Regularity of Form in Organisms

Abstract

ONE of the most interesting and important problems of comparative anatomy is the multiformity and at the same time the regularity of the shapes in the structure of plants and animals. Richard Owen, the eminent British zoologist, had in the past century proposed two principles for comparing the organs and appendages of different animals. The first of these, analogy, characterizes the physiological likeness of organs which perform similar functions, and hence often assume similar shapes, as for instance, the wings of birds and of bats. The second principle, homology, is concerned with organs which, although fundamentally similar in structure, may be different both in form and in function, such as the syringe hole in whales and the nose of other vertebrates. K. Gegenbaur, in his enthusiasm about Darwin's theory of evolution, came to the conclusion that comparative anatomy should be based on the historical method, that is, should study the gradual development of the animal kingdom. Proceeding from this conclusion, he described as homologous such parts of the body as have been derived from a common ancestor and, consequently, are characterized by blood relationship, although they may differ in appearance and shape, as for instance the appendages of the various vertebrates (fishes, birds, mammals). The principle of analogy, however, Gegenbaur declared superfluous for the purposes of comparative anatomy, as having no bearing whatever on the evolution of organisms. By restricting in this way the methodology of comparative anatomy Gegenbaur at the same time also limited the material eligible for comparative study. In common with most of his followers he began to deal almost exclusively with the vertebrates in which the symptoms of homology stand out with exceptional clearness. As this limitation of the very matter and scope of comparative anatomical research seemed unjustified, I have endeavoured to show that analogy is manifest in living nature, in a variety of ways, much more frequently than had hitherto been believed, and that the parallelism observable in the structure of numerous organs in different groups of animals expresses a principle far broader than the principle of homology. The inexhaustible variety of forms apparent in the representatives of the marine fauna makes it clear that the method of limiting comparative anatomical research to the insignificant (from the morphological point of view) group of vertebrates is entirely wrong. On the other hand, if we extend our comparative study of various organs to the whole of the animal realm, we come to the conclusion that most organs exhibiting parallelism are found in forms which are phylogenetically unrelated. Parallelism of form can be observed in various systems of organs. The regularity is most clearly revealed in the shapes of shells and in the structure of organs of photoreception (Figs. 1, 2). Shells and other protective structures are found in widely divergent groups of the animal kingdom-in Protozoa, Polychaeta, Trichoptera, Lepidoptera, Pteropoda, Gastropoda, and Cephalopoda (Fig. 1). They are built from a variety of materials, such as chitin, calcium carbonate, and silica. Moreover, in some cases these substances are secreted by the entire surface of the animal, in other cases by a special mantle, that is by a portion of the body, and in still others (insects), by a few small glands opening at the anterior end of the body. Yet, in unicellular and multicellular forms alike, the protective coverings invariably take the following three typical shapes: