Flotation 243 



being so small as the smallest now existing, or so large 

 as the largest. They grew and multiplied. They 

 differentiated into plants and animals, into large and 

 small, into free-swimming and sedentary. Some be- 

 took themselves to the free life of the open waters and 

 others to more settled habitations on shores. The 

 open- water forms were nomads, forever adrift in the 

 waves: the shoreward forms might find shelter and a 

 qiiiet resting place. 



LIFE IN OPEN WATER 



In the open water there are certain great advantages 

 that lie in minuteness and in buoyancy. These quali- 

 ties determine the ability of organisms to float freely 

 about in the more productive upper strata of water. 

 To descend into the depths is to perish for want of 

 light. So the members of many groups are adapted for 

 floating and drifting about near the surface. These 

 constitute the plancton. 



On the other hand, large size has its advantages when 

 coupled with good ability for swimming and food 

 gathering. In the rough world's strife the battle is 

 usually to the strong. It is the larger, wide-ranging, 

 free-swimming organisms that dominate the life of the 

 open water. These constitute the necton. 



Plancton and necton will be discussed in the next 

 chapter as ecological groups, but in this place we may 

 take note of the two very different sorts of fitness, that 

 they have severally developed for life in the open water, 

 the plancton organisms being fitted for flotation, and 

 the necton for swimming. 



Flotation — All living substance is somewhat heavier 

 than water (i. e. has a specific gravity greater than i) 

 and therefore tends to sink to the bottom. The veloc- 



