A certain amount of material is to be found in a number of books on zoology, and also in a few quite recent works on comparative psychology. Among works of the former class I mention especially that of Brehm, who has reported a considerable number of individual details; of books on comparative psychology, one of the most useful for our purposes is that of Groos,[6] who gives us much valuable information regarding love-games of young animals. I may also point out that in the autobiographies, biographies, memoirs, &c., of celebrated persons, we find much information regarding premature amatory sentiments. Goethe, in his _Wahrheit und Dichtung_, relates that as a boy of ten or so he fell in love with a young Frenchwoman, the sister of his friend Derones. Of Alfred de Musset, his brother and biographer, Paul Musset, records that at the early age of four he was passionately in love with a girl cousin. It is on record that Dante fell in love at the age of nine, Canova at five, and Alfieri at ten. Well known also is the story of Byron's love, at eight years of age, for Mary Duff. Moebius tells us of himself that when a boy of ten he was desperately enamoured of a young married woman. We are told of Napoleon I. that when a boy of nine he fell in love with his father's cousin, a handsome woman of thirty, then on a visit to his home, and that he caressed her in the most passionate manner. Belonging to an earlier day was Felix Platter, the celebrated Swiss physician of the sixteenth century, who tells us in his autobiography that when he was a child he loved to be kissed by a certain young married woman. In _Un Coeur Simple_, Flaubert describes the development of the love-sentiments. "For mankind there is so much love in life. At the age of four we love horses, the sun, flowers, shining weapons, uniforms; at ten we love a little girl, our playmate; at thirteen we love a buxom, full-necked woman. The first time I saw the two breasts of a woman, entirely unclothed, I almost fainted. Finally, at the age of fourteen or fifteen, we love a young girl, who is a little more to us than a sister and a little less than a mistress; and then, at sixteen, we love a woman once more, and marry her."