Read this: The Grey Cup was established in 1909 after being donated by Albert Grey, 4th Earl Grey, The Governor General of Canada as the championship of teams under the CRU for the Rugby Football Championship of Canada. Initially an amateur competition, it eventually became dominated by professional teams in the 1940s and early 1950s. The Ontario Rugby Football Union, the last amateur organization to compete for the trophy, withdrew from competition in 1954. The move ushered in the modern era of Canadian professional football.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): When did professional teams begin to dominate the Canadian football championship?
1940s and early 1950s