INDIRECT ENCROACHMENT ON FEDERAL AUTHORITY 905 nation. Taxes which fall in some degree on instrumentalities of the national government or on the ftuits of interstate commerce have a double aspect. They are imposed on persons or property or occupations or privileges within the geographical jurisdiction of a state and normally within its legal jurisdiction. They also have some effect on operations within the legal jurisdiction of the United States — a legal jurisdiction assumed to be exclusive. One or the other aspect must be legally predominant, since the same tax can- not be both vaHd and invahd. But the necessary legal predominance of one aspect cannot obliterate the existence of the other; and the recognized imperative of cleaving only to one does not carry with it any certain indication of the choice between the two. The choice must be made as the cases arise, and without the aid of any rule of universal application. The rule must be the child and not the parent of the cases. All of the taxes which the Supreme Court has had to consider, from Marshall's day to this, have been demands which it was possi- ble to regard as formally on subjects within the jurisdiction of the state. All have had some effect on interstate commerce or on some operation of the national government. On nearly every crucial question the judges have been in disagreement as to whether the form or the effect should be regarded as controlling. In most im- portant instances this disagreement can be traced to differences of opinion as to the effect to be anticipated from the exercise of state power in question. It may be said, therefore, that the accepted test has always been a judgment on a question of economics, pro- vided it is understood that the judges have been concerned with the economic effect, not of the precise tax before them, but of such a tax levied at the highest rate which a state might be moved to impose. It will not do to accept without qualifications Marshall's statement that "questions of power do not depend on the degree to which it may be exercised," ^ but in general it is true that the court has not forgiven any state tax because the particular rate of levy was so moderate that its effect on national instrumentalities or on interstate commerce was negligible. The disagreement among the judges which has been character- istic of most of the decisions was not present in McCullock v. Mary- land.^ Here the court was unanimous in holding that a Maryland ' Brown v. Maryland, 12 Wheat. (U. S.) 419, 439 (1819). ' Note 2, supra.
