Fairies being creatures of the imagination, it is not possible to classify them by fixed and immutable rules. In the exact sciences, there are laws which never vary, or if they vary, their very eccentricity is governed by precise rules. Even in the largest sense, comparative mythology must demean itself modestly in order to be tolerated in the severe company of the sciences. In presenting his subjects, therefore, the writer in this field can only govern himself by the purpose of orderly arrangement. To secure the maximum of system, for the sake of the student who employs the work for reference and comparison, with the minimum of dullness, for the sake of the general reader, is perhaps the limit of a reasonable ambition. Keightley[8] divides into four classes the Scandinavian elements of popular belief as to fairies, viz.: 1. The Elves; 2. The Dwarfs, or Trolls; 3. The Nisses; and 4. The Necks, Mermen, and Mermaids. How entirely arbitrary this division is, the student of Scandinavian folk-lore at once perceives. Yet it is perhaps as satisfactory as another. The fairies of Wales may be divided into five classes, if analogy be not too sharply insisted on. Thus we have, 1. The Ellyllon, or elves; 2. The Coblynau, or mine fairies; 3. The Bwbachod, or household fairies; 4. The Gwragedd Annwn, or fairies of the lakes and streams; and 5. The Gwyllion, or mountain fairies.