for
species
within the
same
country
could
hardly
have kept
distinct
had they
been
capable of
crossing
freely.
The
importance
of the
fact that
hy- brids
are very
generally
sterile,
has, I
think,
been much
underrated
by some
late
writers.
On the
theory of
natural
selection
the case
is
especially
important,
inasmuch
as the
sterility
of hybrids
could not
possibly
be of any
advantage
to them,
and
therefore
could not
have been
