and
crossing.
Fertility
of
varieties
when
crossed
and of
their
mongrel
offspring
not
universal.
Hybrids
and
mongrels
compared
indepen-
dently of
their
fertility.
Summary.
The view
generally
entertained
by
naturalists
is that
species,
when
inter-
crossed,
have been
specially
endowed
with the
quality of
sterility,
in order
to prevent
the
confusion
of all
organic
forms.
This view
certainly
seems at
first
probable,
