Comparative anatomy

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Comparative anatomy is the study of the evolution of species through similarities and differences in their anatomy. It operates through the “comparison” between the anatomical structures of the different groups of vertebrates and aims to identify and analyze the causes of their shape, their structural organization and their adaptations.

It collects, sorts and interprets the descriptive materials relating to the greatest number of animal-type forms, applying the comparative method to the investigation in order to trace the causes of animal organization, to discover through which processes an immense variety of forms have been created in the scope of the same type, to establish the possible relations of kinship between these forms, etc.

Forerunner of comparative anatomy is Aristotle, who researched the laws of animal organization, drawing inspiration as much from the metaphysical concept of form as an idea of the thing and organism, as well as the concept of purpose. The principle of compensation or correlation of organs, the observations on the homology of various organs in animals of different classes and the concept of the organism as a morphological and physiological unit date back to Aristotle.

After Aristotle, up to the anatomists of the 16th century, the comparative study of animals had above all the subsidiary purpose of knowing the structures and functions of the human body. Morphology and anatomy works, including microscopic ones, developed in the 17th and 18th centuries, becoming the analytical and descriptive material for the subsequent great morphological synthesis, carried out first by G.-L. Buffon and W. Goethe; Buffon, albeit confusedly, anticipated the concepts of unity of the plan of organization of the animal world and of mutability of the species; Goethe, to whom we owe the term ‘morphology’, stated (General Introduction to comparative anatomy, 1795) that the comparative method is the most important means of research for the morphologist, who must aim at the creation of ideal types, to which various forms.

To Goethe we owe the formulation and illustration of the fundamental principles of modern comparative anatomy: the theory of analogues, according to which the same organic materials are found in all animals; the law of fixity of connections, according to which the same parts are found in all animals in the same positions and respective relationships, whatever their function; the law of balance or compensation of organs.

E. Geoffroy Saint-Hilaire, despite having a clear idea of the unity of the vertebrate organization plan, made the mistake of wanting to extend it to invertebrates. At the beginning of the 19th century, G. Cuvier carried out the great reform of morphology, using all the analytical and descriptive material collected up to then. He clearly formulated the principle of the correlation of the parts and that of the subordination of organs, according to which there are organs and systems that are more important and others less, from the functional point of view: they are the first, less subject to variations, which can provide the diagnostic criteria to characterize the various animal types (embranchements), each of which includes all the organisms built according to the same plan.

Cuvier did not admit the variation of species: the kinship of the various forms of the same embranchement was conceived as only ideal and the homologies between the various organs and systems had to be considered. After C. Darwin, the informative criteria and the purposes of the academic year comparata changed radically, above all through the work of C. Gegenbaur and E. Haeckel. The world of living beings was conceived dynamically, as the result of a long transforming process, whereby the structural affinities become evidence of a real kinship not only between the various forms of the same type, but also between type and type.

Gegenbaur precisely defined the concepts of homology and analogy, assigning to comparative anatomy, comparative embryology and paleontology, the study of phylogeny, that is, of the reconstruction of the genealogical trees of the various animal groups. Almost all the morphological studies of the second half of the 19th century. they are inspired by Darwinism. Following the criticism of Darwinian evolutionism, mutationism and the experimental direction in the morphological sciences, the Gegenbaurian and Haeckelian conception of comparative anatomy declined.

Numerous researchers continued to descriptively and comparatively investigate the development and structure of organs and systems, especially vertebrates, neglecting both the genealogical problem and the theory of animal types, and also losing sight of any general and synthetic problem. However, an impressive mass of knowledge and doctrines on animal organization remained, in particular of the chordate type, also important because, independently of any transformistic hypothesis, the study of the comparative anatomy of vertebrates, and in particular of mammals, is of great help in understanding the anatomy of man.

In general, comparative anatomy has increasingly identified with morphology in a broad sense, not only descriptive and comparative, but also experimental, that is, aimed at the causal investigation of the processes that determine or condition the onset of the form. Modernly understood as biological morphology, comparative anatomy has expanded its character as an essentially synthetic science, drawing on almost all the particular biological disciplines, that is, in addition to descriptive anatomy, also histology, embryology, paleontology, to systematic zoology, physiology, genetics and developmental mechanics.

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