California Civil Code
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 sub-agent 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.
If an agent employs a sub-agent without authority, the former is a principal and the latter his agent, and the principal of the former has no connection with the latter.
A sub-agent, lawfully appointed, represents the principal in like manner with the original agent; and the original agent is not responsible to third persons for the acts of the sub-agent.