No one knows how many invasions were broken up by the forests and fens of England before the Romans came with their colonising methods, and linked their scattered camps together by means of paved highways, great roads destined to be used for many centuries, and by many raids and armies. The earliest prehistoric tribes came along a bridge of land by which England was united to France; they found in their course some of the nature-made bridges (p. 114), and the spoor and tracks of formidable animals, such as the mastodon and the mammoth. Much later invasions, also prehistoric, must have come over the sea in boats, for the bridge of land had the history of most bridges, the water swallowed it up; but every boat may be regarded as a floating bridge which is moved from place to place, so that a pontist when he studies the sea-borne invasions keeps in touch with his favourite subject. On their arrival in England the later prehistoric colonists found that most of the nature-made bridges had been copied, and that a great many footpaths and tracks rambled from settlements to watering-places and through the forests where huntsmen risked their lives in a sport of habit. The men of the Bronze Period were supplanted in Europe by a race more powerful, whose clenched fists needed larger sword-handles; it was a race of manly and swaggering nomads, strong and fierce; and yet, as Darwin believed, their success in the war of life may have been aided still more by their superiority in the arts. Can we fix a date for the introduction of bronze into the British Isles? Here is a matter of opinion; but, according to Sir John Evans, the most likely date is separated from the Christian era by about 1400 years, perhaps 200 years less. Iron belongs to a much later time. Probably, in the fourth century B.C., it was known as a metal in South Britain; and about a century later it began to supersede bronze in the manufacture of cutting implements.[4]