The laws of marriage and divorce in the various States and Territories cannot therefore be ascertained from a single legislative or judicial source. The law of the several jurisdictions consists not only of legislative enactments, but of judicial construction and interpretation of such legislation. Fortunately the tendency is toward uniformity of legislation among the States, especially on the important subject of marriage and divorce, and such differences as exist are pointed out substantially in this chapter when each State or Territory is considered separately. The Congress, or national legislature, has power to legislate only upon such subjects as the Federal Constitution marks out for it, and all powers not granted to the Federal government remain with the several States. The regulation of marriage and divorce is one of the most important domestic concerns which remains within the jurisdiction of a State. Article IV., Section 3, of the Constitution of the United States expressly grants to Congress exclusive power to prescribe laws for the Territories of the United States. Just as each State has a separate judicial system so the Federal Government has its separate courts, which have no power to interfere with the proceedings or judgments of the State courts unless some principle of the Federal Constitution or a national law is challenged. ESSENTIALS TO MARRIAGE.--There are three requisites to a lawful marriage in all of the States and Territories of the United States. These are: 1. First, that the marriage is _monogamous_. That is, the Federal courts and the courts of the several States only recognize as a true marriage one which in addition to being valid in other respects is a voluntary union of one man and one woman for life to the exclusion of all others. 2. The parties must be competent according to the _lex loci contractus_, or the law where the contract was concluded. 3. There must be free consent on the part of both of the contracting parties. INTERSTATE COMITY.--As Wharton points out in his "Conflict of Laws," marriage is not merely a contract but an international institution of Christendom. Often complications arise out of some difference between the law of marriage and divorce in the State where a marriage is concluded, or a divorce effected, and the law of the State where one or both of the parties may after the marriage or divorce acquire a domicile. The guiding rule in such cases is that if a marriage or divorce is valid in the State or Territory where it was concluded or effected, it is valid in all of the States and Territories of the United States. PROOF OF MARRIAGE.--There are various methods of proving the existence of a marriage.