28 INTRODUCTION TO EVOLUTION 



Somewhere along the line we should place such gliding forms as the 

 wrongly named "flying" squirrel. It will be noted that the diagram repre- 

 sents the lines leading to arboreal and to aerial as not entirely independ- 

 ent. A single line is shown emerging from terrestrial and then dividing into 

 the two branches. This arrangement was made to suggest the probability 

 that the ancestors of flying mammals lived in trees, i.e., that life in trees 

 preceded flight. Perhaps gliding formed the transitional type of locomotion 

 between climbing and true flight. 



Continuing around the diagram in a clockwise direction we come to the 

 line ending in cursorial. This term refers to mammals, like horses and 

 antelopes, which have developed limbs suitable to rapid movement over 

 the surface of the ground. Part way along this line we should place ani- 

 mals with less strongly modified limbs, such as wolves, foxes, hyenas, 

 lions. 



A line leading downward ends with the term fossorial, applying to bur- 

 rowing mammals. Some of these, like the moles, have modified their fore- 

 limbs into such specialized and powerful digging organs that they are 

 poorly adapted for locomotion on the surface of the ground. Others, like 

 pocket gophers and badgers, are expert diggers but have retained limb 

 structures enabling them to move readily on the surface. 



Finally, a line leads to the term aquatic. At the end of this line we find 

 such mammals as whales and porpoises, with limbs so strongly modified 

 for life in the water that they cannot move about on land. Part way along 

 the line we should place seals, sea lions, and walruses, mammals with 

 limbs strongly modified for life in the water yet retaining some ability to 

 move about on land. Still nearer the center on this same line we should 

 place such accomplished swimmers as otters and polar bears, mammals 

 equally at home in water or on land. 



All the mammals mentioned as belonging on one of the radiating lines 

 have limbs more or less adapted for some particular mode of locomotion. 

 All lines start from a common center representing the short, pentadactyl 

 limbs of terrestrial mammals. From this center, evolutionary lines radiate 

 out in various directions. Hence adaptive radiation is evolution in several 

 directions starting from a common ancestral type. 



What is the relationship of adaptive radiation to homology and analogy? 

 All the limbs mentioned are homologous to each other, since they are all 

 variations of the pentadactyl limb. But for the most part a given limb is 

 only analogous to others on the same radiating branch of the diagram. 

 Thus the leg of the antelope is analogous to the leg of the horse, since 

 they have the same function. Furthermore, limbs of animals on one radiat- 

 ing branch of the diagram are not analogous to limbs of animals on other 



