HOMOLOGY. 



45 



form and function." In other words, those organs or parts 

 in different animals are ho7nologous^ which agree with one 

 another morphologically in their fundamental sinicture^ quite 

 irrespective of what functions they discharge in the economy. 

 Thus the arm of man, the fore-leg of the dog, and the wing 

 of a bird, are constructed upon the same morphological 

 type, and are therefore homologous (fig. 13). They are 

 not, however, analogous, since they perform wholly different 

 functions, the first being an organ of prehension, the second 

 devoted to terrestrial progression, and the third an organ of 

 flight. There are, however, many cases in which organs in 



r .J 



ABC 



Fig. 13. — A Arm of Man ; B Fore-leg of Dog ; C Wing of Bird : h Humerus ; 

 r Radius ; « Ulna ; c Carpus ; ttic Metacarpus ; / Phalanges. 



different animals are not only constructed of the same 

 essential parts, but also discharge the same functions, thus 

 coming to be both homologous and analogous. 



Besides the homologies which subsist between organs in 

 different animals, there are two kinds of homology which 

 may be present in the different parts of the same animal, 



