Are you willing to stretch your search to include what used to be called "planetary romance?" If so, then pick almost anything by Edgar Rice Burroughs.
He wrote many series, so the first books in series that I can recall deploying this trope are...
- Tarzan of the Apes
- A Princess of Mars
- At the Earth's Core
(In the later books, the children of Tarzan and John Carter fall into the same patterns)
Stand-alones include
The Monster Men and
The Oakdale Affair.
Best of all, they're all PD, and available here in the MR library.