Cuvier was a very important figure in scientific circles in Paris during the early 19th century. He helped establish the fields of comparative anatomy and paleontology by comparing living animals with fossils. He proved extinction was a fact. He was the most influential person believing in catastrophism in geology in the early 19th century. His most famous work is the "Règne animal distribué d'après son organisation" 1817; translated into English as "The Animal Kingdom".