30 INTRODUCTION TO EVOLUTION 



tion, rather than parallel evolution. Thus the whales and their relatives 

 and an extinct reptile, Ichthyosaurus, became extremely fishlike in gen- 

 eral body form (Fig. 3.5). This attainment of similar form by animals as 

 distantly related as are a reptile and a mammal forms an example of 



FIG. 3.5. Convergent evolution exhibited by a fish (shark), a rep- 

 tile {Ichthyosaurus), and a mammal (dolphin), all strongly adopted 

 for aquatic life. (From Lull, The Ways of Life, Harper & Brothers, 

 1947, p. 47.) 



convergent evolution. The wings of bee, bird, and bat afford another ex- 

 ample of convergence. 



In summary we may point out that parallel and convergent evolution 

 lead to production of analogous similarities. On the other hand, homolo- 

 gous similarities are indications of the persistence of ancestral structure 

 throughout all the vicissitudes of evolutionary change. 



Homology in Skull Structure 



Use of forelimbs for illustrative purposes in the preceding discussion was 

 dictated by the clarity with which the several points could be shown and 

 by the relative ease with which the structures could be understood by read- 



