Echidnas come as two species in two separate genera and with quite different habits. Echidnas are a successful and at least modestly varied group. Frequently, a suite of characters undergoes a complete transformation before other characters change at all. The concept of mosaic evolution dictates that organs will evolve in different ways to meet varying selective pressures. We would not deny Cuvier's inference today, but only his initial premise of tight and ubiquitous correlation.
Evolution is mosaic in character, proceeding at different rates in different structures.
An animal's parts are largely dissociable, thus permitting historical change to proceed. The former means that the soft body parts might have been evolving invisibly while the parts which fossilized stayed the same. The latter means that natural selection prevented change by eliminating all the innovations, sometimes for periods of millions of years and despite changing environmental conditions that ought to have encouraged adaptive innovation. Natural selection appears here in its formulation as a tautology with rather too much explanatory power, an invisible all-purpose explanation for whatever change or lack of change happened to occur. Mosaic evolution is a wide open concept.