Like all Celtic tribes in all ages, the Britons and the Gael showed small capacity for union. They dwelt apart in many separate tribes, though sometimes a great and warlike chief would compel one or two of his neighbours to do him homage. But such kingdoms usually fell to pieces at the death of the warrior who had built them up. After the kings and chiefs, the most important class among the Celts was that of the Druids, a caste of priests and soothsayers, who possessed great influence over the people. They it was who kept up the barbarous sacrifices which we have already mentioned. Although tribal wars were incessant, yet the Britons had learnt some of the arts of peace, and traded with each other and with the Celts across the Channel. For the tin of Cornwall it would seem that they made barter with the adventurous traders who pushed their way across Gaul from the distant Mediterranean to buy that metal, which was very rare in the ancient world. The Britons used money of gold and of tin, on which they stamped a barbarous copy of the devices on the coins of Philip, the great King of Macedonia, whose gold pieces found their way in the course of trade even to the shores of the Channel. The fact that they had discovered the advantages of a coinage proves sufficiently that they were no longer mere savages. [Sidenote: =Invasion of Julius Cæsar.=] We have no materials for constructing a history of the ancient Celtic inhabitants of Britain till the middle of the first century before Christ, when the great Roman conqueror, Julius Cæsar, who had just subdued northern Gaul, determined to cross the straits and invade Britain. He wished to strike terror into its inhabitants, for the tribes south of the Thames were closely connected with their kinsmen on the other side of the Channel, and he suspected them of stirring up trouble among the Gauls. Cæsar took over two legions and disembarked near Romney (B.C. 55). The natives thronged down to the shore to oppose him, but his veterans plunged into the shallows, fought their way to land, and beat the Britons back into the interior. He found, however, that the land would not be an easy conquest, for all the tribes of the south turned out in arms against him. Therefore he took his legions back to Gaul as the autumn drew on, vowing to return in the next year. In B.C. 54 he brought over an army twice as large as his first expedition, and boldly pushed into the interior. Cassivelaunus, the greatest chief of eastern Britain, roused a confederacy of tribes against him; but Cæsar forced the passage of the Thames, and burnt the great stockaded village in the woods beyond that river, where his enemy dwelt. Many of the neighbouring princes then did him homage; but troubles in Gaul called him home again, and he left the island, taking with him naught save a few hostages and a vague promise of tribute and submission from the kings of Kent. [Sidenote: =Commerce with Europe.=]