Homology, Analogy and Plasis

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Abstract
The thesis is that functional considerations are inherently involved in structural inquiries, and there can be no final "morphology" without physiology. The origin of the present profound separation between animal physiology and morphology is traced to the institution for classificatory purposes of the distinction between homology and analogy. When biologists decided to discount analogies (defined in terms of function) and to concentrate upon homologies (defined solely in terms of structure), physiology appeared to be excluded from participation in problems of structure. It is to be noted, however, that physiology has 2 aspects and the word "function" 2 entirely distinct meanings, the older being the use or application of a part, the more modern being its essential (physico-chemical or causal) mechanism. Analogy was defined in terms of the older meaning of the word function. Nor does anyone seem to have inquired whether functional similarity in the modern sense coincides with homology. During the Darwinian period the homology conception acquired a historical in addition to its positional connotation. Lankester analyzed this new "homology" into homog-eny and homoplasy. Spemann, criticizing the term from the standpoint of experimental embryology, finds no place for it in the causal sphere of investigation. Nevertheless, when the matter is critically examined in the light of examples, it transpires that one of the sharpest tests of homology (in Lankester''s sense of homogeny) is identity in respect of intrinsic functional mechanism. In the study of evolution, too, the origin of modifications in the intrinsic functional mechanism of parts is an affair of fundamental importance. The analogies, or cases of convergence, offer fertile though as yet unexploited material for functional study of structure. The word plasis, adopted from Lankester, implies the causal component in adaptation. Adaptations are either simple application of existing structures to new uses, the relevant parts remaining homologous throughout; or new inventions with novel intrinsic functional mechanism, the parts occasionally exhibiting analogy but never homology with other structures.

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