in his Natural History considered the word to contain the Greek noun drýs (δρύς), "oak-tree" and the Greek suffix -idēs (-ιδης). Both Old Irish druí and Middle Welsh dryw could refer to the wren, possibly connected with an association of that bird with augury in Irish and Welsh tradition (see also Wren Day). Practices and doctrines Sources by ancient and medieval writers provide an idea of the religious duties and social roles involved in being a druid. Societal role and training The Greco-Roman and the vernacular Irish sources agree that the druids played an important part in pagan Celtic society. In his description, Julius Caesar wrote that they were one of the two most important social groups in the region (alongside the equites, or nobles) and were responsible for organizing worship and sacrifices, divination, and judicial procedure in Gallic, British, and Irish societies. He wrote that they were exempt from military service and from paying taxes, and had the power to excommunicate people from religious festivals, making them social outcasts. Two other classical writers, Diodorus Siculus and Strabo, wrote about the role of druids in Gallic society, stating that the druids were held in such respect that if they intervened between two armies they could stop the battle. Pomponius Mela was the first author to say that the druids' instruction was secret and took place in caves and forests. Druidic lore consisted of a large number of verses learned by heart, and Caesar remarked that it could take up to twenty years to complete the course of study. What was taught to druid novices anywhere is conjecture: of the druids' oral literature, not one certifiably ancient verse is known to have survived, even in translation. All instruction was communicated orally, but for ordinary purposes, Caesar reports, the Gauls had a written language in which they used Greek letters. In this he probably draws on earlier writers; by the time of Caesar, Gaulish inscriptions had moved from Greek script to Latin script. Sacrifice Greek and Roman writers frequently made reference to the druids as practitioners of human sacrifice. Caesar says those who had been found guilty of theft or other criminal offences were considered preferable for use as sacrificial victims, but when criminals were in short supply, innocents would be acceptable. A form of sacrifice recorded by Caesar was the burning alive of victims in a large wooden effigy, now often