The Civil War has been characterized as "an appeal from the judgments of Marshall to the arbitrament of war." Its outcome restored the concept of the National Government as a territorial sovereign, present within the States by the superior mandate of the American People, and entitled to "execute on every foot of American soil the powers and functions that belong to it." ¹ These powers and functions are, moreover, today undergoing constant enlargement. No one now doubts that in any clash between national and state power it is national power which is entitled to be defined first, and few persons question that it ought to be defined in the light of Marshall's principle, that a Constitution designed for ages to come must be "adapted to the various crises of human affairs." ¹ Justice Bradley in ex parte Siebold, 100 U. S., 371. It is only when we turn to that branch of Constitutional Law which defines governmental power in relation to private rights that we lose touch with Marshall's principles. As we have seen, he dealt in absolutes: either power was given to an unlimited extent or it was withheld altogether. Today, however, the dominant rule in this field of Constitutional Law is the "rule of reason." In the last analysis, there are few private rights which are not subordinate to the general welfare; but, on the other hand, legislation which affects private rights must have a reasonable tendency to promote the general welfare and must not arbitrarily invade the rights of particular persons or classes. Inasmuch as the hard and fast rules of an age when conditions of life were simpler are no longer practicable under the more complex relationships of modern times, there is today an inevitable tendency to force these rules to greater flexibility. ¹ ¹ Notwithstanding what is said above, it is also true that the modern doctrine of "the police power" owes something to Marshall's interpretation of the "necessary and proper" clause in M'Culloch vs. Maryland, which is frequently offered nowadays as stating the authoritative definition of "a fair legislative discretion" in relation to private rights. Indeed this ingenious transposition was first suggested in Marshall's day. See Cowen (N. Y.), 585. But it never received his sanction and does not represent his point of view.