The Creatures That Time Forgot is about what could happens to humanity if left to survive and evolve on an inhospitable planet. It is a really well told story in terms of answering an interesting question in a unique way. There are a few instances of a men writing women thing I don't like where the main or only attribute of the love interest to be described is her breasts. Not my favorite science fiction story I've ever read, but nowhere close to the worst.