” through the strata, varies greatly.  Some species are limited to a stratum a few feet in thickness; some may range through an entire formation and be found but little modified in still higher beds.  A formation may thus often be divided into zones, each characterized by its own peculiar species.  As a rule, the simpler organisms have a longer duration as species, though not as individuals, than the more complex.