An agent, unless specially forbidden by his principal to do so, can delegate his powers to another person in any of the following cases, and in no others:
(1) When the act to be done is purely mechanical;
(2) When it is such as the agent cannot himself, and the subagent can, lawfully perform;
(3) When it is the usage of the place to delegate such powers; or
(4) When such delegation is specially authorized by the principal.