Generally speaking, the right of a foreign corporation to do business within a State is in the nature of a license which that State may revoke or modify at discretion. Where, however, the foreign corporation, relying upon an existing law to the effect that certain charges will not, for a certain period at least, be imposed upon it, has entered the State for the transaction of business there, a contract to that effect is held to exist between it and the State, the obligation of which the latter may not impair. Thus in American Smelting, etc., Co. v. Colorado11 it was held that "a contract right to do business in the State during the corporate lifetime of domestic corporations without being subject to any greater liabilities than were or might be imposed upon domestic corporations was acquired by a foreign corporation by virtue of its admission into the State of Colorado with the right to do business therein under the then-existing laws of that State, which, inter alia, subjected foreign corporations coming into the State to the liabilities restrictions, and duties which then were or might thereafter be imposed upon domestic corporations of like character, and that such right was unconstitutionally impaired by an act of the State, exacting from such corporation an annual tax or license fee in double the amount of that imposed upon domestic corporations."

9 125 U. S. 190; 8 Sup. Ct. Rep. 723; 31 L. ed. 654.

10 Williams v. Wingo, 177 U. S. 601; 20 Sup. Ct. Rep. 793; 44 L. ed. 905; Fanning v. Gregoire, 16 How. 524; 14 L. ed. 1043.