I am saying that Darwin is open source -- BSD, to be precise. The BSD license allows proprietary software to be grafted on without restriction, so Apple has attached its frameworks (API set) to the core.

Why do you think Apple was able to transition to Intel hardware so relatively easy ?

Take away Darwin, and see how useable our Mac is. There are loads of people that load X11 on top of Darwin on Apple hardware, to make nice useable unix boxes.

R2-E2, 2G Prius.
Highway/City/Husband/Wife MPG: 56.5, as of 12/2005, 26K miles

Jac Nasser, Ford President: "We are planning to launch a hybrid version of
this car [P2000] within this year [1998]. We will also make FCEV available in
2004."