What is the test for granting custody of a child to a third party?

It is now settled that the custody of a child may not be given to a third person in the absence of a finding that the parent seeking such custody is unfit. Stewart v. Stewart, 41 Cal.2d 447, 260 P.2d 44; In re Guardianship of Smith, 42 Cal.2d 91, 265 P.2d 888, 37 A.L.R.2d 867. It is equally well settled that an appellate court is required to consider only the evidence most strongly favoring the respondent and may not reverse a judgment unless there is no substantial evidence which, with the inferences reasonably to be drawn

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