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230 EVOLUTION or MAMMALIAN MOLAR TEETH 



development are much more general and important modes of evolution 

 than is commonly supposed. By parallelism is meant the independent 

 acquisition of similar structure in forms [i.e., animals] which are 

 them selves nearly related, and by convergence such acquisition in 

 forms [i.e., animals] which are not closely related, and thus in one or 

 more respects come to be more nearly alike than were their 

 ancestors." 



The term " homoplasy " (Lankester) has been long used by the 

 writer and others in a somewhat similar sense, but it is not equivalent 

 either to " parallelism " or " convergence." As will be seen below, 

 the fundamental idea is different, because homoplasy always involves 

 homology, while parallelism and convergence may or may not involve 

 homology. 



ANALOGY IN EVOLUTION. 

 Analogous Variation (Darwin). Similar congenital variation in more or less 



distantly related animals and plants. 

 Parallelism. Independent similar development of related animals, plants, 



and organs. 

 Convergence. Independent similar development of unrelated animals, bringing 



them apparently closer together. 



Homoplasy (Lankester) (1 Homomorphy, Fiirbringer). Independent similar 

 development of homologous organs or regions giving rise to similar 

 new parts. 



In brief, analogy embraces similar changes due to similar adaptation 

 in function both in homologous and in non-homologous organs, both in 

 related and in unrelated animals. 



THE LIMITED SIGNIFICANCE OF HOMOLOGY. 



Owen ('43, p. 379), Lankester ('70), and Fiirbringer have especially 

 defined and elaborated the very ancient conception of homology, as 

 employed by Oken, Geoffroy St. Hilaire, and Vicq d'Azyr : homology 

 (Owen, '43), "the same organ in different animals under every variety 

 of form and function"; homogeny (Lankester, '70): "Structures which 

 are genetically related, in so far as they have a single representative 

 in a common ancestor, may be called homogenous." E. B. Wilson ('95, 

 pp. 101-124) has shown that the comparative anatomical test of 

 homology is more reliable than the embryological. Gegenbaur ('98, 

 pp. 23-25) has given a full presentation of the distinctions as the 

 basis of comparative anatomy; in his recent great work ('98, p. 23) 



groups. As employed by Scott in his essay "On the Mode of Evolution in the Mam- 

 malia" ('91, pp. 363-867), "parallelism" is used in a very broad sense as affecting the 

 skeleton and teeth, on the principle "that identical modifications of structure, con- 

 stituting evolution of types, have supervened on distinct lines of descent." as embracing 

 not only single characters but whole series of them. 



