1. Similar characteristics in two animals that are a product of descent from a common ancestor rather than a product of a similar environment.
2. The correspondence of a part or organ of one animal with a similar part or organ of another one, determined by agreement in derivation and development from a like primitive origin, as the foreleg of a quadruped, the wing of a bird, and the pectoral fin of a fish.
3. In chemistry, a similarity in compounds having the same fundamental structure but differing in constituents by a regular succession of changes; such as, the alcohols.