Titus Livius (Livy), The History of Rome, Book 5
Rev. Canon Roberts, Ed.

34. About the passage of the Gauls into Italy we have received the following account. Whilst Tarquinius Priscus was king of Rome, the supreme power amongst the Celts, who formed a third part of the whole of Gaul, was in the hands of the Bituriges; they used to furnish the king for the whole Celtic race. [2] Ambigatus was king at that time, a man eminent for his own personal courage and prosperity as much as for those of his dominions. During his sway the harvests were so abundant and the population increased so rapidly in Gaul that the government of such vast numbers seemed almost impossible. [3] He was now an old man, and anxious to relieve his realm from the burden of over-population. With this view he signified his intention of sending his sister's sons Bellovesus and Segovesus, both enterprising young men, to settle in whatever locality the gods should by augury assign to them. [4] They were to invite as many as wished to accompany them, sufficient to prevent any nation from repelling their approach. When the auspices were taken, the Hercynian forest was assigned to Segovesus; to Bellovesus the gods gave the far pleasanter way into Italy. [5] He invited the surplus population of six tribes —the Bituriges, the Averni, the Senones, the Aedui, the Ambarri, the Carnutes, and the Aulerci. [6] Starting with an enormous force of horse and foot, he came to the Tricastini.

Beyond stretched the barrier of the Alps, and I am not at all surprised that they appeared insurmountable, for they had never yet been surmounted by any route, as far at least as unbroken memory reaches, unless you choose to believe the fables about Hercules. [7] Whilst the mountain heights kept the Gauls fenced in as it were there, and they were looking everywhere to see by what path they could cross the peaks which reached to heaven and so enter a new world, they were also prevented from advancing by a sense of religious obligation, for news came that some strangers in quest of territory were being attacked by the Salyi. [8] These were Massilians who had sailed from Phocaea. The Gauls, looking upon this as an omen of their own fortunes, went to their assistance and enabled them to fortify the spot where they had first landed, without any interference from the Salyi. [9] After crossing the Alps by the passes of the Taurini and the valley of the Douro, they defeated the Tuscans in battle not far from the Ticinus, and when they learnt that the country in which they had settled belonged to the Insubres, a name also borne by a canton of the Haedui, they accepted the omen of the place and built a city which they called Mediolanum.