Man in the Light of Evolution 



steam. But the fire box needs an indraft to bring 

 in air, and a flue to carry off products of com- 

 bustion. Similarly the muscle requires an ex- 

 cretory organ like our kidney to remove some 

 of the waste and a means of gaining oxygen. 

 Gills soon appear. A muscle contracts only un- 

 der the stimulus of a nervous impulse. Hence 

 the nerve cells multiply and gather in groups, 

 ganglia and cords. The moving anirnal can 

 make good use of and requires sense organs for 

 its guidance, to perceive its food, to avoid dan- 

 ger. These react on the foremost ganglia of the 

 body. They form a brain. But a true brain 

 or head was attained only by descendants of 

 worms. A circulatory system will soon appear 

 to hurry food and oxygen to all the cells of the 

 body and to carry off their waste. To build 

 all these organs, and to unite them in one com- 

 pact and mobile body, was no easy problem. It 

 was attained slowly and only after many com- 

 parative failures. 



Worms lifted life to a plane far higher than 

 that of coelenterates. After their appearance 

 only muscular and seeing forms could hope to 

 play any leading part in the world. They de- 

 veloped weapons of offense and defense. Life 



28 



