COMPARATIVE ANATOMY 93 



been reduced in the bird's hand. We find the explanation 

 of this resemblance when we recognize that the bird and 

 the dog are descended from common ancestors in which the 

 leg was used for walking ; that the dog has perfected the 

 limb for walking, while the bird has modified and adapted it 

 for the very different use, flying. The two organs are funda- 

 mentally alike because they are modifications of the same 

 thing. They are superficially different because they are 

 used for very different purposes. This fundamental re- 

 semblance founded on common descent is called homology, 

 and the phenomena of homology, no less than those of 

 taxonomy, lend much support to the evolution theory, being 

 intelligible in the light of that theory, while without this 

 theory they have no meaning to us. \Ve might multiply 

 almost indefinitely illustrations of homology based on ge- 

 netic relationship ; the illustration given, however, will 

 show the line of evidence as well as is needed for our 

 purpose. 



Vestigial structures. 



Among the most interesting of the anatomical evi- 

 dences of evolution are the vestigial organs found in so 



O O 



many animals and plants, organs once normally developed 

 and functional, but now reduced, and, so far as we can 

 judge, functionally insignificant. Certain snakes have very 

 slightly developed hind limbs, reminding us of the fact that 

 they are descended from forms which had well-developed 

 limbs, their present limbless condition being secondary 

 (Fig. 15). Whales also have vestiges of hind limbs, in the 

 form of certain small bones lying beneath the skin and not 

 in any way functional (Fig. 16). They are vestiges of the 



