CHAPTER I 

 INTRODUCTION 



By henry B. ward 



Professor of Zoology in the University of Illinois 



On the surface of the globe, water and life are intimately asso- 

 ciated. As water grows scantier life becomes more restricted until 

 with the total failure of water life also disappears. In regions where 

 water is very scarce the few organisms that exist have learned to 

 store water or to discharge vital functions with a minimum supply 

 and thus to meet the natural defects of the situation. 



The hydrosphere, or the total water mass on the globe, forms the 

 subject of study for hydrography which is readily subdivided into 

 (i) oceanography, that deals with the vast continuous mass of 

 salt water in the ocean, and (2) limnology, which treats of the vari- 

 ous fresh-water units. The term limnology is sometimes re- 

 stricted in its application to the more stable bodies such as lakes 

 and ponds, in which case rheology is used to cover various types of 

 flowing waters. All fresh water is distributed over the surface of 

 the land and variably grouped into separate series of systems 

 connected with each other only through the ocean to which each 

 system is joined. The rare desert systems, such as terminate in 

 the Carson Sink or the Dead Sea, are exceptional in having no 

 present connection with the ocean. 



Fresh water is deposited on the land in the form chiefly of rain 

 or snow, and tends ultimately to reach the sea, though first and 

 last a considerable part is taken up by evaporation and goes back 

 directly into the atmosphere. Much of the precipitation soaks 

 into the ground to reappear elsewhere in springs or by seepage 

 to feed ponds and streams. Activity or rate of movement dis- 

 tinguishes two classes of water bodies: the flowing water of streams 

 and the temporarily quiet water of lakes. The latter almost 

 always form parts of stream systems and have thereby an inti- 

 mate connection with the ocean that is of fundamental importance 

 in determining the origin of fresh-water organisms. 



