In the main he copied Marinus whose work he revised and supplemented in some points, but he failed to realize the peninsular shape of India, erroneously exaggerated the size of Taprobane (Ceylon), and suggested that the Indian Ocean had no connexion with the western ocean, but formed Mare Clausum.
In Mare clausum (1635) John Selden endeavoured to prove that the sea was practically as capable of appropriation as territory.
The claim that Bering Sea was mare clausum was abandoned, but it was asserted that Russia had formerly exercised therein rights of exclusive jurisdiction which had passed to the United States, and they relied inter alia upon the ukase of 1821, b y which foreign vessels had been forbidden to approach within too Italian miles of the coasts of Russian America.
It was not till many years afterwards that the jealousies between England and Holland gave importance to the novel doctrine broached in the tract by Grotius, a doctrine which Selden set himself to refute in his Mare clausum (1632).
