An oral authorization is sufficient for any purpose, except that an authority to enter into a contract required by law to be in writing can only be given by an instrument in writing. Ca. Civ. Code § 2309 Enacted 1872.
An estate in real property, other than an estate at will or for a term not exceeding one year, can be transferred only by operation of law, or by an instrument in writing, subscribed by the party disposing of the same, or by his agent thereunto authorized by writing. Ca. Civ. Code § 1091 Enacted 1872.
When an attorney in fact executes an instrument transferring an estate in real property, he must subscribe the name of his principal to it, and his own name as attorney in fact. Ca. Civ. Code § 1095 Enacted 1872.
No unauthorized act can be made valid, retroactively, to the prejudice of third persons, without their consent. Ca. Civ. Code § 2313 Enacted 1872.