Complex sentence: The renewed interest of Romanticism in the Old North had political implications and the Geatish Society, of which poet and historian Erik Gustaf Geijer was a founder-member, popularized this myth to a great extent.
Split into simpler sentences: The renewed interest of Romanticism in the Old North had political implications. The Geatish Society, of which Geijer was a member, popularized this myth to a great extent.