THEORIES AND MECHANISM OF EVOLUTION 



661 



(Fig. 26-7). They all produced wings same three groups (reptiles, birds, and 



which were essentially the same, although mammals ) which returned to the water and 



varying slightly in minor details. None of became modified in many ways for an 



them are perfect instruments of Hight, yet aquatic life ( Fig. 26-7 ) . The primitive rep- 



they are sufficiently good to permit both tile Ichthyosaurus took on the shape of a 



the birds and the bats very real success, the modern fish and its appendages became 



Fig. 26-6. The front appendage of vertebrates illustrates divergent evolution. The hypothetical vertebrate 

 appendage v/as a five-toed structure which has been modified in many ways as represented by the 

 arm of man, the front leg of the cat, the wing of the bird, the flipper of the whale, and the front leg 

 of the horse. Each modification satisfies the needs of the particular animal living in an environment 

 that requires such an appendage for survival. 



former more than the latter. The pterodac- fin-like. The whale, a mammal, developed 



tyls became extinct for unknown reasons, fish-like appendages and body form. The 



but it seems unlikely that the ability to fly penguin, a bird, lost its ability to fly but 



was a contributing factor. became a proficient swimmer, using its 



Another case of convergent evolution is wings as flippers, 

 illustrated by still other members of these Finally divergent evolution is nicely il- 



