CLASSIFICATION AND DISTRIBUTION 369 



example of analogous organs, may be mentioned the teeth in 

 the mouth of vertebrates and the peculiar teeth found inside 

 the stomach of the lobster. These organs are both used for 

 grinding food; but they are not homologous organs, since their 

 structure is so different. The teeth are bony organs arising 

 from the bones of the skull, which are themselves developed from 

 the mesoderm of the embryo; the teeth of the lobster are of 

 horny texture, and are developed from the ectoderm of the em- 

 bryo which is folded inward to line the stomach. Numerous 

 other examples of analogous organs might be given, for it 

 frequently happens that different animals use for the same pur- 

 pose organs that have quite a different origin and structure. 



Explanation of Homology and Analogy. Analogous organs 

 sometimes show much similarity, as in the shape of the wings 

 of the bird and butterfly, and sometimes very little. When 

 they do show a likeness it is explained by the fact that similar 

 necessities of life have forced the development of similar struc- 

 ture. For example, both the vertebrates and the lobster are 

 obliged to masticate their food, and both have consequently 

 developed hard cutting and grinding surfaces for the purpose. 

 There is, therefore, some similarity in the form of the organs; 

 but there is no necessity for similarity in structure, and in the 

 two cases different parts of the body have been utilized for the 

 purpose. 



The likeness between homologous organs, however, requires 

 a very different explanation, because here we find a similarity 

 in structure in spite of differences in function. We cannot explain 

 the similarity in structure by any similarity of conditions. 

 Although the wing of the bird and the arm of man are adapted 

 to wholly different functions and have developed different shapes 

 and motions, they are, in spite of this difference, formed upon the 

 same plan, with an identical structure. The explanation must 

 be something more fundamental than mere similarity in use. 

 Naturalists to-day account for likeness in homologous organs by 

 the theory of descent, saying that two animals with homologous 



