The "compromise" forms are likely symbionts. Having one organ be carbon based while another is silicon based strains my suspenders of disbelief.

> It's that "better tolerance" that seems to be the sticking point. One
> of the early posters (I think it was Tim) pointed out that this did
> not necessarily follow from simply being silicon-based.

No, it does not.

The frequency and intensity of contacts between the two biospheres determines the rate of evolutionary change.

It can be very difficult for an organism to evolve away traits that were useful before conditions changed.

There appears to be no biological analogue of quantum tunnelling with regard to the 'fitness landscape' - organisms can't easily lose a fitness maximising trait e.g. vision, aerobic metabolism.