The concept of Homology is fundamental to the field of comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Homology is evaluated strictly in an evolutionary context. That is, organs in two species are homologous only if the same structure was present in their last common ancestor. Organs as disparate as a bat's wing, a seal's flipper, a cat's limb and a human arm have a common underlying anatomy which was present in their last common ancestor and so therefore are homologous as forelimbs
