lotka: efficiency in organic evolution 361 



it depends on the properties or characteristics of the type of 

 organism. 



These properties may be divided into two classes, viz., those 

 which influence r through h, the birthrate per head, and those 

 which do so through d, the deathrate per head. 



The latter set of properties can be further subdivided into a 

 class which we may group together collectively as constituting 

 the means of passive resistance of the organism,^ as exemplified, 

 for instance, by such defensive structures as the shell of the 

 tortoise; and another class which we may similarly denote col- 

 lectively as the means of active opposition to circumstances un- 

 favorable to the life of the individual and the species. These 

 latter means comprise that system of sense and motor organs, 

 with a controlling and coordinating nerve-apparatus, which is 

 characteristic of animals, especially of the higher animals, and 

 notably of man. 



These facts are summarized in tabular form below: 



Index of Fitness 



r = (b - d) 



depends upon 



External conditions Characteristic prop- 



erties of organism 



Properties which Properties which 



determine b, the determine d, the 



birthrate deathrate 



Passive resistance Active opposition 



For the purposes of our present discussion we shall regard the 

 external conditions as given, and shall concern ourselves only 

 with the properties of the type of organism under consideration. 



' For details the reader must be referred to the previous paper cited. 



