CHAPTER VI. 



THE STMMBTKT OP ATSTIMAL POEMS. 



The determining condition of animal symmetry is 

 the power of movement, which renders the vast majo- 

 rity of animal forms the possessors of bilateral sym- 

 metry. That is to say, animals usually consist of two 

 similar halves arranged right and left of an axis. This 

 axis is the direction of forward movement : the end of 

 the axis that moves forward is the head, the end that 

 follows is the tail; by reference to which, respectively, 

 we distinguish the succession of intermediate parts as 

 comparatively anterior or posterior. One surface pro- 

 gresses along the surface traversed, and one is turned 

 to the sky : the former is the ventral or belly surface, 

 the latter, the dorsal or back surface. There are a 

 few animals, including the human, in which these 

 terms are slightly modified in meaning, because the 

 animal has an upright posture ; but they still refer 

 equally correctly to the arrangement of its parts. It 

 is very easy to understand how animals formed in this 

 way obtain the idea of space in three dimensions, which 

 seems to us at first sight the only way of measuring 

 space, though other ideas might occur to animals 

 built on a diS'erent plan. 



81 Q 



