A trustee cannot delegate his office or any of his duties either to a co-trustee or to a stranger, unless ( a) the instrument of trust so provides, or (b) the delegation is in the regular course of business, or (c) the delegation is necessary, or ( d) the beneficiary, being competent to contract, consents to the delegation.

   Explanation.—The appointment of an attorney or proxy to do an act merely ministerial and involving no independent discretion is not a delegation within the meaning of this section.

Illustration.

   (a)  A bequeaths certain property to B and C on certain trusts to be executed by them or the survivor of them or the assigns of such survivor. B dies. C may bequeath the trust -property to D and E upon the trusts of A’s will.

   (b)  A is a trustee of certain property with power to sell the same. A may employ an auctioneer to effect the sale.

   (c)  A bequeaths to B fifty houses let at monthly rents in trust to collect the rents and pay them to C. B may employ a proper person to collect these rents.