74 



FIRST YEAR SCIENCE 



that substances which readily sink in the upper part of 

 the sea sink to the bottom no matter how deep the water 

 may be, as the bottom is so little denser than the top. 



The substances that are dissolved in water mix thor- 

 oughly together. In isolated bodies of water there are 

 often great differences in the amount and kinds of dis- 

 solved materials, but over the whole ocean from top to 

 bottom the composition of the water is practically uniform. 

 From previous experiments we have learned some of 

 the chief physical properties of water, so perhaps we can 

 understand the different effects that water and land have 

 had upon the development and activities of living things 

 upon the globe. Some water animals move about easily 



to get their food, but 

 others have it brought 

 to them in solution and 

 so obtain it without 

 muscular effort. The 

 air that they breathe 

 is in solution and they 

 cannot as easily obtain 

 a large quantity of it as 

 can the land animals. 

 Since the energy of all 

 animals depends upon 

 the amount of oxygen they use in their bodies, the water 

 animals are generally less energetic than the land ani- 

 mals. Since they also have such an easy time in moving 

 or floating about to get the things they need they have 

 not developed as high organisms as the land animals. 



Water is readily moved by the winds and becomes a 

 means of cutting down the land and carrying away its 

 material. When heated by the sun or any other source 

 of heat it evaporates and, rising into the air, floats away 



CORALS. 



These are fixed animals whose food is 



brought to them in solution by the 



ocean currents. 



