of regulation. Ever since Willson _v._ Black-Bird Creek Marsh Co., 2
Pet. 245, and Cooley _v._ Board of Wardens, 12 How. 299, it has been
recognized that, in the absence of conflicting legislation by Congress,
there is a residuum of power in the state to make laws governing matters
of local concern which nevertheless in some measure affect interstate
commerce or even, to some extent, regulate it.[783] Thus the states may
regulate matters which, because of their number and diversity, may never
be adequately dealt with by Congress.[784] When the regulation of
matters of local concern is local in character and effect, and its
impact on the national commerce does not seriously interfere with its
operation, and the consequent incentive to deal with them nationally is
slight, such regulation has been generally held to be within state
authority.[785]

"But ever since Gibbons _v._ Ogden, 9 Wheat. 1, the states have not been
deemed to have authority to impede substantially the free flow of
commerce from state to state, or to regulate those phases of the
national commerce which, because of the need of national uniformity,
demand that their regulation, if any, be prescribed by a single
authority.[786] Whether or not this long-recognized distribution of
power between the national and the state governments is predicated upon
the implications of the commerce clause itself,[787] or upon the
presumed intention of Congress, where Congress has not spoken,[788] the
result is the same.

"In the application of these principles some enactments may be found to
be plainly within and others plainly without state power. But between
these extremes lies the infinite variety of cases, in which regulation
of local matters may also operate as a regulation of commerce, in which
reconciliation of the conflicting claims of state and national power is
to be attained only by some appraisal and accommodation of the competing
demands of the state and national interests involved.[789]

"For a hundred years it has been accepted constitutional doctrine that
the commerce clause, without the aid of Congressional legislation, thus
affords some protection from state legislation inimical to the national
commerce, and that in such cases, where Congress has not acted, this
Court, and not the state legislature, is under the commerce clause the
final arbiter of the competing demands of state and national
interests.[790]

"Congress has undoubted power to redefine th