ARTICLES 417 



have spoken of the second law of thermodynamics broadly 

 as the law of evolution. 



In inorganic physical systems irreversible processes are 

 attended with a decrease in certain functions of the variables 

 defining the state of the system. Stable equilibrium is char- 

 acterised by the fact that these functions assume a minimum 

 value. 



In the case of organic systems we have not, in general, 

 any such definite criteria for irreversibility or for equilibrium. 



In the present contribution a broad -formulation of evolu- 

 tion, organic or otherwise, is presented in analytical form. 

 From this it is shown that, for certain cases, functions of the 

 variables X and the parameters P defining the state of the 

 system, and of the coefficients a defining its characteristic 

 properties, can be indicated, which have the property, in the 

 neighbourhood of stable equilibrium, of diminishing in the 

 (irreversible) process of the evolution of the system, and of 

 assuming a minimum when stable equilibrium is established. 



In these cases, therefore, it is possible to define in exact 

 terms the direction of evolution, whereas the descriptions 

 ordinarily given of this direction (passage from lower to higher, 

 from simpler to more complex forms, etc.) are vague or in- 

 accurate. 



By the way of example, the principles thus established 

 are applied to the system of equations given by Sir Ronald 

 Ross to represent the history (evolution) of a system com- 

 prising a human population, mosquitoes, and malaria para- 

 sites. 



