180 THE FITNESS OF THE ENVIRONMENT 



— to say nothing of the establishment of 

 liquid currents within the body, — are all 

 related to osmotic pressure. The forces in- 

 volved in such processes are large, and osmotic 

 phenomena assume a special importance 

 wherever colloidal systems occur. It appears 

 to be certain that osmotic pressure is now se- 

 curely established as one of the fundamental 

 factors in the physico-chemical organization of 

 the living mechanism, and one of the constant 

 conditions, like concentration of the several 

 constituents, alkalinity, temperature, etc., 

 whose preservation is of vital importance. 



n 



THE CIRCULATION OF WATER 



There are a number of causes which bring 

 about ocean currents. In the tropics high 

 temperature causes a far greater evaporation 

 of water than can be offset by rainfall and the 

 flow of rivers ; near the poles this relation is 

 reversed. Hence water must steadily flow 

 from high to low latitudes, there to evaporate 

 and complete the cycle in the atmosphere and 

 on the land. In polar regions the cold water 

 sinks and penetrates along the bottom of the 

 sea in great deep currents to the tropics. 



