But it seems possible to go farther than this. Of human sacrifices offered on these occasions the most unequivocal traces, as we have seen, are those which, about a hundred years ago, still lingered at the Beltane fires in the Highlands of Scotland, that is, among a Celtic people who, situated in a remote corner of Europe, enjoying practical independence, and almost completely isolated from foreign influence, had till then conserved their old heathenism better than any other people in the West of Europe. It is significant, therefore, that human sacrifices by fire are known, on unquestionable evidence, to have been systematically practised by the Celts. The earliest description of these sacrifices is by Julius Caesar. As conqueror of the hitherto independent Celts of Gaul, Caesar had ample opportunity of observing the national Celtic religion and manners, while these were still fresh and crisp from the native mint and had not yet been fused in the melting-pot of Roman civilisation. With his own notes Caesar appears to have incorporated the observations of a Greek explorer, by name Posidonius, who travelled in Gaul about fifty years before Caesar carried the Roman arms to the English Channel. The Greek geographer Strabo and the historian Diodorus seem also to have derived their descriptions of the Celtic sacrifices from the work of Posidonius, but independently of each other and of Caesar, for each of the three derivative accounts contains some details which are not to be found in either of the others. By combining them, therefore, we can restore the original account of Posidonius with some certainty, and thus obtain a picture of the sacrifices offered by the Celts of Gaul at the close of the second century B.C.(748) The following seem to have been the main outlines of the custom. Condemned criminals were reserved by the Celts in order to be sacrificed to the gods at a great festival which took place once in every five years. The more there were of such victims, the greater was believed to be the fertility of the land.(749) When there were not enough criminals to furnish victims, captives taken in war were sacrificed to supply the deficiency. When the time came the victims were sacrificed by the Druids or priests. Some were shot down with arrows, some were impaled, and some were burned alive in the following manner. Colossal images of wicker-work or of wood and grass were constructed; these were filled with live men, cattle, and animals of other kinds; fire was then applied to the images, and they were burned with their living contents.