278 THE FOUR INSEPARABLE FACTORS OF EVOLUTION 



INTRODUCTION. 



How the Problem is Approached. 



The total assemblage of the conditions that precede the appearance of 

 " characters ' ' in living animals is the subject of this contribution. To continue 

 the paraphrase of John Stuart Mill's argument, we have no right to give the name 

 of cause to one of these conditions exclusive of the others. Mill's doctrine that 

 the cause is the "sum total of the conditions, positive and negative, taken to- 

 gether, the whole of the contingencies of every description, which being realized, 

 the consequent invariably follows," is the doctrine on which the Law of the 

 Four Inseparable causes or Factors of Evolution is founded. 



Out of the analysis of all the series of conditions which accompany evolution 

 it appears that these phenomena fall into four groups which center respectively 

 around the conditions that we term Environment, Ontogeny, Heredity, and 



Selection. 



In the causes and effects resident in these complexes of conditions are to be 

 discovered the origins of new characters and the transformations of existing 



characters. 



This contribution to biology is associated with the name of Joseph Leidy, 

 one of the most distinguished members of this Academy, through the fact that 

 he was one of the first to make known the extinct family of perissodactyl quad- 

 rupeds now popularly termed the Titanotheres. In attempting to monograph this 

 family from very abundant materials, the author of the present memoir found 

 that these materials presented a rare opportunity of contributing what appear 

 to be new data on the two chief phenomena of evolution above mentioned, 

 namely : the origins of new characters, the transformations of existing characters. 

 To understand these origins and transformations the author was led to the 

 above-mentioned analysis of all the evolutionary phenomena and conditions. 

 This analysis resulted in the conclusion that all these phenomena are comprised, 

 as indicated above, under four relations or complexes of causes and effects. 

 These relations are external and internal as follows : 



External : Environment = the sum of all external conditions. . , , 



Internal : Ontogeny = the sum of all conditions of individual, bodily, or somatic deveiop- 



Internal : Heredity 

 External: Selection 



ment. 



the sum of all germinal or blastic forces and conditions. 



the sum of all competitive relations between individuals in me 



struggle for existence. 



■ « • 



The true conception of individual animals or plants, of varieties, of species, 

 is that they are all complexes of the relations of these four sets of conditions, 

 which for the sake of simplicity may be called Factors, although the word factor 

 does not exactly express our meaning. A factor is defined as "one of severa 

 circumstances, elements, or influences which tend to the production of a given 



1 Osborn, H. F., The Ideas and Terms of Modern Philosophical Anatomy, Science, N. S., vo . 

 XXI, No. 547, June 23, 1905, pp. 959-961. Jour, of Philosophy, Psychology and Scientific Methods, v 

 II, No. 17, Aug. 17, 1905, pp. 455-458 (condensed). 



