Xos. G22-023] MIGRATION A FACTOR IN EVOLUTION 483 



the underlying relations that the stresses, exerted by 

 agents, will reinforce, overcome or balance one another, 

 and influence the end result of change. This is a quanti- 

 tative law. The inertia of the process of adjustment, and 

 the inertia of equilibria, should be recalled (cf. Newton's 

 first law) in this connection. The conception of inertia 

 appears to have been almost neglected in biology. 



2. Bancroft 's law, that systems tend to change to mini- 

 mize external disturbance, is a general law which appears 

 to apply to the interaction of all systems. This is a quali- 

 tative law, which should be of great practical value. 



3. The phase rule, according to Henderson ('13, pp. 

 257-258) is that the 



condition of equilibrium in any material system depends upon the num- 

 ber of its components, tbe number of its pbases, temperature, pressure. 



The quantitative character of this rule makes its appli- 

 cation one of great difficulty, but it will serve as a guide 

 or model for the organization of problems, and suggests 

 the form into which experimental data should be organ- 

 ized, and secured for testing its application and validity. 



The following extracts from Findlay ('04, pp. 8-18) 

 will assist in gaining some of the general ideas involved 

 in this subject: 



homogeneous, but marked off in space and separated from the other por- 



