SEGMENTAL BILATERAL SYMMETRY 239 



Gravity is victorious, and the larger part of the bird's 

 existence is spent on the earth, or on the water. For moving 

 on the surface of the earth the bird has two symmetrical 

 legs, and as in the case of the wings this symmetry reflects 

 the call for undeviating responsive movement towards any 

 point of attraction, or away from any source of repulsion. 



In the case of quadrupeds, it may be said that, with 

 the exception of such as live in trees or live an amphibious 

 existence, the only possible form of deviation is lateral — 

 to the right or the left. The body-mass is, as it were, slung 

 on four upright supports, symmetrically paired, with the 

 abdominal contents invariably sagging downwards in the 

 middle towards the earth's surface. 



Man is after all but a quadruped who has learnt to go 

 about constantly on his hind legs, and moving as he does 

 on the earth's surface the deviation he has to avoid or correct 

 is lateral. It is interesting to note that the situation of 

 the cortical centres in man's brain tend to confirm the 

 supposition already mentioned, that the direction of habitual 

 force-lines impinging on the evolving primitive Fish could 

 determine the direction of the main nerve-tracts. For if 

 we turn to Fig. 76, and compare it with the next below, 

 we can see that the lines from the region of the posterior 

 limbs, or fins, pass to the mesial and anterior part of the 

 opposite hemisphere ; those from the anterior limbs to a 

 point farther out and behind ; while any from the head 

 region would pass to a still more external or posterior part 

 of the cortex. 



