MERISTIC VARIATION 



35 



The right hand and arm are made upon the same plan as the 

 left, but could not replace them because they would not fit ; the 

 one is the reverse of the other. 



Reflected in the mirror the right hand seems to be the left, 

 but it is an illusion, for the right side is the negative or optical 

 image of the left with all its elements reversed. Hence a part 

 upon the one side could not replace the corresponding part upon 

 the other. It is its counterpart, not its duplicate as among leaves 

 and petals. Thus we arrive at the distinction between the com- 

 plexity of bilateral symmetry and the simplicity of radial symmetry. 

 It is also significant that bilateral symmetry is characteristic of 

 higher animal life, and radial symmetry of lower animals and plants. 



Dorsal and ventral surfaces. The fundamental fact at the 

 bottom of bilateral symmetry is the distinction between dorsal 

 and ventral surfaces, necessitating differences in the quadrants 

 that are forced to work in opposition to each other. 1 



Indian corn and the grass family generally are as distinctly 

 bilateral as is the horse or man, yet there is no thought of dorsal 

 and ventral differences, and hence no distinction between right 

 and left. 



For example, let the leaves of a corn plant and the arms of a 

 man extend east and west. Then the north and south sides of 

 the corn plant will be alike. Not so with the man. In the one 

 direction (we will say the south) will be his spinal column and 

 the general framework of the body ; in the other (to the north) 

 will be his face, his nose, his eyes, and all the -active external 

 parts. Moreover his hands are made to oppose each other and to 

 work together with this (the ventral) side of the body. There is 

 therefore bilateral symmetry in one direction but not in the other. 



With the corn plant the case is different. It does not move 

 from place to place, and it presents its plain sides indifferently 

 to the world. Accordingly no distinctions similar to dorsal and 

 ventral are possible. 2 



1 The word "opposition " is here used not in the sense of " antagonism " but 

 as " placed opposite and working with " ; as, The thumb is " opposed " to the 

 other members of the hand, thereby making a working unit. 



2 None of these distinctions should be confused with homologous parts or 

 with analogous parts, nor should the ideas be confounded. 



Homologous parts belong to two individuals, not one, and they are such as 

 bear corresponding structural relations to their respective organisms, suggesting 



