7O EVOLUTION OF TO-DAY. 



can be modified so as to be in accordance with all 

 of the facts mentioned. The force of the evolution 

 argument here is not in the impossibility of making 

 some other theory to fit the facts, but in the fact 

 that (i) all other theories are supernatural explana- 

 tions rather than natural, and (2) that they are 

 unnecessary, for the simple theory of descent is 

 entirely sufficient to explain the classification of the 

 organic world. 



Application of the Theory to the Explanation of Ho- 

 mology Homology in General Serial Homology. 



Let us now turn to some of the minor applica- 

 tions of the argument and see how the theory 

 explains some special features. First let us notice 

 the explanation of homology in general. Homolo- 

 gous organs are those whose structures are alike, 

 though their functions may be entirely different ; for 

 instance, the short useless tail of the hog and the 

 highly developed prehensile tail of the monkey are 

 homologous ; or the arm of a man and the wing of 

 a bird. Analogous organs are those whose func- 

 tions are alike, though their structures may be very 

 different ; as the bird's wing and the butterfly's 

 wing. Analogous likenesses are plainly of second- 

 ary importance, existing because the organs in 

 question have similar functions. Homologous like- 

 ness is of much more significance, since it exists in 

 spite of differences in function and clearly indicates 

 some fundamental likeness between animals. The 

 whole subject of comparative anatomy or morphol- 

 ogy is the study of homologies of structure. When 



