222 
COMPARATIVE ZOOLOGY. 
groups of animals, Insects and Birds are the most con¬ 
stant in their dimensions. 
Every organism has its own special law of growth: a 
Fish and an Oyster, though born in the same locality, de¬ 
velop into very different forms. Yet a symmetry of plan 
underlies the structure of all animals. In the embryo, 
this symmetry of the two ends, as well as the two sides, 
is nearly perfect; but it is subsequently interfered with 
to adapt the animal to its special conditions of life. It is 
a law that an animal grows equally in those directions in 
which the incident forces are equal. The Polyp, rooted 
to the rocks, is subjected to like conditions on all sides, 
and, therefore, it has no right and left, or fore and hind 
parts. The lower forms, generally, are more or less geo¬ 
metrical figures: spheroidal, as the Sea-urchin; radiate, 
as the Star-fish; and spiral, as many Foraminifers. The 
higher animals are subjected to a greater variety of con¬ 
ditions. Thus, a Fish, always going through the water 
head foremost, must show considerable difference between 
the head and the hinder end; or a Turtle, moving over 
the ground with the same surface always down, must have 
distinct dorsal and ventral sides. 
Nevertheless, there is a striking likeness between the 
two halves or any two organs situated on opposite sides 
of an axis. And, first, a bilateral symmetry is most com¬ 
mon. It is best exhibited by the Articulates and Verte¬ 
brates, but nearly all animals can be clearly divided into 
right and left sides — in other words, they appear to be 
double. A vertical plane would divide into two equal 
parts our brain, spinal cord, vertebral column, organs of 
sight, hearing, and smell; our teeth, jaws, limbs, lungs, 
etc. In fact, the two halves of every egg are identical. 
There are many exceptions: the heart and liver of the 
higher Vertebrates are eccentric; the nervous system of 
Mollusks is scattered; the hemispheres of the human 
