This section is from the book "The Law Of Quasi Contracts", by Frederic Campbell Woodward. Also available from Amazon: The Law of Quasi Contracts.
In so far as the right of a tort-feasor to indemnity or contribution has been recognized, it is enforced both in equity and at law. When there is no express contract, it is commonly rested upon the theory of an implied contract between the parties. Indeed, in an English admiralty case it is declared, in substance, that unless the circumstances raise an implied promise the right to indemnity does not exist.2 But in some cases, as for example where the wrong consists of a mere unintentional neglect of duty, there can hardly be said to be an implication of a genuine promise of indemnity or contribution. In such cases, the obligation may well be rested upon quasi contractual principles, for in so far as one tort-feasor pays what in equity and good conscience another tort-feasor ought to pay, the latter receives a benefit at the expense of the former, the retention of which is unjust.
1 Austin, etc., R. Co. v. Faust, 1911, Tex. Civ. App. ; 133 S. W. 449, 453.
2 The Englishman and The Australia,  P. 212.