“Genius is not a possession of the limited few, but exists in some degree in everyone. Where there is natural growth, a full and free play of faculties, genius will manifest itself.”

As quoted by Martha Graham, in Dance Observer, Volumes 24-27 (1957), p. 5

January 5, 1856
Journals (1838-1859)

Three Times Three (1936)

Source: 1890s, The Principles of Psychology (1890), Ch. 19
Source: The Writings of William James

True genius without heart is a thing of nought - for not great understanding alone, not intelligence alone, nor both together, make genius. Love! Love! Love! that is the soul of genius. - Nikolaus Joseph von Jacquin, entry in Mozart's souvenir album (1787-04-11) from Mozart: A Life by Maynard Solomon [Harper-Collins, 1966, ISBN 0-060-92692-9], p. 312.
Misattributed

Le génie n'est que l'enfance retrouvée à volonté, l'enfance douée maintenant, pour s'exprimer, d'organes virils et de l'esprit analytique qui lui permet d'ordonner la somme de matériaux involontairement amassée.
III: "L'artiste, homme du monde, homme des foules et enfant" http://fr.wikisource.org/wiki/L%E2%80%99Artiste%2C_homme_du_monde%2C_homme_des_foules_et_enfant
Le peintre de la vie moderne (1863)

Réflexions sur les passions et sur les goûts (1741).
Translation reported in Harbottle's Dictionary of quotations French and Italian (1904), p. 131.