8 



THEORY OF EVOLUTION 





The liaiid 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 



6 '°°- 



Fig. 3. lAmh skeletons of extinct and living animals, show- 

 ing the homologous hones: 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 hack hones of these same ani- 

 mals; in the hrain; in the digestive system: in 

 the heart and hlood vessels; in the muscles. 

 Each of these systems is very complex, but 



