at
masses of sediment being deposited on subsiding areas, our formations
have been almost necessarily accumulated at wide and irregularly
intermittent intervals of time; 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 an ever 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 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; and organisms already
differing would vary in a different manner. For instance, it is
possible, if all our fantail-pigeons were destroyed, that fanciers might
make a new breed hardly distinguishable from the present breed; but if
the parent rock-pigeon were likewise destroyed, and under nature we have
every reason to believe that parent forms are generally supplanted
and exterminated by their improved offspring, it is incredible that a
fantail, identical with the existing breed, could be raised from any
other species of pigeon, or even from any other well established race
of the domestic pigeon, for the successive variations would almost
certainly be in some degree different, and the newly-formed variety
would probably inherit from its progenitor some 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, when it has once disappeared, never reappears; that is, 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 the theory. For all the species of
the same group, however long it may have lasted, are the modified
descen