CHAPTER XXV 



SEGMENTAL BILATERAL SYMMETRY 



(continued) 



While the development of ventral ballast was proceeding 

 in the evolving primitive Fish, other factors were working 

 to produce bilateral symmetry of form. The bilaterally 

 symmetrical brain was developing in response to a demand 

 for symmetry of power in all it controlled, and this brain 

 symmetry had its own influence in producing and main- 

 taining that of the Fish's body and appendages. 



The development of a brain was one of the results of 

 terminal compression (page 193). For a given part of the 

 developing Fish was constantly obliged to lead the way 

 when the organism moved through the water, and thus 

 became the region which received with fullest force all 

 attracting or repelling influences acting along the line of 

 its course. This leading or anterior part was, in fact, called 

 on to register and distribute the influences to the rest of 

 the body, and, as an inevitable result of the water-resistance 

 encountered during movement, the anterior segments were 

 massed together as they developed, and the ancestrally 

 derived ganglia took shape as the registering and controlling 

 brain-mass. 



The young primitive Fish would be stimulated to develop 

 its locomotive powers through Food-attraction, and also 

 through sources of repulsion ; and while the weight of the 

 abdominal viscera would, as we have seen, assist in the 

 maintenance of physical equilibrium, it is clear that this 

 could be easily upset by suddenly acting external forces, 

 and that means would require to evolve to correct this. 

 Moreover, if the locomotive appendages formed by the Fish 

 were not capable of exerting symmetrical power, they them- 

 selves would upset equilibrium. 



229 



