Complex sentence: This story, incorporated into Stephenson Percy Smith's "Great Fleet" hypothesis, was widely believed during the early 20th century, but was discredited in the 1960s and 1970s.
Split into simpler sentences: This story, incorporated into Stephenson Percy Smith's "Great Fleet" hypothesis, was widely believed during the early 20th century. The earlier hypothesis was discredited in the 1960s and 1970s.