I wonder, what was the first non-trivial graph class for which there was a forbidden (induced) subgraph characterisation ?
Of course, bipartite graph is one example but I am considering it as trivial because of the simplicity of the proof of sufficiency.
Another example can be planar graphs, but does Kuratowski's Theorem qualify as a forbidden subgraph characterisation ?
Do you think that the Boland-Lekkerkerker characterisation of interval graph can qualify as the first instance of non-trivial forbidden induced subgraph characterisation? Or does there exist such an example which appeared before ?