Complex sentence: Zapffe first set out his ideas in Den sidste Messias (en. "The Last Messiah") (1933); later a more systematic defence was given in his philosophical treatise "Om det tragiske" (en. "On the tragic") (1941).
Split into simpler sentences: Zapffe first set out his ideas in Den sidste Messias (en. "The Last Messiah") (1933). Later Zapffe gave a more systematic defence in his philosophical treatise "Om det tragiske" (en. "On the tragic") (1941).