Simplify the following text for a Grade 8 native English-speaking person.
Original: He is well known for establishing extinction as a fact , being the most influential proponent of catastrophism in geology in the early 19th century , and opposing the evolutionary theories of Lamarck and Geoffroy Saint-Hilaire .
Simplified: He is well known because he proved extinction was a fact . He was the most influential person believing in catastrophism in geology in the early 19th century .