In members of the same class the average amount of change, during long
and equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of long-enduring fossiliferous formations depends on great
masses of sediment having been deposited on areas whilst subsiding, our
formations have been almost necessarily accumulated at wide and
irregularly intermittent intervals; consequently the amount of organic
change exhibited by the fossils embedded in consecutive formations is
not equal. Each formation, on this view, does not mark a new and
complete act of creation, but only an occasional scene, taken almost at
hazard, in a slowly changing drama.
We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and
inorganic, should recur. For though the offspring of one species might
be adapted (and no doubt this has occurred in innumerable instances) to
fill the exact place of another species in the economy of nature, and
thus supplant it; yet the two forms the old and the new would not be
identically the same; for both would almost certainly inherit different
characters from their distinct progenitors. For instance, it is just
possible, if our fantail-pigeons were all destroyed, that fanciers, by
striving during long ages for the same object, might make a new breed
hardly distinguishable from our present fantail; but if the parent
rock-pigeon were also destroyed, and in nature we have every reason to
believe that the parent-form will generally be supplanted and
exterminated by its improved offspring, it is quite incredible that a
fantail, identical with the existing breed, could be raised from any
other species of pigeon, or even from the other well-established races
of the domestic pigeon, for the newly-formed fantail would be almost
sure to inherit from its new progenitor some slight characteristic
differences.
Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single
species, changing more or less quickly, and in a greater or lesser
degree. A group does not reappear after it has once disappeared; or its
existence, as long as it lasts, is continuous. I am aware that there
are some apparent exceptions to this rule, but the exceptions are
surprisingly few, so few, that E. Forbes, Pictet, and Woodward (though
all strongly opposed to such views as I maintain) admit its truth; and
the rule strictly accords with my theory. For as all the species of the
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