Evolution And Devolution

HG Wells imagined:

in a paradisal Terrestrial environment, social classes devolving into distinct species, Morlocks and Eloi, the former eating the latter;

Martians degenerating into machine-protected brains living by extracting blood from humanoid organisms;

Selenites artificially adapting members of their species solely to perform specific industrial tasks.

Aldous Huxley imagined future humanity adapting its members for socially stratified roles.

Olaf Stapledon imagined:

eighteen successive human species;

some artificially adapted to inhabit Venus and Neptune;

Neptunians devolving into animality and later re-evolving intelligence.

Poul Anderson continued such evolutionary speculations by imagining extrasolar planetary environments, then explaining why, e.g., his fictional Diomedeans and Ythrians had evolved as they did in their particular environments. Thus, Anderson is Wellsian and Stapledonian. In his Harvest Of Stars future history, artificially adapted and specialized human beings have become redundant and are ghettoized.

SM Stirling's alternative histories include one in which Mars and Venus are humanly habitable because they were somehow terraformed a long time ago but I have yet to read any installments of this series.

(I did not want to go upstairs to check how many future human species Stapledon had imagined so I googled it instead. I will devolve into a brain attached to a finger pressing a keypad.)

Addendum, 18 Mar: Regular readers might notice that I have added a sentence to the paragraph about Anderson. It is difficult to remember every relevant detail while posting. Today is likely to be taken up with chores and preparation for a Latin class tomorrow, thus less time for posting.