The _order_ in which the metals are discovered, the leading problem in what may be called the natural history of metallurgy, is far more dependant upon induction. Induction, however, has given the priority to copper, just as is expected from the comparative reducibility of its ores--lead and gold being put out of the question. So that it is not so much the general fact of the order of succession in respect to the Stone, Copper, and Iron periods that the laudable investigations of British archæologists have established as the nature of the concomitant details, the modifications of the periods themselves, and the exact character of their sequence. Under each of these heads there is much worth notice. The difference between the shape and size of the skulls of the Stone and Bronze periods has been broadly asserted--perhaps it has been exaggerated, at any rate it has formed the basis of an hypothesis. The substitution of a Bronze for a Copper period in Britain is an important modification, mainly attributable to the existence of tin. The comparative completeness of the sequence is interesting. It by no means follows that it should be regular. In Norway there is no Bronze period at all; but Bone and Stone in the first instance, and Iron immediately afterwards. FOOTNOTES: [1] _See_ Wilson's "Pre-historic Annals of Scotland." [2] This is well worked out by Mr. Wilson, in his "Pre-historic Annals of Scotland."--Pp. 238 &c. CHAPTER II. AUTHORITIES FOR THE EARLIEST HISTORICAL PERIOD.--HERODOTUS.-- ARISTOTLE.--POLYBIUS.--ONOMACRITUS.--DIODORUS SICULUS.--STRABO.-- FESTUS AVIENUS.--ULTIMATE SOURCES.--DAMNONII.--PHŒNICIAN TRADE.-- THE ORGIES.--SOUTH-EASTERN BRITONS OF CÆSAR.--THE DETAILS OF HIS ATTACKS.--THE CALEDONIANS OF GALGACUS. The _extant_ writers anterior to the time of Julius Cæsar, in whose works notice of the British islands are to be found, are, at most, but four in number. They are all, of course, Greek. Herodotus is the earliest. He writes "of the extremities of Europe towards the west, I cannot speak with certainty ... nor am I acquainted with the islands Cassiterides, from which tin is brought to us."[3]--iii. 115. Aristotle is the second. "Beyond the Pillars of Hercules," he tells us, "the ocean flows round the earth; in this ocean, however, are two islands, and those very large, called Britannic, Albion and Ierne, which are larger than those beforementioned, and lie beyond the Celti; and other two not less than these, Taprobane, beyond the Indians, lying obliquely in respect of the main land, and that called Phebol, situate over against the Arabic Gulf; moreover not a few small islands, around the Britannic Isles and Iberia, encircle as with a diadem this earth; which we have already said to be an island."--_De Mundo_, §. 3.