acquired
by the
continued
preservation
of
successive
profitable
degrees of
sterility.
I hope,
however,
to be able
to show
that
sterility
is not a
specially
acquired
or endowed
quality,
but is
incidental
on other
acquired
differences.
In
treating
this
subject,
two
classes of
facts, to
a large
extent
fundamentally
different,
have
generally
been
confounded
together;
namely,
the
sterility
of two
species
when first
crossed,
and the
sterility
