92 EVOLUTION 



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 ner- 

 vous cells would be most developed in that 

 much-stimulated, and otherwise over-edu- 

 cated, head region. But a brain always 

 arises from the sinking in of ectodermic cells 

 from the surface of the embryo, and its be- 

 ginning in the cerebral ganglion of the sim- 

 plest 'worms' is thus in part explained. 

 It is difficult to over-estimate the importance 

 of the establishment of an anterior brain 

 a chief motor and sensory and co-ordinating 

 nerve-centre and the consequent evolution 

 of a head. 



THE BEGINNINGS OF BEHAVIOUR. Jen- 

 nings has shown that some unicellular ani- 

 mals "behave " in a very definite way. They 

 are not mere automata which rush about as 

 long as their spring keeps unrolling, and they 

 are more than the mere slaves of stimulus. 

 There are some, it is true, which seem to 

 have only one kind of reaction to every kind 



