GREAT STEPS IN EVOLUTION 91 



and female, but it also determines, either 

 from the start, or after maleness and female- 

 ness have been partly established, what par- 

 ticular expression will be given to a whole 

 series of secondary characters, both struc- 

 tural and functional, whether a masculine 

 or a feminine expression. 



THE BEGINNINGS OF BRAINS. In most 

 sponges and coelenterates (such as jelly-fish, 

 sea-anemones and coral polyps) the body 

 has radial symmetry. That is to say, it is 

 the same all round, it has no right nor left, it 

 can be cut into symmetrical halves along 

 many different vertical planes. This kind of 

 symmetry is well suited for sedentary life, 

 like that of sea-anemones, which wait for 

 food to come within the scope of their sting- 

 ing and grasping tentacles; or for an easy- 

 going life, like that of jelly-fishes, which live 

 in the very uniform environment of the open 

 sea where all directions mean very much the 

 same. 



For conditions of more active and strenu- 

 ous life, however, where it is important to 

 chase the food, to flee from enemies, to pur- 

 sue mates, and so on, radial symmetry is 

 unsuitable, and it is replaced by bilateral 

 symmetry. This acquisition of head end and 

 tail end, of right side and left side, was doubt- 

 less of enormous importance, both in itself 



