And just what was that "philosophical approach that Darwin undertook?"
Darwin was not a philosopher and never intended to be one. As far as I understand, he was a naturalist who proposed a strictly scientific, empiric theory.
Philosophy does not deal with scientific theories, IMHO. There are no philosophical discussions about the atomic theory of the structure of matter, or about the theory of gravity, or about the theory of electromagnetism, or about the hypothesis (or theory) of clonal selection in immunology, etc.
Philosophy deals with issues like, what exists (ontology), how do we learn about the world and about ourselves (epistemology), what is good and evil (ethics), and what is beautiful (aesthetics). Where do you want to fit the theory of biological evolution?
Best wishes,
Richard