That the Britons then possessed no naval force fitted to compete with the Saxons is clear, from the fact that their contests with them were invariably carried on by land, and that no effectual resistance by sea was offered to the vast numbers of Saxons and Angles who poured into the island. It is, indeed, not improbable that the Romans, when they assumed the government of Britain, discouraged the inhabitants from the creation of a fleet, which might endanger their possession of the island, a fact which would naturally account for the inferiority of the Britons when matched against invaders, who, from the nature of their previous lives, were more at home on the sea than on land. [Sidenote: Accession of Offa, A.D. 755.] One only of the kings of the Heptarchy, Offa of Mercia,[470] exerted himself to resuscitate the declining trade and commerce of Britain. Offa, having in view the raising of a naval force to defend his dominions, encouraged his subjects to fit out ships, and to carry goods in their own bottoms instead of trusting for transport to so large an extent as they had hitherto done, to the vessels of foreign nations. But his praiseworthy efforts were only transient in their character. Some of the English traders who resorted to the continent (including Rome and Venice), in order to evade payment of the customs exacted from them in their transit through France, pretended to be pilgrims on their journey to the imperial city, whose baggage was exempted from duties. This attempt to evade the duties was discovered, and reported to Charlemagne by his collectors of customs; the payment of the duties was enforced, and the goods of the English merchants, consisting chiefly of works in gold and silver, for which English workmen were then famous, and which were in great demand in Italy,[471] were seized and confiscated until the pleasure of the emperor was known. The merchants appealed to Offa for redress, who, by way of retaliation, laid an embargo upon the French shipping frequenting his ports. Thus differences arose between Charlemagne and Offa which continued for some years, and thus the improvement in commerce which, since the time of the Roman dominion in Britain, had been well-nigh extinguished, was nipped almost in its bud. These differences, combined with the incessant wars then waged between the Anglo-Saxons and their remote ancestors, the Danes and Norwegians or Northmen, for the supremacy of the Northern Ocean, prevented for the time all hope of any permanent improvement in the maritime commerce of Britain. [Sidenote: Restrictions on trade and commerce.]