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.
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