Analogy and Homology.—In biology those organs or parts in different animals are said to be analogous which, however different their origin, have a general similarity of form and especially of function; while those are called homologous which, however different their general appearance, and however different their function, yet may, by close examination and extensive comparison, be shown to be modifications of one another—to be, in fact, originally the same part modified for different purposes. In the former the parts compared look and behave as if they were the same, but are not; in the latter they look and behave entirely differently, but are, in fact, the same part in disguise.

We can best make this plain by examples. The wing of a bird and the wing of a butterfly are analogous organs. They have the same function—i.e., flying; and this function necessitates the same general form of a flat plane. But they are not at all homologous; they are not at all the same organ or part. They certainly have never been formed one out of the other by modification. But the wing of a bird, the fore-paw of a reptile or mammal, the wing of a bat, and the arm and hand of a man, though so different in form and function, are homologous parts.