232 THE EVOLUTION OF CONTINUITY 



like a torpedo in shape, and had a brain-equivalent in the 

 form of a steel nose ; and if this were to receive obliquely 

 the attracting force of a strong magnet, then the object 

 would swing forwards into the direct line of force and con- 

 tinue to move without deviation. But the Fish's movement 

 is not passive response, and the attracting force calls for 

 active fish movements. 



If we confine our attention to the lateral fins as pro- 

 pelling agencies, it is clear that the Fish in the last figure, 

 if called to swing to the right to get in the main line of 

 attraction, would have to drive with the fins of the left side, 

 or that furthest from the source of attraction. Conversely, 

 if it had to swing to the left, it would have to work the 

 fins of the right side of the body. But in either case the 

 immediate effect would be that the Fish would swing beyond 



Via. 71 



the straight line of attraction unless something prevented 

 this. The next figure shows how swinging past the straight 

 line is obviated. 



Thus it will be evident that the Fish's brain would be 

 called on to have two similar parts which would evenly 

 balance each other and so prevent deviation ; and which 

 could also correct deviation when this occurred. 



An interesting point is to be noticed here. If responsive 

 movement to the right involves left fin movement, and 

 vice versa, it is clear that the messages sent from the brain 

 to the fins must cross the body ; in other words, that the 

 motor nerve-tracts would require to be crossed. 



This is the logical conclusion, and the fact is that in 

 the typical Fish, as in all true vertebrates (these having 

 evolved through some primitive piscine type), a crossing 

 of main motor nerve-paths always takes place. In Man, 



