Thanks to Kelly
McNeill for this link.
“Apple’s Macintosh OS X operating system, when combined with
it’s Open Source initiative, presents us with the following
interesting conundrum:”
“How can an operating system contain another complete operating
system? Mac OS X has as its foundation an Open Source project that
Apple is calling Darwin. Basically, Darwin is BSD Unix on a Mach
2.5 kernel with some Apple modifications and additions. Darwin is a
complete, stand-alone operating system.”