MR. BUCHANAN AND THE SUCCESSION. – One of our black republican contemporaries indulges in a long tirade of abuse and misrepresentation concerning the expectations, calculations and schemes of Mr. Buchanan as a Presidential candidate. He is thus represented as involved in the Southern ultra programme of “the spoliation of Mexico,” the “quiet revival of the African slave trade,” &c. And this, too, when equally violent denunciations of Mr. Buchanan come from the fire-eating Southern filibusters and slave traders. The Richmond Enquirer which, as the organ of Governor Wise, demands the Congressional protection of slavery in the Territories, denounces the President as identified with the black republican free soil notions of Eli Thayer. Thus denounced on each side of the slavery line as bought and sold by the other, we must come to the conclusion that Mr. Buchanan has not made himself the tool of either Northern abolitionists or Southern fire-eaters. In fact, in these very denunciations from Northern and Southern ultras we have the clearest proofs of the even-handed justice which has characterized his administration. And so, in reference to the succession, to whatever extent sectional demagogues may be befogged, the administration has only to pursue to the end the even tenor of its way. And this we have no doubt is the programme of Mr. Buchanan, and the whole of it.