What is the fine-grained topic of the following text: After a mediocre career as a player and manager, he spent half a century in the front office as one of professional baseball’s greatest visionary executives.With the St. Louis Cardinals in the 20’s and 30’s, Rickey invented the modern farm system, promoting a new way of training and developing players. Only return the topic.
Rickey's contributions to professional baseball as an executive and innovator