Serially Homologous Research Articles

The milk‐molars of Perissodactyla, with remarks on molar occlusion.

SUMMARY. A survey is made of the milk‐molars of Perissodactyla and Condylarthra with a view to determining the evolutionary trends. Molarization of the milk‐molars takes place in a variety of ways, which may be grouped into three main types: (a) the condylarthran type, found also in Plagiolophus, (b) the type found in the Equidae and Brontotheriidae, and (c) that found in the Tapiroidea and Rhinocerotoidea. Each of these types has a characteristic order of development of the cusps. The condylarthran type is considered to be the most primitive. From a study of the wear of the teeth, it is concluded that the occlusal movement in all Perissodactyla is transverse (ectal). The characteristic features of occlusion in the various families are discussed. During molarization the talonid becomes intercalated between two trigonids, the intercalation taking place at a relatively late stage in type (a), earliest in type (b) and at an intermediate stage in type (c). The buccal parts of the trigonid and talonid become functional before the lingual parts. The Palaeotheriidae (excluding Pachynolophus and Propachynolophus) are regarded as a distinct family that retains condylarth features of the milk dentition to a late date, and cannot be derived from Hyracotherium. With the exception of Plagiolophus, they show in their milk‐molars transitional stages leading to the conditions found in other perissodactyls. The hypothesis is put forward that the order of development of the cusps depends upon their different sensitivities to the graded morphogenetic conditions in the developing jaw. Correlated evolution of the upper and lower teeth suggests that occlusion has a morphogenetic basis which affects both jaws. The concepts of homology due to descent from a common ancestral structure, homology due to parallel evolution, and serial homology are considered to be closely related and not sharply separable in practice.

The Relationship between Serial and Special Homology and Organic Similarities

The Relationship between Serial and Special Homology and Organic Similarities

Homology and Analogy: A Century After the Definitions of "Homologue" and "Analogue" of Richard Owen

" NALOGUE. A part or organ in one animal which has the same function as another part or organ in a different animal." ~~~"Homologue. The same organ in different animals under every variety of form and function." These definitions are to be found in the glossary to Owen's Lectures on the Comparative Anatomy and Physiology of the Invertebrate Animals delivered at the Royal College of Surgeons and published in 1843. In spite of the fact that these terms form an essential part of the foundation of comparative zoology there is still much confusion in their usage. However complex the morphological and physiological characteristics of animals may be there is no excuse for a lack of precise descriptions and clear-cut comparisons of them. It is time that the implications and relations of both terms be critically examined so that we can decide upon, and adopt, their most effective meanings. Owen wrote (1866, Vol. I, p. xiii), "Terms are the tools of the teacher; and only an inferior hand persists in toiling with a clumsy instrument when a better one lies within his reach." There could be no more appropriate celebration of the centenary of Owen's definitions than the wise adoption of these terms as he defined them, for such action would inevitably lead us to a clearer and more effective analysis of the data of comparative zoology. The need for reexamination of the implications of homology and analogy can be quickly shown. In the first place, the term homology was broadened by Owen and others beyond the meaning published in 1843 so that it came to refer not only to "the same organ in different animals..." (Owen's special homology (1846, 1848)) but to the essential similarities of organs along the chief axis of the body known as serial homology (Owen's homotypy (1846, 1848)) and even to the most general structural resemblances of organisms to their archetype (Owen's general homology (1846, 1848)). These extensions in meaning have led to a deplorably loose and relatively meaningless usage of the term. Thus today many texts in zoology use the term indiscriminately to refer to special or serial homology, without realizing that these two kinds of structural resemblance have an almost totally unlike significance to the zoologist. Obviously serial homologies have a very different bearing on the problems of taxonomy and genetic relationships than do special homologies for the former are intra-individual structural expressions, and no question of genetic relationship of individual organisms is involved. On the other hand, special homology most commonly signifies genetic relationship. Gegenbaur's words well express the common view, "Blood relationship becomes dubious exactly as the proof of homologies is uncertain" (1878, p. 63). In the second place, the term analogy has had, and still has, widely differing usages. Thus, in brief, analogy has meant to zoologists (1) a similarity in function of organs whether homologous or not (Owen, Lankester, Gegenbaur, Haeckel), (2) a similarity in use of organs which are nonhomologous (MacLeay, Darwin, and most modern texts), and (3) an essential similarity in structure (Geofroy Saint-Hilaire, Cuvier, Dumeril, and other French 19th century zoologists). But if the meanings of analogy have been so various, what of the relations of the terms homology and analogy to each other? Here the human instinct for bungling terms has reached its fullest expression! It has been so bad that even the same writer has contradicted himself in a single paragraph, as shown, for example, in the following quotation from Sir William Turner (Encyclopedia Britannica, 9th ed., 1899. New American Supplement, VoL I, p. 830, article Anatomy).