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 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 cases are certainly exceptional; the general rule being
a gradual increase in number, till the group reaches its maximum, and
then, sooner or later, it gradually decreases. If the number of
the species of a genus, or the number of the genera of a family, be
repre