192 THE HAUNTS OF LIFE 



the great seaweeds might be gradually trans- 

 formed into terrestrial plants. Who knows? 



THE THREE GREAT INVASIONS OF THE 

 DRY LAND 



In the conquest of the dry land we can dis- 

 tinguish three great invasions or colonisations. 

 The first was the Worm-Invasion, led by sim- 

 ple worms such as the land-planarians, which 

 had begun the profitable habit of moving with 

 one end of the body always in front. In ma- 

 rine animals of comparatively low degree, 

 such as jelly-fishes and swimming-bells, sea- 

 anemones and corals, the symmetry of the 

 body is more or less radial, that is to say, there 

 is no right or left, no head- or tail-end. The 

 body can be cut into two almost identical 

 halves along many different planes. Radial 

 symmetry may be illustrated by an orange and 

 by the circular plate it rests on. It is well 

 suited for easy-going life, for drifting in the 

 sea, or for waiting for food to drop into the 

 mouth. But certain worms acquired bilateral 

 symmetry, moving with one end of the body 

 always in front. This was better suited for 

 quick and definite movements, such as are 



