From the quotations which have been made, the general rule governing the construction of state law by the federal courts is sufficiently clear. We have now to consider the exceptions which have been made to its application.

First of all it is to be observed that the rule itself would appear to be one not so much of imperative constitutional necessity, as of comity adopted by the federal courts from a proper sense of the respect due to the States whose law they are supposed to administer, and that, therefore, the provision of section 721 of the Revised Statutes states a purely statutory and not a constitutional requirement.