SEGMENTAL BILATERAL SYMMETRY 231 



away ; and it does so in as straight a line as possible. This 

 is what the force of repulsion demands. The Fish could 

 naturally deviate abruptly from this straight line to move 

 in a new one at the call of some laterally situated new 

 source of repulsion, or one of attraction ; but whether this 

 occurred or not, the point to note is that repulsion from one 

 source or direction is equivalent to attraction in the opposite 

 direction. This may seem rather obvious, but it is none 

 the less important (Fig. 70). 



Naturally the deviation to be corrected could be either 

 upwards or downwards, or to the right or left, but we shall 



»A. 



Fig. 70. — To illustrate how repulsion from one direction is 

 equivalent to attraction in the opposite one. 1, 2, repulsion 

 acting from E on the brain B of the organism, causes its attrac 

 tion in the opposite direction along a straight line. From 

 this line, however, there might be deviation at the call of some 

 definite source of attraction, as in 3. 



confine our remarks to the correction of lateral deviation 

 when the Fish is moving on an even keel in a given plane. 

 This correction would demand the formation of a bilaterally 

 symmetrical brain. 



If we draw the developing Fish as lying obliquely across 

 a line of attraction, as in Fig. 71, it is clear that the force 

 would be most strongly exerted on the right side of the 

 anterior end of the organism ; and similarly, in 2, it would 

 be most strongly felt on the left side. In either case the 

 immediate result would be asymmetrical power response. 



If the Fish were a rigid inanimate object something 



