There really is no other way to describe Isaacson's characterization of the relationship between NeXTStep and Mac OS X: It's false. OS X was not Mac OS with a kernel transplant — it was an evolution of NeXTStep with some accommodations for Mac users and developers. This is why you could get OpenStep programs running with little more than a recompile, but Mac programs that hadn't specifically been designed with compatibility in mind had to run in an OS 9 VM. The Finder is the most significant carry-over from OS 9 to OS X, but the OS X version was a Carbon program written completely from scratch to mimic the behavior of the OS 9 Finder because the OS was so fundamentally different. Meanwhile, NeXT's Mail, TextEdit and Preview chugged along just fine (not to mention third-party OpenStep apps like OmniWeb and Create).