EVOLUTION AS SEEN IN THE STRUCTURE OF MODERN ANIMALS 25 



housedress of cotton. The silk dress is not the "ancestor" of the cotton one, 

 or even its "sister" or "cousin" in any literal sense. Similarly, according to 

 the theory of special creation different species of animals are not genet- 

 ically related to one another even though they may exhibit similarites of 

 structure. 



Most modern biologists do not find this explanation satisfying. For one 

 thing, it is really not an explanation at all; it amounts to saying, "Things 

 are this way because they are this way." Furthermore, it removes the 

 subject from scientific inquiry. One can do no more than speculate as to 

 why the Creator chose to follow one pattern in creating diverse animals 

 rather than to use differing patterns. 



Hence most modern biologists explain the origin of similarities which 

 have no relation to similar functions in a different manner. They are con- 

 vinced that the similarity exists because the animals concerned inherited 

 the structure from an ancestor which they shared in common. We have 

 seen that the vertebrate forelimbs, for example, appear to be modifications 

 of a five-fingered (pentadactyl) limb having one upper-arm bone (hu- 

 merus), two lower-arm bones (radius and ulna), wristbones (carpals), 

 and metacarpals and phalanges arranged to form five fingers (Fig. 3.2). 

 Why are such diverse limbs as those of man, bat, bird, whale, horse, and 

 so on, all modifications of this pattern? The evolutionary explanation is 

 that these animals all inherited the limb pattern from an ancestor which 

 had that pentadactyl limb in more or less typical form. When the descend- 

 ants of this ancestor took to life in the water, to locomotion through the air, 

 or to running over hard ground they made over what they had in the way 

 of limbs to serve the new functions. But despite the reconstruction neces- 

 sary the indelible traces of the inherited pattern still remain. Thus, in 

 contrast to the theory of special creation, the theory of creation by evolu- 

 tion maintains that different animals are related to each other in the sense 

 of direct inheritance. 



In our discussion we have noted two types of similarity. Similarity con- 

 nected with similar functioning we have ascribed to analogy. We shall 

 find useful a term for similarities not connected with similarities of func- 

 tion: the word homology. Two organs in different animals are analogous 

 if they are used for the same purpose; two organs in different animals are 

 homologous if they have the same fundamental structure, whether or not 

 they are used for the same purpose. 



These terms can be readily illustrated in connection with the forelimbs 

 just discussed. We have seen that the wing of an insect is analogous to the 

 wing of the bird; i.e., both wings are used for flight. The insect wing is not 



