Homologous Organs

HOMOLOGOUS ORGANS (from Greek homologos—agreeing, corresponding), a name for morphologically similar organs, i.e., organs of identical origin, developing from identical primordia and exhibiting similar morphological relationships. The term "homology" was introduced by the English anatomist R. Owen to distinguish it from analogy, or functional similarity. Since the establishment of the evolutionary theory, morphological similarity is explained by the blood relationship of organisms, indicating an origin from the same organ of a more or less distant common ancestor. Homologous organs can exhibit varying degrees of perfection, can possess different forms, and accordingly perform different functions. In all these changes, however, a certain common basis remains, if not in the adult animal, then in the embryo, indicating that we are dealing with the result of the transformation of one and the same organ. Relationships with other organs are preserved particularly firmly, which is therefore the most reliable criterion of homology. Homologous, for example, are the forelimbs of vertebrates, despite all the differences in their function: the walking limb of an amphibian or reptile, the wing of a bird or bat, the swimming flipper of a whale, the digging paw of a mole, and the grasping hand of a human. In all these cases, these limbs develop in a completely similar way from very similar primordia, exhibiting in general a similar plan of structure from the same bones, muscles, nerves, and vessels. The connections with other parts of the body—with the shoulder girdle and its musculature—are also identical, and the blood supply by branches of the subclavian artery and innervation by nerves of the brachial plexus are similar. The same applies to the vertebrate skull, consisting in general of the same bones, the brain, etc. In some cases, adaptation to different functions can change the structure of homologous organs so much that only careful study, especially of embryonic development, can reveal the true homology. Thus, for example, the stapes of the mammalian ear is homologous to the upper part of the hyoid arch (the "suspensorium") of fish, the incus is homologous to the posterior part of the lower jaw of other vertebrates (the "articular" bone), and the malleus to the primary upper jaw (the "quadrate" bone), which is proven without much difficulty by the study of the history of mammalian development and data from comparative anatomy and paleontology. A distinction is made between complete homology, when the compared organ exhibits, even if in a modified form, all parts in a typical relationship. Such homology is, of course, encountered more often in closely related forms: e.g., the forelimb of many monkeys and humans, the hind limb of most mammals and humans. Homology is incomplete if one of the compared organs has extra parts added from the outside or, conversely, lacks certain parts. Thus, in the forelimb of birds, two digits are missing compared, for example, to the limb of a human, while the shank of birds, conversely, is enlarged by the fusion of a series of metatarsal bones.

I. Schmalhausen.