Curiously interesting is the hypothesis concerning the realistic origin of the Tylwyth Teg, which was put forth at the close of the last century by several writers, among them the Rev. Peter Roberts, author of the 'Collectanea Cambrica.' This hypothesis precisely accounts for the fairies anciently as being the Druids, in hiding from their enemies, or if not they, other persons who had such cause for living concealed in subterraneous places, and venturing forth only at night. 'Some conquered aborigines,' thought Dr. Guthrie; while Mr. Roberts fancied that as the Irish had frequently landed hostilely in Wales, 'it was very possible that some small bodies of that nation left behind, or unable to return, and fearing discovery, had hid themselves in caverns during the day, and sent their children out at night, fantastically dressed, for food and exercise, and thus secured themselves.' But there were objections to this presumption, and the Druidical theory was the favourite one. Says Mr. Roberts: 'The fairy customs appeared evidently too systematic, and too general, to be those of an accidental party reduced to distress. They are those of a consistent and regular policy instituted to prevent discovery, and to inspire fear of their power, and a high opinion of their beneficence. Accordingly tradition notes, that to attempt to discover them was to incur certain destruction. "They are fairies," says Falstaff: "he that looks on them shall die." They were not to be impeded in ingress or egress; a bowl of milk was to be left for them at night on the hearth; and, in return, they left a small present in money when they departed, if the house was kept clean; if not, they inflicted some punishment on the negligent, which, as it was death to look on them, they were obliged to suffer, and no doubt but many unlucky tricks were played on such occasions. Their general dress was green, that they might be the better concealed; and, as their children might have betrayed their haunts, they seem to have been suffered to go out only in the night time, and to have been entertained by dances on moonlight nights. These dances, like those round the Maypole, have been said to be performed round a tree; and on an elevated spot, mostly a tumulus, beneath which was probably their habitation, or its entrance. The older persons, probably, mixed as much as they dared with the world; and, if they happened to be at any time recognised, the certainty of their vengeance was their safety. If by any chance their society was thinned, they appear to have stolen children, and changed feeble for strong infants. The stolen children, if beyond infancy, being brought into their subterraneous dwellings, seem to have had a soporific given them, and to have been carried to a distant part of the country; and, being there allowed to go out merely by night, mistook the night for the day, and probably were not undeceived until it could be done securely. The regularity and generality of this system shows that there was a body of people existing in the kingdom distinct from its known inhabitants, and either confederated, or obliged to live or meet mysteriously; and their rites, particularly that of dancing round a tree, probably an oak, as Herne's, etc., as well as their character for truth and probity, refer them to a Druidic origin. If this was the case, it is easy to conceive, as indeed history shows, that, as the Druids were persecuted by the Romans and Christians, they used these means to preserve themselves and their families, and whilst the country was thinly peopled, and thickly wooded, did so successfully; and, perhaps, to a much later period than is imagined: till the increase of population made it impossible. As the Druidical was one of the most ancient religions, so it must have been one of the first persecuted, and forced to form a regular plan of security, which their dwelling in caves may have suggested, and necessity improved.'