58 THE STRUGGLE FOR EXISTENCE 



out two principal conditions which must be realized in a mathematical 

 investigation of the struggle for existence in order to avoid serious 

 errors and the consequent disillusionment as to the very direction of 

 work. These conditions are: (1) The equations of the growth of 

 populations must be expressed in terms of the populations themselves, 

 i.e., in terms of the number of individuals, or rather of the biomass, 

 constituting a definite population. It must always be kept in mind 

 that even in such a science as physical chemistry it is only after the 

 course of chemical reactions has been quantitatively formulated in 

 terms of the reactions themselves, that the attempt has been made 

 to explain some of them on the ground of the kinetic theory of gases. 

 (2) The quantitative expression of the growth of population must go 

 hand in hand with a direct study of the factors which control growth. 

 Only in those cases, where the results deduced from equations are 

 confirmed by the data obtained through entirely different methods, by a 

 direct study of the factors limiting growth, can we be sure of the correct- 

 ness of the quantitative theories. 



