176 THE FITNESS OF THE ENVIRONMENT 



except that in a measure potassium may be 

 replaced by rubidium and caesium, and chlo- 

 rine by bromine. Moreover the relative con- 

 centrations are of the highest importance. 

 Thus it has become clear that the remark- 

 able relative and absolute constancy of the 

 chemical composition of sea water is biolog- 

 ically far more important than formerly 

 could be surmised. This characteristic of 

 the ocean undoubtedly fits it for living 

 things as they exist. 



It is further to be noted that the salinity of 

 sea water is proportional to its osmotic pres- 

 sure. This important property also is there- 

 fore nearly constant. 



When a solution is inclosed in a membrane, 

 a bladder, for example, and the latter is im- 

 mersed in water, both water and dissolved 

 substance pass through the wall of the mem- 

 brane. Ordinarily, however, the water will 

 move much more rapidly than the dissolved 

 substance, hence the volume of the solution 

 will increase, and hydrostatic pressure will 

 be established. If a well-supported mem- 

 brane of cupric ferrocyanide be substituted 

 for the bladder, the process will be modified, 

 in that water alone, not the dissolved sub- 

 stance, can pass through the membrane, 

 which is accordingly termed semipermeable. 



