Basically the entire job of a historian is to determine the credibility of old sources, so they can interpret all the data and come to the most accurate conclusion about what happened.
Unsubstantiated conjecture by Victorian historians shouldn't really be treated as a "source" in the first place by actual historians.
Theodore Andrea Cook wasn't a historian. Xe was a writer for the Daily Telegraph, amongst other things, who wrote about sports such as fencing and rowing; and who was also an art critic.
Theodore Andrea Cook wasn't writing in the Victorian Era. _Spirals in Nature and Art_ was a 20th century work, in the Edwardian Era. _The Curves of Life_ was from the subsequent Georgian Era.
Theodore Andrea Cook is the earliest person found espousing this hypothesis. This is, as far as anyone has determined, Theodore Andrea Cook's own original hypothesis, based upon zero evidence. That is certainly what the text of _Spirals_ implies.