"The administration of private justice between the citizens of the same state, the supervision of agriculture and of other concerns of a similar nature, all those things in short which are proper to be provided for by local legislation, can never be desirable cares of a general jurisdiction . . . the attempt to exercise these powers would be as troublesome as it would be nugatory; and the possession of them, for that reason, would contribute nothing to the dignity, to the importance, or to the splendour of the national government."