ANALOGY AND HOMOLOGY 165 



the key to relationships. Animals that fly, for example, includ- 

 ing bats, birds, and insects, all have wings, and might be classi- 

 fied in one group as " beasts of the air." But study of bats and 

 birds shows that they belong to two entirely different classes, the 

 bats having wings like the arms and fingers of a mammal and 

 the mammary glands of the mammals, while birds have espe- 

 cially modified fore limbs, entirely different bone structure and 

 other organs, which place them in the class Aves. Birds and 

 insects are also different, both in the character of the wings 

 and in the absence of an internal bony skeleton in the latter. 

 While the functions of wings of birds and insects are the same, 

 their anatomy shows an entirely different mode of origin and 

 different secondary structures. In such cases the organs are said 

 to be analogous. When organs have the same ancestry, that is, 

 when they come from some common part of an ancestral type, 

 they are said to be homologous. The wings of a bird have had 

 the same ancestry as the fore-legs of beasts or the arms of man; 

 so too have the wings of a bat hence arms, fore-legs of beasts, 

 and wings of bat or bird are homologous structures. It is quite 

 otherwise with the wings of a bee or fly. These have had an 

 entirely different ancestry from the wings of a bird and are 

 not homologous with the latter. Wings of different insects, 

 however, are homologous. 



Homology, or genetic relationship of organs and structures 

 in general, is the ground principle of classification of species. 

 Two organs on different animals may be homologous whether 

 they perform the same functions or not, and conversely, the 

 same functions may be performed by organs not homologous. 

 The study of homologies therefore is one of the most important 

 in comparative anatomy and in taxonomy. The walking legs 

 of vertebrates and those of a lobster are the same in function 

 and are analogous organs, but no one would compare them 

 morphologically, and they are not homologous in any sense. It 

 is quite otherwise, however, with the legs of lobsters, of crabs 

 and of shrimps, which are homologous, having had a like origin. 

 All of the appendages of a lobster or crab, furthermore, although 

 they have widely different functions and are quite different in 



