What is the legal test used to determine fit and proper in a custody application?

The proposed adoptive parents refer me to King v. Low, supra, which they suggest supersedes D. (W.) v. P. (G.), supra, and replaces the fit and proper test with the best interest tests. That case reviews the evolution of the common law with respect to the applicable test to be applied to a mother of an illegitimate child when contesting custody. It does indeed give authority to the general principle that the parens patriae jurisdiction of the court should be exercised in the best interest of the child. In my view, it does not, however, create a jurisdiction to grant guardianship where that jurisdiction does not otherwise exist or where it is inconsistent with the statute that deals with guardianship jurisdiction. Nor in my opinion does it supersede D. (W.) v. P. (G.). King v. Low deals with an application for return of a child, not an order for guardianship. It recognizes the need to deal within the applicable legislative enactment.