As a result of the fact that in women the sexual emotions tend not to develop great intensity until submitted to powerful stimulation, we find that the maximum climax of sexual emotion tends to fall somewhat later in a woman's life than in a man's. Among animals generally there appears to be frequently traceable a tendency for the sexual activities of the male to develop at a somewhat earlier age than those of the female. In the human, species we may certainly trace the same tendency. As the great physiologist, Burdach, pointed out, throughout nature, with the accomplishment of the sexual act the part of the male in the work of generation comes to an end; but that act represents only the beginning of a woman's generative activity. A youth of 20 may often display a passionate ardor in love which is very seldom indeed found in women who are under 25. It is rare for a woman, even though her sexual emotions may awaken at puberty or earlier, to experience the great passion of her life until after the age of 25 has been passed. In confirmation of this statement, which is supported by daily observation, it may be pointed out that nearly all the most passionate love-letters of women, as well as their most passionate devotions, have come from women who had passed, sometimes long passed, their first youth. When Heloise wrote to Abelard the first of the letters which have come down to us she was at least 32. Mademoiselle Aissé's relation with the Chevalier began when she was 32, and when she died, six years later, the passion of each was at its height. Mary Wollstonecraft was 34 when her love-letters to Imlay began, and her child was born in the following year. Mademoiselle de Lespinasse was 43 when she began to write her letters to M. de Guibert. In some cases the sexual impulse may not even appear until after the period of the menopause has been passed.[173] In Roman times Ovid remarked (_Ars Amatoria_, lib. ii) that a woman fails to understand the art of love until she has reached the age of 35. "A girl of 18," said Stendhal (_De l'Amour_, ch. viii), "has not the power to crystallize her emotions; she forms desires that are too limited by her lack of experience in the things of life, to be able to love with such passion as a woman of 28." "Sexual needs," said Restif de la Bretonne (_Monsieur Nicolas_, vol. xi, p. 221), "often only appears in young women when they are between 26 and 27 years of age; at least, that is what I have observed."