in biology, similarity of function and superficial resemblance of structures that have different origins. For example, the wings (wing) of a fly, a moth, and a bird are analogous because they developed independently as adaptations to a common function—flying. The presence of the analogous structure, in this case the wing, does not reflect evolutionary closeness among the organisms that possess it. Analogy is one aspect of evolutionary biology and is distinct from homology (q.v.), the similarity of structures as a result of similar embryonic origin and development, considered strong evidence of common descent.

In many cases analogous structures, or analogues, tend to become similar in appearance by a process termed convergence. An example is the convergence of the streamlined form in the bodies of squid, shark, seal, porpoise, penguin, and ichthyosaur, animals of diverse ancestry. Physiological processes and behaviour patterns may also exhibit analogous convergence. Egg-guarding behaviour in the cobra, the stickleback, the octopus, and the spider is thought to have evolved independently among those animals, which are quite distant in their biological relationships.

Many New World cacti and African euphorbias are similar in appearance, being succulent, spiny, water-storing, and adapted to desert conditions generally. They are classified, however, in two separate and distinct families, sharing characteristics that have evolved independently in response to similar environmental challenges.

(from Greek ana logon, “according to a ratio”), originally, a similarity in proportional relationships. It may be a similarity between two figures (e.g., triangles) that differ in scale or between two quantities, one of which, though unknown, can be calculated if its relation to the other is known to be similar to that in which two other known quantities stand. Thus, if 2:4::4:x, it can be seen that x = 8. Another form of analogy noted by the Greeks is that of inferring similarity of function, known as “educing the correlate.” Aristotle (Topics, i, 17) stated the formulas of these two kinds of analogy: “As A is to B, so C is to D”; and “As A is in B, so C is in D.”

Plato employed a functional analogy when he argued that the Idea of the Good makes knowledge possible in the intelligible world just as the Sun makes vision possible in the perceptual world. Here a relationship not yet understood is analogous to one already familiar.

In the European Middle Ages it was believed that the universe forms an ordered structure such that the macrocosmic pattern of the whole is reproduced in the microcosmic pattern of the parts so that it is possible to draw inferences by analogy from the one to the other. Thus, the law of nature conceived in the juridical sense, which prescribes the fitting order of human relationships, could be assimilated to the physical sense of law, which describes the order obtaining in the natural world. Because the natural world exhibits hierarchical degrees of subordination, it was argued, human relationships should also exhibit such subordination. Such parallels were held to constitute arguments and not merely allegorical illustrations; it was contended, for instance, that, as there were two luminaries to light the world and two authorities set over man (the papacy and the empire), then, as the Moon's light is reflected from the Sun, so the imperial authority must be derived from the papal. Dante, in his De monarchia (c. 1313), while claiming that it is light and not authority that the empire derives from the papacy, nevertheless accepted the principles on which such arguments are built.

In social and political discussion, analogies may elucidate some unfamiliar point in terms of what is more familiar. Thus, biological analogies may suggest that a community has an “organic” relationship. Such analogies are misleading, however, insofar as they overlook the fact that individual members of the community also have purposes, rights, and responsibilities of their own. In employing the method of analogy, it should always be possible to show that the resemblances noted bear relevantly on the point to be established, whereas the differences are irrelevant. In many cases it is difficult to be sure of this distinction, and arguments from analogy are therefore precarious unless supported by considerations that can be established independently.