  We show that the Persson-Brener theory of crack propagation in viscoelastic
solids gives a viscoelastic fracture energy factor $G/G_0 = 1+f$ which is
nearly the same as the viscoelastic factor obtained using the cohesive-zone
model. We also discuss finite-size effects and comment on the use of crack
propagation theories for "solids" with a viscoelastic modulus that vanishes at
zero frequency.
