Grounds of decree.

    Such decree may be made on any of the following grounds:---
         (1) that the respondent was impotent at the time of the marriage and at the time of the institution of the suit;
         (2) that the parties are within the prohibited degree of consanguinity (whether natural or legal) or affinity;
         (3) that either party was a lunatic or idiot at the time of the marriage;
         (4) that the former husband or wife of either party was living at the time of the marriage, and the marriage with such former husband or wife was then in force.
         Nothing in this section shall affect the ¹[jurisdiction of the District Court] to make decrees of nullity of marriage on the ground that the consent of either party was obtained by force or fraud.