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 descendants one from the other, and all from
a common progenitor.
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