The overall result has been to make entrance to professional geological journals harder for amateurs, a result that has been reinforced by the widespread introduction of refereeing, first by national journals in the 19th century and then by several local geological journals in the 20th century. 

请问这句话中的first by national journals in the 19th century and then by several local geological journals in the 20th century.修饰refereeing【推荐制度】应该怎么翻译，这里的by的意思是什么?