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COMPARATIVE ZOOLOGY 



female is sometimes larger than the male, as of the nau- 

 tilus, spider, and eagle. The higher the class, the more 

 uniform the size. Of all groups of animals, insects and 

 birds are the most constant 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 syrrimetry of the two ends, as well as the 

 two sides, is nearly perfect ; but it is subsequently inter- 

 fered 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 con- 

 ditions on all sides, and, therefore, it has no right and 

 left, or fore and hind parts. The lower forms, generally, 

 are more or less geometrical figures : spheroidal, as the 

 sea urchin ; radiate, as the starfish ; and spiral, as many 

 foraminifers. The higher animals are subjected to a 

 greater variety of conditions. Thus, a fish, always go- 

 ing through the water head foremost, must show con- 

 siderable 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 arthropods 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. 



