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. Source: CivC 1877, § 1380; CL 1887, § 4003; RCivC 1903, § 1699; RC 1919, § 1281; SDC 1939, § 3.0212. See Cal Civ Code, § 2349.