The last I checked, it was weirdly difficult to buy a copy of Last and First Men and be certain you were getting the complete book as it was intended to be read by the author.
Oddly, this article is "adapted from Matthew De Abaitua’s introduction to Olaf Stapledon’s "Last and First Men," which says "abridged" on the cover. I wonder what that's all about.
It’s freely available from Project Gutenberg.
https://www.gutenberg.org/ebooks/79003
The edition referenced in the article removes the last 4 chapters (13–16)
Off topic, but what kind of ego does it take to write an introduction that gets your name on the cover of an abridged edition? Certainly, a writer might conceive of a work as having 10 chapters and being complete, only for a publisher to demand 3 more to satisfy the expectations of the market or some literary superstition. But having been written, the text exists. It seems extremely presumptuous to me for a literary critic to take on the curatorial role and say 'you don't need to read the last quarter of the book, it's boring/repetitive.'