You make a first-approximation distinction between high level and low level theories, which I think is helpful. But where would you see Haldane's famous 'rabbit in the Precambrian' fitting into that? Would that really challenge the high level theory of universal common ancestry, which is what I think Haldane was implying, or do you think it would only falsify a certain set of lower level theories, for example the precise place of the Mammalia in the tree of life?I think it would take more than a Precambrian rabbit to discredit the high level model of common ancestry. High level models do not depend directly on any particular observation, and they usually require more than just one or two contradictions (no matter how spectacular) to discredit them. I suspect that a true Precambrian rabbit fossil would call for a radical re-evaluation of the fossil record and the evolutionary timescale. That would seriously shake what we think we know about evolution, but the I think that biogeography and comparative biology remain compelling evidence of common ancestry. That should say something about creationists obsessed with "out of place" fossils: Even if they were real (which I seriously doubt), they don't really do that much for a "case for creation" or against evolution.
The second concerns something that Mark Ridley says in his textbook 'Evolution' (Blackwell, Third Edition, pp.8-9). He suggests that it was Cuvier and his followers, in their zeal to crush Lamarck, who ensured that species fixity was the prevailing view by the time Darwin came along. My question is: what do you think of that?I think he oversimplifies the state of biology before Darwin. There were plenty of people who toyed with the idea of evolution in the early nineteenth century. As Adrian Desmond argues in his Politics of Evolution, these tended to be politically radical types that were frowned upon by the establishment in England. Others were more conservative, suggesting that there was a kind of limited evolution (e.g. within genera). See Paul Garner's paper from the Genesis Kinds book or my own on species variability [PDF] for more information.
Having said all that, I don't want to leave the impression that species fixity was unimportant, but I definitely think attributing it to the influence of Cuvier is missing an important ingredient. I suspect that British natural theology was the far greater factor in the widespread acceptance of species fixity during the first half of the nineteenth century. For example, in the introduction to The Wisdom of God Manifested in the Works of Creation, John Ray wrote, "...by the Works of Creation, in the Title, I mean the Works created by God at first, and by Him conserv'd to this Day in the same State and Condition in which they were at first made" (I'm quoting from a 1717 copy). Likewise, according to William Paley, the attributes of the Creator are revealed in creation just as it is today, even with death and suffering. He wrote, "Immortality upon this earth is out of the question. Without death there could be no generation, no sexes, no parental relation, i.e. as things are constituted, no animal happiness" (happiness gained by fulfilling God's intentions is the ultimate good in Paley's utilitarian philosophy). This implies that the Fall of man did not bring animal death into the world, and therefore things are the same now as they always were. I suspect that if we looked more carefully at the natural theology literature, we could find other explicit and implicit endorsements of stasis.