296 THE FOUR INSEPARABLE FACTORS OF EVOLUTION. 



"Romanes drew attention to the effects of cessation of selection, which involves the perpetuat' 

 of mediocrity. Weismann laid stress on the promiscuous interbreeding which results, and termed ^ 

 panmixia. The dwindling of vestigial organs was attributed by both authors in large degree t 

 panmixia with cessation of selection. Lankester and others contend that, in the absence of oth ° 

 factors, there could be no dwindling beyond the existing birth-mean of the species, and economy f 

 growth and germinal selection have been suggested as additional factors. Pearson claims to hav 

 demonstrated, mathematically, that ' panmixia without active reversal of natural selection does not 

 lead to degeneration' (on the basis of Galton's law, p. v., of ancestral heredity). Cf. Amixia." 



Soon after the enunciation of the doctrine of the continuity of the germ 

 plasm by Weismann there was extended discussion of all the phenomena of 

 selection. At this time, in the reaction from Lamarckism, the effects of environ- 

 ment and ontogeny were considered of subordinate importance and the efficiency 

 of heredity and of selection as the all-important factors of evolution was the 

 dominant biological teaching. 



II. INTERACTION OF THE FOUR FACTORS. 



Having considered the special action of each of the four factors we may now 

 take up the interactions. Each factor has its especial role in these interactions. 

 Thus ontogeny, or the soma, alone enjoys what may be called the experience of 

 environment and selection; through ontogeny alone heredity is tested; change of 

 habits, ontogeny, and consequent modification, in the experience of new environ- 

 ment, leads the way ultimately to changes in heredity. On the contrary, a 

 saltation or mutation in heredity, the sudden appearance of a new habit or of a 

 new function, will immediately be reflected in a new course of ontogenic changes 

 and perhaps in the choice of a new environment. 



What may be called the prevailing relations of the four factors in respect to 

 the origin and history of new characters and habits would appear to be as follows : 



Factors, 



Chief Action as to Characters. 



Environment Initiation of new conditions in ontogeny and heredity. 



Ontogeny Adjustment of heredity to environment, experiment with hereditary predis- 

 positions, ontogenic origins of characters. 



Heredity Genesis, continuous or discontinuous origins of new characters. 



Selection Fixation of characters and determination of the trend or direction from which 



new characters accumulate. 



I. Life. 



II. Conditions of Life. 



Including all characters new and old and Including the exciting, originating and 



the exciting and originating causes of many eliminating causes of many characters belonging 

 characters. ' • 



in I. 



Purely Internal Relations. Purely External External and Internal 



Relations. Relations. 



, * , , » , 



Heredity. Ontogeny. Environment. Selection. 



Entire life of the Entire life of the Entire surroundings of Struggle for exist- 



germ cell and germ body cells from the individual life. ence. Survival 01 



plasm, including fertilization to (A) organic, social or individual lite ana 



fertilization, germi- death. of kind. Animal multiplication oy 



nal variation, etc. and plant life not of progeny. 



kind. Nutritive. (B) 

 inorganic. 



These interactions may be very simply expressed as follows: 



* 



