I guess you didn't read the very chapter you speak of earlier?
"In looking for the gradations by which an organ in any species has been perfected, we ought to look exclusively to its lineal ancestors; but this is scarcely ever possible, and we are forced in each case to look to species of the same group, that is to the collateral descendants from the same original parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent, in an unaltered or little altered condition."--Chapter 6, Origin of Species, Charles Darwin
That is what we see with the evolution of the irreducibly complex mammalian middle ear. We see the collateral descendants that have preserved the ancestral position of these bones as they evolved the IC middle ear.