Numerical simulations with an anelastic, spherical, axisymmetric mantle convection model have been conducted to address the question of the radial mixing length in the general circulation of the mantle. Continuing debate centers on the question as to whether or not the 670 km seismic discontinuity (which we now understand to exist as a consequence of an endothermic phase change of mantle mineralogy from the spinel phase to a mixture of perovskite and periclase) in combination with the 400 km discontinuity (associated with the exothermic phase change from olivine to spinel) will impose a sufficient barrier to the circulation so as to induce layering. We argue herein that the mantle must currently be converting in a partially layered style but that the degree of layering is highly time dependent. Moreover, in the perhaps not too distant past the propensity to layering was greater, possibly to the extent that soon after planetary formation mantle mixing occurred in two distinct reservoirs. As the planet cooled and the Rayleigh number fell, we suggest that the circulation was transformed from the layered state to the partially layered state that obtains today.