Original: Enumerative graph theory then rose from the results of Cayley and the fundamental results published by Pólya between 1935 and 1937 and the generalization of these by De Bruijn in 1959.
Simplified: Enumerative graph theory developed from Cayleys results, a scientific publication done by Pólya between 1935 and 1937 and the generalization of these by De Bruijn in 1959.