14 GENERAL PRINCIPLES OF ZOOLOGY. 



had taken place between Christianity and natural science. As a 

 criterion of the progress made in comparison with the earlier 

 centuries, a mere glance at the illustrations is sufficient. Any one 

 will at the first glance recognize the difference between the shabby 

 drawings of an Aldrovandus and the masterly figures of a Lyonet 

 or a Rosel von Rosenhof. 



Period of Comparative Anatomy. Thus through the zeal of 

 numerous men filled with a love of nature a store of anatomical 

 facts was collected, which needed only a mental reworking ; and 

 this mental reworking was brought about, or at least entered upon, 

 by the great comparative anatomists who lived at the end of the 

 eighteenth and the beginning of the nineteenth century. Among 

 these the French zoologists Lamarck, Savigny, Geoffroy St. Hilaire, 

 Cuvier, and the Germans Meckel and Goethe are especially to be 

 named. 



Correlation of Parts. When the various animals were com- 

 pared with one another with reference to their structure there was 

 obtained a series of important fundamental laws, particularly the 

 law of the Correlation of Parts and the law of the Homology of 

 Organs. The former established the fact that there exists a 

 dependent relation between the organs of the same animal, that 

 local changes in one single organ also lead to corresponding 

 changes at some distant part of the body, and that therefore from 

 the constitution of certain parts an inference can be drawn as to 

 the constitution of another part of the body. Cuvier particularly 

 made use of this principle in reconstructing the form of extinct 

 animals. 



Homology and Analogy. Still more important was the theory 

 of the Homology of Organs. In the organs of animals a distinction 

 was drawn between an anatomical and a physiological character; 

 the anatomical character is the sum of all the anatomical features, 

 as found in form, structure, position, and mode of connection of 

 organs; the physiological character is their function. Anatomically 

 similar organs in closely related animals will usually have the same 

 functions, as, for example, the liver of all vertebrates has the 

 function of producing gall; here anatomical and physiological 

 characteristics go hand in hand. But this need not necessarily be 

 the case; very often it may happen that one and the same function 

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

 respiration of vertebrates is carried on in fishes by gills, in mammals 

 by lungs. Conversely, anatomically similar organs may have 

 different functions, as the lungs of mammals and the swim-bladder 



