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.
It does not shape our vision. It laces no constraints on our expectations of nature. Since it allows anything, mosaic evolution is not a scientific theory. Instead of finding character traits which are obviously transitional we find them to be either basically reptilian or basically mammalian, so that although the monotremes are a puzzle in terms of typology they afford little evidence for believing that any of the basic character traits of the mammals were achieved gradually in the way evolution envisages. Then, in 1824, Meckel discovered mammary glands.