Before Darwin, homology was defined morphologically and explained by reference to ideal archetypes -- that is, to intelligent design. Darwin reformulated biology in naturalistic* rather than teleological terms, and explained homology as the result of descent with modification from a common ancestor. Descent with modification, however, renders design unnecessary only if it is due entirely tonaturalistic mechanisms. Two such mechanisms have been proposed, genetic programs and developmental pathways, but neither one fits the evidence. Without an empirically demonstrated naturalistic mechanism to account for homology, design remains a possibility which can only be excluded on the basis of questionable philosophical assumptions.
* In this article, "naturalism" and "naturalistic" refer to the philosophical doctrine that nature is the whole of reality, and that intelligent causation does not qualify as a scientific explanation. Read More ›