Clarik's Theorem

Clarik's Theorem stated that "societies dominated by a single intelligent life form, no matter how culturally disparate or variously organised, will always retain, within certain parametres, the essential accoutrements required for the existence of that life form."

This was given as an explanation for the similarity of the clothes, technology, equipment, languages, and so on, for a given species (for example, humans or Chelonians) over widely different points in their history, even millions or billions of years apart. (PROSE: The Well-Mannered War)