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 tho 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 Fisii, 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 Hlateral 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 



