  We get the same degeneracy relation between $w_0$ and $w_a$ for the tachyon
fields as for quintessence and phantom fields. Our results show that the
dynamics of scalar fields with different origins becomes indistinguishable when
the equation of state parameter $w$ does not deviate too far away from -1, and
the time variation $w'$ satisfies the same bound for the same class of models.
For the tachyon fields, a limit on $w'$ exists due to the Hubble damping and we
derived the generic bounds on $w'$ for different classes of models. We may
distinguish different models in the phase plane of $w-w'$. The current
constraints on $w$ and $w'$ are consistent with all classes of models. We need
to improve the constraint on $w'$ by 50% to distinguish different models.
