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COMPARATIVE ZOOLOGY 



By serial homology is meant the homology existing be- 

 tween 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 

 developed 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 butterfly 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 analo- 

 gous to the air chambers 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 organism are so closely connected and mutually 

 dependent that the character of one must receive its 

 stamp from the character of all the rest. Thus, from a 

 single tooth it may be inferred that the animal had a 

 skeleton and spinal cord, and that it was a carnivorous, 

 hot-blooded mammal. Certain structures always coexist. 

 Animals with two occipital condyles, and non-nucleated 

 blood corpuscles, suckle their young, i.e., they are mam- 



