The Eternal Savage by Edgar Rice Burroughs (1914) is a one-off / sort-of-Tarzan-spin-off that was serialized in 1914/1915 in All-Story Weekly and originally titled The Eternal Lover. It forms a pair with The Mad King, which I haven't read, but which sounds very fun and quite a bit different from many of Burroughs other books.
The book is divided into two parts. In the first, Victoria and Barney Custer, a brother and sister from Beatrice, Nebraska (!!) are staying with Lord Greystoke and his wife in Africa as part of a hunting party. Those in the know will remember that Lord Greystoke is Tarzan. Her whole life, Victoria has been frightened of earthquakes and visited with dreams about a hunky, half-naked cave man that she feels must be her only love. While the group is out hunting, the countryside is hit by a large earthquake which happens to open up a cave that was sealed by a similar earthquake 100,000 years ago. Oddly enough, the occupant of the cave, a hunky, half-naked cave man named Nu, Son of Nu, is still alive and very confused by his new surroundings. A combination of fate and a series of misunderstandings lead the time-crossed lovers together, then apart, then together again, and ultimately Victoria needs to decide if she should follow her heart and stay with Nu or reject him and stay with her brother and his rich friends.
But then!: Another earthquake happens and we are rocked back to 100,000 years ago where Victoria's counterpart, Nat-ul is waiting for her lover Nu to return to her with the head of a saber-toothed tiger that he promised to defeat before they became mates. A complicated combination of fate and a series of misunderstandings lead the lovers together, then apart, then together again in a very familiar pattern. Interspersed with the misunderstandings are lots of lion attacks, chases, and sniffing the air for the scent of one's mate.
In the end we are met with yet another earthquake and a slightly disappointing return to Victoria's life where the whole thing (spoiler alert) was just a dream (double spoiler alert) OR WAS IT!
In many ways, this plot is similar to The Monster Men, which I read a couple months ago: the smart and spunky heroine who still needs a lot of saving, the tough and straightforward hero who hasn't been corrupted by modernity, lots of lots of chasing and fighting, etc. In fact, the plot is similar to a lot of Burroughs books and a lot of serialized adventure novels in general, but who am I to argue with a successful formula?
Like much of Burroughs, one needs to accept a certain amount of sexism and racism with the story, and I could see an entire dissertation being written on gender roles and the issue of race in this book alone (for example, it is mentioned multiple times that Nu is white, although he was living in Africa 100,000 years ago and the current inhabitants are certainly black). The reader also has to suspend her disbelief when, for example, Nu is staying at Tarzan's house (Tarzan of the jungle who can speak to the apes and all that) and there are zero interactions between the two of them and not even any mention of how Tarzan might be able to help out with Nu's situation.
I'll leave you with this gem to help you start your dissertation: "Such reveries made Nu very sad, for he loved Nat-ul just as you or I would love -- just as normal white men have always loved -- with a devotion that placed the object of his affection upon a pedestal before which he was happy to bow down and worship. His passion was not of the brute type of the inferior races which oftentimes solemnizes the marriage ceremony with a cudgel and ever places the woman in the position of an inferior and a chattel." (p. 65)
[read the whole thing here!]