Nos. 622-623] MIGRATION A FACTOR IN EVOLUTION 487 



contact between these two methods and to fuse them into 

 one cycle Bancroft's should also be quantitatively ex- 

 pressed (the interacting systems). The importance of 

 this is evident. I have not seen attention called to this 

 fact, or the intimate relation between these two laws. 



2. The number of components, phases, independent 

 variables, concentrations, etc., to which the phase rule 

 applies, is also one of the most marked features of bio- 

 logical problems in its dealing with the relation of ani- 

 mals to diverse environmental conditions and media. 



3. The analysis of biological problems into cycles of 

 action, systems, and agencies, is a necessary simplifica- 

 tion of the biological problems, and is preliminary to the 

 determination of the number of components, phases, and 

 concentrations which are involved in the application of 

 the phase rule to equilibria, and to Bancroft's law of 

 their development. Even in case of biological problems 

 which have not been reduced to quantity this model of 

 dynamic relations should be of much assistance in clari- 

 fying working plans, especially in associational studies. 



4. Improvements in the dynamic theory will probably 

 simplify its application to biology. The detailed non- 

 mathematical expression of these correlations will facili- 

 tate their wider use in biology, and it is also equally evi- 

 dent that with an adequate mathematical equipment the 

 biologist's application of these ideas would be greatly 

 facilitated. 



The phase rule has been so valuable chemically that a 

 special effort should be made to use it as much as possible 

 as a model in biological work. Mellor ( '04, pp. 183-184) 

 says: "Gibbs' phase rule is the best system extant for 

 the classification of equilibria— chemical and physical. 

 All changes, both physical changes of state and changes 

 of chemical composition, are found to depend upon the 

 same general laws." Henderson ('13, p. 260) remarks: 

 "There can be no doubt that, when feasible, the ideal 

 method— from the physico-chemical point of view— to de- 



