In
The
Origin
of
Species
,
Darwin
provided
abundant
evidence
that
life
on
Earth
has
evolved
over
time
,
and
he
proposed
natural
selection
as
the
primary
mechanism
for
that
change
.
He
observed
that
individuals
____________________
in
their
inherited
traits
and
that
selection
ads
on
such
differences
,
leading
to
____________________
change
.
Although
Darwin
realised
that
variation
in
heritable
traits
is
a
prerequisite
for
____________________
,
he
did
not
know
precisely
how
organisms
pass
heritable
traits
to
their
____________________
.
Just
a
few
years
after
Darwin
published
The
Origin
of
Species
,
Gregor
Mendel
wrote
a
groundbreaking
____________________
on
inheritance
in
pea
plants
.
____________________
that
paper
,
Mendel
proposed
a
model
of
inheritance
in
which
organisms
transmit
____________________
heritable
units
(
now
called
genes
)
to
their
offspring
.
Although
Darwin
did
not
know
about
genes
,
Menders
paper
set
the
stage
____________________
understanding
the
genetic
differences
on
____________________
evolution
is
based
.
