HISTORY OF ZOOLOGY 11 



is possessed by organs anatomically different; as, for example, the res- 

 piration is carried on in fishes by gills, in mammals by lungs. Con- 

 versely, anatomically similar organs may have different functions, as 

 the lungs of mammals and the swim-bladder of fishes; similar organs 

 may also undergo a change of function from one group to another; the 

 hydrostatic apparatus of fishes has come to be the seat of respiration in 

 the mammals. Organs with like functions physiologically equivalent 

 organs are called 'analogous'; organs of like anatomical constitution 

 anatomically equivalent organs are called 'homologous.' It is the 

 task of comparative anatomy to discover in the various parts of 

 animals those which are homologous, and to follow the changes in 

 them conditioned by a change of function. 



Cuvier. The foremost representative of comparative anatomy was 

 Georges Dagobert Cuvier. His investigations extended to* the coelenter- 

 ates, molluscs, arthropods, and vertebrates, living and fossil. He 

 collected his extensive observations into his two chief works "Le regne 

 animal distribue d'apres son organization" and "Lefons d'anatomie 

 comparee." Of epoch-making importance was his little pamphlet "Sur 

 un rapprochement a etablir entre les differentes classes des animaux," 

 in which he founded his celebrated type theory, and which introduced a 

 reform of classification. The Cuvierian division, the starting-point for 

 all later classifications, differed from all the earlier systems in that the 

 classes of mammals, birds, reptiles, and fishes were united in a higher 

 grade under the name, introduced by Lamarck, of 'vertebrate animals'; 

 and the so-called 'invertebrate animals' were divided into three similar 

 grades, each equal to that of the vertebrate animals, viz., Mollusca, 

 Articulata, and Radiata. Cuvier called these grades standing above the 

 classes, provinces or chief branches (embranchements). But still more 

 important are the differences which appear in the structural basis of 

 the system. Instead of, like the earlier systematists, using a few ex- 

 ternal characteristics for the division, Cuvier built upon the totality of 

 internal organization, as expressed in the relative positions of the most 

 important organs, especially the position of the nervous system, as 

 determining the arrangement of the other organs. Thus for the first time 

 comparative anatomy was employed in the formation of a natural system 

 of animals. 



Cuvier found prevalent the theory that all animals formed a single 

 connected series ascending from the lowest infusorian to man ; within this 

 series the position of each animal was determined by the degree of its 

 organization. On the other hand Cuvier taught that the animal kingdom 

 consisted of several co-ordinated unities, the types, which exist inde- 



