GENERAL ZOOLOGY AND COMPARATIVE ANATOMY 23 



The structure and functions of his organs are 

 only to be fully understood by comparison with 

 those of lower animals. 



The leading principles of comparative study 

 are Homology and Analogy. In biology those 

 organs or parts in different animals are said to 

 be analogous which, however different their 

 origin, have a general similarity of form and 

 especially of function, while those are called 

 homologous which, however different their gen- 

 eral appearance and however various their func- 

 tions, are but modifications of the same part al- 

 tered for different purposes. For example, the 

 wing of the bird and the wing of the butterfly 

 are analogous organs, for they look somewhat 

 alike and have the same function, — flying; but 

 they are not homologous, for they are not the 

 same in structure and are dissimilar in origin. 

 But the forelimbs of all Vertebrates, whether the 

 fcrepaws of a reptile or a mammal, the wings of 

 a bird or bat, the arm of a man, the flipper of a 

 whale, — though so different in form and function, 

 — are homologous parts. They have the same 

 general structure, are composed of the same 

 pieces and undoubtedly have the same origin; 

 they are but modifications of the same structure 

 for different functions. They are homologous 

 but not analogous parts. Again, the lungs of a 

 mammal and the gills of a fish are analogous or- 



