238 THE EVOLUTION OF CONTINUITY 



apparent that a crossed motor tract leading to a right fin 

 would, if this drove with vigour, cause the Fish to move 

 away from the source. The fin would have to backwater 

 to help in the proper response. Clearly the brain is such 

 a complicated organ, and its communicating paths so 

 imperfectly known, and the body's responses so multiple 

 and complex, and mutually influential, that no more than 

 superficial suggestions have been offered above. But our 

 main point remains to be emphasised, namely, that the 

 avoidance, or correction, of deviation in fish locomotion 



Fig. 78. — Diagram of fissures, convolutions, and functional 

 areas of human right hemisphere. (After Jacobson and Steward. ) 



would demand a bilaterally symmetrical nervous system, 

 and that local habitually acting sources of attraction and 

 repulsion calling for undeviating response would be the 

 ground factors in the production of the duplicate brain. 



The bilateral symmetry of birds and quadrupeds has 

 clearly been derived in the first place from the primitive 

 Fish. Like the fish, the typical bird is able to leave the 

 surface of the earth and move through its surrounding 

 atmosphere in spite of the direct pull of Gravity ; the 

 difference being that the atmosphere of the bird is air, and 

 wings take the place of lateral fins. On the whole, however, 



