10 GENERAL rRINICPLES OF ZOOLOGY 



The great service of the men named above consists chiefly in that they 

 broke away from the thraldom of jjook-learning and, relying alone upon 

 their own eyes and their own judgment, regained the blessing of inde- 

 pendent and unbiased observation. They spread the interest in ol)ser- 

 vation of nature so that in the eightecntli century the number of natural- 

 histor}' writings had increased enormously. There were busy with the 

 study of insect structure and development, de Geer in Sweden, Reaumur 

 in France, Lyonet in Belgium, Riisel von Rosenliof in Germany; the 

 latter besides wrote a monograph on the indigenous batrachia, which is 

 still worth reading. The investigation of the infusoria formed a favorite 

 occupation for Wrisberg, von Gleichen-Russwurm, Schaffer, Eichhorn, 

 and O. F. Mi.iller. As a criterion of the progress made, a mere glance 

 at the illustrations is sufficient. An)' one will at a glance recognize the 

 difference between the shabby drawings of an Aldrovandus and the 

 masterly ligures of a Lvonet or a Rosel von Rcisenhof. 



Peroid of Comparative Anatomy. — Thus through tlie zeal of 

 numerous men a store of anatomical facts was collected, which needed 

 only a mental reworking; and this was brought al)out, or at least entered 

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

 eighteenth and the beginning of the nineteenth centurv'. Among these 

 the French zoologists Lamarck, Savigny, Geoffrey St. Hilaire, Cuvier, and 

 the Germans Meckel and Goethe are especially to be named. 



Correlation of Parts. — When the various animals were compared 

 with one another with reference to their structure there was obtained a 

 series of fundamental laws, particularly the law of the Correlation of 

 Parts and the law of the Homology of Organs. The former showed that 

 there exists a dependent relation between the organs of the same animal; 

 that local changes of one organ lead to corresponding changes at some 

 distant part of the bod)-, and that therefore from the structure of certain 

 parts an inference can be drawn as to tlie constitution of another part of 

 the body. Cuvier particularly made use of this principle in reconstructing 

 e.xtinct animals. 



Homology and Analogy. — Still more important was the idea 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 form, structure, ]xisition, 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 produces gall; here 

 anatomical and physiological characteristics go hand in haml. But this 

 need not be the case; very often it may happen that the same function 



