tions 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 {316} 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 same group have descended from some one
species, it is clear that as long as any species of the group have appeared
in the long succession of ages, so long must its members have continuously
existed, in order to have generated either new and modified or the same old
and unmodified forms. Species of the genus Lingula, for instance, must have
continuously existed by an unbroken succession of generations, from the
lowest Silurian stratum to the present day.

We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give an
explanation of this fact, which if true would have been fatal to my views.
But such