8 THEORY OF EVOLUTION 



The hand and the arm of man are similar to 

 the hand and arm of the ape. We find the 

 same plan in the forefoot of the rat, the ele- 

 phant, the horse and the opossum. We can 

 identify the same parts in the forefoot of the 

 lizard, the frog (fig. 3), and even, though less 



FIG. 3. Limb skeletons of extinct and living animals, show- 

 ing the homologous bones: 1, salamander; 2, frog; 3, turtle; 

 4, Aetosaurus; 5, Pleisiosaurus ; 6, Ichthyosaurus; 7, Meso- 

 saurus; 8, duck. (After Jordan and Kellogg.) 



certainly, in the pectoral fins of fishes. Com- 

 parison does not end here. We find similarities 

 in the skull and back bones of these same ani- 

 mals; in the brain; in the digestive system; in 

 the heart and blood vessels; in the muscles. 

 Each of these systems is very complex, but 



