26 INTRODUCTION TO EVOLUTION 



homologous to the bird wing, however, since the structures of the two 

 wings differ greatly. The wing of the bird is analogous to the wing of the 

 bat, since they are both used for flight. In this case, moreover, the two 

 wings are also homologous, since they both have the same fundamental 

 structure, both being modifications of the pentadactyl limb. For the same 

 reason the leg of the horse is homologous to the wing of the bird, although 

 the leg of the horse is not analogous to the wing of the bird, since the two 

 limbs are used for such different functions. Thus organs in different ani- 

 mals may be analogous but not homologous, analogous and homologous, 

 or homologous but not analogous. 



According to the most generally accepted interpretation, homologous 

 structures owe their fundamental similarities to common ancestry. They 

 are indications, remaining in modern animals, of what the ancestors of 

 these animals were like. In a sense, all modern animals are "made-over" 

 animals — the made-over versions of their ancestors. And just as a made- 

 over garment if examined closely may reveal some indications of its 

 former state, so modern animals reveal to a discerning eye what the char- 

 acteristics of their ancestors must have been. For this reason the discovery 

 and analysis of homologous structures forms one of the most powerful 

 tools used in tracing the evolutionary histories and relationships of ani- 

 mals. 



In this discussion we have illustrated homology with examples in which 

 the fundamental similarities are easily seen. In all fairness we should men- 

 tion that tracing homologies is frequently difficult. For example, there is 

 convincing evidence that the "hammer" and "anvil" (malleus and incus) 

 of the chain of three little bones in our middle ear are homologous to two 

 bones which formed the articulation of the lower jaw to the skull in our 

 reptilian ancestors (quadrate and articular bones; see p. 190). In this case 

 careful investigations of modern animals, of embryonic development and 

 of fossil forms were all needed before the homology became evident. 



Adaptive Radiation 



The concept of adaptive radiation may be illustrated by the limb struc- 

 ture of mammals. Mammalian hmbs, like those of other vertebrates aside 

 from fishes, are modifications of the pentadactyl limb. Primitive, ancestral 

 mammals are believed to have been short-legged, five-fingered creatures 

 living on the ground but having limbs not strongly modified for any par- 

 ticular type of locomotion. Such animals are called terrestrial in Fig. 3.4; 

 insect-eating (insectivorous) mammals such as the shrews form modern 



