Once again via Social Fiction:
Xiangqi appeares to be an old inbetween state of the ur-game that branched into current day go and chess. The board is go-based, the rules chess based (checkmate etc). Comparative boardology will one day reveal its exact position in the tree of games.
The wikipedia entry has some intersting details that could reveal the way local changes in the rules and boards might evolve a game into a new, if you like, specie.