According to [this article][1] from the BBC's *Science Focus*

> Charles Darwin thought that the 28-day human menstrual cycle was
> evidence that our ancestors lived on the seashore and needed to
> synchronise with the tides.

Did Darwin ever say such a thing?

The closest I have been able to find from a cursory search is [this letter][2] from T.H. Huxley in the Darwin Correspondence Project, where Huxley writes

> Fancy lunacy & menstruation coming out of the primary fact that one’s
> nth. ancestor lived between tide-marks! I declare it’s the grandest
> suggestion I have heard of for an age

Is there any evidence that Darwin suggested this?


  [1]: https://www.sciencefocus.com/the-human-body/is-it-coincidental-that-the-human-menstrual-cycle-is-about-the-same-length-as-the-moon-cycle/
  [2]: http://www.darwinproject.ac.uk/letter/?docId=letters/DCP-LETT-7494.xml