ARTICLES 409 



where X x = mass of H 2 ; X* — mass of 2 ; X 3 = mass of 

 H 2 ; p = pressure ; T = temperature ; t = time. 



Among reactions ordinarily considered, the form of the 

 functions F is commonly given by the law of mass action. 



In the case of organic evolution, we have a system com- 

 prising a number of different species of organisms, and also 

 other components. We may denote the mass of these several 

 species and other components, S x , S?, . . . S n by Xi, X* . . . X n . 

 The state of the system may be thought of as defined by 

 statement of the values of these variables X ; and, further, 

 of certain parameters, Pi, P 2 . . . P f , which measure climatic 

 conditions, extension-in-area, etc., of the system ; lastly, of 

 certain other parameters, Q x , Qi . . , Q k , which define the 

 character of each species, this character being, in general, 

 variable with time as evolution proceeds. 



It is reasonable to make the very broad supposition that 

 the progress of redistribution of matter (evolution) in this 

 system takes place in accordance with a system of differential 

 equations : 



~~jf — Fi(Xi, Xi, . . . X„ ; Pi, Pi, . . . P t 1 ; Qi, Qi, . . . Q k ) 

 d lt = F ^ Xl > *■'■•■" X " ; Pl > P *' * ' * Pi ; Qlf ° 2 '--- Qk) 



— F n {Xi, X2, . . . X n ; Pi, P 2 , . . . P f ; Qi, Qi, ■ • • Qk) , 



(2) 



dt 



which is merely a more general form of the system (1). 



The role played in evolution by the changes in the para- 

 meters Q — i.e., the changes in the character of the evolving 

 species of organisms — is of such absorbing interest that in 

 common conception this alone has almost come to be regarded 

 as evolution. 



In a systematic treatment of the subject from the point of 

 view here set forth, however, it will be seen that this matter of 

 the change of the parameters Q forms only one special sub- 

 sidiary problem within the broader general problem. Further- 

 more, this subsidiary problem repeats, within its own borders, 

 the general character of the broader problem. 



This becomes apparent when we define the " character ' 

 of a species in a statistical way. Such definition takes the 

 following form : 



Let the total mass of the species S 4 be X { . 



Let pi, p2 . . . p r be parameters, measuring certain char- 

 acteristics {e.g., stature, mass, etc.) of individual units (organ- 

 isms) of the species. 

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