218 COMPARATIVE ZOOLOGY. 



having a like origin and development, and therefore the 

 same essential structure, whatever their form or function, 

 are said to be homologous ; while parts or organs corre- 

 sponding in use are called analogous. By serial homol- 

 ogy is meant the hoinology existing between successive 

 parts of one animal. 



The following are examples of homology: the arms of 

 Man, the fore- legs of a Horse, the paddles of a Whale, 

 the wings of a Bird, the front flippers of a Turtle, and the 

 pectoral fins of a Fish; the proboscis of a Moth, and the 

 jaws of a Beetle; the shell of a Snail, and both valves of 

 a Clam. The wings of the Bird, Flying Squirrel, and Bat 

 are hardly homologous, since the wing of the first is de- 

 veloped from the fore-limb only; that of the Squirrel is 

 an extension of the skin between the fore and hind limbs ; 

 while in the Bat the skin stretches between the fingers, 

 and then down the side to the tail. Examples of serial 

 homology: the arms and legs of Man; the upper and 

 lower set of teeth ; the parts of the vertebral column, 

 however modified; the scapular and pelvic arches; the 

 humerus and femur; carpus and tarsus; the right and left 

 sides of most Animals; the dorsal and anal fins of Fishes. 

 The legs of a Lobster and Lizard, the wings of a Butter- 

 fly and Bird, the gills of a Fish, and the lungs of other 

 Vertebrates, are analogous. The air-bladder of a Fish is 

 homologous with a lung, and analogous to the air-cham- 

 bers of the Nautilus. 



In the midst of the great variety of form and structure 

 in the animal world, a certain harmony reigns. Not only 

 are different species so related as to suggest a descent 

 from the same ancestor, but the parts of any one organ- 

 ism are so closely connected and mutually dependent that 

 the character of one must receive its stamp from the char- 

 acter of all the rest. Thus, from a single tooth it may be 

 inferred that the animal had a skeleton and spinal cord. 



