298 THE FOUR INSEPARABLE FACTORS OF EVOLUTION 



known 



Known uiugciicwo mw. Yet a survey of recent discussion among biologists as to the theory of evolution 

 shows broad lines of division into several schools of opinion strictly according to the factor from which 

 the subject has been approached. It is true that, conceiving any one of these principal factors as 

 separable, we become involved in endless difficulties; conceiving them as inseparable and continuously 

 interacting under natural conditions, we reach the only true conception of the evolution process 

 Of these four factors selection is the only one which can be experimentally removed through the agency 

 of man; heredity, ontogeny and environment may be modified but they can not be removed. 



"I shall not stop here to demonstrate, as I shall do elsewhere, that changes may be initiated or 



« „ n 4- nmn ..r +K™,ifrh onv nno <\f flipsp fmiT f actors : I shall state simDlv that under wrtoin «;,.„„.>. 



find a gateway through any one 01 inese iour lauiuio, a suau Diawouujjij mat uuuer certain circum- 

 stances heredity, under other circumstances ontogeny, under still others environment, or finally 

 under selection, a new order of adjustments begins in animals and plants and a new series of characters 

 appears. When such a new order sets in through any one of these factors a readjustment of all the 

 others sooner or later ensues." 



The second statement of the law, made before the Society of Zoologists at 

 the New Haven meeting, December, 1907, was an amplification of the first. 1 

 It was as follows : 



"The actual state of living nature is that of the inseparable and continuous action of these several 

 factors as expressed in the following most fundamental biological law: 



life and evolution of 



f these factors 



/« 



"Representing these processes respectively by the capital letters H, O, E, S, life and evolution 



may be represented by the formula: 



H X X E X S. 



"This formula roughly expresses the intimate nexus which exists between all these processes, 

 a nexus which is quite consistent with the fact that each has also its separate part in life and in evo- 

 lution. The multiplication sign, X, is to be interpreted in the active and passive sense of influencing 

 and influenced by. As examples of what is meant by this formula we may cite such principles as 

 the following: (1) that heredity is conditioned by ontogeny and environment; (2) that selection operates 

 on the product of heredity, ontogeny and environment; (3) that ontogeny initiates many changes 

 which are subsequently taken up by heredity; (4) that of the four processes involved in life and evo- 

 lution heredity is by far the most conservative and stable, among other reasons because it is embodied 

 in a form of matter (germ-plasm) least subject to changing external influences; that ontogeny, on 



the other hand, is the most unstable. . 



"In contrast to the graphic representations of the original extreme hypothesis of Weismann, in 

 which heredity is represented as a continuous current more or less isolated from ontogeny and environ- 

 ment there may be presented the following diagram. . .. 



"This diagram brings out the real cause of the difficulties which arise, as illustrated below {i-V, 

 when we attempt to determine at what point in the chain of processes a new character is set in motion, 

 in course of investigation of the initiation or origin of new characters." 



*s 



" Diagram illustrating the reciprocal influences of heredity, ontogeny, environment, and selection. 

 Arrows across the circle would represent these relations still more completely." 



* The Four Inseparable Factors of Evolution. Theory of their Distinct and Combi ^ ed J i( ^. 

 in the Transformation of the Titanotheres, an Extinct Family of Hoofed Animals in the Order r*er 

 dactyla. Science, N. S., vol. XXVII, No. 682, Jan. 24, 1908, pp. 148-150. 



