1658. Moonlit Night
The potential threat of a monster species that had the same ancestor as the merfolk had made Walker think a lot. He knew that he had to rest while there was a chance. Such a threat could appear anywhere at any time or not at all. It wasn't a predictable thing unless he had some sort of future sight skills and even if he did, anyone with those skills would be the first to tell everyone of the massive downsides that came with them.
The idea that a species could branch off over time was simple. It was clear in the horned rabbits and the rabbit jacks. They had once been from the same species. Even some monsters changed over time, their ancestors being regular animals without the mana that made them monsters. But that was unique.