No. 624] MIGRATION A FACTOR IN EVOLUTION 57 



may look upon these three physical systems as a result 

 of existing temperature conditions of the earth. Increase 

 the temperature to a certain amount and the atmosphere 

 would be rarefied, the hydrosphere would disappear into 

 the atmosphere as water vapor, and the solid earth would 

 become fluid. Or, reverse the process, beginning with its 

 present state, and should the earth cool progressively, 

 the hydrosphere would become frozen to the solid phase 

 and atmosphere would likewise be transformed to 

 the solid state, and all these systems would become one. 

 The present resolved and differentiated state is thus de- 

 pendent on the present temperature conditions. The re- 

 lation of equilibria between these three systems is one of 

 the major problems for the application of the phase rule, 

 and their methods of interaction is an unlimited field for 

 the application of Bancroft's law, and both of these are 

 of the greatest importance to all concerned with the gross 

 environments of organisms. 



In view of the dominating influence of temperature, we 

 must not overlook the fact that temperature is only one 

 of the essential conditions of life. It is important to ob- 

 serve that the present stratum of the earth's surface 

 where organisms live is a remarkably narrow one, and 

 only moderate departure above or below the condition in 

 this stratum at once becomes limiting factors to organic 

 activity. Chamberlin ('06, pp. 1-2) states this impres- 

 sively as follows : 



riuioe on the 

 that affecting 

 ndred million 



The maintenance of such a dynamic system of equilib- 

 rium of the environment and of the organism, and the in- 

 ertia of their systems— the tendency to continue or per- 



