Mysterious? More impossible. Too many mainland (& different mainlands) creatures for it to be natural, which spoiled it a bit for me. Even given when it was written, it didn't make sense in that way. (That islands were limited in the species was known.)
The building technology from the ground up was fun, but rather underestimated how hard it would be.
The ending seemed very rushed and improbable, even compared to the rest.
On the whole, although it's impressive for its time, I didn't enjoy the book. Perhaps a better (or more modern) translation would have helped. There were at least a couple of occasions where a translation problem caught my eye - just from the poor quality of the English. But it may be that the impossibilities just distracted me too much from the story.