236 THE EVOLUTION OF CONTINUITY 



would be those crossing to the opposite side of the brain 

 (Fig. 75). 



Naturally it is to be remembered that our evolving Fish 

 had as an ancestral gift a nervous system somewhat as in 



Figj 75. — Diagram illustrating how tactile impressions behind 

 the brain would demand crossed sensory tracts to conduct 

 them to the brain. If we imagine the evolving primitive Fish to 

 be constantly receiving tactile impressions on the surface of 

 its body at different points, as in the figure, we can see that 

 the main lines of force would be always across the body, and 

 that of those passing in the direction of the brain the strongest 

 would be the ones reaching the hemisphere of the opposite side. 



Fig. 61 on page 192, and that this plan would, as it were, 

 only require modification in its attempts at realisation. 

 But our suggestion is that the method of this modification 



Fig. 76. — A suggested explanation of sensory decussation in 

 the cord and medulla, c, cord ; P, P, points of decussation in 

 the cord ; D, great decussation in the medulla ; s, the spreading 

 out on the cortex of the brain ; jj, l,, lines of force whose direc- 

 tion may have determined the course of the nerve-tracts ; B, 

 brain. 



was fundamentally due to the impressions which the evolv- 

 ing organism came habitually to receive. Is it not possible 

 that the decussation of force lines as pictured in the last 



