THE NEW EVOLUTION 



up the living portion of the bodies of the plants and 

 animals. Since water, and especially sea water, is 

 nearly as heavy as protoplasm, it naturally follows 

 that plants and animals may live suspended in it main- 

 taining a position at any desired distance beneath the 

 surface with little effort on their part. A drop or two 

 of oil or a little gas or some other minor adaptation 

 is sufficient to enable small animals or plants to main- 

 tain hydrostatic equilibrium. This simple fact is of 

 immense importance. 



As in the case of plants on land, the plants living 

 in the sea require sunlight to enable them to build 

 up their tissues. All sea plants, therefore, are con- 

 fined to a thin surface layer of water of not more than 

 six hundred feet in depth, below which there is not 

 sufficient light to permit their growth. 



Within this thin illuminated surface layer of water 

 there are found along the shores attached firmly to the 

 bottom many different kinds of algas, commonly 

 known as sea-weeds, some of which are very large. 

 On muddy bottoms in quiet bays and estuaries the eel- 

 grass and some other types of flowering plants belong- 

 ing to the pond-weed family (Najadacex) often occur 

 in great abundance, rooted in the mud. But the sum 

 total of all the marine plants growing along the shores 

 in water of less than six hundred feet in depth yields 

 only a small fraction of the vegetable material which 

 is required to support the animals of the sea. 



How, then, are the sea animals supported? Most 

 of the vegetation in the sea is in the form of micro- 

 scopic plants which are quite invisible to the naked 



[6^] 



