My Second Mac

We're talking about two slightly different things here, though- 68000 (while it did not have provisions for easily running 6800 code) had a common-mode so that programs built on the 68040 could run on the 68000, but you can also have code that requires instructions (or other things) added on later 68000-series processors, similar to Intel (variable 286-style segments, the copy-on-write provision of the 80486, etc.). The transition between 68000 and PPC was more like the Intel gap between the x86 and the 960 - they are completely different processors with no common heritage (the common heritage piece was provided by Apple in firmware). I haven't worked with them, but AFAIK the new embedded versions of the 68000 (ColdFire?) maintain the tradition of working with earlier 68ks (except in a few cases).