The well-known cases of mathematical, musical, and artistic prodigies and somnambulists with prophetic gifts, who nevertheless appear to be perfectly imbecile apart from their special talents, are interesting examples of the transition from madness to genius. The solving of equations of the fourth and fifth degree or mental calculations involving the multiplication or division of a large number of figures, are difficult operations for normal persons; yet individuals barely able to read and write, and often afflicted with insanity or imbecility, have been known to possess marvellous mathematical faculties. Imualdi was a cretin, and Dase, Juller, Buxton, Mondeur, and Prolongeau, men of feeble intellect. Among the inmates of asylums, we may find cretins and idiots that are able to play on a whistle any melody they have heard. The drawings of cats, executed by a Norwegian cretin, have been deemed worthy of a place among the treasures of art-galleries and museums. Such cases prove that the possession of one highly developed faculty does not imply a corresponding development of all the intellectual powers. Unintelligent, unbalanced, or even mentally deficient women, when in a somnambulistic or hypnotic state, are able to predict future events, an impossible feat for normal persons, or to discover the whereabouts of objects hidden at a distance, a marvellous phenomenon, which can be explained only by presuming the existence of a far-seeing vision, and the working of a powerful synthetic process resembling the inspirations of genius. Although not a difficult task to prove the existence of traits of genius in mentally diseased persons, the bringing to light of instances of insanity in men of genius was a much simpler matter. These instances, carefully classified, form the longest and most important part of _The Man of Genius_, but it is not necessary to give space to any of these instances here. The proofs of the connection between genius and insanity were supplemented by data supplied by the physical examination of a number of geniuses, compared with insane subjects, and a careful investigation of the ethnical, social, and geographical causes which influence the formation of both types. All the facts elicited demonstrated their complete analogy. But my father's studies did not stop short at the discovery of this analogy, or that of the sources whence the diverse varieties of genius spring, which is perhaps the most interesting part of the book, or even at the application of the new doctrines for the purpose of clearing up obscure points in history and shedding light on the lives of great men. He pursued his investigations until he found the keystone of the edifice reared by insanity and genius--epilepsy.