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 homology 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, 
