Speculations

Future histories enhance one another. Although Olaf Stapledon's Last And First Men and Poul Anderson's Technic History present two mutually incompatible fictional futures, both works occupy different regions of that vast intellectual and imaginative space encompassing all futuristic speculations. When sf readers encounter either of Anderson's two intelligent flying species, the Diomedeans or the Ythrians, some might remember Stapledon's equally winged Venerians - or his devolved, then re-evolved, Neptunians. Stepping just outside the realm of future histories as such, HG Wells' two extra-terrestrial races, the Selenites and the Martians, stand up well against any of the (many) intelligent species imagined by Wells' many successors. Martians, Merseians (of the Roidhunate) and kzinti are all threats to humanity. 

In time travel fiction, Anderson's Time Patrol timecycles read like higher tech - and mass-produced - Time Machines whereas his ultimately evolved Danellians contrast sharply with Wells' devolved Morlocks and Eloi. I suggest that the Danellians encountered by Manse Everard during the twentieth century can be described as "extra-temporals," the time travel equivalents of extra-terrestrials. They originate from a later era (our descendants) rather than from another planet (no relations). And these "Danellians" - their name never explained - also contrast sharply with Stapledon's mentally time travelling Neptunians who, when exploring their past, are surprised to learn that their ancestors had migrated from Venus and, before that, from Earth. Comparisons of alternative speculations generate a sense of comprehensiveness.

Googling for images of Last And First Men, I was surprised to learn that this novel has recently been filmed.