THE FOUR INSEPARABLE FACTORS OF EVOLUTION. 307 



APPENDIX. 



Note on the Quantitative Representation of the Factors of Evolution. 



By William K. Gregory. 



Let F represent the average condition of a given part in the ancestral species 

 (may refer to area, volume, degree of saturation of color, strength of muscle or 

 any characteristic capable of quantitative determination). 



Let F n represent the average condition of the same characteristic in a derived 

 species or descendant of F. 



Then F n — F is the measure of evolution of the part, and 10Q(F„-F)/F is the 

 percentage increment of F n over F. 



Hence F n = F + (P/100)F, where P/100 represents the total percentage 

 increment of F n over F. 



Suppose that this total percentage increment is made up of the following 

 partial increments: 



H (heredity), that part of the total percentage increment which may be 

 ascribed hypothetically to an orthogenetic or germinal tendency to increase, 

 diminish or maintain the original condition. 



S (selection), that part of the total percentage increment which may be 

 ascribed hypothetically to the selective effect of environment upon a germinally 

 unstable race. 



(ontogeny), all those parts of the total percentage increment which may be 

 ascribed hypothetically to metabolic changes arising in the soma, evoked either 

 by use and disuse or by physiological correlation during individual development. 



E (environment), that part of the total percentage increment which may be 

 ascribed hypothetically to the action of environment. (See page 301.) 



Substituting these symbols in the formula F n = F -f (P/100)F we have 



Fn = F +' H + s + ° + E 



whence 



100 



H + S + + E = 100 



iU 





In palaeontological researches one is frequently able to find the numerical 

 value of (F„ - F)/F and hence to obtain an exact measure of H + S + + E 

 taken together. 



