Concepts of Homology and Analogy

Boyden urges that we return to Owen's original, century-old definitions of homology and analogy-analogy to signify a similarity of structures in use, without regard to fundamental agreement in structure or evolution; that is, without regard to homology; and homology to indicate a basic agreement in structure and relative position, without regard to functional similitude. But the concept of homology has evolved through the century, particularly under the influence of the doctrine of evolution. Homology has come to signify an agreement in evolutionary derivation and in embryonic development. The traditional and persistent limitation of the homology concept to structures has overemphasized morphology and is inconsistent with the modern view that structures and functions are inseparably associated. It is desirable that the term homology be regarded as applicable also to intrinsic functional mechanisms, reflexes, behavior patterns and other functional attributes, when they indicate a common heritage. Attempts to dissociate "serial homologies" from "true or special homologies," and various proposals to break down homology into a series of relations each with a separate name, have largely been abortive and are not regarded as wise. A consideration of homologies between diffuse elements helps to clarify the general picture. Endless inconsistencies would arise from the application of suggested restrictions of the homology concept. Homologous relationships are very complex. There are many degrees of specificity in homology. Two characters may be homologous from one point of view but not so from another. Where to draw the line is a function of the special needs in any comparative study. Many complexities in homological relations come to mind when we review the facts of anatomy, variation, genetics and experimental embryology, and the uncertainties may be accentuated in studying closely related kinds of animals. Such irregularities encumber almost all biological concepts, but destroy neither their validity nor their usefulness. What may be called independent homologies-more or less identical but independent modifications of homologous structures or functions, agreeing with typical homologies in all respects other than time of origin-provide a troublesome transition between homologies and analogies. There are genetic discontinuities of various kinds and degrees in the lineage of characters. A modern concept of analogy is that of the agreement in use of characters (structures or functions), which have had a distinct evolutionary origin. There seems to be no special need for a return to a century-old concept of either analogy or homology. Both concepts-of homology and of analogy-should be recognized as involving grave complexities. These ideas involve the same sort of inexactitude that is inherent in such principles as those expressed in Haeckel's biogenetic law and Dollo's law of irreversible evolution. Precision in any of these concepts should not be allowed to transcend the complexities that exist. Despite these difficulties, the concepts of homology and analogy retain, in their evolved, modern meaning, a place of importance and value in biology.