GREAT STEPS IN EVOLUTION 93 



For conditions of more active and strenuous 

 life, however, where it is important to chase 

 the food, to flee from enemies, to pursue mates, 

 and so on, radial symmetry is unsuitable, and 

 it is replaced by bilateral symmetry. This ac- 

 quisition of head end and tail end, of right side 

 and left side, was doubtless of enormous im- 

 portance, both in itself and in its consequences, 

 which include our knowing our right hand 

 from our left. 



It is likely that certain " worms " were the 

 first animals definitely to abandon the more 

 primitive radial symmetry, to begin moving 

 with one part of the body always in front, to 

 acquire head and sides. And if one end of 

 the body constantly experienced the first 

 impressions of external objects, it seems 

 reasonable to suppose that sensitive and nerv- 

 ous cells would be most developed in that 

 much-stimulated, and otherwise over-educated, 

 head region. But a brain always arises from 

 the sinking in of ectodermic cells from the 

 surface of the embryo, and its beginning in 

 the cerebral ganglion of the simplest " worms " 

 is thus in part explained. It is difficult to 

 over-estimate the importance of the establish- 

 ment of an anterior brain a chief motor and 

 sensory and co-ordinating nerve-centre and 

 the consequent evolution of a head. 



