As regards the arts and traffic in the towns, they were chiefly carried on by guilds, which had their origin, as Dr. Brentano has shown with great probability, in separate families, who combined to keep up their own trade secrets as a family affair. In time, however, the guilds grew into regular organisations, having their own code of rules and laws, many of which (as at Cambridge, Exeter, and Abbotsbury) we still possess. It is possible that the families of craftsmen may at first have been Romanised Welsh inhabitants of the cities; for all the older towns–London, Canterbury, York, Lincoln, and Rochester–were almost certainly inhabited without interruption from the Roman period onward. But in any case the guilds seem to have grown out of family compacts, and to have retained always the character of close corporations. There must have been considerable division of the various trades even before the Conquest, and each trade must have inhabited a separate quarter; for we find at Winchester, or elsewhere, in the reign of Æthelred, Fellmonger, Horsemonger, Fleshmonger, Shieldwright, Shoewright, Turner, and Salter Streets. The exact amount of the population of England cannot be ascertained, even approximately; but we may obtain a rough approximation from the estimates based upon Domesday Book. It seems probable that at the end of the Conqueror's reign, England contained 1,800,000 souls. Allowing for the large number of persons introduced at the Conquest, and for the natural increase during the unusual peace in the reigns of Cnut, of Eadward the Confessor, and, above all, of William himself, we may guess that it could not have contained more than a million and a quarter in the days of Eadgar. London may have had a population of some 10,000; Winchester and York of 5,000 each; certainly that of York at the date of Domesday could not have exceeded 7,000 persons, and we know that it contained 1,800 houses in the time of Eadward the Confessor.