PRIMARY FUNCTIONS OF THE ORGANISM 47 



organs of allied and connected functions are com- 

 bined into systems, some of which, like the nervous 

 and circulatory systems, pervade the whole organism. 

 It is of great interest and value to compare animals 

 and plants with respect to the degree of specializa- 

 tion of their parts ; for such a comparison often re- 

 veals relationships. In making such comparisons it is 

 sometimes found that organs carrying out the same 

 function, such as the wing of a bird and that of a 

 butterfly, are of a very diverse origin and structure. 

 On the other hand, the wing of a bird and the foreleg 

 of a dog, in spite of the apparently very different 

 functions which each performs, have each the same 

 origin relative to the rest of the body and the same 

 general internal structure. Such a similarity we 

 call homology, and we speak of the two parts as 

 homologous, whereas the similarity of function be- 

 tween the wing of a bird and of a butterfly we speak 

 of as analogy, and the parts as analogous. 



