CHAPTER II 

 CONDITIONS OF EXISTENCE 



By victor E. SHELFORD 



Assistant Professor of Zoology, University of Illinois. Biohsist Illinois State Laboratory 

 of Natural History 



Conditions of existence are of importance only in so far as they 

 affect the life and death processes of organisms. The present 

 knovdedge of such effects is far from complete and there is justifi- 

 cation for noting in detail only those conditions which observation 

 and experiment have shown to be important. Nevertheless if no 

 scientific observations or experiments had ever been made upon 

 organisms, water and its properties would occupy an important 

 place in a discussion of conditions of existence of aquatic life. 



Water possesses certain thermal properties and certain charac- 

 teristic relations to other substances which put it in a class quite 

 apart from the vast majority of chemical substances (Henderson). 

 The thermal properties of water are such as to make it a very fit 

 condition of existence for organisms. In raising the temperature 

 of water one degree centigrade, several times as much heat is ab- 

 sorbed as in the case of various other common substances, except 

 Kving matter itself. This property moderates both winter and 

 summer temperatures to which aquatic organisms are subjected 

 (Birge). Ice melts at fully a hundred degrees lower than the fus- 

 ing point of other common environmental substances and the latent 

 heat of melting ice is proportionately high. Thus in melting, ice 

 absorbs large quantities of heat and in freezing water gives off this 

 heat again. This further modifies the aquatic cHmate as compared 

 with one that might be afforded by some other substance. The 

 latent heat of evaporation of water is also relatively high and this 

 tends to prevent the evaporation of all the water from the surface 

 of the land. 



The expansion of water on freezing is one of its most important 



