531] Congressional regulation of commerce, however, does not have to be
uniform. The uniformity rule is a test of the invalidity of State
legislation affecting commerce, not the validity of Congressional
legislation regulating commerce. Clark Distilling Co. _v._ W.M.R. Co.,
242 U.S. 311, 327 (1917); Currin _v._ Wallace, 306 U.S. 1, 14 (1939);
Prudential Ins. Co. _v._ Benjamin, 328 U.S. 408 (1946).

[532] Simpson _v._ Shepard, 230 U.S. 352 (1913).

[533] Ibid. 400-402.

[534] McCarroll _v._ Dixie Greyhound Lines, 309 U.S. 176, 188-189
(1940). F.D.G. Ribble's _State and National Power Over Commerce_
(Columbia University Press, 1937) is an excellent study both of the
Court's formulas and of the arbitral character of its task in this field
of Constitutional Law. On the latter point, see especially Chapters X
and XII. The late Chief Justice Stone took repeated occasion to stress
the "balancing" and "adjusting" role of the Court when applying the
commerce clause in relation to State power. _See_ his words in South
Carolina State Highway Dept. _v._ Barnwell Bros., 303 U.S. 177, 184-192
(1938); California _v._ Thompson, 313 U.S. 109, 113-116 (1941); Parker
_v._ Brown, 317 U.S. 341, 362-363 (1943); and Southern Pacific _v._
Arizona, 325. U.S. 761, 766-770 (1945). _See also_ Justice Black for the
Court in United States _v._ South-Eastern Underwriters Assoc., 322 U.S.
533, 548-549 (1944).

[535] 12 Wheat. 419 (1827).

[536] Compare, for example, May _v._ New Orleans, 178 U.S. 496 (1900);
and the recent case of Hooven & Allison Co. _v._ Evatt, 324 U.S. 652
(1945). In the latter case the benefits of the original package doctrine
were extended to imports from the Philippine Islands title to which did
not vest in the importer until their arrival in the United States.

[537] Freeman _v._ Hewit, 329 U.S. 249, 251 (1946).

[538] Philadelphia & R.R. Co. _v._ Pennsylvania (State Freight Tax
Case), 15 Wall. 232 (1873).

[539] Headnotes. Said the Court: "The rule has been asserted with great
clearness, that whenever the subjects over which a power to regulate
commerce is asserted are in their nature national, or admit of one
uniform system or plan of regulation, they may justly be said to be of
such a nature as to require exclusive legislation by Congress. Surely
transportation of passengers or merchandise through a State, or from one
State to another, is of this nature. It is of national importance that
over that subject there shou